Ototoxicity: bioprotective mechanisms

Expression of the kidney injury molecule 1 in the rat cochlea and induction by cisplatin

Cisplatin is a widely used chemotherapeutic agent whose dose-limiting side effects include ototoxicity and nephrotoxicity. Recent evidence indicates that cisplatin induces the expression of a novel protein, kidney injury molecule-1, in the renal proximal tubular epithelium to aid in regeneration. In this study, we determined whether kidney injury molecule-1 is expressed in the cochlea and is induced by cisplatin. Using reverse transcriptase polymerase chain reaction techniques, we have now identified kidney injury molecule-1 in the rat cochlea and in three different mouse transformed hair cell lines. Administration of cisplatin to rats produced hearing loss and induced kidney injury molecule-1 mRNA in the rat cochlea. Pretreatment of rats with lipoic acid, a scavenger of reactive oxygen species, significantly reduced cisplatin-induced hearing loss and kidney injury molecule-1 expression. Cisplatin also increased the expression of cochlear NOX3 mRNA, a member of the superoxide generating NADPH oxidase family of proteins recently identified in the cochlea, inhibition of which decreased kidney injury molecule-1 expression. Polymerase chain reaction performed on different regions of the cochlea indicated the presence of kidney injury molecule-1 mRNA in the lateral wall, organ of Corti and spiral ganglion. This distribution was confirmed by immunocytochemistry. Taken together, these data identify kidney injury molecule-1 as a novel cochlear injury molecule, whose expression is regulated by reactive oxygen species generated via the NADPH oxidase pathway.

Lateral line hair cell maturation is a determinant of aminoglycoside susceptibility in zebrafish (Danio rerio)

Developmental differences in hair cell susceptibility to aminoglycoside-induced cell death has been observed in multiple species. Increased sensitivity to aminoglycosides has been temporally correlated with the onset of mechanotransduction-dependent activity. We have used in vivo fluorescent vital dye markers to further investigate the determinants of aminoglycoside induced hair cell death in the lateral line of zebrafish (Danio rerio). Labeling hair cells of the lateral line in vivo with the dyes FM 1-43, To-Pro-3, and Yo-Pro-1 served as reliable indicators of hair cell viability. Results indicate that hair cell maturation is a determinant of developmental differences in susceptibility. The age dependent differences in susceptibility to aminoglycosides are independent of the onset of mechanotransduction-dependent activity as measured by FM 1-43 uptake and independent of hair cell ability to take up fluorescently conjugated aminoglycosides.

TRPV1 regulators mediate gentamicin penetration of cultured kidney cells

Transient receptor potential (TRP) receptors are, typically, calcium-permeant cation channels that transduce environmental stimuli. Both kidney epithelial and inner ear sensory cells express TRPV1, are mechanosensors and accumulate the aminoglycoside antibiotic gentamicin. Recently, we showed that Texas Red-conjugated gentamicin (GTTR) enters kidney cells via an endosome-independent pathway. Here, we used GTTR to investigate this non-endocytotic mechanism of gentamicin uptake. In serum-free buffers, GTTR penetrated MDCK cells within 30 s and uptake was modulated by extracellular, multivalent cations (Ca2+, La3+, Gd3+) or protons. We verified the La3+ modulation of GTTR uptake using immunocytochemical detection of unconjugated gentamicin. Membrane depolarization, induced by high extracellular K+ or valinomycin, also reduced GTTR uptake, suggesting electrophoretic permeation through ion channels. GTTR uptake was enhanced by the TRPV1 agonists, resiniferatoxin and anandamide, in Ca2+-free media. Competitive antagonists of the TRPV1 cation current, iodo-resiniferatoxin and SB366791, also enhanced GTTR uptake independently of Ca2+, reinforcing these antagonists' potential as latent agonists in specific situations. Ruthenium Red blocked GTTR uptake in the presence or absence of these TRPV1-agonists and antagonists. In addition, GTTR uptake was blocked by RTX in the presence of more physiological levels (2 mM) of Ca2+. Thus gentamicin enters cells via cation channels, and gentamicin uptake can be modulated by regulators of the TRPV1 channel.

