HGF activates Brk. (a). In vitro kinase assays were performed using HaCaT cells. Serum-starved HaCaT cells were treated with either vehicle (water) or 50 ng/ml HGF for 15 to 60 min. Whole cell lysates were subjected to Brk immunoprecipitation and in vitro kinase assays using purified recombinant Sam68 as a substrate. Densitometry of bands represent phosphorylated Brk and Sam68 proteins indicated by arbitrary units (a.u.). Brk kinase assays were performed under similar conditions and treatments as in (a) using MDA-MB-231 (b) and T47D (c) breast cancer cells.