The fibrinolytic system in renal disease
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[edit] The Kidney
The main function of our kidneys is maintaining the delicate internal balance called homeostasis. In order to do so, our kidneys filtrate approximately 1700 liters of blood daily in order to excrete waste products in the urine1. In the average human adult, each kidney is about 11 cm long and 5 cm thick, weighing 130-150 grams. The smallest functional unit of the kidney is the nephron, which consists of the glomerulus, proximal tubule, Henle’s loop, distal tubule and collecting duct. Each human kidney contains about 400,000 to 1,200,000 nephrons2. Nephrons regulate water and solute in the body by first filtering the blood under pressure, and then reabsorbing indispensable fluid and essential molecules back into the blood while secreting other metabolites. Reabsorption and secretion are accomplished with both cotransport and countertransport mechanisms established along the nephron3. Apart from excreting waste products, the kidney plays an essential role in the regulation of blood pressure through water and salt management, the maintenance of plasma acid balance, and the secretion of hormones such as erythropoietin, renin, prostaglandins and vitamin D3.
[edit] Renal diseases that we have studied
[edit] Renal transplant rejection
A variety of kidney diseases can cause a progressive deterioration of kidney function. Currently, an estimated number of 40,000 Dutch citizens are affected with one or other kidney disease4. When approximately 90-95% of normal kidney function is lost, the patient enters a state that is referred to as end stage renal disease (ESRD), necessitating renal replacement therapy in order to sustain vital functions. In The Netherlands, the number of patients that is dependent on dialysis treatment is currently ±5,5004. For these patients, the therapy of choice remains renal transplantation. In 2005, 561 renal transplantations were performed in The Netherlands4. Despite meticulous mismatch-screening, the transplant is readily recognized and attacked by the recipient’s immune system, requiring aggressive immunosuppressive therapies to safeguard the allograft. The development of such therapeutic regimens has resulted in a dramatic improvement of patient and graft survival curves over the last three decades5. However, acute renal allograft rejection still occurs in 10-20% of patients after cadaveric renal transplantation, and is responsible for graft loss in up to 4% of cases in the first year after transplantation6. Acute rejection is essentially an inflammatory disorder, which can be mediated by two different immunologic pathways: T-cell-mediated (or cellular) rejection and antibody-mediated (or humoral) rejection. These two pathways have distinct immunopathologic characteristics but frequently occur simultaneously7. The histopathologic changes that characterize acute cellular rejection include interstitial infiltration with T-cells, tubulitis and endothelialitis. Acute antibody-mediated rejection is characterized histopathologically by congestion, neutrophil influx in the glomerular and peritubular capillaries, and in severe cases arteriolar fibrinoid necrosis, interstitial hemorrhage and fibrin thrombi in glomerular capillaries. These features are present to a varying extent. In general, tubulitis is more commonly seen with cellular than with humoral rejection8. The type and severity of rejection is determined by renal biopsy. The histomorphologic changes are usually graded according to the Banff classification of renal allograft pathology9. So far, no reliable biomarkers have been found that can predict the occurrence or estimate the severity of acute rejection in a non-invasive fashion.
[edit] Ischemia-reperfusion injury
Acute renal failure (ARF) is classically defined as a sudden decline in kidney function, resulting in an increase of blood urea nitrogen and serum creatinine. ARF is a life-threatening condition, which is often the result of renal ischemia, followed by reperfusion. Ischemia-reperfusion (I/R) injury occurs in shock, sepsis, vascular surgery and during renal transplantation procedures. For patients in shock, ARF is associated with a mortality rate of more than 50% and the incidence of this major clinical problem has been rising for the past 30 years, resulting in a serious financial burden to society10. Although knowledge on the pathophysiology of I/R injury has been accumulating, its treatment remains largely supportive11. The characteristic morphological alterations of tubular epithelial cells (TEC) upon I/R have been described as early as 189112. With short periods of ischemia-reperfusion, TEC lose polarity. With more severe or sustained ischemia, the epithelial cell is irreversibly damaged resulting in necrosis or apoptosis (Figure 1) and intraluminal cast formation. During recovery from I/R injury, surviving TEC de-differentiate and start to proliferate, eventually replacing the irreversibly injured tubular epithelial cells and restoring tubular integrity and function13 (Figure 1).
