Promotes Insulin Sensitivity Research Articles

TET2 facilitates PPARγ agonist–mediated gene regulation and insulin sensitization in adipocytes

Emerging evidence indicates that epigenetic mechanisms like DNA methylation directly contribute to metabolic regulation. For example, we previously demonstrated that de novo DNA methyltransferase Dnmt3a plays a causal role in the development of adipocyte insulin resistance. Recent studies suggest that DNA demethylation plays an important role in the developmental process of adipocytes. However, little is known about whether DNA demethylase ten-eleven translocation (TET) proteins regulate the metabolic functions of adipocytes. MethodsThe expression of Tet genes was assessed in the fractionated adipocytes of chow- and high fat diet–fed C57/Bl6 mice using qPCR and western blotting. The effect of Tet2 gain- or loss-of-function in fully mature 3T3-L1 adipocytes in the presence/absence of Rosiglitazone (Rosi) and TNF-α on insulin sensitivity was using the insulin-stimulated glucose uptake and insulin signaling assays. Gene expression and DNA methylation analyses of PPARγ target genes was performed in the same setting. In addition, PPARγ reporter assays, co-immunoprecipitation assays, PPARγ ChIP-PCR analyses were performed. ResultsWe found that adipose expression of TET2, alone among its family members, was significantly reduced in diet-induced insulin resistance. TET2 gain-of-function was sufficient to promote insulin sensitivity while loss-of-function was necessary to facilitate insulin sensitization in response to the PPARγ agonist Rosiglitazone (Rosi) in cultured adipocytes. Consistent with this, TET2 was required for Rosi-dependent gene activation of certain PPARγ targets accompanied by changes in DNA demethylation at the promoter regions. Furthermore, TET2 was necessary to sustain PPARγ binding to target loci upon activation with Rosi via physical interaction with PPARγ. ConclusionsOur data demonstrate that TET2 works as an epigenetic regulator of Rosi-mediated insulin sensitization and transcriptional regulation in adipocytes.

Sirt1 activator induces proangiogenic genes in preadipocytes to rescue insulin resistance in diet-induced obese mice

Sirt1 plays an important role in regulating glucose and lipid metabolism in obese animal models. Impaired adipose tissue angiogenesis in the obese state decreases adipogenesis and thereby contributes to glucose intolerance and lipid metabolism. However, the mechanism by which Sirt1 activation affects obesity-associated impairments in angiogenesis in the adipose tissue is not fully understood. Here, we show that SRT1720 treatment induces angiogenic genes in cultured 3T3-L1 preadipocytes and ex vivo preadipocytes. siRNA-mediated knockdown of Sirt1 in 3T3-L1 preadipocytes downregulated angiogenic genes in the preadipocytes. SRT1720 treatment upregulated metabolically favorable genes and reduced inflammatory gene expressions in the adipose tissue of diet-induced obese (DIO) mice. Collectively, these findings suggest a novel role of SRT1720-induced Sirt1 activation in the induction of angiogenic genes in preadipocytes, thereby reducing inflammation and fibrosis in white adipose tissue (WAT) and promoting insulin sensitivity.

Chicoric acid mitigates impaired insulin sensitivity by improving mitochondrial function.

Mitochondrial dysfunction is associated with insulin resistance. Although chicoric acid (CA) is known to have beneficial effects on insulin sensitivity, the involvement of mitochondrial function has not been elucidated yet. Here, we investigated the effect of CA on insulin resistance and mitochondrial dysfunction. In palmitate-induced insulin-resistant C2C12 myotubes, CA improved impaired glucose uptake and insulin signaling pathways, along with enhanced mitochondrial membrane potential and oxygen consumption. CA treatment in diet-induced obese mice ameliorated glucose tolerance and increased insulin sensitivity. CA treatment also recovered the dysregulated expression of glucose metabolism-related genes in the high-fat-fed mice. CA significantly increased the mitochondrial DNA content, citrate synthase, and ATP content, as well as the expression of genes related to mitochondrial biogenesis and oxidative phosphorylation in the liver and skeletal muscle in high-fat- fed obese mice. These findings suggested that CA attenuates insulin resistance and promotes insulin sensitivity by enhancing mitochondrial function.

