Increased concentrations of conjugated bile acids are associated with osteoporosis in PSC patients

Primary sclerosing cholangitis (PSC) is an idiopathic cholestatic liver disease characterized by chronic inflammation and progressive fibrosis of intra- and extrahepatic bile ducts. Osteoporosis is a frequent comorbidity in PSC, and we could previously demonstrate that IL17-dependent activation of bone resorption is the predominant driver of bone loss in PSC. Since we additionally observed an unexpected heterogeneity of bone mineral density in our cohort of 238 PSC patients, the present study focused on a comparative analysis of affected individuals with diagnosed osteoporosis (PSC^OPO, n = 10) or high bone mass (PSC^HBM, n = 7). The two groups were not distinguishable by various baseline characteristics, including liver fibrosis or serum parameters for hepatic function. In contrast, quantification of serum bile acid concentrations identified significant increases in the PSC^OPO group, including glycoursodeoxycholic acid (GUDCA), an exogenous bile acid administered to both patient groups. Although cell culture experiments did not support the hypothesis that an increase in circulating bile levels is a primary cause of PSC-associated osteoporosis, the remarkable differences of endogenous bile acids and GUDCA in the serum of PSC^OPO patients strongly suggest a yet unknown impairment of biliary metabolism and/or hepatic bile acid clearance in this patient subgroup, which is independent of liver fibrosis.