Fusion estrogen receptor proteins: toward the development of receptor-based agonists and antagonists

Estrogen-induced signaling mediated by estrogen receptors (ERs) is also aff ected by aberrant ERs that act as constitutively active or dominant negativ e modulators. Variant ERs can contribute to carcinogenesis and to the loss of estrogen responsiveness, rendering antiestrogen therapy ineffective. Det ermining target gene response during co-synthesis of different ER species i s difficult, because dimers formed in the presence of more than one ER spec ies are a heterogenous population of homo- or heterodimers. We engineered a hornofusion ER alpha as a prototype single-chain receptor by genetically c onjugating two ER monomers into a covalently fused single-chain protein to obtain a homogeneous population. This permits analysis of symmetrical or as ymmetrical mutations that simulate variant homo- and heterodimers. Although a monomer, the homofusion receptor exhibited similar biochemical and funct ional properties to the dimeric ER alpha. We used activation function-2 (AF 2) defective mutants as a model in either one or both receptor domains for a dominant-negative phenotype by suppressing the reporter activity induced by the WT receptor. When co-expressed with ER alpha, the fusion variant def icient in both AF2 functions suppressed the reporter activity effectively i nduced by ER alpha. These results show the utility of fusion receptors as m odels for generation of receptor-based agonists and antagonists. (C) 2001 E lsevier Science Ireland Ltd. All rights reserved.