Downstream targets of Lmo4 are modulated by cisplatin in the inner ear of Wistar rats

Quantitative profiling of cochlear synaptosomal proteins in cisplatin-induced synaptic dysfunction

The disruption of ribbon synapses in the cochlea impairs the transmission of auditory signals from the cochlear sensory receptor cells to the auditory cortex. Although cisplatin-induced loss of ribbon synapses is well-documented, and studies have reported nitration of cochlear proteins after cisplatin treatment, yet the underlying mechanism of cochlear synaptopathy is not fully understood. This study tests the hypothesis that cisplatin treatment alters the abundance of cochlear synaptosomal proteins, and selective targeting of nitrative stress prevents the associated synaptic dysfunction. Auditory brainstem responses of mice treated with cisplatin showed a reduction in amplitude and an increase in latency of wave I, indicating cisplatin-induced synaptic dysfunction. The mass spectrometry analysis of cochlear synaptosomal proteins identified 102 proteins that decreased in abundance and 249 that increased in abundance after cisplatin treatment. Pathway analysis suggested that the dysregulated proteins were involved in calcium binding, calcium ion regulation, synapses, and endocytosis pathways. Inhibition of nitrative stress by co-treatment with MnTBAP, a peroxynitrite scavenger, attenuated cisplatin-induced changes in the abundance of 27 proteins. Furthermore, MnTBAP co-treatment prevented the cisplatin-induced decrease in the amplitude and increase in the latency of wave I. Together, these findings suggest a potential role of oxidative/nitrative stress in cisplatin-induced cochlear synaptic dysfunction.

lmo4a Contributes to Zebrafish Inner Ear and Vestibular Development via Regulation of the Bmp Pathway

BACKGROUND

In vertebrates, the development of the inner ear is a delicate process, whereas its relating molecular pathways are still poorly understood. LMO4, an LIM domain-only transcriptional regulator, is drawing an increasing amount of interest for its multiple roles regarding human embryonic development and the modulation of ototoxic side effects of cisplatin including cochlear apoptosis and hearing loss. The aim of the present study is to further explore the role of lmo4a in zebrafish inner ear development and thus explore its functional role.

METHODS

The Spatial Transcript Omics DataBase was referred to in order to evaluate the expression of lmo4a during the first 24 h of zebrafish development. In situ hybridization was applied to validate and extend the expression profile of lmo4a to 3 days post-fertilization. The morpholino (MO) knockdown and CRISPR/Cas9 knockout (KO) of lmo4a was applied. Morphological analyses of otic vesical, hair cells, statoacoustic ganglion and semicircular canals were conducted. The swimming pattern of lmo4a KO and MO zebrafish was tracked. In situ hybridization was further applied to verify the expression of genes of the related pathways. Rescue of the phenotype was attempted by blockage of the bmp pathway via heat shock and injection of Dorsomorphin.

RESULTS

lmo4a is constitutively expressed in the otic placode and otic vesicle during the early stages of zebrafish development. Knockdown and knockout of lmo4a both induced smaller otocysts, less hair cells, immature statoacoustic ganglion and malformed semicircular canals. Abnormal swimming patterns could be observed in both lmo4a MO and KO zebrafish. eya1 in preplacodal ectoderm patterning was downregulated. bmp2 and bmp4 expressions were found to be upregulated and extended in lmo4a morphants, and blockage of the Bmp pathway partially rescued the vestibular defects.

CONCLUSIONS

We concluded that lmo4a holds a regulative effect on the Bmp pathway and is required for the normal development of zebrafish inner ear. Our study pointed out the conservatism of LMO4 in inner ear development between mammals and zebrafish as well as shed more light on the molecular mechanisms behind it. Further research is needed to distinguish the relationships between lmo4 and the Bmp pathway, which may lead to diagnostic and therapeutic approaches towards human inner ear malformation.

