Myeloid-Derived Suppressor Cells Recruited by Chemokine (C-C Motif) Ligand 3 Promote the Progression of Breast Cancer via Phosphoinositide 3-Kinase-Protein Kinase B-Mammalian Target of Rapamycin Signaling

Purpose: Numerous studies have shown that the frequency of myeloid-derived suppressor cells (MDSCs) is associated with tumor progression, metastasis, and recurrence. Chemokine (C-C motif) ligand 3 (CCL3) may be secreted by tumor cells and attract MDSCs into the tumor microenvironment. In the present study, we aimed to explore the molecular mechanisms whereby CCL3 is involved in the interaction of breast cancer cells and MDSCs.

Methods: The expression of CCL3 and its receptors was investigated using real-time polymerase chain reaction, western blotting, and enzyme-linked immunosorbent assay. The cell counting Kit-8, wound healing, and transwell assays were performed to study cell growth, migration, and invasion. Cell cycling, apoptosis, and the frequency of MDSCs were investigated through flow cytometry. Transwell assays were used for co-culture and chemotaxis detection. Markers of the epithelial-mesenchymal transition (EMT) were determined with western blotting. The role of CCL3 in vivo was studied via tumor xenograft experiments.

Results: CCL3 promoted cell proliferation, migration, invasion, and cycling, and inhibited apoptosis of breast cancer cells in vitro. Blocking CCL3 in vivo inhibited tumor growth and metastases. The frequency of MDSCs in patients with breast cancer was higher than that in healthy donors. Additionally, MDSCs might be recruited by CCL3. Co-culture with MDSCs activated the phosphoinositide 3-kinase-protein kinase B-mammalian target of rapamycin (PI3K-Akt-mTOR) pathway and promoted the EMT in breast cancer cells, and their proliferation, migration, and invasion significantly increased. These changes were not observed when breast cancer cells with CCL3 knockdown were co-cultured with MDSCs.

Conclusion: CCL3 promoted the growth of breast cancer cells, and MDSCs recruited by CCL3 interacted with these cells and then activated the PI3K-Akt-mTOR pathway, which led to EMT and promoted the migration and invasion of the cells.