Enhanced lysosomal acidification leads to increased chloroquine accumulation in CHO cells expressing the pfmdr1 gene

Antimalarial Drug Resistance and Novel Targets for Antimalarial Drug Discovery

Malaria is among the most devastating and widespread tropical parasitic diseases in which most prevalent in developing countries. Antimalarial drug resistance is the ability of a parasite strain to survive and/or to multiply despite the administration and absorption of medicine given in doses equal to or higher than those usually recommended. Among the factors which facilitate the emergence of resistance to existing antimalarial drugs: the parasite mutation rate, the overall parasite load, the strength of drug selected, the treatment compliance, poor adherence to malaria treatment guideline, improper dosing, poor pharmacokinetic properties, fake drugs lead to inadequate drug exposure on parasites, and poor-quality antimalarial may aid and abet resistance. Malaria vaccines can be categorized into three categories: pre-erythrocytic, blood-stage, and transmission-blocking vaccines. Molecular markers of antimalarial drug resistance are used to screen for the emergence of resistance and assess its spread. It provides information about the parasite genetics associated with resistance, either single nucleotide polymorphisms or gene copy number variations which are associated with decreased susceptibility of parasites to antimalarial drugs. Glucose transporter PfHT1, kinases (Plasmodium kinome), food vacuole, apicoplast, cysteine proteases, and aminopeptidases are the novel targets for the development of new antimalarial drugs. Therefore, this review summarizes the antimalarial drug resistance and novel targets of antimalarial drugs.

Elucidating the Pivotal Immunomodulatory and Anti-Inflammatory Potentials of Chloroquine and Hydroxychloroquine

Chloroquine (CQ) and hydroxychloroquine (HCQ) are derivatives of 4-aminoquinoline compounds with over 60 years of safe clinical usage. CQ and HCQ are able to inhibit the production of cytokines such as interleukin- (IL-) 1, IL-2, IL-6, IL-17, and IL-22. Also, CQ and HCQ inhibit the production of interferon- (IFN-) α and IFN-γ and/or tumor necrotizing factor- (TNF-) α. Furthermore, CQ blocks the production of prostaglandins (PGs) in the intact cell by inhibiting substrate accessibility of arachidonic acid necessary for the production of PGs. Moreover, CQ affects the stability between T-helper cell (Th) 1 and Th2 cytokine secretion by augmenting IL-10 production in peripheral blood mononuclear cells (PBMCs). Additionally, CQ is capable of blocking lipopolysaccharide- (LPS-) triggered stimulation of extracellular signal-modulated extracellular signal-regulated kinases 1/2 in human PBMCs. HCQ at clinical levels effectively blocks CpG-triggered class-switched memory B-cells from differentiating into plasmablasts as well as producing IgG. Also, HCQ inhibits cytokine generation from all the B-cell subsets. IgM memory B-cells exhibits the utmost cytokine production. Nevertheless, CQ triggers the production of reactive oxygen species. A rare, but serious, side effect of CQ or HCQ in nondiabetic patients is hypoglycaemia. Thus, in critically ill patients, CQ and HCQ are most likely to deplete all the energy stores of the body leaving the patient very weak and sicker. We advocate that, during clinical usage of CQ and HCQ in critically ill patients, it is very essential to strengthen the CQ or HCQ with glucose infusion. CQ and HCQ are thus potential inhibitors of the COVID-19 cytokine storm.

V-ATPase (Vacuolar ATPase) Activity Required for ABCA1 (ATP-Binding Cassette Protein A1)-Mediated Cholesterol Efflux

Objective- We have shown that ABCA1 (ATP-binding cassette protein A1) mediates unfolding of the apoA1 (apolipoprotein A1) N-terminal helical hairpin during apoA1 lipidation. Others have shown that an acidic pH exposes the hydrophobic surface of apoA1. We postulated that the V-ATPase (vacuolar ATPase) proton pump facilitates apoA1 unfolding and promotes ABCA1-mediated cholesterol efflux. Approach and Results- We found that V-ATPase inhibitors dose-dependently decreased ABCA1-mediated cholesterol efflux to apoA1 in baby hamster kidney cells and RAW264.7 cells; and similarly, siRNA knockdown of ATP6V₀C inhibited ABCA1-mediated cholesterol efflux to apoA1 in RAW264.7 cells. Although ABCA1 expression did not alter total cellular levels of V-ATPase, ABCA1 increased the cell surface levels of the V₀A1 and V₁E1 subunits of V-ATPase. We generated a fluorescein isothiocyanate/Alexa647 double-labeled fluorescent ratiometric apoA1 pH indicator whose fluorescein isothiocyanate/Alexa647 emission ratio decreased as the pH drops. We found that ABCA1 induction in baby hamster kidney cells led to acidification of the cell-associated apoA1 pH indicator, compared with control cells without ABCA1 expression. The V-ATPase inhibitor bafilomycin A1 dose-dependently inhibited the apoA1 pH shift in ABCA1-expressing cells, without affecting the levels of cell-associated apoA1. However, we were not able to detect ABCA1-mediated extracellular proton release. We showed that acidic pH facilitated apoA1 unfolding, apoA1 solubilization of phosphatidycholine:phosphatidyserine liposomes, and increased lipid fluidity of these liposomes. Conclusions- Our results support a model that ABCA1 recruits V-ATPase to the plasma membrane where V-ATPase mediates apoA1 acidification and membrane remodeling that promote apoA1 unfolding and ABCA1-mediated HDL (high-density lipoprotein) biogenesis and lipid efflux.

