Basal-lateral transport and transcellular flux of methyl alpha-D-glucoside across LLC-PK1 renal epithelial cells

Sensitivity and mechanisms of taxol-resistant prostate adenocarcinoma cells to Vernonia amygdalina extract

Prostate cancer (PC) patients once Paclitaxel (TAX) treatment responsive later develop hormone refractory PC, thus becoming TAX-insensitive. This underscores the urgent need to develop novel anti-PC therapies. Vernonia amygdalina (VA) could be one such candidate agent. We have shown that androgen-independent PC-3 cells are sensitive to VA treatment in vitro. VA extract (0.01, 0.1 and 1 mg/ml) inhibited DNA synthesis by 12%, 45% (p<0.05), and 73% (p<0.01) respectively. In contrast, TAX (0.01, 0.1, and 1 μM) failed to significantly affect cell growth, suggesting TAX resistance. We tested molecular mechanisms which may lend to the observed PC-3 cell VA sensitivity/TAX resistance. Though both VA and TAX stimulated MAPK activity, VA's induction was more intense, but transient, compared to TAX's sustained action. NF-κB activation was inhibited on average by 50% by either 1 mg/ml VA or 1 μM TAX. VA extract caused 35% and 45% increases in c-Myc activity at 10 and 60 min intervals respectively, with the highest stimulation attained 1h after treatment. In contrast, similar levels were attained by TAX rapidly (within 5 min) and were sustained compared to the slow/multi-phasic action of VA. VA extract treatments had no effect on AKT gene expression, while TAX treatments yielded a four-fold (P<0.01) increase; and P-glycoprotein (P-gp) activity was inhibited by VA and stimulated by TAX, compared to control (basal ATPase activity). This study shows that TAX-resistant PC-3 cells are sensitive to VA, perhaps explained by differential regulatory patterns of MAPK, c-Myc, AKT, and Pgp activities/expressions.

Zinc enhancement of LLC-PK(1) renal epithelial barrier function

BACKGROUND AND AIMS

Earlier work by our group and others has documented improvement of epithelial barrier function in human gastrointestinal models. Here we tested zinc's ability to improve a renal epithelial model. Our aim was to compare the functional and structural effects of zinc on the tight junctional (TJ) complexes of these two very distinct epithelial cell types. Zinc's ability to achieve barrier enhancement in very different epithelial cell types by action upon distinct molecular targets in each epithelial model may suggest a fundamental general role for supplemental zinc in epithelial barrier improvement throughout the body.

METHODS

Cell layers were exposed to 50 or 100 μM zinc on both cell surfaces for 48 h followed by measurement of transepithelial electrical resistance (Rt) and transepithelial (14)C-mannitol flux (Jm). TJ proteins in cell layers were analyzed by Western immunoblot.

RESULTS AND CONCLUSIONS

Zinc supplementation improved the basal TJ barrier function of LLC-PK1 renal cell layers, exemplified by increased Rt and decreased Jm. These zinc-induced changes were also accompanied by decreased NaCl dilution potentials. Of the tight junctional proteins that were tested (occludin, claudins 1, 2, 3, 4, and 5, and tricellulin), we did not observe a zinc-induced change in abundance of any of them, in detergent-soluble fractions of lysates of confluent differentiated cell layers. However, examination of cytosolic fractions showed concentration-dependent increases in the levels of claudins -2 and -4 in this compartment as a result of supplemental zinc. The effects of supplemental zinc on the tight junctional complexes and barrier properties of this renal epithelial model are contrasted with zinc effects on the CACO-2 gastrointestinal model.

Zinc supplementation modifies tight junctions and alters barrier function of CACO-2 human intestinal epithelial layers

BACKGROUND

Zinc deficiency is known to result in epithelial barrier leak in the GI tract. Precise effects of zinc on epithelial tight junctions (TJs) are only beginning to be described and understood. Along with nutritional regimens like methionine-restriction and compounds such as berberine, quercetin, indole, glutamine and rapamycin, zinc has the potential to function as a TJ modifier and selective enhancer of epithelial barrier function.

AIMS

The purpose of this study was to determine the effects of zinc-supplementation on the TJs of a well-studied in vitro GI model, CACO-2 cells.

METHODS

Barrier function was assessed electrophysiologically by measuring transepithelial electrical resistance (Rt), and radiochemically, by measuring transepithelial (paracellular) diffusion of 14C-D-mannitol and 14C-polyethyleneglycol. TJ composition was studied by Western immunoblot analyses of occludin, tricellulin and claudins-1 to -5 and -7.

RESULTS

Fifty- and 100-μM zinc concentrations (control medium is 2 μM) significantly increase Rt but simultaneously increase paracellular leak to D-mannitol. Claudins 2 and 7 are downregulated in total cell lysates, while occludin, tricellulin and claudins-1, -3, -4 and -5 are unchanged. Claudins-2 and -7 as well as tricellulin exhibit decreased cytosolic content as a result of zinc supplementation.

CONCLUSIONS

Zinc alters CACO-2 TJ composition and modifies TJ barrier function selectively. Zinc is one of a growing number of "nutraceutical" substances capable of enhancing epithelial barrier function, and may find use in countering TJ leakiness induced in various disease states.

