Metformin protects against gentamicin-induced hair cell death in vitro but not ototoxicity in vivo

Metformin Protects Against Noise-Induced Hearing Loss in Male Mice

HYPOTHESIS

Metformin treatment will protect mice from noise-induced hearing loss (NIHL).

BACKGROUND

We recently identified metformin as the top-ranking, Food and Drug Administration-approved drug to counter inner ear molecular changes induced by permanent threshold shift-inducing noise. This study is designed to functionally test metformin as a potential otoprotective drug against NIHL.

METHODS

Male and female B6CBAF1/J mice were obtained at 7 to 8 weeks of age. A cohort of the females underwent ovariectomy to simulate menopause and eliminate the effect of ovarian-derived estrogens. At 10 weeks of age, mice underwent a permanent threshold shift-inducing noise exposure (102.5 or 105 dB SPL, 8-16 kHz, 2 h). Auditory brainstem response (ABR) thresholds were obtained at baseline, 24 h after noise exposure, and 1 week after noise exposure. Mice were administered metformin (200 mg/kg/d) or a saline control in their drinking water after the baseline ABR and for the remainder of the study. After the 1-week ABR, mice were euthanized and cochlear tissue was analyzed.

RESULTS

Metformin treatment reduced the 1-week ABR threshold shift at 16 kHz ( p < 0.01; d = 1.20) and 24 kHz ( p < 0.01; d = 1.15) as well as outer hair cell loss in the 32-45.5 kHz range ( p < 0.0001; d = 2.37) in male mice. In contrast, metformin treatment did not prevent hearing loss or outer hair cell loss in the intact or ovariectomized female mice.

CONCLUSIONS

Metformin exhibits sex-dependent efficacy as a therapeutic for NIHL. These data compel continued investigation into metformin's protective effects and demonstrate the importance of evaluating the therapeutic efficacy of drugs in subjects of both sexes.

Metformin beyond an anti-diabetic agent: A comprehensive and mechanistic review on its effects against natural and chemical toxins

In addition to the anti-diabetic effect of metformin, a growing number of studies have shown that metformin has some exciting properties, such as anti-oxidative capabilities, anticancer, genomic stability, anti-inflammation, and anti-fibrosis, which have potent, that can treat other disorders other than diabetes mellitus. We aimed to describe and review the protective and antidotal efficacy of metformin against biologicals, chemicals, natural, medications, pesticides, and radiation-induced toxicities. A comprehensive search has been performed from Scopus, Web of Science, PubMed, and Google Scholar databases from inception to March 8, 2023. All in vitro, in vivo, and clinical studies were considered. Many studies suggest that metformin affects diseases other than diabetes. It is a radioprotective and chemoprotective drug that also affects viral and bacterial diseases. It can be used against inflammation-related and apoptosis-related abnormalities and against toxins to lower their effects. Besides lowering blood sugar, metformin can attenuate the effects of toxins on body weight, inflammation, apoptosis, necrosis, caspase-3 activation, cell viability and survival rate, reactive oxygen species (ROS), NF-κB, TNF-α, many interleukins, lipid profile, and many enzymes activity such as catalase and superoxide dismutase. It also can reduce the histopathological damages induced by many toxins on the kidneys, liver, and colon. However, clinical trials and human studies are needed before using metformin as a therapeutic agent against other diseases.

Metformin Reduces the Risk of Hearing Loss: A Retrospective Cohort Study

OBJECTIVE

To compare the risk of hearing loss with regard to metformin exposure.

STUDY DESIGN

Retrospective cohort.

SETTING

Taiwan's National Health Insurance database.

METHODS

We enrolled 292,071 ever users and 18,200 never users of metformin with new-onset diabetes mellitus from 1999 to 2005 and followed them for hearing loss from January 1, 2006, to December 31, 2011. Hazard ratios (HRs) weighted by propensity score were estimated.

