Functional blockade of platelet-derived growth factor receptor-beta but not of receptor-alpha prevents vascular smooth muscle cell accumulation in fibrous cap lesions in apolipoprotein E-deficient mice

Epac1 Deficiency Attenuated Vascular Smooth Muscle Cell Migration and Neointimal Formation

OBJECTIVE

Vascular smooth muscle cell (SMC) migration causes neointima, which is related to vascular remodeling after mechanical injury and atherosclerosis development. We previously reported that an exchange protein activated by cAMP (Epac) 1 was upregulated in mouse arterial neointima and promoted SMC migration. In this study, we examined the molecular mechanisms of Epac1-induced SMC migration and the effect of Epac1 deficiency on vascular remodeling in vivo.

APPROACH AND RESULTS

Platelet-derived growth factor-BB promoted a 2-fold increase in SMC migration in a primary culture of aortic SMCs obtained from Epac1(+/+) mice (Epac1(+/+)-ASMCs), whereas there was only a 1.2-fold increase in Epac1(-/-)-ASMCs. The degree of platelet-derived growth factor-BB-induced increase in intracellular Ca(2+) was smaller in Fura2-labeled Epac1(-/-)-ASMCs than in Epac1(+/+)-ASMCs. In Epac1(+/+)-ASMCs, an Epac-selective cAMP analog or platelet-derived growth factor-BB increased lamellipodia accompanied by cofilin dephosphorylation, which is induced by Ca(2+) signaling, whereas these effects were rarely observed in Epac1(-/-)-ASMCs. Furthermore, 4 weeks after femoral artery injury, prominent neointima were formed in Epac1(+/+) mice, whereas neointima formation was significantly attenuated in Epac1(-/-) mice in which dephosphorylation of cofilin was inhibited. The chimeric mice generated by bone marrow cell transplantation from Epac1(+/+) into Epac1(-/-) mice and vice versa demonstrated that the genetic background of vascular tissues, including SMCs rather than of bone marrow-derived cells affected Epac1-mediated neointima formation.

CONCLUSIONS

These data suggest that Epac1 deficiency attenuates neointima formation through, at least in part, inhibition of SMC migration, in which a decrease in Ca(2+) influx and a suppression of cofilin-mediated lamellipodia formation occur.

In-Vivo Higher Plasma Levels of Platelet-Derived Growth Factor and Matrix Metalloproteinase-9 in Coronary Artery at the Very Onset of Myocardial Infarction with ST-Segment Elevation

OBJECTIVE

Platelet-derived growth factor (PDGF) induces matrix metalloproteinase (MMP), which is regarded as a biomarker of plaque rupture or vulnerability. The aim of this study is to investigate those interactions in human coronary arteries at the onset of ST-segment elevation myocardial infarction (STEMI).

METHODS

Thirty-two patients with STEMI who underwent primary percutaneous coronary intervention (PCI) were enrolled in this study. Plasma levels of PDGF-BB and MMP-9 were measured from infarct-related artery (IRA) and from femoral artery (FA) during PCI.

RESULTS

Plasma levels of PDGF-BB and MMP-9 in the IRA were significantly higher than those in the FA (PDGF-BB: median 3130 pg/ml, IQR (interquartile range): 2020 to 4375 pg/ml vs. median 2605 pg/ml, IQR: 1305 to 3290 pg/ml, p <0.01, MMP-9: median 49 ng/ml, IQR: 35 to 100 ng/ml vs. median 42 ng/ml, IQR: 27 to 78 ng/ml, p = 0.04, IRA and FA, respectively).

CONCLUSIONS

This in vivo study demonstrated that PDGF-BB with MMP-9 seems to play a role in coronary plaque instability in acute phase of STEMI.

PDGFRβ signalling regulates local inflammation and synergizes with hypercholesterolaemia to promote atherosclerosis

Platelet-derived growth factor (PDGF) is a mitogen and chemoattractant for vascular smooth muscle cells (VSMCs). However, the direct effects of PDGF receptor β (PDGFRβ) activation on VSMCs have not been studied in the context of atherosclerosis. Here, we present a new mouse model of atherosclerosis with an activating mutation in PDGFRβ. Increased PDGFRβ signaling induces chemokine secretion and leads to leukocyte accumulation in the adventitia and media of the aorta. Furthermore, PDGFRβ^D849V amplifies and accelerates atherosclerosis in hypercholesterolemic ApoE^−/− or Ldlr^−/− mice. Intriguingly, increased PDGFRβ signaling promotes advanced plaque formation at novel sites in the thoracic aorta and coronary arteries. However, deletion of the PDGFRβ-activated transcription factor STAT1 in VSMCs alleviates inflammation of the arterial wall and reduces plaque burden. These results demonstrate that PDGFRβ pathway activation has a profound effect on vascular disease and support the conclusion that inflammation in the outer arterial layers is a driving process for atherosclerosis.

The pericyte as a cellular regulator of penile erection and a novel therapeutic target for erectile dysfunction

Pericytes are known to play critical roles in vascular development and homeostasis. However, the distribution of cavernous pericytes and their roles in penile erection is unclear. Herein we report that the pericytes are abundantly distributed in microvessels of the subtunical area and dorsal nerve bundle of mice, followed by dorsal vein and cavernous sinusoids. We further confirmed the presence of pericytes in human corpus cavernosum tissue and successfully isolated pericytes from mouse penis. Cavernous pericyte contents from diabetic mice and tube formation of cultured pericytes in high glucose condition were greatly reduced compared with those in normal conditions. Suppression of pericyte function with anti-PDGFR-β blocking antibody deteriorated erectile function and tube formation in vivo and in vitro diabetic condition. In contrast, enhanced pericyte function with HGF protein restored cavernous pericyte content in diabetic mice, and significantly decreased cavernous permeability in diabetic mice and in pericytes-endothelial cell co-culture system, which induced significant recovery of erectile function. Overall, these findings showed the presence and distribution of pericytes in the penis of normal or pathologic condition and documented their role in the regulation of cavernous permeability and penile erection, which ultimately explore novel therapeutics of erectile dysfunction targeting pericyte function.

Coronary Artery Disease Associated Transcription Factor TCF21 Regulates Smooth Muscle Precursor Cells That Contribute to the Fibrous Cap

Recent genome wide association studies have identified a number of genes that contribute to the risk for coronary heart disease. One such gene, TCF21, encodes a basic-helix-loop-helix transcription factor believed to serve a critical role in the development of epicardial progenitor cells that give rise to coronary artery smooth muscle cells (SMC) and cardiac fibroblasts. Using reporter gene and immunolocalization studies with mouse and human tissues we have found that vascular TCF21 expression in the adult is restricted primarily to adventitial cells associated with coronary arteries and also medial SMC in the proximal aorta of mouse. Genome wide RNA-Seq studies in human coronary artery SMC (HCASMC) with siRNA knockdown found a number of putative TCF21 downstream pathways identified by enrichment of terms related to CAD, including “vascular disease,” “disorder of artery,” and “occlusion of artery,” as well as disease-related cellular functions including “cellular movement” and “cellular growth and proliferation.” In vitro studies in HCASMC demonstrated that TCF21 expression promotes proliferation and migration and inhibits SMC lineage marker expression. Detailed in situ expression studies with reporter gene and lineage tracing revealed that vascular wall cells expressing Tcf21 before disease initiation migrate into vascular lesions of ApoE^-/- and Ldlr^-/- mice. While Tcf21 lineage traced cells are distributed throughout the early lesions, in mature lesions they contribute to the formation of a subcapsular layer of cells, and others become associated with the fibrous cap. The lineage traced fibrous cap cells activate expression of SMC markers and growth factor receptor genes. Taken together, these data suggest that TCF21 may have a role regulating the differentiation state of SMC precursor cells that migrate into vascular lesions and contribute to the fibrous cap and more broadly, in view of the association of this gene with human CAD, provide evidence that these processes may be a mechanism for CAD risk attributable to the vascular wall.

Coronary artery disease (CAD) is responsible for the majority of deaths in the Western world, and is due in part to environmental and metabolic factors. However, half of the risk for developing heart disease is genetically predetermined. Genome-wide association studies in human populations have identified over 100 sites in the genome that appear to be associated with CAD, however, the mechanisms by which variation in these regions are responsible for predisposition to CAD remain largely unknown. We have begun to study a gene that contributes to CAD risk, the TCF21 gene. Through genomic studies we show that this gene is involved in processes related to alterations in vascular gene expression, and in particular those related to the smooth muscle cell biology. With cell culture models, we show that TCF21 regulates the differentiation state of this cell type, which is believed critical for vascular disease. Using mouse genetic models of atherosclerotic vascular disease we provide evidence that this gene is expressed in precursor cells that migrate into the disease lesions and contribute to the formation of the fibrous cap that is believed to stabilize these lesions and prevent heart attacks.

