Pathways of retinoic acid- or arsenic trioxide-induced PML/RARalpha catabolism, role of oncogene degradation in disease remission

Design and Synthesis of Thiourea-Conjugating Organic Arsenic D-Glucose with Anticancer Activities

Organic arsenic compounds such as p-aminophenylarsine oxide (p-APAO) are easier for structural optimization to improve drug-like properties such as pharmacokinetic properties, therapeutic efficacy, and target selectivity. In order to strengthen the selectivity of 4-(1,3,2-dithiarsinan-2-yl) aniline 7 to tumor cell, a thiourea moiety was used to strengthen the anticancer activity. To avoid forming a mixture of α/β anomers, the strategy of 2-acetyl's neighboring group participation was used to lock the configuration of 2,3,4,6-tetra-O-acetyl-β-d-glucopyranosyl isothiocyanate from 2,3,4,6-tetra-O-acetyl-α-d-glucopyranosyl bromide. 1-(4-(1,3,2-dithiarsinan-2-yl) aniline)-2-N-(2,3,4,6-tetra-O-acetyl-β-d-glucopyranos-1-yl)-thiourea 2 can increase the selectivity of human colon cancer cells HCT-116 (0.82 ± 0.06 μM vs. 1.82 ± 0.07 μM) to human embryonic kidney 293T cells (1.38 ± 0.01 μM vs. 1.22 ± 0.06 μM) from 0.67 to 1.68, suggesting a feasible approach to improve the therapeutic index of arsenic-containing compounds as chemotherapeutic agents.

A machine learning model identifies M3-like subtype in AML based on PML/RARα targets

The typical genomic feature of acute myeloid leukemia (AML) M3 subtype is the fusion event of PML/RARα, and ATRA/ATO-based combination therapy is current standard treatment regimen for M3 subtype. Here, a machine-learning model based on expressions of PML/RARα targets was developed to identify M3 patients by analyzing 1228 AML patients. Our model exhibited high accuracy. To enable more non-M3 AML patients to potentially benefit from ATRA/ATO therapy, M3-like patients were further identified. We found that M3-like patients had strong GMP features, including the expression patterns of M3 subtype marker genes, the proportion of myeloid progenitor cells, and deconvolution of AML constituent cell populations. M3-like patients exhibited distinct genomic features, low immune activity and better clinical survival. The initiative identification of patients similar to M3 subtype may help to identify more patients that would benefit from ATO/ATRA treatment and deepen our understanding of the molecular mechanism of AML pathogenesis.

Understanding a high-risk acute myeloid leukemia by analyzing the interactome of its major driver mutation

The WHO classifies t(6;9)-positive acute myeloid leukemia (AML) as a subgroup of high-risk AML because of its clinical and biological peculiarities, such as young age and therapy resistance. t(6;9) encodes the DEK/NUP214 fusion oncoprotein that targets only a small subpopulation of bone marrow progenitors for leukemic transformation. This distinguishes DEK/NUP214 from other fusion oncoproteins, such as PML/RARα, RUNX1/ETO, or MLL/AF9, which have a broad target population they block differentiation and increase stem cell capacity. A common theme among most leukemogenic fusion proteins is their aberrant localization compared to their wild-type counterparts. Although the actual consequences are widely unknown, it seems to contribute to leukemogenesis most likely by a sequester of interaction partners. Thus, we applied a global approach to studying the consequences of the aberrant localization of t(6;9)-DEK/NUP214 for its interactome. This study aimed to disclose the role of localization of DEK/NUP214 and the related sequester of proteins interacting with DEK/NUP214 for the determination of the biology of t(6;9)-AML. Here we show the complexity of the biological consequences of the expression of DEK/NUP214 by an in-depth bioinformatic analysis of the interactome of DEK/NUP214 and its biologically dead mutants. DEK/NUP214's interactome points to an essential role for aberrant RNA-regulation and aberrant regulation of apoptosis and leukocyte activation as a significant determinant of the phenotype of t(6;9)-AML. Taken together, we provide evidence that the interactome contributes to the aberrant biology of an oncoprotein, providing opportunities for developing novel targeted therapy approaches.

TRIM8 modulates the EWS/FLI oncoprotein to promote survival in Ewing sarcoma

Fusion-transcription factors (fusion-TFs) represent a class of driver oncoproteins that are difficult to therapeutically target. Recently, protein degradation has emerged as a strategy to target these challenging oncoproteins. The mechanisms that regulate fusion-TF stability, however, are generally unknown. Using CRISPR-Cas9 screening, we discovered tripartite motif-containing 8 (TRIM8) as an E3 ubiquitin ligase that ubiquitinates and degrades EWS/FLI, a driver fusion-TF in Ewing sarcoma. Moreover, we identified TRIM8 as a selective dependency in Ewing sarcoma compared with >700 other cancer cell lines. Mechanistically, TRIM8 knockout led to an increase in EWS/FLI protein levels that was not tolerated. EWS/FLI acts as a neomorphic substrate for TRIM8, defining the selective nature of the dependency. Our results demonstrate that fusion-TF protein stability is tightly regulated and highlight fusion oncoprotein-specific regulators as selective therapeutic targets. This study provides a tractable strategy to therapeutically exploit oncogene overdose in Ewing sarcoma and potentially other fusion-TF-driven cancers.

Excellent Early Outcomes of Combined Chemotherapy With Arsenic Trioxide for Stage 4/M Neuroblastoma in Children: A Multicenter Nonrandomized Controlled Trial

This nonrandomized, multicenter cohort, open-label clinical trial evaluated the efficacy and safety of combined chemotherapy with arsenic trioxide (ATO) in children with stage 4/M neuroblastoma (NB). We enrolled patients who were newly diagnosed with NB and assessed as stage 4/M and received either traditional chemotherapy or ATO combined with chemotherapy according to their own wishes. Twenty-two patients were enrolled in the trial group (ATO combined with chemotherapy), and 13 patients were enrolled in the control group (traditional chemotherapy). Objective response rate (ORR) at 4 weeks after completing induction chemotherapy was defined as the main outcome, and adverse events were monitored and graded in the meantime. Data cutoff date was December 31, 2019. Finally, we found that patients who received ATO combined with chemotherapy had a significantly higher response rate than those who were treated with traditional chemotherapy (ORR: 86.36% vs. 46.16%, p=0.020). Reversible cardiotoxicity was just observed in three patients who were treated with ATO, and no other differential adverse events were observed between the two groups. ATO combined with chemotherapy can significantly improve end-induction response in high-risk NB, and our novel regimen is well tolerated in pediatric patients. These results highlight the superiority of chemotherapy with ATO, which creates new opportunity for prolonging survival. In addition, this treatment protocol minimizes therapeutic costs compared with anti-GD2 therapy, MIBG, and proton therapy and can decrease the burden to families and society. However, we also need to evaluate more cases to consolidate our conclusion.

