Quinone reductase induction as a biomarker for cancer chemoprevention

Abstract

Chemoprevention involves the use of natural or synthetic substances to reduce the risk of developing cancer. Strategies for protecting cells from initiation events include decreasing metabolic enzymes responsible for generating reactive species (phase I enzymes) while increasing phase II enzymes that can deactivate radicals and electrophiles known to intercede in normal cellular processes. Reduction of electrophilic quinones by quinone reductase is an important detoxification pathway. Following evaluation of approximately 3000 plant and marine organism extracts, the number characterized as "active" was established in the range of 12% of the total, and over 60 active compounds have been isolated as quinone reductase inducers. One of them, isoliquiritigenin (1), isolated from tonka bean, was shown to be a monofunctional inducer by having similar quinone reductase inducing ability in wild-type Hepa 1c1c7 cells and two mutant cell lines. To further investigate the mechanism of induction, HepG2 human hepatoma cells stably transfected with ARE-luciferase plasmid were used. Isoliquiritigenin (1) significantly induced the luciferase activity in a dose-dependent manner. On the basis of these results, a full-term cancer chemoprevention study was conducted with 7,12-dimethylbenz[a]anthracene (DMBA)-treated female Sprague-Dawley rats. Dietary administration of 1 increased tumor latency. Based on these promising preliminary results, additional mechanistic studies are underway, as well as full-term carcinogenesis studies with chronic administration schedules.

Figure 1

Isoliquiritigenin (1) induces the luciferase activity in HepG2 cells stably transfected with an ARE-luciferase plasmid. Transfected cells were treated with 1.9–30 μM isoliquiritigenin (1), 10 μM 4′-bromoflavone (4′BF), 10 μM sulforaphane (S), or DMSO (0.5% final concentration) as control (C) and then analyzed for chemiluminescence using the luciferase assay system from Promega. Results are shown as a fold induction relative to the level observed in the control. Results are the means of three determinations ± SD. *Significantly different from control values, determined by Student’s t-test (p < 0.05).

Figure 2

Effect of dietary isoliquiritigenin (1) on percent incidence of observable mammary tumors (A), number of tumors (B), and body weight (C). Female Sprague-Dawley rats were given a single i.g. dose of 7,12-dimethylbenz(a)anthracene (DMBA) on day 0. The rat treatment groups were (○) DMBA in sesame oil; (■) DMBA and 2500 mg/kg diet of isoliquiritigenin; (▲) DMBA and 5000 mg/kg diet of isoliquiritigenin; and (×) 5000 mg/kg diet of isoliquiritigenin. Isoliquiritigenin was included in the diet during the period of 7 days prior to DMBA administration (−7) to 7 days post-DMBA administration (+7). During the remainder of the experimental period, unsupplemented diet was given to the animals (20 rats/group).