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Wy-14,643-induced hypomethylation of the c-myc gene in mouse liver
R Ge1, W Wang, P M Kramer
1Department of Pathology, Medical College of Ohio, 3055 Arlington Avenue, Toledo, Ohio 43614-5806, USA.
Abstract:
The carcinogenic activity of Wy-14,643 in mouse liver appears to be nongenotoxic and could involve a decrease in DNA methylation. The mechanism for Wy-14,643-induced decrease in DNA methylation is proposed to involve increased cell proliferation followed by prevention of the methylation of the newly synthesized DNA. To investigate this mechanism, female B6C3F1 mice were administered daily by oral gavage 50 mg/kg Wy-14,643. Mice were sacrificed at 2, 5, 8, 24, 26, 29, 32, 36, 48, 72, and 96 h after the first dose. Some mice also received 450 mg/kg methionine by ip injection at 30 min after administering Wy-14,643. Hypomethylation of the c-myc gene first occurred at 48 h after the first dose of Wy-14,643. Cell proliferation determined by the Proliferating Cell Nuclear Antigen (PCNA)-Labeling Index started to increase at 36 h and peaked at 72h. Wy14,643 did not affect the liver concentration of either S-adenosyl methionine (SAM) or S-adenosyl homocysteine (SAH). Methionine prevented and reversed the hypomethylation of the c-myc gene induced by Wy-14,643. However, the increased levels of SAM and SAH returned to control levels prior to the prevention by methionine of Wy-14,643-induced hypomethylation. Furthermore, methionine did not prevent Wy-14,643-induced increase in the PCNA-Labeling Index. The activity of nuclear DNA methyltransferase (DNA MTase) was increased at 72 and 96 h after administering Wy14,643. Wy14,643 also increased the activity of DNA MTase when added in vitro to nuclear extracts. The results are consistent with Wy-14,643 decreasing the methylation of the c-myc gene by a mechanism that includes enhancement of cell proliferation followed by prevention of the methylation of the newly synthesized DNA. However, the results indicate that Wy-14,643 does not prevent methylation by decreasing either the availability of SAM or the activity of DNA MTase.
Insights
Wy-14,643 causes liver cancer in mice by decreasing DNA methylation, linked to increased cell proliferation. Methionine counteracts this hypomethylation, but the exact mechanism requires further study.
Area of Science:
- Toxicology
- Molecular Biology
- Epigenetics
Background:
- Wy-14,643 is a peroxisome proliferator-activated receptor alpha (PPARα) agonist known to induce liver tumors in rodents.
- Its carcinogenic activity is considered nongenotoxic, suggesting epigenetic mechanisms like DNA methylation alterations are involved.
Purpose of the Study:
- To investigate the proposed mechanism of Wy-14,643-induced hypomethylation, involving increased cell proliferation and subsequent prevention of new DNA methylation.
- To determine the role of S-adenosyl methionine (SAM) and DNA methyltransferase (DNA MTase) activity in this process.
Main Methods:
- Female B6C3F1 mice were administered Wy-14,643 (50 mg/kg) orally, with sacrifice at various time points (2-96 h).
- Methionine (450 mg/kg) was administered intraperitoneally to assess its effect on hypomethylation and proliferation.
- DNA methylation of the c-myc gene, PCNA-Labeling Index, SAM and SAH levels, and DNA MTase activity were measured.
Main Results:
- Hypomethylation of the c-myc gene was observed at 48 h post-Wy-14,643 administration.
- PCNA-Labeling Index increased significantly starting at 36 h, peaking at 72 h, indicating enhanced cell proliferation.
- Methionine prevented and reversed c-myc hypomethylation but did not affect the proliferation increase or initial SAM/SAH levels. DNA MTase activity increased at later time points.
Conclusions:
- Wy-14,643 induces c-myc hypomethylation through a mechanism involving increased cell proliferation and impaired methylation of newly synthesized DNA.
- This effect is not mediated by decreased SAM availability or reduced DNA MTase activity.
- Methionine can reverse the hypomethylation, suggesting a complex interplay between proliferation and epigenetic regulation.