Emetine regulates the alternative splicing of caspase 9 in tumor cells

Exons 3 to 6 in the caspase 9 gene undergo alternative splicing in which the larger caspase 9 splice variant promotes apoptosis, in contrast to the dominant negative anti-apoptotic splice variant, the smaller caspase 9b. In this study, the regulation of the alternative splicing of caspase 9 pre-mRNA was examined in response to Emetine. Treatment of C33A cells, breast cancer MCF-7 cells and MCF-7/Adr cells with Emetine dihydrochloride upregulated the level of smaller caspase 9b mRNA and concomitantly decreased the mRNA level of larger caspase 9 in a dose- and time-dependent manner, indicating that Emetine desensitizes C33A, MCF-7 and MCF-7/Adr to cell death. In contrast, treatment of PC3 cells, a prostate cancer cell line, manifested an opposite effect: a greater production of the larger caspase 9 mRNA with a concomitant decrease of caspase 9b mRNA. Pretreatment with calyculin A, an inhibitor of protein phosphatase 1 (PP1) and protein phosphatase 2A (PP2A) blocked Emetine-induced alternative splicing in cells, in contrast to okadaic acid, a specific inhibitor of PP2A, demonstrating a PP1-mediated mechanism. These results suggest that the various splicing patterns of the caspase 9 gene that are regulated by chemotherapy reagents may contribute to the resistance or sensitization of the tumors to other cell death inducers.