Introduction: Tumor-associated macrophages of the M2 phenotype promote tumor proliferation and are associated with a poor prognosis in patients with various malignancies. We previously reported that large numbers of M2 macrophages have been found in the peritoneal cavity of gastric cancer patients with peritoneal dissemination. As M2 macrophages in the peritoneal cavity contribute to tumor progression, they are considered promising targets in patients who have gastric tumors with peritoneal dissemination. Paclitaxel (PTX) is an antineoplastic agent derived from the bark of the Pacific yew. Low dose PTX was shown to block tumor-induced polarization of conventional dendritic cells (DCs) into immunosuppressive regulatory DCs. However, it has not yet been determined whether PTX can modulate TAMs of the M2 phenotype. This study assessed whether PTX suppresses M2 macrophages, by acting as a Toll-like receptor 4 (TLR4) agonist.
Methods: Macrophages derived from the THP-1 monocytic cell line and peripheral blood mononuclear cell (PBMC)-derived macrophages were cultured with gastric cancer cells in medium containing PTX, at a concentration that did not affect cell proliferation. The effects of PTX on macrophage expression of CD204, a marker of M2 macrophages, and NOS2, a marker of M1 macrophages, were evaluated by western blotting. The ability of PTX to stimulate intranuclear translocation of NF-kB was determined by measuring expression of the p65 subunit of NF-kB.
Results: In THP-1 macrophages, low-dose PTX (1 nM, 5 nM) inhibited the expression of CD204, enhanced the expression of NOS2, and significantly suppressed the phosphorylation of Stat3, which is essential for the M2 phenotype. Low-dose PTX also inhibited CD204 expression in primary macrophages derived from PBMCs. PTX treatment of THP-1 macrophages for 1 hour induced marked intranuclear translocation of NF-kB p65.
Conclusion: Low-dose PTX inhibited the M2 phenotype and induced the M1 phenotype via TLR4 signaling, suggesting that low dose PTX can alter macrophage phenotype, whereas clinical doses can kill cancer cells. These results suggest that the anti-cancer effects of PTX are due both to its cytotoxic and immunomodulatory activities.