Albumin transport and processing by the proximal tubule: physiology and pathophysiology

PURPOSE OF REVIEW: Significant epidemiological and clinical trial evidence supports the association between increased urinary albumin excretion, cardiovascular events and renal failure. An increase in albumin excretion has traditionally been considered to reflect a 'glomerular' leak of protein; however, it is now recognized that significant tubular reabsorption of albumin occurs under physiological conditions that may be modified by genetic determinants, systemic disease and drug therapies. RECENT FINDINGS: The endocytosis of albumin by the proximal tubule is a highly regulated process depending on protein-protein interactions between several membrane proteins and scaffolding and regulatory molecules. The elucidation of these interactions is an ongoing research focus. There is also mounting evidence for a transcytotic pathway for retrieval of albumin from the tubular filtrate. The molecular basis for the role of albuminuria in both interstitial renal disease and cardiovascular pathology continues to be defined. The clinical implications of albuminuria due to a glomerular leak vs. reduced tubular reabsorption of albumin are, however, now under consideration. In particular, the prognostic implication of microalbuminuria induced by the more potent 3-hydroxy-3-methylglutaryl coenzyme A reductase inhibitors is under study. SUMMARY: The currently defined mechanisms underpinning the tubular reabsorption of albumin, how these are modified by pathology and pharmacology, and the clinical implications are the subject of this review.     

The ability of statins to inhibit bone resorption is directly related to their inhibitory effect on HMG-CoA reductase activity.

Statins, which are inhibitors of 3-hydroxy-3-glutaryl-coenzyme A (HMG-CoA) reductase, decrease the hepatic biosynthesis of cholesterol by blocking the mevalonate pathway. Nitrogen-containing bisphosphonate drugs also inhibit the mevalonate pathway, preventing the production of the isoprenoids, which consequently results in the inhibition of osteoclast formation and osteoclast function. Therefore, we hypothesized that statins could affect bone metabolism in vivo through effects on osteoclastic bone resorption. In vitro, cerivastatin inhibited the parathyroid hormone (PTH)-stimulated bone resorption. Using a panel of 40 statin analogs, which showed variable effects on HMG-CoA reductase activity, we found that the ability of compounds to inhibit bone resorption is directly related to HMG-CoA reductase activity. However, in the thyro-parathyrodectomy (TPTX) model for bone resorption in the rat in vivo, cerivastatin did not prevent experimentally induced increases in bone resorption. The lack of effect of cerivastatin in this model is not related to a limited penetration of the target tissue (bone marrow), because a significant effect on HMG-CoA reductase activity was demonstrated in the total rat bone marrow cell extracts of rats posttreatment in vivo. Furthermore, cerivastatin inhibited protein prenylation in osteoclasts isolated from the rabbit bone marrow of rabbits after treatment in vivo. In contrast to other studies, none of the statins tested showed anabolic effects in parietal bone explant cultures. Taken together, we conclude that statins inhibit bone resorption in vitro, which correlates directly with the potency of the compounds for inhibition of HMG-CoA reductase activity. However, cerivastatin does not affect bone resorption in the rat TPTX model in vivo.     

Proteinuria and progression of glomerular diseases

One of the major challenges of nephrology is to develop therapeutic strategies to halt the progression of kidney diseases. In clinical settings, nephrotic-range proteinuria correlates with the rate of progression, particularly in glomerular diseases. Hence, the degree of proteinuria has been utilized to monitor the response to treatment as well as to predict outcome. However, the pathophysiology of proteinuria-induced progression remains unknown. Albumin accounts for the majority of the protein in nephrotic urine and as a result of this clinical observation studies have focused on understanding the adverse effects of albumin overload in the kidney. Albumin is internalized by receptor-mediated endocytosis in proximal tubule cells via low density lipoprotein (LDL) type receptor, megalin. Albumin at high concentrations mimicking nephrotic milieu has resulted in the upregulation of pro-inflammatory/fibrogenic genes and apoptosis in proximal tubule cells in in vivo and in vitro models of albumin overload. These properties of albumin on proximal tubule cells may explain extensive tubulointerstitial fibrosis and tubular atrophy observed in end-stage kidney disease. In addition to tubular toxicity, podocytes respond to proteinuric states by cytoskeletal alterations and loss of the differentiation marker synaptopodin. Identifying the molecular network of proteins involved in albumin handling will enable us to manipulate the specific signaling pathways and prevent damage caused by proteinuria.     

