No abstract

C holesterol is vital for the survival and growth of mammalian cells. More than a membrane constituent, cholesterol is a precursor to bile acids and steroid hormones, which can initiate or promote colon, breast and prostate cancers 1-3 . Cholesterol can also modulate signalling pathways involved in tumourigenesis and cancer progression by covalently modifying proteins including hedgehog and smoothened 4,5 , and by facilitating the formation of specialized membrane microdomains 6,7 . Brief overview of cholesterol metabolismEvery mammalian cell can synthesize cholesterol through the mevalonate pathway (Fig. 1a). Two acetyl-CoA molecules in the cytosol condense, thus forming acetoacetyl-CoA, which reacts with the third acetyl-CoA and yields 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA). HMG-CoA is reduced to mevalonate by HMG-CoA reductase (HMGCR), the primary rate-limiting enzyme in cholesterol biosynthesis. A series of enzymatic reactions convert mevalonate to farnesyl pyrophosphate (FPP), a precursor of sterols and all non-sterol isoprenoids. The condensation of two FPP molecules to squalene commits the process to sterol production. FPP also gives rise to geranylgeranyl pyrophosphate (GGPP), and both FPP and GGPP can prenylate and activate several oncogenic proteins such as Ras 8 . Squalene is then oxidized by squalene epoxidase (SQLE) to 2,3-epoxysqualene, which is cyclized to lanosterol. In the next steps, lanosterol follows the Bloch pathway, the Kandutsch-Russell pathway or a hybrid pathway before it is finally converted to cholesterol.Beyond de novo cholesterol biosynthesis, most cells acquire cholesterol from low-density lipoprotein (LDL) taken up from the circulation via LDL receptor (LDLR)-mediated endocytosis 9 . Enterocytes absorb dietary cholesterol from the intestinal lumen in a process involving the cholesterol transporter NPC1L1, the clathrin adaptor NUMB and the adaptor protein LIMA1 (refs. 10-12 ). Cholesterol within the cell is dynamically transported, reaching the destined membranes for structural and functional needs 13 . Cholesterol in excess of the current cellular demand is either exported from the cell by ATP-binding cassette (ABC) transporters, Reprogrammed cholesterol metabolism in cancer cellsHallmark features of cancer cholesterol metabolism. As fastproliferating cells, cancer cells require high levels of cholesterol for membrane biogenesis and other functional needs. For example, the cholesterol-derived oncometabolite 6-oxo-cholestan-3β,5α-diol, which is enriched in patients with breast cancer, binds glucocorticoid receptors and subsequently promotes tumour growth 19 . In general, cholesterol metabolism substantially contributes to cancer progression, including cell proliferation, migration and invasion 20-23 .Cholesterol metabolism produces essential membrane components as well as metabolites with a variety of biological functions. In the tumour microenvironment, cell-intrinsic and cell-extrinsic cues reprogram cholesterol metabolism and consequently promote tumourigenesis. Cholesterol-de...

The endoplasmic reticulum enzyme 3-hydroxy-3-methylglutaryl-CoA reductase produces mevalonate, which is converted to sterols and to other products, including geranylgeraniol groups attached to proteins. The enzyme is known to be ubiquitinated and rapidly degraded when sterols and nonsterol end products of mevalonate metabolism accumulate in cells. Here, we use RNA interference to show that sterol-accelerated ubiquitination of reductase requires Insig-1 and Insig-2, membrane-bound proteins of the endoplasmic reticulum that were shown previously to accelerate degradation of reductase when overexpressed by transfection. Alanine substitution experiments reveal that binding of reductase to Insigs and subsequent ubiquitination require the tetrapeptide sequence YIYF in the second membrane-spanning helix of reductase. The YIYF peptide is also found in the sterol-sensing domain of SCAP, another protein that binds to Insigs in a sterolstimulated fashion. When lysine 248 of reductase is substituted with arginine, Insig binding persists, but the reductase is no longer ubiquitinated and degradation is markedly slowed. Lysine 248 is predicted to lie immediately adjacent to a membrane-spanning helix, suggesting that a membrane-bound ubiquitin transferase is responsible. Finally, we show that Insig-dependent, sterolstimulated degradation of reductase is further accelerated when cells are also supplied with the 20-carbon isoprenoid geranylgeraniol, but not the 15-carbon farnesol, raising the possibility that the nonsterol potentiator of reductase regulation is a geranylgeranylated protein.The enzyme 3-hydroxy-3-methylglutaryl-CoA reductase (HMG-CoA reductase) 1 catalyzes the two-step reduction of HMG-CoA to mevalonate, a crucial intermediate in the synthesis of cholesterol and nonsterol isoprenoids (1). Mevalonatederived products include ubiquinone, heme, and the farnesyl and geranylgeranyl groups that become attached to many proteins, directing them to membranes. HMG-CoA reductase is subject to tight regulation by a multivalent feedback mechanism mediated by nonsterol and sterol end products of mevalonate metabolism (2). These end products decrease reductase activity by inhibiting transcription of the reductase gene, blocking translation of the reductase mRNA, and accelerating degradation of the reductase protein. The transcriptional effects are mediated through the action of sterol regulatory elementbinding proteins (SREBPs), membrane-bound transcription factors that enhance transcription of genes encoding cholesterol biosynthetic enzymes and the low density lipoprotein receptor (3). The translational effects are mediated by nonsterol mevalonate-derived isoprenoids, which act by an undefined mechanism (4). The accelerated degradation of reductase is mediated both by sterols and nonsterol end products of mevalonate metabolism (4, 5) and appears to be carried out by the 26 S proteasome (6, 7). Hamster HMG-CoA reductase is an endoplasmic reticulum (ER) resident glycoprotein of 887 amino acids and consists of two distinct domains ...

Sterol-regulated ubiquitination is an obligatory step in ER-associated degradation (ERAD) of HMG CoA reductase, a rate-limiting enzyme in cholesterol synthesis. Accelerated degradation of reductase, one of several strategies animal cells use to limit production of cholesterol, requires sterol-induced binding of the enzyme to ER membrane proteins called Insigs. Once formed, the reductase-Insig complex is recognized by a putative membrane-associated ubiquitin ligase (E3) that mediates the reductase ubiquitination reaction. Here, we show that gp78, a membrane bound E3, binds to Insig-1 and is required for sterol-regulated ubiquitination of reductase. In addition, gp78 couples regulated ubiquitination to degradation of reductase by binding to VCP, an ATPase that plays a key role in recognition and degradation of ERAD substrates. The current results identify gp78 as the E3 that initiates sterol-accelerated degradation of reductase, and Insig-1 as a bridge between gp78/VCP and the reductase substrate.

Cholesterol is dynamically transported among organelles, which is essential for multiple cellular functions. However, the mechanism underlying intracellular cholesterol transport has remained largely unknown. We established an amphotericin B-based assay enabling a genome-wide shRNA screen for delayed LDL-cholesterol transport and identified 341 hits with particular enrichment of peroxisome genes, suggesting a previously unappreciated pathway for cholesterol transport. We show dynamic membrane contacts between peroxisome and lysosome, which are mediated by lysosomal Synaptotagmin VII binding to the lipid PI(4,5)P2 on peroxisomal membrane. LDL-cholesterol enhances such contacts, and cholesterol is transported from lysosome to peroxisome. Disruption of critical peroxisome genes leads to cholesterol accumulation in lysosome. Together, these findings reveal an unexpected role of peroxisome in intracellular cholesterol transport. We further demonstrate massive cholesterol accumulation in human patient cells and mouse model of peroxisomal disorders, suggesting a contribution of abnormal cholesterol accumulation to these diseases.