Abstract 538: Rassf1A Inhibits Cardiac Hypertrophy through Mst1
RAS-association domain family 1, isoform A (Rassf1A), a Ras binding protein, negatively regulates cell growth in non-cardiac cells. Since Rassf1A activates mammalian STE 20-like kinase 1 (Mst1), a kinase which promotes apoptosis and inhibits growth of cardiac myocytes, we examined whether Rassf1A negatively regulates cardiac hypertrophy through Mst1. Rassf1A is upregulated by transverse aortic constriction (TAC) (1.3 fold) in the mouse heart. In order to determine the function of endogenous Rassf1A, we used Rassf1A −/− (KO) mice. Although KO did not show an obvious baseline cardiac phenotype at 3 months of age, left ventricular weight (LVW) /tibial length (TL) (8.7 ± 0.7 vs 5.7 ± 0.2, p < 0.05, n = 7, 5) and myocyte size (1.5 fold, p < 0.05, n = 4, 3) were significantly greater in KO than in wild type mice (WT) at ages 7–10 months. The LV ejection fraction (LVEF) was significantly lower in KO than in WT (61 ± 2% vs 70 ± 1%, p < 0.05, n = 7, 6), and the extent of LV fibrosis was greater in KO. In response to TAC for one week, LVW/TL (9.3 ± 0.4 vs 7.6 ± 0.3, p < 0.05, n = 6, 8) and myocyte size (1.4 fold, p < 0.05, n = 3, 3) were significantly greater in KO than in WT at 3 months of age. Furthermore, more severe LV fibrosis was observed in KO than in WT, and lung weight/TL was greater in KO than in WT (15.3 ± 1.3 vs 9.6 ± 0.7, p < 0.05, n = 6, 8). These findings suggest that endogenous Rassf1A negatively regulates cardiac hypertrophy at baseline and after TAC. Adenovirus-mediated expression of Rassf1A in cultured cardiac myocytes phosphorylated Mst1 and significantly inhibited phenylephrine (PE)-induced increases in myocytes size (p < 0.05). Expression of dominant negative (DN) Mst1 reversed the anti-hypertrophic effect of Rassf1A (PE 1.5 fold, PE + Rassf1A 0.9 fold, PE + Rassf1A + DN Mst1 1.5 fold, n = 6, 6, 5). Although Rassf1A physically associates with Mst1, Rassf1A L308P failed to associate with Mst1. Expression of Rassf1A L308P failed to inhibit PE-induced cardiac hypertrophy (PE + Rassf1A L308P 1.7 fold), suggesting that interaction with Mst1 is required for the anti-hypertrophic effect of Rassf1A. These results suggest that Rassf1A acts as a negative regulator of cardiac hypertrophy. Furthermore, the anti-hypertrophic effect of Rassf1A is mediated through direct interaction with and activation of Mst1.