Dynamic transcriptional regulatory complexes, including E2F4, p107, p130, and Sp1, control fibroblast growth factor receptor 1 gene expression during myogenesis.

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.