Transforming growth factor-beta receptor types I and II are expressed in renal tubules and are increased after chronic unilateral ureteral obstruction

A Klotho-derived peptide protects against kidney fibrosis by targeting TGF-β signaling

Loss of Klotho, an anti-aging protein, plays a critical role in the pathogenesis of chronic kidney diseases. As Klotho is a large transmembrane protein, it is challenging to harness it as a therapeutic remedy. Here we report the discovery of a Klotho-derived peptide 1 (KP1) protecting kidneys by targeting TGF-β signaling. By screening a series of peptides derived from human Klotho protein, we identified KP1 that repressed fibroblast activation by binding to TGF-β receptor 2 (TβR2) and disrupting the TGF-β/TβR2 engagement. As such, KP1 blocked TGF-β-induced activation of Smad2/3 and mitogen-activated protein kinases. In mouse models of renal fibrosis, intravenous injection of KP1 resulted in its preferential accumulation in injured kidneys. KP1 preserved kidney function, repressed TGF-β signaling, ameliorated renal fibrosis and restored endogenous Klotho expression. Together, our findings suggest that KP1 recapitulates the anti-fibrotic action of Klotho and offers a potential remedy in the fight against fibrotic kidney diseases.

A novel role of LRP5 in tubulointerstitial fibrosis through activating TGF-β/Smad signaling

Previous studies by us and others demonstrated that activation of Wnt/β-catenin signaling plays a pathogenic role in chronic kidney diseases (CKD). Wnt co-receptor LRP5 variants are reported to associate with autosomal dominant polycystic kidney disease; but their exact roles in this disease and renal fibrosis have not been explored. Here, we observed the upregulation of LRP5 in the renal tubules of both type 1 and type 2 diabetic models and of an obstructive nephropathy model. In the obstructed kidneys, Lrp5 knockout significantly ameliorated tubulointerstitial fibrosis and tubular injury without changing Wnt/β-catenin signaling. Instead, decreased levels of TGF-β1 and TGF-β receptors (TβRs) were detected in Lrp5 knockout kidneys, followed by attenuated activation and nuclear translocation of Smad2/3 in the renal tubules, suggesting a regulatory effect of LRP5 on TGF-β/Smad signaling. In consistent with this hypothesis, LRP5 overexpression resulted in enhanced TGF-β/Smad signaling activation in renal tubule epithelial cells. Furthermore, LRP5 was co-immunoprecipitated with TβRI and TβRII, and its extracellular domain was essential for interacting with TβRs and for its pro-fibrotic activity. In addition to stabilizing TβRs, LRP5 increased the basal membrane presentation and TGF-β1-induced internalization of these receptors. Notably, TGF-β1 also induced LRP5 internalization. These findings indicate that LRP5 promotes tubulointerstitial fibrosis, at least partially, via direct modulation of TGF-β/Smad signaling, a novel, Wnt-independent function.

Diverse Role of TGF-β in Kidney Disease

Inflammation and fibrosis are two pathological features of chronic kidney disease (CKD). Transforming growth factor-β (TGF-β) has been long considered as a key mediator of renal fibrosis. In addition, TGF-β also acts as a potent anti-inflammatory cytokine that negatively regulates renal inflammation. Thus, blockade of TGF-β inhibits renal fibrosis while promoting inflammation, revealing a diverse role for TGF-β in CKD. It is now well documented that TGF-β1 activates its downstream signaling molecules such as Smad3 and Smad3-dependent non-coding RNAs to transcriptionally and differentially regulate renal inflammation and fibrosis, which is negatively regulated by Smad7. Therefore, treatments by rebalancing Smad3/Smad7 signaling or by specifically targeting Smad3-dependent non-coding RNAs that regulate renal fibrosis or inflammation could be a better therapeutic approach. In this review, the paradoxical functions and underlying mechanisms by which TGF-β1 regulates in renal inflammation and fibrosis are discussed and novel therapeutic strategies for kidney disease by targeting downstream TGF-β/Smad signaling and transcriptomes are highlighted.

Targeting Anti-TGF-β Therapy to Fibrotic Kidneys with a Dual Specificity Antibody Approach

Targeted delivery of a therapeutic agent to a site of pathology to ameliorate disease while limiting exposure at undesired tissues is an aspirational treatment scenario. Targeting diseased kidneys for pharmacologic treatment has had limited success. We designed an approach to target an extracellular matrix protein, the fibronectin extra domain A isoform (FnEDA), which is relatively restricted in distribution to sites of tissue injury. In a mouse unilateral ureteral obstruction (UUO) model of renal fibrosis, injury induced significant upregulation of FnEDA in the obstructed kidney. Using dual variable domain Ig (DVD-Ig) technology, we constructed a molecule with a moiety to target FnEDA and a second moiety to neutralize TGF-β After systemic injection of the bispecific TGF-β + FnEDA DVD-Ig or an FnEDA mAb, chemiluminescent detection and imaging with whole-body single-photon emission computed tomography (SPECT) revealed significantly higher levels of each molecule in the obstructed kidney than in the nonobstructed kidney, the ipsilateral kidney of sham animals, and other tissues. In comparison, a systemically administered TGF-β mAb accumulated at lower concentrations in the obstructed kidney and exhibited a more diffuse whole-body distribution. Systemic administration of the bispecific DVD-Ig or the TGF-β mAb (1-10 mg/kg) but not the FnEDA mAb attenuated the injury-induced collagen deposition detected by immunohistochemistry and elevation in Col1a1, FnEDA, and TIMP1 mRNA expression in the obstructed kidney. Overall, systemic delivery of a bispecific molecule targeting an extracellular matrix protein and delivering a TGF-β mAb resulted in a relatively focal uptake in the fibrotic kidney and reduced renal fibrosis.

