Increases in the activity of the solubilized rat liver nuclear RNA polymerases following partial hepatectomy

Kinetic parameters of Thymidine kinase and DNA synthesis during liver regeneration: role of thyroid hormones

The role of thyroid hormones in DNA synthesis and in the activity of Thymidine kinase (TK), a key regulatory enzyme of DNA synthesis was studied in proliferating hepatocytes in vivo. Liver regeneration after partial hepatectomy was used as a model for controlled cell division in rats having different thyroid status - euthyroid, hypothyroid and 3,3',5'-triiodo-L-thyronine (T3)-treated hypothyroid. Partial hepatectomy caused a significant elevation of DNA synthesis (p<0.01) in all the three groups compared to their sham-operated counterparts. Hypothyroid hepatectomised animals showed significantly lower (p<0.01) level of DNA synthesis than euthyroid hepatectomised animals. A single subcutaneous dose of T3 to hypothyroid sham-operated animals resulted in a significant increase (p<0.01) of DNA synthesis in the intact liver. This was comparable to the level of DNA synthesis occurring in regenerating liver of euthyroid animals. In hypothyroid hepatectomised animals, T3 showed an additive effect on DNA synthesis and this group exhibited maximum level of DNA synthesis (p<0.01). Studies of the kinetic parameters of TK show that the Michelis-Menten constant, (K(m)) of TK for thymidine was altered by the thyroid status. K(m) increased significantly (p<0.01) in untreated hypothyroid animals when compared to the euthyroid rats. T3 treatment of hypothyroid animals reversed this effect and this group showed the lowest value for K(m) (p<0.01). Thus our results indicate that thyroid hormones can influence DNA synthesis during liver regeneration and they may regulate the activity of enzymes such as Thymidine kinase which are important for DNA synthesis and hence cell division.

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.

RNA synthesis in regenerating mouse liver evaluated by incorporation of [methyl-14C]methionine and by determination of RNA polymerase activity

The synthesis of RNA during mouse liver regeneration was studied by two different methods at 24 and 48 h after partial hepatectomy. Total chromatin-bound RNA polymerase activity showed an increase of 32% at 24 h after partial hepatectomy. At 48 h a slight increase in total activity was also observed in regenerating liver, but the difference was not significant. The increase in total RNA polymerase activity was due to a rise in RNA polymerase I plus III activity. This enzyme activity was increased at both 24 and 48 h. The increase was 57% at 24 h and 51% at 48 h. When [methyl-14C]methionine was used for labelling of methyl groups in rRNA, there was an increased specific radioactivity of this class of RNA at both 24 h and 48 h. The increases were 263 and 103% at 24 and 48 h respectively. Thus both methods revealed an increased synthesis of rRNA during mouse liver regeneration. The results are discussed in relation to previous results from this laboratory [Yngner, Carlberg, Lewan & Engelbrecht (1979) Hoppe-Seyler's Z. Physiol. Chem. 360, 1069-1074; Yngner, Engelbrecht, Lewan & Annerfeldt (1979) Biochem. J. 178, 1-8; Yngner, Bengtsson, Carlberg, Engelbrecht & Wieslander (1980) Exp. Cell. Biol. 48, 393-403], which have shown that the incorporation of orotic acid or uridine into RNA is not increased in mouse liver regenerating after partial hepatectomy.

Effects of partial hepatectomy on RNA polymerase activities in mouse liver

Chromatin-bound and poly[d(A-T)]dependent RNA polymerase I plus III and II activities of mouse liver were analysed 24 and 48 hr after partial hepatectomy. Chromatin-bound RNA polymerase I plus III activity showed an increase of 57% at 24 hr and 51% at 48 hr after partial hepatectomy. There was a decrease in chromatin-bound RNA polymerase II activity of 15% at 24 hr and 34% at 48 hr after partial hepatectomy. There was no significant changes in poly[d(A-T)]dependent RNA polymerase activities. Heparin caused an approximately 10-fold increase in chromatin-bound RNA polymerase II activity. The stimulation by heparin was significantly increased 48 h after partial hepatectomy. Anaesthesia and/or surgery had great influence on RNA polymerase activities. At 24 hr after operation, chromatin-bound RNA polymerase I plus III and II activities were depressed, and the liver cell chromatin was more susceptible to stimulation by heparin.

Glucocorticoids directly affect the synthesis of ribosomal RNA in rat-liver cells

Administration of the glucocorticoid dexamethasone to rats significantly stimulated the synthesis of ribosomal RNA by hepatic nuclei in vitro, as determined by hybridization to plasmid DNA containing the appropriate gene from X. laevis. This effect was also seen in rats in which RNA polymerase B had previously been blocked by the administration of alpha-amanitin. RNA synthesized in response to the hormone hybridized to both the 5' and 3' end of the gene for the precursor to ribosomal RNA. We conclude from these data that the steroid directly affects transcription of the ribosomal gene without obligatory participation of RNA polymerase B and hence the intermittent production of mRNA or its translation products.

Inhibition of RNA and protein synthesis by ethanol in regenerating rat liver: evidence for transcriptional inhibition of protein synthesis

The effect of ethanol on RNA and protein synthesis was studied in regenerating rat liver. Partial hepatectomy stimulated the activity of ornithine decaroxylase (ODC) and that of tyrosine aminotransferase (TAT) as well as the synthesis of RNA. The intubation of ethanol (3g/kg) 1 hr before the operation inhibited the stimulation of ODC and TAT when measured 4 hrs after the operation. Ethanol administration also inhibited RNA synthesis. By using inhibitors of alcohol dehydrogenase and aldehyde dehydrogenase it was shown that the inhibition of RNA synthesis (and also that of protein synthesis) was most probably a direct effect of ethanol itself. When RNA synthesis was measured in vivo in purified nuclei, ethanol inhibited the incorporation of nucleotides to RNA. It is suggested that ethanol inhibits protein synthesis in regenerating rat liver at the transcriptional level by interfering in nuclei with the synthesis of RNA.

