Figure 1From: Old drugs, old problems: where do we stand in prediction of rheumatoid arthritis responsiveness to methotrexate and other synthetic DMARDs? Methotrexate mode of action. Methotrexate (MTX) is actively transported into the cell by the reduced folate carrier 1 (RFC1; also known as SLC19A1) and is then polyglutamated by folylpolyglutamate synthetase (FPGS) to form MTX polyglutamates (MTX PG), which are kept inside the cell [221] and are responsible for MTX anti-inflammatory intracellular actions [17, 174]. Glutamates can be removed by γ-glutamyl hydrolase (GGH) and MTX monoglutamate is rapidly effluxed from the cell via membrane transporters of the ATP-binding cassette (ABC) family [222], especially ABCC1-4 and ABCG2 [223, 224]. Inside the cell, MTX PG exert their anti-inflammatory actions through inhibition of essential enzymes of the folate pathway: dihydrofolate reductase (DHFR) [225], blocking the conversion of dihydrofolate (DHF) to tetrahydrofolate (THF) and ultimately leading to depletion of methionine and decreased DNA methylation; thymidylate synthase (TYMS) [226, 227], interfering with de novo pyrimidine synthesis; and 5-aminoimidazole-4-carbox-amide ribonucleotide (AICAR) transformylase (ATIC) [148, 228], an enzyme of the de novo purine synthesis, causing accumulation of AICAR, which will finally result in increased secretion of adenosine, a strong anti-inflammatory mediator [229, 230]. The enzyme 5,10-methylene-tetrahydrofolate reductase (MTHFR) is not directly inhibited by MTX, but is affected by it because of its action in the folate pathway [176]. ADA, adenosine deaminase; AMPd, adenosine monophosphate deaminase; dTMP, deoxythymidine monophosphate; dUMP, deoxyuridine monophosphate; FAICAR, 10-formyl 5-aminoimidazole-4-carboxamide ribonucleotide; IMP, inosine monophosphate; Methyl-THF, 5-methyl-tetrahydrofolate; Methylene-THF, 5,10-methylene-tetrahydrofolate; MS, methionine synthase; SHMT, serine hydroxymethil transferase.Back to article page