Manibusan Mary K, Odin Marc, Eastmond David A
Office of Pesticide Programs, U.S. Environmental Protection Agency, Washington, DC 20460, United States.
J Environ Sci Health C Environ Carcinog Ecotoxicol Rev. 2007 Jul-Sep;25(3):185-209. doi: 10.1080/10590500701569398.
Under the 2005 U.S. EPA Guidelines for Carcinogen Risk Assessment (1), evaluations of carcinogens rely on mode of action data to better inform dose response assessments. A reassessment of carbon tetrachloride, a model hepatotoxicant and carcinogen, provides an opportunity to incorporate into the assessment biologically relevant mode of action data on its carcinogenesis. Mechanistic studies provide evidence that metabolism of carbon tetrachloride via CYP2E1 to highly reactive free radical metabolites plays a critical role in the postulated mode of action. The primary metabolites, trichloromethyl and trichloromethyl peroxy free radicals, are highly reactive and are capable of covalently binding locally to cellular macromolecules, with preference for fatty acids from membrane phospholipids. The free radicals initiate lipid peroxidation by attacking polyunsaturated fatty acids in membranes, setting off a free radical chain reaction sequence. Lipid peroxidation is known to cause membrane disruption, resulting in the loss of membrane integrity and leakage of microsomal enzymes. By-products of lipid peroxidation include reactive aldehydes that can form protein and DNA adducts and may contribute to hepatotoxicity and carcinogenicity, respectively. Natural antioxidants, including glutathione, are capable of quenching the lipid peroxidation reaction. When glutathione and other antioxidants are depleted, however, opportunities for lipid peroxidation are enhanced. Weakened cellular membranes allow sufficient leakage of calcium into the cytosol to disrupt intracellular calcium homeostasis. High calcium levels in the cytosol activate calcium-dependent proteases and phospholipases that further increase the breakdown of the membranes. Similarly, the increase in intracellular calcium can activate endonucleases that can cause chromosomal damage and also contribute to cell death. Sustained cell regeneration and proliferation following cell death may increase the likelihood of unrepaired spontaneous, lipid peroxidation- or endonuclease-derived mutations that can lead to cancer. Based on this body of scientific evidence, doses that do not cause sustained cytotoxicity and regenerative cell proliferation would subsequently be protective of liver tumors if this is the primary mode of action. To fulfill the mode of action framework, additional research may be necessary to determine alternative mode(s) of action for liver tumors formed via carbon tetrachloride exposure.
根据2005年美国环境保护局（EPA）的致癌物风险评估指南（1），致癌物评估依赖于作用模式数据，以便更好地为剂量反应评估提供信息。对典型肝毒性致癌物四氯化碳的重新评估，为在评估中纳入其致癌作用的生物学相关作用模式数据提供了契机。机制研究表明，四氯化碳通过细胞色素P450 2E1（CYP2E1）代谢为高反应性自由基代谢物，在假定的作用模式中起关键作用。主要代谢物三氯甲基自由基和三氯甲基过氧自由基具有高反应性，能够与细胞大分子发生局部共价结合，尤其倾向于与膜磷脂中的脂肪酸结合。这些自由基通过攻击膜中的多不饱和脂肪酸引发脂质过氧化，引发自由基链式反应序列。已知脂质过氧化会导致膜破坏，导致膜完整性丧失和微粒体酶泄漏。脂质过氧化的副产物包括活性醛，它们可分别形成蛋白质和DNA加合物，并可能分别导致肝毒性和致癌性。包括谷胱甘肽在内的天然抗氧化剂能够淬灭脂质过氧化反应。然而，当谷胱甘肽和其他抗氧化剂耗尽时，脂质过氧化的机会就会增加。细胞膜受损会使足够的钙泄漏到细胞质中，从而破坏细胞内钙稳态。细胞质中高钙水平会激活钙依赖性蛋白酶和磷脂酶，进一步加剧膜的分解。同样，细胞内钙的增加会激活核酸内切酶，导致染色体损伤并导致细胞死亡。细胞死亡后持续的细胞再生和增殖可能会增加未修复的自发突变、脂质过氧化或核酸内切酶衍生突变导致癌症的可能性。基于这一科学证据，如果这是主要作用模式，那么不会引起持续细胞毒性和再生细胞增殖的剂量随后将对肝肿瘤具有保护作用。为了完善作用模式框架，可能需要进行更多研究，以确定通过四氯化碳暴露形成肝肿瘤的替代作用模式。
