Age-related hearing loss and its potential drug candidates: a systematic review

BACKGROUND

Age-related hearing loss (ARHL) is one of the main illnesses afflicting the aged population and has a significant negative impact on society, economy, and health. However, there is presently no appropriate therapeutic treatment of ARHL due to the absence of comprehensive trials.

OBJECTIVES

The goal of this review is to systematically evaluate and analyze recent statistics on the pathologic classifications, risk factors, treatment strategies, and drug candidates of ARHL, including that from traditional Chinese medicine (TCM), to provide potential new approaches for preventing and treating ARHL.

METHODS

Literature related to ARHL was conducted in databases such as PubMed, WOS, China National Knowledge Infrastructure (CNKI), and Wanfang from the establishment of the database to Jan, 2023. The pathology, causal factor, pathophysiological mechanism, treatment strategy, and the drug candidate of ARHL were extracted and pooled for synthesis.

RESULTS

Many hypotheses about the etiology of ARHL are based on genetic and environmental elements. Most of the current research on the pathology of ARHL focuses on oxidative damage, mitochondrial dysfunction, inflammation, cochlear blood flow, ion homeostasis, etc. In TCM, herbs belonging to the kidney, lung, and liver meridians exhibit good hearing protection. Seven herbs belonging to the kidney meridian, 9 belonging to the lung meridian, and 4 belonging to the liver meridian were ultimately retrieved in this review, such as Polygonum multiflorum Thunb., Panax ginseng C.A. Mey, and Pueraria lobata (Willd.) Ohwi. Their active compounds, 2,3,4',5-Tetrahydroxystilbene-2-O-D-glucoside, ginsenoside Rb1, and puerarin, may act as the molecular substance for their anti-ARHL efficacy, and show anti-oxidative, neuroprotective, anti-inflammatory, anti-apoptotic, or mitochondrial protective effects.

CONCLUSION

Anti-oxidants, modulators of mitochondrial function, anti-inflammation agents, vasodilators, K+ channel openers, Ca2+ channel blockers, JNK inhibitors, and nerve growth factors/neurotrophic factors all contribute to hearing protection, and herbs are an important source of potential anti-ARHL drugs.

Ameliorative effect of taxifolin on gentamicin-induced ototoxicity via down-regulation of apoptotic pathways in mouse cochlear UB/OC-2 cells

BACKGROUND

Taxifolin is a flavanonol with efficacious cytoprotective properties, such as anti-inflammatory, antioxidant, anticancer, hepatoprotective, and nephroprotective effects. However, the potential protective effects of taxifolin against gentamicin-induced ototoxicity have not been confirmed. In this study, the possible mechanisms underlying the effects of taxifolin on gentamicin-induced death of UB/OC-2 cochlear cells were investigated.

METHODS

Mouse cochlear UB/OC-2 cells with or without taxifolin pretreatment were exposed to gentamicin, and the effects on cytotoxicity, reactive oxygen species (ROS) production, mitochondrial permeability transition, and apoptotic marker expression were examined using biochemical techniques, flow cytometry, western blotting, and fluorescent staining.

RESULTS

Little or no apparent effect of taxifolin on cell viability was observed at concentrations less than 40 μM. Further investigations showed that gentamicin significantly inhibited cell viability in a concentration-dependent manner. Pretreatment with taxifolin attenuated gentamicin-induced lactate dehydrogenase release, as well as cellular cytotoxicity. In addition, taxifolin significantly prevented gentamicin-induced cell damage by decreasing ROS production, stabilizing mitochondrial membrane potential, and downregulating the mitochondrial pathway of apoptosis.

CONCLUSION

In summary, pretreatment with taxifolin is effective for mitigating gentamicin-induced apoptotic cell death mediated by the mitochondrial pathway. Our data suggest that taxifolin provides a new approach to combat gentamicin-induced ototoxicity.

