Association of Shiga toxin glycosphingolipid receptors with membrane microdomains of toxin-sensitive lymphoid and myeloid cells

Glycosphingolipids (GSLs) of the globo-series constitute specific receptors for Shiga toxins (Stxs) released by certain types of pathogenic Escherichia coli strains. Stx-loaded leukocytes may act as transporter cells in the blood and transfer the toxin to endothelial target cells. Therefore, we performed a thorough investigation on the expression of globo-series GSLs in serum-free cultivated Raji and Jurkat cells, representing B- and T-lymphocyte descendants, respectively, as well as THP-1 and HL-60 cells of the monocyte and granulocyte lineage, respectively. The presence of Stx-receptors in GSL preparations of Raji and THP-1 cells and the absence in Jurkat and HL-60 cells revealed high compliance of solid-phase immunodetection assays with the expression profiles of receptor-related glycosyltransferases, performed by qRT-PCR analysis, and Stx2-caused cellular damage. Canonical microdomain association of Stx GSL receptors, sphingomyelin, and cholesterol in membranes of Raji and THP-1 cells was assessed by comparative analysis of detergent-resistant membrane (DRM) and nonDRM fractions obtained by density gradient centrifugation and showed high correlation based on nonparametric statistical analysis. Our comprehensive study on the expression of Stx-receptors and their subcellular distribution provides the basis for exploring the functional role of lipid raft-associated Stx-receptors in cells of leukocyte origin.     

Shiga toxin glycosphingolipid receptors in microvascular and macrovascular endothelial cells: differential association with membrane lipid raft microdomains

Vascular damage caused by Shiga toxin (Stx)-producing Escherichia coli is largely mediated by Stxs, which in particular, injure microvascular endothelial cells in the kidneys and brain. The majority of Stxs preferentially bind to the glycosphingolipid (GSL) globotriaosylceramide (Gb3Cer) and, to a lesser extent, to globotetraosylceramide (Gb4Cer). As clustering of receptor GSLs in lipid rafts is a functional requirement for Stxs, we analyzed the distribution of Gb3Cer and Gb4Cer to membrane microdomains of human brain microvascular endothelial cells (HBMECs) and macrovascular EA.hy 926 endothelial cells by means of anti-Gb3Cer and anti-Gb4Cer antibodies. TLC immunostaining coupled with infrared matrix-assisted laser desorption/ionization (IR-MALDI) mass spectrometry revealed structural details of various lipoforms of Stx receptors and demonstrated their major distribution in detergent-resistant membranes (DRMs) compared with nonDRM fractions of HBMECs and EA.hy 926 cells. A significant preferential partition of different receptor lipoforms carrying C24:0/C24:1 or C16:0 fatty acid and sphingosine to DRMs was not detected in either cell type. Methyl-β-cyclodextrin (MβCD)-mediated cholesterol depletion resulted in only partial destruction of lipid rafts, accompanied by minor loss of GSLs in HBMECs. In contrast, almost entire disintegration of lipid rafts accompanied by roughly complete loss of GSLs was detected in EA.hy 926 cells after removal of cholesterol, indicating more stable microdomains in HBMECs. Our findings provide first evidence for differently stable microdomains in human endothelial cells from different vascular beds and should serve as the basis for further exploring the functional role of lipid raft-associated Stx receptors in different cell types.     

Secretory response of endothelin-1 in cultured human glomerular microvascular endothelial cells to shear stress

