Insulin Signaling In Adipocytes Research Articles

Natural Compound α-PGG and Its Synthetic Derivative 6Cl-TGQ Alter Insulin Secretion: Evidence for Diminishing Glucose Uptake as a Mechanism.

PurposePreviously we showed that natural compound α-penta-galloyl-glucose (α-PGG) and its synthetic derivative 6-chloro-6-deoxy-1,2,3,4-tetra-O-galloyl-α-D-glucopyranose (6Cl-TGQ) act to improve insulin signaling in adipocytes by increasing glucose transport. In this study, we investigated the mechanism of actions of α-PGG and 6Cl-TGQ on insulin secretion.MethodsMouse islets and/or INS-1832/13 beta-cells were used to test the effects of our compounds on glucose-stimulated insulin secretion (GSIS), intracellular calcium [Ca2+]i using fura-2AM, glucose transport activity via a radioactive glucose uptake assay, intracellular ATP/ADP, and extracellular acidification (ECAR) and mitochondrial oxygen consumption rates (OCAR) using Seahorse metabolic analysis.ResultsBoth compounds reduced GSIS in beta-cells without negatively affecting cell viability. The compounds primarily diminished glucose uptake into islets and beta-cells. Despite insulin-like effects in the peripheral tissues, these compounds do not act through the insulin receptor in islets. Further interrogation of the stimulus-secretion pathway showed that all the key metabolic factors involved in GSIS including ECAR, OCAR, ATP/ADP ratios, and [Ca2+]i of INS-1832/13 cells were diminished after the compound treatment.ConclusionThe compounds suppress glucose uptake of the beta-cells, which consequently slows down the rates of glycolysis and ATP synthesis, leading to decrease in [Ca2+]i and GSIS. The difference between adipocytes and beta-cells in effects on glucose uptake is of great interest. Further structural and functional modifications could produce new compounds with optimized therapeutic potentials for different target cells. The higher potency of synthetic 6Cl-TGQ in enhancing insulin signaling in adipocytes but lower potency in reducing glucose uptake in beta-cells compared to α-PGG suggests the feasibility of such an approach.

Inflammation promotes adipocyte lipolysis via IRE1 kinase

Obesity associates with inflammation, insulin resistance, and higher blood lipids. It is unclear if immune responses facilitate lipid breakdown and release from adipocytes via lipolysis in a separate way from hormones or adrenergic signals. We found that an ancient component of ER stress, inositol-requiring protein 1 (IRE1), discriminates inflammation-induced adipocyte lipolysis versus lipolysis from adrenergic or hormonal stimuli. Our data show that inhibiting IRE1 kinase activity was sufficient to block adipocyte-autonomous lipolysis from multiple inflammatory ligands, including bacterial components, certain cytokines, and thapsigargin-induced ER stress. IRE1-mediated lipolysis was specific for inflammatory triggers since IRE1 kinase activity was dispensable for isoproterenol and cAMP-induced lipolysis in adipocytes and mouse adipose tissue. IRE1 RNase activity was not associated with inflammation-induced adipocyte lipolysis. Inhibiting IRE1 kinase activity blocked NF-κB activation, interleukin-6 secretion, and adipocyte-autonomous lipolysis from inflammatory ligands. Inflammation-induced lipolysis mediated by IRE1 occurred independently from changes in insulin signaling in adipocytes, suggesting that inflammation can promote IRE1-mediated lipolysis independent of adipocyte insulin resistance. We found no role for canonical unfolded protein responses or ABL kinases in linking ER stress to IRE1-mediated lipolysis. Adiponectin-Cre-mediated IRE1 knockout in mice showed that adipocyte IRE1 was required for inflammatory ligand-induced lipolysis in adipose tissue explants and that adipocyte IRE1 was required for approximately half of the increase in blood triglycerides after a bacterial endotoxin-mediated inflammatory stimulus in vivo. Together, our results show that IRE1 propagates an inflammation-specific lipolytic program independent from hormonal or adrenergic regulation. Targeting IRE1 kinase activity may benefit metabolic syndrome and inflammatory lipid disorders.

