Acute promyelocytic leukemia

Acute Myeloid Leukemia: 2025 Update on Diagnosis, Risk-Stratification, and Management

DISEASE OVERVIEW

Acute myeloid leukemia (AML) is a bone marrow stem cell cancer that is often fatal despite available treatments. Diagnosis, risk assessment, monitoring, and therapeutic management of AML have changed dramatically in the last decade due to increased pathophysiologic understanding, improved assessment technology, and the addition of at least 12 approved therapies.

DIAGNOSIS

The diagnosis is based on the presence of immature leukemia cells in the blood, and/or bone marrow or less often in extra-medullary tissues. New biological insights have been integrated into recent classification systems.

RISK ASSESSMENT

The European Leukemia Network has published risk classification algorithms for both intensively and non-intensively treated patients based on cytogenetic and on molecular findings. Prognostic factors may differ based on the therapeutic approach.

MONITORING

Our increasing ability to quantify lower levels of measurable residual disease (MRD) potentially allows better response assessment, as well as dynamic monitoring of disease status. The incorporation of MRD findings into therapeutic decision-making is rapidly evolving.

RISK ADAPTED THERAPY

The availability of 12 newly approved agents has been welcomed; however, optimal strategies incorporating newer agents into therapeutic algorithms are debated. The overarching approach integrates patient and caregiver goals of care, comorbidities, and disease characteristics.

Single-cell multiomics reveals a gene regulatory circuit driving leukemia cell differentiation

Cancer differentiation therapy aims to induce the maturation of neoplastic cells, but the mechanisms regulating cell fate decisions in oncogenic contexts remain unclear. In this study, we integrated single-cell chromatin accessibility and single-cell transcriptome analyses to explore the regulatory trajectories of a classical PML/RARα+ acute promyeloid leukemia (APL) cell line (NB4) post treatment by all-trans-retinoid acid (ATRA). Our findings indicated that ATRA activated specific PML/RARα-target enhancers to trigger a regulatory circuit composed of a positive feedforward gene regulatory circuit involving two transcription factors, SPI1 and CEBPE. This regulatory circuit was both necessary and sufficient to drive NB4 cells through an intermediate cell fate decision point to initiate terminal granulopoiesis. Moreover, ectopic expression of SPI1 and CEBPE promoted granulocytic differentiation in non-APL leukemia cell lines HL60 and K562. Our study sheds mechanistic insights into the differentiation trajectories induced by ATRA and illustrates a gene regulatory circuit that could be widely applied to promote differentiation of leukemia cells.

Control of Cellular Differentiation Trajectories for Cancer Reversion

Cellular differentiation is controlled by intricate layers of gene regulation, involving the modulation of gene expression by various transcriptional regulators. Due to the complexity of gene regulation, identifying master regulators across the differentiation trajectory has been a longstanding challenge. To tackle this problem, a computational framework, single-cell Boolean network inference and control (BENEIN), is presented. Applying BENEIN to human large intestinal single-cell transcriptome data, MYB, HDAC2, and FOXA2 are identified as the master regulators whose inhibition induces enterocyte differentiation. It is found that simultaneous knockdown of these master regulators can revert colorectal cancer cells into normal-like enterocytes by synergistically inducing differentiation and suppressing malignancy, which is validated by in vitro and in vivo experiments.

Super-resolved analysis of colocalization between replication and transcription along the cell cycle in a model of oncogene activation

To understand how oncogenes affect genome organization, it is essential to visualize fundamental processes such as DNA replication and transcription at high resolution in intact cells. At the same time, it is important to determine the progression of the cell along the cell cycle, as cell cycle regulation is crucial for the control of cell proliferation and oncogenesis. Here, we present a super-resolution imaging-based method to analyze single cell nuclei sorted according to specific phases of the cell cycle. The sorting is based on the evaluation of the number and the intensity of pixels in the replication foci image and the colocalization analysis is based on image cross-correlation spectroscopy (ICCS). We evaluate the colocalization between replication and transcription, at different cell cycle phases, in a model of PML-RARα oncogene activation. We find that colocalization between replication and transcription is higher in cells in early S phase compared to cells in middle and late S phase. When we turn on the PML-RARα oncogene, this colocalization pattern is preserved but we detect an increase of colocalization between replication and transcription in the early S phase which points to an effect of the PML-RARα oncogene on the coordination between replication and transcription.

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.

Fatty acid synthase (FASN) signalome: A molecular guide for precision oncology

The initial excitement generated more than two decades ago by the discovery of drugs targeting fatty acid synthase (FASN)-catalyzed de novo lipogenesis for cancer therapy was short-lived. However, the advent of the first clinical-grade FASN inhibitor (TVB-2640; denifanstat), which is currently being studied in various phase II trials, and the exciting advances in understanding the FASN signalome are fueling a renewed interest in FASN-targeted strategies for the treatment and prevention of cancer. Here, we provide a detailed overview of how FASN can drive phenotypic plasticity and cell fate decisions, mitochondrial regulation of cell death, immune escape and organ-specific metastatic potential. We then present a variety of FASN-targeted therapeutic approaches that address the major challenges facing FASN therapy. These include limitations of current FASN inhibitors and the lack of precision tools to maximize the therapeutic potential of FASN inhibitors in the clinic. Rethinking the role of FASN as a signal transducer in cancer pathogenesis may provide molecularly driven strategies to optimize FASN as a long-awaited target for cancer therapeutics.

Extirpating the cancer stem cell hydra: Differentiation therapy and Hyperthermia therapy for targeting the cancer stem cell hierarchy

Ever since the discovery of cancer stem cells (CSCs), they have progressively attracted more attention as a therapeutic target. Like the mythical hydra, this subpopulation of cells seems to contribute to cancer immortality, spawning more cells each time that some components of the cancer cell hierarchy are destroyed. Traditional modalities focusing on cancer treatment have emphasized apoptosis as a route to eliminate the tumor burden. A major problem is that cancer cells are often in varying degrees of dedifferentiation contributing to what is known as the CSCs hierarchy and cells which are known to be resistant to conventional therapy. Differentiation therapy is an experimental therapeutic modality aimed at the conversion of malignant phenotype to a more benign one. Hyperthermia therapy (HT) is a modality exploiting the changes induced in cells by the application of heat produced to aid in cancer therapy. While differentiation therapy has been successfully employed in the treatment of acute myeloid leukemia, it has not been hugely successful for other cancer types. Mounting evidence suggests that hyperthermia therapy may greatly augment the effects of differentiation therapy while simultaneously overcoming many of the hard-to-treat facets of recurrent tumors. This review summarizes the progress made so far in integrating hyperthermia therapy with existing modules of differentiation therapy. The focus is on studies related to the successful application of both hyperthermia and differentiation therapy when used alone or in conjunction for hard-to-treat cancer cell niche with emphasis on combined approaches to target the CSCs hierarchy.

