Molecular Aspects of Renal Handling of Aminoglycosides and Strategies for Preventing the Nephrotoxicity

Aminoglycosides such as gentamicin and amikacin are the most commonly used antibiotics worldwide in the treatment of Gram-negative bacterial infections. However, serious complications like nephrotoxicity and ototoxicity are dose-limiting factors in the use of aminoglycosides. A relatively large amount of the intravenously administered dose is accumulated in the kidney (about 10% of dose), whereas little distribution of aminoglycosides to other tissues is observed. Aminoglycosides are taken up in the epithelial cells of the renal proximal tubules and stay there for a long time, resulting in nephrotoxicity. Acidic phospholipids are considered as a binding site for aminoglycosides in the brush-border membrane of the proximal tubular cells. More recently, it has been reported that megalin, a giant endocytic receptor abundantly expressed at the apical membrane of renal proximal tubules, plays an important role in binding and endocytosis of aminoglycosides in the proximal tubular cells. The elucidation of the aminoglycoside-binding receptor would help design a strategy to prevent against aminoglycoside-induced nephrotoxicity. In this review, we summarize recent advances in the understandings of the molecular mechanisms responsible for renal accumulation of aminoglycosides, especially megalin-mediated endocytosis. In addition, approaches toward prevention of aminoglycoside-induced nephrotoxicity are discussed, based on the molecular mechanisms of the renal accumulation of aminoglycosides.

Heat-induced chaperone activity of HSP90

The 90-kDa stress protein, HSP90, is a major cytosolic protein ubiquitously distributed in all species. Using two substrate proteins, dihydrofolate reductase (DHFR) and firefly luciferase, we demonstrate here that HSP90 newly acquires a chaperone activity when incubated at temperatures higher than 46 degrees C, which is coupled with self-oligomerization of HSP90. While chemically denatured DHFR refolds spontaneously upon dilution from denaturant, oligomerized HSP90 bound DHFR during the process of refolding and prevented it from renaturation. DHFR was released from the complex with HSP90 by incubating with GroEL/ES complexes in an ATP-dependent manner and refolded into the native form. alpha-Casein inhibited the binding of DHFR to HSP90 and also chased DHFR from the complex with HSP90. These results suggest that HSP90 binds substrates to maintain them in a folding-competent structure. Furthermore, we found that HSP90 prevents luciferase from irreversible thermal denaturation and enables it to refold when postincubated with reticulocyte lysates. This heat-induced chaperone activity of HSP90 associated with its oligomerization may have a pivotal role in protection of cells from thermal damages.

Interaction with P-glycoprotein and transport of erythromycin, midazolam and ketoconazole in Caco-2 cells

The effect of cytochrome P-450 3A (CYP3A) substrates (erythromycin, midazolam) and an inhibitor (ketoconazole) on P-glycoprotein-mediated transport was studied in Caco-2, the human colon adenocarcinoma cell line expressing various functions of differentiated intestinal epithelial cells. The involvement of P-glycoprotein in the transport of these drugs was also examined. The basal-to-apical transport of rhodamine 123, a P-glycoprotein substrate, was inhibited by erythromycin, midazolam and ketoconazole, as well as by P-glycoprotein inhibitors such as verapamil. The apical-to-basal transport of rhodamine 123 was increased by these drugs. The transepithelial transport of erythromycin and midazolam, but not of ketoconazole, was much greater from the basal to apical side than from the apical to basal side. The inhibitory effect of verapamil was observed on the basal to apical transport of erythromycin, but not on midazolam and ketoconazole transport. In conclusion, erythromycin, midazolam and ketoconazole could interact with P-glycoprotein-mediated transport, and P-glycoprotein could be, at least in part, involved in the transport of erythromycin, but not of midazolam and ketoconazole, in the intestinal epithelia.

