P-glycoprotein and cell volume-activated chloride channels

Postoperative peritoneal adhesion: an update on physiopathology and novel traditional herbal and modern medical therapeutics

Postoperative peritoneal adhesion (PPA) is a serious clinical condition that affects the high percentage of patients after abdominal surgery. In this review, we have tried to focus on pathophysiology and different underlying signal pathways of adhesion formation based on recent progress in the molecular and cellular mechanisms. Also, the strategies, developed based on traditional herbal and modern medicines, to prevent and treat the PPA via regulation of the molecular mechanisms were investigated. The search engines such as Google Scholar, PubMed, Scopus, and Science Direct have been used to evaluate the current literature related to the pathogenesis of adhesion formation and novel products. Recently, different mechanisms have been defined for adhesion formation, mainly categorized in fibrin formation and adhesion fibroblast function, inflammation, and angiogenesis. Therefore, the suppression of these mechanisms via traditional and modern medicine has been suggested in several studies. While different strategies with encouraging findings have been developed, most of the studies showed contradictory results and were performed on animals. The herbal products have been introduced as safe and effective agent which can be considered in future preclinical and clinical studies. Although a wide range of therapeutics based on traditional and modern medicines have been suggested, there is no agreement in the efficacy of these methods to prevent or treat adhesion formation after surgeries. Further basic and clinical researches are still needed to propose the efficiency of recommended strategies for prevention and treatment of PPA.

P-glycoprotein Mediates Postoperative Peritoneal Adhesion Formation by Enhancing Phosphorylation of the Chloride Channel-3

P-glycoprotein (P-gp) is encoded by the multidrug resistance (MDR1) gene and is well studied as a multi-drug resistance transporter. Peritoneal adhesion formation following abdominal surgery remains an important clinical problem. Here, we found that P-gp was highly expressed in human adhesion fibroblasts and promoted peritoneal adhesion formation in a rodent model. Knockdown of P-gp expression by intraperitoneal injection of MDR1-targeted siRNA significantly reduced both the peritoneal adhesion development rate and adhesion grades. Additionally, we found that operative injury up-regulated P-gp expression in peritoneal fibroblasts through the TGF-β1/Smad signaling pathway and histone H3 acetylation. The overexpression of P-gp accelerated migration and proliferation of fibroblasts via volume-activated Cl(-) current and cell volume regulation by enhancing phosphorylation of the chloride channel-3. Therefore, P-gp plays a critical role in postoperative peritoneal adhesion formation and may be a valuable therapeutic target for preventing the formation of peritoneal adhesions.

Hydrogel-assisted functional reconstitution of human P-glycoprotein (ABCB1) in giant liposomes

This paper describes the formation of giant proteoliposomes containing P-glycoprotein (P-gp) from a solution of small proteoliposomes that had been deposited and partially dried on a film of agarose. This preparation method generated a significant fraction of giant proteoliposomes that were free of internalized vesicles, making it possible to determine the accessible liposome volume. Measuring the intensity of the fluorescent substrate rhodamine 123 (Rho123) inside and outside these giant proteoliposomes determined the concentration of transported substrates of P-gp. Fitting a kinetic model to the fluorescence data revealed the rate of passive diffusion as well as active transport by reconstituted P-gp in the membrane. This approach determined estimates for the membrane permeability coefficient (Ps) of passive diffusion and rate constants of active transport (kT) by P-gp as a result of different experimental conditions. The Ps value for Rho123 was larger in membranes containing P-gp under all assay conditions than in membranes without P-gp indicating increased leakiness in the presence of reconstituted transmembrane proteins. For P-gp liposomes, the kT value was significantly higher in the presence of ATP than in its absence or in the presence of ATP and the competitive inhibitor verapamil. This difference in kT values verified that P-gp was functionally active after reconstitution and quantified the rate of active transport. Lastly, patch clamp experiments on giant proteoliposomes showed ion channel activity consistent with a chloride ion channel protein that co-purified with P-gp. Together, these results demonstrate several advantages of using giant rather than small proteoliposomes to characterize transport properties of transport proteins and ion channels.

