Ge R, Wang W, Kramer P M, Yang S, Tao L, Pereira M A
Department of Pathology, Medical College of Ohio, 3055 Arlington Avenue, Toledo, Ohio 43614-5806, USA.
Toxicol Sci. 2001 Jul;62(1):28-35. doi: 10.1093/toxsci/62.1.28.
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.
Wy-14,643在小鼠肝脏中的致癌活性似乎是非基因毒性的，可能与DNA甲基化减少有关。Wy-14,643诱导DNA甲基化减少的机制被认为涉及细胞增殖增加，随后阻止新合成DNA的甲基化。为了研究这一机制，给雌性B6C3F1小鼠每日经口灌胃50 mg/kg Wy-14,643。在首次给药后2、5、8、24、26、29、32、36、48、72和96小时处死小鼠。一些小鼠在给予Wy-14,643后30分钟通过腹腔注射接受450 mg/kg蛋氨酸。c-myc基因的低甲基化在首次给予Wy-14,643后48小时首次出现。通过增殖细胞核抗原（PCNA）标记指数测定的细胞增殖在36小时开始增加，并在72小时达到峰值。Wy14,643不影响肝脏中S-腺苷甲硫氨酸（SAM）或S-腺苷同型半胱氨酸（SAH）的浓度。蛋氨酸预防并逆转了Wy-14,643诱导的c-myc基因低甲基化。然而，在蛋氨酸预防Wy-14,643诱导的低甲基化之前，SAM和SAH升高的水平恢复到对照水平。此外，蛋氨酸不能阻止Wy-14,643诱导的PCNA标记指数增加。给予Wy14,643后72和96小时，核DNA甲基转移酶（DNA MTase）的活性增加。当在体外添加到核提取物中时，Wy14,643也增加了DNA MTase的活性。结果与Wy-14,643通过一种机制降低c-myc基因甲基化一致，该机制包括增强细胞增殖，随后阻止新合成DNA的甲基化。然而，结果表明Wy-14,643不会通过降低SAM的可用性或DNA MTase的活性来阻止甲基化。
