POSITIVE REGULATION OF THE VHNF1 PROMOTER BY THE ORPHAN RECEPTORS COUP-TFI EAR3 AND COUP-TFII/ARP1/

vHNF1 (also termed HNF1 beta) is a member of the hepatocyte nuclear fa ctor 1 (HNF1; also termed HNF1 alpha) family of homeodomain-containing transcription factors that interact with a sequence motif found in th e regulatory regions of a large number of genes expressed mainly in th e liver, It has been suggested that vHNF1 plays a role in early differ entiation of specialized epithelia of several endoderm- and mesoderm-d erived organs, with HNF1 playing a role in later stages, In support of this idea, expression of vHNF1 but not HNF1 is induced upon treatment of the embryonal carcinoma cell line F9 with retinoic acid. We have c loned and analyzed the vHNF1 promoter to gain a better understanding o f the regulation of vHNF1 expression and how it relates to the express ion of HNF1, We have identified five sites of DNA-protein interaction within the first 260 bp upstream of the transcription start site, whic h involve at least three different families of transcription factors, Two sites, a distal DR-1 motif and a proximal octamer motif, are the m ost important for promoter activity, The DR-1 motif interacts with sev eral members of the steroid hormone receptor superfamily including HNF 4, COUP-TFI/Ear3, COUP-TFII/Arp1, and RAR alpha/RXR alpha heterodimers , The vHNF1 promoter is transactivated by COUP-TFI/Ear3 and COUP-TFII/ Arp1 and, unlike the vHNF1 promoter, is virtually unaffected by HNF4. Interestingly, the proximal octamer site and not the DR-1 site is requ ired for COUP-TFI/Ear3 and COUP-TFII/Arp1 transactivation of the vHNF1 promoter, COUP-TFI/Ear3 does not bind directly to this proximal octam er site, We present evidence of an interaction between COUP-TFI/Ear3 a nd the octamer-binding proteins in vitro and in the cell, suggesting t hat COUP-TFI and COUP-TFII activate the vHNF1 promoter via an indirect mechanism.