Peihua Lu

β-Amyloid efflux mediated by p-glycoprotein

A large body of evidence suggests that an increase in the brain β‐amyloid (Aβ) burden contributes to the etiology of Alzheimers disease (AD). Much is now known about the intracellular processes regulating the production of Aβ, however, less is known regarding its secretion from cells. We now report that p‐glycoprotein (p‐gp), an ATP‐binding cassette (ABC) transporter, is an Aβ efflux pump. Pharmacological blockade of p‐gp rapidly decrease extracellular levels of Aβ secretion. In vitro binding studies showed that addition of synthetic human Aβ1–40 and Aβ1–42 peptides to hamster mdr1‐enriched vesicles labeled with the fluorophore MIANS resulted in saturable quenching, suggesting that both peptides interact directly with the transporter. Finally, we were able to directly measure transport of Aβ peptides across the plasma membranes of p‐gp enriched vesicles, and showed that this phenomenon was both ATP‐ and p‐gp‐dependent. Taken together, our study suggests a novel mechanism of Aβ detachment from cellular membranes, and represents an obvious route towards identification of such a mechanism in the brain.

Interaction of the P-glycoprotein multidrug transporter (MDR1) with high affinity peptide chemosensitizers in isolated membranes, reconstituted systems, and intact cells.

P-glycoprotein-mediated multidrug resistance can be reversed by the action of a group of compounds known as chemosensitizers. The interactions with P-glycoprotein of two novel hydrophobic peptide chemosensitizers (reversins 121 and 205) have been studied in model systems in vitro, and in a variety of MDR1-expressing intact tumor cells. The reversins bound to purified P-glycoprotein with high affinity (77-154 nM), as assessed by a quenching assay using fluorescently labeled purified protein. The peptides modulated P-glycoprotein ATPase activity in Sf9 insect cell membranes expressing human MDR1, plasma membrane vesicles from multidrug-resistant cells, and reconstituted proteoliposomes. Both peptides induced a large stimulation of ATPase activity; however, higher concentrations, especially of reversin 205, led to inhibition. This pattern was different from that of simple linear peptides, and resembled that of chemosensitizers such as verapamil. In both membrane vesicles and reconstituted proteoliposomes, 1-2 microM reversins were more effective than cyclosporin A at blocking colchicine transport. Reversin 121 and reversin 205 restored the uptake of [3H]daunorubicin and rhodamine 123 in MDR1-expressing cells to the level observed in the drug-sensitive parent cell lines, and also effectively inhibited the extrusion of calcein acetoxymethyl ester from intact cells. In cytotoxicity assays, reversin 121 and reversin 205 eliminated the resistance of MDR1-expressing tumor cells against MDR1-substrate anticancer drugs, and they had no toxic effects in MDR1-negative control cells. We suggest that peptides of the reversin type interact with the MDR1 protein with high affinity and specificity, and thus they may be good candidates for the development of MDR1-modulating agents to sensitize drug resistance in cancer.

Insights into the structure and substrate interactions of the P-glycoprotein multidrug transporter from spectroscopic studies

The P-glycoprotein multidrug transporter is a 170-kDa efflux pump which exports a diverse group of natural products, chemotherapeutic drugs, and hydrophobic peptides across the plasma membrane, driven by ATP hydrolysis. The transporter has been proposed to interact with its drug substrates within the membrane environment; however, much remains to be learned about the nature and number of the drug binding site(s). The two nucleotide binding domains are responsible for ATP binding and hydrolysis, which is coupled to drug movement across the membrane. In recent years, P-glycoprotein has been purified and functionally reconstituted in amounts large enough to allow biophysical studies. The use of spectroscopic techniques has led to insights into both its secondary and tertiary structure, and its interaction with nucleotides and drugs. In this review, we will summarise what has been learned by application to purified P-glycoprotein of fluorescence spectroscopy, circular dichroism spectroscopy and infra-red spectroscopy.