Thyroid hormone receptor α1: a novel regulator of thyroid cancer cell differentiation

Thyroid hormone receptor α1 (TRα1) mediates the genomic actions of thyroid hormone (T3). The biology of TRα1 in growth and development has been well studied, but the functional role of TRα1 in cancers remains to be elucidated. Analysis of the human thyroid cancer database of The Cancer Genome Atlas (TCGA) showed that THRA gene expression is lost in highly dedifferentiated anaplastic thyroid cancer (ATC). We, therefore, explored the effects of TRα1 on the progression of ATC. We stably expressed TRα1 in two human ATC cell lines, THJ-11T (11T-TRα1 #2, #7, and #8) and THJ-16T (16T-TRα1 #3, #4, and #8) cells. We found that the expressed TRα1 inhibited ATC cell proliferation and induced apoptosis. TCGA data showed that THRA gene expression was best correlated with the paired box gene 8 (PAX8). Consistently, we found that the PAX8 expression was barely detectable in parental 11T and 16T cells. However, PAX8 gene expression was elevated in 11T- and 16T-TRα1-expressing cells at the mRNA and protein levels. Using various molecular analyses, we found that TRα1 directly regulated the expression of the PAX8 gene. Single-cell transcriptomic analyses (scRNA-seq) demonstrated that TRα1 functions as a transcription factor through multiple signaling pathways to suppress tumor growth. Importantly, scRNA-seq analysis showed that TRα1-induced PAX8, via its transcription program, shifts the cell landscape of ATC toward a differentiated state. The present studies suggest that TRα1 is a newly identified regulator of thyroid differentiation and could be considered as a potential therapeutic target to improve the outcome of ATC patients.

Characterization and therapeutic perspectives of differentiation-inducing therapy in malignant tumors

Cancer is a major public health issue globally and is one of the leading causes of death. Although available treatments improve the survival rate of some cases, many advanced tumors are insensitive to these treatments. Cancer cell differentiation reverts the malignant phenotype to its original state and may even induce differentiation into cell types found in other tissues. Leveraging differentiation-inducing therapy in high-grade tumor masses offers a less aggressive strategy to curb tumor progression and heightens chemotherapy sensitivity. Differentiation-inducing therapy has been demonstrated to be effective in a variety of tumor cells. For example, differentiation therapy has become the first choice for acute promyelocytic leukemia, with the cure rate of more than 90%. Although an appealing concept, the mechanism and clinical drugs used in differentiation therapy are still in their nascent stage, warranting further investigation. In this review, we examine the current differentiation-inducing therapeutic approach and discuss the clinical applications as well as the underlying biological basis of differentiation-inducing agents.

The Promise of Retinoids in the Treatment of Cancer: Neither Burnt Out Nor Fading Away

Since the introduction of all-trans retinoic acid (ATRA), acute promyelocytic leukemia (APL) has become a highly curable malignancy, especially in combination with arsenic trioxide (ATO). ATRA's success has deepened our understanding of the role of the RARα pathway in normal hematopoiesis and leukemogenesis, and it has influenced a generation of cancer drug development. Retinoids have also demonstrated some efficacy in a handful of other disease entities, including as a maintenance therapy for neuroblastoma and in the treatment of cutaneous T-cell lymphomas; nevertheless, the promise of retinoids as a differentiating therapy in acute myeloid leukemia (AML) more broadly, and as a cancer preventative, have largely gone unfulfilled. Recent research into the mechanisms of ATRA resistance and the biomarkers of RARα pathway dysregulation in AML have reinvigorated efforts to successfully deploy retinoid therapy in a broader subset of myeloid malignancies. Recent studies have demonstrated that the bone marrow environment is highly protected from exogenous ATRA via local homeostasis controlled by stromal cells expressing CYP26, a key enzyme responsible for ATRA inactivation. Synthetic CYP26-resistant retinoids such as tamibarotene bypass this stromal protection and have shown superior anti-leukemic effects. Furthermore, recent super-enhancer (SE) analysis has identified a novel AML subgroup characterized by high expression of RARα through strong SE levels in the gene locus and increased sensitivity to tamibarotene. Combined with a hypomethylating agent, synthetic retinoids have shown synergistic anti-leukemic effects in non-APL AML preclinical models and are now being studied in phase II and III clinical trials.

Potential of natural products in combination with arsenic trioxide: Investigating cardioprotective effects and mechanisms

Over the past few decades, clinical trials conducted worldwide have demonstrated the efficacy of arsenic trioxide (ATO) in the treatment of relapsed acute promyelocytic leukemia (APL). Currently, ATO has become the frontline treatments for patients with APL. However, its therapeutic applicability is severely constrained by ATO-induced cardiac side effects. Any cardioprotective agents that can ameliorate the cardiac side effects and allow exploiting the full therapeutic potential of ATO, undoubtedly gain significant attention. The knowledge and use of natural products for evidence-based therapy have grown rapidly in recent years. Here we discussed the potential mechanism of ATO-induced cardiac side effects and reviewed the studies on cardiac side effects as well as the research history of ATO in the treatment of APL. Then, We summarized the protective effects and underlying mechanisms of natural products in the treatment of ATO-induced cardiac side effects. Based on the efficacy and safety of the natural product, it has a promising future in the development of cardioprotective agents against ATO-induced cardiac side effects.

Acute myeloid leukemia: 2023 update on diagnosis, risk-stratification, and management

DISEASE OVERVIEW

Acute myeloid leukemia (AML) is a frequently fatal bone marrow stem cell cancer characterized by unbridled proliferation of malignant marrow stem cells with associated infection, anemia, and bleeding. An improved understanding of pathophysiology, improvements in measurement technology and at least 10 recently approved therapies have led to revamping the diagnostic, prognostic, and therapeutic landscape of AML.

DIAGNOSIS

One updated and one new classification system were published in 2022, both emphasizing the integration of molecular analysis into daily practice. Differences between the International Consensus Classification and major revisions from the previous 2016 WHO system provide both challenges and opportunities for care and clinical research.

RISK ASSESSMENT AND MONITORING

The European Leukemia Net 2022 risk classification integrates knowledge from novel molecular findings and recent trial results, as well as emphasizing dynamic risk based on serial measurable residual disease assessment. However, how to leverage our burgeoning ability to measure a small number of potentially malignant myeloid cells into therapeutic decision making is controversial.

RISK ADAPTED THERAPY

The diagnostic and therapeutic complexity plus the availability of newly approved agents requires a nuanced therapeutic algorithm which should integrate patient goals of care, comorbidities, and disease characteristics including the specific mutational profile of the patient's AML. The framework we suggest only represents the beginning of the discussion.

