Reduced tumor growth in EP2 knockout mice is related to signaling pathways favoring an increased local anti-tumor immunity in the tumor stroma

Inflammatory signaling through prostaglandin E2 receptor subtype 2 (EP2) is associated with malignant tumor growth in both experimental models and cancer patients. Thus, the absence of EP2 receptors in host tissues appears to reduce tumor growth and systemic inflammation by inducing major alterations in gene expression levels across tumor tissue compartments. However, it is not yet well-established how signaling pathways in tumor tissue relate to simultaneous signaling alterations in the surrounding tumor-stroma, at conditions of reduced disease progression due to decreased host inflammation. In the present study, wild-type tumor cells, producing high levels of prostaglandin E2 (MCG 101 cells, EP2(+/+)), were inoculated into EP2 knockout (EP2(−/−)) and EP2 wild-type (EP2(+/+)) mice. Solid tumors were dissected into tumor- and tumor-stroma tissue compartments for RNA expression microarray screening, followed by metabolic pathway analyses. Immunohistochemistry was used to confirm adequate dissections of tissue compartments, and to assess cell proliferation (Ki-67), prostaglandin enzymes (cyclooxygenase 2) and immunity biomarkers (CD4 and CD8) at the protein level. Microarray analyses revealed statistically significant alterations in gene expression in the tumor-stroma compartment, while significantly less pathway alterations occurred in the tumor tissue compartment. The host knockout of EP2 receptors led to a significant downregulation of cell cycle regulatory factors in the tumor-stroma compartment, while interferon γ-related pathways, chemokine signaling pathways and anti-tumor chemokines [chemokine (C-X-C motif) ligand 9 and 10] were upregulated in the tumor compartment. Thus, such gene alterations were likely related to reduced tumor growth in EP2-deficient hosts. On the whole, pathway analyses of both tumor- and tumor-stroma compartments suggested that absence of host EP2 receptor signaling reduces ‘remodeling’ of tumor microenvironments and increase local immunity, probably by decreased productions of stimulating growth factors, perhaps similar to well-recognized physiological observations in wound healing.