Cancer cell death by programmed necrosis?

A recent paper by Zong et al. describes how alkylating agents kill cells by a process they term "programmed necrosis," induced by excessive activation of PARP resulting in degradation of cytosolic NAD(+) and inhibition of glycolysis. We argue that it is not obvious whether chemotherapy in patients can induce sufficient NAD(+) loss to affect glycolysis; that the "programmed" nature of the necrosis requires more evidence; and that there are mechanisms making cancer cells hypersensitive to DNA damage other than their high rate of aerobic glycolysis.

Base J, found in nuclear DNA of Trypanosoma brucei, is not a target for DNA glycosylases

Base excision repair (BER) is an evolutionarily conserved system which removes altered bases from DNA. The initial step in BER is carried out by DNA glycosylases which recognize altered bases and cut the N-glycosylic bond between the base and the DNA backbone. In kinetoplastid flagellates, such as Trypanosoma brucei, the modified base beta-D-glucosyl-hydroxymethyluracil (J) replaces a small percentage of thymine residues, predominantly in repetitive telomeric sequences. Base J is synthesized at the DNA level via the precursor 5-hydroxymethyluracil (5-HmU). We have investigated whether J in DNA can be recognized by DNA glycosylases from non-kinetoplastid origin, and whether the presence of J and 5-HmU in DNA has required modifications of the trypanosome BER system. We tested the ability of 15 different DNA glycosylases from various origins to excise J or 5-HmU paired to A from duplex oligonucleotides. No excision of J was found, but 5-HmU was excised by AlkA and Mug from Escherichia coli and by human SMUG1 and TDG, confirming previous reports. In a combination of database searches and biochemical assays we identified several DNA glycosylases in T. brucei, but in trypanosome extracts we detected no excision activity towards 5-HmU or ethenocytosine, a product of oxidative DNA damage and a substrate for Mug, TDG and SMUG1. Our results indicate that trypanosomes have a BER system similar to that of other organisms, but might be unable to excise certain forms of oxidatively damaged bases. The presence of J in DNA does not require a specific modification of the BER system, as this base is not recognized by any known DNA glycosylase.

Mechanism of the pharmacokinetic interaction between methotrexate and benzimidazoles: potential role for breast cancer resistance protein in clinical drug-drug interactions

The antifolate drug methotrexate (MTX) is transported by breast cancer resistance protein (BCRP; ABCG2) and multidrug resistance-associated protein1-4 (MRP1-4; ABCC1-4). In cancer patients, coadministration of benzimidazoles and MTX can result in profound MTX-induced toxicity coinciding with an increase in the serum concentrations of MTX and its main metabolite 7-hydroxymethotrexate. We hypothesized that benzimidazoles interfere with the clearance of MTX and/or 7-hydroxymethotrexate by inhibition of the ATP-binding cassette drug transporters BCRP and/or MRP2, two transporters known to transport MTX and located in apical membranes of epithelia involved in drug disposition. First, we investigated the mechanism of interaction between benzimidazoles (pantoprazole and omeprazole) and MTX in vitro in membrane vesicles from Sf9 cells infected with a baculovirus containing human BCRP or human MRP2 cDNA. In Sf9-BCRP vesicles, pantoprazole and omeprazole inhibited MTX transport (IC50 13 microm and 36 microm, respectively). In Sf9-MRP2 vesicles, pantoprazole did not inhibit MTX transport and at high concentrations (1 mm), it even stimulated MTX transport 1.6-fold. Secondly, we studied the transport of pantoprazole in MDCKII monolayers transfected with mouse Bcrp1 or human MRP2. Pantoprazole was actively transported by Bcrp1 but not by MRP2. Finally, the mechanism of the interaction was studied in vivo using Bcrp1-/- mice and wild-type mice. Both in wild-type mice pretreated with pantoprazole to inhibit Bcrp1 and in Bcrp1-/- mice that lack Bcrp1, the clearance of i.v. MTX was decreased significantly 1.8- to 1.9-fold compared with the clearance of i.v. MTX in wild-type mice. The conclusion is as follows: benzimidazoles differentially affect transport of MTX mediated by BCRP and MRP2. Competition for BCRP may explain the clinical interaction between MTX and benzimidazoles.

