Effect of DNA-repair-enzyme modulators on cytotoxicity of L-phenylalanine mustard and cis-diamminedichloroplatinum (II) in mammary carcinoma cells resistant to alkylating drugs

Recent insights into platinum drug resistance in cancer

Cisplatin and its analogs have become important components of chemotherapeutic regimens for the treatment of solid tumors, however, their overall effectiveness is limited by the emergence of drug-resistant tumor cells. Resistance to the platinum drugs is multifactorial consisting of mechanisms that prevent the formation of lethal platinum-DNA adducts and mechanisms that operate downstream of the drug/target interaction to promote cell survival. Continued progress in the study of the drug resistance phenotype as well as the development of new platinum analogs may eventually lead to improved therapies and increased survival rates.

A rationally designed genotoxin that selectively destroys estrogen receptor-positive breast cancer cells

We describe a novel strategy to increase the selective toxicity of genotoxic compounds. The strategy involves the synthesis of bifunctional molecules capable of forming DNA adducts that have high affinity for specific proteins in target cells. It is proposed that the association of such proteins with damaged sites in DNA can compromise protein function and/or DNA repair resulting in increased toxicity. We describe the synthesis of a bifunctional compound consisting of an aniline mustard linked to the 7alpha position of estradiol. This novel compound can form covalent DNA adducts that have high affinity for the estrogen receptor. Breast cancer cells that express high levels of the estrogen receptor showed increased sensitivity to the cytotoxic effects of the new compound.

Repair of DNA interstrand cross-links

DNA interstrand cross-links (ICLs) are very toxic to dividing cells, because they induce mutations, chromosomal rearrangements and cell death. Inducers of ICLs are important drugs in cancer treatment. We discuss the main properties of several classes of ICL agents and the types of damage they induce. The current insights in ICL repair in bacteria, yeast and mammalian cells are reviewed. An intriguing aspect of ICLs is that a number of multi-step DNA repair pathways including nucleotide excision repair, homologous recombination and post-replication/translesion repair all impinge on their repair. Furthermore, the breast cancer-associated proteins Brca1 and Brca2, the Fanconi anemia-associated FANC proteins, and cell cycle checkpoint proteins are involved in regulating the cellular response to ICLs. We depict several models that describe possible pathways for the repair or replicational bypass of ICLs.

Disruption of mouse SNM1 causes increased sensitivity to the DNA interstrand cross-linking agent mitomycin C

DNA interstrand cross-links (ICLs) represent lethal DNA damage, because they block transcription, replication, and segregation of DNA. Because of their genotoxicity, agents inducing ICLs are often used in antitumor therapy. The repair of ICLs is complex and involves proteins belonging to nucleotide excision, recombination, and translesion DNA repair pathways in Escherichia coli, Saccharomyces cerevisiae, and mammals. We cloned and analyzed mammalian homologs of the S. cerevisiae gene SNM1 (PSO2), which is specifically involved in ICL repair. Human Snm1, a nuclear protein, was ubiquitously expressed at a very low level. We generated mouse SNM1(-/-) embryonic stem cells and showed that these cells were sensitive to mitomycin C. In contrast to S. cerevisiae snm1 mutants, they were not significantly sensitive to other ICL agents, probably due to redundancy in mammalian ICL repair and the existence of other SNM1 homologs. The sensitivity to mitomycin C was complemented by transfection of the human SNM1 cDNA and by targeting of a genomic cDNA-murine SNM1 fusion construct to the disrupted locus. We also generated mice deficient for murine SNM1. They were viable and fertile and showed no major abnormalities. However, they were sensitive to mitomycin C. The ICL sensitivity of the mammalian SNM1 mutant suggests that SNM1 function and, by implication, ICL repair are at least partially conserved between S. cerevisiae and mammals.

