Oxalate-induced ceramide accumulation in Madin-Darby canine kidney and LLC-PK1 cells

Dynamics of differentiated-renal epithelial cell monolayer after calcium oxalate injury: The role of cyclooxygenase-2

AIMS

Calcium oxalate (Oxa), constituent of most common kidney stones, damages renal tubular epithelial cells leading to kidney disease. Most in vitro studies designed to evaluate how Oxa exerts its harmful effects were performed in proliferative or confluent non-differentiated renal epithelial cultures; none of them considered physiological hyperosmolarity of renal medullary interstitium. Cyclooxygenase 2 (COX2) has been associated to Oxa deleterious actions; however, up to now, it is not clear how COX2 acts. In this work, we proposed an in vitro experimental system resembling renal differentiated-epithelial cells that compose medullary tubular structures which were grown and maintained in a physiological hyperosmolar environment and evaluated whether COX2 → PGE2 axis (COX2 considered a cytoprotective protein for renal cells) induces Oxa damage or epithelial restitution.

MAIN METHODS

MDCK cells were differentiated with NaCl hyperosmolar medium for 72 h where cells acquired the typical apical and basolateral membrane domains and a primary cilium. Then, cultures were treated with 1.5 mM Oxa for 24, 48, and 72 h to evaluate epithelial monolayer restitution dynamics and COX2-PGE2 effect.

KEY FINDINGS

Oxa completely turned the differentiated phenotype into mesenchymal one (epithelial-mesenchymal transition). Such effect was partially and totally reverted after 48 and 72 h, respectively. Oxa damage was even deeper when COX2 was blocked by NS398. PGE2 addition restituted the differentiated-epithelial phenotype in a time and concentration dependence.

SIGNIFICANCE

This work presents an experimental system that approaches in vitro to in vivo renal epithelial studies and, more important, warns about NSAIDS use in patients suffering from kidney stones.

Ceramides and Acute Kidney Injury

Altered lipid metabolism is a characteristic feature and potential driving factor of acute kidney injury (AKI). Of the lipids that accumulate in injured renal tissues, ceramides are potent regulators of metabolism and cell fate. Up-regulation of ceramide synthesis is a common feature shared across several AKI etiologies in vitro and in vivo. Furthermore, ceramide accumulation is an early event in the natural history of AKI that precedes cell death and organ dysfunction. Emerging evidence suggests that inhibition of ceramide accumulation may improve renal outcomes in several models of AKI. This review examines the landscape of ceramide metabolism and regulation in the healthy and injured kidney. Furthermore, we discuss the body of literature regarding ceramides as therapeutic targets for AKI and consider potential mechanisms by which ceramides drive kidney pathogenesis.

A Rheostat of Ceramide and Sphingosine-1-Phosphate as a Determinant of Oxidative Stress-Mediated Kidney Injury

Reactive oxygen species (ROS) modulate sphingolipid metabolism, including enzymes that generate ceramide and sphingosine-1-phosphate (S1P), and a ROS-antioxidant rheostat determines the metabolism of ceramide-S1P. ROS induce ceramide production by activating ceramide-producing enzymes, leading to apoptosis, while they inhibit S1P production, which promotes survival by suppressing sphingosine kinases (SphKs). A ceramide-S1P rheostat regulates ROS-induced mitochondrial dysfunction, apoptotic/anti-apoptotic Bcl-2 family proteins and signaling pathways, leading to apoptosis, survival, cell proliferation, inflammation and fibrosis in the kidney. Ceramide inhibits the mitochondrial respiration chain and induces ceramide channel formation and the closure of voltage-dependent anion channels, leading to mitochondrial dysfunction, altered Bcl-2 family protein expression, ROS generation and disturbed calcium homeostasis. This activates ceramide-induced signaling pathways, leading to apoptosis. These events are mitigated by S1P/S1P receptors (S1PRs) that restore mitochondrial function and activate signaling pathways. SphK1 promotes survival and cell proliferation and inhibits inflammation, while SphK2 has the opposite effect. However, both SphK1 and SphK2 promote fibrosis. Thus, a ceramide-SphKs/S1P rheostat modulates oxidant-induced kidney injury by affecting mitochondrial function, ROS production, Bcl-2 family proteins, calcium homeostasis and their downstream signaling pathways. This review will summarize the current evidence for a role of interaction between ROS-antioxidants and ceramide-SphKs/S1P and of a ceramide-SphKs/S1P rheostat in the regulation of oxidative stress-mediated kidney diseases.

Trimethylamine N-oxide promotes hyperoxaluria-induced calcium oxalate deposition and kidney injury by activating autophagy

Calcium oxalate (CaOx) is the most common component of kidney stones. Oxidative stress, inflammation and autophagy-induced cell death are the major causes of CaOx crystal deposition and CaOx crystal deposition can further lead to kidney injury. Trimethylamine N-oxide (TMAO), a gut microbiota-derived metabolite, plays an important role in the pathogenesis of many diseases, such as atherosclerosis, diabetes and chronic kidney disease, but the effect of TMAO on hyperoxaluria-induced CaOx crystal deposition and kidney injury remains unknown. We hypothesize that TMAO aggravates CaOx crystal deposition via promoting CaOx-mediated cell death. C57Bl/6 mice were given high-oxalate diet as a model of hyperoxaluria. TMAO was provided via drinking water. Serum TMAO levels increased 15 days after CaOx treatment (6.30 ± 0.17 μmol/L vs. 34.65 ± 8.95 μmol/L). High-oxalate diet induced inflammation, CaOx deposition and kidney injury, which TMAO aggravated. In accordance, TMAO intensified high-oxalate diet induced oxidative stress, autophagy and apoptosis. Moreover, TMAO enhanced CaOx crystal adhesion to HK-2 cells and reduced cell viability (from 88.9 ± 1.6% to 75.0 ± 2.7%). Protein kinase R-like endoplasmic reticulum kinase (PERK) may mediate these TMAO effects, as TMAO promoted PERK phosphorylation. Consistently, PERK knockdown alleviated TMAO-evoked CaOx-autophagy, apoptosis and oxidative stress in HK-2 cells. In conclusion, TMAO can aggravate hyperoxaluria-induced kidney injury by triggering the PERK/ROS pathway, which enhances autophagy, apoptosis and inflammation, and facilitates CaOx crystal deposition in renal tubular cells.

