Human endothelial cells are activated by interferon-gamma plus tumour necrosis factor-alpha to kill intracellular Pseudomonas aeruginosa

Toxoplasma gondii, endothelial cells and schizophrenia: is it just a barrier matter?

Toxoplasma gondii is an obligatory intracellular parasite responsible for causing toxoplasmosis. It is estimated that approximately one-third of the world's population has positive serology for toxoplasmosis. Acute T. gondii infection often results in subtle symptoms because of its nonspecific nature. Owing to immune pressure, parasites tend to encyst and persist in different tissues and organs, such as the brain, chronicling the infection. While most chronically infected individuals do not develop significant symptoms, the parasite can affect the central nervous system (CNS), leading to symptoms that range from dizziness to behavioral changes. To reach the CNS, parasites must overcome the blood-brain barrier, which is composed primarily of endothelial cells. While these cells are typically efficient at separating blood elements from the CNS, in T. gondii infection, they not only permit parasitic colonization of the CNS but also contribute to an inflammatory profile that may exacerbate previously established conditions at both the local CNS and systemic levels. An increasing body of research has demonstrated a potential link between the CNS, infection by T. gondii and the cellular or humoral response to infection, with the worsening of psychiatric conditions, such as schizophrenia. Therefore, continually advancing research aimed at understanding and mitigating the relationship between parasitic infection and schizophrenia is imperative.

Human Brain Endothelial Cell-Derived Extracellular Vesicles Reduce Toxoplasma gondii Infection In Vitro in Human Brain and Umbilical Cord Vein Endothelial Cells

The endothelial layer, formed by endothelial cells, performs crucial functions in maintaining homeostasis. The endothelial integrity and function might be compromised due to various causes, including infection by Toxoplasma gondii, leading to an endothelial dysfunction. Toxoplasma gondii is an Apicomplexa parasite that infects a broad range of animals, including humans. This parasite can invade all nucleated cells, as well as endothelial cells. The interaction between this protozoan and endothelial cells can be mediated by different molecules, such as extracellular vesicles (EVs), which may either favor or hinder the infectious process. To investigate this interaction, we evaluated the infection of T. gondii on human brain microvascular endothelial cells (HBMEC) and human umbilical vein endothelial cells (HUVEC), in addition to assessing transcriptional changes. We also featured the EVs secreted by T. gondii and by infected and non-infected HBMEC and HUVEC. Finally, we evaluated the infection of cells stimulated with EVs of parasitic or cellular origin. Our results demonstrated that HUVEC not only exhibit a higher infection rate than HBMEC but also display a more pro-inflammatory transcriptional profile, with increased expression of interleukin-6 (IL6), interleukin-8 (IL8), and monocyte chemotactic protein-1 (MCP1) following infection. Additionally, we observed few differences in the concentration, distribution, and morphology of EVs secreted by both cell types, although their properties in modulating infection varied significantly. When cells were EVs stimulated, EVs from T. gondii promoted an increase in the HBMEC infection, EVs from infected or uninfected HBMEC reduced the infection, whereas EVs from HUVEC had no effect on the infectious process. In conclusion, our data indicate that T. gondii infection induces distinct changes in different endothelial cell types, and EVs from these cells can contribute to the resolution of the infection.

Unraveling the Impact of Extracellular Vesicle-Depleted Serum on Endothelial Cell Characteristics over Time

Extracellular vesicles (EVs) are produced by all kinds of cells, including endothelial cells. It has been observed that EVs present in fetal bovine serum (FBS), broadly used in cell culture, can be a confounding factor and lead to misinterpretation of results. To investigate this phenomenon, human brain microvascular endothelial cells (HBMECs) were cultured for 2 or 24 h in the presence of EV-depleted FBS (EVdS). Cell death, gene and protein expression, and the presence of EVs isolated from these cells were evaluated. The uptake of EVs, intercellular adhesion molecule 1 (ICAM-1) expression, and monocyte adhesion to endothelial cells exposed to EVs were also evaluated. Our results revealed higher apoptosis rates in cells cultured with EVdS for 2 and 24 h. There was an increase in interleukin 8 (IL8) expression after 2 h and a decrease in interleukin 6 (IL6) and IL8 expression after 24 h of culture. Among the proteins identified in EVs isolated from cells cultured for 2 h (EV2h), several were related to ribosomes and carbon metabolism. EVs from cells cultured for 24 h (EV24h) presented a protein profile associated with cell adhesion and platelet activation. Additionally, HBMECs exhibited increased uptake of EV2h. Treatment of endothelial cells with EV2h resulted in greater ICAM-1 expression and greater adherence to monocytes than did treatment with EV24h. According to our data, HBMEC cultivated with EVdS produce EVs with different physical characteristics and protein levels that vary over time.

