Crambene, a bioactive nitrile derived from glucosinolate hydrolysis, acts via the antioxidant response element to upregulate quinone reductase alone or synergistically with indole-3-carbinol

Abstract

Epidemiological studies show that cruciferous vegetables play a role in dietary protection against cancers. The protective effects of crucifers are thought to be associated with secondary metabolites termed glucosinolates, the hydrolysis products of which upregulate hepatic detoxification enzymes. Crambene, a nitrile product of the glucosinolate progoitrin, increases hepatic quinone reductase (QR) when included in the diet of animals. Here we evaluate the mechanism of upregulation of detoxification enzymes by crambene. The regulatory region of the QR gene contains two response elements, the antioxidant response element (ARE) and the xenobiotic response element (XRE), that respond to glucosinolate hydrolysis products. We compared upregulation of QR mRNA expression by crambene in wild-type and Ah receptor-deficient mouse hepatoma cell lines. Both cell lines showed a similar increase in QR mRNA, suggesting that the Ah receptor-dependent XRE pathway is not required for crambene to act. Transient transfection of HepG2 cells with reporter constructs containing portions of the 5′ regulatory region of the rat QR gene confirmed this, revealing that crambene significantly activated ARE, but not XRE, in a dose-dependent manner. Furthermore, both indole-3-carbinol (I3C) and I3C acid condensates (I3C-A) activated the ARE for QR gene expression whereas only I3C-A activated the XRE at the concentrations studied. In addition, co-treatment with crambene and I3C-A caused synergistic increases in QR transcriptional activity and mRNA levels in HepG2 cells. Based on these findings, we propose that synergistic upregulation of QR is due to co-activation of the ARE and the XRE by crambene and I3C-A.