promoting mechanism of lithocholic acid in colon carcinogenesis induced by monomethylating agents

1.研究了17种胆汁酸对α，β，γ，δ和ε的体外抑制作用。只有石炭酸(LCA)及其衍生物被LCA抑制。Pol-β的抑制方式对LCA的抑制最为敏感。Pol-β的抑制方式与模板-引物型非竞争性，与底物竞争性，KI值为10μ-M。在抑制DNA复制聚合酶方面，Lca对PolDNAα有轻度抑制作用，而对PolDNAδ无抑制作用。Lca对在紫外线诱导的核苷酸切除修复和ε复制中起作用的PolDNA有微弱的抑制作用。结论：1.Lca对线粒体γ复制聚合酶的抑制作用较弱。在人白血病细胞系(K562)中，为了观察胆汁酸对DNA单甲基化药物或紫外线照射所致细胞毒性的影响，在甲磺酸甲酯或紫外线照射后加入胆汁酸。LCA在体外对PolDNA的细胞毒作用和对PolDNA的抑制作用较弱，但对β的细胞毒作用略有增强。LCA可能通过抑制PolDNA的ε而抑制依赖于PolDNA的碱基切除修复。PolDNA是单甲基化药物损伤的修复途径。LCA对烷化剂诱导的结肠癌碱基切除修复的抑制作用可能与其促癌活性有关。

1. Seventeen kinds of bile acids were examined with respect to inhibition of enkaryotic DNA polymerase (pol) α, β, γ, δ, and ε in vitro. Only lithocolic acid (LCA) and its derivatives inhibited by LCA. The inhibition mode of pol β was the most sensitive to inhibition by LCA. The inhibition mode of pol β was non-competitive with DNA template-primer and was competitive with substrate with the Ki value of 10 μM. Chemical structures of C-7 and C-12 positions in the sterol skeleton were important for the inhibitory activity of LCA. With respect to inhibition of DNA replicative polymerases, LCA mildly inhibited pol α, while it did not inhibited pol δ. LCA weakly inhibited pol ε, that plays a role in the UV-induced nucleotide excision repair and DNA replication. LCA weakly inhibited pol γ, a mitchondrial DNA replicative polymerase.2. In a human leukemic cell line (K562), to examine the effect of bile acids on cytotoxicities caused by DNA monomethylating agents or UV irradiation, bile acids were added after the treatment with methyl methanesulfonate or UV irradiation. LCA mildly enhanced cytotoxic effect of methyl methanesulfonate without promoting cytotoxity to pol β and the weak inhibition to pol ε of LCA in vitro.LCA may suppress base-excision repair that is dependent on pol β, a repair pathway of DNA damages caused by monomethylating agents, via the pol β inhibition. The inhibition of base-excision repair by LCA could be correlated with its tumor promoting activity in colon cancer induced by alkylating agents.