Platelet-derived growth factor isoforms decrease insulin-like growth factor I gene expression in rat vascular smooth muscle cells and selectively stimulate the biosynthesis of insulin-like growth factor binding protein 4

B lymphocytes directly contribute to tissue fibrosis in patients with IgG4-related disease

BACKGROUND

IgG₄-related disease (IgG₄-RD) is a fibroinflammatory condition marked by rapid clinical improvement after selective depletion of B lymphocytes with rituximab. This feature suggests that B cells might participate in fibrogenesis and wound healing.

OBJECTIVE

In the present work we aimed to demonstrate that B lymphocytes contribute directly to tissue fibrosis in patients with IgG₄-RD.

METHODS

Total circulating CD19⁺ B lymphocytes, naive B cells, memory B cells, or plasmablasts from patients with IgG₄-RD were cultivated with human fibroblasts. Profibrotic soluble factors and collagen production in cocultures were assessed by using ELISAs and Luminex assays. RNA sequencing and quantitative RT-PCR were used to assess fibroblast activation in the presence of B cells, as well as induction of profibrotic pathways in B-cell subsets. Relevant profibrotic and inflammatory molecules were confirmed in vitro by using functional experiments and on IgG₄-RD tissue sections by using multicolor immunofluorescence studies.

RESULTS

B cells from patients with IgG₄-RD (1) produced the profibrotic molecule platelet-derived growth factor B and stimulated collagen production by fibroblasts; (2) expressed enzymes implicated in extracellular matrix remodeling, such as lysyl oxidase homolog 2; (3) produced the chemotactic factors CCL4, CCL5, and CCL11; and (4) induced production of these same chemokines by activated fibroblasts. Plasmablasts expressed sets of genes implicated in fibroblast activation and proliferation and therefore represent cells with intrinsic profibrotic properties.

CONCLUSION

We have demonstrated that B cells contribute directly to tissue fibrosis in patients with IgG₄-RD. These unanticipated profibrotic properties of B lymphocytes, particularly plasmablasts, might be relevant for fibrogenesis in patients with other fibroinflammatory disorders and for wound-healing processes in physiologic conditions.

Fibronectin glycation increases IGF-I induced proliferation of human aortic smooth muscle cells

The advanced glycation end products, namely AGEs, contribute to long-termed complications of diabetes mellitus, including macroangiopathy, where smooth muscle cells (SMC) proliferation stimulated by platelet-derived growth factor (PDGF) isoforms and insulin-like growth factor-I (IGF-I) plays an important role. The objective of the present study was to investigate the effect of an AGE-modified extracellular matrix protein on IGF-I induced SMC proliferation and on the IGF-I-IGF binding protein 4 (IGFBP-4) axis under basal conditions and after stimulation with PDGF-BB. IGF-I resulted in significantly higher thymidine incorporation in SMC seeded on AGE-modified fibronectin (AGE-FN) in comparison to cells seeded on fibronectin (FN). This augmented proliferation could not be accounted for by increased expression of IGF-IR, by decreased secretion of IGFBP-4, a binding protein that inhibits IGF-I mitogenic effects or by increased IGF-IR autophosphorylation. PDGF-BB did not modulate IGF-IR and IGFBP-4 mRNA expression in any of the substrata, however, this growth factor elicited opposite effects on the IGFBP-4 content in the conditioned media, increasing it in cells plated on FN and diminishing it in cells plated on AGE-FN. These findings suggest that one mechanism by which AGE-modified proteins is involved in the pathogenesis of diabetes-associated atherosclerosis might be by increasing SMC susceptibility to IGF-I mitogenic effects.

REGULATION OF INSULIN-LIKE GROWTH FACTOR (IGF)-BINDING PROTEIN EXPRESSION BY GROWTH FACTORS AND CYTOKINES ALTERS IGF-MEDIATED PROLIFERATION OF POSTNATAL LUNG FIBROBLASTS

Postnatal day 5 is the beginning of septation and the peak of postnatal fibroblast proliferation. The author and colleagues studied fibroblasts from this developmental time period to determine factors that regulate cell proliferation. Exposure of cells to insulin-like growth factor (IGF)-I for 48 hours increased cell number whereas exposure to epithelial growth factor (EGF), platelet-derived growth factor (PDGF)-BB, fibroblast growth factor (FGF)-7, FGF-2, tumor necrosis factor-alpha (TNF-alpha), or interleukin (L)-1beta did not alter cell number. Long[R3]IGF-I (a synthetic IGF analog with reduced affinity for IGF-binding proteins [IGFBPs]) was more potent than IGF-I, with half-maximal stimulation at a dose of 0.6 nM for long[R3]IGF-I compared to 1.5 nM for IGF-I, suggesting that IGFBPs in the conditioned medium (CM) inhibit IGF activity. Addition of exogenous IGFBP-3 inhibited the IGF-stimulated increase in cell number. Addition of IGFBP-4 did not alter IGF activity because IGF-I stimulated proteolysis of IGFBP-4. The expression of mRNA for PAPP-A (a known IGFBP-4 protease) suggests that the clearance of IGFBP-4 is mediated by pregnancy-associated plasma protein (PAPP)-A. Exposure of cells to TNF-alpha or IL-1beta increased IGFBP-3 mRNA abundance and IGFBP-3 protein in CM. PDGF-BB and IL-1beta increased IGFBP-4 protein abundance and PDGF-BB and dibutyryl cAMP increased IGFBP-4 mRNA. The increase in CM IGFBP-3 following TNF-alpha exposure blocked IGF-mediated cell proliferation, suggesting that the growth factor- and cytokine-mediated changes in IGFBP abundance regulate postnatal fibroblast cell proliferation.