Lysosomal Sequestration Impairs the Activity of the Preclinical FGFR Inhibitor PD173074

Knowledge of intracellular pharmacokinetics of anticancer agents is imperative for understanding drug efficacy as well as intrinsic and acquired cellular resistance mechanisms. However, the factors driving subcellular drug distribution are complex and poorly understood. Here, we describe for the first time the intrinsic fluorescence properties of the fibroblast growth factor receptor inhibitor PD1703074 as well as utilization of this physicochemical feature to investigate intracellular accumulation and compartmentalization of this compound in human lung cancer cells. Cell-free PD173074 fluorescence, intracellular accumulation and distribution were investigated using analytical chemistry and molecular biology approaches. Analyses on a subcellular scale revealed selective drug accumulation in lysosomes. Coincubation with inhibitors of lysosomal acidification strongly enhanced PD173074-mediated fibroblast growth factor receptor (FGFR) inhibition and cytotoxicity. In conclusion, intrinsic fluorescence enables analysis of molecular factors influencing intracellular pharmacokinetics of PD173074. Lysosome-alkalinizing agents might represent candidates for rational combination treatment, preventing cancer cell-intrinsic PD173074 resistance based on lysosomal trapping.

A 2-amino quinoline, 5-(3-(2-(7-chloroquinolin-2-yl)ethenyl)phenyl)-8-dimethylcarbamyl-4,6-dithiaoctanoic acid, interacts with PfMDR1 and inhibits its drug transport in Plasmodium falciparum

Malaria is a major disease in the tropics where chemotherapy remains the main mode of treatment and as such the rise and spread of drug-resistant malaria can lead to human tragedy. Two membrane transport proteins, PfMDR1 (Plasmodium falciparum multidrug resistance protein 1) and PfCRT (P. falciparum chloroquine resistance transporter), have been shown to cause resistance to several antimalarials. Both PfMDR1 and PfCRT are localized to the digestive vacuolar membrane and appear to regulate the transport of drugs and physiological metabolites. In this study we have used MK571, a 2-amino quinoline, to explore its interaction with PfMDR1 and PfCRT in chloroquine-sensitive and -resistant strains of P. falciparum. Our results show that chloroquine-resistant strains (e.g., K1, Dd2, and 7G8) are consistently more sensitive to MK571 than chloroquine-sensitive strains (e.g., 3D7, 106/1 and D10). This association, however, was not maintained with the chloroquine-resistant strain FCB which IC50 value was similar to chloroquine-sensitive strains. Moreover, the susceptibility of chloroquine-sensitive and -resistant strains to MK571 does not correlate with mutated PfCRT, nor is it reversible with verapamil; but correlates with mutations in PfMDR1. Furthermore, MK571 appears to target the parasite's digestive vacuole (DV), as demonstrated by the ability of MK571 to: (1) block the accumulation of the fluorescent dye Fluo-4 AM, a PfMDR1 substrate, into the digestive vacuole; (2) reduce the transvacuolar pH gradient; and (3) inhibit the formation of β-hematin in vitro. Moreover, the presence of non-toxic concentrations of MK571 sensitized both chloroquine-sensitive and -resistant parasites to mefloquine and halofantrine, likely by competing against PfMDR1-mediated sequestering of the drugs into the DV compartment and away from the drugs' cytosolic targets. Our data, nevertheless, found only a minimal decrease in MK571 IC50 value in FCB parasite which second pfmdr1 copy was inactivated via gene disruption. Taken together, the findings of this study suggest that MK571 interacts with native and mutant PfMDR1 and modulates the import of drugs or solutes into the parasite's DV and, as such, MK571 may be a useful tool in the characterization of PfMDR1 drug interactions and substrate specificity.

Action myoclonus-renal failure syndrome: diagnostic applications of activity-based probes and lipid analysis

Lysosomal integral membrane protein-2 (LIMP2) mediates trafficking of glucocerebrosidase (GBA) to lysosomes. Deficiency of LIMP2 causes action myoclonus-renal failure syndrome (AMRF). LIMP2-deficient fibroblasts virtually lack GBA like the cells of patients with Gaucher disease (GD), a lysosomal storage disorder caused by mutations in the GBA gene. While GD is characterized by the presence of glucosylceramide-laden macrophages, AMRF patients do not show these. We studied the fate of GBA in relation to LIMP2 deficiency by employing recently designed activity-based probes labeling active GBA molecules. We demonstrate that GBA is almost absent in lysosomes of AMRF fibroblasts. However, white blood cells contain considerable amounts of residual enzyme. Consequently, AMRF patients do not acquire lipid-laden macrophages and do not show increased plasma levels of macrophage markers, such as chitotriosidase, in contrast to GD patients. We next investigated the consequences of LIMP2 deficiency with respect to plasma glycosphingolipid levels. Plasma glucosylceramide concentration was normal in the AMRF patients investigated as well as in LIMP2-deficient mice. However, a marked increase in the sphingoid base, glucosylsphingosine, was observed in AMRF patients and LIMP2-deficient mice. Our results suggest that combined measurements of chitotriosidase and glucosylsphingosine can be used for convenient differential laboratory diagnosis of GD and AMRF.

