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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.

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 B(4) (LTB(4)) 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 LTB(4) 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 LTB(4) 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 LTB(4) via the PDK1−FoxO1 pathway and thereby maintains systemic insulin sensitivity.

Diabetes mellitus is associated with various disorders of the locomotor system including the decline in mass and function of skeletal muscle. The mechanism underlying this association has remained ambiguous, however. We now show that the abundance of the transcription factor KLF15 as well as the expression of genes related to muscle atrophy are increased in skeletal muscle of diabetic model mice, and that mice with muscle-specific KLF15 deficiency are protected from the diabetes-induced decline of skeletal muscle mass. Hyperglycemia was found to upregulate the KLF15 protein in skeletal muscle of diabetic animals, which is achieved via downregulation of the E3 ubiquitin ligase WWP1 and consequent suppression of the ubiquitin-dependent degradation of KLF15. Our results revealed that hyperglycemia, a central disorder in diabetes, promotes muscle atrophy via a WWP1/KLF15 pathway. This pathway may serve as a therapeutic target for decline in skeletal muscle mass accompanied by diabetes mellitus.

Background: Evidence suggests that diabetes is a promoting factor of sarcopenia. Mechanism how the condition accelerates the development of sarcopenia remains ambiguous. Results: In streptozotocin (STZ)-induced diabetic mice, skeletal muscle mass was decreased by ∼15 % within 21 days after the STZ treatment. The protein abundance of transcription factor KLF15 as well as the mRNA abundance of proteins related to muscle atrophy (Foxo3a, Atrogin1 and Murf1) were elevated whereas the mRNA of KLF15 was unaltered in skeletal muscle of STZ-diabetic mice. Decrease in skeletal muscle mass and increase in the mRNA abundance of muscle atrophy-related proteins triggered by STZ-induced diabetes were prevented in muscle-specific KLF15 deficient mice suggesting that KLF15 plays a key role in diabetes-induced muscle atrophy. Treatment of C2C12 cells with high concentration of glucose (25 mmol/l) decreased the ubiquitination of and increased the protein abundance of KLF15. The E3 ubiquitin ligase WWP1 was found to be downregulated in skeletal muscle of STZ- diabetic mice and in C2C12 cells treated with glucose. Overexpression and knockdown of WWP1 in C2C12 cells resulted in the decrease and the increase, respectively, of KLF15 protein without affecting its mRNA expression. The mRNA abundance of WWP1 in skeletal muscle was downregulated and the mass of skeletal muscle was reduced in Akita mice, which develop diabetes due to beta-cell dysfunction. Administration of the SGLT2 inhibitor empagliflozin, which lowers glycemia without affecting insulin levels, to Akita mice upregulated the expression of WWP1 in skeletal muscle and increased the skeletal muscle mass of these mice. Conclusion: Our results suggest that hyperglycemia upregulates the protein abundance of KLF15 via the downregulation of WWP1, which catalyzes the ubiquitination of KLF15. Hyperglycemia-induced upregulation of KLF15 leads to the induction of genes for muscle atrophy-related proteins, which in turn promotes the atrophy of skeletal muscle. Disclosure Y. Hirata: Research Support; Self; Boehringer Ingelheim Pharmaceuticals, Inc. K. Nomura: Research Support; Self; Boehringer Ingelheim Pharmaceuticals, Inc. Y. Senga: Research Support; Self; Boehringer Ingelheim Pharmaceuticals, Inc.. M. Imamura: None. S. Takeda: None. Y. Okada: Research Support; Self; Boehringer Ingelheim Pharmaceuticals, Inc.. S.J. Burden: None. T. Hosooka: Research Support; Self; Boehringer Ingelheim Pharmaceuticals, Inc. W. Ogawa: Research Support; Self; Boehringer Ingelheim Pharmaceuticals, Inc., Abbott.

The transcription factor sterol regulatory element-binding protein 1c (SREBP1c) plays an important role in the control of fatty acid metabolism in the liver. Evidence suggests that mammalian target of rapamycin (mTOR) complex 1 (mTORC1) contributes to the regulation of SREBP1c expression, but signaling downstream of mTORC1 remains unclear. We have now shown that medium rich in branched-chain amino acids stimulates expression of the SREBP1c gene in cultured hepatocytes in a manner sensitive both to rapamycin, a pharmacological inhibitor of mTORC1, and to a short hairpin RNA (shRNA) specific for S6 kinase 1 (S6K1), a downstream effector of mTORC1. The phosphorylation of S6K1 was increased in the liver of obese db/db mice. Furthermore, depletion of hepatic S6K1 in db/db mice with the use of an adenovirus vector encoding S6K1 shRNA resulted in down-regulation of SREBP1c gene expression in the liver as well as a reduced hepatic triglyceride content and serum triglyceride concentration. These results thus suggest that S6K1 regulates SREBP1c expression both in cultured hepatocytes and in mouse liver, and that increased hepatic activity of S6K1 contributes at least in part to the pathogenesis of obesity-induced hepatic steatosis and hypertriglyceridemia.

Gene related to anergy in lymphocytes (GRAIL) is an E3 ubiquitin ligase that regulates energy in T-lymphocytes. Whereas, the relevance of GRAIL to T lymphocyte function is well established, the role of this protein in other cell types remains unknown. Given that GRAIL is abundant in the liver, we investigated the potential function of GRAIL in nutrient metabolism by generating mice in which the expression of GRAIL is reduced specifically in the liver. Adenovirus-mediated transfer of a short hairpin RNA specific for GRAIL mRNA markedly reduced the amounts of GRAIL mRNA and protein in the liver. Blood glucose levels of the mice with hepatic GRAIL deficiency did not differ from those of control animals in the fasted or fed states. However, these mice manifested glucose intolerance in association with a normal increase in plasma insulin levels during glucose challenge. The mice also manifested an increase in the serum concentration of free fatty acids, whereas the serum levels of cholesterol and triglyceride were unchanged. The hepatic abundance of mRNAs for glucose-6-phosphatase, catalytic (a key enzyme in hepatic glucose production) and for sterol regulatory element-binding transcription factor 1 (an important transcriptional regulator of lipogenesis) was increased in the mice with hepatic GRAIL deficiency, possibly contributing to the metabolic abnormalities of these animals. Our results thus demonstrate that GRAIL in the liver is essential for maintenance of normal glucose and lipid metabolism in living animals.