Roles of prostaglandin E2 in the cochlea

Structural Characterization and Anti-Inflammatory Effects of 24-Methylcholesta-5(6), 22-Diene-3β-ol from the Cultured Marine Diatom Phaeodactylum tricornutum; Attenuate Inflammatory Signaling Pathways

In the present investigation, 24-methylcholesta-5(6), 22-diene-3β-ol (MCDO), a major phytosterol was isolated from the cultured marine diatom, Phaeodactylum tricornutum Bohlin, and in vitro and in vivo anti-inflammatory effects were determined. MCDO demonstrated very potent dose-dependent inhibitory effects on the production of nitric oxide (NO) and prostaglandin E₂ (PGE₂) against lipopolysaccharide (LPS)-induced RAW 264.7 cells with minimal cytotoxic effects. MCDO also demonstrated a strong and significant suppression of pro-inflammatory cytokines of interleukin-1β (IL-1β) production, but no substantial inhibitory effects were observed on the production of cytokines, including tumor necrosis factor-α (TNF-α) and interleukin-6 (IL-6) at the tested concentrations against LPS treatment on RAW macrophages. Western blot assay confirmed the suppression of inducible nitric oxide synthase (iNOS) and cyclooxygenase-2 (COX-2) protein expressions against LPS-stimulated RAW 264.7 cells. In addition, MCDO was assessed for in vivo anti-inflammatory effects using the zebrafish model. MCDO acted as a potent inhibitor for reactive oxygen species (ROS) and NO levels with a protective effect against the oxidative stress induced by LPS in inflammatory zebrafish embryos. Collectively, MCDO isolated from the cultured marine diatom P. tricornutum exhibited profound anti-inflammatory effects both in vitro and in vivo, suggesting that this major sterol might be a potential treatment for inflammatory diseases.

Inhibition of macrophage migration inhibitory factor alleviates LPS-induced inflammation response of HEI-OC1 cells via suppressing NF-κB signaling

BACKGROUND

Sudden sensorineural hearing loss (SSNHL) is acute and unexplained. Macrophage migration inhibitory factor (MIF) is a pro-inflammatory cytokine in several inflammatory diseases. However, its role in SSNHL remains elusive.

METHODS

Lipopolysaccharide (LPS) was used to induce the inflammatory response of murine auditory cells, HEI-OC1. Silencing of MIF in HEI-OC1 cells was achieved by transfection of short hairpin RNA against MIF. 740Y-P and IMD0354 were used to stimulate the PI3K pathway and suppress the NF-κB pathway, respectively. RT-qPCR and western blotting were used to examine MIF and cyclooxygenase 2 (COX2) expression in LPS-treated HEI-OC1 cells. ELISA was employed to assess prostaglandin E2 (PGE2) concentrations.

RESULTS

MIF was upregulated in LPS-treated HEI-OC1 cells. MIF knockdown reduced PGE2 synthesis and COX2 expression in LPS-treated HEI-OC1 cells. Moreover, MIF knockdown suppressed activation of the PI3K/AKT and NF-κB pathway in LPS-treated HEI-OC1 cells. Additionally, inhibition of MIF decreased PGE2 production and COX2 expression via inactivation of the NF-κB pathway.

CONCLUSION

Inhibition of MIF alleviated LPS-induced inflammation in HEI-OC1 cells via inactivating the NF-κB signaling, which might provide a better understanding for SSNHL development.