Cleavage of the vaspin N-terminus releases cell-penetrating peptides that affect early stages of adipogenesis and inhibit lipolysis in mature adipocytes

ABSTRACT Vaspin expression and function is related to metabolic disorders and comorbidities of obesity. In various cellular and animal models of obesity, diabetes and atherosclerosis vaspin has shown beneficial, protective and/or compensatory action. While testing proteases for inhibition by vaspin, we noticed specific cleavage within the vaspin N-terminus and sequence analysis predicted cell-penetrating activity for the released peptides. These findings raised the question whether these proteolytic peptides exhibit biological activity. We synthesized various N-terminal vaspin peptides to investigate cell-penetrating activity and analyse uptake mechanisms. Focusing on adipocytes, we performed microarray analysis and functional assays to elucidate biological activities of the vaspin–derived peptide, which is released by KLK7 cleavage (vaspin residues 21-30; VaspinN). Our study provides first evidence that proteolytic processing of the vaspin N-terminus releases cell-penetrating and bioactive peptides with effects on adipocyte biology. The VaspinN peptide increased preadipocyte proliferation, interfered with clonal expansion during the early stage of adipogenesis and blunted adrenergic cAMP-signalling, downstream lipolysis as well as insulin signalling in mature adipocytes. Protease-mediated release of functional N-terminal peptides presents an additional facet of vaspin action. Future studies will address the mechanisms underlying the biological activities and clarify, if vaspin-derived peptides may have potential as therapeutic agents for the treatment of metabolic diseases.

Intermittent Hypoxia Mediates Caveolae Disassembly That Parallels Insulin Resistance Development.

Repetitive complete or incomplete pharyngeal collapses are leading to chronic intermittent hypoxia (CIH), a hallmark feature of obstructive sleep apnea (OSA) syndrome responsible for many metabolic disorders. In humans, an association between OSA and insulin resistance has been found independently of the degree of obesity. Based on our previous work showing that hypoxia applied to adipocytes led to cellular insulin resistance associated with caveolae flattening, we have investigated the effects of CIH on caveolae structuration in adipose tissue. Original exploratory experiences demonstrate that 6 weeks-exposure of lean mice to CIH is characterized by systemic insulin resistance and translates into adipocyte insulin signaling alterations. Chronic intermittent hypoxia also induces caveolae disassembly in white adipose tissue (WAT) illustrated by reduced plasma membrane caveolae density and enlarged caveolae width, concomitantly to WAT insulin resistance state. We show that CIH downregulates caveolar gene and protein expressions, including cavin-1, cavin-2, and EHD2, underlying molecular mechanisms responsible for such caveolae flattening. Altogether, we provide evidences for adipose tissue caveolae disassembly following CIH exposure, likely linked to cavin protein downregulation. This event may constitute the molecular basis of insulin resistance development in OSA patients.

Retinol binding protein 4 primes the NLRP3 inflammasome by signaling through Toll-like receptors 2 and 4.

Adipose tissue (AT) inflammation contributes to systemic insulin resistance. In obesity and type 2 diabetes (T2D), retinol binding protein 4 (RBP4), the major retinol carrier in serum, is elevated in AT and has proinflammatory effects which are mediated partially through Toll-like receptor 4 (TLR4). We now show that RBP4 primes the NLRP3 inflammasome for interleukin-1β (IL1β) release, in a glucose-dependent manner, through the TLR4/MD2 receptor complex and TLR2. This impairs insulin signaling in adipocytes. IL1β is elevated in perigonadal white AT (PGWAT) of chow-fed RBP4-overexpressing mice and in serum and PGWAT of high-fat diet-fed RBP4-overexpressing mice vs. wild-type mice. Holo- or apo-RBP4 injection in wild-type mice causes insulin resistance and elevates PGWAT inflammatory markers, including IL1β. TLR4 inhibition in RBP4-overexpressing mice reduces PGWAT inflammation, including IL1β levels and improves insulin sensitivity. Thus, the proinflammatory effects of RBP4 require NLRP3-inflammasome priming. These studies may provide approaches to reduce AT inflammation and insulin resistance in obesity and diabetes.

