Gene/Protein Disease Symptom Drug Enzyme Compound
Pivot Concepts:   Target Concepts:
Query: EC:4.2.2.7 (heparinase)
 1,270 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

Osteoblast-like cells secrete insulin-like growth factor (IGF) binding protein-5 (IGFBP-5), which may act to enhance IGF-stimulated osteoblast function. We recently demonstrated that carboxyl-truncated IGFBP-5 (IGFBP-5(1-169)) binds to the osteoblast surface and stimulates mitogenesis by a pathway that is independent of IGF action. The present study was conducted to determine the mechanism of osteoblast binding of IGFBP-5, beginning with the assumption that cell surface glycosaminoglycans may mediate the binding of this heparin binding protein. Intact 125I-IGFBP-5 and 125I-IGFBP-5(1-169) exhibited one-site binding to mouse osteoblast monolayers with dissociation constants of 28 and 6 nM for intact 125I-IGFBP-5 and 125I-IGFBP-5(1-169), respectively. Osteoblast binding of intact 125I-IGFBP-5 was inhibited by low heparin concentrations, while 125I-IGFBP-5(1-169) binding was stimulated by heparin. Treatment of cells with heparinase or chlorate to decrease surface glycosaminoglycan density failed to reduce the binding of either form of IGFBP-5. In contrast, pretreatment of cells with IGFBP-5 caused down-regulation of 125I-IGFBP-5 binding. Cross-linking studies revealed that both intact 125I-IGFBP-5 and 125I-IGFBP-5(1-169) bind to proteins in Triton extracts of osteoblast membranes, which were absent in osteoblast-derived matrix. Purification of membrane extracts by IGFBP-5 affinity chromatography revealed a 420-kDa band on reduced SDS-polyacrylamide gels. While the membrane protein internalized both forms of IGFBP-5, heparin treatment inhibited the internalization of intact 125I-IGFBP-5 but stimulated 125I-IGFBP-5(1-169) internalization. These data indicate that IGFBP-5 binds to and is internalized by an osteoblast membrane protein, which does not appear to be a proteoglycan. Glycosaminoglycans, however, modulate the binding and internalization of IGFBP-5 in a way that may preferentially favor the intracellular accumulation of the carboxyl-truncated form.
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PMID:Heparin modulates the binding of insulin-like growth factor (IGF) binding protein-5 to a membrane protein in osteoblastic cells. 749 27

Recent studies have identified a specific membrane protein in osteoblast-like cells which binds intact and carboxy-truncated IGFBP-5 with high affinity. The purpose of the present study was to evaluate the IGFBP-5 binding properties of osteoblast-derived extracellular matrix (ECM), with special interest in determining whether ECM proteoglycans were necessary for IGFBP-5 binding. Neonatal mouse osteoblasts and the ECM of these cells both bound intact [125I]IGFBP-5 and [125I] IGFBP-5(1-169), though the ECM bound both forms with lower affinity when compared to their cellular binding. Treatment of the ECM with heparinase or chondroitinase, to remove glycosaminoglycan (GAG) side-chains of proteoglycans, resulted in 20-34% enhanced binding of intact [125I]IGFBP-5 and a 92-100% enhancement of [125I]IGFBP-5(1-169) binding. Similar enzymatic treatment of osteoblast monolayers had no effect on the binding of either form of [125I]IGFBP-5. These results indicate that GAGs within ECM secreted by neonatal mouse osteoblasts do not mediate the binding of IGFBP-5. This study also shows that intact and carboxy-truncated IGFBP-5 preferentially bind to the osteoblast surface, but that removal of GAGs from osteoblast-derived ECM can increase IGFBP-5 localization to this pericellular space, particularly the carboxy-truncated form of IGFBP-5.
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PMID:Comparison studies of IGFBP-5 binding to osteoblasts and osteoblast-derived extracellular matrix. 881 77

