Fusion proteins of the retinoic acid receptor-alpha recruit histone deacetylase in promyelocytic leukaemia

PML::RARA and GATA2 proteins interact via DNA templates to induce aberrant self-renewal in mouse and human hematopoietic cells

The underlying mechanism(s) by which the PML::RARA fusion protein initiates acute promyelocytic leukemia is not yet clear. We defined the genomic binding sites of PML::RARA in primary mouse and human hematopoietic progenitor cells with V5-tagged PML::RARA, using anti-V5-PML::RARA chromatin immunoprecipitation sequencing and CUT&RUN approaches. Most genomic PML::RARA binding sites were found in regions that were already chromatin-accessible (defined by ATAC-seq) in unmanipulated, wild-type promyelocytes, suggesting that these regions are "open" prior to PML::RARA expression. We found that GATA binding motifs, and the direct binding of the chromatin "pioneering factor" GATA2, were significantly enriched near PML::RARA binding sites. Proximity labeling studies revealed that PML::RARA interacts with ~250 proteins in primary mouse hematopoietic cells; GATA2 and 33 others require PML::RARA binding to DNA for the interaction to occur, suggesting that binding to their cognate DNA target motifs may stabilize their interactions. In the absence of PML::RARA, Gata2 overexpression induces many of the same epigenetic and transcriptional changes as PML::RARA. These findings suggested that PML::RARA may indirectly initiate its transcriptional program by activating Gata2 expression: Indeed, we demonstrated that inactivation of Gata2 prior to PML::RARA expression prevented its ability to induce self-renewal. These data suggested that GATA2 binding creates accessible chromatin regions enriched for both GATA and Retinoic Acid Receptor Element motifs, where GATA2 and PML::RARA can potentially bind and interact with each other. In turn, PML::RARA binding to DNA promotes a feed-forward transcriptional program by positively regulating Gata2 expression. Gata2 may therefore be required for PML::RARA to establish its transcriptional program.

History of Developing Acute Promyelocytic Leukemia Treatment and Role of Promyelocytic Leukemia Bodies

The story of acute promyelocytic leukemia (APL) discovery, physiopathology, and treatment is a unique journey, transforming the most aggressive form of leukemia to the most curable. It followed an empirical route fueled by clinical breakthroughs driving major advances in biochemistry and cell biology, including the discovery of PML nuclear bodies (PML NBs) and their central role in APL physiopathology. Beyond APL, PML NBs have emerged as key players in a wide variety of biological functions, including tumor-suppression and SUMO-initiated protein degradation, underscoring their broad importance. The APL story is an example of how clinical observations led to the incremental development of the first targeted leukemia therapy. The understanding of APL pathogenesis and the basis for cure now opens new insights in the treatment of other diseases, especially other acute myeloid leukemias.

High-Fat Diet Promotes Acute Promyelocytic Leukemia through PPARδ-Enhanced Self-renewal of Preleukemic Progenitors

Risk and outcome of acute promyelocytic leukemia (APL) are particularly worsened in obese-overweight individuals, but the underlying molecular mechanism is unknown. In established mouse APL models (Ctsg-PML::RARA), we confirmed that obesity induced by high-fat diet (HFD) enhances leukemogenesis by increasing penetrance and shortening latency, providing an ideal model to investigate obesity-induced molecular events in the preleukemic phase. Surprisingly, despite increasing DNA damage in hematopoietic stem cells (HSC), HFD only minimally increased mutational load, with no relevant impact on known cancer-driving genes. HFD expanded and enhanced self-renewal of hematopoietic progenitor cells (HPC), with concomitant reduction in long-term HSCs. Importantly, linoleic acid, abundant in HFD, fully recapitulates the effect of HFD on the self-renewal of PML::RARA HPCs through activation of peroxisome proliferator-activated receptor delta, a central regulator of fatty acid metabolism. Our findings inform dietary/pharmacologic interventions to counteract obesity-associated cancers and suggest that nongenetic factors play a key role.

PREVENTION RELEVANCE

Our work informs interventions aimed at counteracting the cancer-promoting effect of obesity. On the basis of our study, individuals with a history of chronic obesity may still significantly reduce their risk by switching to a healthier lifestyle, a concept supported by evidence in solid tumors but not yet in hematologic malignancies. See related Spotlight, p. 47.

The Contribution of Hippocampal All-Trans Retinoic Acid (ATRA) Deficiency to Alzheimer's Disease: A Narrative Overview of ATRA-Dependent Gene Expression in Post-Mortem Hippocampal Tissue

There is accumulating evidence that vitamin A (VA) deficiency contributes to the pathogenesis and progression of Alzheimer's disease (AD). All-trans retinoic acid (ATRA), a metabolite of VA in the brain, serves distinct roles in the human hippocampus. Agonists of retinoic acid receptors (RAR), including ATRA, promote activation of the non-amyloidogenic pathway by enhancing expression of α-secretases, providing a mechanistic basis for delaying/preventing amyloid beta (Aβ) toxicity. However, whether ATRA is actually deficient in the hippocampi of patients with AD is not clear. Here, using a publicly available human transcriptomic dataset, we evaluated the extent to which ATRA-sensitive genes are dysregulated in hippocampal tissue from post-mortem AD brains, relative to age-matched controls. Consistent with ATRA deficiency, we found significant dysregulation of many ATRA-sensitive genes and significant upregulation of RAR co-repressors, supporting the idea of transcriptional repression of ATRA-mediated signaling. Consistent with oxidative stress and neuroinflammation, Nrf2 and NfkB transcripts were upregulated, respectively. Interestingly, transcriptional targets of Nrf2 were not upregulated, accompanied by upregulation of several histone deacetylases. Overall, our investigation of ATRA-sensitive genes in the human hippocampus bolsters the scientific premise of ATRA depletion in AD and that epigenetic factors should be considered and addressed as part of VA supplementation.

Efficacy and Safety of Valproic Acid in Myelodysplastic Syndrome and Acute Myeloid Leukemia; a Narrative Review

Loads of new therapeutic regimes have been turned up to manage Myelodysplastic syndrome (MDS) and acute myeloid leukemia (AML), particularly in elderly patients who are unfit for intensive chemotherapy. Despite accumulating research, the best MDS and AML management approach is indeterminate. Myelodysplastic syndrome implies a group of various hematopoietic stem cell disorders that may progress to acute myeloid leukemia. These disorders are more frequent in older adults. To the high rate of morbidity and abundant toxicities related to the therapeutic approaches, also, the treatment would be challenging. The clinical effectiveness of valproic acid, a histone deacetylase inhibitor, in MDS and AML patients is unknown, even though it has demonstrated positive activities to promote differentiation and apoptosis in cancer cells. We investigated the clinical research on the effects of valproic acid in conjunction with various drugs, including low-dose cytarabine, all-trans retinoic acid, DNA-hypomethylating agents, hydrazine, and theophylline. We conclude that VPA is a safe and effective treatment option for MDS and AML patients, particularly when used in conjunction with all-trans retinoic acid, DNA-hypomethylating drugs, and hydralazine. However, more randomized clinical studies are required to identify an ideal regimen.

YY1 Knockdown Relieves the Differentiation Block and Restores Apoptosis in AML Cells

In this study we analyzed the expression of Yin and Yang 1 protein (YY1), a member of the noncanonical PcG complexes, in AML patient samples and AML cell lines and the effect of YY1 downregulation on the AML differentiation block. Our results show that YY1 is significantly overexpressed in AML patient samples and AML cell lines and that YY1 knockdown relieves the differentiation block. YY1 downregulation in two AML cell lines (HL-60 and OCI-AML3) and one AML patient sample restored the expression of members of the CEBP protein family, increased the expression of extrinsic growth factors/receptors and surface antigenic markers, induced morphological cell characteristics typical of myeloid differentiation, and sensitized cells to retinoic acid treatment and to apoptosis. Overall, our data show that YY1 is not a secondary regulator of myeloid differentiation but that, if overexpressed, it can play a predominant role in myeloid differentiation block.