Elimination of Pseudomonas aeruginosa through Efferocytosis upon Binding to Apoptotic Cells

For opportunistic pathogens such as Pseudomonas aeruginosa, the mucosal barrier represents a formidable challenge. Infections develop only in patients with altered epithelial barriers. Here, we showed that P. aeruginosa interacts with a polarized epithelium, adhering almost exclusively at sites of multi-cellular junctions. In these sites, numerous bacteria attach to an extruded apoptotic cell or apoptotic body. This dead cell tropism is independent of the type of cell death, as P. aeruginosa also binds to necrotic cells. We further showed that P. aeruginosa is internalized through efferocytosis, a process in which surrounding epithelial cells engulf and dispose of extruded apoptotic cells. Intracellularly, along with apoptotic cell debris, P. aeruginosa inhabits an efferocytic phagosome that acquires lysosomal features, and is finally killed. We propose that elimination of P. aeruginosa through efferocytosis is part of a host defense mechanism. Our findings could be relevant for the study of cystic fibrosis, which is characterized by an exacerbated number of apoptotic cells and ineffective efferocytosis.

Pseudomonas aeruginosa is an opportunistic pathogen that infects vulnerable patients, such as those with cystic fibrosis or hospitalized in intensive care units. An advance towards understanding infections caused by P. aeruginosa would be to fully elucidate the mechanisms that operate in the bacteria-epithelial barrier interplay. Here, we showed that P. aeruginosa exhibits a remarkable tropism towards dead cells. As bacteria interact with a polarized epithelium, they attach and aggregate almost exclusively on apoptotic cells extruded from the epithelium, while the rest of the surface seems reluctant to bacterial adhesion. We further showed that P. aeruginosa is internalized by epithelial cells surrounding the infected apoptotic cell through efferocytosis, a process in which apoptotic cells are engulfed and disposed of by other cells. Bacteria are eliminated intracellularly. Our findings may help to understand why contexts such as cystic fibrosis, where apoptotic cells are unusually produced and efferocytosis fails, favor P. aeruginosa colonization.

Antibodies against invasive phenotype-specific antigens increase Mycobacterium avium subspecies paratuberculosis translocation across a polarized epithelial cell model and enhance killing by bovine macrophages

Johne's disease, caused by Mycobacterium avium subspecies paratuberculosis (MAP), is a severe chronic enteritis which affects large populations of ruminants globally. Prevention strategies to combat the spread of Johne's disease among cattle herds involve adhering to strict calving practices to ensure young susceptible animals do not come in contact with MAP-contaminated colostrum, milk, or fecal material. Unfortunately, the current vaccination options available are associated with high cost and suboptimal efficacy. To more successfully combat the spread of Johne's disease to young calves, an efficient method of protection is needed. In this study, we examined passive immunization as a mode of introducing protective antibodies against MAP to prevent the passage of the bacterium to young animals via colostrum and milk. Utilizing the infectious MAP phenotype developed after bacterial exposure to milk, we demonstrate that in vitro opsonization with serum from Johne's-positive cattle results in enhanced translocation across a bovine MDBK polarized epithelial cell monolayer. Furthermore, immune serum opsonization of MAP results in a rapid host cell-mediated killing by bovine macrophages in an oxidative-, nitrosative-, and extracellular DNA trap-independent manner. This study illustrates that antibody opsonization of MAP expressing an infectious phenotype leads to the killing of the bacterium during the initial stage of macrophage infection.

