The chemotactic and mitogenic responses of vascular smooth muscle cells to insulin-like growth factor-I require the activation of ERK1/2

Microvascular smooth muscle cells exhibit divergent phenotypic switching responses to platelet-derived growth factor and insulin-like growth factor 1

OBJECTIVE

Vascular smooth muscle cell (VSMC) phenotypic switching is critical for normal vessel formation, vascular stability, and healthy brain aging. Phenotypic switching is regulated by mediators including platelet derived growth factor (PDGF)-BB, insulin-like growth factor (IGF-1), as well as transforming growth factor-β (TGF-β) and endothelin-1 (ET-1), but much about the role of these factors in microvascular VSMCs remains unclear.

METHODS

We used primary rat microvascular VSMCs to explore PDGF-BB- and IGF-1-induced phenotypic switching.

RESULTS

PDGF-BB induced an early proliferative response, followed by formation of polarized leader cells and rapid, directionally coordinated migration. In contrast, IGF-1 induced cell hypertrophy, and only a small degree of migration by unpolarized cells. TGF-β and ET-1 selectively inhibit PDGF-BB-induced VSMC migration primarily by repressing migratory polarization and formation of leader cells. Contractile genes were downregulated by both growth factors, while other genes were differentially regulated by PDGF-BB and IGF-1.

CONCLUSIONS

These studies indicate that PDGF-BB and IGF-1 stimulate different types of microvascular VSMC phenotypic switching characterized by different modes of cell migration. Our studies are consistent with a chronic vasoprotective role for IGF-1 in VSMCs in the microvasculature while PDGF is more involved in VSMC proliferation and migration in response to acute activities such as neovascularization. Better understanding of the nuances of the phenotypic switching induced by these growth factors is important for our understanding of a variety of microvascular diseases.

Protein kinase B/AKT mediates insulin-like growth factor 1-induced phosphorylation and nuclear export of histone deacetylase 5 via NADPH oxidase 4 activation in vascular smooth muscle cells

Insulin-like growth factor 1 (IGF-1) mediates the generation of reactive oxygen species (ROS) and the activation of growth promoting signaling pathways. Histone deacetylases (HDACs) regulate gene transcription by deacetylating lysine residues in histone and nonhistone proteins and a heightened HDAC activation, notably of HDAC5, is associated with vascular disorders, such as atherosclerosis. Although the contribution of IGF-1 in these pathologies is well documented, its role in HDAC phosphorylation and activation remains unexplored. Here, we examined the effect of IGF-1 on HDAC5 phosphorylation in vascular smooth muscle cells (VSMCs) and identified the signaling pathways involved in controlling HDAC5 phosphorylation and nuclear export. Treatment of A10 VSMCs with IGF-1 enhanced HDAC5 phosphorylation. Blockade of the IGF-1 receptor tyrosine kinase (TK) activity with the specific pharmacological inhibitor, AG1024, significantly inhibited IGF-1-induced HDAC5 phosphorylation, whereas the epidermal growth factor receptor (EGFR) TK antagonist, AG1478, had no effect. Inhibition of the mitogen-activated protein kinase pathway with U0126, SP600125, or SB203580, did not affect HDAC5 phosphorylation, whereas two inhibitors of the phosphoinositide 3-kinase (PI3K)/AKT pathways, wortmannin and SC66, almost completely attenuated IGF-1-induced responses as confirmed by immunoblotting of phospho-HDAC5 and by small interfering RNA (siRNA)-induced AKT silencing. Moreover, the NAD(P)H oxidase (Nox) inhibitor, diphenyleneiodonium (DPI), and Nox4 siRNA, attenuated IGF-1-induced phosphorylation of HDAC5 and AKT. The HDAC5 phosphorylation resulted in its nuclear export, which was reversed by SC66 and DPI. Our results indicate that IGF-1-induced phosphorylation and nuclear export of HDAC5 involve Nox4-dependent ROS generation and PI3K/AKT signaling pathways.

