Thrombin regulates components of the fibrinolytic system in human mesangial cells

Thrombin promotes PAI-1 expression and migration in keratinocytes via ERK dependent Smad linker region phosphorylation

Keratinocyte proliferation and migration is essential during re-epithelialisation for the restoration of the epithelial barrier during skin wound healing. Numerous growth factors are involved in the stimulation of keratinocyte proliferation and migration. The signalling pathways that drive these processes during wound healing are not well defined. This study investigated thrombin-mediated signalling in keratinocytes. The thrombin receptor, protease-activated receptor 1 (PAR-1) is a seven transmembrane G-protein coupled receptor that is known to transactivate the epidermal growth factor receptor (EGFR). Immortalized human keratinocytes (HaCaT cells) were treated with thrombin and selective inhibitors to EGFR and MAP kinases. Whole cell lysates were separated on SDS-PAGE and analysed by Western blot using antibodies against transcription factor Smad2. Quantitative real-time polymerase chain reaction was used to measure the mRNA expression of PAI-1 while scratch wound assays were used to measure keratinocyte migration. Western blot data showed that thrombin mediates PAR-1 transactivation of EGFR and the downstream phosphorylation of the transcription factor Smad2 linker (Smad2L) region. ERK1/2 inhibition by UO126 caused a decrease in Smad2L phosphorylation while the p38 inhibitor SB202190 and JNK inhibitor SP600125 did not. Smad2L Ser250 was specifically phosphorylated by this thrombin mediated pathway while Ser245 and Ser255 were not. Thrombin increased PAI-1 mRNA expression and keratinocyte migration and this was reduced when either EGFR or ERK1/2 were blocked. Taken together these results show that thrombin mediated mRNA expression of PAI-1 in keratinocytes and migration occurs via EGFR transactivation and involves signalling intermediates ERK1/2 and Smad2 and may be a key pathway in skin wound healing.

Interactions of some commonly used drugs with human α-thrombin

Adverse side effects of drugs are often caused by the interaction of drug molecules to targets other than the intended ones. In this study, we investigated the off-target interactions of some commercially available drugs with human α-thrombin. The drugs used in the study were selected from Super Drug Database based on the structural similarity to a known thrombin inhibitor argatroban. Interactions of these drugs with thrombin were initially checked by in silico docking studies and then confirmed by thrombin inhibition assay using a fluorescence microplate-based method. Results show that the three commonly used drugs piperacillin (anti-bacterial), azlocillin (anti-bacterial), and metolazone (anti-hypertensive and diuretic) have thrombin inhibitory activity almost similar to that of argatroban. The Ki values of piperacillin, azlocillin, and metolazone with thrombin are .55, .95, and .62 nM, respectively. The IC50 values of piperacillin, azlocillin, and metolazone with thrombin are 1.7, 2.9, and 1.92 nM, respectively. This thrombin inhibitory activity might be a reason for the observed side effects of these drugs related to blood coagulation and other thrombin activities. Furthermore, these compounds (drugs) may be used as anti-coagulants as such or with structural modifications.

Mizoribine suppresses proliferation of rat glomerular epithelial cells in culture and inhibits increase of monocyte chemoattractant protein-1 and macrophage inflammatory protein-2 stimulated by thrombin

Glomerular crescents play an important role in progressive glomerular injury. The lesions consist of epithelial cells, macrophages and fibrin deposition. Macrophage chemoattractant protin-1 (MCP-1) is a chemoattractant of monocytes, which has a potential of procoagulant activity. Macrophage inflammatory protein-2 (MIP-2) is a chemoattractant of neutrophils and acute necrotizing injury is primarily mediated by neutrophils in crescentic glomerulonephritis. Mizoribine (MZR) is an immunosuppressive drug and it has been used for organ transplantation and treatment of various autoimmune diseases. The aim of this study is to investigate the effects of MZR on glomerular epithelial cells (GEC). Rat GEC were cultured with K1 medium and used from 12th to 14th passage. GEC proliferation was determined by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay. MCP-1 and MIP-2 were quantified by enzyme-linked immunosorbent assay (ELISA) in culture supernatants and mRNA expressions of MCP-1 and MIP-2 were analyzed by real-time reverse transcriptase-polymerase chain reaction (RT-PCR). The proliferation of GEC was suppressed by MZR in a dose-dependent manner in the range of 1.0-100.0 µg/mL. These concentrations of MZR had no toxic effect to GEC. Thrombin (1.0-5.0 U/mL) enhanced the production of MCP-1, MIP-2 and the mRNA expressions of MCP-1 and MIP-2. The stimulatory effect of thrombin was inhibited by addition of MZR (10 µg/mL). It is concluded that MZR may be useful for the treatment of crescentic glomerulonephritis.

