Chimera analysis reveals that fibroblasts and endothelial cells require platelet-derived growth factor receptorbeta expression for participation in reactive connective tissue formation in adults but not during development

Development of Adaptive Immunity and Its Role in Lung Remodeling

Asthma is characterized by airflow limitations resulting from bronchial closure, which can be either reversible or fixed due to changes in airway tissue composition and structure, also known as remodeling. Airway remodeling is defined as increased presence of mucins-producing epithelial cells, increased thickness of airway smooth muscle cells, angiogenesis, increased number and activation state of fibroblasts, and extracellular matrix (ECM) deposition. Airway inflammation is believed to be the main cause of the development of airway remodeling in asthma. In this chapter, we will review the development of the adaptive immune response and the impact of its mediators and cells on the elements defining airway remodeling in asthma.

PDGFRα plays a crucial role in connective tissue remodeling

Platelet derived growth factor (PDGF) plays a pivotal role in the remodeling of connective tissues. Emerging data indicate the distinctive role of PDGF receptor-α (PDGFRα) in this process. In the present study, the Pdgfra gene was systemically inactivated in adult mouse (α-KO mouse), and the role of PDGFRα was examined in the subcutaneously implanted sponge matrices. PDGFRα expressed in the fibroblasts of Pdgfra-preserving control mice (Flox mice), was significantly reduced in the sponges in α-KO mice. Neovascularized areas were largely suppressed in the α-KO mice than in the Flox mice, whereas the other parameters related to the blood vessels and endothelial cells were similar. The deposition of collagen and fibronectin and the expression of collagen 1a1 and 3a1 genes were significantly reduced in α-KO mice. There was a significantly decrease in the number and dividing fibroblasts in the α-KO mice, and those of macrophages were similar between the two genotypes. Hepatocyte growth factor (Hgf) gene expression was suppressed in Pdgfra-inactivated fibroblasts and connective tissue. The findings implicate the role of PDGFRα-dependent ECM and HGF production in fibroblasts that promotes the remodeling of connective tissue and suggest that PDGFRα may be a relevant target to regulate connective tissue remodeling.

Transgenic expression and functional characterization of human platelet derived growth factor BB (hPDGF-BB) in tobacco (Nicotiana tabacum L.)

Recombinant human platelet derived growth factor BB (rhPDGF-BB) is clinically approved for treating diabetic neuropathic ulcers. Plant-based expression systems offer less expensive ways of producing recombinant drugs, which do not require purification for clinical use. From this perspective, rhPDGF-BB is an ideal candidate for expression in plants as it can be applied topically. Here, we report a proof of concept study, in which rhPDGF-BB was expressed in tobacco plants, and its biological activity was tested in vitro. The mature human platelet derived growth factor BB (hPDGF-BB) gene was codon-optimized for tobacco and fused with ER targeting and retention signals, 5' and 3' UTRs of arc5-1 gene along with CaMV 35S promoter, and then, transferred by Agrobacterium-mediated transformation. Gene and protein expression of hPDGF-BB were confirmed by PCR and immunoblot studies. Bioactivity of hPDGF-BB expressed protein was determined by in vitro assays such as proliferation and migration in NIH3T3 cells. Our data reveals that total soluble proteins containing hPDGF-BB from transgenic plants showed a 4.5-fold increase in fibroblast proliferation compared to non-transgenic plants. Furthermore, plant-made rhPDGF-BB induced chemotaxis of treated cells and promoted wound healing in vitro. These results clearly demonstrate that functionally active rhPDGF-BB protein can be produced in plants and might have therapeutic benefits.

Phenotypical differences in connective tissue cells emerging from microvascular pericytes in response to overexpression of PDGF-B and TGF-β1 in normal skin in vivo

Fibrosis is a deleterious consequence of chronic inflammation in a number of human pathologies ultimately leading to organ dysfunction and failure. Two growth factors that are important in blood vessel physiology and tissue fibrosis, platelet-derived growth factor (PDGF)-B and transforming growth factor (TGF)-β1, were investigated. Adenoviral vectors were used to induce transient overexpression of these growth factors in mouse skin. Changes in tissue structure and protein and mRNA expressions were investigated. Both PDGF-B and TGF-β1 could initiate but neither could sustain angiogenesis. Instead, vascular regression was observed. Overexpression of both TGF-β1 and PDGF-B led to a marked macrophage influx and an expansion of the connective tissue cell population. Over time, this effect was sustained in mice treated with TGF-β1, whereas it was partially reversible in mice treated with PDGF-B. On the basis of structure and expression of phenotypical markers, the emerging connective tissue cell population may originate from microvascular pericytes. TGF-β1 induced expansion of connective tissue cells with a myofibroblast phenotype, whereas PDGF-B induced a fibroblast phenotype negative for α-smooth muscle actin. TGF-β1 and PDGF-B overexpressions mediated distinct effects on mRNA transcript levels of fibrillar procollagens, their modifying enzymes, small leucin-rich repeat proteoglycans, and matricellular proteins affecting both the composition and the quantity of the extracellular matrix. This study offers new insight into the effects of PDGF-B and TGF-β1 on the vasculature and connective tissue in vivo.

Effects of the histone deacetylase inhibitor valproic acid on human pericytes in vitro

Microvascular pericytes are of key importance in neoformation of blood vessels, in stabilization of newly formed vessels as well as maintenance of angiostasis in resting tissues. Furthermore, pericytes are capable of differentiating into pro-fibrotic collagen type I producing fibroblasts. The present study investigates the effects of the histone deacetylase (HDAC) inhibitor valproic acid (VPA) on pericyte proliferation, cell viability, migration and differentiation. The results show that HDAC inhibition through exposure of pericytes to VPA in vitro causes the inhibition of pericyte proliferation and migration with no effect on cell viability. Pericyte exposure to the potent HDAC inhibitor Trichostatin A caused similar effects on pericyte proliferation, migration and cell viability. HDAC inhibition also inhibited pericyte differentiation into collagen type I producing fibroblasts. Given the importance of pericytes in blood vessel biology a qPCR array focusing on the expression of mRNAs coding for proteins that regulate angiogenesis was performed. The results showed that HDAC inhibition promoted transcription of genes involved in vessel stabilization/maturation in human microvascular pericytes. The present in vitro study demonstrates that VPA influences several aspects of microvascular pericyte biology and suggests an alternative mechanism by which HDAC inhibition affects blood vessels. The results raise the possibility that HDAC inhibition inhibits angiogenesis partly through promoting a pericyte phenotype associated with stabilization/maturation of blood vessels.

