Endoplasmic reticulum-mitochondria miscommunication is an early and causal trigger of hepatic insulin resistance and steatosis.

BACKGROUND & AIMS: Hepatic insulin resistance in obesity and type 2 diabetes was recently associated with endoplasmic reticulum (ER)-mitochondria miscommunication. These contact sites (mitochondria-associated membranes: MAMs) are highly dynamic and involved in many functions; however, whether MAM dysfunction plays a causal role in hepatic insulin resistance and steatosis is not clear. Thus, we aimed to determine whether and how organelle miscommunication plays a role in the onset and progression of hepatic metabolic impairment. METHODS: We analyzed hepatic ER-mitochondria interactions and calcium exchange in a time-dependent and reversible manner in mice with diet-induced obesity. Additionally, we used recombinant adenovirus to express a specific organelle spacer or linker in mouse livers, to determine the causal impact of MAM dysfunction on hepatic metabolic alterations. RESULTS: Disruption of ER-mitochondria interactions and calcium exchange is an early event preceding hepatic insulin resistance and steatosis in mice with diet-induced obesity. Interestingly, an 8-week reversal diet concomitantly reversed hepatic organelle miscommunication and insulin resistance in obese mice. Mechanistically, disrupting structural and functional ER-mitochondria interactions through the hepatic overexpression of the organelle spacer FATE1 was sufficient to impair hepatic insulin action and glucose homeostasis. In addition, FATE1-mediated organelle miscommunication disrupted lipid-related mitochondrial oxidative metabolism and induced hepatic steatosis. Conversely, reinforcement of ER-mitochondria interactions through hepatic expression of a synthetic linker prevented diet-induced glucose intolerance after 4 weeks' overnutrition. Importantly, ER-mitochondria miscommunication was confirmed in the liver of obese patients with type 2 diabetes, and correlated with glycemia, HbA1c and HOMA-IR index. CONCLUSIONS: ER-mitochondria miscommunication is an early causal trigger of hepatic insulin resistance and steatosis, and can be reversed by switching to a healthy diet. Thus, targeting MAMs could help to restore metabolic homeostasis. LAY SUMMARY: The literature suggests that interactions between the endoplasmic reticulum and mitochondria could play a role in hepatic insulin resistance and steatosis during chronic obesity. In the present study, we reappraised the time-dependent regulation of hepatic endoplasmic reticulum-mitochondria interactions and calcium exchange, investigating reversibility and causality, in mice with diet-induced obesity. We also assessed the relevance of our findings to humans. We show that organelle miscommunication is an early causal trigger of hepatic insulin resistance and steatosis that can be improved by nutritional strategies.

Disruption of calcium transfer from ER to mitochondria links alterations of mitochondria-associated ER membrane integrity to hepatic insulin resistance.

AIMS/HYPOTHESIS: Mitochondria-associated endoplasmic reticulum membranes (MAMs) are regions of the endoplasmic reticulum (ER) tethered to mitochondria and controlling calcium (Ca(2+)) transfer between both organelles through the complex formed between the voltage-dependent anion channel, glucose-regulated protein 75 and inositol 1,4,5-triphosphate receptor (IP3R). We recently identified cyclophilin D (CYPD) as a new partner of this complex and demonstrated a new role for MAMs in the control of insulin's action in the liver. Here, we report on the mechanisms by which disruption of MAM integrity induces hepatic insulin resistance in CypD (also known as Ppif)-knockout (KO) mice. METHODS: We used either in vitro pharmacological and genetic inhibition of CYPD in HuH7 cells or in vivo loss of CYPD in mice to investigate ER-mitochondria interactions, inter-organelle Ca(2+) exchange, organelle homeostasis and insulin action. RESULTS: Pharmacological and genetic inhibition of CYPD concomitantly reduced ER-mitochondria interactions, inhibited inter-organelle Ca(2+) exchange, induced ER stress and altered insulin signalling in HuH7 cells. In addition, histamine-stimulated Ca(2+) transfer from ER to mitochondria was blunted in isolated hepatocytes of CypD-KO mice and this was associated with an increase in ER calcium store. Interestingly, disruption of inter-organelle Ca(2+) transfer was associated with ER stress, mitochondrial dysfunction, lipid accumulation, activation of c-Jun N-terminal kinase (JNK) and protein kinase C (PKC)ε and insulin resistance in liver of CypD-KO mice. Finally, CYPD-related alterations of insulin signalling were mediated by activation of PKCε rather than JNK in HuH7 cells. CONCLUSIONS/INTERPRETATION: Disruption of IP3R-mediated Ca(2+) signalling in the liver of CypD-KO mice leads to hepatic insulin resistance through disruption of organelle interaction and function, increase in lipid accumulation and activation of PKCε. Modulation of ER-mitochondria Ca(2+) exchange may thus provide an exciting new avenue for treating hepatic insulin resistance.

