MECHANISM OF TISSUE-SPECIFIC INDUCTION OF INTERNUCLEOSOMAL DEOXYRIBONUCLEIC-ACID CLEAVAGE ACTIVITY AND APOPTOSIS BY GLUCOCORTICOIDS

Cell death occurring by apoptosis has become widely recognized as an i ntegral component of the life cycle of many cell types. Apoptosis can be induced in many tissues by a wide variety of endogenous signals, in cluding glucocorticoids. However, even though there are glucocorticoid receptors present in almost all cells, only certain lymphoid cells ar e susceptible to glucocorticoid-induced apoptosis. The basis for this selective regulation of programmed cell death is unknown. Internucleos omal chromatin degradation is an integral characteristic of apoptosis, common to all cells undergoing this form of programmed cell death. Th us, we have developed an in vitro assay that recapitulates apoptotic D NA degradation in isolated nuclei and have identified a nuclease activ ity with internucleosomal specificity that is present in nuclear extra cts of thymocytes undergoing glucocorticoid-induced apoptosis. We have now extended these studies to analyze the molecular properties of the crude enzyme and to further elucidate the mechanism of its regulation during the tissue-specific induction of apoptosis. In vitro, optimal internucleosomal cleavage activity was detected in the presence of mil limolar concentrations of calcium and magnesium or manganese. Nuclease activity was inhibited by three agents previously shown to block apop tosis (zinc, aurintricarboxylic acid, and sodium), suggesting that the nuclease we detected in apoptotic thymocytes is responsible for the D NA degradation associated with apoptosis. Size-fractionation analysis of thymocyte nuclear extract under native conditions revealed a protei n with an apparent molecular mass of 22.7 kilodaltons and a sedimentat ion coefficient of 3.5S. Interestingly, when extracts from control thy mocytes, inactive in internucleosomal cleavage activity, were subjecte d to gel filtration or sucrose density gradient fractionation, internu cleosomal cleavage activity was detected. The physical characteristics of this endonuclease activity were identical to those found in thymoc yte extract from glucocorticoid-treated rats. Repressed or latent inte rnucleosomal cleavage activity was also detected in hepatocyte nuclear extracts, but this activity was not activated by glucocorticoid treat ment. These data suggest that glucocorticoid-induced apoptosis involve s cell-specific activation of a latent endonuclease, rather than nucle ase induction.