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Molecular mechanisms of inflammation in obesity-linked insulin resistance

Molecular mechanisms of inflammation in obesity-
linked insulin resistance
A Marette
Department of Anatomy and Physiology, Lipid Research Unit, Laval University Hospital Research Center, Ste-Foy, Que
International Journal of Obesity (2003) 27, S46–S48. doi:10.1038/sj.ijo.0802500
It is now well established that obesity is a chronic
inflammatory disorder. First proposed in pioneering studies
from Hotamisligil and Spiegelman
more than a decade ago,
there is now convincing experimental evidence for the
existence of an inflammatory link between obesity and the
occurrence of the insulin resistance dyslipidemic syndrome
commonly known as the ‘metabolic syndrome’. An ever-
increasing number of molecules that are best known for their
role in immune and inflammatory cells are now considered
as key modulators of energy metabolism in insulin target
tissues. In fact, several of those molecules (eg tumor necrosis
factor (TNF)-a, interleukine (IL)-6 and IL-1) are expressed and
secreted by fat cells in the expanded adipose tissue of obese
subjects. Those immune/inflammatory mediators as well as
adipose-specific cytokines (eg leptin and resistin) dubbed
‘adipokines’ exert their actions via a complex interplay of
signal transduction mechanisms that we are just beginning
to appreciate. The last few years have witnessed a burst of
studies providing strong experimental and genetic support
to the pathogenic role of some inflammatory pathways in
the development of insulin resistance in obesity. The goal of
this session of the symposium was to shed light on key
inflammatory pathways thought to promote insulin resis-
tance and the molecular mechanisms by which they are
believed to interfere with insulin signaling.
The use of salicylates for the treatment of diabetes mellitus
was first reported more than a century ago and the glucose-
lowering action of aspirin was confirmed in the 1950s.
Steven Shoelson presented recent experimental work sup-
porting the hypothesis that high-dose aspirin exerts its
beneficial effects through inhibition of the proinflammatory
pathway Ikappa B kinase (IKK)/NF-kB, a known target of
high-dose salicylates. In a series of elegant studies, Shoelson
and colleagues have found that the IKK/NF-kB pathway is
activated in insulin target tissues of obese insulin-resistant
animals and that high-dose aspirin/salicylates improve
insulin sensitivity and glucose tolerance in such animals.
To gain further genetic evidence for the role of the IKK/NF-
kB axis in obesity-linked insulin resistance, they have
performed additional studies in heterozygous Ikkb
þ /
fed a high-fat diet or crossed with obese ob/ob mice. In both
models, a 50% decrease in the Ikkb gene led to reductions in
blood glucose levels and improved insulin sensitivity. High-
dose salicylates and heterozygous Ikkb gene deletion also
protected against the insulin-resistant effects of lipid infu-
sion in rats. Shoelson concluded his talk by proposing future
directions that are intended to answer important questions
such as the identification of target sites of IKK/NF-kB-
induced insulin resistance. A more provocative question is
whether salicylates should be reconsidered for the treatment
of type 2 diabetic subjects. Recent data confirmed that high-
dose salicylates improve peripheral insulin action in diabetic
patients, but additional studies using lower doses of salsalate,
a less irritating head-to-tail dimer of salicylate, are currently
underway to determine the efficacy of this drug in alleviat-
ing insulin resistance and improving glucose control.
Salsalate may turn out to be an effective insulin sensitizer,
and rejuvenate a 100-y-old concept that salicylates exert
antidiabetic effects.
Inflammatory cytokines also activate the MAP kinase
pathway in several cell types. As reported by Gohka
Hotamisligil at this meeting, c-Jun N-terminal kinase (JNK)
activity is elevated in tissues of dietary and genetic models of
obesity. JNK1 but not JNK2 gene invalidation in mice
protects from high-fat-induced obesity and insulin resis-
It appears that JNK1 may cause insulin resistance by
promoting serine phosphorylation of insulin receptor sub-
strate-1 (IRS-1), which converts this protein into an inhibitor
of insulin signaling. Indeed, JNK1 invalidation reduces IRS-1
phosphorylation on serine 307, a well-known site of
heterologous inhibition of IRS-1 signaling. Genetic evidence
*Correspondence: Dr A Marette, Department of Physiology & Lipid
Research Unit, Laval University Hospital Research Center, 2705, Laurier
Bld, Ste-Foy, Que
bec, Canada G1V 4G2.