The potential impact of drug transporters on nucleoside-analog-based antiviral chemotherapy

Several ATP-binding cassette (ABC) transporters can transport drugs out of cells against steep concentration gradients resulting in resistance to the drugs transported. Recent work has shown that at least three members of the family of human Multidrug Resistance-associated Proteins (MRPs), MRP4, 5 and 8, are able to transport some nucleoside-monophosphate analogs. This can result in resistance to the base, nucleoside or nucleotide precursors of these results, at least in cell lines with high levels of transporter. The affinity of these transporters for the nucleotide analogs studied thus far is relatively low (millimolar rather than micromolar), and this limits their potential impact on the resistance. We briefly review how ABC transporters in general, and MRPs in particular, could affect the disposition and cellular accumulation of antiviral compounds.

Factors affecting the level and localization of the transferrin receptor in Trypanosoma brucei

Transfer of bloodstream-form Trypanosoma brucei variant 221a from calf serum to dog serum-based medium induces acute iron starvation, as the transferrin receptor (Tf-R) of variant 221a binds dog Tf poorly. We show here that transfer to dog serum induces a 3-5-fold increase in Tf-R mRNA and protein within one doubling time (8 h). Because iron stores are still high 8 h after transfer, we infer that the signal for Tf-R overproduction is the decreased availability of cytosolic iron when cellular iron import drops. Up to 30% of the extra Tf-R spills out of the flagellar pocket onto the pellicular surface. Because the 5-fold increase in Tf-R is accompanied by a 5-fold increase in bovine Tf uptake, the up-regulation of Tf-R levels in response to Tf starvation helps the trypanosome to compete for limiting amounts of Tf. We noted that Tf-R levels also vary in calf serum medium. Cells in dense cultures contain up to 5-fold more Tf-R mRNA and protein than in dilute cultures. Only one-tenth of the extra Tf-R reaches the pellicular surface. The increase cannot be explained by a lack of Tf or to cell density sensing but is due to pericellular hypoxia. Our results show that bloodstream-form trypanosomes can regulate the expression of the two Tf-R subunit genes and the localization of their gene products in a flexible manner. This flexibility is made possible by the promoter-proximal position of the two genes in the variant surface glycoprotein expression site.

Interactions between Hepatic Mrp4 and Sult2a as Revealed by the Constitutive Androstane Receptor and Mrp4 Knockout Mice

The ABC transporter, Mrp4, transports the sulfated steroid DHEA-s, and sulfated bile acids interact with Mrp4 with high affinity. Hepatic Mrp4 levels are low, but increase under cholestatic conditions. We therefore inferred that up-regulation of Mrp4 during cholestasis is a compensatory mechanism to protect the liver from accumulation of hydrophobic bile acids. We determined that the nuclear receptor CAR is required to coordinately up-regulate hepatic expression of Mrp4 and an enzyme known to sulfate hydroxy-bile acids and steroids, Sult2a1. CAR activators increased Mrp4 and Sult2a1 expression in primary human hepatocytes and HepG2, a human liver cell line. Sult2a1 was down-regulated in Mrp4-null mice, further indicating an inter-relation between Mrp4 and Sult2a1 gene expression. Based on the hydrophilic nature of sulfated bile acids and the Mrp4 capability to transport sulfated steroids, our findings suggest that Mrp4 and Sult2a1 participate in an integrated pathway mediating elimination of sulfated steroid and bile acid metabolites from the liver.