Long-term retrospective study of retinoic acid combined with arsenic and chemotherapy for acute promyelocytic leukemia

Chemotherapy, all-trans retinoic acid (ATRA), and arsenic are effective options for acute promyelocytic leukemia (APL). We conducted a 20-year retrospective analysis of newly diagnosed (ND) APL patients treated with arsenic, ATRA and mitoxantrone. After achieving complete remission (CR), patients received 3-5 cycles of chemotherapy followed by AS4S4 maintenance for 3 years. Eighty-eight ND APL patients were treated with either oral AS4S4 (n = 42) or arsenic trioxide (ATO) (n = 46). The 8-year overall survival (OS) rate was 100% in the AS4S4 group and 90% in the ATO group. The disease-free survival (DFS) rates were 100% and 87.1% (p = 0.027), respectively. Patients in the ATO group had more side effects. A subsequent cohort of 33 ND APL patients received triple therapy with oral AS4S4, ATRA, and chemotherapy. The 13-year OS and DFS rates were 100% and 90.9%. Our long-term analyses show that APL patients with oral AS4S4 had better outcomes compared to ATO, with no need for hospitalization.

Anti-SARS-CoV2 antibody-mediated cytokine release syndrome in a patient with acute promyelocytic leukemia

BACKGROUND

Passive immunization against SARS-CoV-2 limits viral burden and death from COVID-19; however, it poses a theoretical risk of disease exacerbation through antibody-dependent enhancement (ADE). ADE after anti-SARS-CoV2 antibody treatment has not been reported, and therefore the potential risk and promoting factors remain unknown.

CASE PRESENTATION

A 75-year-old female was admitted to the emergency room with recurrent, unexplained bruises and leukocytopenia, anemia, and thrombocytopenia. Evaluation of a bone marrow biopsy established the diagnosis of an acute promyelocytic leukemia (APL). SARS-CoV-2 RT-PCR testing of nasal and throat swabs on admission was negative. During the routine SARS-CoV-2 testing of inpatients, our patient tested positive for SARS-CoV-2 on day 14 after admission without typical COVID-19 symptoms. Due to disease- and therapy-related immunosuppression and advanced age conferring a high risk of progressing to severe COVID-19, casirivimab and imdevimab were administered as a preemptive approach. The patient developed immune activation and cytokine release syndrome (CRS) occurring within four hours of preemptive anti-SARS-CoV2 antibody (casirivimab/imdevimab) infusion. Immune activation and CRS were evidenced by a rapid increase in serum cytokines (IL-6, TNFα, IL-8, IL-10), acute respiratory insufficiency, and progressive acute respiratory distress syndrome.

DISCUSSION AND CONCLUSION

The temporal relationship between therapeutic antibody administration and the rapid laboratory, radiological, and clinical deterioration suggests that CRS was an antibody-related adverse event, potentially exacerbated by APL treatment-mediated differentiation of leukemic blasts and promyelocytes. This case highlights the need for careful assessment of life-threatening adverse events after passive SARS-CoV-2 immunization, especially in the clinical context of patients with complex immune and hematological landscapes.

Intracranial Hemorrhage Secondary to Newly Diagnosed Acute Promyelocytic Leukemia: A Cautionary Tale

Acute promyelocytic leukemia (APL) typically presents with complications from pancytopenia, generalized weakness, and hemorrhagic findings, with a distinguishing feature being the associated predilection of disseminated intravascular coagulation (DIC). APL is characterized by the halting of cellular differentiation in the promyelocyte stage, and balanced chromosomal translocation t(15;17) (q24;q21) that forms the promyelocytic leukemia-retinoic acid receptor-α (PML-RARA) fusion protein present in 95% of cases. APL has a high rate of early mortality secondary to coagulopathy, lending to the imperative need to begin a differentiation agent as soon as the disease is suspected, with all-trans retinoic acid (ATRA) being the most common differentiation agent. Herein, we present the case of a 32-year-old man presenting with non-specific symptoms of fatigue and scattered bruising, who was found to have an intracranial hemorrhage (ICH) in the setting of suspected APL. This case illuminates the importance of early brain imaging in suspected cases of APL to conceivably lessen the severity of hemorrhagic complications and represents a cautionary tale for similar cases in the future.

Hallmarks of Cancer: New Dimensions

The hallmarks of cancer conceptualization is a heuristic tool for distilling the vast complexity of cancer phenotypes and genotypes into a provisional set of underlying principles. As knowledge of cancer mechanisms has progressed, other facets of the disease have emerged as potential refinements. Herein, the prospect is raised that phenotypic plasticity and disrupted differentiation is a discrete hallmark capability, and that nonmutational epigenetic reprogramming and polymorphic microbiomes both constitute distinctive enabling characteristics that facilitate the acquisition of hallmark capabilities. Additionally, senescent cells, of varying origins, may be added to the roster of functionally important cell types in the tumor microenvironment. SIGNIFICANCE: Cancer is daunting in the breadth and scope of its diversity, spanning genetics, cell and tissue biology, pathology, and response to therapy. Ever more powerful experimental and computational tools and technologies are providing an avalanche of "big data" about the myriad manifestations of the diseases that cancer encompasses. The integrative concept embodied in the hallmarks of cancer is helping to distill this complexity into an increasingly logical science, and the provisional new dimensions presented in this perspective may add value to that endeavor, to more fully understand mechanisms of cancer development and malignant progression, and apply that knowledge to cancer medicine.

The Bone Marrow Microenvironment Mechanisms in Acute Myeloid Leukemia

Bone marrow (BM) is a highly complex tissue that provides important regulatory signals to orchestrate hematopoiesis. Resident and transient cells occupy and interact with some well characterized niches to produce molecular and cellular mechanisms that interfere with differentiation, migration, survival, and proliferation in this microenvironment. The acute myeloid leukemia (AML), the most common and severe hematological neoplasm in adults, arises and develop in the BM. The osteoblastic, vascular, and reticular niches provide surface co-receptors, soluble factors, cytokines, and chemokines that mediate important functions on hematopoietic cells and leukemic blasts. There are some evidences of how AML modify the architecture and function of these three BM niches, but it has been still unclear how essential those modifications are to maintain AML development. Basic studies and clinical trials have been suggesting that disturbing specific cells and molecules into the BM niches might be able to impair leukemia competencies. Either through niche-specific molecule inhibition alone or in combination with more traditional drugs, the bone marrow microenvironment is currently considered the potential target for new strategies to treat AML patients. This review describes the cellular and molecular constitution of the BM niches under healthy and AML conditions, presenting this anatomical compartment by a new perspective: as a prospective target for current and next generation therapies.