Losartan, an angiotensin II type 1 receptor blocker, protects human islets from glucotoxicity through the phospholipase C pathway.

As shown in a large clinical prospective trial, inhibition of the renin-angiotensin system (RAS) can delay the onset of type 2 diabetes in high-risk individuals. We evaluated the beneficial effects of RAS inhibition on ß-cell function under glucotoxic conditions. Human islets from 13 donors were cultured in 5.5 mM (controls) or 16.7 mM glucose [high glucose (HG)] for 4 d with or without losartan (5 µM), a selective AT1R blocker, and/or U73122 (2 µM), a selective PLC inhibitor, during the last 2 d. HG induced RAS activation with overexpression of AT1R (P<0.05) and angiotensinogen (P<0.001) mRNAs. HG increased endoplasmic reticulum (ER) stress markers (P<0.001) such as GRP78, sXBP1, and ATF4 mRNAs and Grp78 protein levels (P<0.01). HG also decreased reticular calcium concentration (P<0.0001) and modified protein expressions of ER calcium pumps with reduction of SERCA2b (P<0.01) and increase of IP3R2 (P<0.05). Losartan prevented these deleterious effects and was associated with improved insulin secretion despite HG exposure. AT1R activation triggers the PLC-IP3-calcium pathway. Losartan prevented the increase of PLC ß1 and γ1 protein levels induced by HG (P<0.05). U73122 reproduced all the protective effects of losartan. AT1R blockade protects human islets from the deleterious effects of glucose through inhibition of the PLC-IP3-calcium pathway.

FTO contributes to hepatic metabolism regulation through regulation of leptin action and STAT3 signalling in liver.

BACKGROUND: The fat mass and obesity associated (FTO) gene is related to obesity and type 2 diabetes, but its function is still largely unknown. A link between leptin receptor-signal transducers and activators of transcription 3 (LepR-STAT3) signalling pathway and FTO was recently suggested in the hypothalamus. Because of the presence of FTO in liver and the role of LepR-STAT3 in the control of hepatic metabolism, we investigated both in vitro and in vivo the potential interrelationship between FTO and LepR-STAT3 signalling pathway in liver and the impact of FTO overexpression on leptin action and glucose homeostasis in liver of mice. RESULTS: We found that FTO protein expression is regulated by both leptin and IL-6, concomitantly to an induction of STAT3 tyrosine phosphorylation, in leptin receptor (LepRb) expressing HuH7 cells. In addition, FTO overexpression in vitro altered both leptin-induced Y705 and S727 STAT3 phosphorylation, leading to dysregulation of glucose-6-phosphatase (G6P) expression and mitochondrial density, respectively. In vivo, liver specific FTO overexpression in mice induced a reducetion of Y705 phosphorylation of STAT3 in nuclear fraction, associated with reduced SOCS3 and LepR mRNA levels and with an increased G6P expression. Interestingly, FTO overexpression also induced S727 STAT3 phosphorylation in liver mitochondria, resulting in an increase of mitochondria function and density. Altogether, these data indicate that FTO promotes mitochondrial recruitment of STAT3 to the detriment of its nuclear localization, affecting in turn oxidative metabolism and the expression of leptin-targeted genes. Interestingly, these effects were associated in mice with alterations of leptin action and hyperleptinemia, as well as hyperglycemia, hyperinsulinemia and glucose intolerance. CONCLUSIONS: Altogether, these data point a novel regulatory loop between FTO and leptin-STAT3 signalling pathways in liver cells, and highlight a new role of FTO in the regulation of hepatic leptin action and glucose metabolism.

