The receptor of advanced glycation end products plays a central role in advanced oxidation protein products-induced podocyte apoptosis

The accumulation of plasma advanced oxidation protein products (AOPPs) is prevalent in chronic kidney disease. We previously showed that accumulation of AOPPs resulted in podocyte apoptosis and their deletion by a cascade of signaling events coupled with intracellular oxidative stress. The transmembrane receptor that specifically transmits the AOPPs' signals to elicit cellular activity, however, remains unknown. Using co-immunoprecipitation and immunofluorescence, we found that AOPPs colocalized and interacted with the receptor of advanced glycation end products (RAGE) on podocytes. Blocking RAGE by anti-RAGE immunoglobulin G or its silencing by siRNA significantly protected podocytes from AOPPs-induced apoptosis both in vitro and in vivo and ameliorated albuminuria in AOPPs-challenged mice. AOPPs-induced activation of nicotinamide adenine dinucleotide phosphate oxidase and the excessive generation of intracellular superoxide were largely inhibited by anti-RAGE immunoglobulin G or RAGE siRNA. Moreover, blockade of RAGE decreased the activation of the p53/Bax/caspase-dependent proapoptotic pathway induced by AOPPs. Thus, AOPPs interact with RAGE to induce podocyte apoptosis and this, in part, may contribute to the progression of chronic kidney disease.

Inhibition of integrin-linked kinase attenuates renal interstitial fibrosis

Integrin-linked kinase (ILK) is an intracellular serine/threonine protein kinase that regulates cell adhesion, survival, and epithelial-to-mesenchymal transition (EMT). In this study, we investigated the kinase activity of ILK during tubular EMT induced by TGF-beta1 and examined the therapeutic potential of an ILK inhibitor in obstructive nephropathy. TGF-beta1 induced a biphasic activation of ILK in renal tubular epithelial cells, with rapid activation starting at 5 min and the second wave of activation peaking at 24 h; the latter paralleled the induction of ILK protein expression. Pharmacologic inhibition of ILK with small-molecule inhibitor QLT-0267 abolished TGF-beta1-induced phosphorylation of Akt and glycogen synthase kinase-3beta, suppressed cyclin D1 expression, and largely restored the expression of E-cadherin and zonula occludens 1. Inhibition of ILK also blocked TGF-beta1-mediated induction of fibronectin, Snail1, plasminogen activator inhibitor 1, and matrix metalloproteinase 2. In a mouse model of obstructive nephropathy, administration of QLT-0267 inhibited beta-catenin accumulation; suppressed Snail1, alpha-smooth muscle actin, fibronectin, vimentin, and type I and type III collagen expression; and reduced total tissue collagen content. Inhibition of ILK did not affect kidney structure or function in normal mice. These findings suggest that increased ILK activity mediates EMT and the progression of renal fibrosis. Pharmacologic inhibition of ILK signaling may hold therapeutic potential for fibrotic kidney diseases.

hepatocyte growth factor is a downstream effector that mediates the antifibrotic action of peroxisome proliferator-activated receptor-gamma agonists

Peroxisome proliferator-activated receptor-gamma (PPAR-gamma) is a ligand-dependent transcription factor that plays an important role in the regulation of insulin sensitivity and lipid metabolism. Evidence shows that PPAR-gamma agonists also ameliorate renal fibrotic lesions in both diabetic nephropathy and nondiabetic chronic kidney disease. However, little is known about the mechanism underlying their antifibrotic action. This study demonstrated that PPAR-gamma agonists could exert their actions by inducing antifibrotic hepatocyte growth factor (HGF) expression. Incubation of mesangial cells with natural or synthetic PPAR-gamma agonists 15-deoxy-Delta12,14-prostaglandin J2 (15d-PGJ2) or troglitazone and ciglitazone suppressed TGF-beta1-mediated alpha-smooth muscle actin, fibronectin, and plasminogen activator inhibitor-1 expression. PPAR-gamma agonists also induced HGF mRNA expression and protein secretion. Transfection studies revealed that 15d-PGJ2 stimulated HGF gene promoter activity, which was dependent on the presence of a novel peroxisome proliferator response element. Treatment of mesangial cells with 15d-PGJ2 induced the binding of PPAR-gamma to the peroxisome proliferator response element in the HGF promoter region. PPAR-gamma agonists also activated c-met receptor tyrosine phosphorylation, induced Smad transcriptional co-repressor TG-interacting factor expression, and blocked TGF-beta/Smad-mediated gene transcription in mesangial cells. Furthermore, ablation of c-met receptor through the LoxP-Cre system in mesangial cells abolished the antifibrotic effect of 15d-PGJ2. PPAR-gamma activation also induced HGF expression in renal interstitial fibroblasts and repressed TGF-beta1-mediated myofibroblast activation. Both HGF and 15d-PGJ2 attenuated Smad nuclear translocation in response to TGF-beta1 stimulation in renal fibroblasts. Together, these findings suggest that HGF may act as a downstream effector that mediates the antifibrotic action of PPAR-gamma agonists.

