Fibronectin production by human tubular cells: the effect of apical protein

Human proximal tubular epithelial cell-derived small extracellular vesicles mediate synchronized tubular ferroptosis in hypoxic kidney injury

Hypoxia is the key pathobiological trigger of tubular oxidative stress and cell death that drives the transition of acute kidney injury (AKI) to chronic kidney disease (CKD). The mitochondrial-rich proximal tubular epithelial cells (PTEC) are uniquely sensitive to hypoxia and thus, are pivotal in propagating the sustained tubular loss of AKI-to-CKD transition. Here, we examined the role of PTEC-derived small extracellular vesicles (sEV) in propagating the 'wave of tubular death'. Ex vivo patient-derived PTEC were cultured under normoxia (21 % O2) and hypoxia (1 % O2) on Transwell inserts for isolation and analysis of sEV secreted from apical versus basolateral PTEC surfaces. Increased numbers of sEV were secreted from the apical surface of hypoxic PTEC compared with normoxic PTEC. No differences in basolateral sEV numbers were observed between culture conditions. Biological pathway analysis of hypoxic-apical sEV cargo identified distinct miRNAs linked with cellular injury pathways. In functional assays, hypoxic-apical sEV selectively induced ferroptotic cell death (↓glutathione peroxidase-4, ↑lipid peroxidation) in autologous PTEC compared with normoxic-apical sEV. The addition of ferroptosis inhibitors, ferrostatin-1 and baicalein, attenuated PTEC ferroptosis. RNAse A pretreatment of hypoxic-apical sEV also abrogated PTEC ferroptosis, demonstrating a role for sEV RNA in ferroptotic 'wave of death' signalling. In line with these in vitro findings, in situ immunolabelling of diagnostic kidney biopsies from AKI patients with clinical progression to CKD (AKI-to-CKD transition) showed evidence of ferroptosis propagation (increased numbers of ACSL4+ PTEC), while urine-derived sEV (usEV) from these 'AKI-to-CKD transition' patients triggered PTEC ferroptosis (↑lipid peroxidation) in functional studies. Our data establish PTEC-derived apical sEV and their intravesicular RNA as mediators of tubular lipid peroxidation and ferroptosis in hypoxic kidney injury. This concept of how tubular pathology is propagated from the initiating insult into a 'wave of death' provides novel therapeutic check-points for targeting AKI-to-CKD transition.

Molecular and functional profiling of apical versus basolateral small extracellular vesicles derived from primary human proximal tubular epithelial cells under inflammatory conditions

Proximal tubular epithelial cells (PTEC) are central players in inflammatory kidney diseases. However, the complex signalling mechanism/s via which polarized PTEC mediate disease progression are poorly understood. Small extracellular vesicles (sEV), including exosomes, are recognized as fundamental components of cellular communication and signalling courtesy of their molecular cargo (lipids, microRNA, proteins). In this study, we examined the molecular content and function of sEV secreted from the apical versus basolateral surfaces of polarized human primary PTEC under inflammatory diseased conditions. PTEC were cultured under normal and inflammatory conditions on Transwell inserts to enable separate collection and isolation of apical/basolateral sEV. Significantly increased numbers of apical and basolateral sEV were secreted under inflammatory conditions compared with equivalent normal conditions. Multi‐omics analysis revealed distinct molecular profiles (lipids, microRNA, proteins) between inflammatory and normal conditions for both apical and basolateral sEV. Biological pathway analyses of significantly differentially expressed molecules associated apical inflammatory sEV with processes of cell survival and immunological disease, while basolateral inflammatory sEV were linked to pathways of immune cell trafficking and cell‐to‐cell signalling. In line with this mechanistic concept, functional assays demonstrated significantly increased production of chemokines (monocyte chemoattractant protein‐1, interleukin‐8) and immuno‐regulatory cytokine interleukin‐10 by peripheral blood mononuclear cells activated with basolateral sEV derived from inflammatory PTEC. We propose that the distinct molecular composition of sEV released from the apical versus basolateral membranes of human inflammatory PTEC may reflect specialized functional roles, with basolateral‐derived sEV pivotal in modulating tubulointerstitial inflammatory responses observed in many immune‐mediated kidney diseases. These findings provide a rationale to further evaluate these sEV‐mediated inflammatory pathways as targets for biomarker and therapeutic development.

Proteinuria as a determinant of renal expression of heme oxygenase-1: studies in models of glomerular and tubular proteinuria in the rat

Heme oxygenase-1 (HO-1), a cytoprotective gene, is commonly induced in renal tubules in the diseased kidney. Because proteinuria is a hallmark for kidney disease, we examined the relationship between proteinuria and tubular induction of HO-1, specifically questioning whether increased trafficking of protein across the renal tubular epithelium, as a consequence of proteinuria, induces tubular expression of HO-1. We examined a model of glomerular proteinuria induced by daily injections of BSA, which is associated with increased tubular uptake of filtered protein, and a model of tubular proteinuria induced by maleate, the latter exhibiting decreased tubular uptake and trafficking of protein. The BSA model of glomerular proteinuria failed to exhibit induction of HO-1; HO-1 was not induced in proximal tubular epithelial cells exposed to BSA. In contrast, in maleate nephropathy wherein tubular uptake of protein is decreased because of generalized proximal tubular injury induced by maleate, HO-1 was strongly induced in proximal tubules; inhibition of HO activity in maleate nephropathy worsened proteinuria, renal histological injury, and apoptosis. In renal proximal tubular epithelial cells, maleate induced HO-1 and caused apoptosis, the latter increased when HO activity was inhibited. From these studies, we conclude that expression of HO-1 in the diseased kidney cannot be ascribed to the tubular uptake and metabolism of protein such as albumin, and that the expression of HO-1 in a model of tubular proteinuria reflects a functionally significant stress response to toxin-induced proximal tubular injury.

