Protein kinase C phosphorylation disengages human and mouse-1a P-glycoproteins from influencing the rate of activation of swelling-activated chloride currents

Enhancement of an outwardly rectifying chloride channel in hippocampal pyramidal neurons after cerebral ischemia

Cerebral ischemia induces delayed, selective neuronal death in the CA1 region of the hippocampus. The underlying molecular mechanisms remain unclear, but it is known that apoptosis is involved in this process. Chloride efflux has been implicated in the progression of apoptosis in various cell types. Using both the inside-out and whole-cell configurations of the patch-clamp technique, the present study characterized an outwardly rectifying chloride channel (ORCC) in acutely dissociated pyramid neurons in the hippocampus of adult rats. The channel had a nonlinear current-voltage relationship with a conductance of 42.26±1.2pS in the positive voltage range and 18.23±0.96pS in the negative voltage range, indicating an outward rectification pattern. The channel is Cl(-) selective, and the open probability is voltage-dependent. It can be blocked by the classical Cl(-) channel blockers DIDS, SITS, NPPB and glibenclamide. We examined the different changes in ORCC activity in CA1 and CA3 pyramidal neurons at 6, 24 and 48h after transient forebrain ischemia. In the vulnerable CA1 neurons, ORCC activity was persistently enhanced after ischemic insult, whereas in the invulnerable CA3 neurons, no significant changes occurred. Further analysis of channel kinetics suggested that multiple openings are a major contributor to the increase in channel activity after ischemia. Pharmacological blockade of the ORCC partly attenuated cell death in the hippocampal neurons. We propose that the enhanced activity of ORCC might contribute to selective neuronal damage in the CA1 region after cerebral ischemia, and that ORCC may be a therapeutic target against ischemia-induced cell death.

The Alterations in the Expression and Function of P-Glycoprotein in Vitamin A-Deficient Rats as well as the Effect of Drug Disposition in Vivo

This study was aimed to investigate whether vitamin A deficiency could alter P-GP expression and function in tissues of rats and whether such effects affected the drug distribution in vivo of vitamin A-deficient rats. We induced vitamin A-deficient rats by giving them a vitamin A-free diet for 12 weeks. Then, Abcb1/P-GP expression was evaluated by qRT-PCR and Western blot. qRT-PCR analysis revealed that Abcb1a mRNA levels were increased in hippocampus and liver. In kidney, it only showed an upward trend. Abcb1b mRNA levels were increased in hippocampus, but decreased in cerebral cortex, liver and kidney. Western blot results were in good accordance with the alterations of Abcb1b mRNA levels. P-GP function was investigated through tissue distribution and body fluid excretion of rhodamine 123 (Rho123), and the results proclaimed that P-GP activities were also in good accordance with P-GP expression in cerebral cortex, liver and kidney. The change of drug distribution was also investigated through the tissue distribution of vincristine, and the results showed a significantly upward trend in all indicated tissues of vitamin A-deficient rats. In conclusion, vitamin A deficiency may alter Abcb1/P-GP expression and function in rat tissues, and the alterations may increase drug activity/toxicity through the increase of tissue accumulation.

Evidence for P-Glycoprotein Involvement in Cell Volume Regulation Using Coulter Sizing in Flow Cytometry

The regulation of cell volume is an essential function that is coupled to a variety of physiological processes such as receptor recycling, excitability and contraction, cell proliferation, migration, and programmed cell death. Under stress, cells undergo emergency swelling and respond to such a phenomenon with a regulatory volume decrease (RVD) where they release cellular ions, and other osmolytes as well as a concomitant loss of water. The link between P-glycoprotein, a transmembrane transporter, and cell volume regulation is controversial, and changes in cells volume are measured using microscopy or electrophysiology. For instance, by using the patch-clamp method, our team demonstrated that chloride currents activated in the RVD were more intense and rapid in a breast cancer cell line overexpressing the P-glycoprotein (P-gp). The Cell Lab Quanta SC is a flow cytometry system that simultaneously measures electronic volume, side scatter and three fluorescent colors; altogether this provides unsurpassed population resolution and accurate cell counting. Therefore, here we propose a novel method to follow cellular volume. By using the Coulter-type channel of the cytometer Cell Lab Quanta SC MPL (multi-platform loading), we demonstrated a role for the P-gp during different osmotic treatments, but also a differential activity of the P-gp through the cell cycle. Altogether, our data strongly suggests a role of P-gp in cell volume regulation.

Lobelin reverses multidrug resistance of human gastric carcinoma cell line SGC7901/VCR

AIM: To investigate the reversal effects of lobeline on multidrug resistance (MDR) of human gastric carcinoma cell line SGC7901/VCR, and to explore the possible mechanisms involved.

METHODS: The human gastric carcinoma cell line SGC7901/VCR that had multidrug resistant phenotype was used in this study. MTT assay was used to determine the IC₅₀ of VCR and 5-Fu for non-treated SGC7901/VCR cells and SGC7901/VCR cells treated with non-toxic concentration of lobeline (10 μmol/L). RT-PCR was used to detect MDR1 mRNA expression in non-treated SGC7901/VCR cells and SGC7901/VCR cells treated with different final concentrations (5, 10, 20, 50, 100 μmol/L) of lobeline. Western blot was used to determine intracellular P-gp protein expression.

