Signaling effects of alpha-thrombin and SFLLRN in rat glioma C6 cells

Thrombin Activity and Thrombin Receptor in Rat Glioblastoma Model: Possible Markers and Targets for Intervention?

High-grade gliomas constitute a group of aggressive CNS cancers that have high morbidity and mortality rates. Despite extensive research, current therapeutic approaches enable survival beyond 2 years in rare cases only. Thrombin and its main CNS target, protease-activated receptor-1, have been implicated in tumor progression and brain edema. Our aim was to study protease-activated receptor-1 (PAR-1) protein expression and thrombin-like activity levels in both in vitro and in vivo models of glioblastoma and correlate them with the volume of the surrounding edema. We measured the presence of PAR-1 protein using fluorescence immunohistochemistry and assessed thrombin activity in various glial and non-glial cell lines and in a CNS-1 glioma rat model using a thrombin-specific fluorescent assay. Thrombin activity was found to be highly elevated in various high-grade glioma cell lines as well as in non-glial malignant cell lines. In the CNS-1 glioma model, the level of PAR-1 fluorescence in the tumor was significantly elevated compared to adjacent regions of reactive gliosis or distant brain areas. The elevated level of thrombin activity observed in the high-grade glioma positively correlated with tumor-induced brain edema. In conclusion, thrombin is secreted from glioma cells and PAR-1 may be a new biological marker for high-grade gliomas.

Rationale for clinical trials of coagulation: reactive drugs in hepatocellular carcinoma

Evidence for the regulation of cancer growth by components of the blood coagulation mechanism provides abundant opportunity for the development of novel hypotheses for the experimental treatment of malignancy. Information available on the heterogeneity in mechanisms of interaction between various cancer cell types, and procoagulant and fibrinolytic pathways, platelets, glycosaminoglycan-regulated growth factors and cell-adhesion molecules indicates that insightful clinical trial design may allow targeting of individual cancer cell types with agents capable of intercepting mechanisms of growth control that are relevant to specific tumor types. This paper reviews the evidence that the common anticoagulant, heparin, inhibits hepatocellular carcinoma cell proliferation and hepatocellular carcinoma tumor dissemination in experimental animals. Clinical trials of heparin performed to date have shown increased tumor response rates and survival in other tumor types. Expression of urokinase-type plasminogen activator by hepatocellular carcinoma cells enhances tumor cell proliferation, motility, invasiveness and metastatic dissemination. Inhibition of the urokinase-type plasminogen activator/plasmin system by protease inhibitors such as aprotinin (Trasylol, Bayer) have shown improvement in the clinical course of certain tumor types. These data suggest that drugs that are well-known in the field of vascular medicine may find a role in the treatment of hepatocellular carcinoma, a common tumor type that has resisted containment by other means.

Thrombin and PAR-1 activating peptide increase iNOS expression in cytokine-stimulated C6 glioma cells

Thrombin (THR) plays a key role in the brain under physiological and pathological conditions. Several of the biological activities of thrombin have been shown to be mainly driven through activation of protease-activated receptor-1 (PAR-1)-type thrombin receptor. Here we have studied the effect of THR and PAR-1-activating peptide (PAR1-AP), SFLLRN, on cytokine-induced expression of inducible nitric oxide (iNOS), a prominent marker of astroglial activation using the rat C6 glioma cells. In this cell line, THR (1-10 U/mL) and PAR1-AP (1-100 microM) induced a significant concentration-dependent increase both of IFN-gamma- (250 U/mL) or TNF-alpha- (500 U/mL) induced NO release. The observed increase of NO production was related to an enhancement of iNOS expression as measured in cell lysates prepared from different treatments by using SDS-PAGE followed by western blot analysis. The effect of THR, but not that of PAR1-AP, was significantly inhibited by hirulog(TM) (60 microg/mL), a specific and stochiometric THR inhibitor or by cathepsin-G (40 mU/mL), an inhibitor of PAR-1. In conclusion our data suggest a role for THR through activation of PAR-1 in the induction of astroglial iNOS, and further support the hypothesis that THR may function as an important pathophysiological modulator of the inflammatory response.

A novel PAR-1-type thrombin receptor signaling pathway: cyclic AMP-independent activation of PKA in SNB-19 glioblastoma cells

Cellular effects of thrombin are mediated by members of a new subfamily of G protein-coupled receptors designated proteinase-activated receptors (PARs) with the prototype PAR-1. Investigation of PAR-1-induced signaling has been shown to be very important in clarifying thrombin's role in cell metabolism, differentiation, and growth. We evaluated connection of PAR-1 with the cAMP/PKA pathway in SNB-19 glioblastoma cells. Alpha-thrombin and the synthetic PAR-1 agonist SFLLRN stimulated PKA as shown by increased PKA activity and translocation of the catalytic PKA alpha subunits (PKA(cat)alpha) into the nucleus. However, no effect on cAMP could be observed. PKA(cat)alpha was found to be associated with nuclear factor-kappa B (NF-kappaB) p65 and its inhibitor protein IkappaB in SNB-19 cells. After PAR-1 stimulation, this association was markedly diminished. We conclude that PAR-1 mediates PKA activation without altering cAMP levels but includes NF-kappaB-associated PKA(cat)alpha in SNB-19 glioblastoma cells. This is the first evidence for a cAMP-independent PKA signaling by a G protein-coupled receptor.