Hepatocyte growth factor exerts its anti-inflammatory action by disrupting nuclear factor-kappaB signaling

Renal inflammation, characterized by the influx of inflammatory cells, is believed to play a critical role in the initiation and progression of a wide range of chronic kidney diseases. Here, we show that hepatocyte growth factor (HGF) inhibited renal inflammation and proinflammatory chemokine expression by disrupting nuclear factor (NF)-kappaB signaling. In vivo, HGF gene delivery inhibited interstitial infiltration of inflammatory T cells and macrophages, and suppressed expression of both RANTES (regulated on activation, normal T cell expressed and secreted) and monocyte chemoattractant protein-1 in a mouse model of obstructive nephropathy. In vitro, HGF abolished RANTES induction in human kidney epithelial cells, which was dependent on NF-kappaB signaling. HGF did not significantly affect the phosphorylation or degradation of IkappaBalpha; it also did not influence the phosphorylation or nuclear translocation of p65 NF-kappaB. However, HGF prevented p65 NF-kappaB binding to its cognate cis-acting element in the RANTES promoter. HGF action was dependent on the activation of the phosphoinositide 3-kinase/Akt pathway, which led to the phosphorylation and inactivation of glycogen synthase kinase (GSK)-3beta. Suppression of GSK-3beta activity mimicked HGF and abolished RANTES expression, whereas ectopic expression of GSK-3beta restored RANTES induction. HGF also induced renal GSK-3beta phosphorylation and inactivation after obstructive injury in vivo. These observations suggest that HGF is a potent anti-inflammatory cytokine that inhibits renal inflammation by disrupting NF-kappaB signaling and may be a promising therapeutic agent for progressive renal diseases.

Wnt Signaling in Kidney Development and Disease

Wnt signal cascade is an evolutionarily conserved, developmental pathway that regulates embryogenesis, injury repair, and pathogenesis of human diseases. It is well established that Wnt ligands transmit their signal via canonical, β-catenin-dependent and noncanonical, β-catenin-independent mechanisms. Mounting evidence has revealed that Wnt signaling plays a key role in controlling early nephrogenesis and is implicated in the development of various kidney disorders. Dysregulations of Wnt expression cause a variety of developmental abnormalities and human diseases, such as congenital anomalies of the kidney and urinary tract, cystic kidney, and renal carcinoma. Multiple Wnt ligands, their receptors, and transcriptional targets are upregulated during nephron formation, which is crucial for mediating the reciprocal interaction between primordial tissues of ureteric bud and metanephric mesenchyme. Renal cysts are also associated with disrupted Wnt signaling. In addition, Wnt components are important players in renal tumorigenesis. Activation of Wnt/β-catenin is instrumental for tubular repair and regeneration after acute kidney injury. However, sustained activation of this signal cascade is linked to chronic kidney diseases and renal fibrosis in patients and experimental animal models. Mechanistically, Wnt signaling controls a diverse array of biologic processes, such as cell cycle progression, cell polarity and migration, cilia biology, and activation of renin-angiotensin system. In this chapter, we have reviewed recent findings that implicate Wnt signaling in kidney development and diseases. Targeting this signaling may hold promise for future treatment of kidney disorders in patients.

Plasminogen activator inhibitor-1 is a transcriptional target of the canonical pathway of Wnt/beta-catenin signaling

Plasminogen activator inhibitor-1 (PAI-1) is a multifunctional glycoprotein that plays a critical role in the pathogenesis of chronic kidney and cardiovascular diseases. Although transforming growth factor (TGF)-beta1 is a known inducer of PAI-1, how it controls PAI-1 expression remains enigmatic. Here we investigated the mechanism underlying TGF-beta1 regulation of PAI-1 in kidney tubular epithelial cells (HKC-8). Surprisingly, overexpression of Smad2 or Smad3 in HKC-8 cells blocked PAI-1 induction by TGF-beta1, whereas knockdown of them sensitized the cells to TGF-beta1 stimulation, suggesting that Smad signaling is not responsible for PAI-1 induction. Blockade of several TGF-beta1 downstream pathways such as p38 MAPK or JNK, but not phosphatidylinositol 3-kinase/Akt and ERK1/2, only partially inhibited PAI-1 expression. TGF-beta1 stimulated beta-catenin activation in tubular epithelial cells, and ectopic expression of beta-catenin induced PAI-1 expression, whereas inhibition of beta-catenin abolished its induction. A functional T cell factor/lymphoid enhancer-binding factor-binding site was identified in the promoter region of the PAI-1 gene, which interacted with T cell factor upon beta-catenin activation. Deletion or site-directed mutation of this site abolished PAI-1 response to beta-catenin or TGF-beta1 stimulation. Similarly, ectopic expression of Wnt1 also activated PAI-1 expression and promoter activity. In vivo, PAI-1 was induced in kidney tubular epithelia in obstructive nephropathy. Delivery of Wnt1 gene activated beta-catenin and promoted PAI-1 expression after obstructive injury, whereas blockade of Wnt/beta-catenin signaling by Dickkopf-1 gene inhibited PAI-1 induction. Collectively, these studies identify PAI-1 as a direct downstream target of Wnt/beta-catenin signaling and demonstrate that PAI-1 induction could play a role in mediating the fibrogenic action of this signaling.

