Melatonin blunts the mitochondrial/NLRP3 connection and protects against radiation‐induced oral mucositis

Abstract

Mucositis is a common and distressing side effect of chemotherapy or radiotherapy that has potentially severe consequences, and no treatment is available. The purpose of this study was to analyze the molecular pathways involved in the development of oral mucositis and to evaluate whether melatonin can prevent this pathology. The tongue of male Wistar rats was subjected to irradiation (X-ray YXLON Y.Tu 320-D03 irradiator; the animals received a dose of 7.5 Gy/day for 5 days). Rats were treated with 45 mg/day melatonin or vehicle for 21 days postirradiation, either by local application into their mouths (melatonin gel) or by subcutaneous injection. A connection between reactive oxygen species, generating mitochondria and the NLRP3 (NLR-related protein 3 nucleotide-binding domain leucine-rich repeat containing receptor-related protein 3) inflammasome, has been reported in mucositis. Here, we show that mitochondrial oxidative stress, bioenergetic impairment and subsequent NLRP3 inflammasome activation are involved in the development of oral mucositis after irradiation and that melatonin synthesized in the rat tongue is depleted after irradiation. The application of melatonin gel restores physiological melatonin levels in the tongue and prevents mucosal disruption and ulcer formation. Melatonin gel protects the mitochondria from radiation damage and blunts the NF-κB/NLRP3 inflammasome signaling activation in the tongue. Our results suggest new molecular pathways involved in radiotherapy-induced mucositis that are inhibited by topical melatonin application, suggesting a potential preventive therapy for mucositis in patients with cancer.

Melatonin-Induced Oncostasis, Mechanisms and Clinical Relevance

“…In addition, melatonin increases the efficacy of anticancer drugs and has multiple protective effects against drug toxicity [167,168]. The data support the clinical use of melatonin in the co-treatment of cancer.…”

Oral Mucositis: Melatonin Gel an Effective New Treatment

“…We recently demonstrated that melatonin is synthesized in oral mucosa, where it is involved in autocrine/paracrine signaling by binding to MT1, MT2, and RORγ receptors, thus suggesting that it plays a role in normal oral mucosal physiology [11]. We found that aralkylamine N-acetyltransferase (AANAT) and acetylserotonin O-methyltransferase (ASMT), the two main enzymes in melatonin synthesis, are expressed in oral mucosa and also in salivary glands.…”

“…Its important anti-inflammatory effects include expression inhibition of iNOS/i-mtNOS, COX-2 and pro-inflammatory cytokines such as IL-1β or TNF-α. Many of these properties are attributed to the inhibition of NF-κB-dependent innate immune pathway activation [69][70], and we recently showed that melatonin blunts NLRP3 inflammasome activation under different experimental conditions [11,[71][72].…”

“…NLRP3 inflammasome assembly leads to caspase-1 activation and subsequent cleavage of pro-cytokines such as pro-IL-1β, pro-IL-18 and pro-IL-33 into their mature form [11][12] resulting in ulceration (ulceration phase). Therefore, the NFκB pathway, mitochondrial dysfunction and subsequent NLRP3 inflammasome activation are the three main players involved in the development of oral mucositis, which amplify the whole inflammatory process via positive feedback loops, thus prolonging tissue injury and ending in the healing phase.…”

“…This is followed by the signaling and amplification phase, during which, once activated by chemotherapy and ROS, NF-κB promotes the expression of multiple pro-inflammatory molecules, including inducible nitric oxide synthase (iNOS), cyclooxygenase-2 (COX-2), TNFα, and pro-IL-1β, and ensures feedback amplification of the NF-κB-dependent signaling pathway [10]. Furthermore, mitochondrial ROS production promotes inflammation by activating a multi-protein cytoplasmic complex, such as the NLRP3 (NACHT, LRR and PYD domains-containing protein 3) inflammasome.NLRP3 inflammasome assembly leads to caspase-1 activation and subsequent cleavage of pro-cytokines such as pro-IL-1β, pro-IL-18 and pro-IL-33 into their mature form [11][12] resulting in ulceration (ulceration phase). Therefore, the NFκB pathway, mitochondrial dysfunction and subsequent NLRP3 inflammasome activation are the three main players involved in the development of oral mucositis, which amplify the whole inflammatory process via positive feedback loops, thus prolonging tissue injury and ending in the healing phase.…”