The characterisation of CDP-diacylglycerol synthases in lipid droplet dynamics and adipocyte development

Abstract

Changes in the cellular dynamics of lipid droplets (LDs) are associated with human metabolic disorders such as obesity. Adipocyte differentiation is the process where preadipocytes differentiate into mature adipocytes. Understanding the molecular mechanisms involved in LD dynamics and adipocyte development will provide insights into how therapeutic treatments can be developed against human metabolic diseases. In mammals, there are two cytidine-diphosphate-diacylglycerol (CDP-DAG) synthases, CDS1 and CDS2. CDP-DAG synthases catalyse the formation of CDP-DAG by utilising phosphatidic acid (PA), the precursor for all phospholipids and triacylglycerols (TAGs). This study has characterised CDS1 and CDS2 as enzymes involved in lipid storage, at both the cellular and systemic level. The down-regulation of CDS1 or CDS2 resulted in the formation of “Supersized” LDs (SLDs) in cultured cells. To identify the impact of knocking down CDS1 and CDS2 on lipid profiles, lipid analysis was carried out on extracted neutral lipids. Mass spectrometry indicated a significant increase in total cellular PA of siCDS1 cells. Fluorescence microscopy revealed that PA strongly accumulates in the endoplasmic reticulum (ER) of HeLa cells down-regulated with CDP-DAG synthases. Importantly, depleting CDS1 almost completely blocked the differentiation of 3T3-L1 preadipocytes, whereas depleting CDS2 had a moderate inhibitory effect on adipocyte differentiation. Furthermore, transient siRNA transfections in mature adipocytes showed that CDP-DAG synthases are involved in the fusion of large LDs. A role for CDS1 and CDS2 in isoproterenol stimulated lipolysis was also revealed, with a decrease in phosphorylated-perilipin when CDS1 or CDS2 was depleted. This study also established a functional interaction between CDS1/2 and Fld1p/Seipin. These findings indicate that CDP-DAG synthases are more than just enzymes involved in phospholipid synthesis. They are regulators of lipid storage and play important roles in LD growth, adipogenesis and adipocyte physiology.