Shear-induced tyrosine phosphorylation in endothelial cells requires Rac1-dependent production of ROS

Redox paradox: effect of N-acetylcysteine and serum on oxidation reduction-sensitive mitogen-activated protein kinase signaling pathways

The thiol reducing agent N-acetylcysteine (NAC) is commonly used as an "antioxidant" in studies examining gene expression, signaling pathways, and outcome in acute and chronic models of lung injury. It is less widely appreciated that NAC can also undergo auto-oxidation and behave as an oxidant. We showed previously that NAC can have opposite effects on the activation of nuclear factor-kappaB depending on whether or not serum is present, and that the effects of NAC in the absence of serum are mimicked by various oxidants. Here we show that in a serum-depleted environment (0.1% fetal bovine serum), NAC substantially inhibited lipopolysaccharide (LPS) activation of the mitogen-activated protein kinases (MAPKs), namely extracellular signal-regulated kinase (ERK), p38mapk, and c-Jun NH2-terminal kinase (JNK). By contrast, in the presence of 10% serum, NAC had no effect on LPS activation of p42 and p44 ERK and in fact enhanced LPS induction of p38mapk and JNK phosphorylation. Because serum can significantly alter the redox state, these findings highlight the importance of the local redox milieu in signal transduction.

Role of endothelium in shear stress-induced constrictions in rat middle cerebral artery

BACKGROUND AND PURPOSE

Luminal shear stress has been reported to constrict cerebral arteries and arterioles of several species. Although the endothelium is not required for this response, it is not known whether the endothelium enhances or attenuates shear stress-induced constrictions.

METHODS

Middle cerebral arteries (MCAs) were isolated from male Long-Evans rats, mounted in a tissue bath, and pressurized to 80 mm Hg in the absence of luminal flow. In some MCAs, the endothelium was selectively loaded with fura 2 for the measurement of endothelial Ca(2+) concentration. Luminal shear stress was increased by adjusting luminal flow while maintaining a constant intraluminal pressure.

RESULTS

After the development of spontaneous tone in MCAs without luminal flow, inside diameters were approximately 190 microm. MCAs constricted approximately 15% when luminal flow was increased to produce a shear stress of 50 dyne/cm(2). The shear stress-induced constrictions were more pronounced in vessels without intact endothelium. Scavenging reactive oxygen species with 4,5-dihydroxy-1,3-benzene disulfonic acid (Tiron) or superoxide dismutase/catalase significantly inhibited the shear stress-induced constrictions in vessels with intact endothelium and in vessels in which the endothelium had been removed. In intact vessels, endothelial Ca(2+) increased 33 nmol/L (from 133+/-11 to 166+/-12 nmol/L) when shear stress was increased to 50 dyne/cm(2). The presence of N(G)-nitro-L-arginine methyl ester (L-NAME), L-NAME+indomethacin, or L-NAME+indomethacin+charybdotoxin had no significant effect on the shear stress-induced constrictions in MCAs with intact endothelium.

CONCLUSIONS

We conclude that the endothelium plays a role in attenuating the shear stress-induced constrictions in rat MCAS: The attenuation does not appear to be by release of NO, prostacyclin, or endothelium-derived hyperpolarizing factor. The endothelium apparently attenuates the constriction by an unknown dilating factor, by a dilating process, or simply by attenuating the mechanical force of the shear stress as it is transmitted to the abluminal side of the vessel.

Activation of a tissue-specific stress response in the aqueous outflow pathway of the eye defines the glaucoma disease phenotype

The glaucomas are a group of optic neuropathies comprising the leading cause of irreversible blindness worldwide. Elevated intraocular pressure due to a reduction in normal aqueous outflow is a major causal risk factor. We found that endothelial leukocyte adhesion molecule-1 (ELAM-1), the earliest marker for the atherosclerotic plaque in the vasculature, was consistently present on trabecular meshwork (TM) cells in the outflow pathways of eyes with glaucomas of diverse etiology. We determined expression of ELAM-1 to be controlled by activation of an interleukin-1 (IL-1) autocrine feedback loop through transcription factor NF-kappaB, and activity of this signaling pathway was shown to protect TM cells against oxidative stress. These findings characterize a protective stress response specific to the eye's aqueous outflow pathways and provide the first known diagnostic indicator of glaucomatous TM cells. They further indicate that common mechanisms contribute to the pathophysiology of the glaucomas and vascular diseases.

