Localization of ribonucleotide reductase in mammalian cells.

The results of immunocytochemical studies using two different monoclonal antibodies against the M1 subunit of ribonucleotide reductase show an exclusively cytoplasmic localization of this subunit both in cultured MDBK and mouse 3T6 cells, and in cells from various rat tissues. By fluorescent light microscopy, there is a diffuse staining of the cytoplasm, while by electron microscopy the immunoreactive material appears to be associated with ribosomes. In the rat tissues, only actively dividing cells show M1-specific immunofluorescence revealing a strong correlation between the presence of protein M1 and DNA synthesis. Therefore M1 immunofluorescence could be used to study cell proliferation in normal, inflammatory or neoplastic tissue. A lesser variation in M1 staining is observed between individual cells in tissue culture, where most cells are positive, but neither here nor in the tissues examined are any cells with nuclear staining detected. We interpret our results to mean that in mammalian cells ribonucleotide reduction takes place in the cytoplasm and from there the deoxyribonucleotides are transported into the nucleus to serve in DNA synthesis.     

Inhibition of mammalian ribonucleotide reductase by cis-diamminedichloroplatinum(II).

Ribonucleotide reductase is a highly regulated, rate-limiting activity in the synthesis of DNA. A previous study has shown that the Escherichia coli enzyme is inhibited by the clinically important antitumor agent cis-diamminedichloroplatinum(II) (DDP), and this has led to the hypothesis that ribonucleotide reductase is an important site of action for this chemotherapeutic agent. This hypothesis has been directly tested in this investigation. We observed that DDP inhibits the mammalian ribonucleotide reductase, with 50% inhibition occurring at 0.3 mM. Unlike the E. coli enzyme where only one of the two protein components is targeted by DDP, we observed that both of the mammalian proteins (R1 and R2) were sites for the inhibitory activity of the drug. Colony-forming experiments, enzyme activity studies, and analyses of R1 and R2 message levels in mutant cell lines containing either high levels of ribonucleotide reductase activity or exhibiting resistance to the cytotoxic effects of DDP were used to further investigate the potential role of ribonucleotide reductase in DDP cytotoxic action and drug resistance. These studies did not support a hypothesis formulated in the earlier investigation that inhibition of ribonucleotide reductase is an important component of DDP cytotoxic activity or that it is a major participant in DDP resistance mechanisms. From a biological point of view, DDP is a very active drug, and in addition to its cytotoxic effects it is capable of inducing a variety of cellular changes. Whether or not the inhibition of mammalian ribonucleotide reductase activity that we have described in this study plays a role in mediating any of these other effects remains to be determined.     

Regulation of ribonucleotide reductase activity in mammalian cells

Mammalian ribonucleotide reductase catalyzes the rate-limiting for the de novo synthesis 2'-deoxyribonucleoside 5'-triphosphates. There is some suggestion that this step may also be the rate-limiting step of DNA synthesis. It is apparent that the level of the enzyme, ribonucleotide reductase, varies through the cell cycle and is highest in those tissues with the greatest proliferation rate. This increase in activity is associated with increased protein synthesis. The purified enzyme has been shown to be subject to strict allosteric regulation by the various nucleoside triphosphates and it has been proposed that allosteric regulation plays an important role in the level of ribonucleotide reductase activity which is expressed. All experimental data relating to this point, however, do not support the role of deoxyribonucleoside triphosphates as a major factor in determining cellular reductase activity during normal cell division. Several naturally occurring factors have been isolated from cells which lower ribonucleotide reductase activity in vitro. These factors have been found in tissues of low growth fraction and appear to be absent or low in tissues or high growth fraction such as tumor, regenerating liver and embryonic tissues. The expression of intracellular ribonucleotide reductase activity is therefore controlled at various levels and by various factors and the prevailing mode of regulation may vary throughout the cell cycle transverse and also in the various types of cells.     

