How Does Proteinuria Cause Progressive Renal Damage?
Mauro Abbate*,
Carla Zoja* and
Giuseppe Remuzzi*,
* Mario Negri Institute for Pharmacological Research and Unit of Nephrology and Dialysis, Azienda Ospedaliera, Ospedali Riuniti di Bergamo, Bergamo, Italy
Address correspondence to: Dr. Giuseppe Remuzzi, "Mario Negri" Institute for Pharmacological Research, Via Gavazzeni, 11, 24125 Bergamo, Italy. Phone: +39-035-319-888; Fax: +39-035-319-331; E-mail: gremuzzi{at}marionegri.it
The possibility that proteinuria may accelerate kidney diseaseprogression to end-stage renal failure has received supportfrom the results of increasing numbers of experimental and clinicalstudies. Evidence indicating that this process occurs throughmultiple pathways, including induction of tubular chemokineexpression and complement activation that lead to inflammatorycell infiltration in the interstitium and sustained fibrogenesis,is reviewed. Macrophages are prominent in the interstitial inflammatoryinfiltrate. This cell type mediates progression of renal injuryto the extent that macrophage numbers in renal biopsy predictrenal survival in patients with chronic renal disease. Chemoattractantsand adhesive molecules for inflammatory cells are upregulatedby excess ultrafiltered protein load of proximal tubular cellsvia activation of NF-B–dependent and NF-B–independentpathways. This mechanism is a potential target for therapeuticapproaches, as shown by beneficial effects of manipulationswith inhibitory molecules of NF-B activation or of chemokinereceptors in experimental studies. Targeting complement synthesisor activation in proximal tubule might offer novel therapeuticopportunities. Finally, proximal tubular cell receptors foruptake of plasma proteins that are under investigation may provideactivation signals on excess tubular protein handling.
The past 20 yr of research in nephrology have yielded substantialinformation on the mechanisms by which persisting dysfunctionof an individual component cell in the glomerulus is generatedand signaled to other glomerular cells and to the tubule. Spreadingof disease is central to processes by which nephropathies ofdifferent types progress to ESRD. Independent of the underlyingcauses, chronic proteinuric glomerulopathies have in commona sustained or permanent loss of selectivity of the glomerularbarrier to protein filtration. Glomerular sclerosis is the progressivelesion beginning at the glomerular capillary wall, the siteof abnormal filtration of plasma proteins. Injury is transmittedto the interstitium favoring the self-destruction of nephronsand eventually of the kidney. This review’s main focusis on underlying mechanisms of tubulointerstitial injury thatare activated by ultrafiltered protein load of tubular epithelialcells.
It needs to be emphasized that this field is relevant to interpretclinical findings and to improve treatment of patients withnondiabetic or diabetic nephropathies. The opinion among nephrologiststhat proteinuria could be a marker only of injury largely hasbeen challenged. The strong predictive value of proteinuriain chronic nephropathies now is firmly established. Baselineproteinuria was an independent predictor of renal outcome inpatients with type 1 diabetes and nephropathy (1) and in patientswho did not have diabetes and entered the Modification of Dietin Renal Disease (MDRD) study (2). In the Ramipril EfficacyIn Nephropathy (REIN) trial (3), urinary protein excretion wasthe only baseline variable that correlated significantly withGFR decline and progression of nondiabetic chronic proteinuricnephropathies to ESRD. Similar evidence was provided recentlyin patients with type 2 diabetes and overt nephropathy (4).Other studies corroborated these data and extended the predictivevalue of proteinuria to risks for overall or cardiovascularmortality (5,6).
Clinical trials consistently showed renoprotective effects ofproteinuria reduction and led to the recognition that the antiproteinurictreatment is instrumental to maximize renoprotection (2,3,7,8).The MDRD study revealed tight association between reductionof proteinuria and decrease in rate of GFR decline (2). Protectionthat was achieved by lowering BP depended on the extent of initialproteinuria. The renoprotection that was conferred by angiotensin-convertingenzyme (ACE) inhibition in the REIN study was mediated by thedrug’s action of reducing urinary protein levels, to theextent that patients who were on ramipril had a better outcomeparalleled by more reduction in proteinuria, whereas BP wascomparable to that of control subjects (3). ACE inhibitor–inducedreduction in proteinuria was the strongest time-dependent covariatepredicting slower progression to ESRD. Finding that the rateof GFR decline correlated negatively with proteinuria reductionand positively with residual proteinuria provided further evidencefor a pathogenetic role of proteinuria (9). Likewise, trialsin type 1 (8,10) and type 2 diabetes (11,12) documented thatwhenever proteinuria is decreased by treatments, progressionto ESRD is reduced. The Reduction of Endpoints in NIDDM withthe Angiotensin II Antagonist Losartan (RENAAL) study (11) in1513 patients with type 2 diabetic nephropathy confirmed thatmore reduction in proteinuria by losartan invariably was associatedwith more renoprotection at comparable levels of BP control.Beneficial cardiovascular effects of losartan also were drivenby effects on urinary protein and largely depended on the amountof residual proteinuria. Similar results were found in the IrbesartanDiabetic Nephropathy Trial (12). Finally, the Angiotensin-Converting-EnzymeInhibition and Progression of Renal Disease study (13,14) confirmedthat proteinuria is a strong risk factor for progression ofchronic renal disease and that patients with more severe renaldisease benefit most from ACE inhibitor therapy. Importantly,in no case from a pooled analysis was there a worsening in proteinuriathat subsequently was associated with an improved outcome (15).
Glomerular Proteinuria as Signal for Interstitial Inflammation: Insights from Proximal Tubular Cells in Culture
Several lines of evidence suggested a role of glomerular ultrafiltrateof plasma proteins or protein-associated factors in chronictubulointerstitial damage. In vitro studies using proximal tubularcells as a model to assess effects of apical exposure to plasmaproteins proved highly valuable to approaching direct causalrelationships. In monolayers of proximal tubular cells, theload with plasma proteins (albumin, IgG, and transferrin) inducedthe synthesis of the vasoconstrictor peptide endothelin-1 (ET-1),a mediator of progressive renal injury by virtue of abilityto stimulate renal cell proliferation and extracellular matrixproduction and to attract monocytes (16). Since 1995, afterthis first report, independent investigators confirmed and extendedthe stimulatory effects of a diversity of plasma proteins onthe expression of proinflammatory and profibrotic mediatorsin renal tubular cells (17–23). Among molecules that attractmonocytes/macrophages and T lymphocytes, monocyte chemoattractantprotein-1 (MCP-1) and RANTES were overexpressed in proximaltubular cells that were challenged with plasma proteins (17,24).Albumin upregulated tubular gene expression and production ofIL-8, a potent chemotactic agent for lymphocytes and neutrophils(18). The release of ET-1 and chemokines in response to proteinswas polarized mainly toward the basolateral compartment of thecell, as to mirror a directional secretion that favored theinterstitial inflammatory reaction that was observed in vivo.Protein overloading of human proximal tubular cells inducedthe synthesis of fractalkine, which in its membrane-anchoredform promotes mononuclear cell adhesion via CX3CR1 receptor(25). Fractalkine mRNA was overexpressed in kidneys of micewith protein overload proteinuria, and the gene product wasdetected in tubular epithelial cells mainly in the basal region.Treatment of mice with an antibody against CX3CR1 limited theinterstitial accumulation of monocytes/macrophages (25).
