ROS Function in Redox Signaling and Oxidative Stress

Reactive oxygen species (ROS) are key signaling molecules that play an important role in the progression of inflammatory disorders. An enhanced ROS generation by polymorphonuclear neutrophils (PMNs) at the site of inflammation causes endothelial dysfunction and tissue injury. The vascular endothelium plays an important role in passage of macromolecules and inflammatory cells from the blood to tissue. Under the inflammatory conditions, oxidative stress produced by PMNs leads to the opening of inter-endothelial junctions and promotes the migration of inflammatory cells across the endothelial barrier. The migrated inflammatory cells not only help in the clearance of pathogens and foreign particles but also lead to tissue injury. The current review compiles the past and current research in the area of inflammation with particular emphasis on oxidative stress-mediated signaling mechanisms that are involved in inflammation and tissue injury.

Reactive Oxygen Species in Inflammation and Tissue Injury

Neutrophils have long been viewed as the final effector cells of an acute inflammatory response, with a primary role in the clearance of extracellular pathogens. However, more recent evidence has extended the functions of these cells. The newly discovered repertoire of effector molecules in the neutrophil armamentarium includes a broad array of cytokines, extracellular traps and effector molecules of the humoral arm of the innate immune system. In addition, neutrophils are involved in the activation, regulation and effector functions of innate and adaptive immune cells. Accordingly, neutrophils have a crucial role in the pathogenesis of a broad range of diseases, including infections caused by intracellular pathogens, autoimmunity, chronic inflammation and cancer.

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Neutrophils in the activation and regulation of innate and adaptive immunity

Physiological roles of mitochondrial reactive oxygen species.

正しい方法がわかる臨床基本手技 : from the New England journal of medicine

Toxaphene, but not beryllium, induces human neutrophil chemotaxis and apoptosis via reactive oxygen species (ROS): involvement of caspases and ROS in the degradation of cytoskeletal proteins.

Modulation of interleukin-15-induced human neutrophil responses by the plant lectin Viscum album agglutinin-I.

The idea that emotional states can influence health has been widely accepted in medicine since Roman times (1). This idea has been validated in the modern era; several neuroendocrine pathways have been elucidated that link neural and immune responses (2). These links function reciprocally: a number of neural signaling pathways trigger the release of immunomodulators, and cells of the nervous system sense inflammation through receptors for immune cytokines. The Hypothalamic-Pituitary-Adrenal Axis “translates” psychological stressors into glucocorticoid synthesis by the adrenal cortex and epinephrine synthesis by the adrenal medulla, leading to an immediate epinephrine-activated vasoactive response. Glucocorticoid-mediated immune modulation may provide the short-term advantage of suppressing inflammation during stress, whereas chronic endogenous production or exogenous administration of glucocorticoids results in immunosuppression. Both the benefits and detriments of these effects are seen in patients placed on glucocorticoid therapy for autoimmune and inflammatory diseases. A recent paper by Yu et al. identifies a novel pathway of crosstalk between neural and immune receptors with the observation that pharmacological agonists of 5-hydroxytryptamine2A (5-HT2A) receptors can block the pro-inflammatory effects of Tumor Necrosis Factor (TNF) on smooth muscle vascular cells (3).

5-Hydroxytryptamine, commonly known as serotonin, is a neurotransmitter with potent effects on many aspects of mood and cognition. Selective serotonin reuptake inhibitors [(SSRIs), see (4)] are thought to mediate their antidepressant effects through enhancing the availability of serotonin in the CNS. Of the seven subtypes of serotonin receptors identified in humans, only the 5-HT2A receptor is involved in cognition. Hallucinogens such as lysergic acid diethylamide (LSD) are thought to exert their actions through 5-HT2A receptors, and atypical antipsychotics and some newer generation anti-depressants are thought to act primarily as 5-HT2A antagonists. For these reasons, the effects of serotonin on the CNS have been intensively studied. However, most serotonin is actually produced in other parts of the body by various cells types such as activated intestinal enterochromaffin cells, mast cells, and platelets. Serotonin regulates numerous biological processes outside the CNS such as bowel motility, platelet aggregation, cardiac function, and bladder control (5). In addition, serotonin has important effects on vascular and uterine smooth muscles, as it is involved in their growth and contraction (6). In the immune system, serotonin activates human monocytes and prevents their apoptosis (7) and modulates cytokine and chemokine production in lipopolysaccharide (LPS)-primed monocytes (8). In vivo, serotonin appears to be pro-inflammatory, as a number of studies have shown depletion of serotonin within the CNS acts to reduce animal models of inflammation such as adjuvant-induced arthritis (9–11).