Mouse chimeras as a system to investigate development, cell and tissue function, disease mechanisms and organ regeneration

Chimeras are organisms composed of at least two genetically distinct cell lineages originating from different zygotes. In the laboratory, mouse chimeras can be produced experimentally; various techniques allow combining different early stage mouse embryos with each other or with pluripotent stem cells. Identification of the progeny of the different lineages in chimeras permits to follow cell fate and function, enabling correlation of genotype with phenotype. Mouse chimeras have become a tool to investigate critical developmental processes, including cell specification, differentiation, patterning, and the function of specific genes. In addition, chimeras can also be generated to address biological processes in the adult, including mechanisms underlying diseases or tissue repair and regeneration. This review summarizes the different types of chimeras and how they have been generated and provides examples of how mouse chimeras offer a unique and powerful system to investigate questions pertaining to cell and tissue function in the developing and adult organism.

Metronomic administration of the drug GMX1777, a cellular NAD synthesis inhibitor, results in neuroblastoma regression and vessel maturation without inducing drug resistance

High-risk neuroblastoma is a rapidly growing tumor with a survival rate below 50%. A new treatment strategy is to administer chemotherapeutic drugs metronomically, i.e., at lower doses and frequent intervals. The aim of the study was to investigate the effects of GMX1777, a chemotherapeutic drug affecting cellular energy metabolism, in a mouse model for high-risk neuroblastoma. Female SCID mice were injected s.c. with MYCN-amplified human neuroblastoma cells and randomized to either treatment with GMX1777 or vehicle. In some animals, treatment was discontinued allowing tumor relapse. Treatment response was evaluated using the pediatric preclinical testing program (PPTP). Immunohistochemistry and qRT-PCR was performed on tumor cryosections to investigate the microscopic and molecular changes in tumors in response to GMX1777. Despite an increase in vessel density, tumor regression and a high group response score according to PPTP criteria was induced by GMX1777 without inducing drug resistance. Treatment resulted in inhibition of tumor cell proliferation, vessel maturation, reduced hypoxia, increased infiltration of MHC class II negative macrophages and expansion of the nonvascular stromal compartment. Decreased stromal VEGF-A and PDGF-B mRNA in response to treatment together with the structural data suggest a "deactivation" or "silencing" of the tumor stroma as a paracrine entity. In conclusion, GMX1777 was highly efficient against high-risk neuroblastoma xenografts through modulation of both the tumor cell and stromal compartment.

Overexpression of platelet-derived growth factor-BB increases tumor pericyte content via stromal-derived factor-1alpha/CXCR4 axis

Platelet-derived growth factor-BB (PDGF-BB) is a well-characterized growth factor displaying potent biological effects on angiogenesis. Recent studies reveal that overexpression of PDGF-BB within tumors results in increased pericyte coverage, suggesting that PDGF-BB signaling is also essential for the cancerous pericyte recruitment process. However, the molecular mechanism underlying this regulation remains obscure. In the current study, we show that tumor-derived PDGF-BB induces SDF-1alpha expression in endothelial cells (EC), resulting in the formation of SDF-1alpha chemotaxis gradient, which coincides with the PDGF-BB-induced pericyte recruitment during angiogenesis. PDGF-BB dramatically up-regulates SDF-1alpha secretion through the activation of PDGFRbeta in tumor-associated ECs, whereas this up-regulation can be substantially inhibited by either blockade of the phosphatidylinositol 3-kinase/Akt/mTOR pathway with chemical inhibitors or the inactivation of HIF-1alpha through small interfering RNA interference. On the other hand, we reveal that SDF-1alpha can increase pericytes motility in vitro. Blockade of the SDF-1alpha/CXCR4 axis prevents the PDGF-BB-induced pericyte recruitment not only in three in vitro recruitment models but also in the PDGF-BB-overexpressing tumor xenograft models. These results highlight that the involvement of SDF-1alpha/CXCR4 axis is essential for the pericyte recruitment within the PDGF-BB-overexpressing tumors and raise the possibility that blockade of the SDF-1alpha/CXCR4 axis may provide a therapeutic synergy with antiangiogenic molecules that selectively target ECs.

Two different PDGF beta-receptor cohorts in human pericytes mediate distinct biological endpoints

How activation of a specific growth factor receptor selectively results in either cell proliferation or cytoskeletal reorganization is of central importance to the field of pathophysiology. In this study, we report on a novel mechanism that explains how this process is accomplished. Our current investigation demonstrates that soluble platelet derived growth factor- (PDGF)-BB activates a cohort of PDGF-beta receptors primarily confined to the lipid raft component of the cell membrane, specifically caveolae. In contrast, cell-bound PDGF-BB delivered via cell-cell contact results in activation and the subsequent up-regulation of a cohort of PDGF beta-receptors primarily confined to the non-lipid raft component of the cell membrane. Individual activation of these two receptor cohorts results in distinct biological endpoints, cytoskeletal reorganization or cell proliferation. Mechanistically, our evidence suggests that PDGF-BB-bearing cells preferentially stimulate the non-lipid raft receptor cohort through interleukin 1beta-mediated inhibition of the lipid raft cohort of receptors, leaving the non-raft receptor cohort operational and preferentially stimulated. In human skin injected with PDGF-BB and in tissue reparative processes PDGF beta-receptors colocalize with the caveolae/lipid raft marker caveolin-1. In contrast, in human skin injected with PDGF-BB-bearing tumor cells and in colorectal adenocarcinoma, activated PDGF beta-receptors do not colocalize with caveolin-1. Thus, growth factor receptors are segregated into specific cell membrane compartments that are preferentially activated through different mechanisms of ligand delivery, resulting in distinct biological endpoints.

