Tissue factor (TF) expression by tumor cells correlates with metastasis clinically and supports metastasis in experimental settings. However, the precise pathways coupling TF to malignancy remain incompletely defined. Here, we show that clot formation by TF indirectly enhances tumor cell survival after arrest in the lung, during experimental lung metastasis, by recruiting macrophages characterized by CD11b, CD68, F4/80, and CX 3 CR1 (but not CD11c) expression. Genetic or pharmacologic inhibition of coagulation, by either induction of TF pathway inhibitor expression or by treatment with hirudin, respectively, abrogated macrophage recruitment and tumor cell survival. Furthermore, impairment of macrophage function, in either Mac1-deficient mice or in CD11b-diphtheria toxin receptor mice in which CD11b-positive cells were ablated, decreased tumor cell survival without altering clot formation, demonstrating that the recruitment of functional macrophages was essential for tumor cell survival. This effect was independent of NK cells. Moreover, a similar population of macrophages was also recruited to the lung during the formation of a premetastatic niche. Anticoagulation inhibited their accumulation and prevented the enhanced metastasis associated with the formation of the niche. Our study, for the first time, links TF induced coagulation to macrophage recruitment in the metastatic process. (Blood. 2012;119(13): 3164-3175) IntroductionClinical and experimental studies over the past 30 years have established that the coagulation system actively supports tumor progression and metastasis. Consistent with these observations, expression of procoagulants by tumor cells, among them tissue factor (TF), cancer procoagulant, 1 and selectin ligands, correlates with advanced disease and poor outcome for multiple cancer types. 2,3 TF (also known as coagulation factor III or CD142) is the protease receptor that initiates coagulation after injury through the extrinsic pathway. Under normal physiologic conditions, TF expression is limited to extravascular sites that only become exposed to blood after trauma. In this case, the exposed TF binds to and activates the blood-borne coagulation factor FVII, triggering clot formation through a cascade of proteolytic events that results in thrombin formation, activation of platelets, and fibrin deposition. 4 In addition to triggering coagulation, the binding of FVIIa to TF activates intracellular signaling pathways through the TF cytoplasmic domain, by activating G-protein-coupled protease activated receptors (PARs), especially PAR2. 4 These signaling pathways support tumor angiogenesis 5,6 and regulate tumor progression. 7 Intracellular signaling pathways can be distinguished experimentally from the extracellular coagulative roles of TF by specific antibodies 7 or deletion of the cytoplasmic domain that eliminates many forms of TF signaling but still triggers coagulation. 6,8,9 TF enhances tumor growth and angiogenesis, [4][5][6][7]10 and specifically plays an important role in some experiment...

Liver metastasis from colorectal cancer is a leading cause of cancer mortality. Myeloid cells play pivotal roles in the metastatic process, but their prometastatic functions in liver metastasis remain incompletely understood. To investigate their role, we simulated liver metastasis in C57BL/6 mice through intrasplenic inoculation of MC38 colon carcinoma cells. Among the heterogeneous myeloid infiltrate, we identified a distinct population of CD11b/Gr1 mid cells different from other myeloid populations previously associated with liver metastasis. These cells increased in number dramatically during establishment of liver metastases and were recruited from bone marrow by tumor-derived CCL2. Liver metastasis of Lewis lung carcinoma cells followed this pattern but this mechanism is not universal as liver colonization by B16F1 melanoma cells did not recruit similar subsets. Inhibition of CCL2 signaling and absence of its cognate receptor CCR2 reduced CD11b/Gr1 mid recruitment and decreased tumor burden. Depletion of the CD11b/Gr1 mid subset in a transgenic CD11b-diphtheria toxin receptor mouse model markedly reduced tumor cell proliferation. There was no evidence for involvement of an adaptive immune response in the prometastatic effects of CD11b/Gr1 mid cells. Additionally, an analogous myeloid subset was found in liver metastases of some colorectal cancer patients. Conclusion: Collectively, our findings highlight the importance of myeloid cells-in this case a selective CD11b/Gr1 mid subsetin sustaining development of colorectal cancer liver metastasis and identify a potential target for antimetastatic therapy. (HEPATOLOGY 2013;57:829-839) M etastatic colorectal cancer (CRC) is a prominent cause of cancer mortality worldwide. 1 Hepatic metastases are found in approximately 15% of CRC patients at primary diagnosis 2 with 14% subsequently developing metastases.3 Development of new treatment modalities for CRC liver metastasis is urgently required and a greater understanding of the biology of this process will help establish new therapeutics aimed at downstaging the disease, improving operability, and prolonging survival.Metastasis is a multistep process involving complex and continuous interactions between tumor cells and the host microenvironment.4 Several myeloid-derived cell types have been shown to play key roles in the metastatic cascade, including intravasation, extravasation, 5 and colonization at secondary sites by stimulating tumor cell proliferation and angiogenesis and suppressing antitumor immunity.6-8 However, delineation of their roles in metastasis is complicated by the heterogeneity of myeloid phenotypes that appears to be both tumor-and organ-selective. Vascular endothelial growth factor receptor 1 (VEGFR1) þ hematopoietic progenitor cells accumulated at premetastatic sites to promote adherence and growth of lung Lewis carcinoma (LLC) and B16F1 tumor cells, 9 while a Mac-1 þ myeloid population with different markers was

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