Protein kinase C-mediated phosphorylation of the human multidrug resistance P-glycoprotein regulates cell volume-activated chloride channels

ABC-ATPases, adaptable energy generators fuelling transmembrane movement of a variety of molecules in organisms from bacteria to humans

The approximately 27 kDa ABC-ATPase, an extraordinarily conserved, unique type of ATPase, acts as a machine to fuel the movement across membranes of almost any type of molecule, from large polypeptides to small ions, via many different membrane-spanning proteins. A particular ABC-ATPase must therefore be tailor-made to function in a complex with its cognate membrane protein, forming a transport pathway appropriate for a specific type of molecule, or in the case of some ABC-transporters, several types of molecule. Molecules to be transported recognise their own transporter, bind and switch on the ATPase, which in turn activates or opens the transport pathway. ABC-dependent transport can be inwards across the membrane, or outwards to the cell exterior, and the ABC-ATPase can fuel transport through pathways which may involve a classical channel (CFTR), a "gateway" mechanism through a proteinacious chamber spanning the bilayer, or conceivably via a pathway at the protein-lipid interface of the outside of the membrane domain. This may be the case for drugs transported by Pgp, a multidrug resistance transporter. In this review, we try to identify the common fundamental principles which unite all ABC-transporters, including the basis of specificity for different transported compounds (allocrites), the interactions between the ATPase and membrane domains, activation of the ATPase and the coupling of consequent conformational changes, to the final movement of an allocrite through a given transport pathway. We discuss the so far limited structural information for the intact ABC-transporter complex and the exciting information from the first crystal structure of an ABC-ATPase. Finally, the action of specific transporters, CFTR (Cl- transport), Pgp, MRP and LmrA, all transporting many different drug molecules and HlyB transporting a large protein toxin are discussed.

Hyposmotically induced amino acid release from the rat cerebral cortex: role of phospholipases and protein kinases

In an evaluation of the contribution of swelling-induced amino acid release, through the regulatory volume decrease (RVD) process, to cerebral ischemic injury, studies of the role of phospholipases and protein kinases in the response to hyposmotic stress were undertaken using an in vivo rat cortical cup model. Hyposmotic stress induced significant releases of aspartate, glutamate, glycine, phosphoethanolamine, taurine and GABA from the rat cerebral cortex. Taurine release was most affected, exhibiting a greater than 9-fold increase during the hyposmotic stimulus. The phospholipase A2 (PLA2) inhibitors 4-bromophenacyl bromide (1 microM) and 7,7-dimethyleicosadienoic acid (5 microM) had no significant effects on hyposmotically induced amino acid release. AACOCF3 (50 microM), an inhibitor of cytosolic PLA2 decreased taurine release to 84% of DMSO controls. The release of the other amino acids was not affected. The phospholipase C inhibitor U73122 (5 microM) had no significant effects on amino acid release. The protein kinase C (PKC) inhibitor chelerythrine (5 microM) significantly reduced hyposmotically induced taurine release to 72% of saline controls but had no significant effects on the other amino acids. Stimulation of PKC with phorbol 12-myristate, 13-acetate (10 microM) did not significantly change taurine, glutamate, glycine or phosphethanolamine release. The releases of aspartate and GABA were enhanced 2 to 3 fold. Phorbol 12,13-didecanoate (10 microM), another potent stimulator of PKC, significantly increased taurine release to 122% of DMSO controls. The releases of aspartate, glutamate and glycine were enhanced 2.5 to 3.5 fold. Similarly, stimulation of protein kinase A with forskolin (100 microM) significantly increased taurine, aspartate, and glycine release 1.5- to 2-fold compared to DMSO controls. In summary, phospholipases may play a minor role in volume regulation. These studies also support the hypothesis that protein kinases play a modulatory role in the RVD response. The results show that although RVD may play a role, additional mechanisms, including phospholipase activation, must be involved in the ischemia-evoked release of excitotoxic amino acids.

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.

Regulation of Hsp27 oligomerization, chaperone function, and protective activity against oxidative stress/tumor necrosis factor alpha by phosphorylation

The small heat shock proteins (sHsps) from human (Hsp27) and mouse (Hsp25) form large oligomers which can act as molecular chaperones in vitro and protect cells from heat shock and oxidative stress when overexpressed. In addition, mammalian sHsps are rapidly phosphorylated by MAPKAP kinase 2/3 at two or three serine residues in response to various extracellular stresses. Here we analyze the effect of sHsp phosphorylation on its quaternary structure, chaperone function, and protection against oxidative stress. We show that in vitro phosphorylation of recombinant sHsp as well as molecular mimicry of Hsp27 phosphorylation lead to a significant decrease of the oligomeric size. We demonstrate that both phosphorylated sHsps and the triple mutant Hsp27-S15D,S78D,S82D show significantly decreased abilities to act as molecular chaperones suppressing thermal denaturation and facilitating refolding of citrate synthase in vitro. In parallel, Hsp27 and its mutants were analyzed for their ability to confer resistance against oxidative stress when overexpressed in L929 and 13.S.1.24 cells. While wild type Hsp27 confers resistance, the triple mutant S15D,S78D,S82D cannot protect against oxidative stress effectively. These data indicate that large oligomers of sHsps are necessary for chaperone action and resistance against oxidative stress whereas phosphorylation down-regulates these activities by dissociation of sHsp complexes to tetramers.

Intracellular pH and multidrug resistance regulate complement-mediated cytotoxicity of nucleated human cells

In previous work (Weisburg, J. H., Curcio, M., Caron, P. C., Raghi, G., Mechetner, E. B., Roepe, P. D., and Scheinberg, D. A. (1996) J. Exp. Med. 183, 2699-2704), we showed that multidrug resistance (MDR) cells created by continuous selection with the vinca alkaloid vincristine (HL60 RV+) or by retroviral infection (K562/human MDR 1 cells) exhibited significant resistance to complement-mediated cytotoxicity (CMC). This resistance was due to the presence of overexpressed P-glycoprotein (P-GP). In this paper, we probe the molecular mechanism of this phenomenon. We test whether the significant elevated intracellular pH (pHi) that accompanies P-GP overexpression is sufficient to confer resistance to CMC and whether this resistance is related to effects on complement function in the cell membrane. Control HL60 cells not expressing P-GP, but comparably elevated in cytosolic pHi by two independent methods (CO2 "conditioning" or isotonic Cl- substitution), are tested for CMC using two different antibody-antigen systems (human IgG and murine IgM; protein and carbohydrate) and two complement sources (rabbit and human). Elevation of pHi by either of these methods or by expression of P-GP confers resistance to CMC. Resistance is not observed when the alkalinization mediated by reverse Cl-/HCO3- exchange upon Cl- substitution is blocked by treatment with dihydro-4,4'-diisothiocyanostilbene-2,2'-disulfonate. Continuous photometric monitoring of 2',7'-bis(carboxyethyl)-5, 6-carboxyfluorescein (BCECF), to assess changes in pHi or efflux of the probe through MAC pores, in single cells or cell populations, respectively, verifies changes in pHi upon CO2 conditioning and Cl- substitution and release of BCECF upon formation of MAC pores. Antibody binding and internalization kinetics are similar in both the parental and resistant cell lines as measured by radioimmunoassay, but flow cytometric data showed that net complement deposition in the cell membrane is both delayed and reduced in magnitude in the MDR cells and in the cells with increased pHi. This interpretation is supported by comparison of BCECF release data for the different cells. Dual isotopic labeling of key complement components shows no significant change in molecular stoichiometry of the MACs formed at different pHi. The results are relevant to understanding clinical implications of MDR, the physiology of P-GP, and the biochemistry of the complement cascade and further suggest that the "drug pump" model of P-GP action cannot account for all of its effects.

