Hormone-dependent nuclear export of estradiol receptor and DNA synthesis in breast cancer cells

In breast cancer cells, cytoplasmic localization of the estradiol receptor alpha (ER alpha) regulates estradiol-dependent S phase entry. We identified a nuclear export sequence (NES) in ER alpha and show that its export is dependent on both estradiol-mediated phosphatidylinositol-3-kinase (PI3K)/AKT activation and chromosome region maintenance 1 (CRM1). A Tat peptide containing the ER alpha NES disrupts ER alpha-CRM1 interaction and prevents nuclear export of ER alpha- and estradiol-induced DNA synthesis. NES-ER alpha mutants do not exit the nucleus and inhibit estradiol-induced S phase entry; ER alpha-dependent transcription is normal. ER alpha is associated with Forkhead proteins in the nucleus, and estradiol stimulates nuclear exit of both proteins. ER alpha knockdown or ER alpha NES mutations prevent ER alpha and Forkhead nuclear export. A mutant of forkhead in rhabdomyosarcoma (FKHR), which cannot be phosphorylated by estradiol-activated AKT, does not associate with ER alpha and is trapped in the nucleus, blocking S phase entry. In conclusion, estradiol-induced AKT-dependent phosphorylation of FKHR drives its association with ER alpha, thereby triggering complex export from the nucleus necessary for initiation of DNA synthesis and S phase entry.