Internalization of basic fibroblast growth factor by CCL39 fibroblast cells. Involvement of heparan sulfate glycosaminoglycans

PGF2alpha increases FGF-2 and FGFR2 trafficking in Py1a rat osteoblasts via clathrin independent and importin beta dependent pathway

Previous studies showed that prostaglandin F2alpha (PGF2alpha) stimulated fibroblast growth factor-2 (FGF-2) and fibroblast growth factor receptor 2 (FGFR2) cytosolic and nuclear accumulation, however, the endocytic pathway has not been elucidated. This study demonstrates that although PGF2alpha increased the formation of clathrin-coated structures in Py1a rat osteoblasts, they were not involved in FGF-2 and FGFR2 trafficking. PGF2alpha increased binding of FGF-2 and FGFR2 and co-localization of reactive sites in addition to nuclear translocation at the nuclear pore complex level. FGF-2 and FGFR2 were in close spatial correlation with importin beta, further supporting nuclear import of the FGF-2/FGFR2 complex. Immunogold and immunofluorescence techniques as well as Western blotting demonstrated increased importin beta protein labeling in response to PGF2alpha. Similar to PGF2alpha, phorbol 12-myristate 13-acetate (PMA) also increased importin beta protein. These data strongly suggest that prostaglandins may regulate osteoblast metabolism via FGF-2/FGFR2/importin beta nuclear trafficking.

Visualization of several binding sites for basic fibroblast growth factor (FGF-2) on fibroblasts by photoaffinity labeling: evidence for intracellular complexes

The internalization of basic fibroblast growth factor (FGF-2) was studied in Chinese hamster lung fibroblasts (CCL39). Recombinant FGF-2 was derivatized with a photoactivable agent, N-hydroxysuccinimidyl-4-azidobenzoate (HSAB), iodinated, and used to visualize intracellular FGF-2-affinity-labeled molecules after internalization at 37 degrees C. Iodinated HSAB-FGF-2 maintained the properties of natural FGF-2 such as affinity for heparin, binding to Bek and Fig receptors, interaction with high- and low-affinity binding sites, and reinitiating of DNA synthesis in CCL39 cells. Affinity-labeling experiments at 4 degrees C with 125I-HSAB-FGF-2 led to the detection of several FGF-cell surface complexes with apparent molecular mass of 80, 100, 125, 150, 170-180, 220, 260, and about 320 kDa by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), whereas two specific bands at 80 and 130-160 kDa were obtained using the homobifunctional cross-linking reagent, disuccinimidyl suberate. When the cells, preincubated with 125I-HSAB-FGF-2 at 4 degrees C and then washed, were shifted to 37 degrees C, irradiation of the internalized labeled FGF-2 led to detection of a similar but fainted profile with one major specific band at 80 kDa. Heparitinase II treatment of the cells reduced binding of 125I-HSAB-FGF-2 to its cell surface sites by 80% and internalization by 55%, indicating the involvement of heparan sulfate proteoglycans in these processes. Among the heparitinase-sensitive bands was the 80-kDa complex.

Nuclear accumulation of exogenous basic fibroblast growth factor in endothelial, fibroblast, and myoblast cell lines results in diverse biological responses

During studies comparing 125I-bFGF internalization between endothelial cells and other cell types, we found, unexpectedly, internalization and nuclear translocation of exogenously added 125I-bFGF in two cell lines: Chinese hamster ovary cells (CHO) and rat L6 myoblasts. These cell lines were previously reported to be devoid of FGF receptors. Furthermore, CHO cells showed a weak mitogenic response to added bFGF, while L6 cells were mitogenically unresponsive. By comparison, coronary venular endothelial cells (CVEC), BALB/c 3T3 fibroblasts, and BHK-21 cells, demonstrated internalization and nuclear translocation of added 125I-bFGF, and mitogenic responsiveness to the growth factor. Insulin alone stimulated DNA synthesis in all cell types, yet augmented bFGF-dependent DNA synthesis only in CVEC, 3T3, and BHK. All five cell types expressed FGF receptors as assessed by covalent crosslinking with 125I-bFGF and immunoblotting with anti-FGF receptor antibodies. Differing rates of cytoplasmic and nuclear accumulation of 125I-bFGF and partial inhibition of internalization by pretreatment of CVEC with chlorate support a recent model that bFGF can internalize by two mechanisms. Insulin did not significantly affect 125I-bFGF internalization or metabolism in any cell type. bFGF treatment resulted in weak inhibition of RNA synthesis in L6 cells. bFGF appears firmly bound to the nuclear matrix as little nuclear-bound 125I-bFGF in CVEC is released by DNAse I or RNAse A digestion, while washes with 0.5 M NaCl result in partial release. Nuclear bFGF may thus be involved in regulation of nuclear events (e.g., gene transcription and/or DNA replication).

Basic fibroblast growth factor (bFGF) acts intracellularly to cause the transdifferentiation of avian neural crest-derived Schwann cell precursors into melanocytes

We previously found that cultured neural crest-derived cells from embryonic quail peripheral nerves, which consist mostly of Schwann cell precursors, gave rise to melanocytes following treatment with basic fibroblast growth factor (bFGF) or 12-O-tetradecanoyl phorbol-13-acetate (TPA). Here, we show that antisense deoxyoligonucleotides targeted against two regions of the bFGF mRNA transcript blocked this TPA-induced transdifferentiation of Schwann cell precursors. Neither sense nor scrambled antisense control oligonucleotides had any effect in this regard. TPA increased bFGF protein expression in cell lysates but not in conditioned media from these cultures, and this expression was localized to the nucleus and cytoplasm. Furthermore, bFGF-neutralizing antibodies and inositol-hexakisphosphate (InsP6) both inhibited pigmentation caused by exogenous bFGF, but had no affect on TPA-induced melanogenesis, suggesting that bFGF is not released by these cells. These data indicate that bFGF is necessary for the TPA-induced transdifferentiation of Schwann cell precursors into melanocytes and that bFGF acts via an intracrine mechanism.