Quantitative increases in DNA binding affinity and positional effects determine 9-cis retinoic acid induced activation of the retinoid X receptorβ homodimer

Retinoid X receptors (RXRs) exert transcriptional activities through heterodimerization with members of the nuclear hormone receptor superfamily. RXRs also act as homodimers and stimulate transcription from an RXR responsive element (RXRE) when bound to 9-cis-retinoic acid (9cRA). Here direct effects of 9cRA have been examined on biochemical and mechanistic parameters of RXRβ. It is shown that 9cRA significantly increases RXRβ homodimer binding affinity to an RXRE (K_d without ligand = 18 nM, K_{d with ligand = 6 nM}), while decreasing significantly the affinity of RXRβ/thyroid hormone receptor (T3Ra) heterodimer binding to the same element. Effects on other response elements are also examined. The RXRβ homodimer was found to contact both halves of the RXRE direct repeat, irrespective of the effect of added ligand, while the RXRβ/T3R(α) heterodimer contacted the element only through a specific half-site. Binding of the homodimer to the element functionally activates RXRβ, since RXRβ enhanced transcription in vitro from a specific template in a ligand-dependent fashion. In agreement, transfection of RXRβ alone (but not RXRβ/TSR(α)) led to liganddependent activation of a reporter containing the RXRE. Taken together, 9cRA facilitates functional activation of the KXRβ homodimer in an element-dependent manner.