Thapsigargin or curcumin does not promote maturation of processing mutants of the ABC transporters, CFTR, and P-glycoprotein

ATP-binding cassette (ABC) drug transporters in the developing blood-brain barrier: role in fetal brain protection

The blood-brain barrier (BBB) provides essential neuroprotection from environmental toxins and xenobiotics, through high expression of drug efflux transporters in endothelial cells of the cerebral capillaries. However, xenobiotic exposure, stress, and inflammatory stimuli have the potential to disrupt BBB permeability in fetal and post-natal life. Understanding the role and ability of the BBB in protecting the developing brain, particularly with respect to drug/toxin transport, is key to promoting long-term brain health. Drug transporters, particularly P-gp and BCRP are expressed in early gestation at the developing BBB and have a crucial role in developmental homeostasis and fetal brain protection. We have highlighted several factors that modulate drug transporters at the developing BBB, including synthetic glucocorticoid (sGC), cytokines, maternal infection, and growth factors. Some factors have the potential to increase expression and function of drug transporters and increase brain protection (e.g., sGC, transforming growth factor [TGF]-β). However, others inhibit drug transporters expression and function at the BBB, increasing brain exposure to xenobiotics (e.g., tumor necrosis factor [TNF], interleukin [IL]-6), negatively impacting brain development. This has implications for pregnant women and neonates, who represent a vulnerable population and may be exposed to drugs and environmental toxins, many of which are P-gp and BCRP substrates. Thus, alterations in regulated transport across the developing BBB may induce long-term changes in brain health and compromise pregnancy outcome. Furthermore, a large portion of neonatal adverse drug reactions are attributed to agents that target or access the nervous system, such as stimulants (e.g., caffeine), anesthetics (e.g., midazolam), analgesics (e.g., morphine) and antiretrovirals (e.g., Zidovudine); thus, understanding brain protection is key for the development of strategies to protect the fetal and neonatal brain.

Interplay between P-Glycoprotein Expression and Resistance to Endoplasmic Reticulum Stressors

Multidrug resistance (MDR) is a phenotype of cancer cells with reduced sensitivity to a wide range of unrelated drugs. P-glycoprotein (P-gp)—a drug efflux pump (ABCB1 member of the ABC transporter gene family)—is frequently observed to be a molecular cause of MDR. The drug-efflux activity of P-gp is considered as the underlying mechanism of drug resistance against P-gp substrates and results in failure of cancer chemotherapy. Several pathological impulses such as shortages of oxygen and glucose supply, alterations of calcium storage mechanisms and/or processes of protein N-glycosylation in the endoplasmic reticulum (ER) leads to ER stress (ERS), characterized by elevation of unfolded protein cell content and activation of the unfolded protein response (UPR). UPR is responsible for modification of protein folding pathways, removal of misfolded proteins by ER associated protein degradation (ERAD) and inhibition of proteosynthesis. However, sustained ERS may result in UPR-mediated cell death. Neoplastic cells could escape from the death pathway induced by ERS by switching UPR into pro survival mechanisms instead of apoptosis. Here, we aimed to present state of the art information about consequences of P-gp expression on mechanisms associated with ERS development and regulation of the ERAD system, particularly focused on advances in ERS-associated therapy of drug resistant malignancies.

L1210 Cells Overexpressing ABCB1 Drug Transporters Are Resistant to Inhibitors of the N- and O-glycosylation of Proteins

Overexpression of P-glycoprotein (P-gp, drug transporter) in neoplastic cells is the most frequently observed molecular cause of multidrug resistance. Here, we show that the overexpression of P-gp in L1210 cells leads to resistance to tunicamycin and benzyl 2-acetamido-2-deoxy-α-d-galactopyranoside (GalNAc-α-O-benzyl). Tunicamycin induces both glycosylation depression and ubiquitination improvement of P-gp. However, the latter is not associated with large increases in molecular mass as evidence for polyubiquitination. Therefore, P-gp continues in maturation to an active membrane efflux pump rather than proteasomal degradation. P-gp-positive L1210 cells contain a higher quantity of ubiquitin associated with cell surface proteins than their P-gp-negative counterparts. Thus, P-gp-positive cells use ubiquitin signaling for correct protein folding to a higher extent than P-gp-negative cells. Elevation of protein ubiquitination after tunicamycin treatment in these cells leads to protein folding rather than protein degradation, resulting at least in the partial lack of cell sensitivity to tunicamycin in L1210 cells after P-gp expression. In contrast to tunicamycin, to understand why P-gp-positive cells are resistant to GalNAc-α-O-benzyl, further research is needed.

Curcumin, bisdemethoxycurcumin and dimethoxycurcumin complexed with cyclodextrins have structure specific effect on the paracellular integrity of lung epithelia in vitro

The phytochemical curcumin may improve translocation of the cystic fibrosis transmembrane regulatory (CFTR) protein in lung epithelium and therefore be helpful in the treatment of cystic fibrosis (CF) symptoms. However, previous studies often use commercial curcumin that is a combination of curcumin, demethoxycurcumin and bisdemethoxycurcumin which could affect the investigated cells differently. In the present study, we investigated the potential difference between curcumin, bisdemethoxycurcumin and dimethoxycurcumin on the epithelial tight junction complex, in the bronchial epithelial cell line VA10, by measuring transepithelial electrical resistance (TER), immunofluorescence and western blotting of tight junction proteins. The curcuminoids were complexed with hydroxypropyl-γ–cyclodextrin for increased solubility and stability. Curcumin (10 µg/ml) increased the TER significantly after 24 h of treatment while four times higher concentration of bisdemethoxycurcumin was required to obtain similar increase in TER as curcumin. Interestingly, dimethoxycurcumin did not increase TER. Curcumin clearly affected the F-actin structures both apically and basolaterally. These results begin to define possible effects of curcuminoids on healthy bronchial epithelia and shows that difference in the phenyl moiety structure of the curcuminoids influences the paracellular epithelial integrity.

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Curcuminoids formulized with cyclodextrin for increased solubility and stability.

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Curcumin increases TER in a concentration dependent manner and causes decrease in apical F-actin staining.

