Regulation of proximal tubular osteopontin in experimental hydronephrosis in the rat

Critical Role of Osteopontin in Maintaining Urinary Phosphate Solubility in CKD

Background

Nephron loss dramatically increases tubular phosphate to concentrations that exceed supersaturation. Osteopontin (OPN) is a matricellular protein that enhances mineral solubility in solution; however, the role of OPN in maintaining urinary phosphate solubility in CKD remains undefined.

Methods

Here, we examined (1) the expression patterns and timing of kidney/urine OPN changes in CKD mice, (2) if tubular injury is necessary for kidney OPN expression in CKD, (3) how OPN deletion alters kidney mineral deposition in CKD mice, (4) how neutralization of the mineral-binding (ASARM) motif of OPN alters kidney mineral deposition in phosphaturic mice, and (5) the in vitro effect of phosphate-based nanocrystals on tubular epithelial cell OPN expression.

Results

Tubular OPN expression was dramatically increased in all studied CKD murine models. Kidney OPN gene expression and urinary OPN/Cr ratios increased before changes in traditional biochemical markers of kidney function. Moreover, a reduction of nephron numbers alone (by unilateral nephrectomy) was sufficient to induce OPN expression in residual nephrons and induction of CKD in OPN-null mice fed excess phosphate resulted in severe nephrocalcinosis. Neutralization of the ASARM motif of OPN in phosphaturic mice resulted in severe nephrocalcinosis that mimicked OPN-null CKD mice. Lastly, in vitro experiments revealed calcium-phosphate nanocrystals to induce OPN expression by tubular epithelial cells directly.

Conclusions

Kidney OPN expression increases in early CKD and serves a critical role in maintaining tubular mineral solubility when tubular phosphate concentrations are exceedingly high, as in late-stage CKD. Calcium-phosphate nanocrystals may be a proximal stimulus for tubular OPN production.

The renin-angiotensin system in lupus: physiology, genes and practice, in animals and humans

Although multiple studies suggest a potential role for angiotensin II in inflammation, most were performed either in vitro or in animals with non-immune-complex-mediated diseases. Extrapolation of these findings to humans, particularly patients with lupus, which involves multiple immunoregulatory pathways, is unclear. In autoimmune-prone MRL/lpr mice, angiotensin-converting-enzyme (ACE) inhibition improved survival although to a lesser degree than cyclophosphamide and diminished the glomerular histopathologic damage, proteinuria, lymphoid hyperplasia, dermatitis, and hypergammaglobulinemia, with a reduction in TGF-beta1 and beta 2 expression in the kidneys and renal chemokine mRNA expression. Spleen levels of IL-4 and IL-10 were also reduced. Uncontrolled studies in patients with treatment-refractory lupus nephritis showed a significant reduction in proteinuria with ACE-inhibitors and Angiotensin receptor blockers treatment. The ‘masking’ effect of ACE-inhibitors should be taken into consideration, as an exacerbation of lupus nephritis may be missed when estimated by the magnitude of proteinuria, which is decreased by these treatments. No single ACE genotype was consistently associated with subsets of SLE patients. In retrospective analyses, ACE-inhibitor use predicted a favourable outcome in 94 cases of pauci-immune vasculitis. The attenuating effect of angiotensin II inhibitors on the progression of chronic renal disease is well recognized. The data on the role of this intervention in lupus is limited.

Cyclic stretch-induced TGF-β1 and fibronectin expression is mediated by β1-integrin through c-Src- and STAT3-dependent pathways in renal epithelial cells

Extracellular matrix (ECM) proteins, including fibronectin, may contribute to the early development and progression of renal interstitial fibrosis associated with chronic renal disease. Recent studies showed that β1-integrin is associated with the development of renal fibrosis in a murine model of unilateral ureteral obstruction (UUO). However, the molecular events responsible for β1-integrin-mediated signaling, following UUO, have yet to be determined. In this study, we investigated the mechanism by which mechanical stretch, an in vitro model for chronic obstructive nephropathy, regulates fibronectin and transforming growth factor-β1 (TGF-β1) expression in cultured human proximal tubular epithelium (HK-2) cells. Mechanical stretch upregulated fibronectin and TGF-β1 expression and activated signal transducer and transcription factor 3 (STAT3) in a time-dependent manner. Stretch-induced fibronectin and TGF-β1 were suppressed by a STAT3 inhibitor, S3I-201, and by small interfering RNA (siRNA) targeting human STAT3 (STAT3 siRNA). Similarly, fibronectin and TGF-β1 expression and STAT3 activation induced by mechanical stretch were suppressed by the Src family kinase inhibitor PP2 and by transfection of HK-2 cells with a dominant-negative mutant of c-Src (DN-Src), whereas PP3, an inactive analog of PP2, had no significant effect. Furthermore, mechanical stretch resulted in increased β1-integrin mRNA and protein levels in HK-2 cells. Furthermore, neutralizing antibody against β1-integrin and silencing of β1-integrin expression with siRNAs resulted in decreased c-Src and STAT3 activation and TGF-β1 and fibronectin expression evoked by mechanical stretch. This work demonstrates, for the first time, a role for β1-integrin in stretch-induced renal fibrosis through the activation of c-Src and STAT3 signaling pathways.

Urinary NGAL, cystatin C, β2-microglobulin, and osteopontin significance in hydronephrotic children

BACKGROUND

Ureteropelvic junction obstruction (UPJO) accounts for 35 % of all congenital hydronephrosis cases. The challenge in managing childhood hydronephrosis is to distinguish obstructive from nonobstructive cases and, thereby, identify patients requiring surgical intervention. This study aimed to examine four urinary proteins as potential biomarkers of obstruction in hydronephrosis.

METHODS

Urine samples from 24 children with UPJO were collected pre-, peri-, and postoperatively, together with urine samples from healthy children. Neutrophil gelatinase-associated lipocalin (NGAL), cystatin C (CyC), β2-microglobulin (β2-M), and osteopontin (OPN) in the samples were measured simultaneously using multiplex sandwich immunoassay technology.

