Thrombin and its receptor in growth control

The activation of proteinase-activated receptor-1 (PAR1) promotes gastric cancer cell alteration of cellular morphology related to cell motility and invasion

Cell motility proceeds by cycles of edge protrusion, adhesion and retraction. Whether these functions are coordinated by biochemical or biomechanical processes is unknown. Tumor invasion and metastasis is directly related to cell motility. We showed that stimulation of proteinase-activated receptor-1 (PAR1) can trigger an array of responses that would promote tumor cell growth and invasion. Thus, we examined aspects of PAR1 activation related to cell morphological change that might contribute to cell motility. We established a PAR1 stably transfected MKN45 gastric cancer cell line (MKN45/PAR1). We examined morphological changes, Rho family activation and overexpression of cytoskeletal protein in cells exposed to PAR1 agonists (α-thrombin and TFLLR-NH2). MKN45/PAR1 grows with an elongated and polarized morphology, extending pseudopodia at the leading edge. However, in the presence of PAR1 antagonist, MKN45/PAR1 did not show any changes in cell shape upon addition of either α-thrombin or TFLLR-NH2. Activated PAR1 induced RhoA and Rac1 phosphorylation, and subsequent overexpression of myosin IIA and filamin B which are stress fiber components that were identified by PMF analysis of peptide mass data obtained by MALDI-TOF/MS measurement. Upon stimulation of MKN45/PAR1 for 24 h with either α-thrombin or TFLLR-NH2, the distribution of both myosin IIA and filamin B proteins shifted to being distributed throughout the cytoplasm to the membrane, with more intense luminescence signals than in the absence of stimulation. These results demonstrate that PAR1 activation induces cell morphological change associated with cell motility via Rho family activation and cytoskeletal protein overexpression, and has a critical role in gastric cancer cell invasion and metastasis.

A novel protease-activated receptor-1 interactor, Bicaudal D1, regulates G protein signaling and internalization

Protease-activated receptor-1 (PAR1) is a G protein-coupled receptor that plays critical roles in cancer, angiogenesis, inflammation, and thrombosis. Proteolytic cleavage of the extracellular domain of PAR1 generates a tethered ligand that activates PAR1 in an unusual intramolecular mode. The signal emanating from the irreversibly cleaved PAR1 is terminated by G protein uncoupling and internalization; however, the mechanisms of PAR1 signal shut off still remain unclear. Using a yeast two-hybrid screen, we identified Bicaudal D1 (BicD1) as a direct interactor with the C-terminal cytoplasmic domain of PAR1. BICD was originally identified as an essential developmental gene associated with mRNA and Golgi-endoplasmic reticulum transport. We discovered a novel function of BicD1 in the modulation of G protein signaling, cell proliferation, and endocytosis downstream of PAR1. BicD1 and its C-terminal CC3 domain inhibited PAR1 signaling to G(q)-phospholipase C-beta through coiled-coil interactions with the cytoplasmic 8th helix of PAR1. Unexpectedly, BicD1 was also found to be a potent suppressor of PAR1-driven proliferation of breast carcinoma cells. The growth-suppressing effects of BicD1 required the ability to interact with the 8th helix of PAR1. Silencing of BicD1 expression impaired endocytosis of PAR1, and BicD1 co-localized with PAR1 and tubulin, implicating BicD1 as an important adapter protein involved in the transport of PAR1 from the plasma membrane to endosomal vesicles. Together, these findings provide a link between PAR1 signal termination and internalization through the non-G protein effector, BicD1.

