Image cytometry of early nuclear events during apoptosis induced by camptothecin in HL-60 leukemic cells

The nuclear morphological alterations occurring during apoptosis induced in human HL-60 cells by camptothecin were analyzed quantitatively by image cytometry. Two separate phases were identified during the apoptotic process. The first phase, observed between 0-2 h of incubation, consisted in the appearance of cells with an apparently decondensed chromatin. This phenomenon was blocked by the inhibitors of DNA fragmentation, TLCK and H7. In contrast, staurosporine and cytochalasin B, which inhibit apoptosis without any effect on DNA fragmentation in this system, did not prevent this morphological change. The second phase, observed after 3 h of culture, corresponded to the appearance of cells with very homogeneous and condensed chromatin. This phenomenon correlated with the detection of typical apoptotic cells with fragmented nuclei and was inhibited by all drugs (TLCK, H7, staurosporine, and cytochalasin B). These observations suggest that image cytometry allows the detection of subvisual microscopic events within the first hour after the induction of an apoptotic process and that the dissection of this process into several different phases might be associated with DNA fragmentation.