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Abstract

Regulation of metabolic fuel homeostasis is a critical function of β-cells, which are located in the islets of Langerhans of the animal pancreas. Impairment of this β-cell function is a hallmark of pancreatic β-cell failure and may lead to development of type 2 diabetes mellitus. β-Cells are essentially "fuel sensors" that monitor and react to elevated nutrient load by releasing insulin. This response involves metabolic activation and generation of metabolic coupling factors (MCFs) that relay the nutrient signal throughout the cell and induce insulin biosynthesis and secretion. Glucose is the most important insulin secretagogue as it is the primary fuel source in food. Glucose metabolism is central to generation of MCFs that lead to insulin release, most notably ATP. In addition, other classes of nutrients are able to augment insulin secretion and these include members of the lipid and amino acid family of nutrients. Therefore, it is important to investigate the interplay between glucose, lipid, and amino acid metabolism, as it is this mixed nutrient sensing that generate the MCFs required for insulin exocytosis. The mechanisms by which these nutrients are metabolized to generate MCFs, and how they impact on β-cell insulin release and function, are discussed in detail in this article.

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... Hay aproximadamente un millón de islotes en el páncreas humano y entre todos reciben el 10% del gasto cardíaco. (7) A esto se añade que como consecuencia de las fenestraciones de los capilares, los islotes reciben diez veces más sangre que las células exocrinas circundantes. Su contacto directo con los capilares les permite medir la concentración de nutrientes en el torrente sanguíneo y responder con la secreción de glucagón o de insulina. ...
... Su contacto directo con los capilares les permite medir la concentración de nutrientes en el torrente sanguíneo y responder con la secreción de glucagón o de insulina. (7,8) Se ha comprobado que ya a la edad de un año las células beta son funcionalmente maduras, aunque todavía no se ha alcanzado la cantidad total. Su respuesta a la estimulación es similar a la de las células beta del adulto, si bien secretan menor cantidad de insulina, en correspondencia con la menor masa del niño. ...
... (7) Los ácidos grasos de cadena larga en el citosol son responsables de la segunda fase de la secreción de insulina. Esto se debe a que con el aumento del potencial energético celular se inhibe la enzima isocítrico deshidrogenasa del ciclo de Krebs, el citrato es transportado hacia el citosol mediante el sistema de transporte de los ácidos tricarboxílicos y activa la enzima acetil-CoA carboxilasa, que sintetiza malonil-CoA.El malonil-CoA inhibe la enzima carnitina palmitil transferasa 1, lo que bloquea la entrada de ácidos grasos a la matriz mitocondrial e incrementa su concentración en el citosol. ...
Article
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Insulin is the main anabolic hormone; molecular mechanisms that regulate its synthesis and secretion were reviewed. The structural and functional features that allow the response of Langerhans islets β cells to glycemia changes, with the corresponding insulin secretion, were described. Secretagogues responsible for its secretion are mentioned and the sequence of glucose stimulated insulin secretion, which is the main one, is explained. Langerhans islets and β cells structural and functional features, that allow glycemia control, are described. The molecular mechanisms of the toxic effect on β cells, caused by prolonged hyperglycemia, are explained. Hypoglycemia triggered by some drugs and alcohol is explained.
... There are various etiologies of congenital hyperinsulinism, which include eleven monogenic forms as well as milder episodes as seen in infants with transient perinatal stress and prematurity [18]. Different amino acids have been known to induce positive as well as negative effects on beta cell insulin secretion [12]. ...
... Leucine is an amino acid whose well known insulinotropic effects are mediated by several different mechanisms, including the production of alpha ketoglutarate and allosteric activation of glutamate dehydrogenase (GDH) [12]. Patients with leucine sensitive hypoglycemia were described as early as in 1960, although the mechanism was only understood after several decades [11]. ...
... Given the common origin of liver and pancreatic cells, one could propose that inadvertent activation of stem cells could lead to hyperplasia of pancreatic islet cells in addition to hepatocytes, leading to hyperinsulinism ( Figure 1). This theory, however, could not explain [12] hyperinsulinism in transient tyrosinemia where toxic metabolites do not accumulate. We found one reported case series of three children with hyperinsulinemic hypoglycemia in the setting of HT1 that responded to diazoxide therapy [1]. ...
Article
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Background The spectrum of disorders associated with hyperinsulinemic hypoglycemia (HHI) has vastly increased over the past 20 years with identification of molecular, metabolic and cellular pathways involved in the regulation of insulin secretion and its actions. Hereditary tyrosinemia (HT1) is a rare metabolic disorder associated with accumulation of toxic metabolites of the tyrosine pathway due to a genetically mediated enzyme defect of fumarylacetoacetate hydrolase. Transient tyrosinemia of the newborn (TTN) is a benign condition with a maturational defect of the enzymes associated with tyrosine metabolism without any genetic abnormalities. Results We describe two rare cases of HHI, one in a patient with HT1 and for the first time, in a patient with TTN. Each of our patients presented in the neonatal period with persistent hypoglycemia that on biochemical evaluation was consistent with HHI. Each patient received diazoxide therapy for 3.5 months and 17 months of life, respectively and HHI resolved thereafter. Conclusion Despite the fact that HHI has been described in HT1 for several decades, no specific mechanism has been delineated. Although we considered the common embryonal origin of the liver and pancreas with the hepatotoxic effect in HT1 also impacting the latter, this was not a possible explanation for TTN. The commonality between our two patients is the accumulation of certain amino acids which are known to be insulinotropic. We therefore hypothesize that the excess of amino acids such as leucine, lysine, valine and isoleucine in our patients resulted in HHI, which was transient. Both patients responded to diazoxide. This novel presentation in TTN and the reassuring response in both HT1 and TTN to diazoxide will be useful to inform physicians about managing HHI in these patients. Further studies are required to delineate the mechanism of HHI in these infants.
... There are several excellent reviews of the current consensus of the mechanisms operative in the ␤-cell that couple oxidative metabolism to insulin secretion (220,255,369,420,452,460), so what is the purpose of the current contribution, particularly since it is written by an observer external to the field? The review is a response to the difficulties that this mitochondrial physiologist has encountered in trying to understand the ␤-cell literature in the same way as one would approach the liver, muscle, or adipose tissue. ...
... In spite of the elegant body of research on adenine nucleotide interactions with the ATPsensitive K ϩ channel (K ATP ), it is sometimes difficult to relate findings to the physiological context of the intact cell. It is generally agreed that the state of many of the coupling factor hypotheses is unsatisfactory, with uncertainties over mechanisms and targets (255,420). Finally, with notable exceptions, e.g., References 510 and 511, there appears to be limited cross-fertilization between electrophysiologists investigating ionic circuitry at the plasma membrane and biochemists focusing on the metabolism of the ␤-cell. ...
... A vast literature has been devoted to attempts to understand the mechanisms underlying this biphasic response, with the elaboration of theories requiring amplification by metabolic coupling factors (e.g., Refs. 218,255,322,420,548). The division of in vitro insulin secretion into two mechanistically distinct phases originated with the observation that glucose appeared to have additional effects on insulin secretion beyond K ATP closure (reviewed in Ref. 219). ...
Article
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The pancreatic β-cell secretes insulin in response to elevated plasma glucose. This review applies an external bioenergetic critique to the central processes of glucose-stimulated insulin secretion, including glycolytic and mitochondrial metabolism, the cytosolic adenine nucleotide pool, and its interaction with plasma membrane ion channels. The control mechanisms responsible for the unique responsiveness of the cell to glucose availability are discussed from bioen-ergetic and metabolic control standpoints. The concept of coupling factor facilitation of secretion is critiqued, and an attempt is made to unravel the bioenergetic basis of the oscillatory mechanisms controlling secretion. The need to consider the physiological constraints operating in the intact cell is emphasized throughout. The aim is to provide a coherent pathway through an extensive, complex, and sometimes bewildering literature, particularly for those unfamiliar with the field.
... Pancreatic b cells express high levels of mitochondrial nicotinamide adenine dinucleotide (NADH)/NADPH (redox) shuttles such as pyruvate/malate, pyruvate/citrate, malate/aspartate, and Gly-3-P shuttles to transport these molecules between cytosol and mitochondria. 9,11 Efficient oxidation of pyruvate in the mitochondria quickly generates NADH and flavin adenine dinucleotide (FADH 2 ), that indirectly stimulate ATP generation through the mitochondrial electron transport chain (ETC) and activity of ATP synthase. The enhanced ATP/ADP ratio induces plasma membrane depolarization by closure of b-cell K ATP channels, and subsequently the opening of L-type VDCC. ...
... The influx of extracellular Ca 2þ leads to insulin exocytosis from a readily releasable pool of insulin-containing vesicles. 4,8,9,11,13,14 ...
... Free fatty acids (FFAs) gain entry to the b cell by freely diffusing through the plasma membrane because of their hydrophobic nature. 9 It is noteworthy that the exposure time of b cells to, and the chemical structure of, FFAs result in different effects on b cells, and in some cases their roles are shifted from protection to toxicity or from secretagogue to secretion inhibitor. 22,23 ...
Chapter
Located in the islets of Langerhans of the pancreas, β cells perform essential hormone secretory functions that affect metabolism. Nutrients are potent regulators of β-cell insulin secretion, which is essential to the organism. In the past few decades, changes in lifestyle, especially related to overnutrition, has increased the global incidence of obesity-associated diabetes. In this scenario, impairment to β-cell function may lead to failure and gradual progression to type 2 diabetes. Multiple molecular pathways have been identified that contribute to dysfunction associated with glucotoxicity and lipotoxicity. Recent in vitro and in vivo studies have led to a better understanding of the complexities of β-cell dysfunction and how it interferes in the pathways of insulin secretion and action. New therapeutic strategies targeting the molecules sensitive and responsive to cell nutrition in the context of diabetes present new opportunities and are presented in the present chapter.
... The fuel secretagogues glucose, amino acids, and fatty acids enter the beta cell via glucose transporters, amino acid transporters, and by diffusion, respectively, and are subsequently metabolized in the cytosol and mitochondria. The metabolism of fuels seems to be the necessary condition for fuel-induced insulin secretion (FIIS) and yields a number of different intermediates and cofactors that mediate the stimulus-secretion coupling process and are collectively termed metabolic coupling factors (MCFs) [10][11][12]. Glucose is the principal fuel secretagogue and induces the so-called glucose stimulated insulin secretion (GSIS) also termed glucose induced insulin secretion (GIIS). GIIS consists of two principal pathways: a triggering and an amplifying pathway. ...
... The other two classes of fuels also generate MCFs that play a central role in the in vivo setting where mixed meals, rather than glucose alone, are sensed by the beta cell. Fatty acids are not sufficient to provide the triggering stimulus and this is especially important in the fasted state when fatty acids are metabolized via beta oxidation and intracellular lipid MCFs do not accumulate [10,11]. Postprandially, glucose inhibits beta oxidation (via malonyl-coenzyme A), provides glycerol triphosphate for esterification, and activates lipolysis, which together with free fatty acids provide MCFs for insulin secretion [10,11]. ...
... Fatty acids are not sufficient to provide the triggering stimulus and this is especially important in the fasted state when fatty acids are metabolized via beta oxidation and intracellular lipid MCFs do not accumulate [10,11]. Postprandially, glucose inhibits beta oxidation (via malonyl-coenzyme A), provides glycerol triphosphate for esterification, and activates lipolysis, which together with free fatty acids provide MCFs for insulin secretion [10,11]. Amino acids are able to induce insulin secretion, especially in certain combinations, and they also importantly augment GIIS. ...
Article
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Beta cells in the pancreatic islets of Langerhans are precise biological sensors for glucose and play a central role in balancing the organism between catabolic and anabolic needs. A hallmark of the beta cell response to glucose are oscillatory changes of membrane potential that are tightly coupled with oscillatory changes in intracellular calcium concentration which, in turn, elicit oscillations of insulin secretion. Both membrane potential and calcium changes spread from one beta cell to the other in a wave-like manner. In order to assess the properties of the abovementioned responses to physiological and pathological stimuli, the main challenge remains how to effectively measure membrane potential and calcium changes at the same time with high spatial and temporal resolution, and also in as OPEN ACCESS Sensors 2015, 15 27394 many cells as possible. To date, the most widespread approach has employed the electrophysiological patch-clamp method to monitor membrane potential changes. Inherently, this technique has many advantages, such as a direct contact with the cell and a high temporal resolution. However, it allows one to assess information from a single cell only. In some instances, this technique has been used in conjunction with CCD camera-based imaging, offering the opportunity to simultaneously monitor membrane potential and calcium changes, but not in the same cells and not with a reliable cellular or subcellular spatial resolution. Recently, a novel family of highly-sensitive membrane potential reporter dyes in combination with high temporal and spatial confocal calcium imaging allows for simultaneously detecting membrane potential and calcium changes in many cells at a time. Since the signals yielded from both types of reporter dyes are inherently noisy, we have developed complex methods of data denoising that permit for visualization and pixel-wise analysis of signals. Combining the experimental approach of high-resolution imaging with the advanced analysis of noisy data enables novel physiological insights and reassessment of current concepts in unprecedented detail.
