Discovery of organosulfur-based selective HDAC8 inhibitors with anti-neuroblastoma activity.

Histone deacetylases (HDACs) are important epigenetic regulators of gene expression and various cellular processes, and are potential targets for anticancer therapy. In particular, HDAC8 is a promising therapeutic target for childhood neuroblastoma. To date, five HDAC inhibitors have been approved as anticancer drugs; however, all are non-selective HDAC inhibitors with various side effects. Furthermore, many promising HDAC inhibitors incorporate hydroxamic acid as a zinc binding group (ZBG), which may be associated with toxicity. Therefore, identification of isoform-selective HDAC inhibitors with novel ZBG is crucial. Here, a series of sulfur-based selective HDAC8 inhibitors featuring a novel ZBG were identified by modifying the early hit, ajoene, a component of garlic. Structure-activity relationship studies uncovered potent and selective HDAC8 inhibitors, and docking studies provided a structural rationale for HDAC8 inhibitory activity. One of the potent compounds, (Z)-1-phenyl-7-(4-methoxyphenyl)-2,3,7-trithiahepta-4-ene-7-oxide (15c), exhibited antiproliferative activity, with a GI50 of 2 µM, against neuroblastoma cell lines. 15c also showed significant in vivo efficacy in a neuroblastoma BE(2)-C xenograft model.

Epigenetic roles of KDM3B and KDM3C in tumorigenesis and their therapeutic implications.

Advances in functional studies on epigenetic regulators have disclosed the vital roles played by diverse histone lysine demethylases (KDMs), ranging from normal development to tumorigenesis. Most of the KDMs are Jumonji C domain-containing (JMJD) proteins. Many of these KDMs remove methyl groups from histone tails to regulate gene transcription. There are more than 30 known KDM proteins, which fall into different subfamilies. Of the many KDM subfamilies, KDM3 (JMJD1) proteins specifically remove dimethyl and monomethyl marks from lysine 9 on histone H3 and other non-histone proteins. Dysregulation of KDM3 proteins leads to infertility, obesity, metabolic syndromes, heart diseases, and cancers. Among the KDM3 proteins, KDM3A has been largely studied in cancers. However, despite a number of studies pointing out their importance in tumorigenesis, KDM3B and KDM3C are relatively overlooked. KDM3B and KDM3C show context-dependent functions, showing pro- or anti-tumorigenic abilities in different cancers. Thus, this review provides a thorough understanding of the involvement of KDM3B and KDMC in oncology that should be helpful in determining the role of KDM3 proteins in preclinical studies for development of novel pharmacological methods to overcome cancer.

HDAC8 Deacetylates HIF-1α and Enhances Its Protein Stability to Promote Tumor Growth and Migration in Melanoma.

Melanoma is the most lethal type of skin cancer, and it causes more than 55,000 deaths annually. Although regional melanoma can be surgically removed, once melanoma metastasizes to other regions of the body, the survival rate drops dramatically. The current treatment options are chemotherapy, immunotherapy, and targeted therapy. However, the low response rate and the development of resistance necessitate the search for a novel therapeutic target in melanoma. Hypoxia-inducible factor-1 α (HIF-1α) is overexpressed in melanoma and plays a crucial role in driving malignant transformation in cancer cells. Here, we identified that histone deacetylase 8 (HDAC8) enhances the protein stability of HIF-1α. HDAC8 directly binds to and deacetylates HIF-1α, thereby promoting its protein stability. This, in turn, upregulates the transcriptional activity of HIF-1α and promotes the expressions of its target genes, such as hexokinase 2 (HK2) and glucose transporter 1 (GLUT1). The inhibition of HDAC8 suppresses the proliferation and metastasis of melanoma cells. Furthermore, HDAC8 is correlated with HIF1A expression and poor prognosis in samples from patients with melanoma. These findings uncover a novel epigenetic mechanism that maintains HIF-1α stability and implicates the potential of HDAC8 inhibitors for melanoma therapy.

Drawing a line between histone demethylase KDM5A and KDM5B: their roles in development and tumorigenesis.