Opportunities for future therapeutic interventions for hyperoxaluria: targeting oxidative stress

INTRODUCTION

Oxalate is a toxic byproduct of metabolism and is normally produced in quantities easily removed from the body. However, under specific circumstances oxalate production is increased resulting in deposition of calcium oxalate (CaOx) crystals in the kidneys as well as other organs causing inflammation and injury. Excessive buildup of crystal deposits in the kidneys causes eventual loss of renal function requiring renal transplantation. Areas covered: Cellular exposure to CaOx crystals induces the production of reactive oxygen species (ROS) with the involvement of renin-angiotensin aldosterone system (RAAS), mitochondria, and NADPH oxidase. Inflammasomes are activated and pro-inflammatory cytokines, such as IL-1β and IL-18 are produced. We reviewed results of experimental and clinical studies of crystal renal epithelial cell interactions with emphasis on cellular injury and ROS production. Expert opinion: Treatment should depend upon the level of hyperoxaluria and whether it is associated with CaOx crystal deposition. Persistent low grade or intermittent hyperoxaluria can be treated with antioxidants, free radical scavengers. Hyperoxaluria associated with CaOx crystal deposition will require administration of angiotensin II receptor blockers, and NADPH oxidase or NLRP3 inflammasome inhibitors. DASH-style diet will be beneficial in both cases.

Downregulated Expression of Solute Carrier Family 26 Member 6 in NRK-52E Cells Attenuates Oxalate-Induced Intracellular Oxidative Stress

Solute carrier family 26 member 6 (Slc26a6), which is mainly expressed in the intestines and kidneys, is a multifunctional anion transporter that is crucial in the transport of oxalate anions. This study is aimed at investigating the effect of Slc26a6 expression on oxalate-induced cell oxidation and crystal formation. Lentivirus transfection was used to upregulate or downregulate Slc26a6 expression in NRK cells. Cell viability and apoptosis, reactive oxygen species (ROS) and malondialdehyde (MDA) generation, and superoxide dismutase (SOD) activity were measured. Crystal adhesion and the cell ultrastructure were observed using light and transmission electron microscopy (TEM). Three groups of rats, normal control, lentivirus-vector, and lentivirus-small interfering RNA (lv-siRNA) groups, were used, and after lentivirus transfection, they were fed 1% ethylene glycol (EG) and 0.5% ammonium chloride (NH₄Cl) for 2 weeks. Dihydroethidium (DHE), terminal deoxynucleotidyl transferase (TdT) deoxyuridine dUTP nick-end labeling (TUNEL), and von Kossa staining were performed, and nuclear factor κB (NFκB) and osteopontin (OPN) expression were measured. In the vitro study, compared to the control group, downregulated Slc26a6 NRK cells showed alleviation of the cell viability decrease, cell apoptosis rate, ROS generation, and SOD activity decrease after oxalate treatment. Crystal adhesion and vesicles were significantly less after oxalate exposure than in the untreated controls. Rats infected with lentivirus-siRNA exhibited attenuated SOD generation, cell apoptosis, and crystal formation in the kidneys. Increased phosphorylation of NFκB and OPN was involved in the pathological process. In conclusion, the results of the present study indicate that reducing the expression of Slc26a6 in the kidney may be a potential strategy for preventing stone formation.

Renal Allograft With Calcium Oxalate Deposition: Association with Urinary Tract Infection and Development of Interstitial Fibrosis

OBJECTIVES

The interaction between calcium oxalate deposition and urinary tract infection is not well established. We aimed to identify the association between these and to determine the role of calcium oxalate deposition on interstitial fibrosis development.

MATERIALS AND METHODS

Renal allograft biopsies of 967 patients were reviewed to identify those with calcium oxalate deposition in the renal allograft, with 27 (2.8%) identified. Follow-up biopsies were conducted to reevaluate for calcium oxalate presence and interstitial fibrosis development. At time of biopsy, presence of urinary tract infection and oxaluria was also examined from medical records.

RESULTS

Mean time for development of calcium oxalate deposition in renal allografts was 1.7 ± 0.4 and 32.7 ± 21.6 months in patients with primary and secondary oxalosis, respectively (P < .001). Of 27 patients with calcium oxalate deposition, 7 (25.9%) showed tubulointerstitial nephritis, with 2 also having urinary tract infection. Four patients (14.8%) had only urinary tract infection. Causes of tubulointerstitial nephritis were secondary to bacterial infection in 2 and secondary to viral infection in 5 patients (2 polyomaviruses, 2 cytomegaloviruses, 1 adenovirus). Time until development of interstitial fibrosis after calcium oxalate deposition was 3.5 ± 2.1 and 10.3 ± 4.1 months in patients with primary and secondary oxalosis, respectively (P = .01). Time until graft loss after calcium oxalate deposition was 9.3 ± 7.8 and 21.8 ± 12 months in those with primary and secondary oxalosis (P < .001), with 1-, 3-, and 5-year kidney graft survival of 43%, 28%, and 0% and 100%, 100%, and 67% in those with primary and secondary oxalosis, respectively.

