Control of proliferation of human vascular endothelial cells. Characterization of the response of human umbilical vein endothelial cells to fibroblast growth factor, epidermal growth factor, and thrombin

Human Umbilical Vein Endothelial Cells as a Versatile Cellular Model System in Diverse Experimental Paradigms: An Ultrastructural Perspective

Human umbilical vein endothelial cells (HUVECs) are primary cells isolated from the vein of an umbilical cord, extensively used in cardiovascular studies and medical research. These cells, retaining the characteristics of endothelial cells in vivo, serve as a valuable cellular model system for understanding vascular biology, endothelial dysfunction, pathophysiology of diseases such as atherosclerosis, and responses to different drugs or treatments. Transmission electron microscopy (TEM) has been a cornerstone in revealing the detailed architecture of multiple cellular model systems including HUVECs, allowing researchers to visualize subcellular organelles, membrane structures, and cytoskeletal elements. Among them, the endoplasmic reticulum, Golgi apparatus, mitochondria, and nucleus can be meticulously examined to recognize alterations indicative of cellular responses to various stimuli. Importantly, Weibel-Palade bodies are characteristic secretory organelles found in HUVECs, which can be easily distinguished in the TEM. These distinctive structures also dynamically react to different factors through regulated exocytosis, resulting in complete or selective release of their contents. This detailed review summarizes the ultrastructural features of HUVECs and highlights the utility of TEM as a pivotal tool for analyzing HUVECs in diverse research frameworks, contributing valuable insights into the comprehension of HUVEC behavior and enriching our knowledge into the complexity of vascular biology.

Plexin-A2 enables the proliferation and the development of tumors from glioblastoma derived cells

The semaphorin guidance factors receptor plexin-A2 transduces sema6A and sema6B signals and may mediate, along with plexin-A4, the anti-angiogenic effects of sema6A. When associated with neuropilins plexin-A2 also transduces the anti-angiogenic signals of sema3B. Here we show that inhibition of plexin-A2 expression in glioblastoma derived cells that express wild type p53 such as U87MG and A172 cells, or in primary human endothelial cells, strongly inhibits cell proliferation. Inhibition of plexin-A2 expression in U87MG cells also results in strong inhibition of their tumor forming ability. Knock-out of the plexin-A2 gene in U87MG cells using CRISPR/Cas9 inhibits cell proliferation which is rescued following plexin-A2 re-expression, or expression of a truncated plexin-A2 lacking its extracellular domain. Inhibition of plexin-A2 expression results in cell cycle arrest at the G2/M stage, and is accompanied by changes in cytoskeletal organization, cell flattening, and enhanced expression of senescence associated β-galactosidase. It is also associated with reduced AKT phosphorylation and enhanced phosphorylation of p38MAPK. We find that the pro-proliferative effects of plexin-A2 are mediated by FARP2 and FYN and by the GTPase activating (GAP) domain located in the intracellular domain of plexin-A2. Point mutations in these locations inhibit the rescue of cell proliferation upon re-expression of the mutated intracellular domain in the knock-out cells. In contrast re-expression of a plexin-A2 cDNA containing a point mutation in the semaphorin binding domain failed to inhibit the rescue. Our results suggest that plexin-A2 may represent a novel target for the development of anti-tumorigenic therapeutics.

Targeting Bronchopulmonary Dysplasia-Associated Pulmonary Hypertension (BPD-PH): Potential Role of the FGF Signaling Pathway in the Development of the Pulmonary Vascular System

More than 50 years after the first description of Bronchopulmonary dysplasia (BPD) by Northway, this chronic lung disease affecting many preterm infants is still poorly understood. Additonally, approximately 40% of preterm infants suffering from severe BPD also suffer from Bronchopulmonary dysplasia-associated pulmonary hypertension (BPD-PH), leading to a significant increase in total morbidity and mortality. Until today, there is no curative therapy for both BPD and BPD-PH available. It has become increasingly evident that growth factors are playing a central role in normal and pathologic development of the pulmonary vasculature. Thus, this review aims to summarize the recent evidence in our understanding of BPD-PH from a basic scientific point of view, focusing on the potential role of Fibroblast Growth Factor (FGF)/FGF10 signaling pathway contributing to disease development, progression and resolution.

Targeting Tumor Endothelial Cells with Nanoparticles

Because angiogenesis is a major contributor to cancer progression and metastasis, it is an attractive target for cancer therapy. Although a diverse number of small compounds for anti-angiogenic therapy have been developed, severe adverse effects commonly occur, since small compounds can affect not only tumor endothelial cells (TECs), but also normal endothelial cells. This low selectivity for TECs has motivated researchers to develop alternate types of drug delivery systems (DDSs). In this review, we summarize the current state of knowledge concerning the delivery of nano DDSs to TECs. Their payloads range from small compounds to nucleic acids. Perspectives regarding new therapeutic targets are also mentioned.

FGF2-induced STAT3 activation regulates pathologic neovascularization

Cell-autonomous endothelial cell (EC) fibroblast growth factor receptor (FGFR) signaling through FGFR1/2 is essential for injury-induced wound vascularization and pathologic neovascularization as in blinding eye diseases such as age-related macular degeneration. Which FGF ligand(s) is critical in regulating angiogenesis is unknown. Utilizing ex vivo models of choroidal endothelial sprouting and in vivo models of choroidal neovascularization (CNV), we demonstrate here that only FGF2 is the essential ligand. Though FGF-FGFR signaling can activate multiple intracellular signaling pathways, we show that FGF2 regulates pathogenic angiogenesis via STAT3 activation. The identification of FGF2 as a critical mediator in aberrant neovascularization provides a new opportunity for developing multi-target therapies in blinding eye diseases especially given the limitations of anti-VEGF monotherapy.

Urinary epidermal growth factor predicts renal prognosis in antineutrophil cytoplasmic antibody-associated vasculitis

INTRODUCTION

The current study aimed to investigate the association between urinary epidermal growth factor (uEGF) and renal disease severity and outcomes in patients with antineutrophil cytoplasmic antibody-associated vasculitis (AAV).

METHODS

Intrarenal EGFmRNA expression was extracted from transcriptomic data of microdissected tubulointerstitial compartments of kidney biopsies of patients with AAV. uEGF was measured in 173 patients with AAV in active stage and 143 in remission, and normalised to urine creatinine excretion (uEGF/Cr). The association between uEGF/Cr (or EGFmRNA) and clinical-pathological parameters was tested using linear regression analysis. The ability of uEGF/Cr to predict renal outcomes was analysed using Cox's regression analysis.

