Effects of glucocorticoid and cycloheximide on the activity and amount of RNA polymerase I in nuclei of rat liver

Oxytocin synthesis and secretion from bovine corpora lutea exposed in vitro to cycloheximide and colchicine

Two experiments were conducted to study the effects of cycloheximide and colchicine on prostaglandin F2 alpha (PGF2 alpha)-induced secretion and synthesis of oxytocin in bovine luteal tissue in vitro. Corpora lutea were collected from beef heifers on Day 8 of the estrous cycle. In Experiment 1, incorporation of [14C]-leucine into oxytocin synthesized and secreted by luteal slices after exposure to PGF2 alpha, cycloheximide and cycloheximide plus PGF2 alpha was examined. In Experiment 2, synthesis and secretion of oxytocin were evaluated in luteal slices incubated with colchicine and PGF2 alpha alone and in combination. Cycloheximide inhibited incorporation of labeled leucine into luteal proteins by more than 90% and no labeled oxytocin was detected in the media or tissue. Prostaglandin F2 alpha induced significant secretion of oxytocin that was not inhibited by cycloheximide. Tissue levels of oxytocin after incubation with cycloheximide and/or PGF2 alpha did not differ and were similar to those of the incubated control. Colchicine alone did not suppress oxytocin secretion and did not alter the ability of PGF2 alpha to induce significant secretion of this nonapeptide. Tissue concentrations of oxytocin after incubation with colchicine and/or PGF2 alpha did not differ. These studies indicate that secretion and replenishment of luteal oxytocin in vitro is not contingent upon de novo protein synthesis. Inability of colchicine to suppress oxytocin secretion and synthesis may have been due to the short duration of exposure of luteal tissue to the drug.

Albumin gene expression is down-regulated by albumin or macromolecule infusion in the rat

A novel feedback regulatory mechanism operating on transcription of the albumin gene is described in the rat. In 1946, it was proposed that circulating colloids, including serum albumin, may affect the synthesis and/or secretion of albumin in the liver. The molecular basis for this proposed regulation has now been investigated by adding oncotically active macromolecules to the circulation of normal or genetically albumin-deficient Nagase analbuminemic rats (NAR) and analyzing the hepatic expression of genes, including albumin after 24 h. The transcription rate of the albumin gene was higher in NAR than in normal rats and was dramatically reduced by raising serum albumin to 1.6 g/dl. Intravenous infusion of albumin into normal rats also decreased transcriptional activity of the albumin gene by 50-60%, and this decrease correlated with changes in serum colloid osmotic pressure after albumin infusion. Inhibition of albumin gene transcription was also observed upon intravenous infusion of other protein or nonprotein macromolecules, such as gamma-globulin and dextran. This down-regulation appears to control the steady-state level of albumin mRNA in the liver. Aside from a concomitant decrease in apo E gene transcription after albumin or macromolecule infusion, there was no change in the transcription rate of other genes, including those exhibiting liver-preferred or -specific expression (e.g., tyrosine amino-transferase, cytochrome P-450, alpha 1-antitrypsin, apolipoproteins A-I and B, and transferrin) or general cellular expression (e.g., alpha-tubulin, pro alpha 2 collagen, and beta-actin). Feedback regulation of albumin gene expression by serum colloids may serve as a specific homeostatic mechanism to maintain the steady-state level of total protein in the circulation.

Effect of dexamethasone on nucleolar casein kinase II activity and phosphorylation of nucleolin in lymphosarcoma P1798 cells

Ribosomal RNA (rRNA) synthesis in murine P1798 lymphosarcoma cells is reversibly inhibited by glucocorticoids. The effects of dexamethasone upon nucleolin phosphorylation and upon the amount and activity of casein kinase II have been examined. P1798 cells were exposed to 0.1 microM dexamethasone for 36 h. Cells were labeled in vivo with [32P]orthophosphate followed by immunoprecipitation with anti-nucleolin antibody. Nucleolin phosphorylation was reduced by 60% in dexamethasone-treated cells. Nucleoli were isolated and labeled with [gamma-32P]ATP in vitro. Nucleolin protein was reduced to 40% of control in nuclei from dexamethasone-treated cells. Nucleolin phosphorylation was reduced to 20% of control. Nucleolar casein kinase II activity and protein were also reduced (30-55% and 35-50% of control, respectively) by treatment with dexamethasone. Cycloheximide (10 micrograms/ml for 3 h) reduced the amount and activity of casein kinase II, but did not cause a decrease in nucleolin protein. These observations are discussed relative to the hypothesis that glucocorticoids regulate the amount or activity of proteins of short biological half-life that are involved in the regulation of rRNA synthesis.

Neither the endogenous nor a functional steroid hormone receptor binding site transactivate the ribosomal RNA gene promoter in vitro

The mammalian ribosomal RNA gene promoters exhibit a conserved sequence between positions +1 and +16 that shows a high degree of homology to the response element for glucocorticoids and progestins (GRE/PRE). These sequences bind specifically the glucocorticoid receptor and the progesterone receptor (PR) albeit with lower affinity than a canonical GRE/PRE. Because steroid hormones are known to affect expression of the ribosomal genes, we tested the influence of hormone receptors on the activity of the ribosomal RNA gene promoter in a cell-free transcription assay. Preparations of PR that induce transcription from the mouse mammary tumour virus (MMTV) promoter do not stimulate but slightly inhibit transcription from the ribosomal RNA gene promoter. This weak negative effect is not mediated through binding to the hypothetical GRE/PRE as a mutant promoter that does not bind receptor is equally repressed. Introduction of the functional MMTV GRE/PRE upstream of the basal ribosomal RNA gene promoter does not enhance its transcription in the presence of an active PR. Thus, RNA polymerase I transcription cannot be stimulated in vitro by cis elements and regulatory proteins that are active in RNA polymerase II transcription.

