Mixed Inhibition of P450 3A4 as a Chemoprotective Mechanism against Aflatoxin B1-Induced Cytotoxicity with cis-Terpenones

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• 作者：Qibing Zhou ; Lin Zhang ; Miguel A. Zuniga ; Robert M. Tombes ; and Jennifer K. Stewart
• 刊名：Chemical Research in Toxicology
• 出版年：2008
• 出版时间：March 2008
• 年：2008
• 卷：21
• 期：3
• 页码：732-738
• 全文大小：711k
• 年卷期：v.21,no.3(March 2008)
• ISSN：1520-5010

文摘

We recently reported the protective effect of 2-hydroxy-cis-terpenone (HCT) against aflatoxin B₁ (AFB1)-induced cytotoxicity in human HepG2 liver cells (Zhou et al. Chem. Res. Toxicol. 2006, 19, 1415–1419); however, the mechanism was not clear. In this paper, the chemoprotective mechanism was investigated with liver microsomes and purified P450 3A4 enzyme. HCT showed effective inhibition of the metabolic conversion of AFB1 in liver microsomes at 40 µM, and more importantly, the inhibition of the carcinogenic exo-AFB1-epoxide formation from AFB1. Further study indicated the direct inhibition of purified P450 3A4 enzyme activity by HCT with an IC₅₀ value of 20 µM. Under aqueous conditions, HCT was slowly converted to an oxidized product OHCT, which exhibits similar inhibitory effects on both P450 3A4 and the metabolic conversion and carcinogenic activation of AFB1 with liver microsomes as those of HCT. Enzyme mechanism studies revealed that OHCT acted as a mixed inhibitor of P450 3A4 with K_i and K_i′ at 17.6 ± 5.6 and 7.6 ± 1.5 µM, respectively. Finally, OHCT showed no cytotoxicity at 60 µM in HepG2 liver cells and effective chemoprotection at 40 and 60 µM against AFB1 (2 µM) induced cytotoxicity. In contrast, ketoconazole alone exhibited 20% cell mortality at 20 µM, while chemoprotection with ketoconazole against 2 µM AFB1 in HepG2 was observed at 10 and 20 µM, which was much higher than the 1 µM concentration used in the inhibitory assays of P450 3A4 activity and AFB1 metabolism with liver microsomes.