Chloroquine-mediated lysosomal dysfunction enhances the anticancer effect of nutrient deprivation

PURPOSE

To investigate the ability of chloroquine, a lysosomotropic autophagy inhibitor, to enhance the anticancer effect of nutrient deprivation.

METHODS

Serum-deprived U251 glioma, B16 melanoma and L929 fibrosarcoma cells were treated with chloroquine in vitro. Cell viability was measured by crystal violet and MTT assay. Oxidative stress, apoptosis/necrosis and intracellular acidification were analyzed by flow cytometry. Cell morphology was examined by light and electron microscopy. Activation of AMP-activated protein kinase (AMPK) and autophagy were monitored by immunoblotting. RNA interference was used for AMPK and LC3b knockdown. The anticancer efficiency of intraperitoneal chloroquine in calorie-restricted mice was assessed using a B16 mouse melanoma model.

RESULTS

Chloroquine rapidly killed serum-starved cancer cells in vitro. This effect was not mimicked by autophagy inhibitors or LC3b shRNA, indicating autophagy-independent mechanism. Chloroquine-induced lysosomal accumulation and oxidative stress, leading to mitochondrial depolarization, caspase activation and mixed apoptotic/necrotic cell death, were prevented by lysosomal acidification inhibitor bafilomycin. AMPK downregulation participated in chloroquine action, as AMPK activation reduced, and AMPK shRNA mimicked chloroquine toxicity. Chloroquine inhibited melanoma growth in calorie-restricted mice, causing lysosomal accumulation, mitochondrial disintegration and selective necrosis of tumor cells.

CONCLUSION

Combined treatment with chloroquine and calorie restriction might be useful in cancer therapy.

A novel role for α-tocopherol transfer protein (α-TTP) in protecting against chloroquine toxicity

Chloroquine (CQ) is a widely prescribed anti-malarial agent and is also prescribed to treat autoimmune diseases. Clinical treatment with CQ is often accompanied by serious side effects such as hepatitis and retinopathy. As a weak base, CQ accumulates in intracellular acidic organelles, raises the pH, and induces osmotic swelling and permeabilization of acidic organelles, which account for CQ-induced cytotoxicity. We reported previously that CQ treatment caused α-tocopherol transfer protein (α-TTP), a gene product of familial vitamin E deficiency, to change its location from the cytosol to the surface of acidic organelles. Here we show that α-TTP plays a novel role in protecting against CQ toxicity both in vitro and in vivo. In the presence of CQ, rat hepatoma McARH7777 cells, which do not express α-TTP endogenously, showed more severe cytotoxicity, such as larger vacuolation of acidic organelles and caspase activation, than α-TTP transfectant cells. Similarly, α-TTP knockout mice showed more severe CQ toxicity, such as hepatotoxicity and retinopathy, than wild-type mice. These effects were not ameliorated by vitamin E supplementation. In contrast to bafilomycin A1 treatment, which prevents CQ accumulation in cells by raising the pH of acidic organelles, α-TTP expression prevented CQ accumulation without affecting the pH of acidic organelles. Taken together, our data suggest that α-TTP protects against CQ toxicity by preventing CQ accumulation in acidic organelles through a mechanism distinct from vitamin E transport.

A genome-wide screen identifies yeast genes required for protection against or enhanced cytotoxicity of the antimalarial drug quinine

Quinine is used in the treatment of Plasmodium falciparum severe malaria. However, both the drug's mode of action and mechanisms of resistance are still poorly understood and subject to debate. In an effort to clarify these questions, we used the yeast Saccharomyces cerevisiae as a model for pharmacological studies with quinine. Following on a previous work that examined the yeast genomic expression program in response to quinine, we now explore a genome-wide screen for altered susceptibility to quinine using the EUROSCARF collection of yeast deletion strains. We identified 279 quinine-susceptible strains, among which 112 conferred a hyper-susceptibility phenotype. The expression of these genes, mainly involved in carbohydrate metabolism, iron uptake and ion homeostasis functions, is required for quinine resistance in yeast. Sixty-two genes whose deletion leads to increased quinine resistance were also identified in this screen, including several genes encoding ribosome protein subunits. These well-known potential drug targets in Plasmodium are associated with quinine action for the first time in this study. The suggested involvement of phosphate signaling and transport in quinine tolerance was also studied, and activation of phosphate starvation-responsive genes was observed under a mild-induced quinine stress. Finally, P. falciparum homology searches were performed for a selected group of 41 genes. Thirty-two encoded proteins possess homologs in the parasite, including subunits of a parasitic vacuolar H(+)-ATPase complex, ion and phosphate importers, and several ribosome protein subunits, suggesting that the results obtained in yeast are good candidates to be transposed and explored in a P. falciparum context.

Biomarkers in the diagnosis of lysosomal storage disorders: proteins, lipids, and inhibodies

A biomarker is an analyte indicating the presence of a biological process linked to the clinical manifestations and outcome of a particular disease. In the case of lysosomal storage disorders (LSDs), primary and secondary accumulating metabolites or proteins specifically secreted by storage cells are good candidates for biomarkers. Clinical applications of biomarkers are found in improved diagnosis, monitoring disease progression, and assessing therapeutic correction. These are illustrated by reviewing the discovery and use of biomarkers for Gaucher disease and Fabry disease. In addition, recently developed chemical tools allowing specific visualization of enzymatically active lysosomal glucocerebrosidase are described. Such probes, coined inhibodies, offer entirely new possibilities for more sophisticated molecular diagnosis, enzyme replacement therapy monitoring, and fundamental research.

