Alternatively spliced tissue factor induces angiogenesis through integrin ligation

Abstract

The initiator of coagulation, full-length tissue factor (flTF), in complex with factor VIIa, influences angiogenesis through PAR-2. Recently, an alternatively spliced variant of TF (asTF) was discovered, in which part of the TF extracellular domain, the transmembrane, and cytoplasmic domains are replaced by a unique C terminus. Subcutaneous tumors produced by asTF-secreting cells revealed increased angiogenesis, but it remained unclear if and how angiogenesis is regulated by asTF. Here, we show that asTF enhances angiogenesis in matrigel plugs in mice, whereas a soluble form of flTF only modestly enhances angiogenesis. asTF dose-dependently upregulates angiogenesis ex vivo independent of either PAR-2 or VIIa. Rather, asTF was found to ligate integrins, resulting in downstream signaling. asTF-αVβ3 integrin interaction induces endothelial cell migration, whereas asTF-dependent formation of capillaries in vitro is dependent on α6β1 integrin. Finally, asTF-dependent aortic sprouting is sensitive to β1 and β3 integrin blockade and a TF-antibody that disrupts asTF-integrin interaction. We conclude that asTF, unlike flTF, does not affect angiogenesis via PAR-dependent pathways but relies on integrin ligation. These findings indicate that asTF may serve as a target to prevent pathological angiogenesis.

The development of blood vessels out of existing vessels, termed angiogenesis, occurs in embryonic development, wound healing, and cancer (1). Angiogenesis depends on initial tip cell migration from the existing vessel, followed by migration of stalk cells, which divide and form a capillary (2), and the recruitment of pericytes aligning the capillary. Angiogenesis is regulated by such proteins as vascular endothelial growth factor (VEGF), metalloproteinases, and interleukin-8 (1, 3, 4), but also depends on integrins. In particular, β1- and β3-type integrins play a role in endothelial and pericyte migration as well as capillary formation (5, 6).

Upon vessel damage, tissue factor (TF) together with factor VII(a), generates factor Xa, thrombin, and fibrin, yielding a hemostatic plug (7). TF is indispensable for life since TF-deficient embryos die in utero (8). Moreover, TF affects tumor angiogenesis in colorectal cancer and breast cancer models, through TF:VIIa-dependent protease-activated receptor-2 (PAR-2) activation (9–11), resulting in expression of VEGF, IL-8, MMP-7, and CXCL-1 (10–13). In addition, TF binds α3β1 and α6β1 integrins, and disruption of this complex downregulates pro-angiogenic signaling and suppresses tumor growth in vivo (11, 14).

Recently, a TF isoform, asTF, which results from alternative splicing, was discovered, in which the transmembrane and cytoplasmic domains are replaced by a unique 40 amino acid, C-terminal domain, rendering asTF soluble (15). asTF incorporates in thrombi and asTF secreted by endothelium exhibits pro-coagulant activity (16). While some groups failed to detect pro-coagulant activity of asTF in transfected cells (15–17), native asTF can be pro-coagulant (16) and is expressed in malignancies such as pancreatic and lung cancer (18, 19). Hobbs et al. demonstrated that pancreatic cancer cells transfected to express asTF produce more blood vessels (20), but it remained mechanistically unclear if and how angiogenesis is regulated by asTF. One possibility is that asTF stimulates cancer cells to produce angiogenic factors, but it is also plausible that asTF enhances angiogenesis via paracrine stimulation of endothelial cells. Moreover, the role of VIIa, PAR-2 activation and downstream coagulation activation remains obscure.

In this study, we report that asTF enhances angiogenesis in vivo, ex vivo, and in vitro independent of downstream coagulation factors or activation of PAR-2, but dependent on αvβ3 and α6β1 integrin function.

Results

asTF Induces Angiogenesis In Vivo and Ex Vivo.

To study a potential effect of asTF on angiogenesis, we evaluated the properties of recombinant asTF. Purified asTF migrated as a single band of approximately 26 kDa, corresponding to its predicted molecular weight, on SDS/PAGE (Fig. S1A). The preparation reacted with a monoclonal antibody against the N-terminal part (9C3), but not with antibodies against the C-terminal part of full-length TF (flTF) (10H10, 5G9) (Fig. S1B), confirming that the expressed protein was indeed asTF. Analogous immunoreactivity of these antibodies was observed with asTF expressed in eukaryotic cells (Fig. S1C).