ATP-sensitive potassium channels: a model of heteromultimeric potassium channel/receptor assemblies

ATP-sensitive K+ channels (KATP channels) play important roles in many cellular functions by coupling cell metabolism to electrical activity. By cloning members of the novel inwardly rectifying K+ channel subfamily Kir6.0 (Kir6.1 and Kir6.2) and the receptors for sulfonylureas (SUR1 and SUR2), researchers have clarified the molecular structure of KATP channels. KATP channels comprise two subunits: a Kir6.0 subfamily subunit, which is a member of the inwardly rectifying K+ channel family; and a SUR subunit, which is a member of the ATP-binding cassette (ABC) protein superfamily. KATP channels are the first example of a heteromultimeric complex assembled with a K+ channel and a receptor that are structurally unrelated to each other. Since 1995, molecular biological and molecular genetic studies of KATP channels have provided insights into the structure-function relationships, molecular regulation, and pathophysiological roles of KATP channels.

Characterization of intracellular pH gradients in human multidrug-resistant tumor cells by means of scanning microspectrofluorometry and dual-emission-ratio probes

Multidrug-resistant cells are believed to contain a plasma-membrane-efflux pump which is hypothesized to expel anticancer drugs from the cytosol to the cell exterior. Many of these drugs are classified as weak bases whose binding to intracellular targets is pH-dependent. Slight alterations in intracellular pH gradients have been shown to affect accumulation, endocytosis and secretion of drugs. In this study, we developed a new method based on confocal spectral imaging analysis to determine intracellular pH gradients in sensitive and MDR tumor cells. Fluorescein isothiocyanate (FITC) and tetramethylrhodamine conjugated to dextran (FRD) and SNAFL-calcein-AM were used to determine pH in acidic compartments. Carboxy-SNARF1-AM was used to examine cytosolic pH. We observed that sensitive (HL60, K562, CEM and MCF7) cells exhibit lower acidity of the subcellular organelles than that corresponding to drug-resistant derivatives. Moreover, results obtained with carboxy-SNARF1-AM show that resistant cells display a more alkaline cytosolic pH. This results in a considerably larger pH gradient between the vesicular compartments and the cytosol of resistant cells than of sensitive cells. The lower pH gradient observed in sensitive cells may be related to a disruption in the organization of the trans-Golgi network (TGN). In drug-resistant cells, the organization of TGN appears compact. In addition, confocal microscopic analysis of cells labelled with FRD and SNAFL-calcein showed that sensitive cells contain a lower number of acidified vesicles. This suggest a diminished capacity of these cells to remove protonated drugs from the cytoplasm to secretory compartments followed by their secretion through the activity of the secretory and recycling pathways.

Evidence for altered ion transport in Saccharomyces cerevisiae overexpressing human MDR 1 protein

Recently [Hoffman, M. M., and Roepe, P. D. (1997) Biochemistry 36, 11153-11168] we presented evidence for a novel Na+- and Cl--dependent H+ transport process in LR73/hu MDR 1 CHO transfectants that likely explains pHi, volume, and membrane potential changes in eukaryotic cells overexpressing the hu MDR 1 protein. To further explore this process, we have overexpressed human MDR 1 protein in yeast strain 9.3 following a combination of approaches used previously [Kuchler, K., and Thorner, J. (1992) Proc. Natl. Acad. Sci. U.S.A. 89, 2302-2306; Ruetz, S., et al. (1993) Proc. Natl. Acad. Sci. U.S.A. 90, 11588-11592]. Thus, a truncated hu MDR 1 cDNA was cloned behind a tandem array of sterile 6 (Ste6) and alchohol dehydrogenase (Adh) promoters to create the yeast expression vector pFF1. Valinomycin resistance of intact cells and Western blot analysis with purified yeast plasma membranes confirmed the overexpression of full length, functional, and properly localized hu MDR 1 protein in independently isolated 9.3/pFF1 colonies. Interestingly, relative valinomycin resistance and growth of the 9.3/hu MDR 1 strains are found to strongly depend on the ionic composition of the growth medium. Atomic absorption reveals significant differences in intracellular K+ for 9.3/hu MDR 1 versus control yeast. Transport assays using [3H]tetraphenylphosphonium ([3H]TPP+) reveal perturbations in membrane potential for 9.3/hu MDR 1 yeast that are stimulated by KCl and alkaline pHex. ATPase activity of purified plasma membrane fractions from yeast strains and LR73/hu MDR 1 CHO transfectants constructed previously [Hoffman, M. M., et al. (1996) J. Gen. Physiol. 108, 295-313] was compared. MDR 1 ATPase activity exhibits a higher pH optimum and different salt dependencies, relative to yeast H+ ATPase. Inside-out plasma membrane vesicles (ISOV) fabricated from 9.3/hu MDR 1 and control strains were analyzed for formation of H+ gradients +/- verapamil. Similar pharmacologic profiles are found for verapamil stimulation of MDR 1 ATPase activity and H+ pumping in 9.3/hu MDR 1 ISOV. In sum, these experiments strongly support the notion that hu MDR 1 catalyzes H+ transport in some fashion and lowers membrane potential in yeast when K+ contributes strongly to that potential. In the accompanying paper [Santai, C. T., Fritz, F., and Roepe, P. D. (1999) Biochemistry 38, XXXX-XXXX] the effects of ion gradients on H+ transport by hu MDR 1 are examined.

Expression and purification of the first nucleotide-binding domain and linker region of human multidrug resistance gene product: comparison of fusions to glutathione S-transferase, thioredoxin and maltose-binding protein

Many membrane proteins that belong to the ATP-binding cassette (ABC) superfamily are clinically important, including the cystic fibrosis transmembrane conductance regulator, the sulphonylurea receptor and P-glycoprotein (multidrug resistance gene product; MDR1). These proteins contain two multispanning transmembrane domains, each followed by one nucleotide-binding domain (NBD) and a linker region distal to the first NBD. ATP hydrolysis by the NBDs is critical for ABC protein function; the linker region seems to have a regulatory role. Previous attempts to express soluble NBDs and/or linker regions without detergent solubilization, or to purify NBDs at high yields as soluble fusion proteins, have been unsuccessful. Here we present a system for the expression in Escherichia coli of the first NBD of MDR1 followed by its linker region (NBD1MLD). A comparison of the expressions of NBD1MLD fused to glutathione S-transferase, thioredoxin and maltose-binding protein (MBP) shows that a high level of expression in the soluble fraction (approx. 8% of total E. coli protein) can be achieved only for MBP-NBD1MLD. The addition of a proteolytic thrombin site just proximal to the N-terminal end of NBD1MLD allows the cleavage of NBD1MLD from MBP, which can be easily purified with retention of its ATPase activity. In summary, success was obtained only when using an MBP fusion protein vector containing a thrombin proteolytic site between MBP and NBD1MLD. The approach described here could be generally applicable to solving the problems of expression and purification of NBDs/linker regions of ABC proteins.

Are ion-exchange processes central to understanding drug-resistance phenomena?

Drug resistance in malarial parasites is arguably the greatest challenge currently facing infectious disease research. In addressing this problem, researchers have been intrigued by similarities between drug-resistant malarial parasites and tumour cells. For example, it was originally thought that the role of pfMDR (Plasmodium falciparum multidrug resistance) proteins was central in conferring antimalarial multidrug resistance. However, recent work has questioned the precise role of MDR proteins in multidrug resistance. In addition, recent ground-breaking work in identifying mutations associated with antimalarial drug resistance might have led to identification of yet another parallel between drug-resistant tumour cells and malarial parasites, namely, intriguing alterations in transmembrane ion transport, discussed here by Paul Roepe and James Martiney. This further underscores an emerging paradigm in drug-resistance research.

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.

Moonlighting proteins

The idea of one gene--one protein--one function has become too simple because increasing numbers of proteins are found to have two or more different functions. The multiple functions of such moonlighting proteins add another dimension to cellular complexity and benefit cells in several ways. However, cells have had to develop sophisticated mechanisms for switching between the distinct functions of these proteins.

