Acute effects of hemodialysis on cytokine transcription profiles: evidence for C-reactive protein-dependency of mediator induction

Chronic microinflammation increases cardiovascular morbidity in chronic hemodialysis (HD) patients. Previously published studies are controversial with respect to acute effects of HD treatment on up- or downregulation of cytokine protein levels. Twenty-nine chronic HD patients were hemodialysed for 4 h with a 4008 dialyser using high-flux membranes. Patients were separated into a low (up to 1 mg/dl) and a high (1.1 to 5.5 mg/dl) C-reactive protein (CRP) group. Blood was drawn before HD and 240 min after initiation of HD. Acute changes of transcript levels encoding pro- and anti-inflammatory mediators were analyzed in RNA stabilized immediately from blood leukocytes using microarray analysis (n=1) and quantitative real-time polymerase chain reaction (PCR) (Light Cycler) (n=29). In both patient groups, HD treatment significantly increased the transcript levels of several pro-inflammatory cytokines, such as tumor necrosis factor alpha and interleukin-8 (IL-8), and chemokine receptors such as C-X-C chemokine receptor type 4, C-C chemokine receptor type 7, and the fractakine receptor CX3C chemokine receptor 1. In the low CRP group, the increase of transcript levels for anti-inflammatory IL-1-receptor antagonist and of the receptor for the anti-inflammatory cytokines IL-10 and interferon gamma was significantly more pronounced than in the high CRP group. Subgroup analysis revealed no difference between diabetic vs non-diabetic patients. These observations point towards a marked influence of a routine hemodialysis treatment on transcription in leukocytes of pro- and anti-inflammatory cytokines and receptors relevant for microinflammation. Diminished upregulation of receptors for anti-inflammatory factors in HD patients with high CRP levels could contribute to enhanced microinflammation in those patients.

Urokinase-type plasminogen activator (uPA) is not essential for epithelial sodium channel (ENaC)-mediated sodium retention in experimental nephrotic syndrome

AIM

In nephrotic syndrome, aberrantly filtered plasminogen (plg) is converted to active plasmin by tubular urokinase-type plasminogen activator (uPA) and thought to lead to sodium retention by proteolytic activation of the epithelial sodium channel (ENaC). This concept predicts that uPA is an important factor for sodium retention and that inhibition of uPA might be protective in nephrotic syndrome.

METHODS

Activation of amiloride-sensitive currents by uPA and plg were studied in Xenopus laevis oocytes expressing murine ENaC. In doxorubicin-induced nephrotic mice, uPA was inhibited pharmacologically by amiloride and genetically by the use of uPA-deficient mice (uPA^-/- ).

RESULTS

Experiments in Xenopus laevis oocytes expressing murine ENaC confirmed proteolytic ENaC activation by a combination of plg and uPA which stimulated amiloride-sensitive currents with concomitant cleavage of the ENaC γ-subunit at the cell surface. Treatment of nephrotic wild-type mice with amiloride inhibited urinary uPA activity, prevented urinary plasmin formation and sodium retention. In nephrotic mice lacking uPA (uPA^-/- ), urinary plasmin formation from plg was suppressed and urinary uPA activity absent. However, in nephrotic uPA^-/- mice, sodium retention was not reduced compared to nephrotic uPA^+/+ mice. Amiloride prevented sodium retention in nephrotic uPA^-/- mice which confirmed the critical role of ENaC in sodium retention.

CONCLUSION

uPA is responsible for the conversion of aberrantly filtered plasminogen to plasmin in the tubular lumen in vivo. However, uPA-dependent plasmin generation is not essential for ENaC-mediated sodium retention in experimental nephrotic syndrome.

Plasma kallikrein activates the epithelial sodium channel in vitro but is not essential for volume retention in nephrotic mice

AIM

Recent work has demonstrated that activation of the epithelial sodium channel (ENaC) by aberrantly filtered serine proteases causes sodium retention in nephrotic syndrome. The aim of this study was to elucidate a potential role of plasma kallikrein (PKLK) as a candidate serine protease in this context.

