TRANSLATION OF CUG-INITIATED BUT NOT AUG-INITIATED FORMS OF HUMAN FIBROBLAST GROWTH-FACTOR-2 IS ACTIVATED IN TRANSFORMED AND STRESSED CELLS

Four isoforms of the human fibroblast growth factor 2 (FGF-2), with di fferent intracellular localizations and distinct effects on cell pheno type, result from alternative initiations of translation at three CUG and one AUG start codons. We showed here by Western immunoblotting and immunoprecipitation that the CUG-initiated forms of FGF-2 were synthe sized in transformed cells, whereas ''normal'' cells almost exclusivel y produced the AUG-initiated form. CUG-initiated FGF-2 was induced in primary skin fibroblasts in response to heat shock and oxidative stres s. In transformed cells and in stressed fibroblasts, CUG expression wa s dependent on cis-elements within the 5' region of FGF-2 mRNA and was not correlated to mRNA level, indicating a translational regulation U V cross-linking experiments revealed that CUG expression was linked to the binding of several cellular proteins to FGF-2 mRNA 5' region. Sin ce translation of FGF-2 mRNA was previously shown to occur by internal ribosome entry, a nonclassical mechanism already described for picorn aviruses, the cross-linking patterns of FGF-2 and picornavirus mRNAs w ere compared. Comigration of several, proteins, including a p60, was o bserved. However, this p60 was shown to be different from the p57/PTB internal entry factor, suggesting a specificity towards FGF-2 mRNA. We report here a process of translational activation of the FGF-2 CUG-in itiated forms in direct relation with trans-acting factors specific to transformed and stressed cells. These data favor a critical role of C UG-initiated FGF-2 in cell transformation and in the stress response.