Advances in histone deacetylase inhibitors in targeting glioblastoma stem cells

Uracil- and Pyridine-Containing HDAC Inhibitors Displayed Cytotoxicity in Colorectal and Glioblastoma Cancer Stem Cells

Cancer stem cells (CSCs) are a niche of highly tumorigenic cells featuring self-renewal, activation of pluripotency genes, multidrug resistance, and ability to cause cancer relapse. Seven HDACi (1-7), showing either hydroxamate or 2'-aminoanilide function, were tested in colorectal cancer (CRC) and glioblastoma multiforme (GBM) CSCs to determine their effects on cell proliferation, H3 acetylation levels and in-cell HDAC activity. Two uracil-based hydroxamates, 5 and 6, which differ in substitution at C5 and C6 positions of the pyrimidine ring, exhibited the greatest cytotoxicity in GBM (5) and CRC (6) CSCs, followed by the pyridine-hydroxamate 2, with 2- to 6-fold higher potency than the positive control SAHA. Finally, increased H3 acetylation as well as HDAC inhibition directly in cells by selected 2'-aminoanilide 4 and hydroxamate 5 confirmed target engagement. Further investigation will be conducted into the broad-spectrum anticancer properties of the most potent derivatives and their effects in combination with approved, conventional anticancer drugs.

Suberanilohydroxamic acid (SAHA), a HDAC inhibitor, suppresses the effect of Treg cells by targeting the c-Myc/CCL1 pathway in glioma stem cells and improves PD-L1 blockade therapy

PURPOSE

A strong immunosuppressive tumor microenvironment (TME) represents the major barrier responsible for the failure of current immunotherapy approaches in treating Glioblastoma Multiforme (GBM). Within the TME, the regulatory T cells (Tregs) exert immunosuppressive effects on CD8+ T cell - mediated anti-cancer immune killing. Consequently, targeting and inhibiting their immunosuppressive function emerges as an effective therapeutic strategy for GBM. The present study aimed to investigate the mechanisms and effects of Suberanilohydroxamic Acid (SAHA), a histone deacetylase inhibitor, on immunosuppressive Tregs.

METHODS

The tumor-infiltrating immune cells in the immunocompetent GBM intracranial implanted xenograft mouse model were analyzed by immunohistochemistry and flow cytometry techniques. The mRNA expressions were assessed through the RT-qPCR method, while the related protein expressions were determined using western blot, ELISA, immunofluorescence (IF), and flow cytometry techniques. The relationship between c-Myc and C-C motif Chemokine Ligand 1 (CCL1) promotor was validated through a dual-luciferase reporter assay system and chromatin immunoprecipitation.

RESULTS

SAHA suppressed effectively tumor growth and extended significantly overall survival in the immunocompetent GBM intracranial xenograft mouse model. Additionally, it promoted the infiltration of CD8+ T lymphocytes while suppressed the infiltration of CD4+ CD25+ Tregs. Furthermore, SAHA enhanced anti-PD-L1 immune therapy in the intracranial xenograft of mice. Mechanistically, SAHA exerted its effects by inhibiting histone deacetylase 2 (HDAC2), thereby suppressing the binding between c-Myc and the CCL1 promotor.

CONCLUSION

SAHA inhibited the binding of c-Myc with the CCL1 promoter and then suppressed the transcription of CCL1.Additionally, it effectively blocked the interplay of CCL1-CCR8, resulting in reduced activity of Tregs and alleviation of tumor immunosuppression.

The tumor-associated fibrotic reactions in microenvironment aggravate glioma chemoresistance

Malignant gliomas are one of the most common and lethal brain tumors with poor prognosis. Most patients with glioblastoma (GBM) die within 2 years of diagnosis, even after receiving standard treatments including surgery combined with concomitant radiotherapy and chemotherapy. Temozolomide (TMZ) is the first-line chemotherapeutic agent for gliomas, but the frequent acquisition of chemoresistance generally leads to its treatment failure. Thus, it's urgent to investigate the strategies for overcoming glioma chemoresistance. Currently, many studies have elucidated that cancer chemoresistance is not only associated with the high expression of drug-resistance genes in glioma cells but also can be induced by the alterations of the tumor microenvironment (TME). Numerous studies have explored the use of antifibrosis drugs to sensitize chemotherapy in solid tumors, and surprisingly, these preclinical and clinical attempts have exhibited promising efficacy in treating certain types of cancer. However, it remains unclear how tumor-associated fibrotic alterations in the glioma microenvironment (GME) mediate chemoresistance. Furthermore, the possible mechanisms behind this phenomenon are yet to be determined. In this review, we have summarized the molecular mechanisms by which tumor-associated fibrotic reactions drive glioma transformation from a chemosensitive to a chemoresistant state. Additionally, we have outlined antitumor drugs with antifibrosis functions, suggesting that antifibrosis strategies may be effective in overcoming glioma chemoresistance through TME normalization.