Selective Promotion of Plasminogen Activator Inhibitor-1 Secretion by Activation of Proteinase-Activated Receptor-1 in Cultured Human Brain Microvascular Pericytes: Comparison with Endothelial Cells

Microvessels are composed of endothelial cells that cover the luminal surface and pericytes that wrap around and along endothelial cells. In the present study, we investigated the regulation of fibrinolysis in cultured human brain microvascular pericytes and endothelial cells after exposure to thrombin. It was found that the regulation in pericytes is different from that in endothelial cells. Specifically, thrombin increases the secretion of fibrinolytic proteins (tissue- and urokinase-type plasminogen activators and plasminogen activator inhibitor-1), resulting in an enhancement of plasminogen activator activity in endothelial cells, whereas the proteinase increases the secretion of only plasminogen activator inhibitor-1 by activation of proteinase-activated receptor-1 and induces suppression of plasminogen activator activity in pericytes. The present data suggest that thrombin regulation of fibrinolytic activity in endothelial cells and pericytes may be involved in the removal of intravascular thrombi and the maintenance of hemostasis, respectively, in human brain microvessels.

jun-N-terminal kinase regulates thrombin-induced PAI-1 gene expression in proximal tubular epithelial cells

BACKGROUND

Interstitial activation of the coagulation cascade is a common finding in acute and chronic tubulointerstitial damage. We previously demonstrated that thrombin may induce proximal tubular epithelial cells (PTEC) proliferation and regulate, through plasminogen activator inhibitor (PAI)-1 and urokinase-type plasminogen activator (u-PA), their profibrotic activity. The signaling pathways leading to these effects are still unknown. The PAI-1 promoter contains several activator protein-1 (AP-1) consensus sequences. AP-1 activation is induced by different agonists through jun-N-terminal kinase (JNK). Thus, we investigated the role of the JNK-AP-1 axis on thrombin-induced PAI-1 and u-PA expression in immortalized PTEC and its modulation by PKC and src, two key signaling enzymes.

METHODS

JNK and src activation was investigated by Western blotting, PAR-1 cellular surface expression by flow cytometry, PAI-1 and u-PA gene expression by Northern blotting, AP-1 activation by transient transfection, and DNA synthesis by (3)H-thymidine uptake.

RESULTS

Thrombin and PMA induced a time-dependent increase of JNK phosphorylation in immortalized PTEC that was inhibited by PKC down-regulation. Both thrombin and PMA caused AP-1 activation, significantly reduced by src inhibition. Phorbol 12-myristate 13-acetate (PMA), indeed, induced an increase in src phosphorylation. Both PMA- and thrombin-stimulated PAI-1 gene expression was abolished by JNK, protein kinase C (PKC), and src inhibition, and this effect was regulated at the trascriptional level. PKC, but not src or JNK inhibition, abolished thrombin-elicited u-PA expression. Finally, JNK inhibition did not influence thrombin-induced proliferation.

CONCLUSION

Our data suggest that thrombin activates the JNK-AP-1 axis in a PKC- and src-dependent manner in PTEC. This axis, necessary for thrombin-stimulated PAI-1 expression, but not for its fibrinolytic and regenerative effect, may represent a therapeutic target in acute and chronic tubulointerstitial damage.

Type 1 plasminogen activator inhibitor deficiency aggravates the course of experimental glomerulonephritis through overactivation of transforming growth factor beta

Type 1 plasminogen activator inhibitor (PAI-1) is the primary inhibitor of tissue-type plasminogen activator (tPA) and urokinase-type plasminogen activator (uPA). Whereas PAI-1 is not expressed in normal kidneys, it is strongly induced in glomerular diseases and thus could promote the local accumulation of fibrin. To study the role of PAI-1 in the development of inflammatory glomerular injury, passive antiglomerular basement membrane (GBM) glomerulonephritis (GN) was induced in PAI-1 knockout mice and in wild-type mice of the same genetic background. Unexpectedly, PAI-1 deficiency was associated with an early and severe exacerbation of glomerular injury: Infiltration by CD4 T cells, proportion of fibrinous crescents, and renal function impairment were significantly more pronounced in PAI-1 -/- mice. Interestingly, activation of transforming growth factor (TGF)- beta, which is known to be dependent on the PA/plasmin system in vitro, was dramatically enhanced in the kidneys in the absence of PAI-1. Moreover, administration of neutralizing antibodies against TGF-beta significantly attenuated the disease in PAI-1 -/- mice. This suggests that the poor outcome of GN in PAI-1 -/- mice is consecutive to an uncontrolled activation of TGF-beta and confers PAI-1 with a new, immunomodulatory role.

