Acute renal failure. III. The role of growth factors in the process of renal regeneration and repair

Administered mesenchymal stem cells protect against ischemic acute renal failure through differentiation-independent mechanisms

Severe acute renal failure (ARF) remains a common, largely treatment-resistant clinical problem with disturbingly high mortality rates. Therefore, we tested whether administration of multipotent mesenchymal stem cells (MSC) to anesthetized rats with ischemia-reperfusion-induced ARF (40-min bilateral renal pedicle clamping) could improve the outcome through amelioration of inflammatory, vascular, and apoptotic/necrotic manifestations of ischemic kidney injury. Accordingly, intracarotid administration of MSC (∼ 106/animal) either immediately or 24 h after renal ischemia resulted in significantly improved renal function, higher proliferative and lower apoptotic indexes, as well as lower renal injury and unchanged leukocyte infiltration scores. Such renoprotection was not obtained with syngeneic fibroblasts. Using in vivo two-photon laser confocal microscopy, fluorescence-labeled MSC were detected early after injection in glomeruli, and low numbers attached at microvasculature sites. However, within 3 days of administration, none of the administered MSC had differentiated into a tubular or endothelial cell phenotype. At 24 h after injury, expression of proinflammatory cytokines IL-1β, TNF-α, IFN-γ, and inducible nitric oxide synthase was significantly reduced and that of anti-inflammatory IL-10 and bFGF, TGF-α, and Bcl-2 was highly upregulated in treated kidneys. We conclude that the early, highly significant renoprotection obtained with MSC is of considerable therapeutic promise for the cell-based management of clinical ARF. The beneficial effects of MSC are primarily mediated via complex paracrine actions and not by their differentiation into target cells, which, as such, appears to be a more protracted response that may become important in late-stage organ repair.

Molecular mechanisms of acute renal failure following ischemia/reperfusion

Acute renal failure (ARF) necessitating renal replacement therapy is a common problem associated with high morbidity and mortality in the critically ill. Hypotension, followed by resuscitation, is the most common etiologic factor, mimicked by ischemia/reperfusion (I/R) in animal models. Although knowledge of the pathophysiology of ARF in the course of this condition is increasingly detailed, the intracellular and molecular mechanisms leading to ARF are still incompletely understood. This review aims at describing the role of cellular events and signals, including collapse of the cytoskeleton, mitochondrial and nuclear changes, in mediating cell dysfunction, programmed cell death (apoptosis), necrosis and others. Insight into the molecular pathways in the various elements of the kidney, such as vascular endothelium and smooth muscle and tubular epithelium leading to cell damage upon I/R will, hopefully, open new therapeutic modalities, to mitigate the development of ARF after hypotensive episodes and to promote repair and resumption of renal function once ARF has developed.

Cytoprotection by darbepoetin/epoetin alfa in pig tubular and mouse mesangial cells

BACKGROUND

Erythropoietin has recently been found to have cytoprotective effects in the central nervous system (CNS) and retina. The purpose of this study was to determine if darbepoetin alfa (DA) has cytoprotective properties in renal tissues.

METHODS

DA was studied in LLC/PK1 and mesangial cells. Renal cellular injury was induced in different experiments by prostaglandin D2 synthase (PGDS), camptothecin, hydrogen peroxide, and hypoxia. Cellular proliferation and apoptosis were measured [apoptosis by terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate (dUTP) nick end-labeling (TUNEL) assay or by caspase-3 activity]. In a separate experiment, an inactive form of erythropoietin alfa was used to study receptor effects.

RESULTS

DA protected against the antiproliferative effects of PGDS. In both LLC/PK1 (TUNEL and caspase-3) and mesangial cells (TUNEL), DA reduced the apoptotic stimulus of PGDS. Epoetin alfa was also found to reduce apoptosis. In LLC/PK1 cells, DA reduced apoptosis induced by camptothecin, but not hydrogen peroxide. DA reduced LLC/PK1 apoptosis induced by hypoxia when added 24 hours before hypoxia, but not when given concurrent with the hypoxic stimulus. Erythropoietin inactive did not protect against PGDS-induced apoptosis.

CONCLUSION

DA has renal antiapoptotic effects for both toxic and hypoxic stimuli. The effect may be mediated via the Erythropoietin receptor.