The clinically relevant CHK1 inhibitor MK-8776 induces the degradation of the oncogenic protein PML-RARα and overcomes ATRA resistance in acute promyelocytic leukemia cells

Acute promyelocytic leukemia (APL) is a hematological disease characterized by the expression of the oncogenic fusion protein PML-RARα. The current treatment approach for APL involves differentiation therapy using all-trans retinoic acid (ATRA) and arsenic trioxide (ATO). However, the development of resistance to therapy, occurrence of differentiation syndrome, and relapses necessitate the exploration of new treatment options that induce differentiation of leukemic blasts with low toxicity. In this study, we investigated the cellular and molecular effects of MK-8776, a specific inhibitor of CHK1, in ATRA-resistant APL cells. Treatment of APL cells with MK-8776 resulted in a decrease in PML-RARα levels, increased expression of CD11b, and increased granulocytic activity consistent with differentiation. Interestingly, we showed that the MK-8776-induced differentiating effect resulted synergic with ATO. We found that the reduction of PML-RARα by MK-8776 was dependent on both proteasome and caspases. Specifically, both caspase-1 and caspase-3 were activated by CHK1 inhibition, with caspase-3 acting upstream of caspase-1. Activation of caspase-3 was necessary to activate caspase-1 and promote PML-RARα degradation. Transcriptomic analysis revealed significant modulation of pathways and upstream regulators involved in the inflammatory response and cell cycle control upon MK-8776 treatment. Overall, the ability of MK-8776 to induce PML-RARα degradation and stimulate differentiation of immature APL cancer cells into more mature forms recapitulates the concept of differentiation therapy. Considering the in vivo tolerability of MK-8776, it will be relevant to evaluate its potential clinical benefit in APL patients resistant to standard ATRA/ATO therapy, as well as in patients with other forms of acute leukemias.

Modulating retinoid-X-receptor alpha (RXRA) expression sensitizes chronic myeloid leukemia cells to imatinib in vitro and reduces disease burden in vivo

Introduction: The ligand-activated transcription factors, nuclear hormone receptors (NHRs), remain unexplored in hematological malignancies except for retinoic acid receptor alpha (RARA). Methods: Here we profiled the expression of various NHRs and their coregulators in Chronic myeloid leukemia (CML) cell lines and identified a significant differential expression pattern between inherently imatinib mesylate (IM)-sensitive and resistant cell lines. Results: Retinoid-X-receptor alpha (RXRA) was downregulated in CML cell lines inherently resistant to IM and in primary CML CD34⁺ cells. Pre-treatment with clinically relevant RXRA ligands improved sensitivity to IM in-vitro in both CML cell lines and primary CML cells. This combination effectively reduced the viability and colony-forming capacity of CML CD34⁺ cells in-vitro. In-vivo, this combination reduced leukemic burden and prolonged survival. Overexpression (OE) of RXRA inhibited proliferation and improved sensitivity to IM in-vitro. In-vivo, RXRA OE cells showed reduced engraftment of cells in the bone marrow, improved sensitivity to IM, and prolonged survival. Both RXRA OE and ligand treatment markedly reduced BCR::ABL1 downstream kinase activation, activating apoptotic cascades and improving sensitivity to IM. Importantly, RXRA OE also led to the disruption of the oxidative capacity of these cells. Conclusion: Combining IM with clinically available RXRA ligands could form an alternative treatment strategy in CML patients with suboptimal response to IM.

Hyperthermia promotes degradation of the acute promyelocytic leukemia driver oncoprotein ZBTB16/RARα

The acute promyelocytic leukemia (APL) driver ZBTB16/RARα is generated by the t(11;17) (q23;q21) chromosomal translocation, which is resistant to combined treatment of all-trans retinoic acid (ATRA) and arsenic trioxide (ATO) or conventional chemotherapy, resulting in extremely low survival rates. In the current study, we investigated the effects of hyperthermia on the oncogenic fusion ZBTB16/RARα protein to explore a potential therapeutic approach for this variant APL. We showed that Z/R fusion protein expressed in HeLa cells was resistant to ATO, ATRA, and conventional chemotherapeutic agents. However, mild hyperthermia (42 °C) rapidly destabilized the ZBTB16/RARα fusion protein expressed in HeLa, 293T, and OCI-AML3 cells, followed by robust ubiquitination and proteasomal degradation. In contrast, hyperthermia did not affect the normal (i.e., unfused) ZBTB16 and RARα proteins, suggesting a specific thermal sensitivity of the ZBTB16/RARα fusion protein. Importantly, we found that the destabilization of ZBTB16/RARα was the initial step for oncogenic fusion protein degradation by hyperthermia, which could be blocked by deletion of nuclear receptor corepressor (NCoR) binding sites or knockdown of NCoRs. Furthermore, SIAH2 was identified as the E3 ligase participating in hyperthermia-induced ubiquitination of ZBTB16/RARα. In short, these results demonstrate that hyperthermia could effectively destabilize and subsequently degrade the ZBTB16/RARα fusion protein in an NCoR-dependent manner, suggesting a thermal-based therapeutic strategy that may improve the outcome in refractory ZBTB16/RARα-driven APL patients in the clinic.

Proteomics-based trapping with single or multiple inactive mutants reproducibly profiles histone deacetylase 1 substrates

Histone deacetylase 1 (HDAC1) plays a key role in diverse cellular processes. With the aberrant expression of HDAC1 linked to many diseases, including cancers, HDAC inhibitors have been used successfully as therapeutics. HDAC1 has been predominantly associated with histone deacetylation and gene expression. Recently, non-histone substrates have revealed diverse roles of HDAC1 beyond epigenetics. To augment discovery of non-histone substrates, we introduced "substrate trapping" to enrich HDAC1 substrates using an inactive mutant. Herein, we performed a series of proteomics studies to test the robustness of HDAC1 substrate trapping. Based on our recent results documenting that different HDAC1 mutants preferentially bound different substrates, which suggested that multiple mutants could be used for efficient trapping, trapping with three single point mutants simultaneously identified several potential substrates uniquely compared to a single mutant alone. However, a greater number of biologically interesting hits were observed using only a single mutant, which suggests that the C151A HDAC1 mutant is the optimal trap. Importantly, comparing independent trials with a single mutant performed by different experimentalists and HEK293 cell populations, trapping was robust and reproducible. Based on the reproducible trapping data, carnosine N-methyltransferase 1 (CARNMT1) was validated as an HDAC1 substrate. The data document that mutant trapping is an effective method for discovery of unanticipated HDAC substrates. SIGNIFICANCE: Histone deacetylase (HDAC) proteins are well established epigenetic transcriptional regulators that deacetylate histone substrates to control gene expression. More recently, deacetylation of non-histone substrates has linked HDAC activity to functions outside of epigenetics. Given the use of HDAC inhibitor drugs as anti-cancer therapeutics, understanding the full functions of HDAC proteins in cell biology is essential to future drug design. To discover unanticipated non-histone substrates and further characterize HDAC functions, inactive mutants have been used to "trap" putative substrates, which were identified with mass spectrometry-based proteomics analysis. Here multiple trapping studies were performed to test the robustness of using inactive mutants and proteomics for HDAC substrate discovery. The data confirm the value of trapping mutants as effective tools to discover HDAC substrates and link HDAC activity to unexpected biological functions.

Multi-omics and machine learning reveal context-specific gene regulatory activities of PML::RARA in acute promyelocytic leukemia

The PML::RARA fusion protein is the hallmark driver of Acute Promyelocytic Leukemia (APL) and disrupts retinoic acid signaling, leading to wide-scale gene expression changes and uncontrolled proliferation of myeloid precursor cells. While known to be recruited to binding sites across the genome, its impact on gene regulation and expression is under-explored. Using integrated multi-omics datasets, we characterize the influence of PML::RARA binding on gene expression and regulation in an inducible PML::RARA cell line model and APL patient ex vivo samples. We find that genes whose regulatory elements recruit PML::RARA are not uniformly transcriptionally repressed, as commonly suggested, but also may be upregulated or remain unchanged. We develop a computational machine learning implementation called Regulatory Element Behavior Extraction Learning to deconvolute the complex, local transcription factor binding site environment at PML::RARA bound positions to reveal distinct signatures that modulate how PML::RARA directs the transcriptional response.

Defined Human Leukemic CD34+ Liquid Cultures to Study HDAC/Transcriptional Repressor Complexes

Defined human primary cell model systems with growth dependence on oncogenes are highly requested to investigate tumor pathogenesis and to validate pharmacological inhibitors that specifically target oncoproteins and their executing protein complex partners. In acute myeloid leukemia (AML), transcription factors such as RUNX1 and MLL1, which are important for normal blood cell development, frequently harbor mutations including chromosomal translocations with other coding genes, resulting in tumor-promoting gain-of-function fusion proteins. These oncoproteins completely modify transcriptional programs, thereby inducing malignant cell phenotypes. A common theme of the chimeric gene products is their physical interaction with a variety of chromatin-modifying effector molecules, including histone acetyltransferases (HATs) and histone deacetylases (HDACs). These aberrant multiprotein machineries disturb gene expression and promote malignant cell growth. In this chapter, we briefly summarize the current understanding regarding AML-associated oncogene-driven human CD34+ blood progenitor cell expansion in ex vivo liquid cultures. We provide a step-by-step protocol to establish oncogene-induced human CD34+ blood progenitor cell cultures suitable to analyze the impact of transcriptional repressor/HDAC activity in these human AML cell models.

