Poly(ADP-ribose) polymerase is affected early by thyroid state during liver regeneration in rats

Poly(ADP-ribose) polymerase (PARP), a nuclear enzyme involved in DNA synthesis, DNA repair, and cell replication and transformation, also plays a role in the early steps of liver regeneration induced by partial hepatectomy (PH). PARP and DNA topoisomerase I (Topo I) activities and de novo DNA synthesis were studied during liver regeneration in rats with altered thyroid state. Hepatic PARP activity, evaluated as [(32)P]NAD incorporated into isolated liver nuclei, was inhibited in hyperthyroid rats and increased in hypothyroid animals. In both euthyroid and hyperthyroid rats PARP activity was rapidly stimulated, peaking 6 h after PH. In hypothyroid animals, an early decrease in activity was found, at a minimum of 6 h after PH, followed by an early onset of DNA synthesis. An inverse relationship between PARP and Topo I activities was a shared feature among euthyroid, hypothyroid, and hyperthyroid rats. Together these data show that, in replicating hepatocytes, thyroid hormones exert a regulatory role on PARP activity, which reflects the control of a number of nuclear proteins involved in DNA metabolism.     

Effect of autonomic denervation on DNA synthesis during liver regeneration after partial hepatectomy

The regulatory role of autonomic nerves in liver regeneration after partial hepatectomy was studied in rats by bilateral subdiaphragmatic vagotomy or splanchnicectomy. 1. In control rats the wet weight of the regenerating liver was restored to approximately 80% of the preoperative weight 72 h after partial hepatectomy. Restoration of the liver weight was significantly impaired in vagotomy rats, but not in splanchnicectomy. Increases in the DNA and protein contents of the regenerating liver were also suppressed by vagotomy. 2. Hepatic DNA synthesis, measured as the incorporation of [methyl-3H]thymidine into DNA at various times after partial hepatectomy, was significantly less in vagotomized rats, and slightly more in splanchnicectomized rats than in control rats. The onset of DNA synthesis triggered by partial hepatectomy was also delayed by vagotomy. 3. The increases in activities of hepatic aspartate transcarbamoylase and thymidine kinase, the key enzymes in synthesis of pyrimidine nucleotides via the de novo and salvage pathways respectively, during liver regeneration, were significantly suppressed and retarded in vagotomized rats. Conversely, splanchnicectomy tended to stimulate these enzyme inductions after partial hepatectomy. 4. During starvation the plasma insulin level decreased after partial hapatectomy in control and vagotomized rats, as in sham-operated rats, but showed a transient increase 6 h after partial hepatectomy in splanchnicectomized rats. It is concluded that vagotomy inhibits and delays DNA synthesis and proliferation of liver cells after partial hepatectomy, whereas splanchnicectomy tends to stimulate these processes. The data also suggest that parasympathetic innervation of the liver may play an important regulatory role in liver regeneration.     

Effects of protein-deprivation on the regeneration of rat liver after partial hepatectomy.

Rats maintained on a protein-free diet for 3 days have an altered time course of hepatic DNA synthesis during liver regeneration. The delay in DNA synthesis is eliminated by the administration of casein hydrolysate (given as late as 6h after partial hepatectomy), but not by glucose or incomplete amino acid mixtures. Despite the change in the timing of DNA synthesis, the increases in hepatic amino acid pools, which take place at the earliest stages of the regenerative process, occur in a normal pattern in the regenerating liver of rats fed the protein-free diet. Protein-deprived rats have increased protein synthesis and decreased rates of protein degradation in the liver in response to partial hepatectomy, but these adaptations do not prevent a lag in protein accumulation and low protein/RNA ratios. The regenerating livers of these animals show a deficit in the accumulation of cytoplasmic polyadenylated mRNA as well as a smaller proportion of free polyribosomes. It is suggested that the deficit in free polyribosomes found in the regenerating liver of protein-deprived rats might be a consequence of the slow accumulation of mRNA species coding for intracellular proteins.     