Control of CFTR channel gating by phosphorylation and nucleotide hydrolysis

Control of CTFR Channel Gating by Phosphorylation and Nucleotide Hydrolysis. Physiol. Rev. 79, Suppl.: S77-S107, 1999. - The cystic fibrosis transmembrane conductance regulator (CFTR) Cl- channel is the protein product of the gene defective in cystic fibrosis, the most common lethal genetic disease among Caucasians. Unlike any other known ion channel, CFTR belongs to the ATP-binding cassette superfamily of transporters and, like all other family members, CFTR includes two cytoplasmic nucleotide-binding domains (NBDs), both of which bind and hydrolyze ATP. It appears that in a single open-close gating cycle, an individual CFTR channel hydrolyzes one ATP molecule at the NH2-terminal NBD to open the channel, and then binds and hydrolyzes a second ATP molecule at the COOH-terminal NBD to close the channel. This complex coordinated behavior of the two NBDs is orchestrated by multiple protein kinase A-dependent phosphorylation events, at least some of which occur within the third large cytoplasmic domain, called the regulatory domain. Two or more kinds of protein phosphatases selectively dephosphorylate distinct sites. Under appropriately controlled conditions of progressive phosphorylation or dephosphorylation, three functionally different phosphoforms of a single CFTR channel can be distinguished on the basis of channel opening and closing kinetics. Recording single CFTR channel currents affords an unprecedented opportunity to reproducibly examine, and manipulate, individual ATP hydrolysis cycles in a single molecule, in its natural environment, in real time.

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.

Role of organelle pH in tumor cell biology and drug resistance

Multidrug resistance is a generic term for the variety of strategies that tumor cells develop to evade the cytotoxic effects of anticancer drugs. It is characterized by decreased cellular sensitivity, not only to the drug(s) employed in chemotherapy but also to a broad spectrum of drugs with neither obvious common targets nor structural homology. It is one of the major obstacles to the successful treatment of tumors. This review concentrates on some of the physiological changes observed in drug-sensitive and drug-resistant tumor cell lines that could account for their relative sensitivities to chemotherapeutics. These changes suggest alternative strategies for combating tumor cells in general and multidrug-resistant cells in particular.

ATP-sensitive potassium channels: structures, functions, and pathophysiology

ATP-sensitive potassium channels (KATP channels) play important roles in various tissues by coupling cell metabolic status to electrical activity. Recently, molecular biological and electrophysiological techniques have revealed the molecular basis of the KATP channels to be a complex of the Kir6.0 subunit, a member of the inwardly rectifying K+ channel subfamily Kir6.0, and the sulfonylurea receptor (SUR) subunit, a member of ATP-binding cassette (ABC) superfamily; the functional diversity of the various KATP channels is being determined by a combination of the Kir6.0 subunit (Kir6.1 or Kir6.2) and the SUR subunit (SUR1 or SUR2) comprising it. Recent studies of the KATP channels have suggested mechanisms of KATP channel regulation and pathophysiology and also a new model in which ABC proteins regulate the functional expression of ion channels.

Involvement of PKC-alpha in regulatory volume decrease responses and activation of volume-sensitive chloride channels in human cervical cancer HT-3 cells

1. The present study was carried out to identify the specific protein kinase C (PKC) isoform involved in regulatory volume decrease (RVD) responses, and to investigate the signal transduction pathways underlying the activation of volume-sensitive chloride channels in human cervical cancer HT-3 cells. The role of Ca2+ in RVD and in the activation of chloride currents was also studied. 2. The time course of RVDs was prolonged by microinjection of PKC-alpha antibody but not by PKC-beta or PKC-gamma antibody, and also by exposure to Ca2+-free medium, in particular when combined with microinjection of EDTA. Immunofluorescence staining showed that hypotonic superfusion evoked the translocation of PKC-alpha to the cell membrane, whereas PKC-beta or PKC-gamma remained unaffected. The translocation of PKC-alpha was observed a few minutes after hypotonic stress, reaching peak intensity at 30 min, and returned to the cytoplasm 60 min after hypotonic exposure. Western blot analyses showed an increased PKC-alpha level in terms of intensity and phosphorylation in the cell membrane, while neither PKC-beta nor PKC-gamma was activated upon hyposmotic challenge. 3. Whole-cell patch-clamp studies demonstrated that neomycin and PKC blockers such as staurosporine and H7 inhibited volume-sensitive chloride currents. The inhibitory effect of neomycin on chloride currents can be reversed by the PKC activator phorbol 12-myristate, 13-acetate (PMA). Moreover, the PKC inhibitor and PKC-alpha antibody, but not PKC-beta or PKC-gamma antibody, significantly attenuated the chloride currents. The activation of volume-sensitive chloride currents were insensitive to the changes of intracellular Ca2+ but required the presence of extracellular Ca2+. 4. Our results suggest the involvement of PKC-alpha and extracellular Ca2+ in RVD responses and the activation of volume-sensitive chloride channels in HT-3 cells.

Activation of protein kinase Calpha couples cell volume to membrane Cl- permeability in HTC hepatoma and Mz-ChA-1 cholangiocarcinoma cells

Physiological increases in liver cell volume lead to an adaptive response that includes opening of membrane Cl- channels, which is critical for volume recovery. The purpose of these studies was to assess the potential role for protein kinase C (PKC) as a signal involved in cell volume homeostasis. Studies were performed in HTC rat hepatoma and Mz-ChA-1 human cholangiocarcinoma cells, which were used as model hepatocytes and cholangiocytes, respectively. In each cell type, cell volume increases were followed by: 1) translocation of PKC from cytosolic to particulate (membrane) fractions; 2) a 10- to 40-fold increase in whole-cell membrane Cl- current density; and 3) partial recovery of cell volume. In HTC cells, the volume-dependent Cl- current response (-46 +/- 5 pA/pF) was inhibited by down-regulation of PKC (100 nmol/L phorbol 12-myristate 13-acetate for 18 hours [PMA]; -1.97 +/- 1.5 pA/pF), chelation of cytosolic Ca2+ (2 mmol/L EGTA; -5.3 +/- 4.0 pA/pF), depletion of cytosolic adenosine triphosphate (ATP) (3 U/mL apyrase; -12.58 +/- 1. 45 pA/pF), and by the putative PKC inhibitor, chelerythrine (25 micromol/L; -7 +/- 3 pA/pF). In addition, PKC inhibition by chelerythrine and calphostin C (500 nmol/L) prevented cell volume recovery from swelling. Similar results were obtained in Mz-ChA-1 biliary cells. These findings indicate that swelling-induced activation of PKC represents an important signal coupling cell volume to membrane Cl- permeability in both hepatic and biliary cell models.

Purification and reconstitution of human P-glycoprotein

Human Pgp from the vinblastine-resistant cell line, KB-V1, can be purified by sequential conventional chromatography on DEAE-sepharose CL-6B resin followed by a wheat germ agglutinin column. By including glycerol (osmolyte protectant) and lipid during the solubilization and chromatography procedures most of the biological activity of Pgp can be retained. The activity of Pgp in the detergent extract or in the concentrated column fractions is stable for at least 8-10 months when stored at -80 degrees. However, repeated cycles of freezing and thawing of fractions result in considerable loss of activity. We have purified Pgp from KB-C1 (a subclone of KB 3-1 that is resistant to 1 microgram/ml colchicine) by following the same protocol. When this method was used for purification of Pgp from MDR1-transfected NIH 3T3 transfectants (N3-V2400, grown in the presence of 2.4 micrograms/ml vinblastine), the protein was eluted with 0.1 M NaCl from the DEAE-Sepharose CL-6B column as usual. However, during WGA lectin chromatography, the protein was eluted with a lower concentration of sugar (0.1 M instead of 0.25 M NAG). This altered elution pattern appears to be due to a difference in the glycosylation of human Pgp in mouse NIH 3T3 cells. This is consistent with the observation that human Pgp expressed in NIH 3T3 cells migrates faster compared to the protein from KB-V1 cells on 8-10% acrylamide gel. Similarly, other workers have purified Chinese hamster Pgp either by a single-step chromatography on Reactive Red 120 agarose or by a combination of anion exchange and immunoaffinity chromatography (see the article by Senior et al. for the purification and properties of ATPase activity of Chinese hamster Pgp). The high level of drug-stimulated ATP hydrolysis by Pgp (Table I), like other ion-transporting ATPases, indicates that this is a high-capacity pump that can function as an effective multidrug transporter. This is further supported by the qualitative demonstration of ATP-dependent vinblastine transport in proteoliposomes reconstituted with pure Pgp (see Fig. 2). Thus, these experiments provide strong evidence that purified Pgp retains its activity and that it functions as an ATP-dependent drug transporter.

