Cocoa polyphenols attenuate hydrogen peroxide-induced inhibition of gap-junction intercellular communication by blocking phosphorylation of connexin 43 via the MEK/ERK signaling pathway

Cocoa, a good source of dietary antioxidative polyphenols, exhibited anticarcinogenic activity in animal models, but the molecular mechanisms of the chemopreventive potential of cocoa remain unclear. Inhibition of gap-junction intercellular communication (GJIC) is strongly related to tumorigenesis. Cocoa polyphenol extracts (CPE) dose dependently attenuated hydrogen peroxide (H2O2)-induced inhibition of GJIC in rat liver epithelial (RLE) cells. CPE inhibited the H2O2-induced phosphorylation and internalization of connexin 43, which is a regulating protein of GJIC in RLE cells. The H2O2-induced accumulation of reactive oxygen species and activation of extracellular signal-regulated kinase were inhibited by CPE treatment. However, CPE did not block H2O2-induced phosphorylation of p38 mitogen-activated protein kinase. An ex vivo kinase assay demonstrated that CPE inhibited the H2O2-induced mitogen-activated protein kinase/extracellular signal-regulated kinase kinase (MEK) 1 activity in RLE cell lysates. Ex vivo pull-down assay data revealed that CPE directly bound with MEK1 to inhibit MEK1 activity. These results indicate that CPE protects against the H2O2-induced inhibition of GJIC through antioxidant activity and direct inhibition of MEK activity, which may contribute to its chemopreventive potential.