Specific interaction between E2F1 and Sp1 regulates the expression of murine CTP:phosphocholine cytidylyltransferase alpha during the S phase

CTP:phosphocholine cytidylyltransferase alpha (CCTα) is a key enzyme for phosphatidylcholine biosynthesis in mammalian cells. This enzyme plays an essential role in all processes that require membrane biosynthesis such as cell proliferation and viability. Thus, CCTα activity and expression fluctuate during the cell cycle to achieve PtdCho requirements. We demonstrated, for the first time, that CCTα is localized in the nucleus in cells transiting the S phase, whereas it is localized in the cytoplasm of G0-arrested cells, suggesting a specific role of nuclear CCTα during the S phase. We also investigated how E2F1 influences the regulation of the CCTα-promoter during the S phase; we demonstrated that E2F1 is necessary, but not sufficient, to activate CCTα expression when this factor is over-expressed. However, when E2F1 and Sp1 were over-expressed, the transcription from the CCTα-promoter reporter construct was super-activated. Transient transfection studies demonstrated that E2F1 could super-activate Sp1-dependent transcription in a promoter containing only the Sp1 binding sites “B” or “C”, and that Sp1 could activate Sp1-dependent transcription in a promoter containing the E2F site, thus, further demonstrating a functional interaction of these factors. In conclusion, the present results allowed us to portray the clearest picture of the CCTα-gene expression in proliferating cells, and understand the mechanism by which cells coordinate cell cycle progression with the requirement for phosphatidylcholine.