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

BACKGROUND

Oxalate exposure produces oxidant stress in renal epithelial cells leading to death of some cells and adaptation of others. The pathways involved in these diverse actions remain unclear, but appear to involve activation of phospholipase A2 (PLA2) and redistribution of membrane phospholipids. The present studies examined the possibility that oxalate actions may also involve increased accumulation of ceramide, a lipid-signaling molecule implicated in a variety of pathways, including those leading to apoptotic cell death.

METHODS

Ceramide accumulation was examined in renal epithelial cells from pig kidney (LLC-PK1 cells) and from dog kidney [Madin-Darby canine kidney (MDCK cells)] using the diacylglycerol kinase assay. Sphingomyelin degradation was assessed by monitoring the disappearance of 3H-sphingomyelin from cells that had been prelabeled with [3H]-choline. The effects of oxalate were compared with those of other oxidants (peroxide, xanthine/xanthine oxidase), other organic acids (formate and citrate), and a known activator of sphingomyelinase in these cells [tumor necrosis factor-alpha (TNF-alpha)]. Separate studies determined whether oxalate-induced accumulation of ceramide could be blocked by pretreatment with antioxidants [Mn (III) tetrakis (1-methyl-4-pyridyl) porphyrin (Mn TMPyP, a superoxide dismutase mimetic) or N-acetylcysteine (NAC; an antioxidant)], with an inhibitor of ceramide synthase [fumonisin B1 (FB1)] or with an inhibitor of PLA2 [arachidonyl trifluoromethylketone (AACOCF3)].

RESULTS

Oxalate exposure produced a significant time- and concentration-dependent increase in cellular ceramide. A reciprocal decrease in 3H-sphingomyelin was observed under these conditions. Increases in cellular ceramide levels were also observed after treatment with other oxidants (hydrogen peroxide, and xanthine/xanthine oxidase), activators of sphingomyelinase (TNF-alpha), exogenous sphingomyelinase, or arachidonic acid. Formate produced similar (albeit smaller) effects, and citrate did not. The oxidant-induced increases in ceramide were attenuated by pretreatment with NAC (a glutathione precursor) and MnTMPyP (a superoxide dismutase mimetic), suggesting a role for cellular redox states. The oxalate-induced increase in ceramide was also attenuated by pretreatment with AACOCF3, suggesting a role for PLA2. Pretreatment with FB1 produced a small but statistically insignificant attenuation of the response to oxalate.

CONCLUSIONS

Oxalate exposure produces a marked accumulation of ceramide in renal epithelial cells by a process that is redox sensitive and mediated in part by activation of PLA2. Since cellular sphingomyelin decreased as ceramide increased, it seems likely that oxalate actions are mediated, at least in part, by an increase in sphingomyelinase activity, although alterations in ceramide synthase are also possible. Further study is required to define the steps involved in oxalate actions and to determine the extent to which ceramide signaling mediates oxalate actions.

Oxalate toxicity in LLC-PK1 cells: role of free radicals

Oxalate, the most common constituent of kidney stones, is an end product of metabolism that is excreted by the kidney. During excretion, oxalate is transported by a variety of transport systems and accumulates in renal tubular cells. This process has been considered benign; however, recent studies on LLC-PK1 cells suggested that high concentrations of oxalate are toxic, inducing morphological alterations, increases in membrane permeability to vital dyes and loss of cells from the monolayer cultures. The present studies examined the basis for oxalate toxicity, focusing on the possibility that oxalate exposure might increase the production/availability of free radicals in LLC-PK1 cells. Free radical production was monitored in two ways, by monitoring the reduction of nitroblue tetrazolium to a blue reaction product and by following the conversion of dihydrorhodamine 123 (DHR) to its fluorescent derivative, rhodamine 123. Such studies demonstrated that oxalate induces a concentration-dependent increase in dye conversion by a process that is sensitive to free radical scavengers. Specifically, addition of catalase or superoxide dismutase blocked the oxalate-induced changes in dye fluorescence/absorbance. Addition of these free radical scavengers also prevented the oxalate-induced loss of membrane integrity in LLC-PK1 cells. Thus it seems likely that free radicals are responsible for oxalate toxicity. The levels of oxalate that induced toxicity in LLC-PK1 cells (350 microM) was only slightly higher than would be expected to occur in the renal cortex. These considerations suggest that hyperoxaluria may contribute to the progression of renal injury in several forms of renal disease.

