The cellular action of growth factors, among them basic fibroblast growth factor (bFGF), is mediated by their interaction with a cell surface receptor, but the mechanism of transfer of mitogenic (or other) signals to the nucleus has not been identified. In this work, we show that bFGF is translocated to and accumulated in the nucleolus. Furthermore, the nucleolar localization of bFGF is correlated with a stimulation of transcription of ribosomal genes during Gq-.Gl transition induced by bFGF alone in adult bovine aortic endothelial cells (ABAE cells). Stimulation of ribosomal gene transcription is preceded by a significant increase of the major nonhistone nucleolar protein, nucleolin. In vitro, the growth factor has a direct effect on the enhancement of RNA polymerase I activity in isolated nuclei from quiescent sparse (GO) ABAE cells. The direct action of bFGF on the level of ribosomal gene transcription could correspond to an additional growthsignaling pathway, mediated by this growth factor.The family of fibroblast growth factors (FGFs) includes the factors described as endothelial cell growth factor, chondrosarcoma growth factor, and heparin-binding growth factors (1). Preliminary physical analysis of some of these mitogens has suggested their classification in two groups: acidic fibroblast growth factors (aFGFs) (2) and basic fibroblast growth factors (bFGFs) (3). In vitro, aFGFs and bFGFs are potent mitogens for a wide variety of mesoderm-and neuroectoderm-derived cells, including vascular and capillary endothelial cells (4) and, as in vivo (5), they induce the angiogenic response (6).The cellular action of FGFs is exerted through its interaction with specific cell surface receptors (7,8), but the intervening steps and the mechanism of transfer of mitogenic (or other) signals to the nucleus, leading to the "ple-otropic response" required to bring quiescent cells into full proliferation, are at present unknown.The proliferation state and ribosome biogenesis, which involve a series of coordinated nucleolar events, among them the transcription of ribosomal genes (rDNA), are closely related. The level of transcription of rDNA is modulated by cell growth conditions, growth-promoting hormones (9), and growth factors (10). A specific nucleolar protein, nucleolin, was shown in different eukaryotic cells to play a direct role in the control of the synthesis of the precursor to ribosomal RNA (pre-rRNA) and assembly of ribosomes (11,12). Barely detectable in resting cells, nucleolin represents up to 5% of nucleolar proteins in exponentially growing cells. In vitro, run-off experiments with rDNA as template have shown that endoproteolytic cleavage of phosphorylated nucleolin controls rDNA transcription (13).In this report, we have focused on the effects of bFGF on the reinitiation of ribosome biogenesis in cells undergoing the Gy-3G1 transition. We show by immunocytochemistry using a monospecific polyclonal anti-bFGF antibody that the reinitiation of pre-rRNA synthesis is preceded by the accumulation of nucleo...

Primary cultures of adult bovine aortic endothelial (ABAE) cells require bFGF to grow. G1‐arrested cells, obtained after 48 h without serum and bFGF, were found to enter S phase and grow synchronously for at least two generations on addition of bFGF. In growing cells bFGF was detected both in the cytoplasm (90%) and in the nucleus (10%) where it accumulates in the nucleolus. It was not detected in the nucleus of confluent cells. bFGF uptake was continuous in the cytoplasm throughout the cell cycle with a maximum in G2, while nuclear uptake occurred only in late G1. Cytoplasmic bFGF (18.4 kd) is cleaved into a 16.5 kd peptide in G1 (t1/2 = 30 min). In the nucleus the 18.4 kd form was the only one detected 2 h following bFGF addition and was then cleaved into the 16.5 kd in early S phase. These results are consistent with the possibility that in addition to the classical pathway of signal transduction, bFGF is directly translocated to the nucleus in late G1, and could play a role in replication and/or in transcription of rDNA.

The presence of fibroblast growth factor-2 (FGF-2) in the nucleus has now been reported both in vitro and in vivo, but its nuclear functions are unknown. Here, we show that FGF-2 added to nuclear extract binds to protein kinase CK2 and nucleolin, a CK2 natural substrate. Added to baculovirus-infected cell extracts overexpressing CK2 or its isolated subunits, FGF-2 binds to the enzyme through its regulatory ␤ subunit. Using purified proteins, FGF-2 is shown to directly interact with CK2 and to stimulate CK2 activity toward nucleolin. Furthermore, a mitogenic-deficient FGF-2 mutant protein has an impaired ability to interact with CK2 and to stimulate CK2 activity using nucleolin as substrate. We propose that in growing cells, one function of nuclear FGF-2 is to modulate CK2 activity through binding to its regulatory ␤ subunit.The fibroblast growth factors (FGFs) 1 family includes nine polypeptides of which FGF-1 and FGF-2 (acidic and basic FGF) are prototype members (reviewed in Refs. 1 and 2). FGF-2 exerts its pleiotropic effects in cell growth and differentiation through a dual receptor system consisting of four different high-affinity transmembrane receptor tyrosine kinases and low-affinity binding sites corresponding to heparan sulfate proteoglycans (reviewed in Refs. 2 and 3). In addition, FGF-1 and FGF-2 are translocated from outside the cell to the nucleus (reviewed in Ref.2). Many other growth factors have been detected in the cell nucleus (reviewed in Refs. 4 and 5). In the case of Schwannomaderived growth factor and FGF-1, nuclear localization is necessary for mitogenic activity (6 -8). For FGF-2, nuclear translocation is correlated to cell proliferation, since it is no longer recovered in the nucleus of confluent cells (9, 10). Therefore, these data strongly suggest that FGF-2, as well as other growth factors, plays specific, but still unknown, nuclear functions in addition to classical signaling through cell surface receptors.We reported previously that in synchronized ABAE growing cells, a large increase of ribosomal genes transcription is tightly correlated to the nuclear translocation of FGF-2 and especially to an accumulation of the growth factor in the nucleolus. In contrast, in quiescent cells ribosomal genes transcription is 20-fold lower, and FGF-2 is exclusively found in the cytoplasm (9, 10). Upon addition of FGF-2 to nuclei of serum-starved cells, a 6-fold increase of ribosomal genes transcription is observed together with an increase in phosphorylation of nuclear proteins, essentially of nucleolin (9, 11); These data suggest that a nuclear function of FGF-2 could be to regulate ribosomal genes transcription by modulating the phosphorylation level of nuclear and nucleolar proteins as nucleolin. Nucleolin that is the major component of nucleolus is also a major substrate for protein kinase CK2 in rapidly proliferating tissues, and its phosphorylation level is correlated to the rate of ribosome biogenesis (reviewed in Ref. 12). CK2 is a serine/threonine protein kinase present in both the cyt...