Role of phospholipids in the pathophysiology of the gut-liver axis

Abstract

Phospholipids represent essential components of bile. Together with bile acids and cholesterol, phospholipids form “mixed micelles”. If sufficient amounts of phospholipids are available, no simple bile acid micelles are present, with prevention of bile acid toxicity and cholesterol crystallization. Apart from their effects in bile, bile acid-phospholipid micelles are also very important for cholesterol solubilization and absorption in the intestine. Intestinal absorption of excess cholesterol is crucial in the pathogenesis of hypercholesterolemia and cardiovascular disease. Finally, some exogenous compounds (e.g. some drugs, like NSAIDs) interact with mixed micelles. In this thesis, we focused on in vitro physical-chemical interactions between phospholipids, bile acids, cholesterol and some exogenous compounds. We also investigated molecular biologic aspects of phospholipids in animal models. We specifically evaluated the following: 1. Treatment and prevention of cholesterol crystallization by Aramchol, a new synthetic bile acid-fatty acid compound (FABAC). 2. Protective effects of phospholipids against toxic effects of non steroidal anti-inflammatory drugs (NSAIDs) in the gastrointestinal tract. 3. Effects of phospholipids on intestinal cholesterol absorption. 4. Role of phospholipids on formation and progression of colon cancer. Employing complementary in vitro model systems, we found no decrease of cholesterol crystallization with Aramchol, nor effects on micellar cholesterol solubilization and micelle-vesicle transitions. Furthermore, we found that adding Aramchol increased the cytotoxicity of bile acid micelles. These deleterious effects were prevented by simultaneous addition of phospholipids. Regarding NSAIDs, we found that indomethacin alone did not exert any toxic effects in vitro. Nevertheless, addition of this NSAID to simple bile acid micelles increased toxicity. Similar to Aramchol, adding phosphatidylcholine strongly reduced toxicity. As for intestinal cholesterol absorption, we showed in vitro that incorporation of phosphatidylcholine or sphingomyelin caused marked retention of cholesterol in the apical compartment of a transwell system with CaCo2 cells, concomitant with a decrease of ABC-A1 mRNA and protein levels. ABC-A1 protein is responsible for transfer of cholesterol from enterocyte to HDL particles. Incorporation of lyso-phosphatidylcholine strongly decreased apical retention of cholesterol, with concomitant increased expression of ABC-A1 and increased transfer of cholesterol to HDL at the basolateral compartment. Our findings may be relevant for prevention and treatment of hypercholesterolemia and cardiovascular disease. Finally, we explored whether phospholipids could be involved in the regeneration of intestinal mucosa and the occurrence of intestinal tumors. We availed of the mdr2 “knockout” mouse (with absent phospholipids in bile) and two complementary models for intestinal cancer (based on administration of carcinogen resp. deletion of the APC gene). In the mdr2 “knockout” mice, numbers and sizes of the intestinal tumors were much less than in control mice. This protective effect was abrogated by feeding a diet containing phospholipids to the mdr2 “knockout” mice. Our findings may be relevant for prevention and treatment of intestinal tumors. In summary, in this thesis we explored physical-chemical aspects and molecular biological effects of phospholipids in complementary in vitro model systems and animal models. This work may increase insight in pathophysiological processes and lead to better potential for prevention and treatment of human disease.