IGF-I regulates HT1080 fibrosarcoma cell migration through a syndecan-2/Erk/ezrin signaling axis

Fibrosarcoma is a tumor of mesenchymal origin, originating from fibroblasts. IGF-I is an anabolic growth factor which exhibits significant involvement in cancer progression. In this study, we investigated the possible participation of syndecan-2 (SDC-2), a cell membrane heparan sulfate (HS) proteoglycan on IGF-I dependent fibrosarcoma cell motility. Our results demonstrate that SDC-2-deficient HT1080 cells exhibit attenuated IGF-I-dependent chemotactic migration (p < 0.001). SDC-2 was found to co-localize to IGF-I receptor (IGF-IR) in a manner dependent on IGF-I activity (P ≤ 0.01). In parallel, the downregulation of SDC-2 significantly inhibited both basal and due to IGF-I action ERK1/2 activation, (p < 0.001). The phosphorylation levels of ezrin (Thr567), which is suggested to act as a signaling bridge between the cellular membrane receptors and actin cytoskeleton, were strongly enhanced by IGF-I at both 1h and 24h (p < 0.05; p < 0.01). The formation of an immunoprecipitative complex revealed an association between SDC2 and ezrin which was enhanced through IGF-I action (p < 0.05). Immunoflourescence demonstrated a co-localization of IGF-IR, SDC2 and ezrin upregulated by IGF-I action. IGF-I enhanced actin polymerization and ezrin/actin specific localization to cell membranes. Finally, treatment with IGF-I strongly increased SDC2 expression at both the mRNA and protein level (p < 0.001). Therefore, we propose a novel SDC2-dependent mechanism, where SDC2 is co-localized with IGF-IR and enhances its' IGFI-dependent downstream signaling. SDC2 mediates directly IGFI-induced ERK1/2 activation, it recruits ezrin, contributes to actin polymerization and ezrin/actin specific localization to cell membranes, ultimately facilitating the progression of IGFI-dependent fibrosarcoma cell migration.

The role of insulin-like growth factor 1 and its receptor in the formation and development of colorectal carcinoma

OBJECTIVE

To investigate the role of insulin-like growth factor (IGF)-1 and its receptor (IGF1R) in the formation and development of colorectal carcinoma.

METHODS

Colorectal tissue and matching serum samples were collected from patients with adenomatous polyps or carcinoma and healthy control subjects. IGF1R mRNA levels were determined via quantitative real-time reverse transcription-polymerase chain reaction. Serum IGF1 was quantified using enzyme-linked immunosorbent assay.

RESULTS

Serum IGF1 concentrations and mucosal IGF1R mRNA levels were significantly higher in patients with adenomatous polyps (n = 24) or carcinoma (n = 13) compared with healthy control subjects (n = 13). There was a significant positive correlation between serum IGF1 and mucosal IGF1R mRNA in patients with adenomatous polyps.

CONCLUSIONS

High circulating IGF1 concentrations and mucosal IGF1R expression may play important roles in both the formation and development of colorectal carcinoma. IGF1 and its receptor may be activated before carcinogenesis, and may promote the growth and malignant transformation of adenomatous polyps. IGF1 and IGF1R may be useful biomarkers for evaluating the stage and risk of carcinogenesis.

Drug discovery for improvement of chronic kidney disease and cardiovascular disease

Chronic kidney disease (CKD) has been increasingly recognized as a major public health problem in the world. Recent studies have showed that CKD is an independent risk factor for the occurrence of cardiovascular disease (CVD). Reactive oxygen species (ROS), generated by reduction-oxidation actions, have been generated by reduction-oxidation actions, recognized as the important chemical mediators that regulate signal transduction in various cells including vascular smooth muscle cells (VSMC) and mesangial cells (MC). It has been showed that increase in ROS generation may relate to a risk for CVD and CKD. In addition, ROS mediate activation of mitogen-activated protein (MAP) kinases, extracellular signal-regulated kinase 1/2, c-Jun N-terminal kinase, p38, and big MAP kinase 1, in various cells leading to change in gene expressions. Control of the oxidative stress and ROS-mediated alterations of signaling molecules including MAP kinases may provide new therapeutic strategy against CKD and CVD. In this review, we summarize mainly our data regarding the pharmacological effects of renin-angiotensin-aldosterone system blockers, bioflavonoids and adiponectin in VSMC and MC. Also we review the data on a possible new class drug against oxidative stress to improve CKD and CVD.

