Cardioprotective and Cardiotoxic Effects of Quercetin and Two of Its In Vivo Metabolites on Differentiated H9c2 Cardiomyocytes

Whilst mitotic rat embryonic cardiomyoblast‐derived H9c2 cells have been widely used as a model system to study the protective mechanisms associated with flavonoids, they are not fully differentiated cardiac cells. Hence, the aim of this study was to investigate the cardioprotective and cardiotoxic actions of quercetin and two of its major in vivo metabolites, quercetin 3‐glucuronide and 3′‐O‐methyl quercetin, using differentiated H9c2 cells. The differentiated cardiomyocyte‐like phenotype was confirmed by monitoring expression of cardiac troponin 1 after 7 days of culture in reduced serum medium containing 10 nM all‐trans retinoic acid. Quercetin‐induced cardiotoxicity was assessed by monitoring MTT reduction, lactate dehydrogenase (LDH) release, caspase 3 activity and reactive oxygen species production after prolonged flavonoid exposure (72 hr). Cardiotoxicity was observed with quercetin and 3′‐O‐methyl quercetin, but not quercetin 3‐glucuronide. Cardioprotection was assessed by pre‐treating differentiated H9c2 cells with quercetin or its metabolites for 24 hr prior to 2‐hr exposure to 600 μM H₂O_2, after which oxidative stress‐induced cell damage was assessed by measuring MTT reduction and LDH release. Cardioprotection was observed with quercetin and 3′‐O‐methyl quercetin, but not with quercetin 3‐glucuronide. Quercetin attenuated H₂O₂‐induced activation of ERK1/2, PKB, p38 MAPK and JNK, but inhibitors of these kinases did not modulate quercetin‐induced protection or H₂O₂‐induced cell death. In summary, quercetin triggers cardioprotection against oxidative stress‐induced cell death and cardiotoxicity after prolonged exposure. Further studies are required to investigate the complex interplay between the numerous signalling pathways that are modulated by quercetin and which may contribute to the cardioprotective and cardiotoxic effects of this important flavonoid.