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

Targeting ribonucleotide reductase for cancer therapy

INTRODUCTION

Ribonucleotide reductase (RR) is a unique enzyme, because it is responsible for reducing ribonucleotides to their corresponding deoxyribonucleotides, which are the building blocks required for DNA replication and repair. Dysregulated RR activity is associated with genomic instability, malignant transformation and cancer development. The use of RR inhibitors, either as a single agent or combined with other therapies, has proven to be a promising approach for treating solid tumors and hematological malignancies.

AREAS COVERED

This review covers recent publications in the area of RR, which include: i) the structure, function and regulation of RR; ii) the roles of RR in cancer development; iii) the classification, mechanisms and clinical application of RR inhibitors for cancer therapy and iv) strategies for developing novel RR inhibitors in the future.

EXPERT OPINION

Exploring the possible nonenzymatic roles of RR subunit proteins in carcinogenesis may lead to new rationales for developing novel anticancer drugs. Updated information about the structure and holoenzyme models of RR will help in identifying potential sites in the protein that could be targets for novel RR inhibitors. Determining RR activity and subunit levels in clinical samples will provide a rational platform for developing personalized cancer therapies that use RR inhibitors.

The use of thiols by ribonucleotide reductase

Ribonucleotide reductase (RNR) catalyzes the rate-limiting de novo synthesis of 2'-deoxyribonucleotides from the corresponding ribonucleotides and thereby provides balanced deoxyribonucleotide pools required for error-free DNA replication and repair. The essential role of RNR in DNA synthesis and the use of DNA as genetic material has made it an important target for the development of anticancer and antiviral agents. The most well known feature of the universal RNR reaction in all kingdoms of life is the involvement of protein free radicals. Redox-active cysteines, thiyl radicals, and thiol redox proteins of the thioredoxin superfamily play major roles in the catalytic mechanism. The involvement of cysteine residues in catalysis is common to all three classes of RNR. Taking account of the recent progress in this field of research, this review focuses on the use of thiols in the redox mechanism of RNR enzymes.

Enhancement of the anti-tumor activity of S-1 by low-dose cisplatin in mice bearing the sarcoma-180 model

The combination of S-1, consisting of 1 mol/l tegafur, 0.4 mol/l 5-chloro-2,4-dihydroxypyridine and 1 mol/l potassium oxonate, plus low-dose cisplatin has showed promising anti-tumor activities in experimental and clinical studies. The aim of this study was to investigate the mechanism of this combination chemotherapy. Mice bearing sarcoma-180 cells were divided into groups of seven animals each - Group A: no treatment; Group B: 5-fluorouracil (5-FU) 10 mg/kg continuous i.p. infusion; Group C: S-1 10 mg/kg p.o.; Group D: cisplatin 0.2 mg/kg i.p.; Group E: B+D; Group F: C+D. Treatments were given for 5 consecutive days, and then anti-tumor activity, the concentration of 5-FU, the thymidylate synthase inhibition rate (TSIR) and the level of 5-FU incorporated into RNA (F-RNA) in tumor tissue were evaluated. Anti-tumor activity in Group F was higher than in any other group. A significantly higher concentration of 5-FU in tumor was detected in the S-1-treated groups (C and F) than in the 5-FU-treated groups (B and E). No differences in TSIR were observed between the groups treated with 5-FU or S-1 with or without cisplatin; however, the F-RNA level in Group F was about 1.24 times significantly higher than that in Group C. Group F showed the highest anti-tumor activity, with increasing intratumoral levels of 5-FU and F-RNA, but not that of TSIR. These results suggested that the superior anti-tumor activity obtained by S-1+cisplatin might be associated with an incorporation of 5-FU into RNA.

Mechanisms of synergism between cisplatin and gemcitabine in ovarian and non-small-cell lung cancer cell lines

2',2'-Difluorodeoxycytidine (gemcitabine, dFdC) and cis-diammine-dichloroplatinum (cisplatin, CDDP) are active agents against ovarian cancer and non-small-cell lung cancer (NSCLC). CDDP acts by formation of platinum (Pt)-DNA adducts; dFdC by dFdCTP incorporation into DNA, subsequently leading to inhibition of exonuclease and DNA repair. Previously, synergism between both compounds was found in several human and murine cancer cell lines when cells were treated with these drugs in a constant ratio. In the present study we used different combinations of both drugs (one drug at its IC25 and the other in a concentration range) in the human ovarian cancer cell line A2780, its CDDP-resistant variant ADDP, its dFdC-resistant variant AG6000 and two NSCLC cell lines, H322 (human) and Lewis lung (LL) (murine). Cells were exposed for 4, 24 and 72 h with a total culture time of 96 h, and possible synergism was evaluated by median drug effect analysis by calculating a combination index (CI; CI < 1 indicates synergism). With CDDP at its IC25, the average CIs calculated at the IC50, IC75 IC90 and IC95 after 4, 24 and 72 h of exposure were < 1 for all cell lines, indicating synergism, except for the CI after 4 h exposure in the LL cell line which showed an additive effect. With dFdC at its IC25, the CIs for the combination with CDDP after 24 h were < 1 in all cell lines, except for the CIs after 4 h exposure in the LL and H322 cell lines which showed an additive effect. At 72 h exposure all CIs were < 1. CDDP did not significantly affect dFdCTP accumulation in all cell lines. CDDP increased dFdC incorporation into both DNA and RNA of the A2780 cell lines 33- and 79-fold (P < 0.01) respectively, and tended to increase the dFdC incorporation into RNA in all cell lines. In the AG6000 and LL cell lines, CDDP and dFdC induced > 25% more DNA strand breaks (DSB) than each drug alone; however, in the other cell lines no effect, or even a decrease in DSB, was observed. dFdC increased the cellular Pt accumulation after 24 h incubation only in the ADDP cell line. However, dFdC did enhance the Pt-DNA adduct formation in the A2780, AG6000, ADDP and LL cell lines (1.6-, 1.4-, 2.9- and 1.6-fold respectively). This increase in Pt-DNA adduct formation seems to be related to the incorporation of dFdC into DNA (r = 0.91). No increase in DNA platination was found in the H322 cell line. dFdC only increased Pt-DNA adduct retention in the A2780 and LL cell lines, but decreased the Pt-DNA adduct retention in the AG6000 cell line. In conclusion, the synergism between dFdC and CDDP appears to be mainly due to an increase in Pt-DNA adduct formation possibly related to changes in DNA due to dFdC incorporation into DNA.

