Developmentally controlled transcriptional regulation of myogenic cell proliferation and differentiation via expression of the fibroblast growth factor receptor 1 (FGFR1) gene is positively regulated by Sp1 and negatively regulated by E2F4-based transcriptional complexes. We report that p107 and p130 formed transcriptional complexes with E2F4 on the FGFR1 promoter and repressed FGFR1 gene transcription in myogenic cells. However, in Drosophila melanogaster SL2 cells, only p107 was able to repress Sp1-mediated transactivation of the FGFR1 promoter. Gel shift assays using transfected myoblast nuclear extracts showed that ectopic p107 reduced Sp1 occupancy of the proximal Sp binding site of the FGFR1 promoter, and coimmunoprecipitation studies indicated that Sp1 interacts with p107 but not with p130. Gel shift assays also demonstrated that Sp1 interacted with p107 in E2F4-p107 transcriptional complexes in myoblasts. The nature of the repressor transcriptional complex was altered in differentiated muscle fibers by the relative loss of the E2F4-p107-Sp1 transcription complex and replacement by the repressor E2F4-p130 complex. These findings demonstrate that activation and repression of FGFR1 gene transcription is governed by interplay between Sp1, p107, p130, and E2F4 in distinct transcriptional complexes during skeletal muscle development.
Fibroblast growth factor receptors (FGFRs)1 have diverse functional roles in mitogenesis, angiogenesis, cell migration, differentiation, mesoderm induction, bone growth and limb development (1). In skeletal muscle, FGFR1 mediates the mitogenic activity initiated by FGF1 and FGF2. During skeletal myogenesis, FGFR1 gene expression is positively regulated in proliferating myoblasts and negatively regulated in differentiated muscle fibers. The functional significance of regulated expression of the FGFR1 gene during myogenesis is demonstrated by overexpression in vivo. Chick embryos overexpressing wild-type FGFR1 displayed delayed myoblast differentiation and muscle fiber formation. On the contrary, chick embryos overexpressing a dominant-negative form of FGFR1 displayed premature muscle fiber formation with decreased muscle mass (2, 3).Although regulation of FGFR1 gene expression is important for normal growth and development of skeletal muscles, the molecular mechanism governing its transcription is poorly understood. Positive regulation of FGFR1 gene expression in proliferating myoblasts is governed by the Sp1 transcription factor. The chicken FGFR1 promoter contains two functional, distal Sp1 binding sites, and the proximal region contains three Sp1 binding sites, all of which are essential for full promoter activity in proliferating myoblasts (4, 5). Negative transcriptional regulators control FGFR1 promoter activity as FGFR1 gene expression declines during myogenic differentiation. We recently identified E2F4 as a negative regulator of FGFR1 gene expression in skeletal muscle cells (6). Its repressor activity was mediated by E2F4 binding to a proximal cis-element at Ϫ65 bp. Howev...