DNA synthesis, mitotic index, drug-metabolising systems and cytogenetic analysis in regenerating rat liver. Comparison with bone marrow test after 'in vivo' treatment with cyclophosphamide

Rat-liver cells can be used to reveal "in vivo" clastogenic activity of indirect mutagens, provided that they are stimulated to divide by partial hepatectomy. In order to characterize the rat-liver metabolic capacity in such experimental conditions, several biochemical parameters were measured during the first 54-66 h of liver regeneration in Sprague-Dawley male rats, subjected to a partial hepatectomy. The levels of cytochrome P-450, the activities of styrene monooxygenase, epoxide hydrolase and glutathione-S-epoxide transferase were chosen as markers. All the enzymatic activities and the level of cytochrome P-450 decreased during the first 12 h after the hepatectomy to about 50% of the activities of the sham-operated rats considered as controls. Subsequent recovery of the metabolic capacity was not observed. DNA synthesis and the mitotic index were measured to find the most suitable time for metaphase analysis. DNA synthesis and the number of metaphases were maximal at, respectively, 22-25 and 28-31 h after partial removal of the liver. The sensitivity to clastogenic damage induced by "in vivo" treatment with cyclophosphamide (CPA) was assayed in regenerating liver cells by chromosome-aberration analysis. Different doses, ranging from 5 to 30 mg/kg b.w., were given i.p. to the rats 17 h before or 7 h after partial hepatectomy. Liver cells were collected 31 h after surgery. Clastogenic damage was greater when the drug was administered to the animals after the hepatectomy (24 h of exposure) than before (48 h of exposure). The sensitivity to CPA-induced damage was compared with a bone marrow cell test carried out on non-hepatectomized rats treated in the same way. The results indicated that in these conditions regenerating liver cells are more sensitive than bone marrow cells to the induction of chromosome aberrations by CPA.     

Specific inhibition of the synthesis of putrescine and spermidine by 1,3-diaminopropane in rat liver in vivo.

Chronic administration of 1,3-diaminopropane, a compound inhibiting mammalian ornithine decarboxylase (EC 4.1.1.17) in vivo, effectively prevented the large increases in the concentration of putrescine that normally occur during rat liver regeneration. Furthermore, repeated injections of diaminopropane depressed by more than 85% ornithine decarboxylase activity in rat kidney. Administration of diaminopropane 60 min before partial hepatectomy only marginally inhibited ornithine decarboxylase activity at 4 h after the operation. However, when the compound was given at the time of the operation (4 h before death), or any time thereafter, it virtually abolished the enhancement in ornithine decarboxylase activity in regenerating rat liver remnant. An injection of diaminopropane given 30 to 60 min after operation, but not earlier or later, depressed S-adenosyl-L-methionine decarboxylase activity (EC 4.1.1.50) 4 h after partial hepatectomy. Diaminopropane likewise inhibited ornithine decarboxylase activity during later periods of liver regeneration. In contrast to early regeneration, a total inhibition of the enzyme activity was only achieved when the injection was given not earlier than 2 to 3 h before the death of the animals. Diaminopropane also exerted an acute inhibitory effect on adenosylmethionine decarboxylase activity in 28-h regenerating liver whereas it invariably enhanced the activity of tyrosine aminotransferase (EC 2.6.1.5), used as a standard enzyme of short half-life. Treatment of the rats with diaminopropane entirely abolished the stimulation of spermidien synthesis in vivo from [14C]methionine 4 h after partial hepatectomy or after administration of porcine growth hormone. Both partial hepatectomy and the treatment with growth hormone produced a clear stimulation of hepatic RNA synthesis, the extent of which was not altered by injections of diaminopropane in doses sufficient to prevent any enhancement of ornithine decarboxylase activity and spermidine synthesis.     

