Avenanthramides, polyphenols from oats, inhibit IL-1β-induced NF-κB activation in endothelial cells

The chronic inflammation of arterial walls is associated with the development of atherosclerosis. Earlier we reported that avenanthramide (Avn)s-enriched extract of oats (AvnsO) significantly suppressed interleukin (IL)-1β-stimulated secretion of proinflammatory cytokines, such as IL-6, IL-8, and MCP-1, by human aortic endothelial cells (HAEC). The main objective of the current study was to determine if the mechanism of inhibitory effect of these polyphenols from oats on the expression of proinflammatory cytokines is mediated through modulation of nuclear factor κB (NF-κB)-dependent transcription. Confluent HAEC monolayers were treated for 24 h with AvnsO, and synthetically prepared Avn-c suppressed IL-β-stimulated activation of NF-κB in a concentration-dependent manner. CH3-Avn-c, a synthetically prepared methyl ester derivative of Avn-c with a high biological potency, significantly and dose dependently decreased mRNA expression and secretion of IL-6, IL-8, and MCP-1 by HAEC as determined by real-time RT-PCR and ELISA, and it inhibited IL-1β- and TNFα-stimulated NF-κB activation as determined by a NF-κB DNA binding assay and a NF-κB luciferase reporter assay. AvnsO and Avn-c as well as CH3-Avn-c also inhibited the NF-κB-dependent reporter gene expression activated by TNFR-associated factor 2 and 6 (TRAF2, TRAF6) and NFκB-inducing kinase (NIK). CH3-Avn-c also significantly and dose dependently decreased the phosphorylation level of IκB kinase (IKK) and IκB, and prevented IκB degradation as measured by Western blotting. In addition, CH3-Avn-c markedly increased the overall levels of high mass ubiquitin-conjugated protein levels while it mildly inhibited proteasome activity. These observations suggest that Avns, unique polyphenols from oats, decrease the expression of endothelial proinflammatory cytokines at least in part through inhibition of NF-κB activation by inhibiting the phosphorylation of IKK and IκB, and by suppressing proteasome activity.