Whether asTF induced angiogenesis was tested in a matrigel plug assay. Matrigel supplemented with asTF, VEGF, or buffer control was injected into C57BL/6 mice, and angiogenesis was analyzed 7 days later. Both asTF and VEGF enhanced angiogenesis compared to the buffer control (Fig. 1A), showing that asTF can influence angiogenesis in the absence of tumor cells. Similar results were obtained in an ex vivo aortic sprouting model (21) in which segmented aortas isolated from C57BL/6 mice were implanted in matrigel containing asTF, VEGF, or control buffer (Fig. 1B). Similar ex vivo experiments revealed that asTF concentration-dependently enhanced the formation of sprouts at concentrations as low as 1 nM (Fig. 1C). Active site-blocked VIIa (VIIai) or hirudin, inhibitors of VIIa and thrombin, respectively, did not inhibit asTF-dependent sprouting, indicating that generation of coagulation proteases was not required (Fig. 1C). Moreover, adding asTF in combination with VIIa did not enhance aortic sprouting compared to asTF alone (Fig. 1D), supporting our finding that asTF induced angiogenesis independent of coagulation activation. Presence of asTF in the matrigel, but not in the media placed on top of the matrigel, enhanced angiogenesis, suggesting that asTF's presence in the extracellular matrix is required (Fig. 1E). Recombinant asTF was produced in E. coli, thus LPS contamination could potentially influence sprout formation. However, inclusion of the LPS inhibitor polymyxin B did not affect aortic sprouting induced by asTF (Fig. S2A), and similar sprouting was observed in a serum-free system (Fig. S2B), a condition that does not allow for efficient LPS signaling (22).

Fig. 1.

asTF induces angiogenesis in vivo and ex vivo. (A) Matrigel containing 100 nM asTF, 50 ng/mL VEGF, or buffer control was injected s.c. into mice. FITC-dextran was tail vein-injected before sacrifice. Extracted plugs were examined using MZ 16FA stereo microscope and DFC 420C camera (Leica), and the number of invading vessels was counted. The graph shows quantification of this experiment (n = 10 ± SEM) (B) Aortic segments from C57BL/6 mice were implanted into matrigel supplemented with solvent control, asTF, or VEGF. Outgrowing sprouts were visualized and counted on day 5. The graph shows quantification of this experiment (n = 8 ± SEM). (C) Aortic segments in matrigel supplemented with solvent control or various concentrations of asTF. The coagulation inhibitors VIIai (100 nM) or hirudin (500 nM) were included in some of the conditions. (D) Aortic segments in matrigel supplemented with 100 nM VIIa, 100 nM asTF, or the combination of asTF and VIIa. (E) Sprouting occurs in asTF-containing matrigel, but not when media overlying the matrigel contains asTF.

asTF-Dependent Angiogenesis Is Not Dependent on PAR-2.

Next, we examined the mechanism behind asTF-induced angiogenesis. Under basal conditions, endothelial cells express low levels of PAR-2, but these levels go up during angiogenesis (10). flTF in complex with VIIa was previously shown to promote PAR-2-dependent angiogenesis, presumably through activation of, among others, the MAP kinase signal transduction pathway. To study the influence of asTF on PAR-2 activation in endothelial cells, we used an endothelial cell line (ECRF) adenovirally transduced to express PAR-2, after which these cells were stimulated with asTF, VIIa, or the combination of asTF and VIIa. A truncated soluble form of flTF (sTF) in complex with VIIa and a PAR-2 agonist, SLIGRL, were used as positive controls. VIIa, asTF, or the combination of asTF and VIIa were not able to induce phosphorylation of MAP kinase, whereas VIIa in complex with sTF or SLIGRL elicited robust phosphorylation within 10 min (Fig. 2A). Similar results were obtained using primary endothelial cells (HUVECs, Fig. S3).

Fig. 2.

asTF does not activate PAR-2. (A) PAR-2-expressing ECRF cells were stimulated with 100 nM asTF ± 100 nM VIIa, VIIa alone, 100 nM truncated flTF (sTF) + VIIa, or SLIGRL (100 μM). MAPK phosphorylation was determined by Western blotting. (B) Cells were serum starved and stimulated with asTF ± VIIa, VIIa alone, or 15% FCS. Cell proliferation was assessed using MTT assays. (C) Aortic segments from wild-type or PAR-2-/- C57BL/6 mice were implanted in matrigel supplemented with solvent control, asTF, or sTF, and the number of sprouts was determined as described.