Aberrant phase separation and cancer

Eukaryotic cells are intracellularly divided into numerous compartments or organelles, which coordinate specific molecules and biological reactions. Membrane-bound organelles are physically separated by lipid bilayers from the surrounding environment. Biomolecular condensates, also referred to membraneless organelles, are micron-scale cellular compartments that lack membranous enclosures but function to concentrate proteins and RNA molecules, and these are involved in diverse processes. Liquid-liquid phase separation (LLPS) driven by multivalent weak macromolecular interactions is a critical principle for the formation of biomolecular condensates, and a multitude of combinations among multivalent interactions may drive liquid-liquid phase transition (LLPT). Dysregulation of LLPS and LLPT leads to aberrant condensate and amyloid formation, which causes many human diseases, including neurodegeneration and cancer. Here, we describe recent findings regarding abnormal forms of biomolecular condensates and aggregation via aberrant LLPS and LLPT of cancer-related proteins in cancer development driven by mutation and fusion of genes. Moreover, we discuss the regulatory mechanisms by which aberrant LLPS and LLPT occur in cancer and the drug candidates targeting these mechanisms. Further understanding of the molecular events regulating how biomolecular condensates and aggregation form in cancer tissue is critical for the development of therapeutic strategies against tumorigenesis.

Emerging role of tumor cell plasticity in modifying therapeutic response

Resistance to cancer therapy is a major barrier to cancer management. Conventional views have proposed that acquisition of resistance may result from genetic mutations. However, accumulating evidence implicates a key role of non-mutational resistance mechanisms underlying drug tolerance, the latter of which is the focus that will be discussed here. Such non-mutational processes are largely driven by tumor cell plasticity, which renders tumor cells insusceptible to the drug-targeted pathway, thereby facilitating the tumor cell survival and growth. The concept of tumor cell plasticity highlights the significance of re-activation of developmental programs that are closely correlated with epithelial-mesenchymal transition, acquisition properties of cancer stem cells, and trans-differentiation potential during drug exposure. From observations in various cancers, this concept provides an opportunity for investigating the nature of anticancer drug resistance. Over the years, our understanding of the emerging role of phenotype switching in modifying therapeutic response has considerably increased. This expanded knowledge of tumor cell plasticity contributes to developing novel therapeutic strategies or combination therapy regimens using available anticancer drugs, which are likely to improve patient outcomes in clinical practice.

Targeting positive feedback between BASP1 and EGFR as a therapeutic strategy for lung cancer progression

Rationale: Brain metastasis in patients with lung cancer is life-threatening. However, the molecular mechanism for this catastrophic disease remains elusive, and few druggable targets are available. Therefore, this study aimed to identify and characterize proteins that could be used as therapeutic targets.

Methods: Proteomic analyses were conducted to identify differentially expressed membrane proteins between brain metastatic lung cancer cells and primary lung cancer cells. A neuronal growth-associated protein, brain acid soluble protein 1 (BASP1), was chosen for further investigation. The clinical relevance of BASP1 in lung adenocarcinoma was first assessed. Tyrosine kinase activity assays and in vitro and in vivo functional assays were conducted to explore the oncogenic mechanisms of BASP1.

Results: The protein levels of BASP1 were positively associated with tumor progression and poor prognosis in patients with lung adenocarcinoma. Membrane-bound BASP1 increased EGFR signaling and stabilized EGFR proteins by facilitating their escape from the ubiquitin-proteasome pathway. Reciprocally, activation of EGFR recruited more BASP1 to the plasma membrane, generating a positive feedback loop between BASP1 and EGFR. Moreover, the synergistic therapeutic effects of EGFR tyrosine kinase inhibitor and arsenic trioxide led to a reduction in the level of BASP1 protein observed in lung cancer cells with acquired resistance to EGFR inhibitors.

Conclusions: The reciprocal interaction between BASP1 and EGFR facilitates EGFR signaling in brain metastatic lung cancer. Targeting the newly identified BASP1-EGFR interaction could open new venues for lung cancer treatment.

Proteomic characterization of arsenic and cadmium exposure in bladder cells

RATIONALE

Accumulating evidence has linked prolonged exposure to heavy metals to cancer occurrence in the urinary system. However, the specific biological mechanisms responsible for the association of heavy metals with the unusually high incidence of upper tract urothelial carcinoma in Taiwan are complex and incompletely understood.

METHODS

To elucidate the specific biological mechanism and identify molecular indicators of the unusually high association of upper tract urothelial carcinoma with heavy metal exposure, protein expression following the treatment of T24 human bladder carcinoma and RT4 human bladder papilloma cell line models with arsenic (As) and cadmium (Cd) was studied. Proteomic changes in these cell models were integrated with data from a human bladder cancer (BLCA) tissue proteome to identify possible protein indicators of heavy metal exposure.

RESULTS

After mass spectrometry based proteomic analysis and verification by Western blotting procedures, we identified 66 proteins that were up-regulated and 92 proteins that were down-regulated in RT4 cell extracts after treatment with As or Cd. Some 52 proteins were up-regulated and 136 proteins were down-regulated in T24 cell extracts after treatment with Cd. We further confirmed that down-expression of the PML (promyelocytic leukemia) protein was sustained for at least 75 days after exposure of bladder cells to As. Dysregulation of these cellular proteins by As was associated with three biological pathways. Immunohistochemical analyses of paraffin-embedded BLCA tissue slides confirmed that PML protein expression was decreased in BLCA tumor cells compared with adjacent noncancerous epithelial cells.

CONCLUSIONS

These data suggest that PML may play an important role in the pathogenesis of BLCA and may be an indicator of heavy metal exposure in bladder cells.

[PML isoforms and TGF-β response]

PML/TRIM19 is the organizer of PML nuclear bodies (NB), a multiprotein complex associated to the nuclear matrix, which recruit a large number of proteins involved in various cellular processes. Alternative splicing from a single PML gene generates 6 nuclear PML isoforms (PMLI to PMLVI) and one cytoplasmic isoform, PMLVII. Murine PML-null primary cells are resistant to TGF-β-induced apoptosis. Cytoplasmic PML is an essential activator of TGF-β signaling by increasing the phosphorylation of transcription factors SMAD2/3 while nuclear PML plays a role in TGF-β-induced caspase 8 activation and apoptosis. TGF-β targets nuclear PML by inducing its conjugation to SUMO. In the nucleus, PML is mainly expressed in the nucleoplasm with a small fraction in the nuclear matrix. In response to TGF-β, PML and caspase 8 shift to the nuclear matrix, where both PML and caspase 8 colocalise within PML NBs. Here, we review the implication of cytoplasmic and nuclear PML isoforms in TGF-β response.

