| Abstract: The increased activity of xanthine/xanthine oxidase (X/XO) has been suggested as a risk factor for heart disease and herbal polyphenols exhibits cardioprotection in vitro and in vivo. To understand the cardioprotective action mechanisms of polyphenol quercetin and hydroxytyrosol, the expression levels of stress-responsive proteins were studied in X/XO-induced toxicity model of H9c2 cardiomyocyocytes. Pretreatment with each polypenol (0.1–10 µg/ml; 24 h) enhanced viability (p < 0.01; MTT test) and inhibited reactive oxygen species (ROS) generation (p < 0.001; H2DCFDA assay) against 12 h exposure to a free radical generating system, X (0.5 mM) and XO (5 mU/ml). Western blotting experiments showed that X/XO increases the phosphorylation of downstream substrate of p38, MAPK-activated protein kinase 2 (MAPKAPK-2), p44/42-MAPK (Erk1/2) and cleaved caspase-3 (p < 0.001, vs. Control), however inhibits the levels of phosphorylated c-Jun and Hsp27 (p < 0.01, vs. Control). Pretreatment with quercetin or hydroxytyrosol attenuated the phosphorylation of MAPKAPK-2 and cleaved caspase-3 in X/XO-exposed cells (p < 0.01, vs. X/XO). Hydroxytyrosol enhanced the reduction of phosphorylation of a transcriptional target c-Jun and led to overphosphorylation in protective proteins, p44/42-MAPK and Hsp27 in X/XO-exposed cells (p < 0.01, vs. X/XO). Our data suggest that quercetin and hydroxytyrosol protects cardiomyocytes against X/XO-induced oxidative toxicity by diminishing intracellular ROS and the regulation of stress-sensitive protein kinase cascades and transcription factors.
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General Physiology and Biophysics 
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