Biomedical subjects

D R Clemmons

Publications and source records attributed to D R Clemmons.

At least 73 records · Page 4Linked to original sources

PubMed1996

Interaction between the insulin-like growth factor family and the integrin receptor family in tissue repair processes. Evidence in a rabbit ear dermal ulcer model.

We have determined previously that IGF-I is dependent on the presence of IGF binding protein-1 (IGFBP-1) to act as a wound healing agent. We sought to determine the mechanism whereby IGFBP-1 is able to enhance IGF-I bioactivity. As IGFBP-1 binds both the alpha5beta1 integrin as well as IGF-I in vitro, we asked which of the following interactions were important: (a) the ability of IGFBP-1 to interact with an integrin receptor, and/or (b) the binding of IGF-I by IGFBP-1. We used an IGF-1 analogue (des(1-3)IGF-I) with a > 100-fold reduction in affinity for IGFBP-1 as well as an IGFBP-1 mutant (WGD-IGFBP-1) which does not associate with the alpha5beta1 integrin to selectively abrogate each of these interactions. We also tested the ability of IGFBP-2, a related binding protein which has an arginine-glycine-aspartate sequence but does not associate with integrin family members, to enhance IGF-I bioactivity. Full-thickness dermal wounds were created on rabbit ears; various combinations of native IGF-I, native IGFBP-1, native IGFBP-2, and their respective analogues/mutants were applied to each wound. Wounds were harvested 7 d later for analysis. Only native IGF-I in combination with native IGFBP-1 was effective as a wound healing agent, enhancing reepithelialization and granulation tissue deposition by 64+/-5 and 83+/-12% over controls (P = 0.008 and 0.016, respectively). The same doses of IGF-I/WGD-IGFBP-1, des(1-3)IGF-I/IGFBP-1, and IGF-I/IGFBP-2 were ineffective. We propose that IGF-I physically interacts with IGFBP-1 and that IGFBP-1 also binds to an integrin receptor, most likely the alpha5beta1 integrin. This interaction is unique to IGFBP-1 as the closely related IGFBP-2 had no effect, a finding consistent with its inability to bind to integrin receptors. Our results suggest that activation of both the IGF-I receptor and the alpha5beta1 integrin is required for IGF-I to stimulate wound healing.

Animals↗

PubMed1996

Identification of the extracellular matrix binding sites for insulin-like growth factor-binding protein 5.

Fibroblast extracellular matrix (ECM) contains two forms of insulin-like growth factor-binding proteins (IGFBPs), IGFBP-3 and IGFBP-5. These studies were undertaken to identify the regions within IGFBP-5 that mediate its binding to fibroblast ECM. Synthetic peptides were prepared that were homologous with two regions of basic amino acids within IGFBP-5 (Arg201-Arg218 and Ala131-Thr141). Increasing concentrations of both peptides competed with IGFBP-5 for binding to ECM but the Arg201-Arg218 peptide was more potent. Mutagenesis was used to define the effect of substituting for these basic residues on ECM binding. Substitution for two peptide B residues K134A and R136A reduced binding by 40%. Substitution of a single basic residue within the peptide A region (K211N) reduced binding to ECM by 49%. Substitution for K211N, K134A, and R136A reduced binding by 52%. More extensive substitutions in the peptide A region, e.g. K211N,R214A,K217A,R218N, resulted in a greater (e.g. 88%) decrease. The positional location of basic residues appeared to be more important than the total number of substitutions since the mutant K202N,K206A,R207A had a 79% reduction in ECM binding. Two basic regions of IGFBP-5 contribute to its binding to ECM, but the region containing amino acids 201-218 has a greater contribution. ECM binding is mediated by charged residues and acts to stabilize IGFBP-5 by protecting it from proteolysis.

Amino Acid Sequence↗

PubMed1996

Ligand occupancy of the alpha-V-beta3 integrin is necessary for smooth muscle cells to migrate in response to insulin-like growth factor.