Berberine Hydrochloride Protects Cytokine-Induced Inflammation through Multiple Pathways

Berberine hydrochloride (BBR) has been reported to improve insulin resistance. However, the underlying mechanism remains largely elusive. We therefore investigate the anti-inflammatory role of BBR and the cell signaling pathways that involved. C2C12 myoblasts were treated with IL-1β (10ng/mL) alone or in combination with TNF-α (50 ng/mL) in the presence or absence of BBR (10 µM) for 24 hours. Western blot analysis was used to assess the protein level. In vivo study is conducted using C57BL/6 mice fed with high fat high sucrose (HFHS) diet to induce insulin resistance. Skeletal muscle was collected after 4 weeks treatment with either 5 mg/kg BBR or vehicle. BBR reduced activity of iNOS, and stress-related kinases, including p38 MAPK, NF-kB, SAP/JNK, MMK4 and C-JUN that was induced by cytokines in C2C12 myoblast. Furthermore, BBR reversed cytokine mediated suppression of AMPK-α, augmented activity of SIRT-1 and PGC-1α. Antioxidant enzyme or transcription factor, nuclear factor like 2 (NRF-2) were also elevated after BBR treatment. In addition, BBR improved insulin signaling in C2C12 myoblast. HFHS diet induced iNOS expression was reversed by BBR in skeletal muscle. Augmentation of the activity of uncoupled protein-3 (UCP-3) and PPARα in HFHS diet skeletal muscle was observed. Our results demonstrate that BBR ameliorates cytokine induced inflammation and oxidative stress. BBR improves mitochondrial function, promotes insulin sensitivity. The anti-inflammatory effect of BBR in skeletal muscle may mediated through multiple pathways including activation of AMPKα-SIRT-1-PGC-1α pathway, and inhibition of MMK4- SAP/JUNK-C-JUN pathway. BBR is a potential medication in treating obesity induced insulin resistance. Disclosure A. Poudel: None. J. Zhou: None. S. Massey: None. L. Li: None.

Loss of CEACAM1 in Endothelial Cells Contributes to the Development of Cardiac Fibrosis

Carcinoembryonic Antigen-related Cell Adhesion Molecule 1 (CEACAM1) has been implicated in various physiological processes in the vascular systems, including blood vessel formation, and endothelial barrier function. Consistent with a role for hepatic CEACAM1 in promoting insulin sensitivity by mediating insulin clearance and maintaining physiologic plasma insulin levels, we have recently identified a role for hepatic CEACAM1 in preventing cardiac hypertrophy and dysfunction. The aim of this study was to elucidate the contribution of endothelial cell CEACAM1 in cardiac function and fibrosis that has remained elusive. To this end, we generated an endothelial cell-specific Ceacam1 knockout mouse line (VECadCc1−/−). Gomori Trichrome Blue Staining revealed a significant perivascular and interstitial fibrosis in the heart of VECadCc1−/− mice compared to their control littermates. Consistently, the mRNA levels of markers of fibrosis [collagen 1A1, α-SMA and fibroblast-specific protein1 (FSP1)] were elevated with a reciprocal decrease of endothelial cell makers (CD31), indicating endothelial mesenchymal transition (EndMT) in these mice. Cardiac fibrosis was associated with increased activation of the TGF-β1-Smad2/3 signaling pathway. Immunofluorescence staining co-localized CD31 with α-SMA in the myocardial perivascular section of VECadCc1−/− hearts. Moreover, mRNA level of α-SMA, FSP1, SNAIL and SLUG were up-regulated while that of CD31 was down-regulated in mouse heart endothelial cells (MHEC) isolated from VECadCc1−/− mice. The Akt/eNOS pathway was basally inactivated, leading to lower nitric oxide (NO) level in MHEC isolated from VECadCc1−/− mice. Moreover, loss of endothelial CEACAM1 caused reduced association with SHP-2 phosphatase and consequently, reduction in insulin-stimulated IRS-1/Akt/eNOS activation.These studies identified an important role for the loss of CEACAM1 in endothelial cells as a mechanism underlying cardiac fibrosis. Disclosure H.T. Muturi: None. H.E. Ghadieh: None. S.S. Khuder: None. S. Jash: None. R. Abu Helal: None. N.A. Daniels: None. J. Liu: None. V. Puri: None. G. Vazquez: None. R. Gupta: None. S.M. Najjar: None.

