Examining the mechanism of phase 2 enzyme induction by the chemopreventive agent, oltipraz, and its major metabolite, PPD

1563

Oltipraz [4-methyl-5-(2-pyrazinyl)-1, 2-dithiole-3-thione] is a member of a large class of compounds, dithiolethiones, known to possess chemopreventive properties. Currently, oltipraz is in Phase II chemoprevention clinical trials in Qidong, People’s Republic of China. Oltipraz possesses its cancer chemopreventive activity through its ability to induce the transcription of phase 2 detoxifying enzymes. It induces enzymes such as glutathione S-transferase, NAD(P)H: quinone reductase (NQO1), and epoxide hydrolase in vitro and in vivo. The precise mechanism by which oltipraz initiates transcription is unknown. It has been postulated that oltipraz or its metabolite(s) activates transcription through a mechanism involving reactive oxygen species (ROS), which themselves are capable of inducing gene expression. The experimental results to be presented will aid in determining the involvement of ROS in the mechanism of enzyme induction of oltipraz and an active metabolite of oltipraz, 6-mercapto-7-methyl-2H-pyrrolo[1,2-a]pyrazine-8-thione (PPD).¹ The potency of PPD to induce NQO1 activity is similar to that of oltipraz.¹ These results are promising, but do not determine the mechanism by which transcription of phase 2 enzyme genes are initiated. It has been suggested that the initiation of transcription involves the Keap1 protein, conceivably by direct interaction with oltipraz or its metabolite(s) and/or with the involvement of ROS. In vitro studies reveal that in the presence of thiol, metal ions, and molecular oxygen, oltipraz cleaves supercoiled plasmid DNA in a thiol dependent manner.² Since a function of glutathione (GSH) is to protect cells from ROS, experiments were designed to examine the effects of lowered GSH on NQO1 activity induced by oltipraz and PPD. Results reveal an enhanced induction of NQO1. Other experiments with added N-acetylcysteine (NAC), a known antioxidant, reveal a reverse effect on NQO1 activity in which NAC prevents enzyme induction. These and related experimental results to be presented will examine the basis of GSH and NAC effects on enzyme induction exerted by oltipraz and PPD. ¹ Petzer, J. P., Navamal, M., Johnson, J. K., Kwak, M.-K., Kensler, T. W., and Fishbein, J. C. (2003) Phase 2 Enzyme Induction by the Major Metabolite of Oltipraz. Chem. Res. Toxicol. 16(11), 1463-1469. ² Kim, Woongki, and Gates, Kent S. (1997) Evidence for Thiol-Dependent Production of Oxygen Radicals by 4-methyl-5-pyrazinyl-3H-1,2-dithiole-3-thione (Oltipraz) and 3H-1,2-dithiole-3-thione: Possible Relevance to the Anticarcinogenic Properties of 1,2-Dithiole-3-thione. Chem. Res. Toxicol. 10, 296-301.