Efficiency of resveratrol in the prevention and treatment of age-related hearing loss

Age-related hearing loss (ARHL) is a major public health concern, which is characterized by gradual, progressive sensorineural hearing loss and deterioration of sound localization, with no effective treatment available to date. The aim of the present study was to evaluate the efficacy of resveratrol to prevent and treat ARHL. For this purpose, 32 male C57BL/6 mice were assigned to four groups: Early treatment, late treatment, control and sham control. The experiment lasted for 15 months. Treatment was started at three months of age in the early treatment group and at sixth months in the late treatment group. The auditory brainstem response test was performed once every three months. At the end of the study period, inducible nitric oxide synthase (iNOS), cyclooxygenase (COX)-2, NF-κB, Bcl-2, Bcl-xL, Bax, Bcl-2 homologous antagonist/killer (Bak), caspase-3 and caspase-9 levels in the cochlear tissues of the animals were analyzed by reverse transcription-quantitative PCR. Hearing thresholds of the mice in the early treatment group were better than those in the other groups (P<0.001) at the end of the study. However, hearing levels in the late treatment group were not significantly different from those in the control groups (P>0.05), although mean thresholds were lower. The threshold shift in the early treatment group was significantly lower at all frequencies when compared with those in the control groups (P<0.001). The mRNA expression levels of pro-apoptotic genes Bax and Bak were lower (P<0.05), anti-apoptotic genes Bcl-2 and Bcl-xL were higher (P<0.05), NF-κB, COX-2 and iNOS as genes that have a role in inflammation and caspase-3 and caspase-9 as genes with a vital role in apoptosis were lower (P<0.05) in the early treatment group when compared with the late treatment and control groups. These results suggested that resveratrol is effective in the prevention of ARHL, particularly when started prior to the beginning of hearing loss.

Resveratrol noncompetitively inhibits glycine receptor-mediated currents in neurons of rat central auditory neurons

Resveratrol, a naturally occurring stilbene found in red wine, is known to modulate the activity of several types of ion channels and membrane receptors, including Ca²⁺, K⁺, and Na⁺ ion channels. However, little is known about the effects of resveratrol on some important receptors, such as glycine receptors and GABA_A receptors, in the central nervous system (CNS). In the present study, the effects of resveratrol on glycine receptor or GABA_A receptor-mediated currents in cultured rat inferior colliculus (IC) and auditory cortex (AC) neurons were studied using whole-cell voltage-clamp recordings. Resveratrol itself did not evoke any currents in IC neurons but it reversibly decreased the amplitude of glycine-induced current (I_Gly) in a concentration-dependent manner. Resveratrol did not change the reversal potential of I_Gly but it shifted the concentration-response relationship to the right without changing the Hill coefficient and with decreasing the maximum response of I_Gly. Interestingly, resveratrol inhibited the amplitude of I_Gly but not that of GABA-induced current (I_GABA) in AC neurons. More importantly, resveratrol inhibited GlyR-mediated but not GABA_AR-mediated inhibitory postsynaptic currents in IC neurons using brain slice recordings. Together, these results demonstrate that resveratrol noncompetitively inhibits I_Gly in auditory neurons by decreasing the affinity of glycine to its receptor. These findings suggest that the native glycine receptors but not GABA_A receptors in central neurons are targets of resveratrol during clinical administrations.

OVO homologue-like 1 promotes osteoblast differentiation through BMP2 expression

OVO homologue-like 1 (OVOL1) encodes a C2H2 zinc finger protein and is an evolutionarily conserved gene in mammals. The OVOL1 expression is required for development. However, the function of OVOL1 in bone metabolism remains unreported. Here, we show for the first time the role of OVOL1 in osteoblast differentiation. To determine the role of OVOL1 in osteogenic differentiation, we analyzed OVOL1 expression in the preosteoblastic cell line. OVOL1 messenger RNA expression was induced during osteoblast differentiation. In addition, OVOL1 overexpression enhanced the expression of osteogenic genes including bone morphogenetic protein 2 (BMP2), the inhibitor of DNA binding 1 (Id1), distal-less homeobox 5 (Dlx5), runt-related transcription factor 2 (Runx2), osteocalcin (OC), and alkaline phosphatase (ALP). Moreover, mineralization of the extracellular matrix was increased by OVOL1 overexpression in MC3T3-E1 cells. Furthermore, knockdown of the OVOL1 experiment demonstrated that OVOL1 is required for osteoblast differentiation. Collectively, these results suggest that OVOL1 function as an important regulator of osteoblast differentiation by inducing BMP2 expression in MC3T3-E1 cells.