CONCLUSIONS

Calcium oxalate deposits increased the risk of urinary tract infection and tubulointerstitial nephritis, with bacteria inducing increased presence of calcium oxalate deposition in a renal allograft. Calcium oxalate deposition had a significant influence on interstitial fibrosis development, therefore negatively affecting graft survival.

Cytoprotective and anti-apoptotic role of Terminalia arjuna on oxalate injured renal epithelial cells

Urolithiasis is one of the painful multifactorial disorders caused by metabolic abnormalities influencing the composition of body fluids and urine. The bark of Terminalia arjuna (T. arjuna), very well known in Ayurveda for the treatment of cardiovascular diseases, possesses antioxidant and diuretic activity. The present study was undertaken to investigate the antiurolithiatic efficacy of aqueous extract of bark of T. arjuna on oxalate-induced injury to renal tubular epithelial cells. Madin-Darby canine kidney (MDCK) cells were exposed to 2 mM oxalate for 48 h to evaluate the protective effect of T. arjuna aqueous extract on cell viability, CaOx crystal adherence and apoptotic changes caused by oxalate. The results confirmed that oxalate injured MDCK cells were protected by T. arjuna extract. On treatment with a range concentrations, the cell viability increased in a concentration dependent manner. Moreover, the extract prevented the interaction of the calcium oxalate (CaOx) crystals with the cell surface and reduced the number of apoptotic cells. The current data suggests that T. arjuna bark confers a cytoprotective role and based on our results it could be a potential candidate from natural plant sources against urolithiasis.

In vitro studies reveal antiurolithic effect of Terminalia arjuna using quantitative morphological information from computerized microscopy

PURPOSE

For most cases, urolithiasis is a condition where excessive oxalate is present in the urine. Many reports have documented free radical generation followed by hyperoxaluria as a consequence of which calcium oxalate (CaOx) deposition occurs in the kidney tissue. The present study is aimed to exam the antilithiatic potency of the aqueous extract (AE) of Terminalia arjuna (T. arjuna).

MATERIALS AND METHODS

The antilithiatic activity of Terminalia arjuna was investigated in vitro nucleation, aggregation and growth of the CaOx crystals as well as the morphology of CaOx crystals using the inbuilt software 'Image-Pro Plus 7.0' of Olympus upright microscope (BX53). Antioxidant activity of AE of Terminalia arjuna bark was also determined in vitro.

RESULTS

Terminalia arjuna extract exhibited a concentration dependent inhibition of nucleation and aggregation of CaOx crystals. The AE of Terminalia arjuna bark also inhibited the growth of CaOx crystals. At the same time, the AE also modified the morphology of CaOx crystals from hexagonal to spherical shape with increasing concentrations of AE and reduced the dimensions such as area, perimeter, length and width of CaOx crystals in a dose dependent manner. Also, the Terminalia arjuna AE scavenged the DPPH (2, 2-diphenyl-1-picrylhydrazyl) radicals with an IC50 at 13.1µg/mL.

CONCLUSIONS

The study suggests that Terminalia arjuna bark has the potential to scavenge DPPH radicals and inhibit CaOx crystallization in vitro. In the light of these studies, Terminalia arjuna can be regarded as a promising candidate from natural plant sources of antilithiatic and antioxidant activity with high value.

Ceramide-induced apoptosis in renal tubular cells: a role of mitochondria and sphingosine-1-phoshate

Ceramide is synthesized upon stimuli, and induces apoptosis in renal tubular cells (RTCs). Sphingosine-1 phosphate (S1P) functions as a survival factor. Thus, the balance of ceramide/S1P determines ceramide-induced apoptosis. Mitochondria play a key role for ceramide-induced apoptosis by altered mitochondrial outer membrane permeability (MOMP). Ceramide enhances oligomerization of pro-apoptotic Bcl-2 family proteins, ceramide channel, and reduces anti-apoptotic Bcl-2 proteins in the MOM. This process alters MOMP, resulting in generation of reactive oxygen species (ROS), cytochrome C release into the cytosol, caspase activation, and apoptosis. Ceramide regulates apoptosis through mitogen-activated protein kinases (MAPKs)-dependent and -independent pathways. Conversely, MAPKs alter ceramide generation by regulating the enzymes involving ceramide metabolism, affecting ceramide-induced apoptosis. Crosstalk between Bcl-2 family proteins, ROS, and many signaling pathways regulates ceramide-induced apoptosis. Growth factors rescue ceramide-induced apoptosis by regulating the enzymes involving ceramide metabolism, S1P, and signaling pathways including MAPKs. This article reviews evidence supporting a role of ceramide for apoptosis and discusses a role of mitochondria, including MOMP, Bcl-2 family proteins, ROS, and signaling pathways, and crosstalk between these factors in the regulation of ceramide-induced apoptosis of RTCs. A balancing role between ceramide and S1P and the strategy for preventing ceramide-induced apoptosis by growth factors are also discussed.

NADPH oxidase as a therapeutic target for oxalate induced injury in kidneys

A major role of the nicotinamide adenine dinucleotide phosphate (NADPH) oxidase family of enzymes is to catalyze the production of superoxides and other reactive oxygen species (ROS). These ROS, in turn, play a key role as messengers in cell signal transduction and cell cycling, but when they are produced in excess they can lead to oxidative stress (OS). Oxidative stress in the kidneys is now considered a major cause of renal injury and inflammation, giving rise to a variety of pathological disorders. In this review, we discuss the putative role of oxalate in producing oxidative stress via the production of reactive oxygen species by isoforms of NADPH oxidases expressed in different cellular locations of the kidneys. Most renal cells produce ROS, and recent data indicate a direct correlation between upregulated gene expressions of NADPH oxidase, ROS, and inflammation. Renal tissue expression of multiple NADPH oxidase isoforms most likely will impact the future use of different antioxidants and NADPH oxidase inhibitors to minimize OS and renal tissue injury in hyperoxaluria-induced kidney stone disease.

