Platelets enhance the ability of bone-marrow mesenchymal stem cells to promote cancer metastasis.

Background: Bone marrow-derived mesenchymal stem cells (BM-MSCs) have been identified to be closely associated with cancer progression. Our previous experimental results showed that BM-MSCs promote tumor growth and metastasis of gastric cancer through paracrine-soluble cytokines or exosomes. However, the elements that affect the role of BM-MSCs in promoting tumor metastasis are not clear. It is known that thrombocytosis in cancer patients is very common. Recently, platelets are recognized to play a critical role in tumor progression.

Purpose: This study aims to observe the effect of BM-MSCs which were co-cultured with platelets on tumor cell metastasis.

Methods: Platelet aggregation rate and the expression of P-selectin of platelets co-incubated with conditioned medium of SGC-7901 cells and BM-MSCs were detected by flow cytometry and platelet aggregometer. We also analyzed the change of BM-MSCs after co-incubation with platelets or platelets which were treated with SGC-7901 cells using transwell assay and Western blot analysis. The proliferation and migration ability and expression of VEGF, c-Myc, and sall-4 in SGC-7901 cells treated with medium of BM-MSCs which were co-cultured with platelets were detected. SGC-7901 cells were injected into Balb/c nude mice and the extent of lung metastasis was observed. Both in vitro and in vivo assays were used to analyze the effect of platelets on enhancing the ability of BM-MSCs to promote cancer metastasis.

Results: Results suggested that BM-MSCs and tumor cells can promote platelet aggregation rate and the expression of P-selectin. The protein levels of α-smooth muscle actin, vimentin, and fibroblast activation protein in BM-MSCs were higher after co-incubation with platelets, and SB431542 was used to confirm the effect of TGF-β on transdifferentiation of BM-MSCs into cancer-associated fibroblasts. Medium of BM-MSCs treated with platelets enhanced the proliferation and migration ability of SGC-7901 cells. More lung metastases were found in mice which were injected with SGC-7901 cells treated with conditioned medium from BM-MSCs co-incubated with platelets.

Conclusion: Tumor cells and BM-MSCs activate platelets which can change the characteristics of BM-MSCs through secretion of TGF-β. Moreover, we found that platelets enhanced the effect of BM-MSCs on tumor metastasis, which suggested a potential target and approach for gastric cancer therapy.