Maresin 1 regulates insulin signaling in human adipocytes as well as in adipose tissue and muscle of lean and obese mice.

Maresin 1 (MaR1) is a DHA-derived pro-resolving lipid mediator. The present study aimed to characterize the ability of MaR1 to prevent the alterations induced by TNF-α on insulin actions in glucose uptake and Akt phosphorylation in cultured human adipocytes from overweight/obese subjects, as well as to investigate the effects of MaR1 acute and chronic administration on Akt phosphorylation in absence/presence of insulin in white adipose tissue (WAT) and skeletal muscle from lean and diet-induced obese (DIO) mice. MaR1 (0.1nM) prevented the inhibitory effect of TNF-α on insulin-stimulated 2-Deoxy-D-glucose uptake and Akt phosphorylation in human adipocytes. Acute treatment with MaR1 (50μg/kg, 3h, i.p.) induced Akt phosphorylation in WAT and skeletal muscle of lean mice. However, MaR1 did not further increase the stimulatory effect of insulin on Akt activation. Interestingly, intragastric chronic treatment with MaR1 (50μg/kg, 10days) in DIO mice reduced the hyperglycemia induced by the high fat diet (HFD) and improved systemic insulin sensitivity. In parallel, MaR1 partially restored the impaired insulin response in skeletal muscle of DIO mice and reversed HFD-induced lower Akt phosphorylation in WAT in non-insulin-stimulated DIO mice while did not restore the defective Akt activation in response to acute insulin observed in DIO mice. Our results suggest that MaR1 attenuates the impaired insulin signaling and glucose uptake induced by proinflammatory cytokines. Moreover, the current data support that MaR1 treatment could be useful to reduce the hyperglycemia and the insulin resistance associated to obesity, at least in part by improving Akt signaling.

Protective effect of the hydroalcoholic extract from Lampaya medicinalis Phil. (Verbenaceae) on palmitic acid- impaired insulin signaling in 3T3-L1 adipocytes

BackgroundObesity is strongly associated with insulin resistance (IR). IR at the molecular level may be defined as a diminished activation of insulin signaling-related molecules (IRS-1/Akt/AS160) as well as reduced glucose uptake. Subject with obesity have elevated plasma levels of saturated fatty acids, such as palmitic acid (PA), which triggers insulin signaling disruption in vivo and in vitro. Infusions of Lampaya medicinalis Phil. (Verbenaceae) are used in folk medicine of Northern Chile to counteract inflammatory diseases. Hydroethanolic extracts of lampaya (HEL) contain considerable amounts of flavonoids that may explain the biological activity of the plant. The aim of this study was to assess whether HEL exposure protects against PA-disrupted insulin signaling and glucose uptake in adipocytes. MethodsCytotoxicity of a range of HEL concentrations (0.01–10 μg/mL) was evaluated in 3T3-L1 adipocytes. Cells were exposed or not to 0.1 μg/mL of HEL before adding 0.65 mM PA or vehicle and incubated with 100 nM insulin (or vehicle) for 15 min. Phosphorylation of Tyr-IRS-1, Ser-Akt, Thr-AS160 was evaluated by Western blot. Glucose uptake was assessed using the 2-NBDG analogue. ResultsHEL was not cytotoxic at any concentration assessed. PA-induced reduction in insulin-stimulated phosphorylation of IRS-1, Akt and AS160 and glucose uptake were abolished by co-treatment with HEL. ConclusionThese findings give new insights about the effect of HEL ameliorating PA- impaired IRS-1/Akt/AS160 pathway and glucose uptake in adipocytes. More studies should focus on lampaya, since might represent a preventive approach in individuals whose circulating PA levels contribute to IR.

The guanine nucleotide exchange factor FLJ00068 activates Rac1 in adipocyte insulin signaling.

