Multiple feedback mechanisms converge on the polytopic, ER (endoplasmic reticulum)-localized enzyme HMG CoA reductase (HMGCR), which catalyzes a key step in synthesis of cholesterol and essential nonsterol isoprenoids including Fpp (farnesyl pyrophosphate) and GGpp (geranylgeranyl pyrophosphate). Fpp and GGpp can become attached to many cellular proteins and are utilized in synthesis of dolichol, ubiquinone, heme, and vitamin K2. One mechanism for feedback control of HMGCR involves sterol-induced ubiquitination, which marks the enzyme for extraction across ER membranes and subsequent release into the cytosol for proteasome- mediated ERAD (ER-associated degradation). GGpp augments ERAD of HMGCR by enhancing its membrane extraction. We recently discovered that GGpp inhibits sterol-induced binding of HMGCR to UBIAD1 (UbiA prenyltransferase domain-containing protein-1), which utilizes GGpp to synthesize vitamin K2. This inhibition allows for membrane extraction of HMGCR and ER-to-Golgi transport of UBIAD1. Mutations in UBIAD1 cause SCD (Schnyder corneal dystrophy), an autosomal dominant eye disease characterized by corneal accumulation of cholesterol. SCD-associated UBIAD1 resists GGpp-induced release from HMGCR and becomes trapped in the ER where it blocks HMGCR ERAD. Building on discoveries summarized above, we are now poised to 1) elucidate mechanisms through which sterols promote ubiquitination of HMGCR; 2) determine the molecular structure of HMGCR; 3) establish how ubiquitinated HMGCR is removed from ER membranes for ERAD; and 4) ascertain the physiological significance of HMGCR ERAD. Collectively, these studies will provide key information regarding mechanisms by which polytopic HMGCR is removed from ER membranes and delivered cytosolic proteasomes for ERAD. In addition, these studies have significant clinical implications. HMGCR is the target of statins, widely prescribed drugs that lower plasma LDL-cholesterol and reduce cardiovascular disease. However, statins trigger responses that cause accumulation of HMGCR, which blunts their clinical effects. This increase results in part, from slowed ERAD of HMGCR. Thus, elucidating mechanisms for HMGCR ERAD holds promise for development of new therapies that increase the efficacy of statins and further reduce heart attacks.
Project Relevance The key enzyme in cholesterol synthesis, HMG CoA reductase (HMGCR), is controlled through multiple mechanisms, including regulation of protein degradation. This grant will investigate mechanisms through which sterols modulate HMGCR degradation. Completion of proposed studies will provide insight into mechanisms for HMGCR degradation and into therapies to reduce the incidence of cardiovascular disease and heart attacks.
