Binding of transcription factors to widely-separated cis-regulatory elements of the murine FGF-4 gene

Embryonal carcinoma (EC) cells and their embryo-derived counterparts, embryonic stem (ES) cells, have been used extensively to study the transcriptional regulation of the fibroblast growth factor-4 (FGF4) gene. The FGF 4 gene is expressed in EC cells and ES cells, but it is repressed in their retinoic acid (RA) induced differentiated counterparts. Previous studies have shown that the transcription of the FGF-4 gene is controlled by cis-regulatory elements located in the 5' flanking region of the gene, and by a powerful enhancer located approximately 3 kb downstream from the transcription start site. In the current study, gel mobility shift analysis was used to examine the binding of nuclear proteins to cis-regulatory elements involved in the transcription of the EGF-4 gene. We demonstrate that the transcription factors Sp1 and Sp3 in nuclear extracts prepared from EC cells bind to three Sp1 motifs, one located in the downstream enhancer, and two located in the 5' flanking region of the gene. We also show that Sp1 and Sp3 bind to each of the Sp1 motifs when nuclear extracts prepared from EC derived differentiated cells are used. In contrast, differentiation of EC cells and ES cells drastically reduces the ability of nuclear factors to bind to an octamer motif and an adjacent High Mobility Group (HMG) motif, which have been shown previously to play essential roles in the functioning of the FGF-4 enhancer. Together, these findings provide a mechanistic explanation of how the distant FGF 4 enhancer promotes transcription of this gene in EC cells and ES cells, and how diferentiation of these cells represses transcription of the FGF-4 gene.