HETERODIMERIZATION AMONG THYROID-HORMONE RECEPTOR, RETINOIC ACID RECEPTOR, RETINOID-X RECEPTOR, CHICKEN OVALBUMIN UPSTREAM PROMOTER TRANSCRIPTION FACTOR, AND AN ENDOGENOUS LIVER PROTEIN

Thyroid hormone receptor (TR) binds to DNA as a monomer, homodimer, and heterodimer with nuclear proteins. We have confirmed that the TR can heterodimerize with retinoid X receptors (RXRs)-alpha and -beta, and have found that another member of the nuclear receptor superfamily, chicken ovalbumin upstream promoter transcription factor (COUP-TF), also formed heterodimers with the TR in the context of binding to a palindromic thyroid hormone-responsive element (TREp). The interaction between COUP-TF and the TR was confirmed using specific antibodies which supershifted the COUP-TF/TR DNA complexes. The complex between the TR and the major TR heterodimerization partner in liver was unaffected by antibodies to COUP-TF and RXRbeta, but was supershifted by an anti-RXRalpha antibody, indicating that the liver protein is highly related to RXRalpha. Indeed, the TR/RXR and TR/liver protein heterodimers contact the same guanidine residues in TREp. The retinoic acid receptor (RAR) also heterodimerized with COUP-TF as well as with RXRalpha, RXRbeta, and the TR heterodimerization partner in liver. In contrast to its ability to heterodimerize with the TR and RAR, we did not detect heterodimers between COUP-TF and either RXRalpha, RXRbeta, or the liver nuclear protein in the context of binding to the TREp. These results show that the major TR heterodimerization partner in liver is highly related to RXRalpha, but that other nuclear receptors such as COUP-TF can heterodimerize with the TR and RAR, suggesting that selective protein-protein interactions may be involved in the tissue and target gene specificities of hormone action.