Celastraceae Sesquiterpenes as a New Class of Modulators That Bind Specifically to Human P-Glycoprotein and Reverse Cellular Multidrug Resistance

Overexpression of ABCB1 (MDR1) P-glycoprotein, a multidrug efflux pump, is one mechanism by which tumor cells may develop multidrug resistance (MDR), preventing the successful chemotherapeutic treatment of cancer. Sesquiterpenes from Celastraceae family are natural compounds shown previously to reverse MDR in several human cancer cell lines and Leishmania strains. However, their molecular mechanism of reversion has not been characterized. In the present work, we have studied the ability of 28 dihydro-β-agarofuran sesquiterpenes to reverse the P-glycoprotein-dependent MDR phenotype and elucidated their molecular mechanism of action. Cytotoxicity assays using human MDR1-transfected NIH-3T3 cells allowed us to select the most potent sesquiterpenes reversing the in vitro resistance to daunomycin and vinblastine. Flow cytometry experiments showed that the above active compounds specifically inhibited drug transport activity of P-glycoprotein in a saturable, concentration-dependent manner (Ki down to 0.24 ± 0.01 μmol/L) but not that of ABCC1 (multidrug resistance protein 1; MRP1), ABCC2 (MRP2), and ABCG2 (breast cancer resistance protein; BCRP) transporters. Moreover, sesquiterpenes inhibited at submicromolar concentrations the P-glycoprotein-mediated transport of [3H]colchicine and tetramethylrosamine in plasma membrane from CHRB30 cells and P-glycoprotein-enriched proteoliposomes, supporting that P-glycoprotein is their molecular target. Photoaffinity labeling in plasma membrane and fluorescence spectroscopy experiments with purified protein suggested that sesquiterpenes interact with transmembrane domains of P-glycoprotein. Finally, sesquiterpenes modulated P-glycoprotein ATPase-activity in a biphasic, concentration-dependent manner: they stimulated at very low concentrations but inhibited ATPase activity as noncompetitive inhibitors at higher concentrations. Sesquiterpenes from Celastraceae are promising P-glycoprotein modulators with potential applications in cancer chemotherapy because of their MDR reversal potency and specificity for P-glycoprotein.

Characterization of Fluorescent Sterol Binding to Purified Human NPC1

Mutations in the NPC1 gene cause Niemann-Pick type C disease, which appears to result from a defect in intracellular cholesterol trafficking. NPC1 is a member of the resistance-nodulation-cell division (RND) permease superfamily and contains a sterol-sensing domain, yet its cellular function and the identity of its substrates remain unknown. FLAG-tagged human NPC1 was purified from NPC1-expressing Chinese hamster ovary cells by solubilization in 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonic acid (CHAPS), followed by affinity chromatography. Purified NPC1 in detergent solution appeared to be oligomeric as determined by gel filtration fast protein liquid chromatography and was photolabeled by an azido-cholesterol derivative. Fluorescent cholesterol analogs, including dehydroergosterol, cholestatrienol, and 22-(N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)amino)-23,24-bisnor-5-cholen-3β-ol (NBD-cholesterol), displayed enhanced fluorescence upon binding to NPC1 and also resulted in saturable, concentration-dependent quenching of NPC1 intrinsic Trp fluorescence. The apparent binding affinity for these three sterols was in the 0.5-6 μm range. Binding of NBD-cholesterol to NPC1 at low detergent concentration (2 mm CHAPS) was of high apparent affinity (0.5-0.6 μm) and occurred rapidly (<1 min). However, binding of a BODIPY-labeled cholesterol derivative was very slow, requiring ∼3 h to reach equilibrium. The apparent NBD-cholesterol binding affinity was greatly reduced at higher detergent concentration. The stoichiometry of NBD-cholesterol binding to NPC1 was ∼1. Various sterols, including native cholesterol and 25-hydroxycholesterol, inhibited NBD-cholesterol binding, suggesting that they compete for binding to the protein. Dynamic quenching studies showed that bound NBD-cholesterol was almost completely shielded from the aqueous medium, suggesting that it is buried in a deep hydrophobic pocket in NPC1. The use of fluorescent cholesterol analogs provides novel information on the molecular properties of the sterol-binding site in the full-length NPC1 protein.

