Thymocyte apoptosis induced by phosphorylation of histones is associated with the change in chromatin structure to allow easy accessibility of DNase

The inhibitors of protein phosphatase such as calyculin A and okadaic acid induce the apoptotic cell death in rat thymocytes. To clarify the molecular mechanism of these inhibitor-induced apoptosis, the effect of calyculin A on DNA fragmentation in the isolated nuclei were studied. A significant increase in DNA fragmentation was observed in the nuclei prepared from the cells treated with calyculin A that caused histone hyperphosphorylation. No changes of the activities of caspase-8 and -3 were observed in the extract from the cells treated with calyculin A. The circular dichroism analysis of soluble chromatin from calyculin A-treated thymocyte nuclei indicated that phosphorylation of histones decreased its alpha-helical content. Thus, the change in the chromatin structure may be due to the chemical modification of histones. Moreover, the structural change in chromatin preceded DNA fragmentation in the nuclei. Therefore, these results suggest that the change of chromatin structure allow easy accessibility of nuclear DNase to chromosomal DNA.