A MARCH6 and IDOL E3 Ubiquitin Ligase Circuit Uncouples Cholesterol Synthesis from Lipoprotein Uptake in Hepatocytes

Abstract

Cholesterol synthesis and lipoprotein uptake are tightly coordinated to ensure that the cellular level of cholesterol is adequately maintained. Hepatic dysregulation of these processes is associated with pathological conditions, most notably cardiovascular disease. Using a genetic approach, we have recently identified the E3 ubiquitin ligase MARCH6 as a regulator of cholesterol biosynthesis, owing to its ability to promote degradation of the rate-limiting enzymes 3-hydroxy-3-methyl-glutaryl coenzyme A reductase (HMGCR) and squalene epoxidase (SQLE). Here, we present evidence for MARCH6 playing a multifaceted role in the control of cholesterol homeostasis in hepatocytes. We identify MARCH6 as an endogenous inhibitor of the sterol regulatory element binding protein (SREBP) transcriptional program. Accordingly, loss of MARCH6 increases expression of SREBP-regulated genes involved in cholesterol biosynthesis and lipoprotein uptake. Unexpectedly, this is associated with a decrease in cellular lipoprotein uptake, induced by enhanced lysosomal degradation of the low-density lipoprotein receptor (LDLR). Finally, we provide evidence that induction of the E3 ubiquitin ligase IDOL represents the molecular mechanism underlying this MARCH6-induced phenotype. Our study thus highlights a MARCH6-dependent mechanism to direct cellular cholesterol accretion that relies on uncoupling of cholesterol synthesis from lipoprotein uptake. Cholesterol is an essential constituent of cellular membranes and signaling pathways and is a precursor of sterol-derived molecules (1). Yet elevated levels of cholesterol are toxic to cells, and dysregulated cholesterol metabolism is associated, most evidently, with development of cardiovascular disease. As such, multiple transcriptional networks and posttranscriptional processes regulate the synthesis, uptake, and efflux of cholesterol. Transcriptionally, these processes are largely governed by the opposing actions of the transcription factors sterol regulatory element binding proteins (SREBPs) and the liver X receptors (LXRs) (2-6). Upon sensing low cholesterol levels in the endoplasmic reticulum (ER), SREBPs are processed into their mature, transcriptionally active form. This results in induction of the full set of genes required for de novo biosynthesis of cholesterol via the mevalonate pathway and of the low-density lipoprotein receptor (LDLR) that is required for uptake of LDL-derived cholesterol (7, 8). In contrast, LXRs, members of the nuclear receptor family, are activated when cellular cholesterol levels are elevated. Once activated by their cognate oxysterol ligands, LXRs induce cholesterol efflux pathways (e.g., via the transporters ABCA1 and ABCG1) and limit LDL uptake by inducing expression of the E3 ubiquitin ligase (E3)-inducible degrader of the LDLR (IDOL) (6, 9, 10). The coordinated action of these two transcription factor families ensures that cellular cholesterol is adequately maintained at an appropriate level.Next to transcriptional regulation, posttranscriptional mechanisms are ...

MARCH6 and TRC8 facilitate the quality control of cytosolic and tail‐anchored proteins

“…The identification of overlapping roles for TRC8 and MARCH may explain seemingly contradictory reports regarding TRC8 and MARCH6 substrates. For example, TRC8 was initially implicated in the regulation of SREBP-1/2 [57,58], but loss of MARCH6 was recently shown to increase SREBP-1/2 signalling [59]. Our identification of SREBP-1/2 ( Fig 6A) as a potential MARCH6/TRC8 substrate may reconcile these findings.…”

N-terminal acetylation and the N-end rule pathway control degradation of the lipid droplet protein PLIN2

“…With respect to LD and lipid metabolism, TEB4 mediates degradation of not only PLIN2, but also squalene monooxygenase, a rate-limiting enzyme in cholesterol synthesis (43,44,46). In addition, Doa10 (a yeast homolog of mammalian TEB4) is shown to spatially target LD proteins that are integrated into the ER membrane (39).…”

Emerging lysosomal pathways for quality control at the endoplasmic reticulum

“…The I1061T NPC1 mutant has been described to accumulate in the ER lumen and to be degraded by two independent pathways functioning in a complementary fashion [58]. It is degraded in part by ER-phagy in a FAM134B-and autophagy-dependent process; in part, it is triaged through ERAD by the proteasome via MARCH6, an E3 ubiquitin ligase involved in the control of cholesterol homoeostasis [58,61].…”