Characterization of phosphorylation-defective mutants of human P-glycoprotein expressed in mammalian cells

Protein Kinases and Cross-talk between Post-translational Modifications in the Regulation of Drug Transporters

Drug transporters are modulators for drug absorption, distribution, and excretion. Key drug transporters including P-glycoprotein and breast cancer resistance protein of the ABC superfamily; organic anion transporting polypeptide 1B1 and 1B3, organic anion transporter 1 and 3, and organic cation transporter 2, as well as multidrug and toxin extrusion 1 and 2 of the SLC superfamily have been recommended by regulatory agencies to be investigated and evaluated in drug-drug interaction (DDI) studies due to their important roles in determining the efficacy, toxicity and DDI of various drugs. Drug transporters are subjected to multiple levels of control and post-translational modifications (PTMs) provide rapid and versatile ways of regulation. Under pathologic and/or pharmacological conditions, PTMs may be altered in the cellular system, leading to functional changes of transporter proteins. Phosphorylation is by far the most actively investigated form of PTMs in the regulation of transporters. Further, studies in recent years also found that protein kinases coordinate with other PTMs for the dynamic control of these membrane proteins. Here we summarized the regulation of major drug transporters by protein kinases and their cross-talking with other PTMs that may generate a complex regulatory network for fine-tuning the function of these important drug processing modulators. SIGNIFICANCE STATEMENT: Kinases regulate drug transporters in versatile manners; Kinase regulation cross-talks with other PTMs, forming a complex network for transporter regulation; Pathological and/or pharmacological conditions may alter PTMs and affect transporter function with different molecular mechanisms.

Possible Regulation of P-glycoprotein Function by Adrenergic Agonists in a Vascular-luminal Perfused Preparation of Small Intestine

Although the functions of small intestine are largely regulated by enteric nervous system (ENS), an independent intrinsic innervation, as well as central nervous system (CNS), the neural regulation of drug absorption from the small intestine still remains to be clarified. To obtain some information on it, the effect of adrenergic agonists on P-glycoprotein (P-gp) function was investigated by utilizing a vascular-luminal perfused rat small intestine. Adrenaline significantly decreased the secretion of rhodamine-123 (R-123) into the intestinal lumen, but dibutyryl cAMP (DBcAMP) significantly enhanced R-123 secretion. The inhibition study with quinidine clearly indicated that the decrease in secretory clearance of R-123 by adrenaline or the increase by DBcAMP would be attributed to the decrease or increase in P-gp activity, respectively. Expression levels of P-gp in whole mucosal homogenates were not changed at all by any chemicals examined, but those on brush border membrane (BBM) of intestinal epithelial cells were significantly decreased or increased by adrenaline or DBcAMP, respectively. Furthermore, changes in P-gp activity caused by adrenergic agonists and DBcAMP were significantly correlated with changes in expression level of P-gp in BBM, suggesting that the trafficking of P-gp from cytosolic pool to BBM would be regulated by adrenergic agonists and DBcAMP.

The complexities of PKCα signaling in cancer

Protein kinase C α (PKCα) is a ubiquitously expressed member of the PKC family of serine/threonine kinases with diverse functions in normal and neoplastic cells. Early studies identified anti-proliferative and differentiation-inducing functions for PKCα in some normal tissues (e.g., regenerating epithelia) and pro-proliferative effects in others (e.g., cells of the hematopoietic system, smooth muscle cells). Additional well documented roles of PKCα signaling in normal cells include regulation of the cytoskeleton, cell adhesion, and cell migration, and PKCα can function as a survival factor in many contexts. While a majority of tumors lose expression of PKCα, others display aberrant overexpression of the enzyme. Cancer-related mutations in PKCα are uncommon, but rare examples of driver mutations have been detected in certain cancer types (e. g., choroid gliomas). Here we review the role of PKCα in various cancers, describe mechanisms by which PKCα affects cancer-related cell functions, and discuss how the diverse functions of PKCα contribute to tumor suppressive and tumor promoting activities of the enzyme. We end the discussion by addressing mutations and expression of PKCα in tumors and the clinical relevance of these findings.

Molecular Regulation of Canalicular ABC Transporters

The ATP-binding cassette (ABC) transporters expressed at the canalicular membrane of hepatocytes mediate the secretion of several compounds into the bile canaliculi and therefore play a key role in bile secretion. Among these transporters, ABCB11 secretes bile acids, ABCB4 translocates phosphatidylcholine and ABCG5/G8 is responsible for cholesterol secretion, while ABCB1 and ABCC2 transport a variety of drugs and other compounds. The dysfunction of these transporters leads to severe, rare, evolutionary biliary diseases. The development of new therapies for patients with these diseases requires a deep understanding of the biology of these transporters. In this review, we report the current knowledge regarding the regulation of canalicular ABC transporters' folding, trafficking, membrane stability and function, and we highlight the role of molecular partners in these regulating mechanisms.

Effect of ABC transporter expression and mutational status on survival rates of cancer patients

ATP-binding cassette (ABC) transporters mediate multidrug resistance in cancer. In contrast to DNA single nucleotide polymorphisms in normal tissues, the role of mutations in tumors is unknown. Furthermore, the significance of their expression for prediction of chemoresistance and survival prognosis is still under debate. We investigated 18 tumors by RNA-sequencing. The mutation rate varied from 27,507 to 300885. In ABCB1, three hotspots with novel mutations were in transmembrane domains 3, 8, and 9. We also mined the cBioPortal database with 11,814 patients from 23 different tumor entities. We performed Kaplan-Meier survival analyses to investigate the effect of ABC transporter expression on survival rates of cancer patients. Novel mutations were also found in ABCA2, ABCA3, ABCB2, ABCB5, ABCC1-6, and ABCG2. Mining the cBioPortal database with 11,814 patients from 23 different tumor entities validated our results. Missense and in-frame mutations led to altered binding of anticancer drugs in molecular docking approaches. The ABCB1 nonsense mutation Q856* led to a truncated P-glycoprotein, which may sensitize tumors to anticancer drugs. The search for ABC transporter nonsense mutations represents a novel approach for precision medicine.. Low ABCB1 mRNA expression correlated with significantly longer survival in ovarian or kidney cancer and thymoma. In cancers of breast, kidney or lung, ABC transporter expression correlated with different tumor stages and human populations as further parameters to refine strategies for more individualized chemotherapy.

A dual PI3 kinase/mTOR inhibitor BEZ235 reverses doxorubicin resistance in ABCB1 overexpressing ovarian and pancreatic cancer cell lines

BACKGROUND

Multi-drug resistance (MDR) develops because cancer cells evade toxicity of several structurally unrelated drugs. Besides other mechanisms, MDR is linked to the overexpression of ATP Binding Cassette (ABC), transporters, among which ABCB1 is the best characterized one. Since overactivation of PI3K/Akt/mTOR plays a pivotal role in the growth of human cancers, we hypothesized whether dual PI3K and mTOR inhibitor, BEZ235 (BEZ, dactolisib) reverses resistance to doxorubicin (DOX).

METHODS

Ovarian (A2780) and pancreatic (MiaPaca2) cancer cells were used to generate DOX-resistant clones by overexpressing ABCB1 or stepwise treatment of DOX. Intracellular accumulation of DOX was measured by flow cytometry after treatment with BEZ.

RESULTS

BEZ treatment caused an increase in intracellular levels of DOX which was almost identical to the naïve parental cell lines. BEZ was found to be a weak substrate for ABCB1 as demonstrated by minimal increase in ATPase activity. BEZ treatment caused a dose-dependent decrease in cell viability in combination with DOX, which was associated with an increase in cleaved PARP expression in the drug resistant clones.

CONCLUSIONS

These results suggest that BEZ is a non-substrate inhibitor of ABCB1 and is able to effectively re-sensitize cells overexpressing ABCB1 to the effects of DOX.

GENERAL SIGNIFICANCE

Dual PI3 Kinase/mTOR inhibitor, BEZ, has the potential to reverse MDR in cancer patients.

