Calcium oxalate dihydrate crystal induced changes in glycoproteome of distal renal tubular epithelial cells

Calcium oxalate dihydrate (COD) crystals can adhere onto the apical surface of renal tubular epithelial cells. This process is associated with crystal growth and aggregation, resulting in kidney stone formation. Glycoproteins have been thought to play roles in response to crystal adhesion. However, components of the glycoproteome that are involved in this cellular response remain largely unknown. Our present study therefore aimed to identify altered glycoproteins upon COD crystal adhesion onto tubular epithelial cells representing distal nephron, the initiating site of kidney stone formation. Madin-Darby Canine Kidney (MDCK) cells were maintained in culture medium with or without COD crystals for 48 h (n = 5 flasks per group). Cellular proteins were extracted, resolved by 2-DE and visualized by SYPRO Ruby total protein stain, whereas glycoproteins were detected by Pro-Q Emerald glycoprotein dye. Spot matching and quantitative intensity analysis revealed 16 differentially expressed glycoprotein spots, whose corresponding total protein levels were not changed by COD crystal adhesion. These altered glycoproteins were successfully identified by Q-TOF MS and/or MS/MS analyses, and potential glycosylation sites were identified by the GlycoMod tool. For example, glycoforms of three proteasome subunits (which have a major role in regulating cell-cell dissociation) were up-regulated, whereas a glycoform of actin-related protein 3 (ARP3) (which plays an important role in cellular integrity) was down-regulated. These coordinated changes implicate that COD crystal adhesion induced cell dissociation and declined cellular integrity in the distal nephron. Our findings provide some novel insights into the pathogenic mechanisms of kidney stone disease at the molecular level, particularly cell-crystal interactions.     

Proteomic analysis of calcium oxalate monohydrate crystal-induced cytotoxicity in distal renal tubular cells

Calcium oxalate monohydrate (COM) is the major crystalline component found in kidney stones and its adhesion to renal tubular cells provokes tubular injury, which in turn enhances COM crystal adhesion. However, COM-induced toxic effects in these tubular cells remain largely unknown. We performed a proteomics study to characterize changes in the cellular proteome in MDCK distal renal tubular cells after an exposure to high-dose (1000 microg/mL) COM crystals for 48 h, at which percentage of cell death was significantly increased. Proteins were extracted from MDCK cells cultured with COM-containing or COM-free medium ( n = 5 individual flasks per group), resolved in individual 2-D gels, and stained with SYPRO Ruby fluorescence dye. Quantitative and statistical analyses revealed 53 proteins whose abundance levels were altered (25 were increased, whereas other 28 were decreased) by COM-induced toxicity. Among these, 50 were successfully identified by quadrupole time-of-flight (Q-TOF) mass spectrometry (MS) and/or tandem MS (MS/MS) analyses. The proteomic data were clearly confirmed by 2-D Western blot analysis. While three chaperones (GRP78, Orp150 and Hsp60) were increased, other proteins involved in protein biosynthesis, ATP synthesis, cell cycle regulator, cellular structure, and signal transduction were decreased. These data provide some novel mechanistic insights into the molecular mechanisms of COM crystal-induced tubular toxicity.     

Large-scale identification of calcium oxalate monohydrate crystal-binding proteins on apical membrane of distal renal tubular epithelial cells

Adhesion of calcium oxalate monohydrate (COM) crystals onto apical surface of renal tubular epithelial cells is a crucial mechanism for crystal retention, leading to kidney stone formation. Various proteins on apical membrane may bind to COM crystals; however, these crystal-binding proteins remained unidentified. The present study therefore aimed to identify COM crystal-binding proteins on apical membrane of distal renal tubular epithelial cells. Madin-Darby Canine Kidney (MDCK) cells were cultivated to be polarized epithelial cells and apical membrane was isolated from these cells using a peeling method established recently. Enrichment and purity of isolated apical membrane were confirmed by Western blot analysis for specific markers of apical (gp135) and basolateral (Na(+)/K(+)-ATPase) membranes. Proteins derived from the isolated apical membrane were then resuspended in artificial urine and incubated with COM crystals. The bound proteins were eluted, resolved by SDS-PAGE, and analyzed by Q-TOF MS and MS/MS, which identified 96 proteins. Among these, expression and localization of annexin II on apical surface of MDCK cells were confirmed by Western blot analysis, immunofluorescence staining, and laser-scanning confocal microscopic examination. Finally, the function of annexin II as the COM crystal-binding protein was successfully validated by COM crystal-binding assay. This large data set offers many opportunities for further investigations of kidney stone disease and may lead to the development of new therapeutic targets.     