Proinflammatory mediators disrupt glucose homeostasis in airway surface liquid

The glucose concentration of the airway surface liquid (ASL) is much lower than that in blood and is tightly regulated by the airway epithelium. ASL glucose is elevated in patients with viral colds, cystic fibrosis, chronic obstructive pulmonary disease, and asthma. Elevated ASL glucose is also associated with increased incidence of respiratory infection. However, the mechanism by which ASL glucose increases under inflammatory conditions is unknown. The aim of this study was to investigate the effect of proinflammatory mediators (PIMs) on the mechanisms governing airway glucose homeostasis in polarized monolayers of human airway (H441) and primary human bronchial epithelial (HBE) cells. Monolayers were treated with TNF-α, IFN-γ, and LPS during 72 h. PIM treatment led to increase in ASL glucose concentration and significantly reduced H441 and HBE transepithelial resistance. This decline in transepithelial resistance was associated with an increase in paracellular permeability of glucose. Similar enhanced rates of paracellular glucose flux were also observed across excised trachea from LPS-treated mice. Interestingly, PIMs enhanced glucose uptake across the apical, but not the basolateral, membrane of H441 and HBE monolayers. This increase was predominantly via phloretin-sensitive glucose transporter (GLUT)-mediated uptake, which coincided with an increase in GLUT-2 and GLUT-10 abundance. In conclusion, exposure of airway epithelial monolayers to PIMs results in increased paracellular glucose flux, as well as apical GLUT-mediated glucose uptake. However, uptake was insufficient to limit glucose accumulation in ASL. To our knowledge, these data provide for the first time a mechanism to support clinical findings that ASL glucose concentration is increased in patients with airway inflammation.

A fluorescence method for measurement of glucose transport in kidney cells

BACKGROUND

Diabetes may alter renal glucose reabsorption by sodium (Na(+))-dependent glucose transporters (SGLTs). Radiolabeled substrates are commonly used for in vitro measurements of SGLT activity in kidney cells. We optimized a method to measure glucose uptake using a fluorescent substrate, 2-(N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)amino)-2-deoxyglucose (2-NBDG).

METHODS

Uptake buffers for 2-NBDG were the same as for (14)C-labeled α-methyl-d-glucopyranoside ([(14)C]AMG). Cell lysis buffer was optimized for fluorescence of 2-NBDG and Hoechst DNA stain. Uptake was performed on cultures of primary mouse kidney cells (PMKCs), the LLC-PK(1) proximal tubule cell line, or COS-7 cells transiently overexpressing mouse SGLT1 or SGLT2 by incubating cells at 37°C in buffer containing 50-200 μM 2-NBDG. Microscopy was performed to visualize uptake in intact cells, while a fluorescence microplate reader was used to measure intracellular concentration of 2-NBDG ([2-NBDG](i)) in cell homogenates.

RESULTS

Fluorescent cells were observed in cultures of PMKCs and LLC-PK(1) cells exposed to 2-NBDG in the presence or absence of Na(+). In LLC-PK(1) cells, 2-NBDG transport in the presence of Na(+) had a maximum rate of 0.05 nmol/min/μg of DNA. In these cells, Na(+)-independent uptake of 2-NBDG was blocked with the GLUT inhibitor, cytochalasin B. The Na(+)-dependent uptake of 2-NBDG decreased in response to co-exposure to the SGLT substrate, AMG, and it could be blocked with the SGLT inhibitor, phlorizin. Immunocytochemistry showed overexpression of SGLT1 and SGLT2 in COS-7 cells, in which, in the presence of Na(+), [2-NBDG](i) was fivefold higher than in controls.

CONCLUSION

Glucose transport in cultured kidney cells can be measured with the fluorescence method described in this study.

Glucose homeostasis across human airway epithelial cell monolayers: role of diffusion, transport and metabolism

Glucose in airway surface liquid (ASL) is maintained at low concentrations compared to blood glucose. Using radiolabelled [(3)H]-D: -glucose and [(14)C]-L: -glucose, detection of D: - and L: -glucose by high-performance liquid chromatography and metabolites by nuclear magnetic resonance, we found that glucose applied to the basolateral side of H441 human airway epithelial cell monolayers at a physiological concentration (5 mM) crossed to the apical side by paracellular diffusion. Transepithelial resistance of the monolayer was inversely correlated with paracellular diffusion. Appearance of glucose in the apical compartment was reduced by uptake of glucose into the cell by basolateral and apical phloretin-sensitive GLUT transporters. Glucose taken up into the cell was metabolised to lactate which was then released, at least in part, across the apical membrane. We suggest that glucose transport through GLUT transporters and its subsequent metabolism in lung epithelial cells help to maintain low glucose concentrations in human ASL which is important for protecting the lung against infection.

Absence of Na+/sugar cotransport activity in Barrett’s metaplasia

AIM: To evaluate the presence of Na⁺-dependent, active, sugar transport in Barrett's epithelia as an intestinal biomarker, based on the well-documented, morphological intestinal phenotype of Barrett's esophagus (BE).

