Cell repair after liver injury. Stimulation of RNA synthesis, engaged polymerases, number of RNA transcribing molecules, and elongation rate in postischemic liver nuclei

Patterns of RNA and protein synthesis in post-ischemic livers

The re-establishment of the blood supply to a formerly ischemic liver lobe, before the "point of no return" of the tissue is reached, induces a series of changes in protein and RNA metabolism that are functional to the repair of the damage suffered by the cells. Among these events there is the increase in synthesis of a group of proteins known as heat-shock (or stress) proteins, which are also induced in liver cells by different kinds of oxidative stress. The increase in synthesis of these proteins is largely due to the activation of their genes: some of these genes are also activated in cells stimulated to grow. These observations suggest a link between oxidative stress, repair of cell damage and cell multiplication.

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.

Temporal profiles of proteins responsive to transient ischemia

The responses of long and short half-lived proteins to ischemia were measured in rat brain during 6 days of recovery from 30 min of transient forebrain ischemia produced by four-vessel occlusion. At the end of the ischemic interval, the neocortical activities of four vulnerable enzymes [ornithine (ODC) and S-adenosylmethionine (SAMDC) decarboxylases, and RNA polymerases I and II] were unchanged, but within 30 min of reperfusion, their activities dropped by 25-50%. The loss of substance P in the striatum and substantia nigra was slower, reaching about 50% by 12 h. On the other hand, the activities of 5 long half-lived enzymes did not change in the neocortex at 5 and 15 h of reperfusion and regional protein concentrations were essentially unaffected over 6 days survival. The rate and extent of normalization of the amounts or activities of the vulnerable proteins varied. RNA polymerase II and ODC activities were restored within 4 h, and ODC showed a biphasic increase in activity, with peaks at 10 h and 2-3 days. RNA polymerase I and SAMDC activities were restored by 18 h and 5 days, respectively, whereas substance P concentrations did not completely recover, even at 6-15 days. The greater the regional reduction of blood flow during ischemia, the larger the net change (gain or loss) of SAMDC or ODC activity and the longer the time required to normalize the activities of these enzymes. The average rate of proteolysis, assessed by measuring the rate of clearance of 14C from protein prelabeled with [14C]bicarbonate, was abnormal during the first 2 days of reperfusion. Postischemic changes in both protein synthesis and degradation could affect the amounts of some of the proteins responsive to transient ischemia.

Mechanism of enhanced RNA synthesis in acute-phase rat liver and its relationship to chromatin structure

Nuclei isolated from the liver of rats undergoing an acute inflammatory reaction induced by turpentine treatment show increased RNA synthesis. This increase is essentially determined by a faster polyribonucleotide-elongation rate while the number of transcribing polymerase molecules is unchanged. The sensitivity of chromatin to micrococcal-nuclease digestion and the composition of chromosomal proteins are not affected by the acute-phase process. Therefore the increased RNA synthesis by liver nuclei from acutely inflamed rats does not seem to correlate with major changes in chromatin structure.

Cell repair after liver injury: an analysis of some metabolic conditions required for the stimulation of RNA synthesis in postischemic liver nuclei

Nuclei isolated from liver cells recovering from reversible (non-necrogenic) ischemia show an increased RNA synthesis. The postischemic effect is not abolished by previous adrenalectomy and occurs in fasted as well as in fed animals. Treatment with cycloheximide, at a dose that severely inhibits protein synthesis without primary effects on RNA synthesis, suppresses the postischemic stimulation of RNA synthesis even if cycloheximide is administered when stimulation is already well developed. Postischemic liver nuclei respond only weakly to the additional stimulation of RNA synthesis caused by the presence of cytosol and albumin in the incubation medium.

RNA synthesis and number of transcribing polymerase molecules in ischemic liver nuclei

Nuclei isolated from ischemic liver show a consistent reduction of their RNA synthesis. The reduction persists at high ionic strength and in the presence of heparin, when RNA synthesis in vitro is fully activated and occurs in the presence as well as in the absence of alpha-amanitin. Both the number of transcribing polymerase molecules and the rate of elongation of initiated polynucleotide chains seem to be equally affected.

Effect of post-ischaemic recovery on albumin synthesis and relative amount of translatable albumin messenger RNA in rat liver

In liver cells recovering from reversible ischaemia, total protein synthesis by postmitochondrial supernatant and membrane-bound and free polyribosomes is not different from that in sham-operated controls. However, the relative proportion of specific proteins is changed, since the incorporation of [3H]leucine in vivo into liver albumin, relative to incorporation into total protein, as determined by precipitation of labelled albumin with the specific antibody, decreases by 40-50% in post-ischaemic livers. Cell-free synthesis by membrane-bound polyribosomes and poly(A)-enriched RNA isolated from unfractionated liver homogenate shows that the decrease in albumin synthesis in liver of rats recovering from ischaemia is due to the relative decrease in translatable albumin mRNA.