RESULTS

Compared with controls, NGAL and β2-M were significantly increased in urine from patients with obstructed kidneys at the time of surgery. This increase was followed by a decrease and stabilization to the same level as that of the controls. Furthermore, age was negatively correlated with preoperative urinary concentrations of CyC, β2-M, and OPN.

CONCLUSIONS

This study confirms increased concentrations of NGAL and β2-M in urine from obstructed kidneys and should be tested in larger studies to ascertain their ability to identify obstruction and to determine the importance of age-adjusted reference values.

Congenital ureteropelvic junction obstruction: human disease and animal models

Ureteropelvic junction (UPJ) obstruction is the most frequently observed cause of obstructive nephropathy in children. Neonatal and foetal animal models have been developed that mimic closely what is observed in human disease. The purpose of this review is to discuss how obstructive nephropathy alters kidney histology and function and describe the molecular mechanisms involved in the progression of the lesions, including inflammation, proliferation/apoptosis, renin-angiotensin system activation and fibrosis, based on both human and animal data. Also we propose that during obstructive nephropathy, hydrodynamic modifications are early inducers of the tubular lesions, which are potentially at the origin of the pathology. Finally, an important observation in animal models is that relief of obstruction during kidney development has important effects on renal function later in adult life. A major short-coming is the absence of data on the impact of UPJ obstruction on long-term adult renal function to elucidate whether these animal data are also valid in humans.

Role of inflammation in túbulo-interstitial damage associated to obstructive nephropathy

Obstructive nephropathy is characterized by an inflammatory state in the kidney, that is promoted by cytokines and growth factors produced by damaged tubular cells, infiltrated macrophages and accumulated myofibroblasts. This inflammatory state contributes to tubular atrophy and interstitial fibrosis characteristic of obstructive nephropathy. Accumulation of leukocytes, especially macrophages and T lymphocytes, in the renal interstitium is strongly associated to the progression of renal injury. Proinflammatory cytokines, NF-κB activation, adhesion molecules, chemokines, growth factors, NO and oxidative stress contribute in different ways to progressive renal damage induced by obstructive nephropathy, as they induce leukocytes recruitment, tubular cell apoptosis and interstitial fibrosis. Increased angiotensin II production, increased oxidative stress and high levels of proinflammatory cytokines contribute to NF-κB activation which in turn induce the expression of adhesion molecules and chemokines responsible for leukocyte recruitment and iNOS and cytokines overexpression, which aggravates the inflammatory response in the damaged kidney. In this manuscript we revise the different events and regulatory mechanisms involved in inflammation associated to obstructive nephropathy.

Alterations in renal cilium length during transient complete ureteral obstruction in the mouse

The renal cilium is a non-motile sensory organelle that has been implicated in the control of epithelial phenotype in the kidney. The contribution of renal cilium defects to cystic kidney disease has been the subject of intense study. However, very little is known of the behaviour of this organelle during renal injury and repair. Here we investigate the distribution and dimensions of renal cilia in a mouse model of unilateral ureteral obstruction and reversal of ureteral obstruction. An approximate doubling in the length of renal cilia was observed throughout the nephron and collecting duct of the kidney after 10 days of unilateral ureteral obstruction. A normalization of cilium length was observed during the resolution of renal injury that occurs following the release of ureteral obstruction. Thus variations in the length of the renal cilium appear to be a previously unappreciated indicator of the status of renal injury and repair. Furthermore, increased cilium length following renal injury has implications for the specification of epithelial phenotype during repair of the renal tubule and duct.

Angiotensin II receptor blocker and statins lower elevated levels of osteopontin in essential hypertension--results from the EUTOPIA trial

BACKGROUND

Osteopontin is a pleiotropic cytokine that has been implicated as a key factor in the development of atherosclerosis, a major complication of hypertension. We have earlier shown that olmesartan reduces mediators of vascular inflammation in patients with hypertension and cardiovascular disease. We aimed at studying the effect of olmesartan and/or pravastatin on osteopontin plasma levels, and the association between vascular inflammation markers and osteopontin in hypertensive patients.

METHODS

We assessed a panel of vascular inflammation markers and osteopontin during 12 weeks of therapy with 20mg olmesartan (n=94) or placebo (n=96) in a prospective, double-blind, multi-center study in patients with essential hypertension (re-evaluation of the EUTOPIA trial blood samples). Pravastatin (20mg) was added to the double-blind therapy at week 6 in both arms. The association of demographic variables and inflammation markers with osteopontin has been analyzed as well.

RESULTS

Baseline osteopontin plasma concentrations in the study population were elevated compared to healthy controls (32.85+/-19.04ng/mL vs. 23.82+/-3.69ng/mL, p=0.027). Mono-therapy with olmesartan and co-therapy with pravastatin reduced levels of circulating osteopontin (p<0.001). The addition of pravastatin to the placebo treatment-arm resulted in a reduction of osteopontin levels as well (p<0.01). osteopontin plasma levels correlated with VCAM-1 (r=0.27; p=0.0002), ICAM-1 (r=0.18; p=0.015), IL-6 (r=0.35; p<0.0001) and hsCRP (r=0.22; p=0.0022).

CONCLUSION

We show, for the first time, that olmesartan significantly decreases osteopontin concentrations. Co-therapy with pravastatin also reduces osteopontin levels. Elevated osteopontin levels in hypertensive patients correlate with adhesion molecules and inflammation markers.

Increased E-cadherin expression in the ligated kidney following unilateral ureteric obstruction

E-cadherin expression in the kidney is used as a surrogate marker of epithelial mesenchymal transition for the testing of various antifibrotic strategies. Here we reexamined E-cadherin expression in the kidneys of rats with unilateral ureteric obstruction, which was previously reported to decrease in parallel with the development of tubulointerstitial disease in this widely used experimental model of renal fibrosis and epithelial mesenchymal transition. E-cadherin mRNA expression was consistently increased both acutely (hours) and chronically (days) in the ligated kidney compared to the cognate non-ligated kidney. Increased E-cadherin protein levels were also found in the ligated kidney particularly in dilated tubular segments. Simulation of early pressure changes in the ligated kidney by mechanical stretch of human renal epithelial cells in culture did not alter E-cadherin expression. Porcine LLCPK-1 cells subjected to hypotonic stretch, however, did have increased E-cadherin mRNA and protein levels, responses that were not prevented by transforming growth factor-beta, a cytokine that promotes epithelial mesenchymal transition. Our findings question the utility of E-cadherin as a marker of epithelial mesenchymal transition in this model of renal fibrosis.

