Oxidized LDL activates STAT1 and STAT3 transcription factors: possible involvement of reactive oxygen species

Activation of JAK2 by the oxidative stress generated with oxidized low-density lipoprotein

Atherosclerosis includes a series of cellular and molecular responses characteristic of an inflammatory disease. We provide evidence that cupric-ion-oxidized LDL (CuLDL) or endothelial cell-oxidized LDL (ELDL) induced the activation by Tyr-phosphorylation of JAK2, one of the Janus kinase involved upstream of STATs in the JAK/STAT pathway of cytokine transduction. Oxidized LDL (OxLDL) also initiated STAT1 and STAT3 Tyr-phosphorylation and translocation to the nucleus, with a more marked effect for the extensively modified CuLDL. Genistein, a nonspecific Tyr-kinase inhibitor, and AG490, a specific inhibitor of JAKs, markedly prevented the CuLDL-induced enhancement of STAT1 and STAT3 Tyr-phosphorylation and DNA-binding activity, suggesting that JAKs are the main kinases involved in STATs' activation by oxidized LDL. In addition, the lipid extract of CuLDL increased the intracellular levels of lipid peroxidation products and the Tyr-phosphorylation of JAK2, STAT1, and STAT3, whereas the antioxidant vitamin E prevented all these effects. These results demonstrate that OxLDL induces the activation by Tyr-phosphorylation of JAK2, STAT1, and STAT3 by generation of an intracellular oxidative stress by means of its lipid peroxidation products, and thus include JAK2 within the range of oxidative stress-activated kinases.

Activation of the JAK/STAT pathway by ceramide in cultured human fibroblasts

Endogenous ceramide (CER) was generated by treatment of cultured fibroblasts with sphingomyelinase (SMase) from Bacillus cereus. A 30 min treatment with 0.1-0.3 U/ml SMase induced a dose-dependent increase in the intracellular level of CER. The activation of the transcription factors signal transducer and activator of transcription (STAT) 1 and STAT3 by SMase was investigated by determination of the phosphorylation state by immunoblot, and of DNA binding activity by electrophoretic mobility shift assay. SMase treatment induced a dose-dependent Tyr-phosphorylation of STAT1/3. SMase also enhanced STAT1/3 DNA binding activity in a dose-dependent manner. Concomitantly, SMase enhanced the Tyr-phosphorylation of Janus kinase (JAK) 2, a Tyr-kinase localized upstream of STATs in the JAK/STAT pathway. The Tyr-kinase inhibitor genistein and the JAK inhibitor AG490 both prevented JAK2 Tyr-phosphorylation, together with STAT1 and STAT3 Tyr-phosphorylation and binding activity. The SMase-induced increase in STAT1/3 binding activity was prevented by methyl-beta-cyclodextrin, a cholesterol binding agent that causes a loss of compartmentalization of the molecules located in caveolae. This increase was also prevented by the MEK inhibitor PD98059, thus demonstrating the role of the MEK/ERK pathway in this system. Besides ERK, SMase activated other signaling kinases such as JNK and p38. Exogenous natural CER also activated STAT1/3 binding activity, which indicates that most probably, endogenous CER is the second messenger involved in the effect of SMase. These results describe a crosstalk between the SMase/CER and the JAK/STAT signaling pathways and include JAK2 within the range of CER-activated intracellular kinases.

Oxidant stress and endothelial cell dysfunction

Reactive oxygen species (ROS) are generated at sites of inflammation and injury, and at low levels, ROS can function as signaling molecules participating as signaling intermediates in regulation of fundamental cell activities such as cell growth and cell adaptation responses, whereas at higher concentrations, ROS can cause cellular injury and death. The vascular endothelium, which regulates the passage of macromolecules and circulating cells from blood to tissues, is a major target of oxidant stress, playing a critical role in the pathophysiology of several vascular diseases and disorders. Specifically, oxidant stress increases vascular endothelial permeability and promotes leukocyte adhesion, which are coupled with alterations in endothelial signal transduction and redox-regulated transcription factors such as activator protein-1 and nuclear factor-kappaB. This review discusses recent findings on the cellular and molecular mechanisms by which ROS signal events leading to impairment of endothelial barrier function and promotion of leukocyte adhesion. Particular emphasis is placed on the regulation of cell-cell and cell-surface adhesion molecules, the actin cytoskeleton, key protein kinases, and signal transduction events.