RESULTS: Treatment with 10 μmol/L lobeline increased the chemotherapy sensitivity of SGC7901/VCR cells, and the IC₅₀ of VCR declined from 16.55 μg/L ± 0.13 μg/L to 7.27 μg/L ± 0.65 μg/L, with a reversion index of 2.28. The IC₅₀ of 5-Fu declined from 11.01 μg/L ± 0.43 μg/L to 9.53 μg/L ± 0.79 μg/L, and the reversion index is 1.16. RT-PCR analysis demonstrated that SGC7901/VCR cells highly expressed MDR1 mRNA, and lobeline decreased MDR1 mRNA expression in a concentration-dependent manner (P < 0.05). Western blot analysis indicated that SGC7901/VCR cells highly expressed P-glycoprotein, and lobeline decreased P-glycoprotein expression in a concentration-dependent manner (P < 0.05).

CONCLUSION: Lobeline can reverse the multidrug resistance of SGC7901/VCR cell line and increase chemotherapy sensitivity of SGC7901/VCR cell line to VCR and 5-Fu possibly via mechanism associated with inhibiting the expression of P-glycoprotein.

Tissue-specific alterations in expression and function of P-glycoprotein in streptozotocin-induced diabetic rats

AIM

To investigate the changes of expression and function of P-glycoprotein (P-GP) in cerebral cortex, hippocampus, liver, intestinal mucosa and kidney of streptozocin-induced diabetic rats.

METHODS

Diabetic rats were prepared via a single dose of streptozocin (65 mg/kg, ip). Abcb1/P-GP mRNA and protein expression levels in tissues were evaluated using quantitative real time polymerase chain reaction (QRT-PCR) analysis and Western blot, respectively. P-GP function was investigated via measuring tissue-to-plasma concentration ratios and body fluid excretion percentages of rhodamine 123.

RESULTS

In 5- and 8-week diabetic rats, Abcb1a mRNA levels were significantly decreased in cerebral cortices and intestinal mucosa, but dramatically increased in hippocampus and kidney. In liver, the level was increased in 5-week diabetic rats, and decreased in 8-week diabetic rats. Abcb1b mRNA levels were increased in cerebral cortex, hippocampus and kidney, but reduced in liver and intestinal mucosa in the diabetic rats. Western blot results were in accordance with the alterations of Abcb1a mRNA levels in most tissues examined. P-GP activity was markedly decreased in most tissues of diabetic rats, except kidney tissues.

CONCLUSION

Alterations in the expression and function of Abcb1/P-GP under diabetic conditions are tissue specific, Abcb1 specific and diabetic duration-dependent.

Combined effects of epileptic seizure and phenobarbital induced overexpression of P-glycoprotein in brain of chemically kindled rats

BACKGROUND AND PURPOSE

The multidrug resistance of epilepsy may result from the overexpression of P-glycoprotein, but the mechanisms are unclear. We investigated whether the overexpression of P-glycoprotein in the brains of subjects with pharmacoresistant epilepsy resulted from both drug effects and seizure activity.

EXPERIMENTAL APPROACH

Kindled rats were developed by injecting a subconvulsive dose of pentylenetetrazole (33 mg.kg(-1).day(-1), i.p.) for 28 days. Groups were then treated with an oral dose of phenobarbital (45 mg x kg(-1) x day(-1)) for 40 days. In accord with behavioural observations, P-glycoprotein activity in brain was assessed using brain-to-plasma concentration ratios of rhodamine 123. P-glycoprotein levels in the brain regions were further evaluated using RT-PCR and Western blot analysis. The distribution of phenobarbital in the brain was assessed by measuring phenobarbital concentrations 1 h following its oral administration.

KEY RESULTS

The kindling significantly increased P-glycoprotein activity and expression. Good associations were found among P-glycoprotein activity, expression and phenobarbital concentration in the hippocampus. Short-term treatment with phenobarbital showed good anti-epileptic effect; the maximum effect occurred on day 14 when overexpression of P-glycoprotein was reversed. Continuous treatment with phenobarbital had a gradually reduced anti-epileptic effect and on day 40, phenobarbital exhibited no anti-epileptic effect; this was accompanied by both a re-enhancement of P-glycoprotein expression and decreased phenobarbital concentration in the hippocampus. P-glycoprotein function and expression were also increased in age-matched normal rats treated with phenobarbital.

CONCLUSIONS AND IMPLICATIONS

The overexpression of P-glycoprotein in the brain of subjects with pharmacoresistant epilepsy is due to a combination of drug effects and epileptic seizures.

