Application of stable isotope labels for metabolomics in studies in fatty liver disease

The progression of nonalcoholic fatty liver disease (NAFLD) increases the risks of cirrhosis and cardiovascular disease. Marked alteration of both cytosolic and mitochondrial metabolism, and in combination with insulin resistance, increases hepatic glucose production. Utilization of stable isotope tracers to study liver metabolism offers deep insight into rearrangements of metabolic pathways and substrate-product relationships under the conditions leading to fatty liver and induced by diseases, drugs, toxins, or genetic manipulations. Isotope tracing untargeted metabolomics (ITUM) recently emerged as a powerful platform in which the label can be tracked in an untargeted fashion, revealing the penetration of substrates into metabolic pathways, even at low abundance. Here, we describe a protocol that can be utilized to study the changes in utilization of any labeled substrate toward a wide range of metabolites either in isolated liver cells or whole liver tissue under conditions mimicking various stages of fatty liver disease. Furthermore, a routine protocol for extraction, separation, and mass spectrometric detection of isotopically labeled metabolites in an untargeted or targeted fashion. An informatic approach to analyze stable isotope untargeted metabolomic datasets is also described.