NADPH oxidase activity is required for endothelial cell proliferation and migration

NADPH oxidase has been shown to play an important role in cardiovascular biology. The goal of the present study was to determine whether NADPH oxidase activity is important for endothelial cell growth and migration. In proliferation assays, growth factor- or serum-induced DNA synthesis in three different types of human endothelial cells was abrogated by inhibitors of NADPH oxidase, but not by inhibitors of xanthine oxidase or nitric oxide synthase. Moreover, vascular endothelial growth factor-induced migration of human endothelial cells was suppressed in the presence of NADPH oxidase inhibitors. These results support a potential role for NADPH oxidase in mediating angiogenesis.

Rac-related GTP-binding protein in elicitor-induced reactive oxygen generation by suspension-cultured soybean cells

Plant cells produce reactive oxygen species (ROS) in response to many stimuli. However, the mechanism of ROS biosynthesis remains unclear. We have explored the hypothesis that the superoxide burst in plants mechanistically resembles the oxidative burst in neutrophils. First we have confirmed that ROS production, which occurs in suspension-cultured soybean (Glycine max) cells in response to hypo-osmotic shock, is inhibited by diphenylene iodonium, an inhibitor of the flavin-dependent oxidase of neutrophils. Because a Rac family G protein is an essential regulator of this NADPH oxidase, and because many plant homologs of Rac have been cloned, we next examined whether Rac-like proteins might be involved in the oxidative burst in the soybean cells. We identified a Rac-like 21-kD soybean protein that cross-reacts with antibodies to human Rac and garden pea Rop and also binds [gamma-(35)S] GTP, a diagnostic trait of small G proteins. This Rac-related protein translocated from the cytosol to microsomes during the oxidative burst. Moreover, soybean cells transiently transformed with either a dominant negative (RacN17) or a dominant positive (RacV12) form of Rac1 showed the anticipated altered responses to three different stimuli: hypo-osmotic shock, oligo-GalUA, and harpin. In response to these stimuli, cells transformed with RacN17 produced less ROS and cells transformed with RacV12 generated more ROS than control cells. These results strongly suggest that a Rac-related protein participates in the regulation of ROS production in soybean cells, possibly via activation of an enzyme complex similar to the NADPH oxidase of phagocytes in animal systems.

Modulation of protein kinase activity and gene expression by reactive oxygen species and their role in vascular physiology and pathophysiology

Emerging evidence indicates that reactive oxygen species, especially superoxide and hydrogen peroxide, are important signaling molecules in cardiovascular cells. Their production is regulated by hormone-sensitive enzymes such as the vascular NAD(P)H oxidases, and their metabolism is coordinated by antioxidant enzymes such as superoxide dismutase, catalase, and glutathione peroxidase. Both of these reactive oxygen species serve as second messengers to activate multiple intracellular proteins and enzymes, including the epidermal growth factor receptor, c-Src, p38 mitogen-activated protein kinase, Ras, and Akt/protein kinase B. Activation of these signaling cascades and redox-sensitive transcription factors leads to induction of many genes with important functional roles in the physiology and pathophysiology of vascular cells. Thus, reactive oxygen species participate in vascular smooth muscle cell growth and migration; modulation of endothelial function, including endothelium-dependent relaxation and expression of a proinflammatory phenotype; and modification of the extracellular matrix. All of these events play important roles in vascular diseases such as hypertension and atherosclerosis, suggesting that the sources of reactive oxygen species and the signaling pathways that they modify may represent important therapeutic targets.