The human multidrug resistance protein MRP4 functions as a prostaglandin efflux transporter and is inhibited by nonsteroidal antiinflammatory drugs

Prostaglandins are involved in a wide variety of physiological and pathophysiological processes, but the mechanism of prostaglandin release from cells is not completely understood. Although poorly membrane permeable, prostaglandins are believed to exit cells by passive diffusion. We have investigated the interaction between prostaglandins and members of the ATP-binding cassette (ABC) transporter ABCC [multidrug resistance protein (MRP)] family of membrane export pumps. In inside-out membrane vesicles derived from insect cells or HEK293 cells, MRP4 catalyzed the time- and ATP-dependent uptake of prostaglandin E1 (PGE1) and PGE2. In contrast, MRP1, MRP2, MRP3, and MRP5 did not transport PGE1 or PGE2. The MRP4-mediated transport of PGE1 and PGE2 displayed saturation kinetics, with Km values of 2.1 and 3.4 microM, respectively. Further studies showed that PGF1alpha, PGF2alpha, PGA1, and thromboxane B2 were high-affinity inhibitors (and therefore presumably substrates) of MRP4. Furthermore, several nonsteroidal antiinflammatory drugs were potent inhibitors of MRP4 at concentrations that did not inhibit MRP1. In cells expressing the prostaglandin transporter PGT, the steady-state accumulation of PGE1 and PGE2 was reduced proportional to MRP4 expression. Inhibition of MRP4 by an MRP4-specific RNA interference construct or by indomethacin reversed this accumulation deficit. Together, these data suggest that MRP4 can release prostaglandins from cells, and that, in addition to inhibiting prostaglandin synthesis, some nonsteroidal antiinflammatory drugs might also act by inhibiting this release.

Evidence for two interacting ligand binding sites in human multidrug resistance protein 2 (ATP binding cassette C2)

Multidrug resistance protein 2 (MRP2) belongs to the ATP binding cassette family of transporters. Its substrates include organic anions and anticancer drugs. We have used transport assays with vesicles derived from Sf9 insect cells overproducing MRP2 to study the interactions of drugs, organic anions, and bile acids with three MRP2 substrates: estradiol-17-beta-d-glucuronide (E217betaG), methotrexate, and glutathione-S-dinitrophenol. Complex inhibition and stimulation patterns were obtained, different from those observed with the related transporters MRP1 and MRP3. In contrast to a previous report, we found that the rate of E217betaG transport by MRP2 increases sigmoidally with substrate concentration indicative of homotropic cooperativity. Half-maximal transport was obtained at 120 microm E217betaG, in contrast to values < 20 microm for MRP1 and 3. MRP2 stimulators, such as indomethacin and sulfanitran, strongly increased the affinity of MRP2 for E217betaG (half-maximal transport rates at 65 and 16 microm E217betaG, respectively) and shifted the sigmoidal dependence of transport rate on substrate concentration to a more hyperbolic one, without substantially affecting the maximal transport rate. Sulfanitran also stimulated MRP2 activity in cells, i.e. the transport of saquinavir through monolayers of Madin-Darby canine kidney II cells. Some compounds that stimulate E217betaG transport, such as penicillin G or pantoprazole, are not detectably transported by MRP2, suggesting that they allosterically stimulate transport without being cotransported with E217betaG. We propose that MRP2 contains two similar but nonidentical ligand binding sites: one site from which substrate is transported and a second site that regulates the affinity of the transport site for the substrate.

Characterization of the MRP4- and MRP5-mediated transport of cyclic nucleotides from intact cells

Cyclic nucleotides are known to be effluxed from cultured cells or isolated tissues. Two recently described members of the multidrug resistance protein family, MRP4 and MRP5, might be involved in this process, because they transport the 3',5'-cyclic nucleotides, cAMP and cGMP, into inside-out membrane vesicles. We have investigated cGMP and cAMP efflux from intact HEK293 cells overexpressing MRP4 or MRP5. The intracellular production of cGMP and cAMP was stimulated with the nitric oxide releasing compound sodium nitroprusside and the adenylate cyclase stimulator forskolin, respectively. MRP4- and MRP5-overexpressing cells effluxed more cGMP and cAMP than parental cells in an ATP-dependent manner. In contrast to a previous report we found no glutathione requirement for cyclic nucleotide transport. Transport increased proportionally with intracellular cyclic nucleotide concentrations over a calculated range of 20-600 microm, indicating low affinity transport. In addition to several classic inhibitors of organic anion transport, prostaglandins A(1) and E(1), the steroid progesterone and the anti-cancer drug estramustine all inhibited cyclic nucleotide efflux. The efflux mediated by MRP4 and MRP5 did not lead to a proportional decrease in the intracellular cGMP or cAMP levels but reduced cGMP by maximally 2-fold over the first hour. This was also the case when phosphodiesterase-mediated cyclic nucleotide hydrolysis was inhibited by 3-isobutyl-1-methylxanthine, conditions in which efflux was maximal. These data indicate that MRP4 and MRP5 are low affinity cyclic nucleotide transporters that may at best function as overflow pumps, decreasing steep increases in cGMP levels under conditions where cGMP synthesis is strongly induced and phosphodiesterase activity is limiting.

