Transforming growth factor-β response elements of the skeletal α-actin gene: Combinatorial action of serum response factor, YY1, and the SV40 enhancer-binding protein, TEF-1

Skeletal α-actin (SkA) is representative of the cardiac genes that are expressed at high levels in embryonic myocardium, down-regulated after birth, and reactivated by trophic signals including type β-transforming growth factors (TGFβ). To investigate the molecular basis for cardiac-restricted and TGFβ-induced SkA transcription, we have undertaken a mutational analysis of the SkA promoter in ventricular myocytes, with emphasis on the role of three nominal serum response elements. Serum response factor (SRF) and the bifunctional factor YY1 are the predominant cardiac proteins contacting the proximal SRE (SRE1). Mutations of SRE1 that prevent recognition by SRF and YY1, or SRF alone, virtually abolish SkA transcription in both TGFβ- and vehicle-treated cells; mutation of distal SREs was ineffective. A mutation which selectively abrogates YY1 binding increases both basal and TGFβ-dependent expression, substantiating the predicted role of YY1 as an inhibitor of SRF effects. However, efficient SkA transcription requires combinatorial action of SRE1 with consensus sites for Sp1 and the SV40 enhancer-binding protein, TEF-1. As isolated motifs, either SRE1- or TEF-1-binding sites function as TGFβ response elements. Induction of the SkA promoter by TGFβ required SRF and TEF-1 in concert, unlike other pathways for TGFβ-dependent gene expression.