A single amino acid substitution in Staphylococcus aureus dihydrofolate reductase determines trimethoprim resistance 1 1 Edited by T.Richmond
Glenn E. DaleClemens BrogerAllan D' ArcyPeter HartmanRonald DeHoogtSynèse JolidonI. KompišAlexander M. LabhardtHanno LangenHans H. LocherMalcolm G. P. PageDietrich StüberRudolf ThenBeat WipfChristian Oefner
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Dihydrofolate reductase
Ternary complex
Trimethoprim
Antifolate
Previous studies from this laboratory have shown that the addition of leucovorin to tumor cells dissociates methotrexate, but not methotrexate polyglutamates, from dihydrofolate reductase (L. H. Matherly, D. W. Fry, and I. D. Goldman, Cancer Res., 43: 2694-2699, 1983). To further assess the importance of these interactions to leucovorin rescue, antifolate growth inhibition toward L1210 cells in the presence of leucovorin was correlated with the metabolism of (6S)-5-formyl tetrahydrofolate to dihydrofolate as a measure of dihydrofolate reductase activity. Growth inhibition (greater than 95%) by methotrexate (5-10 microM) following its intracellular polyglutamylation during a 3-h preexposure, or by continuous treatment with high levels of the lipophilic antifolate, trimetrexate (1 microM), was only slightly diminished by 10 microM leucovorin (15-25%). High-pressure liquid chromatographic analyses of the derivatives formed from radiolabeled (6S)-5-formyl tetrahydrofolate under these conditions showed an incomplete conversion to dihydrofolate and metabolism to predominantly 10-formyl tetrahydrofolate. Neither of the antifolates interfered appreciably with the metabolism of the folate derivatives to polyglutamates. Growth inhibition in the presence of leucovorin correlated with the accumulation of dihydrofolate (1.5-2.2 nmol) from radiolabeled (6S)-5-formyl tetrahydrofolate, reflecting continued suppression of dihydrofolate reductase activity at these drug concentrations. With lower equitoxic levels of the trimetrexate (7.5 nM), the provision of leucovorin allowed for a restoration of cell growth to a level greater than 90% of control. Under these conditions, control levels of dihydrofolate (0.2 nmol) were formed from radiolabeled cofactor, consistent with sustained dihydrofolate reductase activity. These findings support a role for the activation of dihydrofolate reductase as an important component of the reversal of the effects of diaminoantifolates by leucovorin, presumably by a competitive displacement of drug from the enzyme. Since no displacement occurs in cells which have accumulated methotrexate polyglutamates, or in the presence of high levels of trimetrexate, it appears that the concentration of unbound drug within cells is a significant determinant of the extent of this competitive binding interaction. From these considerations, the high levels of methotrexate polyglutamates that accumulate in sensitive tumors relative to bone marrow and gastrointestinal cells would appear to represent an important factor for the selectivity of leucovorin rescue in vivo.
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Antifolates targeting dihydrofolate reductase (DHFR) are antimalarial compounds that have long been used for malaria treatment and chemoprevention (inhibition of infection from mosquitoes to humans). Despite their extensive applications, a thorough understanding of antifolate activity against hepatic malaria parasites, especially resistant parasites, has yet to be achieved.
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Dihydrofolate reductase (DHFR) catalyzes folic acid reduction and recycles dihydrofolate generated during dTMP biosynthesis to tetrahydrofolate. DHFR is the main target of methotrexate, the most widely used agent for antifolate therapy. Nevertheless, the emergence of methotrexate-resistance has greatly impeded the curative potential of this drug. Therefore, drugs with improved efficacy are still in demand, as well as an efficient in vitro assay system and animal model for antifolate drug discovery. The aim of this study is to evaluate the suitability of using zebrafish DHFR as an alternative assay system for antifolate drug discovery. The cDNAs encoding zebrafish and human DHFR were cloned, overexpressed, and purified. Similar structural and kinetic properties were revealed between zebrafish and human recombinant DHFRs. The susceptibilities of both enzymes to known DHFR inhibitors, including methotrexate and trimethoprim, and compounds with antifolate potential, such as polyphenols, are also comparable. In addition, the DHFR-mediated dihydrofolate reduction was significantly inhibited by its own substrate folic acid. An unexpected tissue-specific distribution of DHFR was observed with the highest level present in ova and brains of zebrafish. DHFR is also abundant in zebrafish embryos of early stages and decreased abruptly after 3 days postfertilization. The substantial resemblance between zebrafish and human DHFRs, as demonstrated in this study, provides compelling evidence supporting the use of zebrafish DHFR as an in vitro assay system for folate-related studies and drug discovery.
