We are exploring the role of growth factors in the proliferative control of normal and transformed airway epithelial cells. An in vitro model of rat tracheal epithelial cell (RTE) neoplastic progression is being used to examine TGFalpha and TGFbeta signalling pathways. TGFalpha binds to the EGF receptor and is mitogenic while TGFbeta binds to unique receptors and inhibits the growth of many cells, including those of the tracheo-bronchial tract. Normal RTE cells in culture are highly growth factor dependent and require EGF, while transformed cell variants are EGF independent. Both normal and transformed cells express TGFalpha mRNA transcripts, but expression is significantly higher in most neoplastic cell lines. TGFalpha is detected in media conditioned by transformed cells by RIA and Western blotting and addition of TGFalpha antibodies to the media inhibits growth. Tyrphostin, an EGF receptor tyrosine kinase inhibitor also inhibits growth of transformed cells, presumably by blocking ligand induced mitogenic signals. These results indicate that TGFalpha acts as an autocrine growth regulator of transformed RTE cells. Normal and transformed RTE cells also express TGFbeta transcripts, however, transformed cells secrete significantly less of this negative growth factor. Preliminary evidence suggests that transformed cells are less competent in activating latent TGFbeta than normal cells. TGFbeta responsiveness appears to be highly regulated in normal RTE cells since cells in late log and plateau phases of growth are far less TGFbeta responsive than cells in early growth phase. In contrast, several transformed cell lines are either completely unresponsive or markedly hypo-responsive to TGFbeta growth inhibition, regardless of growth state. Whether this difference between normal and transformed RTE cells is due to altered TGFbeta processing, TGFbeta binding or post-receptor signalling is under investigation.
