Nigel Turner

Garvan Institute of Medical Research, דרלינגהרסט, New South Wales, Australia

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Publications (102)521.33 Total impact

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    ABSTRACT: Diets enriched in fructose (FR) increase lipogenesis in the liver, leading to hepatic lipid accumulation and the development of insulin resistance. Previously, we have shown that in contrast to other mouse strains, BALB/c mice are resistant to high fat diet-induced metabolic deterioration, potentially due to a lack of ectopic lipid accumulation in the liver. In this study we have compared the metabolic response of BALB/c and C57BL/6 (BL6) mice to a fructose-enriched diet. Both strains of mice increased adiposity in response to FR-feeding, while only BL6 mice displayed elevated hepatic triglyceride (TAG) accumulation and glucose intolerance. The lack of hepatic TAG accumulation in BALB/c mice appeared to be linked to an altered balance between lipogenic and lipolytic pathways, while the protection from fructose-induced glucose intolerance in this strain was likely related to low levels of ER stress, a slight elevation in insulin levels and an altered profile of diacylglycerol species in the liver. Collectively these findings highlight the multifactorial nature of metabolic defects that develop in response to changes in the intake of specific nutrients and the divergent response of different mouse strains to dietary challenges.
    Preview · Article · Dec 2015 · Scientific Reports
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    ABSTRACT: An important regulator of fatty acid oxidation (FAO) is the allosteric inhibition of CPT-1 by malonyl-CoA produced by the enzyme acetyl-CoA carboxylase 2 (ACC2). Initial studies suggested that deletion of ACC2 increased fat oxidation and reduced adipose tissue mass but in an independently generated strain of ACC2 knockout mice we observed increased whole body and skeletal muscle FAO and a compensatory increase in muscle glycogen stores without changes in glucose tolerance, energy expenditure or fat mass in young mice (12-16 weeks). The aim of the current study was to determine any effect of age or housing at thermoneutrality (29°C; which reduces total energy expenditure), on the phenotype of ACC2 knockout mice. At 42-54 weeks of age, male wild type (WT) and ACC2-/- mice had similar body weight, fat mass, muscle triglyceride content and glucose tolerance. Consistent with younger ACC2-/-mice, aged ACC2-/- mice showed increased whole body FAO (24hr average RER=0.95±0.02 and 0.92±0.02 for WT and ACC2-/- respectively, p<0.05) and skeletal muscle glycogen content (+60%, p<0.05) without any detectable change in whole body energy expenditure. Hyperinsulinemic-euglycemic clamp studies revealed no difference in insulin action between groups, with similar glucose infusion rates and tissue glucose uptake. Housing ACC2-/- mice at 29°C did not alter body composition, glucose tolerance, or the effects of fat feeding compared to WT. These results confirm that manipulation of ACC2 may alter FAO in mice but this has little impact on body composition or insulin action.
    No preview · Article · Dec 2015 · Journal of Endocrinology
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    ABSTRACT: Intramuscular fat accumulation, skeletal muscle mitochondrial dysfunction and oxidative stress have all been implicated in the development of human insulin resistance and type 2 diabetes mellitus (T2DM).
    No preview · Article · Nov 2015
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    ABSTRACT: Null mutations of the Niemann-Pick type C1 ( NPC1 ) gene cause NPC disease, a lysosomal storage disorder characterised by cholesterol accumulation in late endosome and lysosomes. Nascent or mutated NPC1 is degraded through the ubiquitin-proteosome pathway, but how NPC1 degradation is regulated is currently unknown. Here, we demonstrated a link between NPC1 degradation and the Akt/mTOR (mechanistic target of rapamycin) signaling pathway in cervical cancer cell lines. We provided evidence that activated Akt/mTOR pathway increased NPC1 degradation by ~ 50% in C33A cells when compared to SiHa or HeLa cells. NPC1 degradation in C33A cells was reversed when Akt/mTOR activation was blocked by specific inhibitors or when mTORC1 (mTOR complex 1) was disrupted by Raptor knockdown. Importantly, inhibition of the Akt/mTOR pathway led to decreased NPC1 ubiquitination in C33A cells, pointing to a role of Akt/mTOR in the proteasomal degradation of NPC1. Moreover, we found that NPC1 depletion in several cancer cell lines inhibited cell proliferation and migration. Our results uncover Akt as a key regulator of NPC1 degradation, and link NPC1 to cancer cell proliferation and migration. Copyright 2015 The Author(s).
