Molecular and functional characterization of bile acid transport in human hepatoblastoma HepG2 cells

Bile acids are taken up into human liver by Na+-dependent and Na+-independent transport mechanisms. In hepatocarcinogenesis, numerous liver-specific functions are lost and the uptake of organic anions is markedly reduced. We have investigated the molecular and functional derangements of bile acid transport in the human hepatoblastoma cell line HepG2. Uptake of [3H]-taurocholic acid was saturable and entirely Na+ independent, with the kinetic characteristics of the human liver organic anion transporting polypeptide (OATP). OATP, but not the Na+-dependent bile acid transporter (Na+-taurocholate-cotransporting polypeptide [NTCP]), was detectable by reverse-transcription polymerase chain reaction (RT-PCR) analysis of HepG2 RNA. The level of OATP expression in HepG2 cells was determined by Northern blot analysis and was found to be 40% in comparison with normal liver. Transfection of an OATP-derived phosphorothioate (PTO)-antisense oligonucleotide into HepG2 cells resulted in 77% inhibition of temperature-dependent bile acid uptake. Injection of HepG2 messenger RNA (mRNA) into Xenopus laevis oocytes significantly stimulated Na+-independent taurocholate uptake, indicating the expression of a bile acid transport protein. We conclude that bile acid uptake into human hepatoblastoma HepG2 cells is mediated by the multi-specific organic anion transporting polypeptide OATP. Therapeutic strategies employing bile acid-derived cytostatic agents for the treatment of hepatocellular carcinomas may therefore depend upon the expression of the Na+-independent bile acid transporter OATP in hepatic malignancies.