Synergistic stimulatory effect of glucocorticoid, EGF and insulin on the synthesis of ribosomal RNA and phosphorylation of nucleolin in primary cultured rat hepatocytes

Effects of dexamethasone, EGF and insulin on the synthesis of rRNA and phosphorylation of nucleolin in primary cultures of adult rat hepatocytes were studied. Hepatocytes were incubated for 8 h with EGF (20 ng/ml) plus insulin (0.1 microM) and/or for 20 h with dexamethasone (1 microM) before the end of incubation. The incorporation of [3H]uridine into acid-insoluble materials and the nuclear activity of RNA polymerase I were stimulated approx. 2-fold with EGF plus insulin and these were further enhanced 2-3-times by dexamethasone, although dexamethasone alone exerted no stimulation. When hepatocytes were incubated with [32P]orthophosphate, similar enhancement by these hormones was also observed in the phosphorylation of a nucleolar protein, nucleolin, which was detected by immunoprecipitation with anti-nucleolin antibodies. The amount of nucleolin was slightly increased by EGF plus insulin in the presence of dexamethasone, but scarcely changed by treatment with EGF plus insulin or dexamethasone alone. Cycloheximide inhibited RNA synthesis to a greater or lesser degree in the case of all hepatocytes which were cultured with or without these hormonal treatments. These results indicate that the in vivo effect of glucocorticoid on rRNA synthesis and nucleolin phosphorylation in liver is primarily a direct action on parenchymal cells and requires other growth factors such as EGF and insulin.

Activated ribosomal RNA synthesis in regenerated rat liver upon inhibition of protein synthesis

Cycloheximide (Cyh), administered at a dose of 5 mg/kg body wt blocks protein synthesis in normal rat liver (NRL) and regenerating rat liver (RRL). The rate of synthesis of 45S pre-rRNA in RRL, studied after RNA labelling in vivo is activated 2.8 times. Pre-r RNA synthesis in RRL is more sensitive to the stopped translation, but never falls down to the level in NRL. The major contribution to the rDNA transcription activation in RRL comes from the 20-fold increase in the number of pol I molecules engaged in the transcription, the elongation rate being 1.4-fold accelerated. Cyh quenches partially the enhanced rDNA transcription in RRL: the number of pol I molecules and their elongation rate are about 1.7-fold and 1.5-fold higher, respectively, than the corresponding values in NRL after Cyh treatment. The results show that two different mechanisms control the number and the rate of initiation and elongation of RNA polymerase I in rat liver; one of them depends on continuous protein synthesis and can be inactivated by Cyh, the other is Cyh resistant.

Response of skeletal muscle RNA polymerases I and II to tumour growth

The muscle wasting which occurs in animals bearing a transplantable tumour is accompanied by a decrease in the level of protein synthesis and a loss in RNA. This paper examines the behaviour of RNA polymerases I and II (EC 2.7.7.6) in nuclei isolated from skeletal muscle of rats bearing a Walker 256 carcinoma. Marked decreases were observed in template-engaged RNA polymerase I and II activities and in free RNA polymerase I activity. Free RNA polymerase II activity was unaltered. When assays were carried out at high (NH4)2SO4 concentration or in the presence of heparin the diminished RNA polymerase I activity was still apparent, but heparin and high ionic strength overcame the inhibition of RNA polymerase II. Loss of RNA polymerase I activity was associated with a decrease in the number of template-engaged enzyme molecules and in the polynucleotide elongation rate. The number of template-engaged RNA polymerase II molecules was unaltered by tumour growth, but the polynucleotide elongation rate was significantly reduced. No evidence was obtained for any alteration in ribonuclease activity in nuclei or whole muscles of tumour-bearing rats. These results demonstrate an effect of the tumor on transcription in skeletal muscle of its host.

Serum albumin

The liver manufactures albumin at a massive rate and decreases production in times of environmental, nutritional, toxic and trauma stress. Osmotic pressure is a basic evolutionary regulatory factor, and hormonal control over albumin production has been demonstrated. Where and why new or old albumin is degraded are questions which have not been clarified, although the vascular endothelium may well be the degradative site. Albumin is important as a transport protein, as a measure of evolution and as a model to study secretion following synthesis without the intervening steps of glycosylation. Investigations as to how this protein enters the endoplasmic membrane may well answer some of the questions concerning signal peptide insertion (288). The role of the urea cycle intermediate ornithine and its participation in polyamine synthesis, which has a positive effect on albumin synthesis, is under study. Likewise, the inverse relation between acute-phase protein synthesis and albumin synthesis regulated by interleukin 1 and other cytokines will merit further study. These are a few of the concepts which will be tested in the future.

Relationship between RNA polymerase I activity and ornithine decarboxylase in rat liver tissues

In order to examine the relationship between RNA polymerase I and ornithine decarboxylase (ODC), three lines of experiments were performed, with the following results. The glucocorticoid-induced increase of RNA polymerase I in rat liver nuclei was not abolished by administration of inhibitors of ODC synthesis and activity, namely 1,3-diaminopropane and 2-difluoromethylornithine respectively. Anti-ODC antibody did not cross-react with RNA polymerase I solubilized from rat liver nucleoli, indicating the absence of a common protein sequence in these enzymes. The ODC preparation which was treated with transglutaminase in the presence of putrescine could not stimulate the activity of RNA polymerase I in nuclei of liver and prostate. All these results suggest that the increases in ODC protein or activity are not a prerequisite to the increase in RNA polymerase I after hormonal or physiological stimuli, but rather that the increases in both enzymes are separate responses to the primary stimuli.