METHODS: We examined uptake of the nonmeta-bolizable glucose analogue, alpha-methyl-D-glucoside (AMG), a substrate for the entire sodium glucose cotransporter (SGLT) family of transport proteins. During upper endoscopy, patients with BE or with uncomplicated gastroesophageal reflux disease (GERD) allowed for duodenal, gastric fundic, and esophageal mucosal biopsies to be taken. Biopsies were incubated in bicarbonate-buffered saline (KRB) containing 0.1 mmol/L ¹⁴C-AMG for 60 min at 20°C. Characterized by abundant SGLT, duodenum served as a positive control while gastric fundus and normal esophagus, known to lack SGLT, served as negative controls.

RESULTS: Duodenal biopsies accumulated 249.84 ± 35.49 (SEM) picomoles AMG/&mgr;g DNA (n = 12), gastric fundus biopsies 36.20 ± 6.62 (n = 12), normal esophagus 12.10 ± 0.59 (n = 3) and Barrett’s metaplasia 29.79 ± 5.77 (n = 8). There was a statistical difference (P < 0.01) between biopsies from duodenum and each other biopsy site but there was no statistically significant difference between normal esophagus and BE biopsies. 0.5 mmol/L phlorizin (PZ) inhibited AMG uptake into duodenal mucosa by over 89%, but had no significant effect on AMG uptake into gastric fundus, normal esophagus, or Barrett’s tissue. In the absence of Na⁺ (all Na⁺ salts replaced by Li⁺ salts), AMG uptake in duodenum was decreased by over 90%, while uptake into gastric, esophageal or Barrett’s tissue was statistically unaffected.

CONCLUSION: Despite the intestinal enterocyte phenotype of BE, Na⁺-dependent, sugar transport activity is not present in these cells.

Restriction of sulfur-containing amino acids alters claudin composition and improves tight junction barrier function

Restriction of sulfur-containing amino acids (SCAA) has been shown to elicit a similar increase in life span and decrease in age-related morbidity as caloric restriction. The singular importance of epithelial barrier function in both physiological homeostasis and prevention of inflammation raised the issue of examining the effect of SCAA restriction on epithelial tight junction structure and permeability. Using a well-described in vitro, epithelial model, the LLC-PK(1) renal epithelial cell line, we studied the effects of SCAA restriction in culture medium. Reduction of methionine by 90%, cysteine by 50%, and total elimination of cystine resulted in dramatically lower intracellular pools of these amino acids and their metabolite, taurine, but the intracellular pools of the non-SCAA were all elevated. Cell growth and differentiation were maintained, and both confluent cell density and transepithelial short circuit current were unaffected. Certain tight junctional proteins, such as occludin and claudins-1 and -2 were not altered. However, claudins-3 and -7 were significantly decreased in abundance, whereas claudins-4 and -5 were markedly increased in abundance. The functional result of these structural changes was improved barrier function, as evidenced by increased transepithelial electrical resistance and decreased transepithelial (paracellular) diffusion of D-mannitol.

Ras mutation impairs epithelial barrier function to a wide range of nonelectrolytes

Although ras mutations have been shown to affect epithelial architecture and polarity, their role in altering tight junctions remains unclear. Transfection of a valine-12 mutated ras construct into LLC-PK1 renal epithelia produces leakiness of tight junctions to certain types of solutes. Transepithelial permeability of D-mannitol increases sixfold but transepithelial electrical resistance increases >40%. This indicates decreased paracellular permeability to NaCl but increased permeability to nonelectrolytes. Permeability increases to D-mannitol (Mr 182), polyethylene glycol (Mr 4000), and 10,000-Mr methylated dextran but not to 2,000,000-Mr methylated dextran. This implies a "ceiling" on the size of solutes that can cross a ras-mutated epithelial barrier and therefore that the increased permeability is not due to loss of cells or junctions. Although the abundance of claudin-2 declined to undetectable levels in the ras-overexpressing cells compared with vector controls, levels of occludin and claudins 1, 4, and 7 increased. The abundance of claudins-3 and -5 remained unchanged. An increase in extracellular signal-regulated kinase-2 phosphorylation suggests that the downstream effects on the tight junction may be due to changes in the mitogen-activated protein kinase signaling pathway. These selective changes in permeability may influence tumorigenesis by the types of solutes now able to cross the epithelial barrier.

Effect of nitric oxide on cytotoxicity of Taxol: enhanced Taxol transcellular permeability

The present studies were aimed at testing the hypothesis that nitric oxide (NO) may enhance Taxol-induced cytotoxicity in carcinoma cells by increasing influx of Taxol into intracellular compartments. Prostate carcinoma cells (PC-3, LNCaP) and neuroblastoma cells (SKNDZ, CHP212) were used to investigate both transmembrane permeability and cytotoxicity of Taxol in the presence and absence of S-nitrosocaptopril (CapNO), a nitric oxide donating compound. The order of permeability rate of Taxol across the four cell lines was SKNDZ>LNCaP>PC-3>CHP212. Pretreatment of the cell lines with CapNO (100 microM) enhanced permeability of Taxol across prostate PC-3 and LNCaP cells, but not neuroblastoma SKNDZ and CHP212 cells. Taxol inhibited cell growth at nanomolar levels with IC(50)s of 0.21, 17.4, 96.4 and 842.9 nM corresponding to SKNDZ, PC-3, LNCaP and CHP212 cells, respectively. However, CapNO inhibited proliferation of the four cell lines at millimolar levels with IC(50)s ranging from 0.3 to 1.1 mM. Enhancing effect of CapNO (100 microM) on Taxol cytotoxicity were found in PC-3 and LNCaP cells, but not in SKNDZ and CHP212. The findings suggest that the cytotoxic potency of Taxol is mainly dependent upon the cell membrane permeabilization to Taxol, and the enhancing effect of CapNO on Taxol-induced cytotoxicity is primarily mediated via the increased influx of Taxol by NO into intracellular compartments, while NO-induced cytotoxicity cannot be excluded.