Single injection of naked plasmid encoding hepatocyte growth factor prevents cell death and ameliorates acute renal failure in mice

Hepatocyte growth factor (HGF) is a pleiotrophic factor that plays an important role in tissue repair and regeneration after injury. The expression of both HGF and its c-met receptor genes is rapidly upregulated after acute renal injury induced by folic acid. In this study, the role of exogenous HGF in preventing acute renal failure by systemic administration of naked plasmid containing human HGF cDNA driven under the cytomegalovirus promoter (pCMV-HGF) was examined in mice. Intravenous injection of pCMV-HGF plasmid produced substantial levels of human HGF protein in mouse kidneys. Simultaneous injection of HGF plasmid DNA significantly ameliorated renal dysfunctions and accelerated recovery from the acute injury induced by folic acid. Of interest, preadministration of HGF plasmid 24 h before folic acid injection dramatically protected renal epithelial cells from both apoptotic and necrotic death and preserved the structural and functional integrity of renal tubules. Expression of HGF transgene activated protein kinase B/Akt kinase and preserved prosurvival Bcl-xL protein expression in vivo. These results indicate that a single, intravenous injection of naked plasmid containing HGF gene not only promotes renal regeneration after injury but also protects tubular epithelial cells from the initial injury and cell death in the first place. These data suggest that HGF gene therapy may provide a new avenue for exploring a novel therapeutic strategy for clinical acute renal failure.

Tubule-Derived Wnts Are Required for Fibroblast Activation and Kidney Fibrosis

Cell-cell communication via Wnt ligands is necessary in regulating embryonic development and has been implicated in CKD. Because Wnt ligands are ubiquitously expressed, the exact cellular source of the Wnts involved in CKD remains undefined. To address this issue, we generated two conditional knockout mouse lines in which Wntless (Wls), a dedicated cargo receptor that is obligatory for Wnt secretion, was selectively ablated in tubular epithelial cells or interstitial fibroblasts. Blockade of Wnt secretion by genetic deletion of Wls in renal tubules markedly inhibited myofibroblast activation and reduced renal fibrosis after unilateral ureteral obstruction. This effect associated with decreased activation of β-catenin and downstream gene expression and preserved tubular epithelial integrity. In contrast, fibroblast-specific deletion of Wls exhibited little effect on the severity of renal fibrosis after obstructive or ischemia-reperfusion injury. In vitro, incubation of normal rat kidney fibroblasts with tubule-derived Wnts promoted fibroblast proliferation and activation. Furthermore, compared with kidney specimens from patients without CKD, biopsy specimens from patients with CKD also displayed increased expression of multiple Wnt proteins, predominantly in renal tubular epithelium. These results illustrate that tubule-derived Wnts have an essential role in promoting fibroblast activation and kidney fibrosis via epithelial-mesenchymal communication.

Urokinase receptor deficiency accelerates renal fibrosis in obstructive nephropathy

The urokinase cellular receptor (uPAR) recognizes the N-terminal growth factor domain of urokinase-type plasminogen activator (uPA) and is expressed by several cell types. The present study was designed to test the hypothesis that uPAR regulates the renal fibrogenic response to chronic injury. Groups of uPAR wild-type (+/+) and deficient (-/-) mice were investigated between 3 and 14 d after unilateral ureteral obstruction (UUO) or sham surgery. Not detected in normal kidneys, uPAR mRNA was expressed in response to UUO in the +/+ mice. By in situ hybridization, uPAR mRNA transcripts were detected in renal tubules and interstitial cells of the obstructed uPAR+/+ kidneys. The severity of renal fibrosis, based on the measurement of total collagen (13.5 +/- 1.5 versus 9.8 +/- 1.0 microg/mg kidney on day 14; -/- versus +/+) and interstitial area stained by Masson trichrome (22 +/- 4% versus 14 +/- 3% on day 14; -/- versus +/+) was significantly greater in the uPAR-/- mice. In the absence of uPAR, renal uPA activity was significantly decreased compared with the wild-type animals after UUO (62 +/- 20 versus 135 +/- 13 units at day 3 UUO; 74 +/- 17 versus 141 +/- 16 at day 7 UUO; 98 +/- 20 versus 165 +/- 10 at day 14 UUO; -/- versus +/+). In contrast, renal expression of several genes that regulate plasmin activity were similar in both genotypes, including uPA, tPA, PAI-1, protease nexin-1, and alpha2-antiplasmin. Worse renal fibrosis in the uPAR-/- mice appears to be TGF-beta-independent, as TGF-beta activity was actually reduced by 65% in the -/- mice despite similar renal TGF-beta1 mRNA levels. Significantly lower levels of the major 2.3-kb transcript and the 69-kd active protein of hepatocyte growth factor (HGF), a known anti-fibrotic growth factor, in the uPAR-/- mice suggests a potential link between HGF and the renoprotective effects of uPAR. These data suggest that renal uPAR attenuates the fibrogenic response to renal injury, an outcome that is mediated in part by urokinase-dependent but plasminogen-independent functions.

