Normal rat kidney proximal tubule cells in primary and multiple subcultures

Serum-free culture of rat proximal tubule cells with enhanced function on chitosan

The proximal tubule performs a variety of important renal functions and is the major site for nutrient reabsorption. The purpose of this study is to culture rat renal proximal tubule cells (PTCs) on chitosan without serum to maintain a transcellular pathway to transport water and ions effectively without loss of highly differentiated cell function. The effect of chitosan, which is structurally similar to glycosaminoglycans, in the absence of serum on the primary cultured PTCs was compared that of collagen with or without serum. Two days after seeding, more tubule fragments and higher PTC viability were observed on chitosan than on collagen with or without serum. Proliferation marker Ki-67 immunostaining and phosphorylated extracellular-regulated kinase (ERK) expression results displayed similar proliferation capability of PTCs established on chitosan without serum and collagen with 2% fetal bovine serum after 4 days of incubation. When grown to confluence, PTCs formed a monolayer with well-organized tight junctions and formation of domes on chitosan without serum. Moreover, evaluation of the transepithelial electrical resistance showed that both chitosan and serum were involved in the modification of water and ion transport in confluent cells. By showing the direct suppression of PTC growth and dome formation treated with heparinase, we demonstrated that the interaction between cell surface heparin sulfate proteoglycan and chitosan played an important role in PTC proliferation and differentiation. A successful primary culture of PTCs has now been produced on chitosan in serum-free culture condition, which offers potential applications for chitosan in renal tissue engineering.

Aluminum citrate prevents renal injury from calcium oxalate crystal deposition

Calcium oxalate monohydrate crystals are responsible for the kidney injury associated with exposure to ethylene glycol or severe hyperoxaluria. Current treatment strategies target the formation of calcium oxalate but not its interaction with kidney tissue. Because aluminum citrate blocks calcium oxalate binding and toxicity in human kidney cells, it may provide a different therapeutic approach to calcium oxalate-induced injury. Here, we tested the effects of aluminum citrate and sodium citrate in a Wistar rat model of acute high-dose ethylene glycol exposure. Aluminum citrate, but not sodium citrate, attenuated increases in urea nitrogen, creatinine, and the ratio of kidney to body weight in ethylene glycol-treated rats. Compared with ethylene glycol alone, the addition of aluminum citrate significantly increased the urinary excretion of both crystalline calcium and crystalline oxalate and decreased the deposition of crystals in renal tissue. In vitro, aluminum citrate interacted directly with oxalate crystals to inhibit their uptake by proximal tubule cells. These results suggest that treating with aluminum citrate attenuates renal injury in rats with severe ethylene glycol toxicity, apparently by inhibiting calcium oxalate's interaction with, and retention by, the kidney epithelium.

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.

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.

Characterization of the collagen phenotype of rabbit proximal tubule cells in culture

Studies were performed to characterize the collagen phenotype of cultured rabbit proximal tubule (RPT) epithelial cells grown on plastic and on the reconstituted basement membrane preparation, Matrigel. When grown on a plastic substratum, RPT cells display a cobblestone appearance characteristic of glomerular epithelial cells. While initially forming an interlocking network of cells after subculture on Matrigel, this pattern of culture morphology rapidly develops into one characterized by isolated, organized groups of cells. Notwithstanding the effects of Matrigel on culture morphology, total cellular proliferation was reduced only 25% when RPT cells were grown on this substrate. Greater than 90% of the collagen synthesized by RPT cells grown on plastic was secreted into the culture medium. Qualitative analysis by SDS-PAGE revealed components exhibiting electrophoretic mobilities corresponding to the chains present in type IV and type I collagens. Quantitative analysis by CM-Trisacryl chromatography established that approximately 2/3 of the total collagen synthesized by RPT cells grown on plastic was type IV and approximately 1/3 type I. Quantitative analysis of the collagens produced by RPT cells grown on Matrigel again indicated the synthesis of only type IV and type I molecules but in a slightly more equal ratio of both collagen types and in the ratio of secreted to cell-associated molecules. However, the total amount of collagen synthesized by RPT cells grown on Matrigel was reduced to approximately 1% of the level synthesized by the cells grown on plastic. On plastic, approximately 3/4 of the type I collagen produced was recovered as the type I homotrimer, but on Matrigel type I homotrimers represented only approximately 55% of the total type I collagen synthesized. On Matrigel, the majority of the type IV collagen was recovered as heterotrimers containing alpha1(IV) and alpha2(IV) chains. In contrast, RTP cells grown on plastic predominantly produced type IV homotrimers containing only the alpha1(IV) chain. These data represent the initial report describing the collagens produced by nonimmortalized cultured proximal tubule cells. The finding that a significant amount of the total collagen synthesized was type IV (basement membrane) collagen, regardless of culture substrate, suggests that the RPT cells have maintained a significant degree of differentiation in culture, and thus establishes RPT cells as an appropriate model for investigating ECM changes in proximal tubule cells that occur in kidney disease. Finally, the observation that culture of RPT cells on a reconstituted basement membrane preparation results in a significant reduction in total collagen production and alterations in the molecular forms of type IV and type I molecules synthesized indicates that integrity of the tubular basement membrane may represent an important component in preventing the development of tubulointerstitial fibrosis.