Nitrative Stress and Auditory Dysfunction

Nitrative stress is increasingly recognized as a critical mediator of apoptotic cell death in many pathological conditions. The accumulation of nitric oxide along with superoxide radicals leads to the generation of peroxynitrite that can eventually result in the nitration of susceptible proteins. Nitrotyrosine is widely used as a biomarker of nitrative stress and indicates oxidative damage to proteins. Ototoxic insults, such as exposure to noise and ototoxic drugs, enhance the generation of 3-nitrotyrosine in different cell types in the cochlea. Nitrated proteins can disrupt critical signaling pathways and eventually lead to apoptosis and loss of sensory receptor cells in the cochlea. Accumulating evidence shows that selective targeting of nitrative stress attenuates cellular damage. Anti-nitrative compounds, such as peroxynitrite decomposition catalysts and inducible nitric oxide synthase inhibitors, prevent nitrative stress-mediated auditory damage. However, the role of nitrative stress in acquired hearing loss and its potential significance as a promising interventional target is yet to be fully characterized. This review provides an overview of nitrative stress mechanisms, the induction of nitrative stress in the auditory tissue after ototoxic insults, and the therapeutic value of targeting nitrative stress for mitigating auditory dysfunction.

Nimodipine Treatment Protects Auditory Hair Cells from Cisplatin-Induced Cell Death Accompanied by Upregulation of LMO4

Ototoxicity is one of the main dose-limiting side effects of cisplatin chemotherapy and impairs the quality of life of tumor patients dramatically. Since there is currently no established standard therapy targeting hearing loss in cisplatin treatment, the aim of this study was to investigate the effect of nimodipine and its role in cell survival in cisplatin-associated hearing cell damage. To determine the cytotoxic effect, the cell death rate was measured using undifferentiated and differentiated UB/OC−1 and UB/OC−2 cells, after nimodipine pre-treatment and stress induction by cisplatin. Furthermore, immunoblot analysis and intracellular calcium measurement were performed to investigate anti-apoptotic signaling, which was associated with a reduced cytotoxic effect after nimodipine pre-treatment. Cisplatin’s cytotoxic effect was significantly attenuated by nimodipine up to 61%. In addition, nimodipine pre-treatment counteracted the reduction in LIM Domain Only 4 (LMO4) by cisplatin, which was associated with increased activation of Ak strain transforming/protein kinase B (Akt), cAMP response element-binding protein (CREB), and signal transducers and activators of transcription 3 (Stat3). Thus, nimodipine presents a potentially well-tolerated substance against the ototoxicity of cisplatin, which could result in a significant improvement in patients’ quality of life.

Lmo4 Deficiency Enhances Susceptibility to Cisplatin-Induced Cochlear Apoptosis and Hearing Loss

Cisplatin, a potent chemotherapeutic drug, induces ototoxicity, which limits its clinical utility. Cisplatin-induced oxidative stress plays a causal role in cochlear apoptosis while the consequent nitrative stress leads to the nitration of LIM domain only 4 (LMO4), a transcriptional regulator, and decreases its cochlear expression levels. Here, we show a direct link between cochlear LMO4 and cisplatin-induced hearing loss by employing a Lmo4 conditional knockout mouse model (Lmo4^lox/lox; Gfi1^Cre/+). Hair cell-specific deletion of Lmo4 did not alter cochlear morphology or affect hearing thresholds and otoacoustic emissions, in the absence of apoptotic stimuli. Cisplatin treatment significantly elevated the auditory brainstem response thresholds of conditional knockouts, across all frequencies. Moreover, deletion of Lmo4 compromised the activation of STAT3, a downstream target that regulates anti-apoptotic machinery. Immunostaining indicated that the expression of phosphorylated STAT3 was significantly decreased while the expression of activated caspase 3 was significantly increased in Lmo4 deficient hair cells, post-cisplatin treatment. These findings suggest an otoprotective role of LMO4 as cisplatin-induced decrease in cochlear LMO4 could compromise the LMO4/STAT3 cellular defense mechanism to induce ototoxicity.

Cisplatin-induced hair cell loss in zebrafish neuromasts is accompanied by protein nitration and Lmo4 degradation