Investigation of the molecular mechanisms underlying chemokineupregulation in proximal tubular cells on protein challengehad initial focus on the activation of transcriptional NF-B(24). Other studies confirmed the pathway (26,27) and revealedreactive oxygen as a second messenger (18,28). Protein overloadelicited rapid generation of hydrogen peroxide in human proximaltubular cells, an effect that, together with NF-B activation,was prevented by antioxidants (28). Specific inhibitors of proteinkinase C (PKC) prevented hydrogen peroxide generation, NF-Bactivation (28), and MCP-1 and IL-8 gene upregulation that wasinduced by protein overload (18), suggesting a cascade of signalsfrom PKC-dependent oxygen radical generation to nuclear translocationof NF-B and consequent gene upregulation. A link also has beenmade between induction of NF-B activity by protein load andmitogen-activated protein kinases, including p38 (25) and extracellularsignal–regulated kinase 1 and 2 (ERK1/ERK2) (29) thatare involved in chemokine synthesis. In support of the notionof protein overload as a key activator of signaling in proximaltubule is the finding that albumin activated the signal transducerand activator of transcription (STAT) proteins in cultured proximaltubular cells (30). Because the STAT pathway is the principalmechanism that converts the signal from a wide array of cytokinesand growth factors into gene expression programs that regulatecell proliferation, differentiation, survival, and apoptosis(31), it was suggested that albumin may stimulate proximal tubularcells in the manner of a cytokine (32).
Which proteins play a predominant role as activator of tubularcells still is unanswered. Despite evidence that albumin overloadelicits several responses by tubular cells in vitro, it hasbeen argued that albumin per se may not be toxic to the proximaltubular epithelium. Compounds that are bound to albumin, suchas free fatty acids (FFA), instead have been implicated to becausative in proinflammatory activation or injury of culturedproximal tubular cells (33). Kees-Folts et al. (34), by studyingthe specific response of cultured tubular cells to albumin-boundmolecules, found that the tubular metabolism of albumin-boundfatty acids could generate macrophage chemotactic activity,whereas delipidated albumin produced little such activity. Itwas suggested that fatty acids can be released during degradationof albumin, HDL, or LDL. Lipid metabolites then would exertintracellular effects on second messenger systems with impairmentor promotion of epithelial cell growth. Conversely, the exposureof proximal tubular cells to lipidated albumin resulted in chemokineoverexpression at levels similar to delipidated albumin (17).Arici et al. (35) found that, among various fatty acids, oleicacid and linoleic acid exerted the most toxic and profibrogeniceffects in human proximal tubular cells in culture. In studiesof an in vivo model of overload proteinuria, animals that receivedan injection of FFA-replete BSA had higher levels of macrophageinfiltration and tubulointerstitial damage as compared withthe groups that received an injection of FFA-depleted BSA (36–38).Of note, potentially toxic substances, drugs, or haptens mayact in a similar manner. Thus, compounds that are bound to freelyultrafiltered small proteins, such as cadmium to metallothionein,exert proximal tubule cell cytotoxicity upon receptor-mediatedinternalization and release into endosomal/lysosomal compartments(39). These studies collectively indicate that the ability ofalbumin to act as a carrier enhances the proinflammatory activationof proximal tubular cells.
Further studies are required to assess the impact of reportedchanges in the expression of other genes in the proximal tubularcell, such as V5 integrin (40) and Na+/H+ exchanger (41) inresponse to albumin. In vivo gene expression profile analysisof proximal tubules from mice with protein overload proteinuriaidentified 2000 genes that were differentially regulated byexcess proteins. More than half of them were upregulated (42).They included thymic shared antigen-1, the fibroblast-associatedgene GS188, and glia maturation factor-B, a protein that originallywas purified as a neurotrophic factor (43). The expression ofglia maturation factor-B was induced in renal proximal tubularcells of mice with protein overload proteinuria (43). Proximaltubular cells that overexpressed glia maturation factor B acquiredmore susceptibility to death by sustained oxidative stress throughp38-pathway activation.
A controversial issue is related to the concentrations of albuminthat were used in various in vitro studies. Burton et al. (44)found that the apical exposure of human proximal tubular cellsto 1 mg/ml albumin or transferrin did not increase MCP-1 orPDGF-AB release, an effect that instead was observed after exposureto a human serum fraction (40 to 100 kD) in the molecular weightrange similar to albumin and transferrin. Studies that reportedthe effects of protein overload on NF-B activation showed responsesfrom 0.5 mg/ml in some experiments (17) and usually >2.5(17) or >5 mg/ml (16,18). The latter concentration seemsto far exceed the concentration reached in the proteinuric ultrafiltratein vivo (45,46). Conversely, other studies found significantET-1 and TGF- upregulation (19) or enhanced collagen secretion(23) with as low as 0.1 mg/ml albumin (19,23) or 0.01 mg/mlglobulins (23). A source of uncertainty also has been the suggestionthat the normal proximal tubule might have great capacity tohandle increasing amounts of proteins before tubular injurycould develop (reviewed by Gekle [46]). More important, however,the interrelationship between protein uptake and gene expressionin proximal tubule has been established (47). Low-affinity receptorsfor normal uptake of albumin have been identified on the brushborder region of proximal tubular cells. Megalin, a 600-kD transmembraneglycoprotein that belongs to the LDL receptor family, bindsalbumin, insulin, prolactin, and vitamin-binding proteins andmay act as a signal, at least partly, through phosphorylationof its cytoplasmic tail (48). Megalin is suggested to facilitateinternalization and intracellular trafficking of cubilin, a460-kD protein that binds albumin, transferrin, IgG light chains,and receptor-associated protein but lacks a transmembrane domain.Regulated intramembrane proteolysis, a process that links receptor-mediatedendocytosis with intracellular signaling events, has been suggestedto underlie the transcriptional regulation of specific genesin the proximal tubule (47). The first step in this processis the constitutive or ligand binding–dependent ectodomainshedding of the receptor by proteases. The transmembrane domain–containingfragment is the substrate for the -secretase activity of a multimolecularcomplex of proteins, including the so-called presenilins, thatmediates intramembrane proteolysis. A soluble fragment of thereceptor thereby is formed and trafficked to the nucleus toregulate gene expression. In the rat kidney, the brush borderexhibits both -secretase activity and presenilin-1 expression(47,49). In a cell line that was derived from opossum proximaltubule (OK cells), metalloprotease activity mediated ectodomainshedding of megalin, producing a C-terminal fragment the samesize as a major fragment of megalin found in kidney (47). Inhibitionof endocytosis in proximal tubular cell lines did prevent increasesin NF-B DNA-binding activity (50) and in secretion of collagencaused by albumin overload (23).
The proximal tubule bears other receptors for ultrafilteredproteins, such as Ig (51) and complement molecules (52). Thefunctional role of such receptors has not been established.It is likely that filtered proteins other than or in additionto albumin induce tubular dysfunction and injury in conditionsof nonselective proteinuria, in which large molecular weightproteins are a significant component. In contrast, relativelyselective albuminuria induces delayed mononuclear cell infiltration(53) and usually is associated with no or mild chronic tubulointerstitialinjury. In this respect, the case of minimal-change diseasehas been considered sometimes an exception to the rule thatinterstitial infiltrates develop with time in proteinuric glomerulopathies.In addition, however, in minimal-change disease, a substantialpercentage of patients respond to steroids, and the regressionof proteinuria prevents inflammation and renal function deterioration(54). Patients who have this disease and the nephrotic syndromeand experience only a few relapses also are protected from renaldamage, whereas patients who respond initially to glucocorticoidsbut have frequent relapses tend to develop interstitial injuryand focal glomerulosclerosis. Finally, patients who have nephroticsyndrome with mild disease, are drug resistant, and have permanentproteinuria undergo renal function deterioration over time.