Small molecule-induced epigenomic reprogramming of APL blasts leading to antiviral-like response and c-MYC downregulation

Acute promyelocytic leukemia (APL) is an aggressive subtype of acute myeloid leukemia (AML) in which the PML/RARα fusion protein exerts oncogenic activities by recruiting repressive complexes to the promoter of specific target genes. Other epigenetic perturbations, as alterations of histone H3 lysine 9 trimethylation (H3K9me3), have been frequently found in AMLs and are associated with leukemogenesis and leukemia progression. Here, we characterized the epigenomic effects of maltonis, a novel maltol-derived molecule, in APL cells. We demonstrate that maltonis treatments induce a profound remodulation of the histone code, reducing global H3K9me3 signal and modulating other histone post-translational modifications. Transcriptomic and epigenomic analyses revealed that maltonis exposure induces changes of genes expression associated with a genomic redistribution of histone H3 lysine 4 trimethylation (H3K4me3) and lysine 27 acetylation (H3K27ac). Upregulation of interferon alpha and gamma response and downregulation of c-MYC target genes, in function of c-MYC reduced expression (monitored in all the hematopoietic neoplasms tested), represent the most significant modulated pathways. These data demonstrate the ability of maltonis to epigenetically reprogram the gene expression profile of APL cells, inducing an intriguing antiviral-like response, concomitantly with the downregulation of c-MYC-related pathways, thus making it an attractive candidate for antileukemic therapy.

Noncanonical EZH2 drives retinoic acid resistance of variant acute promyelocytic leukemias

Cancer cell heterogeneity is a major driver of therapy resistance. To characterize resistant cells and their vulnerabilities, we studied the PLZF-RARA variant of acute promyelocytic leukemia, resistant to retinoic acid (RA), using single-cell multiomics. We uncovered transcriptional and chromatin heterogeneity in leukemia cells. We identified a subset of cells resistant to RA with proliferation, DNA replication, and repair signatures that depend on a fine-tuned E2F transcriptional network targeting the epigenetic regulator enhancer of zeste homolog 2 (EZH2). Epigenomic and functional analyses validated the driver role of EZH2 in RA resistance. Targeting pan-EZH2 activities (canonical/noncanonical) was necessary to eliminate leukemia relapse-initiating cells, which underlies a dependency of resistant cells on an EZH2 noncanonical activity and the necessity to degrade EZH2 to overcome resistance. Our study provides critical insights into the mechanisms of RA resistance that allow us to eliminate treatment-resistant leukemia cells by targeting EZH2, thus highlighting a potential targeted therapy approach. Beyond RA resistance and acute promyelocytic leukemia context, our study also demonstrates the power of single-cell multiomics to identify, characterize, and clear therapy-resistant cells.

Mouse promyelocytic leukemia zinc finger protein (PLZF) regulates hepatic lipid and glucose homeostasis dependent on SIRT1

Previous studies have demonstrated that promyelocytic leukemia zinc finger protein (PLZF) promotes the expression of gluconeogenic genes and hepatic glucose output, which leads to hyperglycemia. However, the role played by PLZF in regulating lipid metabolism is not known. In this study, we aimed to examine the function of PLZF in regulating hepatic lipid and glucose homeostasis and the underlying mechanisms. The expression of PLZF was determined in different mouse models with regard to non-alcoholic fatty liver disease (NAFLD). In the next step, adenoviruses that express PLZF (Ad-PLZF) or PLZF-specific shRNA (Ad-shPLZF) were utilized to alter PLZF expression in mouse livers and in primary hepatocytes. For the phenotype of the fatty liver, histologic and biochemical analyses of hepatic triglyceride (TG), serum TG and cholesterol levels were carried out. The underlying molecular mechanism for the regulation of lipid metabolism by PLZF was further explored using luciferase reporter gene assay and ChIP analysis. The results demonstrated that PLZF expression was upregulated in livers derived from ob/ob, db/db and diet-induced obesity (DIO) mice. Liver PLZF-overexpressing C57BL/6J mice showed fatty liver phenotype, liver inflammation, impaired glucose tolerance and insulin sensitivity. On the other hand, hepatic PLZF knockdown in db/db and DIO mice alleviated hepatic steatosis. Of note, we found that PLZF activates SREBP-1c gene transcription through binding directly to the promoter fragment of this gene, which would induce a repressor-to-activator conversion depending on its interaction with SIRT1 in the role played by PLZF in the transcription process through deacetylation. Thus, PLZF is identified as an essential regulator of hepatic lipid and glucose metabolism, where the modulation of its liver expression could open up a therapeutic path for treating NAFLD.

Impact of Histone Modifications and Their Therapeutic Targeting in Hematological Malignancies

Hematologic malignancies are a large and heterogeneous group of neoplasms characterized by complex pathogenetic mechanisms. The abnormal regulation of epigenetic mechanisms and specifically, histone modifications, has been demonstrated to play a central role in hematological cancer pathogenesis and progression. A variety of epigenetic enzymes that affect the state of histones have been detected as deregulated, being either over- or underexpressed, which induces changes in chromatin compaction and, subsequently, affects gene expression. Recent advances in the field of epigenetics have revealed novel therapeutic targets, with many epigenetic drugs being investigated in clinical trials. The present review focuses on the biological impact of histone modifications in the pathogenesis of hematologic malignancies, describing a wide range of therapeutic agents that have been discovered to target these alterations and are currently under investigation in clinical trials.

How retinoic acid and arsenic transformed acute promyelocytic leukemia therapy

Acute promyelocytic leukemia (APL) is associated with severe coagulopathy leading to rapid morbidity and mortality if left untreated. The definitive diagnosis of APL is made by identifying a balanced reciprocal translocation between chromosomes 15 and 17. This t(15;17) results in a fusion transcript of promyelocytic leukemia (PML) and retinoic acid receptor alpha (RARA) genes and the expression of a functional PML/RARA protein. Detection of a fused PML/RARA genomic DNA sequence using fluorescence in situ hybridization (FISH) or by detection of the PML/RARA fusion transcript via reverse transcriptase polymerase chain reaction (RT-PCR) has revolutionized the diagnosis and monitoring of APL. Once confirmed, APL is cured in over 90% of cases, making it the most curable subtype of acute leukemia today. Patients with low-risk APL are successfully treated using a chemotherapy-free combination of all-trans retinoic acid and arsenic trioxide (ATO). In this review, we explore the work that has gone into the modern-day diagnosis and highly successful treatment of this once devastating leukemia.

A direct comparison between AML1-ETO and ETO2-GLIS2 leukemia fusion proteins reveals context-dependent binding and regulation of target genes and opposite functions in cell differentiation

The ETO-family transcriptional corepressors, including ETO, ETO2, and MTGR1, are all involved in leukemia-causing chromosomal translocations. In every case, an ETO-family corepressor acquires a DNA-binding domain (DBD) to form a typical transcription factor—the DBD binds to DNA, while the ETO moiety manifests transcriptional activity. A directly comparative study of these “homologous” fusion transcription factors may clarify their similarities and differences in regulating transcription and leukemogenesis. Here, we performed a side-by-side comparison between AML1-ETO and ETO2-GLIS2, the most common fusion proteins in M2-and M7-subtypes of acute myeloid leukemia, respectively, by inducible expression of them in U937 leukemia cells. We found that, although AML1-ETO and ETO2-GLIS2 can use their own DBDs to bind DNA, they share a large proportion of genome-wide binding regions dependent on other cooperative transcription factors, including the ETS-, bZIP- and bHLH-family proteins. AML1-ETO acts as either transcriptional repressor or activator, whereas ETO2-GLIS2 mainly acts as activator. The repressor-versus-activator functions of AML1-ETO might be determined by the abundance of cooperative transcription factors/cofactors on the target genes. Importantly, AML1-ETO and ETO2-GLIS2 differentially regulate key transcription factors in myeloid differentiation including PU.1 and C/EBPβ. Consequently, AML1-ETO inhibits, but ETO2-GLIS2 facilitates, myeloid differentiation of U937 cells. This function of ETO2-GLIS2 is reminiscent of a similar effect of MLL-AF9 as previously reported. Taken together, this directly comparative study between AML1-ETO and ETO2-GLIS2 in the same cellular context provides insights into context-dependent transcription regulatory mechanisms that may underlie how these seemingly “homologous” fusion transcription factors exert distinct functions to drive different subtypes of leukemia.

