Molecular Mechanisms of Complex Mixture Toxicity

描述（由申请人提供）：有毒或致癌化合物的环境暴露很少是由单一毒物的作用引起的。更常见的情况是，有毒物质是多种化学实体的复杂混合物，其数量从少数几种（如许多职业接触）到数千种（如香烟烟雾）不等。通常，这些混合物包括致癌金属和多环芳烃（PAH）的组合。其中，六价铬（一种致癌金属，通常与多环芳烃一起被发现为环境共污染物）和苯并[a]芘（BaP）（一种典型的多环芳烃）是ATSDR/EPA优先清单中的前20种有害物质。该基金资助的研究的长期目标是了解暴露于BaP和铬（VI）二元混合物对健康的不良影响的机制。我们已经证明，高剂量急性铬处理通过交联HDAC1干扰了生产转录复合物的组装。DNMT1复合物作用于启动子染色质，抑制bap诱导的组蛋白H3和H4基因转激活所建立的磷酸化、乙酰化和甲基化表观遗传标记。这些变化抑制了RNA聚合酶II向Cyp1a1启动子的募集，阻断了bap诱导的基因表达，并刺激了BPDE-DNA加合物的形成。同样，持续暴露于低剂量铬会导致组蛋白标记和信号转导途径的逐渐改变，从而累积影响基因表达和沉默。具体而言，环境相关剂量的铬（VI）对BaP介导的CYP1A1、CYP1B1和CYP1A2细胞色素P450基因的诱导作用相反，抑制肿瘤抑制基因的表达，诱导促凋亡基因的表达，这使得我们假设环境相关的铬（VI）和BaP的双重混合物对BaP的毒性和致癌性具有协同作用。基于我们的研究结果，该基金未来5年的目标是：(1)阐明低剂量、长期暴露于铬影响bap诱导基因表达的表观遗传机制；(2)评估长期暴露于环境相关的饮用水铬剂量对BaP毒性和致癌性的影响。基因表达的表观遗传修饰是暴露于铬（单独或与多环芳烃联合）的发育和致癌结果的关键因素。尽管p16ink4a启动子甲基化仅占该人群癌症的三分之一，但在患肺癌的铬暴露工人中发现了肿瘤抑制基因p16ink4a启动子的超甲基化，这表明可能存在其他铬靶点和癌症发展的其他机制。从这项研究中获得的知识将对这些流行病学发现的医学翻译产生重大影响，并通过确定有助于减少疾病发病率的分子靶点，将大大有助于制定治疗和预防措施。

DESCRIPTION (provided by applicant): Environmental exposures to toxic or carcinogenic compounds rarely result from the action of single toxicants. More often, the toxic agent is a complex mixture of chemical entities in numbers ranging from a few, such as in many occupational exposures, to several thousand, as in cigarette smoke. Frequently, these mixtures include a combination of carcinogenic metals and polycyclic aromatic hydrocarbons (PAH). Of these, hexavalent chromium, a carcinogenic metal often found as an environmental co-contaminant with PAHs, and benzo[a]pyrene (BaP), a prototypical PAH, are among the top 20 hazardous substances in the ATSDR/EPA priority list. The long-range goal of the research funded by this grant has been to develop an understanding of the mechanisms responsible for the adverse health effects of exposure to binary mixtures of BaP and chromium(VI). We have shown that high-dose acute chromium treatment interferes with the assembly of productive transcriptional complexes by cross-linking HDAC1.DNMT1 complexes to promoter chromatin and inhibiting phosphorylation, acetylation and methylation epigenetic marks established by BaP-induced gene transactivation in histones H3 and H4. These changes inhibit recruitment of RNA polymerase II to the Cyp1a1 promoter, block BaP-inducible gene expression and stimulate the formation of BPDE-DNA adducts. Similarly, sustained exposure to low doses of chromium leads to gradual changes in histone marks and signal transduction pathways that cumulatively affect gene expression and silencing. Specifically, environmentally relevant doses of chromium(VI) have opposite effects on the BaP-mediated induction of CYP1A1, CYP1B1 and CYP1A2 cytochrome P450 genes, repress tumor suppressor gene expression and induce expression of pro-apoptotic genes, leading us to the hypothesis that environmentally relevant binary mixtures of chromium(VI) and BaP have a synergistic effect on BaP toxicity and carcinogenicity. Based on our findings, the objectives for the next 5 years of this grant are, (1) to elucidate the epigenetic mechanisms by which low-dose, long-term exposure to chromium affects BaP-inducible gene expression; (2) to evaluate how chronic exposure to environmentally relevant chromium doses in drinking water affects BaP toxicity and carcinogenicity. Epigenetic modification of gene expression is a key element of the developmental and carcinogenic outcomes of exposure to chromium, alone or in combination with PAHs. Hypermethylation of the promoter of the tumor suppressor p16ink4a gene has been found in chromium-exposed workers who developed lung cancer, although p16ink4a promoter methylation accounts for only one third of cancer in this population, suggesting that there may be additional chromium targets and alternative mechanisms underlying cancer development. The knowledge derived from this research will have a major impact on the medical translation of these epidemiological findings and, by identifying molecular targets useful to reduce disease incidence, will significantly contribute to the development of therapeutic and preventative measures. PUBLIC HEALTH RELEVANCE: Environmental exposure to hexavalent chromium in drinking water and cigarette smoking and occupational exposure in the workplace are often compounded with concomitant exposures to aromatic hydrocarbon procarcinogens, resulting in health problems including lung, stomach and intestinal tract tumors. The objective of this grant proposal is to evaluate how mixtures of hexavalent chromium and aromatic hydrocarbons cooperate to cause greater toxicity and carcinogenicity than the sum of the toxicities and carcinogenicities of the two compounds. The knowledge derived from this research will identify molecular targets to reduce disease incidence and will significantly contribute to the development of therapeutic and preventative measures with major impact on the treatment of the diseases caused by these agents.