Contribution of organic anion transporting polypeptides to bile acid uptake in the Caco-2 cell monolayer and gastrointestinal tract

The apical sodium-dependent bile acid transporter (ASBT, encoded by SLC10A2) is the sole transporter responsible for bile acid absorption in the human intestinal tract and is a therapeutic target for dyslipidemia. Here, we investigated the mechanisms of bile acid uptake in Caco-2 cells, a widely used in vitro model of intestinal absorption. Initial uptake assays revealed biphasic kinetics in bile acid uptake, suggesting the involvement of multiple transporters. At low substrate concentrations, uptake in Caco-2 cells showed marked sodium dependence, and the kinetic parameters were consistent with those observed in taurocholic acid (TC) uptake by ASBT-expressing cells, identifying ASBT-mediated TC uptake as one component of uptake. The second component showed pronounced pH dependence in Caco-2 monolayers, prompting us to focus on organic anion-transporting polypeptides (OATPs; SLCO family), which are pH-dependent transporters of amphiphilic substrates and mediate bile acid transport in other organs. To identify transporters beyond ASBT, we also examined human OATPs. OATP1A2 and OATP1B3 were identified as candidate transporters contributing to TC uptake in Caco-2 cell monolayers, and OATP1B3 was localized to the apical membrane. These findings indicate that bile acid uptake in Caco-2 cells is mediated by multiple transporters beyond ASBT, underscoring the need for cautious interpretation of bile acid transport studies using this model. Expression of OATP1B3 on the apical side of human colon-derived organoid monolayers, which exhibited TC uptake activity, suggests a potential role in colonic bile acid transport.