Fibrinolytic proteins in apoptotic human umbilical vein endothelial cells

Small Molecule Inhibitors of Plasminogen Activator Inhibitor-1 Elicit Anti-Tumorigenic and Anti-Angiogenic Activity

Numerous studies have shown a paradoxical positive correlation between elevated levels of plasminogen activator inhibitior-1 (PAI-1) in tumors and blood of cancer patients with poor clinical outcome, suggesting that PAI-1 could be a therapeutic target. Here we tested two orally bioavailable small molecule inhibitors of PAI-1 (TM5275 and TM5441) for their efficacy in pre-clinical models of cancer. We demonstrated that these inhibitors decreased cell viability in several human cancer cell lines with an IC₅₀ in the 9.7 to 60.3 μM range and induced intrinsic apoptosis at concentrations of 50 μM. In vivo, oral administration of TM5441 (20 mg/kg daily) to HT1080 and HCT116 xenotransplanted mice increased tumor cell apoptosis and had a significant disruptive effect on the tumor vasculature that was associated with a decrease in tumor growth and an increase in survival that, however, were not statistically significant. Pharmacokinetics studies indicated an average peak plasma concentration of 11.4 μM one hour after oral administration and undetectable levels 23 hours after administration. The effect on tumor vasculature in vivo was further examined in endothelial cells (EC) in vitro and this analysis indicated that both TM5275 and TM5441 inhibited EC branching in a 3D Matrigel assay at concentrations where they had little effect on EC apoptosis. These studies bring novel insight on the activity of PAI-1 inhibitors and provide important information for the future design of inhibitors targeting PAI-1 as therapeutic agents in cancer.

A biphasic endothelial stress-survival mechanism regulates the cellular response to vascular endothelial growth factor A

Vascular endothelial growth factor A (VEGF-A) is an essential cytokine that regulates endothelial function and angiogenesis. VEGF-A binding to endothelial receptor tyrosine kinases such as VEGFR1 and VEGFR2 triggers cellular responses including survival, proliferation and new blood vessel sprouting. Increased levels of a soluble VEGFR1 splice variant (sFlt-1) correlate with endothelial dysfunction in pathologies such as pre-eclampsia; however the cellular mechanism(s) underlying the regulation and function of sFlt-1 are unclear. Here, we demonstrate the existence of a biphasic stress response in endothelial cells, using serum deprivation as a model of endothelial dysfunction. The early phase is characterized by a high VEGFR2:sFlt-1 ratio, which is reversed in the late phase. A functional consequence is a short-term increase in VEGF-A-stimulated intracellular signaling. In the late phase, sFlt-1 is secreted and deposited at the extracellular matrix. We hypothesized that under stress, increased endothelial sFlt-1 levels reduce VEGF-A bioavailability: VEGF-A treatment induces sFlt-1 expression at the cell surface and VEGF-A silencing inhibits sFlt-1 anchorage to the extracellular matrix. Treatment with recombinant sFlt-1 inhibits VEGF-A-stimulated in vitro angiogenesis and sFlt-1 silencing enhances this process. In this response, increased VEGFR2 levels are regulated by the phosphatidylinositol-3-kinase and PKB/Akt signaling pathways and increased sFlt-1 levels by the ERK1/2 signaling pathway. We conclude that during serum withdrawal, cellular sensing of environmental stress modulates sFlt-1 and VEGFR2 levels, regulating VEGF-A bioavailability and ensuring cell survival takes precedence over cell proliferation and migration. These findings may underpin an important mechanism contributing to endothelial dysfunction in pathological states.

Angiogenesis in cancer: Anti-VEGF escape mechanisms

It is now widely accepted that tumor-angiogenesis plays a crucial role in tumor growth, tumor propagation and metastasis formation. Among several angiogenic activators, the vascular endothelial growth factor (VEGF) and its receptors represent one of the major inducers of tumor angiogenesis. Thus, this system has become the focus of therapeutic interventions, which led to the approval of the anti-VEGF blocking antibody bevacizumab and the VEGFR-2 pathway inhibitors pazopanib, sorafenib and sunitinib. However, not every cancer patient benefits from such treatment or finally becomes resistant to anti-VEGF approaches; others are suffering from adverse effects. Thus, there is an urgent need for a better understanding of VEGF-independent mechanisms leading to angiogenesis in cancer. This review focuses on anti-VEGF escape mechanisms of tumor cells and its microenvironment.