Acute Promyelocytic Leukemia: Update on the Mechanisms of Leukemogenesis, Resistance and on Innovative Treatment Strategies

This review highlights new findings that have deepened our understanding of the mechanisms of leukemogenesis, therapy and resistance in acute promyelocytic leukemia (APL). Promyelocytic leukemia-retinoic acid receptor α (PML-RARa) sets the cellular landscape of acute promyelocytic leukemia (APL) by repressing the transcription of RARa target genes and disrupting PML-NBs. The RAR receptors control the homeostasis of tissue growth, modeling and regeneration, and PML-NBs are involved in self-renewal of normal and cancer stem cells, DNA damage response, senescence and stress response. The additional somatic mutations in APL mainly involve FLT3, WT1, NRAS, KRAS, ARID1B and ARID1A genes. The treatment outcomes in patients with newly diagnosed APL improved dramatically since the advent of all-trans retinoic acid (ATRA) and arsenic trioxide (ATO). ATRA activates the transcription of blocked genes and degrades PML-RARα, while ATO degrades PML-RARa by promoting apoptosis and has a pro-oxidant effect. The resistance to ATRA and ATO may derive from the mutations in the RARa ligand binding domain (LBD) and in the PML-B2 domain of PML-RARa, but such mutations cannot explain the majority of resistances experienced in the clinic, globally accounting for 5-10% of cases. Several studies are ongoing to unravel clonal evolution and resistance, suggesting the therapeutic potential of new retinoid molecules and combinatorial treatments of ATRA or ATO with different drugs acting through alternative mechanisms of action, which may lead to synergistic effects on growth control or the induction of apoptosis in APL cells.

Acute Promyelocytic Leukemia: A History over 60 Years-From the Most Malignant to the most Curable Form of Acute Leukemia

Acute promyelocytic leukemia (APL) is a distinct subtype of acute myeloid leukemia (AML) that is cytogenetically characterized by a balanced reciprocal translocation between chromosomes 15 and 17, which results in the fusion of the promyelocytic leukemia (PML) and retinoic acid receptor alpha (RARα) genes. Because patients with APL present a tendency for severe bleeding, often resulting in an early fatal course, APL was historically considered to be one of the most fatal forms of acute leukemia. However, therapeutic advances, including anthracycline- and cytarabine-based chemotherapy, have significantly improved the outcomes of APL patients. Due to the further introduction of all-trans retinoic acid (ATRA) and-more recently-the development of arsenic trioxide (ATO)-containing regimens, APL is currently the most curable form of AML in adults. Treatment with these new agents has introduced the concept of cure through targeted therapy. With the advent of revolutionary ATRA-ATO combination therapies, chemotherapy can now be safely omitted from the treatment of low-risk APL patients. In this article, we review the six-decade history of APL, from its initial characterization to the era of chemotherapy-free ATRA-ATO, a model of cancer-targeted therapy.

TGF-β induces PML SUMOylation, degradation and PML nuclear body disruption

ProMyelocytic Leukemia (PML) protein is essential for the formation of nuclear matrix-associated organelles named PML nuclear bodies (NBs) that act as a platform for post-translational modifications and protein degradation. PML NBs harbor transiently and permanently localized proteins and are associated with the regulation of several cellular functions including apoptosis. There are seven PML isoforms, six nuclear (PMLI-VI) and one cytoplasmic (PMLVII), which are encoded by a single gene via alternative RNA splicing. It has been reported that murine PML-null primary cells are resistant to TGF-β-induced apoptosis and that cytoplasmic PML is an essential activator of TGF-β signaling. The role and the fate of interferon (IFN)-enhanced PML NBs in response to TGF-β have not been investigated. Here we show that IFNα potentiated TGF-β-mediated apoptosis in human cells. IFNα or ectopic expression of PMLIV, but not of PMLIII, enhanced TGF-β-induced caspase 8 activation. In response to TGF-β, both PMLIII and PMLIV were conjugated to SUMO and shifted from the nucleoplasm to the nuclear matrix, however only PMLIV, via its specific C-terminal region, interacted with caspase 8 and recruited it within PML NBs. This process was followed by a caspase-dependent PML degradation and PML NB disruption. Taken together, these findings highlight the role of PML NBs in the enhancement by IFN of TGF-β-induced apoptosis and caspase 8 activation.

Autophagy as a molecular target for cancer treatment

Autophagy is an evolutionarily conserved catabolic mechanism, by which eukaryotic cells recycle or degrades internal constituents through membrane-trafficking pathway. Thus, autophagy provides the cells with a sustainable source of biomolecules and energy for the maintenance of homeostasis under stressful conditions such as tumor microenvironment. Recent findings revealed a close relationship between autophagy and malignant transformation. However, due to the complex dual role of autophagy in tumor survival or cell death, efforts to develop efficient treatment strategies targeting the autophagy/cancer relation have largely been unsuccessful. Here we review the two-faced role of autophagy in cancer as a tumor suppressor or as a pro-oncogenic mechanism. In this sense, we also review the shared regulatory pathways that play a role in autophagy and malignant transformation. Finally, anti-cancer therapeutic agents used as either inhibitors or inducers of autophagy have been discussed.

RNF8 is responsible for ATRA resistance in variant acute promyelocytic leukemia with GTF2I/RARA fusion, and inhibition of the ubiquitin-proteasome pathway contributes to the reversion of ATRA resistance

Background

GTF2I-RARA is a newly identified RARA fusion gene in variant acute promyelocytic leukemia (APL) patients with t(7;17)(q11;q21). Clinical manifestation in the patient showed that it is a sort of ATRA-insensitive oncogene and is different from the classic PML-RARA in terms of therapeutic reaction.

Methods

To reveal the functional characteristics and regulating mechanism of the GTF2I-RARA fusion gene, we established a GTF2I-RARA-transfected HL60 cell model and examined its sensitivity to ATRA by western blot, MTT assay, flow cytometry, and Wright-Giemsa staining. Coimmunoprecipitation and confocal microscopy were used to examine the binding of GTF2I-RARA and transcriptional corepressors. We also performed ChIP-seq to search for potential target genes. Immunoprecipitation, ubiquitination assay, western blot, luciferase assay, and real-time PCR were used to analyze the effects of RNF8 on RARA. Flow cytometry and Wright-Giemsa staining were used to study the effect of MG132 and ATRA on the GTF2I-RARA-transfected HL60 cell model.

