p38γ and p38δ regulate postnatal cardiac metabolism through glycogen synthase 1.

During the first weeks of postnatal heart development, cardiomyocytes undergo a major adaptive metabolic shift from glycolytic energy production to fatty acid oxidation. This metabolic change is contemporaneous to the up-regulation and activation of the p38γ and p38δ stress-activated protein kinases in the heart. We demonstrate that p38γ/δ contribute to the early postnatal cardiac metabolic switch through inhibitory phosphorylation of glycogen synthase 1 (GYS1) and glycogen metabolism inactivation. Premature induction of p38γ/δ activation in cardiomyocytes of newborn mice results in an early GYS1 phosphorylation and inhibition of cardiac glycogen production, triggering an early metabolic shift that induces a deficit in cardiomyocyte fuel supply, leading to whole-body metabolic deregulation and maladaptive cardiac pathogenesis. Notably, the adverse effects of forced premature cardiac p38γ/δ activation in neonate mice are prevented by maternal diet supplementation of fatty acids during pregnancy and lactation. These results suggest that diet interventions have a potential for treating human cardiac genetic diseases that affect heart metabolism.

The lack of functional nicotinamide nucleotide transhydrogenase only moderately contributes to the impairment of glucose tolerance and glucose-stimulated insulin secretion in C57BL/6J vs C57BL/6N mice.

AIMS/HYPOTHESIS: Nicotinamide nucleotide transhydrogenase (NNT) is involved in mitochondrial NADPH production and its spontaneous inactivating mutation (NntTr [Tr, truncated]) is usually considered to be the main cause of the lower glucose tolerance of C57BL/6J vs C57BL/6N mice. However, the impact of this mutation on glucose tolerance remains disputed. Here, we singled out the impact of NntTr from that of other genetic variants between C57BL/6J and C57BL/6N mice on mitochondrial glutathione redox state (EGSH), glucose-stimulated insulin secretion (GSIS) and glucose tolerance. METHODS: Male and female N5BL/6J mice that express wild-type Nnt (NntWT) or NntTr (N5-WT and N5-Tr mice) on the C57BL/6J genetic background were obtained by crossing N5BL/6J NntWT/Tr heterozygous mice. C57BL/6J and C57BL/6N mice were from Janvier Labs. The Nnt genotype was confirmed by PCR and the genetic background by whole genome sequencing of one mouse of each type. Glucose tolerance was assessed by IPGTT, ITT and fasting/refeeding tests. Stimulus-secretion coupling events and GSIS were measured in isolated pancreatic islets. Cytosolic and mitochondrial EGSH were measured using the fluorescent redox probe GRX1-roGFP2 (glutaredoxin 1 fused to redox-sensitive enhanced GFP). RESULTS: The Nnt genotype and genetic background of each type of mouse were confirmed. As reported previously in C57BL/6N vs C57BL/6J islets, the glucose regulation of mitochondrial (but not cytosolic) EGSH and of NAD(P)H autofluorescence was markedly improved in N5-WT vs N5-Tr islets, confirming the role of NNT in mitochondrial redox regulation. However, ex vivo GSIS was only 1.2-1.4-times higher in N5-WT vs N5-Tr islets, while it was 2.4-times larger in C57BL/6N vs N5-WT islets, questioning the role of NNT in GSIS. In vivo, the ITT results did not differ between N5-WT and N5-Tr or C57BL/6N mice. However, the glucose excursion during an IPGTT was only 15-20% lower in female N5-WT mice than in N5-Tr and C57BL/6J mice and remained 3.5-times larger than in female C57BL/6N mice. Similar observations were made during a fasting/refeeding test. A slightly larger (~30%) impact of NNT on glucose tolerance was found in males. CONCLUSIONS/INTERPRETATION: Although our results confirm the importance of NNT in the regulation of mitochondrial redox state by glucose, they markedly downsize the role of NNT in the alteration of GSIS and glucose tolerance in C57BL/6J vs C57BL/6N mice. Therefore, documenting an NntWT genotype in C57BL/6 mice does not provide proof that their glucose tolerance is as good as in C57BL/6N mice.

