Serine 28 phosphorylation of NRIF3 confers its co-activator function for estrogen receptor-alpha transactivation

NRIF3 is an estrogen-inducible nuclear receptor coregulator that stimulates estrogen receptor-alpha (ER alpha) transactivation functions and associates with the endogenous ER and its target gene promoter. p21-activated protein kinase 1 (Pak1) phosphorylates ER alpha at Ser305 and this modification is important in ER alpha transactivation function. Although ERa transactivation functions are regulated by co-activator activity of NRIF3, it remains unclear whether Pak1 could impact ER functions via a posttranslational modi. cation of NRIF3. Here, we report that Pak1 phosphorylates NRIF3 at Serine28 and that NRIF3 binds to Pak1 in vitro and in vivo. We found that NRIF3 phosphorylation, co-activator activity and association with ER alpha increased following Pak1 phosphorylation of NRIF3's Ser28 and that activated ER alpha-Ser305 and NRIF3-Ser28 cooperatively support transactivation of ER alpha. NRIF3 expression increased significantly in cells with inducible Pak1 expression. We found that NRIF3 and ER alpha interaction, subcellular localization and ER alpha transactivation activity all increased in cells expressing the Pak1 phosphorylation-mimicking mutant NRIF3-Ser28Glu. Consistently, the NRIF3-Ser28Glu mutant exhibited an enhanced recruitment to the endogenous ER target genes and increased expression following estrogen stimulation. Finally, breast cancer cells with stable overexpression of NRIF3 showed increased proliferation and enhanced anchorage-independent growth. These findings suggest that NRIF3-Ser28 is a physiologic target of Pak1 signaling and contributes to the enhanced NRIF3 co-activator activity, leading to coordinated potentiation of ER alpha transactivation, its target gene expression and estrogen responsiveness of breast cancer cells.