Protease-activated receptor-2 expression in IgA nephropathy: a potential role in the pathogenesis of interstitial fibrosis

An increasing body of evidence suggests that proteases may play a key role in the pathogenesis of tissue fibrosis. Protease-activated receptor-2 (PAR-2) is cleaved and activated by trypsin-like proteolytic enzymes, including tryptase and activated coagulation factor X (FXa). Both these soluble mediators have been demonstrated, directly or indirectly, at the interstitial level in progressive renal diseases, including IgA nephropathy (IgAN). PAR-2 mRNA and protein levels were investigated by RT-PCR and immunohistochemistry, respectively, in 17 biopsies from IgAN patients and 10 normal kidneys. PAR-2 expression was also evaluated, by RT-PCR and western blotting, in cultured human mesangial and proximal tubular cells. Finally, gene expression of plasminogen activator inhibitor-1 (PAI-1) and TGF-beta, two powerful fibrogenic factors, was evaluated in FXa-, trypsin-, and PAR-2 activating peptide-stimulated human proximal tubular cells by Northern blot. In normal kidneys, PAR-2 gene expression was barely detectable, whereas in IgAN biopsies the mRNA levels for this protease receptor were strikingly increased and directly correlated with the extent of interstitial fibrosis. Immunohistochemical staining demonstrated that PAR-2 protein expression in IgAN biopsies was mainly localized in the proximal tubuli and within the interstitial infiltrate. Proximal tubular cells in culture expressed PAR-2. Activation of this receptor by FXa in tubular cells induced a striking increase in intracellular calcium concentration. In addition, incubation of both cell lines with trypsin, FXa, or PAR-2 activating peptide caused a marked upregulation of PAI-1 gene expression that was not counterbalanced by an increased expression of plasminogen activators. Finally, PAR-2 activation induced a significant upregulation of TGF-beta gene and protein expression in both mesangial and tubular cells. On the basis of our data, we can suggest that PAR-2 expressed by renal resident cells and activated by either mast cell tryptase or FXa may induce extracellular matrix deposition modifying the PAI-1/PA balance and inducing TGF-beta expression. These molecular mechanisms may underlie interstitial fibrosis in IgAN.

Tissue factor pathway inhibitor production by human proximal tubular epithelial cells in culture

Fibrin deposition in the peritubular capillaries and along the tubular basement membrane is commonly observed in several renal diseases and suggests the involvement of blood coagulation in tubulointerstitial damage. It has been demonstrated that tissue factor (TF) is present in tubular epithelial cells of animal models of nephritis. Tissue factor pathway inhibitor (TFPI) regulates the extrinsic pathway of blood coagulation through its ability to inhibit TF activity and it is now thought to be produced mainly by the vascular endothelial cells. We examined whether human proximal tubular epithelial cells (PTEC) could produce TFPI and attempted to clarify the regulatory factors affecting TFPI production. Cultured human PTEC were used. The procoagulant activity (PCA) in PTEC lysate was quantified by measurement of the one-stage recalcification time. TFPI in the cell supernatants was measured by ELISA. The mRNA of TF and TFPI in PTEC was analyzed by reverse transcriptase polymerase chain reaction (RT-PCR). PCA which is compatible with TF activity was present in the PTEC lysate. TF mRNA and TFPI mRNA were detected in PTEC. The amount of TFPI increased over time in the cell supernatants. Immnoblot analysis revealed 40 kD protein of TFPI, and TFPI antigen was demonstrated in PTEC by immunofluorescence. The concentration of TFPI was significantly increased following incubation with thrombin and heparin in a dose- and time-dependent manner, although the amount of TFPI mRNA was not changed. Our study showed that TFPI is produced in cultured PTEC and added one more cell type that produced TFPI other than endothelial cells. Thrombin and heparin stimulated TFPI secretion from PTEC. TFPI of PTEC may act against generation of thrombin and tubular fibrin formation induced by tissue factor activation. The augmentation of TFPI secretion by heparin may play an important role in the modulation of anticoagulant properties of PTEC.

Prognostic value of plasminogen activator inhibitor type 1 mRNA in microdissected glomeruli from transplanted kidneys

BACKGROUND

Plasminogen activator inhibitor type 1 (PAI-1) exerts antifibrinolytic and profibrotic activities. Inside the glomerulus, PAI-1 is mainly synthesized by mesangial cells. We hypothesized that thrombin, via its receptor protease activated receptor type 1 (PAR-1), present on the membrane of glomerular cells, is an important mediator of PAI-1 synthesis.

METHODS

Using the technique of Peten et al., we microdissected the glomeruli of 23 kidney transplanted patients admitted in our department from 1993 to 1997, and we followed-up these patients for up to 5 years, with sometimes iterative renal biopsies. With this technique, we also microdissected the glomeruli of three patients who have had a nephrectomy for cancer (control patients). We investigated mRNA expression of the PAI-1, the thrombin receptor PAR-1, the alpha2 chain of type IV (alpha2 IV) collagen, and of a housekeeping gene (cyclophilin) by reverse transcription-polymerase chain reaction. The results were correlated with the renal function and the histological findings classified into acute rejection (9 biopsies), chronic rejection (22 biopsies), or normal (8 biopsies).

