Control of the estrogen-like actions of the tamoxifen-estrogen receptor complex by the surface amino acid at position 351

Tamoxifen is a valuable therapeutic agent with applications in the treatmen t and prevention of breast cancer. However, the development of drug resista nce limits the usefulness of tamoxifen therapy. One form of drug resistance in breast cancer is tamoxifen-stimulated growth. We have addressed a mecha nism how the tamoxifen-estrogen receptor (ER) complex can convert from bein g a blocking to stimulatory signal in breast cancer. We have described an e ffective assay system to study the action of antiestrogen-ER complex throug h the activation of transforming growth factor alpha gene in situ. The MDA- MB-231 breast cancer cells were stably transfected with cDNAs for wtER (D35 1), mutant Asp351Tyr ER (D351Y) and mutant Asp351Gly ER (D351G). The D351Y ER can enhance the estrogenic properties of 4OHT and change the pharmacolog y of raloxifene by converting it from antiestrogen to estrogen. We hypothes ized that alterations in the charge of amino acid (aa) 351, and changes in the interaction with the side chain of an antiestrogen, are critical for th e subsequent estrogenicity of the complex. Our goal was (1) to modulate the estrogenicity of the antiestrogen-ER complex by different aa substitutions at position 351 and (2) to examine the role of alterations in the side cha in of antiestrogens on the estrogenicity of the complex. Substitution of ty rosine for aspartate at aa351 results in increased estrogenicity for a seri es of tamoxifen derivatives-ER complexes and the conversion of EM 652-ER an d GW 7604-ER complexes from antiestrogenic to estrogen-like. Substitution o f glycine for aspartate at aa 351 results in the conversion of 4OHT-ER comp lex from estrogen-like to antiestrogenic. We propose that the side chain of antiestrogens either neutralizes or displaces the charge at aspartate 351 thereby removing a charged site for the opportunistic binding of a novel co activator. If no charge is present (D351G) then no coactivator can bind and the complex with any antiestrogen is not estrogen-like. However, if the ch arge is extended beyond the reach of an antiestrogen side chain (D351Y), th en the coactivators bind and compounds are estrogen-like. The establishment of a relationship between the structure of the antiestrogen-ER complex and its function will enhance the development of novel compounds with unique b iological activities and potentially avoid premature drug resistance. (C) 2 001 Elsevier Science Ltd. All rights reserved.