HER2 is gene amplified or over-expressed in 20-25% of breast cancers resulting in elevated HER2 activation. Trastuzumab (Herceptin), a humanized monoclonal antibody, targets activated HER2 and is clinically effective in HER2-over-expressing breast cancers. However, despite prolonged survival, treated breast cancer patients develop resistance. Resistance to trastuzumab occurs upon inactivation of HER2 regulatory proteins or upon up-regulation of alternative receptors. In particular, elevated levels of EGFR, present in estrogen receptor (ER) positive, trastuzumab-resistant BT-474 xenografts caused, a trastuzumab-resistant phenotype (Ritter et al. Clin Cancer Res 13:4909-4919, 2007). However, the role of EGFR in acquired trastuzumab resistance in ER negative cell models is not well defined. In this study, SKBR3 cell line clones expressing EGFR were generated to examine the role of EGFR over-expression on trastuzumab sensitivity in an, ER-negative breast carcinoma cell line. A stable clone, SKBR3/EGFR (clone 4) expressing moderate levels of EGFR remained sensitive to trastuzumab, whereas a stable clone, SKBR3/EGFR (clone 5) expressing high levels of EGFR, became resistant to trastuzumab. Depletion of EGFR by EGFR small-interfering RNAs in the SKBR3/EGFR (clone 5) reversed trastuzumab resistance. However, the SKBR3/EGFR (clone 5) cell line remained sensitive to lapatinib, an EGFR/HER2 inhibitor. Biochemical analysis using co-immunoprecipitation and proximity-based quantitative VeraTag assays demonstrated that high levels of EGFR phosphorylation, EGFR/EGFR homo-dimerization, and EGFR/HER2 hetero-dimerization were present in the trastuzumab-resistant cells. We conclude that EGFR over-expression can mediate trastuzumab resistance in both ER positive and ER negative cells and hypothesize that a threshold level of EGFR, in the absence of autocrine ligand production, is required to induce the resistant phenotype.

2533 Background: HER2 overexpression is associated with accelerated disease progression and poor prognosis in breast cancer. Trastuzumab, a monoclonal antibody targeting the extracellular domain of HER2, is effective in the treatment of metastatic breast cancer. However, most patients treated with trastuzumab eventually develop clinical resistance. To investigate the role of HER-family receptors in trastuzumab resistance, we measured HER-family receptor expression, dimerization, and phosphorylation in trastuzumab susceptible and resistant cell lines. Methods: Cell lysates from trastuzumab susceptible and resistant BT474 and SKBR3 cell lines were obtained from the Arteaga and Esteva laboratories. Proximity-based, multiplexed assays were used to detect and quantify HER1, HER2, and HER3 expression and phosphorylation levels, as well as HER1/HER1, HER1/HER2, HER1/HER3, HER2/HER2, and HER2/HER3 dimers. Samples were incubated with a mixture of HER specific antibodies conjugated either with fluorescent reporter tags (eTags), or biotin, which binds a reporter tag releasing agent (chemical scissor). Reporter molecules are released based on proximity to the scissor in a photochemical reaction and separated by capillary gel electrophoresis. Results: In comparison to trastuzumab susceptible parental cell lines, both SKBR3 and BT474 trastuzumab-resistant cell lines displayed upregulated HER1 expression. Resistant BT474 cell lines exhibited markedly increased levels of HER1/HER2 heterodimers. Increases in HER2 phosphorylation in the trastuzumab resistant SKBR3 cell line were observed, consistent with previous studies implicating trastuzumab in the induction of HER2 phosphorylation. Total HER2 and HER3 levels were similar in trastuzumab susceptible and resistant BT474 cell lines. Conclusions: The development of trastuzumab resistance in these cell line models correlated with HER1 expression and the appearance of HER1:HER2 dimers. Since signaling initiated by such heterodimers is ineffectively antagonized by trastuzumab, these data suggest that selection for proliferative signaling mediated by HER1:HER2 dimers may represent a mechanism of trastuzumab resistance in breast cancer. No significant financial relationships to disclose.