Regulation of insulin-like growth factor-I and its receptor in rat aorta after balloon denudation. Evidence for local bioactivity

Insulin and insulin-like growth factor-I cause coronary vasorelaxation in vitro

Insulin and insulin-like growth factor-I (IGF-I) may play a role in the modulation of coronary artery tone, yet there are few data regarding their vasoactive effects on the coronary vascular bed. We evaluated the vasorelaxation effects of insulin and IGF-I on porcine coronary epicardial vessels in vitro and elucidated possible mechanisms. Porcine epicardial arteries were contracted with 10(-7) mol/L endothelin-1 and relaxed with cumulative concentrations of either insulin or IGF-I (10(-12) to 10(-7) mol/L). The above experiments were repeated in vessels without endothelium. Vessels were also incubated with the nitric oxide synthase inhibitor NG-monomethyl-L-arginine (L-NMMA; 10(-4) mol/L) with and without 10(-3.5) mol/L L-arginine, the potassium channel blocker tetraethylammonium (TEA; 10(-2) mol/L), and the guanylyl cyclase inhibitor 1H-[1,2,4]oxadiazolo[4,3,-alpha]quinoxalin-1-one (ODQ; 10(-5.5) mol/L); vessels were then contracted with endothelin-1 and relaxed with insulin or IGF-I. Insulin and IGF-I were also added after contraction with 60 mmol/L KCl. Insulin and IGF-I caused a similar decrease in coronary epicardial tension after contraction with endothelin-1 (relaxation of 28+/-4% [n=7] and 25+/-3% [n=8] with insulin and IGF-I, respectively; P<0.0001 for both peptides). Removal of the endothelium did not affect these responses. Incubation with L-NMMA, but not ODQ, attenuated the vasorelaxation response to insulin and IGF in vessels without endothelium. L-Arginine did not reverse this effect of L-NMMA. KCl and TEA attenuated the vasorelaxation effect of both insulin and IGF-I. Thus, both insulin and IGF-I caused non-endothelium-dependent coronary vasorelaxation in vitro, probably through a mechanism involving the activation of potassium channels. These findings suggest that insulin and IGF-I participate in the regulation of coronary vasomotor tone.

Insulin-like growth factor-1 stimulates vascular smooth muscle cell proliferation in rat aorta in vivo

IGF-I is a mitogen for vascular smooth muscle cells (SMC) in vitro and enhances SMC proliferation in vivo in diabetic rats. In this study, we examined the effect of IGF-I on SMC proliferation in vivo in normal rats. Recombinant human IGF-I (0.87 and 3.1 mg/kg/day), was infused via osmotic minipumps in normal rats starting 3 days before they were subjected to aortic injury with a balloon catheter. IGF-I at an infusion rate of 3.1 mg/kg/day caused a significant increase in 3H-thymidine incorporation into DNA (+53%, P < 0.01) in the rat aortic intima-media 2 days after injury and DNA content (+13%, P < 0.05) after 11 days. The elastin and collagen contents were not changed by IGF-I infusion after 11 days. Body weight increased slightly while blood glucose was not affected. At an infusion rate of 0.87 mg/kg/day, IGF-I had no significant effects. These results suggest that circulating levels of IGF-I can stimulate SMC proliferation in vivo in normal rats but only at high concentrations.

Differential expression and biological effects of insulin-like growth factor-binding protein-4 and -5 in vascular smooth muscle cells

