A comparative analysis of drug-induced DNA effects in a nitrogen mustard resistant cell line expressing sensitivity to nitrosoureas

In vitro evaluation of dimethane sulfonate analogues with potential alkylating activity and selective renal cell carcinoma cytotoxicity

We identified five structurally related dimethane sulfonates with putative selective cytotoxicity in renal cancer cell lines. These compounds have a hydrophobic moiety linked to a predicted alkylating group. A COMPARE analysis with the National Cancer Institute Anticancer Drug Screen standard agent database found significant correlations between the IC50 of the test compounds and the IC50 of alkylating agents (e.g., r = 0.68, P < 0.00001 for chlorambucil). In this report, we examined whether these compounds had activities similar to those of conventional alkylating agents. In cytotoxicity studies, chlorambucil-resistant Walker rat carcinoma cells were 4- to 11-fold cross-resistant to the test compounds compared with 14-fold resistant to chlorambucil. To determine effects on cell cycle progression, renal cell carcinoma (RCC) line 109 was labeled with bromodeoxyuridine prior to drug treatment. Complete cell cycle arrest occurred in cells treated with an IC90 dose of NSC 268965. p53 protein levels increased as much as 5.7-fold in RCC line 109 and as much as 20.4-fold in breast cancer line MCF-7 following an 18-hour drug exposure. Finally, DNA-protein cross-links were found following a 6-hour pretreatment with all compounds. Thus, the dimethane sulfonate analogues have properties expected of some alkylating agents but, unlike conventional alkylating agents, appear to possess activity against RCC.

The role of DNA repair in nitrogen mustard drug resistance

The nitrogen mustards are an important class of DNA cross-linking agents, which are utilized in the treatment of many types of cancer. Unfortunately, resistance often develops in the treatment of patients and the tumor either never responds to or becomes refractory to these agents. Resistance to the nitrogen mustards in murine and human tumor cells has been reported to be secondary to alterations in (i) the transport of these agents, (ii) their reactivity, (iii) apoptosis and (iv) altered DNA repair activity. In the present review, we will discuss the role of DNA repair in nitrogen mustard resistance in cancer. The nitrogen mustards' lethality is based on the induction of DNA interstrand cross-links (ICLs). Two DNA repair pathways are known to be involved in removal of ICLs: non-homologous DNA end-joining (NHEJ) and Rad51-related homologous recombinational repair (HRR). The reports discussed here lead us to hypothesize that low NHEJ activity defines a hypersensitive state, while high NHEJ activity, along with increased HRR activity, contributes to the resistant state in chronic lymphocytic leukemia. Studies on human epithelial tumor cell lines suggest that HRR rather than NHEJ plays a role in nitrogen mustard sensitivity.

The properties of total adducts and interstrand crosslinks in the DNA of cells treated with CB 1954. Exceptional frequency and stability of the crosslink

CB 1954 (5-(aziridin-1-yl)-2,4-dinitrobenzamide) becomes, upon bioactivation, a difunctional alkylating agent. It can be up to a 100,000-fold more cytotoxic in cells that are able to bioactivate it than in those that cannot. This increase in cytotoxicity is much greater than would be predicted from the conversion of a monofunctional alkylating agent to a difunctional one. We now show that the interstrand crosslink formed in the DNA of CB 1954-sensitive cells has some unusual properties. In Walker cells, which are able to activate CB 1954, the interstrand crosslink is the major adduct and can constitute up to 70% of the total adducts. These crosslinks are only poorly excised, as are those produced in V79 cells (which are themselves unable to activate CB 1954) by co-culturing them with Walker cells. Also, CB 1954 is approximately 10-fold more reactive toward the DNA of Walker cells than V79 cells. These observations may explain the extent of the increase in cytotoxicity accompanying the bioactivation of CB 1954.