KAT6A and ENL form an epigenetic transcriptional control module to drive critical leukemogenic gene expression programs

Epigenetic programs are dysregulated in acute myeloid leukemia (AML) and help enforce an oncogenic state of differentiation arrest. To identify key epigenetic regulators of AML cell fate, we performed a differentiation-focused CRISPR screen in AML cells. This screen identified the histone acetyltransferase KAT6A as a novel regulator of myeloid differentiation that drives critical leukemogenic gene expression programs. We show that KAT6A is the initiator of a newly-described transcriptional control module in which KAT6A-catalyzed promoter H3K9ac is bound by the acetyllysine reader ENL, which in turn cooperates with a network of chromatin factors to induce transcriptional elongation. Inhibition of KAT6A has strong anti-AML phenotypes in vitro and in vivo, suggesting that KAT6A small molecule inhibitors could be of high therapeutic interest for mono or combinatorial differentiation-based treatment of AML.

Deep learning for diagnosis of acute promyelocytic leukemia via recognition of genomically imprinted morphologic features

Acute promyelocytic leukemia (APL) is a subtype of acute myeloid leukemia (AML), classified by a translocation between chromosomes 15 and 17 [t(15;17)], that is considered a true oncologic emergency though appropriate therapy is considered curative. Therapy is often initiated on clinical suspicion, informed by both clinical presentation as well as direct visualization of the peripheral smear. We hypothesized that genomic imprinting of morphologic features learned by deep learning pattern recognition would have greater discriminatory power and consistency compared to humans, thereby facilitating identification of t(15;17) positive APL. By applying both cell-level and patient-level classification linked to t(15;17) PML/RARA ground-truth, we demonstrate that deep learning is capable of distinguishing APL in both discovery and prospective independent cohort of patients. Furthermore, we extract learned information from the trained network to identify previously undescribed morphological features of APL. The deep learning method we describe herein potentially allows a rapid, explainable, and accurate physician-aid for diagnosing APL at the time of presentation in any resource-poor or -rich medical setting given the universally available peripheral smear.

Thrombocytosis in a patient with acute promyelocytic leukemia during treatment with all-trans retinoic acid and arsenic trioxide

Thrombocytosis is an unusual side effect of all-trans retinoic acid (ATRA) treatment that occurs in some patients with acute promyelocytic leukemia (APL).

OGP46 Induces Differentiation of Acute Myeloid Leukemia Cells via Different Optimal Signaling Pathways

Acute myelogenous leukemia (AML) is characterized by blockage of cell differentiation leading to the accumulation of immature cells, which is the most prevalent form of acute leukemia in adults. It is well known that all-trans retinoic acid (ATRA) and arsenic trioxide (ATO) are the preferred drugs for acute promyelocytic leukemia (APL). However, they can lead to irreversible resistance which may be responsible for clinical failure after complete remission (CR). Moreover, the differentiation therapy of ATRA-based treatment has not been effective against AML with t(8;21) translocation. Here we aimed to identify the differentiation effect of OGP46 on AML cell lines (HL-60, NB4, and Kasumi-1) and explore its possible mechanisms. We found that OGP46 has significant inhibitory activity against these cells by triggering cell differentiation with cell-cycle exit at G1/G0 and inhibited the colony-formation capacity of the AML cells. It was shown that OGP46 induced the differentiation of NB4 cells via the transcriptional misregulation in cancer signaling pathway by PML-RARα depletion, while it was attributed to the hematopoietic cell lineage and phagosome pathway in Kasumi-1 cells, which are all critical pathways in cell differentiation. These results highlight that OGP46 is an active agent not only in the APL cell line NB4 but also in AML-M2 cell lines, especially Kasumi-1 with t(8;21) translocation. Therefore, OGP46 may be a potential compound for surmounting the differentiation blockage in AML.

Nanosized Drug Delivery Systems for Breast Cancer Stem Cell Targeting

Breast cancer stem cells (BCSCs), also known as breast cancer initiating cells, are reported to be responsible for the initiation, progression, therapeutic resistance, and relapse of breast cancer. Conventional therapeutic agents mainly kill the bulk of breast tumor cells and fail to eliminate BCSCs, even enhancing the fraction of BCSCs in breast tumors sometimes. Therefore, it is essential to develop specific and effective methods of eliminating BCSCs that will enhance the efficacy of killing breast tumor cells and thereby, increase the survival rates and quality of life of breast cancer patients. Despite the availability of an increasing number of anti-BCSC agents, their clinical translations are hindered by many issues, such as instability, low bioavailability, and off-target effects. Nanosized drug delivery systems (NDDSs) have the potential to overcome the drawbacks of anti-BCSC agents by providing site-specific delivery and enhancing of the stability and bioavailability of the delivered agents. In this review, we first briefly introduce the strategies and agents used against BCSCs and then highlight the mechanism of action and therapeutic efficacy of several state-of-the-art NDDSs that can be used to treat breast cancer by eliminating BCSCs.

The important roles of protein SUMOylation in the occurrence and development of leukemia and clinical implications

Leukemia is a severe malignancy of the hematopoietic system, which is characterized by uncontrolled proliferation and dedifferentiation of immature hematopoietic precursor cells in the lymphatic system and bone marrow. Leukemia is caused by alterations of the genetic and epigenetic regulation of processes underlying hematologic malignancies, including SUMO modification (SUMOylation). Small ubiquitin-like modifier (SUMO) proteins covalently or noncovalently conjugate and modify a large number of target proteins via lysine residues. SUMOylation is a small ubiquitin-like modification that is catalyzed by the SUMO-specific activating enzyme E1, the binding enzyme E2, and the ligating enzyme E3. SUMO is covalently linked to substrate proteins to regulate the cellular localization of target proteins and the interaction of target proteins with other biological macromolecules. SUMOylation has emerged as a critical regulatory mechanism for subcellular localization, protein stability, protein-protein interactions, and biological function and thus regulates normal life activities. If the SUMOylation process of proteins is affected, it will cause a cellular reaction and ultimately lead to various diseases, including leukemia. There is growing evidence showing that a large number of proteins are SUMOylated and that SUMOylated proteins play an important role in the occurrence and development of various types of leukemia. Targeting the SUMOylation of proteins alone or in combination with current treatments might provide powerful targeted therapeutic strategies for the clinical treatment of leukemia.

Limiting Self-Renewal of the Basal Compartment by PKA Activation Induces Differentiation and Alters the Evolution of Mammary Tumors

Differentiation therapy utilizes our understanding of the hierarchy of cellular systems to pharmacologically induce a shift toward terminal commitment. While this approach has been a paradigm in treating certain hematological malignancies, efforts to translate this success to solid tumors have met with limited success. Mammary-specific activation of PKA in mouse models leads to aberrant differentiation and diminished self-renewing potential of the basal compartment, which harbors mammary repopulating cells. PKA activation results in tumors that are more benign, exhibiting reduced metastatic propensity, loss of tumor-initiating potential, and increased sensitivity to chemotherapy. Analysis of tumor histopathology revealed features of overt differentiation with papillary characteristics. Longitudinal single-cell profiling at the hyperplasia and tumor stages uncovered an altered path of tumor evolution whereby PKA curtails the emergence of aggressive subpopulations. Acting through the repression of SOX4, PKA activation promotes tumor differentiation and represents a possible adjuvant to chemotherapy for certain breast cancers.