Microtubule-targeting anticancer drug eribulin induces drug efflux transporter P-glycoprotein

This study examined the effects of microtubule-targeting anticancer drugs (paclitaxel, cabazitaxel, and eribulin) on the expression of drug efflux transporter P-glycoprotein, which is encoded by MDR1. Paclitaxel and eribulin induced MDR1 promoter activity in a concentration-dependent manner, while cabazitaxel had little effect in human intestinal epithelial LS174T cells. Overexpression of the nuclear receptor pregnane X receptor (PXR) gene (NR1I2) enhanced paclitaxel- and eribulin-induced MDR1 activation, but expression of the nuclear receptor co-repressor silencing mediator for retinoid and thyroid receptors (SMRT) gene (NCOR2) repressed MDR1 activation. Eribulin increased the mRNA and protein expression of P-glycoprotein in LS174T cells. Cellular uptake of rhodamine 123 and calcein-acetoxymethyl ester (calcein-AM), P-glycoprotein substrates, decreased in paclitaxel- or eribulin-treated LS174T cells. Eribulin also increased MDR1 promoter activity in human breast cancer MCF7 cells. The results suggest that the microtubule-targeting anticancer drug eribulin can induce the drug efflux transporter P-glycoprotein via PXR in human intestinal and breast cancer cells and thus influence the efficacy of anticancer drugs.

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Eribulin activates the P-glycoprotein gene (MDR1) promoter in human intestinal LS174T and breast cancer MCF7 cells.

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Eribulin increases mRNA and protein expression of P-glycoprotein in human intestinal LS174T cells.

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Eribulin can induce P-glycoprotein and modulate the efficacy of anticancer drugs.

Nonsynonymous Mutations in Linker-2 of the Pdr5 Multidrug Transporter Identify a New RNA Stability Element

Analysis of synonymous mutations established that although the primary amino acid sequence remains unchanged, alterations in transcription and translation can result in significant phenotypic consequences. We report the novel observation that a series of nonsynonymous mutations in an unconserved stretch of amino acids found in the yeast multidrug efflux pump Pdr5 increases expression, thus enhancing multidrug resistance. Cycloheximide chase experiments ruled out the possibility that the increased steady-state level of Pdr5 was caused by increased protein stability. Quantitative-RT PCR experiments demonstrated that the mutants had levels of PDR5 transcript that were two to three times as high as in the isogenic wild-type strain. Further experiments employing metabolic labeling of mRNA with 4-thiouracil followed by uracil chasing showed that the half-life of PDR5 transcripts was specifically increased in these mutants. Our data demonstrate that the nucleotides encoding unconserved amino acids may be used to regulate expression and suggest that Pdr5 has a newly discovered RNA stability element within its coding region.

Linker Domains: Why ABC Transporters 'Live in Fragments no Longer'

ATP-binding cassette (ABC) transporters are membrane proteins present in all kingdoms of life. We have considered the disordered region that connects the N- and C-terminal halves in many eukaryotic ABC transporters, allowing all four consensus functional domains to be linked. The recent availability of structures of ABC transporters containing linker regions has allowed us to identify the start and end points of the connectors as well as hinting at their localisation. We address questions such as: Where did the linker regions come from? Why do some ABC transporters have connectors and others not? What are the rules and roles of the linker regions? What are the consequences of mutations in these connector regions for disease in humans?

Midostaurin Reverses ABCB1-Mediated Multidrug Resistance, an in vitro Study

Overexpression of ABC transporters in cancer cells is an underlying mechanism of multidrug resistance (MDR), leading to insensitive response to chemotherapeutic strategies. Thus, MDR is often results in treatment failure in the clinic. In this study, we found midostaurin, a Food and Drug Administration (FDA)-approved anti-leukemia drug, can antagonize ATP-binding cassette subfamily B member 1 (ABCB1)-mediated MDR. Our results indicated that midostaurin has the capacity to antagonize ABCB1-mediated MDR, while no significant reversal effect was found on ATP-binding cassette subfamily G member 2 (ABCG2)-mediated MDR. Our subsequent resistance mechanism studies showed that midostaurin directly inhibited the efflux function of the ABCB1 transporter without alteration of the expression level or the subcellular localization of ABCB1 transporter. In addition, midostaurin inhibited the ATPase activity of ABCB1 transporter in a dose-dependent manner. Moreover, our in silico docking study predicted that midostaurin could interact with the substrate-binding sites of ABCB1 transporter. This novel finding could provide a promising treatment strategy that co-administrating midostaurin with anticancer drugs in the clinic could overcome MDR and improve the efficiency of cancer treatment.

Post-translational modifications of transporters

Drug transporter proteins are critical to the distribution of a wide range of endogenous compounds and xenobiotics such as hormones, bile acids, peptides, lipids, sugars, and drugs. There are two classes of drug transporters- the solute carrier (SLC) transporters and ATP-binding cassette (ABC) transporters -which predominantly differ in the energy source utilized to transport substrates across a membrane barrier. Despite their hydrophobic nature and residence in the membrane bilayer, drug transporters have dynamic structures and adopt many conformations during the translocation process. Whereas there is significant literature evidence for the substrate specificity and structure-function relationship for clinically relevant drug transporters proteins, there is less of an understanding in the regulatory mechanisms that contribute to the functional expression of these proteins. Post-translational modifications have been shown to modulate drug transporter functional expression via a wide range of molecular mechanisms. These modifications commonly occur through the addition of a functional group (e.g. phosphorylation), a small protein (e.g. ubiquitination), sugar chains (e.g. glycosylation), or lipids (e.g. palmitoylation) on solvent accessible amino acid residues. These covalent additions often occur as a result of a signaling cascade and may be reversible depending on the type of modification and the intended fate of the signaling event. Here, we review the significant role in which post-translational modifications contribute to the dynamic regulation and functional consequences of SLC and ABC drug transporters and highlight recent progress in understanding their roles in transporter structure, function, and regulation.

Biochemical studies on the structure-function relationship of major drug transporters in the ATP-binding cassette family and solute carrier family

Human drug transporters often play key roles in determining drug accumulation within cells. Their activities are often directly related to therapeutic efficacy, drug toxicity as well as drug-drug interactions. However, the progress for interpretation of their crystal structures is relatively slow. Hence, conventional biochemical studies together with computer modeling became useful manners to reveal essential structures of these membrane proteins. Over the years, quite a few structure-function relationship information had been obtained for members of the two major transporter families: the ATP-binding cassette family and the solute carrier family. Critical structural features of drug transporters include transmembrane domains, post-translational modification sites and domains for cell surface assembly and protein-protein interactions. Alterations at these important sites may affect protein stability, trafficking to the plasma membrane and/or ability of transporters to interact with substrates.

Protein Kinases C-Mediated Regulations of Drug Transporter Activity, Localization and Expression

Drug transporters are now recognized as major actors in pharmacokinetics, involved notably in drug–drug interactions and drug adverse effects. Factors that govern their activity, localization and expression are therefore important to consider. In the present review, the implications of protein kinases C (PKCs) in transporter regulations are summarized and discussed. Both solute carrier (SLC) and ATP-binding cassette (ABC) drug transporters can be regulated by PKCs-related signaling pathways. PKCs thus target activity, membrane localization and/or expression level of major influx and efflux drug transporters, in various normal and pathological types of cells and tissues, often in a PKC isoform-specific manner. PKCs are notably implicated in membrane insertion of bile acid transporters in liver and, in this way, are thought to contribute to cholestatic or choleretic effects of endogenous compounds or drugs. The exact clinical relevance of PKCs-related regulation of drug transporters in terms of drug resistance, pharmacokinetics, drug–drug interactions and drug toxicity remains however to be precisely determined. This issue is likely important to consider in the context of the development of new drugs targeting PKCs-mediated signaling pathways, for treating notably cancers, diabetes or psychiatric disorders.

