Hypoxia truncates and constitutively activates the key cholesterol synthesis enzyme squalene monooxygenase

Abstract

Cholesterol synthesis is both energy- and oxygen-intensive, yet relatively little is known of the regulatory effects of hypoxia on pathway enzymes. We previously showed that the rate-limiting and first oxygen-dependent enzyme of the committed cholesterol synthesis pathway, squalene monooxygenase (SM), can undergo partial proteasomal degradation that renders it constitutively active. Here, we show hypoxia is a physiological trigger for this truncation, which occurs through a two-part mechanism: (1) increased targeting of SM to the proteasome via stabilization of the E3 ubiquitin ligase MARCHF6 and (2) accumulation of the SM substrate, squalene, which impedes the complete degradation of SM and liberates its truncated form. This preserves SM activity and downstream pathway flux during hypoxia. These results uncover a feedforward mechanism that allows SM to accommodate fluctuating substrate levels and may contribute to its widely reported oncogenic properties.

HepG2 cells with knockouts ofCYP51A1,DHCR24orSC5Dfrom cholesterol synthesis accumulate sterols influencing distinct regulatory pathways

“…Cholesterol metabolism represents an important drug targeting pathway (8, 9). Targeting this highly oxygen-dependent pathway can also alter other physiological processes (10, 11)…”

“…Cholesterol metabolism represents an important drug targeting pathway (8,9). Targeting this highly oxygen-dependent pathway can also alter other physiological processes (10,11) Much less is known about the functions of non-polar sterols from the late part of cholesterol synthesis (sterol intermediates) that are structurally similar to cholesterol (See Supp. Fig.…”

“…Other important roles are connected to cell division and proliferation, with sufficient cholesterol levels being necessary for cell cycle progression from G2 to the M phase (6,7). Consequently, cholesterol metabolism represents a drug targeting pathway (8,9), which can also alter other physiological processes (10) and cholesterol metabolism is also a highly oxygen-dependent process (11).…”

The constitutively active form of a key cholesterol synthesis enzyme is lipid droplet-localized and upregulated in endometrial cancer tissues

“…Both responses depend on the lipid-sensing N-terminal regulatory domain of SM (SM-N100), which is embedded in the ER membrane. Accumulated squalene also promotes the partial rather than complete proteasomal degradation of SM, disrupting the SM-N100 domain and liberating a long-lived and constitutively active enzyme variant (truncated SM or trunSM) ( 9 , 10 ). Previous work showed that SM truncation is stimulated during hypoxia, which preserves enzymatic activity and downstream flux through cholesterol synthesis to maintain cell viability ( 10 , 11 ).…”

“…Accumulated squalene also promotes the partial rather than complete proteasomal degradation of SM, disrupting the SM-N100 domain and liberating a long-lived and constitutively active enzyme variant (truncated SM or trunSM) ( 9 , 10 ). Previous work showed that SM truncation is stimulated during hypoxia, which preserves enzymatic activity and downstream flux through cholesterol synthesis to maintain cell viability ( 10 , 11 ). However, other functional or directly pathophysiological consequences await discovery.…”