Treatment with Mammalian Ste-20-like Kinase 1/2 (MST1/2) Inhibitor XMU-MP-1 Improves Glucose Tolerance in Streptozotocin-Induced Diabetes Mice.

Diabetes mellitus (DM) is one of the major causes of death in the world. There are two types of DM-type 1 DM and type 2 DM. Type 1 DM can only be treated by insulin injection whereas type 2 DM is commonly treated using anti-hyperglycemic agents. Despite its effectiveness in controlling blood glucose level, this therapeutic approach is not able to reduce the decline in the number of functional pancreatic ß cells. MST1 is a strong pro-apoptotic kinase that is expressed in pancreatic ß cells. It induces ß cell death and impairs insulin secretion. Recently, a potent and specific inhibitor for MST1, called XMU-MP-1, was identified and characterized. We hypothesized that treatment with XMU-MP-1 would produce beneficial effects by improving the survival and function of the pancreatic ß cells. We used INS-1 cells and STZ-induced diabetic mice as in vitro and in vivo models to test the effect of XMU-MP-1 treatment. We found that XMU-MP-1 inhibited MST1/2 activity in INS-1 cells. Moreover, treatment with XMU-MP-1 produced a beneficial effect in improving glucose tolerance in the STZ-induced diabetic mouse model. Histological analysis indicated that XMU-MP-1 increased the number of pancreatic ß cells and enhanced Langerhans islet area in the severe diabetic mice. Overall, this study showed that MST1 could become a promising therapeutic target for diabetes mellitus.

PI3K-resistant GSK3 controls adiponectin formation and protects from metabolic syndrome.

Metabolic syndrome is characterized by insulin resistance, obesity, and dyslipidemia. It is the consequence of an imbalance between caloric intake and energy consumption. Adiponectin protects against metabolic syndrome. Insulin-induced signaling includes activation of PI3 kinase and protein kinase B (PKB)/Akt. PKB/Akt in turn inactivates glycogen synthase kinase (GSK) 3, a major regulator of metabolism. Here, we studied the significance of PI3K-dependent GSK3 inactivation for adiponectin formation in diet-induced metabolic syndrome. Mice expressing PI3K-insensitive GSK3 (gsk3(KI)) and wild-type mice (gsk3(WT)) were fed a high-fat diet. Compared with gsk3(WT) mice, gsk3(KI) mice were protected against the development of metabolic syndrome as evident from a markedly lower weight gain, lower total body and liver fat accumulation, better glucose tolerance, stronger hepatic insulin-dependent PKB/Akt phosphorylation, lower serum insulin, cholesterol, and triglyceride levels, as well as higher energy expenditure. Serum adiponectin concentration and the activity of transcription factor C/EBPα controlling the expression of adiponectin in adipose tissue was significantly higher in gsk3(KI) mice than in gsk3(WT) mice. Treatment with GSK3 inhibitor lithium significantly decreased the serum adiponectin concentration of gsk3(KI) mice and abrogated the difference in C/EBPα activity between the genotypes. Taken together, our data demonstrate that the expression of PI3K-insensitive GSK3 stimulates the production of adiponectin and protects from diet-induced metabolic syndrome.

Role of PTP1B in POMC neurons during chronic high-fat diet: sex differences in regulation of liver lipids and glucose tolerance.