Hepatitis B virus X protein promotes renal epithelial-mesenchymal transition in human renal proximal tubule epithelial cells through the activation of NF-κB

Hepatitis B virus (HBV)-associated glomerulo-nephritis is the most common extra-hepatic disorder occurring with hepatitis B virus infection. In the present study, we hypothesized that HBV X protein (HBx) may play a critical role in renal interstitial fibrosis, as HBx has been shown to induce epithelial-mesenchymal transition (EMT) in renal cells. For this purpose, we successfully transfected HBx plasmid into human renal proximal tubule epithelial cells (HK-2 cells). We found that transfection with HBx plasmid significantly downregulated E-cadherin expression and upregulated α-smooth muscle actin, collagen I and fibronectin expression in a time- and concentration-dependent manner (at the lower concentrations and earlier time points). HBx also increased nuclear factor-κB (NF-κB) phosphorylation in a time- and concentration-dependent manner (again at the lower concentrations and earlier time points); however, it did not alter the phosphorylation of Smad2, Smad3, p38, phosphoinositide 3-kinase (PI3K) or extracellular signal-regulated kinase (ERK). Thus, the findings of this study demonstrate that HBx promotes EMT in renal HK-2 cells, and the potential underlying mechanisms may involve the activation of the NF-κB signaling pathway.

Identification of bone morphogenetic protein 9 (BMP9) as a novel profibrotic factor in vitro

Upregulated synthesis of extracellular matrix (ECM) proteins by myofibroblasts is a common phenomenon in the development of fibrosis. Although the role of TGF-β in fibrosis development has been extensively studied, the involvement of other members of this superfamily of cytokines, the bone morphogenetic proteins (BMPs) in organ fibrosis has given contradictory results. BMP9 is the main ligand for activin receptor-like kinase-1 (ALK1) TGF-β1 type I receptor and its effect on fibrosis development is unknown. Our purpose was to study the effect of BMP9 in ECM protein synthesis in fibroblasts, as well as the involved receptors and signaling pathways. In cultured mice fibroblasts, BMP9 induces an increase in collagen, fibronectin and connective tissue growth factor expression, associated with Smad1/5/8, Smad2/3 and Erk1/2 activation. ALK5 inhibition with SB431542 or ALK1/2/3/6 with dorsomorphin-1, inhibition of Smad3 activation with SIS3, and inhibition of the MAPK/Erk1/2 with U0126, demonstrates the involvement of these pathways in BMP9-induced ECM synthesis in MEFs. Whereas BMP9 induced Smad1/5/8 phosphorylation through ALK1, it also induces Smad2/3 phosphorylation through ALK5 but only in the presence of ALK1. Summarizing, this is the first study that accurately identifies BMP9 as a profibrotic factor in fibroblasts that promotes ECM protein expression through ALK1 and ALK5 receptors.

TGF-β/BMP proteins as therapeutic targets in renal fibrosis. Where have we arrived after 25 years of trials and tribulations?

The understanding of renal fibrosis in chronic kidney disease (CKD) remains as a challenge. More than 10% of the population of developed countries suffer from CKD. Proliferation and activation of myofibroblasts and accumulation of extracellular matrix proteins are the main features of kidney fibrosis, a process in which a large number of cytokines are involved. Targeting cytokines responsible for kidney fibrosis development might be an important strategy to face the problem of CKD. The increasing knowledge of the signaling pathway network of the transforming growth factor beta (TGF-β) superfamily members, such as the profibrotic cytokine TGF-β1 or the bone morphogenetic proteins (BMPs), and their involvement in the regulation of kidney fibrosis, has stimulated numerous research teams to look for potential strategies to inhibit profibrotic cytokines or to enhance the anti-fibrotic actions of other cytokines. The consequence of all these studies is a better understanding of all these canonical (Smad-mediated) and non-canonical signaling pathways. In addition, the different receptors involved for signaling of each cytokine, the different combinations of type I-type II receptors, and the presence and function of co-receptors that can influence the biological response have been also described. However, are these studies leading to suitable strategies to block the appearance and progression of kidney fibrosis? In this review, we offer a critical perspective analyzing the achievements using the most important strategies developed up till now: TGF-β antibodies, chemical inhibitors of TGF-β receptors, miRNAs and signaling pathways and BMP agonists with a potential role as therapeutic molecules against kidney fibrosis.

GQ5 Hinders Renal Fibrosis in Obstructive Nephropathy by Selectively Inhibiting TGF-β-Induced Smad3 Phosphorylation

TGF-β1, via Smad-dependent or Smad-independent signaling, has a central role in the pathogenesis of renal fibrosis. This pathway has been recognized as a potential target for antifibrotic therapy. Here, we identified GQ5, a small molecular phenolic compound isolated from the dried resin of Toxicodendron vernicifluum, as a potent and selective inhibitor of TGF-β1-induced Smad3 phosphorylation. In TGF-β1-stimulated renal tubular epithelial cells and interstitial fibroblast cells, GQ5 inhibited the interaction of Smad3 with TGF-β type I receptor (TβRI) by blocking binding of Smad3 to SARA, suppressed subsequent phosphorylation of Smad3, reduced nuclear translocation of Smad2, Smad3, and Smad4, and downregulated the transcription of major fibrotic genes such as α-smooth muscle actin (α-SMA), collagen I, and fibronectin. Notably, intraperitoneal administration of GQ5 in rats immediately after unilateral ureteral obstruction (UUO) selectively inhibited Smad3 phosphorylation in UUO kidneys, suppressed renal expression of α-SMA, collagen I, and fibronectin, and resulted in impressive renal protection after obstructive injury. Late administration of GQ5 also effectively attenuated fibrotic lesions in obstructive nephropathy. In conclusion, our results suggest that GQ5 hinders renal fibrosis in rats by selective inhibition of TGF-β1-induced Smad3 phosphorylation.