Role of megalin in renal handling of aminoglycosides

The role of megalin in tissue distribution of aminoglycosides was examined in normal rats and maleate-treated rats that shed megalin from the renal brush-border membrane. In normal rats, amikacin administered intravenously accumulated most abundantly in the renal cortex, followed by the renal medulla. No amikacin was detected in other tissues. Tissue distributions of amikacin were well correlated with megalin levels in each tissue. Bolus administration of gentamicin increased urinary excretion of megalin ligands (vitamin D binding protein and calcium), suggesting the competition between gentamicin and these megalin ligands in renal tubules. Ligand blotting showed that binding of (45)Ca(2+) to megalin was inhibited by aminoglycosides. Both megalin levels and amikacin accumulation in renal cortex were decreased by maleate injection. Then, amikacin accumulation recovered proportionate to megalin levels. These findings suggest that megalin is involved in the renal cortical accumulation of aminoglycosides in vivo. In addition, the interaction between aminoglycosides and calcium in the kidney may be due to the competition among these compounds to bind to megalin.

Clathrin-mediated endocytosis of FITC-albumin in alveolar type II epithelial cell line RLE-6TN

We examined mechanisms of FITC-albumin uptake by alveolar type II epithelial cells using cultured RLE-6TN cells. Alkaline phosphatase activity and the expression of cytokeratin 19 mRNA, which are characteristic features of alveolar type II epithelial cells, were detected in RLE-6TN cells. The uptake of FITC-albumin by the cells was time and temperature dependent and showed the saturation kinetics of high- and low-affinity transport systems. FITC-albumin uptake was inhibited by native albumin, by chemically modified albumin, and by metabolic inhibitors and bafilomycin A1, an inhibitor of vacuolar H+-ATPase. Confocal laser scanning microscopic analysis after FITC-albumin uptake showed punctate localization of fluorescence in the cells, which was partly localized in lysosomes. FITC-albumin taken up by the cells gradually degraded over time, as shown by fluoroimage analyzer after SDS-PAGE. The uptake of FITC-albumin by RLE-6TN cells was not inhibited by nystatin, indomethacin, or methyl-β-cyclodextrin (inhibitors of caveolae-mediated endocytosis) but was inhibited by phenylarsine oxide and chlorpromazine (inhibitors of clathrin-mediated endocytosis) in a concentration-dependent manner. Uptake was also inhibited by potassium depletion and hypertonicity, conditions known to inhibit clathrin-mediated endocytosis. These results indicate that the uptake of FITC-albumin in cultured alveolar type II epithelial cells, RLE-6TN, is mediated by clathrin-mediated but not by caveolae-mediated endocytosis, and intracellular FITC-albumin is gradually degraded in lysosomes. Possible receptors involved in this endocytic system are discussed.

Mutually dependent localization of megalin and Dab2 in the renal proximal tubule

Disabled-2 (Dab2) is a cytoplasmic adaptor protein that binds to the cytoplasmic tail of the multiligand endocytic receptor megalin, abundantly expressed in renal proximal tubules. Deletion of Dab2 induces a urinary increase in specific plasma proteins such as vitamin D binding protein and retinol binding protein (Morris SM, Tallquist MD, Rock CO, and Cooper JA. EMBO J 21: 1555-1564, 2002). However, the subcellular localization of Dab2 in the renal proximal tubule and its function have not been fully elucidated yet. Here, we report the characterization of Dab2 in the renal proximal tubule. Immunohistocytochemistry revealed colocalization with megalin in coated pits and vesicles but not in dense apical tubules and the brush border. Kidney-specific megalin knockout almost abolished Dab2 staining, indicating that Dab2 subcellular localization requires megalin in the proximal tubule. Reciprocally, knockout of Dab2 led to a redistribution of megalin from endosomes to microvilli. In addition, there was an overall decrease in levels of megalin protein observed by immunoblotting but no decrease in clathrin or alpha-adaptin protein levels or in megalin mRNA. In rat yolk sac epithelial BN16 cells, Dab2 was present apically and colocalized with megalin. Introduction of anti-Dab2 antibody into BN16 cells decreased the internalization of 125I-labeled receptor-associated protein, substantiating the role of Dab2 in megalin-mediated endocytosis. The present study shows that Dab2 is localized in the apical endocytic apparatus of the renal proximal tubule and that this localization requires megalin. Furthermore, the study suggests that the urinary loss of megalin ligands observed in Dab2 knockout mice is caused by suboptimal trafficking of megalin, leading to decreased megalin levels.

