HDAC Inhibitor, Valproic Acid, Induces p53-Dependent Radiosensitization of Colon Cancer Cells

Abstract

Agents that inhibit histone deacetylases (HDAC inhibitors) have been shown to enhance radiation response. The aim of this study was to evaluate the effects of low, minimally cytotoxic concentrations of the HDAC inhibitor, valproic acid (VPA), on radiation response of colorectal cancer cells. Cell lines LS174T and an isogenic pair of HCT116, which differed only for the presence of wild-type p53, were exposed to ionizing radiation (IR) alone, VPA alone, or the combination. Clonogenic survival, g-H2AX induction, apoptosis, changes in mitochondrial membrane potential, and mitochondrial levels of p53 and Bcl-2 family proteins were assessed. In vivo studies monitored tumor growth suppression after therapy in mice bearing HCT116=p53 þ=þ and HCT116=p53 À=À tumor xenografts. VPA led to radiosensitization, which was dependent on p53 status. A decrease in clonogenic survival, an increase in apoptosis, and an increase in levels of g-H2AX were observed after VPAþIR, compared to IR alone, in wild-type p53 cells (LS174T and HCT116=p53 þ=þ ), as opposed to p53 null cells (HCT116=p53 À=À). Exposure to VPA resulted in enhancement of IR-induced mitochondrial localizations of Bax and Bcl-xL, mitochondrial membrane potential, and cytochrome c release only in wild-type p53 cell lines. VPA also enhanced tumor growth suppression after IR only in wild-type p53 xenografts. These data suggest that VPA may have an important role in enhancing radiotherapy response in colorectal cancer, particularly in tumors with the wildtype p53 genotype.

Valproic acid, a histone deacetylase inhibitor, enhances radiosensitivity in esophageal squamous cell carcinoma

“…These studies show that VPA in vitro enhances radiosensitivity in malignant glioma (20,22), colorectal carcinoma (21), leukemia (23,24,26,27) and retinoblastoma cells (25). Moreover, a corresponding enhancement of therapeutic efficacy in vivo was also reported when VPA was combined with irradiation (20,21). To our knowledge, there are currently four on-going phase I clinical trials assessing the combination of HDAC inhibitors and radiotherapy: VPA with radiotherapy for pediatric glioma, VPA with radiotherapy for pediatric refractory solid and central nervous system (CNS) tumors, VPA or hydralazine with CRT for cervical cancer, and vorinostat with palliative radiotherapy (28)(29)(30)(31).…”

“…In mammalian cells, DSBs are repaired mostly by either homologous recombination (HR) or non-homologous end-joining (NHEJ). Several HDAC inhibitors including VPA can downregulate DNA repair proteins related to HR (Rad51) (33)(34)(35) and NHEJ (Ku70, Ku86 and DNA-PKcs) (21,(34)(35)(36)(37)(38)(39)(40). Munchi et al (38) noted that sodium butylate suppresses Ku70, Ku86 and DNA-PKcs at mRNA and protein levels.…”

“…It has been reported that VPA enhances radiosensitivity in various cancer cell lines in vivo and in vitro (20)(21)(22)(23)(24)(25)(26)(27). VPA offers considerable promise as a therapeutic agent for esophageal cancer.…”

Valproic acid and its inhibition of tumor growth in systemic malignancies: beyond gliomas

“…These effects are especially seen when it is used in combination with IIF. The dual combination enhances Bax expression while Bcl-2 expression is markedly reduced [7]. TIMP-1 activity is also enhanced at the same time, while MMP2 activity is considerably reduced.…”

Influence of the novel histone deacetylase inhibitor panobinostat (LBH589) on the growth of ovarian cancer

“…Both transcriptional and nontranscriptional mechanisms of action have been investigated [15]. Pre-clinical studies using cell lines from a variety of gynecologic [16][17][18] and other malignancies [19,20] have demonstrated that natural and synthetic HDAC inhibitors can inhibit tumor cell growth in vitro and in vivo through cell cycle arrest as well as the induction of mitotic defects through histone mediated and histone independent interactions [15]. Investigations utilizing in vitro and in vivo models of ovarian cancer have demonstrated that HDAC inhibition synergizes with conventional chemotherapies to induce potent cytotoxic effects supporting the potential use of this combination in the clinic [18,[21][22][23][24][25].…”