Intermittent Pressure-Loading Increases Transforming Growth Factor-Beta-1 Secretion from Renal Tubular Epithelial Cells: In vitro Vesicoureteral Reflux Model

BACKGROUND

To investigate the effect of hydrodynamic pressure mimicking vesicoureteral reflux on renal tubular epithelial cells in vitro, we constructed an intermittent pressure-loading (IPL) model of Madin-Darby canine kidney (MDCK) cells.

MATERIALS AND METHODS

Three grades of pressure were loaded onto the MDCK cells intermittently. The concentration of cytokines in the supernatant, the amount of the protein and its mRNA in the MDCK cells were studied, respectively.

RESULTS

After 24 h, the concentration of transforming growth factor-beta1 (TGF-beta1) increased under intense IPL conditions (100 and 200 cm H2O) in the 15-min IPL group (p<0.05, p<0.01). The amount of cellular level of TGF-beta1 protein and its mRNA did not show any significant increase within 24 h under the present conditions. The concentration of monocyte chemoattractant peptide-1 (MCP-1) was not significantly different from that of the control.

CONCLUSION

These data suggest that the early TGF-beta1 secretion phenomenon without change in gene expression is the case in the renal tubular epithelial cells under certain intermittent pressure-loading conditions.

Exploring mechanisms involved in renal tubular sensing of mechanical stretch following ureteric obstruction

Tubular mechanical stretch is the key primary insult in obstructive nephropathy. This review addresses how the renal tubular epithelium senses and responds to mechanical stretch. Using data from renal and nonrenal systems, we describe how sensing of stretch initially occurs via the activation of ion channels and subsequent increases in intracellular calcium levels. Calcium influxes activate a number of adaptive and proinjury responses. Key among these are 1) the activation of Rho, consequent cytoskeletal rearrangements, and downstream increases in focal adhesion assembly; and 2) phospholipase activation and resultant mitogen-activated protein kinase activation. These early signaling events culminate in adaptive cellular coupling to the extracellular matrix, a process termed the cell strengthening response. Direct links can be made between increased expression of genes involved in the development of obstructive nephropathy and initial sensing of mechanical stretch. The review illustrates the repercussions of mechanical stretch as a renal stress stimulus, specific to ureteric obstruction, and provides an insight into how tubular responses to mechanical stretch are ultimately implicated in the development of obstructive nephropathy.

Osteopontin regulates renal apoptosis and interstitial fibrosis in neonatal chronic unilateral ureteral obstruction

Congenital obstructive nephropathy is a major cause of renal insufficiency in children. Osteopontin (OPN) is a phosphoprotein produced by the kidney that mediates cell adhesion and migration. We investigated the role of OPN in the renal response to unilateral ureteral obstruction (UUO) in neonatal mice. OPN null mutant (-/-) and wild-type (+/+) mice were subjected to sham operation or UUO within the first 2 days of life. At 7 and 21 days of age, fibroblasts (fibroblast-specific protein (FSP)-1), myofibroblasts (alpha-smooth muscle actin (SMA)), and macrophages (F4/80) were identified by immunohistochemical staining. Apoptotic cells were detected by terminal deoxy transferase uridine triphosphate nick end-labeling technique and interstitial collagen by Masson trichrome or picrosirius red stain. Compared to sham-operated or contralateral kidneys, obstructed kidneys showed increases in all parameters by 7 days, with further increases by 21 days. After 21 days UUO, there was an increase in tubular and interstitial apoptosis in OPN -/- mice as compared to +/+ animals (P<0.05). However, FSP-1- and alpha-SMA-positive cells and collagen in the obstructed kidney were decreased in OPN -/- compared to +/+ mice (P<0.05), whereas the interstitial macrophage population did not differ between groups. We conclude that OPN plays a significant role in the recruitment and activation of interstitial fibroblasts to myofibroblasts in the progression of interstitial fibrosis in the developing hydronephrotic kidney. However, OPN also suppresses apoptosis. Future approaches to limit the progression of obstructive nephropathy in the developing kidney will require targeting of specific renal compartments.

Upregulation of osteopontin gene expression in diabetic rat proximal tubular cells revealed by microarray profiling

Progression of diabetic nephropathy appears directly related to renal tubulointerstitial injury, but the involved genes are incompletely delineated. To identify such genes, DNA microarray analysis was performed with RNA from renal proximal tubules (RPTs) of streptozotocin-induced diabetic Wistar rats, spontaneously diabetic BioBreeding rats, and rat immortalized renal proximal tubular cells (IRPTCs) exposed to high glucose (25 mM) medium for 2 weeks. Osteopontin (OPN) mRNA expression was quantified by real time-quantitative polymerase chain reaction (RT-qPCR) or conventional reverse transcriptase-polymerase chain reaction (RT-PCR). OPN mRNA expression was upregulated (5-70-fold increase) in diabetic rat RPTs and in IRPTCs chronically exposed to high glucose compared to control RPTs and IRPTCs. High glucose, angiotensin II, phorbol 12-myristate 13-acetate and transforming growth factor-beta 1 (TGF-beta1) stimulated OPN mRNA expression in IRPTCs in a dose- and time-dependent manner. This effect was inhibited by tiron, taurine, diphenylene iodinium, losartan, perindopril, calphostin C, or LY 379196 but not PD123319. IRPTCs overexpressing dominant-negative protein kinase C-beta 1 (PKC-beta1) cDNA or antisense TGF-beta1 cDNA prevented the high glucose effect on OPN mRNA expression. We concluded that high glucose-mediated increases in OPN gene expression in diabetic rat RPTs and IRPTCs are mediated, at least in part, via reactive oxygen species generation, intrarenal rennin-angiotensin system activation, TGF-beta1 expression, and PKC-beta1 signaling.

