Functional expression of organic anion transporters in hepatic organoids reconstructed by rat small hepatocytes

Small hepatocytes (SHs) are hepatic progenitor cells with hepatic characteristics. They can proliferate to form colonies in culture and change their morphology from flat to rising/piled-up with bile canaliculi (BC), which results in maturation. In this study, we examined whether SHs could express hepatic transporters with polarity, whether the transporters could transport organic anion substrates into BC, and whether the secreted substances could be recovered from BC. Immunocytochemistry and RT-PCR were carried out. [(3)H]-labeled estrogen derivatives were used to measure the functions of the transporters in SHs isolated from normal and multidrug resistance-associated protein (Mrp) 2-deficient rats. The results showed that organic anion-transporting proteins (Oatps) 1 and 2, Na(+)-dependent taurocholate co-transporting polypeptide (Ntcp), Mrp2, and bile-salt export pump (Bsep) were well expressed in rising/piled-up cells and that their expression was correlated to that of hepatocyte nuclear factor 4alpha. Although small SHs expressed not Oatps and Mrp2 but Mrp3, rising/piled-up SHs expressed Oatp1 and 2 and Mrp2 proteins in the sinusoidal and BC membranes, respectively. On the other hand, breast cancer resistant protein (Bcrp) and Mrp3 expression decreased as SHs matured. The substrate transported via Oatps and Mrp2 was secreted into BC and it accumulated in both BC and cyst-like structures. The secreted substrate could be efficiently recovered from BC reconstructed by SHs derived from a normal rat, but not from an Mrp2-deficient rat. In conclusion, SHs can reconstitute hepatic organoids expressing functional organic anion transporters in culture. This culture system may be useful to analyze the metabolism and excretion mechanisms of drugs.