PGE2/EP4 antagonism enhances tumor chemosensitivity by inducing extracellular vesicle‐mediated clearance of cancer stem cells

Cells expressing mesenchymal/basal phenotypes in tumors have been associated with stem cell properties. Cancer stem cells (CSCs) are often resistant to conventional chemotherapy. We explored overcoming mesenchymal CSC resistance to chemotherapeutic agents. Our goal was to reduce CSC numbers in vivo, in conjunction with chemotherapy, to reduce tumor burden. Analysis of clinical samples demonstrated that COX‐2/PGE2/EP4 signaling is elevated in basal‐like and chemoresistant breast carcinoma and is correlated with survival and relapse of breast cancer. EP4 antagonism elicts a striking shift of breast cancer cells from a mesenchymal/CSC state to a more epithelial non‐CSC state. The transition was mediated by EP4 antagonist‐induced extracellular vesicles [(EVs)/exosomes] which removed CSC markers, mesenchymal markers, integrins, and drug efflux transporters from the CSCs. In addition, EP4 antagonism‐induced CSC EVs/exosomes can convert tumor epithelial/non‐CSCs to mesenchymal/CSCs able to give rise to tumors and to promote tumor cell dissemination. Because of its ability to induce a CSC‐to‐non‐CSC transition, EP4 antagonist treatment in vivo reduced the numbers of CSCs within tumors and increased tumor chemosensitivity. EP4 antagonist treatment enhances tumor response to chemotherapy by reducing the numbers of chemotherapy‐resistant CSCs available to repopulate the tumor. EP4 antagonism can collaborate with conventional chemotherapy to reduce tumor burden. What's new? Cyclooxygenase‐2 (COX‐2) inhibitors block prostaglandin E2 synthesis and are associated with reduced tumor frequency and progression. Nonetheless, they can have limiting side effects, and their utility in the treatment of established cancers remains uncertain. Here, COX‐2‐expressing human breast cancer cells were found to exhibit characteristics similar to those of chemotherapy‐resistant cancer stem cells (CSCs). Selective inhibition of PGE2 receptor 4 (EP4) triggered the release of extracellular vesicles containing CSC proteins, resulting in a loss of stem cell properties. The findings suggest that direct EP4 inhibition could enhance chemotherapeutic approaches without evoking the side effects of broader COX‐2 inhibition.