Evidence that inhibition of tubular cell apoptosis protects against renal damage and development of fibrosis following ureteric obstruction

Ureteric obstruction is frequently encountered in primary care urology and can lead to damage to the ipsilateral kidney. Relief of all types of obstruction generally leads to the normalization of any deterioration in renal function noted at diagnosis. However, some evidence from animal models suggests that obstruction can cause progressive deleterious effects on renal function and blood pressure control, especially in the presence of preexisting pathologies such as essential hypertension. The last 10 years have seen a proliferation of studies in rodents wherein complete unilateral ureteric obstruction has been used as a model of renal fibrosis. However, the relevance of the findings to human obstructive uropathy has, in many cases, not been the primary aim. In this review, we outline the major events linking damage to the renal parenchyma and cell death to the evolution of fibrosis following obstruction. Special focus is given to the role of apoptosis as a major cause of cell death during and post-complete ureteric obstruction. Several interventions that reduce tubular apoptosis are discussed in terms of their ability to prevent subsequent progression to end-organ damage and fibrosis.

Modulation of osteopontin in proteinuria-induced renal interstitial fibrosis

Proteinuria is associated with macrophage-dependent interstitial fibrosis (IF). Osteopontin (OPN), a macrophage chemoattractant, may be involved in the transition of proteinuria to IF but protective properties have also been reported. To elucidate whether OPN may be involved in the proteinuria-induced cascade of tubulointerstitial damage, renal expression of OPN was studied during the development of proteinuria-induced renal damage and during anti-proteinuric intervention with ACE inhibition (ACEi). First, the temporal relationships between proteinuria, interstitial OPN induction, and IF in adriamycin nephrosis (AN), a model of chronic proteinuria-induced renal damage, were studied. Second, the effect of anti-proteinuric treatment on OPN expression was investigated. The time course of OPN induction and markers of renal damage was studied in rats with unilateral AN at 6-week intervals until week 30. In a second study, a renal biopsy was taken 6 weeks after induction of bilateral AN; subsequently, rats were treated with ACEi until termination (week 12). In unilateral AN, proteinuria developed gradually and stabilized at week 10. In proteinuric kidneys, OPN expression was induced from week 12 onwards. Simultaneously, a progressive increase in interstitial macrophages, alpha-smooth muscle actin (alpha-SMA), collagen type III, and focal glomerulosclerosis (FGS) was observed. In bilateral AN, ACEi reduced proteinuria and OPN protein and stabilized fibrosis. In untreated animals, OPN mRNA increased, with stable OPN protein and fibrosis and increased FGS. Thus, in AN, development of proteinuria is followed by up-regulation of OPN along with markers of renal damage. The up-regulation of OPN is reversible by anti-proteinuric treatment without a corresponding reduction in fibrosis. Whereas these data are consistent with a role for OPN in the cascade of transition from proteinuria to fibrosis, intervention with ACEi showed that reduction of OPN does not attenuate established fibrosis.

Tacrolimus and Cyclosporinein vitro and in vivo Induce Osteopontin mRNA and Protein Expression in Renal Tissues

The mechanism of immunosuppression-linked nephrotoxicity in organ transplantation remains to be solved. Expression of osteopontin (OPN), a multifunctional secreted glycoprotein, has been associated with various forms of renal injuries. In this study, using in vitro and in vivo models, we examined the effects of cyclosporine (CsA) and tacrolimus (TAC) on OPN mRNA and protein expression. We also examined if CsA- and TAC-induced OPN expression is dependent on transforming growth factor (TGF)-beta expression. For in vivo experiments mice and rats were injected with CsA (25 mg/kg) and TAC (0.2 mg/kg). For in vitro experiments, human proximal tubular epithelial (PTE) cells were treated with CsA and TAC for 4 h. To study the in vivo effect of TGF-beta on OPN mRNA, mice were injected with recombinant TGF-beta protein (3 mg/kg). The expression of OPN was also studied in CsA-treated PTE cells with and without anti-TGF-beta antibody. At the end of in vitro and in vivo treatments, RNA was isolated from kidney tissue and renal cells reverse transcribed to cDNA and amplified for OPN mRNA. Using immunochemistry and Western blot analysis OPN protein expression was also studied in vivo and in vitro, respectively. Both in vitro and in vivo treatment with CsA and TAC resulted in significantly increased OPN mRNA and protein expression. TGF-beta treatment in vivo also resulted in a significantly increased OPN mRNA expression and anti-TGF-beta antibody but not the control antibody in vivo in CsA-treated mice, and in vitro in CsA-treated PTE cells inhibited OPN mRNA expression. OPN may contribute to the CsA- and TAC-induced nephrotoxicity in organ transplant recipients and the increased OPN expression might be mediated by TGF-beta.

Perinatal obstructive nephropathy

Significant advances have been made recently in elucidating the cellular consequences of urinary tract obstruction during renal development. Urinary tract obstruction impairs growth and maturation of the kidney, and can also cause renal maldevelopment. This includes a reduction in the number of nephrons, tubular atrophy, and progressive interstitial fibrosis. Apoptosis (programmed cell death) accounts for much of the loss of tubular epithelial cells. Factors contributing to apoptosis include stretching of cells in dilated tubules, altered renal production of growth factors, and infiltration of the renal interstitium by macrophages. Two major controversies remain regarding the surgical management of congenital obstructive nephropathy: first, which fetuses with bladder outlet obstruction should undergo prenatal intervention, and second, which infants should undergo early pyeloplasty for ureteropelvic junction obstruction? Even after successful surgery for congential obstructive nephropathy, all patients should be followed for hypertension, proteinuria, or renal deterioration.

Renal damage is not improved by blockade of endothelin receptors in primary renin-dependent hypertension

OBJECTIVE

Secondary activation of the renin-angiotensin system plays a major role in the progression of chronic nephropathies, and blockade of endothelin (ET) receptors has been shown to confer nephroprotection in experimental models of proteinuric renal disease. We tested the nephroprotective potential of selective endothelin A receptor (ETA) and non-selective ETA and endothelin B (ETA/B) receptor blockade in the TGR(mRen2)27 transgenic rat model with renin-dependent hypertension (Ren2).

