DNA Synthesis In Liver Research Articles

DNA synthesis in nuclei and nuclear matrices of regenerating rat liver: Effect of whole-body gamma irradiation

Partial hepatectomy (PH) of rats (Wistar strain) resulted in acceleration of DNA synthesis in liver which reached a maximum at 36 h after PH. Whole-body radiation exposure (10 Gy) of the rats at 12 h after PH completely arrested this stimulation in DNA synthesis. The elevation of DNA synthetic rate in response to PH and complete obliteration of this stimulation by whole-body radiation exposure were found to be the reflection of levels of DNA polymerase-alpha in nuclei and nuclear matrices isolated from the rat livers. Studies based on assays of DNA polymerase in nuclei and nuclear matrices, with and without exogenous DNA template (activated calf thymus DNA), revealed that whole-body irradiation blocked induction of DNA polymerase-alpha and, in turn, assembling of DNA polymerizing apparatus. Irradiation of nuclei (suspended in buffer) in vitro at doses as high as 500 Gy did not have any inhibitory effect on DNA polymerase-alpha activity.

Effects of single ethanol administration on hepatic ornithine decarboxylase induction and polyamine metabolism

The effects of a single ethanol administration on ornithine decarboxylase induction, polyamine metabolism and DNA synthesis in rat liver after partial hepatectomy were studied. Ethanol given 1 hr before partial hepatectomy at the dose of 2, 3 or 5 gm/kg body wt inhibited the increase in ornithine decarboxylase activity and that in the putrescine level in the liver 4 hr after partial hepatectomy. The hepatectomy increased the amount of ornithine decarboxylase messenger RNA expressed, and this amount was unaffected by ethanol administration. Further, ethanol did not accelerate the degradation of ornithine decarboxylase 4 hr after partial hepatectomy, indicating that the inhibition of ornithine decarboxylase activity caused by ethanol was not caused by a decrease in the ornithine decarboxylase messenger RNA level or by the acceleration of ODC degradation. The single dose of ethanol inhibited [3H]thymidine incorporation into the hepatic DNA 24 hr after partial hepatectomy. The suppression of [3H]thymidine incorporation was partially reversed by the administration of putrescine. These results suggested that ethanol inhibits the increase in ornithine decarboxylase activity after transcription, suppressing the accumulation of putrescine, which prevents DNA synthesis in response to hepatectomy. (Hepatology 1991;14:696-700.)

Effects of single ethanol administration on hepatic ornithine decarboxylase induction and polyamine metabolism

The effects of a single ethanol administration on ornithine decarboxylase induction, polyamine metabolism and DNA synthesis in rat liver after partial hepatectomy were studied. Ethanol given 1 hr before partial hepatectomy at the dose of 2, 3 or 5 gm/kg body wt inhibited the increase in ornithine decarboxylase activity and that in the putrescine level in the liver 4 hr after partial hepatectomy. The hepatectomy increased the amount of ornithine decarboxylase messenger RNA expressed, and this amount was unaffected by ethanol administration. Further, ethanol did not accelerate the degradation of ornithine decarboxylase 4 hr after partial hepatectomy, indicating that the inhibition of ornithine decarboxylase activity caused by ethanol was not caused by a decrease in the ornithine decarboxylase messenger RNA level or by the acceleration of ODC degradation. The single dose of ethanol inhibited [3H]thymidine incorporation into the hepatic DNA 24 hr after partial hepatectomy. The suppression of [3H]thymidine incorporation was partially reversed by the administration of putrescine. These results suggested that ethanol inhibits the increase in ornithine decarboxylase activity after transcription, suppressing the accumulation of putrescine, which prevents DNA synthesis in response to hepatectomy.