RESULTS

In patients with AAV, intrarenal EGFmRNA expression was significantly associated with estimated glomerular filtration rate (eGFR)(log2) at time of biopsy (β=0.63, p<0.001). The level of uEGF/Cr was significantly higher in patients in remission than in patients with active disease, both when looking at patients with sequential measurements (2.75±1.03vs 2.08±0.98, p<0.001) and in cross-sectional comparison. uEGF/Cr level was positively associated with eGFR(log2) at time of sampling in both active and remission stage (β=0.60, p<0.001; β=0.74, p<0.001, respectively). Patients with resistant renal disease had significantly lower uEGF/Cr levels than responders (1.65±1.22vs 2.16±1.26, p=0.04). Moreover, after adjusting for other potential predictors, uEGF/Cr was independently associated with composite endpoint of end-stage renal disease or 30% reduction of eGFR (HR 0.61, 95% CI 0.45 to 0.83, p=0.001).

CONCLUSION

Lower uEGF/Cr levels are associated with more severe renal disease, renal resistance to treatment and higher risk of progression to composite outcome in patients with AAV.

plexin-A4/plexin-D1 complexes convey semaphorin-3C signals to induce cytoskeletal collapse in the absence of neuropilins

Class-3 semaphorin guidance factors bind to receptor complexes containing neuropilin and plexin receptors. A semaphorin may bind to several receptor complexes containing somewhat different constituents, resulting in diverse effects on cell migration. U87MG glioblastoma cells express both neuropilins and the four class-A plexins. They respond by cytoskeletal collapse and cell contraction to sema3A or sema3B but fail to contract in response to Sema3C, Sema3D, Sema3G or sema3E even when class-A plexins are over-expressed in the cells. In-contrast, expression of recombinant plexin-D1 enabled contraction in response to these semaphorins. Surprisingly, unlike sema3D and sema3G, sema3C also induced the contraction and repulsion of plexin-D1 expressing U87MG cells in which both neuropilins were knocked-out using CRISPR/cas9. In the absence of neuropilins the EC-50 of sema3C was 5.5 fold higher, indicating that the neuropilins function as enhancers of plexin-D1 mediated sema3C signaling but are not absolutely required for sema3C signal transduction. Interestingly, in the absence of neuropilins, plexin-A4 formed complexes with plexin-D1, and was required in addition to plexin-D1 to enable sema3C induced signal transduction.

Coculture of mesenchymal stem cells and endothelial cells enhances host tissue integration and epidermis maturation through AKT activation in gelatin methacryloyl hydrogel-based skin model

A major challenge for clinical use of skin substitutes is insufficient host tissue integration leading to loosening and partial necrosis of the implant. In this present study, a three-dimensional (3D) coculture system constructed using human umbilical cord mesenchymal stem cells (uc-MSCs) and umbilical vein endothelial cells (HUVECs) encapsulated in gelatin methacryloyl (GelMA) hydrogels was evaluated to determine the outcomes of cell-cell interactions in vitro and in vivo. The results revealed that GelMA hydrogels displayed minor cytotoxicity on both cell types. An uc-MSC:HUVEC ratio of 50:50 demonstrated the highest cell proliferation and expression of angiogenic markers. The supplement of basic fibroblast growth factors (bFGF) in coculture system further induced cell proliferation and gene expression in vitro. In vivo transplantation of this cocultured constructs efficiently enhanced the implant and host tissue integration. Additionally, the proliferation of keratinocytes was well maintained on GelMA hydrogels and the gene expression related to cell proliferation and differentiation was significantly increased in coculture system comparing to monoculture. Mechanistically, AKT signaling pathways were activated in cocultures. Our findings suggest that coculturing MSC and EC in GelMA hydrogels could be a promising approach to substantially improve the integration of exogenous skin substitutes and host tissues.

STATEMENT OF SIGNIFICANCE

In this study, the co-culture of uc-MSCs and HUVECs in photocrosslinkable GelMA hydrogels significantly enhanced host tissue integration. Cell proliferation, ECM deposition and angiogenic genes expression were all substantially improved in vitro and the excellent host tissue integration into the implanted tissue was observed in vivo. When served as a dermal layer, the scaffold with co-cultured cells enhanced the proliferation and differentiation of keratinocytes. AKT signaling was proved to be involved in the regulation of cell survival and fate determination. This work demonstrated the importance of 3D cell co-culture to facilitate host tissue integration that can be a promising approach for long-term survival of skin substitutes.

Exposure to Fine Particulate Air Pollution Is Associated With Endothelial Injury and Systemic Inflammation

RATIONALE

Epidemiological evidence indicates that exposures to fine particulate matter air pollution (PM_2.5) contribute to global burden of disease, primarily as a result of increased risk of cardiovascular morbidity and mortality. However, mechanisms by which PM_2.5 exposure induces cardiovascular injury remain unclear. PM_2.5-induced endothelial dysfunction and systemic inflammation have been implicated, but direct evidence is lacking.

OBJECTIVE

To examine whether acute exposure to PM_2.5 is associated with endothelial injury and systemic inflammation.

METHODS AND RESULTS

Blood was collected from healthy, nonsmoking, young adults during 3 study periods that included episodes of elevated PM_2.5 levels. Microparticles and immune cells in blood were measured by flow cytometry, and plasma cytokine/growth factors were measured using multiplexing laser beads. PM_2.5 exposure was associated with the elevated levels of endothelial microparticles (annexin V⁺/CD41^-/CD31⁺), including subtypes expressing arterial-, venous-, and lung-specific markers, but not microparticles expressing CD62⁺. These changes were accompanied by suppressed circulating levels of proangiogenic growth factors (EGF [epidermal growth factor], sCD40L [soluble CD40 ligand], PDGF [platelet-derived growth factor], RANTES [regulated on activation, normal T-cell-expressed and secreted], GROα [growth-regulated protein α], and VEGF [vascular endothelial growth factor]), and an increase in the levels of antiangiogenic (TNFα [tumor necrosis factor α], IP-10 [interferon γ-induced protein 10]), and proinflammatory cytokines (MCP-1 [monocyte chemoattractant protein 1], MIP-1α/β [macrophage inflammatory protein 1α/β], IL-6 [interleukin 6], and IL-1β [interleukin 1β]), and markers of endothelial adhesion (sICAM-1 [soluble intercellular adhesion molecule 1] and sVCAM-1 [soluble vascular cellular adhesion molecule 1]). PM_2.5 exposure was also associated with an inflammatory response characterized by elevated levels of circulating CD14⁺, CD16⁺, CD4⁺, and CD8⁺, but not CD19⁺ cells.