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Higher concentration required for bisdemethoxycurcumin to increase TER compared to curcumin.

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Dimethoxycurcumin did not increase TER.

Roscovitine is a proteostasis regulator that corrects the trafficking defect of F508del-CFTR by a CDK-independent mechanism

BACKGROUND AND PURPOSE

The most common mutation in cystic fibrosis (CF), F508del, causes defects in trafficking, channel gating and endocytosis of the CF transmembrane conductance regulator (CFTR) protein. Because CF is an orphan disease, therapeutic strategies aimed at improving mutant CFTR functions are needed to target the root cause of CF.

EXPERIMENTAL APPROACH

Human CF airway epithelial cells were treated with roscovitine 100 μM for 2 h before CFTR maturation, expression and activity were examined. The mechanism of action of roscovitine was explored by recording the effect of depleting endoplasmic reticulum (ER) Ca(2+) on the F508del-CFTR/calnexin interaction and by measuring proteasome activity.

KEY RESULTS

Of the cyclin-dependent kinase (CDK) inhibitors investigated, roscovitine was found to restore the cell surface expression and defective channel function of F508del-CFTR in human CF airway epithelial cells. Neither olomoucine nor (S)-CR8, two very efficient CDK inhibitors, corrected F508del-CFTR trafficking demonstrating that the correcting effect of roscovitine was independent of CDK inhibition. Competition studies with inhibitors of the ER quality control (ERQC) indicated that roscovitine acts on the calnexin pathway and on the degradation machinery. Roscovitine was shown (i) to partially inhibit the interaction between F508del-CFTR and calnexin by depleting ER Ca(2+) and (ii) to directly inhibit the proteasome activity in a Ca(2+) -independent manner.

CONCLUSIONS AND IMPLICATIONS

Roscovitine is able to correct the defective function of F508del-CFTR by preventing the ability of the ERQC to interact with and degrade F508del-CFTR via two synergistic but CDK-independent mechanisms. Roscovitine has potential as a pharmacological therapy for CF.

Drug rescue distinguishes between different structural models of human P-glycoprotein

There is no high-resolution crystal structure of the human P-glycoprotein (P-gp) drug pump. Homology models of human P-gp based on the crystal structures of mouse or Caenorhabditis elegans P-gps show large differences in the orientation of transmembrane segment 5 (TM5). TM5 is one of the most important transmembrane segments involved in drug–substrate interactions. Drug rescue of P-gp processing mutants containing an arginine at each position in TM5 was used to identify positions facing the lipid or internal aqueous chamber. Only the model based on the C. elegans P-gp structure was compatible with the drug rescue results.

Protein homeostasis as a therapeutic target for diseases of protein conformation

Protein misfolding and aggregation are widely implicated in an increasing number of human diseases providing for new therapeutic opportunities targeting protein homeostasis (proteostasis). The cellular response to proteotoxicity is highly regulated by stress signaling pathways, molecular chaperones, transport and clearance machineries that function as a proteostasis network (PN) to protect the stability and functional properties of the proteome. Consequently, the PN is essential at the cellular and organismal level for development and lifespan. However, when challenged during aging, stress, and disease, the folding and clearance machineries can become compromised leading to both gain-of-function and loss-of-function proteinopathies. Here, we assess the role of small molecules that activate the heat shock response, the unfolded protein response, and clearance mechanisms to increase PN capacity and protect cellular proteostasis against proteotoxicity. We propose that this strategy to enhance cell stress pathways and chaperone activity establishes a cytoprotective state against misfolding and/or aggregation and represents a promising therapeutic avenue to prevent the cellular damage associated with the variety of protein conformational diseases.

Detection of glycomic alterations induced by overexpression of p-glycoprotein on the surfaces of L1210 cells using sialic acid binding lectins

P-glycoprotein (P-gp) overexpression is the most frequently observed cause of multidrug resistance in neoplastic cells. In our experiments, P-gp was expressed in L1210 mice leukemia cells (S cells) by selection with vincristine (R cells) or transfection with the gene encoding human P-gp (T cells). Remodeling of cell surface sugars is associated with P-gp expression in L1210 cells as a secondary cellular response. In this study, we monitored the alteration of cell surface saccharides by Sambucus nigra agglutinin (SNA), wheat germ agglutinin (WGA) and Maackia amurensis agglutinin (MAA). Sialic acid is predominantly linked to the surface of S, R and T cells via α-2,6 branched sugars that tightly bind SNA. The presence of sialic acid linked to the cell surface via α-2,3 branched sugars was negligible, and the binding of MAA (recognizing this branch) was much less pronounced than SNA. WGA induced greater cell death than SNA, which was bound to the cell surface and agglutinated all three L1210 cell-variants more effectively than WGA. Thus, the ability of lectins to induce cell death did not correlate with their binding efficiency and agglutination potency. Compared to S cells, P-gp positive R and T cells contain a higher amount of N-acetyl-glucosamine on their cell surface, which is associated with improved WGA binding. Both P-gp positive variants of L1210 cells are strongly resistant to vincristine as P-gp prototypical drug. This resistance could not be altered by liberalization of terminal sialyl residues from the cell surface by sialidase.

Effects of cellular, chemical, and pharmacological chaperones on the rescue of a trafficking-defective mutant of the ATP-binding cassette transporter proteins ABCB1/ABCB4

The ATP-binding cassette transporter ABCB4 is a phosphatidylcholine translocator specifically expressed at the bile canalicular membrane in hepatocytes, highly homologous to the multidrug transporter ABCB1. Variations in the ABCB4 gene sequence cause progressive familial intrahepatic cholestasis type 3. We have shown previously that the I541F mutation, when reproduced either in ABCB1 or in ABCB4, led to retention in the endoplasmic reticulum (ER)/Golgi. Here, Madin-Darby canine kidney cells expressing ABCB1-GFP were used as a model to investigate this mutant. We show that ABCB1-I541F is not properly folded and is more susceptible to in situ protease degradation. It colocalizes and coprecipitates with the ER chaperone calnexin and coprecipitates with the cytosolic chaperone Hsc/Hsp70. Silencing of calnexin or overexpression of Hsp70 have no effect on maturation of the mutant. We also tested potential rescue by chemical and pharmacological chaperones. Thapsigargin and sodium 4-phenyl butyrate were inefficient. Glycerol improved maturation and exit of the mutant from the ER. Cyclosporin A, a competitive substrate for ABCB1, restored maturation, plasma membrane expression, and activity of ABCB1-I541F. Cyclosporin A also improved maturation of ABCB4-I541F in Madin-Darby canine kidney cells. In HepG(2) cells transfected with ABCB4-I541F cDNA, cyclosporin A allowed a significant amount of the mutant protein to reach the membrane of bile canaliculi. These results show that the best strategy to rescue conformation-defective ABCB4 mutants is provided by pharmacological chaperones that specifically target the protein. They identify cyclosporin A as a potential novel therapeutic tool for progressive familial intrahepatic cholestasis type 3 patients.