Molecular analysis of the multidrug transporter, P-glycoprotein

Inherent or acquired resistance of tumor cells to cytotoxic drugs represents a major limitation to the successful chemotherapeutic treatment of cancer. During the past three decades dramatic progress has been made in the understanding of the molecular basis of this phenomenon. Analyses of drug-selected tumor cells which exhibit simultaneous resistance to structurally unrelated anti-cancer drugs have led to the discovery of the human MDR1 gene product, P-glycoprotein, as one of the mechanisms responsible for multidrug resistance. Overexpression of this 170 kDa N-glycosylated plasma membrane protein in mammalian cells has been associated with ATP-dependent reduced drug accumulation, suggesting that P-glycoprotein may act as an energy-dependent drug efflux pump. P-glycoprotein consists of two highly homologous halves each of which contains a transmembrane domain and an ATP binding fold. This overall architecture is characteristic for members of the ATP-binding cassette or ABC superfamily of transporters. Cell biological, molecular genetic and biochemical approaches have been used for structure-function studies of P-glycoprotein and analysis of its mechanism of action. This review summarizes the current status of knowledge on the domain organization, topology and higher order structure of P-glycoprotein, the location of drug- and ATP binding sites within P-glycoprotein, its ATPase and drug transport activities, its possible functions as an ion channel, ATP channel and lipid transporter, its potential role in cholesterol biosynthesis, and the effects of phosphorylation on P-glycoprotein activity.

ABC transporters in ophthalmic disease

ABC transporters have been implicated in a variety of human diseases. The ABCR gene and its protein have been linked to Stargardt's disease, fundus flavimaculatus, cone-rod dystrophy, retinitis pigmentosa, and age-related macular degeneration. The genetic and molecular pathways involved in the pathogenesis of ABCR-related ophthalmic conditions will be explored. Future diagnostic and therapeutic objectives for these diseases will also be discussed.

Verapamil Induces Upregulation of P-glycoprotein Expression on Human Monocyte Derived Dendritic Cells

Overexpression of P-glycoprotein, a transmembrane drug efflux pump that mediates efflux of chemotherapeutic agents contributes to drug resistance in many leukaemia and other cancerous cells. Non-malignant cells including leukocytes also express P-glycoprotein, but physiologic functions for P-glycoprotein are poorly defined. Recently, P-glycoprotein expression has been described in human mononuclear phagocytes and Langerhans cells. It has been shown to play a role in phagocytic cell transmigration through endothelial-lined vessels in an ablumenal-lumenal direction, a process that mimics their migration into lymphatic vessels. Using the monoclonal antibody 4E3, and the P-glycoprotein antagonist, verapamil, the expression of P-glycoprotein on human monocyte-derived dendritic cells was evaluated. Dendritic cells used in this study were CD1a+, CD11c+, CD14-, CD80+, CD83+, CD86+ and MHC-II(High). The expression of these markers increased significantly as the cells matured. P-glycoprotein expression was upregulated as the dendritic cells matured as well as in the presence of the "inflammatory stress" of the pathogenic bacteria Strept. pyogenes. Addition of verapamil or Strept. pyogenes to the culture medium during the final 24 hours significantly upregulated P-glycoprotein expression. Immortalized cell lines did not upregulate P-glycoprotein in the presence of verapamil. Evaluation of other normal cells showed that P-glycoprotein upregulation in the presence of verapamil was also a characteristic of macrophages. This novel observation of the upregulation of P-glycoprotein in the presence of verapamil appears to be a characteristic of activated myeloid derived antigen presenting cells and suggest that P-glycoprotein is essential for these cells as when it is blocked, they respond by increasing expression of this protein. In summary, this work describes that human dendritic cells generated from plastic-adherent monocytes rapidly upregulate expression of P-glycoprotein as they mature, and in the presence of inflammatory stress and the pharmacological agent verapamil, which blocks P-glycoprotein activity, suggesting that P-glycoprotein may play a role in activation as well as in migration of dendritic cells.