Real-time monitoring of cell mechanical changes induced by endothelial cell activation and their subsequent binding with leukemic cell lines

Endothelial cell (EC) activation and their subsequent binding with different cells have various mechanical consequences that, if monitored real time, can serve as a functional biomarker of many pathophysiological response mechanisms. This work presents an innovative and facile strategy to conduct such monitoring using quartz crystal microbalance (QCM), thereby relating the shifts in its frequency and motional resistance to morphological changes upon cell-cell and cell-substrate interactions. By activating ECs with TNF-α and then characterizing their binding with HL-60 and KG-1 leukemia cells, we are able to induce the mechanical changes in ECs especially in the region of cell-substrate contact which resulted in dynamically coupled mass and viscoelastic changes representing the extent of both activation and binding. The activated ECs suffered a decrease of cellular contact area, leading to positive frequency shift and decreased motional resistance. The binding of leukemia cells onto pre-activated ECs exerted a mechanical force to regain the cell surface contact which resulted in the obvious QCM responses opposite to that of activation, and proportional to the number of cells added, in spite of the fact that these added cells are extremely outside the extinction boundary of the shear wave generated by QCM. Different cell lines demonstrate different attachment behavior, which was detected by the QCM. Despite these variations are quite subtle, yet the sensitivity of the technique for dynamic changes at the interface makes them detectable. Moreover, the reproducibility of the generated data determined at each step by deviation measurements (<10%) in response plot was very high despite the high possible heterogeneity in cell populations. The results are explained on the basis of simple theoretical and physical models, although, the development of a more quantitative and precise model is underway in our laboratory.

The "Mevalonate hypothesis": a cholesterol-independent alternative for the etiology of atherosclerosis

The “cholesterol hypothesis” is the leading theory to explain the cause of atherosclerosis. The “cholesterol hypothesis” assumes that plasma (LDL) cholesterol is an important causal factor for atherosclerosis.

However, data of at least seven placebo controlled randomized prospective trials with various cholesterol lowering drugs show that plasma cholesterol lowering does not necessarily lead to protection against cardiovascular disease. Therefore an alternative hypothesis for the etiology of cardiovascular disease is formulated. This alternative hypothesis, the “mevalonate hypothesis”, assumes that after stimulation of the mevalonate pathway in endothelial cells by inflammatory factors, these cells start producing cholesterol and free radicals. In this hypothesis, only the latter play a role in the etiology of atherosclerosis by contributing to the formation of oxidized cholesterol which is a widely accepted causal factor for atherosclerosis.

Regardless of how the mevalonate pathway is activated (by withdrawal of statin drugs, by inflammatory factors or indirectly by reduced intracellular cholesterol levels) in all these cases free radical production is observed as well as cardiovascular disease. Since in the “mevalonate hypothesis” cholesterol is produced at the same time as the free radicals causing atherosclerosis, this hypothesis provides an explanation for the correlation which exists between cardiovascular disease and plasma cholesterol levels. From an evolutionary perspective, concomitant cholesterol production and free radical production in response to inflammatory factors makes sense if one realizes that both activities potentially protect cells and organisms from infection by gram-negative bacteria.

In conclusion, data have been collected which suggest that activation of the mevalonate pathway in endothelial cells is likely to be a causal factor for atherosclerosis. This “mevalonate hypothesis” provides a better explanation for results obtained from recent clinical studies with cholesterol lowering drugs than the “cholesterol hypothesis”. Furthermore, this hypothesis explains how cholesterol can be correlated with cardiovascular disease without being a causal factor for it. Finally it provides a logical explanation for the etiology of this disease.

Soluble CD163 promotes recognition, phagocytosis and killing of Staphylococcus aureus via binding of specific fibronectin peptides

CD163 is a multi-ligand scavenger receptor exclusively expressed by monocytes and macrophages, which is released after their activation during sepsis (sCD163). The biological relevance of sCD163, however, is not yet clear. We now demonstrate that sCD163 exhibits direct antimicrobial effects by recognizing a specific subfragment ((6) F1(1) F2(2) F2(7) F1) of fibronectin (FN) bound to staphylococcal surface molecules. Moreover, contact with staphylococci promotes sCD163-shedding from monocyte surface via induction of metalloproteinases ADAM10 and ADAM17. sCD163 subsequently binds to Staphylococcus aureus via FN peptides and strongly amplifies phagocytosis as well as killing by monocytes and to a lesser extend by neutrophils. This mechanism exhibits additional paracrine effects because staphylococci additionally opsonized by sCD163 induce higher activation and more efficient killing activity of non-professional phagocytes like endothelial cells. Targeting pathogen-bound FN by sCD163 would be a very sophisticated strategy to attack S. aureus as any attempt of the pathogen to avoid this defence mechanism will automatically bring about loss of adherence to the host protein FN, which is a pivotal patho-mechanism of highly invasive staphylococcal strains. Thus, we report a novel function for sCD163 that is of particular importance for immune defence of the host against S. aureus infections.