DESIGN

Ren2 animals were treated between 10 and 30 weeks of age with the selective ETA receptor antagonist darusentan (Ren2-ETA) and the ETA/B receptor antagonist Lu420627 (Ren2-ETA/B), and compared with transgene negative Sprague-Dawley (SD) controls. Since the elevated systolic blood pressure in Ren2 was not affected in either Ren2-ETA or Ren2-ETA/ETB, an additional Ren-2 group was treated with a non-antihypertensive dose of the angiotensin II type 1 receptor blocker eprosartan (Ren2-AT1).

RESULTS

During the 20-week observation period 35% of untreated Ren2, 30% of Ren2-ETA/B, 50% of Ren2-ETA, and 83% of Ren2-AT1 animals survived compared with 100% of SD rats. Renal endothelin-1 mRNA expression and proteinuria (4.1-fold) were significantly elevated in Ren2 compared with SD rats (P < 0.05, respectively). Proteinuria was normalized to SD control levels in Ren2-AT1 (P < 0.05) but increased further in Ren2-ETA (7.7-fold) and Ren2-ETA/B (15-fold) (P < 0.05, respectively). Glomerulosclerosis, tubulointerstitial damage and renal osteopontin mRNA expression were reduced in Ren2-AT1 (P < 0.05, respectively) but remained unchanged or increased further in Ren2-ETA and Ren2-ETA/B compared with Ren2.

CONCLUSION

ET receptor blockade fails to improve renal damage and mortality in primary renin-dependent hypertension.

Long-term treatment with ramipril attenuates renal osteopontin expression in diabetic rats

BACKGROUND

Osteopontin (OPN) mediates progressive renal injury in various renal diseases by attracting macrophages, and its expression is regulated by the renin-angiotensin system (RAS). We studied the association between OPN expression and tubulointerstitial injury, and investigated the effect of ramipril on OPN expression in an animal model of non-insulin-dependent diabetes mellitus (NIDDM): Otsuka Long-Evans Tokushima Fatty (OLETF) rats.

METHODS

Control (Long-Evans Tokushima Otsuka, LETO) and diabetic (OLETF) rats were treated with ramipril (3 mg/kg in drinking water) or vehicle for nine months, starting at 20 weeks of age. Systolic blood pressure, body weight, urinary protein excretion and oral glucose tolerance tests (OGTT) were monitored periodically. Renal function, histology (glomerulosclerosis, tubulointerstitial fibrosis, and ED-1-positive cells as a measure of macrophage infiltration), and expressions of OPN and transforming growth factor-beta1 (TGF-beta1) were evaluated at the end of the study.

RESULTS

Compared with the LETO rats, OLETF rats showed declines in creatinine clearance rate, increases in urinary protein excretion and systolic blood pressure, and development of glomerulosclerosis, tubulointerstitial fibrosis, and inflammatory cell infiltration (all P < 0.05). Blocking angiotensin II with ramipril significantly improved all of these parameters (all P < 0.01). At the molecular level, expressions of OPN and TGF-beta1 were up-regulated in the OLETF rats, and were markedly suppressed following ramipril treatment. The sites of strong OPN mRNA and protein expressions were localized to areas of renal injury. Of note, the expression of OPN mRNA was strongly correlated with the number of ED-1-positive cells (r = 0.560, P = 0.01) and the tubulointerstitial fibrosis score (r = 0.500, P < 0.05).

CONCLUSIONS

Up-regulation of OPN expression may play a role in tubulointerstitial injury associated with diabetic nephropathy, and blockade of the RAS by ramipril may confer renoprotection by decreasing OPN expression in non-insulin-dependent diabetic nephropathy.

Reduced postischemic macrophage infiltration and interstitial fibrosis in osteopontin knockout mice

BACKGROUND

Osteopontin (OPN) is a phosphoprotein that is up-regulated in several experimental models of renal disease, including ischemia/reperfusion injury. OPN has been described as a macrophage chemoattractant, may serve as a survival factor for tubular cells, and is implicated in the development of tubulointerstitial fibrosis. However, the precise role of this protein in renal pathophysiology remains unclear.

METHODS

OPN knockout and wild-type mice were subjected to 30 minutes of warm renal ischemia combined with a contralateral nephrectomy, and sacrificed at six different time points, ranging from 12 hours to seven days after reperfusion. Besides functional and morphological parameters of postischemic acute renal failure (ARF), macrophage infiltration, apoptosis and expression of collagen types I and IV were investigated.

RESULTS

Postischemic ARF in OPN knockouts and wild-types showed a similar course and severity, without significant differences in either functional or morphological disease parameters. However, macrophage infiltration was significantly diminished in OPN knockouts after five and seven days, in cortex as well as in the outer stripe of the outer medulla (OSOM). Furthermore, OPN knockout mice showed significantly enhanced apoptosis in the injury phase and significantly less collagen I and IV expression in the regeneration phase of postischemic ARF.

CONCLUSIONS

There was no influence of OPN protein on the severity or course of functional impairment or morphological injury in the first seven days after an ischemic insult to the kidney. However, our results demonstrate that OPN favors macrophage recruitment to the postischemic kidney, inhibits apoptosis, and stimulates the development of renal fibrosis after an acute ischemic insult.