The effect of omega-3 FAs on tumour angiogenesis and their therapeutic potential

Omega-3 fatty acid (omega-3 FA) consumption has long been associated with a lower incidence of colon, breast and prostate cancers in many human populations. Human trials have demonstrated omega-3 FA to have profound anti-inflammatory effects in those with cancer. In vitro and small animal studies have yielded a strong body of evidence establishing omega-3 FA as having anti-inflammatory, anti-apoptotic, anti-proliferative and anti-angiogenic effects. This review explores the evidence and the mechanisms by which omega-3 FA may act as angiogenesis inhibitors and identifies opportunities for original research trialling omega-3 FAs as anti-cancer agents in humans. The conclusions drawn from this review suggest that omega-3 FAs in particular eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) found principally in oily fish have potent anti-angiogenic effects inhibiting production of many important angiogenic mediators namely; Vascular Endothelial Growth Factor (VEGF), Platelet-Derived Growth Factor (PDGF), Platelet-Derived Endothelial Cell Growth Factor (PDECGF), cyclo-oxygenase 2 (COX-2), prostaglandin-E2 (PGE2), nitric oxide, Nuclear Factor Kappa Beta (NFKB), matrix metalloproteinases and beta-catenin.

Integrin alpha1beta1 is involved in the differentiation into myofibroblasts in adult reactive tissues in vivo

Connective tissue cell activation is of importance during reactive conditions such as solid tumour growth, wound healing and pannus formation in rheumatoid arthritis. Here, we have compared connective tissue cells of mesenchymal origin in human tissues from these conditions and their normal counterparts using a panel of cell-type-specific markers. In particular, we investigated variations of integrin expression among connective tissue cell phenotypes. Connective tissue cell populations were defined based on their association with the microvasculature and their expression of activation markers. The phenotype of these cells varied according to the type of pathological connective tissue examined. Our morphological data from human tissues suggested that the alpha(1)beta(1) integrin, a collagen/laminin receptor, is involved in the differentiation of precursor cells into myofibroblasts. To mechanistically investigate this hypothesis, we employed experimental models for carcinoma growth and wound healing utilizing alpha(1) integrin-deficient mice. The data confirmed that the alpha(1)beta(1) integrin is of importance not only for the differentiation of mesenchymal cells into myofibroblasts but also for the neovascularization and connective tissue organization and emphasize the importance of myofibroblasts in the pathophysiology of tissue repair, inflammation and tumour growth.

Platelet-derived growth factor-beta receptor activation is essential for fibroblast and pericyte recruitment during cutaneous wound healing

Connective tissue remodeling provides mammals with a rapid mechanism to repair wounds after injury. Inappropriate activation of this reparative process leads to scarring and fibrosis. Here, we studied the effects of platelet-derived growth factor receptor-beta blockade in vivo using the platelet-derived growth factor receptor (PDGFR)-beta inhibitor imatinib mesylate on tissue repair. After 7 days, healing of wounds was delayed with significantly reduced wound closure and concomitant reduction in myofibroblast frequency, expression of fibronectin ED-A, and collagen type I. Using a collagen type I transgenic reporter mouse, we showed that inhibiting PDGFR-beta activation restricted the distribution of collagen-synthesizing cells to wound margins and dramatically reduced cell proliferation in vivo. By 14 days, treated wounds were fully closed. Blocking PDGFR-beta signaling did not prevent the differentiation of myofibroblasts in vitro but potently inhibited fibroblast proliferation and migration. In addition, PDGFR-beta inhibition in vivo was accompanied by abnormal microvascular morphogenesis reminiscent of that observed in PDGFR-beta-/- mice with significantly reduced immunostaining of the pericyte marker NG2. Imatinib treatment also inhibited pericyte proliferation and migration in vitro. This study highlights the significance of PDGFR-beta signaling for the recruitment, proliferation, and functional activities of fibro-blasts and pericytes during the early phases of wound healing.

Exposure of mice to arsenic and/or benzo[a]pyrene does not increase the frequency of Aprt-deficient cells recovered from explanted skin of Aprt heterozygous mice

Exposure to inorganic arsenic in drinking water is linked to cancer in humans, but the mechanism of arsenic-induced cancer is not clear. Arsenic is not a powerful point mutagen, but can cause chromosome malsegregation and mitotic recombination, two events that can cause loss of tumor suppressor alleles and thereby contribute to the evolution of cancerous cells. To determine whether arsenic increases the frequency of allele loss due to either malsegregation or mitotic recombination in vivo, Aprt(+/-) hybrid mice were exposed to sodium arsenite (10 mg/L) in their drinking water for 10 weeks. To determine whether arsenic enhances the action of a known mutagen, half of the arsenic-treated mice were exposed to benzo[a]pyrene (BaP) for 8 weeks by skin painting (500 nmoles/week). Cells were taken from painted dorsal skin and cultured in the presence of 2,6-diaminopurine (DAP), to select colonies lacking adenosine phosphoribosyl transferase (Aprt) activity. The frequency of DAP-resistant (DAP(r)) colonies varied substantially within the treatment groups, but there was no significant difference between the groups. Analysis of DNA from DAP(r) colonies suggested that mitotic recombination contributed to the loss of wild-type Aprt allele. Whether arsenic or BaP enhanced or diminished the frequency of this process could not be deduced from these data.

