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 transf ormation of cardiac endothelium in early chick heart development is in duced by EDTA-soluble (ES) extracellular molecules synthesized by the myocardium of specific regions, i.e., the outflow tract (OT) and atrio ventricular (AV) canal. Polyclonal antibodies (ES3) prepared against t hese molecules recognized two major bands, 28 and 46 kDa, in immunoblo ts and blocked the transformation of OT endothelial cells into mesench yme in a three-dimensional collagen gel culture system. The studies of Potts ct al. (Proc. Natl. Acad. Sci. USA 88, 1516-1520 (1991)) and Po tts and Runyan (Dev. Biol. 134, 392-401 (1989)) indicate that transfor ming growth factor (TGF)-beta expression is necessary for the formatio n of mesenchyme from cardiac endothelium. In this study, we used ES3 a ntibodies to test the hypothesis that TGF-beta expression by transform ing endothelial cells is regulated by ES antigens. OT and AV endotheli al cells treated with embryonic cardiocyte conditioned medium (CCM), w hich elicits epithelial-mesenchymal transformation, were shown by immu nohistochemistry to increase expression of TGF-beta 1-like protein imm ediately prior to and during their transformation in culture. Endothel ium from a nontransforming region of the heart (i.e., ventricle) did n ot express detectable levels of TGF-beta under similar conditions. The staining pattern for TGF-beta 1-like protein was characterized by a d istinct particulate or granular distribution within the Golgi and cyto plasm and at cell surfaces. However, when endothelial transformation w as blocked by immunoadsorption of ES proteins from CCM, increased stai ning for TGF-beta was not observed. These findings suggest an inductiv e relationship between myocardially derived ES proteins and TGF-B expr ession by chick heart endothelial cells which is requisite for their t ransformation into cushion mesenchyme. (C) 1994 Academic Press, Inc.