IGF-1 and atherothrombosis: relevance to pathophysiology and therapy

IGF-1 (insulin-like growth factor-1) plays a unique role in the cell protection of multiple systems, where its fine-tuned signal transduction helps to preserve tissues from hypoxia, ischaemia and oxidative stress, thus mediating functional homoeostatic adjustments. In contrast, its deprivation results in apoptosis and dysfunction. Many prospective epidemiological surveys have associated low IGF-1 levels with late mortality, MI (myocardial infarction), HF (heart failure) and diabetes. Interventional studies suggest that IGF-1 has anti-atherogenic actions, owing to its multifaceted impact on cardiovascular risk factors and diseases. The metabolic ability of IGF-1 in coupling vasodilation with improved function plays a key role in these actions. The endothelial-protective, anti-platelet and anti-thrombotic activities of IGF-1 exert critical effects in preventing both vascular damage and mechanisms that lead to unstable coronary plaques and syndromes. The pro-survival and anti-inflammatory short-term properties of IGF-1 appear to reduce infarct size and improve LV (left ventricular) remodelling after MI. An immune-modulatory ability, which is able to suppress 'friendly fire' and autoreactivity, is a proposed important additional mechanism explaining the anti-thrombotic and anti-remodelling activities of IGF-1. The concern of cancer risk raised by long-term therapy with IGF-1, however, deserves further study. In the present review, we discuss the large body of published evidence and review data on rhIGF-1 (recombinant human IGF-1) administration in cardiovascular disease and diabetes, with a focus on dosage and safety issues. Perhaps the time has come for the regenerative properties of IGF-1 to be assessed as a new pharmacological tool in cardiovascular medicine.

Changes in proteomic features induced by insulin on vascular smooth muscle cells from spontaneous hypertensive rats in vitro

Hyperinsulinemia is a risk factor in atherosclerosis formation that it stimulated vascular smooth muscle cells (VSMCs) proliferation and migration. To understand the underlying molecular mechanism involved in the processes of cellular response to insulin, VSMCs from Wistar-Kyoto rat (WKY) and spontaneous hypertensive rat (SHR) were isolated and cultured, and its proteome was comparatively analyzed with normal control by two-dimensional gel electrophoresis (2-DE). Results showed that the proliferation of VSMCs from SHR be more sensitive to insulin stimulation than that VSMCs from WKY. The detectable spots ranged from 537 to 608 on the gels in VSMCs of SHR, and 413 ± 31 spots in VSMCs of WKY. The different expressed protein spots in VSMCs of SHR were then isolated and measured by matrix-assisted desorption/ionization time of flight mass spectrometry (MALDI-TOF-MS). A total of 18 spots showed a sharp clear spectrum, and 13 spots matched with the known proteins from database. These proteins were mainly involved in cytoskeleton, glycometabolism, and post-translational processes. Among these proteins, OPN and matrix gla protein were up-regulated expression proteins, while α-SM actin was down-regulated. Furthermore, these preliminarily identified proteins confirmed by RT-PCR and western blotting analysis were coincident with the changes in 2-DE check. In addition, the cytoskeleton changes and migration rate of VSMCs from SHR treated by insulin increased significantly. The results showed that insulin plays a crucial role in activating proliferation and migration of VSMCs, by regulating the phenotype switch of VSMCs.

