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Abstract
Lipid droplets (LDs) are primary lipid storage organelles which have remained evolutionarily conserved in prokaryotes such as Gram negative bacteria up to higher eukaryotes including humans. Recent studies have emphasised the crucial role of LDs in human metabolic diseases including obesity, diabetes and hepatic steatosis, which are health issues that are becoming increasingly problematic in developed countries. Identifying the molecular mechanisms involved in adipogenesis and LD expansion will provide targets for the development of therapeutic treatment against these diseases. This study reports on enzymes involved in de novo triacylglycerol (TAG) synthesis, with particular emphasis on the role of GPAT and seipin in mediating the growth of LDs and influencing adipocyte development. It also identifies an evolutionarily conserved function of GPAT and seipin in yeast and mammalian cells. Four key enzymes catalyse glycerol phosphate pathway; these include glycerol-3-phosphate acyltransferase (GPAT), 1-acyl-sn-glycerol-3-phosphate O-acyltransferase (AGPAT), phosphatidate phoshatase (PAP) and diglyceride acyltransferase (DGAT). Seipin is another major regulator of LD dynamics and adipogenesis and has been implicated with controlling cellular levels of phosphatidic acid (PA), a phospholipid precursor that also governs TAG synthesis. The over-expression of GPAT in yeast and mammalian cells resulted in the development of a “supersized” LDs (SLD) phenotype, whereas the over-expression of seipin did not influence LD size in yeast and mammalian cells. However, seipin depletion resulted in the formation of SLDs. Interestingly, the over-expression of both seipin and GPAT in yeast and mammalian cells revealed a rescue of the SLD phenotype. Moreover, the simultaneous deletion GPAT and seipin in 3T3-L1 pre-adipocytes also abolished the SLD phenotype. Furthermore, GPAT3 and GPAT4 over-expression in 3T3-L1 pre-adipocytes has been established to inhibit adipocyte differentiation. Adipogenesis was not inhibited through over-expression of catalytically inactive GPAT3 and GPAT4 mutants, identifying the role of GPAT activity in adipocyte development. Findings from this study establish a functional relationship between seipin, GPAT3 and GPAT4 that is evolutionarily conserved in yeast and mammalian cells. It also identifies the role of yeast AGPAT, PAP and DGAT in regulating LD expansion.