Non-apoptogenic killing of hela cervical carcinoma cells after short exposure to the alkylating agent N-methyl-N'-nitro-N-nitrosoguanidine (MNNG)

We examined the action of N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) on HeLa cells and compared it with that of cisplatin (CP). MNNG directly killed a substantial number of cells within 1 hour and resulted in strong DNA-damage as evidenced by Comet measurements. Despite appearance of DNA lesions, p53 protein was not activated. Analysis of HeLa cells treated with MNNG for 1h, 3h and 6h by flow cytometry and by Hoechst staining did not reveal any sub-G(1) cell population and chromatin condensation/fragmentation characteristic for apoptosis, respectively. Also, no biochemical changes typical for apoptosis such as activation of caspase-3 or release of cytochrome C from mitochondria were detected. Inactivation of PARP-1 reduced the direct cytotoxicity exerted by MNNG. Our results showing that despite appearance of severe DNA lesions after short exposure of HeLa cells to MNNG neither activation of p53 response nor induction of apoptosis occurred implicate that generation of strong DNA damage is not sufficient to stabilize p53 protein in HeLa cells. Our data unequivocally show that the conscientious determination of the type of cell death induced by genotoxic agents is necessary. The assessment of the changes based on at least a few independent criteria is required to discriminate between apoptosis and necrosis. Since the alkylating agents generate DNA strand breaks, the recruitment of methods based on determination of DNA cleavage such as DNA ladder or TUNEL assay for evaluation of apoptosis is not adequate.