Astrocyte-mediated regulation of multidrug resistance p-glycoprotein in fetal and neonatal brain endothelial cells: age-dependent effects

Brain endothelial cells (BECs) form a major component of the blood-brain barrier (BBB). In late gestation, these cells express high levels of the multidrug transporter p-glycoprotein (P-gp; encoded by Abcb1), which prevents the passage of an array of endogenous factors and xenobiotics into the fetal brain. P-gp levels in the BECs increase dramatically in late gestation, coincident with astrocyte differentiation. However, the role of astrocytes in modulating P-gp in the developing BBB is unknown. We hypothesized that factors produced by astrocytes positively regulate P-gp in BECs. Astrocytes and BECs were isolated from fetal and postnatal guinea pigs. Levels of Abcb1 mRNA and P-gp were increased in BECs co-cultured with astrocytes compared to BECs in monoculture. Moreover, postnatal astrocytes enhanced P-gp function in fetal BECs but fetal astrocytes had no effect on postnatal BECs. These effects were dependent on secreted proteins with a molecular weight in the range of 3-100 kDa. LC/MS-MS revealed significant differences in proteins secreted by fetal and postnatal astrocytes. We propose that astrocytes are critical modulators of P-gp at the developing BBB. As such, aberrations in astrocyte maturation, observed in neurodevelopmental disorders, will likely decrease P-gp at the BBB. This would allow increased transfer of P-gp endogenous and exogenous substrates into the brain, many of which have neurodevelopmental consequences.

Extracellular galectin-3 programs multidrug resistance through Na+/K+-ATPase and P-glycoprotein signaling

Galectin-3 (Gal-3, LGALS3) is a pleotropic versatile, 29-35 kDa chimeric gene product, and involved in diverse physiological and pathological processes, including cell growth, homeostasis, apoptosis, pre-mRNA splicing, cell-cell and cell-matrix adhesion, cellular polarity, motility, adhesion, activation, differentiation, transformation, signaling, regulation of innate/adaptive immunity, and angiogenesis. In multiple diseases, it was found that the level of circulating Gal-3 is markedly elevated, suggesting that Gal-3-dependent function is mediated by specific interaction with yet an unknown ubiquitous cell-surface protein. Recently, we showed that Gal-3 attenuated drug-induced apoptosis, which is one of the mechanisms underlying multidrug resistance (MDR). Here, we document that MDR could be mediated by Gal-3 interaction with the house-keeping gene product e.g., Na+/K+-ATPase, and P-glycoprotein (P-gp). Gal-3 interacts with Na+/K+-ATPase and induces the phosphorylation of P-gp. We also find that Gal-3 binds P-gp and enhances its ATPase activity. Furthermore Gal-3 antagonist suppresses this interaction and results in a decrease of the phosphorylation and the ATPase activity of P-gp, leading to an increased sensitivity to doxorubicin-mediated cell death. Taken together, these findings may explain the reported roles of Gal-3 in diverse diseases and suggest that a combined therapy of inhibitors of Na+/K+-ATPase and Gal-3, and a disease specific drug(s) might be superior to a single therapeutic modality.

Enzastaurin inhibits ABCB1-mediated drug efflux independently of effects on protein kinase C signalling and the cellular p53 status

The PKCβ inhibitor enzastaurin was tested in parental neuroblastoma and rhabdomyosarcoma cell lines, their vincristine-resistant sub-lines, primary neuroblastoma cells, ABCB1-transduced, ABCG2-transduced, and p53-depleted cells. Enzastaurin IC₅₀s ranged from 3.3 to 9.5 μM in cell lines and primary cells independently of the ABCB1, ABCG2, or p53 status. Enzastaurin 0.3125 μM interfered with ABCB1-mediated drug transport. PKCα and PKCβ may phosphorylate and activate ABCB1 under the control of p53. However, enzastaurin exerted similar effects on ABCB1 in the presence or absence of functional p53. Also, enzastaurin inhibited PKC signalling only in concentrations ≥ 1.25 μM. The investigated cell lines did not express PKCβ. PKCα depletion reduced PKC signalling but did not affect ABCB1 activity. Intracellular levels of the fluorescent ABCB1 substrate rhodamine 123 rapidly decreased after wash-out of extracellular enzastaurin, and enzastaurin induced ABCB1 ATPase activity resembling the ABCB1 substrate verapamil. Computational docking experiments detected a direct interaction of enzastaurin and ABCB1. These data suggest that enzastaurin directly interferes with ABCB1 function. Enzastaurin further inhibited ABCG2-mediated drug transport but by a different mechanism since it reduced ABCG2 ATPase activity. These findings are important for the further development of therapies combining enzastaurin with ABC transporter substrates.

Verapamil inhibits tumor progression of chemotherapy-resistant pancreatic cancer side population cells

Tumor side population (SP) cells display stem-like properties that can be modulated by treatment with the calcium channel blocker verapamil. Verapamil can enhance the cytotoxic effects of chemotherapeutic drugs and multi-drug resistance by targeting the transport function of the P-glycoprotein (P-gp). This study focused on the therapeutic potential of verapamil on stem-like SP tumor cells, and further investigated its chemosensitizing effects using L3.6pl and AsPC-1 pancreatic carcinoma models. As compared to parental L3.6pl cells (0.9±0.22%), L3.6pl gemcitabine-resistant cells (L3.6pl_Gres) showed a significantly higher percentage of SP cells (5.38±0.99%) as detected by Hoechst 33342/FACS assays. The L3.6pl_Gres SP cells showed stable gemcitabine resistance, enhanced colony formation ability and increased tumorigenicity. Verapamil effectively inhibited L3.6pl_Gres and AsPC-1 SP cell proliferation in vitro. A pro-apoptotic effect of verapamil was observed in L3.6pl cells, but not in L3.6pl_Gres cells, which was linked to their differential expression of P-gp and equilibrative nucleoside transporter-1 (ENT-1). In an orthotopic pancreatic cancer mouse model, both low and high dose verapamil was shown to substantially reduce L3.6pl_Gres-SP cell tumor growth and metastasis, enhance tumor apoptosis, and reduce microvascular density.

Bafetinib (INNO-406) reverses multidrug resistance by inhibiting the efflux function of ABCB1 and ABCG2 transporters

ATP-Binding Cassette transporters are involved in the efflux of xenobiotic compounds and are responsible for decreasing drug accumulation in multidrug resistant (MDR) cells. Discovered by structure-based virtual screening algorithms, bafetinib, a Bcr-Abl/Lyn tyrosine kinase inhibitor, was found to have inhibitory effects on both ABCB1- and ABCG2-mediated MDR in this in-vitro investigation. Bafetinib significantly sensitized ABCB1 and ABCG2 overexpressing MDR cells to their anticancer substrates and increased the intracellular accumulation of anticancer drugs, particularly doxorubicin and [³H]-paclitaxel in ABCB1 overexpressing cells; mitoxantrone and [³H]-mitoxantrone in ABCG2 overexpressing cells, respectively. Bafetinib stimulated ABCB1 ATPase activities while inhibited ABCG2 ATPase activities. There were no significant changes in the expression level or the subcellular distribution of ABCB1 and ABCG2 in the cells exposed to 3 μM of bafetinib. Overall, our study indicated that bafetinib reversed ABCB1- and ABCG2-mediated MDR by blocking the drug efflux function of these transporters. These findings might be useful in developing combination therapy for MDR cancer treatment.

