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

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.