Proteinuria and growth factors in the development of tubulointerstitial injury and scarring in kidney disease

PURPOSE OF REVIEW

There are ongoing debates as to the role and mechanisms of proteinuria in tubulointerstitial fibrogenesis. Moreover, recent experimental findings have allowed for further insights into mediators and interactions between cells in the renal interstitium during fibrogenesis.

RECENT FINDINGS

Proteinuria or albuminuria are likely just markers for the glomerular ultrafiltration and tubular actions of ultrafiltered, biologically active growth factors which 'activate' tubular cells causing basolateral secretion of chemokines and cytokines. Chemokines attract and activate macrophages. Tubular cell-derived platelet-derived growth factor (PDGF) and macrophage-derived transforming growth factor-beta cause fibroblast proliferation. Several growth factors contribute to their transition into extracellular matrix-producing myofibroblasts. This cascade of events provides targets for some currently available and several novel therapies.

SUMMARY

Albuminuria or glomerular proteinuria appear to be markers but ultrafiltered, bioactive growth factors are culprits in proteinuria-associated interstitial fibrosis. Interactions of tubular cells with macrophages and fibroblasts in the interstitium via defined growth factor/cytokines provide opportunities for therapeutic interventions.

Early proinflammatory activation of renal tubular cells by normal and pathologic IgG

BACKGROUND/AIMS

To verify whether human IgG induces proinflammatory activation of human proximal tubular epithelial cells (PTEC) independent of the metabolic overload of protein reabsorption.

METHODS

Cultured PTEC were incubated with normal IgG, IgG from systemic lupus erythematosus (SLE) patients, albumin or transferrin. IL-6 secretion and extracellular regulated kinase (ERK) activation (dual-phosphorylated ERK) were measured by ELISA and by Western blotting of PTEC extracts, respectively; renal biopsy specimens from patients with IgG and non-IgG proteinuria were analyzed by immunohistochemistry and in situ hybridization to detect ERK-P and IL-6.

RESULTS

Normal and SLE IgG, but not albumin or transferrin, induced an early significant increase in IL-6 secretion by PTECs. Also ERK activation was found after 1-hour incubation of PTEC with IgG, but not with control medium and albumin-treated PTEC. Activated ERK and IL-6 were found to colocalize in tubular cells in the kidney specimens of patients with IgG proteinuria only.

CONCLUSION

IgG-dependent early activation of ERK and increased IL-6 secretion in PTEC suggest that IgG filtered during nonselective proteinuria may play a specific role in tubulointerstitial disease. Such a role could be particularly relevant in diseases associated with abnormal IgG pool compositions, such as SLE. Preliminary results on human renal biopsy specimens suggest that our in vitro observations may also be relevant in vivo.

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.

High molecular weight plasma proteins induce apoptosis and Fas/FasL expression in human proximal tubular cells

BACKGROUND

In proteinuria, proximal tubular epithelial cells (PTECs) are exposed to abnormally high protein concentrations, eventually leading to tubular atrophy and end-stage renal disease. The mode of cell death leading to tubular atrophy in proteinuria has not been fully established. This study examines the role of protein overload on apoptosis, necrosis and cell proliferation in primary cultures of human PTECs using plasma protein fractions representative of selective and non-selective proteinuria. The involvement of the Fas/Fas ligand (FasL) system was also investigated.

METHODS

Plasma was collected from healthy volunteers and fractionated into albumin-rich (30-100 kDa), high molecular weight (100-440 kDa) and combined (30-440 kDa) fractions. PTECs were exposed to 10 mg/ml of the protein fractions for 24, 48 and 72 h. Apoptosis was measured using fluorescein isothiocyanate (FITC)-annexinV and TUNEL. Necrosis was measured using propidium iodide, metabolic activity by MTT and cell proliferation by bromodeoxyuridine incorporation. Fas and FasL expression was analysed by western blotting.

RESULTS

Exposure to the 100-440 and 30-440 kDa fractions produced significant increases in apoptosis at all time points, whereas PTECs exposed to the 30-100 kDa fraction were not significantly different from control cells. There were no changes in the rates of necrosis as a result of protein loading. A significant reduction in metabolic activity was observed in PTECs exposed to the 100-440 and 30-440 kDa fractions, but not to the 30-100 kDa fraction. Cell proliferation was significantly reduced by 24 h in cells exposed to the 100-440 and 30-440 kDa fractions. By 48 and 72 h, all the three fractions had inhibited cell proliferation. PTECs exposed to the 100-440 and the 30-440 kDa fractions showed a significant upregulation in the expression of Fas and FasL. Overall, the high molecular weight fraction was more 'toxic' than the albumin-rich or combined fraction.

