Expression of EGFRvIII in ovarian cancer cells leads to constitutive activation of erbB2

Abstract

2071

Overexpression of the epidermal growth factor receptor (EGFR) and erbB2 occurs frequently in ovarian cancer and is associated with poor patient prognosis. A constitutively active mutant EGFR, EGFRvIII, has been detected at a high frequency in many human tumors, including those of the ovary. There is evidence of a physical association between EGFRvIII and erbB2. The possibility of activation of erbB2 as a result of EGFRvIII’s constitutive activity was evaluated by examining erbB2 level and its phosphorylation status. EGFRvIII was introduced into the epithelial ovarian cancer cell line OVCA 433, and the protein and mRNA levels of the erbB family members were evaluated by western blots, RT-PCR, and real-time PCR. Western blot analysis for the erbB family members revealed erbB2 protein levels were higher in the parental and control cells than in EGFRvIII-expressing cells. The lower erbB2 protein level in EGFRvIII-expressing cells did not correlate with a decreased level of its transcript, indicating a possible downregulation of erbB2 as a result of its constitutive activation and dimerization with EGFRvIII. ErbB2 phosphorylation showed that unstimulated parental and control cells displayed a low, basal level of phosphorylated erbB2. Stimulation of these cells with EGF led to down-regulation of erbB2. However, the increased ratio of phosphorylated erbB2 to total erbB2 in EGF-stimulated cells indicated increased phosphorylation of erbB2. Phosphorylated erbB2 was detected in cell lysates of EGFRvIII expressing cells. However, phosphorylated erbB2 was detected in EGFRvIII-expressing cells at a lower level than that of parental and control cells, which might be the result of down-regulation of erbB2. Inhibition of EGFRvIII’s catalytic activity with the EGFR inhibitor, AG1478, resulted in decreased phosphorylation of erbB2. In addition, the ratio of phopho-erbB2 to total erbB2 as detected by western blot analysis was higher in untreated EGFRvIII-expressing cells in comparison to AG1478-treated cells (ratio of 1.42 vs. 0.10). Thus, the phosphorylation of erbB2 was reversible by a specific EGFR inhibitor in EGFRvIII-expressing ovarian cancer cells. Our data suggested that down-regulation of erbB2 is a result of its constitutive activation by EGFRvIII.