SLN overexpression upregulates mitochondrial dynamics and promotes oxidative metabolism during caloric restriction.

Obesity results from increased caloric intake, physical inactivity and is a major contributor to type 2 diabetes (T2D). Exercise and caloric restriction (CR) are the most effective interventions to prevent obesity, T2D and sarcopenia. By increasing fatty acid oxidation (FAO), CR promotes insulin sensitivity and improves mitochondrial function. Our laboratory has identified Sarcolipin (SLN) as an important player in muscle thermogenesis. SLN‐binding to SERCA promotes uncoupling of Ca2+ transport, causing futile cycling of the SERCA pump, increasing energy expenditure. SLN overexpression in glycolytic muscle leads to increased mitochondrial biogenesis and FAO, resembling an oxidative phenotype. Therefore, we sought to determine if SLN overexpression (1) promotes resistance to fatigue and increased exercise performance and (2) improves the beneficial effects of CR in skeletal muscle mitochondrial metabolism. Wild type (WT), Sarcolipin knockout (SLN−/−) and Sarcolipin overexpression (SLNOE) mice were submitted to 8 weeks of exercise training at 60% of maximal effort capacity. After exercise, oxygen consumption, maximal force and resistance to fatigue were measured in isolated muscles. Electronic transport chain (ETC) and oxidative phosphorylation (OXPHOS) related proteins were analyzed in muscle and adipose tissue. Exercise‐induced increase in ETC proteins of white adipose tissue was blunted in SLN−/− mice. Our results show that SLNOE are resistant to fatigue and show better functional recovery as compared with WT and SLN−/−. Oxygen consumption is also increased in tibialis anterior after exercise in SLNOE, indicating that SLN overexpression in glycolytic muscle improves FAO and promotes endurance capacity. To induce CR, 3 month‐old male WT, SLN−/− and SLNOE mice were submitted to 8 weeks of 40% CR protocol, and body weight was measured weekly. Glucose tolerance test and insulin sensitivity measurements showed that SLNOE improved CR‐induced insulin sensitivity and glucose tolerance. Western blot analysis revealed that CR‐induced increase in ETC proteins (Complex I to V) and mitochondrial content (Tfam) is SLN‐dependent. Secondly, SLN increases mitochondrial fusion (Mfn1 and Opa1) and fission (Drp1) proteins and, as expected, CR increased markers of autophagy (LC3‐ II). Our data indicates that SLN plays an important role in metabolic adaptations in muscle by modulating mitochondrial dynamics and promoting oxidative metabolism. We propose that SLN can contribute to CR induced metabolic benefits in muscle.Support or Funding InformationNIDDK ‐R01 DK098240This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

A feedback control of insulin signaling by transcription factor HMG box‐containing protein 1 (HBP1) in adipocytes

Adipocytes are sensitive to insulin control of cellular glucose utilization, thereby facilitating the maintenance of blood glucose concentration. Previously we demonstrated that transcription factor HBP1 is highly expressed in differentiated adipocytes, which, together with CCAAT/enhancer‐binding proteins C/EBPβ and C/EBPα, regulates adipocyte differentiation. To further gain insight into the biological role of HBP1 in mature adipocytes, here, we employed mouse embryonic fibroblast (MEF)‐derived adipocytes to test the hypothesis that HBP1 might regulate insulin sensitivity in adipocytes. First, suppression of HBP1 by siRNA‐mediated gene silencing significantly attenuated glucose uptake as shown by 2‐NBDG glucose uptake assay. We then examined the insulin‐signaling pathway to unravel the molecular mechanism by which HBP1 commands adipocytes to take up glucose. Under serum starvation, the transcription of insulin receptor (InR) was increased, whereas HBP1 silencing down‐regulated the expression of InR in adipocytes, indicating that HBP1 is able to promote insulin sensitivity under insulin insufficient state. However, the addition of insulin (2 nM for 6 h) led to a significant decrease in the transcription of HBP1, InR, and IRS1/2 (insulin receptor substrate 1/2). Moreover, HBP1 siRNA led to increased level of p‐S307 IRS‐1 that is associated with blockade of the insulin signaling, and also resulted in an unexpected induction of glucose transporter 4 (GLUT4). The upregulation of GLUT4 may function as a compensation mechanism for the defective insulin signaling caused by HBP1 silencing. Taken as a whole, our results reveal a key feedback control mechanism for HBP1 in regulating insulin signaling and glucose metabolism.Support or Funding InformationThis work was supported by the grant to MOST 105‐2320‐B‐039‐044‐MY2 C‐Y Huang.This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Peroxiredoxin 2 mediates insulin sensitivity of skeletal muscles through regulation of protein tyrosine phosphatase oxidation.