Effects of RIalpha overexpression on cisplatin sensitivity in human ovarian carcinoma cells

Our laboratory has found that Chinese hamster ovary (CHO) and mouse Y1 adrenocortical carcinoma PKA mutants with a defective R subunit, but not altered C subunits, exhibit increased resistance to cisplatin as well as other DNA-damaging agents. The mechanism of resistance may be associated with increased recognition of the cisplatin-damaged DNA and protein binding to the DNA lesion, thus enhancing DNA repair in the RI alpha mutants. These data suggest that mutation of RI alpha may confer resistance to cisplatin by affecting DNA repair activity. In the present study, we overexpressed RI alpha in human ovarian carcinoma A2780 cells to demonstrate that RI alpha can modulate cellular sensitivity to cisplatin. Retroviral-infected A2780 cells overexpressing wild-type RI alpha cDNA displayed a four- to eightfold greater sensitivity to cisplatin compared with parental cells. Overexpression of RI alpha in the CP70 cisplatin-resistant derivative of A2780 also increased the sensitivity of these cells to cisplatin. Therefore, enhanced expression of the RI alpha subunit of PKA sensitizes cells to the cytotoxic effects of this DNA-damaging agent. These data suggest that RI alpha may act directly, independent of the C subunit, to influence cellular sensitivity to cisplatin. Therefore, modulation of RI alpha expression or its functional status by pharmacological agents may potentially reverse cisplatin resistance in tumors.

Growth inhibition and cytotoxicity induced by Bowman-Birk inhibitor concentrate in cisplatin-resistant human ovarian cancer cells

Bowman-Birk inhibitor (BBI) is a soybean-derived anticarcinogenic protease inhibitor that was shown to potentiate the cytotoxicity of cisplatin in our previous studies. To assess the potential of BBI as a sensitizing agent for the chemotherapy of cisplatin-resistant cancers, we evaluated the effects of a soybean concentrate enriched in BBI (known as BBI concentrate or BBIC) on cell growth and clonogenic survival of a human ovarian cancer cell line, A2780, and its cisplatin-resistant sublines, C30, and C200. The presence of BBI and BBIC in the cell culture, medium reduced the clonogenic survival of the A2780, C30, and C200 cells in a dose-dependent manner and enhanced cisplatin-induced growth inhibition and/or cytotoxicity. BBIC alone showed greater inhibitory effects on growth in the cisplatin-resistant cell lines. These results suggest that BBI and BBIC could be useful agents for the treatment of cancers, especially with cisplatin, in tumors resistant to this important anticancer agent.

Cisplatin resistance in cyclic AMP-dependent protein kinase mutants

The emergence of cisplatin resistance poses a major problem to the successful treatment of a variety of human malignancies. Therefore, understanding the molecular mechanisms that underlie cisplatin resistance could significantly improve the clinical efficacy of this cytotoxic agent. Various studies have described that cellular sensitivity to cisplatin can be influenced by several signal transduction pathways. In this review, we examine the role of the cyclic AMP-dependent protein kinase (PKA) in the modulation of drug resistance in cancer. By a somatic mutant genetic approach, the role of PKA in the development of resistance to chemotherapeutic agents has been investigated. A series of mutants with decreased PKA activity was examined for their sensitivity to cisplatin. PKA mutants with defective regulatory (RIalpha) subunits, but not altered catalytic (C) subunits, exhibit increased resistance to cisplatin, as well as other DNA-damaging agents. Furthermore, since RIalpha subunit mutants show enhanced DNA repair we, therefore, hypothesize that functional inactivation of PKA may result in increased recognition and repair of cisplatin lesions. Alternatively, it seems likely that mutation of the RIalpha subunit may affect cellular sensitivity to various anticancer drugs, suggesting that the RIalpha subunit may have other physiological functions in addition to inhibiting the kinase activity of the C subunit. Therefore, exploitation of cyclic AMP levels or functional alteration of the R subunit may potentiate the cytotoxicity of chemotherapeutic agents and circumvent drug resistance in cancer. More importantly, the altered pattern and mechanism of drug resistance may offer the opportunity to investigate novel regulatory functions of the RIalpha subunit of PKA.

