Tobacco budworm dihydrofolate reductase is a promising target for insecticide discovery

Cloning, recombinant expression and inhibitor profiles of dihydrofolate reductase from the Australian sheep blow fly, Lucilia cuprina

While dihydrofolate reductase (DHFR) is an important drug target in mammals, bacteria and protozoa, no inhibitors of this enzyme have been developed as commercial insecticides. We therefore examined the potential of this enzyme as a drug target in an important ectoparasite of livestock, the Australian sheep blow fly, Lucilia cuprina (Diptera: Calliphoridae) (Wiedemann). The non-specific DHFR inhibitors aminopterin and methotrexate significantly inhibited the growth of L. cuprina larvae, with IC50 values at µg levels. Trimethoprim and pyrimethamine were 5-30-fold less active. Relative IC50 values for the inhibition of recombinant L. cuprina DHFR by various inhibitors were in accordance with their relative effects on larval growth. The active-site amino acid residues of L. cuprina DHFR differed by between 34% and 50% when compared with two mammalian species, as well as two bacteria and two protozoa. There were significant charge and size differences in specific residues between the blow fly and human DHFR enzymes, notably the L. cuprina Asn21, Lys31 and Lys63 residues. This study provides bioassay evidence to highlight the potential of blow fly DHFR as an insecticide target, and describes differences in active site residues between blow flies and other organisms which could be exploited in the design of blow fly control chemicals.

Antimicrobial Dihydrofolate Reductase Inhibitors - Achievements and Future Options: Review

Despite all progress made in the fight against infections caused by bacteria, fungi, protozoa or viruses, there is a need for more and new active agents. Intensive efforts are currently directed against many new and attractive targets, and are hoped to result in new useful agents. The opportunities offered by some known and validated targets are, however, by far not exhausted. Dihydrofolate reductase (DHFR, EC 1.5.1.3) attracted much attention over several decades, which yielded several useful agents. There are excellent chances for new drugs in this field, and they are thought to increase by limiting the spectrum of activity. Whereas trimethoprim seems to present the optimum which can be achieved for a broad spectrum antibacterial agent, specific agents could probably be designed for well defined groups or specific organisms, such as staphylococci among the bacteria, or for a number of parasites, such as Plasmodium falciparum, the fungus Pneumocystis carinii, and several protozoa, such as Trypanosoma, Toxoplasma, and others. This would even extend to herbicides or specific plant pathogens. Achievements and current efforts directed against new DHFR-inhibitors are reviewed, considering only the most recent literature.

Cloning, expression and enzymatic properties analysis of dihydrofolate reductase gene from the silkworm, Bombyx mori

Tetrahydrobiopterin (BH4) is an essential cofactor for aromatic acid hydroxylases, which control the levels of monoamine neurotransmitters. BH4 deficiency has been associated with many neuropsychological disorders. Dihydrofolate reductase (DHFR) can catalyze 7,8-dihydrobiopterin to 5,6,7,8-tetrahydrobiopterin (BH4) in the salvage pathway of BH4 synthesis from sepiapterin (SP), a major pigment component contained in the integument of silkworm Bombyx mori mutant lemon (lem) in high concentration. In this study, we report the cloning of DHFR gene from the silkworm B. mori (BmDhfr) and identification of enzymatic properties of BmDHFR. BmDhfr is located on scaffold Bm_199 with a predicted gene model BGIBMGA013340, which encodes a 185-aa polypeptide with a predicted molecular mass of about 21 kDa. Biochemical analyses showed that the recombinant BmDHFR protein exhibited high enzymatic activity and suitable parameters to substrate. Together with our previous studies on SP reductase of B. mori (BmSPR) and the lem mutant, it may be an effective way to industrially extract SP from the lem silkworms in large scale to produce BH4 in vitro by co-expressing BmSPR and BmDHFR and using the extracted SP as a substrate in the future.

Novel boron-containing, nonclassical antifolates: synthesis and preliminary biological and structural evaluation

Two boron-containing, ortho-icosahedral carborane lipophilic antifolates were synthesized, and the crystal structures of their ternary complexes with human dihydrofolate reductase (DHFR) and dihydronicotinamide adenine dinucleotide phosphate were determined. The compounds were screened for activity against DHFR from six sources (human, rat liver, Pneumocystis carinii, Toxoplasma gondii, Mycobacterium avium, and Lactobacillus casei) and showed good to modest activity against these enzymes. The compounds were also tested for antibacterial activity against L. casei, M. tuberculosis H37Ra, and three M. avium strains and for cytotoxic activity against seven different human tumor cell lines. Antibacterial and cytotoxic activity was modest, with one sample, the closo-carborane 4, showing about 10-fold greater activity. The less toxic nido-carborane 2 was also tested as a candidate for boron neutron capture therapy, but showed poor tumor retention and low selectivity ratios for boron distribution in tumor tissue versus normal tissue.

Drosophila dihydrofolate reductase mutations confer antifolate resistance to mammalian cells

Antifolates, such as methotrexate, are used to inhibit dihydrofolate reductase (DHFR), an enzyme essential for the biosynthesis of thymidylate, purines, and several amino acids. DHFR sequences corresponding to mutations found in a methotrexate resistant Drosophila S3 cell line (L30Q), a methotrexate resistant fly population (K31P, Q134K), as well as predicted in silico (L22R) were expressed in Chinese Hamster Ovary (CHO) cells. The L30Q and L22R DHFRs both conferred resistance to methotrexate. L22R DHFR provided approximately 200-fold resistance to methotrexate when compared to wild-type Drosophila DHFR allowing CHO(L22R) cells to divide in 10 microM methotrexate, a level of resistance not previously observed in any mammalian system. Constructs using this substitution in combination with other Drosophila DHFR specific residues would make excellent candidates for gene therapy and genetic markers in the treatment of certain human disorders.

Molecular cloning and expression of the dihydrofolate reductase (DHFR) gene from adult buffalo fly (Haematobia irritans exigua): effects of antifolates

The folate analogues methotrexate, aminopterin and pyrimethamine were toxic when fed in a blood meal to adult buffalo flies (Haematobia irritans exigua), but aminopterin caused greater mortality than methotrexate, while trimethoprim was not toxic to adult flies. This is the first recorded instance of mortality in adult insects caused by ingestion of folate analogues. In order to investigate the mechanism of this toxicity, the dihydrofolate reductase (DHFR) gene was cloned from adult buffalo fly cDNA using a PCR-based approach. The full-length DHFR coding sequence (BF-DHFR) was 887 bp and contained an open reading frame encoding a protein of 188 amino acids. The deduced protein sequence identities between BF-DHFR and the other known insect DHFR sequences were: Drosophila melanogaster, 75%; Aedes albopictus, 54%; Heliothis virescens, 43%. The BF-DHFR gene has a single 52 bp intron, an organization more similar to Dipteran species (Drosophila and Aedes). The cDNA encoding BF-DHFR was inserted into an Escherichia coli expression vector and the recombinant protein was expressed to levels representing about 25% of total cell protein. The active enzyme was purified by affinity chromatography on methotrexate-agarose and displayed a relatively low affinity (IC50 = 30 nm) for methotrexate.