REGULATION OF AVIAN PRECARDIAC MESODERM DEVELOPMENT BY INSULIN AND INSULIN-LIKE GROWTH-FACTORS

Endoderm within the heart forming regions of vertebrate embryos has pr onounced effects on myocardial cell development. Previous studies have suggested that these effects are mediated by soluble growth factors, in particular fibroblast growth factor 2 (FGF-2) and activin-A. Since both insulin and insulin-like growth factors (IGFs) are present in dev eloping avian embryos at the time of heart formation, we have investig ated the potential role of these molecules in promoting development of premyocardial cells in quail. Culture of precardiac mesoderm explants from stage 5 quail embryos in medium containing insulin, IGF-I, or IG F-II increased proliferation of premyocardial cells, with maximal stim ulation observed at approximately 25 nM for each ligand. A direct comp arison of the proliferative response of precardiac mesoderm to endoder m, fetal calf serum, insulin, IGF-I, IGF-II, activin-A, and FGF-2 show ed that FGF-2 and activin-A increased proliferation of premyocardial c ells approximately 2-fold, while insulin, IGF-I, and IGF-II stimulated proliferation approximately 3-fold. Insulin and IGF-I enhanced the ra te of myocyte differentiation, similar to previously reported effects of endoderm. In contrast, exposure of precardiac mesoderm explants to transforming growth factor beta (TGF beta) reduced proliferation of pr emyocardial cells and moderated the proliferative effects of IGF-I. TG F beta did not block the differentiation of stage 5 premyocardial cell s. Reverse transcription-polymerase chain reaction (RT-PCR) analyses s howed that mRNAs encoding insulin, IGF-II, insulin receptor, and IGF-I receptor were present in both precardiac mesoderm and endoderm, as we ll as in the forming heart at stage 8. Since premyocardial cells can s urvive and differentiate in a defined medium lacking these factors, pr ecardiac mesoderm may produce IGF-II and insulin at levels that are su fficient to stimulate myocyte development. Taken together, these resul ts suggest that insulin and/or IGF-II may promote cardiac development in vivo by both autocrine and paracrine mechanisms. Cardiogenesis may therefore be promoted by the combined action of several classes of gro wth factors. (C) 1996 Wiley-Liss, Inc.