Impact of Genetic Polymorphisms of ABCB1 (MDR1, P-Glycoprotein) on Drug Disposition and Potential Clinical Implications: Update of the Literature

ATP-binding cassette transporter B1 (ABCB1; P-glycoprotein; multidrug resistance protein 1) is an adenosine triphosphate (ATP)-dependent efflux transporter located in the plasma membrane of many different cell types. Numerous structurally unrelated compounds, including drugs and environmental toxins, have been identified as substrates. ABCB1 limits the absorption of xenobiotics from the gut lumen, protects sensitive tissues (e.g. the brain, fetus and testes) from xenobiotics and is involved in biliary and renal secretion of its substrates. In recent years, a large number of polymorphisms of the ABCB1 [ATP-binding cassette, sub-family B (MDR/TAP), member 1] gene have been described. The variants 1236C>T (rs1128503, p.G412G), 2677G>T/A (rs2032582, p.A893S/T) and 3435C>T (rs1045642, p.I1145I) occur at high allele frequencies and create a common haplotype; therefore, they have been most widely studied. This review provides an overview of clinical studies published between 2002 and March 2015. In summary, the effect of ABCB1 variation on P-glycoprotein expression (messenger RNA and protein expression) and/or activity in various tissues (e.g. the liver, gut and heart) appears to be small. Although polymorphisms and haplotypes of ABCB1 have been associated with alterations in drug disposition and drug response, including adverse events with various ABCB1 substrates in different ethnic populations, the results have been majorly conflicting, with limited clinical relevance. Future research activities are warranted, considering a deep-sequencing approach, as well as well-designed clinical studies with appropriate sample sizes to elucidate the impact of rare ABCB1 variants and their potential consequences for effect sizes.

Regulations of P-Glycoprotein/ABCB1/MDR1 in Human Cancer Cells

Multidrug resistance (MDR) in cancer cells is a phenotype whereby cells display reduced sensitivity to anticancer drugs, based on a variety of mechanisms, including an increase in drug efflux, the reduction of drug uptake, the activation of cell growth and survival signaling, the promotion of DNA repair, and the inhibition of apoptosis signaling. Increased expression of the plasma membrane drug efflux pumps, the ATP-binding cassette (ABC) transporters, is involved in MDR. P-Glycoprotein/ABCB1 is a member of the ABC transporter family, and facilitates the efflux of various anticancer drugs, including anthracyclines, vinca alkaloids, epipodophyllotoxins, taxanes, and kinase inhibitors, from cells. P-Glycoprotein is also expressed in normal tissues and cells, including the kidney, liver, colon, and adrenal gland, to transport and/or secrete substrates and at the blood-brain, blood-placenta, and blood-testis barriers to protect these tissues from toxic substances. To understand the mechanistic functions of P-glycoprotein and to overcome MDR, investigators have identified the substrates and competitive inhibitors of P-glycoprotein. Recently, we and other groups reported associations between cellular signaling pathways and the expression, stability, degradation, localization, and activity of P-glycoprotein. The present review summarizes the currently available information about the transcriptional and posttranslational regulation of P-glycoprotein expression and function.

Inhibition of the multidrug resistance P-glycoprotein: time for a change of strategy?

P-glycoprotein (P-gp) is a key player in the multidrug-resistant phenotype in cancer. The protein confers resistance by mediating the ATP-dependent efflux of an astonishing array of anticancer drugs. Its broad specificity has been the subject of numerous attempts to inhibit the protein and restore the efficacy of anticancer drugs. The general strategy has been to develop compounds that either compete with anticancer drugs for transport or act as direct inhibitors of P-gp. Despite considerable in vitro success, there are no compounds currently available to "block" P-gp-mediated resistance in the clinic. The failure may be attributed to toxicity, adverse drug interaction, and numerous pharmacokinetic issues. This review provides a description of several alternative approaches to overcome the activity of P-gp in drug-resistant cells. These include 1) drugs that specifically target resistant cells, 2) novel nanotechnologies to provide high-dose, targeted delivery of anticancer drugs, 3) compounds that interfere with nongenomic transfer of resistance, and 4) approaches to reduce the expression of P-gp within tumors. Such approaches have been developed through the pursuit of greater understanding of resistance mediators such as P-gp, and they show considerable potential for further application.

Molecular analysis of the multidrug transporter, P-glycoprotein

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

Evidence for modulatory sites at the lipid-protein interface of the human multidrug transporter P-glycoprotein

The human multidrug transporter P-glycoprotein (Pgp or ABCB1) sets up pharmacological barriers to many clinically important drugs, a therapeutic remedy for which has yet to be formulated. For the rational design of mechanism-based inhibitors (or modulators), it is necessary to map the potential sites for modulator interaction and understand their modes of communication with the other functional domains of Pgp. In this study, combining directed mutagenesis with homology modeling, we provide evidence of two modulator-specific sites at the lipid protein interface of Pgp. Targeting 21 variant positions in the COOH-terminal transmembrane (TM) regions, we find residues M948 (in TM11) and F983, M986, V988, and Q990 (all four in TM12) critically involved in substrate-site modulation by a thioxanthene-based allosteric modulator cis-(Z)-flupentixol. Interestingly, for ATP-site modulation by the same modulator, only two (M948 and Q990) of those four residues appear indispensable, together with two additional residues, T837 and I864 in TM9 and TM10, respectively, suggesting independent modes of communication linking the allosteric site with the substrate binding and ATPase domains. None of the seven residues identified prove to be critical for modulation of the substrate or ATP sites by Pgp modulators that are transported by the pump, such as cyclosporin A or verapamil, indicating their specificity for cis-(Z)-flupentixol. On the other hand, ATP-site modulation by verapamil proves to be highly sensitive to replacement at positions F716 (in TM7) and I765 (in TM8), and to a more moderate extent at I764 and L772 (both in TM8). Homology modeling based on the known crystal structures of the bacterial multidrug transporter SAV1866 and the mouse Pgp homologue maps the identified residues primarily at the lipid-protein interface of Pgp, in two spatially distinct modulator-specific clusters. The two modulatory sites demonstrate negative synergism in influencing ATP hydrolysis, consolidating their spatial distinctness. Because Pgp is known to recruit drug molecules directly from the lipid bilayer, identification of modulatory sites at the lipid-protein interface and at the same time outside the conventional central drug binding cavity is mechanistically revealing.

Synthesis and structure-activity evaluation of isatin-β-thiosemicarbazones with improved selective activity toward multidrug-resistant cells expressing P-glycoprotein

Cancer multidrug resistance (MDR) mediated by ATP-binding cassette (ABC) transporters presents a significant unresolved clinical challenge. One strategy to resolve MDR is to develop compounds that selectively kill cells overexpressing the efflux transporter P-glycoprotein (MDR1, P-gp, ABCB1). We have previously reported structure-activity studies based around the lead compound NSC73306 (1, 1-isatin-4-(4'-methoxyphenyl)-3-thiosemicarbazone, 4.3-fold selective). Here we sought to extend this work on MDR1-selective analogues by establishing whether 1 showed "robust" activity against a range of cell lines expressing P-gp. We further aimed to synthesize and test analogues with varied substitution at the N4-position, and substitution around the N4-phenyl ring of isatin-β-thiosemicarbazones (IBTs), to identify compounds with increased MDR1-selectivity. Compound 1 demonstrated MDR1-selectivity against all P-gp-expressing cell lines examined. This selectivity was reversed by inhibitors of P-gp ATPase activity. Structural variation at the 4'-phenyl position of 1 yielded compounds of greater MDR1-selectivity. Two of these analogues, 1-isatin-4-(4'-nitrophenyl)-3-thiosemicarbazone (22, 8.3-fold selective) and 1-isatin-4-(4'-tert-butyl phenyl)-3-thiosemicarbazone (32, 14.8-fold selective), were selected for further testing and were found to retain the activity profile of 1. These compounds are the most active IBTs identified to date.

