The estrogen receptor (ER) functions as a transcription factor that mediates the effects of estrogen. ER alpha, which plays a crucial role in the development and progression of breast cancer, is activated by estrogen binding, leading to receptor phosphorylation, dimerization, and recruitment of co-activators and chaperons to the estrogen-bound receptor complex. The 14-3-3 proteins bind to target proteins via phosphorylation and influence many cellular events by altering their subcellular localization or acting as a chaperone. However, regulation of ER alpha expression and transactivation by the 14-3-3 proteins has not been reported. We demonstrate that 14-3-3 beta functions as a positive regulator of ER alpha through a direct protein-protein interaction in an estrogen-dependent manner. Ectopic expression of 14-3-3 beta stimulated ER alpha-mediated transcriptional activity in MCF-7 breast cancer cells. Enhanced ER alpha transcriptional activity due to 14-3-3 beta increased the expressions of the endogenous ER alpha target genes, leading to proliferation of breast cancer cells. We suggest that 14-3-3 beta has oncogenic potential in breast cancer via binding to ER alpha and activation of the transcriptional activity of ER alpha. (C) 2011 Elsevier Inc. All rights reserved.