Effect of α-difluoromethylornithine on polyamine and DNA synthesis in regenerating rat liver: Reversal of inhibition of DNA synthesis by putrescine

The possibility that α-difluoromethylornithine, a specific, irreversible inhibitor of ornithine decarboxylase could be used to prevent the rise in hepatic putrescine and spermidine content following partial hepatectomy was tested. Administration of α-difluoromethylornithine at a dose of 400 mg/kg every 4 h reduced hepatic putrescine to <2 nmol/g, but had only a small effect on the rise in spermidine seen at 28 h after partial hepatectomy. Such treatment also reduced the rise in DNA synthesis produced by partial hepatectomy by up to 70%. The inhibitory effect towards DNA synthesis could be reversed by administration of putrescine which increased the hepatic putrescine content to about 30–40% of that in the regenerating control livers. These results suggest that accumulation of putrescine rather than spermidine is needed for DNA synthesis after partial hepatectomy. They also suggest that part, but not all of the rise in putrescine normally seen in the liver after partial hepatectomy is needed for the enhanced DNA synthesis associated with liver regeneration. Experiments with lower doses of α-difluoromethylornithine showed that a substantial part of the rise in hepatic ornithine decarboxylase activity could be abolished without affecting either the rise in spermidine content or the increase in DNA synthesis after partial hepatectomy.     

Effect of colchicine and vincristine on DNA synthesis in regenerating rat liver

The increases in the activities of hepatic thymidylate synthetase and thymidine kinase were significantly suppressed at 24 h after 70% partial hepatectomy in rats which had been administered a microtubule disrupter, colchicine or vincristine. The decrease of these enzymic activities was accompanied by a reduction of DNA content in 24 h regenerating liver. The immunoblotting assay showed that the depression of the thymidylate synthetase activity by the injection of colchicine or vincristine was due to the decrease of the enzyme protein. These results indicate that colchicine and vincristine inhibit the DNA synthesis during liver regeneration by inhibiting the induction of the key enzyme in DNA synthesis.     

Nicotinamide nucleotide synthesis in regenerating rat liver

1. The concentrations and total content of the nicotinamide nucleotides were measured in the livers of rats at various times after partial hepatectomy and laparotomy (sham hepatectomy) and correlated with other events in the regeneration process. 2. The NAD content and concentration in rat liver were relatively unaffected by laparotomy, but fell to a minimum, 25 and 33% below control values respectively, 24h after partial hepatectomy. NADP content and concentration were affected similarly by both laparotomy and partial hepatectomy, falling rapidly and remaining depressed for up to 48h. 3. The effect of injecting various doses of nicotinamide on the liver DNA and NAD 18h after partial hepatectomy was studied and revealed an inverse correlation between NAD content and DNA content. 4. Injections of nicotinamide at various times after partial hepatectomy revealed that the ability to synthesize NAD from nicotinamide was impaired during the first 12h, rose to a peak at 26h and fell again by 48h after partial hepatectomy. 5. The total liver activity of NAD pyrophosphorylase (EC 2.7.7.1) remained at or slightly above the initial value for 12h after partial hepatectomy and then rose continuously until 48h after operation. The activity of NMN pyrophosphorylase (EC 2.4.2.12) showed a similar pattern of change after partial hepatectomy, but was at no time greater than 5% of the activity of NAD pyrophosphorylase. 6. The results are discussed with reference to the control of NAD synthesis in rapidly dividing tissue. It is suggested that the availability of cofactors and substrates for NAD synthesis is more important as a controlling factor than the maximum enzyme activities. It is concluded that the low concentrations of nicotinamide nucleotides in rapidly dividing tissues are the result of competition between NAD synthesis and nucleic acid synthesis for common precursor and cofactors.     

