Distinct and redundant functions of histone deacetylases HDAC1 and HDAC2 in proliferation and tumorigenesis

Histone deacetylases (HDACs) are negative regulators of gene expression and have been implicated in tumorigenesis and tumor progression. Therefore, HDACs are promising targets for antitumor drugs. However, the relevant isoforms of the 18 members encompassing HDAC family have not been identified. Studies utilizing either gene targeting or knockdown approaches reveal both specific and redundant functions of the closely related class I deacetylases HDAC1 and HDAC2 in the control of proliferation and differentiation. Combined ablation of HDAC1 and HDAC2 in different cell types led to a severe proliferation defects or enhanced apoptosis supporting the idea that both enzymes are relevant targets for tumor therapy. In a recent study on the role of HDAC1 in teratoma formation we have reported a novel and surprising function of HDAC1 in tumorigenesis. In this tumor model HDAC1 attenuates proliferation during teratoma formation. In the present work we discuss new findings on redundant and unique functions of HDAC1 and HDAC2 as regulators of proliferation and tumorigenesis and potential implications for applications of HDAC inhibitors as therapeutic drugs.Key words: tumor therapy, HDAC inhibitor, teratoma, chromatin, epigenetics, proliferation, histone acetylation, tumorigenesis     

Role of class I and class II histone deacetylases in carcinoma cells using siRNA

The role of the individual histone deacetylases (HDACs) in the regulation of cancer cell proliferation was investigated using siRNA-mediated protein knockdown. The siRNA for HDAC3 and HDAC1 demonstrated significant morphological changes in HeLa S3 consistent with those observed with HDAC inhibitors. SiRNA for HDAC 4 or 7 produced no morphological changes in HeLa S3 cells. HDAC1 and 3 siRNA produced a concentration-dependent inhibition of HeLa cell proliferation; whereas, HDAC4 and 7 siRNA showed no effect. HDAC3 siRNA caused histone hyperacetylation and increased the percent of apoptotic cells. These results demonstrate that the Class I HDACs such as HDACs 1 and 3 are important in the regulation of proliferation and survival in cancer cells. These results and the positive preclinical results with non-specific inhibitors of the HDAC enzymes provide further support for the development of Class I selective HDAC inhibitors as cancer therapeutics.     

Histone deacetylase inhibitors: Potential in cancer therapy

The role of histone deacetylases (HDAC) and the potential of these enzymes as therapeutic targets for cancer, neurodegenerative diseases and a number of other disorders is an area of rapidly expanding investigation. There are 18 HDACs in humans. These enzymes are not redundant in function. Eleven of the HDACs are zinc dependent, classified on the basis of homology to yeast HDACs: Class I includes HDACs 1, 2, 3, and 8; Class IIA includes HDACs 4, 5, 7, and 9; Class IIB, HDACs 6 and 10; and Class IV, HDAC 11. Class III HDACs, sirtuins 1–7, have an absolute requirement for NAD⁺, are not zinc dependent and generally not inhibited by compounds that inhibit zinc dependent deacetylases. In addition to histones, HDACs have many nonhistone protein substrates which have a role in regulation of gene expression, cell proliferation, cell migration, cell death, and angiogenesis. HDAC inhibitors (HDACi) have been discovered of different chemical structure. HDACi cause accumulation of acetylated forms of proteins which can alter their structure and function. HDACi can induce different phenotypes in various transformed cells, including growth arrest, apoptosis, reactive oxygen species facilitated cell death and mitotic cell death. Normal cells are relatively resistant to HDACi induced cell death. Several HDACi are in various stages of development, including clinical trials as monotherapy and in combination with other anti‐cancer drugs and radiation. The first HDACi approved by the FDA for cancer therapy is suberoylanilide hydroxamic acid (SAHA, vorinostat, Zolinza), approved for treatment of cutaneous T‐cell lymphoma. J. Cell. Biochem. 107: 600–608, 2009. © 2009 Wiley‐Liss, Inc.     

Distinct and redundant functions of histone deacetylases HDAC1 and HDAC2 in proliferation and tumorigenesis

Histone deacetylases (HDACs) are negative regulators of gene expression and have been implicated in tumorigenesis and tumor progression. Therefore, HDACs are promising targets for anti-tumor drugs. However, the relevant isoforms of the 18 members encompassing HDAC family have not been identified. Studies utilizing either gene targeting or knockdown approaches reveal both specific and redundant functions of the closely related class I deacetylases HDAC1 and HDAC2 in the control of proliferation and differentiation. Combined ablation of HDAC1 and HDAC2 in different cell types led to a severe proliferation defects or enhanced apoptosis supporting the idea that both enzymes are relevant targets for tumor therapy. In a recent study on the role of HDAC1 in teratoma formation we have reported a novel and surprising function of HDAC1 in tumorigenesis. In this tumor model HDAC1 attenuates proliferation during teratoma formation. In the present work we discuss new findings on redundant and unique functions of HDAC1 and HDAC2 as regulators of proliferation and tumorigenesis and potential implications for applications of HDAC inhibitors as therapeutic drugs.     