METHODS

We analysed PKLK in the urine of patients with chronic kidney disease (CKD, n = 171) and investigated its ability to activate human ENaC expressed in Xenopus laevis oocytes. Moreover, we studied sodium retention in PKLK-deficient mice (klkb1^-/- ) with experimental nephrotic syndrome induced by doxorubicin injection.

RESULTS

In patients with CKD, we found that PKLK is excreted in the urine up to a concentration of 2 μg mL^-1 which was correlated with albuminuria (r = .71) and overhydration as assessed by bioimpedance spectroscopy (r = .44). PKLK increased ENaC-mediated whole-cell currents, which was associated with the appearance of a 67 kDa γ-ENaC cleavage product at the cell surface consistent with proteolytic activation. Mutating a putative prostasin cleavage site in γ-ENaC prevented channel stimulation by PKLK. In a mouse model for nephrotic syndrome, active PKLK was present in nephrotic urine of klkb1^+/+ but not of klkb1^-/- mice. However, klkb1^-/- mice were not protected from ENaC activation and sodium retention compared to nephrotic klkb1^+/+ mice.

CONCLUSION

Plasma kallikrein is detected in the urine of proteinuric patients and mice and activates ENaC in vitro involving the putative prostasin cleavage site. However, PKLK is not essential for volume retention in nephrotic mice.

Plasminogen deficiency does not prevent sodium retention in a genetic mouse model of experimental nephrotic syndrome

AIM

Sodium retention is the hallmark of nephrotic syndrome (NS) and mediated by the proteolytic activation of the epithelial sodium channel (ENaC) by aberrantly filtered serine proteases. Plasmin is highly abundant in nephrotic urine and has been proposed to be the principal serine protease responsible for ENaC activation in NS. However, a proof of the essential role of plasmin in experimental NS is lacking.

METHODS

We used a genetic mouse model of NS based on an inducible podocin knockout (Bl6-Nphs2^tm3.1Antc *Tg(Nphs1-rtTA*3G)^8Jhm *Tg(tetO-cre)^1Jaw or nphs2^Δipod ). These mice were crossed with plasminogen deficient mice (Bl6-Plg^tm1Jld or plg^-/- ) to generate double knockout mice (nphs2^Δipod *plg^-/- ). NS was induced after oral doxycycline treatment for 14 days and mice were followed for subsequent 14 days.

RESULTS

Uninduced nphs2^Δipod *plg^-/- mice had normal kidney function and sodium handling. After induction, proteinuria increased similarly in both nphs2^Δipod *plg^+/+ and nphs2^Δipod *plg^-/- mice. Western blot revealed the urinary excretion of plasminogen and plasmin in nphs2^Δipod *plg^+/+ mice which were absent in nphs2^Δipod *plg^-/- mice. After the onset of proteinuria, amiloride-sensitive natriuresis was increased compared to the uninduced state in both genotypes. Subsequently, urinary sodium excretion dropped in both genotypes leading to an increase in body weight and development of ascites. Treatment with the serine protease inhibitor aprotinin prevented sodium retention in both genotypes.

CONCLUSIONS

This study shows that mice lacking urinary plasminogen are not protected from ENaC-mediated sodium retention in experimental NS. This points to an essential role of other urinary serine proteases in the absence of plasminogen.

