Nuclear Activities of Basic Fibroblast Growth Factor: Potentiation of Low-Serum Growth Mediated by Natural or Chimeric Nuclear Localization Signals

Abstract

Human basic fibroblast growth factor (FGF-2) occurs in four isoforms: a low molecular weight (LMW FGF-2, 18 kDa) and three high molecular weight (HMW FGF-2, 22, 22.5, and 24 kDa) forms. LMW FGF-2 is primarily cytoplasmic and functions in an autocrine manner, whereas HMW FGF-2s are nuclear and exert activities through an intracrine, perhaps nuclear, pathway. Selective overexpression of HMW FGF-2 forms in fibroblasts promotes growth in low serum, whereas overexpression of LMW FGF-2 does not. The HMW FGF-2 forms have two functional domains: an amino-terminal extension and a common 18-kDa amino acid sequence. To investigate the role of these regions in the intracrine signaling of HMW FGF-2, we produced stable transfectants of NIH 3T3 fibroblasts overexpressing either individual HMW FGF-2 forms or artificially nucleartargeted LMW FGF-2. All of these forms of FGF-2 localize to the nucleus/nucleolus and induce growth in low serum. The nuclear forms of FGF-2 trigger a mitogenic stimulus under serum starvation conditions and do not specifically protect the cells from apoptosis. These data indicate the existence of a specific role for nuclear FGF-2 and suggest that LMW FGF-2 represents the biological messenger in both the autocrine/paracrine and intracrine FGF-2 pathways. INTRODUCTIONBasic fibroblast growth factor (FGF-2) 1 is a member of a large family of heparin-binding growth factors. Thus far 19 members (Nishimura et al., 1999) have been described. These proteins affect various biological processes ranging from cell proliferation to plasminogen activation, integrin expression, cell migration, embryonic development, and cell differentiation. FGFs also may be involved in tumor angiogenesis and malignant transformation (Burgess and Maciag, 1989;Rifkin and Moscatelli, 1989;Basilico and Moscatelli, 1992;Mason, 1994).The biological functions of the FGFs are mediated by their interaction with both high-and low-affinity plasma membrane receptors (Baird, 1994). A family consisting of four high-affinity tyrosine kinase FGFreceptors has been identified (Basilico and Moscatelli, 1992;Jaye et al., 1992). The interaction of FGF-2 with its plasma membrane high-affinity receptors induces autophosphorylation of the receptor and initiates the phosphorylation of tyrosine residues in cytosolic substrates (Fantl et al., 1993). The low-affinity receptors consist of heparan sulfate proteoglycans and are thought to provide a mechanism both to concentrate ligand and to present FGF dimers to tyrosine kinase receptors Baird, 1994) The prototypic members of the FGF family, FGF-1 and -2, lack a signal sequence for secretion, although the proteins are released and have been visualized in ‡ Corresponding author. E-mail address: aresem01@mcrcr.med. nyu.edu. 1 Abbreviations used: FGF-2, basic fibroblast growth factor; HMW FGF-2, high molecular weight FGF-2; LMW FGF-2, low molecular weight FGF-2.© 1999 by The American Society for Cell Biology 1429 the ECM of different tissues (Abraham et al., 1986;Jaye et al., 1986). The mechanism of FGF-1 and FGF-2 r...

Differential Intranuclear Localization of Fibroblast Growth Factor-2 Isoforms and Specific Interaction with the Survival of Motoneuron Protein

“…However, because the 18-kDa isoform localizes also to the nucleus, a second NLS in the 18-kDa "core" sequence can be expected. The individual isoforms were reported to localize to the nucleoli (18,16). The aim of the present study was to analyze the localization of FGF-2 isoforms in more detail in other nuclear substructures by means of tagging with fluorescent proteins (EGFP and DsRed) and to elucidate the molecular mechanisms responsible for the nuclear localization of FGF-2 isoforms.…”

Improving protein delivery of fibroblast growth factor-2 from bacterial inclusion bodies used as cell culture substrates

“…Arese and co-workers demonstrated that cell proliferation of the NIH3T3 cell line at low serum concentrations is stimulated by the HMW forms of the FGF-2 protein, being associated with a nuclear action [23]. In addition they showed that a chimeric LMW FGF-2 (fused to an NLS peptide) was capable to emulate HMW forms activity [23].…”

“…In addition they showed that a chimeric LMW FGF-2 (fused to an NLS peptide) was capable to emulate HMW forms activity [23]. In our study, FGF-2 IBs when provided as modified cell culture surfaces were able to enhance cell growth under serum starvation conditions, considering that this activity has been previously linked to a nuclear activity and our IBs are formed by the cytosolic LMW form of the protein it is feasible to speculate that FGF-2 IBs were internalized, as it have been described for other IB types [18;19], delivering the forming protein to the nucleus, the way in which this process takes place remains still unclear and deserves further studies.…”

“…A part from this passive protein delivery IBs have recently shown able to be engineered the same sequence as the, LMW but present an extra peptide that has been described as a nuclear localization signal. Thus, the three HMW FGF-2 forms act at the nuclear level while the LMW form is mainly cytoplasmatic [23]. In this regard, the 18 KDa FGF-2 is secreted and triggers signaling cascades via its interaction with the FGF-2 receptor [24], differentiating FGF-2 action in an extracellular triggered activity (LMW) and an intracellular action (HMW).…”

The G-rich promoter and G-rich coding sequence of basic fibroblast growth factor are the targets of thalidomide in glioma

“…However, because the addition of recombinant human bFGF only partially rescued the loss of the soft agar colony-forming ability of knockdown cells (Supplementary Table S3), 3 the contribution of intracrine signaling of HMW bFGFs to cell transformation was postulated. Although the precise role of nuclear bFGF in glioma cells is unclear, stimulation of fibroblast growth in low serum by nuclear bFGF has been reported (42). In addition, the nuclear accumulation of bFGFs or the increased ratio of HMW bFGFs to LMW bFGF has been reported as an indicator for tumor progression (43).…”