Insulin signaling is essential for regulating glucose homeostasis. Numerous studies have demonstrated that reactive oxygen species (ROS) affect insulin signaling, and low ROS levels can act as a signal to regulate cellular function. Peroxiredoxins (Prxs) are highly abundant and widely expressed antioxidant enzymes. However, it is unclear whether antioxidant enzymes, such as Prx2, mediate insulin signaling. The aim of our study was to investigate the influence of Prx2 deficiency on insulin signaling. Our western blot results showed that Prx2 deficiency enhanced insulin signaling and increased oxidation of protein tyrosine phosphatase 1B (PTP1B) and phosphatase and tensin homologue (PTEN) in mouse embryonic fibroblasts (MEFs) treated with insulin. In addition, we assessed ROS levels with a Cytosol-HyPer H2O2 sensor. As a result, increased ROS levels and Akt activation were decreased by N-acetyl-cysteine (Nac), which acted as an antioxidant in Prx2-deficient MEFs. Body weight measurements and glucose tolerance test (GTT) revealed significant body weight reduction and increase in glucose clearance in Prx2-/- mice fed a high-fat diet. Interestingly, glucose transporter type 4 (GLUT4) was significantly higher in Prx2-/- mice than in wild-type mice according to western blotting results. Western blotting also revealed that Akt phosphorylation was higher in Prx2-/- MEFs and muscle tissue than in wild-type. Together, our findings indicate that increased ROS due to Prx2 deficiency promotes insulin sensitivity and glucose clearance in skeletal muscles by increasing protein tyrosine phosphatase (PTPs) oxidation. These results provide novel insights into the fundamental mechanisms of insulin signaling induced by Prx2 deficiency and suggest that ROS-based therapeutic strategies can be used to suppress insulin resistance.

Leptin-induced basal Akt phosphorylation and its implication in exercise-mediated improvement of insulin sensitivity

Physical exercise is an efficient therapeutical tool in the management of insulin resistance (IR) and related metabolic diseases. Leptin, the well-known obesity hormone and the absence of which leads to IR, showed controversial effects on IR as research continues. Thus, in this study, a detailed investigation of the effect of leptin on exercise-mediated improvement of insulin sensitivity and its underlying mechanism was carried out. Using a rat model of chronic or acute swimming exercise training, we found that serum leptin increased 1 h after either acute exercise or the last session of chronic exercise, when impaired insulin action was observed in previous reports. However, chronic exercise reducd basal serum leptin levels and promoted insulin sensitivity compared with sedentary controls or rats subjected to one bout of aerobic exercise. Our animal results indicated the potential linkage between leptin and insulin sensitivity, which is further investigated in the skeletal muscle L6 cells. Leptin treatment in L6 cells promoted the basal levels of insulin signaling as well as glucose uptake, while blocking JAK2 signaling with either pharmacological intervention (JAK2 inhibitor AG490) or genetic manipulation (siRNA knockdown) decreased the basal levels of insulin signaling. Furthermore, leptin treatment inhibited insulin-stimulated insulin signaling and glucose uptake, while blocking JAK2 signaling restored leptin-attenuated insulin sensitivity. Taken together, our results demonstrated that reduced serum leptin, at least in part, contributes to exercise-mediated improvement of insulin sensitivity, indicating JAK2 as a potent therapeutical target of insulin resistance.