Smooth muscle cells (SMCs) have been shown to migrate in response to insulin-like growth factor I (IGF-I). However, the mechanism mediating this response has not been determined. The migration rates of porcine and human vascular SMCs were assessed in a monolayer wounding assay. IGF-I and IGF-II induced increases of 141% and 97%, respectively, in the number of cells that migrated in 4 days. The presence of 0.2% fetal bovine serum in the culture medium was necessary for the IGFs to stimulate migration over uncoated plastic surfaces. However, if vitronectin was used as the substratum, IGF-I stimulated migration by 162% even in the absence of serum. To determine the role of integrins in mediating this migration, SMC surface proteins were labeled with 125I and immunoprecipitated with specific anti-integrin antibodies. Integrins containing alpha-V (vitronectin receptor), alpha5 (fibronectin receptor), and alpha3 (collagen/laminin receptor) subunits were the most abundant. IGF-I treatment caused a 73% reduction in alpha5-integrin subunit protein and a 25% increase in alpha-V subunit. More importantly, ligand binding of alpha-V-beta3 was increased by 2.4-fold. We therefore examined whether the function of the alpha-V-beta3 integrin was important for IGF-I-mediated migration. The disintegrin kistrin was shown by affinity crosslinking to specifically bind with high affinity to alpha-V-beta3 and not to alpha5-beta1 or other abundant integrins. The related disintegrin echistatin specifically inhibited 125I-labeled kistrin binding to alpha-V-beta3, while a structurally distinct disintegrin, decorsin, had 1000-fold lower affinity. The addition of increasing concentrations of either kistrin or echistatin inhibited IGF-I-induced migration, whereas decorsin had a minimal effect. The potency of these disintegrins in inhibiting IGF-I-induced migration paralleled their apparent affinity for the alpha-V integrin. Furthermore, an alpha-V-beta3 blocking antibody inhibited SMC migration by 80%. In summary, vitronectin receptor activation is a necessary component of IGF-I-mediated stimulation of smooth muscle migration, and alpha-V-beta3 integrin antagonists appear to be important reagents for modulating this process.

Animals↗

PubMed1996

Substitution of specific amino acids in insulin-like growth factor (IGF) binding protein 5 alters heparin binding and its change in affinity for IGF-I response to heparin.

Heparin binding to insulin-like growth factor (IGF)-binding protein 5 (IGFBP-5) leads to a 17-fold decrease in its affinity for IGF-I, and a region that contains several basic amino acids (Arg201-Arg218) may be involved in this affinity shift. In the present study, mutagenesis was used to analyze the effect of substitutions for basic amino acids in the Arg201-Arg218 region of IGFBP-5 on heparin-binding and the heparin-induced affinity shift. Nine mutant forms were prepared. Their association constants (Ka) for IGF-I were similar to native IGFBP-5. When 10 microg/ml of heparin was added, the Ka of native IGFBP-5 decreased 17-fold, and the Ka of the K134A/R136A mutant decreased 16-fold. In contrast, substitutions for specific basic amino acids in the Arg2O1-Arg218 region decrease the affinity shift to 1.1-3.2-fold. Lys 211 was especially important. When a mutant containing that single substitution was tested, heparin caused only a 2.5-fold reduction in IGF-I affinity. Affinity cross-linking studies showed that heparin was equipotent in inhibiting the formation of 125I-IGF-I-K134A/Rl36A mutant complexes compared to native IGFBP-5. In contrast, heparin had minimal effects on the formation of complexes between 125I-IGF-I and the other mutants. The heparin-binding activity of each mutant was determined. Four mutants, R201A/K202N, K202A/K206A/R207A, R201A/K202N/K206N/K208N, and K211N/R214A/K217A/R218A, had reduced heparin binding compared to native IGFBP-5. The other five mutants, including the K21IN mutant, showed no change in heparin binding. The four mutants with reduced heparin binding could be dissociated from heparin-Sepharose with much lower NaCl concentrations, indicating that they had reduced affinity. These findings suggest that Arg201 Lys202, LysS206, and Arg214 are important for heparin binding. In contrast, LyS211 is not important for the binding of IGFBP-5 to heparin, but substitution for it reduced the heparin-induced affinity shift.

Affinity Labels↗

PubMed1996

Characterization of insulin-like growth factor binding protein-1 kinases from human hepatoma cells.

The phosphorylation of insulin-like growth factor binding protein-I (IGFBP-1) alters its binding affinity for insulin-like growth factor I (IGF-I) and thus regulates the bioavailability of IGF-I for binding to the IGF-I receptor. The kinase(s) responsible for the phosphorylation of IGFBP-1 has not been identified. This study was designed to characterize the IGFBP-1 kinase activity in HepG2 human hepatoma cells, a cell line that secretes IGFBP-1 primarily as phosphorylated isoforms. IGFBP-1 kinase activity was partially purified from detergent extracts of the cells by phosphocellulose chromatography and gel filtration. Two kinases of approximate M(r) 150,000 (peak I kinase) and M(r) 50,000 (peak II kinase) were identified. Each kinase phosphorylated IGFBP-1 at serine residues that were phosphorylated by intact HepG2 cells. The kinases were distinct based on their differential sensitivity to inhibition by heparin (IC50 = 2.5 and 16.5 micrograms/ml, peak I and II kinase, respectively) and inhibition by the isoquinoline sulfonamide CKI-7 (IC50 = 50 microM and 100 microM, peak I and II kinase, respectively). In addition, a tenfold molar excess of nonradioactive GTP relative to [gamma-32P]ATP lowered the incorporation of 32P into IGFBP-1 by 80% when the reaction was catalyzed by the peak I kinase, whereas GTP had no effect on the reaction catalyzed by the peak II kinase. In the presence of polylysine, IGFBP-1 was radiolabeled by the partially purified kinase activity when [gamma-32P]GTP served as the phosphate donor indicating the presence of casein kinase II activity. Furthermore, IGFBP-1 was phosphorylated by purified casein kinase I and casein kinase II at sites phosphorylated by the peak I and II kinases. Our data suggest that at least two kinases could be responsible for the phosphorylation of IGFBP-1 in intact HepG2 cells and that the kinases are related to the casein kinase family of protein kinases.

