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 differentiation and disease, proof of in vivo function requires specific 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 cytoplasmic serine/threonine kinase domain. We previously reported that kinase-deficient T beta RII (Delta kT beta RII) blocks TGF-beta-dependent transcription in cardiac myocytes. It is controversial whether both receptors are needed in all cells for gene regulation by TGF beta or whether they mediate distinct subsets of TGF-beta-dependent events. To resolve this uncertainty, TGF-beta-dependent transcription was investigated in cardiac myocytes versus mink lung epithelial cells. 1) Delta kT beta 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 acid recognition motifs, are competent for binding but not transcriptional activation. Each inactive receptor inhibits TGF beta-dependent transcription in both cell types. 3) Kinase-deficient T beta RI (Delta kT beta RI) likewise impairs TGF beta-dependent transcription, less completely than Delta kT beta RII; kinase-deficient activin type I receptor has no effect. 4) TGF-beta-binding proteins in cardiac cells and Mv1Lu 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 lines (that T beta RII and T beta RI are necessary in tandem for the TGF beta-signaling complex to regulate transcription) is valid for cardiac myocytes, the cell type most prominently affected in TGF beta-deficient animals.