Renal cell regeneration following oxidant exposure: inhibition by TGF-beta1 and stimulation by ascorbic acid

Renal toxicity caused by diethylene glycol: an overview

Diethylene glycol (DEG) is nephrotoxic, potentially resulting in high morbidity and mortality. Its main nephrotoxic by-product is diglycolic acid (DGA). This narrative overview summarizes selected literature with a focus on clinical findings, pathophysiology, diagnosis including morphological features of renal biopsies, and management. The kidney injury in DEG poisoning is secondary to proximal tubular necrosis caused by DGA. Marked vacuolization and edema of epithelial cells obstruct the lumen, reducing urine flow and, consequently, resulting in anuria and uremia. The clinical alterations due to DEG poisoning are dose-dependent. Patients may present with gastrointestinal symptoms and anion gap metabolic acidosis, followed by renal failure, and, later, encephalopathy and neuropathy. Although this three-phase pattern has been described, signs and symptoms may be overlapping. Data about DEG intoxication is scarce. Sometimes the diagnosis is challenging. The management includes supportive care, gastric decontamination, correction of acid-base disorders, and hemodialysis. The understanding of the metabolic processes related to DEG poisoning may contribute to its management, preventing death, serious sequels, or irreversible lesions.

Role of Renal Hypoxia in the Progression From Acute Kidney Injury to Chronic Kidney Disease

Over the past 20 years, there has been an increased appreciation of the long-term sequelae of acute kidney injury (AKI) and the potential development of chronic kidney disease (CKD). Several pathophysiologic features have been proposed to mediate AKI to CKD progression including maladaptive alterations in tubular, interstitial, inflammatory, and vascular cells. These alterations likely interact to culminate in the progression to CKD. In this article we focus primarily on evidence of vascular rarefaction secondary to AKI, and the potential mechanisms by which rarefaction occurs in relation to other alterations in tubular and interstitial compartments. We further focus on the potential that rarefaction contributes to renal hypoxia. Consideration of the role of hypoxia in AKI to CKD transition focuses on experimental evidence of persistent renal hypoxia after AKI and experimental maneuvers to evaluate the influence of hypoxia, per se, in progressive disease. Finally, consideration of methods to evaluate hypoxia in patients is provided with the suggestion that noninvasive measurement of renal hypoxia may provide insight into progression in post-AKI patients.

Regeneration and bioengineering of the kidney: current status and future challenges

The prevalence of chronic kidney disease continues to outpace the development of effective treatment strategies. For patients with advanced disease, renal replacement therapies approximate the filtration functions of the kidney at considerable cost and inconvenience, while failing to restore the resorptive and endocrine functions. Allogeneic transplantation remains the only restorative treatment, but donor shortage, surgical morbidity and the need for lifelong immunosuppression significantly limit clinical application. Emerging technologies in the fields of regenerative medicine and tissue engineering strive to address these limitations. We review recent advances in cell-based therapies, primordial allografts, bio-artificial organs and whole-organ bioengineering as they apply to renal regeneration. Collaborative efforts across these fields aim to produce a bioengineered kidney capable of restoring renal function in patients with end-stage disease.

Inhibitory action of Wnt target gene osteopontin on mitochondrial cytochrome c release determines renal ischemic resistance

Certain determinants of ischemic resistance in the Brown Norway rat strain have been proposed, but no studies to date have focused on the role of the Wnt pathway in the ischemic resistance mechanism. We performed a comparative genomic study in Brown Norway vs. Sprague-Dawley rats. Selective manipulations of the Wnt pathway in vivo and in vitro allowed us to study whether the action of the Wnt pathway on apoptosis through the regulation of osteopontin was critical to the maintenance of inherent ischemic resistance mechanisms. The results revealed a major gene upregulation of the Wnt family in Brown Norway rats after renal ischemia-reperfusion. Manipulation of the Wnt signaling cascade by selective antibodies increased mitochondrial cytochrome c release and caspase 3 activity. The antiapoptotic role of Wnt was mediated by osteopontin, a direct Wnt target gene. Osteopontin was reduced by Wnt antibody administration in vivo, and osteopontin gene silencing in vitro significantly increased mitochondrial cytochrome c release. The overexpression of Wnt pathway genes detected in Brown Norway rats is critical in the maintenance of their inherent ischemic resistance. Activation of the Wnt signaling cascade reduces mitochondrial cytochrome c release and caspase 3 activity through the action of osteopontin.