Amino Acid Sequence↗

PubMed1996

Insulin-like growth factor-I (IGF-I) regulates IGF-binding protein-5 synthesis through transcriptional activation of the gene in aortic smooth muscle cells.

Previous studies have shown that porcine aortic smooth muscle cells (SMCs) secrete two insulin-like growth factor-binding proteins (IGFBP), IGFBP-2 and -4, and that these IGFBPs modulate IGF-I-stimulated SMC proliferation and migration. In this study we demonstrate that porcine SMCs express IGFBP-5 mRNA and synthesize and secrete the protein. In this cell type, the biosynthesis of IGFBP-5 is up-regulated by IGF-I. This increase in IGFBP-5 synthesis is accompanied by an increase in the steady-state mRNA levels. The induction of IGFBP-5 mRNA by IGF-I is time- and dose-dependent and requires de novo protein synthesis. IGF-II and insulin also increase IGFBP-5 mRNA levels at high doses. An IGF-I analog with normal affinity for the IGF-I receptor but reduced affinity for IGFBPs evokes a similar increase. Another analog that binds to IGFBPs but not to the receptor has no effect, indicating that this effect of IGF-I is mediated through the IGF-I receptor. The IGF-I-induced IGFBP-5 gene expression is cell type-specific because IGF-I had no such effect in other cell types examined. Nuclear run-on assays revealed that IGF-I increased transcription rate of the IGFBP-5 gene, while IGF-I did not change the IGFBP-5 mRNA stability. Furthermore, the IGFBP-5 promoter was 3.5-fold more active in directing expression of the luciferase reporter gene in IGF-I-treated aortic SMCs as compared to control cells, whereas the luciferase activity remained the same in control- and IGF-I-treated fibroblasts. These results suggest that IGF-I up-regulates IGFBP-5 synthesis by transcriptionally activating the IGFBP-5 gene in aortic SMCs.

Amino Acid Sequence↗

PubMed1996

Binding of insulin-like growth factor (IGF) I or II to IGF-binding protein-2 enables it to bind to heparin and extracellular matrix.

Insulin-like growth factor (IGF)-binding protein-2 (IGFBP-2) is secreted by several cell types that also secrete IGF-I or IGF-II. The binding of IGF-I or IGF-II to IGFBP-2 has been shown to alter their actions. Although IGFBP-2 is not an abundant component of the extracellular matrix (ECM), it is easily localized by immunohistochemical staining in basement membranes of several epithelial cell types. We have previously shown that IGFBP-5 associates with glycosaminoglycans and binds to proteoglycans that are contained in ECM and basement membranes. In those studies we were unable to demonstrate an association of IGFBP-2 with glycosaminoglycans. In this study we report that IGFBP-2 binds to heparin if IGF-I or IGF-II is also included in the incubation buffer. IGFBP-1, -3, -4, or -5 did not have increased heparin binding in the presence of IGF-I or IGF-II. The binding of IGFBP-2 to heparin was detectable using an IGF-I to IGFBP-2 molar ratio of 2:1. Binding to heparin-Sepharose could be inhibited by soluble heparin or heparan sulfate, but not by glycosaminoglycans that do not contain O-linked sulfate groups in the 2 or 3 position of the iduronic acid ring. Binding was also inhibited by a synthetic IGFBP-5 peptide that contained a heparin-binding domain, but not by a peptide with an identical charge to mass ratio that does not bind to heparin. Binding appeared to be physiologically significant, as IGFBP-2 bound to human fibroblast ECM if IGF-I or IGF-II was added. IGF-II was more potent than IGF-I in facilitating the binding interaction, and des(1-3)-IGF-I or insulin had no effect, suggesting that IGF binding to IGFBP-2 is required for this effect to be detected. In summary, IGFBP-2 binding to glycosaminoglycans is dependent upon binding of IGF-I and IGF-II to IGFBP-2. This suggests that IGFBP-2 undergoes a conformational change that exposes a glycosaminoglycan-binding domain. This could provide a mechanism for focally concentrating IGFBP-2 in the pericellular environment.

Amino Acid Sequence↗

PubMed1996

Characterization and determination of the relative abundance of two types of insulin-like growth factor binding protein-5 proteases that are secreted by human fibroblasts.