Insulin therapy restores impaired function and expression of P-glycoprotein in blood-brain barrier of experimental diabetes

We aimed to investigate effects of insulin on function and expression of P-glycoprotein (P-GP) in the blood-brain barrier of streptozotocin (STZ)-induced diabetic rats. Brain-to-plasma concentration ratio of vincristine (VCR) in rats was used as an indicator of in vivo function of P-GP. Western blot and quantitative real time-polymerase chain reaction were used to determine protein levels of P-GP and its mdr1a/mdr1b mRNA levels, respectively, in cerebral cortex of rats. In vitro effects of insulin on function and expression of P-GP in primarily cultured rat brain microvessel endothelial cells (rBMECs) were evaluated using rhodamine 123 (Rho123) uptakes and Western blot, respectively. The results showed that 3- and 5-week insulin treatment alleviated the impaired efflux function, expression and mdr1a/mdr1b mRNA levels of P-GP in cerebral cortex of diabetic rats. The 3- and 5-week insulin treatments also significantly enhanced P-GP levels and mdr1a/mdr1b mRNA levels in the cerebral cortex of normal rats. Addition of insulin to the insulin-deficient diabetic rat serum normalized the impaired function and expression of P-GP in rBMECs cultured in diabetic rat serum. When incubated with normal culture medium containing different levels of insulin, the rBMECs exhibited the enhanced P-GP levels and the reduced Rho123 uptake in a concentration-dependent manner. So we may conclude that appropriate level of insulin plays an important role in maintaining the normal function of BBB through regulating the function and expression of P-GP in the diabetic and normal rats.

Extracellular acidification elicits a chloride current that shares characteristics with ICl(swell)

A Cl− current activated by extracellular acidification, ICl(pHac), has been characterized in various mammalian cell types. Many of the properties of ICl(pHac) are similar to those of the cell swelling-activated Cl− current ICl(swell): ion selectivity (I− > Br− > Cl− > F−), pharmacology [ ICl(pHac) is inhibited by 4,4′-diisothiocyanostilbene-2,2′-disulfonic acid (DIDS), 1,9-dideoxyforskolin (DDFSK), diphenylamine-2-carboxylic acid (DPC), and niflumic acid], lack of dependence on intra- or extracellular Ca2+, and presence in all cell types tested. ICl(pHac) differs from ICl(swell) in three aspects: 1) its rate of activation and inactivation is very much more rapid, currents reaching a maximum in seconds rather than minutes; 2) it exhibits a slow voltage-dependent activation in contrast to the fast voltage-dependent activation and time- and voltage-dependent inactivation observed for ICl(swell); and 3) it shows a more pronounced outward rectification. Despite these differences, study of the transition between the two currents strongly suggests that ICl(swell) and ICl(pHac) are related and that extracellular acidification reflects a novel stimulus for activating ICl(swell) that, additionally, alters the biophysical properties of the channel.

Role of volume-stimulated osmolyte and anion channels in volume regulation by mammalian sperm

The ability to maintain cellular volume is an important general physiological function. Swelling induced by hypotonic stress results in the opening of channels, through which ions exit with accompanying water loss (regulatory volume decrease, RVD). RVD has been shown to occur in mammalian sperm, primarily through the opening of quinine-sensitive potassium channels. However, as yet, direct evidence for the participation of anion channels in sperm RVD has been lacking. The chloride channel type ClC-3 is believed to be involved in RVD in other cell types. Using electronic cell sizing for cell volume measurement, the following results were obtained. (i) The anion channel blockers 5-nitro-2-(3-phenylpropylamino)-benzoic acid (NPPB), tamoxifen and 4,4'-diisothiocyanostilbene-2,2'-disulphonic acid (DIDS) increased hypotonic swelling in concentration-dependent fashion, whereas verapamil (P-glycoprotein inhibitor) had little effect. The most potent, NPPB and DIDS, blocked RVD without affecting cell membrane integrity at effective concentrations. (ii) When gramicidin was included to dissipate Na+/K+ gradients, major secondary swelling was observed under hypotonic conditions. This secondary swelling could be reduced by NPPB, and suppressed completely by replacing chloride in the medium with sulphate, an ion which does not pass through chloride channels. It was deduced that the initial hypotonic swelling activated an anion channel through which chloride ions could then enter freely down a concentration gradient, owing to the lack of a counter-gradient of potassium. (iii) Taurine, an osmolyte often involved in RVD, does not appear to play a role in sperm RVD because lengthy preincubation with taurine did not alter sperm RVD response. Our observations provide direct evidence that a chloride channel (possibly ClC-3) is involved in the process of volume regulation in mammalian sperm.

Selective and persistent upregulation of mdr1b mRNA and P-glycoprotein in the parahippocampal cortex of chronic epileptic rats

There is recent evidence that increased expression of multidrug transporters, such as P-glycoprotein (P-gp), may lead to reduced antiepileptic drug (AED) concentrations in the brain, shortly after status epilepticus (SE), thereby suggesting a possible mechanism for drug-resistance. To get insights on whether increased P-gp expression is a consequence of the initial insult, or evolves more gradually as a result of recurrent spontaneous seizures, we used a rat model of temporal lobe epilepsy in which spontaneous seizures develop after an electrically induced SE. We investigated the temporal and region-specific expression of two isoforms of the multidrug resistance gene (mdr1a and mdr1b, both encoding for P-gp) in two regions within the temporal lobe (the dentate gyrus (DG) and the parahippocampal cortex (PHC)). Using real-time PCR, we found that the mdr1b isoform was increased in the temporal lobe, 1 week after SE; however, this increase was reversible in dentate gyrus while it persisted in the parahippocampal cortex of chronic epileptic rats. Mdr1b upregulation was related to the occurrence of spontaneous seizures, since this isoform was unchanged in rats that were stimulated, but that did not develop SE (non-SE). The mdr1a isoform was transiently upregulated in the dentate gyrus. P-gp immunostaining was enhanced in endothelial and glia-like cells, 1 week after SE. In chronic epileptic rats, the number of strongly P-gp positive glia-like cells was much lower than 1 week after SE, and it was mainly present in the most ventral part of the temporal lobe. These cells were in close apposition to strongly stained blood vessels. These findings show that both mdr1a and mdr1b are induced by SE, although the increase in mdr1b isoform was more persistent. More importantly, increased P-gp expression is still present in chronic epileptic rats.

