Downregulation of the taurine transporter TauT during hypo-osmotic stress in NIH3T3 mouse fibroblasts.
Bottom Line: Taurine influx is reduced following reduction in osmolarity, keeping the extracellular Na(+) concentration constant.TonEBP activity is unaltered, whereas TauT transcription as well as TauT activity are significantly reduced under hypo-osmotic conditions.Acute exposure to ROS reduces taurine uptake as a result of modulated TauT transport kinetics.
Affiliation: Department of Biology, Section of Cellular and Developmental Biology, University of Copenhagen, The August Krogh Building, Universitetsparken 13, 2100 Copenhagen Ø, Denmark.Show MeSH
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Mentions: Taurine uptake in NIH3T3 mouse fibroblasts was previously shown to be totally Na+-dependent and eliminated in the presence of the taurine analog β-alanine (Voss et al. 2004), indicating that taurine uptake in the fibroblasts is mediated by TauT. From Fig. 1a, b it is seen that reduction in the total extracellular Na+ concentration from 150 to 80 mM for 4 h, keeping osmolarity constant with sucrose, results in a significant reduction in the taurine influx in NIH3T3 cells to 56% of the iso-osmotic value (compare “iso-osmotic” to “low Na+ iso-osmotic”). From Fig. 1a, b it is also seen that reduction in the extracellular osmolarity from 335 to 200 mOsm, keeping the extracellular Na+ concentration constant at 80 mM, leads to an additional 50% reduction in taurine uptake (compare “low Na+ iso-osmotic” to “low Na+ hypo-osmotic”). Hence, TauT activity is reduced by 4 h exposure to hypo-osmotic conditions due to reduction in the extracellular Na+ concentration as well as reduction in the extracellular osmolarity, i.e., osmotic cell swelling. This is similar to observations in Ehrlich ascites tumor cells (Hoffmann and Lambert 1983). To test whether reduced expression or membrane localization of TauT is responsible for the decreased taurine uptake under long-term hypo-osmotic conditions (4 h), we compared taurine uptake in cells exposed to 4 h reduction in the extracellular osmolarity with cells exposed acutely to hypo-osmotic conditions. From Fig. 1b it is seen that taurine uptake is reduced to the same extent following 4 h and acute reduction in the extracellular Na+ concentration (compare dark gray bars at 4 h and acute) and in extracellular osmolarity (compare light gray bars at 4 h and acute). As acute and 4 h reduction in Na+ and osmolarity give the same reduction in influx, it is suggested that the reduction in taurine uptake is most likely caused by direct inhibition of TauT. Similarly, it was previously shown in Ehrlich ascites cells that the regulation of the activity of another osmoregulatory transporter, NKCC1, by changes in osmolarity is not related to the number of transport molecules present in the membrane (Hoffmann et al. 1986).Fig. 1
Affiliation: Department of Biology, Section of Cellular and Developmental Biology, University of Copenhagen, The August Krogh Building, Universitetsparken 13, 2100 Copenhagen Ø, Denmark.