Synergistic Activation of ERα by Estrogen and Prolactin in Breast Cancer Cells Requires Tyrosyl Phosphorylation of PAK1

Serine/threonine kinase PAK1 is activated by estrogen and plays an important role in breast cancer. However, the integration of PAK1 into the estrogen response is not fully understood. In this study, we investigated the mechanisms underlying the hormone-induced activation of estrogen receptor (ER alpha, ESR1). We show that estrogen activated PAK1 through both the ER alpha and GPER1 membrane receptors. Estrogen-dependent activation of PAK1 required the phosphorylation of tyrosine residues by Etk/Bmx and protein kinase A (PKA) within an assembled signaling complex comprising pTyr-PAK1, Etk/Bmx, the heterotrimer G-protein subunits G beta 1, G gamma 2, and/or G gamma 5, PAK-associated guanine nucleotide exchange factor (beta PIX, ARHGEF7), and PKA. Moreover, the PKA RII beta subunit is a direct target of PAK1, and thus in response to estrogen, the activated pTyr-PAK1 complex reciprocally potentiated PKA activity, suggesting a positive feedback mechanism. We also demonstrate that PKA phosphorylated Ser305-ER alpha in response to estrogen, but pTyr-PAK1 phosphorylated Ser305-ER alpha in response to prolactin (PRL), implying that maximal ER alpha phosphorylation is achieved when cells are exposed to both PRL and estrogen. Furthermore, S305-ER alpha activation led to enhanced phosphorylation of Ser118-ER alpha and promoted cell proliferation and tumor growth. Together, these data strongly support a critical interplay between PRL and estrogen via PAK1 and suggest that ligand-independent activation of ER alpha through PRL/PAK1 may impart resistance to antiestrogen therapies. (C) 2016 AACR.