Transactivation via RAR/RXR-Sp1 interaction: Characterization of binding between Sp1 and GC box motif

Modulation of Spl activity by nuclear receptors is a novel mechanism by which fat-soluble hormones regulate gene expression. We previously established that upon autoinduction of RARs by RA, RARs/RXRs physically interact with Spl, potentiate Spl binding to the GC box motifs, and thus enhance transactivation of the urokinase promoter, which lacks a canonical RAR-responsive element/RXR-responsive element. Here, we examined whether a similar mechanism might participate in transcriptional regulation of other key RA-inducible genes in endothelial cells and characterized binding between Spl and GC box motifs. Northern blot analyses showed that in addition to urokinase, after induction of RARs, RA upregulates GC-rich region-dependent mRNA expression of transglutaminase, TGF beta1, and types I and II TGF beta receptors. RA failed to alter the expression of Spl at both mRNA and protein levels. Reporter and gel shift assays and Western blot analyses suggested that either RA-treatment or RAR/RXR-over-expression enhances transactivation of these genes through a GC-rich region and strengthens the affinity of Spl to GC box motifs, accompanying a potential conformational change of Spl as reflected in its increased immunogenicity. Detailed analyses of the GC box motifs within the urokinase and other promoters indicate that interaction between RAR/RXR and Spl does not occur in the presence of nonfunctional GC box motifs containing five tandem purine or pyrimidine bases at the 3'-flanking region of hexanucleotide core sequence. These findings provide insight into the molecular mechanisms underlying RARE/RXRE-independent transactivation of RA-inducible gene promoters.