RESULTS

Hearing loss was newly diagnosed in 10,085 ever users and 1072 never users. Their respective incidence rates (per 100,000 person-years) were 738.09 and 1366.83. The HR comparing ever-to-never users was 0.534 (95% confidence interval [CI]: 0.501-0.569]. The HR (95% CI) for the first (<27.07 months), second (27.07-59.13 months), and third (>59.13 months) tertiles of cumulative duration of metformin therapy were 0.912 (0.852-0.975), 0.544 (0.508-0.582), and 0.275 (0.255-0.295), respectively; and were 0.900 (0.841-0.962), 0.531 (0.496-0.569), and 0.293 (0.273-0.315), respectively, for the first (<796.70 g), second (796.70-2020.15 g), and third (>2020.15 g) tertiles of cumulative dose. The magnitude of risk reduction became more remarkable in corresponding to the increasing tertiles of the defined daily dose prescribed. Subtype analyses suggested that the risk reduction was more significant for sensorineural than conductive hearing loss. Findings derived from a propensity score-matched cohort did not substantially change the conclusions, and the risk reduction for mixed hearing loss was not statistically significant in the matched cohort as significantly observed in the unmatched cohort.

CONCLUSION

The risk of hearing loss is reduced in a dose-response pattern in patients who use metformin.

Induced Pluripotent Stem Cells, a Stepping Stone to In Vitro Human Models of Hearing Loss

Hearing loss is the most prevalent sensorineural impairment in humans. Yet despite very active research, no effective therapy other than the cochlear implant has reached the clinic. Main reasons for this failure are the multifactorial nature of the disorder, its heterogeneity, and a late onset that hinders the identification of etiological factors. Another problem is the lack of human samples such that practically all the work has been conducted on animals. Although highly valuable data have been obtained from such models, there is the risk that inter-species differences exist that may compromise the relevance of the gathered data. Human-based models are therefore direly needed. The irruption of human induced pluripotent stem cell technologies in the field of hearing research offers the possibility to generate an array of otic cell models of human origin; these may enable the identification of guiding signalling cues during inner ear development and of the mechanisms that lead from genetic alterations to pathology. These models will also be extremely valuable when conducting ototoxicity analyses and when exploring new avenues towards regeneration in the inner ear. This review summarises some of the work that has already been conducted with these cells and contemplates future possibilities.

Biguanide Pharmaceutical Formulations and the Applications of Bile Acid-Based Nano Delivery in Chronic Medical Conditions

Biguanides, particularly the widely prescribed drug metformin, have been marketed for many decades and have well-established absorption profiles. They are commonly administered via the oral route and, despite variation in oral uptake, remain commonly prescribed for diabetes mellitus, typically type 2. Studies over the last decade have focused on the design and development of advanced oral delivery dosage forms using bio nano technologies and novel drug carrier systems. Such studies have demonstrated significantly enhanced delivery and safety of biguanides using nanocapsules. Enhanced delivery and safety have widened the potential applications of biguanides not only in diabetes but also in other disorders. Hence, this review aimed to explore biguanides’ pharmacokinetics, pharmacodynamics, and pharmaceutical applications in diabetes, as well as in other disorders.

Chemotherapy-induced hearing loss: the applications of bio-nanotechnologies and bile acid-based delivery matrices

Advancement in the prevention of chemotherapy-induced hearing loss has proposed new nano-based delivery matrices that can target inner ear regions most damaged by chemotherapy. Chemotherapy agents (e.g., cisplatin) induce increased reactive oxygen species formation in the inner ear that damage sensory hair cells and result in irreversible hearing impairment. Exogenous antioxidants (e.g., Probucol and metformin) have been shown to block the formation of these reactive oxygen species. Delivery of these drugs in effective concentrations remains a challenge. Microencapsulation in combination with drug excipients provides one technique to effectively deliver these drugs. This paper investigates the use of probucol and metformin in combination with drug excipients for novel, inner ear, delivery.