Regulation of dedifferentiation and redifferentiation in renal proximal tubular cells by the epidermal growth factor receptor

Repair of injured renal epithelium is thought to be mediated by surviving renal proximal tubular cells (RPTC) that must dedifferentiate to allow the proliferation and migration necessary for epithelial regeneration. RPTC then redifferentiate to restore tubular structure and function. Current models suggest that epidermal growth factor receptor (EGFR) activation is required for dedifferentiation characterized by enhanced vimentin expression, decreased N-cadherin expression, spindle morphology, and loss of apical-basal polarity after injury. Because an in vitro model of RPTC redifferentiation has not been reported, and the mechanism(s) of redifferentiation has not been determined, we used rabbit RPTC in primary cultures to address these issues. H2O2 induced the dedifferentiated phenotype that persisted >48 h; redifferentiation occurred spontaneously in the absence of exogenous growth factors after 72 to 120 h. Phosphorylation of two tyrosine residues of EGFR increased 12 to 24 h, peaked at 24 h, and declined to basal levels by 48 h after injury. EGFR inhibition during dedifferentiation restored epithelial morphology and apical-basal polarity, and it decreased vimentin expression to control levels 24 h later. In contrast, exogenous epidermal growth factor addition increased vimentin expression and potentiated spindle morphology. p38 mitogen-activated protein kinase (MAPK) and transforming growth factor (TGF)-beta receptor inhibitors did not affect redifferentiation after H2O2 injury. Similar results were observed in a mechanical injury model. These experiments represent a new model for the investigation of RPTC redifferentiation after acute injury and identify a key regulator of redifferentiation: EGFR, independent of p38 MAPK and the TGF-beta receptor.

Changes in gene expression in human renal proximal tubule cells exposed to low concentrations of S-(1,2-dichlorovinyl)-l-cysteine, a metabolite of trichloroethylene

Epidemiology studies suggest that there may be a weak association between high level exposure to trichloroethylene (TCE) and renal tubule cell carcinoma. Laboratory animal studies have shown an increased incidence of renal tubule carcinoma in male rats but not mice. TCE can undergo metabolism via glutathione (GSH) conjugation to form metabolites that are known to be nephrotoxic. The GSH conjugate, S-(1,2-dichlorovinyl)glutathione (DCVG), is processed further to the cysteine conjugate, S-(1,2-dichlorovinyl)-l-cysteine (DCVC), which is the penultimate nephrotoxic species. We have cultured human renal tubule cells (HRPTC) in serum-free medium under a variety of different culture conditions and observed growth, respiratory control and glucose transport over a 20 day period in medium containing low glucose. Cell death was time- and concentration-dependent, with the EC(50) for DCVG being about 3 microM and for DCVC about 7.5 microM over 10 days. Exposure of HRPTC to sub-cytotoxic doses of DCVC (0.1 microM and 1 microM for 10 days) led to a small number of changes in gene expression, as determined by transcript profiling with Affymetrix human genome chips. Using the criterion of a mean 2-fold change over control for the four samples examined, 3 genes at 0.1 microM DCVC increased, namely, adenosine kinase, zinc finger protein X-linked and an enzyme with lyase activity. At 1 microM DCVC, two genes showed a >2-fold decrease, N-acetyltransferase 8 and complement factor H. At a lower stringency (1.5-fold change), a total of 63 probe sets were altered at 0.1 microM DCVC and 45 at 1 microM DCVC. Genes associated with stress, apoptosis, cell proliferation and repair and DCVC metabolism were altered, as were a small number of genes that did not appear to be associated with the known mode of action of DCVC. Some of these genes may serve as molecular markers of TCE exposure and effects in the human kidney.

Molecular markers of trichloroethylene-induced toxicity in human kidney cells

Difficulties in evaluation of trichloroethylene (TRI)-induced toxicity in humans and extrapolation of data from laboratory animals to humans are due to the existence of multiple target organs, multiple metabolic pathways, sex-, species-, and strain-dependent differences in both metabolism and susceptibility to toxicity, and the lack or minimal amount of human data for many target organs. The use of human tissue for mechanistic studies is thus distinctly advantageous. The kidneys are one target organ for TRI and metabolism by the glutathione (GSH) conjugation pathway is responsible for nephrotoxicity. The GSH conjugate is processed further to produce the cysteine conjugate, S-(1,2-dichlorovinyl)-l-cysteine (DCVC), which is the penultimate nephrotoxic species. Confluent, primary cultures of human proximal tubular (hPT) cells were used as the model system. Although cells in log-phase growth, which are undergoing more rapid DNA synthesis, would give lower LD(50) values, confluent cells more closely mimic the in vivo proximal tubule. DCVC caused cellular necrosis only at relatively high doses (>100 muM) and long incubation times (>24 h). In contrast, both apoptosis and enhanced cellular proliferation occurred at relatively low doses (10-100 muM) and early incubation times (2-8 h). These responses were associated with prominent changes in expression of several proteins that regulate apoptosis (Bcl-2, Bax, Apaf-1, Caspase-9 cleavage, PARP cleavage) and cellular growth, differentiation and stress response (p53, Hsp27, NF-kappaB). Effects on p53 and Hsp27 implicate function of protein kinase C, the mitogen activated protein kinase pathway, and the cytoskeleton. The precise pattern of expression of these and other proteins can thus serve as molecular markers for TRI exposure and effect in human kidney.

