SUMMARY Heart failure (HF) is driven by the interplay between master regulatory transcription factors and dynamic alterations in chromatin structure. While pathologic gene transactivation in this context is known to be associated with recruitment of histone acetyl-transferases and local chromatin hyperacetylation, the role of epigenetic reader proteins in cardiac biology is unknown. We therefore undertook a first study of acetyl-lysine reader proteins, or bromodomains, in HF. Using a chemical genetic approach, we establish a central role for BET-family bromodomain proteins in gene control during HF pathogenesis. BET inhibition potently suppresses cardiomyocyte hypertrophy in vitro and pathologic cardiac remodeling in vivo. Integrative transcriptional and epigenomic analyses reveal that BET proteins function mechanistically as pause-release factors critical to activation of canonical master regulators and effectors that are central to HF pathogenesis and relevant to the pathobiology of failing human hearts. This study implicates epigenetic readers in cardiac biology and identifies BET co-activator proteins as therapeutic targets in HF.

Nonischemic cardiomyopathy (NICM) resulting from long-standing hypertension, valvular disease, and genetic mutations is a major cause of heart failure worldwide. Recent observations suggest that myeloid cells can impact cardiac function, but the role of tissue-intrinsic vs. tissue-extrinsic myeloid cells in NICM remains poorly understood. Here, we show that cardiac resident macrophage proliferation occurs within the first week following pressure overload hypertrophy (POH; a model of heart failure) and is requisite for the heart's adaptive response. Mechanistically, we identify Kruppel-like factor 4 (KLF4) as a key transcription factor that regulates cardiac resident macrophage proliferation and angiogenic activities. Finally, we show that blood-borne macrophages recruited in late-phase POH are detrimental, and that blockade of their infiltration improves myocardial angiogenesis and preserves cardiac function. These observations demonstrate previously unappreciated temporal and spatial roles for resident and nonresident macrophages in the development of heart failure.

Ex vivo culture has been proposed as a means to augment and repair autologous cells in patients with chronic diseases, but the mechanisms governing improvement in cell function are not well understood. Although microRNAs (miRs) are increasingly appreciated as key regulators of cellular function, a role for these factors in CD34+ cell-mediated angiogenesis has not been elucidated. Vascular endothelial growth factor (VEGF) was previously shown to induce expression of certain miRs associated with angiogenesis in endothelial cells and promote survival and number of vascular colony forming units of haematopoietic stem cells (HSCs). We sought to evaluate the role of VEGF in expansion and angiogenic function of CD34+ cells and to identify specific miRs associated with angiogenic properties of expanded cells. Umbilical cord blood CD34+ cells were effectively expanded (18- to 22-fold) in culture medium containing stem cell factor (SCF), Flt-3 ligand (Flt-3), thrombopoietin (TPO) and interleukin-6 (IL-6) with (postEX/+VEGF) and without VEGF (postEX/noVEGF). Tube formation in matrigel assay and tissue perfusion/capillary density in mice ischaemic hindlimb were significantly improved by postEX/+VEGF cells compared with fresh CD34+ and postEX/noVEGF cells. MiR-210 expression was significantly up-regulated in postEX/+VEGF cells. MiR-210 inhibitor abrogated and 210 mimic recapitulated the pro-angiogenic effects by treatment of postEX/+VEGF and postEX/noVEGF cells respectively. Collectively, these observations highlight a critical role for VEGF in enhancing the angiogenic property of expanded cells, and identify miR-210 as a potential therapeutic target to enhance CD34+ stem cell function for the treatment of ischaemic vascular disease.