Tunicamycin depresses P-glycoprotein glycosylation without an effect on its membrane localization and drug efflux activity in L1210 cells

P-glycoprotein (P-gp), also known as ABCB1, is a member of the ABC transporter family of proteins. P-gp is an ATP-dependent drug efflux pump that is localized to the plasma membrane of mammalian cells and confers multidrug resistance in neoplastic cells. P-gp is a 140-kDa polypeptide that is glycosylated to a final molecular weight of 170 kDa. Our experimental model used two variants of L1210 cells in which overexpression of P-gp was achieved: either by adaptation of parental cells (S) to vincristine (R) or by transfection with the human gene encoding P-gp (T). R and T cells were found to differ from S cells in transglycosylation reactions in our recent studies. The effects of tunicamycin on glycosylation, drug efflux activity and cellular localization of P-gp in R and T cells were examined in the present study. Treatment with tunicamycin caused less concentration-dependent cellular damage to R and T cells compared with S cells. Tunicamycin inhibited P-gp N-glycosylation in both of the P-gp-positive cells. However, tunicamycin treatment did not alter either the P-gp cellular localization to the plasma membrane or the P-gp transport activity. The present paper brings evidence that independently on the mode of P-gp expression (selection with drugs or transfection with a gene encoding P-gp) in L1210 cells, tunicamycin induces inhibition of N-glycosylation of this protein, without altering its function as plasma membrane drug efflux pump.

Molecular pathways for intracellular cholesterol accumulation: common pathogenic mechanisms in Niemann-Pick disease Type C and cystic fibrosis

It has been less than two decades since the underlying genetic defects in Niemann-Pick disease Type C were first identified. These defects impair function of two proteins with a direct role in lipid trafficking, resulting in deposition of free cholesterol within late endosomal compartments and a multitude of effects on cell function and clinical manifestations. The rapid pace of research in this area has vastly improved our overall understanding of intracellular cholesterol homeostasis. Excessive cholesterol buildup has also been implicated in clinical manifestations associated with a number of genetically unrelated diseases including cystic fibrosis. Applying knowledge about anomalous cell signaling behavior in cystic fibrosis opens prospects for identifying similar previously unrecognized disease pathways in Niemann-Pick disease Type C. Recognition that Niemann-Pick disease Type C and cystic fibrosis both impair cholesterol regulatory pathways also provides a rationale for identifying common therapeutic targets.

Partial correction of cystic fibrosis defects with PLGA nanoparticles encapsulating curcumin

Cystic fibrosis (CF) is a common life threatening genetic disease (incidence: approximately 1 in 2500 live births). CF is caused by mutations in CFTR, a chloride channel involved in epithelial secretion of fluid and electrolytes. The most common mutation entails the deletion of a phenylalanine in position 508 that causes protein misfolding and abnormal CFTR processing. The DeltaF508 mutation accounts for approximately 70% of all CF alleles, and about 90% of CF patients carry at least one copy of DeltaF508 CFTR. Curcumin, a natural constituent of Curcuma longa (turmeric spice), is a nontoxic low-affinity SERCA (sarco (endo)plasmic reticulum calcium ATPase) pump inhibitor thought to permit DeltaF508 CFTR escape from the ER. The compound has been shown to be capable of correcting the defect in cell lines and mice expressing DeltaF508 CFTR. In this work, poly lactic-co-glycolic acid (PLGA) nanoparticles encapsulating curcumin were synthesized and used to treat two different CF mouse strains in an effort to correct the defects associated with CF by improving bioavailability of the compound, which has previously been a challenge in treatment with curcumin. Our results suggest that oral administration of PLGA nanoparticles encapsulating curcumin enhances the effects of curcumin therapy in CF mice, as compared to delivery of nonencapsulated curcumin.

Bcl-2 suppresses sarcoplasmic/endoplasmic reticulum Ca2+-ATPase expression in cystic fibrosis airways: role in oxidant-mediated cell death

RATIONALE

Modulation of the activity of sarcoendoplasmic reticulum calcium ATPase (SERCA) can profoundly affect Ca(2+) homeostasis. Although altered calcium homeostasis is a characteristic of cystic fibrosis (CF), the role of SERCA is unknown.

OBJECTIVES

This study provides a comprehensive investigation of expression and activity of SERCA in CF airway epithelium. A detailed study of the mechanisms underlying SERCA changes and its consequences was also undertaken.

METHODS

Lung tissue samples (bronchus and bronchiole) from subjects with and without CF were evaluated by immunohistochemistry. Protein and mRNA expression in primary non-CF and CF cells was determined by Western and Northern blots.

MEASUREMENTS AND MAIN RESULTS

SERCA2 expression was decreased in bronchial and bronchiolar epithelia of subjects with CF. SERCA2 expression in lysates of polarized tracheobronchial epithelial cells from subjects with CF was decreased by 67% as compared with those from subjects without CF. Several non-CF and CF airway epithelial cell lines were also probed. SERCA2 expression and activity were consistently decreased in CF cell lines. Adenoviral expression of mutant F508 cystic fibrosis transmembrane regulator gene (CFTR), inhibition of CFTR function pharmacologically (CFTR(inh)172), or stable expression of antisense oligonucleotides to inhibit CFTR expression caused decreased SERCA2 expression. In CF cells, SERCA2 interacted with Bcl-2, leading to its displacement from caveolae-related domains of endoplasmic reticulum membranes, as demonstrated in sucrose density gradient centrifugation and immunoprecipitation studies. Knockdown of SERCA2 using siRNA enhanced epithelial cell death due to ozone, hydrogen peroxide, and TNF-alpha.

