Transcriptional activation of fibroblast growth factor 1.B promoter is mediated through an 18-base pair cis-acting...

Four different transcripts encoding fibroblast growth factor 1 (FGF-1, also known as aFGF) have been previously identified in our laboratory. Among them, FGF-1.B is the major transcript expressed specifically in the neuronal cells in brain tissue. Using the transient transfection experiment in a glioblastoma cell line, U1240MG, that expresses 1.B, we previously identified two regulatory regions (RR1 and RR2) in the brain-specific promoter, FGF-1.B. In the present study, we showed that the minimal region required for the DNA-protein interaction in RR2 resides in an 18-base pair (-484 to -467) sequence, by using DNase I footprinting and methylation interference studies and electrophoretic mobility shift assays. This minimal cis-acting element was found to be sufficient in enhancing the reporter activity driven by the heterologous herpes simplex virus thymidine kinase promoter in the 1.B-positive U1240MG cell line. This enhancing effect, however, was not detected in a glioblastoma cell line, U1242MG, which is negative for 1.B expression. By electrophoretic mobility shift assays, we also identified a specific DNA-protein complex, namely complex I, which is specific for 1.B-positive cell lines and human brain tissue. By in situ UV cross-linking experiment, we further showed that complex I contains two major DNA-binding proteins of apparent molecular masses of 37 and 98 kDa. Our results suggest that the formation of complex I, resulting from the heterodimerization of a 37-kDa protein (1.B-specific) and a 98-kDa protein (ubiquitous) may likely be a prerequisite for the enhanced expression of 1.B transcript in neuronal cells.