Internalization of calcium oxalate crystals by renal tubular cells: a nephron segment-specific process?

Resveratrol inhibits calcium oxalate crystal growth, reduces adhesion to renal cells and induces crystal internalization into the cells, but promotes crystal aggregation

Resveratrol is a natural phenolic compound that belongs to stilbenoid group found in diverse plants. Health benefits and therapeutic potentials of resveratrol have been widely recognized in various diseases. In kidney stone disease, it can alleviate oxalate-induced hyperproduction of free radicals in renal epithelial cells. Nevertheless, its direct effects on calcium oxalate (CaOx) crystal, which is the major stone component, remained unclear. This study therefore addressed the direct effects of resveratrol (at 1, 10 or 100 μM) on each step of CaOx kidney stone formation. The results revealed that resveratrol had no significant effects on CaOx crystallization. However, resveratrol significantly decreased CaOx crystal growth and adhesion to renal epithelial cells at all concentrations, and induced crystal internalization into the cells (a process related to crystal degradation by endolysosomes) in a concentration-dependent manner. On the other hand, resveratrol promoted crystal aggregation. These data indicate that resveratrol serves as a dual modulator on CaOx stone formation. While it inhibits CaOx stone development by reducing crystal growth and adhesion to renal cells and by inducing crystal internalization into the cells, resveratrol promotes crystal aggregation, which is one of the mechanisms leading to kidney stone formation.

Corn Silk Polysaccharides with Different Carboxyl Contents Reduce the Oxidative Damage of Renal Epithelial Cells by Inhibiting Endocytosis of Nano-calcium Oxalate Crystals

OBJECTIVE

Renal epithelial cell injury and cell-crystal interaction are closely related to kidney stone formation.

METHODS

This study aims to explore the inhibition of endocytosis of nano-sized calcium oxalate monohydrate (nano-COM) crystals and the cell protection of corn silk polysaccharides (CCSPs) with different carboxyl contents (3.92, 7.75, 12.90, and 16.38%). The nano-COM crystals protected or unprotected by CCSPs were co-cultured with human renal proximal tubular epithelial cells (HK-2), and then the changes in the endocytosis of nano-COM and cell biochemical indicators were detected.

RESULTS

CCSPs could inhibit the endocytosis of nano-COM by HK-2 cells and reduce the accumulation of nano-COM in the cells. Under the protection of CCSPs, cell morphology is restored, intracellular superoxide dismutase levels are increased, lipid peroxidation product malondialdehyde release is decreased, and mitochondrial membrane potential and lysosomal integrity are increased. The release of Ca²⁺ ions in the cell, the level of cell autophagy, and the rate of cell apoptosis and necrosis are also reduced. CCSPs with higher carboxyl content have better cell protection abilities.

CONCLUSION

CCSPs could inhibit the endocytosis of nano-COM crystals and reduce cell oxidative damage. CCSP3, with the highest carboxyl content, shows the best biological activity.

Human kidney stones: a natural record of universal biomineralization

GeoBioMed - a new transdisciplinary approach that integrates the fields of geology, biology and medicine - reveals that kidney stones composed of calcium-rich minerals precipitate from a continuum of repeated events of crystallization, dissolution and recrystallization that result from the same fundamental natural processes that have governed billions of years of biomineralization on Earth. This contextual change in our understanding of renal stone formation opens fundamentally new avenues of human kidney stone investigation that include analyses of crystalline structure and stratigraphy, diagenetic phase transitions, and paragenetic sequences across broad length scales from hundreds of nanometres to centimetres (five Powers of 10). This paradigm shift has also enabled the development of a new kidney stone classification scheme according to thermodynamic energetics and crystalline architecture. Evidence suggests that ≥50% of the total volume of individual stones have undergone repeated in vivo dissolution and recrystallization. Amorphous calcium phosphate and hydroxyapatite spherules coalesce to form planar concentric zoning and sector zones that indicate disequilibrium precipitation. In addition, calcium oxalate dihydrate and calcium oxalate monohydrate crystal aggregates exhibit high-frequency organic-matter-rich and mineral-rich nanolayering that is orders of magnitude higher than layering observed in analogous coral reef, Roman aqueduct, cave, deep subsurface and hot-spring deposits. This higher frequency nanolayering represents the unique microenvironment of the kidney in which potent crystallization promoters and inhibitors are working in opposition. These GeoBioMed insights identify previously unexplored strategies for development and testing of new clinical therapies for the prevention and treatment of kidney stones.

Excretion of urine extracellular vesicles bearing markers of activated immune cells and calcium/phosphorus physiology differ between calcium kidney stone formers and non-stone formers

BACKGROUNDS

Previous studies have demonstrated that excretion of urinary extracellular vesicles (EVs) from different nephron segments differs between kidney stone formers and non-stone formers (NSFs), and could reflect pathogenic mechanisms of urinary stone disease. In this study we quantified selected populations of specific urinary EVs carrying protein markers of immune cells and calcium/phosphorus physiology in calcium oxalate stone formers (CSFs) compared to non-stone formers (NSFs).

METHODS

Biobanked urine samples from CSFs (n = 24) undergoing stone removal surgery and age- and sex- matched NSFs (n = 21) were studied. Urinary EVs carrying proteins related to renal calcium/phosphorus physiology (phosphorus transporters (PiT1 and PiT2), Klotho, and fibroblast growth factor 23 (FGF23); markers associated with EV generation (anoctamin-4 (ANO4) and Huntington interacting protein 1 (HIP1)), and markers shed from activated immune cells were quantified by standardized and published method of digital flow cytometry.

RESULTS

Urine excretion of calcium, oxalate, phosphorus, and calcium oxalate supersaturation (SS) were significantly higher in CSFs compared to NSFs (P < 0.05). Urinary excretion of EVs with markers of total leukocytes (CD45), neutrophils (CD15), macrophages (CD68), Klotho, FGF23, PiT1, PiT2, and ANO4 were each markedly lower in CSFs than NSFs (P < 0.05) whereas excretion of those with markers of monocytes (CD14), T-Lymphocytes (CD3), B-Lymphocytes (CD19), plasma cells (CD138 plus CD319 positive) were not different between the groups. Urinary excretion of EVs expressing PiT1 and PiT2 negatively (P < 0.05) correlated with urinary phosphorus excretion, whereas excretion of EVs expressing FGF23 negatively (P < 0.05) correlated with both urinary calcium and phosphorus excretion. Urinary EVs with markers of HIP1 and ANO4 correlated negatively (P < 0.05) with clinical stone events and basement membrane calcifications on papillary tip biopsies.