We previously reported that human fibroblasts secrete a protease into their conditioned medium that cleaves insulin-like growth factor binding protein-5 (IGFBP-5) into non-IGF-I binding fragments. Because the protease activity in the fibroblast medium has characteristics of both serine and metalloproteases, the activity was purified and analyzed to determine whether it retained serine or metalloprotease properties. The protease was purified by heparin Sepharose affinity chromatography followed by alpha1 antichymotrypsin affinity or gelatin agarose chromatography. The heparin Sepharose purified material degraded IGFBP-5 into 22-, 17-, and 16-kDa fragments. Amino acid sequencing showed that the 22-kDa fragment contained the amino-terminus of the protein. The protease activity in the fibroblast conditioned medium that was purified by heparin Sepharose was inhibited by both serine and metalloprotease inhibitors. To attempt to separate these activities, the heparin Sepharose purified activity was further purified by gelatin agarose chromatography. The IGFBP-5 protease activity that did not bind to gelatin agarose was inhibited by serine protease inhibitors, such as 3,4 dichloroisocoumain (3,4 DCI), whereas tissue inhibitor metalloprotease-1 (TIMP-1) had minimal activity. When this same pool of protease activity that had been eluted from heparin Sepharose was applied to an alpha1 antichymotrypsin peptide affinity column, the protease activity that bound to the column was inhibited by 3,4 dichloroisocoumain, but was not inhibited by TIMP-1. In contrast, the activity that did not adhere to this column was inhibited by TIMP-1. IGFBP-5 zymography showed that the Mr estimate of the protease that was inhibited by serine protease inhibitors was 92 kDa, whereas gelatin zymography showed that the metalloproteases had Mr estimates of 72, 69, and 55 kDa. When the protease activity in the crude conditioned medium was analyzed by zymography, almost all of the detectable protease had an Mr estimate of 92 kDa, suggesting that the metalloproteases that were detected in the partially purified fractions were inactive in the medium. In summary, fibroblasts secrete a 92-kDa protease that cleaves IGFBP-5 into 22-, 17-, and 16-kDa fragments. The protease inhibitor specificity results, chromatographic characteristics, and zymographic analyses suggest that this is a serine protease. Although metalloproteases are secreted by these cells, the 92-kDa serine protease is the predominate form of activity in the conditioned medium that cleaves IGFBP-5.

Cells, Cultured↗

PubMed1996

Assessment of growth hormone (GH) secretion in men with adult-onset GH deficiency compared with that in normal men--a clinical research center study.

It is not known how patients who acquire GH deficiency (GHD) in adulthood differ in measures of GH secretion from normal adults. To characterize measures of GH secretion in such patients compared to those in normal subjects, we studied 23 men (median age, 51 yr; range, 32-62 yr) with adult-onset pituitary disease, defined as GH-deficient based on having no detectable GH response to two pharmacological agents, and 17 normal men. Patients less than 50 yr old received insulin (0.1 U/kg, i.v.) and clonidine (0.15 mg, orally), whereas those 50 yr of age or older as well as normal controls received arginine (30 g, i.v.) and clonidine. Patients were compared to normal men by investigating GH sampling every 10 min for 24 h and serum levels of insulin-like growth factor I (IGF-I), IGF-binding protein 2 (IGFBP-2), IGFBP-3, and GH-binding protein. Frequent venous sampling of GH was analyzed in terms of mean 24-h levels, pooled 24-h GH, mean levels over the 12 h between 2000-0800 h (mean overnight GH level), and pulse analysis (pulses per 24 h and pulse amplitude) by the Pulsar computer program. Although there were significant differences between the two groups for almost all measures of GH secretion, overlap between the groups was always present. GH levels measured using a highly sensitive chemiluminescence assay on 24-h pools derived from frequent sampling displayed the least overlap between the two groups, as only 2 of 17 normal controls overlapped with the GHD patients. The pooled 24-h GH level using this technique was significantly lower in patients with GHD than in controls (0.117 +/- 0.021 vs. 0.861 +/- 0.098 micrograms/L; P < 0.0001). In the analysis of frequent GH sampling using a standard immunoradiometric assay, mean overnight GH levels provided the best separation between the two groups, as all 23 patients had values of 0.6 microgram/L or less, and 13 of 17 normal controls had values greater than 0.6 microgram/L. The mean overnight GH level in patients was 0.6 +/- 0.0 microgram/L compared to 1.0 +/- 0.1 microgram/L in controls (P < 0.0001). The mean 24-h GH level in patients was 0.5 +/- 0.0 microgram/L compared to 0.8 +/- 0.1 microgram/L in normal controls (P < 0.0001). GH pulse frequency and pulse amplitude were also reduced in patients with GHD compared to those in normal controls [1.7 +/- 0.5 vs. 5.1 +/- 0.5 pulses/24 h (P < 0.0001) and 0.6 +/- 0.1 vs. 2.8 +/- 0.4 microgram/L (P < 0.0001), respectively]. The mean serum IGF-I level was significantly lower in patients with GHD than in normal controls (106.7 +/- 8.0 vs. 218.7 +/- 16.7 microgram/L; P < 0.0001). Three of 23 patients overlapped with control values. Mean serum levels of IGFBP-3 and the serum IGF-I/IGFBP-2 ratio were also significantly lower in patients than in controls, but values overlapped substantially. We conclude that overlap occurs on measures of GH secretion between normal men and men identified as GH deficient despite a stringent definition of GHD. The best separation was obtained using pooled 24-h GH levels determined by a highly sensitive chemiluminescence assay.

