Prostaglandin E2 Promotes Colorectal Cancer Stem Cell Expansion and Metastasis in Mice

Abstract

BACKGROUND & AIMS Inflammation may contribute to formation, maintenance, and expansion of cancer stem cells (CSCs), which have the capacity for self-renewal, differentiation, and resistance to cytotoxic agents. We investigated the effects of the inflammatory mediator prostaglandin E2 (PGE2) on colorectal CSC development and metastasis in mice and the correlation between levels of PGE2 and CSC markers in human colorectal cancer (CRC) specimens. METHODS Colorectal carcinoma specimens and matched normal tissues were collected from patients at the Mayo Clinic (Scottsdale, AZ) and analyzed by mass spectrometry and quantitative PCR. Human primary CRC cells and mouse tumor cells were isolated using microbeads and flow cytometry and analyzed in sphere-formation and flow cytometry assays. LS-174T cells were sorted by flow cytometry (for CD133+CD44+ and CD133−CD44− cells) and also used in these assays. NSG mice were given cecal or subcutaneous injections of LS-174T or human primary CRC cells. ApcMin/+ mice and NSG mice with orthotopic cecal tumors were given vehicle (controls), PGE2, celecoxib, or/and Ono-AE3-208. PGE2 downstream signaling pathways were knocked down with small hairpin RNAs, expressed from lentiviral vectors in LS-174T cells, or blocked with inhibitors in human primary CRC cells. RESULTS Levels of PGE2 correlated with colonic CSC markers (CD133, CD44, LRG5, and SOX2 mRNAs) in human colorectal carcinoma samples. Administration of PGE2 to ApcMin/+ mice increased tumor stem cells and tumor burden, compared with controls. NSG mice given PGE2 had increased numbers of cecal CSCs and liver metastases, compared with controls, after intracecal injection of LS-174T or human primary CRC cells. Alternatively, celecoxib, an inhibitor of prostaglandin-endoperoxide synthase 2 (PTGS2 or COX2), reduced polyp numbers in Apc Min/+ mice, liver metastasis in NSG mice with orthotopic tumors, and numbers of CSCs in ApcMin/+ and NSG mice. Inhibitors or knockdown of PGE receptor 4 (PTGER4 or EP4), PI3K p85α, ERK1, or nuclear factor (NF)-κB reduced PGE2-induced sphere formation by and expansion of LS-174T and/or human primary CRC cells. Knockdown of ERK1 or PI3K p85α also attenuated PGE2-induced activation of NF-κB in LS-174T cells. An EP4 antagonist reduced the ability of PGE2 to induce CSC expansion in orthotopic tumors and to accelerate formation of liver metastases. Knockdown experiments showed that NF-κB was required for PGE2 induction of CSCs and metastasis in mice. CONCLUSIONS PGE2 induces CSC expansion by activating NF-κB, via EP4–PI3K and EP4–MAPK signaling, and promotes formation of liver metastases in mice. The PGE2 signaling pathway might therefore be targeted therapeutically to slow colorectal cancer progression.