Electrical properties of cultured renal tubular cells (OK) grown in confluent monolayers

Predictors of timely diagnostic follow-up after an abnormal Pap test among Hispanic women seeking care in El Paso, Texas

BACKGROUND

Diagnostic follow-up of women with an abnormal Pap test is necessary to resolve the risk developing cervical cancer. The purpose of this study is to describe patient characteristics associated with timely receipt of a diagnostic colposcopy after an abnormal Pap test among Hispanic women in El Paso, a Texas-Mexico border city.

METHODS

We conducted a retrospective chart review of Hispanic patients seen at an academic colposcopy clinic following an abnormal Pap test. An optimal diagnostic interval to colposcopy was based on a National Breast and Cervical Cancer Early Detection Program (NBCCEDP) quality indicator and was defined as receipt of colposcopy within 90 days or less from the date of an abnormal Pap test. Risk ratios (RR) were calculated by building a generalized linear model fit using a Poisson distribution, log link, and robust variance.

RESULTS

Overall, 177 of the 270 women (65.6%) received follow-up within an optimal diagnostic interval. After adjusting for other variables in the model, women who were 30 years of age or older were 32% more likely to have an optimal interval than younger women (adjusted RR = 1.32, P < 0.01). High school graduates were less likely than more educated women to have an optimal interval (adjusted RR = 0.68, P < 0.01). Participation in the NBCCEDP was not associated with receipt of follow-up within an optimal diagnostic interval.

CONCLUSIONS

Compared with women with greater educational attainment, high school graduates were less likely to receive follow-up within an optimal diagnostic interval, as were younger (≤ 30 years) women compared with older women. Participation in the NBCCEDP was not associated with receipt of care within an optimal diagnostic interval.

Application of physiological shear stress to renal tubular epithelial cells

Renal tubular epithelial cells are consistently exposed to flow of glomerular filtrate that creates fluid shear stress at the apical cell surface. This biophysical stimulus regulates several critical renal epithelial cell functions, including transport, protein uptake, and barrier function. Defining the in vivo mechanical conditions in the kidney tubule is important for accurately recapitulating these conditions in vitro. Here we provide a summary of the fluid flow conditions in the kidney and how this translates into different levels of fluid shear stress down the length of the nephron. A detailed method is provided for measuring fluid flow in the proximal tubule by intravital microscopy. Devices to mimic in vivo fluid shear stress for in vitro studies are discussed, and we present two methods for culture and analysis of renal tubule epithelial cells exposed physiological levels of fluid shear stress. The first is a microfluidic device that permits application of controlled shear stress to cells cultured on porous membranes. The second is culture of renal tubule cells on an orbital shaker. Each method has advantages and disadvantages that should be considered in the context of the specific experimental objectives.

Renal Production, Uptake, and Handling of Circulating αKlotho

αKlotho is a multifunctional protein highly expressed in the kidney. Soluble αKlotho is released through cleavage of the extracellular domain from membrane αKlotho by secretases to function as an endocrine/paracrine substance. The role of the kidney in circulating αKlotho production and handling is incompletely understood, however. Here, we found higher αKlotho concentration in suprarenal compared with infrarenal inferior vena cava in both rats and humans. In rats, serum αKlotho concentration dropped precipitously after bilateral nephrectomy or upon treatment with inhibitors of αKlotho extracellular domain shedding. Furthermore, the serum half-life of exogenous αKlotho in anephric rats was four- to five-fold longer than that in normal rats, and exogenously injected labeled recombinant αKlotho was detected in the kidney and in urine of rats. Both in vivo (micropuncture) and in vitro (proximal tubule cell line) studies showed that αKlotho traffics from the basal to the apical side of the proximal tubule via transcytosis. Thus, we conclude that the kidney has dual roles in αKlotho homeostasis, producing and releasing αKlotho into the circulation and clearing αKlotho from the blood into the urinary lumen.