Potential use of n-3 PUFAs to prevent oxidative stress-derived ototoxicity caused by platinum-based chemotherapy

Platinum-based compounds are widely used for the treatment of different malignancies due to their high effectiveness. Unfortunately, platinum-based treatment may lead to ototoxicity, an often-irreversible side effect without a known effective treatment and prevention plan. Platinum-based compound-related ototoxicity results mainly from the production of toxic levels of reactive oxygen species (ROS) rather than DNA-adduct formation, which has led to test strategies based on direct ROS scavengers to ameliorate hearing loss. However, favorable clinical results have been associated with several complications, including potential interactions with chemotherapy efficacy. To understand the contribution of the different cytotoxic mechanisms of platinum analogues on malignant cells and auditory cells, the particular susceptibility and response of both kinds of cells to molecules that potentially interfere with these mechanisms, is fundamental to develop innovative strategies to prevent ototoxicity without affecting antineoplastic effects. The n-3 long-chain polyunsaturated fatty acids (n-3 PUFAs) have been tried in different clinical settings, including with cancer patients. Nevertheless, their use to decrease cisplatin-induced ototoxicity has not been explored to date. In this hypothesis paper, we address the mechanisms of platinum compounds-derived ototoxicity, focusing on the differences between the effects of these compounds in neoplastic versus auditory cells. We discuss the basis for a strategic use of n-3 PUFAs to potentially protect auditory cells from platinum-derived injury without affecting neoplastic cells and chemotherapy efficacy.

Prostaglandin E1 reduces the keratinocyte toxicity of sorafenib by maintaining signal transducer and activator of transcription 3 (STAT3) activity and enhancing the cAMP response element binding protein (CREB) activity

Hand-foot skin reaction (HFSR) is a common side effect of multiple tyrosine kinase inhibitors (mTKIs). HFSR can necessitate dose reductions or interruption of therapy owing to its negative effect on the quality of life. Therefore, effective use of mTKIs requires measures to prevent HFSR. We evaluated the effect of prostaglandin E₁ (PGE₁) on HFSR, because PGE₁ is already used to treat bed sores and skin ulcers and has established angiogenic and antiproliferative effects in keratinocytes. We found that the pathogenesis of sorafenib-induced HFSR is characterized by a decrease in levels of a phosphorylated signal transducer and activator of transcription 3 (STAT3). We investigated the effect of PGE₁ on the sorafenib-mediated reduction in phosphorylated STAT3 levels in HaCaT human epidermal keratinocytes. In cells treated with sorafenib, phosphorylated STAT3 levels decreased in a concentration-dependent manner, and this effect was blocked in cells treated with sorafenib and PGE₁. Furthermore, the expression of phosphorylated STAT3, the antiapoptotic proteins myeloid cell leukemia-1 (Mcl-1) and survivin decreased in cells pretreated with an inhibitor of cAMP response element binding protein (CREB). Cell viability increased in cells treated with sorafenib and PGE₁ compared with that in cells treated with sorafenib alone, and these effects were not observed in STAT3 knockdown HaCaT cells. Collectively, these findings indicate that PGE₁ blocks the inhibitory effects of sorafenib on cell growth by maintaining the activity of STAT3 and enhancing the CREB activity. Therefore, PGE₁ might represent an effective treatment for the prevention of sorafenib-induced HFSR.

Inhibitory effect of triterpenoids from Dillenia serrata (Dilleniaceae) on prostaglandin E2 production and quantitative HPLC analysis of its koetjapic acid and betulinic acid contents

The crude methanol extracts and fractions of the root and stem barks of Dillenia serrata Thunb. showed 64% to 73% inhibition on the production of prostaglandin E₂ (PGE₂) in lipopolysaccharide-induced human whole blood using a radioimmunoassay technique. Three triterpenoids isolated from the root bark of the plant, koetjapic (1), 3-oxoolean-12-en-30-oic (2), and betulinic (3) acids, exhibited significant concentration-dependent inhibitory effects on PGE₂ production with IC₅₀ values of 1.05, 1.54, and 2.59 μM, respectively, as compared with the positive control, indomethacin (IC₅₀ = 0.45 μM). Quantification of compounds 1 and 3 in the methanol extracts and fractions were carried out by using a validated reversed-phase high performance liquid chromatography (RP-HPLC) method. The ethyl acetate fraction of the stem bark showed the highest content of both compound 1 (15.1%) and compound 3 (52.8%). The strong inhibition of the extracts and fractions on cyclooxygenase-2 (COX-2) enzymatic activity was due to the presence of their major constituents, especially koetjapic and betulinic acids.