Stargardt's disease and the ABCR gene

Stargardt's disease is an autosomal recessive form of juvenile macular degeneration. The clinical presentation, relevant ancillary tests, and classic histologic features will be reviewed. The role of genetic mutations in the pathophysiology of Stargardt's disease will also be explored. Stargardt's disease is caused by mutations in the ABCR (ABCA4) gene on chromosome 1. Mutations in this gene have also been attributed to some cases of cone-rod dystrophy, retinitis pigmentosa, and age-related macular degeneration. The genetic and molecular pathways that produce Stargardt's disease will be discussed. Future diagnostic and therapeutic objectives for this visually disabling condition will also be presented.

Relationships between cystic fibrosis transmembrane conductance regulator, extracellular nucleotides and cystic fibrosis

Cystic fibrosis (CF) is one of the most common lethal autosomal recessive genetic diseases in the Caucasian population, with a frequency of about 1 in 3000 livebirths. CF is due to a mutation in the cystic fibrosis transmembrane conductance regulator (CFTR) gene encoding the CFTR protein, a cyclic adenosine 5'-monophosphate (cAMP)-regulated chloride channel localized in the apical membrane of epithelial cells. CFTR is a multifunctional protein which, in addition to be a Cl-channel, is also a regulator of multiple ion channels and other proteins. In particular CFTR has been reported to play a role in the outflow of adenosine 5'-triphosphate (ATP) from cells, but this remains controversial. Extracellular nucleotides are signaling molecules that regulate ion transport and mucociliary clearance by acting on P2 nucleotide receptors, in particular the P2Y(2) receptor. Nucleotides activating the P2Y(2) receptor represent thus one pharmacotherapeutic strategy to treat CF disease, via improvement of mucus hydration and mucociliary clearance in airways. Phase II clinical trials have recently shown that aerosolized denufosol (INS37217, Inspire(R)) improves pulmonary function in CF patients: denufosol was granted orphan drug status and phase III trials are planned. Here, we review what is known about the relationship between extracellular nucleotides and CFTR, the role of extracellular nucleotides in epithelial pathophysiology and their putative role as therapeutic agents.

ATP-binding cassette (ABC) transporters in normal and pathological lung

ATP-binding cassette (ABC) transporters are a family of transmembrane proteins that can transport a wide variety of substrates across biological membranes in an energy-dependent manner. Many ABC transporters such as P-glycoprotein (P-gp), multidrug resistance-associated protein 1 (MRP1) and breast cancer resistance protein (BCRP) are highly expressed in bronchial epithelium. This review aims to give new insights in the possible functions of ABC molecules in the lung in view of their expression in different cell types. Furthermore, their role in protection against noxious compounds, e.g. air pollutants and cigarette smoke components, will be discussed as well as the (mal)function in normal and pathological lung. Several pulmonary drugs are substrates for ABC transporters and therefore, the delivery of these drugs to the site of action may be highly dependent on the presence and activity of many ABC transporters in several cell types. Three ABC transporters are known to play an important role in lung functioning. Mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene can cause cystic fibrosis, and mutations in ABCA1 and ABCA3 are responsible for respectively Tangier disease and fatal surfactant deficiency. The role of altered function of ABC transporters in highly prevalent pulmonary diseases such as asthma or chronic obstructive pulmonary disease (COPD) have hardly been investigated so far. We especially focused on polymorphisms, knock-out mice models and in vitro results of pulmonary research. Insight in the function of ABC transporters in the lung may open new ways to facilitate treatment of lung diseases.

Chloride channels and hepatocellular function: prospects for molecular identification

Hepatocytes possess chloride channels at the plasma membrane and in multiple intracellular compartments. These channels are required for cell volume regulation and acidification of intracellular organelles. Evidence also supports a role of chloride channels in modulation of apoptosis and cell growth. Swelling- and Ca(2+)-activated chloride channels have been identified in hepatocyte plasma membranes, and chloride channels have been observed in the membranes of lysosomes, endosomes, Golgi, endoplasmic reticulum, mitochondria, and the nucleus. This review summarizes the functions of these channels and discusses the specific channel molecules they may represent. Chloride channel molecules shown to be expressed in hepatocytes include members of the ClC channel family (ClC-2, ClC-3, ClC-5, and ClC-7), members of the newly identified CLIC family of intracellular chloride channels (CLIC-1 and CLIC-4), the mitochondrial voltage-dependent anion channel, and a newly identified intracellular channel, MCLC (Mid-1 related chloride channel). Current understanding does not include a molecular identification of most of the observed channel functions, but details of the molecular properties of these channel molecules should allow future identification and further understanding of chloride channel function in hepatocytes.

