Reactive oxygen species (ROS)-induced cardiac cell injury via expression changes 
of multiple genes plays a critical role in the pathogenesis of numerous heart 
diseases. MicroRNAs (miRNAs) comprise a novel class of endogenous, small, 
noncoding RNAs that negatively regulate about 30% of the genes in a cell via 
degradation or translational inhibition of their target mRNAs. Currently, the 
effects of ROS on miRNA expression and the roles of miRNAs in ROS-mediated 
injury on cardiac myocytes are uncertain. Using quantitative real-time RT-PCR 
(qRT-PCR), we demonstrated that microRNA-21 (miR-21) was upregulated in cardiac 
myocytes after treatment with hydrogen peroxide (H(2)O(2)). To determine the 
potential roles of miRNAs in H(2)O(2)-mediated gene regulation and cellular 
injury, miR-21 expression was downregulated by miR-21 inhibitor and upregulated 
by pre-miR-21. H(2)O(2)-induced cardiac cell death and apoptosis were increased 
by miR-21 inhibitor and was decreased by pre-miR-21. Programmed cell death 4 
(PDCD4) that was regulated by miR-21 and was a direct target of miR-21 in 
cardiac myocytes. Pre-miR-21-mediated protective effect on cardiac myocyte 
injury was inhibited in H(2)O(2)-treated cardiac cells via adenovirus-mediated 
overexpression of PDCD4 without miR-21 binding site. Moreover, Activator protein 
1 (AP-1) was a downstream signaling molecule of PDCD4 that was involved in 
miR-21-mediated effect on cardiac myocytes. The results suggest that miR-21 is 
sensitive to H(2)O(2) stimulation. miR-21 participates in H(2)O(2)-mediated gene 
regulation and functional modulation in cardiac myocytes. miR-21 might play an 
essential role in heart diseases related to ROS such as cardiac hypertrophy, 
heart failure, myocardial infarction, and myocardial ischemia/reperfusion 
injury.