Structure-function relationships of the raloxifene-estrogen receptor-α complex for regulating transforming growth factor-α expression in breast cancer cells

Amino acid Asp-351 in the ligand binding domain of estrogen receptor a (ERα) plays an important role in regulating the estrogen-like activity of selective estrogen receptor modulator-ERα complexes. 4-Hydroxytamoxifen is a full agonist at a transforming growth factor α target gene in situ in MDA-MB-231 human breast cancer cells stably transfected with the wild-type ERα. In contrast, raloxifene (Ral), which is also a selective estrogen receptor modulator, is a complete antiestrogen in this system. Because D351G ERα allosterically silences activation function-1 activity in the 4-hydroxytamoxifen-ERα complex with the complete loss of estrogen-like activity, we examined the converse interaction of amino acid 351 and the piperidine ring of the antiestrogen side chain of raloxifene to enhance estrogen-like action. MDA-MB-231 cells were either transiently or stably transfected with Asp-351 (the wild type), D351E, D351Y, or D351F ERα expression vectors. Profound differences in the agonist and antagonist actions of Ral·ERα complexes were noted only in stable transfectants. The agonist activity of the Ral·ERα complex was enhanced with D351E and D351Y ERα, but raloxifene lost its agonist activity with D351F ERα. The distance between the piperidine nitrogen of raloxifene and the negative charge of amino acid 351 was critical for estrogen-like actions. The role of the piperidine ring in neutralizing Asp-351 was addressed using compound R1h, a raloxifene derivative replacing the nitrogen on its piperidine ring with a carbon to form cyclohexane. The derivative was a potent agonist with wild type ERα. These results support the concept that the side chain of raloxifene shields and neutralizes the Asp-351 to produce an antiestrogenic ERα complex. Alteration of either the side chain or its relationship with the negative charge at amino acid 351 controls the estrogen-like action at activating function 2b of the selective estrogen receptor modulator ERα complex.