Suramin promotes proliferation and scattering of renal epithelial cells

Primary cultures of renal proximal tubules are known to recapitulate several early events in the process of renal regeneration following injury. In this study, we show that suramin, a polysulfonated naphthylurea, stimulates outgrowth, scattering, and proliferation of primary cultures of renal proximal tubule cells (RPTC). These responses were comparable to those produced by epidermal growth factor (EGF). However, AG-1478 [4-(3'-chloroanilino)-6,7-dimethoxy-quinazoline], a specific inhibitor of the EGF receptor, blocked EGF but not suramin-induced RPTC outgrowth, scattering, and proliferation. Suramin stimulated phosphorylation of Akt, a downstream kinase of phosphoinositide 3-kinase (PI3K), extracellular signaling-regulated kinase 1/2 (ERK1/2), and Src, but not the EGF receptor. Blockade of Src, but not the EGF receptor, inhibited Akt and ERK1/2 phosphorylation. Furthermore, inactivation of PI3K with LY294002 [2-(4morpholinyl)-8-phenyl-4H-1-benzopyran-4-one] blocked suramin-induced RPTC outgrowth, scattering, and proliferation, whereas blockade of ERK1/2 had no effect. These data identify novel effects of suramin in RPTC outgrowth, scattering, and proliferation. Furthermore, suramin-induced outgrowth, scattering, and proliferation of RPTC are through Src-mediated activation of the PI3K pathway but not ERK1/2 or the EGF receptor.

Resistance to ischemic acute renal failure in the Brown Norway rat: a new model to study cytoprotection

BACKGROUND

An in vivo model of intrinsic resistance to ischemia could be invaluable to define how specific pathways to injury or putative protectors from injury affect the severity of acute renal failure (ARF). The purpose of this study was to determine whether separate rat strains had differential sensitivity to renal ischemia, characterize the extent of protection, and begin to define differences in gene expression that might impact on the severity of ARF.

METHODS

The sensitivity to 45 minutes of renal ischemia in Sprague-Dawley rat (SD) was compared with 2 lines of Brown-Norway rats (BN/Mcw, BN/Hsd). Constitutive and inducible stress protein expression was compared between strains.

RESULTS

At 24 hours' reperfusion, SD rats had higher creatinine (3.4 mg/dL), elevated Na and water excretion, and proximal tubule necrosis. Both strains of BN rats were resistant to loss of renal function (Scr = 0.9 mg/dL at 24 hours' reflow) and had preserved renal morphology. BN rats had no redistribution of Na,K-ATPase into detergent-soluble cortical extracts found early (15 minutes) after ischemia in SD rats. Hsc73 expression did not differ between strains and was not induced by ischemia. Compared with SD, induction of Hsp25 and 72 by renal ischemia was blunted in both BN strains. Constitutive Hsp25 was higher in both BN-Mcw and BN-Hsd compared with SD rat kidney. Constitutive Hsp72 was significantly higher only in BN-Mcw kidneys. Immunohistochemistry showed baseline Hsp72 and 25 expression was increased in proximal tubules of BN-Mcw versus SD.

CONCLUSION

BN rat kidney is resistant to ischemic injury and provides a new model for studying cytoprotective mechanisms. Initial study of strain-specific gene expression suggests particular stress proteins are among the potential mechanisms contributing to protection against ARF.

