Fig. 3. Requirement for ErbB3 for optimal EGFR activation in vivo and vitro.

a Tumor cell lysates obtained from tumor nodules containing persister cells after 5 weeks of treatment of anti-ErbB3 antibody treatment shows reduced levels of p-EGFR (Y1068) in ErbB3-antibody-treated mice compared to vehicle treatment. b Representative immunoblots of protein lysates from conditionally reprogrammed cells (CRCs) from four PDX models show reduced levels of p-EGFR after six days of incubation with anti-ErbB3 antibody (n = 3) or c three days after transient siRNA-mediated knockdown of ErbB3 (n = 3) or d neuregulin-1 (n = 3). Antibodies to both Y1068 and Y1045 were used. Relative decreases in phosphoprotein levels in control vs. experimental groups were quantified by photodensitometry after normalization to total EGFR or ErbB3. Squamous cells were separated from Swiss 3T3 feeder cells by differential trypsanization prior to preparation of protein lysates