Linda May

Contribution of Host-Derived Tissue Factor to Tumor Neovascularization

Objective—The role of host-derived tissue factor (TF) in tumor growth, angiogenesis, and metastasis has hitherto been unclear and was investigated in this study. Methods and Results—We compared tumor growth, vascularity, and responses to cyclophosphamide (CTX) of tumors in wild-type (wt) mice, or in animals with TF levels reduced by 99% (low-TF mice). Global growth rate of 3 different types of transplantable tumors (LLC, B16F1, and ES teratoma) or metastasis were unchanged in low-TF mice. However, several unexpected tumor/context-specific alterations were observed in these mice, including: (1) reduced tumor blood vessel size in B16F1 tumors; (2) larger spleen size and greater tolerance to CTX toxicity in the LLC model; (3) aborted tumor growth after inoculation of TF-deficient tumor cells (ES TF−/−) in low-TF mice. TF-deficient tumor cells grew readily in mice with normal TF levels and attracted exclusively host-related blood vessels (without vasculogenic mimicry). We postulate that this complementarity may result from tumor-vascular transfer of TF-containing microvesicles, as we observed such transfer using human cancer cells (A431) and mouse endothelial cells, both in vitro and in vivo. Conclusions—Our study points to an important but context-dependent role of host TF in tumor formation, angiogenesis and therapy.

Inferior Vena Cava Ligation Rapidly Induces Tissue Factor Expression and Venous Thrombosis in Rats

Objective—Although stasis is important in the pathogenesis of deep vein thrombosis (DVT), how it contributes to thrombogenesis is largely unknown. To gain mechanistic insight, we used a rat model of inferior vena cava (IVC) ligation. Methods and Results—Rats were subjected to IVC ligation for 15 to 60 minutes. Ligation resulted in rapid IVC dilatation and by 60 minutes, thrombi were detected in all rats. Small thrombi were detected in the IVC of most rats after 15 minutes of ligation. Thrombi were rich in fibrin, contained aggregated platelets as well as trapped leukocytes and red cells, and most originated at sites of localized endothelial denudation. Immunohistochemical analysis revealed tissue factor (TF)-expressing leukocytes within the thrombi and adherent to the vessel wall. Despite a largely intact vessel wall, endothelial cells also stained for TF. The expression of TF colocalized with that of protein disulfide isomerase (PDI), an enzyme implicated in TF decryption. Conclusions—These findings suggest that the rapid development of DVT after IVC ligation reflects a combination of stasis-induced vein wall injury and enhanced TF expression in endothelial cells and leukocytes. Because TF expression occurs so soon after ligation, new synthesis is unlikely. Instead, stasis-induced venous dilatation with or without exposure of subendothelial TF, may be responsible for vessel wall TF expression. Colocalization of TF and PDI raises the possibility that PDI-mediated TF decryption plays a role in the pathogenesis of DVT.

Contrasting effects of VEGF gene disruption in embryonic stem cell‐derived versus oncogene‐induced tumors

Previous gene targeting studies have implicated an indispensable role of vascular endothelial growth factor (VEGF) in tumor angiogenesis, particularly in tumors of embryonal or endocrine origin. In contrast, we report here that transformation of VEGF‐deficient adult fibroblasts (MDF528) with ras or neu oncogenes gives rise to highly tumorigenic and angiogenic fibrosarcomas. These aggressive VEGF‐null tumors (528ras, 528neu) originated from VEGF−/− embryonic stem cells, which themselves were tumorigenically deficient. We also report that VEGF production by tumor stroma has a modest role in oncogene‐driven tumor angiogenesis. Both ras and neu oncogenes down‐regulated at least two endogenous inhibitors of angiogenesis [pigment epithelium derived factor (PEDF) and thrombospondin 1 (TSP‐1)]. This is functionally important as administration of an antiangiogenic TSP‐1 peptide (ABT‐526) markedly inhibited growth of VEGF−/− tumors, with some ingress of pericytes. These results provide the first definitive genetic demonstration of the dispensability of tumor cell‐derived VEGF in certain cases of ‘adult’ tumor angiogenesis, and thus highlight the importance of considering VEGF‐independent as well as VEGF‐dependent pathways when attempting to block this process pharmacologically.

The role of tumor-and host-related tissue factor pools in oncogene-driven tumor progression

Oncogenic events play an important role in cancer-related coagulopathy (Trousseau syndrome), angiogenesis and disease progression. This can, in part, be attributed to the up-regulation of tissue factor (TF) and release of TF-containing microvesicles into the pericellular milieu and the circulation. In addition, certain types of host cells (stromal cells, inflammatory cells, activated endothelium) may also express TF. At present, the relative contribution of host- vs tumor-related TF to tumor progression is not known. Our recent studies have indicated that the role of TF in tumor formation is complex and context-dependent. Genetic or pharmacological disruption of TF expression/activity in cancer cells leads to tumor growth inhibition in immunodeficient mice. This occurred even in the case of xenotransplants of human cancer cells, in which TF overexpression is driven by potent oncogenes (K-ras or EGFR). Interestingly, the expression of TF in vivo is not uniform and appears to be influenced by many factors, including the level of oncogenic transformation, tumor microenvironment, adhesion and the coexpression of markers of cancer stem cells (CSCs). Thus, minimally transformed, but tumorigenic embryonic stem (ES) cells were able to form malignant and angiogenic outgrowths in the absence of TF. However, these tumors were growth inhibited in hosts (mice) with dramatically reduced TF expression (low-TF mice). Depletion of host TF also resulted in changes affecting vascular patterning of some, but not all types of tumors. These observations suggest that TF may play different roles growth and angiogenesis of different tumors. Moreover, both tumor cell and host cell compartments may, in some circumstances, contribute to the functional TF pool. We postulate that activation of the coagulation system and TF signaling, may deliver growth-promoting stimuli (e.g. fibrin, thrombin, platelets) to dormant cancer stem cells (CSCs). Functionally, these influences may be tantamount to formation of a provisional (TF-dependent) cancer stem cell niche. As such these changes may contribute to the involvement of CSCs in tumor growth, angiogenesis and metastasis.

Diverse roles of tissue factor-expressing cell subsets in tumor progression.

Oncogenic upregulation of tissue factor (TF) and release of TF-containing microvesicles play an important role in cancer-related coagulopathy (Trousseaus syndrome), angiogenesis, and disease progression. In addition, certain types of host cells (stromal cells, inflammatory cells, activated endothelium) may also express TF. Although the relative contribution of host-related versus tumor-related TF to tumor progression is not known, our recent studies indicate that the role of both sources of TF in tumor formation is complex and context-dependent. Disruption of TF expression/activity in cancer cells leads to tumor growth inhibition in immunodeficient mice, even in cases where TF overexpression is driven by potent oncogenes ( K-RAS or EGFR). Interestingly, TF expression in vivo appears to be influenced by many factors, including the level of oncogenic transformation, tumor microenvironment, and differentiation from cancer stem-like cells. We postulate that activation of TF signaling and coagulation may deliver growth-promoting stimuli (e.g., fibrin, thrombin, platelets) to dormant cancer stem cells (CSCs). Functionally, these influences may be tantamount to formation of a provisional (TF-dependent) cancer stem cell niche. As such, these changes may contribute to the involvement of CSCs in tumor growth, angiogenesis, and metastasis.