Promoter of FGF8 reveals a unique regulation by unliganded RARalpha

We previously reported that retinoids were inducing a complete switch in the expression of two isoforms from the fgf8 gene. In order to gain insight into the transcriptional mechanisms possibly involved in this regulation, we cloned and sequenced a fragment of genomic DNA encompassing 6 kb of the region 5' upstream of the fgf8 coding sequence and investigated its promoter elements. A comprehensive series of biochemical and cellular experiments determined two distinct functional regions cis-responsive to retinoids and/or their receptors: (i) a canonical RARE (type DR2) which is the cis target of a RARalpha-RXRalpha liganded heterodimer; and (ii) a completely novel type of response element composed of two half-binding sites separated by 87 nucleotides, which we demonstrate to be the target of an unliganded RARalpha homodimer phosphorylated on the Ser77 residue. Combined activities of these cis and trans-acting factors support a model of a complex regulation of fgf8 expression: by alternative binding to two distinct promoter elements, phosphorylated-unliganded-RARalpha homodimer or its liganded form have two distinct and mutually exclusive trans-activating activities, explaining the expression of two different isoforms of fgf8.