Involvement of ribonucleotide reductase in cellular differentiation

Role of antimetabolites of purine and pyrimidine nucleotide metabolism in tumor cell differentiation

Transformed cells are characterized by imbalances in metabolic routes. In particular, different key enzymes of nucleotide metabolism and DNA biosynthesis, such as CTP synthetase, thymidylate synthase, dihydrofolate reductase, IMP dehydrogenase, ribonucleotide reductase, DNA polymerase, and DNA methyltransferase, are markedly up-regulated in certain tumor cells. Together with the concomitant down-modulation of the purine and pyrimidine degradation enzymes, the increased anabolic propensity supports the excessive proliferation of transformed cells. However, many types of cancer cells have maintained the ability to differentiate terminally into mature, non-proliferating cells not only in response to physiological receptor ligands, such as retinoic acid, vitamin D metabolites, and cytokines, but also following exposure to a wide variety of non-physiological agents such as antimetabolites. Interestingly, induction of tumor cell differentiation is often associated with reversal of the transformation-related enzyme deregulations. An important class of differentiating compounds comprises the antimetabolites of purine and pyrimidine nucleotide metabolism and nucleic acid synthesis, the majority being structural analogs of natural nucleosides. The CTP synthetase inhibitors cyclopentenylcytosine and 3-deazauridine, the thymidylate synthase inhibitor 5-fluoro-2'-deoxyuridine, the dihydrofolate reductase inhibitor methotrexate, the IMP dehydrogenase inhibitors tiazofurin, ribavirin, 5-ethynyl-1-beta-D-ribofuranosylimidazole-4-carboxamide (EICAR) and mycophenolic acid, the ribonucleotide reductase inhibitors hydroxyurea and deferoxamine, and the DNA polymerase inhibitors ara-C, 9-(2-phosphonylmethoxyethyl)adenine (PMEA), and aphidicolin, as well as several nucleoside analogs perturbing the DNA methylation pattern, have been found to induce tumor cell differentiation through impairment of DNA synthesis and/or function. Thus, by selectively targeting those anabolic enzymes that contribute to the neoplastic behavior of cancer cells, the normal cellular differentiation program may be reactivated and the malignant phenotype suppressed.

Lack of correlation between deoxyribonucleotide pool sizes, spontaneous mutation rates and malignant potential in Chinese hamster ovary cells

To examine the relationship between altered spontaneous mutation rates and malignant characteristics of cells, two hydroxyurea-resistant Chinese hamster ovary cell lines, with alterations in ribonucleotide reductase, were examined for their rates of spontaneous mutation to 6-thioguanine and ouabain resistance, tumor growth rates and their ability to form experimental lung metastases. The most resistant cell line, HR-R2T, showed no changes in the rate of spontaneous mutation to 6-thioguanine or ouabain resistance compared to the parental wild-type cell line; however, the mutant line formed lung metastases in experimental metastasis assays with BALB/c nu/nu mice, and exhibited metastatic abilities significantly different from the wild-type population. Furthermore, the HR-R2T population did not show imbalances in any of the deoxyribonucleoside triphosphate pool sizes, which are frequently observed in cells altered in ribonucleotide reductase activity. The second hydroxyurea-resistant line, HNR-AT, had gross alterations in dCTP and dGTP pools and although the rate of spontaneous mutation to 6-thioguanione resistance was unaltered, it showed a moderate decrease in the rate of spontaneous mutation to ouabain resistance when compared to the parental wild-type population. Interestingly, the HNR-AT cell line did not form any lung metastases in the experimental metastasis assay. Both mutant cell lines, HR-R2T, and HNR-AT, had increased tumor growth rates in C57 BALB/c "beige" nude (nu/nu) mice as compared to the parental wild-type population. In total, the results obtained with the two mutant cell lines question the association of altered mutation rates with increased metastatic potential. Although several explanations are possible for the altered malignant properties exhibited by HR-R2T and HNR-AT cells, it is interesting to note that the results are consistent with earlier suggestions that changes in ribonucleotide reductase may accompany modifications in the malignant characteristics of cells.

