ALTERNATIVE TRANSLATION OF HUMAN FIBROBLAST GROWTH-FACTOR-2 MESSENGER-RNA OCCURS BY INTERNAL ENTRY OF RIBOSOMES

Alternative initiations of translation of the human fibroblast growth factor 2 (FGF-2) mRNA, at three CUG start codons and one AUG start cod on, result in the synthesis of four isoforms of FGF-2. This process ha s important consequences on the fate of FGF-2: the CUG-initiated produ cts are nuclear and their constitutive expression is able to induce ce ll immortalization, whereas the AUG-initiated product, mostly cytoplas mic, can generate cell transformation. Thus, the different isoforms pr obably have distinct targets in the cell. We show here that translatio n initiation of the FGF-2 mRNA breaks the rule of the cap-dependent ri bosome scanning mechanism. First, translation of the FGF-2 mRNA was sh own to be cap independent in vitro. This cap-independent translation r equired a sequence located between nucleotides (nt) 192 and 256 from t he 5' end of the 318-nt-long 5' untranslated region. Second, expressio n of bicistronic vectors in COS-7 cells indicated that the FGF-2 mRNA is translated through a process of internal ribosome entry mediated by the mRNA leader sequence. By introducing additional AUG codons into t he RNA leader sequence, we localized an internal ribosome entry site t o between nt 154 and 318 of the 5' untranslated region, just upstream of the first CUG. The presence of an internal ribosome entry site in t he FGF-2 mRNA suggests that the process of internal translation initia tion, by controlling the expression of a growth factor, could have a c rucial role in the control of cell proliferation and differentiation.