Phenotypic variability in hyperphosphatasia with seizures and neurologic deficit (Mabry syndrome)

Hyperphosphatasia with neurologic deficit (Mabry syndrome) was first described in a single family (OMIM#239300) by Mabry et al. [ 1970 ]. Although considered rare at the time, more than 20 individuals with the triad of developmental disability, seizures, and hyperphosphatasia have been identified world‐wide. The 1‐6 mannosyltransferase 2, phosphatidylinositol glycan V (PIGV) gene has been found to be disrupted in some patients with the additional feature of brachytelephalangy. In the present report we identify three patients compound homozygous for PIGV mutations. Two siblings were found to be compound heterozygotes for c.467G > A and c.494C > A in exon 3 of PIGV (the c.494C > A PIGV variant is novel). A third patient with similar phenotype, was a compound heterozygote for the known c.1022C > A/c.1022C > T (p.Ala341Glu/p.Ala341Val) mutation. This patient was also noted to have lysosomal storage in cultured fibroblasts. In contrast, the fourth patient who had no apparent hand abnormality, was found to be heterozygous for a previously unclassified c.1369C > T mutation in exon 4 of the PIGV gene, resulting in a p.Leu457Phe substitution in the catalytic domain of the enzyme. Unless this variant has a dominant negative effect, however, it seems likely that another GPI biosynthesis gene variant may contribute to the disorder, possibly through digenic inheritance. Since slightly fewer than half of the nine cases presented in this report and our previous report [Thompson et al., 2010 ] have PIGV mutations, we suggest that other genes critical to GPI anchor biosynthesis are likely to be disrupted in some patients.

Synthesis and Evaluation of Strychnos Alkaloids as MDR Reversal Agents for Cancer Cell Eradication

Natural products represent the fourth generation of multidrug resistance (MDR) reversal agents that resensitize MDR cancer cells overexpressing P-glycoprotein (Pgp) to cytotoxic agents. We have developed an effective synthetic route to prepare various Strychnos alkaloids and their derivatives. Molecular modeling of these alkaloids docked to a homology model of Pgp was employed to optimize ligand-protein interactions and design analogues with increased affinity to Pgp. Moreover, the compounds were evaluated for their (1) binding affinity to Pgp by fluorescence quenching, and (2) MDR reversal activity using a panel of in vitro and cell-based assays and compared to verapamil, a known inhibitor of Pgp activity. Compound 7 revealed the highest affinity to Pgp of all Strychnos congeners (Kd=4.4μM), the strongest inhibition of Pgp ATPase activity, and the strongest MDR reversal effect in two Pgp-expressing cell lines. Altogether, our findings suggest the clinical potential of these synthesized compounds as viable Pgp modulators justifies further investigation.

Interaction of the P-Glycoprotein Multidrug Transporter with Sterols.

The ABC transporter P-glycoprotein (Pgp, ABCB1) actively exports structurally diverse substrates from within the lipid bilayer, leading to multidrug resistance. Many aspects of Pgp function are altered by the phospholipid environment, but its interactions with sterols remain enigmatic. In this work, the functional interaction between purified Pgp and various sterols was investigated in detergent solution and proteoliposomes. Fluorescence studies showed that dehydroergosterol, cholestatrienol, and NBD-cholesterol interact intimately with Pgp, resulting in both quenching of protein Trp fluorescence and enhancement of sterol fluorescence. Kd values indicated binding affinities in the range of 3-9 μM. Collisional quenching experiments showed that Pgp-bound NBD-cholesterol was protected from the external milieu, resonance energy transfer was observed between Pgp Trp residues and the sterol, and the fluorescence emission of bound sterol was enhanced. These observations suggested an intimate interaction of bound sterols with the transporter at a protected nonpolar site. Cholesterol hemisuccinate altered the thermal unfolding of Pgp and greatly stabilized its basal ATPase activity in both a detergent solution and reconstituted proteoliposomes of certain phospholipids. Other sterols, including dehydroergosterol, did not stabilize the basal ATPase activity of detergent-solubilized Pgp, which suggests that this is not a generalized sterol effect. The phospholipid composition and cholesterol hemisuccinate content of Pgp proteoliposomes altered the basal ATPase and drug transport cycles differently. Sterols may interact with Pgp and modulate its structure and function by occupying part of the drug-binding pocket or by binding to putative consensus cholesterol-binding (CRAC/CARC) motifs located within the transmembrane domains.

