| Abstrakt: |
3-Hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) synthase, a key regulatory enzyme in the pathway for endogenous cholesterol synthesis, is a target for negative feedback regulation by cholesterol. The promoter for HMG-CoA synthase contains two binding sites for the sterol regulatory element-binding proteins (SREBPs). When cellular sterol levels are low, the SREBPs are released from the endoplasmic reticulum membrane, allowing them to translocate to the nucleus and activate SREBP target genes. In all SREBP-regulated promoters studied to date, additional co-regulatory transcription factors are required. In the HMG-CoA synthase promoter there are several potential co-regulatory transcription factor binding sites, including an inverted CCAAT box. A similar element has been shown to function with SREBP to mediate sterol regulation of another gene involved in cholesterol metabolism, farnesyl diphosphate synthase. Here, we show that CCAAT binding factor/nuclear factor Y (CBF/NF-Y) binding to the CCAAT box is required for sterol-regulated transcription of HMG-CoA synthase. The SREBP sites and the inverted CCAAT box are normally separated by 17 base pairs, and we show that increasing this distance results in a decrease in the level of transcriptional regulation by sterols. Furthermore, we provide evidence that there is a direct interaction between CBF/NF-Y and the basic helix-loop-helix-zipper region of SREBP. Interestingly, this interaction does not occur efficiently with any of the isolated subunits and appears to require all three nonidentical CBF/NF-Y subunits in a preassembled complex. Since CBF/NF-Y only binds to DNA when all three subunits are in a complex, this would prevent SREBP from forming nonproductive associations with the individual subunits. |
