Inhibition of HDAC

In human colon, prostate, and breast cancer cells, SFN has demonstrated inhibition of HDAC activity.In xenograft models of prostate cancer and osteosarcoma, SFN has inhibited HDAC activity, increased acetylation levels of H3 and H4, and a slowing of growth." " In vivo results confirm that SFN is an effective chemoprotective... 

Inhibition of HDACs has become an interesting approach for anticancer therapy. Therefore, two approaches, gene knockdown in cells and knockout in animals, have been developed and widely used for functional studies of specific HDAC isoforms. 

The family of HDAC enzymes has been named after their first substrate identified, i.e., the nuclear histone proteins. Histone proteins (H2A, H2B, H3 and H4) form an octamer complex, around which the DNA helix is wrapped in order to establish a condensed chromatin structure. The acetylation status of histones is in a dynamic equilibrium governed by histone acetyl transferases (HATs), which acetylate and HDACs which are responsible for the deacetylation of histone tails (Fig. 1). Inhibition of the HDAC enzyme promotes the acetylation of nucleosome histone tails, favoring a more transcriptionally competent chromatin structure, which in turn leads to altered expression of genes involved in cellular processes such as cell prohferation, apoptosis and differentiation. Inhibition of HDAC activity results in the activation of only a limited set of pre-programmed genes microarray experiments have shown that 2% of all genes are activated by structmally different HDAC inhibitors [1-5]. In recent years, a growing number of additional nonhistone HDAC substrates have been identified, which will be discussed in more detail below. 

HDACs are zinc metalloproteases involved in the acetylation of histone. Inhibition of HDACs represents a new strategy in human cancer therapy since these enzymes play a fundamental role in regulating gene expression and chromatin assembly. Along this line, inhibition of HD AC by fluoroketones has been studied. The inhibition power of fluoroketones toward HDACs is comparable to that of hydroxamates, which are the classical inhibitors of metalloproteases. These fluoroketones exhibit antiproliferative activities on tumor cell lines (Figure 7.44). ... 

Inhibition of HDACs is one key mechanism to reactivate the expression of these misregulated genes. The astounding tumor specificity of many HDAC inhibitors relays the potential for many of these new compounds for the treatment of cancer and perhaps other disorders. There are five classes of HDAC inhibitors (reviewed in Refs. 51 and 52) including (i) short-chain fatty adds such as sodium- -butyrate (ii) hydroxyamic acids, such as trichostatin A (TSA), suberoylanilide hydroxamic add (SAHA), m-carboxycinnamic acid bishydroxamic acid (CBHA), azelaic bishydroxamic acid (ABHA), and... 

In general, transcriptional activators can bind and recruit HATs while transcriptional repressors and corepressors interact with HDACs. The unwinding of DNA offhistones by lysine acetylation is conceptually helpful for understanding the action of HATs and HDACs. It is, nevertheless, a simplistic and incomplete explanation for the way in which these enzymes control gene expression. For example, in some cases [4] inhibition of HDACs can lead to a counterintuitive decrease in gene expression. It is likely that the overall pattern of histone modification (of which acetylation is but one example) represents... 

Histone deacetylase inhibitors, peptides that inhibit histone deacetylases (HDAC). Inhibition of HDAC effects cell-cycle arrest and induces differentiation. Therefore, HDAC is considered a target for new types of pharmaceuticals for treatment of cancers, and several naturally occurring or synthetic peptides have been reported as lead structures. The most prominent family is a series of cyclic tetrapeptides containing the non-proteinogenic amino acid Aoe [(2S,9S)-2-amino-8-oxo-9,10-epoxydecanoic... 

Histone deacetylases (HDAC) are a class of enzymes that catalyze the removal of acetyl groups from the A -acetylated lysine residues of histone. Histones are the major protein components of chromatin, act as spools around which DNA winds, and play an important role in gene regulation. As certain tumors overexpress HDAC, inhibition of HDAC results in accumulation of acetylated histones, thereby causing cell cycle arrest and apoptosis. 

Table 4.3 IC50 values (nM) for inhibition of HDACs from HeLa nuclear extracts and individual HDAC isoforms (where available) by thiol forms of members of the FK228 family of depsipeptide natural products and growth inhibition of the MCF7 breast cancer cell line by the prodrug natural products (data taken from ref. 68, 74 and 75).