Inhibition of human leukaemic thymidylate kinase and L1210 ribonucleotide reductase by dinucleotides of adenosine and thymidine and their phosphonate analogues

Dinucleotides of adenosine and thymidine in the ApnT series (n = 3,4,5 and 6) and their corresponding phosphonate analogues, where a methylene group replaces the oxygen between the alpha and beta phosphorus atoms adjacent to thymidine, have been evaluated as inhibitors of human leukaemic thymidylate kinase (dTMP kinase, EC 2.7.4.9) and ribonucleotide reductase (EC 1.17.4.1) from L1210 cells. Ap3T, Ap4T, Ap2cpT and Ap3cpT were poor inhibitors of both enzymes. Ap5T, Ap6T and their phosphonate analogues were potent inhibitors of dTMP kinase, possibly acting as bisubstrate analogues (IC50 values: Ap5T, 7.9 microM; Ap4cpT, 5.8 microM; Ap6T, 5.4 microM; Ap4cpT, 4.0 microM). For CDP reductase, where these compounds may bridge activity/effector sites on the M1 subunit of the enzyme, Ap5T and Ap6T were inhibitors with IC50 values of 14.4 microM and 20.3 microM respectively. The phosphonate series of compounds was far less active. The thymidine moiety of the compounds was essential for inhibition since Ap5A was inactive against both enzymes. dTTP, although a poor inhibitor of thymidylate kinase was a potent negative effector of CDP reductase (IC50, 19.3 microM). Significantly, Ap5T was not hydrolysed to release dTTP under the conditions of the assay. These studies show that the activities of both enzymes may be modulated by nucleotide analogues.

Altered expression of ribonucleotide reductase and role of M2 gene amplification in hydroxyurea-resistant hamster, mouse, rat, and human cell lines

Five hamster, mouse, and rat cell lines resistant to the cytotoxic effects of hydroxyurea have been characterized. All cell lines contained increased ribonucleotide reductase activity, elevated levels of the M2 component of ribonucleotide reductase as judged by electron paramagnetic resonance spectroscopy, and increased copies of M2 mRNA as determined by Northern blot analysis. Two species of M2 mRNA were detected in rodent cell lines, a high-molecular-weight species of approximately 3.4 kb in hamster and rat cells and about 2.1 kb in mouse cells. The low molecular-weight M2 mRNA was about 1.6 kb in all rodent lines. Northern blot analysis showed that the mRNA for the other component of ribonucleotide reductase, M1, was not markedly elevated in the drug-resistant cells and existed as a single 3.1-kb species. Four of the five resistant lines contained an M2 gene amplification as determined by Southern blot analysis, providing direct evidence to support earlier suggestions that hydroxyurea resistance is often accompanied by amplification of a ribonucleotide reductase gene. An increase in gene dosage was detected even in cells exhibiting only modest drug-resistance properties. No evidence for amplification of the M1 gene of ribonucleotide reductase was found. In keeping with these observations with drug-resistant rodent lines, a human (HeLa) cell line resistant to hydroxyurea was also found to contain increased levels of two M2 mRNA species (about 3.4 and 1.6 kb) and exhibited M2 gene amplification. One hamster cell line resembled the other resistant rodent lines in cellular characteristics but did not show amplification of either the M1 or M2 gene, providing an example of a drug-resistant mechanism in which an elevation of M2 mRNA has occurred without a concomitant increase in M2 gene copy number.