CONCLUSIONS

Reduced SERCA2 expression may alter calcium signaling and apoptosis in CF. These findings decrease the likelihood of therapeutic benefit of SERCA inhibition in CF.

CFTR function and prospects for therapy

Mutations in the gene coding for the cystic fibrosis transmembrane conductance regulator (CFTR) epithelial anion channel cause cystic fibrosis (CF). The multidomain integral membrane glycoprotein, a member of the adenine nucleotide-binding cassette (ABC) transporter family, conserved in metazoan salt-transporting tissues, is required to control ion and fluid homeostasis on epithelial surfaces. This review considers different therapeutic strategies that have arisen from knowledge of CFTR structure and function as well as its biosynthetic processing, intracellular trafficking, and turnover.

Role of N-linked oligosaccharides in the biosynthetic processing of the cystic fibrosis membrane conductance regulator

The epithelial chloride channel CFTR is a glycoprotein that is modified by two N-linked oligosaccharides. The most common mutant CFTR protein in patients with cystic fibrosis, DeltaF508, is misfolded and retained by ER quality control. As oligosaccharide moieties of glycoproteins are known to mediate interactions with ER lectin chaperones, we investigated the role of N-linked glycosylation in the processing of wild-type and DeltaF508 CFTR. We found that N-glycosylation and ER lectin interactions are not major determinants of trafficking of wild-type and DeltaF508 from the ER to the plasma membrane. Unglycosylated CFTR, generated by removal of glycosylation sites or treatment of cells with the N-glycosylation inhibitor tunicamycin, did not bind calnexin, but did traffic to the cell surface and exhibited chloride channel activity. Most importantly, unglycosylated DeltaF508 CFTR still could not escape quality control in the early secretory pathway and remained associated with the ER. However, the absence of N-linked oligosaccharides did reduce the stability of wild-type CFTR, causing significantly more-rapid turnover in post-ER compartments. Surprisingly, the individual N-linked carbohydrates do not play equivalent roles and modulate the fate of the wild-type protein in different ways in its early biosynthetic pathway.

Failure of bone morphogenetic protein receptor trafficking in pulmonary arterial hypertension: potential for rescue

Heterozygous germline mutations in the gene encoding the bone morphogenetic protein type II receptor cause familial pulmonary arterial hypertension (PAH). We previously demonstrated that the substitution of cysteine residues in the ligand-binding domain of this receptor prevents receptor trafficking to the cell membrane. Here we demonstrate the potential for chemical chaperones to rescue cell-surface expression of mutant BMPR-II and restore function. HeLa cells were transiently transfected with BMPR-II wild type or mutant (C118W) receptor constructs. Immunolocalization studies confirmed the retention of the cysteine mutant receptor mainly in the endoplasmic reticulum. Co-immunoprecipitation studies of Myc-tagged BMPR-II confirmed that the cysteine-substituted ligand-binding domain mutation, C118W, is able to associate with BMP type I receptors. Furthermore, following treatment with a panel of chemical chaperones (thapsigargin, glycerol or sodium 4-phenylbutyrate), we demonstrated a marked increase in cell-surface expression of mutant C118W BMPR-II by FACS analysis and confocal microscopy. These agents also enhanced the trafficking of wild-type BMPR-II, though to a lesser extent. Increased cell-surface expression of mutant C118W BMPR-II was associated with enhanced Smad1/5 phosphorylation in response to BMPs. These findings demonstrate the potential for rescue of mutant BMPR-II function from the endoplasmic reticulum. For the C118W mutation in the ligand-binding domain of BMPR-II, cell-surface rescue leads to at least partial restoration of BMP signalling. We conclude that enhancement of cell-surface trafficking of mutant and wild-type BMPR-II may have therapeutic potential in familial PAH.

Chloride channels as drug targets

Chloride channels represent a relatively under-explored target class for drug discovery as elucidation of their identity and physiological roles has lagged behind that of many other drug targets. Chloride channels are involved in a wide range of biological functions, including epithelial fluid secretion, cell-volume regulation, neuroexcitation, smooth-muscle contraction and acidification of intracellular organelles. Mutations in several chloride channels cause human diseases, including cystic fibrosis, macular degeneration, myotonia, kidney stones, renal salt wasting and hyperekplexia. Chloride-channel modulators have potential applications in the treatment of some of these disorders, as well as in secretory diarrhoeas, polycystic kidney disease, osteoporosis and hypertension. Modulators of GABA(A) (gamma-aminobutyric acid A) receptor chloride channels are in clinical use and several small-molecule chloride-channel modulators are in preclinical development and clinical trials. Here, we discuss the broad opportunities that remain in chloride-channel-based drug discovery.

The role of the UPS in cystic fibrosis

CF is an inherited autosomal recessive disease whose lethality arises from malfunction of CFTR, a single chloride (Cl^-) ion channel protein. CF patients harbor mutations in the CFTR gene that lead to misfolding of the resulting CFTR protein, rendering it inactive and mislocalized. Hundreds of CF-related mutations have been identified, many of which abrogate CFTR folding in the endoplasmic reticulum (ER). More than 70% of patients harbor the ΔF508 CFTR mutation that causes misfolding of the CFTR proteins. Consequently, mutant CFTR is unable to reach the apical plasma membrane of epithelial cells that line the lungs and gut, and is instead targeted for degradation by the UPS. Proteins located in both the cytoplasm and ER membrane are believed to identify misfolded CFTR for UPS-mediated degradation. The aberrantly folded CFTR protein then undergoes polyubiquitylation, carried out by an E1-E2-E3 ubiquitin ligase system, leading to degradation by the 26S proteasome. This ubiquitin-dependent loss of misfolded CFTR protein can be inhibited by the application of ‘corrector’ drugs that aid CFTR folding, shielding it from the UPS machinery. Corrector molecules elevate cellular CFTR protein levels by protecting the protein from degradation and aiding folding, promoting its maturation and localization to the apical plasma membrane. Combinatory application of corrector drugs with activator molecules that enhance CFTR Cl^- ion channel activity offers significant potential for treatment of CF patients.

