Functional analysis of chimeric proteins constructed by exchanging homologous domains of two P-glycoproteins conferring distinct drug resistance profiles

Human-Mouse Chimeras with Normal Expression and Function Reveal That Major Domain Swapping Is Tolerated by P-Glycoprotein (ABCB1)

The efflux transporter P-glycoprotein (P-gp) plays a vital role in the transport of molecules across cell membranes and has been shown to interact with a panoply of functionally and structurally unrelated compounds. How human P-gp interacts with this large number of drugs has not been well understood, although structural flexibility has been implicated. To gain insight into this transporter's broad substrate specificity and to assess its ability to accommodate a variety of molecular and structural changes, we generated human-mouse P-gp chimeras by the exchange of homologous transmembrane and nucleotide-binding domains. High-level expression of these chimeras by BacMam- and baculovirus-mediated transduction in mammalian (HeLa) and insect cells, respectively, was achieved. There were no detectable differences between wild-type and chimeric P-gp in terms of cell surface expression, ability to efflux the P-gp substrates rhodamine 123, calcein-AM, and JC-1, or to be inhibited by the substrate cyclosporine A and the inhibitors tariquidar and elacridar. Additionally, expression of chimeric P-gp was able to confer a paclitaxel-resistant phenotype to HeLa cells characteristic of P-gp-mediated drug resistance. P-gp ATPase assays and photo-cross-linking with [(125)I]iodoarylazidoprazosin confirmed that transport and biochemical properties of P-gp chimeras were similar to those of wild-type P-gp, although differences in drug binding were detected when human and mouse transmembrane domains were combined. Overall, chimeras with one or two mouse P-gp domains were deemed functionally equivalent to human wild-type P-gp, demonstrating the ability of human P-gp to tolerate major structural changes.

Identification of a Cryptic Bacterial Promoter in Mouse (mdr1a) P-Glycoprotein cDNA

The efflux transporter P-glycoprotein (P-gp) is an important mediator of various pharmacokinetic parameters, being expressed at numerous physiological barriers and also in multidrug-resistant cancer cells. Molecular cloning of homologous cDNAs is an important tool for the characterization of functional differences in P-gp between species. However, plasmids containing mouse mdr1a cDNA display significant genetic instability during cloning in bacteria, indicating that mdr1a cDNA may be somehow toxic to bacteria, allowing only clones containing mutations that abrogate this toxicity to survive transformation. We demonstrate here the presence of a cryptic promoter in mouse mdr1a cDNA that causes mouse P-gp expression in bacteria. This expression may account for the observed toxicity of mdr1a DNA to bacteria. Sigma 70 binding site analysis and GFP reporter plasmids were used to identify sequences in the first 321 bps of mdr1a cDNA capable of initiating bacterial protein expression. An mdr1a M107L cDNA containing a single residue mutation at the proposed translational start site was shown to allow sub-cloning of mdr1a in E. coli while retaining transport properties similar to wild-type P-gp. This mutant mdr1a cDNA may prove useful for efficient cloning of mdr1a in E. coli.

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.

Overexpression, purification, and functional characterization of ATP-binding cassette transporters in the yeast, Pichia pastoris

The ATP-binding cassette (ABC) transporter superfamily is a large gene family that has been highly conserved throughout evolution. The physiological importance of these membrane transporters is highlighted by the large variety of substrates they transport, and by the observation that mutations in many of them cause heritable diseases in human. Likewise, overexpression of certain ABC transporters, such as P-glycoprotein and members of the multidrug resistance associated protein (MRP) family, is associated with multidrug resistance in various cells and organisms. Understanding the structure and molecular mechanisms of transport of the ABC transporters in normal tissues and their possibly altered function in human diseases requires large amounts of purified and active proteins. For this, efficient expression systems are needed. The methylotrophic yeast Pichia pastoris has proven to be an efficient and inexpensive experimental model for high-level expression of many proteins, including ABC transporters. In the present review, we will summarize recent advances on the use of this system for the expression, purification, and functional characterization of P-glycoprotein and two members of the MRP subfamily.