Studies on the effect of glycoprotein processing inhibitors on fusion of L6 myoblast cell lines

The effect of oligosaccharide processing inhibitors on the fusion of L6 myoblasts was studied. The glucosidase inhibitors, castanospermine, 1-deoxynojirimycin and N-methyl-deoxynojirimycin were potent inhibitors of myoblast fusion, as was the mannosidase II inhibitor, swainsonine. Inhibition of fusion was reversed when inhibitors were removed. However, the mannosidase I inhibitor, 1-deoxymannojirimycin did not inhibit fusion. Changes in cell membrane oligosaccharide structure were followed by monitoring the binding of concanavalin A (conA) and wheat germ agglutinin (WGA) to cell surface membranes in cells treated with processing inhibitors. All the processing inhibitors resulted in increased binding of conA and decreased binding of WGA; this is consistent with the known mechanisms of inhibition of the inhibitors used in the study. Inhibition of fusion by the processing inhibitors also resulted in reduced activities of creatine phosphokinase, an enzyme used as a marker for biochemical differentiation during fusion. Treatment of a non-differentiating conA-resistant cell line with processing inhibitors did not induce fusion, but the cells did show altered lectin-binding properties. The main conclusion drawn from these studies is that cell surface glycoproteins probably containing the mannose (Man)9 structure are important for the fusion reaction.

Characterization of a mouse cell line selected for hydroxyurea resistance by a stepwise procedure: drug-dependent overproduction of ribonucleotide reductase activity

Hydroxyurea was used as a selective agent in culture, to isolate by a stepwise procedure, a unique mouse L cell line called LHF which exhibited a stable resistance to high concentrations of drug (5 mM). LHF cells contained an elevation in ribonucleotide reductase activity which depended upon whether cells were previously cultured in the presence or absence of hydroxyurea. M1 immunoprecipitation and M2 titration experiments indicated that both ribonucleotide reductase subunits were elevated in drug-resistant cells. Interestingly, a very large drug-dependent change in the M2 activity (about a 100-fold) was observed. Studies on enzyme activity with cycloheximide and actinomycin D indicated that the hydroxyurea-dependent increase in activity required de novo protein synthesis and transcriptional activity. These results are different from other ribonucleotide reductase overproducing cell lines previously described, and indicate that hydroxyurea modulates enzyme activity by an interesting mechanism.

Ribonucleotide reductase: an intracellular target for the male antifertility agent, gossypol

Gossypol is a yellow phenolic compound which reversibly inhibits spermatogenesis making it one of the few effective male antifertility drugs. The cytotoxic effects of gossypol have been associated with its ability to irreversibly inhibit DNA synthesis by a previously unknown mechanism. The results of this study indicate that gossypol is a potent inhibitor of ribonucleotide reductase the rate limiting enzyme activity in DNA synthesis. Furthermore, in agreement with these enzyme studies, DNA synthesis in a hydroxyurea resistant cell line with high levels of ribonucleotide reductase activity showed increased resistance to gossypol when compared to wild type cells with normal levels of reductase activity. Ribonucleotide reductase is the first specific site of action documented for gossypol which can explain its recently described antiproliferative, cell cycle and toxic effects.

Ribonucleotide reductase activity during the senescence of normal human diploid fibroblasts in culture

Changes in ribonucleotide reductase occur during the senescence of normal human diploid fibroblasts in culture. Enzyme activity is significantly lower in cells and extracts at high mean population doublings (MPD) as compared to fibroblasts at low MPD. Although many of the kinetic properties of the enzyme remain unaltered in cells at high MPD, changes in the extent and kinetic mechanism of inhibition of CDP and ADP reductase activity by dATP, the overall negative effector of ribonucleotide reductase, were observed. These results and a previous observation that the four deoxyribonucleotide pools are markedly altered during in vitro senescence of human diploid fibroblasts, provide evidence for a link between ribonucleotide reduction, deoxyribonucleotide pools, and the establishment of the non-proliferative or senescent state.

On the importance of deoxyribonucleotide pools in the senescence of cultured human diploid fibroblasts

Previous studies have indicated that ribonucleotide reductase may participate in a mechanism of cellular senescence, which involves modifications in deoxyribonucleotide pools, the products of the reductase reaction. Since very little information about the levels of these pools in senescing cells is currently available, an analysis of deoxyribonucleotide levels was carried out in young, old and very old normal human diploid fibroblasts, as well as in a variant human fibroblast strain with an altered replicative life span. These studies indicated that there are marked age-related perturbations in these pools which imply that they may be fundamentally important in a process determining the nonproliferative or senescent state.