Molecular Mechanisms of Complex Mixture Toxicity

描述（由申请人提供）：环境中暴露于有毒或致癌化合物很少是由单一毒物的作用引起的。更常见的是，有毒物质是一种复杂的化学物质混合物，数量从几个（如许多职业暴露）到几千个（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的组合。其中，六价铬是一种致癌金属，通常与多环芳烃一起被发现是环境共污染物，苯并[a]芘（BaP）是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。该补助金资助的研究的长期目标是了解导致接触BaP和铬（VI）二元混合物对健康产生不良影响的机制。我们已经表明，高剂量的急性铬处理干扰生产性转录复合物的组装，通过交联HDAC1.DNMT1复合物的启动子染色质和抑制磷酸化，乙酰化和甲基化的表观遗传标记建立BaP诱导的组蛋白H3和H4中的基因反式激活。这些变化抑制了RNA聚合酶II向Cyp 1a 1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。类似地，持续暴露于低剂量的铬导致组蛋白标记和信号转导途径的逐渐变化，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CYP 1A 1，CYP 1B 1和CYP 1A 2细胞色素P450基因的诱导具有相反的作用，抑制肿瘤抑制基因的表达，并诱导促凋亡基因的表达，这使我们假设，环境相关的铬（VI）和BaP的二元混合物对BaP毒性和致癌性具有协同作用。根据我们的研究结果，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）评估饮用水中环境相关剂量的慢性暴露于铬如何影响BaP毒性和致癌性。基因表达的表观遗传修饰是暴露于铬（单独或与多环芳烃组合）的发育和致癌结果的关键因素。肿瘤抑制基因p16 ink 4a启动子的超甲基化已被发现在铬暴露的工人谁发展肺癌，虽然p16 ink 4a启动子甲基化仅占三分之一的癌症在这个人群中，这表明可能有额外的铬目标和替代机制的癌症发展。从这项研究中获得的知识将对这些流行病学调查结果的医学翻译产生重大影响，并通过确定有助于降低疾病发病率的分子靶标，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.