Dual roles for cholesterol in mammalian cells

Abstract

The structural features of sterols required to support mammalian cell growth have not been fully defined. Here, we use mutant CHO cells that synthesize only small amounts of cholesterol to test the capacity of various sterols to support growth. Sterols with minor modifications of the side chain (e.g., campesterol, ␤-sitosterol, and desmosterol) supported long-term growth of mutant cells, but sterols with more complex modifications of the side chain, the sterol nucleus, or the 3-hydroxy group did not. After 60 days in culture, the exogenous sterol comprised >90% of cellular sterols. Inactivation of residual endogenous synthesis with the squalene epoxidase inhibitor NB-598 prevented growth in ␤-sitosterol and greatly reduced growth in campesterol. Growth of cells cultured in ␤-sitosterol and NB-598 was restored by adding small amounts of cholesterol to the medium. Surprisingly, enantiomeric cholesterol also supported cell growth, even in the presence of NB-598. Thus, sterols fulfill two roles in mammalian cells: (i) a bulk membrane requirement in which phytosterols can substitute for cholesterol and (ii) other processes that specifically require small amounts of cholesterol but are not enantioselective.ent-cholesterol ͉ phytosterols ͉ NB-598 S terols are essential components of eukaryote membranes. Their incorporation enhances the packing of the acyl chains of phospholipids in the hydrophobic phase of the bilayer, increases its mechanical strength, and reduces its permeability (1). Despite this crucial role, most animal species (e.g., nematodes and arthropods) cannot make sterols and so must get them from the diet. Vertebrates, by contrast, make sterols de novo from acetyl-coenzyme A and so do not require exogenous sterols. Accordingly, studies to probe sterol requirements of eukaryotes have used in invertebrates (2, 3), protazoans (4, 5), or yeast strains defective in sterol synthesis (6-8), so that the sterol composition of the organism can be controlled exogenously.Although phytosterols can account for a substantial portion of total dietary sterols (approximately one-third in humans), vertebrates systematically exclude them from the body. Cholesterol predominates in the membranes of most animals and is virtually the exclusive sterol of vertebrates. Invertebrates such as insects typically convert phytosterols to cholesterol by dealkylating C24 in the side chain, thereby furnishing cholesterol needed by membranes and preventing accumulation of noncholesterol sterols. Mammals and other vertebrates can either make sterols de novo or get cholesterol from the diet. Accumulation of other dietary sterols in these animals is prevented by the action of two ATP-binding cassette transporters, ABCG5 and ABCG8 (9), which function as a heterodimer to limit intestinal absorption and facilitate biliary excretion of noncholesterol sterols (10, 11). Thus, cholesterol comprises the great majority of vertebrate sterols, even in animals ingesting large quantities of phytosterols.The biological basis for selection of cholesterol as t...

Squalene epoxidase, located on chromosome 8q24.1, is upregulated in 8q+ breast cancer and indicates poor clinical outcome in stage I and II disease

“…It is assumed that the reaction catalysed by SQLE is the rate limiting step in cholesterol biosynthesis, in post-HMG CoA regulation (Astruc et al, 1977;Tabacik et al, 1981). This is supported by the fact that cell growth inhibited by a specific SQLE inhibitor (NB-598) could be rescued by small amounts of cholesterol added to the culture medium (Xu et al, 2005).…”

Desmosterol, the main sterol in rabbit semen: distribution among semen subfractions and its role in the in vitro spermatozoa acrosome reaction and motility

“…This could be due to the very similar chemical structures of both the sterols. Xu et al [29] have demonstrated that desmosterol may efficiently replace cholesterol in the cell membrane without compromising the functional properties because there are compensatory changes in membrane composition.…”

Squalene monooxygenase – a target for hypercholesterolemic therapy

“…In vertebrates, it is an important structural component of cellular membranes, precursor of oxysterols, steroid hormones and bile acids. It regulates embryogenesis and development (Xu et al , 2005 ). Cellular cholesterol is strictly controlled by multiple pathways (Chang et al , 2006 ;Goldstein et al , 2006 ).…”