Expression and function of P-glycoprotein in rats with glycerol-induced acute renal failure

The effect of glycerol-induced acute renal failure on P-glycoprotein expression and function was evaluated in rats. The in vivo function of P-glycoprotein was evaluated by measuring renal secretory and biliary clearance and brain distribution of rhodamine 123 (Rho-123), a P-glycoprotein substrate, under a steady-state plasma concentration. In acute renal failure rats, the P-glycoprotein level increased 2.5-fold in the kidney, but not in the liver and brain. In contrast, P-glycoprotein function in these tissues was suppressed. Interestingly, not only the renal but also the biliary clearance of Rho-123 was correlated with the glomerular filtration rate. In Caco-2 cells, plasma from renal failure rats exhibited a greater inhibitory effect on P-glycoprotein-mediated transport of Rho-123 than did plasma from control rats. In conclusion, P-glycoprotein function was systemically suppressed in acute renal failure, even though the level of P-glycoprotein remained unchanged or rather increased. This may be due to the accumulation of some endogenous P-glycoprotein substrates/modulators in the plasma in disease states.

Modulation of function of multidrug resistance associated-proteins by Kaempferia parviflora extracts and their components

In this study, the effects of extracts and flavone derivatives from the rhizome of Kaempferia parviflora on multidrug resistance associated-proteins (MRP)-mediated transport in A549 cells were examined. The cells employed express MRP1 and MRP2, but not P-glycoprotein. The cellular accumulation of calcein, an MRP substrate, was significantly increased by various MRP inhibitors without being affected by verapamil, a typical P-glycoprotein inhibitor. Ethanol and aqueous extracts from K. parviflora rhizome increased the accumulation of calcein and doxorubicin in A549 cells in a concentration-dependent manner. The inhibitory potency of the ethanol extract for MRP function was greater than that of the aqueous extract. Among six flavone derivatives isolated from K. parviflora rhizome, 5,7-dimethoxyflavone exhibited a maximal stimulatory effect on the accumulation of doxorubicin in A549 cells. The accumulation of doxorubicin was increased by four flavone derivatives without 5-hydroxy group, but not by the other two flavone derivatives with 5-hydroxy group. In addition, 5,7-dimethoxyflavone and 3,5,7,3',4'-pentamethoxyflavone decreased resistance to doxorubicin in A549 cells. These findings indicate that extracts and flavone derivatives from the rhizome of K. parviflora suppress MRP function, and therefore may be useful as modulators of multidrug resistance in cancer cells.

Effects of Kaempferia parviflora extracts and their flavone constituents on P-glycoprotein function

The purpose of this study was to examine the effects of extracts and flavone derivatives from the rhizome of Kaempferia parviflora on P-glycoprotein (P-gp)-mediated transport in LLC-GA5-COL150, a transfectant cell line of a porcine kidney epithelial cell line LLC-PK1 with human MDR1 cDNA. Ethanol extract obtained from Kaempferia parviflora rhizome significantly increased the accumulation of rhodamine 123 and daunorubicin, P-gp substrates, in LLC-GA5-COL150 cells, but not in LLC-PK1 cells. The aqueous extract also increased the accumulation in LLC-GA5-COL150 cells with lower potency than the ethanol extract. The effects of flavone derivatives isolated from the rhizome of Kaempferia parviflora on P-gp function were examined. Among six flavones tested, 3,5,7,3',4'-pentamethoxyflavone most potently increased the accumulation of rhodamine 123 and daunorubicin in LLC-GA5-COL150 cells in a concentration-dependent manner. In addition, 5,7-dimethoxyflavone to lesser degree increased rhodamine 123 accumulation in LLC-GA5-COL150 cells. In contrast, the other four flavone derivatives had no significant effect on the accumulation of rhodamine 123 in LLC-GA5-COL150 cells in a concentration range tested. These results indicate that extracts and flavone derivatives from the rhizome of Kaempferia parviflora can inhibit P-gp function, which may be useful for overcoming P-gp-mediated multidrug resistance and improving the oral bioavailability of anticancer agents.