Structure and pharmacology of swelling-sensitive chloride channels, I(Cl,swell)

Since several years, the interest for chloride channels and more particularly for the enigmatic swelling-activated chloride channel (I(Cl,swell)) is increasing. Despite its well-characterized electrophysiological properties, the I(Cl,swell) structure and pharmacology are not totally elucidated. These channels are involved in a variety of cell functions, such as cardiac rhythm, cell proliferation and differentiation, cell volume regulation and cell death through apoptosis. This review will consider different aspects regarding structure, electrophysiological properties, pharmacology, modulation and functions of these swelling-activated chloride channels.

Identification of HAX-1 as a protein that binds bile salt export protein and regulates its abundance in the apical membrane of Madin-Darby canine kidney cells

ATP-binding cassette (ABC)-type proteins are essential for bile formation in vertebrate liver. BSEP, MDR1, MDR2, and MRP2 ABC transporters are targeted to the apical (canalicular) membrane of hepatocytes where they execute ATP-dependent transport of bile acids, drugs, amphipathic cations, phospholipids, and conjugated organic anions, respectively. Changes in activity and abundance of transporters in the canalicular membrane regulate bile flow; however, little is known regarding cellular proteins that bind ABC transporters and regulate their trafficking. A yeast two-hybrid screen identified HAX-1 as a binding partner for BSEP, MDR1, and MDR2. The interactions were validated biochemically by glutathione S-transferase pull-down and co-immunoprecipitation assays. BSEP and HAX-1 were over-represented in rat liver subcellular fractions enriched for canalicular membrane vesicles, microsomes, and clathrin-coated vesicles. HAX-1 was bound to BSEP, MDR1, and MDR2 in canalicular membrane vesicles and co-localized with BSEP and MDR1 in the apical membrane of Madin-Darby canine kidney (MDCK) cells. RNA interference of HAX-1 increased BSEP levels in the apical membrane of MDCK cells by 71%. Pulse-chase studies indicated that HAX-1 depletion did not affect BSEP translation, post-translational modification, delivery to the plasma membrane, or half-life. HAX-1 depletion resulted in an increased peak of metabolically labeled apical membrane BSEP at 4 h and enhanced retention at 6 and 9 h. HAX-1 also interacts with cortactin. Expression of dominant negative cortactin increased steady state levels of BSEP 2-fold in the apical membrane of MDCK cells, as did expression of dominant negative EPS15. These findings suggest that HAX-1 and cortactin participate in BSEP internalization from the apical membrane.

Cell volume regulation and swelling-activated chloride channels

Maintenance of a constant volume is essential for normal cell function. Following cell swelling, as a consequence of reduction of extracellular osmolarity or increase of intracellular content of osmolytes, animal cells are able to restore their original volume by activation of potassium and chloride conductances. The loss of these ions, followed passively by water, is responsible for the homeostatic response called regulatory volume decrease (RVD). Activation of a chloride conductance upon cell swelling is a key step in RVD. Several proteins have been proposed as candidates for this chloride conductance. The status of the field is reviewed, with particular emphasis on ClC-3, a member of the ClC family which has been recently proposed as the chloride channel involved in cell volume regulation.

Fundamental role of ClC-3 in volume-sensitive Cl- channel function and cell volume regulation in AGS cells

Volume regulation is essential for cell function, but it is unknown which channels are involved in a regulatory volume decrease (RVD) in human gastric epithelial cells. Exposure to a hypotonic solution caused the increase in AGS cell volume, followed by the activation of a current. The reversal potential of the swelling-induced current suggested that Cl- was the primary charge carrier. The selectivity sequence for different anions was I- > Br- > Cl- > F- > gluconate. This current was inhibited by flufenamate, DIDS, tamoxifen, and 5-nitro-2-(3-phenylpropylamino)benzoate. Intracellular dialysis of three different anti-ClC-3 antibodies abolished or attenuated the Cl- current and disrupted RVD, whereas the current and RVD was unaltered by anti-ClC-2 antibody. Immunoblot studies demonstrated the presence of ClC-3 protein in Hela and AGS cells. RT-PCR analysis detected expression of ClC-3, MDR-1, and pICln mRNA in AGS cells. These results suggest a fundamental role of endogenous ClC-3 in the swelling-activated Cl- channels function and cell volume regulation in human gastric epithelial cells.