Adult↗

PubMed1996

Recombinant human insulin-like growth factor I increases insulin sensitivity and improves glycemic control in type II diabetes.

Insulin resistance is a major factor in the pathophysiology of type II diabetes and a major impediment to successful therapy. The identification of treatments that specifically target insulin resistance could improve diabetes management significantly. Since IGFs exert insulin-like actions and increase insulin sensitivity when administered at supraphysiological doses, we determined the effect of 6 weeks of recombinant human IGF-I (rhIGF-I) administration on insulin resistance and glycemic control in obese insulin-resistant patients with type II diabetes. A total of 12 patients with type II diabetes were recruited for the study. Subcutaneous administration of rhIGF-I (100 micrograms/kg b.i.d.) significantly lowered blood glucose. Fructosamine declined from 369 to 299 mumol/l by 3 weeks of administration and then declined further to 271 at the end of 5 weeks. Glycosylated hemoglobin, which was 10.4% pretreatment, declined to 8.1% at the end of therapy. Mean 24-h blood glucose during a modal day was 14.71 +/- 4.5 mmol/l pretreatment and declined to 9.1 +/- 3.21 mmol/l by the end of treatment. These improvements in glycemia were associated with a decrease in serum insulin levels. Mean insulin concentrations declined from 108.0 to 57.0 pmol/l during the modal day measurements and from 97.2 to 72.0 pmol/l during the mixed-meal tolerance test. Changes in glycemia were accompanied by a marked increase in insulin sensitivity. The insulin sensitivity index (SI) calculated from a frequently sampled intravenous glucose tolerance test (FSIVGTT) after the method of Bergman et al. (Bergman RN, Finegold DT, Ader M: Assessment of insulin sensitivity in vivo. Endocr Rev 6:45-86, 1985) increased 3.4-fold. Furthermore, the improvement in glycemic control was accompanied by a change in body composition with a 2.1% loss in body fat as calculated by dual energy x-ray absorptiometry without change in total body weight. Significant side effects were present in some subjects, although nine subjects were able to complete at least 4.5 weeks of the protocol and six subjects completed the entire 6 weeks. Supraphysiological IGF-I concentrations were maintained throughout the study, increasing from 206 micrograms/l in the control period to 849 micrograms/l at the end of 6 weeks of rhIGF-I treatment. The increase in IGF-I levels was accompanied by a significant increase in IGF binding protein-2 levels, a slight reduction in IGF binding protein-3 levels, and an increase in levels of IGF binding protein-1. In summary, IGF-I significantly lowered blood glucose as reflected by short-term and long-term indexes of glycemic control and increased insulin sensitivity. It remains to be determined whether a dosage can be administered that avoids significant side effects and still achieves reasonable glycemic control.

Adult↗

PubMed1995

Transcription factor AP-2 regulates human insulin-like growth factor binding protein-5 gene expression.

Insulin-like growth factor binding protein-5 (IG-FBP-5) is an important modulator of IGF actions. IG-FBP-5 mRNA is abundant in human fibroblasts and is regulated by cAMP. To understand the molecular mechanism underlying this cell type-specific expression and regulation, we isolated the 5'-flanking region of the human IGFBP-5 gene and fused it to a promoter-less reporter plasmid encoding luciferase. Transient transfection of the construct into fibroblasts displayed both constitutive and cAMP-induced promoter activity in an orientation-specific manner. Sequence analysis revealed the existence of distal and proximal consensus AP-2 recognition sites located 5' from the TATA box. Both sequences bound specifically to human AP-2 in vitro by gel shift mobility assay. The possible role of AP-2 was examined by cotransfection of AP-2-deficient HepG2 cells with the IGFBP-5 promoter construct and a human AP-2 expression construct. Cotransfection with AP-2 significantly elevated IGFBP-5 promoter activity. This trans-activation was IGFBP-5 promoter and AP-2 specific. In AP-2 abundant fibroblasts, expression of AP-2B, a dominant-negative inhibitor of AP-2, suppressed IGFBP-5 promoter activity. In HepG2 cells, AP-2B alone had no significant effect, but the AP-2-induced activation of promoter activity was inhibited by AP-2B in a dose-dependent manner. The relative functional importance of the putative AP-2 binding sites was examined using a number of deletion mutants and point mutations. When the first two distal CCCCACCC-like putative AP-2 sites were deleted or mutated, there was no change in AP-2-induced trans-activation. Deletion or mutation of the proximal GCCNNNGGC-like sequences, however, abolished the AP-2-induced activation. These results suggest that AP-2 regulates the IGFBP-5 gene expression through the proximal GCCNNNGGC-like sequences. This AP-2-mediated trans-activation contributes at least in part to the constitutively high expression of IGFBP-5 in fibroblasts and to the cAMP responsiveness of this gene.

