Search for novel agents for cancer chemoprevention continues. One approach to cancer chemoprevention is to inhibit initiation stage of carcinogenesis by blocking the oxidative DNA damage caused as a consequence of carcinogenic insult. Using bovine arotic pulmonary endothelial cells we recently showed that a novel cytoprotective antioxidant MCGA3 (3-O-caffeoyl-1-methylquinic acid), isolated from bamboo leaves (Phyllostachys edulis) induces heme oxygenase-1 (HO-1) and ferritin without any prooxidant potential, and that this effect plays an important role in blocking ROS-mediated cytotoxic response (Kweon et al. Free Radic. Biol. Med. 36: 40-52, 2004). To begin to have relevance of this novel finding to the human situation, this study was designed to elucidate the mechanism of HO-1 induction by MCGA3 in human umbilical vascular endothelial cells (HUVECs). There is increasing evidence that the Keap1-Nrf2 complex is a key molecular target of chemopreventive phase 2 enzyme inducers. We hypothesized that the antioxidant MCGA3 may activate Nrf2 transcription and induce other ARE-dependent genes because it contains an electrophilic unsaturated carbonyl and ortho-dihydroxyl groups, which can disrupt Keap1-Nrf2 complex. To achieve this, we examined mRNA transcription of the selected detoxifying genes using Northern blot analysis and the Nrf2 translocation in MCGA3-treated HUVECs. We found that MCGA3 treatment of HUVECs upregulates phase II detoxifying genes including γ-glutamyl cysteine lyase (γ-GCS), glutathine reductase (GR) and glutathione peroxidase (GPX). Moreover, the cytoprotective effect of MCGA3 was found to be directly associated with upregulated expression of γ-GCS, GR and GPX. To understand the underlying mechanism of the MCGA3-mediated ARE-dependent gene regulation, the nuclear level of Nrf2-Keap1 and intracellular level of GSH/GSSG were measured. MCGA3 significantly increased both nuclear Nrf2 and intracellular GSH. Interestingly, N-acetylcysteine (NAC) and glutathione (GSH) resulted in a significant decrease in the MCGA3-induced HO-1 transcription, Nrf2 translocation, and the cytoprotective activity, suggesting that GSH/GSSG redox regulation by MCGA3 is involved in the Nrf2 translocation. The nuclear Nrf2 translocation by MCGA3 was found to be decreased by JNK inhibitor, SP600125, but inhibitors of MAPK and PI3K pathways were not inhibitory. MCGA3, unlike NAC and GSH, also lowered the level of GSSG at the initial stage (2 h), indicating the increase of reductive GSH/GSSG equilibrium. These findings suggest that MCGA3 upregulates various detoxifying genes through the Nrf2 transcriptional activation and the MCGA3-mediated GSH/GSSG redox regulation is crucial in the protection of the cells from oxidative stress-induced necrosis and carcinogenesis.

[Proc Amer Assoc Cancer Res, Volume 46, 2005]