Are calcium oxalate crystals involved in the mechanism of acute renal failure in ethylene glycol poisoning?

INTRODUCTION

Ethylene glycol (EG) poisoning often results in acute renal failure, particularly if treatment with fomepizole or ethanol is delayed because of late presentation or diagnosis. The mechanism has not been established but is thought to result from the production of a toxic metabolite.

METHODS

A literature review utilizing PubMed identified papers dealing with renal toxicity and EG or oxalate. The list of papers was culled to those relevant to the mechanism and treatment of the renal toxicity associated with either compound. ROLE OF METABOLITES: Although the "aldehyde" metabolites of EG, glycolaldehyde, and glyoxalate, have been suggested as the metabolites responsible, recent studies have shown definitively that the accumulation of calcium oxalate monohydrate (COM) crystals in kidney tissue produces renal tubular necrosis that leads to kidney failure. In vivo studies in EG-dosed rats have correlated the severity of renal damage with the total accumulation of COM crystals in kidney tissue. Studies in cultured kidney cells, including human proximal tubule (HPT) cells, have demonstrated that only COM crystals, not the oxalate ion, glycolaldehyde, or glyoxylate, produce a necrotic cell death at toxicologically relevant concentrations. COM CRYSTAL ACCUMULATION: In EG poisoning, COM crystals accumulate to high concentrations in the kidney through a process involving adherence to tubular cell membranes, followed by internalization of the crystals. MECHANISM OF TOXICITY: COM crystals have been shown to alter membrane structure and function, to increase reactive oxygen species and to produce mitochondrial dysfunction. These processes are likely to be involved in the mechanism of cell death.

CONCLUSIONS

Accumulation of COM crystals in the kidney is responsible for producing the renal toxicity associated with EG poisoning. The development of a pharmacological approach to reduce COM crystal adherence to tubular cells and its cellular interactions would be valuable as this would decrease the renal toxicity not only in late treated cases of EG poisoning, but also in other hyperoxaluric diseases such as primary hyperoxaluria and kidney stone formation.

Lindane-induced generation of reactive oxygen species and depletion of glutathione do not result in necrosis in renal distal tubule cells

Lindane is a chlorinated hydrocarbon pesticide, currently used in prescription shampoos and lotions to treat scabies and lice infestations. Lindane is known to be nephrotoxic; however, the mechanism of action is not well understood. In other organ systems, lindane produces cellular damage by generation of free radicals and oxidative stress. Morphological changes were observed in lindane-treated Madin-Darby canine kidney (MDCK) cells indicative of apoptosis. Lindane treatment induced time-dependent reactive oxygen species (ROS) generation. Onset of ROS generation correlated with an initial increase in total glutathione (GSH) levels above control values, with a subsequent decline in a time-dependent manner. This decline may be attributed to quenching of free radicals by GSH, thereby decreasing the cellular stores of this antioxidant. Necrotic injury was assessed by measuring lactate dehydrogenase (LDH) leakage from the cell after lindane exposure. No significant LDH leakage was noted for all concentrations tested over time. Generation of ROS and alterations in cellular protective mechanisms did not result in necrotic injury in MDCK cells, which corresponds with our morphological findings of lindane-induced apoptotic changes as opposed to necrosis in MDCK cells. Thus, lindane exposure results in oxidative damage and alterations in antioxidant response in renal distal tubule cells, followed by cell death not attributed to necrotic injury.

Oxidative stress, lipid rafts, and macrophage reprogramming

Oxidant stress, induced under a variety of conditions, is known to lead to the molecular reprogramming of the tissue-fixed macrophage. This reprogramming is associated with an altered response to subsequent inflammatory stimuli, such as lipopolysaccharide (LPS), leading to enhanced liberation of proinflammatory chemokines and cytokines. Due to this altered response, dysregulated immunity ensues, leading to the development of clinical syndromes such as multiple organ dysfunction syndrome (MODS). Although the mechanisms responsible for this altered macrophage activity by oxidant stress remains complex and poorly elucidated, it appears, based on recent research, that early and direct alterations within lipid rafts are responsible. This early and direct interaction with lipid rafts by oxidants leads to the mobilization of annexin VI from lipid raft constructs, leading to the release of calcium. This increased cytosolic concentration of this secondary messenger, in turn, results in the activation of calcium-dependent kinases, leading to further alterations in lipid raft lipids and eventually lipid raft proteins. Due to these lipid raft compositional changes, preassembly of receptor complexes occur, leading to enhanced proinflammatory activation. Within this review, the complexity of oxidant-induced reprogramming within the tissue fixed macrophage as currently understood is explained.

Urinary macromolecules and renal tubular cell protection from oxalate injury: Comparison of normal subjects and recurrent stone formers

AIM

To determine whether urinary macromolecules (UMM), which are the high molecular weight substances in urine, can provide protection against the oxalate-associated injury to the renal tubular cells.

METHODS

UMM were extracted from 24-h urine of 12 healthy adult male volunteers and 13 recurrent-stone-former male patients. Urine parameters in relation to urolithiasis were measured, including the level of glycosaminoglycans (GAG) in the UMM. Madin-Darby canine kidney (MDCK) cells were used to evaluate the protective activity of UMM from oxalate-induced cytotoxicity by LDH release measurement and methyl-thiazolyl tertrazolium (MTT) assay.

