To understand cholesterol-mediated regulation of human fatty acid synthase promoter I, we tested various 5 -deletion constructs of promoter I-luciferase reporter gene constructs in HepG2 cells. The reporter gene constructs that contained only the Sp-1-binding site (nucleotides ؊82 to ؊74) and the two tandem sterol regulatory elements (SREs; nucleotides ؊63 to ؊46) did not respond to cholesterol. Only the reporter gene constructs containing a nuclear factor-Y (NF-Y) sequence, the CCAAT sequence (nucleotides ؊90 to ؊86), an Sp-1 sequence, and the two tandem SREs responded to cholesterol. The NF-Y-binding site, therefore, is essential for cholesterol response. Mutating the SREs or the NF-Y site and inserting 4 bp between the Sp-1-and NF-Y-binding sites both resulted in a minimal cholesterol response of the reporter genes. Electrophoretic mobility-shift assays using anti-SRE-binding protein (SREBP) and anti-NF-Ya antibodies confirmed that these SREs and the NF-Y site bind the respective factors. We also identified a second Sp-1 site located between nucleotides ؊40 and ؊30 that can substitute for the mutated Sp-1 site located between nucleotides ؊82 and ؊74. H umans, like other animals, derive long-chain fatty acids either from food or by de novo synthesis from acetyl-CoA. De novo synthesis requires the participation of two multifunctional enzymes, fatty acid synthase (FAS; EC 2.3.1.85) and acetyl-CoA carboxylase (EC 6.4.1.2) (1). The regulation of animal FAS by diet and hormones is well documented (2). A fat-free carbohydrate-rich diet induces the synthesis of FAS and, consequently, an increase in the metabolic levels of long-chain fatty acids (1, 2). The levels of FAS are controlled by the rate of transcription and by the stability of its mRNA (2, 3). The regulation of animal FAS mRNA levels by the thyroid hormone and by insulin has been demonstrated (2, 3).Recent studies on cholesterol-mediated regulation of rat acetyl-CoA carboxylase (4, 5), rat FAS promoter I (6, 7), and stearoyl-CoA desaturase 1 (8, 9) suggest that lipogenesis and sterol synthesis and uptake are all coordinately regulated by cholesterol. Sterol-mediated regulation is facilitated by sterol regulatory element (SRE)-binding proteins (SREBPs) (10-14). SREBPs were originally characterized as transcription factors that regulate the genes involved in cholesterol uptake from plasma, such as the low-density lipoprotein receptor (15,16), and the genes involved in the biosynthetic pathway of cholesterol and lipids, such as 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) synthase, HMG-CoA reductase (17, 18), farnesyl diphosphate (FPP) synthase (19,20), squalene synthase (21, 22), and glycerol-3-phosphate acyltransferase (23).SREBPs are a novel family of membrane-bound transcription factors that contain the basic helix-loop-helix leucine zipper (24). When sterol levels are low, the membrane-bound precursor form of SREBP undergoes proteolytic cleavage in a two-step process that releases the N-terminal SREBP domain, which then enters the nucleus and binds to the SRE...