Functional Analysis of HDACs in Tumorigenesis

HDACs, originally described as histone modifiers, have recently been demonstrated to modify a variety of other proteins that are involved in diverse cellular processes unrelated to the chromatin environment. This includes deacetylation of nonhistone targets involved in multiple signaling pathways. In this regard, a considerable number of reports have analyzed the role of nonspecific inhibition of HDACs through pan-HDACi in cancer as well as processes of immune regulation. However, with pan-HDACi there is a lack of understanding about the exact contribution of inhibition of each individual HDAC, which makes the rational design of improved drug candidates extremely difficult. Additionally, current approaches using nonselective HDACi in the clinic have critical limitations, including pan-HDACi which elicit poor activity in solid tumors and cardiac toxicity, class I HDACi which activate multiple apoptotic pathways, limiting its use for longer periods of time, and class I-HDAC6i that evidenced a number of adverse effects in initial clinical trials. Therefore, there is a growing interest in the identification of more selective HDACi, and the subsequent development of accurate functional tests to identify the effectiveness and selectivity of these inhibitors. In this chapter, we are describing some selected methodologies to identify the individual activities of HDACs. In addition, we present specific methods to identify enzymatic and nonenzymatic molecular targets of HDACs.