Gonzales A J, Christensen J G, Preston R J, Goldsworthy T L, Tlsty T D, Fox T R
Curriculum in Toxicology, University of North Carolina, Chapel Hill 27709, USA.
Carcinogenesis. 1998 Jul;19(7):1173-83. doi: 10.1093/carcin/19.7.1173.
Non-genotoxic chemical carcinogens are capable of inducing tumors in rodents without interacting with or directly altering the genetic material. Since a preponderance of evidence suggests that cancer results from the accumulation of genetic alterations, the mechanisms by which many non-genotoxic carcinogens induce genotoxic events remain unclear. The present study investigated whether the mitogenic, non-genotoxic carcinogen phenobarbital (PB) could alter cell-cycle checkpoint controls, thereby indirectly leading to the accumulation of genetic damage. Initial studies involved characterizing cell-cycle checkpoint responses to DNA damage in freshly isolated B6C3F1 mouse hepatocytes. These cells responded to bleomycin-induced DNA damage by arresting in G1 and G2. Cell-cycle arrest was coupled with p53 protein induction; however, p21WAF1 protein levels remained unchanged. Studies that utilized hepatocytes isolated from C57BL p53-/- mice showed that the DNA damage-induced G1 cell-cycle arrest was dependent on p53 function, but cell-cycle arrest in G2 was not affected by loss of p53. PB was able to delay and attenuate the G1 checkpoint response without altering G2 checkpoint function. A reduction in p53 protein, but not transcript levels, was observed in hepatocytes exposed to PB. Additionally, PB delayed and attenuated p53 protein induction during DNA damage, which suggests that changes in the p53 protein may be contributing to the attenuated G1 checkpoint response caused by PB. Altered G1 checkpoint function represents an epigenetic mechanism by which phenobarbital may prevent the detection and repair of DNA damage and indirectly increase the frequency of genotoxic events above that occurring spontaneously. Abrogation of checkpoint controls may, thus, play an important mechanistic role in mitogenic, non-genotoxic chemical carcinogenesis.
非遗传毒性化学致癌物能够在不与遗传物质相互作用或直接改变遗传物质的情况下，在啮齿动物中诱发肿瘤。由于大量证据表明癌症是由遗传改变的积累引起的，许多非遗传毒性致癌物诱发遗传毒性事件的机制仍不清楚。本研究调查了促有丝分裂的非遗传毒性致癌物苯巴比妥（PB）是否会改变细胞周期检查点控制，从而间接导致遗传损伤的积累。初步研究包括表征新鲜分离的B6C3F1小鼠肝细胞对DNA损伤的细胞周期检查点反应。这些细胞通过在G1期和G2期停滞来响应博来霉素诱导的DNA损伤。细胞周期停滞与p53蛋白诱导相关；然而，p21WAF1蛋白水平保持不变。利用从C57BL p53-/-小鼠分离的肝细胞进行的研究表明，DNA损伤诱导的G1期细胞周期停滞依赖于p53功能，但G2期的细胞周期停滞不受p53缺失的影响。PB能够延迟和减弱G1检查点反应，而不改变G2检查点功能。在暴露于PB的肝细胞中观察到p53蛋白减少，但转录水平未改变。此外，PB在DNA损伤期间延迟并减弱了p53蛋白诱导，这表明p53蛋白的变化可能导致了PB引起的G1检查点反应减弱。改变的G1检查点功能代表了一种表观遗传机制，通过该机制苯巴比妥可能会阻止DNA损伤的检测和修复，并间接增加遗传毒性事件的发生频率，使其高于自发发生的频率。因此，检查点控制的废除可能在促有丝分裂的非遗传毒性化学致癌作用中发挥重要的机制作用。