Base Sequence↗

PubMed1995

Proteolytic cleavage of insulin-like growth factor binding protein 4 (IGFBP-4). Localization of cleavage site to non-homologous region of native IGFBP-4.

Insulin-like growth factor binding protein 4 (IGFBP-4) is a 24-kDa protein that binds insulin-like growth factor 1 (IGF-1) and IGF-2 with high affinity and inhibits IGF action in vitro. We recently described a protease produced by the B104 neuronal cell line that cleaves IGFBP-4, yielding an approximate 16-kDa immunoreactive protein that binds IGFs with reduced affinity. We analyzed fragments produced by exposing pure IGFBP-4 to the protease to determine potential cleavage sites. Electrospray mass spectrometry and amino acid sequencing indicated the 16-kDa fragment spanned the NH2 terminus of native IGFBP-4 through Lys-120. There was evidence for an additional proteolytic fragment beginning at amino acid 132 and continuing to the COOH terminus. Proteolysis could be blocked by a synthetic peptide that spanned amino acids 117-126 but not by peptides that contained flanking sequences 111-120 or 125-135. Mutagenesis was used to alter the basic residue at position 120. The expressed mutant IGFBP-4 (K120A) was relatively resistant to cleavage, strongly suggesting that residues 120-121 represent the cleavage site. This region of IGFBP-4 is not homologous with other IGFBPs, explaining the apparent specificity of the protease for IGFBP-4. The 16-kDa IGFBP-4 fragment no longer inhibited IGF-1-stimulated thymidine uptake in vitro, suggesting that proteolytic processing of IGFBP-4 may have important functional consequences in vivo.

Amino Acid Sequence↗

PubMed1995

Regulation of insulin-like growth factor (IGF) binding protein-2 synthesis and degradation by platelet-derived growth factor and the IGFs is enhanced by serum deprivation in vascular smooth muscle cells.

Growth factors such as platelet-derived growth factor (PDGF) and insulin-like growth factor (IGF-I) stimulated proliferation and migration of vascular smooth muscle cells (SMC). IGF-I bioactivity is modulated by high-affinity binding proteins (IGFBP) which are important regulators of these processes. Porcine vascular SMC synthesize IGFBP-2 and IGFBP-4 in vitro. In the present study, levels of IGFBP-2 in conditioned media (CM) were increased approximately 1.6 to 2.2-fold when cells were exposed to PDGF (20 ng/ml) or insulin (5 micrograms/ml) for 24 hr following a 24 hr incubation in serum-free media, or following a 72 hr exposure to either growth factor. Similar increases in IGFBP-2 mRNA levels were observed. Exposure of cells to PDGF for 24 hr without prior serum deprivation resulted in smaller (47 +/- 11%) increases in IGFBP-2 protein levels but failed to alter mRNA levels. IGF-I, FGF, TGF-beta and EGF failed to increase IGFBP-2 using either experimental paradigm. In contrast, IGFBP-2 protein levels were consistently decreased (75 +/- 14%) after 72 hr of exposure to IGF-II without corresponding decreases in IGFBP-2 mRNA levels. Immunoprecipitation of [35S]methionine-labeled IGFBP-2 indicated that this decrease was not due to a decrease in synthesis of IGFBP-2. Immunoblot analysis of CM from cells treated with IGF-II indicated that the decrease in intact protein corresponded with an increase in two non-IGF binding IGFBP-2 fragments of 22 and 14 kD. Increased abundance of these fragments was also observed following IGF-I exposure, although corresponding decreases in intact IGFBP-2 were not usually observed. The relative abundance of these fragments did not appear to be affected by treatment with PDGF or insulin. In contrast to IGFBP-2, regulation of the levels of IGFBP-4 in CM did not appear to be altered by serum deprivation. Insulin consistently increased IGFBP-4 mRNA and protein levels under all situations. PDGF tended to increase IGFBP-4 protein levels, although this effect was less consistent and not as great as the increase observed with insulin. Treatment with IGF-I or -II consistently decreased IGFBP-4 levels in CM but tended to increase their mRNA levels under all situations. These data indicate that insulin, PDGF, and the IGFs regulate both IGFBP-2 and IGFBP-4. While PDGF and insulin stimulate IGFBP-2 and 4 synthesis, the IGFs appear to activate protease(s) which regulate IGFBP-2 and -4 levels post-translationally.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗

PubMed1995

Role of insulin-like growth factor binding proteins in the control of IGF actions.