Regulation of ribonucleotide reductase activity in intact mammalian cells

An intact cell assay system based upon Tween-80 permeabilization was used to investigate the regulation of ribonucleotide reductase activity in Chinese hamster ovary cells. Models used to explain the regulation of the enzyme have been based upon work carried out with cell-free extracts, although there is concern that the properties of such a complex enzyme would be modified by extraction procedures. We have used the intact cell assay system to evaluate, within whole cells, the current model of ribonucleotide reductase regulation. While some of the results agree with the proposals of the model, others do not. Most significantly, it was found that ribonucleotide reductase within the intact cell could simultaneously bind the nucleoside triphosphate activators for both CDP and ADP reductions. According to the model based upon studies with cell-free preparations, the binding of one of these nucleotides should exclude the binding of others. Also, studies on intracellular enzyme activity in the presence of combinations of nucleotide effectors indicate that GTP and perhaps dCTP should be included in a model for ribonucleotide reductase regulation. For example, GTP has the unique ability to modify through activation both ADP and CDP reductions, and synergistic effects were obtained for the reduction of CDP by various combinations of ATP and dCTP. In general, studies with intact cells suggest that the in vivo regulation of ribonucleotide reductase is more complex than predicted from enzyme work with cell-free preparations. A possible mechanism for the in vivo regulation of ribonucleotide reductase, which combines observations of enzyme activity in intact cells and recent reports of independent substrate-binding subunits in mammalian cells is discussed.     

DNA-fork displacement rates in cultured mammalian cells with mutations in ribonucleotide reductase

DNA-replication fork displacement rates were measured in mouse S49 lymphosarcoma cell lines and in derivatives of those cell lines. One of the derivatives lacks dCMP deaminase activity and two others bear defined mutations in ribonucleotide reductase. We also examined a revertant cell line that was selected from one of the ribonucleotide reductase mutants and has regained normal ribonucleotide reductase activity. Our results show a correlation between decreased fork-displacement rates and alterations in ribonucleotide reductase, suggesting a possible involvement of this enzyme in the replication apparatus.     

Altered ribonucleotide reductase activity in mammalian tissue culture cells resistant to hydroxyurea

Four Chinese hamster ovary cell lines and one mouse L cell line have been isolated which are resistant to the cytotoxic effects of hydroxyurea and guanazole. These five cell lines contain an altered ribonucleotide reductase activity as judged by a decreased sensitivity to the inhibitory action of both drugs. This is strong evidence that ribonucleotide reductase is one of the lethal sites of action for these two antitumour agents. The results are also consistent with the view that mammalian cell variants can arise from structural gene mutations.     

Partial purification and characterization of the ribonucleotide reductase induced by herpes simplex virus infection of mammalian cells.

In this report we confirm and further characterize the induction of a novel ribonucleotide reductase after herpes simplex virus infection of mammalian cells. Induction of the enzyme was observed at a multiplicity of infection of 1 PFU/cell or greater and was found to be maximal (three- to sixfold the activity in mock-infected controls at 6 to 8 h postinfection at a multiplicity of infection of 10 PFU/cell. Partial purification and subsequent characterization of the reductase activity from infected cells demonstrated the existence of two enzymes which could be separated by precipitation with ammonium sulfate. One of the activities precipitated at between 35 and 55% salt saturation, as did the enzyme from control cells, whereas the novel activity precipitated at 0 to 35% saturation. This latter enzyme was similar to the herpes simplex virus-induced reductase described by others in its lack of requirement for Mg2 and its resistance to inhibition by dTTP and dATP; in addition, we found that it was inhibited by ATP, whereas the enzyme from control cells displayed an absolute requirement for the nucleotide. Both enzymes were equally inhibited by pyridoxal phosphate and showed similar cold and heat stability. The enzyme induced by herpes simplex virus infection, however, was much more labile than the control enzyme upon purification.     

Models of the regulation of ribonucleotide reductase and their evaluation in intact mammalian cells

Enzymological studies have shown that ribonucleotide reductase is allosterically inhibited or stimulated by endproduct deoxyribonucleoside triphosphates, and various investigators have put forward schematized models of the regulation of this enzyme; the various models have in practice been considered to be equivalent or at least very similar. Here each of eleven published models is critically examined, and it is shown that there are important differences among them that have not previously been appreciated. In addition, studies using intact mammalian cells have generally been taken as being consistent with the enzymological data, but this point has not received much systematic attention. The available data are examined critically, and the implications of multiple, partially different models are considered.     