Reversible dimers of the atypical antipsychotic quetiapine inhibit p-glycoprotein-mediated efflux in vitro with increased binding affinity and in situ at the blood-brain barrier.

The multidrug resistance transporter P-glycoprotein (P-gp) is highly expressed in the capillary endothelial cells of the blood-brain barrier (BBB) where it functions to limit the brain penetration of many drugs, including antipsychotic agents used to treat schizophrenia. Therefore, in an effort to inhibit the transporter, we designed dimers of the antipsychotic drug and P-gp substrate quetiapine (QT), linked by variable length tethers. In P-gp overexpressing cells and in human brain capillary endothelial hCMEC/D3 cells, the dimer with the shortest tether length (QT2C2) (1) was the most potent inhibitor showing >80-fold better inhibition of P-gp-mediated transport than monomeric QT. The dimers, which are linked via ester moieties, are designed to revert to the therapeutic monomer once inside the target cells. We demonstrated that the addition of two sterically blocking methyl groups to the linker (QT2C2Me2, 8) increased the half-life of the molecule in plasma 10-fold as compared to the dimer lacking methyl groups (QT2C2, 1), while retaining inhibitory potency for P-gp transport and sensitivity to cellular esterases. Experiments with purified P-gp demonstrated that QT2C2 (1) and QT2C2Me2 (8) interacted with both the H- and R-binding sites of the transporter with binding affinities 20- to 30-fold higher than that of monomeric QT. Using isolated rat brain capillaries, QT2C2Me2 (8) was a more potent inhibitor of P-gp transport than QT. Lastly, we showed that QT2C2Me2 (8) increased the accumulation of the P-gp substrate verapamil in rat brain in situ three times more than QT. Together, these results indicate that the QT dimer QT2C2Me2 (8) strongly inhibited P-gp transport activity in human brain capillary endothelial cells, in rat brain capillaries, and at the BBB in an animal model.

A semisynthetic taxane Yg-3-46a effectively evades P-glycoprotein and β-III tubulin mediated tumor drug resistance in vitro

Tumor resistance, especially that mediated by P-glycoprotein (P-gp) and β-III tubulin, is a major obstacle to the efficacy of most microtubule-targeting anticancer drugs in clinics. A novel semisynthetic taxane, 2-debenzoyl-2-(3-azidobenzyl)-10-propionyldocetaxel (Yg-3-46a) was shown to be highly cytotoxic to breast cancer cell lines MCF-7 and MCF/ADR which overexpressed P-gp via long term culture with doxorubicin, and cervical cancer cell lines Hela and Hela/βIII which overexpressed βIII-tubulin via stable transfection with TUBB3 gene. siRNA transfection experiments also confirmed that Yg-3-46a can circumvent P-gp and β-III tubulin mediated drug resistance. In addition, its cytotoxicity was lower than that of paclitaxel in the human mammary cell line HBL-100 and the human telomerase-immortalized retinal pigment epithelium cell line (hTERT-RPE1), suggesting a better safety margin for this compound in vivo. It exhibited more potent microtubule polymerization ability than paclitaxel in vitro, and also induced G2/M phase arrest in MCF-7/ADR cells. Moreover, it was found to induce apoptosis in MCF-7/ADR cells through the caspase-dependent death-receptor pathway by enhancing levels of Fas and FasL, and activating caspase-8 and 3. Yg-3-46a was found to be a poorer substrate of P-gp compared to paclitaxel, in both binding and ATPase experiments, which is likely responsible for its ability to circumvent P-gp mediated multidrug resistance (MDR). All of these results indicate that Yg-3-46a is a novel microtubule-stabilizing agent that has the potential to evade drug resistance mediated by P-gp and β-III tubulin overexpression.