Histone deacetylases and Cancer

The below-listed papers have been reviewed and their summary is illustrated to construct the essence of histone deacetylases in cancer progression.  

HDAC inhibitor as novel anticancer therapeutics. 

Many of the drugs inhibiting HDACs are in clinical phase-2 and phase-3 trials while one drug for T-Cell lymphoma has been proven by the FDA. It affects DNA histone interaction at the N-terminal of core histone-protein and histone-histone interaction. Its interactive non-histone protein partners are tubulin, Hsp90, beta-catenin, etc. Its high expression leads to cancer, as it suppresses p53 which is a tumor suppressor and due to translocation, HDAC is fused with other proteins (repressor or activator) which recruit it to the gene promoter site. HDAC is divided into four classes (Class 1, Class 2(a, b), Class 3, and Class 4) and 2 families (based on homology) which are the Rdp3/Hda1 family and Sirtuin family (NAD+ Dependent Sir2). Family members show sequence similarity in the deacetylase domain but no homology at all, so different HDAC families are sensitive to different inhibitors. HDACi inhibits angiogenesis by repressing MMPs, and VEGF expression, so inhibition of metastasis step. Knockdown of HDAC expression in tumor cell cause tumor growth disruption such as HDAC1 knockout causing mice death during embryogenesis, HDAC2 knockout showed reduced tumor growth, and HDAC3 knockout was observed as a lethal mutation in embryo development with increased apoptosis. This indicated the essential function of HDAC1 in proliferation control and ‘CDK’ inhibition repression and HDAC3 role in S-phase progression and DNA damage control.

Besides regulating gene expression through the deacetylation of histone proteins by HDAC, they also regulate different cell functions through the deacetylation of non-histone proteins. Such as HDAC-6 deacetylates tubulin, Hsp90, and Cortactin. Hydroxamates HDACi but not romidepsin could be inhibited by Bcl overexpression in lymphoma indicating differential activity (45). Resveratrol (SIRT 1 activator) and Tenovin (SIRT1, 2 inhibitors) could be used as anti-cancer drugs which induce apoptosis in cancer cells. Tubacin specifically targets HDAC-6-inducing apoptosis in myeloma cells.

2. HDAC Interactome Network.

HDAC plays a role in the development of neurodegenerative disease (e.g. Huntington's disease), embryonic development, metastatic cancer, poly q disease, etc. Lysine residue in the N-terminal chain of the core histone protein is deacetylated and Bromodomain, PhD-finger domain of these proteins are recognized as a modified signal of transcription repression or expression. Joshi et al identified 200 novel proteins interacting with HDAC by using label-free affinity purification, mass spectrometry, and computational analysis. SILAC was also done to ascertain the HDAC interaction. HDAC-11 is deregulated during its effect in RNA splicing of introns with SMN protein and complex which lead to the development of spinal muscular atrophy phenotype. 

3. HDAC-6 interacts with and deacetylase tubulin and microtubules in vivo.

HDAC-6 plays a role in mitosis as it is enriched near the mitotic spindle and upregulated during cytokinesis. HDACs are recruited to promoter sequences as a part of the transcription factor complex. Acetylation and deacetylation along with methylation, ubiquitination, and phosphorylation of histone proteins regulate gene expression. Whereas acetylation and deacetylation of non-histone proteins regulate protein stability and indirectly protein function such as deacetylation of an e-amino group of Lys40 residue by HDAC-6 regulate tubulin function. All HDACs proteins have one catalytic “hdac” domain, N-terminal and C-terminal zinc finger domain. Whereas HDAC-6 has two catalytic “hdac” domains but in HDAC-10 one of the two hdac domains is incomplete. Overexpression of HDAC-6 hypo acetylates tubulin and its inhibition of hyperacetylated tubulin. In in-vitro condition, all wild-type colonies (NIH3T3 and HEK293T cell line) containing normal HDAC showed growth on selective media but only 11 cDNA clones were able to grow to contain double mutant HDAC cDNA (H216A, H611A). This indicated that there is a specific domain that is necessary for HDAC activity and the catalytic domain is ‘hdac’ (DD1 and DD2 in HDAC-6) which was analyzed when b-tubulin was precipitated with HDAC-6 having DD1 and DD2 domain. But no precipitation is seen in tubulin interaction with HDAC-6 lacking DD1 and DD2 domain (containing only N or C-terminal). Also, catalytic active centers in the hdac domain do not affect the interaction. HDAC-6 mutant lacking one hdac domain unable to deacetylate tubulin but efficiently interact with tubulin. TSA inhibits the tubulin deacetylation activity of HDAC-6 but it does not affect the interaction. HDAC-6 changes subcellular localization in the presence of stabilizers (taxol) or disruptors (nocodazole) of interactions. Also, taxol (GTP and microtubule stabilizer) is important for stabilizing HDAC-6-microtubule interaction which becomes low in taxol absence. DM1A used for precipitating alpha-tubulin did not disrupt alpha and beta-tubulin interaction, both are precipitated simultaneously. But TU2.1 disrupted this interaction as only beta-tubulin precipitated along with its antibody. This indicates that beta-tubulin might be necessary for interaction with HDAC-6. Latrunculin-B (inhibitor of actin assembly) does not affect the HDAC-6 microtubule interaction. This implies the role of HDAC-6 in mitosis (co-localization with mitotic spindle) and microtubule networking but not in histone deacetylation. No tubulin hyper-acetylation was seen when HEK293T or NIH3T3 cells were treated with Sodium butyrate (inhibitor of HDACs except HDAC-6) but treating cells with TSA (HDAC-6 inhibitor) ensures hyper-acetylation. Also wild type but not mutated HDAC-6 and normal HDAC-5, HDAC-1 is able to deacetylate tubulin in in-vitro deacetylation assay. This strongly indicates the role of HDAC-6 in tubulin deacetylation. HDAC-6 activity is regulated by ‘hdac’ domain interaction and subcellular localization through nuclear import and export signals. HDAC-6 mutant ES cells do not show alteration in morphology from the wild type cell but growth was slower and colony count was low with high expression of p21 and p27 (‘CDK inhibitors having antiproliferative activity). HDAC could be the cause of developing cancer by mediating the deacetylation of histone protein repressing tumor suppressor genes and deacetylation of non-histone proteins (e.g. alpha-tubulin and Hsp90) thus deregulating cellular function.