Metabolic endotoxemia caused by dysbiosis-induced elevated lipopolysaccharide (LPS) levels increases susceptibility to cholesterol gallstone formation in mice

Abstract

Cholesterol gallstone formation is a complex process intervened by genetic and environmental factors. Clinical and animal studies have clearly demonstrated that alterations in the gut microbiota are associated with the formation of cholesterol gallstones, suggesting that gut dysbiosis may be a high risk factor for gallstone disease. However, the mechanisms underlying the roles of gut microbiota in the pathogenesis of gallstone formation are not fully understood. Western diet high in fat and cholesterol can increase the proportion of LPS-expressing bacteria in the gut, resulting in a 0.5- to 2-fold increase in plasma LPS levels and low-grade chronic systemic inflammation, also known as metabolic endotoxemia. Our aims are to investigate whether metabolic endotoxemia caused by dysbiosis-induced elevated LPS levels plays a critical role in increasing gallstone formation by disrupting biliary cholesterol metabolism in mice. Methods: The biliary and gallstone phenotypes were studied in male C57BL/6 mice fed a lithogenic diet and treated with LPS (from Escherichia coli 055:B5) at 7.5 μg/day (~300 μg/kg/day) or 0.9% NaCl (as control) for 8 wk. Plasma and bile LPS concentrations were determined by ELISA. Results: Our findings show for the first time that in metabolic endotoxemia (plasma LPS=77.6±8.3 ng/mL), LPS is present in hepatic bile (476±334 pg/mL) of mice treated with LPS at 7.5 μg/day. However, LPS cannot be detected in bile of control mice receiving no LPS. Model bile studies show that LPS reduces cholesterol solubility in bile by disturbing the physical state of cholesterol carriers, i.e., biliary vesicles. Metabolic endotoxemia also disrupts biliary cholesterol homeostasis by inhibiting expression of hepatic cholesterol 7α-hydroxylase and the classical pathway of bile acid biosynthesis via the LPS-stimulated Toll like receptor 4 (TLR4) signaling cascade in the liver. These abnormalities enhance hepatic cholesterol hypersecretion and promote the aggregation and fusion of unilamellar vesicles to form unstable multilamellar vesicles in bile. As a result, this accelerates heterogenous cholesterol crystallization, thereby leading to rapid formation of numerous classical plate-like cholesterol monohydrate crystals in supersaturated gallbladder bile. Metabolic endotoxemia via the LPS/TLR4 pathway also impairs gallbladder emptying and refilling, causing gallbladder stasis. These abnormalities promote the accumulation of excessive mucin gels and rapid growth and agglomeration of solid cholesterol monohydrate crystals into biliary sludge, a precursor of gallstones. Conclusions: Metabolic endotoxemia induced by gut dysbiosis is a risk environmental factor, and its lithogenic mechanism involves driving the initiation of supersaturated bile and heterogeneous cholesterol crystallization, two key steps in the early stages of cholesterol gallstone formation. NIH R01 DK126369 This is the full abstract presented at the American Physiology Summit 2023 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.