Result

We confirmed resistance of GTF2I-RARA to ATRA. Compared with PML-RARA, GTF2I-RARA has a higher affinity to HDAC3 under ATRA treatment. Using the ChIP-sequencing approach, we identified 221 GTF2I-RARA binding sites in model cells and found that the RING finger protein 8 (RNF8) is a target gene of GTF2I-RARA. RNF8 participates in disease progression and therapy resistance in APL with the GTF2I-RARA transcript. Elevated RNF8 expression promotes the interaction between RARA and RNF8 and induces RARA Lys-48 linkage ubiquitylation and degradation, resulting in attenuated transcriptional activation of RARA.

Conclusion

Our results suggest that RNF8 is a key GTF2I-RARA downstream event. Using the combination of MG132 and ATRA to treat GTF2I-RARA-HL60 cells, a synergistic effect leading to GTF2I-RARA-HL60 cell differentiation was confirmed. Taken together, the targeting of RNF8 may be an alternative choice for treatment in variant APL with GTF2I-RARA fusion.

Electronic supplementary material

The online version of this article (10.1186/s12935-019-0803-4) contains supplementary material, which is available to authorized users.

FLT3-ITD impedes retinoic acid, but not arsenic, responses in murine acute promyelocytic leukemias

Acute promyelocytic leukemia (APL) is often associated with activating FLT3 signaling mutations. These are highly related to hyperleukocytosis, a major adverse risk factor with chemotherapy-based regimens. APL is a model for oncogene-targeted therapies: all-trans retinoic acid (ATRA) and arsenic both target and degrade its ProMyelocytic Leukemia/Retinoic Acid Receptor α (PML/RARA) driver. The combined ATRA/arsenic regimen now cures virtually all patients with standard-risk APL. Although FLT3-internal tandem duplication (ITD) was an adverse risk factor for historical ATRA/chemotherapy regimens, the molecular bases for this effect remain unknown. Using mouse APL models, we unexpectedly demonstrate that FLT3-ITD severely blunts ATRA response. Remarkably, although the transcriptional output of initial ATRA response is unaffected, ATRA-induced PML/RARA degradation is blunted, as is PML nuclear body reformation and activation of P53 signaling. Critically, the combination of ATRA and arsenic fully rescues therapeutic response in FLT3-ITD APLs, restoring PML/RARA degradation, PML nuclear body reformation, P53 activation, and APL eradication. Moreover, arsenic targeting of normal PML also contributes to APL response in vivo. These unexpected results explain the less favorable outcome of FLT3-ITD APLs with ATRA-based regimens, and stress the key role of PML nuclear bodies in APL eradication by the ATRA/arsenic combination.

What has traditional Chinese medicine delivered for modern medicine?

The field of Traditional Chinese Medicine (TCM) represents a vast and largely untapped resource for modern medicine. Exemplified by the success of the antimalarial artemisinin, the recent years have seen a rapid increase in the understanding and application of TCM-derived herbs and formulations for evidence-based therapy. In this review, we summarise and discuss the developmental history, clinical background and molecular basis of an action for several representative TCM-derived medicines, including artemisinin, arsenic trioxide, berberine and Salvia miltiorrhiza or Danshen. Through this, we highlight important examples of how TCM-derived medicines have already contributed to modern medicine, and discuss potential avenues for further research.

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.

PML: Regulation and multifaceted function beyond tumor suppression

Promyelocytic leukemia protein (PML) was originally identified as a fusion partner of retinoic acid receptor alpha in acute promyelocytic leukemia patients with the (15;17) chromosomal translocation, giving rise to PML–RARα and RARα–PML fusion proteins. A body of evidence indicated that PML possesses tumor suppressing activity by regulating apoptosis, cell cycle, senescence and DNA damage responses. PML is enriched in discrete nuclear substructures in mammalian cells with 0.2–1 μm diameter in size, referred to as alternately Kremer bodies, nuclear domain 10, PML oncogenic domains or PML nuclear bodies (NBs). Dysregulation of PML NB formation results in altered transcriptional regulation, protein modification, apoptosis and cellular senescence. In addition to PML NBs, PML is also present in nucleoplasm and cytoplasmic compartments, including the endoplasmic reticulum and mitochondria-associated membranes. The role of PML in tumor suppression has been extensively studied but increasing evidence indicates that PML also plays versatile roles in stem cell renewal, metabolism, inflammatory responses, neural function, mammary development and angiogenesis. In this review, we will briefly describe the known PML regulation and function and include new findings.

Carbohydrate-conjugated 4-(1,3,2-dithiarsolan-2-yl)aniline as a cytotoxic agent against colorectal cancer

Arsenic trioxide (As₂O₃) has been approved for the treatment of acute promyelocytic leukemia (APL); however, its use in the treatment of solid tumors is limited due to its pharmacokinetic properties. Organic arsenic compounds provide better options for pharmaceutical optimization. p-Aminophenyl arsenoxide (p-APAO), an organic arsenic compound, was found to interact with the promyelocytic leukemia–retinoic acid receptor alpha (PML–RARα) fusion protein in a similar manner to arsenic trioxide. Analogs of p-APAO such as 4-(1,3,2-dithiarsolan-2-yl)aniline (p-APDTAs) were recently found to show improved cytotoxicity toward several solid tumor cell lines with lower toxicity to normal cells. Here, we synthesized a carbohydrate-conjugated 4-(1,3,2-dithiarsolan-2-yl)aniline (p-APDTAs) and showed that it exhibited reduced cytotoxicity to normal cells, suggesting a feasible approach to improve the therapeutic index of arsenic-containing compounds as chemotherapeutic agents.

We synthesized a carbohydrate-conjugated 4-(1,3,2-dithiarsolan-2-yl)aniline. It exhibited reduced cytotoxicity to normal cells, suggesting a feasible approach to improve the therapeutic index of arsenic-containing compounds as chemotherapeutic agents.

Acute Promyelocytic Leukemia: A Paradigm for Oncoprotein-Targeted Cure

Recent clinical trials have demonstrated that the immense majority of acute promyelocytic leukemia (APL) patients can be definitively cured by the combination of two targeted therapies: retinoic acid (RA) and arsenic. Mouse models have provided unexpected insights into the mechanisms involved. Restoration of PML nuclear bodies upon RA- and/or arsenic-initiated PML/RARA degradation is essential, while RA-triggered transcriptional activation is dispensable for APL eradication. Mutations of the arsenic-binding site of PML/RARA, but also PML, have been detected in therapy-resistant patients, demonstrating the key role of PML in APL cure. PML nuclear bodies are druggable and could be harnessed in other conditions.