In contrast to the apparently pro-inflammatory role of serotonin as a neurotransmitter, Yu et al. (3) show that activation of 5-HT2A receptors on smooth muscle cells with the synthetic agonist (R)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane [(R)-DOI] suppresses multiple responses to TNF. (R)-DOI repressed the TNF-mediated induction of adhesion molecules and cytokine production at concentrations in the low picomolar range, which might be considered “superpotent” because this is below the nanomolar affinity measured for (R)-DOI at 5-HT2A receptors. (R)-DOI suppressed TNFα-mediated induction of mRNA for proinflammatory markers, such as VCAM-1, ICAM-1, and IL-6, nitric oxide synthase activity, and translocation of NF-κB all at this very low concentration. These effects were restricted to 5-HT2A receptors, as 5-HT2B and 5-HT2C receptor-selective agonists were ineffective in suppressing TNFα-induced inflammation. (R)-DOI was also compared with three other 5-HT2A receptor agonists: the phenethylamine 2C-BCB [(4-bromo-3,6-dimethoxybenzocyclobuten-1-yl) methylamine]; and two indolealkylamines: LA-SS-Az [(2′S,4′S)-(+)-9,10-didehydro-6-methylergoline-8β-(trans-2,4-dimethylazetidide)] and LSD. These three agents suppressed TNF-induced ICAM-1, V-CAM-1, and IL-6 expression, but with less potency than (R)-DOI. In addition, (R)-DOI was able to significantly inhibit the effects of TNFα when administered up to four hours after TNFα, indicating possibly therapeutic effects of (R)-DOI on already established chronic inflammation.

Yu et al. (3) only hint at the signal transduction pathways underlying blockade of TNF signaling by the 5-HT2A agonists. TNF has two receptors, TNFR1 (p55) and TNFR2 (p75), but most of the pro-inflammatory effects of TNF are mediated through TNFR1, suggesting that 5-HT2A agonists likely act on TNFR1. Inhibitor studies suggested that the effects of 5-HT2A agonists are mediated through the activity of protein kinase C family members. TNFR1 signaling was affected by inhibition of NF-κB translocation to the nucleus, suggesting a direct effect of (R)-DOI on early events in TNFR1 signaling. The primary TNFR1 signaling complex consists of the adapter protein TNFR1-associated death domain protein (TRADD), the ring finger–containing proteins TNF receptor-associated factor 1 (TRAF1) and TRAF2, and the protein kinase receptor-interacting serine–threonine protein kinase (RIP), which act together to activate the IκB Kinase complex to trigger degradation of the inhibitor of NF-κB (IκB) and release of NF-κB subunits into the nucleus where they function as transcription factors (Figure 1). Interestingly, protein kinase C (PKC) agonists, such as Phorbol 12-Myristate 13-Acetate (PMA, also termed TPA) can block TNF signaling and inhibit assembly of the TNFR1 proximal signaling complex (12).



Figure 1

Potential mechanism of crosstalk between the tumor necrosis factor receptor 1 (TNFR1) and serotonin subtype 2A (5-HT2A) receptor signaling. The primary TNFR1 signaling complex consists of the adapter protein TRADD, the ring finger–containing protein ...



Although these results suggest that 5-HT2A agonists might be investigated further as an anti-inflammatory agent, there are a number of concerns that limit interpretation of these results. The suppressive effects of (R)-DOI on induction of pro-inflammatory genes were only reported for mRNAs. It is certainly necessary to determine whether these effects operate at the protein level as well. Most experiments were done with (R)-DOI, or other synthetic 5-HT2A receptor agonists. It would be useful to know whether these effects are seen with serotonin itself and whether they could be reversed by a 5-HT2A receptor antagonist such as ketanserin or by using cells deficient in the 5-HT2A receptor. Additionally, it is essential to test whether the observed effects of (R)-DOI on TNFα-induced inflammation also apply to other cell types, especially primary human cells responsive to TNF such as hepatocytes, endothelial cells, and monocytes.

