ABSTRACT
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.
Subject(s)
3-Phosphoinositide-Dependent Protein Kinases , Adipocytes/metabolism , Arachidonate 5-Lipoxygenase/metabolism , Forkhead Box Protein O1 , Insulin Resistance , 3-Phosphoinositide-Dependent Protein Kinases/genetics , 3-Phosphoinositide-Dependent Protein Kinases/metabolism , Animals , Cells, Cultured , Forkhead Box Protein O1/genetics , Forkhead Box Protein O1/metabolism , Insulin Resistance/genetics , Insulin Resistance/physiology , Leukotriene B4/metabolism , Male , Mice , Mice, Knockout , Signal Transduction/geneticsABSTRACT
Clinical and animal studies have suggested a possible beneficial effect of sodium-glucose cotransporter 2 (SGLT2) inhibitors on nonalcoholic fatty liver disease (NAFLD) including nonalcoholic steatohepatitis (NASH). Although SGLT2 inhibitors have been shown to reduce hepatic fat deposition in association with loss of body weight, the mechanism of this action has remained unknown. We here show that the SGLT2 inhibitor canagliflozin ameliorated fatty liver and hyperglycemia without affecting body weight or epididymal fat weight in obese diabetic KKAy mice. Lipidomics analysis based on liquid chromatography and tandem mass spectrometry revealed that canagliflozin treatment increased the amounts of prostaglandin E2 (PGE2) and resolvin E3 in the liver of these mice. We also found that PGE2 attenuated fat deposition in mouse primary hepatocytes exposed to palmitic acid. Our results thus suggest that PGE2 may play an important role in the amelioration of hepatic fat deposition by canagliflozin, with elucidation of its mechanism of action potentially providing a basis for the development of new therapeutics for NAFLD-NASH.
Subject(s)
Canagliflozin/pharmacology , Diabetes Mellitus, Experimental/physiopathology , Dinoprostone/metabolism , Liver/drug effects , Non-alcoholic Fatty Liver Disease/drug therapy , Obesity/drug therapy , Sodium-Glucose Transporter 2/chemistry , Animals , Cells, Cultured , Diet, High-Fat , Insulin Resistance , Liver/metabolism , Male , Mice , Mice, Obese , Non-alcoholic Fatty Liver Disease/metabolism , Non-alcoholic Fatty Liver Disease/pathology , Obesity/metabolism , Obesity/pathology , Sodium-Glucose Transporter 2/metabolism , Sodium-Glucose Transporter 2 Inhibitors/pharmacologyABSTRACT
AIMS/INTRODUCTION: Brown adipose tissue (BAT) utilizes large amounts of fuel for thermogenesis, but the mechanism by which fuel substrates are switched in response to changes in energy status is poorly understood. We have now investigated the role of Kruppel-like factor 15 (KLF15), a transcription factor expressed at a high level in adipose tissue, in the regulation of fuel utilization in BAT. MATERIALS AND METHODS: Depletion or overexpression of KLF15 in HB2 differentiated brown adipocytes was achieved by adenoviral infection. Glucose and fatty acid oxidation were measured with radioactive substrates, pyruvate dehydrogenase complex activity was determined with a colorimetric assay, and gene expression was examined by reverse transcription and real-time polymerase chain reaction analysis. RESULTS: Knockdown of KLF15 in HB2 cells attenuated fatty acid oxidation in association with downregulation of the expression of genes related to this process including Acox1 and Fatp1, whereas it increased glucose oxidation. Expression of the gene for pyruvate dehydrogenase kinase 4 (PDK4), a negative regulator of pyruvate dehydrogenase complex, was increased or decreased by KLF15 overexpression or knockdown, respectively, in HB2 cells, with these changes being accompanied by a respective decrease or increase in pyruvate dehydrogenase complex activity. Chromatin immunoprecipitation showed that Pdk4 is a direct target of KLF15 in HB2 cells. Finally, fasting increased expression of KLf15, Pdk4 and genes involved in fatty acid utilization in BAT of mice, whereas refeeding suppressed Klf15 and Pdk4 expression. CONCLUSIONS: Our results implicate KLF15 in the regulation of fuel switching between glucose and fatty acids in response to changes in energy status in BAT.