Identification of renal progenitor-like tubular cells that participate in the regeneration processes of the kidney

The present study was conducted to explore renal progenitor-like cells that are actively engaged in tubular regeneration after injury. For addressing this issue, the existence of label-retaining cells (LRC; slow-cycling cells) in normal rat kidneys by in vivo bromodeoxyuridine (BrdU) labeling was examined. LRC were scattering among renal epithelial tubular cells of normal rat kidneys. During the recovery after renal ischemia, LRC underwent cell division and most of them became positive for proliferating cell nuclear antigen. In contrast, proliferating cell nuclear antigen-positive but BrdU-negative tubular cells were rarely observed, suggesting that cells proliferating during tubular regeneration are essentially derived from LRC. At an early phase of tubular regeneration, descendants of LRC expressed a mesenchymal marker, vimentin, and eventually became positive for an epithelial marker, E-cadherin, after multiple cell divisions. These findings suggested that LRC function as a source of regenerating cells to replace injured cells. Collectively, it was concluded that LRC are renal progenitor-like tubular cells that provide regenerating cells, which actively proliferate and eventually differentiate into epithelial cell, during tubular regeneration. It may be possible to regenerate renal tubules in vivo through the activation of LRC.

Requirement of the epidermal growth factor receptor in renal epithelial cell proliferation and migration

We showed that renal proximal tubular cells (RPTC) can proliferate and migrate following plating and oxidant or mechanical injury in the absence of exogenous growth factors; however, the mechanisms of this response remain unclear. We examined whether epidermal growth factor receptor (EGFR) signaling is activated following plating and mechanical injury and mediates RPTC proliferation and migration. EGFR, Akt [a target of phosphoinositide-3-kinase (PI3K)], and ERK1/2 were activated after plating and mechanical injury, and their phosphorylation was further enhanced by addition of exogenous EGF. Inactivation of the EGFR with the selective inhibitor AG-1478 completely blocked phosphorylation of EGFR, Akt, and ERK1/2 and blocked cell proliferation and migration after plating and injury. Inhibition of PI3K with LY-294002 blocked Akt phosphorylation and proliferation, whereas U-0126 blocked ERK1/2 phosphorylation but had no effect on proliferation. Furthermore, p38 was phosphorylated following mechanical injury and the p38 inhibitor SB-203580 blocked p38 phosphorylation and cell migration. In contrast, neither PI3K nor ERK1/2 inhibition blocked cell migration. These results show that EGFR activation is required for RPTC proliferation and migration and that proliferation is mediated by PI3K, whereas migration is mediated by p38.

Molecular mechanisms of renal tissue repair in survival from acute renal tubule necrosis: role of ERK1/2 pathway

Our earlier studies with S-(1,2-dichlorovinyl)-L-cysteine (DCVC) showed that prior administration of a low priming dose of 15 mg/kg, i.p. to mice, given 72 hours before administration of a normally lethal dose of DCVC (75 mg/kg, i.p.) led to renal tubule necrosis, however sustained renal tubule regeneration was observed and these mice recovered from renal failure and survived. The objective of the present study was to investigate the role of extracellular signal-regulated kinase (ERK) pathway in this autoprotection model. Following the priming dose of DCVC, IL-6 protein and mRNA increased markedly as early as 1 hour after dosing, peaking at 3 hours with a 1.5-fold increase in plasma. Immunocytochemistry on kidney sections using specific antibodies against TGF-alpha, HB-EGF, EGFr, IGF-1Rbeta, Grb-2, and phospho-p44/42 MAP kinase (ERK1/2) revealed a significantly higher staining of these molecules 3 to 72 hours after dosing, indicating up regulation of the ERK pathway. Following a lethal dose of DCVC (75 mg/kg) the early increase in these signaling molecules was not sustained, being markedly reduced 24 and 36 hours after dosing, leading to inhibition of S-phase DNA synthesis, cell division and renal tubule repair. In contrast, prior treatment with a low dose of DCVC, followed by a high dose led to a sustained stimulation of the renal ERK pathway, renal tubule regeneration and recovery from acute renal failure. These results suggest that a sustained activation of the ERK1/2 pathway may be a key factor in enabling a continued renal tubule repair and hence protection from the progressive phase of DCVC-induced acute renal tubular necrosis in the mouse.