The activities of insulin-like growth factor (IGF)-I and -II are regulated by IGF-binding proteins (IGFBPs). Cleavage of IGFBP-4 by the metalloproteinase pregnancy-associated plasma protein-A (PAPP-A) causes release of bound IGF and has been established in several biological systems including the human reproductive system. Using flow cytometry, we first demonstrate that PAPP-A reversibly binds to the cell surface of several cell types analyzed. Heparin and heparan sulfate, but not dermatan or chondroitin sulfate, effectively compete for PAPP-A surface binding, and because incubation of cells with heparinase abrogated PAPP-A adhesion, binding is probably mediated by a cell surface heparan sulfate proteoglycan. Furthermore, the proteolytic activity of PAPP-A is preserved while bound to cells, suggesting that adhesion functions to target its activity to the vicinity of the IGF receptor, decreasing the probability that released IGF is captured by another IGFBP molecule before receptor binding. This mechanism potentially functions in both autocrine and paracrine regulation, as PAPP-A need not be synthesized in a cell to which it adheres. A truncated PAPP-A variant without the five short consensus repeats in the C-terminal third of the 1547-residue PAPP-A subunit, lacked surface binding. We also show that PAPP-A2, a recently discovered IGFBP-5 proteinase with homology to PAPP-A, does not bind cells. This finding allowed further mapping of the PAPP-A adhesion site to short consensus repeat modules 3 and 4 by the expression and analysis of nine PAPP-A/PAPP-A2 chimeras. Interestingly, the proteolytically inactive, disulfide-bound complex of PAPP-A and the proform of eosinophil major basic protein (proMBP), PAPP-A.proMBP, shows only weak surface binding, probably because the adhesion site of PAPP-A is occupied by heparan sulfate, known to be covalently bound to proMBP. This hypothesis was further substantiated by demonstrating that heparinase treatment of PAPP-A.proMBP restores surface binding. We finally propose a model in which IGF bioactivity is regulated by reversible cell surface binding of PAPP-A, which in turn is regulated by proMBP.
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PMID:Cell surface targeting of pregnancy-associated plasma protein A proteolytic activity. Reversible adhesion is mediated by two neighboring short consensus repeats. 1237 Jan 76

Insulin-like growth factor (IGF)-I is a pleiotropic hormone that regulates vascular smooth muscle cell (VSMC) migration, proliferation, apoptosis, and differentiation. These actions are mediated by the IGF-I receptor. How activation of the same receptor by the same ligand leads to these diverse cellular responses is not well understood. Here we describe a novel mechanism specifying VSMC responses to IGF-I stimulation, distinctive for the pivotal roles of local IGF-binding proteins (IGFBPs). The role of local IGFBPs was indicated by comparing the activities of IGF-I and des-1-3-IGF-I, an IGF-I analog with reduced binding affinity to IGFBPs. Compared with IGF-I, des-1-3-IGF-I was more potent in stimulating DNA synthesis but much less potent in inducing directed migration of VSMCs. When the effects of individual IGFBPs were tested, IGFBP-2 and IGFBP-4 were found to inhibit IGF-I-stimulated DNA synthesis and migration. IGFBP-5 had an inhibitory effect on IGF-I-stimulated DNA synthesis, but it strongly potentiated IGF-I-induced VSMC migration. By using a non-IGF-binding IGFBP-5 mutant and an IGF-I-neutralizing antibody, it was demonstrated that IGFBP-5 also stimulates VSMC migration in an IGF-independent manner. This effect of IGFBP-5 was inhibited by soluble heparin and by treating cells with heparinase. Mutation of the heparin-binding motif of IGFBP-5 reduced its migration promoting activity. These findings suggest that local IGFBPs are important determinants of cellular responses to IGF-I stimulation, and a key player in this paradigm is IGFBP-5. IGFBP-5 not only modulates IGF-I actions, but it also stimulates cell migration by interacting with cell-surface heparan sulfate proteoglycans.
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PMID:Regulation of vascular smooth muscle cell responses to insulin-like growth factor (IGF)-I by local IGF-binding proteins. 1291 28