Control of lipid metabolism by phosphorylation-dependent degradation of the SREBP family of transcription factors by SCFFbw7

The sterol regulatory element binding protein (SREBP) family of transcription factors controls cholesterol and lipid metabolism. The nuclear forms of these proteins are rapidly degraded by the ubiquitin-proteasome pathway, but the signals and factors required for this are unknown. Here, we identify a phosphodegron in SREBP1a that serves as a recognition motif for the SCFFbw7 ubiquitin ligase. Fbw7 interacts with nuclear SREBP1a and enhances its ubiquitination and degradation in a manner dependent on the phosphorylation of T426 and S430 by GSK-3. Fbw7 also degrades nuclear SREBP1c and SREBP2, and inactivation of endogenous Fbw7 results in stabilization of nuclear SREBP1 and -2, enhanced expression of SREBP target genes, enhanced synthesis of cholesterol and fatty acids, and enhanced receptor-mediated uptake of LDL. Thus, our results suggest that Fbw7 may be a major regulator of lipid metabolism through control of the phosphorylation-dependent degradation of the SREBP family of transcription factors.