CONCLUSIONS

Episodic PM_2.5 exposures are associated with increased endothelial cell apoptosis, an antiangiogenic plasma profile, and elevated levels of circulating monocytes and T, but not B, lymphocytes. These changes could contribute to the pathogenic sequelae of atherogenesis and acute coronary events.

Epidermal Growth Factor in Cutaneous Repair Processes

Donor areas on patients undergoing plastic surgery were treated with epidermal growth factor (EGF) to stimulate tissue regeneration. Mor phometric analyses showed that cutaneous explants, treated with EGF, pre sented a higher number of microvessels ( p < 0.01) and mesenchimal cells ( p < 0.01), than untreated areas, while EGF did not improve epidermal thick ening. By electron microscopy, better histoarchitectural maturation of cellular and extracellular dermal compartments was observed after EGF application. This work provides evidence that EGF does stimulate skin repair by promoting the formation of mature cutaneous tissue.

Tipping the balance from angiogenesis to fibrosis in CKD

Chronic progressive renal fibrosis leads to end-stage renal failure many patients with chronic kidney disease (CKD). Loss of the rich peritubular capillary network is a prominent feature, and seems independent of the specific underlying disease. The mechanisms that contribute to peritubular capillary regression include the loss of glomerular perfusion, as flow-dependent shear forces are required to provide the survival signal for endothelial cells. Also, reduced endothelial cell survival signals from sclerotic glomeruli and atrophic or injured tubule epithelial cells contribute to peritubular capillary regression. In response to direct tubular epithelial cell injury, and the inflammatory reaction that ensues, capillary pericytes dissociate from their blood vessels, also reducing endothelial cell survival. In addition, direct inflammatory injury of capillary endothelial cells, for instance in chronic allograft nephropathy, also contributes to capillary dropout. Chronic tissue hypoxia, which ensues from the rarefaction of the peritubular capillary network, can generate both an angiogenic and a fibrogenic response. However, in CKD, the balance is strongly tipped toward fibrogenesis. Understanding the underlying mechanisms for failed angiogenesis in CKD and harnessing endothelial-specific survival and pro-angiogenic mechanisms for therapy should be our goal if we are to reduce the disease burden from CKD.

FGF2 and insulin signaling converge to regulate cyclin D expression in multipotent neural stem cells

The ex vivo expansion of stem cells is making major contribution to biomedical research. The multipotent nature of neural precursors acutely isolated from the developing central nervous system has been established in a series of studies. Understanding the mechanisms regulating cell expansion in tissue culture would support their expanded use either in cell therapies or to define disease mechanisms. Basic fibroblast growth factor (FGF2) and insulin, ligands for tyrosine kinase receptors, are sufficient to sustain neural stem cells (NSCs) in culture. Interestingly, real-time imaging shows that these cells become multipotent every time they are passaged. Here, we analyze the role of FGF2 and insulin in the brief period when multipotent cells are present. FGF2 signaling results in the phosphorylation of Erk1/2, and activation of c-Fos and c-Jun that lead to elevated cyclin D mRNA levels. Insulin signals through the PI3k/Akt pathway to regulate cyclins at the post-transcriptional level. This precise Boolean regulation extends our understanding of the proliferation of multipotent NSCs and provides a basis for further analysis of proliferation control in the cell states defined by real-time mapping of the cell lineages that form the central nervous system.

Endothelial cell FGF signaling is required for injury response but not for vascular homeostasis

Endothelial cells (ECs) express fibroblast growth factor receptors (FGFRs) and are exquisitely sensitive to FGF signals. However, whether the EC or another vascular cell type requires FGF signaling during development, homeostasis, and response to injury is not known. Here, we show that Flk1-Cre or Tie2-Cre mediated deletion of FGFR1 and FGFR2 (Fgfr1/2(Flk1-Cre) or Fgfr1/2(Tie2-Cre) mice), which results in deletion in endothelial and hematopoietic cells, is compatible with normal embryonic development. As adults, Fgfr1/2(Flk1-Cre) mice maintain normal blood pressure and vascular reactivity and integrity under homeostatic conditions. However, neovascularization after skin or eye injury was significantly impaired in both Fgfr1/2(Flk1-Cre) and Fgfr1/2(Tie2-Cre) mice, independent of either hematopoietic cell loss of FGFR1/2 or vascular endothelial growth factor receptor 2 (Vegfr2) haploinsufficiency. Also, impaired neovascularization was associated with delayed cutaneous wound healing. These findings reveal a key requirement for cell-autonomous EC FGFR signaling in injury-induced angiogenesis, but not for vascular homeostasis, identifying the EC FGFR signaling pathway as a target for diseases associated with aberrant vascular proliferation, such as age-related macular degeneration, and for modulating wound healing without the potential toxicity associated with direct manipulation of systemic FGF or VEGF activity.

The role of the plexin-A2 receptor in semaphorin-3A and semaphorin-3B signal transduction

Class-3 semaphorins are anti-angiogenic and anti-tumorigenic guidance factors that bind to neuropilins which in turn associate with class-A plexins to transduce semaphorin signals. To study the role of the plexin-A2 receptor in semaphorin signaling, we silenced its expression in endothelial cells and in glioblastoma cells. The silencing did not affect sema3A signaling which depended on neuropilin-1, plexin-A1 and plexin-A4, but abolished completely sema3B signaling which required in addition plexin-A4 and one of the two neuropilins. Interestingly, over-expression of plexin-A2 in plexin-A1 or plexin-A4 silenced cells restored responses to both semaphorins although it nullified their ability to differentiate between them, suggesting that when over-expressed plexin-A2 is functionally interchangeable with other class-A plexins. In-contrast, although plexin-A4 over-expression restored sema3A signaling in plexin-A1 silenced cells, it failed to restore sema3B signaling in plexin-A2 silenced cells. It follows that the identity of plexins in functional semaphorin receptors can be flexible depending on their expression level. Our results suggest that changes in the expression of plexins induced by microenvironmental cues can trigger differential responses of different populations of migrating cells to encountered gradients of semaphorins.

Pathologies at the nexus of blood coagulation and inflammation: thrombin in hemostasis, cancer, and beyond

Thrombin is the protease involved in blood coagulation. Its deregulation can lead to hemostatic abnormalities, which range from subtle subclinical to serious life-threatening coagulopathies, i.e., during septicemia. Additionally, thrombin plays important roles in many (patho)physiological conditions that reach far beyond its well-established role in stemming blood loss and thrombosis, including embryonic development and angiogenesis but also extending to inflammatory processes, complement activation, and even tumor biology. In this review, we will address thrombin's broad roles in diverse (patho)physiological processes in an integrative way. We will also discuss thrombin as an emerging major target for novel therapies.