Potentiation of glycine responses by dideoxyforskolin and tamoxifen in rat spinal neurons

Dideoxyforskolin, a forskolin analogue unable to stimulate adenylate cyclase, and tamoxifen, an antioestrogen widely used against breast cancer, are both known to block some Cl- channels. Their effects on Cl- responses to glycine or GABA have been tested here by using whole-cell recording from cultured spinal neurons. Dideoxyforskolin (4 or 16 microm) and tamoxifen (0.2-5 microm) both potentiate responses to low glycine concentrations. They also induce blocking effects, predominant at high glycine concentrations. At 5 microm, tamoxifen increased responses to 15 microm glycine by a factor >4.5, reaching 20 in some neurons. Potentiation by extracellular dideoxyforskolin or tamoxifen persisted after intracellular application of the modulator and was not due to Zn2+ contamination. Potentiation by tamoxifen also persisted in a Ca2+-free extracellular solution, after intracellular Ca2+ buffering and protein kinase C blockade. Thus, the critical sites of action are not intracellular. The EC50 for glycine was lowered 6.6-fold by 5 microm tamoxifen. The kinetics and voltage-dependence of the effects of tamoxifen on glycine responses support the idea that this hydrophobic drug may act from a site located within the membrane. Tamoxifen (5 micro m) also increased responses to 2 micro m GABA by a factor of 3.5, but barely affected peak responses to 20 microm GABA. The demonstration that tamoxifen affects some of the main inhibitory receptors should be useful for better evaluating its neurological effects. Furthermore, the results identify a new class of molecules that potentiate glycine receptor function.

The Entamoeba histolytica EhPgp5 (MDR-like) protein induces swelling of the trophozoites and alters chloride-dependent currents in Xenopus laevis oocytes

Entamoeba histolytica, the protozoan responsible for human amoebiasis, presents the multidrug resistant phenotype due to the expression of the E. histolytica P-glycoproteins EhPgpl and EhPgp5. Here, we studied the protein EhPgp5 encoded by the EhPgp5 gene in emetine-sensitive trophozoites transfected with the pEhNEOPgp5 plasmid carrying the EhPgp5 gene. The transfected trophozoites increased their drug resistance slightly, but became bigger and globular. To investigate other EhPgp5 functions further, we microinjected the EhPgp5 mRNA in Xenopus laevis oocytes. Microinjected oocytes expressed EhPgp5 protein in their membranes and exhibited an ion current not present in the control oocytes. The antisense EhPgp5AS transcript, co-injected with the EhPgp5 mRNA, abolished the exogenous current, showing its specificity. Exogenous current was outward during depolarizing pulses. Reduction of the extracellular Cl- concentration displayed a reversible decrease of the current amplitude. Niflumic acid, 4,4-diisothiocyanatostilbene-2, 2'-disulfonic acid, and other Cl- channel blockers abolished the exogenous current, which was poorly modified by verapamil and changes in osmolarity of the medium. Our results suggest that the EhPgp5 protein could function as a Cl- current inductor and as a coadjuvant factor to avoid drug accumulation in the cell.

Mrp2 modulates the activity of chloride channels in isolated hepatocytes

Adenosine triphosphate binding cassette family transport proteins are important organic ion transporters in hepatocytes but these molecules may also exhibit other functions. In the present study we have measured the effects of substrates of the canalicular organic ion transporter multidrug resistance associated protein 2 (Mrp2) on chloride channel activation and cell volume regulation. We found that substrates such as leukotriene D(4), 17-beta-estradiol glucuronide, and the leukotriene inhibitor MK-571 accelerated the activation of chloride channels by cell swelling and activated chloride channels in cytokine-pretreated hepatocytes. Two conjugated estrogens that are not Mrp2 substrates did not produce this effect. Hepatocytes derived from a strain of transport-deficient rats (TR(-)), which lack Mrp2 expression, showed none of these substrate effects. Coincident with their ability to activate channels, the Mrp2 substrates increased the rate of volume regulatory decrease by approximately 50% (P <.01), confirming that enhanced channel activation under this condition stimulated volume regulation. In TR-hepatocytes the Mrp2 substrate had no effect on volume regulation. In conclusion, Mrp2 plays a role in regulation of chloride channel function by reducing the lag time necessary for channel activation and consequently accelerating the process of cell volume regulation. Substrates of Mrp2 affect the ability of the protein to interact with chloride channels. These findings represent an alternative function of Mrp2 in hepatocytes.

Volume-activated chloride currents contribute to the resting conductance and invasive migration of human glioma cells

We used an in vitro model for glioma cell invasion (transwell migration assay) and patch-clamp techniques to investigate the role of volume-activated Cl(-) currents (I(Cl,Vol)) in glioma cell invasion. Hypotonic solutions ( approximately 230 mOsm) activated outwardly rectifying currents that reversed near the equilibrium potential for Cl(-) ions (E(Cl)). These currents (I(Cl,Vol)) were sensitive to several known Cl(-) channel inhibitors, including DIDS, tamoxifen, and 5-nitro-2-(3-phenylpropylamino)-benzoate (NPPB). The IC(50) for NPPB inhibition of I(Cl,Vol) was 21 microm. Under isotonic conditions, NPPB (165 microm) blocked inward currents (at -40 mV) and increased input resistance in both standard whole-cell recordings and amphotericin perforated-patch recordings. Reducing [Cl(-)](o) under isotonic conditions positively shifted the reversal potential of whole-cell currents. These findings suggest a significant resting Cl(-) conductance in glioma cells. Under isotonic and hypotonic conditions, Cl(-) channels displayed voltage- and time-dependent inactivation and had an I(-) > Cl(-) permeability. To assess the potential role of these channels in cell migration, we studied the chemotactic migration of glioma cells toward laminin or vitronectin in a Boyden chamber containing transwell filters with 8 microm pores. Inhibition of I(Cl,Vol) with NPPB reduced chemotactic migration in a dose-dependent fashion with an IC(50) of 27 microm. Time-lapse video microscopy during patch-clamp recordings revealed visible changes in cell shape and/or movement that accompanied spontaneous activation of I(Cl,Vol), suggesting that I(Cl,Vol) is activated during cell movement, consistent with the effects of NPPB in migration assays. We propose that I(Cl,Vol) contributes to cell shape and volume changes required for glioma cell migration through brain tissue.

