ToxSci Advance Access published online on March 6, 2007
Toxicological Sciences, doi:10.1093/toxsci/kfm039
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Concentration dependence of the mechanisms of tributyltin-induced apoptosis
a Graduate School of Biomedical Sciences, Hiroshima University, 1-2-3, Kasumi, Minami-ku, Hiroshima, 734-8553, Japan b Center for Quantum Life Sciences, Hiroshima University, 1-2-3, Kasumi, Minami-ku, Hiroshima, 734-8553, Japan
Corresponding Author: Dr. Yaichiro KOTAKE Corresponding Author E-mail: yaichiro{at}hiroshima-u.ac.jp
Received December 11, 2006; revision received February 7, 2007; accepted February 8, 2007
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Abstract |
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Tributyltin chloride, an endocrine-disrupting chemical, has been used as a heat stabilizer, agricultural pesticide and component of antifouling paints. In this study, we investigated the concentration-dependence of the mechanisms of tributyltin cytotoxicity in PC12 cells. Exposure of PC12 cells to both 500 nM and 2 µM tributyltin increased the number of cells showing nuclear fragmentation, a typical apoptotic feature and activated caspase-3. The peak Ca2+ concentration in 2 µM tributyltin-treated cells was higher than that in 500 nM tributyltin-treated cells. The intracellular Ca2+ increase induced by 2 µM tributyltin was mediated by Ca2+ release from both inositol-1,4,5-trisphosphate receptor and ryanodine receptor, while the Ca2+ increase induced by 500 nM tributyltin was mediated through the voltage-dependent calcium channel. Next, we investigated whether the mechanisms leading to cell death after Ca2+ increase were different. Reactive oxygen species were involved only in 2 µM tributyltin-induced cell death, while c-jun N-terminal kinase mediated only 500 nM tributyltin-induced toxicity. Thus, caspase-dependent apoptosis caused by 2 µM tributyltin was mediated by a large Ca2+ increase via inositol-1,4,5-trisphosphate receptor and ryanodine receptor, followed by generation of reactive oxygen species. Apoptosis caused by 500 nM tributyltin was mediated by a moderate Ca2+ increase through the voltage-dependent calcium channel, followed by phosphorylation of c-jun N-terminal kinase. These results suggest that apoptosis by TBT is induced via distinct pathways depending on the TBT concentration, and we showed a rare example that upstream mechanisms of apoptosis are distinct depending on strength of toxic insult.
Key Words: tributyltin; calcium; apoptosis; reactive oxygen species; c-jun N-terminal kinase; caspase.