Sustained expression of naked plasmid DNA encoding hepatocyte growth factor in mice promotes liver and overall body growth

To understand the physiological functions of exogenous hepatocyte growth factor (HGF) on normal adult animals, we delivered human HGF gene into mice by a hydrodynamics-based in vivo gene transfection approach using a naked plasmid vector. Systemic administration of naked plasmid containing HGF cDNA driven under cytomegalovirus promoter (pCMV-HGF) by rapid injection via the tail vein produced a remarkable level of human HGF protein in the circulation, beginning to appear at 4 hours and peaking at 12 hours following injection. Tissue distribution studies identified the liver as the organ with the highest level of transgene expression. Through weekly repeated injections of plasmid vector, we achieved sustained, long-term, high levels of exogenous HGF expression in mice for 8 weeks. Increases of more than 31% and 16% in liver and body weights were found, respectively, in the mice that received pCMV-HGF plasmid compared with that given the control vector for 8 weeks. Expression of exogenous HGF in vivo activated mitogen-activated protein kinases and induced proliferating cell nuclear antigen expression in normal adult liver and kidneys. These data suggest that systemic administration of naked plasmid vector is a convenient, safe, and highly efficient approach to introduce and maintain exogenous HGF gene expression in vivo in a controllable fashion. Our results also indicate that long-term expression of human HGF in mice markedly activates growth-related signal transduction events, promotes cell proliferation, and leads to liver and overall body growth in whole adult animals.

Intravenous Administration of Hepatocyte Growth Factor Gene Ameliorates Diabetic Nephropathy in Mice

Diabetic nephropathy is characterized by progressive loss of renal function, persistent proteinuria, and relentless accumulation of extracellular matrix leading to glomerulosclerosis and interstitial fibrosis. This study investigated the potential effects of long-term expression of exogenous hepatocyte growth factor (HGF) on normal and diabetic kidneys. Intravenous injection of human HGF gene via naked plasmid vector resulted in abundant HGF protein specifically localized in renal glomeruli, despite an extremely low level of transgene mRNA in the kidney. In uninephrectomized mice made diabetic with streptozotocin, delivery of exogenous HGF gene ameliorated the progression of diabetic nephropathy. HGF attenuated urine albumin and total protein excretion in diabetic mice. Exogenous HGF also mitigated glomerular mesangial expansion, reduced fibronectin and type I collagen deposition, and prevented interstitial myofibroblast activation. In addition, HGF prevented kidney cells from apoptotic death in the glomeruli and tubulointerstitium. Moreover, expression of HGF inhibited renal expression of TGF-beta1 and reduced urine level of TGF-beta1 protein. Therefore, despite the effects of HGF on diabetic nephropathy being controversial, these observations suggest that supplementation of HGF is beneficial in ameliorating diabetic renal insufficiency in mice.

The modulation of caveolin-1 expression controls satellite cell activation during muscle repair

We have previously shown that caveolin-1, the principal structural protein component of caveolar membrane domains, inhibits cellular proliferation and induces cell cycle arrest. We demonstrate here for the first time that caveolin-1 is expressed in satellite cells but not in mature muscle fibers. Satellite cells are quiescent myogenic precursors that, after muscle injury, become mitotically active, proliferate, and fuse together or, to existing myofibers, to form new muscle fibers. We show that down-regulation of caveolin-1 expression occurs in satellite cells/myogenic precursor cells (MPCs) during muscle regeneration and that hepatocyte growth factor, which is produced after muscle injury, down-regulates caveolin-1. We also demonstrate that down-regulation of endogenous caveolin-1 expression activates ERK and that activation of the p42/44 MAP kinase pathway is necessary to promote muscle regeneration. Finally, we show that overexpression of caveolin-1 inhibits muscle repair mechanisms both in vitro and in vivo. Taken together, these results propose caveolin-1 as a novel regulator of satellite cell functions and suggest that the following signaling pathway modulates satellite cell activation during muscle repair: injured fibers release HGF --> HGF down-regulates caveolin-1 protein expression --> down-regulation of caveolin-1 activates ERK --> activation of ERK promotes muscle repair by stimulating the proliferation and migration of MPCs toward the wounded area.

