Archives

  • 2018-07
  • 2018-10
  • 2018-11
  • 2019-04
  • 2019-05
  • 2019-06
  • 2019-07
  • 2019-08
  • 2019-09
  • 2019-10
  • 2019-11
  • 2019-12
  • 2020-01
  • 2020-02
  • 2020-03
  • 2020-04
  • 2020-05
  • 2020-06
  • 2020-07
  • 2020-08
  • 2020-09
  • 2020-10
  • 2020-11
  • 2020-12
  • 2021-01
  • 2021-02
  • 2021-03
  • 2021-04
  • 2021-05
  • 2021-06
  • 2021-07
  • 2021-08
  • 2021-09
  • 2021-10
  • 2021-11
  • 2021-12
  • 2022-01
  • 2022-02
  • 2022-03
  • 2022-04
  • 2022-05
  • 2022-06
  • 2022-07
  • 2022-08
  • 2022-09
  • 2022-10
  • 2022-11
  • 2022-12
  • 2023-01
  • 2023-02
  • 2023-03
  • 2023-04
  • 2023-05
  • 2023-06
  • 2023-08
  • 2023-09
  • 2023-10
  • 2023-11
  • 2023-12
  • 2024-01
  • 2024-02
  • 2024-03
  • MTX and MTXPGs block the

    2019-09-21

    MTX and MTXPGs block the activity of the key enzyme DHFR (Fig. 1), which converts folates to their active forms – dihydrofolate (DHF) and tetrahydrofolate (THF). MTXPGs also potently inhibit thymidylate synthase (TS). Furthermore, during dTMP synthesis, TS utilizes the cofactor 5,10-methylene THF, which serves as a donor of the ‐CH2OH group. As a result of this reaction, 5,10-methylene THF is oxidized to DHF, which cannot be reduced back to THF due to the inhibition of DHFR [21]. In addition, MTXPGs and DHF polyglutamates that are accumulated after DHFR inhibition exert an inhibitory effect on GAR transformylase (GART). MTXPGs further inhibit 5-aminoimidazole-4-carboxamide ribonucleotide (AICAR) transformylase (ATIC). The inhibition of ATIC promotes the accumulation of AICAR, a potent inhibitor of adenosine deaminase (ADA) [7]. Overall, it is well known that MTX interferes with purine and pyrimidine synthesis, which is required for DNA replication and cell proliferation [30]. The inhibition of DHFR and other enzymes by MTX results in the depletion of reduced forms of DHF and nucleotides, which strongly affects the proliferation of treated cell populations and also induces cell death [33], [35]. However, there are also many non-DHFR-mediated effects of MTX (Fig. 1), which are discussed below.
    Oxidative stress Although the cytotoxic effects of MTX are often induced by nucleotide depletion, non-DHFR-mediated effects of MTX are also important, as MTX can interfere with glyoxalase and antioxidant systems. It has been shown that MTX affects α-oxoaldehyde metabolism. The inhibition of glyoxalase I (Glo1) by MTX leads to the accumulation of methylglyoxal, a highly reactive α-oxoaldehyde, which causes glycation of biomolecules. This action contributes to the anticancer activity and toxicity of MTX [4]. Regarding oxidative stress, some reports show that MTX-induced anti-proliferation and pro-apoptotic effects depend on alterations of the intracellular reactive oxygen species (ROS) levels [13], [34]. Indirect evidence for MTX-induced actions through increased ROS production was demonstrated by studying the role of ornithine decarboxylase. The proposed mechanism of action is that MTX indirectly inhibits polyamine-producing enzymes. As a consequence, decreased polyamine production leads to increased intracellular ROS levels [51]. Furthermore, MTX is able to induce both apoptosis, through oxidative stress by reducing NO and increasing caspase-3 levels [8], and oxidative DNA damage, which can be lethal to tumor lb100 with defects in the MSH2 DNA mismatch repair gene [23].
    Cell differentiation It has also been described that MTX acts as a strong differentiation factor for immature and undifferentiated monocytic cells [39] and is able to induce differentiation in human keratinocytes [38]. Moreover, MTX has the ability to trigger cellular differentiation in tumor cells, including human and rat choriocarcinoma cells, HL-60 human promyelocytic cells and other human leukemia cell lines, LA-N-1 human neuroblastoma cells, HT29 colon cancer cells and A549 human lung adenocarcinoma cells [30]. Recently, it has been described that in human melanoma cells, MTX promotes differentiation and prevents invasion [37], whereas it also induces differentiation of human osteosarcoma cells [44]. These effects of MTX may be caused by the depletion of thymine deoxyribonucleotides [38] or by the depletion of purines [42]. Although a reduction of nucleotides seems to be a clear explanation of this effect, it is hard to distinguish the precise mechanisms by which induced differentiation is achieved. Non-DHFR-mediated mechanisms of MTX can also be involved in the differentiation process. For example, MTX can induce a decrease of S-adenosylmethionine (SAM) by multiple mechanisms, and reduction in SAM concentrations may contribute to the decrease in cell proliferation as well as to the induction of differentiation by MTX [42]. Interestingly, it has been found that SAM is a key regulator for maintaining undifferentiated pluripotent stem cells and regulates their differentiation [41]. An important feature of MTX is its ability to affect the expression of genes or proteins that are directly or indirectly involved in the differentiation process [15], [44]. To be more specific, some differentiation markers of epithelial cells, e.g., E-cadherin, involucrin, and filaggrin, are selectively induced by MTX in human squamous cell carcinoma cell lines [2]. MTX also promotes E-cadherin expression in the SW620 colorectal adenocarcinoma cell line and in the SK-MEL-28 melanoma cell line [15]. Another study showed that MTX upregulates the mRNA and protein expression of MITF (microphthalmia-associated transcription factor), which prevents the invasion of human melanoma cells. Importantly, through this effect of MTX, human melanoma cells are further sensitized to a tyrosinase-processed antifolate pro-drug 3-O-(3,4,5-trimethoxybenzoyl)-(‐)-epicatechin (TMECG), which inhibits DHFR; thus, an MTX and TMECG combination treatment effectively induces apoptosis in human melanoma cells [37]. In human osteosarcoma cell lines, MTX affects the expression of genes involved in all-trans retinoic acid (ATRA) metabolism and in the regulation of gene expression. The combined treatment of osteosarcoma cells with MTX and ATRA subsequently enhances matrix mineralization, which is considered a marker of osteogenic differentiation [44].