Plasminogen activator inhibitor type 1 interacts with alpha3 subunit of proteasome and modulates its activity

Plasminogen activator inhibitor type-1 (PAI-1), a multifunctional protein, is an important physiological regulator of fibrinolysis, extracellular matrix homeostasis, and cell motility. Recent observations show that PAI-1 may also be implicated in maintaining integrity of cells, especially with respect to cellular proliferation or apoptosis. In the present study we provide evidence that PAI-1 interacts with proteasome and affects its activity. First, by using the yeast two-hybrid system, we found that the α3 subunit of proteasome directly interacts with PAI-1. Then, to ensure that the PAI-1-proteasome complex is formed in vivo, both proteins were coimmunoprecipitated from endothelial cells and identified with specific antibodies. The specificity of this interaction was evidenced after transfection of HeLa cells with pCMV-PAI-1 and coimmunoprecipitation of both proteins with anti-PAI-1 antibodies. Subsequently, cellular distribution of the PAI-1-proteasome complexes was established by immunogold staining and electron microscopy analyses. Both proteins appeared in a diffuse cytosolic pattern but also could be found in a dense perinuclear and nuclear location. Furthermore, PAI-1 induced formation of aggresomes freely located in endothelial cytoplasm. Increased PAI-1 expression abrogated degradation of degron analyzed after cotransfection of HeLa cells with pCMV-PAI-1 and pd2EGFP-N1 and prevented degradation of p53 as well as IκBα, as evidenced both by confocal microscopy and Western immunoblotting.

Altered hemostatic balance and endothelial activation in pregnant women with familial hypercholesterolemia

INTRODUCTION

Patients with familial hypercholesterolemia (FH) are prone to premature cardiovascular disease. During pregnancy plasma lipids reach higher absolute values in FH than in healthy women. Pregnancy is associated with activation of coagulation and possibly also of vascular endothelium, which might further increase the risk of cardiovascular disease in FH. However, whether hemostatic and endothelial activation markers are increased in pregnant FH women compared with non-FH pregnancies, is unknown.

MATERIALS AND METHODS

Activation markers of hemostasis and endothelium were analyzed in blood samples collected prospectively from 22 heterozygous FH women during pregnancy and compared with those of a reference group of 149 healthy, pregnant women.

RESULTS

A procoagulant pattern was detected in both groups, but was more evident among FH women at least partly due to their enhanced thrombin generation, and because tissue factor pathway inhibitor type 1 increased in the reference group only. Furthermore, plasminogen activator inhibitor type 2 antigen increased more in FH than in the reference group. Whereas C-reactive protein, intercellular adhesion marker-1 and E-selectin did not change appreciably, vascular endothelial cell adhesion molecule 1 rose markedly in FH.

CONCLUSION

Increased lipid levels as well as a net procoagulant activity and an enhanced endothelial activation possibly confer additional risks of cardiovascular disease among pregnant FH women.

Ultrastructural changes in endothelium during apoptosis indicate low microembolic potential

BACKGROUND

Apoptotic endothelium has been suggested to have microthrombotic and microembolic potential. While some describe pro-coagulant activities and platelet binding, others demonstrate maintained fibrinolytic protein and anti-platelet aggregatory activity. Canalicular fragmentation is unique to apoptotic endothelium and is suggested to facilitate size reduction of apoptotic cells to reduce microembolic potential. Despite the potential importance of apoptotic microemboli, there are no reports characterizing changes in cell size and shape during endothelial apoptosis.

METHODS

Here, we describe transmission and scanning electron microscopic studies of apoptotic endothelium and compare changes seen with apoptotic HL-60 cells incapable of canalicular fragmentation.

RESULTS

We demonstrate reduced endothelial size (p<0.05) with progressive apoptosis relative to apoptotic HL-60 cells. Mechanical stress accelerated size reduction of apoptotic endothelium(P< 0.01) but did not affect the size of apoptotic HL-60 cells. Mechanical stress also increased circularity in apoptotic endothelium (p<0.01), suggested to facilitate passage through small vessels. Earlier work indicated that canaliculi form through plasma membrane invagination, but we report fusion of small vesicles contributing to canalicular growth, while canaliculi fuse to form large vacuoles and also dilate at late stages of apoptosis.