CONCLUSION

Increased apoptosis and decreased cell proliferation are the major mechanisms of cell death in human PTECs in response to protein overload. These effects may be mediated at least in part by overexpression of the Fas/FasL system. The severity of such changes is largely determined by the high molecular weight fraction (100-440 kDa) rather than the albumin-rich fraction.

The role of bradykinin in the antifibrotic actions of perindoprilat on human mesangial cells

BACKGROUND

Angiotensin-converting enzyme inhibitors (ACE-I) protect against the development of glomerulosclerosis using mechanisms partly dissociated from their systemic antihypertensive action. The aim of the current study was to delineate the mechanism of action underlying the antifibrotic effects of the ACE-I perindoprilat in the context of macrophage-mediated scarring in human mesangial cells.

METHODS

Mesangial cells were treated with macrophage-conditioned medium (MPCM) in the presence or absence of the ACE-I perindoprilat.

RESULTS

Forty micromol/L perindoprilat reduced MPCM-induced mesangial cell fibronectin levels by 19.4 +/- 0.6% (P < 0.001). Immunoprecipitation of 35S-methionine biosynthetically labeled fibronectin and Northern analysis suggested that the decrease in fibronectin levels was not caused by reduced synthesis. MPCM stimulated the production of matrix metalloproteinases (MMP) 2, 3, and 9 in mesangial cells; however, these were not significantly altered by ACE-I treatment, and neither was production of their tissue inhibitor of metalloproteinases (TIMP-1). Addition of exogenous bradykinin to MPCM-treated mesangial cells resulted in a 22.5 +/- 1.4% (P < 0.02) reduction in secreted fibronectin levels, while semiquantitative reverse transcription-polymerase chain reaction (RT-PCR) and Southern blotting demonstrated that bradykinin B2 receptor expression was up regulated by 71 +/- 30% in MPCM-stimulated mesangial cells in response to ACE-I treatment (P= 0.032). Moreover, the bradykinin B2 receptor antagonist HOE 140 attenuated the beneficial effects of perindoprilat. MPCM-stimulated mesangial cell protein expression levels of plasminogen activator system components tissue plasminogen activator (tPA) and plasminogen activator inhibitor-1 (PAI-1) were altered after treatment with ACE-I.

CONCLUSION

These results suggest that ACE-I-induced renoprotection, in the context of macrophage-stimulated mesangial cell scarring, is mediated, at least in part, via the actions of bradykinin.

Monocyte adhesion to mesangial matrix modulates cytokine and metalloproteinase production

BACKGROUND

Monocytes migrate into the glomerular mesangium during acute inflammatory renal disease, differentiate into macrophages, and may play a key role in the development and progression of glomerular scarring. Treatment strategies that inhibit monocyte infiltration ameliorate glomerular injury in animal models. Mesangial matrix contains several potential monocyte-binding domains that may contribute to monocyte entrapment and modulate cell activation.

METHODS

Adhesion of peripheral blood-derived monocytes to matrix synthesized by human mesangial cells and to individual matrix proteins was assessed by colorimetry of nuclear staining with crystal violet. Monoclonal antibodies were used to identify the cell-surface integrins and matrix ligands involved. Monocyte proliferation was assessed by 3H-thymidine incorporation and cytokine production using enzyme-linked immunosorbent assay (ELISA). Secretion of metalloproteinases and their inhibitors was determined by zymography and ELISA, respectively.

RESULTS

Monocytes bound to matrix synthesized by mesangial cells. Prestimulation of mesangial cells with tumor necrosis factor-alpha (TNF-alpha) and transforming growth factor-beta (TGF-beta) enhanced matrix fibronectin content (P < 0.001) and monocyte binding (P < 0.001). Blocking antibodies to fibronectin, as well as to the integrins very late antigen-4 (VLA-4) and VLA-5, reduced monocyte adhesion to mesangial matrix by approximately 50%. Incubation of monocytes with matrix, fibronectin, laminin and collagen IV enhanced production of interleukin-1beta (IL-1beta), interleukin-6 (IL-6), TNF-alpha and metalloproteinase-9 (MMP-9) when compared to cells incubated in plastic wells. However, there was no apparent difference in proliferation rate and no change in production of metalloproteinase inhibitors.

CONCLUSION

Monocyte activation within the glomerulus may be mediated by binding to mesangial matrix components, particularly fibronectin. Matrix-mediated activation enhances production of inflammatory cytokines and matrix-degrading enzymes.

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.