The molecular biology and therapeutic potential of Nrf2 in leukemia

NF-E2-related factor 2 (Nrf2) transcription factor has contradictory roles in cancer, which can act as a tumor suppressor or a proto-oncogene in different cell conditions (depending on the cell type and the conditions of the cell environment). Nrf2 pathway regulates several cellular processes, including signaling, energy metabolism, autophagy, inflammation, redox homeostasis, and antioxidant regulation. As a result, it plays a crucial role in cell survival. Conversely, Nrf2 protects cancerous cells from apoptosis and increases proliferation, angiogenesis, and metastasis. It promotes resistance to chemotherapy and radiotherapy in various solid tumors and hematological malignancies, so we want to elucidate the role of Nrf2 in cancer and the positive point of its targeting. Also, in the past few years, many studies have shown that Nrf2 protects cancer cells, especially leukemic cells, from the effects of chemotherapeutic drugs. The present paper summarizes these studies to scrutinize whether targeting Nrf2 combined with chemotherapy would be a therapeutic approach for leukemia treatment. Also, we discussed how Nrf2 and NF-κB work together to control the cellular redox pathway. The role of these two factors in inflammation (antagonistic) and leukemia (synergistic) is also summarized.

Sox9 directs divergent epigenomic states in brain tumor subtypes

Epigenetic dysregulation is a universal feature of cancer that results in altered patterns of gene expression that drive malignancy. Brain tumors exhibit subtype-specific epigenetic alterations; however, the molecular mechanisms responsible for these diverse epigenetic states remain unclear. Here, we show that the developmental transcription factor Sox9 differentially regulates epigenomic states in high-grade glioma (HGG) and ependymoma (EPN). Using our autochthonous mouse models, we found that Sox9 suppresses HGG growth and expands associated H3K27ac states, while promoting ZFTA-RELA (ZR^FUS) EPN growth and diminishing H3K27ac states. These contrasting roles for Sox9 correspond with protein interactions with histone deacetylating complexes in HGG and an association with the ZR^FUS oncofusion in EPN. Mechanistic studies revealed extensive Sox9 and ZR^FUS promoter co-occupancy, indicating functional synergy in promoting EPN tumorigenesis. Together, our studies demonstrate how epigenomic states are differentially regulated in distinct subtypes of brain tumors, while revealing divergent roles for Sox9 in HGG and EPN tumorigenesis.

All-Trans-Retinoic Acid Combined With Valproic Acid Can Promote Differentiation in Myeloid Leukemia Cells by an Autophagy Dependent Mechanism

Acute myeloid leukemia (AML) is an aggressive blood cancer with an overall survival of 30%. One form of AML, acute promyelocytic leukemia (APL) has become more than 90% curable with differentiation therapy, consisting of all-trans-retinoic acid (ATRA) and arsenic trioxide (ATO). Application of differentiation therapy to other AML subtypes would be a major treatment advance. Recent studies have indicated that autophagy plays a key role in the differentiation of ATRA-responsive APL cells. In this study, we have investigated whether differentiation could be enhanced in ATRA resistant cells by promoting autophagy induction with valproic acid (VPA). ATRA sensitive (NB4) and resistant leukemia cells (NB4R and THP-1) were co-treated with ATRA and valproic acid, followed by assessment of autophagy and differentiation. The combination of VPA and ATRA induced autophagic flux and promoted differentiation in ATRA-sensitive and -resistant cell lines. shRNA knockdown of ATG7 and TFEB autophagy regulators impaired both autophagy and differentiation, demonstrating the importance of autophagy in the combination treatment. These data suggest that ATRA combined with valproic acid can promote differentiation in myeloid leukemia cells by mechanism involving autophagy.

Co-targeting leukemia-initiating cells and leukemia bulk leads to disease eradication

According to a hierarchical model, targeting leukemia-initiating cells (LICs) was speculated to achieve complete remission (CR) or cure. Nonetheless, increasing evidence emphasized the plasticity of differentiated blasts undergoing interconversion into LICs. We exploited murine models of acute promyelocytic leukemia (APL), a subtype of acute myeloid leukemia driven by the promyelocytic leukemia/retinoic acid receptor (PML-RARα) oncofusion protein, which recruits histone deacetylase (HDAC)-containing complexes. We studied APLs with different LIC frequencies and investigated the effect of two HDAC inhibitors: valproic acid (VPA), with relative selectivity towards class I HDAC enzymes and vorinostat/suberoylanilide hydroxamic acid (SAHA) (pan-HDAC inhibitor) in combination with all-trans retinoic acid (ATRA), on the bulk APL cells and APL LICs. Indeed, we found that while VPA differentiates the bulk APL cells, SAHA selectively targets LICs. ATRA + VPA + SAHA combination efficiently induced CR in an APL model with lower LIC frequency. Substituting ATRA with synthetic retinoids as etretinate which promotes APL differentiation without downregulating PML/RARα compromised the therapeutic benefit of ATRA + VPA + SAHA regimen. Altogether, our study emphasizes the therapeutic power of co-targeting the plasticity and heterogeneity of cancer -herein demonstrated by tackling LICs and bulk leukemic blasts - to achieve and maintain CR.

Deep learning identifies Acute Promyelocytic Leukemia in bone marrow smears

BACKGROUND

Acute promyelocytic leukemia (APL) is considered a hematologic emergency due to high risk of bleeding and fatal hemorrhages being a major cause of death. Despite lower death rates reported from clinical trials, patient registry data suggest an early death rate of 20%, especially for elderly and frail patients. Therefore, reliable diagnosis is required as treatment with differentiation-inducing agents leads to cure in the majority of patients. However, diagnosis commonly relies on cytomorphology and genetic confirmation of the pathognomonic t(15;17). Yet, the latter is more time consuming and in some regions unavailable.

METHODS

In recent years, deep learning (DL) has been evaluated for medical image recognition showing outstanding capabilities in analyzing large amounts of image data and provides reliable classification results. We developed a multi-stage DL platform that automatically reads images of bone marrow smears, accurately segments cells, and subsequently predicts APL using image data only. We retrospectively identified 51 APL patients from previous multicenter trials and compared them to 1048 non-APL acute myeloid leukemia (AML) patients and 236 healthy bone marrow donor samples, respectively.

RESULTS

Our DL platform segments bone marrow cells with a mean average precision and a mean average recall of both 0.97. Further, it achieves high accuracy in detecting APL by distinguishing between APL and non-APL AML as well as APL and healthy donors with an area under the receiver operating characteristic of 0.8575 and 0.9585, respectively, using visual image data only.

CONCLUSIONS

Our study underlines not only the feasibility of DL to detect distinct morphologies that accompany a cytogenetic aberration like t(15;17) in APL, but also shows the capability of DL to abstract information from a small medical data set, i. e. 51 APL patients, and infer correct predictions. This demonstrates the suitability of DL to assist in the diagnosis of rare cancer entities. As our DL platform predicts APL from bone marrow smear images alone, this may be used to diagnose APL in regions were molecular or cytogenetic subtyping is not routinely available and raise attention to suspected cases of APL for expert evaluation.

History of Acute Promyelocytic Leukemia

In this article, we discuss the history of acute promyelocytic leukemia (APL) from the pre-therapeutic era, which began after its recognition by Hillestad in 1947 as a nosological entity, to the present day. It is a paradigmatic history that has transformed the “most malignant leukemia form” into the most curable one. The identification of a balanced reciprocal translocation between chromosomes 15 and 17, resulting in fusion between the promyelocytic leukemia gene and the retinoic acid receptor alpha, has been crucial in understanding the mechanisms of leukemogenesis, and responsible for the peculiar response to targeted therapy with all-trans retinoic acid (ATRA) and arsenic trioxide (ATO). We review the milestones that marked successive therapeutic advances, beginning with the introduction of the first successful chemotherapy in the early 1970s, followed by a subsequent incorporation of ATRA and ATO in the late 1980s and early 1990s which have revolutionized the treatment of this disease. Over the past two decades, treatment optimization has relied on the combination of ATRA, ATO, and chemotherapy according to risk-adapted approaches, which together with improvements in supportive therapy have paved the way for cure for most patients with APL.