Immunohistochemistry Evaluation of the Effect in Vivo of Tumor Necrosis Factor (TNF)-alpha on Blood Vessel Density in Murine Fibrosarcoma

Purpose: Angiogenesis is essential for tumor growth and metastases, thus bestowing obvious importance upon methodologies which could enable its inhibition.

Materials: C57BL/6 female mice bearing a subcutaneous (s.c.) MCA205 fibrosarcoma were used.

Methods: Ten mice were divided equally into two groups. One group was injected intraperitoneally (i.p.) with 10 μg tumor necrosis factor-α (TNF-α and the other (controls) with Hanks balanced salt solution (HBSS). Tumor growth was monitored at least twice weekly. The number of endothelial cells in the blood microvessels was assessed by immunohistostaining on paraffin-embedded tumor tissue sections using vascular endothelial growth factor (VEGF) and Factor 8 antibodies. Expression of the p53 gene was similarly assessed by immunohistostaining.

Results: Injection of 10 μg TNF-α into the tumor-bearing mice reduced the number of endothelial cells in the blood microvessels by 46% on day 3 post-injection which was accompanied by an increase (by 37%) in the expression of p53 in these cells. It also inhibited tumor growth compared to the HBSS-injected group starting at 17 days post-cytokine injection.

Discussion: The antitumor in vivo effect exerted by TNF-α on established murine sarcoma s.c. tumors may be due to an earlier effect of the cytokine on the tumor's blood microvessels, probably through an apoptotic mechanism involving the p53 gene.

Gamma interferon confers resistance to infection with Staphylococcus aureus in human vascular endothelial cells by cooperative proinflammatory and enhanced intrinsic antibacterial activities

Vascular endothelium is an exposed target in systemic endovascular Staphylococcus aureus infections. We reported earlier that the proinflammatory and procoagulant activities of primary human umbilical vein endothelial cells (ECs) after binding and ingestion of S. aureus organisms provide the cells effective means for leukocyte-mediated bacterial elimination. Expanding on this, we now show that these ECs exhibit a modest intrinsic capacity for eliminating intracellular S. aureus that was influenced by cytokines relevant to S. aureus infections. Using various EC infection assays, we showed that gamma interferon (IFN-gamma), applied to cultures of ECs prior to or after infection with S. aureus, markedly reduced the level of infection, illustrated by lower percentages of S. aureus-infected ECs and less intracellular bacteria per infected cell. IFN-gamma-activated ECs had unaltered abilities to bind S. aureus and processed ingested bacteria by a seemingly conventional phagocytic pathway. IFN-gamma treatment rescued EC monolayers from severe injury by virulent clinical S. aureus strains or excessive bacterial numbers. Mechanistically, IFN-gamma controls S. aureus infection via IFN-gamma receptor, most likely through stimulation of intrinsic endothelial antibacterial mechanisms but independent of processes that deprive bacteria of intracellular L-tryptophan or iron. The antibacterial activity of IFN-gamma-stimulated ECs coincided with sustained or slightly elevated endothelial proinflammatory responses that supported monocyte recruitment. In conclusion, we identify IFN-gamma as a potent regulatory Th1 cytokine possessing exclusive abilities to augment intrinsic antistaphylococcal effector mechanisms in human ECs without ablating the S. aureus-induced proinflammatory EC responses and, as such, coordinating a protective efficacy of ECs against blood-borne S. aureus infection.