A cell-type-specific atlas of the inner ear transcriptional response to acoustic trauma

Noise-induced hearing loss (NIHL) results from a complex interplay of damage to the sensory cells of the inner ear, dysfunction of its lateral wall, axonal retraction of type 1C spiral ganglion neurons, and activation of the immune response. We use RiboTag and single-cell RNA sequencing to survey the cell-type-specific molecular landscape of the mouse inner ear before and after noise trauma. We identify induction of the transcription factors STAT3 and IRF7 and immune-related genes across all cell-types. Yet, cell-type-specific transcriptomic changes dominate the response. The ATF3/ATF4 stress-response pathway is robustly induced in the type 1A noise-resilient neurons, potassium transport genes are downregulated in the lateral wall, mRNA metabolism genes are downregulated in outer hair cells, and deafness-associated genes are downregulated in most cell types. This transcriptomic resource is available via the Gene Expression Analysis Resource (gEAR; https://umgear.org/NIHL) and provides a blueprint for the rational development of drugs to prevent and treat NIHL.

mTOR Signaling in the Inner Ear as Potential Target to Treat Hearing Loss

Hearing loss affects many people worldwide and occurs often as a result of age, ototoxic drugs and/or excessive noise exposure. With a growing number of elderly people, the number of people suffering from hearing loss will also increase in the future. Despite the high number of affected people, for most patients there is no curative therapy for hearing loss and hearing aids or cochlea implants remain the only option. Important treatment approaches for hearing loss include the development of regenerative therapies or the inhibition of cell death/promotion of cell survival pathways. The mammalian target of rapamycin (mTOR) pathway is a central regulator of cell growth, is involved in cell survival, and has been shown to be implicated in many age-related diseases. In the inner ear, mTOR signaling has also started to gain attention recently. In this review, we will emphasize recent discoveries of mTOR signaling in the inner ear and discuss implications for possible treatments for hearing restoration.

Effects of Moringa Extract on Aminoglycoside-Induced Hair Cell Death and Organ of Corti Damage

HYPOTHESIS

Moringa extract, a naturally occurring anti-oxidant, protects against aminoglycoside-induced hair cell death and hearing loss within the organ of Corti.

BACKGROUND

Reactive oxygen species (ROS) arise primarily in the mitochondria and have been implicated in aminoglycoside-induced ototoxicity. Mitochondrial dysfunction results in loss of membrane potential, release of caspases, and cell apoptosis. Moringa extract has not previously been examined as a protective agent for aminoglycoside-induced ototoxicity.

METHODS

Putative otoprotective effects of moringa extract were investigated in an organotypic model using murine organ of Corti explants subjected to gentamicin-induced ototoxicity. Assays evaluated hair cell loss, cytochrome oxidase expression, mitochondrial membrane potential integrity, and caspase activity.

RESULTS

In vitro application of moringa conferred significant protection from gentamicin-induced hair cell loss at dosages from 25 to 300 μg/mL, with dosages above 100 μg/mL conferring near complete protection. Assays demonstrated moringa extract suppression of ROS, preservation of cytochrome oxidase activity, and reduction in caspase production.

CONCLUSION

Moringa extract demonstrated potent antioxidant properties with significant protection against gentamicin ototoxicity in cochlear explants.

Metformin reduces c-Fos and ATF3 expression in the dorsal root ganglia and protects against oxaliplatin-induced peripheral sensory neuropathy in mice

Oxaliplatin is a third-generation platinum drug commonly used as the first line treatment of metastatic colorectal cancer. Oxaliplatin-based anticancer regimens course with dose-limiting neurotoxicity. The pharmacological strategies used to manage such side effect are not totally effective. Metformin is an anti-diabetic drug that is described to negatively modulate painful diabetic neuropathy. Then, this study aimed to assess the effect of metformin in the oxaliplatin-induced peripheral sensory neuropathy in mice. For that purpose, Swiss male mice were injected with oxaliplatin (1, 2 or 4 mg/kg, i.v., twice a week with a total of nine injections) alone or in combination with daily administration of metformin (250 mg/kg, p.o.). Thermal and mechanical nociceptive tests were performed once a week for five weeks. Then, the animals were euthanized on day 35 post-first injection of oxaliplatin and the dorsal root ganglia were harvested for the assessment of c-Fos and ATF3 expressions. Oxaliplatin caused a nociceptive response accompanied by the increased expression of c-Fos and ATF3 in the dorsal root ganglia and spinal cord. In addition, the oxaliplatin-associated nociception was significantly attenuated by metformin (P < 0.05), which also reduced the expression of c-Fos and ATF3 (P < 0.05). Therefore, metformin protected from the peripheral sensory neuropathy induced by oxaliplatin, which was confirmed by the reduction of c-Fos and ATF3 expression, two known neuronal activation and damage markers, respectively.

