ABSTRACT
The bromodomain and extraterminal (BET) family member BRD4 is pivotal in the pathogenesis of cardiac hypertrophy. BRD4 induces hypertrophic gene expression by binding to the acetylated chromatin, facilitating the phosphorylation of RNA polymerases II (Pol II) and leading to transcription elongation. The present study identified a novel post-translational modification of BRD4: poly(ADP-ribosyl)ation (PARylation), that was mediated by poly(ADP-ribose)polymerase-1 (PARP1) in cardiac hypertrophy. BRD4 silencing or BET inhibitors JQ1 and MS417 prevented cardiac hypertrophic responses induced by isoproterenol (ISO), whereas overexpression of BRD4 promoted cardiac hypertrophy, confirming the critical role of BRD4 in pathological cardiac hypertrophy. PARP1 was activated in ISO-induced cardiac hypertrophy and facilitated the development of cardiac hypertrophy. BRD4 was involved in the prohypertrophic effect of PARP1, as implied by the observations that BRD4 inhibition or silencing reversed PARP1-induced hypertrophic responses, and that BRD4 overexpression suppressed the anti-hypertrophic effect of PARP1 inhibitors. Interactions of BRD4 and PARP1 were observed by co-immunoprecipitation and immunofluorescence. PARylation of BRD4 induced by PARP1 was investigated by PARylation assays. In response to hypertrophic stimuli like ISO, PARylation level of BRD4 was elevated, along with enhanced interactions between BRD4 and PARP1. By investigating the PARylation of truncation mutants of BRD4, the C-terminal domain (CTD) was identified as the PARylation modification sites of BRD4. PARylation of BRD4 facilitated its binding to the transcription start sites (TSS) of hypertrophic genes, resulting in enhanced phosphorylation of RNA Pol II and transcription activation of hypertrophic genes. The present findings suggest that strategies targeting inhibition of PARP1-BRD4 might have therapeutic potential for pathological cardiac hypertrophy.
ABSTRACT
BACKGROUND: Left ventricular hypertrophy(LVH) is one of the complications of hypertension and has been known as an independent risk factor of cardiovascular complications. Recently, it has been reported that hypertensive patients with LVH had the most advanced extracardiac target-organ damage compared with other groups. Previous reports have shown that mean plasma atrial natriuretic peptide(ANP) and brain natriuretic peptide(BNP) levels in hypertensive patients are higher than in normotensive subjects. Therefore, in this study, we investigated the relationships between the plasma ANP and BNP levels and the degree of LVH in hypertensive patients and in normotensive subjects and also investigated the clinical significance of measurement of plasma ANP and BNP levels. METHODS: In all study subjects, left ventricle mass index(LVMI) and left ventricle geometry were measured by M-mode echocardiography. Measurements were made by the recommendations of the American Society of Echocardiography. Plasma ANP and BNP levels were measured by radioimmunoassay method. RESULTS: 1) 57% of the hypertensive patients had eccentric hypertrophy and 6% had concentric hypertrophy. 2) LV mass and LVMI of normotensive subjects and hypertensive patients were 169+/-53 g, 229+/-64 g and 99+/-27.3 g/m2, 142+/-37.7 g/m2, respectively(P<0.05). 3) There were statistically significant correlations between blood pressure and LVMI in all subjects(r=.43, P<0.05). 4) Plasma ANP levels were significantly increased in hypertensive patients than normotensive subjects (28.2+/-14.3 pg/mL and 42.8+/-26 pg/mL, respectively; P<0.05). 5) Plasma BNP levels were significantly increased in hypertensive patients than normotensive subjects (18.4+/-5.4 pg/mL and 36.5+/-26 pg/mL; respectively, P<0.05). 6) Plasma BNP levels were significantly increased in 63% of the hypertensive patients with LVH(P<0.05). 7) There were statistically significant correlations between blood pressure and plasma ANP and BNP levels(ANP:r=.39, p<0.05, BNP:r=.31, P<0.05). CONCLUSIONS: Plasma ANP and BNP levels were increased in the hypertensive patients but only plasma BNP levels were significantly increased in the hypertensive patients with LVH. Measurement of plasma BNP levels may be useful for early detection of LVH, an independent risk factor of cardiovascular complications. Therefore intensive blood pressure control in these patients may reduce cardiovascular morbidity and mortality.