Histone deacetylase 6 acts upstream of DNA damage response activation to support the survival of glioblastoma cells

The molecular characteristics of DNA damage and repair related to P53 mutation for predicting the recurrence and immunotherapy response in hepatocellular carcinoma

Hepatocellular carcinoma (HCC) is one of the leading causes of cancer-related deaths globally, owing to its high recurrence rate of 50 to 70% within five years. Despite known associations of certain DNA damage and repair (DDR) genes with tumor recurrence and drug resistance, a comprehensive understanding of DDR pathways' role in predicting HCC recurrence and therapeutic responses remains elusive. Addressing this gap could offer significant advancements in prognostic and therapeutic strategies for HCC. This study used 769 RNA sequencing samples from public datasets and 53 samples from Xiangya Hospital for DDR model training and validation. It came out that DDR pathways were significantly enriched in samples with P53 mutations. Next, among the 173 combinations of algorithms and parameters, CoxBoost + RSF, Lasso [fold = 10] + RSF, and Lasso [fold = 50] + RSF demonstrated the best performance. The average AUC values of 1 to 5 years and the average concordance index (C-index) value were around 0.7. The risk scores were increased in tumors with recurrence, P53 mutation, and higher TNM stages. High-risk groups, characterized by enriched DDR pathways, exhibited lower CD8 + T cell infiltration and poorer responses to immunotherapy using atezolizumab and bevacizumab, emphasizing the potential of DDR signatures as valuable prognostic and therapeutic biomarkers. In conclusion, the DDR signatures associated with P53 mutations can predict recurrence and therapeutic response in HCC, highlighting their potential as prognostic and therapeutic biomarkers.

Mechanisms of HDACs in cancer development

Histone deacetylases (HDACs) are a class of epigenetic regulators that play pivotal roles in key biological processes such as cell proliferation, differentiation, metabolism, and immune regulation. Based on this, HDAC inhibitors (HDACis), as novel epigenetic-targeted therapeutic agents, have demonstrated significant antitumor potential by inducing cell cycle arrest, activating apoptosis, and modulating the immune microenvironment. Current research is focused on developing highly selective HDAC isoform inhibitors and combination therapy strategies tailored to molecular subtypes, aiming to overcome off-target effects and resistance issues associated with traditional broad-spectrum inhibitors. This review systematically elaborates on the multidimensional regulatory networks of HDACs in tumor malignancy and assesses the clinical translation progress of next-generation HDACis and their prospects in precision medicine, providing a theoretical framework and strategic reference for the development of epigenetic-targeted antitumor drugs.

The influence of homologous recombination repair on temozolomide chemosensitivity in gliomas

Gliomas represent a prevalent form of primary brain tumors, with temozolomide (TMZ) serving as the established first-line therapeutic option. Nevertheless, the effectiveness of TMZ is hindered by the development of chemoresistance. Recent investigations have underscored the correlation of homologous recombination repair (HRR), a pivotal mechanism responsible for mending DNA double-strand breaks, with TMZ resistance in glioma treatment. This review centers on elucidating the significance of HRR in the management of gliomas, with a particular emphasis on pivotal molecules implicated in the HRR process, including RAD51, ATM, ATR, and newly identified small molecules that impact HRR. Modulating the expression of these genes can effectively restrain pathways such as ATM/CHK2, ATR/CHK1, and PI3K/AKT, subsequently augmenting the sensitivity of gliomas to TMZ. Noteworthy efforts have been directed towards exploring inhibitors of these pathways in recent research endeavors, culminating in encouraging outcomes. In conclusion, the involvement of HRR in glioma resistance unveils novel therapeutic avenues, with targeting crucial molecules in the HRR pathway, holding promise for enhancing the effectiveness of TMZ therapy.

HDAC6 inhibition through WT161 synergizes with temozolomide, induces apoptosis, reduces cell motility, and decreases β-catenin levels in glioblastoma cells

