Involvement of Transient Receptor Potential Cation Channel Member A1 activation in the irritation and pain response elicited by skin-lightening reagent hydroquinone

Pharmacologic Inhibition of Transient Receptor Potential Ion Channel Ankyrin 1 Counteracts 2-Chlorobenzalmalononitrile Tear Gas Agent-Induced Cutaneous Injuries

Deployment of the tear gas agent 2-chlorobenzalmalononitrile (CS) for riot control has significantly increased in recent years. The effects of CS have been believed to be transient and benign. However, CS induces severe pain, blepharospasm, lachrymation, airway obstruction, and skin blisters. Frequent injuries and hospitalizations have been reported after exposure. We have identified the sensory neuronal ion channel, transient receptor potential ankyrin 1 (TRPA1), as a key CS target resulting in acute irritation and pain and also as a mediator of neurogenic inflammation. Here, we examined the effects of pharmacologic TRPA1 inhibition on CS-induced cutaneous injury. We modeled CS-induced cutaneous injury by applying 10 μl CS agent [200 mM in dimethyl sulfoxide (DMSO)] to each side of the right ears of 8- to 9-week-old C57BL/6 male mice, whereas left ears were applied with solvent only (DMSO). The TRPA1 inhibitor HC-030031 or A-967079 was administered after CS exposure. CS exposure induced strong tissue swelling, plasma extravasation, and a dramatic increase in inflammatory cytokine levels in the mouse ear skin. We also showed that the effects of CS were not transient but caused persistent skin injuries. These injury parameters were reduced with TRPA1 inhibitor treatment. Further, we tested the pharmacologic activity of advanced TRPA1 antagonists in vitro. Our findings showed that TRPA1 is a crucial mediator of CS-induced nociception and tissue injury and that TRPA1 inhibitors are effective countermeasures that reduce key injury parameters when administered after exposure. Additional therapeutic efficacy studies with advanced TRPA1 antagonists and decontamination strategies are warranted. SIGNIFICANCE STATEMENT: 2-Chlorobenzalmalononitrile (CS) tear gas agent has been deployed as a crowd dispersion chemical agent in recent times. Exposure to CS tear gas agents has been believed to cause transient acute toxic effects that are minimal at most. Here we found that CS tear gas exposure causes both acute and persistent skin injuries and that treatment with transient receptor potential ion channel ankyrin 1 (TRPA1) antagonists ameliorated skin injuries.

IL-33/ST2 induces neutrophil-dependent reactive oxygen species production and mediates gout pain

Objective: Gout, induced by monosodium urate (MSU) crystal deposition in joint tissues, provokes severe pain and impacts life quality of patients. However, the mechanisms underlying gout pain are still incompletely understood.

Methods: We established a mouse gout model by intra-articularly injection of MSU crystals into the ankle joint of wild type and genetic knockout mice. RNA-Sequencing, in vivo molecular imaging, Ca²⁺ imaging, reactive oxygen species (ROS) generation, neutrophil influx and nocifensive behavioral assays, etc. were used.

Results: We found interleukin-33 (IL-33) was among the top up-regulated cytokines in the inflamed ankle. Neutralizing or genetic deletion of IL-33 or its receptor ST2 (suppression of tumorigenicity) significantly ameliorated pain hypersensitivities and inflammation. Mechanistically, IL-33 was largely released from infiltrated macrophages in inflamed ankle upon MSU stimulation. IL-33 promoted neutrophil influx and triggered neutrophil-dependent ROS production via ST2 during gout, which in turn, activated transient receptor potential ankyrin 1 (TRPA1) channel in dorsal root ganglion (DRG) neurons and produced nociception. Further, TRPA1 channel activity was significantly enhanced in DRG neurons that innervate the inflamed ankle via ST2 dependent mechanism, which results in exaggerated nociceptive response to endogenous ROS products during gout.

Conclusions: We demonstrated a previous unidentified role of IL-33/ST2 in mediating pain hypersensitivity and inflammation in a mouse gout model through promoting neutrophil-dependent ROS production and TRPA1 channel activation. Targeting IL-33/ST2 may represent a novel therapeutic approach to ameliorate gout pain and inflammation.