Suppressive role of E3 ubiquitin ligase FBW7 in type I diabetes in non-obese diabetic mice through mediation of ubiquitination of EZH2

The current study tried to uncover the molecular mechanism of E3 ubiquitin ligase F-box and WD repeat domain-containing 7 (FBW7) in a heritable autoimmune disease, type I diabetes (T1D). After streptozotocin-induced T1D model establishment in non-obese diabetic (NOD) mouse, the protein expression of FBW7, enhancer of zeste homolog 2 (EZH2), and Zinc finger and BTB domain containing 16 (ZBTB16) was quantified. Next, splenocytes and pancreatic beta cells were isolated to measure the production of pro-inflammatory cytokines in splenocytes, as well as islet beta-cell apoptosis. Additionally, the stability of EZH2 induced by FBW7 was analyzed by cycloheximide chase assay. The binding affinity of FBW7 and EZH2 and the consequence of ubiquitination were monitored by co-immunoprecipitation assay. Last, a chromatin immunoprecipitation assay was employed to analyze the accumulation of EZH2 and H3K27me3 at the ZBTB16 promoter region. Our study demonstrated downregulated FBW7 and ZBTB16 and upregulated EZH2 in diabetic NOD mice. Overexpression of FBW7 in the NOD mice inhibited pro-inflammatory cytokine release in the splenocytes and the apoptosis of islets beta cells. FBW7 destabilized EZH2 and accelerated ubiquitin-dependent degradation. EZH2 and H3K27me3 downregulated the ZBTB16 expression by accumulating in the ZBTB16 promoter and methylation. FBW7 upregulates the expression of ZBTB16 by targeting histone methyltransferase EZH2 thus reducing the occurrence of T1D.

KDM5A suppresses PML-RARα target gene expression and APL differentiation through repressing H3K4me2

Epigenetic abnormalities are frequently involved in the initiation and progression of cancers, including acute myeloid leukemia (AML). A subtype of AML, acute promyelocytic leukemia (APL), is mainly driven by a specific oncogenic fusion event of promyelocytic leukemia-RA receptor fusion oncoprotein (PML-RARα). PML-RARα was reported as a transcription repressor through the interaction with nuclear receptor corepressor and histone deacetylase complexes leading to the mis-suppression of its target genes and differentiation blockage. Although previous studies were mainly focused on the connection of histone acetylation, it is still largely unknown whether alternative epigenetics mechanisms are involved in APL progression. KDM5A is a demethylase of histone H3 lysine 4 di- and tri-methylations (H3K4me2/3) and a transcription corepressor. Here, we found that the loss of KDM5A led to APL NB4 cell differentiation and retarded growth. Mechanistically, through epigenomics and transcriptomics analyses, KDM5A binding was detected in 1889 genes, with the majority of the binding events at promoter regions. KDM5A suppressed the expression of 621 genes, including 42 PML-RARα target genes, primarily by controlling the H3K4me2 in the promoters and 5' end intragenic regions. In addition, a recently reported pan-KDM5 inhibitor, CPI-455, on its own could phenocopy the differentiation effects as KDM5A loss in NB4 cells. CPI-455 treatment or KDM5A knockout could greatly sensitize NB4 cells to all-trans retinoic acid-induced differentiation. Our findings indicate that KDM5A contributed to the differentiation blockage in the APL cell line NB4, and inhibition of KDM5A could greatly potentiate NB4 differentiation.

Variant Acute Promyelocytic Leukemia Presenting Without Auer Rods Highlights the Need for Correlation with Cytogenetic Data in Leukemia Diagnosis

Variant acute promyelocytic leukemia (vAPL) is a rare leukemia characterized by rearrangement between RARα and a non-PML partner gene. This type of leukemia can be difficult to recognize by histomorphologic evaluation, particularly in patients with few or no Auer rods, and by flow cytometry, but it can be identified by distinct cytogenetic features. Herein, we report on a patient with vAPL with t(11;17)(q23;q21) who presented an initial diagnostic challenge. Detailed flow cytometry findings are presented for this rare entity. Our case study also presents novel treatment (chemotherapy in combination with venetoclax) chosen based on mechanistic data from preclinical studies.

Hyperthermia Selectively Destabilizes Oncogenic Fusion Proteins

The PML/RARα fusion protein is the oncogenic driver in acute promyelocytic leukemia (APL). Although most APL cases are cured by PML/RARα-targeting therapy, relapse and resistance can occur due to drug-resistant mutations. Here we report that thermal stress destabilizes the PML/RARα protein, including clinically identified drug-resistant mutants. AML1/ETO and TEL/AML1 oncofusions show similar heat shock susceptibility. Mechanistically, mild hyperthermia stimulates aggregation of PML/RARα in complex with nuclear receptor corepressors leading to ubiquitin-mediated degradation via the SIAH2 E3 ligase. Hyperthermia and arsenic therapy destabilize PML/RARα via distinct mechanisms and are synergistic in primary patient samples and in vivo, including three refractory APL cases. Collectively, our results suggest that by taking advantage of a biophysical vulnerability of PML/RARα, thermal therapy may improve prognosis in drug-resistant or otherwise refractory APL. These findings serve as a paradigm for therapeutic targeting of fusion oncoprotein-associated cancers by hyperthermia.

SIGNIFICANCE

Hyperthermia destabilizes oncofusion proteins including PML/RARα and acts synergistically with standard arsenic therapy in relapsed and refractory APL. The results open up the possibility that heat shock sensitivity may be an easily targetable vulnerability of oncofusion-driven cancers.See related commentary by Wu et al., p. 300.

Differentiation syndrome with lower-intensity treatments for acute myeloid leukemia

Differentiation Syndrome (DS) has been identified in a subset of patients undergoing treatment with novel classes of differentiating therapies for acute myeloid leukemia (AML) such as IDH and FLT3 inhibitors. While DS is a well-known treatment-related complication in acute promyelocytic leukemia (APL), efforts are still ongoing to standardize diagnostic and treatment parameters for DS in AML. Though the rates of incidence vary, many of the signs and symptoms of DS are common between APL and AML. So, DS can lead to fatal complications in AML, but prompt management is usually effective and rarely necessitates interruption or discontinuation of AML therapy.

C/EBPα is indispensable for PML/RARα-mediated suppression of long non-coding RNA NEAT1 in acute promyelocytic leukemia cells

Better understanding of the transcriptional regulatory network in acute promyelocytic leukemia (APL) cells is critical to illustrate the pathogenesis of other types of acute myeloid leukemia. Previous studies have primarily focused on the retinoic acid signaling pathway and how it is interfered with by promyelocytic leukemia/retinoic acid receptor-α (PML/RARα) fusion protein. However, this hardly explains how APL cells are blocked at the promyelocytic stage. Here, we demonstrated that C/EBPα bound and transactivated the promoter of long non-coding RNA NEAT1, an essential element for terminal differentiation of APL cells, through C/EBP binding sites. More importantly, PML/RARα repressed C/EBPα-mediated transactivation of NEAT1 through binding to NEAT1 promoter. Consistently, mutation of the C/EBP sites or deletion of retinoic acid responsive elements (RAREs) and RARE half motifs abrogated the PML/RARα-mediated repression. Moreover, silencing of C/EBPα attenuated ATRA-induced NEAT1 upregulation and APL cell differentiation. Finally, simultaneous knockdown of C/EBPα and C/EBPβ reduces ATRA-induced upregulation of C/EBPε and dramatically impaired NEAT1 activation and APL cell differentiation. In sum, C/EBPα binds and transactivates NEAT1 whereas PML/RARα represses this process. This study describes an essential role for C/EBPα in PML/RARα-mediated repression of NEAT1 and suggests that PML/RARα could contribute to the pathogenesis of APL through suppressing C/EBPα targets.