Human multipotent mesenchymal stromal cells inhibit proliferation of PBMCs independently of IFNgammaR1 signaling and IDO expression

Multipotent mesenchymal stromal cells (MSCs) inhibit proliferation, helper, and effector functions in most if not all peripheral blood mononuclear cell (PBMC) subpopulations in vitro. The molecular mechanism is widely thought to imply tryptophan degradation by the interferon-gamma (IFNgamma)-induced expression of indoleamine 2,3-dioxygenase (IDO). However, IDO inhibitors were not able to restore proliferation of PBMCs in each case. Moreover, human MSCs with an IFNgamma receptor 1 (R1) defect inhibited proliferation of HLA-mismatched PBMCs to a similar extent as control MSCs. In contrast to healthy MSCs, IFNgammaR1-deficient MSCs showed no detectable mRNA for IDO-neither in the absence nor in the presence of recombinant human IFNgamma, nor in coculture with HLA-mismatched PBMCs. Based on gene expression profiling, we were able to show that insulin-like growth factor (IGF)-binding proteins contribute to the inhibitory mechanism of MSCs. Taken together, human MSCs exert important immunomodulatory functions in the absence of IFNgammaR1 signaling and IDO, partially accounted for by IGF-binding proteins.

Intravenous IGF-I receptor antisense reduces IGF-IR expression and diminishes pressor responses to angiotensin II in conscious normotensive rats

Given the variety of cardiovascular effects of insulin-like growth factor-I (IGF-I), we investigated the effects of a functional deficit in IGF-I signalling in the conscious rat cardiovascular system using intravenous IGF-I receptor antisense (AS, 0.5 nmol) treatment.Insulin-like growth factor-I receptor (IGF-IR) immunoreactivity was reduced in IGF-IR AS-treated tail arteries. Western immunoblot analysis demonstrated a decrease in cardiac IGF-IR in IGF-IR AS-treated rats; treatment reduced the expression of IGF-IR to 83+/-6% of that in samples from vehicle-treated rats, compared to 99+/-3% for a control, full-mismatch oligonucleotide (MM-18) or 100% (vehicle).IGF-IR AS treatment had no effect on resting blood pressure during the 14-day treatment period. Pressor responses (as measured by increase in systolic arterial pressure) to angiotensin II (AngII) gradually decreased over 2 weeks treatment with IGF-IR AS (5 x 0.5 nmol per intravenous injection, 2 weeks), and were significantly reduced at treatment day 14 compared to day 7 (2.7-fold rightward shift). IGF-IR AS treatment caused a significant rightward shift in the angiotensin II (AngII) dose-response compared to both vehicle and full-mismatch treated rats (4.0-fold shift compared to vehicle, P<0.01, n=6-14). There was a significant decrease in cardiac angiotensin II type 1 receptor (AT(1)R) expression in AS-treated rats compared to vehicle-treated rats; cardiac AT(1)R was decreased to 80+/-6% in comparison to 100%. AT(1)R immunoreactivity was also reduced in IGF-IR AS-treated tail arteries.IGF-IR AS treatment resulted in structural changes in both the heart and aortae, with small but significant differences observed between left ventricle/bodyweight ratios of AS and both vehicle- and MM-18-treated rats (n=8, P<0.05). Aortic cross-sectional areas of AS-treated rats were significantly lower than MM-18- and vehicle-treated rats (27.4+/-5.7% reduction of vehicle-treated samples, n=8, P<0.01). The results of this study suggest that an induced loss of IGF-IR, while not affecting resting blood pressure, has a predominantly inhibitory effect on vascular response to vasoconstrictor agents including angiotensin II. This may occur through downstream effects on AT1R expression, via modulation of the expression of receptors for other vasoactive signalling molecules, or via changes in myocyte proliferation.

Insulin-like growth factor binding protein-5 (IGFBP-5) interacts with thrombospondin-1 to induce negative regulatory effects on IGF-I actions

Insulin-like growth factor binding protein-5 (IGFBP-5) and thrombospondin-1 (TS-1) are both present in extracellular matrix (ECM). Both proteins have been shown to bind to one another with high affinity. The purpose of these studies was to determine how the interaction between IGFBP-5 and TS-1 modulates IGF-I actions in porcine aortic smooth muscle cells (pSMC) in culture. The addition of increasing concentrations of TS-1 to pSMC cultures enhanced the protein synthesis and cell migration responses to IGF-I; whereas the addition of IGFBP-5 alone resulted in minimal changes. In contrast, the addition of IGFBP-5 to cultures that were also exposed to IGF-I and TS-1 resulted in inhibition of protein synthesis. When the cell migration response was assessed, the response to IGF-I plus TS-1 was also significantly inhibited by the addition of IGFBP-5, whereas 1.0 microg/ml of IGFBP-5 alone had no effect on the response to IGF-I. To determine the molecular mechanism by which this inhibition occurred, a mutant form of IGFBP-5 that does not bind to IGF-I was tested. This mutant was equipotent compared to native IGFBP-5 in its ability to inhibit both protein synthesis and cell migration responses to IGF-I plus TS-1 thus excluding the possibility that IGFBP-5 was inhibiting the response to TS-1 and IGF-I by inhibiting IGF-I binding to the IGF-I receptor. To determine if an interaction between TS-1 and IGFBP-5 was the primary determinant of the inhibitory effect of IGFBP-5, an IGFBP-5 mutant that bound poorly to TS-1 was utilized. The addition of 1.0 microg/ml of this mutant did not inhibit the protein synthesis or cell migration responses to IGF-I plus TS-1. To determine the mechanism by which IGFBP-5 binding to TS-1 inhibited cellular responses to TS-1 plus IGF-I, TS-1 binding to integrin associated protein (IAP) was assessed. The addition of IGFBP-5 (1.0 microg/ml) inhibited TS-1-IAP association. In contrast, a mutant form of IGFBP-5 that bound poorly to TS-1 had a minimal effect on TS-1 binding to IAP. Further analysis showed that IGFBP-5 addition altered the ability of TS-1 to modulate the SHPS-1/IAP interaction. When the IGFBP-5 mutant that did not bind to IGF-I was incubated with TS-1 and IGF-I, it inhibited the capacity of TS-1 to enhance the IGF-I receptor phosphorylation and MAP kinase activation in response to IGF-I. In contrast, the IGFBP-5 mutant that did not bind to TS-1 had no effect on IGF-I stimulated IGF-I receptor phosphorylation or MAP kinase activation. These results indicate that IGFBP-5 inhibits the binding of TS-1 to IAP, and this results in an alteration of the ability of TS-1 to modulate the disruption of the IAP/SHPS-1 interaction which leads to attenuation of the ability of TS-1 to enhance cellular responsiveness to IGF-I.

