Cellular and molecular mechanisms of renal fibrosis

Renal fibrosis, particularly tubulointerstitial fibrosis, is the common final outcome of almost all progressive chronic kidney diseases. Renal fibrosis is also a reliable predictor of prognosis and a major determinant of renal insufficiency. Irrespective of the initial causes, renal fibrogenesis is a dynamic and converging process that consists of four overlapping phases: priming, activation, execution and progression. Nonresolving inflammation after a sustained injury sets up the fibrogenic stage (priming) and triggers the activation and expansion of matrix-producing cells from multiple sources through diverse mechanisms, including activation of interstitial fibroblasts and pericytes, phenotypic conversion of tubular epithelial and endothelial cells and recruitment of circulating fibrocytes. Upon activation, matrix-producing cells assemble a multicomponent, integrin-associated protein complex that integrates input from various fibrogenic signals and orchestrates the production of matrix components and their extracellular assembly. Multiple cellular and molecular events, such as tubular atrophy, microvascular rarefaction and tissue hypoxia, promote scar formation and ensure a vicious progression to end-stage kidney failure. This Review outlines our current understanding of the cellular and molecular mechanisms of renal fibrosis, which could offer novel insights into the development of new therapeutic strategies.

Epithelial to mesenchymal transition in renal fibrogenesis: pathologic significance, molecular mechanism, and therapeutic intervention

Mature tubular epithelial cells in adult kidney can undergo epithelial-to-mesenchymal transition (EMT), a phenotypic conversion that is fundamentally linked to the pathogenesis of renal interstitial fibrosis. Emerging evidence indicates that a large proportion of interstitial fibroblasts are actually originated from tubular epithelial cells via EMT in diseased kidney. Moreover, selective blockade of EMT in a mouse genetic model dramatically reduces fibrotic lesions after obstructive injury, underscoring a definite importance of EMT in renal fibrogenesis. Tubular EMT is proposed as an orchestrated, highly regulated process that consists of four key steps: (1) loss of epithelial cell adhesion; (2) de novo alpha-smooth muscle actin expression and actin reorganization; (3) disruption of tubular basement membrane; and (4) enhanced cell migration and invasion. Of the many factors that regulate EMT in different ways, transforming growth factor-beta1 is the most potent inducer that is capable of initiating and completing the entire EMT course, whereas hepatocyte growth factor and bone morphogenetic protein-7 act as EMT inhibitors both in vitro and in vivo. Multiple intracellular signaling pathways have been implicated in mediating EMT, in which Smad/integrin-linked kinase may play a central role. This article attempts to provide a comprehensive review of recent advances on understanding the pathologic significance, molecular mechanism, and therapeutic intervention of EMT in the setting of chronic renal fibrosis.

Renal fibrosis: new insights into the pathogenesis and therapeutics

Renal fibrosis is the inevitable consequence of an excessive accumulation of extracellular matrix that occurs in virtually every type of chronic kidney disease. The pathogenesis of renal fibrosis is a progressive process that ultimately leads to end-stage renal failure, a devastating disorder that requires dialysis or kidney transplantation. In a simplistic view, renal fibrosis represents a failed wound-healing process of the kidney tissue after chronic, sustained injury. Several cellular pathways, including mesangial and fibroblast activation as well as tubular epithelial-mesenchymal transition, have been identified as the major avenues for the generation of the matrix-producing cells in diseased conditions. Among the many fibrogenic factors that regulate renal fibrotic process, transforming growth factor-beta (TGF-beta) is one that plays a central role. Although defective matrix degradation may contribute to tissue scarring, the exact action and mechanisms of the matrix-degrading enzymes in the injured kidney have become increasingly complicated. Recent discoveries on endogenous antifibrotic factors have evolved novel strategies aimed at antagonizing the fibrogenic action of TGF-beta/Smad signaling. Many therapeutic interventions appear effective in animal models; however, translation of these promising results into humans in the clinical setting remains a daunting task. This mini-review attempts to highlight the recent progress in our understanding of the cellular and molecular pathways leading to renal fibrosis, and discusses the challenges and opportunities in developing therapeutic strategies.

