P1.02-064 MET-Dependent Activation of STAT3 as Mediator of Resistance to MEK Inhibitors in KRAS-Mutant Lung Cancer

Abstract

Targeting the MAPK pathway by MEK inhibition results in limited activity in patients with KRAS-mutant non-small cell lung cancer (NSCLC). The lack of effectiveness may be associated with activation of other effectors including STAT3, as well as MEK inhibition relief from negative feedback loops. Indeed, in KRAS-mutant colorectal cancer, MEK inhibition decreases the activity of the metalloprotease ADAM17, which normally inhibits MET signaling and STAT3 activation by promoting shedding of MET endogenous antagonist, soluble “decoy” MET. Herein, we explore the MET-dependent activation of STAT3 as a mediator of resistance to MEK inhibitors, and whether MET or STAT3 inhibitors can synergistically increase MEK-inhibitor-induced growth inhibition in KRAS-mutant NSCLC cells in vitro. Cell viability was assessed by MTT (thiazolyl blue) assay after treatment with the allosteric MEK inhibitor, selumetinib, the small-molecule dual inhibitor of the MET and ALK receptor tyrosine kinases, crizotinib, and evodiamine, an alkaloid isolated from the dried, unripe Evodia rutaecarpa (Juss.) Benth fruit, that exerts an anticancer effect by inhibiting STAT3. RNA was isolated from four KRAS cell lines and the STAT3 and MET mRNA expression analysis was performed by TaqMan based qRT-PCR. Western blotting was used to assess the effect of selumetinb on ERK, AKT and STAT3 phosphorylation. We first evaluated the efficacy of the MEK inhibitor selumetinib in our KRAS-mutant NSCLC cell line panel using an MTT cell proliferation assay. H460 cells were relatively insensitive to selumetinib. Following 48-hour treatment with selumetinib, ERK1/2 and AKT phosphorylation were suppressed but a rebound activation of STAT3 occurred in H460 cells. We next investigated whether MET expression was related to the feedback activation of STAT3 signaling following MEK inhibitor treatment. We compared gene expression profiles of the H460 cell line before and after treatment with selumetinib. Interestingly, we found significant upregulation of MET and STAT3 mRNA expression after seven days of selumetinib treatment. To further interrogate the relationship between MEK inhibition and MET-mediated STAT3 reactivation, H460 cells were treated with the combination of selumetinib and crizotinib or selumetinib and evodiamine. A 72-hour exposure to both combinations resulted in a clear cell synergism, as measured by the combination index (CI) analysis, with a CI of 0.79 and 0.78 respectively. Collectively our results showed that the feedback STAT3 activation induced by MET, mitigates the effect of MEK inhibition, and provides rationale for further assessment of combined MEK and MET or STAT3 inhibition in KRAS-mutant NSCLC.