Host knockout of E-prostanoid 2 receptors reduces tumor growth and causes major alterations of gene expression in prostaglandin E2-producing tumors

Abstract

Prostaglandin E₂ (PGE₂) is elevated in a variety of malignant tumors and has been shown to affect several hallmarks of cancer. Accordingly, the PGE₂ receptor, E-prostanoid 2 (EP2), has been reported to be associated with patient survival and reduced tumor growth in EP2-knockout mice. Thus, the aim of the present study was to screen for major gene expression alterations in tumor tissue growing in EP2-knockout mice. EP2-knockout mice were bred and implanted with EP2 receptor-expressing and PGE₂-producing epithelial-like tumors. Tumor tissue and plasma were collected and used for analyses with gene expression microarrays and multiplex enzyme-linked immunosorbent assays. Tumor growth, acute phase reactions/systemic inflammation and the expression of interleukin-6 were reduced in EP2-knockout tumor-bearing mice. Several hundreds of genes displayed major changes of expression in the tumor tissue when grown in EP2-knockout mice. Such gene alterations involved several different cellular functions, including stemness, migration and cell signaling. Besides gene expression, several long non-coding RNAs were downregulated in the tumors from the EP2-knockout mice. Overall, PGE₂ signaling via host EP2 receptors affected a large number of different genes involved in tumor progression based on signaling between host stroma and tumor cells, which caused reduced tumor growth.

Keywords: EP2 receptor; EP2-knockout; cancer; cyclooxygenase; prostaglandin E2; tumor growth.

Figure 1.

Tumor wet weight in EP2-knockout mice (−/−) versus wild-type mice (+/+) at 10 days post-tumor implantation [EP2^−/− (n=7), 2.6 ± 0.17; EP2^+/+ (n=8), 4.0 ± 0.36; *P<0.05]. EP2, E-prostanoid 2.

Figure 2.

Systemic inflammation was decreased in EP2^−/− tumor-bearing mice compared with wild-type mice, as reflected by plasma levels (ng/ml) of A2M and SAP, and determined by the acute phase Luminex^® assay (EP2^−/−, n=7; EP2^+/+, n=8). EP2, E-prostanoid 2; A2M, α-2-macroglobulin; SAP, serum amyloid protein/pentraxin-2; HPTGN, haptoglobin.

Figure 3.

Plasma concentrations of cytokines in EP2-knockout and wild-type tumor-bearing mice. Levels of IL6 (pg/ml) were significantly reduced in EP2^−/− mice (EP2^−/−, n=7; EP2^+/+, n=8). EP2, E-prostanoid 2; IL, interleukin; IFN, interferon; TNF, tumor necrosis factor; VEGF, vascular endothelial growth factor.

Figure 4.

Summary of a speculative interpretation of the findings in the present study, as well as knowledge from the literature. Host EP2-knockout reduced systemic inflammation and plasma cytokine levels, as well as tumor gene expression, resulting in significantly reduced tumor growth in the EP2-knockout mice. These alterations indicated that stroma-tumor interactions based on IL-6 and inflammatory signaling were important for tumor growth, perhaps associated with an undiscovered ‘factor x’, as suggested by the present illustration. EP2, E-prostanoid 2; A2M, α-2-macroglobulin; SAP, serum amyloid protein/pentraxin-2; HPTGN, haptoglobin; IL, interleukin.