TRIBUTYLTIN AND DEXAMETHASONE INDUCE APOPTOSIS IN RAT THYMOCYTES BY MUTUALLY ANTAGONISTIC MECHANISMS

The nuclei of apoptotic thymocytes can be identified by flow cytometry as a subpopulation exhibiting reduced DNA content. We observed that rat thymocyte cultures exposed to 1.0-2.5 mu M tri-n-butyltin methoxide (TBT) exhibited a rapid time- and concentration-dependent induction of apoptosis, with > 85% of cells exhibiting reduced DNA content within 1 hr after exposure to 2.0-2.5 mu M TBT. In contrast, exposure to 1.0 mu M dexamethasone phosphate (DEX) resulted in a gradual time-dependent increase to similar to 45% induction of apoptosis by 6 hr versus similar to 15% spontaneous induction in controls. However, simultaneous exposure to TBT and DEX resulted in a decreased response: although TBT concentrations between 0.1 and 0.5 mu M did not induce apoptosis, they reduced the ability of DEX to initiate apoptosis; while at TBT concentrations greater than or equal to 1.0 mu M, simultaneous exposure to DEX substantially decreased the extent of TBT-induced apoptosis and cytotoxicity. Furthermore, while treatment with the protein synthesis inhibitor cycloheximide or the protein kinase C inhibitor H-7 completely blocked DEX-induced apoptosis, neither significantly reduced induction of apoptosis by TBT. Taken together, the toxicant-specific differences in the timing and extent of apoptotic induction and the dissimilar responses to CHX and H-7 suggest that TBT and DEX initiate endonuclease-mediated apoptotic cell death through different mechanisms. Moreover, the ability of each agent to retard the action of the other suggests that these mechanisms are directly or indirectly antagonistic. (C) 1994 Academic Press, Inc.