Since angiogenesis is dependent on endothelial cell proliferation, we also determined whether asTF induced proliferation in PAR-2 transduced cells. FCS induced significant cell proliferation, but incubation with asTF, VIIa or the combination of asTF and VIIa were without effect (Fig. 2B and Fig. S3). In agreement, we found that aortic segments isolated from PAR-2-/- mice displayed the same increase in asTF-dependent sprout formation as wild-type segments, although basal sprouting in PAR-2-/- segments was lower (Fig. 2C). Thus, the effect of asTF on angiogenesis is not dependent on VIIa-mediated activation of PAR-2. Importantly, the effect of asTF on sprouting was much more profound than that observed using sTF, either in wild-type aortas or PAR-2-/- aortas (Fig. 2C), suggesting that asTF is the main TF isoform that mediates angiogenesis.

asTF Binds Endothelial Integrins.

Since flTF was previously shown to bind integrins, the effect of asTF on angiogenesis may be dependent on integrin ligation. To study this, tissue culture plates were coated with asTF and blocked with BSA, after which ECRF cells were seeded. ECRF cells bound time-dependently to asTF-coated plates, whereas cells bound very poorly to BSA-only treated plates (Fig. 3A). Cells bound to asTF displayed enhanced phosphorylation of Focal Adhesion Kinase (FAK), p42/p44 MAP kinase, p38 MAP kinase, and Akt—events commonly elicited by integrin ligation (Fig. 3B). Inclusion of polymyxin B did not influence cell adhesion to asTF (Fig. S4A). Depletion of asTF from our preparation using nickel-NTA abolished cell adhesion, whereas incubation of asTF preparations with non-nickel bound control beads did not prevent cell adhesion (Fig. S4B). Moreover, cell adhesion to asTF was fully inhibited in the presence of the anti-TF antibody 6B4, which interferes with TF-integrin binding (14) (Fig. 3C). Thus, enhancement of cell adhesion was entirely dependent on the presence of asTF. The use of sTF yielded similar results in this assay.

Fig. 3.

asTF binds integrins on endothelial cells. (A) Cells were seeded on BSA- or asTF-coated culture wells and adhered cells were counted at the indicated times. (B) Cells were left to adhere to BSA- or asTF-coated wells for the indicated times, lysed, and the lysates were checked for FAK, p42/22 MAPK, p38 MAPK, and c-Akt phosphorylation on Western blot. Total levels of these proteins were assessed to verify equal loading. (C) Culture wells were coated with 50 μg/mL asTF or sTF and cell adhesion was assayed in the absence/presence of 100 μg/mL 6B4. (D) Endothelial cells were preincubated with integrin-blocking antibodies and seeded onto BSA- or asTF-coated wells. Adhered cells were counted.

To identify the responsible integrins, ECRF cells were preincubated with specific blocking antibodies. β2 or β4 blockade did not inhibit cell adhesion to asTF, β3 blockade modestly inhibited cell adhesion, and β1 blockade resulted in a 50% reduction of cell adhesion (Fig. 3D). The combination of β1 and β3 blockade reduced cell adhesion to basal levels. Similar results were obtained using a murine endothelial cell line (Fig. S4C), validating the use of human asTF in murine models such as the matrigel plug assay and the aortic ring assays. In conclusion, both β1 and β3 integrins bind to asTF to mediate cell adhesion.

asTF Induces Endothelial Cell Migration.

During angiogenesis, endothelial tip cells initially migrate out of the intima and subsequently, endothelial cell proliferation and differentiation result in capillary formation. Using a transwell assay, we assessed whether asTF induces cell migration. asTF induced a 4-fold upregulation in ECRF migration when present in the lower compartment, and cells appeared rounded (Fig. 4A). When the lower sides of transwells were coated with asTF, a 10-fold upregulation of cell migration was observed while cells displayed a flattened morphology.