Insulin-like growth factor-I (IGF-I) plays an important role in regulating vascular smooth muscle cell (VSMC) proliferation, migration, and apoptosis. The bioactivity of IGF-I is modulated by a group of high affinity, specific binding proteins (IGF-binding proteins; IGFBPs) that are present in the interstitial fluid. Previously, we have reported that porcine VSMCs synthesize and secrete IGF-I and several forms of IGFBPs, including IGFBP-2, IGFBP-4, and IGFBP-5. In this study, we examined the role of autocrine/paracrine secreted IGF-I in controlling the expression of IGFBP-4 and IGFBP-5 as well as the effects of these IGFBPs in modulating the cellular replication response to IGF-I. The concentrations of IGFBP-4 in the conditioned medium increased significantly from <50 ng/ml to 742 +/- 105 ng/ml. This increase was associated with a decrease in the activity of an IGF-I-regulated IGFBP-4 protease. In contrast, the synthesis of IGFBP-5 was inversely correlated with culture density, and its concentration decreased from 792 +/- 91 to 44 +/- 14 ng/ml. IGFBP-5 mRNA in sparse cultures was 3-fold higher compared with those in confluent cultures. This culture density-dependent change in IGFBP-5 mRNA correlated closely with endogenous IGF-I levels. Since treatment of VSMC with exogenous IGF-I increased IGFBP-5 mRNA levels, we neutralized the effect of endogenously secreted IGF-I with an anti-IGF-I antibody to determine if it would alter IGFBP-5 mRNA abundance. This resulted in a 4.4-fold decrease in IGFBP-5 mRNA levels. When added together with IGF-I, exogenous IGFBP-4 inhibited IGF-I-induced DNA synthesis in a concentration-dependent manner. IGFBP-5, on the other hand, potentiated the effect of IGF-I. Therefore, IGFBP-4 and IGFBP-5 appear to be differentially regulated by autocrine/paracrine IGF-I through distinct mechanisms. These two proteins, in turn, play opposing roles in modulating IGF-I action in stimulating VSMC proliferation.

The effect of extracellular matrix proteins on porcine smooth muscle cell insulin-like growth factor (IGF) binding protein-5 synthesis and responsiveness to IGF-I

The aim of this study was to determine if cultured porcine vascular smooth muscle cells (pSMCs) that had been maintained on different extracellular matrix proteins had an alteration in their expression of insulin-like growth factor binding protein-5 (IGFBP-5) and their responsiveness to insulin-like growth factor-I (IGF-I). When pSMCs were plated on fibronectin, they synthesized 6.0 +/- 1.2-fold more IGFBP-5 than did cells maintained on laminin and type IV collagen. IGF-I increased IGFBP-5 gene expression 3-fold in the cells plated on fibronectin. The addition of an RGD peptide and echistatin to pSMC cultures that had been plated on fibronectin inhibited IGFBP-5 mRNA expression. The addition of an antibody against alpha2beta1 integrin partially reversed the inhibitory effects of laminin and type IV collagen on IGFBP-5 expression. Cells maintained on fibronectin had a 5.0 +/- 1.1-fold greater DNA synthesis response to IGF-I compared with those maintained on laminin/type IV collagen, and echistatin significantly inhibited the DNA synthesis response of the fibronectin-maintained cells to IGF-I. The anti-alpha2beta1 antibody partially reversed the inhibitory effect of laminin and type IV collagen on IGF-I-stimulated DNA synthesis. The addition of IGFBP-5 to cultures plated on laminin and type IV collagen significantly increased their response to IGF-I. Atherosclerotic plaques from pig aorta contained abundant fibronectin and had increased IGFBP-5 mRNA (4.5 +/- 1.5-fold) compared with tissue from the normal vessel wall that had a low fibronectin content. These results indicate that fibronectin, laminin, and type IV collagen have major effects on IGFBP-5 expression and on IGF-I-stimulated pSMC responses and that these effects are mediated by their respective integrins.

Serum total IGF-I, free IGF-I, and IGFB-1 levels in an elderly population: relation to cardiovascular risk factors and disease

Recently, a method to measure free insulin-like growth factor-I (IGF-I) levels has been developed. Free IGF-I levels may have greater physiological and clinical relevance than total (bound and free) IGF-I. The associations between the circulating IGF-I/IGF binding protein (IGFBP) system and cardiovascular disorders was studied. In a cross-sectional study of 218 healthy persons (103 men, 115 women) aged 55 to 80 years, fasting serum (total and free) IGF-I and IGFBP-1 levels, lipid profile, insulin, and glucose were measured. In addition, blood pressure, body mass index (BMI), and waist-hip ratio (WHR) were measured. Ultrasonography of both carotid arteries was performed to investigate the presence of atherosclerotic lesions. A history of angina pectoris, the presence of a possible or definite myocardial infarction on the ECG, and plaques in the carotid arteries were used as indicators of presence of cardiovascular signs and symptoms. Free IGF-I was inversely related to serum triglycerides (P=.04, adjusted for age and sex). Mean free IGF-I levels in subjects without signs or symptoms of cardiovascular diseases were significantly higher than in those with at least one cardiovascular symptom or sign (P=.002, adjusted for age and sex). Free IGF-I levels were also higher in subjects who had no atherosclerotic plaques in the carotid arteries (P=.02, adjusted for age and sex) and who had never smoked (P=.02, adjusted for age and sex). IGFBP-1 showed an inverse relation with insulin, BMI, and WHR and a positive relation with HDL cholesterol. The associations between IGFBP-1 levels and HDL cholesterol, WHR, and BMI remained significant after adjustment for fasting insulin levels. High fasting serum free IGF-I levels are associated with a decreased presence of atherosclerotic plaques and coronary artery disease and lower serum triglycerides, whereas high fasting IGFBP-1 levels are associated with a more favorable cardiovascular risk profile. The findings suggest that the IGF-I/IGFBP system is related to cardiovascular risk factors and atherosclerosis.