Insulin stimulates glucose uptake via the translocation of the glucose transporter GLUT4 to the plasma membrane in adipocytes. Several lines of evidence suggest that the small GTPase Rac1 plays an important role in insulin-stimulated glucose uptake in skeletal muscle and adipocytes. The purpose of this study is to investigate the mechanisms whereby Rac1 is regulated in adipocyte insulin signaling. Here, we show that knockdown of the guanine nucleotide exchange factor FLJ00068 inhibits Rac1 activation and GLUT4 translocation by insulin and a constitutively activated form of the protein kinase Akt2. Furthermore, constitutively activated FLJ00068 induced Rac1 activation and Rac1-dependent GLUT4 translocation. Collectively, these results suggest the involvement of FLJ00068 downstream of Akt2 in insulin-stimulated glucose uptake signaling in adipocytes.

Mechanisms of intermittent hypoxia-mediated macrophage activation - potential therapeutic targets for obstructive sleep apnoea.

Intermittent hypoxia (IH) plays a key role in the pathogenesis of insulin resistance (IR) in obstructive sleep apnoea (OSA). IH induces a pro-inflammatory phenotype of the adipose tissue with M1 macrophage polarisation, subsequently impeding adipocyte insulin signalling, and these changes are in striking similarity to those seen in obesity. However, the detailed molecular mechanisms of IH-induced macrophage polarisation are unknown and identification of same should lead to the identification of novel therapeutic targets. In the present study, we tested the hypothesis that IH acts through similar mechanisms as obesity, activating Toll-like-receptor (TLR)4/nuclear factor κ-light-chain-enhancer of activated B cells (NF-κB) and nucleotide-binding domain (NOD)-like receptor protein 3 (NLRP3) signalling pathways leading to the upregulation and secretion of the key cytokines interleukin (IL)-1β and IL-6. Bone-marrow derived macrophages (BMDMs) from lean and obese C57BL/6 male mice were exposed to a state-of-the-art in vitro model of IH. Independent of obesity, IH led to a pro-inflammatory M1 phenotype characterised by increased inducible nitric oxide synthase and IL-6 mRNA expression, robust increase in NF-κB DNA-binding activity and IL-6 secretion. Furthermore, IH significantly increased pro-IL-1β mRNA and protein expression and mature IL-1β secretion compared to control treatment. Providing mechanistic insight, pre-treatment with the TLR4 specific inhibitor, TAK-242, prevented IH-induced M1 polarisation and upregulation of IL-1β mRNA and pro-IL-1β protein expression. Moreover, IH-induced increase in IL-1β secretion was prevented in BMDMs isolated from NLRP3 knockout mice. Thus, targeting TLR4/NF-κB and NLRP3 signalling pathways may provide novel therapeutic options for metabolic complications in OSA.

JUP/plakoglobin is regulated by salt-inducible kinase 2, and is required for insulin-induced signalling and glucose uptake in adipocytes

BackgroundSalt-inducible kinase 2 (SIK2) is abundant in adipocytes, but downregulated in adipose tissue from individuals with obesity and insulin resistance. Moreover, SIK isoforms are required for normal insulin signalling and glucose uptake in adipocytes, but the underlying molecular mechanisms are currently not known. The adherens junction protein JUP, also termed plakoglobin or γ-catenin, has recently been reported to promote insulin signalling in muscle cells. ObjectiveThe objective of this study was to analyse if JUP is required for insulin signalling in adipocytes and the underlying molecular mechanisms of this regulation. MethodsCo-expression of SIK2 and JUP mRNA levels in adipose tissue from a human cohort was analysed. siRNA silencing and/or pharmacological inhibition of SIK2, JUP, class IIa HDACs and CRTC2 was employed in 3T3-L1- and primary rat adipocytes. JUP protein expression was analysed by western blot and mRNA levels by qPCR. Insulin signalling was evaluated by western blot as levels of phosphorylated PKB/Akt and AS160, and by monitoring the uptake of 3H-2-deoxyglucose. ResultsmRNA expression of SIK2 correlated with that of JUP in human adipose tissue. SIK2 inhibition or silencing resulted in downregulation of JUP mRNA and protein expression in 3T3-L1- and in primary rat adipocytes. Moreover, JUP silencing reduced the expression of PKB and the downstream substrate AS160, and consequently attenuated activity in the insulin signalling pathway, including insulin-induced glucose uptake. The known SIK2 substrates CRTC2 and class IIa HDACs were found to play a role in the SIK-mediated regulation of JUP expression. ConclusionsThese findings identify JUP as a novel player in the regulation of insulin sensitivity in adipocytes, and suggest that changes in JUP expression could contribute to the effect of SIK2 on insulin signalling in these cells.