Glioblastoma multiforme (GBM) accounts for 70% of all primary malignancies of the central nervous system. Current treatment strategies involve surgery followed by chemotherapy with temozolomide (TMZ); however, the median survival after treatment is approximately 15 months. Many GBM cases develop resistance to TMZ, resulting in a poor prognosis for patients, which underscores the urgent need for novel therapeutic approaches. One promising avenue is the inhibition of histone deacetylase 6 (HDAC6), an enzyme that deacetylates α-tubulin and is increasingly recognized as a potential pharmacological target in cancer. In GBM specifically, HDAC6 overexpression has been linked to poor prognosis and chemoresistance. In this study, we demonstrate that HDAC6 protein levels are elevated in GBM and evaluate the effects of the novel selective HDAC6 inhibitor, WT161, on U251, U87, and T98G cells to assess its potential to revert the malignant phenotype. Our results show a significant increase in acetylated α-tubulin levels, suppression of cell growth, cell cycle arrest at the G2/M phase, and decreased clonogenicity of 2D-cultured GBM cells. Additionally, WT161 acted synergistically with TMZ, induced apoptosis and enhanced TMZ-induced apoptosis. Notably, HDAC6 inhibition resulted in reduced cell migration and invasion, associated with decreased β-catenin levels. When cultured in 3D conditions, WT161-treated T98G spheroids were sensitized to TMZ and exhibited reduced migration. Finally, HDAC6 inhibition altered the metabolome, particularly affecting metabolites associated with lipid peroxidation. In conclusion, our data reveal, for the first time, the efficacy of the selective HDAC6 inhibitor WT161 in a preclinical GBM setting.

HDAC6 and USP9X Control Glutamine Metabolism by Stabilizing GS to Promote Glioblastoma Tumorigenesis

Glioblastoma (GBM) is the most common and the deadliest brain cancer. Glutamine anabolism mediated by glutamine synthetase (GS) is beneficial for GBM cell growth, especially under glutamine deprivation. However, the molecular mechanism underlying GS homeostasis in GBM remains undisclosed. Here, it is reported that histone deacetylase 6 (HDAC6) promotes GS deacetylation, stabilizing it via ubiquitin-mediated pathway. It is found that deubiquitination of GS is modulated by ubiquitin-specific peptidase 9, X-linked (USP9X). USP9X stabilizes GS by removing its K48-linked polyubiquitination on lysine 91 and 103. Accordingly, targeting HDAC6 and USP9X in vitro and in vivo represses GBM tumorigenesis by decreasing GS stability. Metabolic analysis shows that silencing HDAC6 and USP9X disrupts de novo nucleotide synthesis, thereby attenuating GBM cell growth. Furthermore, GS modulation by targeting HDAC6 and USP9X restrains the self-renewal capacity. These results suggest that HDAC6 and USP9X are crucial epigenetic enzymes that promote GBM tumorigenesis by modulating glutamine metabolism.

Repurposing Linezolid in Conjunction with Histone Deacetylase Inhibitor Access in the Realm of Glioblastoma Therapies

Since decades after temozolomide was approved, no effective drugs have been developed. Undoubtedly, blood-brain barrier (BBB) penetration is a severe issue that should be overcome in glioblastoma multiforme (GBM) drug development. In this research, we were inspired by linezolid through structural modification with several bioactive moieties to achieve the desired brain delivery. The results indicated that the histone deacetylase modification, referred to as compound 1, demonstrated promising cytotoxic effects in various brain tumor cell lines. Further comprehensive mechanism studies indicated that compound 1 induced acetylation, leading to DNA double-strand breaks, and induced the ubiquitination of RAD51, disrupting the DNA repair process. Furthermore, compound 1 also exhibited dramatic improvement in the orthotopic GBM mouse model, demonstrating its efficacy and satisfying BBB penetration. Therefore, the reported compound 1, provided with an independent therapeutic pathway, satisfying elongation in survival and tumor size reduction, and the ability to penetrate the BBB, was potent to achieve further development.

HDAC6 Facilitates PRV and VSV Infection by Inhibiting Type I Interferon Production

HDAC6 modulates viral infection through diverse mechanisms. Here, we investigated the role of HDAC6 in influencing viral infection in pig cells with the aim of exploiting the potential antiviral gene targets in pigs. Using gene knockout and overexpression strategies, we found that HDAC6 knockout greatly reduced PRV and VSV infectivity, whereas HDAC6 overexpression increased their infectivity in PK15 cells. Mechanistic studies identified HDAC6 as a DNA damage inhibitor in PK15 cells. HDAC6 overexpression attenuated DNA damage levels, which can further reduce type I IFN production to promote viral infection. Conversely, HDAC6 deficiency can limit viral infection by increasing DNA damage-mediated type I IFN production. This work demonstrates that HDAC6 affects the infection process of multiple viruses by modulating type I IFN production, highlighting a regulatory role of HDAC6 linking host immune response and viral infection levels in pig cells.