Volume-activated chloride currents contribute to the resting conductance and invasive migration of human glioma cells

We used an in vitro model for glioma cell invasion (transwell migration assay) and patch-clamp techniques to investigate the role of volume-activated Cl(-) currents (I(Cl,Vol)) in glioma cell invasion. Hypotonic solutions ( approximately 230 mOsm) activated outwardly rectifying currents that reversed near the equilibrium potential for Cl(-) ions (E(Cl)). These currents (I(Cl,Vol)) were sensitive to several known Cl(-) channel inhibitors, including DIDS, tamoxifen, and 5-nitro-2-(3-phenylpropylamino)-benzoate (NPPB). The IC(50) for NPPB inhibition of I(Cl,Vol) was 21 microm. Under isotonic conditions, NPPB (165 microm) blocked inward currents (at -40 mV) and increased input resistance in both standard whole-cell recordings and amphotericin perforated-patch recordings. Reducing [Cl(-)](o) under isotonic conditions positively shifted the reversal potential of whole-cell currents. These findings suggest a significant resting Cl(-) conductance in glioma cells. Under isotonic and hypotonic conditions, Cl(-) channels displayed voltage- and time-dependent inactivation and had an I(-) > Cl(-) permeability. To assess the potential role of these channels in cell migration, we studied the chemotactic migration of glioma cells toward laminin or vitronectin in a Boyden chamber containing transwell filters with 8 microm pores. Inhibition of I(Cl,Vol) with NPPB reduced chemotactic migration in a dose-dependent fashion with an IC(50) of 27 microm. Time-lapse video microscopy during patch-clamp recordings revealed visible changes in cell shape and/or movement that accompanied spontaneous activation of I(Cl,Vol), suggesting that I(Cl,Vol) is activated during cell movement, consistent with the effects of NPPB in migration assays. We propose that I(Cl,Vol) contributes to cell shape and volume changes required for glioma cell migration through brain tissue.

The multidrug resistance P-glycoprotein. Oligomeric state and intramolecular interactions

The human multidrug resistance P-glycoprotein (P-gp), a member of the ATP-binding cassette (ABC) superfamily of transporters, is frequently responsible for the failure of chemotherapy by virtue of its ability to export hydrophobic cytotoxic drugs from cells. Elucidating the inter- and intramolecular interactions of this protein is critical to understanding its cellular function and mechanism of action. Toward this end, we have used both biochemical and genetic techniques to probe potential oligomerization interactions of P-gp. Differentially epitope-tagged P-gp molecules did not co-immunoprecipitate when co-expressed in HEK293 cells or when co-translated in vitro, demonstrating that P-gp is monomeric in both the presence and absence of detergents. The two cytoplasmic domains of P-gp did not interact with each other in vivo when co-expressed as gene fusions in yeast. In contrast, the homologous domains of the transporter associated with antigen processing (TAP), which reside on separate polypeptides and must form a heterodimeric transporter (TAP1/TAP2), did interact in this system, suggesting a role for these domains in TAP dimerization. Implications for understanding the subunit organization of ABC transporters are discussed.

Human ClC-3 is not the swelling-activated chloride channel involved in cell volume regulation

Volume regulation is essential for normal cell function. A key component of the cells' response to volume changes is the activation of a channel, which elicits characteristic chloride currents (I(Cl, Swell)). The molecular identity of this channel has been controversial. Most recently, ClC-3, a protein highly homologous to the ClC-4 and ClC-5 channel proteins, has been proposed as being responsible for I(Cl, Swell). Subsequently, however, other reports have suggested that ClC-3 may generate chloride currents with characteristics clearly distinct from I(Cl, Swell). Significantly different tissue distributions for ClC-3 have also been reported, and it has been suggested that two isoforms of ClC-3 may be expressed with differing functions. In this study we generated a series of cell lines expressing variants of ClC-3 to rigorously address the question of whether or not ClC-3 is responsible for I(Cl, Swell). The data demonstrate that ClC-3 is not responsible for I(Cl, Swell) and has no role in regulatory volume decrease, furthermore, ClC-3 is not activated by intracellular calcium and fails to elicit chloride currents under any conditions tested. Expression of ClC-3 was shown to be relatively tissue-specific, with high levels in the central nervous system and kidney, and in contrast to previous reports, is essentially absent from heart. This distribution is also inconsistent with the previous proposed role in cell volume regulation.