Innovative pharmaceutical approaches for the management of inner ear disorders

The sense of hearing is essential for permitting human beings to interact with the environment, and its dysfunctions can strongly impact on the quality of life. In this context, the cochlea plays a fundamental role in the transformation of the airborne sound waves into electrical signals, which can be processed by the brain. However, several diseases and external stimuli (e.g., noise, drugs) can damage the sensorineural structures of cochlea, inducing progressive hearing dysfunctions until deafness. In clinical practice, the current pharmacological approaches to treat cochlear diseases are based on the almost exclusive use of systemic steroids. In the last decades, the efficacy of novel therapeutic molecules has been proven, taking advantage from a better comprehension of the pathological mechanisms underlying many cochlear diseases. In addition, the feasibility of intratympanic administration of drugs also permitted to overcome the pharmacokinetic limitations of the systemic drug administration, opening new frontiers in drug delivery to cochlea. Several innovative drug delivery systems, such as in situ gelling systems or nanocarriers, were designed, and their efficacy has been proven in vitro and in vivo in cochlear models. The current review aims to describe the art of state in the cochlear drug delivery, highlighting lights and shadows and discussing the most critical aspects still pending in the field.

The effect of resveratrol on the histologic characteristics of the cochlea in diabetic rats

OBJECTIVES/HYPOTHESIS

The aim of this study was to investigate changes in the cochlea and the potential dose-dependent effects of resveratrol (RSV) against diabetes mellitus (DM) ototoxicity.

STUDY DESIGN

Animal model.

METHODS

Twenty-four male Wistar albino rats were divided into four groups. Baseline distortion product otoacoustic emission (DPOAE) measurements were evaluated. Group I was the control group, group II was made diabetic with single-dose streptozotocin, and groups III and IV were rendered diabetic as group II and administered 10 and 20 mg RSV, respectively, intraperitoneally for 4 weeks. All animals were sacrificed after repeated DPOAE measurement. Apoptosis was investigated using caspase-3, Bax (Bcl-associated X protein), and TUNEL (terminal deoxynucleotidyl transferase dUTP nick end labeling) staining.

RESULTS

The DPOAE values in the diabetic group were found to be significantly lower compared with the other groups at 5,714 Hz and 8,000 Hz (P < .05). No significant difference in otoacoustic emission was detected in the comparison of the RSV doses (P > .05). The histopathologic investigation using caspase-3, Bax, and TUNEL staining showed that the mean rank of the diabetic group was significantly higher compared with the RSV10, RSV20, and control groups (DM > RSV10 > RSV20 > control) (P < .05).

CONCLUSIONS

These results imply that RSV administration offered statistically significant protection for the cochleas of rats against diabetes. This protective effect improved histologically with higher doses.

LEVEL OF EVIDENCE

NA Laryngoscope, 129:E1-E6, 2019.

The expression of oxidative stress response genes is modulated by a combination of resveratrol and N-acetylcysteine to ameliorate ototoxicity in the rat cochlea

Aminoglycoside antibiotics are used widely in medicine despite their ototoxic side-effects. Oxidative stress and inflammation are key mechanisms determining the extent and severity of the damage. Here we evaluate the protective effect of a treatment with resveratrol plus N-acetylcysteine on the ototoxic actions of kanamycin and furosemide in the rat. Resveratrol (10 mg/kg) and N-acetylcysteine (400 mg/kg) were administered together to Wistar rats on 5 consecutive days. The second day, a concentrated solution of kanamycin and furosemide was placed on the round window to induce ototoxicity. Hearing was assessed by recording auditory brainstem responses before and 5, 16 and 23 days after the beginning of the treatment. Cochlear samples were taken at day 5 (end of the treatment) and at day 23, and targeted PCR arrays or RT-qPCR were performed to analyze oxidative balance and inflammation related genes, respectively. In addition, the cytoarchitecture and the presence of apoptosis, oxidative stress and inflammation markers were evaluated in cochlear sections. Results indicate that administration of resveratrol plus N-acetylcysteine reduced the threshold shifts induced by ototoxic drugs at high frequencies (≈10 dB), although this protective effect fades after the cessation of the treatment. Gene expression analysis showed that the treatment modulated the expression of genes involved in the cellular oxidative (Gpx1, Sod1, Ccs and Noxa1) and inflammatory (Il1b, Il4, Mpo and Ncf) responses to injury. Thus, co-administration of resveratrol and NAC, routinely used individually in patients, could reduce the ototoxic secondary effects of aminoglycosides.

