OVER-EXPRESSION OF CUG-INITIATED OR AUG-INITIATED FORMS OF BASIC FIBROBLAST GROWTH-FACTOR IN CARDIAC MYOCYTES RESULTS IN SIMILAR EFFECTS ONMITOSIS AND PROTEIN-SYNTHESIS BUT DISTINCT NUCLEAR MORPHOLOGIES

Initiation of translation from alternate codons in the same mRNA resul ts in multiple forms of basic fibroblast growth factor (bFGF). High mo lecular weight species of bFGF make use of leucine translation initiat ion sites located upstream of the methionine residue used to produce t he 18 kiloDalton (kDa) form. Although the addition of exogenous 18 kDa bFGF is known to stimulate DNA synthesis and proliferation of several cell types including embryonic chicken cardiac myocytes, little is kn own about the role of high molecular weight forms of bFGF. We modified the rat bFGF cDNA to yield high (22/21.5 kDa) or low (18 kDa) molecul ar weight species of bFGF. Expression of 22/21.5 kDa or 18 kDa bFGF in transfected embryonic chicken ventricular myocyte cultures was confir med by protein blotting. Expression of both high and low molecular wei ght species of bFGF was associated with (i) a three-fold increase in o verall thymidine incorporation as well as cardiomyocyte labelling inde x (fraction of cardiomyocyte nuclei incorporating tritiated thymidine) ; (ii) a two- to three-fold increase in cell number; (iii) an eight-fo ld increase in protein synthesis; and (iv) a three-fold decrease in my osin accumulation. Subcellular Localization of bFGF in the transfected myocyte cultures was also assessed by immunofluorescence microscopy. Over-expression of cDNAs yielding high molecular weight bFGF resulted in predominantly nuclear bFGF staining. By contrast, both cytoplasmic and nuclear staining were observed following oner-expression of 18 kDa bFGF. Over-expression of 22/21.5 kDa bFGF was associated with the for mation of multiple DNA-containing ''clumps'' resembling condensed chro matin in cardiac myocyte nuclei. These DNA ''clumps'' were not observe d in cardiac myocyte cultures overexpressing 18 kDa bFGF. These data i ndicate that over-expression of high as well as low molecular weight f orms of bFGF can stimulate cardiac myocyte proliferative potential and decrease myosin accumulation. However, these forms possess distinct s ubcellular localizations and can have different biological functions i n the nucleus.