Role of cysteinyl leukotriene signaling in a mouse model of noise-induced cochlear injury

Noise-induced hearing loss is one of the most common types of sensorineural hearing loss. In this study, we examined the expression and localization of leukotriene receptors and their respective changes in the cochlea after hazardous noise exposure. We found that the expression of cysteinyl leukotriene type 1 receptor (CysLTR1) was increased until 3 d after noise exposure and enhanced CysLTR1 expression was mainly observed in the spiral ligament and the organ of Corti. Expression of 5-lipoxygenase was increased similar to that of CysLTR1, and there was an accompanying elevation of CysLT concentration. Posttreatment with leukotriene receptor antagonist (LTRA), montelukast, for 4 consecutive days after noise exposure significantly decreased the permanent threshold shift and also reduced the hair cell death in the cochlea. Using RNA-sequencing, we found that the expression of matrix metalloproteinase-3 (MMP-3) was up-regulated after noise exposure, and it was significantly inhibited by montelukast. Posttreatment with a MMP-3 inhibitor also protected the hair cells and reduced the permanent threshold shift. These findings suggest that acoustic injury up-regulated CysLT signaling in the cochlea and cochlear injury could be attenuated by LTRA through regulation of MMP-3 expression. This study provides mechanistic insights into the role of CysLTs signaling in noise-induced hearing loss and the therapeutic benefit of LTRA.

Immunological role of prostaglandin E2 production in mouse auditory cells in response to LPS

The effect of LPS on the production of prostaglandin E2 (PGE2) in mouse HEI-OC1 auditory cells was examined. HEI-OC1 auditory cells constitutively produce a small amount of PGE2. LPS augmented the PGE2 production via enhanced cyclooxygenase 2 (COX2) expression. LPS-induced augmentation of COX2 expression was dependent on up-regulation of COX2 mRNA expression. LPS induced the production of TNF-α, but not IL-1β· An anti-TNF-α neutralizing Ab significantly inhibited PGE2 production and COX2 mRNA expression in response to LPS. LPS-induced PGE2 production was prevented by a series of pharmacological signaling inhibitors to NF-κB and MAPKs. Pam3CSK4 as a TLR2 ligand, as well as LPS as a TLR4 ligand, augmented the PGE2 production. However, poly I:C as a TLR3 ligand, imiquimod as a TLR7 ligand and CpG DNA as a TLR9 ligand did not augment it. HEI-OC1 cells expressed TLR2, TLR4 and TLR9, but not TLR3 or TLR7. The putative role of LPS-induced PGE2 production in auditory cells is discussed.

Prostaglandin E receptor subtype EP4 agonist serves better to protect cochlea than prostaglandin E1

OBJECTIVE

The present study aimed to examine whether an E-prostanoid receptor 4 (EP4) agonist has superior protective effects to those of prostaglandin E1 (PGE1) in a guinea pig model of noise trauma.

METHODS

Drugs were locally applied on the round window membrane of guinea pig cochleae, followed by exposure of the test animals to intense noise. Protective effects mediated by an EP4 agonist were compared with those mediated by PGE1. Auditory function was monitored by measurements of the auditory brainstem response (ABR), and histological damage was assessed by immunohistochemical analysis of cochlear specimens.

RESULTS

Animals treated with an EP4 agonist exhibited significantly better hearing recovery than those pretreated with PGE1. Histologically, the numbers of remaining outer hair cells in cochleae treated with the EP4 agonist were significantly higher than in those treated with PGE1.

CONCLUSION

The selective activation of EP4 has a stronger protective effect on cochleae against noise trauma than does the broad activation of EPs by PGE1.