Role of platelet-derived growth factor/platelet-derived growth factor receptor axis in the trafficking of circulating fibrocytes in pulmonary fibrosis

Circulating fibrocytes have been reported to migrate into the injured lungs, and contribute to fibrogenesis via CXCL12-CXCR4 axis. In contrast, we report that imatinib mesylate prevented bleomycin (BLM)-induced pulmonary fibrosis in mice by inhibiting platelet-derived growth factor receptor (PDGFR), even when it was administered only in the early phase. The goal of this study was to test the hypothesis that platelet-derived growth factor (PDGF) might directly contribute to the migration of fibrocytes to the injured lungs. PDGFR expression in fibrocytes was examined by flow cytometry and RT-PCR. The migration of fibrocytes was evaluated by using a chemotaxis assay for human fibrocytes isolated from peripheral blood. The numbers of fibrocytes triple-stained for CD45, collagen-1, and CXCR4 were also examined in lung digests of BLM-treated mice. PDGFR mRNA levels in fibrocytes isolated from patients with idiopathic pulmonary fibrosis were investigated by real-time PCR. Fibrocytes expressed both PDGFR-α and -β, and migrated in response to PDGFs. PDGFR inhibitors (imatinib, PDGFR-blocking antibodies) suppressed fibrocyte migration in vitro, and reduced the number of fibrocytes in the lungs of BLM-treated mice. PDGF-BB was a stronger chemoattractant than the other PDGFs in vitro, and anti-PDGFR-β-blocking antibody decreased the numbers of fibrocytes in the lungs compared with anti-PDGFR-α antibody in vivo. Marked expression of PDGFR-β was observed in fibrocytes from patients with idiopathic pulmonary fibrosis compared with healthy subjects. These results suggest that PDGF directly functions as a strong chemoattractant for fibrocytes. In particular, the PDGF-BB-PDGFR-β biological axis might play a critical role in fibrocyte migration into the fibrotic lungs.

The matricellular protein Cyr61 is a key mediator of platelet-derived growth factor-induced cell migration

Platelet-derived growth factor (PDGF), a potent chemoattractant, induces cell migration via the MAPK and PI3K/Akt pathways. However, the downstream mediators are still elusive. In particular, the role of extracellular mediators is largely unknown. In this study, we identified the matricellular protein Cyr61, which is de novo synthesized in response to PDGF stimulation, as the key downstream mediator of the ERK and JNK pathways, independent of the p38 MAPK and AKT pathways, and, thereby, it mediates PDGF-induced smooth muscle cell migration but not proliferation. Our results revealed that, when Cyr61 was newly synthesized by PDGF, it was promptly translocated to the extracellular matrix and physically interacted with the plasma membrane integrins α6β1 and αvβ3. We further demonstrate that Cyr61 and integrins are integral components of the PDGF signaling pathway via an "outside-in" signaling route to activate intracellular focal adhesion kinase (FAK), leading to cell migration. Therefore, this study provides the first evidence that the PDGF-induced endogenous extracellular matrix component Cyr61 is a key mediator in modulating cell migration by connecting intracellular PDGF-ERK and JNK signals with integrin/FAK signaling. Therefore, extracellular Cyr61 convergence with growth factor signaling and integrin/FAK signaling is a new concept of growth factor-induced cell migration. The discovered signaling pathway may represent an important therapeutic target in growth factor-mediated cell migration/invasion-related vascular diseases and tumorigenesis.

Role of cAMP-phosphodiesterase 1C signaling in regulating growth factor receptor stability, vascular smooth muscle cell growth, migration, and neointimal hyperplasia

RATIONALE

Neointimal hyperplasia characterized by abnormal accumulation of vascular smooth muscle cells (SMCs) is a hallmark of occlusive disorders such as atherosclerosis, postangioplasty restenosis, vein graft stenosis, and allograft vasculopathy. Cyclic nucleotides are vital in SMC proliferation and migration, which are regulated by cyclic nucleotide phosphodiesterases (PDEs).

OBJECTIVE

Our goal is to understand the regulation and function of PDEs in SMC pathogenesis of vascular diseases.

METHODS AND RESULTS

We performed screening for genes differentially expressed in normal contractile versus proliferating synthetic SMCs. We observed that PDE1C expression was low in contractile SMCs but drastically elevated in synthetic SMCs in vitro and in various mouse vascular injury models in vivo. In addition, PDE1C was highly induced in neointimal SMCs of human coronary arteries. More importantly, injury-induced neointimal formation was significantly attenuated by PDE1C deficiency or PDE1 inhibition in vivo. PDE1 inhibition suppressed vascular remodeling of human saphenous vein explants ex vivo. In cultured SMCs, PDE1C deficiency or PDE1 inhibition attenuated SMC proliferation and migration. Mechanistic studies revealed that PDE1C plays a critical role in regulating the stability of growth factor receptors, such as PDGF receptor β (PDGFRβ) known to be important in pathological vascular remodeling. PDE1C interacts with low-density lipoprotein receptor-related protein-1 and PDGFRβ, thus regulating PDGFRβ endocytosis and lysosome-dependent degradation in an low-density lipoprotein receptor-related protein-1-dependent manner. A transmembrane adenylyl cyclase cAMP-dependent protein kinase cascade modulated by PDE1C is critical in regulating PDGFRβ degradation.

CONCLUSIONS

These findings demonstrated that PDE1C is an important regulator of SMC proliferation, migration, and neointimal hyperplasia, in part through modulating endosome/lysosome-dependent PDGFRβ protein degradation via low-density lipoprotein receptor-related protein-1.

Gallium-68-labeled affibody molecule for PET imaging of PDGFRβ expression in vivo

Platelet-derived growth factor receptor β (PDGFRβ) is a transmembrane tyrosine kinase receptor involved, for example, in angiogenesis. Overexpression and excessive signaling of PDGFRβ has been observed in multiple malignant tumors and fibrotic diseases, making this receptor a pharmaceutical target for monoclonal antibodies and tyrosine kinase inhibitors. Successful targeted therapy requires identification of responding patients. Radionuclide molecular imaging would enable determination of the PDGFRβ status in all lesions using a single noninvasive repeatable procedure. Recently, we have demonstrated that the affibody molecule Z09591 labeled with (111)In can specifically target PDGFRβ-expressing tumors in vivo. The use of positron emission tomography (PET) as an imaging technique would provide superior resolution, sensitivity, and quantitation accuracy. In this study, a DOTA-conjugated Z09591 was labeled with the generator-produced positron emitting radionuclide (68)Ga (T1/2 = 67.6 min, Eβ + max = 1899 keV, 89% β(+)). (68)Ga-DOTA-Z09591 retained the capacity to specifically bind to PDGFRβ-expressing U-87 MG glioma cells. The half-maximum inhibition concentration (IC50) of (68)Ga-DOTA-Z09591 (6.6 ± 1.4 nM) was somewhat higher than that of (111)In-DOTA-Z09591 (1.4 ± 1.2 nM). (68)Ga-DOTA-Z09591 demonstrated specific (saturable) targeting of U-87 MG xenografts in immunodeficient mice. The tumor uptake at 2 h after injection was 3.7 ± 1.7% IA/g, which provided a tumor-to-blood ratio of 8.0 ± 3.1. The only organ with higher accumulation of radioactivity was the kidney. MicroPET imaging provided high-contrast imaging of U-87 MG xenografts. In conclusion, the (68)Ga-labeled affibody molecule Z09591 is a promising candidate for further development as a probe for imaging PDGFRβ expression in vivo using PET.

