ENHANCEMENT OF CA2-DEPENDENT ENDONUCLEASE ACTIVITY IN L1210 CELLS DURING APOPTOSIS INDUCED BY 1-BETA-D-ARABINOFURANOSYLCYTOSINE - POSSIBLE INVOLVEMENT OF ACTIVATING FACTOR(S)()

Internucleosomal DNA fragmentation and morphological changes in nuclei typical of apoptosis were observed in L1210 cells incubated with 1.0 mu g/ml of 1-beta-D-arabinofuranosylcytosine (ara-C). To investigate t he mechanisms involved, we examined the activities of endogenous endon ucleases in nuclei and cytoplasm. Both fractions of control cells cont ained Ca2+-dependent endonuclease which was capable of mediating inter nucleosomal DNA fragmentation. The assay system using two kinds of tar get substrates, i.e., nuclear chromatin of CCRF-CEM cells and naked DN A purified from the same cells, revealed that the activity of Ca2+-dep endent endonuclease was enhanced in the crude nuclear extracts of cell s treated with 1.0 mu g/ml of ara-C for 24 h or 48 h. The activity was extracted more easily from ara-C-treated cells than control cells wit hout sonication of the nuclear fraction. On the other hand, in the cyt oplasmic fraction of the cells, the activity towards naked DNA was unc hanged, whereas that towards nuclear chromatin was clearly enhanced. T hese results suggest that internucleosomal DNA fragmentation induced b y ara-C treatment is associated with enhancement and activation of con stitutively expressed Ca2+-dependent endonuclease in L1210 cells.