... At the center of energy balance in mammals is insulin, a peptide hormone that maintains glucose homeostasis and is involved in lipid and protein metabolism (Wilcox, 2005;Keane and Newsholme, 2014). In the broadest terms, glucose enters the bloodstream, which triggers beta cells in the pancreas to secrete insulin, which in turn, promotes the uptake of surplus glucose by muscle or adipose tissues for storage as glycogen or lipids (Keane and Newsholme, 2014). ...
... At the center of energy balance in mammals is insulin, a peptide hormone that maintains glucose homeostasis and is involved in lipid and protein metabolism (Wilcox, 2005;Keane and Newsholme, 2014). In the broadest terms, glucose enters the bloodstream, which triggers beta cells in the pancreas to secrete insulin, which in turn, promotes the uptake of surplus glucose by muscle or adipose tissues for storage as glycogen or lipids (Keane and Newsholme, 2014). Insulin also inhibits lipolysis (Sears and Perry, 2015). ...
Article
Full-text available
Feast-fast cycles allow animals to live in seasonal environments by promoting fat storage when food is plentiful and lipolysis when food is scarce. Fat-storing hibernators have mastered this cycle over a circannual schedule, by undergoing extreme fattening to stockpile fuel for the ensuing hibernation season. Insulin is intrinsic to carbohydrate and lipid metabolism and is central to regulating feast-fast cycles in mammalian hibernators. Here, we examine glucose and insulin dynamics across the feast-fast cycle in fat-tailed dwarf lemurs, the only obligate hibernator among primates. Unlike cold-adapted hibernators, dwarf lemurs inhabit tropical forests in Madagascar and hibernate under various temperature conditions. Using the captive colony at the Duke Lemur Center, we determined fasting glucose and insulin, and glucose tolerance, in dwarf lemurs across seasons. During the lean season, we maintained dwarf lemurs under stable warm, stable cold, or fluctuating ambient temperatures that variably included food provisioning or deprivation. Overall, we find that dwarf lemurs can show signatures of reversible, lean-season insulin resistance. During the fattening season prior to hibernation, dwarf lemurs had low glucose, insulin, and HOMA-IR despite consuming high-sugar diets. In the active season after hibernation, glucose, insulin, HOMA-IR, and glucose tolerance all increased, highlighting the metabolic processes at play during periods of weight gain versus weight loss. During the lean season, glucose remained low, but insulin and HOMA-IR increased, particularly in animals kept under warm conditions with daily food. Moreover, these lemurs had the greatest glucose intolerance in our study and had average HOMA-IR values consistent with insulin resistance (5.49), while those without food under cold (1.95) or fluctuating (1.17) temperatures did not. Remarkably low insulin in dwarf lemurs under fluctuating temperatures raises new questions about lipid metabolism when animals can passively warm and cool rather than undergo sporadic arousals. Our results underscore that seasonal changes in insulin and glucose tolerance are likely hallmarks of hibernating mammals. Because dwarf lemurs can hibernate under a range of conditions in captivity, they are an emerging model for primate metabolic flexibility with implications for human health.
... Previous work in humans Keane and Newsholme, 2014), and in dairy cows have reported a negative association between fatty acids concentration and pancreatic insulin secretion. In the present study, differences in insulin secretion could not be explained by differences in fatty acids concentration because basal fatty acids concentration was comparable between HY and LY goats on the day of IVGTT, ITT, and AST. ...
... In the present study, differences in insulin secretion could not be explained by differences in fatty acids concentration because basal fatty acids concentration was comparable between HY and LY goats on the day of IVGTT, ITT, and AST. However, Keane and Newsholme (2014) stated that prolonged exposure to elevated concentrations of fatty acids might chronically decrease glucose-stimulated insulin secretion in rats and human pancreatic islets. Considering that HY goats had greater plasma fatty acids concentration during the weeks preceding the metabolic challenges, the lesser BCF and AIR in HY goats might reflect long-term effects of increased fatty acids concentration on pancreatic insulin secretory capacity. ...
Thesis
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In dairy animals, the transition period, which spans from 3 weeks before to 3 weeks after parturition, is the most stressful time in their productive lives. This period is characterized by drastic physiological, metabolic, and endocrine adaptations to accommodate parturition and lactogenesis. Goats unable to adapt to this challenging time are more susceptible to infections and metabolic diseases, which might have a substantial impact on maternal health and productive efficiency beyond the transition period. An in-depth understanding of the biology of the transition period is essential for developing optimized strategies that could enhance milk yield without compromising herd health and welfare. While there is ample published information regarding the transition period in dairy cattle, to date, the endocrine and metabolic status of periparturient dairy goats has only been vaguely described in the literature. Therefore, the overall goal of this dissertation was to expand on previous knowledge of hormonal and metabolic regulation of energy metabolism during the transition period and to explore factors that might aggravate the metabolic burden of pregnancy and lactation in periparturient dairy goats. Thus, a series of studies were conducted in a large commercial goat dairy farm in Australia to 1) determine the effects of month of kidding, parity number, and litter size on lactation curves of dairy goats raised in intensive systems; 2) characterize temporal variations in circulating levels of selected hormones and metabolites involved in energy balance regulation during the transition period; 3) investigate the effects of level of milk production, parity number and litter size on maternal metabolic profile; 4) determine whether higher plasma concentrations of markers of negative energy balance are associated with inferior productive performance; and 5) determine whether differential productivity is related to differences in nutrient partitioning between high- and low-yielding goats. In the first part of this study, an analysis of the production data revealed that goats kidding in spring, in third/fourth parity, or carrying multiple fetuses produce more milk than their counterparts. Interestingly, although the month of kidding had the most significant impact on the shape of lactation curves, the magnitude of such impact increased with increasing parity number. Also, based on the concentration of key biomarkers of energy metabolism analyzed during this time, it was possible to conclude that nutritional deficit was increased with increasing milk yield, parity, and litter size (listed in order of importance) and that both pregnancy and lactation were less able to elicit lipomobilization in primiparous compared with multiparous goats. Further, the likelihood of early removal from the milking herd was significantly increased in goats with elevated blood levels of beta-hydroxybutyrate (BHB). On the other hand, contrasting studies in dairy cows, a positive association was observed between blood levels of non-esterified fatty acids (NEFA) and milk yield. Nevertheless, it was unclear what role, if any, the endocrine system played in the differential productivity in early lactation observed between high- and low-yielding goats. Therefore, in the second part of this study, goats of high and low milk yield were subjected to 3 metabolic challenges (glucose, insulin, and adrenocorticotropin hormone infusions) to determine if differential productivity is related to differences in some aspects of the regulation of nutrient partitioning in dairy goats. The results suggested that differences in milk yield, and overall production efficiency in early lactation, are primarily due to differences in insulin secretion and clearance rates rather than related to differences in peripheral tissue responsiveness to the effects of catabolic and anabolic hormones. In summary, the research within this thesis provides the first comprehensive overview of both lactation performance and the metabolic status of Australian dairy goats. Collectively, the novel findings presented here contribute to further the current understanding of various aspects of the regulation of energy metabolism in periparturient dairy goats. Just as important, this study also provides the local industry with robust and relevant information on the effects of several factors on the productive and metabolic responses of dairy goats during the transition period. Such information can assist with the optimization of farming practices and breeding plans, thereby accelerating increments in the national herd productivity.
... The hormone insulin is synthesized and secreted by pancreatic beta cells [1,2]. The commonly accepted principle is that insulin is secreted in a basal/bolus pattern, with the latter predominately released upon rising blood glucose (most likely from a meal) stimulus [3,4]. The mechanism by which bolus insulin is secreted from the pancreas is well established [5]; however, little is known about the mechanisms by which basal insulin is secreted. ...
... These enter the mitochondria to undergo a series of redox reactions to yield adenosine triphosphate (ATP) via the electron transport chain (ETC) oxidative phosphorylation (OxPhos) machinery coupled to ATPase. The ETC-OxPhos complexes are situated on the cristae of the inner mitochondrial membrane (IMM) [3]. Beta cells express low levels of lactate dehydrogenase, indicating a "preference" to fully oxidise glucose via OxPhos generating maximal ATP (~36 ATP/glucose molecule), as opposed to via the fermentation pathway (~2 ATP/glucose molecule) [18,19]. ...
Article
Full-text available
Unlike bolus insulin secretion mechanisms, basal insulin secretion is poorly understood. It is essential to elucidate these mechanisms in non-hyperinsulinaemia healthy persons. This establishes a baseline for investigation into pathologies where these processes are dysregulated, such as in type 2 diabetes (T2DM), cardiovascular disease (CVD), certain cancers and dementias. Chronic hyperinsulinaemia enforces glucose fueling, depleting the NAD+ dependent antioxidant activity that increases mitochondrial reactive oxygen species (mtROS). Consequently, beta-cell mitochondria increase uncoupling protein expression, which decreases the mitochondrial ATP surge generation capacity, impairing bolus mediated insulin exocytosis. Excessive ROS increases the Drp1:Mfn2 ratio, increasing mitochondrial fission, which increases mtROS; endoplasmic reticulum-stress and impaired calcium homeostasis ensues. Healthy individuals in habitual ketosis have significantly lower glucagon and insulin levels than T2DM individuals. As beta-hydroxybutyrate rises, hepatic gluconeogenesis and glycogenolysis supply extra-hepatic glucose needs, and osteocalcin synthesis/release increases. We propose insulin’s primary role is regulating beta-hydroxybutyrate synthesis, while the role of bone regulates glucose uptake sensitivity via osteocalcin. Osteocalcin regulates the alpha-cell glucagon secretory profile via glucagon-like peptide-1 and serotonin, and beta-hydroxybutyrate synthesis via regulating basal insulin levels. Establishing metabolic phenotypes aids in resolving basal insulin secretion regulation, enabling elucidation of the pathological changes that occur and progress into chronic diseases associated with ageing.
... Previous work in humans (Wilcox, 2005;Keane and Newsholme, 2014) and in dairy cows (De Koster and Opsomer, 2013;Cincović et al., 2018) have reported a negative association between fatty acids concentration and pancreatic insulin secretion. In the present study, differences in insulin secretion could not be explained by differences in fatty acids concentration because basal fatty acids concentration was comparable between HY and LY goats on the day of IVGTT, ITT, and AST. ...
... In the present study, differences in insulin secretion could not be explained by differences in fatty acids concentration because basal fatty acids concentration was comparable between HY and LY goats on the day of IVGTT, ITT, and AST. However, Keane and Newsholme (2014) stated that Zamuner et al.: METABOLIC DIFFERENCES AND MILK YIELD prolonged exposure to elevated concentrations of fatty acids might chronically decrease glucose-stimulated insulin secretion in rats and human pancreatic islets. Considering that HY goats had greater plasma fatty acids concentration during the weeks preceding the metabolic challenges, the lesser BCF and AIR in HY goats might reflect long-term effects of increased fatty acids concentration on pancreatic insulin secretory capacity. ...
Article
Full-text available
This experiment aimed to examine endocrine and metabolic responses to glucose, insulin, and adrenocorticotropin (ACTH) infusions in early-lactation dairy goats of different levels of milk production (LMP). Goats were grouped as either high (HY; 4.0 L/d, n = 13) or low milk yield (LY; 2.4 L/d, n = 13). Individual milk yield (L/d) and dry matter intake (DMI; kg/d) were measured daily. Concentration (mM) of glucose, fatty acids, and β-hydroxybutyrate, percent of milk fat and protein, body weight (BW; kg), and body condition score (BCS) were assessed weekly (from 2-6 wk postpartum). An intravenous glucose tolerance test (IVGTT), an insulin tolerance test (ITT), and an ACTH stimulation test were carried out at 43, 44, and 45 ± 0.7 d in milk, respectively. The HY goats had greater milk yield (+67%), energy-corrected milk (ECM; +70%), DMI (+28%), ratio of ECM output to metabolic BW (+67%), and feed efficiency (+25%), but lesser BCS than LY goats (2.4 vs. 2.6). The DMI (% of BW) was moderately correlated with ECM (r = 0.70) and negatively correlated with BCS (r = -0.57). At the time of the IVGTT, HY goats had lesser basal insulin and glucose than LY goats. However, results from IVGTT and ITT indicate that the sensitivity of peripheral tissues to insulin was unaffected by LMP. Compared with LY, HY goats had lesser insulin secretion (-52%) and greater insulin clearance rate (+47%) after glucose infusion. The ITT and ACTH stimulation test results show that both the growth hormone response to insulin and the cortisol response to ACTH were unaffected by LMP. Also, basal plasma concentrations of GH and cortisol were not correlated with glucose and fatty acids concentrations or any performance traits. Collectively, our results suggest that differences between HY and LY goats, concerning milk yield and feed efficiency, were probably more closely related to differences in insulin secretion and clearance than to differences in peripheral tissue responsiveness to the effects of catabolic and anabolic hormones.
