ToxSci Advance Access originally published online on March 6, 2007
Toxicological Sciences 2007 97(2):438-447; doi:10.1093/toxsci/kfm039
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Concentration Dependence of the Mechanisms of Tributyltin-Induced Apoptosis
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* Graduate School of Biomedical Sciences
Center for Quantum Life Sciences, Hiroshima University, 1-2-3, Kasumi, Minami-ku, Hiroshima 734-8553, Japan
1 To whom correspondence should be addressed. Fax: +81-82-257-5329. E-mail: yaichiro{at}hiroshima-u.ac.jp.
Received December 11, 2006; accepted February 8, 2007
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Abstract |
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Tributyltin chloride (TBT), 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 500nM and 2µM tributyltin increased the number of cells showing nuclear fragmentation, a typical apoptotic feature, and activated caspase-3. The peak Ca2+ concentration in 2µM tributyltin–treated cells was higher than that in 500nM tributyltin–treated cells. The intracellular Ca2+ increase induced by 2µM tributyltin was mediated by Ca2+ release from both inositol 1,4,5-trisphosphate receptor and ryanodine receptor, while the Ca2+ increase induced by 500nM tributyltin was mediated through the voltage-dependent calcium channel (VDCC). Next, we investigated whether the mechanisms leading to cell death after Ca2+ increase were different. Reactive oxygen species (ROS) were involved only in 2µM tributyltin–induced cell death, while c-jun N-terminal kinase (JNK) mediated only 500nM tributyltin–induced toxicity. Thus, caspase-dependent apoptosis caused by 2µM tributyltin was mediated by a large Ca2+ increase via inositol 1,4,5-trisphosphate receptor and ryanodine receptor, followed by generation of ROS. Apoptosis caused by 500nM tributyltin was mediated by a moderate Ca2+ increase through the VDCC, followed by phosphorylation of JNK. 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.