Multiple forms of endonuclease activity linked with radiation induced apoptosis in C4-1 cervical carcinoma cells

Irradiation of C4-1 cervical carcinoma cells induced apoptosis, as determined by their morphology and the presence of oligonucleosomal DNA fragmentation, with the formation of 5'- P and 3'-OH termini. Extracts of nuclear proteins from both control and irradiated cells possessed similar metallodependent endonucleolytic activity which cleaved target plasmid DNA with the same specificity as that found in apoptotic cells. Fractionation of the nuclear extracts revealed that the predominant endonuclease activity of unirradiated cells was a protein of approximately 40 kDa. After irradiation, the predominant activity was found to be associated with a 70 kDa fraction, with a reduction in the 40 kDa form. The activity of each endonuclease was found to be Ca2+ and Mg2+ dependent. It is proposed that the changes in molecular weight observed for these enzymes may be linked to the final step in apoptosis execution, irreversible chromatin fragmentation, and thus offer a potentially novel target for manipulating the effector pathway of apoptosis in these cells.