Previous studies have established that the regulation of gene expression is dependent upon the nucleosomal integrity of nuclear DNA. To a large extent, this integrity is dictated by the acetylation status of the core histone particles. The acetylation of histones is, in turn, controlled by the combined activity of specific acetylases and deacetylases. Moreover, disruption of histone acetylases and deacetylases has been linked to a wide variety of human cancers. For this reason, the recent availability of potent and specific histone deacetylase inhibitors has provoked a great deal of interest amongst cancer biologists, oncologists and pharmacologists. Within the past 2- 3 years, several novel histone deacetylase inhibitors have been reported, many of which have already been tested in vivo in mouse models of cancer. In this review we focus on the rationale behind the use of histone deacetylase inhibitors as anticancer agents. Moreover, we review some of the recent findings pertaining to the use of these compounds as anticancer agents.

Use of specific histone deacetylase inhibitors has revealed critical roles for the histone deacetylases (HDAC) in controlling proliferation. Although many studies have correlated the function of HDAC inhibitors with the hyperacetylation of histones, few studies have specifically addressed whether the accumulation of acetylated histones, caused by HDAC inhibitor treatment, is responsible for growth inhibition. In the present study we show that HDAC inhibitors cause growth inhibition in normal and transformed keratinocytes but not in normal dermal fibroblasts. This was despite the observation that the HDAC inhibitor, suberic bishydroxamate (SBHA), caused a kinetically similar accumulation of hyperacetylated histones. This cell type-specific response to SBHA was not due to the inactivation of SBHA by fibroblasts, nor was it due to differences in the expression of specific HDAC family members. Remarkably, overexpression of HDACs 1, 4, and 6 in normal human fibroblasts resulted in cells that could be growth-inhibited by SBHA. These data suggest that, although histone acetylation is a major target for HDAC inhibitors, the accumulation of hyperacetylated histones is not sufficient to cause growth inhibition in all cell types. This suggests that growth inhibition, caused by HDAC inhibitors, may be the culmination of histone hyperacetylation acting in concert with other growth regulatory pathways.Analysis of histone-modifying enzymes such as the histone acetyltransferases (HATs) 1 and deacetylases has resulted in significant advances in our understanding of transcriptional regulation (1-4). These studies have resulted in a model of transcription in which transcriptionally competent genes are transcribed or repressed dependent upon their ability to recruit either HATs or histone deacetylases to the promoter (4). In these models, recruited histone acetyltransferases associate with transcription factor complexes (5-8), resulting in the acetylation of nucleosomal histones, relaxation of nucleosomal integrity, and hence transcription. Conversely, transcriptional repression occurs when histone deacetylases (and cofactors) are recruited to DNA-bound transcription factors, resulting in the removal of acetyl groups from NH 2 -terminal lysines causing a "tightening" of nucleosomal integrity and a suppression of transcription (9 -13).The isolation and synthesis of new and potent inhibitors of histone deacetylase enzymes (HDACs) has allowed us to identify some of the biological outcomes resulting from manipulation of histone deacetylase activity. For example, it is now established that treatment of cells in vitro and in vivo with HDAC inhibitors can result in specific functional outcomes such as cell cycle arrest (14 -16), apoptosis/cell death (17-19), or differentiation (19 -21). These outcomes to a large extent are cell type-specific and have raised the potential that the HDAC inhibitors may represent a new and important class of anticancer therapeutic agents (4).HDAC inhibitors comprise a diverse range of unrelated compounds that...

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