Response of renal tubular cells to differential types and doses of calcium oxalate crystals: Integrative proteome network analysis and functional investigations

We have previously identified changes in the cellular proteome of renal tubular cells induced by low‐dose (100 μg/mL) and high‐dose (1000 μg/mL) calcium oxalate monohydrate (COM) and dihydrate (COD) crystals. However, the functional significance of such expression data remained unclear. In this study, we performed comparative analyses and functional investigations of four proteomic datasets to define potential mechanisms by which renal tubular cells responded to differential crystal types and doses. The data showed that high‐dose induced greater changes than low‐dose, whereas COM induced more changes than COD. Luciferin–luciferase ATP assay revealed increased intracellular ATP level by high‐dose of both COM and COD. OxyBlot assay and Western blotting showed accumulated intracellular oxidized proteins but decreased ubiquitinated proteins by high‐dose of both crystals. Flow cytometric analysis of cell death showed that high‐dose of both crystals, particularly COM, significantly increased cell death. Also, crystal adhesion assay showed higher degree of cell–crystal adhesion in high‐dose and COM when compared to low‐dose and COD, respectively. Finally, pretreatment of epigallocatechin‐3‐gallate revealed a protective effect on COM/COD crystals‐induced oxidative stress and cell‐crystal adhesion. Collectively, these data may provide a better understanding of cellular responses of renal tubular cells to COM/COD crystals in kidney stone disease.     

Surface heat shock protein 90 serves as a potential receptor for calcium oxalate crystal on apical membrane of renal tubular epithelial cells

Adhesion of calcium oxalate monohydrate (COM) crystals on renal tubular epithelial cells is a crucial step in kidney stone formation. Finding potential crystal receptors on the apical membrane of the cells may lead to a novel approach to prevent kidney stone disease. Our previous study identified a large number of crystal-binding proteins on the apical membrane of MDCK cells. However, their functional role as potential crystal receptors had not been validated. The present study aimed to address the potential role of heat shock protein 90 (HSP90) as a COM crystal receptor. The apical membrane was isolated from polarized MDCK cells by the peeling method and recovered proteins were incubated with COM crystals. Western blot analysis confirmed the presence of HSP90 in the apical membrane and the crystal-bound fraction. Immunofluorescence staining without permeabilization and laser-scanning confocal microscopy confirmed the surface HSP90 expression on the apical membrane of the intact cells. Crystal adhesion assay showed that blocking surface HSP90 by specific anti-HSP90 antibody and knockdown of HSP90 by small interfering RNA (siRNA) dramatically reduced crystal binding on the apical surface of MDCK cells (by approximately 1/2 and 2/3, respectively). Additionally, crystal internalization assay revealed the presence of HSP90 on the membrane of endocytic vesicle containing the internalized COM crystal. Moreover, pretreatment of MDCK cells with anti-HSP90 antibody significantly reduced crystal internalization (by approximately 1/3). Taken together, our data indicate that HSP90 serves as a potential receptor for COM crystals on the apical membrane of renal tubular epithelial cells and is involved in endocytosis/internalization of the crystals into the cells.     

Protective Cellular Mechanism of Estrogen Against Kidney Stone Formation: A Proteomics Approach and Functional Validation

Females have less incidence/prevalence of kidney stone disease than males. Estrogen thus may serve as the protective factor but with unclear mechanism. This study explores cellular mechanism underlying such stone preventive mechanism of estrogen. Madin darby canine kidney (MDCK) renal tubular cells are incubated with or without 20 nm 17β‐estradiol for 7 days. Comparative proteomics reveals 58 differentially expressed proteins in estrogen‐treated versus control cells that are successfully identified by nanoLC–ESI–Q‐TOF‐MS/MS. Interestingly, these altered proteins are involved mainly in “binding and receptor,” “metabolic process,” and “migration and healing” networks. Functional investigations demonstrate reduction of calcium oxalate (CaOx) crystal‐binding capability of the estrogen‐treated cells consistent with the decreased levels of annexin A1 and α‐enolase (the known CaOx crystal‐binding receptors) on the cell surface. High‐calcium and high‐oxalate challenge initially enhances surface expression of annexin A1 and α‐enolase, respectively, both of which return to their basal levels by estrogen. Additionally, estrogen reduces intracellular ATP level and promotes cell migration and tissue healing. Taken together, estrogen causes changes in cellular proteome of renal tubular cells that lead to decreased surface expression of CaOx crystal receptors, reduced intracellular metabolism, and enhanced cell proliferation and tissue healing, all of which may contribute, at least in part, to stone prevention.     

