1,25(OH)2-vitamin D3 upregulates glucose uptake mediated by SIRT1/IRS1/GLUT4 signaling cascade in C2C12 myotubes.

This study examined the hypothesis that 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) upregulates the insulin-independent signaling cascade of glucose metabolism. C2C12 myotubes were treated with high glucose (HG, 25 mM) and 1,25(OH)2D3 (0-50 nM). 1,25(OH)2D3 supplementation upregulated both insulin-independent (SIRT1) and insulin-dependent (p-IRS) signaling molecules, and stimulated the GLUT4 translocation, and glucose uptake in HG-treated myotubes. The effect of 1,25(OH)2D3 on IRS1 phosphorylation, GLUT4 translocation, and glucose uptake was attenuated in SIRT1-knockdown myotubes. Treatment with 1,25(OH)2D3, coupled with insulin, enhanced GLUT4 translocation and glucose uptake compared to treatment with either insulin or 1,25(OH)2D3 alone in HG-treated myotubes, which suggests that insulin-independent signaling molecules can contribute to the higher glucose metabolism observed in 1,25(OH)2D3 and insulin-treated cells. The data, therefore, suggest that 1,25(OH)2D3 increases glucose consumption by inducing SIRT1 activation, which in turn increases IRS1 phosphorylation and GLUT4 translocation in myotubes.