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HER2 is an oncogene overexpressed in about 20% of breast cancers and defines a sub-class of patients that benefit from the HER2 targeted therapy trastuzumab. HER2 signaling could be mediated by HER2 homodimers and/or heterodimers with other HER family receptors. A significant role for heterodimers in oncogenic signaling could have strategic implications for development of HER2 targeted therapies. To further explore the role of HER1 and HER3 in HER2+ breast cancer, we used an RNAi approach to knockdown HER family receptors in several HER2+ breast cancer cell lines. Knockdown of HER2 resulted in a significant decrease in cell proliferation as measured by a [3H]-thymidine incorporation assay. Interestingly, knockdown of HER3 decreased proliferation by the same magnitude as knocking down HER2. In contrast, HER1 knockdown had no effect on proliferation. The knockdown efficiency of each receptor, including HER1, was confirmed by Western blot. To determine the effects of HER3 knockdown in vivo, we generated a doxycycline-inducible HER3 shRNA cell line from BT474-M1 cells. In vitro studies have confirmed that HER3 is knocked down in this cell line by day 3 of treatment with doxycycline and correlates with a decrease in proliferation. Preliminary data using this cell line suggest that induction of HER3 knockdown in both 3-dimensional spheroids in vitro and in xenografts results in a significant reduction in tumor growth. The HER2 targeted antibody pertuzumab is known to disrupt HER2/3 heterodimers, which may have relevance in HER2+ breast cancer given the importance of HER3. Looking forward, we are exploring the combination of trastuzumab with pertuzumab as well as trastuzumab with HER3 knockdown in both in vitro 3D cultures and in vivo xenograft models. Our data provide direct evidence of a prominent role for HER3 in HER2-mediated cellular transformation and may provide a rationale for exploring the combination of trastuzumab and pertuzumab in the clinic.

98th AACR Annual Meeting-- Apr 14-18, 2007; Los Angeles, CA