Grape seed extract upregulates p21 (Cip1) through redox‐mediated activation of ERK1/2 and posttranscriptional regulation leading to cell cycle arrest in colon carcinoma HT29 cells

Abnormalities in cell cycle progression provide unlimited replicative potential to cancer cells, and therefore targeting of key cell cycle regulators could be a sound cancer chemopreventive strategy. Earlier, we found that grape seed extract (GSE) increases Cip/p21 protein level and inhibits growth and induces apoptosis in human colon carcinoma HT29 cells both in vitro and in vivo. However, the mechanism of GSE-induced p21 upregulation and its role in biological efficacy of GSE are not known, which were investigated here. GSE treatment of HT29 cells resulted in a strong dose- and time-dependent phosphorylation of extracellular signal regulated kinase 1/2 (ERK1/2), consistent with p21 induction. The inhibition of sustained ERK1/2 activation by GSE using pharmacological inhibitors abrogated GSE-induced p21 upregulation. Furthermore, pretreatment of cells with N-acetylcysteine inhibited GSE-induced ERK1/2 phosphorylation as well as p21 upregulation, suggesting the involvement of GSE-induced oxidative stress as an upstream event. Consistent with this, GSE also decreased intracellular level of reduced glutathione. Next, we determined whether GSE-induced signaling regulates p21 expression at transcriptional and/or translational levels. GSE was found to increase the stability of p21 message with resultant increase in p21 protein level, but it did not alter the protein stability to a great extent. Importantly, knock-down of p21 abrogated GSE-induced G(1) arrest suggesting that p21 induction by GSE is essential for its G(1) arrest effect. Together, our results for the first time identify a central role of p21 induction and associated mechanism in GSE-induced cell cycle arrest in HT29 cells.