The Role Of Toll-like Receptor4in Tumor Metastasis

Metastasis constitutes the main cause of cancer-related death and a major challenge in today’s cancer management. The metastatic process consists of a series of events:cell detachment from the primary tumor mass, migration into and transport along the bloodstream, and, finally, tumor cell arrest and proliferation within the distant tissuel, but the details of metastasis and mechanisms governing this clinically important process remain poorly understood. Considering metastasis is a highly inefficient process, as less than0.1%of tumor cells that penetrate the circulation end up forming metastatic colonies, to survive in the blood is crucial for successful metastasis. Platelets are reported to play important roles for promoting tumor cell survival in the bloodstream via various mechanisms, and the platelet-tumor cell interaction is vital to tumor metastasis.TLR4sits at the interface of microbial and sterile inflammation by responding to both bacterial endotoxin and multiple endogenous ligands. Studies focusing on the endotoxin and tumor cell TLR4have already shown that endotoxin is a potent promoter of tumor metastasis when infused systemically; and tumor cell TLR4activation by endotoxin resulted in EMT of tumor cells and augmented metastasis. Importantly, TLR4can detect endogenous damage-associated molecular patterns (DAMPs) including the widely expressed alarmin protein HMGB1. It is reported that the action of HMGB1, which is released by dying tumor cells, on TLR4expressed by dendritic cells led to the activation of tumor antigen-specific T-cell immunity, resulting in tumor suppression. However, whether TLR4is involved in tumor metastasis is unclear.Platelets express all the components of the LPS receptor signaling complex (including TLR4, CD14, MD2and myeloid differentiation primary response protein88(MYD88)). During sepsis, TLR4expressed on platelets serves as a bridge between platelets and other immune cells. The recognition of pathogen molecules by TLR4on platelets is linked to activation of innate immune system and contributes to pathogen recognition and to the inhibition of pathogen spreading in vivo. In vitro studies showed that platelet binds to fibrinogen under flow conditions in a TLR4dependent manner; LPS stimulates platelet secretion and potentiates platelet aggregation through a TLR4/MyD88-dependent pathway. In spite of the roles of platelet TLR4in immune regulation, there is no study about platelet TLR4in tumor progression and metastasis. In the present study, we investigated the role of TLR4in tumor metastasis by using experimental metastasis model; we also studied the mechanisms by which TLR4affected metastasis by using the platelet-tumor cell coculture model. We demonstrate that host TLR4deficiency significantly diminished experimental lung metastasis without affecting primary tumor growth. Bone marrow transplantation experiment showed that ablation of TLR4in myeloid cells conferred protection against metastasis. Inhibition of platelet activation led to reduced metastasis in WT but not in TLR4-/-mice, suggesting that TLR4on platelets is of great importance in metastasis. In vitro studies using co-culture system revealed that the ability of platelet adhesion to tumor cells and TGFβ1secretion were impaired due to TLR4deficiency. Further investigation showed that the levels of high-mobility group boxl (HMGB1) in tumor-bearing mice were substantially elevated. HMGB1neutralization using antibody significantly inhibited tumor cell-platelet interaction in a TLR4-dependent manner. Importantly, HMGB1neutralization significantly inhibited tumor metastasis in WT but not in TLR4-/-mice.Our data impose the importance of the TLR4on platelets in the tumor metastasis. Our data showed that interaction between tumors cell-derived HMGB1and TLR4on platelets lead to platelet activation, enhance tumor cell-platelet interactions, and thus promoting tumor metastasis. As depletion of platelets or anti-platelet activation using antibodies or pharmacological methods may have side effects such as hemorrhage, inhibition of TLR4activation by inhibiting HMGB1released from tumor cells may be an ideal target to prevent and treat tumor metastasis.