Effect of nanonization on absorption of 301029: ex vivo and in vivo pharmacokinetic correlations determined by liquid chromatography/mass spectrometry

PURPOSE

To compare Caco-2 monolayer permeability and in vivo bioavailability of microparticle with nanoparticle 301029, a thiadiazole derivative, and to determine whether nanonization could improve oral bioavailability of the poorly soluble compound.

METHODS

The mean particle size of 301029 was reduced from 7 microm to 280 nm by pearl milling. In the ex vivo assay, both microparticle and nanoparticle 301029 at the same concentration were separately added to apical side and were collected from basolateral side of Caco-2 monolayer. In the bioavailability study, the two particle sizes of 301029 were orally administered to rats, respectively, and blood samples were collected. Nanoparticle 301029 in culture medium and rat serum was detected by a liquid chomatography-mass spectrometer (LC/MS) coupled with atmospheric pressure chemical ionization (APCI).

RESULTS

Permeability rate and permeated amounts of nanoparticle 301029 across the Caco-2 monolayer were about four times higher than those of microparticle 301029. In a pharmacokinetic study, nanoparticle 301029 showed Tmax about 1 h, whereas the microparticle 301029 showed Tmax at 4 h. The Cmax and AUC of nanoparticle 301029 were 3- to 4-fold greater than those of microparticle 301029, resulting in a significant increase in oral bioavailability of 301029 as compared with microparticle 301029. The ex vivo permeability and in vivo pharmacokinetic data indicate that nanoparticle formulation improves both absorption rate and absorption extent of the poorly soluble drug.

CONCLUSIONS

Nanoparticle formulation enhances both Caco-2 monolayer permeability and rat oral bioavailability of the poorly soluble 301029. The result also demonstrates a close correlation between ex vivo Caco-2 permeability model and in vivo gastrointestinal absorption.

In vitro and in vivo assessment of cellular permeability and pharmacodynamics of S-nitrosylated captopril, a nitric oxide donor

1. The present studies were aimed at testing the hypothesis that S-nitrosylated captopril (CapNO), a novel crystalline nitric oxide (NO) donor, readily permeates both in vitro and in vivo endothelial monolayers, resulting in its pharmacodynamic effects. 2. CapNO and Captopril (Cap) were added to apical side of endothelial monolayers formed on microporous membranes, and the permeated drugs were collected from basolateral side and detected by a HPLC method. The permeability coefficient (P(app); cm sec(-1)) of CapNO across the endothelial monolayers was 6.0 x 10(-5), higher than that of Cap (3.13 x 10(-5)), indicating the enhancement effect of the attached NO group in CapNO on cellular permeability. The P(app) of CapNO and Cap across Caco-2 cells were 3.15 x 10(-5) and 1.53 x 10(-5), respectively. The low P(app) of CapNO to Caco-2 cells may be attributed to the high membrane resistance of Caco-2 cells. 3. A bolus injection of CapNO to epicardial coronary artery of chronically-instrumented awake dogs caused significant increases in coronary blood flow and coronary diameters dose-dependently without significant changes in aortic pressure. In contrast, the equimolar doses of Cap did not produce haemodynamic responses. 4. Intravenous CapNO caused an instant increase in the regional cerebral blood flow determined by H(2)-clearance, whereas the equimolar doses of Cap did not enhance the cerebral blood flow. 5. These results conclude that the NO group, an active component of CapNO, enhances both in vitro and in vivo endothelial permeability to the entire compound, resulting in instant increases in blood flow and vascular diameters. In contrast, the equimolar Cap does not have the instant vascular effects.

Independent organic cation transport activity of Na(+)-L-carnitine cotransport system in LLC-PK(1) cells

We investigated expression of the Na(+)-L-carnitine cotransport system and its role in transport of tetraethylammonium in a kidney epithelial cell line, LLC-PK(1). L-Carnitine uptake in the LLC-PK(1) cells was markedly stimulated in the presence of Na(+). The uptake was saturable, with Michaelis constant and maximal uptake velocity values of 7.8 microM and 153.7 pmol x mg protein(-1) x 15 min(-1), respectively. Cationic drugs such as tetraethylammonium, cimetidine, and quinidine inhibited L-carnitine uptake. The basolateral-to-apical transport of [(14)C]tetraethylammonium was enhanced markedly in the presence of an H(+) gradient on the apical side at a pH of 5.9. Under the conditions in which Na(+)/L-carnitine cotransport activity was saturable by the addition of 100 microM L-carnitine to the apical-side medium, the basolateral-to-apical transcellular transport of [(14)C]tetraethylammonium was unaffected. These results suggested that the Na(+)-L-carnitine cotransporter is expressed in the apical membranes of LLC-PK(1) cells, and is not responsible for efflux of tetraethylammonium from the cells. Transport of tetraethylammonium appeared to be mediated predominantly by an H(+)/organic cation antiporter in the apical membranes.