Protein tyrosine phosphatase 1B (PTP1B) is a negative regulator of leptin receptor signaling and may contribute to leptin resistance in diet-induced obesity. Although PTP1B inhibition has been suggested as a potential weight loss therapy, the role of specific neuronal PTP1B signaling in cardiovascular and metabolic regulation and the importance of sex differences in this regulation are still unclear. In this study, we investigated the impact of proopiomelanocortin (POMC) neuronal PTP1B deficiency in cardiometabolic regulation in male and female mice fed a high-fat diet (HFD). When compared with control mice (PTP1B flox/flox), male and female mice deficient in POMC neuronal PTP1B (PTP1B flox/flox/POMC-Cre) had attenuated body weight gain (males: -18%; females: -16%) and fat mass (males: -33%; female: -29%) in response to HFD. Glucose tolerance was improved by 40%, and liver lipid accumulation was reduced by 40% in PTP1B/POMC-Cre males but not in females. When compared with control mice, deficiency of POMC neuronal PTP1B did not alter mean arterial pressure (MAP) in male or female mice (males: 112 ± 1 vs. 112 ± 1 mmHg in controls; females: 106 ± 3 vs. 109 ± 3 mmHg in controls). Deficiency of POMC neuronal PTP1B also did not alter MAP response to acute stress in males or females compared with control mice (males: Δ32 ± 0 vs. Δ29 ± 4 mmHg; females: Δ22 ± 2 vs. Δ27 ± 4 mmHg). These data demonstrate that POMC-specific PTP1B deficiency improved glucose tolerance and attenuated diet-induced fatty liver only in male mice and attenuated weight gain in males and females but did not enhance the MAP and HR responses to a HFD or to acute stress.

ß Cell-specific deletion of guanylyl cyclase A, the receptor for atrial natriuretic peptide, accelerates obesity-induced glucose intolerance in mice.

BACKGROUND: The cardiac hormones atrial (ANP) and B-type natriuretic peptides (BNP) moderate arterial blood pressure and improve energy metabolism as well as insulin sensitivity via their shared cGMP-producing guanylyl cyclase-A (GC-A) receptor. Obesity is associated with impaired NP/GC-A/cGMP signaling, which possibly contributes to the development of type 2 diabetes and its cardiometabolic complications. In vitro, synthetic ANP, via GC-A, stimulates glucose-dependent insulin release from cultured pancreatic islets and ß-cell proliferation. However, the relevance for systemic glucose homeostasis in vivo is not known. To dissect whether the endogenous cardiac hormones modulate the secretory function and/or proliferation of ß-cells under (patho)physiological conditions in vivo, here we generated a novel genetic mouse model with selective disruption of the GC-A receptor in ß-cells. METHODS: Mice with a floxed GC-A gene were bred to Rip-CreTG mice, thereby deleting GC-A selectively in ß-cells (ß GC-A KO). Weight gain, glucose tolerance, insulin sensitivity, and glucose-stimulated insulin secretion were monitored in normal diet (ND)- and high-fat diet (HFD)-fed mice. ß-cell size and number were measured by immunofluorescence-based islet morphometry. RESULTS: In vitro, the insulinotropic and proliferative actions of ANP were abolished in islets isolated from ß GC-A KO mice. Concordantly, in vivo, infusion of BNP mildly enhanced baseline plasma insulin levels and glucose-induced insulin secretion in control mice. This effect of exogenous BNP was abolished in ß GC-A KO mice, corroborating the efficient inactivation of the GC-A receptor in ß-cells. Despite this under physiological, ND conditions, fasted and fed insulin levels, glucose-induced insulin secretion, glucose tolerance and ß-cell morphology were similar in ß GC-A KO mice and control littermates. However, HFD-fed ß GC-A KO animals had accelerated glucose intolerance and diminished adaptative ß-cell proliferation. CONCLUSIONS: Our studies of ß GC-A KO mice demonstrate that the cardiac hormones ANP and BNP do not modulate ß-cell's growth and secretory functions under physiological, normal dietary conditions. However, endogenous NP/GC-A signaling improves the initial adaptative response of ß-cells to HFD-induced obesity. Impaired ß-cell NP/GC-A signaling in obese individuals might contribute to the development of type 2 diabetes.

Hypothalamic prolyl endopeptidase (PREP) regulates pancreatic insulin and glucagon secretion in mice.