Distinct epigenetic modifiers ensure coordinated control over genes that govern a myriad of cellular processes. Growing evidence shows that dynamic regulation of histone methylation is critical for almost all stages of development. Notably, the KDM5 subfamily of histone lysine-specific demethylases plays essential roles in the proper development and differentiation of tissues, and aberrant regulation of KDM5 proteins during development can lead to chronic developmental defects and even cancer. In this review, we adopt a unique perspective regarding the context-dependent roles of KDM5A and KDM5B in development and tumorigenesis. It is well known that these two proteins show a high degree of sequence homology, with overlapping functions. However, we provide deeper insights into their substrate specificity and distinctive function in gene regulation that at times divert from each other. We also highlight both the possibility of targeting KDM5A and KDM5B to improve cancer treatment and the limitations that must be overcome to increase the efficacy of current drugs.

Pathological Role of HDAC8: Cancer and Beyond.

Histone deacetylase 8 (HDAC8) is a class I HDAC that catalyzes the deacetylation of histone and non-histone proteins. As one of the best-characterized isoforms, numerous studies have identified interacting partners of HDAC8 pertaining to diverse molecular mechanisms. Consequently, deregulation and overexpression of HDAC8 give rise to diseases. HDAC8 is especially involved in various aspects of cancer progression, such as cancer cell proliferation, metastasis, immune evasion, and drug resistance. HDAC8 is also associated with the development of non-cancer diseases such as Cornelia de Lange Syndrome (CdLS), infectious diseases, cardiovascular diseases, pulmonary diseases, and myopathy. Therefore, HDAC8 is an attractive therapeutic target and various HDAC8 selective inhibitors (HDAC8is) have been developed. Here, we address the pathological function of HDAC8 in cancer and other diseases, as well as illustrate several HDAC8is that have shown anti-cancer effects.

HDAC8-Selective Inhibition by PCI-34051 Enhances the Anticancer Effects of ACY-241 in Ovarian Cancer Cells.

HDAC6 is overexpressed in ovarian cancer and is known to be correlated with tumorigenesis. Accordingly, ACY-241, a selective HDAC6 inhibitor, is currently under clinical trial and has been tested in combination with various drugs. HDAC8, another member of the HDAC family, has recently gained attention as a novel target for cancer therapy. Here, we evaluated the synergistic anticancer effects of PCI-34051 and ACY-241 in ovarian cancer. Among various ovarian cancer cells, PCI-34051 effectively suppresses cell proliferation in wild-type p53 ovarian cancer cells compared with mutant p53 ovarian cancer cells. In ovarian cancer cells harboring wild-type p53, PCI-34051 in combination with ACY-241 synergistically represses cell proliferation, enhances apoptosis, and suppresses cell migration. The expression of pro-apoptotic proteins is synergistically upregulated, whereas the expressions of anti-apoptotic proteins and metastasis-associated proteins are significantly downregulated in combination treatment. Furthermore, the level of acetyl-p53 at K381 is synergistically upregulated upon combination treatment. Overall, co-inhibition of HDAC6 and HDAC8 through selective inhibitors synergistically suppresses cancer cell proliferation and metastasis in p53 wild-type ovarian cancer cells. These results suggest a novel approach to treating ovarian cancer patients and the therapeutic potential in developing HDAC6/8 dual inhibitors.

Heterochromatin Protein 1: A Multiplayer in Cancer Progression.

Dysregulation of epigenetic mechanisms as well as genomic mutations contribute to the initiation and progression of cancer. In addition to histone code writers, including histone lysine methyltransferase (KMT), and histone code erasers, including histone lysine demethylase (KDM), histone code reader proteins such as HP1 are associated with abnormal chromatin regulation in human diseases. Heterochromatin protein 1 (HP1) recognizes histone H3 lysine 9 methylation and broadly affects chromatin biology, such as heterochromatin formation and maintenance, transcriptional regulation, DNA repair, chromatin remodeling, and chromosomal segregation. Molecular functions of HP1 proteins have been extensively studied, although their exact roles in diseases require further study. Here, we comprehensively review the studies that have revealed the altered expression of HP1 and its functions in tumorigenesis. In particular, the distinctive effects of each HP1 subtype, namely HP1α, HP1ß, and HP1γ, have been thoroughly explored in various cancer types. We also highlight how HP1 can serve as a potential biomarker for cancer prognosis and therapeutic target for cancer patients.

Glutamine Synthetase as a Therapeutic Target for Cancer Treatment.