High calcium enhances calcium oxalate crystal binding capacity of renal tubular cells via increased surface annexin A1 but impairs their proliferation and healing

Hypercalciuria is associated with kidney stone formation and impaired renal function. However, responses of renal tubular cells upon exposure to high-calcium environment remain largely unknown. We thus performed a proteomic analysis of altered proteins in renal tubular cells induced by high-calcium and evaluated functional significance of these changes. MDCK cells were maintained with or without 20 mM CaCl(2) for 72 h. Cellular proteins were then analyzed by two-dimensional electrophoresis (2-DE) (n = 5 gels derived from 5 independent culture flasks per group). Spot matching and quantitative intensity analysis revealed 20 protein spots (from a total of 700) that were differentially expressed between the two groups. These altered proteins were then identified by Q-TOF-MS and MS/MS analyses, including those involved in calcium binding, protein synthesis, carbohydrate metabolism, lipid metabolism, cell proliferation, mitosis regulation, apoptosis, cell migration, oxidative stress, and ion transport. Protein network analysis and functional validation revealed that high-calcium-exposed cells had 36.5% increase in calcium oxalate monohydrate (COM) crystal-binding capacity. This functional change was consistent to the expression data in which annexin A1 (ANXA1), a membrane-associated calcium-binding protein, was markedly increased on the apical surface of high-calcium-exposed cells. Pretreatment with anti-ANXA1 antibody could neutralize this increasing crystal-binding capacity. Moreover, high-calcium exposure caused defects in cell proliferation and wound healing. These expression and functional data demonstrate the enhanced crystal-binding capacity but impaired cell proliferation and wound healing in renal tubular cells induced by high-calcium. Taken together, these phenomena may contribute, at least in part, to the pathogenic mechanisms of hypercalciuria-induced nephrolithiasis and impaired renal function. Our in vitro study offers several candidates for further targeted functional studies to confirm their relevance in hypercalciuria and kidney stone disease in vivo.     

Renal tubular cell membranes inhibit growth but promote aggregation of calcium oxalate monohydrate crystals

Cell membranes have been proposed to serve as promoters for calcium oxalate monohydrate (COM) kidney stone formation. However, direct evidence to demonstrate the modulatory effects of renal tubular cell membranes on COM crystals does not currently exist. We thus examined the effects of intact MDCK cells and their fragmented membranes on COM crystal growth, aggregation and transformation. COM crystals were generated in the absence (control) or presence of intact MDCK cells or their membrane fragments. Intact MDCK cells and their membrane fragments significantly inhibited COM crystal growth (22.6% and 25.2% decreases in size, respectively) and significantly reduced COM total crystal mass (23.1% and 25.6% decreases, respectively). In contrast, both of them markedly promoted crystal aggregation (1.9-fold and 3.2-fold increases, respectively). Moreover, both intact cells and membrane fragments could transform COM to calcium oxalate dihydrate (COD) crystals. Finally, COM crystal growth inhibitory activities of both membrane forms were successfully confirmed by a spectrophotometric oxalate-depletion assay. Our data provide the first direct evidence to demonstrate the dual modulatory effects of MDCK membranes on COM crystals. Although growth of individual COM crystals was inhibited, their aggregation was promoted. These findings provide additional insights into the mechanisms of COM kidney stone formation.     