Entry of aminoglycosides into renal tubular epithelial cells via endocytosis-dependent and endocytosis-independent pathways

Aminoglycoside antibiotics such as gentamicin and amikacin are well recognized as a clinically important antibiotic class because of their reliable efficacy and low cost. However, the clinical use of aminoglycosides is limited by their nephrotoxicity and ototoxicity. Nephrotoxicity is induced mainly due to high accumulation of the antibiotics in renal proximal tubular cells. Therefore, a lot of studies on characterization of the renal transport system for aminoglycosides so far reported involved various in-vivo and in-vitro techniques. Early studies revealed that aminoglycosides are taken up through adsorptive endocytosis in renal epithelial cells. Subsequently, it was found that megalin, a multiligand endocytic receptor abundantly expressed on the apical side of renal proximal tubular cells, can bind aminoglycosides and that megalin-mediated endocytosis plays a crucial role in renal accumulation of aminoglycosides. Therefore, megalin has been suggested to be a promising molecular target for the prevention of aminoglycoside-induced nephrotoxicity. On the other hand, recently, some reports have indicated that aminoglycosides are transported via a pathway that does not require endocytosis, such as non-selective cation channel-mediated entry, in cultured renal tubular cells as well as cochlear outer hair cells. In this commentary article, we review the cellular transport of aminoglycosides in renal epithelial cells, focusing on endocytosis-dependent and -independent pathways.

Targeted prevention of renal accumulation and toxicity of gentamicin by aminoglycoside binding receptor antagonists

Receptor-mediated endocytosis plays an important role in accumulation of aminoglycosides in renal proximal tubule. To prevent aminoglycoside-induced nephrotoxicity following concentrated accumulation of gentamicin in the kidney, effect of cationic proteins and their peptide fragments, which could inhibit gentamicin binding to its binding receptor(s), was investigated. Among several substrates for megalin, an endocytic receptor responsible for renal accumulation of aminoglycosides, cytochrome c potently inhibited gentamicin accumulation in renal cortex. Concentration-dependent inhibition by cytochrome c on gentamicin uptake was also observed in OK kidney epithelial cells expressing megalin. In addition, gentamicin-induced increase in urinary excretion of N-acetyl-beta-d-glucosaminidase (NAG), a marker of renal tubular damage, was significantly reduced by cytochrome c. We next attempted to find a peptide fragment with lower molecular size showing inhibitory effect on gentamicin uptake. Cyto79-88 inhibited gentamicin uptake in OK cells, but had little effect on renal accumulation of gentamicin in mice in vivo. On one hand, a peptide fragment of neural Wiskott-Aldrich syndrome protein (N-WASP), which interacts with acidic phospholipids like aminoglycosides, inhibited gentamicin accumulation not only in OK cells but also in mouse kidney. These results show that substrates and/or their peptide fragments for aminoglycoside binding receptor such as megalin might be useful for preventing aminoglycoside-induced nephrotoxicity.

Endocytic uptake of FITC-albumin by human alveolar epithelial cell line A549

The uptake mechanism of FITC-labeled albumin (FITC-albumin) was examined in human alveolar epithelial cell line A549. FITC-albumin uptake by A549 cells was time- and temperature-dependent, and was markedly suppressed at 4°C compared with that at 37°C. The uptake was saturable, and was mediated by a high-affinity, low-capacity system and by a low-affinity, high-capacity system. In the following experiments, we focused on the low-affinity system. FITC-albumin uptake was markedly inhibited by metabolic inhibitors and by a vacuolar H⁺-ATPase, bafilomycin A₁. The uptake was inhibited by clathrin-mediated endocytosis inhibitors (phenylarsine oxide and chlorpromazine). Potassium depletion and hypertonicity that inhibit clathrin-mediated endocytosis also decreased FITC-albumin uptake. On the other hand, caveolae-mediated endocytosis inhibitors (indomethacin and nystatin) did not affect FITC-albumin uptake. In addition, FITC-albumin uptake was inhibited by macropinocytosis inhibitors such as 5-(N-ethyl-N-isopropyl) amiloride. These results suggest that the low-affinity system of FITC-albumin uptake is mediated by endocytosis in A549 cells, predominantly via a clathrin-mediated pathway. Macropinocytosis, but not caveolae-mediated endocytosis, may also be involved. Considering our previous findings, albumin may be transported by a similar mechanism and/or pathway in rat and human alveolar epithelial cells.

Comparison of Albumin Uptake in Rat Alveolar Type II and Type I-like Epithelial Cells in Primary Culture

PURPOSE

To elucidate and compare the activity and mechanism of albumin uptake in primary cultured alveolar type II and type I-like epithelial cells.