Regulation of smooth muscle cells in development and vascular disease: current therapeutic strategies

Vascular smooth muscle cells (SMCs) exhibit extensive phenotypic diversity and rapid growth during embryonic development, but maintain a quiescent, differentiated state in adult. The pathogenesis of vascular proliferative diseases involves the proliferation and migration of medial vascular SMCs into the vessel intima, possibly reinstating their embryonic gene expression programs. Multiple mitogenic stimuli induce vascular SMC proliferation through cell cycle progression. Therapeutic strategies targeting cell cycle progression and mitogenic stimuli have been developed and evaluated in animal models of atherosclerosis and vascular injury, and several clinical studies. Recent discoveries on the recruitment of vascular progenitor cells to the sites of vascular injury suggest new therapeutic potentials of progenitor cell-based therapies to accelerate re-endothelialization and prevent engraftment of SMC-lineage progenitor cells. Owing to the complex and multifactorial nature of SMC regulation, combinatorial antiproliferative approaches are likely to be used in the future in order to achieve maximal efficacy and reduce toxicity.

Vascular smooth muscle cells in cerebral aneurysm pathogenesis

Vascular smooth muscle cells (SMC) maintain significant plasticity. Following environmental stimulation, SMC can alter their phenotype from one primarily concerned with contraction to a pro-inflammatory and matrix remodeling phenotype. This is a critical process behind peripheral vascular disease and atherosclerosis, a key element of cerebral aneurysm pathology. Evolving evidence demonstrates that SMCs and phenotypic modulation play a significant role in cerebral aneurysm formation and rupture. Pharmacological alteration of smooth muscle cell function and phenotypic modulation could provide a promising medical therapy to inhibit cerebral aneurysm progression. This study reviews vascular SMC function and its contribution to cerebral aneurysm pathophysiology.

Targeting the PDGF signaling pathway in the treatment of non-malignant diseases

Platelet-derived growth factor (PDGF) is a family of mesenchymal mitogens with important functions during the embryonal development and in the control of tissue homeostasis in the adult. The PDGF isoforms exert their effects by binding to α-and β-tyrosine kinase receptors. Overactivity of PDGF signaling has been linked to the development of certain malignant and non-malignant diseases, including atherosclerosis and various fibrotic diseases. Different types of PDGF antagonists have been developed, including inhibitory monoclonal antibodies and DNA aptamers against PDGF isoforms and receptors, and receptor tyrosine kinase inhibitors. Beneficial effects have been recorded using such inhibitors in preclinical models and in patients with certain malignant as well as non-malignant diseases. The present communication summarizes the use of PDGF antagonists in the treatment of non-malignant diseases.

Origins and properties of dental, thymic, and bone marrow mesenchymal cells and their stem cells

Mesenchymal cells arise from the neural crest (NC) or mesoderm. However, it is difficult to distinguish NC-derived cells from mesoderm-derived cells. Using double-transgenic mouse systems encoding P0-Cre, Wnt1-Cre, Mesp1-Cre, and Rosa26EYFP, which enabled us to trace NC-derived or mesoderm-derived cells as YFP-expressing cells, we demonstrated for the first time that both NC-derived (P0- or Wnt1-labeled) and mesoderm-derived (Mesp1-labeled) cells contribute to the development of dental, thymic, and bone marrow (BM) mesenchyme from the fetal stage to the adult stage. Irrespective of the tissues involved, NC-derived and mesoderm-derived cells contributed mainly to perivascular cells and endothelial cells, respectively. Dental and thymic mesenchyme were composed of either NC-derived or mesoderm-derived cells, whereas half of the BM mesenchyme was composed of cells that were not derived from the NC or mesoderm. However, a colony-forming unit-fibroblast (CFU-F) assay indicated that CFU-Fs in the dental pulp, thymus, and BM were composed of NC-derived and mesoderm-derived cells. Secondary CFU-F assays were used to estimate the self-renewal potential, which showed that CFU-Fs in the teeth, thymus, and BM were entirely NC-derived cells, entirely mesoderm-derived cells, and mostly NC-derived cells, respectively. Colony formation was inhibited drastically by the addition of anti-platelet-derived growth factor receptor-β antibody, regardless of the tissue and its origin. Furthermore, dental mesenchyme expressed genes encoding critical hematopoietic factors, such as interleukin-7, stem cell factor, and cysteine-X-cysteine (CXC) chemokine ligand 12, which supports the differentiation of B lymphocytes and osteoclasts. Therefore, the mesenchymal stem cells found in these tissues had different origins, but similar properties in each organ.

Sp1, acetylated histone-3 and p300 regulate TRAIL transcription: mechanisms of PDGF-BB-mediated VSMC proliferation and migration

We recently reported that TNF-related apoptosis-inducing ligand (TRAIL) is important in atherogenesis, since it can induce vascular smooth muscle cell (VSMC) proliferation and arterial thickening following injury. Here we show the first demonstrate that TRAIL siRNA reduces platelet-derived growth factor-BB (PDGF-BB)-stimulated VSMC proliferation and migration. PDGF-BB-inducible VSMC proliferation was completely inhibited in VSMCs isolated from aortas of TRAIL(-/-) mice; whereas inducible migration was blocked compared to control VSMCs. TRAIL transcriptional control mediating this response is not established. TRAIL mRNA, protein and promoter activity was increased by PDGF-BB and subsequently inhibited by dominant-negative Sp1, suggesting that the transcription factor Sp1 plays a role. Sp1 bound multiple Sp1 sites on the TRAIL promoter, including two established (Sp1-1 and -2) and two novel Sp1-5/6 and -7 sites. PDGF-BB-inducible TRAIL promoter activity by Sp1 was mediated through these sites, since transverse mutations to each abolished inducible activity. PDGF-BB stimulation increased acetylation of histone-3 (ac-H3) and expression of the transcriptional co-activator p300, implicating chromatin remodelling. p300 overexpression increased TRAIL promoter activity, which was blocked by dominant-negative Sp1. Furthermore, PDGF-BB treatment increased the physical interaction of Sp1, p300 and ac-H3, while chromatin immunoprecipitation studies revealed Sp1, p300 and ac-H3 enrichment on the TRAIL promoter. Taken together, our studies demonstrate for the first time that PDGF-BB-induced TRAIL transcriptional activity requires the cooperation of Sp1, ac-H3 and p300, mediating increased expression of TRAIL which is important for VSMC proliferation and migration. Our findings have the promising potential for targeting TRAIL as a new therapeutic for vascular proliferative disorders.

Interleukin-1β modulates smooth muscle cell phenotype to a distinct inflammatory state relative to PDGF-DD via NF-κB-dependent mechanisms

Smooth muscle cell (SMC) phenotypic modulation in atherosclerosis and in response to PDGF in vitro involves repression of differentiation marker genes and increases in SMC proliferation, migration, and matrix synthesis. However, SMCs within atherosclerotic plaques can also express a number of proinflammatory genes, and in cultured SMCs the inflammatory cytokine IL-1β represses SMC marker gene expression and induces inflammatory gene expression. Studies herein tested the hypothesis that IL-1β modulates SMC phenotype to a distinct inflammatory state relative to PDGF-DD. Genome-wide gene expression analysis of IL-1β- or PDGF-DD-treated SMCs revealed that although both stimuli repressed SMC differentiation marker gene expression, IL-1β distinctly induced expression of proinflammatory genes, while PDGF-DD primarily induced genes involved in cell proliferation. Promoters of inflammatory genes distinctly induced by IL-1β exhibited over-representation of NF-κB binding sites, and NF-κB inhibition in SMCs reduced IL-1β-induced upregulation of proinflammatory genes as well as repression of SMC differentiation marker genes. Interestingly, PDGF-DD-induced SMC marker gene repression was not NF-κB dependent. Finally, immunofluorescent staining of mouse atherosclerotic lesions revealed the presence of cells positive for the marker of an IL-1β-stimulated inflammatory SMC, chemokine (C-C motif) ligand 20 (CCL20), but not the PDGF-DD-induced gene, regulator of G protein signaling 17 (RGS17). Results demonstrate that IL-1β- but not PDGF-DD-induced phenotypic modulation of SMC is characterized by NF-κB-dependent activation of proinflammatory genes, suggesting the existence of a distinct inflammatory SMC phenotype. In addition, studies provide evidence for the possible utility of CCL20 and RGS17 as markers of inflammatory and proliferative state SMCs within atherosclerotic plaques in vivo.

