Different effects of cyclosporine a and FK506 on potassium transport systems in MDCK cells

BACKGROUND

Hyperkalemia and metabolic acidosis are common manifestations in patients receiving the immunosuppressive agent cyclosporine A (CsA) and the recently introduced FK506. We compared the acute toxic and antiproliferative effects as well as the effects on the transport activity of Na(+)/K(+)-ATPase and Na(+)/K(+)/2Cl(-) cotransporter of CsA and FK506 in an established cell line of distal/collecting tubule origin (MDCK cells).

METHODS

MDCK cells were exposed to various concentrations of CsA or FK506 and the effects on cell viability (MTT test and neutral red uptake), plasma membrane integrity (lactate dehydrogenase (LDH) release) and cell proliferation (bromodeoxyuridine (BrdU) incorporation) were compared. For transport studies, after confluence, MDCK cells were exposed to CsA or FK506 for 48 h in the presence and absence of aldosterone. Ouabain- and bumetanide-sensitive (86)Rubidium uptake measurements were used to study the activity of the Na(+)/K(+)-ATPase and Na(+)/K(+)/2Cl(-) cotransporter at the surface of intact cells.

RESULTS

After 24 h of exposure CsA reduced the number of viable cells to 50% at 30 microM, whereas for FK506 2-3 times higher concentrations had to be employed. Similarly, LDH release was stimulated tenfold by 30 microM CsA but only fourfold by 70 microM FK506. In contrast, DNA synthesis was affected at lower concentrations of FK506 than of CsA. In cells treated for 24 h BrdU incorporation was significantly inhibited by 3 microM FK506, whereas a similar inhibition required 10 microM CsA. The transport activity of Na(+)/K(+)-ATPase and of Na(+)/K(+)/2Cl(-) cotransporter were significantly decreased (37 and 63%, respectively) on CsA administration (8 microM). In CsA-treated cells the K(+) channel blockers barium (1 mM), TEA (10 mM) and quinine (1 mM) did not further inhibit the transport activities suggesting that CsA might also act via inhibition of K(+) channels. FK506 at 8 microM had no effect on Na(+)/K(+)-ATPase transport activity but stimulated Na(+)/K(+)/2Cl(-) cotransporter activity by 59%. The stimulatory effect was abolished by K(+) channel blockers indicating that recycling of K(+) might increase by FK506. The simultaneous presence of aldosterone (5 microM) protected the cells from the inhibitory effect of CsA on Na(+)/K(+)-ATPase and Na(+)/K(+)/2Cl(-) cotransporter activity. The stimulatory effect of FK506 on the Na(+)/K(+)/2Cl(-)cotransporter activity was completely abolished in the presence of aldosterone.

CONCLUSIONS

Both CsA and FK506 showed acute toxicity in MDCK cells in vitro with the effects of FK506 being less pronounced. CsA and FK506 had different effects on the in vivo transport rates of the Na(+)/K(+)-ATPase and the Na(+)/K(+)/2Cl(-) cotransporter; CsA inhibited the activity of the Na(+)/K(+)-ATPase and the Na(+)/K(+)/2Cl(-) cotransporter whereas FK506 stimulated the activity of Na(+)/K(+)/2Cl(-) cotransporter. These effects were abolished by the application of aldosterone.

Identification of a region critically involved in the interaction of phlorizin with the rabbit sodium-D-glucose cotransporter SGLT1

In order to define potential interaction sites of SGLT1 with the transport inhibitor phlorizin, mutagenesis studies were performed in a hydrophobic region of loop 13 (aa 604-610), located extracellularly, close to the C-terminus. COS 7 cells were transiently transfected with the mutants and the kinetic parameters of alpha-methyl-D-glucopyranoside (AMG) uptake into the cells were investigated. Replacement of the respective amino acids with lysine reduced the maximal uptake rate: Y604K showed 2.2%, L606K 48.4%, F607K 15.1%, C608K 13.1%, G609K 14.1%, and L610K 17.2% of control. In all mutants the apparent K(i) for phlorizin increased at least by a factor of 5 compared to the wild-type K(i) of 4.6 +/- 0.7 micromol/l; most striking changes were observed for Y604K (K(i) = 75.3 +/- 19.0 micromol/l) and C608K (K(i) = 83.6 +/- 13.9 micromol/l). Replacement of these amino acids with a nonpolar amino acid instead of lysine such as in Y604F, Y604G and C608A showed markedly higher affinities for phlorizin. In cells expressing the mutants the apparent affinity of AMG uptake for the sugar was not statistically different from that of the wild type (Km = 0.8 +/- 0.2 mmol/l). These studies suggest that the region between amino acids 604 and 610 is involved in the interaction between SGLT1 and phlorizin, probably by providing a hydrophobic pocket for one of the aromatic rings of the aglucone moiety of the glycoside.

Investigations of the in vitro transport of human milk oligosaccharides by a Caco-2 monolayer using a novel high performance liquid chromatography-mass spectrometry technique

Complex lactose-derived oligosaccharides belong to the main components of human milk and are believed to exert multiple functions in the breast-fed infant. Therefore, we investigated the transepithelial transport of human milk oligosaccharides over Caco-2 monolayers. Main human milk oligosaccharides (HMOs) in the apical, basolateral, or intracellular compartment were separated by high performance liquid chromatography using a Hypercarb(TM) column and analyzed on line by mass spectrometry. This method allowed the identification and quantification of these components in intra- and extracellular fractions without prior purification. Using this technique we were able to show that acidic and neutral HMOs cross the epithelial barrier. The transepithelial flux of neutral, but not acidic, oligosaccharides was temperature-sensitive and partly inhibited by brefeldin A and bafilomycin A. Furthermore, net flux from the apical to the basolateral compartment was only observed for the neutral components. Similarly, apical cellular uptake was only found for neutral components but not for acidic oligosaccharides. Intracellular concentrations of neutral HMOs were significantly increased by inhibitors of transcytosis such as brefeldin A, N-ethylmaleimide, or bafilomycin A. The cellular uptake was saturable, and an apparent K(m) for lacto-N-fucopentaose I of 1.7 +/- 0.1 mmol/liter and for lacto-N-tetraose of 1.8 +/- 0.4 mmol/liter was determined. Furthermore, the uptake of lacto-N-fucopentaose I could be inhibited by the addition of the stereoisomer lacto-N-fucopentaose II but not by lacto-N-tetraose. These findings suggest that neutral HMOs are transported across the intestinal epithelium by receptor-mediated transcytosis as well as via paracellular pathways, whereas translocation of acidic HMOs solely represents paracellular flux.

Apoptosis induced in HepG2 cells by short exposure to millimolar concentrations of ethanol involves the Fas-receptor pathway

PURPOSE

We previously found that ethanol-induced apoptosis is associated with an activation of caspase-3. However, the initial triggering of this process is yet unknown. Therefore, the present study was designed to determine whether the Fas-receptor pathway plays a role in the initiation by ethanol of human hepatocellular carcinoma (HepG2) cell apoptosis.

METHODS

HepG2 cells were incubated with or without 1 mM ethanol for 24 h. Apoptosis was assessed by DNA fragmentation and caspase-8 activity. Selective inhibitors of caspase-8 and caspase-9 were used to analyze the role of both caspases on apoptosis. Soluble human Fas ligand (Fas-L) was determined by enzyme-linked immunosorbent assay (ELISA). A fluorescent dye was used to investigate the permeability of the mitochondrial outer membrane. A recombinant Fas fusion protein was used to inhibit the activation of Fas receptors. Human anti-Fas-L antibody was employed to neutralize Fas-L released from the cells.

RESULTS

Caspase-8 activity increased significantly threefold (P < 0.005) after 12 h incubation of HepG2 cells with 1 mM ethanol whereas no change was observed in control cells. Incubation with caspase-8 inhibitor completely prevented apoptosis induced by ethanol (P < 0.001). In contrast, a caspase-9 inhibitor did not significantly reduce apoptosis. The permeability of the outer mitochondrial membrane was not altered. Neutralization of Fas-receptors by Fas fusion proteins completely attenuated ethanol-induced apoptosis in HepG2 cells treated with ethanol.

CONCLUSIONS

These findings show that apoptosis induced by low concentrations of ethanol in human HepG2 cells is associated with Fas-receptor activation and subsequent caspase-8 activation. Triggering of apoptosis through Fas-receptors represents a mechanism of action different from that observed with high concentrations of ethanol.