Caki-1 cells represent an in vitro model system for studying the human proximal tubule epithelium

BACKGROUND/AIMS

The human proximal tubule (PT) epithelium is distinguished from other nephron segments via several unique characteristics. Studies assessing PT epithelium increasingly employ cell lines, bypassing the complexity of primary cell cultures. However, few human model systems exist for studying PT cells in vitro. The current work involves an intensive characterization of Caki-1 cells, a commercially available human renal cell line.

METHODS

Caki-1 cells were validated as a representative model system for PT cell research via morphological, physiological and biochemical investigations including light and transmission electron microscopy, transepithelial electrical resistance (TER) measurements and the detection of PT markers.

RESULTS

Morphologically, these cells form a polarized monolayer with apical located microvilli and multiple mitochondria per cell. Low TER ranging from 2 to 28 Omega cm(2) was determined for Caki-1 cells, characteristic of the 'leaky' PT epithelium in vivo. Expression of the PT markers: NHE3, GGT, DPP IV, APM and AP were present in Caki-1 cells. Two epithelial markers, E-cadherin and Na(+)/K(+)-ATPase, were additionally observed.

CONCLUSION

The current work is a concise summary which confirms that Caki-1 cells represent well-differentiated polarized PT cells in vitro, regardless of its cancerous origin and multiple passaging. They prove to be a significant contribution to the field of PT research.

Epithelial barrier characteristics and expression of cell adhesion molecules in proximal tubule-derived cell lines commonly used for in vitro toxicity studies

Recent studies indicate that the actions of several nephrotoxic substances involve alterations in the function of cell adhesion molecules and changes in the paracellular permeability of the proximal tubule. In light of these findings, there is a need for appropriate in vitro model systems to study these phenomenae in greater detail. In the present study, the transepithelial resistance (TER), paracellular permeability of 14C-mannitol and immunofluorescent labeling of cell adhesion molecules (E-cadherin, N-cadherin, ZO-1, occludin, and claudins-2 and -7) were evaluated in several proximal tubule-derived cell lines that have been commonly used as model systems for in vitro toxicity studies. The cell lines studied included: LLC-PK1, OK, NRK-52E and HK-2, along with commercially available primary cultures of human renal proximal tubule epithelial cells (HRPTE). LLC-PK1 cells developed the highest TER followed by the OK cells and NRK-52E cells. The other cell lines failed to develop a TER even after 2 weeks in culture. There was a direct correlation between TER and ability to restrict paracellular movement of 14C-mannitol. Labeling studies showed that the LLC-PK1 and NRK-52E cells expressed high levels of E-cadherin while the HRPTE cells expressed lower levels. OK cells expressed moderate levels of N-cadherin while LLC-PK1 and NRK-52E cells expressed lower levels in isolated patches of cells. All cell lines expressed moderate-high levels of ZO-1. LLC-PK1 also expressed the tight-junction proteins occludin and claudin-7; OK cells also expressed moderate levels of occludin. All other cell lines had weak claudin-7 and occludin labeling. None of the cell lines expressed claudin-2. These results show that the LLC-PK1, OK and NRK-52E cell lines exhibit characteristics that most closely resembled those of the proximal tubule in vivo, and they indicate that these cell lines would be appropriate models for studying the effects of toxicants on cell-cell junctions and cell adhesion molecules.

Cubilin- and megalin-mediated uptake of albumin in cultured proximal tubule cells of opossum kidney

BACKGROUND

Reabsorption of albumin from the glomerular filtrate occurs via receptor-mediated endocytosis in the proximal tubule. This process is initiated by binding of albumin in apical clathrin-coated pits, followed by endocytosis and degradation in lysosomes. Although binding sites have been characterized by kinetic studies, the receptors responsible for the binding of albumin have not been fully identified. Two giant glycoproteins, cubilin and megalin, constitute important endocytic receptors localized to the kidney proximal tubule.

