Effects of HRGβ1 and gefitinib on erbB receptor dimerization patterns and associated downstream signalling activity. (a) Western blot (WB) analysis of phosphorylated epidermal growth factor receptor (EGFR), phosphorylated erbB2, phosphorylated tyrosine (Tyr) and total erbB3 protein expression following immunoprecipitation with total erbB3 antibody in tamoxifen-resistant MCF-7 (Tam-R) cells prior to and following treatment with either gefitinib (1 μM) or vehicle control for 1 hour followed by HRGβ1 (10 ng/ml) for 5 minutes. (b) WB analysis of total and phosphorylated EGFR, erbB2, AKT and ERK1/2 protein expression in Tam-R cells prior to and following incubation with either gefitinib (1 μM) or vehicle control for 1 hour followed by either HRGβ1 (10 ng/ml) or vehicle control for 5 minutes. Tamoxifen was also present in all studies. Data are representative of at least three separate experiments.