Histone H2A.z Is Essential for Cardiac Myocyte Hypertrophy but Opposed by Silent Information Regulator 2α

Abstract

In this study we have shown that the histone variant H2A.z is up-regulated during cardiac hypertrophy. Upon its knockdown with RNA interference, hypertrophy and the underlying increase in growth-related genes, protein synthesis, and cell size were down-regulated. During attempts to understand the mode of regulation of H2A.z, we found that overexpression of silent information regulator 2alpha (Sir2␣) specifically induced down-regulation of H2A.z via NAD-dependent activity. This effect was reversed by the proteasome inhibitor epoxomicin, suggesting a Sir2␣-mediated ubiquitin/proteasome-dependent mechanism for degradation of H2A.z. An increase in Sir2␣ also resulted in a dose-dependent reduction of the response to hypertrophic stimuli, whereas its inhibition resulted in enhanced hypertrophy and apoptosis. We have shown that Sir2␣ directly deacetylates H2A.z. Mutagenesis proved that lysines 4, 7, 11, and 13 do not play a role in the stability of H2A.z, whereas Lys-15 was indispensable. Meanwhile, Lys-115 and conserved, ubiquitinatable Lys-121 are critical for Sir2␣-mediated degradation. Fusion of the C terminus of H2A.z (amino acids 115-127) to H2A.x or green fluorescence protein conferred Sir2␣-inducible degradation to the former protein only. Because H2A.x and H2A.z have conserved N-tails, this implied that both the C and N termini are critical for mediating the effect of Sir2␣. In short, the results suggest that H2A.z is required for cardiac hypertrophy, where its stability and the extent of cell growth and apoptosis are moderated by Sir2␣. We also propose that Sir2␣ is involved in deacetylation of H2A.z, which results in ubiquitination of Lys-115 and Lys-121 and its degradation via a ubiquitin/proteasomedependent pathway.H2A.z is an essential histone variant. Disruption of H2A.z in mice is embryonic lethal at a very early stage (E 4.5) (1). Likewise, H2A.z is vital for the development of Drosophila melanogaster (2) and is indispensable in Tetrahymena thermophila (3). In yeast, disruption of H2A.z results in slow growth and formamide sensitivity and is not rescued by the major H2A but is rescued by T. thermophila H2A.z variant (4). These reports provide evidence that H2A.z has a vital and non-redundant role and that it is conserved among species.One of the mechanisms for transcriptional regulation involves the replacement of core histones with more specialized variants. A report by Abbott et al. (5) shows that nucleosomes harboring H2A.z are destabilized, being less compact than nucleosomes harboring the core histone H2A. This is also supported by the crystal structure of a nucleosome core containing H2A.z that exhibits subtle differences compared with the H2A nucleosomes, which account for instability between the H2A.z-H2B dimer and the H3-H4 tetramer (6). This is consistent with a role for H2A.z in nucleosomal remodeling to reduce its structural compactness, thereby facilitating the binding of transcription factors. It should be noted though that this might not be the sole function of H2A.z in the cell. It has...

Factors controlling cardiac myosin-isoform shift during hypertrophy and heart failure

“…Another point worth noting about the effects of PARP1 is that mild PARP1 activation has the potential to relax the chromatin structure, leading to activation of gene transcription [143]. PARP1 has also been shown to control the activity of a member of the class-III HDACs (SIRT1), which has been shown to have an antihypertrophy potential, thus further indicating that PARP1 may have a role in the regulation of MHC isoform switch during hypertrophy [139,144]. The role of PARP1 in cardiac disorders and that of its co-regulators in gene regulation have recently been reviewed [134,145,146].…”

“…In addition, the effect of class-III HDACs (sirtuins, SIRT), which are also called longevity factors, needs to be examined for their potential to regulate MHC gene expression [150]. These proteins (SIRTs) have been shown to have anti-hypertrophic potential [144]. Also, many factors such as PARP1, Ku70, Nf-kB, and p300 that are known to participate in MHC gene regulation, and in the evolution of pathologic hypertrophy, have been shown to be targets of SIRT-dependent deacetylation [139,[151][152][153].…”

Calorie restriction and the exercise of chromatin

“…Some contradictory data have been published regarding the localization of each among different cell types: In 293 and HeLa cells, the 44-kDa form is nuclear and the 28-kDa form is present in both the nucleus and mitochondria (Scher et al 2007), whereas in mouse cardiomyocytes the 44-kDa form is found in the nucleus, mitochondria, and cytoplasm, and the 28-kDa form is exclusively mitochondrial (Sundaresan et al 2008). These differences are not surprising though, given that in rat cardiomyocytes SirT1-a nuclear protein in most cell lines-localizes to the nucleus in the embryo, to the cytoplasm in newborns, and to both the nucleus and cytoplasm in adults (Chen et al 2006). Nuclear SirT3 localizes to certain presently uncharacterized foci, and under stress conditions (e.g., UV irradiation, genotoxic stress, or SirT3 overexpression), relocalizes completely to mitochondria (Scher et al 2007).…”

Nucleocytoplasmic Shuttling of the NAD+-dependent Histone Deacetylase SIRT1

“…For example, it is localized exclusively to the nucleus in male germ cells and in COS-7 cells transfected with SIRT1 cDNA (16,17). However, the cytoplasmic localization of SIRT1 has been reported recently in murine pancreatic ␤-cells and neonatal rat cardiomyocytes (19,20). Dynamic changes in the subcellular localization of Drosophila Sir2 during embryonic development have also been reported (21).…”