mdr2 encodes P-glycoprotein expressed in the bile canalicular membrane as determined by isoform-specific antibodies

Functional dissection of P-glycoprotein nucleotide-binding domains in chimeric and mutant proteins. Modulation of drug resistance profiles

We wished to determine if the two nucleotide-binding domains (NBD) of P-glycoprotein are functionally equivalent and interchangeable, and if not, which segments and amino acids are important for proper function of each NBD within the context of the C- or N-terminal P-glycoprotien halves. For this, we constructed and tested the biological activity in yeast and mammalian cells of a series of chimeric mdr3 cDNAs in which discrete domains of the N-terminal NBD (NBD1) were replaced by the homologous segments of the C-terminal NBD (NBD2). Although most NBD1 segments could be replaced without loss of P-glycoprotein function, exchange of small segments near the Walker B motif caused a dramatic reduction in Adriamycin, actinomycin D, and colchicine resistance in LR73 cells, as well as in FK506 resistance and STE6 complementation in yeast. Site-directed mutagenesis identified amino acid positions 522-525 (ERGA-->DKGT) and 578 (Thr-->Cys) as essential for proper function of NBD1 in the context of the N-terminal half P-glycoprotein. In addition, the observed phenotype of the mutants (altered drug resistance profile) suggests that these residues may participate directly or indirectly in substrate interactions and are possibly implicated in signal transduction from NBDs to transmembrane domains, the primary sites of drug binding in P-glycoprotein.

Rifampicin enhances anti-cancer drug accumulation and activity in multidrug-resistant cells

Rifampicin, a semi-synthetic antibiotic used in the treatment of tuberculosis and belonging to the chemical class of rifamycins, was examined for its effect on anti-cancer drug accumulation and activity in multidrug resistant cells overexpressing P-glycoprotein (P-gp). Rifampicin was shown to strongly enhance vinblastine accumulation in both rat hepatoma RHC1 and human leukemia K562 R7 multidrug resistant cells, but had no effect in rat SDVI drug-sensitive liver cells. By contrast, two other rifamycins, rifamycins B and SV, had no or only minor effect on vinblastine accumulation in RHC1 cells. Efflux experiments revealed that rifampicin was able, like the well-known chemosensitizer agent verapamil, to decrease export of vinblastine out of resistant cells. Rifampicin, when used at a concentration close to plasma concentrations achievable in humans (25 microM), was able to increase sensitivity of RHC1 cells to both vinblastine and doxorubicin. Rifampicin was also demonstrated to inhibit P-gp radiolabeling by the photoactivable P-gp ligand azidopine, thereby suggesting that the antituberculosis compound can interfere directly with P-gp drug binding sites. These results thus indicate that rifampicin was able to down-modulate P-gp-associated resistance through inhibition of P-gp function.

Characterization of the promoter region of the human MDR3 P-glycoprotein gene

The human MDR3 (or MDR2) P-glycoprotein is probably involved in the transport of phospholipids from liver hepatocytes into bile (Smit et al. (1993) Cell 75, 451-462). In accordance with this function, MDR3 is highly expressed in human liver, but lower mRNA levels were also found in adrenal, heart, muscle and cells of the B-cell compartment. We have cloned and analyzed the MDR3 promoter region. It is GC-rich, and contains neither a TATA nor a CAAT box, but it does contain multiple putative SP1 binding sites, features also found in so-called housekeeping genes. RNase protection and primer extension analyses indicate that the MDR3 gene has multiple transcription start sites in a GC-rich region with considerable homology to the putative mouse mdr2 promoter. A 3 kb genomic fragment containing the MDR3 start sites directs transcription of a chloramphenicol acetyltransferase (CAT) reporter gene upon transient transfection in the human hepatoma cell line HepG2. This transcription is orientation dependent, and stimulated by a SV40 enhancer, indicating that the 3 kb insert contains the core promoter elements of the MDR3 gene. The promoter region contains several consensus sequences where known or putative liver-specific (C/EBP, HNF5) or lymphoid specific (Pu.1, ets-1) transcription factors may bind.

