Separate metabolic pathways leading to DNA fragmentation and apoptotic nuclear chromatin condensation (original) (raw)

Apoptosis is the predominant form of cell death observed in a variety of physiological and pathological conditions such as cancer involution, insect metamorphosis, the development of the immune and nervous systems, and embryogeuesis. The typical nuclear changes taking place in apoptotic cells include extensive condensation of chromatin and internucleosomal DNA fragmentation into units of 200 base pairs. However, the mechanisms responsible for both chromatin condensation and DNA fragmentation have yet to be duddated. In this study, micrococcal nudease and the divalent cations, Ca z+ and Mg z+, were applied to isolated nuclei in an attempt to reconstitute in vitro the digestion of genomic DNA associated with apoptosis. Micrococcal nudease was found to induce a typical pattern of DNA fragmentation, but did not give rise to chromatin condensation, whereas Ca z +/Mg 2 + induced both chromatin condensation and DNA fragmentation in isolated mouse liver nuclei. When the endonuclease inhibitor ZnCh was used, the DNA fragmentation induced by Ca z +/Mg 2 + in nuclei could be completdy inhibited, but chromatin condensation still occurred. For comparison, intact liver cells were treated with valinomycin, a potassium ionophore, which gave rise to an atypical cell death, with chromatin condensation appearing without DNA fragmentation. Our results suggest that endonuclease activation in apoptosis is neither necessary nor suff~deut to induce chromatin condensation, and that DNA fragmentation and chromatin condensation may be triggered through separate pathways during apoptosis.