    Full-text · Article · Aug 2015 · Biochemical Journal
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    Sanket Joshi · Menghan Liu · Nigel Turner
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    ABSTRACT: Diabetes is a disease involving metabolic derangements in multiple organs. While the spectrum of diabetic complications has been known for years, recent evidence suggests that diabetes could also contribute to the initiation and propagation of certain cancers. The mechanism(s) underlying this relationship are not completely resolved but likely involve changes in hormone and nutrient levels, as well as activation of inflammatory and stress-related pathways. Interestingly, some of the drugs used clinically to treat diabetes also appear to have antitumour effects, further highlighting the interaction between these two conditions. In this contribution we review recent literature on this emerging relationship and explore the potential mechanisms that may promote cancer in diabetic patients.
    Full-text · Article · May 2015
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    ABSTRACT: SIRT1 is a NAD+-dependent deacetylase thought to regulate cellular metabolic pathways in response to alterations in nutrient flux. In the current study we investigated whether acute changes in SIRT1 expression affect markers of muscle mitochondrial content and also determined whether SIRT1 influenced muscle insulin resistance induced by acute glucose oversupply. In male Wistar rats either SIRT1 or a deacetylase inactive mutant form (H363Y) was electroprated into the tibialis cranialis (TC) muscle. The other leg was electroporated with an empty control vector. One week later, glucose was infused and hyperglycaemia was maintained at ~11mM. After 5 hours, 11mM glucose induced significant insulin resistance in skeletal muscle. Interestingly, overexpression of either SIRT1 or SIRT1 (H363Y) for 1 week did not change markers of mitochondrial content or function. SIRT1 or SIRT1 (H363Y) overexpression had no effect on the reduction in glucose uptake and glycogen synthesis in muscle in response to hyperglycemia. Therefore we conclude that acute increases in SIRT1 protein have little impact on mitochondrial content and that overexpressing SIRT1 does not prevent the development of insulin resistance during hyperglycaemia.
    Preview · Article · Mar 2015 · PLoS ONE
  • Nigel Turner · Rebecca L. Robker
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    ABSTRACT: Insulin resistance is a key defect associated with obesity, type 2 diabetes and other metabolic diseases. While a number of factors have been suggested to cause defects in insulin action, there is a very strong association between inappropriate lipid deposition in insulin target tissues and the development of insulin resistance. In recent times, a large number of studies have reported changes in markers of mitochondrial metabolism in insulin-resistant individuals, leading to the theory that defects in mitochondrial substrate oxidation are responsible for the buildup of lipid intermediates and the development of insulin resistance. The primary support for the mitochondrial theory of insulin resistance comes from studies in skeletal muscle; however, there is recent evidence in murine models that mitochondrial dysfunction in oocytes may also play a role. Oocytes from obese or insulin-resistant mice have been shown to exhibit abnormalities in many different mitochondrial parameters, including mitochondrial morphology and membrane potential. Here we review the findings regarding the link between mitochondrial dysfunction and insulin resistance, and propose that abnormalities in mitochondrial metabolism in oocytes may predispose to the development of obesity and insulin resistance and thus contribute to the inter-generational programming of metabolic disease.
    No preview · Article · Jan 2015 · Molecular Human Reproduction
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    Magdalene K Montgomery · Nigel Turner
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    ABSTRACT: Mitochondrial dysfunction has been implicated in the development of insulin resistance, however a large variety of association and intervention studies, as well as genetic manipulations in rodents have reported contrasting results. Indeed, even 39 years after the first publication describing a relationship between insulin resistance and diminished mitochondrial function, it is still unclear if a direct relationship exists, and more importantly if changes in mitochondrial capacity are a cause or consequence of insulin resistance. This review will take a journey through the past and summarize the debate about the occurrence of mitochondrial dysfunction and its possible role in causing decreased insulin action in obesity and type 2 diabetes. Evidence will be presented from studies in various human populations, as well as rodents with genetic manipulations of pathways known to affect mitochondrial function and insulin action. Finally, we will discuss if mitochondria are a potential target for the treatment of insulin resistance.
