Protective roles of alpha-lipoic acid in rat model of mitochondrial DNA4834bp deletion in inner ear

The association between dietary total antioxidant capacity and hearing loss: results from the Tehran employees Cohort Study

BACKGROUND

Despite numerous studies that have explored the association between individual antioxidants or specific combinations and the risk of hearing loss, there is lack of information regarding the relationship between dietary total antioxidant capacity (dTAC) and hearing loss. The conflicting results on this association further highlight the need for more research in this area. This study aims to investigate the association between overall dietary antioxidant intake and the risk of hearing loss among Iranian adults.

METHODS

This cross-sectional study recruited 3443 adult participants aged between 19 and 67 years (with an average age of 41.4 years ± 8.8) who were employed at Tehran University of Medical Sciences in Iran. Participants underwent dietary assessment using a validated Food Frequency Questionnaire (FFQ). The hearing status of each participant was evaluated by a licensed audiologist in a soundproof room, using diagnostic audiometry that adhered to American National Standards specifications and followed standard audiometric clinical procedures. The dietary total antioxidant capacity (dTAC) was calculated using the Ferric Reducing-Antioxidant Power (FRAP) values.

RESULTS

43.6% of male participants had hearing loss, while 26.8% among female participants. After accounting for various confounding factors, no significant association was observed between higher levels of dTAC and reduced odds of hearing loss in the overall population. However, among men under the age of 40, higher levels of dTAC were associated with decreased odds of hearing loss, even after adjusting for several covariates (OR: 0.56, 95% CI: 0.31-1.02, Ptrend = 0.02). This relationship was not evident in men over 40 years of age or among women.

CONCLUSION

The study found an inverse relationship between higher antioxidant intake in the diet and lower odds of hearing loss, specifically among men who were 40 years old or younger. However, this relationship was not observed in the overall population or among women. Additional prospective studies are necessary to validate these results.

Mitochondria-Targeted Antioxidants for Treatment of Hearing Loss: A Systematic Review

Mitochondrial dysfunction is associated with the etiologies of sensorineural hearing loss, such as age-related hearing loss, noise- and ototoxic drug-induced hearing loss, as well as hearing loss due to mitochondrial gene mutation. Mitochondria are the main sources of reactive oxygen species (ROS) and ROS-induced oxidative stress is involved in cochlear damage. Moreover, the release of ROS causes further damage to mitochondrial components. Antioxidants are thought to counteract the deleterious effects of ROS and thus, may be effective for the treatment of oxidative stress-related diseases. The administration of mitochondria-targeted antioxidants is one of the drug delivery systems targeted to mitochondria. Mitochondria-targeted antioxidants are expected to help in the prevention and/or treatment of diseases associated with mitochondrial dysfunction. Of the various mitochondria-targeted antioxidants, the protective effects of MitoQ and SkQR1 against ototoxicity have been previously evaluated in animal models and/or mouse auditory cell lines. MitoQ protects against both gentamicin- and cisplatin-induced ototoxicity. SkQR1 also provides auditory protective effects against gentamicin-induced ototoxicity. On the other hand, decreasing effect of MitoQ on gentamicin-induced cell apoptosis in auditory cell lines has been controversial. No clinical studies have been reported for otoprotection using mitochondrial-targeted antioxidants. High-quality clinical trials are required to reveal the therapeutic effect of mitochondria-targeted antioxidants in terms of otoprotection in patients.

Protective Effects of Silymarin Against Age-Related Hearing Loss in an Aging Rat Model

Age-related hearing loss (ARHL) is one of the most common chronic degenerative disorders. Several studies have indicated that supplementation with some antioxidants can slow down the progression of ARHL. Despite several lines of evidence about the potent antioxidant and anti-aging effects of silymarin, its protective effect against ARHL has not evaluated yet. The aim of the current study was to investigate the effects of silymarin in prevention of ARHL in a d-Galactose-induced aging rat model for the first time. 45 male wistar rats aged 3-month old were divided into 5 groups: group 1, 2 and 3 received 500 mg/kg/day d-Gal plus 100, 200 and 300 mg/kg/day silymarin respectively for 8 weeks, placebo group received 500 mg/kg/day d-Gal plus propylene glycol as placebo, and control group received normal saline during this period of time. Auditory brainstem responses were measured at several frequencies (4, 6, 8, 12 and 16 kHz) before and after the intervention. Placebo group and group 3 showed significant ABR threshold increase across frequencies of 4, 6, 16 kHz compared with the other groups (P < 0.05). However, rats treated with silymarin 100 and 200 mg/kg/day plus d-Gal did not show any significant ABR threshold shifts. Similarly, ABR amplitude of P2 at 4, 8 kHz and P1, P4 at 4 kHz in the placebo group and group 3 were decreased significantly compared with other groups (P < 0.05). However, no significant differences are found in ABR absolute and inter-peak latencies between groups (P > 0.05). The findings indicates that silymarin with doses of 100 and 200 mg/kg/day has protective effect against ARHL and it can be supplemented into the diet of older people to slow down the progression of age-related hearing loss.

