Molecular Mechanisms of Complex Mixture Toxicity

描述(由申请人提供)：环境中接触有毒或致癌化合物的情况很少是由单一毒物的作用引起的。更常见的情况是，有毒物质是一种复杂的化学物质混合物，数量从几个到数千个不等，如在许多职业接触中，如香烟烟雾。通常，这些混合物包括致癌金属和多环芳烃(PAH)的组合。其中，六价铬是一种致癌金属，经常被发现是多环芳烃的环境共同污染物，苯并[a]芘(BaP)是一种典型的多环芳烃，属于ATSDR/EPA优先考虑的前20种危险物质。这项由这笔赠款资助的研究的长期目标是了解暴露于BaP和六价铬(VI)二元混合物中对健康不利影响的机制。我们已经证明，高剂量的急性铬处理通过将HDAC1.DNMT1复合体与启动子染色质交联，并抑制由BaP诱导的组蛋白H3和H4中的基因反式激活而建立的磷酸化、乙酰化和甲基化表观遗传标记，从而干扰了生产性转录复合体的组装。这些变化抑制了RNA聚合酶II对Cyp1a1启动子的招募，阻止了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逐渐改变，从而累积影响基因表达和沉默。具体地说，环境相关剂量的铬(VI)对BaP介导的细胞色素P450基因的诱导具有相反的作用，抑制肿瘤抑制基因的表达，诱导促凋亡基因的表达，从而使我们假设环境相关的铬(VI)和BaP的二元混合物对BaP的毒性和致癌性具有协同作用。根据我们的发现，这笔赠款未来5年的目标是：(1)阐明低剂量、长期接触铬影响BaP诱导基因表达的表观遗传学机制；(2)评估长期接触饮用水中环境相关剂量的铬对BaP毒性和致癌性的影响。基因表达的表观遗传修饰是单独或与多环芳烃联合暴露于铬的发育和致癌后果的关键因素。在患肺癌的铬暴露工人中发现了肿瘤抑制基因p16INK4a基因启动子的高甲基化，尽管p16INK4a启动子甲基化仅占该人群癌症的三分之一，这表明可能存在额外的铬靶标和癌症发生的替代机制。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.