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 myocardial growth and development and in particular cardiac myocyte proliferation. 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 mouse heart. In this report, we have examined the ability of long and short FGFR-1 isoforms to signal a mitogenic response. Assessment of RNA from rat myoblast H9c2 cells by reverse transcriptase-polymerase chain reaction and RNA blotting revealed that they were deficient in transcripts corresponding to long and short FGFR-1 species. Hybrid genes containing the cDNAs coding for long and short FGFR-1 isoforms directed by the myosin light chain-2 promoter and simian virus 40 enhancer sequences, were used to transiently transfect H9c2 cells. Total tyrosine phosphorylation was increased 2.0 and 2.6 fold in H9c2 cells transfected with the long and short FGFR-1 isoforms, respectively, compared to 'control' transfected H9c2 cells. This was accompanied by a 2.1 and 2.0 fold increase in DNA synthesis, as measured by tritiated thymidine incorporation, in H9c2 cells expressing the long and short FGFR-1 isoforms, respectively. To assess effects on proliferation, H9c2 cells were stably transfected with the myosin light chain-2/FGFR-1 cDNA genes. The rate of proliferation was increased 1.6 and 3.1 fold in H9c2 cells stably expressing the long and short FGFR-1 isoforms, respectively, compared 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 capable of signalling a mitogenic response.