Control of activation of liver RNA polymerase I occurring after re-feeding of protein-depleted mice

Transcription of liver ribosomal RNA: differential effect of 5-fluorouracil depending on the nutritional state of mice

The effect of 5-fluorouracil (5-FUra) on RNA transcription in mice liver cells was studied using animals exposed to different nutritional conditions as a model of normal, nondividing cells. There are two levels of rRNA transcription in mouse liver: a basal level found in mice fed on a complete diet (control mice) and a higher level, two- to three-fold increased over the basal, found in mice fed on a protein-depleting diet for about 3 days and refed on a complete diet for at least 5 hr (refed mice). The rRNA transcription was measured as the activity of RNA polymerase I in isolated liver nuclei. It was found that the intraperitoneal administration of 5-FUra (30 mg/kg body wt) to refed mice rapidly decreases the higher level of rRNA transcription towards the basal level. This is accomplished through a decrease of the initiation frequency of rRNA chains by RNA polymerase I. 5-FUra however, does not affect the basal level of rRNA transcription in liver from mice fed on a complete diet. Under this condition the drug does not affect the initiation frequency of rRNA chains. The effect of 5-FUra on rRNA transcription in refed mice is not mediated by an inhibition of protein synthesis.

Increased formation and keeping of ribosomes during dietary recovery of mouse kidney

In the kidney of 5-day protein-depleted mice there is a decrease of 23% in the rRNA mass. When these animals are fed with a complete diet, rRNA content is restored to its normal value after 24 h of refeeding. The mechanisms that underlie this phenomenon were studied. It was found that the activity of rRNA polymerase I in the nuclei of kidneys from refed mice showed an increase of about twofold compared with the activity in normal and protein-depleted nuclei. The in vivo incorporation of a large dose (nontrace) of [14C]orotic acid into rRNA was also twofold enhanced in kidneys from refed mice. Ribosome degradation (measured by the disappearance of radioactivity from either ribosomal proteins or rRNA previously labeled by the injection of NaH14CO3 and [14C]orotic acid to the mice, respectively) stopped during the 1st day after refeeding. The estimation of the difference between the rRNA synthesis rate and the net rRNA increase also demonstrated a decrease in the rRNA degradation rate in refed mice.

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

The activity of the template-engaged form of RNA polymerase I from livers of adrenalectomized rats was about 50-60% of that of normal control rats, and increased about 2-fold at 6 h after the administration of dexamethasone. However, no change was found in the activity of the 'free' form of RNA polymerase I or the template-engaged form of RNA polymerase II. Immunochemical studies using guinea-pig anti-(RNA polymerase I) serum disclosed that the total number of RNA polymerase I molecules did not vary during the treatment with dexamethasone. Cycloheximide caused a rapid decrease in the template-engaged form of RNA polymerase I activity in normal rats and in dexamethasone-treated (6 h) adrenalectomized rats, to the value in adrenalectomized rats, but affected it only slightly in adrenalectomized rats. The elongation rate of rRNA-precursor synthesis in liver nuclei was not affected by a change in the concentration of circulating dexamethasone. From these results, it is concluded that about half the rRNA-precursor synthesis in rat liver is regulated by glucocorticoids, probably through the synthesis of short-lived protein(s) which may play a role in conversion of the 'dormant' form of RNA polymerase I into the 'engaged' form.

Nuclear RNA polymerase activity in tumor-host livers

This study has evaluated changes in RNA synthesis in livers under the distant influence of a malignant tumor. A transplantable-induced sarcoma (MCG 101), transplanted on inbred adult mice (C57BL/6J), was used. Activities of DNA-dependent RNA polymerase (EC 2.7.7.6) were measured in relation to RNA content and translational activity. Liver nuclei from freely fed sarcoma-bearing mice had increased RNA synthesis. As a consequence of this, RNA content per DNA was increased in liver tissue. This was independent of depressed food intake and malnutrition. Elevated RNA synthesis, proportional to the tumor burden was due to an increased proportion of chromatin-engaged RNA polymerase I and II activities. RNA polymerase III activity (template-engaged form) was unchanged when evaluated in isolated nuclei, but appeared to be increased in partially purified extracts of nuclei. RNA content in tumor-host liver was a composite of increased levels of rRNA and tRNA, whereas the levels of poly(A)+ mRNA could not be measured as increased. Overall translational activities in vitro of mRNA from liver tissue of tumor-bearing, pair-weighed, and freely fed tumor-free controls were qualitatively and quantitatively different. mRNA from tumor-bearing mice directed an increased synthesis, particularly of larger proteins (above 55,000 daltons) compared with control animals. The results support the conclusion that previous evidence of elevated net protein synthesis in tumor-host liver is accompanied by increased transcription of genes coding for RNA and also for some or several hepatic proteins.

Transcription of ribosomal RNA is differentially controlled in resting and growing BALB/c 3T3 cells

Shortly after serum-deprived BALB/c 3T3 fibroblasts are stimulated to grow in medium containing 10% calf serum, the RNA polymerase I activity in permeabilized cells shows a two-fold increase over the values observed in either serum-deprived or density-inhibited resting cells. Inhibition of protein synthesis by pactamycin or cycloheximide specifically reduces the enhanced RNA polymerase I activity in serum-stimulated cultures without affecting the values in resting cells. On the other hand, inhibition of rRNA processing by the nucleoside analogs 5-fluoruridine and toyocamycin decreases the rate of 45S rRNA transcription in serum-stimulated cells but has no effect on the values found in resting cultures. These data suggest that the regulation of rRNA transcription occurs by two different mechanisms, depending on the growth state of the cell. One mechanism, in serum-stimulated cells, is dependent on a continuous protein synthesis and a correct 45S rRNA processing; the other, in resting cells, is independent of these two parameters.