Estrogen receptors (
ERalpha and
ERbeta) are
ligand-regulated
transcription factors that play critical roles in the development and progression of
breast cancer by regulating target genes involved in cellular proliferation. The transcriptional activity of
ERalpha and
ERbeta is known to be modulated by cofactor
proteins. We used a yeast two-hybrid system and identified NFAT3 as a novel
ERbeta-
binding protein. NFAT3 interacted with
ERalpha and
ERbeta both in vitro and in mammalian cells in a
ligand-independent fashion. NFAT3 bound specifically to the
ERbeta region containing the activation function-1 domain, a
ligand-independent transactivation domain. Overexpression of NFAT3 enhanced both
ERalpha and
ERbeta transcriptional activities in a
ligand-independent manner and up-regulated downstream
estrogen-responsive genes including pS2 and
cathepsin D. Reduction of endogenous NFAT3 with NFAT3
small interfering RNA or overexpression of NFAT3 deletion mutants that lack the ER-binding sites reduced the NFAT3 coactivation of
ERalpha and
ERbeta. NFAT3 increased binding of
ERalpha to the
estrogen-responsive
element and was recruited to endogenous
estrogen-responsive promoters. NFAT3 was expressed differentially in many
breast cancer cell lines and overexpressed in a subset of
breast cancer patients. Knockdown of endogenous NFAT3 reduced the growth of human
breast cancer ZR75-1 cells in a
ligand-independent manner. Taken together, these results suggest that NFAT3 may play important roles in ER signaling and represent a novel target for
breast cancer therapy.