Increased expression level of Hsp70 in the inner ears of mice by exposure to low frequency noise

Previous studies showed that people in urban areas are possibly exposed to 60-110 dB of low frequency noise (LFN) defined as noise of ≤100 Hz in their daily life. Previous studies also showed increased health risks by exposure to high levels (130-140 dB) of LFN in animals. However, little is known about the health effects of exposure to an ordinary level of LFN. We biochemically and immunohistochemically assessed the effects of exposure to inaudible LFN for mice (12 h/day of 100 Hz LFN at 95 dB for 5 days), at a level to which people are possibly exposed in daily life, on a murine inner ear by targeting 9 stress-reactive molecules. There was more than a 5-fold increased transcript level of heat shock protein 70 (Hsp70) in the whole inner ear exposed to LFN. However, the transcript levels of the other 8 stress-reactive molecules including Hsp27 and Hsp90 were comparable in LFN-exposed and unexposed murine inner ears. Only the transcript level of Cebpβ among the previously reported 4 transcriptional activators for Hsp70 expression was more than 3-fold increased by LFN exposure. Hsp70 transcript expression levels in the inner ears 3 days after LFN exposure were comparable to those in unexposed inner ears. The protein level of Hsp70, but not the levels of Hsp27 and Hsp90, was also increased in the vestibule by LFN exposure. However, hearing levels as well as expression levels of Hsp70 protein in the cochleae were comparable in LFN-exposed mice and unexposed mice. Our results demonstrated that the inner ear might be one of the organs that is negatively affected by stress from inaudible LFN exposure. Moreover, LFN exposure might increase Hsp70 expression level via Cebpβ in the inner ear. Thus, Hsp70 and Cebpβ levels could be candidates of biomarkers for response to LFN exposure.