Ribonucleic acid synthesis in the myocardium of spontaneously hypertensive rats. Quantification of transcribing ribonucleic acid polymerases

Cardiac hypertrophy accompanies the progressive rise in blood pressure in spontaneously hypertensive rats. The role of endogenous RNA polymerases in this process was examined in nuclei from isolated cardiac myocytes of 20-week-old spontaneously hypertensive rats and normotensive Wistar-Kyoto controls. Both template-engaged (involved in transcription) and free (loosely attached to endogenous template, transcribing only with exogenous templates) RNA polymerases were increased in spontaneously hypertensive rats. In addition, the ratio of RNA polymerases I/II was lower in the spontaneously hypertensive rats for both functional pools of the enzyme. Endogenous transcribing RNA polymerases were quantified by t.l.c. of RNA-hydrolysis products. Increased numbers of enzyme molecules were present in nuclei from spontaneously hypertensive rats, without appreciable change in the rate of polyribonucleotide-chain elongation. These results could not be explained by differences in the activities of contaminating phosphatases or ribonucleases, nor by changes in endogenous nucleoside pools or recoveries of labelled nucleosides. Enhanced myocardial RNA synthesis in the spontaneously hypertensive rats at the stage of established cardiac hypertrophy is associated with increased numbers of RNA polymerase molecules. This increase may, in turn, reflect altered chromatin structure, resulting in increased polymerase binding and/or chain initiation.

Ribosome biosynthesis is not necessary for initiation of DNA replication

Synthesis of mature 28-S ribosomal RNA and 60-S ribosomal subunits is inhibited in baby hamster kidney (BHK) cell line ts 422E at non-permissive temperature (39 degrees C). This leads to a 66% decrease of total ribosomes per cell, a marked imbalance between the large and small ribosomal subunits in the cytoplasm and a decrease of cells per dish after prolonged culture at 30 degrees C. However, inhibition of ribosome synthesis does not affect progression of cells through the G1 period of the cell division cycle, the length of the pre-replicative period, and the rate of entry of cells into S phase. In contrast to culture at non-permissive temperature, culture of BHK ts 422E cells in the presence of 0.04 micrograms/ml actinomycin D at 33 degrees C inhibits markedly the entry into S period. It is concluded that low doses of actinomycin D exert their inhibitory effect on cell growth by preventing maturation and transport of mRNA rather than by interfering with ribosome synthesis. Microfluorometric analysis revealed only slight differences in the distribution of BHK ts 422E cells in G1, S and G2 phases of the cycle either when cultured at 33 degrees C or at 39 degrees C. When too few ribosomes per cell are produced in BHK ts 422E cells at 39 degrees C, cells do not seem to be arrested reversibly at a specific point of the cell cycle but rather to die at random.

RNA synthesis by nuclei and nucleoli from ischemic liver cells

Nuclei and nucleoli were isolated from rat livers subjected to an interruption of the blood supply for periods of different duration, as well as after restoration of the blood supply. They were assayed for RNA synthesis under conditions of diverse ionic strengths, and in the presence of an exogenous template, such as poly d (A-T), and actinomycin to inactivate the endogenous template; alpha-amanitin was made used of to distinguish polymerase I and polymerase II dependent RNA synthesis. Nuclei and nucleoli from ischemic livers showed a severe impairment of RNA synthesis, which is likely to be due to decreased initiation frequency of the engaged polymerases, while free polymerases were essentially unchanged. Both form I and II polymerase were equally involved. After restoration of the blood supply RNA synthesis recovered with an overshooting well above normal levels of activity, lasting for at least 24 hours. Increased RNA synthesis was not followed by thymidine incorporation into DNA.

Ribosomal RNA synthesis in rat liver cells

DNA-dependent RNA polymerase A was analyzed in rat liver cells after partial hepatectomy, which is known to enhance the synthesis of ribosomal RNA. In agreement with previous results, it was found that the extractable enzyme activity increased after partial hepatectomy and could be quenched by the subsequent administration of cycloheximide, which suggests that continuous protein synthesis is required for the maintenance of the observed stimulation. However, administration of alpha-amanitin to hepatectomized rats did not quench the enzyme activity. These results cast doubt on the hypothesis that the increased activity of RNA polymerase A observed after partial hepatectomy is mediated by RNA products synthesized by polymerase B. Moreover, data are reported which indicate that ribosomal RNA may be regulated directly, without the obligatory involvement of HnRNA.

An autoradiographic study of the 3H-uridine and 3H-thymidine incorporation in the regenerating mouse liver

An autoradiographic study was made of the 3H-uridine incorporation into RNA and DNA in nucleus and cytoplasm of parenchymal cells in the regenerating liver of the mouse after a pulse time of 2 hr. After a decreased uptake of precursor into the parenchymal nucleus during the first 6 hr compared with the normal value, incorporation increased and was maximal at 36 hr; normal values were restored at 72 hr. The cytoplasmic labelling, after an initial small decrease, reached a maximum at 12 hr; this changed to normal 48 hr after hepatectomy. RNase-digestion of the liver sections left a small incorporation in both nucleus and cytoplasm: presumably DNA. This incorporation is maximal at 12 hr over the nucleus and at 24 hr over the cytoplasm. After a 2 hr pulse of 3H-thymidine, there was a marked uptake of the precursor into DNA about 24 hr after hepatectomy. This was maximal at 48 hr and reached normal values at 72 hr. A small amount of incorporation of 3H-thymidine into DNA was seen immediately after the operation, and this population of weakly labelled nuclei was still rather large 72 hr later.