Fig. 4.

asTF induces αvβ3-dependent endothelial cell migration. (A) Transwell inserts were coated with BSA (negative control), 50 μg/mL asTF, or 1% gelatin (positive control). Endothelial cells were left to migrate for 5 h. Cells were also seeded in uncoated inserts and migration was induced by placing 100 nM asTF into the lower well. Note the rounded-up morphology of cells migrating toward a gradient of asTF versus the flattened morphology of cells migrating through asTF-coated inserts. The graph on the right shows a quantification of these results (n = 9). (B) Pericytes were seeded into asTF-coated inserts or inserts coated with a pericyte extracellular matrix protein mix (Cell Systems) and left to migrate for 5 h. (C) Effect of asTF versus sTF coating on migration of ECRF cells. (D) Endothelial cells were preincubated with β integrin-blocking antibodies and seeded into asTF-coated transwell inserts. Migration was assessed as described above. (E) Endothelial cells were preincubated with β3 or αvβ3-blocking antibodies. asTF-induced cell migration was assessed as described. Inclusion of PM in the assay did not inhibit migration. (F) Effects of MAPK pathway inhibitor PD98059 (20 μM), p38 MAPK inhibitor SB203580 (10 μM), or the PI-3 kinase inhibitor LY294002 (10 μM) on asTF-induced migration. (G) Effect of c-Akt inhibitor (10 μM) on asTF-induced migration.

Pericytes also contribute to angiogenesis. We found that asTF did not facilitate pericyte migration, whereas a mix of extracellular matrix proteins optimized for pericyte binding supported potent migration (Fig. 4B). Thus, asTF does not induce pericyte migration, but selectively leads to migration of endothelial cells. Interestingly, endothelial migration was reduced on sTF when compared to asTF, which may explain the lower angiogenic potential of sTF (Fig. 4C).

The identity of integrins involved in asTF-enhanced migration was determined using integrin-blocking antibodies. β1, β2, and β4 blocking antibodies, as well as a control antibody, were unsuccessful in blocking migration, but β3 blockade fully inhibited migration (Fig. 4D). An anti-αvβ3 antibody (LM609) also blocked asTF-dependent migration, demonstrating that αvβ3 is required for asTF-dependent migration (Fig. 4E). Similarly, αvβ3 blockade inhibited migration of primary HUVECs (Fig. S5A). It is important to note that αvβ3 integrin blockade by LM609 induces apoptosis in endothelial cells (23), which may be responsible for the inhibiting effect on asTF-induced migration, but apoptosis was not observed (Fig. S5B). Polymyxin B did not affect migration, confirming that possible traces of LPS did not contribute to this effect (Fig. 4E).

To investigate the signal transduction pathways involved in asTF-dependent migration, we preincubated cells with specific kinase inhibitors. The p38 MAP kinase inhibitor SB203580 and the PI3 kinase inhibitor LY294002, but not the p42/p44 MAP kinase inhibitor, completely abolished migration (Fig. 4F). An Akt inhibitor only partially inhibited asTF-dependent migration (Fig. 4G), suggesting that PI3-kinase acts through Akt-dependent and independent pathways.

asTF Enhances Endothelial Capillary Formation.

Next, we determined asTF's potential to support capillary formation by seeding ECRF cells on matrigel supplemented with asTF or control buffer. asTF enhanced the formation of capillaries 2.5-fold (Fig. 5 A and B). When asTF was added to the medium, capillary formation was inhibited, most likely by binding and blocking the integrins required for capillary formation on matrigel. Surprizingly, β1 but not β3 blockade inhibited capillary formation to subbasal levels in ECRF and primary HUVECs (Fig. 5B and Fig. S6). Apparently, endothelial cell migration and capillary formation induced by asTF were dependent on different integrin heterodimers. Blockade of α3 and α6 integrin subunits that were previously shown to interact with flTF (10) was also tested. Integrin α6 blockade inhibited asTF-induced capillary formation to subbasal levels (Fig. 5C). In contrast, α3 blockade modestly inhibited capillary formation, suggesting that α3 integrins do not play a major role in this process.

