In vitro exposure of a human
testicular teratoma continuous cell line to fractionated X-irradiation resulted in the expression of resistance to
cisplatin. In two independently-derived sublines, designated SUSA-DXR13 and SUSA-DXR10 resulting from treatment with either 13 fractions of 1.5 Gy (dose required to reduce survival by 1 log) or 10 fractions of 3 Gy (dose required to reduce survival by 2 logs) respectively, the IC50 values for
cisplatin were 2- and 3.1-fold higher than that of the parental cell line. These sublines were cross-resistant to
carboplatin (approximately 2-fold) but not to
adriamycin and they showed unaltered radiosensitivities. The SUSA-DXR10 subline expressed some cross-resistance to
mitomycin C and
melphalan but none to
Carmustine (
BCNU). Total
glutathione content was significantly reduced in both SUSA-DXR10 and SUSA-DXR13 cells, but the activities of associated
enzymes, including the
glutathione S-
transferases,
peroxidase and
reductase were not modified significantly in the resistant sublines. Resistance in the SUSA-DXR10 subline was associated with significantly decreased 195mcisplatin uptake (p less than 0.01), but this was not reflected in a reduced level of
drug bound to the
DNA. The formation and removal of four
platinum-
DNA adducts were immunochemically quantitated. Immediately following
drug treatment there was a higher level of total platination of the
DNA in the resistant subline indicative of increased tolerance to DNA damage. After an 18 hr post treatment incubation, there was an indication of some repair capacity in this SUSA-DXR10 cell line, which was not apparent in the parental cells. Neither the parental nor the SUSA-DXR10 cell line was proficient in the repair of the major adduct Pt-GG, whereas both lines repaired the monofunctional adduct and the adduct Pt(GMP)2. SUSA-DXR10 cells were also able to repair the intrastrand adduct Pt-AG and interstrand crosslinks, unlike the repair deficient parental cells. Higher levels of interstrand crosslinks were characteristic of the SUSA-DXR10 subline. These observations therefore implicate both enhanced repair and increased tolerance of DNA damage as mechanisms of resistance to
cisplatin resulting from in vitro exposure of a human
teratoma cell line to fractionated X-irradiation.