Commutators of PAR-1 signaling in cancer cell invasion reveal an essential role of the Rho-Rho kinase axis and tumor microenvironment

We recently reported that proteinase-activated receptors type I (PAR-1) are coupled to both negative and positive invasion pathways in colonic and kidney cancer cells cultured on collagen type I gels. Here, we found that treatments with the cell-permeant analog 8-Br-cGMP and the soluble guanylate cyclase activator BAY41-2272, and Rho kinase (ROK) inhibition by Y27632 or a dominant negative form of ROK lead to PAR-1-mediated invasion through differential Rac1 and Cdc42 signaling. Hypoxia or the counteradhesive matricellular protein SPARC/BM-40 (SPARC: secreted protein acidic rich in cysteine) overexpressed during cancer progression also commutated PAR-1 to cellular invasion through the cGMP/protein kinase G (PKG) cascade, RhoA inactivation, and Rac1-dependent or -independent signaling. Cultured primary cancer cells isolated from peritoneal and pleural effusions from patients with colon cancer or other malignant tumors harbored PAR-1, as shown by RT-PCR and FACS analyses. These malignant effusions also contained high levels of activated thrombin and fibrin, and induced a proinvasive response in HCT8/S11 human colorectal cancer cells. Our data underline the essential role of the tumor microenvironment and of several commutators targeting cGMP/PKG signaling and the RhoA-ROK axis in the control of PAR-1 proinvasive activity and metastatic potential of cancer cells in distant organs and peritoneal or pleural cavities. We also add new insights into the mechanisms linking the coagulation mediators thrombin and PAR-1 in the context of blood coagulation disorders and venous thrombosis often observed in cancer patients, as described in 1865 by Armand Trousseau.

Protease-activated receptor-1-induced calcium signaling in gingival fibroblasts is mediated by sarcoplasmic reticulum calcium release and extracellular calcium influx

Thrombin is a serine protease activated during injury and inflammation. Thrombin and other proteases generated by periodontal pathogens affect the behavior of periodontal cells via activation of protease-activated receptors (PARs). We noted that thrombin and PAR-1 agonist peptide stimulated intracellular calcium levels ([Ca2+]i) of gingival fibroblasts (GF). This increase of [Ca2+]i was inhibited by EGTA and verapamil. U73122 and neomycin inhibited thrombin- and PAR-1-induced [Ca2+]i. Furthermore, 2-APB (75-100 microM, inositol triphosphate [IP3] receptor antagonist), thapsigargin (1 microM), SKF-96365 (200 microM) and W7 (50 and 100 microM) also suppressed the PAR-1- and thrombin-induced [Ca2+]i. However, H7 (100, 200 microM) and ryanodine showed little effects. Blocking Ca2+ efflux from mitochondria by CGP37157 (50, 100 microM) inhibited both thrombin- and PAR-1-induced [Ca2+]i. Thrombin induced the IP3 production of GF within 30-seconds of exposure, which was inhibited by U73122. These results indicate that mitochondrial calcium efflux and calcium-calmodulin pathways are related to thrombin and PAR-1 induced [Ca2+]i in GF. Thrombin-induced [Ca2+]i of GF is mainly due to PAR-1 activation, extracellular calcium influx via L-type calcium channel, PLC activation, then IP3 binding to IP3 receptor in sarcoplasmic reticulum, which leads to intracellular calcium release and subsequently alters cell membrane capacitative calcium entry.

Thrombin protease-activated receptor-1 signals through Gq- and G13-initiated MAPK cascades regulating c-Jun expression to induce cell transformation

Although the ability of G protein-coupled receptors to stimulate normal and aberrant cell growth has been intensely investigated, the precise nature of the molecular mechanisms underlying their transforming potential are still not fully understood. In this study, we have taken advantage of the potent mitogenic effect of thrombin and the focus-forming activity of one of its receptors, protease-activated receptor-1, to dissect how this receptor coupled to Galphai, Galphaq/11, and Galpha12/13 transduces signals from the membrane to the nucleus to initiate transcriptional events involved in cell transformation. Using endogenous and transfected thrombin receptors in NIH 3T3 cells, ectopic expression of muscarinic receptors coupled to Galphaq and Galphai, and chimeric G protein alpha subunits and murine fibroblasts deficient in Galphaq/11, and Galpha12/13, we show here that, although coupling to Galphai is sufficient to induce ERK activation, the ability to couple to Galphaq and/or Galpha13 is necessary to induce c-jun expression and cell transformation. Furthermore, we show that Galphaq and Galpha13 can initiate the activation of MAPK cascades, including JNK, p38, and ERK5, which in turn regulate the activity of transcription factors controlling expression from the c-jun promoter. We also present evidence that c-Jun and the kinases regulating its expression are integral components of the transforming pathway initiated by protease-activated receptor-1.