RESULTS

A significant up-regulation of PAI-1 and alpha2 IV collagen mRNA was observed in acute rejection (P<0.05) when compared to normal kidneys. A positive correlation exists between alpha2 IV collagen mRNA level and the degree of cellular infiltration. A negative correlation was found between the level of mRNA of PAR-1 and the degree of vascular thrombosis (P=0.005) and glomerulosclerosis (P=0.04). A positive correlation was found between the degradation of renal function and the mRNA level of PAI-1 at the time of the renal biopsy (P<0.05).

CONCLUSIONS

These results suggest that glomerular PAI-1 mRNA may be predictive of the long-term renal graft function.

Cultured rat glomerular epithelial cells show gene expression and production of transforming growth factor-beta: expression is enhanced by thrombin

BACKGROUND

Glomerular crescents play an important role in progressive glomerular injury. The lesions consist of epithelial cells, macrophages, and deposits of fibrin and extracellular matrix. Transforming growth factor beta (TGF-beta) contributes to the modulation of cell growth and extracellular matrix synthesis. Thrombin is involved in fibrin formation in crescents. The purpose of this study was to examine whether glomerular epithelial cells (GEC) could produce TGF-beta, and if so, to clarify the role of TGF-beta in GEC proliferation. We also investigated whether thrombin could modulate the production of TGF-beta and extracellular matrix by GEC.

METHODS

Bioassay using the TGF-beta-dependent mink pulmonary epithelial cell line (CCL-64), immunoblot analysis, and reverse transcriptase polymerase chain reaction (RT-PCR) were used to demonstrate TGF-beta production by rat GEC. TGF-beta gene expression was examined by RT-PCR in GEC incubated with thrombin, and type IV collagen and fibronectin were quantified by enzyme immunoassay in culture supernatants of GEC incubated with thrombin or TGF-beta.

RESULTS

TGF-beta activity was demonstrated in GEC supernatants by bioassay. Immunoblot analysis of concentrated culture supernatants using anti-TGF-beta antibody revealed a 12.5-kDa protein, which was compatible with TGF-beta. Concentrated GEC supernatants inhibited GEC proliferation as well as porcine TGF-beta. RT-PCR demonstrated TGF-beta gene expression in GEC. Thrombin (0.5-5.0 U/ml) enhanced TGF-beta mRNA expression in a dose-dependent manner. Thrombin (5.0 U/ml) and porcine TGF-beta (5.0 ng/ml) stimulated the production of type IV collagen and fibronectin by GEC.

CONCLUSIONS

Rat GEC produce TGF-beta in vitro. Thrombin may participate in the progression of glomerulosclerosis in crescentic glomerulonephritis through the stimulation of TGF-beta production by GEC.

Plasminogen activator inhibitor type 1 is a potential target in renal fibrogenesis

Plasminogen activator inhibitor type 1 is a potential target in renal fibrogenesis. The progression of renal lesions to fibrosis involves several mechanisms, among which the inhibition of extracellular matrix (ECM) degradation appears to play an important role. Two interrelated proteolytic systems are involved in matrix degradation: the plasminogen activation system and the matrix metalloproteinase system. The plasminogen activator inhibitor type 1 (PAI-1), as the main inhibitor of plasminogen activation, regulates fibrinolysis and the plasmin-mediated matrix metalloproteinase activation. PAI-1 is also a component of the ECM, where it binds to vitronectin. PAI-1 is not expressed in the normal human kidney but is strongly induced in various forms of kidney diseases, leading to renal fibrosis and terminal renal failure. Thrombin, angiotensin II, and transforming growth factor-beta are potent in vitro and in vivo agonists in increasing PAI-1 synthesis. Several experimental and clinical studies support a role for PAI-1 in the renal fibrogenic process occurring in chronic glomerulonephritis, diabetic nephropathy, focal segmental glomerulosclerosis, and other fibrotic renal diseases. Experimental models of renal diseases in PAI-1-deficient animals are in progress, and preliminary results indicate a role for PAI-1 in renal fibrogenesis. Inhibition of PAI-1 activity or of PAI-1 synthesis by specific antibodies, peptidic antagonists, antisense oligonucleotides, or decoy oligonucleotides has been obtained in vitro, but needs to be evaluated in vivo for the prevention or the treatment of renal fibrosis.

Thrombin is a novel regulator of hexokinase activity in mesangial cells

BACKGROUND

Hexokinase (HK) activity is fundamentally important to cellular glucose uptake and metabolism. Phorbol esters increase both HK activity and glucose utilization in cultured mesangial cells via a protein kinase C (PKC)- and extracellular signal-regulated kinases 1 and 2 (ERK1/2)-dependent mechanism. In adult kidneys, increased HK activity has been reported in both glomerular injury and in diabetes, but the mechanisms responsible for these changes are unknown. Thrombin, a known activator of both PKC and ERK1/2, is increased in the settings of renal injury and diabetes. Thus, thrombin may contribute to the observed changes in HK activity in vivo.

