Vorinostat, a histone deacetylase inhibitor, enhances the response of human tumor cells to ionizing radiation through prolongation of γ-H2AX foci

Abstract

Vorinostat (suberoylanilide hydroxamic acid) is the prototype of a family of hybrid polar compounds that can induce growth arrest in transformed cells and shows promise for the treatment of cancer. Vorinostat specifically binds to and inhibits the activity of histone deacetylases resulting in acetylation of nucleosomal histones and an activation of gene transcription. Because histone deacetylases modulate chromatin structure and gene expression, both of which can influence radioresponse, this study was designed to examine the capacity of Vorinostat to influence radiation response in human tumor cells and investigate the mechanism underlying these interactions. Vorinostat induced hyperacetylation of histone H4 in a dose-dependent manner. We tested its ability to radiosensitize three human tumor cell lines (A375, MeWo, and A549) using clonogenic cell survival assays. Clonogenic cell survival assay showed that Vorinostat significantly radiosensitized all three tumor cell lines, substantially reducing the surviving fraction at 2 Gy. We examined potential mechanisms that may contribute to the enhanced radiation response induced by Vorinostat. Vorinostat and radiation alone did not induce apoptosis in the melanoma cell line. However, enhanced apoptosis was observed when cells were exposed to both Vorinostat and radiation, suggesting that Vorinostat renders tumor cells more susceptible to radiation-induced apoptosis. Results from DNA damage repair analysis in cultured A375 cells showed that Vorinostat had a strong inhibitory effect on the nonhomologous end joining pathway after radiation. A detailed examination of the involvement of the DNA repair pathway following Vorinostat treatment showed that Vorinostat reduced the expression of the repair-related genes Ku70, Ku80, and Rad50 in A375 cells as detected by Western blot analysis. We also examined ;-H2AX phosphorylation as a predictive marker of radiotherapy response to Vorinostat and observed that the combination of Vorinostat and radiation caused a prolongation of expression of DNA repair proteins such as ;-H2AX. Overall, we conclude that Vorinostat enhances tumor radioresponse by multiple mechanisms that may involve antiproliferative growth inhibition and effects on DNA repair after exposure to radiation.

p21Waf1is required for cellular senescence but not for cell cycle arrest induced by the HDAC inhibitor sodium butyrate

“…The fact that γH2AX foci accumulate in long-term treated cells can indicate that HDACI are able either to down-modulate DNA repair or to cause accumulation of γH2AX foci. [15][16][17] Gene expression profiling also demonstrated that a subset of genes monitoring cell cycle, DNA replication and damage repair are downregulated after HDACI exposure. 17 Taken together, these data serve as evidence that accumulation of senescence-associated γH2AX foci can be a trigger of hyperactivation of pathways involved in cell senescence, as was recently proposed on a mouse model, 18 or its consequence.…”

Histone deacetylase inhibitors and genomic instability

“…In mouse xenograft models, the combination of HDACI treatment (MS-275 [42], valproic acid [43], LBH589 [44], LAQ824 [45]and AN-9 [46]) with radiation therapy, results in a greater delay in tumor growth than that accounted for by a simple additive effect of the corresponding individual treatments. HDAC inhibition by LBH589 [44], TSA [47], depsipeptide [48] and SAHA [49] stabilizes and enhances IR-induced phosphorylated histone H2AX nuclear foci, classical markers of DSBs, suggesting that HDACIs inhibit DSB repair and/or render DNA more susceptible to IR-induced damage. HDACIs facilitate radiation-induced killing in tumor cells and at the same time appear to protect healthy tissues from cutaneous radiation syndrome.…”

Pharmaco-epigenomics: discovering therapeutic approaches and biomarkers for cancer therapy

“…Molecular mechanisms underlying this radiosensitizing potential are not yet fully understood, but can be partially explained by the silencing of DNA-repair genes. The HDAC inhibitor sodium butyrate is reported to aggravate radiation-induced damage and enhance apoptosis by inactivation of repair-related genes, such as Ku70 and Ku86 (Munshi et al, 2006). Similarly, treatment with another HDAC inhibitor, Vorinostat, can prolong the appearance of repair foci identified by phosphorylated Histone 2AX (g-H2AX), which is indicative of reduced repair efficiency and increased radiosensitivity (Munshi et al, 2006).…”