Breast cancer stem-like cells can promote metastasis by activating platelets and down-regulating antitumor activity of natural killer cells

ObjectiveTo investigate whether cancer stem cells (CSCs) more efficiently activating platelets and evading immune surveillance than non-CSCs thus promoting metastasis.MethodsWe enriched and identified sphereforming cells (SFCs) and coincubated washed platelets with several platelet activators including collagen, 4T1 and SFCs. Platelet-coating tumor cells, platelet activation and TGF-β1 release were analyzed. Then natural kell cells (NK) were incubated with supernatants of different activated platelet samples what we called sample release (SR). The degranulation assay and NKG2D expression on NK cells were conducted by flow cytometry. Finally tissue factor (TF) expression of SFCs or 4T1 were evaluated by western blot.ResultsBreast cancer cell line 4T1 could form spheres in serum-free medium at low adherence. Sphere-forming cells expressed high levels of the CD24−/lowCD44 + stem cell phenotype. Both sphere-forming cells or 4T1 were coated with abundant platelets while sphere-forming cells induced significantly higher expression of platelet activating receptor CD62p than 4T1 did (P < 0.01). And sphere-forming cells induced platelets to produce more TGF-β1 than 4T1 did (P < 0.01). Furthermore, sample releases induced by sphere-forming cells caused more vigorous inhibition of NK cells antitumor reactivity (P < 0.05) and reduced NKG2D expression (P < 0.01). The final results showed that sphere-forming cells expressed higher levels of TF than 4T1 (P< 0.05).ConclusionOur findings indicate that CSCs could efficiently activate platelets, induce platelets to secrete more TGF-β1, decrease NKG2D expression and inhibit antitumor activity of NK cell, compared with 4T1. And higher levels of TF expression of CSCs may account for this correlation of CSCs and platelets.