Tissue transglutaminase promotes PDGF/PDGFR-mediated signaling and responses in vascular smooth muscle cells

Although the pivotal role of platelet derived growth factor (PDGF)-mediated signaling in vascular diseases was demonstrated, the pathophysiological mechanisms driving its over-activation remain incompletely understood. Tissue transglutaminase (tTG) is a multifunctional protein expressed in the vasculature, including smooth muscle cells (SMCs), and implicated in several vascular pathologies. The goal of this study is to define the regulation of PDGF-BB/PDGFRβ-induced signaling pathways and cell responses by tTG in vascular SMCs. We find that in human aortic SMCs, shRNA-mediated depletion and over-expression of tTG reveals its ability to down-regulate PDGFRβ levels and induce receptor clustering. In these cells, tTG specifically amplifies the activation of PDGFRβ and its multiple downstream signaling targets in response to PDGF-BB. Furthermore, tTG promotes dedifferentiation and increases survival, proliferation, and migration of human aortic SMCs mediated by this growth factor. Finally, PDGF-BB stimulates tTG expression in human aortic SMCs in culture and in the blood vessels in response to injury. Together, our results show that tTG in vascular SMCs acts as a principal enhancer within the PDGF-BB/PDGFRβ signaling axis involved in phenotypic modulation of these cells, thereby suggesting a novel role for this protein in the progression of vascular diseases.

Epigenetic control of smooth muscle cell differentiation and phenotypic switching in vascular development and disease

The vascular smooth muscle cell (SMC) in adult animals is a highly specialized cell whose principal function is contraction. However, this cell displays remarkable plasticity and can undergo profound changes in phenotype during repair of vascular injury, during remodeling in response to altered blood flow, or in various disease states. There has been extensive progress in recent years in our understanding of the complex mechanisms that control SMC differentiation and phenotypic plasticity, including the demonstration that epigenetic mechanisms play a critical role. In addition, recent evidence indicates that SMC phenotypic switching in adult animals involves the reactivation of embryonic stem cell pluripotency genes and that mesenchymal stem cells may be derived from SMC and/or pericytes. This review summarizes the current state of our knowledge in this field and identifies some of the key unresolved challenges and questions that we feel require further study.

Wild-type LRP6 inhibits, whereas atherosclerosis-linked LRP6R611C increases PDGF-dependent vascular smooth muscle cell proliferation

Vascular smooth muscle cell (VSMC) proliferation is an important event in atherosclerosis and other vasculopathies. PDGF signaling is a key mediator of SMC proliferation, but the mechanisms that control its activity remain unclear. We previously identified a mutation in LDL receptor-related protein 6 (LRP6), LRP6(R611C), that causes early atherosclerosis. Examination of human atherosclerotic coronary arteries showed markedly increased expression of LRP6 and colocalization with PDGF receptor β (PDGFR-β). Further investigation showed that wild-type LRP6 inhibits but LRP6(R611C) promotes VSMC proliferation in response to PDGF. We found that wild-type LRP6 forms a complex with PDGFR-β and enhances its lysosomal degradation, functions that are severely impaired in LRP6(R611C). Further, we observed that wild-type and mutant LRP6 regulate cell-cycle activity by triggering differential effects on PDGF-dependent pathways. These findings implicate LRP6 as a critical modulator of PDGF-dependent regulation of cell cycle in smooth muscle and indicate that loss of this function contributes to development of early atherosclerosis in humans.

Targeting non-malignant disorders with tyrosine kinase inhibitors

Receptor and non-receptor tyrosine kinases are involved in multiple proliferative signalling pathways. Imatinib, one of the first tyrosine kinase inhibitors (TKIs) to be approved, revolutionized the treatment of chronic myelogenous leukaemia, and other TKIs with different spectra of kinase inhibition are used to treat renal cell carcinoma, non-small-cell lung cancer and colon cancer. Studies also support the potential use of TKIs as anti-proliferative agents in non-malignant disorders such as cardiac hypertrophy, and in benign-proliferative disorders including pulmonary hypertension, lung fibrosis, rheumatoid disorders, atherosclerosis, in-stent restenosis and glomerulonephritis. In this Review, we provide an overview of the most recent developments--both experimental as well as clinical--regarding the therapeutic potential of TKIs in non-malignant disorders.

Hypoxia enhances platelet-derived growth factor signaling in the pulmonary vasculature by down-regulation of protein tyrosine phosphatases

RATIONALE

Platelet-derived growth factor (PDGF) plays a pivotal role in the pathobiology of pulmonary hypertension (PH) because it promotes pulmonary vascular remodeling. PH is frequently associated with pulmonary hypoxia.

OBJECTIVES

To investigate whether hypoxia alters PDGF β receptor (βPDGFR) signaling in the pulmonary vasculature.

METHODS

The impact of chronic hypoxia on signal transduction by the βPDGFR was measured in human pulmonary arterial smooth muscle cells (hPASMC) in vitro, and in mice with hypoxia-induced PH in vivo.

MEASUREMENTS AND MAIN RESULTS

Chronic hypoxia significantly enhanced PDGF-BB-dependent proliferation and chemotaxis of hPASMC. Pharmacologic inhibition of PI3 kinase (PI3K) and PLCγ abrogated these events under both normoxia and hypoxia. Although hypoxia did not affect βPDGFR expression, it increased the ligand-induced tyrosine phosphorylation of the receptor, particularly at binding sites for PI3K (Y751) and PLCγ (Y1021). The activated βPDGFR is dephosphorylated by protein tyrosine phosphatases (PTPs). Interestingly, hypoxia decreased expression of numerous PTPs (T cell PTP, density-enhanced phosphatase-1, PTP1B, and SH2 domain-containing phosphatase-2), resulting in reduced PTP activity. Hypoxia-inducible factor (HIF)-1α is involved in this regulation of gene expression, because hypoxia-induced βPDGFR hyperphosphorylation and PTP down-regulation were abolished by HIF-1α siRNA and by the HIF-1α inhibitor 2-methoxyestradiol. βPDGFR hyperphosphorylation and PTP down-regulation were also present in vivo in mice with chronic hypoxia-induced PH.

CONCLUSIONS

Hypoxia reduces expression and activity of βPDGFR-antagonizing PTPs in a HIF-1α-dependent manner, thereby enhancing receptor activation and proliferation and chemotaxis of hPASMC. Because hyperphosphorylation of the βPDGFR and down-regulation of PTPs occur in vivo, this mechanism likely has significant impact on the development and progression of PH and other hypoxia-associated diseases.

New therapeutic possibilities for vein graft disease in the post-edifoligide era

Vein graft neointimal hyperplasia involves proliferation and migration of vascular smooth muscle cells into the vessel intima, and ultimately engenders accelerated atherosclerosis and vein graft failure. Since a myriad of stimuli provoke smooth muscle cell proliferation, molecular therapies for vein graft disease have targeted mechanisms fundamental to all cell proliferation - the 'cell-cycle' machinery. Preclinically, the most successful of these therapies has been edifoligide (E2F decoy), a double-stranded oligodeoxynucleotide that binds to the transcription factor known as E2F. Recently, PRoject of Ex vivo vein GRaft Engineering via Transfection (PREVENT) III and IV demonstrated that edifoligide failed to benefit human vein grafts employed to treat lower-extremity ischemia and coronary heart disease, respectively. The clinical failure of edifoligide calls into question previous models of vein graft disease and lends credence to recent animal studies demonstrating that vein graft arterialization substantially involves the immigration into the vein graft of a variety of vascular progenitor cells. Future vein graft disease therapies will likely target not only proliferation of graft-intrinsic cells, but also immigration of graft-extrinsic cells.

Development of a fully human anti-PDGFRbeta antibody that suppresses growth of human tumor xenografts and enhances antitumor activity of an anti-VEGFR2 antibody

Platelet-derived growth factor receptor beta (PDGFRbeta) is upregulated in most of solid tumors. It is expressed by pericytes/smooth muscle cells, fibroblast, macrophage, and certain tumor cells. Several PDGF receptor-related antagonists are being developed as potential antitumor agents and have demonstrated promising antitumor activity in both preclinical and clinical settings. Here, we produced a fully human neutralizing antibody, IMC-2C5, directed against PDGFRbeta from an antibody phage display library. IMC-2C5 binds to both human and mouse PDGFRbeta and blocks PDGF-B from binding to the receptor. IMC-2C5 also blocks ligand-stimulated activation of PDGFRbeta and downstream signaling molecules in tumor cells. In animal studies, IMC-2C5 significantly delayed the growth of OVCAR-8 and NCI-H460 human tumor xenografts in nude mice but failed to show antitumor activities in OVCAR-5 and Caki-1 xenografts. Our results indicate that the antitumor efficacy of IMC-2C5 is primarily due to its effects on tumor stroma, rather than on tumor cells directly. Combination of IMC-2C5 and DC101, an anti-mouse vascular endothelial growth factor receptor 2 antibody, resulted in significantly enhanced antitumor activity in BxPC-3, NCI-H460, and HCT-116 xenografts, compared with DC101 alone, and the trend of additive effects to DC101 treatment in several other tumor models. ELISA analysis of NCI-H460 tumor homogenates showed that IMC-2C5 attenuated protein level of vascular endothelial growth factor and basic fibroblast growth factor elevated by DC101 treatment. Finally, IMC-2C5 showed a trend of additive effects when combined with DC101/chemotherapy in MIA-PaCa-2 and NCI-H460 models. Taken together, these results lend great support to the use of PDGFRbeta antagonists in combination with other antiangiogenic agents in the treatment of a broad range of human cancers.