Altered Regulation of adipomiR Editing with Aging.

Adipose dysfunction with aging increases risk to insulin resistance and other chronic metabolic diseases. We previously showed functional changes in microRNAs involved in pre-adipocyte differentiation with aging resulting in adipose dysfunction. However, the mechanisms leading to this dysfunction in microRNAs in adipose tissue (adipomiRs) during aging are not well understood. We determined the longitudinal changes in expression of adipomiRs and studied their regulatory mechanisms, such as miRNA biogenesis and editing, in an aging rodent model, with Fischer344 × Brown-Norway hybrid rats at ages ranging from 3 to 30 months (male/females, n > 8). Expression of adipomiRs and their edited forms were determined by small-RNA sequencing. RT-qPCR was used to measure the mRNA expression of biogenesis and editing enzymes. Sanger sequencing was used to validate editing with aging. Differential expression of adipomiRs involved in adipocyte differentiation and insulin signaling was altered with aging. Sex- and age-specific changes in edited adipomiRs were observed. An increase in miRNA biogenesis and editing enzymes (ADARs and their splice variants) were observed with increasing age, more so in female than male rats. The adipose dysfunction observed with age is attributed to differences in editing of adipomiRs, suggesting a novel regulatory pathway in aging.

Obesity-related hypoxia via miR-128 decreases insulin-receptor expression in human and mouse adipose tissue promoting systemic insulin resistance.

BackgroundInsulin resistance in visceral adipose tissue (VAT), skeletal muscle and liver is a prominent feature of most patients with obesity. How this association arises remains poorly understood. The objective of this study was to demonstrate that the decrease in insulin receptor (INSR) expression and insulin signaling in VAT from obese individuals is an early molecular manifestation that might play a crucial role in the cascade of events leading to systemic insulin resistance.MethodsTo clarify the role of INSR and insulin signaling in adipose tissue dysfunction in obesity, we first measured INSR expression in VAT samples from normal-weight subjects and patients with different degrees of obesity. We complemented these studies with experiments on high-fat diet (HFD)-induced obese mice, and in human and murine adipocyte cultures, in both normoxic and hypoxic conditions.FindingsAn inverse correlation was observed between increasing body mass index and decreasing INSR expression in VAT of obese humans. Our results indicate that VAT-specific downregulation of INSR is an early event in obesity-related adipose cell dysfunction, which increases systemic insulin resistance in both obese humans and mice. We also provide evidence that obesity-related hypoxia in VAT plays a determinant role in this scenario by decreasing INSR mRNA stability. This decreased stability is through the activation of a miRNA (miR-128) that downregulates INSR expression in adipocytes.InterpretationWe present a novel pathogenic mechanism of reduced INSR expression and insulin signaling in adipocytes. Our data provide a new explanation linking obesity with systemic insulin resistance.FundingThis work was partly supported by a grant from Nutramed (PON 03PE000_78_1) and by the European Commission (FESR FSE 2014-2020 and Regione Calabria).

Exposure to Low Doses of Dechlorane Plus Promotes Adipose Tissue Dysfunction and Glucose Intolerance in Male Mice.

The prevalence of type 2 diabetes (T2D) continues to increase worldwide. It is well established that genetic susceptibility, obesity, overnutrition and a sedentary life style are risk factors for the development of T2D. However, more recently, studies have also proposed links between exposure to endocrine-disrupting chemicals (EDCs) and altered glucose metabolism. Human exposure to environmental pollutants that are suspected to have endocrine disruptor activity is ubiquitous. One such chemical is Dechlorane Plus (DP), a flame retardant, that is now detected in humans and the environment. Here we show that exposure of mice to low, environmentally relevant doses of DP promoted glucose intolerance in mice fed a high-fat diet independent of weight gain. Furthermore, DP had pronounced effects on the adipose tissue, where it induced the development of hypertrophied white adipose tissue (WAT), and increased serum levels of resistin, leptin, and plasminogen activator inhibitor-1. In addition, DP exposure induced "whitening" of brown adipose tissue (BAT), and reduced BAT uncoupling protein 1 expression. Importantly, some of these effects occurred even when the mice were fed a regular, low-fat, diet. Finally, WAT adipogenic markers were reduced with DP treatment in the WAT. We also show that DP directly inhibited insulin signaling in murine adipocytes and human primary subcutaneous adipocytes in vitro. Taken together, our results show that the exposure to low and environmentally relevant levels of DP may contribute to the development of T2D.

