The form of folate affects the mechanisms of methotrexate resistance in Enterococcus faecium

Abstract

Several mechanisms have been described to explain the resistance of cells to methotrexate (MTX); however, the basis for the heterogeneity of mechanisms has been obscure. It was hypothesized that the type of MTX resistance in a single species can be influenced by the form of extracellular folate supplied during the development of resistance. Two strains of MTX-resistant Enterococcus faecium were developed by transferring the bacteria to media containing increasing concentrations of MTX in the presence of constant concentrations of either 5-formyl-5,6,7,8-tetrahydropteroylglutamic acid (5-HCO-H,PteGlu) or pteroylglutamic acid (PteGlu). These resistant strains were designated E. faecium/MTX/5-HCO-H4PteG I u and E. faeciumlMTXIPteG I u, respectively. The mechanisms of MTX resistance included : (1) increased folic acid reductase (FAR) activity in both resistant strains but increased dihydrofolate reductase (DHFR) activity only in E. faeciumlMTXIPteGlu; (2) decreased synthesis and intracellular retention of MTX containing two glutamyl residues; (3) decreased uptake of MTX accompanied by decreased uptake of folates; and (4) reduction of folate-binding capacity. Among these, the form of folate present in the media during the development of resistance affected DHFR and FAR activities and the transport of folates. These findings, together with data from other laboratories, suggest that it may be important t o use a reduced form of folate, a more physiological form than oxidized PteGlu, in the media during the development of resistance for the study of the mechanisms of MTX resistance in cultured cells.

Comparison of Serum and Red Blood Cell Folate Microbiologic Assays for National Population Surveys

“…There have been at least 2 known changes that could contribute to these effects: the properties of the microorganism could have been slightly changed in the process of rendering it antibiotic resistant and subculturing the wild-type microorganism in L2 over the last 40 y could have changed its properties. Tamura et al (18) have shown that the form of folate used during the development of methotrexate resistance affects the response of the assay organism to folates. A similar phenomenon could have occurred with the chloramphenicol-resistant microorganism, making it respond slightly better to reduced folates.…”

Higher Plant Plastids and Cyanobacteria Have Folate Carriers Related to Those of Trypanosomatids

“…1B). Although pABA can cross membranes by simple diffusion (18), folate and pteridine fluxes are likely to be carrier-mediated, as in animals (19 -22) and bacteria (23)(24)(25). Plants, therefore, presumably have several as yet undiscovered organellar folate or pteridine transporters (Fig.…”

Folate overproduction inLactobacillus plantarumWCFS1 causes methotrexate resistance

“…Prolonged exposure of target cells to folate antagonists such as methotrexate and trimethoprim might result in resistance. In cancer cells, several mechanisms were described such as: reduction of the folylpolyglutamate synthetase activity, amplification of the gene coding for DHFR or a decrease in the binding affinity for methotrexate on the DHFR (Tamura et al, 1997). However, to date, folate overproduction has not been considered or described as a mode of resistance against methotrexate.…”