International Journal of Obesity (2003) 27, S46S48
2003 Nature Publishing Group All rights reserved 0307-0565/03
linking increased JNK activity to human diabetes was also
discussed. Indeed, loss-of-function mutations in JIP1, a
natural inhibitor of JNK, causes type 2 diabetes in humans.
Hotamisligil concluded his talk on some evolutionary
concepts, alluding to the fat body of the fruit fly Drosophila
melanogaster. This tissue concomitantly functions as a
hepatic, adipose, and hematopoietic/immune organ and
key proteins, including the fly equivalent of JNK, assume
both metabolic and inflammatory functions. This offers a
selective advantage in providing a strong immune response
to overcome infections, but it brings about an equally strong
metabolic response (eg insulin resistance), which, in the
context of plenty, favors the expression of contemporary
metabolic diseases such as diabetes and atherosclerosis.
Proinflammatory cytokines may also cause insulin resis-
tance by transcriptional mechanisms. Recent studies have
shown that cytokine-induced expression of inducible nitric
oxide synthase (iNOS)
and the suppressors of cytokine
signaling family (SOCS),
can lead to impaired insulin
signaling in muscle and fat tissues. In his talk, Emmanuel
Van Obberghen summarized recent work from his group
showing that SOCS3 is overexpressed in fat of murine
models of acquired and genetic obesity and that exposure
to TNF-a also induces SOCS3 expression in this tissue.
Conversely, SOCS3 levels are decreased in the adipose tissue
of ob/ob mice lacking TNF- a receptors. Hyperinsulinemia
may also be involved in the negative action of SOCS3 on
insulin signaling since insulin itself can promote its activa-
tion by tyrosine phosphorylation. SOCS3 may interfere with
insulin signaling by competing with the ability of the insulin
receptor to increase IRS-1 tyrosine phosphorylation and
downstream signaling to PI 3-kinase,
or by favoring IRS-1/2
degradation through the ubiquitin-mediated degradation
pathway as recently reported by White and colleagues.
Obberghen also brought into perspective the fact that many
insulin-resistant factors or pathways converge at the level of
IRS proteins, either by uncoupling IRS-mediated downstream
signals, and/or through promoting their degradation, at least
in part by enhancing their serine phosphorylation.
Yehiel Zick’s presentation brought us even deeper into the
signal transduction mechanisms leading to ser/thr phos-
phorylation of IRS proteins. He reminded us that ser/thr
phosphorylation is not necessarily negative as the insulin-
responsive protein kinase Akt/PKB also phosphorylates
serines within the PTB domain of IRS-1 but that this protects
from the action of protein tyrosine phosphatases and keeps
IRS-1 in a tyrosine phosphorylated active state.
In contrast,
other insulin-sensitive kinases, such as the mammalian
target of rapamycin (mTOR),
as well as atypical PKCz,
can also phosphorylate IRS-1 on serine residues and
terminate insulin signaling at the level of PI 3-kinase. This
may represent an efficient feedback regulatory mechanism of
insulin signaling and the starting point may be located at the
level of PI 3-kinase itself since its inhibitor, wortmannin,
blocks the negative effects of the above IRS serine kinases. In
inflammatory conditions such as obesity-linked diabetes,
this feedback control loop may over-ride the normal
activation of insulin signaling and thus promote insulin
resistance. Interestingly, IKKb has been shown to be a
substrate of PKCz and to be activated by a functional PKCz,
leading to the hypothesis that IKKb may be a downstream
mediator of PKCz-mediated inhibition of insulin signaling.
This is supported by the observation that TNF-a activates
both PKCz and IKKb. Zick concluded by asking an important
question, that is, are the same or different pathways being
utilized by insulin and inflammatory mediators to regulate
the activity of IRS serine kinases? Clearly, overlapping
pathways are likely to contribute to the modulation of ser/
thr phosphorylation of IRS proteins and the contribution of
each of those pathways may well vary between individual
target tissues.
This symposium thus identified key signal transduction
pathways activated by inflammatory mediators that can
interfere with insulin receptor signaling in insulin target
cells. A majority of these complex and inter-related pathways
appear to converge at the level of IRS-1, by promoting its
serine phosphorylation to mediate heterologous inhibition
of IRS-1 signaling to counter-regulate the insulin response.