Protease-resistant form of insulin-like growth factor-binding protein 5 is an inhibitor of insulin-like growth factor-I actions on porcine smooth muscle cells in culture

IGFs are pleiotrophic mitogens for porcine smooth muscle cells (pSMC) in culture. The effects of IGFs on cells are modulated by various insulin-like growth factor-binding proteins (IGFBP). IGFBP-5 is synthesized by pSMC and binds to the extracellular matrix. However, IGFBP-5 is also secreted into conditioned medium of cultured cells and is cleaved into fragments by a concomitantly produced protease. These fragments have reduced affinity for the IGFs and cleavage makes it difficult to assess the role of intact IGFBP-5. To study the consequence of accumulation of intact IGFBP-5 in medium, we determined the cleavage site in IGFBP-5 and prepared a protease resistant mutant. Amino acid sequencing of purified IGFBP-5 fragments suggested Arg138-Arg139 as the primary cleavage site. Arg138-Arg139-->Asn138-Asn139 mutations were introduced to create protease-resistant IGFBP-5, which has the same affinity for IGF-I as the native protein. This mutant IGFBP-5 remained intact even after 24 h of incubation and it inhibited several IGF-I actions when added to pSMC culture medium. The mutant IGFBP-5 (500 ng/ml) decreased IGF-I stimulated cellular DNA synthesis by 84%, protein synthesis by 77%, and it inhibited IGF-I stimulated migration of pSMC by 77%. It also inhibited IGF-I stimulation of IRS-1 phosphorylation. In contrast, the same amount of native IGFBP-5 did not inhibit IGF-I actions. The significance of inhibitory effects of the protease resistant IGFBP-5 was further demonstrated in pSMC transfected with mutant or native IGFBP-5 cDNAs. The mutant IGFBP-5 accumulated in culture medium of transfected cells, while native IGFBP-5 was degraded into fragments, PSMC overexpressing the mutant IGFBP-5 also responded poorly to IGF-I compared with mock transfected cells. IGF-I (5 ng/ml) increased [35S]methionine incorporation into control cells by 36% above the basal level, but it did not significantly change (4%) in pSMC cultures that were producing the mutant IGFBP-5. In conclusion, the accumulation of protease-resistant IGFBP-5 in the medium was inhibitory to IGF-I actions on pSMC. This suggests that proteolysis can prevent IGFBP-5 from acting as an inhibitor of IGF-I-stimulated effects and that it serves as an important mechanism for regulating cellular responsiveness to IGF-I.

Targeted overexpression of IGF-I evokes distinct patterns of organ remodeling in smooth muscle cell tissue beds of transgenic mice

