Karlsruhe Institute of Technology, Campus North, Institute of Toxicology and Genetics, Hermann-von-Helmholtz-Platz 1, 76344, Eggenstein-Leopoldshafen, Germany.
Arch Toxicol. 2012 Feb;86(2):329-37. doi: 10.1007/s00204-011-0757-3. Epub 2011 Oct 9.
Polycyclic aromatic hydrocarbons are ubiquitous environmental pollutants formed during incomplete combustion of organic material. For example benzo[a]pyrene (B[a]P) is a constituent and contaminant of cigarette smoke, automobile exhaust, industrial waste and even food products. B[a]P is carcinogenic to rodents and humans. B[a]P induces its own metabolism, which generates different metabolites such as the highly reactive electrophilic genotoxin and ultimal carcinogen B[a]P-7,8-dihydrodiol-9,10-epoxide (BPDE). BPDE can bind to nucleophilic macromolecules such as proteins and DNA and causes mutations. Multiple defence mechanisms have evolved to protect the cell from DNA damage. Specific signalling pathways operate to detect and repair different kinds of lesions. In case, the damage is poorly removed expansion of damaged cells can be counteracted, e.g., by the inhibition of proliferation or triggering apoptosis. Examples of damage sensors and transducers are stress-activated protein kinases (SAPKs) and the tumour suppressor protein p53. Here, we studied the role of p53 and the pro-apoptotic protein BAX in BPDE-induced cell death by using wild-type- or knock-out-human colon carcinoma cells. As reported previously, we could reconfirm a critical role of p53 in BPDE-induced apoptosis. Furthermore, induced levels of total p53 and its transcriptional target p21 declined at higher BPDE concentrations correlating with reduced rates of apoptosis. Interestingly, increased phosphorylation of p53 at serine 15 remained elevated at higher BPDE concentrations thus disconnecting p53 phosphorylation from downstream apoptosis. Hence, phosphorylation of p53 seems not only to be a more sensitive biomarker of BPDE exposure but might serve other functions unrelated to apoptosis. In addition, we identify BAX as a novel and essential factor to trigger the intrinsic pathway of apoptosis in response to BPDE. Furthermore, BPDE in parallel activates the SAPKs p38 and JNK, which are as well involved in apoptosis. Although several routes of mutual regulation of p53 and SAPK have been described, we present evidence that the SAPK pathway in response to genotoxic stress can unexpectedly operate independently of p53 and controls apoptosis by a novel mechanism possibly downstream of caspases.
多环芳烃是有机物质不完全燃烧过程中形成的普遍存在的环境污染物。例如，苯并[a]芘（B[a]P）是香烟烟雾、汽车尾气、工业废物甚至食品中的一种成分和污染物。B[a]P 对啮齿动物和人类具有致癌性。B[a]P 诱导其自身代谢，生成不同的代谢物，如具有高反应性的亲电遗传毒素和最终致癌的 B[a]P-7,8-二氢二醇-9,10-环氧化物（BPDE）。BPDE 可以与亲核大分子如蛋白质和 DNA 结合，并导致突变。细胞已经进化出多种防御机制来保护自身免受 DNA 损伤。特定的信号通路可以检测和修复不同类型的损伤。如果损伤不能很好地去除，受损细胞的扩增可以得到抑制，例如通过抑制增殖或触发细胞凋亡。损伤传感器和转导子的例子是应激激活蛋白激酶（SAPK）和肿瘤抑制蛋白 p53。在这里，我们使用野生型或敲除人结肠癌细胞研究了 p53 和促凋亡蛋白 BAX 在 BPDE 诱导的细胞死亡中的作用。如前所述，我们可以再次确认 p53 在 BPDE 诱导的细胞凋亡中起着关键作用。此外，在较高的 BPDE 浓度下，总 p53 及其转录靶标 p21 的诱导水平下降，与细胞凋亡率降低相关。有趣的是，p53 丝氨酸 15 的磷酸化水平在较高的 BPDE 浓度下仍然升高，从而使 p53 磷酸化与下游凋亡脱钩。因此，p53 的磷酸化似乎不仅是 BPDE 暴露的更敏感的生物标志物，而且可能具有与凋亡无关的其他功能。此外，我们确定 BAX 是触发 BPDE 反应的内在凋亡途径的新的和必需的因素。此外，BPDE 同时激活 SAPK p38 和 JNK，它们也参与凋亡。尽管已经描述了 p53 和 SAPK 之间相互调节的几种途径，但我们提供的证据表明，针对遗传毒性应激的 SAPK 途径可以出人意料地独立于 p53 运作，并通过可能下游于半胱天冬酶的新机制控制凋亡。
