DNA REPAIR/REPLICATION IN CHEMICAL CARCINOGENESIS

本建议是我们整体研究计划的一部分。 对致癌分子机制的理解 一般的过程，特别是启动步骤。其基本原理是 详细的实验是从我们早先的观察中得出的，即(I) 细胞增殖伴随着致癌物损伤的DNA的复制， 在修复某些关键病变之前，是 启动；(Ii)体内致癌物损伤DNA的复制 重组事件和(Iii)这种复制的DNA被at去甲基化 至少20-40%。鉴于DNA甲基化在基因中的重要性 表达和分化，我们接受了这样的假设 这种低甲基化DNA的产生可能是 细胞增殖发挥了它的作用。因此，这些实验详细说明了 在本项目申请中使用5-氮杂胞苷和5-氮杂脱氧胞苷 已知的抑制甲基化的药物，并研究它们对 化学物质诱发肝癌的起始步骤。在第一个 我们将通过一系列实验来证实我们之前的观察 N-甲基-N-亚硝脲(MNU)诱导的肝DNA复制 在复制的杂交种中发生低甲基化，并试图将 MNU诱导的DNA甲基化程度。这个 第二个系列将决定(一)不同药物给药的效果 致癌物后5-氮胞苷和5-氮杂脱氧胞苷的剂量 引发现象；(Ii)这些类似物的作用是否 通过整合到DNA中而介导的以及(Iii)启动的 5-氮杂胞苷模型诱导的肝细胞向肝细胞分化 癌症。在这方面取得的初步成果令人鼓舞。 这一假说的一个吸引人的特点是它溶解了 致癌物诱导的相关损伤的化学多样性 强调它们的相似之处在于能够诱导小鼠体内的低甲基化 DNA此外，这一假说还可以通过实验进行验证。

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.