The solubility of calcium oxalate in tissue culture media

An in vitro model system based on calcium- and phosphate ion-induced hMSC spheroid mineralization

A challenge in regenerative medicine is creating the three-dimensional organic and inorganic in vitro microenvironment of bone, which would allow the study of musculoskeletal disorders and the generation of building blocks for bone regeneration. This study presents a microwell-based platform for creating spheroids of human mesenchymal stromal cells, which are then mineralized using ionic calcium and phosphate supplementation. The resulting mineralized spheroids promote an osteogenic gene expression profile through the influence of the spheroids' biophysical environment and inorganic signaling and require less calcium or phosphate to achieve mineralization compared to a monolayer culture. We found that mineralized spheroids represent an in vitro model for studying small molecule perturbations and extracellular mediated calcification. Furthermore, we demonstrate that understanding pathway signaling elicited by the spheroid environment allows mimicking these pathways in traditional monolayer culture, enabling similar rapid mineralization events. In sum, this study demonstrates the rapid generation and employment of a mineralized cell model system for regenerative medicine applications.

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.

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.

Calcium oxalate crystals in fetal bovine serum: implications for cell culture, phagocytosis and biomineralization studies in vitro

Cell culture methods and models are key investigative tools for cell and molecular biology studies. Fetal bovine serum (FBS) is commonly used as an additive during cell culture since its constituents promote cell survival, proliferation and differentiation. Here we report that commercially available FBS from different major suppliers consistently contain precipitated, calcium oxalate crystals-either in the monohydrate (COM) or dihydrate (COD) form. Mineral structure and phase identification of the crystals were determined by X-ray diffraction, chemical composition by energy-dispersive X-ray microanalysis, and imaging and measurement of crystal growth steps by atomic force microscopy-all identified and confirmed crystallographic parameters for COM and COD. Proteins binding to the crystals were identified by immunoblotting, revealing the presence of osteopontin and fetuin-A (alpha(2)HS-glycoprotein)--known regulators of crystal growth found in serum. Macrophage cell cultures exposed to calcium oxalate crystals showed internalization of the crystals by phagocytosis in a process that induced disruption of cell-cell adhesion, release of reactive oxygen species and membrane damage, events that may be linked to the release of inflammatory cytokines by these cells into the culture media. In conclusion, calcium oxalate crystals found in commercially available FBS are toxic to cells, and their presence may confound results from in vitro studies where, amongst others, phagocytosis, biomineralization, renal cell and molecular biology, and drug and biomaterial testing are being examined.

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.

Crystal Retention in Renal Stone Disease: A Crucial Role for the Glycosaminoglycan Hyaluronan?

The mechanisms that are involved in renal stone disease are not entirely clear. In this article, the various concepts that have been proposed during the past century are reviewed briefly and integrated into current insights. Much attention is dedicated to hyaluronan (HA), an extremely large glycosaminoglycan that may play a central role in renal stone disease. The precipitation of poorly soluble calcium salts (crystal formation) in the kidney is the inevitable consequence of producing concentrated urine. HA is a major constituent of the extracellular matrix in the renal medullary interstitium and the pericellular matrix of mitogen/stress-activated renal tubular cells. HA is an excellent crystal-binding molecule because of its size, negative ionic charge, and ability to form hydrated gel-like matrices. Crystal binding to HA leads to crystal retention in the renal tubules (nephrocalcinosis) and to the formation of calcified plaques in the renal interstitium (Randall's plaques). It remains to be determined whether one or both forms of renal crystal retention are involved in the development of kidney stones (nephrolithiasis).

Neurotoxicity of oxaliplatin and cisplatin for dorsal root ganglion neurons correlates with platinum-DNA binding

Cisplatin has been in use for 40 years, primarily for treatment of ovarian and testicular cancer. Oxaliplatin is the only effective treatment for metastatic colorectal cancer. Neurotoxicity occurs in up to 30% of patients and is dose-limiting for both drugs. The neuropathy is characterized by selective sensory loss in the extremities. Cisplatin treatment is associated with high levels of Pt-DNA binding and apoptosis of dorsal root ganglion (DRG) neurons. In this study, we directly compared the effects of oxaliplatin on DRG in vitro. Compared with cisplatin, oxaliplatin formed fewer Pt-DNA adducts following 6, 12, 24, and 48h (0.007ng Pt/mug DNA, 0.012ng/microg, 0.011ng/microg, 0.011ng/microg versus 0.014ng/microg, 0.022ng/microg, 0.041ng/microg, 0.030ng/microg), respectively. These findings closely correlated with data on cell survival where equimolar concentrations of oxaliplatin induced less cell death than cisplatin. Oxaliplatin-induced DRG death was associated with the morphological characteristics of apoptosis defined by 4'-6-diamidino-2-phenylindole and annexin/propidium iodide staining. Death was completely inhibited by the caspase inhibitor z-VAD-fmk. Our results demonstrate that both compounds cause apoptosis of DRG neurons but compared to cisplatin, oxaliplatin forms fewer Pt-DNA adducts and is less neurotoxic to DRG neurons in vitro.

