BINDING OF TYPE-II NUCLEAR RECEPTORS AND ESTROGEN-RECEPTOR TO FULL AND HALF-SITE ESTROGEN RESPONSE ELEMENTS IN-VITRO

The mechanism by which retinoids, thyroid hormone (T-3) and estrogens modulate the growth of breast cancer cells is unclear, Since nuclear t ype II nuclear receptors, including retinoic acid receptor (RAR), reti noid X receptor (RXR) and thyroid hormone receptor (TR), bind direct r epeats (DR) of the estrogen response elements (ERE) half-site (5'-AGGT CA-3'), we examined the ability of estrogen receptor (ER) versus type II nuclear receptors, i.e. RAR alpha, beta and gamma, RXR beta, TR alp ha and TR beta, to bind various EREs in vitro, ER bound a consensus ER E, containing a perfectly palindromic 17 bp inverted repeat (IR), as a homodimer, In contrast, ER did not bind to a single ERE half-site, Li kewise, ER did not bind two tandem (38 bp apart) half-sites, but low E R binding was detected to three tandem copies of the same half-site, R AR alpha, beta or gamma bound both ERE and half-site constructs as a h omodimer, RXR beta did not bind full or half-site EREs, nor did RXR be ta enhance RAR alpha binding to a full ERE, However, RAR alpha and RXR beta bound a half-site ERE cooperatively forming a dimeric complex, T he RAR alpha-RXR beta heterodimer bound the Xenopus vitellogenin B1 es trogen responsive unit, with two non-consensus EREs, with higher affin ity than one or two copies of the full or half-site ERE, Both TR alpha and TR beta bound the full and the half-site ERE as monomers and homo dimers and cooperatively as heterodimers with RXR beta. We suggest tha t the cellular concentrations of nuclear receptors and their ligands, and the nature of the ERE or half-site sequence and those of its flank ing sequences determine the occupation of EREs in estrogen-regulated g enes in vivo.