FROM VACUOLAR GS-X PUMPS TO MULTISPECIFIC ABC TRANSPORTERS

While the concept of H+-coupling has dominated studies of energy-dependent organic solute transport in plants for over two decades, recent studies have demonstrated the existence of a group of organic solute transporters, belonging to the ATP-binding cassette (ABC) superfamily, that are directly energized by MgATP rather than by a transmembrane H+-electrochemical potential difference. Originally identified in microbial and animal cells, the ABC superfamily is one of the largest and most widespread protein families known. Competent in the transport of a broad range of substances including sugars, peptides, alkaloids, inorganic anions, and lipids, all ABC transporters are constituted of one or two copies each of an integral membrane sector and cytosolically oriented ATP-binding domain. To date, two major subclasses, the multidrug resistance-associated proteins (MRPs) and multidrug resistance proteins (MDRs) (so named because of the phenotypes conferred by their animal prototypes), have been identified molecularly in plants. However, only the MRPs have been defined functionally. This review therefore focuses on the functional capabilities, energetics, organization, and regulation of the plant MRPs. Otherwise known as GS-X pumps, or glutathione-conjugate or multispecific organic anion Mg2+-ATPases, the MRPs are considered to participate in the transport of exogenous and endogenous amphipathic anions and glutathionated compounds from the cytosol into the vacuole. Encoded by a multigene family and possessing a unique domain organization, the types of processes that likely converge and depend on plant MRPs include herbicide detoxification, cell pigmentation, the alleviation of oxidative damage, and the storage of antimicrobial compounds. Additional functional capabilities might include channel regulation or activity, and/or the transport of heavy metal chelates. The identification of the MRPs, in particular, and the demonstration of a central role for ABC transporters, in general, in plant function not only provide fresh insights into the molecular basis of energy-dependent solute transport but also offer the prospect for manipulating and investigating many fundamental processes that have hitherto evaded analysis at the transport level.

Defective acidification in human breast tumor cells and implications for chemotherapy

Multidrug resistance (MDR) is a significant problem in the treatment of cancer. Chemotherapeutic drugs distribute through the cyto- and nucleoplasm of drug-sensitive cells but are excluded from the nucleus in drug-resistant cells, concentrating in cytoplasmic organelles. Weak base chemotherapeutic drugs (e.g., anthracyclines and vinca alkaloids) should concentrate in acidic organelles. This report presents a quantification of the pH for identified compartments of the MCF-7 human breast tumor cell line and demonstrates that (a) the chemotherapeutic Adriamycin concentrates in acidified organelles of drug-resistant but not drug-sensitive cells; (b) the lysosomes and recycling endosomes are not acidified in drug-sensitive cells; (c) the cytosol of drug-sensitive cells is 0.4 pH units more acidic than the cytosol of resistant cells; and (d) disrupting the acidification of the organelles of resistant cells with monensin, bafilomycin A1, or concanamycin A is sufficient to change the Adriamycin distribution to that found in drug-sensitive cells, rendering the cell vulnerable once again to chemotherapy. These results suggest that acidification of organelles is causally related to drug resistance and is consistent with the hypothesis that sequestration of drugs in acidic organelles and subsequent extrusion from the cell through the secretory pathways contribute to chemotherapeutic resistance.

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

1. Whole-cell, swelling-activated Cl- currents, ICl(swell), were characterized in Chinese hamster ovary (CHO) cells and found to exhibit time-dependent inactivation at depolarizing potentials, tamoxifen and dideoxyforskolin sensitivity, and an anion permeability sequence: SCN- > I- > Br- > Cl- > F- > gluconate-. 2. CHO cells permanently transfected with either the human MDR1 or mouse mdr1a cDNAs demonstrated an increased rate of activation of ICl(swell) compared with parental cells or those permanently transfected with the mouse mdr1b cDNA. However, no differences in the magnitude of the currents were observed at steady state. 3. Pretreatment with the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA) did not affect ICl(swell) in MDR1 or mdr1a permanently transfected CHO cells. In contrast, pretreatment with TPA reduced ICl(swell) in MDR1(G185V)-expressing transfected NIH3T3 fibroblasts. Subsequently, the CHO cell lines were shown to contain significantly reduced levels of protein kinase C (PKC), suggesting that PKC concentrations might be limiting in these cell lines, at least under whole-cell patch clamp conditions. 4. Addition of purified PKC to the pipette solution, followed by a pretreatment with TPA, reduced the rate of ICl(swell) activation in human Pgp- and mouse Pgp1a-expressing CHO cells to the levels observed in parental and mouse Pgp1b-expressing cells. This confirms that PKC is limiting in these cells under whole-cell, patch clamp conditions. Furthermore, these results suggest that PKC-mediated phosphorylation of human Pgp and mouse Pgp1a disengages the influence which these Pgps have on ICl(swell). 5. These studies also demonstrate a functional distinction between the two mouse homologues, Pgp1a and Pgp1b. Although both can function as drug efflux pumps, only Pgp1a can act like human Pgp to influence ICl(swell).

Leishmania sp.: growth and survival are impaired by ion channel blockers

In the present work we examined the effect of ion transport blockers on the growth and viability of Leishmania sp. and on the infection of macrophages by the parasite. 4-aminopyridine and glibenclamide block voltage-dependent and K+ ATP channels, respectively; amiloride is used to detect Na+ channels and Na+/H+ antiporters; and anthracene-9-carboxylic acid affects chloride channels. The EC50 for promastigote cultures of three strains of the Leishmania subgenus, namely, Leishmania (Leishmania) NR, Leishmania (Leishmania) amazonensis LTB0016, and Leishmania (Leishmania) major, at their stationary phase of growth, were, respectively, 39, 46, and 464 microM for 4-aminopyridine; 7, 0.8, and 10 microM for glibenclamide and 66, 170, and 10 microM for anthracene-9-carboxylic acid. The amiloride EC50 for NR was 264 microM and 10 microM for L. (L.) major, but was never reached for LTB0016. Higher concentrations of the drugs impaired the exponential growth of Leishmania promastigotes. These results suggest the susceptibility of Leishmania sp. to blockers associated with K+ and Cl- and to Na+ or Na+/H+ transport systems. Blockade of such systems might have impaired the survival of the parasites as promastigotes. In addition, it affected the persistence of parasites in host cells. Although the infection of the macrophage cell line J774 and peritoneal-exudate macrophages was not significantly decreased by concentrations of the drugs around the promastigotes' EC50, the survival of intracellular parasites decreased significantly in the presence of these drugs without affecting the viability of the macrophages. Some blockers consistently gave small EC50 and significantly decreased the infection process as well as the survival of intracellular parasites. Thus, elucidation of their mechanism of action in Leishmania is relevant, since they could represent a potential subject for the development of leishmanicidal drugs.

