OATP8/1B3-mediated cotransport of bile acids and glutathione: an export pathway for organic anions from hepatocytes?
In cholestasis, the accumulation of organic anions in hepatocytes is reduced by transporters (multidrug resistance-associated proteins and OSTalpha-OSTbeta) able to extrude them across the basolateral membrane. Here we investigated whether organic anion-transporting polypeptides (OATPs) may contribute to this function. Xenopus laevis oocytes expressing human carboxylesterase-1 efficiently loaded cholic acid (CA) methyl ester, which was cleaved to CA and exported. Expression of OATP8/1B3 enhanced CA efflux, which was trans-activated by taurocholate but trans-inhibited by reduced (GSH) and oxidized (GSSG) glutathione. Moreover, taurocholate and estradiol 17beta-D-glucuronide, but not bicarbonate and glutamate, cis-inhibited OATP8/1B3-mediated bile acid transport, whereas glutathione cis-stimulated this process, which involved the transport of glutathione itself with a stoichiometry of 2:1 (GSH/bile acid). No cis-activation by glutathione of OATP-C/1B1 was found. Using real time quantitative reverse transcription-PCR, the absolute abundance of OATP-A/1A2, OATP-C/1B1, and OATP8/1B3 mRNA in human liver biopsies was measured. In healthy liver, expression levels of OATP-C/1B1 were approximately 5-fold those of OATP8/1B3 and >100-fold those of OATP-A/1A2. This situation was not substantially modified in several cholestatic liver diseases studied here. In conclusion, although both OATP-C/1B1 and OATP8/1B3 are highly expressed, and able to transport bile acids, their mechanisms of action are different. OATP-C/1B1 may be involved in uptake processes, whereas OATP8/1B3 may mediate the extrusion of organic anions by symporting with glutathione as a normal route of exporting metabolites produced by hepatocytes or preventing their intracellular accumulation when their vectorial traffic toward the bile is impaired.
Briz
Oscar
O
Research Unit, University Hospital, Salamanca, Spain.
Romero
Marta R
MR
Martinez-Becerra
Pablo
P
Macias
Rocio I R
RI
Perez
Maria J
MJ
Jimenez
Felipe
F
San Martin
Francisco G
FG
Marin
Jose J G
JJ
D000818:Animals; D001647:Bile Acids and Salts / Q000378:metabolism*; D001692:Biological Transport; D002265:Carboxylic Ester Hydrolases / Q000096:biosynthesis; D020355:Cholates / Q000378:metabolism; D004958:Estradiol / Q000031:analogs & derivatives / Q000737:chemistry; D005978:Glutathione / Q000378:metabolism*; D022781:Hepatocytes / Q000378:metabolism*; D006801:Humans; D009865:Oocytes / Q000378:metabolism; D029362:Organic Anion Transporters, Sodium-Independent / Q000378:metabolism / Q000502:physiology*; D051381:Rats; D000074542:Solute Carrier Organic Anion Transporter Family Member 1B3; D013656:Taurocholic Acid / Q000494:pharmacology; D014982:Xenopus laevis
D001647:Bile Acids and Salts; D020355:Cholates; D029362:Organic Anion Transporters, Sodium-Independent; C421209:SLCO1B3 protein, human; D000074542:Solute Carrier Organic Anion Transporter Family Member 1B3; C025483:estradiol-17 beta-glucuronide; D004958:Estradiol; D013656:Taurocholic Acid; C037034:methyl cholate; D002265:Carboxylic Ester Hydrolases; C474609:CES1 protein, human; D005978:Glutathione