Protection from Radiation-Induced Pulmonary Fibrosis by Peripheral Targeting of Cannabinoid Receptor-1

Abstract

Radiation-induced pulmonary fibrosis (RIF) is a severe complication of thoracic radiotherapy that limits its dose, intensity, and duration. The contribution of the endocannabinoid signaling system in pulmonary fibrogenesis is not known. Using a wellestablished mouse model of RIF, we assessed the involvement of cannabinoid receptor-1 (CB 1 ) in the onset and progression of pulmonary fibrosis. Female C57BL/6 mice and CB 1 knockout mice generated on C57BL/6 background received 20 Gy (2 Gy/min) single-dose thoracic irradiation that resulted in pulmonary fibrosis and animal death within 15 to 18 weeks. Some C57BL/6 animals received the CB 1 peripherally restricted antagonist AM6545 at 1 mg/kg intraperitoneally three times per week. Animal survival and parameters of pulmonary inflammation and fibrosis were evaluated. Thoracic irradiation (20 Gy) was associated with marked pulmonary inflammation and fibrosis in mice and high mortality within 15 to 18 weeks after exposure. Genetic deletion or pharmacological inhibition of CB 1 receptors with a peripheral CB 1 antagonist AM6545 markedly attenuated or delayed the lung inflammation and fibrosis and increased animal survival. Our results show that CB 1 signaling plays a key pathological role in the development of radiation-induced pulmonary inflammation and fibrosis, and peripherally restricted CB 1 antagonists may represent a novel therapeutic approach against this devastating complication of radiotherapy/irradiation.Keywords: radiation-induced pulmonary fibrosis; cannabinoid receptor 1; thoracic irradiation; peripherally restricted CB 1 antagonist; AM6545 Clinical RelevanceWe report for the first time the involvement of cannabinoid receptor 1 (CB 1 )-mediated signaling in the onset and progression of radiation-induced pulmonary fibrosis (RIF). We were able to delay the onset of RIF by genetic targeting of CB 1 receptors as well as by its pharmacological inhibition. Thus, pharmacological targeting of CB 1 receptors with peripherally restricted CB 1 antagonists void of central nervous system complications may represent a novel strategy to prevent the development of RIF.

Cannabinoid CB1 receptor overactivity contributes to the pathogenesis of idiopathic pulmonary fibrosis

“…Pharmacological inhibition or genetic deletion of Cnr1 was reported to mitigate PF and improve survival in mice following thoracic irradiation (26), but a similar role of CB 1 R in IPF and BL-PF has not yet been investigated. On the other hand, the involvement of iNOS in human IPF and in BL-PF was suggested in several prior studies (11,15).…”

“…In contrast, rimonabant at concentrations of up to 10 μM did not inhibit iNOS activity in the same in vitro assay (Supplemental Figure 4B). The peripherally restricted CB 1 R antagonist AM6545 was pharmacologically characterized earlier (32), and its antifibrotic efficacy was recently shown on prevention of radiation-induced PF (26). We next compared the antifibrotic efficacy of MRI-1867, rimonabant, and AM6546 by chronic administration of a 10 mg/kg daily oral dose for rimonabant and MRI-1867 or intraperitoneal dose for AM6545 to mice with bleomycin-induced fibrosis.…”

“…Endocannabinoids are bioactive lipids that act on cannabinoid receptors, CB 1 R and CB 2 R, that also recognize and mediate the effects of marijuana (16). Endocannabinoids acting via CB 1 R promote fibrosis progression in multiple organs, including liver (17)(18)(19)(20)(21), kidney (22,23), heart (24), and skin (25), and CB 1 R has been linked to radiation-induced PF in mice (26). In addition to promoting fibrosis, activation of CB 1 R is proinflammatory in chronic inflammatory diseases (27,28), whereas the brain-penetrant CB 1 R antagonist/inverse agonist rimonabant mitigates liver fibrosis in animal models (17).…”

A Curious Case of Inhalation Fever Caused by Synthetic Cannabinoid

“…Both synthetic cannabinoids and delta-9 tetrahydrocannabinol act on the endocannabinoid receptors, the cannabinoid (CB) receptor-1 that is found in the central nervous system and CB-2 receptors in the immune system [ 13 , 14 ]. A study by Bronova et al showed CB-1 receptor-mediated signalling of pulmonary fibrogenesis in radiation-exposed mice [ 16 ]. Transient receptor potential vanilloid 4 (TRPV-4) that can be activated by endogenous N-acylamine cannabinoids and fatty acid-derived products related to anandamide during lung injury (chlorine gas or intratracheal hydrochloric acid).…”

Endocannabinoid System in the Airways

“…Endocannabinoids are shown to promote the progression of fibrosis in liver [122][123][124], kidney [125,126], heart [127], and skin [128]. Studies in mice suggest that CB 1 receptors are associated with radiation-induced pulmonary fibrosis [129]. Cinar et al performed a detailed study in order to identify the contribution of CB 1 receptors in lung fibrosis, and have shown that both the genetic deletion of CB 1 receptors or their pharmacological inhibition with a CB 1 antagonist can attenuate lung inflammation and fibrosis, and hence increase animal survival in a mouse model of radiation-induced pulmonary fibrosis [130].…”