Relationship between the microsomal epoxide hydrolase and the hepatocellular transport of bile acids and xenobiotics

Recently two different bile-acid carriers for the hepatocellular sodium-dependent uptake of taurocholate have been described. The first transport system was isolated and characterized by functional expression cloning in Xenopus laevis oocytes. The corresponding cDNA clone, named Ntcp for Na+/taurocholate co-transporting polypeptide, codes for a protein of 362 amino acids and shows no similarity to previously known sequences. The transport function of this carrier system is well documented by expression in Xenopus laevis oocytes and by transient and stably transfected cell lines. In addition, several lines of evidence implied that the well-known xenobiotic-metabolizing enzyme microsomal epoxide hydrolase (mEH, EC 3.3.2.3) is also able to mediate sinusoidal uptake of taurocholate. Furthermore, it was claimed that the same enzyme also mediates the uptake of the conjugated bile acid into the smooth endoplasmic reticulum (ER). No direct proof of the transport function of mEH by its heterologous expression has yet been published. In the present work we used a stable transfected cell line that expressed high levels of heterologous mEH for uptake studies of various bile acids and the loop diuretic bumetanide. The uptake of the conjugated bile acid taurocholate, of the non-conjugated bile acid cholate and of the organic anion bumetanide was measured in the transfected as well as in the non-transfected parental cell line. These organic anions represent the main substrates of the known transport systems for organic anions in the rat liver. The results show that the microsomal epoxide hydrolase is unable to transport taurocholate, cholate or bumetanide. Furthermore, Western-blot analysis revealed the expression of mEH in hepatoma tumor cell lines, which show no transport activity for these organic anions. These results show that it is unlikely that mEH can mediate the transport of these substrates.