Surviving TEC possess an amazing ability to regenerate and proliferate after I/R. In recent years, the signal transduction pathways that control the dynamical balance between cell death or cell survival have been identified to a certain extent21. Activation of the phosphoinositide 3-kinases (PI3K)/Akt and extracellular signal-regulated protein kinase (ERK)/mitogen-activated protein kinase (MAPK) pathways generally support TEC regeneration and proliferation22, 23, whereas initiation of c-Jun N-terminal kinase (JNK) and p38MAPK pathways typically result in TEC apoptosis21.
[edit] Acute pyelonephritis
Urinary tract infections (UTI) are among the most common bacterial infections, affecting approximately 50% of women at one point in their lifetime24, while 10% of women experience recurrent episodes of UTI24. In addition, UTI occur frequently in the pediatric patient population; 8% of girls and 2% of boys under the age of 7 years develop at some time acute pyelonephritis25. In most patients, these infections remain limited to the lower urinary tract and are manifest by asymptomatic bacteriuria only. Although cystitis is usually uncomplicated, the upper urinary tract may become involved by ascending infection. Pyelonephritis is defined as infection of the renal parenchyma and pelvicaliceal system. Acute pyelonephritis is a clinical syndrome characterized by flank pain, chills and fever, and variable symptoms of dysuria, urgency, and frequency26. If left untreated, recurrent pyelonephritis can lead to scarring of the renal parenchyma, which is responsible for up to 24% of children that develop ESRD27. Urinary infections can be caused by several bacterial species, the majority of which belong to normal perineal flora. Although the overwhelming majority of UTI are caused by strains of Escherichia coli (E. coli), most of the remainder are caused by Enterobacter, Enterococcus, Proteus mirabilis, and Klebsiella species. Also Gram-positive organisms, including group B Streptococcus and Staphylococcus saprophyticus are isolated with increasing frequency in elderly, diabetics and immune compromised patients28. Specific serogroups of uropathogenic E. coli are identified as the causative organism in approximately 80% of community-acquired UTI29. Uropathogenic E. coli have a number of virulence factors that facilitate colonization and invasion of urinary epithelium. Some of these include adhesins, such as S-pili, P-fimbriae and type 1-fimbriae, which enhance binding to epithelial cells within the urinary tract30. Other E. coli serogroups express an increase in K antigen production which helps protect the microorganism from leukocyte phagocytosis30. A complete list of identifiable virulence factors is beyond the scope of this introductory chapter. Course and outcome of UTIs depend largely on the balance between bacterial virulence factors and host defense mechanisms. Important, though non-specific, host defense mechanisms against UTI comprise regular bladder voiding, urine components that can act as anti-adherence factors (e.g. Tamm-Horsfall protein), shedding of urothelial cells upon bacterial infestation and antimicrobial peptides such as β-defensin 1, which is expressed by tubular epithelial cells31. Once bacterial adhesion has been achieved, complement activation and rapid production of pro-inflammatory cytokines and chemokines result in recruitment of neutrophils that subsequently ingest the invading microbes32-34. Actual killing of bacteria happens through generation of an oxidative burst by neutrophils. Reactive oxygen species (ROS) are the result of activation of the nicotinamide adenine dinucleotide phosphate (NADPH) oxidase, which is responsible for transferring electrons from NADPH to O2, resulting in the formation of superoxide anion (O2-) and generation of hydrogen peroxide (H2O2)35. Furthermore, neutrophil myeloperoxidase (MPO) has the ability to react with O2- and H2O2 to generate the extremely cytotoxic hypochlorous acid (HOCl), chlorine, chloramines, hydroxyl radicals and ozone (O3)36.
[edit] The Fibrinolytic System
Haemostasis or blood coagulation is essential to sustain vital functions in organisms with a closed high-pressure circulatory system. After injury to the vasculature, the haemostatic system is called into action in order to prevent blood loss. Formation of a fibrin-rich clot at the site of vessel injury is a highly complex process that is orchestrated by the coagulation cascade. This cascade comprises an intricate system of serine proteases that, once activated, leads to the formation of thrombin which subsequently cleaves fibrinogen into fibrin37.
