ROLE FOR STEROL REGULATORY ELEMENT-BINDING PROTEIN IN THE REGULATION OF FARNESYL DIPHOSPHATE SYNTHASE AND IN THE CONTROL OF CELLULAR-LEVELSOF CHOLESTEROL AND TRIGLYCERIDE - EVIDENCE FROM STEROL REGULATION-DEFECTIVE CELLS

In order to define the factors involved in the regulation of farnesyl diphosphate (FPP) synthase, we used sterol regulation-defective (SRD) cell lines that constitutively express either high (SRD-2) or low (SRD -6) levels of transcriptionally active sterol regulatory element bindi ng protein (SREBP). FPP synthase mRNA levels were high in SRD-2 cells and low in SRD-6 cells and were unaffected by the addition or removal of sterols from the media. In contrast, the mRNA levels in parental CH O-7 cells were regulated by sterols. SRD-2, SRD-6, and CHO-7 cells wer e also transiently transfected with plasmids containing FPP synthase p romoter-reporter genes. Reporter gene activity was significantly highe r in SRD-2 cells than in either SRD-6 or CHO-7 cells, consistent with a higher rate of transcription of the reporter gene in SRD-2 cells. Th e high expression of the reporter gene in SRD-2 cells was not observed when the FPP synthase promoter contained a three base pair mutation w ithin an SREBP binding site, termed sterol regulatory element-3 (SRE-3 ). These observations are consistent with the hypothesis that high lev els of transcription of the FPP synthase gene are dependent on the ava ilability of transcriptionally active SREBP. We also demonstrate that the incorporation of radioactive acetate into both cholesterol and fat ty acids was enhanced in SRD-2 cells as compared to CHO-7 or SRD-6 cel ls. Finally, we demonstrate that the concentrations of cholesterol, ch olesteryl ester, and triglyceride were all significantly elevated in S RD-2 cells. We conclude that SREBP is involved not only in the regulat ion of FPP synthase and cholesterogenesis but also in fatty acid and t riglyceride synthesis.