Non-invasive single cell pH measurements in the isolated perfused pancreas

1. A new method was developed for non-invasive investigations of intracellular pH (pHi) regulation in different cell types of the isolated perfused pancreas using a confocal laser scanning technique. 2. After removal of the rat pancreas the coeliac artery was cannulated and the splenic segment of the pancreas was perfused with dextran (5%)-Ringer solution at a constant flow rate of 2 mL/min. In a temperature-controlled (37 degrees C) chamber, pH regulation was studied using the pH-sensitive fluorescent dye 2',7'-bis-(2-carboxyethyl)-5-(-6)-carboxyfluorescein (BCECF) with a confocal microscope (MRC-600; Bio-Rad, Hercules, CA, USA). 3. Image analysis permitted the identification and comparison of different cell types with a pHi of 7.26+/-0.1 in acinar cells and of 7.02+/-0.1 in endothelial cells. Increasing PCO2 from 5 to 20% resulted in a rapid decrease in pHi. Omission of sodium from the perfusate resulted in a smooth decline in pHi. Both decreases were found to be fully reversible. Increasing PCO2 under sodium-free conditions also resulted in a drop of pHi that was, however, not fully reversible, suggesting involvement of the Na+/H+ exchanger in the regulation of pHi in the intact organ. 4. The above method completely preserves tissue integrity and, therefore, allows the study of pH regulation in different cell types of the pancreas simultaneously and without interference with their functional arrangement. The technique should be of specific value to investigate experimental disease states of the pancreas.

Calcium signalling during RVD of kidney cells

The balance of a high extracellular osmolarity in the kidney medulla is mainly based on an accumulation of organic osmolytes in the cells. The regulation of cell volume during hypotonic conditions results in a release of organic osmolytes - a process that is partly calcium-dependent. Using calcium-sensitive fluorescent dye and confocal laser scanning microscopy, we have investigated calcium signalling during regulatory volume decrease (RVD) in kidney cells. In rat inner medullary collecting duct (IMCD) cells in primary culture, hypotonic stress induced a calcium release from intracellular stores that preceded calcium entry from the extracellular milieu. Hyposmotic stress had no effect on the cellular IP(3) content. Preincubation with 100 micromol/l ETYA (a non-metabolizible derivative of arachidonic acid), however, reduced the calcium response to hypotonic stress as well as the RVD. Blocker of voltage-dependent calcium channels (verapamil, diltiazem, and nifedipine) in the concentration of 40 micromol/l reduced partly the calcium response. SKF-96365, an inhibitor of receptor-mediatedcalcium channels, also attenuated the calcium influx. In conclusion, swelling of IMCD cells increases intracellular calcium by release from intracellular stores and entry across the cell membranes. The signalling involves arachidonic acid metabolism.

Regulation of sorbitol efflux in different renal medullary cells: similarities and diversities

It is well accepted that organic osmolytes, including sorbitol, play a major role in the volume regulation of renal medullary cells. The signal leading to an activation of release channels during RVD is, however, poorly understood. Hypotonicity induced sorbitol efflux was investigated in freshly isolated rat inner medullary collecting duct (IMCD) cells and in rabbit medullary thick ascending limb of Henle's loop (TALH) cells biochemically or using labeled sorbitol. The time course of release was compared with changes in cell volume, measured by confocal microscopy, and alterations in cell calcium (Ca(i)) determined by Fura 2 technology. In IMCD cells sorbitol release, volume decrease and Ca(i) transients show a close temporal correlation. In addition increases in Ca(i) without volume changes stimulate sorbitol efflux. In TALH cells sorbitol release starts after a significant lag time and reaches a maximum when cell volume is already partially restored. The same discrepancy is observed with regard to changes in Ca(i) and sorbitol efflux. These studies suggest that in IMCD cells changes in Ca(i) are the main regulator for the sorbitol permeability of the plasma membrane. The sorbitol channel present in TALH cells seems to operate predominantly independently of Ca(i). Despite this diversity in signal transduction the sorbitol channels in both renal cell types appear, however, not to be stretch-activated.

Inhibition by mercuric chloride of Na-K-2Cl cotransport activity in rectal gland plasma membrane vesicles isolated from Squalus acanthias

The rectal gland of the dogfish shark is a model system for active transepithelial transport of chloride. It has been shown previously that mercuric chloride, one of the toxic environmental pollutants, inhibits chloride secretion in this organ. In order to investigate the mechanism of action of HgCl(2) at a membrane-molecular level, plasma membrane vesicles were isolated from the rectal gland and the effect of mercury on the activity of the Na-K-2Cl cotransporter was investigated in isotope flux studies. During a 30 s exposure HgCl(2) inhibited cotransport activity in a dose-dependent manner with an apparent K(i) of approx. 50 microM. The inhibition was complete after 15 s, partly reversible by dilution of the incubation medium and completely attenuated upon addition of reduced glutathione. The extent of inhibition by mercury depended on the ionic composition of the medium. The sensitivity of the cotransporter was highest when only the high affinity binding sites for sodium and chloride were saturated. Organic mercurials such as p-chloromercuribenzoic acid and p-chloromercuriphenylsulfonic acid at 100 microM did not inhibit the cotransporter, similarly exposure of the vesicles to 10 mM H(2)O(2) or 1 mM dithiothreitol for 30 min at 15 degrees C did not change cotransport activity. Transport activity was, however, reduced by 45.9+/-2.5% after an incubation with 3 mM N-ethylmaleimide for 20 min. Blocking free amino groups by N-hydroxysuccinimide or biotinamidocapronate-N-hydroxysulfosuccinimide had no effect. Investigations on the sidedness of the plasma membrane vesicles, employing the asymmetry of the (Na+K)-ATPase, demonstrated a right-side-out orientation in which the former extracellular face of the membrane is exposed to the incubation medium. In addition, extracellular mercury (5x10(-5) M) inhibited bumetanide-sensitive rubidium uptake into T84 cells by 48.5+/-7.1% after a 2 min incubation period. This inhibition was reversible in a manner similar to that observed in the plasma membrane vesicles. These studies suggest that in isolated rectal gland plasma membrane vesicles the Na-K-2Cl cotransporter (sNKCC1) exposes functionally relevant mercury binding sites at its external surface. These sites represent probably cysteines, the accessibility and/or sensitivity of which depends on the functional state of the transporter.

Evolution of the Na-P(i) cotransport systems

Membrane transport systems for P(i) transport are key elements in maintaining homeostasis of P(i) in organisms as diverse as bacteria and human. Two Na-P(i) cotransporter families with well-described functional properties in vertebrates, namely NaPi-II and NaPi-III, show conserved structural features with prokaryotic origin. A clear vertical relationship can be established among the mammalian protein family NaPi-III, a homologous system in C. elegans, the yeast system Pho89, and the bacterial P(i) transporter Pit. An alternative lineage connects the mammalian NaPi-II-related transporters with homologous proteins from Caenorhabditis elegans and Vibrio cholerae. The present review focuses on the molecular evolution of the NaPi-II protein family. Preliminary results indicate that the NaPi-II homologue cloned from V. cholerae is indeed a functional P(i) transporter when expressed in Xenopus oocytes. The closely related NaPi-II isoforms NaPi-IIa and NaPi-IIb are responsible for regulated epithelial Na-dependent P(i) transport in all vertebrates. Most species express two different NaPi-II proteins with the exception of the flounder and Xenopus laevis, which rely on only a single isoform. Using an RT-PCR-based approach with degenerate primers, we were able to identify NaPi-II-related mRNAs in a variety of vertebrates from different families. We hypothesize that the original NaPi-IIb-related gene was duplicated early in vertebrate development. The appearance of NaPi-IIa correlates with the development of the mammalian nephron.

Functional asymmetry of the sodium-D-glucose cotransporter expressed in yeast secretory vesicles

The sodium-D-glucose cotransporter (SGLT1) was expressed in a yeast mutant strain NY 17 (sec6-4) that accumulates secretory vesicles at a nonpermissive temperature because of a block in the delivery of these vesicles to the plasma membrane. By differential centrifugation a microsomal fraction enriched in secretory vesicles was prepared with a high specific activity of the vanadate-sensitive H+-ATPase and invertase. In this membrane fraction one protein band of an apparent molecular weight of 55 kDa representing the nonglycosylated SGLT1 protein could be detected by immunochemical analysis. In addition, higher molecular weight protein bands probably representing dimers and aggregates were found. In transport studies with the microsomes D-glucose fluxes showed asymmetric properties: efflux experiments revealed the typical properties of the SGLT1 such as sodium dependence, inhibition by phlorizin and potential dependence. Influx of D-glucose showed no dependence on sodium and was not inhibited by phlorizin. Furthermore, the transporter exhibited a striking asymmetry with regard to the D-glucose affinity and the sugar specificity. These results suggest that the orientation of the SGLT1 expressed in yeast secretory vesicles is, indeed, inverted with regard to its configuration in the plasma membrane of epithelial cells. Moreover, there are striking functional differences between the periplasmic and cytoplasmic face of the transporter.

Cytotoxicity of millimolar concentrations of ethanol on HepG2 human tumor cell line compared to normal rat hepatocytes in vitro

PURPOSE

The antiproliferative effect of high concentrations of ethanol (80-100 mmol) on liver carcinoma is well known. However, the high concentrations of ethanol affect both tumor cells and normal hepatocytes. The present study was designed to determine the effect of low ethanol concentrations (0-10 mmol) on cell proliferation and cell death (apoptosis and necrosis) in a human tumor cell line HepG2 and in normal rat hepatocytes.