Pecam-1 is a modulator of stat family member phosphorylation and localization: lessons from a transgenic mouse

PECAM-1 (CD31) is a member of the immunoglobin (Ig) superfamily of cell adhesion molecules whose expression is restricted to hematopoietic and vascular cells. PECAM-1 can recruit adapter and signaling molecules via its immunoreceptor tyrosine activation motif (ITAM), suggesting that PECAM-1 plays a role in signal transduction pathways. To study the involvement of PECAM-1 in signaling cascades in vivo, we used the major histocompatibility (MHC) I gene promoter to target ectopic PECAM-1 expression in transgenic mice. We noted an attenuation of mammary gland development at early stages of virgin ductal branching morphogenesis. STAT5a, a modulator of milk protein gene expression during lactation, was localized to the nuclei of ductal epithelial cells of 6-week-old virgin PECAM-1 transgenics, but not in control mice. This correlated with decreases in ductal epithelial cell proliferation and induction of p21, an inhibitor of cell cycle progression. Using in vitro model systems we demonstrated PECAM-1/STAT5a association and found that residue Y701 in PECAM-1's cytoplasmic tail is important for PECAM-1/STAT5 association and that PECAM-1 modulates increases in STAT5a tyrosine phosphorylation levels. We suggest that by serving as a scaffolding, PECAM-1 can bring substrates (STAT5a) and enzymes (a kinase) into close proximity, thereby modulating phosphorylation levels of selected proteins, as previously noted for beta-catenin.

Mildly oxidized low density lipoprotein activates multiple apoptotic signaling pathways in human coronary cells

Apoptosis of arterial cells induced by oxidized low density lipoproteins (OxLDL) is thought to contribute to the progression of atherosclerosis. However, most data on apoptotic effects and mechanisms of OxLDL were obtained with extensively oxidized LDL unlikely to occur in early stages of atherosclerotic lesions. We now demonstrate that mildly oxidized LDL generated by incubation with oxygen radical-producing xanthine/xanthine oxidase (X/XO) induces apoptosis in primary cultures of human coronary endothelial and SMC, as determined by TUNEL technique, DNA laddering, and FACS analysis. Apoptosis was markedly reduced when X/XO-LDL was generated in the presence of different oxygen radical scavengers. Apoptotic signals were mediated by intramembrane domains of both Fas and tumor necrosis factor (TNF) receptors I and II. Blocking of Fas ligand (FasL) reduced apoptosis by 50% and simultaneous blocking of FasL and TNF receptors by 70%. Activation of apoptotic receptors was accompanied by an increase of proapoptotic and a decrease in antiapoptotic proteins of the Bcl-2 family and resulted in marked activation of class I and II caspases. Mildly oxidized LDL also activated MAP and Jun kinases and increased p53 and other transcription factors (ATF-2, ELK-1, CREB, AP-1). Inhibitors of Map and Jun kinase significantly reduced apoptosis. Our results provide the first evidence that OxLDL-induced apoptosis involves TNF receptors and Jun activation. More important, they demonstrate that even mildly oxidized LDL formed in atherosclerotic lesions may activate a broad cascade of oxygen radical-sensitive signaling pathways affecting apoptosis and other processes influencing the evolution of plaques. Thus, we suggest that extensive oxidative modifications of LDL are not necessary to influence signal transduction and transcription in vivo.

Oxidized LDL induces an oxidative stress and activates the tumor suppressor p53 in MRC5 human fibroblasts

It is now well established that oxidized LDL (OxLDL) is involved in the progression of the atheromatous plaque via several mechanisms, including its cytotoxicity toward the arterial wall. Our study demonstrates that a 4-h incubation of cultured human fibroblasts with 25-75 microg/ml OxLDL induced a dose-dependent increase in the intracellular levels of reactive oxygen species (ROS) and lipid peroxidation end products (TBARS). This effect was markedly prevented by the antioxidant vitamin E. The lipid extract of OxLDL partially reproduced the action of the LDL particle itself. Concomitantly, OxLDL enhanced the DNA binding activity of p53 measured by electrophoretic mobility shift assay, and the intracellular protein level of p53 determined by immunoblot analysis. Cycloheximide prevented the OxLDL-induced augmentation in both p53 binding activity and intracellular level. Again, the lipid extract of OxLDL reproduced the effect of OxLDL on p53 binding activity, whereas vitamin E prevented it. These results indicate that OxLDL initiates an intracellular oxidative stress by means of its lipid peroxidation products, leading to the activation of the tumour suppressor p53 by enhancement of p53 protein synthesis. This effect might be related to the cytotoxic effect of OxLDL since the activation of p53 is known to lead to cell cycle arrest, necrosis or apoptosis.

Inhibition of LDL oxidation and oxidized LDL-induced cytotoxicity by dihydropyridine calcium antagonists

PURPOSE

The antioxidant activity of dihydropyridine calcium channel antagonists was evaluated based on LDL oxidation kinetics, oxidative cell injury associated with reactive species generation, and increases in free intracellular calcium (Ca2+) levels. Interactions with ascorbic acid were studied under conditions representative of LDL oxidation in plasma and tissue.

METHODS

Analysis of antioxidant activity utilized measurements of one-electron oxidation potentials and scavenging of peroxy radical-mediated oxidation. LDL antioxidant potency was determined spectrophotometrically using copper-mediated oxidation kinetics in the absence and presence of 100 microM ascorbic acid. Prevention of oxidant-induced endothelial cell injury was determined from the formation of reactive oxygen species generation and increases in intracellular free calcium concentrations following addition of oxidized LDL or linoleic acid hydroperoxide.

