Abstract 311: HER2/neu oncogene induced tumors in mammary epithelial cell COX-2 deficient mice have disease delaying modifications of tumor microenvironment

Cyclooxygenase 2 (COX-2) enzyme, which catalyzes the synthesis of several pro- and anti-tumorigenic prostanoids, is associated with more than 40% of breast cancers. COX-2 inhibitors decrease the risk of tumorigenesis and breast cancer recurrence, however, systemic COX-2 inhibition is associated with a serious cardiovascular hazard that is unrelated to the anti-tumorigenic benefits. Targeting the COX-2 pathway in cancer chemoprevention or therapy, therefore, requires refined mechanistic understanding of how COX-2 and its main pro-tumorigenic product PGE2, act in the complex tumor microenvironment to support tumor growth. To that end, we engineered mice on a pure FVB/N background that lack COX-2 only in mammary epithelial cells (MECs). Mice transgenic for COX-2 exons 5 and 6, flanked by loxP sites (COX-2flox/flox (wild type; WT)), were crossed with mice expressing cre recombinase under control of the mouse mammary tumor virus (mmtv) promoter (Cremmtv) to achieve MEC-targeted deletion of COX-2 (COX-2MECKO). Unlike global COX-2 deficiency, mice lacking COX-2 in MECs were viable, fertile and disease-free making them highly suitable for cancer studies. WT and COX-2MECKO mice were crossed with mice that express activated HER2/neu oncogene also under control of the mmtv promoter (HER2/neummtv), to drive mammary tumor development. This is a valid molecular model for human breast cancer as amplification of HER2/neu proto-oncogene occurs in 10-30% of breast cancers. Tumor onset was significantly delayed in HER2/neummtvCOX-2MECKO mice compared to their HER2/neummtv WT littermates (median disease free survival 23 and 29 weeks, respectively (p=0.006, n=12-17)). Tumor multiplicity was also decreased (p=0.043) in HER2/neummtv COX-2MEC KO mice (3.857 ± 1.388; n=7) vs. HER2/neummtv WT (6.929 ± 0.9804; n=14). Tumors from HER2/neummtv WT showed higher expression of mRNA for the vascular endothelial growth factor (VEGF) receptors VEGFR2 and VEGFR3 by real time PCR (Q-PCR). Elevated CD31 mRNA and protein levels were evident in HER2/neummtv WT tumors by Q-PCR and immunohistochemistry, indicating limited vascularization in the HER2/neummtvCOX-2MECKO mice. In addition, cell sorting by flow cytometry revealed increased infiltration of HER2/neummtv COX-2MECKO tumors by CD3+ CD4+ lymphocytes as well as CD3+ CD8+ cytotoxic T cells. These data suggest a paracrine role for mediators, most likely PGE2, derived from COX-2 in mammary epithelium in promoting and tumor supportive microenviro