Stimulation of proximal tubular cell apoptosis by albumin-bound fatty acids mediated by peroxisome proliferator activated receptor-gamma

In nephrotic syndrome, large quantities of albumin enter the kidney tubule. This albumin carries with it a heavy load of fatty acids to which the proximal tubule cells are exposed at high concentration. It is postulated that exposure to fatty acids in this way is injurious to proximal tubule cells. This study has examined the ability of fatty acids to interact with peroxisome proliferator-activated receptors (PPAR) in primary cultures of human proximal tubule cells. Luciferase reporter assays in transiently transfected human proximal tubule cells were used to show that albumin bound fatty acids and other agonists activate PPARgamma in a dose-dependent manner. One of the consequences of this activation is apoptosis of the cells as determined by changes in cell morphology, evidence of PARP cleavage, and appearance of DNA laddering. Overexpression of PPARgamma in these cells also results in enhanced apoptosis. Both fatty acid-induced PPAR activation and apoptosis in these cells can be blocked by PPAR response element decoy oligonucleotides. Activation of PPARgamma by the specific agonist PGJ(2) is associated with inhibition of cell proliferation, whereas activation by albumin bound fatty acids is accompanied by increased proliferation. However, the net balance of apoptosis/proliferation favors deletion of cells. These results implicate albumin-bound fatty acids as important mediators of tubular injury in nephrosis and provide fresh impetus for pursuit of lipid-lowering strategies in proteinuric renal disease.

Fibronectin expression in proximal tubules from ischemic rat kidneys without reperfusion

The expression of fibronectin (FN), one of the extracellular matrix proteins, was studied in isolated renal proximal tubules in a in vivo rat model of unilateral renal ischemia without reperfusion. FN is involved in cell-extracellular matrix interactions and defective cell-extracellular matrix interactions have been hypothesized to contribute to ischemic renal failure. The expression of FN was investigated by reverse transcription-polymerase chain reaction (RT-PCR), Elisa and Western blot. Isolated proximal tubules from control and post-ischemic rat kidneys were used. ATP, intracellular calcium content, and alkaline phosphatase were also measured to describe the effects associated to 40 min of ischemia. Control tubules expressed FN. Forty minutes of ischemia promoted diminished ATP levels and phosphatase alkaline activity, and increased intracellular calcium in isolated proximal tubules. An increased abundance of FN was observed by ischemic tubules as compared with control tubules. To determine quantitatively the value of FN content, ELISA method was performed. The ischemic tubules also expressed higher amount of FN mRNA. Three amplification products were obtained from both ischemic and control proximal tubular cDNA. The relative amounts of each of the obtained products were the same, strongly suggesting that the augmentation of the FN gene transcription during ischemia is not associated to a modification in the splicing pattern. Moreover, this expression is increased after 40 min of ischemia, not followed by reperfusion.

Effects of glucose dialysate on extracellular matrix production by human peritoneal mesothelial cells (HPMC): the role of TGF-beta

BACKGROUND

Dialysate glucose has been implicated in the loss of peritoneal membrane function seen in long-term CAPD patients.

METHODS

In order to investigate this in vitro, human peritoneal mesothelial cells (HPMC) were cultured in a 50:50 mix of dialysis solution and M199 for 12 h. The dialysate was laboratory manufactured and designed to be identical in composition to PD4 (LAB). The final glucose concentration ranged between 5 and 40 mmol/l. Experiments were conducted in the presence and absence of an anti-transforming growth factor-beta (TGF-beta) antibody. Cell viability was measured by lactate dehydrogenase (LDH) release. Fibronectin (FN) and TGF-beta protein were measured by ELISA, and FN gene expression was measured by Northern analysis. Separately, the effects of recombinant TGF-beta(1) added to M199: dialysate at 5 mmol/l glucose were investigated.

RESULTS

Forty millimoles per litre d-glucose LAB caused a decrease in cell viability, as evidenced by an increase in LDH release (6.0+/-1.3 vs 2.6+/-0.7%). This effect was dependent on osmolality. Forty millimoles per litre d-glucose LAB stimulated a 15.4+/-4.6% increase in FN, a 46.5+/-18.3% increase in TGF-beta protein (both P<0.05), and 1.4+/-0.09-fold increase in FN mRNA compared with 5 mmol/l d-glucose LAB. Exogenous TGF-beta 0-1 ng/ml induced a dose-dependent increase in FN protein (280+/-45% increase at TGF-beta 1 ng/ml, P<0.0001), and FN mRNA levels (10.0+/-1.8-fold at TGF-beta 1 ng/ml). The increase in FN in response to 40 mmol/l glucose was significantly reduced by anti-TGF-beta antibody to levels not different from control (93.8+/-6.6%, P<0.05 vs no Ab).

CONCLUSIONS

These data suggest that the pro-fibrotic effect of glucose dialysate on HPMC is mediated through stimulation of TGF-beta, which promotes FN gene expression and protein production.

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

BACKGROUND

The cause of tubulointerstitial pathology in glomerular disease is uncertain. Proteinuria may be a causative factor and has been shown to increase the turnover of tubular cells in a rat model of proteinuria. We investigated whether increased tubular cell proliferation occurs in human proteinuric renal disease. A human cell culture model was used to investigate the effects of proteins on tubular cell turnover further.

METHODS

Tubular proliferation in renal biopsies from patients with membranous nephropathy (MN) and minimal change nephropathy (MCN) was assessed by in situ hybridization for histone mRNA. Proliferation was measured in polarized human tubular cell cultures using tritiated thymidine, following addition of proteins to the apical medium. Toxicity was assessed by lactate dehydrogenase (LDH) release and monolayer permeability to inulin.