HMG-CoA reductase inhibitors induce apoptosis in pericytes

Pericytes, which surround endothelial cells in precapillary arterioles, capillaries, and postcapillary venules, are important for the development, maturation, and maintenance of the vascular system. Pericytes are also pluripotent cells that can differentiate into a variety of mesenchymal cells including smooth muscle cells and osteoblasts. Possibly because of their vasculature regulating activities and ability to differentiate in situ, pericytes are implicated in several diseases with vascular complications, including diabetic retinopathy, as well as Reynaud's Syndrome, central nervous system dementias, and vascular calcification among others. Statin drugs, which block the conversion of HMG-CoA to mevalonate in the cholesterol synthesis pathway, are known to have apoptotic and growth inhibitory effects on cells in vitro and complex pleiotropic effects on cells and tissues in vivo. Recently, evidence has emerged that statin drug use in human patients results in a significant 20% reduction in cancer incidence. It is not known whether these results are due to direct statin action on normal tissue, growth inhibitory/pro-apoptotic effects on tumor cells, and/or effects on angiogenesis. Because of the role of pericytes in angiogenesis and the effects of statins on cancer incidence, we tested the direct effects of statins on pericytes. Specifically, we demonstrate that 3 statins, simvastatin, lovastatin, and mevastatin induce dose-dependent apoptosis in the TR-PCT1 pericyte cell line, that simvastatin (empirically shown to be the most potent of the 3 statins) induces similar levels of apoptosis in freshly isolated pericytes, and that simvastatin-induced apoptosis in pericytes is cholesterol, caspase-3, and caspase-7 mediated.

An MT1-MMP-PDGF receptor-beta axis regulates mural cell investment of the microvasculature

Platelet-derived growth factor (PDGF)/PDGFRbeta-dependent investment of the vascular endothelium by mural cells (i.e., pericytes and vascular smooth muscle cells; VSMCs) is critical for normal vessel wall structure and function. In the developing vasculature, mural cell recruitment is associated with the functionally undefined expression of the type I transmembrane proteinase, membrane-type 1 matrix metalloproteinase (MT1-MMP). In this paper, using VSMCs and tissues isolated from gene-targeted mice, we identify MT1-MMP as a PDGF-B-selective regulator of PDGFRbeta-dependent signal transduction and mural cell function. In VSMCs, catalytically active MT1-MMP associates with PDGFRbeta in membrane complexes that support the efficient induction of mitogenic signaling by PDGF-B in a matrix metalloproteinase inhibitor-sensitive fashion. In contrast, MT1-MMP-deficient VSMCs display PDGF-B-selective defects in chemotaxis and proliferation as well as ERK1/2 and Akt activation that can be rescued in tandem fashion following retroviral transduction with the wild-type protease. Consistent with these in vitro findings, MT1-MMP-deficient brain tissues display a marked reduction in mural cell density as well as abnormal vessel wall morphology similar to that reported in mice expressing PDGF-B or PDGFRbeta hypomorphic alleles. Together, these data identify MT1-MMP as a novel proteolytic modifier of PDGF-B/PDGFRbeta signal transduction that cooperatively regulates vessel wall architecture in vivo.

Endogenous thrombospondin-1 is not necessary for proliferation but is permissive for vascular smooth muscle cell responses to platelet-derived growth factor

We have reexamined the role of endogenous thrombospondin-1 (TSP1) in growth and motility of vascular smooth muscle cells (SMCs). Based on the ability of aortic-derived SMCs isolated from TSP1 null mice and grown in the absence of exogenous TSP1 to grow at comparable rates and to a slightly higher density than equivalent cells from wild-type mice, TSP1 is not necessary for their growth. Low concentrations of exogenous TSP1 stimulate growth of TSP1 null SMCs, but higher doses of TSP1 or its C-terminal domain are inhibitory. However, SMCs from TSP1 null mice are selectively deficient in chemotactic and proliferative responses to platelet-derived growth factor and in outgrowth in three-dimensional cultures. Recombinant portions of the N- and C-terminal domains of TSP1 stimulate SMC chemotaxis through different integrin receptors. Based on these data, the relative deficiency in SMC outgrowth during an ex vivo angiogenic response of muscle tissue from TSP1 null mice is probably due to restriction of platelet-derived growth factor dependent SMC migration and/or proliferation.

PDGF and cardiovascular disease

Platelet-derived growth factor (PDGF) was identified in a search for serum factors that stimulate smooth muscle cell (SMC) proliferation. During the development of lesions of atherosclerosis that can ultimately lead to vessel occlusion, SMC are stimulated by inflammatory factors to migrate from their normal location in the media. They accumulate within the forming lesion where they contribute to lesion expansion by proliferation and deposition of extracellular matrix. Different genetic manipulations in vascular cells combined with various inhibitory strategies have provided strong evidence for PDGF playing a prominent role in the migration of SMC into the neointima following acute injury and in atherosclerosis. Other activities of PDGF identified in vivo suggest additional functions for PDGF in the pathogenesis of cardiovascular disease.

Signal transduction pathways as novel therapy targets in lung cancer

Cytotoxic therapy for lung-cancer patients has only moderately improved during the last decades. Simultaneously, efforts of intensive research to increase our understanding of the molecular basis of lung cancer have been undertaken. The cancer cell has been characterised by several genetic changes that lead to altered cellular functions. In addition, multiple factors of the cancer-cell environment further affect the tumour cell via various receptors and subsequent signaling pathways. The increased knowledge of cellular signaling offers the opportunity to develop novel substances that target specific pathway molecules. In the current review, some of the most essential receptors and signaling pathways involved in lung cancer will be described. In conjunction, examples of novel target-specific agents that have already found their way into clinical trials will be discussed.