Once tissue integrity has been restored, the now redundant fibrin clots are dissolved by the fibrinolytic system (Figure 2), which thus plays an important role in the maintenance of vascular patency.
[edit] The mechanism of fibrinolysis
The key protease in the fibrinolytic cascade is plasmin (Figure 2), which degrades fibrin into fibrin degradation products (FDPs). Plasmin is formed through conversion of its proenzyme, plasminogen (Figure 2). Two physiological plasminogen activators have been recognized: tissue-type plasminogen activator (tPA) and urokinase-type plasminogen activator (uPA)38. The latter interacts with its own highly specific receptor (uPAR, CD87). Other proteolytic effects of the fibrinolytic system involve the orchestration of tissue remodelling. Plasmin and tPA can activate matrix metalloproteinases (MMP) that have the ability to break down extracellular matrix (ECM) components (Figure 2). In addition, plasmin, uPA and tPA are capable of transforming pro-hepatocyte growth factor (HGF) into active HGF39, as well as activating latent transforming growth factor-β (TGF-β)40. Inhibition of the fibrinolytic system can take place at the level of the plasminogen activators by specific plasminogen activator inhibitors (PAI) or at the level of plasmin or the plasmin-fibrin interaction, by α2-antiplasmin and the thrombin-activatable fibrinolysis inhibitor (TAFI) respectively41. Inhibition of MMP is executed by tissue inhibitors of MMP (TIMP)38.
[edit] Tissue-type plasminogen activator (tPA)
tPA is a two-chain serine protease, that is converted out of its single-chain precursor form by plasmin. In contrast to the single-chain precursors of most serine proteases, single-chain tPA is enzymatically active42. The catalytic site of tPA is involved in most functions of the enzyme, such as fibrin-specific plasminogen activation and binding to endothelial cell receptors. tPA has a rather low affinity for plasminogen in the absence of fibrin. The presence of fibrin enhances the activation rate of plasminogen dramatically43. Also in the vicinity of cell surfaces the activity of tPA is markedly enhanced44. Many cell types, among which monocytes and neutrophils have been described to bind plasminogen activators and plasminogen45. About 95% of tPA in the circulation is bound in an inactivated form to PAI-138. Opposed to its functional cousin uPA, no specific receptor for tPA has been described thus far. tPA can bind non specifically to activation receptors, especially annexin A2, and clearance receptors, most notably the low density lipoprotein receptor-related protein-1 (LRP-1). Annexin A2, is a widely expressed cell membrane Ca2+/phospholipid binding protein that is present on endothelial cells and can bind both tPA and plasminogen, thus enhancing the activation of plasminogen by tPA46. LRP-1 is a scavenging receptor that is critically involved in the clearance of its numerous structurally different ligands but has the ability to act as a signalling receptor as well47. Although tPA-LRP-1 interaction is primarily considered to be a clearance mechanism, binding of tPA to LRP-1 can activate cell signalling cascades in several cell types such as fibroblasts and smooth muscle cells48, 49.
[edit] Urokinase plasminogen activator (uPA) and its receptor (uPAR)
uPA is produced by several cell types, among which endothelial cells, monocytes and macrophages, as single-chain pro-uPA, which needs cleavage by e.g. plasmin to generate the two-chain uPA. Both pro-uPA and uPA display enzymatic activity, although the activator activity of pro-uPA in the presence of fibrin and plasminogen is only 0.4% to 5% that of uPA50. Binding of uPA to uPAR leads to strongly enhanced generation of plasmin, due to higher efficiency of plasminogen activation, the conversion of pro-uPA to uPA by the generated plasmin, and the protection of cell-associated plasmin from inhibition by α2-antiplasmin51. uPAR is a protein of 50 to 60 kD (dependent on its degree of glycosylation), which is anchored to the cell membrane by a glycosylphosphatidylinositol (GPI) moiety. The uPAR molecule is composed of three homologous, differently folded structural domains, of which the N-terminal domain accounts for binding pro-uPA and uPA52. uPAR is expressed in a wide variety of cells and tissues; very few human cell types do not express uPAR, such as non-activated lymphocytes and the HEK-293 cell line53. In addition to cell-bound uPAR, a soluble form of uPAR occurs both in vitro (in the supernatants of cell cultures) and in vivo (in bodily fluids)54. Shedding of uPAR can be mediated by GPI-specific phospholipases. Probably other, hitherto unidentified, proteases may be involved in uPAR shedding as well. With respect to fibrinolysis, an important function of uPAR is to localize uPA at the cell surface, thus provoking pericellular fibrinolysis and proteolysis. In recent years it has become increasingly clear that uPAR is capable of exerting a plethora of cellular responses that include cell differentiation, cell adhesion and cell proliferation in a non-proteolytic fashion53. Some of the manifold functions of uPAR will be addressed further on in this chapter.
