Role of phosphatidylcholine saturation in preventing bile salt toxicity to gastrointestinal epithelia and membranes

Abstract

The mechanism which protects the biliary and intestinal mucosa from the detergent properties of bile acids is not fully understood. We employed three contrasting in vitro model systems (human red blood cells, polarized intestinal [Caco-2] cells, and synthetic liposomes), to compare the efficacy of saturated and unsaturated phosphatidylcholine (PC) to protect cells and membranes from bile salt injury. Hemolysis of red blood cells, electrical resistance across confluent monolayers of Caco-2 cells, and disruption of synthetic PC liposomes were assessed after incubation with varying concentrations of bile salt (sodium deoxycholate) alone or in the presence of saturated or unsaturated PC. The hemolytic activity of deoxycholate on red blood cells was observed at > or =2 mM, and could be blocked by equimolar concentration or greater of both saturated or unsaturated PC. In contrast, exposure of Caco-2 cells to deoxycholate at > or =0.8 mM induced a maximal decrease in resistance, which was reversed by > or =0.8 mM unsaturated PC or 5 mM saturated PC. Similarly, synthetic liposomes were permeabilized by 0.8 mM deoxycholate and were protected by a lower concentration of unsaturated PC (2 mM) than saturated (5 mM). Cells can show variable resistance to bile salt toxicity. Extracellular PC, especially in the unsaturated state, can directly protect cell and artificial membranes from bile salt injury. These findings support a role for biliary PC in the formation of mixed micelles that have low cytotoxic properties.