Increased gene specific repair of cisplatin induced interstrand crosslinks in cisplatin resistant cell lines, and studies on carrier ligand specificity

Development of resistance to cisplatin in previously treatment-responsive malignancies is a major obstacle to successful treatment. Enhanced DNA repair as well as enhanced replicative bypass of DNA adducts have been suggested to play a role in the development of resistance to cisplatin. However, the relative contribution of these mechanisms is unknown. Second generation platinum compounds containing the 1,2-diaminocyclohexane (dach) carrier ligand have been of particular interest in the studies of resistance mechanisms since they have been effective in treatment of cells resistant to cisplatin. We have investigated the formation and repair of interstrand crosslinks (ICL) in the mouse leukemia cell line L1210/0 and its carrier ligand specific resistant derivatives L1210/DDP and L1210/DACH after treatment with ethylenediamine (en)-Pt and diaminocyclohexane (dach)-Pt compounds. ICL in the overall genome were examined using a modification of the alkaline elution assay. A Southern blot technique was employed for the study of ICL in specific regions of the genome. In the overall genome we found decreased formation of ICL with either -en or -dach carrier ligands in the two resistant cell lines without carrier ligand specificity. Some carrier ligand specificity of ICL formation was observed in the dihydrofolate reductase (DHFR) gene, but it did not correlate with the carrier ligand specificity of resistance. At the level of the overall genome there was no difference in repair of ICL between the sensitive and the two resistant cell lines. When measured in the DHFR gene, however, there was enhanced repair of ICL in the two resistant cell lines compared with the sensitive cell line. The enhanced repair at the level of the gene did not display any carrier ligand specificity.

Enhanced replicative bypass of platinum-DNA adducts in cisplatin-resistant human ovarian carcinoma cell lines

We have examined the relationship between cis-diamminedichloroplatinum(II) (cisplatin) resistance and replicative bypass in the human ovarian carcinoma cell lines 2008, A2780, and their respective cisplatin-resistant derivatives C13* and A2780/DDP. Replicative bypass is defined as the ability of a replication complex to proceed past a DNA adduct known to block or stall the complex during synthesis. Previous studies in our laboratory have shown a 3-4-fold increase in the replicative bypass of platinum-DNA adducts in platinum-resistant murine leukemia cell lines [G. R. Gibbons et al, Carcinogenesis (Lond.), 12: 2253-2257, 1991]. To test for this effect in the human lines, we used a steady-state replication assay which measures the inhibition of DNA chain elongation (based on the incorporation of [3H]thymidine into nascent DNA strands) as a function of the number of platinum-DNA adducts present on the DNA following cisplatin treatment. With this technique we demonstrated a 4.5-fold increase in the replicative bypass ability of the C13* line compared to the 2008 line and a 2.3-fold increase in the bypass ability of the A2780/DDP line compared to the A2780 line. To confirm these results, we performed a pulse-chase replication assay on the 2008 and C13* lines. This assay differs from the first in that DNA chain elongation is measured in a time-dependent manner. With the pulse-chase assay we observed a 4.8-fold increase in the replicative bypass ability of the C13* line compared to the 2008 line. We then examined the specificity of this enhanced bypass by repeating the steady-state assay with the 2008 and C13* lines using as damaging agents 1,2-diaminocyclohexanedichloroplatinum(II), UV radiation (producing pyrimidine dimers), and benzo(a)pyrene-7,8-diol-9,10-epoxide. In both cell lines, 1,2-diaminocyclohexanedichloroplatinum(II)-DNA adducts caused a greater inhibition of DNA chain elongation than cisplatin-DNA adducts. The level of enhanced bypass of 1,2-diaminocyclohexanedichloroplatinum(II)-DNA adducts in the resistant line was 2.1-fold (approximately 2-fold less than the level of enhanced bypass observed with cisplatin-DNA adducts). There was no evidence of enhanced bypass in the resistant line when cells were treated with UV light or benzo(a)pyrene-7,8-diol-9,10-epoxide. These results indicate that the bypass response in the C13* line has some degree of specificity for cisplatin adducts. The specificity of bypass in these cell lines coincided well with the specificity of resistance to each agent.(ABSTRACT TRUNCATED AT 400 WORDS)