The frusemide test: simple screening test for renal acidification defect in urolithiasis

Urinary pH was first labelled as a risk factor for patients with recurrent calcium stone disease of the renal tract in 1978. The standard diagnostic test for patients with a history of stones, who appear to have urinary acidification defects is the ammonium chloride test. However, the performance of this test results in significant morbidity, particularly nausea and vomiting, and as a result there is little enthusiasm for screening for such defects. We describe a screening test that may be used to determine which patients require more definitive testing. It involves an oral dose of frusemide (40 mg) followed by half-hourly urine sampling for pH. For the detection of a renal tubular acidification defect, the frusemide test had sensitivity = 100%, specificity = 82%, predictive value of a positive result = 40%, predictive value of a negative result = 100%, screening efficiency = 84%, and there was no morbidity. During the ammonium chloride test 80% of the patients vomited or felt very nauseated.

A new approach to studying inhibitors of calcium oxalate crystal growth

The nucleation and crystal growth of calcium oxalate (CaOx) were studied at pH 5.5 using turbidimetric measurements at 620 nm of suspensions produced by mixing calcium chloride and sodium oxalate (initial conditions: Ca, 3 x 10(-3) M; Ox, 0.5 x 10(-3) M). CaOx crystallization kinetics were defined first by the induction time ti and then by the slope of turbidity as a function of time during the interval corresponding to a correlation coefficient r2 > 0.99. The technique described requires only a small amount of material, is quick, convenient, and can be used to study inhibitors of CaOx crystallization by comparing ti and the rate of crystal growth in the presence and absence of inhibitors. The effects on CaOx crystal growth of several low molecular weight compounds, i.e. di- and tricarboxylic acids, were examined. The majority of these compounds were inhibitors of crystal growth, the greatest effect being seen with citric acid (50% inhibition in the presence of 1.5 x 10(-3) M citric acid), isocitric acid (50% inhibition in the presence of 0.75 x 10(-3) M isocitric acid) and pyrophosphate (30% inhibition in presence of 0.15 x 10(-3) M pyrophosphate). The inhibitors' behaviour regarding the medium was studied without any assumptions about their possible mechanisms of action. Measurements of ionized calcium before and after the reaction, as well as the observation of crystals by scanning electron microscopy, allowed us to formulate the hypothesis that the effect of citric acid and tartaric acid can be attributed mainly to ion pairing, in contrast to that of pyrophosphate and the other carboxylic acids.

Gastrointestinal citrate absorption in nephrolithiasis

Gastrointestinal absorption of citrate was measured in stone patients with idiopathic hypocitraturia to determine if citrate malabsorption could account for low urinary citrate. Citrate absorption was measured directly from recovery of orally administered potassium citrate (40 mEq.) in the intestinal lavage fluid, using an intestinal washout technique. In 7 stone patients citrate absorption, serum citrate levels, peak citrate concentration in serum and area under the curve were not significantly different from those of 7 normal subjects. Citrate absorption was rapid and efficient in both groups, with 96 to 98% absorbed within 3 hours. The absorption of citrate was less efficient from a tablet preparation of potassium citrate than from a liquid preparation, probably due to a delayed release of citrate from wax matrix. However, citrate absorption from solid potassium citrate was still high at 91%, compared to 98% for a liquid preparation. Thus, hypocitraturia is unlikely to be due to an impaired gastrointestinal absorption of citrate in stone patients without overt bowel disease.

The effect of a standard oral calcium load given in two different forms on plasma ionized calcium and serum PTH

A standard dose (1 g, 25.4 mmol) of calcium was administered both as calcium chloride in syrup and as Calcium-Sandoz Syrup (calcium glubionate and calcium galactogluconate) to 10 volunteers. Both dosage forms caused a significant rise in ionized calcium and a significant fall in concentration of serum PTH, the calcium chloride producing significantly greater changes. The urinary excretion of calcium measured over 5 h after calcium chloride was double that after Calcium-Sandoz. Calcium chloride, as expected, also gave rise to a decreased urine pH. Calcium chloride would be the better choice for an oral PTH suppression test.

An animal model to study the effects of diet on risk factors of calcium stone formation

The effects of diet on the formation of calcium oxalate stones were studied in 150-day-old rats by measuring the diuresis, pH, and urinary and blood concentrations of promoting and inhibitory substances. An increase in phosphate (promoting) and magnesium (inhibiting), and a decrease in citrate (inhibiting) and pH were found in the urine of rats fed with a high protein diet. No differences were observed in the plasma concentrations of calcium, magnesium, and phosphate. These results confirm epidemiological and clinical studies in humans that have shown that any diet rich in protein can change the concentrations of these substances in urine.