Tenascin-C Is a Major Component of the Fibrogenic Niche in Kidney Fibrosis

Kidney fibrosis initiates at certain focal sites in which the fibrogenic niche provides a specialized microenvironment that facilitates fibroblast activation and proliferation. However, the molecular identity of these fibrogenic niches is poorly characterized. Here, we determined whether tenascin-C (TNC), an extracellular matrix glycoprotein, is a component of the fibrogenic niche in kidney fibrosis. In vivo, TNC expression increased rapidly in kidneys subjected to unilateral ureteral obstruction or ischemia/reperfusion injury and predominantly localized at the foci rich in fibroblasts in renal interstitium. In vitro, TNC selectively promoted renal interstitial fibroblast proliferation, bromodeoxyuridine incorporation, and the expression of proliferation-related genes. The mitogenic activity of TNC required the integrin/focal adhesion kinase/mitogen-activated protein kinase signaling cascade. Using decellularized extracellular matrix scaffolds, we found that TNC-enriched scaffolds facilitated fibroblast proliferation, whereas TNC-deprived scaffolds inhibited proliferation. Matrix scaffold prepared from fibrotic kidney also promoted greater ex vivo fibroblast proliferation than did scaffolds prepared from healthy kidney. Conversely, small interfering RNA-mediated knockdown of TNC in vivo repressed injury-induced fibroblast expansion and renal fibrosis. These studies identify TNC as a major constituent of the fibrogenic niche that promotes fibroblast proliferation, and illustrate a pivotal role for the TNC-enriched microenvironment in kidney fibrogenesis.

Activation of Wnt/beta-catenin pathway during hepatocyte growth factor-induced hepatomegaly in mice

Hepatocyte growth factor (HGF) and beta-catenin both play a crucial role in stimulating hepatocyte proliferation, but whether these 2 pathways cooperate in inducing hepatocyte proliferation is unclear. We have previously reported that beta-catenin forms a complex with c-Met (HGF receptor) that undergoes dissociation because of beta-catenin tyrosine phosphorylation on stimulation by HGF. It is also known that delivery of the human HGF gene cloned in a plasmid under a CMV promoter results in hepatomegaly in mice. In addition, recently characterized beta-catenin transgenic mice also showed hepatomegaly. The present study was based on the hypothesis that HGF-induced hepatomegaly is mediated, at least in part, by activation of the Wnt/beta-catenin pathway. Here we report that delivery of the human HGF gene delivery in mice led to hepatomegaly via beta-catenin activation in the liver in 1- and 4-week studies. The mechanisms of beta-catenin activation in the 1-week study included loss of c-Met-beta-catenin association as well as canonical beta-catenin activation, leading to its nuclear translocation. In the 4-week study, beta-catenin activation was observed via canonical mechanisms, whereas the c-Met-beta-catenin complex remained unchanged. In both studies there was an associated increase in the E-cadherin-beta-catenin association at the membrane. In addition, we generated liver-specific beta-catenin knockout mice, which demonstrated significantly smaller livers. HGF gene delivery failed to induce hepatomegaly in these beta-catenin conditionally null mice. In conclusion, beta-catenin- and HGF-mediated signaling pathways cooperate in hepatocyte proliferation, which may be crucial in liver development, regeneration following partial hepatectomy, and pathogenesis of hepatocellular carcinoma.

Tubule-derived exosomes play a central role in fibroblast activation and kidney fibrosis

Extracellular vesicles such as exosomes are involved in mediating cell-cell communication by shuttling an assortment of proteins and genetic information. Here, we tested whether renal tubule-derived exosomes play a central role in mediating kidney fibrosis. The production of exosomes was found to be increased in the early stage of unilateral ureteral obstruction, ischemia reperfusion injury or 5/6 nephrectomy models of kidney disease. Exosome production occurred primarily in renal proximal tubular epithelium and was accompanied by induction of sonic hedgehog (Shh). In vitro, upon stimulation with transforming growth factor-β1, kidney proximal tubular cells (HKC-8) increased exosome production. Purified exosomes from these cells were able to induce renal interstitial fibroblast (NRK-49F) activation. Conversely, pharmacologic inhibition of exosome secretion with dimethyl amiloride, depletion of exosome from the conditioned media or knockdown of Shh expression abolished the ability of transforming growth factor-β1-treated HKC-8 cells to induce NRK-49F activation. In vivo, injections of tubular cell-derived exosomes aggravated kidney injury and fibrosis, which was negated by an Shh signaling inhibitor. Blockade of exosome secretion in vivo ameliorated renal fibrosis after either ischemic or obstructive injury. Furthermore, knockdown of Rab27a, a protein that is essential for exosome formation, also preserved kidney function and attenuated renal fibrotic lesions in mice. Thus, our results suggest that tubule-derived exosomes play an essential role in renal fibrogenesis through shuttling Shh ligand. Hence, strategies targeting exosomes could be a new avenue in developing therapeutics against renal fibrosis.