Combinatorial functionomics identifies HDAC6-dependent molecular vulnerability of radioresistant head and neck cancer

BACKGROUND

Radiotherapy is the primary treatment modality for most head and neck cancers (HNCs). Despite the addition of chemotherapy to radiotherapy to enhance its tumoricidal effects, almost a third of HNC patients suffer from locoregional relapses. Salvage therapy options for such recurrences are limited and often suboptimal, partly owing to divergent tumor and microenvironmental factors underpinning radioresistance. In this study, we utilized a combinatorial functionomics approach, the Quadratic Phenotypic Optimization Platform (QPOP), to rationally design drug pairings that exploit the molecular fingerprint and vulnerability of established in vitro isogenic radioresistant (RR)-HNC models.

METHODS

A QPOP-specific protocol was applied to RR-HNC models to rank and compare all possible drug combinations from a 12-drug set comprising standard chemotherapy, small molecule inhibitors and targeted therapies specific to HNC. Drug combination efficacy was evaluated by computing combination index scores, and by measuring apoptotic response. Drug targeting was validated by western blot analyses, and the Comet assay was used to quantify DNA damage. Enhanced histone deacetylase inhibitor (HDACi) efficacy in RR models was further examined by in vivo studies, and genetic and chemical inhibition of major Class I/II HDACs. Regulatory roles of HDAC6/SP1 axis were investigated using immunoprecipitation, gel shift and ChIP-qPCR assays. Comparative transcriptomic analyses were employed to determine the prognostic significance of targeting HDAC6.

RESULTS

We report the therapeutic potential of combining panobinostat (pan-HDAC inhibitor) with AZD7762 (CHK1/2 inhibitor; AstraZeneca) or ionizing radiation (IR) to re-sensitize RR-HNC cells and showed increased DNA damage underlying enhanced synergy. We further refined this RR-specific drug combination and prioritized HDAC6 as a targetable dependency in reversing radioresistance. We provide mechanistic insights into HDAC6-mediated regulation via a crosstalk involving SP1 and oncogenic and repair genes. From two independent patient cohorts, we identified a four-gene signature that may have discriminative ability to predict for radioresistance and amenable to HDAC6 inhibition.

CONCLUSION

We have uncovered HDAC6 as a promising molecular vulnerability that should be explored to treat RR-HNC.

The Role of HDAC6 in Glioblastoma Multiforme: A New Avenue to Therapeutic Interventions?

Despite the great advances in basic research results, glioblastoma multiforme (GBM) still remains an incurable tumour. To date, a GBM diagnosis is a death sentence within 15-18 months, due to the high recurrence rate and resistance to conventional radio- and chemotherapy approaches. The effort the scientific community is lavishing on the never-ending battle against GBM is reflected by the huge number of clinical trials launched, about 2003 on 10 September 2024. However, we are still far from both an in-depth comprehension of the biological and molecular processes leading to GBM onset and progression and, importantly, a cure. GBM is provided with high intratumoral heterogeneity, immunosuppressive capacity, and infiltrative ability due to neoangiogenesis. These features impact both tumour aggressiveness and therapeutic vulnerability, which is further limited by the presence in the tumour core of niches of glioblastoma stem cells (GSCs) that are responsible for the relapse of this brain neoplasm. Epigenetic alterations may both drive and develop along GBM progression and also rely on changes in the expression of the genes encoding histone-modifying enzymes, including histone deacetylases (HDACs). Among them, HDAC6-a cytoplasmic HDAC-has recently gained attention because of its role in modulating several biological aspects of GBM, including DNA repair ability, massive growth, radio- and chemoresistance, and de-differentiation through primary cilia disruption. In this review article, the available information related to HDAC6 function in GBM will be presented, with the aim of proposing its inhibition as a valuable therapeutic route for this deadly brain tumour.

Effects of a novel HDAC6-selective inhibitor's radiosensitization on cancer cells

BACKGROUND

The radiation sensitivity of tumor cells is a critical determinant of their therapeutic response to radiotherapy. Histone deacetylase 6 (HDAC6), beyond its known role in modulating tubulin acetylation and influencing cell motility, is also involved in the DNA damage response, potentially enhancing tumor cell radiosensitivity. Targeted HDAC6 inhibitors have shown substantial promise in preclinical studies aimed at increasing radiosensitivity and inhibiting cellular migration.