METHODS

Primary cultures of normal rat hepatocytes and HepG2 cells cultures were used. Cells were incubated with increasing ethanol concentrations or without ethanol (control group) for 24 h and analyzed immediately (group I) or after an additional incubation time of 48 h without additional ethanol application (group II). Cell proliferation was determined by assessing 5-bromo-2'-deoxyuridine (BrdU) incorporation. Apoptosis was assessed by means of DNA fragmentation and cysteine aspartate-specific protease (caspase-3) activity. Necrosis was analyzed by quantification of lactate dehydrogenase (LDH) release into culture medium.

RESULTS

Twenty-four h exposure to 1 mmol ethanol inhibited cell proliferation in HepG2 cells by 75% (P < 0.05), while it remained unaltered in rat hepatocytes. The effect of ethanol persisted for another 48 h where cell proliferation was 5% of control in HepG2 cells and 70% of control in rat hepatocytes (P < 0.005). After 24 h incubation with 1 mmol ethanol 28% of HepG2 cells and 12% of rat hepatocytes showed DNA fragmentation as sign of apoptosis (P < 0.001). In group II 39% of HepG2 cells and 26% of rat hepatocytes were apoptotic (P < 0.001). Caspase-3 activation progressively increased after ethanol treatment in HepG2 cells and rat hepatocytes. The first significant difference was observed after 4 h (activity in HepG2 was 68% higher than in rat hepatocytes) and was maximum after 10 to 12 h where the activity in HepG2 was 180% of the activity in rat hepatocytes. Lactate dehydrogenase release into culture medium as an indicator of necrosis in HepG2 cells, increased from 0.5% in group I to 12% in group II, and from 0.1% to 8% in rat hepatocytes (P < 0.005). Increasing ethanol concentration to 10 mmol increased necrosis to 75% in HepG2 cells, and to 45% in rat hepatocytes (P < 0.05) whereas the effects on cell proliferation and apoptosis were not significantly different.

CONCLUSIONS

Small ethanol concentrations (equivalent to 1 mmol) inhibit cell proliferation and increase apoptosis more strongly in HepG2 cells than in normal rat hepatocytes. These findings suggest the use of 1 mmol ethanol as a treatment for hepatocellular carcinoma because this mainly affects tumor cells but not surrounding normal tissue.

Cellular uptake of dietary flavonoid quercetin 4'-beta-glucoside by sodium-dependent glucose transporter SGLT1

Although it has been suggested that the intestinal glucose transporter may actively absorb dietary flavonoid glucosides, there is a lack of direct evidence for their transport by this system. In fact, our previous studies with the human Caco-2 cell model of intestinal absorption demonstrated that a major dietary flavonoid, quercetin 4'-beta-glucoside, is effluxed by apically expressed multidrug resistance-associated protein-2, potentially masking evidence for active absorption. The objective of this study was to test the hypothesis that quercetin 4'-beta-glucoside is a substrate for the intestinal sodium-dependent D-glucose cotransporter SGLT1. Cellular uptake of quercetin 4'-beta-glucoside was examined with Caco-2 cells and SGLT1 stably transfected Chinese hamster ovary cells (G6D3 cells). Although quercetin 4'-beta-glucoside is not absorbed across Caco-2 cell monolayers, examination of the cells by indirect fluorescent microscopy as well as by HPLC analysis of cellular content revealed cellular accumulation of this glucoside after apical loading. Consistent with previous observations, the accumulation of quercetin 4'-beta-glucoside in both Caco-2 and G6D3 cells was markedly enhanced in the presence of multidrug resistance-associated protein inhibition. Uptake of quercetin 4'-beta-glucoside was greater in SGLT1-transfected cells than in parental Chinese hamster ovary cells. Uptake of the glucoside by Caco-2 and G6D3 cells was sodium-dependent and was inhibited by the monovalent ionophore nystatin. In both Caco-2 and G6D3 cells, quercetin 4'-beta-glucoside uptake was inhibited by 30 mM glucose and 0.5 mM phloridzin. These results demonstrate for the first time that quercetin 4'-beta-glucoside is transported by SGLT1 across the apical membrane of enterocytes.

Vitalism and synthesis of urea. From Friedrich Wöhler to Hans A. Krebs

In 1828, Friedrich Wöhler, a German physician and chemist by training, published a paper that describes the formation of urea, known since 1773 to be a major component of mammalian urine, by combining cyanic acid and ammonium in vitro. In these experiments the synthesis of an organic compound from two inorganic molecules was achieved for the first time. These results weakened significantly the vitalistic hypothesis on the functioning of living cells, although Wöhler, at that time, was more interested in the chemical consequences of isomerism than in the philosophical implications of his finding. However, the chemical synthesis observed by Wöhler does not represent the reaction which is employed in the mammalian liver for urea synthesis. The mechanism of this process was elucidated by the German physician Hans A. Krebs and his medical student Kurt Henseleit in 1932 and was shown to include the ornithine cycle. This 'urea cycle' is only observed in living cells; this apparently vitalistic phenomenon is caused by the compartmentalization of the various enzymatic reactions in mitochondria and cytosol, respectively.

Short exposure to millimolar concentrations of ethanol induces apoptotic cell death in multicellular HepG2 spheroids

PURPOSE

We have shown previously that 1 mM ethanol reduces cell proliferation and increases apoptosis in monolayers of human hepatocellular carcinoma (HepG2) cells. However, in vivo liver tumors are usually three-dimensional and multicellular. The purpose of this study was therefore to determine the effect of ethanol in multicellular tumor spheroids (MCTS) as a model system in vitro.

METHODS

After the application of 1 mM ethanol for 24 h and 48 h, viable, apoptotic and necrotic cells within MCTS were stained with specific fluorescent dyes, and their amount and distribution within the MCTS were assessed by confocal laser scanning microscopy. To evaluate the effect on HepG2 cell migration and cell proliferation, the outgrowth potential after 1 week in culture was evaluated.

RESULTS

As assessed by YO-PRO-1 staining, ethanol increased the number of apoptotic cells from 21.5 units (U) in control spheroids to 364 U and 482.2 U after 24 h and 48 h in ethanol-treated spheroids, respectively (P < 0.001). Merocyanine staining fluorescence increased from 10.7 U in the control to 122 U after 24 h and 293.2 U after 48 h (P < 0.001). Cell viability, as determined by staining with the acetoxymethyl ester of calcein, decreased from 578.5 U in the control to 236 U and 73.4 U after 24 h and 48 h of ethanol exposure respectively (P < 0.001). Necrosis showed an increase from 2 U in control to 24.9 after 24 h and 54 U after 48 h. MCTS treated with ethanol showed almost complete inhibition of outgrowth potential after 1 week in culture, compared to controls (P < 0.005).

CONCLUSIONS

Small concentrations of ethanol (1 mM) induced apoptosis in HepG2 MCTS with a concomitant inhibition on outgrowth potential, accompanied with a low degree of necrosis. These findings suggest that low concentrations of ethanol may already be sufficient for the treatment of hepatocellular carcinoma.

Probing the conformation of the sugar transport inhibitor phlorizin by 2D-NMR, molecular dynamics studies, and pharmacophore analysis

Sodium/D-glucose cotransport, one of the prototypes for sodium gradient-driven symport systems in kidney and intestine, is known to be inhibited by aromatic and aliphatic glucosides (Diedrich, D. F. Biochim. Biophys. Acta 1963, 71, 688-700; Diedrich, D. F. Arch. Biochem. Biophys. 1966, 117, 248-256; Kipp, H.; et al. Biochim. Biophys. Acta 1996, 1282, 124-130; Ramaswamy, K.; et al. Biochim. Biophys. Acta 1976, 433, 32-38). The conformation in which the most potent inhibitor, phlorizin, interacts with the transport protein was investigated with different approaches. Phlorizin consists of the glucose moiety and two aromatic rings (A and B) joined by an alkyl spacer. First the interaction of these various parts of the molecule was determined by two-dimensional (2D) solution NMR. From the 2D-NOESY (nuclear Overhauser effect) measurements spatial distances (up to 5 A) between various interacting H atoms could be detected. Using these values as distance constraints, conformations of phlorizin were calculated and analyzed by the valence force-field method. As a result, a set of conformations could be obtained. The most probable phlorizin conformation shows a nearly perpendicular arrangement of the two aromatic rings (A and B) with the ring B situated above the sugar ring. A very similar conformation could be found by using molecular dynamics simulations when water was chosen as the solvent. This phlorizin conformation in aqueous solution then served as a template for conformational analysis of various phlorizin derivatives. The resulting conformations of derivatives were taken as input to establish a pharmacophore model using the DISCO calculation. As a result, the essential elements of phlorizin for interaction with its binding pocket could be deduced: namely hydrogen bonding via hydroxyl groups of the pyranoside at C(2), C(3), C(4), and C(6) and at C(4) and C(6) of aromatic ring A and hydrophobic interactions via the pyranoside ring and aromatic ring A. Finally, from these conformational features of the pharmacophore the dimension of the phlorizin binding site on the sodium/D-glucose cotransporter was estimated to be 17 x 10 x 7 A(3).