Fig. 5.

asTF enhances α6β1-dependent endothelial capillary formation. (A) Endothelial cells were seeded on matrigel supplemented with solvent control or 100 nM asTF. Photographs were taken after O/N incubation. (B) Cells were seeded on matrigel containing solvent control or asTF. Alternatively, asTF was added tot the medium containing the cells and the mix was added onto a matrigel layer without asTF. Cells were also preincubated with β integrin blocking antibodies and seeded onto asTF-containing matrigel. Total capillary length was quantified. (C) Cells were preincubated with α3 or α6-blocking antibodies and seeded onto asTF-containing matrigel. Non-treated cells were also seeded onto asTF-cotaining matrigel in the presence of VIIai or hirudin. (D) Cells were preincubated with α3 or α6-blocking antibodies in the presence or absence of β3-blocking antibody before seeding of cells onto immobilized asTF. Adhered cells were counted after 4 h. (E) Effect of PD98059, SB203580 or LY294002 on asTF-induced capillary formation. (F) asTF-dependent phosphorylation of MAPK, p38, and Akt in capillary-forming endothelial cells. Cells were left to form capillaries for the times indicated on matrigel in the presence/absence of 100 nM asTF.

Matrigel primarily consists of collagen and laminin, thus α3 and α6 may also participate in the attachment of cells to matrigel. To verify whether the inhibition of capillary formation by α3 and α6 blockade was due to diminished adhesion to asTF, we tested α3 and α6 blockade on cell adhesion to asTF. α6, but not α3, blockade partially inhibited cell binding to asTF (Fig. 5D). Cell adhesion was reduced to basal levels upon inclusion of a β3 blocking antibody, supporting the conclusion that endothelial cells employ different integrin heterodimers, namely α6β1 and αvβ3, but only require α6β1 integrin for asTF-induced capillary formation. Preincubation of ECRF cells with SB203580, LY294002 and PD98059 showed that asTF-induced capillary formation was dependent on p42/p44 MAP kinase and PI3 kinase action, but not the promigratory p38 MAP kinase cascade (Fig. 5E), demonstrating that distinct integrin-dependent pathways are involved in asTF-dependent migration and capillary formation. In support of that, cells allowed to form capillaries on matrigel supplemented with asTF showed higher levels of p42/p44 and Akt, but not p38 phosphorylation.

Similar to what was observed in aortic sprouting experiments, VIIai, hirudin, and polymyxin B did not influence capillary formation (Fig. 5 C and E), demonstrating that asTF-dependent capillary formation was not dependent on coagulation activation and/or LPS contamination.

asTF-Enhanced Aortic Sprouting Is Dependent on Integrins.

We next assessed the relative contributions of β1 and β3 integrins to asTF-dependent sprout formation in matrigel, using integrin blocking antibodies. Blockade of β1 inhibited basal sprouting and asTF-induced sprouting to similar levels (Fig. 6A), suggesting that both matrigel and asTF ligation of integrins was inhibited. Blockade of β3 did not affect basal sprouting, but reduced asTF-dependent sprouting to basal levels. This is expected because matrigel contains relatively little β3-ligating matrix proteins. 6B4, which inhibits asTF-integrin interaction, similarly reduced asTF-dependent aortic sprouting (Fig. 6B).

Fig. 6.

asTF induces integrin-dependent angiogenesis in vivo and ex vivo. (A) Wild-type aortic segments were implanted in solvent control-containing matrigel or asTF-containing matrigel. Murine β1 or β3-blocking antibodies were added to the matrigel. The number of sprouts was determined as described. (B) Wild-type aortic segments were implanted in solvent control-containing matrigel or matrigel containing 6B4-preincubated (100 μg/mL) asTF. (C) Wild-type aortic segments in fibrin supplemented with various concentrations of asTF or sTF. (D) Wild-type aortic segments in fibrin ± 100 nM asTF, in the presence/absence of 50 μg/mL integrin-blocking antibodies or 100 μg/mL 6B4. (E) In vivo matrigel plug assay; matrigel was supplemented with control buffer VEGF, asTF or sTF. asTF and sTF were also preincubated with 100 μg/mL 6B4 before addition to the matrigel. Vessels were counted as described above.

asTF is found in thrombi and could therefore potentiate angiogenesis in a fibrin matrix. asTF dose-dependently upregulated sprouting in a fibrin milieu, similarly to that observed in matrigel (Fig. 6C); sTF also enhanced sprouting, but at much higher concentrations. Inclusion of 6B4 and blockade of β1 and β3 integrins led to inhibition of asTF-dependent sprouting (Fig. 6D). In contrast to what was observed in matrigel, β3 but not β1 blockade inhibited sprouting to subbasal levels, reflecting the requirement of β3 integrin activation by fibrin. The asTF/integrin-blocking antibody 6B4 reduced vessel formation in vivo, and although sTF modestly induced vessel formation, 6B4 did not have an effect (Fig. 6E). Thus, integrins are instrumental in asTF-dependent sprouting in matrigel and fibrin and in vivo angiogenesis.

