Wensheng Lang

The use of stable isotope-labeled glycerol and oleic acid to differentiate the hepatic functions of DGAT1 and -2

Diacylglycerol acyltransferase (DGAT) catalyzes the final step in triglyceride (TG) synthesis. There are two isoforms, DGAT1 and DGAT2, with distinct protein sequences and potentially different physiological functions. To date, the ability to determine clear functional differences between DGAT1 and DGAT2, especially with respect to hepatic TG synthesis, has been elusive. To dissect the roles of these two key enzymes, we pretreated HepG2 hepatoma cells with 13C3-D5-glycerol or 13C18-oleic acid, and profiled the major isotope-labeled TG species by liquid chromatography tandem mass spectrometry. Selective DGAT1 and DGAT2 inhibitors demonstrated that 13C3-D5-glycerol-incorporated TG synthesis was mediated by DGAT2, not DGAT1. Conversely, 13C18-oleoyl-incorporated TG synthesis was predominantly mediated by DGAT1. To trace hepatic TG synthesis and VLDL triglyceride (VLDL-TG) secretion in vivo, we administered D5-glycerol to mice and measured plasma levels of D5-glycerol-incorporated TG. Treatment with an antisense oligonucleotide (ASO) to DGAT2 led to a significant reduction in D5-glycerol incorporation into VLDL-TG. In contrast, the DGAT2 ASO had no effect on the incorporation of exogenously administered 13C18-oleic acid into VLDL-TG. Thus, our results indicate that DGAT1 and DGAT2 mediate distinct hepatic functions: DGAT2 is primarily responsible for incorporating endogenously synthesized FAs into TG, whereas DGAT1 plays a greater role in esterifying exogenous FAs to glycerol.

Biotransformation of Geldanamycin and 17-Allylamino-17-Demethoxygeldanamycin by Human Liver Microsomes: Reductive versus Oxidative Metabolism and Implications

Comparative metabolite profiling of geldanamycin and 17-allylamino-17-demethoxygeldanamycin (17AAG) using human liver microsomes in normoxia and hypoxia was conducted to understand their differential metabolic fates. Geldanamycin bearing a 17-methoxy group primarily underwent reductive metabolism, generating the corresponding hydroquinone under both conditions. The formed hydroquinone resists further metabolism and serves as a reservoir. On exposure to oxygen, this hydroquinone slowly reverts to geldanamycin. In the presence of glutathione, geldanamycin was rapidly converted to 19-glutathionyl geldanamycin hydroquinone, suggesting its reactive nature. In contrast, the counterpart (17AAG) preferentially remained as its quinone form, which underwent extensive oxidative metabolism on both the 17-allylamino sidechain and the ansa ring. Only a small amount (<1%) of 19-glutathione conjugate of 17AAG was detected in the incubation of 17AAG with glutathione at 37°C for 60 min. To confirm the differential nature of quinone-hydroquinone conversion between the two compounds, hypoxic incubations with human cytochrome P450 reductase at 37°C and direct injection analysis were performed. Approximately 89% of hydroquinone, 5% of quinone, and 6% of 17-O-demethylgeldanamycin were observed after 1-min incubation of geldanamycin, whereas about 1% of hydroquinone and 99% of quinone were found in the 60-min incubation of 17AAG. The results provide direct evidence for understanding the 17-substituent effects of these benzoquinone ansamycins on their phase I metabolism, reactivity with glutathione, and acute hepatotoxicity.

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.

The IC50 Concept Revisited

A major strategy used in drug design is the inhibition of enzyme activity. The ability to accurately measure the concentration of the inhibitor which is required to inhibit a given biological or biochemical function by half is extremely important in ranking compounds. Since the concept of the half maximal inhibitory concentration (IC50) is used extensively for studying reversible inhibition enzymatic reactions, it is important to clearly understand the experimental design and the mathematical modeling techniques used to generate IC50 values. The most important part of the experimental design is to measure the rate of production of [P] during the linear phase of the time course of the reaction and to prove that the enzyme- catalyzed reaction is reversible. The most important part of the mathematical modeling is to select the correct model and to have a firm understanding on how to handle outliers in the data. These topics are discussed in greater detail along with a discussion on how much quantitative and mechanistic information can be reasonably deduced from an experiment.

