Inhibitory roles of NRF2 and an oleanolic triterpenoid on adipocyte differentiation and obesity

Nuclear factor (erythroid-derived 2)-like 2 (Nrf2) and the aryl hydrocarbon receptor (AhR) are transcription factors that control expression of a wide range of enzymes involved in xenobiotic metabolism. This study first demonstrates that Nrf2 regulates expression of AhR and subsequently modulates transcription of downstream cytochrome P450s Cyp1a1 and Cyp1b1 . Quantitative RT-PCR studies using Nrf2 -/- mouse embryonic fibroblasts (MEFs) demonstrated that Nrf2 genotype affects constitutive mRNA levels of AhR. 1-[2-cyano-3,12-dioxooleana-1,9(11)-dien-28-oyl]imidazole (CDDO-Im), a pharmacologic activator of Nrf2 signaling, upregulated mRNA expression of Cyp1a1 and Cyp1b1 in an Nrf2 genotype-dependant manner. Luciferase reporter assay and chromatin immunoprecipitation (ChIP) confirmed a direct binding of Nrf2 to one Antioxidant Response Element (ARE) found in the -230 bp region of the promoter of Ahr.;Recent studies have shown importance of the AhR signaling pathway in negative regulation of adipocyte differentiation. Our hypothesis that Nrf2 inhibits adipocyte differentiation through cross-talk with AhR has been tested in MEFs undergoing adipocyte differentiation. Indeed, adipocyte differentiation was markedly accelerated in Nrf2-/- MEFs, while it was delayed in Keap1-/- MEFs compared to their congenic wild-type cells. Nrf2-/- MEFs were rescued from differentiation by ectopic expression of Ahr, confirming that Nrf2 inhibits adipocyte differentiation through interaction with AhR.;Having confirmed the negative role of Nrf2 in adipogenesis in MEFs, effects of CDDO-Im treatment on obesity and related diseases were evaluated in mouse models. CDDO-Im effectively prevented increases in body weight gain in mice fed a high-fat, obesogenic diet but not a control diet. This effect was associated with a reduction of adipose mass and decreased lipid accumulation following CDDO-Im treatment in the mice fed a high-fat diet. Furthermore, CDDO-Im treatment improved glucose tolerance in high-fat-fed and streptozotocin-treated mice. Indirect calorimetry and quantitative RT-PCR confirmed that energy expenditure and fat oxidation were upregulated while expression and activity of fatty acid synthesis enzymes were downregulated following CDDO-Im treatment. Collectively, these results raise the possibility of the use of CDDO-Im for prevention of obesity, diabetes, and fatty liver.