Characterization of the transport of nucleoside analog drugs by the human multidrug resistance proteins MRP4 and MRP5

The human multidrug resistance proteins MRP4 and MRP5 are organic anion transporters that have the unusual ability to transport cyclic nucleotides and some nucleoside monophosphate analogs. Base and nucleoside analogs used in the chemotherapy of cancer and viral infections are potential substrates. To assess the possible contribution of MRP4 and MRP5 to resistance against these drugs, we have investigated the transport mediated by MRP4 and MRP5. In cytotoxicity assays, MRP4 conferred resistance to the antiviral agent 9-(2-phosphonomethoxyethyl)adenine (PMEA) and high-performance liquid chromatography analysis showed that, like MRP5, MRP4 transported PMEA in an unmodified form. MRP4 also mediated substantial resistance against other acyclic nucleoside phosphonates, whereas MRP5 did not. Apart from low-level MRP4-mediated cladribine resistance, the cytotoxicity of clinically used anticancer nucleosides was not influenced by overexpression of MRP4 or MRP5. In contrast, MRP5 mediated efflux of the pyrimidine-based antiviral 2',3'-dideoxynucleoside 2',3'-didehydro-2',3'-dideoxythymidine 5'-monophosphate (d4TMP) and its phosphoramidate derivative alaninyl-d4TMP from cells loaded with the 2',3'-didehydro-2',3'-dideoxythymidine prodrugs cyclosaligenyl-d4TMP and aryloxyphosphoramidate d4TMP (So324), respectively. Moreover, only inside-out membrane vesicles derived from MRP5-overexpressing cells accumulated alaninyl-d4TMP. Cellular efflux and vesicular uptake studies were carried out to further compare transport mediated by MRP4 and MRP5 and showed that dipyridamole, dilazep, nitrobenzyl mercaptopurine riboside, sildenafil, trequinsin and MK571 inhibited MRP4 more than MRP5, whereas cyclic nucleotides and monophosphorylated nucleoside analogs were equally poor inhibitors of both pumps. These results strongly suggest that the affinity of MRP4 and MRP5 for nucleotide-based substrates is low.

Delineation of the regulated Variant Surface Glycoprotein gene expression site domain of Trypanosoma brucei

The African trypanosome Trypanosoma brucei is protected in the bloodstream of the mammalian host by a dense Variant Surface Glycoprotein (VSG) coat. Although an individual cell has hundreds of VSG genes, the active VSG is transcribed in a mutually exclusive fashion from one of about twenty telomeric VSG expression sites. Expression sites are regulated domains flanked by 50 bp repeat arrays and extensive tracts of repetitive elements. We have integrated exogenous rDNA and expression site promoters upstream of the 50 bp repeats of the VO2 VSG expression site. Transcription from both types of exogenous promoter is downregulated and comparable to promoters targeted into the VSG Basic Copy arrays. We show that the upstream exogenous rDNA promoter escapes VSG expression site control, as switching the downstream VO2 VSG expression site on and off does not affect its activity. Therefore, the 50 bp repeat arrays appear to be the boundary of the regulated expression site domain.

