Defective caspase-3 activation and caspase-independent apoptosis in UV-irradiated HeLa S3 cells

Full text: Following exposure to radiation, most hematopoietic cells show a typical morphological characteristic of apoptosis and die quickly before the next mitosis. In contrast, death of non-hematopoietic cells, such as human tumor cells and fibroblasts, occurs after one or several cell divisions. Recently, it has been reported that many tumor cell lines die in interphase 12 h or longer after irradiation with relatively high doses of UV or ionizing radiation. However, the relationship between delayed interphase cell death and apoptosis is not clear. In this study, we used two kinds of cells, mouse lymphoma 3SB and human leukemic Jurkat cells and two human carcinoma cell lines (HeLa S3 and MCF-7). When irradiated with UV, 3SB and Jurkat cells showed an extensive release of cytochrome c from mitochondria and exhibited a clear production of oligonucleosomal DNA fragments within 3 h of incubation after irradiation. Simultaneously, activation of caspase-9 and its downstream caspases was detected. In the case of HeLa S3 and MCF-7 cells, DNA fragmentation could be detected at 24 or 48 h of post-irradiation incubation, but relatively early release of cytochrome c was observed (within 6 h after exposure). Interestingly, UV-irradiated HeLa S3 cells exhibited extremely low levels of caspase-3 like activity, similar to those in caspase-3-deficient MCF-7 cells, suggesting that the inhibition of apoptosis in HeLa S3 cells occurs downstream of cytochrome c release. A similar cell type-specific apoptosis was also observed when irradiated with γ-rays. To confirm the existence of caspase-independent apoptosis, we examined the effect of a caspase inhibitor, Z-VAD-FMK, on the induction of DNA fragmentation by UV exposure, and found that Z-VAD-FMK completely blocked DNA fragmentation in 3SB and Jurkat cells but did not suppress the delayed production of oligonuclesomal DNA fragments in HeLa S3 and MCF-7 cells. These data indicate that the delayed form of apoptosis in HeLa S3 cells occurs in a caspase-independent pathway