Regulation of Hepatic Cholesterol Homeostasis Through Accelerated Degradation of HMG CoA Reductase

Cholesterol biosynthesis is rigorously controlled by negative feedback regulation. This reaction occurs, in part, through sterol-accelerated degradation of HMG CoA reductase (HMGCR), which catalyzes the rate-limiting step in cholesterol biosynthesis. The molecular mechanisms for the degradation of HMGCR have been actively investigated; however, the physiological relevance of the degradative regulation in animals is unclear. The current study investigates the role of sterol-accelerated degradation of HMGCR in overall regulation of HMGCR protein and cholesterol homeostasis in the liver. This was achieved by utilizing two mouse models: (1) liver-specific transgenic mice expressing the membrane domain of HMGCR, which is necessary and sufficient for sterol-regulated degradation of HMGCR in cultured cells and (2) knock-in mice expressing mutant HMGCR that is resistant to sterol-induced ubiquitination. These models were subjected to various feeding regimens known to modulate Insig and Scap, key players in feedback regulation of HMGCR. Cholesterol replenishment accelerates degradation of HMGCR in the liver of transgenic animals, whereas deprivation of sterols by lovastatin administration suppresses degradation of HMGCR. Ubiquitination-resistant HMGCR accumulated in the liver and resulted in the elevation of hepatic cholesterol, indicating degradation plays a significant role in the in vivo regulation of the enzyme and cholesterol homeostasis. This study further explored the physiological settings other than changing cholesterol status that may modulate the degradation of HMGCR in the two mouse models. As cholesterol synthesis is an oxygen-consumptive process, I determined the link between oxygen sensing and feedback control of cholesterol synthesis. In cultured human fibroblasts, stabilization of oxygen-sensitive transcription factor, hypoxia-inducible factor-1α (HIF-1α) directly activates transcription of INSIG-2 gene. Insig-2 inhibits cholesterol synthesis by mediating sterol-induced ubiquitination and subsequent degradation of HMGCR. Hepatic levels of Insig-2 mRNA are enhanced in mouse models of hypoxia. Moreover, pharmacologic stabilization of HIF-1α in liver stimulates HMGCR degradation through a reaction that requires the protein's prior ubiquitination and the presence of Insig-2. These results indicate that HIF-mediated induction of Insig-2 and degradation of HMGCR are physiologically relevant events in the cellular adaptation to hypoxic stress. Overall, the current study provides evidence supporting the physiological significance of the accelerated degradation of HMGCR in cholesterol homeostasis.