RESULTS

Increased tubular cell histone mRNA expression occurred in biopsies in MN (3.0-fold increase, P < 0.002) and MCN (3.6-fold increase, P < 0.02). Serum proteins added to the medium on human tubular cell cultures increased thymidine uptake (1.3-fold, P < 0.005), LDH release (1.5-fold, P < 0.001), and monolayer permeability (1.7-fold, P < 0.005). The effects were reproduced by a fraction of molecular weight 40 to 100 kD, but not by pure albumin or transferrin.

CONCLUSIONS

Proliferation of tubular cells is associated with proteinuria in vivo and is caused by proteins in cell culture. Toxicity of proteins to tubular cells and increased cell turnover may contribute to tubulointerstitial pathology in glomerular disease.

Transferrin but not albumin mediates stimulation of complement C3 biosynthesis in human proximal tubular epithelial cells

Complement is increasingly implicated in the pathogenesis of progressive renal disease resulting from persistent proteinuria. We have previously shown that apical serum proteins stimulate C3 in cultured human proximal tubular epithelial cells (PTECs), and that the stimulant is a nonalbumin compound of 30 to 100 kd. We postulated in this study that transferrin and apotransferrin, also important components of proteinuric urine in this molecular-weight range, might be the culprit. Human PTECs were obtained by differential sieving of renal cortical tissue from the normal pole of tumor nephrectomy specimens and characterized to be predominantly of proximal tubular origin. Complement C3 messenger RNA (mRNA) expression was analyzed in confluent growth-arrested PTEC monolayers in media containing different concentrations (2.5 to 20 mg/mL) of transferrin by reverse transcription and polymerase chain reaction. Pure human albumin was used as a control protein. C3 protein secretion was detected and quantified by a sandwich enzyme-linked immunosorbent assay on cell culture supernatants after distinct time points. Transferrin enhanced the rate of C3 secretion in a dose-dependent manner, reaching maximal stimulation at doses of 10 mg/mL. Selected experiments using the Transwell technique showed that C3 release was predominantly apical in the resting state. The addition of 10 mg/mL of transferrin apically but not basolaterally stimulated both apical and basolateral C3 secretion and increased the basolateral-apical ratio of C3 secretion from 0.45 +/- 0.16 to 0.93 +/- 0.24 (P: < 0.02). Constitutive C3 mRNA expression was upregulated by transferrin in a time- and dose-dependent fashion, reaching a peak after 24 hours. A similar degree of C3 upregulation was reproduced when iron-poor transferrin, apotransferrin, was used instead. These results indicate that C3 synthesis in PTECs is upregulated by transferrin, for which protein rather than iron moiety may account for the observed effects. These findings provide evidence linking proteinuria with overexpression of tubular complement.

Progressive renal disease: does the quality of the proteinuria matter or only the quantity?

Proteinuria is now accepted to be not just a sign of renal disease but also a contributory factor to the development of progressive tubulointerstitial fibrosis. Excellent correlations between the degree of proteinuria and rate of decline of glomerular filtration rate have been demonstrated. What has been investigated less is whether the type of protein found in the urine is important. Using transformed and primary human proximal tubular epithelial cells, we have investigated the binding of albumin and retinol binding protein to plasma membrane preparations and studied the response of the intact cells to increasing concentrations of these same proteins. We have preliminary evidence for differences in the pattern of binding of these two proteins to the plasma membrane receptors and also for differential release of pro-inflammatory cytokines from intact cells. These in vitro results, along with those of other groups, and some recent clinical findings suggest that the quality of proteinuria may play a role in the early development of interstitial fibrosis. Furthermore, the use of such in vitro model systems based on human proximal epithelial cell culture can provide a means of evaluating the potential significance of different markers of tubular damage.

Predominant role for C5b-9 in renal ischemia/reperfusion injury

Previous work has indicated that complement is a mediator of ischemia/reperfusion (I/R) injury. To investigate the components of complement responsible for this effect, we examined a model of renal I/R injury in C3-, C4-, C5-, and C6-deficient mice. We occluded the renal arteries and veins (40-58 minutes) and, after reperfusion (0-72 hours), assessed renal structural and functional injury. C3-, C5-, and C6-deficient mice were protected from renal I/R injury, whereas C4-deficient mice were not protected. C6-deficient mice treated with antibody to block C5a generation showed no additional protection from I/R injury. Reconstitution with C6 alone restored the I/R injury in C6-deficient mice. Tubular epithelial cells were the main structures damaged by complement-mediated attack, and, in contrast, the renal vessels were spared. Neutrophil infiltration and myeloperoxidase activity were reduced in C-deficient mouse kidney, but by a similar extent in C3-deficient and C6-deficient mice. We conclude that the membrane attack complex of complement (in which C5 and C6 participate) may account for the effect of complement on mouse renal I/R injury. Neither C5a-mediated neutrophil infiltration nor the classic pathway, in which C4 participates, appears to contribute to I/R injury in this model. By contrast with other organs, such as the heart, the primary effect of complement in the ischemic area is on the parenchymal cell rather than the vascular endothelial cell. The membrane attack complex of complement is a potential target for prevention of I/R injury in this model.

