Immunomodulatory effects of histone deacetylase inhibitors

Histone deacetylase inhibitors (HDACi) have emerged as a new generation of anticancer therapeutics. The classical broad-spectrum HDACi typically alter the cell cycle distribution and induce cell death, apoptosis and differentiation in malignant and transformed cells. This provides the basis for the clinical potential of HDACi in cancer therapy. Currently two compounds, suberoylanilide hydroxamic acid (SAHA, Vorinostat, Zolinza™) and depsipeptide (Romidepsin, Istodax™) have been approved for by the US Food and Drug Administration for the treatment of refractory cutaneous T-cell lymphoma. Apart from clinical application in oncology, HDACi have also been investigated as potential therapeutics for various pathologies including those of the central nervous system (such as Huntington's disease and multiple sclerosis), cardiac conditions (particularly hypertrophy), arthritis and malaria. Further, evidence is accumulating for potent immunomodulatory effects of classical HDACi which is the focus of this review. We review the antiinflammatory effects of HDACi and in particular findings implicating regulation of the innate and adaptive immune systems by HDAC enzymes. The recent findings highlighting the immunomodulatory function of HDAC11 which relates to balancing immune activation versus tolerance are also discussed.

Controversies surrounding the clinical potential of cinnamon for the management of diabetes

Obesity levels have increased significantly in the past five decades and are predicted to continue rising, resulting in important health implications. In particular, this has translated to an increase in the occurrence of type II diabetes mellitus (T2D). To alleviate associated problems, certain nutraceuticals have been considered as potential adjuncts or alternatives to conventional prescription drugs. Cinnamon, a commonly consumed spice originating from South East Asia, is currently being investigated as a potential preventative supplement and treatment for insulin resistance, metabolic syndrome and T2D. Extensive in vitro evidence has shown that cinnamon may improve insulin resistance by preventing and reversing impairments in insulin signalling in skeletal muscle. In adipose tissue, it has been shown that cinnamon increases the expression of peroxisome proliferator-activated receptors including, PPARγ. This is comparable to the action of commonly used thiazolinediones, which are PPAR agonists. Studies have also shown that cinnamon has potent anti-inflammatory properties. However, numerous human clinical trials with cinnamon have been conducted with varying findings. While some studies have showed no beneficial effect, others have indicated improvements in cholesterol levels, systolic blood pressure, insulin sensitivity and postprandial glucose levels with cinnamon. However, the only measurement consistently improved by cinnamon consumption is fasting glucose levels. While it is still premature to suggest the use of cinnamon supplementation based on the evidence, further investigation into mechanisms of action is warranted. Apart from further characterization of genetic and epigenetic changes in model systems, systematic large-scale clinical trials are required. In this study, we discuss the mechanisms of action of cinnamon in the context of T2D and we highlight some of the associated controversies.

Disparity of histone deacetylase inhibition on repair of radiation-induced DNA damage on euchromatin and constitutive heterochromatin compartments

Epigenetic regulation of chromatin structure is central to the process of DNA repair. A well-characterized epigenetic feature is the dynamic phosphorylation of the histone H2AX (gammaH2AX) and mobilization of double strand break (DSB) recognition and repair factors to the site. How chromatin structure is altered in response to DNA damage and how such alterations influence DSB repair mechanisms are currently relevant issues. Despite the clear link between histone deacetylases (HDACs) and radiosensitivity, how histone hyperacetylation influence DSB repair remains poorly understood. We have determined the structure of chromatin is a major factor determining radiosensitivity and repair in human cells. Trichostatin A (TSA) enhances radiosensitivity with dose modification factors of 1.2 and 1.9 at 0.2 and 1 microM, respectively. Cells treated with TSA causing hyperacetylation and remodelling on euchromatic alleles coexist with gammaH2AX accumulation in radiosensitized cells. Formation of gammaH2AX on heterochromatin was significantly reduced even when cells were treated with TSA, suggesting that chromatin structure and histone hyperacetylation are pronounced features of radiation sensitivity and repair in euchromatic regions.

Chromatin modifications and DNA double-strand breaks: the current state of play

The packaging and compaction of DNA into chromatin is important for all DNA-metabolism processes such as transcription, replication and repair. The involvement of chromatin modifications in transcriptional regulation is relatively well characterized, and the distinct patterns of chromatin transitions that guide the process are thought to be the result of a code on the histone proteins (histone code). In contrast to transcription, the intricate link between chromatin and responses to DNA damage has been given attention only recently. It is now emerging that specific ATP-dependent chromatin remodeling complexes (including the Ino80, Swi/Snf and RSC remodelers) and certain constitutive (methylation of lysine 79 of histone H3) and DNA damage-induced covalent histone modifications (the most well characterized being the rapid phosphorylation of histone H2A) facilitate responses to double-strand breaks. Indeed, evidence is already accumulating for a DNA repair-specific histone code. In this review, the recent advances in our understanding of the relationship between chromatin modifications and double-strand break signaling and repair is discussed.

