Nobiletin and its colonic metabolites suppress colitis‐associated colon carcinogenesis by downregulating iNOS, inducing anti‐oxidative enzymes and arresting cell cycle progression

Abstract

Nobiletin (NOB) is a major citrus polymethoxyflavone with various beneficial biological activities. We reported previously that dietary NOB significantly inhibited colitis-associated colon carcinogenesis in azoxymethane (AOM)/dextran sulfate sodium (DSS)-treated mice, and the chemopreventive effects were associated with NOB metabolites found in the mouse colonic tissues. Specifically, we found that the total colonic level of the three major metabolites was about 20-fold higher than that of NOB in mice after long-term feeding of NOB. In this study, to better understand the role of colonic metabolites of NOB, we determined the anti-inflammation and anti-cancer effects of a mixture of NOB and its major metabolites (NOB-Met) at the concentrations equivalent to those found in colonic tissues of NOB-fed mice. The results demonstrated that NOB-Met effectively decreased the expression level of iNOS, increased the level of HO-1 and NQO1, and upregulated Nrf2 signaling pathway in lipopolysaccharide-stimulated macrophages. NOB-Met also caused a significant cell cycle arrest in human colon cancer cells. Validation study confirmed that dietary NOB led to the effects similar to those described above in the colon of AOM/DSS-treated mice. Specifically, dietary NOB significantly reduced the level of iNOS, up-regulated Nrf2-dependent enzymes, and profoundly modulated key signaling proteins resulting in decreased cell cycle progression in the colonic tissue of AOM/DSS-treated mice. Overall, our findings demonstrated that dietary NOB led to the presence of NOB and its metabolites in the colonic tissue, which suppressed colitis-associated colon carcinogenesis via downregulating iNOS, inducing anti-oxidative enzymes and arresting cell cycle progression. Support or Funding InformationThis work was partially supported by a NIH grant (CA139174), an AICR grant (10A044), and a USDA Special Grant on bioactive food components. (A) Effects of dietary NOB on the expression level of iNOS in the tumor of AOM/DSS-treated mice colon. (B) Effects of dietary NOB on the expression levels of HO-1, NQO1 and Nrf2 in whole cell lysate, and Nrf2 in nuclear/cytoplasmic fraction in the colonic mucosa of AOM/DSS-treated mice.(C) Effects of dietary NOB on the expression levels of cell cycle related signaling proteins in the colonic mucosa of AOM/DSS-treated mice.