Turnover of human tubular cells exposed to proteins in vivo and in vitro

C5b-9 does not mediate tubulointerstitial injury in experimental acute glomerular disease characterized by selective proteinuria

AIM: To determine whether complement membrane attack complex (C5b-9) has a pathogenic role in tubulointerstitial injury in a renal disease model characterized by acute highly selective proteinuria.

METHODS: Protein-overload nephropathy (PON) was induced in adult female Piebald-Viral-Glaxo rats with or without complement C6 deficiency (C6^- and C6⁺) by daily intraperitoneal injections of bovine serum albumin (BSA, 2 g/d), and examined on days 2, 4 and 8.

RESULTS: Groups with PON developed equivalent levels of heavy proteinuria within 24 h of BSA injection. In C6⁺ rats with PON, the tubulointerstitial expression of C5b-9 was increased and localized predominantly to the basolateral surface of tubular epithelial cells (TECs), whereas it was undetectable in C6^- animals. TEC proliferation (as assessed by the number of BrdU+ cells) increased by more than 50-fold in PON, peaking on day 2 and declining on days 4 to 8. There was a trend for a reduction in the number of BrdU+ TECs on day 4 in the C6^- PON group (P = 0.10 compared to C6⁺) but not at any other time-point. Kidney enlargement, TEC apoptosis (TUNEL⁺ cells) and markers of tubular injury (tubule dilatation, loss of TEC height, protein cast formation) were not altered by C6 deficiency in PON. Interstitial monocyte (ED-1+ cell) accumulation was partially reduced in C6^- animals with PON on day 4 (P = 0.01) but there was no change in myofibroblast accumulation.

CONCLUSION: These data suggest that C5b-9 does not mediate tubulointerstitial injury in acute glomerular diseases characterized by selective proteinuria.

Mycophenolic acid inhibits the phosphorylation of nuclear factor‑κB and Akt in renal tubular epithelial cells

Renal tubulointerstitial injury induced by albumin overload is a critical stage during the progression of renal interstitial fibrosis and progression of chronic renal diseases. Inosine‑5'‑monophosphate dehydrogenase inhibitor mycophenolate mofetil (MMF), a pro‑drug of mycophenolic acid (MPA), is known to attenuate the progression of renal interstitial fibrosis; however, the underlying molecular mechanisms of the anti‑fibrotic effects of derivatives of MMF have not yet been studied. The present study assessed the effects of the MPA on renal tubular epithelial cells. Transforming growth factor beta 1 (TGF‑β1) has been indicated to have a central role in the underlying molecular mechanisms of renal fibrosis; furthermore, nuclear transcription factor‑κB (NF‑κB) is a transcription factor associated with the production of inflammatory cytokines, cell proliferation and apoptosis. In addition, the Akt signaling pathway has important roles in cell proliferation, differentiation, metabolism and apoptosis. The present study subjected the NRK52E rat kidney epithelial‑derived cell line to albumin overload, which resulted in an increase in TGF‑β1 production as well as phosphorylation of Akt and the binding activity of NF‑κB to the promoter region of the TGF‑β1 gene, which was, however, reduced following pre‑incubation of the cells with MPA. In addition, the effects of albumin were partially blocked by Ly294002, a specific inhibitor of Akt. In conclusion, the results of the present study suggested that MPA may exert its anti‑fibrotic effects by inhibiting the upregulation of TGF‑β1 and the activation of NF‑κB following albumin overload, which may be partly dependent on the Akt pathway.

The effect of progressive glomerular disease on megalin-mediated endocytosis in the kidney

BACKGROUND

A well-characterized dog model of the X-linked collagen disease Alport syndrome (XLAS) was used to study the effect of progressive glomerular disease on megalin-mediated endocytosis. In XLAS, altered structure and function of the glomerular basement membrane induces a progressive proteinuric nephropathy.

METHODS

The investigation was performed in male XLAS dogs and age-matched normal male littermates. The urine profile and megalin-mediated endocytosis in the proximal tubule of six healthy and six XLAS dogs were examined at 2, 4, 6, 8 and 10 months of age using SDS-PAGE, immunoblotting and immunohistochemistry.

RESULTS

Gradually increasing urinary excretion of proteins over time and a reduced content of the same proteins in proximal tubule cells were found. Besides the glomerular component of the proteinuria, a significant tubular component was seen, which is due to a progressive change in the uptake of low-molecular-weight (LMW) ligands by megalin. Furthermore, the protein overload present in the lumen of the proximal tubule exceeds the reabsorption capacity of megalin and the co-receptor cubilin and results in a combined low- and high-molecular-weight (HMW) proteinuria. Also, a shift in the distribution of lysosomes was seen in the XLAS dogs suggesting changes in the lysosomal degradation pattern in response to the altered endocytosis.

