Estrogen activates PAK1. (A) MCF-7 and T47D cells overexpressing PAK1 WT or PAK1 Y3F were incubated with/without E2. Changes in cell numbers at day 7 are shown as percentages of the veh-treated cell number. *, p <0.05 compared with control cells in the same condition, n=3. (B) PAK1 WT or Y3F MCF-7 clones were treated with PRL (left), or E2 (right). PAK1 was IP’d and subjected to an in vitro kinase assay with H4 histone as a substrate. PAK1 kinase activity was normalized by IP’d PAK1 for each lane and plotted. (C) PAK1 WT or Y3F clones were treated with veh, PRL, E2, PRL+E2 or HRG and assessed as in B. *, p<0.05, n=3.