Effect of dietary iron overload on electromotility of outer hair cells in male mice

Objective To observe the role of dietary iron overload-induced insulin resistance in hearing function and cochlear outer hair cell electromotility in male mice.Methods Twelve C57Bl/6 mice were randomly assigned to an iron overload group fed with iron-enriched diet(iron content:8.39±1.03 g/kg feed)and a control group with normal diet (iron content:8.26±2.67 mg/kg feed) according to their body weight.Blood glucose levels of the mice were measured at baseline and 4,8,12 weeks after the treatment.Hemoglobin,serum iron,ferritin,total iron binding capacity,and transferrin saturation were determined at 16 weeks after the treatment;then glucose tolerance test,insulin tolerance test,distortion product otoacoustic emission (DPOAE) task were performed.The basilar membrane of left and right cochlea of the mice were separated for the detections of prestin distribution in cochlear outer hair cells with immumofluorescence method and prestin expression with Western blot,respectively.Results Compared to the mice of the control group,the mice of the iron overload group showed significantly increased serum iron (64.34±9.01 mol/L),serum ferritin (166.03±9.16 ng/mL),transferrin saturation (75.72±6.12%),blood glucose (176.3±20.6 mg/dl),and homeostasis model assessment-insulin resistance (HOMA-IR) index (0.89±0.43)(P < 0.05 for all) and significantly decreased DPOAE threshold shifts (13.75±4.10 dB,19.52±4.36 dB,14.49±4.95 dB at 1,2,4 kHz respectively)(P < 0.05).The prestin was distributed in membrane and some parts of cytoplasm and significantly up-regulated in cochlear outer hair cells of the iron overload mice compared to that of the control mice(P < 0.01).Conclusion Dietary iron overload could induce insulin resistance and result in increased expression of prestin in cochlear outer hair cells,which in turn disturbs DPOAE leading to hearing impairment in mice.