Protective effect of T-type calcium channel blocker flunarizine on cisplatin-induced death of auditory cells

Changes in intracellular Ca2+ level are involved in a number of intracellular events, including triggering of apoptosis. The role of intracellular calcium mobilization in cisplatin-induced hair cell death, however, is still unknown. In this study, the effect of calcium channel blocker flunarizine (Sibelium), which is used to prescribe for vertigo and tinnitus, on cisplatin-induced hair cell death was investigated in a cochlear organ of Corti-derived cell line, HEI-OC1, and the neonatal (P2) rat organ of Corti explant. Cisplatin induced apoptotic cell death showing nuclear fragmentation, DNA ladder, and TUNEL positive in both HEI-OC1 and primary organ of Corti explant. Flunarizine significantly inhibited the cisplatin-induced apoptosis. Unexpectedly, flunarizine increased the intracellular calcium ([Ca2+]i) levels of HEI-OC1. However, the protective effect of flunarizine against cisplatin was not mediated by modulation of intracellular calcium level. Treatment of cisplatin resulted in ROS generation and lipid peroxidation in HEI-OC1. Flunarizine did not attenuate ROS production but inhibited lipid peroxidation and mitochondrial permeability transition in cisplatin-treated cells. This result suggests that the protective mechanism of flunarizine on cisplatin-induced cytotoxicity is associated with direct inhibition of lipid peroxidation and mitochondrial permeability transition.

The use of zebrafish for assessing ototoxic and otoprotective agents

Zebrafish and other fish exhibit hair cells in the lateral-line neuromasts which are structurally and functionally similar to mammalian inner ear hair cells. To facilitate drug screening for ototoxic or otoprotective agents, we report a straightforward, quantitative in vivo assay to determine potential ototoxicity of drug candidates and to screen otoprotective agents in zebrafish larva. In this study, a fluorescent vital dye, DASPEI (2-(4-(dimethylamino)styryl)-N-ethylpyridinium iodide), was used to stain zebrafish hair cells in vivo and morphometric analysis was performed to quantify fluorescence intensity and convert images to numerical endpoints. Various therapeutics, including gentamicin, cisplatin, vinblastine sulfate, quinine, and neomycin, which cause ototoxicity in humans, also resulted in hair cell loss in zebrafish. In addition, protection against cisplatin-induced ototoxicity was observed in zebrafish larva co-treated with cisplatin and different antioxidants including, glutathione (GSH), allopurinol (ALO), N-acetyl l-cysteine (l-NAC), 2-oxothiazolidine-4-carboxylate (OTC) and d-methionine (d-MET). Our data indicate that results of ototoxicity and otoprotection in zebrafish correlated with results in humans, supporting use of zebrafish for preliminary drug screening.

[Stem cells for the treatment of hearing loss]

One of the greatest challenges in the treatment of inner ear disorders is to find a cure for the hearing loss caused by the loss of cochlear hair cells or spiral ganglion neurons. The recent discovery of stem cells in the adult inner ear that are capable of differentiating into hair cells, as well as the finding that embryonic stem cells can be converted into hair cells, raise hope for the future development of stem-cell-based treatments.

American ginseng berry extract and ginsenoside Re attenuate cisplatin-induced kaolin intake in rats

PURPOSE

Cisplatin, a chemotherapeutic agent, causes significant nausea and vomiting. It is postulated that cisplatin-induced oxidant stress may be responsible for these symptoms. We tested whether pretreatment with American ginseng berry extract (AGBE), an herb with potent antioxidant capacity, and one of its active antioxidant constituents, ginsenoside Re, could counter cisplatin-induced emesis using a rat pica model.