Characterization of the free radical of mammalian ribonucleotide reductase

Mouse fibroblast 3T6 cells, selected for resistance to hydroxyurea, were shown to overproduce protein M2, one of the two nonidentical subunits of mammalian ribonucleotide reductase. Packed resistant cells gave an EPR signal at 77 K very much resembling the signal given by the tyrosine-free radical of the B2 subunit of Escherichia coli ribonucleotide reductase. Also, the M2-specific free radical was shown to be located at a tyrosine residue. Of the known tyrosine-free radicals of ribonucleotide reductases from E. coli, bacteriophage T4 infected E. coli and pseudorabies virus infected mouse L cells, the M2-specific EPR signal is most closely similar to the signal of the T4 radical. The small differences in the low temperature EPR signals between these four highly conserved tyrosine-free radical structures can be explained by slightly different angles of the beta-methylene group in relation to the plane of the aromatic ring of tyrosine, reflecting different conformations of the polypeptide chain around the tyrosines. The pronounced difference in microwave saturation between the E. coli B2 tyrosine radical EPR signal and the M2 signal could be due to their different interactions with unspecific paramagnetic ions or with the antiferromagnetically coupled iron pair, shown to be present in the E. coli enzyme and postulated also for the mammalian enzyme. A difference in the iron-radical center between the bacterial and mammalian ribonucleotide reductase is also observed in the ability to regenerate the free radical structure. In contrast to the B2 radical, the M2 tyrosine free radical could be regenerated by merely adding dithiothreitol in the presence of O2 to a cell extract where the radical had previously been destroyed by hydroxyurea treatment.     

Evidence that mammalian ribonucleotide reductase is a nuclear membrane associated glycoprotein

Epitope-specific antibodies to the M1 and M2 subunits of mammalian ribonucleotide reductase were prepared using peptides predicted to have a high antigenic index. Western blotting demonstrated that the anti-M1 antibody was specific for the 89-kilodalton M1 subunit (and its degradation fragments) and the anti-M2 antibody specifically recognized the 45-kilodalton M2 subunit. Both antibodies inhibited the CDP-reductase activity of the holoenzyme. Using these antibodies, both the M1 and M2 subunits were shown to be localized in the cytoplasm and in the nuclear regions of a number of cell types, including B77 avian sarcoma virus transformed NRK cells, T51B rat liver cells, 5123tc hepatoma cells, and rat liver cells in vivo. In addition, the M1 subunit was found to be localized as a halo around isolated rat liver nuclei. Biochemical analysis of the cytoplasmic fraction of liver cells and a Triton X-100 wash of nuclei from these cells confirmed the location of the enzyme activity in these cellular compartments. The M1 subunit appears to be glycosylated, as indicated by its retention on a Affi-Gel-concanavalin A affinity column. Therefore, in mammalian cells ribonucleotide reductase appears to be not only in the cytoplasm, but is also associated with the nuclear membrane or nuclear lamina. The activity of the enzyme in the membrane fraction changes dynamically during the cell cycle.     

Structure-based optimization of peptide inhibitors of mammalian ribonucleotide reductase

Mammalian ribonucleotide reductase (mRR), a potential target for cancer intervention, is composed of two subunits, mR1 and mR2, whose association is critical for enzyme activity. In this article we describe the structural features of the mRR-inhibitor Ac-F-c[ELAK]-DF (Peptide 3) while bound to the mR1 subunit as determined by transferred NOEs. Peptide 3 is a cyclic analogue of the N-acetylated form of the heptapeptide C-terminus of the mR2 subunit (Ac-FTLDADF), which is the link between the two subunits and previously shown to be the minimal sequence inhibitor mRR by competing with mR2 for binding to mR1. Structural refinement employing an ensemble-based, full-relaxation matrix approach resulted in two structures varying in the conformations of F(1) and the cyclic lactam side chains of E(2) and K(5). The remainder of the molecule, both backbone and side chains, is extremely well-defined, with an RMSD of 0.54 A. The structural features of this conformationally constrained analogue provide unique insight into the requirements for binding to mR1, critical for further inhibitor development.     

