Brk promotes p38 MAPK signaling in vitro. A, Brk expression sensitizes HC11 cells to prolactin treatment and increased basal p38 MAPK phosphorylation. Transiently transfected HC11 cells treated with 500 ng/mL prolactin were subjected to Western blotting for STAT5 and p38 MAPK phosphorylation. B, Stable expression of Brk in HMEC increases basal p38 MAPK activation. C, Brk knockdown in T47D breast cancer cells decreases EGF induced p38 MAPK phosphorylation. Whole cell lysates were subjected to Western blotting for p38 MAPK phosphorylation and/or STAT5 phosphorylation.