Regulation of hepatic detoxification enzymes by glucosinolate breakdown products in cruciferous vegetables

Numerous epidemiological studies show that cruciferous vegetables play a role in protection against cancer. These protective effects are thought to be associated with secondary metabolites, termed glucosinolates within the vegetables. Breakdown products of the glucosinolates are considered the active components responsible for upregulating hepatic detoxification enzymes that detoxify carcinogens or their reactive metabolites before they can damage DNA and initiate cancer. Monofunctional inducers, which upregulate phase II enzymes only, are believed to trigger a cellular signal that activates gene transcription of these enzymes through the antioxidant response element (ARE). In contrast, bifunctional inducers that upregulate both phase I and II enzymes via xenobiotic response element (XRE) for gene transcription.; At the start of my research, I looked to see if there was an additive or synergistic effect in upregulation of phase II detoxification enzymes by two glucosinolate breakdown products, crambene and indole-3-carbinol (I3C) in vivo. The result was a synergistic increase in quinone reductase (QR) and glutathione-S-transferase (GST) enzyme activities and their mRNA levels, suggesting that whole foods, containing combinations of active breakdown products, may be more effective than individual components. Next the molecular mechanism for the synergism was examined at the level of transcriptional regulation by using crambene and 13C/I3C acid condensation products separately and in combination examine their effects on QR expression using a CAT (Chloramphenicol Acetyl Iransferase) reporter assay system in vitro. In this study, crambene was shown to be a monofunctional inducer, upregulating QR transcription by activating the ARE. 13C/I3C acid condensation products known as bifunctional inducers, showed upregulation of QR gene expression through activation of both the ARE and the XRE. Therefore, we hypothesized that the molecular mechanism of synergism could result from co-activation of ARE and XRE by crambene and 13C/I3C acid condensation products.; Lastly, this study focused on whether the glucosinolate breakdown products could inhibit TPA-induced AP-1 binding activity in HepG2 human hepatoma cells in vitro using a reporter construct containing the TRE (TPA response element) consensus. Dose-dependent inhibition of TRE expression was observed with various concentrations of the active glucosinolate breakdown products including crambene, sulforaphane, BC, and phenethylisothiocyanante (PEITC) on TPA-induced AP-1 binding activity measured by electophoretic mobility shift assay.; In conclusion, there is a molecular interaction between crambene and I3C that explains the synergism in phase II enzyme induction after exposure to combinations of these two components. In addition to this, active glucosinolate breakdown products studied here also have anti-promotional effects through inhibition of AP-1 induced tumor promotion in vitro.