Publication history: Republished from Current BioData's Targeted Proteins database (TPdb; ).

Maintaining low Ca2+ level in the endoplasmic reticulum restores abnormal endogenous F508del-CFTR trafficking in airway epithelial cells

The most common mutation in cystic fibrosis, F508del, results in cystic fibrosis transmembrane conductance regulator protein (CFTR) that is retained in the endoplasmic reticulum (ER). Retention is dependent on chaperone proteins, many of which, like calnexin, require calcium for optimal activity. Here, we show that a limited and a maintained ER calcium level is sufficient to inhibit the F508del-CFTR/calnexin interaction and to restore the cAMP-dependent CFTR chloride transport, thus showing the correction of abnormal trafficking. We used Western blot analysis, iodide efflux and calcium measurement techniques applied to the human airway epithelial cystic fibrosis cell line CF15 (F508del/F508del). The inhibition of ER calcium pump, with thapsigargin, curcumin, 2,5-di(t-butyl)hydroquinone or cyclopiazonic acid, maintains a threshold levels of calcium that is correlated to the recovery of endogenous F508del-CFTR transport activity. In particular, cyclopiazonic acid restores a 2-aminoethyoxydiphenyl borate-sensitive F508del-CFTR trafficking with an EC50 of 915 nm. By contrast, the 1,4,5-trisphosphate or IP3 receptor activators, i.e., ATP and histamine, while transiently emptying the ER intracellular calcium store, did not affect the trafficking of F508del-CFTR. Our data suggest that decreasing the ER calcium level is not sufficient to restore the defective trafficking of F508del-CFTR, whereas decreasing and also maintaining low ER calcium level allow correction of defective biosynthetic pathway of endogenous F508del-CFTR in human airway epithelial cells.

Additive effect of multiple pharmacological chaperones on maturation of CFTR processing mutants

The most common cause of CF (cystic fibrosis) is the deletion of Phe(508) (DeltaF508) in the CFTR [CF TM (transmembrane) conductance regulator] chloride channel. One major problem with DeltaF508 CFTR is that the protein is defective in folding so that little mature protein is delivered to the cell surface. Expression of DeltaF508 CFTR in the presence of small molecules known as correctors or pharmacological chaperones can increase the level of mature protein. Unfortunately, the efficiency of corrector-induced maturation of DeltaF508 CFTR is probably too low to have therapeutic value and approaches are needed to increase maturation efficiency. We postulated that expression of DeltaF508 CFTR in the presence of multiple correctors that bound to different sites may have an additive effect on maturation. In support of this mechanism, we found that expression of P-glycoprotein (CFTR's sister protein) processing mutants in the presence of two compounds that bind to different sites (rhodamine B and Hoechst 33342) had an additive effect on maturation. Therefore we tested whether expression of DeltaF508 CFTR in the presence of combinations of three different classes of corrector molecules would increase its maturation efficiency. It was found that the combination of the quinazoline VRT-325 together with the thiazole corr-2b or bisaminomethylbithiazole corr-4a doubled the steady-state maturation efficiency of DeltaF508 CFTR (approx. 40% of total CFTR was mature protein) compared with expression in the presence of a single compound. The additive effect of the correctors on DeltaF508 CFTR maturation suggests that they directly interact at different sites of the protein.

Curcumin induced suicidal erythrocyte death

The natural nutrient component Curcumin with anti-inflammatory and antitumor activity has previously been shown to stimulate apoptosis of several nucleated cell types. The present study has been performed to explore whether Curcumin could similarly induce suicidal death of erythrocytes or eryptosis, which is characterized by cell shrinkage and cell membrane scrambling with phosphatidylserine exposure at the erythrocyte surface. Phosphatidylserine exposing cells are phagocytosed and thus rapidly cleared from circulating blood. Erythrocyte membrane scrambling may be triggered by increase of cytosolic Ca(2+) activity or formation of ceramide. To test for eryptosis, erythrocyte phosphatidylserine exposure has been estimated from annexin V binding, and erythrocyte volume from forward scatter in FACS analysis. Exposure of erythrocytes to Curcumin (= 1 microM) increased annexin V binding and decreased forward scatter, pointing to phosphatidylserine exposure at the cell surface and cell shrinkage. According to Fluo3 fluorescence Curcumin increased cytosolic Ca(2+) activity and according to immunofluorescence Curcumin increased ceramide formation. As shown previously, hypertonic shock (addition of 550mM sucrose), chloride removal and glucose depletion decreased the forward scatter and increased annexin V binding. The effects on annexin binding were enhanced in the presence of Curcumin. Exposure to Curcumin did, however, not significantly enhance the shrinking effect of hypertonic shock or Cl(-) removal and reversed the shrinking effect of glucose withdrawal. The present observations disclose a proeryptotic effect of Curcumin which may affect the life span of circulating erythrocytes.

Curcumin induces electrical activity in rat pancreatic beta-cells by activating the volume-regulated anion channel

Curcumin, the principal active component of turmeric, is reported to exert a number of therapeutic actions, including a hypoglycaemic/antidiabetic action. The underlying mechanisms to this action are essentially unknown. We have investigated the hypothesis that a direct stimulatory action on the pancreatic beta-cell could contribute towards the hypoglycaemic activity of this compound. Electrical and ion channel activity were recorded in rat beta-cells using the patch-clamp technique. beta-Cell volume was measured using a video-imaging technique. Insulin release was measured from intact islets by radioimmunoassay. Curcumin (2-10 microM) activated the volume-regulated anion channel in beta-cells. Single channel studies indicated that activation was the result of increased channel open probability. This effect was accompanied by depolarisation of the cell membrane potential, the generation of electrical activity and enhanced insulin release. Curcumin also decreased beta-cell volume, presumably reflecting loss of Cl(-) (and hence water) as a result of anion channel activation. These findings are consistent with the suggestion that Cl(-) fluxes play an important role in regulating beta-cell function. The stimulation of beta-cell function by curcumin could contribute to the hypoglycaemic actions of this compound, and these findings identify a novel potential therapeutic target for the treatment of type 2 diabetes mellitus.