New insights into epithelial-mesenchymal transition in kidney fibrosis

Epithelial-mesenchymal transition (EMT), a process by which differentiated epithelial cells undergo a phenotypic conversion that gives rise to the matrix-producing fibroblasts and myofibroblasts, is increasingly recognized as an integral part of tissue fibrogenesis after injury. However, the degree to which this process contributes to kidney fibrosis remains a matter of intense debate and is likely to be context-dependent. EMT is often preceded by and closely associated with chronic interstitial inflammation and could be an adaptive response of epithelial cells to a hostile or changing microenvironment. In addition to tubular epithelial cells, recent studies indicate that endothelial cells and glomerular podocytes may also undergo transition after injury. Phenotypic alteration of podocytes sets them in motion to functional impairment, resulting in proteinuria and glomerulosclerosis. Several intracellular signal transduction pathways such as TGFbeta/Smad, integrin-linked kinase (ILK) and Wnt/beta-catenin signaling are essential in controlling the process of EMT and presently are potential targets of antifibrotic therapy. This review highlights the current understanding of EMT and its underlying mechanisms to stimulate further discussion on its role, not only in the pathogenesis of renal interstitial fibrosis but also in the onset of podocyte dysfunction, proteinuria, and glomerulosclerosis.

Dissection of key events in tubular epithelial to myofibroblast transition and its implications in renal interstitial fibrosis

Myofibroblast activation is a key event playing a critical role in the progression of chronic renal disease. Emerging evidence suggests that myofibroblasts can derive from tubular epithelial cells by an epithelial to mesenchymal transition (EMT); however, the details regarding the conversion between these two cell types are poorly understood. Here we dissect the key events during the process of EMT induced by transforming growth factor-beta1. Incubation of human tubular epithelial cells with transforming growth factor-beta1 induced de novo expression of alpha-smooth muscle actin, loss of epithelial marker E-cadherin, transformation of myofibroblastic morphology, and production of interstitial matrix. Time-course studies revealed that loss of E-cadherin was an early event that preceded other alterations during EMT. The transformed cells secreted a large amount of matrix metalloproteinase-2 that specifically degraded tubular basement membrane. They also exhibited an enhanced motility and invasive capacity. These alterations in epithelial phenotypes in vitro were essentially recapitulated in a mouse model of renal fibrosis induced by unilateral ureteral obstruction. Hence, these results indicate that tubular epithelial to myofibroblast transition is an orchestrated, highly regulated process involving four key steps including: 1) loss of epithelial cell adhesion, 2) de novo alpha-smooth muscle actin expression and actin reorganization, 3) disruption of tubular basement membrane, and 4) enhanced cell migration and invasion.

Wnt/beta-catenin signaling promotes renal interstitial fibrosis

Wnts compose a family of signaling proteins that play an essential role in kidney development, but their expression in adult kidney is thought to be silenced. Here, we analyzed the expression and regulation of Wnts and their receptors and antagonists in normal and fibrotic kidneys after obstructive injury. In the normal mouse kidney, the vast majority of 19 different Wnts and 10 frizzled receptor genes was expressed at various levels. After unilateral ureteral obstruction, all members of the Wnt family except Wnt5b, Wnt8b, and Wnt9b were upregulated in the fibrotic kidney with distinct dynamics. In addition, the expression of most Fzd receptors and Wnt antagonists was also induced. Obstructive injury led to a dramatic accumulation of beta-catenin in the cytoplasm and nuclei of renal tubular epithelial cells, indicating activation of the canonical pathway of Wnt signaling. Numerous Wnt/beta-catenin target genes (c-Myc, Twist, lymphoid enhancer-binding factor 1, and fibronectin) were induced, and their expression was closely correlated with renal beta-catenin abundance. Delivery of the Wnt antagonist Dickkopf-1 gene significantly reduced renal beta-catenin accumulation and inhibited the expression of Wnt/beta-catenin target genes. Furthermore, gene therapy with Dickkopf-1 inhibited myofibroblast activation; suppressed expression of fibroblast-specific protein 1, type I collagen, and fibronectin; and reduced total collagen content in the model of obstructive nephropathy. In summary, these results establish a role for Wnt/beta-catenin signaling in the pathogenesis of renal fibrosis and identify this pathway as a potential therapeutic target.

Wnt/beta-catenin signaling promotes podocyte dysfunction and albuminuria

Podocyte dysfunction, one of the major causes of proteinuria, leads to glomerulosclerosis and end stage renal disease, but its underlying mechanism remains poorly understood. Here we show that Wnt/beta-catenin signaling plays a critical role in podocyte injury and proteinuria. Treatment with adriamycin induced Wnt and activated beta-catenin in mouse podocytes. Overexpression of Wnt1 in vivo activated glomerular beta-catenin and aggravated albuminuria and adriamycin-induced suppression of nephrin expression, whereas blockade of Wnt signaling with Dickkopf-1 ameliorated podocyte lesions. Podocyte-specific knockout of beta-catenin protected against development of albuminuria after injury. Moreover, pharmacologic activation of beta-catenin induced albuminuria in wild-type mice but not in beta-catenin-knockout littermates. In human proteinuric kidney diseases such as diabetic nephropathy and focal segmental glomerulosclerosis, we observed upregulation of Wnt1 and active beta-catenin in podocytes. Ectopic expression of either Wnt1 or stabilized beta-catenin in vitro induced the transcription factor Snail and suppressed nephrin expression, leading to podocyte dysfunction. These results suggest that targeting hyperactive Wnt/beta-catenin signaling may represent a novel therapeutic strategy for proteinuric kidney diseases.