Carbohydrate and oxygen metabolism during hepatocellular proliferation: A study in perfused livers from mirex-treated rats

Liver regeneration after partial hepatectomy is accompanied by altered hepatic intermediary metabolism. Because the organochlorine compound mirex also causes liver cell growth, the purpose of this study was to investigate hepatic carbohydrate and oxygen metabolism in perfused livers from mirex-treated rats and to localize cell proliferation in this model. Pretreatment with mirex (100 mg/kg, intragastrically) increased liver/body weight ratios and DNA synthesis in livers of fed rats, effects that were markedly diminished in livers of fasted rats. This finding shows that liver growth caused by mirex, as is the case after partial hepatectomy, is hindered when animals are deprived of food. Furthermore, perfused livers from mirex-treated rats had depleted glycogen stores but significantly elevated oxygen uptake compared with livers from control rats. Increases in oxygen uptake and hepatocellular proliferation were observed mostly in periportal regions of the liver lobule. In regenerating livers, most DNA synthesis was reported to also occur in these regions of the liver lobule. Taken together, these data show that liver cell growth caused by mirex is accompanied by changes in hepatic intermediary metabolism and sublobular proliferation similar to those observed after partial hepatectomy.

Carbohydrate and oxygen metabolism during hepatocellular proliferation: A study in perfused livers from mirex-treated rats

Liver regeneration after partial hepatectomy is accompanied by altered hepatic intermediary metabolism. Because the organochlorine compound mirex also causes liver cell growth, the purpose of this study was to investigate hepatic carbohydrate and oxygen metabolism in perfused livers from mirex-treated rats and to localize cell proliferation in this model. Pretreatment with mirex (100 mg/kg, intragastrically) increased liver/body weight ratios and DNA synthesis in livers of fed rats, effects that were markedly diminished in livers of fasted rats. This finding shows that liver growth caused by mirex, as is the case after partial hepatectomy, is hindered when animals are deprived of food. Furthermore, perfused livers from mirex-treated rats had depleted glycogen stores but significantly elevated oxygen uptake compared with livers from control rats. Increases in oxygen uptake and hepatocellular proliferation were observed mostly in periportal regions of the liver lobule. In regenerating livers, most DNA synthesis was reported to also occur in these regions of the liver lobule. Taken together, these data show that liver cell growth caused by mirex is accompanied by changes in hepatic intermediary metabolism and sublobular proliferation similar to those observed after partial hepatectomy.

Possible endocrine control by Hepatocyte Growth Factor of liver regeneration after partial hepatectomy

Hepatocyte Growth Factor (HGF), which is a most potent growth factor for primary cultured hepatocytes, may act as a trigger for liver regeneration. After 70% of the rat liver was removed, HGF activity in the remnant liver began to increase within 24 h. In parallel with the activity, the HGF mRNA level in the remnant liver increased at 12 h after the operation and reached a maximum at 24 h. Increases in HGF activity and in the mRNA level were much lower and later than those in the liver of rats with hepatitis induced with CCl 4. However, the first increase in HGF activity in the plasma of hepatectomized rats was noted 3 h after the resection, that is much earlier than the initial DNA synthesis in the remnant liver. Thus, while HGF production was induced in the remnant liver during regeneration after partial hepatectomy, the initial trigger may not be the liver-derived HGF, rather, it may be HGF derived from extrahepatic organs, via blood circulation.

Hepatocyte growth factor (hepatopoietin A) rapidly increases in plasma before DNA synthesis and liver regeneration stimulated by partial hepatectomy and carbon tetrachloride administration

