The role of AGPAT2 in lipid droplet biogenesis and insulin signalling

Abstract

1-Acylglycerol-3-Phosphate O-Acyltransferase 2 (AGPAT2) is an intermediate enzyme esterifying the sn-2 position of lysophosphatidic acid (LPA) to make phosphatidic acid (PA) in the de novo synthesis of glycerophospholipids (GPLs) and triacylglycerol (TAG). It has been reported that the absence of AGPAT2 elevated PA levels resulting in the increase of ER surface tension, thus altering the growth of LDs. However, the molecular link in between AGPAT2 and lipid droplet biogenesis remains obscure in the cells. Here, I showed that AGPAT2 controlled lipid droplet (LD) growth and maturation. AGPAT2 deficiency resulted in the formation of supersized lipid droplets and the accumulation of numerous nascent LDs. Furthermore, the depletion of glycerol-3-phostphate 3/4 (GPAT3/4), or the over-expression of CDP-diacylglycerol synthases (CDS1/2) and Seipin decreased the size of LDs in AGPAT2-deficient cells.This thesis also identified a novel regulatory mechanism to explain the elevated PA levels in AGPAT2-deficient cells. It was interesting to observe that the fluorescence intensity of CDS1/2 in AGPAT2-deficient cells was reduced significantly. Furthermore, CDS2, but not CDS1, was found to be the binding partner of AGPAT2 among all AGPAT family members. Most importantly, CDS2 protein level and CDS activity were decreased significantly in AGPAT2-deficient cells and tissues. In all, our results unveil a novel mechanism by which AGPAT2 deficiency facilitates CDS2 degradation and thus leads to an increase of cellular PA. Finally, this thesis also reported that the change of PA and all down-stream lipid intermediates by over-expressing of GPAT3 and AGPAT10 decreased AKT phosphorylation, causing insulin resistance. This thesis may contribute a mechanistic link between the effect of altered AGPATs levels and CGL1 or diabetes