A Cross-Sectional Quantitative Metabolomics Study Evidencing the Metabolic Signature in Six Organs during a 14-Week High-Fat High-Sucrose and Standard Diet in Mice.

Obesity is a risk factor for many diseases, such as type 2 diabetes and cardiovascular diseases. In line with the need for precision medicine, the search for biomarkers reporting the progression of obesity- and diet-associated disorders is urgent. We used NMR to determine the metabolomics profile of key organs (lung, liver, heart, skeletal muscle, kidney, and brain) and serum from male C57Bl/6J mice (5 weeks old) fed for 6, 10, and 14 weeks on a high-fat and high-sucrose diet (HFHSD) vs. a standard diet (STD). We determined metabolite concentrations in the organs at each time point, which allowed us to discriminate age- and diet-related effects as well as the interactions between both, highlighting the need to evaluate the influence of age as a confounding factor on metabolic signatures. Notably, the analysis revealed the influence of time on metabolite concentrations in the STD condition, probably reflecting the juvenile-to-adult transition. Variations impacted the liver and lung metabolites, revealing the strong influence of the HFHS diet on normal metabolism maturation during youth.

Human Pancreatic Islets React to Glucolipotoxicity by Secreting Pyruvate and Citrate.

Progressive decline in pancreatic beta-cell function is central to the pathogenesis of type 2 diabetes (T2D). Here, we explore the relationship between the beta cell and its nutritional environment, asking how an excess of energy substrate leads to altered energy production and subsequent insulin secretion. Alterations in intracellular metabolic homeostasis are key markers of islets with T2D, but changes in cellular metabolite exchanges with their environment remain unknown. We answered this question using nuclear magnetic resonance-based quantitative metabolomics and evaluated the consumption or secretion of 31 extracellular metabolites from healthy and T2D human islets. Islets were also cultured under high levels of glucose and/or palmitate to induce gluco-, lipo-, and glucolipotoxicity. Biochemical analyses revealed drastic alterations in the pyruvate and citrate pathways, which appear to be associated with mitochondrial oxoglutarate dehydrogenase (OGDH) downregulation. We repeated these manipulations on the rat insulinoma-derived beta-pancreatic cell line (INS-1E). Our results highlight an OGDH downregulation with a clear effect on the pyruvate and citrate pathways. However, citrate is directed to lipogenesis in the INS-1E cells instead of being secreted as in human islets. Our results demonstrate the ability of metabolomic approaches performed on culture media to easily discriminate T2D from healthy and functional islets.

Dietary oxidized n-3 PUFA induce oxidative stress and inflammation: role of intestinal absorption of 4-HHE and reactivity in intestinal cells.

Dietary intake of long-chain n-3 PUFA is now widely advised for public health and in medical practice. However, PUFA are highly prone to oxidation, producing potentially deleterious 4-hydroxy-2-alkenals. Even so, the impact of consuming oxidized n-3 PUFA on metabolic oxidative stress and inflammation is poorly described. We therefore studied such effects and hypothesized the involvement of the intestinal absorption of 4-hydroxy-2-hexenal (4-HHE), an oxidized n-3 PUFA end-product. In vivo, four groups of mice were fed for 8 weeks high-fat diets containing moderately oxidized or unoxidized n-3 PUFA. Other mice were orally administered 4-HHE and euthanized postprandially versus baseline mice. In vitro, human intestinal Caco-2/TC7 cells were incubated with 4-hydroxy-2-alkenals. Oxidized diets increased 4-HHE plasma levels in mice (up to 5-fold, P < 0.01) compared with unoxidized diets. Oxidized diets enhanced plasma inflammatory markers and activation of nuclear factor kappaB (NF-κB) in the small intestine along with decreasing Paneth cell number (up to -19% in the duodenum). Both in vivo and in vitro, intestinal absorption of 4-HHE was associated with formation of 4-HHE-protein adducts and increased expression of glutathione peroxidase 2 (GPx2) and glucose-regulated protein 78 (GRP78). Consumption of oxidized n-3 PUFA results in 4-HHE accumulation in blood after its intestinal absorption and triggers oxidative stress and inflammation in the upper intestine.