Exenatide Prevents Diet-induced Hepatocellular Injury in A CEACAM1-Dependent Mechanism.

The Carcinoembryonic Antigen-Related Cell Adhesion Molecule 1 (CEACAM1) promotes insulin sensitivity by inducing insulin clearance and reducing de novo lipogenesis in the liver. Consistently, Cc1-/- mice with null deletion of Ceacam1 gene exhibit hyperinsulinemia and insulin resistance, in addition to steatohepatitis. They also exhibit early pericellular fibrosis. Redelivering Ceacam1 to the liver reverses the altered metabolism and histopathology of Cc1-/- mice. Exenatide, a long-acting glucagon-like peptide-1 receptor agonist, induces Ceacam1 transcription and consequently, reverses impaired insulin clearance and insulin resistance caused by high-fat intake. Additionally, it reverses fat accumulation in the liver. The current studies show that exenatide also restored the activities of alanine transaminase and aspartate aminotransferase, and reversed the inflammatory and oxidative stress response to high-fat diet in wild-type, but not in Cc1-/- mice. Exenatide also prevented diet-induced activation of the TGFβ/Smad2/Smad3 pro-fibrogenic pathways, and normalized the mRNA levels of pro-fibrogenic genes in wild-type, but not in Cc1-/- mice. Together, the data demonstrate that exenatide prevented diet-induced pro-fibrogenesis and hepatocellular injury in a CEACAM1-dependent mechanism.

Cell Type-Specific Immunomodulation Induced by Helminthes: Effect on Metainflammation, Insulin Resistance and Type-2 Diabetes.

Recent epidemiological studies have documented an inverse relationship between the decreasing prevalence of helminth infections and the increasing prevalence of metabolic diseases ("metabolic hygiene hypothesis"). Chronic inflammation leading to insulin resistance (IR) has now been identified as a major etiological factor for a variety of metabolic diseases other than obesity and Type-2 diabetes (metainflammation). One way by which helminth infections such as filariasis can modulate IR is by inducing a chronic, nonspecific, low-grade, immune suppression mediated by modified T-helper 2 (Th2) response (induction of both Th2 and regulatory T cells) which can in turn suppress the proinflammatory responses and promote insulin sensitivity (IS). This article provides evidence on how the cross talk between the innate and adaptive arms of the immune responses can modulate IR/sensitivity. The cross talk between innate (macrophages, dendritic cells, natural killer cells, natural killer T cells, myeloid derived suppressor cells, innate lymphoid cells, basophils, eosinophils, and neutrophils) and adaptive (helper T [CD4+] cells, cytotoxic T [CD8+] cells and B cells) immune cells forms two opposing circuits, one associated with IR and the other associated with IS under the conditions of metabolic syndrome and helminth-mediated immunomodulation, respectively.

Endothelial APLNR regulates tissue fatty acid uptake and is essential for apelin's glucose-lowering effects.

Treatment of type 2 diabetes mellitus continues to pose an important clinical challenge, with most existing therapies lacking demonstrable ability to improve cardiovascular outcomes. The atheroprotective peptide apelin (APLN) enhances glucose utilization and improves insulin sensitivity. However, the mechanism of these effects remains poorly defined. We demonstrate that the expression of APLNR (APJ/AGTRL1), the only known receptor for apelin, is predominantly restricted to the endothelial cells (ECs) of multiple adult metabolic organs, including skeletal muscle and adipose tissue. Conditional endothelial-specific deletion of Aplnr (AplnrECKO ) resulted in markedly impaired glucose utilization and abrogation of apelin-induced glucose lowering. Furthermore, we identified inactivation of Forkhead box protein O1 (FOXO1) and inhibition of endothelial expression of fatty acid (FA) binding protein 4 (FABP4) as key downstream signaling targets of apelin/APLNR signaling. Both the Apln-/- and AplnrECKO mice demonstrated increased endothelial FABP4 expression and excess tissue FA accumulation, whereas concurrent endothelial Foxo1 deletion or pharmacologic FABP4 inhibition rescued the excess FA accumulation phenotype of the Apln-/- mice. The impaired glucose utilization in the AplnrECKO mice was associated with excess FA accumulation in the skeletal muscle. Treatment of these mice with an FABP4 inhibitor abrogated these metabolic phenotypes. These findings provide mechanistic insights that could greatly expand the therapeutic repertoire for type 2 diabetes and related metabolic disorders.

