Dexamethasone protects organ of corti explants against tumor necrosis factor-alpha-induced loss of auditory hair cells and alters the expression levels of apoptosis-related genes.

Dinh CT, Haake S, Chen S, Hoang K, Nong E, Eshraghi AA, Balkany TJ, Van De Water TR
Neuroscience. 2008 157 (2): 405-13

PMID: 18838114 · DOI:10.1016/j.neuroscience.2008.09.012

OBJECTIVE - Determine the molecular mechanism(s) behind tumor necrosis factor-alpha (TNFalpha)-induced loss of auditory hair cells and the ability of dexamethasone base (DXMb) to protect against TNFalpha ototoxicity.

METHODS - Hair cell counts: Three-day-old rat organ of Corti explants were cultured under three different conditions: 1) untreated-control; 2) TNFalpha (2 mug/ml); and 3) TNFalpha (2 mug/ml)+DXMb (70 mug/ml) for 4 days, fixed, and stained with FITC-phalloidin. Hair cells were counted in the basal and middle turns. Gene expression: total RNA was extracted from the three different groups of explants at 0, 12, 24 and 48 h. Using quantitative real-time RT-PCR, mRNAs were transcribed into cDNAs and amplification was performed using primers for rat ss-actin (housekeeping gene), TNFR1, Bcl-2, Bax, and Bcl-xl.

RESULTS - DXMb protected explant hair cells from TNFalpha-induced loss. Bax gene expression was greater in TNFalpha-exposed explants compared with TNFalpha+DXMb-treated explants at 48 h (P=0.023), confirmed by the increase in the Bax/Bcl-2 ratio at 48 h (P<0.001). These results correlated with increased TNFR1 expression at 24 h (P=0.038). DXMb otoprotection in TNFalpha-exposed cultures was accompanied by an up-regulation of Bcl-xl at both the 24 (P<0.001) and 48 h time points (P=0.030) and up-regulation of Bcl-2 expression at 24 h (P=0.018). DXMb treatment also prevented increases in the expression levels of Bax, TNFR1, and the Bax/Bcl-2 ratio that occurred in untreated TNFalpha-exposed explants.

CONCLUSIONS - TNFalpha's ototoxicity may be mediated through an up-regulation of Bax and TNFR1 expression as well as an increase in the Bax/Bcl-2 ratio. DXMb protects the organ of Corti against TNFalpha ototoxicity by up-regulating Bcl-2 and Bcl-xl expression and by inhibiting TNFalpha-induced increases in Bax, TNFR1, and the Bax/Bcl-2 ratio. These results support the use of local dexamethasone treatment to conserve hearing following a trauma.

MeSH Terms (18)

Analysis of Variance Animals Animals, Newborn Apoptosis bcl-2-Associated X Protein bcl-X Protein Dexamethasone Gene Expression Glucocorticoids Hair Cells, Auditory Organ Culture Techniques Organ of Corti Proto-Oncogene Proteins c-bcl-2 Rats Rats, Sprague-Dawley Receptors, Tumor Necrosis Factor, Type I Time Factors Tumor Necrosis Factor-alpha

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