METHODS

In rats, exposure to emetic stimuli such as cisplatin causes significant kaolin intake, a phenomenon called pica. We therefore measured cisplatin-induced kaolin intake as an indicator of the emetic response. Rats were pretreated with vehicle, AGBE (dose range 50-150 mg/kg, IP) or ginsenoside Re (2 and 5 mg/kg, IP). Rats were treated with cisplatin (3 mg/kg, IP) 30 min later. Kaolin intake, food intake, and body weight were measured every 24 h for 120 h. Additionally, the free radical scavenging activity of AGBE was measured in vitro using ESR spectroscopy.

RESULTS

A significant dose-response relationship was observed between increasing doses of pretreatment with AGBE and reduction in cisplatin-induced pica. Kaolin intake was maximally attenuated by AGBE at a dose of 100 mg/kg. Food intake also improved significantly at this dose (P<0.05). Pretreatment with ginsenoside Re (5 mg/kg) also decreased kaolin intake (P<0.05). In vitro studies demonstrated a concentration-response relationship between AGBE and its ability to scavenge superoxide and hydroxyl radicals.

CONCLUSION

Pretreatment with AGBE and its major constituent, Re, attenuated cisplatin-induced pica, and demonstrated potential for the treatment of chemotherapy-induced nausea and vomiting. Significant recovery of food intake further strengthens the conclusion that AGBE may exert an antinausea/antiemetic effect.

The protective effect of erdosteine against ototoxicity induced by cisplatin in rats

The elimination of cisplatin ototoxicity is an ongoing concern. This experimental study was undertaken to investigate the effect of oral erdosteine in ameliorating cisplatin-induced ototoxicity. Twenty-eight adult female Wistar albino rats were randomly divided into four equal groups. Group A received an oral carrier vehicle of the drug erdosteine with 0.2 ml of 0.9% saline. Group B was administered only erdosteine (per oral 10 mg/kg twice a day) for 6 days. Group C was injected with cisplatin intraperitoneally (i.p.) on day 0 (16 mg/kg body weight), once only. Group D was given erdosteine (per oral 10 mg/kg/day) 1 day before and for 5 days consecutively after cisplatin injection (16 mg/kg, i.p.). Distortion product otoacoustic emissions (DPOAEs) were elicited in different frequency regions, ranging from 1,001 to 6,299 Hz as DPgram and input/output (I/O) functions from the control and experimental animals. All experimental animals were killed under general anesthesia on day 5, following the last otoacoustic emission measurements. Prior to death, blood samples were drawn for measurement of superoxide dismutase, xanthine oxidase (XO), malondialdehyde and nitric oxide. Initial DPgram and I/O function baseline measurements were similar in all groups prior to any drug administration ( P>0.05). On day 5, intra-subject measurement parameters of DPgrams and I/O functions in the cisplatin group showed significant deterioration ( P <0.05). The other groups revealed no differences between their pre- and post-test drug administration DPgrams and I/O functions at any test frequency ( P>0.05). Comparison of the amplitudes of DPgrams and I/O functions between the cisplatin and control groups showed significant changes ( P <0.05). Biochemical studies noted an increased XO activity following cisplatin administration ( P <0.007). The other biochemical results did not show significant differences between the study and control groups. This study demonstrates that, in rats, erdosteine is protective for cochlear function against the disruptive effects of cisplatin as measured by DPOAEs.

Stem cells as therapy for hearing loss

One of the greatest challenges in the treatment of inner-ear disorders is to find a cure for the hearing loss that is caused by the loss of cochlear hair cells or spiral ganglion neurons. The recent discovery of stem cells in the adult inner ear that are capable of differentiating into hair cells, as well as the finding that embryonic stem cells can be converted into hair cells, raise hope for the future development of stem-cell-based treatment regimens. Here, we propose different approaches for using stem cells to regenerate the damaged inner ear and we describe the potential obstacles that translational approaches must overcome for the development of stem-cell-based cell-replacement therapies for the damaged inner ear.