The control of nucleic acid and nicotinamide nucleotide synthesis in regenerating rat liver

1. The effect of injecting nicotinamide on the incorporation of [(14)C]orotate into the hepatic nucleic acids of rats after partial hepatectomy was investigated. 2. At 3h after partial hepatectomy the rapid incorporation of [(14)C]orotate into RNA, and at 20h after partial hepatectomy the incorporation of [(14)C]orotate into both RNA and DNA, were inhibited in a dose-dependent fashion by the previous injection of nicotinamide. 3. The injection of nicotinamide at various times before the injection of [(14)C]orotate at 20h after partial hepatectomy revealed an inhibition of the incorporation of orotate into RNA and DNA which was non-linear with respect to the duration of nicotinamide pretreatment. 4. The induction of a hepatic ATP depletion by ethionine demonstrated that the synthesis of hepatic NAD and NADP in partially hepatectomized rats was more susceptible to an ATP deficiency than in control rats. 5. The total hepatic activity of ribose phosphate pyrophosphokinase (EC 2.7.6.1) was assayed at various times after partial hepatectomy and found to be only marginally greater than the maximum rate of hepatic NAD synthesis induced in vivo by nicotinamide injection between 12 and 24h after partial hepatectomy. 6. It is suggested that a competition exists between NAD synthesis and purine and pyrimidine nucleotide synthesis for available ATP and particularly 5-phosphoribosyl 1-pyrophosphate. In regenerating liver the competition is normally in favour of the synthesis of nucleic acid precursors, at the expense of NAD synthesis. This situation may be reversed by the injection of nicotinamide with a subsequent inhibition of nucleic acid synthesis.     

Regulation of DNA primase activity by phosphorylation of histone-H1 in regenerating rat liver

The biological importance of histone H1 was investigated in relation to the cell cycle using liver regeneration in rat. Histone H1 was extracted from the regenerating rat liver at various intervals after partial hepatectomy and the number of phosphate residues was measured. The inhibitory effect of the extracted histone H1 on DNA primase was assayed. The activities of DNA polymerase-alpha, DNA primase and DNA synthesis were also determined in the regenerating rat liver. It was found that: 1) phosphate residue in histone H1 from normal rat liver was between 2-3 mol/mol of histone H1. 2) The number of phosphate residues did not change for the first 16h after partial hepatectomy. 3) A dramatic sudden increase of phosphate residues was detected at 18h after partial hepatectomy. 4) The high levels of phosphate residues remained constant thereafter up to 50h. 5) DNA primase activity was less inhibited by highly phosphorylated than by slightly phosphorylated histone H1. It seems probable that phosphorylation of histone H1 is needed for the releasing of DNA primase activity from its inhibited state, which would start DNA synthesis together with DNA polymerase-alpha.     

Specific inhibition of the synthesis of putrescine and spermidine by 1,3-diaminopropane in rat liver in vivo

Chronic administration of 1,3-diaminopropane, a compound inhibiting mammalian ornithine decarboxylase (EC 4.1.1.17) in vivo, effectively prevented the large increases in the concentration of putrescine that normally occur during rat liver regeneration. Furthermore, repeated injections of diaminopropane depressed by more than 85% ornithine decarboxylase activtivity in rat kidney.Adminsitration of diaminopropane 60 min before partial hepatectomy only marginally inhibited orthine decarboxylase activity at 4 h after the operation. However, when the compound was given at the time of the operation (4 h before death), or any time thereafter, it virtually abolished the enhancement in ornithine decarboxylase activity in regenerating rat liver remnant.An injection of diaminopropane given 30 to 60 min after operation, but not earlier or later, depressed $\text{S-}\text{adenosyl}\text{-}\text{l}\text{-}\text{methionine}$ decarboxylase activity (EC 4.1.1.50) 4 h after partial hepatectomy.Diaminopropane likewise inhibited ornithine decarboxylase activity during later periods of liver regeneration. In contrast to early regeneration, a total inhibition of the enzyme activity was only achieved when the injection was given not earlier than 2 to 3 h before the death of the animals.Diaminopropane also exerted an acute inhibitory effect on adenosylmethionine decarboxylase activity in 28-h regenerating liver whereas it invariably enhanced the activity of tyrosine aminotransferase (EC 2.6.1.5), used as a standard enzyme of short half-life.Treatment of the rats with diaminopropane entirely abolished the stimulation of spermidien synthesis in vivo from [¹⁴C] methionine 4 h after hepatectomy or after administration of porcine growth hormone.Both partial hepatectomy and the treatment with growth hormone produced a clear stimulation of hepatic RNA synthesis, the extent of which was not altered by injections of diaminopropane in doses sufficient to prevent any enhancement of ornitine decarboxylase activity and spemedicine synthesis.     