Rosiglitazone binds to RXRα to induce RXRα tetramerization and NB4 cell differentiation

Rosiglitazone is a ligand of peroxisome proliferation-activated receptor gamma (PPARγ). However, it exerts biological activities and therapeutic effects through both PPARγ-dependent and independent mechanisms. In this study, we defined that rosiglitazone was also a ligand of retinoid X receptor alpha (RXRα) and displayed RXRα-dependent activities. We found that rosiglitazone directly bound to the ligand binding domain (LBD) of RXRα and induced RXRα/LBD tetramerization. Rosiglitazone inhibited the agonist-induced transcriptional activity of RXRα homodimers and heterodimers likely through inhibiting RXRα homo- and hetero-dimerization. In acute promyelocytic leukemia (APL) NB4 cells, rosiglitazone inhibited cell proliferation and induced cell differentiation, resulting from inhibiting RXRα/PML-RARα complex formation and down-regulating PML-RARα. Together, our study identified RXRα as a novel target of rosiglitazone and RXRα mediating the anti-APL activity of rosiglitazone.

PML-RARα interaction with TRIB3 impedes PPARγ/RXR function and triggers dyslipidemia in acute promyelocytic leukemia

Although dyslipidemia commonly occurs in patients with acute promyelocytic leukemia (APL) in response to anti-APL therapy, the underlying mechanism and the lipid statuses of patients with newly diagnosed APL remain to be addressed.

Methods: We conducted a retrospective study to investigate the lipid profiles of APL patients. PML-RARα transgenic mice and APL cells-transplanted mice were used to assess the effects of APL cells on the blood/liver lipid levels. Subsequently, gene set enrichment analysis, western blot and dual luciferase reporter assay were performed to examine the role and mechanism of PML-RARα and TRIB3 in lipid metabolism regulation in APL patients at pretreatment and after induction therapy.

Results: APL patients exhibited a higher prevalence of dyslipidemia before anti-APL therapy based on a retrospective study. Furthermore, APL cells caused secretion of triglycerides, cholesterol, and PCSK9 from hepatocytes and degradation of low-density lipoprotein receptors in hepatocytes, which elevated the lipid levels in APL cell-transplanted mice and Pml-Rarα transgenic mice. Mechanistically, pseudokinase TRIB3 interacted with PML-RARα to inhibit PPARγ activity by interfering with the interaction of PPARγ and RXR and promoting PPARγ degradation. Thus, reduced PPARγ activity in APL cells decreased leptin but increased resistin expression, causing lipid metabolism disorder in hepatocytes and subsequent dyslipidemia in mice. Although arsenic/ATRA therapy degraded PML-RARα and restored PPARγ expression, it exacerbated dyslipidemia in APL patients. The elevated TRIB3 expression in response to arsenic/ATRA therapy suppressed PPARγ activity by disrupting the PPARγ/RXR dimer, which resulted in dyslipidemia in APL patients undergoing therapy. Indeed, the PPAR activator not only enhanced the anti-APL effects of arsenic/ATRA by suppressing TRIB3 expression but also reduced therapy-induced dyslipidemia in APL patients.

Conclusion: Our work reveals the critical role of the PML-RARα/PPARγ/TRIB3 axis in the development of dyslipidemia in APL patients, potentially conferring a rationale for combining ATRA/arsenic with the PPAR activator for APL treatment.

Interconversion between Tumorigenic and Differentiated States in Acute Myeloid Leukemia

Tumors are composed of phenotypically heterogeneous cancer cells that often resemble various differentiation states of their lineage of origin. Within this hierarchy, it is thought that an immature subpopulation of tumor-propagating cancer stem cells (CSCs) differentiates into non-tumorigenic progeny, providing a rationale for therapeutic strategies that specifically eradicate CSCs or induce their differentiation. The clinical success of these approaches depends on CSC differentiation being unidirectional rather than reversible, yet this question remains unresolved even in prototypically hierarchical malignancies, such as acute myeloid leukemia (AML). Here, we show in murine and human models of AML that, upon perturbation of endogenous expression of the lineage-determining transcription factor PU.1 or withdrawal of established differentiation therapies, some mature leukemia cells can de-differentiate and reacquire clonogenic and leukemogenic properties. Our results reveal plasticity of CSC maturation in AML, highlighting the need to therapeutically eradicate cancer cells across a range of differentiation states.

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.

Identification and characterization of a phenyl-thiazolyl-benzoic acid derivative as a novel RAR/RXR agonist

Objective

To identify an agonist of RXRα and RARα with reduced undesired profiles of all-trans retinoic acid for differentiation-inducing therapy of acute promyelocytic leukemia (APL), such as its susceptibility to P450 enzyme, induction of P450 enzyme, increased sequestration by cellular retinoic acid binding protein and increased expression of P-glycoprotein, a virtual screening was performed.

Results and conclusion

In this study, a phenyl-thiazolyl-benzoic acid derivative (PTB) was identified as a potent agonist of RXRα and RARα. PTB was characterized in nuclear receptor binding, reporter gene, cell differentiation and cell growth assays. PTB bound directly to RXRα and RARα, but not to PPARα, δ(β) or γ. PTB fully activated reporter genes with enhancer elements for RXRα/RXRα, and partially activated reporter genes with enhancer elements for RARα/RXRα, PPARδ(β) and PPARγ. Furthermore, PTB induced differentiation and inhibited the growth of human APL cells. Thus, PTB is a novel dual agonist of RXRα and RARα and works as both a differentiation inducer and a proliferation inhibitor to leukemic cells.

Bone Marrow Findings in Patients With Acute Promyelocytic Leukemia Treated With Arsenic Trioxide

OBJECTIVES

Increasingly, acute promyelocytic leukemia (APL) is treated with a combination of all-trans retinoic acid (ATRA) and arsenic trioxide (ATO). This study characterizes bone marrow findings after ATRA/ATO therapy.

METHODS

Bone marrow biopsies from 16 patients treated with ATRA/ATO and seven patients treated with ATRA/chemotherapy (CTX) for APL were evaluated.

RESULTS

In ATRA/ATO cases, the marrow was likely to be hypercellular (79%) with a decreased myeloid:erythroid (M:E) ratio (88%), megaloblastoid maturation of erythroid precursors (100%), erythroid atypia (75%), and increased (88%) and atypical (75%) megakaryocytes. Significant myeloid atypia was only seen in extensive residual disease. The ATRA/CTX cases were less likely to be hypercellular (38%), have a M:E ratio of 1:1 or less (0%), exhibit significant erythroid atypia (0%), or have increased (0%) or atypical (38%) megakaryocytes.

