Abstract
We have previously isolated the human FGF-1 gene in order to elucidate the molecular basis of its gene expression. The gene spans over 100 kbp and encodes multiple transcripts expressed in a tissue- and cell-specific manner. Two variants of FGF-1 mRNA (designated FGF-1.A and 1.B), which differ in their 5' untranslated region, were identified in our laboratory. Recently, two novel variants of FGF-1 mRNA (designated FGF-1.C and 1.D) have been isolated. In this study we used RNase protection assays to demonstrate expression of FGF-1.D mRNA in human fibroblasts and vascular smooth muscle cells and to show that promoter 1D has multiple transcription start sites. A single-strand nuclease-sensitive region has also been identified in the promoter 1D region that may have implications in chromatin conformation and transcriptional regulation of this promoter. Using Northern blot hybridization analyses, a previous study demonstrated a significant increase of FGF-1 mRNA levels in cultured saphenous vein smooth muscle cells in response to serum and phorbol ester. Here we confirm these results by RNase protection analysis and show that FGF-1.C mRNA is significantly increased in response to these stimuli. RNase protection assays indicate that promoter 1C has one major start site. The phorbol ester effect suggests that a protein kinase C-dependent signalling pathway may be involved in this phenomenon. Our results point to a dual promoter usage of the FGF-1 gene in vascular smooth muscle cells. Thus, normal growing cells primarily utilize promoter 1D. In contrast, quiescent cells, when exposed to serum or phorbol ester, utilize a different FGF-1 promoter, namely promoter 1C. Overall, these phenomena suggest mechanisms for increased production of FGF-1 that may play a role in inflammatory settings, wound healing, tissue repair, and neovascularization events and processes via autocrine and paracrine mechanisms. Our findings suggest that different FGF-1 promoters may respond to different physiological conditions and stimuli, in reference to the cell type or tissue milieu, resulting in ultimate production of the FGF-1 protein.