Alternative translation initiation of human fibroblast growth factor 2 mRNA controlled by its 3'-untranslated region involves a Poly(A) switch and a translational enhancer.

Five fibroblast growth factor 2 (FGF-2) isoforms are synthesized from human FGF-2 mRNA by a process of alternative initiation of translation. The regulation of FGF-2 isoform expression by the mRNA 5823-nucleotide-long 3'-untranslated region containing eight alternative polyadenylation sites was examined. Because previous studies had shown that FGF-2 expression was regulated in primary cells but not in transformed cells, primary human skin fibroblasts were used in this study. Using an approach of cell transfection with synthetic reporter mRNAs, a novel translational enhancer (3'-TE) was identified in the 1370-nucleotide mRNA segment located upstream from the eighth poly(A) site. Deletion mutagenesis showed that the 3'-TE was composed of two domains with additive effects. The 3'-TE exhibited the unique feature of modulating the use of FGF-2 alternative initiation codons, which favored the relative expression of CUG-initiated isoforms. Interestingly, the use of an alternative polydenylation site removing the 3'-TE was detected in skin fibroblasts in response to heat shock and cell density variations. At high cell densities, 3'-TE removal was correlated with a loss of CUG-initiated FGF-2 expression. These data show that the FGF-2 mRNA 3'-untranslated region is able to modulate FGF-2 isoform expression by the coupled processes of translation activation and alternative polyadenylation.