The Epigenetic Modifier,Valproic Acid,Enhances Radiation Sensitivity

Valproic acid is an established therapeutic for a variety of seizure disorders and incertain cases for depression and anxiety. In addition, valproic acid has been shown topossess histone deacetylase inhibition activity and is currently being investigated asan anti-cancer agent, either alone or in combination with other conventional cancertherapies such as ionizing radiation. In this study, we investigated whether valproicacid modulates cellular responses to radiation in human erythroleukemic, K562 cells.Hyperacetylation of nuclear histones 3 and 4 was used to correlate the effects ofvalproic acid to inhibition of histone deacetylase activity, clonogenic survival,apoptosis and apoptosis. The findings from the clonogenic survival and caspaseinduction assays indicated that pre-treatment of cells with valproic acid for 24 hours,markedly enhanced radiation induced cell-death and apoptosis in K562 cells,respectively. Mechanisms involving drug-mediated cytotoxicity and changes in cellcycle distribution were associated with the radiation sensitizing properties of valproicacid, particularly at the higher concentrations. Overall, our findings are consistentwith the general consensus that HDAC inhibitors efficiently sensitize cancer cells tothe effects of ionizing radiation and support the idea of developing clinically relevantcombinations of HDAC inhibitors and radiotherapy.     

Protective effects of valproic acid against airway hyperresponsiveness and airway remodeling in a mouse model of allergic airways disease

Airway remodeling and airway hyperresponsiveness are major aspects of asthma pathology that are not targeted optimally by existing anti-inflammatory drugs. Histone deacetylase inhibitors have a wide range of effects that may potentially abrogate aspects of remodeling. One such histone deacetylase inhibitor is valproic acid (2-propylvaleric acid). Valproic acid is used clinically as an anti-epileptic drug and is a potent inhibitor of class I histone deacetylases but also inhibits class II histone deacetylases. We used valproic acid as a molecular model of histone deacetylase inhibition in vivo in chronic allergic airways disease mice with airway remodeling and airway hyperresponsiveness. Wild-type Balb/c mice with allergic airways disease were treated with valproic acid or vehicle control. Airway inflammation was assessed by bronchoalveolar lavage fluid cell counts and examination of lung tissue sections. Remodeling was assessed by morphometric analysis of histochemically stained slides and lung function was assessed by invasive plethysmography measurement of airway resistance. Valproic acid treatment did not affect inflammation parameters; however, valproic acid treatment resulted in reduced epithelial thickness as compared to vehicle treated mice

(p < 0.01), reduced subepithelial collagen deposition (p < 0.05) and attenuated airway hyperresponsiveness (p < 0.05 and p < 0.01 for the two highest doses of methacholine, respectively). These findings show that treatment with valproic acid can reduce structural airway remodeling changes and hyperresponsiveness, providing further evidence for the potential use of histone deacetylase inhibitors for the treatment of asthma.     

Protective effects of valproic acid against airway hyperresponsiveness and airway remodeling in a mouse model of allergic airways disease

Airway remodeling and airway hyperresponsiveness are major aspects of asthma pathology that are not targeted optimally by existing anti-inflammatory drugs. Histone deacetylase inhibitors have a wide range of effects that may potentially abrogate aspects of remodeling. One such histone deacetylase inhibitor is valproic acid (2-propylvaleric acid). Valproic acid is used clinically as an anti-epileptic drug and is a potent inhibitor of class I histone deacetylases but also inhibits class II histone deacetylases. We used valproic acid as a molecular model of histone deacetylase inhibition in vivo in chronic allergic airways disease mice with airway remodeling and airway hyperresponsiveness. Wild-type Balb/c mice with allergic airways disease were treated with valproic acid or vehicle control. Airway inflammation was assessed by bronchoalveolar lavage fluid cell counts and examination of lung tissue sections. Remodeling was assessed by morphometric analysis of histochemically stained slides and lung function was assessed by invasive plethysmography measurement of airway resistance. Valproic acid treatment did not affect inflammation parameters; however, valproic acid treatment resulted in reduced epithelial thickness as compared to vehicle treated mice (p < 0.01), reduced subepithelial collagen deposition (p < 0.05) and attenuated airway hyperresponsiveness (p < 0.05 and p < 0.01 for the two highest doses of methacholine, respectively). These findings show that treatment with valproic acid can reduce structural airway remodeling changes and hyperresponsiveness, providing further evidence for the potential use of histone deacetylase inhibitors for the treatment of asthma.     

Histone deacetylase inhibitors protect against and mitigate the lethality of total-body irradiation in mice

It was hypothesized that histone deacetylase (HDAC) inhibitors may increase survival after total-body irradiation (TBI) based on previous reports demonstrating that HDAC inhibitors stimulate the proliferation of bone marrow stem cells. Using the time for mice to lose 20% or more of their weight as the end point, two HDAC inhibitors, valproic acid and trichostatin-A, were found to reduce lethality in a dose-dependent manner. HDAC inhibitors were effective at reducing lethality when given either 24 h before or 1 h after TBI. The results indicate that HDAC inhibitors have potential for protecting against and mitigating radiation-induced lethality.     