Will broad-spectrum histone deacetylase inhibitors be superseded by more specific compounds?

Histone deacetylase (HDAC) inhibitors can induce differentiation, cell cycle and growth arrest or in certain cases apoptosis in cancer cells. In a remarkably short period of time, especially considering that their mechanism of action remains largely undefined, HDAC inhibitors have realized both success and failure as therapeutics for cancer in clinical trials. Notably, the pleiotropic HDAC inhibitors, suberoylanilide hydroxamic acid (SAHA) and depsipeptide, have shown efficacy in a wide range of cancers, in particular for cutaneous T-cell lymphoma (CTCL), and are progressing in phase II clinical studies. However, evidence is accumulating that specific HDAC enzymes are important with respect to clinical efficacy, calling the usefulness of the classical inhibitors into question. Class I enzymes are being heralded as the most clinically relevant, however, this is still controversial and much of the information is in the private domain. Nevertheless, the potential to alter the expression of a more focused, disease-related subset of genes and to limit adverse effects has prompted the development of isoform-specific HDAC inhibitors. Here, we consider the growing view that broad-spectrum HDAC inhibitors may be superseded by more specific compounds.

Modulation of cellular radiation responses by histone deacetylase inhibitors

Histone deacetylase (HDAC) inhibitors are emerging as a new class of targeted cancer chemotherapeutics. Several HDAC inhibitors are currently in clinical trials and promising anticancer effects at well-tolerated doses have been observed for both hematologic and solid cancers. HDAC inhibitors have been shown to induce cell-cycle and growth arrest, differentiation and in certain cases apoptosis in cell cultures and in vivo. However, it is known that these compounds induce varying responses in different cells and biological settings, and identifying their precise mechanisms of action is an area of great interest. Important findings are continually expanding our understanding of the cellular effects of HDAC inhibitors and recent studies will be briefly outlined in this review. In addition to their intrinsic anticancer properties, numerous studies have demonstrated that HDAC inhibitors can modulate cellular responses to other cytotoxic modalities including ionizing radiation, ultraviolet radiation and chemotherapeutic drugs. Hence, there is a growing interest in potential clinical use of HDAC inhibitors in combination with conventional cancer therapies. In this review, the interaction of HDAC inhibitors with other anticancer agents is discussed. The focus of the article is on the different mechanisms by which HDAC inhibitors enhance the sensitivity of cells to the effects of ionizing radiation.

Double-strand breaks: signaling pathways and repair mechanisms

Double-strand breaks arise frequently in the course of endogenous - normal and pathological - cellular DNA metabolism or can result from exogenous agents such as ionizing radiation. It is generally accepted that these lesions represent one of the most severe types of DNA damage with respect to preservation of genomic integrity. Therefore, cells have evolved complex mechanisms that include cell-cycle arrest, activation of various genes, including those associated with DNA repair, and in certain cases induction of the apoptotic pathway to respond to double-strand breaks. In this review we discuss recent progress in our understanding of cellular responses to DNA double-strand breaks. In addition to an analysis of the current paradigms of detection, signaling and repair, insights into the significance of chromatin remodeling in the double-strand break-response pathways are provided.

Cytotoxicity of an 125I-labelled DNA ligand

The subcellular distribution and cytotoxicity of a DNA-binding ligand [125I]-Hoechst 33258 following incubation of K562 cells with the drug was investigated. The ability of a radical scavenger, dimethyl sulphoxide, to protect cells from the 125I-decay induced cell death was also studied. Three different concentrations and specific activities of the drug were used to provide different ligand : DNA binding ratios. The results demonstrated a trend toward improved delivery of the ligand to the nucleus and to chromatin at higher ligand concentrations, with concomitant increased sensitivity to 125I-decay induced cytotoxicity and decreased protection by dimethyl sulphoxide. This correlation of radiobiological parameters with subcellular drug distribution is consistent with the classical dogma that attributes cytotoxicity to DNA double-stranded breakage in the vicinity of the site of decay, where the high LET nature of the damage confers minimal sensitivity to radical scavenging.