Smooth muscle cells (SMC) of the vascular wall, bladder, myometrium, and gastrointestinal and respiratory tracts retain the ability to proliferate postnatally, which enables adaptive responses to injury, hormonal, or mechanical stimulation. SMC growth is regulated by a number of mesenchymal growth factors, including insulin-like growth factor I (IGF-I). To explore the function of IGF-I on SMC in vivo, the mouse SMC alpha-actin promoter fragment SMP8 (-1074 bp, 63 bp of 5'UT and 2.5 kb of intron 1) was cloned upstream of rat IGF-I cDNA, and the fusion gene microinjected to fertilized eggs of the FVB-N mouse strain. Mating of hemizygous mice with controls produced about 50% transgenic offspring, with equal sex distribution. Transgenic IGF-I mRNA expression was confined to SMC-containing tissues, with the following hierarchy: bladder > stomach > aorta = uterus > intestine. There was no transgene expression in skeletal muscle, heart, or liver. Radioimmunoassayable IGF-I content was increased by 3.5- to 4-fold in aorta, and by almost 10-fold in bladder of transgenic mice at 5 and 10 wk, with no change in plasma IGF-I levels. Wet weight of bladder, stomach, intestine, uterus, and aorta was selectively increased, with no change in total body or carcass weight of transgenic animals. In situ hybridization showed that transgene expression was exquisitely targeted to the smooth muscle layers of the arteries, veins, bladder, ureter, stomach, intestine, and uterus. Paracrine overproduction of IGF-I resulted in hyperplasia of the muscular layers of these tissues, manifesting in remarkably different phenotypes in the various SMC beds. Whereas the muscular layer of the bladder and stomach exhibited a concentric thickening, the SMC of the intestine and uterus grew in a longitudinal fashion, resulting in a marked lengthening of the small bowel and of the uterine horns. This report describes the first successful targeting of expression of any functional protein capable of modifying the phenotype of SMC in transgenic mice. IGF-I stimulates SMC hyperplasia, leading to distinct patterns of organ remodeling in the different tissue environments.

Lanreotide reduces serum free and total insulin-like growth factor-I after angioplasty

BACKGROUND

Several experiments point to a participating role of insulin-like growth factor-I (IGF-I) in the vascular events leading to restenosis after percutaneous transluminal coronary angioplasty (PTCA).

METHODS AND RESULTS

We measured fasting serum total (extractable) IGF-I in 553 patients in a controlled clinical trial. Half of the patients received continuous subcutaneous infusion of the somatostatin analogue lanreotide from the day before (baseline) and for 4 days after PTCA. We also measured ultrafiltrated serum free IGF-I and IGF-II, total IGF-II, IGF-binding protein-1 (IGFBP-1), IGFBP-3, and insulin in a subgroup of 18 placebo-treated and 20 lanreotide-treated patients. Total IGF-I had decreased by 7% (P < .0001) 1 day after initiation of lanreotide infusion and stayed reduced, whereas no early changes occurred in placebo-treated patients. The same pattern was observed in the subgroup. Free IGF-I decreased significantly from baseline by 22% to 27% (P < .05) in lanreotide-treated patients and increased insignificantly by 10% to 30% (P = .054) in placebo-treated patients. IGFBP-1 increased (P < .05) in both groups postoperatively, but levels in lanreotide-treated patients exceeded (P < .05) those of placebo-treated patients. Lanreotide treatment resulted in minor reductions (P < .05) in free and total IGF-II and IGFBP-3, whereas insulin was unaltered.

CONCLUSIONS

Lanreotide administration acutely decreases circulating total and free IGF-I, the latter relatively more, and increases IGFBP-1. These alterations in the IGF system may participate in the improvement of the long-term outcome after PTCA noted with lanreotide treatment.

Lack of effect of recombinant human superoxide dismutase on cold ischemia-induced arteriosclerosis in syngeneic rat aortic transplants

Prolonged cold ischemia time and the generation of free oxygen radicals during reperfusion are risk factors for allograft arteriosclerosis. Growth factors are the main pro-proliferative mediators of smooth muscle cells in classical and in allograft arteriosclerosis. Superoxide dismutase is an enzyme that catalyzes the dismutation of superoxide anions into hydrogen peroxide. This study was designed to investigate which smooth muscle cell growth factor contribute to the formation of arteriosclerosis in syngenic vascular grafts with prolonged ischemia time, and whether perioperative intravenous administration of recombinant human superoxide dismutase (rh-SOD) prevents arteriosclerosis in these grafts. DA aortas were transplanted into DA recipients. One group of transplants was made with a short ex vivo ischemia time (15 min), while the other group transplant grafts was stored for 24 hr in cold saline. In addition to morphometric quantitation of the histological alterations, RNA isolated from grafts with short cold ischemia time in a semiquantitative polymerase chain reaction specific for various known smooth muscle cell growth factors. Syngeneic grafts with prolonged cold ischemia time showed severe intimal thickening and prominent medical necrosis, which were not seen in control groups. Approximately 3-fold levels of insulin-like growth factor-1 were found in ischemic syngeneic grafts compared with non-ischemic syngenic grafts, whereas epidermal growth factor levels were slightly lower. No changes in other growth factor mRNAs were found. Perioperative treatment with rh-SOD did not have significant effect on the extent of intimal thickening nor on the intensity of medial necrosis in grafts with prolonged ischemia time, and administration of rh-SOD did not change the expression level of insulin-like growth factor-1 in the grafts, either.