Action of EGF and PGE2on basolateral organic anion uptake in rabbit proximal renal tubules and hOAT1 expressed in human kidney epithelial cells

We recently showed that, in a proximal tubule cell line (opossum kidney cells), epithelial growth factor (EGF) stimulates basolateral organic anion transport (OAT) via ERK1/2, arachidonic acid, phospholipase A2, and generation of prostaglandins. PGE2binds the prostanoid receptor and, thus, activates adenylate cyclase and PKA, which stimulate basolateral organic anion uptake. In the present study, we investigated whether this regulatory cascade is also true 1) for ex vivo conditions in isolated renal proximal (S2) tubules from rabbit and 2) in a human renal epithelial cell line stably expressing human OAT1 (IHKE-hOAT1). EGF activated ERK1/2 in S2 tubules and IHKE-hOAT1, and, in both cases, inhibition of ERK activation (by U-0126) abolished this stimulation. In S2 tubules and IHKE-hOAT1, EGF led to an increase of organic anion uptake, which again was inhibited by U-0126. PGE2stimulated basolateral organic anion uptake in rabbit S2 tubules and IHKE-hOAT1. EGF- and PGE2-mediated stimulation of organic anion uptake was abolished by inhibition of PKA in rabbit S2 tubules and IHKE-hOAT1, respectively. We conclude that 1) stimulation of basolateral organic anion uptake by EGF or PGE2is a widespread (if not general) regulatory mechanism, 2) the signal transduction pathway involved seems to be general, 3) stimulation of basolateral organic anion uptake by EGF or PGE2is also present under ex vivo conditions and, thus, is not a cell culture artifact, 4) activation of OAT1 is sufficient to explain the stimulatory effects of EGF and PGE2in opossum kidney cells and rabbit S2 segments, and 5) stimulation of basolateral OAT1 by EGF or PGE2is also important in humans and, thus, may have clinical implications.

The story so far: Molecular regulation of the heme oxygenase-1 gene in renal injury

Heme oxygenases (HOs) catalyze the rate-limiting step in heme degradation, resulting in the formation of iron, carbon monoxide, and biliverdin, the latter of which is subsequently converted to bilirubin by biliverdin reductase. Recent attention has focused on the biological effects of product(s) of this enzymatic reaction, which have important antioxidant, anti-inflammatory, and cytoprotective functions. Two major isoforms of the HO enzyme have been described: an inducible isoform, HO-1, and a constitutively expressed isoform, HO-2. A third isoform, HO-3, closely related to HO-2, has also been described. Several stimuli implicated in the pathogenesis of renal injury, such as heme, nitric oxide, growth factors, angiotensin II, cytokines, and nephrotoxins, induce HO-1. Induction of HO-1 occurs as an adaptive and beneficial response to these stimuli, as demonstrated by studies in renal and non-renal disease states. This review will focus on the molecular regulation of the HO-1 gene in renal injury and will highlight the interspecies differences, predominantly between the rodent and human HO-1 genes.

Ischemia-reperfusion induces G-CSF gene expression by renal medullary thick ascending limb cells in vivo and in vitro

Ischemic acute renal failure involves not only the kidney but also extrarenal organs such as the bone marrow that produces inflammatory cells. By ELISA and RNase protection assays, we now show that renal ischemia-reperfusion increases serum concentrations of granulocyte macrophage colony-stimulating factor (G-CSF) protein and increases both G-CSF mRNA and protein in the ischemic kidney. In situ hybridization localized the increased G-CSF mRNA to tubule cells, including medullary thick ascending limb cells (mTAL), in the outer medulla. We also show that mTAL produce G-CSF protein and increase G-CSF mRNA after stimulation by reactive oxygen species in vitro. The production of G-CSF by the kidney after ischemia-reperfusion provides a means of communication from the injured kidney to the bone marrow. This supports the known inflammatory response to ischemia.

Developmental approaches to kidney tissue engineering

Recent advances in our understanding of the developmental biology of the kidney, as well as the establishment of novel in vitro model systems, have potential implications for kidney tissue engineering. These advances include delineation of the roles of a number of growth factors in the developmental programs of branching morphogenesis and mesenchymal differentiation, a new understanding of the roles of the extracellular matrix, identification of potential “renal” stem cells, the ex vivo propagation and subsequent recombination of isolated components of the kidney, and successful transplantation of renal primordia into adult hosts. This review will examine these advances in the context of approaches to tissue engineering. Finally, novel approaches that synthesize advances in both cell-based and organ-based approaches are proposed.

