Glycerol-3-phosphate acyltransferase isoforms: hepatic triacylgycerol synthesis and the development of insulin resistance

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  • March 21, 2019
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  • Nagle, Cynthia A.
    • Affiliation: Gillings School of Global Public Health, Department of Nutrition
Abstract
  • Elucidation of the metabolic pathways of triacylglycerol synthesis is critical to the understanding of chronic metabolic disorders such as obesity, cardiovascular disease, and diabetes. Glycerol-sn-3-phosphate acyltransferase (GPAT) and sn-1-acylglycerol-3- phosphate acyltransferase (AGPAT) catalyze the first and second steps in de novo triacylglycerol synthesis. These enzymes have multiple isoforms with different subcellular locations and tissue distributions. The specific function of each isoform and its product in the regulation of TAG synthesis and intracellular signaling pathways is not completely understood. This dissertation describes two separate projects. The first project examines the role of GPAT1 and de novo glycerolipid synthesis in the development of hepatic steatosis and hepatic insulin resistance. The second project describes the identification of a novel GPAT isoform, GPAT4. In the first project, we used an adenoviral construct to overexpress glycerol-sn-3- phosphate acyltransferase-1 (Ad-GPAT1) to determine whether increased glycerolipid flux can, by itself, cause hepatic insulin resistance. We found that hepatic overexpression of GPAT1 caused hepatic TAG accumulation and hyperlipidemia without affecting body weight or adiposity. This GPAT1-mediated increase in hepatic de novo glycerolipid synthesis caused both systemic and hepatic insulin resistance, which was associated with elevated hepatic diacylglyerol and lysophosphatidic acid content, suggesting a role for these lipid metabolites in the development of hepatic insulin resistance. We identified a potential DAG/protein kinase C-mediated mechanism for hepatic insulin resistance, and we did not find any evidence for an immune mediated mechaninsm. In the second project, we characterized the acyltransferase activity of AGPAT6. AGPAT6 was one of eight AGPAT isoforms identified through sequence homology, but the enzyme activity for AGPAT6 had not been confirmed. The NEM-sensitive GPAT specific activity was 65% lower in liver and brown adipose tissue from Agpat6-/- mice than in tissues from wild type mice, but the AGPAT specific activity was similar in wild type and Agpat6-/- mice. Cos-7 cells transfected with AGPAT6 had higher NEM-sensitive GPAT specific activity and no change in AGPAT activity. Thus, we have identified a novel NEM sensitive GPAT isoform that is expressed in liver and brown adipose tissue. We have named this isoform GPAT4.
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  • Coleman, Rosalind
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