Williams G M, Iatropoulos M J, Wang C X, Jeffrey A M, Thompson S, Pittman B, Palasch M, Gebhardt R
American Health Foundation, Valhalla, New York 10595, USA.
Toxicol Sci. 1998 Oct;45(2):152-61. doi: 10.1006/toxs.1998.2514.
The dose responses for several effects of low-level limited exposures to 2-acetylaminofluorene (AAF) in the livers of male Fischer 344 rats were measured and a subsequent phenobarbital tumor promotion regimen was used to manifest initiation of carcinogenesis. Three doses over a 10-fold range yielding cumulative total exposures of 0.126, 0.42, and 1.26 mmol AAF/kg body weight were achieved by daily intragastric instillation for up to 12 weeks with interim terminations. This was followed by 24 weeks administration of 500 ppm phenobarbital (PB) in the diet to promote liver tumor development. At 12 weeks at the end of AAF administration, all exposures produced adducts in liver DNA, measured by 32P postlabeling, and the level of adducts increased with exposure, except that the high exposure did not produce a dose proportional increase. Measurement of arylsulfotransferase activity, a key enzyme in the metabolic activation of AAF, revealed that in livers from the high exposure animals, the enzyme was inhibited. To assess for toxicity, the centrilobular zone of glutamine synthetase-positive hepatocytes was quantified immunohistochemically at 12 weeks. The area of the zone was reduced in the high exposure group and there was a trend to reduction in relationship to exposure. The two lower exposures to AAF produced no increase in cell proliferation, whereas the high exposure resulted in a marked increase, about 8-fold over controls. Initiation was assessed by induction of hepatocellular altered foci (HAF) that expressed the placental form of glutathione S-transferase. AAF induced HAF in the high exposure group, 9-fold at 8 weeks and 170-fold at 12 weeks compared to controls. In rats maintained on PB for 24 weeks after exposure, the multiplicity of HAF increased in controls and comparably in the low and mid exposure groups, but remained at the about the same high level in the high exposure group. The high exposure produced a substantial incidence of benign neoplasms by 12 weeks, and with promotion by 36 weeks, all rats developed hepatocellular neoplasia. In the mid exposure group, only one adenoma occurred at 36 weeks in 17 rats, while in the low exposure group, no liver tumor occurred in 23 rats. Thus, these findings document nonlinearities for some of the effects of AAF, with supralinear effects at the high exposure for cell proliferation and induction of HAF, and a no-observed-effect level for induction of promotable liver neoplasms at the lowest cumulative exposure of 0.126 mmol/kg, in spite of the formation of DNA adducts. We conclude that the effects of this DNA-reactive hepatocarcinogen leading to initiation exhibit nonlinearities and possible thresholds.
测定了雄性Fischer 344大鼠肝脏中低水平有限接触2-乙酰氨基芴（AAF）的几种效应的剂量反应，并采用随后的苯巴比妥肿瘤促进方案来显现致癌作用的起始。通过每日胃内灌胃长达12周并进行中期终止，实现了在10倍范围内的三个剂量，累积总暴露量分别为0.126、0.42和1.26 mmol AAF/千克体重。随后在饮食中给予500 ppm苯巴比妥（PB）24周以促进肝肿瘤发展。在AAF给药12周结束时，通过32P后标记法测量，所有暴露组的肝脏DNA中均产生了加合物，加合物水平随暴露量增加而升高，但高暴露组未产生剂量成比例的增加。对AAF代谢活化中的关键酶芳基磺基转移酶活性的测量表明，高暴露动物肝脏中的该酶受到抑制。为评估毒性，在12周时通过免疫组织化学法定量谷氨酰胺合成酶阳性肝细胞的中央小叶区。高暴露组中该区域面积减小，且与暴露量呈减少趋势。较低的两个AAF暴露组细胞增殖未增加，而高暴露组导致细胞增殖显著增加，约为对照组的8倍。通过诱导表达谷胱甘肽S-转移酶胎盘形式的肝细胞改变灶（HAF）来评估起始作用。与对照组相比，高暴露组中AAF在8周时诱导HAF增加9倍，在12周时增加170倍。在暴露后给予PB 24周的大鼠中，对照组和低、中暴露组中HAF的数量均增加，但高暴露组仍保持在大致相同的高水平。高暴露组在12周时产生了大量良性肿瘤，经36周促进后，所有大鼠均发生肝细胞肿瘤。在中暴露组中，17只大鼠在36周时仅出现1例腺瘤，而在低暴露组中，23只大鼠未出现肝肿瘤。因此，这些发现证明了AAF某些效应的非线性，高暴露时细胞增殖和HAF诱导呈超线性效应，并且在最低累积暴露量0.126 mmol/kg时，尽管形成了DNA加合物，但诱导可促进性肝肿瘤的未观察到效应水平。我们得出结论，这种导致起始作用的DNA反应性肝癌致癌物的效应表现出非线性和可能的阈值。