MATERIALS AND METHODS

Type II epithelial cells isolated from rat lungs were cultured for 2 days at 5 x 10(6) cells/35-mm dish or for 6 days at 2 x 10(6) cells/35-mm dish. The mRNA expression of marker genes and FITC-albumin uptake were examined.

RESULTS

The cells cultured for 2 days exhibited cuboidal type II epithelial morphology with lamellar bodies inside the cells, while the cells cultured for 6 days exhibited squamous type I epithelial morphology. These morphological characteristics were consistent with the changes in mRNA expression pattern of marker genes. FITC-albumin uptake in both cells was temperature-dependent and was inhibited by metabolic inhibitors and bafilomycin A1. The rate of uptake was much higher in type II cells than type I-like cells. In both cells, FITC-albumin uptake was inhibited by clathrin mediated-endocytosis inhibitors, but not by caveolae mediated-endocytosis inhibitors.

CONCLUSIONS

These findings indicate that albumin in alveolar lining fluid is internalized into type II and type I epithelial cells via clathrin-mediated endocytosis, and the rate of albumin uptake is higher in type II cells than type I cells.

Nanoparticulation of poorly water soluble drugs using a wet-mill process and physicochemical properties of the nanopowders

In order to improve the dissolution and oral absorption properties of poorly water soluble drugs such as omeprazole, albendazole and danazol, various dispersing agents were added to prepare nanopowder formulations using an ULTRA APEX MILL, which is a wet-mill instrument, and their physicochemical properties were evaluated. Using Pluronic F-108 or F-68 as dispersing agents, slurries containing drug particles having nanometer size were obtained for all model drugs tested. Omeprazole, a heat labile drug, was not degraded by wet-milling and the omeprazole nanoparticles in a milled slurry did not aggregate for 24 h after wet-milling. After lyophilization of these milled slurries containing drug nanoparticles, fine solid white nanopowders were obtained. Scanning electron microscopy (SEM) suggested that the model drugs were milled into nanometer size. X-ray powder diffraction (XRPD) patterns and Differential Scanning Calorimetry (DSC) curves confirmed that all milled drug nanopowders were crystalline, although milling of albendazole nanopowder transformed it to another crystal form. Wet-milling using an ULTRA APEX MILL offers a highly effective approach to produce stable drug nanopowders and is a very useful tool for bioavailability enhancement of poorly water soluble and heat labile drugs.

Purification and cloning of a thermostable manganese catalase from a thermophilic bacterium

We have purified a heat-stable catalase from a thermophilic bacterium, Thermus species strain YS 8-13. The enzyme was purified 160-fold from crude cellular extracts and possessed a specific activity of 8000 units/mg at 65 degrees C. The purified enzyme displayed the highest activity at pH 7 to 10 and temperatures around 85 degrees C. The catalase was determined to be a manganese catalase, based on results from atomic absorption spectra and inhibition experiments using sodium azide. The enzyme was composed of six identical subunits of molecular weight 36,000. Amino acid sequences determined from the purified protein were used to design oligonucleotide primers, which were in turn used to clone the coding gene. The nucleotide sequence of a 1.4-kb fragment of Thermus sp. YS 8-13 genomic DNA containing a 909-bp open reading frame was determined. The gene encoded a 302-residue polypeptide of deduced molecular weight 33,303. The deduced amino acid sequence displayed a region-specific homology with the sequences of the manganese catalase from a mesophilic organism, Lactobacillus plantarum.

Single molecular observation of the interaction of GroEL with substrate proteins

To understand the mechanism of GroEL-assisted protein folding, we observed the interaction of fluorescence-labeled GroEL with fluorescence-labeled substrate proteins at the single molecule level by total internal reflection fluorescence microscopy. GroEL with a A133C mutation in the equatorial domain was labeled with a fluorescent dye, tetramethylrhodamine. As substrate proteins, we used the largely denatured and partly denatured forms of bovine beta-lactoglobulin, both labeled with another fluorescent dye, Cy5. The complexes formed by GroEL with these substrates were characterized by size-exclusion gel chromatography. The recovered complexes were then observed by fluorescence microscopy. For both substrates, agreement of the fluorescent spots for tetramethylrhodamine and Cy5 indicated formation of the complex at the single molecule level. Similar observation of macroscopic binding by size-exclusion chromatography and microscopic binding by the fluorescence microscopy was done for the folding intermediate of Cy5-labeled bovine rhodanese. The fluorescence microscopy opens a new avenue for studying the interaction of GroEL with substrate proteins.

