Nancy A. Noble

Transforming Growth Factor β in Tissue Fibrosis

Progressive fibrosis in the kidney, liver, lung, heart, bone marrow, and skin is both a major cause of suffering and death and an important contributor to the cost of health care. All of this is li...

Angiotensin II stimulates extracellular matrix protein synthesis through induction of transforming growth factor-beta expression in rat glomerular mesangial cells.

Angiotensin II (Ang II) has been implicated in the development of progressive glomerulosclerosis, but the precise mechanism of this effect remains unclear. In an experimental model, we have shown previously that TGF-beta plays a key role in glomerulosclerosis by stimulating extracellular matrix protein synthesis, increasing matrix protein receptors, and altering protease/protease-inhibitor balance, thereby inhibiting matrix degradation. We hypothesized that Ang II contributes to glomerulosclerosis through induction of TGF-beta. Ang II treatment of rat mesangial cells in culture increased TGF-beta and matrix components biglycan, fibronectin, and collagen type I at both the mRNA and protein levels in a time- and dose-dependent manner. Saralasin, a competitive inhibitor of Ang II, prevented the stimulation. Ang II also promoted conversion of latent TGF-beta to the biologically active form. Coincubation of mesangial cells with Ang II and neutralizing antibody to TGF-beta blocked the Ang II-induced increases in matrix protein expression. Continuous in vivo administration of Ang II to normal rats for 7 d resulted in 70% increases in glomerular mRNA for both TGF-beta and collagen type I. These results indicate that Ang II induces mesangial cell synthesis of matrix proteins and show that these effects are mediated by Ang II induction of TGF-beta expression. This mechanism may well contribute to glomerulosclerosis in vivo.

Interactions of Transforming Growth Factor-β and Angiotensin II in Renal Fibrosis

Overproduction of transforming growth factor-beta clearly underlies tissue fibrosis in numerous experimental and human diseases. Transforming growth factor-betas powerful fibrogenic action results from simultaneous stimulation of matrix protein synthesis, inhibition of matrix degradation, and enhanced integrin expression that facilitates matrix assembly. In animals, overexpression of transforming growth factor-beta by intravenous injection, transient gene transfer, or transgene insertion has shown that the kidney is highly susceptible to rapid fibrosis. The same seems true in human disease, where excessive transforming growth factor-beta has been demonstrated in glomerulonephritis, diabetic nephropathy, and hypertensive glomerular injury. A possible explanation for the kidneys particular susceptibility to fibrosis may be the recent discovery of biologically complex interactions between the renin-angiotensin system and transforming growth factor-beta. Alterations in glomerular hemodynamics can activate both the renin-angiotensin system and transforming growth factor-beta. Components of the renin-angiotensin system act to further stimulate production of transforming growth factor-beta and plasminogen activator inhibitor leading to rapid matrix accumulation. In volume depletion, transforming growth factor-beta is released from juxtaglomerular cells and may act synergistically with angiotensin II to accentuate vasoconstriction and acute renal failure. Interaction of the renin-angiotensin system and transforming growth factor-beta has important clinical implications. The protective effect of inhibition of the renin-angiotensin system in experimental and human kidney diseases correlates closely with the suppression of transforming growth factor-beta production. This suggests that transforming growth factor-beta, in addition to blood pressure, should be a therapeutic target. Higher doses or different combinations of drugs that block the renin-angiotensin system or entirely new drug strategies may be needed to achieve a greater antifibrotic effect.