... High concentrations of FAA in HM is a distinctive trait present in humans. Free AA in HM are readily available and have potential effects in cells and tissues through specific receptors [19][20][21]. A recent report that studied the association between FAA in HM, total AA (free and bound amino acids, TAAs), and total protein levels with infant gender showed that breast-milk intended for infant females had higher protein and TAAs content during the first 3 months of lactation [14]. ...
... It is possible that BCAA have positive effects on muscle protein anabolism [26] and contribute to fast growth of infants; BCAA stimulate the mTORC1 intracellular pathway associated with skeletal muscle protein synthesis [27]. Also, Leu, Gln, and Ala can favor anabolism through insulin release and growth functions improving infant's growth [19]. These observations suggest that higher concentration of free BCAA are present in HM of infants with faster growth. ...
Article
Full-text available
Background: There is a growing interest regarding the physiological role of free amino acids (FAA) present in human milk (HM). Recent studies show FAA in HM could be influenced by infants' gender and could have an important role in their growth and development. We studied the concentrations of FAA in HM and potential associations with infants' gender and their patterns of growth in a cohort of Ecuadorian women. Methods: Human milk samples were collected after approximately eight hours of overnight fast within one week (colostrum), 2 weeks (transition milk), and 2 and/or 4 months (mature milk) after parturition. Free AA were determined by cation-exchange chromatography separation. Results: We observed significantly higher concentrations of Glu 14.40 (1.35, 27.44), Gly 1.82 (0.24, 3.4), Cys 0.36 (0.03, 0.68), and Tyr 0.24 (0.02, 0.46) in HM intended for boys. Free Glu, Gly, Cys, and Tyr concentrations increased with time of lactation. In addition, there were higher concentrations of Glu 28.62 (1.78, 55.46) and Ala 7.16 (1.26, 13.06) in HM for children that presented faster weight gain than for those with slower gain. Conclusions: The present results showed that there are differences in FAA levels in HM intended for male and fast-growing children.
... Impairment of ␤-cell function is a hallmark of the worldwide epidemic, diabetes mellitus (DM) (Keane and Newsholme, 2014;Rutter et al., 2015). Insulin resistance (IR) occurs in metabolic syndrome (MetS) and obesity (Koleva et al., 2013;Merino et al., 2015) which is a major risk factor for type 2 diabetes mellitus (T2DM) (Li et al., 2013). ...
... ␤-Cell senses glucose, amino acids and lipids which act as insulin secretagogue (Keane and Newsholme, 2014). The metabolic cycle of free fatty acids (FFA) in the form of lipolysis and lipogenesis plays a crucial role in insulin secretion (Prentki and Madiraju, 2012). ...
Article
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Abstract Hydroxychloroquine (HCQ) has been demonstrated to reduce the risk to develop diabetes mellitus (DM). However no previous experimental study had investigated its effect on the structure of the endocrine pancreas, islets of Langerhans (IOL), in insulin resistance (IR). In addition, the mechanism by which HCQ can prevent DM is not well understood. In this study, we hypothesized that the possible favorable outcome of HCQ may be partly achieved by its molecular effect on the endothelial stress markers as well as on the imparied balance of the adipokines that usually accompanies IR. A total of 54 rats were divided equally into; control, high fat diet (HFD) and HFD + HCQ groups (received standard chow, HFD and HFD + HCQ respectively). After 12 weeks, samples from pancreas as well as visceral adipose tissue (VAT) were histologically studied for the consequent changes. In the HFD group, there were mild degenerative changes and expansion of the IOL accompanied with a significantly increased (p < 0.05) β-cell area%, mass, proliferation and neogenesis as well as a significantly decreased (p < 0.05) α-cell area% compared with the other groups. On combining HCQ with HFD, reversal of these changes along with correction of the impaired adipokines levels (leptin, adiponectin, resistin, visfatin and lipocalin-2) and significant decrease (p < 0.05) of the vascular endothelial stress markers (sE-selectin, sICAM and sVICAM) were manifested compared with the HFD group. Therefore, HCQ favorable effects in IR may be attributed to relieving of the endothelial stress as well as normalization of the skewed balance of adipokines.
... Many of these agents act by binding to G protein-coupled receptors (GPCRs), and the activation of phospholipase C (PLC)/protein kinase C (PKC) or adenylate cyclase (AC)/protein kinase A (PKA) pathways which rely on Ca 2+ for signaling (27)(28)(29)(30)). An extended model ( Figure 1B) recognizes both the role of other agents in modulating insulin secretion, and the critically important role of Ca 2+ in other organelles. ...
Article
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Early in the development of Type 2 diabetes (T2D), metabolic stress brought on by insulin resistance and nutrient overload causes β-cell hyperstimulation. Herein we summarize recent studies that have explored the premise that an increase in the intracellular Ca²⁺ concentration ([Ca²⁺]i), brought on by persistent metabolic stimulation of β-cells, causes β-cell dysfunction and failure by adversely affecting β-cell function, structure, and identity. This mini-review builds on several recent reviews that also describe how excess [Ca²⁺]i impairs β-cell function.
... La despolarización celular resultante permite que entre Ca2+, lo cual desencadena exocitosis de gránulos que contienen insulina. Además, parte importante de la secreción de insulina en respuesta a la ingesta oral puede atribuirse a hormonas entéricas (5,6). ...
Article
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La diabetes mellitus tipo 2 constituye una condición clínica debilitante, degenerativa y multifacética de alta prevalencia a nivel mundial. Dada la complejidad de su fisiopatología y las variadas opciones terapéuticas que existen esta enfermedad presenta un desafío para el médico general, se hace imperativo describir comprensiblemente esta patología para mejorar la resolutividad de ésta en atención primaria. Tras una búsqueda bibliográfica exhaustiva de 103 estudios publicados hasta el año 2010, se identificaron los aspectos más importantes tanto de la fisiología, fisiopatología, complicaciones y terapéuticas de esta patología. La resistencia a la insulina (RI) es una condición metabólica central en la etiopatogenia de esta patología donde se logra reconocer de manera clásica tanto la pérdida de la acción periférica de la insulina por parte de los diferentes tejidos, así como defectos en la secreción de insulina conllevando estados de hiperglucemia constantes asociados tanto a complicaciones agudas como crónicas caracterizadas por provocar disfunción y fallo en diferentes órganos. Es de conocimiento general que parte importante de los resultados en el manejo de esta patología se logran con cambios en el estilo de vida que van desde modificaciones en la dieta a cambios en el patrón de actividad física con pérdida de peso corporal. No obstante, existe a su vez una amplia gama de terapias farmacológicas orientadas a controlar estados hiperglucémicos ante la falla de la terapia no farmacológica. Dentro de este mismo contexto varias son las dianas y objetivos terapéuticos en el tratamiento del diabético tipo 2, sin embargo, todas confluyen en el control metabólico de los estados de hiperglucemia y la prevención de sus complicaciones.
... dry base (Wangrimen, 2019). Protein and amino acids stimulate insulin secretion (Keane & Newsholme, 2014). Insulin is a hormone produced by pancreatic cells to lower blood glucose levels (Tortora & Derrickson, 2016). ...
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Cookies are food products that are often a consumption. High glycemic index (GI) cookies consumption causes increased diabetes mellitus (DM) cases. In addition, high GI cookies can improve obesity. DM and obesity lead to other complications and degenerative diseases. The low GI value cookies need to produce for diabetes mellitus prevention. This research is a preliminary study to produce raw material flour for low GI cookies. Heat moisture treatment (HMT) was applied to modify the Canna flour. Canna flour increased the moisture content to 34.39% wet base (w.b.), then dried at 92°C for 20.6 h. HMT process increased the resistance starch, soluble and insoluble fiber. Treated Ganyong flour has 2.16 % dry base (d.b.) soluble fiber, 30.19 % d.b., and 28.03% d.b. The result showed that HMT increased total dietary and soluble fiber. Those parameters are essential to low GI cookies for diabetes mellitus prevention. HMT also increased the whiteness index of Ganyong flour.
... The pancreatic islets are a heterogeneous mixture of endocrine cells and non-endocrine support cells that maintain homeostatic blood glucose levels via balanced hormone secretion. The beta cells make up (50-75%) of the islet cell mass in humans, and 60-80% in mice [ Figure 1] [3][4][5] , and are the sole source of insulin in the body 6 . Insulin release, triggered by increased blood glucose 7,8 , lowers glycaemia through the net effect of decreased glycogenolysis and gluconeogenesis at the liver and skeletal muscle and increased uptake of glucose in the liver, skeletal muscle, and adipose tissue 9,10 [ Figure 2]. ...
Article
The pancreatic islet is a complex mini organ composed of a variety of endocrine cells and their support cells, which together tightly control blood glucose homeostasis. Changes in glucose concentration are commonly regarded as the chief signal controlling insulin-secreting beta cells, glucagon-secreting alpha cells and somatostatin-secreting delta cells. However, each of these cell types is highly responsive to a multitude of endocrine, paracrine, nutritional and neural inputs, which collectively shape the final endocrine output of the islet. Here, we review the principal inputs for each islet-cell type and the physiological circumstances in which these signals arise, through the prism of the insights generated by the transcriptomes of each of the major endocrine-cell types. A comprehensive integration of the factors that influence blood glucose homeostasis is essential to successfully improve therapeutic strategies for better diabetes management. Pancreatic islets are heterogeneous clusters of endocrine cells responsible for glucose homeostasis. Here Noguchi and Huising review the main stimuli for each islet-cell type and their response, guided by insights from islet-cell transcriptomes.
... Insulin decreases blood glucose levels. However, long-term high blood glucose levels impair insulin secretion by pancreatic b-cells and a loss of cell mass and insulin biosynthesis capacity can occur ( Keane and Newsholme 2014;Mohan et al. 2015;Rutter et al. 2015). Similarly, sustained hyperglycemia can lead to dysregulated glucagon secretion by pancreatic a-cells and elevated glucagon concentration ( ). ...
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Type 2 diabetes mellitus (T2DM) is the most prevalent disease and becoming a serious public health threat worldwide. It is a severe endocrine metabolic disorder that has the ability to induce serious complications in all kinds of organs. Although mechanisms of anti-diabetics have been described before, we focus here on the cellular and physiological mechanisms involved in the modulation of insulin and glucose blood levels. As obesity and inflammation are intimately associated with the development of T2DM, their possible relationships are also described. The effects of gut microbiota on insulin resistance have been recently investigated in clinical trials, and we discuss the potential mechanisms by which gut microbiota may improve glucose handling, especially via the metabolism of ingested phytochemicals. Among the historically supported effects of phytochemicals, their therapeutic potential for T2DM leads to consider these natural products as an important pool for the identification of novel anti-diabetic drug leads. This current research extends the descriptions of anti-diabetic effects of plants that are used in traditional medicines or as nutraceuticals. The objective of the present review is to make a systematic report on glucose metabolism in T2DM as well as to explore the relationships between natural phytochemicals and glucose handling.
... Even though impaired GSIS caused by long-term exposure of kisspeptin to β-cells were clearly demonstrated via transgenic mice 10 , further study of kisspeptin-reduced insulin secretion is needed to illustrate the precise mechanisms at work. The amount of insulin secretion is determined by strict control steps including biosynthesis and degradation 31,32 . Namely, abnormalities in insulin biosynthesis or degradation result in insulin secretion dysfunction due to unbalanced insulin storage. ...
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Previous studies have demonstrated the important role of kisspeptin in impaired glucose-stimulated insulin secretion (GSIS). In addition, it was reported that the activation of autophagy in pancreatic β-cells decreases insulin secretion by selectively degrading insulin granules. However, it is currently unknown whether kisspeptin suppresses GSIS in β-cells by activating autophagy. To investigate the involvement of autophagy in kisspeptin–regulated insulin secretion, we overexpressed Kiss1 in NIT-1 cells to mimic the long-term exposure of pancreatic β-cells to kisspeptin during type 2 diabetes (T2D). Interestingly, our data showed that although kisspeptin potently decreases the intracellular proinsulin and insulin ((pro)insulin) content and insulin secretion of NIT-1 cells, autophagy inhibition using bafilomycin A1 and Atg5 siRNAs only rescues basal insulin secretion, not kisspeptin-impaired GSIS. We also generated a novel in vivo model to investigate the long-term exposure of kisspeptin by osmotic pump. The in vivo data demonstrated that kisspeptin lowers GSIS and (pro)insulin levels and also activated pancreatic autophagy in mice. Collectively, our data demonstrated that kisspeptin suppresses both GSIS and non-glucose-stimulated insulin secretion of pancreatic β-cells, but only non-glucose-stimulated insulin secretion depends on activated autophagic degradation of (pro)insulin. Our study provides novel insights for the development of impaired insulin secretion during T2D progression.
... CB1R ablation in beta cells leads to a metabolic shift and reduces ROS production CB1R impacts mitochondrial metabolism in muscle and neurons [32,33], and ATP generation via glucose metabolism is required for insulin secretion [34]. We analysed mitochondrial metabolism in β-CB1R −/− islets by measuring the OCR and ECAR (Fig. 6a-c). ...