    Preview · Article · Nov 2014 · Endocrine Connections
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    ABSTRACT: Grb10 is an intracellular adaptor protein which binds directly to several growth factor receptors, including those for insulin and insulin-like growth factor receptor-1 (IGF-1), and negatively regulates their actions. Grb10-ablated (Grb10-/-) mice exhibit improved whole body glucose homeostasis and an increase in muscle mass associated specifically with an increase in myofiber number. This suggests that Grb10 may act as a negative regulator of myogenesis. In this study, we investigated in vitro, the molecular mechanisms underlying the increase in muscle mass and the improved glucose metabolism. Primary muscle cells isolated from Grb10-/- mice exhibited increased rates of proliferation and differentiation compared to primary cells isolated from wild-type mice. The improved proliferation capacity was associated with an enhanced phosphorylation of Akt and ERK in the basal state and changes in the expression of key cell cycle progression markers involved in regulating transition of cells from the G1 to S phase (e.g., retinoblastoma (Rb) and p21). The absence of Grb10 also promoted a faster transition to a myogenin positive, differentiated state. Glucose uptake was higher in Grb10-/- primary myotubes in the basal state and was associated with enhanced insulin signaling and an increase in GLUT4 translocation to the plasma membrane. These data demonstrate an important role for Grb10 as a link between muscle growth and metabolism with therapeutic implications for diseases, such as muscle wasting and type 2 diabetes. J. Cell. Physiol. © 2014 Wiley Periodicals, Inc.
    Full-text · Article · Nov 2014 · Journal of Cellular Physiology
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    ABSTRACT: There is a strong association between aging, diet, and immunity. The effects of macronutrients and energy intake on splanchnic and hepatic lymphocytes were studied in 15 month old mice. The mice were ad-libitum fed 1 of 25 diets varying in the ratios and amounts of protein, carbohydrate, and fat over their lifetime. Lymphocytes in liver, spleen, Peyers patches, mesenteric lymph nodes, and inguinal lymph nodes were evaluated using flow cytometry. Low protein intake reversed aging changes in splenic CD4 and CD8 T cells, CD4:CD8 T cell ratio, memory/effector CD4 T cells and naïve CD4 T cells. A similar influence of total caloric intake in these ad-libitum fed mice was not apparent. Protein intake also influenced hepatic NK cells and B cells, while protein to carbohydrate ratio influenced hepatic NKT cells. Hepatosteatosis was associated with increased energy and fat intake and changes in hepatic Tregs, effector/memory T, and NK cells. Hepatic NK cells were also associated with body fat, glucose tolerance, and leptin levels while hepatic Tregs were associated with hydrogen peroxide production by hepatic mitochondria. Dietary macronutrients, particularly protein, influence splanchnic lymphocytes in old age, with downstream associations with mitochondrial function, liver pathology, and obesity-related phenotype.
    Full-text · Article · Oct 2014 · The Journals of Gerontology Series A Biological Sciences and Medical Sciences
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    ABSTRACT: Excess dietary lipid generally leads to fat deposition and impaired glucose homeostasis, but consumption of fish oil (FO) alleviates many of these detrimental effects. The beneficial effects of FO are thought to be mediated largely via activation of the nuclear receptor peroxisomal-proliferator-activated receptor α (PPARα) by omega-3 polyunsaturated fatty acids and the resulting upregulation of lipid catabolism. However, pharmacological and genetic PPARα manipulations have yielded variable results. We have compared the metabolic effects of FO supplementation and the synthetic PPARα agonist Wy-14,643 (WY) in mice fed a lard-based high-fat diet. In contrast to FO, WY treatment resulted in little protection against diet-induced obesity and glucose intolerance, despite upregulating many lipid metabolic pathways. These differences were likely due to differential effects on hepatic lipid synthesis, with FO decreasing and WY amplifying hepatic lipid accumulation. Our results highlight that the beneficial metabolic effects of FO are likely mediated through multiple independent pathways.
    Full-text · Article · Jul 2014 · Scientific Reports
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    ABSTRACT: Lipid deposition in the liver is associated with metabolic disorders including fatty liver disease, type II diabetes, and hepatocellular cancer. The enzymes acetyl-CoA carboxylase 1 (ACC1) and ACC2 are powerful regulators of hepatic fat storage; therefore, their inhibition is expected to prevent the development of fatty liver. In this study we generated liver-specific ACC1 and ACC2 double knockout (LDKO) mice to determine how the loss of ACC activity affects liver fat metabolism and whole-body physiology. Characterization of LDKO mice revealed unexpected phenotypes of increased hepatic triglyceride and decreased fat oxidation. We also observed that chronic ACC inhibition led to hyper-acetylation of proteins in the extra-mitochondrial space. In sum, these data reveal the existence of a compensatory pathway that protects hepatic fat stores when ACC enzymes are inhibited. Furthermore, we identified an important role for ACC enzymes in the regulation of protein acetylation in the extra-mitochondrial space.