Anthelmintics are substrates and activators of nematode P glycoprotein

P glycoproteins (Pgp), members of the ABC transporter superfamily, play a major role in chemoresistance. In nematodes, Pgp are responsible for resistance to anthelmintics, suggesting that they are Pgp substrates, as they are in mammalian cells. However, their binding to nematode Pgp and the functional consequences of this interaction have not been investigated. Our study showed that levamisole and most of the macrocyclic lactones (MLs) are Pgp substrates in nematodes. Ivermectin, although a very good substrate in mammalian cells, is poorly transported. In contrast to their inhibitory effect on mammalian Pgp, these drugs had a stimulatory effect on the transport activity of the reference Pgp substrate rhodamine 123 (R123) in the nematode. This may be due to a specific sequence of nematode Pgp, which shares only 44% identity with mammalian Pgp. Other factors, such as the affinity of anthelmintics for Pgp and their concentration in the Pgp microenvironment, could also differ in nematodes, as suggested by the specific relationship observed between the octanol-water partition coefficient (log P) of MLs and R123 efflux. Nevertheless, some similarities were also observed in the functional activities of the mammalian and nematode Pgp. As in mammalian cells, substrates known to bind the H site (Hoechst 33342 and colchicine) activated the R site, resulting in an increased R123 efflux. Our findings thus show that ML anthelmintics, which inhibit Pgp-mediated efflux in mammals, activate transport activity in nematodes and suggest that several substituents in the ML structure are involved in modulating the stimulatory effect.

Rapid loss of blood-brain barrier P-glycoprotein activity through transporter internalization demonstrated using a novel in situ proteolysis protection assay

Blood-brain barrier (BBB) P-glycoprotein activity is rapidly reduced by vascular endothelial growth factor (VEGF) acting via Src and by tumor necrosis factor-alpha acting via protein kinase C (PKC)beta1. To probe underlying mechanism(s), we developed an in vivo, immunoblot-based proteinase K (PK) protection assay to assess the changes in the P-glycoprotein content of the BBB's luminal membrane. Infusion of PK into the brain vasculature selectively cleaved luminal membrane P-glycoprotein, leaving intracellular proteins intact. Intracerebroventricular injection of VEGF partially protected P-glycoprotein from proteolytic cleavage, consistent with transporter internalization. Activation of PKCbeta1 did not protect P-glycoprotein. Thus, VEGF and PKCbeta1 reduce P-glycoprotein activity by distinct mechanisms.

Pim-1 kinase protects P-glycoprotein from degradation and enables its glycosylation and cell surface expression

The oncogenic serine/threonine kinase Pim-1 phosphorylates and activates the ATP-binding cassette transporter breast cancer resistance protein (ABCG2). The ABC transporter P-glycoprotein (Pgp; ABCB1) also contains a Pim-1 phosphorylation consensus sequence, and we hypothesized that Pim-1 also regulates Pgp. Pgp is exported from the endoplasmic reticulum (ER) as a 150-kDa species that is glycosylated to 170-kDa Pgp, translocates to the cell surface, and mediates drug efflux; alternatively, 150-kDa Pgp is cleaved to a 130-kDa proteolytic product by ER proteases or undergoes ubiquitination and proteasomal degradation. Pim-1 and Pgp interaction was studied in GST pull-down and phosphorylation in in vitro kinase assays. Pim-1 knockdown and inhibition effects on Pgp expression were studied by immunoblotting and flow cytometry and on Pgp stability by immunoblotting after cycloheximide treatment. Pim-1 directly interacted with and phosphorylated Pgp in intact cells and in vitro. Pim-1 knockdown or inhibition decreased cellular and cell surface 170-kDa Pgp, in association with both transient increase in 130-kDa Pgp and increased Pgp ubiquitination and proteasomal degradation. Pim-1 inhibition also decreased expression of 150-kDa Pgp in the presence of the glycosylation inhibitor 2-deoxy-d-glucose. Finally, Pim-1 inhibition sensitized Pgp-overexpressing cells to doxorubicin. Thus, Pim-1 regulates Pgp expression by protecting 150-kDa Pgp from proteolytic and proteasomal degradation and enabling Pgp glycosylation and cell surface translocation and thus Pgp-mediated drug efflux. Pim-1 inhibitors are entering clinical trials and may provide a novel approach to abrogating drug resistance.

PET imaging of multidrug resistance in tumors using 18F-fluoropaclitaxel

The failure of solid tumors to respond to chemotherapy is a complicated and clinically frustrating issue. The ability to predict which tumors will respond to treatment could reduce the human and monetary costs of cancer therapy by allowing pro-active selection of a chemotherapeutic to which the tumor does not express resistance. PET/CT imaging with a radiolabeled form of paclitaxel, F-18 fluoropaclitaxel (FPAC), may be able to predict the uptake of paclitaxel in solid tumors, and as a substrate of P-glycoprotein, it may also predict which tumors exhibit multidrug resistance (MDR), a phenotype in which tumors fail to respond to a wide variety of chemically unrelated chemotherapeutic agents. This article reviews the synthetic, preclinical and early human data obtained during the development phase of this promising new radiopharmaceutical.

Functional role of the linker region in purified human P-glycoprotein

Human P-glycoprotein (P-gp), which conveys multidrug resistance, is an ATP-dependent drug efflux pump that transports a wide variety of structurally unrelated compounds out of cells. P-gp possesses a 'linker region' of approximately 75 amino acids that connects two homologous halves, each of which contain a transmembrane domain followed by a nucleotide-binding domain. To investigate the role of the linker region, purified human P-gp was cleaved by proteases at the linker region and then compared with native P-gp. Based on a verapamil-stimulated ATP hydrolase assay, size-exclusion chromatography analysis and a thermo-stability assay, cleavage of the P-gp linker did not directly affect the preservation of the overall structure or the catalytic process in ATP hydrolysis. However, linker cleavage increased the k(cat) values both with substrate (k(sub)) and without substrate (k(basal)), but decreased the k(sub)/k(basal) values of all 10 tested substrates. The former result indicates that cleaving the linker activates P-gp, while the latter result suggests that the linker region maintains the tightness of coupling between the ATP hydrolase reaction and substrate recognition. Inspection of structures of the P-gp homolog, MsbA, suggests that linker-cleaved P-gp has increased ATP hydrolase activity because the linker interferes with a conformational change that accompanies the ATP hydrolase reaction. Moreover, linker cleavage affected the specificity constants [k(sub)/K(m(D))] for some substrates (i.e. linker cleavage probably shifts the substrate specificity profile of P-gp). Thus, this result also suggests that the linker region regulates the inherent substrate specificity of P-gp.

A synonymous polymorphism in a common MDR1 (ABCB1) haplotype shapes protein function

The MDR1 (ABCB1) gene encodes a membrane-bound transporter that actively effluxes a wide range of compounds from cells. The overexpression of MDR1 by multidrug-resistant cancer cells is a serious impediment to chemotherapy. MDR1 is expressed in various tissues to protect them from the adverse effect of toxins. The pharmacokinetics of drugs that are also MDR1 substrates also influence disease outcome and treatment efficacy. Although MDR1 is a well-conserved gene, there is increasing evidence that its polymorphisms affect substrate specificity. Three single nucleotide polymorphisms (SNPs) occur frequently and have strong linkage, creating a common haplotype at positions 1236C>T (G412G), 2677G>T (A893S) and 3435C>T (I1145I). The frequency of the synonymous 3435C>T polymorphism has been shown to vary significantly according to ethnicity. Existing literature suggests that the haplotype plays a role in response to drugs and disease susceptibility. This review summarizes recent findings on the 3435C>T polymorphism of MDR1 and the haplotype to which it belongs. A possible molecular mechanism of action by ribosome stalling that can change protein structure and function by altering protein folding is discussed.

