Vasoactive intestinal peptide receptor-1 (VPAC1) is the high-affinity receptor of vasoactive intestinal peptide (VIP), a major regulator of bile secretion. To better define the level at which VPAC1 stimulates bile secretion, we examined its expression in the different cell types participating in bile formation (i.e., hepatocytes, bile duct, and gallbladder epithelial cells). Because VPAC1 expression was previously shown to be regulated by nuclear receptors, we tested the hypothesis that it may be regulated by the farnesoid X receptor (FXR). Quantitative RT-PCR and immunoblot analyses of cell isolates indicated that VPAC1 is expressed in all three cell types lining the human biliary tree, with predominant expression in the gallbladder. In primary cultures of human gallbladder epithelial cells, VIP induced cAMP production and chloride secretion. Analysis of the VPAC1 gene revealed the presence of potential FXR response element sequences, and both FXR and RXR␣ expressions were detected in gallbladder epithelial cells. In these cells, the FXR pharmacological agonist GW4064 upregulated VPAC1 expression in a dose-dependent manner, and this effect was antagonized by the RXR␣ ligand, 9-cis retinoic acid. T he regulation of bile secretion occurs at different levels of the biliary tree. Bile is formed primarily in hepatocyte canaculi by an osmotic process resulting from active bile salt secretion. 1 In bile ducts and in the gallbladder, bile is then modified by the absorption or secretion of water and ions. 2,3 Thus, bile delivered to the intestine results from vectorial transport occurring in the different epithelial cell types lining the biliary tree, i.e., hepatocytes, intrahepatic bile duct, and gallbladder epithelial cells. Among regulatory peptides, the vasoactive intestinal peptide (VIP) induces bile secretion by stimulating transport activities both in hepatocytes and in biliary epithelial cells. In hepatocytes, VIP induces an increase in bile salt-dependent bile secretion. 4 In bile ducts, VIP stimulates a bicarbonaterich secretion with higher potency than all other secretagogues, including secretin. 4 In the gallbladder, VIP is also a potent inducer of fluid and anion secretion. 5,6 In most systems, VIP effects are mediated by cyclic adenosine monophosphate (cAMP) production after the activation of highor low-affinity receptors, namely, the vasoactive intestinal peptide receptor-1 (VPAC1) and the vasoactive intestinal peptide receptor-2 (VPAC2), respectively. 7,8 Among these two types of VIP receptors, only VPAC1 is expressed in the liver. 7 In cancer cell lines, VPAC1 gene transcription is regulated by members of the nuclear receptor superfamily. [9][10][11]