    Full-text · Article · Jul 2014 · Molecular Metabolism
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    ABSTRACT: A critical feature of obesity is enhanced insulin secretion from pancreatic β-cells, enabling the majority of individuals to maintain glycaemic control despite adiposity and insulin resistance. Surprisingly, the factors coordinating this adaptive β-cell response with adiposity have not been delineated. Here we show that fatty acid binding protein 4 (FABP4/aP2) is an adipokine released from adipocytes under obesogenic conditions, such as hypoxia, to augment insulin secretion. The insulinotropic action of FABP4 was identified using an in vitro system that recapitulates adipocyte to β-cell endocrine signalling, with glucose-stimulated insulin secretion (GSIS) as a functional readout, coupled with quantitative proteomics. Exogenous FABP4 potentiated GSIS in vitro and in vivo, and circulating FABP4 levels correlated with GSIS in humans. Insulin inhibited FABP4 release from adipocytes in vitro, in mice and in humans, consistent with feedback regulation. These data suggest that FABP4 and insulin form an endocrine loop coordinating the β-cell response to obesity.
    Full-text · Article · Jul 2014 · Molecular Metabolism
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    ABSTRACT: The fundamental questions of what represents a macronutritionally balanced diet and how this maintains health and longevity remain unanswered. Here, the Geometric Framework, a state-space nutritional modeling method, was used to measure interactive effects of dietary energy, protein, fat, and carbohydrate on food intake, cardiometabolic phenotype, and longevity in mice fed one of 25 diets ad libitum. Food intake was regulated primarily by protein and carbohydrate content. Longevity and health were optimized when protein was replaced with carbohydrate to limit compensatory feeding for protein and suppress protein intake. These consequences are associated with hepatic mammalian target of rapamycin (mTOR) activation and mitochondrial function and, in turn, related to circulating branched-chain amino acids and glucose. Calorie restriction achieved by high-protein diets or dietary dilution had no beneficial effects on lifespan. The results suggest that longevity can be extended in ad libitum-fed animals by manipulating the ratio of macronutrients to inhibit mTOR activation.
    Full-text · Article · Mar 2014 · Cell metabolism
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    Jayne Alexandra Barbour · Nigel Turner
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    ABSTRACT: Mitochondrial dysfunction has been implicated in the aetiology of many complex diseases, as well as the ageing process. Much of the research on mitochondrial dysfunction has focused on how mitochondrial damage may potentiate pathological phenotypes. The purpose of this review is to draw attention to the less well-studied mechanisms by which the cell adapts to mitochondrial perturbations. This involves communication of stress to the cell and successful induction of quality control responses, which include mitophagy, unfolded protein response, upregulation of antioxidant and DNA repair enzymes, morphological changes, and if all else fails apoptosis. The mitochondrion is an inherently stressful environment and we speculate that dysregulation of stress signaling or an inability to switch on these adaptations during times of mitochondrial stress may underpin mitochondrial dysfunction and hence amount to pathological states over time.
    Preview · Article · Jan 2014 · International Journal of Cell Biology
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    ABSTRACT: Ever since eukaryotes subsumed the bacterial ancestor of mitochondria, the nuclear and mitochondrial genomes have had to closely coordinate their activities, as each encode different subunits of the oxidative phosphorylation (OXPHOS) system. Mitochondrial dysfunction is a hallmark of aging, but its causes are debated. We show that, during aging, there is a specific loss of mitochondrial, but not nuclear, encoded OXPHOS subunits. We trace the cause to an alternate PGC-1α/β-independent pathway of nuclear-mitochondrial communication that is induced by a decline in nuclear NAD(+) and the accumulation of HIF-1α under normoxic conditions, with parallels to Warburg reprogramming. Deleting SIRT1 accelerates this process, whereas raising NAD(+) levels in old mice restores mitochondrial function to that of a young mouse in a SIRT1-dependent manner. Thus, a pseudohypoxic state that disrupts PGC-1α/β-independent nuclear-mitochondrial communication contributes to the decline in mitochondrial function with age, a process that is apparently reversible.
    Full-text · Article · Dec 2013 · Cell
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    ABSTRACT: Fatty acids are essential elements of all cells and have significant roles as energy substrates, components of cellular structure and signalling molecules. The storage of excess energy intake as fat in adipose tissue is an evolutionary advantage aimed at protecting against starvation but in much of today's world humans are faced with an unlimited availability of food and the excessive accumulation of fat is now a major risk for human health especially the development of type 2 diabetes. Since first recognition of the association between fat accumulation, reduced insulin action and increased risk of type 2 diabetes, several mechanisms have been proposed to link excess fatty acid availability to reduced insulin action, some of them competing or contradictory. This review summarises the evidence for these mechanisms in the context of excess dietary fatty acids generating insulin resistance in muscle, the major tissue involved in insulin-stimulated disposal of blood glucose. It also outlines potential problems with models and measurements that may hinder, as well as help improve our understanding of the links between fatty acids and insulin action.
