Cytoprotective Roles of Oat Avenathramides Against Inflammation and Cellular Stress

Abstract

Natural polyphenols have been considered as promising anti-aging compounds not only for their antioxidant activity, many of them also function as signaling mediators that modulate cellular pathways involved in cytoprotection. Avenanthramides (Avns), a group of polyphenols found exclusively in oats, are natural antioxidants associated with human health promotion. In this study, avenanthramide A, B and C (Avn A, B and C), the three most abundant Avns in oats, were identified and quantified from oat phenolic-rich extracts of ten oat varieties. In addition, in vitro antioxidant activities of oat extracts and Avn A, B and C were evaluated. It was found that Avn C had the highest in vitro antioxidant activity among the three Avns. To investigate the cytoprotective activity of Avn C, normal human skin fibroblasts (2DD) were treated with Avn C followed by exposure to extracellular stress and its ability to reduce cellular damage was determined. Pre-treatment of cells with Avn C reduced hydrogen peroxide (H2O2)-induced oxidative stress significantly as demonstrated by decreased intracellular free radical levels and antioxidant gene transcripts. Avn C pre-treatment also resulted in decreased levels of gene transcripts encoding pro-inflammatory cytokines in response to H2O2 or tumor necrosis factor α (TNF-α) stimulation. This reduction in cytokine gene transcription occurred concomitantly with reduced phosphorylated nuclear factor-κB (NF-κB) p65, indicating reduced pro-inflammatory response. To better understand the mechanisms of actions, the impact of Avn C on cellular signaling pathways was investigated on Avn C-treated 2DD cells without exposure to stress. We found that Avn C induced heme oxygenase-1 (HO-1) expression through increased DNA-Nrf2 binding activity. Also, it reduced basal levels of pro-inflammatory cytokines through decreased DNA-NF-κB binding activity. Those mechanism pathways are independent of free radical scavenging and strongly associated with oxidation, inflammation and the aging process. Moreover, anti-proliferative effect of Avn C on 2DD cells was observed via mechanisms independent of autophagy activation. Collectively, our findings suggest that Avn C protects normal human skin fibroblasts against oxidative stress and inflammatory response through Nrf2/HO-1 activation and NF-κB inhibition and further imply that Avn C may be a potential nutraceutical for health promotion and disease prevention.