METHODS

In the present study, we examined the colocalization of cubilin and megalin in the endocytic pathway and the relationship between the uptake of albumin and the expression of cubilin and megalin in opossum kidney (OK) proximal tubule cells by immunocytochemistry and immunoblotting.

RESULTS

OK cells expressed both cubilin and megalin. The light microscope labeling patterns for cubilin and megalin were almost identical and were mainly located at the surface area of the cells. Cubilin and megalin were also shown to colocalize on cell surface microvilli, in coated pits, and in endocytic compartments at the electron microscope level. Endocytosed bovine serum albumin (BSA) was identified exclusively in cells expressing megalin and cubilin. Uptake of BSA-FITC was saturable and inhibited by receptor-associated protein (RAP) and by intrinsic factor-vitamin B12 complex (IF-B12) at high concentrations. Significant inhibition was also observed by specific antibodies to cubilin, and megalin and cubilin antisense oligonucleotides likewise significantly reduced albumin uptake. Egg albumin did not affect the uptake of BSA.

CONCLUSION

The present observations suggest that the two receptors cubilin and megalin are both involved in the endocytic uptake of albumin in renal proximal tubule cells.

The paracellular permeability of opossum kidney cells, a proximal tubule cell line

The paracellular permeability of opossum kidney cells, a proximal tubule cell line.

BACKGROUND

The regulation of the unusually leaky paracellular pathway of the proximal tubule is poorly understood partially because of the lack of an appropriate in vitro cell model. In this study, we determined whether the paracellular permeability of opossum kidney (OK) cells would resemble that of the in vivo proximal tubule epithelium.

METHODS

The parental and subclonal OK cells and, for comparison, LLC-PK1 cells were cultured on permeable Transwell supports. The apparent paracellular permeability coefficient (Papp) for the extracellular marker 3H-mannitol was determined.

RESULTS

The Papp of OK cell sheets (12.17 x10-6 cm/sec) was remarkably close to the previously reported Papp of rat proximal tubules. The Papp of LLC-PK1 cells, another proximal tubule cell line, however, was approximately 20-fold lower than that of both OK cells and the in vivo proximal tubule. Phorbol 12-myristate 13-acetate, a protein kinase C activator, enhanced the Papp of OK cell sheets. The characteristic response of paracellular permeability to Ca2+ switch was demonstrated in OK cell sheets. Slight variations of Papp among several OK subclones were observed. Basal to apical Papp was uniformly higher than apical to basal Papp, independent of cell subtype. This rectification was attenuated by inhibition of active transport.

CONCLUSIONS

OK cell sheets cultured on Transwell supports possess a leaky paracellular pathway resembling that of the proximal tubule epithelium in vivo.

Nitric oxide enhances paracellular permeability of opossum kidney cells

BACKGROUND

Nitric oxide (NO) has been shown to be a paracrine/autocrine regulator of proximal tubular transport. In this study, we investigated the effect of NO on the paracellular permeability of opossum kidney (OK) cells, a proximal tubule cell line that possesses a leaky paracellular pathway resembling that of the in vivo proximal tubule.

METHODS

Paracellular permeability of OK cells cultured on permeable supports was measured as the apparent paracellular permeability coefficient (Papp) for 3[H]-D-mannitol. Changes in cell viability, cellular adenosine triphosphate (ATP) content, cGMP levels, and lipid peroxidation were assessed.

RESULTS

Incubation with 2 mM sodium nitroprusside (SNP), an NO donor, for 24 hours significantly enhanced the Papp of OK cell sheets by 30.6 +/- 7.0% (N = 8, P < 0.05). This effect was largely blunted by hemoglobin, a NO scavenger. Cell viability was not compromised. This effect of SNP was concomitant with a moderate reduction of cellular ATP content, an increase in lipid peroxidation, and an increase in cellular cGMP levels. The antioxidant superoxide dismutase (SOD) significantly attenuated the effect of SNP on cellular ATP content and blunted the increase in Papp caused by SNP. A soluble guanylate cyclase inhibitor did not affect the effect of SNP on the Papp.

