Involvement of basic fibroblast growth factor NH2 terminus in nuclear accumulation

The human basic Fibroblast Growth Factor (bFGF) gene was shown to encode four polypeptides by an alternative use of initiation codons (three CUG and one AUG). In this report, we present a comparative study of the fate and intracellular localization of individual bFGF isoforms. For this purpose, we have produced the various bFGF isoforms in E. coli and purified them to homogeneity: the 210 amino acid form initiated at CUG1 that contains a nuclear localization sequence (NLS), the 155 amino acid form (AUG-mediated initiation) and the 146 amino acid form (processed form extracted from tissues). While the different bFGFs were taken up by the cell with equal efficiency, more of the 210 amino acid form accumulated in the nucleus and represented 36% of the internalized bFGF compared with 15% in the others. A chimeric protein containing the minimal SV40 Large T NLS (SV40NLS) fused to the 155 amino acid bFGF form (SVbFGF) behaves like the native 155 amino acid form, indicating that nuclear accumulation of exogenous bFGF is not mediated by the NLS-associated function. These results suggest that the amino-terminal part of the 210 amino acid bFGF contains a sequence responsible for its nuclear retention. Bioactivities of the different forms were tested on adult bovine aortic endothelial (ABAE) cells. The bFGF degradation pathways, mitogenic activity and stimulation of rRNA synthesis appeared to be the same for all bFGFs but the stimulation of plasminogen activator was enhanced by the 210 amino acid form and correlated with nuclear accumulation.