Abstract P5-19-01: Targeting the HER3-phosphatidyl inositol-3 kinase pathway in breast cancers

Abstract

Abstract Background: Using a mouse model of HER3 ablation in the mammary epithelium, we previously showed that HER3 is required for the genesis of transgenic HER2-overexpressing breast cancers. However, increasing evidence suggests that transgenic models of HER2 overexpression result in luminal B-like mammary tumors, which resemble human breast tumors that express estrogen receptor (ER) but are often not responsive to endocrine therapy. Furthermore, we found that HER3 is required in the untransformed mouse mammary epithelium for differentiation and survival of luminal cells, but not basal cells. Herein we report that HER3 drives growth and survival of luminal breast cancers in a manner that parallels its role in maintaining the untransformed luminal epithelium in the breast. Results: Expression profiling of a large human breast cancer dataset demonstrated that HER3 expression was highest in Luminal breast cancers as compared to different molecular subtypes of breast cancers, including HER2-enriched, claudin-low, and basal-like breast cancers. In luminal breast cancer cell lines MCF7, ZR75-1, and MDA-MB-361, HER3 targeting using siRNA or a monoclonal antibody against HER3 decreased Akt, S6, and Erk1/2 phosphorylation, cell growth and cell survival under conditions of estrogen deprivation. Luminal breast tumor xenografts treated with U3-1287, a humanized monoclonal anti-ErbB3 antibody, or EZN-3920, an anti-sense oligonucleotide against ErbB3, decreased tumor growth and phosphatidyl inositol-3 kinase (PI3K)/mTOR signaling in vivo. Interestingly, luminal breast tumor xenografts responded to the ER inhibitor fulvestrant with increased HER3 expression and phosphorylation, and with increased PI3K/mTOR signaling as shown by enhanced S6 phosphorylation. Fulvestrant-mediated upregulation of HER3 was also seen in matched clinical breast tumor specimens harvested before treatment and after 21 days of fulvestrant treatment. AMG-888 blocked fulvestrant-induced HER3 and phospho-S6 upregulation in vivo, resulting in regression of luminal breast tumor xenografts. Conclusions: These data suggest that HER3 targeting may improve the outcome of luminal breast cancers treated with endocrine therapy, and are consistent with our findings that a HER3-dependent expression signature positively correlates with relapse in tamoxifen-treated patients. Taken together, these results define a novel role for HER3 in luminal breast cancer cells, and support future investigations into therapeutic strategies to target HER3 in luminal breast cancers. Citation Information: Cancer Res 2012;72(24 Suppl):Abstract nr P5-19-01.