Western blot analysis. (a) Phosphorylated and total 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 either lipid and control siRNA (C si) mix (100 nM) or lipid and IRS 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 phosphorylated Akt and ERK1/2 protein levels in (b) MCF-7 and (c) T47D cells resulting from the HRGβ1-primed IRS-1 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; In T47D cells * P ≤ 0.001 versus Control siRNA.