Post-translational control of the long and winding road to cholesterol

Abstract

The synthesis of cholesterol requires more than 20 enzymes, many of which are intricately regulated. Post-translational control of these enzymes provides a rapid means for modifying flux through the pathway. So far, several enzymes have been shown to be rapidly degraded through the ubiquitin-proteasome pathway in response to cholesterol and other sterol intermediates. Additionally, several enzymes have their activity altered through phosphorylation mechanisms. Most work has focused on the two rate-limiting enzymes: 3-hydroxy-3-methylglutaryl CoA reductase and squalene monooxygenase. Here, we review current literature in the area to define some common themes in the regulation of the entire cholesterol synthesis pathway. We highlight the rich variety of inputs controlling each enzyme, discuss the interplay that exists between regulatory mechanisms, and summarize findings that reveal an intricately coordinated network of regulation along the cholesterol synthesis pathway. We provide a roadmap for future research into the post-translational control of cholesterol synthesis, and no doubt the road ahead will reveal further twists and turns for this fascinating pathway crucial for human health and disease.

Keywords: E3 ubiquitin ligase; HMGCR; SM; acetylation; cholesterol; cholesterol metabolism; cholesterol regulation; cholesterol synthesis; phosphorylation; post-translational modification; post-translational modification (PTM); post-translational regulation; proteasome; protein degradation; ubiquitin ligase; ubiquitination; ubiquitylation (ubiquitination).

Figure 1.

Branches and alternative end products of the cholesterol synthesis pathway. Cholesterol synthesis proceeds via the early pathway, which converts acetyl-CoA to lanosterol, and the post-lanosterol pathway, which converts lanosterol to cholesterol through the Bloch or modified Kandutsch–Russell pathways. Double-headed arrows indicate multiple enzymatic steps. For the effect of cholesterol on the post-translational regulation of pathway enzymes, refer to Fig. 2. Alternative end products are formed by branches of the cholesterol synthesis pathway. The early pathway intermediate farnesyl diphosphate is the precursor to isoprenoids such as geranylgeranyl pyrophosphate, which augments the sterol-induced degradation of HMGCR. The Kandutsch–Russell pathway intermediate 7-dehydrocholesterol is the precursor to vitamin D, which promotes the degradation of the downstream enzyme DHCR7 in keratinocytes. Cholesterol itself can be converted to other products such as bile acids, steroid hormones, and oxysterols.

Figure 2.

Post-translational regulation of the long and winding cholesterol synthesis pathway. Cholesterol synthesis can be divided into the early pathway and post-lanosterol pathway, which in turn proceeds via the Bloch pathway or the Kandutsch–Russell pathway (containing C24-saturated derivatives of Bloch pathway intermediates). Depicted is the more widely utilized modified Kandutsch–Russell pathway, which bypasses upstream intermediates and downstream entry points. For simplicity, chemical structures are shown only for the sterol intermediates at the start and end of the Bloch and modified Kandutsch–Russell pathways. Red broken lines and inhibitory arrows indicate intermediates and E3 ligases that promote the degradation of HMGCR, whereas the red solid lines indicate intermediates that promote the degradation of other cholesterol synthesis enzymes (inhibitory arrows) or stabilize MARCHF6 (arrowheads). Note that for clarity, we have indicated only the major contributors to degradation of each enzyme. It is also likely that additional intermediates promote degradation of some enzymes, but these have yet to be explored. The E3 ubiquitin ligase MARCHF6 targets the enzymes indicated in purple. For more details, please refer to the text.

Figure 3.

Post-translational modifications on cholesterol synthesis enzymes. For each enzyme, length (aa) and sequence were obtained from UniProt (19) (the identifiers are listed in Table 1). Hydrophobicity was determined by the percentage of hydrophobic residues, grand average of hydropathy (gravy) score (where a positive score indicates hydrophobic and a negative score indicates hydrophilic), and predicted number of transmembrane domains (mem, determined by TOPCONS (96), as a measure of the likelihood of membrane association). The post-translational modifications for each enzyme were obtained from PhosphoSite (14) and presented as the total percentage of modified residues per enzyme (PTM), and the percentage of each specific residue that has a particular modification (e.g. %S indicates the percentage of serine residues in that enzyme that are known to be phosphorylated). Median values are for the cholesterol synthesis enzymes. Proteome values are the median values based on a median length of ∼400 amino acids (97) with amino acid distribution as per UniProt (19). For the percentage of modified residues, all modifications from PhosphoSite (14) were compared with the composition of the proteome. The intensity of shading reflects increasing values in the mem column and collectively for the PTM columns. For more details, please see Table S1. Ub, ubiquitination; Ac, acetylation.