Animal Models of Diet-induced Hypercholesterolemia

Another series of experiments was conducted to investigate the role of phospholipids in secretion of biliary cholesterol. Disruption of the Abcb4 (also known as Mdr2 ) gene in mice led to a severe liver disease. Phospholipids were undetectable in the bile of homozygous Abcb4 knockout ( Abcb4-/- ) mice, while the levels in heterozygous Abcb4 knockout ( Abcb4+/- ) mice were half of those in nontransgenic mice. Interestingly, Abcb4-/- mice secreted extremely low levels of cholesterol into bile, but cholesterol levels in flowing bile from Abcb4+/- mice were similar to those in nontransgenic mice [41]. Langheim et al. [46] investigated whether overex‐ pression of ABCG5 and ABCG8 could restore biliary cholesterol secretion in Abcb4-/- mice by breeding Abcb4-/- mice with hG5G8Tg mice to generate Abcb4-/- ; hG5G8Tg mice. The Abcb4-/- ; hG5G8Tg mice also secreted very low levels of biliary cholesterol, which were similar to the levels in Abcb4-/- mice. Taken together, these results indicate that biliary cholesterol secretion requires a minimal concentration of phospholipids in the bile. Biliary phospholipid secretion in the liver serves two purposes. One is to protect the canalicular membrane of hepatocytes exposed to high concentrations of bile acids from damage by the detergent action of bile acids. Secretion of phospholipids by the liver into the bile reduces bile salt micelles to extract phospholipids from the membranes of hepatocytes. The other is to make phospholipids available for incorporation into pure bile salt micelles to form bile salt mixed micelles. Solubility of cholesterol in bile salt mixed micelles is greater than that in pure bile salt micelles, thus phospholipid secretion prevents formation of gallstones [47]. and biliary phospholipids Bile samples collected from gall bladders analyzed to determine whether there are any differences in the secretion of cholesterol into bile by low and high responders fed the HCHF diet. The results revealed differences in biliary cholesterol concentration and biliary phospholipid concentration in gall bladder bile. Choles‐ cholesterol homeostasis causes high responders to develop fatty livers and atherosclerotic lesions. Cholesterol accumulates in the livers of high responders as a result of impaired biliary cholesterol secretion. After 4 weeks of HCHF diet, serum levels of liver enzymes (alanine aminotransferase, aspartate aminotransferase, and γ– glutamyltransferase) and bilirubin were significantly elevated, indicating high responders had liver injury. Histology revealed steatosis, inflammation and ballooned hepatocytes in their livers after 8 weeks of HCHF diet. The pathologic condition in the liver worsened as high responders continued to consume the HCHF diet. In one study in which high and low responders were fed the HCHF diet for 24 weeks, livers of high responders were markedly enlarged compared to those of low responders. The enlarged livers had an increase in free cholesterol (2-fold), esterified cholesterol (11-fold) and triglycerides (2-fold), but no significant increase in free fatty acids. Low responders did not display any significant morphological changes in the liver after 24 weeks on the HCHF diet. Pro‐ longed HCHF dietary challenge caused high responders to develop fibrosis in addition to steatosis, inflammation and ballooned cells. Liver fibrosis is a characteristic feature of the severe form of nonalcoholic fatty liver disease known as nonalcoholic steatohepatitis.