More potent linear peptide inhibitors of mammalian ribonucleotide reductase

Mammalian ribonucleotide reductase (mRR) is a chemotherapeutic target. The enzyme is composed of two subunits (mR1 and mR2) and is inhibited by Ac-FTLDADF (denoted P7), corresponding to the C-terminus of mR2, which disrupts mRR quaternary structure by competing with mR2 for binding to mR1. The tripeptide FmocWFF acts similarly. Here we report on the use of small, focused libraries to identify Fmoc derivatives of tetra and hexapeptides having comparable or considerably higher activities than P7 toward inhibition of mRR.     

Nucleoside 5'-diphosphates as effectors of mammalian ribonucleotide reductase

It was found that nucleoside 5'-diphosphates could serve as effectors of ribonucleotide reductase. ADP was an activator of CDP reduction; ADP reduction was activated by dGDP; GDP reduction was activated by dTDP. Conversely, dADP inhibited the reduction of CDP, UDP, GDP, and ADP; dGDP inhibited UDP and GDP reductions; and dTDP inhibited UDP reduction. The inhibition of UDP reduction by dADP, dTDP, and dGDP was at least equal to that observed for dATP, dTTP, and dGTP, respectively. In these experiments with the nucleoside diphosphates as effectors, high-pressure liquid chromatography analysis of the reaction mixtures showed that no nucleoside 5'-triphosphates were found during the reaction period which could account for the effects seen with the nucleoside diphosphates as effectors. Further experiments were carried out in which adenyl-5'-yl imidodiphosphate was used as the positive effector of CDP and UDP reductions in place of ATP. Under these conditions, CDP and UDP reductions were inhibited by dADP, dTDP, and dGDP to the same extent observed in the presence of ATP. ADP served not only as a substrate for ribonucleotide reductase but also as an activator of CDP and UDP reductions. The direct products (dNDPs) also served as positive and negative effectors. Dixon plots indicated that the dNDPs were acting as noncompetitive inhibitors with respect to the substrate. ADP increased the sedimentation velocity of the ribonucleotide reductase in a manner similar to ATP. These data are consistent with the allosteric effects seen with the nucleoside 5'-triphosphates. Additionally, from the thorough study of the role of effectors on UDP reduction, it is clear that UDP reduction was most sensitive to the negative effectors dATP, dADP, dTTP, dTDP, dGTP, and dGDP.     

Amplification of the genes for both components of ribonucleotide reductase in hydroxyurea resistant mammalian cells

Ribonucleotide reductase catalyzes the formation of deoxyribonucleotides from ribonucleoside diphosphate precursors, and is a rate-limiting step in the synthesis of DNA. The enzyme consists of two dissimilar subunits usually called M1 and M2. The antitumor agent, hydroxyurea, is a specific inhibitor of DNA synthesis and acts by destroying the tyrosyl free radical of the M2 subunit of ribonucleotide reductase. Two highly drug resistant cell lines designated HR-15 and HR-30 were isolated by exposing a population of mouse L cells to increasing concentrations of hydroxyurea. HR-15 and HR-30 cells contained elevated levels of ribonucleotide reductase activity, and were 68 and 103 times, respectively, more resistant than wild type to the cytotoxic effects of hydroxyurea. Northern and Southern blot analysis indicated that the two drug resistant lines contained elevated levels of M2 mRNA and M2 gene copy numbers. Similar studies with M1 specific cDNA demonstrated that HR-15 and HR-30 cell lines also contained increased M1 message levels, and showed M1 gene amplification. Mutant cell lines altered in expression and copy numbers for both the M1 and M2 genes are useful for obtaining information relevant to the regulation of ribonucleotide reductase, and its role in DNA synthesis and cell proliferation.     

Subunit M2 of mammalian ribonucleotide reductase. Characterization of a homogeneous protein isolated from M2-overproducing mouse cells

The M2 subunit of mammalian ribonucleotide reductase was purified to homogeneity from hydroxyurea-resistant, M2-overproducing mouse cells. The purification procedure involved affinity chromatography on an anti-tubulin antibody-Sepharose column and high performance gel permeation chromatography. The pure protein is a dimer of Mr = 88,000, containing stoichiometric amounts of a non-heme iron center and a tyrosyl free radical. The radical is destroyed by hydroxyurea but can readily be regenerated on incubation of the radical-free protein alone with iron-dithiothreitol in the presence of air. The ability to spontaneously regenerate the tyrosyl radical distinguishes protein M2 from the corresponding subunit of Escherichia coli ribonucleotide reductase, protein B2, but apart from that the two proteins are very similar.     