Curcumin enhances cystic fibrosis transmembrane regulator expression by down-regulating calreticulin

Curcumin has been reported to correct cystic fibrosis caused by the DeltaF508 mutation of the cystic fibrosis transmembrane regulator (CFTR) but its mechanistic action remains unclear. We have recently demonstrated that the ER chaperone calreticulin (CRT) negatively regulates the CFTR cell surface expression and activity. Thus, we aimed at determining whether CRT mediates the effect of curcumin on CFTR. We show here that the treatment with curcumin of Chinese hamster ovary cells suppressed CRT expression and increased wild-type CFTR but did not affect DeltaF508 CFTR expression. However, we determined that although curcumin did not augment DeltaF508 CFTR expression, it enhanced the functional competence of DeltaF508 CFTR induced by 26 degrees C incubation. Knock down of CRT by siRNA at low-temperature had a similar effect. Our findings suggest that the positive effect of curcumin on CFTR expression is mediated through the down-regulation of CRT, a negative regulator of CFTR.

Functional analysis of the extracellular cysteine residues in the human organic anion transporting polypeptide, OATP2B1

Organic anion transporting polypeptide (OATP) superfamily member 2B1 (OATP2B1) mediates the uptake of steroid hormone precursors and selected drugs in the placenta, liver, mammary gland, brain, and intestine. This action is modulated by sulfhydryl reagents. Common to all OATPs is a large extracellular loop between transmembrane domains IX and X with 10 conserved cysteines. To elucidate the structure-function relationship of this cysteine rich ectodomain, a truncated OATP2B1 lacking 10 extracellular cysteines (OATP2B1(Delta489-557)) and 10 OATP2B1 mutants containing individual Cys-to-Ala substitutions were generated and expressed in CHO-K1 cells. The immunolocalization, cell-surface expression, transport activity, and free cysteine labeling with N-biotinoylaminoethylmethane-thiosulfonate of mutant proteins and wild-type OATP2B1 were compared. OATP2B1(Delta489-557) accumulated intracellularly. Nine Cys-to-Ala substitutions, C489A, C495A, C504A, C516A, C520A, C539A, C541A, C553A, and C557A, were misprocessed, appearing predominantly as core-glycosylated, 60-kDa proteins and as 180-kDa complexes. Only C493A was a fully glycosylated 75-kDa protein expressed at the cell surface. Thapsigargin partially corrected the misprocessing of mutants. Compared with OATP2B1, C493A and C557A transported estrone-3-sulfate and dehydroepiandrosterone sulfate less efficiently, whereas all other mutants were functionally impaired. MTSEA labeled free cysteines in all Cys-to-Ala mutants but not in OATP2B1, suggesting that all 10 extracellular cysteines are normally disulfide-bonded. Our findings show that the trafficking and function of OATP2B1 is vulnerable to changes in the cysteine residues of extracellular loop IX-X.

Specific Rescue of Cystic Fibrosis Transmembrane Conductance Regulator Processing Mutants Using Pharmacological Chaperones

Most mutants of the cystic fibrosis transmembrane conductance regulator (CFTR) that cause severe symptoms of cystic fibrosis do not reach the cell surface because they are defective in folding. Many CFTR folding mutants, however, including the DeltaF508 mutant found in more than 90% of cystic fibrosis patients, are potentially functional at the cell surface if they can be induced to fold correctly. In a previous study (Mol Pharm 2:407-413, 2005), we reported that a quinazoline derivative (CFcor-325) could rescue CFTR processing mutants. The corrector was not specific however, as it could also rescue a processing mutant of CFTR's sister protein, the multidrug resistance P-glycoprotein. The goal of this study was to test whether it was possible to specifically rescue CFTR processing mutants using a pharmacological chaperone. In this article, we report that two compounds, 4-methyl-2-(5-phenyl-1H-pyrazol-3-yl)-phenol (CFpot-532) and 2-phenylamino-4-(4-ethylene-phenyl)-thiazole (corr-2b) could rescue CFTR processing mutants such as DeltaF508 CFTR but not a P-glycoprotein processing mutant. The compound CFpot-532 also acts as a potentiator of DeltaF508 CFTR channel activity. Therefore, the results suggest that the mechanism whereby CFpot-532 and corr-2b promote folding of CFTR processing mutants is through direct interaction with the CFTR mutant proteins. The compound CFpot-532 could be a particularly useful lead compound for treatment of cystic fibrosis because it is both a CFTR channel potentiator as well as a specific pharmacological chaperone.

Recent progress in understanding the mechanism of P-glycoprotein-mediated drug efflux

P-glycoprotein (P-gp) is an ATP-dependent drug pump that can transport a broad range of hydrophobic compounds out of the cell. The protein is clinically important because of its contribution to the phenomenon of multidrug resistance during AIDS/HIV and cancer chemotherapy. P-gp is a member of the ATP-binding cassette (ABC) family of proteins. It is a single polypeptide that contains two repeats joined by a linker region. Each repeat has a transmembrane domain consisting of six transmembrane segments followed by a hydrophilic domain containing the nucleotide-binding domain. In this mini-review, we discuss recent progress in determining the structure and mechanism of human P-glycoprotein.

CFTR is required for PKA-regulated ATP sensitivity of Kir1.1 potassium channels in mouse kidney

The cystic fibrosis transmembrane conductance regulator (CFTR) Cl- channel plays vital roles in fluid transport in many epithelia. While CFTR is expressed along the entire nephron, its function in renal tubule epithelial cells remains unclear, as no specific renal phenotype has been identified in cystic fibrosis. CFTR has been proposed as a regulator of the 30 pS, ATP-sensitive renal K channel (Kir1.1, also known as renal outer medullar K [ROMK]) that is critical for K secretion by cells of the thick ascending limb (TAL) and distal nephron segments responsive to aldosterone. We report here that both ATP and glibenclamide sensitivities of the 30 pS K channel in TAL cells were absent in mice lacking CFTR and in mice homozygous for the deltaF508 mutation. Curcumin treatment in deltaF508-CFTR mice partially reversed the defect in ATP sensitivity. We demonstrate that the effect of CFTR on ATP sensitivity was abrogated by increasing PKA activity. We propose that CFTR regulates the renal K secretory channel by providing a PKA-regulated functional switch that determines the distribution of open and ATP-inhibited K channels in apical membranes. We discuss the potential physiological role of this functional switch in renal K handling during water diuresis and the relevance to renal K homeostasis in cystic fibrosis.