Deletion of the PDGFR-beta gene affects key fibroblast functions important for wound healing

This study provides new perspectives of the unique aspects of platelet-derived growth factor beta-receptor (PDGFR-beta) signaling and biological responses through the establishment of a mutant mouse strain in which two loxP sequences were inserted into the introns of PDGFR-beta genome sequences. Isolation of skin fibroblasts from the mutant mice and Cre recombinase transfection in vitro induced PDGFR-beta gene deletion (PDGFR-betaDelta/Delta). The resultant depletion of the PDGFR-beta protein significantly attenuated platelet-derived growth factor (PDGF)-BB-induced cell migration, proliferation, and protection from H2O2-induced apoptosis of the cultured PDGFR-betaDelta/Delta dermal fibroblasts. PDGF-AA and fetal bovine serum were mitogenic and anti-apoptotic but were unable to induce the migration in PDGFR-beta Delta/Delta fibroblasts. Concerning the PDGF signaling, PDGF-BB-induced phosphorylation of Akt, ERK1/2, and JNK, but not p38, decreased in PDGFR-betaDelta/Delta fibroblasts, but PDGF-AA-induced signaling was not altered. Overexpression of the phospholipid phosphatases, SHIP2 and/or PTEN, inhibited PDGF-BB-induced phosphorylation of Akt and ERK1/2 in PDGFR-betaDelta/Delta fibroblasts but did not affect that of JNK and p38. These results indicate that disruption of distinct PDGFR-beta signaling pathways in PDGFR-betaDelta/Delta dermal fibroblasts impaired their proliferation and survival, but completely inhibits migratory response, and that PDGF-BB-induced phosphorylation of Akt and ERK1/2 possibly mediated by PDGFR-alpha is regulated, at least in part, by the lipid phosphatases SHIP2 and/or PTEN. Thus, the PDGFR-beta function on dermal fibroblasts appears to be critical in PDGF-BB action for skin wound healing and is clearly distinctive from that of PDGFR-alpha in the ligand-induced biological responses and the underlying properties of cellular signaling.

Measuring hemodynamic changes during mammalian development

The pathogenesis of many congenital cardiovascular diseases involves abnormal flow within the embryonic vasculature that results either from malformations of the heart or defects in the vasculature itself. Extensive genetic and genomic analysis in mice has led to the identification of an array of mutations that result in cardiovascular defects during embryogenesis. Many of these mutations cause secondary effects within the vasculature that are thought to arise because of altered fluid dynamics. Presumably, cardiac defects disturb or reduce flow and thereby lead to the disruption of the mechanical signals necessary for proper vascular development. Unfortunately, a precise understanding of how flow disruptions lead to secondary vasculature defects has been hampered by the inadequacy of existing analytical tools. Here, we used a fast line-scanning technique for the quantitative analysis of hemodynamics during early organogenesis in mouse embryos, and we present a model system for studying cellular responses during the formation and remodeling of the mammalian cardiovascular system. Flow velocity profiles can be measured as soon as a heart begins to beat even in newly formed vessels. These studies establish a link between the pattern of blood flow within the vasculature and the stage of heart development and also enable analysis of the influence of mechanical forces during development.

Angiogenesis of gastrointestinal tumours and their metastases – a target for intervention?

Angiogenesis is an obligatory event for the growth of tumours beyond 2 mm in diameter, above which simple oxygen diffusion can no longer support the rapid proliferation of malignant cells. Angiogenesis is a fine balance between inhibitory and stimulatory factors, the knowledge of which offers novel targets for the treatment of gastrointestinal neoplasia. A literature search of Pubmed and Medline databases was undertaken, using the keywords colorectal cancer, pancreatic cancer, gastrointestinal cancer, angiogenesis and anti-angiogenesis therapy. It was found that angiogenesis in primary tumours is a sequential and highly complex cascade of molecular events resulting in the rapid exponential growth of the tumour. Hepatic metastases of primary tumours may be less reliant on traditional angiogenic pathways, by co-opting pre-existing hepatic vasculature. Research into angiogenesis has revealed many different sites that can be targeted by agents such as tyrosine kinase inhibitors. Many anti-angiogenic agents are undergoing preclinical evaluation, with only a few entering phase I and phase III clinical trials. However, early results suggest that anti-angiogenic therapy could be an important adjunct to conventional chemotherapy treatment of gastrointestinal neoplasia.

Platelet-derived growth factor plays a critical role to convert bone marrow cells into glomerular mesangial-like cells

BACKGROUND

Despite increasing interest in bone marrow-derived stem cells, little is known about critical factors that determine their fates both in vitro and in vivo. Recently, we have reported that bone marrow is a reservoir for glomerular mesangial cells in rats. To find a key factor responsible for the differentiation of bone marrow-derived cells into mesangial cells, we established a new culture system of rat bone marrow, which is based on serial replating and differential attachment to collagen types I and IV.

METHODS

Bone marrow cells that did not adhere to collagen type I within 24 hours were transferred to collagen type IV-coated dishes. Then, the cells attached to collagen type IV in the following 24 hours were maintained in the presence of 2% horse serum, 200 ng/mL of platelet-derived growth factor (PDGF)-BB, and 1 micromol/L of all-trans retinoic acid. In vivo effect of PDGF-B was also examined by introducing human PDGF-B gene into glomeruli.

RESULTS

After cultivation under the above condition for 7 days, approximately 14% of cells expressed Thy-1 and desmin, both of which are markers for rat mesangial cells. Thy-1++/desmin+ cells were stellate-shaped, and contracted in response to angiotensin II. When human PDGF-B gene was overexpressed in the glomeruli of chimeric rats whose bone marrow was transplanted from enhanced green florescent protein (EGFP) transgenic rats, the number of EGFP+ mesangial cells increased. This effect was canceled by prior introduction of a neutralizing molecule that is composed of PDGF receptor-beta ligand binding site and IgG-Fc.

CONCLUSION

These results indicate that PDGF-B plays a critical role to direct bone marrow-derived cells toward mesangial-like cells both in vitro and in vivo.

