Scaffolding protein connector enhancer of kinase suppressor of Ras 1 (CNKSR1) regulates MAPK inhibition responsiveness in pancreas cancer via crosstalk with AKT signaling.

Combinatorial molecular therapy in pancreatic ductal adenocarcinoma (PDAC) has yielded largely disappointing results in clinical testing to-date as a multitude of adaptive resistance mechanisms is making selection of patients via molecular markers which capture essential, intersecting signaling routes challenging. Here, we report the scaffolding protein connector enhancer of kinase suppressor of Ras 1 (CNKSR1) as mediator of resistance to mitogen-activated protein kinase (MEK) inhibition. MEK inhibition in CNKSR1high cancer cells induces translocation of CNKSR1 to the plasma membrane where the scaffolding protein interacts with and stabilizes the phosphorylated form of AKT. CNKSR1-mediated AKT activation following MEK inhibition was associated with increased cellular p-PRAS40 levels and reduced nuclear translocation and cellular levels of FoxO1, a negative regulator of AKT signaling. In clinical PDAC specimens, high cytoplasmatic CNKSR1 levels correlated with increased cellular phospho-AKT and mTOR levels. Pharmacological co-blockade of AKT and MEK ranked top in induced synergies with MEK inhibition in CNKSR1high pancreas cancer cells among other inhibitor combinations targeting known CNKSR1 signaling. In vivo, CNKSR1high pancreatic tumors treated with AKT and MEK inhibitors showed improved outcome in the combination arm compared to single agent treatment, an effect not observed in CNKSR1low models. Our results identify CNKSR1 as regulator of adaptive resistance to MEK inhibition by promoting crosstalk to AKT signaling via a scaffolding function for the phosphorylated form of AKT. CNSKR1 expression might be a possible molecular marker to enrich patients for future AKT-MEK inhibitor precision medicine studies. Implications: The CNKSR1 scaffold, identified within a RNAi screen as a novel mediator of resistance to MEK inhibition in pancreas cancer, connects MAPK pathway and AKT signaling and may be adopted as a biomarker to select patients for combined MEK AKT blockade.