Deletion of the Vitamin D Receptor in Skeletal Muscle is Associated with Improved Glucose Tolerance and Reduced Muscle Function

Skeletal muscle insulin resistance contributes to the pathogenesis of type 2 diabetes mellitus. Epidemiological studies have demonstrated that vitamin D deficiency is associated with the presence of type 2 diabetes in humans. To probe the role of the vitamin D receptor (VDR), an intracellular and nuclear protein that mediates the bioactivity of the active vitamin D ligand, 1,25-dihydroxyvitamin D, in skeletal muscle mediated glucose disposal, we deleted the skeletal muscle VDR in adult mice. We assessed glucose disposal, insulin concentrations and sensitivity, and skeletal muscle gene expression in Vdr knockout and wild-type Vdr mice. Following the induction of a skeletal muscle-specific Crerecombinase with tamoxifen in adult male Vdrfl/fl mice, efficient deletion of the Vdrwas demonstrated. In skeletal muscle-specific Vdrknockout mice, basal glucose concentrations were similar to those in wild-type Vdrmice. In Vdrknockout mice, blood glucose concentrations decreased more rapidly following the administration of intraperitoneal glucose than in control mice expressing wild-type Vdr (Figure 1A and Figure 1B). The AUC for glucose concentrations in Vdrfl/fl .actin iCre mice treated with tamoxifen was reduced by 20% (p=0.015) compared to the AUC of blood glucose concentrations prior to tamoxifen treatment. Pre-and post-treatment AUC's for blood glucose in Vdrfl/fl .actin iCre mice treated with vehicle and in Vdrfl/fl mice treated with tamoxifen were unchanged (p=0.97,0.23). Plasma insulin concentrations were identical in knockout and wild-type Vdr mice following a bolus of glucose, demonstrating that deletion of the Vdr in skeletal muscle did not influence the release of insulin in response to glucose (one-way ANOVA, p=0.45). Insulin sensitivity was similar in Vdrknockout and wild-type Vdrmice. Gene expression analysis of skeletal muscle mRNA derived from Vdrknockout mice and wild-type mice demonstrated an increase in the expression of genes associated with oxidative phosphorylation, fatty acid oxidation and adipogenesis pathways that could potentially alter glucose utilization. We conclude that selective deletion of the Vdr in the skeletal muscle of adult mice results in more efficient glucose disposal than in wild-type Vdrmice; basal and glucose-stimulated insulin serum concentrations are identical in knock-out and wild type VDR mice with no changes in insulin sensitivity. The data suggest that modulation of the vitamin-D endocrine system is unlikely to improve glucose utilization.