Therapeutic potential for protein kinase C inhibitor in vascular restenosis

Vascular restenosis, an overreaction of biological response to injury, is initialized by thrombosis and inflammation. This response is characterized by increased smooth muscle cell migration and proliferation. Available pharmacological treatments include anticoagulants, antiplatelet agents, immunosuppressants, and antiproliferation agents. Protein kinase C (PKC), a large family of serine/threonine kinases, has been shown to participate in various pathological stages of restenosis. Consequently, PKC inhibitors are expected to exert a wide range of pharmacological activities therapeutically beneficial for restenosis. In this review, the roles of PKC isozymes in platelets, leukocytes, endothelial cells, and smooth muscle cells are discussed, with emphasis given to smooth muscle cells. We will describe cellular and animal studies assessing prevention of restenosis with PKC inhibitors, particularly targeting -α, -β, -δ, and -ζ isozymes. The delivery strategy, efficacy, and safety of such PKC regulators will also be discussed.

The insulin-like growth factor family: molecular mechanisms, redox regulation, and clinical implications

Insulin-like growth factor (IGF)-induced signaling networks are vital in modulating multiple fundamental cellular processes, such as cell growth, survival, proliferation, and differentiation. Aberrations in the generation or action of IGF have been suggested to play an important role in several pathological conditions, including metabolic disorders, neurodegenerative diseases, and multiple types of cancer. Yet the exact mechanism involved in the pathogenesis of these diseases by IGFs remains obscure. Redox pathways involving reactive oxygen species (ROS) and reactive nitrogen species (RNS) contribute to the pathogenetic mechanism of various diseases by modifying key signaling pathways involved in cell growth, proliferation, survival, and apoptosis. Furthermore, ROS and RNS have been demonstrated to alter IGF production and/or action, and vice versa, and thereby have the ability to modulate cellular functions, leading to clinical manifestations of diseases. In this review, we provide an overview on the IGF system and discuss the potential role of IGF-1/IGF-1 receptor and redox pathways in the pathophysiology of several diseases.

Adiponectin inhibits insulin-like growth factor-1-induced cell migration by the suppression of extracellular signal-regulated kinase 1/2 activation, but not Akt in vascular smooth muscle cells

Adiponectin, an adipocyte-derived hormone, has been proposed to show antiatherogenic properties through the inhibitory effects against various growth factors. Insulin-like growth factor-1 (IGF-1) is one of the potent mitogens, which has been considered to play important roles in both atherogenesis and plaque stabilization in accordance to the phase of atherosclerosis. The aim of this study is to elucidate the adiponectin effects on IGF-1-induced cell migration and its intracellular signaling pathways in vascular smooth muscle cells (VSMCs). In this study, we assessed cell migration and several kinase activities in cultured rat aortic smooth muscle cells (RASMCs). Adiponectin pretreatment suppressed IGF-1-induced cell migration and extracellular signal-regulated kinase (ERK)1/2 activation, which is one of the major mediators for IGF-1-induced cell migration. In RASMCs, adiponectin and 5-aminoimidazole-4-carboxamide riboside (AICAR), a 5'-AMP-activated protein kinase (AMPK) activator, stimulated AMPK activation. AMPK activation by AICAR inhibited IGF-1-induced ERK1/2 activation and cell migration in RASMCs. On the other hand, phosphorylation of Akt and Bad, proapoptotic molecules of the Bcl-2 family, which were increased by IGF-1 stimulation, was not diminished by the pretreatment with adiponectin. It was shown that adiponectin inhibited IGF-1-induced VSMC migration through suppression of ERK1/2 activation, which might be implicated in AMPK activation. Furthermore, adiponectin selectively inhibited ERK1/2 pathway, not Akt-Bad pathway, stimulated by IGF-1. From these findings, it was implied that adiponectin suppressed IGF-1-induced VSMC migration and its signaling selectivity.