[edit] Plasminogen activator inhibitor type 1 (PAI-1)
PAI-1 is a single-chain glycoprotein which belongs to the serpin superfamily. PAI-1 is a specific inhibitor of tPA and uPA, which is present in the circulation under normal circumstances. PAI-1 inhibits tPA and uPA by formation of a 1:1 reversible complex. PAI-1 forms a covalent uPA-PAI-1 complex which inhibits uPA activity55 and is rapidly internalized and degraded56. This process requires low density lipoprotein receptor-related protein (LRP) and uPAR57. Many cells are capable of PAI-1 production, including endothelial cells, platelets, monocytes and adipocytes58. Plasma PAI-1 levels are known to be increased in many different disease states, such as acute myocardial infarction, obesity and diabetes58. Upon vascular injury, platelets become activated, thereby releasing large amounts of PAI-1. This effect is thought to limit fibrinolysis and to stabilize the primary blood clot. Apart from proteinase inhibition, PAI-1 is known to play an important role in regulating cell adhesion and tissue remodelling through its binding to the ECM component vitronectin, thereby competing with integrins and uPAR59. The other physiological plasminogen activator inhibitors, PAI-2 and PAI-3, are beyond the scope of this thesis.
[edit] The fibrinolytic system in inflammation
Until quite recently, the fibrinolytic system was considered to be involved in fibrin homeostasis solely. In recent years however, evidence has accumulated showing that the fibrinolytic system plays a very active role at the intersection between fibrinolysis, inflammation and immunity60. These very diverse functions are at least partly independent from the classical proteolytic tasks of the various components of the fibrinolytic system. On the one hand the fibrinolytic system is required to prevent extracellular fibrin deposition, thus acting as an anti-inflammatory factor. On the other hand it is required to mount proper innate and adaptive immune responses, thus functioning as a pro-inflammatory agent.
[edit] tPA
It has become increasingly clear that tPA can be an active modulator of inflammatory responses in a variety of inflammatory conditions49, 61-68. To this, both pro- and anti-inflammatory effects of tPA have been described, depending on the disease model and target organ. In experimental arthritis, tPA-/- mice showed more severe disease and more neutrophil influx, accompanied by larger fibrin depositions in the joints67, 68. Partly in line with these findings, tPA-/- mice displayed more footpad oedema after local carrageenan injection than wild type (WT) mice. This effect could be abolished by treatment with exogenous tPA69. A comparable anti-inflammatory effect of tPA has been described in a model of IL-1 induced lung oedema70. In a model of cerebral ischemia-reperfusion, tPA-deficient mice exhibited approximately 50% smaller cerebral infarcts than WT mice, whereas administration of exogenous tPA resulted in larger infarcts64. In the abovementioned studies, the effects of tPA seem largely mediated through its plasminogen activating activity. Recently however, it has been shown that tPA has the ability to act as a cytokine; through interaction with LRP-1 tPA can activate signalling cascades in fibroblasts, resulting in gene transcription49. A similar cytokine-like behaviour of tPA has been described in the brain, where tPA mediates activation of microglia (the immunocompetent cells of the central nervous system) by signaling through annexin A271, again independent from its classical proteolytic activity. Recently, Renckens et al. have described a protective role for tPA during E. coli-induced abdominal sepsis. tPA-/- mice showed impaired defense during E. coli peritonitis, caused by a reduced migratory capacity of tPA deficient neutrophils. The protective function of endogenous tPA was independent of plasmin since plasminogen-/- mice displayed the same phenotype as WT mice66.