Reduction of endoplasmic reticulum stress using chemical chaperones or Grp78 overexpression does not protect muscle cells from palmitate-induced insulin resistance.

Endoplasmic reticulum (ER) stress is proposed as a novel link between elevated fatty acids levels, obesity and insulin resistance in liver and adipose tissue. However, it is unknown whether ER stress also contributes to lipid-induced insulin resistance in skeletal muscle, the major tissue responsible of insulin-stimulated glucose disposal. Here, we investigated the possible role of ER stress in palmitate-induced alterations of insulin action, both in vivo, in gastrocnemius of high-palm diet fed mice, and in vitro, in palmitate-treated C(2)C(12) myotubes. We demonstrated that 8 weeks of high-palm diet increased the expression of ER stress markers in muscle of mice, whereas ex-vivo insulin-stimulated PKB phosphorylation was not altered in this tissue. In addition, exposure of C(2)C(12) myotubes to either tuncamycine or palmitate induced ER stress and altered insulin-stimulated PKB phosphorylation. However, alleviation of ER stress by either TUDCA or 4-PBA treatments, or by overexpressing Grp78, did not restore palmitate-induced reduction of insulin-stimulated PKB phosphorylation in C(2)C(12) myotubes. This work highlights that, even ER stress is associated with palmitate-induced alterations of insulin signaling, ER stress is likely not the major culprit of this effect in myotubes, suggesting that the previously proposed link between ER stress and insulin resistance is less important in skeletal muscle than in adipose tissue and liver.

Inhibition of xanthine oxidase reduces hyperglycemia-induced oxidative stress and improves mitochondrial alterations in skeletal muscle of diabetic mice.

Reactive oxygen species (ROS) have been widely implicated in the pathogenesis of diabetes and more recently in mitochondrial alterations in skeletal muscle of diabetic mice. However, so far the exact sources of ROS in skeletal muscle have remained elusive. Aiming at better understanding the causes of mitochondrial alterations in diabetic muscle, we designed this study to characterize the sites of ROS production in skeletal muscle of streptozotocin (STZ)-induced diabetic mice. Hyperglycemic STZ mice showed increased markers of systemic and muscular oxidative stress, as evidenced by increased circulating H(2)O(2) and muscle carbonylated protein levels. Interestingly, insulin treatment reduced hyperglycemia and improved systemic and muscular oxidative stress in STZ mice. We demonstrated that increased oxidative stress in muscle of STZ mice is associated with an increase of xanthine oxidase (XO) expression and activity and is mediated by an induction of H(2)O(2) production by both mitochondria and XO. Finally, treatment of STZ mice, as well as high-fat and high-sucrose diet-fed mice, with oxypurinol reduced markers of systemic and muscular oxidative stress and prevented structural and functional mitochondrial alterations, confirming the in vivo relevance of XO in ROS production in diabetic mice. These data indicate that mitochondria and XO are the major sources of hyperglycemia-induced ROS production in skeletal muscle and that the inhibition of XO reduces oxidative stress and improves mitochondrial alterations in diabetic muscle.

Mitochondrial dysfunction results from oxidative stress in the skeletal muscle of diet-induced insulin-resistant mice.

Mitochondrial dysfunction in skeletal muscle has been implicated in the development of type 2 diabetes. However, whether these changes are a cause or a consequence of insulin resistance is not clear. We investigated the structure and function of muscle mitochondria during the development of insulin resistance and progression to diabetes in mice fed a high-fat, high-sucrose diet. Although 1 month of high-fat, high-sucrose diet feeding was sufficient to induce glucose intolerance, mice showed no evidence of mitochondrial dysfunction at this stage. However, an extended diet intervention induced a diabetic state in which we observed altered mitochondrial biogenesis, structure, and function in muscle tissue. We assessed the role of oxidative stress in the development of these mitochondrial abnormalities and found that diet-induced diabetic mice had an increase in ROS production in skeletal muscle. In addition, ROS production was associated with mitochondrial alterations in the muscle of hyperglycemic streptozotocin-treated mice, and normalization of glycemia or antioxidant treatment decreased muscle ROS production and restored mitochondrial integrity. Glucose- or lipid-induced ROS production resulted in mitochondrial alterations in muscle cells in vitro, and these effects were blocked by antioxidant treatment. These data suggest that mitochondrial alterations do not precede the onset of insulin resistance and result from increased ROS production in muscle in diet-induced diabetic mice.