Histone Deacetylase Inhibitor Valproic Acid Promotes the Differentiation of Human Induced Pluripotent Stem Cells into Hepatocyte-Like Cells

In this study, we aimed to elucidate the effects and mechanism of action of valproic acid on hepatic differentiation from human induced pluripotent stem cell-derived hepatic progenitor cells. Human induced pluripotent stem cells were differentiated into endodermal cells in the presence of activin A and then into hepatic progenitor cells using dimethyl sulfoxide. Hepatic progenitor cells were matured in the presence of hepatocyte growth factor, oncostatin M, and dexamethasone with valproic acid that was added during the maturation process. After 25 days of differentiation, cells expressed hepatic marker genes and drug-metabolizing enzymes and exhibited drug-metabolizing enzyme activities. These expression levels and activities were increased by treatment with valproic acid, the timing and duration of which were important parameters to promote differentiation from human induced pluripotent stem cell-derived hepatic progenitor cells into hepatocytes. Valproic acid inhibited histone deacetylase activity during differentiation of human induced pluripotent stem cells, and other histone deacetylase inhibitors also enhanced differentiation into hepatocytes. In conclusion, histone deacetylase inhibitors such as valproic acid can be used to promote hepatic differentiation from human induced pluripotent stem cell-derived hepatic progenitor cells.     

Valproic acid synergistically enhances the cytotoxicity of gossypol in DU145 prostate cancer cells: an iTRTAQ-based quantitative proteomic analysis

Gossypol (GOS), a BH3 mimetic, has been investigated as a sensitizing co-therapy to radiation and chemotherapy in treatment of metastatic prostate cancer. In this study, we found that valproic acid (VPA), a histone deacetylase inhibitor (HDACI), counteracted the suppressive effect of GOS on histone H3 acetylation and enhanced the cytotoxicity of GOS to DU145 prostate cancer cells. Significant synergistic effects were observed in combined GOS and VPA treatment, culminating in more DNA damage and cell death. The iTRAQ-based quantitative proteomic analysis revealed differential proteomic profiles in cells treated with VPA, GOS or their combination. In GOS-treated cells, oxidative phosphorylation-related proteins were depressed and endoplasmic reticulum stress markers were upregulated. In the presence of VPA, the GOS-induced mitochondrial stress was further enhanced since glycolysis- and hypoxia-associated proteins were upregulated, suggesting a disruption of energy metabolism in these cells. Furthermore, the DNA damage repair ability of cells co-treated with GOS and VPA was also decreased, as evidenced by the downregulation of DNA damage repair proteins and the enhancement of DNA fragmentation and cell death. These findings suggest that GOS in combination with an HDACI has the potential to increase its clinical efficacy in the treatment of prostate cancer.     

Sensing of ionizing radiation-induced DNA damage by ATM through interaction with histone deacetylase.

The ATM gene is mutated in individuals with ataxia telangiectasia, a human genetic disease characterized by extreme sensitivity to radiation. The ATM protein acts as a sensor of radiation-induced cellular damage and contributes to cell cycle regulation, signal transduction, and DNA repair; however, the mechanisms underlying these functions of ATM remain largely unknown. Binding and immunoprecipitation assays have now shown that ATM interacts with the histone deacetylase HDAC1 both in vitro and in vivo, and that the extent of this association is increased after exposure of MRC5CV1 human fibroblasts to ionizing radiation. Histone deacetylase activity was also detected in immunoprecipitates prepared from these cells with antibodies to ATM, and this activity was blocked by the histone deacetylase inhibitor trichostatin A. These results suggest a previously unanticipated role for ATM in the modification of chromatin components in response to ionizing radiation.     

Sequence-specific potentiation of topoisomerase II inhibitors by the histone deacetylase inhibitor suberoylanilide hydroxamic acid

Acetylation of histones leads to conformational changes of DNA. We have previously shown that the histone deacetylase (HDAC) inhibitor, suberoylanilide hydroxamic acid (SAHA), induced cell cycle arrest, differentiation, and apoptosis. In addition to their antitumor effects as single agents, HDAC inhibitors may cause conformational changes in the chromatin, rendering the DNA more vulnerable to DNA damaging agents. We examined the effects of SAHA on cell death induced by topo II inhibitors in breast cancer cell lines. Topo II inhibitors stabilize the topo II-DNA complex, resulting in DNA damage. Treatment of cells with SAHA promoted chromatin decondensation associated with increased nuclear concentration and DNA binding of the topo II inhibitor and subsequent potentiation of DNA damage. While SAHA-induced histone hyperacetylation occurred as early as 4 h, chromatin decondensation was most profound at 48 h. SAHA-induced potentiation of topo II inhibitors was sequence-specific. Pre-exposure of cells to SAHA for 48 h was synergistic, whereas shorter pre-exposure periods abrogated synergy and exposure of cells to SAHA after the topo II inhibitor resulted in antagonistic effects. Synergy was not observed in cells with depleted topo II levels. These effects were not limited to specific types of topo II inhibitors. We propose that SAHA significantly potentiates the DNA damage induced by topo II inhibitors; however, synergy is dependent on the sequence of drug administration and the expression of the target. These findings may impact the clinical development of combining HDAC inhibitors with DNA damaging agents.