Generation of reactive oxygen species, a critical factor in cisplatin-induced ototoxicity, leads to the formation of peroxynitrite, which in turn results in the nitration of susceptible proteins. Previous studies indicated that LMO4, a transcriptional regulator, is the most abundantly nitrated cochlear protein after cisplatin treatment and that LMO4 nitration facilitates ototoxicity in rodents. However, the role of this mechanism in regulating cisplatin-induced hair cell loss in non-mammalian models is unknown. As the mechanosensory hair cells in the neuromasts of zebrafish share many features with mammalian inner ear and is a good model for studying ototoxicity, we hypothesized that cisplatin treatment induces protein nitration and Lmo4 degradation in zebrafish hair cells, thereby facilitating hair cell loss. Immunostaining with anti-parvalbumin revealed a significant decrease in the number of hair cells in the neuromast of cisplatin treated larvae. In addition, cisplatin treatment induced a significant decrease in the expression of Lmo4 protein and a significant increase in nitrotyrosine levels, in the hair cells. The cisplatin-induced changes in Lmo4 and nitrotyrosine levels strongly correlated with hair cell loss, implying a potential link. Furthermore, a significant increase in the expression of activated Caspase-3 in zebrafish hair cells, post cisplatin treatment, suggested that cisplatin-induced decrease in Lmo4 levels is accompanied by apoptosis. These findings suggest that nitrative stress and Lmo4 degradation are important factors in cisplatin-induced hair cell loss in zebrafish neuromasts and that zebrafish could be used as a model to screen the otoprotective efficacy of compounds that inhibit protein nitration.

An integrative approach to cisplatin chronic toxicities in mice reveals importance of organic cation-transporter-dependent protein networks for renoprotection

Cisplatin (CDDP) is one of the most important chemotherapeutic drugs in modern oncology. However, its use is limited by severe toxicities, which impair life quality after cancer. Here, we investigated the role of organic cation transporters (OCT) in mediating toxicities associated with chronic (twice the week for 4 weeks) low-dose (4 mg/kg body weight) CDDP treatment (resembling therapeutic protocols in patients) of wild-type (WT) mice and mice with OCT genetic deletion (OCT1/2^-/-). Functional and molecular analysis showed that OCT1/2^-/- mice are partially protected from CDDP-induced nephrotoxicity and peripheral neurotoxicity, whereas ototoxicity was not detectable. Surprisingly, proteomic analysis of the kidneys demonstrated that genetic deletion of OCT1/2 itself was associated with significant changes in expression of proinflammatory and profibrotic proteins which are part of an OCT-associated protein network. This signature directly regulated by OCT consisted of three classes of proteins, viz., profibrotic proteins, proinflammatory proteins, and nutrient sensing molecules. Consistent with functional protection, CDDP-induced proteome changes were more severe in WT mice than in OCT1/2^-/- mice. Laser ablation-inductively coupled plasma-mass spectrometry analysis demonstrated that the presence of OCT was not associated with higher renal platinum concentrations. Taken together, these results redefine the role of OCT from passive membrane transporters to active modulators of cell signaling in the kidney.

Inhibition of protein nitration prevents cisplatin-induced inactivation of STAT3 and promotes anti-apoptotic signaling in organ of Corti cells

JAK/STAT pathway is one among the several oxidative stress-responsive signaling pathways that play a critical role in facilitating cisplatin-induced ototoxicity. Cisplatin treatment decreases the levels of cochlear LMO4, which acts as a scaffold for IL6-GP130 protein complex. Cisplatin-induced nitration and degradation of LMO4 could destabilize this protein complex, which in turn could compromise the downstream STAT3-mediated cellular defense mechanism. Here, we investigated the link between cisplatin-induced nitrative stress and STAT3-mediated apoptosis by using organ of Corti cell cultures. SRI110, a peroxynitrite decomposition catalyst that prevented cisplatin-induced decrease in LMO4 levels and ototoxicity, was used to inhibit nitrative stress. Immunoblotting and immunostaining indicated that cisplatin treatment decreased the expression levels, phosphorylation, and nuclear localization of STAT3 in UB/OC1 cells. Inhibition of nitration by SRI110 co-treatment prevented cisplatin-induced inactivation of STAT3 and promoted its nuclear localization. SRI110 co-treatment reversed the cisplatin-induced changes in the expression levels of Bcl2l1, Ccnd1, Jak2, Jak3, and Src and significantly attenuated the changes in the expression levels of Cdkn1a, Egfr, Fas, Il6st, Jak1, Stat3, and Tyk2. Collectively, these results suggest that the inhibition of cisplatin-induced nitration prevents the inactivation of STAT3, which in turn enables the transcription of anti-apoptotic genes and thereby helps to mitigate cisplatin-induced toxicity.