ATP-dependent transport of the linear renin-inhibiting peptide EMD 51921 by canalicular plasma membrane vesicles of rat liver: evidence of drug-stimulatable ATP-hydrolysis

Certain peptide drugs, such as the linear hydrophobic renin-inhibitor EMD 51921, are rapidly eliminated via the bile. At the sinosoidal membrane of liver cells EMD 51921 is taken up via a sodium-independent carrier-mediated mechanism, competing for the uptake of bile acids. Until now, the mechanisms of biliary excretion of EMD 51921 were unknown. In this study we describe an ATP-dependent transport system for the enzymatically and metabolically stable hydrophobic linear renin-inhibiting peptide EMD 51921. The ATP-dependent uptake into the osmotic reactive intravesicular space is saturable (Km 12 microM, Vmax 663 pmol/min per mg protein), temperature dependent and specifically requires ATP. Transport is inhibited by vanadate but not by ouabain, EGTA or NaN3, and does not function in basolateral plasma membrane vesicles. Transport is not altered in canalicular membrane vesicles isolated from Tr- rats lacking the canalicular ATP-dependent transport of cysteinyl leukotrienes and related anions. Transport is inhibited by taurocholate, a typical substrate of the canalicular ATP-dependent bile acid transporter, but also by vincristine and daunomycin, substrates of P-glycoproteins. EMD 51921, however, only inhibits the uptake of taurocholate, whereas the transport of daunomycin is not influenced. Taurocholate and EMD 51921 are mutually non- or un-competitive transport inhibitors. Incubation of rat liver canalicular membranes with micromolar concentrations of EMD 51921 resulted in a 1.8-2.5-fold increase in the rate of ATP-hydrolysis. In contrast, ATP-hydrolysis was not affected by fragments of the peptide that are not transported in an ATP-dependent manner. The apparent Km value (EMD) for ATP-hydrolysis is 68 microM. Vmax is 0.032 U/mg protein. ATPase activity is pH dependent. Stimulation of ATP-hydrolysis is inhibited by vanadate, NEM, hydroxymercuribenzoate and ascorbate, but is not affected by ouabain, EGTA or NaN3. EMD 51921 does not stimulate the ATPase activity of the Na+/K(+)-ATPase isolated from kidney medulla. The EMD-stimulatable ATPase seems to be distinct from the glutathione-S-conjugate stimulatable ATPase and the mdr 1a/b gene products and differs in its characteristics from that of the canalicular ecto-ATPase.

The human MDR3 P-glycoprotein promotes translocation of phosphatidylcholine through the plasma membrane of fibroblasts from transgenic mice

The mouse mdr2 P-glycoprotein (P-gp) and its human MDR3 homologue are present in high concentrations in the canalicular membrane of hepatocytes. Mice lacking this protein are unable to secrete phosphatidylcholine (PC) into bile, suggesting that this P-gp is a PC translocator. We have tested this in fibroblasts from transgenic mice expressing the MDR3 gene under a vimentin promoter. Transgenic and control fibroblasts were incubated with [14C]choline to label PC. When the labeled cells were incubated with a PC transfer protein and acceptor liposomes, transfer of radioactive PC was enhanced in transgenic cells relative to the wild type controls. We conclude that the MDR3 P-glycoprotein is able to promote the transfer of PC from the inner to the outer leaflet of the plasma membrane, supporting the idea that this protein functions as a PC flippase.

Effect of colchicine and heat shock on multidrug resistance gene and P-glycoprotein expression in rat liver

The multidrug resistance genes encode plasma membrane glycoproteins named P-glycoproteins, that act as an ATP-dependent drug efflux pump and decrease the cytosolic concentration of chemotherapeutic agents. It has been hypothesized that in rat liver, this protein may have a physiological role as a biliary transporter of xenobiotics and endobiotics. Some human tumor cell lines turn on the human multidrug resistance gene in response to high temperature and after exposure to toxic chemicals. Accordingly, it has been proposed that the human multidrug resistance gene is a heat shock gene. We have assessed whether two environmental stresses, heat shock or acute exposure to cytotoxic drugs (colchicine, vincristine, vinblastine and daunomycin), induce changes in the expression of multidrug resistance genes in the rat. Total cellular RNA extracted from rat liver was hybridized to a labeled human multidrug resistance gene cDNA probe. Temperature upshift did not increase the steady-state of mdr mRNA levels in the tissues studied, suggesting that the mdr genes are not activated as part of a heat shock response. The mdr mRNA levels increased in rat liver as early as 3 h after a single injection of colchicine, reached a peak (500%; p < 0.05) after around 24 h and returned to constitutive levels after 48 h. Changes in the relative content of mdr mRNA were not detected in kidney, adrenal gland and small bowel, suggesting that the in vivo induction of the mdr gene in the liver is a tissue-specific response. The other cytotoxic drugs that were tested did not increase the steady-state of mdr mRNA levels.(ABSTRACT TRUNCATED AT 250 WORDS)

The role of the canalicular multispecific organic anion transporter in the disposal of endo- and xenobiotics

Bile is an important excretory route for the elimination of amphiphilic organic anions, and hepatocytes are the primary secretory units of bile formation. The hepatocytic basolateral and canalicular membranes are equipped with various carrier proteins. Transport across the canalicular membrane represents a major concentrative step. Various ATP-dependent transporters have been identified, such as a multispecific organic anion transporter (canalicular multispecific organic ion transporter, cMOAT), a bile acid transporter and several P-glycoproteins. TR- rats, which lack cMOAT activity, have been valuable in defining the substrate specificity of cMOAT. A wide range of glucuronide-, glutathione- and sulfate-conjugates are transported by this system.

