MMP9 and lung cancer: BCAFs can produce a variety of MMPs that induce invasion, migration and metastasis of breast cancer cells [39], and HGF antagonists have been shown to downregulate MMP-9 activity in lung cancer cells [40], leading us to speculate that HGF may play an important role in the secretion of MMPs by BCAFs to directly or indirectly induce invasion and metastasis of breast cancer.