Activation of NF-kappaB is necessary for the restoration of the barrier function of an epithelium undergoing TNF-alpha-induced apoptosis

Tumor necrosis factor-alpha (TNF) induces apoptosis in confluent LLC-PK1 epithelial cells, but also activates NF-kappaB, a negative regulator of apoptosis. The presence of increased TNF-induced apoptosis causes a transient increase in epithelial permeability, but the epithelial barrier function recovers, as assessed by measuring the transepithelial electrical resistance, the paracellular flux of mannitol and by the electron microscopic evaluation of the penetration of the electron-dense dye ruthenium red across the tight junctions. The integrity of the epithelial cell layer is maintained by rearrangement of non-apoptotic cells in the monolayer and by the phagocytosis of apoptotic fragments. To study the role of NF-kappaB in an epithelium exposed to TNF, NF-kappaB was inhibited in LLC-PK1 epithelial cells with either the dietary compound, curcumin, or by transfection with a dominant negative mutant inhibitor I kappaB alpha. Replacement of serine 32 and 36 by alanine has been shown to prevent its phosphorylation and degradation, blocking NF-kappaB activation. Inhibition of NF-kappaB altered the morphology of TNF-induced apoptotic cells, which showed lack of fragmentation and membrane blebbings, and absence of phagocytosis by neighboring cells. TNF treatment of NF-kappaB-inhibited cells also caused altered distribution of the tight junction-associated protein ZO-1, increased epithelial leakiness, and impaired the recovery of the epithelial barrier function, which normally occurs 6 hours after TNF treatment of LLC-PK1 cells. These data demonstrate that NF-kappaB activation is required for the maintenance of the barrier function of an epithelium undergoing TNF-induced apoptosis.

Overexpression of protein kinase C-delta increases tight junction permeability in LLC-PK1 epithelia

The Ca2+-independent delta-isoform of protein kinase C (PKC-delta) was overexpressed in LLC-PK1 epithelia and placed under control of a tetracycline-responsive expression system. In the absence of tetracycline, the exogenous PKC-delta is expressed. Western immunoblots show that the overexpressed PKC-delta is found in the cytosolic, membrane-associated, and Triton-insoluble fractions. Overexpression of PKC-delta produced subconfluent and confluent epithelial morphologies similar to that observed on exposure of wild-type cells to the phorbol ester 12-O-tetradecanoylphorbol-13-acetate. Transepithelial electrical resistance (RT) in cell sheets overexpressing PKC-delta was only 20% of that in cell sheets incubated in the presence of tetracycline, in which the amount of PKC-delta and RT were similar to those in LLC-PK1 parental cell sheets. Overexpression of PKC-delta also elicited a significant increase in transepithelial flux of D-[14C]mannitol and a radiolabeled 2 x 10(6)-molecular-weight dextran, suggesting with the RT decrease that overexpression increased paracellular, tight junctional permeability. Electron microscopy showed that PKC-delta overexpression results in a multilayered cell sheet, the tight junctions of which are almost uniformly permeable to ruthenium red. Freeze-fracture electron microscopy indicates that overexpression of PKC-delta results in a more disorganized arrangement of tight junctional strands. As with LLC-PK1 cell sheets treated with 12-O-tetradecanoylphorbol-13-acetate, the reduced RT, increased D-mannitol flux, and tight junctional leakiness to ruthenium red that are seen with PKC-delta overexpression suggest the involvement of PKC-delta in regulation of tight junctional permeability.

Oxalate transport and calcium oxalate renal stone disease

Hyperoxaluria is considered to play a crucial role in calcium oxalate (CaOx) renal stone disease. The amount of oxalate excreted into the urine depends on intestinal absorption, endogenous production, renal clearance and renal tubular transport. Since a primary disorder has not been found so far in most CaOx stone formers and since oxalate is freely filtered at the glomerulus, most studies are presently focussed on alterations in epithelial oxalate transport pathways. Oxalate can be transported across an epithelium by the paracellular (passive) and transcellular (active) pathway. Oxalate transport across cellular membranes is mediated by anion-exchange transport proteins. A defect in the structure of these transport proteins could explain augmented transcellular oxalate transport. Little is known about the physiological regulation of oxalate transport. In this review cellular transport systems for oxalate will be summarized with special attention for the progress that has been made to study oxalate transport in a model of cultured renal tubule cells. Better understanding of the physiological processes that are involved in oxalate transport could yield information on the basis of which it might be possible to design new approaches for an effective treatment of CaOx stone disease.

