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Cardiac hypertrophy is characterized by increased myofibrillogenesis. Angiotensin II (Ang-II) is an essential mediator of the pressure overload-induced cardiac hypertrophy in part through RhoA/ROCK (small GTPase/Rho-associated coiled-coil containing protein kinase) pathway. FHOD3 (formin homology 2 domain containing 3), a cardiac-restricted member of diaphanous-related formins, is crucial in regulating myofibrillogenesis in cardiomyocytes. FHOD3 maintains inactive through autoinhibition by an intramolecular interaction between its C- and N-terminal domains. Phosphorylation of the 3 highly conserved residues (1406S, 1412S, and 1416T) within the C terminus (CT) of FHOD3 by ROCK1 is sufficient for its activation. However, it is unclear whether ROCK-mediated FHOD3 activation plays a role in the pathogenesis of Ang-II-induced cardiac hypertrophy. In this study, we detected increases in FHOD3 expression and phosphorylation in cardiomyocytes from Ang-II-induced rat cardiac hypertrophy models. Valsartan attenuated such increases. In cultured neonate rat cardiomyocytes, overexpression of phosphor-mimetic mutant FHOD3-DDD, but not wild-type FHOD3, resulted in myofibrillogenesis and cardiomyocyte hypertrophy. Expression of a phosphor-resistant mutant FHOD3-AAA completely abolished myofibrillogenesis and attenuated Ang-II-induced cardiomyocyte hypertrophy. Pretreatment of neonate rat cardiomyocytes with ROCK inhibitor Y27632 reduced Ang-II-induced FHOD3 activation and upregulation, suggesting the involvement of ROCK activities. Silencing of ROCK2, but not ROCK1, in neonate rat cardiomyocytes, significantly lessened Ang-II-induced cardiomyocyte hypertrophy. ROCK2 can directly phosphorylate FHOD3 at both 1412S and 1416T in vitro and is more potent than ROCK1. Both kinases failed to phosphorylate 1406S. Coexpression of FHOD3 with constitutively active ROCK2 induced more stress fiber formation than that with constitutively active ROCK1. Collectively, our results demonstrated the importance of ROCK2 regulated FHOD3 expression and activation in Ang-II-induced myofibrillogenesis, thus provided a novel mechanism for the pathogenesis of Ang-II-induced cardiac hypertrophy. [ABSTRACT FROM AUTHOR]