Protein kinase C-epsilon modulates mitochondrial function and active Na+ transport after oxidant injury in renal cells

The aim of this study was to determine whether protein kinase C-epsilon (PKC-epsilon) is involved in the repair of mitochondrial function and/or active Na+ transport after oxidant injury in renal proximal tubular cells (RPTC). Sublethal injury was produced in primary cultures of RPTC using tert-butylhydroperoxide (TBHP), and the recovery of functions was examined. PKC-epsilon was activated three- to fivefold after injury. Active PKC-epsilon translocated to the mitochondria. Basal oxygen consumption (Qo2), uncoupled Qo2, and ATP production decreased 58, 60, and 41%, respectively, at 4 h and recovered by day 4 after injury. At 4 h, complex I-coupled respiration decreased 50% but complex II- and IV-coupled respirations were unchanged. Inhibition of PKC-epsilon translocation using a peptide selective inhibitor, PKC-epsilonV1-2, reduced decreases in basal and uncoupled Qo2 values and increased complex I-linked respiration in TBHP-injured RPTC at 4 h of recovery. Furthermore, PKC-epsilonV1-2 prevented decreases in ATP production in injured RPTC. Na+-K+-ATPase activity and ouabain-sensitive 86Rb+ uptake were decreased by 60 and 53%, respectively, at 4 h of recovery. Inhibition of PKC-epsilon activation prevented a decline in Na+-K+-ATPase activity and reduced decreases in ouabain-sensitive 86Rb+ uptake. We conclude that during early repair after oxidant injury in RPTC 1) PKC-epsilon is activated and translocated to mitochondria; 2) PKC-epsilon activation decreases mitochondrial respiration, electron transport rate, and ATP production by reducing complex I-linked respiration; and 3) PKC-epsilon mediates decreases in active Na+ transport and Na+-K+-ATPase activity. These data show that PKC-epsilon activation after oxidant injury in RPTC is involved in the decreases in mitochondrial function and active Na+ transport and that inhibition of PKC-epsilon activation promotes the repair of these functions.

Protein kinase C mediates repair of mitochondrial and transport functions after toxicant-induced injury in renal cells

Previously, we have shown that renal proximal tubular cells (RPTCs) recover physiological functions after injury induced by the oxidant tert-butylhydroperoxide (TBHP), but not by the nephrotoxic cysteine conjugate dichlorovinyl-l-cysteine (DCVC). This study examined the role of protein kinase C (PKC) in the repair of RPTC functions after sublethal injury produced by these toxicants. Total PKC activity decreased 65 and 86% after TBHP and DCVC exposures, respectively, and recovered in TBHP-injured but not in DCVC-injured RPTCs. Mitochondrial function, active Na+ transport, and Na+-dependent glucose uptake decreased after toxicant exposure and recovered in TBHP- but not in DCVC-injured RPTCs. PKC inhibition decreased the repair of RPTC functions after TBHP injury. PKC activation promoted recovery of mitochondrial function and active Na+ transport in TBHP- and DCVC-injured RPTCs but had no effect on recovery of Na+-dependent glucose uptake. We conclude that in RPTCs, 1) total PKC activity decreases after TBHP and DCVC injury and recovers after TBHP but not after DCVC exposure, 2) recovery of PKC activity precedes the return of physiological functions after oxidant injury, 3) PKC inhibition decreases recovery of physiological functions, and 4) PKC activation promotes recovery of mitochondrial function and active Na+ transport but not Na+-dependent glucose uptake. These results suggest that the repair of renal functions is mediated through PKC-dependent mechanisms and that cysteine conjugates may inhibit renal repair, in part, through inhibition of PKC signaling.

Mechanisms of renal cell repair and regeneration after acute renal failure

In many cases, acute renal failure (ARF) is the result of proximal tubular cell injury and death and can arise in a variety of clinical situations, especially following renal ischemia and drug or toxicant exposure. Although much research has focused on the cellular events leading to ARF, less emphasis has been placed on the mechanisms of renal cell repair and regeneration, although ARF is reversed in over half of those who acquire it. Studies using in vivo and in vitro models have demonstrated the importance of proliferation, migration, and repair of physiological functions of injured renal proximal tubular cells (RPTC) in the reversal of ARF. Growth factors have been shown to produce migration and proliferation of injured RPTC, although the specific mechanisms through which growth factors promote renal regeneration in vivo are unclear. Recently, interactions between integrins and extracellular matrix proteins such as collagen IV were shown to promote the repair of physiological functions in injured RPTC. Specifically, collagen IV synthesis and deposition following cellular injury restored integrin polarity and promoted repair of mitochondrial function and active Na(+) transport. Furthermore, exogenous collagen IV, but not collagen I, fibronectin, or laminin, promoted the repair of physiological functions without stimulating proliferation. These findings suggest the importance of establishing and/or maintaining collagen IV-integrin interactions in the stimulation of repair of physiological functions following sublethal cellular injury. Furthermore, the pathway that stimulates repair is distinct from that of proliferation and migration and may be a viable target for pharmacological intervention.

