The E3 Ubiquitin Ligase MARCH6 Degrades Squalene Monooxygenase and Affects 3-Hydroxy-3-Methyl-Glutaryl Coenzyme A Reductase and the Cholesterol Synthesis Pathway

Themevalonate pathway is used by cells to produce sterol and nonsterol metabolites and is subject to tight metabolic regulation. We recently reported that squalene monooxygenase (SM), an enzyme controlling a rate-limiting step in cholesterol biosynthesis, is subject to cholesterol-dependent proteasomal degradation. However, the E3-ubiquitin (E3) ligase mediating this effect was not established. Using a candidate approach, we identify the E3 ligase membrane-associated RING finger 6 (MARCH6, also known as TEB4) as the ligase controlling degradation of SM.We find that MARCH6 and SM physically interact, and consistent with MARCH6 acting as an E3 ligase, its overexpression reduces SM abundance in a RING-dependent manner. Reciprocally, knock-down of MARCH6 increases the level of SM protein and prevents its cholesterol-regulated degradation. Additionally, this in-creases cell-associated SM activity but is unexpectedly accompanied by increased flux upstream of SM. Prompted by this obser-vation, we found that knockdown of MARCH6 also controls the level of 3-hydroxy-3-methyl-glutaryl coenzyme A reductase (HMGCR) in hepatocytes andmodel cell lines. In conclusion, MARCH6 controls abundance of both SM and HMGCR, establish-ing it as a major regulator of flux through the cholesterol synthesis pathway. Themevalonate pathway leading to cholesterol synthesis is con-trolled transcriptionally and, for more rapid shutdown, post-translationally (1). The third step in the pathway, catalyzed by 3-hydroxy-3-methyl-glutaryl coenzyme A reductase (HMGCR), is generally regarded as the rate-limiting step in cholesterol syn