Cytokines in Kidney Disease: The Role of Transforming Growth Factor-β

Cytokines such as transforming growth factor-beta (TGF-beta) are peptide factors that regulate embryogenesis, development, inflammation, tissue repair, and carcinogenesis. Growing evidence indicates that dysregulation of cytokine actions may underlie the pathogenesis of serious autoimmune, degenerative, and fibrotic diseases. Studies in a model of acute mesangial proliferative glomerulonephritis show that overproduction of TGF-beta is the cause of pathologic accumulation of extracellular matrix in the nephritic glomeruli. Transforming growth factor-beta acts to increase matrix production, inhibit matrix degradation, and modulate matrix receptors in the glomerulonephritic rats. It may also play a role in the glomerular matrix build-up that is a central feature of diabetic nephropathy. Elevated expression of TGF-beta mRNA and TGF-beta protein were found in the glomeruli of diabetic rats along with increased levels of proteoglycans and other matrix components that are known to be induced by TGF-beta. The study of human diabetic glomeruli has also showed markedly elevated levels of TGF-beta protein. Glomeruli from normal kidneys and nonprogressive kidney disorders were negative. The striking ability of TGF-beta to cause exuberant matrix formation may be due to the fact that TGF-beta can induce its own production by resident cells at a site of injury. Thus, the potential for TGF-beta to do harm may be due to this autoinduction mechanism whereby TGF-beta expression can become chronic, creating a vicious circle. As the role that TGF-beta plays in chronic fibrotic diseases becomes better understood, it is likely that TGF-beta inhibitors will become important future drugs for treating these conditions.(ABSTRACT TRUNCATED AT 250 WORDS)

A mutant, noninhibitory plasminogen activator inhibitor type 1 decreases matrix accumulation in experimental glomerulonephritis

In fibrotic renal disease, elevated TGF-beta and angiotensin II lead to increased plasminogen activator inhibitor type 1 (PAI-1). PAI-1 appears to reduce glomerular mesangial matrix turnover by inhibiting plasminogen activators, thereby decreasing plasmin generation and plasmin-mediated matrix degradation. We hypothesized that therapy with a mutant human PAI-1 (PAI-1R) that binds to matrix vitronectin but does not inhibit plasminogen activators, would enhance plasmin generation, increase matrix turnover, and decrease matrix accumulation in experimental glomerulonephritis. Three experimental groups included normal, untreated disease control, and PAI-1R-treated nephritic rats. Plasmin generation by isolated day 3 glomeruli was dramatically decreased by 69%, a decrease that was reversed 43% (P < 0.02) by in vivo PAI-1R treatment. At day 6, animals treated with PAI-1R showed significant reductions in proteinuria (48%, P < 0.02), glomerular staining for periodic acid-Schiff positive material (33%, P < 0.02), collagen I (28%, P < 0.01), collagen III (34%, P < 0.01), fibronectin (48%, P < 0.01), and laminin (41%, P < 0.01), and in collagen I (P < 0.01) and fibronectin mRNA levels (P < 0.02). Treatment did not alter overexpression of TGF-beta1 and PAI-1 mRNAs, although TGF-beta1 protein was significantly reduced. These observations strongly support our hypothesis that PAI-1R reduces glomerulosclerosis by competing with endogenous PAI-1, restoring plasmin generation, inhibiting inflammatory cell infiltration, decreasing local TGF-beta1 concentration, and reducing matrix accumulation.

In vivo interactions of TGF-β and extracellular matrix

TGF-beta, a multifunctional cytokine, plays an important role in embryogenesis and in regulating repair and remodeling following tissue injury. Many of the biological actions of TGF-beta are mediated by widespread effects on deposition of extracellular matrix. TGF-beta stimulates the synthesis of individual matrix components including proteoglycans, collagens and glycoproteins. TGF-beta also blocks matrix degradation by decreasing the synthesis of proteases and increasing the synthesis of protease inhibitors. Finally, TGF-beta increases the synthesis of matrix receptors and alters their relative proportions on the surface of cells in a manner that could facilitate adhesion to matrix. All of these events have largely been demonstrated in vitro in cultured cells. In an experimental model of glomerulonephritis we have shown that TGF-beta is responsible for the accumulation of pathological matrix in the glomeruli following immunological injury. Furthermore, all three of TGF-betas actions on extracellular matrix--increased synthesis, decreased degradation and modulation of receptors--have now been documented to be involved in matrix deposition in vivo in this model. Administration of the proteoglycan decorin suppressed TGF-beta-induced matrix deposition in the nephritic glomeruli, thus confirming a physiological role for decorin as a regulator of TGF-beta. Inhibitors of TGF-beta may be important future drugs in treating fibrotic diseases caused by overproduction of TGF-beta.

