Resolvins inhibit breast tumor progression by countering cancer stem cells

It is now increasingly appreciated that post‐treatment progression and recurrence after chemotherapy is not solely due to selection of mutant cells but an active response by surviving tumor cells in the injured tumor bed. Two mechanisms have been implicated: the direct induction by cytotoxic stress inflicted by treatment of stem‐like state in the non‐killed cells and the generation of pro‐tumorigenic inflammation following massive cell death, tissue destruction and accumulation of cellular debris. Both mechanisms are promoted by the lipid autacoid, prostaglandin E 2 (PGE 2 ), released by cancer cells following cytotoxic treatment, which stimulates stemness by converting non‐killed cells to cancer stem cells (CSCs). PGE 2 also is a potent stimulator of tumor‐promoting inflammation and can cause immunosuppression. Thus, any treatment that causes tumor regression (which is required to reduce the tumor burden) via cell death is inherently a double‐edged sword . To solve this dilemma immanent to cancer treatment, we hypothesized that these two effects, the tumor mass reduction and the tumor‐enhancing activity of cytotoxic treatment can be decoupled using specialized pro‐resolving mediators (SPMs) to mitigate the stemness response and tumor promoting activity of cell‐debris. SPM are endogenous, pro‐resolving, and anti‐inflammatory lipid autacoids that promote the resolution of inflammation. We postulate that their biological function may also reduce CSCs in the tumor cells triggered by the tissue destruction. Therefore, we examined whether resolvins, a family of SPMs, may reduce stemness in two breast tumor models. Mice were inoculated with a sub‐threshold inoculum of EO771 breast tumor cells and treated with chemotherapy or targeted therapy, which trigger breast tumor growth and dormancy escape. Flow cytometry measurement of CD44 and CD24 confirmed that chemotherapy (cisplatin) and anti‐estrogen therapy (tamoxifen) increased the percentage of stem‐like breast cells (CD44 high CD24 low ) in EO771 tumor cells. We isolated the dead putative CSC (CD44 high CD24 low ) cells by FACS and co‐injected these cells with the sub‐threshold inoculum of 10000 EO771 cells into mice. Dead CSCs (CD44 high CD24 low ) isolated from tamoxifen‐treated breast tumor EO771 debris stimulated primary EO771 breast tumor growth in a dose‐dependent manner. Resolvins (e.g. RvD2) inhibited the spontaneous MMTV‐PyMT breast tumor growth at 15 nanogram/day by inhibiting stemness. Cancer stem cell markers (e.g. CD44) and PGE 2 were suppressed in resolvin‐treated tumors. Thus, in addition to stimulating clearance of debris, resolvins prevent transition into a stem‐like state and reduce stemness in breast cancer. Our studies suggest that resolvins represents a novel modality for combination therapy with standard cell killing chemotherapy or targeted therapy, that does not simply “double‐down” on the cell killing potency but by eliminating the intrinsic double‐sword activity of cytotoxic treatment.