Plasminogen Activator Inhibitor-1 in Chronic Kidney Disease (2006)

In 1984, Loskutoff et al. (1) purified plasminogen activator inhibitor-1 (PAI-1) from conditioned media of cultured endothelial cells. This 50-kd glycoprotein is the primary physiologic inhibitor of the serine proteases tissue-type and urokinase-type plasminogen activators (tPA and uPA, respectively). It now is known to mediate important biologic activities that extend far beyond fibrinolysis through interactions with its co-factor, vitronectin (also known as protein S), and with the urokinase receptor (uPAR) and its co-receptors (2,3). Plasma PAI-1 levels increase in response to stress as an acute-phase protein. Usually present in trace amounts, plasma PAI-1 levels increase in several chronic inflammatory states that are associated with chronic kidney disease (CKD), and it may contribute to the pathogenesis of the accelerated vascular disease in this patient population (4–7). Liver and adipose tissue seem to be the primary sources of plasma PAI-1 (8,9). Other inhibitors of plasminogen activation exist; protease nexin-1 and α-2 anti-plasmin can be produced by the kidney. Although PAI-1 normally is not produced in kidneys, synthesis by both resident and intrarenal inflammatory cells occurs in several acute and chronic disease states (Table 1). In the past decade, a growing body of experimental evidence that has derived largely from animal models supports the view that PAI-1 is a powerful fibrosis-promoting molecule and is a promising therapeutic target for new drugs and biologics to combat the current CKD epidemic (Table 2). Exactly how PAI-1 promotes renal fibrosis is not understood completely. Recent studies suggest that in addition to its ability to inhibit serine protease activity within vascular and extracellular compartments, PAI-1 directly modulates cellular behavior, leading to a vicious cycle of inflammatory cell recruitment, fibroblast activation, and scar tissue accumulation. PAI-1 Is Present in Most Aggressive Kidney Diseases Acute/Thrombotic Diseases Thrombotic microangiopathy (TMA) is a pathologic lesion that is characterized by fibrin deposition in the microvasculature, often involving glomeruli and renal arterioles. TMA characterizes renal diseases that are caused by hemolytic uremic syndrome, preeclampsia, scleroderma, malignant hypertension, and the antiphospholipid antibody syndrome. Glomerular PAI-1 deposition is a feature of TMA (10). In children who have Escherichia coli 0157:H7 infection and later develop hemolytic uremic syndrome (11), plasma PAI-1 levels increase before the onset of renal disease. Plasma PAI-1 activity correlates with renal disease severity and long-term outcome (12–15). One pediatric study found that PAI-1 activity also increases during acute renal failure of other causes (16). Animal hemolytic uremic syndrome models that should provide specific insights into the pathogenetic role of PAI-1 are under development (17,18). The extent to which renal PAI-1 is produced locally or derived from the plasma pool has not been determined. PAI-1 expression is a feature of preeclampsia (19) and scleroderma, although experimental data suggest that PAI-1 is not essential in the latter (20,21). Radiation nephropathy is characterized in its early phase by TMA with PAI-1 generation, and it often progresses to sclerosis. Angiotensin or aldosterone inhibition reduces PAI-1 levels and the severity of glomerular sclerosis in experimental radiation nephropathy (22,23). Crescentic Glomerulonephritis Crescents develop as a result of segmental breaks of the glomerular basement membrane (GBM), often in association with fibrinoid necrosis. In human crescentic glomerulonephritis, PAI-1 is detected both in areas of glomerular necrosis and in crescents (24,25). Parietal epithelial cells are a source of PAI-1 and uPA in human crescentic glomerulonephritis (26). In experimental models of anti-GBM crescentic nephritis, PAI-1 is produced, whereas tPA levels are suppressed, leading to decreased net glomerular fibrinolytic activity and prolonged fibrin deposition (27). A functional role for PAI-1 in the pathogenesis of anti-GBM crescentic glomerulonephritis was established by elegant studies in genetically engineered mice (Table 2) (28). The PAI-1−/− mice developed fewer glomerular crescents and glomerular fibrin deposits and reduced collagen accumulation long term. In contrast, mice that were engineered to overexpress PAI-1 formed more crescents along with more extensive fibrin deposits and extensive collagen accumulation. Genetic manipulations that reduce serine protease activity, either plasminogen deficiency or combined uPA and tPA deficiency, also lead to aggressive injury with more extensive crescents, necrosis, and fibrin deposition (29). Isolated tPA deficiency causes glomerular injury that is intermediate between wild-type and combination PA knockout mice, whereas isolated uPA deficiency results in fewer