The role of endothelial nitric oxide synthase (eNOS) in endothelial activation: insights from eNOS-knockout endothelial cells

The role of endothelial nitric oxide synthase (eNOS) in endothelial activation: insights from eNOS-knockout endothelial cells
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    The role of endothelial nitric oxide synthase (eNOS) in endothelial activation: insights from eNOS-knockout endothelial cells Peter J. Kuhlencordt 1 , Eva Rosel 1 , Robert E. Gerszten 2 , Manuel Morales-Ruiz 3, 4 , David Dombkowski 5 , William J. Atkinson 6 , Fred Han 2 , Frederic Preffer  5 ,  Anthony Rosenzweig 2 , William C. Sessa 3 , Michael A. Gimbrone, Jr. 6 , Georg Ertl 1 , and Paul L. Huang 2 1 Department of Medicine, University of Wuerzburg, D97080; Germany; 2 Cardiovascular Research Center, Division of Cardiology, Massachusetts General Hospital and Harvard Medical School, Boston, MA 02114 USA; 3 Department of Pharmacology and Molecular Cardiobiology Program, Yale University School of Medicine, New Haven, Connecticut 06536, USA; 4 Hormonal Laboratory, Hospital Clinic Universitari, University of Barcelona, Barcelona 09036, Spain; 5 Department of Pathology, Massachusetts General Hospital and Harvard Medical School, Boston, MA 02114 USA;  6 Vascular Research Division, Brigham and Women’s Hospital and Harvard Medical School, Boston, Massachusetts 02115, USA Contact information: Paul L. Huang, M.D, Ph.D. Cardiovascular Research Center, Massachusetts General Hospital-East 149 East 13 th  Street Charlestown, MA 02129 Telephone: (617) 724-9849 Fax: (617) 726-5806 e-mail: Running Head: Targeted deletion of eNOS and endothelial cell activation Key words: vascular biology, atherosclerosis, mouse models Articles in PresS. Am J Physiol Cell Physiol (January 28, 2004). 10.1152/ajpcell.00546.2002 Copyright (c) 2004 by the American Physiological Society.    2 Abstract  The objective of this study was to determine whether absence of endothelial nitric oxide synthase (eNOS) affects the expression of cell surface adhesion molecules in endothelial cells. Murine lung endothelial cells (MLEC’s) were prepared by immunomagnetic bead selection from wild-type and eNOS knockout mice. Wild-type cells expressed eNOS, but eNOS knockout cells did not. Expression of neuronal NOS and inducible NOS was not detectable in cells of either genotype. Upon stimulation confluent wild-type MLEC’s produced significant amounts of NO compared to L-NMMA-treated wild-type cells. eNOS knockout and wild-type cells showed no difference in the expression of E-selectin, P-selectin, Intracellular Adhesion Molecule-1 and Vascular Cell  Adhesion Molecule-1 measured by flow cytometry on the surface of Platelet Endothelial Cell Adhesion Molecule-1 (PECAM-1/CD31) positive cells. Both eNOS knockout and wild-type cells displayed the characteristics of resting endothelium. Adhesion studies in a parallel plate laminar flow chamber showed no difference in leukocyte-endothelial cell interactions between the two genotypes. Cytokine treatment induced endothelial cell adhesion molecule expression and increased leukocyte-endothelial cell interactions in both genotypes. We conclude that in resting murine endothelial cells, absence of endothelial production of NO by itself does not initiate endothelial cell activation or promote leukocyte-endothelial cell interactions. We propose that eNOS derived NO does not chronically suppress endothelial cell activation in an autocrine fashion, but serves to counterbalance signals that mediate activation.    3 Introduction The changes in the morphology and gene expression pattern of endothelial cells at sites of inflammation and cell-mediated immune responses have been described as the process of endothelial cell activation (11, 12, 33). The activated state is characterized by an increase in the surface expression of adhesion molecules, which regulate leukocyte-endothelial cell interactions. NO is a homeostatic regulator of vascular tone, and pharmacologic inhibition of NO synthesis or disruption of the eNOS gene significantly increases blood pressure (15, 27). Moreover, NO inhibits platelet aggregation in vitro  and modulates leukocyte adhesion in the microcirculation (17, 27). The mechanisms by which NO modulates leukocyte-endothelial interactions are not known. One possibility is a direct effect of NO on the regulation of expression of adhesion molecules and cytokines by the transcription factor NF κ   B (39). NO induces transcription of I κ   B α , an inhibitor of NF κ   B, thus stabilizing the inhibitory NF κ   B/I κ   B α  complex in the cytosol (35). A second possibility is that NO may protect cells from oxidative stress because it interacts rapidly with superoxide, which acts as a proadhesive molecule (3, 7, 9, 36, 37). Hence, NO may indirectly influence leukocyte-endothelial cell interactions by counterbalancing oxygen free radicals, the latter being the actual effector molecules that initiate vascular pathology. NO donors and NOS inhibitors have been employed to study the importance of NO in regulation of endothelial gene expression in vitro . NO donors, which release NO independent of NOS enzymes, decrease cytokine-induced endothelial activation in HUVEC’s (5). NO also modulates NF κ   B activation by I κ   B α . These effects require high concentrations of NO donors not likely to be produced by eNOS under physiologic conditions (35), or the expression of inducible NOS (iNOS) in mononuclear cells (39).   