Macropinocytosis is the Major Mechanism for Endocytosis of Calcium Oxalate Crystals into Renal Tubular Cells

During an initial phase of kidney stone formation, the internalization of calcium oxalate (CaOx) crystals by renal tubular cells has been thought to occur via endocytosis. However, the precise mechanism of CaOx crystal endocytosis remained unclear. In the present study, MDCK renal tubular cells were pretreated with inhibitors specific to individual endocytic pathways, including nystatin (lipid raft/caveolae-mediated), cytochalasin D (actin-dependent or macropinocytosis), and chlorpromazine (CPZ; clathrin-mediated) before exposure to plain (non-labeled), or fluorescence-labeled CaOx monohydrate (COM) crystals. Quantitative analysis by flow cytometry revealed that pretreatment with nystatin and CPZ slightly decreased the crystal internalization, whereas the cytochalasin D pretreatment caused a marked decrease in crystal uptake. Immunofluorescence study and laser-scanning confocal microscopic examination confirmed that the cytochalasin D-pretreated cells had dramatic decrease of the internalized crystals, whereas the total number of crystals interacted with the cells was unchanged (crystals could adhere but were not internalized). These data have demonstrated for the first time that renal tubular cells endocytose COM crystals mainly via macropinocytosis. These novel findings will be useful for further tracking the endocytosed crystals inside the cells during the course of kidney stone formation.     

Accumulation of 8-oxoguanine in the cellular DNA and the alteration of the OGG1 expression during ischemia-reperfusion injury in the rat kidney

During ischemia-reperfusion (I/R) injury in the rat kidney, apoptosis was observed in the distal tubules of the cortico-medullary region and outer medulla (OM) while severe necrosis was seen in the proximal straight tubules of the OM. The majority of these changes disappeared within 2 weeks. We examined the contents of 8-oxo-2'-deoxyguanosine (8-oxo-dG), which is a major type of oxidative damage in DNA, in the rat kidney during I/R injury, and also investigated the expression level of the OGG1 gene encoding the 8-oxoguanine DNA glycosylase. High-performance liquid chromatography with an MS/MS analysis of the nuclear DNA revealed an immediate accumulation of 8-oxo-dG in the nuclear DNA prepared from the cortex and OM of the kidney 1h after I/R, and an immunohistochemical analysis demonstrated the immediate accumulation of 8-oxo-dG in the nuclei of renal tubular cells both in the cortex and OM. A delayed increase of cytoplasmic staining with anti-8-oxo-dG was observed only in the cortico-medulla and OM, where the cytoplasmic staining in the proximal tubular cells is higher than in the distal tubular cells. The level of cytoplasmic staining representing 8-oxo-dG in mitochondrial DNA, peaked at 6h after I/R and preceded the necrosis of proximal tubular cells in the OM. An RNase protection assay showed a high level of OGG1 mRNA in the normal kidney, and the level decreased within 3h only in the OM, and increased thereafter 1-7 days of I/R both in the cortex and OM. In situ hybridization showed higher levels of OGG1 mRNA expression in the renal tubules in the OM than in the cortex of the normal kidney, which decreased rapidly within 3h of I/R. Thus, the accumulation of 8-oxo-dG in the mitochondrial DNA rather than in nuclear DNA is likely to be involved in the pathogenic responses such as necrosis of renal tubular cells during I/R injury of the kidney, together with an altered level of OGG1 expression.     

Protein Network Analysis and Functional Studies of Calcium Oxalate Crystal‐Induced Cytotoxicity in Renal Tubular Epithelial Cells

Our previous expression study has reported a set of proteins with altered levels in renal tubular cells after exposure to calcium oxalate monohydrate (COM) crystals, which are the main composition of kidney stones. However, their functional significance remained largely unknown. In this study, protein network analysis revealed that the significantly altered proteins induced by COM crystals were involved mainly in three main functional networks, including i) cell proliferation and wound healing; ii) oxidative stress and mitochondrial function; and iii) cellular junction complex and integrity. Cell proliferation and wound healing assays showed that the COM‐treated cells had defective proliferation and tissue healing capability, respectively. Oxyblot analysis demonstrated accumulation of the oxidized proteins, whereas intracellular ATP level was significantly increased in the COM‐treated cells. Additionally, level of zonula occludens‐1 (ZO‐1), a tight junction protein, was significantly decreased, consistent with the significant declines in transepithelial resistance (TER) and level of RhoA signaling molecule in the COM‐treated cells. These findings indicate significant perturbations in mitochondrial and oxidative stress axis that cause defective cell proliferation, tissue healing capability, junctional protein complex, and cellular integrity of renal tubular epithelial cells exposed to COM crystals that may play important roles in kidney stone pathogenesis.     