RESULTS

Considering urinary parameters, citrate was significantly higher in urine from normal subjects than stone-former subjects; the other parameters show no differences between the groups. Total UMM and the level of GAG in the UMM were also significantly higher in the normal subject group. Compared with normal subject and stone-former subject UMM, after cells were treated with the UMM and then exposed to oxalate solution, LDH release was significantly higher in stone-former group. In the MTT assay, we found that more viable cells were observed after treatment with UMM compared to control in both groups. Moreover, UMM from the normal subjects showed higher protective activity against oxalate-related cytotoxicity than UMM from the stone-former subjects.

CONCLUSION

UMM protected renal epithelial cells from oxalate-related injury. This protective activity was found to be higher in normal subject UMM than stone-former UMM. Among other factors, a higher concentration of GAG and citrate in normal subject UMM might affect some parts in this finding.

Intracellular events in the initiation of calcium oxalate stones

This review summarizes our current understanding of intracellular events in the initiation of kidney stone formation, focusing on results from studies using renal epithelial cells in vitro. Such studies have shown that oxalate - either in crystalline or in soluble form - triggers a spectrum of responses in renal cells that favor stone formation, including alterations in membrane surface properties that promote crystal attachment and alterations in cell viability that provide debris for crystal nucleation. Activation of cytosolic PLA2 appears to play an important role in oxalate actions, triggering a signaling cascade that generates several lipid mediators (arachidonic acid, AA; lysophosphatidylcholine, Lyso-PC; ceramide) that act on key intracellular targets (mitochondria, nucleus). The net effect is increased production of reactive oxygen molecules (that in turn affect other cellular processes), an increase in cell death and an induction of a number of genes in surviving cells, some of which may promote proliferation for replacement of damaged cells, or may promote secretion of urinary macromolecules that serve to modulate crystal formation. A scheme is provided that explains how such oxalate-induced alterations could initiate stone formation in vivo.

Hyperoxaluria-induced oxidative stress and antioxidants for renal protection

Renal cellular exposure to oxalate (Ox) and/or CaOx crystals leads to the production of reactive oxygen species (ROS), development of oxidative stress followed by injury and inflammation. Renal injury and inflammation appear to play a significant role in stone formation. ROS are produced from many sources and involve a variety of signaling pathways. Tissue culture and animal model studies show that treatments with anti-oxidants and free radical scavengers reduce Ox/CaOx crystal induced injuries. In addition, CaOx crystal deposition in kidneys is significantly reduced by treatments with antioxidants and free radical scavengers, indicating their efficacy. These results point towards a great potential for the therapeutic application of antioxidants and free radical scavengers to reduce stone recurrence particularly after shock wave lithotripsy, which is itself known to generate ROS and cause renal damage.

Oxalate toxicity in renal cells

Exposure to oxalate, a constituent of the most common form of kidney stones, generates toxic responses in renal epithelial cells, including altered membrane surface properties and cellular lipids, changes in gene expression, disruption of mitochondrial function, formation of reactive oxygen species and decreased cell viability. Oxalate exposure activates phospholipase A2 (PLA2), which increases two lipid signaling molecules, arachidonic acid and lysophosphatidylcholine (Lyso-PC). PLA2 inhibition blocks, whereas exogenous Lyso-PC or arachidonic acid reproduce many of the effects of oxalate on mitochondrial function, gene expression and cell viability, suggesting that PLA2 activation plays a role in mediating oxalate toxicity. Oxalate exposure also elicits potentially adaptive or protective changes that increase expression of proteins that may prevent crystal formation or attachment. Additional adaptive responses may facilitate removal and replacement of dead or damaged cells. The presence of different inflammatory cells and molecules in the kidneys of rats with hyperoxaluria and in stone patients suggests that inflammatory responses play roles in stone disease. Renal epithelial cells can synthesize a variety of cytokines, chemoattractants and other molecules with the potential to interface with inflammatory cells; moreover, oxalate exposure increases the synthesis of these molecules. The present studies demonstrate that oxalate exposure upregulates cyclooxygenase-2, which catalyzes the rate-limiting step in the synthesis of prostanoids, compounds derived from arachidonic acid that can modify crystal binding and may also influence inflammation. In addition, renal cell oxalate exposure promotes rapid degradation of IkappaBalpha, an endogenous inhibitor of the NF-kappaB transcription factor. A similar response is observed following renal cell exposure to lipopolysaccharide (LPS), a bacterial cell wall component that activates toll-like receptor 4 (TLR4). While TLRs are primarily associated with immune cells, they are also found on many other cell types, including renal epithelial cells, suggesting that TLR signaling could directly impact renal function. Prior exposure of renal epithelial cells to oxalate in vitro produces endotoxin tolerance, i.e. a loss of responsiveness to LPS and conversely, prior exposure to LPS elicits a similar heterologous desensitization to oxalate. Renal cell desensitization to oxalate stimulation may have profound effects on the outcome of renal stone disease by impairing protective responses.