Chimera analysis supports a predominant role of PDGFRbeta in promoting smooth-muscle cell chemotaxis after arterial injury

The carotid artery shows a common response to many forms of injury, including a rapid activation of smooth muscle cell (SMC) proliferation in the media and migration of SMCs into the intima to form a neointima. Platelet-derived growth factor (PDGF) is believed to play a role in this response to injury, but it has proven difficult to distinguish whether it is stimulating cell migration or cell proliferation, and whether the action is direct or indirect. To determine this, we created chimeric mice composed of both wild-type (WT) and marked PDGF receptor beta (PDGFRbeta)-deficient cells, and determined the consequences of PDGFRbeta expression for SMC participation in response to ligation of the left common carotid artery. The proportion of PDGFRbeta-/- SMCs increased 4.5-fold in the media and decreased 1.8-fold during formation of the neointima, consistent with migration of WT SMCs out of the media and into the intima, leaving the PDGFRbeta-/- cells behind. The fibrotic reaction in the adventitia, which does not involve cell migration, did not result in any change in relative abundance of WT and PDGFRbeta-deficient fibroblasts. We conclude that the most significant direct role of PDGFRbeta is to mediate responses that involve cell migration rather than proliferation.

Biology of angiogenesis in tumors of the gastrointestinal tract

The realization that the growth and spread of tumors are dependent on angiogenesis has created new avenues of research designed to help us to better understand cancer biology and to facilitate the development of new therapeutic strategies. However, the process of angiogenesis consists of multiple, sequential, and interdependent steps with a myriad of positive and negative regulators of angiogenesis being involved. The survival of tumors and thus their metastases are dependent upon the balance of endogenous angiogenic and anti-angiogenic factors such that the outcome favors increased angiogenesis. Several growth factors have been identified that regulate angiogenesis in cancers of the gastrointestinal tract. These include pro-angiogenic factors like vascular endothelial growth factor (VEGF) and anti-angiogenic factors, i.e., thrombospondin. The following review provides a brief overview about the most important factors that are involved in the angiogenic process in tumors derived from colon, stomach, and pancreas. A thorough understanding of the role these factors play in the angiogenic process may lead to the development of novel therapeutic antineoplastic strategies.

Blockade of platelet-derived growth factor or its receptors transiently delays but does not prevent fibrous cap formation in ApoE null mice

Platelet-derived growth factor (PDGF) is a potent stimulant of smooth muscle cell migration and proliferation in culture. To test the role of PDGF in the accumulation of smooth muscle cells in vivo, we evaluated ApoE -/- mice that develop complex lesions of atherosclerosis. Fetal liver cells from PDGF-B-deficient embryos were used to replace the circulating cells of lethally irradiated ApoE -/- mice. One month after transplant, all monocytes in PDGF-B -/- chimeras are of donor origin (lack PDGF), and no PDGF-BB is detected in circulating platelets, primary sources of PDGF in lesions. Although lesion volumes are comparable in the PDGF-B +/+ and -/- chimeras at 35 weeks, lesions in PDGF-B -/- chimeras contain mostly macrophages, appear less mature, and have a reduced frequency of fibrous cap formation as compared with PDGF-B +/+ chimeras. However, after 45 weeks, smooth muscle cell accumulation in fibrous caps is indistinguishable in the two groups. Comparison of elicited peritoneal macrophages by RNase protection assay shows an altered cytokine and cytokine receptor profile in PDGF-B -/- chimeras. ApoE -/- mice were also treated for up to 50 weeks with a PDGF receptor antagonist that blocks all three PDGF receptor dimers. Blockade of the PDGF receptors similarly delays, but does not prevent, accumulation of smooth muscle and fibrous cap formation. Thus, elimination of PDGF-B from circulating cells or blockade of PDGF receptors does not appear sufficient to prevent smooth muscle accumulation in advanced lesions of atherosclerosis.

Stable expression of angiopoietin-1 and other markers by cultured pericytes: phenotypic similarities to a subpopulation of cells in maturing vessels during later stages of angiogenesis in vivo

Pericytes have been difficult cells to study because they do not maintain their characteristic phenotype in vitro, and they begin to express fibroblast markers after only a few days in culture. We now report methods for the isolation, purification, culture, and repurification of human dermal pericytes from mixed cell populations using an immunoaffinity-magnetic bead approach coupled with the 3G5 IgM monoclonal antibody that is specific for a pericyte surface ganglioside. These purified cells could be expanded in culture, and they maintained their pericyte phenotype for up to 8 days. In addition, they strongly expressed angiopoietin-1 (Ang-1) but not angiopoietin-2, Tie-1, or Tie-2; in contrast, dermal microvascular endothelial cells exhibited a reciprocal expression pattern. These findings are important because the close proximity of endothelial cells and pericytes has often made it difficult to determine with certainty the specific cell type(s) that expressed each of these proteins in situ. Extending our in vitro findings to two models of angiogenesis in vivo, we demonstrated a subpopulation of Ang-1-expressing cells that appeared in maturing microvessels during later stages of cutaneous wound healing and vascular permeability factor/vascular endothelial growth factor-induced angiogenesis. Our results provide strong evidence that Ang-1 is expressed by pericytes in vitro and in vivo and that the role proposed for Ang-1 in vessel maturation in development can be extended to vessel maturation after angiogenesis in adult tissues.

Hepatic stellate cell proliferation is an early platelet-derived growth factor-mediated cellular event in rat cholestatic liver injury

SUMMARY

After liver injury, hepatic stellate cells (HSC) undergo a pleiotropic response termed "activation" that also occurs in culture models and ultimately leads to the conversion of HSC into myofibroblasts expressing smooth muscle alpha-actin (alpha-SMA). The onset of HSC proliferation in primary culture coincides with the induction of platelet-derived growth factor receptor-beta (PDGFR-beta) expression, while platelet-derived growth factor (PDGF) is the most potent mitogen for culture-activated HSC. Yet, the mechanisms and the stage of activation required for HSC proliferation in the intact liver are still uncertain. In the present study, we analyzed the proliferative response of HSC to rat cholestatic liver injury and the role of PDGF in this response. After in vivo incorporation of bromodeoxyuridine (BrdU), pure vitamin A-containing HSC were isolated at different time points after bile duct ligation (BDL) or sham operation and were analyzed by means of flow cytometry. The induction of HSC proliferation, as ascertained by BrdU incorporation, occurred between 24 and 48 hours and reached a plateau as soon as 48 hours after BDL. Flow cytometry and immunoblot analyses of HSC indicated that the induction of proliferation in HSC coincided with the up-regulation of PDGFR-beta protein on their surface but preceded that of alpha-SMA. A dose-dependent inhibition of PDGF-BB-induced HSC proliferation by STI571, a PDGF receptor tyrosine kinase inhibitor, was documented in vitro. Daily intraperitoneal injections of STI571 (20 mg/kg) caused a 60% reduction in BrdU positive isolated HSC and in the amount of desmin-immunoreactive sinusoidal cells on liver tissue sections in 48-hour bile duct-ligated rats. These results indicate that cholestatic liver injury elicits an early proliferative response in HSC that is mainly mediated by PDGF, and which precedes HSC phenotypic conversion into myofibroblasts.

