Tissue factor- and factor X-dependent activation of protease-activated receptor 2 by factor VIIa

Protease-activated receptor 2 (PAR2) is expressed by vascular endothelial cells and other cells in which its function and physiological activator(s) are unknown. Unlike PAR1, PAR3, and PAR4, PAR2 is not activatable by thrombin. Coagulation factors VIIa (FVIIa) and Xa (FXa) are proteases that act upstream of thrombin in the coagulation cascade and require cofactors to interact with their substrates. These proteases elicit cellular responses, but their receptor(s) have not been identified. We asked whether FVIIa and FXa might activate PARs if presented by their cofactors. Co-expression of tissue factor (TF), the cellular cofactor for FVIIa, together with PAR1, PAR2, PAR3, or PAR4 conferred TF-dependent FVIIa activation of PAR2 and, to lesser degree, PAR1. Responses to FXa were also observed but were independent of exogenous cofactor. The TF/FVIIa complex converts the inactive zymogen Factor X (FX) to FXa. Strikingly, when FX was present, low picomolar concentrations of FVIIa caused robust signaling in cells expressing TF and PAR2. Responses in keratinocytes and cytokine-treated endothelial cells suggested that PAR2 may be activated directly by TF/FVIIa and indirectly by TF/FVIIa-generated FXa at naturally occurring expression levels of TF and PAR2. These results suggest that PAR2, although not activatable by thrombin, may nonetheless function as a sensor for coagulation proteases and contribute to endothelial activation in the setting of injury and inflammation. More generally, these findings highlight the potential importance of cofactors in regulating PAR function and specificity.

Binding of factor VIIa to tissue factor on keratinocytes induces gene expression

Binding of the zymogen serine protease Factor VII (FVII) to its cellular cofactor tissue factor (TF) triggers blood coagulation. Several recent reports have suggested that the formation of this complex may serve additional functions. We have used cDNA arrays to study differential gene expression in response to the interaction of activated FVII (FVIIa) with TF on a human keratinocyte cell line. Of 931 mRNA species observed up to 6 h after FVIIa (10 nM) addition, 24 were significantly up-regulated in what may resemble a wound-type response. Responders included mRNA species coding for transcription regulators (c-fos, egr-1, ETR101, BTEB2, c-myc, fra-1, and tristetraproline), growth factors (amphiregulin, hbEGF, CTGF, and FGF-5), proinflammatory cytokines (IL-1beta, IL-8, LIF, and MIP2alpha), proteins involved in cellular reorganization/migration (RhoE, uPAR, and collagenases 1 and 3), and others (PAI-2, cyclophilin, GADD45, Jagged1, and prostaglandin E(2) receptor). The transcriptional response to FVIIa was abrogated by antibodies to TF and left unaffected by hirudin. The pattern of genes induced suggests that the FVIIa.TF complex may play an active role in early wound repair as well as hemostasis. The former is a novel function ascribed to the complex that may also be contributing to the pathophysiology of unwarranted TF expression.

Coagulation factors VIIa and Xa induce cell signaling leading to up-regulation of the egr-1 gene

Intracellular signaling induced by the coagulation factors (F) VIIa and Xa is poorly understood. We report here studies on these processes in a human keratinocyte line (HaCaT), which is a constitutive producer of tissue factor (TF) and responds to both FVIIa and FXa with elevation of cytosolic Ca(2+), phosphorylation of extracellular signal-regulated kinase (Erk) 1/2, p38(MAPK), and c-Jun N-terminal kinase, and up-regulation of transcription of the early growth response gene-1 (egr-1). Using egr-1 as end point, we observed with both agonists that phosphatidylinositol-specific phospholipase C and the mitogen-activated protein kinase/Erk kinase/Erk pathway were mediators of the responses. The responses to FVIIa were TF-dependent and up-regulation of egr-1 mRNA did not require presence of the TF cytoplasmic domain. Antibodies to EPR-1 and factor V had no effect on the response to FXa. We have provided evidence that TF is not the sole component of the FVIIa receptor. The requirement for proteolytic activity of both FVIIa and FXa suggests that protease-activated receptors may be involved. We now report evidence suggesting that protease-activated receptor 2 or a close homologue may be a necessary but not sufficient component of this particular signal transduction pathway. The up-regulation of egr-1 describes one way by which the initiation of blood coagulation may influence gene transcription. The ability of these coagulation proteases to induce intracellular signals at concentrations at or below the plasma concentrations of their zymogen precursors suggests that these processes may occur also in vivo.

