Functional expression cloning and characterization of the hepatocyte Na+/bile acid cotransport system.

TLDR

Liver parenchymal cells continuously extract high amounts of bile acids from portal blood plasma via a Na⁺/bile acid cotransport system. Sequence analysis of the cDNA revealed an open reading frame of 1086 nucleotides coding for a 362‑amino‑acid protein (~39 kDa) with five N‑linked glycosylation sites and seven putative transmembrane domains. A cDNA encoding the rat liver bile acid uptake system was isolated by expression cloning in Xenopus laevis oocytes; the transporter is strictly sodium‑dependent, glycosylated, has an apparent mass of 33–35 kDa, can be inhibited by various non‑bile‑acid organic compounds, and its mRNA is expressed in liver, kidney, intestine, and multiple mammalian species.

Abstract

Liver parenchymal cells continuously extract high amounts of bile acids from portal blood plasma. This uptake process is mediated by a Na+/bile acid cotransport system. A cDNA encoding the rat liver bile acid uptake system has been isolated by expression cloning in Xenopus laevis oocytes. The cloned transporter is strictly sodium-dependent and can be inhibited by various non-bile-acid organic compounds. Sequence analysis of the cDNA revealed an open reading frame of 1086 nucleotides coding for a protein of 362 amino acids (calculated molecular mass 39 kDa) with five possible N-linked glycosylation sites and seven putative transmembrane domains. Translation experiments in vitro and in oocytes indicate that the transporter is indeed glycosylated and that its polypeptide backbone has an apparent molecular mass of 33-35 kDa. Northern blot analysis with the cloned probe revealed crossreactivity with mRNA species from rat kidney and intestine as well as from liver tissues of mouse, guinea pig, rabbit, and man.