A direct protein-protein interaction is involved in the cooperation between thyroid hormone and retinoic acid receptors and the transcription factor GHF-1.

The nuclear receptors for thyroid hormone (TRs) and retinoic acid (RARs and RXRs) cooperate with the pituitary-specific transcription factor GHF-1 to activate the rat growth hormone (GH) gene. The GH promoter contains a hormone response element (HRE), which binds TR/RXR and RAR/RXR heterodimers, located close to two binding sites for GHF-1. GHF-1 inhibits binding of TR/RXR and RAR/RXR heterodimers to an isolated HRE. Similarly, the receptors inhibit binding of GHF-1 to its cognate site. These results suggest the existence of direct protein to protein interactions between the receptors and the pituitary transcription factor. This was confirmed by in vitro binding studies with GST fusion proteins, which demonstrated a strong association of GHF-1 with RXR and a weaker interaction with RAR and TR. GHF-1 and the receptor heterodimers form a ternary complex with a fragment of the rat GH promoter, which contains binding sites for both, and GHF-1 increases receptor binding to the promoter when present in limiting conditions. These results suggest that the synergistic activation of the rat GH gene involves protein-DNA interactions as well as a physical association between the nuclear receptors and the pituitary-specific transcription factor GHF-1.