Steroid and bile acid conjugates are substrates of human multidrug-resistance protein (MRP) 4 (ATP-binding cassette C4)

Human multidrug-resistance protein (MRP) 4 transports cyclic nucleotides and when overproduced in mammalian cells mediates resistance to some nucleoside analogues. Recently, it has been shown that Mrp4 is induced in the livers of Fxr ((-/-)) mice, which have increased levels of serum bile acids. Since MRP4, like MRP1-3, also mediates transport of a model steroid conjugate substrate, oestradiol 17-beta-D-glucuronide (E(2)17betaG), we tested whether MRP4 may be involved in the transport of steroid and bile acid conjugates. Bile salts, especially sulphated derivatives, and cholestatic oestrogens inhibited the MRP4-mediated transport of E(2)17betaG. Inhibition by oestradiol 3,17-disulphate and taurolithocholate 3-sulphate was competitive, suggesting that these compounds are MRP4 substrates. Furthermore, we found that MRP4 transports dehydroepiandrosterone 3-sulphate (DHEAS), the most abundant circulating steroid in humans, which is made in the adrenal gland. The ATP-dependent transport of DHEAS by MRP4 showed saturable kinetics with K (m) and V (max) values of 2 microM and 45 pmol/mg per min, respectively (at 27 degrees C). We further studied the possible involvement of other members of the MRP family of transporters in the transport of DHEAS. We found that MRP1 transports DHEAS in a glutathione-dependent manner and exhibits K (m) and V (max) values of 5 microM and 73 pmol/mg per min, respectively (at 27 degrees C). No transport of DHEAS was observed in membrane vesicles containing MRP2 or MRP3. Our findings suggest a physiological role for MRP1 and MRP4 in DHEAS transport and an involvement of MRP4 in transport of conjugated steroids and bile acids.

Transport of bile acids in multidrug-resistance-protein 3-overexpressing cells co-transfected with the ileal Na+-dependent bile-acid transporter

Many of the transporters involved in the transport of bile acids in the enterohepatic circulation have been characterized. The basolateral bile-acid transporter of ileocytes and cholangiocytes remains an exception. It has been suggested that rat multidrug resistance protein 3 (Mrp3) fulfills this function. Here we analyse bile-salt transport by human MRP3. Membrane vesicles from insect ( Spodoptera frugiperda ) cells expressing MRP3 show time-dependent uptake of glycocholate and taurocholate. Furthermore, sulphated bile salts were high-affinity competitive inhibitors of etoposide glucuronide transport by MRP3 (IC50 approximately 10 microM). Taurochenodeoxycholate, taurocholate and glycocholate inhibited transport at higher concentrations (IC50 approximately 100, 250 and 500 microM respectively). We used mouse fibroblast-like cell lines derived from mice with disrupted Mdr1a, Mdr1b and Mrp1 genes to generate transfectants that express the murine apical Na+-dependent bile-salt transporter (Asbt) and MRP3. Uptake of glycocholate by these cells is Na+-dependent, with a K(m) and V(max) of 29+/-7 microM and 660 +/- 63 pmol/min per mg of protein respectively and is inhibited by several organic-aniontransport inhibitors. Expression of MRP3 in these cells limits the accumulation of glycocholate and increases the efflux from cells preloaded with taurocholate or glycocholate. In conclusion, we find that MRP3 transports both taurocholate and glycocholate, albeit with low affinity, in contrast with the high-affinity transport by rat Mrp3. Our results suggest that MRP3 is unlikely to be the principal basolateral bile-acid transporter of ileocytes and cholangiocytes, but that it may have a role in the removal of bile acids from the liver in cholestasis.

The expression level determines the surface distribution of the transferrin receptor in Trypanosoma brucei

The transferrin receptor (TfR) of Trypanosoma brucei is a heterodimer attached to the surface membrane by a glycosylphosphatidylinositol (GPI) anchor. The TfR is restricted to the flagellar pocket, a deep invagination of the plasma membrane. The membrane of the flagellar pocket and the rest of the cell surface are continuous, and the mechanism that selectively retains the TfR in the pocket is unknown. Here, we report that the TfR is retained in the flagellar pocket by a specific and saturable mechanism. In bloodstream-form trypanosomes transfected with the TfR genes, TfR molecules escaped flagellar pocket retention and accumulated on the entire surface, even at modest (threefold) overproduction levels. Similar surface accumulation was observed when the TfR levels were physiologically upregulated threefold when trypanosomes were starved for transferrin. These results suggest that the TfR flagellar pocket retention mechanism is easily saturated and that control of the expression level is critical to maintain the restricted surface distribution of the receptor.

