[Heterocomplex formation between high and low affinity FGF receptors is mediated by the formation of a FGF dimer].
2
Citation
0
Reference
10
Related Paper
Keywords:
Internalization
Cell surface receptor
Cite
Internalization
Cell surface receptor
Cite
Citations (2)
Since 1989, the receptors for fibroblast growth factors (FGFs) were cloned and characterized as a subgroup of the family of receptor tyrosine kinases. Four FGF receptor genes were identified, all of which encode membrane-bound glycoproteins containing three immunoglobulin (Ig) -like domains at the extracellular region, where only two of these domains are involved in ligand binding. Three unique features characterize the FGF receptors: 1) overlapping recognition and redundant specificity, where one receptor may bind with a similar affinity several of the seven known FGFs and one FGF may bind similarly to several distinct receptors. 2) The binding of FGFs to their receptors is dependent on the interaction of FGF with cell surface heparan sulfate proteoglycans. 3) A multitude of isoforms of cell-bound or secreted receptors are produced by the same gene. The gene structure of these receptors revealed two major mechanisms that are responsible for the formation of the diverse forms: alternative mRNA splicing, resulting in deletions or alternate exons usage, and internal polyadenylation, resulting in truncated products. These are reminiscent of mechanisms that also operate in the immunoglobulin family to generate diversity and to produce either secreted or cell-bound molecules. Tissue-specific alternative splicing in FGF receptors allows for the generation of two distinct receptors from a single gene because alternative exons determine the sequence of the COOH-terminal half of the third Ig-like domain involved in ligand binding. This represents a novel genetic mechanism to generate receptor diversity and specificity and to increase receptor repertoire.
Cite
Citations (442)
Cell surface receptor
Cite
Citations (2,441)
Internalization
Cell surface receptor
Cell membrane
Cite
Citations (352)
Bovine capillary endothelial (BCE) cells were incubated at 4 degrees C with 5 ng/ml 125I-basic fibroblast growth factor (bFGF) to equilibrate 125I-bFGF with high affinity cell surface receptors and low affinity matrix binding sites. 67% of the added 125I-bFGF bound to the matrix and 7% bound to receptors. The fate of bound bFGF was followed after cells were incubated in bFGF-free medium and were shifted to 37 degrees C to restore cell metabolism. 125I-bFGF bound to receptors decreased rapidly while the amount of 125I-bFGF bound to matrix was reduced more slowly. The rapid decrease in receptor-bound 125I-bFGF appeared to be due to a down-regulation of bFGF receptors; cells that had been treated for 5 h with bFGF had 60% fewer high affinity receptors than untreated cells. Despite the initial high level of 125I-bFGF binding to matrix, most of this 125I-bFGF was mobilized and metabolized by the cells. 125I-bFGF was internalized by the cells at 37 degrees C, leading to a constant accumulation of 125I-bFGF within the cell. Internalized bFGF was rapidly cleaved from an 18-kD form to a 16-kD form. The 16-kD form was more slowly degraded with a half-life of approximately 8 h. Degradation of internalized 125I-bFGF was inhibited by chloroquine, suggesting that the digestion occurred in a lysosomal compartment. The role of matrix binding sites in the internalization process was investigated. Binding to matrix sites seemed not to be directly involved in the internalization process, since addition of heparin at a concentration that blocked 95% of the binding to matrix had no effect on the initial rate of internalization of bFGF. BCE cells also released a substance that competed for the binding of bFGF to matrix but not to receptors. This substance bound to DEAE-cellulose and was sensitive to heparinase treatment, suggesting that it was a heparinlike molecule. Thus, heparinlike molecules produced by BCE cells can modulate the cellular interaction with bFGF. Matrix-associated heparinlike molecules bind bFGF which can later be metabolized by the cell, and secreted heparinlike molecules release bFGF from matrices.
Internalization
Matrix (chemical analysis)
Cell surface receptor
Cite
Citations (277)
The effect of heparin on the rate of binding of basic fibroblast growth factor (bFGF) to high affinity (receptor) and low affinity (heparan sulfate) binding sites on endothelial cells and CHO cells transfected with FGF receptor-1 or FGF receptor-2 was investigated. Radiolabeled bFGF bound rapidly to both high and low affinity sites on all three types of cells. Addition of 10 micrograms/ml heparin eliminated binding to low affinity sites and decreased the rate of binding to high affinity sites to about 30% of the rate observed in the absence of heparin. However, the same amount of 125I-bFGF bound to high affinity sites at equilibrium in the presence and absence of heparin. The effect of heparin on the initial rate of binding to high affinity sites was related to the log of the heparin concentration. Depletion of the cells of heparan sulfates by treatment with heparinase also decreased the initial rate of binding to high affinity receptors. These results suggest that cell-surface heparan sulfates facilitate the interaction of bFGF with its receptor by concentrating bFGF at the cell surface. Dissociation rates for receptor-bound and heparan sulfate-bound bFGF were also measured. Dissociation from low affinity sites was rapid, with a half-time of 6 min for endothelial cell heparan sulfates and 0.5 min for Chinese hamster ovary heparan sulfates. In contrast, dissociation from receptors was slow, with a half-time of 46 min for endothelial cell receptors, 2.5 h for FGF receptor-1, and 1.4 h for FGF receptor-2. These results suggest that degradative enzymes may not be needed to release bFGF from the heparan sulfates in instances where receptors and heparan sulfate-bound bFGF are in close proximity because dissociation from heparan sulfates occurs rapidly enough to allow bFGF to bind to unoccupied receptors by laws of mass action.