Despite these concerns, it is tempting to speculate on the clinical implications of this study. Single-nucleotide genetic polymorphisms in the 5-HT2A receptor gene have been associated with rheumatoid arthritis (13), although the genome-wide significance of this association is not clear. Although not tested in this study, one might predict from these results that patients on 5-HT2A-blocking agents would become hypersensitive to the effects of TNF and possibly develop arthritic symptoms. In fact, in a recent retrospective survey of drug reactions, Dahlqvist and colleagues found a 45-fold excess rate of joint complaints in patients given 5-HT2A-blocking antidepressants, such as mianserin, nefazodone, and mirtazapin compared to patients given SSRIs (14). The same group observed that the density of serotonin 5-HT2A receptors was decreased in rheumatoid arthritis patients (15), suggesting a possible role for reduced expression of the 5-HT2A receptor in the sensitization of cells to TNF in rheumatoid arthritis. One might also speculate that SSRIs could be anti-inflammatory, although there is little evidence of this in the literature, and SSRIs would increase availability of serotonin for all subgroups of receptors, not just 5-HT2A.

Stimulating 5-HT2A receptors to suppress inflammation would clearly require the development of agents that do not affect the CNS, as 5-HT2A agonists such as LSD are well known to induce hallucinations and altered mental status. Also, the discovery that (R)-DOI was able to potently inhibit the effects of TNFα even for up to two hours after the administration of TNFα indicate that potential therapies could not only be aimed at preventing inflammation but also treating inflammatory injury that has already occurred or is ongoing. However, it would be important to determine the duration of the effects and whether receptor desensitization occurs, because treatment with 5-HT2A agonists would most likely be needed for much longer periods of time in patients with chronic inflammatory joint disease. Whether these results bear fruit in terms of therapeutic agents, they add one more thread to the intriguing web of interconnections that link our immune and nervous systems.






Martin Pelletier, PhD, received his doctorate in 2005 in Virology and Immunology at INRS-Institut Armand-Frappier (Laval, Quebec, Canada) for his research in cytokine-mediated neutrophil activation by interleukin-15 and interleukin-21. As a postdoctoral ...











Richard Siegel’s interest in immunology, autoimmunity, and apoptosis began as an MD/PhD student at the University of Pennsylvania School of Medicine, from which he graduated in 1993. He trained in Internal Medicine and Rheumatology at Hospital ...

/pdf/wishing-away-inflammation-new-links-between-serotonin-and-3eukmm4syw.pdf

Wishing Away Inflammation? New Links between Serotonin and TNF Signaling

Bisphenol A, bisphenol S and their glucuronidated metabolites modulate glycolysis and functional responses of human neutrophils.

IL-15 has been found to activate NF-kappaB in various types of cells. However, the role of this transcription factor in IL-15- and IL-21-stimulated murine bone marrow (BM) cells is unclear. In this study, we demonstrated that both IL-15 and IL-21 are capable of delaying BM cell factor deprivation-induced apoptosis, but only IL-15 induced their proliferation. Following separation of BM cells into myeloid (CD11b(+)) and lymphoid (CD11b(-)) cell populations, we found that IL-15, but not IL-21, significantly induced proliferation in both cell populations. Both cytokines significantly delayed apoptosis, but only in CD11b(-) BM cells. IL-15Ralpha, CD122 (IL-2/15Rbeta), and common gamma-chains (CD132) were expressed in both populations, whereas IL-21Ralpha was expressed only in CD11b(-) BM cells. In addition, we demonstrated that IL-15-induced BM cell proliferation was significantly inhibited in NF-kappaBp50(-/-) mice when compared with littermate controls. The ability of IL-15 and IL-21 to delay BM cell apoptosis was slightly inhibited in NF-kappaBp50(-/-) mice, whereas the antiapoptotic effect of LPS was markedly reversed. We conclude that IL-15, but not IL-21, induces BM cell proliferation and that both cytokines delay BM cell apoptosis. These biological activities were preferentially observed in CD11b(-) BM cells. Using NF-kappaBp50(-/-) mice, we demonstrated for the first time that NF-kappaB plays a greater role in IL-15-induced cell proliferation than in IL-15- and IL-21-induced suppression of apoptosis.

Martin Pelletier

Papers

Toxaphene, but not beryllium, induces human neutrophil chemotaxis and apoptosis via reactive oxygen species (ROS): involvement of caspases and ROS in the degradation of cytoskeletal proteins.

Modulation of interleukin-15-induced human neutrophil responses by the plant lectin Viscum album agglutinin-I.

Wishing Away Inflammation? New Links between Serotonin and TNF Signaling

Bisphenol A, bisphenol S and their glucuronidated metabolites modulate glycolysis and functional responses of human neutrophils.

Differential effects of IL-15 and IL-21 in myeloid (CD11b+) and lymphoid (CD11b-) bone marrow cells.