The role of endoglin in kidney fibrosis

Tubulointerstitial fibrosis and glomerulosclerosis, are a major feature of end stage chronic kidney disease (CKD), characterised by an excessive accumulation of extracellular matrix (ECM) proteins. Transforming growth factor beta-1 (TGF-β1) is a cytokine with an important role in many steps of renal fibrosis such as myofibroblast activation and proliferation, ECM protein synthesis and inflammatory cell infiltration. Endoglin is a TGF-β co-receptor that modulates TGF-β responses in different cell types. In numerous cells types, such as mesangial cells or myoblasts, endoglin regulates negatively TGF-β-induced ECM protein expression. However, recently it has been demonstrated that 'in vivo' endoglin promotes fibrotic responses. Furthermore, several studies have demonstrated an increase of endoglin expression in experimental models of renal fibrosis in the kidney and other tissues. Nevertheless, the role of endoglin in renal fibrosis development is unclear and a question arises: Does endoglin protect against renal fibrosis or promotes its development? The purpose of this review is to critically analyse the recent knowledge relating to endoglin and renal fibrosis. Knowledge of endoglin role in this pathology is necessary to consider endoglin as a possible therapeutic target against renal fibrosis.

Transforming growth factor-β1 receptor inhibition preserves glomerulotubular integrity during ureteral obstruction in adults but worsens injury in neonatal mice

Unilateral ureteral obstruction (UUO), a widely used model of chronic kidney disease and congenital obstructive uropathy, causes proximal tubular injury and formation of atubular glomeruli. Because transforming growth factor-β1 (TGF-β1) is a central regulator of renal injury, neonatal and adult mice were subjected to complete UUO while under general anesthesia and treated with vehicle or ALK5 TGF-β1 receptor inhibitor (IN-1130, 30 mg·kg(-1)·day(-1)). After 14 days, glomerulotubular integrity and proximal tubular mass were determined by morphometry of Lotus tetragonolobus lectin distribution, and the fraction of atubular glomeruli was determined by serial section analysis of randomly selected individual glomeruli. Glomerular area, macrophage infiltration, fibronectin distribution, and interstitial collagen were measured by morphometry. Compared with placebo, inhibition of TGF-β1 by IN-1130 decreased apoptosis and formation of atubular glomeruli, prevented parenchymal loss, increased glomerular area and glomerulotubular integrity, and increased proximal tubule fraction of the adult obstructed kidney parenchyma from 17 to 30% (P < 0.05, respectively). IN-1130 decreased macrophage infiltration and fibronectin and collagen deposition in the adult obstructed kidney by ∼50% (P < 0.05, respectively). In contrast to these salutary effects in the adult, IN-1130 caused widespread necrosis in obstructed neonatal kidneys. We conclude that whereas IN-1130 reduces obstructive injury in adult kidneys through preservation of glomerulotubular integrity and proximal tubular mass, TGF-β1 inhibition aggravates obstructive injury in neonates. These results indicate that while caution is necessary in treating congenital uropathies, ALK5 inhibitors may prevent nephron loss due to adult kidney disease.

How does TGF-β mediate tubulointerstitial fibrosis?

Tubulointerstitial fibrosis mediates the development of end-stage renal disease from renal injuries of all etiologies and is considered an important predictor of renal survival. Transforming growth factor-β (TGF-β) is one of the most important growth factors that promotes tubulointerstitial fibrosis, but the mechanisms whereby this occurs are not well defined. This is because TGF-β has pleiotropic effects that depend on the target cell type. This review discusses how TGF-β signaling in each of the relevant cell types (eg, tubular epithelium, fibroblasts) may contribute to tubulointerstitial fibrosis progression and suggests ways in which future research can improve our understanding of TGF-β-mediated tubulointerstitial fibrosis.

CCN2/CTGF increases expression of miR-302 microRNAs, which target the TGFβ type II receptor with implications for nephropathic cell phenotypes

Signalling interplay between transforming growth factor-β (TGFβ) and CCN2 [also called connective tissue growth factor (CTGF)] plays a crucial role in the progression of diabetic nephropathy and has been implicated in cellular differentiation. To investigate the potential role of microRNAs (miRNAs) in the mediation of this signalling network, we performed miRNA screening in mesangial cells treated with recombinant human CCN2. Analysis revealed a cohort of 22 miRNAs differentially expressed by twofold or more, including members of the miR-302 family. Target analysis of miRNA to 3'-untranslated regions (3'-UTRs) identified TGFβ receptor II (TβRII) as a potential miR-302 target. In mesangial cells, decreased TβRII expression was confirmed in response to CCN2 together with increased expression of miR-302d. TβRII was confirmed as an miR-302 target, and inhibition of miR-302d was sufficient to attenuate the effect of CCN2 on TβRII. Data from the European Renal cDNA Biopsy Bank revealed decreased TβRII in diabetic patients, suggesting pathophysiological significance. In a mouse model of fibrosis (UUO), miR-302d was increased, with decreased TβRII expression and aberrant signalling, suggesting relevance in chronic fibrosis. miR-302d decreased TGFβ-induced epithelial mesenchymal transition (EMT) in renal HKC8 epithelial cells and attenuated TGFβ-induced mesangial production of fibronectin and thrombospondin. In summary, we demonstrate a new mode of regulation of TGFβ by CCN2, and conclude that the miR-302 family has a role in regulating growth factor signalling pathways, with implications for nephropathic cell fate transitions.