Matrix (chemical analysis)
Cite
Citations (115)
We have investigated the interaction of basic fibroblast growth factor (bFGF) with its receptors and heparan sulfate proteoglycans (HSPG). It has been suggested that in the absence of HSPG, cells are not able to bind bFGF or respond to treatment with bFGF. In our studies, Balb/c3T3 fibroblasts were treated with 50 mM sodium chlorate to completely inhibit (99%) sulfation of proteoglycans. We found that bFGF was able to bind, be internalized, and stimulate DNA synthesis in the absence of HSPG in a dose-dependent manner. bFGF bound to its receptors on chlorate-treated cells with a lower apparent affinity and no change in receptor number. To determine if this decreased affinity bFGF-receptor interaction is functional, we quantitatively analyzed bFGF internalization and stimulation of DNA synthesis in control and chlorate-treated cells. Endocytotic rate constants (ke) for chlorate-treated and control cells were ke = 0.078 ± 0.022 min−1 and ke = 0.043 ± 0.012 min−1, respectively, suggesting that the process of bFGF internalization is not dramatically altered by HSPG. bFGF stimulated DNA synthesis to the same maximal level under both conditions, but chlorate-treated cells were significantly less responsive at low bFGF doses (∼10-fold increase in ED50). The differences observed for control and chlorate-treated cells in the dose-response curves for stimulation of DNA synthesis and receptor binding correlated directly, suggesting that receptors are equally capable of eliciting a mitogenic signal under both conditions. It is unlikely that these results are due to residual HSPG since heparinase (I and III) digestion of chlorate-treated cells had little effect. Although the presence of HSPG on the cell surface increases the affinity of bFGF for its receptors, our observations suggest that HSPG are not "absolutely" required for binding, internalization, or stimulation of mitogenic activity. We have investigated the interaction of basic fibroblast growth factor (bFGF) with its receptors and heparan sulfate proteoglycans (HSPG). It has been suggested that in the absence of HSPG, cells are not able to bind bFGF or respond to treatment with bFGF. In our studies, Balb/c3T3 fibroblasts were treated with 50 mM sodium chlorate to completely inhibit (99%) sulfation of proteoglycans. We found that bFGF was able to bind, be internalized, and stimulate DNA synthesis in the absence of HSPG in a dose-dependent manner. bFGF bound to its receptors on chlorate-treated cells with a lower apparent affinity and no change in receptor number. To determine if this decreased affinity bFGF-receptor interaction is functional, we quantitatively analyzed bFGF internalization and stimulation of DNA synthesis in control and chlorate-treated cells. Endocytotic rate constants (ke) for chlorate-treated and control cells were ke = 0.078 ± 0.022 min−1 and ke = 0.043 ± 0.012 min−1, respectively, suggesting that the process of bFGF internalization is not dramatically altered by HSPG. bFGF stimulated DNA synthesis to the same maximal level under both conditions, but chlorate-treated cells were significantly less responsive at low bFGF doses (∼10-fold increase in ED50). The differences observed for control and chlorate-treated cells in the dose-response curves for stimulation of DNA synthesis and receptor binding correlated directly, suggesting that receptors are equally capable of eliciting a mitogenic signal under both conditions. It is unlikely that these results are due to residual HSPG since heparinase (I and III) digestion of chlorate-treated cells had little effect. Although the presence of HSPG on the cell surface increases the affinity of bFGF for its receptors, our observations suggest that HSPG are not "absolutely" required for binding, internalization, or stimulation of mitogenic activity.
Cite
Citations (112)
During feeding by infected mosquitoes, malaria sporozoites are injected into the host's bloodstream and enter hepatocytes within minutes. The remarkable target cell specificity of this parasite may be explained by the presence of receptors for the region II-plus of the circumsporozoite protein (CS) on the basolateral domain of the plasma membrane of hepatocytes. We have now identified these receptors as heparan sulfate proteoglycans (HSPG). The binding of CS to the receptors is abolished by heparitinase treatment, indicating that the recognition of region II-plus is via the glycosaminoglycan chains. We have purified and partially characterized the CS-binding HSPGs from HepG2 cells. They have a molecular weight of 400,000-700,000, are tightly associated with the plasma membrane, and are released from the cell surface by very mild trypsinization, a property which the CS receptors share with the syndecan family of proteoglycans.
Trypsinization
Cell surface receptor
Internalization
Circumsporozoite protein
Cite
Citations (343)