CONCLUSIONS

The present study shows that the increased glomerular permeability and the subsequently altered megalin-mediated and megalin-dependent cubilin-mediated endocytosis lead to a partial LMW proteinuria and partial HMW proteinuria.

Sirolimus Therapy of Focal Segmental Glomerulosclerosis Is Associated With Nephrotoxicity

To evaluate the safety and efficacy of sirolimus in treating patients with focal segmental glomerulosclerosis (FSGS), we performed a phase 2, open-label clinical trial. Inclusion criteria were adults and children 13 years and older with biopsy-proven idiopathic FSGS, proteinuria with protein of 3.5 g/d or greater while on angiotensin antagonist therapy, glomerular filtration rate (GFR) of 30 mL/min/1.73 m(2) or greater (>or=0.50 mL/s), and failure to achieve sustained remission with at least 1 immunosuppressive agent. Eligible patients received sirolimus doses adjusted to achieve trough levels of 5 to 15 ng/mL during the first 4 months and 10 to 20 ng/mL for the subsequent 8 months. The primary outcome was decrease in proteinuria, expressed as complete remission (protein < 0.3 g/d) or partial remission (protein >or= 50% decrease and <3.5 g/d). Six adult patients with FSGS were enrolled in the study; they had median disease duration of 4.0 years, mean age of 39 +/- 11 years, mean baseline Modification of Diet in Renal Disease-estimated GFR of 52 +/- 15 mL/min/1.73 m(2) (0.87 +/- 0.25 mL/s), and median baseline proteinuria with protein of 6.6 g/d (interquartile range, 4.2 to 9.4). Five patients had received cyclosporine. No patient experienced a complete or partial remission. Sirolimus therapy was stopped prematurely in 5 patients for the following reasons: (1) precipitous decrease in GFR in 4 patients after 7 to 9 months of therapy with a greater than 2-fold increase in proteinuria in 3 patients and (2) hypertriglyceridemia with triglyceride levels greater than 1,600 mg/dL (>18 mmol/L) at 5 months in 1 patient. Because of a rapid decrease in GFR with worsening proteinuria, the protocol was closed to further recruitment. We conclude that sirolimus may be associated with nephrotoxicity in some patients with FSGS, particularly those with prolonged disease duration and prior cyclosporine therapy.

Therapeutic measures in proteinuric nephropathy

The level of proteinuria is one of the most important risk factors for progressive renal function loss in renal diseases. Any therapeutic measure that reduces proteinuria will slow or halt the progression of proteinuric nephropathies. Blockade of the renin-angiotensin-aldosterone system (RAAS) with angiotensin-converting enzyme (ACE) inhibitors or AT1-receptor antagonists (ARA) is currently the most powerful available antiproteinuric treatment. Recent investigations point out that blockade of RAAS at other levels (e.g., aldosterone or renin antagonists) could also induce a significant decrease in proteinuria. Because angiotensin II is also generated from angiotensin I by enzymes other than ACE, ARA would provide a more effective blockade of angiotensin II; however, ACE inhibition increases plasma levels of substances such as bradykinin and N-acetyl-seryl-aspartyl-lysyl-proline, which have strong antifibrotic properties. These differential effects of ACE inhibitors and ARA are the rationale for combined administration of both agents, which in clinical studies has demonstrated a significantly higher antiproteinuric and renoprotective effect than by either drug alone. Salt and protein restriction, as well as cautious use of diuretics, can also increase the antiproteinuric effect of RAAS blockade. Treatment with statins or other lipid-lowering agents leads to reduction in proteinuria levels, as some meta-analyses have demonstrated. Smoking is associated with an increased risk for the appearance of proteinuria, so cessation of smoking should be mandatory in proteinuric renal diseases. Recent studies have highlighted an epidemic increase of obesity-related proteinuric glomerulopathies; weight loss is effective not only in this condition, but also in overweight patients with proteinuric nephropathies of other etiologies.

Rapamycin worsens renal function and intratubular cast formation in protein overload nephropathy

BACKGROUND

Rapamycin (sirolimus) is associated with functional nephrotoxicity in some patients with nephrotic glomerular diseases but the pathophysiologic mechanisms are not known. This study investigated the effects of rapamycin on renal function and structure in protein overload nephropathy.