DOCA and TGF-beta induce early growth response gene-1 (Egr-1) expression

Renal fibrosis is characterized by excessive accumulation of extracellular matrix proteins. Recent findings show that transforming growth factor-beta (TGF-beta) induces a rapid but transient expression of early growth response gene-1 (Egr-1) by skin fibroblasts. The present study aims to define the role of Egr-1 in mineralocorticoid-induced renal fibrosis. Therefore, we transiently transfected immortalized human renal fibroblasts (TK188) with recombinant Egr-1 and analysed the transcription of several pro-fibrotic genes (Coll1A1, Coll1A2, osteopontin, TIMP-1, and CTGF). We also examined Egr-1 expression and the regulation of pro-fibrotic genes in DOCA- (deoxycorticosterone acetate) and TGF-beta-treated renal fibroblasts. Finally, we compared Egr-1 gene expression in DOCA/high salt-induced fibrotic kidneys and untreated mice. Egr-1 transfection of TK188 fibroblasts induced the expression of TIMP-1 and osteopontin mRNA. Similar results were obtained after DOCA-activation of TK188 cells. Stimulation of TK188 with TGF-beta, but not with DOCA, resulted in elevated Coll1A1/Coll1A2 and CTGF levels. Co-stimulation with DOCA and TGF-beta was followed by enhanced Egr-1, Coll1A1, TIMP-1, and CTGF transcription. In conclusion, both DOCA and TGF-beta alone or in combination synergistically induced Egr-1 expression by human renal fibroblasts. DOCA induction of TIMP-1/osteopontin is Egr-1 dependent, whereas TGF-beta appears to induce Coll1A1 and CTGF by an Egr-1 independent pathway. In vivo analyses revealed significantly higher Egr-1 transcript levels in DOCA/high salt-induced fibrotic kidneys compared to untreated mice. Thus, we show for the first time that Egr-1 might participate in DOCA-induced renal fibrosis.

Experimental nephrotic syndrome leads to proteolytic activation of the epithelial Na+ channel in the mouse kidney

Proteolytic activation of the renal epithelial Na⁺ channel (ENaC) involves cleavage events in its α- and γ-subunits and is thought to mediate Na⁺ retention in nephrotic syndrome (NS). However, the detection of proteolytically processed ENaC in kidney tissue from nephrotic mice has been elusive so far. We used a refined Western blot technique to reliably discriminate full-length α-ENaC and γ-ENaC and their cleavage products after proteolysis at their proximal and distal cleavage sites (designated from the NH₂-terminus), respectively. Proteolytic ENaC activation was investigated in kidneys from mice with experimental NS induced by doxorubicin or inducible podocin deficiency with or without treatment with the serine protease inhibitor aprotinin. Nephrotic mice developed Na⁺ retention and increased expression of fragments of α-ENaC and γ-ENaC cleaved at both the proximal cleavage site and, more prominently, the distal cleavage site, respectively. Treatment with aprotinin but not with the mineralocorticoid receptor antagonist canrenoate prevented Na⁺ retention and upregulation of the cleavage products in nephrotic mice. Increased expression of cleavage products of α-ENaC and γ-ENaC was similarly found in healthy mice treated with a low-salt diet, sensitive to mineralocorticoid receptor blockade. In human nephrectomy specimens, γ-ENaC was found in the full-length form and predominantly cleaved at its distal cleavage site. In conclusion, murine experimental NS leads to aprotinin-sensitive proteolytic activation of ENaC at both proximal and, more prominently, distal cleavage sites of its α- and γ-subunit, most likely by urinary serine protease activity or proteasuria.NEW & NOTEWORTHY This study demonstrates that murine experimental nephrotic syndrome leads to aprotinin-sensitive proteolytic activation of the epithelial Na⁺ channel at both the α- and γ-subunit, most likely by urinary serine protease activity or proteasuria.

Proteolytic activation of the epithelial sodium channel (ENaC) by factor VII activating protease (FSAP) and its relevance for sodium retention in nephrotic mice