Cytoplasmic Phospholipase A2Deletion Enhances Colon Tumorigenesis

Cellular pools of free arachidonic acid are tightly controlled through enzymatic release of the fatty acid and subsequent utilization by downstream enzymes including the cyclooxygenases. Arachidonic acid cleavage from membrane phospholipids is accomplished by the actions of phospholipase A(2) (PLA(2)). Upon release, free arachidonic acid provides substrate for the synthesis of eicosanoids. However, under certain conditions, arachidonic acid may participate in ceramide-mediated apoptosis. Disruption of arachidonic acid homeostasis can shift the balance of cell turnover in favor of tumorigenesis, via overproduction of tumor-promoting eicosanoids or alternatively by limiting proapoptotic signals. In the following study, we evaluated the influence of genetic deletion of a key intracellular phospholipase, cytoplasmic PLA(2) (cPLA(2)), on azoxymethane-induced colon tumorigenesis. Heterozygous and null mice, upon treatment with the organotropic colon carcinogen, azoxymethane, developed a significant (P < 0.05) increase in colon tumor multiplicity (7.2-fold and 5.5-fold, respectively) relative to their wild-type littermates. This enhanced tumor sensitivity may be explained, in part, by the attenuated levels of apoptosis observed by terminal deoxynucleotidyl transferase-mediated nick end labeling staining within the colonic epithelium of heterozygous and null mice ( approximately 50% of wild type). The lower frequency of apoptotic cells corresponded with reduced ceramide levels (69% and 46% of wild-type littermates, respectively). Remarkably, increased tumorigenesis resulting from cPLA(2) deletion occurred despite a significant reduction in prostaglandin E(2) production, even in cyclooxygenase-2-overexpressing tumors. These data contribute new information that supports a fundamental role of cPLA(2) in the control of arachidonic acid homeostasis and cell turnover. Our findings indicate that the proapoptotic role of cPLA(2) in the colon may supercede its contribution to eicosanoid production in tumor development.

Citrate provides protection against oxalate and calcium oxalate crystal induced oxidative damage to renal epithelium

PURPOSE

Oxalate and calcium oxalate (CaOx) crystals are injurious to renal epithelial cells. The injury is caused by the production of reactive oxygen species (ROS). Citrate is a well-known inhibitor of CaOx crystallization and as such it is one of the major therapeutic agents prescribed. Since citrate increases cellular reduced nicotinamide adenine dinucleotide phosphate and glutathione (GSH), we hypothesized that exogenously administered citrate should act as an antioxidant and protect cells from oxalate induced injury.

MATERIALS AND METHODS

We exposed LLC-PK1 and MDCK cells to 500 microM/ml oxalate or 150 mug/cm calcium oxalate crystals for 30, 60 and 180 minutes with or without 3 mg/ml citrate in the medium. We determined cell viability by lactate dehydrogenase release and trypan blue exclusion, ROS involvement by changes in hydrogen peroxide and GSH, and lipid peroxidation by quantifying 8-isoprostane.

RESULTS

The presence of citrate was associated with significant decrease in lactate dehydrogenase release (p <0.001) and staining with trypan blue (p <0.05). In addition, there was a significant increase in GSH (p <0.005) and a decrease in the production of hydrogen peroxide (p <0.05) and 8-isoprostane (p <0.0005) secretion into the culture medium when citrate was present in the medium.

CONCLUSIONS

Citrate protects cells from oxalate and CaOx crystal induced injury by preventing lipid peroxidation through a decrease in ROS production. The results provide additional data for the beneficial role of citrate therapy for CaOx nephrolithiasis.

The cytotoxicity of oxalate, metabolite of ethylene glycol, is due to calcium oxalate monohydrate formation

Oxalate is a minor, but important metabolite of ethylene glycol and has been directly linked with acute and subchronic renal toxicity in ethylene glycol poisoning. Numerous studies have characterized the cytotoxicity of oxalate as including plasma membrane damage and organelle injury. Oxalate has two forms in vivo: oxalate ions and calcium oxalate monohydrate (COM) crystals that readily form in the presence of calcium. The present study was designed to compare the cytotoxicity of the oxalate ion and COM crystals in human and rat cells. In rat red blood cells, the oxalate ion did not increase hemolysis, while COM crystals produced hemolysis with a concentration-dependent increase. In human proximal tubule (HPT) cells in culture, COM suspensions, at concentrations >3 mM but with no oxalate ion, caused cytotoxicity as evidenced by the release of lactate dehydrogenase (LDH) into media. Cytotoxicity was not observed in HPT cells treated with oxalate solutions that contained no COM because EDTA prevented its formation. The cytotoxic effects of COM to HPT cells were potentiated by acidosis (pH 6.5), but not by glycolate, the major metabolite of ethylene glycol. The toxicity of COM to HPT cells and to proximal tubule cells from Wistar and F-344 rats, compared using both ethidium homodimer uptake and LDH leakage, increased in human and rat cells in a concentration-dependent manner. Rat cells were more sensitive to COM than HPT cells, but there were no apparent differences between the effects in Wistar cells and F-344 cells. These results demonstrate that COM crystals, and not the oxalate ion, are responsible for the membrane damage and cell death observed in normal human and rat PT cells and suggest that COM accumulation in the kidney is responsible for the renal toxicity associated with ethylene glycol exposure.

Mitochondrial dysfunction is a primary event in renal cell oxalate toxicity

BACKGROUND

In cultured renal epithelial cells, exposure to oxalate, a constituent of many kidney stones, elicits a cascade of responses that often leads to cell death. Oxalate toxicity is mediated via generation of reactive oxygen species (ROS) in a process that depends at least in part upon lipid signaling molecules that are generated through membrane events that culminate in phospholipase A2 (PLA2) activation. The present studies asked whether mitochondria, a major site of ROS production, were targets of oxalate toxicity, and if so, whether mitochondrial responses to oxalate were mediated by PLA2 activation.