The future challenge is to integrate reductionist models into
reality and describe networks of signal transduction path-
ways in complex biosystems. A detailed knowledge of this
complexity is a daunting task that will require a multifaceted
approach, which is, nevertheless, necessary for the develop-
ment of effective means for the treatment of insulin
resistance, a pathognomonic syndrome in chronic metabolic
disorders such as obesity, diabetes, and associated cardiovas-
cular diseases.
I would like to express my gratitude to Dr Yves Deshaies for
editing and critically revising several manuscripts of this
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Inflammatory mechanisms in obesity
A Marette
International Journal of Obesity
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Inflammatory mechanisms in obesity
A Marette
International Journal of Obesity
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Patients with SARS-CoV-2 infections experience lymphopenia and inflammatory cytokine storms in the severe stage of the disease, leading to multi-organ damage. The exact pattern of immune system changes and their condition during the disease process is unclear. The available knowledge has indicated that the NF-kappa-B pathway, which is induced by several mediators, has a significant role in cytokine storm through the various mechanisms. Therefore, identifying the state of the immune cells and the dominant mechanisms for the production of cytokines incorporated in the cytokine storm can be a critical step in the therapeutic approach. On the other hand, some studies identified a higher risk for diabetic patients. Diabetes mellitus exhibits a close association with inflammation and increases the chance of developing COVID-19. Patients with diabetes mellitus have shown to have more virus entry, impaired immunity response, less viral elimination, and dysregulated inflammatory cytokines. The parallel analysis of COVID-19 and diabetes mellitus pathogenesis has proposed that the control of the inflammation through the interfering with the critical points of major signaling pathways may provide the new therapeutic approaches. In recent years, the role of Dipeptidyl Peptidase 4 (DPP4) in chronic inflammation has been proved. Numerous immune cells express the DPP4 protein. DPP4 regulates antibody production, cytokine secretion, and immunoglobulin class switching. DPP4 inhibitors like sitagliptin reduce inflammation intensity in different states. Following the accumulating data, we hypothesize that sitagliptin might reduce COVID-19 severity. Sitagliptin, an available DPP4 inhibitor drug, showed multidimensional anti-inflammatory effects among diabetic patients. It reduces the inflammation mostly by affecting on NF-kappa-B signaling pathway. Under the fact that inflammatory mediators are active in individuals with COVID-19, blocking the predominant pathway could be helpful.
... Both fat compartments have different health implications, where excess visceral fat is associated with type 2 diabetes [43], insulin resistance [44], inflammatory diseases [45], and other obesity-related diseases [46]. Hence, from a clinical perspective, preferential loss of visceral fat may be metabolically advantageous [47]. ...
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... Accumulation of intracellular lipid, receptor downregulation induced by insulin itself, increased FFAs and various circulating peptides (adiponectin, resistin, IL-6) produced by adipocytes are among major factors that provide a molecular link between obesity and IR [46,49]. Inflammatory mediators (TNF-α, IL-1, and IL-6) and adipose-specific cytokines (leptin and resistin) can interfere with insulin receptor signaling at target cells by promoting serine phosphorylation to mediate heterologous inhibition of IRS-1 signaling thereby counter-regulating the insulin response [63]. Experiments on mice suggested that components of the extracellular matrix, including collagen, convert extracellular signals via their cytoplasmic domain and binding to intracellular integrin-binding proteins [64]. ...
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... The origin of this inflammation could be related to adipose tissue expansion because the expression of pro-inflammatory cytokines including TNFα and IL-1β are increased in this tissue in the obese state. In this regard, increased macrophage population in adipose tissue is thought to be responsible for this elevated production of cytokines [3,4]. ...
... Adipose tissue can be classified according to its location and function, e.g., visceral fat versus subcutaneous fat and WAT versus brown adipose tissue (BAT). Among these types, excess visceral adipose tissue has been shown to be important in obesity-related diseases and in diabetes [9][10][11]. Currently, little is known regarding the roles of peripheral nerves in adipose tissue and their changes in diabetes with respect to metabolic outcomes or peripheral neuropathies. ...
