Silencing and reactivation of the selective estrogen receptor modulator-estrogen receptor alpha complex

4-Hydroxytamoxifen (4-OHT), a selective estrogen receptor modulator, is an agonist at a transforming growth factor-alpha (TGF-alpha) target gene in si tu in MDA-MB-231 human breast cancer cells stably transfected with mild-typ e human ER alpha. In contrast, raloxifene (Ral) is a complete antiestrogen silencing activation function (AF) 1 and AF2 in this system. A natural muta tion D351YER alpha enhances 4-OHT agonist activity and changes Ral-like com pounds from antagonists to partial agonists, We reasoned that: either the c onformation of the Ral-D351YER alpha is altered, thereby reactivating AF2 i n the ligand binding domain, or the change at amino acid 351 allosterically reactivates AF1 in the Ral-D351YER alpha complex. Unlike the estradiol-ER alpha complex, agonist activity of 4-OHT and raloxifene through ER alpha an d D351YER alpha were not attributed to coactivator (such as SRC-1, AIB1) bi nding to the ligand binding domain. We conclude that the classic AF2 is not responsible for the agonist activities of 4-OHT-ER alpha, 4-OHT-D351YER al pha, and Ral-D351YER alpha, To address the role of AF1, stable transfectant s of ER alpha or D351YER alpha with an AF1 deletion (D351 Delta AF1, D351Y Delta AF1) mere generated in MDA-MB-231 cells. Additionally, D538A/E542A/D5 45A triple mutations within helix 12 (D351-3m, D351Y3m) or the COOH-termina l 537 deletion (D351 Delta 537) were tested. The agonist activities of 4-OH T and raloxifene were lost in these stable transfectants, but antiestrogeni c action was retained. The reactivation of an estrogen-like property of the Ral-ER alpha complex through AF1,vith the D351Y mutation illustrates a nov el allosteric mechanism for the selective estrogen receptor modulator ER al pha complex.