Low intensity laser therapy speeds wound healing in hemophilia by enhancing platelet procoagulant activity

Our group has previously shown that cutaneous wound healing is delayed and histologically abnormal in a mouse model of hemophilia. Hemostasis is not only required to stop bleeding at the time of wounding, but also produces bioactive substances that promote appropriate inflammatory and proliferative responses during healing. Low intensity laser therapy (LILT) has been reported to enhance impaired wound healing in a variety of animal and human studies. The current studies were conducted to test the hypothesis that LILT can improve healing in a hemophilia B mouse model. Three daily treatments with 12 J/sq cm of 650 nm laser illumination reduced the time to closure of a 3-mm cutaneous punch biopsy wound in the hemophilic mice. All wounds were closed at 13 days in the sham-treated hemophilic mice, compared with 10 days in the LILT-treated hemophilic mice, and 9 days in wild-type mice. While LILT can speed healing by enhancing proliferation of cutaneous cells, we found that an additional mechanism likely contributes to the efficacy of LILT in the hemophilic mice. LILT enhanced the mechanical rigidity and platelet activity of clots formed from human platelet-rich plasma. Illumination of isolated platelets increased the mitochondrial membrane potential and enhanced binding of coagulation factors to the surface of activated platelets. Thus, while LILT can directly promote proliferative responses during healing, it also appears to enhance hemostasis in an animal model with impaired coagulation. These data suggest that trials of LILT as an adjunct to the usual hemostatic therapies in hemophilia are warranted.

Wound healing in haemophilia--breaking the vicious cycle

Our group has been studying how haemostasis interacts with repair processes and also how to optimize treatment of bleeding disorders in a mouse model of haemophilia B. We have found that cutaneous wounds heal more slowly in haemophilic mice than in wild-type mice, and also exhibit histological abnormalities, even after closure of the skin defect. The haemophilic wounds showed reduced influx of inflammatory cells and increased angiogenesis. Even after surface closure, the haemophilic animals experienced repeated episodes of re-bleeding and progressive accumulation of iron in the wound bed and deeper tissues. A dose of replacement or bypassing therapy sufficient to establish initial haemostasis did not normalize wound healing. In fact, daily dosing for 7 days was required to normalize wound closure. Thus, normal healing requires adequate haemostatic function for an extended period of time. We have hypothesized that this is because angiogenesis during healing predisposes to bleeding, especially in the setting where haemostasis is impaired. Thus, normalizing haemostasis, until the process of angiogenesis has resolved, may be required to prevent re-bleeding and additional tissue damage.

Platelet functions beyond hemostasis

Although their central role is in the prevention of bleeding, platelets probably contribute to diverse processes that extend beyond hemostasis and thrombosis. For example, platelets can recruit leukocytes and progenitor cells to sites of vascular injury and inflammation; they release proinflammatory and anti-inflammatory and angiogenic factors and microparticles into the circulation; and they spur thrombin generation. Data from animal models suggest that these functions may contribute to atherosclerosis, sepsis, hepatitis, vascular restenosis, acute lung injury, and transplant rejection. This article represents an integrated summary of presentations given at the Fourth Annual Platelet Colloquium in January 2009. The process of and factors mediating platelet-platelet and platelet-leukocyte interactions in inflammatory and immune responses are discussed, with the roles of P-selectin, chemokines and Src family kinases being highlighted. Also discussed are specific disorders characterized by local or systemic platelet activation, including coronary artery restenosis after percutaneous intervention, alloantibody-mediated transplant rejection, wound healing, and heparin-induced thrombocytopenia.

A chimeric epidermal growth factor with fibrin affinity promotes repair of injured keratinocyte sheets

The aim of the present study is to create a novel chimeric protein of epidermal growth factor (EGF) with fibrin affinity and demonstrate its potential for repairing injured tissues by immobilization to fibrin. The chimeric protein (FBD-EGF) was produced by the fusion of the fibronectin fibrin-binding domain (FBD) to EGF. It showed dose-dependent binding to fibrin and its binding was stable for at least 7days, while native EGF showed little affinity. FBD-EGF promoted the growth of fibroblasts and keratinocytes in the fibrin-bound state as well as in the soluble state. Its activity was further studied in a keratinocyte culture system in which fibrin was exposed upon injury of cell sheets. Fibrin-bound FBD-EGF promoted growth of the sheets over the injured area at a significantly faster rate (approximately eightfold) than native EGF (p<0.01). Wounds 2mm wide were closed in 7-9days. This repair process was inhibited by anti-EGF. Keratinocytes proliferated more extensively in the leading edges of sheets contacting fibrin with FBD-EGF, approximately 1.7-fold more than in the adjacent regions. These results imply that the stable binding of chimeric EGF to fibrin is effective for the repair of injured keratinocyte sheets, suggesting a potential use in tissue engineering.

The scientific contributions of M. Judah Folkman to cancer research

Dr Judah Folkman was frequently described as a highly compassionate physician who served his patients not only by performing surgery and offering them comfort and reassurance, but also by working tirelessly in the laboratory to find new approaches to the treatment of disease. His dedication to understanding the role of angiogenesis, the formation of new blood vessels, in human disease has given rise to new treatments for several diseases, including inflammatory diseases, vision-threatening diseases of the eye and, as will be emphasized in this Perspective, cancer.

Regulation of nestin expression by thrombin and cell density in cultures of bone mesenchymal stem cells and radial glial cells

BACKGROUND

Bone marrow stromal cells and radial glia are two stem cell types with neural phenotypic plasticity. Bone marrow mesenchymal stem cells can differentiate into osteocytes, chondrocytes and adipocytes, but can also differentiate into non-mesenchymal cell, i.e. neural cells in appropriate in vivo and in vitro experimental conditions. Likewise, radial glial cells are the progenitors of many neurons in the developing cortex, but can also generate astrocytes. Both cell types express nestin, an intermediate filament protein which is the hallmark of neural precursors.