Mdr1b facilitates p53-mediated cell death and p53 is required for Mdr1b upregulation in vivo

The mdr1b gene is thought to be a "stress-responsive" gene, however it is unknown if this gene is regulated by p53 in the whole animal. Moreover, it is unknown if overexpression of mdr1b affects cell survival. The dependence of mdr1b upon p53 for upregulation was evaluated in p53 knockout mice. Wild-type (wt) or p53-/- mice were treated singly or in combination with gamma irradiation (IR) and/or the potent DNA damaging agent, diethylnitrosoamine (DEN). Both IR and DEN induced mdr1b in wild-type animals, but not in the p53-/- mice. IR also upregulated endogenous mdr1b in the H35 liver cell line, and the mdr1b promoter was activated by IR and activation correlated with p53 levels; moreover activation required an intact p53 binding site. Colony survival studies revealed that co-transfection of both mdr1b and p53 dramatically reduced colony numbers compared to cells transfected with either p53 or mdr1b alone and cells microinjected with both mdr1b and p53 had a more dramatic loss in viability compared to cells injected with either expression vector alone. Further studies using acridine orange and ethidium bromide to measure apoptosis revealed that mdr1b caused apoptosis and this was enhanced by p53, however the increased apoptosis required a functional p53 transactivation domain. These studies indicate that mdr1b is a downstream target of p53 in the whole animal and expression of mdr1b facilitates p53-mediated cell death.

Biophysical properties of ClC-3 differentiate it from swelling-activated chloride channels in Chinese hamster ovary-K1 cells

ClC-3 is a highly conserved voltage-gated chloride channel, which together with ClC-4 and ClC-5 belongs to one subfamily of the larger group of ClC chloride channels. Whereas ClC-5 is localized intracellularly, ClC-3 has been reported to be a swelling-activated plasma membrane channel. However, recent studies have shown that native ClC-3 in hepatocytes is primarily intracellular. Therefore, we reexamined the properties of ClC-3 in a mammalian cell expression system and compared them with the properties of endogenous swelling-activated channels. Chinese hamster ovary (CHO)-K1 cells were transiently transfected with rat ClC-3. The resulting chloride currents were Cl(-) > I(-) selective, showed extreme outward rectification, and lacked inactivation at positive voltages. In addition, they were insensitive to the chloride channel blockers, 5-nitro-2-(3-phenylpropylamino)-benzoic acid (NPPB) and 4, 4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS) and were not inhibited by phorbol esters or activated by osmotic swelling. These properties are identical to those of ClC-5 but differ from those previously attributed to ClC-3. In contrast, nontransfected CHO-K1 cells displayed an endogenous swelling-activated chloride current, which was weakly outward rectifying, inactivated at positive voltages, sensitive to NPPB and DIDS, and inhibited by phorbol esters. These properties are identical to those previously attributed to ClC-3. Therefore, we conclude that when expressed in CHO-K1 cells, ClC-3 is an extremely outward rectifying channel with similar properties to ClC-5 and is neither activated by cell swelling nor identical to the endogenous swelling-activated channel. These data suggest that ClC-3 cannot be responsible for the swelling-activated chloride channel under all circumstances.

A role for P-glycoprotein in regulating cell death

P-glycoprotein (P-gp) is an energy dependent drug pump responsible for multidrug resistance (MDR) in human cancers. While it is irrefutable that P-gp can efflux xenobiotics out of cells, the biological function of P-gp in multicellular organisms has yet to be firmly established. The question of what, if anything, P-gp does when not effluxing drugs has been raised by recent reports indicating that P-gp may regulate apoptosis, chloride channel activity, cholesterol metabolism and immune cell function. There is now a lively debate regarding the possible role of P-gp in regulating cell differentiation, proliferation and survival.

Anion transport in heart

Anion transport proteins in mammalian cells participate in a wide variety of cell and intracellular organelle functions, including regulation of electrical activity, pH, volume, and the transport of osmolites and metabolites, and may even play a role in the control of immunological responses, cell migration, cell proliferation, and differentiation. Although significant progress over the past decade has been achieved in understanding electrogenic and electroneutral anion transport proteins in sarcolemmal and intracellular membranes, information on the molecular nature and physiological significance of many of these proteins, especially in the heart, is incomplete. Functional and molecular studies presently suggest that four primary types of sarcolemmal anion channels are expressed in cardiac cells: channels regulated by protein kinase A (PKA), protein kinase C, and purinergic receptors (I(Cl.PKA)); channels regulated by changes in cell volume (I(Cl.vol)); channels activated by intracellular Ca(2+) (I(Cl.Ca)); and inwardly rectifying anion channels (I(Cl.ir)). In most animal species, I(Cl.PKA) is due to expression of a cardiac isoform of the epithelial cystic fibrosis transmembrane conductance regulator Cl(-) channel. New molecular candidates responsible for I(Cl.vol), I(Cl.Ca), and I(Cl.ir) (ClC-3, CLCA1, and ClC-2, respectively) have recently been identified and are presently being evaluated. Two isoforms of the band 3 anion exchange protein, originally characterized in erythrocytes, are responsible for Cl(-)/HCO(3)(-) exchange, and at least two members of a large vertebrate family of electroneutral cotransporters (ENCC1 and ENCC3) are responsible for Na(+)-dependent Cl(-) cotransport in heart. A 223-amino acid protein in the outer mitochondrial membrane of most eukaryotic cells comprises a voltage-dependent anion channel. The molecular entities responsible for other types of electroneutral anion exchange or Cl(-) conductances in intracellular membranes of the sarcoplasmic reticulum or nucleus are unknown. Evidence of cardiac expression of up to five additional members of the ClC gene family suggest a rich new variety of molecular candidates that may underlie existing or novel Cl(-) channel subtypes in sarcolemmal and intracellular membranes. The application of modern molecular biological and genetic approaches to the study of anion transport proteins during the next decade holds exciting promise for eventually revealing the actual physiological, pathophysiological, and clinical significance of these unique transport processes in cardiac and other mammalian cells.