Functional communications between the apical and equatorial domains of GroEL through the intermediate domain

The Escherichia coli GroEL subunit consists of three domains with distinct functional roles. To understand the role of each of the three domains, the effects of mutating a single residue in each domain (Y203C at the apical, T89W at the equatorial, and C138W at the intermediate domain) were studied in detail, using three different enzymes (enolase, lactate dehydrogenase, and rhodanese) as refolding substrates. By analyzing the effects of each mutation, a transfer of signals was detected between the apical domain and the equatorial domain. A signal initiated by the equatorial domain triggers the release of polypeptide from the apical domain. This trigger was independent of nucleotide hydrolysis, as demonstrated using an ATPase-deficient mutant, and, also, the conditions for successful release of polypeptide could be modified by a mutation in the apical domain, suggesting that the polypeptide release mechanism of GroEL is governed by chaperonin-target affinities. Interestingly, a reciprocal signal from the apical domain was suggested to occur, which triggered nucleotide hydrolysis in the equatorial domain. This signal was disrupted by a mutation in the intermediate domain to create a novel ternary complex in which GroES and refolding protein are simultaneously bound in a stable ternary complex devoid of ATPase activity. These results point to a multitude of signals which govern the overall chaperonin mechanism.

Unfolding and refolding of Escherichia coli chaperonin GroES is expressed by a three-state model

The guanidine-hydrochloride (Gdn-HCl) induced unfolding and refolding characteristics of the co-chaperonin GroES from Escherichia coli, a homoheptamer of subunit molecular mass 10,000 Da, were studied by using intrinsic fluorescence, 1-anilino-8-naphthalene sulfonate (ANS) binding, and size-exclusion HPLC. When monitored by tyrosine fluorescence, the unfolding reaction of GroES consisted of a single transition, with a transition midpoint at around 1.0 M Gdn-HCl. Interestingly, however, ANS binding and size-exclusion HPLC experiments strongly suggested the existence of an intermediate state in the transition. In order to confirm the existence of an intermediate state between the native heptameric and unfolded monomeric states, a tryptophan residue was introduced into the interface of GroES subunits as a fluorescent probe. The unfolding reaction of GroES I48W as monitored by tryptophyl fluorescence showed a single transition curve with a transition midpoint at 0.5 M Gdn-HCl. This unfolding transition curve as well as the refolding kinetics were dependent on the concentration of GroES protein. CD spectrum and size-exclusion HPLC experiments demonstrated that the intermediates assumed a partially folded conformation at around 0.5 M Gdn-HCl. The refolding of GroES protein from 3 M Gdn-HCl was probed functionally by measuring the extent of inhibition of GroEL ATPase activity and the enhancement of lactate dehydrogenase refolding yields in the presence of GroEL and ADP. These results clearly demonstrated that the GroES heptamer first dissociated to monomers and then unfolded completely upon increasing the concentration of Gdn-HCl, and that both transitions were reversible. From the thermodynamic analysis of the dissociation reaction, it was found that the partially folded monomer was only marginally stable and that the stability of GroES protein is governed mostly by the association of the subunits.

Cisplatin resistance in human lung cancer cells is linked with dysregulation of cell cycle associated proteins

AIMS

Cisplatin (CDDP) is a platinum-based drug that is widely used in cancer chemotherapy, but the development of resistance in tumor cells is a major weakness of these treatments. Several mechanisms have been proposed to explain cisplatin resistance, and disruption of certain cellular pathways could modulate drug sensitivity to cisplatin. A lower level of cross-resistance to cisplatin leads to better outcomes in clinical use.

MAIN METHODS

Cross-resistance was assessed using cisplatin resistant lung cancer cell line A549/CDDP. Cell cycle analysis was used to examine the effect of cisplatin on cell signaling pathways regulating G2/M transition in cisplatin resistant cells.

