Effect of bile on the intestinal bile-acid binding protein (I-BABP) expression. In vitro and in vivo studies.

Enterocytes actively transport bile acids from the ileal lumen to the portal blood. This physiological process greatly contributes to maintaining the bile acid homeostasis. However, little is known about the molecular mechanisms involved in this transport system. The effect of bile on gene expression of the intestinal bile-acid binding protein (I-BABP) expressed in the enterocytes was studied in vivo, using the by-pass method, and in vitro, using organ culture of ileum explants and Caco-2 cell line. The low cytosolic I-BABP concentration and I-BABP mRNA level found in diverted ileum was totally recovered when bile was added in the ileal lumen. Northern blot analysis of the ileal explants revealed a dose-dependent increase in the I-BABP mRNA in the presence of bile. In Caco-2 cells, the I-BABP transcript was dramatically increased in the presence of human bile while it was undetectable in the control cultures. These data offer the first evidence that biliary components regulate the I-BABP gene expressed in the enterocytes.