Effect of benzodiazepines on the epithelial and neuronal high-affinity glutamate transporter EAAC1

EAAC1-mediated glutamate transport concentrates glutamate across plasma membranes of brain neurons and epithelia. In brain, EAAC1 provides a presynaptic uptake mechanism to terminate the excitatory action of released glutamate and to keep its extracellular concentration below toxic levels. Here we report the effect of well known anxiolytic compounds, benzodiazepines, on glutamate transport in EAAC1-stably transfected Chinese hamster ovary (CHO) cells and in EAAC1-expressing Xenopus laevis oocytes. Functional properties of EAAC1 agreed well with already reported characteristics of the neuronal high-affinity glutamate transporter (Km D-Asp,CHO cells: 2.23+/-0.15 microM; Km D-Asp,oocytes: 17.01+/-3.42 microM). In both expression systems, low drug concentrations (10-100 microM) activated substrate uptake (up to 200% of control), whereas concentrations in the millimolar range inhibited (up to 50%). Furthermore, the activation was more pronounced at low substrate concentrations (1 microM), and the inhibition was attenuated. The activity of other sodium cotransporters such as the sodium/D-glucose cotransporter SGLT1, stably transfected in CHO cells, was not affected by benzodiazepines. In electrophysiological studies, these drugs also failed to change the membrane potential of EAAC1-expressing Xenopus laevis oocytes. These results suggest a direct action on the glutamate transporter itself without modifying the general driving forces. Thus, in vivo low concentrations of benzodiazepines may reduce synaptic glutamate concentrations by increased uptake, providing an additional mechanism to modulate neuronal excitability.

Probing transmembrane topology of the high-affinity Sodium/Glucose cotransporter (SGLT1) with histidine-tagged mutants

To reexamine the existing predictions about the general membrane topology of the high-affinity Na+/glucose cotransporter (SGLT1) and in particular of the large loop at the C-terminal region, a small 6 x Histidine-tag was introduced at different positions of the SGLT1 sequence by site-directed mutagenesis. Eleven His-SGLT1 mutants were constructed and were transiently transfected into COS-7 cells. As demonstrated by immunofluorescent labeling with antipeptide antibodies against SGLT1, all mutants were expressed and inserted into the plasma membrane. Only mutants with the tag in the N-terminal region and the C-terminal region retained Na+/glucose cotransport activity at 0.1 mM D-glucose. The arrangement of the His-tag in the membrane was analyzed by indirect immunofluorescence, using a monoclonal antihistidine antibody. In nonpermeabilized cells the His-tag could be detected at the N-terminal end (insertion at aa 5) and at the C-terminal end (replacement between aa 584-589 and between aa 622-627), suggesting that these portions of the polypeptide are accessible from the extracellular space. Furthermore, an epitope-specific antibody directed against aa 606-630 reacted strongly with the cell surface. To support this topology intact stably transfected SGLT1 competent CHO cells were partially digested with an immobilized trypsin and subsequently subjected to electrophoresis and Western blot analysis. The size of the digestion product suggests that extravesicular trypsin removed the extracellular loop that contains the amino acid residues 549-664. Thus our results indicate that the last large loop (about aa 541-aa 639) towards the C-terminal end faces the cell exterior where it might be involved in substrate recognition.

Effects of doxorubicin, mitomycin C, and ethanol on Hep-G2 cells in vitro

There are conflicting results for experiments aimed at determining whether anticancer drug therapy of human hepatocellular carcinoma prolongs the survival rate effectively. The purpose of this study was to assess the effect of low concentrations of doxorubicin, mitomycin C, and ethanol on cell replication (cell number and proliferation), and cell apoptosis of cultured human hepatocellular carcinoma (Hep-G2) cells. After 1 day of exposure doxorubicin inhibited cell replication initially by 72%, but a partial recovery of the cell number was observed. Mitomycin C inhibited to the same extent but without recovery. Ethanol reduced the cell number even further, the maximum inhibition (12 days after exposure) being 96.4%. After 3 days of exposure all three agents stopped cell replication at a level of 2%-4% of the control (P < 0.001). Cell apoptosis was activated most strikingly by mitomycin C (5 microg/ml) after 1 day of exposure and by ethanol (150 microl/ml) after 3 days of exposure. Two-way repeated-measures analysis of variance showed statistically significant differences, with ethanol being the most significant followed by mitomycin C doxorubicin, and the control (P < 0.01). Thus, a low dose of ethanol combined with an exposure time of up to 3 days appears to be an effective regimen to control growth of human hepatocellular carcinoma cells in vitro. The strong induction of apoptosis by ethanol might be of additional benefit for a local application in vivo.

High-level expression of Na+/D-glucose cotransporter (SGLT1) in a stably transfected Chinese hamster ovary cell line

The coding region of the high affinity Na+/d-glucose cotransporter (SGLT1) was inserted into the eukaryotic expression vector GFP-N1 under the control of a CMV promoter. The plasmid was then stably transfected into a Chinese hamster ovary cell line (CHO). Transcription and synthesis of SGLT1 were proved by Northern and Western blot analyses. Transport activities of the transfected cells (G6D3) were examined by measuring the sodium-dependent uptake of alpha-methyl[14C]d-glucoside (AMG). Kinetic analysis revealed a Vmax of 10.3 nmol/min/mg (total cell protein) and a Km of 0.26+/-0.09 mM, respectively. The concentration of phlorizin required to inhibit AMG uptake by 50% in the presence of 0.1 mM AMG was 2.35+/-1.84 microM. Electrophysiological studies showed that AMG induces a significant depolarization of membrane voltage in stably transfected CHO cells, suggesting an electrogenic Na-AMG symport. Immunoprecipitation with an antipeptide antibody yielded a nearly homogeneous polypeptide with a molecular mass of about 72 kDa. The amount of SGLT1 present in the CHO cell plasma membranes represents at least 1% of membrane protein, which is about 30-100 times higher than in natural sources, such as renal brush border membranes. In conclusion, the stably transfected G6D3 cells with a markedly high SGLT1 expression can serve as a promising model for studying cellular events related to Na+/d-glucose cotransport and for analyzing the structure and function of the cotransporter itself.

Translated anti-sense product of the Na/phosphate co-transporter (NaPi-II)

The homeostasis of Pi in marine teleosts is maintained by renal Pi secretion as well as by Pi reabsorption. A Na/Pi co-transport system belonging to the NaPi-II protein family is instrumental in tightly controlled renal Pi handling in mammals and fish. We have isolated an NaPi-II related cDNA from winter flounder. It was cloned from a female gonad cDNA library and is 624 bp long. The transcript is expressed in female and male flounder gonads as well as in kidney and intestine, although at very low levels. RNase H digestion experiments revealed an opposite orientation of the transcript with regard to NaPi-II-related mRNA. The anti-sense orientation was confirmed by genomic sequence analysis and Southern blotting. Alluding to the sense transcript, the anti-sense transcript was denoted IPAN. The open reading frame of IPAN encodes a basic protein of 68 amino acid residues. Immunohistochemistry confined the anti-sense related protein, Ipan, to a submembranous compartment of immature oocytes, suggesting a role in oocyte development. In kidney and intestine Ipan is partly co-localized with the Na/Pi co-transporter, implying a regulatory function for the anti-sense protein. However, direct protein-protein interaction could not be established. The existence of a putative open reading frame in other species extends the biological significance of the novel protein.

Properties of a polarized primary culture from rat renal inner medullary collecting duct (IMCD) cells

A primary culture from rat renal IMCD cells was established to investigate the permeability characteristics of the luminal and contraluminal plasma membranes of the papillary collecting duct in vitro. Freshly isolated IMCD cells were grown on filters in a special "epithelial cell" medium. Confluency was proved with an epithelial volt/ohm meter. After 7 d of culture the transepithelial resistance reached more than 1000 omega x cm2. A polarization of the cells with regard to a basolateral localization of a lactate efflux system, and an L-alanine transport system was achieved. The hypotonicity-activated release systems for the organic osmolytes sorbitol and betaine were also located basolaterally, whereas taurine, glycerophosphorylcholine, and myo-inositol left the cells at both cell poles but with different capacity. Morphological observations revealed also that the monolayer was well differentiated. Thus, a model of a renal collecting duct epithelium was established which can be used to analyze polarized and differentiated transport processes across the epithelial cells and their plasma membranes.

