HGF and cancer: In the present study, we demonstrated the following: (1) co-inoculation of mice with lung cancer cells and lung fibroblasts results in higher susceptibility to tumorigenesis and higher tumor progression in those mice, (2) lung fibroblast-conditioned medium contributes to lung cancer cell survival through, at least in part, the production of HGF, (3) inhibition of HGF/Met signaling can suppress cancer cell survival and tumor progression, and (4) lung cancer cells stimulate HGF production in lung fibroblasts, as summarized in Fig. 7.