Adipose tissue resident macrophages have important roles in the maintenance of tissue homeostasis and regulate insulin sensitivity for example by secreting pro-inflammatory or anti-inflammatory cytokines. Here, we show that M2-like macrophages in adipose tissue regulate systemic glucose homeostasis by inhibiting adipocyte progenitor proliferation via the CD206/TGFβ signaling pathway. We show that adipose tissue CD206⁺ cells are primarily M2-like macrophages, and ablation of CD206⁺ M2-like macrophages improves systemic insulin sensitivity, which was associated with an increased number of smaller adipocytes. Mice genetically engineered to have reduced numbers of CD206⁺ M2-like macrophages show a down-regulation of TGFβ signaling in adipose tissue, together with up-regulated proliferation and differentiation of adipocyte progenitors. Our findings indicate that CD206⁺ M2-like macrophages in adipose tissues create a microenvironment that inhibits growth and differentiation of adipocyte progenitors and, thereby, control adiposity and systemic insulin sensitivity.

Nicotinamide adenine dinucleotide (NAD) is an important coenzyme that participates in various energy metabolism pathways, including glycolysis, β-oxidation, and oxidative phosphorylation. Besides, it is a required cofactor for post-translational modifications such as ADP-ribosylation and deacetylation by poly (ADP-ribose) polymerases (PARPs) and sirtuins, respectively. Thus, NAD regulates energy metabolism, DNA damage repair, gene expression, and stress response through these enzymes. Numerous studies have shown that NAD levels decrease with aging and under disturbed nutrient conditions, such as obesity. Additionally, a decline in NAD levels is closely related to the development of various metabolic disorders, including diabetes and fatty liver disease. In addition, many studies have revealed that administration of NAD precursors, such as nicotinamide mononucleotide (NMN) and nicotinamide riboside (NR), efficiently increase NAD levels in various tissues and prevent such metabolic diseases. These NAD precursors are contained in natural foods, such as cow milk, vegetables, and meats. Therefore, altered NAD metabolism can be a practical target for nutritional intervention. Recently, several human clinical trials using NAD precursors have been conducted to investigate the safety, pharmacokinetics, and efficacy against metabolic disorders such as glucose intolerance. In this review, we summarize current knowledge on the implications of NAD metabolism in metabolic diseases and discuss the outcomes of recent human clinical trials.

In recent years, several oral antidiabetic drugs with new mechanisms of action have become available, expanding the number of treatment options. Sodium/glucose cotransporter-2 (SGLT2) inhibitors are a new class of oral antidiabetic drugs with an insulin-independent mechanism promoting urinary glucose excretion. We report the results of a combined Phase 2 and 3 clinical study (Japic CTI-101349) of the SGLT2 inhibitor tofogliflozin (CSG452, RG7201) in Japanese patients with type 2 diabetes mellitus. The efficacy and safety of tofogliflozin were assessed in this multicenter, placebo-controlled, randomized, double-blind parallel-group study involving 230 patients with type 2 diabetes mellitus with inadequate glycemic control on diet/exercise therapy. Between 30 October 2010 and 28 February 2012, patients at 33 centers were randomized to either placebo (n = 56) or tofogliflozin (10, 20, or 40 mg; n = 58 each) orally, once daily for 24 weeks. The primary efficacy endpoint was the change from baseline in HbA_1c at week 24. Overall, 229 patients were included in the full analysis set (placebo: n = 56; tofogliflozin 10 mg: n = 57; tofogliflozin 20 and 40 mg: n = 58 each). The least squares (LS) mean change (95% confidence interval) from baseline in HbA_1c at week 24 was −0.028% (−0.192 to 0.137) in the placebo group, compared with −0.797% (−0.960 to −0.634) in the tofogliflozin 10 mg group, −1.017% (−1.178 to −0.856) in the tofogliflozin 20 mg group, and −0.870% (−1.031 to −0.709) in the tofogliflozin 40 mg group (p < 0.0001 for the LS mean differences in all tofogliflozin groups vs placebo). There were also prominent decreases in fasting blood glucose, 2-h postprandial glucose, and body weight in all tofogliflozin groups compared with the placebo group. The main adverse events were hyperketonemia, ketonuria, and pollakiuria. The incidence of hypoglycemia was low. Furthermore, most adverse events were classified as mild or moderate in severity. Tofogliflozin 10, 20, or 40 mg administered once daily as monotherapy significantly decreased HbA_1c and body weight, and was generally well tolerated in Japanese patients with type 2 diabetes mellitus. Phase 3 studies were recently completed and support the findings of this combined Phase 2 and 3 study. This study was registered in the JAPIC clinical trials registry (ID: Japic%CTI-101349 ).

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.

Astaxanthin is a member of the carotenoid family that is found abundantly in marine organisms, and has been gaining attention in recent years due to its varied biological/physiological activities. It has been reported that astaxanthin functions both as a pigment, and as an antioxidant with superior free radical quenching capacity. We recently reported that astaxanthin modulated mitochondrial functions by a novel mechanism independent of its antioxidant function. In this paper, we review astaxanthin’s well-known antioxidant activity, and expand on astaxanthin’s lesser-known molecular targets, and its role in mitochondrial energy metabolism.

Adipose tissue hypoxia is an important feature of pathological adipose tissue expansion. Hypoxia-inducible factor-1α (HIF-1α) in adipocytes reportedly induces oxidative stress and fibrosis, rather than neoangiogenesis via vascular endothelial growth factor (VEGF)-A. We previously reported that macrophages in crown-like structures (CLSs) are both hypoxic and inflammatory. In the current study, we examined how macrophage HIF-1α is involved in high-fat diet (HFD)–induced inflammation, neovascularization, hypoxia, and insulin resistance using mice with myeloid cell–specific HIF-1α deletion that were fed an HFD. Myeloid cell–specific HIF-1α gene deletion protected against HFD-induced inflammation, CLS formation, poor vasculature development in the adipose tissue, and systemic insulin resistance. Despite a reduced expression of Vegfa in epididymal white adipose tissue (eWAT), the preadipocytes and endothelial cells of HIF-1α–deficient mice expressed higher levels of angiogenic factors, including Vegfa, Angpt1, Fgf1, and Fgf10 in accordance with preferable eWAT remodeling. Our in vitro study revealed that lipopolysaccharide-treated bone marrow–derived macrophages directly inhibited the expression of angiogenic factors in 3T3-L1 preadipocytes. Thus, macrophage HIF-1α is involved not only in the formation of CLSs, further enhancing the inflammatory responses, but also in the inhibition of neoangiogenesis in preadipocytes. We concluded that these two pathways contribute to the obesity-related physiology of pathological adipose tissue expansion, thus causing systemic insulin resistance.