Cholesterol-Dependent Degradation of Squalene Monooxygenase, a Control Point in Cholesterol Synthesis beyond HMG-CoA Reductase

SummaryExquisite control of cholesterol synthesis is crucial for maintaining homeostasis of this vital yet potentially toxic lipid. Squalene monooxygenase (SM) catalyzes the first oxygenation step in cholesterol synthesis, acting on squalene before cyclization into the basic steroid structure. Using model cell systems, we found that cholesterol caused the accumulation of the substrate squalene, suggesting that SM may serve as a flux-controlling enzyme beyond 3-hydroxy-3-methylglutaryl-coenzyme A reductase (HMGR, considered as rate limiting). Cholesterol accelerated the proteasomal degradation of SM which required the N-terminal domain, partially conserved in vertebrates but not in lower organisms. Unlike HMGR, SM degradation is not mediated by Insig, 24,25-dihydrolanosterol, or side-chain oxysterols, but rather by cholesterol itself. Importantly, SM's N-terminal domain conferred cholesterol-regulated turnover on heterologous fusion proteins. Furthermore, proteasomal inhibition almost totally eliminated squalene accumulation, highlighting the importance of this degradation mechanism for the control of SM and suggesting this as a possible control point in cholesterol synthesis.

Graphical AbstractFigure optionsDownload full-size imageDownload high-quality image (194 K)Download as PowerPoint slideHighlights► Cholesterol treatment induced accumulation of squalene in a variety of cell types ► This occurred posttranslationally and was reversed by proteasomal inhibition ► Cholesterol-regulated turnover was mediated by the N-terminal domain of SM ► Unlike HMGR, SM degradation did not require Insig or side-chain oxysterols