Walker rat carcinoma cells are exceptionally sensitive to cis-diamminedichloroplatinum(II) (cisplatin) and other difunctional agents but not defective in the removal of platinum-DNA adducts

Preferential energy- and potential-dependent accumulation of cisplatin-gutathione complexes in human cancer cell lines (GLC4 and K562): A likely role of mitochondria

cis-Diamminedichloroplatinum(II) (CDDP) is an important chemotherapeutic agent used in the treatment of a wide variety of solid tumors. We have recently shown that aquated forms of cisplatin (aqua-Pt) rapidly accumulate in K562 and GLC4 cultured cells, in comparison to CDDP. Thus, when cells are incubated with aquated forms of cisplatin a gradient of concentration is observed after a short time, approximately 40 min, with an intracellular concentration of aqua-Pt of 20-30 times higher than that of extracellular aqua-Pt. The same gradient of concentration is observed when cells are incubated with CDDP but it takes a longer time, i.e., about 24 h. Therefore, the question arises as to the identity of the intracellular sites of accumulation of aqua-Pt. Using several agents to modulate membrane potential, acidic compartment pH and/or ATP level, we obtained evidence that aqua-Pt may accumulate rapidly inside mitochondria as this accumulation is energy- and membrane-potential-dependent. However, aqua-Pt complexes are not characterized by a delocalized charge and a lipophilic character that would permit their movement through the inner membrane. Therefore, it is suggested that intracellular aqua-Pt reacts rapidly with glutathione with the resultant complex being transported inside the mitochondria via one of the known glutathione transporters, i.e., dicarboxylate and/or 2-oxoglutarate transporters present in the inner membrane.

Circumvention of acquired tetraplatin resistance in a human ovarian carcinoma cell line by a novel trans platinum complex, JM335 [trans ammine (cyclohexylamine) dichloro dihydroxo platinum (IV)]

Acquired resistance to tetraplatin [d,1-trans-1,2-diaminocy-clohexane tetrachloroplatinum (IV)] has been generated in vitro in the human ovarian carcinoma cell line PXN94; the derived line, PXN94tetR, was 24-fold resistant to tetraplatin. Intracellular tetraplatin accumulation was reduced in PXN94tetR compared with PXN94 by an average of 1.3-fold across the concentration range 1-100 microM (2 hr exposure). There was no significant difference in glutathione levels between the 2 cell lines. PXN94tetR was 1.6-fold more resistant to cadmium chloride than PXN94, suggesting that metallothionein levels may be elevated. However, no significant difference was observed between PXN94 and PXN94tetR in the levels of total platinum bound to DNA or DNA interstrand cross-links immediately after tetraplatin exposure (10-100 microM x 2 hr). There was also no significant difference between the 2 cell lines in the rate of removal of total platinum or interstrand cross-links from DNA following 2 hr exposure to 25 microM tetraplatin. Hence the major mechanism of acquired tetraplatin resistance in PXN94tetR appears to be increased tolerance of platinum-DNA adducts. PXN94tetR was partially cross-resistant to the bifunctional alkylating agents melphalan, chlorambucil and mitomycin C. Partial cross-resistance was also observed to Adriamycin, bleomycin, etoposide, 5-fluorouracil and vinblastine; however, no elevation in P-glycoprotein levels was apparent in PXN94tetR. No cross-resistance was observed to taxotere. PXN94tetR was partially cross-resistant to cisplatin, carboplatin and several novel cis platinum complexes. In contrast, resistance was completely circumvented by the novel trans platinum complex JM335 [trans ammine (cyclohexylamine) dichloro dihydroxo platinum (IV)].

DNA excision-repair synthesis is enhanced in a murine leukemia L1210 cell line resistant to cisplatin

Among various molecular mechanisms of cell resistance to antitumor agents such as cisplatin, it has recently been suggested that enhanced DNA-repair activity might be involved in the resistant phenotype of cell lines. Mouse leukemia-cisplatin-resistant cell lines L1210/10 (adapted in vitro) and L1210/DDP (adapted in vivo) have been reported to exhibit an increase DNA-repair activity, as determined by host-cell reactivation after transformation with damaged plasmids. In this paper, excision-repair activity was monitored by an in-vitro assay allowing quantification of DNA-repair synthesis in cell extracts from resistant and sensitive parental cells (L1210/10 versus L1210/0 and L1210/DDP versus L1210/S). Experimental conditions for optimal repair-synthesis activity were found to be different from these reported with human cell-line extracts. L1210/S sensitive cell line, grown in vivo by a weekly intraperitoneal graft in mice, displayed a repair activity about fourfold lower than the same cell line maintained in vitro or than L1210/0 cell grown in vitro. The repair activity was found similar in a L1210/10 and L1210/0 cell lines, but it was enhanced in L1210/DDP resistant cell line when compared with its parental line.

