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Gibran Farook Butt, Abubakar J Habib, Benjamin M Davis, Shereen Nizari, Joana Margarida Galvao, Mark Tilley, Eduardo Maria Normando, Li Guo, M Francesca Cordeiro, Glaucoma & Retinal Neurodegnrtn Res Grp; Vitamin E - TPGS inhibits P-glycoprotein in retinal cells through modulation of the membrane dipole potential - more harm than good? An in-vitro evaluation. Invest. Ophthalmol. Vis. Sci. 2014;55(13):1709.
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Vitamin E -TPGS (TPGS) is an excipient used to solubilize lipophilic molecules and improve bioavailability, the latter achieved through inhibition of the multidrug efflux channel P-glycoprotein (P-gp). In RGC-5 cells, we investigated whether TPGS (TPGS) inhibits P-glycoprotein activity through modulation of the cell membrane dipole potential. The effect of TPGS on cell viability was also investigated, when it is co-administered with Dimethyl sulfoxide (DMSO) a cellular insult. Finally, we investigated whether the administration of CoQ10 affected cell viability where TPGS and DMSO were co-administered.
All experiments were performed in the RGC-5 cell line. P-gp activity was assessed using the lipophilic probe and P-gp substrate Calcein AM. Changes in membrane dipole potential were detected through wavelength ratiometric fluorescence measurements using the electrochromic probe Di-8-anepps. The MTT assay was used to measure cell viability.
TPGS was found to significantly inhibit RGC-5 P-gp activity in a dose dependent manner, EC50 3.748 μg/ml TPGS vs 0.47 μg/ml verapamil (control inhibitor). The presence of TPGS altered the membrane dipole potential, with results suggesting saturation of this effect at concentrations as low as 10 μg/ml. DMSO reduced cell viability in a dose dependent manner. Co-administration of DMSO and TPGS to RGC-5s lead to significantly greater reduction in cell viability compared to DMSO alone. In cell cultures treated with CoQ10/TPGS and DMSO the cell viability was greater in a fixed concentration range compared to TPGS and DMSO alone.
DMSO is known to inhibit the antioxidant glutathione, which is located in cell mitochondria. This leads to an increase in cytotoxic compounds such as reactive oxygen species (ROS) and consequently oxidized lipids. P-gp is responsible for the removal of oxidized lipids and cytotoxic drug molecules from the cell and is located in the plasma and mitochondrial membranes. The inhibition of P-gp is shown here to decrease cell viability. The anti oxidant CoQ10 demonstrates a neuroprotective effect in this context, proposed to take effect in two possible ways: by reducing ROS; and by minimizing mitochondrial dysfunction.
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