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Present study investigated the morphologic changes of autonomic nerves in the adipose tissue in diabetic animal model. Male obese type 2 diabetic db/db mice and age matched non-diabetic db/m control mice were used. Epididymal adipose tissue from diabetic db/db mice with that from control heterozygous db/m mice was compared using confocal microscopy-based method to visualize intact whole adipose tissue. Immunohistochemistry with tyrosine hydroxylase for sympathetic (SP), choline acetyltransferase for parasympathetic (PSP), and protein gene product 9.5 (PGP 9.5) for whole autonomic nerves was performed. The quantity of immunostained portion of SP, PSP, and PGP 9.5 stained nerve fibers showed decreased trend in diabetic group; however, the ratio of SP/PSP of adipose tissue was higher in diabetic group compared with control group as follows (0.70±0.30 vs. 0.95±0.25, P<0.05; normal vs. diabetic, respectively). Both SP and PSP nerve fibers were observed in white adipose tissue and PSP nerve fibers were suggested as more decreased in diabetes based on our observation.
... It appears that the type of fat storage may also contribute to AT toxicant accumulation. There are two major areas where AT deposits: 1) visceral AT (vAT), which surrounds internal organs and is generally considered to contribute to obesity-related diseases (275) (196) (257), and 2) subcutaneous AT (scAT), located beneath the skin. These AT locations can display unique structural features and properties that may influence the kinetics of toxicants. ...
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We examine the role of adipose tissue, typically considered an energy storage site, as a potential site of toxicant accumulation. Although the production of most persistent organic pollutants (POPs) was banned years ago, these toxicants persist in the environment due to their resistance to biodegradation and widespread distribution in various environmental forms (e.g., vapor, sediment, and water). As a result, human exposure to these toxicants is inevitable. Largely due to their lipophilicity, POPs bioaccumulate in adipose tissue, resulting in greater body burdens of these environmental toxicants with obesity. POPs of major concern include polychlorinated biphenyls (PCBs), polychlorinated dibenzo-p-dioxins and furans (PCDDs/PCDFs), and polybrominated biphenyls and diphenyl ethers (PBBs/PBDEs), among other organic compounds. In this review, we (i) highlight the physical characteristics of toxicants that enable them to partition into and remain stored in adipose tissue, (ii) discuss the specific mechanisms of action by which these toxicants act to influence adipocyte function, and (iii) review associations between POP exposures and the development of obesity and diabetes. An area of controversy relates to the relative potential beneficial versus hazardous health effects of toxicant sequestration in adipose tissue. © 2017 American Physiological Society. Compr Physiol 7:1085-1135, 2017.
... Consistently with our observations, numerous studies have shown that the mentioned cytokines induce adipogenesis. 26,[31][32][33] In particular, we reported that IL1b level in untreated diabetic cells is overexpressed compared to control cells. To this regard this finding correlates with reduced IRS-1 gene expression only in diabetic cells under our experimental conditions. ...
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Adipose tissue dysfunction represents a hallmark of diabetic patients and is a consequence of the altered homeostasis of this tissue. Mesenchymal stem cells (MSCs) and their differentiation into adipocytes contribute significantly in maintaining the mass and function of adult adipose tissue. The aim of this study was to evaluate the differentiation of MSCs from patients suffering type 2 diabetes (dASC) and how such process results in hyperplasia or rather a stop of adipocyte turnover resulting in hypertrophy of mature adipocytes. Our results showed that gene profile of all adipogenic markers is not expressed in diabetic cells after differentiation indicating that diabetic cells fail to differentiate into adipocytes. Interestingly, delta like 1, peroxisome proliferator-activated receptor alpha, and interleukin 1β were upregulated whereas Sirtuin 1 and insulin receptor substrate 1 gene expression were found downregulated in dASC compared to cells obtained from healthy subjects. Taken together our data indicate that dASC lose their ability to differentiate into mature and functional adipocytes. In conclusion, our in vitro study is the first to suggest that diabetic patients might develop obesity through a hypertrophy of existing mature adipocytes due to failure turnover of adipose tissue. Impact statement In the present manuscript, we evaluated the differentiative potential of mesenchymal stem cells (MSCs) in adipocytes obtained from healthy and diabetic patients. This finding could be of great potential interest for the field of obesity in order to exploit such results to further understand the pathophysiological processes underlying metabolic syndrome. In particular, inflammation in diabetic patients causes a dysfunction in MSCs differentiation and a decrease in adipocytes turnover leading to insulin resistance.