Functional significance of cell volume regulatory mechanisms

To survive, cells have to avoid excessive alterations of cell volume that jeopardize structural integrity and constancy of intracellular milieu. The function of cellular proteins seems specifically sensitive to dilution and concentration, determining the extent of macromolecular crowding. Even at constant extracellular osmolarity, volume constancy of any mammalian cell is permanently challenged by transport of osmotically active substances across the cell membrane and formation or disappearance of cellular osmolarity by metabolism. Thus cell volume constancy requires the continued operation of cell volume regulatory mechanisms, including ion transport across the cell membrane as well as accumulation or disposal of organic osmolytes and metabolites. The various cell volume regulatory mechanisms are triggered by a multitude of intracellular signaling events including alterations of cell membrane potential and of intracellular ion composition, various second messenger cascades, phosphorylation of diverse target proteins, and altered gene expression. Hormones and mediators have been shown to exploit the volume regulatory machinery to exert their effects. Thus cell volume may be considered a second message in the transmission of hormonal signals. Accordingly, alterations of cell volume and volume regulatory mechanisms participate in a wide variety of cellular functions including epithelial transport, metabolism, excitation, hormone release, migration, cell proliferation, and cell death.

A comparison of the phenomenology and genetics of multidrug resistance in cancer cells and quinoline resistance in Plasmodium falciparum

Plasmodium falciparum is the causative agent of the most deadly form of human malaria. Chemotherapy traditionally has been the main line of defense against this parasite, and chloroquine, the drug of choice, has been one of the most successful drugs ever developed. Unfortunately, the evolution and spread of resistance to chloroquine and other quinoline-containing drugs means that these compounds are now virtually useless in many endemic areas. Future prospects for the use of quinoline compounds improved considerably when it was demonstrated that chloroquine resistance could be circumvented in vitro by a number of structurally and functionally unrelated compounds such as verapamil and desipramine. The phenomenon of resistance reversal by compounds such as verapamil is also a key feature of drug resistance in mammalian cells, and this has raised the possibility that the underlying mechanisms of drug resistance of the two cell types could be similar. This hypothesis has prompted a large number of studies into the genetics and biochemistry of resistance to quinoline-containing drugs in P. falciparum. Both the genetic and the biochemical studies have raised issues of controversy and stimulated much debate. These issues are discussed in this review, in the context of a comparison with the genetics and biochemistry of multidrug resistance in mammalian cells.

Molecular identification of a volume-regulated chloride channel

A volume-regulated chloride current (ICl.vol) is ubiquitously present in mammalian cells, and is required for the regulation of electrical activity, cell volume, intracellular pH, immunological responses, cell proliferation and differentiation. However, the molecule responsible for ICl.vol has yet to be determined. Although three putative chloride channel proteins expressed from cloned genes (P-glycoprotein, pICln and ClC-2 ) have been proposed to be the molecular equivalent of ICl.vol, neither P-glycoprotein nor pICln is thought to be a chloride channel or part thereof, and the properties of expressed ClC-2 channels differ from native ICl.vol. Here we report that functional expression in NIH/3T3 cells of a cardiac clone of another member of the ClC family, ClC-3, results in a large basally active chloride conductance, which is strongly modulated by cell volume and exhibits many properties identical to those of ICl.vol in native cells. A mutation of asparagine to lysine at position 579 at the end of the transmembrane domains of ClC-3 abolishes the outward rectification and changes the anion selectivity from I- > Cl- to Cl- > I- but leaves swelling activation intact. Because ClC-3 is a channel protein belonging to a large gene family of chloride channels, these results indicate that ClC-3 encodes ICl.vol in many native mammalian cells.

Cl- and K+ conductances activated by cell swelling in primary cultures of rabbit distal bright convoluted tubules

Ionic currents induced by cell swelling were characterized in primary cultures of rabbit distal bright convoluted tubule (DCTb) by the whole cell patch-clamp technique. Cl- currents were produced spontaneously by whole cell recording with an isotonic pipette solution or by exposure to a hypotonic stress. Initial Cl- currents exhibited outwardly rectifying current-voltage relationship, whereas steady-state currents showed strong decay with depolarizing pulses. The ion selectivity sequence was I- = Br- > Cl- >> glutamate. Currents were inhibited by 0.1 mM 5-nitro-2-(3-phenylpropylamino) benzoic acid and 1 mM 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid and strongly blocked by 1 mM diphenylamine-2-carboxylate. Currents were insensitive to intracellular Ca2+ but required the presence of extracellular Ca2+. They were not activated in cells pretreated with 200 nM staurosporine, 50 microM LaCl3, 10 microM nifedipine, 100 microM verapamil, 5 microM tamoxifen, and 50 microM dideoxyforskolin. Staurosporine, tamoxifen, verapamil, or the absence of external Ca2+ was without effect on the fully developed Cl- currents. Osmotic shock also activated K+ currents in Cl- free conditions. These currents were time independent, activated at depolarized potentials, and inhibited by 5 mM BaCl2. The activation of Cl- and K+ currents by an osmotic shock may be implicated in regulatory volume decrease in DCTb cells.

Polymorphic expression of multidrug resistance mRNA in lung parenchyma of nonpregnant and pregnant rats: a comparison to cystic fibrosis mRNA expression

Multidrug resistance (MDR1b) and cystic fibrosis transmembrane conductance regulator (CFTR) proteins are members of the "ATP-binding cassette" superfamily of transporters. They are associated with chloride channel activities and ATP secretion and have complementary patterns of expression in several organs. In the rat uterus, CFTR expression is replaced by MDR1b expression during pregnancy. We have studied whether expression of MDR1b and CFTR also vary in the lung during pregnancy. No variations in MDR1b or CFTR mRNA levels during pregnancy were detected. However, there was an unusual degree of variation in MDR1b mRNA expression in lung parenchyma between animals in both the control group and the pregnant group. If present among humans, polymorphic expression of MDR1 in lung parenchyma may explain part of the differences in lung symptomatology observed in the CF patients carrying the same mutation.

Analysis of ion transport perturbations caused by hu MDR 1 protein overexpression

In previous work [Luz et al. (1994) Biochemistry 33, 7239-7249; Roepe et al. (1994) Biochemistry 33, 11008-11015] we measured changes in Cl- and HCO3--dependent pHi regulation for LR73 Chinese hamster ovary fibroblasts overexpressing mu MDR 1 protein. However, only one clonal cell line overexpressing the protein but not previously exposed to chemotherapeutic drug (i.e., a "true" transfectant) was examined, since very few MDR cell lines of this nature have been constructed. Recently [Hoffman et al. (1996) J. Gen. Physiol. 108, 295-313] we derived a series of true LR73/hu MDR 1 transfectants that are valuable for defining the MDR phenotype mediated by MDR protein alone, without the additional complexities introduced by exposing cells to chemotherapeutic drugs. Several independently derived clones from these and additional transfection experiments exhibit expression of MDR protein that is higher than that found in other true transfectants, and that is similar to the highest level of overexpression yet recorded for drug selected MDR cells. We examined altered Cl--dependent pHi regulation for these clones using improved single-cell photometry (SCP) techniques. Short-term isotonic Cl- substitution experiments performed in the presence of CO2/HCO3- reveal that mild overexpression of hu MDR 1 protein alters anion exchange (Cl-/HCO3- exchange or AE) for LR73 cells, as expected on the basis of previous work [Luz et al. (1994) Biochemistry 33, 7239-7249]. Interestingly, we now find that several independently selected high-level MDR 1 overexpressing clones acidify quite extensively upon isotonic exchange of Cl- and then rapidly alkalinize upon restoring normal [Cl-]. These data suggest that MDR protein may effectively compete against AE. The MDR protein effect is not dependent on HCO3-/CO2 or K+, is partially inhibited by verapamil, is completely inhibited by substituting K+ or N-methylglucamine (NMG+) for Na+ in the SCP perfusate but is not affected by 100 microM levels of amiloride, bumetanide, chlorothiazide, or stilbene. ATP depletion inhibits the MDR 1 effect. We are unable to restore normal AE activity for the MDR clones via manipulation of Cl- or HCO3- gradients. We thus suggest that MDR protein overexpression provides a novel Na+- and Cl--dependent pathway for transmembrane H+ transport. From analysis of ion dependency and inhibitor sensitivities, we conclude the transport is not via altered regulation of any known K+/H+, Na+/H+, or Cl-/HCO3- antiporters, Na+:K+:2Cl-, Na+:K+:2HCO3-, K+:HCO3-, or Na+:HCO3- co-transporters, or any combination of these. Thus, it appears to represent a novel ATP and Na+-dependent Cl-/H+ antiport process that (1) may be directly mediated by the MDR protein, (2) may represent the modulation of one or more currently unidentified ion transport proteins by MDR protein, (3) may be due to some combination of direct ion transport and regulation of ion transport, or (4) may represent unusual passive H+ movement in response to a novel Cl--dependent electrical perturbation that occurs during our Cl- substitution protocol. The results have important implications for understanding drug resistance mediated by MDR 1 overexpression, as well as the physiologic function of endogenously expressed MDR protein.

