CBF-beta mitigates PI3K-alpha specific inhibitor killing through PIM1 in PIK3CA mutant gastric cancer
PIK3CA is the second most mutated gene in cancer leading to aberrant PI3K/AKT/mTOR signaling and increased translation, proliferation, and survival. Some 4-25% of gastric cancers display activating PIK3CA mutations including 80% of EBV-associated GCs. Small molecules including pan-PI3K and dual PI3K/mTOR inhibitors have shown moderate success clinically, due to broad on-target/off-tissue effects. Thus, isoform specific and mutant selective inhibitors have been of significant interest. However, drug resistance is a problem and has affected success of new drugs. There has been a concerted effort to define mechanisms of resistance and identify potent combinations in many tumor types, though gastric cancer is comparatively understudied. In this study we identified modulators of the response to the PI3Ka-specific inhibitor, BYL719, in PIK3CA mutant GCs. We found that loss of NEDD9 or inhibition of BCL-XL conferred hyper-sensitivity to BYL719, through increased cell cycle arrest and cell death, respectively. Additionally, we discovered that loss of CBFB conferred resistance to BYL719. CBFB loss led to up-regulation of the protein kinase PIM1, which can phosphorylate and activate several overlapping downstream substrates as AKT thereby maintaining pathway activity in the presence of PI3Ka inhibition. The addition of a pan-PIM inhibitor re-sensitized resistant cells to BYL719. Our data provide clear mechanistic insights into PI3Ka inhibitor response in PIK3CA mutant gastric tumors and can inform future work as mutant selective inhibitors are in development for diverse tumor types. Overall design: To determine the mechanism of PI3Ka inhibitor resistance in PIK3CA mutant gastric cancer cells, we generated AGS cell lines with developed resistance to BYL719. We additionally generated AGS cells with genetic knockout of CBFB. We then performed gene expression profiling analysis using data obtained from RNA-seq of WT, CBFB KO, and BYL719-resistant cell lines basally and following 24 hour acute treatment with 1 uM BYL719.