In Vivo Evidence Linking Proteinuria and the Interstitial Inflammatory Reaction in Chronic Proteinuric Nephropathy
The link between proteinuria and mononuclear cell accumulationinto the interstitium via activation of transcription factorsand overexpression of chemokines has received consistent supportfrom studies in experimental models. In rats with overload proteinuria,upregulation of MCP-1 and osteopontin in tubular epithelialcells was associated with an interstitial inflammatory reaction(55). NF-B activity also increased in tubular epithelial cells(56). In this model, anti–MCP-1 gene therapy reduced interstitialinflammation and fibrosis and tubular damage (57). In nephropathiesof nonimmune (five-sixths nephrectomy) or immune origin (passiveHeymann nephritis), inflammatory cell infiltrates developedin the vicinity of tubules that were engaged in protein reabsorptionand overexpressing osteopontin (58). In the same models, proteinuriaover time was associated with increased NF-B activity, paralleledby overexpression of MCP-1 mRNA that preceded the accumulationof monocytes/macrophages and T lymphocytes in the renal interstitium(59). Therapy with an ACE inhibitor, acting to reduce proteinuria,almost suppressed NF-B activation and MCP-1 upregulation andlimited the interstitial accumulation of mononuclear cells inboth models (59). The ACE inhibitor also reduced proteinuria,osteopontin upregulation, and interstitial macrophage infiltrationin rats with Adriamycin nephrosis. The treatment, however, couldnot reverse established fibrosis in this model (60).
MCP-1 expression that is driven by excess plasma protein signalingconsistently seems to represent an important pathway for progressionof injury in humans (61). A strong relationship was found betweenproteinuria and MCP-1–mediated interstitial damage ina prospective study of patients who underwent renal biopsy forchronic renal disease (61). High levels of albuminuria, urinaryMCP-1, and interstitial macrophages were predictive of doublingin serum creatinine and/or end-stage renal failure. In previousreports, the analysis of renal biopsy specimens from patientswith severe proteinuria revealed NF-B activation in tubularcells, which significantly correlated with the magnitude ofproteinuria. Concomitant upregulation of MCP-1, RANTES, andosteopontin was found in tubular epithelial cells, with thestrongest expression in patients with progressive nephropathy(62,63). These findings are in strong support of the predictionthat the MCP-1 pathway, as suggested by in vitro studies, mediatesinterstitial macrophage accumulation that is responsible forfurther injury. Findings of NF-B activation and MCP-1 upregulationin proximal tubular cells have been reported in patients withdiabetes (64). The role of macrophage accumulation into therenal interstitium in human diabetes is not established. However,recent data in db/db type 2 diabetic mice (65) and in MCP-1–/–mice with streptozotocin-induced diabetes (66) provide evidencefor chemokine-mediated macrophage accumulation and associatedtubulointerstitial injury.
Targeting NF-B activation seems to be an effective means ofinterrupting the process of tubulointerstitial injury, as documentedin rats with Adriamycin-induced nephropathy that was treatedlong term with the NF-B inhibitor pyrrolidine dithiocarbamate(67) and in rats that had overload proteinuria and underwentgene transfer of truncated IB (68). In addition, a series ofreports indicated that interference with the chemokine pathwaywas protective in models of proteinuric nephropathies. NakedDNA vaccination against MCP-1 and RANTES ameliorated the progressionof renal disease in rats with Adriamycin nephropathy (69). Theprotective mechanism possibly involved the production of autoantibodiesagainst MCP-1 and RANTES, with consequent reduction in renalinfiltration by and activation of effector cells. Blockade ofchemokine receptors with specific small molecule antagonistswas shown to reduce interstitial leukocyte accumulation andsubsequent fibrosis in mouse models of chronic kidney disease(70).
Key Role for the Intrarenal Activation of Complement
Complement activation is a powerful mechanism underlying tubularand interstitial injury via cytotoxic, proinflammatory, andfibrogenic effects. Abnormal C3 and C5b-9 staining in proximaltubular cells and along the brush border is a long known featureboth in human chronic proteinuric diseases and experimentalmodels. In vitro, proximal tubular cells activate serum complementvia an alternative pathway, leading to C5b-9 fixation on cellsurface (71) as well as cytoskeletal changes and productionof superoxide anion, hydrogen peroxide, and cytokines (IL-6and TNF-) (72). Studies in rats with puromycin aminonucleoside–inducednephrosis showed beneficial effects of complement depletionor C6 deficiency against tubulointerstitial damage associatedwith proteinuria (73,74). Evidence subsequently was providedthat C3 and ultrafiltered plasma proteins co-localized to proximaltubular cells that were exposed to filtered protein overloadin rats with remnant kidney since stages that preceded the accumulationof monocyte/macrophages into the interstitium (75). Treatmentwith ACE inhibitor limited the excess load of both C3 and plasmaproteins in proximal tubular cells that may significantly contributeto promote the recruitment of inflammatory cells (75). Protectiveeffects by C6 deficiency (76) revealed the role of activatedcomplement as a key mediator of tubulointerstitial injury inthis model.
Glomerular permeability dysfunction of proteinuric nephropathiesallows complement factors to be ultrafiltered abnormally acrossthe altered glomerular barrier into the Bowman’s spaceand tubular lumen. Plasma-derived C3 (molecular weight 180 kd)is likely to reflect more loss of glomerular permselectivityand to enhance cell dysfunction in the presence of abnormallyfiltered plasma proteins. Renal tubular cells also synthesizeC3 and other complement factors (77) in ways that may have criticalimportance in disease, as found in experimental renal transplantrejection (78) and postischemic acute renal failure (79). Therefore,both excess ultrafiltration and proximal tubular cell synthesisof complement could underlie complement-mediated injury in chronicproteinuric renal disease (Figure 1). Recent findings of C3mRNA upregulation and C3 accumulation in proximal tubular cellsin kidneys of mice with protein overload proteinuria are insupport of a role for the local synthesis of complement (80).Complement is an important effector of interstitial mononuclearcell infiltration and fibrogenesis in this model, as shown bysignificant attenuation of injury in C3-deficient mice (80).A direct role for protein overload as a stimulus was indicatedby findings that the exposure of cultured proximal tubular cellsto total serum proteins at the apical surface upregulated C3mRNA expression and protein biosynthesis (20). Serum fractionationexperiments identified substances that were responsible forsuch effects in the molecular size range of 30 to 100 kd. Thisfraction contains proteins that pass the glomerular barrierin proteinuric states. The addition of albumin alone did notreproduce the effects that were observed with whole serum, whereasthe incubation with apical transferrin caused C3 mRNA overexpressionand both apical and basolateral C3 secretion (20). C3 was upregulatedto similar degrees in response to iron-poor transferrin or apotransferrin,indicating that transferrin protein rather than iron moietycould act as stimulus.
Figure 1. Mechanisms underlying the activation of inflammatory and fibrogenic pathways in proximal tubular epithelial cells by ultrafiltered protein load. As a consequence of proteinuria, the intrarenal activation of the complement cascade may promote injury through the formation of membrane attack complex and biologically active products, such as C3a, that interact with specific receptors. Monocytes/macrophages contribute to fibrosis by release of TGF-, which stimulates myofibroblast formation and collagen deposition and epithelial mesenchymal transformation. The latter process could be induced in an autocrine manner by TGF- of proximal tubular cell origin.