Drug-resistant malaria: molecular mechanisms and implications for public health

Resistance to antimalarial drugs has often threatened malaria elimination efforts and historically has led to the short-term resurgence of malaria incidences and deaths. With concentrated malaria eradication efforts currently underway, monitoring drug resistance in clinical settings complemented by in vitro drug susceptibility assays and analysis of resistance markers, becomes critical to the implementation of an effective antimalarial drug policy. Understanding of the factors, which lead to the development and spread of drug resistance, is necessary to design optimal prevention and treatment strategies. This review attempts to summarize the unique factors presented by malarial parasites that lead to the emergence and spread of drug resistance, and gives an overview of known resistance mechanisms to currently used antimalarial drugs.

The ABCs of multidrug resistance in malaria

Expanding drug resistance could become a major problem in malaria treatment, as only a limited number of effective antimalarials are available. Drug resistance has been associated with single nucleotide polymorphisms and an increased copy number of multidrug resistance protein 1 (MDR1), an ATP-binding cassette (ABC) protein family member. Many ABC transport proteins are membrane transporters that actively translocate a wide range of structurally and functionally diverse amphipathic compounds. The Plasmodium falciparum ABC family consists of 16 members and current knowledge of their physiological function and contribution to antimalarial drug resistance is limited. Here, we give an overview of the Plasmodium ABC family members with reference to their possible role in multidrug resistance.

The patterns of mutation and amplification of Plasmodium falciparum pfcrt and pfmdr1 genes in Thailand during the year 1988 to 2003

The study investigated the patterns of pfmdr1 and pfcrt genetic polymorphisms in Plasmodium falciaprum isolates collected from Thailand during the periods 1988-1993 (35 isolates), and 2003 (21 isolates). Pfcrt polymorphisms were almost universal for the mutations at codons K76T, A220S, Q271E, N326S, and R371I. All parasites displayed the chloroquine (CQ)-resistant phenotypes. This data suggested that pfcrt gene was sufficient to CQ resistance but did not mediate level of resistance. The prevalence [number of isolates (%)] of pfmdr1 polymorphisms at codons N86Y, Y184F, S1034C, N1042D and D1246Y were five (9%), 48 (86%), ten (18%), and 15 (27%), respectively. All isolates carried the wild-type nucleotide at position 1246. Results support the role of pfmdr1 in modulating susceptibilities of the P. falciparum to CQ, QN, and MQ. The frequencies of the S1034C and N1042D pfmdr1 polymorphisms and number of gene copy were significantly different in isolates collected during the two periods, with a trend of increasing prevalence of wild-type genotypes and number of gene copy from 1988 to 2003. The prominent pattern of pfmdr1 at codons 86/184/1034/1042/1246 was NFSND, with prevalence increasing from 40% to 95% during the 10-year period.

Transcriptomic profiling of the Saccharomyces cerevisiae response to quinine reveals a glucose limitation response attributable to drug-induced inhibition of glucose uptake

Quinine has been employed in the treatment of malaria for centuries and is still used against severe Plasmodium falciparum malaria. However, its interactions with the parasite remain poorly understood and subject to debate. In this study, we used the Saccharomyces cerevisiae eukaryotic model to better understand quinine's mode of action and the mechanisms underlying the cell response to the drug. We obtained a transcriptomic profile of the yeast's early response to quinine, evidencing a marked activation of genes involved in the low-glucose response (e.g., CAT8, ADR1, MAL33, MTH1, and SNF3). We used a low inhibitory quinine concentration with no detectable effect on plasma membrane function, consistent with the absence of a general nutrient starvation response and suggesting that quinine-induced glucose limitation is a specific response. We have further shown that transport of [(14)C]glucose is inhibited by quinine, with kinetic data indicating competitive inhibition. Also, tested mutant strains deleted for genes encoding high- and low-affinity hexose transporters (HXT1 to HXT5, HXT8, and HXT10) exhibit resistance phenotypes, correlating with reduced levels of quinine accumulation in the mutants examined. These results suggest that the hexose transporters are facilitators of quinine uptake in S. cerevisiae, possibly through a competitive inhibition mechanism. Interestingly, P. falciparum is highly dependent on glucose uptake, which is mediated by the single-copy transporter PfHT1, a protein with high homology to yeast's hexose transporters. We propose that PfHT1 is an interesting candidate quinine target possibly involved in quinine import in P. falciparum, an uptake mechanism postulated in recent studies to occur through a still-unidentified importer(s).

Membrane transport proteins of the malaria parasite

The malaria parasite-infected erythrocyte is a multi-compartment structure, incorporating numerous different membrane systems. The movement of nutrients, metabolites and inorganic ions into and out of the intraerythrocytic parasite, as well as between subcellular compartments within the parasite, is mediated by transporters and channels - integral membrane proteins that facilitate the movement of solutes across the membrane bilayer. Proteins of this type also play a key role in antimalarial drug resistance. Genes encoding transporters and channels account for at least 2.5% of the parasite genome. However, ascribing functions and physiological roles to these proteins, and defining their roles in drug resistance, is not straightforward. For any given membrane transport protein, a full understanding of its role(s) in the parasitized erythrocyte requires a knowledge of its subcellular localization and substrate specificity, as well as some knowledge of the effects on the parasite of modifying the sequence and/or level of expression of the gene involved. Here we consider recent work in this area, describe a number of newly identified transport proteins, and summarize the likely subcellular localization and putative substrate specificity of all of the candidate membrane transport proteins identified to date.

