Buetler T M, Bammler T K, Hayes J D, Eaton D L
University of Washington, Department of Environmental Health, Seattle, 98195, USA.
Cancer Res. 1996 May 15;56(10):2306-13.
Oltipraz (OPZ) is currently being considered for human use to protect against aflatoxin B1 (AFB)-induced hepatocarcinogenesis based on its proven protective effect in rats. The effectiveness of this treatment presumes that orthologous cytochrome P450 and glutathione S-transferase (GST) isozymes metabolize AFB in humans as they do in rats. In this study, alterations in the expression of multiple forms of cytochrome P450 and GST were evaluated after treatment with OPZ, as well as other known P450 inducers, including 3-methylcholanthrene, pregnenolone-16alpha-carbonitrile, and ciprofibrate. Evidence is presented that the male-specific rat CYP 3A2, an orthologue of human CYP 3A4, may be primarily responsible for AFB activation in rat liver at both high and low AFB substrate concentrations. The CYP 1A2 enzyme does not appear to play a role in AFB activation in rat liver at any substrate concentration, whereas the major human P450 enzyme capable of activating AFB at a low substrate concentration has been identified as CYP 1A2. Surprisingly, we found that the CYP 1A2 steady-state mRNA level and the CYP 1A2-associated methoxyresorufin-O-demethylase activity were induced approximately 3- and 2-fold, respectively, by OPZ in rat liver. However, because CYP 1A2 does not appear to participate in AFB activation, induction of CYP 1A2 may be insignificant for AFB-induced hepatocarcinogenesis in rat models. In the rat, a heterodimeric alpha class GST enzyme containing the Yc2 subunit is the only polypeptide characterized to date in this species with high catalytic activity for the conjugation of activated AFB with glutathione. The GST Yc2 steady-state mRNA level was induced 5-fold by OPZ treatment. This induction was mirrored by significant increases in both the corresponding protein level and AFB-8,9-epoxide-conjugating enzyme activity, which may contribute significantly to protection against AFB-induced carcinogenesis in the rat. Investigations from this and other laboratories have not revealed any evidence for a Yc2-like GST isozyme with high AFB-8,9-epoxide-conjugating activity in human liver. We have also been unable to demonstrate that the two major human alpha class GST isozymes, A1-1 and A2-2, purified from bacteria expressing the corresponding cDNAs, exhibit any significant AFB-8,9-epoxide-conjugating activity. Our results suggest that humans may not be protected to the same extent as rats against AFB-induced hepatocarcinogenesis by treatment with OPZ and that further investigations are needed to establish the usefulness of OPZ for protection against human exposure to AFB.
奥替普拉（OPZ）目前正被考虑用于人类，以预防黄曲霉毒素B1（AFB）诱导的肝癌发生，这是基于其在大鼠中已证实的保护作用。这种治疗方法的有效性假定直系同源细胞色素P450和谷胱甘肽S-转移酶（GST）同工酶在人类中代谢AFB的方式与在大鼠中相同。在本研究中，评估了用OPZ以及其他已知的P450诱导剂（包括3-甲基胆蒽、孕烯醇酮-16α-腈和环丙贝特）处理后多种形式的细胞色素P450和GST表达的变化。有证据表明，雄性特异性大鼠CYP 3A2（人类CYP 3A4的直系同源物）可能在高和低AFB底物浓度下主要负责大鼠肝脏中AFB的激活。在任何底物浓度下，CYP 1A2酶似乎都不参与大鼠肝脏中AFB的激活，而能够在低底物浓度下激活AFB的主要人类P450酶已被鉴定为CYP 1A2。令人惊讶的是，我们发现OPZ在大鼠肝脏中分别诱导CYP 1A2稳态mRNA水平和与CYP 1A2相关的甲氧基试卤灵-O-脱甲基酶活性约3倍和2倍。然而，由于CYP 1A2似乎不参与AFB的激活，CYP 1A2的诱导对于大鼠模型中AFB诱导的肝癌发生可能并不重要。在大鼠中，含有Yc2亚基的异二聚体α类GST酶是该物种迄今为止唯一具有高催化活性将活化的AFB与谷胱甘肽结合的多肽。OPZ处理使GST Yc2稳态mRNA水平诱导了5倍。相应蛋白质水平和AFB-8,9-环氧化物结合酶活性的显著增加反映了这种诱导，这可能对预防大鼠中AFB诱导的致癌作用有显著贡献。来自本实验室和其他实验室的研究尚未发现任何证据表明人类肝脏中存在具有高AFB-8,9-环氧化物结合活性的Yc2样GST同工酶。我们也未能证明从表达相应cDNA的细菌中纯化的两种主要人类α类GST同工酶A1-1和A2- 2表现出任何显著的AFB-8,9-环氧化物结合活性。我们的结果表明，用OPZ治疗时，人类可能无法像大鼠那样在相同程度上免受AFB诱导的肝癌发生的影响，需要进一步研究以确定OPZ在预防人类接触AFB方面的有用性。