Loss of Klotho contributes to kidney injury by derepression of Wnt/β-catenin signaling

Aging is an independent risk factor for CKD, but the molecular mechanisms that link aging and CKD are not well understood. The antiaging protein Klotho may be an endogenous antagonist of Wnt/β-catenin signaling, which promotes fibrogenesis, suggesting that loss of Klotho may contribute to CKD through increased Wnt/β-catenin activity. Here, normal adult kidneys highly expressed Klotho in the tubular epithelium, but various models of nephropathy exhibited markedly less expression of Klotho. Loss of Klotho was closely associated with increased β-catenin in the diseased kidneys, suggesting an inverse correlation between Klotho and canonical Wnt signaling. In vitro, both full-length and secreted Klotho bound to multiple Wnts, including Wnt1, Wnt4, and Wnt7a. Klotho repressed gene transcription induced by Wnt but not by active β-catenin. Furthermore, Klotho blocked Wnt-triggered activation and nuclear translocation of β-catenin, as well as the expression of its target genes in tubular epithelial cells. Investigating potential mediators of Klotho loss in CKD, we found that TGF-β1 suppressed Klotho expression and concomitantly activated β-catenin; conversely, overexpression of Klotho abolished fibrogenic effects of TGF-β1. In two mouse models of CKD induced by unilateral ureteral obstruction or adriamycin, in vivo expression of secreted Klotho inhibited the activation of renal β-catenin and expression of its target genes. Secreted Klotho also suppressed myofibroblast activation, reduced matrix expression, and ameliorated renal fibrosis. Taken together, these results suggest that Klotho is an antagonist of endogenous Wnt/β-catenin activity; therefore, loss of Klotho may contribute to kidney injury by releasing the repression of pathogenic Wnt/β-catenin signaling.

Role for integrin-linked kinase in mediating tubular epithelial to mesenchymal transition and renal interstitial fibrogenesis

Under pathologic conditions, renal tubular epithelial cells can undergo epithelial to mesenchymal transition (EMT), a phenotypic conversion that is believed to play a critical role in renal interstitial fibrogenesis. However, the underlying mechanism that governs this process remains largely unknown. Here we demonstrate that integrin-linked kinase (ILK) plays an important role in mediating tubular EMT induced by TGF-beta1. TGF-beta1 induced ILK expression in renal tubular epithelial cells in a time- and dose-dependent manner, which was dependent on intracellular Smad signaling. Forced expression of ILK in human kidney proximal tubular epithelial cells suppressed E-cadherin expression and induced fibronectin expression and its extracellular assembly. ILK also induced MMP-2 expression and promoted cell migration and invasion in Matrigel. Conversely, ectopic expression of a dominant-negative, kinase-dead form of ILK largely abrogated TGF-beta1-initiated tubular cell phenotypic conversion. In vivo, ILK was markedly induced in renal tubular epithelia in mouse models of chronic renal diseases, and such induction was spatially and temporally correlated with tubular EMT. Moreover, inhibition of ILK expression by HGF was associated with blockade of tubular EMT and attenuation of renal fibrosis. These findings suggest that ILK is a critical mediator for tubular EMT and likely plays a crucial role in the pathogenesis of chronic renal fibrosis.