Plasmodium falciparum: Genome wide perturbations in transcript profiles among mixed stage cultures after chloroquine treatment

A genomic approach was taken to study the effect of chloroquine (CQ) on Plasmodium falciparum cultures in multiple cell states, following short and long exposures to drug at varying concentrations. Six hundred genes from numerous functional groups were responsive to CQ amongst all cell states assayed in a micro-array analysis; however, the amplitude of fold-change was low in the majority of cases. Moreover, alterations in specific, functionally related cascades could not be discerned, leading us to believe there is no single signature response to CQ at the transcript level in P. falciparum. Instead, cell cycle changes appear to have a more pronounced effect on gene expression; only a fraction of the drug responsive loci (approximately 5%) were shared between two separate starting cultures that varied in staging profile in the current study, as well as a previous published analysis using SAGE technology [Gunasekera, A.M., Patankar, S., Schug, J., Eisen, G.,Wirth, D.F., 2003. Drug-induced alterations in gene expression of the asexual blood forms of Plasmodium falciparum. Molecular Microbiology 50, 1229-1239]. These findings are important to report, given the striking contrast to similar studies in other model eukaryotic organisms.

[Metallocenes and malaria: a new therapeutic approach]

Rapid development of significant resistance to antimalarial drugs has been a major force driving research to identify and develop new compounds. The use of synthetic organometallic complexes seems to be promising for treatment of malaria infections. Recent progress in identification and development of new drugs promises to lead to a much greater range of antimalarial agents. Organometallic complexes and metalloporphyrins have shown in vitro activity against Plasmodium falciparum. Ferroquine (ferrocenyl chloroquine) is more active than chloroquine against strains and isolates of P. falciparum and shows efficacy against murine parasites.

Contribution of the pfmdr1 gene to antimalarial drug-resistance

The emergence of drug-resistance poses a major obstacle to the control of malaria. A homolog of the major multidrug-transporter in mammalian cells was identified, Plasmodium falciparum multidrug resistance protein-1, pfmdr1, also known as the P-glycoprotein homolog 1, Pgh-1. Several studies have demonstrated strong, although incomplete, associations between resistance to the widely used antimalarial drug chloroquine and mutation of the pfmdr1 gene in both laboratory and field isolates. Genetic studies have confirmed a link between mutation of the pfmdr1 gene and chloroquine-resistance. Although not essential for chloroquine-resistance, pfmdr1 plays a role in modulating levels of resistance. At the same time it appears to be a significant component in resistance to the structurally related drug quinine. A strong association has been observed between possession of the wildtype form of pfmdr1, amplification of pfmdr1 and resistance to hydrophobic drugs such as the arylaminoalcohol mefloquine and the endoperoxide artemisinin derivatives in field isolates. This is supported by genetic studies. The arylaminoalcohol and endoperoxide drugs are structurally unrelated drugs and this resistance resembles true multidrug resistance. Polymorphism in pfmdr1 and gene amplification has been observed throughout the world and their usefulness in predicting resistance levels is influenced by the history of drug selection of each population.

ATPase activity of purified plasma membranes and digestive vacuoles from Plasmodium falciparum

The ATPase activity of the human malaria parasite, Plasmodium falciparum was investigated using two experimental systems, (i) digestive vacuoles, and (ii) purified plasma membranes isolated from a chloroquine-sensitive and a chloroquine-resistant strain. No correlation between the level of ATPase activity and chloroquine sensitivity could be detected. In both systems, the ATPase activity of the chloroquine-resistant and -sensitive strain was decreased in the presence of the P-glycoprotein inhibitor vanadate. Susceptibility to inhibition by vanadate together with the lack of effect of ouabain implies a P-type ATPase activity in the plasma membrane. Furthermore, the inhibition of Fac8 ATPase activity by oligomycin both in the digestive vacuoles and the plasma membranes would be consistent with higher levels of Pgh1 in Fac8. Our data are consistent with the presence of a V-type H+-ATPase in the parasite food vacuole. Bafilomycin A1 and N-ethylmaleimide decreased the vacuolar ATPase activity in both chloroquine-resistant and -sensitive strains. Interestingly, a 30% decrease was observed between the ATPase activity of plasma membranes isolated from Fac8 and D10 in the presence of bafilomycin A1, suggesting the presence of a V-type ATPase in D10 plasma membrane that is underexpressed or altered in the plasma membrane of the chloroquine-resistant Fac8. The chemosensitisers tested had no effect on the ATPase activity of chloroquine-resistant P. falciparum in both systems suggesting that their activity is not mediated through an ATP-dependent mechanism. No effect was observed on the vacuolar ATPase activity in the presence of the antimalarials tested indicating that an ATP-dependent transport has not been activated.