Transcription Repressor Protein ZBTB25 Associates with HDAC1-Sin3a Complex in Mycobacterium tuberculosis-Infected Macrophages, and Its Inhibition Clears Pathogen by Autophagy

Downregulation of host gene expression is a key strategy employed by intracellular pathogens for their survival in macrophages and subsequent pathogenesis. In a previous study, we have shown that histone deacetylase 1 (HDAC1) levels go up in macrophages infected with Mycobacterium tuberculosis, and it hypoacetylates histone H3 at the promoter of IL-12B gene, leading to its downregulation. We now show that after infection with M. tuberculosis, HDAC1 is phosphorylated, and the levels of phosphorylated HDAC1 (pHDAC1) increase significantly in macrophages. We found that transcriptional repressor protein zinc finger and BTB domain 25 (ZBTB25) and transcriptional corepressor Sin3a associate with the HDAC1 silencing complex, which is recruited to the promoter of IL-12B to downregulate its expression in infected macrophages. Knocking down of ZBTB25 enhanced release of IL-12p40 from infected macrophages. Inhibition of HDAC1 and ZBTB25 promoted colocalization of M. tuberculosis and LC3 (microtubule-associated protein 1A/1B-light chain 3) in autophagosomes. Induction of autophagy resulted in the killing of intracellular M. tuberculosis. Enhanced phosphorylation of JAK2 and STAT4 was observed in macrophages upon treatment with HDAC1 and ZBTB inhibitors, and inhibition of JAK2/STAT4 negated the killing of the intracellular pathogen, suggesting their role in the autophagy-mediated killing of intracellular M. tuberculosis. In view of the emergence of drug resistance in M. tuberculosis, host-directed therapy is an attractive alternative strategy to combat tuberculosis (TB). HDACs have been proposed to be host targets for TB treatment. Our study indicates that ZBTB25, a functional subunit of the HDAC1/Sin3a repressor complex involved in IL-12B suppression, could be an alternative target for host-directed anti-TB therapy.

IMPORTANCE Following infection with M. tuberculosis, levels of HDAC1 go up in macrophages, and it is recruited to the promoter of IL-12B where it hypoacetylates histone H3, leading to the downregulation of the gene. Here, we show that host transcriptional repressor protein ZBTB25 and transcriptional corepressor Sin3a associate with HDAC1 in the silencing complex. Knocking down of ZBTB25 prevented the recruitment of the complex to the promoter and consequently enhanced the gene expression and the release of IL-12p40 from infected macrophages. Pharmacological inhibition of ZBTB25 in infected macrophages resulted in the induction of autophagy and killing of intracellular M. tuberculosis. Drug-resistant TB is a serious challenge to TB control programs all over the world which calls for finding alternative therapeutic methods. Host-directed therapy is gaining significant momentum in treating infectious diseases. We propose that ZBTB25 is a potential target for host-directed treatment of TB.

CUDC-101 overcomes arsenic trioxide resistance via caspase-dependent promyelocytic leukemia-retinoic acid receptor alpha degradation in acute promyelocytic leukemia

Although arsenic trioxide (ATO) treatment has transformed acute promyelocytic leukemia (APL) from the most fatal to the most curable hematological cancer, many high-risk APL patients who fail to achieve a complete molecular remission or relapse become resistant to ATO. Herein, we report that 7-(4-(3-ethynylphenylamino)-7-methoxyquinazolin-6-yloxy)-N-hydroxyheptanamide (CUDC-101) exhibits specific anticancer effects on APL and ATO-resistant APL in vitro and in vivo, while showing negligible cytotoxic effect on the noncancerous cells including normal CD34 cells and bone marrow mesenchymal stem cells from APL patients. Further mechanistic studies show that CUDC-101 triggers caspase-dependent degradation of the promyelocytic leukemia-retinoic acid receptor alpha fusion protein. As a result, APL and ATO-resistant APL cells undergo apoptosis upon CUDC-101 treatment and this apoptosis-inducing effect is even stronger than that of ATO. Finally, using a xenograft mouse model, we demonstrated that CUDC-101 significantly represses leukemia development in vivo. In conclusion, these results suggested that CUDC-101 can serve as a potential candidate drug for APL, particularly for ATO-resistant APL.

Transcriptional and Metabolic Dissection of ATRA-Induced Granulocytic Differentiation in NB4 Acute Promyelocytic Leukemia Cells

Acute promyelocytic leukemia (APL) is a hematological disease characterized by a balanced reciprocal translocation that leads to the synthesis of the oncogenic fusion protein PML-RARα. APL is mainly managed by a differentiation therapy based on the administration of all-trans retinoic acid (ATRA) and arsenic trioxide (ATO). However, therapy resistance, differentiation syndrome, and relapses require the development of new low-toxicity therapies based on the induction of blasts differentiation. In keeping with this, we reasoned that a better understanding of the molecular mechanisms pivotal for ATRA-driven differentiation could definitely bolster the identification of new therapeutic strategies in APL patients. We thus performed an in-depth high-throughput transcriptional profile analysis and metabolic characterization of a well-established APL experimental model based on NB4 cells that represent an unevaluable tool to dissect the complex mechanism associated with ATRA-induced granulocytic differentiation. Pathway-reconstruction analysis using genome-wide transcriptional data has allowed us to identify the activation/inhibition of several cancer signaling pathways (e.g., inflammation, immune cell response, DNA repair, and cell proliferation) and master regulators (e.g., transcription factors, epigenetic regulators, and ligand-dependent nuclear receptors). Furthermore, we provide evidence of the regulation of a considerable set of metabolic genes involved in cancer metabolic reprogramming. Consistently, we found that ATRA treatment of NB4 cells drives the activation of aerobic glycolysis pathway and the reduction of OXPHOS-dependent ATP production. Overall, this study represents an important resource in understanding the molecular “portfolio” pivotal for APL differentiation, which can be explored for developing new therapeutic strategies.

Therapeutic strategies targeting somatic stem cells: Chemical approaches

Therapeutic modulation of fate and behavior of somatic stem cells can generate safe and functional cells ex vivo for cell-based therapy, or to repair and regenerate damaged tissues in vivo. Chemical approaches involving small molecules have provided promising approaches for modulating cellular fate and function. These strategies offer opportunities that support regenerative medicine. Here, we discuss strategies targeting somatic stem cells through chemical approaches, highlighting their progression as well as future prospects.

Verification of a ZBTB16 variant in polycystic ovary syndrome patients

RESEARCH QUESTION

This study investigated whether rs1784692 is a risk factor for polycystic ovary syndrome (PCOS) in Han Chinese women.

DESIGN

A case-control study was conducted in Han Chinese women, involving 526 PCOS patients and 522 control participants. A TaqMan MGB probe assay was used to genotype the variant rs1784692. Dominant and additive models were employed for genotype-phenotype association analysis in the PCOS and control samples.

RESULTS

The minor allele C of rs1784692 is protective against PCOS (odds ratio [OR] 0.556, 95% confidence interval [CI] 0.408-0.759, P = 1.83 × 10-4), even after adjustment for body mass index (BMI) and age (ORadj 0.539, 95% CI 0.391-0.743, Padj= 1.62 × 10-4). Genotype-phenotype analysis of the dominant model showed that mean BMI in the CC+CT group was higher than in the TT group in the PCOS group (27.12 ± 5.82 versus 24.57 ± 4.52, P = 1.0 × 10-3), but not in the control groups, indicating that the minor allele C of rs1784692 associates with BMI in women with PCOS. The mean LH (luteinizing hormone) concentration in the CC+CT group was lower than in the TT group in PCOS and control participants (9.33 ± 5.08 versus 10.93 ± 5.91, P = 0.036; 4.39 ± 1.66 versus 4.89 ± 2.07, P = 0.021). Genotype-phenotype analysis of additive model showed that mean BMI in TC group was higher than in the TT group in PCOS patients compared with control participants (27.14 ± 5.81 versus 24.57 ± 4.52, P = 3.06 × 10-3).

CONCLUSIONS

The SNP rs1784692 in gene ZBTB16 is protective against PCOS but is associated with increased BMI in Han Chinese women with PCOS.

Synergistic Enhancement of Cancer Therapy Using HDAC Inhibitors: Opportunity for Clinical Trials

Chemotherapy is one of the most established and effective treatments for almost all types of cancer. However, the elevated toxicity due to the non-tumor-associated effects, development of secondary malignancies, infertility, radiation-induced fibrosis and resistance to treatment limit the effectiveness and safety of treatment. In addition, these multiple factors significantly impact quality of life. Over the last decades, our increased understanding of cancer epigenetics has led to new therapeutic approaches and the promise of improved patient outcomes. Epigenetic alterations are commonly found in cancer, especially the increased expression and activity of histone deacetylases (HDACs). Dysregulation of HDACs are critical to the development and progression of the majority of tumors. Hence, HDACs inhibitors (HDACis) were developed and now represent a very promising treatment strategy. The use of HDACis as monotherapy has shown very positive pre-clinical results, but clinical trials have had only limited success. However, combinatorial regimens with other cancer drugs have shown synergistic effects both in pre-clinical and clinical studies. At the same time, these combinations have enhanced the efficacy, reduced the toxicity and tumor resistance to therapy. In this review, we will examine examples of HDACis used in combination with other cancer drugs and highlight the synergistic effects observed in recent preclinical and clinical studies.