Synchronized Ca2+ oscillations induced in Madin Darby canine kidney cells by bradykinin and thrombin but not by ATP

In an earlier report, we described synchronous Ca2+ oscillations in globally stimulated, subconfluent MDCK cells [Røttingen J-A, Enden T., Camerer E., Iversen J-G., Prydz H. Binding of human factor VIIa to tissue factor induces cytosolic Ca2+ signals in J82 cells, transfected COS-1 cells, Madin-Darby canine kidney cells and in human endothelial cells induced to synthesize tissue factor. J Biol Chem 1995; 270: 4650-4660]. In order to elucidate the mechanisms behind these oscillations, we have analyzed the fluctuations in cytosolic Ca2+ in single, Fura-2 loaded, MDCK cells grown to subconfluence, after stimulation with bradykinin, thrombin and ATP. All three agonists gave rise to an initial Ca2+ spike followed by oscillations or transients. Both the initial and subsequent spikes appeared to be due mainly to release of Ca2+ from internal stores, since they remained after Ca2+ influx was impeded by either La3+ or by chelation of extracellular Ca2+ with EGTA. The secondary spikes were apparently synchronized when the cells were (permanently and globally) stimulated with bradykinin or thrombin, but each cell seemed to oscillate independently when stimulated in the same way with ATP. Synchronized secondary spikes arose with a constant frequency and amplitude, independent of agonist concentration in contrast to most Ca2+ oscillations observed. Pretreatment of the cells with octanol to block gap junctions, or with EGTA or La3+ to inhibit Ca2+ influx, abolished the synchronization induced by bradykinin or thrombin. We observed that in the MDCK cell layer there are some "pacemaker' cells and hypothesize that these have a higher sensitivity for the agonists than their neighboring cells. From these pacemakers, an intercellular Ca2+ wave can be seen to spread to adjacent cells in the presence of intact gap junctions, thereby initiating concurrent transients in all cells. The Ca2+ wave is amplified by release from internal stores, probably owing to the bell-shaped Ca2+ activation curve of the IP3 receptor and by subsequent Ca2+ influx through Ca2+ release activated channels.

Coagulation factors VII and X induce Ca2+ oscillations in Madin-Darby canine kidney cells only when proteolytically active

We have recently reported that the activated serine protease and blood coagulation Factor VII (FVIIa) can induce Ca2+ oscillations in Madin-Darby canine kidney cells. We now demonstrate a similar response by Madin-Darby canine kidney cells to the active coagulation Factor X (FXa), which is also a serine protease and a substrate of the tissue factor (TF).FVIIa complex in the initiation of the coagulation cascade. The phosphatidyl inositol-specific phospholipase C inhibitor U73122 inhibited the signals elicited by both FVIIa and FXa. Lack of sensibility to the tyrosine kinase inhibitors herbimycin A, genistein, and the tyrphostin AG18 and discordance between TF expression and FVIIa responsiveness argued against TF acting as a cytokine-like receptor, with tyrosine kinase-mediated activation by FVIIa. As demonstrated using the protease inhibitor benzamidine and by specific active site inhibition with 1,5-dansyl-Glu-Gly-Arg chloromethyl ketone, both FVIIa and FXa lost their ability to elicit a calcium response when devoid of their proteolytic activity. Consistent with this, the native (zymogen) form of Factor X did not induce Ca2+ transients. Homologous but not heterologous inhibition of FVIIa- and FXa-evoked Ca2+ signals by 1,5-dansyl-Glu-Gly-Arg chloromethyl ketone-inactivated FVIIa and FXa suggested that each factor had its own specific cell surface anchoring receptor. The two coagulation factors did not show homologous desensitization as seen for thrombin stimulation. Studies with hirudin excluded involvement of the established activation pathway through thrombin itself. Lack of desensitization of the response to FVIIa or FXa by thrombin ruled out any involvement of proteinase activated receptor-1 (PAR-1), the thrombin receptor. We speculate that FXa and FVIIa may work via a receptor (possibly common) analogous to PAR-1 or its functional homologue PAR-2. Although TF is essential for the FVIIa-induced signaling event, its role in the phosphatidyl inositol-specific phospholipase C-mediated Ca2+ signal may be in anchoring FVIIa to the cell surface rather than in transmembrane signal mediation.