CONCLUSIONS

Urinary excretion of EVs derived from specific types of activated immune cells and EVs with proteins related to calcium/phosphorus regulation differed between CSFs and NSFs. Further validation of these and other populations of urinary EVs in larger cohort could identify biomarkers that elucidate novel pathogenic mechanisms of calcium stone formation in specific subsets of patients.

Effects of Selenized Astragalus Polysaccharide on the Adhesion and Endocytosis of Nanocalcium Oxalate Dihydrate after the Repair of Damaged HK-2 Cells

An oxidative damage model of human proximal renal epithelial cells (HK-2) was established using oxalate damage. The repair effects of Astragalus polysaccharide (APS) and selenized APS (Se-APS) on damaged HK-2 cells were investigated. Differences in the adhesion and endocytosis of HK-2 cells to calcium oxalate dihydrate crystals with a size of approximately 100 nm before and after APS and Se-APS repair were also explored. The results showed that after being repaired by APS and Se-APS, HK-2 cells exhibited increased cell viability, restored cell morphology, reduced reactive oxygen species level, increased mitochondrial membrane potential, reduced phosphatidylserine eversion, and osteopontin expression. Moreover, the amount of adherent crystals on the cell surface decreased, but the amount of endocytic crystals increased. At the same concentration, Se-APS exhibited better repair effects on the damaged HK-2 cells than APS. All these findings revealed that Se-APS may be a potential drug candidate for inhibiting the formation of kidney stones.

Comparison of the adhesion and endocytosis of calcium oxalate dihydrate to HK-2 cells before and after repair by Astragalus polysaccharide

This work investigated the effects of repairing injured renal proximal tubular epithelial (HK-2) cells by using three Astragalus polysaccharides (APS) with different molecular weights and the adhesion and endocytosis of HK-2 cells to the calcium oxalate dihydrate (COD) nanocrystals before and after repair to develop new products that can protect against kidney stones. HK-2 cells cultured in vitro were injured with 2.6 mmol/L oxalic acid to establish a damaged cell model. Three kinds of APS (APS0, APS1, and APS2 with molecular weights of 11.03, 4.72, and 2.60 kDa, respectively) were used to repair the damaged cells. The changes in the adhesion and endocytosis of 100 nm COD crystals to cells before and after the repair were detected. After the repair of HK-2 cells by the APS, the speed of wound healing of the damaged HK-2 cells increased, and the amount of phosphatidylserine (PS) ectropion decreased. In addition, the proportion of cells with adhered COD crystals decreased, whereas the proportion of cells with internalized crystals increased. As a result of the repair activity, APS can inhibit the adhesion and promote the endocytosis of COD nanocrystals to damaged cells. APS1, which had a moderate molecular weight, displayed the strongest abilities to repair the cells, inhibit adhesion, and promote endocytosis. Thus, APS, particularly APS1, may serve as potential green drugs for preventing kidney stones.

A human proximal tubule-on-a-chip to study renal disease and toxicity

Renal disease is a global problem with unsustainable health-care costs. There currently exists a lack of accurate human renal disease models that take into account the complex microenvironment of these tissues. Here, we present a reusable microfluidic model of the human proximal tubule and glomerulus, which allows for the growth of renal epithelial cells in a variety of conditions that are representative of renal disease states including altered glomerular filtration rate, hyperglycemia, nephrolithiasis, and drug-induced nephrotoxicity (cisplatin and cyclosporine). Cells were exposed to these conditions under fluid flow or in traditional static cultures to determine the effects of a dynamic microenvironment on the pathogenesis of these renal disease states. The results indicate varying stress-related responses (α-smooth muscle actin (α-SMA) expression, alkaline phosphatase activity, fibronectin, and neutrophil gelatinase-associated lipocalin secretion) to each of these conditions when comparing cells that had been grown in static and dynamic conditions, potentially indicating more realistic and sensitive predictions of human responses and a requirement for a more complex "fit for purpose" model.

Cell cycle shift from G0/G1 to S and G2/M phases is responsible for increased adhesion of calcium oxalate crystals on repairing renal tubular cells at injured site

Renal tubular cell injury can enhance calcium oxalate monohydrate (COM) crystal adhesion at the injured site and thus may increase the stone risk. Nevertheless, underlying mechanism of such enhancement remained unclear. In the present study, confluent MDCK renal tubular cell monolayers were scratched to allow cells to proliferate and repair the injured site. At 12-h post-scratch, the repairing cells had significant increases in crystal adhesion capacity and cell proliferation as compared to the control. Cell cycle analysis using flow cytometry demonstrated that the repairing cells underwent cell cycle shift from G0/G1 to S and G2/M phases. Cyclosporin A (CsA) and hydroxyurea (HU) at sub-toxic doses caused cell cycle shift mimicking that observed in the repairing cells. Crystal-cell adhesion assay confirmed the increased crystal adhesion capacity of the CsA-treated and HU-treated cells similar to that of the repairing cells. These findings provide evidence indicating that cell cycle shift from G0/G1 to S and G2/M phases is responsible, at least in part, for the increased adhesion of COM crystals on repairing renal tubular cells at the injured site.

Effect of Crystal Shape and Aggregation of Calcium Oxalate Monohydrate on Cellular Toxicity in Renal Epithelial Cells

Renal epithelial cell injury is a key step in inducing kidney stone formation. This injury induced by crystallites with different shapes and aggregation states has been receiving minimal research attention. To compare the shape and aggregation effects of calcium oxalate crystals on their toxicity, we prepared calcium oxalate monohydrate (COM) crystals with the morphology of a hexagonal lozenge, a thin hexagonal lozenge, and their corresponding aggregates. We then compared their toxicities toward human kidney proximal tubular epithelial (HK-2) cells. All four shapes of COM crystals caused cell-membrane rupture, upregulated intracellular reactive oxygen, and decreased mitochondrial membrane potential. This series of phenomena ultimately led to necrotic cell death. The overall damage in cells was determined in terms of both exterior and interior damage. Crystals with a large Ca2+ ion-rich (1̅01) active face showed the greatest toxicity in HK-2 cells and the largest extent of adhesion onto the cell surface. Crystals with sharp edges easily caused cell-membrane ruptures. The aggregation of sharp crystals aggravated cell injury, whereas the aggregation of blunt crystals weakened cell injury. Therefore, crystal shapes and aggregation states were important factors that affected crystal toxicity in renal epithelial cells. All of these findings elucidated the relationship between the physical properties of crystals and cytotoxicity and provided theoretical references for inhibiting stone formation.

