Cardiomyocyte BRAF is a key signalling intermediate in cardiac hypertrophy in mice
Authors: Alharbi Hajed O. , Hardyman Michelle A. , Cull Joshua J. , Markou Thomais , Cooper Susanna T.E. , Glennon Peter E. , Fuller Stephen J. , Sugden Peter H. , Clerk Angela 
Published: September 2022

Cardiac hypertrophy is necessary for the heart to accommodate an increase in workload. Physiological, compensated hypertrophy (e.g. with exercise) is reversible and largely due to cardiomyocyte hypertrophy. Pathological hypertrophy (e.g. with hypertension) is associated with additional features including increased fibrosis and can lead to heart failure. RAF kinases (ARAF/BRAF/RAF1) integrate signals into the ERK1/2 cascade, a pathway implicated in cardiac hypertrophy, and activation of BRAF in cardiomyocytes promotes compensated hypertrophy. Here, we used mice with tamoxifen-inducible cardiomyocyte-specific BRAF knockout (CM-BRAFKO) to assess the role of BRAF in hypertension-associated cardiac hypertrophy induced by angiotensin II (AngII; 0.8 mg/kg/d, 7 d) and physiological hypertrophy induced by phenylephrine (40 mg/kg/d, 7 d). Cardiac dimensions/function were assessed by echocardiography with histological assessment of cellular changes. AngII promoted cardiomyocyte hypertrophy and increased fibrosis within the myocardium (interstitial) and around the arterioles (perivascular) in male mice; cardiomyocyte hypertrophy and interstitial (but not perivascular) fibrosis were inhibited in mice with CM-BRAFKO. Phenylephrine had a limited effect on fibrosis, but promoted cardiomyocyte hypertrophy and increased contractility in male mice; cardiomyocyte hypertrophy was unaffected in mice with CM-BRAFKO, but the increase in contractility was suppressed and fibrosis increased. Phenylephrine induced a modest hypertrophic response in female mice and, in contrast to the males, tamoxifen-induced loss of cardiomyocyte BRAF reduced cardiomyocyte size, had no effect on fibrosis and increased contractility. The data identify BRAF as a key signalling intermediate in both physiological and pathological hypertrophy in male mice, and highlight the need for independent assessment of gene function in females.

Clinical perspectives

Background. BRAF is a key signalling intermediate that causes cancer and is upregulated in heart failure, but its role in physiological and pathological cardiac hypertrophy remains to be established.

Summary. Cardiomyocyte BRAF is required in male mice for hypertrophy and contributes to interstitial fibrosis in hypertension induced by angiotensin II, but it increases contractility and suppresses fibrosis in physiological hypertrophy induced by α1-adrenergic receptor stimulation with phenylephrine. Differences between males and females are highlighted in the phenylephrine response.

Potential significance of results to human health and disease. BRAF is a key signalling node in both pathological and physiological hypertrophy: inhibiting BRAF may be beneficial in pathological hypertrophy and the data have implications for repurposing of RAF inhibitors developed for cancer; inhibiting BRAF in physiological hypertrophy may result in increased fibrosis and using RAF inhibitors in this context could be detrimental in the longer term.