Secretion of chemokines and cytokines by human tubular epithelial cells in response to proteins

BACKGROUND

Chronic interstitial scarring contributes to the progression of renal failure in glomerular disease but its cause is unknown. The development of proteinuria could stimulate tubular cells to release cytokines, chemoattractants and matrix proteins into the interstitium, thus contributing to interstitial disease.

METHODS

Polarized human tubular epithelial cells were grown on permeable supports and exposed to serum proteins on their apical surface. The release of tumour necrosis factor alpha(TNFalpha), platelet derived growth factor (PDGF) and monocyte chemoattractant protein-1 (MCP-1) by the cells was measured using immunoassays.

RESULTS

Under control conditions there was polarized release of PDGF-AB with predominant basolateral secretion (basolateral to apical ratio 4.7+/-1.6). MCP-1 release was less polarized (ratio 1. 7+/-0.5). TNFalpha was not detected. Exposure of the cells to normal human serum proteins on their apical side increased basolateral release of PDGF-AB (1.7+/-0.4 fold) and MCP-1 (2.4+/-0.2 fold). Fractionation of the serum showed that this effect on human tubular epithelial cells was reproduced by a fraction of molecular weight 40-100 kDa. The predominant proteins in this fraction were albumin and transferrin but these purified proteins alone did not alter secretion of PDGF-AB or MCP-1.

CONCLUSION

This data demonstrates that human tubular cells exposed to proteins, which would be filtered in glomerular disease, produce inflammatory mediators with the potential to stimulate inflammation and scarring in the interstitium of the kidney.

Induction of monocyte chemoattractant protein-1 by albumin is mediated by nuclear factor kappaB in proximal tubule cells

The transcription and translation of monocyte chemoattractant protein-1 (MCP-1), a CC chemokine, are increased in proximal tubule epithelial cells (PTC) stimulated with pathophysiologically relevant concentrations of albumin. The purpose of this study was to investigate whether nuclear factor kappaB (NFkappaB)/Rel proteins play a role in albumin-induced MCP-1 transcription. Confluent monolayers of rat PTC in primary culture were stimulated with delipidated bovine serum albumin. NFkappaB, the NFkappaB inhibitory protein (IkappaB), and MCP-1 transcription were assessed using electrophoretic mobility shift assays, Western immunoblotting, semiquantitative reverse transcription-PCR, and ribonuclease protection assays. Activation of NFkappaB by delipidated bovine serum albumin (15 mg/ml) was detectable within 2 h, maximal after 8 h, and maintained for at least 16 h of continuous exposure. Supershift analysis showed that the activated proteins were composed of p50/p50, p50/p65, and p50/c-Rel dimers. dimers. Cytoplasmic IkappaBalpha levels were decreased 30 min after stimulation and returned to unstimulated levels by 4 to 8 h. IkappaBbeta levels were decreased at 2 h and there was no recovery until 8 h. Inhibition of NFkappaB with pharmacologic agents (N-tosyl-phenylalanine chloromethyl ketone and dexamethasone) and an antisense oligonucleotide to the rat p65 subunit of NFkappaB significantly reduced MCP-1 transcription. The 3.6-kb 5' flanking region of the rat MCP-1 gene was cloned and sequenced, and two putative kappaB binding sites were identified within the enhancer region. Therefore, albumin increased NFkappaB and reduced IkappaB levels in PTC, and MCP-1 expression was dependent on NFkappaB activation. It is concluded that the activation of NFkappaB/Rel proteins modulates chemokine production in PTC in response to albumin and is likely to have an important role in the mediation of tubulointerstitial injury in proteinuric renal disease.

Renal proximal tubular cell fibronectin accumulation in response to glucose is polyol pathway dependent

BACKGROUND

Thickening and reduplication of the tubular basement membrane have been reported as early events in diabetic nephropathy. In this study, we have examined the polar requirements of proximal tubular cells for the d-glucose-stimulated accumulation of fibronectin and the mechanism by which this occurred, with particular emphasis on the polyol pathway.

METHODS

To determine the polarity of fibronectin generation in response to glucose, LLC-PK1 cells were grown on porous tissue culture inserts. Monolayer confluence was determined by serial measurement of transepithelial resistance. Confluent cells were growth arrested by serum deprivation, and all experiments were performed under serum-free conditions.