The shear-induced secretory response of endothelin-1 (ET-1) by human microvascular endothelial cells was studied using paired human glomerular microvascular endothelial cell (HGMEC) cultured monolayers exposed to steady-state laminar shear stress for up to 10 hours. The first cell monolayer was subjected to a shear stress of 0.65 N m-2 and the second, 1.3 N m-2. ET-1 secretion was determined by radioimmunoassay. Over 10 hours of shear, the total cumulative secretion of ET-1 was 237.4 pg/cm2 for the monolayer exposed to 1.3 N m-2 and 143.6 pg/cm2 for the monolayer exposed to 0.65 N m-2. The average ET-1 secretion rate was 20.90 +/- 2.15 and 12.45 +/- 1.05 pg/cm2.h at 0.65 N m-2 and 1.3 N m-2, respectively. The results showed that ET-1 secretion varied with the time of shear in a nonlinear fashion. Although the level of shear stress affected the absolute value of ET-1 cumulative secretion and secretion rate, the major secretion period for both monolayers occurred between 2.0 and 8.0 hours, with the peak secretion rate occurring at approximately 5 hours. Thus, the response of cultured human microvascular endothelial cells to shear stress differed from that of large vessel endothelial cell cultures in terms of ET-1 secretion. In addition to the level of shear stress, the time of shear was also an important determinant of ET-1 secretion. Consequently, the heterogeneity of vascular endothelial cells and the time of shear should both be considered in future research on the secretion of vascular endothelial cell cultures.     

Shiga toxins induce autophagy leading to differential signalling pathways in toxin-sensitive and toxin-resistant human cells

The bacterial virulence factors Shiga toxins (Stxs) are expressed by Shigella dysenteriae serotype 1 and certain Escherichia coli strains. Stxs are protein synthesis inhibitors and induce apoptosis in many cell types. Stxs induce apoptosis via prolonged endoplasmic reticulum stress signalling to activate both extrinsic and intrinsic pathways in human myeloid cells. Studies have shown that autophagy, a lysosome-dependent catabolic process, may be associated with activation of pro-survival or death processes. It is currently unknown if autophagy contributes to apoptosis or protects cells from Stxs. To study cellular responses to Stxs, we intoxicated toxin-sensitive cells (THP-1 and HK-2 cells), and toxin-resistant cells (primary human monocyte-derived macrophages) and examined toxin intracellular trafficking and autophagosome formation. Stxs translocated to different cell compartments in toxin-resistant versus toxin-sensitive cells. Confocal microscopy revealed autophagosome formation in both toxin-resistant and toxin-sensitive cells. Proteolytic cleavage of Atg5 and Beclin-1 plays pivotal roles in switching non-cytotoxic autophagy to cell death signalling. We detected cleaved forms of Atg5 and Beclin-1 in Stx-treated toxin-sensitive cells, while cleaved caspases, calpains, Atg5 and Beclin-1 were not detected in toxin-resistant primary human monocytes and macrophages. These findings suggest that toxin sensitivity correlates with caspase and calpain activation, leading to Atg5 and Beclin-1 cleavage.     

Escherichia coli outer membrane protein A adheres to human brain microvascular endothelial cells

Escherichia coli K1 is the most common gram-negative bacterium causing neonatal meningitis. The outer membrane protein A (OmpA) assembles a beta-barrel structure having four surface-exposed loops in E. coli outer membrane. OmpA of meningitis-causing E. coli K1 is shown to contribute to invasion of the human brain microvascular endothelial cells (HBMEC), the main cellular component of the blood-brain barrier (BBB). However, the direct evidence of OmpA protein interacting with HBMEC is not clear. In this study, we showed that OmpA protein, solubilized from the outer membrane of E. coli, adhered to HBMEC surface. To verify OmpA interaction with the HBMEC, we purified N-terminal membrane-anchoring beta-barrel domain of OmpA and all surface-exposed loops deleted OmpA proteins, and showed that the surface-exposed loops of OmpA were responsible for adherence to HBMEC. These findings indicate that the OmpA is the adhesion molecule with HBMEC and the surface-exposed loops of OmpA are the determinant of this interaction.     