Renal expression and activity of the germinal center kinase SK2

Rat fetal kidney mRNA was analyzed by RT-PCR to identify protein kinases. This screening demonstrated expression of a protein kinase consistent with SK2, a group II germinal center kinase and homolog of human Ste20-like kinase (SLK). SK2 mRNA, protein expression, and kinase activity were increased in rat fetal kidney homogenates (embryonic days 17-21) compared with adult controls. In adult kidneys subjected to cross-clamping of the renal artery, followed by reperfusion, SK2 mRNA, protein expression, and kinase activity were increased compared with untreated contralateral controls. By immunohistochemistry, SK2 expression was evident mainly in the cytoplasm of tubular epithelial cells in fetal and adult kidneys. There was also some expression in developing and mature podocytes, but staining of the interstitium was negative. In cultured renal tubular epithelial cells, SK2 kinase activity was increased after incubation with serum, or after exposure to chemical anoxia plus reexposure to glucose. Stable overexpression of SLK reduced cell proliferation and increased apoptosis and exacerbated apoptosis and necrosis after chemical anoxia plus reexposure to glucose. Thus SK2 is a renal epithelial protein kinase whose expression and activity are increased during development and recovery from acute renal failure, where tubular epithelial regeneration may recapitulate developmental processes. The actions of SK2 appear to be antiproliferative and may facilitate cell injury.

Molecular mechanisms of recovery from acute renal failure

OBJECTIVES

Despite technological advances in renal replacement therapy over the past few years, acute renal failure in the intensive care unit remains associated with high morbidity and mortality rates. In this article I review recent research aimed at elucidating mechanisms of renal recovery from acute injury.

DESIGN

Review of the literature.

CONCLUSIONS

A number of peptide growth hormones are reviewed, including epidermal growth factor, insulin-like growth factor-1, thyroxine, hepatocyte growth factor, and bone morphogenetic protein-7 promote renal regeneration in model systems. Unfortunately, despite promising studies in animal models of toxin and ischemia-induced acute tubular necrosis, human studies have not shown any clinical benefit. However, several of these molecules have not been studied in clinical trials. Existing pharmacologic strategies have a limited role in renal recovery. Finally, several recent studies have focused on the effects of renal replacement therapy on renal recovery, but additional studies are needed to confirm and extend these results.

Role of extracellular matrix in kidney development and repair

Extracellular matrix (ECM) molecules and their receptors exert a dynamic role in cell-matrix interactions during kidney development and repair processes. They provide a physical substratum for the spatial organization of the cells, but also regulate cell growth and proliferation by interacting with growth factors. In addition, they can regulate signal transduction pathways by binding to integrins or by modulating the activity of signaling molecules such as Wnts. ECM and ECM-related molecules control multiple (if not all) steps of kidney development, including ureteric bud branching morphogenesis, mesenchymal condensation, nephron formation, terminal differentiation of renal tubules, and glomerular basement membrane assembly. Their role still needs to be better documented in renal repair. The emergence of conditionally mutated mice for basement membrane components will provide a useful tool to demonstrate further the involvement of ECM and ECM-related proteins in development and repair.

Nephrogenesis is induced by partial nephrectomy in the elasmobranch Leucoraja erinacea