Epithelial-to-mesenchymal transition is a potential pathway leading to podocyte dysfunction and proteinuria

Podocyte dysfunction plays an essential role in the pathogenesis of proteinuria and glomerulosclerosis. However, the mechanism underlying podocyte dysfunction in many common forms of chronic kidney diseases remains poorly understood. Here we tested the hypothesis that podocytes may undergo epithelial-to-mesenchymal transition after injury. Conditionally immortalized mouse podocytes were incubated with transforming growth factor (TGF)-beta1, a potent fibrogenic cytokine that is up-regulated in the diseased kidney. TGF-beta1 suppressed the slit diaphragm-associated protein P-cadherin, zonula occludens-1, and nephrin, a change consistent with loss of the epithelial feature. Meanwhile, TGF-beta1 induced the expression of the intermediate filament protein desmin and interstitial matrix components fibronectin and collagen I. Furthermore, TGF-beta1 promoted the expression and secretion of matrix metalloproteinase-9 by podocytes. Functionally, TGF-beta1 increased albumin permeability across podocyte monolayers, as demonstrated by a paracellular albumin influx assay. The expression of Snail, a key transcriptional factor that has been implicated in initiating epithelial-to-mesenchymal transition, was induced by TGF-beta1, and ectopic expression of Snail suppressed P-cadherin and nephrin in podocytes. In vivo, in addition to loss of nephrin and zonula occludens-1, mesenchymal markers such as desmin, fibroblast-specific protein-1, and matrix metalloproteinase-9 could be observed in glomerular podocytes of diabetic nephropathy. These results suggest that podocyte dedifferentiation and mesenchymal transition could be a potential pathway leading to their dysfunction, thereby playing a role in the genesis of proteinuria.

Blockage of tubular epithelial to myofibroblast transition by hepatocyte growth factor prevents renal interstitial fibrosis

Activation of alpha-smooth muscle actin-positive myofibroblast cells is a key event in the progression of chronic renal diseases that leads to end-stage renal failure. Although the origin of these myofibroblasts in the kidney remains uncertain, emerging evidence suggests that renal myofibroblasts may derive from tubular epithelial cells by a process of epithelial to mesenchymal transition. It was demonstrated that hepatocyte growth factor (HGF) exhibited a remarkable ability to block this phenotypic transition both in vitro and in vivo. HGF abrogated the alpha-smooth muscle actin expression and E-cadherin depression triggered by transforming growth factor-beta1 in tubular epithelial cells in a dose-dependent manner. HGF also blocked morphologic transformation of tubular epithelial cells and inhibited the expression and extracellular deposition of fibronectin. In a mouse model of renal fibrosis disease induced by unilateral ureteral obstruction, transforming growth factor-beta type I receptor expression was specifically increased in renal tubules, and myofibroblastically phenotypic transition of the tubules was evident in vivo. Remarkably, injections of exogenous HGF blocked myofibroblast activation and drastically prevented renal interstitial fibrosis in the obstructed kidneys. These results suggest that tubular epithelial to myofibroblast conversion may play an important role in the pathogenesis of renal fibrosis and that blocking this phenotypic transition could provide a novel therapeutic strategy for the treatment of fibrotic diseases.

Blockade of Wnt/β-catenin signaling by paricalcitol ameliorates proteinuria and kidney injury

Recent studies implicate Wnt/β-catenin signaling in podocyte dysfunction. Because vitamin D analogs can inhibit β-catenin in other tissues, we tested whether the vitamin D analog paricalcitol could ameliorate podocyte injury, proteinuria, and renal fibrosis in adriamycin (ADR) nephropathy. Compared with vehicle-treated controls, paricalcitol preserved expression of nephrin, podocin, and WT1; prevented proteinuria; and reduced glomerulosclerotic lesions induced by ADR. Paricalcitol also inhibited expression of proinflammatory cytokines, reduced renal infiltration of monocytes/macrophages, hampered activation of renal myofibroblasts, and suppressed expression of the fibrogenic TGF-β1, CTGF, fibronectin, and types I and III collagen. Selective suppression of renal Wnt4, Wnt7a, Wnt7b, and Wnt10a expression after ADR accompanied these renoprotective effects of paricalcitol. Significant upregulation of β-catenin, predominantly in podocytes and tubular epithelial cells, accompanied renal injury; paricalcitol largely abolished this induction of renal β-catenin and inhibited renal expression of Snail, a downstream effector of Wnt/β-catenin signaling. Administration of paricalcitol also ameliorated established proteinuria. In vitro, paricalcitol induced a physical interaction between the vitamin D receptor and β-catenin in podocytes, which led to suppression of β-catenin-mediated gene transcription. In summary, these findings suggest that paricalcitol prevents podocyte dysfunction, proteinuria, and kidney injury in adriamycin nephropathy by inhibiting Wnt/β-catenin signaling.