TGF-β1 → SMAD/p53/USF2 → PAI-1 transcriptional axis in ureteral obstruction-induced renal fibrosis

Chronic kidney disease constitutes an increasing medical burden affecting 26 million people in the United States alone. Diabetes, hypertension, ischemia, acute injury, and urological obstruction contribute to renal fibrosis, a common pathological hallmark of chronic kidney disease. Regardless of etiology, elevated TGF-β1 levels are causatively linked to the activation of profibrotic signaling pathways initiated by angiotensin, glucose, and oxidative stress. Unilateral ureteral obstruction (UUO) is a useful and accessible model to identify mechanisms underlying the progression of renal fibrosis. Plasminogen activator inhibitor-1 (PAI-1), a major effector and downstream target of TGF-β1 in the progression of several clinically important fibrotic disorders, is highly up-regulated in UUO and causatively linked to disease severity. SMAD and non-SMAD pathways (pp60(c-src), epidermal growth factor receptor [EGFR], mitogen-activated protein kinase, p53) are required for PAI-1 induction by TGF-β1. SMAD2/3, pp60(c-src), EGFR, and p53 activation are each increased in the obstructed kidney. This review summarizes the molecular basis and translational significance of TGF-β1-stimulated PAI-1 expression in the progression of kidney disease induced by ureteral obstruction. Mechanisms discussed here appear to be operative in other renal fibrotic disorders and are relevant to the global issue of tissue fibrosis, regardless of organ site.

Effect of combination therapy with enalapril and the TGF-β antagonist 1D11 in unilateral ureteral obstruction

In unilateral ureteral obstruction (UUO), the kidney is characterized by increased fibrosis and apoptosis. Both transforming growth factor-β (TGF-β) and ANG II have been implicated, and ANG II may mediate its effects through TGF-β. Previous studies demonstrated amelioration of renal damage when either TGF-β or ANG II has been individually targeted. In this study, we sought to determine whether combining 1D11 (monoclonal antibody to TGF-β) and an ACE inhibitor, enalapril, would be more effective in UUO than either individual treatment, as has been shown in diabetic and glomerulonephritic models. Rats underwent UUO and were given either control monoclonal antibody, 1D11 or enalapril, or 1D11/enalapril combination, for 14 days. Kidneys were harvested and examined for fibrosis [trichrome; collagen (real-time PCR, Sircol assay) and fibroblast-specific protein expression (immunohistochemistry), apoptosis (TUNEL), macrophage infiltration (immunohistochemistry), and TGF-β expression (real-time PCR and tubular localization with immunohistochemistry)]. UUO was found to induce fibrosis, apoptosis, macrophage infiltration, and TGF-β expression in the obstructed kidney. Administration of either 1D11 or enalapril individually significantly decreased all these changes; when 1D11 and enalapril were combined, there was little additive effect, and the combination did not provide full protection against damage. The results demonstrate that, for the most part, combination therapy is not additive in UUO. This could be due to the continued presence of a physical obstruction or to biochemical differences between UUO and other renal disease models. Furthermore, it suggests that other targets may be amenable to pharmacological manipulation in UUO.

IN-1130, a novel transforming growth factor-β type I receptor kinase (ALK5) inhibitor, suppresses renal fibrosis in obstructive nephropathy

The transforming growth factor-beta (TGF-beta) plays a central role in the progression of renal fibrosis. TGF-beta transduces its signal through the activin receptor-like kinase (ALK)5. IN-1130, a novel small molecule ALK5 inhibitor, inhibited the purified kinase domain of ALK5-mediated Smad3 phosphorylation with an IC(50) value of 5.3 nM. IN-1130 proved to be highly selective in a panel of 27 serine/threonine and tyrosine kinases including p38alpha mitogen-activated protein kinase. We evaluated the efficacy of IN-1130 to block renal fibrogenesis induced by unilateral ureteral obstruction (UUO) in rats. Either vehicle (saline) or IN-1130 (10 and 20 mg/kg/day) was intraperitoneally administered to UUO rats for 7 and 14 days. Phosphorylated Smad2 (pSmad2) and markers of fibrosis were analyzed in kidney tissues. In UUO control kidneys, interstitial fibrosis including tubular atrophy, loss and dilation, inflammatory cell infiltration, and fibroblast cell proliferation was prominent. These morphological changes were notably reduced by IN-1130 treatment. IN-1130 decreased levels of TGF-beta1 messenger RNA (mRNA), type I collagen mRNA, and pSmad2, compared to UUO control rats. As determined by measuring the hydroxyproline content, total kidney collagen amount was increased in UUO control kidneys, but significantly reduced by IN-1130 treatment, which was comparable to results of histochemical staining for collagen. IN-1130 also suppressed the expression of alpha-smooth muscle actin (alpha-SMA) and fibronectin in UUO kidneys. Our results show that IN-1130 suppressed the fibrogenic process of UUO, further underscoring the potential clinical benefits of IN-1130 in the treatment of renal fibrosis.