In vivo , leukocyte-endothelial cell interactions have been studied in splanchnic ischemia/reperfusion using intravital microscopy. Superfusion with NO donors reduces neutrophil-endothelial cell interactions, possibly by acutely scavenging oxygen radicals (10). NOS inhibitors increase leukocyte-endothelial cell interactions due to degranulation of mast cells and increased superoxide production (16). In addition, intravital microscopy of the mesenteric circulation of eNOS knockout mice showed enhanced leukocyte adhesion to the vascular endothelium associated with increased    4 surface expression of P-selectin (10, 22). Cultured endothelial cells exposed to ischemia/reperfusion rapidly express P-selectin, which leads to neutrophil adherence (32, 34). This condition is reversible by administration of superoxide dismutase, a scavenger of superoxide radicals (13, 40). Thus, the increased adhesion seen in eNOS knockout mice could be caused by increased superoxide production not counterbalanced by eNOS-dependent production of the superoxide scavenger NO. In a different study by Sanz and colleagues, also employing in vivo microscopy, baseline adhesion of leucocytes to the microvasculature of eNOS knockout animals was unchanged. These investigators find that neuronal NOS compensates for the loss of eNOS at baseline (38). However, leukocyte recruitment elicited by oxidative stress was more pronounced in eNOS knockout, than in wild-type animals, suggesting that nNOS did not completely compensate for eNOS deficiency. Reduction of endothelium-derived, eNOS-dependent NO production has been reported after ischemia/reperfusion injury, in sepsis, in hyperlipidemia and in atherosclerosis (8, 19, 20, 23-25, 31). We previously showed that genetic deficiency of eNOS increases intimal proliferation in response to vessel injury and increases atherosclerosis in hypercholesterolemic apolipoprotein-E knockout mice (2, 18, 29). To date, ample evidence suggests that NO modulates leukocyte-endothelial interactions and vascular pathology. However, uncertainty remains whether NO is required to continuously suppress endothelial cell activation. In this study, we apply a reliable novel cell culture technique to isolate endothelial cells from eNOS knockout mice in order to study the effects of complete NO deprivation on endothelial homeostasis. Using this strategy, we avoid problems with pharmacologic inhibition of NOS and interference with other cell types “in vivo”. Our major finding is that endothelial activation does not result from eNOS deficiency in and of itself, raising the possibility that eNOS derived NO counterbalances signals mediating endothelial activation. Materials and methods  All procedures performed conform with MGH policies and the NIH guidelines for care and use of laboratory animals.    5 Materials:  Flow cytometry antibodies (Pharmingen) Biotin conjugated CD31, clone MEC 13.3; R-PE-CD54, clone 3E2; FITC-CD62P, clone RB40.34; FITC-CD106, clone 429; R-PE-CD62E, clone 10E9.6. Streptavidin PerCP (Becton Dickinson). Amplex red   (Molecular probes). Generation of eNOS knockout mice:  eNOS deficient mice were generated by targeted deletion in our laboratory (15). The mice were backcrossed for ten generations to C57BL6 and latter strain served as the wild-type control. Cell culture: Microvascular endothelial cells were isolated from lungs of animals 3 to 4 months old. For each experiment, primary cultures of both genotypes were started simultaneously. Animals were sacrificed by cervical dislocation and lungs were collected in ice-cold Dulbecco’s Modified Eagle medium (D-MEM). Peripheral lung tissue was minced and digested for 1 hour at 37 ° C in 0.1% Collagenase-A (Boehringer Mannheim). The digest was passed through a blunt 14-gauge needle and filtered through a 130 µ m steel mesh. Cells were pelleted at 300x g   and resuspended in “MLEC-medium” (37 ° C) containing 20% FBS, 35% D-MEM, 35% F12, endothelial mitogen (50 µ g/ml; Biomedical Technologies Inc.), L-glutamine (2mmol/l), heparin (100 µ g/ml) and penicillin/streptomycin (100U/100 µ g/ml) and plated in 0.1% gelatin coated T75 flasks. Cells were washed after 24 hours and cultured for 2-4 days. Magnetic beads were coated with anti-mouse CD102 (Pharmigen, clone 3C4) antibody (5 µ g/4 × 10 6  beads: DYNABEADS M-450; DYNAL). Per flask, 4 × 10 6 beads were added and incubated for 1 hour at 4 ° C. Cells were trypsinized and selected in a magnetic field for 10 min. Cultures were grown to confluence and selected twice before plating for experiments. Following this procedure cells used in the experiments were in average ten days in culture. Acetylated Low Density Lipoprotein labelling:  Labelling was done according to the manufacturer’s protocol. Measurement of NO release:  We analyzed the release of NO using chemiluminescent detection for nitrite (NO 2- ) according to a previously published protocol (28). Net NO per µg of protein was calculated after subtracting background levels of NO found in the media.
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