Abstract 503: Prostaglandin E2 produced by tumor cells or by the host tumor microenvironment is not completely abolished by aspirin or celecoxib and limits the ability of the host immune system to control tumor growth

Prostaglandins are lipid autacoids derived from arachidonic acid (AA). AA released from the cell membrane is metabolized by the sequential actions of prostaglandin G/H synthase or cyclooxygenase (COX) and respective synthases, such as mPGES-1, which forms the most abundant prostaglandin, PGE2. Prostaglandin biosynthesis is blocked by nonsteroidal anti-inflammatory drugs (NSAIDs), which inhibit the activity of either one or both cyclooxygenases. Besides their role in homeostasis, prostanoids can modulate immune responses and contribute to chronic inflammation associated with tumor initiation, vascularization, and growth. The prostanoid pathway is activated in many solid tumors, including breast cancer, and high levels of PGE2 have been associated with aggressive tumors and metastatic spread. The objective of this work was to define the pathways and cellular compartments that contribute to tumor PGE2 levels and their role in tumor growth and immune evasion. The anti-tumor impact of classical NSAIDs combined with immune checkpoint antibodies PD1 or CTLA4 (ICPAb) was also evaluated. 4T1 or CT26 murine tumors were grown in immune competent mice to confirm the presence of PGE2 in tumors. Mice were dosed with NSAIDs Aspirin or Celecoxib monotherapy, given at maximal tolerated dose, or combined with ICPAb. While ICPAb reduced tumor growth, nor Aspirin neither celecoxib was effective in monotherapy or combination with ICPAb. Interestingly, PGE2 concentration in tumors was reduced by NSAIDs but not abolished. To examine the production of PGE2 by tumors or host, we have used CRISPR/cas9 mutagenesis to generate tumor lines lacking various enzymes required for PGE2 production. Those lines were combined with mice carrying null mPGES1 alleles. We show that, while the host tumor microenvironment contributes to overall PGE2 levels in the tumor, large amounts of PGE2 are produced by the tumor epithelial cells. Despite high expression of Ptgs1 in these tumor lines, PGE2 production was dependent on COX-2 expression. Similarly, while 4T1 tumors express all three PGE2 synthases at relatively high levels, we found that mPGES1 was solely responsible for all PGE2 released from epithelial cells. Examining several independent Ptgs1/2 null 4T1 tumor lines, we show that loss of Ptgs1/2 results in decreased tumor growth in immune competent mice but not in an allogenic Rag1/2-/- line. Knowledge of tumor PGE2 metabolic and functional pathways is a critical first step in designing the mos