Adipose and Liver Function in Primate Offspring with Insulin Resistance Induced by Estrogen Deprivation in Utero

We recently demonstrated that offspring delivered to baboons deprived of estrogen during the second half of gestation exhibited insulin resistance. Therefore, because skeletal muscle accounts for >80% of insulin dependent glucose disposal, we suggested that estrogen in utero programs factors in fetal skeletal muscle important for insulin sensitivity in offspring. However, liver and adipose are also sites of insulin action and adipose insulin resistance can increase serum free fatty acid (FFA) levels and thereby reduce skeletal muscle insulin sensitivity. Therefore, in the current study we determined whether estrogen-deprived offspring exhibit normal adipose and hepatic function. The fasting serum levels of adiponectin, leptin, glucose, and analytes of liver function as well as the basal levels of serum FFA were similar in offspring of estrogen replete/suppressed baboons. Moreover, the normal glucose-induced decline in serum FFA levels measured in untreated offspring was also measured in offspring of letrozole-treated baboons. Fetal serum levels of adiponectin and leptin in late gestation also were similar and expression of nitrotyrosine negligible in fetal liver and adipose of untreated and letrozole-treated animals. These results indicate that offspring of letrozole-treated baboons have normal adipose and liver function and do not exhibit adipose insulin resistance. Therefore, we suggest that the insulin resistance observed in estrogen-deprived offspring primarily reflects a decline in insulin-stimulated glucose clearance by skeletal muscle and which supports our original suggestion that estrogen in utero programs factors in fetal skeletal muscle that promote insulin sensitivity in offspring.

Enhancement of Glucose Uptake by Meso-Dihydroguaiaretic Acid through GLUT4 Up-Regulation in 3T3-L1 Adipocytes.

Type 2 diabetes is characterized by insulin resistance, which leads to increased blood glucose levels. Adipocytes are involved in the development of insulin resistance, resulting from the dysfunction of the insulin signaling pathway. In this study, we investigated whether meso-dihydroguaiaretic acid (MDGA) may modulate glucose uptake in adipocytes, and examined its mechanism of action. MDGA enhanced adipogenesis through up-regulation of peroxisome proliferator-activated receptor γ and CCAAT/enhancer-binding protein α in 3T3-L1 adipocytes partially differentiated with sub-optimal concentrations of insulin. MDGA also increased glucose uptake by stimulating expression and translocation of glucose transporter 4 (GLUT4) in adipocytes. These results suggest that MDGA may increase GLUT4 expression and its translocation by promoting insulin sensitivity, leading to enhanced glucose uptake.

The bradykinin-cGMP-PKG pathway augments insulin sensitivity via upregulation of MAPK phosphatase-5 and inhibition of JNK.

Bradykinin (BK) promotes insulin sensitivity and glucose uptake in adipocytes and other cell types. We demonstrated that in rat adipocytes BK enhances insulin-stimulated glucose transport via endothelial nitric oxide synthase, nitric oxide (NO) generation, and decreased activity of the mitogen-activated protein kinase (MAPK) JNK (c-Jun NH2-terminal kinase). In endothelial cells, NO increases soluble guanylate cyclase (sGC) activity, which, in turn, activates protein kinase G (PKG) by increasing cGMP levels. In this study, we investigated whether BK acts via the sGC-cGMP-PKG pathway to inhibit the negative effects of JNK on insulin signaling and glucose uptake in rat adipocytes. BK augmented cGMP concentrations. The BK-induced enhancement of insulin-stimulated glucose uptake was mimicked by the sGC activator YC-1 and a cell-permeable cGMP analog, CPT-cGMP, and inhibited by the sGC inhibitor ODQ and the PKG inhibitor KT 5823. Transfection of dominant-negative PKG reduced the BK augmentation of insulin-induced Akt phosphorylation. The activation of JNK and ERK1/2 by insulin was attenuated by BK, which was mediated by the sGC-cGMP-PKG pathway. Whereas insulin-stimulated phosphorylation of upstream activators of JNK and ERK, i.e., MKK4 and MEK1/2, was unaffected, BK augmented insulin-mediated induction of MKP-5 mRNA and protein levels. Furthermore, zaprinast, a phosphodiesterase inhibitor, enhanced cGMP and MKP-5 and prolonged the action of BK. These data indicate that BK enhances insulin action by inhibition of negative feedback by JNK and ERK via upregulation of MKP-5, mediated by the sGC-cGMP-PKG signaling pathway.

