Western blot analysis. (a) Total and phosphorylated insulin-like growth factor type I receptor (IGF-IR), insulin receptor substrate 1 (IRS-1), erbB3, Akt and extracellular-signal regulated kinase 1/2 (ERK1/2) protein expression following incubation of MCF-7 and T47D cells in medium containing either the specific IGF-IR/IR tyrosine kinase inhibitor 4-anilino-5-bromo-2-[4-(2-hydroxy-3-(N, N-dimethylamino)propoxy)anilino]pyrimidine (ABDP) (1 μM) or appropriate vehicle control for 24 hours and subsequently challenged with increasing concentrations of heregulin β1 (HRGβ1) (0.1 to 10 ng/ml) or vehicle control for 5 minutes. Densitometric analysis of phosphorylated Akt and ERK1/2 protein levels in (b) MCF-7 and (c) T47D cells treated with HRGβ1 in the absence and presence of ABDP. The results are expressed as the means ± standard errors of the mean of at least three separate experiments from the 1 ng/ml HRGβ1-primed groups. (d) Western blot analysis of total IGF-IR, erbB3 and IRS-1 expression following immunoprecipitation (IP) with total IRS-1 antibody in MCF-7 and T47D cells incubated for 24 hours in medium containing ABDP (1 μM) or vehicle control. * P ≤ 0.05 versus HRGβ1 for both cell lines.