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Aims/hypothesis: The cannabinoid 1 receptor (CB1R) regulates insulin sensitivity and glucose metabolism in peripheral tissues. CB1R is expressed on pancreatic beta cells and is coupled to the G protein Gαi, suggesting a negative regulation of endogenous signalling in the beta cell. Deciphering the exact function of CB1R in beta cells has been confounded by the expression of this receptor on multiple tissues involved in regulating metabolism. Thus, in models of global genetic or pharmacological CB1R blockade, it is difficult to distinguish the indirect effects of improved insulin sensitivity in peripheral tissues from the direct effects of inhibiting CB1R in beta cells per se. To assess the direct contribution of beta cell CB1R to metabolism, we designed a mouse model that allows us to determine the role of CB1R specifically in beta cells in the context of whole-body metabolism. Methods: We generated a beta cell specific Cnr1 (CB1R) knockout mouse (β-CB1R-/-) to study the long-term consequences of CB1R ablation on beta cell function in adult mice. We measured beta cell function, proliferation and viability in these mice in response to a high-fat/high-sugar diet and induction of acute insulin resistance with the insulin receptor antagonist S961. Results: β-CB1R-/-mice had increased fasting (153 ± 23% increase at 10 weeks of age) and stimulated insulin secretion and increased intra-islet cAMP levels (217 ± 33% increase at 10 weeks of age), resulting in primary hyperinsulinaemia, as well as increased beta cell viability, proliferation and islet area (1.9-fold increase at 10 weeks of age). Hyperinsulinaemia led to insulin resistance, which was aggravated by a high-fat/high-sugar diet and weight gain, although beta cells maintained their insulin secretory capacity in response to glucose. Strikingly, islets from β-CB1R-/-mice were protected from diet-induced inflammation. Mechanistically, we show that this is a consequence of curtailment of oxidative stress and reduced activation of the NLRP3 inflammasome in beta cells. Conclusions/interpretation: Our data demonstrate CB1R to be a negative regulator of beta cell function and a mediator of islet inflammation under conditions of metabolic stress. Our findings point to beta cell CB1R as a therapeutic target, and broaden its potential to include anti-inflammatory effects in both major forms of diabetes. Data availability: Microarray data have been deposited at GEO (GSE102027).
... However, because FAM3A-induced Akt activation is partially repressed by the P2 receptor inhibition but completely blocked by CaM inhibition, it is likely that other mechanism is also involved in FAM3Amediated Akt phosphorylation in addition to P2 receptor signaling transduction. In pancreatic islet β cells, an increase in intracellular ATP levels can close ATP-sensitive potassium (K ATP ) channels to open L-type calcium channels, resulting in an influx of extracellular calcium [38][39][40][41]. Several lines of evidences have suggested that the opening of L-type calcium channels is also affected by K ATP channel in hepatocytes [42][43][44]. ...
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Non-alcoholic fatty liver disease (NAFLD) and diabetes are severe public health issues worldwide. The Family with sequence similarity 3 (FAM3) gene family consists of four members designated as FAM3A, FAM3B, FAM3C and FAM3D, respectively. Recently, there had been increasing evidence that FAM3A, FAM3B and FAM3C are important regulators of glucose and lipid metabolism. FAM3A expression is reduced in the livers of diabetic rodents and NAFLD patients. Hepatic FAM3A restoration activates ATP-P2 receptor-Akt and AMPK pathways to attenuate steatosis and hyperglycemia in obese diabetic mice. FAM3C expression is also reduced in the liver under diabetic condition. FAM3C is a new hepatokine that activates HSF1-CaM-Akt pathway and represses mTOR-SREBP1-FAS pathway to suppress hepatic gluconeogenesis and lipogenesis. In contrast, hepatic expression of FAM3B, also called PANDER, is increased under obese state. FAM3B promotes hepatic lipogenesis and gluconeogenesis by repressing Akt and AMPK activities, and activating lipogenic pathway. Under obese state, the imbalance among hepatic FAM3A, FAM3B and FAM3C signaling networks plays important roles in the pathogenesis of NAFLD and type 2 diabetes. This review briefly discussed the latest research progress on the roles and mechanisms of FAM3A, FAM3B and FAM3C in the regulation of hepatic glucose and lipid metabolism.
... In the pancreatic β-cell, ATP-sensitive potassium (K ATP ) channels link glucose metabolism and insulin secretion and are essential to the normal regulation of plasma glucose and other nutrients [1,2]. At low glucose, intracellular [ATP]/[ADP] is low, and K ATP channels are open, hyperpolarizing the cell membrane. ...
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ATP-sensitive potassium channels (KATP channels) are critical nutrient sensors in many mammalian tissues. In the pancreas, KATP channels are essential for coupling glucose metabolism to insulin secretion. While orthologous genes for many components of metabolism–secretion coupling in mammals are present in lower vertebrates, their expression, functionality and ultimate impact on body glucose homeostasis are unclear. In this paper, we demonstrate that zebrafish islet β-cells express functional KATP channels of similar subunit composition, structure and metabolic sensitivity to their mammalian counterparts. We further show that pharmacological activation of native zebrafish KATP using diazoxide, a specific KATP channel opener, is sufficient to disturb glucose tolerance in adult zebrafish. That β-cell KATP channel expression and function are conserved between zebrafish and mammals illustrates the evolutionary conservation of islet metabolic sensing from fish to humans, and lends relevance to the use of zebrafish to model islet glucose sensing and diseases of membrane excitability such as neonatal diabetes.
... In rainbow trout, the decreased mRNA levels of FAS and CPT1c in BB after treatment with oleate or octanoate (Librán-Pérez et al., 2012) suggest that components of putative fatty acid sensing systems respond in BB to increased fatty acid levels. This response could modulate insulin secretion from this tissue, as reported in mammals (Keane and Newsholme, 2014), with the main difference that in fish fatty acid sensing systems are also responsive to a MCFA like octanoate. This mechanism appear to be mainly the result of a direct action of fatty acid in β-cells (Librán-Pérez et al., 2013a) though an indirect action by previous hypothalamic sensing mediated by vagal and/or splanchnic outflow cannot be discarded (Librán-Pérez et al., 2015c). ...
Article
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Evidence obtained in recent years in a few species, especially rainbow trout, supports the presence in fish of nutrient sensing mechanisms. Glucosensing capacity is present in central (hypothalamus and hindbrain) and peripheral [liver, Brockmann bodies (BB, main accumulation of pancreatic endocrine cells in several fish species), and intestine] locations whereas fatty acid sensors seem to be present in hypothalamus, liver and BB. Glucose and fatty acid sensing capacities relate to food intake regulation and metabolism in fish. Hypothalamus is as a signaling integratory center in a way that detection of increased levels of nutrients result in food intake inhibition through changes in the expression of anorexigenic and orexigenic neuropeptides. Moreover, central nutrient sensing modulates functions in the periphery since they elicit changes in hepatic metabolism as well as in hormone secretion to counter-regulate changes in nutrient levels detected in the CNS. At peripheral level, the direct nutrient detection in liver has a crucial role in homeostatic control of glucose and fatty acid whereas in BB and intestine nutrient sensing is probably involved in regulation of hormone secretion from endocrine cells.
... The β-cell in the Langerhan's islet from the pancreas is the site of production of the hormone insulin [86]. In order to maintain normoglycemia, β-cells secrete insulin in response to different stimuli [87,88]. ...
Article
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Diabetes is a complex metabolic disorder triggered by the deficient secretion of insulin by the pancreatic β-cell or the resistance of peripheral tissues to the action of the hormone. Chronic hyperglycemia is the major consequence of this failure, and also the main cause of diabetic problems. Indeed, several clinical trials have agreed in that tight glycemic control is the best way to stop progression of the disease. Many anti-diabetic drugs for treatment of type 2 diabetes are commercially available, but no ideal normoglycemic agent has been developed yet. Moreover, weight gain is the most common side effect of many oral anti-diabetic agents and insulin, and increased weight has been shown to worsen glycemic control and increase the risk of diabetes progression. In this sense, the inorganic salt sodium tungstate (NaW) has been studied in different animal models of metabolic syndrome and diabetes, proving to have a potent effect on normalizing blood glucose levels and reducing body weight, without any hypoglycemic action. Although the liver has been studied as the main site of NaW action, positive effects have been also addressed in muscle, pancreas, brain, adipose tissue and intestine, explaining the effective anti-diabetic action of this salt. Here, we review NaW research to date in these different target organs. We believe that NaW deserves more attention, since all available anti-diabetic treatments remain suboptimal and new therapeutics are urgently needed.
... It is worthwhile to note that, high glucose is able to synergetically exacerbate the cellular dysfunction caused by lipotoxicity in the context of diabetes. It is well established in pancreatic β-cell that in the fasting state, fatty acids are primarily transferred into mitochondria for β-oxidation [18]. This is rapidly reversed upon carbohydrate uptake. ...
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Glucolipotoxicity is one of the critical causal factors of diabetic complications. Whether mesenchymal stem cells (MSCs) have effects on glucolipotoxicity in human umbilical vein endothelial cells (HUVECs) and mechanisms involved are unclear. Thirty mM glucose plus 100 μM palmitic acid was used to induce glucolipotoxicity in HUVECs. MSCs and HUVECs were co-cultured at the ratio of 1:5 via Transwell system. The mRNA expressions of inflammatory factors were detected by RT-qPCR. The productions of reactive oxygen species (ROS), cell cycle and apoptosis were analyzed by flow cytometry. The tumor necrosis factor-α stimulated protein 6 (TSG-6) was knockdown in MSCs by RNA interference. High glucose and palmitic acid remarkably impaired cell viability and tube formation capacity, as well as increased the mRNA expression of inflammatory factors, ROS levels, and cell apoptosis in HUVECs. MSC co-cultivation ameliorated these detrimental effects in HUVECs, but no effect on ROS production. Moreover, TSG-6 was dramatically up-regulated by high glucose and fatty acid stimulation in both MSCs and HUVECs. TSG-6 knockdown partially abolished the protection mediated by MSCs. MSCs had protective effects on high glucose and palmitic acid induced glucolipotoxicity in HUVECs, and TSG-6 secreted by MSCs was likely to play an important role in this process.
... A thorough discussion of metabolomics applied to diabetes in general [7,8] or of all current mechanistic models of islet function [9] and dysfunction [2,10] Abbreviations: 6PGDH, 6-phosphogluconate dehydrogenase; K ATP , ATP-sensitive K + channel; ER, endoplasmic reticulum; G6PDH, glucose-6-phosphate dehydrogenase; GC, gas chromatography; GHB, c-hydroxybutyric acid; GSIS, glucose-stimulated insulin secretion; ICDc, cytosolic NADP-dependent isocitrate dehydrogenase; LC, liquid chromatography; MS, mass spectrometry; NMR, nuclear magnetic resonance; PPP, pentose phosphate pathway; PUFA, polyunsaturated fatty acid; T1D, type 1 diabetes; T2D, type 2 diabetes; TCA, tricarboxylic acid cycle; TOF, time-of-flight mass spectrometry; ZMP, 5-aminoimidazole-4-carboxamide ribonucleotide. ...
... Glucose is a primary stimulator of insulin secretion in pancreatic b-cells and modulates the effects of incretins and acetylcholine (Ashcroft & Rorsman 1989, Rasmussen et al. 1990, Rorsman 1997, Newsholme et al. 2014). We have shown recently that T1R3, a subunit of the sweet taste receptor (Nelson et al. 2001, Temussi 2007), functions as a glucose-sensing receptor in b-cells (Malaisse 2014, Nakagawa et al. 2014, Kojima et al. 2015). ...
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Glucose activates the glucose-sensing receptor T1R3 and facilitates its own metabolism in pancreatic β-cells. An inhibitor of this receptor would be helpful in elucidating the physiological function of the glucose-sensing receptor. The present study was conducted to examine whether or not lactisole can be used as an inhibitor of the glucose-sensing receptor. In MIN6 cells, in a dose-dependent manner, lactisole inhibited insulin secretion induced by sweeteners, acesulfame-K, sucralose and glycyrrhizin. The IC50 was approximately 4 mmol/l. Lactisole attenuated the elevation of cytoplasmic Ca2+ concentration ([Ca2+]c) evoked by sucralose and acesulfame-K but did not affect the elevation of intracellular cAMP concentration ([cAMP]c) induced by these sweeteners. Lactisole also inhibited the action of glucose in MIN6 cells. Thus, lactisole significantly reduced elevations of intracellular [NADH] and intracellular [ATP] induced by glucose, and also inhibited glucose-induced insulin secretion. To further examine the effect of lactisole on T1R3, we prepared HEK293 cells stably expressing mouse T1R3. In these cells, sucralose elevated both [Ca2+]c and [cAMP]c. Lactisole attenuated the sucralose-induced increase in [Ca2+]c but did not affect the elevation of [cAMP]c. Finally, lactisole inhibited insulin secretion induced by a high concentration of glucose in mouse islets. These results indicate that the mouse glucose-sensing receptor was inhibited by lactisole. Lactisole may be useful in assessing the role of the glucose-sensing receptor in mouse pancreatic β-cells.