Thrombin induces NO release from cultured rat microglia via protein kinase C, mitogen-activated protein kinase, and NF-kappa B

Microglia, brain resident macrophages, become activated in brains injured due to trauma, ischemia, or neurodegenerative diseases. In this study, we found that thrombin treatment of microglia induced NO release/inducible nitric-oxide synthase expression, a prominent marker of activation. The effect of thrombin on NO release increased dose-dependently within the range of 5-20 units/ml. In immunoblot analyses, inducible nitric-oxide synthase expression was detected within 9 h after thrombin treatment. This effect of thrombin was significantly reduced by protein kinase C inhibitors, such as Go6976, bisindolylmaleimide, and Ro31-8220. Within 15 min, thrombin activated three subtypes of mitogen-activated protein kinases: extracellular signal-regulated kinase, p38, and c-Jun N-terminal kinase/stress-activated protein kinase. Inhibition of the extracellular signal-regulated kinase pathway and p38 reduced the NO release of thrombin-treated microglia. Thrombin also activated nuclear factor kappaB (NF-kappaB) within 5 min, and N-acetyl cysteine, an inhibitor of NF-kappaB, reduced NO release. However, thrombin receptor agonist peptide (an agonist of protease activated receptor-1 (PAR-1)), could not mimic the effect of thrombin, and cathepsin G, a PAR-1 inhibitor, did not reduce the effect of thrombin. These results suggest that thrombin can activate microglia via protein kinase C, mitogen-activated protein kinases, and NF-kappaB but that this occurs independently of PAR-1.

Meizothrombin, an intermediate of prothrombin activation, stimulates human glioblastoma cells by interaction with PAR-1-type thrombin receptors

Thrombin induces well-characterized effects on normal and neoplastic brain cells by interaction with protease-activated receptor (PAR)-type thrombin receptors. However, nothing is known about the function of intermediate enzymes of prothrombin activation recently shown to evoke PAR-1-mediated signaling in smooth muscle cells. Therefore, we investigated the effect of recombinant human meizothrombin (rMT), one of thrombin's catalytically active precursor enzymes in the prothrombin cleavage cascade, on calcium mobilization in human SNB-19 glioblastoma cells. By using reverse-transcription polymerase chain reaction, immunofluorescence studies with a monoclonal anti-PAR-1 antibody and calcium measurements, SNB-19 cells were shown to express functional PAR-1-type thrombin receptors. PAR-1 is not only a receptor for thrombin in SNB-19 cells but was also activated by rMT very effectively. Under the conditions used in our experiments, SNB-19 cells stimulated with thrombin after rMT challenge were unable to elicit a new calcium response and vice versa. In addition, both rMT and thrombin induced no further calcium signal after that observed with the PAR-1-activating peptide SFLLRN. Therefore, rMT and thrombin seem to activate calcium signaling by similar mechanisms including PAR-1. Our results demonstrate rMT as a potent activator of PAR-1-type thrombin receptors in SNB-19 glioblastoma cells, suggesting a function of catalytically active thrombin precursor enzymes in cells of glial origin.

Presence of the proteinase-activated receptor-2 (PAR-2) in human brain tumor cells--trypsin- and SLIGRL-induced calcium response in primary cultured meningiomas

This study focused on proteinase-activated receptor-2 (PAR-2) in primary cultured human meningioma cells. Stimulation of these cells with the serine proteinase trypsin resulted in a dose-dependent transient calcium response. Since the specific PAR-2 agonist peptide SLIGRL also induced [Ca2+]i mobilization in human meningioma cells and successive application of SLIGRL and trypsin elicited no new calcium signal we conclude that trypsin-induced calcium signaling is mediated by PAR-2 in human meningioma cells. To our knowledge, this is the first report describing functional PAR-2-type receptors in human brain tumor cells.

Thrombin has a bimodal effect on glioma cell growth

Using rat glioma C6 cells as a model, we have found a bimodal effect of alpha-thrombin on cell growth. In C6 cells treated with alpha-thrombin at concentrations from 0.02 nM to 1.0 nM, inhibition of cell proliferation was noted. Because the thrombin receptor agonist peptide TRAP-6 also induced inhibition of cell proliferation and the thrombin receptor antagonist peptide T1 prevented the inhibitory effect of alpha-thrombin on C6 glioma cell growth, thrombin receptor involvement in antiproliferative action of alpha-thrombin in C6 glioma cells is highly likely. However, stimulation of cell proliferation observed when C6 cells were treated with alpha-thrombin at higher doses (> 1.0 nM) seems to be mediated by as yet undefined thrombin receptor-independent biochemical mechanisms.