EXPRESSION OF FIBROBLAST GROWTH-FACTOR RECEPTOR-1 IN RAT-HEART H9C2 MYOBLASTS INCREASES CELL-PROLIFERATION

Basic fibroblast growth factor (FGF-2) plays an important role in myoc ardial growth and development and in particular cardiac myocyte prolif eration. FGF-2 exerts its effects by binding to cell surface receptors (FGFR-1) of the tyrosine kinase family. We have detected the presence of both long and short isoforms of FGFR-1 in embryonic and adult mous e heart. In this report, we have examined the ability of long and shor t FGFR-1 isoforms to signal a mitogenic response. Assessment of RNA fr om rat myoblast H9c2 cells by reverse transcriptase-polymerase chain r eaction and RNA blotting revealed that they were deficient in transcri pts corresponding to long and short FGFR-1 species. Hybrid genes conta ining the cDNAs coding for long and short FGFR-1 isoforms directed by the myosin light chain-2 promoter and simian virus 40 enhancer sequenc es, were used to transiently transfect H9c2 cells. Total tyrosine phos phorylation was increased 2.0 and 2.6 fold in H9c2 cells transfected w ith the long and short FGFR-1 isoforms, respectively, compared to 'con trol' transfected H9c2 cells. This was accompanied by a 2.1 and 2.0 fo ld increase in DNA synthesis, as measured by tritiated thymidine incor poration, in H9c2 cells expressing the long and short FGFR-1 isoforms, respectively. To assess effects on proliferation, H9c2 cells were sta bly transfected with the myosin light chain-2/FGFR-1 cDNA genes. The r ate of proliferation was increased 1.6 and 3.1 fold in H9c2 cells stab ly expressing the long and short FGFR-1 isoforms, respectively, compar ed to 'control' H9c2 cells. In contrast to non transfected H9c2 cells, treatment of H9c2 cells stably expressing long FGFR-1 with FGF-2 for 24 h resulted in a slight increase (1.3 fold, p < 0.02) in cell number . However, a greater response (1.5 fold, p < 0.0005) was observed with H9c2 cells stably expressing short FGFR-1 after treatment with FGF-2. These results suggest that both long and short FGFR-1 isoforms are ca pable of signalling a mitogenic response.