METHODS

Thrombin and thrombin receptor agonists were tested for the ability to increase HK activity and glucose metabolism in murine mesangial (SV40 MES 13) cells. ERK1/2 activation was also evaluated in parallel. Thrombin inhibition (hirudins), PKC depletion, Ser-Thr kinase inhibition (H-7), MEK1/2 inhibition (PD98059), pertussis toxin (PTX), and general inhibitors of transcription or translation were then tested for the ability to attenuate these effects.

RESULTS

Thrombin (>/=0.01 U/mL) mimicked the effect of phorbol esters, increasing HK activity> 50% within 12 to 24 hours (P < 0.05). This effect was inhibited by hirudins, mimicked by thrombin receptor agonists, and accompanied by increased Glc utilization. H-7, PD98059, and general inhibitors of transcription or translation-but not PTX-prevented thrombin-induced HK activity at 24 hours. PKC depletion and PD98059 also blocked the associated phosphorylation and activation of ERK1/2.

CONCLUSIONS

Thrombin increases mesangial cell HK activity via a PTX-insensitive mechanism involving thrombin receptor activation, PKC-dependent activation of ERK1/2, and both ongoing gene transcription and de novo protein synthesis. As such, thrombin is a novel regulator of HK activity in mesangial cells and may play a role in coupling renal injury to metabolism.

Tissue factor, plasminogen activator inhibitor-1, and thrombin receptor expression in human crescentic glomerulonephritis

Glomerular fibrin deposition is a common histological feature of crescentic glomerulonephritis (CGN). Tissue factor (TF) is the most powerful activator of the coagulation system, whereas plasminogen activator inhibitor (PAI)-1 is a key modulator of the fibrinolytic pathway. Thrombin, released locally as the final step of the coagulation cascade and trapped within the fibrin clots, can induce the activation of glomerular cells, through the interaction with a specific receptor. To investigate the mechanisms underlying coagulation cascade activation and fibrin deposition and the role of this phenomenon in the pathogenesis of human CGN, TF, PAI-1, and thrombin receptor expression were studied in CGN biopsy specimens. Glomerular TF gene and protein expression were strikingly increased in CGN, in particular within the crescents and in the mesangial area, with the same distribution of fibrin deposits. Interestingly, very few infiltrating mononuclear cells were stained in TF immunohistochemistry. To better evaluate the involvement of monocytes in TF expression, TF mRNA and CD68 protein were studied by an in situ hybridization/immunohistochemistry combined technique. Only 16% of the cells expressing TF mRNA were CD68 positive. However, most of the TF signal was localized in the proximity of monocytes, suggesting that soluble mediator(s) released by these cells could induce TF expression. Indeed, interleukin-1 (IL-1), one of the main monocyte-derived cytokines, upregulated TF mRNA levels in cultured human mesangial cells in a time-dependent manner. Moreover, a striking increase in IL-1 expression was present within the cellular crescents in CGN biopsy specimens. Finally, we observed a marked upregulation of both PAI-1 and thrombin receptor mRNA levels in CGN with a pattern resembling TF and fibrin distribution. Surprisingly, thrombin receptor protein expression was strikingly downregulated in CGN, suggesting its continuous activation and degradation. In conclusion, we can hypothesize that TF and PAI-1, mainly expressed by resident cells, may play a pivotal role in the development and preservation of fibrin deposits in CGN. In addition, thrombin, released locally and accumulated within the fibrin clots, may represent a pathogenetic mediator of crescentic lesions.

Thrombin stimulates synthesis of type IV collagen and tissue inhibitor of metalloproteinases-1 by cultured human mesangial cells

Glomerular accumulation of extracellular matrix (ECM) is the common pathologic feature following glomerular injury, and the alteration in the synthesis and degradation of ECM may be involved in the glomerular accumulation of ECM. Glomerular fibrin formation occurs in various forms of human and experimental glomerulonephritis, and it may play an important role in progressive glomerular injury. Thrombin, a multifunctional serine proteinase that is generated at the site of vascular injury, has central functions in hemostasis and it also shows various biologic effects. In this study, it is hypothesized that thrombin may alter the production and the degradation of type IV collagen, which is an important component of ECM in the glomeruli. Human mesangial cells (HMC) were cultured, and the levels of type IV collagen, tissue inhibitor of metalloproteinase-1 (TIMP-1), and matrix metalloproteinase-2 (MMP-2) in the culture supernatants were measured by enzyme immunoassay using specific antibodies. MMP-2 activity was also evaluated by zymography using polyacrylamide/ sodium dodecyl sulfate gel-containing gelatin. Thrombin increased the production of type IV collagen and TIMP-1 in a dose-and time-dependent manner, but it did not increase MMP-2. Thrombin also stimulated the gene expressions of the type IV collagen and TIMP-1 in HMC in a dose- and time-dependent manner. Thrombin treated with diisopropylfluorophosphate, a serine proteinase inhibitor, did not show any of these effects. Hirudin, a natural thrombin inhibitor, and anti-transforming growth factor-beta-neutralizing antibody inhibited the stimulating effect of thrombin. These findings suggest that thrombin may contribute to the excessive accumulation of ECM and progression of glomerulosclerosis through an increase of type IV collagen production and a decreased matrix degradation presumably via a transforming growth factor-beta-dependent mechanism.

