Abstract
Cholesterol is a nonpolar lipid dietary constituent, absorbed from the small intestine, transported in blood and taken up by the liver. In bile, the sterol is solubilized in mixed micelles by bile salts and phospholipids. In case of supersaturation, cholesterol is kept in vesicles with phospholipid or phase-separated as
... read more
crystals. Accurate isolation of micellar and vesicular lipid carriers in model systems is important for the study of the cholesterol crystallization and gallstone formation. A new method for separating lipid carriers in model biles has been proposed in this thesis. Also, the key role of bile salts and phospholipids in cholesterol solubilization in bile is described, together with their modulating/protecting effects on cholesterol supersaturation and crystallization.
Apart from cholesterol crystallization, this thesis also focused on the process of nascent bile formation at the level of the hepatocyte canalicular membrane. Phosphatidylcholine is the predominant phospholipid in bile, although both sphingomyelin and phosphatidylcholine are the major structural phospholipids in the outer leaflet of the hepatocyte canalicular membrane. Different physical-chemical characteristics of the two phospholipids may relate to their different distribution in vivo. We have studied by means of spectrophotometry and by sequential state-of-the-art cryo-transmission electron microscopy, effects of including sphingomyelin within phosphatylcholine-containing vesicles (with and without cholesterol) on vesicle ? micelle phase transitions that were induced by addition of bile salts. Cholesterol plays a key role in formation of pathophysiologically relevant phosphatidylcholine plus sphingomyelin-containing bilayers and in modulating interactions between vesicles and detergent bile salts.
The entero-hepatic circulation and the detergent activity of the bile salts are discussed int he last part of the thesis. Bile salts may present as “good” or “bad” guys. The good aspect is their effect in intestinal lipid absorption, bile formation and cholesterol solubilization. The “bad” aspect is the potential cytotoxicity induced by the detergent bile salts. The presence of phospholipids within bile salt micelles protects against bile salt-induced cytotoxicity. When compared to egg yolk phosphatidylcholine, sphingomyelin offered greatly enhanced protection against bile salt-induced cytotoxicity compared to egg yolk phosphatidylcholine. Last, epidemiological and experimental studies have indicated a consistent correlation between increased risk of colon cancer and elevated levels of fecal bile salts in Western populations that consume high-fat diets. Deoxycholate, a secondary bile salts present in the colon, may act as a tumor promoter, inducing hyperproliferation and increasing the number of tumors elicited by complete carcinogens. Appreciable amounts of dietary sphingomyelin appear to reach to the colon. In CaCo-2 cells, we have shown that pathophysiologically relevant concentrations of this bile salt induced dose-dependent apoptosis in the early stages of incubation, but hyperproliferation at later stages. Sphingomyelin dose-dependently inhibited apoptosis as well as hyperproliferation, presumably through a local effect on deoxycholate activity. These findings may have important implications for colon cancer prevention.
show less