Glycosyl transfer from nucleotide sugars to C85- and C55-polyprenyl and retinyl phosphates by microsomal subfractions and Golgi membranes of rat liver.

Compared with normally fed animals, rats fed on a low-protein diet for 3 days exhibit a considerable delay in DNA synthesis after partial hepatectomy. In the regenerating livers of these animals (a) the timing of the first peak of ornithine decarboxylase activity is not altered and (b) the second peak of enzyme activity is delayed by a few hours, but polyamine concentrations are similar to those of normally fed rats. The results suggest that regardless of the possible effect of polyamines on DNA synthesis, the time course of ornithine decarboxylase activity appears to be independent of the onset of DNA replication in regenerating livers.     

Effect of propylthiouracil on liver regeneration in rats after partial hepatectomy.

The effect of propylthiouracil (PTU) on the growth activity of intact liver and liver regenerating after partial (65-70%) hepatectomy (PH) was studied in rats. PTU (Propycil, Kali-Chemie, FRG) was dissolved in drinking water (1 g PTU per litre) and this was given to the rats, as their sole source of fluids, three days before PH and then up to the end of the experiment. In rats given PTU, marked inhibition of liver DNA synthesis and the mitotic activity of hepatocytes was found after PH. This effect was potentiated to some extent by partial inanition of the experimental animals given PTU, as demonstrated in a paired feeding test in control rats. PTU inhibition of DNA synthesis in intact and regenerating liver also took effect in thyroidectomized rats, even with substitution (thyroid hormone) therapy. The experiments demonstrated that the effect of propylthiouracil on DNA synthesis in the liver is mediated primarily by way of its direct effect on the liver.     

Intracellular distribution of transglutaminase activity during rat liver regeneration

Transglutaminase and ornithine decarboxylase activities have been assayed at intervals after partial hepatectomy in regenerating liver cells fractionated to obtain nuclear, cytoplasmic-particulate, and cytoplasmic-soluble fractions. Ornithine decarboxylase activity, localized entirely in the cytoplasmic fractions, undergoes a dramatic induction during the first 4 h after partial hepatectomy and remains elevated. This induction is very sensitive to inhibition by cycloheximide and actinomycin D, as previously reported. Transglutaminase activity is localized in both the cytoplasm and the nucleus with the highest specific activity in the nucleus. Nuclear transglutaminase activity approximately doubles in the first 2 h of liver regeneration, apparently as a result of a translocation of enzyme from the cytoplasm to the nucleus. Inhibitor studies indicate that the translocation is not dependent upon protein or RNA synthesis. In the first 2 h, actinomycin D slightly activates transglutaminase activity in the cytoplasmic-particulate and nuclear fractions. Only at 4 h after the onset of regeneration do actinomycin D and cycloheximide show some inhibition of transglutaminase activity indicating de novo synthesis at this time. A broad increase of transglutaminase activity occurs from hours 12–16 to hour 32 after partial hepatectomy in the nuclear and cytoplasmic-particulate fraction. These data suggest the existence of a function for transglutaminase in the nucleus of rat liver cells.     

Hepatocytes in heterokaryons can suppress entry into the S-period of fibroblast nuclei from murine NIH 3T3 cells