Temporal changes in renal endoglin and TGF-beta1 expression following ureteral obstruction in rats

Chronic renal disease is characterized by the accumulation of extracellular matrix proteins in the kidney and a loss of renal function. Tubulointerstitial fibrosis has been reported to play an important role in the progression of chronic renal diseases. Transforming growth factor-beta1 (TGF-beta1) is a profibrotic cytokine playing a major contribution to fibrotic kidney disease. Endoglin is a membrane glycoprotein of the TGF-beta1 receptor system. The aim of this work was to determine the time-course expression of renal type I and IV collagens, endoglin and TGF-beta1 in a rat model of induced tubulointerstitial fibrosis at 1, 3, 10 and 17 days after unilateral ureteral obstruction (UUO). In 17 days-ligated (L)-renal samples, a marked interstitial fibrosis was detected by Masson's trichromic and Sirius red staining, accompanied by an increase in type I collagen expression as shown by immunohistochemical analysis. Northern blot studies revealed a progressive increase in collagen alpha2(I), TGF-beta1 and endoglin mRNA expression in L kidneys when compared with the corresponding non-ligated (NL) kidneys from the animals subjected to left UUO. Seventeen days after UUO, significant increases in collagen alpha2(I), collagen alpha1(IV), TGF-beta1 and endoglin mRNA levels were detected in L kidneys vs NL kidneys. Significantly higher levels of the protein endoglin were found in L kidneys than in NL kidneys 10 and 17 days following obstruction. A marked increase expression for endoglin and TGF-beta1 was localized in renal interstitium by immunohistochemical studies 17 days after obstruction. In conclusion, this work reports the upregulation of endoglin coincident to that of its ligand TGF-beta1 in the kidneys of rats with progressive tubulointerstitial fibrosis induced by UUO.

Mediators of fibrosis and apoptosis in obstructive uropathies

Upper urinary tract obstruction, regardless of its cause, often poses a significant clinical challenge to the urologist. Renal cellular and molecular events that occur in response to upper urinary tract obstruction result in a progressive and permanent loss in renal function when left untreated. These pathologic changes include the development of renal fibrosis, tubular atrophy, interstitial inflammation, and apoptotic renal cell death. Several cytokines and growth factors have been identified as major contributors to obstruction-induced renal fibrosis and apoptotic cell death, most notably transforming growth factor-b1 (TGF-b1), angiotensin II, nuclear factor-kB (NF-kB), and tumor necrosis factor-a (TNF-a). This review examines the challenges of upper urinary tract obstruction and the role of these mediators in obstruction-induced renal injury.

Smad3 deficiency attenuates renal fibrosis, inflammation,and apoptosis after unilateral ureteral obstruction

BACKGROUND

Transforming growth factor-beta (TGF-beta) has been implicated in the development of renal fibrosis induced by unilateral ureteral obstruction (UUO). However, there is little information on signaling pathways mediating TGF-beta activity involved in molecular and cellular events leading to renal fibrosis induced by UUO. In this study, we sought to determine whether Smad3, a major signaling component of TGF-beta, mediated renal fibrosis induced by UUO.

METHODS

Renal fibrosis, inflammation, and apoptosis induced by UUO were macroscopically and histologically compared between wild-type mice and Smad3 null mice.

RESULTS

Gross appearance of the kidney after UUO showed relatively intact kidney in Smad3 null mice [Smad3(-/-) mice] when compared with that of wild-type mice [Smad3(+/+) mice]. Renal interstitial fibrosis based on the interstitial area stained with Aniline-blue or Sirius red solution was significantly attenuated in the obstructed kidney of Smad3(-/-) mice when compared with that of Smad3(+/+) mice. Deposition of type I and type III collagens were also significantly reduced in the obstructed kidney of Smad3(-/-) mice. In addition, the numbers of myofibroblasts, macrophages, and CD4/CD8 T cells infiltrated into the kidney after UUO were significantly attenuated in the obstructed kidney of Smad3(-/-) mice when compared with that of Smad3(+/+) mice. Furthermore, terminal deoxynucleotidyltransferase-mediated deoxyuridine triphosphate (dUTP) nick-end labeling (TUNEL) staining after UUO showed significantly reduced number of tubular apoptotic cells in the obstructed kidney of Smad3(-/-) mice when compared with that of Smad3(+/+) mice. Endogenous Smad pathway was activated in the obstructed kidney after UUO in wild-type mice as judged by the increase of phosphorylated Smad2 or phosphorylated Smad2/3-positive cells in renal interstitial area.

CONCLUSION

Smad3 deficiency attenuated renal fibrosis, inflammation, and apoptosis after UUO, suggesting that Smad3 was a key molecule mediating TGF-beta activity leading to real fibrosis after UUO.

Acute Nephrotoxicity of Tacrolimus and Sirolimus in Renal Isografts: Differential Intragraft Expression of Transforming Growth Factor-??1 and ??-Smooth Muscle Actin

BACKGROUND

Renal dysfunction early after kidney transplantation has multiple causes including ischemia-reperfusion (I/R) injury and drug-induced nephrotoxicity. This study assesses the acute nephrotoxicity of tacrolimus (Tac) and sirolimus (Sir) in a rat renal isograft model.

METHODS

Lewis renal isografts and uninephrectomized rats that did not undergo transplantation were treated with various doses of Tac (0.5-5.0 mg/kg/d) or Sir (0.5-6.5 mg/kg/d). Kidneys were examined on day 14 by routine histology and immunohistochemistry for transforming growth factor (TGF)-beta1 and alpha-smooth muscle actin (SMA).