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The structural features and lipid solubility of four different classes of antifolate compounds were compared for their inhibition of dihydrofolate reductase (DHFR) and growth in a normal and methotrexate (MTX)-resistant 3T6 mouse cell line. All of the compounds have been shown previously to have antifolate activity. The resistant cell line has a 7-fold increase in DHFR activity with normal transport, but an altered affinity for MTX. All the antifolates were equally effective in inhibiting DHFR and growth in the parent cell line. Inhibition of partially purified DHFR from the resistant cells increased with changes in lipid solubility and structure of the compounds, compared to the parent DHFR. These data demonstrate that the resistant cells may be more sensitive to the structurally dissimilar antifolates than to MTX and lend importance to further development of this type of antifolate. These results suggest that these compunds may be useful in circumventing antifolate resistance due to alterations in target enzyme concentration and drug-enzyme affinity, as well as drug transport.
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Abstract Changes in the mechanisms of folate incorporation were studied in cells treated with low concentrations of methotrexate in order to evaluate their contribution to the development of resistance to antifolate drugs. The uptake of methotrexate via reduced‐folate system, the membrane‐associated high‐affinity folate binding capacity and the activity, levels and affinity for methotrexate of dihydrofolate reductase were measured in L5178 murine leukemic lymphoblasts and in a subline, MTX/R16, 16 times more resistant to methotrexate which was isolated after a short exposure to the antifolate. Various simultaneous changes were characterized in MTX/R16 cells which co‐participated in the development of resistance: a decreased affinity of the carrier for methotrexate uptake via the reduced‐folate system of entry, the increase of dihydrofolate reductase activity and levels and a two‐fold increased expression of a membrane‐associated high‐affinity folate‐binding protein (mFBP). The increase of the mFBP expression, besides ensuring the growth of resistant cells by its contribution to the reduced folate intake, also participates in the methotrexate resistance by the internalization of folate cofactor which would compete with methotrexate hindering the effective inhibition of dihydrofolate reductase by the antifolate.
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The therapy of Pneumocystis carinii (PC) pneumonia is often unsuccessful, particularly in patients with acquired immune deficiency syndrome (AIDS). Because of difficulties in growing the organism in vitro or obtaining purified organisms, current treatment choices have been made with little information on the metabolic effects of therapeutic agents on PC. This report quantitates the effects of the commonly used antifolates as well as the classic antineoplastic antifolate methotrexate and a lipid-soluble analogue, trimetrexate, on the target enzyme, dihydrofolate reductase (DHFR), in the PC organisms. Trimethoprim and pyrimethamine were found to be weak inhibitors (ID50 = 39,600 and 2,800 nM, respectively), while methotrexate and trimetrexate were potent reductase inhibitors (ID50 = 1.4 and 26.1 nM, respectively). transport studies with radiolabeled compounds showed that compounds with the classic folate structure (methotrexate and leucovorin) were not taken up by the intact PC organisms. In contrast, trimetrexate exhibited rapid uptake. These results suggest a major therapeutic advantage may be gained by combining a potent, readily transported PC DHFR inhibitor such as trimetrexate with the reduced folate leucovorin to achieve a highly potent antiprotozoan effect while preventing toxicity to mammalian cells.
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Dihydrofolate reductase (DHFR) is a potential drug target for Trypanosoma brucei, a human parasite, which is the causative agent for African sleeping sickness. No drug is available against this target, since none of the classical antifolates such as pyrimethamine (PYR), cycloguanil, or trimethoprim are effective as selective inhibitors of T. brucei DHFR (TbDHFR). In order to design effective drugs that target TbDHFR, co-crystal structures with bound antifolates were studied. On comparison with malarial Plasmodium falciparum DHFR (PfDHFR), the co-crystal structures of wild-type TbDHFR reveal greater structural similarities to a mutant PfDHFR causing antifolate resistance than the wild-type enzyme. TbDHFR imposes steric hindrance for rigid inhibitors like PYR around Thr86, which is equivalent to Ser108Asn of the malarial enzymes. In addition, a missing residue on TbDHFR active-site loop together with the presence of Ile51 widens its active site even further than the structural effect of Asn51Ile, which is observed in PfDHFR structures. The structural similarities are paralleled by the similarly poor affinities of the trypanosomal enzyme for rigid inhibitors. Mutations of TbDHFR at Thr86 resulted in 10-fold enhancement or 7-fold reduction in the rigid inhibitors affinities for Thr86Ser or Thr86Asn, respectively. The co-crystal structure of TbDHFR with a flexible antifolate WR99210 suggests that its greater affinity result from its ability to avoid such Thr86 clash and occupy the widened binding space similarly to what is observed in the PfDHFR structures. Natural resistance to antifolates of TbDHFR can therefore be explained, and potential antifolate chemotherapy of trypanosomiasis should be possible taking this into account.
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