METHODS

A new HDAC inhibitor, named OXHA, was designed by substituting the phenyl cap of SAHA with an N,5-diphenyloxazole-2-carboxamide group. The inhibitory activity of OXHA was evaluated via in vitro enzymatic assays. Its effects on tumor cell migration and radiosensitization potential were assessed using scratch wound healing assays, micronucleus formation, and clonogenic survival assays.

RESULT

Enzymatic assays confirmed OXHA's selective inhibition of HDAC6. Compared to SAHA, OXHA significantly increased α-tubulin acetylation while minimally impacting histone H3 acetylation, indicating a high selectivity for HDAC6. In combination with X-ray irradiation, OXHA markedly impaired wound healing in A549 and HepG2 cells, enhanced micronucleus formation, and reduced clonogenic survival across multiple tumor lines.

CONCLUSION

OXHA exhibits potent and selective HDAC6 inhibition, effectively impeding tumor cell migration and enhancing radiosensitivity across multiple cell lines. These findings suggest that OXHA has strong potential as a therapeutic strategy to improve radiotherapy efficacy.

Chemotherapy-Induced Peripheral Neuropathy: A Recent Update on Pathophysiology and Treatment

Chemotherapy-induced peripheral neuropathy (CIPN) is a major long-lasting side effect of some chemotherapy drugs, which threatens cancer survival rate. CIPN mostly affects sensory neurons and occasionally motor neurons, causing numbness, tingling, discomfort, and burning pain in the upper and lower extremities. The pathophysiology of CIPN is not completely understood; however, it is believed that chemotherapies induce peripheral neuropathy via directly damaging mitochondria, impairing the function of ion channels, triggering immunological mechanisms, and disrupting microtubules. The treatment of CIPN is a medical challenge, and there are no approved pharmacological options. Currently, duloxetine and other antidepressants, antioxidant, anti-inflammatory, and ion-channel targeted therapies are commonly used in clinics to relieve the symptoms of CIPN. Several other types of drugs, such as cannabinoids, sigma-1 receptor antagonists, and nicotinamides ribose, are being evaluated in preclinical and clinical studies. This paper summarizes the information related to the physiology of CIPN and medicines that could be used for treating this condition.

Targeting histone deacetylase 6 (HDAC6) to enhance radiation therapy in meningiomas in a 2D and 3D in vitro study

BACKGROUND

External radiation therapy (RT) is often a primary treatment for inoperable meningiomas in the absence of established chemotherapy. Histone deacetylase 6 (HDAC6) overexpression, commonly found in cancer, is acknowledged as a driver of cellular growth, and inhibiting HDACs holds promise in improving radiotherapeutic efficacy. Downregulation of HDAC6 facilitates the degradation of β-catenin. This protein is a key element in the Wnt/β-catenin signalling pathway, contributing to the progression of meningiomas.

METHODS

In order to elucidate the associations and therapeutic potential of HDAC6 inhibitors (HDAC6i) in conjunction with RT, we administered Cay10603, HDAC6i, to both immortalised and patient-derived meningioma cells prior to RT in this study.

FINDINGS

Our findings reveal an increase in HDAC6 expression following exposure to RT, which is effectively mitigated with pre-treated Cay10603. The combination of Cay10603 with RT resulted in a synergistic augmentation of cytotoxic effects, as demonstrated through a range of functional assays conducted in both 2D as well as 3D settings; the latter containing syngeneic tumour microenvironment (TME). Radiation-induced DNA damage was augmented by pre-treatment with Cay10603, concomitant with the inhibition of β-catenin and minichromosome maintenance complex component 2 (MCM2) accumulation within the nucleus. This subsequently inhibited c-myc oncogene expression.

INTERPRETATION

Our findings demonstrate the therapeutic potential of Cay10603 to improve the radiosensitisation and provide rationale for combining HDAC6i with RT for the treatment of meningioma.

FUNDING

This work was funded by Brain Tumour Research Centre of Excellence award to C Oliver Hanemann.

A novel HDAC6 inhibitor interferes microtubule dynamics and spindle assembly checkpoint and sensitizes cisplatin-induced apoptosis in castration-resistant prostate cancer

BACKGROUND

Metastatic castration-resistant prostate cancer (CRPC), the most refractory prostate cancer, inevitably progresses and becomes unresponsive to hormone therapy, revealing a pressing unmet need for this disease. Novel agents targeting HDAC6 and microtubule dynamics can be a potential anti-CRPC strategy.