[edit] uPA and uPAR
The often synergistic roles of uPA and uPAR in inflammatory processes are well-established. The importance of uPA and its receptor in the inflammatory response has been especially studied by means of infectious disease models in uPA(R)-deficient mice, which have demonstrated that uPA and uPAR favour the inflammatory response chiefly by promoting inflammatory cell activation and migration rather than through their fibrinolytic function. In a model of pulmonary Cryptococcus neoformans infection, uPA-/- mice displayed severe impairment of inflammatory cell recruitment towards the site of infection, resulting in uncontrolled disseminated infection, and death72. uPA proved to be obligatory for the generation of a type 1 immune response to pulmonary cryptococcal infection73. On the other hand, uPA requirement depends on the type of microorganism that causes the infection, since uPA-/- mice had an unremarkable host defense during Pseudomonas aeruginosa pneumonia, with similar neutrophil recruitment to the lung as WT mice74. In addition, uPA-/- macrophages and neutrophils have impaired antimicrobial activity in vitro73, 75, whereas uPA-/- T cells show a diminished T cell receptor-mediated lymphocyte proliferation and activation upon stimulation with concanavalin A76. uPAR has the ability to promote inflammatory cell migration in two ways. Apart from its ability to concentrate proteolytic activity on the cell surface77, uPAR can enable cell migration in a non-proteolytic fashion through interaction with several integrins78, most notably the β2-integrins on monocytes and neutrophils79. Interaction of GPI-linked uPAR with transmembrane molecules such as integrins and G protein coupled receptors (GPCRs) results in initiation of various signaling cascades, including PI3K/Akt and MEK/ERK mediated signaling, which leads to cell adhesion, proliferation and migration56. Several studies have shown that uPAR indeed favors inflammatory cell migration; uPAR-/- mice showed less influx of neutrophils in bronchoalveolar lavage fluid during Pseudomonas aeruginosa- and Streptococcus pneumoniae-induced pneumonia74, 80. Furthermore, uPAR-/- mice had attenuated cerebrospinal fluid pleocytosis during pneumococcal meningitis81. In addition, uPAR-/- mice display decreased neutrophil migration into the peritoneal cavity during lipopolysaccharide (LPS)-induced peritonitis82. Also in blistering skin diseases and rheumatoid arthritis uPAR has been implicated in inflammatory cell migration83, 84.
[edit] PAI-1
Besides its role in regulating fibrinolysis, PAI-1 plays a role in a variety of processes dependent on plasmin activity. Studies with transgenic mice have revealed a functional role for PAI-1 in processes that involve ECM turnover such as wound healing, atherosclerosis, and fibrosis. In these disease models, the antiproteolytic activity of PAI-1 results in wound healing delay85, increased atherosclerotic plaque growth58 and increased bleomycin-induced pulmonary fibrosis86. With respect to inflammatory processes, PAI-1 has been shown to act as an acute phase protein, with marked elevation of plasma PAI-1 levels upon surgery, myocardial infarction and during sepsis87, 88. Enhanced plasma levels of PAI-1 account for the so-called fibrinolytic shut down during sepsis, that is partly responsible for the occurrence of diffuse intravascular coagulation (DIC)88. In addition, in sepsis patients PAI-1 activity is a sensitive predictor of lethality89, 90 and the relatively common functional insertion/deletion (4G/5G) polymorphism in the promoter region of the PAI-1 gene, which causes elevated levels of PAI-1 in the circulation, is associated with poor outcome of meningococcal sepsis91. PAI-1 regulates (inflammatory) cell migration by three separate, although not mutually exclusive pathways: 1) modulating ECM degradation via regulation of plasmin levels; 2) regulating cell adhesion; and 3) modifying the formation, or maintenance, of chemoattractant gradients92. PAI-1 can inhibit cell bound uPA, resulting in reduced pericellular proteolysis and a subsequent decrease in cell migration93. Furthermore, PAI-1 inhibits integrin- and vitronectin-mediated cell migration independently of its function as an inhibitor of plasminogen activation, by competing for vitronectin binding to integrins59. On the other hand, the de-adhesive action of PAI-1 by inactivation of the cell-integrin-ECM interaction, may result in an increase of cell mobility as well94. Recently it has been shown that PAI-1 in macrophages functions as a master switch between cell adhesion and detachment by inducing internalization of the CD11b/CD18-LRP complex, thus promoting macrophage migration95. In addition, PAI-1 acts in vitro as a chemotactic factor for macrophages96. Another mechanism via which PAI-1 can influence inflammatory cell migration, is the modification of chemotactic gradients92. PAI-1 inhibits the constitutive shedding of endothelial IL-8-heparan sulphate-syndecan-1 complexes, resulting in enhanced IL-8-mediated transendothelial neutrophil migration97. Which one of the abovementioned mechanisms predominates during inflammatory processes, depends strongly on the cellular milieu and the applied experimental set up. Recently it was demonstrated that PAI-1 not only influences inflammatory cell migration, but neutrophil activity as well. Kwak et al. described that addition of PAI-1 to LPS-stimulated neutrophils results in enhanced nuclear translocation of NF-κB and increased production of the proinflammatory cytokines IL-1β, TNF-α and MIP-2, in a JNK dependent fashion98, providing further evidence for the pro-inflammatory properties of PAI-1.
