High-content assays for evaluating cellular and hepatic diacylglycerol acyltransferase activity

Acyl-CoA:diacylglycerol acyltransferase (DGAT) catalyzes the terminal step in triglyceride (TG) synthesis using diacylglycerol (DAG) and fatty acyl-CoA as substrates. In the liver, the production of VLDL permits the delivery of hydrophobic TG from the liver to peripheral tissues for energy metabolism. We describe here a novel high-content, high-throughput LC/MS/MS-based cellular assay for determining DGAT activity. We treated endogenous DGAT-expressing cells with stable isotope-labeled ¹³C₁₈]oleic acid. The ¹³C₁₈]oleoyl-incorporated TG and DAG lipid species were profiled. The TG synthesis pathway assay was optimized to a one-step extraction, followed by LC/MS/MS quantification. Further, we report a novel LC/MS/MS method for tracing hepatic TG synthesis and VLDL-TG secretion in vivo by administering ¹³C₁₈]oleic acid to rats. The ¹³C₁₈]oleic acid-incorporated VLDL-TG was detected after one-step extraction without conventional separation of TG and recovery by derivatizing ¹³C₁₈]oleic acid for detection. Using potent and selective DGAT1 inhibitors as pharmacological tools, we measured changes in ¹³C₁₈]oleoyl-incorporated TG and DAG and demonstrated that DGAT1 inhibition significantly reduced ¹³C₁₈]oleoyl-incorporated VLDL-TG. This DGAT1-selective assay will enable researchers to discern differences between the roles of DGAT1 and DGAT2 in TG synthesis in vitro and in vivo.