Profiling Rat Brain Monoacylglycerol Lipase Activity Using an Ammonia-Adduct Enhanced Selected Ion Monitoring Liquid-ChromatographyPositive Electrospray Ionization Mass Spectrometry Assay

2-Arachidonoylglycerols (2-AG) is one of the major endocannabinoids in the central nervous system. Upregulation of central endocannabinoids signaling by selective inhibition of brain monoacylglycerol lipase (MAGL) activity is a potential therapeutic approach in pain, obesity, and diabetes modulation to name a few. Thus, a sensitive and reliable analytical method for determination of endocannabinoid levels in the brain is essential for the discovery of MAGL inhibitors. Accurate measurement of monoacylglycerols (MAG) levels, using liquid chromatography positive electrospray ionization mass spectrometry (LC / +ESI / MS) and tandem MS, is a challenge since the brain endocannabinoids are not only susceptible to stress in the in-life phase of the study, but also prone to postmortem metabolism, acyl migration (i.e., conversion from 2-MAG to 1(3)-MAG), metal adduct ion formation and chemical hydrolysis generating the same products as those by the target enzyme. To avoid these artifacts, we have developed a simple LC / +ESI / MS method for direct detection of ammonium-adduct cations of the major MAGs in selected ion monitoring mode (SIM). For the in vitro MAGL inhibition assay, a LC isocratic elution was used for baseline separation of MAGs and their acyl migration isomers produced during the 37oC incubation with rat brain homogenate. To minimize the postmortem metabolism and isomerization of MAGs for in-vivo studies, rat brain was homogenized directly in four milliliters of ethanol for every gram of brain tissue and a linear LC gradient elution was applied for broad endocannabinoid profiling. The SIM LC / +ESI / MS method is shown to be useful for in-vitro brain evaluation of inhibitory potency of MAGL and fatty acid amide hydrolase (FAAH) inhibitors and for in-vivo brain assessment for target engagement studies.

Novel LC/MS/MS and High-Throughput Mass Spectrometric Assays for Monoacylglycerol Acyltransferase Inhibitors:

Monoacylglycerol acyltransferase enzymes (MGAT1, MGAT2, and MGAT3) convert monoacylglycerol to diacylglycerol (DAG). MGAT1 and MGAT2 are both implicated in obesity-related metabolic diseases. Conventional MGAT enzyme assays use radioactive substrates, wherein the product of the MGAT-catalyzed reaction is usually resolved by time-consuming thin layer chromatography (TLC) analysis. Furthermore, microsomal membrane preparations typically contain endogenous diacylglycerol acyltransferase (DGAT) from the host cells, and these DGAT activities can further acylate DAG to form triglyceride (TG). Our mass spectrometry (liquid chromatography–tandem mass spectrometry, or LC/MS/MS) MGAT2 assay measures human recombinant MGAT2-catalyzed formation of didecanoyl-glycerol from 1-decanoyl-rac-glycerol and decanoyl-CoA, to produce predominantly 1,3-didecanoyl-glycerol. Unlike 1,2-DAG, 1,3-didecanoyl-glycerol is proved to be not susceptible to further acylation to TG. 1,3-Didecanoyl-glycerol product can be readily solubilized and directly subjected to high-throughput mass spectrometry (HTMS) without further extraction in a 384-well format. We also have established the LC/MS/MS MGAT activity assay in the intestinal microsomes from various species. Our assay is proved to be highly sensitive, and thus it allows measurement of endogenous MGAT activity in cell lysates and tissue preparations. The implementation of the HTMS MGAT activity assay has facilitated the robust screening and evaluation of MGAT inhibitors for the treatment of metabolic diseases.