Ribonucleotide reductase: a possible target for orotic acid induced mitoinhibition in normal hepatocytes in primary culture

The regulation of ribonucleoside diphosphate reductase by the tumor promoter orotic acid in normal rat liver in vivo

Our earlier studies have shown that in normal hepatocytes, orotic acid (OA) inhibits DNA synthesis induced by several growth factors in vitro and after two-thirds partial hepatectomy (PH) in vivo. As in the normal liver OA induces an imbalance in nucleotide pools (specifically, an increase in uridine nucleotides, including deoxyuridine nucleotides, and a decrease in adenosine nucleotides, including ATP) and creation of this imbalance is crucial for the mitoinhibitory effects of OA, we hypothesized that ribonucleoside diphosphate reductase (RNR), a key enzyme in DNA synthesis that is regulated by nucleotide/deoxynucleotide levels, might be one of the targets for the inhibition of DNA synthesis by OA. To test this hypothesis, we subjected male Fischer 344 rats (130-150 g) to two-thirds PH in the absence or in the presence of OA (a 300-mg tablet of OA methyl ester implanted intraperitoneally at the time of two-thirds PH). The rats were killed at different times later, and their livers were processed for analysis of levels of RNR enzyme activity, protein, and mRNA transcripts. The results obtained indicated that treatment with OA resulted in a near-100% inhibition of RNR induced by two-thirds PH in rat liver, as monitored by enzyme activity and protein level. Furthermore, this inhibition was paralleled by a decrease in the mRNA transcripts for both the M1 and M2 subunits of RNR. Nuclear run-off assays indicated that this decrease in the levels of mRNA transcripts could not be attributed to an effect on transcription. However, administration of OA 20 h after two-thirds PH, when RNR mRNA transcripts were maximally induced, resulted in increased degradation of the RNR M1 and M2 subunits. Taken together, these results indicate that OA treatment decreases RNR levels induced by two-thirds PH, at the levels of enzyme activity, protein, and mRNA transcripts, and the decreased levels of mRNA transcripts appeared to be due to increased degradation of the transcripts.

Inhibition of in vivo rat liver regeneration by 2-acetylaminofluorene affects the regulation of cell cycle-related proteins

The effects of dietary 2-acetylaminofluorene (2-AAF) on cell cycle-related proteins was studied in regenerating livers from male Wistar rats. The levels of cyclins, cyclin dependent kinases (cdks), and related proteins were studied at different times during the first cell cycle after partial hepatectomy (PH). The frequency of proliferation cell nuclear antigen (PCNA)-positive nuclei, a marker of S phase progression, was almost zero during the first 27 hours after PH in the mitoinhibited 2-AAF-treated rats, while about 50% of the nuclei were labeled 24 hours after PH in control animals. Accordingly, Western blot tests showed markedly elevated PCNA protein levels from 18 hours to the end of S phase in untreated animals but no upregulation in response to 2-AAF. Compared with control animals, animals treated with 2-AAF showed increased levels of cdk 4 and cyclin D3 from 12 and 15 hours after PH, respectively, and altered cyclicity in cyclin D3 expression. No effects on cyclin E were observed, while the increase in cdk 2 levels in control animals during late G1/S (15-27 hours) was abolished by 2-AAF. p53 was induced by 2-AAF treatment during the same period, with a peak at 24 hours. The protein detected with p21 antibodies was highly expressed in unstimulated hepatocytes in control animals, and further increased by 2-AAF. The expression was sustained until 15 hours after PH in control rats while 2-AAF-treated animals lacked detectable protein during this period; however, a transient increase was observed at 21 hours. Thus, 2-AAF affects several parameters of cell cycle regulation of possible relevance for its inhibitory effects on hepatocyte proliferation in vivo.

Transient inhibition by orotic acid does not abolish the in vivo response of rat hepatocytes to a direct mitogen, lead nitrate

BACKGROUND

Orotic acid (OA) is able to inhibit hepatocyte proliferation in vivo induced by 2/3 partial hepatectomy. The present studies were aimed at establishing: (i) whether OA also inhibits hepatocyte proliferation induced by a direct mitogen and, if so (ii) whether the stimulus provided by the mitogen is still expressed following transient inhibition by OA.

METHODS/RESULTS

In the first experiment male Wistar rats were injected with either lead nitrate (100 mumol/kg, i.v.) or saline and 20 h later some animals receiving the mitogen were also implanted with a 400-mg OA tablet (as OA-methyl ester. i.p.). Multiple injections of 3H-thymidine were given to each rat (50 microCi each, 6 h apart, i.p.) until 2 h before killing. All groups were killed 3 days after the initial treatment. Results indicated that OA almost completely inhibited hepatocyte DNA synthesis and labelling induced by lead nitrate (e.g. labelling index was 1.9 +/- 0.5% in the saline-treated group, 44.7 +/- 4.0% in the lead nitrate group and 1.4 +/- 0.3% in the group receiving lead nitrate + OA). Based on the above results, in a second experiment rats were given a similar dose of lead nitrate and a subset of animals was implanted 20 h later with a 400-mg OA tablet, as previously described. Multiple doses of 3H-thymidine were again given to each rat (20 microCi each, 6 h apart) until 2 h before killing. Animals from both groups were killed at 3, 6 or 8 days after lead nitrate. Results indicated that, while at day 3 lead nitrate-induced DNA synthesis was effectively inhibited by OA, at day 6 the proliferative response was resumed in the group receiving OA. Cumulative labelling index over 6 days was 30.3 +/- 1.4 in rats given the mitogen alone and 52.1 +/- 2.2 in the group exposed to lead nitrate + OA.

CONCLUSIONS

These data indicate that: (i) OA is also able to inhibit hepatocyte proliferation induced by a direct mitogen such as lead nitrate; this, in turn, suggests that its inhibitory effect is not unique to the stimulus elicited by partial hepatectomy. (ii) The proliferative response triggered by the mitogen is not abolished by the transient (3-4 days) inhibitory phase imposed by OA. Possible mechanisms underlying these effects are considered in the discussion.

Sequential histopathological analysis of hepatocarcinogenesis in rats during promotion with orotic acid

In the present study, sequential histopathological changes during hepatocarcinogenesis promoted by orotic acid were examined. Male Fischer 344 rats were given 1,2-dimethylhydrazine.2HCl (100 mg/kg, i.p.) 18 h after 2/3 partial hepatectomy. After 1 week of recovery, they were divided into 2 groups; group 1 was continued on a semisynthetic basal diet while the group 2 received the basal diet containing 1% orotic acid. Rats were sacrificed after 5, 10, 20, 29, 40 and 53 weeks of promotion. Histopathological analysis indicated that emergence of hepatocellular carcinomas was preceded first by foci of morphologically and histochemically altered hepatocytes and then by the appearance of hepatocyte nodules. Clear cell foci, eosinophilic ground glass foci and gamma-glutamyltransferase positive foci were detectable after 5 weeks in initiated rats fed orotic acid. Hepatocyte nodules developed in 56% of the rats after 20 weeks of promotion, while the first hepatocellular carcinoma was observed in one rat sacrificed after 29 weeks of orotic acid promotion. Cancer incidence steadily increased with the duration of the orotic acid treatment and 59% developed hepatocellular carcinomas with 30% metastasis to lungs by 53 weeks of promotion. A relevant feature of this model is that during exposure to orotic acid no liver hyperplasia, nor bile duct or oval cell proliferation were seen and the liver architecture in the tissue surrounding focal lesions was well preserved throughout the sequence.