Epoxygenase-driven angiogenesis in human lung microvascular endothelial cells

Angiogenesis is one of the most recent physiological functions attributed to products of cytochrome P-450 (CYP450) enymes. To test this at a molecular level in human cells, we used a cloned cDNA for the human endothelial enzyme CYP450 2C9 (CYP2C9) to study growth as well as differentiation of human microvascular endothelial cells from the lung (HMVEC-L). Using adenoviral vectors overexpressing mRNA for CYP2C9, we show that the presence of CYP2C9 doubles thymidine incorporation and stimulates proliferation of primary cultures of endothelial cells compared with Ad5-GFP (control) in 24 h. In addition, there is a significant increase of tube formation in Matrigel after infection of HMVEC-L with Ad5-2C9 than with Ad5-GFP. More interestingly, Ad5-2C9 expressing the antisense product of CYP2C9 (2C9AS) inhibited tube formation compared with both Ad5-GFP as well as the Ad5-2C9 constructs. Finally, we tested the most abundant arachidonic acid metabolite of CYP2C9, 14,15-epoxyeicosatrienoic acid, which induced angiogenesis in vivo when embedded in Matrigel plugs and implanted in adult rats. These data support an important role for CYP2C9 in promoting angiogenesis.     

Regulation of sterol transport in human microvascular endothelial cells

In cultured human dermal microvessel endothelial cells, the rate of efflux (about twofold greater than for fibroblasts under equivalent conditions) was coupled to an equivalent high rate of sterol net transport from the cells to the medium. This net transport was linked with esterification via lecithin:cholesterol acyltransferase. Since the use of free sterol by plasma transferase is constant, such increased net transport indicates that endothelial cells are highly efficient, in competition with plasma lipoproteins, in supplying free sterol for esterification. These results indicate the marked ability of endothelial cells to regulate and maintain their sterol balance in the face of high sterol levels to which these cells are uniquely exposed in human plasma.     

Spontaneous immortalization of human dermal microvascular endothelial cells

AIM: To establish and characterize a spontaneously immortalized human dermal microvascular endothelial cell line, iHDME1.METHODS: We developed a spontaneous immortalization method. This approach is based on the application of optimized culture media and culture conditions without addition of any exogenous oncogenes or carcinogens. Using this approach, we have successfully established a microvascular endothelial cell line, iHDME1, from primary human dermal microvascular endothelial cells. iHDME1 cells have been maintained in culture dishes for more than 50 passages over a period of 6 mo. Using a GFP expressing retrovirus, we generated a GFP-stable cell line (iHDME1-GFP).RESULTS: iHDME1 retain endothelial morphology and uniformly express endothelial markers such as VEGF receptor 2 and VE-cadherin but not α-smooth muscle actin (α-SM-actin) and cytokeratin 18, markers for smooth muscle cells and epithelial cells respectively. These cells retain endothelial properties, migrate in response to VEGF stimulation and form 3-D vascular structures in Matrigel, similar to the parental cells. There is no significant difference in cell cycle profile between the parental cells and iHDME1 cells. Further analysis indicates enhanced stemness in iHDME1 cells compared to parental cells. iHDME1 cells display elevated expression of CD133 and hTERT.CONCLUSION: iHDME1 cells will be a valuable source for studying angiogenesis.     

Th1 cytokines regulate adenosine receptors and their downstream signaling elements in human microvascular endothelial cells

We and others have shown that adenosine, acting at its receptors, is a potent modulator of inflammation and angiogenesis. To better understand the regulation of adenosine receptors during these processes we studied the effects of IL-1, TNF-alpha, and IFN-gamma on expression and function of adenosine receptors and select members of their coupling G proteins in human dermal microvascular endothelial cells (HMVEC). HMVEC expressed message and protein for A(2A) and A(2B), but not A(1) or A(3) receptors. IL-1 and TNF-alpha treatment increased message and protein expression of A(2A) and A(2B) receptor. IFN-gamma treatment also increased the expression of A(2B) receptors, but decreased expression of A(2A) receptors. Resting HMVEC and IFN-gamma-treated cells showed minimal cAMP response to the selective A(2A) receptor agonist 2-[2-(4-chlorophenyl)ethoxy]adenosine (MRE0094). In contrast, MRE0094 stimulated a dose-dependent increase in cAMP levels in TNF-alpha-treated cells that was almost completely blocked by the A(2A) receptor antagonist ZM-241385 (4-[2-[7-amino-2-(2-furyl)[1,2,4]triazolo-[2,3-a][1,3,5]triazin-5-ylamino]ethyl]phenol). The nonselective adenosine receptor agonist 5'-(N-ethylcarboxamido)adenosine increased cAMP levels in both TNF-alpha- and IFN-gamma-treated cells, but not control cells, and its effect was only partially reversed by ZM-241385 in TNF-alpha-treated cells and not affected in IFN-gamma-treated cells. HMVEC expressed a higher level of G protein beta1 isoform than beta4 isoform. Although none of the cytokines tested affected G(beta1) expression, both IL-1 and TNF-alpha significantly up-regulated G(beta4) expression. These findings indicate that inflammatory cytokines modulate adenosine receptor expression and function on HMVECs and suggest that the interaction between proinflammatory cytokines and adenosine receptors may affect therapeutic responses to anti-inflammatory drugs that act via adenosine-dependent mechanisms.     