Renal parenchymal cells express a limited repertoire of receptors,including CR1, CR3, and CD88, that may bind complement proteinsthat are present in the ultrafiltrate. To which extent the stimulationof complement receptors on tubular cells could influence progressiverenal disease has not been established. The C3a receptor hasbeen found to be highly expressed both by normal murine andby human kidney epithelial cells and to mediate altered geneexpression in cultured proximal tubular cells (52). Among thosegenes, both TGF- and collagen I were upregulated by C3a in vitro,suggesting the induction of pathways underlying inflammationand fibrosis. In kidneys of proteinuric mice with Adriamycinnephrosis, deposition of C3 and enhanced expression of collagentype IV and of its chaperone, heat-shock protein 47, showedtopographic relationships at sites of tubulointerstitial damage(81). The gene expression of collagen and heat-shock protein47 also was upregulated in primary cultures of mouse proximaltubular epithelial cells in response to C5b-9, further suggestingthat complement activation on tubular cells can stimulate thefibrotic process directly. Other genes, such as pyrin (a genethat is responsible for familial Mediterranean fever) and Gulpwere negatively regulated in tubular cells that were exposedto C3a, possibly reflecting modulatory effects on the inflammatoryresponse and, respectively, on the clearance of apoptotic cells(52).
One important aspect of research in the complement field isthe availability of complement inhibitory molecules that mayblock effector mechanisms even in the presence of persistentproteinuria. A strategy for specific delivery of complementinhibitors to the proximal tubule was designed using recombinantproteins that consisted of a carrier antibody against brushborder antigen and a complement inhibitory molecule (sCrry andtCD59) in rats with puromycin aminonucleoside nephrosis (82).Inhibitors that were given intraperitoneally to proteinuricrats localized to proximal tubuli and protected against tubulointerstitialinjury and renal dysfunction as assessed on day 11 after diseaseinduction. This approach would obviate the need to inhibit complementsystemically and its function in innate immunity.
Profibrogenic Signaling from Proximal Tubular Cells in Response to Protein Overload
Local recruitment of macrophages by tubular cells that are loadedwith ultrafiltered plasma proteins may contribute to interstitialfibrosis by engaging matrix-producing interstitial myofibroblasts.Macrophages also regulate matrix accumulation via release ofgrowth factors such as TGF- and PDGF, ET-1, and PAI-1. TGF-stimulates the transformation of interstitial cells into myofibroblasts.In addition, proximal tubular epithelial cells communicate withinterstitial fibroblasts to promote fibrogenesis via paracrinerelease of TGF- (Figure 1). In rats with remnant kidneys atday 14, after the onset of proteinuria, TGF- mRNA was upregulatedin proximal tubular cells in parallel with early accumulationof -smooth muscle actin (-SMA)-positive myofibroblasts, whichwere localized strictly to the peritubular interstitium, suggestingthat interstitial fibroblasts are the initial target of profibrogenicsignals elicited by protein overreabsorption (83). In remnantkidneys at day 30, staining for -SMA also was visualized inproximal tubules, indicating that a phenotypic transformationof tubular cells occurs in a relatively late stage. Treatmentof these rats with an ACE inhibitor at the same time limitedexcess protein overload and interstitial inflammatory cell infiltrationand abrogated the abnormal TGF-1 gene expression in tubularcells that in all likelihood was responsible for the recruitmentof myofibroblasts in the surrounding areas. ACE inhibitor exertsbeneficial effects in the glomerulus primarily by preservingthe permselective barrier to proteins (84), thereby limitingproteinuria and filtered protein-dependent inflammatory andfibrogenic signals. The ACE inhibitor also may act locally bypreventing nonhemodynamic effects of angiotensin II (AngII)via apical angiotensin receptors on tubular cells, includingrenal cell proliferation and TGF-1 expression (85). However,the toxic effect of protein trafficking on renal disease progressionin mice with overload proteinuria is not dependent on AngIIthrough the major angiotensin type 1 receptor isoform (86).Certainly, proteinuria and direct effects of AngII on tubularcells are not mutually exclusive targets of the drug’saction.
TGF- remains the most important cytokine for renal fibrogenesis,and it has been identified as a major stimulus in the processof epithelial-mesenchymal transition (EMT) of tubular epithelialcells (87). Studies have focused on the signaling pathways thatare activated during TGF-–induced EMT. It was shown thatTGF- induced Smad2 phosphorylation in a tubular epithelial cellline and that overexpression of an inhibitory Smad protein,Smad7, inhibited TGF-–induced Smad2 activation, therebypreventing EMT and collagen synthesis (88). The same pathwaywas shown to mediate renal fibrosis in rats with ureteral obstruction(89). An endogenous antagonist of TGF-1–induced EMT hasbeen identified as bone morphogenic protein-7 (BMP-7), a memberof the TGF- superfamily whose genetic deletion in mice leadsto severe impairment of kidney development (90). BMP-7 reversedTGF-1–induced EMT through a Smad-dependent reinductionof E-cadherin, an adhesive junction protein that maintains thestructural integrity and polarity of epithelial cells (91).Systemic administration of recombinant BMP-7 repaired severelydamaged tubular epithelial cells and reversed renal injury inmice with nephrotoxic serum nephritis (91). Wu et al. (92) recentlyprovided the interesting evidence that the immunosuppressiveagent rapamycin was able to block EMT, as demonstrated by partialrestoration of E-cadherin expression and inhibition of the denovo expression of -SMA in cultured tubular epithelial cellsafter TGF-1 stimulation. Importantly, in the rat model of unilateralureteral obstruction, treatment with rapamycin attenuated tubulointerstitialdamage and limited -SMA expression and collagen deposition inthe interstitium. Moreover, rapamycin decreased the infiltrationof inflammatory cells and inhibited renal TGF-1 expression (92).In rats with passive Heymann nephritis that was acceleratedby severe renal mass reduction, rapamycin also was shown toameliorate proteinuria-associated tubulointerstitial inflammationand fibrosis by preventing the overexpression of MCP-1, TGF-,and PDGF in the kidney (93).
In vitro studies have highlighted further the causal relationand mechanisms underlying the fibrogenic reaction. Albumin challengeof cultured proximal tubular cells resulted in increased geneexpression and production of TGF- (19). Of interest, albuminupregulated TGF- receptor type II transcription, synthesis,and surface expression in proximal tubular cells, which couldamplify the matrix-stimulatory actions of TGF- on tubular cells,thereby contributing to the development of tubulointerstitialfibrosis (94). Another in vitro study demonstrated that fattyacid–free albumin stimulated the accumulation of extracellularcollagen type IV, laminin, and fibronectin by proximal tubularcells through a posttranscriptional mechanism (95). Findingthat albumin caused an increase in the levels of tissue inhibitorsof metalloproteinases 1 (TIMP-1) and TIMP-2 suggested that adecrease in degradation rather than an increase in protein expressioncould be responsible for the increased accumulation of extracellularmatrix protein components in response to albumin load. Recentfindings suggested that dose-dependent TGF- secretion by proximaltubular cells that were exposed to albumin requires activationof mitogen-activated protein kinase signaling pathway upon albuminbinding to surface receptors (96). The response would be independentof albumin endocytosis, because inhibitors of endocytosis suchas simvastatin and the megalin ligand, receptor-associated protein,failed to inhibit albumin-induced TGF- secretion.