    Preview · Article · Dec 2013 · Journal of Endocrinology
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    ABSTRACT: Accompanying the high rates of vitamin D deficiency observed in many countries, there is increasing interest in the physiological functions of vitamin D. Vitamin D is recognised to exert extra-skeletal actions in addition to its classic roles in bone and mineral homeostasis. Here we review the evidence for vitamin D's actions in muscle on the basis of observational studies, clinical trials and basic research. Numerous observational studies link vitamin D deficiency with muscle weakness and sarcopaenia. Randomised trials predominantly support an effect of vitamin D supplementation and the prevention of falls in older or institutionalised patients. Studies have also examined the effect of vitamin D in athletic performance, both inferentially by UV radiation and directly by vitamin D supplementation. Effects of vitamin D in muscle metabolic function, specifically insulin sensitivity, are also addressed in this review. At a mechanistic level, animal studies have evaluated the roles of vitamin D and associated minerals, calcium and phosphate, in muscle function. In vitro studies have identified molecular pathways by which vitamin D regulates muscle cell signalling and gene expression. This review evaluates evidence for the various roles of vitamin D in skeletal muscle and discusses controversies that have made this a dynamic field of research. This article is protected by copyright. All rights reserved.
    Full-text · Article · Nov 2013 · Clinical Endocrinology
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    ABSTRACT: There is emerging evidence for reduced muscle function in children with neurofibromatosis type 1 (NF1). We have examined three murine models featuring NF1 deficiency in muscle to study the effect on muscle function as well as any underlying pathophysiology. The Nf1+/– mouse exhibited no differences in overall weight, lean tissue mass, fiber size, muscle weakness as measured by grip strength or muscle atrophy-recovery with limb disuse, although this model lacks many other characteristic features of the human disease. Next, muscle-specific knockout mice (Nf1muscle−/−) were generated and they exhibited a failure to thrive leading to neonatal lethality. Intramyocellular lipid accumulations were observed by electron microscopy and Oil Red O staining. More mature muscle specimens lacking Nf1 expression taken from the limb-specific Nf1Prx1−/− conditional knockout line showed a 10-fold increase in muscle triglyceride content. Enzyme assays revealed a significant increase in the activities of oxidative metabolism enzymes in the Nf1Prx1−/− mice. Western analyses showed increases in the expression of fatty acid synthase and the hormone leptin, as well as decreased expression of a number of fatty acid transporters in this mouse line. These data support the hypothesis that NF1 is essential for normal muscle function and survival and are the first to suggest a direct link between NF1 and mitochondrial fatty acid metabolism.
    No preview · Article · Oct 2013 · Human Molecular Genetics
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    ABSTRACT: α-Actinin-3 deficiency occurs in approximately 16% of the global population due to homozygosity for a common nonsense polymorphism in the ACTN3 gene. Loss of α-actinin-3 is associated with reduced power and enhanced endurance capacity in elite athletes and nonathletes due to "slowing" of the metabolic and physiological properties of fast fibers. Here, we have shown that α-actinin-3 deficiency results in increased calcineurin activity in mouse and human skeletal muscle and enhanced adaptive response to endurance training. α-Actinin-2, which is differentially expressed in α-actinin-3-deficient muscle, has higher binding affinity for calsarcin-2, a key inhibitor of calcineurin activation. We have further demonstrated that α-actinin-2 competes with calcineurin for binding to calsarcin-2, resulting in enhanced calcineurin signaling and reprogramming of the metabolic phenotype of fast muscle fibers. Our data provide a mechanistic explanation for the effects of the ACTN3 genotype on skeletal muscle performance in elite athletes and on adaptation to changing physical demands in the general population. In addition, we have demonstrated that the sarcomeric α-actinins play a role in the regulation of calcineurin signaling.
    Full-text · Article · Oct 2013 · The Journal of clinical investigation

Publication Stats

4k Citations
521.33 Total Impact Points

Institutions

  • 2007-2015
    • Garvan Institute of Medical Research
      • Cancer Research Program
      דרלינגהרסט, New South Wales, Australia
  • 2012-2014
    • University of New South Wales
      • • Department of Pharmacology
      • • St Vincent's Clinical School
      Kensington, New South Wales, Australia
  • 2003-2010
    • University of Wollongong
      • • School of Health Sciences
      • • Department of Biomedical Science
      City of Greater Wollongong, New South Wales, Australia