Phosphorylation site mutations in the human multidrug transporter modulate its drug-stimulated ATPase activity

In the human multidrug transporter (MDR1), three serine residues located in the "linker" region of the protein are targets of in vivo phosphorylation. These three serines, or all eight serines and threonines in the linker, were substituted by alanines (mutants 3A and 8A) or with glutamic acids (mutants 3E and 8E). The wild-type and mutant proteins were expressed in baculovirus-infected Spodoptera frugiperda (Sf9) ovarian insect cells, and the vanadate-sensitive, drug-stimulated ATPase activity was measured in isolated membrane preparations. The maximum drug-stimulated MDR1-ATPase activity was similar for the wild-type and the mutant proteins. However, wild-type MDR1, which is known to be phosphorylated in Sf9 membranes, and the 3E and 8E mutants, which mimic the charge of phosphorylation, achieved half-maximum activation of MDR1-ATPase activity at lower verapamil, vinblastine, or rhodamine 123 concentrations than the nonphosphorylatable 3A and 8A variants. For some other drugs (e.g. valinomycin or calcein acetoxymethylester) activation of the MDR1-ATPase for any of the mutants was indistinguishable from that of the wild-type protein. Kinetic analysis of the data obtained for the 3A and 8A MDR1 variants indicated the presence of more than one drug interaction site, exhibiting an apparent negative cooperativity. This phenomenon was not observed for the wild-type or the 3E and 8E MDR1 proteins. The dependence of the MDR1-ATPase activity on ATP concentration was identical in the wild-type and the mutant proteins, and Hill plots indicated the presence of more than one functional ATP-binding site. These results suggest that phosphorylation of the linker region modulates the interaction of certain drugs with MDR1, especially at low concentrations, although phosphorylation does not alter the maximum level of MDR1-ATPase activity or its dependence on ATP concentration.

Volume expansion-sensing outward-rectifier Cl- channel: fresh start to the molecular identity and volume sensor

The maintenance of a constant volume in the face of extracellular and intracellular osmotic perturbation is essential for the normal function and survival of animal cells. Osmotically swollen cells restore their volume, exhibiting a regulatory volume decrease by releasing intracellular K+, Cl-, organic solutes, and obligated water. In many cell types, the volume regulatory effluxes of Cl- and some organic osmolytes are known to be induced by swelling-induced activation of anion channels that are characterized by their moderate outward rectification, cytosolic ATP dependency, and intermediate unitary conductance (10-100 pS). Recently, simultaneous measurements of cell size by light microscopy and whole cell Cl- current have shown that the Cl- current density is proportionally increased with an increase in the outer surface area, which is mainly achieved through unfolding of membrane invaginations by volume expansion. Thus this anion channel can somehow sense volume expansion and can be called the volume expansion-sensing outwardly rectifying (VSOR) anion channel. Its molecular identity and activation mechanism are yet to be elucidated. Three cloned proteins, ClC-2, P-glycoprotein, and pIcln, have been proposed as candidates for the VSOR anion channel. The unitary conductance, voltage dependency, anion selectivity, pH dependency, and pharmacology of the VSOR anion channel are distinct from the ClC-2 Cl- channel, which is also known to be sensitive to volume changes. Recent patch-clamp studies in combination with molecular biological techniques have shown that P-glycoprotein is not itself the channel protein but is a regulator of its volume sensitivity. Although there is still debate about another candidate protein, pIcln, the most recent study has suggested that this is likely to be a regulator of some other distinct Cl- channel. Identification of the VSOR anion channel protein per se, its volume-sensing mechanism, and its accessory/regulatory proteins at the molecular level is currently a subject of utmost physiological importance.

Characterization of acidic vesicles in multidrug-resistant and sensitive cancer cells by acridine orange staining and confocal microspectrofluorometry

To study the pH gradient status through membranes of acidic vesicles, either in sensitive or in multidrug-resistant living cancer cells, we monitored the fluorescence-emission spectra of acridine orange. Successive stainings with a pH-sensitive dye and AO showed that low-pH organelles were stained red by AO. In these compartments, high AO concentrations are driven by the pH gradient through membrane vesicles. The resulting rise in the dye's oligomeric/monomeric ratio induced an increase in the red/green (655-nm/530-nm) emission intensity ratio. Therefore, the accumulation of AO in acidic organelles was appraised by determination of the contribution of the red emission intensity (R%) in each emission spectrum, using laser scanning confocal microspectrofluorometry. In vesicles of multidrug-resistant K562-R cells, R% is significantly higher (72 +/- 10%) than the value (48 +/- 8%) from K562-sensitive cells (p < 0.001). This result is interpreted as a more important accumulation of AO in acidic cytoplasmic structures of resistant cells, which induces a shift from AO monomers (green emission) to self-associated structures (red emission). Equilibration of the pH gradient through acidic organelles was performed by addition of weak bases and carboxylic ionophores. Ammonium chloride (0.1 mM), methylamine (0.1 mM), monensine (10 microM), or nigericine (0.3 microM) all suppressed the initial difference of local AO accumulation between both cell lines. These agents decreased the red emission intensity for the resistant cell line but not for the sensitive one. The same effects were induced by 50 microM verapamil, a pleiotropic drug-resistance modulator. Our data allow the hypothesis of a higher pH gradient through membranes of acidic organelles, which would be a potential mechanism of multidrug resistance via the sequestration of weak bases inside these organelles.

Hepatocellular ATP-binding cassette protein expression enhances ATP release and autocrine regulation of cell volume

In a model liver cell line, recovery from swelling is mediated by a sensitive autocrine pathway involving conductive release of ATP, P2 receptor stimulation, and opening of membrane Cl- channels (Wang, Y., Roman, R. M., Lidofsky, S. D., and Fitz, J. G. (1996) Proc. Natl. Acad. Sci. U. S. A. 93, 12020-12025). However, the mechanisms coupling changes in cell volume to ATP release are not known. Based on evidence that certain ATP-binding cassette (ABC) proteins may function as ATP channels or channel regulators, we evaluated the potential role of ABC proteins by comparing ATP release and volume regulation in rat HTC and HTC-R hepatoma cells, the latter of which overexpress Mdr proteins. In both cell types, Cl- current activation (ICl-swell) and volume recovery following swelling were dependent on conductive ATP efflux. The rate of volume recovery was approximately 6-fold faster in HTC-R cells compared with HTC cells. This effect is likely due to enhanced ABC protein-dependent ATP release since (i) ICl-swell and cell volume recovery were eliminated by inhibition of P-glycoprotein transport (20 microM verapamil and 15 microM cyclosporin A); (ii) swelling-induced Cl- current density was similar in both cell types (approximately -50 pA/pF; not significant); and (iii) ATP conductance measured by whole-cell techniques was increased approximately 3-fold in HTC-R cells compared with HTC cells. Moreover, HTC-R cells exhibited enhanced survival during hypotonic stress. By modulating ATP release, hepatic ABC proteins may play a key role in the cellular pathways coupling changes in cell volume to ion permeability and secretion.

