Abstract
A140
We have previously shown that the synthetic triterpenoid 1-[2-cyano-3,12-dioxooleana-1,9(11)-dien-28-oyl]imidazole or CDDO-Im, is an effective and extremely potent chemopreventive agent against aflatoxin-induced hepatic tumorigenesis in rats. This activity is in part due to induction of hepatic Keap1-Nrf2-regulated cytoprotective and detoxication genes. While CDDO-Im can act through Keap1-Nrf2 signaling, it is not known what other signaling pathways may be important in the chemopreventive efficacy of CDDO-Im. This study seeks to address this question by coupling global gene expression analysis at a low dose of CDDO-Im with analysis of a transgenic mouse model of Nrf2 hyperactivation. Conditional Keap1 knockout mice were compared to genetic control “wild-type” mice in order to identify the full set of genes which are modulated through constitutive Nrf2 hyperactivation created by deletion of Keap1 in the liver. Hepatic global gene expression in wild-type mice treated with CDDO-Im was then compared to vehicle controls in order to identify genes modulated by CDDO-Im irrespective of genotype influence. Wild-type mice were treated with CDDO-Im (30 µmol/kg body weight, p.o.), which is known to afford chemoprotection without eliciting toxicity. Hepatic global gene expression changes induced by CDDO-Im were then compared with genes modulated in the conditional Keap1 knockout mice to determine how much of the action of CDDO-Im is mediated through Keap1-Nrf2 signaling. This analysis showed that approximately 30% of CDDO-Im modulated genes are Keap1-dependent. The CDDO-Im modulated and Keap1-dependent genes were primarily involved in xenobiotic metabolism, glutathione metabolism, protein ubiquitination, and lipid metabolism. The remaining 70% of CDDO-Im modulated and Keap1-independent genes were involved in many functions; two of the largest categories include regulation of cell cycle and apoptosis. Of these, Transforming Growth Factor β (TGF-β) signaling pathways were significantly altered. Transcripts of bone morphogenic protein 6 (Bmp6) were increased 3.4-fold. In addition, transcripts of the genes encoding activin/inhibin beta subunits were significantly modulated (Inhbb, 2.4-fold; Inhba, -3.3-fold; and Inhbe, -2.0-fold). Furthermore, CDDO-Im treatment resulted in reduced expression of genes involved in suppression of TGF-β signaling. Previous in vitro studies have shown that CDDO-Im activates TGF-β signaling in leukemia cell lines. These studies confirm that activation of TGF-β signaling occurs in vivo and at doses relevant to chemoprevention. Additional studies are necessary to determine the functional consequences of these gene expression changes. Further analysis of the microarray data is ongoing to determine other molecular pathways that participate in the chemopreventive activity of CDDO-Im and to provide a clearer picture of interactions between them. Supported by CA94076, ES03819, and Reata Pharmaceuticals.
Sixth AACR International Conference on Frontiers in Cancer Prevention Research-- Dec 5-8, 2007; Philadelphia, PA