[Control of circadian activity of cholesterol-7 alpha-hydroxylase by glucocorticosteroids]

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.

Cell and molecular biology of the assembly and secretion of apolipoprotein B-containing lipoproteins by the liver

Triglycerides are one of the most efficient storage forms of free energy. Because of their insolubility in biological fluids, their transport between cells and tissues requires that they be assembled into lipoprotein particles. Genetic disruption of the lipoprotein assembly/secretion pathway leads to several human disorders associated with malnutrition and developmental abnormalities. In contrast, patients displaying inappropriately high rates of lipoprotein production display increased risk for the development of atherosclerotic cardiovascular disease. Insights provided by diverse experimental approaches describe an elegant biological adaptation of basic chemical interactions required to overcome the thermodynamic dilemma of producing a stable emulsion vehicle for the transport and tissue targeting of triglycerides. The mammalian lipoprotein assembly/secretion pathway shows an absolute requirement for: (1) the unique amphipathic protein: apolipoprotein B, in a form that is sufficiently large to assemble a lipoprotein particle containing a neutral lipid core; and, (2) a lipid transfer protein (microsomal triglyceride transfer protein-MTP). In the endoplasmic reticulum apolipoprotein B has two distinct metabolic fates: (1) entrance into the lipoprotein assembly pathway within the lumen of the endoplasmic reticulum; or, (2) degradation in the cytoplasm by the ubiquitin-dependent proteasome. The destiny of apolipoprotein B is determined by the relative availability of individual lipids and level of expression of MTP. The dynamically varied expression of cholesterol-7alpha-hydroxylase indirectly influences the rate of lipid biosynthesis and the assembly and secretion lipoprotein particles by the liver.

Regulation of bile acid biosynthesis

This article provides a review of the pathways through which cholesterol is degraded to bile acids. Regulation of key enzymes in the bile acid biosynthestic pathways is discussed. The important role of these pathways in the maintenance of cholesterol homeostasis and the possible therapeutic implications for the treatment of hypercholesterolemia are emphasized.

Circadian synchronization of liver regeneration in adult rats: the role played by adrenal hormones

The role played by the adrenal hormones in the regulation of liver proliferation in adult rats was investigated under various experimental conditions. In untreated control groups, cell growth was very low and endogenous corticosterone levels showed a clearly-defined circadian rhythm with a peak in the evening. Adrenalectomy depressed the level of endogenous corticosterone immediately and the growth rate of the liver increased significantly. We were able to prevent this effect by repeated injections of corticosterone at physiological doses. After a 1/3 hepatectomy and a sham-operation, the corticosterone blood level maintained its normal circadian pattern with the exception of a transient increase during the first two post-operative hours. After a hepatectomy of this kind, a negative correlation was found to exist between the adrenal hormone level and the waves of DNA synthesis; the subsequent mitoses appeared in two successive circadian waves of decreasing amplitude, a maximum value being reached in the morning. In rats submitted to a 1/3 hepatectomy and an adrenalectomy simultaneously, the endogenous corticosterone level fell significantly after a post-operative peak. The regenerating pattern was completely different from that induced by 1/3 hepatectomy alone. The rise in the labelling index began earlier and rose to significantly higher values; it was then followed by a single large mitotic wave without any circadian rhythm. These results favour the hypothesis that adrenal hormones have a significant effect on the negative control of liver regeneration. Circadian changes in the corticosterone level were responsible for the nycthemeral pattern observed in the regenerating liver after a partial hepatectomy. The results show a marked inhibition of the G1-S transition, particularly in the evening, when the endogenous corticosterone concentration was at its highest. Also discussed is the relationship between corticoids and 'chalones', which synergetically inhibit the passage from G0 into the cell cycle.

Simultaneous measurement of cholesterol 7 alpha-hydroxylase activity by reverse-phase high-performance liquid chromatography using both endogenous and exogenous [4-14C]cholesterol as substrate

The HPLC-spectrophotometric method (T. Ogishima and K. Okuda (1986) Anal. Biochem. 158, 228-232) for measuring cholesterol 7 alpha-hydroxylase activity was modified by using a C-18 reverse-phase column to separate 7 alpha-hydroxy-4-cholesten-3-one and 4-cholesten-3-one and by adding 7 beta-hydroxycholesterol to each reaction mixture as an internal recovery standard. With this method, we were able to simultaneously measure cholesterol 7 alpha-hydroxylase activity using endogenous cholesterol and exogenous [4-14C]cholesterol as substrate. Rat liver cytosol differentially stimulated (286%) the 7 alpha-hydroxylation of exogenous [4-14C]-cholesterol. In contrast, total cholesterol 7 alpha-hydroxylase activity was stimulated only 35% by cytosol. This method should prove useful for studying mechanisms of cholesterol delivery to cholesterol 7 alpha-hydroxylase.