... The β-cell in the Langerhan's islet from the pancreas is the site of production of the hormone insulin [86]. In order to maintain normoglycemia, β-cells secrete insulin in response to different stimuli [87,88]. ...
Article
Full-text available
Diabetes is a complex metabolic disorder triggered by the deficient secretion of insulin by the pancreatic β-cell or the resistance of peripheral tissues to the action of the hormone. Chronic hyperglycemia is the major consequence of this failure, and also the main cause of diabetic problems. Indeed, several clinical trials have agreed in that tight glycemic control is the best way to stop progression of the disease. Many anti-diabetic drugs for treatment of type 2 diabetes are commercially available, but no ideal normoglycemic agent has been developed yet. Moreover, weight gain is the most common side effect of many oral anti-diabetic agents and insulin, and increased weight has been shown to worsen glycemic control and increase the risk of diabetes progression. In this sense, the inorganic salt sodium tungstate (NaW) has been studied in different animal models of metabolic syndrome and diabetes, proving to have a potent effect on normalizing blood glucose levels and reducing body weight, without any hypoglycemic action. Although the liver has been studied as the main site of NaW action, positive effects have been also addressed in muscle, pancreas, brain, adipose tissue and intestine, explaining the effective anti-diabetic action of this salt. Here, we review NaW research to date in these different target organs. We believe that NaW deserves more attention, since all available anti-diabetic treatments remain suboptimal and new therapeutics are urgently needed.
... Moreover, cells in our body could not utilize glucose without insulin. All defects in both insulin production and resistance to its action underline all types of Diabetes mellitus [22] . Diabetes mellitus is an important metabolic disease which causes defects in the secretion and synthesis of insulin and can be associated with the third-largest cause of death in industrialized countries [23]. ...
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Insulin secretion from pancreatic β cells is essential for glucose homeostasis. An insufficient response to the demand for insulin results in diabetes. We previously showed that β cell-specific deletion of Zfp148 (β-Zfp148KO) improves glucose tolerance and insulin secretion in mice. Here, we performed Ca2+ imaging of islets from β‑Zfp148KO and control mice fed both a chow and a Western-style diet. β-Zfp148KO islets demonstrated improved sensitivity and sustained Ca2+ oscillations in response to elevated glucose levels. β-Zfp148KO islets also exhibited elevated sensitivity to amino acid-induced Ca2+ influx under low glucose conditions, suggesting enhanced mitochondrial phosphoenolpyruvate-dependent (PEP-dependent), ATP-sensitive K+ channel closure, independent of glycolysis. RNA-Seq and proteomics of β-Zfp148KO islets revealed altered levels of enzymes involved in amino acid metabolism (specifically, SLC3A2, SLC7A8, GLS, GLS2, PSPH, PHGDH, and PSAT1) and intermediary metabolism (namely, GOT1 and PCK2), consistent with altered PEP cycling. In agreement with this, β-Zfp148KO islets displayed enhanced insulin secretion in response to l-glutamine and activation of glutamate dehydrogenase. Understanding pathways controlled by ZFP148 may provide promising strategies for improving β cell function that are robust to the metabolic challenge imposed by a Western diet.
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Insulin plays roles in lipid uptake, lipolysis, and lipogenesis, in addition to controlling blood glucose levels. Excessive circulating insulin is associated with adipose tissue expansion and obesity, yet a causal role for hyperinsulinemia in the development of mammalian obesity has proven controversial, with many researchers suggesting it as a consequence of insulin resistance. Recently, evidence that specifically reducing hyperinsulinemia can prevent and reverse obesity in animal models has been presented. Our experiments, and others in this field, question the current dogma that hyperinsulinemia is a response to obesity and/or insulin resistance. In this review, we discuss preclinical evidence in the context of the broader literature and speculate on the possibility of clinical translation of alternative approaches for treating obesity.
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Pancreatic islet β cells secrete insulin in response to nutrient secretagogues, like glucose, dependent on calcium influx and nutrient metabolism. One of the most intriguing qualities of β cells is their ability to use metabolism to amplify the amount of secreted insulin independent of further alterations in intracellular calcium. Many years studying this amplifying process have shaped our current understanding of β cell stimulus-secretion coupling; yet, the exact mechanisms of amplification have been elusive. Recent studies utilizing metabolomics, computational modeling, and animal models have progressed our understanding of the metabolic amplifying pathway of insulin secretion from the β cell. New approaches will be discussed which offer in-roads to a more complete model of β cell function. The development of β cell therapeutics may be aided by such a model, facilitating the targeting of aspects of the metabolic amplifying pathway which are unique to the β cell.
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Islets of Langerhans are the regulators of in vivo blood glucose levels through the secretion of endocrine hormones. Amino acids, released from various cells within islets or from intrapancreatic neurons, are hypothesized to further adjust hormone secretions. In contrast to the well-accepted mechanism of glucose-stimulated insulin secretion, several questions remain as to the function of amino acids in the regulation of hormone release from islets. To understand the autocrine and paracrine roles that amino acids play in islet physiology, a microfluidic system was developed to perform online monitoring of the secretion profiles of amino acids from 2 - 5 islets. The device contained an islet chamber with the ability to perfuse stimulants, and an amino acid measurement system with derivatization and electrophoretic separation integrated on a single microchip. The setup was optimized to allow -15 kV to be applied to the device for high efficiency and rapid separations of derivatized amino acids. The compositions of the derivatization and separation buffers were optimized to prevent precipitations in the channels, which allowed continuous monitoring of secretion for over 2 hours. With this method, 10 amino acids were resolved with limits of detection ranging from 1 - 20 nM. When murine islets were perfused with 3 mM glucose, the secretion rates of 9 amino acids were measured and ranged from 30 to 400 fmol islet-1 min-1. As the glucose concentration was increased to 20 mM, the dynamic changes of amino acids were monitored. The biological relevance of the amino acid secretions was verified using 2,4-dinitrophenol as an inhibitor of the proton motive force. The microfluidic system was also used to measure dynamic changes of amino acid release from human islets, which showed different release profiles compared to their murine counterparts.
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Pigment epithelium-derived factor (PEDF) is a multifunctional glycoprotein, associated with lipid catabolism and insulin resistance. In the present study, PEDF increased chronic and acute insulin secretion in a clonal rat β-cell line BRIN-BD11, without alteration of glucose consumption. PEDF also stimulated insulin secretion from primary mouse islets. Seahorse flux analysis demonstrated that PEDF did not change mitochondrial respiration and glycolytic function. The cytosolic presence of the putative PEDF receptor - adipose triglyceride lipase (ATGL) - was identified, and ATGL associated stimulation of glycerol release was robustly enhanced by PEDF, while intracellular ATP levels increased. Addition of palmitate or ex vivo stimulation with inflammatory mediators induced β-cell dysfunction, effects not altered by the addition of PEDF. In conclusion, PEDF increased insulin secretion in BRIN-BD11 and islet cells, but had no impact on glucose metabolism. Thus elevated lipolysis and enhanced fatty acid availability may impact insulin secretion following PEDF receptor (ATGL) stimulation.
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Human liver acts as a homeostatic controller for maintaining the normal levels of plasma metabolite concentrations by uptake, utilization, storage and synthesis of essential metabolites. These hepatic functions are orchestrated through a multilevel regulation composed of metabolic, signaling and transcriptional networks. Plasma macronutrients namely, glucose, amino acids and fatty acids are known to influence these regulatory mechanisms to facilitate homeostasis. We composed a regulatory circuit that elicits the design principle behind the metabolic regulation in liver. We have developed a detailed dynamic model for hepatic metabolism incorporating the regulatory mechanisms at signaling and transcriptional level. The model was analyzed to capture the behavior of hepatic metabolic fluxes under various combinations of plasma macronutrient levels. The model was used to rationalize and explain the experimental observations of metabolic dysfunctions through regulatory mechanisms. We addressed the key questions such as, how high carbohydrate diet increases cholesterol and why a high protein diet would reduce it; how high fat and high protein diet increases gluconeogenesis leading to hyperglycemia; how TCA (tricarboxylic acid) cycle is impaired through diet induced insulin resistance; how high fat can impair plasma ammonia balance; how high plasma glucose can lead to dyslipidemia and fatty liver disease etc. The analysis indicates that higher levels (above 2.5-3 fold) of macronutrient in plasma results in impairment of metabolic functions due to perturbations in the regulatory circuit. While higher glucose levels saturate the rate of plasma glucose uptake, higher amino acids activate glucagon and inhibit IRS (Insulin receptor substrate) through S6K (S6 kinase), whereas higher fatty acid levels inhibit IRS through DAG-PKC (diacylglycerol and protein kinase C)and TRB3 activation. Moreover the ATP-ADP ratio is reduced under such conditions and β-oxidation is up-regulated through activation of PPARα (peroxisome proliferator-activated receptor alpha) leading to reduced anabolic capacity and increased cataplerosis in TCA cycle. The above factors together decrease insulin sensitivity and enhances glucagon effect through underlying signaling and transcriptional network leading to insulin resistance in liver. Such a metabolic state is known to result in diabetes and non-alcoholic fatty liver disease.
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Prior studies demonstrated increased plasma immunoglobulin E (IgE) in diabetic patients, but the direct participation of IgE in diabetes or obesity remains unknown. This study found that plasma IgE levels correlated inversely with body weight, body mass index, and body fat mass among a population of randomly selected obese women. IgE receptor FcϵR1-deficient (Fcer1a(-/-)) mice and diet-induced obesity (DIO) mice demonstrated that FcϵR1 deficiency in DIO mice increased food intake, reduced energy expenditure, and increased body weight gain, but improved glucose tolerance and glucose-induced insulin secretion. White adipose tissue (WAT) from Fcer1a(-/-) mice showed increased expression of phospho-AKT, C/EBPα, PPARγ, Glut4, and Bcl-2, but reduced UCP1 and phospho-JNK expression, tissue macrophage accumulation, and apoptosis, suggesting that IgE reduces adipogenesis and glucose uptake, but induces energy expenditure, adipocyte apoptosis, and WAT inflammation. In 3T3-L1 cells, IgE inhibited the expression of C/EBPα and PPARγ, and preadipocyte adipogenesis, and induced adipocyte apoptosis. IgE reduced 3T3-L1 cell expression of Glut4, phospho-AKT, and glucose uptake, which concurred with improved glucose tolerance in Fcer1a(-/-) mice. This study established two novel pathways of IgE in reducing body weight gain in DIO mice by suppressing adipogenesis and inducing adipocyte apoptosis, while worsening glucose tolerance by reducing Glut4 expression, glucose uptake, and insulin secretion.
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Metabolic syndrome (MetS) is associated with abdominal obesity, hyperlipidemia, insulin resistance, and type 2 diabetes mellitus, and increases the risk of cardiovascular disease. Given the complex multifactorial pathogenesis of MetS, qualified animal models are currently seriously limited for researchers. The aim of our study was to develop a MetS model in juvenile rhesus monkeys (Macaca mulatta). Rhesus monkeys (1-year-old) fed a high-fat diet (15 % fat, 2 % cholesterol) were used as the HF group (n = 6), and those on a normal diet (5 % fat) were used as the control group (n = 4). After being fed a high-fat diet for approximately 12 months, 2 monkeys (HF + STZ group) were injected with low-dose streptozotocin (STZ, 25 mg/kg) twice, with a 7 days interval, and were then fed the same diet continuously for another 24 months. After 36 months of treatment, the high-fat diet monkeys, including the HF and HF + STZ groups, had acquired increased body weights, abnormal serum lipids, and impaired glucose tolerance compared to the control group. In addition, much more marked metabolic changes were observed in the two monkeys of the HF + STZ group, particularly in terms of high-blood glucose level and insulin resistance. Morphological observation of biopsies of liver and pancreatic tissues showed decreased islet number and mass and decreased insulin staining in the monkeys of the HF + STZ group. In addition, Oil red O staining suggested remarkable accumulation of lipid droplets in the hepatocytes. Our study suggested that a long-term high-fat diet followed with a low-dose STZ was able to induce MetS in juvenile rhesus monkeys with faster pathophysiological progress compared with high-fat diet induction alone. Our primary data showed that this method may have potentials to develop MetS animal model in non-human primates.
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Activation of the innate immune system in obesity is a risk factor for the development of type 2 diabetes. The aim of the current study was to investigate the notion that increased numbers of macrophages exist in the islets of type 2 diabetes patients and that this may be explained by a dysregulation of islet-derived inflammatory factors. Increased islet-associated immune cells were observed in human type 2 diabetic patients, high-fat-fed C57BL/6J mice, the GK rat, and the db/db mouse. When cultured islets were exposed to a type 2 diabetic milieu or when islets were isolated from high-fat-fed mice, increased islet-derived inflammatory factors were produced and released, including interleukin (IL)-6, IL-8, chemokine KC, granulocyte colony-stimulating factor, and macrophage inflammatory protein 1alpha. The specificity of this response was investigated by direct comparison to nonislet pancreatic tissue and beta-cell lines and was not mimicked by the induction of islet cell death. Further, this inflammatory response was found to be biologically functional, as conditioned medium from human islets exposed to a type 2 diabetic milieu could induce increased migration of monocytes and neutrophils. This migration was blocked by IL-8 neutralization, and IL-8 was localized to the human pancreatic alpha-cell. Therefore, islet-derived inflammatory factors are regulated by a type 2 diabetic milieu and may contribute to the macrophage infiltration of pancreatic islets that we observe in type 2 diabetes.