The BCR/ABL tyrosine kinase induces production of reactive oxygen species in hematopoietic cells

The BCR/ABL oncogene causes chronic myelogenous leukemia, a myeloproliferative disorder characterized by clonal expansion of hematopoietic progenitor cells and myeloid cells. It is shown here that transformation of the hematopoietic cell lines Ba/F3, 32Dcl3, and MO7e with BCR/ABL results in an increase in reactive oxygen species (ROS) compared with quiescent, untransformed cells. The increase in ROS was directly due to BCR/ABL because it was blocked by the ABL-specific tyrosine kinase inhibitor STI571. Oxidative stress through ROS is believed to have many biochemical effects, including the potential ability to inhibit protein-tyrosine phosphatases (PTPases). To understand the significance of increased production of ROS, a model system was established in which hydrogen peroxide (H(2)O(2)) was added to untransformed cells to mimic the increase in ROS induced constitutively by BCR/ABL. H(2)O(2) substantially reduced total cellular PTPase activity to a degree approximately equivalent to that of pervanadate, a well known PTPase inhibitor. Further, stimulation of untransformed cells with H(2)O(2) or pervanadate increased tyrosine phosphorylation of each of the most prominent known substrates of BCR/ABL, including c-ABL, c-CBL, SHC, and SHP-2. Treatment of the BCR/ABL-expressing cell line MO7/p210 with the reducing agents pyrrolidine dithiocarbamate or N-acetylcysteine reduced the accumulation of ROS and also decreased tyrosine phosphorylation of cellular proteins. Further, treatment of MO7e cells with H(2)O(2) or pervanadate increased the tyrosine kinase activity of c-ABL. Drugs that alter ROS metabolism or reactivate PTPases may antagonize BCR/ABL transformation.

The small G-protein Rac mediates depolarization-induced superoxide formation in human endothelial cells

Superoxide anions impair nitric oxide-mediated responses and are involved in the development of hypertensive vascular hypertrophy. The regulation of their production in the vascular system is, however, poorly understood. We investigated whether changes in membrane potential that occur in hypertensive vessels modulate endothelial superoxide production. In cultured human umbilical vein endothelial cells, changes in membrane potential were induced by high potassium buffer, the non-selective potassium channel blocker tetrabutylammonium chloride (1 mm), and the non-selective cation ionophore gramicidin (1 micrometer). Superoxide formation was significantly elevated to a similar degree by all three treatments (by approximately 60%, n = 23, p < 0.01), whereas hyperpolarization by the K(ATP) channel activator Hoe234 (1 micrometer) significantly decreased superoxide formation. Depolarization also induced an increased tyrosine phosphorylation of several not yet identified proteins (90-110 kDa) and resulted in a significant increase in membrane association of the small G-protein Rac. Accordingly, the Rac inhibitor Clostridium difficile toxin B blocked the effects of depolarization on superoxide formation. The tyrosine kinase inhibitor genistein (30 micrometer, n = 15) abolished depolarization-induced superoxide formation and also prevented depolarization-induced Rac translocation associated with it. It is concluded that depolarization is an important stimulus of endothelial superoxide production, which involves a tyrosine phosphorylation-dependent translocation of the small G-protein Rac.

Platelet-derived growth factor-induced H(2)O(2) production requires the activation of phosphatidylinositol 3-kinase

Autophosphorylation of the platelet-derived growth factor (PDGF) receptor triggers intracellular signaling cascades as a result of recruitment of Src homology 2 domain-containing enzymes, including phosphatidylinositol 3-kinase (PI3K), the GTPase-activating protein of Ras (GAP), the protein-tyrosine phosphatase SHP-2, and phospholipase C-gamma1 (PLC-gamma1), to specific phosphotyrosine residues. The roles of these various effectors in PDGF-induced generation of H(2)O(2) have now been investigated in HepG2 cells expressing various PDGF receptor mutants. These mutants included a kinase-deficient receptor and receptors in which various combinations of the tyrosine residues required for the binding of PI3K (Tyr(740) and Tyr(751)), GAP (Tyr(771)), SHP-2 (Tyr(1009)), or PLC-gamma1 (Tyr(1021)) were mutated to Phe. PDGF failed to increase H(2)O(2) production in cells expressing either the kinase-deficient mutant or a receptor in which the two Tyr residues required for the binding of PI3K were replaced by Phe. In contrast, PDGF-induced H(2)O(2) production in cells expressing a receptor in which the binding sites for GAP, SHP-2, and PLC-gamma1 were all mutated was slightly greater than that in cells expressing the wild-type receptor. Only the PI3K binding site was alone sufficient for PDGF-induced H(2)O(2) production. The effect of PDGF on H(2)O(2) generation was blocked by the PI3K inhibitors LY294002 and wortmannin or by overexpression of a dominant negative mutant of Rac1. These results suggest that a product of PI3K is required for PDGF-induced production of H(2)O(2) in nonphagocytic cells, and that Rac1 mediates signaling between the PI3K product and the putative NADPH oxidase.