Dual targeting of Raf and VEGF receptor 2 reduces growth and metastasis of pancreatic cancer through direct effects on tumor cells, endothelial cells, and pericytes

The Ras/Raf/MEK pathway represents an important oncogenic signaling pathway in gastrointestinal malignancies, including pancreatic cancer. Although activating B-Raf mutations are infrequent in pancreatic cancer, we hypothesized that targeting Raf could be valuable for therapy of this cancer entity. Moreover, as vascular endothelial growth factor receptor 2 (VEGFR2) is involved in tumor angiogenesis, we sought to investigate the effects of dual inhibition of Raf and VEGFR2 on pancreatic tumor growth, vascularization, and metastasis. Effects of a Raf/VEGFR2 inhibitor (NVP-AAL881) on pancreatic cancer cells, endothelial cells, and vascular smooth muscle cells were determined by Western blotting, 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide analysis, and migration assays, respectively. Changes in the expression of VEGF-A or survivin were investigated by ELISA and/or real-time PCR. The growth-inhibitory effects of Raf/VEGFR2 inhibition were additionally evaluated in orthotopic tumor models. Results showed that various Raf isoforms were activated in pancreatic cancer cells and NVP-AAL881 diminished the activation of MEK, Akt, Erk, and also STAT3. Moreover, dual inhibition of Raf/VEGFR2 significantly reduced VEGF expression and impaired cancer cell migration. Importantly, besides blocking VEGF-induced Erk and SAPK phosphorylation in endothelial cells, the Raf inhibitor diminished STAT3 phosphorylation, independent of a VEGFR2 blockade, and reduced the expression of survivin. In addition, cell proliferation and migration of both endothelial cells and vascular smooth muscle cells were significantly reduced. In vivo, blocking Raf/VEGFR2 significantly inhibited orthotopic tumor growth and vascularization and reduced cancer metastasis. In conclusion, blocking Raf exerts growth-inhibitory effects on pancreatic tumor cells, endothelial cells, and pericytes and elicits antiangiogenic properties. Dual targeting of Raf and VEGFR2 appears to be a valid strategy for therapy of pancreatic cancer.

Neuraminidase-1, a subunit of the cell surface elastin receptor, desialylates and functionally inactivates adjacent receptors interacting with the mitogenic growth factors PDGF-BB and IGF-2

We recently established that the elastin-binding protein, which is identical to the spliced variant of beta-galactosidase, forms a cell surface-targeted complex with two proteins considered "classic lysosomal enzymes": protective protein/cathepsin A and neuraminidase-1 (Neu1). We also found that cell surface-residing Neu1 can desialylate neighboring microfibrillar glycoproteins and facilitate the deposition of insoluble elastin, which contributes to the maintenance of cellular quiescence. Here we provide evidence that cell surface-residing Neu1 contributes to a novel mechanism that limits cellular proliferation by desialylating cell membrane-residing sialoglycoproteins that directly propagate mitogenic signals. We demonstrated that treatment of cultured human aortic smooth muscle cells (SMCs) with either a sialidase inhibitor or an antibody that blocks Neu1 activity induced significant up-regulation in SMC proliferation in response to fetal bovine serum. Conversely, treatment with Clostridium perfringens neuraminidase (which is highly homologous to Neu1) decreased SMC proliferation, even in cultures that did not deposit elastin. Further, we found that pretreatment of aortic SMCs with exogenous neuraminidase abolished their mitogenic responses to recombinant platelet-derived growth factor (PDGF)-BB and insulin-like growth factor (IGF)-2 and that sialidosis fibroblasts (which are exclusively deficient in Neu1) were more responsive to PDGF-BB and IGF-2 compared with normal fibroblasts. Furthermore, we provide direct evidence that neuraminidase caused the desialylation of both PDGF and IGF-1 receptors and diminished the intracellular signals induced by the mitogenic ligands PDGF-BB and IGF-2.

