Characterization of the PRDM16-Dependent Regulation of SREBP1/2

Abstract

In the current report we explore the functional interaction of two important regulators of organismal lipid metabolism, SREBP1/2 and PRDM16. The SREBP family of transcription factors regulate cholesterol and fatty acid synthesis and metabolism, primarily in liver but also in white adipocyte tissue. PRDM16 is a major activator of brown adipogenesis and an inhibitor of white adipogenesis. We demonstrate that PRDM16 interacts with the nuclear forms of both SREBP1 and SREBP2 and map the interaction domains in both proteins. When using SREBP target promoters in promoter-reporter assays, we find that PRDM16 is a negative regulator of SREBP1a, SREBP1c and SREBP2. In these assays, ectopic expression of PRDM16 represses and shRNA-mediated inactivation of PRDM16 enhances the expression of the reporter genes. The PRDM16 sensitivity of these promoters was lost when the SREBP binding sites in the promoters were mutated, suggesting that PRDM16 represses DNA-bound SREBP molecules. This possibility is supported by our observation that PRDM16 interacts with SREBP target promoters in an SREBP-dependent manner in vitro. Inactivation of PRDM16 enhances the expression of well-established SREBP target genes involved in fatty acid and cholesterol synthesis/metabolism. The transcriptional effects of PRDM16 are dependent on nuclear SREBP1/2, illustrated by our observation that the inactivation of PRDM16 fails to affect the expression of SREBP target genes in cells treated with an excess of sterols. The LDL receptor is a clinically important SREBP target gene, and inactivation of PRDM16 enhanced the expression of LDL receptor mRNA, protein, and the uptake of LDL particles. Consequently, PRDM16-deficient cells also accumulated more neutral lipids, which was attenuated by including excess sterols in the media. As expected, ectopic expression of PRDM16 inhibited the adipogenesis of 3T3-L1 preadipocyte cells, accompanied with reduced expression of SREBP target genes. Importantly, inactivation of PRDM16 in 3T3-L1 resulted in the enhanced expression of SREBP target genes. Taken together, our study identifies PRDM16 as a novel inhibitor of SREBP-dependent lipid metabolism.

Date of Award	2025
Original language	American English
Awarding Institution	HBKU College of Health & Life Sciences