Rescue of DeltaF508-CFTR (cystic fibrosis transmembrane conductance regulator) by curcumin: involvement of the keratin 18 network

The most common mutation in the cystic fibrosis transmembrane conductance regulator (CFTR) gene, DeltaF508, causes retention of DeltaF508-CFTR in the endoplasmic reticulum and leads to the absence of CFTR Cl(-) channels in the plasma membrane. DeltaF508-CFTR retains some Cl(-) channel activity so increased expression of DeltaF508-CFTR in the plasma membrane can restore Cl(-) secretion deficiency. Recently, curcumin was shown to rescue DeltaF508-CFTR localization and function. In our previous work, the keratin 18 (K18) network was implicated in DeltaF508-CFTR trafficking. Here, we hypothesized that curcumin could restore a functional DeltaF508-CFTR to the plasma membrane acting via the K18 network. First, we analyzed the effects of curcumin on the localization of DeltaF508-CFTR in different cell lines (HeLa cells stably transfected with wild-type CFTR or DeltaF508-CFTR, CALU-3 cells, or cystic fibrosis pancreatic epithelial cells CFPAC-1) and found that it was significantly delocalized toward the plasma membrane in DeltaF508-CFTR-expressing cells. We also performed a functional assay for the CFTR chloride channel in CFPAC-1 cells treated or not with curcumin and detected an increase in a cAMP-dependent chloride efflux in treated DeltaF508-CFTR-expressing cells. The K18 network then was analyzed by immunocytochemistry and immunoblot exclusively in curcumin-treated or untreated CFPAC-1 cells because of their endogenic DeltaF508-CFTR expression. After curcumin treatment, we observed a remodeling of the K18 network and a significant increase in K18 Ser52 phosphorylation, a site directly implicated in the reorganization of intermediate filaments. With these results, we propose that K18 as a new therapeutic target and curcumin, and/or its analogs, might be considered as potential therapeutic agents for cystic fibrosis.

Rescue of Folding Defects in ABC Transporters Using Pharmacological Chaperones

The ATP-binding cassette (ABC) family of membrane transport proteins is the largest class of transporters in humans (48 members). The majority of ABC transporters function at the cell surface. Therefore, defective folding and trafficking of the protein to the cell surface can lead to serious health problems. The classic example is cystic fibrosis (CF). In most CF patients, there is a deletion of Phe508 in the CFTR protein (DeltaF508 CFTR) that results in defective folding and intracellular retention of the protein (processing mutant). A potential treatment for most patients with CF would be to use a ligand(s) of CFTR that acts a pharmacological chaperone to correct the folding defect. The feasibility of such an approach was first demonstrated with the multidrug transporter P-glycoprotein (P-gp), an ABC transporter, and a sister protein of CFTR. It was found that P-gps with mutations at sites equivalent to those found in CFTR processing mutants were rescued when they were expressed in the presence of drug substrates or modulators of P-gp. These compounds acted as pharmacological chaperones and functioned by promoting interactions among the various domains in the protein during the folding process. Several groups have attempted to identify compounds that could rescue the folding defect in DeltaF508 CFTR. The best compound identified through high-throughout screening is a quinazoline derivative (CFcor-325). Expression of DeltaF508 CFTR as well as other CFTR processing mutants in the presence of 1 muM CFcor-325 promoted folding and trafficking of the mutant proteins to the cell surface in an active conformation. Therefore, CFcor-325 and other quinazoline derivates could be important therapeutic compounds for the treatment of CF.

SERCA pump inhibitors do not correct biosynthetic arrest of deltaF508 CFTR in cystic fibrosis

Deletion of phenylalanine 508 (deltaF508) accounts for nearly 70% of all mutations that occur in the cystic fibrosis transmembrane conductance regulator (CFTR). The deltaF508 mutation is a class II processing mutation that results in very little or no mature CFTR protein reaching the apical membrane and thus no cAMP-mediated Cl- conductance. Therapeutic strategies have been developed to enhance processing of the defective deltaF508 CFTR molecule so that a functional cAMP-regulated Cl- channel targets to the apical membrane. Sarcoplasmic/endoplasmic reticulum calcium (SERCA) inhibitors, curcumin and thapsigargin, have been reported to effectively correct the CF ion transport defects observed in the deltaF508 CF mice. We investigated the effect of these compounds in human airway epithelial cells to determine if they could induce deltaF508 CFTR maturation, and Cl- secretion. We also used Baby Hamster Kidney cells, heterologously expressing deltaF508 CFTR, to determine if SERCA inhibitors could interfere with the interaction between calnexin and CFTR and thereby correct the deltaF508 CFTR misfolding defect. Finally, at the whole animal level, we tested the ability of curcumin and thapsigargin to (1) induce Cl- secretion and reduce hyperabsorption of Na+ in the nasal epithelia of the deltaF508 mouse in vivo, and (2) induce Cl- secretion in intestine (jejunum and distal colon) and the gallbladder of the deltaF508 CF mouse. We conclude that curcumin and thapsigargin failed to induce maturation of deltaF508 CFTR, or induce Cl- secretion, as measured by biochemical and electrophysiologic techniques in a variety of model systems ranging from cultured cells to in vivo studies.

Expression of P-glycoprotein in L1210 cells is linked with rise in sensitivity to Ca2+

L1210/VCR cell line (R) was obtained by adaptation of the L1210 mouse leukaemia cells (S) to vincristine and showed P-glycoprotein (P-gp) mediated multidrug resistance (MDR). R cells were observed to be more sensitive to high external calcium as parental S. More pronounced calcium uptake was observed for R cells. Moreover, differences in intracellular calcium cell localization between S and R cells were found ultrastructurally following a calcium precipitating cytochemical method. In S cells, calcium precipitates were found to be localized predominantly along the cell surface coat and within mitochondria delineating the cristae. In R cells, precipitates were also found inside nuclei, at the border of heterochromatin clumps, and scattered within the cytoplasm. High extracellular calcium did not influence the P-gp mediated extrusion of calcein/AM as P-gp substrate. These results indicate that calcium enters and consequently damages the MDR cells to a higher extent than parental cells.

