Differential modulation of transcriptional activity of oestrogen receptorsby direct protein-protein interactions with retinoid receptors

Control of oestradiol-responsive gene regulation by oestrogen receptors (ER s) may involve complex cross-talk with retinoic acid receptors (RARs) and r etinoid X receptors (RXRs). Recently, we have shown that ER alpha directly interacts with RAR alpha and RXR alpha through their ligand binding domains (LBDs). In the present work, we extend these results by showing that ERP b inds similarly to RAR alpha and RXR alpha but not to the glucocorticoid rec eptor, as demonstrated by the yeast two-hybrid tests and glutathione S-tran sferase pull-down assays. These direct interactions u ere also demonstrated in gel-shift assays, in which the oestrogen response element (ERE) binding by ER alpha was enhanced by the RXR alpha LBD but was abolished by the RAR alpha LBD. In addition, we showed that RAR alpha and RXR alpha bound the E RE as efficiently as ER alpha, suggesting that competition for DNA binding may affect the transactivation function of the ER. In transient transfectio n experiments, co-expression of RAR alpha or RXR alpha, along with ER alpha or ER beta, revealed differential modulation of the ERE-dependent transact ivation, which was distinct from the results when each receptor alone was c o-transfected, Importantly, when the LED of RAR alpha was co-expressed with ER alpha transactivation of ER alpha. on the ER alpha was repressed as eff iciently as when wild-type RAR alpha was co-expressed. Furthermore, ligande d RAR alpha or unliganded RXR alpha enhanced the ER alpha transactivation, suggesting the formation of transcriptionally active heterodimer complexes between the ER and retinoid receptors. Taken together, these results sugges t that direct protein-protein interactions may play major roles in the dete rmination of the biological consequences of cross-talk between ERs and RAR alpha or RXR alpha.