Thrombin regulates insulin-like growth factor-1 receptor transcription in vascular smooth muscle: characterization of the signaling pathway

We have previously demonstrated that thrombin upregulation of insulin-like growth factor-1 receptor (IGF-1R) is essential for thrombin-induced mitogenic signaling. To characterize the mechanisms involved, we studied transcription of the IGF-1R gene in rat aortic smooth muscle cells. Thrombin markedly increased IGF-1R mRNA levels, peaking at 3 hours (112+/-7% above control). This effect was mimicked by the hexapeptide SFFLRN (that functions as a tethered ligand) and was blocked by the thrombin inhibitor hirudin. Nuclear run-on assays indicated that thrombin stimulated IGF-1R gene transcription by 2.1-fold, and this was confirmed with the use of actinomycin D. Thrombin-mediated upregulation of IGF-1R mRNA and protein levels was protein kinase C independent but was completely inhibited by the protein tyrosine kinase inhibitor genistein and by the antioxidants N-acetyl-L-cysteine and pyrrolidinedithiocarbamate, suggesting the involvement of reactive oxygen species. The thrombin-induced increase in IGF-1R mRNA was inhibitable by diphenyleneiodonium chloride but not by other inhibitors of cellular oxidase systems, suggesting that NAD(P)H oxidase was necessary for the increase. Furthermore, inhibitors of the epidermal growth factor receptor kinase, Janus kinase-2 kinase, and Src kinase did not block the effect. Thus, thrombin transcriptionally regulates the IGF-1R gene via a redox-sensitive protein tyrosine kinase-dependent pathway that does not require protein kinase C activation. In view of our prior data indicating that IGF-1R density is a critical determinant of vascular smooth muscle cell growth, our findings have particular relevance to understanding mechanisms whereby growth factors such as thrombin regulate vascular proliferation in vivo.

Thrombomodulin prolongs thrombin-induced extracellular signal-regulated kinase phosphorylation and nuclear retention in endothelial cells

On endothelial cells, thrombin binds to thrombomodulin (TM), an integral membrane-bound glycoprotein, and to protease-activated receptors (PARs). Thrombin binding to TM modulates endothelial cell and smooth muscle cell proliferation mediated through PAR1. We studied the phosphorylation and nuclear translocation of extracellular signal-regulated kinases (ERKs) 1 and 2 in human umbilical vein endothelial cells activated by thrombin. Thrombin and thrombin receptor-activating peptide (TRAP)-induced DNA synthesis were significantly inhibited by PD98059, an inhibitor of ERK phosphorylation. Immunoblots of phosphorylated ERKs (pERKs) and immunocytochemical studies of pERK localization revealed differences in the signal generated by thrombin and TRAP. After a short activation (15 minutes), the phosphorylation and the intracellular localization of pERKs were the same with the 2 agonists. After 4 hours, however, pERKs were visualized in the nuclei of thrombin-activated cells but barely detectable in TRAP-activated cells. Moreover, after 4 hours, the pERKs were visualized in the nuclei of cells stimulated by TRAP in the presence of a thrombin mutant that bound to TM, whereas they were around the nuclei in cells stimulated by thrombin in the presence of a monoclonal antibody preventing thrombin binding to TM. The results demonstrate that ERKs are involved in human umbilical vein endothelial cell DNA synthesis mediated by PAR agonists, that the duration of pERK nuclear retention is in inverse ratio to the mitogenic response, and that in addition to its role in the regulation of blood coagulation, TM acts as a thrombin receptor that modulates the duration of pERK nuclear retention and cell proliferation in response to thrombin.