glomerular macrophages, but, otherwise, disease severity is similar to the wild-type mice, suggesting that tPA is the primary glomerular plasminogen activator. A different outcome was observed in a passive model of anti-GBM glomerulonephritis, in which PAI-1−/− mice developed more severe renal injury that was attributed to plasminogen activator–dependent activation of TGF-β (30). It was postulated further that enhanced TGF-β activity influenced CD4+ T cell responses, leading to disease exacerbation. It is possible that differences in the mouse strain may account for some of these divergent observations in the genetically engineered mouse studies. It is remarkable, however, that whenever renal plasmin activity is measured in these mice, it is not found to differ significantly from that of wild-type mice. Proliferative Glomerulonephritis PAI-1 mRNA and protein are increased in both human and animal models of proliferative glomerulonephritis, particularly when lesions are severe with fibrin and crescent formation (31). In human disease, increased PAI-1 levels correlate with level of proteinuria rather than with degree of proliferation. In a study of 80 patients, the highest glomerular PAI-1 levels were found in FSGS and membranous nephropathy (32). In the rat Habu snake venom proliferative glomerulonephritis model, PAI-1 is increased and localized to mesangial lesions at early time points, suggesting a possible role of PAI-1 in cell migration (33). In the rat mesangial proliferative model of anti–Thy-1 antibody–mediated glomerulonephritis, PAI-1, along with growth factors such as TGF-β and PDGF, are increased, and interventions that decrease injury also lower PAI-1 (34,35). Conversely, injecting recombinant tPA in this rat model increases plasmin generation, with subsequent decrease in matrix accumulation (36). PAI-1 inhibition by a mutant PAI-1 that binds matrix vitronectin but does not inhibit plasminogen activator results in significant reduction in extracellular matrix (ECM) accumulation (37). This treatment also was linked to increased glomerular plasmin activity and enhanced ECM degradation. However, mRNA levels for genes encoding ECM proteins also were decreased, perhaps as a consequence of decreased macrophage infiltration and plasmin-independent effects. PAI-1 in Chronic Progressive Renal Disease In addition to its effects on fibrinolysis that may promote thrombotic and necrotizing renal lesions, PAI-1 has complex interactions with matrix proteins that enhance matrix accumulation in several glomerular disease states (38–40). In humans, PAI-1 is prominent in atherosclerotic lesions and in sclerotic glomeruli that are damaged as a consequence of hypertensive nephrosclerosis, diabetic nephropathy, and chronic allograft nephropathy. Plasma PAI-1 levels are increased in patients with insulin resistance and obesity as well as overt diabetes (38–40). Within the kidney, PAI-1 protein is prominent in Kimmelstiel-Wilson nodules, often associated with fragmented red blood cells in regions of local injury and mesangiolysis (41). Because adipose tissue is an important source of PAI-1, it also may be important in the genesis of the nephropathy of obesity that may develop even in the absence of diabetes (42). Although rodent models of diabetes do not develop robust glomerular sclerosis or interstitial fibrosis, there is evidence to support a role for PAI-1 in the pathogenesis of diabetic nephropathy. In the mouse model of mild diabetic injury that is induced by streptozotocin injection, PAI-1−/− mice have reduced albuminuria and fibronectin levels compared with wild-type diabetic mice (43). In the db/db mouse model, PAI-1 deficiency also was associated with reduced albuminuria and kidney collagen levels, but a high mortality rate was observed (44). In early streptozotocin-induced nephropathy in rats, spironolactone therapy reduced PAI-1 and TGF-β expression levels and matrix deposition (45). In another study, PAI-1−/− mice failed to develop the diabetic phenotype that was induced in wild-type mice by feeding a high-fat diet (46). Furthermore, PAI-1−/− adipocytes were functionally distinct. They produced lower levels of several hormones that have been identified as key mediators of the dysmetabolic syndrome, including resistin, peroxisome proliferator–activated receptor-γ, and adiponectin (47). These studies suggest complex roles for PAI-1 in the pathogenesis of diabetes and its complications. PAI-1 also is increased in most experimental models of glomerulosclerosis, including cyclosporin-induced glomerulosclerosis, radiation nephropathy, various models of FSGS, chronic anti–Thy-1 nephritis, diabetic nephropathy, aging, and chronic allograft nephropathy (38,48). Two powerful fibrogenic systems are potent PAI-1 inducers: TGF-β and the renin-angiotensinaldosterone system. Because angiotensin, PAI-1, and TGF-β often are coexpressed in chronic renal disease, it has been challenging to decipher the