Abstract
Abstract Introduction: Although recent studies showed that HER3 plays a key role in the pathophysiology of HER2-amplified breast cancers, its role in other cancers driven by HER2 amplification remains unknown. This study aimed to define the importance of HER3 signaling in other types of HER2-amplified cancer. Method: The expression and signaling activity of HER2, HER3, and downstream pathway proteins were studied in cell panels representing HER2-amplified cancers of the breast, bladder, colon and rectal, stomach, esophagus, lung, tongue, and endometrium along with controls lacking HER2 amplification. The time-dependent compensatory upregulation of HER3 in response to HER2 inhibitor therapy was assayed by western blotting and the apoptotic effects of HER2 inhibition were assayed by flow cytometry. The functional significance of HER3 in promoting tumorigenic growth in xenograft assays was determined in selected HER2-amplified cancer cell types by shRNA knockdown. Results: Many HER2-amplified cancers of the breast (HCC1569, HCC1954, HCC1419, UACC732, SUM190), bladder (CLS-439), stomach (N87, OE19), esophagus (TE4, KYSE410), lung (Calu3), and endometrium (USPC-ARC1, USPC-ARC2) showed HER3 activation, whereas HER2-nonamplified cancers of the same organs did not. With some exceptions, all HER2-amplified cancer cells undergo apoptosis when HER2 signaling is inhibited by lapatinib. Lapatinib induced apoptosis correlated with HER3 activation. Tumor growth was reduced upon HER3 knockdown in HER2-amplified cancers other than breast cancers, particularly cancers with constitutive HER3 phosphorylation. Significance: These results suggest that HER3 plays a functionally significant role in HER2 amplified cancers other than breast cancers, and may underlie resistance to HER2-targeting in these cancers. Combined HER2-HER3 targeting may be of value across the spectrum of different types of cancers. Citation Format: Avisek Majumder, Veronica Steri, Fernando Salangsang, Mark Moasser. The role of HER3 in HER2-amplified cancers other than breast cancers [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr LB-326.
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