Regulation of vascular smooth muscle cell responses to insulin-like growth factor (IGF)-I by local IGF-binding proteins

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.

Targeted expression of a protease-resistant IGFBP-4 mutant in smooth muscle of transgenic mice results in IGFBP-4 stabilization and smooth muscle hypotrophy

The insulin-like growth factor-binding protein 4 (IGFBP-4), the most abundant IGF-binding protein produced by rodent smooth muscle cells (SMC), is degraded by specific protease(s) potentially releasing IGF-I for local bioactivity. IGFBP-4 protease(s) recognizes basic residues within the midregion of the molecule. We constructed a mutant IGFBP-4 with the cleavage domain substitution 119-KHMAKVRDRSKMK-133 to 119-AAMAAVADASAMA-133. Myc-tagged native and IGFBP-4.7A retained equivalent IGF-I binding affinity. Whereas native IGFBP-4 was cleaved by SMC-conditioned medium, IGFBP-4.7A was completely resistant to proteolysis. To explore the function of the protease-resistant IGFBP-4 in vivo, expression of the mutant and native proteins was targeted to SMC of transgenic mice by means of a smooth muscle alpha-actin promoter. Transgene expression was confined to SMC-rich tissues in all lines. Bladder and aortic immunoreactive IGFBP-4/transgene mRNA ratios in SMP8-BP4.7A mice were increased by 2- to 4-fold relative to SMP8-BP4 mice, indicating that the IGFBP-4.7A protein was stabilized in vivo. SMP8-BP4.7A mice had lower aortic, bladder, and stomach weight and intestinal length relative to SMP8-BP4 counterparts matched for protein expression by Western blotting. Thus, IGFBP-4.7A results in greater growth inhibition than equivalent levels of native IGFBP-4 in vivo, demonstrating a role for IGFBP-4 proteolysis in the regulation of IGF-I action.

Aberrant expression and activation of insulin-like growth factor-1 receptor (IGF-1R) are mediated by an induction of IGF-1R promoter activity and stabilization of IGF-1R mRNA and contributes to growth factor independence and increased survival of the pancreatic cancer cell line MIA PaCa-2

In the present study we investigated the mechanisms responsible for and the biological consequences of the constitutive activation of the insulin-like growth factor-1 receptor (IGF-1R) in the MIA PaCa-2 cells. An aberrant increase in the expression and activation of the IGF-1R was observed during the transition of growth states from exponential to quiescent. The increase in IGF-1R expression is preceded by an increase in IGF-1R mRNA transcript and is associated with an increase in the IGF-1R promoter activity. Inhibition of de novo transcription by actinomycin D increased the stability of IGF-1R mRNA in exponentially growing cells, thereby increasing the expression of IGF-1R to a level similar to that seen in quiescent cells. Increased IGF-1R signaling mediated the growth factor independence of quiescent MIA PaCa-2 cells through the constitutive activation of mitogen-activated protein kinase (MAPK). Exogenous IGF-1 increased cell proliferation and activated MAPK and AKT signaling pathways. The resistance of cells to apoptosis by IGF-1R signaling was mediated through MAPK and phosphatidylinositol 3-kinase (PI3K) pathways and a yet unidentified pathway(s). Thus, aberrant regulation of IGF-1R signaling is required for resistance to apoptosis and growth factor independence of MIA PaCa-2 cells. This likely protects cells from unfavorable conditions and allows cells to rapidly re-enter the cell cycle when conditions are favorable.

IGF-binding protein-4 expression and IGF-binding protein-4 protease activity are regulated coordinately in smooth muscle during postnatal development and after vascular injury

Recent studies support a critical role for the paracrine IGF/IGF-binding protein system in the regulation of vascular smooth muscle cell growth. In this study we have explored the hypothesis that the abundance of individual IGF-binding proteins in smooth muscle is subject to regulation during postnatal life and in response to injury. IGF-binding protein-2 was the predominant binding protein secreted by neonatal rat vascular smooth muscle cells, whereas IGF-binding protein-4 was most prevalent in adult vascular smooth muscle cells coincident with increased IGF-binding protein-4 protease activity. After arterial injury, IGF-binding protein-4 mRNA increased, associated with greater IGF-binding protein-4 proteolytic activity, resulting in stable steady state levels of the IGF-binding protein-4 protein. Expression of pregnancy-associated plasma protein A mRNA, recently identified as an IGF-binding protein-4 protease, was expressed at higher levels in adult than neonatal vascular smooth muscle cell lines, but did not change significantly after arterial injury. The peak of immunoreactive pregnancy-associated plasma protein A from hydrophobic interaction chromatography fractions of smooth muscle cell-conditioned medium coincided, but did not fully overlap, with the fractions containing maximal IGF-binding protein-4 protease activity. In conclusion, our data point to a developmental switch from IGF-binding protein-2 to IGF-binding protein-4 in vascular smooth muscle cells postnatally. Moreover, IGF-binding protein-4 expression is coregulated with IGF-binding protein-4 protease activity, suggesting that biosynthesis and degradation of this binding protein are coordinated events important for regulating biological activity of IGF-I.