The impact of multiple drug resistance (MDR) proteins on chemotherapy and drug discovery

Transportation of molecules across the cell membrane in living organisms is a critical aspect of life. Transportation includes importation of nutrients from the environment and exportation of toxic compounds. When export includes therapeutic compounds, then the practice of clinical medicine may become compromised. Often efflux of therapeutic compounds is mediated by a large superfamily of proteins referred to as multidrug resistance (MDR) proteins. The initial sections of this chapter are focused on MDR proteins and their negative impact on clinical medicine in cancer chemotherapy as well as infectious diseases mediated by bacteria, fungi and parasites. A brief description of major classes of MDR proteins found in microbes is followed by a more exhaustive treatment of ABC transporters in lower eukaryotes and parasites as well as cancerous mammalian cells. Later sections deal with potential and real positive aspects and applications brought about by a growing knowledge of MDR proteins. Examples described include improved antibiotic production, leveraging MDR proteins in drug discovery, new therapeutic options, dual therapy in treatment of cancer and infectious diseases, and finally MDR proteins as targets for new classes of therapeutic compounds.

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.

P-Glycoprotein in cell cultures: a combined approach to study expression, localisation, and functionality in the confocal microscope

Madin Darby canine kidney (MDCK) cells transfected with the multidrug resistance mdr1 gene, MDR1-MDCK (Pastan et al., 1988, Proc. Natl. Acad. Sci. USA 85 4486-4470), were used in a combined approach to study expression, localisation and functionality of the P-glycoprotein (P-gp) membrane transporter in the same cell culture preparations. Cells were characterised with regard to their growth curve, transepithelial electrical resistance (TEER), and cytoarchitecture. Efflux of the P-gp substrate rhodamine123 (rho123) was monitored with confocal laser scanning microscopy (CLSM). The transfected cells grew in multilayers. After reaching confluence they exhibited a complete tight junction (TJ) network. P-gp was strongly expressed at the uppermost apical surface of the multilayer already after 4 days in culture. The lower cell layers were not clearly polarised. P-gp-mediated transport could be followed by efflux of the fluorescent rho123 from the cells into the apical extracellular space. Verapamil, a P-gp inhibitor, significantly decreased efflux. For MDCK parent cells the rho123 assay was negative up to about day 20, and only at later times (day 25) low P-gp activity was detected. These results clearly show that despite the fact that the transfected cells form irregular layers, they provide a good model for screening of P-gp substrates and inhibitors.

Regulation of volume-activated chloride channels by P-glycoprotein: phosphorylation has the final say!

P-glycoprotein (Pgp) is a transmembrane transporter causing efflux of a number of chemically unrelated drugs and is responsible for resistance to a variety of anticancer drugs during chemotherapy. Pgp overexpression in cells is also associated with volume-activated chloride channel activity; Pgp is thought to regulate such activity. Reversible phosphorylation is a possible mechanism for regulating the transport and chloride channel regulation functions of Pgp. Protein kinase C (PKC) is a good candidate for inducing such phosphorylation. Hierarchical multiple phosphorylation (e.g. of different serines and with different PKC isoforms) may shuttle the protein between its different states of activity (transport or channel regulation). Cell volume changes may trigger phosphorylation of Pgp at sites causing inhibition of transport. The possible regulation of chloride channels by Pgp and the potential involvement of reversible phosphorylation in such regulation is reviewed.