An enzyme-linked immunosorbent assay was used to measure the level of hepatocyte growth factor in rat plasma at various times after two-thirds partial hepatectomy or CCl4 administration. An initial 17-fold rise and 13-fold rise in the level of hepatocyte growth factor was observed 2 hr after partial hepatectomy and CCl4 treatment, respectively, well before the onset of DNA synthesis in the liver. The peaks of DNA synthesis in remnant livers and livers exposed to CCl4 occurred at 24 hr and 48 hr, respectively, as determined by 5-bromo-2′-deoxyuridine labeling and [3H]thymidine uptake by the liver. A later peak level (17-fold above control) of hepatocyte growth factor at 24 hr after CCl4 treatment coincided with strong immunostaining of damaged or necrotic hepatocytes around central veins with an antibody to hepatocyte growth factor. This suggests a later intrahepatic origin of the signals for liver regeneration after hepatotoxic injury subsequent to the early extrahepatic production of hepatocyte growth factor at 2 hr after CCl4 administration. The absence of staining in the liver remnants in partially hepatectomized rats implies that the increase in hepatocyte growth factor seen in the plasma is caused by production at extrahepatic site(s). Possible sources include the pancreas, brain, thyroid and salivary glands, and Brunner's glands of the duodenum. Norepinephrine also increases in plasma as early as 2 hr after hepatectomy. In vitro, [3H]thymidine incorporation into hepatocyte DNA in the presence of hepatocyte growth factor is greater if 10−5 mol/L norepinephrine is also present in the media. In contrast with other growth factors whose production by hepatocytes coincides with the peak of DNA synthesis (transforming growth factor α and acidic fibroblast growth factor), the hepatocyte growth factor increase in plasma occurs long before DNA synthesis and coincides with the observed increase in plasma norepinephrine at 2 to 4 hr after two-thirds partial hepatectomy. The concurrent elevation in plasma levels of the strongest hepatocyte mitogen (hepatocyte growth factor) and the strongest hepatocyte mitogenic amplifier (norepinephrine) probably serves to provide the mitogenic signal that initiates the first round of DNA synthesis during liver regeneration. (HEPATOLOGY 1991;13:743–749.)

Hepatocyte growth factor (hepatopoietin A) rapidly increases in plasma before DNA synthesis and liver regeneration stimulated by partial hepatectomy and carbon tetrachloride administration

An enzyme-linked immunosorbent assay was used to measure the level of hepatocyte growth factor in rat plasma at various times after two-thirds partial hepatectomy or CCl4 administration. An initial 17-fold rise and 13-fold rise in the level of hepatocyte growth factor was observed 2 hr after partial hepatectomy and CCl4 treatment, respectively, well before the onset of DNA synthesis in the liver. The peaks of DNA synthesis in remnant livers and livers exposed to CCl4 occurred at 24 hr and 48 hr, respectively, as determined by 5-bromo-2'-deoxyuridine labeling and [3H]thymidine uptake by the liver. A later peak level (17-fold above control) of hepatocyte growth factor at 24 hr after CCl4 treatment coincided with strong immunostaining of damaged or necrotic hepatocytes around central veins with an antibody to hepatocyte growth factor. This suggests a later intrahepatic origin of the signals for liver regeneration after hepatotoxic injury subsequent to the early extrahepatic production of hepatocyte growth factor at 2 hr after CCl4 administration. The absence of staining in the liver remnants in partially hepatectomized rats implies that the increase in hepatocyte growth factor seen in the plasma is caused by production at extrahepatic site(s). Possible sources include the pancreas, brain, thyroid and salivary glands, and Brunner's glands of the duodenum. Norepinephrine also increases in plasma as early as 2 hr after hepatectomy. In vitro, [3H]thymidine incorporation into hepatocyte DNA in the presence of hepatocyte growth factor is greater if 10(-5) mol/L norepinephrine is also present in the media.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of central administration of beta-endorphin on brain and liver DNA synthesis in preweanling rats