Establishment and characterization of arsenic trioxide resistant KB/ATO cells

Arsenic trioxide (ATO) is used as a chemotherapeutic agent for the treatment of acute promyelocytic leukemia. However, increasing drug resistance is reducing its efficacy. Therefore, a better understanding of ATO resistance mechanism is required. In this study, we established an ATO-resistant human epidermoid carcinoma cell line, KB/ATO, from its parental KB-3-1 cells. In addition to ATO, KB/ATO cells also exhibited cross-resistance to other anticancer drugs such as cisplatin, antimony potassium tartrate, and 6-mercaptopurine. The arsenic accumulation in KB/ATO cells was significantly lower than that in KB-3-1 cells. Further analysis indicated that neither application of P-glycoprotein inhibitor, breast cancer resistant protein (BCRP) inhibitor, or multidrug resistance protein 1 (MRP1) inhibitor could eliminate ATO resistance. We found that the expression level of ABCB6 was increased in KB/ATO cells. In conclusion, ABCB6 could be an important factor for ATO resistance in KB/ATO cells. The ABCB6 level may serve as a predictive biomarker for the effectiveness of ATO therapy.

TRIM proteins and diseases

Ubiquitination is one of the posttranslational modifications that regulates a number of intracellular events including signal transduction, protein quality control, transcription, cell cycle, apoptosis and development. The ubiquitin system functions as a garbage machine to degrade target proteins and as a regulator for several signalling pathways. Biochemical reaction of ubiquitination requires several enzymes including E1, E2 and E3, and E3 ubiquitin ligases play roles as receptors for recognizing target proteins. Most of the tripartite motif (TRIM) proteins are E3 ubiquitin ligases. Recent studies have shown that some TRIM proteins function as important regulators for a variety of diseases including cancer, inflammatory diseases, infectious diseases, neuropsychiatric disorders, chromosomal abnormalities and developmental diseases. In this review, we summarize the involvement of TRIM proteins in the aetiology of various diseases.

p53 as an Effector or Inhibitor of Therapy Response

Although integrity of the p53 signaling pathway in a given tumor was expected to be a critical determinant of response to therapies, most clinical studies failed to link p53 status and treatment outcome. Here, we present two opposite situations: one in which p53 is an essential effector of cure by targeted leukemia therapies and another one in advanced breast cancers in which p53 inactivation is required for the clinical efficacy of dose-dense chemotherapy. If p53 promotes or blocks therapy response, therapies must be tailored on its status in individual tumors.

Synergistic targeted therapy for acute promyelocytic leukaemia: a model of translational research in human cancer

Acute promyelocytic leukaemia (APL), the M3 subtype of acute myeloid leukaemia, was once a lethal disease, yet nowadays the majority of patients with APL can be successfully cured by molecularly targeted therapy. This dramatic improvement in the survival rate is an example of the advantage of modern medicine. APL is characterized by a balanced reciprocal chromosomal translocation fusing the promyelocytic leukaemia (PML) gene on chromosome 15 with the retinoic acid receptor α (RARα) gene on chromosome 17. It has been found that all-trans-retinoic acid (ATRA) or arsenic trioxide (ATO) alone exerts therapeutic effect on APL patients with the PML-RARα fusion gene, and the combination of both drugs can act synergistically to further enhance the cure rate of the patients. Here, we provide an insight into the pathogenesis of APL and the mechanisms underlying the respective roles of ATRA and ATO. In addition, treatments that lead to more effective differentiation and apoptosis of APL cells, including leukaemia-initiating cells, and more thorough eradication of the disease will be discussed. Moreover, as a model of translational research, the development of a cure for APL has followed a bidirectional approach of 'bench to bedside' and 'bedside to bench', which can serve as a valuable example for the diagnosis and treatment of other malignancies.

New strategies in acute promyelocytic leukemia: moving to an entirely oral, chemotherapy-free upfront management approach

Incorporation of all-trans retinoic acid (ATRA) and arsenic trioxide (ATO) into the management paradigms of acute promyelocytic leukemia (APL) has markedly improved outcomes. Significant progress occurred in understanding the molecular pathogenesis of APL. ATO, in contrast with ATRA, is capable of eradicating the APL-initiating cells and can result in cure. Preclinical and clinical data confirmed the synergy of ATO and ATRA, and the ATRA-ATO combination was proved noninferior to a standard ATRA-chemotherapy regimen in patients with non-high-risk APL. Oral formulations of arsenic exhibited excellent activity in advanced clinical testing and their combinations with ATRA offer an opportunity for a completely oral, chemotherapy-free regimen for curing APL. Nonetheless, significant challenges remain. Reducing early death due to bleeding complications is an important area of unmet need. Data suggest that delays in initiation of ATRA upon suspecting APL continue to occur in the community and contribute to early mortality. Questions remain about the optimal place and schedule of arsenic in the therapeutic sequence and the role of the oral formulations. Refining the role of minimal residual disease in directing treatment decisions is important. Development of novel targeted agents to treat relapsed disease requires deeper understanding of the secondary resistance mechanisms to ATRA and ATO.

Retinoic acid receptors: from molecular mechanisms to cancer therapy

Retinoic acid (RA), the major bioactive metabolite of retinol or vitamin A, induces a spectrum of pleiotropic effects in cell growth and differentiation that are relevant for embryonic development and adult physiology. The RA activity is mediated primarily by members of the retinoic acid receptor (RAR) subfamily, namely RARα, RARβ and RARγ, which belong to the nuclear receptor (NR) superfamily of transcription factors. RARs form heterodimers with members of the retinoid X receptor (RXR) subfamily and act as ligand-regulated transcription factors through binding specific RA response elements (RAREs) located in target genes promoters. RARs also have non-genomic effects and activate kinase signaling pathways, which fine-tune the transcription of the RA target genes. The disruption of RA signaling pathways is thought to underlie the etiology of a number of hematological and non-hematological malignancies, including leukemias, skin cancer, head/neck cancer, lung cancer, breast cancer, ovarian cancer, prostate cancer, renal cell carcinoma, pancreatic cancer, liver cancer, glioblastoma and neuroblastoma. Of note, RA and its derivatives (retinoids) are employed as potential chemotherapeutic or chemopreventive agents because of their differentiation, anti-proliferative, pro-apoptotic, and anti-oxidant effects. In humans, retinoids reverse premalignant epithelial lesions, induce the differentiation of myeloid normal and leukemic cells, and prevent lung, liver, and breast cancer. Here, we provide an overview of the biochemical and molecular mechanisms that regulate the RA and retinoid signaling pathways. Moreover, mechanisms through which deregulation of RA signaling pathways ultimately impact on cancer are examined. Finally, the therapeutic effects of retinoids are reported.