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The role currently ascribed to the accumulation of l-arginine in the pancreatic islet B-cell as a determinant of its insulinotropic action was reevaluated by comparing the uptake and the metabolic, ionic, electric, and secretory effects of the cationic amino acid with those of its more positively charged methyl ester in rat pancreatic islets. The response to l-arginine methyl ester differed from that evoked by the unesterified amino acid by a lower uptake and oxidation, lack of inhibitory action on d-glucose metabolism, more severe inhibition of the catabolism of endogenous l-glutamine, inhibition of 45Ca net uptake, decrease in both 86Rb outflow from prelabeled islets perifused at normal extracellular Ca2+ concentration and 45Ca efflux from prelabeled islets perifused in the absence of extracellular Ca2+, and delayed and lesser insulinotropic action. These findings reinforce the view that the carrier-meadiated entry of l-arginine into the islet B-cells, with resulting depolarization of the plasma membrane, represents the essential mechanism for stimulation of insulin release by this cationic amino acid.
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Growing evidence indicates that the regulation of intracellular reactive oxygen species (ROS) and reactive nitrogen species (RNS) levels is essential for maintaining normal β-cell glucose responsiveness. While long-term exposure to high glucose induces oxidative stress in β cells, conflicting results have been published regarding the impact of ROS on acute glucose exposure and their role in glucose stimulated insulin secretion (GSIS). Although β cells are considered to be particularly vulnerable to oxidative damage, as they express relatively low levels of some peroxide-metabolizing enzymes such as catalase and glutathione (GSH) peroxidase, other less known GSH-based antioxidant systems are expressed in β cells at higher levels. Herein, we discuss the key mechanisms of ROS/RNS production and their physiological function in pancreatic β cells. We also hypothesize that specific interactions between RNS and ROS may be the cause of the vulnerability of pancreatic β cells to oxidative damage. In addition, using a hypothetical metabolic model based on the data available in the literature, we emphasize the importance of amino acid availability for GSH synthesis and for the maintenance of β-cell function and viability during periods of metabolic disturbance before the clinical onset of diabetes.
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The escalating epidemic of obesity has driven the prevalence of both type 1 and 2 diabetes mellitus to historically high levels. Chronic low-grade inflammation, which is present in both type 1 and type 2 diabetics, contributes to the pathogenesis of insulin resistance. The accumulation of activated innate immune cells in metabolic tissues results in release of inflammatory mediators, in particular, IL-1β and TNFα, which promote systemic insulin resistance and β-cell damage. In this article, we discuss the central role of innate immunity and, in particular, the macrophage in insulin sensitivity and resistance, β-cell damage, and autoimmune insulitis. We conclude with a discussion of the therapeutic implications of this integrated understanding of diabetic pathology.
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Type 2 diabetes is a complex metabolic disorder characterized by high blood glucose in the context of insulin resistance and relative insulin deficiency by β-cell failure. Even if the mechanisms underlying the pathogenesis of β-cell failure are still under investigation, recent increasing genetic, experimental, and clinical evidence indicate that hyperactivation of the unfolded protein response (UPR) to counteract metabolic stresses is closely related to β-cell dysfunction and apoptosis. Signaling pathways of the UPR are "a double-edged sword" that can promote adaptation or apoptosis depending on the nature of the ER stress condition. In this paper, we summarized our current understanding of the mechanisms and components related to ER stress in the β-cell pathogenesis of type 2 diabetes.
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In the brain, glutamate is an extracellular transmitter that mediates cell-to-cell communication. Prior to synaptic release it is pumped into vesicles by vesicular glutamate transporters (VGLUTs). To inactivate glutamate receptor responses after release, glutamate is taken up into glial cells or neurons by excitatory amino acid transporters (EAATs). In the pancreatic islets of Langerhans, glutamate is proposed to act as an intracellular messenger, regulating insulin secretion from β-cells, but the mechanisms involved are unknown. By immunogold cytochemistry we show that insulin containing secretory granules express VGLUT3. Despite the fact that they have a VGLUT, the levels of glutamate in these granules are low, indicating the presence of a protein that can transport glutamate out of the granules. Surprisingly, in β-cells the glutamate transporter EAAT2 is located, not in the plasma membrane as it is in brain cells, but exclusively in insulin-containing secretory granules, together with VGLUT3. In EAAT2 knock out mice, the content of glutamate in secretory granules is higher than in wild type mice. These data imply a glutamate cycle in which glutamate is carried into the granules by VGLUT3 and carried out by EAAT2. Perturbing this cycle by knocking down EAAT2 expression with a small interfering RNA, or by over-expressing EAAT2 or a VGLUT in insulin granules, significantly reduced the rate of granule exocytosis. Simulations of granule energetics suggest that VGLUT3 and EAAT2 may regulate the pH and membrane potential of the granules and thereby regulate insulin secretion. These data suggest that insulin secretion from β-cells is modulated by the flux of glutamate through the secretory granules.
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Neonatal diabetes mellitus (NDM) can be caused by gain-of-function ATP-sensitive K(+) (K(ATP)) channel mutations. This realization has led to sulfonylurea therapy replacing insulin injections in many patients. In a murine model of K(ATP)-dependent NDM, hyperglycemia and consequent loss of β-cells are both avoided by chronic sulfonylurea treatment. Interestingly, K(ATP) mutations may underlie remitting-relapsing, transient, or permanent forms of the disease in different patients, but the reason for the different outcomes is unknown. To gain further insight into disease progression and outcome, we examined the effects of very early intervention by injecting NDM mice with high-dose glibenclamide for only 6 days, at the beginning of disease onset, then after the subsequent progression with measurements of blood glucose, islet function, and insulin sensitivity. Although ∼70% of mice developed severe diabetes after treatment cessation, ∼30% were essentially cured, maintaining near-normal blood glucose until killed. Another group of NDM mice was initiated on oral glibenclamide (in the drinking water), and the dose was titrated daily, to maintain blood glucose <200 mg/dL. In this case, ∼30% were also essentially cured; they were weaned from the drug after ∼4 weeks and again subsequently maintained near-normal blood glucose. These cured mice maintain normal insulin content and were more sensitive to insulin than control mice, a compensatory mechanism that together with basal insulin secretion may be sufficient to maintain near-normal glucose levels. At least in a subset of animals, early sulfonylurea treatment leads to permanent remission of NDM. These cured animals exhibit insulin-hypersensitivity. Although untreated NDM mice rapidly lose insulin content and progress to permanently extremely elevated blood glucose levels, early tight control of blood glucose may permit this insulin-hypersensitivity, in combination with maintained basal insulin secretion, to provide long-term remission.
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Long-chain fatty acids amplify insulin secretion from the pancreatic beta-cell. The G-protein-coupled receptor GPR40 is specifically expressed in beta-cells and is activated by fatty acids; however, its role in acute regulation of insulin secretion in vivo remains unclear. To this aim, we generated GPR40 knockout (KO) mice and examined glucose homeostasis, insulin secretion in response to glucose and Intralipid in vivo, and insulin secretion in vitro after short- and long-term exposure to fatty acids. Our results show that GPR40 KO mice have essentially normal glucose tolerance and insulin secretion in response to glucose. Insulin secretion in response to Intralipid was reduced by approximately 50%. In isolated islets, insulin secretion in response to glucose and other secretagogues was unaltered, but fatty acid potentiation of insulin release was markedly reduced. The Galpha(q/11) inhibitor YM-254890 dose-dependently reduced palmitate potentiation of glucose-induced insulin secretion. Islets from GPR40 KO mice were as sensitive to fatty acid inhibition of insulin secretion upon prolonged exposure as islets from wild-type animals. We conclude that GPR40 contributes approximately half of the full acute insulin secretory response to fatty acids in mice but does not play a role in the mechanisms by which fatty acids chronically impair insulin secretion.
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Anaplerosis, the net synthesis in mitochondria of citric acid cycle intermediates, and cataplerosis, their export to the cytosol, have been shown to be important for insulin secretion in rodent beta cells. However, human islets may be different. We observed that the enzyme activity, protein level, and relative mRNA level of the key anaplerotic enzyme pyruvate carboxylase (PC) were 80-90% lower in human pancreatic islets compared with islets of rats and mice and the rat insulinoma cell line INS-1 832/13. Activity and protein of ATP citrate lyase, which uses anaplerotic products in the cytosol, were 60-75% lower in human islets than in rodent islets or the cell line. In line with the lower PC, the percentage of glucose-derived pyruvate that entered mitochondrial metabolism via carboxylation in human islets was only 20-30% that in rat islets. This suggests human islets depend less on pyruvate carboxylation than rodent models that were used to establish the role of PC in insulin secretion. Human islets possessed high levels of succinyl-CoA:3-ketoacid-CoA transferase, an enzyme that forms acetoacetate in the mitochondria, and acetoacetyl-CoA synthetase, which uses acetoacetate to form acyl-CoAs in the cytosol. Glucose-stimulated human islets released insulin similarly to rat islets but formed much more acetoacetate. β-Hydroxybutyrate augmented insulin secretion in human islets. This information supports previous data that indicate beta cells can use a pathway involving succinyl-CoA:3-ketoacid-CoA transferase and acetoacetyl-CoA synthetase to synthesize and use acetoacetate and suggests human islets may use this pathway more than PC and citrate to form cytosolic acyl-CoAs.
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The endoplasmic reticulum (ER) as an intracellular Ca(2+) store not only sets up cytosolic Ca(2+) signals, but, among other functions, also assembles and folds newly synthesized proteins. Alterations in ER homeostasis, including severe Ca(2+) depletion, are an upstream event in the pathophysiology of many diseases. On the one hand, insufficient release of activator Ca(2+) may no longer sustain essential cell functions. On the other hand, loss of luminal Ca(2+) causes ER stress and activates an unfolded protein response, which, depending on the duration and severity of the stress, can reestablish normal ER function or lead to cell death. We will review these various diseases by mainly focusing on the mechanisms that cause ER Ca(2+) depletion.
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The dysfunction of pancreatic β-cell and the reduction in β-cell mass are the decisive events in the progression of type 2 diabetes. There is increasing evidence that cytokines play important roles in the procedure of β-cell failure. Cytokines, such as IL-1β, IFN-γ, TNF-α, leptin, resistin, adiponectin, and visfatin, have been shown to diversely regulate pancreatic β-cell function. Recently, islet-derived cytokine PANcreatic DERived factor (PANDER or FAM3B) has also been demonstrated to be a regulator of islet β-cell function. The change in cytokine profile in islet and plasma is associated with pancreatic β-cell dysfunction and apoptosis. In this paper, we summarize and discuss the recent studies on the effects of certain important cytokines on pancreatic β-cell function. The imbalance in deleterious and protective cytokines plays pivotal roles in the development and progression of pancreatic β-cell dysfunction under insulin-resistant conditions.
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Chronic exposure of pancreatic β-cells to saturated non-esterified fatty acids can lead to inhibition of insulin secretion and apoptosis. Several previous studies have demonstrated that saturated fatty acids such as PA (palmitic acid) are detrimental to β-cell function compared with unsaturated fatty acids. In the present study, we describe the effect of the polyunsaturated AA (arachidonic acid) on the function of the clonal pancreatic β-cell line BRIN-BD11 and demonstrate AA-dependent attenuation of PA effects. When added to β-cell incubations at 100 μM, AA can stimulate cell proliferation and chronic (24 h) basal insulin secretion. Microarray analysis and/or real-time PCR indicated significant AA-dependent up-regulation of genes involved in proliferation and fatty acid metabolism [e.g. Angptl (angiopoietin-like protein 4), Ech1 (peroxisomal Δ3,5,Δ2,4-dienoyl-CoA isomerase), Cox-1 (cyclo-oxygenase-1) and Cox-2, P<0.05]. Experiments using specific COX and LOX (lipoxygenase) inhibitors demonstrated the importance of COX-1 activity for acute (20 min) stimulation of insulin secretion, suggesting that AA metabolites may be responsible for the insulinotropic effects. Moreover, concomitant incubation of AA with PA dose-dependently attenuated the detrimental effects of the saturated fatty acid, so reducing apoptosis and decreasing parameters of oxidative stress [ROS (reactive oxygen species) and NO levels] while improving the GSH/GSSG ratio. AA decreased the protein expression of iNOS (inducible NO synthase), the p65 subunit of NF-κB (nuclear factor κB) and the p47 subunit of NADPH oxidase in PA-treated cells. These findings indicate that AA has an important regulatory and protective β-cell action, which may be beneficial to function and survival in the 'lipotoxic' environment commonly associated with Type 2 diabetes mellitus.
