Despite improvements in preventive, diagnostic, and therapeutic approaches, lung cancer remains the leading cause of cancer-related deaths in the U.S. There are currently no effective therapies for those diagnosed with later stage lung cancer. Recent efforts have focused on targeting specific lung cancer-related growth pathways, such as the hepatocyte growth factor (HGF)/c-Met signaling pathway. HGF/c-Met signaling plays a critical role in mediating proliferation, angiogenesis, invasion, and inflammatory responses in non-small cell lung cancer (NSCLC). This signaling pathway also confers resistance to therapies targeting other growth factor pathways such as epidermal growth factor receptor (EGFR) and vascular endothelial growth factor (VEGF). This study focuses on two aspects of HGF/c-Met signaling relevant to lung cancer: HGF stimulated c-Met angiogenic response and EGFR ligand-induced c-Met pro-cancer signaling. We previously reported airway expression of a human HGF transgene (HGF TG) produced mice that were more susceptible to lung tumorigenesis induced by a tobacco carcinogen. Here we show untreated HGF TG mice display enhanced vascularization and lymph vessel formation in the lungs compared to wild-type (WT) littermates, as ascertained by microvessel density. Whole lung microarray analysis consistently found significant decreases in expression of genes involved in angiogenesis including the VEGF family of genes (Vegfa,Vegfb, Vegfc, Vegfd/Figf) at 10, 20, and 40 weeks of age in HGF TG animals compared to WT littermates. HGF TG lung tumors derived from carcinogen treated HGF TG mice demonstrated reduction in VEGF genes with an increased expression of five Cxcl family genes including Cxcl1 and Cxcl2 (murine forms of IL-8). Thus, increased vascularization produced by airway over-expression of HGF may occur through direct activation of c-Met on endothelial cells and expression of inflammatory mediators, rather than induction of VEGF pathways. Ligand-independent delayed and prolonged activation of c-Met has also been demonstrated previously by our laboratory via cross-talk from the EGFR pathway. Here we show that prolonged activation of STAT3 by EGFR-ligands is dependent on delayed c-Met activation. Inhibition of c-Met, by PHA665752, eliminates EGFR stimulated activation of STAT3 at delayed time points. The failure of Src inhibition, by PP2, to block delayed phospho-STAT3, and the co-immunoprecipitation of STAT3 with c-Met 8-24hours post EGF stimulation, suggests STAT3 is directly activated by c-Met. These data reinforce the idea that delayed c-Met activation is utilized by EGFR to potentiate its full biological effects through STAT3. Both ligand-dependent transient and ligand-independent delayed c-Met activation appear to be important in lung tumorigenesis. The findings of this study support future development of novel HGF/c-Met and EGFR combination therapies in NSCLC