METHODS

Rats with protein overload nephropathy [induced by bovine serum albumin (BSA), 2.1 g by daily intraperitoneal injection, day 0 to day 3] received daily intraperitoneal injections of either vehicle [dimethyl sulfoxide (DMSO)], rapamycin (0.2 mg/kg, an inhibitor of mammalian target of rapamycin), or roscovitine (3.5 mg/kg, a small molecule cyclin-dependent kinase inhibitor) (N= 9 each) from day -3 to day 3.

RESULTS

In protein overload nephropathy, rapamycin caused severe acute renal failure and mild hypercholesterolemia (both P < 0.05). Rapamycin dramatically increased intratubular cast formation, and proximal tubular epithelial cells were swollen and engorged with increased cytoplasmic protein droplets. The number of 5-bromo-2'-deoxyuridine (BrdU)-positive tubular epithelial cells increased by more than 20-fold on day 3 in protein overload nephropathy, and this was attenuated by 65% with rapamycin (P < 0.05), whereas roscovitine was ineffective. Rapamycin increased the protein expression of p27(kip1) in tubular epithelial cells, but did not alter D-type cyclin expression or apoptosis.

CONCLUSION

Rapamycin caused a specific pattern of acute renal injury characterized by increased intratubular cast formation in protein overload nephropathy. This could be due to disruption of a potentially important compensatory mechanism in nephrotic glomerular diseases involving tubular epithelial cell protein endocytosis and proliferation.

Effect of human anti-DNA antibodies on proximal renal tubular epithelial cell cytokine expression: implications on tubulointerstitial inflammation in lupus nephritis

This study aimed to investigate the effects of human anti-DNA antibodies (Ab) from patients with lupus on renal proximal tubular epithelial cells (PTEC), focusing on alterations in cell morphology and proinflammatory cytokine synthesis. Immunohistochemistry showed increased tubulointerstitial IL-6 expression and IgG deposition in renal biopsies from patients with diffuse proliferative lupus nephritis, not observed in controls or membranous lupus nephritis, which correlated with the severity of inflammatory cell infiltration. Sera from patients with lupus nephritis contained IgG that bound to cultured PTEC. Such binding increased with disease activity and correlated with the level of anti-DNA Ab. Incubation of PTEC with anti-DNA Ab that were isolated during active (active Ab) or inactive (inactive Ab) disease induced IL-6 synthesis, both apically and from the basolateral aspect. This was accompanied by altered cell morphology, increased cell proliferation (P < 0.05), and lactate dehydrogenase release (P < 0.05). The binding of inactive Ab and active Ab to PTEC resulted in differential and sequential upregulation of TNF-alpha, IL-1beta, and IL-6 secretion (P < 0.05). Early induction of TNF-alpha was observed with active Ab; the two then acted synergistically to induce IL-6 secretion. Exposure of PTEC to inactive Ab was associated with modest induction of TNF-alpha, which was not involved in downstream induction of other proinflammatory peptides. These data suggest distinct immunopathogenetic mechanisms during disease flare or remission. Conditioned media from human mesangial cells acted synergistically with anti-DNA Ab to induce cytokine secretion in PTEC. Results from these studies underscore the pivotal role of PTEC in the pathogenesis of tubulointerstitial inflammation and fibrosis in lupus nephritis.

Why is proteinuria an ominous biomarker of progressive kidney disease?

Progressive tubule injury and interstitial fibrosis frequently accompany glomerulopathies associated with proteinuria. Clinical experience indicates that higher levels of proteinuria prior to, as well as after initiation of treatment predict more rapid decline in renal function and more pronounced tubulointerstitial injury. It has been proposed that filtration of potentially tubulotoxic plasma proteins is responsible for the observed correlations between proteinuria and progression (i.e., proteinuria is a cause and not only a consequence of progressive renal injury). Numerous attempts have been made to identify the species of putative tubulotoxic proteins in this progressive injury process, but much uncertainty persists. These uncertainties stem from nonphysiologic exposure of apical cell surfaces to proteins in vitro, the extremely high concentrations of various proteins tested in vitro, and the nonuniformity of end points measured. Furthermore, there is often a lack of correlation between in vitro and in vivo findings, and a lack of uniformity of results even for seemingly similar in vitro experiments. Less controversy is evident in the potential pathways whereby injured tubules evoke a tubulointerstitial inflammatory and fibrotic response, with many in vivo models serving to incriminate excessive cytokine and chemokine production, infiltration of various inflammatory cells, and the balance between apoptosis and cell proliferation. Despite many years of concerted efforts, we believe it is still unclear whether proteinuria is a cause (and if so, which species of protein), or only a consequence of progressive renal injury. Nevertheless, pending the resolution of these uncertainties by more decisive and unambiguous experimentation, the strongly predictive inverse relationship between level of proteinuria and long-term renal survival currently justifies aggressive antiproteinuric treatment strategies, with a goal of reducing protein excretion rate to the lowest level possible without the induction of symptoms or undue risk.

