Sirtuin Upregulation in the Cochlea During Honokiol-Mediated Protection Against Cisplatin Ototoxicity

Background/Objectives: Cisplatin-induced hearing loss (CIHL) is a major global health sequela of chemotherapy, affecting over half a million cancer patients annually. We previously reported that honokiol, a multifunctional molecule compatible with cisplatin, protects against CIHL and upregulates sirtuin 3. The sirtuin family is composed of seven highly conserved yet differentially expressed members that are critical enzymes for oxidative stress regulation in different cellular compartments. Therefore, this study aims to further verify the expression and upregulation of the entire sirtuin family during honokiol-mediated hearing protection against CIHL in a tumor-bearing mouse model. Methods: Adult female mice carrying the mouse mammary tumor virus polyomavirus middle T agent oncogene were used. These mice developed palpable tumors spontaneously at the age of 70.8 ± 10.3 days. CIHL was induced using a three-cycle (14 days/cycle) low-dose (4 mg/kg/day for 4 days per cycle, intraperitoneal injection) cisplatin treatment mimicking chemotherapy regimens. Honokiol (5–20 mg/kg) was administered one hour before cisplatin. Cochlear function was evaluated by auditory brainstem response threshold shifts and synaptic changes. Immunostaining and RT-qPCR were performed to verify the upregulation of sirtuin family members (1, 2, 3, 5, and 7) during these processes. Results: CIHL was observed in the high-frequency range (24–32 kHz), which was mitigated by honokiol treatment. The optimal dose of honokiol for oto-protection and animal survival was 10 mg/kg. Fluorescence signals of sirtuins 1, 2, 3, 5, and 7 increased in cochleae from honokiol-treated mice. RT-qPCR confirmed a significant upregulation of sirtuin 3 mRNA with honokiol treatment. Conclusions: Sirtuin family upregulation was involved in honokiol-mediated hearing protection against CIHL. The dose–response curve of honokiol lays the foundation for potential translational applications for human studies.