Intrinsic regulation of c-Met induction by STAT3 in nasopharyngeal cancer (NPC): a point for therapeutic intervention for aggressive NPC

2149 Overexpression of c-Met, a key player in cancer invasion and metastasis, is believed to be involved in the malignant progression of nasopharyngeal cancer (NPC). The mechanism for its overexpression in human cancers remains unclear. Hepatocyte growth factor (HGF), the ligand of c-Met, activates downstream signaling of c-Met, as well as inducing upregulation of c-Met expression in some epithelial cancers. This HGF-induced c-Met upregulation may further augment the invasiveness of c-Met-overexpressing cancers. Here, we hypothesize that STAT3, an intrinsic downstream signaling molecule of c-Met, is crucial for the regulation of HGF-induced c-Met overexpression in NPC. In NPC cell lines (e.g. HONE-1 and HK1), HGF induced a dose-dependent expression of c-Met precursor protein at 48 hrs, with a maximal induction at 50 ng/ml of HGF. To investigate the involvement of STAT3 in regulating c-Met expression, two representative NPC cell lines, HONE-1 and CNE-2, were transfected with STAT3 siRNA. Specific downregulation of STAT3 by STAT3 siRNA (300 pmoles), but not control siRNA, was accompanied by significant inhibition of c-Met expression in both NPC cell lines up to day 4 post-transfection. Both c-Met protein and mRNA expression was inhibited up to 70% (by Western blotting and quantitative RT-PCR, respectively). Treatment of HONE-1 and CNE-2 cells with a STAT3 inhibitor, JSI-124 (Curcubitacin I), also resulted in significant inhibition of HGF-induced c-Met protein expression (>50% inhibition) at 48 hrs. The contribution of two other major downstream signaling molecules of c-Met in c-Met regulation was found to be less significant, as activation of AKT by transfection with an activated AKT mutant (myr-AKT1) did not alter c-Met expression up to day 3, and treatment with a MEK-1/-2 inhibitor (U0126, at 20 μM, for 48 hrs) reduced c-Met protein expression by only 30 %. Our novel finding indicates that STAT3 serves as a major intrinsic regulator of c-Met expression in NPC. Therefore, targeting such an intrinsic regulator, STAT3, (and most likely, targeting MAPK as well), may offer additional advantages to current c-Met targeting strategies by attenuating both c-Met induction, as well as, c-Met signaling.This is supported by our finding that: 1) HGF-induced NPC proliferation was completely and efficiently abrogated by targeting STAT3 (by JSI-124 at 200nM for 48 hrs), 2) HGF-induced cell motility and migration (by wound healing and cell scattering assays) was significantly inhibited by JSI-124 (200nM, 24 hrs) in HONE-1. Our findings may have broad implications for various cancers and provide a model for efficient c-Met targeting in cancer. Financial Support: Start-up fund, Department of Clinical Oncology, CUHK (to VWYL)