Opposite sorting of tissue factor in human umbilical vein endothelial cells and Madin-Darby canine kidney epithelial cells

Tissue factor (TF) is a 48-kD transmembrane glycoprotein that triggers the extrinsic pathway of blood coagulation by interacting with the plasma coagulation factor VII (FVII). TF is also a true receptor in that a cellular signal is generated when activated FVII (FVIIa) binds to TF. For both of these functions, the cellular surface distribution of TF is important, since FVII is primarily available on the apical side of vascular endothelial cells and on the basolateral side of epithelial cells lining the internal and external surfaces. We show that in endothelial cells, TF (both antigen and procoagulant activity) is sorted to the apical surface, whereas in wild-type and stably transfected Madin-Darby canine kidney epithelial cells (MDCK), which form tight junctions and express TF constitutively, TF antigen is on the basolateral surface. No significant clotting activity is detectable on this surface. Truncated TF (cytoplasmic tail residues 246 to 263 deleted) is sorted as wild-type in MDCK cells.

Notes on the cell biology of tissue factor

Tissue factor is both a trigger of blood coagulation and a true receptor inducing an intracellular signal upon binding of its ligand factor VII to its extracellular parts. When induced in endothelial cells more than 75% of tissue factor antigen was found on the apical side of the cell.

Binding of human factor VIIa to tissue factor induces cytosolic Ca2+ signals in J82 cells, transfected COS-1 cells, Madin-Darby canine kidney cells and in human endothelial cells induced to synthesize tissue factor

Tissue factor (TF) is the most potent trigger of blood clotting known. It activates factor VII (FVII) thereby initiating a cascade of proteolytic reactions resulting in thrombin production. The cloning of TF revealed its structural characteristics to be those of a receptor related to the class 2 cytokine receptor superfamily, but until now no intracellular signal has been discovered related to binding of the ligand (FVIIa) to the putative receptor. We have studied possible intracellular signaling effects of the FVIIa-TF interaction by measuring cytosolic free Ca2+ in single fura-2-loaded cells and found that 200 nM FVIIa caused Ca2+ transients in about 30% of human umbilical vein endothelial cells treated with interleukin-1 beta to express TF, compared to below 5% in uninduced cells. A gradual increase of the basal Ca2+ level was also caused by binding of FVIIa. In the human bladder carcinoma cell line J82, which has a high constitutive TF activity, similar results were found. An antibody neutralizing TF activity decreased the response rate to control levels. COS-1 cells which do not make TF did not respond to FVIIa as opposed to COS-1 cells expressing TF after transfection with a human TF cDNA construct. The canine kidney cell line MDCK, a constitutive TF producer, responded especially well; up to 100% of the cells examined showed Ca2+ oscillations which were dose dependent with regard to frequency, latency, maximal amplitude, and recruitment of responding cells. The frequency was reduced by inhibition of Ca2+ influx with 100 microM LaCl3. In confluent MDCK cells the Ca2+ oscillations were synchronous, constituting the first evidence of a synchronous cytosolic Ca2+ oscillator generated by global application of agonist. Thus, TF mediates a cytosolic Ca2+ signal upon interaction with its ligand FVIIa, thereby suggesting a more complex biological role for TF.