Inhibiting the function of ABCB1 and ABCG2 by the EGFR tyrosine kinase inhibitor AG1478

The tyrphostin 4-(3-chloroanilino)-6,7-dimethoxyquinazoline (AG1478) is a potent and specific EGFR tyrosine kinase inhibitor (TKI); its promising pre-clinical results have led to clinical trials. Overexpression of ATP-binding cassette (ABC) transporters such as ABCB1, ABCC1 and ABCG2 is one of the main causes of multidrug resistance (MDR) and usually results in the failure of cancer chemotherapy. However, the interaction of AG1478 with these ABC transporters is still unclear. In the present study, we have investigated this interaction and found that AG1478 has differential effects on these transporters. In ABCB1-overexpressing cells, non-toxic doses of AG1478 were found to partially inhibit resistance to ABCB1 substrate anticancer drugs as well as increase intracellular accumulation of [3H]-paclitaxel. Similarly, in ABCG2-overexpressing cells, AG1478 significantly reversed resistance to ABCG2 substrate anticancer drugs and increased intracellular accumulation of [3H]-mitoxantrone as well as fluorescent compound BODIPY-prazosin. AG1478 also profoundly inhibited the transport of [3H]-E(2)17betaG and [3H]-methotrexate by ABCG2. We also found that AG1478 slightly stimulated ABCB1 ATPase activity and significantly stimulated ABCG2 ATPase activity. Interestingly, AG1478 did not inhibit the photolabeling of ABCB1 or ABCG2 with [125I]-iodoarylazidoprazosin. Additionally, AG1478 did not alter the sensitivity of parental, ABCB1- or ABCG2-overexpressing cells to non-ABCB1 and non-ABCG2 substrate drug and had no effect on the function of ABCC1. Overall, we conclude that AG1478 is able to inhibit the function of ABCB1 and ABCG2, with a more pronounced effect on ABCG2. Our findings provide valuable contributions to the development of safer and more effective EGFR TKIs for use as anticancer agents in the clinic.

Utilization of membrane vesicle preparations to study drug-ABC transporter interactions

BACKGROUND

The last 15 years have marked an expansion in our understanding of how ABC transporters modulate the pharmacokinetic properties of drugs. Assays based on different membrane preparations were one of the first methods developed to study ABC transporters. Later, they turned out to be valuable tools to gain insight into the nature of drug-ABC transporter interactions.

OBJECTIVES

Membranes prepared from different sources have been used and characterized; based on the biochemical characteristics of the transport process, a number of different assay types have been developed.

METHODS

This review focuses on the current experiences on how different membrane-based assays can be utilized in pharmaceutical R&D. Sources of membrane preparations, available assay types and correlation studies between different in-vitro and in-vivo methods are discussed.

RESULTS/CONCLUSION

Membrane-based assays are valuable tools in drug discovery to characterize drug-ABC transporter interactions.

PhosphoBlast, a computational tool for comparing phosphoprotein signatures among large datasets

Identification of specific protein phosphorylation sites provides predicative signatures of cellular activity and specific disease states such as cancer, diabetes, Alzheimer disease, and rheumatoid arthritis. Recent progress in phosphopeptide isolation technology and tandem mass spectrometry has provided the means to identify thousands of phosphorylation sites from a single biological sample. These advances now make it possible to profile global changes in the phosphoproteome at an unprecedented level. However, although this technology is generating a wealth of information, there is currently no efficient means to identify phosphoprotein signatures shared among large phosphoprotein databases. Identification of common phosphoprotein signatures found in biologically relevant systems and their conservation throughout evolution would provide valuable insight into mechanisms of signal transduction and cell function. Here we describe the development of a computational program (PhosphoBlast) that can rapidly match thousands of phosphopeptides that share phosphorylation sites within and across species. PhosphoBlast analysis of several large phosphoprotein datasets from the literature revealed common phosphorylation signatures shared across diverse experimental platforms and species. Moreover PhosphoBlast is a powerful analysis tool to identify specific phosphosite mutations. Comparison of the mouse and human phosphoproteomes revealed more than 130 specific phosphoamino acid mutations, some of which are predicted to alter protein function. Further analysis revealed that known phosphorylated amino acids are more evolutionally conserved than the Ser/Thr/Tyr amino acids not known to be phosphorylated. Together our results demonstrate that PhosphoBlast is a versatile mining tool capable of identifying related phosphorylation signatures and phosphoamino acid mutations among complex proteomics datasets in a highly efficient and accurate manner. PhosphoBlast will aid in the informatics analysis of the phosphoproteome and the identification of phosphoprotein biomarkers of disease.

Targeting the protein kinase C family: are we there yet?

Protein kinase C (PKC) comprises a family of serine/threonine kinases that are involved in the transduction of signals for cell proliferation, differentiation, apoptosis and angiogenesis. Unsurprisingly, disruption of PKC regulation is implicated in tumorigenesis and drug resistance. PKC function is complex in this context owing to the differing roles of individual isozymes within the cell and across tumour types. Therapeutically targeting PKC isozymes is not new; however, with many of the early PKC inhibitor cytotoxic drug combinations being discarded at the phase II level, and recent phase III studies in non-small-cell lung cancer proving negative, what's going wrong?

The P-Glycoprotein (ABCB1) Linker Domain Encodes High-Affinity Binding Sequences to α- and β-Tubulins

P-Glycoprotein (or ABCB1) has been shown to cause multidrug resistance in tumor cell lines selected with lipophilic anticancer drugs. ABCB1 encodes a duplicated molecule with two hydrophobic and hydrophilic domains linked by a highly charged region of approximately 90 amino acids, the "linker domain" with as yet unknown function(s). In this report, we demonstrate a role for this domain in binding to other cellular proteins. Using overlapping hexapeptides that encode the entire amino acid sequence of the linker domain of human ABCB1, we show a direct and specific binding between sequences in the linker domain and several intracellular proteins. Three different polypeptide sequences [617EKGIYFKLVTM627 (LDS617-627), 657SRSSLIRKRSTRRSVRGSQA676 (LDS657-676), and 693PVSFWRIMKLNLT705 (LDS693-705)] in the linker domain interacted tightly with several proteins with apparent molecular masses of approximately 80, 57, and 30 kDa. Interestingly, only the 57 kDa protein (or P57) interacted with all three different sequences of the linker domain. Purification and partial N-terminal amino acid sequencing of P57 showed that it encodes the N-terminal amino acids of alpha- and beta-tubulins. The identity of the P57 interacting protein as tubulins was further confirmed by Western blotting using monoclonal antibodies to alpha- and beta-tubulin. Taken together, the results of this study provide the first evidence for ABCB1 protein interaction mediated by sequences in the linker domain. These findings are likely to provide further insight into the functions of ABCB1 in normal and drug resistant tumor cells.

Characterization of P-glycoprotein and multidrug resistance proteins in rat kidney and intestinal cell lines

The activity of P-glycoprotein (Pgp/MDR1/ABCB1) and multidrug resistance proteins (MRP/ABCC) influence the pharmacokinetics and bioavailability of many drugs. Few suitable cell lines for the study of drug transport exist. Additional non-human cell lines may help clarify species differences and contribute to the current knowledge of drug transport. The aim of the present study was to characterize three rat epithelial cell lines for transporter expression and activity. Transporter expression was assessed in intestinal IEC-6 and renal GERP and NRK-52E cells using RT-PCR and Western blot analysis. Pgp and Mrp transport activity were analyzed by measuring calcein accumulation and glutathione-S-bimane efflux, respectively. The three cell lines showed Pgp expression and Pgp-dependent transport, both decreasing with culture time after reaching confluency. Besides Pgp, cells expressed Mrp1, Mrp3, Mrp4, and Mrp5, while Mrp2 and Mrp6 were absent. In addition, they showed temperature- and Mrp-dependent efflux of glutathione-S-bimane. Exposure to a panel of different inhibitors showed that this efflux was probably mediated by Mrp4. In conclusion, the three rat epithelial cell lines investigated showed Pgp and Mrp expression and transport. Mrp dependent transport was most likely mediated by Mrp4. In future, these cell lines may be used as in vitro models to study drug transport.

