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Sensory hair cells of the inner ear are sensitive to death from aging, noise trauma, and ototoxic drugs. Ototoxic drugs include the aminoglycoside antibiotics and the antineoplastic agent cisplatin. Exposure to aminoglycosides results in hair cell death that is mediated by specific apoptotic proteins, including c-Jun N-terminal kinase (JNK) and caspases. Induction of heat shock proteins (Hsps) is a highly conserved stress response that can inhibit JNK- and caspase-dependent apoptosis in a variety of systems. We have previously shown that heat shock results in a robust upregulation of Hsps in the hair cells of the adult mouse utricle in vitro. In addition, heat shock results in significant inhibition of both cisplatin- and aminoglycoside-induced hair cell death. In our system, Hsp70 is the most strongly induced Hsp, which is upregulated over 250-fold at the level of mRNA 2 h after heat shock. Therefore, we have begun to examine the role of Hsp70 in mediating the protective effect of heat shock. To determine whether Hsp70 is necessary for the protective effect of heat shock against aminoglycoside-induced hair cell death, we utilized utricles from Hsp70.1/3 (-/-) mice. While heat shock inhibited gentamicin-induced hair cell death in wild-type utricles, utricles from Hsp70.1/3 (-/-) mice were not protected. In addition, we have examined the role of the major heat shock transcription factor, Hsf1, in mediating the protective effect of heat shock. Utricles from Hsf1 (-/-) mice and wild-type littermates were exposed to heat shock followed by gentamicin. The protective effect of heat shock on aminoglycoside-induced hair cell death was only observed in wild-type mice and not in Hsf1 (-/-) mice. To determine whether Hsp70 is sufficient to protect hair cells, we have utilized transgenic mice that constitutively overexpress Hsp70. Utricles from Hsp70-overexpressing mice and wild-type littermates were cultured in the presence of varying neomycin concentrations for 24 h. The Hsp70-overexpressing utricles were significantly protected against neomycin-induced hair cell death at moderate to high doses of neomycin. This protective effect was achieved without a heat shock. Taken together, these data indicate that Hsp70 and Hsf1 are each necessary for the protective effect of heat shock against aminoglycoside-induced death. Furthermore, overexpression of Hsp70 alone significantly inhibits aminoglycoside-induced hair cell death.

Hsp70 Inhibits Aminoglycoside-Induced Hair Cell Death and is Necessary for the Protective Effect of Heat Shock