Farnesol-Induced Apoptosis in Oral Squamous Carcinoma Cells is Mediated by MRP1 Extrusion and Depletion of Intracellular Glutathione

Abstract

Farnesol is a key intermediate in the sterol biosynthesis pathway in eukaryotic cells that has exhibited significant anti-cancer and antimicrobial activity. We have shown that farnesol triggers apoptosis in oral squamous carcinoma cells (OSCCs) and in the fungal pathogen Candida albicans via a classical apoptotic process. However, the exact mechanism of farnesol cytotoxicity in eukaryotic cells has not been fully elucidated. In the cell, hydrophobic xenobiotics conjugate with glutathione, an antioxidant crucial for cellular detoxification against damaging compounds. This process results in the formation of glutathione S-conjugates which act as substrates for export by ATP-binding cassette transporters (ABC transporter) and are extruded from the cell. This study was undertaken to validate the hypothesis that farnesol conjugation with intracellular glutathione coupled with multidrug resistance-associated protein 1 (MRP1) - mediated extrusion of glutathione-farnesol conjugates and oxidized glutathione results in total glutathione depletion, oxidative stress and ultimately cell death. The combined findings demonstrated that farnesol exposure resulted in significant decrease in intracellular glutathione levels concomitant with intracellular Reactive oxygen species (ROS) accumulation and decrease in cell viability. However, addition of exogenous glutathione maintained intracellular levels and enhanced cell viability. Furthermore, gene and protein expression studies demonstrated significant up-regulation of MRP1 in cells treated with farnesol. However, MRP1 blocking and monoclonal antibody specific inhibition of MRP1 enhanced cell tolerance to farnesol. This is the first study describing the involvement of MRP1-mediated glutathione efflux as a mechanism for farnesol-induced apoptosis in OSCCs. Understanding of the mechanisms underlying farnesol-cytotoxicity in eukaryotic cells may lead to the development of this redox-cycling agent as an alternative chemotherapeutic target.