METHODS

Cell proliferation was examined in CRPC PC-3 and DU-145 cells using sulforhodamine B assay and anchorage-dependent colony formation assay. Flow cytometric analysis of propidium iodide staining was used to determine cell-cycle progression. Cell-based tubulin polymerization assay and confocal immunofluorescence microscopic examination determine microtubule assembly/disassembly status. Protein expressions were determined using Western blot analysis.

RESULTS

A total of 82 novel derivatives targeting HDAC6 were designed and synthesized, and Compound 25202 stood out, showing the highest efficacy in blocking HDAC6 (IC50, 3.5 nM in enzyme assay; IC50, 1.0 μM in antiproliferative assay in CRPC cells), superior to tubastatin A (IC50, 5.4 μM in antiproliferative assay). The selectivity and superiority of 25202 were validated by examining the acetylation of both α-tubulin and histone H3, detecting cell apoptosis and HDACs enzyme activity assessment. Notably, 25202 but not tubastatin A significantly decreased HDAC6 protein expression. 25202 prolonged mitotic arrest through the detection of cyclin B1 upregulation, Cdk1 activation, mitotic phosphoprotein levels, and Bcl-2 phosphorylation. Compound 25202 did not mimic docetaxel in inducing tubulin polymerization but disrupted microtubule organization. Compound 25202 also increased the phosphorylation of CDC20, BUB1, and BUBR1, indicating the activation of the spindle assembly checkpoint (SAC). Moreover, 25202 profoundly sensitized cisplatin-induced cell death through impairment of cisplatin-evoked DNA damage response and DNA repair in both ATR-Chk1 and ATM-Chk2 pathways.

CONCLUSION

The data suggest that 25202 is a novel selective and potent HDAC6 inhibitor. Compound 25202 blocks HDAC6 activity and interferes microtubule dynamics, leading to SAC activation and mitotic arrest prolongation that eventually cause apoptosis of CRPC cells. Furthermore, 25202 sensitizes cisplatin-induced cell apoptosis through impeding DNA damage repair pathways.

Dual inhibition of CYP17A1 and HDAC6 by abiraterone-installed hydroxamic acid overcomes temozolomide resistance in glioblastoma through inducing DNA damage and oxidative stress

Glioblastoma (GBM) is a highly aggressive and treatment-resistant brain tumor, necessitating novel therapeutic strategies. In this study, we present a mechanistic breakthrough by designing and evaluating a series of abiraterone-installed hydroxamic acids as potential dual inhibitors of CYP17A1 and HDAC6 for GBM treatment. We established the correlation of CYP17A1/HDAC6 overexpression with tumor recurrence and temozolomide resistance in GBM patients. Compound 12, a dual inhibitor, demonstrated significant anti-GBM activity in vitro, particularly against TMZ-resistant cell lines. Mechanistically, compound 12 induced apoptosis, suppressed recurrence-associated genes, induced oxidative stress and initiated DNA damage response. Furthermore, molecular modeling studies confirmed its potent inhibitory activity against CYP17A1 and HDAC6. In vivo studies revealed that compound 12 effectively suppressed tumor growth in xenograft and orthotopic mouse models without inducing significant adverse effects. These findings highlight the potential of dual CYP17A1 and HDAC6 inhibition as a promising strategy for overcoming treatment resistance in GBM and offer new hope for improved therapeutic outcomes.

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.

RNA methylation, homologous recombination repair and therapeutic resistance

Homologous recombination (HR) repair of DNA double-strand breaks (DSBs) is critical for maintaining genomic integrity and stability. Defects in HR increase the risk of tumorigenesis. However, many human tumors exhibit enhanced HR repair capabilities, consequently endowing tumor cells with resistance to DNA-damaging chemotherapy and radiotherapy. This review summarizes the role of RNA methylation in HR repair and therapeutic resistance in human tumors. We also analyzed the interactions between RNA methylation and other HR-modulating modifications including histone acetylation, histone deacetylation, ubiquitination, deubiquitination, protein arginine methylation, and gene transcription. This review proposes that targeting RNA methylation is a promising approach to overcoming HR-mediated therapeutic resistance.