Thromboxane A2 modulates the fibrinolytic system in glomerular mesangial cells

We examined the effects of thromboxane A2 (TxA2) on the activities of the plasminogen-plasmin system in glomerular mesangial cells. When mesangial cells are exposed to the TxA2 agonist U-46619, a substantial increase in production of plasminogen activator inhibitor-1 (PAI-1) protein is observed that is significantly greater than that induced by 10% serum alone. This increase in PAI-1 protein production is accompanied by an increase in steady-state levels of PAI-1 mRNA. This stimulation is specifically mediated by TxA2 (thromboxane prostanoid, TP) receptors, since U-46619 also stimulates PAI-1 expression in cells that are transfected with TP receptors, and this stimulation of PAI-1 production is completely blocked by the TxA2 receptor antagonist, SQ-29,548. Despite the increase in PAI-1 production, there was net stimulation of plasmin activity in the medium of mesangial cells that had been exposed to U-46619. Furthermore, U-46619 also caused an increase in tissue plasminogen activator (tPA) mRNA levels. Thus TxA2 stimulates the production of PAI-1 and plasminogen activators by mesangial cells through a receptor-dependent mechanism. In inflammatory renal diseases, the balance of these effects may modulate glomerular thrombosis and renal fibrosis.

Thrombomodulin is synthesized by human mesangial cells

Thrombomodulin (TM), an endothelial receptor for thrombin, endowed with a powerful anticoagulant activity, plays an important role in the antithrombogenicity of the vascular endothelium. Its presence within the human renal glomerulus is already known but was thought to be only endothelial. We looked for TM expression in human mesangial cells (MC), both in situ, in freshly prepared glomeruli, and in primary culture. Both fresh and cultured MC were strongly reactive for TM by immunocytochemical methods. Total TM antigen measured on MC lysates and surface TM activity on MC were 0.292 +/- 0.075 ng/mg of cellular proteins and 1.20 +/- 0.02 pmole of activated protein C/min/mg of cellular proteins, respectively. As shown by the presence of numerous transcripts detected by in situ hybridization, TM was shown to be synthesized by MC in vivo and in culture. The synthesis of active TM by both endothelial and mesangial cells within the renal glomerulus stresses the importance of its role in maintaining renal hemostatic equilibrium, and sheds some light on the conflicting reports of TM over- and underexpression in glomerulopathies to open a new field for investigation.

Specific receptor binding of renin on human mesangial cells in culture increases plasminogen activator inhibitor-1 antigen

Some proteases possess a membrane receptor that focalizes their proteolytic activity on the cell surface and may mediate a proliferative effect, such as urokinase on glomerular epithelial cells. Since some hypertensive states are associated with high concentrations of renin and proliferation of arteriolar smooth muscle cells, we asked whether renin, an aspartyl-protease, would bind to mesangial cells that are smooth-muscle derived cells, which would induce their proliferation. The binding of 125I labeled recombinant human renin (125I-R) was studied on human primary mesangial cells and mesangial cells immortalized by transfection with SV40-T antigen. At 37 degrees C, the binding of 125I-R was time dependent and reached a plateau after two hours. 125I-R was found to bind in a saturable and specific manner with a Kd = 0.4 nM and 1 nM and 8,000 and 2,000 binding sites/cell, for primary and immortalized cells, respectively. When binding experiments were performed in the presence RO 42-5892, a synthetic inhibitor of renin, RO 42-5892 could inhibit the specific binding of labeled renin only at concentrations 1,000 times superior to the IC 50, indicating that the renin-mesangial receptor interaction did not depend on the active site of renin. Analysis by SDS-PAGE and autoradiography of cross-linking experiments of 125I-R bound to a membrane preparation showed a band of approximately 110 to 120 kDa, suggesting a Mr of 70 to 80 kDa for the renin receptor. Incubation of mesangial cells with 100 nM renin for 24 hours provoked a 100% increase of 3H thymidine incorporation that was not accompanied by an increase of the cell number, even after a seven day period of incubation. However, the incubation of mesangial cells with renin for 24 hours induced a significant increase (170% of control, P = 0.04) of plasminogen activator inhibitor-1 (PAI1) antigen in the conditioned medium. In conclusion, we have shown that human mesangial cells in culture express a specific receptor for renin, and that the binding of renin increases 3H thymidine incorporation independently of renin enzymatic activity. The absence of cell proliferation, the increase of 3H thymidine incorporation and the increase of PAI1 antigen suggest that the binding of renin can induce mesangial cell activation, which is reflected by a change in the fibrinolytic capacity of the cells. The role of this receptor remains to be determined in nephropathies and hypertensive states associated with high plasma/tissue renin concentrations, hypertrophy of mesangial or smooth muscle cells and extracellular matrix remodeling.