The significance of glutamine in cancer metabolism has been extensively studied. Cancer cells consume an excessive amount of glutamine to facilitate rapid proliferation. Thus, glutamine depletion occurs in various cancer types, especially in poorly vascularized cancers. This makes glutamine synthetase (GS), the only enzyme responsible for de novo synthesizing glutamine, essential in cancer metabolism. In cancer, GS exhibits pro-tumoral features by synthesizing glutamine, supporting nucleotide synthesis. Furthermore, GS is highly expressed in the tumor microenvironment (TME) and provides glutamine to cancer cells, allowing cancer cells to maintain sufficient glutamine level for glutamine catabolism. Glutamine catabolism, the opposite reaction of glutamine synthesis by GS, is well known for supporting cancer cell proliferation via contributing biosynthesis of various essential molecules and energy production. Either glutamine anabolism or catabolism has a critical function in cancer metabolism depending on the complex nature and microenvironment of cancers. In this review, we focus on the role of GS in a variety of cancer types and microenvironments and highlight the mechanism of GS at the transcriptional and post-translational levels. Lastly, we discuss the therapeutic implications of targeting GS in cancer.

HDAC6-Selective Inhibitor Overcomes Bortezomib Resistance in Multiple Myeloma.

Although multiple myeloma (MM) patients benefit from standard bortezomib (BTZ) chemotherapy, they develop drug resistance, resulting in relapse. We investigated whether histone deacetylase 6 (HDAC6) inhibitor A452 overcomes bortezomib resistance in MM. We show that HDAC6-selective inhibitor A452 significantly decreases the activation of BTZ-resistant markers, such as extracellular signal-regulated kinases (ERK) and nuclear factor kappa B (NF-κB), in acquired BTZ-resistant MM cells. Combination treatment of A452 and BTZ or carfilzomib (CFZ) synergistically reduces BTZ-resistant markers. Additionally, A452 synergizes with BTZ or CFZ to inhibit the activation of NF-κB and signal transducer and activator of transcription 3 (STAT3), resulting in decreased expressions of low-molecular-mass polypeptide 2 (LMP2) and LMP7. Furthermore, combining A452 with BTZ or CFZ leads to synergistic cancer cell growth inhibition, viability decreases, and apoptosis induction in the BTZ-resistant MM cells. Overall, the synergistic effect of A452 with CFZ is more potent than that of A452 with BTZ in BTZ-resistant U266 cells. Thus, our findings reveal the HDAC6-selective inhibitor as a promising therapy for BTZ-chemoresistant MM.

HDAC6-selective inhibitors enhance anticancer effects of paclitaxel in ovarian cancer cells.

Histone deacetylase 6 (HDAC6)-selective inhibitors are potent anticancer agents that are gaining increasing attention and undergoing various developments. These have been approved or are under clinical trials for use with other anticancer agents, such as pomalidomide, anti-programmed death-ligand 1 antibody and paclitaxel, for various types of cancer, including solid tumors. In the present study, a second generation HDAC6-selective inhibitor, ACY-241 (citarinostat), and a novel inhibitor, A452, exhibited synergistic anticancer effects with paclitaxel in AT-rich interaction domain 1A-mutated ovarian cancer in vitro. Co-treatment of paclitaxel and the two HDAC6 inhibitors synergistically decreased cell growth and viability of TOV-21G. Furthermore, the protein expression levels of pro-apoptotic markers, such as poly(ADP-ribose) polymerase, cleaved caspase-3, Bak and Bax, were increased, whereas the expression levels of anti-apoptotic markers, such as Bcl-xL and Bcl-2, were decreased synergistically. Treatment with all drug combinations increased the portion of apoptotic cells in fluorescence-activated cell sorting analysis. These results demonstrated synergy between paclitaxel and HDAC6-selective inhibitors, providing further impetus for clinical trials of combination therapy using HDAC6-selective inhibitors, not only in ovarian cancer but also in other tumors.

Histone demethylase KDM4C controls tumorigenesis of glioblastoma by epigenetically regulating p53 and c-Myc.

