Date of Award
Master of Science
Cisplatin is a widely used chemotherapeutic drug for the treatment of solid tumors. However, the drug accumulates in the cochlea, and damages inner ear structures, resulting in bilateral andpermanent hearing loss. Previous data from our laboratory indicate that activation of the transient receptor potential vanilloid 1 (TRPV1) receptor (by capsaicin) increases the NOX3 isoform of NADPH oxidase, leading to the generation of reactive oxygen species (ROS) in the cochlea, transient cochlear inflammation and transient hearing loss. We also demonstrated that the transient inflammation was produced by ROS-mediated activation of signal transducer and activator of transcription 1 (STAT1). Surprisingly, over time, this response desensitizes and capsaicin was subsequently able to protect against cisplatin ototoxicity. The goal of this study was to determine the mechanism of otoprotection against cisplatin ototoxicity following the administration of capsaicin. For this study we utilize both an immortalized organ of Corti outer hair cells and rat cochlea. Capsaicin (2.5 µM) increased both Ser727 p-STAT1 and Tyr705 p-STAT3 implicating its role in inflammation. Expression of cannabinoid receptors were observed in UB/OC-1 cells as well as rat outer hair cells (OHCs). However, inhibition of CB2 receptors (by AM630) reduced capsaicin-mediated Tyr705 p-STAT3, but had little effect on Ser727 STAT1. Capsaicin protected UB/OC-1 cells against cisplatin-induced apoptosis. This protection was reversed by CB2 antagonist but potentiated by TRPV1 inhibition. Significant cell death was observed following treatment of UB/OC-1 cells with AM630 alone, underscoring the importance of CB2 receptors in survival of these cells. CB2 agonist, JWH, significantly increased the protective signal, STAT3. Furthermore, capsaicin-mediated protection was reversed by the inhibition of STAT3, implicating STAT3 in otoprotection. In animal studies, oral administration of capsaicin (0.5% solution) induced transient inflammation but led to a long term recovery. Animals pre-treated with oral capsaicin were protected against cisplatin-induced hearing loss as compared to vehicle-treated animals, suggesting protection against hearing loss. Capsaicin increased the expression of both CB1 and CB2 receptors in the organ of Corti, which might confer the long term protective actions of this agent against hearing loss. In rats pretreated with AM630, the protective action of capsaicin was abolished. We conclude that otoprotection mediated by capsaicin is produced by activation of CB receptors in the cochlea which are coupled to both STAT1 and STAT3 activation. However, our data support the conclusion that activation of STAT3 confers the otoprotective action of capsaicin. In contrast, activation of STAT1 by capsaicin could contribute to the transient inflammatory response previously observed in vivo. The net protective action of capsaicin could result from an increase in the STAT3/STAT1 ratio of cells in the cochlea, which antagonizes the ability of cisplatin lower this ratio and promote cell death.
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