BDNF and lung cancer: We have previously found higher levels of proBDNF in the media of A549 cells (p53 wild-type) than in H1299 cells (p53-null) and reported that the ratio of proBDNF/mature BDNF was not affected by MMP2 knockdown but increased in the media of both cell lines upon knockdown of MMP9, suggesting that MMP9 regulates the cytotoxic effects induced by proBDNF in lung cancer cells [35].