Myofibrillogenesis regulator-1 (MR-1) is a novel homo gene identified from a human skeletal muscle cDNA library that interacts with contractile proteins and exists in human myocardial myofibrils. The present study investigated MR-1 protein expression in hypertrophied myocardium and the involvement of MR-1 in cardiac hypertrophy. Cardiac hypertrophy was induced by abdominal aortic stenosis (AAS) in Sprague-Dawley (SD) rats. Left ventricle (LV) hypertrophy was assessed by the ratio of wet weight of the LV to the whole heart weight (LV/HW) or body weight (LV/BW). Rat MR-1 (rMR-1) expression in the myocardium was detected by immunohistochemical and Western blotting analysis. Hypertrophy was induced by angiotensin II (Ang II) incubation in cultured neonatal rat cardiomyocytes. The effect of rMR-1 RNA interference (RNAi) on Ang II-induced hypertrophy was studied by transfection of the cardiomyocytes with an RNAi plasmid pSi-1 that targets rMR-1. Hypertrophy in cardiomyocytes was assessed by ³H-leucine incorporation and myocyte size. The rMR-1 protein expression in cardiomyocytes was detected by Western blotting. We found that AAS resulted in a significant increase in LV/HW and LV/BW ratios, by 89% and 86% (P<0.01), respectively. Immunohistochemistry and Western blot analyses demonstrated upregulated rMR-1 protein expression in hypertrophic myocardium. Ang II induced a 24% increase in ³H-leucine incorporation and a 65.8% increase in cell size compared to that in control cardiomyocytes (P<0.01), which was prevented by treatment with losartan, an AT1 receptor inhibitor, or transfection with pSi-1. rMR-1 expression increased in Ang II-induced hypertrophied cardiomyocytes, and pSi-1 transfection abolished the upregulation. These findings suggest that MR-1 is associated with cardiac hypertrophy in rats in vivo and in vitro.