Platelet-derived growth factor B-chain of hematopoietic origin is not necessary for granulation tissue formation and its absence enhances vascularization

The hypothesis that wound repair is augmented by delivery of platelet-derived growth factor (PDGF) from platelets and macrophages is an attractive extrapolation from the known activities of PDGF in cell culture and in vivo. To test this hypothesis in mice, we prepared hematopoietic chimeras, in which the hematopoietic system of a normal adult mouse was replaced by the hematopoietic system of a PDGF B-chain -/- or +/+ donor. We initiated local granulation tissue formation either by implanting small surgical sponges to elicit a foreign body granulation tissue response, or by ligating the left common carotid to form an organized thrombus. We found that the absence of hematopoietic PDGF B-chain did not decrease the extent of granulation tissue or vascular lesion formation, and that the vascularization of both lesions increased by approximately 100%. We conclude that PDGF B-chain from cells of hematopoietic origin, including platelets and macrophages, is not important for granulation tissue formation, and that it reduces vascularization of granulation issue, probably through disabling of the short-range chemotactic gradients of PDGF that are important for recruiting pericytes/smooth muscle cells to the endothelium of new vessels.

Developmental roles of platelet-derived growth factors

Platelet-derived growth factor (PDGF) was originally identified in platelets and in serum as a mitogen for fibroblasts, smooth muscle cells (SMC) and glia cells in culture. PDGF has since expanded to a family of dimers of at least four gene products, whose biological actions are mediated through two receptor tyrosine kinases, PDGFRs. The present review summarizes and discusses the biological functions of PDGFs and PDGFRs in developmental processes, mainly as revealed through genetic analysis in mice. Such studies have demonstrated multiple critical roles of PDGFs and PDGFRs in embryonic and postnatal development. PDGFs seem to act upon specific populations of progenitor cells that give rise to several different cell types with distinct functions in a variety of developmental processes. Analogies are seen between the cell functions and the developmental processes controlled by PDGFs. This suggests that ancestral PDGF and PDGFR expression patterns and functions may have been iterated in related sets of morphogenetic processes in the course of evolution.

Aspirin effects on gastric epithelial cell proliferation and cytokine expression

Aspirin is known to cause gastric injury and to delay ulcer healing. The effects of aspirin on gastric epithelial cell function are heterogeneous; in contrast to injuring the mucosa, aspirin may also act beneficially by inducing adaptation; a mechanism that is poorly understood. We aimed to document the effects of different doses of aspirin on gastric epithelial cell function defined as proliferation, and secretion as well as mRNA expression of cytokines. Furthermore, we studied the effects of aspirin pretreatment on cytokine secretion as a potential element of gastric adaptation. The proliferative activity of three different gastric epithelial cell lines (AGS, KATO III, RGM-1) was assessed by (3)H-thymidine incorporation; secretion of growth factors PDGF-AB and VEGF into culture supernatant was documented by ELISA. mRNA transcripts of both cytokines were quantified by real time RT-PCR. Low doses of aspirin did not alter the proliferative dynamics in two of the three studied cell lines; high doses abolished proliferation. Secretion of PDGF-AB and VEGF increased during the first days of low dose aspirin exposition; higher concentrations led to a depletion of cytokines after an initial liberation in the case of VEGF, mRNA of which was also dose-dependently increased by aspirin. Seven-day pretreatment with low amounts of aspirin did not alter the secretory response of the epithelia caused by higher doses of this drug. The secretion of cytokines and proliferation of gastric epithelial cells are adversely effected by aspirin in a similarly dose-dependent fashion as the intended effects of this drug on platelet function and pain relief.

In vivo chemotactic properties and spatial expression of PDGF in developing mesenteric microvascular networks

The recruitment of perivascular cells to developing microvessels is a key component of microvessel assembly. Whereas platelet-derived growth factor (PDGF) signaling is critical for this process during embryonic development, its role from the postnatal stages through adulthood remains unclear. We investigated the potential role of PDGF signaling during microvessel assembly by measuring in vivo the migration of labeled fibroblasts to PDGF in mesenteric connective tissue and by examining PDGF-B and PDGF receptor-beta (PGDFR-beta) expression in microvascular networks during normal maturation. PDGF-B homodimer (PDGF-BB; 30 ng/ml) application elicited a significant (P < 0.05) increase (7.8 +/- 4.1 cells) in labeled fibroblasts within 100 microm of the source micropipette after 2 h. PDGF-A homodimer (30 ng/ml) application and control solution did not elicit directed migration. PDGF-B was expressed in microvessel endothelium and smooth muscle, whereas PDGFR-beta was expressed in endothelium, smooth muscle, and interstitial fibroblasts. Given that PDGF-BB elicits fibroblast migration in the mesentery and that PDGF-B and PDGFR-beta are expressed in a pattern that indicates paracrine signaling from microvessels to the interstitium, the results are consistent with a role for PDGF-B in perivascular cell recruitment to microvessels.

