Abstract and Introduction
Background and Aims: Bile acid (BA) dysregulation is related to not only metabolic diseases but also nonalcoholic fatty liver disease (NAFLD). We investigated whether circulating BA levels are altered according to the histological severity of NAFLD independent of metabolic derangements.
Methods: Global metabolic profiling and targeted BA analysis using sera collected from biopsy-proven no-NAFLD (n = 67), nonalcoholic fatty liver (NAFL) (n = 99), and nonalcoholic steatohepatitis (NASH, n = 75) subjects were performed sequentially. Circulating metabolome analysis integrated with the hepatic transcriptome was performed to elucidate the mechanistic basis of altered circulating BA profiles after stratification by obesity (body mass index ≤ 25 kg/m2). Circulating BA alterations were also validated in an independent validation cohort (29 no-NAFLD, 70 NAFL and 37 NASH).
Results: Global profiling analysis showed that BA was the metabolite significantly altered in NASH compared to NAFL. Targeted BA analysis demonstrated that glyco-/tauro-conjugated primary BAs were commonly increased in nonobese and obese NASH, while unconjugated primary BAs increased only in nonobese NASH. These characteristic primary BA level changes were maintained even after stratification according to diabetes status and were replicated in the independent validation cohort. Compared to nonobese NAFL patients, nonobese NASH patients exhibited upregulated hepatic expression of CYP8B1.
Conclusions: BA metabolism is dysregulated as the histological severity of NAFLD worsens, independent of obesity and diabetes status; dysregulation is more prominent in nonobese NAFLD patients. Metabolome-driven omics approach provides new insight into our understanding of altered BA metabolism associated with individual phenotypes of NAFLD.
Bile acids (BAs), end-products of cholesterol metabolism, contribute to the solubilization and digestion of dietary lipids, and play a role in regulating hepatic lipid and glucose metabolism.[1,2] Primary BAs are synthesized in the liver and conjugated to glycine or taurine for secretion into the bile. Conjugated primary BAs are then released into the small intestine to promote the absorption of lipids and converted into secondary BAs by the intestinal microbiota. BAs are subsequently reabsorbed into the liver by a process termed enterohepatic circulation to inhibit BA synthesis. The accumulation of BAs may induce hepatotoxicity; therefore, BA metabolism is tightly regulated through negative feedback.[3–5] Altered BA composition has been reported not only in the serum[3,6,7] but also in the stool of patients with nonalcoholic fatty liver disease (NAFLD). Circulating primary BA levels consistently increase as the histological severity of NAFLD increases.[3,6,7] The alteration in BA conjugation and the primary-to-secondary BA ratio are also affected by NAFLD severity;[3,6,7] however, the association between NAFLD severity and the primary-to-secondary BA ratio has not been consistent among studies.[6,7]
BA metabolism is closely associated with obesity and diabetes mellitus (DM) as well as with NAFLD.[3,6,7] In addition, nutritional or pharmacological regulation of BA metabolism can affect not only liver fat content[11,12] but also body weight. As NAFLD is frequently accompanied by obesity,[13,14] obesity and NAFLD may have a synergistic effect on alterations in BA metabolism. The association between NAFLD and BA metabolism has been reported in the context of obese NAFLD subjects (mean body mass index 32–34 kg/m2).[3,6,7] The alteration in BA metabolism in nonobese NAFLD subjects may provide new insights into an independent association between NAFLD and BA metabolism. Recently, patients with lean NAFLD have been reported to have higher serum secondary BA and fibroblast growth factor 19 (FGF19) levels than obese NAFLD patients. However, in that study, lean NAFLD subjects showed significantly less severe NAFLD pathology than obese NAFLD subjects (mean NAFLD activity score, 3 vs. 4).
To elucidate the alteration in BA metabolism according to NAFLD severity independent of obesity, we enrolled nonobese subjects with biopsy-proven NAFLD and analysed their circulating BA levels according to NAFLD severity. In addition, disparities in circulating BA alterations between nonobese nonalcoholic fatty liver (NAFL) and nonalcoholic steatohepatitis (NASH) patients and between obese NAFL and NASH patients were investigated to determine whether there is an independent association between circulating BA alterations and NASH in the absence of obesity.
Liver International. 2021;41(12):2892-2902. © 2021 Blackwell Publishing