Cell density influences insulin-like growth factor I gene expression in a cell type-specific manner

The effect of cellular density on insulin-like growth factor I (IGF-I) gene expression was characterized in several tumor-derived cell lines. IGF-I messenger RNA (mRNA) transcripts increased more than 200-fold when C6 glioma cells grew to postconfluence. IGF-I receptor and beta-actin mRNAs were induced by 6- and 2-fold, respectively, as a function of confluence. IGF-I mRNA transcripts in GH3 and SK-N-MC cells increased about 4- to 5-fold in confluent cultures compared with sparse cultures. In OVCAR-3 cells, the IGF-I mRNA level remained constant as the cell density increased. Transient transfection experiments were performed with IGF-I exon 1 promoter/luciferase fusion constructs in C6 cells. The luciferase activity of a construct containing exon 1 sequence between +75 and +282 (the most 5' transcription initiation site was designated +1) was stimulated by 2.5-fold in dense cultures compared with that in sparse cultures of C6 cells. Luciferase activities of other constructs containing at least 282 bp of exon 1 sequence were also stimulated about 2- to 4-fold by cell density. However, 3' deletion to +192 led to loss of the cell density stimulatory effect. In contrast, luciferase activities of IGF-I promoter constructs were not altered by cell density in SK-N-MC cells. When the conditioned medium of low density C6 cultures was exchanged with that of high density cultures, the IGF-I mRNA level remained the same. In summary, cell density has a cell type- and gene type-specific effect on IGF-I gene expression. A cell density response element(s) may be located between +192 and +282 of the exon 1 promoter region in C6 cells.

Insulin-like growth factor (IGF)-I regulates IGF-binding protein-5 gene expression through the phosphatidylinositol 3-kinase, protein kinase B/Akt, and p70 S6 kinase signaling pathway

Expression of the insulin-like growth factor-binding protein 5 (IGFBP-5) gene in vascular smooth muscle cells is up-regulated by IGF-I through an IGF-I receptor-mediated mechanism. In this study, we studied the possible involvement of the mitogen-activated protein kinase (MAPK) and PI 3-kinase signaling pathways in mediating IGF-I-regulated IGFBP-5 gene expression. The addition of Des(1-3)IGF-I, an IGF analog with reduced affinity to IGFBPs, resulted in a transient activation of p44 and p42 MAPK. Inhibition of the MAPK activation by PD98059, however, did not affect IGF-I-stimulated IGFBP-5 expression. Des(1-3)IGF-I treatment also strongly activated PI 3-kinase. This activation was probably mediated through IRS-1, because IGF-I stimulation resulted in a significant increase in IRS-1- but not IRS-2-associated PI 3-kinase activity. This activation occurred within 5 min and was sustained at high levels for over 6 h. Likewise, Des(1-3)IGF-I caused a long lasting activation of PKB/Akt and p70(s6k). When LY294002 and wortmannin, two specific inhibitors of PI 3-kinase, were added with Des(1-3)IGF-I, the IGF-I-regulated IGFBP-5 expression was negated. The addition of rapamycin, which inhibits IGF-I-induced p70(s6k) activation, significantly inhibited IGF-I-regulated IGFBP-5 gene expression. These results suggest that the action of IGF-I on IGFBP-5 gene expression requires the activation of the PI 3-kinase-PKB/Akt-p70(s6k) pathway but not the MAPK pathway in vascular smooth muscle cells.

Down-regulation of protein kinase C inhibits insulin-like growth factor I-induced vascular smooth muscle cell proliferation, migration, and gene expression

Insulin-like growth factor-I (IGF-I) plays an important role in regulating vascular smooth muscle cell (VSMC) proliferation, directed migration, differentiation, and apoptosis. The signaling mechanisms used by IGF-I to elicit these actions, however, are not well defined. In this study, we examined the role(s) of protein kinase C (PKC) in mediating the IGF-I actions in cultured porcine VSMCs. Out of the eleven known members of PKC family, PKC-alpha, -betaI, -epsilon, -eta, -lambda, -theta, and -zeta, were detectable by Western immunoblot analysis in these cells. Further analysis indicated that the subcellular distribution of several PKC isoforms is regulated by IGF-I. While IGF-I stimulated membrane translocation of PKC-eta, -epsilon, and -zeta and regulated the cytosolic levels of PKC-betaI, it had no such effect on PKC-alpha and -lambda. To examine whether PKC activation is required for the IGF-I-regulated biological responses, phorbol myristate acetate (PMA) and GF109203X were used to down-regulate or inhibit PKC activity. Both PMA (1 microM) and GF109203X (20 microM) nearly completely suppressed the total PKC activity after a 30-min incubation (> 90%), and this inhibition lasted for at least 24 h. Down-regulation or inhibition of PKC activity abolished the IGF-I-induced DNA synthesis, migration and IGFBP-5 gene expression. In contrast, the IGFBP-5 expression induced by forskolin was unaffected by PKC down-regulation or inhibition, suggesting that PKC activation is required for the IGF-regulated but not the cAMP-regulated events. Because the actions of IGF-I on DNA synthesis and IGFBP-5 gene expression in VSMCs have been shown to be mediated through the phosphatidylinositol 3-kinase (PI3 kinase) signaling pathway in porcine VSMCs, the potential role of PKC in IGF-I-induced activation of PI3 kinase and PKB/Akt were examined. Treatment with either PMA or GF109203X did not significantly affect the effects of IGF-I on PI3 kinase activation or PKB/Akt phosphorylation. These results indicated that PKC-betaI, -eta, -epsilon, and -zeta may play an essential role(s) in IGF-I regulation of VSMC migration, DNA synthesis and gene expression, and that these PKC isoforms may either act independently of the PI3 kinase pathway or act further downstream of PKB/Akt in the IGF signaling network.

Interaction of angiotensin II and the insulin-like growth factor system in vascular smooth muscle cells

We studied the effects and interactions of ANG II and the insulin-like growth factor (IGF) system in cultured rat aortic smooth muscle cells. ANG II (1 microM) and IGF-I (10 nM) stimulated both DNA and protein synthesis. The effects of the two peptides in combination were additive or more than additive. The AT(1) receptor antagonist losartan (10 and 100 microM) blocked their synergistic effect on DNA synthesis. IGF binding protein (IGFBP)-1 inhibited the effect of IGF-I but not that of ANG II on DNA synthesis. IGF-I stimulated gene expression of IGFBP-2 and IGFBP-4. ANG II decreased IGF-I, IGFBP-2, and IGFBP-4 transcripts but increased the IGF-I receptor transcript. IGF-I and ANG II in combination had similar effects on gene expression as ANG II alone. The IGFBP-2 and IGFBP-4 peptides could be detected in the conditioned medium. Our results show that ANG II and IGF-I have synergistic effects on vascular smooth muscle cells and that they interact in several ways.