Differential Effect of Glucose on ER-Mitochondria Ca2+ Exchange Participates in Insulin Secretion and Glucotoxicity-Mediated Dysfunction of ß-Cells.

Glucotoxicity-induced ß-cell dysfunction in type 2 diabetes is associated with alterations of mitochondria and the endoplasmic reticulum (ER). Both organelles interact at contact sites, defined as mitochondria-associated membranes (MAMs), which were recently implicated in the regulation of glucose homeostasis. The role of MAMs in ß-cells is still largely unknown, and their implication in glucotoxicity-associated ß-cell dysfunction remains to be defined. Here, we report that acute glucose treatment stimulated ER-mitochondria interactions and calcium (Ca2+) exchange in INS-1E cells, whereas disruption of MAMs altered glucose-stimulated insulin secretion (GSIS). Conversely, chronic incubations with high glucose of either INS-1E cells or human pancreatic islets altered GSIS and concomitantly reduced ER Ca2+ store, increased basal mitochondrial Ca2+, and reduced ATP-stimulated ER-mitochondria Ca2+ exchanges, despite an increase of organelle interactions. Furthermore, glucotoxicity-induced perturbations of Ca2+ signaling are associated with ER stress, altered mitochondrial respiration, and mitochondria fragmentation, and these organelle stresses may participate in increased organelle tethering as a protective mechanism. Last, sustained induction of ER-mitochondria interactions using a linker reduced organelle Ca2+ exchange, induced mitochondrial fission, and altered GSIS. Therefore, dynamic organelle coupling participates in GSIS in ß-cells, and over time, disruption of organelle Ca2+ exchange might be a novel mechanism contributing to glucotoxicity-induced ß-cell dysfunction.

Regulation of Mitochondria-Associated Membranes (MAMs) by NO/sGC/PKG Participates in the Control of Hepatic Insulin Response.

Under physiological conditions, nitric oxide (NO) produced by the endothelial NO synthase (eNOS) upregulates hepatic insulin sensitivity. Recently, contact sites between the endoplasmic reticulum and mitochondria named mitochondria-associated membranes (MAMs) emerged as a crucial hub for insulin signaling in the liver. As mitochondria are targets of NO, we explored whether NO regulates hepatic insulin sensitivity by targeting MAMs. In Huh7 cells, primary rat hepatocytes and mouse livers, enhancing NO concentration increased MAMs, whereas inhibiting eNOS decreased them. In vitro, those effects were prevented by inhibiting protein kinase G (PKG) and mimicked by activating soluble guanylate cyclase (sGC) and PKG. In agreement with the regulation of MAMs, increasing NO concentration improved insulin signaling, both in vitro and in vivo, while eNOS inhibition disrupted this response. Finally, inhibition of insulin signaling by wortmannin did not affect the impact of NO on MAMs, while experimental MAM disruption, using either targeted silencing of cyclophilin D or the overexpression of the organelle spacer fetal and adult testis-expressed 1 (FATE-1), significantly blunted the effects of NO on both MAMs and insulin response. Therefore, under physiological conditions, NO participates to the regulation of MAM integrity through the sGC/PKG pathway and concomitantly improves hepatic insulin sensitivity. Altogether, our data suggest that the induction of MAMs participate in the impact of NO on hepatocyte insulin response.

A comprehensive survey of the laminins and collagens type IV expressed in mouse Leydig cells and their regulation by LH/hCG.

