Role of the liver-enriched transcription factor hepatocyte nuclear factor 1 in transcriptional regulation of the factor V111 gene.

摘要

Coagulation factor VIII is an essential cofactor required for normal hemostatic function. A deficiency in factor VIII results in the bleeding disorder hemophilia A. Despite the fact that the factor VIII gene was cloned a decade ago, the mechanisms which control its transcription remain unresolved. In our studies, we have characterized 12 protein binding sites within the factor VIII promoter by DNase I protection assays performed with rat liver nuclear extracts. Three of these elements (sites 1 to 3) are situated within the 5' untranslated region of the gene, while three other sites (sites 4 to 6) lie within the first 100 bp upstream of the transcriptional start site. We have identified an additional site (site 7) approximately 300 bp upstream from site 6, as well as a cluster of five sites in a 250-bp region which terminates approximately 1 kb from the transcriptional start site. Seven of these binding sites (sites 2, 3, 4, 6, 7, 9, and 10) bind members of the C/EBP family of transcription factors. DBP also binds to five of these sites (sites 3, 4, 6, 7, and 9). Utilizing transient transfection studies in HepG2 cells, we have shown that deletion of the factor VIII promoter sequences distal to nucleotide -44 results in a significant but small increase in promoter activity. The activity of each of the various 5' deletion constructs is significantly enhanced by cotransfection of C/EBPalpha and D-site-binding protein expression plasmids, while cotransfection of both C/EBPalpha and C/EBPbeta plasmids resulted in a further enhancement of transactivation. These studies also provide evidence of a repressor element located between nucleotides -740 and -1002. Since the minimal promoter sequence (-44 to +148) maintains the transcriptional activity of the full-length promoter sequence, we proceeded to identify additional factors binding to sites 1 to 4. Competition studies revealed that a ubiquitous transcription factor, NF-Y, binds to site 4, while the liver-enriched transcription factor hepatocyte nuclear factor I (HNF-1) binds to site 1. Mutation analysis of the minimal promoter demonstrated that HNF-1 is critical for activating transcription of the factor VIII gene in vitro. Our results also suggest that the multiple upstream elements that we have identified may act as a backup regulatory region in the event of disruption of the HNF-1 element in the 5' untranslated region.