Taurocholate inhibits the glucocorticoid-induced rise of 3-hydroxy-3-methylglutaryl-CoA reductase in primary culture of hepatocytes

Negative regulation by dexamethasone of fluvastatin-inducible CYP2B expression in primary cultures of rat hepatocytes: role of CYP3A

Fluvastatin (Fluva), a synthetic inhibitor of 3-hydroxy-3-methylglutaryl coenzyme A reductase, induces CYP2B1/2 in rat liver and primary cultured rat hepatocytes. However, the overall profile of CYP induction, which includes induction of CYP4A, suggests that Fluva is not a typical "phenobarbital (PB)-like" inducer. Several treatments affecting diverse cell signaling pathways have been reported to modify PB-inducible CYP2B expression in primary cultured rat hepatocytes. We examined the effects of selected treatments on the ability of Fluva to induce CYP2B1/2 mRNA. Only dexamethasone (Dex) produced effects on Fluva-inducible CYP2B1/2 mRNA expression that differed from those produced on PB-inducible CYP2B1/2 mRNA expression. Dex concentrations up to 10(-7) M of potentiated PB (10(-4) M)-mediated CYP2B1/2 mRNA induction, while higher Dex concentrations produced a progressive reduction in PB-induced CYP2B1/2 mRNA levels. By contrast, Dex concentrations up to 10(-8) M had no effect on Fluva (3 x 10(-5) M)-induced CYP2B1/2 mRNA levels, while Dex concentrations of 10(-7) M and higher markedly suppressed Fluva-mediated CYP2B1/2 mRNA induction. The concentrations of several glucocorticoids that produced suppression of Fluva-induced CYP2B1/2 mRNA levels were the same concentrations that induced CYP3A mRNA. Treatment with pregnenolone 16 alpha-carbonitrile also produced a concentration-dependent suppression of Fluva-induced CYP2B1/2 mRNA levels. Dex-mediated suppression of Fluva-induced CYP2B1/2 mRNA was concentration-dependently reversed when hepatocytes were cotreated with troleandomycin, a selective CYP3A inhibitor. The amounts of Fluva detected in culture medium and cells were reduced significantly when hepatocytes were incubated with Dex. However, Dex-mediated suppression of Fluva-induced CYP2B1/2 mRNA expression was not overcome when hepatocytes were incubated with Fluva concentrations greater than 3 x 10(-5) M, suggesting that mechanisms other than CYP3A-catalyzed metabolism may contribute to Dex-mediated suppression of Fluva-induced CYP2B1/2 expression.

The enzyme inducers 3-methylcholanthrene and phenobarbital affect the activities of glucocorticoid hormone-regulated enzymes in rat liver and kidney

3-Methylcholanthrene, an inducer of P448-type cytochromes (mostly 1A1 and 1A2), and phenobarbital, an inducer of P450-type cytochromes (mostly 2B1 and 2B2), are prototypical for the actions of many xenobiotics. They cause endocrine disruption by affecting, among others, steroid hormone levels. Rats were treated with single bolus doses of 3-methylcholanthrene or phenobarbital, and enzyme activities that are controlled by glucocorticoids were measured in liver and kidney. The activities of the cytosolic enzymes L-alanine aminotransferase, indoleamine 2,3-dioxygenase (L-tryptophan pyrrolase), phosphoenolpyruvate carboxykinase, L-serine dehydratase and L-tyrosine aminotransferase were affected in a similar fashion: an initial activity reduction followed by two overshoots of activity 1 and 2 days after dosing. 3-Hydroxy-3-methylglutaryl coenzyme A reductase, the microsomal key enzyme of sterol synthesis, responded with a temporary reduction of activity only and evidently lost its diurnal rhythm. The time course of these changes is most likely caused by a combination of sub-physiological levels of glucocorticoids plus changes of other regulatory hormones elicited by feed intake, postprandial state, etc. A possible role for a combined action of the arylhydrocarbon (Ah) and glucocorticoid receptors in the effects of 3-methylcholanthrene is also suggested.