Eicosanoid-mediated proinflammatory activity of Pseudomonas aeruginosa ExoU

As Pseudomonas aeruginosa ExoU possesses two functional blocks of homology to calcium-independent (iPLA(2)) and cytosolic phospholipase A(2) (cPLA(2)), we addressed the question whether it would exhibit a proinflammatory activity by enhancing the synthesis of eicosanoids by host organisms. Endothelial cells from the HMEC-1 line infected with the ExoU-producing PA103 strain exhibited a potent release of arachidonic acid (AA) that could be significantly inhibited by methyl arachidonyl fluorophosphonate (MAFP), a specific PLA(2) inhibitor, as well as significant amounts of the cyclooxygenase (COX)-derived prostaglandins PGE(2) and PGI(2). Cells infected with an isogenic mutant defective in ExoU synthesis did not differ from non-infected cells in the AA release and produced prostanoids in significantly lower concentrations. Infection by PA103 induced a marked inflammatory response in two different in vivo experimental models. Inoculation of the parental bacteria into mice footpads led to an early increase in the infected limb volume that could be significantly reduced by inhibitors of both COX and lipoxygenase (ibuprofen and NDGA respectively). In an experimental respiratory infection model, bronchoalveolar lavage (BAL) from mice instilled with 10(4) cfu of PA103 exhibited a marked influx of inflammatory cells and PGE(2) release that could be significantly reduced by indomethacin, a non-selective COX inhibitor. Our results suggest that ExoU may contribute to P. aeruginosa pathogenesis by inducing an eicosanoid-mediated inflammatory response of host organisms.

Implications of oxidative stress in the cytotoxicity of Pseudomonas aeruginosa ExoU

ExoU PLA2-like activity has been shown to account for membrane lysis and acute death of infected cells. Translocation of effector proteins by the type III secretion systems depends on close contact between microbial and host cells. Our finding that both the ExoU-producing PA103 Pseudomonas aeruginosa and its mutant obtained by deletion of exoU adhered poorly to endothelial cells (EC) led to the hypothesis that, in some cells, the amount of injected toxin may not be enough to induce cell lysis but cells would suffer from a long-term effect of ExoU intoxication. To address this question, cells were exposed to both bacteria for 1 h and then treated with gentamicin-containing medium, to eliminate infecting microorganisms. After 24 h, the percentage of viable EC in PA103-infected cultures was significantly lower than in cultures exposed to the mutant, as determined by the MTT assay. Cell death was not likely to depend on the ExoU lytic activity since cell labeling with propidium iodide was similar in cultures infected with both bacterial strains. Bacterial cytotoxicity was significantly reduced by MAFP, a specific inhibitor of cPLA2 and iPLA2. Since the PLA2 activity on membrane phospholipids generates free fatty acid, including arachidonic acid (AA), we next compared the bacterial ability to release AA from infected EC. PA103 was shown to induce a potent AA release that was inhibited by MAFP. AA oxidation by oxygenases generates eicosanoids, known to induce both cell death and proliferation. However neither inhibitors of cyclooxygenases (ibuprofen) nor lipoxygenases (NDGA) reduced the ExoU toxicity. Since non-enzymatic oxidation of AA generates reactive radicals, we next investigated the PA103 ability to induce oxidative stress in infected cells. FACS analysis of cell labeling with the C-11 fluor probe and with anti-4-hydroxynonel antibody revealed a significant peroxidation of cell membrane lipids. These results, together with our finding that PA103-infected EC death was significantly attenuated by alpha-tocopherol, led to the conclusion that AA-induced oxidative stress may be another mechanism of cell damage in the course of infection by ExoU-producing P. aeruginosa.

Pseudomonas aeruginosa‐induced production of free radicals by IFNγ plus TNFα‐activated human endothelial cells: mechanism of host defense or of bacterial pathogenesis?

We have previously shown that human umbilical vein endothelial cells (HUVEC) can be activated by IFNgamma plus TNFalpha to kill intracellular (IC) Pseudomonas aeruginosa through production of reactive oxygen intermediate, but the cumulative effects of cytokine activation and bacterial infection on host cells has not been extensively addressed. In this study we investigated the fate of IFNgamma plus TNFalpha-activated HUVEC that have harboured IC bacteria for up to 24 h. At 10 h, the endothelial cell killing of P. aeruginosa isolates exceeded 90%. IC bacteria enhanced the expression of inducible nitric oxide synthase (iNOS) and induced overproduction of NO and superoxide by infected HUVEC. P. aeruginosa IC infection also induced a slight decrease in the cellular level of reduced glutathione (GSH). Overproduction of NO correlated with a marked peroxidation of plasma membrane lipids and decline in HUVEC viability. Treatment of cells with the antioxidant alpha-lipoic acid significantly increased the survival of infected cells. Our data suggest that with the failure of adequate scavenger mechanisms, oxidant radicals overproduced in response to bacterial infection were highly toxic to host cells. Therefore, instead of contributing to defence against infectious agents, the upregulation of free radicals production by endothelial cells in response to cytokine activation would be detrimental to the host.

