Lack of a liver‐specific apolipoprotein (apo)A‐V in bile promotes cholesterol gallstone formation by disrupting biliary cholesterol homeostasis in mice

Abstract

BackgroundApoA‐V is synthesized exclusively by the liver and secreted into the circulation, involving in plasma lipoprotein and lipid metabolism. Although its concentration in human plasma is extremely low (114–258 ng/mL), 1,000 to 10,000‐fold lower than that of apoB‐100 and apoA‐I, respectively, human and mice lacking functional apoA‐V suffer from hypertriglyceridemia. Because our new results found that apoA‐V is also secreted into bile, we hypothesize that it plays an important role in regulating biliary cholesterol metabolism, and its deficiency reduces cholesterol solubility in bile, promoting cholesterol crystallization and gallstone formation.MethodsTo explore the lithogenic mechanisms of biliary apoA‐V deficiency, male apoA‐V knockout (KO) and wild‐type (WT) mice (n=10 per group) were fed a lithogenic diet for 4 wk. Gallstones and cholesterol crystallization in bile were examined by physical‐chemical methods. Hepatic lipid output was determined by biliary secretion studies. Gene expression and protein content were analyzed by qRT‐PCR, Western blots, and ELISA.ResultsCompared to the chow diet, feeding the lithogenic diet significantly (P<0.01) reduced hepatic apoA‐V mRNA and protein levels by 55% and its bile concentrations from 45.7±12.4 ng/mL to 16.7±6.0 ng/mL in WT mice. ApoA‐V was associated only with vesicles, but not micelles, in bile of WT mice. Biliary cholesterol, but not phospholipid or bile salt, output was higher (P<0.05) in KO mice (29.0±8.5 μmol/h/kg) than in WT mice (20.4±5.5 μmol/h/kg), leading to cholesterol‐supersaturated bile. The deficiency of apoA‐V in bile reduced biliary cholesterol solubility and promoted cholesterol crystallization through the liquid crystalline pathway by enhancing biliary unilamellar vesicle aggregation and fusion to form unstable multilamellar vesicles in KO mice. After 50 ng/mL of apoA‐V protein isolated from WT mice was added to supersaturated model bile, cholesterol crystallization was significantly delayed than that added the protein at 0 ng/mL. This implies that apoA‐V plays a key role in regulating cholesterol solubility in bile by modulating the physical state of cholesterol carriers. In addition, the lack of apoA‐V impaired gallbladder emptying and refilling function, thus promoting the accumulation of excess mucin gel and the formation of biliary sludge, a precursor of gallstones. These abnormalities enhanced the growth and agglomeration of solid cholesterol crystal into microlithiasis. After 4 wk of feeding, gallstone prevalence was higher (P<0.05) in KO mice (60%) than in WT mice (25%).ConclusionsThese findings clearly demonstrate that the liver‐specific protein apoA‐V plays a critical role in the regulation of biliary cholesterol metabolism and transport, and its lack in bile dramatically enhances cholelithogenesis by disrupting biliary cholesterol homeostasis. Increasing biliary apoA‐V may be a novel and efficient approach for the prevention of cholesterol gallstones.Support or Funding InformationThis work was supported in part by research grants DK101793, DK106249 and AA03557, all from the National Institutes of Health (US Public Health Service).This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.