Activation of the mammalian target of rapamycin complex 1 is both necessary and sufficient to stimulate eukaryotic initiation factor 2Bvarepsilon mRNA translation and protein synthesis

In a previous study we demonstrated a requirement for activation of mTORC1 in the stimulation of eIF2Bepsilon mRNA translation in skeletal muscle in response to resistance exercise. Although that study established the necessity of mTORC1 activation, the experimental model used did not lend itself readily to address the question of whether or not mTORC1 activation was sufficient to produce the response. Therefore, the present study was designed to address the sufficiency of mTORC1 activation, using cultures of Rat2 fibroblasts in which mTORC1 signaling was repressed by serum/leucine-depletion and stimulated by repletion of leucine and/or IGF-1. Repletion with leucine and IGF-1 caused a shift of eIF2Bepsilon mRNA into actively translating polysomes and a stimulation of new eIF2Bepsilon protein synthesis, but had no effect on mRNAs encoding the other four eIF2B subunits. Stimulation of eIF2Bepsilon translation was reversed by pre-treatment with the mTORC1 inhibitor rapamycin. Exogenous overexpression of FLAG-Rheb, a proximal activator of mTORC1, also caused a re-distribution of eIF2Bepsilon mRNA into polysomes and a stimulation of eIF2Bepsilon protein synthesis. The stimulation of eIF2Bepsilon mRNA translation occurred in the absence of any effect on eIF2Bepsilon mRNA abundance. RNAi-mediated knockdown of eIF2Bepsilon resulted in reduced cellular proliferation, a result that phenocopied the known cytostatic effect of mTORC1 repression. Overall the results demonstrate that activation of mTORC1 is both necessary and sufficient to stimulate eIF2Bepsilon mRNA translation and that this response may represent a novel mechanism through which mTORC1 can affect mRNA translation initiation, rates of protein synthesis, and cellular growth/proliferation.

Insulin-like growth factors control cell migration in health and disease

Insulin-like growth factors I and II (IGF-I and IGF-II) have an ancient origin and play essential roles in fundamental biological processes. Although IGFs are principally known for their roles in regulating cell growth and survival, their ability to influence cell motility is just as significant. In the past 20 years, research has provided indisputable evidence for the regulatory role of IGFs in the migration of various cell types. Cell migration is crucial for reproduction, development, and tissue regeneration; IGFs play an important role in coordinating these processes. Moreover, studies continue to uncover the IGFs' role in stimulating cancer cell migration, invasion and metastasis. This review surveys current knowledge on the cell migration-modulating properties of IGFs and the biochemical pathways by which these peptides regulate cell movement in both physiological and pathological conditions.

IGF-I and IGF-II stimulate directed cell migration of bone-marrow-derived human mesenchymal progenitor cells

Insulin-like growth factors (IGFs) are known to be key regulators of bone growth, remodeling, and repair. Since all these processes depend on the recruitment of cells with the potential to be committed to the osteoblastic lineage, we studied possible effects of IGF-I and -II on migration of human mesenchymal progenitor cells (MPC) using a modified Boyden chamber assay. The results were compared to those of primary osteoblasts and in vitro-osteogenic-differentiated MPC. IGF-I and -II stimulated cell migration of all these cell populations in a dose-dependent manner from 1 to 100ng/mL. The maximal chemotactic index (CI) was 4-5 for MPC and primary osteoblasts and about 3 for in vitro-differentiated MPC. Checkerboard analysis revealed that IGFs stimulated true directed cell migration (chemotaxis) and not simply chemokinesis. Addition of an antibody against the type I IGF receptor (alphaIR3) completely abolished (MPC) or markedly reduced (primary osteoblasts) the chemotactic effects of each of the IGFs. IGFBP-3 itself had no direct effect, while IGFBP-5 stimulated MPC migration at concentrations of 80 and 160ng/mL. Parallel application of IGFBP-3 had borderline inhibitory effects while the addition of 40ng/mL of IGFBP-5 enhanced the chemotactic effect of IGF-I on MPC. In conclusion, our results show that IGF-I and -II are chemotactic factors for MPC and indicate that IGFBP-5 both modulates the IGF-I effect and directly stimulates migration of human mesenchymal progenitor cells.