Cisplatin-induced inhibition of receptor-mediated endocytosis of protein in the kidney

BACKGROUND: Administration of cisplatin, cis-diamminedichloroplatinum (II) (CDDP), causes a severe impairment of renal function, including increases in urinary excretion of proteins. We recently found that CDDP inhibits vacuolar H+-ATPase, which plays an important role in receptor-mediated endocytosis in the renal proximal tubules. Therefore, CDDP-induced proteinuria may be due to an inhibition of the receptor-mediated endocytosis in the renal proximal tubules following a decrease in vacuolar H+-ATPase activity by the drug. METHODS: Effects of CDDP on receptor-mediated endocytosis of albumin in opossum kidney (OK) epithelial cells, and on urinary excretion of albumin and vitamin D binding protein, which are reabsorbed in the renal proximal tubules by endocytosis, in rats were examined. RESULTS: CDDP inhibited uptake of fluorescein-isothiocyanate (FITC)-albumin, a receptor-mediated endocytosis marker, by OK cells in a time- and concentration-dependent fashion. In contrast, CDDP treatment did not affect the uptake of FITC-inulin, a fluid-phase endocytosis marker. CDDP caused a decrease in the affinity and in the maximal velocity of FITC-albumin uptake. The adenosine 5'-triphosphate (ATP) content in OK cells was not changed by CDDP at concentrations that inhibited FITC-albumin uptake. The endosomal pH in OK cells was increased by CDDP treatment. Administration of CDDP to rats increased the urinary excretion of albumin and vitamin D binding protein. CONCLUSIONS: These results suggest that CDDP decreases the receptor-mediated endocytosis of protein following the inhibition of vacuolar H+-ATPase in the renal proximal tubules, and the inhibition of receptor-mediated endocytosis would be the mechanisms underlying the proteinuria induced by CDDP.     

Renal tubular transport and catabolism of proteins and peptides

The kidney plays an important role in the metabolism of proteins and peptides. Current evidence indicates that only the proximal tubule possesses the mechanism for degradation or transport of these substances and reabsorption of metabolic products. Proteins and large polypeptides filtered at the glomerulus are absorbed from proximal tubular fluid by luminal endocytosis into apical vacuoles. These fuse with primary lysosomes, where hydrolysis occurs followed by diffusion of metabolites out of the cells and into the blood. Recent evidence indicates that small linear peptides are handled by a different mechanism. It is likely that small peptides are degraded at the luminal surface of the brush border of proximal tubules, which contains many hydrolytic enzymes, by the process of membrane or contact digestion with reabsorption of the breakdown products. The probable biological significance of proximal tubular mechanisms for handling of proteins and peptides are conservation of amino acids, inactivation of toxic substances, and participation in the regulation of the circulating level of protein and peptide hormones.     

Cubilin- and megalin-mediated uptake of albumin in cultured proximal tubule cells of opossum kidney

BACKGROUND: Reabsorption of albumin from the glomerular filtrate occurs via receptor-mediated endocytosis in the proximal tubule. This process is initiated by binding of albumin in apical clathrin-coated pits, followed by endocytosis and degradation in lysosomes. Although binding sites have been characterized by kinetic studies, the receptors responsible for the binding of albumin have not been fully identified. Two giant glycoproteins, cubilin and megalin, constitute important endocytic receptors localized to the kidney proximal tubule. METHODS: In the present study, we examined the colocalization of cubilin and megalin in the endocytic pathway and the relationship between the uptake of albumin and the expression of cubilin and megalin in opossum kidney (OK) proximal tubule cells by immunocytochemistry and immunoblotting. RESULTS: OK cells expressed both cubilin and megalin. The light microscope labeling patterns for cubilin and megalin were almost identical and were mainly located at the surface area of the cells. Cubilin and megalin were also shown to colocalize on cell surface microvilli, in coated pits, and in endocytic compartments at the electron microscope level. Endocytosed bovine serum albumin (BSA) was identified exclusively in cells expressing megalin and cubilin. Uptake of BSA-FITC was saturable and inhibited by receptor-associated protein (RAP) and by intrinsic factor-vitamin B12 complex (IF-B12) at high concentrations. Significant inhibition was also observed by specific antibodies to cubilin, and megalin and cubilin antisense oligonucleotides likewise significantly reduced albumin uptake. Egg albumin did not affect the uptake of BSA. CONCLUSION: The present observations suggest that the two receptors cubilin and megalin are both involved in the endocytic uptake of albumin in renal proximal tubule cells.     