Ascorbic acid promotes recovery of cellular functions following toxicant-induced injury

We have shown that renal proximal tubular cells (RPTC) recover cellular functions following sublethal injury induced by the oxidant t-butylhydroperoxide but not by the nephrotoxic cysteine conjugate dichlorovinyl-L-cysteine (DCVC). This study investigated whether L-ascorbic acid phosphate (AscP) promotes recovery of RPTC functions following DCVC-induced injury. DCVC exposure (200 microM; 100 min) resulted in 60% RPTC death and loss from the monolayer at 24 h independent of physiological (50 microM) or pharmacological (500 microM) AscP concentrations. Likewise, the DCVC-induced decrease in mitochondrial function (54%), active Na(+) transport (66%), and Na(+)-K(+)-ATPase activity (77%) was independent of the AscP concentration. Analysis of Na(+)-K(+)-ATPase protein expression and distribution in the plasma membrane using immunocytochemistry and confocal laser scanning microscopy revealed the loss of Na(+)-K(+)-ATPase protein from the basolateral membrane of RPTC treated with DCVC. DCVC-injured RPTC cultured in the presence of 50 microM AscP did not proliferate nor recover their physiological functions over time. In contrast, RPTC cultured in the presence of 500 microM AscP proliferated, recovered all examined physiological functions, and the basolateral membrane expression of Na(+)-K(+)-ATPase by day 4 following DCVC injury. These results demonstrate that pharmacological concentrations of AscP do not prevent toxicant-induced cell injury and death but promote complete recovery of mitochondrial function, active Na(+) transport, and proliferation following toxicant-induced injury.

Differential effects of EGF on repair of cellular functions after dichlorovinyl-L-cysteine-induced injury

This study examined the repair of renal proximal tubule cellular (RPTC) functions following sublethal injury induced by the nephrotoxicant S-(1,2-dichlorovinyl)-L-cysteine (DCVC). DCVC exposure resulted in 31% cell death and loss 24 h following the treatment. Monolayer confluence recovered through migration/spreading but not proliferation after 6 days. Basal, uncoupled, and ouabain-sensitive oxygen consumption (QO2) decreased 47, 76, and 62%, respectively, 24 h after DCVC exposure. Na+-K+-ATPase activity and Na+-dependent glucose uptake were inhibited 80 and 68%, respectively, 24 h after DCVC exposure. None of these functions recovered over time. Addition of epidermal growth factor (EGF) following DCVC exposure did not prevent decreases in basal, uncoupled, and ouabain-sensitive QO2 values and Na+-K+-ATPase activity but promoted their recovery over 4-6 days. In contrast, no recovery of Na+-dependent glucose uptake occurred in the presence of EGF. These data show that: 1) DCVC exposure decreases mitochondrial function, Na+-K+-ATPase activity, active Na+ transport, and Na+-dependent glucose uptake in sublethally injured RPTC; 2) DCVC-treated RPTC do not proliferate nor regain their physiological functions in this model; and 3) EGF promotes recovery of mitochondrial function and active Na+ transport but not Na+-dependent glucose uptake. These results suggest that cysteine conjugates may cause renal dysfunction, in part, by decreasing RPTC functions and inhibiting their repair.

Recovery of cellular functions following oxidant injury

This study investigated the recovery of renal proximal tubule cellular (RPTC) functions following oxidant-induced sublethal injury. tert-Butylhydroperoxide (TBHP) treatment resulted in 24% cell death and loss 4 h following the exposure. The remaining sublethally injured RPTC proliferated, and monolayer DNA content returned to control values on day 4 following TBHP exposure. Basal oxygen consumption (Qo2) and ATP content in sublethally injured RPTC were decreased 64 and 63%, respectively, at 4 h and returned to control values on day 6. Net lactate consumption decreased 71% at 4 h and returned to control values on day 4. In contrast, net glutamine consumption increased 2.7-fold at 4 h and returned to control values on day 6. Ouabain-sensitive Qo2, Na(+)-K(+)-adenosinetriphosphatase (Na(+)-K(+)-ATPase) activity, and Na(+)-coupled glucose transport were inhibited 77, 88, and 83%, respectively, at 4 h and recovered to control values on day 6. These data show that 1) mitochondrial function, Na(+)-K(+)-ATPase activity, active Na+ transport, and Na(+)-coupled glucose transport are decreased in sublethally injured RPTC following oxidant exposure and are repaired over time; 2) monolayer regeneration precedes the recovery of mitochondrial and transport functions, and 3) sublethal injury and subsequent regeneration are associated with alterations in metabolic substrate utilization. These results suggest that oxidant-induced sublethal injury to RPTC may contribute to renal dysfunction and that RPTC can repair and regain cellular functions following oxidant injury.