A PAI-1 Mutant, PAI-1R, Slows Progression of Diabetic Nephropathy

Plasminogen activator inhibitor-1 (PAI-1) has been implicated in renal fibrosis. In vitro, PAI-1 inhibits plasmin generation, and this decreases mesangial extracellular matrix turnover. PAI-1R, a mutant PAI-1, increases glomerular plasmin generation, reverses PAI-1 inhibition of matrix degradation, and reduces disease in experimental glomerulonephritis. This study sought to determine whether short-term administration of PAI-1R could slow the progression of glomerulosclerosis in the db/db mouse, a model of type 2 diabetes in which mesangial matrix accumulation is evident by 20 wk of age. Untreated uninephrectomized db/db mice developed progressive albuminuria and mesangial matrix expansion between weeks 20 and 22, associated with increased renal mRNA encoding alpha1(I) and (IV) collagens and fibronectin. Treatment with PAI-1R prevented these changes without affecting body weight, blood glucose, glycosylated hemoglobin, creatinine, or creatinine clearance; therefore, PAI-1R may prevent progression of glomerulosclerosis in type 2 diabetes.

Infusion of angiotensin-(1–7) reduces glomerulosclerosis through counteracting angiotensin II in experimental glomerulonephritis

Recent identification of a counterregulatory axis of the renin-angiotensin system, called angiotensin-converting enzyme 2-angiotensin-(1-7) [ANG-(1-7)]-Mas receptor, may offer new targets for the treatment of renal fibrosis. We hypothesized that therapy with ANG-(1-7) would improve glomerulosclerosis through counteracting ANG II in experimental glomerulonephritis. Disease was induced in rats with the monoclonal anti-Thy-1 antibody, OX-7. Based on a three-dose pilot study, 576 microg x kg(-1) x day(-1) ANG-(1-7) was continuously infused from day 1 using osmotic pumps. Measures of glomerulosclerosis include semiquantitative scoring of matrix proteins stained for periodic acid Schiff, collagen I, and fibronectin EDA+ (FN). ANG-(1-7) treatment reduced disease-induced increases in proteinuria by 75%, glomerular periodic acid Schiff staining by 48%, collagen I by 24%, and FN by 25%. The dramatic increases in transforming growth factor-beta1, plasminogen activator inhibitor-1, FN, and collagen I mRNAs seen in disease control animals compared with normal rats were all significantly reduced by ANG-(1-7) administration (P < 0.05). These observations support our hypothesis that ANG-(1-7) has therapeutic potential for reversing glomerulosclerosis. Several results suggest ANG-(1-7) acts by counteracting ANG II effects: 1) renin expression in ANG-(1-7)-treated rats was dramatically increased as it is with ANG II blockade therapy; and 2) in vitro data indicate that ANG II-induced increases in mesangial cell proliferation and plasminogen activator inhibitor-1 overexpression are inhibited by ANG-(1-7) via its binding to a specific receptor known as Mas.

Therapeutic strategies to halt renal fibrosis

Angiotensin II blockade has become a standard anti-fibrotic therapy in renal diseases because it slows progression to end-stage renal disease. However, current data support the notion that angiotensin II blockade alone cannot stop progressive fibrotic disease. Of an increasing number of therapies showing efficacy in animal studies, antibodies to transforming growth factor beta are the most thoroughly studied and are likely to be effective in human clinical trials. However, hints exist in the literature suggesting that no single agent will effectively halt renal fibrosis and that combinations of agents will be required.

Transforming growth factor-β in human glomerular injury

Overexpression of the cytokine transforming growth factor-beta has been identified as the key mediator of chronic progressive kidney fibrosis in experimental and human kidney diseases. The renoprotective effects of angiotensin II blockade and low-protein diets have recently been linked to downregulation of transforming growth factor-beta production. Neutralizing the actions of transforming growth factor-beta by decorin gene therapy appears to be a highly effective new approach in the treatment of renal fibrosis.