Protective Effects of Pistacia lentiscus L. fruit extract against calcium oxalate monohydrate induced proximal tubular injury

ETHNOPHARMACOLOGICAL RELEVANCE

The world prevalence of kidney stones is increasing and plants are frequently used to treat urolithiasis. Pistacia lentiscus L, a plant which freely grows around the Mediterranean basin areas, is widely used for various pathologies. P. lentiscus has an important impact as it has economical value on top of its pharmacological interest. Decoctions of its aerial parts and/or resin are used to treat kidney stones.

AIM OF THE STUDY

To in vitro assess the potential nephroprotective effect of Pistacia lentiscus ethanolic fruit extract (PLEF) on proximal tubular cells in response to the adhesion of calcium oxalate monohydrate (COM) crystals.

MATERIALS AND METHODS

Human Kidney [HK]-2 cells were incubated with and without COM in the presence or absence of PLEF. Cell viability was measured by the resazurin assay. The expression of E-cadherin was analyzed by PCR. The extracellular production of H₂O₂ was measured by Amplex® Red H₂O₂ Assay. The numbers of detached or non-adherent COM crystals in the presence of PLEF were microscopically captured and counted using ImageJ software. The interaction of PLEF with COM and the effect of PLEF on crystal size were analyzed by flow cytometry. The spectrophotometric measurement of turbidity was performed for assessing the COM concentration.

RESULTS

PLEF incubated with COM was able to increase the cell viability. The decrease of E-cadherin expression after incubation with COM was counteracted by PLEF. Overproduction of H₂O₂ induced by COM was also inhibited by PLEF. Observations using flow cytometry showed that interactions between PLEF and the COM crystals occurred. PLEF was also effective in reducing the particles size and in lowering COM concentration.

CONCLUSION

Our data show that COM tubulotoxicity can be significantly reversed by PLEF -at least in part- via an inhibition of COM crystals adhesion onto the apical membrane. This early beneficial effect of PLEF needs to be further investigated as a useful strategy in nephrolithiasis prevention.

Acute and chronic exposure to high levels of glucose modulates tight junction-associated epithelial barrier function in a renal tubular cell line

AIMS

Type 2 diabetes mellitus (T2DM) is one of the most prevalent diseases worldwide. Diabetic nephropathy (DN) is a complication of diabetes and the mechanisms underlying onset and progression of this disease are not fully understood. It has been shown that hyperglycemia is an independent factor to predict the development of DN in individuals with T2DM, however, a link between high plasma glucose levels and renal tubular injuries in DN remains unknown. In this study, we investigated the effect of high levels of glucose (i.e. 180 or 360mg/dL) for up to 24h (acute) or over 72h (chronic) upon tight junction (TJ)-mediated epithelial barrier integrity of the kidney tubular cell line, MDCK.

METHODS/KEY FINDINGS

High levels of glucose (180 and 360mg/dL) induced a decrease in transepithelial electrical resistance associated with an increase in TJ cation selectivity at 24h or in TJ permeability to a paracellular marker, Lucifer Yellow, at 72h-exposure when compared to control group (exposed to 100mg/dL glucose). Immunofluorescence analyses showed that glucose treatment induced a significant decrease in the tight junctional content of claudins-1 and -3 as well as a significant increase in claudin-2 (particularly at 24h-exposure) and a time-dependent change in occludin/ZO-1 junctional content. The analyses of total cell content of these junctional proteins by Western blot did not reveal significant changes, except in claudin-2 expression.

SIGNIFICANCE

Our data suggest that high levels of glucose induce time-dependence changes in TJ structure in MDCK monolayers, suggesting a possible link between hyperglycemia-induced tubular epithelial barrier disruption and diabetic nephropathy.

Response surface methodology based extraction of Tribulus terrestris leads to an upsurge of antilithiatic potential by inhibition of calcium oxalate crystallization processes

Tribulus terrestris has significant antilithiatic efficacy established via both in vitro as well as in vivo studies and is used in numerous anti-urolithiatic herbal formulations viz. Cystone, Uriflow, Uritone and Neeri. However, to fully utilize its antilithiatic potential, the influence of different extraction parameters on antilithiatic ability of T. terrestris aqueous extract needs elucidation. Thus, the current study was undertaken using statistically optimized extraction conditions for aqueous extract preparation. Response surface methodology was employed to observe the influence of three variables i.e. temperature (°C), time (h) and solid: liquid ratio (S: L) on the extraction yield (%) and protein content (mg/g) of T. terrestris aqueous extract. RSM results revealed that the high S:L ratio, low temperature and reduced incubation time were optimal conditions for aqueous extraction. Under such extraction conditions the protein content reached the value of 26.6±1.22 mg/g and the obtained extraction yield was 27.32±1.62%. The assessment of antilithiatic activity of 4 selected extracts (AE1-4), revealed enhanced nucleation and aggregation inhibition of calcium oxalate crystals with AE1 and AE2, which in addition significantly altered the size and morphology of calcium oxalate monohydrate (COM) crystals compared to AE3 and AE4. In vitro cell culture based studies on renal epithelial cells (MDCK, NRK-52E and PK 15) proved that the AE1 showed higher cytoprotective potency by increasing cell viability as compared to the oxalate treated group. The free radical scavenging activity of aqueous extract lowered the reactive oxygen specie’s induced damage and potentially reduced the signals of programmed cell death due to oxalate injury. In addition, modulation of the COM crystal morphology was enhanced by AE1 as compared to AE2. The FTIR and GC-MS analysis of AE1, showed the presence of biomolecules which could aid in the attenuation of lithiatic process. In the light of these results the utility of the RSM approach to fully optimize the antilithiatic potential of T. terrestris cannot be undermined.