207-OR: Liver-Preadipocyte Cross Talk Reverses Lipodystrophy-Induced Insulin Resistance

Insulin and IGF1 receptors (IR/IGF1R) control adipose tissue development. We previously demonstrated that mice with an inducible adipocyte-specific knockout of IR/IGFR (Ai-DKO) develop severe lipodystrophy, insulin resistance, glucose intolerance, hepatosteatosis, islet hyperplasia with hyperinsulinemia. This phenotype resolves over 10-30 days due to proliferation of preadipocytes (PAds) and regeneration of adipose tissue. Here, we sought to identify circulating factors that trigger PAds proliferation by treating primary wild type PAds with serum from either WT or Ai-DKO mice. To exclude the hyperglycemia effect, we also incubated cells with serum from WT and Ai-DKO mice treated with the SGLT2 inhibitor Remogliflozin (Remo). Serum from either, chow or Remo-treated Ai-DKO mice, induced primary PAds proliferation of approximately 30%. Serum proteomics analysis revealed that Cathepsin D and multiple apolipoproteins, including C2 and C3, were significantly upregulated in Ai-DKO serum (FC>1 and FDR<0.1). Cathepsin D is known to induce fibroblast proliferation, however, when serum Cathepsin D was neutralized with its antibody, we observed an increase of 25% in PAds proliferation, indicating that, in this context, this protein has the potential to block proliferation. Apolipoproteins are mainly secreted by the liver. We found that ex-vivo liver culture conditioned media from Ai-DKO mice induced PAds proliferation in approximately 40% compared to control. Striking, this phenomenon was mimicked when PAds were treated with purified Apo C2 or C3. Taken together, our data suggest that loss of insulin signaling in mature adipocytes initiates a complex crosstalk between liver and preadipocytes resulting in PAds proliferation and adipose tissue regeneration. Further characterization of these factors will provide important insights into adipose tissue biology during insulin resistant states and contribute to reverse the metabolic syndrome associated with lipodystrophy. Disclosure B. Brandao: None. M. Sakaguchi: None. T.M. Batista: None. W. Qian: None. C. Kahn: Advisory Panel; Self; ERX Pharmaceuticals, Kaleido Biosciences. Consultant; Self; AntriaBio, Flagship Pioneering, Sana-Cobalt. Funding National Institutes of Health (R01DK082659-11)

1948-P: STING in Hematopoietic Cells Promotes Obesity-Associated Adipose Tissue Inflammation and Systemic Insulin Resistance in Mice

Stimulator of interferon genes (STING) is a mediator of innate immunity. Previous research has validated that STING promotes macrophage-stimulated nonalcoholic fatty liver disease. However, it remains to be elucidated how STING regulates the pathogenesis of white adipose tissue (WAT) inflammation, which is a causal factor of obesity-associated systemic insulin resistance and metabolic dysregulation. We examined the expression of STING in adipose tissue of mice with high fat diet (HFD)-induced obesity. We also determined the effect of STING disruption or presence only in hematopoietic cells on diet-induced WAT inflammation and systemic insulin resistance in chimeric mice. Upon HFD feeding for 12 weeks, C57BL/6J mice revealed overt WAT inflammation and systemic insulin resistance compared with mice fed a low-fat diet. These changes were accompanied with increased STING expression in WAT whereas STING-positive cells were located predominantly within the crown structures. In mice whose STING was disrupted in all cells, the severity of HFD-induced WAT inflammation and systemic insulin resistance was significantly smaller than that in wild type mice. When the role of STING in hematopoietic cells was examined, the severity of HFD-induced WAT inflammation and systemic insulin resistance in chimeric mice whose STING was disrupted only in hematopoietic cells was also smaller than that in control chimeric mice whose STING was intact in all cells. In contrast, the severity of HFD-induced phenotype in mice whose STING was intact only in hematopoietic cells was greater than that in mice whose STING was disrupted in all cells. In cultured cells, STING-driven macrophage factors increased adipocyte proinflammatory responses and decreased adipocyte insulin signaling. Together, these results suggest that increased STING in hematopoietic cells plays a critical role in promoting WAT inflammation and systemic insulin resistance. Disclosure B. Zhu: None. H. Li: None. C. Wu: None. Funding American Diabetes Association (1-17-IBS-145 to C.W.)