Oxalate is toxic to renal tubular cells only at supraphysiologic concentrations

BACKGROUND

Oxalate-induced tissue damage may play an initiating role in the pathophysiology of calcium oxalate nephrolithiasis. The concentration of oxalate is higher in the renal collecting ducts ( approximately 0.1 to 0.5 mmol/L) than in the proximal tubule ( approximately 0.002 to 0.1 mmol/L). In the present investigation, we studied the damaging effect of oxalate to renal proximal and collecting tubule cells in culture.

METHODS

Studies were performed with the renal proximal tubular cell lines, LLC-PK1 and Madin Darby canine kidney II (MDCK-II), and the renal collecting duct cell lines, rat renal cortical collecting duct (RCCD1) and MDCK-I. Confluent monolayers cultured on permeable growth substrates in a two-compartment culture system were apically exposed for 24 hours to relatively low (0.2, 0.5, and 1.0 mmol/L) and high (5 and 10 mmol/L) oxalate concentrations, after which several cellular responses were studied, including monolayer morphology (confocal microscopy), transepithelial electrical resistances (TER), prostaglandin E(2) (PGE(2)) secretion, lactate dehydrogenase (LDH) release, DNA synthesis ([(3)H]-thymidine incorporation), total cell numbers, reactive oxygen species (H(2)O(2)) generation, apoptotic (annexin V and DNA fragmentation), and necrotic (propidium iodide influx) cell death.

RESULTS

Visible morphologic alterations were observed only at high oxalate concentrations. TER was concentration-dependently decreased by high, but not by low, oxalate. Elevated levels of PGE(2), LDH, and H(2)O(2) were measured in both cell types after exposure to high, but not to low oxalate. Exposure to high oxalate resulted in elevated levels of DNA synthesis with decreasing total cell numbers. High, but not low, oxalate induced necrotic cell death without signs of programmed cell death.

CONCLUSION

This study shows that oxalate is toxic to renal tubular cells, but only at supraphysiologic concentrations.

Effects of luminal oxalate or calcium oxalate on renal tubular cells in culture

Oxalate or calcium oxalate crystal-induced tissue damage could be conducive to renal stone disease. We studied the response of renal proximal (LLC-PK1 and MDCK-II) and collecting (RCCD1 and MDCK-I) tubule cell lines to oxalate ions as well as to calcium oxalate monohydrate (COM) crystals. Cells grown on tissue culture plastic or permeable growth substrates were exposed to high (1 mM) and extremely high (5 and 10 mM) oxalate concentrations, or to a relatively large quantity of crystals (146 microg), after which cell morphology, prostaglandin E(2) (PGE(2)) secretion, [(3)H]thymidine incorporation, total cell numbers and various forms of cell death were studied. Morphological alterations, increased PGE(2) secretion, elevated levels of DNA synthesis and necrotic cell death were induced by extremely high, but not by high oxalate. Crystals were rapidly internalized by proximal tubular cells, which stimulated PGE(2) secretion and DNA synthesis and the release of crystal-containing necrotic cells from the monolayer. Crystals did not bind to, were not taken up by, and did not cause marked responses in collecting tubule cells. These results show that free oxalate is toxic only at supraphysiological concentrations and that calcium oxalate is toxic only to renal tubular cells that usually do not encounter crystals. Based on these results, it is unlikely that oxalate anions or calcium oxalate crystals are responsible for the tissue damage that may precede renal stone formation.

Oxalate, calcium and ash intake and excretion balances in fat sand rats (Psammomys obesus) feeding on two different diets

Fat sand rats Psammomys obesus feed exclusively on plants of the family Chenopodiaceae, which contain high concentrations of chloride salts (NaCl, KCl) and oxalate salts. Ingestion of large quantities of oxalate is challenging for mammals because oxalate chelates Ca(2+) cations, reducing Ca(2+) availability. Oxalate is a metabolic end-point in mammalian metabolism, however it can be broken-down by intestinal bacteria. We predicted that in fat sand rats microbial breakdown of oxalate will be substantial due to the high dietary load. In addition, since a high concentration of soluble chloride salts increases the solubility of calcium oxalate in solution, we examined whether a change in the intake of chloride salts affects microbial oxalate breakdown and calcium excretion in fat sand rats. We measured oxalate, calcium and other inorganic matter (ash) intake and excretion in fat sand rats feeding on two different diets: saltbush (Atriplex halimus), their natural diet, and goose-foot (Chenopodium album), a non-native chenopod on which fat sand rats will readily feed and that has a similar oxalate content to saltbush but only 2/3 of the ash content. In animals feeding on both diets, 65-80% of the oxalate ingested did not appear in urine or faeces. In animals consuming the more saline saltbush, significantly more oxalate was apparently degraded (p<0.001), while significantly less oxalate was excreted in urine (p<0.01) and in faeces (p<0.05). We propose, therefore, that fat sand rats rely on symbiotic bacteria to remove a large portion of the oxalates ingested with their diet, and that the high dietary salt intake may play a beneficial role in their oxalate and calcium metabolism.