The cold-induced lipokine 12,13-diHOME promotes fatty acid transport into brown adipose tissue.

Brown adipose tissue (BAT) and beige adipose tissue combust fuels for heat production in adult humans, and so constitute an appealing target for the treatment of metabolic disorders such as obesity, diabetes and hyperlipidemia. Cold exposure can enhance energy expenditure by activating BAT, and it has been shown to improve nutrient metabolism. These therapies, however, are time consuming and uncomfortable, demonstrating the need for pharmacological interventions. Recently, lipids have been identified that are released from tissues and act locally or systemically to promote insulin sensitivity and glucose tolerance; as a class, these lipids are referred to as 'lipokines'. Because BAT is a specialized metabolic tissue that takes up and burns lipids and is linked to systemic metabolic homeostasis, we hypothesized that there might be thermogenic lipokines that activate BAT in response to cold. Here we show that the lipid 12,13-dihydroxy-9Z-octadecenoic acid (12,13-diHOME) is a stimulator of BAT activity, and that its levels are negatively correlated with body-mass index and insulin sensitivity. Using a global lipidomic analysis, we found that 12,13-diHOME was increased in the circulation of humans and mice exposed to cold. Furthermore, we found that the enzymes that produce 12,13-diHOME were uniquely induced in BAT by cold stimulation. The injection of 12,13-diHOME acutely activated BAT fuel uptake and enhanced cold tolerance, which resulted in decreased levels of serum triglycerides. Mechanistically, 12,13-diHOME increased fatty acid (FA) uptake into brown adipocytes by promoting the translocation of the FA transporters FATP1 and CD36 to the cell membrane. These data suggest that 12,13-diHOME, or a functional analog, could be developed as a treatment for metabolic disorders.

Association between osteocalcin gamma-carboxylation and insulin resistance in overweight and obese postmenopausal women

AimIn mice, osteocalcin (OCN) acts as a bone-derived hormone promoting insulin sensitivity and glucose tolerance. In that species, OCN endocrine action is inhibited when its first glutamic acid residue (Glu13) is γ-carboxylated (Gla). The importance of this posttranslational modification for OCN function in human is still unclear. Our objectives were to identify an assay to assess γ-carboxylation of human OCN on its first Glu residue (Glu17) and to test its association with insulin resistance and inflammation profile in overweight women. MethodsSeveral ELISAs were tested to determine their specificity toward various forms of human OCN. Associations between OCN γ-carboxylation and determinants of glucose tolerance, insulin sensitivity, liver function and subclinical inflammation were then investigated in 129 non-diabetic overweight and obese postmenopausal women. ResultsWe identified assays allowing the measurement of total OCN (tOCN) and the ratio of Gla17/tOCN. Circulating Gla17/tOCN levels correlated negatively with insulin sensitivity assessed by hyperinsulinemic-euglyceamic clamp (P=0.02) or insulin sensitivity index derived from oral glucose tolerance test (P=0.00003), and positively with insulin resistance assessed by HOMA-IR (P=0.0005) and with markers of subclinical inflammation and liver enzymes, including C-reactive protein (CRP; P=0.007) and aspartate aminotransferase (AST; P=0.009). Conclusionsγ-carboxylation of OCN on Glu17 residue correlates with insulin resistance and subclinical inflammation, suggesting that γ-carboxylation of OCN negatively regulates its endocrine action in humans.