CRISPR/Cas9-mediated knockout of Lim-domain only four retards organ of Corti cell growth

Lim-domain only 4 (LMO4) plays a critical role in mediating the ototoxic side-effects of cisplatin, a highly effective anti-cancer drug. However, the signaling mechanism by which cochlear LMO4 mediates otopathology is yet to be fully understood. Knockout cell culture models are useful tools for investigating the functional roles of novel genes and delineating associated signaling pathways. Therefore, LMO4 knockout organ of Corti cells were generated by using the CRISPR (clustered regularly interspersed short palindromic repeats)/Cas9 (CRISPR-associated protein 9) system. Successful knockout of LMO4 in UB/OC1 cells was verified by the absence of LMO4 protein bands in immunoblots. Though the Knockout of LMO4 retarded the growth rate and the migratory potential of the cells it did not inhibit their long-term viability as the LMO4 knockout UB/OC1 cells were able to survive, proliferate, and form colonies. In addition, the knockout of LMO4 did not alter the expression of myosin VIIa, a biomarker of hair cells, suggesting that the knockout cells retain important characteristic features of cochlear sensory receptor cells. Thus, the findings of this study indicate that CRISPR/Cas9 system is a simple and versatile method for knocking out genes of interest in organ of Corti cells and that LMO4 knockout UB/OC1 cells are viable experimental models for studying the functional role of LMO4 in ototoxicity.

Epigallocatechin-3-gallate, a prototypic chemopreventative agent for protection against cisplatin-based ototoxicity

Cisplatin-induced ototoxicity is one of the major factors limiting cisplatin chemotherapy. Ototoxicity results from damage to outer hair cells (OHCs) and other regions of the cochlea. At the cellular level, cisplatin increases reactive oxygen species (ROS) leading to cochlear inflammation and apoptosis. Thus, ideal otoprotective drugs should target oxidative stress and inflammatory mechanisms without interfering with cisplatin's chemotherapeutic efficacy. In this study, we show that epigallocatechin-3-gallate (EGCG) is a prototypic agent exhibiting these properties of an effect otoprotective agent. Rats administered oral EGCG demonstrate reduced cisplatin-induced hearing loss, reduced loss of OHCs in the basal region of the cochlea and reduced oxidative stress and apoptotic markers. EGCG also protected against the loss of ribbon synapses associated with inner hair cells and Na⁺/K⁺ ATPase α1 in the stria vascularis and spiral ligament. In vitro studies showed that EGCG reduced cisplatin-induced ROS generation and ERK1/2 and signal transducer and activator of transcription-1 (STAT1) activity, but preserved the activity of STAT3 and Bcl-xL. The increase in STAT3/STAT1 ratio appears critical for mediating its otoprotection. EGCG did not alter cisplatin-induced apoptosis of human-derived cancer cells or cisplatin antitumor efficacy in a xenograft tumor model in mice because of its inability to rescue the downregulation of STAT3 in these cells. These data suggest that EGCG is an ideal otoprotective agent for treating cisplatin-induced hearing loss without compromising its antitumor efficacy.

Targeting nitrative stress for attenuating cisplatin-induced downregulation of cochlear LIM domain only 4 and ototoxicity

Cisplatin-induced ototoxicity remains a primary dose-limiting adverse effect of this highly effective anticancer drug. The clinical utility of cisplatin could be enhanced if the signaling pathways that regulate the toxic side-effects are delineated. In previous studies, we reported cisplatin-induced nitration of cochlear proteins and provided the first evidence for nitration and downregulation of cochlear LIM domain only 4 (LMO4) in cisplatin ototoxicity. Here, we extend these findings to define the critical role of nitrative stress in cisplatin-induced downregulation of LMO4 and its consequent ototoxic effects in UBOC1 cell cultures derived from sensory epithelial cells of the inner ear and in CBA/J mice. Cisplatin treatment increased the levels of nitrotyrosine and active caspase 3 in UBOC1 cells, which was detected by immunocytochemical and flow cytometry analysis, respectively. The cisplatin-induced nitrative stress and apoptosis were attenuated by co-treatment with SRI110, a peroxynitrite decomposition catalyst (PNDC), which also attenuated the cisplatin-induced downregulation of LMO4 in a dose-dependent manner. Furthermore, transient overexpression of LMO4 in UBOC1 cells prevented cisplatin-induced cytotoxicity while repression of LMO4 exacerbated cisplatin-induced cell death, indicating a direct link between LMO4 protein levels and cisplatin ototoxicity. Finally, auditory brainstem responses (ABR) recorded from CBA/J mice indicated that co-treatment with SRI110 mitigated cisplatin-induced hearing loss. Together, these results suggest that cisplatin-induced nitrative stress leads to a decrease in the levels of LMO4, downregulation of LMO4 is a critical determinant in cisplatin-induced ototoxicity, and targeting peroxynitrite could be a promising strategy for mitigating cisplatin-induced hearing loss.

