Western blot showing erbB3, EGFR, erbB2 and insulin receptor substrate 1 protein expression. (a) Immunoprecipitation (IP) with rabbit immunoglobulin G (IgG) (negative control), insulin receptor substrate 1 (IRS-1) or erbB3 antibody for MCF-7 cells treated with either heregulin β1 (HRGβ1) (10 ng/ml) or vehicle control for 5 minutes or up to 20 minutes, (b) immunoprecipitation with either IRS-1 or erbB3 antibody in T47D and BT-474 cells treated with either HRGβ1 (10 ng/ml) or vehicle control for 5 minutes. (c) Western blot analysis of IGF-IR and IRS-1 protein expression following immunoprecipitation with IRS-1 antibody in MCF-7, T47D and BT-474 cells treated with either HRGβ1 (10 ng/ml) or vehicle control for 5 minutes. (d) Densitometric analysis of the loss of IRS-1 association with IGF-IR following HRGβ1 treatment in MCF-7 and T47D cells. The results are expressed as means ± standard errors of the mean of at least three separate experiments. The y-axis represents arbitrary optical densitometric units. In MCF-7 cells * P ≤ 0.01 versus Control; In T47D cells * P ≤ 0.05 versus Control.