[edit] The fibrinolytic system and the kidney
Several studies address the role of the fibrinolytic system in a number of kidney disorders. Most of these reports concern renal diseases in which fibrin is thought to be of pathogenetic importance, such as crescentic glomerulonephritis.
[edit] tPA
In the kidney, tPA is constitutively expressed by endothelial cells, glomerular cells (among which mesangial cells and podocytes) and epithelial cells of the distal collecting duct99. As the principle plasminogen activator within the glomerular compartment100, tPA influences the course of proliferative glomerulonephritis, a disease that invariably is accompanied by glomerular fibrin and matrix deposition101. In the model of anti-glomerular basement membrane (GBM) antibody-mediated crescentic glomerulonephritis, tPA-/- mice display more glomerular fibrin deposits, larger crescents, higher glomerular macrophage counts and higher serum creatinine values than WT mice61. Plasminogen-/- mice had a similar phenotype, indicating that the protective role of tPA in this disease model depends on its role as plasminogen activator61. Conversely, injecting recombinant tPA in this model decreased fibrin deposits and crescent formation and enhanced renal function in rabbits102. In the rat mesangial proliferative model of anti-Thy-1 antibody-mediated glomerulonephritis, recombinant tPA increases plasmin generation, with subsequent decrease in matrix accumulation and proteinuria103. Besides its protective role in glomerular inflammation, there is mounting evidence for a deleterious role of tPA in the tubulointerstitial compartment. In obstructive nephropathy, characterized by extensive tubulointerstitial damage and fibrosis, tPA-/- mice show a reduced deposition of interstitial collagen and fibronectin than WT mice104. tPA deficiency leads to decreased MMP-9 production during obstructive nephopathy, which results in preservation of tubular basement membrane integrity, with less epithelial-to-myofibroblast transition and therefore less fibrosis104. Recently, Hu et al. showed that tPA induces MMP-9 gene expression and protein secretion by renal interstitial fibroblasts49. This effect is independent of the proteolytic activity of tPA, since both WT and non-enzymatic mutant tPA were found to induce MMP-9 expression. As shown by Hu et al., tPA-LRP-1 interaction results in tyrosine phosphorylation on the beta subunit of LRP-1, which is followed by MAPK activation and subsequent MMP-9 gene expression49. In this way tPA has the ability to act, independent from plasmin generation, in a cytokine-like manner.