Thiopurine metabolism and identification of the thiopurine metabolites transported by MRP4 and MRP5 overexpressed in human embryonic kidney cells

Mercaptopurines have been used as anticancer agents for more than 40 years, and most acute lymphoblastic leukemias are treated with 6-mercaptopurine (6MP) or 6-thioguanine (TG). Overexpression of the two related multidrug resistance proteins MRP4 and MRP5 has been shown to confer some resistance against mercaptopurines, which has been attributed to extrusion of mercaptopurine metabolites by these transporters. We have analyzed the mercaptopurine metabolites formed in human embryonic kidney cells and determined which metabolites are extruded by MRP4 and MRP5. Incubation with 6MP led to the formation of thioinosine and thioxanthosine metabolites and we found that thio-IMP was transported by both MRP4 and MRP5; MRP5 showed the highest transport rate. In contrast, only MRP5 transported thioxanthosine monophosphate (tXMP). During incubation with TG, the monophosphorylated form of thioguanosine was transported by both MRP4 and MRP5; the highest transport rate was for MRP4. Similarly, only 6-methyl-thio-IMP was formed during incubation with 6-methyl mercaptopurine riboside. This compound was a substrate for both MRP4 and MRP5; MRP4 showed the highest transport rate. Our results show that all major thiopurine monophosphates important in the efficacy of mercaptopurine treatment are transported by MRP4 and MRP5, although the substrate specificity of the two transporters differs in detail.

Mammalian ABC transporters in health and disease

The ATP-binding cassette (ABC) transporters are a family of large proteins in membranes and are able to transport a variety of compounds through membranes against steep concentration gradients at the cost of ATP hydrolysis. The available outline of the human genome contains 48 ABC genes; 16 of these have a known function and 14 are associated with a defined human disease. Major physiological functions of ABC transporters include the transport of lipids, bile salts, toxic compounds, and peptides for antigen presentation or other purposes. We review the functions of mammalian ABC transporters, emphasizing biochemical mechanisms and genetic defects. Our overview illustrates the importance of ABC transporters in human physiology, toxicology, pharmacology, and disease. We focus on three topics: (a) ABC transporters transporting drugs (xenotoxins) and drug conjugates. (b) Mammalian secretory epithelia using ABC transporters to excrete a large number of substances, sometimes against a steep concentration gradient. Several inborn errors in liver metabolism are due to mutations in one of the genes for these pumps; these are discussed. (c) A rapidly increasing number of ABC transporters are found to play a role in lipid transport. Defects in each of these transporters are involved in human inborn or acquired diseases.

Inhibition of the multidrug resistance protein 1 (MRP1) by peptidomimetic glutathione-conjugate analogs

Inhibition of multidrug resistance protein 1 (MRP1) mediated cytostatic drug efflux might be useful in the treatment of drug resistant tumors. Because the glutathione (GSH) conjugate of ethacrynic acid (EA), GS-EA, is a good substrate of MRP1, GS-EA derivatives are expected to be good inhibitors of MRP1. To study structure-activity relationships of MRP1 inhibition, a series of novel GS-EA analogs was synthesized in which peptide bonds of the GSH backbone were replaced by isosteric groups [Bioorg Med Chem 10:195-205, 2002]. Several of these compounds were effective inhibitors of MRP1-mediated [(3)H]GS-EA and [(3)H]E(2)17betaG transport, as studied in membrane vesicles prepared from MRP1-overproducing Sf9 cells. The modifications of the peptide backbone have distinct implications for recognition by MRP1: the gamma-glutamyl-cysteine peptide bond is important for binding, whereas the cysteinyl-glycine amide does not seem essential. When the gamma-glutamyl-cysteine peptide bond (C-CO-N) is replaced by a urethane isostere (O-CO-N), an effective competitive MRP1-inhibitor (K(i) = 11 microM) is obtained. After esterification of this compound to improve its cellular uptake, it inhibited MRP1-mediated efflux of calcein from 2008 ovarian carcinoma cells overexpressing MRP1. This compound also partially reversed the resistance of these cells to methotrexate. Because the urethane isostere is stable toward gamma-glutamyl transpeptidase-mediated breakdown, it is an interesting lead-compound for the development of in vivo active MRP1 inhibitors.

