After being secreted into bile and
After being secreted into bile and entering the intestine, ∼95% of the bile acids are returned to the liver through the enterohepatic circulation via an active transport and amyloid by a specific bile acid transporter, apical sodium-dependent bile acid transporter predominantly in the distal ileum. As a result, newly synthesized bile acids in the liver contribute only a small fraction (less than 5%) to biliary secretion, which compensate for bile acids that escape intestinal absorption and are lost in the feces. Therefore, biliary bile acids consist of those that are newly synthesized in the liver and those undergoing enterohepatic cycling. The hepatic secretion of biliary bile acids is determined by ABCB11, a bile acid export pump on the canalicular membrane of hepatocytes. Hepatic secretion of bile acids could directly affect phospholipid vesicle secretion, although the molecular mechanism by which bile acid secretion is coupled to cholesterol and phospholipid secretion is still unclear. The relationship between bile acid secretion and cholesterol secretion has been found to be curvilinear. At low bile acid secretion rates (less than 10 μmol/h/kg), more cholesterol is secreted per molecule of bile acid than at higher rates. Although bile acid secretion rates are not usually low in normal subjects, they could diminish during prolonged fasting, during the overnight period, and with substantial bile acid losses, such as with a biliary fistula or ileal resection when the liver cannot sufficiently compensate with increased bile acid synthesis. In contrast, at high bile acid secretion rates—for example, during and after eating—biliary saturation is less than during the interprandial period.
Recently, genetic analysis in mice supports the candidacy of the G protein-coupled receptor 30 (GPR30), a novel estrogen receptor, for a new gallstone gene Lith18. Of special note is that ∼50% of cholesterol is converted to bile acids in the liver each day in humans and in mice. Because GPR30 is localized in the endoplasmic reticulum, but not the nucleus, of hepatocytes, GPR30 activation by estrogen possibly through the epidermal growth factor receptor signaling cascade inhibits hepatic cholesterol 7α-hydroxylase and the classical pathway of bile acid synthesis, thereby leading to the availability of excess cholesterol for hepatic hypersecretion and bile lithogenesis.
Cholesterol nucleation and crystallization in supersaturated bile To systematically study the sequences of cholesterol crystallization, solid cholesterol crystal growth, and gallstone formation, gallbladder bile is carefully investigated at various time points using phase contrast and polarizing light microscopy in mice during the 8-week period of lithogenic diet feeding. Representative photomicrographs of cholesterol crystallization and gallstone formation in mice are shown in Fig. 5. After gallbladder bile becomes supersaturated with cholesterol, i.e., CSI values are greater than 1.0, large amounts of non-birefringent amorphous mucin gel are accumulated in the gallbladder lumen, followed by the formation of numerous liquid crystals. In general, minimally sized, non-birefringent, and scattered small liquid crystals appear first. Non-birefringent aggregated liquid crystals with 1–5 μm of particles in diameter are found subsequently. If CSI values continue to increase in bile, fused liquid crystals are formed, which are birefringent with focal conic Maltese-cross textures and greater than 0.5–1.0 μm in size. In addition, some anhydrous cholesterol crystals are infrequently found. They are denoted as arc-like crystals that are short curved rods and rarely are filamentous, and tubular crystals that often appear to fracture at their ends producing classical cholesterol monohydrate crystals. Typical solid plate-like cholesterol monohydrate crystals are 79.2° and 100.8° angled parallelograms, often with a small notched corner. Mucin gel, a potent pro-nucleating agent, often promotes the growth and agglomeration of solid cholesterol crystals. Amorphous masses of cholesterol monohydrate crystal are defined loosely as agglomerated sheets. Sandy stones are encircled by mucin gel, and individual cholesterol monohydrate crystals are often found to project from the edges of sandy stones. Finally, true gallstones are exhibited with typical round contours and black centers under polarizing light microscopy.