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...