CONCLUSIONS

Bone marrow biopsies from patients treated with ATO have unusual but characteristic features. Despite variability in marrow findings, clinical outcomes were uniformly favorable.

Retinoic acid and arsenic trioxide induce lasting differentiation and demethylation of target genes in APL cells

Acute promyelocytic leukemia (APL) is characterized by arrested differentiation of promyelocytes. Patients treated with all-trans retinoic acid (ATRA) alone experience relapse, while patients treated with ATRA and arsenic trioxide (ATO) are often relapse-free. This suggests sustained changes have been elicited by the combination therapy. To understand the lasting effects of the combination therapy, we compared the effects of ATRA and ATO on NB4 and ATRA-resistant NB4-MR2 APL cells during treatment versus post treatment termination. After treatment termination, NB4 cells treated with ATRA or ATO reverted to non-differentiated cells, while combination-treated cells remained terminally differentiated. This effect was diminished in NB4-MR2 cells. This suggests combination treatment induced more permanent changes. Combination treatment induced higher expression of target genes (e.g., transglutaminase 2 and retinoic acid receptor beta), which in NB4 cells was sustained post treatment termination. To determine whether sustained epigenetic changes were responsible, we quantified the enrichment of histone modifications by chromatin immunoprecipitation, and CpG methylation by bisulfite-pyrosequencing. While ATRA and combination treatment induced similar histone acetylation enrichment, combination treatment induced greater demethylation of target genes, which was sustained. Therefore, sustained demethylation of target genes by ATRA and ATO combination treatment is associated with lasting differentiation and gene expression changes.

Discovery of novel glycogen synthase kinase-3α inhibitors: Structure-based virtual screening, preliminary SAR and biological evaluation for treatment of acute myeloid leukemia

Glycogen synthase kinase 3α (GSK-3α) plays a constitutive role in various physiological processes and has been proved to be a therapeutic target for acute myeloid leukemia (AML). In this paper, by means of computer-aided drug design, we discovered a novel chemical series of GSK-3α inhibitors with an IC₅₀ value of 0.033-2.804 μM. The preliminary structure-activity relationship was concluded and, notably, the most potent and isoform-selective compound G28_14 was identified with IC₅₀ values of 33 nM and 218 nM against GSK-3α and -3β, respectively, exhibiting a nearly ten-fold isoform-selectivity. Further cell viability assays and colony formation assays revealed that G28_14 suppressed cell survival by impairing cell proliferation by up to 90% in two AML cell lines. Moreover, surface marker expression analysis demonstrated that G28_14 induced terminal differentiation with a high level of CD11b, CD11c, and CD14. Western immunoblotting showed that G28_14 isoform-selectively inhibited the phosphorylation of GSK-3α in-cell without activating Wnt/β-catenin signaling. In addition, to elucidate its structure-activity relationship, the binding mode of this chemical series was proposed using molecular docking and molecular dynamics simulations. Taken together, this chemical series is worth developing as differentiation therapies for the treatment of AML.

Cell-of-origin susceptibility to glioblastoma formation declines with neural lineage restriction

The contribution of lineage identity and differentiation state to malignant transformation is controversial. We have previously shown that adult neural stem and early progenitor cells give origin to glioblastoma. Here we systematically assessed the tumor-initiating potential of adult neural populations at various stages of lineage progression. Cell type-specific tamoxifen-inducible Cre recombinase transgenes were used to target glioblastoma-relevant tumor suppressors Nf1, Trp53 and Pten in late-stage neuronal progenitors, neuroblasts and differentiated neurons. Mutant mice showed cellular and molecular defects demonstrating the impact of tumor suppressor loss, with mutant neurons being the most resistant to early changes associated with tumor development. However, we observed no evidence of glioma formation. These studies show that increasing lineage restriction is accompanied by decreasing susceptibility to malignant transformation, indicating a glioblastoma cell-of-origin hierarchy in which stem cells sit at the apex and differentiated cell types are least susceptible to tumorigenesis.

Traffic lights for retinoids in oncology: molecular markers of retinoid resistance and sensitivity and their use in the management of cancer differentiation therapy

For decades, retinoids and their synthetic derivatives have been well established anticancer treatments due to their ability to regulate cell growth and induce cell differentiation and apoptosis. Many studies have reported the promising role of retinoids in attaining better outcomes for adult or pediatric patients suffering from several types of cancer, especially acute myeloid leukemia and neuroblastoma. However, even this promising differentiation therapy has some limitations: retinoid toxicity and intrinsic or acquired resistance have been observed in many patients. Therefore, the identification of molecular markers that predict the therapeutic response to retinoid treatment is undoubtedly important for retinoid use in clinical practice. The purpose of this review is to summarize the current knowledge on candidate markers, including both genetic alterations and protein markers, for retinoid resistance and sensitivity in human malignancies.

The success and the challenge of all-trans retinoic acid in the treatment of cancer

All-trans retinoic acid (ATRA), an active metabolite of vitamin A, plays important roles in cell proliferation, cell differentiation, apoptosis, and embryonic development. The effects of ATRA are mediated by nuclear retinoid receptors as well as non-genomic signal pathway, such as MAPK and PKA. The great success of differentiation therapy with ATRA in acute promyelocytic leukemia (APL) not only improved the prognosis of APL but also spurred the studies of ATRA in the treatment of other tumors. Since the genetic and physiopathological simplicity of APL is not common in human malignancies, the combination of ATRA with other agents (chemotherapy, epigenetic modifiers, and arsenic trioxide, etc) had been extensively investigated in a variety of tumors. In this review, we will discuss in details about ATRA and its role in cancer treatment.

Targeted therapy for fusion-driven high-risk acute leukemia

Despite continued progress in drug development for acute leukemias, outcomes for patients with some subtypes have not changed significantly in the last decade. Recurrent chromosomal translocations have long been recognized as driver events in leukemia, and many of these oncogenic fusions portend high-risk disease. Improved understanding of the molecular underpinnings of these fusions, coupled with novel chemistry approaches, now provide new opportunity for therapeutic inroads into the treatment of leukemia driven by these fusions.

All-Trans Retinoic Acid Enhances Matrix Metalloproteinase 2 Expression and Secretion in Human Myeloid Leukemia THP-1 Cells

All-trans retinoic acid (ATRA) is an effective drug for the induction therapy of acute promyelocytic leukemia. However, the treatment is associated with adverse events such as retinoic acid syndrome (RAS) in some patients, whose histologic characteristics included organ infiltration by leukemic cells. Matrix metalloproteinase 2 (MMP-2) is often upregulated in tumor cells and plays a role in tumor cell migration and invasion by degrading the extracellular matrix. In this study, we examined the possible modulatory effects of ATRA on MMP-2 expression and secretion in human myeloid leukemia cell line THP-1. The cells were treated with various concentrations of ATRA, and MMP-2 expression and secretion were examined. MMP-2 expression and secretion started to increase with ATRA concentration as low as 0.1 nM and gradually increased thereafter. Agonists of retinoic acid receptor (RAR) or retinoid X receptor (RXR) alone could enhance MMP-2 secretion, and RAR or RXR antagonists alone could reverse ATRA-induced MMP-2 secretion. ATRA increased intracellular calcium ion levels, and a calcium-channel blocker inhibited ATRA-induced MMP-2 secretion. Dexamethasone suppressed ATRA-induced MMP-2 secretion. Our results suggest that ATRA enhances MMP-2 expression and secretion in human myeloid leukemia THP-1 cells in a calcium ion dependent manner through RAR/RXR signaling pathways, and this enhanced expression and secretion may be associated with the possible mechanisms of RAS.

