Myocardial regulation of transforming growth factor-beta expression by outflow tract endothelium in the early embryonic chick heart.

We have demonstrated previously that the epithelial-mesenchymal transformation of cardiac endothelium in early chick heart development is induced by EDTA-soluble (ES) extracellular molecules synthesized by the myocardium of specific regions, i.e., the outflow tract (OT) and atrioventricular (AV) canal. Polyclonal antibodies (ES3) prepared against these molecules recognized two major bands, 28 and 46 kDa, in immunoblots and blocked the transformation of OT endothelial cells into mesenchyme in a three-dimensional collagen gel culture system. The studies of Potts et al. (Proc. Natl. Acad. Sci. USA 88, 1516-1520 (1991)) and Potts and Runyan (Dev. Biol. 134, 392-401 (1989)) indicate that transforming growth factor (TGF)-beta expression is necessary for the formation of mesenchyme from cardiac endothelium. In this study, we used ES3 antibodies to test the hypothesis that TGF-beta expression by transforming endothelial cells is regulated by ES antigens. OT and AV endothelial cells treated with embryonic cardiocyte conditioned medium (CCM), which elicits epithelial-mesenchymal transformation, were shown by immunohistochemistry to increase expression of TGF-beta 1-like protein immediately prior to and during their transformation in culture. Endothelium from a nontransforming region of the heart (i.e., ventricle) did not express detectable levels of TGF-beta under similar conditions. The staining pattern for TGF-beta 1-like protein was characterized by a distinct particulate or granular distribution within the Golgi and cytoplasm and at cell surfaces. However, when endothelial transformation was blocked by immunoadsorption of ES proteins from CCM, increased staining for TGF-beta was not observed. These findings suggest an inductive relationship between myocardially derived ES proteins and TGF-beta expression by chick heart endothelial cells which is requisite for their transformation into cushion mesenchyme.