Transcription Initiation And Transcriptional Directionality Of The CpG Island-associated Gene, Insulin-degrading Enzyme

CpG islands are defined as GC-rich genomic regions which have a high frequencyof CpG sites. In mammals, CpG islands are often associated with gene promoters. CpGisland promoters frequently lack common core promoter elements, such as theTATA box, and usually have multiple transcription start sites (called dispersedtranscription initiation). Besides, bidirectional promoters are over-representedin CpG islands. The mechanism of transcription initiation and transcriptionaldirectionality of CpG island promoters remains unclear. Specially, CpG islandpromoters often lack the TATA box, so it is necessary to study whetherTATA-binding protein (TBP) participates in their transcription initiation.Insulin-degrading enzyme (IDE) is a ubiquitous zinc metalloprotease whichdegrades several substrates including insulin. In this paper, we use the IDE geneas a model to investigate the mechanism of transcription initiation from CpGisland promoters. The mouse IDE promoter contains a CpG island and hasmultiple transcription start sites. The IDE core promoter locates within a23-bpregion and contains a binding site of nuclear respiratory factor1(NRF-1). Ourstudies indicate that NRF-1is critical for IDE transcription initiation.Furthermore, the IDE promoter contains no TATA boxes and its transcriptioninitiation is TBP-independent. We speculate that some transcription factors candrive transcription initiation via themselves and play important roles during thetranscriptional regulation of CpG island promoters. We therefore screen avariety of transcription factors, and find that NRF-1, the basic helix-loop-helix/leucine zipper (bHLH/ZIP) family of proteins and the E-twenty six (Ets) familyof proteins have strong transcriptional activity. Besides, these transcriptionfactors direct dispersed transcription initiation, which is consistent with theproperty of CpG island promoters. Finally, we study the transcriptionaldirectionality of the IDE promoter. Although IDE is unidirectionally transcribedaccording to its genomic context, the basic promoter region of mouse IDE hasbidirectional transcriptional activity. Our experiments suggest that an upstreamDNA element in the IDE promoter blocks its antisense transcription. Throughpromoter deletion and mutagenesis analysis, we map this DNA element precisely. In addition, the human IDE promoter contains a similar DNA element.In conclusion, we have investigated the transcription initiation and thetranscriptional directionality of the CpG island-associated gene, IDE. NRF-1mediates the transcription initiation of IDE in a TBP-independent manner. Anupstream promoter element blocks the antisense transcription of IDE. NRF-1,the bHLH/ZIP family and the Ets family have strong transcriptional initiationactivity, and play important roles during the transcriptional regulation of CpGisland promoters. Our studies are helpful to clarify the mechanism oftranscription initiation and transcriptional directionality of CpG islandpromoters.