Metabolism of hydroxy sterols by rat liver

The role of cytochrome P450 in the regulation of cholesterol biosynthesis

A ubiquitously expressed member of the cytochrome P450 superfamily, CYP51, encodes lanosterol 14alpha-demethylase, the first step in the conversion of lanosterol into cholesterol in mammals. The biosynthetic intermediates of lanosterol 14alpha-demethylation are oxysterols, which inhibit HMG-CoA reductase and sterol synthesis in mammalian cells in vitro. These oxysterols (5alpha-lanost-8-en-3beta,32-diol and 3beta-hydroxy-5alpha-lanost-8-en-32-al) are efficiently converted into cholesterol in vitro and are generally considered to be natural cholesterol precursors. When added to hepatocytes in high concentrations, besides their conversion into cholesterol, they are also rapidly metabolized into more polar sterols and into steryl esters. The 15alpha- and 15beta-hydroxy epimers of 5alpha-lanost-8-en-3beta-ol are also rapidly metabolized into more polar sterols and steryl esters but are not converted efficiently into cholesterol. Polar sterol formation from all these oxysterols is dependent on an active form of cytochrome P450. Oxysterols are potent regulators of the activities of transcription factors of the sterol regulatory element-binding protein family and of liver X-receptor alpha. It is proposed that the rapid, cytochrome P450-dependent metabolism of naturally occurring regulatory oxysterols provides a route for their deactivation so that they become incapable of affecting gene transcription. Inhibition of cytochrome P450 by the drug ketoconazole prevents the inactivation of such oxysterols, leading to a prolonged suppression of hepatic HMG-CoA reductase in vivo and in vitro.

Oxysterols: modulators of cholesterol metabolism and other processes

Oxygenated derivatives of cholesterol (oxysterols) present a remarkably diverse profile of biological activities, including effects on sphingolipid metabolism, platelet aggregation, apoptosis, and protein prenylation. The most notable oxysterol activities center around the regulation of cholesterol homeostasis, which appears to be controlled in part by a complex series of interactions of oxysterol ligands with various receptors, such as the oxysterol binding protein, the cellular nucleic acid binding protein, the sterol regulatory element binding protein, the LXR nuclear orphan receptors, and the low-density lipoprotein receptor. Identification of the endogenous oxysterol ligands and elucidation of their enzymatic origins are topics of active investigation. Except for 24, 25-epoxysterols, most oxysterols arise from cholesterol by autoxidation or by specific microsomal or mitochondrial oxidations, usually involving cytochrome P-450 species. Oxysterols are variously metabolized to esters, bile acids, steroid hormones, cholesterol, or other sterols through pathways that may differ according to the type of cell and mode of experimentation (in vitro, in vivo, cell culture). Reliable measurements of oxysterol levels and activities are hampered by low physiological concentrations (approximately 0.01-0.1 microM plasma) relative to cholesterol (approximately 5,000 microM) and by the susceptibility of cholesterol to autoxidation, which produces artifactual oxysterols that may also have potent activities. Reports describing the occurrence and levels of oxysterols in plasma, low-density lipoproteins, various tissues, and food products include many unrealistic data resulting from inattention to autoxidation and to limitations of the analytical methodology. Because of the widespread lack of appreciation for the technical difficulties involved in oxysterol research, a rigorous evaluation of the chromatographic and spectroscopic methods used in the isolation, characterization, and quantitation of oxysterols has been included. This review comprises a detailed and critical assessment of current knowledge regarding the formation, occurrence, metabolism, regulatory properties, and other activities of oxysterols in mammalian systems.

Oxidative metabolism of cholesterol precursors: sensitivity to ketoconazole, an inhibitor of cytochrome P-450

The effects of ketoconazole, an inhibitor of cytochrome P-450, on the metabolism of the cholesterol precursors lanosterol, dihydrolanosterol, lanost-8-en-3 beta,32-diol, and 3 beta-hydroxylanost-8-en-32-al were investigated in subcellular fractions of rat liver and in rat hepatocytes in culture. At low (1-2 microM) concentrations of the drug, the oxidative demethylation of lanosterol was inhibited by about 70% in the subcellular fractions but there was no effect on the metabolism of the 3 beta, 32-diol or the 32-aldehyde. Higher drug concentrations (10-20 microM) were required to inhibit the oxidative metabolism of these cholesterol precursors. Similar results were obtained during longer-term incubations using hepatocytes in culture medium, but higher concentrations of ketoconazole were required to effect the same degree of inhibition of each precursor. In the subcellular fractions, dihydrolanosterol, the 3 beta,32-diol and the 32-aldehyde were each metabolized to more polar sterols, in addition to cholesterol. Ketoconazole also inhibited the formation of these polar substances.

Cholesterol autoxidation 1981-1986

Literature published between 1980 and 1986 dealing broadly with the topic of cholesterol autoxidation is reviewed. The review builds on the detailed 1981 monographic treatment of the topic by the author and covers new items of chemistry, analysis, and metabolism.