Gene/Protein Disease Symptom Drug Enzyme Compound
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Query: EC:1.5.1.3 (dihydrofolate reductase)
 5,819 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

Fetal liver or bone marrow-derived T lymphocyte precursors undergo extensive, developmentally regulated proliferation in response to inductive signals from the thymic microenvironment. We have used neonatal mouse thymocytes size-separated by centrifugal elutriation to study the cell cycle stage-specific expression of several genes associated with cell proliferation. These include genes involved in the biosynthesis of deoxyribonucleotide precursors, such as dihydrofolate reductase (DHFR), thymidylate synthase (TS), and the M1 and M2 subunits of ribonucleotide reductase, as well as c-myc, a cellular oncogene of unknown function. Using nuclear run-on assays, we observed that the transcription rates for these genes, with the exception of TS, are essentially invariant not only throughout the cell cycle in proliferating cells, but also in noncycling (G0) cells. The TS gene showed a transient increase in transcription rate in cells which bordered between a proliferating and nonproliferating status. Studies of an elutriated T cell line, S49.1, yielded similar results, indicating that the process of immortalization has not affected the transcriptional regulation of these genes. Analysis of steady-state mRNA levels using an RNase protection assay demonstrated that the levels of DHFR and TS mRNA accumulate as thymocytes progress through the cell cycle. In contrast, only the M2 subunit of ribonucleotide reductase showed cyclic regulation. Finally, in contrast to cultured cell models, we observed an abrupt fivefold increase in the steady-state level of c-myc mRNA in the transition from G1 to S-phase. We conclude from these studies that the transcriptional regulation of specific genes necessary for cellular proliferation is a minor component of the developmental modulation of the thymocyte cell cycle.
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PMID:A cell cycle analysis of growth-related genes expressed during T lymphocyte maturation. 227 81

Friend murine erythroleukemia (F-MEL) cells were transfected with a plasmid bearing tandemly arranged mouse c-myc antisense and dihydrofolate reductase transcription units. Sixteen clones were isolated, each containing unrearranged c-myc sequences and expressing high levels of antisense transcripts. All antisense clones examined contained reduced amounts of cytoplasmic endogenous c-myc transcripts. The kinetics of reaccumulation of endogenous c-myc mRNA during a 24-h exposure to dimethyl sulfoxide (DMSO) were also retarded and the ultimate transcript levels attained were less than in control cells. Antisense clones grew as well as control F-MEL cells in medium containing 10% fetal calf serum but at only a half and a quarter of the control rates in media containing 5 and 2% serum, respectively. Antisense clones differentiated faster and to a greater degree than control cells following DMSO exposure. myc antisense transcript expression was increased by growing cells in methotrexate, which resulted in an enhanced response to DMSO. Fluorescence-activated cell sorter (FACS) analysis of cellular DNA content indicated that a greater fraction of antisense nuclei contained a G0/G1 2n DNA content following a 24-h exposure to DMSO. When density-arrested antisense clones were diluted into fresh medium to allow reentry into the cell cycle, they incorporated less [3H]thymidine than control cells. FACS analysis showed that this was because only a portion of the cell population was entering S phase. Whereas control cells did not increase in size following release from density arrested antisense cells contained a subpopulation which were initially smaller and which eventually attained the same size as control cells. Quiescent antisense cells thus comprise two populations, each arrested at a different point in G1. Dilutional replating allowed both populations to reenter the cell cycle. We propose a model which postulates that certain minimal myc levels are necessary for cells to traverse G1. Those with insufficient levels, due, for example, to antisense inhibition, are unable to completely traverse G1 during density arrest and synchronize at an earlier point than do control cells. This earlier point may be along the differentiation pathway and may account for the greater responsiveness of antisense cells to DMSO induction. This model postulates that F-MEL cells overexpressing myc fail to differentiate because myc levels are never sufficiently low enough to allow cells to enter the differentiation pathway.
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PMID:c-myc antisense transcripts accelerate differentiation and inhibit G1 progression in murine erythroleukemia cells. 246 42