AKT Isoform-Specific Signals Regulate RNA Processing in Lung Cancer

The AKT serine/threonine kinase is a mediator of extracellular and intracellular signaling that is often aberrantly activated in cancer cells. The three AKT isoforms (AKT1, AKT2, and AKT3) are thought to have some nonoverlapping functions, prompting Sanidas and colleagues to examine whether differential substrate phosphorylation contributes to isoform-specific AKT activity. Phosphoproteomic screening of isogenic cell lines expressing different AKT isoforms identified 25 AKT substrates involved in RNA processing, including IWS1, which was phosphorylated by AKT1 and AKT3 at a conserved phosphorylation site. IWS1 phosphorylation was required for recruitment of SETD2, which trimethylates histone H3 at lysine 36 during transcriptional elongation and creates a docking site for MRG15 and PTB, which then regulate alternative splicing of genes such as fibroblast growth factor receptor 2 (FGFR2). Alternative splicing of FGFR2 generates two isoforms, IIIb and IIIc, of which only IIIc is recognized by FGF2. The IIIc isoform has been linked to FGF2-mediated cell migration, invasion, and epithelial-to- mesenchymal transition. In support of a model in which IWS1 phosphorylation facilitates alternative FGFR2 splicing, silencing of AKT or IWS1 phosphorylation increased the IIIb/IIIc isoform ratio and reduced binding of the splicing regulators MRG15 and PTB at the FGFR2 locus. Moreover, phosphorylation of IWS1 was required for FGF2-mediated migration, invasiveness, and proliferation of lung cancer cells, and IWS1 phosphorylation was required for tumor growth and invasiveness in vivo. Analysis of a panel of human non–small cell lung cancers (NSCLC) revealed that 21 out of 24 tumors, but no normal samples, expressed easily detectable levels of IWS1 and that IWS1 phosphorylation in these tumors was correlated with a decreased IIIb/IIIc ratio, AKT phosphorylation, and AKT3 expression. Together, these results highlight an isoform-specific role for AKT in oncogenesis that involves IWS1-dependent alternative splicing.

Sanidas I, Polytarchou C, Hatziapostolou M, Ezell SA, Kottakis F, Hu L, et al. Phosphoproteomics screen reveals Akt isoform-specific signaling linking RNA processing to lung cancer. Mol Cell 2014 Jan 23 [Epub ahead of print].