Regulation of the expression and subcellular localization of the taurine transporter TauT in mouse NIH3T3 fibroblasts.

The cellular level of the organic osmolyte taurine is a balance between active uptake and passive leak via a volume sensitive pathway. Here, we demonstrate that NIH3T3 mouse fibroblasts express a saturable, high affinity taurine transporter (TauT, Km = 18 microm), and that taurine uptake via TauT is a Na+- and Cl(-)-dependent process with an apparent 2.5 : 1 : 1 Na+/Cl-/taurine stoichiometry. Transport activity is reduced following acute administration of H2O2 or activators of protein kinases A or C. TauT transport activity, expression and nuclear localization are significantly increased upon serum starvation (24 h), exposure to tumour necrosis factor alpha (TNFalpha; 16 h), or hyperosmotic medium (24 h); conditions that are also associated with increased localization of TauT to the cytosolic network of microtubules. Conversely, transport activity, expression and nuclear localization of TauT are reduced in a reversible manner following long-term exposure (24 h) to high extracellular taurine concentration. In contrast to active taurine uptake, swelling-induced taurine release is significantly reduced following preincubation with TNFalpha (16 h) but unaffected by high extracellular taurine concentration (24 h). Thus, in NIH3T3 cells, (a) active taurine uptake reflects TauT expression; (b) TauT activity is modulated by multiple stimuli, both acutely, and at the level of TauT expression; (c) the subcellular localization of TauT is regulated; and (d) volume-sensitive taurine release is not mediated by TauT.