RESULTS

Application of 25 mm d-glucose to either the apical or basolateral aspect of LLC-PK1 cells led to fibronectin accumulation in the basolateral compartment. This reached statistical significance 24 hours following apical addition of glucose (2.6-fold increase compared with 5 mm d-glucose, P = 0.0025, N = 6 vs. N = 4 controls) and 12 hours after the basolateral addition of glucose (2.5-fold increase compared with 5 mm d-glucose, P = 0.03, N = 6 vs. N = 4 controls). Exposure of cells to glucose at either their apical or basolateral aspect leads to accumulation of intracellular glucose and polyol pathway activation, as assessed by sorbitol accumulation. The increase in fibronectin concentration in response to glucose was inhibited by the aldose reductase inhibitor sorbinil. At a dose of 100 micron sorbinil, there was a 59% inhibition of fibronectin accumulation in response to apical glucose (P = 0.004, N = 3 sorbinil vs. N = 4 controls) and a 66% inhibition in response to basolateral glucose (P = 0.008, N = 3 sorbinil vs. N = 4 controls) 48 hours after the addition of the inhibitor. Furthermore, fibronectin accumulation was also demonstrated following both the apical and basolateral addition of 1 mm sorbitol, but not following the addition of 25 mm galactose to either aspect of the cells. Following the addition of sorbitol, there was a 2. 8-fold increase in fibronectin 48 hours after apical stimulation (P = 0.01, N = 3 treated vs. N = 4 control) and a 2.27-fold increase following basolateral stimulation (P = 0.04, N = 3 treated vs. N = 4 control) at 24 hours.

CONCLUSIONS

In summary, these data demonstrate that fibronectin generation in response to glucose was nonpolar in terms of application of glucose but was polar in terms of fibronectin accumulation. The mechanisms of glucose-induced modulation of fibronectin were mediated by polyol pathway activation and were more specifically related to the metabolism of sorbitol to fructose.

Fibronectin augments monocyte adhesion to low-density lipoprotein-stimulated mesangial cells

BACKGROUND

Glomerular monocyte infiltration is an early feature of lipid-mediated renal injury in animal models. Interactions between mesangial and infiltrating mononuclear cells may contribute to the development of glomerular scarring.

METHODS

Adherence of U-937 monocytes to low-density lipoprotein (LDL)- or tumor necrosis factor alpha (TNFalpha)-prestimulated human mesangial cells was assessed by colorimetry of nuclear staining with crystal violet. Blocking antibodies were added to examine the mechanisms of binding. Adhesion molecule expression and fibronectin synthesis were measured by ELISA.

RESULTS

Preincubation of mesangial cells for 24 hours with LDL (100 micrograms/ml) or mildly oxidized (minimally modified) LDL (MM-LDL) increased monocyte adhesion by 207% and 240%, respectively, compared with control nonstimulated cells (100%). TNFalpha (100 U/ml) enhanced binding by 335% and up-regulated intercellular adhesion molecule-1 (ICAM-1) and vascular cell adhesion molecule-1 (VCAM-1) expression by 505% and 179%, respectively, as compared with MM-LDL (120% and 116%) and LDL, which had no effect. Blocking antibodies to these adhesion molecules inhibited monocyte binding to TNFalpha- and, to a lesser extent, MM-LDL-primed mesangial cells, but had no effect after LDL pretreatment. In contrast to TNFalpha, MM-LDL and LDL increased mesangial cell-associated fibronectin, whereas antibodies to fibronectin inhibited monocyte binding to lipoprotein-stimulated but not TNFalpha-stimulated cells.

CONCLUSIONS

Although enhanced monocyte adhesion to TNFalpha- and, to a lesser extent, MM-LDL-stimulated mesangial cells is mediated by changes in ICAM-1 and VCAM-1 expression, both LDL and MM-LDL promote similar cellular interactions as a result of increased fibronectin production.

Apical proteins stimulate complement synthesis by cultured human proximal tubular epithelial cells

There is increasing evidence to suggest that the renal tubular epithelium is important in the pathogenesis of progressive renal failure resulting from persistent proteinuria. The role of complement in the progression of chronic renal failure is not well defined. The purpose of this study was to characterize the production of complement by human proximal tubular epithelial cells exposed to serum proteins at the apical surface. Complement C3 gene expression was analyzed by reverse transcription and PCR. C3 protein biosynthesis was confirmed by metabolic labeling followed by immunoprecipitation and quantified by enzyme-linked immunosorbent assay. In the quiescent state, proximal tubular epithelial cells grown on permeable membrane supports secreted C3 predominantly into the apical medium. The addition of 5 mg/ml serum proteins led to an 8.9-fold increase in basolateral C3 secretion and a 2.1-fold increase in apical C3 secretion, altering the ratio of basolateral: apical C3 secretion from 0.44 +/- 0.16 to 1.87 +/- 0.52. C3 mRNA expression was also upregulated in a time- and dose-dependent manner. Serum fractionation demonstrated that the stimulant responsible for these effects was in the molecular weight range 30 to 100 kD. The observed phenomenon was not reproduced when purified human albumin alone was used as the stimulant. These findings could provide a possible mechanism for the link between proteinuria and interstitial fibrosis. This may have potential implications for strategies directed against complement in retarding the progression of chronic renal failure.

