Figure 1

Cetuximab-resistant (Ctx^R) clones have increased HER family receptor activity. (A) Ctx^R cells have increased HER family activity as well as phosphorylation levels of downstream signaling molecules. Parental (HP) and Ctx^R clones (HC1, HC4, HC8) were harvested and protein lysates were fractionated on SDS-PAGE followed by Immunoblotting for the indicated proteins. α-Tubulin was used as a loading control. (B) Expression of EGFR and HER3 on the plasma membrane is increased in Ctx^R clones compared to cetuximab-sensitive (Ctx^S) cells (HP). The levels of EGFR and HER3 on the cell surface were measured by flow cytometry. (n = 3) (C) EGFR displayed increased association with HER3 in all three Ctx^R clones compared to Ctx^S HP cell line. Cells were harvested and EGFR or HER3 were immunoprecipitated with anti-rabbit EGFR antibody or anti-rabbit HER3 antibody. The immunoprecipitate complexes were fractionated on SDS-PAGE followed by immunoblotting for indicated proteins.