Thrombin, phorbol ester, and cAMP regulate thrombin receptor protein and mRNA expression by different pathways

Human mesangial cells have been used to study the regulation of thrombin receptor protein and mRNA expression during cross-talk between different signal transduction pathways. Persistent activation of thrombin receptor by thrombin led to homologous down-regulation of thrombin receptor protein. However, thrombin receptor mRNA expression was not affected, suggesting that increased receptor degradation is responsible for homologous down-regulation. Chronic activation of protein kinase C by phorbol 12-myristate 13-acetate (PMA) and of adenylylcyclase by prostaglandin E1 (PGE1) resulted in heterologous down-regulation of thrombin receptor protein. In contrast to thrombin, PMA and PGE1 reduced in parallel thrombin receptor mRNA levels to 51% and 24% of control, respectively, indicating that heterologous down-regulation of thrombin receptor protein is, at least in part, due to inhibition of receptor mRNA expression. The mechanisms of heterologous down-regulation of thrombin receptor protein have been studied in detail and compared to homologous down-regulation. PMA-induced down-regulation was completely blocked by GF 109 203 X, an inhibitor of protein kinase C. However, the loss of thrombin receptor induced by thrombin was not prevented by GF 109 203 X, indicating that homologous regulation is not dependent on protein kinase C activation. The heterologous effect of PGE1 was mimicked by 8-bromo-cAMP, isobutylmethylxanthine, and forskolin, suggesting that an increase in intracellular cAMP level is involved in heterologous regulation. Interestingly, heterologous down-regulation induced by PGE1 seems not to require previous internalization of thrombin receptor. These data indicate that thrombin receptor protein and mRNA expression can be regulated in homologous and heterologous ways by different mechanisms.

Characterization of platelet-activating factor synthesis in glomerular endothelial cell lines

Platelet-activating factor synthesis in two transformed lines of glomerular endothelial cells was characterized and contrasted with platelet-activating factor production in macrovascular-derived endothelial cells as well as with glomerular cells of mesenchymal origin. Platelet-activating factor synthesis was assessed in intact cells and in cell-free preparations. Glomerular endothelial cells constitutively synthesize bio-active alkyl-PAF, and this basal activity can be chronically augmented by various inflammatory and thrombotic agents. In contrast, thrombin-mediated platelet-activating factor formation in bovine pulmonary aortic endothelial cells as well as in glomerular mesangial cells is acute and transient. The potential role of anti-inflammatory prostanoids to function as negative feedback modulators of thrombin- or endothelin-mediated platelet-activating factor synthesis was also investigated, as the synthesis of platelet-activating factor is often associated with the formation of these prostanoids. Indomethacin augmented receptor-mediated platelet-activating factor synthesis while prostanoids of the E and I series reduced agonist-stimulated PAF synthesis. In summary, the unique capacity of glomerular endothelial cells to respond to inflammatory stimuli with sustained platelet-activating factor synthesis is a clear indication of this cell's pivotal role in augmenting the inflammatory response in the limited environment of the glomerulus.

A high concentration of glucose alters the production of tPA, uPA and PAI-1 antigens from human mesangial cells

To elucidate a role of tPA, uPA and PAI-1 for the development of diabetic glomerulosclerosis, the effect of high glucose concentration on the production of both basal and thrombin-mediated tPA, uPA and PAI-1 antigens from human mesangial cells was investigated. The culture of mesangial cells in the presence of high glucose (33 mM) for 11 days resulted in an increase in the synthesis of tPA and uPA when compared with that in normal glucose concentration (5 mM). In contrast, the cells grown in high glucose produced less PAI-1 than those in normal glucose. Thrombin stimulated dose-dependently the production of tPA, uPA and PAI-1 from the cells grown in either 5 or 33 mM glucose. However, the magnitude of the increase in tPA, uPA and PAI-1 from the cells grown in high glucose was less than that in normal glucose. These results suggest that the plasmin activity in mesangial cells may increase under a high glucose condition, leading to increased proteolysis of mesangial matrix. In addition, either fibrinolysis or proteolysis mediated by thrombin may be altered by high glucose concentration. Therefore, it is postulated that the turnover of mesangial matrix may be increased in diabetic nephropathy.