We have previously shown that central administration of beta-endorphin results in a reduction of ornithine decarboxylase activity. Ornithine decarboxylase catalyses the rate-limiting step in the biosynthesis of the polyamines putrescine, spermidine and spermine, thought to modulate nucleic acid synthesis. The present study examines the effects of intracisternal injection of beta-endorphin on brain and liver DNA synthesis in preweanling rats. In six-day-old rats, beta-endorphin (0.75 μg/g brain wt) produced approximately a 70% inhibition in brain and liver DNA synthesis 1h after injection, and values were still subnormal in both tissues 10 h later. Subcutaneous administration of beta-endorphin did not alter liver DNA synthesis. Thus, it is most likely that the suppressed liver DNA synthesis observed in animals given beta-endorphin intracisternally is mediated by central mechanisms. Co-administration of naloxone plus beta-endorphin intracisternally prevented the response, indicating an opioid receptormediated phenomenon. Naloxone alone caused small but significant increases in brain and liver DNA synthesis, suggesting a tonic influence on tissue DNA by endogenous opioids in the CNS. Acute inhibition of ornithine decarboxylase activity by alpha-difluoromethylornithine did not alter DNA synthesis, indicating that the decreases in DNA synthesis induced by beta-endorphin are unrelated to the ornithine decarboxylase/polyamine system. The effect appears to be restricted to early development as no significant changes in DNA synthesis were obtained in 20-day-old animals. The results from these studies indicate that CNS beta-endorphin has the ability to influence DNA synthesis in central as well as in peripheral tissues. Since changes in DNA biosynthetic rates reflect alterations in cell replication, perturbations in CNS beta-endorphin levels during early postnatal life could result in significant deficits in development.

Synthetic estrogens and tamoxifen as promoters of hepatocarcinogenesis

Previously, we demonstrated that the synthetic estrogens mestranol and ethinyl estradiol (EE) were strong promoters of hepatocarcinogenesis initiated in intact female rats by prior treatment with diethylnitrosamine (J. D. Yager, H. A. Campbell, D. S. Longnecker, B. D. Roebuck, and M. C. Benoit, Cancer Res. 1984; 44:3862–3869). In subsequent studies designed to elucidate possible mechanisms of promotion by EE, we investigated whether the antiestrogen tamoxifen was antagonistic to the effects of EE (J. D. Yager, B. D. Roebuck, T. L. Paluszcyk, and V. A. Memoli, Carcinogenesis 1986; 7:2007–2014). In these and more recent studies we found that tamoxifen inhibited the stimulatory effects of EE on pituitary size, liver DNA synthesis, and, in cultured hepatocytes, the potentiation by EE of epidermal growth factor-induced DNA synthesis. Furthermore, tamoxifen also inhibited the ability of EE to promote hepatocarcinogenesis. However, paradoxically, tamoxifen alone enhanced the appearance of γ-glutamyl transpeptidase positive foci in diethylnitrosamine-initiated livers indicating that it is a promoter of hepatocarcinogenesis.

Influence of the Calcium and Vitamin d Endocrine System on the “priming” of the Liver for Compensatory Growth

The liver is known to be sensitive to dietary challenge and to reduction in its functional mass. To investigate the influence of the vitamin D endocrine system on the hepatic growth response, liver DNA synthesis (S phase of cell cycle) was primed by dietary manipulation (high carbohydrate/protein-free diet x 3 d followed by high protein diet x 15-18 hr) in animals presenting various calcium and vitamin D status. Data indicate the dietary manipulation increased [3H]thymidine incorporation into DNA following vitamin D3 (D3) or 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) administration (p less than 0.0001) while vitamin D depleted (D-) rats (both hypo and normocalcemic) showed no significant increases over unchallenged rats (p less than 0.0001). Studies on the hepatic EGF receptor indicate that while no significant between-group difference was observed in receptor density or affinity, evaluation of the receptor density in relation to the [3H]thymidine incorporation response revealed a higher receptor density in responders (D3 and 1,25(OH)2D3 supplemented groups) with 30.2 +/- 1.4% maximum binding than in non-responders (hypo and normocalcemic D- groups) with 25.4 +/- 1.8% maximum EGF binding (p less than 0.03); EGF EC50 was found to be 50.2 +/- 4.4 and 63.8 +/- 9.7 ng/ml in responders and non-responders respectively (p = 0.1). These data indicate that vitamin D depletion is accompanied by hyporesponsiveness when challenged by a dietary protocol known to prime the liver for growth inducing stimuli.

