Self-production of tissue factor-coagulation factor VII complex by ovarian cancer cells

Venous thromboembolism (VTE) is a common complication in cancer patients and a significant cause of morbidity and mortality (Chew et al, 2007; Varki, 2007). The risk of VTE is highest for cancers of the ovary, pancreas and liver (Iodice et al, 2008). In fact, ovarian cancer patients closely associate with intravascular thrombosis (Tateo et al, 2005; Satoh et al, 2007), and the development of VTE within 2 years is a strong predictor of death due to this cancer (Rodriguez et al, 2007). Activation of platelets and elevation of plasma tissue factor (TF) levels (Han et al, 2006; Tesselaar et al, 2007) are candidate determinants for the development of thrombosis in cancer patients. Numerous studies have suggested that TF may have an important function in thrombosis in cancer patients. TF primarily presents on the cell surface as a transmembrane protein and functions as a cellular receptor for coagulation factor VII (fVII) circulating in the blood (Furie and Furie, 1988). By binding with TF, fVII is changed to its active form, fVIIa, and subsequently triggers the extrinsic coagulation cascade by activating factor X (Furie and Furie, 1988). Overexpression of TF correlates with a high incidence of VTE in pancreatic (Khorana et al, 2007) and ovarian (Uno et al, 2007) cancer patients. We have also reported that TF is constitutively expressed in many tumour cell lines (Koizume et al, 2006). Functionally active TF also circulates in the blood as a component of cell-derived microparticles (MPs) produced by platelets, cells with a monocyte/macrophage lineage (Lopez et al, 2005) and cancer cells (Amin et al, 2008). It is likely that this circulating TF may contribute to the development of thrombosis. Hypoxia is a condition closely associated with cancer (Dewhirst et al, 2008), including ovarian cancer (Kim et al, 2006). The gene encoding TF is activated under hypoxia by a transcription factor, Egr-1, found in glioblastoma (Rong et al, 2005), melanoma, breast, lung (Amirkhosravi et al, 1998) and cervical cancer cells (Denko et al, 2000). To date, hypoxic induction of TF in ovarian cancer cells has not been reported. In an earlier study, we showed that fVII, primarily synthesised in the liver, was constitutively produced in various non-hepatic cancer cells including ovarian cancer cells (Koizume et al, 2006). Furthermore, fVII transcription was inducible under hypoxic conditions by the hypoxia-inducible factor-2α (HIF-2α)-dependent pathway in ovarian cancer cells (Koizume et al, 2006). Thus, we surmised that fVII and TF induced by hypoxia in ovarian cancer cells, and not fVII derived from circulating blood, may be involved in thrombotic events in ovarian cancer patients. The aim of this study was to examine the possibility that ovarian cancer tissue could intrinsically produce the TF-fVIIa complex independently of fVII supplied from blood plasma, and thus increase the risk of thrombosis. To this end, we investigated the expression of TF and fVII and the secretion of plasma membrane-derived MPs in ovarian cancer cells under both normoxia and hypoxia.