An update on drug design strategies to prevent acquired sensorineural hearing loss

INTRODUCTION

Acute sensorineural hearing loss is a dramatic event for the patient. Different pathologies might result in acute sensorineural hearing loss, such as sudden hearing loss, exposure to medications/drugs or loud sound. Current therapeutic approaches include steroids and hyperbaric oxygen in addition to other methods. Research activities of the past have shed light on the molecular mechanisms involved in damage to hair cells, the synapses at the hair cell spiral ganglion junction and the stria vascularis. Molecular events and signaling pathways which underlie damage to these structures have been discovered. Areas covered: This paper summarizes current research efforts involved in investigating the molecular mechanisms involved in acute sensorineural hearing loss. Expert opinion: While progress has been made in unraveling basic mechanisms involved in acute sensorineural hearing loss, it is difficult to translate basic concepts to the clinic. There are often conflicting data in animal and human studies on the effect of a given intervention. There is also a lack of high quality clinical trials (double blind, placebo controlled and high powered). However, this author is confident that research efforts will pay out and that some of these efforts will translate into new therapeutic options for patients with acute hearing loss.

An Antioxidant Screen Identifies Candidates for Protection of Cochlear Hair Cells from Gentamicin Toxicity

Reactive oxygen species are important elements in ototoxic damage to hair cells (HCs), appearing early in the damage process. Higher levels of natural antioxidants are positively correlated with resistance to ototoxins and many studies have shown that exogenous antioxidants can protect HCs from damage. While a very wide variety of antioxidants with different characteristics and intracellular targets exist, most ototoxicity studies have focused upon one or a few well-characterized compounds. Relatively little research has attempted to determine the comparative efficacy of large variety of different antioxidants. This has been in part due to the lack of translation between cell culture and in vivo measures of efficacy. To circumvent this limitation, we used an in vitro assay based on micro-explants from the basal and middle turns of the neonatal mouse organ of Corti to screen a commercial redox library of diverse antioxidant compounds for their ability to protect mammalian HCs from a high dose of the ototoxic antibiotic gentamicin. The library included several antioxidants that have previously been studied as potential treatments for HC damage, as well as many antioxidants that have never been applied to ototoxicity. The micro-explants were treated with 200 μM gentamicin alone, gentamicin plus one of three dosages of a redox compound, the highest dosage of compound alone, or were untreated. HC counts were determined before the gentamicin insult and at 1, 2, and 3 days afterward to evaluate the HC survival. From a total of 81 antioxidant compounds, 13 exhibited significant protection of HCs. These included members of a variety of antioxidant classes with several novel antioxidants, not previously tested on HCs, appearing to alleviate the damaging gentamicin effect. Some compounds previously shown to be protective of HCs were correspondingly protective in this in vitro screen, while others were not. Finally, one of the three pro-oxidant compounds included in the library as well as six antioxidants exhibited evidence of toxicity in the absence of gentamicin. The results demonstrate the wide variability in the ability of antioxidants to protect HCs from high-dose gentamicin damage, and identify promising candidate leads for further study as potential drug targets.

HIGHLIGHTS

• A medium-throughput assay based on micro-explants of the organ of Corti was developed to screen mammalian cochlear hair cells for protection from damage by ototoxins.• Eighty one antioxidants and 3 pro-oxidants were evaluated for hair cell protection from high-dose gentamicin.• Thirteen antioxidants were significantly protective, while 6 proved to be damaging.• The use of a common assay permitted an evaluation of the relative capacity of different antioxidants for the protection of hair cells.

Paraoxonase Activity and Oxidative Status in Patients with Tinnitus

BACKGROUND AND OBJECTIVES

The aim of this study was to investigate serum paraoxanase-1 (PON) activity, total oxidant status (TOS), total antioxidant status (TAS), and the oxidative stress index (OSI) in tinnitus; and to compare the results with data from healthy subjects.

SUBJECTS AND METHODS

A total of 114 subjects-54 patients with tinnitus and 60 healthy controls were enrolled in this study. Serum PON activity, TOS, TAS, and OSI levels were measured.

RESULTS

In the tinnitus group, TAS, and PON were significantly lower than in the control group (p<0.001). However, the TOS, and OSI levels were significantly higher in the tinnitus group than in the control group (p<0.001).

CONCLUSIONS

According to the data obtained from the present study, patients with tinnitus were exposed to potent oxidative stress. Oxidative stress may be the key contributing factor to the pathogenesis of tinnitus.