Emergence of different macrophage populations in hepatic fibrosis following thioacetamide-induced acute hepatocyte injury in rats

Macrophages may play a role in fibrogenesis. The kinetics and distribution of different macrophage populations were investigated immunohistochemically in hepatic lesions following acute hepatocyte injury induced in F344 rats by a single injection of thioacetamide (TAA) (300 mg/kg body weight, intraperitoneally). Hepatocyte degeneration or necrosis induced by TAA occurred mainly in the perivenular areas of hepatic lobules as early as post-injection (PI) days 1 and 3; fibrotic lesion development began in the damaged areas on day 1, and peaked on day 5; thereafter (PI days 7 and 10), the fibrotic areas decreased and were replaced by regenerated hepatocytes on PI days 15 and 20, indicating a remodelling process. In this rat model, the number of macrophages reacting with ED1 antibody (specific for exudate macrophages), ED2 (recognizing cell membrane antigens of resident macrophages, including Kupffer cells) and OX6 (recognizing MHC class II antigens expressed in antigen-presenting macrophages and dendritic cells) began to increase on PI day 1, peaking on PI day 3. The numbers gradually decreased on PI days 5 and 7; however, the statistically significant increase was maintained in respect of ED1-positive cells up to PI day 20, whereas no significant increase in ED2- and OX6-positive cells remained from PI day 10 onwards. Interestingly, of the ED1-, ED2- and OX6-positive cells, the OX6-positive cells were the least numerous. ED1- and OX6-positive cells appeared exclusively in the injured perivenular areas, whereas ED2-positive cells were present mainly in the mid-zonal areas and in smaller numbers in the perivenular areas. These findings indicated differences in kinetics and distribution between macrophage populations appearing in hepatic fibrosis. In addition, RT-PCR revealed that mRNA expression of osteopontin, a factor for induction and maintenance of macrophages in inflammation, was markedly increased on PI days 5, 7 and 10, suggesting a role in the pathogenesis of hepatic fibrosis.

Expression of osteopontin in rat kidneys: induction during ethylene glycol induced calcium oxalate nephrolithiasis

PURPOSE

Osteopontin is a well-known component of stone matrix and a strong inhibitor of the nucleation, growth and aggregation of calcium oxalate crystals in vitro. To understand its involvement in vivo in calcium oxalate nephrolithiasis we investigated the renal expression and urinary excretion of osteopontin in normal rats, and rats with hyperoxaluria and calcium oxalate crystal deposits in the kidneys.

MATERIALS AND METHODS

Calcium oxalate nephrolithiasis was induced by administering ethylene glycol. Immunohistochemistry and in situ hybridization were done to localize osteopontin and osteopontin messenger RNA in the kidneys, while sensitive reverse transcriptase quantitative competitive template polymerase chain reaction was performed to detect and quantify osteopontin messenger RNA expression. Urinary excretion was determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and Western blot analysis, and then quantified by densitometry of the Western blots.

RESULTS

Osteopontin expression in the kidneys was significantly increased after hyperoxaluria and it increased further after the deposition of calcium oxalate crystals in the kidneys. Urinary excretion of osteopontin increased concomitantly. The results reveal differences in renal responses after exposure to oxalate and calcium oxalate crystals. In normal kidneys osteopontin expression was limited to a small number of cells of the thin limbs of the loop of Henle and papillary surface epithelium. During hyperoxaluria osteopontin expression in the kidneys was increased but still mostly limited to cells of the thin limb and papillary surface epithelium. However, after calcium oxalate crystal deposition osteopontin expression was observed throughout the kidneys, including segments of the proximal tubules.

CONCLUSIONS

In response to exposure to oxalate and calcium oxalate crystals renal epithelial cells increase the production of osteopontin, which may have a significant role in calcium oxalate nephrolithiasis.

Increased osteopontin expression following renal ablation is attenuated by angiotensin type 1 receptor antagonism

Osteopontin is an extracellular matrix protein that is upregulated in renal injury. The aim of this study was to explore the renal expression of osteopontin in a model of progressive renal injury following subtotal nephrectomy (STNx) in rats and the effects of angiotensin type1 (AT1) receptor antagonist irbesartan on osteopontin expression. STNx or a sham operation was performed in 8-week-old Sprague-Dawley rats. STNx rats were given either irbesartan (15 mg/g) or no treatment for 12 weeks. Upregulation of osteopontin mRNA expression was observed in injured renal tubules as assessed by in situ hybridization (42 +/- 8 dpm/mm(2) v.s. control 7.7 +/- 0.6 dpm/mm(2), p < 0.01). Increased osteopontin expression was closely related to infiltration of monocytes/macrophages and increased cellular proliferation. Double immunohistochemical staining demonstrated co-existence of proliferating cell nuclear antigen and osteopontin positive staining in individual cells in kidney sections from STNx rats. The increase in osteopontin expression was inhibited by the AT1 receptor antagonist irbesartan (6.9 +/- 1.2 dpm/mm(2)), associated with attenuation of impaired renal function and pathology as well as decreased monocyte/macrophage infiltration and cellular proliferation. These findings suggest that osteopontin is upregulated in STNx rats and is reduced by AT1 receptor antagonism.

Role of osteopontin in cellular signaling and toxicant injury

Osteopontin (OPN) is a glycosylated phosphoprotein found in all body fluids and in the proteinaceous matrix of mineralized tissues. It can function both as a cell attachment protein and as a cytokine, delivering signals to cells via a number of receptors including several integrins and CD44. Expression of OPN is enhanced by a variety of toxicants, especially those that activate protein kinase C. In its capacity as a signaling molecule, OPN can modify gene expression and promote the migration of monocytes/macrophages up an OPN gradient. It has both inflammatory and anti-inflammatory actions. Some experiments suggest that it may inhibit apoptosis, possibly contributing to the survival of cells in response to toxicant injury. Elevated OPN expression often correlates with malignancy and has been shown to enhance the tumorigenic and/or metastatic phenotype of the cancer cell. Recent studies have revealed that OPN plays critical roles in bone remodeling and cell-mediated immunity.

Monocyte chemoattractant protein-1 and osteopontin differentially regulate monocytes recruitment in experimental glomerulonephritis

BACKGROUND

This study evaluated the mechanisms of monocyte/macrophage (M/M) infiltration in a rat model of anti-glomerular basement membrane glomerulonephritis (GN). We focused on chemokines and osteopontin, which are known regulators of M/M recruitment.

METHODS

Using immunohistology, in situ hybridization, and Northern blotting, the expression levels of chemokines and osteopontin were evaluated in isolated glomeruli and tubules 4, 10, and 20 days after the induction of GN. In vivo blocking experiments were performed by application of neutralizing antibodies against osteopontin and monocyte chemoattractant protein-1 (MCP-1).

