Abstract 491: Cardiomyocyte Braf Promotes Hypertrophy and is Required for Hypertrophic Adaptation to Hypertension in Mice In Vivo, but Raf Inhibitors Have Differential Effects

Abstract

Introduction: Raf kinases lie upstream of ERK1/2 with BRaf having the highest activity. ERK1/2 promote hypertrophy/cardioprotection, but the role of BRaf in the adult heart is unclear. BRaf mutations cause cancer and Raf inhibitors (Rafi) are used but, paradoxically, they can activate ERK1/2 and the cardiac consequences remain to be established. Hypothesis: We hypothesise that BRaf is critical in regulating ERK1/2 signaling in cardiomyocytes and Rafi affect cardiac function. Methods: We studied 8 wk wild-type male C57Bl/6 mice, or mice with cardiomyocyte-specific tamoxifen-inducible knock-in of activating BRaf(V600E) or BRaf knockout (BRafKO). V600E and BRafKO mice received vehicle or tamoxifen (40mg/kg; 6-12 mice per group). Mice were also treated with 0.8 mg/kg/d AngII, 0.5 mg/kg/d SB590885 (SB), 3 mg/kg/d dabrafenib (Dab) or vehicle delivered by osmotic minipumps. Echocardiography was performed and data for each mouse were normalized to the mean of 2 baseline control scans. Kinase activities and mRNA expression were assessed by immunoblotting and qPCR. Results: Knockin of BRaf(V600E) activated ERK1/2 and increased expression of ERK1/2 dependent genes (24h). By 7 d, there was increased ejection fraction (24%) and cardiac output (17%) with increased systolic left ventricular (LV) posterior wall (PW) thickness (13%). Cardiac function normalised by 10 d, but there was significant hypertrophy with increased LVPW in systole/diastole (11-19%) and decreased LV internal diameter. BRafKO alone did not affect cardiac function/dimensions or changes in function induced by AngII, but loss of BRaf inhibited the increase in systolic/diastolic LVPW (30% increase with AngII alone vs 9% with AngII/BRafKO). SB (type I Rafi), but not Dab (type 1.5 Rafi) activated cardiomyocyte ERK1/2 and induced hypertrophy in cultured neonatal rat ventricular myocytes. Consistent with this, SB (not Dab) promoted cardiac hypertrophy in vivo with increased LVPW (3d), but Dab (not SB) inhibited AngII-induced hypertrophy (7 d). Conclusions: Activation of cardiomyocyte BRaf promotes cardiac hypertrophy and cardiomyocyte BRaf is required for AngII-induced hypertrophy. However, Rafi developed for cancer have differential effects on the heart according to their mode of action.