CONCLUSIONS

NO enhances the paracellular permeability of OK cells possibly via mechanisms involving decreases in cellular ATP content.

Potassium transport in opossum kidney cells: effects of Na-selective and K-selective ionizable cryptands, and of valinomycin, FCCP and nystatin

The effects of two ionizable cryptands, the Na-selective (221)C10 and the K-selective (222)C10, and of valinomycin, FCCP and nystatin on K+ fluxes in opossum kidney (OK) cells have been quantified. The Na,K-ATPase (ouabain-sensitive 86Rb influx) was stimulated by nystatin (> or = 20%), and inhibited by the other ionophores (50-80%), by barium (K-channel blocker) (61%) and by amiloride (Na entry blocker) (34%). The Vmax of the Na,K-ATPase phosphatase activity was unmodified by the ionophores, indicating the absence of direct interaction with the enzyme. The ATPi content was unmodified by the inhibitors and nystatin, but was lowered by (221)C10 (47%), (222)C10 (75%), valinomycin (72%) and FCCP (88%). Amiloride was found to partially remove the inhibition caused by (222)C10 (51%) and valinomycin (49%). Rb efflux was stimulated by nystatin (32%), unmodified by valinomycin, and was inhibited by (221)C10 (19%), (222)C10 (19%) and FCCP (10%). Barium (39%) and amiloride (32%) inhibited this efflux and, in their presence, the nystatin effect persisted, whereas that of the other ionophores vanished. At pH 6.4, the Rb efflux decreased by 14% of its value at pH 7.4, with no additional inhibition by cryptands. Cryptands are shown to inhibit the pH-sensitive K+-conductance, probably by inducing a K+-H+ exchange at the plasma membrane, and by uncoupling oxidative phosphorylation by inducing the entry of K+ and H+ (and possibly Ca2+) ions into the mitochondria.

Effects of cell differentiation on ion conductances and membrane voltage in LLC-PK1 cells

LLC-PK1 cells serve as a widely used model for the renal proximal tubule. Until now, little has been found out about their membrane voltage (Vm) and ionic conductances (g). Several studies have shown changes in cell properties during differentiation and ageing. The aim of this study was to examine the relationship between Vm or g and the age of these cells. Therefore, we investigated single cells, subconfluent and confluent monolayers of LLC-PK1 cells aged 1-8 days with the slow-whole-cell patch-clamp technique. The Vm of all cells was -34 +/- 2 mV (n = 75) and the membrane conductance (gm) was 2.3 +/- 0.3 nS (n = 30). Vm in cells aged up to 2 days was -24 +/- 3 mV (n = 22) whereas Vm in cells aged 5-8 days was -50 +/- 3 mV (n = 15). An increase of extracellular K+ from 3.6 to 18.6 mmol/l led to a depolarization in all cells of 4 +/- 1 mV (n = 31) and an increase of gm by 17 +/- 13% (n = 15). Complete replacement of extracellular Na+ by N-methyl-D-glucamine (NMDG) led to a hyperpolarization of 19 +/- 2 mV (n = 38) and gm was lowered by 27 +/- 14% (n = 17). A reduction in extracellular Cl- from 147 to 32 mmol/l showed no significant effect on Vm (n = 16) or gm (n = 11).(ABSTRACT TRUNCATED AT 250 WORDS)