In vitro model for the evaluation of inferior vena cava filters: effect of experimental parameters on thrombus-capturing efficacy of the Vena Tech-LGM filter

PURPOSE

To determine the experimental parameters in an in vitro model that influence the thrombus-capturing efficacy of the Vena Tech-LGM filter.

MATERIALS AND METHODS

The Vena Tech-LGM filter was evaluated in an in vitro model of the vena cava with a computer-controlled flow system with a total of 5,200 thrombi. The influences of the following experimental parameters on the capture rate were analyzed with a multiple logistic regression model: type of testing (single, double, and multiple shot testing), thrombus diameter and length, IVC diameter and orientation, flow quality and quantity, flow velocity, and the length of the prepositioned thrombus.

RESULTS

A significant influence on the capture rate could be demonstrated for the type of testing, the thrombus diameter and length, the IVC diameter, and with double shot testing for the length of the prepositioned thrombus and the IVC orientation. The flow quality and the peak velocity were not significant. Based on these results, a protocol for in vitro testing of IVC filters was designed.

CONCLUSIONS

Experimental parameters influence the thrombus-capturing efficacy of the Vena Tech-LGM filter and should be taken into account when in vitro testing is performed.

Regulation of sorbitol content in cultured porcine urinary bladder epithelial cells

Sorbitol content was determined in porcine urinary bladder epithelial cells immediately after death of the animals and after primary culture of the cells at different osmolalities. In both instances, sorbitol content increased with urine and medium osmolality, respectively. For example, at 300 mosmol/kg the cultured cells contained 0.84 +/- 0.02 nmol/mg protein, at 600 mosmol/kg contained 21.7 +/- 0.95 nmol/mg protein, and at 900 mosmol/kg contained 59.5 +/- 2.8 nmol/mg protein. Similarly, aldose reductase activity rose from 0.27 +/- 0.04 mumol.h-1.mg protein-1 at 300 mosmol/kg to 1.81 +/- 0.16 at 600 mosmol/kg and to 3.02 +/- 0.33 at 900 mosmol/kg. These changes were, however, only observed when NaCl but not when urea was used to augment the medium osmolality, since urea equilibrated across the cell membrane. In contrast, sorbitol release from cells cultured at 900 mosmol/kg was slowest into a 900 mosmol/kg medium and fastest into a 300 mosmol/kg medium (63 +/- 16 nmol/10 min compared with 389 +/- 52 nmol/10 min). These studies demonstrate that the sorbitol content of porcine urinary bladder epithelium is regulated by changes both in sorbitol synthesis and sorbitol release. Thus the regulatory mechanisms in the urinary bladder seem to be similar to those present in the embryological related collecting duct.

Na-P(i) cotransport sites in proximal tubule and collecting tubule of winter flounder (Pleuronectes americanus)

Localization of a recently described and cloned Na-Pi cotransport system from flounder was investigated by reverse transcription-polymerase chain reaction (RT-PCR) of microdissected tubules and by immunocytochemistry of kidney of winter flounder. Histological examination showed a small glomerulus, an extremely short proximal tubule PI with a selective affinity to Lens culinaris agglutinin from lentils, and an extensive second proximal tubule segment PII (> 90% of proximal tubules), consisting of cells with numerous apical clear vesicles and extensive amplification of basolateral cell membranes. PII merged with the collecting tubule/ collecting duct (CT/CD) system without a distal segment. By RT-PCR, PII cells revealed high levels of NaPi-II related RNA; low levels were also observed in CTs. Previously characterized antisera against different epitopes of flounder NaPi-II specifically labeled the basolateral regions of PII and the apical cell portion of CT/CD cells and of some PII cells. These results suggest that tubular secretion of P(i) occurs in PII of teleost fish with modulation of urinary P(i) content in the subsequent CT/CD system.

BCECF in single cultured cells: inhomogeneous distribution but homogeneous response

Using confocal laser scanning microscopy with a dual-wavelength laser system, the behaviour of BCECF [(2',7'-bis-2-carboxyethyl)-5-(and-6)carboxyfluorescein] was investigated in a variety of cell lines. Selection of a small area for monitoring allowed discrimination between various intracellular organelles, whose identity was established by vital staining. It was found that, after loading the cells with BCECF, both the nucleus and the mitochondria showed a higher level of fluorescence than the cytoplasm. Calibration of the pH-sensitivity of these fluorescence signals using the nigericin method yielded identical curves, as did exposure of the cells to NH4Cl. These studies suggest that BCECF, despite its inhomogeneous intracellular distribution, reports the pH of only one cellular compartment, the cytosol.

Biochemistry and physiology of carbohydrates in the renal collecting duct

Using 13C-NMR analysis of cell extracts, enzymatic determination of metabolites and cofactors as well as enzyme assays on cell homogenates aerobic and anaerobic glycolysis, sorbitol formation by aldose reductase, the pentose phosphate shunt, and gluconeogenesis could be identified as the major pathways of D-glucose metabolism in renal inner medullary collecting ducts. In flux studies it was shown that D-glucose enters the collecting duct cells via a sodium-independent, cytochalasin- and phloretin-inhibitable transport system located at the basal-lateral cell side. At the same side sorbitol leaves the cells during regulatory volume decrease in a calcium-calmodulin-dependent fashion. From cell isolation studies it is proposed that sorbitol is taken up by adjacent (interstitial) cells, converted into fructose and then recycled to the collecting duct cells. This cycle might prevent carbohydrate wasting. Thus, IMCD cells exhibit unique aspects of carbohydrate biochemistry and physiology which enable them to function in a surrounding of low oxygen tension, low substrate supply, and extreme changes in extracellular osmolality.

Effect of cyclosporine A on Na+/K(+)-ATPase, Na+/K+/2Cl- cotransporter, and H+/K(+)-ATPase in MDCK cells and two subtypes, C7 and C11

We investigated the influence of cyclosporine A (CsA) on key plasma membrane ion transport systems Na+/K(+)-ATPase, Na+/K+/2Cl- cotransporter, and H+/K(+)-ATPase in MDCK cells and two subtypes, C7 and C11, serving as a model system to study principal (C7) and intercalated (C11) cell properties of the distal nephron. The transport activity of Na+/K(+)-ATPase was significantly decreased in all cell types on CsA administration (8 x 10(-6) M) for 2 days, whereas the protein levels of Na+/K(+)-ATPase alpha-subunit in plasma membranes isolated from MDCK, C7, and C11 cells remained unchanged. The transport activity of Na+/K+/2Cl- cotransporter was significantly inhibited by CsA only in MDCK and C11 cells, but again plasma membrane protein levels were not altered. In contrast, C7 cell plasma membranes showed an increase of transport protein content, although the Na+/K+/2Cl- cotransporter activity was not affected by CsA. The H+/K(+)-ATPase transport activity remained unchanged in all three cell types. These data indicate that in C7 cells CsA might induce insertion of transporters into the plasma membrane, thus compensating the decrease of transport activity observed in MDCK and C11 cells. Furthermore, CsA significantly inhibited cell proliferation at 4 x 10(-6) M for C7 and C11 cells and at 8 x 10(-6) M for MDCK cells. Proliferation was completely abolished at 1.6 x 10(-5) M CsA. After 48 h of CsA incubation, the intracellular sodium concentration increased in all three different cell types; however, it stayed within the physiological range of mammalian cells. We, therefore, suggest that CsA is capable of reducing Na+/K(+)-ATPase and Na+/K+/2Cl- cotransporter activities in cells of the distal nephron, thereby contributing to the hyperkalemia observed in patients treated with CsA.

Sorbitol uptake in plasma membrane vesicles isolated from immortalized rabbit TALH cells: activation by a Ca2+/calmodulin-dependent protein kinase

Apical plasma membrane vesicles were isolated from cultures of immortalized thick ascending limb of Henle's loop (TALH) cells and sorbitol uptake was investigated using a rapid filtration technique. In the presence of Mg2+, Ca2+, ATP, and GTP sorbitol equilibrated within three minutes with the intravesicular space; this uptake was reduced by 75% when the incubation temperature was decreased from 37 degrees C to 4 degrees C. A lower level of uptake was also observed in the presence of 100 microM quinidine and when Ca2+ or ATP were omitted from the medium. Membranes preincubated with Mg2+, Ca2+, ATP, and GTP showed, however, a high sorbitol uptake in ATP-free medium. Staurosporine, but only at high concentrations of 200 nm, inhibited sorbitol uptake when present during the transport experiments or during the preincubation with ATP. Similar results were obtained with 1 microM trifluoperazine. Protein kinase C inhibitory peptide was ineffective whereas 20 nm KT 5926, at low concentrations a specific inhibitor of Ca2+/calmodulin-dependent kinase, attenuated the activation. On the basis of these data we suggest that a Ca2+/calmodulin-dependent kinase is a mediator of regulation of sorbitol plasma membrane permeability in renal medullary cells.

