Abstract 1777: Bnip3 Induces a Global Increase in Histone Acetyltransferase Affecting Contractile Proteins and Promoting Contractile Dysfunction and Ventricular Dilation
INTRODUCTION: Bnip3 is an atypical Bcl2 family member that has been implicated in cardiac myocyte death by apoptosis, necrosis and autophagy. There are conflicting reports on the mechanism of Bnip3-mediated death and whether accessory stimuli are required to activate the death function. Bnip3 has been reported to colocalize with mitochondria, endoplasmic reticulum and nucleus; no function has been attributed to nuclear Bnip3.
HYPOTHESIS: Bnip3 has novel nuclear functions that contribute to cardiac myocyte (CM) responses to ischemia.
METHODS: Nuclear activities of Bnip3 and their consequences were analyzed in isolated neonatal CM and transgenic mice over-expressing Bnip3 in the heart.
RESULTS: Activation of Bnip3 by a combination of hypoxia and acidosis (HA) but not hypoxia alone induced a global increase in the acetylation (Ac) of histones H3 and H4. Histone acetylation increased 17 ±8.2 (H3) and 15±6.3 (H4) fold relative to aerobic incubation. These effects were paralleled by increased histone acetyltransferase (HAT) activity (2.8±0.8 fold). Histone acetylation and HAT activity were significantly reduced by Bnip3 siRNA: H3-Ac (58% ± 0.2%), H4-Ac (63% ± 1.0%), HAT (68 ± 0.55%). Confocal microscopy localized Bnip3 to the nucleus and anti-Bnip3 antibody co-immunoprecipitated H3-Ac and H4-Ac from hypoxia-acidotic CM. CM overexpression of T7-tagged Bnip3 displayed altered morphology with a 2-fold increase in the length:width ratio compared with non-transfected CM (n=50; p<0.01). The latter effect is reminiscent of MEF2c overexpression in CM. Microarray analysis comparing CM after HA treatment ± Bnip3 siRNA revealed 246 genes (out of 4,807 genes) that were differentially regulated by Bnip3. These genes included calcium channels and contractile proteins. Bnip3 transgenic mice expressed 40-fold increased Bnip3 and displayed constitutively increased acetylated histone H3 and H4. These hearts progressed to contractile dysfunction with severe LV dilation without evidence of underlying apoptosis.
CONCLUSIONS: Bnip3 exhibits nuclear localization and pronounced activation of HAT. This correlates with altered gene expression that may lead to contractile dysfunction, LV dilation and heart failure independently of apoptosis.