Oxalate transport and calcium oxalate renal stone disease

Hyperoxaluria is considered to play a crucial role in calcium oxalate (CaOx) renal stone disease. The amount of oxalate excreted into the urine depends on intestinal absorption, endogenous production, renal clearance and renal tubular transport. Since a primary disorder has not been found so far in most CaOx stone formers and since oxalate is freely filtered at the glomerulus, most studies are presently focussed on alterations in epithelial oxalate transport pathways. Oxalate can be transported across an epithelium by the paracellular (passive) and transcellular (active) pathway. Oxalate transport across cellular membranes is mediated by anion-exchange transport proteins. A defect in the structure of these transport proteins could explain augmented transcellular oxalate transport. Little is known about the physiological regulation of oxalate transport. In this review cellular transport systems for oxalate will be summarized with special attention for the progress that has been made to study oxalate transport in a model of cultured renal tubule cells. Better understanding of the physiological processes that are involved in oxalate transport could yield information on the basis of which it might be possible to design new approaches for an effective treatment of CaOx stone disease.

Sodium-independent carrier-mediated inositol transport in cultured renal epithelial (LLC-PK1) cells

In addition to the concentrative, Na(+)-dependent inositol transport system demonstrated in many cell types, carrier-mediated, Na(+)-independent inositol transport is also shown to exist in LLC-PK1 renal epithelia. Inhibition of inositol uptake in Na(+)-free saline by 0.1 mM phloretin, and self-inhibition by net concentrations of inositol exceeding 10 mM, demonstrate the carrier-mediation of the Na(+)-independent uptake and distinguish it from flux through anion channels. The Na(+)-dependent uptake exhibits higher affinity for inositol, as seen by the stronger self-inhibition at lower inositol concentrations in Na+ saline. Kinetic analyses indicate a Km of 178 microM and a Vmax of 2447 pmol/min per microgram DNA for the Na(+)-dependent system, whereas the lower affinity, lower capacity Na(+)-independent system manifests a Km of 5.2 mM and a Vmax of 249 pmol/min per microgram DNA. the Na(+)-independent uptake further differs from the Na(+)-dependent transport by the lack of inhibitory effect of 10 microM glucose, and the greater relative inhibition of phloretin compared to that of phlorizin. Both types of uptake appear to localize predominantly to the basal-lateral cell surface. The Na(+)-independent transport is bidirectional, functioning in efflux as well as influx of inositol.

Polarity of transport of 2-deoxy-D-glucose and D-glucose by cultured renal epithelia (LLC-PK1)

At least two types of glucose transporter exist in cultured renal epithelial cells, a Na(+)-glucose cotransporter (SGLT), capable of interacting with D-glucose but not 2-deoxy-D-glucose (2dglc) and a facilitated transporter (GLUT) capable of interacting with both D-glucose and 2dglc. In order to examine the polarity of transport in cultured renal epithelia, 2dglc and D-glucose uptakes were measured in confluent cultures of LLC-PK1 cells grown on collagen-coated filters that permitted access of medium to both sides of the monolayer. The rates of basolateral uptake of both 1 mM glucose (Km 3.6 mM) and 1 mM 2dglc (Km 1.5 mM) were greater than apical uptake rates and the (apical-to-basolateral)/(basolateral-to-apical) flux ratio was high for glucose (9.4) and low for 2dglc (0.8), thus, confirming the lack of interaction of 2dglc with the apical SGLT. Specific glucose transport inhibitor studies using phlorizin, phloretin and cytochalasin B confirmed the polarised distribution of SGLT and GLUT in LLC-PK1 cells. Basolateral sugar uptake could be altered by addition of insulin (1 mU/ml) which increased 2dglc uptake by 72% and glucose uptake by 50% and by addition of 20 mM glucose to the medium during cell culture which decreased 2dglc uptake capacity at confluence by 30%. During growth to confluence, 2dglc uptake increased to a maximum, then decreased at the time of confluence, coincident with a rise in uptake capacity for alpha-methyl-D-glucoside, a hexose that interacts only with the apical SGLT. It was concluded that the non-metabolisable sugar 2dglc was a useful, specific probe for GLUT in LLC-PK1 cells and that GLUT was localised at the basolateral membrane after confluence.

Transcellular transport of organic cation across monolayers of kidney epithelial cell line LLC-PK

Transcellular transport and the accumulation of [14C]tetraethylammonium, a typical organic cation, by LLC-PK1 cell monolayers grown on microporous membrane filters were studied. Tetraethylammonium was accumulated progressively in the monolayers from the basolateral side and was transported unidirectionally to the apical side. The transcellular transport of tetraethylammonium was saturable, temperature dependent, and sensitive to the pH of the apical side of the monolayers. The apparent Michaelis constant and maximum velocity values for the transport were 67 microM and 222 pmol.mg protein-1.min-1, respectively. Unlabeled tetraethylammonium, amiloride, procainamide, cimetidine, and choline inhibited the basolateral uptake and transcellular transport of [14C]tetraethylammonium. The development of tetraethylammonium transport activity was observed in the differentiating cells. A sulfhydryl reagent inhibited the tetraethylammonium transport at both the basolateral and apical membranes of the LLC-PK1 cells. These findings suggest that these monolayers possess unidirectional transport systems for organic cations, corresponding to the secretion in the renal proximal tubules.