Glioblastoma Therapy: Past, Present and Future

Glioblastoma (GB) stands out as the most prevalent and lethal form of brain cancer. Although great efforts have been made by clinicians and researchers, no significant improvement in survival has been achieved since the Stupp protocol became the standard of care (SOC) in 2005. Despite multimodality treatments, recurrence is almost universal with survival rates under 2 years after diagnosis. Here, we discuss the recent progress in our understanding of GB pathophysiology, in particular, the importance of glioma stem cells (GSCs), the tumor microenvironment conditions, and epigenetic mechanisms involved in GB growth, aggressiveness and recurrence. The discussion on therapeutic strategies first covers the SOC treatment and targeted therapies that have been shown to interfere with different signaling pathways (pRB/CDK4/RB1/P16ink4, TP53/MDM2/P14arf, PI3k/Akt-PTEN, RAS/RAF/MEK, PARP) involved in GB tumorigenesis, pathophysiology, and treatment resistance acquisition. Below, we analyze several immunotherapeutic approaches (i.e., checkpoint inhibitors, vaccines, CAR-modified NK or T cells, oncolytic virotherapy) that have been used in an attempt to enhance the immune response against GB, and thereby avoid recidivism or increase survival of GB patients. Finally, we present treatment attempts made using nanotherapies (nanometric structures having active anti-GB agents such as antibodies, chemotherapeutic/anti-angiogenic drugs or sensitizers, radionuclides, and molecules that target GB cellular receptors or open the blood-brain barrier) and non-ionizing energies (laser interstitial thermal therapy, high/low intensity focused ultrasounds, photodynamic/sonodynamic therapies and electroporation). The aim of this review is to discuss the advances and limitations of the current therapies and to present novel approaches that are under development or following clinical trials.

Glioblastoma Biology, Genetics and Possible Therapies

Glioblastoma is the most aggressive intracranial tumor [...].

Molecular Pathways and Genomic Landscape of Glioblastoma Stem Cells: Opportunities for Targeted Therapy

Glioblastoma (GBM) is an aggressive tumor of the central nervous system categorized by the World Health Organization as a Grade 4 astrocytoma. Despite treatment with surgical resection, adjuvant chemotherapy, and radiation therapy, outcomes remain poor, with a median survival of only 14-16 months. Although tumor regression is often observed initially after treatment, long-term recurrence or progression invariably occurs. Tumor growth, invasion, and recurrence is mediated by a unique population of glioblastoma stem cells (GSCs). Their high mutation rate and dysregulated transcriptional landscape augment their resistance to conventional chemotherapy and radiation therapy, explaining the poor outcomes observed in patients. Consequently, GSCs have emerged as targets of interest in new treatment paradigms. Here, we review the unique properties of GSCs, including their interactions with the hypoxic microenvironment that drives their proliferation. We discuss vital signaling pathways in GSCs that mediate stemness, self-renewal, proliferation, and invasion, including the Notch, epidermal growth factor receptor, phosphatidylinositol 3-kinase/Akt, sonic hedgehog, transforming growth factor beta, Wnt, signal transducer and activator of transcription 3, and inhibitors of differentiation pathways. We also review epigenomic changes in GSCs that influence their transcriptional state, including DNA methylation, histone methylation and acetylation, and miRNA expression. The constituent molecular components of the signaling pathways and epigenomic regulators represent potential sites for targeted therapy, and representative examples of inhibitory molecules and pharmaceuticals are discussed. Continued investigation into the molecular pathways of GSCs and candidate therapeutics is needed to discover new effective treatments for GBM and improve survival.

HDAC Class I Inhibitor Domatinostat Preferentially Targets Glioma Stem Cells over Their Differentiated Progeny

Cancer stem cells (CSCs) are in general characterized by higher resistance to cell death and cancer therapies than non-stem differentiated cancer cells. However, we and others have recently revealed using glioma stem cells (GSCs) as a model that, unexpectedly, CSCs have specific vulnerabilities that make them more sensitive to certain drugs compared with their differentiated counterparts. We aimed in this study to discover novel drugs targeting such Achilles’ heels of GSCs as anti-GSC drug candidates to be used for the treatment of glioblastoma, the most therapy-resistant form of brain tumors. Here we report that domatinostat (4SC-202), a class I HDAC inhibitor, is one such candidate. At concentrations where it showed no or minimal growth inhibitory effect on differentiated GSCs and normal cells, domatinostat effectively inhibited the growth of GSCs mainly by inducing apoptosis. Furthermore, GSCs that survived domatinostat treatment lost their self-renewal capacity. These results suggested that domatinostat is a unique drug that selectively eliminates GSCs not only physically by inducing cell death but also functionally by inhibiting their self-renewal. Our findings also imply that class I HDACs and/or LSD1, another target of domatinostat, may possibly have a specific role in the maintenance of GSCs and therefore could be an attractive target in the development of anti-GSC therapies.

