Loss of RBPMS in ovarian cancer compromises the efficacy of EGFR inhibitor gefitinib through activating HER2/AKT/mTOR/P70S6K signaling

RBPMS may be a tumor suppressor in cancer, but its impact in modulation of drug sensitivity is unclear. This study aimed to investigate the regulatory role of RBPMS in cellular response to EGFR inhibitor gefitinib in ovarian cancer (OC). By western blotting assay, we revealed RBPMS was down-regulated in epithelial ovarian cancer tissues compared to normal control ovarian epithelial tissues. Overexpression of RBPMS inhibited cell viability and proliferation, and conferred gefitinib sensitivity, accompanied by reduced expression of p-EGFR, and vice versa. Proteomic analysis and flow cytometry experiments showed that RBPMS induced S-stage cell cycle arrest in gefitinib-treated OC cells. Co-IP assay suggested that HER2 was a downstream target of RBPMS, and RBPMS negatively regulated HER2 expression. HER2 counteracted the stimulation of RBPMS to cell growth blocking, gefitinib sensitivity and cell cycle arrest. We further demonstrated that RBPMS overexpression suppressed the activation of p-AKT, p-mTOR and p-P70S6K, which was rescued by up-regulation of HER2. The combination of AKT inhibitor MK2206 and gefitinib had a synergistic effect on OC cells with high level of RBPMS. In conclusion, through the direct inhibition of HER2/AKT/mTOR/P70S6K pathway, RBPMS may be a potential therapeutic target for improving gefitinib sensitivity in OC.