Role of Ultrafiltration of Cytokines and Growth Factors in Proteinuric Disease
In addition to albumin, transferrin, and Ig, glomerular proteinuriaresults in ultrafiltration of high molecular weight precursorforms or complexes of growth factor proteins such as insulin-likegrowth factor 1, hepatocyte growth factor, and TGF-1 (97). Micropuncturestudies in proteinuric rats and experiments in cultured proximaltubular cells allowed documentation that growth factors becomeactivated in tubular fluid to interact with tubular cell receptors,thereby causing secretion of collagen types I and IV, MCP-1,and RANTES. Chemokines, upon secretion in the basolateral compartmentof tubular cells, also may stimulate macrophages in the renalinterstitium to secrete TGF-, which in turn is a powerful stimulusfor the expression of extracellular matrix proteins by interstitialmyofibroblasts (98).
Intraglomerular cytokines may originate as a consequence ofinflammation and be conveyed with proteins into the tubularlumen to amplify interstitial injury. The kidney of the axolotl,a primitive amphibian, has provided a tool whereby the effectsof protein toxicity can be assessed in the absence of glomerularinflammation and possibly other, concurrent or alternative mechanismsof progressive injury, such as altered glomerular hemodynamicsand misdirected glomerular filtration (99). Besides closed nephrons,the axolotl kidney contains nephrons that have ciliated peritonealfunnels (nephrostomes) that exhibit free access to the peritonealcavity (100). Injection of bovine serum or human proteins (transferrin,IgG, or LDL but not albumin) into the peritoneal cavity causedmassive accumulation of proteins and lipids within intracellulardroplets in tubular epithelial cells of nephrons with nephrostomes.Progressive focal accumulation of fibrous tissue was noted aroundprotein-storing tubules, with the presence of fibronectin andTGF- both in tubular epithelial cells and in interstitial cells.The axolotl kidney differs significantly from the mammalianorgan. However, these data are in support of the possibilitythat tubular injury and interstitial fibrosis in response toprotein loading can ensue irrespective of the presence of glomerulardamage.
Emerging evidence suggests that proteinuria causes tubular cellapoptosis. In cultured proximal tubular cells, delipidated albumininduced apoptosis in a dose- and duration-dependent manner (101).This phenomenon was associated with activation of Fas-FADD-caspase8 pathway. Peroxisome proliferator activated receptor- seemedto play a role in the apoptotic response to high concentrationsof albumin-bound fatty acids in human proximal tubular cells(102). However, the role of albumin as an inducer of apoptosishas not been established clearly, in that the effect may notoccur unless a high-protein dose is used (103,104). A recentstudy showed that the exposure of proximal tubular cells toa 100- to 440-kD human plasma fraction but not to the 30- to100-kD albumin-rich fraction showed strong upregulation in theexpression of Fas and Fas ligand and apoptotic response, suggestingthat protein load–induced apoptosis can be mediated byhigher molecular weight protein species that are filtered uponmore severe permselective defect (105).
Evidence of apoptotic responses to protein load is not confinedto cultured tubular epithelial cells. Kidneys of rats with albuminoverload proteinuria indeed showed increased numbers of terminaldUTP nick-end labeling–positive apoptotic cells both inthe tubulointerstitial compartment and in the glomeruli. Mostof the positive tubular cells belonged to profiles that expressedAngII type 2 receptor. Findings of reduced phosphorylation ofERK and Bcl-2 were suggested to reflect an AngII type 2 receptor–mediatedmechanism underlying tubular cell apoptosis (106). Proximaltubular cell apoptosis also was found and may contribute toglomerular–tubule disconnection and atrophy in responseto proteinuria in the accelerated model of passive Heymann nephritis(107). Apoptotic cells were detected recently in both proximaland distal tubular profiles in biopsy specimens of patientswith primary FSGS. In support of the pathophysiologic significanceof such observation, a strong positive correlation was foundbetween proteinuria and incidence of tubule cell apoptosis,which was identified as a strong predictor of outcome in thesepatients (108). The prolonged incubation of Madin-Darby caninekidney epithelial (distal/collecting) cells with albumin alsoresulted in the activation of the Fas pathway and apoptosis,extending to the distal nephron the pathogenetic potential ofexcess exposure of tubular cells to ultrafiltered proteins andof the epithelial apoptotic response (108).
In progressive nephropathies, severe dysfunction of the glomerularcapillary barrier to circulating proteins causes protein overloadof tubular epithelial cells and intrarenal activation of complementthat is responsible for spreading of injury to the tubulointerstitium.These pathways are summarized in Table 1 along with other postulatedmechanisms of tubulointerstitial injury. Drugs that block AngIIlimit the abnormal passage of plasma proteins and are renoprotective.The podocyte is the primary site of antiproteinuric action throughstabilization of podocyte–podocyte contacts and preventionof permselective dysfunction at the slit diaphragm. Althoughthe abnormal passage of plasma proteins across the glomerularcapillary wall is likely to be a factor that is responsiblefor further podocyte injury and progression to glomerulosclerosis(84), most of the available data highlight the mechanisms underlyingproximal tubular cell activation and interstitial inflammationand fibrosis. The toxicity of albumin seems to be mediated byits initial endocytic uptake, although the importance of albuminitself versus protein-bound molecules in the induction of irreversibletubular damage is not clear. Other proteins, including ultrafilteredtransferrin and Ig, and the intrarenal complement pathway couldplay a predominant role. Developments in these areas yield furthersupport to design protocols in which drugs against secondarypathways of injury should be tested in association with drugsthat limit the abnormal passage of proteins across the glomerularcapillary barrier.
Breyer JA, Bain RP, Evans JK, Nahman NS Jr, Lewis EJ, Cooper M, McGill J, Berl T: Predictors of the progression of renal insufficiency in patients with insulin-dependent diabetes and overt diabetic nephropathy. The Collaborative Study Group.
Kidney Int 50
: 1651
–1658, 1996[Medline]
Peterson JC, Adler S, Burkart JM, Greene T, Hebert LA, Hunsicker LG, King AJ, Klahr S, Massry SG, Seifter JL: Blood pressure control, proteinuria, and the progression of renal disease. The Modification of Diet in Renal Disease Study.
Ann Intern Med 123
: 754
–762, 1995[Abstract/Free Full Text]
GISEN: Randomized placebo-controlled trial effect of ramipril on decline in glomerular filtration rate and risk of terminal renal failure in proteinuric, non-diabetic nephropathy.
Lancet 349
: 1857
–1863, 1997[CrossRef][Medline]
de Zeeuw D, Remuzzi G, Parving HH, Keane WF, Zhang Z, Shahinfar S, Snapinn S, Cooper ME, Mitch WE, Brenner BM: Proteinuria, a target for renoprotection in patients with type 2 diabetic nephropathy: Lessons from RENAAL.
Kidney Int 65
: 2309
–2320, 2004[CrossRef][Medline]
Tarver-Carr M, Brancati F, Eberhardt MS, Powe N: Proteinuria and the risk of chronic kidney disease (CKD) in the United States.
J Am Soc Nephrol 11
: 168A
, 2000
Irie F, Iso H, Sairenchi T, Fukasawa N, Yamagishi K, Ikehara S, Kanashiki M, Saito Y, Ota H, Nose T: The relationships of proteinuria, serum creatinine, glomerular filtration rate with cardiovascular disease mortality in Japanese general population.