The Walker 256 carcinoma: a cell type inherently sensitive only to those difunctional agents that can form DNA interstrand crosslinks

The Walker 256 rat tumour has been maintained in vivo for over 60 years and until recently was used as a primary screen for new antitumour agents. This screen was particularly useful in identifying difunctional alkylating agents as potentially useful anticancer agents and it would seem that the Walker tumour is composed of cells sensitive towards this type of agent. A cell line (WS) established from the Walker tumour retained the sensitivity of the tumour towards difunctional agents and we have examined its phenotype in comparison to a derived, resistant, cell line (WR). The response of WR cells to a range of cytotoxic agents was similar to other established cell lines whilst WS cells were much more sensitive only towards difunctional reacting agents. There were no significant differences in the binding of these agents to the DNA of WS or WR cells. All the agents towards which WS cells showed sensitivity were, without exception, capable of reacting with DNA in Walker cells and forming DNA-DNA interstrand crosslinks. WS cells were not sensitive to busulphan, BCNU, CCNU or Me-CCNU but these agents did not produce interstrand crosslinks in the DNA of either WS or WR cells. Thus WS cells are intrinsically sensitive to specific DNA damage and this is probably a DNA interstrand crosslink. Hybrid cells produced by fusion of WS with WR cells lacked the inherent sensitivity of the WS cells towards cisplatin; sensitivity was therefore a recessive characteristic. Transfection of WS cells with human DNA also gave rise to 2 cisplatin-resistant clones, although it could not be ascertained if these clones were true transfectants or revertants. The survival of these resistant clones, after treatment with cisplatin, was about the same as WR cells a finding which would be consistent with complementation by a transferred gene or reversion of a single gene defect in WS cells. In their sensitivity only to difunctional compounds and lack of an apparent DNA excision repair defect the phenotype of Walker cells strongly resembles those cells from human patients suffering from Fanconi's anaemia and also of yeast snm1 mutant cells. The mechanisms giving rise to this failure to tolerate specific DNA damage (which seems to involve the inability to recover from the initial inhibition of DNA synthesis and may involve a single defect of a gene involved in the late steps of crosslink repair), do not involve drug uptake, drug binding to DNA, cell size, cell doubling time or DNA excision repair.

Caffeine, aminoimidazolecarboxamide and dicoumarol, inhibitors of NAD(P)H dehydrogenase (quinone) (DT diaphorase), prevent both the cytotoxicity and DNA interstrand crosslinking produced by 5-(aziridin-1-yl)-2,4-dinitrobenzamide (CB 1954) in Walker cells

A form of NAD(P)H dehydrogenase (quinone) (DT diaphorase, menadione reductase (NMOR), phylloquinone reductase, quinone reductase, EC 1.6.99.2) has been isolated from Walker 256 rat carcinoma cells. This enzyme can convert 5-(aziridin-1-yl)-2,4-dinitrobenzamide (CB 1954) to a cytotoxic DNA interstrand crosslinking agent by reduction of its 4-nitro group to the corresponding hydroxylamino species (Knox et al. Biochem Pharmacol, 37: 4661-4669 and 4671-4677, 1988). 2-Phenyl-5(4)-aminoimidazole-4(5)-carboxamide and AICA [5(4)-aminoimidazole-4(5)-carboxamide] have previously been reported to be antagonists of the anti-tumour effects of CB 1954. We have shown that both these compounds are inhibitors of the above enzyme and that AICA protects against both the cytotoxicity and the formation of DNA interstrand crosslinks, produced by CB 1954 in Walker cells. Similarly, known inhibitors of NAD(P)H dehydrogenase (quinone) such as dicoumarol, also reduced the cytotoxicity and DNA-interstrand crosslinking of CB 1954 in Walker cells. Caffeine was shown to be a novel inhibitor of NAD(P)H dehydrogenase (quinone) and also elicited the above protective effects. All of the above inhibitors were also shown to potentiate the toxic effects of menadione against the Walker cell. This quinone is known to be detoxified by NAD(P)H dehydrogenase (quinone) and thus emphasises the ability of these compounds to inhibit this enzyme within the cell.

