Molecular and cellular characterization of drug resistant hamster cell lines with alterations in ribonucleotide reductase

A phase I pharmacodynamic study of GTI-2040, an antisense oligonucleotide against ribonuclotide reductase, in acute leukemias: a California Cancer Consortium study

We performed a phase I study of GTI-2040, an antisense oligonucleotide against ribonucleotide reductase mRNA, on a novel dosing schedule of days 1-4 and 15-18 by continuous infusion to examine efficacy and tolerability in patients with leukemia. A dose of 11 mg/kg/d was safely reached. Dose-limiting toxicities (DLTs) at the higher levels included elevated troponin I and liver function enzymes. There were no objective responses to GTI-2040 in this study; 7/24 patients were able to complete the predetermined three infusion cycles. Pharmacokinetic and pharmacodynamic studies were performed, indicating a trend towards increasing intracellular drug levels and decreasing RRM2 gene expression with increasing doses. This dose schedule may be considered if appropriate combinations are identified in preclinical studies.

The in vivo toxicity of hydroxyurea depends on its direct target catalase

Hydroxyurea (HU) is a well tolerated ribonucleotide reductase inhibitor effective in HIV, sickle cell disease, and blood cancer therapy. Despite a positive initial response, however, most treated cancers eventually progress due to development of HU resistance. Although RNR properties influence HU resistance in cell lines, the mechanisms underlying cancer HU resistance in vivo remain unclear. To address this issue, we screened for HU resistance in the plant Arabidopsis thaliana and identified seventeen unique catalase mutants, thereby establishing that HU toxicity depends on catalase in vivo. We further demonstrated that catalase is a direct HU target by showing that HU acts as a competitive inhibitor of catalase-mediated hydrogen peroxide decomposition. Considering also that catalase can accelerate HU decomposition in vitro and that co-treatment with another catalase inhibitor alleviates HU effects in vivo, our findings suggests that HU could act as a catalase-activated pro-drug. Clinically, we found high catalase activity in circulating cells from untreated chronic myeloid leukemia, offering a possible explanation for the efficacy of HU against this malignancy.

A simple and sensitive ribonucleotide reductase assay

Ribonucleotide reductase (RR) is a key regulatory enzyme in the DNA synthesis pathway and is the target of the cancer chemotherapeutic agent hydroxyurea. The study of RR is significantly hindered by the tedious and labor-intensive nature of enzymatic assay. In this report, we present a novel RR assay in which detection of the deoxyribonucleotides produced by RR occurs via coupling to the DNA polymerase reaction, and is enhanced by using RNase to degrade endogenous RNA. Cell extracts from various cell lines were treated with RNase and then reacted with ATP and radioactive ribonucleotide diphosphate as the substrate. Incorporation of the radioactive substrate [14C]CDP into DNA was linear over 30 min and was linear with the amount of extract, which provided RR activity. The reaction was inhibited by hydroxyurea and required Mg2+ and ATP, suggesting that the assay is specific to RR activity. While RR activities determined by our method and by a conventional method were comparable, this novel method proved to be simpler, faster, more sensitive and less expensive. In addition, assay of the RR activity for multiple samples can easily be performed simultaneously. It is superior to other RR assays in all aspects.

Determination of deoxyribonucleoside triphosphate pool sizes in ribonucleotide reductase cDNA transfected human KB cells

Ribonucleotide reductase (RR) is a rate-limiting enzyme in DNA synthesis, which is responsible for controlling deoxyribonucleoside triphosphate (dNTP) pool size. It has been shown that transfection of RR M2 cDNA in human KB cells (M2-D clone) results in overexpression for the M2 subunit and resistance to hydroxyurea (HU). In this study, dNTP pool assays were performed to measure the pool sizes in six cell lines: two controls, three transfectants, and drug-induced HU-resistant (HUR) cells. Total dNTP levels among the six cell lines rose in the following order: KB wild-type, KB vector-only transfectant, M1 cDNA transfectant, M2 cDNA transfectant, M1/M2 cDNA transfectant, and HU-induced resistant clone. The dCTP levels of the cells mimicked the total dNTP pools on a smaller scale. The significant increases in the dCTP pool sizes of the M2-D, X-D, and HUR clones were proportional to their respective increases in RR activity. Relative to all other transfectants, the M1-D clone demonstrated lower dCTP levels but increased dATP pools. The M1-D clone demonstrated a significant resistance to dNTP inhibition of RR activity compared with the control KB wild-type cells. In contrast, a profound inhibition of dCTP and a decreased sensitivity to dATP inhibition was observed in M2-D, X-D, and HUR clones. In summary, M2 cDNA transfectants and HUR clones had increased RR activity as well as expanded dNTP pools, particularly dCTP, when compared with wild-type KB cells. These data provide evidence for the intertwined relationship between RR activity and dNTP pools.

