CTP : phosphocholine cytidylyltransferase, a new sterol- and SREBP-responsive gene

The CTP:phosphocholine cytidylyltransferase (CT) gene encodes the rate-cont rolling enzyme in the phosphatidylcholine biosynthesis pathway. CT alpha mR NA levels, like farnesyl diphosphate synthase and the LDL receptor, are rep ressed when human or rodent cells are incubated with exogenous sterols and induced when cells are incubated in lipid-depleted medium. A putative stero l response element (SRE) was identified 156 bp upstream of the transcriptio n start site of the CT alpha gene. Electrophoretic mobility shift assays de monstrate that recombinant SREBP-1a binds to the wild-type SRE identified i n the CT alpha promoter but not to oligonucleotides containing two mutation s in the SRE. In other studies, a luciferase reporter construct under the c ontrol of the murine CT alpha proximal promoter was transiently transfected into cells. The activity of the reporter was repressed after addition of s terols to the medium and induced when the cells were incubated in lipid-dep leted medium. The activity of the CT alpha -luciferase reporter was also in duced when cells were cotransfected with plasmids encoding either SREBP-1a or SREBP-2. In contrast, no induction was observed under the same condition s when the CT alpha promoter-reporter gene contained two mutations in the S RE. In addition, the induction of the wild-type CTa promoter-reporter gene that occurs in cells incubated in lipid-depleted medium is attenuated when dominant-negative SREBP is cotransfected into the cells. These studies demo nstrate that transcription of the CT alpha gene is inhibited by sterols and activated by mature forms of SREBP.ie We conclude that SREBP-regulated gen es are involved not only in the synthesis of cholesterol, fatty acids, trig lycerides, and NADPH, but also, as shown here, in the synthesis of phosphol ipids.