CBP IS REQUIRED FOR STEROL-REGULATED AND STEROL REGULATORY ELEMENT-BINDING PROTEIN REGULATED TRANSCRIPTION

Cells were transfected with luciferase reporter genes, under the contr ol of promoters derived from either the farnesyl diphosphate (FPP) syn thase, 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) synthase, HMG-C oA reductase, or low density lipoprotein receptor genes. The increase in luciferase activity that occurred when cells were either incubated in sterol-depleted medium or cotransfected with a cDNA encoding sterol regulatory element-binding protein (SREBP)-1a was prevented by coexpr ession of wild type E1A or a Gal4-CBP (1-451) fusion protein. The inhi bitory effect of E1A was overcome by coexpression of CBP. The increase in reporter gene activity noted above was not affected when the cells were cotransfected with cDNAs that encoded either a mutant E1A that i s unable to interact with the transcriptional activator CBP or Gal4-CB P fusion proteins encoding separate fragments of CBP, which span the r emainder of the CBP molecule, A preformed SREBP-1a:[P-32]DNA complex b ound specifically to membrane-immobilized GST-CBP fusion proteins that contained amino terminal portions of CBP,In order to investigate the role of CBP in the regulation of endogenous genes, we isolated stable transformants that express Ga14-CBP(1-451) in response to added doxycy cline. Induction of endogenous FPP synthase and HMG-CoA synthase mRNAs , in response to cellular cholesterol depletion, was prevented when ce lls expressed Ga14-CBP(1-451), We conclude that when cells are incubat ed in the absence of sterols, the transcriptional activation of the HM G-CoA synthase, HMG-CoA reductase, FPP synthase, and low density lipop rotein receptor genes is dependent on a specific interaction between S REBP, which is bound to the promoter DNA, and the amino terminal domai n (amino acids 1-451) of CBP.