Agpat6 deficiency causes subdermal lipodystrophy and resistance to obesity

Triglycerides are the primary form of energy storage in mammals, and alterations in triglyceride biosynthesis and metabolism can lead to metabolic diseases such as hyperlipidemia, obesity, and lipodystrophy. In eukaryotes, two major pathways exist for triglyceride synthesis: the glycerol phosphate pathway and the monoacylglycerol pathway. The monoacylglycerol pathway functions predominantly in small intestine to generate triglycerides from monoacylglycerol in dietary fat. The glycerol phosphate pathway, in contrast, is considered the main pathway for triglyceride synthesis and occurs in most cells types. In particular, it is responsible for triglyceride synthesis in adipose tissue. Acylation of glycerol phosphate occurs through a step-wise addition of acyl groups, each addition being catalyzed by a distinct enzyme (1, 2). In recent years, the cloning and identification of several of these enzymes has facilitated their molecular characterization, but many questions remain about the physiological functions of these enzymes. To date, only a few mouse models with genetic deficiency in the triglyceride biosynthetic pathway have been reported. Mice lacking the mitochondrial glycerol-3-phosphate acyltransferase (GPAT), which catalyzes the addition of the first acyl group to glycerol-3-phosphate, showed reduced levels of triglycerides in plasma and liver (3). They also had reduced body weight and adipose tissue mass. Deficiency in acyl-coenzyme A:diacylglycerol acyltransferase-1 (DGAT1), responsible for the final step in triglyceride synthesis, leads to reduced adiposity and resistance to diet-induced obesity (4). Dgat1-deficient (Dgat1−/−) mice also showed increased energy expenditure, attributable in part to increased locomotor activity and increased levels of uncoupling protein-1, insulin, and leptin (4–6). Interestingly, Dgat1−/− mice had dry fur and hair loss, which were associated with atrophic sebaceous glands and abnormal lipid composition in the fur (7). Mice deficient in DGAT2 had a marked reduction in triglyceride synthesis and died at birth (8). These studies demonstrated that DGAT2 is essential for viability and that DGAT1 is unable to compensate for a deficiency in DGAT2. Although GPATs and DGATS catalyze the initial and final acylation steps in triglyceride synthesis from glycerol phosphate, the 1-acylglycerol-3-phosphate O-acyltransferase (AGPAT) enzymes act at an intermediate step. The product of the GPAT reaction, lysophosphatidic acid, is again acylated, this time by an AGPAT, to form phosphatidic acid (1, 2). Several AGPAT proteins have been reported and designated AGPAT1 through AGPAT7. An eighth related sequence was identified recently and designated AGPAT8 (accession number {"type":"entrez-protein","attrs":{"text":"NP_766303","term_id":"27370046","term_text":"NP_766303"}}NP_766303; (9)). AGPAT1 and AGPAT2 are well characterized, and their enzymatic activity has been documented (10, 11), whereas the other AGPAT family members were identified based upon sequence homology (12–14) or very modest activity levels (15). Mutations in Agpat2 have been reported in patients with congenital generalized lipodystrophy (16, 17), but no genetically modified animal model of any member of the AGPAT family has been generated. To investigate the importance of AGPAT enzymes in triglyceride biosynthesis and metabolism, we created and characterized a mouse model deficient in one of the most recently described family members, AGPAT6 (previously designated LPAAT-ζ) (13). We identified the mouse Agpat6 gene from sequence tags within the BayGenomics database. We used Agpat6 knockout embryonic stem cells to produce Agpat6−/− mice and then characterized the physiological role of this enzyme in adipose tissue and lipid metabolism.