Multifaceted Influence of Histone Deacetylases on DNA Damage Repair: Implications for Hepatocellular Carcinoma

Hepatocellular carcinoma (HCC) is one of the most commonly diagnosed cancers and a leading cause of cancer-related mortality worldwide, but its pathogenesis remains largely unknown. Nevertheless, genomic instability has been recognized as one of the facilitating characteristics of cancer hallmarks that expedites the acquisition of genetic diversity. Genomic instability is associated with a greater tendency to accumulate DNA damage and tumor-specific DNA repair defects, which gives rise to gene mutations and chromosomal damage and causes oncogenic transformation and tumor progression. Histone deacetylases (HDACs) have been shown to impair a variety of cellular processes of genome stability, including the regulation of DNA damage and repair, reactive oxygen species generation and elimination, and progression to mitosis. In this review, we provide an overview of the role of HDAC in the different aspects of DNA repair and genome instability in HCC as well as the current progress on the development of HDAC-specific inhibitors as new cancer therapies.

Introducing HDAC-Targeting Radiopharmaceuticals for Glioblastoma Imaging and Therapy

Despite recent advances in multimodality therapy for glioblastoma (GB) incorporating surgery, radiotherapy, chemotherapy and targeted therapy, the overall prognosis remains poor. One of the interesting targets for GB therapy is the histone deacetylase family (HDAC). Due to their pleiotropic effects on, e.g., DNA repair, cell proliferation, differentiation, apoptosis and cell cycle, HDAC inhibitors have gained a lot of attention in the last decade as anti-cancer agents. Despite their known underlying mechanism, their therapeutic activity is not well-defined. In this review, an extensive overview is given of the current status of HDAC inhibitors for GB therapy, followed by an overview of current HDAC-targeting radiopharmaceuticals. Imaging HDAC expression or activity could provide key insights regarding the role of HDAC enzymes in gliomagenesis, thus identifying patients likely to benefit from HDACi-targeted therapy.

Targeting HDAC6 to Overcome Autophagy-Promoted Anti-Cancer Drug Resistance

Histone deacetylases (HDACs) regulate gene expression through the epigenetic modification of chromatin structure. HDAC6, unlike many other HDACs, is present in the cytoplasm. Its deacetylates non-histone proteins and plays diverse roles in cancer cell initiation, proliferation, autophagy, and anti-cancer drug resistance. The development of HDAC6-specific inhibitors has been relatively successful. Mechanisms of HDAC6-promoted anti-cancer drug resistance, cancer cell proliferation, and autophagy are discussed. The relationship between autophagy and anti-cancer drug resistance is discussed. The effects of combination therapy, which includes HDAC6 inhibitors, on the sensitivity of cancer cells to chemotherapeutics and immune checkpoint blockade are presented. A summary of clinical trials involving HDAC6-specific inhibitors is also presented. This review presents HDAC6 as a valuable target for developing anti-cancer drugs.

Adapt to Persist: Glioblastoma Microenvironment and Epigenetic Regulation on Cell Plasticity

Glioblastoma (GBM) is the most frequent and aggressive brain tumor, characterized by great resistance to treatments, as well as inter- and intra-tumoral heterogeneity. GBM exhibits infiltration, vascularization and hypoxia-associated necrosis, characteristics that shape a unique microenvironment in which diverse cell types are integrated. A subpopulation of cells denominated GBM stem-like cells (GSCs) exhibits multipotency and self-renewal capacity. GSCs are considered the conductors of tumor progression due to their high tumorigenic capacity, enhanced proliferation, invasion and therapeutic resistance compared to non-GSCs cells. GSCs have been classified into two molecular subtypes: proneural and mesenchymal, the latter showing a more aggressive phenotype. Tumor microenvironment and therapy can induce a proneural-to-mesenchymal transition, as a mechanism of adaptation and resistance to treatments. In addition, GSCs can transition between quiescent and proliferative substates, allowing them to persist in different niches and adapt to different stages of tumor progression. Three niches have been described for GSCs: hypoxic/necrotic, invasive and perivascular, enhancing metabolic changes and cellular interactions shaping GSCs phenotype through metabolic changes and cellular interactions that favor their stemness. The phenotypic flexibility of GSCs to adapt to each niche is modulated by dynamic epigenetic modifications. Methylases, demethylases and histone deacetylase are deregulated in GSCs, allowing them to unlock transcriptional programs that are necessary for cell survival and plasticity. In this review, we described the effects of GSCs plasticity on GBM progression, discussing the role of GSCs niches on modulating their phenotype. Finally, we described epigenetic alterations in GSCs that are important for stemness, cell fate and therapeutic resistance.