Multiple Forms of bFGF: Differential Nuclear and Cell Surface Localization

Abstract

The single copy gene for human basic fibroblast growth factor (bFGF) has been shown to encode not one but multiple proteins of 24, 23, 22 and 18 kD. Although bioactivities of the 18 kD protein are currently used to define bFGF gene function, it is not yet known if the three larger proteins have these same bioactivities or whether they will serve to define new bFGF gene functions. In this report we present a comparative study describing the de novo synthesis, transport, processing and intracellular location of individual bFGF isoforms. Data from cDNA mutagenesis and COS cell expression experiments show that individual isoforms are differentially localized to either the cell surface or to the nucleus. The 24, 23 and 22 kD proteins (CUG-mediated initiation) exclusively localize in the nucleus while the 18 kD protein (AUG-mediated initiation) is preferentially exported onto the cell surface, but is not released into the surrounding culture medium. Specific CUG or AUG translation initiation codons are necessary and sufficient for the synthesis of each isoform examined and thereby, indirectly, mediate differential localization. Since bFGF does not contain the characteristic signals predicted for cell surface or nuclear targeting, our continuing studies will either unmask its functionally equivalent domain(s) or will identify the requisite participation of yet unknown cellular components.

Fibroblast growth factor/fibroblast growth factor receptor system in angiogenesis

“…Experimental evidences point to different functions of FGF2 isoforms in transfected endothelial cells [66], possibly related to differences in their subcellular localization and release. Indeed, high molecular weight FGF2 isoforms contain a nuclear localization sequence and are mostly recovered in the nucleus whereas the 18 kDa FGF2 isoform is mostly cytosolic [67]. The constitutive overexpression of high molecular weight FGF2 isoforms leads to cell immortalization whereas 18 kDa FGF2 overexpression induces a transformed phenotype [68].…”

Nuclear accumulation of FGF-2 is associated with proliferation of human astrocytes and glioma cells

“…The increase in proliferation of SF-767 or U251MG cells transfected with the CMV-HMW plasmid supports the intracellular action of FGF-2. The high molecular weight FGF-2 isoforms do not associate with the plasma membrane and are absent outside the cells (Bugler et al, 1991;Florkiewicz et al, 1991;Bikfalvi et al, 1995). Proliferation of SF-767 cells expressing high molecular weight isoforms of FGF-2 was inhibited by intracellular acting MTA (Maher, 1993), indicating an involvement of FGF receptor tyrosine kinase, but it was resistant to an extracellularly acting FGF-2 binding antagonist, IP6.…”

“…At least four amino terminus extended isoforms of FGF-2 arise from the same gene due to initiation of translation at di erent start codons with di erent cells expressing di erent isoforms (Florkiewicz and Sommer, 1989). All FGF-2 isoforms are detected in both the cytoplasm and the nucleus of expressing cells (Powell and Klagsbrun, 1991;Florkiewicz et al, 1991;Gualandris et al, 1993;Woodward et al, 1992;Vijayan et al, 1993;Stachowiak et al, 1994Stachowiak et al, , 1996a. Nuclear presence of FGF-2 was detected in rat brain astrocytes both in vivo (Woodward et al, 1992) and in vitro (Vijayan et al, 1993) and in cultured human astrocytes and glioma cells (Mo ett et al, 1996;Stachowiak et al, 1996a).…”

Differentially promoted peripheral nerve regeneration by grafted Schwann cells over-expressing different FGF-2 isoforms

“…The FGF-2 protein is expressed in different isoforms, representing different translation products from a single mRNA (Florkiewicz et al, 1991;Florkiewicz and Sommer, 1989). In DRGs, the elevation of the endogenous FGF-2 following injury implied a trophic function of this molecule for sensory neurons.…”