Combination effects of poly(ADP-ribose) polymerase inhibitors and DNA-damaging agents in ovarian tumor cell lines--with special reference to cisplatin

The effects of the poly(ADP-ribose) polymerase inhibitors 4-amino-1,8-naphthalimide (4-ANI), 6(5H)-phenanthridinone (PHD), 1,5-isoquinolinediol (IQD), 3-aminobenzamide (3-AB) or 4-hydroxyquinazoline (4-HYA) on the cytotoxicity of cisplatin were investigated. The human ovarian tumor cell lines SK-OV-3 and OAW 42 and the rat ovarian tumor cell line O-342 as well as its cisplatin (DDP)-resistant subline O-342/DDP were used. Cytotoxicity was determined with the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay. 1-Methyl-3-nitro-1-nitrosoguanidine (MNNG) plus its respective combinations with poly(ADP-ribose) polymerase inhibitors served as positive controls. In addition, the alkylating agents L-threitol-1,4-bismethanesulfonate (DHB) and 1,3-bis(2-chloroethyl)-1-nitrosourea (carmustine) as well as two other DNA-repair inhibitors caffeine and theophylline were included in the investigations. The cytotoxicity of cisplatin could not be increased by 4-ANI, PHD, IQD, 4-HYA or 3-AB in any cell line investigated, while it was increased by caffeine in lines O-342/DDP and SK-OV-3 as well as by theophylline in lines O-342/DDP, SK-OV-3 and OAW 42. The cytotoxicity of MNNG was increased by combination with 4-ANI, PHD, IQD, 4-HYA, 3-AB or theophylline for all lines except OAW42; in the latter line, only 4-ANI, PHD and IQD increased MNNG cytotoxicity. The cytotoxicity of DHB was increased by 4-ANI, PHD, 4-HYA, theophylline and caffeine in line O-342/DDP; by 4-HYA, theophylline and caffeine in line SK-OV-3; and by theophylline and caffeine in line OAW42. The cytotoxicity of carmustine was increased only by 3-AB in two lines (SK-OV-3 and OAW 42). Results are discussed with regard to different DNA-repair mechanisms.

Fludarabine as a modulator of cisplatin activity in human tumour primary cultures and established cell lines

The potential of the purine analogue fludarabine (9-beta-D-arabinofuranosyl-2-fluoroadenine-5' monophosphate) as a modulator of cisplatin cytotoxicity was investigated in four established cell lines and 20 primary cultures of human melanoma and ovarian cancer. Tumour cells were exposed to fludarabine and cisplatin, alone or in combination, for 4 h. Fludarabine did not affect the growth of ovarian cancer cell lines, whereas it induced a marked and dose-dependent inhibition of proliferation in melanoma cell lines. In primary cultures of both histotypes, the purine analogue did not induce appreciable antiproliferative effects. Combined cisplatin-fludarabine treatment caused additive effects in all established cell lines. Conversely, a synergistic effect of the combination was seen in 5 of 10 melanoma and 4 of 10 ovarian cancer primary cultures, with a dose-modifying factor ranging from 2.1 to 3.9 for melanomas and from 4.0 to 7.5 for ovarian cancers, respectively. In the remaining cultures, the interaction between fludarabine and cisplatin was additive. The alkaline filter elution analysis performed on primary cultures showed that the synergistic interaction between the two drugs was paralleled by an increase in the extent and persistence of the cisplatin-induced DNA interstrand crosslinks. Our results indicate that fludarabine can enhance cisplatin cytotoxic activity in human tumour primary cultures from ovarian cancer and malignant melanoma. Such an effect may be partially due to an interference by fludarabine on cisplatin-induced DNA adduct metabolism and repair.

Potentiation of chlorambucil toxicity in B-CLL lymphocytes using the DNA synthesis inhibitors aphidicolin and 1-beta-D-arabinofuranosylcytosine

Previous studies in our laboratory have identified enhanced cross-link repair as a primary mechanism of resistance to nitrogen mustards in B-cell chronic lymphocytic leukemia (B-CLL). To evaluate the therapeutic potential for modulation of DNA repair by aphidicolin and 1-beta-D-arabinofuranosylcytosine (ara-C), we examined the interaction between these two agents and chlorambucil in lymphocytes from untreated and treated-resistant B-CLL patients. We found that both aphidicolin and ara-C displayed synergy with chlorambucil over a range of inhibitor concentrations. This synergy was not restricted to the resistant samples. Our results indicate that these combinations can enhance the potency of chlorambucil in a clinically relevant model and should be considered for further preclinical and, eventually, clinical trials.