RESULTS

Both Tac and Sir demonstrated evidence of nephrotoxicity in the early posttransplant period including increased serum creatinine and morphologic changes in the graft including interstitial inflammation, fibrosis, and tubular vacuolization. Nephrotoxicity was most prominent in the high-dose treatment groups for both drugs and was more severe in transplanted kidneys than in uninephrectomized animals that did not undergo transplantation, suggesting an additive effect of I/R injury and drug nephrotoxicity. Both Tac and Sir increased intragraft TGF-beta1 and alpha-SMA, but there were distinct differences in the patterns of TGF-beta1 expression. Both demonstrated TGF-beta1 in tubular epithelial cells, but Sir was associated with proximal tubular TGF-beta1 localization in a bright granular pattern, whereas Tac was associated with diffuse distal tubular staining.

CONCLUSIONS

Both Tac and Sir may be nephrotoxic in the early posttransplant period, especially at high doses and when combined with I/R injury. Immunohistochemical localization of TGF-beta1 in the tubular cells was distinctly different with each drug, suggesting possible differences in the mechanism(s) of nephrotoxicity requiring further study.

Inflammatory mediators and growth factors in obstructive renal injury

Obstruction of the upper urinary tract poses a significant clinical challenge to the urologist, and the cascade of renal cellular and molecular events triggered by upper urinary tract obstruction result in a progressive, and eventually permanent, loss in renal function. These pathological changes include the development of renal fibrosis, tubular atrophy, interstitial inflammation, and apoptotic renal cell death. A myriad of cytokines and growth factors have been identified as major contributors to obstruction-induced renal fibrosis and apoptotic cell death, including transforming growth factor-beta1, angiotensin II, nuclear factor-kappaB, and tumor necrosis factor-alpha. This review examines the role of these mediators in obstruction-induced renal injury.

Downregulation of Smad Transcriptional Corepressors SnoN and Ski in the Fibrotic Kidney: An Amplification Mechanism for TGF-β1 Signaling

ABSTRACT. TGF-β1 is a profibrotic cytokine that plays a central role in the onset and progression of chronic renal diseases. The activity of TGF-β1 is tightly controlled by multiple mechanisms, in which antagonizing Smad-mediated gene transcription by co-repressors is an important regulatory component. This study examined the expression of Smad transcriptional co-repressors in the fibrotic kidney and investigated their potential functions in controlling TGF-β1 response. Western blot analysis demonstrated that the protein levels of Smad transcriptional co-repressors SnoN and Ski were progressively reduced in a time-dependent manner in the fibrotic kidney induced by unilateral ureteral obstruction in mice, whereas renal Smad abundance was relatively unaltered. Consistently, SnoN and Ski staining was diminished in the nuclei of renal tubular epithelium and interstitium after obstructive injury. In vitro, knockdown of SnoN expression by RNA interference in tubular epithelial cells dramatically sensitized their responsiveness to TGF-β1 stimulation. Conversely, ectopic expression of exogenous SnoN or Ski after transfection conferred tubular epithelial cell resistance to TGF-β1–induced epithelial to myofibroblast transition. Both SnoN and Ski could block Smad-mediated activation of TGF-β1–responsive promoter and exhibited additive effect in abrogating the profibrotic actions of TGF-β1. These results indicate that as a result of loss of Smad transcriptional co-repressors, the profibrotic TGF-β1 signaling in diseased kidney is markedly amplified in a magnitude much greater than previously thought. Therefore, new strategy aimed to increase Smad transcriptional co-repressors expression may be effective in antagonizing TGF-β1 signaling and thereby blocking the progression of chronic renal fibrosis.

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.

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.

Transforming growth factor-beta1 potentiates renal tubular epithelial cell death by a mechanism independent of Smad signaling

Tubular atrophy resulting from epithelial cell loss is one of the characteristic features in the development of chronic renal interstitial fibrosis. Although the trigger(s) and mechanism for tubular cell loss remain undefined, the hyperactive transforming growth factor (TGF)-beta1 signaling has long been suspected to play an active role. Here we demonstrate that although TGF-beta1 did not induce cell death per se, it dramatically potentiated renal tubular cell apoptosis initiated by other death cues in vitro. Pre-incubation of human kidney epithelial cells (HKC) with TGF-beta1 markedly promoted staurosporine-induced cell death in a time- and dose-dependent manner. TGF-beta1 dramatically accelerated the cleavage and activation of pro-caspase-9, but not pro-caspase-8, in HKC cells. This event was followed by an accelerated activation of pro-caspase-3. To elucidate the mechanism underlying TGF-beta1 promotion of tubular cell death, we investigated the signaling pathways activated by TGF-beta1. Both Smad-2 and p38 mitogen-activated protein (MAP) kinase were rapidly activated by TGF-beta1, as demonstrated by the early induction of phosphorylated Smad-2 and p38 MAP kinase, respectively. We found that overexpression of inhibitory Smad-7 completely abolished Smad-2 phosphorylation and activation induced by TGF-beta1 but did not inhibit TGF-beta1-induced apoptosis. However, suppression of p38 MAP kinase with chemical inhibitor SC68376 not only abolished p38 MAP kinase phosphorylation but also obliterated apoptosis induced by TGF-beta1. These results suggest that hyperactive TGF-beta1 signaling potentiates renal tubular epithelial cell apoptosis by a Smad-independent, p38 MAP kinase-dependent mechanism.