Extracellular matrix (ECM) proteins have been shown to alter Leydig cell steroidogenesis in vitro, substantiating the hypothesis that Leydig cell steroidogenic activity and matrix environment are interdependent events. However, the nature of the ECM components synthesized by Leydig cells and their regulation by LH/human chorionic gonadotropin (hCG) remain unknown. Here, we examine the occurrence of the 11 laminin subunits and the 6 alpha chains of collagen IV (COL4A1-6) by RT-PCR in Leydig cells cultured with or without LH/hCG. Leydig cells were a tumor Leydig cell line (mLTC-1) or 8-week-old mice Leydig cells. Based on PCR data, it is suggested that normal Leydig cells may synthesize a maximum of 11 laminin heterotrimers and the 6 alpha chains of collagen IV. They also may synthesize various proteases and inhibitors of the metzincin family. The mLTC-1 cells have a limited repertoire as compared with normal Leydig cells. Interestingly, none of the ten proteases and inhibitors monitored is under LH-hCG regulation whereas every protease and inhibitor of the serine protease family yet identified in Leydig cells is under gonadotropin regulation. In addition, a few laminin and collagen subunit genes are regulated by LH/hCG. These are laminins alpha3 and gamma3 (Lama3 and Lamc3), Col4a3, and Col4a6, which are negatively regulated by LH/hCG in both Leydig cell types, and Col4a4, which was downregulated in primary cultures but not in mLTC-1 cells. Collectively, the present study suggests that Leydig cells modulate in a selective fashion their matrix environment in response to their trophic hormone. This may alter the steroidogenic outcome of Leydig cells.

Disruption of Mitochondria-Associated Endoplasmic Reticulum Membrane (MAM) Integrity Contributes to Muscle Insulin Resistance in Mice and Humans.

Modifications of the interactions between endoplasmic reticulum (ER) and mitochondria, defined as mitochondria-associated membranes (MAMs), were recently shown to be involved in the control of hepatic insulin action and glucose homeostasis, but with conflicting results. Whereas skeletal muscle is the primary site of insulin-mediated glucose uptake and the main target for alterations in insulin-resistant states, the relevance of MAM integrity in muscle insulin resistance is unknown. Deciphering the importance of MAMs on muscle insulin signaling could help to clarify this controversy. Here, we show in skeletal muscle of different mice models of obesity and type 2 diabetes (T2D) a marked disruption of ER-mitochondria interactions as an early event preceding mitochondrial dysfunction and insulin resistance. Furthermore, in human myotubes, palmitate-induced insulin resistance is associated with a reduction of structural and functional ER-mitochondria interactions. Importantly, experimental increase of ER-mitochondria contacts in human myotubes prevents palmitate-induced alterations of insulin signaling and action, whereas disruption of MAM integrity alters the action of the hormone. Lastly, we found an association between altered insulin signaling and ER-mitochondria interactions in human myotubes from obese subjects with or without T2D compared with healthy lean subjects. Collectively, our data reveal a new role of MAM integrity in insulin action of skeletal muscle and highlight MAM disruption as an essential subcellular alteration associated with muscle insulin resistance in mice and humans. Therefore, reduced ER-mitochondria coupling could be a common alteration of several insulin-sensitive tissues playing a key role in altered glucose homeostasis in the context of obesity and T2D.

CCR4-associated factor CAF1 is an essential factor for spermatogenesis.

The CCR4-associated protein CAF1 has been demonstrated to play several roles in the control of transcription and of mRNA decay. To gain further insight into its physiological function, we generated CAF1-deficient mice. They are viable, healthy, and normal in appearance; however, mCAF1(-/-) male mice are sterile. The crossing of mCAF1(+/-) mice gave a Mendelian ratio of mCAF1(+/+), mCAF1(+/-), and mCAF1(-/-) pups, indicating that haploid mCAF1-deficient germ cells differentiate normally. The onset of the defect occurs during the first wave of spermatogenesis at 19 to 20 days after birth, during progression of pachytene spermatocytes to haploid spermatids and spermatozoa. Early disruption of spermatogenesis was evidenced by Sertoli cell vacuolization and tubular disorganization. The most mature germ cells were the most severely depleted, but progressively all germ cells were affected, giving Sertoli cell-only tubes, large interstitial spaces, and small testes. This phenotype could be linked to a defect(s) in germ cells and/or to inadequate Sertoli cell function, leading to seminiferous tubule disorganization and finally to a total disappearance of germ cells. The mCAF1-deficient mouse provides a new model of failed spermatogenesis in the adult that may be relevant to some cases of human male sterility.