Kidney Int 69
: 1264
–1271, 2006[CrossRef][Medline]
Wapstra FH, Navis G, de Jong PE, de Zeeuw D: Prognostic value of the short-term antiproteinuric response to ACE inhibition for prediction of GFR decline in patients with nondiabetic renal disease.
Exp Nephrol 4[Suppl 1]
: 47
–52, 1996
Lewis EJ, Hunsicker LG, Bain RP, Rohde RD: The effect of angiotensin-converting-enzyme inhibition on diabetic nephropathy. The Collaborative Study Group.
N Engl J Med 329
: 1456
–1462, 1993[Abstract/Free Full Text]
Ruggenenti P, Perna A, Remuzzi G: Retarding progression of chronic renal disease: The neglected issue of residual proteinuria.
Kidney Int 63
: 2254
–2261, 2003[CrossRef][Medline]
Bjorck S, Mulec H, Johnsen SA, Norden G, Aurell M: Renal protective effect of enalapril in diabetic nephropathy.
BMJ 304
: 339
–343, 1992[Medline]
Brenner BM, Cooper ME, de Zeeuw D, Keane WF, Mitch WE, Parving H-H, Remuzzi G, Snapinn SM, Zhang Z, Shahinfar S: Effects of losartan on renal and cardiovascular outcomes in patients with type 2 diabetes and nephropathy.
N Engl J Med 345
: 861
–869, 2001[Abstract/Free Full Text]
Lewis EJ, Hunsicker LG, Clarke WR, Berl T, Pohl MA, Lewis JB, Ritz E, Atkins RC, Rohde R, Raz I: Renoprotective effect of the angiotensin-receptor antagonist irbesartan in patients with nephropathy due to type 2 diabetes.
N Engl J Med 345
: 851
–860, 2001[Abstract/Free Full Text]
Jafar TH, Stark PC, Schmid CH, Landa M, Maschio G, Marcantoni C, de Jong PE, de Zeeuw D, Shahinfar S, Ruggenenti P, Remuzzi G, Levey AS: Proteinuria as a modifiable risk factor for the progression of non-diabetic renal disease.
Kidney Int 60
: 1131
–1140, 2001[CrossRef][Medline]
Jafar TH, Schmid CH, Landa M, Giatras I, Toto R, Remuzzi G, Maschio G, Brenner BM, Kamper A, Zucchelli P, Becker G, Himmelmann A, Bannister K, Landais P, Shahinfar S, de Jong PE, de Zeeuw D, Lau J, Levey AS: Angiotensin-converting enzyme inhibitors and progression of nondiabetic renal disease. A meta-analysis of patient-level data.
Ann Intern Med 135
: 73
–87, 2001[Abstract/Free Full Text]
Ruggenenti P, Schieppati A, Remuzzi G: Progression, remission, regression of chronic renal diseases.
Lancet 357
: 1601
–1608, 2001[CrossRef][Medline]
Zoja C, Morigi M, Figliuzzi M, Bruzzi I, Oldroyd S, Benigni A, Ronco P, Remuzzi G: Proximal tubular cell synthesis and secretion of endothelin-1 on challenge with albumin and other proteins.
Am J Kidney Dis 26
: 934
–941, 1995[Medline]
Wang Y, Chen J, Chen L, Tay YC, Rangan GK, Harris DC: Induction of monocyte chemoattractant protein-1 in proximal tubule cells by urinary protein.
J Am Soc Nephrol 8
: 1537
–1545, 1997[Abstract]
Tang S, Leung JCK, Abe K, Wah Chan K, Chan LYY, Mao Chan T, Neng Lai K: Albumin stimulates interleukin-8 expression in proximal tubular epithelial cells in vitro and in vivo.
J Clin Invest 111
: 515
–527, 2003[CrossRef][Medline]
Yard BA, Chorianopoulos E, Herr D, van der Woude FJ: Regulation of endothelin-1 and transforming growth factor-beta1 production in cultured proximal tubular cells by albumin and heparan sulphate glycosaminoglycans.
Nephrol Dial Transplant 16
: 1769
–1775, 2001[Abstract/Free Full Text]
Tang S, Sheerin NS, Zhou W, Brown Z, Sacks SH: Apical proteins stimulate complement synthesis by cultured human proximal tubular epithelial cells.
J Am Soc Nephrol 10
: 69
–76, 1999[Abstract/Free Full Text]
Tang S, Leung JCK, Tsang AWL, Lan HY, Chan TM, Lai KN: Transferrin up-regulates chemokine synthesis by human proximal tubular epithelial cells: Implication on mechanism of tubuloglomerular communication in glomerulopathic proteinuria.
Kidney Int 61
: 1655
–1665, 2002[CrossRef][Medline]
Drumm K, Bauer B, Freudinger R, Gekle M: Albumin induces NF-kappaB expression in human proximal tubule-derived cells (IHKE-1).
Cell Physiol Biochem 12
: 187
–196, 2002[CrossRef][Medline]
Wohlfarth V, Drumm K, Mildenberger S, Freudinger R, Gekle M: Protein uptake disturbs collagen homeostasis in proximal tubule-derived cells.
Kidney Int Suppl 84
: S103
–S109, 2003[Medline]
Zoja C, Donadelli R, Colleoni S, Figliuzzi M, Bonazzola S, Morigi M, Remuzzi G: Protein overload stimulates RANTES production by proximal tubular cells depending on NF-kappaB activation.
Kidney Int 53
: 1608
–1615, 1998[CrossRef][Medline]
Donadelli R, Zanchi C, Morigi M, Buelli S, Batani C, Tomasoni S, Corna D, Rottoli D, Benigni A, Abbate M, Remuzzi G, Zoja C: Protein overload induces fractalkine upregulation in proximal tubular cells through nuclear factor kappaB- and p38 mitogen-activated protein kinase-dependent pathways.
J Am Soc Nephrol 14
: 2436
–2446, 2003[Abstract/Free Full Text]
Wang Y, Rangan GK, Tay Y-C, Harris DCH: Induction of monocyte chemoattractant protein-1 by albumin is mediated by nuclear factor kappaB in proximal tubule cells.
J Am Soc Nephrol 10
: 1204
–1213, 1999[Abstract/Free Full Text]
Drumm K, Lee E, Stanners S, Gassner B, Gekle M, Poronnik P, Pollock C: Albumin and glucose effects on cell growth parameters, albumin uptake and Na(+)/H(+)-exchanger Isoform 3 in OK cells.
Cell Physiol Biochem 13
: 199
–206, 2003[CrossRef][Medline]
Morigi M, Macconi D, Zoja C, Donadelli R, Buelli S, Zanchi C, Ghilardi M, Remuzzi G: Protein overload-induced NF-kappaB activation in proximal tubular cells requires H2O2 through a PKC-dependent pathway.
J Am Soc Nephrol 13
: 1179
–1189, 2002[Abstract/Free Full Text]
Dixon R, Brunskill NJ: Albumin stimulates p44/p42 extracellular-signal-regulated mitogen-activated protein kinase in opossum kidney proximal tubular cells.
Clin Sci (Lond) 98
: 295
–301, 2000[Medline]
Nakajima H, Takenaka M, Kaimori JY, Hamano T, Iwatani H, Sugaya T, Ito T, Hori M, Imai E: Activation of the signal transducer and activator of transcription signaling pathway in renal proximal tubular cells by albumin.
