The histone deacetylase inhibitor PCI-24781 as a putative radiosensitizer in pediatric glioblastoma cell lines

Chloroquine-induced DNA damage synergizes with DNA repair inhibitors causing cancer cell death

Background

Cancer is a global health problem accounting for nearly one in six deaths worldwide. Conventional treatments together with new therapies have increased survival to this devastating disease. However, the persistent challenges of treatment resistance and the limited therapeutic arsenal available for specific cancer types still make research in new therapeutic strategies an urgent need.

Methods

Chloroquine was tested in combination with different drugs (Panobinostat, KU-57788 and NU-7026) in 8 human-derived cancer cells lines (colorectal: HCT116 and HT29; breast: MDA-MB-231 and HCC1937; glioblastoma: A-172 and LN-18; head and neck: CAL-33 and 32816). Drug´s effect on proliferation was tested by MTT assays and cell death was assessed by Anexin V-PI apoptosis assays. The presence of DNA double-strand breaks was analyzed by phospho-H2AX fluorescent staining. To measure homologous recombination efficiency the HR-GFP reporter was used, which allows flow cytometry-based detection of HR stimulated by I-SceI endonuclease-induced DSBs.

Results

The combination of chloroquine with any of the drugs employed displayed potent synergistic effects on apoptosis induction, with particularly pronounced efficacy observed in glioblastoma and head and neck cancer cell lines. We found that chloroquine produced DNA double strand breaks that depended on reactive oxygen species formation, whereas Panobinostat inhibited DNA double-strand breaks repair by homologous recombination. Cell death caused by chloroquine/Panobinostat combination were significantly reduced by N-Acetylcysteine, a reactive oxygen species scavenger, underscoring the pivotal role of DSB generation in CQ/LBH-induced lethality. Based on these data, we also explored the combination of CQ with KU-57788 and NU-7026, two inhibitors of the other main DSB repair pathway, nonhomologous end joining (NHEJ), and again synergistic effects on apoptosis induction were observed.

Conclusion

Our data provide a rationale for the clinical investigation of CQ in combination with DSB inhibitors for the treatment of different solid tumors.

Assessing the Effect of Histone Deacetylase Inhibitors on DNA Double-Strand Break Repair by Nonhomologous End Joining

This book chapter describes a plasmid-based reporter method, first described by Bennardo et al. (2008) that we use in our laboratory for determining the activity of the repair of DNA double-strand breaks by nonhomologous end joining. This method can be used to measure the impact of epigenetic modifiers of the histone deacetylase family on this DNA repair pathway by flow cytometry.

Chloroquine-Induced DNA Damage Synergizes with Nonhomologous End Joining Inhibition to Cause Ovarian Cancer Cell Cytotoxicity

Ovarian cancer (OC) is the most lethal gynecological malignancy; therefore, more effective treatments are urgently needed. We recently reported that chloroquine (CQ) increased reactive oxygen species (ROS) in OC cell lines (OCCLs), causing DNA double-strand breaks (DSBs). Here, we analyzed whether these lesions are repaired by nonhomologous end joining (NHEJ), one of the main pathways involved in DSB repair, and if the combination of CQ with NHEJ inhibitors (NHEJi) could be effective against OC. We found that NHEJ inhibition increased the persistence of γH2AX foci after CQ-induced DNA damage, revealing an essential role of this pathway in the repair of the lesions. NHEJi decreased the proliferation of OCCLs and a strong in vitro synergistic effect on apoptosis induction was observed when combined with CQ. This effect was largely abolished by the antioxidant N-Acetyl-L-cysteine, revealing the critical role of ROS and DSB generation in CQ/NHEJi-induced lethality. We also found that the NHEJ efficiency in OCCLs was not affected by treatment with Panobinostat, a pan-histone deacetylase inhibitor that also synergizes with CQ in OCCLs by impairing homologous recombination. Accordingly, the triple combination of CQ-NHEJi-Panobinostat exerted a stronger in vitro synergistic effect. Altogether, our data suggest that the combination of these drugs could represent new therapeutic strategies against OC.

Differential gene expression-based connectivity mapping identified novel drug candidate and improved Temozolomide efficacy for Glioblastoma

BACKGROUND

Glioblastoma (GBM) has a devastating median survival of only one year. Treatment includes resection, radiation therapy, and temozolomide (TMZ); however, the latter increased median survival by only 2.5 months in the pivotal study. A desperate need remains to find an effective treatment.

