Role of TGFα and its receptor in proliferation of immortalized rat tracheal epithelial cells: Studies with tyrphostin and TGFα antisera

We have shown in the present study and in studies reported previously that preneoplastic and neoplastic rat tracheal epithelial (RTE) cell lines express TGF alpha and do so regardless of the mechanism by which they were transformed. In order to determine whether TGF alpha is an autocrine growth regulator of immortalized RTE cells, we have examined the function of TGF alpha/EGF receptors and the growth requirements for TGF alpha in these cells. The level of immunoprecipitated TGF alpha/EGF receptor protein in immortalized RTE cells was similar to or less than levels in primary RTE cells, indicating that chemically induced transformation of RTE cells does not involve overexpression of TGF alpha/EGF receptors. Scatchard analysis of TGF alpha/EGF receptors in the neoplastic EGV5T cell line revealed the presence of high-affinity (Kd = 0.4 nM) and low-affinity (Kd = 9.8 nM) binding sites. A tyrphostin TGF alpha/EGF receptor tyrosine kinase inhibitor decreased in a dose-dependent manner the proliferation as well as EGF-induced autophosphorylation of the TGF alpha/EGF receptor of transformed RTE cells. The inhibitory effect of tyrphostin on proliferation and receptor kinase activity was attenuated in late log and plateau phase cultures. The phosphotyrosine content of several other EGF-dependent and independent phosphoproteins was also decreased by the tyrphostin. Proliferation of transformed RTE cells was also inhibited when TGF alpha antisera was added to the media of growing cells. These data are consistent with the hypothesis that proliferation of transformed RTE cells involves autocrine regulation by TGF alpha and its receptor.