γ-Synuclein Promotes Cancer Cell Survival and Inhibits Stress- and Chemotherapy Drug-induced Apoptosis by Modulating MAPK Pathways

Abstract Synucleins are a family of highly conserved small proteins predominantly expressed in neurons. Recently we and others have found that γ-synuclein is dramatically up-regulated in the vast majority of late-stage breast and ovarian cancers and that γ-synuclein over-expression can enhance tumorigenicity. In the current study, we have found that γ-synuclein is associated with two major mitogen-activated kinases (MAPKs),i.e. extracellular signal-regulated protein kinases (ERK1/2) and c-JunN-terminal kinase 1 (JNK1), and have shown that over-expression of γ-synuclein leads to constitutive activation of ERK1/2 and down-regulation of JNK1 in response to a host of environmental stress signals, including UV, arsenate, and heat shock. We also tested the effects of γ-synuclein on apoptosis and activation of JNK and ERK in response to several chemotherapy drugs. We have found that γ-synuclein-expressing cells are significantly more resistant to the chemotherapeutic drugs paclitaxel and vinblastine as compared with the parental cells. The resistance to paclitaxel can be partially obliterated when ERK activity is inhibited using a MEK1/2 inhibitor. Activation of JNK and its downstream caspase-3 by paclitaxel or vinblastine is significantly down-regulated in γ-synuclein-expressing cells, indicating that the paclitaxel- or vinblastine-activated apoptosis pathway is blocked by γ-synuclein. In contrast to paclitaxel and vinblastine, etoposide does not activate JNK, and γ-synuclein over-expression has no apparent effect on this drug-induced apoptosis. Taken together, our data indicate that oncogenic activation of γ-synuclein contributes to the development of breast and ovarian cancer by promoting tumor cell survival under adverse conditions and by providing resistance to certain chemotherapeutic drugs.