RESULTS

In nephritic animals, high glomerular MCP-1 and RANTES (regulated upon activation normal T cell expressed and secreted) expression levels were observed on days 4 and 10. The tubular expression of MCP-1, however, was only slightly enhanced. In contrast, tubular osteopontin production was maximally stimulated (day 10) and paralleled with peaks of albuminuria and tubulointerstitial M/M infiltration. Application of an anti-osteopontin antibody ameliorated tubulointerstitial and glomerular M/M recruitment, whereas treatment with an anti-MCP-1 antibody selectively reduced glomerular M/M recruitment. However, tubulointerstitial M/M infiltration remained unchanged.

CONCLUSION

These studies show that chemokines and osteopontin are differentially expressed in glomeruli and tubules in this model of GN. Chemokines play a primary role in the glomeruli, whereas osteopontin has a predominant role in tubulointerstitial M/M recruitment. The roles of chemokines and osteopontin may thus be dependent on the renal compartment and on the disease model.

Renal and vascular injury induced by exogenous angiotensin II is AT1 receptor-dependent

Angiotensin II (Ang II) infusion in rats augments vascular injury in balloon-injured carotid arteries and induces marked vascular and tubulointerstitial injury in kidneys. We examined how the AT1 receptor is modulated and whether blockade of the receptor with losartan could prevent the phenotypic and cellular changes. We also examined the role of the local renin-angiotensin system (RAS) by examining the expression of angiotensin-converting enzyme (ACE) and the effect of treatment with the ACE inhibitor, ramipril. Ang II infusion resulted in systemic hypertension and accelerated intimal and medial thickening in balloon-injured carotid arteries. Renal injury was manifested by proteinuria, glomerular phenotypic changes (mesangial expression of alpha-actin and podocyte expression of desmin), and tubulointerstitial injury with the tubular upregulation of the macrophage-adhesive protein, osteopontin, the interstitial accumulation of macrophages and myofibroblasts, and the deposition of collagen types III and IV. Ang II infusion decreased AT1 receptor number in the renal interstitium but not in glomeruli. Losartan completely blocked the Ang II-mediated hypertension, proteinuria, and injury to both carotid and kidney. Ang II infusion was also associated with an increase in ACE protein in both the proximal tubular brush border as well as at interstitial sites of injury, but despite evidence for activation of the local RAS, treatment with ramipril was without effect. These studies demonstrate that the renal and vascular injury induced by Ang II infusion is mediated by the AT1 receptor despite downregulation of the receptor in the interstitium. In addition, although there is evidence for local RAS activation, the injury appears to be mediated solely by the exogenous Ang II.

Angiotensin II: a regulator of inflammation during renal disease?

It has been recently recognized that besides its vasoactive actions Angiotensin II (Ang II) exerts various immunomodulatory effects that may contribute to renal injury and to the progression of renal disease. Consistent with this concept, Ang II facilitates macrophage recruitment into the kidney either directly or through the-upregulation of different chemotactic molecules such as RANTES (Regulated on Activation Normal T Expressed and Secreted), monocyte chemoattractant protein-1 (MCP-1) and osteopontin. Infiltrating macrophages not only produce a number of cytokines, growth factors and proinflammatory Mediators, but also synthesize Ang II intacellularly which increases tissue levels of the hormone within the kidney. Finally, specific binding sites for Ang II have been demonstrated on macrophages and increasing evidence indicates that Ang II directly modulates many of the cellular functions of these cells during the course of renal disease. Together these data suggest that Ang II plays an important role in modulating inflammatory responses in the kidney.

Osteopontin expression in progressive renal injury in remnant kidney: role of angiotensin II

BACKGROUND

Osteopontin (OPN) is a macrophage chemotactic and adhesion molecule and has been shown to play a role in glomerular and tubulointerstitial injury in several kidney disease models.

METHODS

The present study examined whether OPN expression is involved in the progression of renal disease following subtotal (5/6) nephrectomy (STNx) in rats and whether angiotensin II (Ang II) mediates the up-regulation of renal OPN expression and macrophage accumulation in this model by administering valsartan, an Ang II type I (AT1) receptor antagonist, or ramipril, an angiotensin-converting enzyme (ACE) inhibitor.

RESULTS

In normal and sham-operated rat kidneys, OPN was expressed in a few tubules (<5%) and was absent in glomeruli. Following STNx (weeks 2 to 16), there was substantial up-regulation of OPN mRNA and protein expression in glomeruli [2 to 12 cells/glomerular cross section (gcs)] and tubular epithelial cells (20 to 75% OPN+). The up-regulation of OPN expression was associated with macrophage accumulation within the kidney, severe proteinuria, loss of renal function, and severe histologic damage, including tubulitis and tubulointerstitial fibrosis (all P < 0.001). Treatment with either valsartan or ramipril completely abrogated the up-regulation of OPN mRNA and protein expression in glomeruli and tubules. The reduction in OPN expression was associated with a significant inhibition of macrophage accumulation and progressive renal injury (P < 0.001).

CONCLUSION

An up-regulation of OPN expression may play a role in progressive renal injury following STNx. Inhibition of OPN expression may be one of the mechanisms by which Ang II blockade attenuated renal injury after renal ablation.

