Effects of estradiol and 4-hydroxytamoxifen on the conformation, thermal stability, and DNA recognition of estrogen receptor beta.

Estrogen receptors (ERalpha and ERbeta) are ligand-activated transcription factors. We examined the effects of estradiol (E2), 4-hydroxytamoxifen (HT), and the estrogen response element (ERE) on the helical content and thermal unfolding of ERbeta. A circular dichroism (CD) spectrum of ERbeta showed changes at 210 and 222 nm that were due to the presence of E2, which is indicative of partial unfolding. In contrast, HT did not alter the CD spectrum of ERbeta. The addition of E2 + ERE caused an increase in the alpha-helical content and an increase in the temperature midpoint of folding transition (TM) from 39 +/- 0.7 degrees C to 57.2 +/- 1 degrees C. The addition of E2 + mutant ERE, or E2 + control oligonucleotide, increased the TM of ERbeta to 45 +/- 2 degrees C only. In the presence of HT, ERbeta yielded similar TM values (55-58 degrees C) with ERE, mutant ERE, or control oligodeoxynucleotide. The binding affinity of ERbeta for ERE increased 125.7-fold as a result of the presence of E2, but only 4-fold as a result of HT. These results demonstrate coupled effects of E2 and ERE on ERbeta stability and binding affinity. The increased thermal stability of HT-ERbeta-ERE was associated with reduced specificity of ERbeta-ERE recognition, illustrating profound differences in conformational states of ERbeta induced by E2 and HT.