Potential role of insulin-like growth factor binding protein-4 in the uncoupling of bone turnover in multiple myeloma

Decreased bone formation plays an important role in the development of lytic lesions during the late stage of multiple myeloma (MM). Release of insulin-like growth factor binding protein-4 (IGFBP4) by tumour cells adjacent to bone may inhibit IGF-I-stimulated osteoblast growth and contribute to decreased bone formation. The present study demonstrates that the human MM cell line, ARH-77, expresses IGFBP4 and, to a lesser extent, IGFBP6 mRNA and protein. IGFBP4 expression in myeloma cells may be modulated by cytokines released by stromal cells and T cells in the microenvironment. We tested the effect of recombinant interferon-gamma (INF) on IGFBP4 expression in ARH-77. INF increased IGFBP4 mRNA and protein levels at 12 h, with a decline to baseline by 24 h. In contrast, IGFBP4 was not regulated in response to IL-6, TNF-alpha, PDGF BB, bFGF, TGF-beta or the cAMP agonist, forskolin. In other systems. IGFBP4 may also be regulated post-transcriptionally by a protease that is activated by IGF-I or -II. Conditioned medium from ARH-77 cultures incubated with IGF-I or -II for up to 24 h failed to demonstrate proteolytic activity. Proteolysis was also not observed when conditioned medium containing exogenous rhIGFBP4 was incubated with IGF-I or -II under cell-free conditions. To determine if human myeloma tumours also express IGFBP4, total RNA was isolated from four tumour biopsies. All samples expressed detectable levels of IGFBP4 mRNA. These findings indicate that interferon-gamma may indirectly modulate bone formation via the the release of tumour-derived IGFBP4. suggesting that the immune system may influence bone turnover in MM. Failure of myeloma cells to release protease activity may promote IGFBP4 accumulation in the microenvironment during tumour growth.

Elevated levels of the IGF-binding protein protease MMP-1 in asthmatic airway smooth muscle

We have previously demonstrated that the asthma-associated proinflammatory eicosanoid leukotriene D4 (LTD4) is co-mitogenic with insulin-like growth factors (IGFs) in airway smooth-muscle (ASM) cells in vitro. This synergistic effect of LTD4 and IGF on ASM cell growth involves proteolysis of ASM-produced IGF binding proteins (IGFBPs), which are cell growth-inhibitory proteins. We also identified this IGFBP protease to be the matrix metalloproteinase-1 (MMP-1), and showed that this enzyme had a significant role in modulating IGF action in ASM cells. In the present study, we tested the hypothesis that ASM hyperplasia in vivo involves induction of MMP-1 leading to IGFBP proteolysis. We detected the presence of MMP-1 and measured its levels in human airway tissue sections prepared from nonasthmatic and asthmatic subjects. Six nonasthmatic and six asthmatic airway tissue samples were analyzed for immunoreactive MMP-1 through an immunohistochemical detection method. Both the bronchial and tracheal smooth-muscle cells from different regions of the same sample were examined and documented. The immunostaining for MMP-1 was significantly elevated in both the bronchial and tracheal smooth-muscle cells of the airway sections from asthmatic samples relative to that of the nonasthmatic samples. The differences in levels of MMP-1, IGFBP-2, IGFBP-3, and IGFBP proteolytic activity were quantified using densitometric analyses of the ASM tissue extracts that were separated on sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The MMP-1 levels in the asthmatic airway tissue extracts were 12-fold higher than those found in control samples. In addition, IGFBP-2 and IGFBP-3, which we have previously demonstrated to be proteolytic substrates of MMP-1, were found to be cleaved in asthmatic airway tissue extracts. Furthermore, the asthmatic airway extracts contained IGFBP proteolytic activity that was shown by immunodepletion studies to be due to MMP-1. These observations demonstrate that MMP-1 may play a significant role in inducing ASM hyperplasia and airway obstruction in asthma by modulating the IGF axis.

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.

Overexpression of insulin-like growth factor-binding protein-4 (IGFBP-4) in smooth muscle cells of transgenic mice through a smooth muscle alpha-actin-IGFBP-4 fusion gene induces smooth muscle hypoplasia

Insulin-like growth factor I (IGF-I) has been postulated to function as a smooth muscle cell (SMC) mitogen and to play a role in the pathogenesis of bladder hypertrophy, estrogen-induced uterine growth, and restenosis after arterial angioplasty. IGF-binding protein-4 (IGFBP-4) inhibits IGF-I action in vitro and is the most abundant IGFBP in the rodent arterial wall. To explore the function of this binding protein in vivo, transgenic mouse lines were developed harboring fusion genes consisting of a rat IGFBP-4 complementary DNA cloned downstream of either a -724 bp fragment of the mouse smooth muscle alpha-actin 5'-flanking region (SMP2-BP-4) or -1074 bp, 63 bp of 5'-untranslated region, and 2.5 kb of intron 1 of smooth muscle alpha-actin (SMP8-BP-4). SMP2-BP-4 mice expressed low levels of the exogenous IGFBP-4 messenger RNA (mRNA), which was not specifically targeted to SMC-rich tissue environments, and were therefore not analyzed further. Six SMP8-BP-4 transgenic lines derived from separate founders were characterized. Mating of hemizygous SMP8-BP-4 mice with controls produced about 50% transgenic offspring, with equal sex distribution. Expression of IGFBP-4 mRNA in nontransgenic littermates was maximal in liver and kidney. By contrast, transgenic IGFBP-4 mRNA expression, distinguished because of a smaller transcript size, was confined to SMC-containing tissues, with the following hierarchy: bladder > aorta > stomach = uterus. There was no transgene expression in skeletal muscle, brain, or cardiac myocytes. The abundance of IGFBP-4 measured by Western ligand blotting or by immunoblotting, was 8- to 10-fold higher in aorta and bladder of SMP8-BP-4 mice than in their nontransgenic littermates, with no change in plasma IGFBP-4 levels. Transgenic mice exhibited a significant reduction in wet weight of SMC-rich tissues, including bladder, intestine, aorta, uterus, and stomach, with no change in total body or carcass weight. In situ hybridization showed that transgene expression was targeted exclusively to the muscular layers of the arteries, veins, bladder, ureter, stomach, intestine, and uterus. Overexpression of IGFBP-4 was associated with SMC hypoplasia, a reciprocal phenotype to that of transgenic mice overexpressing IGF-I under control of the same promoter (SMP8-IGF-I). Double transgenic mice derived from mating SMP8-BP-4 with SMP8-IGF-I animals showed a modest decrease in wet weight at selected SMC tissues. Although we cannot exclude that the effects of IGFBP-4 may be IGF independent, these data suggest that IGFBP-4 is a functional antagonist of IGF-I action on SMC in vivo.