Delayed administration of hepatocyte growth factor reduces renal fibrosis in obstructive nephropathy

Hepatocyte growth factor (HGF) is a renotropic protein that elicits antifibrogenic activity by preventing the activation of matrix-producing myofibroblast cells in animal models of chronic renal diseases. However, whether a delayed administration of HGF can still attenuate renal fibrosis remains uncertain. In this study, we examined the therapeutic potential of exogenous HGF on an established renal interstitial fibrosis induced by unilateral ureteral obstruction (UUO). Three days after UUO, the obstructed kidneys displayed interstitial fibrotic lesions with characteristic features of an established renal fibrosis, as manifested by myofibroblast activation, fibronectin overexpression, interstitial matrix deposition, and transforming growth factor-beta1 upregulation. Beginning at this time point, administration of recombinant HGF into mice by intravenous injections for 11 days markedly suppressed the progression of renal interstitial fibrosis. HGF significantly suppressed renal alpha-smooth muscle actin expression, total kidney collagen contents, interstitial matrix components, such as fibronectin, and renal expression of transforming growth factor-beta1 and its type I receptor. Compared with the starting point (3 days after UUO), HGF treatment largely blunted the progression of myofibroblast accumulation and collagen deposition but did not reverse it. Delayed administration of HGF also suppressed the myofibroblastic transdifferentiation from tubular epithelial cells in vitro, as demonstrated by a decline in alpha-smooth muscle actin and fibronectin expression. These results suggest that exogenous HGF exhibits potent therapeutic effects on retarding the progression of an established renal fibrosis.

Endoglin upregulation during experimental renal interstitial fibrosis in mice

The goal of the present study was to evaluate the role of endoglin, a transforming growth factor-beta1 (TGF-beta1) accessory receptor, in the pathogenesis of renal fibrosis. This was achieved by testing a model of tubulo-interstitial fibrosis induced by unilateral ureteral obstruction in endoglin heterozygous (Eng(+/-)) mice. Northern and Western blot analysis revealed that endoglin expression in kidneys of these mice was significantly reduced compared with Eng(+/+) littermates. Pronounced interstitial fibrosis induced by ureteral obstruction was confirmed histologically by Masson's trichromic staining and by increased immunostaining for fibronectin and laminin without significant differences between Eng(+/-) and Eng(+/+) mice. Ureteral obstruction induced significant increases in alpha2(I) and alpha1(IV) collagen, fibronectin, and TGF-beta1 mRNA levels, as well as in total kidney collagen but changes were similar in Eng(+/-) and Eng(+/+) mouse kidneys. Ureteral obstruction also induced a 2-fold increase in endoglin mRNA levels in both Eng(+/+) mice and Eng(+/-) mice, which was confirmed by Western blot analysis. Thus, the present study provides clear evidence that endoglin is upregulated in the kidneys of mice with interstitial fibrosis induced by unilateral ureteral ligation. However, Eng(+/-) mice do not show any changes in the severity of renal disease induced in this model when compared with normal mice, suggesting that the absolute level of endoglin is not critical for the effects of TGF-beta1 in the renal fibrosis process.

TGF-beta and CTGF have overlapping and distinct fibrogenic effects on human renal cells

Transforming growth factor-beta (TGF-beta) and connective tissue growth factor (CTGF) are ubiquitously expressed in various forms of tissue fibrosis, including fibrotic diseases of the kidney. To clarify the common and divergent roles of these growth factors in the cells responsible for pathological extracellular matrix (ECM) deposition in renal fibrosis, the effects of TGF-beta and CTGF on ECM expression in primary human mesangial (HMCs) and human proximal tubule epithelial cells (HTECs) were studied. Both TGF-beta and CTGF significantly induced collagen protein expression with similar potency in HMCs. Additionally, alpha(2)(I)-collagen promoter activity and mRNA levels were similarly induced by TGF-beta and CTGF in HMCs. However, only TGF-beta stimulated collagenous protein synthesis in HTECs. HTEC expression of tenascin-C (TN-C) was increased by TGF-beta and CTGF, although TGF-beta was the more potent inducer. Thus both growth factors elicit similar profibrogenic effects on ECM production in HMCs, while promoting divergent effects in HTECs. CTGF induction of TN-C, a marker of epithelial-mesenchymal transdifferentiation (EMT), with no significant induction of collagenous protein synthesis in HTECs, may suggest a more predominant role for CTGF in EMT rather than induction of excessive collagen deposition by HTECs during renal fibrosis.

Hepatocyte growth factor gene therapy and angiotensin II blockade synergistically attenuate renal interstitial fibrosis in mice

Tubulointerstitial fibrosis is considered to be common endpoint result of many forms of chronic renal diseases. Except for renal replacement, chronic renal fibrosis is presently incurable. This study demonstrates that the combination of hepatocyte growth factor (HGF) gene therapy with inhibition of the renin-angiotensin system produced synergistic beneficial effects leading to dramatic attenuation of renal tubulointerstitial fibrosis in obstructive nephropathy in mice. The combined treatment with human HGF gene and losartan, an angiotensin II (AngII) type I receptor blocker, preserved renal mass and gross morphology of the obstructed kidneys. Although HGF gene therapy alone inhibited the expression of alpha-smooth muscle actin (alpha SMA) by approximately 54% and 60% at day 7 and day 14 after surgery, respectively, its combination with losartan almost completely abolished alpha SMA induction in the obstructed kidneys. The combined therapy also synergistically inhibited the accumulation of interstitial matrix components, such as fibronectin and collagen I, and suppressed renal expression of transforming growth factor-beta1 (TGF-beta1) and its type I receptor. In vitro studies revealed that AngII by itself did not induce alpha SMA, but it drastically potentiated TGF-beta1-initiated alpha SMA expression in tubular epithelial cells. Furthermore, HGF abrogated de novo alpha SMA expression induced by TGF-beta1 plus AngII. These results suggest that many factors are implicated in the pathogenesis of renal interstitial fibrosis; therefore, a combined therapy aimed at simultaneously targeting multiple pathologic pathways may be necessary for halting the progression of chronic renal diseases. These findings may provide the basis for designing future therapeutic regimens for blocking progressive renal fibrosis in patients.