In vitro increase in chloroquine accumulation induced by dihydroethano- and ethenoanthracene derivatives in Plasmodium falciparum-parasitized erythrocytes

The effects of a series of dihydroethano- and ethenoanthracene derivatives on chloroquine (CQ) accumulation in CQ-susceptible strain 3D7 and CQ-resistant clone W2 were assessed. The levels of CQ accumulation increased little or none in CQ-susceptible strain 3D7 and generally increased markedly in CQ-resistant strain W2. At 10 microM, 28 compounds yielded cellular accumulation ratios (CARs) greater than that observed with CQ alone in W2. At 10 microM, in strain W2, 21 of 31 compounds had CQ CARs two or more times higher than that of CQ alone, 15 of 31 compounds had CQ CARs three or more times higher than that of CQ alone, 13 of 31 compounds had CQ CARs four or more times higher than that of CQ alone, and 9 of 31 compounds had CQ CARs five or more times higher than that of CQ alone. At 1 microM, 17 of 31 compounds had CQ CARs two or more times higher than that of CQ alone, 12 of 31 compounds had CQ CARs three or more times higher than that of CQ alone, 6 of 31 compounds had CQ CARs four or more times higher than that of CQ alone, and 3 of 31 compounds had CQ CARs five or more times higher than that of CQ alone. At 1 microM, 17 of 31 compounds were more potent inducers of CQ accumulation than verapamil and 12 of 31 compounds were more potent inducers of CQ accumulation than promethazine. The nature of the basic group seems to be associated with increases in the levels of CQ accumulation. At 1 and 10 microM, 10 of 14 and 13 of 14 compounds with amino group (amines and diamines), respectively, had CARs >or=3, while at 1 and 10 microM, only 1 of the 13 derivatives with amido groups had CARs >or=3. Among 12 of the 31 compounds which were more active inducers of CQ accumulation than promethazine at 1 microM, 10 had amino groups and 1 had an amido group.

Inhibition of mitogen-activated protein kinase signaling by chloroquine

Previously, we demonstrated that the anti-inflammatory drug chloroquine (CQ) inhibited LPS-induced TNF-alpha transcription. To define further the mechanism of CQ, we studied the effect of this drug on mitogen-activated protein kinase signaling pathways involved in regulation of TNF production. CQ interfered with phosphorylation of extracellular signal-regulated kinases (ERK)1/2 and the ERK-activating kinases mitogen-activating protein/ERK kinase (MEK)1/2. Both CQ and PD98059, a MEK1 inhibitor, reduced luciferase reporter activity driven by human TNF promoter sequences. However, CQ appeared to mediate these effects by deactivating Raf, the upstream activator of MEK. These findings were supported by functional data demonstrating that CQ and PD98059 interfered with TNF expression in several human and murine cell types while neither inhibitor blocked TNF production in murine RAW264.7 macrophages, a cell line that does not require MEK-ERK signaling for TNF production. Finally, we evaluated whether CQ could sensitize HeLa cells to undergo anti-Fas-mediated apoptosis, an effect observed when ERK activation is interrupted in this cell line. CQ rendered HeLa cells sensitive to anti-Fas treatment in a manner similar to PD98059. Taken together, these data argue that therapeutic concentrations of CQ interfere with ERK activation by a novel mechanism, an effect that could be responsible, at least in part, for the potent anti-inflammatory effects of this drug.

Chloroquine resistance and the pH of the malaria parasite's digestive vacuole

The molecular mechanisms underlying chloroquine resistance in the malaria parasite are not well understood. Recent studies have focused attention on the role of the pH of the parasite's 'digestive vacuole', an internal acidic compartment in which ingested host cell proteins are degraded. In these studies, vacuolar pH was estimated from measurements of fluorescence from parasites stained with acridine orange. The validity of the method, and the conclusions drawn, have been challenged. Nevertheless, the correlation observed between acridine orange fluorescence and chloroquine resistance is intriguing and may well provide insights into the resistance mechanism. Whether vacuolar pH plays a role in chloroquine resistance remains an open question.

ABC transporters required for endocytosis and endosomal pH regulation inDictyostelium

In Dictyostelium, the RtoA protein links both initial cell-type choice and physiological state to cell-cycle phase. rtoA– cells (containing a disruption of the rtoA gene) generally do not develop past the mound stage, and have an abnormal ratio of prestalk and prespore cells. RtoA is also involved in fusion of endocytic/exocytic vesicles. Cells lacking RtoA, although having a normal endocytosis rate, have a decreased exocytosis rate and endosomes with abnormally low pHs. RtoA levels vary during the cell cycle, causing a cell-cycle-dependent modulation of parameters such as cytosolic pH (Brazill et al., 2000). To uncover other genes involved in the RtoA-mediated differentiation, we identified genetic suppressors of rtoA. One of these suppressors disrupted two genes, mdrA1 and mdrA2, a tandem duplication encoding two members of the ATP binding cassette (ABC) transporter superfamily. Disruption of mdrA1/mdrA2 results in release from the developmental block and suppression of the defect in initial cell type choice caused by loss of the rtoA gene. However, this is not accomplished by re-establishing the link between cell type choice and cell cycle phase. MdrA1 protein is localized to the endosome. mdrA1–/mdrA2– cells (containing a disruption of these genes) have an endocytosis rate roughly 70% that of wild-type or rtoA– cells, whereas mdrA1–/mdrA2–/rtoA– cells have an endocytosis rate roughly 20% that of wild-type. The exocytosis rates of mdrA1–/mdrA2– and mdrA1–/mdrA2–/rtoA– are roughly that of wild-type. mdrA1–/mdrA2– endosomes have an unusually high pH, whereas mdrA1–/mdrA2–/rtoA– endosomes have an almost normal pH. The ability of mdrA1/mdrA2 disruption to rescue the cell-type proportion, developmental defects, and endosomal pH defects caused by rtoA disruption, and the ability of rtoA disruption to exacerbate the endocytosis defects caused by mdrA1/mdrA2 disruption, suggest a genetic interaction between rtoA, mdrA1 and mdrA2.