Role of PGE-type receptor 4 in auditory function and noise-induced hearing loss in mice

This study explored the physiological roles of PGE-type receptor 4 (EP4) in auditory function. EP4-deficient mice exhibited slight hearing loss and a reduction of distortion-product otoacoustic emissions (DPOAEs) with loss of outer hair cells (OHCs) in cochleae. After exposure to intense noise, these mice showed significantly larger threshold shifts of auditory brain-stem responses (ABRs) and greater reductions of DPOAEs than wild-type mice. A significant increase of OHC loss was confirmed morphologically in the cochleae of EP4-deficient mice. Pharmacological inhibition of EP4 had a similar effect to genetic deletion, causing loss of both hearing and OHCs in C57BL/6 mice, indicating a critical role for EP4 signaling in the maintenance of auditory function. Pharmacological activation of EP4 significantly protected OHCs against noise trauma, and attenuated noise-induced hearing loss in C57BL/6 mice. These findings suggest that EP4 signaling is necessary for the maintenance of cochlear physiological function and for cochlear protection against noise-induced damage, in particular OHCs. EP4 might therefore be an effective target for cochlear disease therapeutics.

Physiopathology of the cochlear microcirculation

Normal blood supply to the cochlea is critically important for establishing the endocochlear potential and sustaining production of endolymph. Abnormal cochlear microcirculation has long been considered an etiologic factor in noise-induced hearing loss, age-related hearing loss (presbycusis), sudden hearing loss or vestibular function, and Meniere's disease. Knowledge of the mechanisms underlying the pathophysiology of cochlear microcirculation is of fundamental clinical importance. A better understanding of cochlear blood flow (CoBF) will enable more effective management of hearing disorders resulting from aberrant blood flow. This review focuses on recent discoveries and findings related to the physiopathology of the cochlear microvasculature.

Developmental profiling of spiral ganglion neurons reveals insights into auditory circuit assembly

The sense of hearing depends on the faithful transmission of sound information from the ear to the brain by spiral ganglion (SG) neurons. However, how SG neurons develop the connections and properties that underlie auditory processing is largely unknown. We catalogued gene expression in mouse SG neurons from embryonic day 12, when SG neurons first extend projections, up until postnatal day 15, after the onset of hearing. For comparison, we also analyzed the closely related vestibular ganglion (VG). Gene ontology analysis confirmed enriched expression of genes associated with gene regulation and neurite outgrowth at early stages, with the SG and VG often expressing different members of the same gene family. At later stages, the neurons transcribe more genes related to mature function, and exhibit a dramatic increase in immune gene expression. Comparisons of the two populations revealed enhanced expression of TGFβ pathway components in SG neurons and established new markers that consistently distinguish auditory and vestibular neurons. Unexpectedly, we found that Gata3, a transcription factor commonly associated with auditory development, is also expressed in VG neurons at early stages. We therefore defined new cohorts of transcription factors and axon guidance molecules that are uniquely expressed in SG neurons and may drive auditory-specific aspects of their differentiation and wiring. We show that one of these molecules, the receptor guanylyl cyclase Npr2, is required for bifurcation of the SG central axon. Hence, our dataset provides a useful resource for uncovering the molecular basis of specific auditory circuit assembly events.

Geranylgeranylacetone suppresses noise-induced expression of proinflammatory cytokines in the cochlea

OBJECTIVE

Heat shock transcription factor 1 (HSF1) is a master regulator of heat shock response, and also inhibits expression of inflammatory cytokines directly or indirectly. Here, we examined effects of HSF1 activation on the expression of proinflammatory cytokines in mouse cochlea after exposure to noise.

METHODS

Male CBA/N mice with normal Preyer's reflex were exposed to intense noise for 3h. Three hours after noise exposure, bilateral cochleae were removed and expression of major inflammatory cytokines was examined.

RESULTS

We found that interleukin-6 (IL-6) and interleukin-1β (IL-1β) expression increased significantly after noise exposure, and the expression was suppressed significantly in mice administered with geranylgeranylacetone (GGA), which activates HSF1. Seven days after noise exposure, thresholds for auditory brainstem response were elevated, and GGA administration significantly suppressed this elevation.

CONCLUSION

These results suggest that HSF1-mediated suppression of proinflammatory cytokines in the cochlea by GGA administration could be an important means of inner ear protection.