Rat mammary cancer (spontaneous, radiation, MNU, PhIP, radiation + MNU, radiation + PhIP) (high corn oil diet)

Although various mechanisms have been inferred for combinatorial actions of multiple carcinogens, these mechanisms have not been well demonstrated in experimental carcinogenesis models. We evaluated mammary carcinogenesis initiated by combined exposure to various doses of radiation and chemical carcinogens. Female rats at 7 weeks of age were ¿-irradiated (0.2–2 Gy) and/or exposed to 1-methyl-1-nitrosourea (20 or 40 mg/kg, single intraperitoneal injection) or 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (40 mg/kg/day by gavage for 10 days) and were observed until 50 weeks of age. The incidence of mammary carcinoma increased steadily as a function of radiation dose in the absence of chemicals; mathematical analysis supported an additive increase when radiation was combined with a chemical carcinogen, irrespective of the chemical species and its dose. Hras mutations were characteristic of carcinomas that developed after chemical carcinogen treatments and were overrepresented in carcinomas induced by the combination of radiation and MNU (but not PhIP), indicating an interaction of radiation and MNU at the level of initiation. The expression profiles of seven classifier genes, previously shown to distinguish two classes of rat mammary carcinomas, categorized almost all examined carcinomas that developed after individual or combined treatments with radiation (1 Gy) and chemicals as belonging to a single class; more comprehensive screening using microarrays and a separate test sample set failed to identify differences in gene expression profiles among these carcinomas. These results suggest that a complex, multilevel interaction underlies the combinatorial action of radiation and chemical carcinogens in the experimental model.尽管学界已针对多种致癌物的联合作用机制提出诸多推断，但此类机制尚未在实验性致癌模型中得到充分验证。本研究评估了不同剂量辐射与化学致癌物联合暴露诱导的乳腺致癌作用。将7周龄雌性大鼠接受γ射线照射（剂量范围0.2–2 Gy），并/或单独暴露于1-甲基-1-亚硝基脲（1-methyl-1-nitrosourea, MNU，单次腹腔注射20或40 mg/kg），或2-氨基-1-甲基-6-苯基咪唑[4,5-b]吡啶（2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine, PhIP，以40 mg/kg/天的剂量灌胃连续10天），随后持续观察至50周龄。在未接触化学致癌物的对照组中，乳腺腺癌的发生率随辐射剂量升高呈稳步上升趋势；数学分析显示，当辐射与化学致癌物联合暴露时，致癌发生率呈相加性升高，且该趋势不受化学致癌物种类及其给药剂量的影响。Hras基因突变是化学致癌物诱导的乳腺腺癌的特征性分子改变，且在辐射与MNU联合诱导的腺癌中显著富集（但在PhIP诱导的腺癌中未观察到此现象），表明辐射与MNU在致癌启动阶段存在相互作用。此前已有研究证实，7个分类基因（classifier genes）的表达谱可区分两类大鼠乳腺腺癌；本研究中，经辐射（1 Gy）与化学物单独或联合暴露诱导的所有检测腺癌样本，几乎均可被该7个分类基因的表达谱归为同一类别；通过微阵列（microarrays）开展的更全面转录组筛选及独立样本集测试，均未在上述腺癌中发现基因表达谱的显著差异。上述结果提示，在本实验模型中，辐射与化学致癌物的联合致癌作用依赖于复杂的多层面相互作用。