Prolyl endopeptidase (PREP) has been implicated in neuronal functions. Here we report that hypothalamic PREP is predominantly expressed in the ventromedial nucleus (VMH), where it regulates glucose-induced neuronal activation. PREP knockdown mice (Prep(gt/gt)) exhibited glucose intolerance, decreased fasting insulin, increased fasting glucagon levels, and reduced glucose-induced insulin secretion compared with wild-type controls. Consistent with this, central infusion of a specific PREP inhibitor, S17092, impaired glucose tolerance and decreased insulin levels in wild-type mice. Arguing further for a central mode of action of PREP, isolated pancreatic islets showed no difference in glucose-induced insulin release between Prep(gt/gt) and wild-type mice. Furthermore, hyperinsulinemic euglycemic clamp studies showed no difference between Prep(gt/gt) and wild-type control mice. Central PREP regulation of insulin and glucagon secretion appears to be mediated by the autonomic nervous system because Prep(gt/gt) mice have elevated sympathetic outflow and norepinephrine levels in the pancreas, and propranolol treatment reversed glucose intolerance in these mice. Finally, re-expression of PREP by bilateral VMH injection of adeno-associated virus-PREP reversed the glucose-intolerant phenotype of the Prep(gt/gt) mice. Taken together, our results unmask a previously unknown player in central regulation of glucose metabolism and pancreatic function.

Adult-onset deficiency of acyl CoA:monoacylglycerol acyltransferase 2 protects mice from diet-induced obesity and glucose intolerance.

Acyl-CoA:monoacylglycerol acyltransferase (MGAT) 2 catalyzes triacylglycerol (TAG) synthesis, required in intestinal fat absorption. We previously demonstrated that mice without a functional MGAT2-coding gene (Mogat2(-/-)) exhibit increased energy expenditure and resistance to obesity induced by excess calories. One critical question raised is whether lacking MGAT2 during early development is required for the metabolic phenotypes in adult mice. In this study, we found that Mogat2(-/-) pups grew slower than wild-type littermates during the suckling period. To determine whether inactivating MGAT2 in adult mice is sufficient to confer resistance to diet-induced obesity, we generated mice with an inducible Mogat2-inactivating mutation. Mice with adult-onset MGAT2 deficiency (Mogat2(AKO)) exhibited a transient decrease in food intake like Mogat2(-/-) mice when fed a high-fat diet and a moderate increase in energy expenditure after acclimatization. They gained less weight than littermate controls, but the difference was smaller than that between wild-type and Mogat2(-/-) mice. The moderate reduction in weight gain was associated with reduced hepatic TAG and improved glucose tolerance. Similar protective effects were also observed in mice that had gained weight on a high-fat diet before inactivating MGAT2. These findings suggest that adult-onset MGAT2 deficiency mitigates metabolic disorders induced by high-fat feeding and that MGAT2 modulates early postnatal nutrition and may program metabolism later in life.

Multilevel control of glucose homeostasis by adenylyl cyclase 8.

AIMS/HYPOTHESIS: Nutrient homeostasis requires integration of signals generated by glucose metabolism and hormones. Expression of the calcium-stimulated adenylyl cyclase ADCY8 is regulated by glucose and the enzyme is capable of integrating signals from multiple pathways. It may thus have an important role in glucose-induced signalling and glucose homeostasis. METHODS: We used pharmacological and genetic approaches in beta cells to determine secretion and calcium metabolism. Furthermore, Adcy8 knockout mice were characterised. RESULTS: In clonal beta cells, inhibitors of adenylyl cyclases or their downstream targets reduced the glucose-induced increase in cytosolic calcium and insulin secretion. This was reproduced by knock-down of ADCY8, but not of ADCY1. These agents also inhibited glucose-induced increase in cytosolic calcium and electrical activity in primary beta cells and similar effects were observed after ADCY8 knock-down. Moreover, insulin secretion was diminished in islets from Adcy8 knockout mice. These mice were glucose intolerant after oral or intraperitoneal administration of glucose whereas their levels of glucagon-like peptide-1 remained unaltered. Finally, we knocked down ADCY8 in the ventromedial hypothalamus to evaluate the need for ADCY8 in the central regulation of glucose homeostasis. Whereas mice fed a standard diet had normal glucose levels, high-fat diet exacerbated glucose intolerance and knock-down mice were incapable of raising their plasma insulin levels. Finally we confirmed that ADCY8 is expressed in human islets. CONCLUSIONS/INTERPRETATIONS: Collectively, our findings demonstrate that ADCY8 is required for the physiological activation of glucose-induced signalling pathways in beta cells, for glucose tolerance and for hypothalamic adaptation to a high-fat diet via regulation of islet insulin secretion.