CONCLUSIONS

These observations are consistent with the suggestion that endothelium is adapted to minimize microembolic potential and that canalicular fragmentation contributes to this.

Plasmodium falciparum Malaria: reduction of endothelial cell apoptosis in vitro

Organ failure in Plasmodium falciparum malaria is associated with neutrophil activation and endothelial damage. This study investigates whether neutrophil-induced endothelial damage involves apoptosis and whether it can be prevented by neutralization of neutrophil secretory products. Endothelial cells from human umbilical veins were coincubated with neutrophils from healthy donors and with sera from eight patients with P. falciparum malaria, three patients with P. vivax malaria, and three healthy controls. Endothelial apoptosis was demonstrated by terminal deoxynucleotidyltransferase-mediated dUTP-biotin nick end labeling (TUNEL) and annexin V staining. The rate of apoptosis of cells was markedly increased after incubation with patient serum compared to that with control serum. Apoptosis was most pronounced after incubation with sera from two patients with fatal cases of P. falciparum malaria, followed by sera of survivors with severe P. falciparum malaria and, finally, by sera of patients with mild P. falciparum and P. vivax malaria. Ascorbic acid, tocopherol, and ulinastatin reduced the apoptosis rate, but gabexate mesilate and pentoxifylline did not. Furthermore, in fatal P. falciparum malaria, apoptotic endothelial cells were identified in renal and pulmonary tissue by TUNEL staining. These findings show that apoptosis caused by neutrophil secretory products plays a major role in endothelial cell damage in malaria. The antioxidants ascorbic acid and tocopherol and the protease inhibitor ulinastatin can reduce malaria-associated endothelial apoptosis in vitro.

Vascular endothelial growth factor receptor-2-induced initial endothelial cell migration depends on the presence of the urokinase receptor

The angiogenic response of endothelial cells initiated by different growth factors is accompanied by assembly of cell surface-bound proteolytic machinery as a prerequisite for focal invasion. We have shown previously how the vascular endothelial growth factor (VEGF) initiates proteolysis by activation of pro-urokinase (pro-PA) via the VEGF receptor-2 (VEGFR-2). We now show that the cell surface receptor of the uPA-system, the urokinase receptor (uPAR), is redistributed to focal adhesions at the leading edge of endothelial cells in response to VEGF. VEGF165 and VEGF-E, both interacting with VEGFR-2, but not PlGF exclusively stimulating VEGFR-1, induce within minutes internalization of uPAR via an LDL receptor-like molecule, dependent on generation of active uPA and the presence of plasminogen activator inhibitor-1 (PAI-1). uPAR seems to play a pivotal role in VEGFR-2-induced endothelial cell migration because cleavage of surface uPAR impaired the migratory response of endothelial cells toward VEGF-E, but not toward PlGF.

Induction of contact-dependent endothelial apoptosis by osteosarcoma cells suggests a role for endothelial cell apoptosis in blood-borne metastasis

Although tumour cells are believed to migrate between endothelial cells early in metastasis, the possibility remains that endothelial apoptosis may also contribute to the tumour's breach of the vascular barrier. Although seemingly inconsistent with tumour angiogenesis, one publication describes the induction of contact-dependent apoptosis in cultured endothelium by tumour cells. The cell culture data are, however, open to challenge on technical grounds while there are no confirmatory reports. The present paper describes experiments overcoming these limitations. SAOS-2 human osteosarcoma cells and two rat carcinoma cell lines were co-cultured with human umbilical vein endothelial cells (HUVECs) and cultures labelled by surface lectin histochemistry for endothelium. The HUVEC culture density was determined and SAOS-2 cells, but not rat carcinoma cells, were found significantly to reduce HUVEC survival despite the release of potent growth factors as determined in separate experiments with tumour cell conditioned medium. Lectin labelling combined with light microscopy, transmission electron microscopy, flow cytometry for both lectin binding and DNA content, and DNA gel electrophoresis of SAOS-2/HUVEC co-cultures revealed extensive HUVEC apoptosis. These findings indicate contact-dependent endothelial apoptosis by SAOS-2, while this activity appeared weaker and overwhelmed by HUVEC proliferation with rat carcinoma cells. Importantly, this study supports the suggestion that endothelial apoptosis may be important for metastasis and suggests a complex interplay between endothelial proliferation and apoptosis in tumours.