... The central nervous system and immune system stress outflow pathways are implicated in development of insulin resistance [23]. Mechanisms include marked elevation of counter regulatory hormones (norepinephrine, cortisol) and inflammatory cytokines [23,24]. ...
Insulin resistance can be seen as a molecular and genetic mystery, with a role in the pathophysiology of type 2 diabetes mellitus. It is a basis for a number of chronic diseases like hypertension, dyslipidemia, glucose intolerance, coronary heart disease, cerebral vascular disease along with T2DM, thus the key is to cure and prevent insulin resistance. Critical perspicacity into the etiology of insulin resistance have been gained by the use of animal models where insulin action has been modulated by various transgenic and non-transgenic models which is not possible in human studies. The following review comprises the pathophysiology involved in insulin resistance, various factors causing insulin resistance, their screening and various genetic and non-genetic animal models highlighting the pathological and metabolic characteristics of each.
AIM: PCOS is the most common endocrine disorder of reproductive age women, affecting 4-12%. The condition is associated with reproductive and cardiometabolic complications including obesity, insulin resistance, hyperandrogenism and infertility. Women with PCOS have a two to four-fold higher prevalence of metabolic syndrome (and higher oxidised LDL) and higher concentrations of Anti-Mullerian hormone (AMH) compared to BMI-matched controls. The present study aims to determine whether physical activity reduces AMH and OxLDL levels and therefore improves fertility and cardiometabolic outcomes. METHODS: Women with PCOS (n=14) and healthy controls (n=11) completed an 8-week exercise programme consisting of 3 x 1- hour treadmill sessions per week at 60% VO2max. Blood samples were analysed for AMH and oxidised LDL concentrations using the ELISA method. Pre collected anthropometric and cardiorespiratory fitness markers were also analysed to determine any significant differences in means before and after the intervention. RESULTS: Within the longitudinal PCOS study, mixed two-way ANOVAs found no significant differences between AMH and oxidised LDL levels pre and post intervention in PCOS or controls, despite significant improvements in body mass, BMI, WC and VO2max evident in the PCOS group. CONCLUSION: Results from the PCOS intervention demonstrated no significant improvement of AMH or oxidised LDL levels in women with PCOS or controls following short-term structure moderate intensity exercise training. It did have a significant impact on anthropometric and cardiorespiratory fitness markers. Future research should place importance upon identifying and conducting research on the phenotypes of PCOS separately, particularly in relation to those phenotoypes with hyperandrogenism, as well as conducting exercise interventions over a longer duration.
Mesenteric fat, a depot within the visceral fat, accumulates in cattle during maturation and finishing and may be a potential source of production inefficiency. The aim of this study was to determine whether the genes expressed in the mesenteric fat of steers were associated with body weight gain and feed intake. Sixteen steers chosen by their rank of distance from the bivariate mean for gain and feed intake were used for this study. Mesenteric fat was obtained and evaluated for differences in gene expression. A total of 1831 genes were identified as differentially expressed among steers with variation in feed intake and gain. Many of these genes were involved with metabolic processes such as proteolysis, transcription and translation. In addition, the Gene Ontology annotations including transport and localization were both over-represented among the differentially expressed genes. Pathway analysis was also performed on the differentially expressed genes. The superoxide radical degradation pathway was identified as over-represented based on the differential expression of the genes GPX7, SOD2 and TYRP1, suggesting a potential role for oxidative stress or inflammatory pathways among low gain-high intake animals. GPX7 and SOD2 were in lower transcript abundance, and TYRP1 was higher in transcript abundance among the low gain-high feed intake animals. The retinoate biosynthesis pathway was also enriched due to the differential expression of the genes AKR1C3, ALDH8A1, RDH8, RDH13 and SDR9C7. These genes were all more highly expressed in the low gain-high intake animals. The glycerol degradation and granzyme A signaling pathways were both associated with gain. Three glycerol kinase genes and the GZMA gene were differentially expressed among high vs. low gain animals. Mesenteric fat is a metabolically active tissue, and in this study, genes involved in proteolysis, transcription, translation, transport immune function, glycerol degradation and oxidative stress were differentially expressed among beef steers with variation in body weight gain and feed intake.