METHODS

Effects of oxalate and various lipids on mitochondrial membrane potential (DeltaPsim) were measured in Madin-Darby canine kidney (MDCK) cell monolayers using 5,5',6,6'-tetrachloro 1,1',3,3'-tetraethylbenzimidazolylcarbocyanine iodide (JC-1), a DeltaPsim-sensitive dye. Other studies assayed caspases, serine proteases activated during apoptosis, in response to oxalate or lipid signaling molecules. Additional studies asked whether oxalate or lipids produced by PLA2 activation promoted ROS formation in isolated renal mitochondria.

RESULTS

Oxalate exposure decreased MDCK cell DeltaPsim within 30 minutes, a response attenuated by arachidonyl trifluoromethyl ketone (AACOCF3), an inhibitor of cytosolic PLA2 (cPLA2). Exposure to arachidonic acid or to lysophosphatidylcholine (lyso-PC), lipid products of PLA2 activation, or to ceramide, another lipid signal generated in MDCK cells following oxalate exposure, also depolarized MDCK cell DeltaPsim and increased the number of caspase-positive cells. Isolated renal mitochondria responded to oxalate, arachidonic acid, lyso-PC, and ceramide by increasing their accumulation of ROS, lipid peroxides, and oxidized thiol proteins.

CONCLUSION

These studies suggest that lipid signaling molecules released after oxalate-induced PLA2 activation trigger marked, rapid changes in mitochondrial function that may mediate toxicity in renal epithelial cells.

Prophylactic protection by N-acetylcysteine against the pulmonary injury induced by 2-chloroethyl ethyl sulfide, a mustard analogue

Mustard gas exposure causes adult respiratory distress syndrome associated with lung injury. The purpose of this study was to investigate whether an antioxidant, such as N-acetylcysteine (NAC), has any protective effect. Guinea pigs were given single exposure (0.5-6 mg/kg body weight) of 2-chloroethyl ethyl sulfide (CEES) as a mustard analogue intratracheally and maintained for various lengths of time (1 h to 21 days). Within 1 h of CEES infusion at 4 mg/kg, high levels of tumor necrosis factor alpha (TNF-alpha), ceramides, and nuclear factor kappaB accumulated in lung and alveolar macrophages. Both acid and neutral sphingomyelinases were activated within 4 h. These signal transduction events were associated with alteration in the oxygen defense system. Within 1 h of exposure to CEES (6 mg/kg body weight), there was 10-fold increase in the (125)I-BSA leakage into lung tissue, indicating severe lung injury. Although low level of CEES exposure (0.5 mg/kg body weight) produced symptoms of chemical burn in lung as early as 1 h after exposure, the severity of edema, congestion, hemorrhage, and inflammation increased progressively with time (1 h to 21 days). Feeding of single dose of NAC (0.5 g) by gavage just before the CEES infusion was ineffective to counteract these effects. However, consumption of the antioxidant in drinking water for 3 or 30 days prior to CEES exposure significantly inhibited the induction of TNF-alpha, activation of neutral and acid sphingomyelinases, production of ceramides, activation of caspases, leakage of (125)I-bovine serum albumin ((125)I-BSA) into lung tissue, and histological alterations in lung. Pretreatment with NAC for 3 and 30 days protected against 69-76% of the acute lung injury. Therefore, NAC may be an antidote for CEES-induced lung injury.

Signal transduction events in lung injury induced by 2-chloroethyl ethyl sulfide, a mustard analog

Sulfur mustard has been used as a vesicant chemical warfare agent. To understand the mechanism by which mustard gas exposure causes respiratory damage, we have used 2-chloroethyl ethyl sulfide (CEES) as a mustard analog. Our initial studies have shown that guinea pigs exposed to CEES intratracheally accumulate high levels of TNF-alpha. Accumulation of TNF-alpha leads to activation of both acid and neutral sphingomyelinases, resulting in high accumulation of ceramides, a second messenger involved in cell apoptosis. In addition, NF-kappa B was activated for a short period (1-2 h after exposure) as determined by mobility shift assay. Supershift assays indicated that both p50 and p65 of NF-kappa B were activated due to CEES exposure. However, NF-kappa B rapidly disappeared after 2 h. It is possible that the initial activation of NF-kappa B was an adaptive response to protect the cells from damage since NF-kappa B is known to inhibit TNF-alpha/ceramide-induced cell apoptosis. Since NF-kappa B disappeared after 2 h, the cells continued being damaged owing to accumulation of ceramides and activation of several caspases, leading to apoptosis.

Bcl-xL interrupts oxidative activation of neutral sphingomyelinase

Recent studies demonstrate a role for intracellular oxidation in the regulation of neutral sphingomyelinase (N-SMase). Glutathione (GSH) has been shown to regulate N-SMase in vitro and in cells. However, it has not been established whether the effects of GSH in cells are due to direct action on N-SMase. In this study, treatment of human mammary carcinoma MCF-7 cells with diamide, a thiol-depleting agent, caused a decrease in intracellular GSH and degradation of sphingomyelin (SM) to ceramide. The SM pool hydrolyzed in response to diamide belonged to the bacterial SMase-resistant pool of SM. Importantly, pretreatment of MCF-7 cells with GSH, N-acetylcysteine, an antioxidant, or GW69A, a specific N-SMase inhibitor, prevented diamide-induced degradation of SM to ceramide, suggesting that intracellular levels of GSH regulate the extent to which SM is degraded to ceramide and that this probably involves a GW69A-sensitive N-SMase. Unexpectedly, expression of Bcl-xL prevented tumor necrosis factor-alpha-induced SM hydrolysis and ceramide accumulation but not the decrease in intracellular GSH. Furthermore, Bcl-xL inhibited diamide-induced SM hydrolysis and ceramide accumulation but not the decrease in intracellular GSH. These results suggest that the site of action of Bcl-xL is downstream of GSH depletion and upstream of ceramide accumulation, and that GSH probably does not exert direct physiologic effects on N-SMase.