Exposure of vascular allografts to insulin-like growth factor-I (IGF-I) increases vascular expression of IGF-I ligand and receptor protein and accelerates arteriosclerosis in rats

BACKGROUND

Accelerated arteriosclerosis limits the survival of transplanted hearts. We hypothesized that insulin-like growth factor-I (IGF-I) is crucial in accelerating transplant arteriosclerosis. Recently, we reported that exposure to IGF-I prior to transplantation accelerates transplant arteriosclerosis in the rat aorta allograft model. Here, we studied the mechanism whereby IGF-I exposure accelerates transplant arteriosclerosis.

METHODS

The abdominal aorta was harvested from male Brown Norway rats and exposed to 0, 200, or 500 ng/ml of IGF-I at 37 degrees C for 30 min prior to transplantation to the abdominal position of male Lewis rats. The allografts were harvested 14 days later and processed for immunohistochemical staining for alpha-actin, growth factors (IGF-I, IGF-I receptor, platelet-derived growth factor-BB, and basic fibroblast growth factor), and immunological markers (major histocompatibility complex class II antigen, macrophage, and CD4- and CD8-positive T cells).

RESULTS

By 14 days, the ex vivo IGF-I donor aorta treatment with IGF-I increased in a concentration-dependent manner the expression of IGF-I and IGF-I receptor in both the intima and the adventitia. In contrast, the expression of platelet-derived growth factor-BB was decreased in a concentration-dependent manner in the intima while basic fibroblast growth factor remained unchanged. The cell-mediated immune response was not affected by IGF-I at 14 days after transplantation, which suggests that the immune events associated with acceleration of transplant arteriosclerosis may occur at an earlier time.

CONCLUSION

Acceleration of transplant arteriosclerosis by exposure to IGF-I is associated with increased IGF-I ligand and receptor expression in the allograft vascular wall. These data further suggest that IGF-I may be a major factor in mediating graft arteriosclerosis.

Increased insulin-like growth factor and platelet-derived growth factor system in the pyloric muscle in infantile hypertrophic pyloric stenosis

BACKGROUND

Infantile hypertrophic pyloric stenosis (IHPS) is characterized by hypertrophy of the pyloric muscle. The etiology of IHPS is unknown. The growth of smooth muscle cells (SMC) is regulated by several growth factors. Insulin-like growth factor (IGF) and platelet-derived growth factor (PDGF) act synergistically to stimulate SMC proliferation. The effects of IGF-I and PDGF are mediated via their receptors.

METHODS

Full-thickness muscle biopsy specimens were obtained from eight IHPS patients (age range, 14 to 46 days) at pyloromyotomy and from eight age-matched controls without gastrointestinal disease at autopsy performed within 4 hours after death. Indirect three-step immunohistochemistry was performed using anti-IGF-I, IGF-I receptor alpha (IGF-IR alpha), IGF-IR beta, PDGF-BB and PDGF receptor (PDGF-R) antibodies and visualized by peroxidase staining.

RESULTS

The most striking difference between tissues from IHPS patients and controls was the marked increase in IGF-I, IGF-IR alpha, IGF-IR beta and PDGF-R in the hypertrophic circular muscle layer, and, to a lesser degree, in the longitudinal muscle in pyloric stenosis.

CONCLUSION

The findings suggest that the upregulated local IGF and PDGF systems may play a role in the development of pyloric muscle hypertrophy in IHPS.

IGF-binding protein-2 is induced during development of urinary bladder hypertrophy in the diabetic rat

Because the locally produced insulin-like growth factor-binding proteins (IGFBP) may influence bladder hypertrophy, either directly or by their interaction with insulin-like growth factor I (IGF-I), we studied the IGF system during the development of urinary bladder hypertrophy in rats with streptozotocin-induced diabetes. Messenger RNA for IGF-I, IGFBP-2, and IGFBP-4 was determined by solution hybridization. The bladder wet weight was elevated after 7 days. DNA synthesis was increased and peaked at 2 days, whereas DNA content per bladder wet weight was decreased by 7 days. The IGF-I mRNA did not change during the first 7 days and then decreased, and IGFBP-4 mRNA was increased transiently on day 7. On the other hand, IGFBP-2 mRNA was significantly increased after 1 day (2-fold), peaked by 7 days (6.4-fold), and then declined to approximately 50% above control at the end of experiment. This was associated with an increased IGFBP-2 protein content. Our results suggest that both stretching of the bladder due to diuresis and the diabetic state contribute to changes of the IGF system in the hypertrophying bladder.