Adipogenic healing in adult mice by implantation of hollow devices in muscle

In mammals, wound healing is thought to result in the formation of scar tissue, with the exception of bony healing after fractures. Here we describe a previously unknown pattern of wound healing in which adipose rather than scar tissue is formed. Adipogenesis is normally confined to the embryo, although there are several experimental models for adipogenesis with highly specific dietary, cytokine, matrix, sex, or age requirements. The adipogenic healing described in this work provides a simple and reproducible experimental mouse model for adipogenesis without these limitations. Mice received intramuscular implants of nylon mesh material. Fibrinous material impregnated implants and within 4 weeks was replaced with highly vascular granulation tissue, typical of wound healing. Also consistent with wound healing was a reduction in vascularity of the newly formed tissue over time (P < 0.05). Lipoblasts were prevalent in granulation tissue, reaching a maximum in week 2 (P < 0.001) but falling to very low levels by week 9. These cells matured to adipocytes, with intermediate forms being seen. The identity of lipoblasts and adipocytes was confirmed by Oil Red O staining and electron microscopy. Control experiments confirmed that adipogenesis was independent of the materials used as well as of the sex and age of the animals. Rather, adipogenesis appeared to be due to replacement of fibrinous material in a space created within muscle. It is possible that adipogenic healing represents an adaptation for limiting the formation of restrictive scar tissues within muscle, and that this is the basis for the formation of traumatic lipomas in humans. Also, muscle tissue is replaced by adipose cells, seemingly derived from pluripotential satellite cells, in several degenerative muscle conditions, suggesting a role for adipogenic healing in these conditions.

Umbilical cord endothelial cells expressing large T antigen: comparison with primary cultures and effect of cell age

A number of human endothelial cell lines from umbilical cord cells (HUVECs) have been generated by transfection with SV40 large T and small t antigen sequences. Comparison of these lines with primary cultures of HUVECs has been carried out by monitoring the expression of a number of endothelial cell markers with specific regard to cell age. The secreted levels of the protein plasminogen activator inhibitor (PAI) was found to be significantly reduced in SV40-transfected cells when compared to untransfected controls. Tissue plasminogen activator (tPA) and urokinase (uPA) levels were unchanged. As cells entered crisis, there was a rapid and significant increase in the levels of tPA, uPA, and PAl and this was observed for all clones screened. The endothelial cell marker von Willebrand Factor (vWF) was found intracellularly and was also secreted into the medium. The levels were not altered between transfected and untransfected cells. Angiotensin converting enzyme (ACE) activity was maintained in cell lines at levels found in nonimmortalized HUVECs. Both isoforms (alpha and beta) of IL-1 (interleukin-1) increased as cells approached crisis, and the presence of these cytokines may be responsible for the increased levels of tPA, PAI, and uPA. With one exception, the ability of the transfected cells to produce prostacyclin (PGI2) was lost by all clones.

Elevated plasminogen receptor expression occurs as a degradative phase event in cellular apoptosis

Plasminogen activation (PA) is involved in a variety of extracellular proteolytic events, such as fibrinolysis, cell migration (e.g. angiogenesis, tumour cell invasion, inflammation, wound healing, bacterial invasion), ovulation, tissue remodelling and the activation of other protease classes and growth factors. These diverse roles are due to the specific localization of components of the PA system to extracellular matrices, basement membranes, fibrin and cell surfaces. We have previously reported that PA is dramatically elevated during cycloheximide (CHX)-induced apoptosis in U937 cells due to a concomitant increase in both plasminogen receptors (PLG-R; i.e. specific PLG binding) and cell-surface urokinase plasminogen activator. We now extend this study by showing that the increase in PLG-R (resulting in an increase in specific PLG binding) is a late apoptotic event coincident with propidium iodide uptake and internucleosomal DNA fragmentation but occurring after elevations in phosphatidylserine exposure. Plasminogen was also observed to dramatically increase the rate of CHX-induced apoptosis. We conclude that PA may play a role in the degradative (i.e. late-stage) events of cellular apoptosis.