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 polypeptid e; Ntcp) and a Na+-independent bromosulphophthalein (BSP)/bile acid up take system (organic-anion-transporting polypeptide; oatp) have been c loned from rat liver by using functional expression cloning in Xenopus laevis oocytes. To evaluate the extent to which these cloned transpor ters could account for overall hepatic bile acid and BSP uptake, we us ed antisense oligonucleotides to inhibit the expression of Ntcp and oa tp in Xenopus laevis oocytes injected with total rat liver mRNA. An Nt cp-specific antisense oligonucleotide co-injected with total rat liver mRNA blocked the expression of Na+-dependent taurocholate uptake by a pprox. 95 %. In contrast, an oatp-specific antisense oligonucleotide w hen co-injected with total rat liver mRNA had no effect on the express ion of Na+-dependent taurocholate uptake, but it blocked Na+-independe nt uptake of taurocholate by approx. 80 % and of BSP by 50 %. Assuming similar expression of hepatocellular bile acid and organic anion tran sporters in Xenopus laevis oocytes, these results indicate that Ntcp a nd oatp respectively represent the major, if not the only, Na+-depende nt and Na+-independent taurocholate uptake systems in rat liver. By co ntrast, the cloned oatp accounts for only half of BSP transport, sugge sting that there must be additional, non-bile acid transporting organi c anion uptake systems in rat liver.