Background Skeletal muscle is mainly responsible for insulin‐stimulated glucose disposal. Dysfunction in skeletal muscle metabolism especially during obesity contributes to the insulin resistance. Astaxanthin (AX), a natural antioxidant, has been shown to ameliorate hepatic insulin resistance in obese mice. However, its effects in skeletal muscle are poorly understood. The current study aimed to investigate the molecular target of AX in ameliorating skeletal muscle insulin resistance. Methods We fed 6‐week‐old male C57BL/6J mice with normal chow (NC) or NC supplemented with AX (NC+AX) and high‐fat‐diet (HFD) or HFD supplemented with AX for 24 weeks. We determined the effect of AX on various parameters including insulin sensitivity, glucose uptake, inflammation, kinase signaling, gene expression, and mitochondrial function in muscle. We also determined energy metabolism in intact C2C12 cells treated with AX using the Seahorse XFe96 Extracellular Flux Analyzer and assessed the effect of AX on mitochondrial oxidative phosphorylation and mitochondrial biogenesis. Results AX‐treated HFD mice showed improved metabolic status with significant reduction in blood glucose, serum total triglycerides, and cholesterol (p< 0.05). AX‐treated HFD mice also showed improved glucose metabolism by enhancing glucose incorporation into peripheral target tissues, such as the skeletal muscle, rather than by suppressing gluconeogenesis in the liver as shown by hyperinsulinemic–euglycemic clamp study. AX activated AMPK in the skeletal muscle of the HFD mice and upregulated the expressions of transcriptional factors and coactivator, thereby inducing mitochondrial remodeling, including increased mitochondrial oxidative phosphorylation component and free fatty acid metabolism. We also assessed the effects of AX on mitochondrial biogenesis in the siRNA‐mediated AMPK‐depleted C2C12 cells and showed that the effect of AX was lost in the genetically AMPK‐depleted C2C12 cells. Collectively, AX treatment (i) significantly ameliorated insulin resistance and glucose intolerance through regulation of AMPK activation in the muscle, (ii) stimulated mitochondrial biogenesis in the muscle, (iii) enhanced exercise tolerance and exercise‐induced fatty acid metabolism, and (iv) exerted antiinflammatory effects via its antioxidant activity in adipose tissue. Conclusions We concluded that AX treatment stimulated mitochondrial biogenesis and significantly ameliorated insulin resistance through activation of AMPK pathway in the skeletal muscle.

Platelet-derived growth factor (PDGF) is a key factor in angiogenesis; however, its role in adult obesity remains unclear. In order to clarify its pathophysiological role, we investigated the significance of PDGF receptor β (PDGFRβ) in adipose tissue expansion and glucose metabolism. Mature vessels in the epididymal white adipose tissue (eWAT) were tightly wrapped with pericytes in normal mice. Pericyte desorption from vessels and the subsequent proliferation of endothelial cells were markedly increased in the eWAT of diet-induced obese mice. Analyses with flow cytometry and adipose tissue cultures indicated that PDGF-B caused the detachment of pericytes from vessels in a concentration-dependent manner. M1-macrophages were a major type of cells expressing PDGF-B in obese adipose tissue. In contrast, pericyte detachment was attenuated and vascularity within eWAT was reduced in tamoxifen-inducible conditional -knockout mice with decreases in adipocyte size and chronic inflammation. Furthermore,-knockout mice showed enhanced energy expenditure. Consequently, diet-induced obesity and the associated deterioration of glucose metabolism in wild-type mice were absent in -knockout mice. Therefore, PDGF-B-PDGFRβ signaling plays a significant role in the development of adipose tissue neovascularization and appears to be a fundamental target for the prevention of obesity and type 2 diabetes.

Obesity-associated activation of the renin-angiotensin-aldosterone system is implicated in the pathogenesis of insulin resistance; however, influences of mineralocorticoid receptor (MR) inhibition remain unclear. Therefore, we aimed to clarify the anti-inflammatory mechanisms of MR inhibition using eplerenone, a selective MR antagonist, in C57BL/6 mice fed a high-fat diet (HFD) for 12 weeks. Eplerenone prevented excessive body weight gain and fat accumulation, ameliorated glucose intolerance and insulin resistance and enhanced energy metabolism. In the epididymal white adipose tissue (eWAT), eplerenone prevented obesity-induced accumulation of F4/80CD11cCD206-M1-adipose tissue macrophage (ATM) and reduction of F4/80CD11cCD206-M2-ATM. Interestingly, M1-macrophage exhibited lower expression levels of MR, compared with M2-macrophage, in the ATM of eWAT and -polarized bone marrow-derived macrophages (BMDM). Importantly, eplerenone and MR knockdown attenuated the increase in the expression levels of and in the eWAT and liver of HFD-fed mice and LPS-stimulated BMDM. Moreover, eplerenone suppressed IL1b secretion from eWAT of HFD-fed mice. To reveal the anti-inflammatory mechanism, we investigated the involvement of NLRP3-inflammasome activation, a key process of IL1b overproduction. Eplerenone suppressed the expression of the inflammasome components, and in the eWAT and liver. Concerning the second triggering factors, ROS production and ATP- and nigericin-induced IL1b secretion were suppressed by eplerenone in the LPS-primed BMDM. These results indicate that eplerenone inhibited both the priming and triggering signals that promote NLRP3-inflammasome activation. Therefore, we consider MR to be a crucial target to prevent metabolic disorders by suppressing inflammasome-mediated chronic inflammation in the adipose tissue and liver under obese conditions.

No abstract