Dose-dependent effects of selenite (Se(4+)) on arsenite (As(3+))-induced apoptosis and differentiation in acute promyelocytic leukemia cells

To enhance the therapeutic effects and decrease the adverse effects of arsenic on the treatment of acute promyelocytic leukemia, we investigated the co-effects of selenite (Se⁴⁺) and arsenite (As³⁺) on the apoptosis and differentiation of NB4 cells and primary APL cells. A 1.0-μM concentration of Se⁴⁺ prevented the cells from undergoing As³⁺-induced apoptosis by inhibiting As³⁺ uptake, eliminating As³⁺-generated reactive oxygen species, and repressing the mitochondria-mediated intrinsic apoptosis pathway. However, 4.0 μM Se⁴⁺ exerted synergistic effects with As³⁺ on cell apoptosis by promoting As³⁺ uptake, downregulating nuclear factor-кB, and activating caspase-3. In addition to apoptosis, 1.0 and 3.2 μM Se⁴⁺ showed contrasting effects on As³⁺-induced differentiation in NB4 cells and primary APL cells. The 3.2 μM Se⁴⁺ enhanced As³⁺-induced differentiation by promoting the degradation of promyelocytic leukemia protein–retinoic acid receptor-α (PML–RARα) oncoprotein, but 1.0 μM Se⁴⁺ did not have this effect. Based on mechanistic studies, Se⁴⁺, which is similar to As³⁺, might bind directly to Zn²⁺-binding sites of the PML RING domain, thus controlling the fate of PML–RARα oncoprotein.

Retinoic acid signaling in cancer: The parable of acute promyelocytic leukemia

Inevitably fatal some 40 years, acute promyelocytic leukemia (APL) can now be cured in more than 95% of cases. This clinical success story is tightly linked to tremendous progress in our understanding of retinoic acid (RA) signaling. The discovery of retinoic acid receptor alpha (RARA) was followed by the cloning of the chromosomal translocations driving APL, all of which involve RARA. Since then, new findings on the biology of nuclear receptors have progressively enlightened the basis for the clinical efficacy of RA in APL. Reciprocally, the disease offered a range of angles to approach the cellular and molecular mechanisms of RA action. This virtuous circle contributed to make APL one of the best-understood cancers from both clinical and biological standpoints. Yet, some important questions remain unanswered including how lessons learnt from RA-triggered APL cure can help design new therapies for other malignancies.

Arsenic trioxide inhibits transforming growth factor-β1-induced fibroblast to myofibroblast differentiation in vitro and bleomycin induced lung fibrosis in vivo

Background

Idiopathic pulmonary fibrosis (IPF) is a progressive disease of insidious onset, and is responsible for up to 30,000 deaths per year in the U.S. Excessive production of extracellular matrix by myofibroblasts has been shown to be an important pathological feature in IPF. TGF-β1 is expressed in fibrotic lung and promotes fibroblast to myofibroblast differentiation (FMD) as well as matrix deposition.

Methods

To identify the mechanism of Arsenic trioxide’s (ATO)’s anti-fibrotic effect in vitro, normal human lung fibroblasts (NHLFs) were treated with ATO for 24 hours and were then exposed to TGF-β1 (1 ng/ml) before harvesting at multiple time points. To investigate whether ATO is able to alleviate lung fibrosis in vivo, C57BL/6 mice were administered bleomycin by oropharyngeal aspiration and ATO was injected intraperitoneally daily for 14 days. Quantitative real-time PCR, western blotting, and immunofluorescent staining were used to assess the expression of fibrotic markers such as α-smooth muscle actin (α-SMA) and α-1 type I collagen.

Results

Treatment of NHLFs with ATO at very low concentrations (10-20nM) inhibits TGF-β1-induced α-smooth muscle actin (α-SMA) and α-1 type I collagen mRNA and protein expression. ATO also diminishes the TGF-β1-mediated contractile response in NHLFs. ATO’s down-regulation of profibrotic molecules is associated with inhibition of Akt, as well as Smad2/Smad3 phosphorylation. TGF-β1-induced H₂O₂ and NOX-4 mRNA expression are also blocked by ATO. ATO-mediated reduction in Smad3 phosphorylation correlated with a reduction of promyelocytic leukemia (PML) nuclear bodies and PML protein expression. PML-/- mouse embryonic fibroblasts (MEFs) showed decreased fibronectin and PAI-1 expression in response to TGF-β1. Daily intraperitoneal injection of ATO (1 mg/kg) in C57BL/6 mice inhibits bleomycin induced lung α-1 type I collagen mRNA and protein expression.

Conclusions

In summary, these data indicate that low concentrations of ATO inhibit TGF-β1-induced fibroblast to myofibroblast differentiation and decreases bleomycin induced pulmonary fibrosis.

Treatment of acute promyelocytic leukemia with arsenic trioxide: clinical results and open questions

Acute promyelocytic leukemia (APL) is a rare form of acute myeloid leukemia. The specific translocation t(15;17), which results in the fusion gene PML-RARA is the diagnostic and pathomechanistic hallmark of APL. By combination, treatment consisting of the differentiating agent all-trans retinoic acid (ATRA), which targets this molecular lesion, and cytotoxic chemotherapy, cure can be achieved in over 70% of patients. Recently, arsenic trioxide (ATO) has emerged to be the most active single agent in the treatment of APL. Previous studies employing ATO in relapse settings reported average complete remission rates of 85% and a mean overall survival of over 60%. In recent approaches installing ATO in first-line treatment, ATO-induced response rates comparable to previous combination regimen. The results of these newer studies indicate that the backbone of chemotherapy can be dramatically reduced or completely replaced by ATO and ATRA with similar or even better outcome.