Dexamethasone regulates bile acid synthesis in monolayer cultures of rat hepatocytes by induction of cholesterol 7 alpha-hydroxylase

To study the effect of steroid hormones on bile acid synthesis by cultured rat hepatocytes, cells were incubated with various amounts of these compounds during 72 h and conversion of [4-14C]cholesterol into bile acids was measured. Bile acid synthesis was stimulated in a dose-dependent way by glucocorticoids, but not by sex steroid hormones, pregnenolone or the mineralocorticoid aldosterone in concentrations up to 10 microM. Dexamethasone proved to be the most efficacious inducer, giving 3-fold and 7-fold increases in bile acid synthesis during the second and third 24 h incubation periods respectively, at a concentration of 50 nM. Mass production of bile acids as measured by g.l.c. during the second day of culture (28-52 h) was 2.2-fold enhanced by 1 microM-dexamethasone. No change in the ratio of bile acids produced was observed during this period in the presence of dexamethasone. Conversion of [4-14C]7 alpha-hydroxycholesterol, an intermediate of the bile acid pathway, to bile acids was not affected by dexamethasone. Measurement of cholesterol 7 alpha-hydroxylase activity in homogenates of hepatocytes, incubated with 1 microM-dexamethasone, showed 10-fold and 90-fold increases after 48 and 72 h respectively, as compared with control cells. As with bile acid synthesis from [14C]cholesterol, no change in enzyme activity was found in hepatocytes cultured in the presence of 10 microM steroid hormones other than glucocorticoids. Addition of inhibitors of protein and mRNA synthesis lowered bile acid production and cholesterol 7 alpha-hydroxylase activity and prevented the rise of both parameters with dexamethasone, suggesting regulation at the mRNA level. We conclude that glucocorticoids regulate bile acid synthesis in rat hepatocytes by induction of enzyme activity of cholesterol 7 alpha-hydroxylase.

Differences in the biochemical properties of aldrin epoxidase, a cytochrome P-450-dependent monooxygenase, in various tissues

Aldrin epoxidase, a cytochrome P-450-dependent monooxygenase, was studied in the lung and kidney of male rats. The sensitivity of the liver enzyme activity to different chemicals in vitro was influenced by the treatment of the animals with phenobarbital or methylcholanthrene. These results confirm that more than one form of cytochrome P-450 supports aldrin epoxidase in the liver. The lung and kidney aldrin epoxidase activity was not modified by the administration of chemical inducers to the rats. In vitro, the lung and kidney aldrin epoxidase activities were activated by tetrahydrofurane and progesterone, respectively. The results obtained from the lung and kidney indicate that one single species of cytochrome P-450, associated with aldrin epoxidase, exists in these organs, but it may be a different type, or regulated in a different manner in these tissues.

Correlation of liver growth and function during liver regeneration and hepatocarcinogenesis

We have previously studied some parameters of rat liver activity and compared the kinetics of cell proliferation (normal growth or after partial hepatectomy) with some specific hepatic enzymes. The mutually exclusive relationship between division and tissue function, their specific circadian rhythm as well as the "chalone effect" have been used to characterize the normal homeostatic regulatory mechanism in the liver. The same parameters have been recently determined during chemical carcinogenesis. Adult rats, fed long term with diethylnitrosamine (DENA, 10 mg/kg/day) develop liver carcinoma after 90 days of carcinogen administration. The results show that the relationship between the above parameters is progressively disturbed during the second month of treatment. A minimum of 4 weeks of continuous DENA feeding is found to be necessary for the induction of liver cancers. Giving the carcinogen for a second month decreases the delay before death with cancer. Protracting the treatment after the second month has no further effect either on survival or on cancer induction. The mechanism of carcinogenesis is explained by postulating that preneoplastic lesions evolution would closely depend on the homeostatic control disturbances.