Adenosine triphosphate activates ion permeabilities in biliary epithelial cells

BACKGROUND/AIMS

The biliary epithelium contributes to bile formation through absorption and secretion of fluid and electrolytes. The effects of extracellular nucleotides on membrane ion transport were assessed in isolated bile duct cells from rats and Mz-ChA-1 cells from a human cholangiocarcinoma.

METHODS

The rates of efflux of 125I and 86Rb were used to assess membrane Cl- and K+ permeabilities, respectively. Patch clamp recordings of whole cell currents were used to evaluate the properties of adenosine triphosphate (ATP)-activated currents.

RESULTS

Purinergic receptor agonists ATP and uridine triphosphate stimulated 125I and 86Rb efflux about twofold above basal levels. The effects were reproduced by a nonhydrolyzable analogue of ATP (adenosine 5'-O-[3-thiophosphate]) and were unaffected by an adenosine receptor blocker xanthine amine congener. 125I efflux was also stimulated by adenosine and its receptor agonists 5'-N-ethylcarboxamidoadenosine, N6-(2-phenylisopropyl)adenosine; these effects were inhibited by xanthine amine congener, suggesting a separate adenosine receptor. ATP, adenosine 5'-O-(3-thiophosphate), and uridine triphosphate each stimulated release of Ca2+ from intracellular stores, whereas adenosine had no effect. In whole cell recordings of Mz-ChA-1 cells, ATP activated an early transient outward current consistent with a K+ conductance and a later, sustained inward current consistent with a Cl- conductance.

CONCLUSIONS

Biliary cells possess at least two classes of nucleotide receptors that modulate membrane ion permeability through Ca(2+)-dependent and -independent pathways, and ATP may be involved in the regulation of biliary secretion.

A mechanism for P-glycoprotein action in multidrug resistance: are we there yet?

Multidrug resistance is associated with the overexpression of P-glycoprotein, a membrane glycoprotein. The mechanism by which P-glycoprotein confers to cells the capacity to resist cytotoxic attack by structurally unrelated drugs has remained difficult to decipher. However, the recent functional expression of this group of proteins in the membrane of secretory vesicles from yeast mutants has allowed the systematic analysis of the parameters of drug transport by this protein. Proposed mechanisms of action of P-glycoprotein are discussed in this review by Stephan Ruetz and Philippe Gros.

Identification of two nuclear protein binding sites and their role in the regulation of the murine multidrug resistance mdr1a promoter

Multidrug resistance genes (mdr) that encode P-glycoproteins (P-gp) are transcriptionally regulated in normal tissues and in some multidrug-resistant (MDR) cells. Several lines of evidence suggest that regulation of P-gp overexpression at the transcriptional level is also important in human tumors. In murine MDR cells, mdr1a and/or mdr1b genes are overexpressed and P-gp isoforms are overproduced. To identify the mdr1a promoter regions that are required for transcription, the promoter has been linked to the chloramphenicol acetyltransferase (CAT) gene in transient expression vectors. 5'-Deletions of the promoter sequences have demonstrated that the region between -155 to +89 bp is crucial for basal activity of the mdr1a gene. DNase I footprinting, methylation interference, and gel retardation assays identified two nuclear protein binding sites within these sequences. One of the nuclear protein binding sites contains an 11-bp DNA sequence that interacts with nuclear protein(s) and is conserved in the promoters of the murine mdr1a and mdr1b, hamster pgp1, and human MDR1 genes. The conserved SP1 site (5'-GGGCGGG-3') that is present further downstream was shown to interact with its nuclear factor. These observations suggest that at least part of mdr gene transcriptional regulation is mediated by conserved mdr cis-regulatory elements and common nuclear factors.

ATP-dependent transport of amphiphilic cations across the hepatocyte canalicular membrane mediated by mdr1 P-glycoprotein

The ATP-dependent transport of the three 3H-labeled, amphiphilic cations quinidine, N-(n-pentyl)-quinidinium, and N-(4',4'-azo-n-pentyl)-21-deoxyajmalinium was studied in rat canalicular plasma membrane vesicles. N-Alkylation of quinidine with an n-pentyl residue resulted in a permanently charged cationic substrate for ATP-dependent transport which exhibited a 10-fold higher transport rate relative to quinidine. The Km value was 0.4 microM for N-(n-pentyl)-quinidinium and 5 microM for quinidine. The permanently cationic and photolabile derivative of ajmaline, N-(4',4'-azo-n-pentyl)-21-deoxyajmalinium, was also an efficient substrate and served to label canalicular membrane proteins with molecular masses of 143 kDa and 108 kDa. ATP-dependent transport of the permanently charged amphiphilic cations was inhibited by the P-glycoprotein inhibitors and substrates quinidine, verapamil, and daunorubicin. The data demonstrate that N-alkylation of quinidine and ajmaline results in most efficient substrates for mdr1 P-glycoprotein-mediated ATP-dependent transport.