Krüppel-like factor 4 inhibits proliferation by platelet-derived growth factor receptor beta-mediated, not by retinoic acid receptor alpha-mediated, phosphatidylinositol 3-kinase and ERK signaling in vascular smooth muscle cells

Proliferation inhibition of vascular smooth muscle cells (VSMCs) is governed by the activity of a transcription factor network. Krüppel-like factor 4 (Klf4), retinoic acid receptor (RAR alpha), and platelet-derived growth factor receptor (PDGFR) are expressed in VSMCs and are components of such a network. However, the relationship among them in the regulation of VSMC proliferation remains unknown. Here, we investigated the mechanisms whereby Klf4 mediates the growth inhibitory effects in VSMCs through RAR alpha and PDGFR beta. We demonstrated that Klf4 directly binds to the 5' regulatory region of RAR alpha, down-regulates RAR alpha expression, and specifically inhibits RAR alpha-mediated phosphatidylinositol 3-kinase (PI3K) and ERK signaling in cultured VSMCs induced by the synthetic retinoid Am80. Of particular interest, Klf4 inhibits RAR alpha and PDGFR beta expression while blocking PI3K and ERK signaling induced by Am80 and PDGF-BB, respectively. The anti-proliferative effects of Klf4 on neointimal formation depend largely on PDGFR-mediated PI3K signaling without involvement of the RAR alpha-activated signaling pathways. These findings provide a novel mechanism for signal suppression and growth inhibitory effects of Klf4 in VSMCs. Moreover, the results of this study suggest that Klf4 is one of the key mediators of retinoid actions in VSMCs.

Differential effects of red and white wines on inhibition of the platelet-derived growth factor receptor: impact of the mash fermentation

AIMS

Moderate wine consumption is associated with a significant reduction of cardiovascular mortality. The molecular basis of this phenomenon remains unknown. Platelet-derived growth factor (PDGF) is an important contributor to atherogenesis. We investigated the effects of selected red and white wines on PDGF receptor (PDGFR) signalling in rat and human vascular smooth muscle cells (VSMCs).

METHODS AND RESULTS

All red wines concentration dependently inhibited the ligand-induced tyrosine phosphorylation of the PDGFR, downstream signalling events such as mitogen activated protein (MAP) kinase activation (Erk 1/2) and induction of immediate early genes (Egr-1, c-fos), and PDGF-induced cellular responses, whereas all white wines had no effect. At concentrations achieved after wine consumption in humans, all red wines completely abolished PDGF-dependent VSMC proliferation and migration. Red wines also inhibited PDGFR phosphorylation in vascular tissue, and in human coronary smooth muscle cells. Quantitative analyses of all tested wines and of samples collected at various time points (Days 0-16) of the 'mash fermentation', which is only performed for red wine, revealed that flavonoids of the catechin family, which potently inhibit PDGFR signalling, are extracted from grape seeds and skins during this process and therefore accumulate specifically in red wine. The accumulation of flavonoids correlated with the inhibitory potency of red wines on PDGFR signalling. Furthermore, this procedure could be imitated by incubation of wines with shredded grape seeds, and flavonoid-enriched white wine inhibited the PDGFR as potently as red wines.

CONCLUSION

Only red wines abrogate a critical pathogenic mechanism during atherogenesis, PDGFR signalling, in VSMCs. This effect is mediated by non-alcoholic constituents, which accumulate during the mash fermentation. Our findings offer a molecular explanation for the vasoprotective effects particularly of red wine. Therefore, future epidemiological studies should consider differential protective effects of red and white wine in vivo.

Gene expression profiling of non-alcoholic steatohepatitis using gene set enrichment analysis

AIM

Non-alcoholic steatohepatitis (NASH) is a subset of non-alcoholic fatty liver disease (NAFLD) and sometimes progresses to cirrhosis and liver failure. In this study we analyzed the expression profile of genes and biological pathways involved in NASH in comparison with non-NASH by gene set enrichment analysis (GSEA) employing a DNA microarray technique.

METHODS

mRNA from liver biopsy specimens was collected from a group of NASH patients and a group of non-NASH patients. We analyzed the relative abundance of mRNA using high-density oligonucleotide microarrays containing probes for 54 675 known genes, and investigated the pathogenetic mechanisms of NASH by means of a powerful technique for analyzing molecular profiling data, GSEA.

RESULTS

The results showed that the level of expression of 27 gene sets was significantly higher and the level of expression of 25 gene sets was significantly lower in the NASH samples than in the non-NASH samples. Based on these results we created an online, publicly available, searchable database containing the data for the gene expression profiles of the NASH patients (http://www2.genome.rcast.u-tokyo.ac.jp/___/NASH/NASH_GSEA2/).

CONCLUSION

Our data revealed differences in expression of many gene sets that are involved in the pathogenesis of NASH.

The synergistic inhibition of atherogenesis in apoE-/- mice between pravastatin and the sPLA2 inhibitor varespladib (A-002)

Secretory phospholipase A2 (sPLA2) activity promotes foam cell formation, increases proinflammatory bioactive lipid levels, decreases HDL levels, increases atherosclerosis in transgenic mice, and is an independent marker of cardiovascular disease. The effects of the sPLA2 inhibitor A-002 (varespladib) and pravastatin as monotherapies and in combination on atherosclerosis, lipids, and paraoxonase (PON) activity in apoE(-/-) mice were investigated. Male apoE(-/-) mice were placed on a 12-week high-fat diet supplemented with A-002 alone or combined with pravastatin. Atherosclerotic lesions were examined for size and composition using en face analysis, Movat staining, anti-CD68, and anti-alpha actin antibodies. Plasma lipids and PON activity were measured. A-002 decreased atherosclerotic lesion area by approximately 75% while increasing fibrous cap size by over 200%. HDL levels increased 40% and plasma PON activity increased 80%. Pravastatin monotherapy had no effect on lesion size but when combined with A-002, decreased lesion area 50% and total cholesterol levels 18% more than A-002 alone. A-002, a sPLA2 inhibitor, acts synergistically with pravastatin to decrease atherosclerosis, possibly through decreased levels of systemic inflammation or decreased lipid levels. A-002 treatment also resulted in a profound increase in plasma PON activity and significantly larger fibrous caps, suggesting the formation of more stable plaque architecture.

Involvement of platelet-derived growth factor receptor-beta in maintenance of mesenchyme and sensory epithelium of the neonatal mouse inner ear

Platelet-derived growth factor receptor (PDGFR) signaling has been demonstrated to play a pivotal role in early embryonic development. Although the expression of PDGF in the inner ear has been studied by RT-PCR, how PDGFR is involved there remains largely unclear. In the current study, we used the antagonistic anti-PDGFR-beta antibody, APB5, to investigate the role of PDGFR-beta in the neonatal mouse inner ear. PDGFR-beta was detected immunohistochemically in the mesenchymal tissue adjacent to the sensory epithelium of the inner ear, and a ligand for PDGFR-beta was detected around the sensory epithelium. To determine whether this expression plays a functional role, we injected APB5 into neonates to block the function of PDGFR-beta. Mesenchymal tissue defects and abnormal capillaries with irregular shapes, especially in the cochlear lateral wall, were detected in APB5-treated mice. The results of a TUNEL assay revealed that not only the adjacent mesenchymal cells but also the sensory epithelial cells underwent cell death. These results indicate that PDGFR-beta signals are required for the survival of the capillary and mesenchymal cells in the neonatal mouse inner ear and also indirectly implicate these signals in the survival of the sensory epithelium.