Sex-related differences in diurnal activities and development of hepatic microsomal 3-hydroxy-3-methylglutaryl coenzyme A reductase and cholesterol 7alpha-hydroxylase

The activities of hepatic microsomal 3-hydroxy-3-methylglutaryl CoA reductase and cholesterol 7alpha-hydroxylase were consistently higher (up to 3-fold) in female compared to male rats fed 2% cholestyramine for 8 h daily. In all animals studied, enzymic activities were highest 6 h after feeding began. However, 85% of the rise in cholesterol 7alpha-hydroxylase activity occurred in the 6 h before and 89% of the rise in 3-hydroxy-3-methylglutaryl CoA reductase activity occurred in the 6 h after feeding started. Sex-related differences in both enzymic activities first became apparent at the time of sexual maturity. Enzymic activities before weaning were generally low and a late-suckling (13--20 days) rise in cholesterol 7alpha-hydroxylase was not accompanied by a rise in 3-hydroxy-3-methylglutaryl CoA reductase. For all of these studies we assayed cholesterol 7alpha-hydroxylase at two concentrations of exogenous cholesterol to obviate problems relating to size of the cholesterol pool.

Rat-liver cholesterol 7 alpha-hydroxylase. 1. Development of a new assay based on the enzymic exchange of the tritium located on the 7 alpha position of the substrate

A new assay is described to measure the activity of cholesterol 7alpha-hydroxylase and compared to the conventional 14C method used by other investigators. This method is based on the mechanism of the enzymic hydroxylation, i.e. a direct and stereospecific substitution of the 7alpha-hydrogen by a hydroxyl group. [7alpha-3H]Cholesterol is incubated at 37 degrees C and in the presence of molecular O2, in a medium buffered by postassium phosphate at pH 7.4 and containing liver microsomes (or 9000 X g supernatant), NADPH, MgCl2 and cysteamine. Tween-80 (1.5 mg/ml) is used to introduce enough substrate (300 muM) in the incubation mixture to saturate the enzyme (Km = 100 muM). Under these conditions the tritiated water released into the incubation medium reflects accurately the enzymic activity. The results obtained with this method are similar to the one obtained with a [4-14C]cholesterol technique (r = 0.96; P less than 0.001). The main advantage of the [7alpha-3H]cholesterol method is a complete independence from further metabolism of the first enzymic product, the 7alpha-hydroxycholesterol, the tritiated water representing the entire cholesterol 7alpha-hydroxylase activity.

Rat-liver cholesterol 7alpha-hydroxylase. 3. New results about its circadian rhythm

The establishement of the circadian rhythm of cholesterol 7alpha-hydroxylase activity requires protein and RNA synthesis. The spontaneous decrease of the enzymic activity, at the end of the night, allows us to evaluate a half-life time of about two hours. The half-life time goes up to about four hours when the enzymatic activity decay is measured following cycloheximide administration. This difference suggests that an active mechanism is involved in the control of the enzyme degradation. The daily variation of the enzyme activity is regulated via the hypothalamo-hypophysis-adrenal axis. At the cellular level glucocorticoids are the most likely responsible agent. The hepatic cholesterol 7alpha-hydroxylase variations always parallel the plasmatic corticosterone concentration fluctuations, the latter being by far the most important adrenocortical excretion product. These two rhythms are modified in a similar manner under different physio-pathological conditions, such as the inversion of lighting in the animal room or the inversion of feeding time. Of these two parameters, the moment of food intake is the most important and accounts for the synchronisation of the rhythm in the animals. The rhythm is retained after several days of starvation but its amplitude decreases and the individual variations among the animals increase significantly at each time point.

Cholesterol 7alpha-hydroxylase. 2. Biochemical properties and participation of endogenous cholesterol in the assay in vitro

70% of the microsome-bound cholesterol is directly accessible to cholesterol 7alpha-hydroxylase in an assay in vitro. After 5 min of incubation this endogenous cholesterol makes a single pool with the exogenously added substrate and modifies its specific radioactivity. Thus, an accurate estimation of the enzymic activity should take the participation of endogenous cholesterol into account. Cholesterol 7alpha-hydroxylase activity is enhanced in vitro by thiol-containing substances like mercaptoethanol, dithiothreitol, or cysteamine. On the contrary, the enzymic activity is inhibited by heavy cations (Hg2+, Pb2+, Cu2+, Zn2+), or --SH-blocking agents (mersalic acid p-chloro-mercuribenzoic acid). Several steroids are potent inhibitors (Ki less than Km) of the enzyme, among them pregnenolone and its acetate derivative and the cholesterol closely related 7-oxocholesterol and 7-dehydrocholesterol. The cholesterol esters are neither substrates nor inhibitors of cholesterol 7alpha-hydroxylase. Only a high concentration (1 mM) of biliary acids or of their glyco or tauro derivatives inhibits cholesterol 7alpha-hydroxylase. The quantitatively less important lithocholic acid and deoxycholic acid are the strongest inhibitors; the most common cholic acid does not affect the enzymic activity even at 1 mM.