Signalling pathways in vascular endothelium activated by shear stress: relevance to atherosclerosis

Major advances in our understanding of how endothelial cells sense and respond to haemodynamic forces and, more specifically, to fluid shear stress have been achieved during the past 3 years. These include definition of potential shear stress receptors and multiple signalling pathways that mediate shear stress regulation of gene expression. A few studies have also pointed to the unique effects of complex shear stress on endothelial activation, thus leading to better understanding of the mechanisms that lead to the development of atherosclerosis.

NAD(P)H oxidase: role in cardiovascular biology and disease

Reactive oxygen species have emerged as important molecules in cardiovascular function. Recent work has shown that NAD(P)H oxidases are major sources of superoxide in vascular cells and myocytes. The biochemical characterization, activation paradigms, structure, and function of this enzyme are now partly understood. Vascular NAD(P)H oxidases share some, but not all, characteristics of the neutrophil enzyme. In response to growth factors and cytokines, they produce superoxide, which is metabolized to hydrogen peroxide, and both of these reactive oxygen species serve as second messengers to activate multiple intracellular signaling pathways. The vascular NAD(P)H oxidases have been found to be essential in the physiological response of vascular cells, including growth, migration, and modification of the extracellular matrix. They have also been linked to hypertension and to pathological states associated with uncontrolled growth and inflammation, such as atherosclerosis.

Simulated ischemia in flow-adapted endothelial cells leads to generation of reactive oxygen species and cell signaling

We have previously shown that increased reactive oxygen species (ROS) generation occurs with ischemia in the oxygenated lung and have hypothesized that mechanotransduction is the initiating event. In the present study, we developed an in vitro model of oxygenated ischemia by interrupting medium flow to flow-adapted bovine pulmonary artery endothelial cells in an artificial capillary system. Cellular oxygenation during the "ischemic" period was maintained by perfusing medium over the abluminal surface of porous capillaries. Cells were assessed for ROS generation, nuclear factor-kappaB (NF-kappaB) and activator protein-1 (AP-1) binding activities, and DNA synthesis using dichlorofluorescein fluorescence by flow cytometry and spectrofluorometry, electrophoretic mobility shift assay of nuclear extracts with NF-kappaB-specific or AP-1-specific (32)P-labeled oligonucleotides, and (3)H-thymidine incorporation into DNA. Cells that were flow adapted for 2 to 7 days with 1 to 2 dyne/cm(2) shear stress exhibited a 1.6- to 1.9-fold increase in ROS generation during 1 hour of simulated ischemia compared with continuously perfused cells. This effect was abolished by diphenyleneiodonium chloride (DPI), indicating a role for a flavoprotein such as NADPH oxidase. The increase in ROS generation with ischemia was similar for cells from low and high passages. With ischemia, flow-adapted cells exhibited increases of 1.7-fold in nuclear NF-kappaB and 1.5-fold in nuclear AP-1; these changes were abolished by pretreatment with N-acetylcysteine or DPI. Ischemia for 24 hours resulted in a 1.8-fold increase of (3)H-thymidine incorporation into DNA and a significant increase of cells entering the cell cycle, as indicated by flow cytometry with propidium iodide. We conclude that flow-adapted endothelial cells generate ROS with ischemia that results in activation of NF-kappaB and AP-1 and an increase of DNA synthesis. This effect is not mediated by hypoxia, implicating a role for mechanotransduction in ischemia-mediated cell signaling.