KEY FINDINGS

A549/CDDP cells exhibited cross-resistance to carboplatin, but not oxaliplatin, which is often found in platinum analogues. Flow cytometry showed that nocodazole treatment caused a G2/M block in both A549/CDDP cells and cisplatin susceptible cells. However, A549/CDDP cells escaped the G2/M block following exposure to cisplatin. Activation of the Cdc2/CyclinB complex is required for transition from G2 to M phase, and the inactive form of phosphorylated Cdc2 is activated by Cdc25C dephosphorylation of Tyr15. In the cisplatin-treated susceptible cells, the levels of phosphorylated Cdc2 and Cdc25C were markedly decreased, leading to a loss of Cdc2 activity and G2/M arrest. In A549/CDDP cells, however, Cdc2 activity was supported by the expression of Cdc2 and Cdc25C after the addition of cisplatin, which resulted in G2/M progression.

SIGNIFICANCE

The resistance phenotype of G2/M progression has been correlated with dysregulation of Cdc2 in a human lung cancer cell line selected for cisplatin.

Cisplatin-induced inhibition of receptor-mediated endocytosis of protein in the kidney

BACKGROUND

Administration of cisplatin, cis-diamminedichloroplatinum (II) (CDDP), causes a severe impairment of renal function, including increases in urinary excretion of proteins. We recently found that CDDP inhibits vacuolar H+-ATPase, which plays an important role in receptor-mediated endocytosis in the renal proximal tubules. Therefore, CDDP-induced proteinuria may be due to an inhibition of the receptor-mediated endocytosis in the renal proximal tubules following a decrease in vacuolar H+-ATPase activity by the drug.

METHODS

Effects of CDDP on receptor-mediated endocytosis of albumin in opossum kidney (OK) epithelial cells, and on urinary excretion of albumin and vitamin D binding protein, which are reabsorbed in the renal proximal tubules by endocytosis, in rats were examined.

RESULTS

CDDP inhibited uptake of fluorescein-isothiocyanate (FITC)-albumin, a receptor-mediated endocytosis marker, by OK cells in a time- and concentration-dependent fashion. In contrast, CDDP treatment did not affect the uptake of FITC-inulin, a fluid-phase endocytosis marker. CDDP caused a decrease in the affinity and in the maximal velocity of FITC-albumin uptake. The adenosine 5'-triphosphate (ATP) content in OK cells was not changed by CDDP at concentrations that inhibited FITC-albumin uptake. The endosomal pH in OK cells was increased by CDDP treatment. Administration of CDDP to rats increased the urinary excretion of albumin and vitamin D binding protein.

CONCLUSIONS

These results suggest that CDDP decreases the receptor-mediated endocytosis of protein following the inhibition of vacuolar H+-ATPase in the renal proximal tubules, and the inhibition of receptor-mediated endocytosis would be the mechanisms underlying the proteinuria induced by CDDP.

Interaction of valproic acid and carbapenem antibiotics with multidrug resistance-associated proteins in rat erythrocyte membranes

We recently reported that the decrease in plasma valproic acid (VPA) level by carbapenem antibiotics (CPs) may partly be due to the increased erythrocyte distribution of VPA. In order to clarify the mechanisms underlying altered VPA distribution in erythrocytes, we examined the role of multidrug resistance-associated proteins (Mrps). The uptake of 2,4-dinitrophenyl-S-glutathione (DNP-SG), a substrate of Mrps, by inside-out vesicles (IOVs) prepared from rat erythrocytes was an ATP-dependent, active process. DNP-SG uptake was mediated by high- and low-affinity transport systems, and was inhibited by various Mrp inhibitors such as probenecid and indomethacin. Glutathione stimulated only the high-affinity transport system. VPA inhibited the low-affinity transport of DNP-SG, while panipenem, a CP, inhibited both high- and low-affinity transport. ATP-dependent, Mrp-mediated transport of methotrexate, another Mrp substrate, in IOVs was also observed, and VPA and various CPs inhibited the transport. The uptake of [(3)H]VPA was examined, and found to be ATP-dependent. ATP-dependent uptake of [(3)H]VPA was inhibited by Mrp inhibitors and panipenem, while the inhibition was not observed in the absence of ATP. These results indicate that VPA and CPs interact with Mrp-mediated transport in erythrocyte membranes, and VPA itself is transported by Mrps, which is inhibited by panipenem. Thus, the increased erythrocyte distribution of VPA by CPs observed under in vivo conditions may partly be explained by their interaction with Mrps in erythrocyte membranes.