Intracellular glutathione regulates taurocholate transport in HepG2 cells

The hepatic organic anion transporter 1, Oatp1, was recently demonstrated to function as a GSH exchanger, indicating that hepatic uptake of drugs and xenobiotics may be sensitive to intracellular GSH levels. The present study characterized taurocholate uptake and efflux mechanisms in HepG2 cells and the effects of intracellular GSH on these transport processes. Taurocholate uptake into HepG2 cells was Na(+)-independent, saturable ( K(m) = 82 +/- 16 microM), and was cis-inhibited by bromosulfophthalein and some bile acids. Intracellular GSH depletion inhibited 3H-taurocholate uptake, and, conversely, the release of GSH from HepG2 cells was stimulated in the presence of extracellular taurocholate and other bile acids, consistent with a role for intracellular GSH in stimulating organic anion uptake. Interestingly, efflux of 3H-taurocholate from HepG2 cells was also sensitive to intracellular GSH concentration: efflux was inhibited in cells with lower intracellular GSH and stimulated in cells with higher GSH. RT-PCR analysis revealed that OATP-A, OATP-D, OATP-E, OATP-8, MRP1, MRP2, and MRP3 are expressed in HepG2 cells but that their expression is not altered by the maneuvers used to lower or raise intracellular GSH. These results provide direct evidence that intracellular GSH levels modulate both uptake and efflux of taurocholate and suggest that GSH plays a regulatory role in the hepatobiliary transport of potentially toxic organic compounds.