Changes in serum and hepatic polyamine concentrations after 30%, 70% and 90% partial hepatectomy in rats

Polyamines have been reported to play an important role in stimulating hepatic regeneration after partial hepatectomy. To determine whether changes in systemic or hepatic polyamine concentrations correlate with the extent of the regenerative stimulus, serum and tissue putrescine, spermidine and spermine concentrations were determined in groups of adult male rats (n = 6 to 12/group) 0, 24, 48, and 72 hr after 30%, 70% or 90% partial hepatectomy. Serum putrescine levels were variably increased after partial hepatectomy and did not correlate with hepatic regenerative activity. Serum spermidine levels remained unaltered and spermine levels were undetectable both before and after partial hepatectomy. In hepatic tissue, only putrescine concentrations increased in proportion to the extent of hepatic resection. Moreover, there was a significant correlation between hepatic putrescine concentrations and restitution of liver mass (r = 0.778), DNA synthesis (r = 0.8026) and protein synthesis (r = 0.7034) (p less than 0.05). No significant correlations existed between hepatic spermidine or spermine concentrations and these parameters of hepatic regeneration. The results of this study support the hypothesis that hepatic putrescine plays an important role in stimulating hepatic regeneration after partial hepatectomy.

Hepatotrophic growth factor in blood of mice treated with carbon tetrachloride

The presence of a human hepatocyte growth factor (hHGF)-like DNA-synthesis promoter in platelet-poor serum of mice with liver injury was examined. Activity of the serum for stimulating DNA synthesis in cultured rat hepatocytes was low in untreated or vehicle-treated mice, but markedly increased 24 h after carbon tetrachloride administration and then dropped to normal levels prior to the peak of liver DNA synthesis. The effect of the serum was additive with the maximal effects of mouse and human epidermal growth factors, but not with that of hHGF. The growth-stimulating factor in the mouse serum, like hHGF, had affinity for heparin and was heat-labile. These results indicate that the level of a serum hHGF-like hepatocyte growth factor increased in mice treated with carbon tetrachloride prior to liver regeneration.

Quinoline: Unscheduled DNA synthesis and mitogenesis data from the rat liver in vivo

Quinoline is a specific and potent carcinogen to the rat and mouse liver. Studies are described here in which it was evaluated for its ability to initiate unscheduled DNA synthesis (UDS) in the rat liver in vivo. Although some individual animals showed indications of a marginal response the absence of clear group positive responses and the lack of an obvious dose relationship precluded the classification of quinoline as positive. The analogous NTP non-carcinogen 8-hydroxyquinoline was shown also to be devoid of UDS activity. Quinoline did, however, induce a potent mitogenic response in the rat liver between 24 and 48 hr after oral dosing of 200-500 mg/kg. Under similar conditions of test, 8-hydroxyquinoline was essentially inactive. These data represent a further instance in which mitogenicity in the liver appears to correlate better with carcinogenicity than does genotoxicity; but it may not be that simple, as discussed in the text. A single dose of quinoline was shown to act as a replacement for surgical partial hepatectomy in the liver micronucleus assay described by Tates, consistent with its potent mitogenicity.

Scheduled and Unscheduled DNA Synthesis in Chick Embryo Liver Following X-irradiation and Treatment with DNA Repair Inhibitorsin Vivo

Three hours following X-irradiation of chick embryos with doses of 4 and 8 Gy the in vitro incorporation of tritiated thymidine [( 3H]dT) into DNA (scheduled DNA synthesis, ss) of hepatocytes was reduced to about one-third. Within 24 h after the exposure, ss returned to control values. The return of ss to a normal rate could be strongly inhibited by 2',3'-dideoxythymidine (ddT), and to a lesser extent by 1-beta-D-arabinofuranosylcytosine (araC). In strong contrast to ss, the hydroxyurea (hu)-resistant [3H]dT incorporation (unscheduled DNA synthesis, us) showed a highly significant increase 24 h after treatment of the embryos with araC and/or X-irradiation. Autoradiographic studies revealed no change of total [3H]dT labelling frequency in the whole chick embryo liver 24 h after treatment with araC and/or X-irradiation, but a persistent depression of ss and a simultaneous increase of us. The histological discrimination between affected and non-affected areas argue for a stimulation of DNA synthesis as an antecedent of subsequent mitosis and reparative proliferation adjacent to cell necrosis. It is suggested that the fast recovery of ss in the 12-15-day-old chick embryo is due to an efficient DNA repair system for which DNA polymerase beta is important. The increase of hu-resistance may be an expression of an aberrant DNA synthesis.