METHODS

We used the Connectivity Map (CMap) bioinformatic tool to identify candidates for repurposing based on GBM's specific genetic profile. CMap identified histone deacetylase (HDAC) inhibitors as top candidates. In addition, Gene Expression Profiling Interactive Analysis (GEPIA) identified HDAC1 and HDAC2 as the most upregulated and HDAC11 as the most downregulated HDACs. We selected PCI-24781/abexinostat due to its specificity against HDAC1 and HDAC2, but not HDAC11, and blood-brain barrier permeability.

RESULTS

We tested PCI-24781 using in vitro human and mouse GBM syngeneic cell lines, an in vivo murine orthograft, and a genetically engineered mouse model for GBM (PEPG - PTENflox/+; EGFRvIII+; p16Flox/- & GFAP Cre +). PCI-24781 significantly inhibited tumor growth and downregulated DNA repair machinery (BRCA1, CHK1, RAD51, and O6-methylguanine-DNA- methyltransferase (MGMT)), increasing DNA double-strand breaks and causing apoptosis in the GBM cell lines, including an MGMT expressing cell line in vitro. Further, PCI-24781 decreased tumor burden in a PEPG GBM mouse model. Notably, TMZ + PCI increased survival in orthotopic murine models compared to TMZ + vorinostat, a pan-HDAC inhibitor that proved unsuccessful in clinical trials.

CONCLUSION

PCI-24781 is a novel GBM-signature specific HDAC inhibitor that works synergistically with TMZ to enhance TMZ efficacy and improve GBM survival. These promising MGMT-agnostic results warrant clinical evaluation.

Synergistic effect of Chloroquine and Panobinostat in ovarian cancer through induction of DNA damage and inhibition of DNA repair

Ovarian cancer (OC) is the deadliest gynecologic malignancy, which is mainly due to late-stage diagnosis and chemotherapy resistance. Therefore, new and more effective treatments are urgently needed. The in vitro effects of Panobinostat (LBH), a histone deacetylase inhibitor that exerts pleiotropic antitumor effects but induces autophagy, in combination with Chloroquine (CQ), an autophagy inhibitor that avoid this cell survival mechanism, were evaluated in 4 OC cell lines. LBH and CQ inhibited ovarian cancer cell proliferation and induced apoptosis, and a strong synergistic effect was observed when combined. Deeping into their mechanisms of action we show that, in addition to autophagy modulation, treatment with CQ increased reactive oxygen species (ROS) causing DNA double strand breaks (DSBs), whereas LBH inhibited their repair by avoiding the correct recruitment of the recombinase Rad51 to DSBs. Interestingly, CQ-induced DSBs and cell death caused by CQ/LBH combination were largely abolished by the ROS scavenger N-Acetylcysteine, revealing the critical role of DSB generation in CQ/LBH-induced lethality. This role was also manifested by the synergy found when we combined CQ with Mirin, a well-known homologous recombination repair inhibitor. Altogether, our results provide a rationale for the clinical investigation of CQ/LBH combination in ovarian cancer.

Radiosensitization in Pediatric High-Grade Glioma: Targets, Resistance and Developments

Pediatric high-grade gliomas (pHGG) are the leading cause of cancer-related death in children. These epigenetically dysregulated tumors often harbor mutations in genes encoding histone 3, which contributes to a stem cell-like, therapy-resistant phenotype. Furthermore, pHGG are characterized by a diffuse growth pattern, which, together with their delicate location, makes complete surgical resection often impossible. Radiation therapy (RT) is part of the standard therapy against pHGG and generally the only modality, apart from surgery, to provide symptom relief and a delay in tumor progression. However, as a single treatment modality, RT still offers no chance for a cure. As with most therapeutic approaches, irradiated cancer cells often acquire resistance mechanisms that permit survival or stimulate regrowth after treatment, thereby limiting the efficacy of RT. Various preclinical studies have investigated radiosensitizers in pHGG models, without leading to an improved clinical outcome for these patients. However, our recently improved molecular understanding of pHGG generates new opportunities to (re-)evaluate radiosensitizers in these malignancies. Furthermore, the use of radio-enhancing agents has several benefits in pHGG compared to other cancers, which will be discussed here. This review provides an overview and a critical evaluation of the radiosensitization strategies that have been studied to date in pHGG, thereby providing a framework for improving radiosensitivity of these rapidly fatal brain tumors.

