25-Hydroxyvitamin D Increases Insulin-Stimulated Glucose Uptake by Enhancing Adipocyte Differentiation.

Vitamin D and its receptor (vitamin D receptor; VDR) regulate calcium homeostasis in mammals. Recently, studies have shown that serum concentrations of 25-hydroxyvitamin D (25VD) are negatively associated with insulin resistance and the incidence of type 2 diabetes. In adipose tissues, glucose transporter 4 (GLUT4) contributes to insulin-stimulated glucose uptake; however, the effect of 25VD on glucose uptake in adipocytes remains unclear. We examined the role of 25VD in glucose uptake and the differentiation of adipose-derived stromal cells. Insulin-stimulated glucose uptake in adipocytes was increased by treatment with 25VD and decreased by VDR knockdown. The expression levels of GLUT4 were upregulated by 25VD treatment. 25VD exposure increased the expression of adipocyte differentiation-related genes including peroxisome proliferator-activated receptor γ and CCAAT/enhancer-binding proteins through VDR, thereby enhancing the formation of mature adipocytes. Moreover, 25VD increased the expression levels of 11ß-hydroxysteroid dehydrogenase 1 (HSD11B1), which catalyzes the conversion of cortisone to cortisol in a concentration-dependent manner. 25VD-stimulated adipocyte differentiation was suppressed by HSD11B1 knockdown. Cortisone together with 25VD enhanced adipocyte differentiation, whereas synthesized glucocorticoid dexamethasone-induced adipocyte differentiation is not promoted by 25VD. Overall, these results indicate that 25VD stimulates adipocyte differentiation through the induction of HSD11B1 expression, leading to increased insulin-induced glucose uptake in adipocytes.

Theobromine enhances the conversion of white adipocytes into beige adipocytes in a PPARγ activation-dependent manner.

The adipocytes play an important role in driving the obese-state-white adipose tissue (WAT) stores the excess energy as fat, wherein brown adipose tissue (BAT) is responsible for energy expenditure via the thermoregulatory function of uncoupling protein 1 (UCP1)-the imbalance between these two onsets obesity. Moreover, the anti-obesity effects of brown-like-adipocytes (beige) in WAT are well documented. Browning, the process of transformation of energy-storing into energy-dissipating adipocytes, is a potential preventive strategy against obesity and its related diseases. In the present study, to explore an alternative source of natural products in the regulation of adipocyte transformation, we assessed the potential of theobromine (TB), a bitter alkaloid of the cacao plant, inducing browning in mice (in vivo) and primary adipocytes (in vitro). Dietary supplementation of TB significantly increased skin temperature of the inguinal region in mice and induced the expression of UCP1 protein. It also increased the expression levels of mitochondrial marker proteins in subcutaneous adipose tissues but not in visceral adipose tissues. The microarray analysis showed that TB supplementation upregulated multiple thermogenic and beige adipocyte marker genes in subcutaneous adipose tissue. Furthermore, in mouse-derived primary adipocytes, TB upregulated the expression of the UCP1 protein and mitochondrial mass in a PPARγ ligand-dependent manner. It also increased the phosphorylation levels of PPARγ coactivator 1α without affecting its protein expression. These results indicate that dietary supplementation of TB induces browning in subcutaneous WAT and enhances PPARγ-induced UCP1 expression in vitro, suggesting its potential to treat obesity.