Novel nuclear target for thrombin: activation of the Elk1 transcription factor leads to chemokine gene expression

Thrombin is primarily known for its role in homeostasis and thrombosis. However, this enzyme also plays important roles in wound healing and pathologic situations such as inflammation and tumorigenesis. Among the molecules stimulated by thrombin in these latter processes are the stress response proteins, chemokines. Chemokines are also known for their roles in inflammatory responses and tumor development. These correlative observations strongly suggest that chemokines may be mediators of some of thrombin's functions in these processes. Elucidation of the molecular mechanisms of stimulation of chemokines by thrombin may help to unravel the ways in which their expression can be modulated. Up-regulation of the chemokine 9E3/cCAF by thrombin occurs via its proteolytically activated receptor with subsequent transactivation of the epidermal growth factor receptor tyrosine kinase. This study shows that stimulation by thrombin very rapidly activates this chemokine at the transcriptional level, that 2 Elk1 binding elements located between −534 and −483 bp of the promoter are major thrombin response elements, that activation occurs via the Elk1 transcription factor, and that the latter is directly activated by MEK1/ERK2. The common occurrence of Elk1 binding domains in the promoters of immediate early response genes suggests that it may be characteristically involved in gene activation by stress-inducing agents.

Novel nuclear target for thrombin: activation of the Elk1 transcription factor leads to chemokine gene expression

Thrombin is primarily known for its role in homeostasis and thrombosis. However, this enzyme also plays important roles in wound healing and pathologic situations such as inflammation and tumorigenesis. Among the molecules stimulated by thrombin in these latter processes are the stress response proteins, chemokines. Chemokines are also known for their roles in inflammatory responses and tumor development. These correlative observations strongly suggest that chemokines may be mediators of some of thrombin's functions in these processes. Elucidation of the molecular mechanisms of stimulation of chemokines by thrombin may help to unravel the ways in which their expression can be modulated. Up-regulation of the chemokine 9E3/cCAF by thrombin occurs via its proteolytically activated receptor with subsequent transactivation of the epidermal growth factor receptor tyrosine kinase. This study shows that stimulation by thrombin very rapidly activates this chemokine at the transcriptional level, that 2 Elk1 binding elements located between −534 and −483 bp of the promoter are major thrombin response elements, that activation occurs via the Elk1 transcription factor, and that the latter is directly activated by MEK1/ERK2. The common occurrence of Elk1 binding domains in the promoters of immediate early response genes suggests that it may be characteristically involved in gene activation by stress-inducing agents.

Regulation of the actin cytoskeleton by thrombin in human endothelial cells: role of Rho proteins in endothelial barrier function

Endothelial barrier function is regulated at the cellular level by cytoskeletal-dependent anchoring and retracting forces. In the present study we have examined the signal transduction pathways underlying agonist-stimulated reorganization of the actin cytoskeleton in human umbilical vein endothelial cells. Receptor activation by thrombin, or the thrombin receptor (proteinase-activated receptor 1) agonist peptide, leads to an early increase in stress fiber formation followed by cortical actin accumulation and cell rounding. Selective inhibition of thrombin-stimulated signaling systems, including Gi/o (pertussis toxin sensitive), p42/p44, and p38 MAP kinase cascades, Src family kinases, PI-3 kinase, or S6 kinase pathways had no effect on the thrombin response. In contrast, staurosporine and KT5926, an inhibitor of myosin light chain kinase, effectively blocked thrombin-induced cell rounding and retraction. The contribution of Rho to these effects was analyzed by using bacterial toxins that either activate or inhibit the GTPase. Escherichia coli cytotoxic necrotizing factor 1, an activator of Rho, induced the appearance of dense actin cables across cells without perturbing monolayer integrity. Accordingly, lysophosphatidic acid, an activator of Rho-dependent stress fiber formation in fibroblasts, led to reorganization of polymerized actin into stress fibers but failed to induce cell rounding. Inhibition of Rho with Clostridium botulinum exoenzyme C3 fused to the B fragment of diphtheria toxin caused loss of stress fibers with only partial attenuation of thrombin-induced cell rounding. The implication of Rac and Cdc42 was analyzed in transient transfection experiments using either constitutively active (V12) or dominant-interfering (N17) mutants. Expression of RacV12 mimicked the effect of thrombin on cell rounding, and RacN17 blocked the response to thrombin, whereas Cdc42 mutants were without effect. These observations suggest that Rho is involved in the maintenance of endothelial barrier function and Rac participates in cytoskeletal remodeling by thrombin in human umbilical vein endothelial cells.

