BMP-2 and TGFß2 shared pathways regulate endocardial cell transformation.

ABSTRACT

Valvular heart disease is a major cause of mortality and morbidity. Revealing the cellular processes and molecules that regulate valve formation and remodeling is required to develop effective therapies. A key step in valve formation during heart development is the epithelial-mesenchymal transformation (EMT) of a subpopulation of endocardial cells in the atrioventricular cushion (AVC). The type III transforming growth factor-ß receptor (TGFßR3) regulates AVC endocardial cell EMT in vitro and mesenchymal cell differentiation in vivo. Little is known concerning the signaling mechanisms downstream of TGFßR3. Here we use endocardial cell EMT in vitro to determine the role of 2 well-characterized downstream TGFß signaling pathways in TGFßR3-dependent endocardial cell EMT. Targeting of Smad4, the common mediator Smad, demonstrated that Smad signaling is required for EMT in the AVC and TGFßR3-dependent EMT stimulated by TGFß2 or BMP-2. Although we show that Smads 1, 2, 3, and 5 are required for AVC EMT, overexpression of Smad1 or Smad3 is not sufficient to induce EMT. Consistent with the activation of the Par6/Smurf1 pathway downstream of TGFßR3, targeting ALK5, Par6, or Smurf1 significantly inhibited EMT in response to either TGFß2 or BMP-2. The requirement for ALK5 activity, Par6, and Smurf1 for TGFßR3-dependent endocardial cell EMT is consistent with the documented role of this pathway in the dissolution of tight junctions. Taken together, our data demonstrate that TGFßR3-dependent endocardial cell EMT stimulated by either TGFß2 or BMP-2 requires Smad4 and the activation of the Par6/Smurf1 pathway.