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All living organisms depend on dynamic mechanisms that repeatedly reassess the status of amassed energy, in order to adapt energy supply to demand. The AMP-activated protein kinase (AMPK) alphabetagamma heterotrimer has emerged as an important integrator of signals managing energy balance. Control of AMPK activity involves allosteric AMP and ATP regulation, auto-inhibitory features and phosphorylation of its catalytic (alpha) and regulatory (beta and gamma) subunits. AMPK has a prominent role not only as a peripheral sensor but also in the central nervous system as a multifunctional metabolic regulator. AMPK represents an ideal second messenger for reporting cellular energy state. For this reason, activated AMPK acts as a protective response to energy stress in numerous systems. However, AMPK inhibition also actively participates in the control of whole body energy homeostasis. In this review, we discuss recent findings that support the role and function of AMPK inhibition under physiological and pathological states.
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Pancreatic beta cells are specialised endocrine cells that continuously sense the levels of blood sugar and other fuels and, in response, secrete insulin to maintain normal fuel homeostasis. During postprandial periods an elevated level of plasma glucose rapidly stimulates insulin secretion to decrease hepatic glucose output and promote glucose uptake into other tissues, principally muscle and adipose tissues. Beta cell mitochondria play a key role in this process, not only by providing energy in the form of ATP to support insulin secretion, but also by synthesising metabolites (anaplerosis) that can act, both intra- and extramitochondrially, as factors that couple glucose sensing to insulin granule exocytosis. ATP on its own, and possibly modulated by these coupling factors, triggers closure of the ATP-sensitive potassium channel, resulting in membrane depolarisation that increases intracellular calcium to cause insulin secretion. The metabolic imbalance caused by chronic hyperglycaemia and hyperlipidaemia severely affects mitochondrial metabolism, leading to the development of impaired glucose-induced insulin secretion in type 2 diabetes. It appears that the anaplerotic enzyme pyruvate carboxylase participates directly or indirectly in several metabolic pathways which are important for glucose-induced insulin secretion, including: the pyruvate/malate cycle, the pyruvate/citrate cycle, the pyruvate/isocitrate cycle and glutamate-dehydrogenase-catalysed alpha-ketoglutarate production. These four pathways enable 'shuttling' or 'recycling' of these intermediate(s) into and out of mitochondrion, allowing continuous production of intracellular messenger(s). The purpose of this review is to present an account of recent progress in this area of central importance in the realm of diabetes and obesity research.
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Chronic inflammation is a key player in pathogenesis. The inflammatory cytokine, tumor necrosis factor-alpha is a well known inflammatory protein, and has been a therapeutic target for the treatment of diseases such as Rheumatoid Arthritis and Crohn's Disease. Obesity is a well known risk factor for developing non-insulin dependent diabetes melitus. Adipose tissue has been shown to produce tumor necrosis factor-alpha, which has the ability to reduce insulin secretion and induce insulin resistance. Based on these observations, we sought to investigate the impact of unsaturated fatty acids such as oleic acid in the presence of TNF-alpha in terms of insulin production, the molecular mechanisms involved and the in vivo effect of a diet high in oleic acid on a mouse model of type II diabetes, KKAy. The rat pancreatic beta cell line INS-1 was used as a cell biological model since it exhibits glucose dependent insulin secretion. Insulin production assessment was carried out using enzyme linked immunosorbent assay and cAMP quantification with competitive ELISA. Viability of TNF-alpha and oleic acid treated cells was evaluated using flow cytometry. PPAR-gamma translocation was assessed using a PPRE based ELISA system. In vivo studies were carried out on adult male KKAy mice and glucose levels were measured with a glucometer. Oleic acid and peanut oil high in oleic acid were able to enhance insulin production in INS-1. TNF-alpha inhibited insulin production but pre-treatment with oleic acid reversed this inhibitory effect. The viability status of INS-1 cells treated with TNF-alpha and oleic acid was not affected. Translocation of the peroxisome proliferator- activated receptor transcription factor to the nucleus was elevated in oleic acid treated cells. Finally, type II diabetic mice that were administered a high oleic acid diet derived from peanut oil, had decreased glucose levels compared to animals administered a high fat diet with no oleic acid. Oleic acid was found to be effective in reversing the inhibitory effect in insulin production of the inflammatory cytokine TNF-alpha. This finding is consistent with the reported therapeutic characteristics of other monounsaturated and polyunsaturated fatty acids. Furthermore, a diet high in oleic acid, which can be easily achieved through consumption of peanuts and olive oil, can have a beneficial effect in type II diabetes and ultimately reverse the negative effects of inflammatory cytokines observed in obesity and non insulin dependent diabetes mellitus.
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Evidence has been published that L -alanine may, under appropriate conditions, promote insulin secretion in normal rodent islets and various beta cell lines. Previous results utilising the clonal beta-cell line BRIN-BD11, demonstrated that alanine dramatically elevated insulin release by a mechanism requiring oxidative metabolism. We demonstrate in this paper that addition ofL -alanine had an insulinotropic effect in dispersed primary islet cells. Addition of D -glucose increasedL -alanine consumption in both BRIN-BD11 cells and primary islet cells.L -glutamine consumption in the BRIN-BD11 cell line and primary rat islets was also determined. The consumption rate was in line with that previously reported for cells of the immune system and other glutamine-utilising cells or tIssues. However,L -alanine consumption was at least an order of magnitude higher thanL -glutamine consumption. The metabolism ofL -alanine in the beta-cell may result in stimulation of insulin secretion via generation of metabolic stimulus secretion coupling factors such asL -glutamate.
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Glutamate has been implicated as an intracellular messenger in the regulation of insulin secretion in response to glucose. Here we demonstrate by measurements of cell capacitance in rat pancreatic β-cells that glutamate (1 mM) enhanced Ca2+-dependent exocytosis. Glutamate (1 mM) also stimulated insulin secretion from permeabilized rat β-cells. The effect was dose-dependent (half-maximum at 5.1 mM) and maximal at 10 mM glutamate. Glutamate-induced exocytosis was stronger in rat β-cells and clonal INS-1E cells compared to β-cells isolated from mice and in parental INS-1 cells, which correlated with the expressed levels of glutamate dehydrogenase. Glutamate-induced exocytosis was inhibited by the protonophores FCCP and SF6847, by the vacuolar-type H+-ATPase inhibitor bafilomycin A1 and by the glutamate transport inhibitor Evans Blue. Our data provide evidence that exocytosis in β-cells can be modulated by physiological increases in cellular glutamate levels. The results suggest that stimulation of exocytosis is associated with accumulation of glutamate in the secretory granules, a process that is dependent on the transgranular proton gradient.
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Whey protein hydrolysates (WPHs) represent novel antidiabetic agents that affect glycemia in animals and humans, but little is known about their insulinotropic effects. The effects of a WPH were analyzed in vitro on acute glucose-induced insulin secretion in pancreatic BRIN-BD11 β cells. WPH permeability across Caco-2 cell monolayers was determined in a 2-tiered intestinal model. WPH effects on insulin resistance were studied in vivo following an 8-wk oral ingestion (100 mg/kg body weight) by ob/ob (OB-WPH) and wild-type mice (WT-WPH) compared with vehicle control (OB and WT groups) using a 2 × 2 factorial design, genotype × treatment. BRIN-BD11 cells showed a robust and reproducible dose-dependent insulinotropic effect of WPH (from 0.01 to 5.00 g/L). WPH bioactive constituents were permeable across Caco-2 cell monolayers. In the OB-WPH and WT-WPH groups, WPH administration improved glucose clearance after a glucose challenge (2 g/kg body weight), as indicated by differences in the area under curves (AUCs) (P ≤ 0.05). The basal plasma glucose concentration was not affected by WPH treatment in either genotype. The plasma insulin concentration was lower in the OB-WPH than in the OB group (P ≤ 0.005) but was similar between the WT and WT-WPH groups; the interaction genotype × treatment was significant (P ≤ 0.005). Insulin release from pancreatic islets isolated from the OB-WPH group was greater (P ≤ 0.005) than that from the OB group but did not differ between the WT-WPH and WT groups; the interaction genotype × treatment was not significant. In conclusion, an 8-wk oral administration of WPH improved blood glucose clearance, reduced hyperinsulinemia, and restored the pancreatic islet capacity to secrete insulin in response to glucose in ob/ob mice. Hence, it may be useful in diabetes management.
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Context: Mutations causing genetic defects have been described in many of the enzymes involved in mitochondrial fatty acid oxidation (FAO). Recently, mutations in the penultimate enzyme in the FAO chain have been described that result in quite different symptoms from those normally seen. Patients with mutations in 3-hydroxyacyl-CoA dehydrogenase (HADH) present with protein (leucine)-induced hyperinsulinemic hypoglycemia (HH), suggesting a link between FAO, amino acid metabolism, and insulin secretion. Evidence acquisition and synthesis: Peer-reviewed articles were searched in PubMed with relevance to HADH and disorders of FAO and protein sensitivity. Relevant articles were cited. Recent evidence suggests that mutations in HADH cause HH that is precipitated by protein in a similar manner to the hyperinsulinism/hyperammonemia (HI/HA) syndrome, which is caused by mutations in the GLUD1 gene, encoding the enzyme glutamate dehydrogenase (GDH). Conclusion: Current data suggest that the HH observed in patients with mutations in HADH is precipitated by leucine as seen in the HI/HA syndrome. This is caused by a loss of protein/protein interaction between short-chain HADH (SCHAD, the enzyme coded for by HADH) and GDH, causing an overstimulation of GDH and a rise in cellular ATP and up-regulated insulin secretion. These observations provide new mechanistic insights into the regulation of insulin secretion by fatty acid and amino acid metabolism.
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Pancreatic β cells secrete insulin, the body's only hormone capable of lowering plasma glucose levels. Impaired or insufficient insulin secretion results in diabetes mellitus. The β cell is electrically excitable; in response to an elevation of glucose, it depolarizes and starts generating action potentials. The electrophysiology of mouse β cells and the cell's role in insulin secretion have been extensively investigated. More recently, similar studies have been performed on human β cells. These studies have revealed numerous and important differences between human and rodent β cells. Here we discuss the properties of human pancreatic β cells: their glucose sensing, the ion channel complement underlying glucose-induced electrical activity that culminates in exocytotic release of insulin, the cellular control of exocytosis, and the modulation of insulin secretion by circulating hormones and locally released neurotransmitters. Finally, we consider the pathophysiology of insulin secretion and the interactions between genetics and environmental factors that may explain the current diabetes epidemic. Expected final online publication date for the Annual Review of Physiology Volume 75 is February 10, 2013. Please see http://www.annualreviews.org/catalog/pubdates.aspx for revised estimates.
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Pancreatic β-cell dysfunction plays an important role in the pathogenesis of both type 1 and type 2 diabetes. Insulin, which is produced in β-cells, is a critical regulator of metabolism. Insulin is synthesized as preproinsulin and processed to proinsulin. Proinsulin is then converted to insulin and C-peptide and stored in secretary granules awaiting release on demand. Insulin synthesis is regulated at both the transcriptional and translational level. The cis-acting sequences within the 5' flanking region and trans-activators including paired box gene 6 (PAX6), pancreatic and duodenal homeobox-1(PDX-1), MafA, and Β-2/Neurogenic differentiation 1 (NeuroD1) regulate insulin transcription, while the stability of preproinsulin mRNA and its untranslated regions control protein translation. Insulin secretion involves a sequence of events in β-cells that lead to fusion of secretory granules with the plasma membrane. Insulin is secreted primarily in response to glucose, while other nutrients such as free fatty acids and amino acids can augment glucose-induced insulin secretion. In addition, various hormones, such as melatonin, estrogen, leptin, growth hormone, and glucagon like peptide-1 also regulate insulin secretion. Thus, the β-cell is a metabolic hub in the body, connecting nutrient metabolism and the endocrine system. Although an increase in intracellular [Ca2+] is the primary insulin secretary signal, cAMP signaling-dependent mechanisms are also critical in the regulation of insulin secretion. This article reviews current knowledge on how β-cells synthesize and secrete insulin. In addition, this review presents evidence that genetic and environmental factors can lead to hyperglycemia, dyslipidemia, inflammation, and autoimmunity, resulting in β-cell dysfunction, thereby triggering the pathogenesis of diabetes.
