Alternative splicing of Caspase 9 is modulated by the PI3K/Akt pathway via phosphorylation of SRp30a

Increasing evidence points to the functional importance of alternative splice variations in cancer pathophysiology. Two splice variants are derived from the CASP9 gene via the inclusion (Casp9a) or exclusion (Casp9b) of a four exon cassette. Here we show that alternative splicing of Casp9 is dysregulated in non-small cell lung cancers (NSCLC) regardless of their pathological classification. Based on these findings we hypothesized that survival pathways activated by oncogenic mutation regulated this mechanism. In contrast to K-RasV12 expression, EGFR overexpression or mutation dramatically lowered the Casp9a/9b splice isoform ratio. Moreover, Casp9b downregulation blocked the ability of EGFR mutations to induce anchorage-independent growth. Furthermore, Casp9b expression blocked inhibition of clonogenic colony formation by erlotinib. Interrogation of oncogenic signaling pathways showed that inhibition of PI3K or Akt dramatically increased the Casp9a/9b ratio in NSCLC cells. Finally, Akt was found to mediate exclusion of the exon 3,4,5,6 cassette of Casp9 via the phosphorylation state of the RNA splicing factor SRp30a via serines 199, 201, 227 and 234. Taken together, our findings demonstrate that oncogenic factors activating the PI3Kinase/Akt pathway can regulate alternative splicing of Casp9 via a coordinated mechanism involving the phosphorylation of SRp30a.