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Osmosensitive Taurine Transporter Expression and Activity in Human Corneal Epithelial Cells

¹ From the Department of Integrated Biosciences, Graduate School of Frontier Sciences, University of Tokyo, Kashiwa, Chiba, Japan; and ² State University of New York, College of Optometry, Department of Biological Sciences, New York, New York.

PURPOSE. To characterize in SV40-immortalized human corneal epithelial cells (tHCEC) osmosensitive taurine transporter gene and protein expression as well as its functional activity. To evaluate whether medium supplementation with taurine improves cell viability during a hypertonic challenge.

METHODS. tHCEC were preincubated for up to 48 hours in hypertonic DMEM medium (i.e., up to 500 mosmol/kg). Taurine uptake was monitored through measurements of intracellular [³H]taurine accumulation. Gene and protein expression was detected by Northern and Western blot analyses, respectively. An amino acid analyzer measured intracellular cold taurine content. The live/dead assay evaluated with confocal microscopy determined cell viability.

RESULTS. Na⁺-dependent taurine uptake occurred in an isotonic (310 mosmol/kg) medium. The apparent Michaelis-Menten constant, K_t, for taurine was 4.6 µM, and uptake increased as a function of exposure time and rises in osmolality. Exposure for 12 hours to a 450 mosmol/kg medium increased uptake by 4.1-fold. However, after 48 hours of exposure to this medium, taurine uptake returned to its isotonic level. With time, biphasic changes occurred in taurine transporter gene and protein expression and taurine transport activity as well as elevating intracellular taurine content by 4.5-fold. Taurine medium supplementation for 48 hours improved cell viability.

CONCLUSIONS. tHCEC express Na⁺-dependent osmosensitive taurine transport activity. The hypertonic-induced biphasic effects on gene and protein expression as well as transport activity suggest feedback regulation of these responses. Rises in intracellular taurine do not appear to be essential for osmoregulation, but instead enhance cell survival perhaps through a membrane stabilizer or an antioxidant effect.

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