Functional analysis of drug resistance in Plasmodium falciparum in the post-genomic era

Malaria has plagued humans throughout recorded history and results in the death of over 2 million people per year. The protozoan parasite Plasmodium falciparum causes the most severe form of malaria in humans. Chemotherapy has become one of the major control strategies for this parasite; however, the development of drug resistance to virtually all of the currently available drugs is causing a crisis in the use and deployment of these compounds for prophylaxis and treatment of this disease. The genome sequence of P. falciparum is providing the informational base for the use of whole-genome strategies such as bioinformatics, microarrays and genetic mapping. These approaches, together with the availability of a high-resolution genome linkage map consisting of hundreds of microsatellite markers and the advanced technologies of transfection and proteomics, will facilitate an integrated approach to address important biological questions. In this review we will discuss strategies to identify novel genes involved in the molecular mechanisms used by the parasite to circumvent the lethal effect of current chemotherapeutic agents.

New developments: chloroquine-resistance in Plasmodium falciparum

Chloroquine-resistance is associated with higher malaria mortality in children in Africa where the drug is still widely used. In sensitive strains the drug attacks hemoglobin digestion in the lysosome and prevents detoxification of hemin to hemozoin. Reduced drug uptake is responsible for resistance, which is incompletely associated with changes in lysosome membrane protein PGH1. The report discussed here gives evidence for the role of another lysosome membrane protein, PfCRT, where a change from lysine to threonine in a transmembrane domain determines the change to resistance. Other changes in PfCRT, and to some extent change(s) in PGH1, are believed to compensate for loss of fitness of the modified PfCRT.

Membrane transport in the malaria-infected erythrocyte

The malaria parasite is a unicellular eukaryotic organism which, during the course of its complex life cycle, invades the red blood cells of its vertebrate host. As it grows and multiplies within its host blood cell, the parasite modifies the membrane permeability and cytosolic composition of the host cell. The intracellular parasite is enclosed within a so-called parasitophorous vacuolar membrane, tubular extensions of which radiate out into the host cell compartment. Like all eukaryote cells, the parasite has at its surface a plasma membrane, as well as having a variety of internal membrane-bound organelles that perform a range of functions. This review focuses on the transport properties of the different membranes of the malaria-infected erythrocyte, as well as on the role played by the various membrane transport systems in the uptake of solutes from the extracellular medium, the disposal of metabolic wastes, and the origin and maintenance of electrochemical ion gradients. Such systems are of considerable interest from the point of view of antimalarial chemotherapy, both as drug targets in their own right and as routes for targeting cytotoxic agents into the intracellular parasite.

The ABC transporter genes of Plasmodium falciparum and drug resistance

The seminal observations that (a) chloroquine-resistant Plasmodium falciparum strains accumulate less drug than more sensitive parasites, and (b) chloroquine resistance could be modulated in vitro by the classic multidrug-resistance (MDR) modulator verapamil, suggested not only that parasite resistance to multiple drugs may be similar to the MDR phenotype described in mammalian cancer cells, but that homologous proteins may be involved. These findings prompted search for MDR-like genes in the parasite. To date, three full-length ABC transporter genes have been isolated from P. falciparum: two P-glycoprotein-like homologues, pfmdr1 and pfmdr2, and a homologue of the yeast GCN20 gene, pfgcn20.

Chloroquine interferes with lipopolysaccharide-induced TNF-alpha gene expression by a nonlysosomotropic mechanism

Chloroquine (CQ) is a lysosomotropic weak base with over 60 years of clinical use for the treatment of malaria and rheumatologic disorders. Consistent with its anti-inflammatory properties, CQ has been shown to interfere with TNF-alpha release from mononuclear phagocytes. Because it is unclear how CQ mediates these immunomodulatory effects, we set out to elucidate its mechanism of action. CQ exhibited dose-dependent inhibition of LPS-induced TNF-alpha release from human PBMC at therapeutically attainable concentrations. Additional studies to determine the specificity of this effect showed that although CQ reduced IL-1beta and IL-6 release, secretion of RANTES was unaffected. CQ acted by reducing TNF-alpha mRNA accumulation without destabilizing its mRNA or interfering with NF-kappaB nuclear translocation or p50/p65 isoform composition of DNA-binding complexes. Intracellular cytokine staining indicated that CQ reduced TNF-alpha production pretranslationally without interfering with TNF-alpha processing or release. We utilized bafilomycin A1 pretreatment to block the pH-dependent trapping of CQ in endosomes and lysosomes. Although bafilomycin A1 alone did not interfere with TNF-alpha expression, preincubation augmented the ability of CQ to reduce TNF-alpha mRNA levels, suggesting that CQ did not act by a lysosomotropic mechanism. Using confocal microscopy, we showed that bafilomycin A1 pretreatment resulted in a dramatic redistribution of quinacrine, a fluorescent congener of CQ, from cytoplasmic vacuoles to the nucleus. These data indicate that CQ inhibits TNF-alpha gene expression without altering translocation of NF-kappaB p50/p65 heterodimers. This dose-dependent effect occurs over a pharmacologically relevant concentration range and does not require pH-dependent lysosomotropic accumulation of CQ.