Shape-dependent cellular toxicity on renal epithelial cells and stone risk of calcium oxalate dihydrate crystals

Renal epithelial cell injury causes crystal retention and leads to renal stone formation. However, the effects of crystal shape on cell injury and stone risk remain unclear. This study compared the cytotoxicity degrees of calcium oxalate dihydrate (COD) crystals having different shapes toward human kidney proximal tubular epithelial (HK-2) cells to reveal the effect of crystal shape on cell injury and to elucidate the pathological mechanism of calcium oxalate kidney stones. The effects of exposure to cross-shaped (COD-CS), flower-like (COD-FL), bipyramid (COD-BD), and elongated-bipyramid (COD-EBD) COD crystals on HK-2 cells were investigated by examining the cell viability, cell membrane integrity, cell morphology change, intracellular reactive oxygen species, mitochondrial membrane potential (Δψm), and apoptotic and/or necrotic rate. Crystals with large (100) faces (COD-EBD) and sharp edges (COD-CS) showed higher toxicity than COD-BD and COD-FL, respectively. COD crystal exposure caused cell membrane rupture, upregulated intracellular reactive oxygen, and decreased Δψm. This series of phenomena ultimately led to a high apoptotic rate and a low necrotic rate. Crystals with large active faces have a large contact area with epithelial cell surface, and crystals with sharp edges can easily scratch epithelial cells; these factors could promote crystal adhesion and aggregation, thus increasing stone risk.

Time-dependent subcellular structure injuries induced by nano-/micron-sized calcium oxalate monohydrate and dihydrate crystals

Comparative studies were conducted to investigate the time effect of cell injury induced by nano-sized (50nm) and micron-sized (10μm) calcium oxalate monohydrate (COM) and dihydrate (COD) crystals in African green monkey renal epithelial (Vero) cells. The effects of nano-/micron-sized COM and COD exposure on Vero cells were investigated by detecting the cell viability, cell morphology, LDH release, reactive oxygen species, mitochondrial membrane potential, cell cycle, and cell apoptosis, as well as the intracellular and extracellular crystal distribution. Nano-/micron-sized COM and COD exposure lead to subcellular organelle injury in varying degrees, but the injury sequence of various organelles differed. The time sequence of organelle injury presenting significant variation was described as follows: cell membrane injury (1h)<mitochondrial membrane potential decrease (3h to 6h)≈cell-cycle arrest (3h to 6h)<cell apoptosis (12h). Nano-sized crystals lead organelle injury faster than micron-sized crystals, and COM crystals showed more obvious time-dependent effects than the same-sized COD crystals. This study may provide insights into the damage to renal epithelial cells induced by urinary crystals and the formation mechanism of kidney stones.

Reinjury risk of nano-calcium oxalate monohydrate and calcium oxalate dihydrate crystals on injured renal epithelial cells: aggravation of crystal adhesion and aggregation

BACKGROUND

Renal epithelial cell injury facilitates crystal adhesion to cell surface and serves as a key step in renal stone formation. However, the effects of cell injury on the adhesion of nano-calcium oxalate crystals and the nano-crystal-induced reinjury risk of injured cells remain unclear.

METHODS

African green monkey renal epithelial (Vero) cells were injured with H2O2 to establish a cell injury model. Cell viability, superoxide dismutase (SOD) activity, malonaldehyde (MDA) content, propidium iodide staining, hematoxylin-eosin staining, reactive oxygen species production, and mitochondrial membrane potential (Δψm) were determined to examine cell injury during adhesion. Changes in the surface structure of H2O2-injured cells were assessed through atomic force microscopy. The altered expression of hyaluronan during adhesion was examined through laser scanning confocal microscopy. The adhesion of nano-calcium oxalate monohydrate (COM) and calcium oxalate dihydrate (COD) crystals to Vero cells was observed through scanning electron microscopy. Nano-COM and COD binding was quantitatively determined through inductively coupled plasma emission spectrometry.

RESULTS

The expression of hyaluronan on the cell surface was increased during wound healing because of Vero cell injury. The structure and function of the cell membrane were also altered by cell injury; thus, nano-crystal adhesion occurred. The ability of nano-COM to adhere to the injured Vero cells was higher than that of nano-COD crystals. The cell viability, SOD activity, and Δψm decreased when nano-crystals attached to the cell surface. By contrast, the MDA content, reactive oxygen species production, and cell death rate increased.

CONCLUSION

Cell injury contributes to crystal adhesion to Vero cell surface. The attached nano-COM and COD crystals can aggravate Vero cell injury. As a consequence, crystal adhesion and aggregation are enhanced. These findings provide further insights into kidney stone formation.

Calcium oxalate monohydrate crystals internalized into renal tubular cells are degraded and dissolved by endolysosomes

Interaction between calcium oxalate crystals and renal tubular cells has been recognized as one of the key mechanisms for kidney stone formation. While crystal adhesion and internalization have been extensively investigated, subsequent phenomena (i.e. crystal degradation and dissolution) remained poorly understood. To explore these mechanisms, we used fluorescein isothiocyanate (FITC)-labelled calcium oxalate monohydrate (COM) crystals (1000 μg/ml of crystals/culture medium) to confirm crystal internalization into MDCK (Type II) renal tubular cells after exposure to the crystals for 1 h and to trace the internalized crystals. Crystal size, intracellular and extracellular fluorescence levels were measured using a spectrofluorometer for up to 48 h after crystal internalization. Moreover, markers for early endosome (Rab5), late endosome (Rab7) and lysosome (LAMP-2) were examined by laser-scanning confocal microscopy. Fluorescence imaging and flow cytometry confirmed that FITC-labelled COM crystals were internalized into MDCK cells (14.83 ± 0.85%). The data also revealed a reduction of crystal size in a time-dependent manner. In concordance, intracellular and extracellular fluorescence levels were decreased and increased, respectively, indicating crystal degradation/dissolution inside the cells and the degraded products were eliminated extracellularly. Moreover, Rab5 and Rab7 were both up-regulated and were also associated with the up-regulated LAMP-2 to form large endolysosomes in the COM-treated cells at 16-h after crystal internalization. We demonstrate herein, for the first time, that COM crystals could be degraded/dissolved by endolysosomes inside renal tubular cells. These findings will be helpful to better understand the crystal fate and protective mechanism against kidney stone formation.