Ribonucleotide reductase R2 protein is phosphorylated at serine-20 by P34cdc2 kinase

Ribonucleotide reductase is a rate-limiting enzyme in DNA synthesis and is composed of two different proteins, R1 and R2. The R2 protein appears to be rate-limiting for enzyme activity in proliferating cells, and it is phosphorylated by p34cdc2 and CDK2, mediators of cell cycle transition events. A sequence in the R2 protein at serine-20 matches a consensus sequence for p34cdc2 and CDK2 kinases. We tested the hypothesis that the serine-20 residue was the major p34cdc2 kinase site of phosphorylation. Three peptides were synthesized (from Asp-13 to Ala-28) that contained either the wild type amino acid sequence (Asp-Gln-Gln-Gln-Leu-Gln-Leu-Ser-Pro-Leu-Lys-Arg-Leu-Thr-Leu-Ala, serine peptide) or a mutation, in which the serine residue was replaced with an alanine residue (alanine peptide) or a threonine residue (threonine peptide). Only the serine peptide and threonine peptide were phosphorylated by p34cdc2 kinase. In two-dimensional phosphopeptide mapping experiments of serine peptide and Asp-N endoproteinase digested R2 protein, peptide co-migration patterns suggested that the synthetic phosphopeptide containing serine-20 was identical to the major Asp-N digested R2 phosphopeptide. To further test the hypothesis that serine-20 is the primary phosphorylated residue on R2 protein, three recombinant R2 proteins (R2-Thr, R2-Asp and R2-Ala) were generated by site-directed mutagenesis, in which the serine-20 residue was replaced with threonine, aspartic acid or alanine residues. Wild type R2 and threonine-substituted R2 proteins (R2-Thr) were phosphorylated by p34cdc2 kinase, whereas under the same experimental conditions, R2-Asp and R2-Ala phosphorylation was not detected. Furthermore, the phosphorylated amino acid residue in the R2-Thr protein was determined to be phosphothreonine. Therefore, by replacing a serine-20 residue with a threonine, the phosphorylated amino acid in R2 protein was changed to a phosphothreonine. In total, these results firmly establish that a major p34cdc2 phosphorylation site on the ribonucleotide reductase R2 protein occurs near the N-terminal end at serine-20, which is found within the sequence Ser-Pro-Leu-Lys-Arg-Leu. Comparison of ribonucleotide reductase activities between wild type and mutated forms of the R2 proteins suggested that mutation at serine-20 did not significantly affect enzyme activity.

The enzyme ribonucleotide reductase: target for antitumor and anti-HIV therapy

Ribonucleotide reductase is the rate-limiting enzyme of DNA synthesis, and it has been shown to be linked with malignant transformation and tumor cell proliferation. It was therefore considered as an excellent target for cancer chemotherapy. This article reviews the in vitro and in vivo effects of hydroxyurea the first inhibitor of the enzyme, which is currently used in general clinical practice. In addition, we summarize the results obtained with other inhibitors of the enzyme; for instance, polyhydroxy-substituted benzohydroxamic acid derivatives, a promising group of inhibitors of ribonucleotide reductase that was synthesized by Bart van'T Riet and investigated by our group. In vitro as well as animal data and pharmacokinetic results are reviewed and possible implications for an improvement in the management of various patient groups are outlined.

Redox control of resistance to cis-diamminedichloroplatinum (II) (CDDP): protective effect of human thioredoxin against CDDP-induced cytotoxicity

Thioredoxin is a small ubiquitous protein with multiple biological functions, including cellular defense mechanisms against oxidative stress. In the present study, we investigated the role of human thioredoxin (hTRX) in the acquisition of cellular resistance to cis-diamminedichloroplatinum (II) (CDDP). The expression and activity of hTRX in Jurkat T cells was dose-dependently enhanced by exposure to CDDP, as determined by immunoblot analysis and insulin reducing assay. Furthermore, chloramphenicol acetyltransferase analysis using the hTRX promoter-reporter gene construct revealed that treatment of Jurkat cells with CDDP caused transcriptional activation of the hTRX gene, which might be mediated through increased generation of intracellular reactive oxygen intermediates. To examine the biological significance of hTRX induction, we established hTRX-overexpressing derivatives of L929 fibrosarcoma cells by stable transfection with the hTRX cDNA. The clones, which constitutively expressed the exogenous hTRX, displayed increased resistance to CDDP-induced cytotoxicity, compared with the control clones. After exposure to CDDP, the control cells showed a significant increase in the intracellular accumulation of peroxides, whereas the hTRX-transfected cells did not. Taken together, these results suggest that overexpressed hTRX is responsible for the development of cellular resistance to CDDP, possibly by scavenging intracellular toxic oxidants generated by this anticancer agent.