Mouse liver regeneration after partial hepatectomy can be considered as a spectacular example of controlled tissue increase. In this study serum-deprived (0.2%) resting and serum-stimulated (10%) proliferating NIH 3T3 mouse fibroblasts were fused with primary hepatocytes isolated from normal (intact) and regenerating adult mouse liver at different times after partial hepatectomy (1-15 days) to elucidate mechanisms of liver cell proliferation cessation at the regeneration end. DNA synthesis was investigated in the nuclei of heterokaryons and non-fused cells using radioautography. Hepatocytes isolated from regenerating liver within 1-12 days following operation did not retard the entry of stimulated fibroblast nuclei into the S-period. In contrast, hepatocytes isolated within 15 days after hepatectomy were found to have inhibitory effect on the entry of stimulated fibroblast nuclei into the S-period in heterokaryons. Preincubation of these hepatocytes with cyclocheximide for 2-4 h abolished their ability to suppress DNA synthesis in stimulated fibroblast nuclei in heterokaryons. Possible reasons of inhibitory effect of differentiated cells in heterokaryos are discussed. The data obtained enable us to conclude that the mechanism of proliferative process control in regenerating hepatocytes seems to be stopped being affected by the intracellular growth inhibitors, whose formation depends on protein synthesis.     

Effect of branched chain amino acids on liver regeneration after partial hepatectomy

The authors investigated the effect of the enrichment of commercial amino acid solutions with branched chain amino acids on the development of liver regeneration. Partial (65-70%) hepatectomy was performed on male Wistar rats (140-160 g body weight). Starting with the day of the operation, amino acid solutions normally used in clinical practice and the same solutions enriched with branched chain amino acids were administered by stomach tube; 24, 48 and 96 h after the operation the animals were decapitated. The onset of DNA synthesis was found to be more rapid in animals given the enriched solutions. Once regeneration had started, the stimulant effect of an increased supply of branched chain amino acid on liver regeneration was smaller. Nevertheless, even in the later phase after partial hepatectomy branched chain amino acids had a stronger stimulant effect on liver regeneration after partial hepatectomy than an energy supply in the form of sorbitol.     

Changes in activity and mRNA levels of poly(ADP-ribose) polymerase during rat liver regeneration

ADP-ribosylation of nuclear proteins, catalysed by the enzyme poly(ADP-ribose) polymerase, is involved in the regulation of different cellular processes of DNA metabolism. To further clarify the role of the enzyme during proliferating activity of mammalian cells, we have studied the control of gene expression in regenerating rat liver. The changes in activity and mRNA levels were analysed during the early and late phases of the compensatory model. When enzyme activity was measured in isolated liver nuclei obtained at different times after hepatectomy, two different phases were observed: an early wave occurring before the onset of DNA synthesis, and a second one, starting several hours after the onset of DNA synthesis and returning to control values at later times. The evaluation of the enzymatic level in nuclear extracts and by activity gel analysis showed a more gradual increase starting 1 day after hepatectomy, in concomitance with the peak of DNA synthesis. By using a specific murine cDNA probe, a significant enhancement of mRNA levels for poly(ADP-ribose) polymerase was observed during liver regeneration, slightly preceding the onset of DNA synthesis. The results obtained show that changes in poly(ADP-ribose) polymerase activity, during liver regeneration, are associated both to early events preceding the increase in DNA synthesis and to later phases of the cell proliferation process.     

Acceleration of the onset of liver regeneration by carnitine in partially hepatectomized rats

Immediately and 6 h after removal of 70% of the liver tissue, rats were treated with L-carnitine (Carnitene, Sigma-Tau, Italy) and received an injection of 100, 200 or 1,000 mg/kg b.w. into their femoral vein. The control rats were given the same volume of saline solution. The rats were sacrificed 18, 21, 24 or 30 h after the operation. The development of liver regeneration was evaluated from the incorporation of 14C-thymidine into DNA and from the hepatocyte mitiotic activity. In rats given carnitine in a dose of 100 or 200 mg/kg b.w. significantly higher DNA specific activity values were found 18 and 21 h after partial hepatectomoy and higher hepatocyte mitotic activity values after 30 h. In rats given carnitine in a dose of 1,000 mg/kg b.w., DNA specific activity values 21 h after partial hepatectomy were lower than in the control group. We conclude that L-carnitine, in a dose of 100 or 200 mg/kg b.w. has an enhancing effect on the onset of liver regeneration after 70% hepatectomy.