Bcl-xL overexpression protects from apoptosis induced by HMG-CoA reductase inhibitors in murine tubular cells

BACKGROUND: Hyperplasia is attributed to enhanced tubular cell proliferation with unbalanced cell death. The 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase inhibitors induce apoptosis in a variety of cell lines, including proximal tubular cells. However, the mechanisms by which statins induce apoptosis in tubular cells have not been fully addressed. METHODS: Apoptosis induced by simvastatin was measured in murine tubular cells with and without overexpressing Bcl-xL. Expression of genes implicated in cell death was studied by Northern and Western blot. RESULTS: The treatment of proliferating murine tubular cells (MCT) with simvastatin induced apoptosis in a time- and dose-dependent manner (0.1 to 1 micromol/L). Apoptosis was correlated with Bcl-xL mRNA and protein down-regulation. By contrast, the treatment with simvastatin did not modify the expression of the proapoptotic protein Bax. Simvastatin treatment was associated with cytochrome C release from the mitochondria to the cytosol. We also observed the presence of active caspase 9 and 3 during apoptosis induced by simvastatin. These effects were reversed by mevalonate, farnesylpyrophosphate (FPP), and geranylgeranylpyrophosphate (GGPP), suggesting the involvement of protein prenylation. Simvastatin appears to alter the balance between cell-life and death-promoting genes, as reflected by the decreased Bcl-xL/Bax ratio. Supporting this hypothesis, overexpression of Bcl-xL reduced the amount of apoptosis induced by simvastatin by 80% when compared with control vector-expressing cells. The overexpression of Bcl-xL also prevented the activation of caspase 9 and 3. CONCLUSION: Our results indicate that down-regulation of Bcl-xL expression mediates apoptosis induced by statins in tubular cells. These results may be relevant to the treatment of disorders characterized by altered tubular proliferation.     

Early proximal tubule injury in experimental aristolochic acid nephropathy: functional and histological studies.

BACKGROUND: Aristolochic acid (AA), the plant extract of Aristolochia species, is involved in the onset of progressive tubulointerstitial renal fibrosis in humans. Clinical and in vitro findings have previously suggested that the proximal tubule was the target of AA. METHODS: Using a rat model of AA nephropathy, the proximal tubular lesions induced by daily subcutaneous injections of AA for 35 or 5 days were characterized biochemically and histologically. Urinary excretion of proteins, albumin, low molecular weight proteins, N-acetyl-beta-d-glucosaminidase, alpha-glutathione S-transferase, leucine aminopeptidase and neutral endopeptidase (NEP) was determined and related to histological conventional findings and immunostainings of NEP and megalin. RESULTS: In both protocols, an acute phase of release of urinary markers was observed within the first 3 days of AA treatment in parallel with a significant increase of specific AA-related DNA adducts reflecting early tubular intoxication. A dramatic loss of the proximal tubule brush border was histologically confirmed, while the expression of megalin decreased at the damaged apical epithelium (mainly of the S3 segment). CONCLUSION: Proximal tubule injury occurs early after AA intoxication in rats, with a link between specific AA-DNA adduct formation, decreased megalin expression and inhibition of receptor-mediated endocytosis of low molecular weight proteins, bringing in vivo confirmation of previous in vitro studies.     

Protein uptake disturbs collagen homeostasis in proximal tubule-derived cells

BACKGROUND: Interstitial fibrosis is of major importance for the deterioration of renal function, leading to uremia. Interaction of filtered proteins with proximal tubular cells is important for the onset and development of tubulointerstitial damage. METHODS: We investigated the effects of protein endocytosis on collagen homeostasis and signaling pathways of proximal tubule-derived cells (OK cells, LLC-PK1 cells), which express the endocytic machinery typical for the proximal tubule (megalin and cubilin), and compared it to renal epithelial cells with low endocytic activity (MDCK, IHKE1, NHE3-deficient OK cells). Collagen homeostasis was assessed by proline incorporation, ELISA, and Western blot. Matrix metalloproteinase (MMP) activity was assessed by gelatinase assay. Signaling pathways were monitored by reporter gene assay. RESULTS: Albumin, glycated albumin, fatty acid-free albumin, or globulins led to an increase of secreted collagen (types I, III, and IV) in OK and LLC-PK1 cells. In cells with low protein uptake activity, albumin exposure inhibited collagen secretion. Western blot analysis showed an increase of cellular collagen. MMP activity was significantly decreased by albumin exposure. Furthermore, albumin exposure led to activation of the NF-kappa B-, AP1-, NFAT-, SRE-, and CRE-pathways. Inhibition of NF-kappa B, PKC, or PKA partially reversed the effects of albumin. In addition, inhibition of albumin endocytosis reduced collagen secretion and activation of the signaling pathways. Discussion. The data show that endocytic uptake of proteins disturbs collagen homeostasis in proximal tubular cells. This disturbed matrix homeostasis probably supports the progression of interstitial fibrosis, which is of importance for the development of renal insufficiency.     