A model of the hyperkalemia produced by metabolic acidosis

In both humans and animals, mineral acids predictably result in hyperkalemia, whereas plasma K+ remains normal or may even decrease during organic acidosis. The purpose of these studies was to define the mechanism for these effects in the opossum kidney cell, an established epithelial cell line derived from the renal cortex of the opossum. This cell was chosen because the acid/base transport pathways in this cell type are well defined and because it is one of the few cells known to express K/H antiport, the transport pathway that has been proposed to mediate the hyperkalemia of acidosis. Cell K+ at pH 7.4 averaged 988 +/- 48 nmol/mg protein. Relative to this value (100%), cell K+ increased when buffer pH was increased to pH 8.4 with NaOH (108% +/- 3%) and decreased when buffer pH was acidified with HCl to pH 6.4 (93% +/- 4%), producing a highly significant correlation of cell K+ with buffer pH: cell K+ (% of baseline at pH 7.4) = 6.9 (cell pH) + 49 (r = 0.5, P < 0.004). In contrast, acidification of the buffer to pH 6.4 with either butyric or lactic acid increased cell K+ (115% +/- 4% and 110% +/- 2%, respectively, both P < 0.05 v 7.4 or HCl value). Cell pH acidified in response to HCl at a rate of 0.0053 +/- 0.0007 pH U/s, a significantly slower rate than in response to lactic acid or butyric acid (0.0071 +/- 0.0007 and 0.0091 +/- 0.0007 pH U/s, respectively). Unidirectional ouabain-sensitive 42K+ influx was significantly inhibited by HCl acidosis and less so by the organic acids.(ABSTRACT TRUNCATED AT 250 WORDS)

Phosphate deprivation inhibits NH4+ transport in OK cells

Deprivation of dietary phosphate leads to a decrease in urinary excretion of ammonium in rats which may be mediated through an alteration in ammonium transport in the proximal tubule. In the present study the OK renal epithelial cell line, a model for the proximal tubule, was used to determine if NH4+ transport was changed during acute phosphate deprivation. The intracellular pH after perfusion with NH4Cl solution was used for calculation of intracellular NH4+ concentration. Intracellular [NH4+] increased linearly during the first 2 min of acidification with NH4Cl. NH4+ transport, defined by the initial rate of the increase in intracellular [NH4+], was decreased by 32% (P < 0.005) in phosphate-deprived cells. This transport process was inhibited by barium chloride, but not by DIDS, amiloride or ouabain, suggesting that the NH4+ transport pathway may utilize K+ channels. Acute phosphate deprivation may inhibit NH4+ transport in OK cells by decreasing membrane K+ permeability.

Electrogenic Na-independent HCO3 transport in OK cells

We have shown previously that OK cells recover from an acid load in a medium nominally CO2-free by extruding H via a Na/H exchanger and a passive H-conductive pathway. In this work, the regulation of cell pH (pHi) was studied after addition or withdrawal of CO2/HCO3 (5% CO2, 95 mM HCO3, pH = 8) using the fluoroprobe BCECF. In the presence of Na and amiloride to inhibit Na/H exchange, the recovery of pHi after CO2 entry and CO2 exit were found to depend in part on HCO3 entry and exit, respectively. Efflux of H per se also contributed to restoring pHi after CO2 addition, whereas H influx may have played a smaller role to normalize pHi after CO2 removal. DIDS, 0.5 mM, significantly inhibited both recovery phases of pHi. Removal of Na failed to inhibit the recovery of pHi after CO2 addition and removal. Cl removal also failed to inhibit pHi recovery after CO2 removal. Cell depolarization in the presence of Na moderately stimulated the pHi recovery rate after CO2 addition whereas it markedly inhibited the normalization of pHi after CO2 removal. Cell depolarization in the absence of sodium had only a slight effect to increase pHi recovery after CO2 addition but markedly prevented the pHi recovery after CO2 removal. These results indicate that OK cells lack Na or Cl-dependent HCO3 transport systems. The OK cell possesses a novel stilbene-sensitive electrogenic HCO3 transport system that is involved in the regulation of cell pH.