The insulin-like growth factor binding proteins have been shown to modify IGF actions. IGFBP-5 binds to extracellular matrix (ECM) and its ability to potentiate IGF activity is dependent upon the amount that is ECM associated. To determine the specific regions of IGFBP-5 that are required for ECM association, site directed mutagenesis has been used to prepare several forms of IGFBP-5. Mutants that have had the amino acids between positions 201 and 218 altered have been useful. Mutation of the lysine 211 resulted in no change in the affinity of IGFBP-5 for ECM or heparin Sepharose; however, it resulted in a major reduction in affinity for IGF-I following heparin binding. Other mutations which disrupted heparin binding also resulted in loss of this affinity shift. Most distruptive were mutations of amino acids 211, 214, 217 and 218 and 202, 206 and 207. Mutation of residues 201 plus 202 had some effect, but substitution for 207, 211, 217 and 218 had no effect. When binding to intact ECM was analyzed, similar results were obtained. This suggests that amino acids 202, 206 and 214 are definitely involved in heparin and ECM binding. When binding to proteoglycans such as tenascin and heparin sulfate proteoglycan was analyzed, similar results were obtained. IGFBP-5 also binds to other proteins in ECM, including type IV collagen and plasminogen activator inhibitor-I. Specific antisera for plasminogen activator inhibitor-1 can coprecipitate IGFBP-5. IGFBPs are degraded by specific proteases. Three proteases that degrade IGFBP-2, -4 and -5 have been characterized. They are serine proteases that cleave these proteins at basic residues. Although several well characterized serine proteases cleave IGFBP-4 or -5, the proteases in cell conditioned media appear to be distinct.

Animals↗

PubMed1995

Cell migration: interactions among integrins, IGFs and IGFBPs.

The migratory behaviour of cells is fundamental to diverse biologic processes such as tumor metastasis, development of atherosclerotic plaques, embryonic development and wound healing. We have examined the effects of IGF-I and IGFBPs on the migration of Chinese Hamster ovary (CHO) cells, smooth muscle cells (SMC) and human breast cancer cells (HBC) and have studied the involvement of integrin receptors in migration induced by IGF-I and by IGFBPs. Using a monolayer wounding assay, we determined the effect of IGFBP-1 on SMC to be qualitatively similar to its effect we reported earlier on CHO cells, in that there is a direct stimulation of migration mediated by the alpha 5 beta 1 integrin. IGFBP-2 has no direct effect on SMC migration, and although it also contains the Arg-Gly-Asp sequence, we can detect no integrin binding. Unlike CHO cells, SMC are stimulated to migrate by IGF-I. IGFBP-2 and IGFBP-1 both inhibit this IGF-I receptor-mediated stimulation. We have also studied the migration of HBC using a Boyden chamber apparatus and have shown a potent chemotactic effect of IGF-I. We have investigated the mechanisms for IGF-I stimulation of SMC and HBC migration. IGF-I stimulation of SMC migration requires the presence of either 0.2% serum or vitronectin, because of a requirement for ligand binding by the alpha V beta 3 integrin (vitronectin receptor). MCF-7 HBC migrate toward a concentration gradient of IGF-I, the only growth factor that was able to stimulate these cells to migrate. Integrin ligand binding was also necessary for MCF-7 cells to migrate in response to IGF-I; alpha V beta 5 integrin was required for migration on vitronectin and alpha 2 beta 1 was required on collagen. These studies demonstrate that the stimulation of cell migration by IGFBP-1 and IGF-I involves signaling by members of the integrin family of receptors. The mechanisms by which the IGF-I receptor and integrin receptors interact are not yet known.

Animals↗

PubMed1995

Persistent lipolytic effect of exogenous growth hormone during caloric restriction.

