Mahler J F, Price H C, O'Connor R W, Wilson R F, Eldridge S R, Moorman M P, Morgan D L
National Institute of Environmental Health Sciences, Research Triangle Park, North Carolina 27709, USA.
Toxicol Sci. 1999 Mar;48(1):123-33. doi: 10.1093/toxsci/48.1.123.
Short-term inhalation exposure of B6C3F1 mice to styrene causes necrosis of centrilobular (CL) hepatocytes. However, in spite of continued exposure, the necrotic parenchyma is rapidly regenerated, indicating resistance by regenerated cells to styrene toxicity. These studies were conducted to test the hypothesis that resistance to repeated styrene exposure is due to sustained cell proliferation, with production of hepatocytes that have reduced metabolic capacity. Male mice were exposed to air or 500 ppm styrene (6 h/day); hepatotoxicity was evaluated by microscopic examination, serum liver enzyme levels, and bromodeoxyuridine (BrdU)-labeling index (LI). Metabolism was assessed by measurement of blood styrene and styrene oxide. Both single and repeated exposures to styrene resulted in mortality by Day 2; in mice that survived, there was CL necrosis with elevated BrdU LI at Day 6, and complete restoration of the necrotic parenchyma by Day 15. The BrdU LI in mice given a single exposure had returned to control levels by Day 15. Re-exposure of these mice on Day 15 resulted in additional mortality and hepatocellular necrosis, indicating that regenerated CL cells were again susceptible to the cytolethal effect of styrene following a 14-day recovery. However, in mice repeatedly exposed to styrene for 14 days, the BrdU LI remained significantly increased on Day 15, with preferential labeling of CL hepatocytes with enlarged nuclei (karyomegaly). If repeated exposures were followed by a 10-day recovery period, CL karyomegaly persisted, but the BrdU LI returned to control level and CL hepatocytes became susceptible again to styrene toxicity as demonstrated by additional mortality and acute necrosis after a challenge exposure. These findings indicated a requirement for continued styrene exposure and DNA synthesis in order to maintain this resistant phenotype. Analyses of proliferating-cell nuclear-antigen (PCNA) labeling were conducted to further characterize the cell cycle kinetics of these hepatocytes. The proportion of cells in S-phase was increased by repeated exposure. However, PCNA analysis also revealed an even larger increase in the G1 cell compartment with repeated exposures, without a concurrent increase in G2 phase or in mitotic cell numbers. These data indicate that resistance to styrene-induced necrosis under conditions of repeated exposure is not due to sustained cell turnover and production of new, metabolically inactive cells, but rather is due to some other, as yet unknown, protective phenotype of the regenerated cells.
将B6C3F1小鼠短期吸入苯乙烯会导致小叶中心（CL）肝细胞坏死。然而，尽管持续接触，坏死的实质组织会迅速再生，这表明再生细胞对苯乙烯毒性具有抗性。进行这些研究是为了检验以下假设：对反复接触苯乙烯的抗性是由于持续的细胞增殖，产生了代谢能力降低的肝细胞。雄性小鼠暴露于空气或500 ppm苯乙烯（每天6小时）；通过显微镜检查、血清肝酶水平和溴脱氧尿苷（BrdU）标记指数（LI）评估肝毒性。通过测量血液中的苯乙烯和环氧苯乙烯来评估代谢情况。单次和反复接触苯乙烯都会在第2天导致死亡；在存活的小鼠中，第6天出现CL坏死且BrdU LI升高，到第15天坏死的实质组织完全恢复。单次接触苯乙烯的小鼠的BrdU LI在第15天已恢复到对照水平。在第15天对这些小鼠再次接触会导致额外的死亡和肝细胞坏死，这表明再生的CL细胞在14天恢复后再次对苯乙烯的细胞致死作用敏感。然而，在反复接触苯乙烯14天的小鼠中，第15天BrdU LI仍显著升高，CL肝细胞有优先标记且细胞核增大（核肿大）。如果反复接触后有10天的恢复期，CL核肿大持续存在，但BrdU LI恢复到对照水平，并且如再次接触后出现额外死亡和急性坏死所表明的，CL肝细胞再次对苯乙烯毒性敏感。这些发现表明需要持续接触苯乙烯和进行DNA合成以维持这种抗性表型。进行增殖细胞核抗原（PCNA）标记分析以进一步表征这些肝细胞的细胞周期动力学。反复接触会使处于S期的细胞比例增加。然而，PCNA分析还显示反复接触后G1细胞区室有更大幅度的增加，而G2期或有丝分裂细胞数量没有同时增加。这些数据表明，在反复接触条件下对苯乙烯诱导的坏死的抗性不是由于持续的细胞更新和产生新的、代谢不活跃的细胞，而是由于再生细胞的某种其他尚未知晓的保护表型。
