1α,25(OH)2D3 downregulates gene expression levels of muscle ubiquitin ligases MAFbx and MuRF1 in human myotubes.

Clinical trials involving in patients with osteoporosis have reported that activated vitamin D3 (1α,25(OH)2D3, calcitriol) can prevent falling by acting on the skeletal muscles. However, pharmacological mechanisms of 1α,25(OH)2D3 with respect to skeletal muscle hypertrophy or atrophy are still poorly understood. Therefore, we examined changes in the expression of several related genes in human myotubes to test whether 1α,25(OH)2D3 influences hypertrophy and atrophy of skeletal muscle. Myotubes treated with 1α,25(OH)2D3 increased interleukin-6 (IL-6) expression and inhibited expression of tumor necrosis factor alpha (TNF-α), whereas the expression of insulin-like growth factor-1 (IGF-1) that is involved in muscle hypertrophy was not affected. However, 1α,25(OH)2D3 treatment significantly inhibited the expression of muscle atrophy F-box (MAFbx) and muscle RING finger 1 (MuRF1), ubiquitin ligases involved in muscle atrophy. The analysis of pathways using microarray data suggested that 1α,25(OH)2D3 upregulates AKT-1 by inhibiting the expression of protein phosphatase 2 (PP2A), a phosphatase acting on AKT-1, in the phosphatidylinositol 3-kinase (PI3K)/AKT signaling pathway, thereby inhibiting the expression of ubiquitin ligases. Thus, this study showed that 1α,25(OH)2D3 might have an inhibitory effect on the expression of MAFbx and MuRF1 in skeletal muscle and a suppressive effect on muscle degradation in patients with osteoporosis.