Control of ribonucleotide reductase in heat- and cold-sensitive mammalian cell-cycle mutants

Two heat-sensitive (reversibly arrested in G1 phase at 39.5 degrees C, multiplying at 33 degrees C) and two cold-sensitive (reversibly arrested in G1 phase at 33 degrees C, multiplying at 39.5 degrees C) cell-cycle mutants of the P-815-X2 murine mastocytoma line were tested for ribonucleotide reductase activity, using cells made permeable to nucleotides. After transfer of the heat-sensitive mutant cells to 39.5 degrees C, ribonucleotide reductase activity, similar to thymidine kinase (Schneider, E., Müller, B. and Schindler, R. (1983) Biochim. Biophys. Acta 741, 77-85), but unlike DNA polymerase alpha (Schneider, E., Müller, B. and Schindler, R. (1985) Biochim. Biophys. Acta 825, 375-383), decreased rapidly and in parallel with numbers of cells in S phase, whereas in the cold-sensitive mutant cells brought to 33 degrees C, ribonucleotide reductase activity decreased approx. 8 h later than numbers of DNA-synthesizing cells. When arrested heat- or cold-sensitive mutant cells were returned to the permissive temperature, ribonucleotide reductase activities, similar to DNA polymerase alpha and to thymidine kinase in heat-sensitive mutants, increased essentially in parallel with reentry of cells into S phase, whereas the increase in thymidine kinase activity in the cold-sensitive mutants was previously shown to occur approx. one cell-cycle time later. This indicates that ribonucleotide reductase and thymidine kinase are coordinately expressed in the heat-sensitive, but independently regulated in the cold-sensitive mutants.     

Inactivation of ribonucleotide reductase by nitric oxide.

Ribonucleotide reductase has been demonstrated to be inhibited by NO synthase product(s). The experiments reported here show that nitric oxide generated from sodium nitroprusside, S-nitrosoglutathione and the sydnonimine SIN-1 inhibits ribonucleotide reductase activity present in cytosolic extracts of TA3 mammary tumor cells. Stable derivatives of these nitric oxide donors were either inactive or much less inhibitory. EPR experiments show that the tyrosyl radical of the small subunit of E. Coli or mammalian ribonucleotide reductase is efficiently scavenged by these NO donors.     

DNA precursor pools and ribonucleotide reductase activity: distribution between the nucleus and cytoplasm of mammalian cells.

Nuclear and whole-cell deoxynucleoside triphosphate (dNTP) pools were measured in HeLa cells at different densities and throughout the cell cycle of synchronized CHO cells. Nuclei were prepared by brief detergent (Nonidet P-40) treatment of subconfluent monolayers, a procedure that solubilizes plasma membranes but leaves nuclei intact and attached to the plastic substratum. Electron microscopic examination of monolayers treated with Nonidet P-40 revealed protruding nuclei surrounded by cytoskeletal remnants. Control experiments showed that nuclear dNTP pool sizes were stable during the time required for isolation, suggesting that redistribution of nucleotides during the isolation procedure was minimal. Examination of HeLa whole-cell and nuclear dNTP levels revealed that the nuclear proportion of each dNTP was distinct and remained constant as cell density increased. In synchronized CHO cells, all four dNTP whole-cell pools increased during S phase, with the dCTP pool size increasing most dramatically. The nuclear dCTP pool did not increase as much as the whole-cell dCTP pool during S phase, lowering the relative nuclear dCTP pool. Although the whole-cell dNTP pools decreased after 30 h of isoleucine deprivation, nuclear pools did not decrease proportionately. In summary, nuclear dNTP pools in synchronized CHO cells maintained a relatively constant concentration throughout the cell cycle in the face of larger fluctuations in whole-cell dNTP pools. Ribonucleotide reductase activity was measured in CHO cells throughout the cell cycle, and although there was a 10-fold increase in whole-cell activity during S phase, we detected no reductase in nuclear preparations at any point in the cell cycle.