Lower ototoxicity and absence of hidden hearing loss point to gentamicin C1a and apramycin as promising antibiotics for clinical use

Spread of antimicrobial resistance and shortage of novel antibiotics have led to an urgent need for new antibacterials. Although aminoglycoside antibiotics (AGs) are very potent anti-infectives, their use is largely restricted due to serious side-effects, mainly nephrotoxicity and ototoxicity. We evaluated the ototoxicity of various AGs selected from a larger set of AGs on the basis of their strong antibacterial activities against multidrug-resistant clinical isolates of the ESKAPE panel: gentamicin, gentamicin C1a, apramycin, paromomycin and neomycin. Following local round window application, dose-dependent effects of AGs on outer hair cell survival and compound action potentials showed gentamicin C1a and apramycin as the least toxic. Strikingly, although no changes were observed in compound action potential thresholds and outer hair cell survival following treatment with low concentrations of neomycin, gentamicin and paromomycin, the number of inner hair cell synaptic ribbons and the compound action potential amplitudes were reduced. This indication of hidden hearing loss was not observed with gentamicin C1a or apramycin at such concentrations. These findings identify the inner hair cells as the most vulnerable element to AG treatment, indicating that gentamicin C1a and apramycin are promising bases for the development of clinically useful antibiotics.

Structure Characterization and Otoprotective Effects of a New Endophytic Exopolysaccharide from Saffron

Saffron, a kind of rare medicinal herb with antioxidant, antitumor, and anti-inflammatory activities, is the dry stigma of Crocus sativus L. A new water-soluble endophytic exopolysaccharide (EPS-2) was isolated from saffron by anion exchange chromatography and gel filtration. The chemical structure was characterized by FT-IR, GC-MS, and 1D and 2D-NMR spectra, indicating that EPS-2 has a main backbone of (1→2)-linked α-d-Manp, (1→2, 4)-linked α-d-Manp, (1→4)-linked α-d-Xylp, (1→2, 3, 5)-linked β-d-Araf, (1→6)- linked α-d-Glcp with α-d-Glcp-(1→ and α-d-Galp-(1→ as sidegroups. Furthermore, EPS-2 significantly attenuated gentamicin-induced cell damage in cultured HEI-OC1 cells and increased cell survival in zebrafish model. The results suggested that EPS-2 could protect cochlear hair cells from ototoxicity exposure. This study could provide new insights for studies on the pharmacological mechanisms of endophytic exopolysaccharides from saffron as otoprotective agents.

Toward Cochlear Therapies

Sensorineural hearing impairment is the most common sensory disorder and a major health and socio-economic issue in industrialized countries. It is primarily due to the degeneration of mechanosensory hair cells and spiral ganglion neurons in the cochlea via complex pathophysiological mechanisms. These occur following acute and/or chronic exposure to harmful extrinsic (e.g., ototoxic drugs, noise...) and intrinsic (e.g., aging, genetic) causative factors. No clinical therapies currently exist to rescue the dying sensorineural cells or regenerate these cells once lost. Recent studies have, however, provided renewed hope, with insights into the therapeutic targets allowing the prevention and treatment of ototoxic drug- and noise-induced, age-related hearing loss as well as cochlear cell degeneration. Moreover, genetic routes involving the replacement or corrective editing of mutant sequences or defected genes are showing promise, as are cell-replacement therapies to repair damaged cells for the future restoration of hearing in deaf people. This review begins by recapitulating our current understanding of the molecular pathways that underlie cochlear sensorineural damage, as well as the survival signaling pathways that can provide endogenous protection and tissue rescue. It then guides the reader through to the recent discoveries in pharmacological, gene and cell therapy research towards hearing protection and restoration as well as their potential clinical application.