Profiling and functional analysis of circular RNAs in acute promyelocytic leukemia and their dynamic regulation during all-trans retinoic acid treatment

Circular RNAs (circRNAs) are a novel class of powerful regulators in gene expression and participate in the pathogenesis of many diseases, including cancer. However, little is known about the roles of circRNAs in the development and treatment of acute promyelocytic leukemia (APL). Here we report the expression profiling and function of circRNAs in APL, including their dynamic regulation during all-trans retinoic acid (ATRA)-induced differentiation. We performed two independent ribosomal RNA-minus RNA-sequencing (Ribo-minus RNA-seq) experiments with and without RNase R treatment on APL patient-derived NB4 cells and identified a total of 4313 circRNAs, including 1098 newly identified circRNAs. Detailed analysis showed that circRNAs expressed in APL cells were mostly exon-derived, not by-products during splicing, and could be distinguished from hematopoietic stem cells, neutrophils and lymphocytes. The true presence and stability of circRNAs were verified both in NB4 cells and primary APL patient samples. Moreover, we conducted a time-series analysis of circRNAs on ATRA-treated NB4 cells and uncovered 508 circRNAs with dynamic expression during ATRA treatment, including 246 upregulated and 262 downregulated. Further evidence demonstrated that the majority of circRNAs were regulated independently of their host linear mRNAs. Detailed functional experiments demonstrated that circ-HIPK2, one of the differentially expressed circRNAs, significantly influenced ATRA-induced differentiation of APL cells. Further mechanistic studies revealed that circ-HIPK2 was located in cytoplasm and served as a sponge for differentiation-associated miR-124-3p. Finally, circ-HIPK2 expression in APL patients was significantly lower than that in normal peripheral mononuclear cells and other subtypes of AML, indicating its potential role as an APL biomarker. Our study indicates the biological functions of circRNAs in the development and treatment of APL, and provides a comprehensive circRNA resource for future studies.

2'-Fluoro ribonucleic acid modified DNA dual-probe sensing strategy for enzyme-amplified electrochemical detection of double-strand DNA of PML/RARα related fusion gene

In the study, a novel sensing strategy based on dual-probe mode, which involved two groups of 2'-fluoro ribonucleic acid (2'-F RNA) modified probes, was designed for the detection of synthetic target double-strand DNA (dsDNA) of PML/RARα fusion genes in APL. And each pair of probes contained a thiolated capture probe (C1 or C2) immobilized on one of electrode surfaces in the dual-channel electrochemical biosensor and a biotinylated reporter probe (R1 or R2). The two groups of 2'-F RNA modified probes were separately complementary with the corresponding strand (Sa or Sb) from target dsDNA in order to prevent renaturation of target dsDNA. Through flanking target dsDNA, two "sandwitch" complexes (C1/Sa/R1 and C2/Sb/R2) were separately shaped by capture probes (C1 and C2) and free reporter probes (R1 and R2) in hybridization solution on the surfaces of different electrodes after the thermal denaturation. The biotin-modified enzyme which produced the measurable electrochemical current signal was localized to the surface by affinity binding between biotin with streptavidin. Under the optimal condition, the biosensor was able to detect 84 fM target dsDNA and showed a good specificity in PBS hybridization solution. Otherwise, the investigations of the specificity and sensitivity of the biosensor were carried out further in the mixed hybridization solution containing different kinds of mismatch sequences as interference background. It can be seen that under a certain interference background, the method still exhibited excellent selectivity and specificity for the discrimination between the fully-complementary and the mismatch sequences. The results of our research laid a good basis of further detection research in practical samples.

New synergistic combinations of differentiation-inducing agents in the treatment of acute promyelocytic leukemia cells

Acute promyelocytic leukemia (APL) was considered to be one of the most lethal forms of leukemia in adults before the introduction of the vitamin A metabolite all-trans retinoic acid (ATRA). Surprisingly, it has been confirmed that FICZ (6-Formylindolo (3, 2-b) carbazole) enhances ATRA-induced differentiation. Moreover, a number of studies have demonstrated that anti CD44 monoclonal antibody (mAb) induces to bring back differentiation blockage the leukemic stem cells. The level of differentiation markers including CD11b and CD11c in NB4 cells was assessed by flow cytometry. The induction of apoptosis was also evaluated. We estimated the induction potential of a triple compound of ATRA-FICZ, anti-CD44 maps. The cells showed the gradually increased expression levels of CD11b and CD11c. A mixture of a "CD44 mAb, ATRA and FICZ effectively promoted granulocytic maturation resulting in increased rates of apoptosis. The differences in expression of CD11b and CD11c at 5 μg/ml and 10 μg/ml were significant. These phenomena were highest at 10 μg/ml CD44 mAb concentrations. Synergistic induction differentiation and apoptosis of APL cells by using a co-treatment with novel triple compound are more effective for eradicating blasts and controlling the metastasis. Our results show that the addition of anti-CD44 mAb improves "ATRA-FICZ"-induced differentiation and has potential to reduce usual chemotherapy based treatments. Taken together, this compound may lead to novel clinical applications of differentiation-based approaches for APL and other types of leukemia. Further clinical studies would be recommended to clarify the clinical efficacy.

