A New 34-Kilodalton Isoform of Human Fibroblast Growth Factor 2 Is Cap Dependently Synthesized by Using a Non-AUG Start Codon and Behaves as a Survival Factor

Abstract

Four isoforms of human fibroblast growth factor 2 (FGF-2) result from alternative initiations of translation at three CUG start codons and one AUG start codon. Here we characterize a new 34-kDa FGF-2 isoform whose expression is initiated at a fifth initiation codon. This 34-kDa FGF-2 was identified in HeLa cells by using an N-terminal directed antibody. Its initiation codon was identified by site-directed mutagenesis as being a CUG codon located at 86 nucleotides (nt) from the FGF-2 mRNA 5' end. Both in vitro translation and COS-7 cell transfection using bicistronic RNAs demonstrated that the 34-kDa FGF-2 was exclusively expressed in a cap-dependent manner. This contrasted with the expression of the other FGF-2 isoforms of 18, 22, 22.5, and 24 kDa, which is controlled by an internal ribosome entry site (IRES). Strikingly, expression of the other FGF-2 isoforms became partly cap dependent in vitro in the presence of the 5,823-nt-long 3' untranslated region of FGF-2 mRNA. Thus, the FGF-2 mRNA can be translated both by cap-dependent and IRES-driven mechanisms, the balance between these two mechanisms modulating the ratio of the different FGF-2 isoforms. The function of the new FGF-2 was also investigated. We found that the 34-kDa FGF-2, in contrast to the other isoforms, permitted NIH 3T3 cell survival in low-serum conditions. A new arginine-rich nuclear localization sequence (NLS) in the N-terminal region of the 34-kDa FGF-2 was characterized and found to be similar to the NLS of human immunodeficiency virus type 1 Rev protein. These data suggest that the function of the 34-kDa FGF-2 is mediated by nuclear targets.

Alternative translation of osteopontin generates intracellular and secreted isoforms that mediate distinct biological activities in dendritic cells

“…16). Alternative translation initiation of non-AUG codons can also regulate the localization of proteins, including Fgf2 (18,19), Fgf3 (Int-2) (20), hck (21), and VEGF (22). In the case reported here, the alternative Opn translation initiation site is downstream of the canonical AUG start codon and therefore differs from previously reported instances of alternative translation in which codons locate upstream of the first in-frame AUG codon.…”

Tumour suppressor p53 inhibits human fibroblast growth factor 2 expression by a post-transcriptional mechanism

“…The multiple roles carried out by FGF-2 require a strong and subtle control of its synthesis. Although numerous studies have described transcriptional regulations of FGF-2 expression, such regulations cannot account for the di erential expression of FGF-2 which is synthesized as ®ve molecular forms with di erent localization and functions (Arnaud et al, 1999;Bikfalvi et al, 1997;Florkiewicz and Sommer, 1989;Prats et al, 1989). Expression of the FGF-2 isoforms is translationally regulated by stress and cell density in normal human cells, whereas these isoforms are constitutively expressed in transformed cells (Galy et al, 1999;Vagner et al, 1996).…”

Regulation of translation by upstream translation initiation codons of surfactant protein A1 splice variants

“…Moreover, the weak context of the AUGs cannot verify or discard the notion of an NH 2 -terminally extended precursor protein. Such proteins may be the products of in-frame uAUGs as has been reported for RNase H1 (79), estrogen receptor-␣ (40), FGF-2 (3,84), and C/EBP␣ (60). This form of regulation sometimes associates with sequestration in a specific cellular compartment, but sometimes is the result of response to cellular stress (50,75,86).…”