Limitations of the in vitro repair synthesis assay for probing the role of DNA repair in platinum resistance

Mechanisms of resistance to alkylating agents

Alkylating agents are the most widely used anticancer drugs whose main target is the DNA, although how exactly the DNA lesions cause cell death is still not clear. The emergence of resistance to this class of drugs as well as to other antitumor agents is one of the major causes of failure of cancer treatment. This paper reviews some of the best characterized mechanisms of resistance to alkylating agents. Pre- and post-target mechanisms are recognized, the former able to limit the formation of lethal DNA adducts, and the latter enabling the cell to repair or tolerate the damage. The role in the pre-target mechanisms of reduced drug accumulation and the increased detoxification or activation systems (such as DT-diaphorase, metallothionein, GST/GSH system, etc...) are discussed. In the post-target mechanisms the different DNA repair pathways, tolerance to alkylation damage and the 'downstream' effects (cell cycle arrest and/or apoptosis) are examined.

An XPG DNA repair defect causing mutagen hypersensitivity in mouse leukemia L1210 cells

One of the most widely used antitumor drugs is cis-diamminedichloroplatinum(II) (cisplatin), and mechanisms of cisplatin resistance have been investigated in numerous model systems. Many studies have used mouse leukemia L1210/0 as a reference wild-type cell line, and cisplatin-resistant subclones have been derived from it. Increased DNA excision repair capacity is thought to play a key role in the acquired cisplatin resistance, and this has influenced development of drugs for clinical trials. We report here that the L1210/0 line is in fact severely deficient in nucleotide excision repair of damaged DNA in vivo and in vitro. L1210/0 cell extracts could be complemented by extracts from repair-defective human xeroderma pigmentosum (XP) or rodent excision repair cross-complementing (ERCC) mutant cells, except for XPG/ERCC5 mutants. Purified XPG protein could restore repair proficiency to L1210/0 extracts. Expression of mouse XPG mRNA was similar in all L1210 lines studied, suggesting a point mutation or small alteration of XPG in L1210/0 cells. The DNA repair capacity of a cisplatin-resistant subline, L1210/DDP10, is similar to that of type culture collection L1210 cells and to those of other normal mammalian cell lines. Nucleotide excision repair of DNA is thus clearly important in the intrinsic cellular defense against cisplatin. However, in contrast to what is generally believed, enhancement of DNA repair above the normal level in these rodent cells does not appear to be a mechanism of acquired resistance to the drug.

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.