Endothelial and epithelial cells: general principles of selective vectorial transport

Endothelial and epithelial cells are both barriers and bridges between different compartments. This contribution discusses the general principles of paracellular, transcellular, and transmembrane transport with special emphasis on the relation between asymmetry and net movement of small solutes. Asymmetry of cell membrane transport properties is found in both epithelial and endothelial cell layers and provides the basis for transcellular transport. Furthermore, the asymmetry of membrane transporters such as the blood-brain barrier GLUT1 and the renal sodium-glutamate cotransporter is discussed with regard to their different properties at the extra- and intracellular face. These molecular asymmetries play an important role in the efficiency, direction, and regulation of transport processes across the plasma membranes in endothelial and epithelial cells.

Hypertonicity-induced alkalinization of rat hepatocytes is not involved in activation of Na+ conductance or Na+,K+-ATPase

We investigated whether cell alkalinization via activation of Na+/H+ exchange is involved in the stimulation of Na+ conductance and Na+,K+-ATPase in rat hepatocytes under hypertonic stress. Osmolarity was increased from 300 to 400 mOsm/l at constant extracellular pH (7.4), whereas osmotically induced cell alkalinization (0.3 pH units in HCO3-free solutions) was mimicked by increasing extracellular pH from 7.4 to 7.8 in normosmotic solutions. In intracellular recordings with conventional and ion-sensitive microelectrodes, hypertonic stress led to a transient shift in the voltage response to low Na+ solutions (95% in exchange for choline) by -4.3 +/- 0.8 mV and a continuous increase in cell Na+ from 13.7 +/- 1.8 to 18.6 +/- 3.0 mmol/l within 8 min. In the presence of 10(-5) mol/l amiloride, these effects were reduced by 80 and 90%, respectively. In contrast, increasing pH did not change the voltage responses to low Na+ or cell Na+ concentrations significantly. In addition, application of 2 mmol/l Ba2+ pulses revealed that a sustained membrane hyperpolarization of 15.6 +/- 1.4 mV following intracellular alkalinization exclusively reflects an increase in K+ conductance. Increasing osmolarity at pH 7.4 augmented ouabain-sensitive 86Rb+ uptake from 5.5 +/- 1.1 to 8.5 +/- 1.6 nmol mg protein(-1) min(-1). In normosmotic solution at pH 7.8, 86Rb+ uptake equalled 4.9 +/- 1.6 nmol mg protein(-1) min(-1), which is not significantly different from control. We conclude that, in rat hepatocytes, cell alkalinization under hypertonic stress is not responsible for the activation of Na+ conductance and probably does not participate in the stimulation of Na+,K+-ATPase.

A physiologic in vitro model of the inferior vena cava with a computer-controlled flow system for testing of inferior vena cava filters

RATIONALE AND OBJECTIVES

The authors develop a physiologic in vitro model of the inferior vena cava (IVC) for testing of filters.

METHODS

The model is driven by a centrifugal pump. A computer-controlled electromagnetic valve is used for generation of different flow patterns. Limitation of the pressure increase in case of IVC occlusion is achieved by a bypass circuit. A glycerin solution is used for perfusion. Artificial clots are made from polyacrylamide gel. Data acquisition includes continuous monitoring of flow and difference pressure over the filter and video recording of the testing events.

RESULTS

The model can generate constant and pulsatile flows. The pressure increase can be limited to 70 mm Hg in case of occlusion. Calculation of the flow velocities in the IVC is possible. A classification of thrombus capturing is presented. The testing of most of the results are reproducible.

CONCLUSIONS

The in vitro model simulates the physiologic conditions in the IVC. It can be used for comparative testing of different filters and the evaluation of new filter designs.

Characteristics of renal Na(+)-D-glucose cotransport in the skate (Raja erinacea) and shark (Squalus acanthias)

We have investigated the properties of the skate (Raja erinacea) and shark (Squalus acanthias) kidney Na(+)-D-glucose cotransporters (SGLT) in uptake studies of radiolabeled substrates into isolated renal brush-border membrane vesicles (BBMV). Scatchard plot analysis of the substrate dependence revealed that the Na(+)-D-glucose cotransporter population is homogenous within each species. Skate BBMV showed a relatively high affinity for D-glucose [Michaelis constant (K(m)) = 0.12 mM] with an apparent coupling ratio of approximately 2 Na+ to 1 D-glucose, whereas the shark transporter was much lower in affinity (K(m) = 1.90 mM) and had a lower coupling ratio, more like 1 Na+ to 1 D-glucose. These characteristics resemble the properties of SGLT1 and SGLT2, which are known to coexist in the mammalian kidney. Inhibitor studies using sugar analogs and glucosides suggested structural differences of the D-glucose binding site among these transporters, whereas the hydrophobic transporter domains in the vicinity of the D-glucose binding site appeared to be similar. In the high-affinity skate system, D-glucose was recognized by hydrogen bonds to the hydroxy groups at C-2, C-3, and C-4 and by hydrophobic interaction with the C-6 methylene group. In contrast, the low-affinity shark system seemed to lack the hydrophobic recognition motif for the C-6 methylene group of D-glucose.

Intracellular pH-measurements in rat duodenal mucosa in vitro using confocal laserscan microscopy

An improved technique was developed to measure intracellular pH-changes in in vitro duodenal mucosa. A confocal laserscan microscope was equipped with a second laser to permit dual wavelength excitation measurements employing BCECF (2'7'-bis-2-carboxyethyl-5-(and-6)carboxyfluorescein), a pH-sensitive fluorescent dye. Intact rat duodenal epithelium was mounted in a microperfusion chamber and loaded with BCECF via submucosal injection. Viability of the epithelial cells could be directly monitored by estimating the nucleus-to-cytoplasm ratio of the fluorescence intensity of BCECF. A calibration procedure using isolated duodenal cells, allowed estimation of the apparent intracellular pH. Initial apparent intracellular pH was 7.32 +/- 0.12, identical to that of isolated duodenocytes. Exposure of the duodenal epithelium to 60 mM NH4Cl led to a steady increase in apparent intracellular pH of 0.46 units within 2 min. Luminal application of 0.01 N HCl led to a steady decrease in apparent intracellular pH of 0.53 pH units within 1 min and was followed by a slow increase to baseline level after acid removal. Thus, confocal laserscan microscopy in combination with BCECF allowed noninvasive monitoring of intracellular pH-changes in single cells of an intact duodenal epithelium.

Synthesis of the photoaffinity label [1'-14C]-6C-(azimethyl)octylglucoside and its reaction with isolated renal brush border membranes

We describe the synthesis of the photolabile analog of n-octylglucoside (OG), 6C-(azimethyl)octylglucoside (diazirino-octylglucoside, DOG). This diazirino derivative (lambda max = 335 nm) can be activated with long-wavelength UV to generate a highly reactive carbene, an intermediate suitable for a covalent ligand-receptor linkage. In inhibitor studies on D-glucose uptake into rabbit renal brush border membrane vesicles (BBMV) DOG showed a competitive inhibition with regard to D-glucose with a Ki of 20 +/- 5 microM, which was not significantly different from that of the template compound OG (Ki = 30 +/- 10 microM). On irradiation (lambda max = 350 nm) of BBMV in the presence of [14C]DOG, proteins of 61, 72 to 78, and 95 kDa were labeled by photoinduction. A band of 73 kDa was also positive in Western blots using an anti-Na+/D-glucose cotransporter antibody. Further investigations revealed that the labeled proteins were presumably mainly glucosidases known to be present in high concentration in renal BBMV. By comparison with authentic material one of the labeled proteins (61 kDa) was identified as trehalase (EC 3.2.1.28). These studies suggest that labels such as DOG carrying the reactive group directly at the D-glucose moiety of the molecule can be used to label proteins with carbohydrate recognition sites.