Determination of folate transport pathways in cultured rat proximal tubule cells

Deficiency of the vitamin folic acid has recently been linked with increased incidence of neural tube defects and of cardiovascular disease, through elevated plasma homocysteine levels. The kidney has an important role in conserving folate to counteract development of deficiency. Urinary folate excretion is regulated by the degree of reabsorption of folate by the proximal tubule cell. To evaluate an in vitro model for studies of the regulation of urinary folate excretion, the present studies examined the transport of 5-methyltetrahydrofolate (5-CH3-H4PteGlu), the primary form of folate in the glomerular filtrate, by normal rat proximal tubule (RPT) cells in confluent monolayer cultures. Specific binding of 5-CH3-H4PteGlu to the apical membrane was saturable (K(D) = 27 nM), but intracellular transport was not saturated up to 100 nM concentrations. 5-CH3-H4PteGlu transport was decreased 50% by concentrations of folic acid that completely blocked 5-CH3-H4PteGlu binding by the apical folate receptor. Probenecid (10 mM), an anion exchange (reduced folate carrier) inhibitor, reduced 5CH3-H4PteGlu transport by 50% without significantly affecting binding. Aspirin (3 mM) did not alter 5-CH3-H4PteGlu transport, but significantly enhanced the inhibition due to probenecid. Similarly, indomethacin (5 microM) potentiated the inhibition of 5-CH3-H4PteGlu transport by probenecid. These data suggest that RPT cells take up 5-CH3-H4PteGlu by both the folate receptor and the reduced folate carrier, implying a role for both pathways in regulating urinary folate excretion.

Nephrotoxicity testing in vitro--what we know and what we need to know

The kidney is affected by many chemicals. Some of the chemicals may even contribute to end-stage renal disease and thus contribute considerably to health care costs. Because of the large functional reserve of the kidney, which masks signs of dysfunction, early diagnosis of renal disease is often difficult. Although numerous studies aimed at understanding the mechanisms underlying chemicals and drugs that target various renal cell types have delivered enough understanding for a reasonable risk assessment, there is still an urgent need to better understand the mechanisms leading to renal cell injury and organ dysfunction. The increasing use of in vitro techniques using isolated renal cells, nephron fragments, or cell cultures derived from specific renal cell types has improved our insight into the molecular mechanisms involved in nephrotoxicity. A short overview is given on the various in vitro systems currently used to clarify mechanistic aspects leading to sublethal or lethal injury of the functionally most important nephron epithelial cells derived from various species. Whereas freshly isolated cells and nephron fragments appear to represent a sufficient basis to study acute effects (hours) of nephrotoxins, e.g., on cell metabolism, primary cultures of these cells are more appropriate to study long-term effects. In contrast to isolated cells and fragments, however, primary cultures tend to first lose several of their in vivo metabolic properties during culture, and second to have only a limited life span (days to weeks). Moreover, establishing such primary cultures is a time-consuming and laborious procedure. For that reason many studies have been carried out on renal cell lines, which are easy to cultivate in large quantities and which have an unlimited life span. Unfortunately, none of the lines display a state of differentiation comparable to that of freshly isolated cells or their primary cultures. Most often they lack expression of key functions (e.g., gluconeogenesis or organic anion transport) of their in vivo correspondents. Therefore, the use of cell lines for assessment of nephrotoxic mechanisms will be limited to those functions the lines express. Upcoming molecular biology approaches such as the transduction of immortalizing genes into primary cultures and the utilization of cells from transgenic animals may in the near future result in the availability of highly differentiated renal cells with markedly extended life spans and near in vivo characteristics that may facilitate the use of renal cell culture for routine screening of nephrotoxins.