Anti-inflammatory and immunomodulatory effects of statins

Anti-inflammatory and immunomodulatory effects of statins. 3-Hydroxy-3-methyl-gutaryl coenzyme A (HMG-CoA) reductase inhibitors or statins constitute the most powerful class of lipid-lowering drugs. Clinical trials have demonstrated a marked reduction in cardiovascular mortality in patients treated with statins. However, the benefits observed with statin therapy appear to be related, at least in part, with their cholesterol-lowering independent effects. Extensive research carried out mainly in the last decade suggests that the clinical benefits of these drugs could be related to an improvement in endothelial dysfunction, a reduction in blood thrombogenicity, anti-inflammatory properties, and, recently, immunomodulatory actions. In this sense, statins decrease T cell activation, the recruitment of monocytes and T cells into the arterial wall, and enhance the stability of atherosclerotic lesions. Many of these effects are related with the inhibition of isoprenoid synthesis, which serve as a lipid attachment for a variety of proteins implicated in intracellular signaling. In fact, small G proteins, whose proper membrane localization and function are dependent on isoprenylation, may play an important role in the lipid-lowering independent effects of HMG-CoA reductase inhibitors. This article summarizes the anti-inflammatory and immunomodulatory effects of statins and their participation in the different steps of atherosclerotic lesion formation.     

Statins prevent oxidized LDL-induced injury of glomerular podocytes by activating the phosphatidylinositol 3-kinase/AKT-signaling pathway

The injury of podocytes is associated with alterations of the glomerular size-selective barrier to proteins. In this study, oxidized LDL (oxLDL) but not native LDL induced apoptosis in human cultured podocytes and reduced Akt activity and P-Akt/Akt ratio. Moreover, oxLDL-induced redistribution and loss of nephrin, an adhesion molecule specific for the glomerular slit diaphragm. Nephrin reduction was preceded by inhibition of nephrin tyrosine phosphorylation and of its association with p85 phosphatidylinositol 3-kinase (PI3K). Moreover, three different statins, mevastatin, pravastatin, and simvastatin, inhibited in a dose-dependent manner apoptosis and loss of nephrin induced by oxLDL by stimulating Akt activity. In addition, simvastatin significantly increased the expression of nephrin protein and mRNA by podocytes. The protective effects of statins were blocked by treatment of podocytes with two unrelated pharmacologic inhibitors of PI3K, LY294002 and wortmannin, suggesting a role for PI3K, and by mevalonate, indicating dependency on HMG-CoA reductase activity. Statins directly stimulated Akt phosphorylation ad activity. Finally, oxLDL induced a retraction of cultured podocytes and an increase in the albumin diffusion across their monolayer that was inhibited by treatment with statins. In conclusion, statins reduced the oxLDL-induced apoptosis and loss of nephrin in glomerular podocytes. The statin-induced Akt activation may protect from the loss of nephrin by an inhibition of its redistribution and shedding and by a stimulation of its synthesis. These data provide a rationale for the anti-proteinuric effect of statins.     

Megalin contributes to the early injury of proximal tubule cells during nonselective proteinuria

Megalin, a member of the LDL receptor family, is expressed on the apical membrane of proximal tubules and serves as an endocytic scavenger of filtered proteins and hence might contribute to the tubule injury as a consequence of glomerular disease. To study its role, we crossed megalin knockout mosaic mice (lacking megalin expression in 60% of proximal tubule cells) with NEP25 mice (a transgenic line expressing human CD25 in the podocyte). Treatment of this transgenic mouse with the immunotoxin causes nephrotic syndrome, focal segmental glomerulosclerosis and tubule-interstitial injury. Following this treatment, the double transgenic mice had massive non-selective proteinuria and mild glomerular and tubular injury. Comparison of megalin-containing to megalin-deficient proximal tubule cells within each kidney showed that albumin, immunoglobulin light chain, IgA and IgG were preferentially accumulated in proximal tubule cells expressing megalin. Tubule injury markers such as heme-oxygenase-1, monocyte chemoattractant protein-1 and cellular apoptosis were also preferentially found in these megalin-expressing cells. These results show that megalin plays a pivotal role in the reabsorption of small to large molecular size proteins and provides direct in vivo evidence that reabsorption of filtered proteins triggers events leading to tubule injury.     

