Devereux T R, Anderson M W, Belinsky S A
Laboratory of Biochemical Risk Analysis, National Institute of Environmental Health Sciences, Research Triangle Park, North Carolina 27709.
Cancer Res. 1988 Aug 1;48(15):4215-21.
The molecular dosimetry for O6-methylguanine (O6MG) formation in DNA from rat lung and pulmonary cells was compared following treatment for 4 days with equimolar doses of 4-(N-methyl-N-nitrosamino)-1-(3-pyridyl)-1-butanone (NNK), a potent pulmonary carcinogen or nitrosodimethylamine (NDMA), a weak carcinogen in rat lung. The dose response for O6MG formation from NNK was biphasic; the O6MG to dose ratio, an index of alkylation efficiency, increased dramatically as the dose of carcinogen was decreased. In contrast, the dose-response curve for methylation by NDMA appeared opposite of that for NNK with alkylation efficiency increasing as a function of dose. These results suggested that high and low Km pathways exist for the activation of NNK, whereas only high Km pathways may be involved in NDMA activation. Furthermore, DNA methylation by NNK was cell selective with the highest levels in the Clara cell, whereas methylation by NDMA was not. DNA methylation in the Clara cell was 50-fold greater by NNK than by NDMA at equimolar doses (0.005 mmol/kg). Thus, differences in O6MG formation, specifically the presence of a high affinity pathway in the Clara cell for activation of NNK, may explain why following low dose exposure, NNK is a potent pulmonary carcinogen while NDMA is not. Different cytochrome P-450 isozymes also appear to be involved in the activation of NNK and NDMA. Inhibition of in vitro methylation (with calf thymus DNA and lung microsomes) by antibodies to cytochrome P-450 isozymes provided evidence that a homolog of rabbit cytochrome P-450(2) (cytochrome P-450b) may be important in the activation of NNK in rat lung, whereas cytochrome P-450(5) may activate NDMA. A 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-inducible cytochrome P-450 isozyme (P-450c) may also be involved in the activation of NNK but not NDMA. Treatment with TCDD increased both NNK activation by pulmonary microsomes and the formation of O6MG in Clara cells and type II cells incubated in vitro with NNK. alpha-Naphthoflavone (alpha-NF), a specific inhibitor of cytochrome P-450c reversed the increase in methylation by TCDD-induced microsomes but did not inhibit in vitro activation of NNK using microsomes from untreated rats. However, NNK mediated O6MG formation in Clara cells, but not in type II cells incubated with alpha-NF, was decreased by 21%. These data indicate that both cytochrome P-450b and P-450c are probably involved in the activation of NNK in Clara cells from untreated rats.(ABSTRACT TRUNCATED AT 400 WORDS)
将大鼠肺和肺细胞中的DNA用等摩尔剂量的4-（N-甲基-N-亚硝基氨基）-1-（3-吡啶基）-1-丁酮（NNK，一种强效肺致癌物）或亚硝基二甲胺（NDMA，大鼠肺中的一种弱致癌物）处理4天后，比较了O6-甲基鸟嘌呤（O6MG）形成的分子剂量学。NNK形成O6MG的剂量反应是双相的；O6MG与剂量的比率，即烷基化效率的指标，随着致癌物剂量的降低而急剧增加。相比之下，NDMA甲基化的剂量反应曲线与NNK的相反，烷基化效率随剂量增加。这些结果表明，NNK的激活存在高Km和低Km途径，而NDMA激活可能仅涉及高Km途径。此外，NNK导致的DNA甲基化具有细胞选择性，在克拉拉细胞中水平最高，而NDMA导致的甲基化则没有这种选择性。在等摩尔剂量（0.005 mmol/kg）下，NNK导致的克拉拉细胞中的DNA甲基化比NDMA高50倍。因此，O6MG形成的差异，特别是克拉拉细胞中存在用于激活NNK的高亲和力途径，可能解释了为什么在低剂量暴露后，NNK是一种强效肺致癌物而NDMA不是。不同的细胞色素P-450同工酶似乎也参与了NNK和NDMA的激活。用细胞色素P-450同工酶抗体抑制体外甲基化（用小牛胸腺DNA和肺微粒体）提供了证据，表明兔细胞色素P-450(2)（细胞色素P-450b）的同源物可能在大鼠肺中NNK的激活中起重要作用，而细胞色素P-450(5)可能激活NDMA。一种2,3,7,8-四氯二苯并-p-二恶英（TCDD）诱导的细胞色素P-450同工酶（P-450c）也可能参与NNK的激活，但不参与NDMA的激活。用TCDD处理增加了肺微粒体对NNK的激活以及在体外与NNK一起孵育的克拉拉细胞和II型细胞中O6MG的形成。α-萘黄酮（α-NF），一种细胞色素P-4(5)的特异性抑制剂，逆转了TCDD诱导的微粒体导致的甲基化增加，但不抑制使用未处理大鼠的微粒体对NNK的体外激活。然而，在与α-NF一起孵育的克拉拉细胞中，NNK介导的O6MG形成减少了21%，但在II型细胞中没有减少。这些数据表明，细胞色素P-450b和P-450c可能都参与了未处理大鼠克拉拉细胞中NNK的激活。（摘要截短至400字）