Differential expression of nerve growth factor transcripts in glia and neurons and their regulation by transforming growth factor-beta1

Nerve growth factor (NGF) influences neuronal development, function, and response to injury. Using reverse transcriptase polymerase chain reaction, we find that mouse and rat cortex and spinal cord, and both neurons and glia in culture, express NGF mRNA. In the mouse, NGF is regulated by at least two promoters that govern synthesis of four different transcripts, A through D, that are all expressed in the mouse tissues and cells examined. In contrast, rat NGF expression varies with tissue and with cell type: transcript C is expressed strongly in brain but weakly in spinal cord, and transcript D is undetectable in rat central nervous system (CNS). In addition to species- and tissue-specific expression, NGF transcripts also exhibit cell type-specific expression: transcripts B, C and D are expressed in rat astrocytes but poorly or not at all in rat neurons, identifying glia as an important source of NGF in rat. NGF increases sharply after injury. TGF-beta1, which also increases immediately after injury, induces NGF mRNA and protein in rat and mouse glia but not in neurons. Furthermore, transcripts A, B and D, but not C, are upregulated by TGF-beta1 in mouse glia, whereas in rat glia, the major responsive transcript is C. Thus, there may be multiple TGF-beta1-responsive elements in the NGF promoters located upstream of exons 1 and 3 that may differ between mouse and rat. Moreover, NGF transcripts are differentially expressed in a species-, cell type-, and inducer-specific manner. These results have implications for the use of mice versus rats as models for the study of NGF regulation following CNS injury.

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.

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.

Deregulation of renal transforming growth factor-beta1 after experimental short-term ureteric obstruction in fetal sheep

Renal malformations are the commonest cause of chronic renal failure in children and they are often associated with urinary tract abnormalities that impair fetal urine flow. Up-regulation of transforming growth factor-beta1 (TGF-beta1) occurs after experimental postnatal urinary tract obstruction and we recently reported increased levels of TGF-beta1 in human renal malformations (Yang SP et al, Am J Pathol 2000, 157:1633-1647). These findings led us to propose that obstruction-induced stretch of developing renal epithelia causes up-regulation of TGF-beta1, which then perturbs renal development. In this study, therefore, we examined expression of components of the TGF-beta1 signaling axis in a previously characterized ovine model of fetal short-term urine flow impairment in which complete unilateral ureteric obstruction was induced at 90 days when a few layers of glomeruli had formed. Up-regulation of TGF-beta1 mRNA and protein was observed in obstructed kidneys, compared to sham-operated control organs, after only 10 days. Increased levels of TGF-beta1 receptors I (TGF-betaR1) and II (TGF-betaR2) were also detected on Western blot, and the cytokine and TGF-betaR1 co-localized in disrupted epithelia on immunohistochemistry. De novo expression of alpha-smooth muscle actin, a structural protein up-regulated during TGF-beta1-induced phenotypic switching between human renal dysplastic epithelial and mesenchymal lineages in vitro, was also observed in these aberrant epithelia. These findings implicate increased TGF-beta1 signaling in the early biological changes generated by fetal urinary tract obstruction.

Increased expression of TGF-beta1 but not of its receptors contributes to human obstructive nephropathy

Increased expression of TGF-beta1 but not of its receptors contributes to human obstructive nephropathy.

BACKGROUND

Previous studies have revealed an increased expression of transforming growth factor-beta1 (TGF-beta1) and deposition of extracellular matrix in the kidney of animals with ureteral obstruction. However, these relationships have not been elucidated in the hydronephrotic kidney of humans.

METHODS

We analyzed the tissue expression of extracellular matrix proteins, TGF-beta1, and its receptors in the human kidney with ureteral obstruction by immunohistochemistry and reverse transcription-polymerase chain reaction (RT-PCR). Obstructed kidneys (OBKs) were obtained from patients with ureteral tumors. A kidney specimen from patients with a renal tumor was used as control (CNKs).

RESULTS

The interstitial volume was significantly increased in OBKs in comparison with CNKs. OBKs showed increased deposition of collagen types I and IV and fibronectin in the renal interstitium. RT-PCR revealed overexpression of collagen alpha1(IV) mRNA and fibronectin mRNA in OBKs. OBKs showed a significantly increased mRNA expression of TGF-beta1 in comparison with CNKs. The immunoreactivity for TGF-beta1 increased markedly in the interstitium of OBKs. There was a significant correlation between the TGF-beta1 mRNA level and the interstitial volume. However, there was no significant difference between OBKs and CNKs in the relative mRNA level nor in immunoreactivity for TGF-beta receptors.

CONCLUSIONS

These data suggest that TGF-beta1 may contribute to the interstitial fibrosis found in the human kidney with ureteral obstruction, mainly because of an increase in the expression of this cytokine without significant changes to its receptors.