Induction of transient thermotolerance by heat or other cytotoxic stressors has been reported to confer a moderate degree of drug resistance to tumor cells in vitro. In this study, a genetically stable, heat-resistant mouse B16 melanoma variant (W-H75) was tested for its sensitivity to various cytotoxic and antiproliferative agents. The heat-resistant W-H75 cells displayed a moderate two- to threefold resistance to doxorubicin, VP-16, VM-26, colchicine, cis-dichlorodiammineplatinum(II), HgCl2, and CdCl2. Marginal resistance to 4′(9-acridinylamino)methanesulfon-m-anisidide vinblastine, 1,3-bis(2-chloroethyl)-1-nitrosourea, and NaAsO2 was observed, while no difference in sensitivity to the anticancer drugs, actinomycin D and camptothecin, was observed. Although W-H75 cells were generally more resistant than the parental cells to most of the agents that were tested, they were collaterally sensitive to the antimetabolites methotrexate and 6-mercaptopurine. Resistance of the W-H75 cells to epipodophyllotoxins and anthracyclines was not due to differences in steady-state drug accumulation. For the epipodophyllotoxin VP-16, resistance may be related to a relative decrease in the number of drug-induced DNA strand breaks in W-H75 cells. However, no difference in DNA strand breakage was observed between W-H75 and parental cells which were treated with doxorubicin, suggesting that resistance to this drug occurred by a different mechanism. The possible involvement of glutathione and glutathione S-transferase in resistance was also investigated. The glutathione content in W-H75 cells was 35% higher than that in the parental line. However, glutathione S-transferase activity appeared to be identical in both cell lines. Two other heat-resistant B16 melanoma variants, B-H103 and R-H92, were also tested for sensitivity to doxorubicin and VP-16. In contrast to the W-H75 cells, these two heat-resistant variants were hypersensitive to doxorubicin. The B-H103 cells were also hypersensitive to VP-16. This study suggests that selection for cellular resistance to heat may result in cells that have an altered sensitivity to drugs.

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