[edit] uPA and uPAR
Fibrinolytic activity of human urine has been recognized as early as 1951105. This phenomenon is attributable to large quantities (40 to 80 μg/L) of urokinase plasminogen activator (uPA) in urine, hence the name of this protease. Opposed to tPA, uPA is normally not produced in the glomerulus, but in abundant quantities by TEC99. It is therefore not surprising that uPA plays a limited role in fibrin-mediated glomerular disease. Although uPA-/- and uPAR-/- mice have lower glomerular influx of macrophages in the model of anti-GBM crescentic glomerulonephritis, the extent of fibrin deposits, crescent formation and decrease of renal function is similar as in WT mice61. Although the primary physiological role of uPA within the kidney is unknown, it has been suggested that tubular uPA is involved in sustaining tubular patency by dissolving intraluminal proteinaceous material101, 106. Indeed, urolithiasis patients have lower levels of urinary uPA levels, suggesting a preventive role of uPA in kidney stone formation107. Although tubular uPA expression is up-regulated during obstructive nephropathy96 and uPA theoretically could influence fibrosis through activation of HGF39, 108 and MMPs109, uPA does not influence the course of this disease110. In the kidney, uPAR is expressed by glomerular epithelial and mesangial cells and TEC throughout the nephron111. During crescentic glomerulonephritis, glomerular uPAR expression is up-regulated112. As mentioned above, uPAR directs inflammatory cell migration into the glomerulus in this disease model without influencing course or outcome61. Interaction between uPA and uPAR results in complement anaphylatoxin C5a receptor (C5aR) production by mesangial cells in vitro113. In vivo, intraperitoneal administration of LPS in mice causes uPAR-mediated mesangial C5aR expression, however without a discernible inflammatory response or neutrophil influx113. Also in human thrombotic microangiopathy114 and in streptozotocin-induced diabetic glomerulopathy in rats115, glomerular uPAR expression is elevated. In the tubular compartment, elevated uPAR expression by TEC has been reported in chronic allograft rejection116, acute pyelonephritis117 and obstructive nephropathy118, 119. During chronic obstructive nephropathy, uPAR-/- mice experience accelerated development of renal fibrosis, associated with higher PAI-1 levels and lower plasminogen activator activity118, 119.
[edit] PAI-1
PAI-1 is normally only produced in trace amounts in the kidney, but significant up-regulation of its expression has been reported in a wide variety of both acute and chronic kidney diseases, including thrombotic microangiopathy114, crescentic glomerulonephritis120, diabetic nephropathy121, focal segmental glomerulonephritis (FSGS)122, membranous nephropathy123 and chronic allograft nephropathy116, 124. The presence of fibrin thrombi in the microvasculature, including renal arterioles and glomerular capillaries, forms the hallmark of thrombotic microangiopathy125. In renal biopsies with features of thrombotic microangiopathy, PAI-1 is expressed in glomeruli and in arteriolar walls114. In addition, children infected with E. coli O157:H7, who subsequently develop the haemolytic-uremic syndrome (a form of thrombotic microangiopathy) have significantly higher plasma concentrations of PAI-1 and tPA/PAI-1 complex than children with uncomplicated infection126. In experimental crescentic glomerulonephritis, PAI-1 production results in fibrin deposition, macrophage influx and glomerular injury127. Indeed, PAI-1-/- mice develop fewer crescents than WT mice, whereas PAI-1 overexpressing mice have increased glomerular crescent formation, more glomerular fibrin deposition and increased numbers of infiltrating leukocytes128. In addition to these acute renal disorders, the anti-proteolytic agent PAI-1 has been implicated in a number of chronic kidney diseases, that are associated with matrix accumulation and fibrosis. In diabetic nephropathy, classic Kimmelstiel-Wilson nodules contain PAI-1 protein121. Opposed to human diabetes, currently available mouse models for diabetic nephropathy do not result in overt glomerular sclerosis. However, in streptozotocin induced diabetes, PAI-1-/- mice have reduced levels of fibronectin and lower albuminuria than WT mice129. In addition, spontaneously diabetic PAI-1-/-db/db mice, generated by crossbreeding PAI-1-/- mice with heterozygous leptin receptor deficient db/+ mice, display lower albuminuria and collagen accumulation than PAI-1+/+db/db mice130. In obstructive nephropathy, PAI-1-/- mice are protected against the development of interstitial fibrosis, with lower accumulation of interstitial macrophages and myofibroblasts96. PAI-1 overexpressing mice display the opposite phenotype in this model131. Strikingly, plasmin is harmful rather than protective during obstructive nephropathy132, suggesting that the pro-fibrotic action of PAI-1 occurs independent from plasmin inhibition, but through the promotion of macrophage and myofibroblast migration instead.
[edit] References
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