The independent action of radiation and cisplatin on the survival or recovery of human normal or tumour cells in vitro or in vivo

Recovery from radiation- or cisplatin-induced lethal damage has been studied in euoxic normal human foetal lung fibroblasts (HFL cells) that remain viable for at least 20 days in plateau-phase culture. After a 1 hour treatment with cisplatin the half-time of recovery was about 2 days. By contrast recovery after radiation was more rapid with half-times of approximately 10 h. There was no further measurable recovery after 2 days. With either agent the recovery ratios (RR) were dose-dependent but recovery (following treatment with equitoxic doses of the two agents) was appreciably greater after cisplatin (RR approximately 123 after 40 microM for 1 h) than after radiation (RR approximately 15 after 900 cGy). When radiation (900 cGy) was combined with cisplatin (40 microM for 1 h) the cell survival, measured at 5 days or later times after the treatments, was not significantly less than that predicted by the additive, independent effects of both agents (calculated as the product of their respective effects on cell survival) irrespective of whether cisplatin was given 1 h before, during or for 1 h immediately after radiation. In euoxic, exponentially growing HFL or HeLa cells there was no evidence that combinations of cisplatin and radiation gave more than additive toxic effects in the protocols tested. The combined effects of cisplatin or carboplatin and whole-body irradiation given 45 min later, on human melanoma cells (assessed by their colony-forming abilities in vitro) growing in thymectomised mice, were essentially the same as that predicted by the additive, independent effects of the two agents.

Evaluation of a genotoxicity test measuring DNA-strand breaks in mouse lymphoma cells by alkaline unwinding and hydroxyapatite elution

A rapid genotoxicity test, based on the measurement of the proportion of single- to double-stranded DNA by alkaline unwinding and hydroxyapatite elution in mouse lymphoma cells treated in vitro with various chemicals, was evaluated. Seventy-eight compounds from diverse chemical groups, including commonly tested mutagens, toxic compounds not usually tested for genotoxicity and non-toxic compounds not thought to be genotoxic were tested. The results obtained were compared with those from the mouse lymphoma TK locus forward-mutation assay, providing a basis for assessing the relative sensitivity of the 2 assays using the same cells exposed to chemicals under similar conditions. Clear evidence of DNA-damaging activity was obtained with 43 of the compounds, while 4 gave equivocal results. Of the remaining 31 compounds, 14 were toxic without inducing DNA damage while the rest were non-toxic and did not induce any DNA damage. Results were available from both the alkaline unwinding assay and the mouse lymphoma assay for 61 compounds; they showed a concordance between the 2 assays of 77%. Of the 47 compounds that were positive or equivocal in the alkaline unwinding assay, only carbon tetrachloride and prednisolone were negative in the mouse lymphoma assay, while 12 of the 19 compounds that were negative in the alkaline unwinding assay were positive in the mouse lymphoma assay. These included 3 compounds that interfere with nucleic acid metabolism, and 3 crosslinking agents, which would be expected to produce mutations to a greater extent than strand breaks. The other 6 compounds were anthranilic acid, benzoquinone, p-chloroaniline, diethylmaleate, glucose and procarbazine HCl. Of these only the last is a known carcinogen. It is concluded from the present study that there was good overall agreement between the results of the DNA alkaline unwinding and mouse lymphoma TK locus assays, but that the sensitivity of the alkaline unwinding assay is lower for some classes of compounds. Bearing this in mind, the alkaline unwinding assay is considered suitable as a rapid screen for genotoxic activity in eukaryotic cells.

