Vitamin D Inhibits Adipokine Production and Inflammatory Signaling Through the Vitamin D Receptor in Human Adipocytes

Abstract

Objective: The purpose of this study was to investigate the effects of vitamin D on adipokine expression and inflammation in human adipose tissues and adipocytes and evaluate the molecular mechanisms involved. Methods: Omental and abdominal subcutaneous human adipose tissues were treated with 1,25-dihydroxyvitamin D3 (1,25(OH)2D3), and adipokine levels were measured. Vitamin D effects were measured with or without dexamethasone because glucocorticoids are known to affect vitamin D actions. Using RNA interference, we examined whether the vitamin D receptor (VDR) mediated vitamin D actions on adipokine expression and inflammatory signaling pathways in human adipocytes. Results: mRNA levels and secretion of leptin and IL-6 were suppressed by 1,25(OH)2D3 in omental adipose tissues. Cotreatment with dexamethasone did not affect these inhibitory actions but partially blocked CYP24A1 induction. Similar results were observed in the subcutaneous depot. In addition, 1,25(OH)2D3 suppressed leptin and IL-6 expression as well as nuclear factor-κB and extracellular signal–regulated kinase-1/2 phosphorylation in human adipocytes. Adipokine expression also was decreased by 25-hydroxyvitamin D3 (25(OH)D3), but not vitamin D3. Knockdown of VDR increased the inflammatory signaling activity in the control condition and blocked the inhibitory effects of 1,25(OH)2D3 on adipokine and inflammatory signaling pathways. Conclusion: Vitamin D acts through VDR to inhibit inflammatory pathways and adipokine expression in human adipocytes. Increasing vitamin D status may ameliorate obesity-associated metabolic complications by decreasing adipose tissue inflammation.