RESULTS

In this study, we demonstrate that thrombin, a multifunctional serine protease, stimulates the growth of radial glial cells (RG) and mesenchymal stem cells (MSCs) in a dose-dependent manner. In RG, the mitogenic effect of thrombin is correlated with increased expression of nestin but in MSCs, this mitogenic effect is associated with nestin down-regulation. Both cell types express the PAR-1 type receptor for Thrombin and the effect of Thrombin on both cell types can be mimicked by its analogue TRAP-6 activating specifically this receptor subtype or by serum which contains various amount of thrombin. Moreover, we also demonstrate that serum deprivation-induced expression of nestin in MSCs is inhibited by high cell density (> 50,000 cells/cm2).

CONCLUSION

This work shows that thrombin stimulates the growth of both RG and MSCs and that nestin expression by MSCs and RG is regulated in opposite manner by thrombin in vitro. Thrombin effect is thus associated in both cell types with a proliferating, undifferentiated state but in RG this involves the induction of nestin expression, a marker of immaturity for neural progenitors. In MSCs however, nestin expression, as it corresponds to a progression from the mesenchymal "undifferentiated", proliferating phenotype toward acquisition of a neural fate, is inhibited by the mitogenic signal.

Restoring hemostatic thrombin generation at the time of cutaneous wounding does not normalize healing in hemophilia B

BACKGROUND

We recently reported that wound healing is abnormal in hemophilia B (HB) mice [1]. The wounds show abnormal histology: s.c. hematoma formation; delayed re-epithelialization; delayed macrophage influx; and an increase in wound site angiogenesis.

OBJECTIVE

To test the hypothesis that restoring a hemostatic level of thrombin generation at the time of wounding would allow formation of an adequate platelet/fibrin plug and correct abnormalities of wound healing in HB.

METHODS

We placed a 3-mm cutaneous wound on the back of each HB or wild-type (WT) mouse. Some HB mice were treated just prior to wounding with either human factor IX (FIX) or FVIIa in a dose sufficient to normalize bleeding in a tail bleed model.

RESULTS

The average wound size over time in treated HB animals was intermediate between those in WT and untreated HB mice. However, the time to complete skin closure was not improved by treatment. Hematoma formation was decreased and macrophage influx began earlier in treated than in untreated HB animals. However, treated HB mice had evidence of ongoing low-level bleeding near the wound site, even after closure of the skin defect. Treatment also did not normalize the increased angiogenesis observed in HB mice.

CONCLUSIONS

Restoring initial hemostasis can modulate some of the parameters of wound healing. However, an extended period of adequate hemostatic function is necessary to achieve normal healing, probably because the risk of hemorrhage is increased by vascular remodeling and angiogenesis during the healing process.

Ocular neovascularization: basic mechanisms and therapeutic advances

The vast majority of diseases that cause catastrophic loss of vision do so as a result of ocular neovascularization. During normal retinal vascular development, vascular endothelial cells proliferate and migrate through the extracellular matrix in response to a variety of cytokines, leading to the formation of new blood vessels in a highly ordered fashion. During abnormal neovascularization of the iris, retina, or choroid, angiogenesis is unregulated and usually results in the formation of dysfunctional blood vessels. When these newly formed vessels leak fluid, hemorrhage, or are associated with fibrous proliferation, retinal edema, retinal/vitreous hemorrhage, or traction retinal detachments may occur resulting in potentially catastrophic loss of vision. In this review, we will briefly discuss the scope of the clinical problem and the general underlying principles of angiogenesis. We will focus on recent laboratory advances that have led to the development of therapeutics useful in the treatment of neovascular eye diseases. We will describe compounds currently in pre-clinical development stages as well as the results of clinical trials involving the use of these drugs as treatments for ocular neovascularization.

ErbB receptors in fetal endothelium--a potential linkage point for inflammation-associated neonatal disorders

OBJECTIVE

ErbB receptors and their ligands play crucial roles in development. During late gestation, they might also be involved in the pathogenesis of prematurity-associated disorders. ErbB receptor dimerization leads to a diversity of biologic signals. We studied the expression and localization patterns of erbB receptors in the developing human umbilical endothelial cell system. It is still unclear, whether expression patterns might be developmentally regulated and depend on the cell type studied.

METHODS

Primary human umbilical venous endothelial cells (HUVEC) and arterial endothelial cells (HUAEC) were isolated between 24 and 42 weeks of gestation and used for immunoprecipitation, Western blotting, and confocal microscopy.

RESULTS

All four erbB receptors were present in HUVEC and HUAEC. Expression patterns were similar for cell types at gestational ages examined. ErbB4 always co-precipitated with erbB1 in both cell types independent of the gestational age. Confocal microscopy revealed that all erbB receptors were localized in the nucleus, erbB1 and erbB3 in the nucleoli, while erbB2 and erbB4 spared the nucleolar region. All receptors showed a tendency to co-localize. Growth factor stimulation altered localization patterns. Cellular subfractionation experiments for erbB4 largely confirmed microscopy results. Pretreatment with lipopolysaccharide enhanced this nuclear localization of erbB4, particularly of its intracellular domain.

CONCLUSIONS

All erbB receptors are present in both HUVEC and HUAEC at all gestational ages tested. ErbB receptor expression patterns were independent of the developmental stage of the endothelial cell, at least in the third trimester. We speculate that endothelial erbB receptors might play a role in normal development in mid and late gestation. We also speculate that these findings, together with the known involvement of erbB receptors in development, inflammation, and angiogenesis, will open new avenues for erbB receptor-related research in the pathogenesis of fetal and neonatal inflammation-associated disorders.

Pro-angiogenic cytokines and their role in tumor angiogenesis

The development of solid tumors depends upon an adequate supply of blood. This can be achieved by way of co-option of preexisting blood vessels and by the induction of angiogenesis. During the past 30 years, tumor angiogenesis had been found to play a crucial role in the progression of solid tumors. Tumor angiogenesis was found to be induced by a variety of pro-angiogenic cytokines of which the best characterized is vascular endothelial growth factor (VEGF). Indeed, the first FDA approved anti-angiogenic drug for the treatment of cancer is Avastin, a neutralizing antibody directed against VEGF. This review focuses on cytokines which have been reported to induce tumor angiogenesis.

Increased Serum Vascular Endothelial Growth Factor Following Major Surgical Injury

OBJECTIVES

Vascular endothelial growth factor (VEGF) plays an important role in angiogenesis. We evaluated the changes in serum levels of VEGF following major surgical trauma and postoperative inflammatory complications.