J-binding protein increases the level and retention of the unusual base J in trypanosome DNA

The nuclear DNA of Trypanosoma brucei and other kinetoplastid flagellates contains the unusual base beta-d-glucosyl-hydroxymethyluracil, called J, replacing part of the thymine in repetitive sequences. We have described a 100 kDa protein that specifically binds to J in duplex DNA. We have now disrupted the genes for this J-binding protein (JBP) in T. brucei. The disruption does not affect growth, gene expression or the stability of some repetitive DNA sequences. Unexpectedly, however, the JBP KO trypanosomes contain only about 5% of the wild-type level of J in their DNA. Excess J, randomly introduced into T. brucei DNA by growing the cells in the presence of the J precursor 5-hydroxymethyldeoxyuridine, is lost by simple dilution as the KO trypanosomes multiply, showing that JBP does not protect J against removal. In contrast, cells containing JBP lose excess J only sluggishly. We conclude that JBP is able to activate the thymine modification enzymes to introduce additional J in regions of DNA already containing a basal level of J. We propose that JBP is a novel DNA modification maintenance protein.

Expression of the human DNA glycosylase hSMUG1 in Trypanosoma brucei causes DNA damage and interferes with J biosynthesis

In kinetoplastid flagellates such as Trypanosoma brucei, a small percentage of the thymine residues in the nuclear DNA is replaced by the modified base beta-D-glucosyl-hydroxymethyluracil (J), mostly in repetitive sequences like the telomeric GGGTTA repeats. In addition, traces of 5-hydroxymethyluracil (HOMeUra) are present. Previous work has suggested that J is synthesised in two steps via HOMedU as an intermediate, but as J synthesising enzymes have not yet been identified, the biosynthetic pathway remains unclear. To test a model in which HOMeUra functions as a precursor of J, we introduced an inducible gene for the human DNA glycosylase hSMUG1 into bloodstream form T.brucei. In higher eukaryotes SMUG1 excises HOMeUra as part of the base excision repair system. We show that expression of the gene in T.brucei leads to massive DNA damage in J-modified sequences and results in cell cycle arrest and, eventually, death. hSMUG1 also reduces the J content of the trypanosome DNA. This work supports the idea that HOMeUra is a precursor of J, freely accessible to a DNA glycosylase.

Role of the N-terminal transmembrane region of the multidrug resistance protein MRP2 in routing to the apical membrane in MDCKII cells

In polarized cells, the multidrug resistance protein MRP2 is localized in the apical plasma membrane, whereas MRP1, another multidrug resistance protein (MRP) family member, is localized in the basolateral membrane. MRP1 and MRP2 are thought to contain an N-terminal region of five transmembrane segments (TMD(0)) coupled to 2 times six transmembrane segments via an intracellular loop (L(0)). We previously demonstrated for MRP1 that a mutant lacking TMD(0) but still containing L(0), called L(0)DeltaMRP1, was functional and routed to the lateral plasma membrane. To investigate the role of the TMD(0)L(0) region of MRP2 in routing to the apical membrane, we generated mutants similar to those made for MRP1. In contrast to L(0)DeltaMRP1, L(0)DeltaMRP2 was associated with an intracellular compartment, most likely endosomes. Co-expression with TMD(0), however, resulted in apical localization of L(0)DeltaMRP2 and transport activity. Uptake experiments with vesicles containing L(0)DeltaMRP2 demonstrated that the molecule is able to transport LTC(4). An MRP2 mutant without TMD(0)L(0), DeltaMRP2, was only core-glycosylated and localized intracellularly. Co-expression of DeltaMRP2 with TMD(0)L(0) resulted in an increased protein level of DeltaMRP2, full glycosylation of the protein, routing to the apical membrane, and transport activity. Our results suggest that the TMD(0) region is required for routing to or stable association with the apical membrane.

