Histone deacetylase inhibitor-mediated radiosensitization of human cancer cells: class differences and the potential influence of p53

Chromatin-remodelling mechanisms in cancer

Chromatin-remodelling mechanisms include DNA methylation, histone-tail acetylation, poly-ADP-ribosylation, and ATP-dependent chromatin-remodelling processes. Some epigenetic modifications among others have been observed in cancer cells, namely (1) local DNA hypermethylation and global hypomethylation, (2) alteration in histone acetylation/deacetylation balance, (3) increased or decreased poly-ADP-ribosylation, and (4) failures in ATP-dependent chromatin-remodelling mechanisms. Moreover, these alterations can influence the response to classical anti-tumour treatments. Drugs targeting epigenetic alterations are under development. Currently, DNA methylation and histone deacetylase inhibitors are in use in cancer therapy, and poly-ADP-ribosylation inhibitors are undergoing clinical trials. Epigenetic therapy is gaining in importance in pharmacology as a new tool to improve anti-cancer therapies.

Phenylbutyrate sensitizes human glioblastoma cells lacking wild-type p53 function to ionizing radiation

PURPOSE

Histone deacetylase (HDAC) inhibitors induce growth arrest, differentiation, and apoptosis in cancer cells. Phenylbutyrate (PB) is a HDAC inhibitor used clinically for treatment of urea cycle disorders. Because of its low cytotoxicity, cerebrospinal fluid penetration, and high oral bioavailability, we investigated PB as a potential radiation sensitizer in human glioblastoma cell lines.

METHODS AND MATERIALS

Four glioblastoma cell lines were selected for this study. Phenylbutyrate was used at a concentration of 2 mM, which is achievable in humans. Western blots were used to assess levels of acetylated histone H3 in tumor cells after treatment with PB. Flow cytometry was used for cell cycle analysis. Clonogenic assays were performed to assess the effect of PB on radiation sensitivity. We used shRNA against p53 to study the role of p53 in radiosensitization.

RESULTS

Treatment with PB alone resulted in hyperacetylation of histones, confirmed by Western blot analysis. The PB alone resulted in cytostatic effects in three cell lines. There was no evidence of G(1) arrest, increase in sub-G(1) fraction or p21 protein induction. Clonogenic assays showed radiosensitization in two lines harboring p53 mutations, with enhancement ratios (+/- SE) of 1.5 (+/- 0.2) and 1.3 (+/- 0.1), respectively. There was no radiopotentiating effect in two cell lines with wild-type p53, but knockdown of wild-type p53 resulted in radiosensitization by PB.

CONCLUSIONS

Phenylbutyrate can produce p21-independent cytostasis, and enhances radiation sensitivity in p53 mutant human glioblastoma cells in vitro. This suggests the potential application of combined PB and radiotherapy in glioblastoma harboring mutant p53.

Inhibition of telomerase activity and human telomerase reverse transcriptase gene expression by histone deacetylase inhibitor in human brain cancer cells

The aim of the present study is to investigate the effect of histone deacetylase inhibitor, trichostatin A (TSA) on the cell growth, apoptosis, genomic DNA damage and the expression of telomerase and associated factors in human normal and brain cancer cells. Here, human normal un-transformed fibroblasts (MRC-5), human normal hTERT-immortalised fibroblasts (hTERT-BJ1) and human brain cancer cell lines (glioblastoma cell line, A-172 and medulloblastoma cell line, ONS-76) were treated with 0.5-3.0microM TSA for 24h. Exposure to TSA resulted in apoptosis in a dose-dependent manner in the brain cancer cells. Glioblastoma cell line (A-172) displayed higher sensitivity to TSA-induced cell killing effect and apoptosis than the medulloblastoma cell line (ONS-76). The brain cancer cell lines and hTERT-BJ1 cell line displayed significant inhibition in telomerase activity and hTERT mRNA level after 2microM TSA treatment. Elevated expressions of p53 and p21 with a decrease in cyclin-D level supported the observation on cell cycle arrest following TSA treatment. Upregulation of Bax and cytochrome c correlated with the apoptotic events in TSA-treated cells. This study suggests that telomerase and hTERT might be the primary targets of TSA which may have the potential to be used as a telomerase inhibitor in cancer therapy.

The regulation of cyclin D1 degradation: roles in cancer development and the potential for therapeutic invention

Cyclin D1 is an important regulator of cell cycle progression and can function as a transcriptionl co-regulator. The overexpression of cyclin D1 has been linked to the development and progression of cancer. Deregulated cyclin D1 degradation appears to be responsible for the increased levels of cyclin D1 in several cancers. Recent findings have identified novel mechanisms involved in the regulation of cyclin D1 stability. A number of therapeutic agents have been shown to induce cyclin D1 degradation. The therapeutic ablation of cyclin D1 may be useful for the prevention and treatment of cancer. In this review, current knowledge on the regulation of cyclin D1 degradation is discussed. Novel insights into cyclin D1 degradation are also discussed in the context of ablative therapy. A number of unresolved questions regarding the regulation of cellular cyclin D1 levels are also addressed.

Histone deacetylase inhibitors--turning epigenic mechanisms of gene regulation into tools of therapeutic intervention in malignant and other diseases

Histone deacetylase inhibitors reside among the most promising targeted anticancer agents that are potent inducers of growth arrest, differentiation, and/or apoptotic cell death of transformed cells. In October 2006, the US Food and Drug Administration approved the first drug of this new class, vorinostat (1, Zolinza, Merck). Several histone deacetylase (HDAC) inhibitors more are in clinical trials. HDAC inhibitors have shown significant activity against a variety of hematological and solid tumors at doses that are well tolerated by patients, both in monotherapy as well as in combination therapy with other drugs. This paper reviews the most recent developments in HDAC inhibitor design, particularly in the context of anticancer therapy, and other possible pharmaceutical applications.

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.