Abstract 469: mPGES1 deletion and the mechanisms of tumor growth suppression
Authors: Markosyan Nune , Rech Andrew , Vonderheide Robert H. , FitzGerald Garret A. , Smyth Emer M. 
Published: August 2015

Abstract
The pro-tumor contributions of prostaglandin (PG) E2 are established, as is the clinical efficacy of pharmacological inhibition of COX-2, the rate-limiting enzyme in PGE2 synthesis. In addition to the desired suppression of tumor PGE2, collateral loss of endothelial COX-2-derived PGI2 imposes a cardiovascular hazard that limits clinical use of COX-2 inhibitors. mPGES1, the terminal enzyme in the PGE2 synthesis pathway, is an alternative target to interrupt COX-2-driven events in tumors without elevating cardiovascular risk. We engineered mice transgenic for an activated HER2/neu oncogene to lack mPGES1 globally (mPGES1 KOgl) or only in mammary epithelial cells (mPGES1 KOepi). Abdominal mammary glands from wild type (WT), heterozygous (het), mPGES1 KOgl, and mPGES1 KOepi mice were harvested at 22 weeks of age, paraffin embedded, H&E stained and scanned for whole slide imaging (WSI). Tumor multiplicity was calculated based on the number of lesions on WSI. Adjacent sections were immunostained for Ki67, caspase 3, Factor VIII, CD3, and F4/80, to assess proliferation, apoptosis, angiogenesis, and tumor lymphocyte and macrophage infiltration. Tumor multiplicity was significantly higher in WT compared to either mPGES1 KOgl or mPGES1 KOepi mice. Concordantly, shRNA knock down (KD) of mPGES1 in mammary tumor cells dramatically suppressed their growth as orthotopic tumors in syngeneic immune competent WT hosts: 4 weeks after the injections, 6 out of 6 non-target shRNA control tumors grew successfully while 6 out of 10 mPGES1 KD cells failed to grow. In separate experiments, 14 of 14 non-target shRNA cells formed tumors after 2 weeks while 12 out of 18 mPGES1 KD tumors failed. By immunohistochemistry, no difference was observed in proliferation and apoptosis markers, as well as in the number of tumor infiltrating macrophages and lymphocytes, between WT, mPGES1 KOgl, and mPGES1 KOepi tumors. In contrast to our previous study in mice lacking epithelial COX-2 (COX-2 KOepi), vascularization was not different between mPEGS-1 KOepi and WT tumors suggesting divergence in the anti-tumorigenic mechanisms in mPEGS-1 KOepi and COX-2 KOepi tumors. By flow cytometry there was no difference in infiltrating macrophage and lymphocyte density or functional phenotypes, between WT, mPGES1 KOepi, and mPGES1 KD orthotopic tumors. Similarly, no difference was observed in the number of macrophages and myeloid derived suppressor cells in spleens from mPGES1 sufficient and mPGES1 KD orthotopic tumor bearing mice. In contrast, decreased co-inhibitory molecule PD-1 expressing CD4+ and CD8+ lymphocyte numbers were evident in draining lymph nodes in mice injected with mPGES1 KD cells compared to WT. This finding indicates that the absence of mPGES1-derived PGE2 renders tumor cells less efficient in creating and maintaining an immunosuppressive tumor microenvironment and that targeting mPGES1 may enhance anti-tumor immune mechanisms.
Citation Format: Nune Markosyan, Andrew Rech, Robert H. Vonderheide, Garret A. FitzGerald, Emer M. Smyth. mPGES1 deletion and the mechanisms of tumor growth suppression. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 469. doi:10.1158/1538-7445.AM2015-469