MATERIALS AND METHODS

The serum concentration of VEGF was measured with enzyme-linked immunosorbent assay in 41 patients with esophageal cancer who underwent right transthoracic esophagectomy with extensive lymphadenectomy and in 13 patients with gallstones who underwent less-invasive laparoscopic cholecystectomy for comparison. Serum and plasma samples were obtained before the operation and on postoperative days (PODs) 1, 7, 14, 21, and 28. The changes in serum VEGF levels were compared among groups categorized by age, sex, blood loss volume during operation, amount of transfusion, pathological stage of the tumor, and postoperative inflammatory complications. The correlation between serum VEGF levels and inflammatory factors, such as peripheral blood cell count, interleukin-6 (IL-6), C-reactive protein (CRP), and severity of postoperative inflammatory complications, was also investigated. Furthermore, because platelets are a potential source of serum VEGF, platelet-poor plasma (PPP) was prepared from plasma samples, and the VEGF concentration in PPP was measured to compare with those in sera.

RESULTS

Serum VEGF levels increased significantly postoperatively. After reaching maximal levels on POD 14, VEGF levels gradually decreased until POD 28. The increase in the tranthoracic esophagectomy group was approximately twice that in laparoscopic cholecystectomy group on POD 14. Serum VEGF levels were not correlated with sex, age, blood loss, amount of transfusion, or tumor stage. However, serum VEGF levels were significantly higher in patients with postoperative inflammatory lung complications than in patients without such complications, and the maximal level of serum VEGF correlated with the severity of postoperative lung complications. However, there were no significant correlations between the increase in the level of serum VEGF and that of serum IL-6 or CRP. The increase of platelet counts in the peripheral blood correlated with that of the serum VEGF level, and VEGF levels in PPP were significantly lower than those in sera.

CONCLUSIONS

Serum VEGF levels increased in the angiogenesis phase of wound healing following major surgical injury. Platelets are a potential source of increased serum VEGF levels, whereas inflammatory lung complications might also be related to increased serum VEGF levels.

Thrombin induces tumor growth, metastasis, and angiogenesis: Evidence for a thrombin-regulated dormant tumor phenotype

The association of idiopathic venous thrombosis with occult cancer is generally recognized. However, it has not been fully appreciated that thrombin generated during thrombosis can augment the malignant phenotype. Thrombin activates tumor cell adhesion to platelets, endothelial cells, and subendothelial matrix proteins; enhances tumor cell growth; increases tumor cell seeding and spontaneous metastasis; and stimulates tumor cell angiogenesis. These mechanisms are reviewed. Evidence is also presented to support the hypothesis that thrombin serves to preserve dormant tumor cells in individuals, preventing host eradication. It is proposed that tumor malignancy may be regulated by a procoagulant/anticoagulant axis.

Implantation of a new porous gelatin-siloxane hybrid into a brain lesion as a potential scaffold for tissue regeneration

For brain tissue regeneration, any scaffold for migrated or transplanted stem cells with supportive angiogenesis is important once necrotic brain tissue has formed a cavity after injury such as cerebral ischemia. In this study, a new porous gelatin-siloxane hybrid derived from the integration of gelatin and 3-(glycidoxypropyl) trimethoxysilane was implanted as a three-dimensional scaffold into a defect of the cerebral cortex. The porous hybrid implanted into the lesion remained at the same site for 60 days, kept integrity of the brain shape, and attached well to the surrounding brain tissues. Marginal cavities of the scaffolds were occupied by newly formed tissue in the brain, where newly produced vascular endothelial, astroglial, and microglial cells were found with bromodeoxyuridine double positivity, and the numbers of those cells were dose-dependently increased with the addition of basic fibroblast growth factor (bFGF) and epidermal growth factor (EGF). Extension of dendrites was also found from the surrounding cerebral cortex to the newly formed tissue, especially with the addition of bFGF and EGF. The present study showed that a new porous gelatin-siloxane hybrid had biocompatibility after implantation into a lesion of the central nervous system, and thus provided a potential scaffold for cell migration, angiogenesis and dendrite elongation with dose-dependent effects of additive bFGF and EGF.

The role of endometrium in endometriosis

Endometriosis is defined as the presence of endometrial glands and stroma outside the uterus. Several theories have been proposed to explain the pathogenesis of this disease. According to Sampson's retrograde menstruation theory, endometrial cells are refluxed through the fallopian tubes during the menstruation and implant onto peritoneum or pelvic organs. Since retrograde menstruation is a very common phenomenon among women of reproductive age, there must be other factors that may contribute to the pathophysiology and/or pathogenesis of endometriosis. Genetic predisposition, environmental factors, and alterations in immune and endocrine functions are believed to play significant roles in the establishment and maintenance of endometriosis. Although the eutopic endometriums of women with and without endometriosis are histologically similar, studies revealed that there are many fundamental differences between these two tissues. Invasive properties, decreased apoptosis, alterations in expression of specific gene and proteins, and increased steroid and cytokine production have been identified in eutopic endometrium of women with endometriosis. Furthermore, significant biochemical differences exist even between ectopic and autologous eutopic endometrium. These differences can be explained by the direct effects of an inflammatory peritoneal environment.

The epidermal growth factor receptors and their family of ligands: their putative role in atherogenesis

The epidermal growth factor receptor is a member of type-I growth factor receptor family with tyrosine kinase activity that is activated following the binding of multiple cognate ligands. Several members of the EGF family of ligands are expressed by cells involved in atherogenesis. EGF receptor mediated processes have been well characterised within epithelial, smooth muscle and tumour cell lines in vitro, and the EGF receptor has been identified immunocytochemically on intimal smooth muscle cells within atherosclerotic plaques. There is also limited evidence for the expression of the EGF receptor family on leukocytes, although their function has yet to be clarified. In this review, we will discuss the biological functions of this receptor and its ligands and their potential to modulate the function of cells involved in the atherosclerotic process.

Thrombin-mediated IL-10 up-regulation involves protease-activated receptor (PAR)-1 expression in human mononuclear leukocytes

Thrombin, the key enzyme of the coagulation cascade, exerts cellular effects through activation of the protease-activated receptors (PARs). Interleukin (IL)-10, besides its anti-inflammatory properties, is considered a major denominator of the immunosuppressive effect during human endotoxemia. We have recently shown that thrombin inhibits IL-12 production in human mononuclear cells and that such inhibition is accompanied by IL-10 up-regulation. To our knowledge, there are no data available to show that thrombin mediates IL-10 production by its interactions with PAR-1. We here report that human alpha-thrombin enhances IL-10 expression in human peripheral blood mononuclear cells and in established monocytic cell lines and that this up-regulation requires PAR-1 expression. The use of proteolytically inactive thrombin reveals that such enhancement requires thrombin proteolytic activity. Addition of PAR-1 agonist peptides, such as SFLLRN, results in a significant increase of IL-10 production. PAR-1 expression is required for thrombin-induced IL-10 production, as shown by experiments performed with antisense or sense PAR-1 oligonucleotides. Treatment with thrombin or SFLLRN of monocytic cell lines, such as U937 and Mono Mac-6, results in an increased IL-10 production. This suggests that the observed IL-10 up-regulation may be the result of a direct interaction with monocytes. The observation that thrombin-mediated up-regulation of IL-10 may require the expression of the PAR-1 receptor identifies a new, functional link between inflammation and coagulation. Our results may also contribute to better design therapeutic strategies to treat several disorders, characterized by the presence of inflammatory as well as coagulant responses.

