UC San Diego

Heart failure is a major cause of death and a serious and growing public health problem in the United States. The development of heart failure is often preceded by cardiac hypertrophy, and the presence of hypertrophy strongly correlates with increased risk for failure. It is of great importance to continue gaining insight into the underlying mechanisms responsible for the development of both hypertrophy and heart failure, and even more so, to define signaling pathways that contribute to the transition or progression of hypertrophy to failure. RhoA has been strongly implicated in the development of hypertrophy in cultured primary cardiomyocytes. However, the role of RhoA signaling in the in vivo heart is not as clear. Data from our lab and others suggests that the primary role of RhoA in the heart may be the regulation of cell survival. To directly test whether RhoA can modulate cardiomyocyte survival, an adenoviral construct was used to express a constitutively active RhoA in primary neonatal rat ventricular myocytes. Expression of RhoA resulted in robust activation of FAK, which engaged a protective signaling pathway by binding and activating PI3K and subsequently, Akt. Inhibition of the RhoA effector Rho kinase, FAK, or PI3K abolished the protection afforded by RhoA. Interestingly, however, with prolonged expression/ activation of RhoA, protective responses were lost and frank apoptosis developed. Progression from protection to apoptosis correlated with down regulation of FAK and PI3K/ Akt signaling and induction of pro-apoptotic events. RhoA, specifically acting via Rho kinase, increased the expression of the Bcl-2 family protein Bax. Increases in activated Bax at mitochondria initiate apoptosis through a mitochondrial death pathway characterized by increases in cytochrome c release and caspase-9 and -3 activation. Thus, based on these findings we conclude that RhoA is a pleiotropic molecule that can initiate both protective and apoptotic signaling cascades in cultured cardiomyocytes. To explore the role of increased RhoA signaling in the in vivo myocardium, a tetracycline-inducible line of cardiac specific RhoA transgenic mice have been generated. These mice will be used to examine temporal and dose dependent protective versus deleterious effects of myocardial RhoA expression in a physiological setting