Lack of in vivo function of osteopontin in experimental anti-GBM nephritis

Osteopontin (Opn) is a potent chemoattractant for mononuclear cells that is upregulated in various inflammatory states of the kidney. Opn is believed to contribute to mononuclear cell infiltration and renal injury. The importance of Opn was examined in vivo in rapidly progressive glomerulonephritis in Opn knockout mice. Glomerulonephritis was induced by intravenous injection of rabbit anti-mouse glomerular basement membrane antiserum in mice that had been presensitized to rabbit IgG. Immunologic responsiveness to rabbit IgG (assessed by cutaneous delayed-type hypersensitivity and antibody titers) showed no significant difference between wild-type and Opn -/- mice. Proteinuria was also similar in both groups. Glomerular crescent formation was not different in Opn +/+ and -/- groups (26 +/- 6% versus 29 +/- 7%). Tubulointerstitial infiltration was assessed qualitatively and showed no significant difference between the two genotypes. Formation of thrombi in the glomerular capillaries on a scale from 0 to 3 also showed no significant difference (1.3 +/- 0.3 for Opn +/+ and 1.4 +/- 0.3 for Opn -/- mice). Northern blot analysis of total kidney RNA showed a 5.1-fold increase of Opn expression in Opn +/+ mice compared with untreated controls and the absence of expression in Opn -/- mice, as expected. Regulated upon activation, normal T cell expressed and secreted (RANTES) and monocyte chemoattractant protein-1 mRNA levels were also markedly upregulated with no significant difference between the two strains, excluding compensatory overexpression of these two chemokines in Opn -/- mice. It is concluded that the known upregulation of Opn in murine anti-glomerular basement membrane nephritis does not significantly contribute to the glomerular and tubulointerstitial mononuclear cell infiltration in this model.

Tubular osteopontin expression in human glomerulonephritis and renal vasculitis

Tubulointerstitial change is a common histopathologic feature of acute and chronic glomerular diseases and is more closely correlated than glomerular damage with renal function and subsequent outcome. Monocyte infiltration is presumed to be initiated by chemoattractants and has a pivotal role in tubulointerstitial changes. Osteopontin (OPN) is a candidate as such a chemoattractant and has been shown to recruit monocytes into the interstitium of animal models of renal diseases. In this study, we investigated OPN expression by immunostaining and its correlation with clinical and histopathologic parameters in patients with immunoglobulin A (IgA) nephropathy, diffuse proliferative lupus nephritis (DPLN), and myeloperoxidase-antineutrophil cytoplasmic autoantibody-associated microscopic polyangiitis (MMP). Twenty patients with IgA nephropathy, 12 patients with DPLN, and 14 patients with MMP were studied. OPN expression, which was constitutively observed on the apical membrane of distal tubules, was upregulated in the cytoplasm of proximal and distal tubular epithelium parallel to the degree of interstitial mononuclear cell infiltration in patients with IgA nephropathy, as well as those with DPLN. CD68(+) monocyte infiltration significantly correlated with the degree of OPN expression in the tubular epithelium. Conversely, there was no apparent induction of OPN in the proximal and distal tubular epithelium of patients with MMP despite remarkable monocyte infiltration. In conclusion, these data suggest that inducible expression of OPN in the tubular epithelium seems to be associated with interstitial monocyte infiltration and subsequent tubulointerstitial changes in some forms of human renal diseases.

Angiotensinogen and AT(1) antisense inhibition of osteopontin translation in rat proximal tubular cells

Antisense oligonucleotide inhibition of angiotensinogen and ANG II type 1 receptor (AT(1)) mRNA translation in rat proximal tubules (PT) was examined to provide direct evidence for a role of the renin-angiotensin system (RAS) in upregulated osteopontin expression observed following mechanical cell stretch. Male Sprague-Dawley rats underwent unilateral ureteral obstruction (UUO) under Brevital anesthesia. In situ hybridization and Western blot analysis demonstrated angiotensinogen mRNA and angiotensin converting enzyme (ACE) protein localized to PTs and upregulated in obstructed kidneys, respectively, confirming an increased expression of renal RAS in vivo. In vitro studies were performed to provide mechanistic insight into ANG II-dependent osteopontin expression following mechanical cell stretch, which putatively mimics the increased PT luminal pressure post-UUO. A cationic transfection method was used to introduce either angiotensinogen or AT(1) antisense oligonucleotide into cultured rat PT cells prior to 1 h of cyclic mechanical cell stretch. Northern blot analysis revealed that PT cells subjected to cyclic mechanical stretch with/without prior transfection with a sense oligonucleotide exhibited increased osteopontin mRNA expression compared with unstretched cells. Blockade of either angiotensinogen or AT(1) mRNA translation by antisense oligonucleotide inhibition prior to cell stretch was found to significantly decrease osteopontin mRNA levels 2.4-fold (P<0.004) and 1.6-fold (P<0.001), respectively, compared with values observed in control unstretched cells. This study provides evidence that stretch-induced upregulation of osteopontin mRNA expression is mediated, in part, via production of ANG II. These results lend insight into upregulation of osteopontin via a local PT RAS leading to macrophage infiltration in the tubulointerstitium in experimental hydronephrosis.

Regulatory peptides and their antagonists in nephropathies

Many peptides influence renal function and structure in physiological and pathophysiological situations. Bioactive peptides that regulate renal function and structure encompass various substances including vasopeptides, growth factors, cytokines and peptide hormones. We highlight some novel concepts indicating that the vasoactive peptides angiotensin II and endothelin-1 play a major role in the progression of renal disease. These effects may be amplified by reduced concentration of counteracting natriuretic peptides. In addition, recent evidence suggests that peptides such as leptin, previously not considered to exert any renal effects, may be involved in renal pathophysiology under certain conditions. One of the most imperative tasks in nephrology is to develop innovative strategies to slow the progression of chronic renal disease. Interference with the renal action of bioactive peptides will certainly be part of this strategy.

Osteopontin expressed by renal tubular epithelium mediates interstitial monocyte infiltration in rats

In this study, we have shown that intravenously administered antisense oligodeoxynucleotide (ODN) was demonstrated to be taken up by tubular epithelium, after which it blocked mRNA expression of target genes in normal and nephritic rats. Therefore, we injected osteopontin (OPN) antisense ODN to Goodpasture syndrome (GPS) rats every second day between days 27 and 35, the time when renal OPN expression increased and interstitial monocyte infiltration was aggravated. In parallel to blockade of tubular OPN expression, this treatment significantly attenuated monocyte infiltration and preserved renal plasma flow in GPS rats at day 37, compared with sense ODN-treated and untreated GPS rats. No significant changes were observed in OPN mRNA level by RT-PCR and histopathology of the glomeruli after ODN treatment, which was compatible with an absence of differences in the urinary protein excretion rate. In conclusion, OPN expressed by tubular epithelium played a pivotal role in mediating peritubular monocyte infiltration consequent to glomerular disease.