Thrombin and Its Protease-Activated Receptor-1 (PAR1) Participate in the Endothelial–Mesenchymal Transdifferentiation Process

The serine protease thrombin, independently of its participation in hemostasis and thrombosis, has been involved in tissue repair and remodeling, embryogenesis, angiogenesis, and development and progression of atherosclerosis. Many of these functions appear to be mediated by specific thrombin receptors, particularly the protease-activated receptor-1 (PAR1). In this study, we investigated whether both thrombin and PAR1 were present in the aortic wall of chicken embryos at days 11 and 12 of development. We found that PAR1 was limited to some cells of the intimal thickening and the inner media, whereas thrombin appeared distributed across the aortic wall. We also investigated whether PAR1 was present during endothelial-mesenchymal transdifferentiation (EMT) in vitro. A moderate immunoreactivity was detected in the monolayer of endothelial cells. In contrast, a strong cytoplasmic immunoreactivity was observed in the detaching and migrating cells and those that had acquired mesenchymal characteristics. This PAR1 expression was confirmed by flow cytometry. In this study, the addition of thrombin to arrested endothelial cell cultures was assessed. We found that thrombin stimulated endothelial cell spreading and migration, as no migrating cells were observed in serum-free medium (SFM) condition. Immunolocalization of PAR1 in the thrombin-treated cultures showed strong cytoplasmic immunoreactivity in the monolayers and in spreading and migrating cells, whereas in the SFM condition undetectable PAR1 immunoreactivity was observed. Flow cytometry of these cultures revealed an elevated expression of PAR1 in the presence of thrombin, in contrast to that detected in SFM and complete medium. These data indicate that both thrombin and PAR1 are involved in the remodeling of the aortic wall and intimal thickening formation, and in the endothelial-mesenchymal transdifferentiation process.

Regulation of VEGF-A, VEGFR-I, thrombospondin-1, -2, and -3 expression in a human pituitary cell line (HP75) by TGFbeta1, bFGF, and EGF

Pituitary tumors are highly vascular neoplasms, which suggest an important role of angiogenesis in pituitary tumor growth. We used the human pituitary cell line (HP75) to examine the effects of the growth factors TGFbeta1, bFGF, and EGF on cell growth, and on the regulation of the pro-angiogenic growth factor VEGF-A and the VEGFR-I and the anti-angiogenic molecules thrombospondin (TSP) TSP-1 and TSP-2 along with TSP-3. Real-time RT-PCR was used to measure mRNA levels, and Western blot was used to analyze TSP-1 and TSP-2 protein levels. TGFbeta1 treatment (1 x 10(-9) M) increased VEGF-A mRNA levels significantly (p < 0.05) after 4 and 24 h of treatment. TGF beta1 treatment decreased VEGF-R mRNA levels after 96 h of treatment (p < 0.05). After 96 h of treatment, TSP-1 and TSP-2 mRNA levels were significantly increased (p < 0.05) by TGFbeta1 treatment, which also inhibited HP75 cell growth. Basic FGF also increased TSP-1 mRNA levels after 96 h of treatment, but did not regulate growth of the pituitary tumor cells. Basic FGF and EGF did not modulate changes in VEGF-A mRNA levels after 4 and 24 h of treatment, but EGF increased VEGF-A significantly (p < 0.05) after 96 h of treatment. These results indicate that TGFbeta1 treatment may regulate angiogenesis in pituitary cells by initially increasing levels of pro-angiogenic VEGF-A and then stimulating the anti-angiogenic molecules TSP-1 and TSP-2 levels.

Role of protease-activated receptors in the vascular system

Thrombin is one of the key molecules involved in the development of vascular diseases. Thrombin does not only serve as a coagulation factor, but it also exerts cellular effects by activating protease (proteinase)-activated receptors (PARs), a family of seven-transmembrane G protein-coupled receptors. This study focused on the role of PARs in the vascular system. Among the four members so far identified, PAR-1 and PAR-2 were found to play an important role in the vascular system, while the functional roles of PAR-3 and PAR-4 appear to be mostly limited to platelets. The endothelial cells play a primary role in mediating the vascular effects of PARs under physiological conditions, while PARs of the smooth muscle cells can be induced under pathological conditions, and therefore play a more pathophysiological role. PAR-1 and PAR-2 mediate various vascular effects including regulation of vascular tone, proliferation and hypertrophy of smooth muscle and angiogenesis. Since proteases are activated under pathological conditions such as hemorrhage, tissue damage, and inflammation, PARs are suggested to play a critical role in the development of functional and structural abnormality in the vascular lesion. Understanding the functional role of PARs in the vascular system can thus help in the development of new strategies for the prevention and therapy of vascular diseases.

Angiogenic effects of dual gene transfer of bFGF and PDGF-BB after myocardial infarction

Therapeutic effects of combination of angiogenic growth factors for the treatment of ischemia after myocardial infarction are largely unknown. Plasmids expressing basic fibroblast growth factor (bFGF), platelet-derived growth factor (PDGF-BB) or their combination with a 1:1 mass ratio were injected into hearts with 7-day-old myocardial infarction. Hearts were harvested after 1 and 4 weeks after gene transfer. The major findings in this chronic myocardial infarction model were that bFGF, PDGF-BB and their combination all had a more pronounced angiogenic effect on the arteriolar than the capillary level. bFGF stimulated both capillary and arteriolar growth while PDGF-BB preferentially stimulated arterioles. The combination increased the amount of both capillaries and arterioles and in addition gave rise to stable capillaries compared to single factor transfer but did not further enhance angiogenesis. No cardiovascular side effects were observed after gene transfer.