Arsenic induced epigenetic changes and relevance to treatment of acute promyelocytic leukemia and beyond

Epigenetic alterations regulate gene expression without changes in the DNA sequence. It is well-demonstrated that aberrant epigenetic changes contribute to the leukemogenesis of acute promyelocytic leukemia (APL). Arsenic trioxide (ATO) is one of the most common drugs used in the frontline treatment of APL that act through targeting and destabilizing the PML/RARα oncofusion protein. ATO together with all-trans retinoic acid (ATRA) lead to durable remission of more than 90% non-high-risk APL patients, turning APL treatment into a paradigm of oncoprotein targeted cure. Although relapse and drug resistance in APL are yet to be resolved in the clinic, epigenetic machineries might hold the key to address this issue. Further, ATO also showed promising anticancer activities against a variety of malignancies, but its application is particularly restricted due to limited understanding of the mechanism. Thus, a thorough understanding of epigenetic mechanism behind anti-leukemic effects of ATO would benefit the development of ATO-based anticancer strategy. Role of ATRA on APL associated epigenetic alterations has been extensively studied and reviewed. Recently, accumulating evidence suggest that ATO also induces some epigenetic changes that might favor APL eradication. In this article, we comprehensively discuss arsenic induced epigenetic changes and its relevance in APL treatment and beyond, so as to provide novel insights into overcoming arsenic resistance in APL and promote application of this drug to other malignancies.

Emerging Cancer Epigenetic Mechanisms Regulated by All-Trans Retinoic Acid

All-trans retinoic acid (RA), which is the dietary bioactive derivative obtained from animal (retinol) and plant sources (beta-carotene), is a physiological lipid signal of both embryonic and postembryonic development. During pregnancy, either RA deficiency or an excessive RA intake is teratogenic. Too low or too high RA affects not only prenatal, but also postnatal, developmental processes such as myelopoiesis and mammary gland morphogenesis. In this review, we mostly focus on emerging RA-regulated epigenetic mechanisms involving RA receptor alpha (RARA) and Annexin A8 (ANXA8), which is a member of the Annexin family, as well as ANXA8 regulatory microRNAs (miRNAs). The first cancer showing ANXA8 upregulation was reported in acute promyelocytic leukemia (APL), which induces the differentiation arrest of promyelocytes due to defective RA signaling caused by RARA fusion genes as the PML-RARA gene. Over the years, ANXA8 has also been found to be upregulated in other cancers, even in the absence of RARA fusion genes. Mechanistic studies on human mammary cells and mammary glands of mice showed that ANXA8 upregulation is caused by genetic mutations affecting RARA functions. Although not all of the underlying mechanisms of ANXA8 upregulation have been elucidated, the interdependence of RA-RARA and ANXA8 seems to play a relevant role in some normal and tumorigenic settings.

PML Nuclear Body Biogenesis, Carcinogenesis, and Targeted Therapy

Targeted therapy has become increasingly important in cancer therapy. For example, targeting the promyelocytic leukemia PML protein in leukemia has proved to be an effective treatment. PML is the core component of super-assembled structures called PML nuclear bodies (NBs). Although this nuclear megaDalton complex was first observed in the 1960s, the mechanism of its assembly remains poorly understood. We review recent breakthroughs in the PML field ranging from a revised assembly mechanism to PML-driven genome organization and carcinogenesis. In addition, we highlight that oncogenic oligomerization might also represent a promising target in the treatment of leukemias and solid tumors.

Circular RNAs in Embryogenesis and Cell Differentiation With a Focus on Cancer Development

In the recent years thousands of non-coding RNAs have been identified, also thanks to highthroughput sequencing technologies. Among them, circular RNAs (circRNAs) are a well-represented class characterized by the high sequence conservation and cell type specific expression in eukaryotes. They are covalently closed loops formed through back-splicing. Recently, circRNAs were shown to regulate a variety of cellular processes functioning as miRNA sponges, RBP binding molecules, transcriptional regulators, scaffold for protein translation, as well as immune regulators. A growing number of studies are showing that deregulated expression of circRNAs plays important and decisive actions during the development of several human diseases, including cancer. The research on their biogenesis and on the various molecular mechanisms in which they are involved is going very fast, however, there are still few studies that address their involvement in embryogenesis and eukaryotic development. This review has the intent to describe the most recent progress in the study of the biogenesis and molecular activities of circRNAs providing insightful information in the field of embryogenesis and cell differentiation. In addition, we describe the latest research on circRNAs as novel promising biomarkers in diverse types of tumors.

Pan-HDAC inhibitor panobinostat, as a single agent or in combination with PI3K inhibitor, induces apoptosis in APL cells: An emerging approach to overcome MSC-induced resistance

The emergence of the drugs targeting epigenetic alterations has brought an optimistic outlook for cancer patients and probably put an end into the devastating effects of the disease. Given to the prominent involvement of histone deacetylase (HDAC) enzymes in the formation of neoplastic nature of acute promyelocytic leukemia (APL), this study was aimed to evaluate the suppressive effect of pan-HDAC inhibitor panobinostat on both NB4 and primary APL patients cells, either in the context of mono- or co-culture with mesenchymal stem cells (MSC). Panobinostat effectively reduced the survival of APL cells; however, as compared to NB4, the viability of primary cells was inhibited at higher concentrations. Our results also showed that although HDAC inhibition could merely block the survival signals transduced from MSC, the presence of PI3K inhibitor could robustly reinforce panobinostat cytotoxicity; suggesting that MSC-induced activation of PI3K may attenuate, at least partly, the efficacy of HDAC inhibition in APL cells. In addition, cellular and molecular investigations on NB4 revealed that not only panobinostat induced p21-mediated G1 arrest and ROS-mediated apoptosis, but also exerted a superior cytotoxicity when combined with c-Myc and autophagy inhibitors. Last but not least, we found that panobinostat combined with arsenic trioxide (ATO) prompted a synergistic effect and provided an improved therapeutic value in NB4; proposing that the abrogation of HDAC using panobinostat might be a befitting approach in APL, either as a single agent or in a combined-modal strategy.

Nuclear import of NLS- RARα is mediated by importin α/β

The promyelocytic leukemia-retinoic acid receptor α (PML/RARα) is hypothesized to play a vital role in the pathogenesis of acute promyelocytic leukemia (APL). A previous study has demonstrated that PML/RARα is cleaved by neutrophil elastase (NE) in early myeloid cells, which leads to an increase in the nuclear localization signal (NLS) in RARα and in the incidence of APL. In this study, we explored the effects of NLS-RARα on acute myeloid leukemia (AML) cells and studied the mechanism of its localization. LV-NLS-RARα recombinant lentivirus and negative control LV-NC lentivirus were transfected into HL-60 cells and U937 cells while mutant NLS-RARα were transfected into U937 cells, and all groups were treated with 1α, 25-dihydroxyvitamin D3(1,25D3). The results showed that NLS-RARα was located mainly in the nucleus while mutant NLS-RARα was located in the cytoplasm. Overexpression of NLS-RARα downregulated the expression of CD11b, CD11c, CD14, and three forms of CEBPβ compared to the overexpression of NC and mutant NLS-RARα. It was speculated that the abnormal localization of NLS-RARα was mediated via importin-α/β in the pathogenesis of APL. By producing point mutations in the two NLSs in NLS-RARα, we showed that the nuclear import of NLS-RARα was mainly dependent on the NLS of the RARα portion. Subsequently, we found that importin-α1 (KPNA2)/importin-β1 (KPNB1) participates in the nuclear transport of NLS-RARα. Taken together, abnormal localization of NLS-RARα blocks the differentiation of APL cells, and nuclear localization of NLS-RARα depends on NLS of the RARα portion and is mediated via binding with importin-α/β.