Inducible overexpression of adiponectin receptors highlight the roles of adiponectin-induced ceramidase signaling in lipid and glucose homeostasis

ObjectiveAdiponectin and the signaling induced by its cognate receptors, AdipoR1 and AdipoR2, have garnered attention for their ability to promote insulin sensitivity and oppose steatosis. Activation of these receptors promotes the deacylation of ceramide, a lipid metabolite that appears to play a causal role in impairing insulin signaling. MethodsHere, we have developed transgenic mice that overexpress AdipoR1 or AdipoR2 under the inducible control of a tetracycline response element. These represent the first inducible genetic models that acutely manipulate adiponectin receptor signaling in adult mouse tissues, which allows us to directly assess AdipoR signaling on glucose and lipid metabolism. ResultsOverexpression of either adiponectin receptor isoform in the adipocyte or hepatocyte is sufficient to enhance ceramidase activity, whole body glucose metabolism, and hepatic insulin sensitivity, while opposing hepatic steatosis. Importantly, metabolic improvements fail to occur in an adiponectin knockout background. When challenged with a leptin-deficient genetic model of type 2 diabetes, AdipoR2 expression in adipose or liver is sufficient to reverse hyperglycemia and glucose intolerance. ConclusionThese observations reveal that adiponectin is critical for AdipoR-induced ceramidase activation which enhances hepatic glucose and lipid metabolism via rapidly acting “cross-talk” between liver and adipose tissue sphingolipids.

A Long-lived Mouse Lacking Both Growth Hormone and Growth Hormone Receptor: A New Animal Model for Aging Studies.

Disruption of the growth hormone (GH) signaling pathway promotes insulin sensitivity and is associated with both delayed aging and extended longevity. Two kinds of long-lived mice-Ames dwarfs (df/df) and GH receptor gene-disrupted knockouts (GHRKO) are characterized by a suppressed GH axis with a significant reduction of body size and decreased plasma insulin-like growth factor-1 (IGF-1) and insulin levels. Ames dwarf mice are deficient in GH, prolactin, and thyrotropin, whereas GHRKOs are GH resistant and are dwarf with decreased circulating IGF-1 and increased GH. Crossing Ames dwarfs and GHRKOs produced a new mouse line (df/KO), lacking both GH and GH receptor. These mice are characterized by improved glucose tolerance and increased adiponectin level, which could imply that these mice should be also characterized by additional life-span extension when comparing with GHRKOs and Ames dwarfs. Importantly, our longevity experiments showed that df/KO mice maintain extended longevity when comparing with N control mice; however, they do not live longer than GHRKO and Ames df/df mice. These important findings indicate that silencing GH signal is important to extend the life span; however, further decrease of body size in mice with already inhibited GH signal does not extend the life span regardless of improved some health-span markers.

Protein Kinase Mitogen-activated Protein Kinase Kinase Kinase Kinase 4 (MAP4K4) Promotes Obesity-induced Hyperinsulinemia

Previous studies revealed a paradox whereby mitogen-activated protein kinase kinase kinase kinase 4 (Map4k4) acted as a negative regulator of insulin sensitivity in chronically obese mice, yet systemic deletion of Map4k4 did not improve glucose tolerance. Here, we report markedly reduced glucose-responsive plasma insulin and C-peptide levels in whole body Map4k4-depleted mice (M4K4 iKO) as well as an impaired first phase of insulin secretion from islets derived from M4K4 iKO mice ex vivo. After long-term high fat diet (HFD), M4K4 iKO mice pancreata also displayed reduced β cell mass, fewer proliferating β cells and reduced islet-specific gene mRNA expression compared with controls, although insulin content was normal. Interestingly, the reduced plasma insulin in M4K4 iKO mice exposed to chronic (16 weeks) HFD was not observed in response to acute HFD challenge or short term treatment with the insulin receptor antagonist S961. Furthermore, the improved insulin sensitivity in obese M4K4 iKO mice was abrogated by high exogenous insulin over the course of a euglycemic clamp study, indicating that hypoinsulinemia promotes insulin sensitivity in chronically obese M4K4 iKO mice. These results demonstrate that protein kinase Map4k4 drives obesity-induced hyperinsulinemia and insulin resistance in part by promoting insulin secretion from β cells in mice.