Renal proximal tubular cells in suspension or in primary culture as in vitro models to study nephrotoxicity

The kidney forms a frequent target for xenobiotic toxicity. The complex biochemical mechanisms underlying nephrotoxicity are best studied in vitro provided that reliable and relevant in vitro models are available. Since most nephrotoxicants affect primarily the cells of the proximal tubules (PTC), much effort has been directed towards the development of in vitro models of PTC. This review focuses on the preparation of PTC and the use of these cells. Discussed are important criteria such as the viability (survival time) of the cells and the parameters to assess toxicity. Recent studies have shown that isolated PTC in suspension are especially suitable for studies on the biochemical mechanisms of 'acute' nephrotoxicity, whereas PTC in primary culture may be used to investigate mechanisms of nephrotoxic damage at very low concentrations, upon prolonged exposure. PTC cultured on porous filter membranes provide new possibilities to study toxicity in relation to cell and transport polarity. Primary cell cultures of human PTC have been set up. Although a further characterization of these systems is needed, recent data indicate their usefulness.

Na+-sugar cotransport system as a polarization marker during organization of epithelial membrane

The present study analyzed the changes in Na+-dependent sugar transport and transepithelial electrical resistance as LLC-PK1 cells reorganize into epithelial membranes. Sugar influx increased to reach a maximum 9 h after plating. The increase in the transepithelial electrical resistance, however, showed a significant delay, reaching steady state 15 h after plating. No changes in the electrochemical Na+ gradient were observed during the reorganization of the epithelial membranes. Kinetic analysis and [3H]phlorizin-binding studies showed that the increase in sugar influx resulted from an increase in the number of carriers. Unidirectional sugar influx measurements indicated that the sugar transporters were primarily located at the apical surface of the epithelial cells. These observations are consistent with the hypothesis that the sorting of native proteins occurs intracellularly before their insertion in the apical membrane, or as an alternative that they are randomly inserted, but then immediately sorted such as any carrier could be detected in the basolateral side during the reorganization process. In addition, the results suggest that the functional development of the apical membrane may occur before the complete sealing of the intercellular space during the development of the occluding junctions. Furthermore, development of the sugar transport system and occluding junctions was inhibited by cycloheximide and puromycin but not by actinomycin D, suggesting that the expression of epithelial cell polarization is probably a posttranslational event in the protein synthesis.

Characteristics of cystine uptake by cultured LLC-PK1 cells

The characteristics of the uptake of L-cystine by LLC-PK1 cells were examined. The uptake diminished with time in culture after passage of cells while the uptake of sugar increased. In 48-h-cultured cells at a range of cystine concentrations including physiological levels uptake occurred via a saturable process which was independent of medium sodium concentration and pH. No inhibition of cystine uptake occurred in the presence of lysine which is known to share the cystine transport system in uncultured renal proximal tubule cells and brush-border membrane vesicles. Glutamate was a potent inhibitor of cystine uptake and participated in heteroexchange diffusion with cystine. The cystine-glutamate transport process resembles that of cultured human fibroblasts. The inability of these cells to reflect the genetically determined cystine-lysine system which is altered in the kidney in human cystinuria makes them an inappropriate model of the renal tubule cell cystine transport system. On the other hand, they may provide a model system for examining the factors which determine the presence of the various cystine transport process.

Effects of diacylglycerols on LLC-PK1 renal epithelia: similarity to phorbol ester tumor promoters

As previously shown using phorbol ester tumor promoters (see Mullin and O'Brien: Am. J. Physiol., 251:C597-C602, 1986), diacylglycerols induce leakiness in LLC-PK1 renal epithelial tight junctions. The similarity between phorbol ester and diacylglycerol action includes effects on 1) cell morphology, 2) dome formation, 3) transepithelial resistance and potential difference, 4) transepithelial flux of D-mannitol, and 5) mitogenesis. Four diacylglycerols have been tested: 1,2-dioctanoylglycerol; 1,2-dicaprylglycerol; 1,2-dioleoylglycerol; and 1-oleoyl-2-acetoyl-sn-3-glycerol. Their relative effectiveness depended upon the phenomenon being observed. Unlike phorbol esters, diacylglycerol effects were reversible within hours at 37 degrees C in the continued presence of diacylglycerol, and effects were more pronounced when cell sheets were exposed to diacylglycerols from the basolateral cell surface. Overall, these findings indicate that previous results with phorbol esters may be attributed to the protein kinase C signal transduction system, and this system may therefore exert a role in transepithelial permeability.

Spontaneous reversal of polarity of the voltage across LLC-PK1 renal epithelial cell sheets

