Receptor interconversion model of hormone action. I. ATP-mediated conversion of estrogen receptors from a high to lower affinity state and its relationship to antiestrogen action

Academic Article
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publication date

  • 1986

abstract

  • We have previously reported that the estrogen receptor exists in three distinct states in the oviduct of the estrogen-treated chick. Since two of these forms bind estrogen and possess a number of similar properties, the intrinsic relationship between these two receptor forms has been investigated. We now show that a quantitative conversion of the high affinity (Rx) to the lower affinity (Ry) state can be induced by mild heating (30 degrees C) in the presence of estradiol and ATP, or the synthetic analogue alpha,beta-methylene adenosine triphosphate and the divalent cation Mg2+. Other nucleotides, including ADP, GTP, CTP, cAMP, and cGMP, as well as the nonhydrolyzable analogues beta,gamma-methylene adenosine triphosphate and alpha,beta-methylene adenosine diphosphate are ineffective. The conversion occurs only partially in the absence of estradiol but completely in its presence. The process is not inhibited by the protease inhibitors phenylmethylsulfonyl fluoride and alpha 2-macroglobulin, and when conversion is induced by low concentrations of ATP (1 mM), a time dependent reequilibration back to Rx occurs. These observations and the fact that the pure hormone-binding peptides Rx and Ry have similar molecular weights on sodium dodecyl sulfate-polyacrylamide gels (66,000) confirm that proteolysis is not involved in the conversion. Moreover their physical properties suggest that Rx and Ry exist in alternate conformations, with Ry being favored as a result of an ATP-mediated event involving the gamma-phosphoryl moiety. The biological relevance of the receptor conversion is suggested by studies with the antiestrogen hydroxytamoxifen. This antiestrogen binds to Rx with higher affinity than either estradiol or diethylstilbestrol but with low affinity to Ry. Hydroxytamoxifen also inhibits the conversion of Rx to Ry. Since this antiestrogen is a complete antagonist in the chick oviduct and prevents estradiol-induced stimulation of ovalbumin gene transcription, it is speculated that Rx to Ry conversion is crucial for ovalbumin gene activation and that Rx may act as a transcriptional repressor. Furthermore, since Rx and Ry both bind to nuclei and DNA, it is proposed that the presence of Ry is a better predictor of ovalbumin gene activation than DNA binding alone.