Folic acid deficiency induces premature hearing loss through mechanisms involving cochlear oxidative stress and impairment of homocysteine metabolism

Nutritional imbalance is emerging as a causative factor of hearing loss (HL). Epidemiological studies have linked HL to elevated plasma homocysteine (pHcy) and folate deficiency, and showed that folate supplementation lowers pHcy levels potentially ameliorating age-related HL. The purpose of this study was to address the potential impact of folate deficiency in HL and to unveil the underlying mechanisms. For this purpose, two-month old C57BL/6J-mice (Animalia Chordata Mus musculus) were randomly divided in two groups (n=65 each) that were fed folate-deficient or standard diets for 8 weeks. HPLC analysis demonstrated 7-fold decline in serum folate and 3-fold increase in pHcy levels. Auditory brainstem recordings showed that only folate-deficient mice exhibited severe HL and cochlear TUNEL+-apoptotic cells. RTqPCR and Western-blotting showed reduced levels of enzymes involved in Hcy production and recycling, together with 30% increased protein homocysteinylation. Redox stress was evidenced by decreased expression of Cat, Gpx4 and Gss genes, increased levels of the proteins MnSOD and the NOX-complex adaptor p22phox, and elevated concentrations of glutathione species. Altogether, our findings show for the first time that the relationship between folate-induced hyperhomocysteinemia and premature HL involves impairment of cochlear Hcy metabolism and associated oxidative stress.