Histone acetylation-independent effect of histone deacetylase inhibitors on Akt through the reshuffling of protein phosphatase 1 complexes

Despite advances in understanding the role of histone deacetylases (HDACs) in tumorigenesis, the mechanism by which HDAC inhibitors mediate antineoplastic effects remains elusive. Modifications of the histone code alone are not sufficient to account for the antitumor effect of HDAC inhibitors. The present study demonstrates a novel histone acetylation-independent mechanism by which HDAC inhibitors cause Akt dephosphorylation in U87MG glioblastoma and PC-3 prostate cancer cells by disrupting HDAC-protein phosphatase 1 (PP1) complexes. Of four HDAC inhibitors examined, trichostatin A (TSA) and HDAC42 exhibit the highest activity in down-regulating phospho-Akt, followed by suberoylanilide hydroxamic acid, whereas MS-275 shows only a marginal effect at 5 microm. This differential potency parallels the respective activities in inducing tubulin acetylation, a non-histone substrate for HDAC6. Evidence indicates that this Akt dephosphorylation is not mediated through deactivation of upstream kinases or activation of downstream phosphatases. However, the effect of TSA on phospho-Akt can be rescued by PP1 inhibition but not that of protein phosphatase 2A. Immunochemical analyses reveal that TSA blocks specific interactions of PP1 with HDACs 1 and 6, resulting in increased PP1-Akt association. Moreover, we used isozyme-specific small interfering RNAs to confirm the role of HDACs 1 and 6 as key mediators in facilitating Akt dephosphorylation. The selective action of HDAC inhibitors on HDAC-PP1 complexes represents the first example of modulating specific PP1 interactions by small molecule agents. From a clinical perspective, identification of this PP1-facilitated dephosphorylation mechanism underscores the potential use of HDAC inhibitors in lowering the apoptosis threshold for other therapeutic agents through Akt down-regulation.     

Novel inhibitors of human histone deacetylases: Design,synthesis and bioactivity of 3-alkenoylcoumarines

Histone deacetylases (HDACs) are well-established, promising targets for anticancer therapy due to their critical role in cancer development. Accordingly, an increasing number of HDAC inhibitors displaying cytotoxic effects against cancer cells have been reported. Among them, a large panel of chemical structures was described including coumarin-containing molecules. In this study, we described synthesis and biological activity of new coumarin-based derivatives as HDAC inhibitors. Among eight derivatives, three compounds showed HDAC inhibitory activities and antitumor activities against leukemia cell lines without affecting the viability of peripheral blood mononuclear cells from healthy donors.     

HDAC inhibitors: Targets for tumor therapy,immune modulation and lung diseases

Histone deacetylases (HDACs) are enzymes that play a key role in the epigenetic regulation of gene expression by remodeling chromatin. Inhibition of HDACs is a prospective therapeutic approach for reversing epigenetic alteration in several diseases. In preclinical research, numerous types of HDAC inhibitors were discovered to exhibit powerful and selective anticancer properties. However, such research has revealed that the effects of HDAC inhibitors may be far broader and more intricate than previously thought. This review will provide insight into the HDAC inhibitors and their mechanism of action with special emphasis on the significance of HDAC inhibitors in the treatment of Chronic Obstructive Pulmonary Disease and lung cancer. Nanocarrier-mediated HDAC inhibitor delivery and new approaches for targeting HDACs are also discussed.     

A substrate-independent TR-FRET histone deacetylase inhibitor assay

Developing molecularly targeted therapeutics with minimal off-target effects is facilitated by an understanding of compound selectivity. However, for HDAC inhibitors, a clear understanding of specificity has been challenging. In particular, it has been suggested that use of nonspecific substrates and the presence of multiple HDAC activities in enzyme preparations may complicate interpretation of inhibitor experiments. To overcome these and other potential limitations of activity-based HDAC assays, the authors have developed an assay format based on measurement of the binding affinity of inhibitors rather than measurement of enzyme activity. One advantage of this format is that it does not require use of a substrate and thus ameliorates concerns about lack of specificity of existing substrates. This assay is based on an Alexa Fluor? 647-labeled HDAC inhibitor or "tracer," which binds with a high affinity to Class I and Class IIb HDACs. Unlike activity assays, which can be affected by the presence of residual untagged endogenous HDACs from the host expression system, the signal in this format is dependent on the presence of an epitope tag on the specific HDAC of interest. The authors demonstrate the utility of this method by determining the potencies of commonly used inhibitors for six human HDACs.     

Energy based pharmacophore mapping of HDAC inhibitors against class I HDAC enzymes

Histone deacetylases (HDACs) are important class of enzymes that deacetylate the ε-amino group of the lysine residues in the histone tails to form a closed chromatin configuration resulting in the regulation of gene expression. Inhibition of these HDACs enzymes have been identified as one of the promising approaches for cancer treatment. The type-specific inhibition of class I HDAC enzymes is known to elicit improved therapeutic effects and thus, the search for promising type-specific HDAC inhibitors compounds remains an ongoing research interest in cancer drug discovery. Several different strategies are employed to identify the features that could identify the isoform specificity factors in these HDAC enzymes. This study combines the insilico docking and energy-optimized pharmacophore (e-pharmacophore) mapping of several known HDACi's to identify the structural variants that are significant for the interactions against each of the four class I HDAC enzymes. Our hybrid approach shows that all the inhibitors with at least one aromatic ring in their linker regions hold higher affinities against the target enzymes, while those without any aromatic rings remain as poor binders. We hypothesize the e-pharmacophore models for the HDACi's against all the four Class I HDAC enzymes which are not reported elsewhere. The results from this work will be useful in the rational design and virtual screening of more isoform specific HDACi's against the class I HDAC family of proteins.