Cytotoxic and cytostatic effects of antitumor agents induced at the plasma membrane level

A variety of antitumor agents inhibit cell proliferation by interacting with the plasma membrane. They act as growth factor antagonists, growth factor receptor blockers, interfere with mitogenic signal transduction or exert direct cytotoxic effects. The P-glycoprotein encoded by the MDR1 gene represents a transmembrane protein which catalyzes the efflux of various antitumor agents. This membrane protein is the target of compounds acting as Multi-Drug Resistance (MDR)-modulators. Finally, several established antitumor agents which are considered to represent DNA-targeted drugs, including anthracyclines, platinum complexes and alkylating agents, cause a variety of membrane lesions. Their contribution to the antitumor activity of these drugs is discussed.

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.

Differential sensitivity of histone acetylation in nitrogen-mustard sensitive and resistant cells. Relation to drug uptake, formation and repair of DNA-interstrand cross-links

Cultivation of Ehrlich-ascites tumor cells in the presence of N-mustard leads to a selection of cells with a defective choline carrier. As N-mustard employs the choline carrier for transport, this results in reduced drug uptake and in a decrease in drug sensitivity which is specific for N-mustard. Walker carcinoma cells with a stable pleiotropic resistance to a variety of alkylating agents and adriamycin exhibit no evidence for an impaired drug transport and show the same frequency of DNA-interstrand cross-links as the sensitive parental line. Both sensitive and resistant Walker cells exhibit equal capacities for repair of N-mustard induced DNA-interstrand cross-links. The inhibition of histone acetylation by N-mustard, however, was found to be significantly lower in the resistant Walker or Ehrlich cells compared to sensitive counterparts. Although the difference between N-mustard concentrations leading to half maximal inhibition of histone acetylation in sensitive and resistant cells is considerably smaller than the difference between N-mustard doses required for half maximal inhibition of cell proliferation the data suggest that--besides DNA-DNA cross-linking--the inhibition of histone acetylation has to be considered as an important alternative mechanism responsible for the cytotoxic activity of alkylating agents. Inhibition of histone acetylation is not due an accelerated deacetylation and is predominantly expressed in chromatin fractions soluble in 0.1 M NaCl after digestion with micrococcal nuclease.

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.

The effect of monofunctional or difunctional platinum adducts and of various other associated DNA damage on the expression of transfected DNA in mammalian cell lines sensitive or resistant to difunctional agents

The effects of introducing various DNA damage into pSV2gpt DNA on the subsequent expression of xanthine guanine phosphoribosyltransferase (XGPRT), after its transfection into two Walker 256 cell lines, one which is inherently sensitive only to difunctional agents while the other shows a normal sensitivity, have been examined. Both the sensitive (WS) and the relatively resistant (WR) cell lines were shown to be equally capable of both ligation of DNA double-strand breaks (although the efficiency varied with the actual site of the break) introduced into pSV2gpt and homologous recombination of pSV2gpt fragments (recombination events are thought to be important in the repair of DNA-DNA interstrand crosslinks). Reacting the plasmid with either the difunctional platinum compound, Cisplatin, or the monofunctional reacting Pt(Dien) caused a dose-dependent decrease in the subsequent expression of XGPRT. This decrease was about the same with either agent in either cell line when expressed as a function of dose of drug. However, when the actual binding of platinum to DNA by these compounds was measured, a large difference (due to the higher specific binding of Pt(Dien) to DNA) in the effects of the difunctional, as opposed to the monofunctional agent, was apparent and this was a reflection of the relative cytotoxicities of these compounds towards mammalian cells. Although at doses of Cisplatin equitoxic to WS and WR cells 20-fold less Pt is bound to the DNA of WS cells, no significant difference was seen on the expression of pSV2gpt, reacted with this agent, between WS or WR cells. Based upon a knowledge of the proportions of adducts formed in DNA reacted with Cisplatin, the lesion that inactivates expression of XGPRT was probably the intrastrand crosslink and it was calculated that due to the size of the plasmid, the interstrand crosslink was unlikely to be present at these inactivating doses. It is suggested that the inherent sensitivity of WS cells only to difunctional agents is due to their response to such relatively rare lesions such as a DNA-DNA interstrand crosslink.