Signaling Crosstalk between Tubular Epithelial Cells and Interstitial Fibroblasts after Kidney Injury

BACKGROUND

A wide variety of kidney diseases ultimately lead to tubulointerstitial damage. The initial site of injury is usually the renal tubules, with activation of fibroblasts occurring later. Self-limited disease is characterized by transient cellular activation with timed deactivation and ultimately a return to normal functioning, whereas sustained responses characterize chronic disease and the development of irreversible fibrosis. The underlying molecular and cellular mechanisms of this cascade of events remain an area of active research. Current data overwhelmingly support a role for crosstalk between the tubular epithelium and the interstitial fibroblast that mediates both repair/regeneration and progressive disease. This epithelial-mesenchymal communication (EMC) is regulated by a variety of soluble ligands binding to cell surface receptors to induce intracellular signaling events.

SUMMARY

EMC is an important mechanism whereby tubular epithelium and fibroblasts/mesenchymal cells crosstalk to affect renal physiology and pathology. Numerous soluble factors such as sonic hedgehog, Wnt ligands, transforming growth factor-β, hepatocyte growth factor, connective tissue growth factor, and angiotensin II all participate in bidirectional EMC. Recent studies have also identified exosomes as a vehicle to mediate EMC during kidney injury. In general, while the short-term activity of EMC factors is renoprotective, prolonged activation of these factors leads to chronic disease and fibrosis.

KEY MESSAGES

The discovery of a complex and intricate system of communication between tubular cells and fibroblasts is a new paradigm in our understanding of renal fibrosis. An appreciation of both their regenerative and pathologic functions will inform the development and use of targeted therapeutic interventions.

Kindlin-2 mediates activation of TGF-β/Smad signaling and renal fibrosis

Activation of TGF-β/Smad signaling plays a central role in the pathogenesis of tubulointerstitial fibrosis, but the mechanisms underlying the initial interaction of the TGF-β receptor with Smads, leading to their activation, remain unclear. Here, we found that Kindlin-2, an integrin-binding protein, physically mediated the interaction of the TGF-β type I receptor (TβRI) with Smad3 in human kidney tubular epithelial cells. Kindlin-2 bound to TβRI through its FERM domain and to Smad3 through its N terminus. Overexpression of Kindlin-2 increased TGF-β-induced Smad3 activation. Knockdown of Kindlin-2 significantly suppressed the engagement of TβRI with Smad3 and inhibited TGF-β-induced Smad3 activation, as well as the expression of its target genes. Neither transfection of a Kindlin-2 mutant incapable of binding to β1 integrin nor knockdown of β1 integrin influenced the effect of Kindlin-2 on TGF-β1-induced Smad3 activation, indicating that this effect is independent of integrin. Kindlin-2 expression was markedly increased, predominantly in renal tubular epithelial cells, both in the unilateral ureteral obstruction model of kidney fibrosis and in human tissue exhibiting tubulointerstitial fibrosis. Furthermore, in the unilateral ureteral obstruction model, knocking down Kindlin-2 significantly inhibited activation of TGF-β/Smad signaling, decreased the expression of matrix genes, and ameliorated fibrosis. In summary, Kindlin-2 physically interacts with both TβRI and Smad3, promoting the activation of TGF-β/Smad signaling and contributing to the pathogenesis of tubulointerstitial fibrosis. Blockade of Kindlin-2 might be a rational therapeutic strategy for the treatment of fibrotic kidney diseases.

Hepatocyte growth factor signaling ameliorates podocyte injury and proteinuria

Hepatocyte growth factor (HGF) is a potent antifibrotic protein that inhibits kidney fibrosis through several mechanisms. To study its role in podocyte homeostasis, injury, and repair in vivo, we generated conditional knockout mice in which the HGF receptor, c-met, was specifically deleted in podocytes using the Cre-LoxP system. Mice with podocyte-specific ablation of c-met (podo-met(-/-)) developed normally. No albuminuria or overt pathologic lesions were detected up to 6 months of age, suggesting that HGF signaling is dispensable for podocyte maturation, survival, and function under normal physiologic conditions. However, after adriamycin treatment, podo-met(-/-) mice developed more severe podocyte injury and albuminuria than their control littermates. Ablation of c-met also resulted in more profound suppression of Wilms tumor 1 (WT1) and nephrin expression, and podocyte apoptosis after injury. When HGF was expressed ectopically in vivo, it ameliorated adriamycin-induced albuminuria, preserved WT1 and nephrin expression, and inhibited podocyte apoptosis. However, exogenous HGF failed to significantly reduce albuminuria in podo-met(-/-) mice, suggesting that podocyte-specific c-met activation by HGF confers renal protection. In vitro, HGF was able to preserve WT1 and nephrin expression in cultured podocytes after adriamycin treatment. HGF also protected podocytes from apoptosis induced by a lethal dose of adriamycin primarily through a phosphoinositide 3-kinase (PI3K)/Akt-dependent pathway. Collectively, these results indicate that HGF/c-met signaling has an important role in protecting podocytes from injury, thereby reducing proteinuria.