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.

G-protein coupled and tyrosine kinase receptors: evidence that activation of the insulin-like growth factor I receptor is required for thrombin-induced mitogenesis of rat aortic smooth muscle cells

IGF I is an ubiquitous peptide that activates a membrane tyrosine kinase receptor and has autocrine/paracrine effects on vascular smooth muscle cells. Thrombin activates a G-protein coupled receptor and is also a mitogen for vascular smooth muscle cells. To assess the potential role of IGF I as a mediator of thrombin's effects, we characterized expression of IGF I and of its receptor on vascular smooth muscle cells exposed to thrombin. Thrombin dose-dependently decreased IGF I mRNA levels and caused a delayed decrease in IGF I secretion from vascular smooth muscle cells. This effect was mimicked by the hexapeptide SF-FLRN (that functions as a tethered ligand) and was inhibited by hirudin. In contrast, thrombin doubled IGF I receptor density on vascular smooth muscle cells, without altering binding affinity (Kd). An anti-IGF I antiserum markedly reduced thrombin-induced DNA synthesis, whereas nonimmune serum and an anti-fibroblast growth factor antibody were without effect. Cell counts confirmed these results. Downregulation of IGF I receptors by antisense phosphorothioate oligonucleotides likewise markedly inhibited thrombin-induced DNA synthesis. These data demonstrate that a functional IGF I-IGF I receptor pathway is essential for thrombin-induced mitogenic signaling and support the concept of cross talk between G-protein coupled and tyrosine kinase receptors.

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)

Inhibition of vascular smooth muscle cell growth through antisense transcription of a rat insulin-like growth factor I receptor cDNA

Insulin-like growth factor I (IGF I) is an autocrine/paracrine growth factor that is produced in multiple tissues and is essential for normal developmental growth. Its effects are mediated by activation of a membrane-bound tyrosine kinase receptor, IGF IR. On the basis of the partial rat IGF IR alpha-chain cDNA sequence previously reported, we cloned cDNA encoding the full-length rat IGF IR. The deduced amino acid sequence predicts a 1370-amino acid receptor precursor, which includes signal sequence, a 707-amino acid alpha-chain, a 4-Arg cleavage site, and a 629-amino acid beta-chain. Overall, similarity to human IGF IR is 89% and 98% at the nucleotide and amino acid levels, respectively. Antisense IGF IR expression constructs in vectors incorporating Epstein-Barr virus replicative signals and the cytomegalovirus promoter/enhancer or the inducible human metallothionein IIa promoter/enhancer were assembled and stably transfected into cultured rat aortic smooth muscle cells. Clone CA9 (constitutively expressing abundant antisense IGF IR transcripts), clones MA5 and MA7 (expressing antisense IGF IR transcripts inducibly), and clones ME8 and ME10 (expressing vector alone) were characterized. There was a 57% reduction in IGF IR mRNA levels in clone CA9 after confluence compared with clone ME10. This resulted in a 51% decrease in IGF I binding sites in clone CA9, without a change in binding affinity (Kd), and a 55% and 57% reduction in DNA synthesis rates, basally and in response to 10 ng/mL IGF I, respectively. Clones MA5/MA7 similarly showed a 54% reduction in IGF IR number after confluence following exposure to 100 mumol/L ZnSO4 and a 44% and 58% reduction in DNA synthesis, basally and in response to 10 ng/mL IGF I, respectively. Growth curves indicated that proliferation of clone CA9 in the presence of 10% serum was reduced by 60% compared with clone ME10. Thus, cloning of cDNA encoding the full-length rat IGF IR indicates that this receptor is highly conserved. Antisense targeting of this receptor in vascular smooth muscle cells (VSMCs) demonstrates that a decrease in IGF IR density results in marked inhibition of VSMC proliferation. These findings indicate an important role for this ligand-receptor system in regulating VSMC growth. Specifically, they suggest that modulation of VSMC IGF IR density may be an important mechanism whereby growth of these cells is controlled.