Multixenobiotic resistance as a cellular defense mechanism in aquatic organisms

Multixenobiotic resistance in aquatic organisms exposed to natural toxins or anthropogenic contaminants is a phenomenon analogous to multidrug resistance in mammalian tumor cell lines tolerant of anti-cancer drugs. Multidrug resistance is commonly due to the elevated expression of transmembrane P-glycoproteins (P-gp) which actively transport a wide variety of structurally and functionally diverse compounds. The purpose of this review is to place aquatic ecotoxicological data in context of the larger multidrug resistance field of study. Information on P-glycoproteins structure, mechanism of transport, and substrate specificity gained through traditional mammalian and cell culture models is examined in conjunction with recent work on aquatic species exposed to xenobiotics both in the field and in the laboratory. The physiological function of P-glycoproteins is explored through studies of gene knockout models and expression patterns in normal tissues and tumors. The effect of xenobiotic exposures on P-gp activity and protein titer is examined in wild and captive populations of aquatic invertebrates and vertebrates. Substrate overlap and evidence of co-expression of phase I detoxification enzymes (e.g. cytochromes P450) and P-gp are presented. The role of P-gp chemosensitizers as environmental pollutants and the ecotoxicological consequences of P-gp inhibition are highlighted. The overwhelming evidence suggests that P-glycoproteins provide aquatic organisms with resistance to a wide range of natural and anthropogenic toxins.

Current Clinical Practice: Application of Resistance Reversal Agents in Hematologic Malignancies

The clinical application of resistance reversal drugs for patients with hematologic malignancies is reviewed. The phenomenon of multidrug resistance versus other mechanisms are discussed. The pump-like mechanisms of P-glycoprotein, multidrug resistance associated protein, lung resistance protein and of other ATP binding cassette transporter proteins are reviewed briefly, as well as the important substrate drugs and pump-blocking compounds. The problems associated with resistance protein assays in clinical samples and the concept of prognostic versus therapeutic clinical relevance are described, within the context of selected hematologic malignancies. Toxicities and treatment outcomes of phase II and III trials of reversal agents in lymphoma, multiple myeloma, myelodysplastic syndromes, acute myeloid leukemia and blast phase of chronic myeloid leukemia are reviewed. Finally, current options for on-study management of relapsed or refractory hematologic malignancy patients are discussed.

Selection of keratinocytes transduced with the multidrug resistance gene in an in vitro skin model presents a strategy for enhancing gene expression in vivo

In gene therapy studies, achieving prolonged, high-level gene expression in a significant percentage of cells has been difficult. One solution to enhance expression would be to select for cells expressing both the desired gene and a linked selectable marker gene in a bicistronic vector. As a potential target tissue, the skin is easily accessible for safe topical application of a selecting agent that could lead to significant gene expression in a high percentage of keratinocytes. To test the feasibility of such an approach, a skin raft culture model was developed. Human keratinocytes were transduced with the multidrug resistance (MDR) gene, which confers resistance to a variety of cytostatic and antimitotic compounds, such as colchicine. While growing on acellular dermis, transduced keratinocytes were treated with various doses of colchicine (10-50 ng/ml). Colchicine treatment increased the percentage of keratinocytes expressing MDR to almost 100% in raft cultures, Significantly, keratinocytes in colchicine-treated, MDR-transduced raft cultures were able to proliferate normally and form a stratified, differentiated epidermis. This model suggests that topical selection for MDR-expressing keratinocytes in vivo should be feasible without hampering the biologic integrity of skin. Thus, topical selection leading to enhanced expression of a desired gene, linked to a resistance gene, holds future promise for skin gene therapy.