Immunohistochemical detection of CCl 4-induced, mitosis-related DNA synthesis in livers of trout and rat

Monoclonal antibodies (MAB) directed against 5-bromo, 2′deoxyuridine (BrdU) were used to detect cells in which carbon tetrachloride-induced DNA synthesis had occurred in livers of rainbow trout ( Salmo gairdneri) and of rats. Trout exposed to CCl 4 (single i.p. injection, 2.5 ml/kg) were injected 45 h later with BrdU (50 mg/kg); 3 h after BrdU administration, trout were sacrificed and livers were processed for immunohistochemistry and histopathology. Anti-BrdU immunohistochemical analysis revealed labeling of only occasional hepatocyte nuclei. On the other hand, in rats given a single i.p. injection of CCl 4 (1.5 ml/kg) and injected 21 h later (3 h prior to sacrifice) with BrdU (50 mg/kg), immunohistochemistry revealed labeling of most hepatocyte nuclei. When control animals (not treated with CCl 4) of both species were administered BrdU and processed for immunohistochemistry, no labeled hepatocyte nuclei were detected. Immunohistochemical studies with cultured human fibroblasts treated with N-methyl- N′-, nitro- N-nitroso guanidine (MNNG) and BrdU demonstrated that replicative DNA synthesis was detected, but unscheduled DNA synthesis (UDS or DNA repair synthesis) was not detected. Therefore, MAB directed against BrdU proved useful in detecting chemically-induced replicative changes in liver cells from both trout and rat. Freeze-dried glycol methacrylate (GMA)-embedded tissue yielded excellent immunostaining and morphology. The immunohistochemical reaction appears particularly applicable to analyzing mitotic response and eliminates production of radioactive waste water when analyzing chemically-induced nuclear changes in aquatic species.

Stimulation of liver growth and DNA synthesis by glucosylceramide.

The nature of the growth-stimulating effect of glucosylceramide was studied. Mice were injected intraperitoneally with emulsified glucosylceramide and conduritol B epoxide, an inhibitor of cerebroside glucosidase. Within one or two days, the liver grew 18-24%, as reported. Two enzymes involved in DNA synthesis also increased more than the weight. The total liver activity of thymidine kinase increased 46-73%, and the total activity of ornithine decarboxylase increased as much as 101%. It is suggested that elevated liver levels of glucocerebroside stimulate cell proliferation through a relatively direct mechanism.

Effect of nafenopin and clofibrate on uptake and utilization of labeled thymidine for DNA synthesis in rat liver and kidney

The mitogenic effect of nafenopin and clofibrate in the liver is paralleled by a decreased utilization of [14C]thymidine for kidney DNA synthesis. Analogous changes in the liver and kidney DNA biosynthesis after nafenopin administration occur if [14C]orotic acid is added as a precursor of DNA pyrimidines. There is a time correlation between the uptake of labeled thymidine and its utilization for DNA synthesis in the liver during the initial stages of the mitogenic effect of the drug. Later on--after administration of a low dose of nafenopin or of repeated doses of clofibrate--the specific activity of DNA thymine does not differ from the values observed with the control group: the total radioactivity of the thymine components of the acid-soluble extract is even increased. The existence of a correlation between decreased utilization of [14C]thymidine for DNA synthesis and the total radioactivity of the thymine components of the acid-soluble extract in kidney can be observed only during the early stages after nafenopin administration. Subsequently, when the depression of the specific activity of kidney DNA thymine still persists, the specific activity of the thymine components of the acid-soluble extract does not differ from control values. The utilization of [14C]thymidine for DNA synthesis in the kidney is decreased after repeated clofibrate administration only 24 h after the last dose of the drug; these values later increase and even exceed control values. The total radioactivity of the thymine components of the kidney acid-soluble extract is unchanged.

