Soluble EpEX-Fc binds to EGFR and induces ERK1/2 and AKT.

(A) Bidirectional co-immunoprecipitation (“IP”) of EGFR and EpCAM in whole-cell lysates of FaDu, Cal27, and HCT8 cells using EGFR- and EpCAM-specific antibodies. Isotype control antibody (“IgG”) served as control. Coimmunoprecipitated EGFR and EpCAM were visualized in immunoblotting with specific antibodies (“IB”), with whole-cell lysates as control (“lysate”). Shown are representative results from n = 3 independent experiments. (B) SNs of Cal27, Kyse30, FaDu, and HCT8 cells were immunoprecipitated with EpEX-specific antibodies and separated under reducing (left) and nonreducing native conditions (right), and EpEX was detected with specific antibodies. Antibody HCs and EpEX mono-, di-, and oligomers are indicated. Shown are representative results from n = 3 independent experiments. (C) EpEX-Fc or Fc were incubated with whole-cell lysates of FaDu and Cal27 and immobilized on protein A agarose beads, and protein complexes were separated on SDS-PAGE. Immunoprecipitated proteins were detected by immunoblotting (“IB”) with Fc- and EGFR-specific antibodies. Shown are representative results from n = 3 independent experiments. (D) EGFRex and EpEX were incubated in the presence or absence of cross-linker (BS3). Where indicated, EGF was added. Monomers, dimers, and EGFRex/EpEX complexes are marked. Shown are representative results from n = 3 independent experiments. (E, F) FaDu, Cal27, and Kyse30 cells were kept untreated (control) or were treated with EpEX-Fc, Fc (10 nM), or EGF (1.8 nM) for the indicated time points. Where indicated, cells were additionally treated with Cetuximab (“Cet.”). Phosphorylation of ERK1/2 (E) and AKT (F) was assessed by immunoblotting with specific antibodies. Levels of ERK1/2 and AKT were assessed in parallel. Shown are representative results from n = 3 independent experiments. (G) FaDu and Cal27 cells were kept untreated or were treated with EGF (9 nM), Fc, or EpEX-Fc (10 nM) for 30 min, and phosphorylation of ERK1/2 and AKT was detected by immunofluorescence laser scanning confocal microscopy (ERK1/2 or AKT: green, nuclei: blue [DAPI]). Shown are representative results from n = 3 independent experiments. (H) FaDu, Cal27, and Kyse30 cells were kept untreated (control) or were treated with EpEX-Fc, Fc (10 nM), or EGF (1.8 nM) for the indicated time points (“EGF 30 min”). Where indicated, MEK1 inhibitor AZD6244 or EGFR inhibitor AG1478 were added. Levels of ERK1/2 and AKT were assessed in parallel. Shown are representative results from n = 3 independent experiments. (I) HEK293 cells were transiently transfected with GFP or EGFR expression plasmids and were either kept untreated (control) or were treated with EpEX-Fc, Fc (10 nM), or EGF (1.8 nM) for 30 min. Expression of EGFR and activation of ERK1/2 were assessed by immunoblotting. Levels of ERK1/2 were assessed in parallel. Shown are representative results from n = 3 independent experiments. (J) Expression of EpCAM and EGFR was assessed in HCT8WT and CRISPR-Cas9 EpCAM K.O.1 [48] by immunoblotting. Levels of actin were assessed in parallel. Shown are representative results from n = 3 independent experiments. (K) HCT8WT and CRISPR-Cas9 EpCAM K.O.1 cells were either kept untreated (control) or were treated with EpEX-Fc, Fc (10 nM), or EGF (1.8 nM) for 30 min. Activation of ERK1/2 was assessed by immunoblotting. Levels of ERK1/2 were assessed in parallel. Shown are representative results from n = 3 independent experiments. BS3, bisulfosuccinimidyl suberate; CRISPR-Cas9, clustered regularly interspaced short palindromic repeat/CRISPR-associated 9; EGF, epidermal growth factor; EGFR, EGF receptor; EGFRex, extracellular domain of EGFR; EpCAM, epithelial cell adhesion molecule; EpCAM K.O.1, EPCAM-knockout clone 1; EpEX, extracellular domain of EpCAM; EpICD, intracellular domain of EpCAM; ERK1/2, extracellular signal–regulated kinase 1/2; Fc, fragment crystallizable region; GFP, green fluorescent protein; HC, heavy chain; HCT8WT, HCT8 wild type; HEK293, human embryonic kidney 293; IgG, immunoglobulin G; MW, molecular mass; pAKT, phosphorylated AKT; pERK, phosphorylated ERK; SN, supernatant.