Site-specific interactions of JBP with base and sugar moieties in duplex J-DNA. Evidence for both major and minor groove contacts

Beta-D-Glucosyl-hydroxymethyluracil, also called base J, is an unusually modified DNA base conserved among Kinetoplastida. Base J is found predominantly in repetitive DNA and correlates with epigenetic silencing of telomeric variant surface glycoprotein genes. We have previously identified a J-binding protein (JBP) in Trypanosoma, Leishmania, and Crithidia, and we have shown that it is a structure-specific binding protein. Here we examine the molecular interactions that contribute to recognition of the glycosylated base in synthetic DNA substrates using modification interference, modification protection, DNA footprinting, and photocross-linking techniques. We find that the two primary requirements for J-DNA recognition include contacts at base J and a base immediately 5' of J (J-1). Methylation interference analysis indicates that the requirement of the base at position J-1 is due to a major groove contact independent of the sequence. DNA footprinting of the JBP.J-DNA complex with 1,10-phenanthroline-copper demonstrates that JBP contacts the minor groove at base J. Substitution of the thymine moiety of J with cytosine reduces the affinity for JBP approximately 15-fold. These data indicate that the sole sequence dependence for JBP binding may lie in the thymine moiety of base J and that recognition requires only two specific base contacts, base J and J-1, within both the major and minor groove of the J-DNA duplex.

The architecture of variant surface glycoprotein gene expression sites in Trypanosoma brucei

Trypanosoma brucei evades the immune system by switching between Variant Surface Glycoprotein (VSG) genes. The active VSG gene is transcribed in one of approximately 20 telomeric expression sites (ESs). It has been postulated that ES polymorphism plays a role in host adaptation. To gain more insight into ES architecture, we have determined the complete sequence of Bacterial Artificial Chromosomes (BACs) containing DNA from three ESs and their flanking regions. There was variation in the order and number of ES-associated genes (ESAGs). ESAGs 6 and 7, encoding transferrin receptor subunits, are the only ESAGs with functional copies in every ES that has been sequenced until now. A BAC clone containing the VO2 ES sequences comprised approximately half of a 330 kb 'intermediate' chromosome. The extensive similarity between this intermediate chromosome and the left telomere of T. brucei 927 chromosome I, suggests that this previously uncharacterised intermediate size class of chromosomes could have arisen from breakage of megabase chromosomes. Unexpected conservation of sequences, including pseudogenes, indicates that the multiple ESs could have arisen through a relatively recent amplification of a single ES.

Expression site activation in Trypanosoma brucei with three marked variant surface glycoprotein gene expression sites

The genes for the Variant Surface Glycoprotein (VSG) of Trypanosoma brucei are transcribed in telomeric expression sites (ESs). There are about 20 different ESs per trypanosome nucleus. Usually, only one is active at a time, but trypanosomes can switch the ES that is active at a low rate (<10(-5) per cell per generation). To study activation and silencing of ESs, we have generated a line of T. brucei 427 with three ESs marked with a different drug resistance gene. We show that a selection with any combination of two of these drugs leads to an unstable double-resistant phenotype in which the two ESs containing the corresponding marker genes switch backward and forward at a very high rate (>10(-1) per cell per generation). Unstable triple-resistant trypanosomes were not obtained. We conclude that the unstable rapid-switching state is a natural intermediate in ES switching. It only involves two ESs, whereas the other ESs are not expressed. Furthermore, we show that "inactive" ESs can exist at several different stable levels of activation. Whereas, a "silent" ES shows a low level of expression of promoter proximal sequences, the level of activation can be reversibly increased, leading to partially activated ESs.