Mechanism by which avenanthramide-c, a polyphenol of oats, blocks cell cycle progression in vascular smooth muscle cells

Previously, we reported that avenanthramide-c (Avn-c), one of the major avenanthramides, polyphenols of oats, inhibited the serum-induced proliferation of vascular smooth muscle cells (SMC), which is an important process in the initiation and development of atherosclerosis. In the present study, we further investigated its cell cycle inhibitory mechanism. Rat embryonic aortic smooth muscle cell line A10 was used in this study. Flow cytometry analysis revealed that treatment of A10 cells with 80 μM Avn-c arrested the cell cycle in G1 phase as indicated by an increase in the number of cells in G1 phase and a decrease in the number of cells in S phase. This cell cycle arrest was associated with a decrease in the phosphorylation of retinoblastoma protein (pRb), whose hyperphosphorylation is a hallmark of the G1 to S transition in the cell cycle. The inhibition of pRb phosphorylation with Avn-c was accompanied by a decrease in cyclin D1 expression and an increase in cyclin-dependent kinase inhibitor p21cip1 expression, without significant changes in p27kip1 expression. Furthermore, Avn-c treatment increased the expression level and stability of p53 protein, which could account for the increase of p21cip1 expression. Our results demonstrate for the first time that Avn-c, which is a unique polyphenol found in oats, arrests SMC proliferation at G1 phase by upregulating the p53-p21cip1 pathway and inhibiting pRB phosphorylation. This inhibitory effect of Avn-c on SMC proliferation is an additional indication for the potential health benefit of oat consumption in the prevention of coronary heart disease beyond its known effect through lowering blood cholesterol.