Inhibition of Sp1 functions by its sequestration into PML nuclear bodies

Promyelocytic leukemia nuclear bodies (PML NBs) are comprised of PML and a striking variety of its associated proteins. Various cellular functions have been attributed to PML NBs, including the regulation of gene expression. We report here that induced expression of PML recruits Sp1 into PML NBs, leading to the reduction of Sp1 transactivation function. Specifically, Chromatin immunoprecipitation (ChIP) assay demonstrated that induced expression of PML significantly diminishes the amount of Sp1 binding to its target gene promoter, immunofluorescence staining showed dramatic increase in the co-localization between PML and Sp1 upon induction of PML expression, moreover, PML and Sp1 co-fractionated in the core nuclear matrix. Our study further showed that PML promotes SUMOylation of Sp1 in a RING-motif-dependent manner, SUMOylation of Sp1 facilitates physical interaction between Sp1 and PML and recruitment of Sp1 into the PML NBs, the SUMO binding motif of PML was also important for its interaction with Sp1. The results of this study demonstrate a novel mechanism by which PML regulates gene expression through sequestration of the transcription factor into PML NBs.

The role of PML in hematopoietic and leukemic stem cell maintenance

The tumor suppressor promyelocytic leukemia (PML) was first identified as a component of PML-RARα fusion protein, one of the initiating cytogenetic abnormalities in acute promyelocytic leukemia. PML is now known to have diverse functions regulating the DNA-damage response, apoptosis, senescence, and angiogenesis. Recent investigations have identified PML as a regulator of metabolic pathways in stem cell compartments, including the hematopoietic system, and have provided researchers with new strategies for controlling stem cell maintenance and differentiation. Studies of PML in leukemia-initiating cells demonstrate that PML is also an essential component of their maintenance, which has drawn tremendous attention to PML from scientists in various stem cell fields. Here, we review research into PML and its associated pathways, including recent studies of PML as it relates to stem cell biology, as well as our finding that PML regulates fatty acid oxidation, which is essential to the maintenance of normal hematopoietic stem cells. We also discuss the therapeutic potential of controlling PML-associated pathways. In particular, we describe promising evidence for the use of arsenic trioxide in the treatment of chronic myeloid leukemia.

Acute promyelocytic leukemia: A model of molecular target based therapy

Leukemia, a group of hematological malignancies characterized by clonal expansion of hematopoietic cells with uncontrolled proliferation, decreased apoptosis and blocked differentiation, is one of the most notorious enemies of mankind which accounts for some 300,000 new cases and 222,000 deaths each year worldwide. Leukemia can be divided into acute or chronic, lymphoid or myeloid types, based on the disease progression and hematopoietic lineages involved 5. The responses of leukemia to therapies differ from one type or subtype to another. Hence, to improve the clinical outcome, the therapeutic strategies should be disease pathogenesis-based and individualized. The close collaboration between bench and bedside may not only shed new lights on leukemogenesis, gain insights into therapeutic mechanisms, but also provide opportunities for designing more rational therapies. The development of curative approaches for acute promyelocytic leukemia (APL) may serve as a paradigm.

PML, SUMOylation, and Senescence

Since its discovery, 25 years ago, promyelocytic leukemia (PML) has been an enigma. Implicated in the oncogenic PML/RARA fusion, forming elusive intranuclear domains, triggering cell death or senescence, controlled by and perhaps controlling SUMOylation… there are multiple PML-related issues. Here we review the reciprocal interactions between PML, senescence, and SUMOylation, notably in the context of cellular transformation.

The cell biology of disease: Acute promyelocytic leukemia, arsenic, and PML bodies

Acute promyelocytic leukemia (APL) is driven by a chromosomal translocation whose product, the PML/retinoic acid (RA) receptor α (RARA) fusion protein, affects both nuclear receptor signaling and PML body assembly. Dissection of APL pathogenesis has led to the rediscovery of PML bodies and revealed their role in cell senescence, disease pathogenesis, and responsiveness to treatment. APL is remarkable because of the fortuitous identification of two clinically effective therapies, RA and arsenic, both of which degrade PML/RARA oncoprotein and, together, cure APL. Analysis of arsenic-induced PML or PML/RARA degradation has implicated oxidative stress in the biogenesis of nuclear bodies and SUMO in their degradation.

An emerging role for retinoid X receptor α in malignant hematopoiesis

The retinoid X receptor alpha is the obligatory heterodimerization partner for a range of nuclear hormone receptors, and is required for signaling through the pathways mediated by those receptors. While RXR alpha has critical roles in embryonic development, it appears to be dispensable in adult hematopoiesis. Strikingly, recent evidence has indicated that proper functioning of RXR alpha is necessary for the pathogenesis of acute promyelocytic leukemia (APL), suggesting a novel avenue that can be exploited in the management and treatment of this disease. In this review we highlight recent studies that clarify the role of RXR alpha in normal and malignant hematopoiesis.

Arsenic trioxide for management of acute promyelocytic leukemia: current evidence on its role in front-line therapy and recurrent disease

INTRODUCTION

Acute promyelocytic leukemia (APL), the most rapidly fatal leukemia only two decades ago, has been converted into the most frequently curable leukemia by the advent of all-trans retinoic acid (ATRA) and its combination with anthracycline-based chemotherapy. More recently, arsenic trioxide (ATO) has been shown to be the most effective single agent in this disease and has been approved for the treatment of relapsed patients both in the United States and Europe. Moreover, ATO has been included in the design of several front-line studies, with the aim to reduce therapy-related toxicity while maintaining the potential of cure.

AREAS COVERED

First, this review briefly discusses the mechanisms of action and the toxicity profile of ATO. Furthermore, the reported experience on the use of ATO as single agent or in combinatorial schemes both in relapsed and in newly diagnosed patients with APL is critically reviewed. Finally, the use of this agent in special subsets of patients unfit to receive conventional chemotherapy is discussed, along with its potential role in maintenance therapy.

EXPERT OPINION

While the role of ATO as single agent or in combination with ATRA is well established and recommended by the European LeukemiaNet guidelines as a first option for relapsed patients, the role of the drug in newly diagnosed patients is still uncertain and based only on evidence levels mostly originating from non-randomized trials. The results of ongoing randomized studies should better define the role of ATO in front-line therapy.

How to manage acute promyelocytic leukemia

Acute promyelocytic leukemia (APL) is a unique subtype of acute myeloid leukemia (AML). The prognosis of APL is changing, from the worst among AML as it used to be, to currently the best. The application of all-trans-retinoic acid (ATRA) to the induction therapy of APL decreases the mortality of newly diagnosed patients, thereby significantly improving the response rate. Therefore, ATRA combined with anthracycline-based chemotherapy has been widely accepted and used as a classic treatment. It has been demonstrated that high doses of cytarabine have a good effect on the prevention of relapse for high-risk patients. However, as the indications of arsenic trioxide (ATO) for APL are being extended from the original relapse treatment to the first-line treatment of de novo APL, we find that the regimen of ATRA, combined with ATO, seems to be a new treatment option because of their targeting mechanisms, milder toxicities and improvements of long-term outcomes; this combination may become a potentially curable treatment modality for APL. We discuss the therapeutic strategies for APL, particularly the novel approaches to newly diagnosed patients and the handling of side effects of treatment and relapse treatment, so as to ensure each newly diagnosed patient of APL the most timely and best treatment.

