Identification of a functional vitamin D response element in the murine insig-2 promoter and its potential role in the differentiation of 3T3-L1 preadipocytes

Insulin-induced gene-1 (Insig-1) and its homolog Insig-2 encode closely related proteins of the endoplasmic reticulum that block proteolytic activation of sterol regulatory element binding proteins, membrane-bound transcription factors that activate synthesis of cholesterol and fatty acids in animal cells. These proteins also restrict lipogenesis in mature adipocytes and block differentiation of preadipocytes. Herein, we identified a novel 1α,25-dihydroxyvitamin D3 [1,25-(OH)₂D₃] response element in the promoter region of Insig-2 gene, which specifically binds to the heterodimer of retinoid X receptor and vitamin D receptor (VDR) and directs VDR-mediated transcriptional activation in a 1,25-(OH)₂D ₃-dependent manner. Interestingly, 1,25-(OH)₂D₃ is known to directly suppress the expression of peroxisome proliferator- activated receptor γ2 protein and inhibits adipocyte differentiation of 3T3-L1 preadipocytes and murine bone marrow stromal cells. Consistent with an idea that the antiadipogenic action of 1,25-(OH)₂D₃ may also involve up-regulation of Insig-2, we found that 1,25-(OH)₂D ₃ transiently but strongly induces Insig-2 expression in 3T3-L1 cells. This novel regulatory circuit may also play important roles in other lipogenic cell types that express VDR, and collectively our results suggest an intriguing, new linkage between 1,25-(OH)₂D₃ and lipogenesis.