Neurolysin knockout mice generation and initial phenotype characterization.

The oligopeptidase neurolysin (EC 3.4.24.16; Nln) was first identified in rat brain synaptic membranes and shown to ubiquitously participate in the catabolism of bioactive peptides such as neurotensin and bradykinin. Recently, it was suggested that Nln reduction could improve insulin sensitivity. Here, we have shown that Nln KO mice have increased glucose tolerance, insulin sensitivity, and gluconeogenesis. KO mice have increased liver mRNA for several genes related to gluconeogenesis. Isotopic label semiquantitative peptidomic analysis suggests an increase in specific intracellular peptides in gastrocnemius and epididymal adipose tissue, which likely is involved with the increased glucose tolerance and insulin sensitivity in the KO mice. These results suggest the exciting new possibility that Nln is a key enzyme for energy metabolism and could be a novel therapeutic target to improve glucose uptake and insulin sensitivity.

Intestine-specific deletion of acyl-CoA:monoacylglycerol acyltransferase (MGAT) 2 protects mice from diet-induced obesity and glucose intolerance.

The absorption of dietary fat involves the re-esterification of digested triacylglycerol in the enterocytes, a process catalyzed by acyl-CoA:monoacylglycerol acyltransferase (MGAT) 2. Mice without a functional gene encoding MGAT2 (Mogat2(-/-)) are protected from diet-induced obesity. Surprisingly, these mice absorb normal amounts of dietary fat but increase their energy expenditure. MGAT2 is expressed in tissues besides intestine, including adipose tissue in both mice and humans. To test the hypothesis that intestinal MGAT2 regulates systemic energy balance, we generated and characterized mice deficient in MGAT2 specifically in the small intestine (Mogat2(IKO)). We found that, like Mogat2(-/-) mice, Mogat2(IKO) mice also showed a delay in fat absorption, a decrease in food intake, and a propensity to use fatty acids as fuel when first exposed to a high fat diet. Mogat2(IKO) mice increased energy expenditure although to a lesser degree than Mogat2(-/-) mice and were protected against diet-induced weight gain and associated comorbidities, including hepatic steatosis, hypercholesterolemia, and glucose intolerance. These findings illustrate that intestinal lipid metabolism plays a crucial role in the regulation of systemic energy balance and may be a feasible intervention target. In addition, they suggest that MGAT activity in extraintestinal tissues may also modulate energy metabolism.

Cardiac mTOR rescues the detrimental effects of diet-induced obesity in the heart after ischemia-reperfusion.