Mechanism of cytotoxicity of micron/nano calcium oxalate monohydrate and dihydrate crystals on renal epithelial cells

The toxicity difference and distribution in Vero cells of calcium oxalate with different crystal phases and sizes.

The trigger-maintenance model of persistent mild to moderate hyperoxaluria induces oxalate accumulation in non-renal tissues

Persistent mild to moderate hyperoxaluria (PMMH) is a common side effect of bariatric surgery. However, PMMH's role in the progression to calcium oxalate (CaOx) urolithiasis and its potential effects on non-renal tissues are unknown. To address these points, a trigger + maintenance (T + Mt) model of PMMH was developed in rats (Experiment 1). The trigger was an i.p. injection of PBS (TPBS) or 288 μmol sodium oxalate (T288). Maintenance (Mt) was given via minipumps dispensing PBS or 7.5-30 μmol potassium oxalate/day for 28 days. Urinary oxalate ranged from 7.7 ± 0.8 μmol/day for TPBS + MtPBS to 18.2 ± 1.5 μmol/day for T288 + Mt30 (p ≤ 0.0005). All rats receiving T288 developed CaOx nephrocalcinosis, and many developed 'stones'. This was also true for Mt doses that did not elevate urinary oxalate above that of TPBS + MtPBS (p > 0.1) and for rats that did not have a detectable surge in urinary oxalate post T288. When TPBS was administered, CaOx nephrocalcinosis did not develop regardless of the Mt dose even if urinary oxalate was elevated compared to TPBS + MtPBS (p ≤ 0.0005). One of the risks associated with PMMH is oxalate accumulation within tissues. Hence, in a second set of experiments (Experiment 2) different doses of oxalate (Mt0.05, Mt15, Mt30) labeled with (14)C-oxalate ((14)C-Ox) were administered by minipump for 13 days. Tissues were harvested and (14)C-Ox accumulation assessed by scintillation counting. (14)C-Ox accumulated in a dose dependent manner (p ≤ 0.004) in bone, kidney, muscle, liver, heart, kidney, lungs, spleen, and testis. All these tissues exhibited (14)C-Ox concentrations higher (p ≤ 0.05) than the plasma. Extrapolation of our results to patients suggests that PMMH patients should take extra care to avoid dietary-induced spikes in oxalate excretion to help prevent CaOx nephrocalcinosis or stone development. Monitoring for oxalate accumulation within tissues susceptible to damage by oxalate or CaOx crystals may also be required.

Aluminum citrate blocks toxicity of calcium oxalate crystals by preventing binding with cell membrane phospholipids

BACKGROUND/AIMS

Renal damage from ethylene glycol and primary hyperoxaluria is linked to accumulation of calcium oxalate monohydrate (COM) crystals in the renal proximal tubule (PT). In vitro studies have shown that aluminum citrate (AC), uniquely among citrate salts, blocks COM cytotoxicity to tubular cells. These studies were designed to evaluate the interaction of COM with membrane phospholipids and the ability of AC to reduce COM toxicity by interfering with this interaction.

METHODS

Interaction of COM with phospholipids was assessed using differential scanning calorimetric analysis of structural changes in specific liposomes. Interaction of COM with cell membranes was studied by measuring binding of radiolabeled crystals by human PT (HPT) cells.

RESULTS

Analysis of liposomes prepared from phosphatidylserine (PS) or phosphatidylcholine (PC) showed that COM interfered with the gel-liquid transition of PS liposomes, but not that of PC liposomes. AC reversed the COM-induced changes in liposomal structure. AC inhibited the binding of [(14)C]-COM by HPT cells in a concentration-dependent manner. AC blocked COM binding by interacting with the crystal surface and not the cell membrane.

CONCLUSION

These results indicate that AC blocks the binding of COM by PT cells, and consequently its cytotoxicity, by attaching to the surface of the crystal. Thus, AC, or a related compound that works by the same mechanism, could be a useful adjunct therapy to reduce the renal damage produced by severe hyperoxaluria.

Oral Reference Dose for ethylene glycol based on oxalate crystal-induced renal tubule degeneration as the critical effect

Several risk assessments have been conducted for ethylene glycol (EG). These assessments identified the kidney as the primary target organ for chronic effects. None of these assessments have incorporated the robust database of species-specific toxicokinetic and toxicodynamic studies with EG and its metabolites in defining uncertainty factors used in reference value derivation. Pertinent in vitro and in vivo studies related to one of these metabolites, calcium oxalate, and its role in crystal-induced nephropathy are summarized, and the weight of evidence to establish the mode of action for renal toxicity is reviewed. Previous risk assessments were based on chronic rat studies using a strain of rat that was later determined to be less sensitive to the toxic effects of EG. A recently published 12-month rat study using the more sensitive strain (Wistar) was selected to determine the point of departure for a new risk assessment. This approach incorporated toxicokinetic and toxicodynamic data and used Benchmark Dose methods to calculate a Human Equivalent Dose. Uncertainty factors were chosen, depending on the quality of the studies available, the extent of the database, and scientific judgment. The Reference Dose for long-term repeat oral exposure to EG was determined to be 15 mg/kg bw/d.

Cyclooxygenase 2 and prostaglandin E2 regulate the attachment of calcium oxalate crystals to renal epithelial cells

OBJECTIVE

To determine the roles of endogenous cyclooxygenase 2 and prostaglandin E(2) in crystal-cell binding, which is considered to be an important step in the development of intratubular nephrocalcinosis.

METHODS

An expression plasmid for human cyclooxygenase 2 was introduced into Madin-Darby canine kidney cells using the lipofection method. Cyclooxygenase activity was measured using thin-layer chromatography, and the prostaglandin E(2) concentration was determined with an enzyme immunoassay. In addition, crystal attachment was evaluated with a liquid scintillation counter using [(14)C] calcium oxalate monohydrate crystals, and immunohistochemistry and an enzyme immunoassay were used to analyze and quantify the expression of hyaluronan, a crystal-binding molecule.

