Glucocorticoid regulation of transcription: the role of physiologic concentrations of adrenal glucocorticoids in the diurnal variation of rat liver chromatin template availability

Stimulation of protein and RNA synthesis by methylmercury chloride in the liver of intact and adrenalectomized rats

(1) A single injection of methylmercury chloride in the rat (10-50 mg/kg) increased both in vivo and in vitro rates of 14C-leucine incorporation into the protein of the post-mitochondrial supernatant fraction of the liver. In contrast, no stimulation of protein synthesis was observed in the brain of the methylmercury-treated rats. (2) Methylmercury administration also stimulated RNA polymerase activities in isolated hepatic nuclei, stimulation of Mg-dependent activity being higher than that of Mn-dependent activity. (3) In experiments with adrenalectomized rats, it was found that the stimulatory effect of methylmercury on protein and RNA synthesis in the liver was mediated partly through the adrenal gland. (4) Analysis of serum by starch-block electrophoresis revealed that synthesis of all serum proteins, including albumin and alpha-gamma globulin fractions, was stimulated by methylmercury treatment. (5) These results suggest that the observed effects of methylmercury on the liver depend on mechanisms other than enhancement of the synthesis of acute-phase proteins.

Control of RNA content of developing human placenta

Human placentae obtained early in pregnancy or at full term were examined for RNA content per cell, RNA polymerase types and activities, and chromatin template availability. The RNA:DNA ratio fell from 0.7 at 15 to 20 weeks to 0.4 at 40 weeks of pregnancy. Since RNase activities were similar at both times, the reduction in RNA content was attributed not to increased degradation, but to reduced synthesis. At both stages of pregnancy, about 55 to 60 per cent of the RNA polymerase activity in isolated placental nuclei was accounted for by RNA polymerase II, as judged by suppression of activity with alpha-amanitin and by separation of the extracted polymerases on DEAE-Sephadex. The relative roles of changes in polymerase activity and template availability were measured in nuclei from 20- and 40-week placentae. Nuclei showed 20 per cent greater polymerase activity in full-term than in early placentae, but the template availability of isolated chromatin for transcription by RNA polymerase II was 70 per cent less at full term. We conclude that the reduced amount of RNA per cell in the full-term placenta is due to reduced template availability that more than offsets the slight increase in polymerase activity.

Circadian rhythm of cholesterol-7alpha-hydroxylase and cortisol in the African green monkey (Cercopithecus aethiops)

Cholesterol-7alpha-hydroxylase in African green monkey liver had an apparent Km of 1.65-10(-4) M cholesterol and a pH optimum of 7.4. The amplitudes of the circadian maxima of enzyme activity and serum cortisol levels were significantly greater in vervets than in grivets. Fluctuations in enzyme activity and cortisol levels during the circadian cycle were positively correlated (r = 0.89). Enzyme activities and hormone levels were 2.7-fold lower over a 24-h period in the grivet than in the vervet. Cholesterol feeding reduced the enzyme activity by 40% and serum cortisol was reduced to 38% of control levels at the diurnal peak. Serum glucocorticoids may be important physiological regulators of cholesterol-7alpha-hydroxylase in non-human primates. The concentration of cortisol and its time of release appear to be factors in the hyperresponsive trait of grivets. Genetic differences between vervet and grivet races may account for differences in the amplitude and timing of the circadian rhythm of cholesterol-7alpha-hydroxylase activity possibly influenced by cortisol.

Genetic and hormonal regulation of steroid hydroxylases and drug metabolizing enzymes in rat liver

Two microsomal steroid hydroxylase activities (cholesterol-7alpha-hydroxylase and progesterone-16alpha-hydroxylase) were measured in the livers of Sprague-Dawley and Wistar rats and compared to three other monooxygenase activities (aryl hydrocarbon hydroxylase, p-nitro-anisole-O-demethylase and aminopyrine-N-demethylase). Cholesterol-7alpha-hydroxylase behaves in a very unique manner. It is the only one of the studied enzymes to be more active in the female than in the male, it is very poorly induced by phenobarbital and methylcholanthrene, but responds quickly to the administration of glucocorticoids. In fact, the cholesterol-7alpha-hydroxylase activity presents a very pronounced circadian rhythm which is under the control of the hypothalamo-adrenal axis. Marked differences are also found in the response of the various enzymatic activities to the administration of inducers as well as in their relative activities in untreated male and female animals.

Diurnal variations in endogenous RNA polymerase activity and amounts of nuclear non-histone protein, DNA and cytoplasmic protein in rat liver

From rats fed ad libitum and kept under a 12 + 12 h light/dark regimen, the DNA dependent RNA polymerase activity of liver cell nuclei was determined avery four hours. From identical rats, nuclear non-histone protein and DNA, and cytoplasmic protein was determined by Feulgen-Naphthol Yellow S cytophotometry of isolated liver cells. The minimum: maximum ratio of the RNA polymerase activity is 0.77; the min:max ratio of nuclear non-histone protein is 0.84. These two parameters have identical time courses with a gradual decline during the light period and a sharp rise after the onset of the dark period. The variations in nuclear DNA content, estimated as the amount of Feulgen stain bound, closely parallel those of the RNA polymerase activity and nuclear non-histone protein content (min:max = 0.96). The amount of cytoplasmic protein per cell also varies throughout the day, but its time curve lags behind those of nuclear non-histone content and RNA polymerase activity. These results are consistent with the concept of nuclear non-histone proteins as de-repressors of the DNA template in differentiated, non-proliferating cells, and support the validity of using Feulgen-Naphthol Yellow S cytophotometry of nuclear non-histone proteins as an estimate of gene expression in such cells.