Effect of childhood obesity and obesity-related cardiovascular risk factors on glomerular and tubular protein excretion

There is increasing evidence that obesity may damage the kidney in otherwise healthy individuals. Our study investigated the effect of childhood obesity on urinary albumin and beta-2-microglobulin excretion, and the association of these with obesity-related cardiovascular risk factors. Random morning spot urine samples were collected from clinically healthy obese (n = 86; median age 12.9 years, range 8.9-17.2 years; median weight 80.6 kg, range 46.1-136.8 kg; median body mass index 30.4 kg/m2, range 24.5-43.2 kg/m2) and normal weight children (n = 79; median age 13.5 years, range 10.7-14.9 years; median weight 51.0 kg, range 27.3-72.5 kg; median body mass index 18.2 kg/m2, range 13.2-23.9 kg/m2). The obese children were examined for the presence of common obesity-related cardiovascular risk factors including hyperinsulinaemia, impaired glucose tolerance (IGT), dyslipidaemia, hypercholesterolaemia, and hypertension. Obese children had a significantly higher urinary albumin/creatinine ratio (U-ACR) (median 11.7 mg/g, interquartile range 12.9 mg/g versus median 9.0 mg/g, interquartile range 5.1 mg/g; P = 0.003) and urinary beta-2-microglobulin/creatinine ratio (U-BMCR) (median 63.9 microg/g, interquartile range 34.7 microg/g versus median 34.6 microg/g, interquartile range 44.1 microg/g; P < 0.001) than normal weight children. Among the obese children, the U-ACR was associated with fasting hyperinsulinaemia, IGT, and hypercholesterolaemia (all P < 0.05), and significantly correlated with the fasting (r = 0.23, P < 0.05) and 2-h (r = 0.37, P < 0.001) plasma glucose levels measured during an oral glucose tolerance test. Obese children with no more than one of the features of the metabolic syndrome had significantly lower U-ACRs than obese children with two or more features (median 10.4 mg/g, interquartile range 5.8 mg/g versus median 15.3 mg/g, interquartile range 14.9 mg/g; P < 0.05).

CONCLUSION

According to our results, clinically healthy obese children have a higher degree of albuminuria and beta-2-microglobulinuria than normal weight children, indicating early renal glomerular and tubular dysfunction as a consequence of childhood obesity. The urinary albumin/creatinine ratio in the obese children was associated with certain metabolic derangements linked to obesity, and also with the clustering of features of the metabolic syndrome.

Enlarged nephrons and severe nondiabetic nephropathy in hepatocyte nuclear factor-1beta (HNF-1beta) mutation carriers

BACKGROUND

Mutations in hepatocyte nuclear factor-1beta (HNF-1beta) lead to a syndrome with diabetes and urogenital malformations [maturity onset of diabetes of the young, type 5 (MODY5)]. The aim of this study was to perform a clinicopathologic investigation of the renal disease in members of a Norwegian family with the HNF-1beta mutation R137-K161del.

METHODS

The study was based on long-term clinical observations of five mutation carriers, combined with renal biopsies from four of these. The biopsies were examined by light microscopy, immunohistochemistry, and transmission electron microscopy. The diameter of the glomerulus, proximal and distal tubules, in addition to thickness of the glomerular basement membrane (GBM), were measured in light microscopic slides and transmission electron micrographs. The results were compared with biopsies from adult patients with diabetic glomerulopathy, glomerulonephritis, and/or benign nephrosclerosis, and children with minimal-change glomerulopathy or glomerulonephritis, respectively.

RESULTS

Clinically, there was a wide intrafamilial variation from stable or slightly increasing serum creatinine to progressive renal failure and end-stage renal disease (ESRD). In all cases, the kidney disease was diagnosed prior to diabetes. Hypertrophy of the proximal and distal tubules as well as enlarged glomeruli were found in three of four mutation carriers. Essentially normal nephrons were found in the 10-year-old boy. The thickness of the GBM was considered near normal in all mutation carriers. Oligomeganephronia was found in one patient.

CONCLUSION

Histopathologic and morphometric studies of kidney biopsies from four carriers of an HNF-1beta mutation revealed enlarged glomeruli and tubular structures. Long-term clinical follow-up demonstrated that the renal disease developed prior to and independently of diabetes. Finally, there is a wide phenotypic variation of the renal disease caused by HNF-1beta mutations.