Glomerular fibrinolytic activity in anti-GBM glomerulonephritis in rabbits

Fibrin is an important mediator of injury in severe proliferative forms of glomerulonephritis (GN). Normal glomeruli express fibrinolytic activity, which may protect against the injurious effects of fibrin deposition. Changes in glomerular fibrinolytic activity (GFA) may play an important role in modulating fibrin accumulation in GN. To study the changes in GFA associated with fibrin deposition in GN, autologous phase anti-glomerular basement antibody initiated GN (anti-GBM GN) was studied in rabbits. Net GFA was significantly reduced in association with glomerular fibrin deposition (1.3 +/- 0.8 ng fibrin lysed/10(3) glomeruli/2 hr, normal 57.1 +/- 25.4 ng fibrin lysed/10(3) glomeruli/2 hr, P < 0.02). Reduced GFA in fibrin associated GN was associated with decreased expression of tissue type plasminogen activator (tPA) and increased expression of plasminogen activator inhibitor type-1 (PAI-1) and glomerular macrophage infiltration. In a fibrin independent model of anti-GBM induced GN (heterologous phase), with equivalent injury (proteinuria), net GFA was increased (174 +/- 64 ng fibrin lysed/10(3) glomeruli/2 hr). This was associated with increased tPA and uPA, and decreased PAI-1 in the absence of significant macrophage infiltration. These studies demonstrate that fibrin deposition in GN is associated with a net reduction of GFA, attributable to reduced expression of plasminogen activators and augmentation of PAI-1. Reduction of GFA may potentiate glomerular fibrin deposition and consequent glomerular injury. The association between glomerular macrophage influx and reduction in GFA suggests that this change may be directed by macrophages.

Tumor necrosis factor alpha increases antifibrinolytic activity of cultured human mesangial cells

Tumor necrosis factor alpha (TNF alpha) is likely to exert a major influence in the pathogenesis of glomerulopathies. Besides its proinflammatory properties. TNF alpha interacts with cell growth and synthesis of components of the fibrinolytic system. In this study, we report the effects of recombinant human TNF alpha on the synthesis of tissue-type plasminogen activator (t-PA) and its inhibitor (PAI-1) by human mesangial cells in culture. We first demonstrate that TNF alpha binds specifically to a single class of high affinity receptors (Kd 5.10(-11) M; 1500 receptors/cell). TNF alpha has an antimitogenic effect on human mesangial cells since it decreased DNA synthesis, measured by 3H-thymidine incorporation, in a dose-dependent manner. Release of cytosolic LDH and incorporated 51Cr was not increased by 100 ng/ml TNF alpha as compared with control, indicating that this monokine is not cytotoxic for cultured human mesangial cells. Zymographic analysis and reverse fibrin autography disclosed a 120 kD t-PA-PAI-1 complex and a 50 kD free form of PAI-1 in the supernatants of both unstimulated and TNF-stimulated cells; PAI-1 was released in excess and free t-PA was not observed. TNF alpha (0 to 100 ng/ml) had no effect on t-PA synthesis, but enhanced PAI-1 release in a time- and dose-dependent manner (97% increase of PAI-1 synthesis after a 24 hour incubation). This effect was abolished by cycloheximide, suggesting that protein synthesis was required. Northern blot analysis showed that TNF alpha increased the steady-state PAI-1 mRNA levels in a time-dependent manner, with a maximal effect at two hours.(ABSTRACT TRUNCATED AT 250 WORDS)

Glomerular coagulation system in renal diseases

Glomeruli possess a complex hemostasis system with prothrombotic (procoagulant, antifibrinolytic) and antithrombotic (anticoagulant, fibrinolytic) properties that can act locally on platelet adhesion or aggregation, on plasmatic coagulation pathways, and on fibrinolysis. In vitro, inflammatory mediators, such as TNF, favor glomerular thrombogenic properties through enhancement of thromboplastin synthesis and of plasminogen activator inhibitor PAI-1, and through decrease in thrombomodulin activity. In some diseases, intraglomerular fibrin formation appears to be favored by increased glomerular prothrombotic properties, for example: augmented thromboplastin activity in immune glomerulonephritides and in Shwartzman phenomenon, excessive thromboxane A2 synthesis, and decreased fibrinolytic activity in severe renal allograft rejection. In other diseases glomerular hemostasis appears to function homeostatically, for example, in thrombin-induced disseminated intravascular coagulation with enhancement of fibrinolytic activity favoring fibrin dissolution. Novel methods allowing the study of glomerular hemostatic activities in renal biopsy fragments should help to understand the mechanisms of fibrin deposition in human diseases and to treat it on a logical basis.