J Am Soc Nephrol 15
: 276
–285, 2004[Abstract/Free Full Text]
Rawlings JS, Rosler KM, Harrison DA: The JAK/STAT signaling pathway.
J Cell Sci 117
: 1281
–1283, 2004[Free Full Text]
Brunskill NJ: Albumin signals the coming of age of proteinuric nephropathy.
J Am Soc Nephrol 15
: 504
–505, 2004[Free Full Text]
Schreiner GF: Renal toxicity of albumin and other lipoproteins.
Curr Opin Nephrol Hypertens 4
: 369
–373, 1995[Medline]
Kees-Folts D, Sadow JL, Schreiner GF: Tubular catabolism of albumin is associated with the release of an inflammatory lipid.
Kidney Int 45
: 1697
–1709, 1994[Medline]
Arici M, Brown J, Williams M, Harris KP, Walls J, Brunskill NJ: Fatty acids carried on albumin modulate proximal tubular cell fibronectin production: A role for protein kinase C.
Nephrol Dial Transplant 17
: 1751
–1757, 2002[Abstract/Free Full Text]
Kamijo A, Kimura K, Sugaya T, Yamanouchi M, Hase H, Kaneko T, Hirata Y, Goto A, Fujita T, Omata M: Urinary free fatty acids bound to albumin aggravate tubulointerstitial damage.
Kidney Int 62
: 1628
–1637, 2002[CrossRef][Medline]
Thomas ME, Harris KPG, Walls J, Furness PN, Brunskill NJ: Fatty acids exacerbate tubulointerstitial injury in protein-overload proteinuria.
Am J Physiol Renal Physiol 283
: F640
–F647, 2002[Abstract/Free Full Text]
van Timmeren MM, Bakker SJ, Stegeman CA, Gans RO, van Goor H: Addition of oleic acid to delipidated bovine serum albumin aggravates renal damage in experimental protein-overload nephrosis.
Nephrol Dial Transplant 20
: 2349
–2357, 2005[Abstract/Free Full Text]
Wolff NA, Abouhamed M, Verroust PJ, Thevenod F: Megalin-dependent internalization of cadmium-metallothionein and cytotoxicity in cultured renal proximal tubule cells.
J Pharmacol Exp Ther 318
: 782
–791, 2006[Abstract/Free Full Text]
Peruzzi L, Trusolino L, Amore A, Gianoglio B, Cirina P, Basso G, Emancipator SN, Marchisio PC, Coppo R: Tubulointerstitial responses in the progression of glomerular diseases: Albuminuria modulates alpha v beta 5 integrin.
Kidney Int 50
: 1310
–1320, 1996[Medline]
Klisic J, Zhang J, Nief V, Reyes L, Moe OW, Ambuhl PM: Albumin regulates the Na+/H+ exchanger 3 in OKP cells.
J Am Soc Nephrol 14
: 3008
–3016, 2003[Abstract/Free Full Text]
Nakajima H, Takenaka M, Kaimori J-Y, Nagasawa Y, Kosugi A, Kawamoto S, Imai E, Hori M, Okubo K: Gene expression profile of renal proximal tubules regulated by proteinuria.
Kidney Int 61
: 1577
–1587, 2002[CrossRef][Medline]
Kaimori JY, Takenaka M, Nakajima H, Hamano T, Horio M, Sugaya T, Ito T, Hori M, Okubo K, Imai E: Induction of glia maturation factor-beta in proximal tubular cells leads to vulnerability to oxidative injury through the p38 pathway and changes in antioxidant enzyme activities.
J Biol Chem 278
: 33519
–33527, 2003[Abstract/Free Full Text]
Burton CJ, Combe C, Walls J, Harris KP: Secretion of chemokines and cytokines by human tubular epithelial cells in response to proteins.
Nephrol Dial Transplant 14
: 2628
–2633, 1999[Abstract/Free Full Text]
Zandi-Nejad K, Eddy AA, Glassock RJ, Brenner BM: Why is proteinuria an ominous biomarker of progressive kidney disease?
Kidney Int Suppl 92
: S76
–S89, 2004[Medline]
Gekle M: Renal tubule albumin transport.
Annu Rev Physiol 67
: 573
–594, 2005[CrossRef][Medline]
Biemesderfer D: Regulated intramembrane proteolysis of megalin: Linking urinary protein and gene regulation in proximal tubule?
Kidney Int 69
: 1717
–1721, 2006[CrossRef][Medline]
Birn H, Christensen EI: Renal albumin absorption in physiology and pathology.
Kidney Int 69
: 440
–449, 2006[CrossRef][Medline]
Zou Z, Chung B, Nguyen T, Mentone S, Thomson B, Biemesderfer D: Linking receptor-mediated endocytosis and cell signaling: Evidence for regulated intramembrane proteolysis of megalin in proximal tubule.
J Biol Chem 279
: 34302
–34310, 2004[Abstract/Free Full Text]
Drumm K, Gassner B, Silbernagl S, Gekle M: Inhibition of Na superset+/H superset+ exchange decreases albumin-induced NF-kappaB activation in renal proximal tubular cell lines (OK and LLC-PK1 cells).
Eur J Med Res 6
: 422
–432, 2001[Medline]
Haymann JP, Levraud JP, Bouet S, Kappes V, Hagege J, Nguyen G, Xu Y, Rondeau E, Sraer JD: Characterization and localization of the neonatal Fc receptor in adult human kidney.
J Am Soc Nephrol 11
: 632
–639, 2000[Abstract/Free Full Text]
Braun MC, Reins RY, Li TB, Hollmann TJ, Dutta R, Rick WA, Teng BB, Ke B: Renal expression of the C3a receptor and functional responses of primary human proximal tubular epithelial cells.
J Immunol 173
: 4190
–4196, 2004[Abstract/Free Full Text]
Kikuchi H, Kawachi H, Ito Y, Matsui K, Nosaka H, Saito A, Orikasa M, Arakawa M, Shimizu F: Severe proteinuria, sustained for 6 months, induces tubular epithelial cell injury and cell infiltration in rats but not progressive interstitial fibrosis.
Nephrol Dial Transplant 15
: 799
–810, 2000[Abstract/Free Full Text]
Remuzzi G, Bertani T: Pathophysiology of progressive nephropathies.
N Engl J Med 339
: 1448
–1456, 1998[Free Full Text]
Eddy AA, Giachelli CM: Renal expression of genes that promote interstitial inflammation and fibrosis in rats with protein-overload proteinuria.
Kidney Int 47
: 1546
–1557, 1995[Medline]
Gomez-Garre D, Largo R, Tejera N, Fortes J, Manzarbeitia F, Egido J: Activation of nuclear factor kappaB in tubular epithelial cells of rats with intense proteinuria: Role of angiotensin II and endothelin-1.
Hypertension 37
: 1171
–1178, 2001[Abstract/Free Full Text]
Shimizu H, Maruyama S, Yuzawa Y, Kato T, Miki Y, Suzuki S, Sato W, Morita Y, Maruyama H, Egashira K, Matsuo S: Anti-monocyte chemoattractant protein-1 gene therapy attenuates renal injury induced by protein-overload proteinuria.
J Am Soc Nephrol 14
: 1496
–1505, 2003[Abstract/Free Full Text]
Abbate M, Zoja C, Corna D, Capitanio M, Bertani T, Remuzzi G: In progressive nephropathies, overload of tubular cells with filtered proteins translates glomerular permeability dysfunction into cellular signals of interstitial inflammation.