Human multidrug resistance ABCB and ABCG transporters: participation in a chemoimmunity defense system

In this review we give an overview of the physiological functions of a group of ATP binding cassette (ABC) transporter proteins, which were discovered, and still referred to, as multidrug resistance (MDR) transporters. Although they indeed play an important role in cancer drug resistance, their major physiological function is to provide general protection against hydrophobic xenobiotics. With a highly conserved structure, membrane topology, and mechanism of action, these essential transporters are preserved throughout all living systems, from bacteria to human. We describe the general structural and mechanistic features of the human MDR-ABC transporters and introduce some of the basic methods that can be applied for the analysis of their expression, function, regulation, and modulation. We treat in detail the biochemistry, cell biology, and physiology of the ABCB1 (MDR1/P-glycoprotein) and the ABCG2 (MXR/BCRP) proteins and describe emerging information related to additional ABCB- and ABCG-type transporters with a potential role in drug and xenobiotic resistance. Throughout this review we demonstrate and emphasize the general network characteristics of the MDR-ABC transporters, functioning at the cellular and physiological tissue barriers. In addition, we suggest that multidrug transporters are essential parts of an innate defense system, the "chemoimmunity" network, which has a number of features reminiscent of classical immunology.

Selective toxicity of NSC73306 in MDR1-positive cells as a new strategy to circumvent multidrug resistance in cancer

ATP-binding cassette (ABC) proteins include the best known mediators of resistance to anticancer drugs. In particular, ABCB1 [MDR1/P-glycoprotein (P-gp)] extrudes many types of drugs from cancer cells, thereby conferring resistance to those agents. Attempts to overcome P-gp-mediated drug resistance using specific inhibitors of P-gp has had limited success and has faced many therapeutic challenges. As an alternative approach to using P-gp inhibitors, we characterize a thiosemicarbazone derivative (NSC73306) identified in a generic screen as a compound that exploits, rather than suppresses, P-gp function to induce cytotoxicity. Cytotoxic activity of NSC73306 was evaluated in vitro using human epidermoid, ovarian, and colon cancer cell lines expressing various levels of P-gp. Our findings suggest that cells become hypersensitive to NSC73306 in proportion to the increased P-gp function and multidrug resistance (MDR). Abrogation of both sensitivity to NSC73306 and resistance to P-gp substrate anticancer agents occurred with specific inhibition of P-gp function using either a P-gp inhibitor (PSC833, XR9576) or RNA interference, suggesting that cytotoxicity was linked to MDR1 function, not to other, nonspecific factors arising during the generation of resistant or transfected cells. Molecular characterization of cells selected for resistance to NSC73306 revealed loss of P-gp expression and consequent loss of the MDR phenotype. Although hypersensitivity to NSC73306 required functional expression of P-gp, biochemical assays revealed no direct interaction between NSC73306 and P-gp. This article shows that NSC73306 kills cells with intrinsic or acquired P-gp-induced MDR and indirectly acts to eliminate resistance to MDR1 substrates.

Mismatched siRNAs downregulate mRNAs as a function of target site location

In mammalian cells, RNA interference can be mediated by synthetic duplex RNAs, termed small interfering RNAs (siRNAs), which assist in cleaving completely complementary mRNA transcripts. MicroRNAs (miRNAs) are endogenous small RNAs that assist in translationally repressing mRNAs with regions of partial complementarity, but may also reduce transcript levels. Since miRNAs predominantly interact with the 3' UTRs of transcripts, we sought to ask if mismatched siRNAs mimicking miRNAs affect cognate mRNA levels as a function of target site location. We find that mismatched siRNAs targeting the 3' UTRs of two endogenous transcripts yield a greater reduction in mRNA levels than those targeting the coding region. Our findings demonstrate the importance of target site location within endogenous mRNAs for small RNAs associated with RNAi.

Modulator-induced interference in functional cross talk between the substrate and the ATP sites of human P-glycoprotein

The human P-glycoprotein (Pgp, ABCB1) is an ATP-dependent efflux pump for structurally unrelated hydrophobic compounds, conferring simultaneous resistance to and restricting bioavailability of several anticancer and antimicrobial agents. Drug transport by Pgp requires a coordinated communication between its substrate binding/translocating pathway (substrate site) and the nucleotide binding domains (NBDs or ATP sites). In this study, we demonstrate that certain thioxanthene-based Pgp modulators, such as cis-(Z)-flupentixol and its closely related analogues, effectively disrupt molecular cross talk between the substrate, and the ATP, sites without affecting the basic functional aspects of the two domains, such as substrate recognition, binding, and hydrolysis of ATP and dissociation of ADP following ATP hydrolysis. The allosteric modulator cis-(Z)-flupentixol has no effect on [alpha-(32)P]-8-azido-ATP binding to Pgp under nonhydrolytic conditions or on the K(m) for ATP during ATP hydrolysis. Both hydrolysis of ATP and vanadate-induced [alpha-(32)P]-8-azido-ADP trapping (following [alpha-(32)P]-8-azido-ATP breakdown) by Pgp are stimulated by the modulator. However, the ability of Pgp substrates (such as prazosin) to stimulate ATP hydrolysis and facilitate vanadate-induced trapping of [alpha-(32)P]-8-azido-ADP is substantially affected in the presence of cis-(Z)-flupentixol. Substrate recognition by Pgp as determined by [(125)I]iodoarylazidoprazosin ([(125)I]IAAP) binding both in the presence and in the absence of ATP is facilitated by the modulator, whereas substrate dissociation in response to vanadate trapping is considerably affected in its presence. In the Pgp F983A mutant, which is impaired in modulation by cis-(Z)-flupentixol, the modulator has a minimal effect on substrate-stimulated ATP hydrolysis as well as on substrate dissociation coupled to vanadate trapping. Finally, cis-(Z)-flupentixol has no effect on dissociation of [alpha-(32)P]-8-azido-ADP (or ADP) from vanadate-trapped Pgp, which is essential for subsequent rounds of ATP hydrolysis. Taken together, our results demonstrate a distinct mechanism of Pgp modulation that involves allosteric disruption of molecular cross talk between the substrate, and the ATP, sites without any direct interference with their individual functions.

Biochemical and pharmacological properties of an allosteric modulator site of the human P-glycoprotein (ABCB1)

The drug-transport function of the human P-glycoprotein (Pgp or ABCB1) is inhibited by a number of structurally unrelated compounds, known as modulators or reversing agents. Among them, the thioxanthene derivative flupentixol inhibits Pgp-mediated drug transport by an allosteric mechanism. Unlike most other Pgp modulators, the cis isomer of flupentixol [cis-(Z)-flupentixol] facilitates interaction of Pgp with its transport-substrate [125I]iodoarylazidoprazosin (or [125I]IAAP), yet inhibits transport. In this study, we show that the flupentixol site acts as a common site of interaction for the tricyclic ring-containing modulators thioxanthenes and phenothiazines. The allosteric stimulation of [125I]IAAP binding to Pgp occurs independent of the phosphorylation status of the transporter. Stimulation is retained in purified Pgp reconstituted into proteoliposomes, suggesting no involvement of any other cellular protein in the phenomenon. However, perturbation of the lipid environment of the reconstituted Pgp by nonionic detergent octylglucoside abolishes stimulation by cis-(Z)-flupentixol of [125I]IAAP binding. Extensive trypsin digestion of the [125I]IAAP-labeled Pgp generates a 5.5 kDa fragment with 80% of the stimulated level of labeling associated with it. Sensitivity to inhibition by transport-substrate vinblastine and competitive modulator cyclosporin A suggests that the elevated level of [125I]IAAP binding to the fragment represents a functionally relevant interaction with the substrate site of Pgp. In summary, we demonstrate that allosteric modulation by cis-(Z)-flupentixol is mediated through its interaction with Pgp at a site specific for tricyclic ring-containing Pgp modulators of thioxanthene and phenothiazine backbone, independent of other cellular components and the phosphorylation status of the protein.

Single nucleotide polymorphisms in human P-glycoprotein: its impact on drug delivery and disposition

Drug efflux pumps belong to a large family of ATP-binding cassette transporter proteins. These pumps bind their substrate and export it through the membrane using energy derived from ATP hydrolysis. P-glycoprotein, the main efflux pump in this family, is expressed not only in tumour cells but also in normal tissues with excretory function (liver, kidney and the intestine). It has a broad specificity of substrates and plays an important role in drug delivery and disposition. Recently, genetic screening of P-glycoprotein has yielded multiple single nucleotide polymorphisms, which seem to alter transporter function and expression. This review discusses the various polymorphisms of this gene and its impact on drug disposition and diseases.

