DNA REPAIR/REPLICATION IN CHEMICAL CARCINOGENESIS

目前的建议是我们整个研究计划的一部分， 对了解致癌的分子机制， 一般来说是工艺，特别是起始步骤。 的理由 详细的实验源自我们早期的观察：（i） 细胞增殖伴随致癌物损伤DNA的复制， 在修复某些关键病变之前， 启动;（ii）致癌物损伤的DNA在体内诱导复制 （iii）这种复制的DNA被至少一种重组事件低甲基化， 至少20- 40%。 鉴于DNA甲基化在基因表达中的重要性， 表达和分化，我们已经接受了假设， 这种低甲基化DNA的产生可能是 细胞增殖发挥其作用。 因此，详细的实验 在该项目中，要求使用5-氮杂胞苷和5-氮脱氧胞苷 已知抑制甲基化的药物，并研究它们对 化学物质诱发肝癌的起始步骤。 上 通过一系列实验，我们将证实我们先前的观察结果， N-甲基-N-亚硝基脲（MNU）处理的肝DNA的复制诱导 在复制的杂交低甲基化，并试图关联的程度， DNA甲基化与MNU诱导的起始程度有关。 的 第二个系列将确定（一）管理不同的效果 剂量的5-氮杂胞苷和5-氮杂脱氧胞苷后，致癌物对 引发现象;（ii）这些类似物的效果是否 通过掺入DNA介导，以及（iii）启动的 5-氮杂胞苷模型的肝细胞发育成肝细胞 carcinoma. 在这方面取得的初步成果令人鼓舞。 这个假设的一个吸引人的特点是，它消除了 致癌物诱导的相关病变的化学多样性， 强调它们在能够诱导低甲基化方面的相似性， DNA. 此外，这一假设还有待于实验验证。

The present proposal forms a part of our overall research program oriented towards understanding the molecular mechanisms involved in the carcinogenic process in general and initiation step in particular. The rationale for the experiments detailed is derived from our earlier observations that (i) cell proliferation with attendent replication of carcinogen-damaged DNA, before the repair of certain critical lesions, were key events for initiation; (ii) replication of carcinogen-damaged DNA in vivo induced recombination events and (iii) such replicated DNA is hypomethylated by at least 20-40%. In view of the importance of DNA methylation in gene expression and differentiation, we have entertained the hypothesis that creation of such undermethylated DNA may be one of the mechanisms by which cell proliferation exerts its effect. Accordingly the experiments detailed in this project request make use of 5 azacytidine and 5-azadeoxycytidine agents which are known to inhibit methylation and study their effect on the initiation step of liver carcinogenesis induced by chemicals. In the first series of experiments we will confirm our earlier observations that replication of N-methyl-N-nitrosourea (MNU)-treated liver DNA induces hypomethylation in the replicated hybrid and try to correlate the degree of methylation of the DNA with the extent of initiation induced by MNU. The second series will determine (i) the effect of administration of different doses of 5-azacytidine and 5-aza-deoxycitydine after the carcinogen on the initiation phenomenon; (ii) whether the effect of these analogues is mediated via incorporation into DNA and (iii) whether the initiated hepatocytes by the 5-azacytidine model develop into hepatocellular carcinoma. Preliminary results obtained in this direction are encouraging. An attractive feature of this hypothesis is that it dissolves out the chemical diversity of the carcinogen-induced relevant lesion while emphasizing their similarity in being able to induce hypomethylation in DNA. In addition, the hypothesis is amenable for experimental validation.