Reactive oxygen species enhance mitochondrial function, insulin sensitivity and glucose uptake in skeletal muscle of senescence accelerated prone mice SAMP8.

Whereas reactive oxygen species (ROS) can have opposite impacts on insulin signaling, they have mainly been associated with mitochondrial dysfunction in skeletal muscle. We analyzed the relationship between these three features in skeletal muscle of senescence accelerated mice (SAM) prone (P8), which are characterized by enhanced oxidative stress compared to SAM resistant (R1). Oxidative stress, ROS production, antioxidant system, mitochondrial content and functioning, as well as in vitro and in vivo insulin signaling were investigated in gastrocnemius and quadriceps muscles. In SAMP8 compared to SAMR1, muscle content in carbonylated proteins was two-fold (p < 0.01) and ROS production by xanthine oxidase 70% (p < 0.05) higher. Furthermore, insulin-induced Akt phosphorylation measured in vivo and ex vivo as well as muscle glucose uptake measured ex vivo were significantly higher (p < 0.05). Mitochondrial respiration evidenced uncoupling and higher respiration rates with substrates of complexes II and IV, in agreement with higher maximal activity of complexes II and IV (+ 18% and 62%, respectively, p < 0.05). By contrast, maximal activity of complex I was 22% lower (p < 0.05). All strain differences were corrected after 6 months of N-acetylcysteine (NAC) treatment, thus supporting the involvement of high ROS production in these differences. In conclusion in muscle of SAMP8 compared to SAMR1, high ROS production is associated to higher insulin sensitivity and glucose uptake but to lower mitochondrial complex I activity. These conflicting adaptations, with regards to the resulting imbalance between NADH production and use, were associated with intrinsic adjustments in the mitochondrial respiration chain (mitochondrial uncoupling, enhanced complexes II and IV activity). We propose that these bioenergetics adaptations may help at preserving muscle metabolic flexibility of SAMP8.

Mitochondria-associated endoplasmic reticulum membranes allow adaptation of mitochondrial metabolism to glucose availability in the liver.

Mitochondria-associated endoplasmic reticulum membranes (MAM) play a key role in mitochondrial dynamics and function and in hepatic insulin action. Whereas mitochondria are important regulators of energy metabolism, the nutritional regulation of MAM in the liver and its role in the adaptation of mitochondria physiology to nutrient availability are unknown. In this study, we found that the fasted to postprandial transition reduced the number of endoplasmic reticulum-mitochondria contact points in mouse liver. Screening of potential hormonal/metabolic signals revealed glucose as the main nutritional regulator of hepatic MAM integrity both in vitro and in vivo Glucose reduced organelle interactions through the pentose phosphate-protein phosphatase 2A (PP-PP2A) pathway, induced mitochondria fission, and impaired respiration. Blocking MAM reduction counteracted glucose-induced mitochondrial alterations. Furthermore, disruption of MAM integrity mimicked effects of glucose on mitochondria dynamics and function. This glucose-sensing system is deficient in the liver of insulin-resistant ob/ob and cyclophilin D-KO mice, both characterized by chronic disruption of MAM integrity, mitochondrial fission, and altered mitochondrial respiration. These data indicate that MAM contribute to the hepatic glucose-sensing system, allowing regulation of mitochondria dynamics and function during nutritional transition. Chronic disruption of MAM may participate in hepatic mitochondrial dysfunction associated with insulin resistance.

Ozone exposure triggers insulin resistance through muscle c-Jun N-terminal kinase activation.