Allicin attenuates calcium oxalate crystal deposition in the rat kidney by regulating gap junction function

Renal calculus is a global common urological disease that is closely related to crystal adhesion and renal tubular epithelial cell impairment. Gap junctions (GJs) and their components (connexins and Cxs) are involved in various pathophysiology processes, but their roles in renal calculi progression are not well defined. Our previous RNA microarray analysis suggests that GJs are one of the key predicted pathways involved in the renal calcium oxalate (CaOx) crystal rat model. In the current study, we found that the Cx43 and Cx32 expression and the GJ function decreased significantly after stimulation with CaOx or sodium oxalate (NaOx) in NRK-52E, MDCK, and HK-2 cells, and Cx43 expression also decreased in renal tissues in renal CaOx crystal model rats. Inhibition of Cx43 in NRK-52E cells by small interference RNA significantly increased the CD44 and androgen receptor expression, and the adhesion between CaOx crystals and cells, which were consistent with the function of GJ inhibitors. On the other hand, after GJ function and Cx43 expression were increased by allicin, diallyl disulfide, or diallyl trisulfide, the impairment of NRK-52E cells by NaOx or other GJ inhibitors and the adhesion between CaOx crystals and renal cells decreased significantly. Furthermore, allicin also increased Cx43 expression and inhibited crystal deposition in rat kidneys. Taken together, our results provide a basis that GJs and Cx43 may participate in renal CaOx stone progression and that allicin, together with its analogues, could be potential drugs for renal calculus precaution.     

Cytoskeletal rearrangement and cell death induced by Bothrops alternatus snake venom in cultured Madin-Darby canine kidney cells.

Bothrops snake venoms cause renal damage, with renal failure being the main cause of death in humans bitten by these snakes. In this work, we investigated the cytoskeletal rearrangement and cytotoxicity caused by Bothrops alternatus venom in cultured Madin-Darby canine kidney (MDCK) cells. Incubation with venom (10 and 100 microg/mL) significantly (p <0.05) decreased the cellular uptake of neutral red dye after 1 and 3 h. Venom (100 microg/mL) also markedly decreased the transepithelial electrical resistance (RT) across MDCK monolayers. Staining with rhodamine-conjugated phalloidin revealed disarray of the cytoskeleton that involved the stress fibers at the basal cell surface and focal adhesion-associated F-actin in the cell-matrix contact region. Feulgen staining showed a significant decrease in the number of cells undergoing mitosis and an increase in the frequency of altered nuclei. Scanning electron microscopy revealed a decrease in the number of microvilli and the presence of cells with a fusiform format. Flow cytometry with annexin V and propidium iodide showed that cell death occurred by necrosis, with little apoptosis, a conclusion supported by the lack of DNA fragmentation characteristic of apoptosis. Pretreating the cells with catalase significantly attenuated the venom-induced loss of viability, indicating a possible involvement of H2O2 in the cellular damage; less protection was observed with superoxide dismutase or N omega-nitro-L-arginine methyl ester. These results indicate that Bothrops alternatus venom is cytotoxic to cultured MDCK cells, possibly via the action of reactive oxygen species. This cytotoxicity could contribute to nephrotoxicity after envenoming by this species.     

Preactivation of AMPK by metformin may ameliorate the epithelial cell damage caused by renal ischemia