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Serine/threonine phosphorylation of insulin receptor substrate 1 (IRS-1) has been implicated as a negative regulator of insulin signaling. Prior studies have indicated that this negative regulation by protein kinase C involves the mitogen-activated protein kinase and phosphorylation of serine 612 in IRS-1. In the present studies, the negative regulation by platelet-derived growth factor (PDGF) was compared with that induced by endothelin-1, an activator of protein kinase C. In contrast to endothelin-1, the inhibitory effects of PDGF did not require mitogen-activated protein kinase or the phosphorylation of serine 612. Instead, three other serines in the phosphorylation domain of IRS-1 (serines 632, 662, and 731) were required for the negative regulation by PDGF. In addition, the PDGF-activated serine/threonine kinase called Akt was found to inhibit insulin signaling. Moreover, this inhibition required the same IRS-1 serine residues as the inhibition by PDGF. Finally, the negative regulatory effects of PDGF and Akt were inhibited by rapamycin, an inhibitor of the mammalian target of rapamycin (mTOR), one of the downstream targets of Akt. These studies implicate the phosphatidylinositol 3-kinase/Akt kinase cascade as an additional negative regulatory pathway for the insulin signaling cascade.
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Incubation of cells with insulin leads to a transient rise in Tyr phosphorylation of insulin receptor substrate (IRS) proteins, accompanied by elevation in their Ser(P)/Thr(P) content and their dissociation from the insulin receptor (IR). Wortmannin, a phosphatidylinositol 3-kinase inhibitor, selectively prevented the increase in Ser(P)/Thr(P) content of IRS-1, its dissociation from IR, and the decrease in its Tyr(P) content following 60 min of insulin treatment. Four conserved phosphorylation sites within the phosphotyrosine binding/SAIN domains of IRS-1 and IRS-2 served as in vitro substrates for protein kinase B (PKB), a Ser/Thr kinase downstream of phosphatidylinositol 3-kinase. Furthermore, PKB and IRS-1 formed stable complexes in vivo, and overexpression of PKB enhanced Ser phosphorylation of IRS-1. Overexpression of PKB did not affect the acute Tyr phosphorylation of IRS-1; however, it significantly attenuated its rate of Tyr dephosphorylation following 60 min of treatment with insulin. Accordingly, overexpression of IRS-1(4A), lacking the four potential PKB phosphorylation sites, markedly enhanced the rate of Tyr dephosphorylation of IRS-1, while inclusion of vanadate reversed this effect. These results implicate a wortmannin-sensitive Ser/Thr kinase, different from PKB, as the kinase that phosphorylates IRS-1 and acts as the feedback control regulator that turns off insulin signals by inducting the dissociation of IRS proteins from IR. In contrast, insulin-stimulated PKB-mediated phosphorylation of Ser residues within the phosphotyrosine binding/SAIN domain of IRS-1 protects IRS-1 from the rapid action of protein-tyrosine phosphatases and enables it to maintain its Tyr-phosphorylated active conformation. These findings implicate PKB as a positive regulator of IRS-1 functions.
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Incubation of rat hepatoma Fao cells with insulin leads to a transient rise in Tyr phosphorylation of insulin receptor substrate (IRS) proteins. This is followed by elevation in their P-Ser/Thr content, and their dissociation from the insulin receptor (IR). Wortmannin, a phosphatidylinositol 3-kinase (PI3K) inhibitor, abolished the increase in the P-Ser/Thr content of IRS-1, its dissociation from the IR, and the decrease in its P-Tyr content following 60 min of insulin treatment, indicating that the Ser kinases that negatively regulate IRS-1 function are downstream effectors of PI3K. PKCzeta fulfills this criterion, being an insulin-activated downstream effector of PI3K. Overexpression of PKCzeta in Fao cells, by infection of the cells with adenovirus-based PKCzeta construct, had no effect on its own, but it accelerated the rate of insulin-stimulated dissociation of IR.IRS-1 complexes and the rate of Tyr dephosphorylation of IRS-1. The insulin-stimulated negative regulatory role of PKCzeta was specific and could not be mimic by infecting Fao cells with adenoviral constructs encoding for PKC alpha, delta, or eta. Because the reduction in P-Tyr content of IRS-1 was accompanied by a reduced association of IRS-1 with p85, the regulatory subunit of PI3K, it suggests that this negative regulatory process induced by PKCzeta, has a built-in attenuation signal. Hence, insulin triggers a sequential cascade in which PI3K-mediated activation of PKCzeta inhibits IRS-1 functions, reduces complex formation between IRS-1 and PI3K, and inhibits further activation of PKCzeta itself. These findings implicate PKCzeta as a key element in a multistep negative feedback control mechanism of IRS-1 functions.