Chloroquine - some open questions on its antimalarial mode of action and resistance

During the digestion of its host cell hemoglobin, large amounts of toxic ferriprotoporphyrin IX (FPIX) are generated in the intraerythrocytic malaria parasite. FPIX is detoxified either by being polymerized into hemozoin inside the food vacuole, or through its degradation by glutathione in the cytosol. Chloroquine is able to complex with FPIX, thus inhibiting both processes and thereby generating receptors for its own uptake. These leads to the accumulation of FPIX in the membrane fraction of infected cells that results in membrane permeabilization and disruption of cation homeostasis and concluded in parasite death. Several unresolved questions, such as the site of FPIX:chloroquine complex formation, the role of pH gradient in drug accumulation and resistance, the role of Pgh-1 in resistance, the mode of action of reversers and the involvement of proteins and their mutants in resistance, are discussed. Copyright 1999 Harcourt Publishers Ltd.

High-level chloroquine resistance of Plasmodium berghei is associated with multiple drug resistance and loss of reversal by calcium antagonists

The chloroquine resistance of Plasmodium falciparum is reversed in vitro by numerous compounds, including calcium antagonists, which could enhance the accumulation of the drug in the parasite food vacuole. However, this mechanism of resistance could be insufficient when the resistance level increases. Using in vitro drug trials on strains of Plasmodium berghei displaying various chloroquine-resistance levels, we confirmed previous results obtained in vivo in the chloroquine-resistant strains of P. berghei are cross-resistant to related drugs (amodiaquine, quinine and mefloquine), the resistance levels to these drugs being related to their analogy to chloroquine. Furthermore, we showed that high-level resistant lines were associated with a loss of drug potentiation by verapamil and nicardipine in vivo, but that the reversal rates obtained in vitro are of low significance. We conclude that the parasite is able to escape the activity of these reversing agents.

Pfmdr1 Asn1042Asp and Asp1246Tyr polymorphisms, thought to be associated with chloroquine resistance, are present in chloroquine-resistant and -sensitive Brazilian field isolates of Plasmodium falciparum

Parasite resistance to antimalarial drugs, particularly chloroquine, is the most disturbing problem of malaria chemotherapy. There is evidence that the codon 86Tyr polymorphism of the Pfmdr1 gene is associated with chloroquine resistance in West African Plasmodium falciparum. The association of this and four other coding alterations of the Pfmdr1 gene with chloroquine resistance has not been extensively investigated in South American isolates. In this study, we examined 51 Brazilian P. falciparum isolates for the presence or absence of Asn86Tyr, Asn1042Asp, and Asp1246Tyr polymorphisms. While these isolates were all sensitive in vitro to mefloquine, amodiaquine, and quinine, only 2 (4%) were chloroquine-sensitive. The findings reported here provide the first observations of this kind on a large number of field parasite samples from South America. We show that in vitro chloroquine-resistant and -sensitive strains carry the Asn1042Asp and Asp1246Tyr polymorphisms and provide support for earlier suggestions that Asn86Tyr may be rare or absent in South American P. falciparum.

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.

Active efflux by multidrug transporters as one of the strategies to evade chemotherapy and novel practical implications of yeast pleiotropic drug resistance

Mankind is faced by the increasing emergence of resistant pathogens, including cancer cells. An overview of the different strategies adopted by a variety of cells to evade chemotherapy is presented, with a focus on the mechanisms of multidrug transport. In particular, we analyze the yeast network for pleiotropic drug resistance and assess the potentiality of this system for further understanding of the mechanism of broad specificity and for development of novel practical applications.

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.

The ATP-binding cassette (ABC) gene family of Plasmodium falciparum

Multidrug resistance (MDR) in mammalian tumour cells is mediated by P-glycoproteins. The apparent similarities between MDR and the chloroquine-resistance phenotype (CQR) in Plasmodium falciparum suggests that homologous proteins may be involved. In mammals, P-glycoproteins are encoded by mdr genes that are a subset of a super-family characterized by ATP-binding cassettes (ABC). Three genes, pfmdr1, pfmdr2 and pfef3-rl, have been identified in P. falciparum that have homology to the ABC transporter gene family. Each protein encoded by these genes has a distinct structure, suggesting functional differences between the three. Justin Rubio and Alan Cowman here discuss the structure and possible function of the ABC proteins from P. falciparum and evidence that the protein encoded by the pfmdr1 gene can influence quinoline-containing antimalarial drug-resistance phenotypes.

Functional complementation of the ste6 gene of Saccharomyces cerevisiae with the pfmdr1 gene of Plasmodium falciparum

The pfmdr1 gene has been associated with a drug-resistant phenotype in Plasmodium falciparum, and overexpression of pfmdr1 has been associated with mefloquine- and halofantrine-resistant parasites, but little is known about the functional role of pfmdr1 in this process. Here, we demonstrate that the pfmdr1 gene expressed in a heterologous yeast system functions as a transport molecule and complements a mutation in ste6, a gene which encodes a mating pheromone a-factor export molecule. In addition, the pfmdr1 gene containing two mutations which are associated with naturally occurring chloroquine resistance abolishes this mating phenotype, suggesting that these genetic polymorphisms alter this transport function. Our results support the functional role of pfmdr1 as a transport molecule in the mediation of drug resistance and provide an assay system to address the nature of this transport function.