Role of platelet-derived growth factors in physiology and medicine

Platelet-derived growth factors (PDGFs) and their receptors (PDGFRs) have served as prototypes for growth factor and receptor tyrosine kinase function for more than 25 years. Studies of PDGFs and PDGFRs in animal development have revealed roles for PDGFR-alpha signaling in gastrulation and in the development of the cranial and cardiac neural crest, gonads, lung, intestine, skin, CNS, and skeleton. Similarly, roles for PDGFR-beta signaling have been established in blood vessel formation and early hematopoiesis. PDGF signaling is implicated in a range of diseases. Autocrine activation of PDGF signaling pathways is involved in certain gliomas, sarcomas, and leukemias. Paracrine PDGF signaling is commonly observed in epithelial cancers, where it triggers stromal recruitment and may be involved in epithelial-mesenchymal transition, thereby affecting tumor growth, angiogenesis, invasion, and metastasis. PDGFs drive pathological mesenchymal responses in vascular disorders such as atherosclerosis, restenosis, pulmonary hypertension, and retinal diseases, as well as in fibrotic diseases, including pulmonary fibrosis, liver cirrhosis, scleroderma, glomerulosclerosis, and cardiac fibrosis. We review basic aspects of the PDGF ligands and receptors, their developmental and pathological functions, principles of their pharmacological inhibition, and results using PDGF pathway-inhibitory or stimulatory drugs in preclinical and clinical contexts.

Chronic insulin treatment amplifies PDGF-induced motility in differentiated aortic smooth muscle cells by suppressing the expression and function of PTP1B

Hyperinsulinemia plays a major role in the pathogenesis of vascular disease. Restenosis occurs at an accelerated rate in hyperinsulinemia and is dependent on increased vascular smooth muscle cell movement from media to neointima. PDGF plays a critical role in mediating neointima formation in models of vascular injury. We have reported that PDGF increases the levels of protein tyrosine phosphatase PTP1B and that PTP1B suppresses PDGF-induced motility in cultured cells and that it attenuates neointima formation in injured carotid arteries. Others have reported that insulin enhances the mitogenic and motogenic effects of PDGF in cultured smooth muscle cells and that hyperinsulinemia promotes vascular remodeling. In the present study, we tested the hypothesis that insulin amplifies PDGF-induced cell motility by suppressing the expression and function of PTP1B. We found that chronic but not acute treatment of cells with insulin enhances PDGF-induced motility in differentiated cultured primary rat aortic smooth muscle cells and that it suppresses PDGF-induced upregulation of PTP1B protein. Moreover, insulin suppresses PDGF-induced upregulation of PTP1B mRNA levels, PTP1B enzyme activity, and binding of PTP1B to the PDGF receptor-beta, and it enhances PDGF-induced PDGF receptor phosphotyrosylation. Treatment with insulin induces time-dependent upregulation of phosphatidylinositol 3-kinase (PI3-kinase)-delta and activation of Akt, an enzyme downstream of PI3-kinase. Finally, inhibition of PI3-kinase activity, or its function, by pharmacological or genetic means rescues PTP1B activity in insulin-treated cells. These observations uncover novel mechanisms that explain how insulin amplifies the motogenic capacity of the pivotal growth factor PDGF.

Functions of paracrine PDGF signaling in the proangiogenic tumor stroma revealed by pharmacological targeting

BACKGROUND

Important support functions, including promotion of tumor growth, angiogenesis, and invasion, have been attributed to the different cell types populating the tumor stroma, i.e., endothelial cells, cancer-associated fibroblasts, pericytes, and infiltrating inflammatory cells. Fibroblasts have long been recognized inside carcinomas and are increasingly implicated as functional participants. The stroma is prominent in cervical carcinoma, and distinguishable from nonmalignant tissue, suggestive of altered (tumor-promoting) functions. We postulated that pharmacological targeting of putative stromal support functions, in particular those of cancer-associated fibroblasts, could have therapeutic utility, and sought to assess the possibility in a pre-clinical setting.

METHODS AND FINDINGS

We used a genetically engineered mouse model of cervical carcinogenesis to investigate platelet-derived growth factor (PDGF) receptor signaling in cancer-associated fibroblasts and pericytes. Pharmacological blockade of PDGF receptor signaling with the clinically approved kinase inhibitor imatinib slowed progression of premalignant cervical lesions in this model, and impaired the growth of preexisting invasive carcinomas. Inhibition of stromal PDGF receptors reduced proliferation and angiogenesis in cervical lesions through a mechanism involving suppression of expression of the angiogenic factor fibroblast growth factor 2 (FGF-2) and the epithelial cell growth factor FGF-7 by cancer-associated fibroblasts. Treatment with neutralizing antibodies to the PDGF receptors recapitulated these effects. A ligand trap for the FGFs impaired the angiogenic phenotype similarly to imatinib. Thus PDGF ligands expressed by cancerous epithelia evidently stimulate PDGFR-expressing stroma to up-regulate FGFs, promoting angiogenesis and epithelial proliferation, elements of a multicellular signaling network that elicits functional capabilities in the tumor microenvironment.

CONCLUSIONS

This study illustrates the therapeutic benefits in a mouse model of human cervical cancer of mechanism-based targeting of the stroma, in particular cancer-associated fibroblasts. Drugs aimed at stromal fibroblast signals and effector functions may prove complementary to conventional treatments targeting the overt cancer cells for a range of solid tumors, possibly including cervical carcinoma, the second most common lethal malignancy in women worldwide, for which management remains poor.

Hypercholesterolemia contributes to the development of atherosclerosis and vascular remodeling by recruiting bone marrow-derived cells in cuff-induced vascular injury

Recently, the role of bone marrow (BM)-derived endothelial cells and smooth muscle cells (SMCs) has been extensively studied in the pathogenesis of atherosclerosis. In this study we examined the effect of hypercholesterolemia on cuff-induced intimal thickening in LDL-receptor knockout (LDLR-/-) mice fed with a high-fat diet. We transplanted BM of green fluorescence protein (GFP)-transgenic mice to LDLR-/- mice to identify the cell lineage in the lesion. After BM transplantation mice were fed with a high-fat diet for 4 weeks and were then planted a polyethylene cuff on the right femoral artery. Two weeks after cuff placement, atherosclerotic lesions developed in the intima predominantly consisting of a massive accumulation of foam cells with a number of alpha smooth muscle actin (alphaSMA)- and GFP-positive cells. Adventitial small vessels were positive both for CD31 and GFP. Our data indicate that BM-derived cells can contribute to the development of atherosclerosis in the presence of hypercholesterolemia.

Eplerenone attenuates myocardial fibrosis in the angiotensin II-induced hypertensive mouse: involvement of tenascin-C induced by aldosterone-mediated inflammation

Tenascin-C is an extracellular matrix glycoprotein that is supposed to be a profibrotic molecule in various fibrogenic processes. To elucidate its significance for myocardial fibrosis in the hypertensive heart, we used a mouse model with infusion of angiotensin II and examined results by histology, immunohistochemistry, in situ hybridization, and quantitative real-time reverse transcriptase polymerase chain reaction (RT-PCR). Angiotensin II treatment elevated blood pressure and expression of tenascin-C by interstitial fibroblasts in perivascular fibrotic lesions, and angiotensin II infusion caused accumulation of macrophages. It also upregulated expression of collagen Ialpha2; IIIalpha1; and proinflammatory/profibrotic mediators including transforming growth factor beta (TGFbeta), platelet-derived growth factor alpha (PDGF-A), PDGF-B, and PDGF-receptor alpha, but not IL-1beta and PDGF-receptor beta, in the myocardium. Treatment with an aldosterone receptor antagonist, eplerenone, significantly attenuated angiotensin II-induced fibrosis, expression of tenascin-C, and inflammatory changes without affecting the blood pressure level. In vitro, neither eplerenone nor aldosterone exerted any influence on tenascin-C expression of cardiac fibroblasts, whereas angiotensin II, TGF-beta1, and PDGF significantly upregulated expression of tenascin-C. These results suggest that, in the angiotensin II-induced hypertensive mouse heart: (1) tenascin-C may be involved in the progression of cardiac fibrosis and (2) aldosterone may elicit inflammatory reactions in myocardium, which might, in turn, induce tenascin-C synthesis of fibroblasts through at least 2 pathways mediated by TGF-beta and PDGF-A-B/PDGF-receptor alpha.