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Cisplatin-induced nitrative stress leads to a decrease in the levels of LMO4.

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Downregulation of LMO4 is a critical factor in cisplatin-induced ototoxicity.

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SRI110 appears to be a promising candidate for preventing cisplatin ototoxicity.

Loss of STAT1 protects hair cells from ototoxicity through modulation of STAT3, c-Jun, Akt, and autophagy factors

Hair cell damage is a side effect of cisplatin and aminoglycoside use. The inhibition or attenuation of this process is a target of many investigations. There is growing evidence that STAT1 deficiency decreases cisplatin-mediated ototoxicity; however, the role of STAT function and the molecules that act in gentamicin-mediated toxicity have not been fully elucidated. We used mice lacking STAT1 to investigate the effect of STAT1 ablation in cultured organs treated with cisplatin and gentamicin. Here we show that ablation of STAT1 decreased cisplatin toxicity and attenuated gentamicin-mediated hair cell damage. More TUNEL-positive hair cells were observed in explants of wild-type mice than that of STAT1^−/− mice. Although cisplatin increased serine phosphorylation of STAT1 in wild-type mice and diminished STAT3 expression in wild-type and STAT1^−/− mice, gentamicin increased tyrosine phosphorylation of STAT3 in STAT1^−/− mice. The early inflammatory response was manifested in the upregulation of TNF-α and IL-6 in cisplatin-treated explants of wild-type and STAT1^−/− mice. Expression of the anti-inflammatory cytokine IL-10 was altered in cisplatin-treated explants, upregulated in wild-type explants, and downregulated in STAT1^−/− explants. Cisplatin and gentamicin triggered the activation of c-Jun. Activation of Akt was observed in gentamicin-treated explants from STAT1^−/− mice. Increased levels of the autophagy proteins Beclin-1 and LC3-II were observed in STAT1^−/− explants. These data suggest that STAT1 is a central player in mediating ototoxicity. Gentamicin and cisplatin activate different downstream factors to trigger ototoxicity. Although cisplatin and gentamicin triggered inflammation and activated apoptotic factors, the absence of STAT1 allowed the cells to overcome the effects of these drugs.

Cisplatin-induced apoptosis in auditory, renal, and neuronal cells is associated with nitration and downregulation of LMO4

Cytotoxic effects of cisplatin occur primarily through apoptosis. Though several pro- and anti-apoptotic signaling molecules have been identified to play an important role in mediating the ototoxic, nephrotoxic, and neurotoxic side effects of cisplatin, the underlying mechanism is yet to be fully characterized. We reported that nitration of LIM domain-only 4 (LMO4), a transcriptional regulator, facilitates cochlear apoptosis in cisplatin-induced ototoxicity. However, its role in cisplatin-mediated nephrotoxicity and neurotoxicity is poorly understood. Therefore, HK2 and SH-SY5Y cells were used along with UBOC1 cells, to investigate the perturbations of LMO4 in cisplatin-induced cytotoxicity, in renal, neuronal, and auditory cells, respectively. Cisplatin induced an increase in the expression of active caspase-3, indicating cellular apoptosis, and increased the nitration of proteins, 24 h post treatment. Immunostaining with anti-nitrotyrosine and anti-LMO4 indicated that nitrotyrosine co-localized with LMO4 protein in cisplatin-treated cells. Immunoblotting with anti-LMO4 indicated that cisplatin induced a decrease in LMO4 protein levels. However, a corresponding decrease in LMO4 gene levels was not observed. Inhibition of protein nitration with SRI110, a peroxynitrite decomposition catalyst, attenuated cisplatin-induced downregulation of LMO4. More importantly, overexpression of LMO4 mitigated the cytotoxic effects of cisplatin in UBOC1 cells while a dose-dependent decrease in LMO4 protein strongly correlated with cell viability in UBOC1, HK2, and SH-SY5Y cells. Collectively, these findings suggested a potential role of LMO4 in facilitating the cytotoxic effects of cisplatin in auditory, renal, and neuronal cells.