NO-aspirin modulates carcinogen activation and detoxification enzyme expression in HepG2 human hepatoma cells.

B61

Mounting evidence has suggested a role for non-steroidal anti-inflammatory drugs (NSAIDs) in cancer prevention. However, gastric, renal and cardiovascular toxicity associated with use of these agents greatly diminishes their potential in this regard. Nitric oxide (NO)-donating NSAIDs represent a promising new class of drugs developed to provide a safer alternative than their conventional NSAID counterparts in chemoprevention. While recent findings have begun to demonstrate important activities of these compounds, such as mucosal sparing and enhanced anti-inflammatory action, mechanistic studies of these compounds are largely unexplored. We tested the effects of NO-aspirin 2 (NCX 4040) on the activity and expression of Phase 1 and Phase 2 carcinogen-metabolizing enzymes: the cytochromes P450 (CYP) 1A1, 1A2 and 1B1; glutamate cysteine ligase (GCL); and glutathione-S-transferase (GST). In HepG2 human hepatoma cells, NO-aspirin 2 inhibited 2,3,7,8-tetrachlordibenzo-p-dioxin (TCDD)- and benzo[a]pyrene (BP)-induced CYP activity. Furthermore, in the absence of carcinogens, NO-aspirin 2 inhibited basal CYP enzyme activity. NO-aspirin 2 also had a direct effect on CYP enzyme activity as determined by using individual CYP microsomes. The inhibition of CYP activity was associated with a decrease in TCDD- and BP-induced CYP1A1, 1A2 and 1B1 mRNA expression. This effect was further characterized by demonstrating that NO-aspirin 2 inhibits carcinogen-induced transcription of CYP heterogeneous nuclear RNA expression and XRE-controlled luciferase activity. The fate of carcinogen metabolites depends not only upon activation by CYP enzymes, but also on conjugation/detoxification by Phase 2 enzymes. HepG2 cells treated with NO-aspirin 2 showed an increase in GST and GCL expression. We then compared the effects of NO-aspirin 2 to another NO-releasing compound, diethylenetriamine-NO (DETANO). The inhibitory effects of NO-aspirin 2 on TCDD-induced CYP activity and mRNA expression were much more potent than those of DETANO (IC₅₀ ~5 μM and ~125μM, respectively). These results demonstrate that NO-aspirin 2 may be an effective chemopreventive agent by favorably affecting the balance of Phase 1 and Phase 2 carcinogen metabolism.