Abnormalities of growth factor systems in transformed airway epithelial cells

The role of the peptide growth factors transforming growth factor alpha (TGF alpha) and transforming growth factor beta (TGF beta) in the regulation of proliferation of normal and transformed airway epithelial cells was studied. Normal as well as transformed rat tracheal epithelial (RTE) cell cultures secrete similar amounts of TGF alpha during logarithmic growth. Once normal RTE cell cultures reach the plateau phase of growth, they down-regulate TGF alpha expression at the RNA and protein level; in contrast, transformed cells do not down-regulate TGF alpha. Using neutralizing TGF alpha antiserum and a tyrphostin TGF alpha/EGF receptor tyrosine kinase inhibitor, we show that the secreted TGF alpha is utilized by both normal and transformed cells as an autocrine mitogenic factor. Normal RTE cells are highly sensitive to the growth inhibitory effects of TGF beta, particularly during early phases of logarithmic growth. At late logarithmic and plateau phases of proliferation, cultures of normal RTE cells secrete large amounts of TGF beta. That the endogenous TGF beta is exerting growth inhibitory effects can be demonstrated by adding TGF beta antisera to the cultures which causes a burst of proliferation. Many transformed RTE cell lines exhibit a markedly reduced sensitivity to the growth inhibitory effects of TGF beta. However, the cells remain responsive to regulation of ECM genes by TGF beta. The transformed cell lines examined secrete less than one tenth the amount of TGF beta of normal cells. Our studies show that the autocrine TGF beta growth restraining mechanism is inoperative in many of the RTE cell transformants. We conclude, therefore, that alterations in TGF alpha and TGF beta regulation of cell proliferation are important factors contributing to the abnormal growth behavior of transformed RTE cells.