br Mechanisms of HDAC inhibition

Mechanisms of HDAC inhibition-dependent cardioprotection Multiple preclinical studies have demonstrated potent cardioprotective benefits of HDAC inhibition in murine models of myocardial stress, including I/R [19,25,29,30]. TSA reduces myocardial infarct size by up to 50% [19,25]. In addition, treatment with the structurally distinct HDAC inhibitor Scriptaid (another class I and class II HDAC inhibitor) resulted in nearly identical protection as TSA when compared with Nullscript (negative control), which strongly suggests a class effect [19]. Given that HDAC inhibitors are so effective in targeting reperfusion injury, they provide opportunities to delineate mechanisms of reperfusion injury. Initially, TSA was thought to activate the p38 pathway during I/R to protect myocardial tissue [25]. Further investigation demonstrated that HDAC inhibitors prevent ischemia-induced activation of gene programs in vivo and in vitro that involve hypoxia-inducible factor-1α, cell death, and vascular permeability, thereby reducing vascular leak and myocardial injury [25]. Furthermore, long-term use of TSA promotes myocardial repair and blunts adverse cardiac remodeling by stimulating endogenous cardiac regeneration and neovascularization, which seems to be dependent on c-kit signaling [31]. However, with recent data showing that c-kit+ Toremifene have minimal contribution to cardiomyocytes but mainly contribute to endothelial cells in the heart [32], these data should be interpreted with caution. We have reported that SAHA increases cardiomyocyte autophagic activity within the infarct border zone as measured by LC3-II levels and formation of GFP-LC3 puncta, findings that were verified by electron microscopy [26]. Furthermore, SAHA increased autophagic flux in the infarct border zone assayed using a tandem fluorescence reporter RFP-GFP-LC3 transgenic mouse. In cultured cardiomyocytes subjected to simulated I/R, SAHA pretreatment reduced cell death by 40%. This reduction in cell death correlated with increased autophagic flux in SAHA-treated cells. RNAi-mediated knockdown of ATG7 and ATG5, essential autophagy proteins, abolished SAHA's cardioprotective effects. Based on these findings, we concluded that the cardioprotective effects of SAHA during I/R occur, at least in part, through induction and maintenance of cardiomyocyte autophagic flux [26]. As noted earlier, SAHA plasma concentrations in the rabbits were similar to those achieved in cancer patients [26]. In aggregate, these data support a model in which HDACs participate in the suppression of autophagic flux during myocardial reperfusion injury, and SAHA-dependent suppression of HDAC activity restores autophagic flux, thereby limiting reperfusion injury. In light of this, we submit that SAHA may emerge as an effective therapeutic agent in reperfusion injury, a significant clinical problem that lacks meaningfully, efficacious therapy [4,26,33]. TSA and SAHA are selective inhibitors of class I and II HDAC inhibitors, but non-selective among enzymes within those classes [34]. Other work has shown that the class I-specific HDAC inhibitor entinostat (MS-275) significantly reduced infarct size in an isolated rat heart I/R model [35,36]. It is reported that entinostat increased expression of SOD2 and catalase acting through the transcription factor FoxO3a [35]. Interestingly, selective inhibition of class I HDACs afforded superior cardioprotection when compared with pan-HDAC inhibition in this pretreatment model [35]. The same group tested entinostat delivered at the time of reperfusion. They observed that HDAC1 is present in mitochondria of cardiac myocytes but not those of fibroblasts or endothelial cells [36]. The investigators engineered mitochondria-restricted and mitochondria-excluded HDAC inhibitors and tested both in an ex vivo I/R model. Interestingly, selective inhibition of mitochondrial HDAC1 attenuated I/R injury to the same extent as entinostat, whereas the mitochondria-excluded inhibitor did not. These effects were attributed to a decrease in succinate dehydrogenase (SDHA) activity and subsequent metabolic ROS production in reperfusion [36].