Nuclear-localized CTP:phosphocholine cytidylyltransferase α regulates phosphatidylcholine synthesis required for lipid droplet biogenesis

The reversible association of CTP:phosphocholine cytidylyltransferase alpha (CCT alpha) with membranes regulates the synthesis of phosphatidylcholine (PC) by the CDP-choline (Kennedy) pathway. Based on results with insect CCT homologues, translocation of nuclear CCTa onto cytoplasmic lipid droplets (LDs) is proposed to stimulate the synthesis of PC that is required for LD biogenesis and triacylglycerol (TAG) storage. We examined whether this regulatory mechanism applied to LD biogenesis in mammalian cells. During 3T3-L1 and human preadipocyte differentiation, CCT alpha expression and PC synthesis was induced. In 3T3-L1 cells, CCTa translocated from the nucleoplasm to the nuclear envelope and cytosol but did not associate with LDs. The enzyme also remained in the nucleus during human adipocyte differentiation. RNAi silencing in 3T3-L1 cells showed that CCT alpha regulated LD size but did not affect TAG storage or adipogenesis. LD biogenesis in nonadipocyte cell lines treated with oleate also promoted CCT alpha translocation to the nuclear envelope and/or cytoplasm but not LDs. In rat intestinal epithelial cells, CCT alpha silencing increased LD size, but LD number and TAG deposition were decreased due to oleate-induced cytotoxicity. We conclude that CCT alpha increases PC synthesis for LD biogenesis by translocation to the nuclear envelope and not cytoplasmic LDs.