Apoptosis induced by oxalate in human renal tubular epithelial HK-2 cells

Oxalate is not only considered to be one of the main constituents of urinary stones, but it also has toxic effects on renal tubular epithelial cells, affecting the pathogenesis of nephrolithiasis. We tried to elucidate the effects of oxalate on human renal tubular epithelial cells (HK-2 cells). In addition, we investigated whether the toxic effect of oxalate occurs by apoptosis, and determined the expression of Bcl-2 family and caspase 9 proteins known as apoptosis-related protein. HK-2 cells were incubated with different concentrations of oxalate, and the effect of oxalate on the growth of the cells was assessed by MTT assay. A caspase-3 activity assay and TUNEL assay were performed on HK-2 cells after oxalate exposure in order to evaluate apoptosis. Immunoblot analysis of Bax, Bcl-2, Bcl-xL, and caspase-9 were performed. Oxalate exposure resulted in significant growth inhibition of HK-2 cells as oxalate concentrations increased. The toxic effect of oxalate on HK-2 cells was considered to occur through apoptosis, as suggested by the increase of caspase-3 activity. The percentage of positive nuclei stained using the TUNEL method was 18+/-2.3 in oxalate-treated cells and 2.5+/-0.9 in untreated cells (P<0.05). Bax and caspase-9 protein expression increased significantly as oxalate concentrations increased, but Bcl-2 protein expression decreased. There was no difference in Bcl-xL protein expression among the various concentrations of oxalate. Our results show that oxalate has a toxic effect on HK-2 cells and that this effect is induced by apoptosis, which may be mediated by an intrinsic pathway.

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.

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.

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

BACKGROUND

Crystal retention in the kidney is caused by the interaction between crystals and the cells lining the renal tubules. These interactions involve crystal attachment, followed by internalization or not. Here, we studied the ability of various renal tubular cell lines to internalize calcium oxalate monohydrate (COM) crystals.

METHODS

Crystal-cell interactions are studied by light-, electron-, and confocal microscopy with cells resembling the renal proximal tubule [porcine kidney (LLC-PK1)], proximal/distal tubule [Madin-Darby canine kidney II (MDCK-II)], and distal tubule and/or collecting ducts [(Madin-Darby canine kidney I (MDCK-I), rat cortical collecting duct 1 (RCCD1)]. Crystal-binding strength and internalization are characterized and quantified with radiolabeled COM.

RESULTS

Microscopy studies showed that crystals were firmly embedded in the membranes of LLC-PK1 and MDCK-II cells to be subsequently internalized. On the other hand, crystals bound only loosely to MDCK-I and RCCD1 and were not taken up by these cells. Crystal uptake by LLC-PK1 and MDCK-II, expressed in microg/10(6) cells, is temperature-dependent and gradually increases from 0.88 and 0.15 in 30 minutes, respectively, to 4.70 and 3.85, respectively, after five hours, whereas these values never exceeded background levels in MDCK-I and RCCD1 cells.

CONCLUSION

The adherence of COM crystals to renal cells with properties of the proximal tubule is inevitable and actively followed by their uptake, whereas crystals attached to cells resembling the distal tubule and/or collecting duct are not internalized. Since crystal formation usually occurs in segments beyond the renal proximal tubule, crystal uptake may be of less importance in the etiology of idiopathic calcium oxalate stone disease.

Protective role of heparin/heparan sulfate on oxalate-induced changes in cell morphology and intracellular Ca2+

Alterations in intracellular Ca2+ ([Ca2+]i) are generally associated with cellular distress. Oxalate-induced cell injury of the renal epithelium plays an important role in promoting CaOx nephrolithiasis. However, the degree of change in intracellular free calcium ions in renal epithelial cells during oxalate exposure remains unclear. The aim of this study is to determine whether acute short-term exposure to oxalate produces morphological changes in the cells, induces a change in cytosolic Ca2+ levels in renal tubular epithelial cells and whether the application of extracellular glycosaminoglycans (GAGs) prevents these changes. Cultured Mardin-Darby canine kidney cells were exposed to oxalate, and changes in cytosolic Ca2+ were determined under various conditions. The effect of heparin and heparan sulfate (HS) during oxalate exposure was examined. The change in the GAG contents of the culture medium was also determined. Transmission electron microscopy (TEM) was performed for morphological analysis. The degree of change in cytosolic Ca2+ strongly correlated with oxalate concentration. Cytosolic Ca2+ levels decreased in parallel with an increase in the concentration of oxalate. However, this decrease was strongly inhibited by pretreatment with heparin or HS. TEM revealed cytoplasmic vacuolization, the appearance of flocculent material and mitochondrial damage after oxalate exposure. On the other hand, pretreatment with heparin or HS completely blocked these morphological changes. The present data suggest that acute exposure to a high concentration of oxalate challenges the renal cells, diminishes their viability and induces changes in cytosolic Ca2+ levels. Heparin and HS, which are known as potent inhibitors of CaOx crystallization, may also prevent oxalate-induced cell changes by stabilizing the cytosolic Ca2+ level.