Allosteric modulation of the human P-glycoprotein involves conformational changes mimicking catalytic transition intermediates

The drug transport function of human P-glycoprotein (Pgp, ABCB1) can be inhibited by a number of pharmacological agents collectively referred to as modulators or reversing agents. In this study, we demonstrate that certain thioxanthene-based Pgp modulators with an allosteric mode of action induce a distinct conformational change in the cytosolic domain of Pgp, which alters susceptibility to proteolytic digestion. Both cis and trans-isomers of the Pgp modulator flupentixol confer considerable protection of an 80 kDa Pgp fragment against trypsin digestion, that is recognized by a polyclonal antibody specific for the NH(2)-terminal half to Pgp. The protection by flupentixol is abolished in the Pgp F983A mutant that is impaired in modulation by flupentixols, indicating involvement of the allosteric site in generating the conformational change. A similar protection to an 80 kDa fragment is conferred by ATP, its nonhydrolyzable analog ATPgammaS, and by trapping of ADP-vanadate at the catalytic domain, but not by transport substrate vinblastine or by the competitive modulator cyclosporin A, suggesting different outcomes from modulator interaction at the allosteric site and at the substrate site. In summary, we demonstrate that allosteric interaction of flupentixols with Pgp generates conformational changes that mimic catalytic transition intermediates induced by nucleotide binding and hydrolysis, which may play a crucial role in allosteric inhibition of Pgp-mediated drug transport.

Allosteric modulation bypasses the requirement for ATP hydrolysis in regenerating low affinity transition state conformation of human P-glycoprotein

ATP-dependent drug transport by human P-glycoprotein (Pgp, ABCB1) involves a coordinated communication between its drug-binding site (substrate site) and the nucleotide binding/hydrolysis domain (ATP sites). It has been demonstrated that the two ATP sites of Pgp play distinct roles within a single catalytic turnover; whereas ATP binding or/and hydrolysis by one drives substrate translocation and dissociation, the hydrolytic activity of the other resets the transporter for the subsequent cycle (Sauna, Z. E., and Ambudkar, S. V. (2000) Proc. Natl. Acad. Sci. U. S. A. 97, 2515-2520; Sauna, Z. E., and Ambudkar, S. V. (2001) J. Biol. Chem. 276, 11653-11661). Trapping of ADP (or 8-azido-ADP) and vanadate (ADP.Vi or 8-azido-ADP.Vi) at the catalytic site, following nucleotide hydrolysis, markedly reduces the affinity of Pgp for its transport substrate [125I]iodoarylazidoprazosin ([125I]IAAP), resulting in dissociation of the latter. Regeneration of the [125I]IAAP site requires an additional round of nucleotide hydrolysis. In this study, we demonstrate that certain thioxanthene-based allosteric modulators, such as cis-(Z)-flupentixol and its closely related analogs, induce regeneration of [125I]IAAP binding to vanadate-trapped (or fluoroaluminate-trapped) Pgp without any further nucleotide hydrolysis. Regeneration was facilitated by dissociation of the trapped nucleotide and vanadate. Once regenerated, the substrate site remains accessible to [125I]IAAP even after removal of the modulator from the medium, suggesting a modulator-induced relaxation of a constrained transition state conformation. Consistent with this, limited trypsin digestion of vanadate-trapped Pgp shows protection by cis-(Z)-flupentixol of two Pgp fragments (approximately 60 kDa) recognizable by a polyclonal antiserum specific for the NH2-terminal half. No regeneration was observed in the Pgp mutant F983A that is impaired in modulation by flupentixols, indicating involvement of the allosteric modulator site in the phenomenon. In summary, the data demonstrate that in the nucleotide-trapped low affinity state of Pgp, the allosteric site remains accessible and responsive to modulation by flupentixol (and its closely related analogs), which can reset the high affinity state for [125I]IAAP binding without any further nucleotide hydrolysis.

Absence of N-linked glycosylation does not affect plasma membrane localization of breast cancer resistance protein (BCRP/ABCG2)

Breast cancer resistance protein (BCRP/ABCG2) is an ATP-binding cassette (ABC) multidrug transporter that confers resistance to various anticancer drugs like topotecan and mitoxantrone. To obtain more insight in its cellular functioning, we investigated phosphorylation and N-linked glycosylation of BCRP. In the epithelial Madin-Darby canine kidney (MDCK) cell line, we did not detect phosphorylation of BCRP, in contrast to MRP2, which was phosphorylated. In the ovarian carcinoma cell line T8 also no phosphorylated BCRP was detected. As BCRP in both lines effectively transports drugs, it appears that phosphorylation of BCRP (if it occurs at all) is not needed for drug transport. We further mutated the asparagine residues 418, 557 and 596 in three putative N-linked glycosylation motifs of BCRP to alanines. Mutant proteins were expressed in CHO9 and MDCKII cells by transient transfection and characterized by Western blot and immunofluorescence analysis. We found that only BCRP-N596A and a mutant with all three asparagines mutated (triple mutant) were not glycosylated anymore, indicating that only asparagine 596 is normally glycosylated. The mutation of asparagine 596 (or 418) had little effect on the subcellular localization of BCRP, indicating that N-linked glycosylation is not essential for routing to the plasma membrane. However, BCRP-N557A and the triple mutant were mainly localized intracellularly, probably in the endoplasmic reticulum, suggesting that this mutation disrupted proper routing of BCRP.

Regulation of the stability of P-glycoprotein by ubiquitination

Ubiquitination plays a crucial role in regulating protein turnover. Here we show that ubiquitination regulates the stability of the MDR1 gene product, P-glycoprotein, thereby affecting the functions of this membrane transporter that mediates multidrug resistance. We found that P-glycoprotein was constitutively ubiquitinated in drug-resistant cancer cells. Transfection of multidrug-resistant cells with wild-type ubiquitin or treatment with an N-glycosylation inhibitor increased the ubiquitination of P-glycoprotein and increased P-glycoprotein degradation. Carbobenzoxy-L-leucyl-L-leucyl-L-leucinal (MG-132), a proteasome inhibitor, induced accumulation of ubiquitinated P-glycoprotein, suggesting the involvement of the proteasome in the turnover of the transporter. Treatment of multidrug-resistant cells with 12-O-tetradecanoylphorbol-13-acetate, a phorbol ester that increases the phosphorylation of P-glycoprotein through activation of protein kinase C, or substituting phosphorylation sites of P-glycoprotein by nonphosphorylatable residues did not affect the ubiquitination of the transporter. Enhanced ubiquitination of P-glycoprotein resulted in a decrease of the function of the transporter, as demonstrated by increased intracellular drug accumulation and increased cellular sensitivity to drugs transported by P-glycoprotein. Our results indicate that the stability and function of P-glycoprotein can be regulated by the ubiquitin-proteasome pathway and suggest that modulating the ubiquitination of P-glycoprotein might be a novel approach to the reversal of drug resistance.

Phosphorylation controls Ikaros's ability to negatively regulate the G(1)-S transition

Ikaros is a key regulator of lymphocyte proliferative responses. Inactivating mutations in Ikaros cause antigen-mediated lymphocyte hyperproliferation and the rapid development of leukemia and lymphoma. Here we show that Ikaros's ability to negatively regulate the G(1)-S transition can be modulated by phosphorylation of a serine/threonine-rich conserved region (p1) in exon 8. Ikaros phosphorylation in p1 is induced during the G(1)-S transition. Mutations that prevent phosphorylation in p1 increase Ikaros's ability to impede cell cycle progression and its affinity for DNA. Casein kinase II, whose increased activity in lymphocytes leads to transformation, is a key player in Ikaros p1 phosphorylation. We thus propose that Ikaros's activity as a regulator of the G(1)-S transition is controlled by phosphorylation in response to signaling events that down-modulate its DNA binding activity.