Multiwalled Carbon Nanotubes Prevent Tumor Metastasis Through Switching M2-Polarized Macrophages to M1 via TLR4 Activation.

Targeting tumor-associated macrophages (TAMs) has emerged as a novel therapeutic strategy for cancer metastasis. Here, we investigated whether carboxylated multiwalled carbon nanotubes (MWCNTs-COOH) prevent tumor metastasis through regulating macrophage polarization. In Lewis lung carcinoma (LLC) or B16F10 melanoma-bearing mice, intratracheal instillation of MWCNTs-COOH significantly reduced metastatic burden in the lungs. MWCNTs-COOH promoted the expression of M1 markers (iNOS and CXCR10) and inhibited the expression of M2 markers (CD206 and Arg-1) along with an increased expression of Th1 cytokines (TNF-α and IL-12) and decreased expression of Th2 cytokines (TGF-β and IL-10). Such changes were accompanied with TLR4 mRNA and protein elevation. Importantly, macrophage depletion in mice lungs reversed the anti-metastatic effects of MWCNTs-COOH. In vitro, MWCNTs-COOH switched IL-4/13-treated macrophages to the M1 phenotype and thus prevented the migration and invasion of LLC cells, accompanied by the upregulation of toll-like receptor (TLR)-4/NF-κB p65 signaling. Moreover, TAK-242 (resatorvid), a specific TLR4 inhibitor, reversed the effects of MWCNTs-COOH on macrophage polarization. In summary, MWCNTs-COOH effectively prevent tumor metastasis through skewing M2-polarized macrophages to M1 via activating TLR4/NF-κB signaling. Thus, targeting TAMs by MWCNTs-COOH is a potential therapeutic approach against tumor metastasis.