Cancer cells regulate biomechanical properties of human microvascular endothelial cells

Metastasis is a key event of malignant tumor progression. The capability to metastasize depends on the ability of the cancer cell to migrate into connective tissue, adhere, and possibly transmigrate through the endothelium. Previously we reported that the endothelium does not generally act as barrier for cancer cells to migrate in three-dimensional extracellular matrices (3D-ECMs). Instead, the endothelium acts as an enhancer or a promoter for the invasiveness of certain cancer cells. How invasive cancer cells diminish the endothelial barrier function still remains elusive. Therefore, this study investigates whether invasive cancer cells can decrease the endothelial barrier function through alterations of endothelial biomechanical properties. To address this, MDA-MB-231 breast cancer cells were used that invade deeper and more numerous into 3D-ECMs when co-cultured with microvascular endothelial cells. Using magnetic tweezer measurements, MDA-MB-231 cells were found to alter the mechanical properties of endothelial cells by reducing endothelial cell stiffness. Using spontaneous bead diffusion, actin cytoskeletal remodeling dynamics were shown to be increased in endothelial cells co-cultured with MDA-MB-231 cells compared with mono-cultured endothelial cells. In addition, knockdown of the α5 integrin subunit in highly transmigrating α5β1(high) cells derived from breast, bladder, and kidney cancer cells abolished the endothelial invasion-enhancing effect comparable with the inhibition of myosin light chain kinase. These results indicate that the endothelial invasion-enhancing effect is α5β1 integrin-dependent. Moreover, inhibition of Rac-1, Rho kinase, MEK kinase, and PI3K reduced the endothelial invasion-enhancing effect, indicating that signaling via small GTPases may play a role in the endothelial facilitated increased invasiveness of cancer cells. In conclusion, decreased stiffness and increased cytoskeletal remodeling dynamics of endothelial cells may account for the breakdown of endothelial barrier function, suggesting that biomechanical alterations are sufficient to facilitate the transmigration and invasion of invasive cancer cells into 3D-ECMs.     

Expression of tissue kallikrein and kinin receptors in angiogenic microvascular endothelial cells

Angiogenesis is the sprouting of new capillary blood vessels from pre-existing ones. The kinin family of vasoactive peptides, formed by the serine protease tissue kallikrein from its endogenous multifunctional protein substrate kininogen, is believed to regulate the angiogenic process. The aim of this study was to determine the expression of tissue kallikrein and kinin receptors in an in vitro model of angiogenesis. Microvascular endothelial cells from the bovine mature and regressing corpus luteum were used only if they reacted with known endothelial cell markers. At first the cultured endothelial cells began sprouting, and within four weeks formed three-dimensional, capillary-like structures. Immunolabelling for tissue prokallikrein and the mature enzyme was intense in the angiogenic endothelial cells derived from mature corpora lutea. Immunoreactivity was lower in non-angiogenic endothelial cells and least in angiogenic endothelial cultures of the regressing corpus luteum. Additionally, using specific antisense DIG-labelled probes, tissue kallikrein mRNA was demonstrated in cells of the angiogenic phenotype. Immunolabelled kinin B2 receptors, but not kinin B1 receptors, were visualised on angiogenic endothelial cells. Our results suggest an important regulatory role for kinins in the multiple steps of the angiogenic cascade that may occur in wound healing and cancer cell growth.     