Diet-induced obesity deteriorates the recovery of cardiac function after ischemia-reperfusion (I/R) injury. While mechanistic target of rapamycin (mTOR) is a key mediator of energy metabolism, the effects of cardiac mTOR in ischemic injury under metabolic syndrome remains undefined. Using cardiac-specific transgenic mice overexpressing mTOR (mTOR-Tg mice), we studied the effect of mTOR on cardiac function in both ex vivo and in vivo models of I/R injury in high-fat diet (HFD)-induced obese mice. mTOR-Tg and wild-type (WT) mice were fed a HFD (60% fat by calories) for 12 wk. Glucose intolerance and insulin resistance induced by the HFD were comparable between WT HFD-fed and mTOR-Tg HFD-fed mice. Functional recovery after I/R in the ex vivo Langendorff perfusion model was significantly lower in HFD-fed mice than normal chow diet-fed mice. mTOR-Tg mice demonstrated better cardiac function recovery and had less of the necrotic markers creatine kinase and lactate dehydrogenase in both feeding conditions. Additionally, mTOR overexpression suppressed expression of proinflammatory cytokines, including IL-6 and TNF-α, in both feeding conditions after I/R injury. In vivo I/R models showed that at 1 wk after I/R, HFD-fed mice exhibited worse cardiac function and larger myocardial scarring along myofibers compared with normal chow diet-fed mice. In both feeding conditions, mTOR overexpression preserved cardiac function and prevented myocardial scarring. These findings suggest that cardiac mTOR overexpression is sufficient to prevent the detrimental effects of diet-induced obesity on the heart after I/R, by reducing cardiac dysfunction and myocardial scarring.

Essential roles of PI(3)K-p110beta in cell growth, metabolism and tumorigenesis.

On activation by receptors, the ubiquitously expressed class IA isoforms (p110alpha and p110beta) of phosphatidylinositol-3-OH kinase (PI(3)K) generate lipid second messengers, which initiate multiple signal transduction cascades. Recent studies have demonstrated specific functions for p110alpha in growth factor and insulin signalling. To probe for distinct functions of p110beta, we constructed conditional knockout mice. Here we show that ablation of p110beta in the livers of the resulting mice leads to impaired insulin sensitivity and glucose homeostasis, while having little effect on phosphorylation of Akt, suggesting the involvement of a kinase-independent role of p110beta in insulin metabolic action. Using established mouse embryonic fibroblasts, we found that removal of p110beta also had little effect on Akt phosphorylation in response to stimulation by insulin and epidermal growth factor, but resulted in retarded cell proliferation. Reconstitution of p110beta-null cells with a wild-type or kinase-dead allele of p110beta demonstrated that p110beta possesses kinase-independent functions in regulating cell proliferation and trafficking. However, the kinase activity of p110beta was required for G-protein-coupled receptor signalling triggered by lysophosphatidic acid and had a function in oncogenic transformation. Most strikingly, in an animal model of prostate tumour formation induced by Pten loss, ablation of p110beta (also known as Pik3cb), but not that of p110alpha (also known as Pik3ca), impeded tumorigenesis with a concomitant diminution of Akt phosphorylation. Taken together, our findings demonstrate both kinase-dependent and kinase-independent functions for p110beta, and strongly indicate the kinase-dependent functions of p110beta as a promising target in cancer therapy.

System of matrix metalloproteinases and cytokine secretion in type 2 diabetes mellitus and impaired carbohydrate tolerance associated with arterial hypertension.

The study included patients with type 2 diabetes mellitus and impaired carbohydrate tolerance associated with arterial hypertension, patients with arterial hypertension, and healthy volunteers. We evaluated the levels of matrix metalloproteinases 2 and 9 (MMP-2, MMP-9), tissue inhibitor of metalloproteinase type 1 (TIMP-1), glucose, insulin, C-peptide, glycated hemoglobin, and spontaneous and mitogen-activated cytokine secretion (IL-2, IL4, IL-6, IL-10, IL-17, TNF-α, and IFN-γ). Patients with type 2 diabetes mellitus in combination with arterial hypertension exhibited maximum TIMP-1 levels and TIMP-1/MMP-2, TIMP-1/ MMP-9 ratios as well as enhanced secretion of TNF-α, IL-6, IL-17 and reduced secretion of IL-10 in comparison with healthy individuals. The observed shifts are probably determined the development of systemic hyperinsulinemia in patients suffering from type 2 diabetes mellitus coupled with arterial hypertension.

Hepatocyte-specific deletion of Janus kinase 2 (JAK2) protects against diet-induced steatohepatitis and glucose intolerance.