Intratumoral asTF Concentrations.

To test whether asTF concentrations that induce angiogenesis are pathologically relevant, we assessed intratumoral asTF concentrations in a set of cervical tumors. We chose cervical cancer since high levels of asTF mRNA were observed in tumors from seven patients (Fig. 7A). Ten tumor specimens were homogenized and analyzed for asTF expression by Western blot using a specific anti-asTF antibody. Bands were quantified using an asTF concentration curve. All but one specimen revealed asTF concentrations that fell within the range that induces angiogenesis (Fig. 7B), indicating that the observed asTF-induced angiogenesis is pathobiologically relevant.

Fig. 7.

Cervical tumors express high levels of asTF. (A) mRNA obtained from seven cervival tumors was subjected to RT-PCR using asTF- and β-actin-specific primers. (B) Ten cervical tumor specimens were homogenized and lysed in sample buffer. Lysates were analyzed for asTF expression on Western blot, and asTF bands were quantified using an asTF concentration curve.

Discussion

Here, we show that asTF induces angiogenesis in a matrigel plug assay and aortic sprouting in matrigel and fibrin. asTF induced angiogenesis in a VIIa/PAR-2-independent manner but acted through ligation of β1 and β3 integrins, at concentrations as low as 1 nM. asTF-induced endothelial cell migration and capillary formation was dependent on αvβ3 and α6β1, respectively. Our conclusion that asTF binds integrins is based on the following observations: (i) endothelial cell adhesion to asTF as well as asTF-induced capillary formation and aortic sprouting were potently inhibited by integrin blocking antibodies, (ii) inclusion of asTF in the medium blocked formation of capillaries, most likely by blocking integrins, and (iii) an antibody that inhibits TF-integrin interaction similarly inhibited sprouting and in vivo angiogenesis. Only the combination of β1 and β3 integrin blocking antibodies completely abrogated asTF-dependent cell adhesion, thus both β1 and β3 integrin appear to physically bind to asTF. This distinguishes asTF from matrigel and fibrin which predominantly activate β1 and β3 integrins, respectively. Although the importance of other integrin ligands, for example fibronectin and vitronectin, cannot formally be excluded, they are not the constituents of the tumor extracellular matrix (matrigel) or the provisional angiogenic fibrin matrix, and therefore play no role in our model. Endothelial cell migration was dependent on αvβ3 integrin, p38 MAP kinase, and PI3-kinase, whereas capillary formation was dependent on α6β1, p42/p44 MAP kinase, and PI3-kinase. It is noteworthy that tip cell migration and capillary formation represent different stages of angiogenesis and involves different integrins (24). Concordantly, αvβ3 ligation results in directional migration, whereas β1 ligation induces non-directional migration that is likely to be more important for capillary formation (25). In wound healing, αvβ3 is focally expressed at the tips of invading capillary sprouts whereas β1 integrins are broadly expressed on the entire capillary (26). It appears that asTF-driven migration through αvβ3 integrin mimics endothelial tip cell migration, whereas β1-dependent capillary formation through α6β1 recapitulates the formation of vessels from non-tip cell endothelial cells through migration and differentiation. In aortic sprouting assays, which combine these features, asTF-induced angiogenesis was indeed sensitive to both β1 and β3 blockade.

flTF on tumor cells has also been shown to induce angiogenesis, but in a mechanistically different manner. Like asTF, flTF interacts with integrins, but rather than serving as receptors to TF, integrin α3β1, and α6β1 control flTF-VIIa signaling via PAR-2 (10). The fact that flTF is membrane bound and binds integrins expressed on the same cell surface, whereas asTF is a soluble protein capable of being deposited into the extracellular matrix; that is, at sites not accessible to flTF, suggests flTF and asTF may bind integrins in distinct orientations, thus determining specificity of integrins for these two TF forms. Our results, showing a lack of asTF-α3β1 integrin interaction, support this concept.