Stretch induces upregulation of key tyrosine kinase receptors in microvascular endothelial cells

We previously demonstrated that cyclic stretch of cardiac myocytes activates paracrine signaling via vascular endothelial growth factor (VEGF) leading to angiogenesis. The present study tested the hypothesis that cyclic stretch upregulates tyrosine kinase receptors in rat coronary microvascular endothelial cells (RCMEC) and human umbilical vein endothelial cells (HUVEC). VEGF receptor-2 (Flk-1) protein levels increased in HUVEC and RCMEC in a time-dependent manner, but the increase occurred much earlier in RCMEC than in HUVEC. The enhancement of Flk-1 protein level was not inhibited by addition of VEGF neutralizing antibodies, indicating that VEGF is not involved in stretch-induced Flk-1 expression. VEGF receptor-1 (Flt-1) protein and mRNA were not changed by stretch. However, Tie-2 and Tie-1 protein levels increased in RCMEC. Angiopoietin-1 and -2, the ligands for Tie-2, increased in cardiac myocytes subjected to cyclic stretch but were not affected by stretch in endothelial cells (EC). Stretch or incubation of RCMEC with VEGF increased cell proliferation moderately, whereas stretch + VEGF had an additive effect on proliferation. Mechanical stretch induces upregulation of the key tyrosine kinase receptors Flk-1, Tie-2, and Tie-1 in vascular EC, which underlies the increase in sensitivity of EC to growth factors and, therefore, facilitates angiogenesis. These in vitro findings support the concept that stretch of cardiac myocytes and EC plays a key role in coronary angiogenesis.     

Mechanisms of endotoxin tolerance in human intestinal microvascular endothelial cells

Lipopolysaccharide (endotoxin) tolerance is well described in monocytes and macrophages, but is less well characterized in endothelial cells. Because intestinal microvascular endothelial cells exhibit a strong immune response to LPS challenge and play a critical regulatory role in gut inflammation, we sought to characterize the activation response of these cells to repeated LPS exposure. Primary cultures of human intestinal microvascular endothelial cells (HIMEC) were stimulated with LPS over 6-60 h and activation was assessed using U937 leukocyte adhesion, expression of E-selectin, ICAM-1, VCAM-1, IL-6, IL-8, manganese superoxide dismutase, HLA-DR, and CD86. Effect of repeat LPS stimulation on HIMEC NF-kappaB and mitogen-activated protein kinase (MAPK) activation, generation of superoxide anion, and Toll-like receptor 4 expression was characterized. LPS pretreatment of HIMEC for 24-48 h significantly decreased leukocyte adhesion after subsequent LPS stimulation. LPS pretreatment inhibited expression of E-selectin, VCAM-1, IL-6, and CD86, while ICAM-1, IL-8, and HLA-DR were not altered. Manganese superoxide dismutase expression increased with repeated LPS stimulation, with a reduction in intracellular superoxide. NF-kappaB activation was transiently inhibited by LPS pretreatment for 6 h, but not at later time points. In contrast, p44/42 MAPK, p38 MAPK, and c-Jun N-terminal kinase activation demonstrated inhibition by LPS pretreatment 24 or 48 h prior. Toll-like receptor 4 expression on HIMEC was not altered by LPS. HIMEC exhibit endotoxin tolerance after repeat LPS exposure in vitro, characterized by diminished activation and intracellular superoxide anion concentration, and reduced leukocyte adhesion. HIMEC possess specific mechanisms of immunoregulatory hyporesponsiveness to repeated LPS exposure.     