RhoB regulates PDGFR-beta trafficking and signaling in vascular smooth muscle cells

OBJECTIVE

RhoB is a small GTPase localized at the plasma membrane and endosomes that participates in the regulation of endocytic trafficking of the epidermal growth factor (EGF) receptor and the nonreceptor kinases Src and Akt. This study was performed to determine whether RhoB plays a critical role in trafficking and signaling by the platelet-derived growth factor receptor-beta (PDGFR-beta) in vascular smooth muscle cells.

METHODS AND RESULTS

Cells derived from RhoB knockout mice failed to proliferate in response to PDGF, and downstream signaling was compromised as reflected by reduced phosphorylation of the effector kinases Akt and ERK1/2. In normal cells, PDGF stimulated trafficking of PDGFR-beta into a perinuclear late endosomal compartment and triggered entry of Src, Akt, extracellular signal-regulated kinase (ERK) into the cell nucleus. In contrast, PDGF treatment of RhoB null cells resulted in neither PDGFR-beta trafficking to late endosomes nor nuclear localization of Src, Akt, or ERK. In support of an essential function in these processes, restoring expression of RhoB in null cells rescued these defects and restored cell proliferation in response to PDGF.

CONCLUSIONS

Our findings establish RhoB as a critical regulator of PDGFR-beta trafficking and signaling in vascular smooth muscle cells.

Chlamydia pneumoniae infection of aortic smooth muscle cells reduces platelet-derived growth factor receptor-beta expression

Chlamydia pneumoniae infection may play a role in the pathogenesis of atherosclerosis. In this study, an oligonucleotide microarray was utilized to examine the transcriptional response of human aortic smooth muscle cells (AoSMC) to C. pneumoniae infection. Alteration of mRNA expression in 71 out of 780 genes was detected at 24 h after infection. Among the down-regulated genes, platelet-derived growth factor receptor-beta (PDGFR-beta) was identified as a target for further analysis because the PDGF system is involved in the fibroproliferative response of SMC in atherogenesis. Reverse transcriptase PCR analysis demonstrated that C. pneumoniae inhibits the up-regulation of PDGFR-beta mRNA occurring in AoSMC after mock infection. PDGFR-beta protein synthesis was examined by immunoblotting and fluorescence-activated cell sorting. Compared with mock-infected cells, the amount of receptor protein was reduced at 24, 48, and 72 h after infection. Diminished PDGFR-beta synthesis in infected cultures was accompanied by the suppression of AoSMC growth following PDGF-BB stimulation. The interference of C. pneumoniae with PDGFR-beta expression may result in decreased SMC proliferation in atherosclerotic plaques, thereby affecting the development and stability of advanced lesions.

Mulberry leaf powder prevents atherosclerosis in apolipoprotein E-deficient mice

Mulberry is commonly used to feed silkworms. Here we examined whether a dietary intake of mulberry leaf (ML) could affect atherogenesis in vivo and in vitro. Apolipoprotein E-deficient mice were fed either normal chow (control group) or a diet containing 1% ML powder (ML group) from 6 weeks of age. The mice were sacrificed after 12 weeks. The susceptibility of plasma lipoprotein to oxidation was assessed using diene formation. A significant increase in the lag time of lipoprotein oxidation was detected in the ML group compared with the control group. Furthermore, the ML group showed a 40% reduction in atherosclerotic lesion size in the aortae compared with the control. We also examined the direct anti-oxidative activity of ML in vitro. Aqueous extract of ML had a strong scavenging effect on 1,1-diphenyl-2-picrylhydrazyl and inhibited lipoprotein oxidation. These results confirm that ML contains anti-oxidative substances that might help prevent atherosclerosis.

An antibody directed against PDGF receptor beta enhances the antitumor and the anti-angiogenic activities of an anti-VEGF receptor 2 antibody

Platelet-derived growth factor (PDGF) and its receptors (PDGFR) play important roles in tumorigenesis through stimulating tumor growth and promoting angiogenesis via enhancing pericyte recruitment and vessel maturation. Here we produced a neutralizing antibody, 1B3, directed against mouse PDGFRbeta. 1B3 binds to PDGFRbeta with high affinity (9x10(-11)M) and blocks PDGF-BB from binding to the receptor with an IC(50) of approximately 1.2 nM. The antibody also blocks ligand-stimulated activation of PDGFRbeta and downstream signaling molecules, including Akt and MAPK p42/44, in tumor cells. In animal studies, 1B3 significantly enhanced the antitumor and the anti-angiogenic activities of DC101, an antibody directed against mouse vascular endothelial growth factor receptor 2, in a pancreatic (BxPC-3) and a non-small cell lung (NCI-H460) tumor xenograft models. Treatment with the combination of 1B3 and DC101 in BxPC-3 xenograft-bearing mice resulted in tumor regression in 58% of mice compared to that in 18% of mice treated with DC101 alone. Taken together, these results lend great support to use PDGFRbeta antagonists in combinations with other antitumor and/or anti-angiogenic agents in the treatment of a variety of cancers.

Lipid oxidation products and oxidized low-density lipoproteins impair platelet-derived growth factor receptor activity in smooth muscle cells: implication in atherosclerosis

The platelet-derived growth factor receptor-beta (PDGFRbeta) signaling pathway regulates smooth muscle cell (SMC) migration and proliferation in the vascular wall. Oxidized low-density lipoproteins (oxLDLs) and 4-hydroxynonenal (4-HNE) induce a dual effect on PDGFRbeta signaling. Short-term incubation of SMCs with oxLDLs and 4-HNE induced PDGFRbeta activation. Long-term incubation triggered a desensitization of PDGFR to its own agonist, with a progressive inhibition of PDGFRbeta phosphorylation, associated with increased formation of HNE-PDGFR adducts in SMC and in vivo, in the aortae of apoE-deficient mice. Hydralazine used as carbonyl scavenger prevented PDGFRbeta inhibition in vitro and in vivo In conclusion, PDGFRbeta is a target for 4-HNE, acrolein and oxidative stress and its progressive inhibition may contribute to defective SMC proliferation and decrease the stability of a vulnerable plaque.

Protection of atherogenesis in thromboxane A2 receptor-deficient mice is not associated with thromboxane A2 receptor in bone marrow-derived cells

In the previous study, we generated mice lacking thromboxane A2 receptor (TP) and apolipoprotein E, apoE(-/-)TP(-/-) mice, and reported that the double knockout mice developed markedly smaller atherosclerotic lesions than those in apoE(-/-) mice. To investigate the mechanism responsible for reduced atherosclerosis in apoE(-/-)TP(-/-) mice, we examined the role of TP in bone marrow (BM)-derived cells in the development of the atherosclerotic lesions. When we compared the function of macrophages in apoE(-/-) and in apoE(-/-)TP(-/-) mouse in vitro, there was no difference in the expression levels of cytokines and chemokines after stimulation with lipopolysaccharide. We then transplanted the BM from either apoE(-/-) or apoE(-/-)TP(-/-) mice to either apoE(-/-) or apoE(-/-)TP(-/-) mice after sublethal irradiation. After 12 weeks with high fat diet, we analyzed the atherosclerotic lesion of aortic sinus. When the BM from apoE(-/-) or apoE(-/-)TP(-/-) mice was transplanted to apoE(-/-) mice, the lesion size was almost the same as that of apoE(-/-) mice without BM transplantation. In contrast, when the BM from apoE(-/-) or apoE(-/-)TP(-/-) mice was transplanted to apoE(-/-)TP(-/-) mice, the lesion size was markedly reduced. These results indicate that the protection of atherogenesis in TP(-/-) mice is not associated with TP in BM-derived cells.

The role of platelet-derived growth factor signaling in healing myocardial infarcts

OBJECTIVES

This study sought to examine the role of platelet-derived growth factor (PDGF) signaling in healing myocardial infarcts.

BACKGROUND

Platelet-derived growth factor isoforms exert potent fibrogenic effects through interactions with PDGF receptor (PDGFR)-alpha and PDGFR-beta. In addition, PDGFR-beta signaling mediates coating of developing vessels with mural cells, leading to the formation of a mature vasculature. We hypothesized that PDGFR activation may regulate fibrosis and vascular maturation in healing myocardial infarcts.

METHODS

Mice undergoing reperfused infarction protocols were injected daily with a neutralizing anti-PDGFR-beta antibody (APB5), an anti-PDGFR-alpha antibody (APA5), or control immunoglobulin G, and were killed after 7 days of reperfusion.