Stimulation of cell proliferation by calcium and a calcimimetic compound

Some mesenchymal cells respond to stimulation by specific cations with increased cell proliferation. In the present study we have investigated whether the parathyroid/kidney/brain calcium-sensing receptor (PCaR) can mediate such mitogenic responses. We have expressed the recombinant rat PCaR in CCL39 hamster fibroblasts, which do not express a detectable endogenous cation sensor. The transfected cells responded to increased extracellular calcium concentrations ([Ca2+]e) with strong inositol phosphate (IP) formation, which was insensitive to pertussis toxin treatment of cells. We could not detect negative coupling of the receptor to adenylyl cyclase. The calcimimetic NPS R-568 left-shifted the concentration-response curve for [Ca2+]e-induced IP formation and increased the maximal response. In [3H]thymidine incorporation experiments, increasing [Ca2+]e from 1 to 4 mM was found to stimulate DNA synthesis weakly, but significantly. A strong potentiation of this response was observed in the presence of NPS R-568. [Ca2+]e and NPS R-568 also synergized to increase cell numbers in cultures maintained in defined medium. In contrast to our expectations, no significant stimulation of IP formation or cell proliferation could be observed after stimulation of cells with the reported PCaR agonist gadolinium (Gd3+) or with aluminum (Al3+), which stimulates osteoblast proliferation. Gd3+ actually inhibited IP formation stimulated by increased [Ca2+]e as well as by thrombin and AlF4-, indicating toxicity. However, submaximal receptor stimulation by Gd3+ was evident when intracellular calcium transients were measured in fluo-3-loaded cells. Our data show that PCaR can stimulate cell proliferation when expressed in an appropriate cellular context. However, it is unlikely that PCaR mediates the strong mitogenic effects elicited by the cations Gd3+ and Al3+ observed in osteoblasts.

Tyrosine phosphorylation of the novel protein-tyrosine kinase RAFTK during an early phase of platelet activation by an integrin glycoprotein IIb-IIIa-independent mechanism

A key regulatory event controlling platelet activation is mediated through the phosphorylation of several cellular proteins by protein-tyrosine kinases. The related adhesion focal tyrosine kinase (RAFTK) is a novel cytoplasmic tyrosine kinase and a member of the focal adhesion kinase (FAK) gene family. FAK phosphorylation in platelets is integrin-dependent, occurs in a late stage of platelet activation, and is dependent on platelet aggregation. In this study, we have investigated the involvement of RAFTK phosphorylation during different stages of platelet activation. Treatment of platelets with thrombin induced, in as early as 10 s, a rapid tyrosine phosphorylation of RAFTK in a time- and concentration-dependent manner. Treatment of platelets with thrombin in the absence of stirring or pretreatment of platelets with RGDS peptide prevented platelet aggregation, but not RAFTK phosphorylation. Furthermore, phosphorylation of RAFTK did not require integrin engagement since platelets treated with the 7E3 inhibitory antibodies that block fibrinogen binding to glycoprotein IIb-IIIa did not inhibit RAFTK phosphorylation. Similarly, platelets treated with LIBS6 antibodies, which specifically activate glycoprotein IIb-IIIa, did not induce RAFTK phosphorylation. Stimulation of platelets by several agonists such as collagen, ADP, epinephrine, and calcium ionophore A23187 induced RAFTK phosphorylation. Tyrosine phosphorylation of RAFTK in platelets is regulated by calcium and is mediated through the protein kinase C pathway. Phosphorylation of RAFTK is dependent upon the formation of actin cytoskeleton as disruption of actin polymerization by cytochalasin D significantly inhibited this phosphorylation. The RAFTK protein appears to be proteolytically cleaved by calpain in an aggregation dependent manner upon thrombin stimulation. These results demonstrate that RAFTK is tyrosine-phosphorylated during an early phase of platelet activation by an integrin- independent mechanism and is not dependent on platelet aggregation, suggesting different mechanisms of regulation for FAK and RAFTK phosphorylation during platelet activation.