Proteolytic activation of the epithelial sodium channel (ENaC) by aberrantly filtered serine proteases is thought to contribute to renal sodium retention in nephrotic syndrome. However, the identity of the responsible proteases remains elusive. This study evaluated factor VII activating protease (FSAP) as a candidate in this context. We analyzed FSAP in the urine of patients with nephrotic syndrome and nephrotic mice and investigated its ability to activate human ENaC expressed in Xenopus laevis oocytes. Moreover, we studied sodium retention in FSAP-deficient mice (Habp2^−/−) with experimental nephrotic syndrome induced by doxorubicin. In urine samples from nephrotic humans, high concentrations of FSAP were detected both as zymogen and in its active state. Recombinant serine protease domain of FSAP stimulated ENaC-mediated whole-cell currents in a time- and concentration-dependent manner. Mutating the putative prostasin cleavage site in γ-ENaC (γRKRK178AAAA) prevented channel stimulation by the serine protease domain of FSAP. In a mouse model for nephrotic syndrome, active FSAP was present in nephrotic urine of Habp2^+/+ but not of Habp2^−/− mice. However, Habp2^−/− mice were not protected from sodium retention compared to nephrotic Habp2^+/+ mice. Western blot analysis revealed that in nephrotic Habp2^−/− mice, proteolytic cleavage of α- and γ-ENaC was similar to that in nephrotic Habp2^+/+ animals. In conclusion, active FSAP is excreted in the urine of nephrotic patients and mice and activates ENaC in vitro involving the putative prostasin cleavage site of γ-ENaC. However, endogenous FSAP is not essential for sodium retention in nephrotic mice.

Sodium retention in nephrotic syndrome is independent of the activation of the membrane-anchored serine protease prostasin (CAP1/PRSS8) and its enzymatic activity

Experimental nephrotic syndrome leads to activation of the epithelial sodium channel (ENaC) by proteolysis and promotes renal sodium retention. The membrane-anchored serine protease prostasin (CAP1/PRSS8) is expressed in the distal nephron and participates in proteolytic ENaC regulation by serving as a scaffold for other serine proteases. However, it is unknown whether prostasin is also involved in ENaC-mediated sodium retention of experimental nephrotic syndrome. In this study, we used genetically modified knock-in mice with Prss8 mutations abolishing its proteolytic activity (Prss8-S238A) or prostasin activation (Prss8-R44Q) to investigate the development of sodium retention in doxorubicin-induced nephrotic syndrome. Healthy Prss8-S238A and Prss8-R44Q mice had normal ENaC activity as reflected by the natriuretic response to the ENaC blocker triamterene. After doxorubicin injection, all genotypes developed similar proteinuria. In all genotypes, urinary prostasin excretion increased while renal expression was not altered. In nephrotic mice of all genotypes, triamterene response was similarly increased, consistent with ENaC activation. As a consequence, urinary sodium excretion dropped in all genotypes and mice similarly gained body weight by + 25 ± 3% in Prss8-wt, + 20 ± 2% in Prss8-S238A and + 28 ± 3% in Prss8-R44Q mice (p = 0.16). In Western blots, expression of fully cleaved α- and γ-ENaC was similarly increased in nephrotic mice of all genotypes. In conclusion, proteolytic ENaC activation and sodium retention in experimental nephrotic syndrome are independent of the activation of prostasin and its enzymatic activity and are consistent with the action of aberrantly filtered serine proteases or proteasuria.

Renal effects of the serine protease inhibitor aprotinin in healthy conscious mice

Treatment with aprotinin, a broad-spectrum serine protease inhibitor with a molecular weight of 6512 Da, was associated with acute kidney injury, which was one of the reasons for withdrawal from the market in 2007. Inhibition of renal serine proteases regulating the epithelial sodium channel ENaC could be a possible mechanism. Herein, we studied the effect of aprotinin in wild-type 129S1/SvImJ mice on sodium handling, tubular function, and integrity under a control and low-salt diet. Mice were studied in metabolic cages, and aprotinin was delivered by subcutaneously implanted sustained release pellets (2 mg/day over 10 days). Mean urinary aprotinin concentration ranged between 642 ± 135 (day 2) and 127 ± 16 (day 8) µg/mL . Aprotinin caused impaired sodium preservation under a low-salt diet while stimulating excessive hyperaldosteronism and unexpectedly, proteolytic activation of ENaC. Aprotinin inhibited proximal tubular function leading to glucosuria and proteinuria. Plasma urea and cystatin C concentration increased significantly under aprotinin treatment. Kidney tissues from aprotinin-treated mice showed accumulation of intracellular aprotinin and expression of the kidney injury molecule 1 (KIM-1). In electron microscopy, electron-dense deposits were observed. There was no evidence for kidney injury in mice treated with a lower aprotinin dose (0.5 mg/day). In conclusion, high doses of aprotinin exert nephrotoxic effects by accumulation in the tubular system of healthy mice, leading to inhibition of proximal tubular function and counterregulatory stimulation of ENaC-mediated sodium transport.