J Am Soc Nephrol 9
: 1213
–1224, 1998[Abstract]
Donadelli R, Abbate M, Zanchi C, Corna D, Tomasoni S, Benigni A, Remuzzi G, Zoja C: Protein traffic activates NF-kappaB gene signaling and promotes MCP-1-dependent interstitial inflammation.
Am J Kidney Dis 36
: 1226
–1241, 2000[Medline]
Kramer AB, Ricardo SD, Kelly DJ, Waanders F, van Goor H, Navis G: Modulation of osteopontin in proteinuria-induced renal interstitial fibrosis.
J Pathol 207
: 483
–492, 2005[CrossRef][Medline]
Eardley KS, Zehnder D, Quinkler M, Lepenies J, Bates RL, Savage CO, Howie AJ, Adu D, Cockwell P: The relationship between albuminuria, MCP-1/CCL2, and interstitial macrophages in chronic kidney disease.
Kidney Int 69
: 1189
–1197, 2006[CrossRef][Medline]
Mezzano SA, Droguett MA, Burgos ME, Ardiles LG, Aros CA, Caorsi I, Egido J: Overexpression of chemokines, fibrogenic cytokines, and myofibroblast in human membranous nephropathy.
Kidney Int 57
: 147
–158, 2000[CrossRef][Medline]
Mezzano SA, Barria M, Droguett MA, Burgos ME, Ardiles LG, Flores C, Egido J: Tubular NF-kappaB and AP-1 activation in human proteinuric renal disease.
Kidney Int 60
: 1366
–1377, 2001[CrossRef][Medline]
Mezzano S, Aros C, Droguett A, Burgos ME, Ardiles L, Flores C, Schneider H, Ruiz-Ortega M, Egido J: NF-kappaB activation and overexpression of regulated genes in human diabetic nephropathy.
Nephrol Dial Transplant 19
: 2505
–2512, 2004[Abstract/Free Full Text]
Chow F, Ozols E, Nikolic-Paterson DJ, Atkins RC, Tesch GH: Macrophages in mouse type 2 diabetic nephropathy: Correlation with diabetic state and progressive renal injury.
Kidney Int 65
: 116
–128, 2004[CrossRef][Medline]
Chow FY, Nikolic-Paterson DJ, Ozols E, Atkins RC, Rollin BJ, Tesch GH: Monocyte chemoattractant protein-1 promotes the development of diabetic renal injury in streptozotocin-treated mice.
Kidney Int 69
: 73
–80, 2006[CrossRef][Medline]
Rangan GK, Wang Y, Tay Y-C, Harris DCH: Inhibition of nuclear factor-kappaB activation reduces cortical tubulointerstitial injury in proteinuric rats.
Kidney Int 56
: 118
–134, 1999[CrossRef][Medline]
Takase O, Hirahashi J, Takayanagi A, Chikaraishi A, Marumo T, Ozawa Y, Hayashi M, Shimizu N, Saruta T: Gene transfer of truncated IkappaBa prevents tubulointerstitial injury.
Kidney Int 63
: 501
–513, 2003[CrossRef][Medline]
Wu H, Wang Y, Tay YC, Zheng G, Zhang C, Alexander SI, Harris DC: DNA vaccination with naked DNA encoding MCP-1 and RANTES protects against renal injury in Adriamycin nephropathy.
Kidney Int 67
: 2178
–2186, 2005[CrossRef][Medline]
Anders HJ, Ninichuk V, Schlondorff D: Progression of kidney disease: Blocking leukocyte recruitment with chemokine receptor CCR1 antagonists.
Kidney Int 69
: 29
–32, 2006[CrossRef][Medline]
Biancone L, David S, Della Pietra V, Montrucchio G, Cambi V, Camussi G: Alternative pathway activation of complement by cultured human proximal tubular epithelial cells.
Kidney Int 45
: 451
–460, 1994[Medline]
David S, Biancone L, Caserta C, Bussolati B, Cambi V, Camussi G: Alternative pathway complement activation induces proinflammatory activity in human proximal tubular epithelial cells.
Nephrol Dial Transplant 12
: 51
–56, 1997[Abstract/Free Full Text]
Nomura A, Morita Y, Maruyama S, Hotta N, Nadai M, Wang L, Hasegawa T, Matsuo S: Role of complement in acute tubulointerstitial injury of rats with aminonucleoside nephrosis.
Am J Pathol 151
: 539
–547, 1997[Abstract]
Nangaku M, Pippin J, Couser WG: Complement membrane attack complex (C5b-9) mediates interstitial disease in experimental nephrotic syndrome.
J Am Soc Nephrol 10
: 2323
–2331, 1999[Abstract/Free Full Text]
Abbate M, Zoja C, Rottoli D, Corna D, Perico N, Bertani T, Remuzzi G: Antiproteinuric therapy while preventing the abnormal protein traffic in proximal tubule abrogates protein and complement-dependent interstitial inflammation in experimental renal disease.
J Am Soc Nephrol 10
: 804
–813, 1999[Abstract/Free Full Text]
Nangaku M, Pippin J, Couser WG: C6 mediates chronic progression of tubulointerstitial damage in rats with remnant kidneys.
J Am Soc Nephrol 13
: 928
–936, 2002[Abstract/Free Full Text]
Zhou W, Marsh JE, Sacks SH: Intrarenal synthesis of complement.
Kidney Int 59
: 1227
–1235, 2001[CrossRef][Medline]
Pratt JR, Basheer SA, Sacks SH: Local synthesis of complement component C3 regulates acute renal transplant rejection.
Nat Med 8
: 582
–587, 2002[CrossRef][Medline]
Farrar CA, Zhou W, Lin T, Sacks SH: Local extravascular pool of C3 is a determinant of postischemic acute renal failure.
FASEB J 20
: 217
–226, 2006[Abstract/Free Full Text]
Abbate M, Corna D, Rottoli D, Zanchi C, Cassis P, Morigi M, Zoja C, Remuzzi G: An intact complement pathway is not dispensable for glomerular and tubulointerstitial injury induced by protein overload [Abstract].
J Am Soc Nephrol 15
: 479A
, 2004
Abe K, Li K, Sacks SH, Sheerin NS: The membrane attack complex, C5b-9, up regulates collagen gene expression in renal tubular epithelial cells.
Clin Exp Immunol 136
: 60
–66, 2004[CrossRef][Medline]
B–dependent and NF-
Top
Glomerular Proteinuria as Signal...
In Vivo Evidence Linking...
V
5 integrin (40) and Na+/H+ exchanger (41) in response to albumin. In vivo gene expression profile analysis of proximal tubules from mice with protein overload proteinuria identified 2000 genes that were differentially regulated by excess proteins. More than half of them were upregulated (42). They included thymic shared antigen-1, the fibroblast-associated gene GS188, and glia maturation factor-B, a protein that originally was purified as a neurotrophic factor (43). The expression of glia maturation factor-B was induced in renal proximal tubular cells of mice with protein overload proteinuria (43). Proximal tubular cells that overexpressed glia maturation factor B acquired more susceptibility to death by sustained oxidative stress through p38-pathway activation. 
-secretase activity of a multimolecular complex of proteins, including the so-called presenilins, that mediates intramembrane proteolysis. A soluble fragment of the receptor thereby is formed and trafficked to the nucleus to regulate gene expression. In the rat kidney, the brush border exhibits both 