A growing body of evidence suggests that exposure to traffic-related air pollution is a risk factor for type 2 diabetes. Ozone, a major photochemical pollutant in urban areas, is negatively associated with fasting glucose and insulin levels, but most aspects of this association remain to be elucidated. Using an environmentally realistic concentration (0.8 parts per million), we demonstrated that exposure of rats to ozone induced whole-body insulin resistance and oxidative stress, with associated endoplasmic reticulum (ER) stress, c-Jun N-terminal kinase (JNK) activation, and disruption of insulin signaling in skeletal muscle. Bronchoalveolar lavage fluids from ozone-treated rats reproduced this effect in C2C12 myotubes, suggesting that toxic lung mediators were responsible for the phenotype. Pretreatment with the chemical chaperone 4-phenylbutyric acid, the JNK inhibitor SP600125, or the antioxidant N-acetylcysteine alleviated insulin resistance, demonstrating that ozone sequentially triggered oxidative stress, ER stress, and JNK activation to impair insulin signaling in muscle. This study is the first to report that ozone plays a causative role in the development of insulin resistance, suggesting that it could boost the development of diabetes. We therefore provide a potential mechanism linking pollutant exposure and the increased incidence of metabolic diseases.

Imeglimin normalizes glucose tolerance and insulin sensitivity and improves mitochondrial function in liver of a high-fat, high-sucrose diet mice model.

Imeglimin is the first in a new class of oral glucose-lowering agents currently in phase 2b development. Although imeglimin improves insulin sensitivity in humans, the molecular mechanisms are unknown. This study used a model of 16-week high-fat, high-sucrose diet (HFHSD) mice to characterize its antidiabetic effects. Six-week imeglimin treatment significantly decreased glycemia, restored normal glucose tolerance, and improved insulin sensitivity without modifying organs, body weights, and food intake. This was associated with an increase in insulin-stimulated protein kinase B phosphorylation in the liver and muscle. In liver mitochondria, imeglimin redirects substrate flows in favor of complex II, as illustrated by increased respiration with succinate and by the restoration of respiration with glutamate/malate back to control levels. In addition, imeglimin inhibits complex I and restores complex III activities, suggesting an increase in fatty acid oxidation, which is supported by an increase in hepatic 3-hydroxyacetyl-CoA dehydrogenase activity and acylcarnitine profile and the reduction of liver steatosis. Imeglimin also reduces reactive oxygen species production and increases mitochondrial DNA. Finally, imeglimin effects on mitochondrial phospholipid composition could participate in the benefit of imeglimin on mitochondrial function. In conclusion, imeglimin normalizes glucose tolerance and insulin sensitivity by preserving mitochondrial function from oxidative stress and favoring lipid oxidation in liver of HFHSD mice.

Mitochondria-associated endoplasmic reticulum membrane (MAM) integrity is required for insulin signaling and is implicated in hepatic insulin resistance.

Mitochondria-associated endoplasmic reticulum (ER) membranes (MAMs) are functional domains between both organelles involved in Ca(2+) exchange, through the voltage-dependent anion channel (VDAC)-1/glucose-regulated protein 75 (Grp75)/inositol 1,4,5-triphosphate receptor (IP3R)-1 complex, and regulating energy metabolism. Whereas mitochondrial dysfunction, ER stress, and altered Ca(2+) homeostasis are associated with altered insulin signaling, the implication of MAM dysfunctions in insulin resistance is unknown. Here we validated an approach based on in situ proximity ligation assay to detect and quantify VDAC1/IP3R1 and Grp75/IP3R1 interactions at the MAM interface. We demonstrated that MAM integrity is required for insulin signaling and that induction of MAM prevented palmitate-induced alterations of insulin signaling in HuH7 cells. Disruption of MAM integrity by genetic or pharmacological inhibition of the mitochondrial MAM protein, cyclophilin D (CypD), altered insulin signaling in mouse and human primary hepatocytes and treatment of CypD knockout mice with metformin improved both insulin sensitivity and MAM integrity. Furthermore, ER-mitochondria interactions are altered in liver of both ob/ob and diet-induced insulin-resistant mice and improved by rosiglitazone treatment in the latter. Finally, increasing organelle contacts by overexpressing CypD enhanced insulin action in primary hepatocytes of diabetic mice. Collectively, our data reveal a new role of MAM integrity in hepatic insulin action and resistance, providing a novel target for the modulation of insulin action.