Partial agonistic effect of cetuximab on epidermal growth factor receptor and Src kinase activation in triple‑negative breast cancer cell lines.

Cetuximab is a monoclonal antibody developed to inhibit the binding of growth factors and the subsequent activation of epidermal growth factor receptor (EGFR). Triple‑negative breast cancer (TNBC) is resistant to cetuximab treatment. The aim of the present study was to examine the partial agonistic properties of cetuximab, which not only blocks ligand binding, but also partially triggers EGFR activation, which may lead to cetuximab resistance in TNBC. The phosphorylation of growth factor receptors and their signalling pathways were evaluated by determining the phosphorylation of EGFR, insulin‑like growth factor receptor (IGF‑1R), vascular endothelial growth factor receptor (VEGFR)‑2, Src kinase, phosphoinositide‑3‑kinase (PI3K), extracellular signal‑regulated kinase (ERK1/2) and serine/threonine‑specific protein kinase (Akt) and the degradation of EGFR, and by assessing the morphology and proliferation of MDA‑MB‑231 and MDA‑MB‑468 cells. Cetuximab treatment led to the phosphorylation of EGFR, VEGFR‑2, IGF‑1R and downstream signalling molecules, Src kinase and PI3K in these cells, as well as Akt in the MDA‑MB‑231 cells. The cetuximab‑mediated phosphorylation of IGF‑1R, VEGFR‑2 and Akt was inhibited by the EGFR kinase inhibitor, AG1478, and the Src kinase inhibitor, PP2. Cetuximab treatment led to the degradation of EGFR. The cetuximab‑induced phosphorylation and EGFR degradation were less prominent compared with those induced by EGF. Cetuximab partially inhibited EGF‑mediated responses. Cetuximab, similar with EGF, altered cellular morphology in a serum‑free medium. In both cell lines, the Src kinase inhibitor enhanced the cetuximab‑induced anti‑proliferative response. These results indicate that cetuximab exerts a partial agonistic effect on EGFR, which activates Src kinase and subsequently transactivates IGF‑1R and VEGFR‑2. This partial agonistic property is likely one of the mechanisms underlying the resistance of TNBC to cetuximab.