Ileal Bile Acid-binding Protein, Functionally Associated with the Farnesoid X Receptor or the Ileal Bile Acid Transporter, Regulates Bile Acid Activity in the Small Intestine

Abstract

Bile acids secreted in the small intestine are reabsorbed in the ileum where they activate the nuclear farnesoid X receptor (FXR), which in turn stimulates expression of the ileal bile acid-binding protein (I-BABP). We first hypothesized that I-BABP may negatively regulate the FXR activity by competing for the ligands, bile acids. Reporter assays using stable HEK293 cell lines expressing I-BABP revealed that I-BABP enhances rather than attenuates FXR activity. In these cells I-BABP localizes predominantly in the cytosol and partially in the nucleus, a distribution that does not shift in response to FXR expression. In vitro binding assays reveal that recombinant I-BABP is able to bind 35 S-labeled FXR and that chenodeoxycholic acid (CDCA) stimulates this interaction modestly. When FLAG-tagged FXR was expressed in stable cells, the FXR⅐I-BABP complex in the nuclear extracts was more efficiently immunoprecipitable with anti-FLAG antibodies in the presence of CDCA. These results indicate that I-BABP stimulates FXR activity through a mutual interaction augmented by bile acids. When stable cells were transfected with an expression plasmid of the ileal bile acid transporter (IBAT) essential for the reabsorption of conjugated bile acids, the 14 C-labeled conjugated bile acid, glycocholic acid, was more efficiently imported via IBAT in the presence than absence of I-BABP, whereas no change was observed in 14 C-labeled CDCA uptake, which is independent of IBAT. Immunofluorescent staining analysis revealed that these two proteins co-localize in the vicinity of the plasma membrane in stable cells. Taken together, the current data provide the first evidence that I-BABP is functionally associated with FXR and IBAT in the nucleus and on the membrane, respectively, stimulating FXR transcriptional activity and the conjugated bile acid uptake mediated by IBAT in the ileum.

Intestinal bile acid physiology and pathophysiology

“…Instead, SHP binds to other nuclear receptors such as RXR/RAR (retinoic acid receptor), LRH1 (liver receptor homolog 1) and LXR (liver X receptor), inhibiting their transcriptional effects [11,33] . IBABP seems to interact with ASBT and FXR to promote FXR transcription [34] . An important feature of FXR role in regulation of BA homeostasis is that it is not limited to local effects.…”

Structural determinants of ligand binding in the ternary complex of human ileal bile acid binding protein with glycocholate and glycochenodeoxycholate obtained from solution NMR

“…The synthesis of [23, C 2 ]-labelled GCA and GCDA was achieved by a steroidal side chain degradation and isotopic regeneration strategy [39]. The synthesis of [3,[4][5][6][7][8][9][10][11][12][13] C 2 ]-labelled bile salts is detailed elsewhere [40]. Isotopically enriched and unenriched bile salts were dissolved in tetrahydrofuran.…”

“…In addition to regulating the cellular trafficking of bile salts, BABPs have also been suggested to have a role in regulating the transcriptional activities of nuclear receptors and, by interacting with the ileal bile acid transporter, modulating conjugated bile acid uptake in the ileum [8].…”

Antibacterial Drug Treatment Increases Intestinal Bile Acid Absorption <i>via</i> Elevated Levels of Ileal Apical Sodium-Dependent Bile Acid Transporter but Not Organic Solute Transporter α Protein

“…IBABP is believed to be involved in the transcellular transport of bile acids. 10,11) Bile acids are ultimately shuttled across the ileal enterocyte and are exported across the basolateral membrane into the portal circulation by the organic solute transporter, organic solute transporter (OST)α-OSTβ. 12,13) Bile acid pool sizes are significantly decreased not only in Asbt-null mice but also in Ostα-null mice, indicating that both bile acid transporters are essential for the intestinal bile acid transport.…”