Since the discovery of epidermal growth factor receptor (EGFR) mutations in 2004 as a driver alteration in non-small-cell lung cancer (NSCLC), mono-therapy with EGFR tyrosine kinase inhibitors (TKIs) is the conventional therapy. Growing evidence demonstrates that single EGFR TKI hyper-activates signal transducer and activator of transcription 3 (STAT3) almost immediately after starting treatment.
We conducted clinical and preclinical studies of key components of signaling pathways limiting EGFR TKI efficacy in EGFR mutant NSCLC.
Gefitinib suppressed EGFR, ERK1/2 and AKT phosphorylation but increased STAT3 phosphorylation on the critical tyrosine residue 705 (pSTAT3-Tyr705) in a time and dose-dependent manner in PC9 cells. Gefitinib with TPCA-1 (STAT3 inhibitor) abolished pSTAT3-Tyr705. In addition to STAT3 activation, co-activation of Src-YES-associated protein 1 (YAP1) was also unexpectedly observed in our study. Gefitinib with TPCA-1 blocked STAT3, but not the YAP1 phosphorylation on tyrosine residue 357 by Src family kinases (SFKs). The triple combination of gefitinib, TPCA-1 and AZD0530 (SFK inhibitor) ablated both STAT3 and YAP1 phosphorylation and had greater effect than gefitinib alone or double combinations in vitro and in vivo.
High levels of STAT3 or YAP1 mRNA expression were associated with worse outcome to EGFR TKI in two independent cohorts of EGFR-mutant NSCLC patients. In the initial cohort of 64 patients, progression-free survival (PFS) was shorter among the patients with high STAT3 than among those with low STAT3 (hazard ratio [HR] for disease progression, 3.02; 95% confidence interval [CI], 1.54-5.93; P = 0.0013). PFS was shorter among the patients with high YAP1 than among those with low YAP1 (HR for disease progression, 2.57; 95%CI, 1.30-5.09; P = 0.0067). The results were similar in the validation cohort of 55 patients.
Co-targeting STAT3 and Src-YAP1-NOTCH signaling pathways could be a feasible solution that would have a major impact in prolonging cancer free survival in EGFR mutant NSCLC patients.
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This work was supported by grants from the La Caixa Foundation and Red Tematica de Investigacion Cooperativa en Cancer (RTICC; grant RD12/0036/ 0072), the National Natural Science Foundation of China (No. 81573680), and the Jiangsu Province Funds for Distinguished Young Scientists (No. BK20140049).
All authors have declared no conflicts of interest.