Lipopolysaccharide induces inflammation and facilitates lung metastasis in a breast cancer model via the prostaglandin E2-EP2 pathway

Inflammation is a potent promoter of tumor metastasis. The aim of the present study was to explore the function of systemic inflammation in the formation of lung metastasis of breast cancer cells in a mouse model. BALB/c mice were injected intraperitoneally with lipopolysaccharide (LPS) in order to establish an inflammatory animal model and 4T1 murine breast cancer cells were injected through the tail vein to induce lung metastasis. The levels of proinflammatory cytokines were evaluated by ELISA. Metastases on the surface of the lungs were counted and histologically analyzed by hematoxylin and eosin staining. Angiogenesis in the lungs was examined by CD31 immunofluorescence. Mouse pulmonary endothelial cells (MPVECs) were isolated and used to assay endothelial tube formation and determine the protein expression levels of vascular endothelial growth factor (VEGF) in vitro. Serum levels of VEGF and prostaglandin E2 (PGE2), the number and size of metastatic lesions, and the expression levels of cyclooxygenase-2 were significantly greater in the lungs of LPS-treated mice, as compared with those in control mice threated with phosphate-buffered saline. Blood vessel density was also markedly increased in the LPS-treated mice. These increases were reversed by treatment with celecoxib. In vitro, the protein expression levels of VEGF produced by the PGE2-treated cells were significantly increased in a concentration-dependent manner. In addition, the production of VEGF was increased in response to treatment with the PGE2 receptor (EP2) agonist ONO-AE1-259-01; however, this increase was abrogated by treatment with AH6809, an EP2 receptor antagonist. Treatment with PGE2 or VEGF alone promoted the tube formation of MPVECs and this effect was reversed by treatment with celecoxib. These results demonstrated that PGE2 may regulate the release of VEGF by MPVECs through the EP2 receptor pathway and thereby promoted pulmonary angiogenesis and breast cancer metastasis in a mouse model.