LRP5 and LRP6 in Wnt Signaling: Similarity and Divergence

The canonical Wnt/β-catenin signaling plays a fundamental role in regulating embryonic development, injury repair and the pathogenesis of human diseases. In vertebrates, low density lipoprotein receptor-related proteins 5 and 6 (LRP5 and LRP6), the single-pass transmembrane proteins, act as coreceptors of Wnt ligands and are indispensable for Wnt signal transduction. LRP5 and LRP6 are highly homologous and widely co-expressed in embryonic and adult tissues, and they share similar function in mediating Wnt signaling. However, they also exhibit distinct characteristics by interacting with different protein partners. As such, each of them possesses its own unique functions. In this review, we systematically discuss the similarity and divergence of LRP5 and LRP6 in mediating Wnt and other signaling in the context of kidney diseases. A better understanding of the precise role of LRP5 and LRP6 may afford us to identify and refine therapeutic targets for the treatment of a variety of human diseases.

(Pro)renin Receptor Is an Amplifier of Wnt/β-Catenin Signaling in Kidney Injury and Fibrosis

The (pro)renin receptor (PRR) is a transmembrane protein with multiple functions. However, its regulation and role in the pathogenesis of CKD remain poorly defined. Here, we report that PRR is a downstream target and an essential component of Wnt/β-catenin signaling. In mouse models, induction of CKD by ischemia-reperfusion injury (IRI), adriamycin, or angiotensin II infusion upregulated PRR expression in kidney tubular epithelium. Immunohistochemical staining of kidney biopsy specimens also revealed induction of renal PRR in human CKD. Overexpression of either Wnt1 or β-catenin induced PRR mRNA and protein expression in vitro Notably, forced expression of PRR potentiated Wnt1-mediated β-catenin activation and augmented the expression of downstream targets such as fibronectin, plasminogen activator inhibitor 1, and α-smooth muscle actin (α-SMA). Conversely, knockdown of PRR by siRNA abolished β-catenin activation. PRR potentiation of Wnt/β-catenin signaling did not require renin, but required vacuolar H⁺ ATPase activity. In the mouse model of IRI, transfection with PRR or Wnt1 expression vectors promoted β-catenin activation, aggravated kidney dysfunction, and worsened renal inflammation and fibrotic lesions. Coexpression of PRR and Wnt1 had a synergistic effect. In contrast, knockdown of PRR expression ameliorated kidney injury and fibrosis after IRI. These results indicate that PRR is both a downstream target and a crucial element in Wnt signal transmission. We conclude that PRR can promote kidney injury and fibrosis by amplifying Wnt/β-catenin signaling.

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.

1,25-dihydroxyvitamin D inhibits renal interstitial myofibroblast activation by inducing hepatocyte growth factor expression

BACKGROUND

Vitamin D and its metabolites play an important role in calcium homeostasis, bone remodeling, hormone secretion, cell proliferation, and differentiation. Recent studies also suggest a beneficial role of vitamin D in slowing the progression of chronic renal glomerular diseases. This study investigated the effects and potential mechanism of 1,25-dihydroxyvitamin D(3)[1,25(OH)(2)D(3)] on the regulation of myofibroblast activation from interstitial fibroblast, a critical event in generating alpha-smooth muscle actin (alphaSMA)-positive, matrix-producing effector cells in renal interstitial fibrosis.

METHODS

Normal rat renal interstitial fibroblast cell line (NRK-49F) was used as a model system. Myofibroblast activation was initiated by incubation with transforming growth factor (TGF)-beta1. Expression of alpha-SMA, collagen I, thrombospondin-1, and hepatocyte growth factor (HGF) was assessed by reverse transcription-polymerase chain reaction (RT-PCR), Western blot, and immunostaining, respectively. HGF promoter activity was evaluated by using luciferase reporter assay.

