POST1/C12ORF49 regulates the SREBP pathway by promoting site-1 protease maturation

Sterol-regulatory element binding proteins (SREBPs) are the key transcriptional regulators of lipid metabolism. The activation of SREBP requires translocation of the SREBP precursor from the endoplasmic reticulum to the Golgi, where it is sequentially cleaved by site-1 protease (S1P) and site-2 protease and releases a nuclear form to modulate gene expression. To search for new genes regulating cholesterol metabolism, we perform a genome-wide CRISPR/Cas9 knockout screen and find that partner of site-1 protease (POST1), encoded by C12ORF49, is critically involved in the SREBP signaling. Ablation of POST1 decreases the generation of nuclear SREBP and reduces the expression of SREBP target genes. POST1 binds S1P, which is synthesized as an inactive protease (form A) and becomes fully mature via a two-step autocatalytic process involving forms B'/B and C'/C. POST1 promotes the generation of the functional S1P-C'/C from S1P-B'/B (canonical cleavage) and, notably, from S1P-A directly (non-canonical cleavage) as well. This POST1-mediated S1P activation is also essential for the cleavages of other S1P substrates including ATF6, CREB3 family members and the α/β-subunit precursor of N-acetylglucosamine-1-phosphotransferase. Together, we demonstrate that POST1 is a cofactor controlling S1P maturation and plays important roles in lipid homeostasis, unfolded protein response, lipoprotein metabolism and lysosome biogenesis.

Effects of chrysophanol on expression of SREBPs and lipid metabolism in Huh-7 cells / 药学学报

Rhubarb is a traditional Chinese medicines which possess laxative, lipid-lowering, and weight-loss activities, but the active compounds of lipid-lowering and underlying molecular mechanisms are not yet clear. This study aims to explore the effects of chrysophanol on the mRNA expressions of sterol regulatory element-binding proteins (SREBPs) and lipid metabolism in human liver carcinoma Huh-7 cells, which is one of the active compounds obtained from Rhubarb. A reporter gene assay was used to test the transcription of SREBP. The intracellular triglyceride and total cholesterol contents were measured by using commercially available test kits. The SREBPs target genes expressions were measured by Quantitative Real-Time PCR. Cell viability was evaluated by Cell Counting Kit-8. As the results shown, chrysophanol (40 μmol · L(-1), 16 h) could notably inhibited human SRE promoter activity in a dose-dependent manner and decrease intracellular cholesterol and triglyceride levels. Furthermore, the mRNA expressions of SREBPs target genes were significantly downregulated by chrysophanol treatment. However there are no significant differences on cell viability when compared with the control group. These results suggested that chrysophanol might improve lipid metabolism through suppressing the mRNA expressions of SREBPs target genes to attenuate intracellular lipid accumulation.

Ubiquitin Ligases in Cholesterol Metabolism

To maintain cholesterol homeostasis, the processes of cholesterol metabolism are regulated at multiple levels including transcription, translation, and enzymatic activity. Recently, the regulation of protein stability of some key players in cholesterol metabolism has been characterized. More and more ubiquitin ligases have been identified including gp78, Hrd1, TRC8, TEB4, Fbw7, and inducible degrader of low density lipoprotein receptor. Their working mechanisms and physiological functions are becoming revealed. Here, we summarize the structure, substrates and function of these ubiquitin ligases. Their potential application in drug discovery is also discussed.