Fibroblast growth factor receptor 1 gene expression is required for cardiomyocyte proliferation and is repressed by Sp3

Fibroblast growth factor receptor 1 (FGFR1) is the only high-affinity FGFR in the vertebrate myocardium. FGFR1 is a tyrosine kinase receptor and has a non-redundant role in proliferation and differentiation of cardiomyocytes during embryogenesis. Results presented here demonstrate that FGFR1 gene expression declines as neonatal cardiomyocytes develop into adult cardiomyocytes. Furthermore, silencing FGFR1 gene expression reduced neonatal cardiomyocyte proliferation, indicating that FGFR1 gene expression is required for the optimal proliferative capacity of cardiomyocytes. To determine the mechanism that governs FGFR1 gene expression in cardiomyocytes, sequence analysis of the proximal mouse FGFR1 promoter identified a potential binding site for Sp transcription factors. Mutation of this site increased FGFR1 promoter activity compared to the wild-type promoter, indicating the presence of a negative transcriptional regulator of the FGFR1 promoter at this site in cardiomyocytes. Sp3 expression in neonatal cardiomyocytes and Drosophila SL2 cells reduced FGFR1 promoter activity in a dose-dependent manner. Western blots and immunocytochemistry indicated that Sp3 was present in the nuclear and cytoplasmic compartments of neonatal cardiomyocytes. Chromatin-immunoprecipitation studies verified that endogenous Sp3 in cardiomyocytes interacts with the FGFR1 promoter. Transient chromatin-immunoprecipitation studies using wild-type and mutated FGFR1 promoter constructs in SL2 cells identified the specific Sp3 binding site within the FGFR1 promoter. These studies implicate Sp3 as a negative transcriptional regulator of FGFR1 promoter activity in cardiomyocytes and as a suppressor of cardiomyocyte proliferation.