Effect of antisense oligonucleotides on the expression of hepatocellular bile acid and organic anion uptake systems in Xenopus laevis oocytes

A Na(+)-dependent bile acid (Na+/taurocholate co-transporting polypeptide; Ntcp) and a Na(+)-independent bromosulphophthalein (BSP)/bile acid uptake system (organic-anion-transporting polypeptide; oatp) have been cloned from rat liver by using functional expression cloning in Xenopus laevis oocytes. To evaluate the extent to which these cloned transporters could account for overall hepatic bile acid and BSP uptake, we used antisense oligonucleotides to inhibit the expression of Ntcp and oatp in Xenopus laevis oocytes injected with total rat liver mRNA. An Ntcp-specific antisense oligonucleotide co-injected with total rat liver mRNA blocked the expression of Na(+)-dependent taurocholate uptake by approx. 95%. In contrast, an oatp-specific antisense oligonucleotide when co-injected with total rat liver mRNA had no effect on the expression of Na(+)-dependent taurocholate uptake, but it blocked Na(+)-independent uptake of taurocholate by approx. 80% and of BSP by 50%. Assuming similar expression of hepatocellular bile acid and organic anion transporters in Xenopus laevis oocytes, these results indicate that Ntcp and oatp respectively represent the major, if not the only, Na(+)-dependent and Na(+)-independent taurocholate uptake systems in rat liver. By contrast, the cloned oatp accounts for only half of BSP transport, suggesting that there must be additional, non-bile acid transporting organic anion uptake systems in rat liver.