RESULTS

Cyclooxygenase 2-overexpressing Madin-Darby canine kidney cells produced about 10-fold more prostaglandin E(2) than wild-type Madin-Darby canine kidney cells, and their hyaluronan production was also upregulated. The attachment of calcium oxalate monohydrate crystals to cyclooxygenase 2-overexpressing Madin-Darby canine kidney cells was significantly reduced compared with their attachment to wild-type and mock-transfected Madin-Darby canine kidney cells. Pre-incubation of the cyclooxygenase 2-overexpressing cells, as well as the mock-transfected and wild-type cells with the cyclooxygenase 2 selective inhibitor etodolac, increased the cellular attachment of calcium oxalate monohydrate crystals in a dose-dependent manner.

CONCLUSIONS

These findings suggest that cyclooxygenase 2 expression and the resultant increase in endogenous prostaglandin E(2), leading to increased hyaluronan production, help to prevent nephrocalcinosis by inhibiting the attachment of calcium oxalate monohydrate crystals to the surface of renal epithelial cells.

A case of renal oxalosis in a 3-month-old cat raised under controlled conditions

The kidneys of a 3-month-old female cat were examined. The cat which had been raised under controlled conditions with no history of any poisoning showed progressive weight loss with increases in blood BUN and creatinine concentrations. At necropsy, both kidneys were firm in consistency with formation of focal scars. Histopathologically, widespread deposition of crystals was observed in the renal tubules (in both dilated lumina and degenerative epithelia) accompanying mild interstitial fibrosis with lymphocyte infiltration. The crystals were colorless or basophilic on the hematoxilin and eosin-stained section and could be visualized with polarized light as doubly fractile crystals. The crystals were identified as calcium oxalate crystals by histochemical examinations using von Kossa stain and alizarin red S stain under different conditions and by ultrastructural examination. Judging from the above-mentioned findings, the present renal lesion detected in an infant cat was diagnosed as renal oxalosis which was suspected to be hereditary in nature.

The effect of intracrystalline and surface-bound osteopontin on the degradation and dissolution of calcium oxalate dihydrate crystals in MDCKII cells

In vivo, urinary crystals are associated with proteins located within the mineral bulk as well as upon their surfaces. Proteins incarcerated within the mineral phase of retained crystals could act as a defence against urolithiasis by rendering them more vulnerable to destruction by intracellular and interstitial proteases. The aim of this study was to examine the effects of intracrystalline and surface-bound osteopontin (OPN) on the degradation and dissolution of urinary calcium oxalate dihydrate (COD) crystals in cultured Madin Darby canine kidney (MDCK) cells. [(14)C]-oxalate-labelled COD crystals with intracrystalline (IC), surface-bound (SB) and IC + SB OPN, were generated from ultrafiltered (UF) urine containing 0, 1 and 5 mg/L human milk OPN and incubated with MDCKII cells, using UF urine as the binding medium. Crystal size and degradation were assessed using field emission scanning electron microscopy (FESEM) and dissolution was quantified by the release of radioactivity into the culture medium. Crystal size decreased directly with OPN concentration. FESEM examination indicated that crystals covered with SB OPN were more resistant to cellular degradation than those containing IC OPN, whose degree of disruption appeared to be related to OPN concentration. Whether bound to the crystal surface or incarcerated within the mineral interior, OPN inhibited crystal dissolution in direct proportion to its concentration. Under physiological conditions OPN may routinely protect against stone formation by inhibiting the growth of COD crystals, which would encourage their excretion in urine and thereby perhaps partly explain why, compared with calcium oxalate monohydrate crystals, COD crystals are more prevalent in urine, but less common in kidney stones.

The tubular epithelium in the initiation and course of intratubular nephrocalcinosis

Intratubular nephrocalcinosis is defined as the histological observation of calcium oxalate and/or calcium phosphate deposits retained within the lumen of the renal tubules. As the tubular epithelium is the primary interaction partner of crystals formed in the tubular fluid, the role of the epithelial cells in nephrocalcinosis has been investigated intensively. This review summarizes our current understanding on how the tubular epithelium mechanistically appears to be involved both in the initiation and in the course of nephrocalcinosis, with emphasis on in vivo observations.

The initial and subsequent inflammatory events during calcium oxalate lithiasis

BACKGROUND

Crystallization is believed to be the initiation step of urolithiasis, even though it is unknown where inside the nephron the first crystal nucleation occurs.

METHODS

Direct nucleation of calcium oxalate and subsequent events including crystal retention, cellular damage, endocytosis, and hyaluronan (HA) expression, were tested in a two-compartment culture system with intact human proximal tubular HK-2 cell monolayer.

RESULTS

Calcium oxalate dihydrate (COD) was nucleated and bound onto the apical surface of the HK-2 cells under hypercalciuric and hyperoxaluric conditions. These cells displayed mild cellular damage and internalized some of the adhered crystals within 18h post-COD-exposure, as revealed by electron microscopy. Prolonged incubation in complete medium caused significant damage to disrupt the monolayer integrity. Furthermore, hyaluronan disaccharides were detected in the harvested media, and were associated with HAS-3 mRNA expression.

CONCLUSION

Human proximal cells were able to internalize COD crystals which nucleated directly onto the apical surface, subsequently triggering cellular damage and HAS-3 specific hyaluronan synthesis as an inflammatory response. The proximal tubule cells here demonstrate that it plays an important role in facilitating urolithiasis via endocytosis and creating an inflammatory environment whereby free hyaluronan in tubular fluid can act as crystal-binding molecule at the later segments of distal and collecting tubules.

Renal toxicity of ethylene glycol results from internalization of calcium oxalate crystals by proximal tubule cells

Ethylene glycol exposure can lead to the development of renal failure due to the metabolic formation of calcium oxalate monohydrate (COM) crystals. The renal damage is closely linked to the degree of COM accumulation in the kidney and most likely results from a COM-induced injury to proximal tubule (PT) cells. The present studies have measured the binding and internalization of COM by primary cultures of normal PT cells from humans and from Wistar and Fischer-344 rats in order to examine the roles of these uptake processes in the resulting cytotoxicity. Internalization was determined by incubation of cells with [(14)C]-COM at 37 degrees C, removal of bound COM with an EDTA incubation, followed by solubilization of cells, as well as by transmission electron microscopy of COM-exposed cells. COM crystals were internalized by PT cells in time- and concentration-dependent manners. COM crystals were bound to and internalized by rat cells about five times more than by human cells. Binding and internalization values were similar between PT cells from Wistar and Fischer-344 rats, indicating that a differential uptake of COM does not explain the known strain difference in sensitivity to ethylene glycol renal toxicity. Internalization of COM correlated highly with the degree of cell death, which is greater in rat cells than in human cells. Thus, surface binding and internalization of COM by cells play critical roles in cytotoxicity and explain why rat cells are more sensitive to COM crystals. At the same level of COM accumulation after ethylene glycol exposure or hyperoxaluria in vivo, rats would be more susceptible than humans to COM-induced damage.