A novel hypothesis for the mechanism of action of P-glycoprotein as a multidrug transporter

For years, P-glycoprotein (P-gp) has been purported to be a membrane transporter capable of selectively transporting many (but not all) lipophilic anticancer drugs with diverse chemical structures. Because the alleged functions of P-gp provide a straightforward, near-perfect explanation for the molecular mechanism of multidrug resistance associated with P-gp overexpression. However, the exact molecular mechanism for P-gp's purported function has never been clearly understood since its initial discovery some 20 yr ago. In this paper, I develop a novel working hypothesis regarding the mechanism of P-gp's action and suggest that P-gp is an energy-dependent efflux pump only for certain conjugated metabolites (probably sulfates) of the lipophilic anticancer drugs but not for the parent compounds, as was always claimed. According to this hypothesis, P-gp overexpression in most cases is not the "culprit" but instead an "accomplice" in P-gp-associated multidrug resistance. The culprit is probably the enhanced function of the metabolizing enzymes for the lipophilic anticancer drugs. This hypothesis also predicts that one of the important physiological functions of P-gp is to be part of an intracellular machinery (together with the phase I and II metabolizing enzymes) for the metabolism, detoxification, and disposition of lipophilic endogenous chemicals as well as xenobiotics, including cytotoxic anticancer drugs. There exists a considerable body of circumstantial evidence in the literature that lends strong support to this mechanistic hypothesis of P-gp's action as well as to the predicted physiological functions of P-gp. It will be of considerable interest to examine this novel hypothesis experimentally.

Contribution of the murine mdr1a P-glycoprotein to hepatobiliary and intestinal elimination of cationic drugs as measured in mice with an mdr1a gene disruption

In the mouse, both the mdr1a and the mdr1b gene encode drug-transporting P-glycoproteins. The mdr1a P-glycoprotein is expressed in epithelial cells of, among others, the liver and the intestine. Furthermore, the mdr1b gene product is found in the liver but is not detectable in the intestine. To establish the potential involvement of P-glycoprotein in the elimination of cationic amphiphilic drugs from the body, we investigated biliary, intestinal, and urinary excretion in mice with a homozygous disruption of the mdr1a gene (mdr1a(-/-) mice). These mice are fully viable under laboratory conditions and have normal bile flow. Cumulative biliary excretion (expressed as percent of the intravenously administered dose excreted over a 1-hour period) of several cationic compounds was decreased as follows in mdr1a(-/-) mice compared with the wild-type animals: tri-n-butylmethylammonium (TBuMA), 0.7% versus 2.1%; azidoprocainamide methoiodide (APM), 3.8% versus 7.6%; and vecuronium, 22.7% versus 41.3%. The luminal secretion of both TBuMA and APM in the small intestine was profoundly decreased, respectively 4.6-fold (1.8% vs. 8.2% in the wild-type) and 7.9-fold (1.6% vs. 10.3% in the wild-type) in mdr1a(-/-) mice. Thus mdr1a P-glycoprotein contributes substantially to the removal of a wide variety of cationic agents from the body through intestinal and hepatobiliary secretion, but it evidently acts in concert with other transport system(s). These processes probably provide a protective mechanism limiting the overall rate of absorption as well as the bioavailability of potentially toxic organic amines.

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.

Differential sensitivity of resting and IL-2 activated NK cells to R-verapamil

Natural killer (NK) cells are the first lymphoid population to reconstitute the peripheral blood compartment of immunologically compromised bone marrow transplant (BMT) recipients. Recent data suggest that, among patients transplanted for leukemia, NK cells can prevent or delay disease relapse by mediating a cytotoxic graft vs leukemia (GvL) response. Although the major mechanism by which NK cells mediate target cell lysis involves degranulation and release of cytolytic effector molecules (granzymes, proteoglycans, perforin), accumulating evidence suggests that NK cells possess additional pathways to mediate target cell killing. In fact, it is well recognized that recombinant cytokines such as IL-2 enhance the in vitro cytolytic activity of NK cells. In this study, we observed that the lytic activity mediated by resting and IL-2 activated NK cells against the same target cell appears to occur via two distinct pathways, as distinguished by their differential response to R-verapamil. Specifically, we observed that 25 microM R-verapamil inhibited the lytic activity of resting NK cells against K562 targets by approximately 50%. However, the lytic activity of IL-2 activated NK cells was unaffected by this concentration of R-verapamil. Additional studies suggested that the inhibitory effect of R-verapamil on NK cytotoxic activity was associated with its ability to prevent degranulation of cytotoxic granules. Specifically, R-verapamil inhibited BLT esterase release from resting but not IL-2 activated NK cells. These data suggest that IL-2 activated NK cells can promote target cell lysis by a pathway (possibly degranulation independent) distinct from that used by resting NK cells. We speculate that the target of R-verapamil on resting NK cells is P-glycoprotein (Pgp), an ABC transporter that we recently reported was expressed on NK cells and whose functional activity is known to be inhibited by R-verapamil.