Angiotensin II stimulates transcription of insulin-like growth factor I receptor in vascular smooth muscle cells: role of nuclear factor-kappaB

Increased expression of the IGF-I receptor (IGF-IR) is associated with proliferation and survival of vascular smooth muscle cells (VSMCs). In cultured VSMCs, we reported that angiotensin II (Ang II) increases transcription and expression of IGF-IR. Now, we show that mesenteric arteries of rats infused with Ang II develop thickening and increased IGF-IR expression. To determine how Ang II transcriptionally regulates IGF-IR expression in VSMCs, we generated 5'-end deletions of the IGF-IR promoter and measured Ang II-induced promoter-luciferase activity in VSMCs. Activities from these promoter sequences suggested that the Ang II-responsive region is located between -270 and -135 of the IGF-IR promoter. Using a DNase I foot printing analysis, we identified two putative nuclear factor-kappaB (NF-kappaB)-like sequences located in the same region of the IGF-IR promoter. When we mutated either of these NF-kappaB-like sites, Ang II-induced IGF-IR promoter activity decreased sharply. Electrophoretic mobility gel shift, anti-p50 of NF-kappaB supershift and chromatin immunoprecipitation assays demonstrated that both the p65 and p50 subunits of NF-kappaB will bind to this Ang II response element in the IGF-IR promoter. When we blocked the Ras/MAPK kinase 1 pathway or the inhibitory-kappaB kinase pathway, both Ang II-induced IGF-IR promoter activity and expression of IGF-IR protein significantly declined. Our results indicate that the mechanism by which Ang II stimulates IGF-IR expression in VSMCs involves NF-kappaB binding to NF-kappaB sites in the IGF-IR promoter, leading to expression of IGF-IR through both Ras/MAPK kinase 1-and inhibitory-kappaB kinase-dependent pathways. Because IGF-IR is a major factor associated with thickening of mesenteric vessels, our results provide potential therapeutic targets.

Role of 12-lipoxygenase in hypoxia-induced rat pulmonary artery smooth muscle cell proliferation

The 12-lipoxygenase (12-LO) pathway of arachidonic acid metabolism stimulates cell growth and metastasis of various cancer cells and the 12-LO metabolite, 12(S)-hydroxyeicosatetraenoic acid [12(S)-HETE], enhances proliferation of aortic smooth muscle cells (SMCs). However, pulmonary vascular effects of 12-LO have not been previously studied. We sought evidence for a role of 12-LO and 12(S)-HETE in the development of hypoxia-induced pulmonary hypertension. We found that 12-LO gene and protein expression is elevated in lung homogenates of rats exposed to chronic hypoxia. Immunohistochemical staining with a 12-LO antibody revealed intense staining in endothelial cells of large pulmonary arteries, SMCs (and possibly endothelial cells) of medium and small-size pulmonary arteries and in alveolar walls of hypoxic lungs. 12-LO protein expression was increased in hypoxic cultured rat pulmonary artery SMCs. 12(S)-HETE at concentrations as low as 10(-5) microM stimulated proliferation of pulmonary artery SMCs. 12(S)-HETE induced ERK 1/ERK 2 phosphorylation but had no effect on p38 kinase expression as assessed by Western blotting. 12(S)-HETE-stimulated SMC proliferation was blocked by the MEK inhibitor PD-98059, but not by the p38 MAPK inhibitor SB-202190. Hypoxia (3%)-stimulated pulmonary artery SMC proliferation was blocked by both U0126, a MEK inhibitor, and baicalein, an inhibitor of 12-LO. We conclude that 12-LO and its product, 12(S)-HETE, are important intermediates in hypoxia-induced pulmonary artery SMC proliferation and may participate in hypoxia-induced pulmonary hypertension.

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.