Effect And Mechanism Of C-myc Gene On The Cochleae In Guinea Pigs With Noise Induced Hearing Loss

【Background】With industrialization of our society, acoustic trauma is becoming an important public health problem. Excessive noise exposure inflicts both mechanical and metabolic damage to the inner ear, which has a significant impact on the quality of life and may lead to noise induced hearing loss (NIHL). In humans, loss of cochlear hair cells is permanent, because the auditory cells are considered as terminal cells, without the ability of regeneration. One strategy for preventing NIHL might be to develop therapeutics that block the cellular signaling leading to cell death following trauma. Therefore, manipulating related molecules in mammalian cochleae could provide a new avenue to restore hearing.The proto-oncogene c-myc, an immediate-early gene, encodes a nuclear protein that is known to be a major regulator for the G1 to S transition. It is genenrally accepted that this gene has a major role in multiple processes, including embryonic development, cell proliferation, differentiation, and apoptosis. In addition, it was reported that c-myc might recruit transcription factors when quiescent cells were stimulated to reenter the cell cycle. However, the funtion of c-myc gene in the mammalian cochlea is still unclear. Therefore, it may provide insights into the regeneration of hair cells in mammalian. It is necessary to clarify the function of this gene in the cochlea.【Aims】The study was designed to investigate the function and mechanisms of c-myc gene under noise condition, from the aspect of cochlear morphological changes and auditory function.【Methods】We had established the adenoviral vector of c-myc gene (Ad.c-myc-EGFP), and western blot and frozen section were used to detecte the expression of c-Myc in inner ear. Four days before noise exposure, the adenovirus suspension was delivered into the scala tympani of guinea pigs. The empty adenoviral vector (Ad. EGFP) was injected as control. Then, all subjects were exposed to 110 dB SPL white noise, mainly concentrated on about 4-kHz frequency, for 8 h/day, 7 days consecutively. Auditory thresholds were assessed by auditory brainstem response (ABR), prior to and 7 days following noise exposure. On the seventh days after noise exposure, the cochlear sensory epithelia surface was observed microscopically and the cochleae were taken to study the ultrastructural changes. Then, microarray and bioinformatics were used to investigate the mechanisms underlying this gene's role on noise induced hearing loss, and real-time PCR was used to identify the results of genechip. Tunel staining was used to clarify the role of apoptosis in NIHL. The expression and location of significant genes in the cochlea were examined by immunohistochemistry. Thus, we speculated the molecular mechanism of c-myc gene in the cochlea under noisy condition.【Results】We had successfully established the adenoviral vector of c-myc gene, and it could efficiently upregulate the expression of c-Myc protein. The roles of c-myc gene in the cochlea indicated that auditory threshold shift after noise exposure was higher in the ears treated with Ad.EGFP than that treated with Ad.c-myc-EGFP. Stereocilia loss and the disarrangement of outer hair cells were observed, with greater changes found in the Ad.EGFP group. Aslo, the ultrastructure changes were severe in the Ad. EGFP group, but were not obvious in the Ad.c-myc-EGFP group. As the results of microarray showed, noise might mediate 539 kinds of genes, including 279 kinds of genes significantly upregulated and 260 kinds of genes significantly downregulated. These genes mainly involved in immune system, stress, homeostatic process, rhythm and apoptosis. As identified by real time PCR, noise may upregulate Reg3b,Lcn2,Nob1,Serpina3n,Hamp,Lbp,C3 genes, and downregulated Myh4,F2 genes. The result of tunel staining indicated noise could induce apoptosis of IHCs, OHCs and SGCs. Therefore, we speculated the roles of c-myc gene in the guinea pig's cochlea maybe due to upregulate protective genes or downregulate apoptosis related genes. However, the definite mechnisims had yet to be further clarified. Especially, NOB1 expression was detected mainly in spiral ganglion cells (SGCs), inner hair cells (IHCs) and outer hair cells (OHCs) after noise exposure, but the expression of NOB1 was not detected in normal ones. We speculated that nob1 gene as a transcription factor is likely involved in the regulation of genes within the cochlea, and it is a new regulatory gene of inner ear, its roles need further investigation. 【Conclusions】c-Myc might play an unexpected protective roles in hearing functional and morphological protection from acoustic trauma, through regulation of Reg3b,Lcn2,Nob1,Serpina3n,Hamp,Lbp,C3,Myh4,F2 and apoptosis related genes.