Thrombin enhancement of interleukin-1 expression in mononuclear cells: involvement of proteinase-activated receptor-1

In addition to its central role in blood coagulation and hemostasis, human alpha-thrombin is considered a pro-inflammatory molecule. We have previously demonstrated that differentiated monocytes express the proteolytically activated receptor for thrombin (PAR-1) and that thrombin enhances the release of interleukin (IL)-6 in human monocytes. In the present study we show that thrombin upregulates the production of both IL-1alpha and IL-1beta in phytohemagglutin (PHA)-activated human peripheral blood mononuclear cells (PBMC). Treating PHA-activated PBMC with the PAR-1 activation peptide, SFLLRN, mimics the effects of thrombin on IL-1alpha and IL-1beta production. Thus, it appears that these pro-inflammatory effects induced by thrombin may be mediated through activation of PAR-1. ELISA and RNase protection assays indicate that thrombin and SFLLRN peptide upregulates IL-1 expression at both protein and mRNA levels. Thrombin directly affects monocyte IL-1 expression, since treatment of differentiated U937 cells with thrombin and SFLLRN enhances IL-1 production. These results may help explain how thrombin can enhance IL-1 expression in normal tissue to initiate tissue repair and why thrombin and thrombin-like enzymes may contribute to inflammatory responses observed in several pathophysiological conditions.

Human alpha-thrombin stimulates proliferation of interferon-gamma differentiated, growth-arrested U937 cells, overcoming differentiation-related changes in expression of p21CIP1/WAF1 and cyclin D1

In addition to its central role in blood coagulation and hemostasis, human alpha-thrombin is a growth factor for a variety of cell types. We recently demonstrated that interferon-gamma (IFNgamma)-differentiated U937 cells show increased expression of the proteolytically activated receptor for thrombin (PAR-1) relative to undifferentiated U937. In the present study we show that cell proliferation is inhibited in IFNgamma-differentiated cells relative to undifferentiated U937. Addition of thrombin to the differentiated cells, however, overcomes the inhibition and restores the cells to a highly proliferative state. Ribonuclease protection assays indicate that the IFNgamma-induced growth arrest is associated with an increased expression of the cyclin-dependent kinase inhibitor p21(CIP1/WAF1) and downregulation of cyclin D(1). Treatment of cells with thrombin downregulates p21(CIP1/WAF1) expression in these cells and upregulates cyclin D(1) mRNA expression, thus overcoming the differentiation-related effects in a coordinated manner. Treating differentiated cells with the PAR-1 activation peptide, SFLLRN, stimulates proliferation and has effects similar to those of thrombin on expression of p21(CIP1/WAF1). Thus, it appears that these thrombin stimulated proliferative effects are mediated through activation of PAR-1. These results may help explain how thrombin can overcome growth arrest in normal tissue to initiate tissue repair and why thrombin and thrombin-like enzymes may contribute to unrestricted proliferation observed in certain malignancies.

Angiogenic factors in endometriosis

Similar to tumor metastases, endometriotic implants require neovascularization to establish, grow, and invade. The peritoneal environment is ideally suited to provide a proangiogenic milieu. Nevertheless, endometriotic lesions are found only in a minority of reproductive-age women (approximately 10%) with retrograde menstruation. In this paper, we review the major cytokines, growth factors, steroid hormones, and eicosanoids responsible for angiogenesis in endometriosis. We postulate that interference with angiogenic principles expressed in the peritoneum may constitute novel therapeutic opportunities for the prevention, amelioration, or treatment of pelvic endometriosis.

Advances towards understanding heart valve response to injury

BACKGROUND

Composed of endocardial endothelial, valvular interstitial, cardiac muscle, and smooth muscle cells (SMC), heart valves are prone to various pathologic conditions the morphology of which has been well described. The morphology of diseased valves suggest that the "response to injury" process occurs in these valves, and is associated with an accumulation of interstitial cells and matrix, valvular inflammation and calcification, conditions that lead to dysfunction. The purpose of this study is to describe the current knowledge of the regulation of the valvular "response to injury" process, since we feel that this paradigm is essential to understanding valve disease.

METHODS

The pertinent literature relating to the cell and molecular biology of valvular repair, and specifically interstitial cell function in valve repair, is reviewed.

RESULTS

The cell and molecular biology of valve interstitial cells are poorly understood. Molecules regulating some of the aspects of the "response to injury" process have been studied, however, the signal transduction pathways, gene activation, and interactions of bioactive molecules with each other, with cells, and with the matrix have not been characterized. Initial studies identify the cell and molecular biology of interstitial cells to be an important area of research. Agents that have been studied include nitric oxide (NO) and FGF-2 and several matrix-related proteins including osteopontin. The present review suggests several directions for future study and a working model of valvular repair is presented.

DISCUSSION

The regulation of the "response to injury" process in the human heart valve is still largely unknown. The cell and molecular events and processes that occur in heart valve function and repair remain poorly understood. These events and processes are vital to our understanding of the pathobiology of heart valve disease, and to the successful design of tissue engineered replacement valves.

Bimodal concentration-dependent effect of thrombin on endothelial cell proliferation and growth factor release in culture

BACKGROUND

The role of thrombin in the stimulation of endothelial cell (EC) proliferation is controversial. The aim of this study was to investigate if thrombin regulates cell proliferation and production of platelet-derived growth factor (PDGF), bovine fibroblast growth factor (bFGF), and transforming growth factor beta(1) (TGF-beta(1)) by bovine aortic ECs.

METHODS

ECs, obtained from thoracic aortas of calves, were stimulated with thrombin at various concentrations (from 0.05 to 1.0 IU/ml) in serum free culture. Mitogenic activity of thrombin on ECs was determined by tritiated thymidine uptake. The release of PDGF, bFGF, and TGF-beta(1) was assessed by ELISA. PDGF release was confirmed by Western blot and bFGF and TGF-beta(1) mRNA expression was determined by polymerase chain reaction (PCR).

RESULTS

Thrombin at high concentrations did not cause any increase in EC proliferation after 72 h of culture and induced inhibition of EC proliferation after 96 h and 8 days of culture. It induced a decrease in PDGF release and an increase in TGF-beta(1) release. Thrombin at low concentrations induced a significant increase in EC proliferation at 72 h, 96 h, and 8 days of culture. It induced an increase in PDGF release and a decrease in TGF-beta(1) release. bFGF release was higher than control at all thrombin concentrations. These data were confirmed by Western blot and PCR studies.

CONCLUSIONS

Thrombin regulates EC growth through the inhibition of EC proliferation at high concentrations and through the stimulation of EC proliferation at low physiological concentrations. EC proliferation is partially mediated by autocrine production of PDGF, bFGF, and TGF-beta(1).