Protective effects of resveratrol on cisplatin-dependent inner-ear damage in rats

Cisplatin is a common chemotherapeutic agent used in many solid and hematologic malignancies. The main unwanted effect of cisplatin is ototoxicity, for which no standard treatment has been reported. The present study examined the protective efficacy of resveratrol on cisplatin-dependent ototoxicity through an experimental model. Fifteen rats were randomized into three groups. Group 1 (control group) (n = 5) received intraperitoneal (i.p.) 15 mg/kg cisplatin; group 2 (resveratrol group) (n = 5) received i.p. 100 mg/kg resveratrol, followed by i.p. 15 mg/kg cisplatin; group 3 (n = 5) served as a vehicle group and received i.p. 1 ml dimethyl sulfoxide. All rats underwent the auditory brainstem response (ABR) test before and 72 h after the treatment. Pretreatment ABR values of the groups were not significantly different. The pretreatment hearing threshold values of the groups were 30 ± 6.60 and 28.5 ± 5.29 dB in groups 1 and 2, respectively (p > 0.05). The post-ABR-I and post-ABR-IV values were, respectively, 1.41 ± 0.18 and 5.83 ± 0.16 ms in the control subjects and 1.19 ± 0.22 and 4.58 ± 0.27 ms in the study group. The ABR-I and ABR-IV durations in rats treated with resveratrol were significantly shorter (p < 0.01). A comparison of threshold values shows that the resveratrol-treated rats had significantly lower values than the control rats. After cisplatin injection, ABR I-IV intervals were compared among the groups. The ABR I-IV interval duration was 4.42 ± 0.16 ms in the control group, while the resveratrol-treated rats showed a significantly shorter ABR I-IV interval duration of 3.49 ± 0.27 ms (p < 0.001). Resveratrol attenuated cisplatin-dependent inner-ear damage, as shown by the ABR-I, ABR-IV, ABR I-IV interval, and hearing threshold values. Our results suggest that this natural antioxidant may be effectively used in reducing the unwanted effects of cisplatin on the ear physiology of patients, particularly those undergoing chemotherapy.

Protective role of resveratrol against cisplatin induced ototoxicity in guinea pigs

OBJECTIVE

The aim of this study was to evaluate the effectiveness of systemic administration of resveratrol against cisplatin-induced ototoxicity in guinea pigs.

MATERIALS AND METHODS

Healthy guinea pigs (n=24) were randomly divided into four groups. Group 1 (n=6) received resveratrol+cisplatin, group 2 (n=6) received 4% ethanol+cisplatin, group 3 (n=6) received cisplatin, and group 4 (n=6) received saline. Cisplatin was administered at a dose of 10mg/kg/day on days 14 and 15 of the study. Resveratrol (10mg/kg/day), 4% ethanol, and saline were administered throughout the study. Baseline auditory brainstem responses (ABR) (4 kHz, 8 kHz, and click stimulus) were determined for all groups. ABR was repeated 72 h after the last dose of cisplatin in order to record the threshold shifts. The ABR threshold shifts for the click stimulus, 4-kHz- and 8-kHz-frequency stimuli were compared after drug administration. After follow-up ABRs the animals sacrificed under deep sedation and their cochleae were removed. Left cochleae were immediately harvested for measurement of level of reactive oxygen species (ROS). Right cochleae were prepared for histological changes which were observed by scanning electron microscopy (SEM).

RESULTS

For the all stimulus, there was a significant threshold difference among the groups (p<0.01). Group 3 had a significantly higher threshold shift at all stimuli when compared with groups 1 and 4. There was no significant threshold shifts in all stimuli between groups 2 and 3. The resveratrol-treated group 1 showed preservation of threshold in ABR (p ≤ 0.05). SEM showed that inner and outer hair cells were preserved in the group 1. Level of reactive oxygen species (ROS) were significantly higher in groups 2 and 3 compared with groups 1 and 4 (p ≤ 0.05).

CONCLUSION

These results indicated that systemic administration of resveratrol afforded statistically significant protection to the cochlea of guinea pigs from cisplatin toxicity. Experimental dose of resveratrol injections may have a protective effect against cisplatin ototoxicity in guinea pigs.

Mechanisms of aminoglycoside ototoxicity and targets of hair cell protection

Aminoglycosides are commonly prescribed antibiotics with deleterious side effects to the inner ear. Due to their popular application as a result of their potent antimicrobial activities, many efforts have been undertaken to prevent aminoglycoside ototoxicity. Over the years, understanding of the antimicrobial as well as ototoxic mechanisms of aminoglycosides has increased. These mechanisms are reviewed in regard to established and potential future targets of hair cell protection.