Insig-1 "brakes" lipogenesis in adipocytes and inhibits differentiation of preadipocytes.

We have examined gene expression in the fat tissue of normal mice at the onset of diet-induced obesity. Insulin-induced gene 1 (insig-1) mRNA rose progressively with a high-fat diet and declined on a restricted diet. Because insig-1 binds sterol regulatory element-binding protein cleavage-activating protein in the endoplasmic reticulum, thereby blocking proteolytic processing required for sterol regulatory element-binding protein activation, we tested its influence on lipogenesis. In differentiating 3T3-L1 cells, insig-1 and -2 rose in parallel with aP2 mRNA during differentiation. The mRNA of the lipogenic transcription factor, carbohydrate response element-binding protein, was undetectable in undifferentiated 3T3-L1 preadipocytes but rose dramatically during differentiation in 25 mM, but not in 5 mM, glucose. Transfection of mouse or human insig-1 into 3T3-L1 preadipocytes completely prevented oil red O staining and blocked upregulation of aP2, peroxisome proliferator-activated receptor gamma2, and carbohydrate response element-binding protein, while reducing down-regulation of preadipocyte factor 1. The results suggest that insig-1 expression restricts lipogenesis in mature adipocytes and blocks differentiation in preadipocytes.