Involvement of platelet-derived growth factor in disease: development of specific antagonists

Platelet-derived growth factor (PDGF) is a family of dimeric isoforms that stimulates, e.g., growth, chemotaxis and cell shape changes of various connective tissue cell types and certain other cells. The cellular effects of PDGF isoforms are exerted through binding to two structurally related tyrosine kinase receptors. Ligand binding induces receptor dimerization and autophosphorylation. This enables a number of SH2 domain containing signal transduction molecules to bind to the receptors, thereby initiating various signaling pathways. PDGF isoforms have important roles during the embryonic development, particularly in the formation of connective tissue in various organs. In the adult, PDGF stimulates wound healing. Overactivity of PDGF has been implicated in certain disorders, including fibrotic conditions, atherosclerosis, and malignancies. Different kinds of PDGF antagonists are currently being developed and evaluated in different animal disease models, as well as in clinical trials.

Intimal hyperplasia recurs after removal of PDGF-AB and -BB inhibition in the rat carotid artery injury model

Several antagonists specific for platelet-derived growth factor (PDGF) or its receptors have recently been developed and shown to inhibit intimal hyperplasia formation in various animal models, but data investigating the durability of this intervention is limited. The present study was designed to investigate the potency of PDGF B-chain aptamer, a novel type of PDGF-AB and -BB antagonist, in the rat carotid model and to characterize intermediate-term effects on lesion formation. One hundred thirty-four animals were randomized to aptamer treatment or placebo. Daily treatment with the antagonist resulted in a 50% reduction in lesion size at 2 weeks (P<0.001). The beneficial effect involved increased apoptosis and possibly an interference with smooth muscle cell migration. Discontinuing administration 1 week earlier did not give any significant benefit compared with phosphate-buffered saline-treated controls. When the antagonist was administered for 2 weeks and the vessels analyzed 6 weeks later, the beneficial effect was lost and the treated lesions had a higher intima-media and area-cell ratio compared with the treated lesions in the 2-week-endpoint study. Our findings confirm a role of PDGF B-chain in intimal hyperplasia, but the successful use of PDGF antagonists may require either prolonged treatment or combination therapy with other agents.

Endothelial cells of hematopoietic origin make a significant contribution to adult blood vessel formation

Granulation tissue formation is an example of new tissue development in an adult. Its rich vascular network has been thought to derive via angiogenic sprouting and extension of preexisting vessels from the surrounding tissue. The possibility that circulating cells of hematopoietic origin can differentiate into vascular endothelial cells (ECs) in areas of vascular remodeling has recently gained credibility. However, no quantitative data have placed the magnitude of this contribution into a physiological perspective. We have used hematopoietic chimeras to determine that 0.2% to 1.4% of ECs in vessels in control tissues derived from hematopoietic progenitors during the 4 months after irradiation and hematopoietic recovery. By contrast, 8.3% to 11.2% of ECs in vessels that developed in sponge-induced granulation tissue during 1 month derived from circulating hematopoietic progenitors. This recruitment of circulating progenitors to newly forming vessels would be difficult to observe in standard histological studies, but it is large enough to be encouraging for attempts to manipulate this contribution for therapeutic gain.

Interstitial fibrosis and growth factors

Interstitial pulmonary fibrosis (IPF) is scarring of the lung caused by a variety of inhaled agents including mineral particles, organic dusts, and oxidant gases. The disease afflicts millions of individuals worldwide, and there are no effective therapeutic approaches. A major reason for this lack of useful treatments is that few of the molecular mechanisms of disease have been defined sufficiently to design appropriate targets for therapy. Our laboratory has focused on the molecular mechanisms through which three selected peptide growth factors could play a role in the development of IPF. Hundreds of growth factors and cytokines could be involved in the complex disease process. We are studying platelet-derived growth factor because it is the most potent mesenchymal cell mitogen yet described, transforming growth factor beta because it is a powerful inducer of extracellular matrix (scar tissue) components by mesenchymal cells, and tumor necrosis factor alpha because it is a pleiotropic cytokine that we and others have shown is essential for the development of IPF in animal models. This review describes some of the evidence from studies in humans, in animal models, and in vitro, that supports the growth factor hypothesis. The use of modern molecular and transgenic technologies could elucidate those targets that will allow effective therapeutic approaches.

Brainstem activation of platelet-derived growth factor-beta receptor modulates the late phase of the hypoxic ventilatory response

The early phase of the biphasic ventilatory response to hypoxia in mammals is critically dependent on NMDA glutamate receptor activation within the nucleus of the solitary tract. However, the mechanisms underlying the subsequent development of the typical ventilatory roll-off are unclear and could underlie important roles in the functional and molecular adaptation to oxygen deprivation. Because the growth factor platelet-derived growth factor (PDGF)-BB can modulate the open channel probability of NMDA receptors by activating PDGF-beta receptors, its contribution to hypoxic ventilatory roll-off was examined. Administration of PDGF-BB, but not PDGF-AA, in the nucleus of the solitary tract was associated with significant attenuations of the early hypoxic ventilatory response in conscious rats. Furthermore, marked reductions in the magnitude of hypoxic ventilatory roll-off occurred in mice heterozygous for a mutation in the PDGF-beta receptor. Administration of a PDGF-beta receptor antagonist to wild-type littermates elicited similar declines in hypoxic ventilatory roll-off. The relative abundance of PDGF-beta receptors was confirmed in the nucleus of the solitary tract and other nuclei implicated in the hypoxic ventilatory response. In nucleus of the solitary tract lysates, PDGF-beta receptor tyrosine phosphorylation was temporally correlated with hypoxic ventilatory roll-off formation. Increased PDGF-B chain mRNA expression was induced by hypoxia in the nucleus of the solitary tract, and PDGF-B chain immunoreactivity colocalized with approximately 40% of nucleus of the solitary tract neurons, demonstrating hypoxia-induced c-Fos enhancements. Thus, PDGF-BB release and PDGF-beta receptor activation in the nucleus of the solitary tract are critical components of hypoxic ventilatory roll-off and may have important functional implications in processes underlying survival and acclimatization to hypoxic environments.