RARα2 and PML-RAR similarities in the control of basal and retinoic acid induced myeloid maturation of acute myeloid leukemia cells

Treatment of acute promyelocytic leukemia (APL) with all-trans retinoic acid (ATRA) is the first example of targeted therapy. In fact, the oncogenic fusion-protein (PML-RAR) typical of this leukemia contains the retinoid-nuclear-receptor RARα. PML-RAR is responsible for the differentiation block of the leukemic blast. Besides PML-RAR, two endogenous RARα proteins are present in APL blasts, i.e. RARα1 and RARα2. We developed different cell populations characterized by PML-RAR, RARα2 and RARα1 knock-down in the APL-derived NB4 cell-line. Unexpectedly, silencing of PML-RAR and RARα2 results in similar increases in the constitutive expression of several granulocytic differentiation markers. This is accompanied by enhanced expression of the same granulocytic markers upon exposure of the NB4 blasts to ATRA. Silencing of PML-RAR and RARα2 causes also similar perturbations in the whole genome gene-expression profiles of vehicle and ATRA treated NB4 cells. Unlike PML-RAR and RARα2, RARα1 knock-down blocks ATRA-dependent induction of several granulocytic differentiation markers. Many of the effects on myeloid differentiation are confirmed by over-expression of RARα2 in NB4 cells. RARα2 action on myeloid differentiation does not require the presence of PML-RAR, as it is recapitulated also upon knock-down in PML-RAR-negative HL-60 cells. Thus, relative to RARα1, PML-RAR and RARα2 exert opposite effects on APL-cell differentiation. These contrasting actions may be related to the fact that both PML-RAR and RARα2 interact with and inhibit the transcriptional activity of RARα1. The interaction surface is located in the carboxy-terminal domain containing the D/E/F regions and it is influenced by phosphorylation of Ser-369 of RARα1.

Selenite promotes all-trans retinoic acid-induced maturation of acute promyelocytic leukemia cells

Selective targeting of the PML/RARα oncoprotein demonstrates a successful molecular targeted therapy in acute promyelocytic leukemia (APL) with a typical t(15:17) chromosomal translocation. The zinc-thiolate coordination is critical for structural stability of zinc finger proteins, including the PML moiety of PML/RARα. Based on the known interaction of redox-active selenium compounds with thiolate ligands of zinc, we herein have investigated the abrogatory effects of selenite alone or in combination with all-trans retinoic acid on PML/RARα and the possible effects on differentiation in these cells. At pharmacological concentrations, selenite inhibited the proliferation and survival of APL originated NB4 cells. In combination with ATRA, it potentiated the differentiation of NB4 cells without any differentiating effects of its own as a single agent. Concordant with our hypothesis, PML/RARα oncoprotein expression was completely abrogated by selenite. Increased expression of RARα, PU.1 and FOXO3A transcription factors in the combined treatment suggested the plausible basis for increased differentiation in these cells. We show that selenite at clinically achievable dose targets PML/RARα oncoprotein for degradation and potentiates differentiation of promyelocytic leukemic cells in combination with ATRA. The present investigation reveals the hitherto unknown potential of selenite in targeted abrogation of PML/RARα in APL cells with prospective therapeutic value.

Differentiation therapy revisited

The concept of differentiation therapy emerged from the fact that hormones or cytokines may promote differentiation ex vivo, thereby irreversibly changing the phenotype of cancer cells. Its hallmark success has been the treatment of acute promyelocytic leukaemia (APL), a condition that is now highly curable by the combination of retinoic acid (RA) and arsenic. Recently, drugs that trigger differentiation in a variety of primary tumour cells have been identified, suggesting that they are clinically useful. This Opinion article analyses the basis for the clinical successes of RA or arsenic in APL by assessing the respective roles of terminal maturation and loss of self-renewal. By reviewing other successful examples of drug-induced tumour cell differentiation, novel approaches to transform differentiating drugs into more efficient therapies are proposed.

MEK inhibitors enhance therapeutic response towards ATRA in NF1 associated malignant peripheral nerve sheath tumors (MPNST) in-vitro

OBJECTIVE

Neurofibromatosis type 1 (NF1) is a hereditary tumor syndrome characterized by an increased risk of malignant peripheral nerve sheath tumors (MPNST). Chemotherapy of MPNST is still insufficient. In this study, we investigated whether human tumor Schwann cells derived from NF1 associated MPNST respond to all-trans retinoic acid (ATRA). We analyzed effects of ATRA and MEK inhibitor (MEKi) combination therapy.

METHODS

MPNST cell lines S462, T265, NSF1 were treated with ATRA and MEKi U0126 and PD0325901. We assessed cell viability, proliferation, migration, apoptosis and differentiation as well as mRNA expression of RAR and RXR subtypes and ATRA target genes such as CRABP2, CYP26A1, RARB and PDK1. We also analyzed CRABP2 methylation in cell lines and performed immunohistochemistry of human MPNST specimens.

RESULTS

ATRA therapy reduced viability and proliferation in S462 and T265 cells, accompanied by differentiation, apoptosis and reduced migration. NSF1 cells which lacked RXRG expression did not respond to ATRA. We furthermore demonstrated that ATRA signaling was functional for common targets, and that mRNA expression of CRABP2 and its targets was raised by ATRA therapy, whereas alternative pathways via FABP5 were not induced. Finally, combination of ATRA and MEKi demonstrated additively reduced viability of T265 and S462 cells.

CONCLUSIONS

We observed therapeutic effects in two of three MPNST cell lines pronounced by combination therapy. These data point to a potentially successful treatment of MPNST by combined application of ATRA and MEK inhibitors such as U0126 or PD0325901.

Recurrent Fusion Genes in Leukemia: An Attractive Target for Diagnosis and Treatment

INTRODUCTION

Since the first fusion gene was discovered decades ago, a considerable number of fusion genes have been detected in leukemia. The majority of them are generated through chromosomal rearrangement or abnormal transcription. With the development of techniques, high-throughput sequencing method makes it possible to detect fusion genes systematically in multiple human cancers. Owing to their biological significance and tumor-specific expression, some of the fusion genes are attractive diagnostic tools and therapeutic targets. Tyrosine kinase inhibitors (TKI) targeting BCR-ABL1 fusions have been widely used to treat CML. The combination of ATRA and ATO targeting PML-RARA fusions has proven to be effective in acute promyelocytic leukemia (APL). Moreover, therapy with high dose cytarabine (HDAC) has significantly improved the prognosis of core binding factor (CBF) acute myeloid leukemia (AML) patients. Therefore, studies on fusion genes may benefit patients with leukemia by providing more diagnostic markers and therapies in the future.

CONCLUSION

The presented review focuses on the history of fusion genes, mechanisms of formation, and treatments against specific fusion genes in leukemia.

RaRF confers RA resistance by sequestering RAR to the nucleolus and regulating MCL1 in leukemia cells

Retinoic acid (RA) has broad clinical applications for the treatment of various cancers, particularly acute promyelocytic leukemia. However, RA-based therapy is limited by relapse in patients associated with RA resistance, the mechanism of which is poorly understood. Here, we suggest a new molecular mechanism of RA resistance by a repressor, named RA resistance factor (RaRF). RaRF suppressed transcriptional activity of the RA receptor (RAR) by directly interacting with and sequestering RAR to the nucleolus in response to RA. RaRF was highly expressed in RA-resistant leukemia cells and its expression was strongly correlated with RA sensitivity. MCL1 was upregulated by RA treatment upon RaRF depletion, accompanying leukemic myeloblast differentiation, which is negatively regulated by ectopic RaRF expression. Collectively, we propose that RaRF may be a factor in the resistance mechanism and thus a potential target for leukemia therapy using RA.