INVOLVEMENT OF CA2-MOLECULAR-WEIGHT DNA FRAGMENTS IN THYMOCYTE APOPTOSIS( IN THE FORMATION OF HIGH)

Internucleosomal DNA fragmentation (DNA laddering) and formation of ap optotic bodies have long been considered characteristic features of ap optosis. However, recent work has shown that formation of high molecul ar weight DNA fragments precedes internucleosomal cleavage and may inv olve mechanisms that differ from those responsible for DNA laddering. Here, we show that glucocorticoid treatment of human thymocytes stimul ated the formation of high molecular weight DNA fragments by Ca2+- and endonuclease-mediated mechanisms. Either the removal of Ca2+ from the medium or pretreatment of the cells with the intracellular Ca2+ chela tor, BAPTA-AM, prevented the formation of large DNA fragments. Further , treatment of the thymocytes with the microsomal Ca2+-ATPase inhibito r, thapsigargin, which caused a sustained increase in intracellular Ca 2+ concentration, was in itself sufficient to activate high molecular weight DNA fragmentation. Our results show that Ca2+-dependent mechani sms promote the multistep chromatin cleavage in human thymocyte apopto sis. (C) 1994 Academic Press, Inc.