Combining HDAC and MEK Inhibitors with Radiation against Glioblastoma-Derived Spheres

Glioblastoma stem-like cells (GSLCs) in glioblastoma limit effective treatment and promote therapeutic resistance and tumor recurrence. Using a combined radiation and drug-screening platform, we tested the combination of a histone deacetylase inhibitor (HDACi) and MAPK/ERK kinase inhibitor (MEKi) with radiation to predict the efficacy against GSLCs. To mimic a stem-like phenotype, glioblastoma-derived spheres were used and treated with a combination of HDACi (MS-275) and MEKi (TAK-733 or trametinib) with 4 Gy irradiation. The sphere-forming ability after the combined radiochemotherapy was investigated using a sphere formation assay, while the expression levels of the GSLC markers (CD44, Nestin and SOX2) after treatment were analyzed using Western blotting and flow cytometry. The combined radiochemotherapy treatment inhibited the sphere formation in both glioblastoma-derived spheres, decreased the expression of the GSLC markers in a cell-line dependent manner and increased the dead cell population. Finally, we showed that the combined treatment with radiation was more effective at reducing the GSLC markers compared to the standard treatment of temozolomide and radiation. These results suggest that combining HDAC and MEK inhibition with radiation may offer a new strategy to improve the treatment of glioblastoma.

Epigenetic modulators for brain cancer stem cells: Implications for anticancer treatment

Primary malignant brain tumors are a major cause of morbidity and mortality in both adults and children, with a dismal prognosis despite multimodal therapeutic approaches. In the last years, a specific subpopulation of cells within the tumor bulk, named cancer stem cells (CSCs) or tumor-initiating cells, have been identified in brain tumors as responsible for cancer growth and disease progression. Stemness features of tumor cells strongly affect treatment response, leading to the escape from conventional therapeutic approaches and subsequently causing tumor relapse. Recent research efforts have focused at identifying new therapeutic strategies capable of specifically targeting CSCs in cancers by taking into consideration their complex nature. Aberrant epigenetic machinery plays a key role in the genesis and progression of brain tumors as well as inducing CSC reprogramming and preserving CSC characteristics. Thus, reverting the cancer epigenome can be considered a promising therapeutic strategy. Three main epigenetic mechanisms have been described: DNA methylation, histone modifications, and non-coding RNA, particularly microRNAs. Each of these mechanisms has been proven to be targetable by chemical compounds, known as epigenetic-based drugs or epidrugs, that specifically target epigenetic marks. We review here recent advances in the study of epigenetic modulators promoting and sustaining brain tumor stem-like cells. We focus on their potential role in cancer therapy.

Genetic Alterations in Gliomas Remodel the Tumor Immune Microenvironment and Impact Immune-Mediated Therapies

High grade gliomas are malignant brain tumors that arise in the central nervous system, in patients of all ages. Currently, the standard of care, entailing surgery and chemo radiation, exhibits a survival rate of 14-17 months. Thus, there is an urgent need to develop new therapeutic strategies for these malignant brain tumors. Currently, immunotherapies represent an appealing approach to treat malignant gliomas, as the pre-clinical data has been encouraging. However, the translation of the discoveries from the bench to the bedside has not been as successful as with other types of cancer, and no long-lasting clinical benefits have been observed for glioma patients treated with immune-mediated therapies so far. This review aims to discuss our current knowledge about gliomas, their molecular particularities and the impact on the tumor immune microenvironment. Also, we discuss several murine models used to study these therapies pre-clinically and how the model selection can impact the outcomes of the approaches to be tested. Finally, we present different immunotherapy strategies being employed in clinical trials for glioma and the newest developments intended to harness the immune system against these incurable brain tumors.

Cellular and Molecular Mechanisms Underlying Glioblastoma and Zebrafish Models for the Discovery of New Treatments

Glioblastoma (GBM) is the most common of all brain malignant tumors; it displays a median survival of 14.6 months with current complete standard treatment. High heterogeneity, aggressive and invasive behavior, the impossibility of completing tumor resection, limitations for drug administration and therapeutic resistance to current treatments are the main problems presented by this pathology. In recent years, our knowledge of GBM physiopathology has advanced significantly, generating relevant information on the cellular heterogeneity of GBM tumors, including cancer and immune cells such as macrophages/microglia, genetic, epigenetic and metabolic alterations, comprising changes in miRNA expression. In this scenario, the zebrafish has arisen as a promising animal model to progress further due to its unique characteristics, such as transparency, ease of genetic manipulation, ethical and economic advantages and also conservation of the major brain regions and blood-brain-barrier (BBB) which are similar to a human structure. A few papers described in this review, using genetic and xenotransplantation zebrafish models have been used to study GBM as well as to test the anti-tumoral efficacy of new drugs, their ability to interact with target cells, modulate the tumor microenvironment, cross the BBB and/or their toxicity. Prospective studies following these lines of research may lead to a better diagnosis, prognosis and treatment of patients with GBM.