Western blot analysis. (a) Total and phosphorylated insulin-like growth factor type I receptor (IGF-IR), insulin receptor substrate 1 (IRS-1), Akt, extracellular signal-regulated kinase 1/2 (ERK1/2) and β-actin protein expression following incubation of MCF-7 and T47D cells in medium containing either lipid and control siRNA (C si) mix (100 nM) or lipid and IRS si mix (100 nM) in the absence or presence of 4-anilino-5-bromo-2-[4-(2-hydroxy-3-(N, N-dimethylamino)propoxy)anilino]pyrimidine (ABDP) (1 μM) for 2 days and subsequently challenged with heregulin β1 (HRGβ1) (10 ng/ml) for 5 minutes. Data are representative of at least three separate experiments. The effects of 4-day treatment with HRGβ1 (10 ng/ml), ABDP (0.1 μM) or a combination of HRGβ1 and ABDP on the growth of (b) MCF-7 cells or (c) T47D cells. The results are expressed as the means ± standard errors of the mean of triplicate wells and are representative of at least three separate experiments. *P ≤ 0.01 versus control, †P ≤ 0.001 versus ABDP.