EGFR-HER3 transactivation regulates resistant cell survival through PI3K signaling. (A) The effects of EGFR and HER3 knockdown on steady-state phospho-HER3Y1197 protein expression. The absolute optical density (OD) values attributed to the p-HER3 bands, determined by using the Odyssey Infrared Imaging System, are indicated under each corresponding lane. (B) Western blot analysis with the indicated phosphospecific antibodies showing the effects of HER3 knockdown on EGFRY992 and PI3K-p85Y508 in rBT474 and rSKBR3 cells. (C) Increased steady-state protein levels of cleaved PARP product, and (D) inhibition of cell growth in response to HER3 knockdown in rBT474 and rSKBR3 cells. P < 0.017 (rBT474); P < 0.013 (rSKBR3). Results represent the mean ± standard error of triplicate samples and are representative of three independent experiments.