RESULTS

Felodipine and amlodipine effectively inhibit peroxyl radical-mediated oxidation in lipoproteins and cells that is markedly enhanced in the presence of ascorbic acid. In the presence of ascorbic acid, inhibition of LDL oxidation is over four times greater than in LDL treated without antioxidants, and oxidized LDL and linoleic acid hydroperoxide-induced reactive oxygen species formation is effectively suppressed in cells. Inhibition of intracellular calcium increases was achieved using nM concentrations of felodipine or amlodipine.

CONCLUSIONS

The additive effect for ascorbic acid and the calcium channel antagonist is postulated to involve a combination of peroxide-degrading and peroxyl radical scavenging reactions, demonstrating the importance of lipid peroxides during LDL oxidation and oxidized LDL-induced cytotoxicity. Cytoprotection is associated with inhibition of oxidant-induced increases in intracellular free calcium. Both the cytoprotective and LDL antioxidant activity for these compounds is manifested at concentrations approaching the therapeutic levels found in plasma.

Class A scavenger receptor up-regulation in smooth muscle cells by oxidized low density lipoprotein. Enhancement by calcium flux and concurrent cyclooxygenase-2 up-regulation

Oxidative stress caused by phorbol esters or reactive oxygen up-regulates the class A scavenger receptor (SR-A) in human smooth muscle cells (SMC), which normally do not express this receptor. The increase in SR-A expression correlates with activation of the redox-sensitive transcription factors activating protein-1 c-Jun and CCAAT enhancer-binding protein beta. Here we show that coincubation of SMC with macrophages or oxidized low density lipoproteins (LDL) from macrophage-conditioned medium activates these same regulatory pathways and stimulates SR-A expression. The increased SR-A gene transcription induced by cell-oxidized LDL up-regulated SR-A mRNA and increased by 30-fold the uptake of acetyl LDL, a ligand for the SR-A. Copper-oxidized LDL also increased SR-A receptor expression. Oxidized LDL with a lipid peroxide level of 80-100 nmol/mg of LDL protein and an electrophoretic mobility approximately 1.5 times that of native LDL exhibited the greatest bioactivity. Inhibition of calcium flux suppressed SR-A induction by oxidized LDL. Conversely, calcium ionophore greatly enhanced SR-A up-regulation by oxidized LDL or other treatments that promote intracellular oxidative stress. This enhancement was dependent upon concurrent up-regulation of SMC cyclooxygenase-2 expression and activity and was blocked by the cyclooxygenase-2 inhibitors NS-398 and Resveratrol. In THP-1 cells, oxidized LDL induced monocyte-to-macrophage differentiation and increased SR-A expression. These findings support a role for mildly oxidized LDL in the redox regulation of macrophage differentiation and SR-A expression and suggest that increased vascular oxidative stress may contribute to the formation of both SMC and macrophage foam cells.

Physiological aspects of low-density lipoprotein oxidation

The oxidation of LDL is thought to be a major contributor to the development of atherosclerosis. Considerable descriptive evidence has been accumulated showing that oxidized LDL promotes pro-atherogenic events. However, direct evidence that oxidized LDL causes atherosclerosis is lacking. This article summarizes the results of recent studies that demonstrate how oxidized LDL affects cellular function, and highlights key issues that should be addressed to link LDL oxidation with atherosclerosis.

Lipid oxidation products in cell signaling

The recent research on the impact that oxidative changes of biolipids could have in pathophysiology serves to explain how free radical-driven reactions not only are considered as mere toxicologic events, but also modulators of cell activity and function. Oxidatively modified low-density lipoproteins are known to affect various cellular processes by modulating various molecular pathways and signaling nuclear transcription. Among the lipid oxidation products detectable in ox-LDLs, and also in the atherosclerotic plaques, 4-hydroxynonenal has been widely investigated. This aldehyde was shown to upregulate AP-1 transcription factor, signaling through the MAP kinase pathway, with eventual nuclear localization and induction of a series of genes. Further, oxidation products of cholesterol and cholesterol esters, in ox-LDL are of similar interest, especially in relation to the pathogenesis of fibrosclerotic lesions of the arterial wall.

Oxidative stress and gene regulation

Reactive oxygen species are produced by all aerobic cells and are widely believed to play a pivotal role in aging as well as a number of degenerative diseases. The consequences of the generation of oxidants in cells does not appear to be limited to promotion of deleterious effects. Alterations in oxidative metabolism have long been known to occur during differentiation and development. Experimental perturbations in cellular redox state have been shown to exert a strong impact on these processes. The discovery of specific genes and pathways affected by oxidants led to the hypothesis that reactive oxygen species serve as subcellular messengers in gene regulatory and signal transduction pathways. Additionally, antioxidants can activate numerous genes and pathways. The burgeoning growth in the number of pathways shown to be dependent on oxidation or antioxidation has accelerated during the last decade. In the discussion presented here, we provide a tabular summary of many of the redox effects on gene expression and signaling pathways that are currently known to exist.