Characterization of proteinuria and tubular protein uptake in a new model of oral L-lysine administration in rats

Intravenous infusion of basic amino acids is used experimentally and pharmacologically to prevent renal proximal tubular uptake of filtered proteins. Intravenously injected L-lysine is rapidly cleared from plasma and the effect on tubular protein reabsorption is transient. To obtain a more sustained effect, we developed a model of oral L-lysine administration and characterized this model by analyzing urinary protein excretion and proximal tubule uptake of filtered proteins. Rats placed in metabolic cages were treated with 20 mmol/kg/6 h of L-lysine, glycine, or water. Urines were analyzed for proteins by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, immunoblotting, and radioimmunoassay. Proximal tubule uptake of proteins and expression of apical membrane receptors were investigated by immunocytochemistry. In vitro uptake and receptor expression were studied using a yolk sac cell line. L-lysine administration produced increased urinary excretion of a large number of proteins while the effect on tubular accumulation of selected proteins was variable. L-lysine treatment induced changes in the localization of two receptors responsible for tubular endocytosis of filtered proteins. In conclusion, oral L-lysine treatment induced proteinuria, in particular albuminuria, as efficiently as previous reports on intravenous infusion. The effect on tubular protein accumulation was variable suggesting differential effects on tubular reabsorption and degradation of filtered proteins. Changes in tubular protein handling were accompanied by changes in the localization of the endocytic receptors, megalin, and cubilin. In vitro experiments supported the in vivo observations. The findings suggest that L-lysine may affect receptor trafficking in addition to possible effects on the direct binding of ligands to the receptors.     

Influence of albumin infusion on the urinary excretion of beta2-microglobulin in patients with proteinuria

Most filtered proteins are reabsorbed by the renal proximal tubule by a mechanism that involves binding to the brush border membrane and endocytosis. Under normal conditions the low-molecular-weight protein beta2-microglobulin (beta2M), which is used to detect tubular injury, is reabsorbed almost completely. However, in proteinuric patients an increased urinary excretion of beta2M may not simply reflect tubular damage but might also result from a decreased tubular reabsorption due to competitive mechanisms. To examine the magnitude of such an effect we have studied the renal effects of albumin infusion (40 g in 2 h of a 20% solution) in 10 patients with a glomerular disease and proteinuria >3.5 g/24 h. Before, during and after albumin infusion the GFR (inulin clearance), RPF (PAH clearance), blood pressure and the urinary excretion of albumin, IgG, transferrin and beta2M were measured. Albumin infusion resulted in a slight decrease of the GFR (72 +/- 11 ml/min before and 67 +/- 10 ml/min after infusion), an increase of the RPF (379 +/- 66 ml/min before and 445 +/- 83 ml/min after), a decrease of the filtration fraction (0.20 before and 0.17 after), and hemodilution. After infusion the urinary excretion of albumin increased from 4.5 +/- 0.7 to 8.4 +/- 1.6 mg/min (p < 0.05). The urinary excretion of IgG and transferrin increased, probably reflecting a change in glomerular size-selectivity. In contrast, the urinary excretion of beta2M did not change significantly (baseline 12 +/- 5 microg/min, end 13 +/- 6 microg/min, percentage change 16.8 +/- 11%). To correct for changes in tubular load we calculated the fractional reabsorption of beta2M. The initial rise in albuminuria during infusion did not affect fractional tubular reabsorption (Delta%: 0. 72 +/- 0.52%, median 0.005%). In the period after infusion a slight decrease was noted (median -0.33%, p < 0.01). A decrease in the fractional reabsorption was particularly observed in patients with pre-existing tubular damage. In conclusion: infusion of albumin in proteinuric patients has no clinically relevant effect on the tubular reabsorption of beta2M. Therefore, beta2M is useful as a parameter to detect tubular injury and alterations in tubular handling of proteins in patients with proteinuria and glomerular diseases.     