Speciation of arsenic trioxide metabolites in peripheral blood and bone marrow from an acute promyelocytic leukemia patient

Background

Speciation of arsenic trioxide (ATO) metabolites in clinical samples such as peripheral blood (PB) from acute promyelocytic leukemia (APL) patients has been conducted. However, speciation of arsenicals in bone marrow (BM) has not yet been performed. Profiles of arsenic speciation in plasma of BM were thus investigated and compared with those of PB plasma from a relapsed APL patient. The total arsenic concentrations in high molecular weight fraction (HMW-F) of BM and PB plasma were also determined.

Methods

Response assessment was evaluated by BM aspirate examination and fluorescence in situ hybridization analysis. The analyses of total arsenic concentrations and speciation were preformed by inductively coupled plasma mass spectrometry (ICP-MS), and high-performance liquid chromatography (HPLC)/ICP-MS, respectively.

Results

Response assessment showed that the patient achieved complete remission. The total arsenic concentrations in BM plasma increased with time during the consecutive administration. The PB plasma concentrations of methylated arsenic metabolites substantially increased after the start of administration, while those of inorganic arsenic were still kept at a low level, followed by substantially increase from day-14 after administration. The arsenic speciation profiles of PB plasma were very similar to those of BM plasma. Furthermore, the total arsenic concentrations of HMW-F in BM plasma were much higher than those in PB plasma.

Conclusions

The behaviors of arsenic speciation suggested for the first time that arsenic speciation analysis of PB plasma could be predicative for BM speciation, and showed relatively higher efficiency of drug metabolism in the patient. These results may further provide not only significance of clinical application of ATO, but also a new insight into host defense mechanisms in APL patients undergoing ATO treatment, since HMW proteins-bound arsenic complex could be thought to protect BM from the attack of free arsenic species.

Altered nuclear cofactor switching in retinoic-resistant variants of the PML-RARα oncoprotein of acute promyelocytic leukemia

Acute promyelocytic leukemia (APL) results from a reciprocal translocation that fuses the gene for the PML tumor suppressor to that encoding the retinoic acid receptor alpha (RARα). The resulting PML-RARα oncogene product interferes with multiple regulatory pathways associated with myeloid differentiation, including normal PML and RARα functions. The standard treatment for APL includes anthracycline-based chemotherapeutic agents plus the RARα agonist all-trans retinoic acid (ATRA). Relapse, which is often accompanied by ATRA resistance, occurs in an appreciable frequency of treated patients. One potential mechanism suggested by model experiments featuring the selection of ATRA-resistant APL cell lines involves ATRA-resistant versions of the PML-RARα oncogene, where the relevant mutations localize to the RARα ligand-binding domain (LBD). Such mutations may act by compromising agonist binding, but other mechanisms are possible. Here, we studied the molecular consequence of ATRA resistance by use of circular dichroism, protease resistance, and fluorescence anisotropy assays employing peptides derived from the NCOR nuclear corepressor and the ACTR nuclear coactivator. The consequences of the mutations on global structure and cofactor interaction functions were assessed quantitatively, providing insights into the basis of agonist resistance. Attenuated cofactor switching and increased protease resistance represent features of the LBDs of ATRA-resistant PML-RARα, and these properties may be recapitulated in the full-length oncoproteins.

Targeting PML-RARα and Oncogenic Signaling Pathways by Chinese Herbal Mixture Tien-Hsien Liquid in Acute Promyelocytic Leukemia NB4 Cells

Tien-Hsien Liquid (THL) is a Chinese herbal mixture that has been used worldwide as complementary treatment for cancer patients in the past decade. Recently, THL has been shown to induce apoptosis in various types of solid tumor cells in vitro. However, the underlying molecular mechanisms have not yet been well elucidated. In this study, we explored the effects of THL on acute promyelocytic leukemia (APL) NB4 cells, which could be effectively treated by some traditional Chinese remedies containing arsenic trioxide. The results showed THL could induce G2/M arrest and apoptosis in NB4 cells. Accordingly, the decrease of cyclin A and B1 were observed in THL-treated cells. The THL-induced apoptosis was accompanied with caspase-3 activation and decrease of PML-RARα fusion protein. Moreover, DNA methyltransferase 1 and oncogenic signaling pathways such as Akt/mTOR, Stat3 and ERK were also down-regulated by THL. By using ethyl acetate extraction and silica gel chromatography, an active fraction of THL named as EAS5 was isolated. At about 0.5–1% of the dose of THL, EAS5 appeared to have most of THL-induced multiple molecular targeting effects in NB4 cells. Based on the findings of these multi-targeting effects, THL might be regarding as a complementary and alternative therapeutic agent for refractory APL.

The role of PML in the nervous system

The promyeloctic leukemia protein PML is a tumor suppressor that was originally identified due to its involvement in the (15;17) translocation of acute promyelocytic leukemia. While the majority of early research has focused upon the role of PML in the pathogenesis of leukemia, more recent evidence has identified important roles for PML in tissues outside the hemopoietic system, including the central nervous system (CNS). Here, we review recent literature on the role of PML in the CNS, with particular focus on the processes of neurodevelopment and neurodegeneration, and propose new lines of investigation.

Molecular mechanisms of leukemia-associated protein degradation

Chemical biology, using small molecules as probes to study the cellular signaling network, has developed rapidly in recent years. The interaction between chemistry and biology not only provides new insight into the understanding of cellular activities, but also generates new lead compounds for the treatment of diseases. Transcription factors and kinases such as retinoic acid receptor-alpha (RARα), acute myeloid leukemia 1 (AML1), CAAT/enhancer-binding protein α (C/EBPα), c-myc, and c-abl play important roles in the differentiation of hematopoietic stem/progenitor cells. Abnormalities in these proteins may cause the dysregulation of hematopoiesis and even the occurrence of leukemia. Ubiquitin-mediated protein degradation represents a critical mechanism in regulating the cellular levels and functions of these proteins. Thus, targeting protein degradation has been emerging as an important strategy to conquer malignant diseases. In this review, we will summarize the recent advances in the understanding of the roles of protein degradation in leukemia, with an emphasis on the mechanisms revealed by small molecules.