Inhibitors of Histone Deacetylases; evaluation and target identification of a cardioprotective combination therapy for cancer

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Deterioration of myocardial contractile function is a major limitation for successful cancer therapies. Moreover, cardiac function is strongly associated with cancer-related morbidity and mortality. Anthracyclines, which are well-established anti-cancer drugs and part of many classical therapeutic regimes share profound cardiac side effects. Despite its clinical relevance, there is only very limited knowledge about the downstream targets involved and potential cardioprotective mechanisms of co-therapies. Histone deacetylases (HDACs) work as epigenetic repressors. Recent work has shown that HDAC inhibitors (HDACi) not only successful as therapeutics in various cancer entities but also beneficial for the heart in preclinical models of heart failure. Even though its beneficial potential has been shown, the specific downstream mechanisms of the cardioprotective effects of HDACi are not clarified so far. In our ongoing work, we identified the chaperone 14-3-3 to be hyperacetylated in cardiomyocytes upon treatment with vorinostat, the first FDA approved HDACi. We could show, that hyperacetylation of 14-3-3 leads to reduced binding to class II HDACs, namely HDAC4 and 5. Nuclear HDAC4/5 act as repressors of the transcription factor (TF) myocyte enhancer factor 2 (MEF2), which is required for upregulation of genes that are causative for pathological cardiac remodeling and dysfunction. In addition, we found that vorinostat inhibits the nuclear export of HDAC4/5. Conversely, vorinostat resulted in remarkable suppression of MEF2 in neonatal rat ventricular cardiomyocytes (NRMVs) while knockdown of HDAC4 and HDAC5 attenuated the vorinostat-dependent MEF2 inhibition. These results suggest that the protective effects of vorinostat are partially transmitted by HDAC4/5. Accordingly, we found that cardiomyocyte-specific deletion of HDAC4 increases the susceptibility for cardiac dysfunction upon treatment with the anthracycline ‘doxorubicin’ and upregulates the MEF2 target gene myh7. The myh7-promotor was further identified to be bound to topoisomerase IIb in a doxorubicin-dependent manner. Based on our unpublished work, we propose a crucial role of HDAC-dependent MEF2 inhibition in doxorubicin-induced cardiotoxicity. This effect can be amplified by co-treatment with the HDACi vorinostat. Still, the epigenetic changes caused by vorinostat and potential other downstream targets independent of the TF MEF2 are widely unknown, which is why they require further investigations. The specific aims are: 1: (a) Identification of global HDACi-dependent epigenetic changes in the heart and (b) determination of HDACi-targeted genomic regions in cardiomyocytes. 2: To determine the role of MEF2 in the control of pathological gene regulation upon doxorubicin treatment. 3: To determine potential cardioprotective effects of vorinostat upon co-treatment with anthracyclines in vivo and to elucidate of the downstream mechanism via gain and loss of function experiments in vitro.