Alterations in epithelial cell polarity and in the subcellular distributions of epithelial ion transport proteins are key molecular consequences of acute kidney injury and intracellular energy depletion. AMP-activated protein kinase (AMPK), a cellular energy sensor, is rapidly activated in response to renal ischemia, and we demonstrate that its activity is upregulated by energy depletion in Madin-Darby canine kidney (MDCK) cells. We hypothesized that AMPK activity may influence the maintenance or recovery of epithelial cell organization in mammalian renal epithelial cells subjected to energy depletion. MDCK cells were ATP depleted through a 1-h incubation with antimycin A and 2-deoxyglucose. Immunofluoresence localization demonstrated that this regimen induces mislocalization of the Na-K-ATPase from its normal residence at the basolateral plasma membrane to intracellular vesicular compartments. When cells were pretreated with the AMPK activator metformin before energy depletion, basolateral localization of Na-K-ATPase was preserved. In MDCK cells in which AMPK expression was stably knocked down with short hairpin RNA, preactivation of AMPK with metformin did not prevent Na-K-ATPase redistribution in response to energy depletion. In vivo studies demonstrate that metformin activated renal AMPK and that treatment with metformin before renal ischemia preserved cellular integrity, preserved Na-K-ATPase localization, and led to reduced levels of neutrophil gelatinase-associated lipocalin, a biomarker of tubular injury. Thus AMPK may play a role in preserving the functional integrity of epithelial plasma membrane domains in the face of energy depletion. Furthermore, pretreatment with an AMPK activator before ischemia may attenuate the severity of renal tubular injury in the context of acute kidney injury.     

Nephrotoxic cell death by diclofenac and meloxicam

The nephrotoxicity of diclofenac, a non-steroidal anti-inflammatory drug that inhibits both isoforms of cyclooxygenase (COX) has been reported to be fatal to vultures but this was not so with meloxicam which is COX-2 selective. Our study showed that diclofenac was more toxic than meloxicam to both the proximal tubular LLC-PK1 cells and the distal tubular Madin-Darby canine kidney type II (MDCKII) cells, and that LLC-PK1 cells were more susceptible. Exposure of MDCKII cells to meloxicam caused activation of caspase-9/-3 and release of cytochrome c. These observations together with a positive annexin V-FITC staining implicate an intrinsic mitochondrial cell death pathway by apoptosis. Diclofenac-treated MDCKII cells on the other hand showed extensive propidium iodide staining, suggestive of cell death by necrosis. The mode of cell death in LLC-PK1 cells was however less well-defined with positive annexin V-FITC staining but minimal increase in caspase-3 activity alluding to a caspase-independent pathway.     

alpha 1- and beta 2-adrenergic receptor expression in the Madin-Darby canine kidney epithelial cell line

The Madin-Darby canine kidney (MDCK) cell line, derived from distal tubule/collecting duct, expresses differentiated properties of renal tubule epithelium in culture. We studied the expression of adrenergic receptors in MDCK to examine the role of catecholamines in the regulation of renal function. Radioligand-binding studies demonstrated, on the basis of receptor affinities of subtype-selective adrenergic agonists and antagonists, that MDCK cells have both alpha 1- and beta 2- adrenergic receptors. To determine whether these receptor types were expressed by the same cell, we developed a number of clonal MDCK cell lines. The clonal lines had stable but unique morphologies reflecting heterogeneity in the parent cell line. Some clones expressed only beta 2-adrenergic receptors and were nonmotile, whereas others expressed both alpha 1- and beta 2-receptors and demonstrated motility on the culture substrate at low cell densities. In one clone, alpha- and beta- receptor expression was stable for more than 50 passages. Catecholamine agonists increased phosphatidylinositol turnover by activating alpha- adrenergic receptors and cellular cyclic adenosine monophosphate accumulation by activating beta-adrenergic receptors. Guanine nucleotide decreased the affinity of isoproterenol for the beta 2- receptor but did not alter the affinity of epinephrine for the alpha 1- receptor. These results show that alpha 1- and beta 2-receptors can be expressed by a single renal tubular cell and that the two receptors behave as distinct entities in terms of cellular response and receptor regulation. Heterogeneity of adrenergic receptor expression in MDCK clones may reflect properties of different types of renal tubule cells.     

The MDCK epithelial cell line expresses a cell surface antigen of the kidney distal tubule

Monoclonal antibodies were prepared against the Madin-Darby canine kidney (MDCK) cell line to identify epithelial cell surface macromolecules involved in renal function. Lymphocyte hybrids were generated by fusing P3U-1 myeloma cells with spleen cells from a C3H mouse immunized with MDCK cells. Hybridomas secreting anti-MDCK antibodies were obtained and clonal lines isolated in soft agarose. We are reporting on one hybridoma line that secretes a monoclonal antibody that binds to MDCK cells at levels 20-fold greater than background binding. Indirect immunofluorescence microscopy was utilized to study the distribution of antibody binding on MDCK cells and on frozen sections of dog kidney and several nonrenal tissues. In the kidney the fluorescence staining pattern demonstrates that the antibody recognizes an antigenic determinant that is expressed only on the epithelial cells of the thick ascending limb of Henle's loops and the distal convoluted tubule and appears to be localized on the basolateral plasma membrane. This antigen also has a unique distribution in non-renal tissues and can only be detected on cells known to be active in transepithelial ion movements. These results indicate the probable distal tubule origin of MDCK and suggest that the monoclonal antibody recognizes a cell surface antigen involved in physiological functions unique to the kidney distal tubule and transporting epithelia of nonrenal tissues.     