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.

Intracrystalline urinary proteins facilitate degradation and dissolution of calcium oxalate crystals in cultured renal cells

We have previously proposed that intracrystalline proteins would increase intracellular proteolytic disruption and dissolution of calcium oxalate (CaOx) crystals. Chauvet MC, Ryall RL. J Struct Biol 151: 12-17, 2005; Fleming DE, van Riessen A, Chauvet MC, Grover PK, Hunter B, van Bronswijk W, Ryall RL. J Bone Miner Res 18: 1282-1291, 2003; Ryall RL, Fleming DE, Doyle IR, Evans NA, Dean CJ, Marshall VR. J Struct Biol 134: 5-14, 2001. The aim of this investigation was to determine the effect of increasing concentrations of intracrystalline protein on the rate of CaOx crystal dissolution in Madin-Darby canine kidney (MDCKII) cells. Crystal matrix extract (CME) was isolated from urinary CaOx monohydrate (COM) crystals. Cold and [14C]oxalate-labeled COM crystals were precipitated from ultrafiltered urine containing 0-5 mg/l CME. Crystal surface area was estimated from scanning electron micrographs, and synchrotron X-ray diffraction was used to determine nonuniform strain and crystallite size. Radiolabeled crystals were added to MDCKII cells and crystal dissolution, expressed as radioactive label released into the medium, was measured. Increasing CME content did not significantly alter crystal surface area. However, nonuniform strain increased and crystallite size decreased in a dose-response manner, both reaching saturation at a CME concentration of 3 mg/ and demonstrating unequivocally the inclusion of increasing quantities of proteins in the crystals. This was confirmed by Western blotting. Crystal dissolution also followed saturation kinetics, increasing proportionally with final CME concentration and reaching a plateau at a concentration of approximately 2 mg/l. These findings were complemented by field emission scanning electron microscopy, which showed that crystal degradation also increased relative to CME concentration. Intracrystalline proteins enhance degradation and dissolution of CaOx crystals and thus may constitute a natural defense against urolithiasis. The findings have significant ramifications in biomineral metabolism and pathogenesis of renal stones.

Proposed mechanisms in renal tubular crystal retention

The production of concentrated urine inevitably leads to the precipitation of poorly soluble waste salts in the renal tubular fluid. These crystallization processes are physiologic and without consequences as long as all crystals are excreted with the urine. The retention of crystals in the renal tubules, however, may lead to tubular nephrocalcinosis. Here, we present a brief survey of the possible mechanisms involved in this process, which seems to depend predominantly on the presence of regenerating/(re)differentiating cells in the renal tubules. Crystal binding to the surface of these cells can be mediated by a number of luminal membrane molecules, including acidic fragment of nucleolin-related protein, annexin-II, osteopontin, and hyaluronan.

Effect of urine fractionation on attachment of calcium oxalate crystals to renal epithelial cells: implications for studying renal calculogenesis

Our aim was to determine whether fractionation of human urine affects the attachment of calcium oxalate monohydrate (COM) crystals to renal cells. Urine collected from six healthy subjects was fractionated into sieved (S), centrifuged (C), centrifuged and filtered (CF), or ultrafiltered (UF). Attachment of [(14)C]COM crystals to Madin-Darby canine kidney (MDCK) cells was studied after precoating the crystals or the cells with the urine fractions and by using the same fractions as the binding medium. Protein content of the fractions and precoated crystals was analyzed with SDS-PAGE and Western blotting. All urine fractions inhibited crystal attachment. When fractions from the six urine samples were used to precoat the cells, the median inhibitions of crystal adhesion ( approximately 40%) were not significantly different. Median inhibition after preincubation of crystals was the same for the S, C, and CF fractions ( approximately 40%) but significantly greater than for the UF fraction ( approximately 28%). When fractions were used as the binding medium, median inhibitions decreased from 64% in the S fraction to 47 (C), 42 (CF), and to 29% (UF). SDS-PAGE analysis showed that centrifugation and filtration reduced the amount of Tamm-Horsfall glycoprotein (THG), which was confirmed by Western blotting. Human serum albumin, urinary prothrombin fragment 1, and osteopontin, but not THG, were present in demineralized extracts of the precoated crystals. Fractionation of human urine affects the attachment of COM crystals to MDCK cells. Hence future studies investigating regulation of crystal-cell interactions should be carried out in untreated urine as the binding medium.

Acute oxalate nephropathy: A new etiology for acute renal failure following nonrenal solid organ transplantation

Acute renal insufficiency (ARI) is a frequent complication of nonrenal solid organ transplantation and may be responsible for an unfavorable outcome, particularly if dialysis is required. The etiology of post-transplantation ARI is poorly understood, with only isolated clinical cases being reported, most imputed to drug toxicity. We report here, the first three observations of irreversible ARI associated with acute oxalate nephropathy (AON) in the course of nonrenal organ transplants: a lung transplant and a lung-liver transplant in two patients with mucoviscidosis, and a cardiac transplant. The diagnosis of AON was made histologically. In all three cases, the ARI supervened after prolonged consumption of antibiotics capable of interfering with the colonic flora, and leading to enteric hyperoxaluria. The recognition of AON as a cause of post-transplantation, ARI underlines hyperoxaluria and digestive hyperabsorption of oxalate as specific risk factors for AON and should permit better posttransplant care of these patients.

Crystals cause acute necrotic cell death in renal proximal tubule cells, but not in collecting tubule cells

BACKGROUND

The interaction between renal tubular cells and crystals generated in the tubular fluid could play an initiating role in the pathophysiology of calcium oxalate nephrolithiasis. Crystals are expected to form in the renal collecting ducts, but not in the proximal tubule. In the present investigation, we studied the damaging effect of calcium oxalate crystals on renal proximal and collecting tubule cells in culture.