Protective Effect of Yang Mi Ryung® Extract on Noise-Induced Hearing Loss in Mice

Noise-induced hearing loss (NIHL) results from the damage of the delicate hair cells inside the ear after excessive stimulation of noise. Unlike certain lower animals such as amphibians, fishes, and birds, in humans, hair cells cannot be regenerated once they are killed or damaged; thus, there are no therapeutic options to cure NIHL. Therefore, it is more important to protect hair cells from the noise before the damage occurs. In this study, we report the protective effect of Yang Mi Ryung extract (YMRE) against NIHL; this novel therapeutic property of YMRE has not been reported previously. Our data demonstrates that the hearing ability damaged by noise is markedly restored in mice preadministrated with YMRE before noise exposure, to the level of normal control group. Our study also provides the molecular mechanism underlying the protective effect of YMRE against NIHL by showing that YMRE significantly blocks noise-induced apoptotic cell death and reduces reactive oxygen species (ROS) production in cochleae. Moreover, quantitative polymerase chain reaction (qPCR) analysis demonstrates that YMRE has anti-inflammatory properties, suppressing the mRNA levels of TNFα and IL-1β induced by noise exposure. In conclusion, YMRE could be a useful preventive intervention to prevent hearing impairment induced by the exposure to excessive noise.

Role of antioxidants in prevention of age-related hearing loss: a review of literature

Age-related hearing loss (ARHL), also known as presbycusis, is one of the most prevalent chronic degenerative conditions. It is characterized by a decline in auditory function. ARHL is caused by the interaction of multiple factors, including cochlear aging, environment, genetic predisposition, and health comorbidities. The primary pathology of ARHL includes the hair cells loss, stria vascularis atrophy, and loss of spiral ganglion neurons as well as the changes in central auditory pathways. The research to date suggests that oxidative stress and mitochondrial DNA deletion (mtDNA) play a major role in pathophysiology of ARHL. Therefore, similar to other otological conditions, several studies have also showed that antioxidants can slow ARHL, but some also indicate that antioxidant therapy is not a magic elixir that will prevent or treat hearing loss associated with aging completely, but why? All available clinical trials, including animal and human studies, in English language that examined the protective effects of antioxidants against ARHL were reviewed. Materials were obtained by searching ELSEVIER, PubMed, Scopus, Web of knowledge, Google Scholar databases, Clinical trials, and Cochrane database of systematic reviews. Although ARHL has been shown to be slowed by supplementation with antioxidants, particularly in laboratory animals, a few studies have investigated the effect of interventions against ARHL in humans. High-quality clinical trials are needed to investigate if ARHL can be delayed or prevented in humans. However, it seems that targeting several cell-death pathways is better than targeting the only oxidative stress pathway.

NADPH oxidase 3‑associated oxidative stress and caspase 3‑dependent apoptosis in the cochleae of D‑galactose‑induced aged rats

Oxidative damage to mitochondrial DNA (mtDNA) and cell apoptosis are heavily implicated in aging. Our previous study established a mimetic rat model of aging in the cochleae using D‑galactose (D‑gal), and revealed that chronic injection of D‑gal can increase oxidative stress and mtDNA common deletions (CD). The aim of the present study was to investigate the sources of reactive oxygen species and the occurrence of apoptosis in the cochleae of rats following 8 weeks of D‑gal exposure. The results of the present study indicated that an elevated accumulation of the mtDNA CD and mitochondrial ultrastructural damage occurred in the cochleae of rats injected with D‑gal for 8 weeks. In addition, the levels of 8‑hydroxy‑2‑deoxyguanosine, NADPH oxidase (NOX) 3, P22phox and cleaved caspase 3, and the number of terminal deoxynucleotidyl transferase‑mediated deoxyuridine triphosphate nick‑end‑labelling‑positive cells were increased in the cochleae of D‑gal‑treated rats, compared with the controls. These findings suggested that nitric oxide synthase NOX3‑associated oxidative stress may contribute to the accumulation of mtDNA mutations and activate a caspase 3‑dependent apoptotic signalling pathway in the cochleae during aging. The present study also provided novel insights into the development of age‑associated hearing loss, also termed presbycusis.

Early investigational drugs for hearing loss

INTRODUCTION

Sensorineural hearing loss (HL) is becoming a global phenomenon at an alarming rate. Nearly 600 million people have been estimated to have significant HL in at least one ear. There are several different causes of sensorineural HL included in this review of new investigational drugs for HL. They are noise-induced, drug-induced, sudden sensorineural HL, presbycusis and HL due to cytomegalovirus infections.

AREAS COVERED

This review presents trends in research for new investigational drugs encompassing a variety of causes of HL. The studies presented here are the latest developments either in the research laboratories or in preclinical, Phase 0, Phase I or Phase II clinical trials for drugs targeting HL.

EXPERT OPINION

While it is important that prophylactic measures are developed, it is extremely crucial that rescue strategies for unexpected or unavoidable cochlear insult be established. To achieve this goal for the development of drugs for HL, innovative strategies and extensive testing are required for progress from the bench to bedside. However, although a great deal of research needs to be done to achieve the ultimate goal of protecting the ear against acquired sensorineural HL, we are likely to see exciting breakthroughs in the near future.