Arachidonic acid as a second messenger for hypotonicity-induced calcium transients in rat IMCD cells

In rat inner medullary collecting duct (IMCD) cells in primary culture, hypotonic stress induces Ca2+ transients consisting of an early peak phase caused by a Ca2+ release from intracellular stores and a subsequent plateau phase that involves Ca2+ entry from the extracellular milieu. In the present study, the mechanisms by which cell swelling is transduced into the Ca2+ release were investigated. The free intracellular Ca2+ concentration ([Ca2+]i) was measured using the fluorescent dye fura-2 and cell volume using a confocal laser scanning microscope. In control experiments, after reduction of extracellular osmolarity from 600 to 300 mosmol/l, by omission of sucrose, [Ca2+]i rapidly increased from 106 +/- 9 nmol/l to a peak value of 405 +/- 22 nmol/l (P </= 0.05) and thereafter reached a steady-state of 230 +/- 23 nmol/l. In low-Ca2+ conditions (10 nmol/l), the reduction of osmolarity evoked only a transient increase of [Ca2+]i by 182 +/- 11 nmol/l (P </= 0.05), which reflected Ca2+ release from intracellular stores. Hyposmotic stress had no effect on inositol 1,4,5-triphosphate (IP3) production measured by a [3H]IP3 radioreceptor assay. Preincubation with 100 micromol/l ETYA (a non-metabolisible derivative of arachidonic acid) reduced the Ca2+ response to hyposmotic stress under high and low Ca2+ conditions (87 and 85% inhibition respectively) as well as the regulatory volume decrease (RVD). Extracellular application of arachidonic acid in isotonic medium led to an increase in [Ca2+]i under high and low Ca2+ conditions. Pretreatment of IMCD cells with 50 microg/ml D609 (a phosphatidylcholine-directed phospholipase C inhibitor) or with 200 micromol/l propranolol (a phosphatidate phosphohydrolase inhibitor) reduced the hypotonic Ca2+ response more strongly than pretreatment with 5 micromol/l BPhB (a phospholipase A2 inhibitor). The Ca2+ response was also suppressed after preincubation with 200 micromol/l RHC 80267 (a diacylglycerol lipase inhibitor). Preincubation with 50 ng/ml pertussis toxin (a G-protein inhibitor) reduced the transient component of the Ca2+ response partially. We conclude that G-proteins, phosphatidylcholine-directed phospholipase C, phospholipase A2, diacylglycerol lipase and arachidonic acid, but not IP3, are involved in the mechanisms by which Ca2+ is released from the intracellular stores during RVD in IMCD cells.

Single-channel properties of swelling-activated anion conductance in rat inner medullary collecting duct cells

Single-channel properties of the volume-activated outwardly rectifying Cl- conductance of rat IMCD cells were studied in primary cultures by means of the patch-clamp technique in the whole cell and in the outside-out configuration. Measurements were performed by noise analysis and in single-channel recordings during voltage-induced current inactivation and reactivation and in long-lasting experiments at constant membrane voltages. Unitary conductances could be defined for the voltage range of -100 to -50 mV and between +50 and +120 mV and chord conductances of 34.1 and 76.6 pS, respectively, can be calculated. The overall current-to-voltage relationship very much resembles that of the macroscopic Cl- conductance and the open probability of the activated channel is close to unity (Po = 0.98-0.99). The channel exhibits many similarities to volume-activated outwardly rectifying Cl- channels found in other systems although certain species differences do exist.

Role of G-proteins in the regulation of organic osmolyte efflux from isolated rat renal inner medullary collecting duct cells

Hypotonic shock (change of osmolality from 600 mosmol to 300 mosmol by lowering NaCl concentration) increases the release of organic osmolytes from isolated inner medullary collecting duct (IMCD) cells in the following sequence: taurine > betaine > sorbitol > myo-inositol > glycerophosphorylcholine (GPC). The role of G-proteins in regulating the hypotonicity-induced efflux was analysed by exposing cells to various concentrations of a G-protein inhibitor, pertussis toxin (PTX; 20-200 ng/ml), and a Gialpha-protein stimulator, mastoparan (10-50 microM). PTX diminished the hypotonic release of sorbitol and betaine by 43.2+/-9. 5% and 32.2+/-7.8% (n = 5), respectively. Efflux of GPC, myo-inositol and taurine was not significantly altered. Mastoparan (10 microM) increased osmolyte release under isotonic conditions such that release of betaine was increased 3.8-fold and that of sorbitol 2.1-fold, while GPC, myo-inositol and taurine effluxes were only slightly augmented. Under hypotonic conditions, mastoparan stimulated betaine release (1.86+/-0.2-fold, n = 5) but not that of sorbitol. As tested in connection with sorbitol and betaine release, the effect of mastoparan was abolished by PTX, but not the A23187-evoked sorbitol release. Like mastoparan, arachidonic acid increased the release of sorbitol and betaine under isotonic conditions, but under hypotonic conditions it only increased the release of betaine. As to the role of intracellular Ca2+, hypotonic shock evoked an intracellular Ca2+ peak which could be prevented by PTX. Mastoparan increased intracellular Ca2+ under isotonic conditions, whether the extracellular Ca2+ concentration was low or high. The results indicate that G-proteins are involved in regulating sorbitol and betaine efflux from IMCD cells. The G-proteins regulating sorbitol release are probably involved in generating the proper intracellular Ca2+ signal. Betaine efflux, which is independent of intracellular Ca2+, might be regulated by a G-protein-stimulated release of arachidonic acid. Thus, probably several G-proteins are involved in controlling organic osmolyte efflux from IMCD cells.

Interferons modify in vitro proliferation of human bladder transitional cell carcinoma in the presence of doxorubicin and mitomycin C

PURPOSE

The aim of the present study was to investigate whether interferons with their known antitumor activity modify the response of human bladder carcinoma cells to antitumor drugs.

MATERIALS AND METHODS

We investigated the in vitro effect of doxorubicin, mitomycin C and the interferons alpha and gamma on cell proliferation in human bladder carcinoma cell lines as measured by 5-bromo-2'-deoxy-uridine (BrdU) incorporation.

RESULTS

Exposure of RT 112 (but not EJ 28) cells for 2 hours to doxorubicin (500 ng./ml.) and mitomycin C (200 ng./ml.) reduced the proliferation rate to 85.9 +/- 3.3% (n = 4) and 89.3 +/- 4.0% (n = 4) of control. Treatment for 2 days with interferon alpha and gamma up to the highest concentration (200 U/ml.) showed no effect. The combination of 100 U/ml. interferon alpha and doxorubicin decreased proliferation significantly. At 50 ng./ml. the proliferation rate was decreased to 88.0 +/- 5.7% of control and at 500 ng./ml. to 67.7 +/- 3.1%. Thus interferon alpha seems to increase the sensitivity of the cells to doxorubicin. Cells treated with 20 ng./ml. mitomycin C after pretreatment with interferon alpha showed a dramatic decrease in cell proliferation (from 98.8 +/- 2.1% to 80.2 +/- 4.0% of control). This decrease was similar in the presence of 200 ng./ml. mitomycin C. Thus mitomycin C seems to render cells more sensitive to the antiproliferative action of interferon alpha. Interferon gamma had only minor effects on the response of the cells to doxorubicin or mitomycin C.

CONCLUSIONS

These studies suggest that exposure to interferon alpha increases the efficacy of anticancer drugs in vitro, probably by several mechanisms. Potential consequences of this finding for the therapeutic regime employed for treatment of bladder carcinoma are discussed.

The human gene of a protein that modifies Na(+)-D-glucose co-transport

Recently, a cDNA (pRS1) was cloned from pig kidney cortex that encodes a membrane-associated protein involved in Na(+)-coupled sugar transport. pRS1 alters sugar transport by SGLT1 from rabbit intestine or by SMIT from dog kidney which is homologous to SGLT1. In contrast, pRS1 does not influence transporters from other genetic families. We report the cloning of the intronless human gene hRS1 (6,743 bp), which encodes a 617-amino-acid protein with 74% amino acid identity to pRS1. By fluorescence in situ hybridization, hRS1 was localized to chromosome 1p36.1. The localization to one chromosome and Southern blot analysis of restricted genomic DNA suggest that there is only one RS1-homologous gene in humans. Functionality of hRS1 was demonstrated by co-expression experiments of hRS1 and SGLT1 from human intestine in oocytes from Xenopus laevis. They show that hRS1-protein inhibits Na(+)-D-glucose co-transport expressed by human SGLT1 by decreasing both the Vmax and the apparent Km value of the transporter. The analysis of the 5'-noncoding sequence of hRS1 revealed different enhancer consensus sequences that are absent in the SGLT1 gene, e.g., several consensus sequences for steroid-binding proteins.

Taurine permeation through swelling-activated anion conductance in rat IMCD cells in primary culture

Whole cell recordings were performed on rat inner medullary collecting duct (IMCD) cells in primary culture. With 140 mmol/l CsCl in bath and pipette we find within 10 min a 60-fold increase in membrane conductance from 0.02 +/- 0.003 to 1.2 +/- 0.1 nS/pF when bath osmolarity is decreased from 600 to 500 mosmol/l. The effect is due to the activation of an outwardly rectifying anion conductance with the anion selectivity SCN- > I- > NO-3 > Br- > Cl- > F- > isethionate > gluconate > or = aspartate > or = glutamate. A relative permeability of the organic osmolyte taurine to Cl- (Ptaurine: PCl-) of 0.15 was detected. With taurine in pipette and bath, the channel exhibits a nearly identical activation and sensitivity profile to a variety of anion channel blockers as under symmetrical Cl- conditions. Furthermore, the 50% inhibitory concentration value for the effect of 5-nitro-2-(3-phenylpropylamino)benzoate on both currents is virtually identical. We conclude that hypotonic stress increases the anion conductance of rat IMCD cells and that this anion conductance mediates taurine efflux.

