Procyanidins, polymeric flavan-3-ols, are known to possess antioxidant, antiatherogenic, and anticarcinogenic properties. In the present study, we investigated the role of almond (Prunus amygdalus) skin procyanidins (ASP) in regulating the protein expression of phase II detoxifying and antioxidant enzymes in HepG2 cells and acetaminophen (APAP)-treated hepatotoxic mice. Treatments of ASP significantly induced the expression of phase II enzymes including
Nad(p)h: quinoneoxidoreductase 1, catalase, glutathione peroxidase, and superoxide dismutase in the cells and mice. ASP also potently enhanced the expression of nuclear factor-E2-related factor 2 (Nrf2) and antioxidant response element (ARE)-reporter gene activity in vitro. APAP-induced hepatotoxic markers including AST and ALT in mice were inhibited by ASP administration. However, regulation of upstream kinases by ASP was different between in vitro and in vivo models. Collectively, ASP could induce the activation of Nrf2/ARE-mediated phase II detoxifying/antioxidant enzymes but with differential regulation on upstream kinases between in vitro and in vivo.