Molecular mechanisms of multidrug resistance in cancer chemotherapy

The occurrence of multidrug resistance (MDR) is one of the main obstacles in the successful chemotherapeutic treatment of cancer. MDR cell lines are resistant to the so-called naturally occurring anti-cancer drugs, such as anthracyclines, Vinca alkaloids and epipodophyllotoxins, but are not cross-resistant to alkylating agents, antimetabolites and cisplatin. So far, three separate forms of MDR have been characterized in more detail: classical MDR, non-Pgp MDR and atypical MDR. Although all three MDR phenotypes have much in common with respect to cross-resistance patterns, the underlying mechanisms certainly differ. Atypical MDR is associated with quantitative and qualitative alterations in topoisomerase II alpha, a nuclear enzyme that actively participates in the lethal action of cytotoxic drugs. Atypical MDR cells do not overexpress P-glycoprotein, and are unaltered in their ability to accumulate drugs. In this review we will focus on classical and non-Pgp MDR. The molecular mechanism of classical and non-Pgp MDR is transcriptional activation of membrane-bound transport proteins. These transport proteins belong to the ATP-binding cassette (ABC) superfamily of transport systems. The classical MDR phenotype is characterized by a reduced ability to accumulate drugs, due to activity of an energy-dependent uni-directional, membrane-bound, drug-efflux pump with broad substrate specificity. The classical MDR drug pump is composed of a transmembrane glycoprotein (P-glyco-protein-Pgp) with a molecular weight of 170 kD, and is, in man, encoded by the so-called multidrug resistance (MDR1) gene. Typically, non-Pgp MDR has no P-gly-coprotein expression, yet has about the same cross-resistance pattern as classical MDR. This non-Pgp MDR phenotype is caused by overexpression of the multidrug resistance-associated protein (MRP) gene, which encodes a 190 kD membrane-bound glycoprotein (MRP). MRP probably works by direct extrusion of cytotoxic drugs from the cell and/or by mediating sequestration of the drugs into intracellular compartments, both leading to a reduction in effective intracellular drug concentrations. For the classical MDR phenotype, evidence is accumulating that it plays a role indeed, in clinical drug resistance, especially in some hematological malignancies (acute myeloid leukemia, multiple myeloma and non-Hodgkin's lymphoma) and solid tumors (soft tissue sarcomas and neuroblastoma). The association of MRP with clinical drug resistance has not been elaborated, yet, and studies on MRP expression in human cancer have just begun. We found that overexpression of MRP, as determined by RNase protection assay as well as by immunohistochemistry, occurs in several human cancers, among which are cancer of the lung, esophagus, breast and ovary, and leukemias. Further studies are indicated to establish whether elevated MRP expression at diagnosis is an unfavorable prognostic factor for clinical outcome of chemotherapy.

Purification and reconstitution of functional human P-glycoprotein

The overexpression of the P-glycoprotein, the MDR1 gene product, has been linked to the development of resistance to multiple cytotoxic natural product anticancer drugs in certain cancers and cell lines derived from tumors. P-glycoprotein, a member of the ATP-binding cassette (ABC) superfamily of transporters, is believed to function as an ATP-dependent drug efflux pump with broad specificity for chemically unrelated hydrophobic compounds. We review here recent studies on the purification and reconstitution of P-glycoprotein to elucidate the mechanism of drug transport. P-glycoprotein from the human carcinoma multidrug resistant cell line, KB-V1, was purified by sequential chromatography on anion exchange followed by a lectin (wheat germ agglutinin) column. Proteoliposomes reconstituted with pure protein exhibited high levels of drug-stimulated ATPase activity as well as ATP-dependent [3H]vinblastine accumulation. Both the ATPase and vinblastine transport activities of the reconstituted P-glycoprotein were inhibited by vanadate. In addition, the vinblastine transport was inhibited by verapamil and daunorubicin. These studies provide strong evidence that the human P-glycoprotein functions as an ATP-dependent drug transporter. The development of the reconstitution system and the availability of recombinant protein in large amounts due to recent advances in overexpression of P-glycoprotein in a heterologous expression system should facilitate a better understanding of the function of this novel protein.