Secretory phospholipase A(2) induces apoptosis via a mechanism involving ceramide generation

Secretory phospholipase A(2) (sPLA(2)) plays important roles in cellular signaling and various biological events. In this study, we examined the biological effects and the potential signaling mechanism of purified sPLA(2) in MV1Lu cells. Three types of snake venom sPLA(2) were purified and their enzymatic activities were characterized by using various lipid substrates prepared from [3H]-myristate-labeled cells and by determining their effects on the induction of arachidonic acid (AA) release. The purified sPLA(2) induced apoptosis in Mv1Lu cells in a dose- and time-dependent manner, and was associated with a rapid increase in the intracellular ceramide level. Similar apoptotic effects were observed in Mv1Lu cells treated with exogenous ceramide analog, C(2)- and C(8)-ceramide. Moreover, treatment of cells with sphingomyelinase (SMase), which reduced the intracellular SM level, enhanced the apoptotic response to sPLA(2)s. sPLA(2)s also displayed an inhibitory effect on bradykinin-induced phospholipase D (PLD) activity, which can be imitated by exogenous ceramide. Our data indicate that sPLA(2) induces cell apoptosis via a mechanism involving increased ceramide generation.

Ceramide in apoptosis signaling: relationship with oxidative stress

Ceramide is one of the major sphingosine-based lipid second messengers that is generated in response to various extracellular agents. However, while widespread attention has focused on ceramide as a second messenger involved in the induction of apoptosis, important issues with regard to the mechanisms of ceramide formation and mode of action remain to be addressed. Several lines of evidence suggest that ceramide and oxidative stress are intimately related in cell death induction. This review focuses on the putative relationships between oxidative stress and sphingolipid metabolism in the apoptotic process and discusses the potential mechanisms that connect and regulate the two phenomena.

Renal apoptosis parallels ceramide content after prolonged ureteral obstruction in the neonatal rat

Obstructive nephropathy, the primary cause of renal insufficiency in infants, is characterized by progressive renal apoptosis. Ceramide is a sphingolipid known to stimulate apoptosis in the kidney. We investigated the effects of unilateral ureteral obstruction (UUO) on endogenous renal ceramide content and apoptosis in neonatal and adult rats. Animals were subjected to UUO or sham operation on the first day of life and were studied 3-28 days later. Adult rats were similarly treated and then studied 3 or 14 days later. In additional neonatal rats, the obstruction was removed after 5 days, with study at 14 or 28 days. Renal ceramide content was measured by diacylglycerol kinase assay, and apoptosis was determined by the terminal deoxynucleotidyl transferase dUTP nick-end-labeling technique. Renal ceramide content was 50-fold higher in the 3-day neonatal compared with the adult kidney and 10-fold higher in the 7-day neonatal compared with the adult kidney, but there was no additional effect of UUO on ceramide content at these ages. However, after 14 or 28 days UUO in the neonate, renal ceramide was elevated compared with sham or intact opposite kidneys, and renal apoptosis was directly related to ceramide content (r = 0.99, P < 0.001). Moreover, renal ceramide was reduced by relief of obstruction (P < 0.05). There was less apoptosis in the obstructed kidney of the adult than the neonate, and UUO had no effect on ceramide content at 14 days in the adult. We conclude that prolonged UUO (at least 14 days duration) increases endogenous renal ceramide in the neonatal but not the adult rat. It is likely that this contributes to the prolonged renal apoptotic response of the neonatal obstructed kidney.

Chronic renal failure secondary to oxalate nephropathy: a preventable complication after jejunoileal bypass

Enteric hyperoxaluria is a commonly seen adverse event after the jejunoileal bypass procedure. The increased concentration of urinary oxalate predisposes bypass patients to various renal complications such as nephrolithiasis and oxalate nephropathy. If not diagnosed and appropriately treated, these complications can lead to irreversible renal damage. We describe 3 patients in whom severe renal complications developed with irreversible compromise of renal function after a jejunoileal bypass. Patients who undergo a jejunoileal bypass require lifelong follow-up with close monitoring of their renal function. Marked decline in renal function mandates prompt investigation and aggressive intervention, including reversal of the jejunoileal bypass if necessary. Chronic renal failure secondary to oxalate nephropathy is preventable and treatable but may require conversion of a jejunoileal bypass to a more current form of bypass.

Superoxide potently induces ceramide formation in glomerular endothelial cells

Recent evidence suggests that the sphingolipid-derived second messenger ceramide and oxidative stress are intimately involved in apoptosis induction. Here we report that exposure of microcapillary glomerular endothelial cells to superoxide-generating substances, including hypoxanthine/xanthine oxidase and the redox cyclers DMNQ and menadione results in a dose-dependent and delayed increase in the lipid signaling molecule ceramide. Long-term incubation of endothelial cells for 2-30 h with either DMNQ or hypoxanthine/xanthine oxidase leads to a continuous increase in ceramide levels. In contrast, short-term stimulation for 1 min up to 1 h had no effect on ceramide formation. The DMNQ-induced delayed ceramide formation is dose-dependently inhibited by reduced glutathione, whereas oxidized glutathione was without effect. Furthermore, N-acetylcysteine completely blocks DMNQ-induced ceramide formation. All superoxide-generating substances were found to dose-dependently trigger endothelial cell apoptosis. In addition, glutathione and N-acetylcysteine also prevented superoxide-induced apoptosis and implied that ceramide represents an important mediator of superoxide-triggered cell responses like apoptosis.