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Amino acids have emerged as potent modulators of the mTOR/p70 S6 kinase pathway. The involvement of this pathway in the regulation of insulin-stimulated glucose transport was investigated in the present study. Acute exposure (1 h) to a balanced mixture of amino acids reduced insulin-stimulated glucose transport by as much as 55% in L6 muscle cells. The effect of amino acids was fully prevented by the specific mTOR inhibitor rapamycin. Time course analysis of insulin receptor substrate 1 (IRS-1)-associated phosphatidylinositol (PI) 3-kinase activity revealed that incubation with amino acids speeds up its time-dependent deactivation, leading to a dramatic suppression (-70%) of its activity after 30 min of insulin stimulation as compared with its maximal activation (5 min of stimulation). This accelerated deactivation of PI 3-kinase activity in amino acid-treated cells was associated with a concomitant and sustained increase in the phosphorylation of p70 S6 kinase. In marked contrast, inhibition of mTOR by rapamycin maintained PI 3-kinase maximally activated for up to 30 min. The marked inhibition of insulin-mediated PI 3-kinase activity by amino acids was linked to a rapamycin-sensitive increase in serine/threonine phosphorylation of IRS-1 and a decreased binding of the p85 subunit of PI 3-kinase to IRS-1. Furthermore, amino acids were required for the degradation of IRS-1 during long term insulin treatment. These results identify the mTOR/p70 S6 kinase signaling pathway as a novel modulator of insulin-stimulated glucose transport in skeletal muscle cells.
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Inducible nitric oxide synthase (iNOS) is induced by inflammatory cytokines in skeletal muscle and fat. It has been proposed that chronic iNOS induction may cause muscle insulin resistance. Here we show that iNOS expression is increased in muscle and fat of genetic and dietary models of obesity. Moreover, mice in which the gene encoding iNOS was disrupted (Nos2-/- mice) are protected from high-fat-induced insulin resistance. Whereas both wild-type and Nos2-/- mice developed obesity on the high-fat diet, obese Nos2-/- mice exhibited improved glucose tolerance, normal insulin sensitivity in vivo and normal insulin-stimulated glucose uptake in muscles. iNOS induction in obese wild-type mice was associated with impairments in phosphatidylinositol 3-kinase and Akt activation by insulin in muscle. These defects were fully prevented in obese Nos2-/- mice. These findings provide genetic evidence that iNOS is involved in the development of muscle insulin resistance in diet-induced obesity.
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SOCS (suppressor of cytokine signaling) proteins are inhibitors of cytokine signaling involved in negative feedback loops. We have recently shown that insulin increases SOCS-3 mRNA expression in 3T3-L1 adipocytes. When expressed, SOCS-3 binds to phosphorylated Tyr(960) of the insulin receptor and prevents Stat 5B activation by insulin. Here we show that in COS-7 cells SOCS-3 decreases insulin-induced insulin receptor substrate 1 (IRS-1) tyrosine phosphorylation and its association with p85, a regulatory subunit of phosphatidylinositol-3 kinase. This mechanism points to a function of SOCS-3 in insulin resistance. Interestingly, SOCS-3 expression was found to be increased in the adipose tissue of obese mice, but not in the liver and muscle of these animals. Two polypeptides known to be elevated during obesity, insulin and tumor necrosis factor-alpha (TNF-alpha), induce SOCS-3 mRNA expression in mice. Insulin induces a transient expression of SOCS-3 in the liver, muscle, and the white adipose tissue (WAT). Strikingly, TNF-alpha induced a sustained SOCS-3 expression, essentially in the WAT. Moreover, transgenic ob/ob mice lacking both TNF receptors have a pronounced decrease in SOCS-3 expression in the WAT compared with ob/ob mice, providing genetic evidence for a function of this cytokine in obesity-induced SOCS-3 expression. As SOCS-3 appears as a TNF-alpha target gene that is elevated during obesity, and as SOCS-3 antagonizes insulin-induced IRS-1 tyrosine phosphorylation, we suggest that it is a player in the development of insulin resistance.