Mutations to amino acids located in predicted transmembrane segment 6 (TM6) modulate the activity and substrate specificity of human P-glycoprotein

Site-directed mutagenesis was used to investigate whether amino acids located in the predicted transmembrane segment, TM6 (residues 330-351), of human P-glycoprotein play essential roles in drug transport. Mutant cDNAs were expressed in mouse NIH 3T3 cells and analyzed with respect to their ability to confer resistance to cytotoxic drugs. Four mutations were found to strongly alter the drug resistance profile conferred by P-glycoprotein. Mutation of Val338 to Ala resulted in a mutant P-glycoprotein which conferred enhanced resistance to colchicine and reduced relative resistance to vinblastine. By contrast, mutant Gly341 to Val conferred little resistance to colchicine or doxorubicin, while its ability to confer resistance to vinblastine or actinomycin D was retained. A reduction in the ability of P-glycoprotein to confer resistance to all four drugs was observed for mutant Ala342 to Leu. Mutation of Ser344 to Ala, Thr, Cys, or Tyr resulted in mutant P-glycoproteins which were unable to confer drug resistance. Photolabeling of P-glycoprotein with azidopine in the presence of varying amounts of vinblastine showed that mutation of Ser344 to Tyr required approximately 15-fold more vinblastine to inhibit photolabeling when compared to wild-type enzyme. All of the Ser344 mutants were found to have reduced drug-stimulated ATPase activity relative to wild-type enzyme. These results, together with our previous demonstration that changes to Phe335 affected dissociation of vinblastine, suggest that TM6 may play an important role in drug--protein interaction and coupling of drug binding to ATPase activity.

Functional studies with a full-length P-glycoprotein cDNA encoded by the hamster pgp1 gene

Hamster cells grown in culture may, like human and mouse cells, develop multidrug resistance (MDR) when exposed to certain cytotoxic chemotherapeutic agents. Several phenotypic features that are characteristic of MDR have been described; these include (1) resistance to many structurally and functionally unrelated drugs that have different cellular targets and modes of action; (2) reversal of MDR by certain agents, including verapamil and cyclosporin A; and (3) reduced intracellular drug accumulation relative to that of drug-sensitive cells. In this report we show that the introduction and overexpression of the hamster pgp1 cDNA confers to otherwise drug-sensitive cells an MDR phenotype with these features. Moreover, pgp1 transfectants showed varying degrees of resistance to anthracycline analogues, indicating that structural analogues of commonly used anticancer agents are capable of circumventing drug resistance conferred by pgp.

Modulatory effects on substrate specificity of independent mutations at the serine939/941 position in predicted transmembrane domain 11 of P-glycoproteins

The serine residue located at position 939 and 941 in the predicted transmembrane segment 11 of P-glycoprotein (P-gp) encoded by mouse mdr3 and mdr1, respectively, appears to be important for interaction of chemotherapeutic drugs and reversal agents with P-gp. To further understand the role of this residue in this process and to identify the structural requirements involved, we have replaced this serine residue by alanine, cysteine, threonine, tyrosine, tryptophan, and aspartic acid and tested the effect of these mutations on the overall activity and substrate specificity of mdr1 and mdr3. All mutant proteins could be expressed at high levels in the membrane fractions of LR73 Chinese hamster cells transfected with the corresponding mutant cDNAs. All introduced mutations had limited effect on the capacity of mdr1 and mdr3 to confer resistance to vinblastine. The modulatory effect of mutations on resistance to colchicine, adriamycin, and actinomycin D was more dramatic. The hydroxyl group of serine did not seem essential for interaction with these drugs since mutant mdr1 and mdr3 bearing alanine or cysteine at that position behaved essentially as wild type, while threonine-bearing mutants showed significantly reduced resistance to these drugs. The insertion at that site of residues with bulkier side chains had more complex effects on P-gp function. While introducing tyrosine, tryptophan, or aspartic acid caused an almost complete loss of colchicine and adriamycin resistance in both mdr1 and mdr3, the replacement to tyrosine or tryptophan had the opposite effect on mdr1 and mdr3 for actinomycin D resistance, causing either a 3-fold increase or a 4-8-fold decrease in resistance to this drug, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

The multidrug resistance P-glycoprotein

P-glycoprotein plays an important role in the resistance of cancers to chemotherapy. Thus, an understanding of the mechanism by which it functions, and its 'normal' physiological role, is of clinical relevance as well as intrinsic interest. Considerable progress towards this goal has been made in the last year or so. In addition, the finding that P-glycoprotein is associated with both a channel and a transporter activity has, potentially, far-reaching implications for an understanding of the nature of channels and transporters.