While sterilely monitoring transepithelial voltage (potential difference) across LLC-PK cell sheets over a 24-hr period, we noted that the apical-negative, transepithelial voltage, a key property of the LLC-PK1 renal epithelial cell line, reverses polarity to become apical-positive. This spontaneous change of polarity of electrical potential difference (PD) across LLC-PK1 cell sheets cultured on permeable filters was observed to occur approximately 12 hr after refeeding. Unlike the apical (luminal)-negative PD, the apical-positive PD was insensitive to phlorizin and ouabain. Both were insensitive to the diuretics amiloride, furosemide, and 4-acetamido-4-isothiocynato-stilbene-2,2-disulfonic acid (SITS). A pH gradient existed across apical-positive cell sheets (apical medium more acidic by 0.3 units) but an osmotic gradient did not. Unlike the temperature-sensitive apical-negative PD, the apical positive PD was unaffected by brief exposure to 4 degrees C temperature. Junctional disruptive agents such as the tumor promotor, TPA, dissipated both types of PD with similar time courses. The formation of the apical-positive PD correlated in time with apical glucose levels falling below the reported Km of the Na+-sugar contransporter. A high glycolytic rate per se may not be essential for this PD polarity reversal since the reversal could occur in glucose-free medium with a normal time course and magnitude. The lysis with time of floating cells with consequent release of KCl into the apical compartment was also considered as a possible cause of the polarity reversal, but the turnover of even 2 X 10(6) cells in 12 hr was found not to raise apical KCl sufficiently to produce the polarity shift. Although a significant K+ gradient did not exist across cell sheets with apical-positive PD values, a sizable gradient of Cl- did exist, directed apical to basolateral. This gradient, coupled with anion-selective tight junctions, should contribute to the observed apical positive voltage. The voltage polarity shift seen in these cell cultures with time is not unlike the polarity shift occurring in the renal proximal convoluted tubule, with distance from the glomerulus.

The phorbol ester, TPA, increases transepithelial epidermal growth factor flux

Exposure of cultured kidney epithelial (LLC-PK1) cell sheets to 10(-7) M TPA, a potent tumor promoter and activator of protein kinase C, initiates within minutes a drop in the transepithelial voltage across these sheets. This fall in potential difference correlates with an over 40-fold increase in the transepithelial flux of 1 mM D-mannitol, suggesting that the intercellular junctions have become leaky. Dual labeling experiments with 1 mM D-[14C]mannitol and 10 nM 125I-EGF show that after promoter treatment, a 7-fold increase in net 125I flux accompanies the increase in mannitol flux. Gel filtration and gel electrophoresis indicate that for control cell sheets only 15% of the transited 125I is actually EGF, whereas with TPA-treated cell sheets, 60% of the 125I which passed across is EGF. These percentages permitted determination of actual EGF flux values, and show that TPA treatment engenders a 35-fold increase in transepithelial EGF flux. Diacylglycerols also increase the junctional permeability of these cells, thereby suggesting the involvement of protein kinase C.

Cystine and lysine transport in cultured human renal epithelial cells

The transport of the amino acids, cystine and lysine, was studied in epithelial cell lines propagated from human kidney cortex. Cystine uptake data were reproducible in different cell lines and did not vary over several cell passages of an individual cell line. The transport of this disulfide amino acid was sodium-dependent with kinetic analysis showing one apparent Kt system of 0.09 mmol/L and Vmax of 0.054 mmol/L cell water/min. Studies of the kinetics of lysine transport, however, revealed two uptake systems with apparent high and low affinities with Kt of 0.14 mmol/L and 5 mmol/L and Vmax of 0.041 and 0.167 mmol/L cell water/min, respectively. Glutamate appeared to be the most potent inhibitor of cystine uptake by these cultured human renal cells and this interaction was competitive. Although cystine did not inhibit lysine uptake, arginine and ornithine were shown to be major inhibitors, thus providing evidence for the presence of a shared dibasic amino acid transport system.

Na+-linked active transport of ascorbate into cultured bovine retinal pigment epithelial cells: heterologous inhibition by glucose

The transport of ascorbate into cultured bovine retinal pigment epithelial (RPE) cells is reported. Primary or subcultured RPE cells were incubated in the presence of 10-500 microM L-[carboxyl-14C]-ascorbate for various periods of time. Accumulation of ascorbate into RPE cells followed a saturable active transport with a Km of 125 microM and a Vmax of 28 pmole/micrograms DNA/min. RPE intracellular water was calculated to be 0.8 pL/cell, and the transported cellular ascorbate concentration was 7.5 +/- 0.8 mM. Replacement of 150 mM NaCl in the incubation media with choline-Cl strongly inhibited (80 +/- 8%) ascorbate uptake into cultured RPE cells. Although the depletion of cellular ATP by 2,4-dinitrophenol and the inhibition of Na+-K+-ATPase by ouabain reduced ascorbate transport into RPE significantly, active transport of ascorbate was not entirely inhibited by these metabolic inhibitors. The ascorbate analogue, D-isoascorbate, competitively inhibited ascorbate transport into cultured RPE with a Ki of 12.5 mM. Cells grown in the presence of 5 to 50 mM alpha-D-glucose in the growth media did not differ in their ability to transport ascorbate. In contrast, the presence of alpha-D-glucose or its nonmetabolizable analogues, 3-0-methyl-glucose, alpha-methyl-glucose, and 2-deoxy-glucose, but not L-glucose or beta-D-fructose, in the incubation media inhibited ascorbate transport. myo-Inositol (10 or 20 mM) also inhibited ascorbate transport into RPE cells. The active uptake of ascorbate into cultured RPE cells was primarily coupled to the movement of sodium ion down its electrochemical gradient. A bifunctional, cotransport carrier possessing an ascorbate-binding site and a sodium-binding site may be involved in the ascorbate uptake system. The inhibition of ascorbate uptake by sugars appeared to be heterologous in nature, occurring between two distinct carrier systems, both of which were dependent on the sodium ions.