Folic acid improves inner ear vascularization in hyperhomocysteinemic mice

Abstract

More than 29 million adults in the United States have been diagnosed with hearing loss. Interestingly, elevated homocysteine (Hcy) levels, known as hyperhomocysteinemia (HHcy) is also associated with impaired hearing. However, the associated mechanism remains obscure. The collagen receptor such as discoidin domain receptor 1 and matrix metalloproteinase (MMP) play a significant role in inner ear structure and function. We hypothesize that HHcy increases hearing thresholds by compromise in inner ear vasculature resulted from impaired Hcy metabolism, increased oxidative stress, collagen IVa and collagen la turnover. The treatment with folic acid (FA) protects elevated hearing thresholds and prevents reduction in vessel density by lowering abundant collagen deposition and oxidative stress in inner ear. To test this hypothesis we employed 8 weeks old male wild type (WT), cystathionine-beta-synthase heterozygote knockout (CBS+/−) mice, WT+FA (0.0057 μg/g/day, equivalent to a 400 μg/70 kg/day human dose in drinking water); and CBS(+/−)+FA. The mice were treated for four weeks. The hearing thresholds were determined by recording the auditory brainstem responses. Integrity of vessels was analyzed by perfusion of horseradish peroxidase (HRP) tracer. Endothelial permeability was assessed, which indicated restoration of HRP leakage by FA treatment. A total Hcy level was increased in stria vascularis (SV) and spiral ligament (SL) of CBS+/− mice which was lowered by FA. Interestingly, FA treatment lowered Col IVa Immunostaining by affecting its turnover. The levels of MMP-2, -9, methylenetetrahydrofolate reductase (MTHFR) and cystathione gamma lyase (CSE) were measured by Western blot analysis. The oxidative stress was high in SV and SL of CBS+/− compared to WT however the treatment with FA lowered oxidative stress in CBS+/− mice. These data suggested that hearing loss in CBS+/− mice was primarily due to leakage in inner ear circulation, also partly by induced collagen imbalance, increase in Hey and oxidative stress in inner ear.

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

“…CBS and cystathionase are able to synthesize H 2 S, whose protective role as cochlear vasodilator has been reported in noise‐induced hearing loss (53). A reduced production of H 2 S may contribute to the changes detected in striatal capillaries in FD mice, in concordance with the decreased vessel density previously reported for Cbs +/‐ mice cochleae (24).…”

“…Animal models have generated data that, collectively taken, support a central role for Hcy metabolism in hearing pathophysiology. For example, Cbs +/‐ mutant mice have increased hearing thresholds, high concentrations of Hcy in the stria vascularis and the spiral ligament, oxidative stress, and reduction of vessel density, effects that were prevented by the administration of folic acid in the drinking water (24). Hearing impairment has been also described in Connexin 30 ( Cx30 ‐/‐ ) null mice, where the stria vascularis exhibited down‐ and up‐regulation of Bhmt and Ahcy expression, respectively, and elevated levels of Hcy immunostaining (25).…”

Erythrocyte folate, serum vitamin B12, and hearing loss in the 2003-2004 National Health And Nutrition Examination Survey (NHANES)

“… 14 These results are also consistent with protective effects of folate on hearing and cochlear vascularization that have been observed in animal studies. 15 , 25 , 26 …”

“…Secondly, most studies that included biomarkers had small sample sizes, 22 , 28 did not include minorities or individuals in middle ages, 22 and were mainly conducted before initiation of fortification of foods with folic acid, which began in 1998 in the US. 25 , 26 Studies that include various population subgroups and use more robust measures of nutrient status are needed to further understand whether folate and vitamin B12 are associated with hearing loss in the general population where food fortification with these nutrients has already reached a steady state. As shown in different studies, tissue levels of folate and B12 are good biomarkers of intake 29 , 30 and because erythrocytes live for about 120 days, folate measures in these cells are preferred to serum measures.…”

Involvement of Oxidative Stress, Inflammation, and Glutamate in Ototoxicity, and their Attenuation by Simultaneous Activation of Nrf2 and Elevation of Antioxidant Compounds

“…Folic acid deficiency induced premature hearing loss by increasing the levels of oxidative stress and homocysteine in mice [91]. Indeed, supplementation with folic acid reduced hearing disorders by reducing oxidative stress and homocysteine levels [92].…”