Cochlear detoxification: Role of alpha class glutathione transferases in protection against oxidative lipid damage, ototoxicity, and cochlear aging

Age-related hearing loss (AHL) is the most common form of hearing impairment. AHL is thought to be a multifactorial condition resulting from the interaction of numerous causes including aging, genetics, exposure to noise, and exposure to endogenous and exogenous toxins. Cells possess many detoxification enzymes capable of removing thousands of cytotoxic xenobiotics and endogenous toxins such as 4-hydroxynonenal (4-HNE), one of the most abundant cytotoxic end products of lipid peroxidation. The cellular detoxification system involves three phases of enzymatic detoxification. Of these, the glutathione transferase (GST) detoxification system converts a toxic compound into a less toxic form by conjugating the toxic compound to reduced glutathione by GST enzymes. In this review, we describe the current understanding of the cochlear detoxification system and examine the growing link between GST detoxification, oxidative lipid damage, ototoxicity, and cochlear aging with a particular focus on the alpha-class GSTs (GSTAs). We also describe how exposure to ototoxic drugs, exposure to noise, or aging results in increased 4-HNE levels, how 4-HNE damages various cell components under stress conditions, and how GSTAs detoxify 4-HNE in the auditory system.