Pathophysiology of proteinuria

Proteinuria is consequence of two mechanisms: the abnormal transglomerular passage of proteins due to increased permeability of glomerular capillary wall and their subsequent impaired reabsorption by the epithelial cells of the proximal tubuli. In the various glomerular diseases, the severity of disruption of the structural integrity of the glomerular capillary wall correlates with the area of the glomerular barrier being permeated by "large" pores, permitting the passage in the tubular lumen of high-molecular-weight (HMW) proteins, to which the barrier is normally impermeable. The increased load of such proteins in the tubular lumen leads to the saturation of the reabsorptive mechanism by the tubular cells, and, in the most severe or chronic conditions, to their toxic damage, that favors the increased urinary excretion of all proteins, including low-molecular-weight (LMW) proteins, which are completely reabsorbed in physiologic conditions. Recent clinical studies showed that in patients with glomerular diseases the urinary excretion of some HMW proteins [immunoglobulins G and M (IgG and IgM)] and of some LMW proteins, alpha1-microglobulin, beta2-microglobulin, correlates with the severity of the histologic lesions, and may predict, better than the quantity of proteinuria, the natural course, the outcome, and the response to treatment. It is suggested that some patients have already, at the time of clinical presentation, a structural damage of the glomerular capillary wall (injury of podocytes) and of the tubulointerstitium, the severity and scarce reversibility of which are reliably indicated by an elevated urinary excretion of HMW and LMW proteins.

Albumin stimulates interleukin-8 expression in proximal tubular epithelial cells in vitro and in vivo

Renal tubulointerstitial injury is characterized by inflammatory cell infiltrate; however, the stimuli for leukocyte recruitment are not fully understood. IL-8 is a potent chemokine produced by proximal tubular epithelial cells (PTECs). Whether nephrotic proteins stimulate tubular IL-8 expression remains unknown. Acute exposure of human PTECs to albumin induced IL-8 gene and protein expression time- and dose-dependently. Apical albumin predominantly stimulated basolateral IL-8 secretion. Electrophoretic mobility shift assay demonstrated nuclear translocation of NF-kappaB, and the p65/p50 subunits were activated. NF-kappaB activation and IL-8 secretion were attenuated by the NF-kappaB inhibitors pyrrolidine dithiocarbamate and cell-permeable peptide. Albumin upregulated intracellular reactive oxygen species (ROS) generation, while exogenous H2O2 stimulated NF-kappaB translocation and IL-8 secretion. Albumin-induced ROS generation, NF-kappaB activation, and IL-8 secretion were endocytosis- and PKC-dependent as these downstream events were abrogated by the PI3K inhibitors LY294002 and wortmannin, and the PKC inhibitors GF109203X and staurosporin, respectively. In vivo, IL-8 mRNA expression was localized by in situ hybridization to the proximal tubules in nephrotic kidney tissues. The intensity of IL-8 immunostaining was higher in nephrotic than non-nephrotic subjects. In conclusion, albumin is a strong stimulus for tubular IL-8 expression, which occurs via NF-kappaB-dependent pathways through PKC activation and ROS generation.

Renal damage associated with proteinuria

Experimental studies have demonstrated that proteins filtered by the glomerulus induce a proliferation of proximal tubular cells accompanied by an increased synthesis of many vasoactive and proinflammatory substances. The appearance of interstitial cellular infiltrates, a well-known finding in proteinuric diseases, precedes progressive tubulointerstitial fibrosis. Activation of the transcription factor kappaB (NF-kappaB) plays a pivotal role in the renal damage induced by proteinuria. In this scenario, any therapeutic intervention that reduces proteinuria should be beneficial for the kidney. Drugs that block the renin-angiotensin system [angiotensin converting enzyme (ACE) inhibitors and angiotensin II receptor antagonists (ARA)] have repeatedly shown striking antiproteinuric and renoprotective properties, both in experimental and clinical studies. Studies in patients with type 1 and type 2 diabetic nephropathy as well as in non-diabetic nephropathies have confirmed that the renoprotection obtained with ACEI/ARA is closely related with their antiproteinuric effect and is largely independent of blood pressure changes. However, resistance to the antiproteinuric effect of ACEI/ARA is a common clinical observation. Several therapeutic measures (that is, adequate blood pressure control, early introduction of ACEI/ARA, dietary protein restriction, low salt diets, weight loss in overweight patients, addition of a diuretic, increasing ACEI/ARA dose titrated against proteinuria levels, combined therapy ACEI plus ARA, addition of drugs with antiproteinuric effect such as non-dihydropiridine calcium channel blockers or NSAIDs) may increase the proteinuria reduction induced by ACEI and ARA.