Influence of hormones and growth factors on viability, DNA, and protein content of adult hepatocytes in primary culture

The survival of adult rat hepatocytes in monolayer culture was studied in the presence of different hormones (neurotensin, oxytocin, thyrotropin releasing hormone, luteinizing hormone releasing hormone, cholecalciferol, bradykinin, substance P, aldosterone, melanocyte stimulating hormone, 3,3',5-triiodo-1-thyronine, corticosterone, human growth hormone, glucagon, insulin, progesterone, testosterone, estradiol, and dexamethasone phosphate) or growth factors (fetal bovine serum). For this purpose trypan blue exclusion, lactate dehydrogenase, and DNA and protein content were measured at 24 and 72 h of culture. 10(-7) M Dexamethasone, a mixture of eight hormones, 10% fetal bovine serum, and a combination of the latter two supplements caused a more than 64% higher DNA content at 72 h when compared to control cultures. A striking agreement of these results with changes of lactate dehydrogenase leakage was observed, whereas trypan blue exclusion gave erratic results. Considerable changes of cell arrangement apparently specific for each supplement were observed by low magnification microscopy. It is concluded that glucocorticoids and fetal bovine serum have an outstanding effect on cell viability and that DNA or protein content or both are reliable indicators of cell viability in amitotic cultures.

Regulation and interaction of cholesterol, bile salt and lipoprotein synthesis in liver

The liver is the junction of several inter-organ metabolic cycles which are essential for the homeostasis of mammalian metabolism. Two of these are described in greater detail and their role in control of lipid metabolism will be presented. The fatty acid-triglyceride cycle is of particular importance for our understanding of the mechanisms governing serum lipid levels. This is due to the fact that the lipoprotein secreted by the liver in the course of this metabolic cycle - very low density lipoprotein - has a relatively long half-life in the plasma compartment. Data have been collected from the literature to show that different nutritional and pharmacological stimuli affecting serum lipid levels do so by interfering with the rate of very low density lipoprotein input into the plasma compartment. The enterohepatic circulation of steroids is another cycle which contributes to control of lipid metabolism. Data are presented which show that bile acids, the major steroids circulating in this cycle, exert direct feedback control of hepatic cholesterol synthesis. This characteristic of bile acids may explain why certain bile acids, when given orally, reduce serum cholesterol levels. Several clinical and experimental observations suggest a close relation between bile acid and triglyceride metabolism. It is characterized by an inverse relation between bile acid pool size and serum triglyceride levels. Moreover, a reduction of the bile acid pool size is accompanied by an enhanced hepatic fatty acid and triglyceride synthesis and secretion into blood. The molecular basis and physiological significance of these observations have still to be explored.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of dexamethasone on 3-hydroxy-3-methylglutaryl-coenzyme A reductase activity and cholesterol synthesis in rat liver

Dexamethasone increases reductase activity in cultured liver cells after a lag period of 2 h. The increases of activity are linear from 10(-9) to 10(-5) M dexamethasone, the maximum responses ranging from 2- to 4-fold. The increased reductase activity after dexamethasone treatment is not due to a change of the state of phosphorylation/dephosphorylation of the enzyme nor to an increase of cytosolic activating factor(s) for the reductase. Cholesterol synthesis, measured by incorporation of either [14C]acetate or 3H2O, increases 3-fold after dexamethasone (10(-6) M) treatment, as does the hydroxymethylglutaryl-CoA reductase activity, confirming that this enzyme is rate-controlling for cholesterol synthesis in cultured liver cells as it is in vivo. Dexamethasone (10 micrograms/100 g rat), given after onset of the light cycle, increases reductase activity over control rats at the nadir of the circadian cycle of this enzyme. When given after onset of the dark cycle, dexamethasone does not increase reductase activity over controls at the peak of their circadian cycle. Thus, physiologic doses of glucocorticoids partially reverse the decline in reductase activity due to the circadian rhythm.