The PDK1-FoxO1 signaling in adipocytes controls systemic insulin sensitivity through the 5-lipoxygenase–leukotriene B4 axis

Although adipocytes are major targets of insulin, the influence of impaired insulin action in adipocytes on metabolic homeostasis remains unclear. We here show that adipocyte-specific PDK1 (3'-phosphoinositide-dependent kinase 1)-deficient (A-PDK1KO) mice manifest impaired metabolic actions of insulin in adipose tissue and reduction of adipose tissue mass. A-PDK1KO mice developed insulin resistance, glucose intolerance, and hepatic steatosis, and this phenotype was suppressed by additional ablation of FoxO1 specifically in adipocytes (A-PDK1/FoxO1KO mice) without an effect on adipose tissue mass. Neither circulating levels of adiponectin and leptin nor inflammatory markers in adipose tissue differed between A-PDK1KO and A-PDK1/FoxO1KO mice. Lipidomics and microarray analyses revealed that leukotriene B4 (LTB4) levels in plasma and in adipose tissue as well as the expression of 5-lipoxygenase (5-LO) in adipose tissue were increased and restored in A-PDK1KO mice and A-PDK1/FoxO1KO mice, respectively. Genetic deletion of the LTB4 receptor BLT1 as well as pharmacological intervention to 5-LO or BLT1 ameliorated insulin resistance in A-PDK1KO mice. Furthermore, insulin was found to inhibit LTB4 production through down-regulation of 5-LO expression via the PDK1-FoxO1 pathway in isolated adipocytes. Our results indicate that insulin signaling in adipocytes negatively regulates the production of LTB4 via the PDK1-FoxO1 pathway and thereby maintains systemic insulin sensitivity.

Differential Diabetogenic Effect of Pitavastatin and Rosuvastatin, in vitro and in vivo.

Aim: Most statins increase the risk of new-onset diabetes. Unlike other statins, pitavastatin is reported to exert neutral effects on serum glucose level, but the precise mechanism is unknown.Methods: Eight-week-old male C57BL/6J mice (n = 26) were fed high-fat diet (HFD, 45% fat) with 0.01% placebo, rosuvastatin, or pitavastatin for 12 weeks. Cultured HepG2, C2C12, and 3T3-L1 cells and visceral adipocytes from HFD-fed mice were treated with vehicle or 10 µM statins for 24 h. The effects of pitavastatin and rosuvastatin on intracellular insulin signaling and glucose transporter 4 (GLUT4) translocation were evaluated.Results: After 12 weeks, the fasting blood glucose level was significantly lower in pitavastatin-treated group than in rosuvastatin-treated group (115.2 ± 7.0 versus 137.4 ± 22.3 mg/dL, p = 0.024). Insulin tolerance significantly improved in pitavastatin-treated group as compared with rosuvastatin-treated group, and no significant difference was observed in glucose tolerance. Although plasma adiponectin and insulin levels were not different between the two statin treatment groups, the insulin-induced protein kinase B phosphorylation was weakly attenuated in pitavastatin-treated adipocytes than in rosuvastatin-treated adipocytes. Furthermore, minor attenuation in insulin-induced GLUT4 translocation to the plasma membrane of adipocytes was observed in pitavastatin-treated group.Conclusion: Pitavastatin showed lower diabetogenic effects than rosuvastatin in mice that may be mediated by minor attenuations in insulin signaling in adipocytes.

Long-term dietary administration of diethyl phthalate triggers loss of insulin sensitivity in two key insulin target tissues of mice.