Electroacupuncture Alleviates Paclitaxel-Induced Peripheral Neuropathic Pain in Rats via Suppressing TLR4 Signaling and TRPV1 Upregulation in Sensory Neurons

Paclitaxel-induced peripheral neuropathy is a common adverse effect during paclitaxel treatment resulting in sensory abnormalities and neuropathic pain during chemotherapy and in cancer survivors. Conventional therapies are usually ineffective and possess adverse effects. Here, we examined the effects of electroacupuncture (EA) on a rat model of paclitaxel-induced neuropathic pain and related mechanisms. EA robustly and persistently alleviated paclitaxel-induced pain hypersensitivities. Mechanistically, TLR4 (Toll-Like Receptor 4) and downstream signaling MyD88 (Myeloid Differentiation Primary Response 88) and TRPV1 (Transient Receptor Potential Vallinoid 1) were upregulated in dorsal root ganglion (DRGs) of paclitaxel-treated rats, whereas EA reduced their overexpression. Ca²⁺ imaging further indicated that TRPV1 channel activity was enhanced in DRG neurons of paclitaxel-treated rats whereas EA suppressed the enhanced TRPV1 channel activity. Pharmacological blocking of TRPV1 mimics the analgesic effects of EA on the pain hypersensitivities, whereas capsaicin reversed EA’s effect. Spinal astrocytes and microglia were activated in paclitaxel-treated rats, whereas EA reduced the activation. These results demonstrated that EA alleviates paclitaxel-induced peripheral neuropathic pain via mechanisms possibly involving suppressing TLR4 signaling and TRPV1 upregulation in DRG neurons, which further result in reduced spinal glia activation. Our work supports EA as a potential alternative therapy for paclitaxel-induced neuropathic pain.

Transient receptor potential ankyrin 1 (TRPA1)-mediated toxicity: friend or foe?

Transient receptor potential (TRP) channels have been widely studied during the last decade. New studies uncover new features and potential applications for these channels. TRPA1 has a huge distribution all over the human body and has been reported to be involved in different physiological and pathological conditions including cold, pain, and damage sensation. Considering its role, many studies have been devoted to evaluating the role of this channel in the initiation and progression of different toxicities. Accordingly, we reviewed the most recent studies and divided the role of TRPA1 in toxicology into the following sections: neurotoxicity, cardiotoxicity, dermatotoxicity, and pulmonary toxicity. Acetaminophen, heavy metals, tear gases, various chemotherapeutic agents, acrolein, wood smoke particulate materials, particulate air pollution materials, diesel exhaust particles, cigarette smoke extracts, air born irritants, sulfur mustard, and plasticizers are selected compounds and materials with toxic effects that are, at least in part, mediated by TRPA1. Considering the high safety of TRPA1 antagonists and their efficacy to resolve selected toxic or adverse drug reactions, the future of these drugs looks promising.

ETAR and protein kinase A pathway mediate ET-1 sensitization of TRPA1 channel: A molecular mechanism of ET-1-induced mechanical hyperalgesia

Endothelin-1 (ET-1) is a potent endogenous vasoconstrictor that has been widely known as a pain mediator involved in various pain states. Evidence indicates that ET-1 sensitizes transient receptor potential cation channel, subfamily A, member 1 (TRPA1) in vivo. But the molecular mechanisms still remain unknown. We aim to explore whether ET-1 sensitizes TRPA1 in primary sensory neurons and the molecular mechanisms. Ca2+ imaging, immunostaining, electrophysiology, animal behavioral assay combined with pharmacological experiments were performed. ET-1 sensitized TRPA1-mediated Ca2+ responses in human embryonic kidney (HEK)293 cells as well as in cultured native mouse dorsal root ganglion (DRG) neurons. ET-1 also sensitized TRPA1 channel currents. ET-1 sensitized TRPA1 activated by endogenous agonist H2O2. ETA receptor (ETAR) colocalized with TRPA1 in DRG neurons. ET-1-induced TRPA1 sensitization in vivo was mediated via ETAR and protein kinase A (PKA) pathway in HEK293 cells and DRG neurons. Pharmacological blocking of ETAR, PKA, and TRPA1 significantly attenuated ET-1-induced mechanical hyperalgesia in mice. Our results suggest that TRPA1 acts as a molecular target for ET-1, and sensitization of TRPA1 through ETAR-PKA pathway contributes to ET-1-induced mechanical hyperalgesia. Pharmacological targeting of TRPA1 and ETAR-PKA pathway may provide effective strategies to alleviate pain conditions associated with ET-1.