Antiproliferative effect of elevated glucose in human microvascular endothelial cells

Diabetic microangiopathy has been implicated as a fundamental feature of the pathological complications of diabetes including retinopathy, neuropathy, and diabetic foot ulceration. However, previous studies devoted to examining the deleterious effects of elevated glucose on the endothelium have been performed largely in primary cultured cells of macrovessel origin. Difficulty in the harvesting and maintenance of microvascular endothelial cells in culture have hindered the study of this relevant population. Therefore, the objective of this study was to characterize the effect of elevated glucose on the proliferation and involved signaling pathways of an immortalized human dermal microvascular endothelial cell line (HMEC-1) that possess similar characteristics to their in vivo counterparts. Human dermal microvascular endothelial cells (HMEC-1) were grown in the presence of normal (5 mM) or high D-glucose (20 mM) for 14 days. The proliferative response of HMEC-1 was compared under these conditions as well as the cAMP and PKC pathways by in vitro assays. Elevated glucose significantly inhibited (P < 0.05) HMEC-1 proliferation after 7, 10, and 14 days. This effect was not mimicked by 20 mM mannitol. The antiproliferative effect was more pronounced with longer exposure (1–14 days) to elevated glucose and was irreversible 4 days after a 10-day exposure. The antiproliferative effect was partially reversed in the presence of a PKA inhibitor, Rp-cAMP (10–50 μM), and/or a PKC inhibitor, Calphostin C (10 nM). HMEC-1 exposed to elevated glucose (20 mM) for 14 days caused an increase in cyclic AMP accumulation, PKA, and PKC activity but was not associated with the activation of downstream events such as CRE and AP-1 binding activity. These data support the hypothesis that HMEC-1 is a suitable model to study the deleterious effects of elevated glucose on microvascular endothelial cells. Continued studies with HMEC-1 may prove advantageous in delineation of the molecular pathophysiology associated with diabetic microangiopathy. J. Cell. Biochem. 71:491–501, 1998. © 1998 Wiley-Liss, Inc.     

Cultivation of microvascular endothelial cells from human preputial skin

Summary A procedure is described for the isolation and cultivation of microvascular endothelium from human skin. Neonatal foreskins are pooled, washed, minced, and dissociated by a mixture of collagenase and dispase. Microvascular endothelium, liberated in the form of intact capillary fragments, is incompletely separated from fibroblasts and epidermal cells by sieving through nylon mesh, followed by velocity sedimentation on 5% bovine serum albumin. The endothelium-enriched fraction has been maintained in primary culture for up to 3 weeks. The resulting epithelioid colonies have been characterized morphologically by both light and transmission electron microscopy and manifest all of the structural features that distinguish other, large-vessel endothelia in culture. In addition, immunohistochemical studies using an indirect fluorescent antibody technique demonstrate that these cells contain the endothelium-specific product, Factor VIII antigen. This work was supported by National Institutes of Health Grants AM18904 and AM20571, the RGK Foundation, the Charlotte and Sidney Lifschultz Foundation, the Juvenile Diabetes Foundation, and the South Carolina Geenral Medical Faculty Research Appropriation.     

Origin of periendothelial cells in microvessels derived from human microvascular endothelial cells

In microvessels, periendothelial cells expressing alpha smooth muscle actin (alphaSMA) interact with the endothelial cells and are essential for vessel maturation and stabilization. In adult tissues, the cellular origin of the periendothelial cells is still not clear, in particular in humans. To determine the origin of human periendothelial cells, we used a recently developed 3D co-culture system that mimics human skin connective tissue. This system is composed of normal human dermal fibroblasts (NHDF), human dermal microvascular endothelial cells (HMEC-1), and a collagen matrix. In this system, "microvessels" composed of an endothelial lumen associated with periendothelial cells develop. Using this co-culture system, we (i) labelled fibroblasts with the vital dye CFDA-SE, cultured them with unlabelled endothelial cells, and observed that only endothelium-associated CFDA-SE-labelled cells express alphaSMA; (ii) infected endothelial cells with a retrovirus stably expressing eGFP, cultured them with unlabelled fibroblasts, and observed that cells expressing alphaSMA did not co-express eGFP, but were associated with the eGFP-expressing endothelial cells of the microvessels. Together, these results indicate that periendothelial cells arise by differentiation from fibroblasts and that they require interaction with endothelial cells to do so.