RESULTS

Incubation of rat renal interstitial fibroblasts (NRK-49F) with 1,25(OH)(2)D(3) suppressed TGF-beta1-induced de novo alpha-SMA expression in a dose-dependent manner. 1,25(OH)(2)D(3) also suppressed type I collagen and thrombospondin-1 expression induced by TGF-beta1. Interestingly, 1,25(OH)(2)D(3) induced HGF mRNA expression and protein secretion in renal interstitial fibroblasts. Transfection studies revealed that 1,25(OH)(2)D(3) stimulated HGF gene promoter activity, which was dependent on the presence of vitamin D response element (VDRE). 1,25(OH)(2)D(3) induced the binding of vitamin D receptor to the VDRE in HGF promoter region. Furthermore, 1,25(OH)(2)D(3) was capable of stimulating HGF receptor phosphorylation in renal fibroblasts. Incubation with specific HGF neutralizing antibody largely abolished 1,25(OH)(2)D(3)-mediated suppression of myofibroblast activation.

CONCLUSION

These observations suggest that vitamin D analogue possesses renoprotective activity through suppression of the matrix-producing myofibroblast activation. This action of vitamin D is mediated, at least in part, by up-regulating antifibrotic HGF gene expression in renal interstitial fibroblasts.

Systemic administration of naked plasmid encoding hepatocyte growth factor ameliorates chronic renal fibrosis in mice

The progression of chronic renal diseases is considered as an irreversible process that eventually leads to end-stage renal failure characterized by extensive tissue fibrosis. At present, chronic renal fibrosis is incurable and the incidence of affected patients is on the rise worldwide. In this study, we demonstrate that delivery of hepatocyte growth factor (HGF) gene via systemic administration of naked plasmid vector markedly ameliorated renal fibrosis in an animal model of chronic renal disease induced by unilateral ureteral obstruction. A high level of exogenous HGF protein was detected in the obstructed kidneys following intravenous injection of naked plasmid encoding human HGF. Delivery of human HGF gene induced a sustained activation of extracellular signal-regulated kinases-1 and -2 in the obstructed kidneys. Exogenous HGF expression dramatically inhibited alpha-smooth muscle actin expression, attenuated renal interstitial accumulation and deposition of collagen I and fibronectin. In addition, exogenous HGF suppressed renal expression of pro-fibrogenic cytokine TGF-beta1 and its type I receptor in vivo. These results suggest that systemic administration of naked plasmid vector introduces a high level of exogenous HGF to the diseased kidneys, and that HGF gene transfer may provide a novel therapeutic strategy for amelioration of chronic renal fibrosis in vivo.

Hepatocyte growth factor protects renal epithelial cells from apoptotic cell death

Hepatocyte growth factor (HGF) is a pleiotropic factor that plays an essential role in renal tubular repair and regeneration following injury. Studies indicate that administration of exogenous HGF to animals stimulates renal epithelial cell DNA synthesis and accelerates recovery from acute renal failure (ARF). However, whether increased cell proliferation accounts for all of the beneficial effects of HGF in ARF is unknown. In this study, we demonstrate that HGF protects renal epithelial cells from undergoing apoptotic cell death. Treatment of renal epithelial mIMCD-3 cells with 25 microM cisplatin in the serum-free medium induced significant apoptosis, as assessed by fluorescent Dye H-33342 staining, TUNEL staining, light and electron microscopy, and DNA laddering analysis. However, constitutive expression of HGF by transfection in mIMCD-3 cells resulted in resistance to cisplatin-induced apoptotic death. The survival rate of HGF-producing C1 cells was more than 2-fold greater as compared to control, mIMCD-3 cells following treatment with 25 microM cisplatin for 2 days. These results suggest that HGF may not only activate tubular repair processes but also ameliorate the initial injury by protecting renal epithelial cells from undergoing apoptosis.

The human hepatocyte growth factor receptor gene: complete structural organization and promoter characterization

The hepatocyte growth factor (HGF) receptor is a tyrosine kinase transmembrane protein encoded by the c-met proto-oncogene. Here, we have isolated and characterized human genomic DNA clones containing the entire coding sequence of the HGF receptor. The gene spans approx. 120kb in length and consists of 21 exons interrupted by 20 introns. Two alternative splice sites corresponding to known mRNA transcripts are located at exon 10 and exon 14, respectively. Structural analysis of the 5'-regulatory region reveals that the c-met promoter lacks TATA or CAAT elements but has an extremely high G-C content and multiple Sp1 binding sites. By transfection of a series of chimeric reporter constructs containing a variable region of the c-met promoter and the coding region for chloramphenicol acetyltransferase, we have identified two positive, and one negative regulatory elements that dictate c-met transcription in renal epithelial mIMCD-3 cells at nucleotide positions -2615 to -1621, -223 to -68, and -1621 to -1093, respectively. Moreover, deletion and mutation of the multiple Sp1 sites in the c-met promoter region markedly reduced c-met promoter activity in mIMCD-3 cells, suggesting that the Sp1 sites are essential for establishing the constitutive expression of the c-met gene. These data provide fundamental information on HGF receptor gene organization, as well as on the genomic origin of different receptor isoforms, and should facilitate further studies on the transcriptional regulation of its expression.