independent contributions of each molecule to the sclerotic process (49). However, in the anti–Thy-1 model, combined TGF-β and angiotensin inhibition reduces PAI-1 expression and glomerular matrix accumulation and is more effective than either therapy alone (35). Studies in the β6 knockout mice provide additional insights. The αvβ6 integrin is involved in TGF-β activation. β6−/− mice are protected from fibrosis after unilateral ureteral obstruction (UUO) and do not show increased active TGF-β and PAI-1 like the wild-type mice (50,51). However, PAI-1 expression and fibrosis, but not TGF-β, are restored to wild-type levels by infusing angiotensin, supporting a direct link among angiotensin, PAI-1, and fibrosis. Molecular studies indicate that angiotensin-dependent, TGF-β–independent induction of PAI-1 gene expression involves the AT1 receptor and a glucocorticoid response element in the PAI-1 promoter (40,52). AT1 receptor blockade but not nonspecific antihypertensive treatment reduces renal PAI-1 expression that is induced by angiotensin II (AngII) infusion (53). Aldosterone also enhances angiotensin-induced PAI-1 expression. In a small study of hypertensive human kidney transplant recipients, elevated plasma PAI-1 levels were decreased by treatment with angiotensin receptor blockers but not by nifedipine (54). Additional CKD models are remarkably attenuated when PAI-1 is absent (Table 2). Sclerosis-prone 129Sv mice that are bred to generate a PAI-1 null genotype are completely protected from glomerular scarring and tubulointerstitial fibrosis after 5/6 nephrectomy (Figure 1) (55). In vitro, PAI-1−/− podocytes synthesize less collagen in response to AngII than wild-type cells (56). PAI-1 deficiency also reduces vascular sclerotic lesions that develop in wild-type mice that are exposed to AngII and salt loading or nitric oxide inhibition by L-NAME (57–59). PAI-1 deficiency protects against interstitial fibrosis that is induced by protein overload (60). In the UUO model, PAI-1−/− mice have reduced renal collagen, whereas PAI-1 mice develop fibrosis (Figure 2) PAI-1 levels are increased in mice that develop these TGF-β mice are bred with PAI-1−/− mice, both glomerular and interstitial matrix deposition are reduced of these in studies an that the effects that are associated with PAI-1 deficiency often differences in renal protease activity, whereas the of inflammatory and were during the early response to Renal It was that early renal and may even has been in glomerular diseases that are associated with of the mesangial matrix the acute rat glomerulonephritis and in models of sclerosis In the 5/6 nephrectomy model inhibitors of angiotensin or aldosterone alone or in combination significantly decrease and This is linked to decreased glomerular PAI-1 expression (Figure results are in rats, and glomerular sclerosis are by high AT1 receptor blockade and are associated with decreased PAI-1 levels In the 5/6 nephrectomy model, is associated with increased plasmin activity The of sclerotic glomeruli from that of the mesangial matrix and to be more to perhaps is not in 5/6 that are with The also to the interstitial matrix may during acute diseases such as acute necrosis and syndrome, whereas interstitial fibrosis that is induced by chronic and injury is more to and a mouse UUO study evidence of interstitial matrix when UUO was after The specific that interstitial and glomerular deposits of matrix proteins to be but it is to that PAI-1 be during fibrosis and during of PAI-1 PAI-1 is as a that is seem to be the cell that is of its In PAI-1 in the PAI-1 to a more molecule it with of its The matrix molecule tPA or uPA, or protein a and receptor also known as the α-2 receptor PAI-1 to its when a in the tPA and uPA accumulation at of renal injury may active PAI-1 as a result of The of PAI-1 in tissue is to on vitronectin is PAI-1 expression is and may be induced in a of cells. growth and factors that are in kidney disease pathogenesis have been to PAI-1 expression in identified PAI-1 protein and proteins and have been to decrease PAI-1 expression of PAI-1 Although plasminogen is not produced in the kidney, plasma plasminogen can to plasmin is by or of the plasminogen (Figure The active is to fibrin and thrombotic activity is particularly important within vascular also may be for crescent formation is the extent to which within renal mesangial and interstitial and its in such scar In contrast, fibrin as an early has been an important to fibrosis. However, the severity of fibrosis is not attenuated in mice, that fibrin does not seem to be essential for in the studies have not been to a role in kidney disease. 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It also that of gene such as of other protease may account for some of the current evidence that biologic effects beyond protease inhibition contribute to the ability of PAI-1 to promote fibrosis. 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