PURPOSE: In previous studies in which obese volunteers were calorically restricted to 24, 18, or 12 kcal/kg of ideal body weight (IBW) per day, we observed that the growth-hormone-induced acceleration of body fat loss was variable and that the severity of caloric restriction modulated the magnitude of fat loss and the anabolic response to growth hormone. The present study was undertaken to characterize the effects of caloric restriction to 15 kcal/kg IBW per day on the metabolic response to growth hormone and to determine whether acceleration of body fat loss by growth hormone could be reproduced under conditions predicted to be optimal. PATIENTS AND METHODS: Eleven obese subjects were studied during two 38-day periods of caloric restriction. During one of these periods they received injections of growth hormone, 0.05 mg/kg IBW, for 28 days. Measurements of nitrogen balance, body fat content, insulin like growth factor I (IGF-I), free fatty acids, and glycerol concentrations were performed. RESULTS: Growth hormone injections caused an approximate 2.5-fold increase in IGF-I concentrations so that the mean IGF-I concentration was significantly greater during the injections than during diet alone (growth hormone 69.3 +/- 29.3 nmol/L; diet alone 26.6 +/- 7.6 nmol/L; P < 0.001). Growth hormone also caused nitrogen sparing, and the mean daily nitrogen balance was significantly greater during the injections (growth hormone 36.9 +/- 121.1 mmol/day; diet alone -122.3 +/- 125.9 mmol/day; P < 0.001). This nitrogen-sparing response to growth hormone attenuated over the 4 weeks of the injections. Growth hormone had a persistent lipolytic effect manifested by increases in glycerol concentrations, and body fat loss was greater during injections than during diet alone (fraction of weight lost as fat during injections 0.77 +/- 0.07; diet alone 0.63 +/- 0.06; P < 0.001). CONCLUSION: We conclude that growth hormone exerts anabolic effects that attenuate over time during caloric restriction but maintains its lipolytic effect despite hyperinsulinism and results in accelerated fat loss.

Adipose Tissue↗

PubMed1995

Tubular hypertrophy due to work load induced by furosemide is associated with increases of IGF-1 and IGFBP-1.

We have examined the expression of insulin-like growth factor 1 (IGF-1), IGF binding protein-1 (IGFBP-1), and IGF binding protein-3 (IGFBP-3) in the rat distal nephron during increased cell work load and hypertrophy, induced by the diuretic, furosemide. Furosemide was given for six days to increase distal sodium delivery and uptake. To mitigate salt loss, the animals drank 0.8% NaCl and 0.1% KCl. Control rats were infused with vehicle (0.9% saline) and drank tap water. Furosemide increased urinary volume (13-fold) and sodium excretion (eightfold), and decreased urine osmolarity (fourfold). By immunocytochemistry, staining for IGF-1 and IGFBP-1 was markedly increased in distal convoluted tubules and cortical collecting ducts; both segments also underwent hypertrophy. Increased staining for the peptides was evident early (1 hr, 18 hr) after furosemide, prior to hypertrophy of cells. Whereas transcripts of IGF-1 and IGFBP-3 mRNA showed little or no increase in extracts from furosemide-treated kidney cortices, IGFBP-1 mRNA was increased threefold 18 hours after furosemide. Alterations of IGF-1 and IGFBP-1 were independent of changes in plasma aldosterone, glucocorticoids or arginine vasopressin. That IGFBP-1 mRNA increased threefold without significant changes in IGF-1 mRNA suggests that hypertrophic stimuli might initially induce the synthesis of IGF binding protein followed by the trapping of extracellular IGF-1. The present study raises the possibility of IGF-1 and IGFBP-1 being involved in processes that lead to tubular hypertrophy. IGFBP-1 may regulate these effects by binding to and interaction with IGF-1.

Animals↗

PubMed1995

Porcine aortic smooth muscle cells secrete a serine protease for insulin-like growth factor binding protein-2.

Porcine aortic smooth muscle cells secrete two forms of insulin-like growth factor (IGF) binding proteins (IGFBP-2 and -4), and both forms have been shown to modulate IGF-I actions in this cell type. Recently, we showed that IGFBP-4 inhibited IGF-I action and that the cells produced a protease that cleaved IGFBP-4 into non-IGF binding fragments. After the cleavage of IGFBP-4, the cellular DNA synthesis response to IGF-I was enhanced. This study reports that these cells also secrete a protease for IGFBP-2. Like the IGFBP-4 protease, this protease is also secreted constitutively, but unlike the IGFBP-4 protease, its secretion is enhanced if the cells are serum-deprived for 24 hours before the collection of conditioned medium. The protease cleaved IGFBP-2 into 25- and 16-kD fragments, which had reduced IGF-I binding activity. Protease activity was enhanced by coincubation with IGF-I or IGF-II, and IGF-II was more potent than IGF-I. The protease is a serine protease, since its activity can be inhibited by 3,4-dichloroisocoumarin and aprotinin. It is also inhibited by EDTA, and its activity can be restored with calcium but not zinc. The heparin-binding serpins, specifically, heparin cofactor II and antithrombin III, are inhibitory. Heparin alone also had activity, and the combination of antithrombin III plus heparin caused complete inhibition. The conditioned medium also contained proteolytic activities for IGFBP-4 and -5 but it did not cleave IGFBP-1 and -3.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