Mechanism of inhibition of P-glycoprotein-mediated drug transport by protein kinase C blockers

P-glycoprotein is a membrane ATPase that transports drugs out of cells and confers resistance to a variety of chemically unrelated drugs (multidrug resistance). P-glycoprotein is phosphorylated by protein kinase C (PKC), and PKC blockers reduce P-glycoprotein phosphorylation and increase drug accumulation. These observations suggest that phosphorylation of P-glycoprotein stimulates drug transport. However, there is evidence that PKC inhibitors directly interact with P-glycoprotein, and therefore the mechanism of their effects on P-glycoprotein-mediated drug transport and the possible role of phosphorylation in the regulation of P-glycoprotein function remain unclear. In the present work, we studied the effects of different kinds of PKC inhibitors on drug transport in cells expressing wild-type human P-glycoprotein and a PKC phosphorylation-defective mutant. We demonstrated that PKC blockers inhibit drug transport hy mechanisms independent of P-glycoprotein phosphorylation. Inhibition by the blockers occurs by (i) direct competition with transported drugs for binding to P-glycoprotein, and (ii) indirect inhibition through a pathway that involves PKC inhibition, but is independent of P-glycoprotein phosphorylation. The effects of the blockers on P-glycoprotein phosphorylation do not seem to play an important role, but the PKC-signaling pathway regulates P-glycoprotein-mediated drug transport.

ClC channels: leaving the dark ages on the verge of a new millennium

The field of molecular physiology of ClC chloride channels has witnessed a tremendous surge in knowledge over the past few years; however, fundamental issues such as the stoichiometry of ClC channels and the identification of pore-lining sequences have only recently begun to be addressed. New studies have also provided important insights into the role of ClC channels in cell volume regulation and their function in intracellular organelles.

Phosphorylation of P-glycoprotein by PKA and PKC modulates swelling-activated Cl- currents

Several proteins belonging to the ATP-binding cassette superfamily can affect ion channel function. These include the cystic fibrosis transmembrane conductance regulator, the sulfonylurea receptor, and the multidrug resistance protein P-glycoprotein (MDR1). We measured whole cell swelling-activated Cl- currents (ICl,swell) in parental cells and cells expressing wild-type MDR1 or a phosphorylation-defective mutant (Ser-661, Ser-667, and Ser-671 replaced by Ala). Stimulation of protein kinase C (PKC) with a phorbol ester reduced the rate of increase in ICl,swell only in cells that express MDR1. PKC stimulation had no effect on steady-state ICl,swell. Stimulation of protein kinase A (PKA) with 8-bromoadenosine 3',5'-cyclic monophosphate reduced steady-state ICl, swell only in MDR1-expressing cells. PKA stimulation had no effect on the rate of ICl,swell activation. The effects of stimulation of PKA and PKC on ICl,swell were additive (i.e., decrease in the rate of activation and reduction in steady-state ICl,swell). The effects of PKA and PKC stimulation were absent in cells expressing the phosphorylation-defective mutant. In summary, it is likely that phosphorylation of MDR1 by PKA and by PKC alters swelling-activated Cl- channels by independent mechanisms and that Ser-661, Ser-667, and Ser-671 are involved in the responses of ICl,swell to stimulation of PKA and PKC. These results support the notion that MDR1 phosphorylation affects ICl,swell.

A serine residue in ClC-3 links phosphorylation-dephosphorylation to chloride channel regulation by cell volume

In many mammalian cells, ClC-3 volume-regulated chloride channels maintain a variety of normal cellular functions during osmotic perturbation. The molecular mechanisms of channel regulation by cell volume, however, are unknown. Since a number of recent studies point to the involvement of protein phosphorylation/dephosphorylation in the control of volume-regulated ionic transport systems, we studied the relationship between channel phosphorylation and volume regulation of ClC-3 channels using site-directed mutagenesis and patch-clamp techniques. In native cardiac cells and when overexpressed in NIH/3T3 cells, ClC-3 channels were opened by cell swelling or inhibition of endogenous PKC, but closed by PKC activation, phosphatase inhibition, or elevation of intracellular Ca2+. Site-specific mutational studies indicate that a serine residue (serine51) within a consensus PKC-phosphorylation site in the intracellular amino terminus of the ClC-3 channel protein represents an important volume sensor of the channel. These results provide direct molecular and pharmacological evidence indicating that channel phosphorylation/dephosphorylation plays a crucial role in the regulation of volume sensitivity of recombinant ClC-3 channels and their native counterpart, ICl.vol.