Another important finding is that asTF is a much more potent inducer of angiogenesis that a truncated form of flTF (sTF), probably because asTF is more pro-migratory than sTF, rendering extracellular asTF the principal candidate to engage in integrin-dependent angiogenesis, especially because sTF is not known to occur naturally in vivo. The underlying basis may be the differential integrin activation by secreted asTF and membrane bound flTF. Indeed, asTF differs from flTF such that asTF displays a decreased affinity for VII.

In our experiments, asTF induced angiogenesis at concentrations as low as 1 nM. Interestingly, in a panel of 10 cervical tumors, we found that nine out of 10 tumors contained asTF at levels in the range within which asTF is pro-angiogenic, rendering asTF a potential key player in tumor angiogenesis. In agreement, asTF is expressed in pancreatic cell lines (18), squamous cell carcinoma of the lung (27), and advanced stages of non-small cell lung cancer (19); asTF appears to drive tumor growth in xenograft models (20). It remains controversial whether asTF always displays coagulant activity and can be effectively secreted (16). Regardless of whether asTF is procoagulant in vivo, we did not observe extensive thrombus formation in our in vivo experiments using either asTF or sTF, ruling out that these proteins induce thrombus formation in newly formed vessels. Szotowski et al. (17) showed that asTF can be secreted after appropriate stimulation and asTF is found in normal human plasma, making up approximately 30% of the total bloodborne TF pool (15). Although the cue to asTF secretion in tumors remains to be identified, hypoxia and/or tumor-expressed cytokines may contribute to this phenomenon.

Materials and Methods

Materials, Cell Lines. Protein Expression, and Survival Assay.

Materials, cell lines, protein expression and survival assays are described in detail in the SI Text.

Endothelial Cell Adhesion Assays.

Ninety-six-well plates were coated with 50 μg/mL asTF or 10% BSA as negative control. Uncoated areas were blocked with 10% BSA. Cells (20,000) were seeded per well and plates were incubated at 37 °C for 4 h. In some experiments, cells were preincubated with 50 μg/mL integrin blocking antibodies. Photographs were taken and cells adopting a flattened morphology were counted. Data are shown as mean ± SD, n = 4.

Cell Migration Assays.

Cell migration was assessed using a transwell assay. Polycarbonate membrane transwell inserts (8.0 μm) (Corning Costar, Corning Life Sciences, Corning) were coated with 1% gelatin or asTF (50 μg/mL). Cells (25,000) were seeded per insert after a 20-min pretreatment with integrin blocking mAb's, when appropriate. Cells were allowed to migrate for 5 h at 37 °C; migrated cells were fixed, stained with crystal violet for 10 min, and counted per field of 40× magnification.

In Vitro Capillary Formation Assay.

Ninety-six-well plates were coated with 50 μL matrigel, supplemented with asTF according to the experimental conditions. Endothelial cells (20,000) were seeded after a 20-min pretreatment with blocking antibodies when appropriate, allowed to form capillaries for 18 h (ECRF) or 6 h (HUVECs), and the lengths of tubular networks was measured.

Mouse Aortic Ring Assay.

Mouse thoracic aortas were isolated and cleaned of the surrounding tissue in serum-free RPMI (Invitrogen) containing 50 μg/mL penicillin and 50 μg/mL streptomycin. Dissected aortas were flushed, cut into equal segments, embedded in matrigel, and covered with EBM containing 2% serum and penicillin/streptomycin. Sprouts were counted on day 5. Aortas were also embedded in fibrin that was prepared by mixing 2 mg/mL fibrinogen with 0.1 U/mL thrombin. Aortas were then overlayed with medium containing 5 U/mL hirudin.

Matrigel Plug Assay.

Eight-week-old C57Bl6 mice (n = 10 per group) were anesthetized with isoflurane and injected s.c. into the flank with 0.5 mL ice-cold matrigel. Matrigel was either supplemented with 100 nM asTF or sTF in the presence/absence of 100 μg/mL 6B4 and 50 ng/mL mouse recombinant VEGF or PBS. After 7 days, 150 μL FITC-Dextran (30 mg/mL) was injected into the tail vein. After 15 min the animals were killed, implants were extracted, fixed in 10% formalin, and analyzed on a Leica MZ16 FA stereomicroscope.

Statistics.

Data are mean +/- SD unless otherwise stated. Statistical analysis was performed using Student's t-test. *, P < 0.05; **, P < 0.01; ***, P < 0.001.