Role of tubule epithelial cells in the pathogenesis of tubulointerstitial fibrosis induced by glomerular disease

Tubulointerstitial fibrosis is a final common pathway for progressive renal injury in most 'inflammatory' and 'non-inflammatory' glomerulopathies. Indeed, the level of tubulointerstitial fibrosis correlates closely with the degree of chronic renal dysfunction in these settings. An emerging body of evidence suggests that tubule epithelial cells are dynamic players in the pathogenesis of tubulointerstitial fibrosis. Here we briefly review the potential mechanisms of tubule cell activation in patients with glomerular disease. These mechanisms include: (a) direct involvement of glomerular and tubulointerstitial compartments by the primary disease; (b) secondary activation of tubule epithelial cells by glomerulus-derived cytokines; (c) perturbation of tubule epithelial cell function by plasma proteins and associated moieties filtered in excess through injured glomeruli; (d) tubulointerstitial ischaemia downstream to glomerular injury; and (e) hyperfunction of remnant tubules. Activated tubule epithelial cells are, in turn, a rich source of cytokines, chemokines and other mediators that promote leukocyte recruitment, cytotoxicity and fibrogenesis, thereby establishing a 'vicious cycle' of tubulointerstitial injury. The further delineation of the role played by the tubule epithelial cell in the pathogenesis of tubulointerstitial fibrosis may suggest novel approaches for the treatment of progressive renal diseases.

Fibronectin synthesis by human tubular epithelial cells in culture: effects of PDGF and TGF-beta on synthesis and splicing

BACKGROUND

Enhanced synthesis of extracellular matrix proteins including fibronectin (FN) is associated with the development of sclerosis. In this context we studied FN synthesis by tubular epithelial cells in response to transforming growth factor-beta (TGF-beta) and platelet-derived growth factor (PDGF).

METHODS

FN protein synthesis by human tubular epithelial cells in culture (TEC) was measured by biosynthetic labeling and ELISA. Splicing of FN was assessed by RT-PCR and by Northern blotting.

RESULTS

Cultivated TEC synthesized and released FN, the majority of which was deposited as an unsoluble protein and a minor portion (10 to 15%) was released into the supernatant. TGF-beta and, to a lesser degree, PDGF, up-regulated FN synthesis. All three FN splice variants (EDA, EDB, and IIICS) were produced. PDGF did not influence the splicing. TGF-beta preferentially up-regulated the EDA splice variant, but had no effect on the splicing of the other domains.

CONCLUSIONS

PDGF and TGF-beta both up-regulate FN synthesis of TEC. TGF-beta, but not PDGF, also changed the quality of the de novo synthesized FN, and thus has a different role in the development of sclerosis.

Potential autocrine and paracrine mechanisms of recovery from mechanical injury of renal tubular epithelial cells

The present studies were done to clarify potential pathways of the nephrogenic repair process. Media removed from mechanically injured vascular smooth muscle cells and LLC-PK1 renal tubular epithelial cells significantly stimulated [3H]thymidine uptake and cell number in quiescent LLC-PK1 cells, demonstrating the existence of potential autocrine and paracrine pathways of nephrogenic repair. The effect of mechanical injury resulting in release of one or more growth factors into culture media was also found in the opossum kidney OK renal tubular cell line. The nonspecific peptide growth factor antagonist suramin inhibited the effect of media from injured LLC-PK1 cells to stimulate [3H]thymidine uptake in quiescent LLC-PK1 cells. Exposure of quiescent LLC-PK1 cells to six growth factors, including acidic and basic fibroblastic growth factors (aFGF and bFGF), platelet-derived growth factors AA and BB (PDGF-AA and PDGF-BB), endothelin-2, and hepatocyte growth factor, reproduced the biological responses seen when quiescent LLC-PK1 cells were exposed to media from injured cells. Immunoblotting and enzyme-linked immunosorbent assay experiments demonstrated the presence of aFGF, bFGF, and PDGF-BB but not other candidate growth factors in the media from injured LLC-PK1 cells. A neutralizing antibody directed against bFGF attenuated the effect of media from injured cells to stimulate [3H]thymidine uptake in serum-starved LLC-PK1 cells. These results demonstrate that mechanical injury to renal tubular epithelial cells results in release of aFGF, bFGF, and PDGF-BB into the media and suggests that bFGF may be involved in an autocrine fashion to promote recovery from injury.

Hypoxia induces intercellular adhesion molecule-1 on cultured human tubular cells

The adverse effects of acute renal ischemia are partly mediated through an infiltration of inflammatory cells into the tubulointerstitium. The expression of adhesion molecules such as intercellular adhesion molecule-1 (ICAM-1) by resident renal cells (endothelial cells and tubular cells) may facilitate this process. We investigated whether hypoxia stimulates the expression of ICAM-1 by cultured human proximal tubular cells (HPTC). Hypoxic culture conditions (PO2 < 4 kPa) stimulated the expression of ICAM-1 by HPTC in a time-dependent manner (P < 0.0001) as demonstrated by quantitative flow cytometry analysis. Quantitative PCR demonstrated an increase in ICAM-1 transcription. Re-oxygenation of tubular cells did not increase ICAM-1 expression further. TNF alpha concentration in culture supernatants increased with hypoxia, but blocking experiments demonstrated that TNF alpha was not implicated in hypoxia-induced expression of ICAM-1. Furthermore, the cytokines IL-6 and IL-1 beta were not involved, but the effect of hypoxia was blocked by PDTC, an antioxidant that may inhibit the activation of the transcription factor NF-kappa B. These data demonstrate that hypoxia is a stimulus that induces the synthesis and expression of the adhesion molecule ICAM-1, presumably via the activation of NF-kappa B.