Possible involvement of MAP kinase signaling pathway in tributyltin-induced cytotoxicity

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Effects of tributyltin chloride(TBT) exposure on mitogen-activated protein kinase(MAPK) signaling pathways were examined in CCRF-CEM human T cells, NIH3T3 mouse fibroblasts, PC12 rat pheochromocytoma cells and MCF-7 human breast cancer cells. In all types of cell examined, the levels of phosphorylated form of extracellular signal-regulated protein kinase(ERK), c-Jun N-terminal kinase(JNK) and p38 MAPK increased in a dose-dependent manner. On the other hand, no clear changes were found in the total protein levels of ERK, JNK and p38 MAPK.The p53 tumor suppressor protein has been known to be phosphorylated at Ser15 by MAPKs. In MCF-7 cells exposed to TBT, clear phosphorylation of p53 at Ser15 and other serine residues was not found. Treatment with other heavy metals such as manganese chloride, zinc chloride, mercury chloride and lead chloride also failed to induce p53 phosphorylation. In contrast, DNA damaging agents, cadmium and asbestos fibers (chrysotile and crocidolite), induced marked phosphorylation of p53 at Ser15 in MCF-7 cells and A549 human pulmonary epithelial cells, respectively. Activation of MAPKs by TBT was not involved in p53 phosphorylation at least in MCF-7 cells.In order to clarify the roles of MAPKs activation by heavy metals including TBT, the experimental conditions in which JNK signaling pathway was blocked were established using (1)mouse embryonic stem(ES) cells lacking JNK kinase, SEK1 (MKK4) or MKK7,and (2)a macrocyclic nonaketide compound, LL-Z1640-2. In NIH3T3 cells or PC12 cells pretreated with LL-Z1640-2 at the concentration of 25 ng/ml, which suppressed heavy metal-induced JNK activation, cytotoxicity of TBT or cadmium was not reduced significantly, suggesting that JNK activation by these metals might not be responsible for cellular damages. Further investigations on JNK kinase-deficient ES cells might give clues to understand the functions of TBT and other heavy metals-induced MAPKs activation.