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Activation of free fatty acid receptor 1 (FFAR1; also known as G-protein-coupled receptor 40) by fatty acids stimulated glucose-dependent β-cell insulin secretion in preclinical models. We aimed to assess whether selective pharmacological activation of this receptor by TAK-875 in patients with type 2 diabetes mellitus improved glycaemic control without hypoglycaemia risk. We undertook a phase 2, randomised, double-blind, and placebo-controlled and active-comparator-controlled trial in outpatients with type 2 diabetes who had not responded to diet or metformin treatment. Patients were randomly assigned equally to receive placebo, TAK-875 (6·25, 25, 50, 100, or 200 mg), or glimepiride (4 mg) once daily for 12 weeks. Patients and investigators were masked to treatment assignment. The primary outcome was change in haemoglobin A(1c) (HbA(1c)) from baseline. Analysis included all patients randomly assigned to treatment groups who received at least one dose of double-blind study drug. The trial is registered at ClinicalTrials.gov, NCT01007097. 426 patients were randomly assigned to TAK-875 (n=303), placebo (n=61), and glimepiride (n=62). At week 12, significant least-squares mean reductions in HbA(1c) from baseline occurred in all TAK-875 (ranging from -1·12% [SE 0·113] with 50 mg to -0·65% [0·114] with 6·25 mg) and glimepiride (-1·05% [SE 0·111]) groups versus placebo (-0·13% [SE 0·115]; p value range 0·001 to <0·0001). Treatment-emergent hypoglycaemic events were similar in the TAK-875 and placebo groups (2% [n=7, all TAK-875 groups] vs 3% [n=2]); significantly higher rates were reported in the glimepiride group (19% [n=12]; p value range 0·010-0·002 vs all TAK-875 groups). Incidence of treatment-emergent adverse events was similar in the TAK-875 overall (49%; n=147, all TAK-875 groups) and placebo groups (48%, n=29) and was lower than in the glimepiride group (61%, n=38). TAK-875 significantly improved glycaemic control in patients with type 2 diabetes with minimum risk of hypoglycaemia. The results show that activation of FFAR1 is a viable therapeutic target for treatment of type 2 diabetes. Takeda Global Research and Development.
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Pyruvate is the major product of glycolysis in pancreatic β-cells, and its ultimate metabolic fate depends on the relative activities of two enzymes. The first, pyruvate carboxylase (PC) replenishes oxaloacetate withdrawn from the tricarboxylic acid (TCA) cycle via the carboxylation of pyruvate to form oxaloacetate. Flux via PC is also involved in the formation of NADPH, one of several important coupling factors for insulin secretion. In most tissues, PC activity is enhanced by increased acetyl-CoA. The alternative fate of pyruvate is its oxidative decarboxylation to form acetyl-CoA via the pyruvate dehydrogenase complex (PDC). The ultimate fate of acetyl-CoA carbon is oxidation to CO2 via the TCA cycle, and so the PDC reaction results of the irreversible loss of glucose-derived carbon. Thus, PDC activity is stringently regulated. The mechanisms controlling PDC activity include end-product inhibition by increased acetyl-CoA, NADH and ATP, and its phosphorylation (inactivation) by a family of pyruvate dehydrogenase kinases (PDHKs 1-4). Here we review new developments in the regulation of the activities and expression of PC, PDC and the PDHKs in the pancreatic islet in relation to islet pyruvate disposition and glucose-stimulated insulin secretion (GSIS).
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SLC2A2 encoding glucose transporter -2 (GLUT2) acts as the primary glucose transporter and sensor in rodent pancreatic islets and is widely assumed to play a similar role in humans. In healthy adults SLC2A2 variants are associated with elevated fasting plasma glucose (fpg) concentrations but physiological characterisation does not support a defect in pancreatic beta-cell function. Interspecies differences can create barriers for the follow up of disease association signals. We hypothesised that GLUT2 is not the principal glucose transporter in human beta-cells and that SLC2A2 variants exert their effect on fpg levels through defects in other tissues. SLC2A1-4 (GLUT 1-4) mRNA expression levels were determined in human and mouse islets, beta-cells, liver, muscle and adipose tissue by qRT-PCR whilst GLUT1-3 protein levels were examined by immunohistochemistry. The presence of all three glucose transporters was demonstrated in human and mouse islets and purified beta-cells. Quantitative expression profiling demonstrated that Slc2a2 is the predominant glucose transporter (expression >10 fold higher that Slc2a1) in mouse islets whilst SLC2A1 and SLC2A3 predominate in both human islets and beta-cells (expression 2.8 and 2.7 fold higher than SLC2A2 respectively). Our data therefore suggest that GLUT2 is unlikely to be the principal glucose transporter in human beta-cells and that SLC2A2 defects in other metabolic tissues drive the observed differences in glucose levels between carriers of SLC2A2 variants. Direct extrapolation from rodent to human islet glucose transporter activity is unlikely to be appropriate.
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Proteins requiring post-translational modifications such as N-linked glycosylation are processed in the endoplasmic reticulum (ER). A diverse array of cellular stresses can lead to dysfunction of the ER and ultimately to an imbalance between protein-folding capacity and protein-folding load. Cells monitor protein folding by an inbuilt quality control system involving both the ER and the Golgi apparatus. Unfolded or misfolded proteins are tagged for degradation via ER-associated degradation (ERAD) or sent back through the folding cycle. Continued accumulation of incorrectly folded proteins can also trigger the unfolded protein response (UPR). In mammalian cells, UPR is a complex signaling program mediated by three ER transmembrane receptors: activating transcription factor 6 (ATF6), inositol requiring kinase 1 (IRE1) and double-stranded RNA-activated protein kinase (PKR)-like endoplasmic reticulum kinase (PERK). UPR performs three functions, adaptation, alarm, and apoptosis. During adaptation, the UPR tries to reestablish folding homeostasis by inducing the expression of chaperones that enhance protein folding. Simultaneously, global translation is attenuated to reduce the ER folding load while the degradation rate of unfolded proteins is increased. If these steps fail, the UPR induces a cellular alarm and mitochondrial mediated apoptosis program. UPR malfunctions have been associated with a wide range of disease states including tumor progression, diabetes, as well as immune and inflammatory disorders. This review describes recent advances in understanding the molecular structure of UPR in mammalian cells, its functional role in cellular stress, and its pathophysiology.
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In this work, our aim was to determine whether L-arginine (a known insulinotropic amino acid) can promote a shift of β-cell intermediary metabolism favoring glutathione (GSH) and glutathione disulfide (GSSG) antioxidant responses, stimulus-secretion coupling and functional integrity. Clonal BRIN-BD11 β-cells and mouse islets were cultured for 24 h at various L-arginine concentrations (0-1.15  mmol/l) in the absence or presence of a proinflammatory cytokine cocktail (interleukin 1β, tumour necrosis factor α and interferon γ). Cells were assessed for viability, insulin secretion, GSH, GSSG, glutamate, nitric oxide (NO), superoxide, urea, lactate and for the consumption of glucose and glutamine. Protein levels of NO synthase-2, AMP-activated protein kinase (AMPK) and the heat shock protein 72 (HSP72) were also evaluated. We found that L-arginine at 1.15  mmol/l attenuated the loss of β-cell viability observed in the presence of proinflammatory cytokines. L-arginine increased total cellular GSH and glutamate levels but reduced the GSSG/GSH ratio and glutamate release. The amino acid stimulated glucose consumption in the presence of cytokines while also stimulating AMPK phosphorylation and HSP72 expression. Proinflammatory cytokines reduced, by at least 50%, chronic (24 h) insulin secretion, an effect partially attenuated by L-arginine. Acute insulin secretion was robustly stimulated by L-arginine but this effect was abolished in the presence of cytokines. We conclude that L-arginine can stimulate β-cell insulin secretion, antioxidant and protective responses, enabling increased functional integrity of β-cells and islets in the presence of proinflammatory cytokines. Glucose consumption and intermediary metabolism were increased by L-arginine. These results highlight the importance of L-arginine availability for β-cells during inflammatory challenge.
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A growing body of evidence suggests an inverse relationship between calcium and vitamin D status and dairy food intake and the development of the insulin resistance syndrome (IRS) and type 2 diabetes mellitus (t2DM). Observational studies show a consistent inverse association between dairy intake and the prevalence of IRS and t2DM. In a systematic review of the observational evidence, the odds for developing the IRS was 0.71 (95% CI, 0,57-0.89) for the highest dairy intake (3-4 servings/d) vs. the lowest intake (0.9-1.7 servings/d). Few interventional studies have been conducted to evaluate the effects of dairy food intake on the management of prevention of IRS or t2DM. Intervention studies that have examined the independent effects of dairy intake on specific metabolic components of the IRS including blood pressure and obesigenic parameters have shown favorable effects that support the observational findings albeit the results have been less consistent. Many metabolic and dietary factors appear to influence the degree to which dairy affects IRS metabolic parameters including calcium and vitamin D intake status, BMI, ethnicity and age. Overall, the intake of low-fat dairy products is a feature of a healthy dietary pattern which has been shown to contribute to a significant extent to the prevention of IRS.
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Decreases in both beta-cell function and number can contribute to insulin deficiency in type 2 diabetes. Here, we quantified the beta-cell mass in pancreas obtained at autopsy of 57 type 2 diabetic (T2D) and 52 non-diabetic subjects of European origin. Sections from the body and tail were immunostained for insulin. The beta-cell mass was calculated from the volume density of beta-cells (measured by point-counting methods) and the weight of the pancreas. The pancreatic insulin concentration was measured in some of the subjects. beta-cell mass increased only slightly with body mass index (BMI). After matching for BMI, the beta-cell mass was 41% (BMI < 25) and 38% (BMI 26-40) lower in T2D compared with non-diabetic subjects, and neither gender nor type of treatment influenced these differences. beta-cell mass did not correlate with age at diagnosis but decreased with duration of clinical diabetes (24 and 54% lower than controls in subjects with <5 and >15 years of overt diabetes respectively). Pancreatic insulin concentration was 30% lower in patients. In conclusion, the average beta-cell mass is about 39% lower in T2D subjects compared with matched controls. Its decrease with duration of the disease could be a consequence of diabetes that, with further impairment of insulin secretion, contributes to the progressive deterioration of glucose homeostasis. We do not believe that the small difference in beta-cell mass observed within 5 years of onset could cause diabetes in the absence of beta-cell dysfunction.
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Introduction: Ceramide may be synthesized de novo or generated by sphingomyelinase-dependent hydrolysis of sphingomyelin. Areas covered: The role of ceramide, ceramide-sensitive signaling and ion channels in β-cell apoptosis, lipotoxicity and amyloid-induced β-cell death. Expert opinion: Ceramide participates in β-cell dysfunction and apoptosis after exposure to TNFα, IL-1β and IFN-γ, excessive amyloid and islet amyloid polypeptide or non-esterified fatty acids (lipotoxicity). Knockout of sphingomyelin synthase 1, which converts ceramide to sphingomyelin, leads to impairment of insulin secretion. Increased ceramidase activity or pharmacological inhibition of ceramide synthetase, inhibits β-cell apoptosis. Ceramide contributes to endoplasmatic reticulum (ER) stress, decreased mitochondrial membrane potential in insulin-secreting cells and mitochondrial release of cytochrome c into the cytosol, which are all triggers of apoptotic cell death. Ceramide-dependent signaling involves activation of extracellularly regulated kinases 1 and 2 (ERK1/2), downregulation of Period (Per)-aryl hydrocarbon receptor nuclear translocator (Arnt)-single-minded (Sim) kinase (PASK), activation of okadaic-acid-sensitive protein phosphatase 2A (PP2A) and stimulation of NADPH-oxidase with generation of superoxides and lipid peroxides. Ceramide reduces the activity of voltage gated potassium (Kv)-channels in insulin-secreting cells. The role of ceramide in β-cell survival and function may be therapeutically relevant, because ceramide formation can be suppressed by pharmacological inhibition of ceramide synthetase and/or sphingomyelinase.
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Three novel human insulin-releasing cell lines designated 1.1B4, 1.4E7, and 1.1E7 were generated by electrofusion of freshly isolated of human pancreatic beta cells and the immortal human PANC-1 epithelial cell line. Functional studies demonstrated glucose sensitivity and responsiveness to known modulators of insulin secretion. Western blot, RT-PCR, and immunohistochemistry showed expression of the major genes involved in proinsulin processing and the pancreatic beta cell stimulus-secretion pathway including PC1/3, PC2, GLUT-1, glucokinase, and K-ATP channel complex (Sur1 and Kir6.2) and the voltage-dependent L-type Ca(2+) channel. The cells stained positively for insulin, and 1.1B4 cells were used to demonstrate specific staining for insulin, C-peptide, and proinsulin together with insulin secretory granules by electron microscopy. Analysis of metabolic function indicated intact mechanisms for glucose uptake, oxidation/utilization, and phosphorylation by glucokinase. Glucose, alanine, and depolarizing concentrations of K(+) were all able to increase [Ca(2+)](i) in at least two of the cell lines tested. Insulin secretion was also modulated by other nutrients, hormones, and drugs acting as stimulators or inhibitors in normal beta cells. Subscapular implantation of the 1.1B4 cell line improved hyperglycemia and resulted in glucose lowering in streptozotocin-diabetic SCID mice. These novel human electrofusion-derived beta cell lines therefore exhibit stable characteristics reminiscent of normal pancreatic beta cells, thereby providing an unlimited source of human insulin-producing cells for basic biochemical studies and pharmacological drug testing plus proof of concept for cellular insulin replacement therapy.