Differential expression of collateral sensitivity or resistance to cisplatin in human bladder carcinoma cell lines pre-exposed in vitro to either X-irradiation or cisplatin

Two sublines were derived from a human bladder carcinoma continuous cell line (RT112-P), one by exposure to fractionated X-irradiation (RT112-DXR8) and the other by continuous exposure to cisplatin (RT112-CP). RT112-DXR8 cells were 1.6- to 2-fold more sensitive to cisplatin and 2 analogues, carboplatin and iproplatin, compared with the parental line, whereas RT112-CP cells were 1.6- to 2.8-fold more resistant to these agents. Uptake of 195mcisplatin was elevated 1.4-fold in RT112-DXR8 cells compared with RT112-P cells whereas uptake into RT112-CP cells was similar to that of the parental line. Binding of 195mcisplatin to DNA was similar in all 3 lines. Levels of reduced glutathione were significantly elevated in RT112-CP cells and significantly reduced in RT112-DXR8 cells compared with the parental cells. In addition, activities of glutathione reductase and glutathione peroxidase were higher in RT112-CP cells than in the parental cells whereas the activity of glutathione-S-transferase was similar in all 3 cell lines. A 2.5-fold greater induction of DNA-DNA interstrand crosslinks occurred in RT112-DXR8 cells compared with the parental line, whereas crosslinking in RT112-CP cells, whilst initially similar to that seen in RT112-P cells, was significantly elevated at later times. These findings suggest that mechanisms associated with the expression of resistance and collateral sensitivity to cisplatin may differ.

Drug resistance and DNA repair

DNA repair confers resistance to anticancer drugs which kill cells by reacting with DNA. A review of our current information on the topic will be presented here. Our understanding of the molecular biology of repair of 0(6)-alkylguanine adducts in DNA has advanced as a result of the molecular cloning of the E. coli ada gene but the precise role of this lesion in the cytotoxic effects of alkylating agents in mammalian cells is not completely understood. Less progress has been made in understanding the enzymology and molecular biology of DNA cross-link repair even though such lesions are important for the cytotoxic effects of the widely used bifunctional alkylating agents and platinum compounds. It is evident that drug sensitive or resistant phenotypes are as highly complex as are the effects of DNA damage on cell metabolism and various aspects of these effects are discussed. Few clear correlations have been made between quantitative differences in DNA repair capacity and cellular sensitivity but assays which were developed to measure fidelity and intragenomic heterogeneity in DNA repair are beginning to be applied. Such studies may reveal subtle differences between sensitive and resistant cell lines. The molecular cloning of human DNA repair genes by transfection into drug sensitive rodent cells has been attempted. Some success has been achieved in this area but the functions of the cloned genes have yet to be identified.

CB 1954 (2,4-dinitro-5-aziridinyl benzamide) becomes a DNA interstrand crosslinking agent in Walker tumour cells

Walker tumour cells were shown to be uniquely sensitive to CB 1954 when compared with other cells in vitro. CB 1954 forms DNA-DNA interstrand crosslinks in a time-dependent manner in Walker tumour but not Chinese hamster cells. The absence of interstrand crosslinks in hamster cells was not due to a lack of uptake of drug but rather to a failure to convert (probably by bioreduction) CB 1954 to the required reactive difunctional intermediate.

The unique sensitivity of Walker rat tumour cells to difunctional agents is associated with a failure to recover from inhibition of DNA synthesis and increased chromosome damage

The rate and mode of DNA synthesis was examined by thymidine uptake and by flow cytometry in Walker tumour cells highly sensitive to difunctional agents (WS), and in a derived subline of resistant cells (WR) (Rawlings and Roberts, 1986), following their treatment with sulphur mustard. Both cell lines exhibited the same dose-dependent and progressive depression in rate of DNA synthesis for up to 4 h after treatment. Thereafter the depression in rate of synthesis was partially reversed in the WR cells but DNA synthesis continued to decrease in the WS cells resulting in their slower transit through the S phase and a persistent block in the G2/M phase of the cell cycle. Sensitive cells which finally escaped the block in G2 carried more chromosome aberrations than the corresponding resistant cells. Neither cell line was defective in daughter strand-gap repair. In their sensitivity to difunctional but not to monofunctional compounds, their failure to recover from the early depression of DNA synthesis, their apparent lack of a defect in excision repair and their sensitivity to chromosome aberration induction, the Walker cell phenotype closely resembles that of the human Fanconi's anaemia cell.