A qualitative and quantitative cytochemical assay of dihydrofolate reductase in erythroid cells

Metabolic genes in cancer: their roles in tumor progression and clinical implications

Re-programming of metabolic pathways is a hallmark of physiological changes in cancer cells. The expression of certain genes that directly control the rate of key metabolic pathways including glycolysis, lipogenesis and nucleotide synthesis are drastically altered at different stages of tumor progression. These alterations are generally considered as an adaptation of tumor cells; however, they also contribute to the progression of tumor cells to become more aggressive phenotypes. This review summarizes the recent information about the mechanistic link of these genes to oncogenesis and their potential utility as diagnostic markers as well as for therapeutic targets. We particularly focus on three groups of genes; GLUT1, G6PD, TKTL1 and PGI/AMF in glycolytic pathway, ACLY, ACC1 and FAS in lipogenesis and RRM2, p53R2 and TYMS for nucleotide synthesis. All these genes are highly up-regulated in a variety of tumor cells in cancer patients, and they play active roles in tumor progression rather than expressing merely as a consequence of phenotypic change of the cancer cells. Molecular dissection of their orchestrated networks and understanding the exact mechanism of their expression will provide a window of opportunity to target these genes for specific cancer therapy. We also reviewed existing database of gene microarray to validate the utility of these genes for cancer diagnosis.

Role of antimetabolites of purine and pyrimidine nucleotide metabolism in tumor cell differentiation

Transformed cells are characterized by imbalances in metabolic routes. In particular, different key enzymes of nucleotide metabolism and DNA biosynthesis, such as CTP synthetase, thymidylate synthase, dihydrofolate reductase, IMP dehydrogenase, ribonucleotide reductase, DNA polymerase, and DNA methyltransferase, are markedly up-regulated in certain tumor cells. Together with the concomitant down-modulation of the purine and pyrimidine degradation enzymes, the increased anabolic propensity supports the excessive proliferation of transformed cells. However, many types of cancer cells have maintained the ability to differentiate terminally into mature, non-proliferating cells not only in response to physiological receptor ligands, such as retinoic acid, vitamin D metabolites, and cytokines, but also following exposure to a wide variety of non-physiological agents such as antimetabolites. Interestingly, induction of tumor cell differentiation is often associated with reversal of the transformation-related enzyme deregulations. An important class of differentiating compounds comprises the antimetabolites of purine and pyrimidine nucleotide metabolism and nucleic acid synthesis, the majority being structural analogs of natural nucleosides. The CTP synthetase inhibitors cyclopentenylcytosine and 3-deazauridine, the thymidylate synthase inhibitor 5-fluoro-2'-deoxyuridine, the dihydrofolate reductase inhibitor methotrexate, the IMP dehydrogenase inhibitors tiazofurin, ribavirin, 5-ethynyl-1-beta-D-ribofuranosylimidazole-4-carboxamide (EICAR) and mycophenolic acid, the ribonucleotide reductase inhibitors hydroxyurea and deferoxamine, and the DNA polymerase inhibitors ara-C, 9-(2-phosphonylmethoxyethyl)adenine (PMEA), and aphidicolin, as well as several nucleoside analogs perturbing the DNA methylation pattern, have been found to induce tumor cell differentiation through impairment of DNA synthesis and/or function. Thus, by selectively targeting those anabolic enzymes that contribute to the neoplastic behavior of cancer cells, the normal cellular differentiation program may be reactivated and the malignant phenotype suppressed.

Immunohistochemistry of dihydrofolate reductase in methotrexate-sensitive and -resistant human cell lines by flow cytometry: a comparison with the cytochemical tetrazolium salt method

Dihydrofolate reductase (DHFR, EC 1.5.1.3) is an enzyme involved in the metabolism of nucleic acids; it is also an important target for folate antagonists such as methotrexate (MTX). The distribution and expression of DHFR both in human HeLa BU-25 cell line and in methotrexate-resistant (MTX-R) variant, deriving from the human VA2-B cell line (having the DHFR gene amplified) was studied by tetrazolium salt method and by flow cytometric analysis. The immunohistochemical labelling of DHFR was achieved by using the streptavidinbiotinilated complex technique. DHFR activity was low in the human HeLa BU-25 cell line, while it was very high in the MTX-R cell line; the activity level increased with the increasing concentration of the MTX. The results obtained with cytochemical and immunohistochemical technique were compared. These findings showed that the hyperproduction of DHFR is strictly related with the cells having the DHFR gene amplified. Since MTX resistance is a common finding in the cells of patients with acute leukaemia, these studies may be extended to tumour-bearing patients at onset and following chemotherapy with methotrexate.