Inactive type II and type I receptors for TGFβ are dominant inhibitors of TGFβ-dependent transcription

Abstract

Although transforming growth factor-β (TGFβ) is implicated in differentiation and disease, proof of in viva function requires specific inhibitors of the TGFβ cascade. TGFβ binds a family of type I and type II receptors (TβRI, TβRII), containing a cytoplasmic serine/threonine kinase domain. We previously reported that kinase-deficient TβRII (ΔkTβRII) blocks TGFβ-dependent transcription in cardiac myocytes. It is controversial whether both receptors are needed in all cells for gene regulation by TGFβ or whether they mediate distinct subsets of TGFβ-dependent events. To resolve this uncertainty, TGFβ-dependent transcription was investigated in cardiac myocytes versus mink lung epithelial cells. 1) ΔkTβRII inhibits induction of a TGFβ-responsive reporter gene, in both cell backgrounds. 2) Charged-to-alanine mutations of key residues of the TβRII kinase, including consensus ATP binding and amino acid recognition motifs, are competent for binding but not transcriptional activation. Each inactive receptor inhibits TGFβ-dependent transcription in both cell types. 3) Kinase-deficient TβRI (ΔkTβRI) likewise impairs TGFβ-dependent transcription, less completely than ΔkTβRII; kinase-deficient activin type I receptor has no effect. 4) TGFβ-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 TβRII and TβRI. Thus, the model inferred from TGFβ-resistant cell lines (that TβRII and TβRI are necessary in tandem for the TGFβ-signaling complex to regulate transcription) is valid for cardiac myocytes, the cell type most prominently affected in TGFβ-deficient animals.