PPARα-dependent Insig2a overexpression inhibits SREBP-1c processing during fasting

Peroxisome-proliferator-activated receptor alpha (PPAR alpha) and sterol regulatory element-binding protein (SREBP) play a role in regulating cellular fatty acid and cholesterol homeostasis via fatty acid oxidation and lipogenesis. The control of SREBP processing is regulated by the insulin induced gene (INSIG) 2a protein, which binds SREBP to prevent SREBP translocation to the Golgi apparatus during nutrient starvation in the liver. However, the regulation of SREBP-1c processing by INSIGs during fasting and the regulatory mechanisms of the mouse Insig2a gene expression have not been clearly addressed. In the present study, we found that Insig2a was upregulated by PPAR alpha in mouse livers and primary hepatocytes during fasting, whereas Insig2a mRNA expression was decreased in the livers of refed mice. A PPAR-responsive element between -126 bp and -114 bp in the Insig2a promoter was identified by a transient transfection assay and a chromatin immunoprecipitation assay; its role in regulation by PPARa was characterised using Ppar alpha-null mice. These results suggest that PPAR alpha is a trans-acting factor that enhances Insig2a gene expression, thereby suppressing SREBP-1c processing during fasting.