Efficient purification and reconstitution of P-glycoprotein for functional and structural studies

Plasma membrane P-glycoprotein is known as an ATP-dependent drug efflux pump that confers multidrug resistance to tumor cells. None of the reported purification procedures worked properly for our P-glycoprotein-overproducing cell lines, i.e. murine lymphoid leukemia P388/ADR25, rat hepatoma AS30-D/COL10, and human lymphoblastic leukemia CEM/VLB5 cells. We have thus developed a general procedure for efficient purification of P-glycoprotein by combining solubilization with sodium dodecyl sulfate and chromatography on ceramic hydroxyapatite. This procedure was successful for the three cell lines and yielded 70% of the P-glycoprotein present in the starting plasma membranes with more than 99% purity. After exchanging sodium dodecyl sulfate into dodecyl maltoside and reconstitution into liposomes, purified P-glycoprotein exhibited a specific ATPase activity of about 200 nmol/min/mg, which was very similar to that obtained for P-glycoprotein solubilized and purified with 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonic acid. This ATPase activity was sensitive to orthovanadate inhibition and stimulated by verapamil and other drugs. More importantly, drug transport properties of the reconstituted P-glycoprotein were comparable with those of P-glycoprotein embedded in plasma membranes. Since it is virtually devoid of lipids, this preparation is suitable for both functional and structural investigations.

The role of swelling-induced anion channels during neuronal volume regulation

Regulation of cell volume is an essential function of most mammalian cells. In the cells of the central nervous system, maintenance of cell osmolarity and, hence, volume, is particularly crucial because of the restrictive nature of the skull. Cell volume regulation involves a variety of pathways, with considerable differences between cell types. One common pathway activated during hypo-osmotic stress involves chloride (Cl-) channels. However, hypo-osmotically stimulated anion permeability can be regulated by a diverse array of second messengers. Although neuronal swelling can occur in a number of pathological and nonpathological conditions, our understanding of neuronal volume regulation is limited. This article summarizes our current understanding of the role of anion channels during neuronal volume regulation.

Volume-sensitive chloride currents in primary cultures of human fetal vas deferens epithelial cells

Using the patch-clamp technique, we have identified a large, outwardly rectifying, Cl--selective whole-cell current in primary cultures of human vas deferens epithelial cells. Whole-cell currents were time- and voltage-dependent and displayed inactivation following depolarising pulses >/= 60 mV. Currents were equally permeable to bromide (PBr/PCl = 1.05 +/- 0.04), iodide (PI/PCl = 1. 06 +/- 0.07) and Cl-, but significantly less permeable to gluconate (PGluc /PCl = 0.23 +/- 0.03). Currents spontaneously increased with time after establishing a whole-cell recording, but could be inhibited by exposure to a hypertonic bath solution which reduced inward currents by 68 +/- 4%. Subsequent exposure of the cells to a hypotonic bath solution led to a 418 +/- 110% increase in inward current, indicating that these currents are regulated by osmolarity. 4,4'-Diisothiocyanatostilbene-2,2'-disulphonic acid (100 microM) produced a rapid and reversible voltage-dependent block (60 +/- 5% and 10 +/- 7% inhibition of current, measured at +/- 60 mV, respectively). Dideoxyforskolin (50 microM) also reduced the volume-sensitive Cl- current, but with a much slower time course, by 41 +/- 13% and 32 +/- 16% (measured at +/- 60 mV, respectively). Tamoxifen (10 microM) had no effect on the whole-cell Cl- current. These results suggest that vas deferens epithelial cells possess a volume-sensitive Cl- conductance which has biophysical and pharmacological properties broadly similar to volume-sensitive Cl- currents previously described in a variety of cell types.