The mammalian kidney responds to partial nephrectomy with glomerular and tubular hypertrophy, but without renal regeneration. In contrast, renal regeneration in lower vertebrates is known to occur. Understanding the underlying mechanisms of renal regeneration is highly important; however, a serviceable animal model has not been developed. A neonephrogenic zone has been identified in the European lesser spotted dogfish, Scyliorhinus caniculus (Hentschel H. Am J Anat 190: 309-333, 1991), as well as in the spiny dogfish Squalus acanthias and the little skate, Leucoraja erinacea. The zone features the production of new nephrons complete with a countercurrent system. To analyze this nephrogenic region of elasmobranch fish further, a renal reduction model was established. The neonephrogenic zone in the adult kidney of the little skate resembles the embryonic metanephric kidney and contains stem cell-like mesenchymal cells, tips of the branching collecting duct system, and outgrowth of the arterial system. Four stages of nephron development were analyzed by serial sections and defined: stage I, aggregated mesenchymal cells; stage II, S-shaped body-like structure with high-prismatic epithelial cells; stage III, segmental nephron segregation; stage IV, functioning nephron. The stages were analyzed after partial nephrectomy. In addition, cell proliferation was assessed by incorporation of bromo-deoxyuridine (BrdU). New nephrons developed in animals undergoing partial nephrectomy. Growth was greatly stimulated in the nephrogenic zone, both in the remnant tissue and in the contralateral kidney within 10 wk. Mesenchymal cell aggregates increased significantly per renal cross-section compared with controls (stage I, 0.64 +/- 0.28 versus 0.27 +/- 0.25; P < 0.005; n = 10 animals per group). The same was the case for S-shaped body-like cysts (stage II, 0.24 +/- 0.19 versus 0.08 +/- 0.09; P < 0.02). Cellular proliferation in the neonephrogenic zone of the contralateral kidney was also greatly enhanced (14.42 +/- 3.26 versus 2.64 +/- 1.08 BrdU-positive cells per cross-section, P < 0.001). It is concluded that the skate possesses a nephrogenic zone containing stem cell-like mesenchymal cells during its entire life. Partial nephrectomy induces renal growth by accelerating nephrogenesis. This unique model may facilitate understanding renal regeneration.

Hematopoietic stem cells contribute to the regeneration of renal tubules after renal ischemia-reperfusion injury in mice

Ischemia-reperfusion injury (I/R injury) is a common cause of acute renal failure. Recovery from I/R injury requires renal tubular regeneration. Hematopoietic stem cells (HSC) have been shown to be capable of differentiating into hepatocytes, cardiac myocytes, gastrointestinal epithelial cells, and vascular endothelial cells during tissue repair. The current study tested the hypothesis that murine HSC can contribute to the regeneration of renal tubular epithelial cells after I/R injury. HSC isolated from male Rosa26 mice that express beta-galactosidase constitutively were transplanted into female nontransgenic mice after unilateral renal I/R injury. Four weeks after HSC transplantation, beta-galactosidase-positive cells were detected in renal tubules of the recipients by X-Gal staining. PCR analysis of the male-specific Sry gene and Y chromosome fluorescence in situ hybridization confirmed the presence of male-derived cells in the kidneys of female recipients. Antibody co-staining showed that beta-galactosidase was primarily expressed in renal proximal tubules. This is the first report to show that HSC can differentiate into renal tubular cells after I/R injury. Because of their availability, HSC may be useful for cell replacement therapy of acute renal failure.

Mechanisms of renal cell repair and regeneration after acute renal failure

In many cases, acute renal failure (ARF) is the result of proximal tubular cell injury and death and can arise in a variety of clinical situations, especially following renal ischemia and drug or toxicant exposure. Although much research has focused on the cellular events leading to ARF, less emphasis has been placed on the mechanisms of renal cell repair and regeneration, although ARF is reversed in over half of those who acquire it. Studies using in vivo and in vitro models have demonstrated the importance of proliferation, migration, and repair of physiological functions of injured renal proximal tubular cells (RPTC) in the reversal of ARF. Growth factors have been shown to produce migration and proliferation of injured RPTC, although the specific mechanisms through which growth factors promote renal regeneration in vivo are unclear. Recently, interactions between integrins and extracellular matrix proteins such as collagen IV were shown to promote the repair of physiological functions in injured RPTC. Specifically, collagen IV synthesis and deposition following cellular injury restored integrin polarity and promoted repair of mitochondrial function and active Na(+) transport. Furthermore, exogenous collagen IV, but not collagen I, fibronectin, or laminin, promoted the repair of physiological functions without stimulating proliferation. These findings suggest the importance of establishing and/or maintaining collagen IV-integrin interactions in the stimulation of repair of physiological functions following sublethal cellular injury. Furthermore, the pathway that stimulates repair is distinct from that of proliferation and migration and may be a viable target for pharmacological intervention.

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.

Regenerative biology: the emerging field of tissue repair and restoration

Regenerative biology has now been recognized as a new field with certain aims and goals. One direction of this new field is to understand the basic mechanisms by which tissues can be repaired and restored. The other direction examines the possibility of using this basic knowledge to apply it to medicine with the goal to clinically repair damaged tissues. Regeneration of tissues can occur by the differentiation of stem cells (local or non-local) or by the transdifferentiation of local terminally differentiated cells. While the transdifferentiation aspects are old, during the past few years many data have accumulated regarding the existence of stem cells and their participation in tissue renewal. This review will present an overview of the potential of all vertebrate organs to regenerate and of the basic mechanisms involved.