Paricalcitol attenuates renal interstitial fibrosis in obstructive nephropathy

Deficiency in vitamin D and its active metabolites is a pathologic feature of chronic kidney diseases. Despite that tubular epithelial cells are the major sites of active vitamin D synthesis, little is known about the role of vitamin D in maintaining the structural and functional integrity of tubular epithelium. This study investigated the effects of paricalcitol (19-nor-1,25-hydroxy-vitamin D(2)), a synthetic vitamin D analogue, on obstructive nephropathy, a model that is characterized by predominant tubulointerstitial lesions. Compared with vehicle controls, paricalcitol significantly attenuated renal interstitial fibrosis in mouse kidney after ureteral obstruction, as demonstrated by a reduced interstitial volume, decreased collagen deposition, and repressed mRNA expression of fibronectin and type I and type III collagens. Paricalcitol largely preserved E-cadherin and reduced alpha-smooth muscle actin expression in vivo. In addition, paricalcitol suppressed renal TGF-beta1 and its type I receptor expression, restored vitamin D receptor abundance, and inhibited cell proliferation and apoptosis after obstructive injury. In vitro, paricalcitol abolished TGF-beta1-mediated E-cadherin suppression and alpha-smooth muscle actin and fibronectin induction in tubular epithelial cells, underscoring its ability to block directly the epithelial to mesenchymal transition (EMT). It is interesting that paricalcitol almost completely suppressed renal induction of Snail, a critical transcription factor that is implicated in EMT programming. Furthermore, paricalcitol inhibited the TGF-beta1-mediated Snail induction in vitro, and ectopic expression of Snail repressed E-cadherin promoter activity and downregulated E-cadherin expression in tubular epithelial cells. These studies suggest that paricalcitol is able to ameliorate renal interstitial fibrosis in obstructive nephropathy, possibly by preserving tubular epithelial integrity through suppression of EMT.

Wnt/β-catenin signaling and kidney fibrosis

Wnt/β-catenin signaling is an evolutionarily conserved, highly complex, key developmental pathway that regulates cell fate, organ development, tissue homeostasis, as well as injury and repair. Although relatively silent in normal adult kidney, Wnt/β-catenin signaling is re-activated after renal injury in a wide variety of animal models and in human kidney disorders. Whereas some data point to a protective role of this signaling in healing and repair after acute kidney injury, increasing evidence suggests that sustained activation of Wnt/β-catenin is associated with the development and progression of renal fibrotic lesions. In kidney cells, Wnt/β-catenin promotes the expression of numerous fibrosis-related genes such as Snail1, plasminogen activator inhibitor-1, and matrix metalloproteinase-7. Recent studies also indicate that multiple components of the renin-angiotensin system are the direct downstream targets of Wnt/β-catenin. Consistently, inhibition of Wnt/β-catenin signaling by an assortment of strategies ameliorates kidney injury and mitigates renal fibrotic lesions in various models of chronic kidney disease, suggesting that targeting this signaling could be a plausible strategy for therapeutic intervention. In this mini review, we will briefly discuss the regulation, downstream targets, and mechanisms of Wnt/β-catenin signaling in the pathogenesis of kidney fibrosis.

Paricalcitol inhibits renal inflammation by promoting vitamin D receptor-mediated sequestration of NF-kappaB signaling

Inflammation is a pathologic feature of a variety of chronic kidney diseases. Several lines of evidence suggest a potential anti-inflammatory role for vitamin D in chronic kidney disease, but the underlying mechanism remains unknown. Here, the effect of the synthetic vitamin D analogue paricalcitol on renal inflammation was investigated in a mouse model of obstructive nephropathy. Paricalcitol reduced infiltration of T cells and macrophages in the obstructed kidney. This inhibition of inflammatory cell infiltration was accompanied by a decreased expression of RANTES and TNF-alpha. Induction of RANTES was localized primarily to the tubular epithelium, underscoring a role for tubular cells in renal inflammation. In a human proximal tubular cell line (HKC-8), paricalcitol inhibited RANTES mRNA and protein expression and abolished the ability of tubular cells to recruit lymphocytes and monocytes after TNF-alpha stimulation. Although RANTES induction depended on NF-kappaB signaling, paricalcitol affected neither TNF-alpha-mediated IkappaB alpha phosphorylation and degradation nor p65 NF-kappaB activation and nuclear translocation. Instead, chromatin immunoprecipitation assay showed that paricalcitol abolished the binding of p65 to its cognate cis-acting element in the RANTES promoter. The vitamin D receptor (VDR) and p65 formed a complex in tubular cells after paricalcitol treatment, which inhibited the ability of p65 to trans-activate gene transcription. In vivo, paricalcitol did not block NF-kappaB nuclear translocation after obstructive injury but did increase the expression and nuclear distribution of VDR. These results suggest that paricalcitol inhibits renal inflammatory infiltration and RANTES expression by promoting VDR-mediated sequestration of NF-kappaB signaling.