RESULTS

The PDGF-B, PDGFR-alpha, and PDGFR-beta mRNA expression was induced in reperfused mouse infarcts. Perivascular cells expressing phosphorylated PDGFR-beta were identified in the infarct after 7 days of reperfusion, indicating activation of the PDGF-BB/PDGFR-beta pathway. The PDGFR-beta blockade resulted in impaired maturation of the infarct vasculature, enhanced capillary density, and formation of dilated uncoated vessels. Defective vascular maturation in antibody-treated mice was associated with increased and prolonged extravasation of red blood cells and monocyte/macrophages, suggesting increased permeability. These defects resulted in decreased collagen content in the healing infarct. In contrast, PDGFR-alpha inhibition did not affect vascular maturation, but significantly decreased collagen deposition in the infarct.

CONCLUSIONS

Platelet-derived growth factor signaling critically regulates postinfarction repair. Both PDGFR-beta- and PDGFR-alpha-mediated pathways promote collagen deposition in the infarct. Activation of PDGF-B/PDGFR-beta is also involved in recruitment of mural cells by neovessels, regulating maturation of the infarct vasculature. Acquisition of a mural coat and maturation of the vasculature promotes resolution of inflammation and stabilization of the scar.

Vascular Wall Maturation and Prolonged Angiogenic Effect by Endothelial-Specific Platelet-Derived Growth Factor Expression

BACKGROUND

The implementation of angiogenic gene therapy at clinics is hindered by the transience of the therapeutic effect. Recruiting vascular wall smooth muscle cells, a process termed 'maturation', can stabilize newly formed vessels.

OBJECTIVE

To induce angiogenesis followed by vessel maturation in a murine ischemic limb model by endothelial cell-specific promoter regulated expression of vascular endothelial growth factor (VEGF) and platelet-derived growth factor-BB (PDGF-BB).

METHODS

We constructed adenoviral vectors containing angiogenic factors VEGF and PDGF-B regulated by a modified preproendothelin-1 (PPE-1-3x) promoter and investigated their angiogenic effect in a murine ischemic limb model.

RESULTS

VEGF gene therapy increased perfusion and the vessel density in the limb shortly after expression with PPE-1-3x promoter or cytomegalovirus (CMV) promoter vectors, but only PPE-1-3xVEGF treatment exhibited a sustained effect. Expression of PDGF-B by PPE-1-3x promoter resulted in morphological maturation of the vasculature and further increased the perfusion, while nonspecific expression of PDGF-B with CMV promoter had no therapeutic effect. Regulation of dual therapy with VEGF and PDGF-B by PPE-1-3x promoter resulted in an early-onset, sustained angiogenic effect accompanied by vessel maturation.

CONCLUSIONS

Systemic gene therapy with the angiogenic factors VEGF and PDGF-B under angiogenic- endothelial cell-specific regulation was effective in inducing functionally and morphologically mature vasculature.

Regulation of the platelet-derived growth factor receptor-beta by G protein-coupled receptor kinase-5 in vascular smooth muscle cells involves the phosphatase Shp2

Smooth muscle cell (SMC) proliferation and migration are substantially controlled by the platelet-derived growth factor receptor-beta (PDGFRbeta), which can be regulated by the Ser/Thr kinase G protein-coupled receptor kinase-2 (GRK2). In mouse aortic SMCs, however, we found that prolonged PDGFRbeta activation engendered down-regulation of GRK5, but not GRK2; moreover, GRK5 and PDGFRbeta were coordinately up-regulated in SMCs from atherosclerotic arteries. With SMCs from GRK5 knock-out and cognate wild type mice (five of each), we found that physiologic expression of GRK5 increased PDGF-promoted PDGFRbeta seryl phosphorylation by 3-fold and reduced PDGFRbeta-promoted phosphoinositide hydrolysis, thymidine incorporation, and overall PDGFRbeta tyrosyl phosphorylation by approximately 35%. Physiologic SMC GRK5 activity also increased PDGFRbeta association with the phosphatase Shp2 (8-fold), enhanced phosphorylation of PDGFRbeta Tyr(1009) (the docking site for Shp2), and reduced phosphorylation of PDGFRbeta Tyr(1021). Consistent with having increased PDGFRbeta-associated Shp2 activity, GRK5-expressing SMCs demonstrated greater PDGF-induced Src activation than GRK5-null cells. GRK5-mediated desensitization of PDGFRbeta inositol phosphate signaling was diminished by Shp2 knock-down or impairment of PDGFRbeta/Shp2 association. In contrast to GRK5, physiologic GRK2 activity did not alter PDGFRbeta/Shp2 association. Finally, purified GRK5 effected agonist-dependent seryl phosphorylation of partially purified PDGFRbetas. We conclude that GRK5 mediates the preponderance of PDGF-promoted seryl phosphorylation of the PDGFRbeta in SMCs, and, through mechanisms involving Shp2, desensitizes PDGFRbeta inositol phosphate signaling and enhances PDGFRbeta-triggered Src activation.

Upregulation of intermediate-conductance Ca2+-activated K+ channel (IKCa1) mediates phenotypic modulation of coronary smooth muscle

A hallmark of smooth muscle cell (SMC) phenotypic modulation in atherosclerosis and restenosis is suppression of SMC differentiation marker genes, proliferation, and migration. Blockade of intermediate-conductance Ca(2+)-activated K(+) channels (IKCa1) has been shown to inhibit restenosis after carotid balloon injury in the rat; however, whether IKCa1 plays a role in SMC phenotypic modulation is unknown. Our objective was to determine the role of IKCa1 channels in regulating coronary SMC phenotypic modulation and migration. In cultured porcine coronary SMCs, platelet-derived growth factor-BB (PDGF-BB) increased TRAM-34 (a specific IKCa1 inhibitor)-sensitive K(+) current 20-fold; increased IKCa1 promoter histone acetylation and c-jun binding; increased IKCa1 mRNA approximately 4-fold; and potently decreased expression of the smooth muscle differentiation marker genes smooth muscle myosin heavy chain (SMMHC), smooth muscle alpha-actin (SMalphaA), and smoothelin-B, as well as myocardin. Importantly, TRAM-34 completely blocked PDGF-BB-induced suppression of SMMHC, SMalphaA, smoothelin-B, and myocardin and inhibited PDGF-BB-stimulated migration by approximately 50%. Similar to TRAM-34, knockdown of endogenous IKCa1 with siRNA also prevented the PDGF-BB-induced increase in IKCa1 and decrease in SMMHC mRNA. In coronary arteries from high fat/high cholesterol-fed swine demonstrating signs of early atherosclerosis, IKCa1 expression was 22-fold higher and SMMHC, smoothelin-B, and myocardin expression significantly reduced in proliferating vs. nonproliferating medial cells. Our findings demonstrate that functional upregulation of IKCa1 is required for PDGF-BB-induced coronary SMC phenotypic modulation and migration and support a similar role for IKCa1 in coronary SMC during early coronary atherosclerosis.

VEGF-A and FGF-2 synergistically promote neoangiogenesis through enhancement of endogenous PDGF-B-PDGFRbeta signaling

Combined stimulation with VEGF-A, FGF-2, or PDGF-BB has emerged as a potent strategy for therapeutic angiogenesis, although the mechanisms underlying the synergism of these factors are not well understood. In the present study, we investigated the mechanism of synergism between VEGF-A and FGF-2 by using Matrigel plug assay in vivo and embryonic stem cell (ESC)-derived VEGF receptor 2 (VEGFR2)-positive cells in vitro. Experiments in vitro revealed that, in addition to having direct mitogenic effects, these molecules enhance intercellular PDGF-B signaling in a cell-type specific manner: VEGF-A enhances endothelial PDGF-B expression, whereas FGF-2 enhances mural PDGF receptor beta (PDGFRbeta) expression. Co-stimulation with VEGF-A and FGF-2 caused significant mural cell recruitment in vitro and formation of functional neovasculature in vivo, compared with single-agent stimulation. These effects were abrogated not only by anti-PDGFRbeta neutralizing antibody, but also by exogenous PDGF-BB, which could overwhelm the endogenous PDGF-BB distribution. These findings indicated the importance of preservation of the periendothelial PDGF-BB gradient. Thus, we demonstrated that the directional enhancement of endogenous PDGF-B-PDGFRbeta signaling is indispensable for the synergistic effect of VEGF-A and FGF-2 on neoangiogenesis in adults. The findings provide insights into the mechanisms underlying the effects of co-stimulation by growth factors, which could lead to rational design of therapeutic angiogenic strategies.