Potential therapeutic role for statins in respiratory disease

Statins reduce cholesterol levels by inhibiting 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase and have an established role in the treatment of atherosclerotic disease. Recent research has identified anti-inflammatory properties of statins. Statins appear to reduce the stability of lipid raft formation with subsequent effects on immune activation and regulation, and also prevent the prenylation of signalling molecules with subsequent downregulation of gene expression. Both these effects result in reduced cytokine, chemokine, and adhesion molecule expression, with effects on cell apoptosis or proliferation. This review considers the evidence for the anti-inflammatory properties of statins in the lung, and how these effects are being applied to research into the role of statins as a novel treatment of respiratory diseases.     

Aristolochic acid impedes endocytosis and induces DNA adducts in proximal tubule cells

BACKGROUND: Aristolochic acid (AA), present in Aristolochia plants, appears to be the toxin responsible for Chinese herbs nephropathy (CHN), a rapidly progressive tubulointerstitial nephritis. One of the earliest sign of CHN is the urinary excretion of low-molecular-weight proteins (LMWP), suggesting that AA is toxic to proximal tubules (PT). METHODS: The effects of AA on PT functions including reabsorption of LMWP were investigated on the well-established opossum kidney (OK) cell line, a model for PT, and compared with those of the classical PT toxin cadmium chloride (CdCl2). RESULTS: OK cell monolayers internalized albumin and beta2-microglobulin by receptor-mediated endocytosis, both proteins apparently competing for the same receptor, a complex of megalin and cubulin. The process was significantly impaired by 24-hour preincubation with AA (10 or 20 micromol/L) or CdCl2 (15 micromol/L). Furthermore, 24-hour exposure to AA followed by its removal during one to six days led to a persistent inhibition of the uptake of albumin, in contrast to the substantial recovery observed after CdCl2 removal. Neither AA nor CdCl2 affected cell viability, Na+-glucose cotransport or total rate of protein synthesis. AA significantly decreased megalin expression and formed specific DNA adducts in OK cells, similar to those found in kidneys from CHN patients. CONCLUSIONS: The present data support the involvement of AA in the early PT dysfunction found in CHN; furthermore, they suggest a causal relationship between DNA adduct formation, decreased megalin expression, and inhibition of receptor-mediated endocytosis of LMWP.     

Antiproteinuric therapy while preventing the abnormal protein traffic in proximal tubule abrogates protein- and complement-dependent interstitial inflammation in experimental renal disease

In proteinuric glomerulopathies, the excess traffic of proteins into the renal tubule is a candidate trigger of interstitial inflammatory and immune events leading to progressive injury, and a key target for the renoprotective action of antiproteinuric drugs. Among proteins trafficked to the proximal tubule, the third component of complement (C3) can be activated locally and contribute to inflammation at sites of protein reabsorption. Experiments were performed in rats with renal mass reduction (RMR, 5/6 nephrectomy) with the following aims: (1) to study Ig (IgG) and complement deposition in proximal tubules, and interstitial macrophage infiltration and MHC class II expression at intervals after surgery by double immunofluorescence analysis; (2) to assess whether lisinopril (angiotensin-converting enzyme inhibitor [ACEi], 25 mg/L in the drinking water, from either day 1 or day 7) limited IgG and C3 accumulation and interstitial inflammation at day 30. In 7-d remnant kidneys, intracellular staining for both IgG and C3 was detectable in proximal tubules in focal areas; C3 was restricted to IgG-positive tubular cells, and there were no interstitial ED-1 macrophage and MHC II-positive cellular infiltrates. In 14-d and 30-d remnant kidneys, proximal tubular IgG and C3 staining was associated with the appearance of interstitial infiltrates that preferentially localized to areas of tubules positive for both proteins. RMR rats given ACEi had no or limited increases in levels of urinary protein excretion, tubular IgG, and C3 reactivity, and interstitial cellular infiltrates in kidneys at 30 d, even when ACEi was started from day 7 after surgery. These findings document that (1) in RMR, IgG and C3 accumulation in proximal tubular cells is followed by leukocyte infiltration and MHC II overexpression in the adjacent interstitium; (2) ACEi while preventing proteinuria limits both tubular accumulation of IgG and C3 and interstitial inflammation. The data suggest that ACE inhibition can be renoprotective by limiting the early abnormal protein traffic in proximal tubule and consequent deleterious effects of excess protein reabsorption, including the accumulation and local activation of complement as well as the induction of chemokines and endothelin genes known to promote interstitial inflammation and fibrosis.