Targeted inhibition of β-catenin/CBP signaling ameliorates renal interstitial fibrosis

Because fibrotic kidneys exhibit aberrant activation of β-catenin signaling, this pathway may be a potential target for antifibrotic therapy. In this study, we examined the effects of β-catenin activation on tubular epithelial-mesenchymal transition (EMT) in vitro and evaluated the therapeutic efficacy of the peptidomimetic small molecule ICG-001, which specifically disrupts β-catenin-mediated gene transcription, in obstructive nephropathy. In vitro, ectopic expression of stabilized β-catenin in tubular epithelial (HKC-8) cells suppressed E-cadherin and induced Snail1, fibronectin, and plasminogen activator inhibitor-1 (PAI-1) expression. ICG-001 suppressed β-catenin-driven gene transcription in a dose-dependent manner and abolished TGF-β1-induced expression of Snail1, PAI-1, collagen I, fibronectin, and α-smooth muscle actin (α-SMA). This antifibrotic effect of ICG-001 did not involve disruption of Smad signaling. In the unilateral ureteral obstruction model, ICG-001 ameliorated renal interstitial fibrosis and suppressed renal expression of fibronectin, collagen I, collagen III, α-SMA, PAI-1, fibroblast-specific protein-1, Snail1, and Snail2. Late administration of ICG-001 also effectively attenuated fibrotic lesions in obstructive nephropathy. In conclusion, inhibiting β-catenin signaling may be an effective approach to the treatment of fibrotic kidney diseases.

Sustained Activation of Wnt/β-Catenin Signaling Drives AKI to CKD Progression

AKI is increasingly recognized as a major risk factor for progression to CKD. However, the factors governing AKI to CKD progression are poorly understood. In this study, we investigated this issue using moderate (20 minutes) and severe (30 minutes) ischemia/reperfusion injury (IRI) in mice. Moderate IRI led to acute kidney failure and transient Wnt/β-catenin activation, which was followed by the restoration of kidney morphology and function. However, severe IRI resulted in sustained and exaggerated Wnt/β-catenin activation, which was accompanied by development of renal fibrotic lesions characterized by interstitial myofibroblast activation and excessive extracellular matrix deposition. To assess the role of sustained Wnt/β-catenin signaling in mediating AKI to CKD progression, we manipulated this signaling by overexpression of Wnt ligand or pharmacologic inhibition of β-catenin. In vivo, overexpression of Wnt1 at 5 days after IRI induced β-catenin activation and accelerated AKI to CKD progression. Conversely, blockade of Wnt/β-catenin by small molecule inhibitor ICG-001 at this point hindered AKI to CKD progression. In vitro, Wnt ligands induced renal interstitial fibroblast activation and promoted fibronectin expression. However, activated fibroblasts readily reverted to a quiescent phenotype after Wnt ligands were removed, suggesting that fibroblast activation requires persistent Wnt signaling. These results indicate that sustained, but not transient, activation of Wnt/β-catenin signaling has a decisive role in driving AKI to CKD progression.

Hepatocyte growth factor in kidney fibrosis: therapeutic potential and mechanisms of action

Hepatocyte growth factor (HGF) is a pleiotropic factor that plays an imperative role in tubular repair and regeneration after acute renal injury. Growing evidence indicates that HGF is also an endogenous renoprotective factor that possesses a potent antifibrotic ability. HGF prevents the initiation and progression of chronic renal fibrosis and inhibits transforming growth factor (TGF)-beta(1) expression in a wide variety of animal models. In vitro, HGF counteracts the action of TGF-beta(1) in different types of kidney cells, resulting in blockade of the myofibroblastic activation from interstitial fibroblasts and glomerular mesangial cells, as well as inhibition of the mesenchymal transition from tubular epithelial cells. Recent studies reveal that HGF antagonizes the profibrotic actions of TGF-beta(1) by intercepting Smad signal transduction through diverse mechanisms. In interstitial fibroblasts, HGF blocks activated Smad-2/3 nuclear translocation, whereas it specifically upregulates the expression of the Smad transcriptional corepressor SnoN in tubular epithelial cells. In glomerular mesangial cells, HGF stabilizes another Smad corepressor, TGIF, by preventing it from degradation. Smad corepressors bind to activated Smad-2/3 and sequester their ability to transcriptionally activate TGF-beta target genes. This article reviews recent advances in our understanding of the cellular and molecular mechanisms underlying HGF inhibition of renal fibrosis.

