Bradykinin-12-lipoxygenase-VR1 signaling pathway for inflammatory hyperalgesia

Abstract

The capsaicin-sensitive vanilloid receptor (VR1) was recently shown to play an important role in inflammatory pain (hyperalgesia), but the underlying mechanism is unknown. We hypothesized that pain-producing inflammatory mediators activate capsaicin receptors by inducing the production of fatty acid agonists of VR1. This study demonstrates that bradykinin, acting at B2 bradykinin receptors, excites sensory nerve endings by activating capsaicin receptors via production of 12-lipoxygenase metabolites of arachidonic acid. This finding identifies a mechanism that might be targeted in the development of new therapeutic strategies for the treatment of inflammatory pain. V R1, a cloned capsaicin receptor, is a nonspecific cation channel expressed preferentially in small sensory neurons and activated by the vanilloids, capsaicin and resiniferatoxin (1). Because VR1 is also activated by heat and acid (1, 2), it is now considered to be a molecular sensor that detects a variety of painful stimuli. Indeed, recent experiments performed in mice that lack VR1 demonstrated that the receptor is essential for inflammation-induced heat hyperalgesia (3, 4). Therefore, understanding the cellular mechanisms by which capsaicin receptors are activated by inflammatory mediators may be a key to identifying novel therapeutic targets for pain treatment. Because of the presence of VR1 in sensory neurons and an apparent role in inflammatory hyperalgesia, endogenous activators of VR1 have been suspected. We recently demonstrated that products of the lipoxygenase pathway of arachidonic acid (AA) metabolism can activate capsaicin receptors (5). Among the eicosanoids tested, the 12-lipoxygenase product, 12-hydroperoxyeicosatetraenoic acid (12-HPETE), structurally similar to capsaicin, was the most potent VR1 agonist. Thus, metabolic products of lipoxygenases become candidates for the endogenous capsaicinlike substances. However, the upstream signals that stimulate lipoxygenase and activate VR1 are elusive.Bradykinin (BK) is a potent inflammatory mediator that causes pain and hyperalgesia. BK is known to activate as well as sensitize sensory neurons to other stimuli. Various signaling pathways have been suggested to mediate the sensitizing effect of BK on sensory neurons (6, 7). However, activation mechanism by BK is not known. BK is now known to stimulate the production of AA in sensory neurons (8), a key substrate of lipoxygenases. Therefore, on the basis of previous observations that products of lipoxygenase activate VR1 (5), we hypothesized that BK excites sensory neurons by opening the capsaicin receptor via production of 12-lipoxygenase products of AA metabolism. Materials and MethodsCell Culture. Experiments were carried out according to the Ethical Guidelines of the International Association for the Study of Pain and approved by the research ethics committee for the use of animals of the Seoul National University and the University of California, San Francisco. Thoracic and lumbar dorsal root ganglia (DRGs) were dissected from 1-to 2 day-...

Human Sensory Neuron-specific Mas-related G Protein-coupled Receptors-X1 Sensitize and Directly Activate Transient Receptor Potential Cation Channel V1 via Distinct Signaling Pathways

“…So far, direct TRPV1 activation has been proposed for muscarinic-3 receptors (18) and metabotropic glutamate-5 receptors (55) via DAG production and for the B2R via PIP 2 degradation (17) or LOX activation (45). Herein, we noted that TRPV1 co-expression in F11-MRGPR-X1 cells profoundly enhanced MRGPR-X1-induced Ca 2ϩ signals, Mn 2ϩ influx, and cation currents at RT and constant pH, indicative of direct TRPV1 activation.…”

Nociceptors Lacking TRPV1 and TRPV2 Have Normal Heat Responses

“…That TRPV1 is important in pathological states is further suggested by its ability to respond, and be potentiated by, multiple stimuli including those that are physical (heat and mechanical) and chemical (e.g., vanilloid compounds and acid). In addition, TRPV1 currents can be potentiated by interactions with G-coupled proteins like the bradykinin receptors (Shin et al, 2002;Sugiura et al, 2002;Carr et al, 2003;Ferreira et al, 2004) and the P2Y2 ATP receptor (Moriyama et al, 2003). It is also important to note that behavioral heat hyperalgesia induced by either complete Freund's adjuvant, carrageenan, or ATP is absent in TRPV1 Ϫ/Ϫ mice (although pretreatment responses are normal) (Caterina et al, 2000;Davis et al, 2000;Moriyama et al, 2003).…”

TRP Channels and Pruritus

“…These involve heat ( 43°C), acidosis (pH<5.9) [173] and mostly arachidonic acid derived lipid mediators such as the endocannabinoid anandamide [179,180], the "endovanilloid" N-arachidonoyl-dopamine [181], and lypoxigenase products [182,183]. Besides direct activation mechanisms, several molecules are able to act on TRPV1 via first activating their specific receptors and then initiating downstream signaling pathways leading to the sensitization of TRPV1.…”

“…Indeed, bradykinin [183,184], ATP [185], lipoxygenase products [182,183], prostaglandins [186,187], histamine [188], various neurotrophins (NGF, neurotrophin-3 and -4) [91,102,184], TNF- [189,190], pro-inflammatory chemokines [191], and PAR2 [192,193] were all reported to sensitize TRPV1 via various signal transduction pathways. TRPV1 was shown to be modulated by the protein kinase C (PKC) [183,[194][195][196], phospholipase C (PLC), and phosphatidylinositol 4,5-bisphosphate (PIP 2 ) [184,[197][198][199][200] systems as well as by intracellular ATP [201]. The sensitization process eventually results in the shift of the activation temperature and voltage threshold towards more physiological ranges [195,[202][203][204].…”