Molecular mechanisms of resistance to antifolates, a review.

Abstract

Methotrexate (MTX) is a clinically important antifolate that has been used in combination with other chemotherapeutic agents in the treatment of malignancies including acute lymphocytic leukemia, osteosarcoma, carcinomas of the breast, head and neck, choriocarcinoma and non-Hodgkin's lymphoma. The primary target of MTX is the enzyme dihydrofolate reductase (DHFR) which catalyzes the reduction of folate and 7,8-dihydrofolate to 5,6,7,8-tetrahydrofolate. Understanding of MTX action has revealed how cells acquire resistance to this drug. The four known mechanisms of MTX resistance are a decrease in the uptake of the drug, a decrease in the retention of the drug due to defective polyglutamylation or an increase in polyglutamate breakdown, an increase in the enzyme activity and a decrease in the binding of MTX to DHFR. The molecular basis for some of these mechanisms has been elucidated in MTX resistant cell lines; in particular the occurrence of gene amplification resulting in increased DHFR and point mutations resulting in altered DHFR with reduced affinity for MTX. Cloning of the human folylpolyglutamate synthase gene and the reduced folate transport gene have been reported recently and should facilitate the identification of the molecular basis of these resistant phenotypes. DHFR protein has been shown to regulate its synthesis by exerting an inhibitory influence on its own translation. Addition of MTX relieves this inhibition thus providing a possible molecular explanation for the rapid rise in DHFR activity noted in some cells after MTX administration. Alterations in genes involved in regulating the cell cycle such as cyclin D1 and the retinoblastoma (Rb) gene have also been shown to influence cellular response to MTX. Overexpression of cyclin D1 in HT1080, a human fibrosarcoma cell line, results in decreased MTX sensitivity. The molecular basis of this observation is under investigation. Abnormalities in the Rb gene may also have profound effects on MTX sensitivity. Rb interacts with the family of transcription factors called E2F reducing transcription of genes that contain E2F binding sites in the promoter regions e.g. DHFR. When Rb is deleted or rendered nonfunctional levels of "free" or unbound E2F are high resulting in enhanced transcription of genes such as DHFR. This results in increased DHFR protein and may lead to MTX resistance. As the knowledge regarding mechanisms of resistance increases newer approaches to circumvent such resistance or to target resistant cells can be undertaken.

Elucidating feed‑forward apoptosis signatures in breast cancer datasets: Higher FOS expression associated with a better outcome

“…This feed forward process of apoptosis is exemplified by over-activation of the T-cell receptor signaling pathway in deletion of self-reactive T-cells in the thymus (3) and by treatment of cells with interferon-γ (4), which is pro-proliferative at lower concentrations and pro-apoptotic at higher concentrations. The importance of a specific pro-proliferative TF in apoptosis was elegantly demonstrated many years ago by unexpected tumor development in mice lacking E2F1 (5-7), a classic pro-proliferative TF that stimulates histone gene and dihydrofolate reductase gene expression (8,9), in preparation for S-phase. POU2F1 and RB1 have been shown to regulate the interferon-γ transition from stimulation of proliferation to stimulation of apoptosis (4,10).…”

Good response to methotrexate is associated with a decrease in the gene expression of the drug transporter ABCG2 in patients with rheumatoid arthritis

“…MTX is transported into target cells through a transporter, reduced folate carrier 1 (RFC-1) [15,16], and converted to polyglutamated MTX (MTX-PG) by folylpolyglutamyl synthase (FPGS) [17]. MTX-PG is very stable in the intracellular compartment, where it inhibits dihydrofolate reductase (DHFR) signi cantly more potently than non-polyglutamated MTX [18][19][20][21], resulting in an inhibition of DNA synthesis. MTX-PG is reconverted to MTX by γ-glutamyl hydrolase (GGH) [22,23], and is excluded through transporters such as the ATP-binding cassette sub-family C member 1 (ABCC1)/multidrug-resistance associated protein 1 (MRP-1) [24] and the ATP-binding cassette sub-family G member 2 (ABCG2)/ breast cancer resistance protein (BCRP) [25,26].…”

Genetic polymorphisms in cyclin D1 are associated with risk of renal cell cancer in the Chinese population

“…CCND1 encodes a subunit of the holoenzyme complex that phosphorylates the retinoblastoma protein (Rb) by CDK4 and CDK6. Phosphorylation of Rb releases transcription factors such as E2F to activate the transcription of genes, whose products are required to entry into S phase of the cell cycle [ 13 , 14 ]. In addition, the accumulating evidences discoveried that increased expression of CCND1 disrupted normal cell cycle process and possibly promoted the development of many malignant cancers, including RCC, which might be closely associated with metastases and the poor prognosis [ 15 – 17 ].…”