Role of Hsp90/Akt pathway in the pathogenesis of gentamicin-induced hearing loss

Studies have suggested that gentamicin may induce hair cell apoptosis through the Hsp90/Akt signaling pathway. Nevertheless, the exact mechanisms remain unclear. The following study investigated Hsp90 expression in gentamicin-treated cochleae (in vitro and in vivo) and explored whether the Hsp90/Akt signaling pathway has a role in gentamicin ototoxicity. For in vitro experiments, organotypic cultures from post-natal organ of Corti, collected from post-natal day 2 or 3 (p2-3) CBA/J explants were treated with 0.2 mM gentamicin for 24 h; for the in vivo experiments, 6-week-old male CBA/J mice were injected with gentamicin (150 mg/kg) to induce hearing loss. P-Akt and AKT proteins expression and the levels of Hsp90-Akt complex were examined using immunochemistry and western blot. Our data suggested that Hsp90 expression decreased in the hair ear cells after treatment. In addition, the pAkt and Hsp90/AKT levels significantly decreased in treated mice compared to the control group. To conclude, these results support the idea that the Hsp90/Akt signaling pathway may have an important role in the ototoxic effects of gentamicin.

Protection for medication-induced hearing loss: the state of the science

OBJECTIVE

This review will summarise the current state of development of pharmaceutical interventions (prevention or treatment) for medication-induced ototoxicity.

DESIGN

Currently published literature was reviewed using PubMed and ClinicalTrials.gov to summarise the current state of the science. Details on the stage of development in the market pipeline are provided, along with evidence for clinical safety and efficacy reported.

STUDY SAMPLE

This review includes reports from 44 articles and clinical trial reports regarding agents in clinical or preclinical trials, having reached approved Investigational New Drug status with the Federal Drug Administration.

RESULTS

Vitamins and antioxidants are the most common agents currently evaluated for drug-induced ototoxicity intervention by targeting the oxidative stress pathway that leads to cochlear cell death and hearing loss. However, other strategies, including steroid treatment and reduction of ototoxic properties of the primary drugs, are discussed.

CONCLUSIONS

Retention of hearing during and after a life threatening illness is a major quality-of-life issue for patients receiving ototoxic drugs and their families. The agents discussed herein, while not mature enough at this point, offer great promise towards that goal. This review will provide a knowledge base for hearing providers to inquiries about such options from patients and interdisciplinary care teams alike.

From Otic Induction to Hair Cell Production: Pax2EGFP Cell Line Illuminates Key Stages of Development in Mouse Inner Ear Organoid Model

Producing hair cells of the inner ear is the major goal of ongoing research that combines advances in developmental and stem cell biology. The recent advent of an inner ear organoid protocol-resulting in three-dimensional stem cell-derived tissues resembling vestibular sensory epithelia-has sparked interest in applications such as regeneration, drug discovery, and disease modeling. In this study, we adapted this protocol for a novel mouse embryonic stem cell line with a fluorescent reporter for Pax2 expression. We used Pax2^EGFP/+ organoid formation to model otic induction, the pivotal developmental event when preplacodal tissue adopts otic fate. We found upregulation of Pax2 and activation of ERK downstream of fibroblast growth factor signaling in organoid formation as in embryonic inner ear development. Pax2 expression was evident from the EGFP reporter beginning at the vesicle formation stage and persisting through generation of the sensory epithelium. The native ventralizing signal sonic hedgehog was largely absent from the cell aggregates as otic vesicles began to form, confirming the dorsal vestibular organoid fate. Nonetheless, cochlear- or vestibular-like neurons appeared to delaminate from the derived otic vesicles and formed synaptic contacts with hair cells in the organoids. Cell lines with transcriptional reporters such as Pax2^EGFP/+ facilitate direct evaluation of morphological changes during organoid production, a major asset when establishing and validating the culture protocol.