Functional incorporation of P-glycoprotein into Xenopus oocyte plasma membrane fails to elicit a swelling-evoked conductance

Microinjecton of Xenopus oocytes with P-glycoprotein-containing membranes from multidrug resistant cells following a recently published procedure resulted in the transplantation of the protein to the plasma membrane of the oocytes and was confirmed by Western blot analysis. These oocytes showed a reduced intracellular accumulation of daunomycin, when compared to uninjected oocytes or to those injected with membrane vesicles lacking P-glycoprotein, thus indicating that the protein had been incorporated in a transport-competent form. On the other hand, transplantation of P-glycoprotein to the oocyte membrane did not significantly change either the appearance or the properties of swelling-elicited membrane conductance with respect to those determined in oocytes either uninjected or injected with membranes lacking P-glycoprotein. These results do not support a role for P-glycoprotein as a swelling-activated chloride channel.

P-glycoprotein is not a swelling-activated Cl- channel; possible role as a Cl- channel regulator

1. The whole-cell configuration of the patch-clamp technique was used to determine if P-glycoprotein (Pgp) is a swelling-activated Cl- channel. 2. Hamster pgp1 cDNA was transfected into a mouse fibroblast cell line resulting in expression of functional Pgp in the plasma membrane. This cell line was obtained without exposure to chemotherapeutic agents. 3. Swelling-activated whole-cell Cl- current (ICl,swell) was elicited by lowering the bath osmolality. ICl,swell was characterized in detail in the pgp1-transfected mouse cell line and compared with that of its parental cell line. Expression of Pgp did not modify the magnitude or properties of ICl,swell, except that addition of the anti-Pgp antibody C219 to the pipette solution inhibited this current by 75% only in the Pgp-expressing cells. 4. ICl,swell in the mouse Pgp-expressing cell line was compared with that in a Pgp-expressing hamster fibroblast cell line. The characteristics of ICl,swell (voltage dependence, blocker sensitivity, anion selectivity sequence, requirement for hydrolysable ATP) in Pgp-expressing cells were different between the two cell lines. These results suggest that the channel(s) responsible for ICl,swell are different between the two cell lines. In addition, C219 inhibited ICl,swell in both Pgp-expressing cell lines, even though they seem to express different swelling-activated Cl- channels. 5. We conclude that firstly, Pgp is not a swelling-activated Cl- channel; secondly, it possibly functions as a Cl- channel regulator; and thirdly, ICl,swell is underlined by different Cl- channels in different cells.

Protein kinase C-independent correlation between P-glycoprotein expression and volume sensitivity of Cl- channel

The possible correlation between P-glycoprotein (PGP) and volume-sensitive Cl- channel was examined in a pair of cell lines: a subline of the human epidermoid KB cell (KB-3-1) and the corresponding MDR1-transfected cell line (KB-G2). Western blot analysis and indirect immunofluorescence studies indicated that KB-G2, but not KB-3-1, exhibits the PGP expression. Patch-clamp whole-cell recordings showed that osmotic swelling activates Cl- currents not only in PGP-expressing but also in PGP-lacking cells. The amplitude of the maximal current was indistinguishable between both cells. Activation of protein kinase C (PKC) or loading with a PKC inhibitor failed to affect the swelling-induced activation of the Cl- currents in both cells. The relation between whole-cell Cl- currents and cell size measured simultaneously showed that volume sensitivity of the Cl- channel was augmented by the PGP expression irrespective of the activity of PKC on the plasma membrane. A similar increase in volume sensitivity of the Cl- channel was also induced by the expression of the ATP hydrolysis-deficient PGP mutant, K433M. We conclude that P-glycoprotein does not represent the volume-sensitive Cl- channel but that its expression modulates volume sensitivity of the Cl- channel in a manner independent of its ATPase activity or of the protein kinase C activity.

Dietary salt increases first-pass elimination of oral quinidine

BACKGROUND

Some cytochrome P450 (CYP) enzymes, including CYP3A, are expressed not only in the liver but also in the intestine; the latter may therefore be an important site of drug disposition. Animal data suggests that dietary salt modulates expression of renal CYPs. We therefore hypothesized that intestinal CYP3A may be similarly modulated by dietary salt.

METHODS

The effect of changes in dietary salt on the disposition of two CYP3A substrates, quinidine (administered orally and intravenously) and 14C-erythromycin (administered intravenously) were determined after normal volunteers were given high-salt (400 mEq/day) and low-salt (10 mEq/day) diets for 7 to 10 days each.

RESULTS

Plasma concentrations after oral quinidine were significantly lower during the high-salt phase, with the difference between the two treatments attributable to changes within the first 1 to 4 hours after administration. For example, the area under the plasma concentration-time curve for the first hour after drug administration was 0.56 +/- 0.38 microgram.hr/ml for the high-salt diet compared with 1.57 +/- 0.60 micrograms.hr/ml for the low-salt diet (p < 0.05). Similarly, the peak plasma concentration (Cmax) achieved was lower and the time to reach Cmax was later for the high-salt diet (p < 0.05). In contrast, the terminal phase elimination half-lives were similar for the two diets, and no differences in disposition were found with the intravenous drug. The erythromycin breath test was unaffected by the dietary treatments.

CONCLUSIONS

These results indicate an effect of dietary salt on the presystemic disposition of orally administered quinidine. Although the mechanism(s) of CYP3A activity modulation is unknown, this finding may be important in determining drug availability in conditions associated with abnormal salt homeostasis.

Chloride channels: an emerging molecular picture

Chloride channels are probably found in every cell, from bacteria to mammals. Their physiological tasks range from cell volume regulation to stabilization of the membrane potential, signal transduction, transepithelial transport and acidification of intracellular organelles. These different functions require the presence of many distinct chloride channels, which are differentially expressed and regulated by various stimuli. These include various intracellular messengers (like calcium and cyclic AMP), pH, extracellular ligands and transmembrane voltage. Three major structural classes of chloride channels are known to date, but there may be others not yet identified. After an overview of the general functions of chloride channels, this review will focus on these cloned chloride channels: the CLC chloride channel family, which includes voltage-gated chloride channels, and the cystic fibrosis transmembrane regulator (CFTR), which performs other functions in addition to being a chloride channel. Finally, a short section deals with GABA and glycine receptors. Diseases resulting from chloride channel defects will be specially emphasized, together with the somewhat limited information about how these proteins work at the molecular level.

ABC1, an ATP binding cassette transporter required for phagocytosis of apoptotic cells, generates a regulated anion flux after expression in Xenopus laevis oocytes

The ATP binding cassette transporter ABC1 is a 220-kDa glycoprotein expressed by macrophages and required for engulfment of cells undergoing programmed cell death. Since members of this family of proteins such as P-glycoprotein and cystic fibrosis transmembrane conductance regulator share the ability to transport anions, we have investigated the transport capability of ABC1 expressed in Xenopus oocytes using iodide efflux and voltage-clamp techniques. We report here that ABC1 generates an anion flux sensitive to glibenclamide, sulfobromophthalein, and blockers of anion transporters. The anion flux generated by ABC1 is up-regulated by orthovanadate, cAMP, protein kinase A, and okadaic acid. In other ABC transporters, mutating the conserved lysine in the nucleotide binding folds was found to severely reduce or abolish hydrolysis of ATP, which in turn altered the activity of the transporter. In ABC1, replacement of the conserved lysine 1892 in the Walker A motif of the second nucleotide binding fold increased the basal ionic flux, did not alter the pharmacological inhibitory profile, but abolished the response to orthovanadate and cAMP agonists. Therefore, we conclude that ABC1 is a cAMP-dependent and sulfonylurea-sensitive anion transporter.

