Abstract A100: Role of HER3 ligand-independent activation in EGFR resistance in pancreatic cancer

Abstract

Abstract The purpose of this study was to investigate the role of HER3 ligand-independent activation in EGFR resistance in pancreatic cancer. The expression and activation status of EGFR and HER3 were determined in a panel of 13 human pancreatic cancer cell lines by immunoprecipitation and Western blot analysis. To determine whether HER3 plays a central role in EGFR resistance, the effect of combination treatment with erlotinib and anti-HER3 on signaling pathways was examined in 2 pancreatic cancer cell lines. This was done using a novel anti-HER3 antibody (1A5) capable of inhibiting HER3 activation in a ligand-independent fashion, and a classic anti-HER3 antibody (3D4) primarily blocking ligand-induced HER3 activation. The MEK inhibitor trametinib was used to investigate EGFR and HER3 activation. Combination treatment studies in athymic nude mice bearing BxPC-3 xenografts involved anti-HER3, lapatinib, trametinib, and gemcitabine treatment. The majority of pancreatic cancer cell lines expressed high levels of total EGFR and HER3, and had variable levels of phospho-EGFR (Tyr1068) and phospho-HER3. Erlotinib treatment did not reduce the levels of phospho-EGFR. Although ligand-independent and ligand-dependent anti-HER3 antibodies did not alter the levels of phospho-EGFR, they did reduce the levels of phospho-AKT. Combination treatment with anti-HER3 antibodies and erlotinib or trametinib reduced the levels of phospho-EGFR and phospho-AKT. When MEK/ERK was inhibited by trametinib, both EGFR and HER3 were activated. Treatment of BxPC-3 xenografts with lapatinib, trametinib, anti-HER3, and gemcitabine produced the greatest inhibition of tumor growth as compared to any single agent or combinations of two or three agents. These results show that EGFR is expressed and highly activated in pancreatic cancer cell lines, but the cell lines are resistant to an EGFR inhibitor. They also suggest that ligand-independent activation of HER3 may lead to erlotinib resistance via the AKT pathway. These results reveal a novel mechanism by which HER3 is activated in a ligand-independent fashion via inhibition of the MEK/ERK pathway. This is an important observation as it indicates that HER3 ligand-independent activation must be inhibited when the MEK/ERK pathway is targeted. Overall, these results indicate that HER3 ligand-independent activation is a key target for overcoming EGFR resistance in pancreatic cancer cells, and suggest a combination strategy of anti-HER3 antibodies with either erlotinib or trametinib for the treatment of pancreatic cancer. Citation Format: Donald J. Buchsbaum, Tong Zhou. Role of HER3 ligand-independent activation in EGFR resistance in pancreatic cancer. [abstract]. In: Proceedings of the AACR Special Conference on Pancreatic Cancer: Innovations in Research and Treatment; May 18-21, 2014; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2015;75(13 Suppl):Abstract nr A100.