Multiple genes of the renin-angiotensin system are novel targets of Wnt/β-catenin signaling

Activation of the renin-angiotensin system (RAS) plays an essential role in the pathogenesis of CKD and cardiovascular disease. However, current anti-RAS therapy only has limited efficacy, partly because of compensatory upregulation of renin expression. Therefore, a treatment strategy to simultaneously target multiple RAS genes is necessary to achieve greater efficacy. By bioinformatics analyses, we discovered that the promoter regions of all RAS genes contained putative T-cell factor (TCF)/lymphoid enhancer factor (LEF)-binding sites, and β-catenin induced the binding of LEF-1 to these sites in kidney tubular cells. Overexpression of either β-catenin or different Wnt ligands induced the expression of all RAS genes. Conversely, a small-molecule β-catenin inhibitor ICG-001 abolished RAS induction. In a mouse model of nephropathy induced by adriamycin, either transient therapy or late administration of ICG-001 abolished established proteinuria and kidney lesions. ICG-001 inhibited renal expression of multiple RAS genes in vivo and abolished the expression of other Wnt/β-catenin target genes. Moreover, ICG-001 therapy restored expression of nephrin, podocin, and Wilms' tumor 1, attenuated interstitial myofibroblast activation, repressed matrix expression, and inhibited renal inflammation and fibrosis. Collectively, these studies identify all RAS genes as novel downstream targets of Wnt/β-catenin. Our results indicate that blockade of Wnt/β-catenin signaling can simultaneously repress multiple RAS genes, thereby leading to the reversal of established proteinuria and kidney injury.

Tubule-specific ablation of endogenous β-catenin aggravates acute kidney injury in mice

β-Catenin is a unique intracellular protein functioning as an integral component of the cell-cell adherens complex and a principal signaling protein mediating canonical Wnt signaling. Little is known about its function in adult kidneys in the normal physiologic state or after acute kidney injury (AKI). To study this, we generated conditional knockout mice in which the β-catenin gene was specifically disrupted in renal tubules (Ksp-β-cat-/-). These mice were phenotypically normal with no appreciable defects in kidney morphology and function. In the absence of β-catenin, γ-catenin functionally substituted for it in E-cadherin binding, thereby sustaining the integrity of epithelial adherens junctions in the kidneys. In AKI induced by ischemia reperfusion or folic acid, the loss of tubular β-catenin substantially aggravated renal lesions. Compared with controls, Ksp-β-cat-/- mice displayed higher mortality, elevated serum creatinine, and more severe morphologic injury. Consistently, apoptosis was more prevalent in kidneys of the knockout mice, which was accompanied by increased expression of p53 and Bax, and decreased phosphorylated Akt and survivin. In vitro activation of β-catenin by Wnt1 or stabilization of β-catenin protected tubular epithelial cells from apoptosis, activated Akt, induced survivin, and repressed p53 and Bax expression. Hence, endogenous β-catenin is pivotal for renal tubular protection after AKI by promoting cell survival through multiple mechanisms.

Matrix metalloproteinases in kidney homeostasis and diseases

Matrix metalloproteinases (MMPs) are a family of zinc-dependent endopeptidases that have been increasingly linked to both normal physiology and abnormal pathology in the kidney. Collectively able to degrade all components of the extracellular matrix, MMPs were originally thought to antagonize the development of fibrotic diseases solely through digestion of excessive matrix. However, increasing evidence has shown that MMPs play a wide variety of roles in regulating inflammation, epithelial-mesenchymal transition, cell proliferation, angiogenesis, and apoptosis. We now have robust evidence for MMP dysregulation in a multitude of renal diseases including acute kidney injury, diabetic nephropathy, glomerulonephritis, inherited kidney disease, and chronic allograft nephropathy. The goal of this review is to summarize current findings regarding the role of MMPs in kidney diseases as well as the mechanisms of action of this family of proteases.

