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 microM 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 microM TBT. In contrast, exposure to 1.0 microM dexamethasone phosphate (DEX) resulted in a gradual time-dependent increase to approximately 45% induction of apoptosis by 6 hr versus approximately 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 microM did not induce apoptosis, they reduced the ability of DEX to initiate apoptosis; while at TBT concentrations > or = 1.0 microM, 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.
Zucker
R M
RM
ManTech Environmental Technology, Inc., Research Triangle Park, North Carolina 27709.
Elstein
K H
KH
Thomas
D J
DJ
Rogers
J M
JM
D000704:Analysis of Variance; D000818:Animals; D017209:Apoptosis / Q000187:drug effects*; D002467:Cell Nucleus / Q000187:drug effects; D002470:Cell Survival / Q000187:drug effects; D002478:Cells, Cultured; D004247:DNA / Q000378:metabolism; D003907:Dexamethasone / Q000037:antagonists & inhibitors / Q000494:pharmacology*; D004305:Dose-Response Relationship, Drug; D005434:Flow Cytometry; D008214:Lymphocytes / Q000166:cytology / Q000187:drug effects*; D008297:Male; D051381:Rats; D013950:Thymus Gland / Q000166:cytology / Q000187:drug effects*; D014220:Trialkyltin Compounds / Q000037:antagonists & inhibitors / Q000494:pharmacology*
D014220:Trialkyltin Compounds; C011559:tributyltin; D003907:Dexamethasone; D004247:DNA