Non-alcoholic fatty liver disease (NAFLD) is becoming the leading cause of chronic liver disease and is now considered to be the hepatic manifestation of the metabolic syndrome. However, the role of steatosis per se and the precise factors required in the progression to steatohepatitis or insulin resistance remain elusive. The JAK-STAT pathway is critical in mediating signaling of a wide variety of cytokines and growth factors. Mice with hepatocyte-specific deletion of Janus kinase 2 (L-JAK2 KO mice) develop spontaneous steatosis as early as 2 weeks of age. In this study, we investigated the metabolic consequences of jak2 deletion in response to diet-induced metabolic stress. To our surprise, despite the profound hepatosteatosis, deletion of hepatic jak2 did not sensitize the liver to accelerated inflammatory injury on a prolonged high fat diet (HFD). This was accompanied by complete protection against HFD-induced whole-body insulin resistance and glucose intolerance. Improved glucose-stimulated insulin secretion and an increase in ß-cell mass were also present in these mice. Moreover, L-JAK2 KO mice had progressively reduced adiposity in association with blunted hepatic growth hormone signaling. These mice also exhibited increased resting energy expenditure on both chow and high fat diet. In conclusion, our findings indicate a key role of hepatic JAK2 in metabolism such that its absence completely arrests steatohepatitis development and confers protection against diet-induced systemic insulin resistance and glucose intolerance.

Muscle-specific knock-out of NUAK family SNF1-like kinase 1 (NUAK1) prevents high fat diet-induced glucose intolerance.

NUAK1 is a member of the AMP-activated protein kinase-related kinase family. Recent studies have shown that NUAK1 is involved in cellular senescence and motility in epithelial cells and fibroblasts. However, the physiological roles of NUAK1 are poorly understood because of embryonic lethality in NUAK1 null mice. The purpose of this study was to elucidate the roles of NUAK1 in adult tissues. We determined the tissue distribution of NUAK1 and generated muscle-specific NUAK1 knock-out (MNUAK1KO) mice. For phenotypic analysis, whole body glucose homeostasis and muscle glucose metabolism were examined. Quantitative phosphoproteome analysis of soleus muscle was performed to understand the molecular mechanisms underlying the knock-out phenotype. Nuak1 mRNA was preferentially expressed in highly oxidative tissues such as brain, heart, and soleus muscle. On a high fat diet, MNUAK1KO mice had a lower fasting blood glucose level, greater glucose tolerance, higher insulin sensitivity, and higher concentration of muscle glycogen than control mice. Phosphoproteome analysis revealed that phosphorylation of IRS1 Ser-1097 was markedly decreased in NUAK1-deficient muscle. Consistent with this, insulin signaling was enhanced in the soleus muscle of MNUAK1KO mice, as evidenced by increased phosphorylation of IRS1 Tyr-608, AKT Thr-308, and TBC1D4 Thr-649. These observations suggest that a physiological role of NUAK1 is to suppress glucose uptake through negative regulation of insulin signaling in oxidative muscle.

Hypothalamic glycogen synthase kinase 3ß has a central role in the regulation of food intake and glucose metabolism.

GSK3ß (glycogen synthase kinase 3ß) is a ubiquitous kinase that plays a key role in multiple intracellular signalling pathways, and increased GSK3ß activity is implicated in disorders ranging from cancer to Alzheimer's disease. In the present study, we provide the first evidence of increased hypothalamic signalling via GSK3ß in leptin-deficient Lep(ob/ob) mice and show that intracerebroventricular injection of a GSK3ß inhibitor acutely improves glucose tolerance in these mice. The beneficial effect of the GSK3ß inhibitor was dependent on hypothalamic signalling via PI3K (phosphoinositide 3-kinase), a key intracellular mediator of both leptin and insulin action. Conversely, neuron-specific overexpression of GSK3ß in the mediobasal hypothalamus exacerbated the hyperphagia, obesity and impairment of glucose tolerance induced by a high-fat diet, while having little effect in controls fed standard chow. These results demonstrate that increased hypothalamic GSK3ß signalling contributes to deleterious effects of leptin deficiency and exacerbates high-fat diet-induced weight gain and glucose intolerance.