Characterization of hyaluronic acid interaction with calcium oxalate crystals: implication of crystals faces, pH and citrate

Interaction between hyaluronic acid (HA) present at the surface of tubular epithelial cells and calcium oxalate monohydrate (COM) crystals is thought to play an important role in kidney stone formation. AFM-based force spectroscopy, where HA is covalently attached to AFM-probes, was used to quantify the interaction between HA and the surfaces of COM crystals. The work of adhesion of the HA-probe as well as the rupture force of single HA molecules were quantified in order to understand the molecular regulation of HA binding to COM crystals. Our results reveal that HA adsorbs to the crystal surface in physiological conditions. We also observed increased adhesion when the pH is lowered to a value that increases the risk of kidney stone formation. HA adhesion to the COM crystal surface can be suppressed by citrate, a physiological inhibitor of stone retention currently used in the treatment and prevention of kidney stone formation. Interestingly, we also observed preferential binding of HA onto the [100] face versus the [010] face, suggesting a major contribution of the [100] faces in the crystal retention process at the surface of tubular epithelial cells and the promotion of stone formation. Our results clearly establish a direct role for the glycosaminoglycan HA present at the surface of kidney tubular epithelium in the process of COM crystal retention.     

Cyclosporine A-induced toxicity in two renal cell culture models (LLC-PK1 and MDCK)

Renal damage caused by therapeutic treatment with cyclosporine A has been well documented. Clinical experiences have shown that cyclosporine A nephrotoxicity is determined by interstitial fibrosis with tubular atrophy. However, the exact mechanism by which this drug causes nephrotoxicity has not yet been clarified. This study used an in vitro model in an attempt to identify the cellular mechanisms underlying kidney cyclosporine A damage. We used two cell lines with the characteristics of proximal and distal tubule cells (pig kidney proximal tubular epithelial cell line [LLC-PK1] and Madin–Darby canine kidney cell line [MDCK]. The cell lines were treated with cyclosporine A for 24h. After the treatment, the cells were stained with Trypan Blue to estimate cell viability and processed by histochemical reactions to evaluate their cellular metabolism. Four enzymes (acid phosphatase, alkaline phosphatase, lactate dehydrogenase and succinate dehydrogenase) were considered. The cell viability assay showed that the LLC-PK1 cell line was more sensitive to cyclosporine A than MDCK. Remarkably, the LLC-PK1 cells disappeared with cyclosporine A treatment. As for the hydrolytic enzymes, only acid phosphatases showed an increased positivity in the treated LLC-PK1 cells. Similarly, lactate dehydrogenase showed a different activity histochemically. No statistically significant alterations were observed in the succinate dehydrogenase reaction.The cyclosporine A-treated MDCK cell lines did not show any difference in either their hydrolytic or succinate dehydrogenase enzyme positivity with respect to the control line. In contrast, there was a significant increase in lactate dehydrogenase activity. This study allowed the possible mechanism of cyclosporine A-induced damage in renal tubular cells to be evaluated. The enzymatic changes happened rapidly (during the 24h of treatment), suggesting that this alteration was one of the steps by which cyclosporine A induced toxicity. Moreover, since acid phosphatase is a marker of protein catabolism, the variation in the activity of this enzyme, in the LLC-PK1 line only, showed that cyclosporine can induce alterations leading to cellular toxicity. The modifications in lactate dehydrogenase activity, in both lines, suggested that this drug caused cell stress, inducing the production of lactic acid from glucose in the presence of oxygen. In conclusion, cyclosporine A treatment may force LLC-PK1 and MDCK cells to use anaerobic glycolysis preferentially. Further, these enzyme alterations may represent an epiphenomenon or a consequence of cyclosporine A toxicity.