Roscovitine enhances All-trans retinoic acid (ATRA)-induced leukemia cell differentiation: Novel effects on signaling molecules for a putative Cdk2 inhibitor

All-trans retinoic acid (ATRA)-based differentiation therapy has been unsuccessful in treating t(15;17) negative acute myeloid leukemia (AML) patients, motivating interest in combination therapies using ATRA plus other agents. Using the t (15, 17) negative HL-60 human myeloblastic leukemia model, we find that the cyclin-dependent kinase (CDK) inhibitor, roscovitine, augments signaling by an ATRA-induced macromolecular signalsome that propels differentiation and enhances ATRA-induced differentiation. Roscovitine co-treatment enhanced ATRA-induced expression of pS259- pS289/296/301- pS621-c-Raf, pS217/221-Mek, Src Family Kinases (SFKs) Lyn and Fgr and SFK Y416 phosphorylation, adaptor proteins c-Cbl and SLP-76, Vav, and acetylated 14-3-3 in the signalsome. Roscovitine enhanced ATRA-induced c-Raf interaction with Lyn, Vav, and c-Cbl. Consistent with signalsome hyper-activation, roscovitine co-treatment enhanced ATRA-induced G1/0 arrest and expression of differentiation markers, CD11b, ROS and p47 Phox. Because roscovitine regulated Lyn expression, activation and partnering, a stably transfected Lyn knockdown was generated from wt-parental cells to investigate its function in ATRA-induced differentiation. Lyn-knockdown enhanced ATRA-induced up-regulation of key signalsome molecules, c-Raf, pS259-c-Raf, pS289/296/301-c-Raf, Vav1, SLP-76, and Fgr, but with essentially total loss of pY416-SFK. Compared to ATRA-treated wt-parental cells, differentiation markers p47 phox, CD11b, G1/G0 arrest and ROS production were enhanced in ATRA-treated Lyn-knockdown stable transfectants, and addition of roscovitine further enhanced these ATRA-inducible markers. The Lyn-knockdown cells expressed slightly higher c-Raf, pS259-c-Raf, pS289/296/301-c-Raf, and SLP-76 than wt-parental cells, and this was associated with enhanced ATRA-induced upregulation of Fgr and cell differentiation, consistent with heightened signaling, suggesting that enhanced Fgr may have compensated for loss of Lyn to enhance differentiation in the Lyn-knockdown cells.

PML nuclear body biogenesis and oligomerization-driven leukemogenesis

PML nuclear bodies (NBs), which are increasingly recognized as the central hub of many cellular signaling events, are superassembled spherical complexes with diameters of 0.1–2 μm. Recent studies reveal that RING tetramerization and B1-box polymerization are key factors to the overall PML NBs assembly. The productive RBCC oligomerization allows subsequent PML biogenesis steps, including the PML auto-sumoylation and partners recruitment via SUMO–SIM interactions. In promyelocytic leukemia, the oncoprotein PML/RARα (P/R) inhibits PML NBs assembly and leads to a full-fledged leukemogenesis. In this review, we review the recent progress in PML and acute promyelocytic leukemia fields, highlighting the protein oligomerization as an important direction of future targeted therapy.

All-trans retinoic acid (ATRA)-induced TFEB expression is required for myeloid differentiation in acute promyelocytic leukemia (APL)

OBJECTIVE

In acute promyelocytic leukemia (APL), normal retinoid signaling is disrupted by an abnormal PML-RARα fusion oncoprotein, leading to a block in cell differentiation. Therapeutic concentrations of all-trans-retinoic acid (ATRA) can restore retinoid-induced transcription and promote degradation of the PML-RARα protein. Autophagy is a catabolic pathway that utilizes lysosomal machinery to degrade intracellular material and facilitate cellular re-modeling. Recent studies have identified autophagy as an integral component of ATRA-induced myeloid differentiation.

METHODS

As the molecular communication between retinoid signaling and the autophagy pathway is not defined, we performed RNA sequencing of NB4 APL cells treated with ATRA and examined autophagy-related transcripts.

RESULTS

ATRA altered the expression of >80 known autophagy-related transcripts, including the key transcriptional regulator of autophagy and lysosomal biogenesis, TFEB (11.5-fold increase). Induction of TFEB and its transcriptional target, sequestosome 1 (SQSTM1, p62), is reduced in ATRA-resistant NB4R cells compared to NB4 cells. TFEB knockdown in NB4 cells alters the expression of transcriptional targets of TFEB and reduces CD11b transcript levels in response to ATRA.

CONCLUSIONS

We show for the first time that TFEB plays an important role in ATRA-induced autophagy during myeloid differentiation and that autophagy induction potentiates leukemic cell differentiation (Note: this study includes data obtained from NCT00195156, https://clinicaltrials.gov/show/NCT00195156).

PML/RARa Interferes with NRF2 Transcriptional Activity Increasing the Sensitivity to Ascorbate of Acute Promyelocytic Leukemia Cells

NRF2 (NF-E2 p45-related factor 2) orchestrates cellular adaptive responses to stress. Its quantity and subcellular location is controlled through a complex network and its activity increases during redox perturbation, inflammation, growth factor stimulation, and energy fluxes. Even before all-trans retinoic acid (ATRA) treatment era it was a common experience that acute promyelocytic leukemia (APL) cells are highly sensitive to first line chemotherapy. Since we demonstrated how high doses of ascorbate (ASC) preferentially kill leukemic blast cells from APL patients, we aimed to define the underlying mechanism and found that promyelocytic leukemia/retinoic acid receptor α (PML/RARa) inhibits NRF2 function, impedes its transfer to the nucleus and enhances its degradation in the cytoplasm. Such loss of NRF2 function alters cell metabolism, demarcating APL tissue from both normal promyelocytes and other acute myeloide leukemia (AML) blast cells. Resistance to ATRA/arsenic trioxide (ATO) treatment is rare but grave and the metabolically-oriented treatment with high doses of ASC, which is highly effective on APL cells and harmless on normal hematopoietic stem cells (HSCs), could be of use in preventing clonal evolution and in rescuing APL-resistant patients.

Immunological Effects of Epigenetic Modifiers

Epigenetic alterations are associated with major pathologies including cancer. Epigenetic dysregulation, such as aberrant histone acetylation, altered DNA methylation, or modified chromatin organization, contribute to oncogenesis by inactivating tumor suppressor genes and activating oncogenic pathways. Targeting epigenetic cancer hallmarks can be harnessed as an immunotherapeutic strategy, exemplified by the use of pharmacological inhibitors of DNA methyltransferases (DNMT) and histone deacetylases (HDAC) that can result in the release from the tumor of danger-associated molecular patterns (DAMPs) on one hand and can (re-)activate the expression of tumor-associated antigens on the other hand. This finding suggests that epigenetic modifiers and more specifically the DNA methylation status may change the interaction of chromatin with chaperon proteins including HMGB1, thereby contributing to the antitumor immune response. In this review, we detail how epigenetic modifiers can be used for stimulating therapeutically relevant anticancer immunity when used as stand-alone treatments or in combination with established immunotherapies.

Retinoic Acid Receptors in Acute Myeloid Leukemia Therapy

Retinoic acid (RA) signaling pathways regulate fundamental biological processes, such as cell proliferation, development, differentiation, and apoptosis. Retinoid receptors (RARs and RXRs) are ligand-dependent transcription factors. All-trans retinoic acid (ATRA) is the principal endogenous ligand for the retinoic acid receptor alpha (RARA) and is produced by the enzymatic oxidation of dietary vitamin A, whose deficiency is associated with several pathological conditions. Differentiation therapy using ATRA revolutionized the outcome of acute promyelocytic leukemia (APL), although attempts to replicate these results in other cancer types have been met with more modest results. A better knowledge of RA signaling in different leukemia contexts is required to improve initial designs. Here, we will review the RA signaling pathway in normal and malignant hematopoiesis, and will discuss the advantages and the limitations related to retinoid therapy in acute myeloid leukemia.

Histone Deacetylase Inhibitors in Tumor Immunotherapy

BACKGROUND

With an increasing understanding of the antitumor immune response, considerable progress has been made in the field of tumor immunotherapy in the last decade. Inhibition of histone deacetylases represents a new strategy in tumor therapy and histone deacetylase inhibitors have been recently developed and validated as potential antitumor drugs. In addition to the direct antitumor effects, histone deacetylase inhibitors have been found to have the ability to improve tumor recognition by immune cells that may contribute to their antitumor activity. These immunomodolutory effects are desirable, and their in-depth comprehension will facilitate the design of novel regimens with improved clinical efficacy.

OBJECTIVE

Our goal here is to review recent developments in the application of histone deacetylase inhibitors as immune modulators in cancer treatment.

METHODS

Systemic compilation of the relevant literature in this field.

RESULTS & CONCLUSION

In this review, we summarize recent advances in the understanding of how histone deacetylase inhibitors alter immune process and discuss their effects on various cytokines. We also discuss the challenges to optimize the use of these inhibitors as immune modulators in cancer treatment. Information gained from this review will be valuable to this field and may be helpful for designing tumor immunotherapy trials involving histone deacetylase inhibitors.