Aging and Cholesterol Gallstone Formation: Molecular Mechanisms of Gallstone (Lith) Genes

David Q.H. Wang, M.D., Ph.D. Beth Israel Deaconess Medical Center, Harvard Medical School Contact Dr. Wang

Cholesterol gallstone disease occurs rarely in childhood and adolescence. As epidemiological observations have suggested and as clinical studies have confirmed, the prevalence of cholesterol gallstone disease increases linearly with advancing age and approaches 50% at age 80. Elderly individuals are at high risk for developing gallstone complications and the death rate from surgery is often unacceptably high in patients older than 65. It has been known that cholesterol-supersaturated bile due to excess hepatic secretion of cholesterol into bile is a essential metabolic condition for the formation of cholesterol gallstones. Although it was observed that cholesterol concentration of bile is significantly higher in elderly people, and age is positively correlated with hepatic cholesterol secretion rate, as well as negatively correlated with bile salt synthesis and bile salt pool size in the liver, it has not been established at the molecular level whether aging modifies biliary cholesterol metabolism or how aging influences biliary lipid secretion.

Recently, I studied 9 strains of inbred mice fed a diet containing high fat and cholesterol (plus cholic acid) for 8 weeks, and found that differences in gallstone susceptibility between C57L mice and AKR mice are determined by at least two gallstone (Lith) genes. The first gallstone gene (Lith1) maps to mouse chromosome 2 by a powerful genetic analysis (quantitative trail locus mapping). Compared to gallstone-resistant AKR mice, gallstone-susceptible C57L mice with gallstone (Lith) genes display significantly higher secretion rate of biliary cholesterol, greater cholesterol concentration of gallbladder bile, more rapid cholesterol crystallization, and higher prevalence of gallstones. In response to the diet with high fat and cholesterol (plus cholic acid), AKR mice decrease HMG-CoA reductase activity (hepatic cholesterol biosynthesis), but C57L mice fail to down-regulate this enzyme activity. Down-regulation of cholesterol 7a-hydroxylase and sterol 27-hydroxylase activities (hepatic bile salt synthesis) are significantly more pronounced in C57L mice than in AKR mice. Whereas, acyl-CoA:cholesterol acyltransferase (cholesterol ester synthesis) decreases significantly in C57L mice than in AKR mice. My results suggest that the observed changes in the lipid regulatory enzyme activities are secondary effects of gallstone genes, and are, in part, an integrated response that ensures the continuous supply of hepatic cholesterol for secretion into bile. Furthermore, I found that the sister of P-glycoprotein, a bile salt export pump (bile salt transporter) in the liver, has a high function in C57L mice, as well as is a strong candidate gene for the first gallstone gene (Lith1) because the gene for the sister of P-glycoprotein is localized on mouse chromosome 2 within the similar region for the first gallstone gene.

Therefore, I propose three specific aims to explore the molecular mechanisms responsible for the high incidence of gallstone formation with age using genetically gallstone-susceptible C57L mice compared to gallstone-resistant AKR mice. Aim 1: To examine the influence of aging on cholesterol levels of gallbladder and hepatic biles and the formation of cholesterol gallstones. Aim 2: To explore molecular mechanisms how aging induces hypersecretion of cholesterol and the formation of lithogenic bile. Aim 3: To study whether aging modifies activities of hepatic lipid regulatory enzymes that alter biliary lipid metabolism. The elucidation of the molecular mechanisms underlying influence of aging on gallstone formation, biliary lipid secretion and hepatic lipid regulatory enzyme activities will shed more light on the pathogenesis of cholesterol gallstone disease, as well as open a door for the gene therapy in the human.