This study investigates the role of phosphatidylinositol 3-kinase (PI 3-kinase) and AKT in the activation of estrogen receptor α (ERα) in the absence of estrogen. The authors found that PI 3-kinase increased both estrogen-independent and estrogen-dependent activation functions of ERα, while AKT specifically increased the estrogen-independent activation function (AF-1). The consensus AKT phosphorylation site Ser-167 of ERα was required for AKT-mediated phosphorylation and activation. Inhibiting the PI 3-kinase/AKT pathway reduced ERα phosphorylation and activity. Overexpression of AKT led to up-regulation of estrogen-regulated genes such as pS2, Bcl-2, and macrophage inhibitory cytokine 1, and protected breast cancer cells from tamoxifen-induced apoptosis. These findings suggest that the PI 3-kinase/AKT pathway modulates ERα activity, leading to hormone-independent activation and resistance to anti-estrogen therapy.This study investigates the role of phosphatidylinositol 3-kinase (PI 3-kinase) and AKT in the activation of estrogen receptor α (ERα) in the absence of estrogen. The authors found that PI 3-kinase increased both estrogen-independent and estrogen-dependent activation functions of ERα, while AKT specifically increased the estrogen-independent activation function (AF-1). The consensus AKT phosphorylation site Ser-167 of ERα was required for AKT-mediated phosphorylation and activation. Inhibiting the PI 3-kinase/AKT pathway reduced ERα phosphorylation and activity. Overexpression of AKT led to up-regulation of estrogen-regulated genes such as pS2, Bcl-2, and macrophage inhibitory cytokine 1, and protected breast cancer cells from tamoxifen-induced apoptosis. These findings suggest that the PI 3-kinase/AKT pathway modulates ERα activity, leading to hormone-independent activation and resistance to anti-estrogen therapy.