Over the past years, a growing body of work has linked numerous pervasive environmental chemicals with a multitude of adverse reproductive, developmental, behavioral, and metabolic changes in humans and animal models. Plasticizers include a wide variety of phthalate esters that are extensively used in a host of personal day care and cosmetic products. Many population-based studies have indicated a close association between diethyl phthalate (DEP) and diabetes albeit the mechanisms remain much unexplored. Presently, we report that long-term dietary administration of DEP to adult male Swiss albino mice at two different concentrations mirroring the recommended tolerable doses, severely impaired insulin signaling in hepatocytes and adipocytes. This was concomitant with sustained oxidative stress from the overactivation of NADPH oxidase 2, a major intracellular source of reactive oxygen species, in both the cell types. The present study provides evidences of the onset of insulin resistance in mice after chronic exposure to DEP in diet even at lower levels. This, in turn, can have serious pathological consequences with ultimate manifestations of type 2 diabetes and metabolic syndrome (MetS). Thus, by disrupting the central metabolic function of liver and adipose tissue, the key insulin target tissues, daily exposure to phthalates in plastics can potentially contribute to the alarming prevalence of MetS in recent times.

The Novel Adipokine Gremlin 1 Antagonizes Insulin Action and Is Increased in Type 2 Diabetes and NAFLD/NASH.

The BMP2/4 antagonist and novel adipokine Gremlin 1 is highly expressed in human adipose cells and increased in hypertrophic obesity. As a secreted antagonist, it inhibits the effect of BMP2/4 on adipose precursor cell commitment/differentiation. We examined mRNA levels of Gremlin 1 in key target tissues for insulin and also measured tissue and serum levels in several carefully phenotyped human cohorts. Gremlin 1 expression was high in adipose tissue, higher in visceral than in subcutaneous tissue, increased in obesity, and further increased in type 2 diabetes (T2D). A similar high expression was seen in liver biopsies, but expression was considerably lower in skeletal muscles. Serum levels were increased in obesity but most prominently in T2D. Transcriptional activation in both adipose tissue and liver as well as serum levels were strongly associated with markers of insulin resistance in vivo (euglycemic clamps and HOMA of insulin resistance), and the presence of nonalcoholic fatty liver disease (NAFLD) and nonalcoholic steatohepatitis (NASH). We also found Gremlin 1 to antagonize insulin signaling and action in human primary adipocytes, skeletal muscle, and liver cells. Thus, Gremlin 1 is a novel secreted insulin antagonist and biomarker as well as a potential therapeutic target in obesity and its complications T2D and NAFLD/NASH.

Clinically confirmed DEL-1 as a myokine attenuates lipid-induced inflammation and insulin resistance in 3T3-L1 adipocytes via AMPK/HO-1- pathway

ABSTRACT Regular exercise is the first line of therapy for treating obesity-mediated metabolic disorders, including insulin resistance. It has been reported that developmental endothelial locus-1 (DEL-1) enhances macrophage efferocytosis, resulting in inflammation clearance as well as improves insulin resistance in skeletal muscle. However, the relationship between exercise and DEL-1, and the effects of DEL-1 on insulin signalling in adipocytes have not been fully elucidated to date. Protein expression levels were determined by Western blot analysis. Cells were transfected with small interfering (si) RNA to suppress gene expression. Lipid accumulation levels were detected using Oil red-O staining. Proinflammatory cytokine secretion levels were measured using ELISA. DEL-1 expression levels were induced in the skeletal muscle of people who exercised using microarray analysis. Recombinant DEL-1 augmented AMP-activated protein kinase (AMPK) phosphorylation and haem oxygenase (HO)-1 expression to alleviating inflammation and impairment of insulin signalling in 3T3-L1 adipocytes treated with palmitate. siRNA of AMPK or HO-1 also mitigated the effects of DEL-1 on inflammation and insulin resistance. DEL-1 ameliorates inflammation and insulin resistance in differentiated 3T3-L1 cells via AMPK/HO-1 signalling, suggesting that DEL-1 may be the exercise-mediated therapeutic target for treating insulin resistance and type 2 diabetes.