Phospholipid (PL), cholesterol (CH), and bile acids are the major lipid components of bile. Apolipoprotein (apo) A-I and B are synthesized and secreted by enterocytes as components of intestinal lipoproteins. Purpose: To test the effect of these biliary components on apo A-I and B secretion and lipid synthesis in a newborn piglet intestinal epithelial cell line (IPEC-1). Methods: Differentiated IPEC-1 cells were cultured on collagen-coated filters in Transwell culture plates in serum-free medium. Cells were incubated with various combinations of taurocholate (TC, 1 mM), CH (0.5 mM), and phosphatidylcholine (PC, 0.5 mM) added to the apical medium for 24 hrs (n=5-8 for each group). Cells and basolateral medium were harvested, and apo A-I and B mass was measured by ELISA. 3H-glycerol was also added to determine triglyceride (TG) and PL synthesis. Results: Results for apo A-I mass are shown in the table below [mean (SEM); cell content, ng/μg protein; secretion, ng/μg protein/24 hr; cell content, p NS; secretion, p<0.05 by ANOVA]. No effect was observed for apo B, either for cell content or secretion. Medium lactate dehydrogenase activity as an index of membrane injury was not changed in any group. Dose response studies with TC, PC, and PC+PL demonstrated a plateau of apo A-I secretion at 2.0 mM TC and 1.0 mM PC, alone and in combination. PC, but not TC, caused a 6-fold increase in secretion of radiolabeled PL without effect on cellular labeled PL. PC+TC resulted in a 13-fold increase in labeled PL secretion. Incubation with3 H-PC demonstrated negligible hydrolysis and/or uptake of PC. Conclusions: TC and PC up-regulate secretion of apo A-I, but not apo B, by IPEC-1 cells. PC increases secretion of newly synthesized PL, and the effect is enhanced by TC. These effects do not require PC hydrolysis and/or uptake.

Table 1
Full size table