Pretreatment of Pseudomonas aeruginosa with a sub-MIC of imipenem enhances bactericidal activity of neutrophils

The influence of the pretreatment of Pseudomonas aeruginosa strain O1 (PAO-1) with a sub-minimum inhibitory concentration (MIC) of imipenem on bactericidal activity, phagocytosis, the production of oxygen radical intermediates, and the induction of apoptosis in murine peritoneal neutrophils, as well as the catalase activity in the bacteria in comparison with that of ceftazidime-treated bacteria were studied. Bacteria treated with imipenem at (1/4) MIC were killed at significantly higher rates by neutrophils than ceftazidime-treated and nontreated bacteria. However, antibiotic-treated bacteria showed similar numbers of bacteria-phagocytized neutrophils to those in untreated bacteria. Imipenem pretreatment of bacteria led to an increase in the production of oxygen radical intermediates by neutrophils and the inhibition of neutrophilic apoptosis following incubation, whereas these features did not occur in neutrophils incubated with nontreated and ceftazidime-treated bacteria. The catalase activity of bacteria was not suppressed by pretreatment with either antibiotic at (1/4) MIC. These findings suggest that the exposure of P. aeruginosa to a sub-MIC of imipenem enhances the susceptibility of the bacteria to neutrophilic killing and effectively modifies the physiological activities of neutrophils, but does not decrease bacterial catalase activity. These actions may account for the postantibiotic leukocyte enhancement (PALE) effect of a sub-MIC of imipenem in the host.

End-organ dysfunction in cystic fibrosis: association with angiotensin I converting enzyme and cytokine gene polymorphisms

The clinical course of patients with cystic fibrosis (CF) with functionally similar mutations in the CF transmembrane conductance regulator gene is variable and must therefore relate to secondary genetic and environmental factors. We examined the hypothesis that polymorphisms of certain inflammatory mediator and regulatory genes affect clinical outcome by influencing the degree of end-organ damage. By studying the possible association between clinical outcome and angiotensin I-converting enzyme (ACE) and cytokine genotypes by amplification refractory mutation system-polymerase chain reaction, using stored DNA from 261 white patients with CF, we found that ultrasound features of cirrhosis occurred more frequently in patients with the high-producer (DD) rather than the low-producer (II) ACE genotype (odds ratio [95% confidence interval], 3.7 [1.2 to 12]). Moreover, significant pulmonary dysfunction (age at which FEV1 < 50%) was associated with the high-producer ACE genotype (2.3 [1.2 to 4.5]) and transforming growth factor-beta1 genotype (2.6 [1.0 to 6.8]) as well as with age at first colonization with Pseudomonas aeruginosa (9.1 [1.1 to 72]). We conclude that the high-producer ACE genotype predicts patients with CF who have an increased chance of developing portal hypertension; and high-producer ACE and TGF-beta1 genotypes are secondary genetic factors contributing to pulmonary dysfunction in these patients.

Expression of inducible nitric oxide synthase in human umbilical vein endothelial cells during primary culture

The adaptive response of endothelial cells to stress may lead to the upregulation of nitric oxide (NO) production. Herein, we report inducible nitric oxide synthase (iNOS) induction in primary cultures of human umbilical vein endothelial cells (HUVEC). The enzyme expression was earlier observed in 12-h cultures, reaching maximal levels after 3 days and decreasing when cells become confluent. The time course of NO production by HUVEC paralleled iNOS expression during the whole culture period, indicating that enzyme was functionally active. Conversely, iNOS induction could not be further detected in HUVEC subcultures passed once from cells presenting maximal levels of iNOS expression in the primary culture. Induction of iNOS in HUVEC was not related to lipopolysaccharide contamination, since the enzyme expression was not affected in the presence of polymyxin B added to primary cultures. Further analysis showed that aminoguanidine, a specific iNOS inhibitor, did not affect cell proliferation, suggesting that the NO produced by HUVEC may not be directly related to cell growth. Platelet endothelial cell adhesion molecule-1 expression was upregulated during cell confluence, in contrast to the decrease of iNOS expression and activity. The data suggest that iNOS expression may be a molecular mechanism mediating the adaptive response of endothelial cells to culture environment.