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Inflammation associates with peripheral insulin resistance, which dysregulates nutrient homeostasis and leads to diabetes. Inflammation induces the expression of SOCS proteins. We show that SOCS1 or SOCS3 targeted IRS1 and IRS2, two critical signaling molecules for insulin action, for ubiquitin-mediated degradation. SOCS1 or SOCS3 bound both recombinant and endogenous IRS1 and IRS2 and promoted their ubiquitination and subsequent degradation in multiple cell types. Mutations in the conserved SOCS box of SOCS1 abrogated its interaction with the elongin BC ubiquitin-ligase complex without affecting its binding to IRS1 or IRS2. The SOCS1 mutants also failed to promote the ubiquitination and degradation of either IRS1 or IRS2. Adenoviral-mediated expression of SOCS1 in mouse liver dramatically reduced hepatic IRS1 and IRS2 protein levels and caused glucose intolerance; by contrast, expression of the SOCS1 mutants had no effect. Thus, SOCS-mediated degradation of IRS proteins, presumably via the elongin BC ubiquitin-ligase, might be a general mechanism of inflammation-induced insulin resistance, providing a target for therapy.
Incubation of rat hepatoma Fao cells with insulin leads to a transient rise in Tyr phosphorylation of insulin receptor substrate (IRS) proteins. This is followed by elevation in their P-Ser/Thr content, and their dissociation from the insulin receptor (IR). Wortmannin, a phosphatidylinositol 3-kinase (PI3K) inhibitor, abolished the increase in the P-Ser/Thr content of IRS-1, its dissociation from the IR, and the decrease in its P-Tyr content following 60 min of insulin treatment, indicating that the Ser kinases that negatively regulate IRS-1 function are downstream effectors of PI3K. PKCζ fulfills this criterion, being an insulin-activated downstream effector of PI3K. Overexpression of PKCζ in Fao cells, by infection of the cells with adenovirus-based PKCζ construct, had no effect on its own, but it accelerated the rate of insulin-stimulated dissociation of IR·IRS-1 complexes and the rate of Tyr dephosphorylation of IRS-1. The insulin-stimulated negative regulatory role of PKCζ was specific and could not be mimic by infecting Fao cells with adenoviral constructs encoding for PKC α, δ, or η. Because the reduction in P-Tyr content of IRS-1 was accompanied by a reduced association of IRS-1 with p85, the regulatory subunit of PI3K, it suggests that this negative regulatory process induced by PKCζ, has a built-in attenuation signal. Hence, insulin triggers a sequential cascade in which PI3K-mediated activation of PKCζ inhibits IRS-1 functions, reduces complex formation between IRS-1 and PI3K, and inhibits further activation of PKCζ itself. These findings implicate PKCζ as a key element in a multistep negative feedback control mechanism of IRS-1 functions.
Tumor necrosis factor-alpha (TNF-alpha) has been shown to have certain catabolic effects on fat cells and whole animals. An induction of TNF-alpha messenger RNA expression was observed in adipose tissue from four different rodent models of obesity and diabetes. TNF-alpha protein was also elevated locally and systemically. Neutralization of TNF-alpha in obese fa/fa rats caused a significant increase in the peripheral uptake of glucose in response to insulin. These results indicate a role for TNF-alpha in obesity and particularly in the insulin resistance and diabetes that often accompany obesity.
We show that high doses of salicylates reverse hyperglycemia, hyperinsulinemia, and dyslipidemia in obese rodents by sensitizing insulin signaling. Activation or overexpression of the IκB kinase β (IKKβ) attenuated insulin signaling in cultured cells, whereas IKKβ inhibition reversed insulin resistance. Thus, IKKβ, rather than the cyclooxygenases, appears to be the relevant molecular target. Heterozygous deletion (Ikkβ +/−) protected against the development of insulin resistance during high-fat feeding and in obese Lepob/ob mice. These findings implicate an inflammatory process in the pathogenesis of insulin resistance in obesity and type 2 diabetes mellitus and identify the IKKβ pathway as a target for insulin sensitization.