Transmembrane Protein 68 Functions as an MGAT and DGAT Enzyme for Triacylglycerol Biosynthesis

Triacylglycerol (TG) biosynthesis is an important metabolic process for intracellular storage of surplus energy, intestinal dietary fat absorption, attenuation of lipotoxicity, lipid transportation, lactation and signal transduction in mammals. Transmembrane protein 68 (TMEM68) is an endoplasmic reticulum (ER)-anchored acyltransferase family member of unknown function. In the current study we show that overexpression of TMEM68 promotes TG accumulation and lipid droplet (LD) formation in a conserved active sites-dependent manner. Quantitative targeted lipidomic analysis showed that diacylglycerol (DG), free fatty acid (FFA) and TG levels were increased by TMEM68 expression. In addition, TMEM68 overexpression affected the levels of several glycerophospholipids, such as phosphatidylcholine, phosphatidylethanolamine and phosphatidylinositol, as well as sterol ester contents. TMEM68 exhibited monoacylglycerol acyltransferase (MGAT) and diacylglycerol acyltransferase (DGAT) activities dependent on the conserved active sites in an in vitro assay. The expression of lipogenesis genes, including DGATs, fatty acid synthesis-related genes and peroxisome proliferator-activated receptor γ was upregulated in TMEM68-overexpressing cells. These results together demonstrate for the first time that TMEM68 functions as an acyltransferase and affects lipogenic gene expression, glycerolipid metabolism and TG storage in mammalian cells.

Cell-based assay of MGAT2-driven diacylglycerol synthesis for profiling inhibitors: use of a stable isotope-labeled substrate and high-resolution LC/MS

To demonstrate monoacylglycerol acyltransferase 2 (MGAT2)-mediated enzyme activity in a cellular context, cells of the murine secretin tumor cell-1 line of enteroendocrine origin were used to construct human MGAT2-expressing recombinant cell lines. Low throughput and utilization of radiolabeled substrate in a traditional TLC technique were circumvented by development of a high-resolution LC/MS platform. Monitoring incorporation of stable isotope-labeled D31-palmitate into diacylglycerol (DAG) allowed selective tracing of the cellular DAG synthesis activity. This assay format dramatically reduced background interference and increased the sensitivity and the signal window compared with the TLC method. Using this assay, several MGAT2 inhibitors from different chemotypes were characterized. The described cell-based assay adds a new methodology for the development and evaluation of MGAT2 inhibitors for the treatment of obesity and type 2 diabetes.

Acyltransferase inhibitors: a patent review (2010-present)

INTRODUCTION

Acyltransferase (AT) catalyzes the transfer of an acyl moiety from acyl-coenzyme A (acyl-CoA) to an acceptor. ATs play important roles in the maintenance of homeostasis in the human body and have been linked to various diseases; therefore, several ATs have been proposed as potential targets for the treatment or prevention of such diseases. The AT family includes acyl-CoA:cholesterol AT (ACAT), diacylglycerol AT (DGAT), and monoacylglycerol AT (MGAT) for the metabolism of lipids. Furthermore, recent molecular biological studies revealed the existence of their isozymes with distinct functions in the body.

AREAS COVERED

This review summarized patent filings published between 2010 and the present date that claimed isozyme-selective inhibitors of ACAT, DGAT and MGAT, which are involved in neutral lipid metabolism.

EXPERT OPINION

Isozymes of ACAT, DGAT and MGAT play distinct functions in neutral lipid metabolism in the human body and have been considered as potential therapeutic targets. Accordingly, isozyme-selective inhibitors that could be used in the treatment or prevention of lipid metabolism disorders were searched for. Of these, pyripyropene A derivatives, ACAT2-selective inhibitors, may be potential therapeutics for the treatment of atherosclerosis, homozygous familial hypercholesterolemia and nonalcoholic fatty liver disease.

Beyond triglyceride synthesis: the dynamic functional roles of MGAT and DGAT enzymes in energy metabolism

Monoacyglycerol acyltransferases (MGATs) and diacylglycerol acyltransferases (DGATs) catalyze two consecutive steps of enzyme reactions in the synthesis of triacylglycerols (TAGs). The metabolic complexity of TAG synthesis is reflected by the presence of multiple isoforms of MGAT and DGAT enzymes that differ in catalytic properties, subcellular localization, tissue distribution, and physiological functions. MGAT and DGAT enzymes play fundamental roles in the metabolism of monoacylglycerol (MAG), diacylglycerol (DAG), and triacylglycerol (TAG) that are involved in many aspects of physiological functions, such as intestinal fat absorption, lipoprotein assembly, adipose tissue formation, signal transduction, satiety, and lactation. The recent progress in the phenotypic characterization of mice deficient in MGAT and DGAT enzymes and the development of chemical inhibitors have revealed important roles of these enzymes in the regulation of energy homeostasis and insulin sensitivity. Consequently, selective inhibition of MGAT or DGAT enzymes by synthetic compounds may provide novel treatment for obesity and its related metabolic complications.