Abstract 3599: Ras-Association Domain Family 1 Isoform A (RASSF1A)Inhibits Myocardial Hypertrophy by Blocking Signal Transmission From Ras to Raf1
Understanding the molecular mechanisms of cardiac hypertrophy is key in elucidating the pathogenesis of heart failure. Many factors that positively mediate hypertrophy have been identified in recent years, but little is known about counteracting negative regulators. The tumour suppressor RASSF1A (Ras-Association Domain Family 1 Isoform A) has recently been described as a regulator of cell proliferation and apoptosis in cancer cells. Although we and others have detected RASSF1A expression in the heart, little is known about its (patho-)physiological functions. Here we investigate the role of this molecule in regulating myocardial hypertrophy in mouse and man. Ablation of RASSF1A in mice (RASSF1A KO) significantly enhanced the hypertrophic response to transverse aortic constriction/TAC (64.7% increase in heart weight/body weight ratio in RASSF1A KO mice compared to 30.6% in wild type (WT), n=10, P<0.05) yielding one of the most severe hypertrophic phenotypes described. Histological analysis showed that TAC-stimulated RASSF1A KO mice exhibited a significant increase in cross sectional myocyte area (121% increase in KO vs 22% increase in WT (P<0.01)) and fibrosis (22% fibrosis area in KO vs 11% in WT, P<0.01). Consistent with the in vivo data, overexpression of human RASSF1A in neonatal rat cardiomyocytes markedly reduced the cellular hypertrophic response to phenylephrine stimulation by ~50% compared to control cells as indicated by analysis of cell size, protein synthesis and BNP expression. Affinity precipitation of cardiomyocyte extracts using a GST-fusion peptide corresponding to the Ras-binding domain of Raf1 revealed that RASSF1A inhibited the binding of Raf1 to active Ras and subsequently blocked the activation of the downstream Raf1-ERK1/2 pathway. Furthermore, a significant downregulation of RASSF1A expression by ~50% was observed in failing human hearts (n=3, P<0.05) strongly indicating its involvement in the development of heart failure. Our data establish RASSF1A as a novel inhibitor of cardiac hypertrophy by inhibiting Raf1 activation by Ras. The identification of RASSF1A as a negative regulator of cardiac hypertrophy may provide opportunities for the development of novel therapeutic strategies for heart failure.