Hypertension increases insulin-like growth factor binding protein-4 mRNA levels in rat aorta

Insulin-like growth factor-I (IGF-I) is an endocrine and autocrine/paracrine growth factor. Recently, we have demonstrated that interrenal aortic coarctation in the rat increases IGF-I mRNA levels in the thoracic aorta, consistent with a role for this mitogen in hypertensive vascular remodeling. The effects of IGF-I are modulated by several IGF binding proteins including IGFBP-3, the main circulating carrier of IGF-I, and IGFBP-4, the main IGF binding protein produced by vascular smooth muscle cells in vitro. To obtain insights into the regulation of IGF-I and more specifically to study potential changes in IGF binding proteins in high-renin hypertension, we studied male Sprague-Dawley rats that had undergone abdominal aortic coarctation. Compared with sham-operated rats, the study rats showed a rapid increase in IGFBP-4 mRNA levels in the hypertensive (thoracic) aorta, reaching a plateau at 3 days (2.5-fold increase) and persisting for at least 14 days. In striking contrast, IGFBP-4 mRNA decreased slightly in the normotensive (abdominal) aorta at 14 days. IGFBP-3 mRNA levels did not change in either vascular bed after coarctation. Study of hepatic tissue indicated that in coarcted rats IGFBP-4 and IGFBP-3 mRNA levels decreased transiently (approximately 50% at 7 days compared with sham). Circulating IGF-I in coarcted animals decreased slightly (P = .08), and Western ligand analysis indicated that circulating levels of IGF binding proteins were not altered.(ABSTRACT TRUNCATED AT 250 WORDS)

Vascular endothelial growth factor, platelet-derived growth factor, and insulin-like growth factor-1 promote rat aortic angiogenesis in vitro

The purpose of this study was to evaluate the vasoformative response of isolated vascular explants to a variety of growth factors that have been shown to stimulate angiogenesis. Rings of rat aorta were cultured in collagen gels under serum-free conditions in the presence or absence of vascular endothelial growth factor (VEGF), natural platelet-derived growth factor (PDGF), PDGF-AA, PDGF-BB, insulin-like growth factor-1 (IGF-1), transforming growth factor-alpha (TGF-alpha), transforming growth factor-beta 1 (TGF-beta 1), epidermal growth factor (EGF), interleukin-1 alpha (IL-1 alpha), or hepatocyte growth factor (HGF). The angiogenic response of the rat aorta was stimulated by VEGF, PDGF, PDGF-AA, PDGF-BB, and IGF-1. Maximum stimulatory effects were obtained with VEGF and PDGF-BB. By contrast, TGF-beta 1 and IL-1 alpha had inhibitory activity. No significant effects were observed with TGF-alpha, EGF, or HGF. The vascular outgrowth of VEGF-stimulated cultures was primarily composed of microvessels, whereas that of PDGF- and IGF-1-stimulated cultures contained an increased number of fibroblast-like cells. The inability of TGF-alpha, TGF-beta 1, IL-1 alpha, EGF, and HGF to stimulate rat aortic angiogenesis in serum-free culture suggests that either these factors require the mediatory activity of accessory cells that are not present in the rat aorta model or that blood vessels are heterogeneous in their capacity to respond to different angiogenic factors.

Expression and insulin-like growth factor-dependent proteolysis of insulin-like growth factor-binding protein-4 are regulated by cell confluence in vascular smooth muscle cells