The application of histone deacetylases inhibitors in glioblastoma

The epigenetic abnormality is generally accepted as the key to cancer initiation. Epigenetics that ensure the somatic inheritance of differentiated state is defined as a crucial factor influencing malignant phenotype without altering genotype. Histone modification is one such alteration playing an essential role in tumor formation, progression, and resistance to treatment. Notably, changes in histone acetylation have been strongly linked to gene expression, cell cycle, and carcinogenesis. The balance of two types of enzyme, histone acetyltransferases (HATs) and histone deacetylases (HDACs), determines the stage of histone acetylation and then the architecture of chromatin. Changes in chromatin structure result in transcriptional dysregulation of genes that are involved in cell-cycle progression, differentiation, apoptosis, and so on. Recently, HDAC inhibitors (HDACis) are identified as novel agents to keep this balance, leading to numerous researches on it for more effective strategies against cancers, including glioblastoma (GBM). This review elaborated influences on gene expression and tumorigenesis by acetylation and the antitumor mechanism of HDACis. Besdes, we outlined the preclinical and clinical advancement of HDACis in GBM as monotherapies and combination therapies.

Zinc Dependent Histone Deacetylase Inhibitors in Cancer Therapeutics: Recent Update

BACKGROUND

Histone deacetylases (HDAC) are an important class of enzymes that play a pivotal role in epigenetic regulation of gene expression that modifies the terminal of core histones leading to remodelling of chromatin topology and thereby controlling gene expression. HDAC inhibitors (HDACi) counter this action and can result in hyperacetylation of histones, thereby inducing an array of cellular consequences such as activation of apoptotic pathways, generation of reactive oxygen species (ROS), cell cycle arrest and autophagy. Hence, there is a growing interest in the potential clinical use of HDAC inhibitors as a new class of targeted cancer therapeutics. Methodology and Result: Several research articles spanning between 2016 and 2017 were reviewed in this article and presently offer critical insights into the important strategies such as structure-based rational drug design, multi-parameter lead optimization methodologies, relevant SAR studies and biology of various class of HDAC inhibitors, such as hydroxamic acids, benzamides, cyclic peptides, aliphatic acids, summarising the clinical trials and results of various combination drug therapy till date.

CONCLUSION

This review will provide a platform to the synthetic chemists and biologists to cater the needs of both molecular targeted therapy and combination drug therapy to design and synthesize safe and selective HDAC inhibitors in cancer therapeutics.

Tumor Location and Survival Outcomes in Adult Patients with Supratentorial Glioblastoma by Levels of Toll-Like Receptor 9 Expression

BACKGROUND

Toll-like receptor 9 (TLR9) is a key immunotherapy target for glioblastoma (GBM). This study explored the correlation of TLR9 expression with tumor location and survival outcomes in patients with supratentorial GBM.

METHODS

We retrospectively identified 46 patients with supratentorial GBMs and divided them into those with high TLR9 (TRL9^High) and low TLR9 (TRL9^Low) levels. The 2 groups were compared by patients' ages, sex, preoperative Karnofsky Performance Scale (KPS) score, resection extent, tumor location, progression-free survival (PFS), and overall survival (OS).

RESULTS

The TLR9 expression percentages for the GBM specimens were TRL9^High: 72% (33/46) and TRL9^Low: 28% (13/46). The 2 groups showed no differences in patient age (P = 0.147) and sex (χ² = 0.002, P = 0.966), preoperative KPS score (χ² = 0.033, P = 0.855), or resection extent (χ² = 2.405, P = 0.121). Location differed significantly, with 85% (11/13) of TRL9^Low tumors in the left hemisphere and 45% (15/33) of TRL9^High tumors on the same side (χ² = 5.82, P = 0.016). The TRL9^Low group had a median PFS of 612 days (range, 77-926 days), significantly longer than the TRL9^High group (355 days; range, 105-861 days; P = 0.042). Median OS of the TRL9^Low group (733 days; range, 163-969 days) was also significantly longer than in the TRL9^High group (396 days; range, 135-1024 days; P = 0.020).

CONCLUSIONS

Patients with TRL9^Low supratentorial GBM tend to have longer survival than those with higher TLR9 expression. Such tumors show a location preference for the left hemisphere.