INACTIVE TYPE-II AND TYPE-I RECEPTORS FOR TGF-BETA ARE DOMINANT INHIBITORS OF TGF-BETA-DEPENDENT TRANSCRIPTION

Although transforming growth factor-beta (TGF beta) is implicated in d ifferentiation and disease, proof of in vivo function requires specifi c inhibitors of the TGF beta cascade. TGF beta binds a family of type I and type II receptors (T beta RI, T beta RII), containing a cytoplas mic serine/threonine kinase domain. We previously reported that kinase -deficient T beta RII (Delta kT beta RII) blocks TGF-beta-dependent tr anscription in cardiac myocytes. It is controversial whether both rece ptors are needed in all cells for gene regulation by TGF beta or wheth er they mediate distinct subsets of TGF-beta-dependent events. To reso lve this uncertainty, TGF-beta-dependent transcription was investigate d in cardiac myocytes versus mink lung epithelial cells. 1) Delta kT b eta RII inhibits induction of a TGF beta-responsive reporter gene, in both cell backgrounds. 2) Charged-to-alanine mutations of key residues of the T beta RII kinase, including consensus ATP binding and amino a cid recognition motifs, are competent for binding but not transcriptio nal activation. Each inactive receptor inhibits TGF beta-dependent tra nscription in both cell types. 3) Kinase-deficient T beta RI (Delta kT beta RI) likewise impairs TGF beta-dependent transcription, less comp letely than Delta kT beta RII; kinase-deficient activin type I recepto r has no effect. 4) TGF-beta-binding proteins in cardiac cells and Mv1 Lu cells are comparable by affinity labeling and immunoprecipitation; however, Mv1Lu cells express up to 3-fold higher levels of T beta RII and T beta RI. Thus, the model inferred from TGF beta-resistant cell l ines (that T beta RII and T beta RI are necessary in tandem for the TG F beta-signaling complex to regulate transcription) is valid for cardi ac myocytes, the cell type most prominently affected in TGF beta-defic ient animals.