Insulin-like growth factor (IGF)-I is markedly induced after balloon injury in the rat aorta, where it may serve to mediate vascular repair. Because the bioavailability of IGF-I is modulated by IGF-binding proteins (IGFBPs), we examined the regulation of IGFBPs by IGFs in primary cultures of rat aortic smooth muscle cells (SMCs). Serum-deprived SMC-conditioned medium contains IGFBPs of 38 to 45 kD (only in confluent cultures), 30 kD (possibly IGFBP-2), 28 kD, and 24 kD (IGFBP-4), the latter being the most abundant. IGF-I and IGF-II but not insulin evoked a marked decrease of IGFBP-4 as early as 4 hours after treatment. IGFBP-4 mRNA abundance, however, was entirely unaffected by IGF-I for up to 48 hours. IGF-I analogues with high affinity for the IGF-I receptor and weak affinity for IGFBP paradoxically evoked a small increase in IGFBP-4, probably through a general increase in protein synthesis. IGF-I only minimally decreased IGFBP-4 content in medium of sparse cultures, whereas it completely abolished IGFBP-4 content in conditioned medium of superconfluent SMCs. IGF-I also evoked a concentration-dependent increase in the abundance of IGFBP-3 in confluent, but not sparse, SMCs without affecting IGFBP-3 mRNA. Addition of IGF-I to cell-free medium conditioned by confluent, but not by sparsely cultured, SMCs led to rapid degradation of IGFBP-4. Interestingly, IGFBP-4 mRNA was markedly induced in confluent relative to sparsely grown SMCs in an IGF-I independent fashion. Thus, both biosynthesis and IGF-dependent proteolysis of IGFBP-4 are increased in confluent SMCs. Proteolysis was maximal at 37 degrees C and was abrogated by EDTA and by benzamidine. Phenylmethylsulfonyl fluoride and the plasmin inhibitor bdellin had minor inhibitory activity, whereas aprotinin, angiotensin-converting enzyme inhibitors, and N-ethylmaleimide were without effect. The protease does not affect the structure of IGF-I as determined by reverse-phase high-performance liquid chromatography and size-exclusion chromatography of 125I-IGF-I incubated for up to 24 hours with SMC-conditioned medium containing IGFBP-4. In summary, SMCs elaborate a cation-dependent protease in a confluence-dependent fashion, which degrades bound IGFBP-4 and likely releases free structurally intact IGF-I, presumably to interact with the cell surface receptor and/or other IGFBPs.

Insulin-like growth factor-I and platelet-derived growth factor-BB induce directed migration of human arterial smooth muscle cells via signaling pathways that are distinct from those of proliferation

Directed migration or chemotaxis of arterial smooth muscle cells (SMC) contributes to intimal SMC accumulation, a key event in the development of atherosclerotic lesions and in restenosis after angioplasty. The present study compares and contrasts insulin-like growth factor I (IGF-I) and platelet-derived growth factor (PDGF-BB) as chemoattractants and mitogens for human arterial SMC. Compared with PDGF-BB, IGF-I is a weaker SMC mitogen. Thus, PDGF-BB, but not IGF-I, evokes a strong and rapid activation of mitogen-activated protein (MAP) kinase kinase and MAP kinase. However, IGF-I is a potent stimulator of directed migration of human arterial SMC, as measured in a Boyden chamber assay. The half-maximal concentration for migration is similar to the Kd for IGF-I receptor interaction. An IGF-I receptor-blocking antibody blocks the effects of IGF-I, IGF-II, and insulin, indicating that the effects are indeed mediated through the IGF-I receptor. The maximal effect of IGF-I on directed migration ranges between 50% and 100% of the effect of PDGF-BB, the strongest known chemoattractant for SMC. The ability of IGF-I and PDGF-BB to induce chemotaxis coincides with their ability to stimulate phosphatidylinositol turnover, diacylglycerol formation, and intracellular Ca2+ flux and suggests that these signaling pathways, but not activation of the MAP kinase cascade, are required for chemotaxis of human arterial SMC.

Increase in insulin-like growth factor I in hypertrophying smooth muscle

The present study focuses on the role of the insulin-like growth factor (IGF) system in the development of smooth muscle hypertrophy. Hypertrophy was initiated by partial ligation of portal vein or urethra in female Sprague-Dawley rats weighing approximately 220 g. Levels of mRNA were analyzed by solution hybridization. Seven days after ligation, the wet weight of the portal vein was increased about threefold and the concentration of IGF-I mRNA was increased fourfold. The bladder wet weight was increased twofold 3 days after ligation and fourfold 10 days after ligation. IGF-I mRNA in the bladder was elevated 3-fold after 3 days and 2.5-fold after 10 days, whereas IGF binding protein 2 mRNA was increased approximately 2-fold after 3 days and 5-fold after 10 days. IGF-I receptor mRNA in the hypertrophying bladder remained unchanged. Increased levels of IGF-I were demonstrated with immunohistochemistry in both hypertrophying portal vein and urinary bladder. The results show a specific increase in IGF-I mRNA as well as an increased IGF-I immunoreactivity during hypertrophy of smooth muscle, which suggests that the local IGF-system may play a role in smooth muscle hypertrophy.