Side chain cleavage of hydroxylated sterols by bovine adrenal cortex mitochondria: observation of 25-hydroxylase activity during incubation at pH 7.80

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.

Activation of the orphan nuclear receptor steroidogenic factor 1 by oxysterols

Steroidogenic factor 1 (SF-1), an orphan member of the intracellular receptor superfamily, plays an essential role in the development and function of multiple endocrine organs. It is expressed in all steroidogenic tissues where it regulates the P450 steroidogenic genes to generate physiologically active steroids. Although many of the functions of SF-1 in vivo have been defined, an unresolved question is whether a ligand modulates its transcriptional activity. Here, we show that 25-, 26-, or 27-hydroxycholesterol, known suppressors of cholesterol biosynthesis, enhance SF-1-dependent transcriptional activity. This activation is dependent upon the SF-1 activation function domain, and, is specific for SF-1 as several other receptors do not respond to these molecules. The oxysterols activate at concentrations comparable to those previously shown to inhibit cholesterol biosynthesis, and, can be derived from cholesterol by P450c27, an enzyme expressed within steroidogenic tissues. Recent studies have shown that the nuclear receptor LXR also is activated by oxysterols. We demonstrate that different oxysterols differ in their rank order potency for these two receptors, with 25-hydroxycholesterol preferentially activating SF-1 and 22(R)-hydroxycholesterol preferentially activating LXR. These results suggest that specific oxysterols may mediate transcriptional activation via different intracellular receptors. Finally, ligand-dependent transactivation of SF-1 by oxysterols may play an important role in enhancing steroidogenesis in vivo.

Catalytic properties of cytochrome P-450scc purified from the human placenta: comparison to bovine cytochrome P-450scc

Cytochrome P-450scc was purified from the human placenta by extraction of mitochondria with cholate and Emulgen 911, chromatography on phenyl-Sepharose and DEAE-Sephacel, and ammonium sulphate fractionation. The catalytic properties of the purified human cytochrome P-450scc were analysed in Tween-20 micelles and compared to those of bovine adrenal cytochrome P-450scc analysed in the same system. Both enzymes had the same Km for cholesterol and were stimulated by cardiolipin when the cholesterol concentration was subsaturating. Examination of the rates of pregnenolone synthesis from 20 alpha-hydroxycholesterol, 22R-hydroxycholesterol and 20 alpha, 22R-dihydroxycholesterol by human and bovine cytochromes P-450scc revealed that the first hydroxylation (22R position) was rate-limiting for both in Tween-20 micelles. The rate of the 22R-hydroxylation was further decreased when a 20 alpha-hydroxyl group was already present on the cholesterol side-chain. The second hydroxylation occurred at about the same rate as the third hydroxylation for both enzymes. The rate of side-chain cleavage of 25-hydroxycholesterol by human cytochrome P-450scc in Tween-20 micelles was low, the highest rate being about 1% of the Vmax for cholesterol. Substrate inhibition was seen with high concentrations of 25-hydroxycholesterol. Conversion of 25-hydroxycholesterol to pregnenolone was accompanied by a build-up of products with intact side-chains, which were probably intermediates of the reaction. Side-chain cleavage of 25-hydroxycholesterol by bovine cytochrome P-450scc showed similar characteristics to the human enzyme, except that the highest velocity observed was approx. 25% of the Vmax for cholesterol. Rates of cleavage of 25-hydroxycholesterol by both enzymes were higher in dioleoylphosphatidylcholine vesicles than in Tween-20, but were still well below the Vmax for cholesterol and showed substrate inhibition. This study shows that there is close similarity in catalytic properties between human and bovine cytochromes P-450scc which suggests that the active site of the cytochrome is highly conserved.

Human placental cholesterol side-chain cleavage: enzymatic synthesis of (22R)-20 alpha,22-dihydroxycholesterol

(22R)-20 alpha,22-Dihydroxycholesterol is the second intermediate in the conversion of cholesterol to pregnenolone by cytochrome P450scc in steroidogenic tissues. We report a rapid method for the enzymatic synthesis of (22R)-20 alpha,22-dihydroxycholesterol from (22R)-22-hydroxycholesterol using mitochondria from the human placenta.

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.

Side-chain cleavage of C27-3-oxo-4-ene sterols

In this study various C27 sterols with a 3-oxo-4-ene structure were incubated with adrenal cortex mitochondrial preparations. (22R)-22-Hydroxy-4-cholesten-3-one and (20R,22R)-20,22-dihydroxy-4-cholesten-3-one were found to be converted into progesterone. This suggests the existence of a pathway for adrenal progesterone formation analogous to the normal 3 beta-hydroxy-5-ene pathways. (20S)-20-Hydroxy-4-cholesten-3-one was hydroxylated at C25. 4-Cholesten-3-one, 25-hydroxy-4-cholesten-3-one and (22S)-22-hydroxy-4-cholesten-3-one were not converted to a measurable extent. With 3-oxo-4-ene C27 sterols as substrates, the cholesterol side-chain cleaving enzyme system seems to require the presence of a 22R-hydroxyl group in the substrate. The clinical relevance of these observations is discussed.

LH-dependent steroid production and protein phosphorylation in culture of rat tumour Leydig cells

The possible cause of the decreased responsiveness of cultured Leydig cells to stimulation with LH was investigated with rat tumour Leydig cells cultured at 32 degrees C for 2 days. Pregnenolone production and phosphorylation of proteins were studied in combination with the activity of cyclic AMP-dependent protein kinase. Pregnenolone production in cultured tumour Leydig cells decreased from 45 +/- 16 on day 0 to 14 +/- 7 on day 2 (ng/60 min/mg protein) under basal conditions and from 226 +/- 108 on day 0 to 30 +/- 24 on day 2 (ng/60 min/ng protein) under LH-stimulated conditions (means +/- SD, n = 4). This decrease may be accounted for by decreased capacity of cholesterol side-chain cleavage which decreased (in the presence of 25-hydroxycholesterol) from 1.68 +/- 0.26 on day 0 to 0.74 +/- 0.48 on day 2 (microgram/60 min/mg protein; mean +/- SD, n = 4) and decreased activity of cyclic AMP-dependent protein kinase (as apparent from a direct assay of protein kinase activity and the extent of phosphorylation of LH-dependent phosphoproteins).

Subcellular localization of LH-dependent phosphoproteins and their possible role in regulation of steroidogenesis in rat tumour Leydig cells

Stimulation of rat tumour Leydig cells with LH resulted in phosphorylation of 7 proteins of 17, 22, 24, 33, 43, 57 and 76 kDa, and in dephosphorylation of a single protein of 20 kDa. The subcellular localization of these LH-dependent phosphoproteins in combination with effects of inhibitors of microfilament formation and protein synthesis, suggest that phosphoproteins of 20, 43 and 76 kDa present in the cytosol may be involved in the action of microfilaments, whilst phosphoproteins of 24 and 33 kDa present in microsomes may be involved in specific protein synthesis.