Regulation of apolipoprotein secretion by biliary lipids in newborn swine intestinal epithelial cells.

Biliary lipids, composed of bile acids, cholesterol, and phosphatidylcholine, are a major source of luminal lipid in the small intestine. In the present study in a newborn swine intestinal epithelial cell line (IPEC-1), taurocholate and phosphatidylcholine were found to have no effect on apolipoprotein B (apo B) secretion but did significantly increase the basolateral secretion of apo A-I. This regulation of apo A-I secretion occurred at the pretranslational level for taurocholate and at the posttranslational level for phosphatidylcholine. The regulation of apo A-I secretion by phosphatidylcholine did not involve changes in apo A-I degradation and may involve mobilization of a preformed pool of apo A-I. Cholesterol, whether solubilized with taurocholate or phosphatidylcholine, had no effect on the secretion of either apo B or apo A-I. However, when taurocholate, phosphatidylcholine, and cholesterol were combined, apo B secretion was decreased, and the increase in apo A-I secretion noted with taurocholate and phosphatidylcholine alone was ablated. Another primary bile acid, taurochenodeoxycholate, was found to decrease apo B secretion but had no effect on apo A-I secretion. However, the significance of this effect is uncertain, since this bile acid caused significant cellular membrane injury, as evidenced by increased apical medium lactate dehydrogenase activity. Phosphatidylcholine, but not taurocholate, dramatically increased the basolateral secretion of radiolabeled phospholipid with a modest increase in cellular triglyceride radiolabeling. Furthermore, this effect of phosphatidylcholine on lipid synthesis did not require significant hydrolysis or uptake of the phosphatidylcholine molecule. Studies using radiolabeled taurocholate did not demonstrate active transport of taurocholate by these cells.