Biophysical aspects of P-glycoprotein-mediated multidrug resistance

In the 45 years since Burchenal's observation of chemotherapeutic drug resistance in tumor cells, many investigators have studied the molecular basis of tumor drug resistance and the phenomenon of tumor multidrug resistance (tumor MDR). Examples of MDR in microorganisms have also become topics of intensive study (e.g., Plasmodium falciparum MDR and various types of bacterial MDR) and these emerging fields have, in some cases, borrowed language, techniques, and theories from the tumor MDR field. Serendipitously, the cloning of MDR genes overexpressed in MDR tumor cells has led to elucidation of a large family of membrane proteins [the ATP-binding cassette (ABC) proteins], an important subset of which confer drug resistance in many different cells and microorganisms. In trying to decipher how ABC proteins confer various forms of drug resistance, studies on the structure and function of both murine and human MDR1 protein (also called P-glycoprotein or P-gp) have often led the way. Although various theories of P-gp function have become popular, there is still no precise molecular-level description for how P-gp overexpression lowers intracellular accumulation of chemotherapeutic drugs. In recent years, controversy has developed over whether the protein protects cells by translocating drugs directly (as some type of drug pump) or indirectly (through modulating biophysical parameters of the cell). In this ongoing debate over P-gp function, detailed consideration of biophysical issues is critical but has often been neglected in considering cell biological and pharmacological issues. In particular, P-gp overexpression also changes plasma membrane electrical potential (delta psi zero) and intracellular pH (pHi), and these changes will greatly affect the cellular flux of a large number of compounds to which P-gp overexpression confers resistance. In this chapter, we highlight these biophysical issues and describe how delta psi zero and pHi may in fact be responsible for many MDR-related phenomena that have often been hypothesized to be due to direct drug translocation (e.g., drug pumping) by P-gp.

Altered drug translocation mediated by the MDR protein: direct, indirect, or both?

Overexpression of the MDR protein, or p-glycoprotein (p-GP), in cells leads to decreased initial rates of accumulation and altered intracellular retention of chemotherapeutic drugs and a variety of other compounds. Thus, increased expression of the protein is related to increased drug resistance. Since several homologues of the MDR protein (CRP, ItpGPA, PDR5, sapABCDF) are also involved in conferring drug resistance phenomena in microorganisms, elucidating the function of the MDR protein at a molecular level will have important general applications. Although MDR protein function has been studied for nearly 20 years, interpretation of most data is complicated by the drug-selection conditions used to create model MDR cell lines. Precisely what level of resistance to particular drugs is conferred by a given amount of MDR protein, as well as a variety of other critical issues, are not yet resolved. Data from a number of laboratories has been gathered in support of at least four different models for the MDR protein. One model is that the protein uses the energy released from ATP hydrolysis to directly translocate drugs out of cells in some fashion. Another is that MDR protein overexpression perturbs electrical membrane potential (delta psi) and/or intracellular pH (pHi) and thereby indirectly alters translocation and intracellular retention of hydrophobic drugs that are cationic, weakly basic, and/or that react with intracellular targets in a pHi or delta psi-dependent manner. A third model proposes that the protein alternates between drug pump and Cl- channel (or channel regulator) conformations, implying that both direct and indirect mechanisms of altered drug translocation may be catalyzed by MDR protein. A fourth is that the protein acts as an ATP channel. Our recent work has tested predictions of these models via kinetic analysis of drug transport and single-cell photometry analysis of pHi, delta psi, and volume regulation in novel MDR and CFTR transfectants that have not been exposed to chemotherapeutic drugs prior to analysis. This paper reviews these data and previous work from other laboratories, as well as relevant transport physiology concepts, and summarizes how they either support or contradict the different models for MDR protein function.

Homologues of the human multidrug resistance genes MRP and MDR contribute to heavy metal resistance in the soil nematode Caenorhabditis elegans

Acquired resistance of mammalian cells to multiple chemotherapeutic drugs can result from enhanced expression of the multidrug resistance-associated protein (MRP), which belongs to the ABC transporter superfamily. ABC transporters play a role in the protection of organisms against exogenous toxins by cellular detoxification processes. We have identified four MRP homologues in the soil nematode Caenorhabditis elegans, and we have studied one member, mrp-1, in detail. Using an mrp::lacZ gene fusion, mrp-l expression was found in cells of the pharynx, the pharynx-intestinal valve and the anterior intestinal cells, the rectum-intestinal valve and the epithelial cells of the vulva. Targeted inactivation of mrp-l resulted in increased sensitivity to the heavy metal ions cadmium and arsenite, to which wild-type worms are highly tolerant. The most pronounced effect of the mrp-1 mutation is on the ability of animals to recover from temporary exposure to high concentrations of heavy metals. Nematodes were found to be hypersensitive to heavy metals when both the MRP homologue, mrp-1, and a member of the P-glycoprotein (Pgp) gene family, pgp-1, were deleted. We conclude that nematodes have multiple proteins, homologues of mammalian proteins involved in the cellular resistance to chemotherapeutic drugs, that protect them against heavy metals.

Effects of membrane potential versus pHi on the cellular retention of doxorubicin analyzed via a comparison between cystic fibrosis transmembrane conductance regulator (CFTR) and multidrug resistance (MDR) transfectants

Recently (Wei et al., Biophys J 69: 883-895, 1995), several 3T3/hu cystic fibrosis transmembrane conductance regulator (CFTR) transfectant clones were found to exhibit a low-level multidrug resistance (MDR) phenotype. This phenotype is similar, but not identical to that found for MDR transfectants not previously exposed to chemotherapeutic drugs. Both MDR and CFTR transfectants are depolarized (exhibit lower plasma membrane delta psi ), but the former have alkaline pHi whereas the latter are acidic. It has been proposed (Roepe et al., Biochemistry 32: 11042-11056, 1993) that both decreased delta psi and increased pHi contribute to altered cellular retention of chemotherapeutic drugs in MDR tumor cells, but the relative contribution of each to altered cellular drug accumulation, drug retention, and drug efflux has not been studied in detail. We therefore examined doxorubicin transport for hu CFTR and mu MDR 1 transfectants using sensitive continuous monitoring of fluorescence techniques. Both CFTR and MDR transfectants exhibited significantly reduced doxorubicin accumulation, relative to drug-sensitive control cells. Plots of the initial rate of accumulation versus doxorubicin concentration were linear for the control cells and the CFTR and MDR transfectants between 0.1 to 0.5 microM drug, but better fit by a quadratic between 0.1 to 1.5 microM drug. The slopes of these curves were proportional to measured delta psi. Low-level selection of either CFTR or MDR transfectants with chemotherapeutic drug did not decrease further the initial rate of drug accumulation or change delta psi. Accumulation experiments for control cells performed in the presence of various concentrations of K+ further suggests that the rate of accumulation is related to delta psi. By measuring the kinetics of doxorubicin release for CFTR and MDR transfectants preloaded with drug, we concluded that alkaline pHi perturbations are more important for determining relative intracellular binding efficiency. We also concluded, similar to the case previously made for MDR protein (Roepe, Biochemistry 31: 12555-12564, 1992) that CFTR overexpression does not enhance the rate of drug efflux. These data better define the role of lowered delta psi and elevated pHi in altering the cellular retention of doxorubicin in MDR tumor cells.

Volume-activated Cl- channels

1. An increase in cell volume activates, in most mammalian cells, a Cl- current, ICl,vol. This current is involved in a variety of cellular functions, such as the maintenance of a constant cell volume, pH regulation, and control of membrane potential. It might also play a role in the regulation of cell proliferation and in the processes that control transition from proliferation to differentiation. This review focuses on various aspects of this current, including its biophysical characterisation and its functional role for various cell processes. 2. Volume-activated Cl- channels show all outward rectification. Iodide is more permeable than chloride. In some cell types, ICl,vol inactivates at positive potentials. Single channel conductance can be divided mainly into two groups: small (< 5 pS) and medium conductance channels (around 50 pS). 3. The pharmacology and modulation of these channels are reviewed in detail, and suggest the existence of an heterogeneous family of multiple volume-activated Cl- channels. 4. Molecular candidates for this channel (i.e. ClC-2, a member of the ClC-family of voltage-dependent Cl- channels, the mdr-1 encoded P-glycoprotein, the nucleotide-sensitive pICln protein and phospholemman) will be discussed.