Western blot analysis. (a) Phosphorylated and total erbB3, insulin receptor substrate 1 (IRS-1), Akt, extracellular-signal regulated kinase 1/2 (ERK1/2) and β-actin protein expression in MCF-7 and T47D cells incubated in medium supplemented with (a) lipid and control siRNA (C si) mix (100 nM) or lipid and erbB3 siRNA (3 si) mix (100 nM) for 4 days and subsequently challenged with either heregulin β1 (HRGβ1) (10 ng/ml) or vehicle control alone for 5 minutes. Densitometric analysis of (b) phosphorylated IRS-1 Y612 and Y896 protein levels in MCF-7 and (c) T47D cells resulting from HRGβ1-primed erbB3 siRNA- versus control siRNA-treated groups. The results are expressed as means ± standard errors of the mean of at least three separate experiments. In MCF-7 cells * P ≤ 0.01 versus Control siRNA for both phosphorylation sites; In T47D cells * P ≤ 0.01 versus Control siRNA for pY612 IRS-1 and * P ≤ 0.001 versus Control siRNA for pY896 IRS-1.