ATP-dependent export pumps and their inhibition by cyclosporins

Cyclosporins are potent tools to inhibit several primary-active, ATP-dependent export carriers. This has been demonstrated in membrane vesicle transport assays for CsA and for its non-immunosuppressive analog PSC 833. Inhibition in the low micromolar and in the nanomolar concentration range is shown for the three distinct ATP-dependent export carriers in the liver canalicular membrane mediating the secretion into bile of leukotrienes (LTC4, other cysteinyl leukotrienes, and related conjugates), bile salts (taurocholate), and amphiphilic, mostly cationic substances (daunorubicin and other P-glycoprotein substrates). Competitive inhibition by cyclosporins is most potent for ATP-dependent taurocholate transport with Ki values of 0.2 and 0.6 microM for CsA and PSC 833, respectively. This inhibition is in agreement with in vivo studies in the rat demonstrating a block at the canalicular membrane in the hepatobiliary elimination of labeled taurocholate. The data suggest that cholestasis, as a side effect during CsA therapy, is largely due to inhibition of the ATP-dependent bile salt export carrier in the canalicular membrane. Inhibition by cyclosporins is less effective with respect to ATP-dependent leukotriene transport, both during biosynthetic release from mastocytoma cells and during hepatobiliary excretion. The Ki values for the former were 4.5 and 30 microM, and the Km/Ki ratios only 0.015 and 0.002 for CsA and PSC 833, respectively. Distinct transporters are inhibited by the cyclosporins with different potency and structurally modified cyclosporins may serve to induce preferential inhibition of a selected transporter. This is illustrated by the inhibition of the multidrug export carrier with daunorubicin as substrate using PSC 833 as inhibitor with a Ki value of 0.3 microM in an in vitro membrane transport system.

Homozygous disruption of the murine mdr2 P-glycoprotein gene leads to a complete absence of phospholipid from bile and to liver disease

Two types of P-glycoprotein have been found in mammals: the drug-transporting P-glycoproteins and a second type, unable to transport hydrophobic anticancer drugs. The latter is encoded by the human MDR3 (also called MDR2) and the mouse mdr2 genes, and its tissue distribution (bile canalicular membrane of hepatocytes, B cells, heart, and muscle) suggests a specialized metabolic function. We have generated mice homozygous for a disruption of the mdr2 gene. These mice develop a liver disease that appears to be caused by the complete inability of the liver to secrete phospholipid into the bile. Mice heterozygous for the disrupted allele had no detectable liver pathology, but half the level of phospholipid in bile. We conclude that the mdr2 P-glycoprotein has an essential role in the secretion of phosphatidylcholine into bile and hypothesize that it may be a phospholipid transport protein or phospholipid flippase.

Differential expression of P-glycoprotein genes in primary rat hepatocyte culture

The multidrug resistance (MDR)-associated protein, P-glycoprotein (Pgp), is expressed on the bile canalicular surface of hepatocytes, where it is thought to function in the detoxification of xenobiotics or in the transport of specific metabolites. Several studies have shown that Pgp expression in rat liver can be perturbed in vivo and in vitro; however, it is not known which of the 3 Pgp genes (class I, II, or III) are involved. In rodents, the class I and II Pgp genes have been shown to mediate MDR while the class III gene apparently does not. In this report, we have used gene-specific probes generated from the 3'-untranslated regions of the three rat Pgp genes (Deuchars et al.: Biochim. Biophys. Acta, 1130:157-165, 1992) to investigate Pgp gene expression in primary rat hepatocytes. We observed that the class II Pgp mRNA, the least abundant in the intact liver, is dramatically increased in culture over a 48 h period, while the class I Pgp showed only a modest increase in mRNA level. In contrast, the class III Pgp mRNA, which is the most abundant in the intact liver, exhibited a gradual decline. In rat liver hepatocytes, different culture conditions, as well as drugs such as cytochalasin D and colchicine, appear to affect the level of the class II Pgp gene expression. Moreover, under all these conditions, there is a strong correlation between the level of the class II Pgp and cytoskeletal (actin and tubulin) mRNAs. Thus, there may be a common mechanism regulating the expression of cytoskeletal protein genes and the class II Pgp gene. These findings have implications for our understanding of the regulation of Pgp gene expression in normal and malignant tissues.

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.