The nitrogen of the acetamido group of colchicine modulates P-glycoprotein-mediated multidrug resistance

The substituents of drug molecules and the specific amino acid residues of P-glycoprotein (P-gp) implicated in drug/protein interactions are largely unknown. We have used a series of colchicine analogs modified on the A, B, and C rings to identify the discrete chemical groups on the colchicine molecule that are required for recognition by P-gp. For this, the toxicity of these analogs was tested on independent cell clones expressing either of the two mouse mdr genes, mdr1 and mdr3, known to confer multidrug resistance. Modifications of the methoxy groups on the A and C rings modulated cellular toxicity but had no effect on P-gp recognition; however, modifications at the C7 position of the B ring, in particular the removal of the nitrogen atom of the acetamido group, had a dramatic effect. Analogs bearing a hydrogen at that position were not substrates for P-gp. The importance of the nitrogen at C7 was independently verified in thiocolchicine and allocolchicine analogs similarly modified, although overall levels of resistance to these compounds were somewhat reduced compared to their colchicine counterparts. The study of allocolchicine congeners bearing a six-carbon C ring and of two other analogs completely lacking a B ring suggested that intact B and C rings were important for interaction with P-gp. These results suggest that the structural determinants for cytotoxicity (tubulin binding) and P-gp recognition map to nonoverlapping sites in the colchicine analogs analyzed. Examination of calculated molar refractivities (CMR) revealed that only compounds showing CMR values greater than 9.7 were P-gp substrates.(ABSTRACT TRUNCATED AT 250 WORDS)

Differing patterns of cross-resistance resulting from exposures to specific antitumour drugs or to radiation in vitro

This article reviews the patterns of cross-resistance identified in various P-glycoprotein-mediated and non-P-glycoprotein-mediated drug resistant mammalian tumour cell lines. The differing patterns of cross-resistance and the variable levels of resistance expressed are summarised and discussed. Although the mechanism by which P-glycoprotein can recognise and transport a large group of structurally-unrelated substrates remains to be defined, the recent evidence indicating that membrane associated domains participate in substrate recognition and binding is summarised, and other possible explanations for these variable cross-resistance patterns are considered. Amongst the non-P-glycoprotein-overexpressing multidrug resistant cell lines, two subsets are clearly identifiable, one lacking and the other expressing cross-resistance to the Vinca alkaloids. Resistance mechanisms implicated in these various sublines and possible explanations for their differing levels and patterns of cross-resistance are summarised. Clinical resistance is identified in patients following treatment not only with antitumour drugs, but also after radiotherapy. Experimental data providing a biological basis for this observation are summarised. A distinctive multiple drug resistance phenotype has been identified in tumour cells following exposure in vitro to fractionated X-irradiation characterised by: the expression of resistance to the Vinca alkaloids and the epipodophyllotoxins but not the anthracyclines and overexpression of P-glycoprotein which is post-translationally regulated, but without any concomitant overexpression of P-glycoprotein mRNA. Finally, the possible clinical relevance of these variable patterns of cross-resistance to the antitumour drugs commonly used in the clinic is considered.

Molecular analysis of the multidrug transporter

The multidrug resistance gene product, P-glycoprotein or the multidrug transporter, confers multidrug resistance to cancer cells by maintaining intracellular levels of cytotoxic agents below a killing threshold. P-glycoprotein is located within the plasma membrane and is thought to act as an energy-dependent drug efflux pump. The multidrug transporter represents a member of the ATP-binding cassette superfamily of transporters (or traffic ATPases) and is composed of two highly homologous halves, each of which harbors a hydrophobic transmembrane domain and a hydrophilic ATP-binding fold. This review focuses on various biochemical and molecular genetic approaches used to analyze the structure, function, and mechanism of action of the multidrug transporter, whose most intriguing feature is its ability to interact with a large number of structurally and functionally different amphiphilic compounds. These studies have underscored the complexity of this membrane protein which has recently been suggested to assume alternative topological and quaternary structures, and to serve multiple functions both as a transporter and as a channel. With respect to the multidrug transporter activity of P-glycoprotein, progress has been made towards the elucidation of essential amino acid residues and/or polypeptide regions. Furthermore, the drug-stimulatable ATPase activity of P-glycoprotein has been established. The mechanism of drug transport by P-glycoprotein, however, is still unknown and its physiological role remains a matter of speculation.