REDUCED EXPRESSION OF THE ICE-RELATED PROTEASE CPP32 IS ASSOCIATED WITH RADIATION-INDUCED CISPLATIN RESISTANCE IN HELA-CELLS

Low-dose fractionated gamma-irradiation (three cycles of 5 x 2 Gy) ind uced cisplatin resistance in HeLa cells, The drug resistance was modes t (R-1 of about 2) and stable, similar to that found previously in mur ine cells after irradiation. In the drug-resistant HeLa-C3 cells, flow cytometric analysis revealed a decreased number of apoptotic cells co mpared with the parental cells. Drug resistance was associated with co nsiderably enhanced expression of the p53 suppressor protein in HeLa-C 3 cells after cisplatin exposure but seemed not to be regulated by the bcl-2-dependent pathway. Cisplatin resistance correlated with reduced expression of ICE-related proteases (interleukin-l P-converting enzym e). Basal levels of the 45-kDa precursor ICE protein were reduced in H eLa-C3 cells, while those of the mature 60-kDa heterotetramer were sim ilar. The CPP32 protease, a member of the ICE family with structural h omology but different substrate specificity, was expressed at a lowere d level. After drug exposure, there was a slight increase of CPP32 in HeLa-C3 cells, equivalent to about 45% of the level attained in the pa rental cells. This is in contrast to the CPP32 levels measured after i rradiation, which were similar in sensitive and in resistant cells. As the radiosensitivity is unchanged in both cell lines, these results s uggest that cisplatin resistance in HeLa-C3 cells is associated with a lterations of a CPP32-linked apoptotic pathway, which is affected by t he damage caused by cisplatin but not by irradiation. Whether these ch anges are dependent on the observed p53 modifications is now being stu died in resistant clones.