Receptor-mediated endocytosis of albumin in cultured opossum kidney cells: a model for proximal tubular protein reabsorption

The accumulation of fluorescein (FITC)-labelled bovine albumin was measured against the extracellular-fluid-phase marker FITC-inulin within confluent monolayers of the opossum kidney cell line OK. Fluorescence and electron microscopic pictures show that FITC-albumin is taken up by endocytosis and appears in a vesicular intracellular distribution. The uptake of FITC-albumin was quantified by measuring the cell-adherent fluorescence fluorimetrically. FITC-albumin uptake shows a time- and concentration-dependent saturation kinetics in contrast to the non-saturable FITC-inulin uptake, and exceeds the latter more than tenfold at low concentrations. Half-maximum saturation occurs at 20-30 mg/l. Initial FITC-albumin uptake/mg protein is stimulated by cell maturation, being six-to sevenfold higher in the confluent than in the subconfluent state, while FITC-inulin uptake is unchanged. Both an elevation of ambient osmolality to 600-750 mOsm/kg and disruption of the cytoskeleton by cytochalasin B (0.1 mmol/l) reduce initial FITC-albumin uptake by 50%-60% in a non-additive fashion. Albumin endocytosis is reduced both in acidic (pH 5.4) and alkaline (pH 8.4) medium, but does not depend on extracellular sodium, calcium or chloride. High concentrations of fetal calf serum or unlabelled albumin reduce FITC-albumin endocytosis dose-dependently. The present study is the first to investigate both the protein uptake and the fluid-phase endocytosis in a cultured proximal tubular cell line, using these cells as a model system-for proximal tubular protein reabsorption.

Transepithelial resistance of ciliary epithelial cells in culture: functional modification by protamine and extracellular calcium

1. Bovine pigmented and human non-pigmented ciliary epithelial cells were cultured on porous filter supports to obtain measurements of transepithelial electrical parameters. 2. The non-pigmented cells showed maximal transepithelial resistance of 15-30 omega cm2 from the third to seventh day in culture. 3. The pigmented ciliary cells reached maximal resistances of 9-20 omega cm2 after the fourth day in culture. 4. The transepithelial resistances of the cultured epithelia were functionally increased by protamine. This effect could be reversed by heparin. 5. We conclude that the range of resistances in cultured ciliary epithelial cells is the same as in whole ciliary preparations. Thus, cultured ciliary epithelial cells can be used for studies on transepithelial transport.

pHi-dependent membrane conductance of proximal tubule cells in culture (OK): differential effects on K(+)- and Na(+)-conductive channels

Confluent monolayers of the established opossum kidney cell line were exposed to NH4Cl pulses (20 mmol/liter) during continuous intracellular measurements of pH, membrane potential (PDm) and membrane resistance (R'm) in bicarbonate-free Ringer. The removal of extracellular NH4Cl leads to an intracellular acidification from a control value of 7.33 +/- 0.08 to 6.47 +/- 0.03 (n = 7). This inhibits the absolute K conductance (gK+), reflected by a decrease of K+ transference number from 71 +/- 3% (n = 28) to 26 +/- 6% (n = 5), a 2.6 +/- 0.2-fold rise of R'm, and a depolarization by 24.2 +/- 1.5 mV (n = 52). In contrast, intracellular acidification during a block of gK+ by 3 mmol/liter BaCl2 enhances the total membrane conductance, being shown by R'm decrease to 68 +/- 7% of control and cell membrane depolarization by 9.8 +/- 2.8 mV (n = 17). Conversely, intracellular alkalinization under barium elevates R'm and hyperpolarizes PDm. The replacement of extracellular sodium by choline in the presence of BaCl2 significantly hyperpolarizes PDm and increases R'm, indicating the presence of a sodium conductance. This conductance is not inhibited by 10(-4) mol/liter amiloride (n = 7). Patch-clamp studies at the apical membrane (excised inside-out configuration) revealed two Na(+)-conductive channels with 18.8 +/- 1.4 pS (n = 10) and 146 pS single-channel conductance. Both channels are inwardly rectifying and highly selective towards Cl-. The low-conductive channel is 4.8 times more permeable for Na+ than for K+. Its open probability rises at depolarizing potentials and is dependent on the pH of the membrane inside (higher at pH 6.5 than at pH 7.8).