Inhibition of the membrane localization of p21 ras proteins by lovastatin in tumor cells possessing a mutated N-ras gene

Mutated ras genes are found in a variety of human tumors. For biological activity the gene product p21 ras needs to be bound to the cell membrane by a farnesyl residue. Treatment of tumor cells with lovastatin reduces the availability of farnesyl pyrophosphate for the modification of the ras proteins. The membrane localization of p21 ras has been reduced by 30-36% after the tumor cells have grown in the presence of 10 microM lovastatin for 7 days. The extent of the inhibition depends on the growth kinetics of the cell lines.

Essential role of DNA-PKcs and plasminogen for the development of doxorubicin-induced glomerular injury in mice

Susceptibility to doxorubicin-induced nephropathy (DIN), a toxic model for the induction of proteinuria in mice, is related to the single-nucleotide polymorphism (SNP) C6418T of the Prkdc gene encoding for the DNA-repair enzyme DNA-PKcs. In addition, plasminogen (Plg) has been reported to play a role in glomerular damage. Here, we investigated the interdependence of both factors for the development of DIN. Genotyping confirmed the SNP of the Prkdc gene in C57BL/6 (Prkdc^C6418/C6418) and 129S1/SvImJ (Prkdc^T6418/T6418) mice. Intercross of heterozygous 129SB6F1 mice led to 129SB6F2 hybrids with Mendelian inheritance of the SNP. After doxorubicin injection, only homozygous F2 mice with Prkdc^T6418/T6418 developed proteinuria. Genetic deficiency of Plg (Plg^−/−) in otherwise susceptible 129S1/SvImJ mice led to resistance to DIN. Immunohistochemistry revealed glomerular binding of Plg in Plg^+/+ mice after doxorubicin injection involving histone H2B as Plg receptor. In doxorubicin-resistant C57BL/6 mice, Plg binding was absent. In conclusion, susceptibility to DIN in 129S1/SvImJ mice is determined by a hierarchical two-hit process requiring the C6418T SNP in the Prkdc gene and subsequent glomerular binding of Plg.

This article has an associated First Person interview with the first author of the paper.

Summary: Susceptibility to doxorubicin-induced nephropathy in 129S1/SvImJ mice is determined by a hierarchical two-hit process requiring the C6418T single-nucleotide polymorphism in the Prkdc gene and subsequent glomerular binding of plasminogen.

Impact of aldosterone deficiency on the development of diuretic resistance in mice

The effect of diuretics can be limited by stimulation of counter-regulatory mechanisms, eventually leading to diuretic resistance. It is thought that the mineralocorticoid aldosterone might contribute to the development of diuretic resistance. To test this, we challenged genetically modified mice with or without a deletion of the gene coding for the aldosterone synthase (AS) with furosemide, hydrochlorothiazide (HCT) and triamterene. Urinary excretion was studied in metabolic cages; kidneys were studied for expression of sodium transporters. In both genotypes, a 4-day treatment with HCT via drinking water (400 mg/l) induced a similar natriuresis and modest loss of body weight < 10%. In contrast, furosemide (125 mg/l) and triamterene (200 mg/l) via drinking water stimulated a significantly higher natriuresis and body weight loss in AS-/- mice and in addition, triamterene caused massive hyperkalemia > 9 mM and acidosis (pH < 7.0). In AS+/+ mice, plasma aldosterone concentration tended to increase under furosemide and HCT administration, while triamterene induced a robust ~ sixfold increase. In the kidney, apical targeting and proteolytic activation of the epithelial sodium channel ENaC were stimulated in AS+/+ mice under triamterene treatment, an effect that was diminished in AS-/- mice. In conclusion, aldosterone is essentially involved in the development of diuretic resistance to ENaC blockade by triamterene and to a lesser extent to furosemide. In contrast, resistance to HCT was independent of aldosterone.