METHODS

Studies were performed with the renal proximal tubular cell lines, porcine proximal tubular cells (LLC-PK(1)) and Madin-Darby canine kidney II (MDCK-II) and the renal collecting duct cell lines, RCCD(1) and MDCK-I. Confluent monolayers cultured on permeable growth substrates in a two-compartment culture system were apically exposed to calcium oxalate monohydrate crystals, after which several cellular responses were studied, including monolayer morphology (confocal microscopy), transepithelial electrical resistances (TER), prostaglandin E(2) (PGE(2)) secretion, DNA synthesis ([(3)H]-thymidine), total cell numbers, reactive oxygen species [hydrogen peroxide (H(2)O(2))] generation, apoptotic (annexin V and DNA fragmentation), and necrotic (propidium iodide influx) cell death.

RESULTS

Crystals were rapidly taken up by proximal tubular cells and induced a biphasic response. Within 24 hours approximately half of the cell-associated crystals were released back into the apical fluid (early response). Over the next 2 weeks half of the remaining internalized crystals were eliminated (late response). The early response was characterized by morphologic disorder, increased synthesis of PGE(2), H(2)O(2), and DNA and the release of crystal-containing cells from the monolayers. These released cells appeared to be necrotic, but not apoptotic cells. Scrape-injured monolayers generated even higher levels of H(2)O(2) than those generated in response to crystals. During the late response, crystals were gradually removed from the monolayers without inflammation-mediated cell death. Crystals did not bind to, were not taken up by, and did not cause marked responses in collecting tubule cells.

CONCLUSION

This study shows that calcium oxalate crystals cause acute inflammation-mediated necrotic cell death in renal proximal tubular cells, but not in collecting tubule cells. The crystal-induced generation of reactive oxygen species by renal tubular cells is a general response to tissue damage and the increased levels of DNA synthesis seem to reflect regeneration rather than growth stimulation. As long as the renal collecting ducts are not obstructed with crystals, these results do not support an important role for crystal-induced tissue injury in the pathophysiology of calcium oxalate nephrolithiasis.

Calcium phosphate and calcium oxalate crystal handling is dependent upon CLC-5 expression in mouse collecting duct cells

Defects in an intracellular chloride channel CLC-5 cause Dent's disease, an inherited kidney stone disorder. Using a collecting duct model, mIMCD-3 cells, we show expression of dimeric mCLC-5. Transient transfection of antisense CLC-5 reduces CLC-5 protein expression. Binding of both calcium phosphate (hydroxyapatite) and calcium oxalate monohydrate (COM) crystals overlaid onto mIMCD-3 cultures was affected by altered CLC-5 expression. Calcium phosphate crystal agglomerations (>10 microm) were minimal in control (9%) and sense (13%) CLC-5-transfected cells, compared to 66% of antisense CLC-5-transfected cells (P<0.001). Small calcium phosphate crystals (<10 microm) were found associated with 45% of sense CLC-5-treated cells, of which the majority (11/14 cells) appeared to be internalised within the cell. Calcium oxalate agglomerations (>10 microm) were also largely absent for controls or sense mCLC-5 transfectants (11% and 9% of cells, respectively) with COM crystal agglomerates predominating in antisense CLC-5 transfectants (66%, P<0.0001). We conclude that collecting duct cells with reduced CLC-5 expression lead to a tendency to form calcium crystal agglomeration, which may help explain the nephrocalcinosis and nephrolithiasis seen in Dent's disease.

The influence of oxalate on renal epithelial and interstitial cells

Most renal stones in humans are composed of calcium oxalate. An increase in urinary oxalate levels has been shown to result in renal epithelial cell injury and crystal retention. However, the underlying mechanisms are unclear. Although the localization of primary stone formation and the associated cells playing the pivotal role in stone formation are still unknown, renal epithelial cells and interstitial cells seem to be involved in this process. The aim of this study was to evaluate the effects of oxalate on distinct renal epithelial and endothelial cells as well as fibroblasts. The first part focused on the toxicity of oxalate on the cells and a potential time- and dose-dependency. In the second part, renal epithelial cells were cultured in a two-compartment model to examine the vulnerability of the tubular or basolateral side to oxalate. LLCPK1, MDCK, renal fibroblast and endothelial cell lines were cultured under standard conditions. In part 1, cells were grown in standard culture flasks until confluent layers were achieved. Sodium oxalate was delivered at final concentrations of 1, 2 and 4 mM to either the apical or basolateral side (plain medium was delivered to the contralateral side). Cell survival was assessed microscopically by trypan blue staining after 1, 2 and 4 h. The influence of oxalate on proliferation and apoptosis induction was also investigated. In the second part, MDCK and LLCPK1 cells were grown in 6-well plates until confluent layers were achieved. Sodium oxalate at the above concentrations was applied, to either the apical or basolateral side and plain medium was delivered to the opposite side. The same protocol was then followed as in part 1. Part 1: sodium oxalate led to a time- and concentration-dependent decline in cell survival that was comparable in LLCPK1 and MDCK. Non-tubular cell lines like fibroblasts and endothelial cells were significantly more vulnerable to oxalate. These observations were reflected by significant impairment to cell proliferation. We could not demonstrate an induction of apoptosis in any cell line. Part 2: both cell lines were more vulnerable to oxalate on the basolateral side. This effect was more pronounced in MDCK cells at high oxalate concentrations (4 mM). Cells are apparently more resistant on the apical (tubular) side. Our results show that sodium oxalate has a negative effect on the growth and survival of renal epithelial cells and, to a greater extent, also fibroblasts and endothelial cells. We could not demonstrate any induction of apoptotic processes which implies a direct induction of cell necrosis. The finding of interstitial calcification and the proximity of tubules, vessels and interstitial cells make involvement of non-tubular renal cells in tissue calcification processes possible. Renal epithelial cells are apparently more vulnerable to oxalate on their basolateral side. Therefore, calcification processes within the interstitium may exert pronounced toxic effects to these cells, leading to inflammation and necrosis. These observations further support the idea of the interstitium as a site of primary stone formation.