Aim: To investigate whether OATP isoforms may contribute to prevent accumulation of bile acids in hepatocytes during cholestasis.

Methods: Transporter expression in Xenopus laevis oocytes was used for functional studies. The absolute abundance of OATP-C/1B1 and OATP8/1B3 mRNA in liver biopsies was measured by real-time quantitative RT-PCR.

Results: Oocytes expressing human carboxyl-esterase-1 were loaded with cholic acid (CA)-methyl ester, which was cleaved to CA and then effluxed. Expression of OATP8/1B3 enhanced CA efflux, which was trans-activated by tauro-CA, but trans-inhibited by reduced (GSH) and oxidized (GSSG) glutathione. The bile acid uptake mediated by OATP8/1B3, but not OATP-C/1B1, was cis-stimulated by glutathione, but not by bicarbonate, glutamate or estradiol 17b-glucuronide. This stimulation involved GSH transport and was due to increased Vmax (Km was not significantly modified). Stoichiometry of OATP8/1B3-mediated cotransport was 2:1 (GSH/bile acid). Efflux of tauro-CA microinjected in oocytes expressing OATP8/1B3 was faster when these were loaded with GSH, whose efflux was also enhanced by OATP8/1B3. In human liver samples obtained from 42 patients with several cholestatic liver diseases the expression of OATP8/1B3 was not impaired. Approximately 5:1 ratio between mRNA levels of OATP-C/1B1 and OATP8/1B3 was found.

Conclusion: Although both OATP-C/1B1 and OATP8/1B3 are able to transport bile acids, their mechanism of action and physiological role are probably different. Whereas OATP-C/1B1 may be involved in uptake processes, OATP8/1B3 may mediate the extrusion across the basal membrane of organic anions by symport with glutathione, to prevent their intracellular accumulation when their vectorial traffic toward the bile is impaired.