Overexpression of transfected human ribonucleotide reductase M2 subunit in human cancer cells enhances their invasive potential

The ribonucleotide reductase (RR) gene has been associated with malignant transformation and metastatic potential. In this report, the significance of the expression of RR mRNA and enzymatic activity to the invasive potential was examined by Boyden chamber invasion assay. Our results suggest that overexpression of RR M2 mRNA and RR enzymatic activity correlates to an increase in cell invasive potential. The drug-induced HURs clone expressed a higher level RR M2 mRNA and enzyme activity which contributes significantly to the 3-fold increase in invasive potential of the cells observed relative to the KB wild-type control. On the contrary, the HUr revertant clone decreased the RR M2 mRNA level and enzymatic activity, concomitantly decreasing their invasive potential. This phenomenon is most likely due to the return of RR to levels comparable to that of the KB wild-type cells. To confirm that this observation was not of a drug-resistance phenotype associated with multiple gene alterations, the panel of RR transfectants (M1-D transfected M1 subunit cDNA, M2-D transfected M2 subunit cDNA, X-D transfected M1/M2 cDNA) characterized in a previous study were also tested in the invasion assay. The M2-D clone expressed 6-fold higher RR M2 mRNA and RR activity and also demonstrated 6-fold higher invasive potential in vitro than either the parental or vector only transfected cell line (KB-V). The X-D clone demonstrated 3-fold higher M2 mRNA expression and revealed 4-fold higher invasive potential than control cells. The M1-D clone, in contrast, expressed a baseline level of RR M2 mRNA and higher M1 mRNA. In contrast to the X-D and M2-D cells, the invasive potential of M1-D reached an even lower level in the invasive assay than the control. These results, therefore, suggest that RR M2 overexpression plays an important role in a tumor's invasiveness.

Mammalian drug resistant mutants with multiple gene amplifications: genes encoding the M1 component of ribonucleotide reductase, the M2 component of ribonucleotide reductase, ornithine decarboxylase, p5-8, the H-subunit of ferritin and the L-subunit of ferritin

Hydroxyurea was used to select two very highly drug resistant cell lines, designated HR-15 and HR-30. Both drug resistant lines contained elevated levels of ribonucleotide reductase activity. Northern and Southern blot analysis indicated that the two drug resistant lines contained increased levels of mRNA for the two components, M1 and M2, of ribonucleotide reductase, and M1 and M2 gene amplifications. Alterations in M1 and M2 protein levels were also evident in Western blot analysis. Further studies of HR-15 and HR-30 cells by Northern and Southern blot analysis showed that the drug resistant cell lines had elevated levels of ornithine decarboxylase mRNA and p5-8 mRNA, as well as increased ornithine decarboxylase and p5-8 gene copy numbers, respectively. Furthermore, characterization of HR-15 and HR-30 drug-resistant cell lines revealed increased mRNA levels for both H- and L-ferritin. Both cell lines exhibited by Southern blot analysis, amplification of the H- and L-ferritin genes. Increases in the cellular levels of H- and L-ferritin subunit proteins were also observed in both HR-15 and HR-30 cells, by Western blot analysis. This is the first description of mutant cell lines containing this complex combination of modified gene expressions and gene amplifications. The alterations exhibited by these lines confirm and extend present models of hydroxyurea resistance, are in agreement with and help substantiate models of ribonucleotide reductase regulation and provide interesting links between the expressions of several cellular activities important in proliferation.

Metabolism of pyrimidine analogues and their nucleosides

The pyrimidine antimetabolite drugs consist of base and nucleoside analogues of the naturally occurring pyrimidines uracil, thymine and cytosine. As is typical of antimetabolites, these drugs have a strong structural similarity to endogenous nucleic acid precursors. The structural differences are usually substitutions at one of the carbons in the pyrimidine ring itself or substitutions at on of the hydrogens attached to the ring of the pyrimidine or sugar (ribose or deoxyribose). Despite the differences noted above, these analogues, can still be taken up into cells and then metabolized via anabolic or catabolic pathways used by endogenous pyrimidines. Cytotoxicity results when the antimetabolite either is incorporated in place of the naturally occurring pyrimidine metabolite into a key molecule (such as RNA or DNA) or competes with the naturally occurring pyrimidine metabolite for a critical enzyme. There are four pyrimidine antimetabolites that are currently used extensively in clinical oncology. These include the fluoropyrimidines fluorouracil and fluorodeoxyuridine, and the cytosine analogues, cytosine arabinoside and azacytidine.

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.