Tamm-Horsfall protein as a marker of tubular maturation

Tamm-Horsfall protein (THP), a glycoprotein with a molecular weight of 95 kilodaltons, is produced and secreted in the ascending loop of Henle. To evaluate the measurement of THP in the assessment of fetal renal development and function, we stained fetal kidney sections for THP and measured THP concentrations in 129 amniotic fluid samples from healthy pregnancies, together with other parameters such as transferrin, albumin, alpha 1- and beta 2-microglobulin. After the 16th week of gestation THP could be detected immunohistochemically in the distal tubular cells, but was not consistently detected by sandwich enzyme immunoassay until after the 20th week of gestation (detection limit 50 ng/ml). Between the 15th and 19th week of gestation THP was only detected occasionally, but after the 20th week of gestation the concentration increased significantly reaching levels of 0.4-4 mg/l at term. The THP concentration was lower in samples taken directly before birth than in the corresponding first urine after birth, indicating that THP is produced from the fetal kidney only and does not pass the placental barrier. This pattern was different from other proteins studied. Transferrin and albumin were significantly lower in the first urine voided, microglobulins remained unchanged, and the creatinine concentration increased. This indicates that maternal to fetal exchange or transport is likely for most of the other proteins. Measurement of THP concentrations, in addition to other proteins in the amniotic fluid, can improve fetal renal assessment, but because the range of THP concentrations is wide accurate predictions are still not possible.

Interactions of alkylglucosides with the renal sodium/D-glucose cotransporter

To study the nature of the glucose binding pocket of the renal Na+/D-glucose cotransporter, we have evaluated the inhibitory potency of various alkylglucosides (AG) on sodium-dependent D-glucose uptake into hog kidney brush border membrane vesicles (BBMV). Inhibition at 0.1 mM AG level was found to be strongly dependent on the anomeric configuration, on the length and on the flexibility of the side chain. Beta-n-AG inhibited transport significantly more effectively compared to the corresponding alpha-anomer (n-octylglucoside: alpha-anomer 15%, beta-anomer 84%) and AG with an unsaturated n-alkenyl side chain were significantly less effective inhibitors than the corresponding saturated compound (cis/trans 3-n-beta-hexenylglucoside 53% and 32%, beta-n-hexylglucoside 76%). A series of beta-n-AG increasing in side chain length from 1 to 13 carbon atoms revealed a global maximum in the inhibition pattern when beta-AG with side chains ranging from 8 to 11 carbon atoms were used, thus beta-methylglucoside inhibited glucose transport by 13%, beta-n-nonylglucoside by 92%, and beta-n-tridecylglucoside showed no effect. Kinetic analysis of inhibition by beta-n-octylglucoside revealed a fully competitive type of inhibition with an apparent K(i) of 10 +/- 2 microM. n-Octylglucoside at 0.1 mM did not inhibit sodium-dependent L-alanine uptake; similarly, n-octylmannoside at 0.1 mM level did not affect D-glucose uptake. These results suggest that the inhibition of sodium-dependent D-glucose uptake was, at least in the concentration range tested (up to 0.1 mM), not due to a detergent effect of AG, but due to interaction with the carrier. Optimum interaction requires a beta-anomer with a glycosidic bond that places the alkyl chain into an equatorial position with regard to the D-glucose molecule and the two main determinants of the sugar recognition site C2 and C3. In addition, the alkyl chain has to be highly flexible. The alkyl chains thus apparently interact with hydrophobic sites at the carrier in a slightly coiled conformation, thereby AG with a chain length up to 6 carbon atoms interact only with one hydrophobic site, AG with higher chain length probably with two sites.

Osmoregulation in the renal papilla: membranes, messengers and molecules

This contribution summarizes recent progress in the understanding of the molecular basis of the release of organic osmolytes that occurs when inner medullary cells are confronted with a drop in osmolarity in their environment. For sorbitol release across the basolateral membrane an increase in intracellular calcium seems to be the prominent signal, initiated by G-protein activation, followed by phosphatidylcholine phospholipase activation and generation of arachidonic acid. The increase in betaine permeability is also G-protein dependent but calcium independent, and is restricted to the basal-lateral cell face. Myo-inositol and glycerophosphorylcholine efflux are calcium and G-protein independent and occur both across the apical and basolateral membrane, although to a different extent. Taurine release is also calcium and G-protein independent; a swelling-activated anion channel at the basolateral membrane represents the major efflux pathway.

Na-Pi cotransport in flounder: same transport system in kidney and intestine

The cloning of a renal Na-Pi contransport in system from winter flounder (P eudopleuronectes americanus) has recently been reported. We used this information to answer the questions 1) what is the distribution of the transport protein along the nephron? and 2) how are renal and intestinal transporters related? The distribution of the flounder NaPi-II protein was tested using two antisera raised against partial sequences (amino acids 1-14 and 388-441) of the transporter. Antibody-specific fluorescence was detected at the basolateral membrane of epithelial cells in the proximal tubular segment PII. Two clones corresponding to the renal Na-Pi cotransporter were isolated from a flounder intestinal cDNA library. Their functional properties were determined using Xenopus laevis oocytes. The apparent affinities for Pi [Michaelis constant (K(m)) = 0.063 mM] and Na (K(m) = 45.3 mM), as well as the pH dependency (increasing transport activity with increasing pH), showed the same characteristics in both intestinal and the renal systems. Sequence analysis revealed that the two intestinal clones were 100% homologous to the renal cDNA, Flounder NaPi-II-specific immunofluorescence was observed predominantly at the apical membrane on intestinal cross sections. We report the cloning and expression of the first intestinal Na-Pi cotransport system. This transporter belongs to the small group of proteins that exhibit the same function in the apical and the basolateral membranes of different cells.

Electrodiffusive transport of Mg across renal membrane vesicles of the rainbow trout Oncorhynchus mykiss

The mechanism of tubular Mg transport was investigated in membrane vesicles (MV) of trout kidneys prepared by differential centrifugation with sucrose. MV consisted largely of brush-border membranes, as indicated by high enrichments of brush-border membrane enzymes. Although measured transport of 28 Mg included a binding component, most membrane transport was into or out of an osmotically active space. There was no evidence for amiloride-sensitive Na/Mg exchange, nor was Mg uptake affected by the carboxyl group reagents trimethyloxonium tetrafluoroborate, glycine methyl ester.HCl-1-ethyl-3- (3-dimethyl-aminopropyl)carbodiimide, and N,N'-dicyclohexyl carbodiimide or the Ca channel modulators D-600, verapamil, diltiazem, and BAY K 8644. However, Mg uptake increased in the presence of inside-negative voltages generated by inward gradients of the permeant anions NO3, SCN, and Cl or by outward gradients of K (plus valinomycin). Alkaline-earth cations displayed the selectivity sequence VII (Mg > Ca > Sr > Ba) for cis-inhibition of 28 Mg uptake. Mg efflux was trans-inhibited by La and Gd, and Mg uptake was cis-inhibited by Mn. The sulfhydryl group reagents p- chloromercuribenzoic acid and p-chloromercuriphenylsulfonate stimulated Mg uptake and efflux. These results reveal an electrodiffusive pathway for Mg transport in trout renal MV.

Transport and metabolism of L-glutamate during oxygenation, anoxia, and reoxygenation of rat cardiac myocytes

The intracellular glutamate concentration of oxygenated, isolated adult rat heart cells incubated with 0.15 mM glutamate amounts to 2.89 +/- 0.6 mM. Under these conditions the velocity of glutamate transport was 24.3 +/- 1.6 pmol.min-1.mg protein-1 and occurs via a high-affinity carrier characterized by an apparent affinity (K(m)) value of 0.18 +/- 0.03 mM. At high glutamate concentrations ( > 1mM) this high-affinity transport system is superimposed by additional uptake processes of a low affinity but a high capacity for glutamate. The 1.6-fold increased uptake of glutamate observed during 30 min of anoxic incubation of cardiomyocytes does not prevent an intracellular decrease in this amino acid to a concentration of 0.49 mM. After 15 min reoxygenation of cardiomyocytes the intracellular glutamate content increases to the control values of oxygenated cells. Only 2.4% of the glutamate increase after reoxygenation is due to the transport o glutamate from the incubation medium. The competitive inhibitor of transaminases, aminooxyacetate, prevents both the observed intracellular decrease in glutamate during anoxia and the increase in intracellular glutamate after reoxygenation of cardiomyocytes. Half of the amino groups needed for the synthesis of glutamate originate from intracellular alanine, which increases during anoxia and is metabolized during reoxygenation of cardiomyocytes. The velocity of the glutamate uptake of cardiomyocytes incubated in a medium containing 10 mM L-glutamate amounted to 728 +/- 140 pmol.min-1.mg protein-1. During anoxic incubation of cardiomyocytes at this high extracellular glutamate concentration, the intracellular glutamate breakdown may be compensated by a simultaneous uptake of this amino acid via the transport processes characterized by a high capacity