Differential effects of 1α,25-dihydroxycholecalciferol on MCP-1 and adiponectin production in human white adipocytes

Obesity is characterized by a low-grade inflammation in white adipose tissue (WAT), which promotes insulin resistance. Low serum levels of 1 alpha,25-dihydroxycholecalciferol (DHCC) associate with insulin resistance and higher body mass index although it is unclear whether vitamin D supplementation improves insulin sensitivity. We investigated the effects of DHCC on adipokine gene expression and secretion in adipocytes focusing on two key factors with pro-inflammatory [monocyte chemoattractant protein-1 (MCP-1/CCL2)] and anti-inflammatory [adiponectin (ADIPOQ)] effects. Pre-adipocytes were isolated from human subcutaneous WAT and cultured until full differentiation. Differentiated adipocytes were either pre-treated with DHCC (10(-7) M) and subsequently incubated with tumor necrosis factor-alpha (TNF alpha, 100 ng/mL) or concomitantly incubated with TNF alpha/DHCC. MCP1 and adiponectin mRNA expression was measured by RT-PCR and protein release by ELISA. DHCC was not toxic and did not affect adipocyte morphology or the mRNA levels of adipocyte-specific genes. TNF alpha induced a significant increase in CCL2 mRNA and protein secretion, while DHCC alone reduced CCL2 mRNA expression (similar to 25%, p < 0.05). DHCC attenuated TNF alpha-induced CCL2 mRNA expression in both pre-incubation (similar to 15%, p < 0.05) and concomitant (similar to 60%, p < 0.01) treatments. TNF alpha reduced ADIPOQ mRNA (similar to 80%) and secretion (similar to 35%). DHCC alone decreased adiponectin secretion to a similar degree (similar to 35%, p < 0.05). Concomitant treatment with DHCC/TNF alpha for 48 h had an additive effect, resulting in a pronounced reduction in adiponectin secretion (similar to 70%). DHCC attenuates MCP-1 and adiponectin production in human adipocytes, thereby reducing the expression of both pro- and anti-inflammatory factors. These effects may explain the difficulties so far in determining the role of DHCC in insulin sensitivity and obesity in humans.