Pyrroloquinoline quinone, a novel protein tyrosine phosphatase 1B inhibitor, activates insulin signaling in C2C12 myotubes and improves impaired glucose tolerance in diabetic KK-A(y) mice.

Insulin resistance is a pathological hallmark of type 2 diabetes mellitus and is characterized by defects in insulin signaling. Protein tyrosine phosphatase 1B (PTP1B) negatively regulates insulin signaling by tyrosine dephosphorylation of insulin receptor, and increased activity and expression of PTP1B is implicated in the pathogenesis of insulin resistance. Therefore, inhibition of PTP1B is anticipated to improve insulin resistance in type 2 diabetic subjects. Pyrroloquinoline quinone (PQQ), a redox cofactor for bacterial dehydrogenases, inhibits PTP1B to oxidatively modify the catalytic cysteine through its redox cycling activity. Here, we report that PQQ induces the ligand-independent activation of insulin signaling by inhibiting cellular PTP1B and enhances glucose uptake through the translocation of glucose transporter 4 in mouse C2C12 myotubes. Furthermore, we demonstrated that oral administration of PQQ improved impaired glucose tolerance in type 2 diabetic KK-A(y) mice. Our results strongly suggest that PQQ can be useful in anti-diabetic treatment for type 2 diabetic subjects.

Contribution of insulin and Akt1 signaling to endothelial nitric oxide synthase in the regulation of endothelial function and blood pressure.

Impaired insulin signaling via phosphatidylinositol 3-kinase/Akt to endothelial nitric oxide synthase (eNOS) in the vasculature has been postulated to lead to arterial dysfunction and hypertension in obesity and other insulin resistant states. To investigate this, we compared insulin signaling in the vasculature, endothelial function, and systemic blood pressure in mice fed a high-fat (HF) diet to mice with genetic ablation of insulin receptors in all vascular tissues (TTr-IR(-/-)) or mice with genetic ablation of Akt1 (Akt1-/-). HF mice developed obesity, impaired glucose tolerance, and elevated free fatty acids that was associated with endothelial dysfunction and hypertension. Basal and insulin-mediated phosphorylation of extracellular signal-regulated kinase 1/2 and Akt in the vasculature was preserved, but basal and insulin-stimulated eNOS phosphorylation was abolished in vessels from HF versus lean mice. In contrast, basal vascular eNOS phosphorylation, endothelial function, and blood pressure were normal despite absent insulin-mediated eNOS phosphorylation in TTr-IR(-/-) mice and absent insulin-mediated eNOS phosphorylation via Akt1 in Akt1-/- mice. In cultured endothelial cells, 6 hours of incubation with palmitate attenuated basal and insulin-stimulated eNOS phosphorylation and NO production despite normal activation of extracellular signal-regulated kinase 1/2 and Akt. Moreover, incubation of isolated arteries with palmitate impaired endothelium-dependent but not vascular smooth muscle function. Collectively, these results indicate that lower arterial eNOS phosphorylation, hypertension, and vascular dysfunction following HF feeding do not result from defective upstream signaling via Akt, but from free fatty acid-mediated impairment of eNOS phosphorylation.

Mitochondrial transhydrogenase--a key enzyme in insulin secretion and, potentially, diabetes.

Two recent studies have shown that the glucose intolerance and impaired insulin secretion of the C57BL/6J mouse strain results from oxidative stress due to a mutated nicotinamide nucleotide transhydrogenase. Reproduction of this phenotype, by mutating the same enzyme in another strain with normal glucose tolerance, suggests that the mechanism of the transhydrogenase-dependent inhibition of insulin secretion involves a partial uncoupling by the UCP2 protein. These exciting findings raise important questions, not least their potential relevance for human diabetes.