Prolactin as a mammalian mitogen and tumor promoter

Cellular proliferation and differentiation of the mammalian mammary gland requires a medley of hormones including the anterior pituitary hormone, PRL. Recent evidence extends the role of PRL as a mammalian mitogen to cells in several physiological systems not directly involved in reproductive functions, such as liver and lymphocytes. PRL administration induces biochemical markers expressed during the G 1 phase of cell cycle and activates DNA synthesis in rat liver. Chronic PRL treatment causes hepatomegaly, reflecting its stimulation of the proliferative process. In vitro, a lactogen-dependent cell line, the Nb2 rat node lymphoma cell, serves as a useful paradigm to study PRL action on mitogenesis. These cells, when cultured in the presence of lactogens, proliferate in a dose-dependent manner. The effects of various pharmacological agents on discrete phases of the cell cycle may be readily assessed in these cells since PRL-stimulated entry into cycle is signalled by an elevation of ODC activity at 6 hr and entry into S-phase at 6–12 hr. The parallel effects of phorbol ester tumor promoters and PRL on cell cycle progression in Nb2 lymphoma cells and in hepatic proliferation suggest that PRL may likewise mediate proliferation in aberrant growth conditions such as neoplasia. The data presented support the hypothesis that PRL is capable of promoting hepatocarcinogenesis. Its chronic administration after a hepatic initiating agent stimulated the development of histochemical and biochemical markers characteristic of preneoplasia. Further, the effect of PRL was comparable to that of the hepatocarcinogen when either was administered alone. Thus, hyperprolactinemia may serve to promote the development of hepatic tumors. Phorbol esters are thought to promote tumorigenesis by directly activating PKC. In the Nb2 lymphoma cell model, tumor promoting phorbol esters mimic the effects of PRL. Similarily, PRL-stimulated enzyme induction in liver is mirrored by phorbol ester treatment, and inhibitors of PKC block PRL-stimulated mitogenesis in Nb2 cells. Further, PRL or TPA administration to rats causes translocation of PKC activity from the hepatic cytosol to the membrane fraction, reflecting kinase activation. Therefore, PRL activation of PKC appears to be a physiological phenomenon of general significance, occurring as the result of lactogen receptor stimulation and serving to transmit intracellular signals linked to the regulation of mitogenesis. Further study is required to more fully define the scope of PRL-mediated mitogenic actions as well as its effects on the expression of differentiated products in tissues and cells.

Liver DNA alkylation after a single carcinogenic dose of dimethylnitrosamine to newborn and adult CFW Swiss mice

N-nitrosodimethylamine N-demethylase activity, DNA alkylation, capacity for O 6-methylguanine repair and cell proliferation were measured in livers of newborn and adult CFW mice after a single carcinogenic dose of DMNA. DNA alkylation was found in newborn and adult mouse livers but it was significantly higher in the newborn. 6- and 7-methyl substitutions of guanine were identified by HPLC analysis in newborn and in adult mouse livers. Metabolic 14C incorporation into adenine and guanine was observed only in liver DNA of newborns. O 6-methylguanine levels were higher in newborn than adult mice after a single i.p. dose of [ 14C]DMNA. Liver DNA repair capacity measured as O 6-meG-DNA methyltransferase was higher in adults than in newborns. De novo liver DNA synthesis was more inhibited by DMNA pretreatment in newborn than in adult mice. The relationship between these parameters and the greater neonatal liver tumor susceptibility is discussed.