All four members of this class of HDAC have been demonstrated to be sensitive to HDAC-specific inhibitors (HDACi) such as trichostatin A (TSA). It is noteworthy that the HDACi Valproic acid (VPA), in addition to selectively inhibiting the catalytic activity of class I HDACs, induces proteasomal degradation of HDAC2, in contrast to other inhibitors such as TSA (Kramer et al, 2003). 

Figure 4. Therapeutic strategies to counteract CBP loss of function. CBP loss of function leads to a decrease in histone acetylation levels as well as a decrease in CBP-dependent transcription. Two main approaches can be tested to reverse diis process either resetting HAT functionality or resetting global acetylation levels widi die use of HDAC inhibitors. Whereas both strategies would increase histone acetylation levels, HDAC inhibition would act on a broad range of genes, while CBP activation (overexpression or by a pharmacological approach) would specifically target bodi CBP-dependent histone acetylation and transcription. The structure of some of the HDACi that have been tested in different models, such as small fatty acids and hydroxamic acids, are represented in the boxes...

Flavone has been shown to be effective at 170 /iM concentration to inhibit HDAC-1 activity, and also to induce histone H3 acetylation in NB4 APL cell lines. Flavone s mechanism of HDAC inhibition is currently unknown, but it has been proposed that similar to the inhibitors of kinases, it might occupy the exit channel or an alternative, shallow binding site found in HDAC with possible effects of causing allosteric modulation. This blockage or allosteric modulation would promote inhibition. ... 

Apicidin (Fig. 10) was first isolated from fimgus Fusarium pallidoroseum ATCC 74289, MF6040, from Acacia sp. (collected from Santa Rosa National Park Costa Rica) as an antiprotozoal agent. It was found that apicidin was active against wide range of protozoans specially Apicomplexan parasites. The underlying mechanism for this cidal activity was found out to be HDAC inhibition of protozoas. It was also observed that they were very potent HDAC inhibitors when compared to similar cyclopeptide inhibitors known at that time (Table 5). 

After 5 years in the FDA s fast track development program, Romidepsin (Fig. 18) was approved by the FDA for refractory cutaneous T-cell lymphoma on November 6,2009. In the literature, romidepsin has also been called depsipeptide, FK228, FR901228, and NSC-630176. It was isolated from bacterial fermentation extracts from Chromobacterium violaceum and is a potent inhibitor of HDAC. In some human cancer cell lines, romidepsin inhibits HDACs at levels ten times that of TSA. 

Normally corticosteroids are used for COPD treatment. The corticosteroids bind to glucocorticoid receptors and enter the nucleus of the cells, where it recruits HDAC-2. HDAC-2 deacetylates the chromatin and represses gene expression of inflammatory genes. Thus, low levels of HDAC-2 is detrimental to the treatment of COPD. Antioxidants (for removal of superoxides) or iNOS inhibitor or theophylline/curcumin for PI3K inhibition are some of the options to restore the required HDAC-2 level. 

The use of plants for medicinal purposes is an ancient practice. Nature, with its wealth of traditional knowledge has been the source of inspiration for numerous drugs currently used for the improvement of life as well as treatment for a cure. Considering the beneficial role of many plants and fruits, they were included in the human diets. In many instances, the knowledge of the underlying mechanism of action of a particular natural product is incomplete. Continuous investigation can lead to new mechanisms and new structures, which may open up entirely new windows and perspectives. For instance, before the discovery of apicidin and bispyri-dinium diene, it was believed that unless there is a classical chelator for zinc ion, it cannot be a HDAC inhibitor. SAHA has been approved by FDA, which is inspired from the natural product trichostatin. The natural product, romidepsin has also been approved by FDA and many are on clinical trials. Currently, isozyme-selective inhibition for HDAC is at its nascent stage. The invention of some novel molecules or invention of some novel natural product structures with synthetic modifications will solve the problem. 

Benzamides constitute a fourth dass of HDAC inhibitors. One example, MS-275, is a phenylenediamine derivative that exhibits robust HDAC inhibition in patients with advanced myeloid leukemia as well as refractory solid tumors or lymphoma in Phase I studies [72]. MS-275 is currently in Phase II trials. In a recent study aimed at optimizing the benzamide scaffold, several bis-(aryl) type analogs were synthesized and evaluated for their activity against a panel of HDACs [85]. Moradei et al. found that a thienyl substitution para to the free amino group in the phenylenediamine core rendered inhibitors specific for HDACsl, 2 with potency superior to that of MS-275. Isoform-specific inhibitors should aid in dissecting the roles of HDACs in normal cellular fundioning and cancer. 

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