Curcumin modulation of Na,K-ATPase: phosphoenzyme accumulation, decreased K+ occlusion, and inhibition of hydrolytic activity

1 Curcumin, the major constitute of tumeric, is an important nutraceutical that has been shown to be useful in the treatment of many diseases. As an inhibitor of the sarcoplasmic reticulum Ca(2+)-ATPase, curcumin was shown to correct cystic fibrosis (CF) defects in some model systems, whereas others have reported no or little effects on CF after curcumin treatment, suggesting that curcumin effect is not due to simple inhibition of the Ca(2+)-ATPase. 2 We tested the hypothesis that curcumin may modulate other members of the P(2)-type ATPase superfamily by studying the effects of curcumin on the activity and kinetic properties of the Na,K-ATPase. 3 Curcumin treatment inhibited Na,K-ATPase activity in a dose-dependent manner (K(0.5) approximately 14.6 microM). Curcumin decreased the apparent affinity of Na,K-ATPase for K(+) and increased it for Na(+) and ATP. Kinetic analyses indicated that curcumin induces a three-fold reduction in the rate of E1P --> E2P transition, thereby increasing the steady-state phosphoenzyme level. Curcumin treatment significantly abrogated K(+) occlusion to the enzyme as evidenced from kinetic and proteolytic cleavage experiments. Curcumin also significantly decreased the vanadate sensitivity of the enzyme. 4 Thus, curcumin partially blocks the K(+) occlusion site, and induces a constitutive shift in the conformational equilibrium of the enzyme, towards the E1 conformation. 5 The physiological consequences of curcumin treatment previously reported in different epithelial model systems may, at least in part, be related to the direct effects of curcumin on Na,K-ATPase activity.

Hemin induces active chloride secretion in Caco-2 cells

Enterocytes maintain fluid-electrolyte homeostasis by keeping a tight barrier and regulating ion channels. Carbon monoxide (CO), a product of heme degradation, modulates electrolyte transport in kidney and lung epithelium, but its role in regulating intestinal fluid-electrolyte homeostasis has not been studied. The major source of endogenous CO formation comes from the degradation of heme via heme oxygenase. We hypothesized that heme activates electrolyte transport in intestinal epithelial cells. Basolateral hemin treatment increased baseline Caco-2 cell short-circuit currents (I(sc)) twofold (control = 1.96 +/- 0.14 microA/cm(2) vs. hemin = 4.07 +/- 0.16 microA/cm(2), P < 0.01); apical hemin had no effect. Hemin-induced I(sc) was caused by Cl- secretion because it was inhibited in Cl- -free medium, with ouabain, 5-nitro-2-(3-phenylpropylamino) benzoic acid (NPPB), or DIDS. Apical electrogenic Na+ channel inhibitor benzamil had no effect on hemin-induced I(sc). Hemin did not alter the ability of Caco-2 cells to respond maximally to forskolin, but a soluble guanylate cyclase inhibitor, [1,2,4]oxadiazolo-[4,3-a]quinoxalin-1-one (ODQ) inhibited the effects of hemin. A CO-releasing molecule, tricarbonyldichlororuthenium II, induced active Cl- secretion that was also inhibited with ODQ. We conclude that hemin induces active Cl- secretion in Caco-2 cells via a cGMP-dependent pathway. These effects are probably the consequence of CO formation. Heme and CO may be important regulators of intestinal fluid-electrolyte homeostasis.

Chronic exposure to EGF affects trafficking and function of ENaC channel in cystic fibrosis cells

Using the whole-cell patch-clamp technique, we identified an amiloride (AMI)-sensitive Na(+) current in cystic fibrosis cells, JME/CF15, growing in standard medium. The reversal potential of this current depended on Na(+) concentrations and the cation selectivity was much higher for Na(+) than for K(+), indicating that the current is through ENaC channels. In contrast, cells from EGF-containing medium lacked AMI-sensitive Na(+) currents. In permeabilized cells growing in EGF-containing medium, alphaENaC was mainly detected in a perinuclear region, while in cells from standard medium it was distributed over the cell body. Western-blot analysis showed that in standard medium cells expressed fast-migrating EndoH-insensitive and slow-migrating EndoH-sensitive alphaENaC fractions, while in cells growing in the presence of EGF, alphaENaC was only detected as the fast-migrating EndoH-insensitive fraction. Long-term incubation of cells with EGF resulted in an increased basal Ca(2+) level, [Ca(2+)](i). A similar increase of [Ca(2+)](i) was also observed in the presence of 2muM thapsigargin, resulting in inhibition of ENaC function. Thus, in JME/CF15 cells inhibition of the ENaC function by chronic incubation with EGF is a Ca(2+)-mediated process that affects trafficking and surface expression of ENaC channels.

Curcumin Stimulates Cystic Fibrosis Transmembrane Conductance Regulator Cl– Channel Activity

Compounds that enhance either the function or biosynthetic processing of the cystic fibrosis transmembrane conductance regulator (CFTR) Cl(-) channel may be of value in developing new treatments for cystic fibrosis (CF). Previous studies suggested that the herbal extract curcumin might affect the processing of a common CF mutant, CFTR-DeltaF508. Here, we tested the hypothesis that curcumin influences channel function. Curcumin increased CFTR channel activity in excised, inside-out membrane patches by reducing channel closed time and prolonging the time channels remained open. Stimulation was dose-dependent, reversible, and greater than that observed with genistein, another compound that stimulates CFTR. Curcumin-dependent stimulation required phosphorylated channels and the presence of ATP. We found that curcumin increased the activity of both wild-type and DeltaF508 channels. Adding curcumin also increased Cl(-) transport in differentiated non-CF airway epithelia but not in CF epithelia. These results suggest that curcumin may directly stimulate CFTR Cl(-) channels.