Nuclear receptors modulate the interaction of Sp1 and GC-rich DNA via ternary complex formation

Binding sites for transcription factor Sp1 have been implicated in the tran scriptional regulation of several genes by hormones or vitamins, and here w e show that a GC-rich element contributes to the retinoic acid response of the interleukin 1 beta promoter. To explain such observations, it has been proposed that nuclear receptors can interact with Sp1 bound to GC-rich DNA. However, evidence supporting this model has remained indirect. So far, nuc lear receptors have not been detected in a complex with Sp1 and GC-rich DNA , and the expected ternary complexes in non-denaturing gels were not seen. In search for these missing links we found that nuclear receptors [retinoic acid receptor (RAR), thyroid hormone receptor (TR), vitamin D-3 receptor, peroxisome-proliferator-activated receptor and retinoic X receptor] induce an electrophoretic mobility increase of Sp1-GC-rich DNA complexes. Concomit antly, binding of Sp1 to the GC-box is enhanced. It is proposed that nuclea r receptors may partially replace Sp1 in homo-oligomers at the GC-box. RARs and Sp1 can also combine into a complex with a retinoic acid-response elem ent. The presence of RAR and Sp1 in complexes with either cognate site was revealed in supershift experiments. The C-terminus of Sp1 interacts with nu clear receptors. Both the ligand- and DNA-binding domains of the receptor a re important for complex formation with Spl and GC-rich DNA. In spite of si milar capacity to form ternary complexes, RAR but not TR up-regulated an Sp l-driven reporter in a ligand-dependent way. Thus additional factors limit the transcriptional response mediated by nuclear receptors and Sp1.