Influence of PDGF-BB on proliferation and transition through the MyoD-myogenin-MEF2A expression program during myogenesis in mouse C2 myoblasts

We have previously demonstrated that PDGF-BB enhances proliferation of C2 myoblasts. This has led us to examine whether the mitogenic influence of PDGF-BB in the C2 model correlates with modulation of specific steps associated with myogenic differentiation. C2 myoblasts transiting through these differentiation specific steps were monitored via immunocytochemistry. We show that the influence of PDGF on enhancing cell proliferation correlates with a delay in the emergence of cells positive for sarcomeric myosin. We further monitored the influence of PDGF-BB on differentiation steps preceding the emergence of myosin+ cells. We demonstrate that mononucleated C2 cells first express MyoD (MyoD+/myogenin- cells) and subsequently, myogenin. Cells negative for both MyoD and myogenin (the phenotype preceding the MyoD+ state) were present at all times in culture and comprised the majority, if not all, of the cells which responded mitogenically to PDGF. Additionally, the frequency of the MyoD+/myogenin+ cell phenotype was reduced in cultures receiving PDGF, suggesting that PDGF can modulate the transition of the cells into the myogenin+ state. We determined that many of the myogenin+ cells subsequently become MEF2A+ and this phenomenon is not influenced by PDGF-BB. FGF-2 also enhanced the proliferation of C2 myoblasts and suppressed the appearance of the myogenin+ cells, but did not influence the subsequent transition into the MEF2A+ state. The study raises the possibility that PDGF-BB and FGF-2 might delay the transition of the C2 cells into the MyoD+/myogenin+ state by depressing a paracrine signal that enhances differentiation.

Differentiated properties and proliferation of arterial smooth muscle cells in culture

The smooth muscle cell is the sole cell type normally found in the media of mammalian arteries. In the adult, it is a terminally differentiated cell that expresses cytoskeletal marker proteins like smooth muscle alpha-actin and smooth muscle myosin heavy chains, and contracts in response to chemical and mechanical stimuli. However, it is able to revert to a proliferative and secretory active state equivalent to that seen during vasculogenesis in the fetus, and this is a prerequisite for the involvement of the smooth muscle cell in the formation of atherosclerotic and restenotic lesions. A similar transition from a contractile to a synthetic phenotype occurs when smooth muscle cells are established in culture. Accordingly, an in vitro system has been used extensively to study the regulation of differentiated properties and proliferation of these cells. During the first few days after seeding, the cells are reorganized structurally with a loss of myofilaments and formation of a widespread endoplasmic reticulum and a prominent Golgi complex. In parallel, they lose their contractility and instead become competent to divide in response to a large variety of mitogens, including platelet-derived growth factor (PDGF) and basic fibroblast growth factor (bFGF). After entering the cell cycle, they start to produce these and other mitogens on their own, and continue to replicate in the absence of exogenous stimuli for a restricted number of generations. Furthermore, they start to secrete extracellular matrix components such as collagen, elastin, and proteoglycans. The mechanisms that control this change in morphology and function of the smooth muscle cells are still poorly understood. Adhesive proteins such as fibronectin and laminin apparently have an important role in determining the basic phenotypic state of the cells and exert their effects via integrin receptors. The proliferative and secretory activities of the cells are influenced by a multitude of growth factors, cytokines, and other molecules. Although much work remains before an integrated view of this regulatory machinery can be achieved, there is no doubt that the cell culture technique has contributed substantially to our knowledge of smooth muscle differentiation and growth. At the same time, it has been crucial in exploring the role of these cells in vascular disease and developing new therapeutic strategies to cope with major causes of human death and disability.

The insulin-like growth factor system in vascular smooth muscle: interaction with insulin and growth factors

Vascular smooth muscle cells (SMCs) occur throughout the vascular tree and have important physiological functions. They are also involved in pathological processes such as development and progression of atherosclerotic lesions, restenosis following angioplasty, and in hypertension. This review is focused on the role of the insulin-like growth factor (IGF) system in proliferation, migration, and hypertrophy of vascular SMCs and its interaction with insulin and other growth factors. The IGF-I receptor is highly expressed in SMCs in intact arteries and in cultured SMCs and is activated by binding of IGF-I to the two alpha-subunits. Insulin and IGF-II from the circulation can interact with the IGF-I receptor at higher concentrations. Insulin receptors are few or absent in SMCs and circulating insulin concentrations in vivo are probably too low for a direct action of insulin on the IGF-I receptor in SMCs. Receptor activation initiates a number of signal transduction pathways. Increased phosphatidylinositol turnover and calcium mobilization correlates with actin filament reorganization and stimulation of directed migration of the SMC in a gradient of IGF-I. The effects of IGF-I receptor activation on signal transduction pathways (eg, the MAP kinase cascade) implicated in DNA synthesis and proliferation are weak and this correlates with the meager mitogenic activity of IGF-I in SMC. Several components of the IGF-system in SMC are regulated by growth factors such as platelet-derived growth factor (PDGF)-BB and basic fibroblast growth factor (bFGF).(ABSTRACT TRUNCATED AT 250 WORDS)

Differentiation of MC3T3-E1 osteoblasts is associated with temporal changes in the expression of IGF-I and IGFBPs

We examined the relationship between osteoblast maturation and temporal changes in the secretion of IGF-I and the IGF-binding proteins (IGFBPs) in the MC3T3-E1 model of osteoblast development. IGF-I was present at low levels in conditioned media in proliferating preosteoblasts (3.7 +/- 1.7 ng/micrograms DNA and 3.9 +/- 0.6 at culture (days 3 and 9) and increased progressively in postmitotic differentiating osteoblasts, reaching a maximal concentration of 13.1 +/- 1.5 ng/micrograms DNA by day 25 of culture. We also observed an increase in IGF-I mRNA expression. Using Western ligand blot and immunoblot techniques, we found that IGFBP-2, -4, and -5 also displayed temporal differences in expression during MC3T3-E1 development. We observed a sustained increase in IGFBP-2, -4, and -5 mRNA expression between days 10-14, coincident with the onset of differentiation. IGFBP-2 and IGFBP-4 protein concentrations increased in parallel with IGFBP mRNA expression, but IGFBP-5 levels peaked between days 8-14 of culture, and declined thereafter in spite of persistent IGFBP-5 mRNA levels. These findings suggest complex transcriptional and post-transcriptional regulation of IGFBP metabolism during osteoblast development. Thus, IGF-I and IGFBP production are regulated during osteoblast development. In turn, time-dependent changes in IGF-I and modulation of IGF-I bioavailability by IGFBPs may regulate the osteoblastic developmental sequence.