Activin A: an autocrine regulator of cell growth and differentiation in renal proximal tubular cells

BACKGROUND

Activin A is involved in tubular regeneration after ischemia/reperfusion injury. The present study was conducted to examine the role of activin A in cell growth, apoptosis and differentiation of tubular cells.

METHODS

We performed cell proliferation assays (MTT assay, [3H]-thymidine incorporation) and apoptosis detection assays (nuclear staining, DNA ladder formation, TUNEL staining) using LLC-PK1 cells. Expression of activin and activin receptor in LLC-PK1 cells also were examined by real-time polymerase chain reaction (PCR) and immunostaining. Stable cell lines expressing the truncated type II activin receptor were generated and the phenotype of these cells was analyzed.

RESULTS

Activin A inhibited DNA synthesis and cell growth in a dose-dependent manner and induced apoptosis in LLC-PK1 cells. The expression level of mRNA for the activin betaA subunit was markedly increased when the growth was stimulated. The expression of the type II activin receptor was observed in LLC-PK1 cells. The growth rate of cells expressing dominantly negative activin receptor was significantly faster than that of non-transfected cells. The expression level and pattern of cytokeratin and vimentin in these cells were quite different compared to non-transfected cells. When cultured in collagen gel, these cells formed multiple processes, which was not observed in non-transfected cells. Finally, the expression of Pax-2 was markedly elevated in these cells.

CONCLUSIONS

Activin A acts as an autocrine inhibitor of cell growth, an inducer of apoptosis, and an important modulator of differentiation in cultured proximal tubular cells.

Role of apoptosis in the pathogenesis of acute renal failure

Renal tubular cells die by apoptosis as well as necrosis in experimental models of ischemic and toxic acute renal failure as well as in humans with acute tubular necrosis. It is not yet possible, however, to determine the relative contribution of these two forms of cell death to loss of renal tubular cells in acute tubular necrosis. The beneficial effect of administering growth factors to animals with acute tubular necrosis is probably related to the potent antiapoptotic (survival) effects of growth factors as well as to their proliferative effects. Rapamycin inhibits both of these effects of growth factors and delays the recovery of renal function after acute tubular necrosis by inhibiting renal tubular cell regeneration and by increasing renal tubular cell loss by apoptosis. The administration of caspase inhibitors ameliorates ischemia-reperfusion injury in multiple organs including the kidney. However, the extent to which this protective effect of caspase inhibition is caused by reduced intrarenal inflammation, or by amelioration of renal tubular cell loss due to apoptosis, remains uncertain. In addition to caspase inhibition, the apoptotic pathway offers many potential targets for therapeutic interventions to prevent renal tubular cell apoptosis.

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.

The role of the activin-follistatin system in the developmental and regeneration processes of the kidney

Regeneration processes in many tissues are modulated by various factors, which are involved in their organogenesis. Activin A, a member of the TGF-beta superfamily, inhibits branching tubulogenesis of the kidney in organ culture system as well as in in vitro tubulogenesis model. On the other hand, follistatin, an antagonist activin A, reverses the effect of activin A on kidney development, induces branching tubulogenesis, and also promotes tubular regeneration after ischemia/reperfusion injury by blocking the action of endogenous activin A. The activin-follistatin system is one of the important regulatory systems modulating developmental and regeneration processes of the kidneys.

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.

Reactive oxygen species and acute renal failure

Acute renal failure is commonly due to acute tubular necrosis (ATN), the latter representing an acute, usually reversible loss of renal function incurred from ischemic or nephrotoxic insults occurring singly or in combination. Such insults instigate a number of processes-hemodynamic alterations, aberrant vascular responses, sublethal and lethal cell damage, inflammatory responses, and nephron obstruction-that initiate and maintain ATN. Eventually, reparative and regenerative processes facilitate the resolution of renal injury and the recovery of renal function. Focusing mainly on ischemic ATN, this article reviews evidence indicating that the inordinate or aberrant generation of reactive oxygen species (ROS) may contribute to the initiation and maintenance of ATN. This review also discusses the possibility that ROS may instigate adaptive as well as maladaptive responses in the kidney with ATN, and raises the possibility that ROS may participate in the recovery phase of ATN.