Wnt/β-catenin/RAS signaling mediates age-related renal fibrosis and is associated with mitochondrial dysfunction

Renal fibrosis is the common pathological feature in a variety of chronic kidney diseases. Aging is highly associated with the progression of renal fibrosis. Among several determinants, mitochondrial dysfunction plays an important role in aging. However, the underlying mechanisms of mitochondrial dysfunction in age-related renal fibrosis are not elucidated. Herein, we found that Wnt/β-catenin signaling and renin-angiotensin system (RAS) activity were upregulated in aging kidneys. Concomitantly, mitochondrial mass and functions were impaired with aging. Ectopic expression of Klotho, an antagonist of endogenous Wnt/β-catenin activity, abolished renal fibrosis in d-galactose (d-gal)-induced accelerated aging mouse model and significantly protected renal mitochondrial functions by preserving mass and diminishing the production of reactive oxygen species. In an established aging mouse model, dickkopf 1, a more specific Wnt inhibitor, and the mitochondria-targeted antioxidant mitoquinone restored mitochondrial mass and attenuated tubular senescence and renal fibrosis. In a human proximal tubular cell line (HKC-8), ectopic expression of Wnt1 decreased biogenesis and induced dysfunction of mitochondria, and triggered cellular senescence. Moreover, d-gal triggered the transduction of Wnt/β-catenin signaling, which further activated angiotensin type 1 receptor (AT1), and then decreased the mitochondrial mass and increased cellular senescence in HKC-8 cells and primary cultured renal tubular cells. These effects were inhibited by AT1 blocker of losartan. These results suggest inhibition of Wnt/β-catenin signaling and the RAS could slow the onset of age-related mitochondrial dysfunction and renal fibrosis. Taken together, our results indicate that Wnt/β-catenin/RAS signaling mediates age-related renal fibrosis and is associated with mitochondrial dysfunction.

Hepatocyte growth factor attenuates liver fibrosis induced by bile duct ligation

Hepatic fibrosis is a common outcome of a variety of chronic liver diseases. Here we evaluated the therapeutic efficacy of hepatocyte growth factor (HGF) on liver fibrosis induced by bile duct ligation (BDL) and investigated potential mechanisms. Mice underwent BDL, followed by intravenous injections of naked HGF expression plasmid or control vector. HGF gene therapy markedly ameliorated hepatic fibrotic lesions, as demonstrated by reduced alpha-smooth muscle actin (alphaSMA) expression, attenuated deposition of type I and type III collagen, and normalized total hydroxyproline content. HGF also suppressed transforming growth factor-beta1 (TGF-beta1) expression. Interestingly, colocalization of alphaSMA and cytokeratin-19 in bile duct epithelium was observed, suggesting the possibility of biliary epithelial to myofibroblast transition after BDL. Cells that were still positive for cytokeratin-19 but actively producing type I collagen were found in the biliary epithelia and periductal region. Laminin staining revealed an impaired basement membrane of the bile duct epithelium in diseased liver. These lesions were largely prevented by HGF administration. In vitro, treatment of human biliary epithelial cells with TGF-beta1 induced alphaSMA and fibronectin expression and suppressed cytokeratin-19. HGF abolished the phenotypic conversion of biliary epithelial cells induced by TGF-beta1. These results suggest that HGF ameliorates hepatic biliary fibrosis in part by blocking bile duct epithelial to mesenchymal transition.

Tissue-type plasminogen activator acts as a cytokine that triggers intracellular signal transduction and induces matrix metalloproteinase-9 gene expression

Tissue-type plasminogen activator (tPA), a serine protease well known for generating plasmin, has been demonstrated to induce matrix metalloproteinase-9 (MMP-9) gene expression and protein secretion in renal interstitial fibroblasts. However, exactly how tPA transduces its signal into the nucleus to control gene expression is unknown. This study investigated the mechanism by which tPA induces MMP-9 gene expression. Both wild-type and non-enzymatic mutant tPA were found to induce MMP-9 expression in rat kidney interstitial fibroblasts (NRK-49F), indicating that the actions of tPA are independent of its proteolytic activity. tPA bound to the low density lipoprotein receptor-related protein-1 (LRP-1) in NRK-49F cells, and this binding was competitively abrogated by the LRP-1 antagonist, the receptor-associated protein. In mouse embryonic fibroblasts (PEA-13) lacking LRP-1, tPA failed to induce MMP-9 expression. Furthermore, tPA induced rapid tyrosine phosphorylation on the beta subunit of LRP-1, which was followed by the activation of Mek1 and its downstream Erk-1 and -2. Blockade of Erk-1/2 activation by the Mek1 inhibitor abolished MMP-9 induction by tPA in NRK-49F cells. Conversely, overexpression of constitutively activated Mek1 induced Erk-1/2 phosphorylation and MMP-9 expression. In mouse obstructed kidney, tPA, LRP-1, and MMP-9 were concomitantly induced in the renal interstitium. Collectively, these results suggest that besides its classical proteolytic activity, tPA acts as a cytokine that binds to the cell membrane receptor LRP-1, induces its tyrosine phosphorylation, and triggers intracellular signal transduction, thereby inducing specific gene expression in renal interstitial fibroblasts.