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)

c-myb affects intracellular calcium handling in vascular smooth muscle cells

The protooncogene c-myb is responsible for elevating intracellular calcium concentration ([Ca2+]i) at the G1/S interface in vascular smooth muscle cells (VSMC). However, the molecular components of this pathway are undefined, and the biological effects of increased levels of divalent cation are unknown. We have demonstrated that growth-arrested c-myb-transfected VSMC, compared with wild type VSMC, exhibit a fourfold increased number of insulin-like growth factor I (IGF-I) receptors, increased amount of secreted IGF-I activity, and a twofold increased level of [Ca2+]. The c-myb transfected cells, compared with wild type cells, also possess a twofold increased rate of calcium influx and a twofold decreased rate of calcium efflux. The elevated calcium influx rate of transfected cells is decreased to that of wild type cells with IGF-I neutralizing antibody, whereas the decreased calcium efflux rate of transfected cells is increased to that of wild type cells with antisense c-myb oligonucleotides. Proliferating wild type VSMC exhibit an increased calcium influx rate in late G1, which is dependent on production of augmented amounts of IGF-I activity but not increased levels of IGF-I receptors. The wild type VSMC also show a decreased calcium efflux rate at the same point in the cell cycle, which is dependent on expression of c-myb. The treatment of wild type cells with antisense c-myb or IGF-I receptor oligonucleotides induces a late G1 block in cell proliferation, which can be overcome by exposure to the calcium ionophore, 4-bromo-A-27318, in amounts sufficient to raise [Ca2+]i to levels observed at the G1/S interface. We conclude that IGF-I/IGF-I receptors and c-myb are involved in control of [Ca2+]i at the G1/S interface by separately regulating the rates of calcium influx and efflux and that elevated levels of divalent cation are necessary for progression of VSMC into the S phase of the cell cycle.

Fibroblast growth factor regulates insulin-like growth factor-binding protein production by vascular smooth muscle cells

Insulin-like growth factor I is an important mitogen for vascular smooth muscle cells, and its effects are regulated by several binding proteins. Western ligand blotting of conditioned medium from rat aortic smooth muscle cells detected a 24 kDa binding protein and a 28 kDa glycosylated variant of this protein, consistent with insulin-like growth factor binding protein-4 by size. Low amounts of a glycosylated 38 to 42 kDa doublet (consistent with binding protein-3) and a 31 kDa non-glycosylated protein also were present. Basic fibroblast growth factor markedly increased secretion of the 24 kDa binding protein and its 28 kDa glycosylated variant. This effect was dose- and time-dependent and was inhibited by co-incubation with cycloheximide. Crosslinking of [125I]-insulin-like growth factor I to cell monolayers revealed no surface-associated binding proteins, either basally or after agonist treatment. Induction of binding protein production by fibroblast growth factor at sites of vascular injury may be important in vascular proliferative responses in vivo.

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.

Vascular smooth muscle cells synthesize two forms of insulin-like growth factor binding proteins which are regulated differently by the insulin-like growth factors

Vascular smooth muscle cells (SMC) synthesize insulin-like growth factor-I (IGF-I), which is a mitogen for this cell type in vitro. Since IGF binding proteins (IGFBP) modulate IGF bioactivity, we determined which IGFBPs were secreted by porcine SMC. Porcine SMC secreted 34,000 and 24,000 M(r) forms of IGFBPs which were identified as IGFBP-2 and IGFBP-4, respectively, by immunoblotting. Northern blot analysis showed single transcripts of 1.6 kb and 2.4 kb for IGFBP-2 and IGFBP-4, respectively. Secretion of IGFBP-2 was not regulated to a significant degree, with insulin, IGF-II, IGF-I, forskolin, and dibutyryl cyclic adenosine monophosphate (cAMP) inducing minimal changes in IGFBP-2 secretion of less than 30% by radioimmunoassay (RIA). Insulin increased (2.8 +/- 0.1-fold) the abundance of IGFBP-4 protein in conditioned media (CM) and increased IGFBP-4 mRNA levels. Growth factors for SMC such as platelet-derived growth factor (PDGF), fibroblast growth factor (FGF), epidermal growth factor (EGF), and transforming growth factor beta-1 (TGF beta-1) were without effect on either IGFBP-2 or -4. IGF-I treatment decreased the amount of IGFBP-4 present in CM, but a corresponding decrease in IGFBP-4 mRNA levels was not observed. In order to determine if IGFBP-4 could modulate IGF-I bioactivity, IGFBP-4 was added to pSMCs with and without IGF-I. IGF-I alone (20 ng/ml) induced a 1.6 to threefold increase in 3H-thymidine incorporation. Addition of IGFBP-4 (between 50 and 250 ng/ml) to cultures containing IGF-I (20 ng/ml) had no effect on DNA synthesis compared to that observed with IGF-I alone, while 500 ng/ml consistently caused a small decrease (15 +/- 5%; mean +/- SE). Immunoblotting of the CM obtained at the end of the 3H-thymidine assay showed a loss of intact IGFBP-4 in the cultures containing IGF-I. This corresponded with an increase in the abundance of a 16,000 M(r) immunoreactive fragment that did not bind IGF-I. Coincubation with insulin had no effect on the amount of IGFBP-4 that was converted to fragment, suggesting that the reaction was dependent upon IGF-I binding to IGFBP-4. In contrast, addition of IGFBP-4 (500 ng/ml) to human fibroblast cultures with IGF-I (20 ng/ml) almost completely inhibited the stimulatory effect of IGF-I on DNA synthesis and no increase in fragment was detected in the CM. In summary, SMC secrete IGFBP-2 and IGFBP-4, both of which have been shown to regulate IGF-mediated DNA synthesis.(ABSTRACT TRUNCATED AT 400 WORDS)