N-acetyl-cysteine prevents age-related hearing loss and the progressive loss of inner hair cells in γ-glutamyl transferase 1 deficient mice

Genetic factors combined with oxidative stress are major determinants of age-related hearing loss (ARHL), one of the most prevalent disorders of the elderly. Dwarf grey mice, Ggt1^dwg/dwg, are homozygous for a loss of function mutation of the γ-glutamyl transferase 1 gene, which encodes an important antioxidant enzyme critical for the resynthesis of glutathione (GSH). Since GSH reduces oxidative damage, we hypothesized that Ggt1^dwg/dwg mice would be susceptible to ARHL. Surprisingly, otoacoustic emissions and cochlear microphonic potentials, which reflect cochlear outer hair cell (OHC) function, were largely unaffected in mutant mice, whereas auditory brainstem responses and the compound action potential were grossly abnormal. These functional deficits were associated with an unusual and selective loss of inner hair cells (IHC), but retention of OHC and auditory nerve fibers. Remarkably, hearing deficits and IHC loss were completely prevented by N-acetyl-L-cysteine, which induces de novo synthesis of GSH; however, hearing deficits and IHC loss reappeared when treatment was discontinued. Ggt1^dwg/dwgmice represent an important new model for investigating ARHL, therapeutic interventions, and understanding the perceptual and electrophysiological consequences of sensory deprivation caused by the loss of sensory input exclusively from IHC.

An evaluation of US patent 2015065565 (A1) for a new class of SGLT2 inhibitors for treatment 1 of type II diabetes mellitus

INTRODUCTION

Type 2 diabetes mellitus (T2DM) is a growing and serious global health problem. Pharmacological inhibition of the sodium-glucose cotransporter-2 (SGLT2; SLC5A2) increases urinary glucose excretion, decreasing plasma glucose levels in an insulin-independent manner. Agents that inhibit SGLT2 have recently become available for clinical therapy of T2DM.

AREAS COVERED

The patent claims a new class of SGLT2 inhibitors: derivatives of dioxa-bicyclo[3.2.1]octane-2,3,4-triol (including ertugliflozin; PF-04971729). The invention describes the design, synthesis and pharmacological tests related to ertugliflozin, which could ultimately lead to efficacious therapy for T2DM alone or in combination with other anti-diabetic agents.

EXPERT OPINION

Ertugliflozin is likely to be of great clinical significance in the near future. Continued analysis of ertugliflozin derivatives to now validate safe and efficacious treatment of T2DM in a larger number of clinical subjects over an extended period is needed to further support clinical utility. Identification, and discussion, of likely contra-indications is also needed.

XBP1 mitigates aminoglycoside-induced endoplasmic reticulum stress and neuronal cell death

Here we study links between aminoglycoside-induced mistranslation, protein misfolding and neuropathy. We demonstrate that aminoglycosides induce misreading in mammalian cells and assess endoplasmic reticulum (ER) stress and unfolded protein response (UPR) pathways. Genome-wide transcriptome and proteome analyses revealed upregulation of genes related to protein folding and degradation. Quantitative PCR confirmed induction of UPR markers including C/EBP homologous protein, glucose-regulated protein 94, binding immunoglobulin protein and X-box binding protein-1 (XBP1) mRNA splicing, which is crucial for UPR activation. We studied the effect of a compromised UPR on aminoglycoside ototoxicity in haploinsufficient XBP1 (XBP1^+/−) mice. Intra-tympanic aminoglycoside treatment caused high-frequency hearing loss in XBP1^+/− mice but not in wild-type littermates. Densities of spiral ganglion cells and synaptic ribbons were decreased in gentamicin-treated XBP1^+/− mice, while sensory cells were preserved. Co-injection of the chemical chaperone tauroursodeoxycholic acid attenuated hearing loss. These results suggest that aminoglycoside-induced ER stress and cell death in spiral ganglion neurons is mitigated by XBP1, masking aminoglycoside neurotoxicity at the organismal level.