Glioblastoma is the most lethal brain tumor and its pathogenesis remains incompletely understood. KDM4C is a histone H3K9 demethylase that contributes to epigenetic regulation of both oncogene and tumor suppressor genes and is often overexpressed in human tumors, including glioblastoma. However, KDM4C's roles in glioblastoma and the underlying molecular mechanisms remain unclear. Here, we show that KDM4C knockdown significantly represses proliferation and tumorigenesis of glioblastoma cells in vitro and in vivo that are rescued by overexpressing wild-type KDM4C but not a catalytic dead mutant. KDM4C protein expression is upregulated in glioblastoma, and its expression correlates with c-Myc expression. KDM4C also binds to the c-Myc promoter and induces c-Myc expression. Importantly, KDM4C suppresses the pro-apoptotic functions of p53 by demethylating p53K372me1, which is pivotal for the stability of chromatin-bound p53. Conversely, depletion or inhibition of KDM4C promotes p53 target gene expression and induces apoptosis in glioblastoma. KDM4C may serve as an oncogene through the dual functions of inactivation of p53 and activation of c-Myc in glioblastoma. Our study demonstrates KDM4C inhibition as a promising therapeutic strategy for targeting glioblastoma.

Combination of ACY-241 and JQ1 Synergistically Suppresses Metastasis of HNSCC via Regulation of MMP-2 and MMP-9.

Overexpression of histone deacetylase 6 (HDAC6) and bromodomain-containing protein 4 (BRD4) is related to aggressiveness of head and neck squamous carcinoma (HNSCC). Based on studies that HDAC6 and BRD4 are potential therapeutic targets of HNSCC, we hypothesized that the combination treatment of BET inhibitor JQ1 and HDAC6-selective inhibitor ACY-241 could exhibit synergistic anticancer effects in human papillomavirus (HPV)-positive and HPV-negative HNSCC cells. In this study, HNSCC cell growth and viability were measured by CCK-8 assay, apoptosis was analyzed by flow cytometry, and metastasis was studied by wound healing and transwell assays. Furthermore, immunoblotting is conducted to investigate proteins that modulate apoptosis or metastasis. Here, we report that the combination of ACY-241 and JQ1 shows synergistic cell growth inhibition, viability reduction, and apoptosis induction in HNSCC cells through inactivation of AKT and NF-κB signaling. Importantly, we demonstrate that combined treatment of ACY-241 and JQ1 synergistically suppresses TNF-α-induced migration and invasion via dysregulating matrix metalloproteinase (MMP)-2, MMP-9, and MT1-MMP. Overall, the combination of ACY-241 and JQ1 significantly suppresses proliferation and metastasis in HPV-positive and HPV-negative HNSCC. Collectively, these findings suggest that the co-inhibition of BET and HDAC6 can be a new therapeutic strategy in HNSCC.

Advances in Histone Demethylase KDM3A as a Cancer Therapeutic Target.

Lysine-specific histone demethylase 3 (KDM3) subfamily proteins are H3K9me2/me1 histone demethylases that promote gene expression. The KDM3 subfamily primarily consists of four proteins (KDM3A-D). All four proteins contain the catalytic Jumonji C domain (JmjC) at their C-termini, but whether KDM3C has demethylase activity is under debate. In addition, KDM3 proteins contain a zinc-finger domain for DNA binding and an LXXLL motif for interacting with nuclear receptors. Of the KDM3 proteins, KDM3A is especially deregulated or overexpressed in multiple cancers, making it a potential cancer therapeutic target. However, no KDM3A-selective inhibitors have been identified to date because of the lack of structural information. Uncovering the distinct physiological and pathological functions of KDM3A and their structure will give insight into the development of novel selective inhibitors. In this review, we focus on recent studies highlighting the oncogenic functions of KDM3A in cancer. We also discuss existing KDM3A-related inhibitors and review their potential as therapeutic agents for overcoming cancer.

Advances in histone demethylase KDM4 as cancer therapeutic targets.

The KDM4 subfamily H3K9 histone demethylases are epigenetic regulators that control chromatin structure and gene expression by demethylating histone H3K9, H3K36, and H1.4K26. The KDM4 subfamily mainly consists of four proteins (KDM4A-D), all harboring the Jumonji C domain (JmjC) but with differential substrate specificities. KDM4A-C proteins also possess the double PHD and Tudor domains, whereas KDM4D lacks these domains. KDM4 proteins are overexpressed or deregulated in multiple cancers, cardiovascular diseases, and mental retardation and are thus potential therapeutic targets. Despite extensive efforts, however, there are very few KDM4-selective inhibitors. Defining the exact physiological and oncogenic functions of KDM4 demethylase will provide the foundation for the discovery of novel potent inhibitors. In this review, we focus on recent studies highlighting the oncogenic functions of KDM4s and the interplay between KDM4-mediated epigenetic and metabolic pathways in cancer. We also review currently available KDM4 inhibitors and discuss their potential as therapeutic agents for cancer treatment.