Role of the promyelocytic leukaemia protein in cell death regulation

The Role of ND10 Nuclear Bodies in Herpesvirus Infection: A Frenemy for the Virus?

Nuclear domains 10 (ND10), a.k.a. promyelocytic leukemia nuclear bodies (PML-NBs), are membraneless subnuclear domains that are highly dynamic in their protein composition in response to cellular cues. They are known to be involved in many key cellular processes including DNA damage response, transcription regulation, apoptosis, oncogenesis, and antiviral defenses. The diversity and dynamics of ND10 residents enable them to play seemingly opposite roles under different physiological conditions. Although the molecular mechanisms are not completely clear, the pro- and anti-cancer effects of ND10 have been well established in tumorigenesis. However, in herpesvirus research, until the recently emerged evidence of pro-viral contributions, ND10 nuclear bodies have been generally recognized as part of the intrinsic antiviral defenses that converge to the incoming viral DNA to inhibit the viral gene expression. In this review, we evaluate the newly discovered pro-infection influences of ND10 in various human herpesviruses and analyze their molecular foundation along with the traditional antiviral functions of ND10. We hope to shed light on the explicit role of ND10 in both the lytic and latent cycles of herpesvirus infection, which is imperative to the delineation of herpes pathogenesis and the development of prophylactic/therapeutic treatments for herpetic diseases.

Acetylation of SUMO1 Alters Interactions with the SIMs of PML and Daxx in a Protein-Specific Manner

The interactions between SUMO proteins and SUMO-interacting motif (SIM) in nuclear bodies formed by the promyelocytic leukemia (PML) protein (PML-NBs) have been shown to be modulated by either phosphorylation of the SIMs or acetylation of SUMO proteins. However, little is known about how this occurs at the atomic level. In this work, we examined the role that acetylation of SUMO1 plays on its binding to the phosphorylated SIMs (phosphoSIMs) of PML and Daxx. Our results demonstrate that SUMO1 binding to the phosphoSIM of either PML or Daxx is dramatically reduced by acetylation at either K39 or K46. However, acetylation at K37 only impacts binding to Daxx. Structures of acetylated SUMO1 variants bound to the phosphoSIMs of PML and Daxx demonstrate that there is structural plasticity in SUMO-SIM interactions. The plasticity observed in these structures provides a robust mechanism for regulating SUMO-SIM interactions in PML-NBs using signaling generated post-translational modifications.

MICONIDINE acetate, a new selective and cytotoxic compound with synergic potential, induces cell cycle arrest and apoptosis in leukemia cells

Plants are important sources of biologically active compounds and they provide unlimited opportunities for the discovery and development of new drug leads, including new chemotherapeutics. Miconidin acetate (MA) is a hydroquinone derivative isolated from E. hiemalis. In this study we demonstrated that MA was cytotoxic against acute leukemia (AL), solid tumor cells and cancer stem cells, with the strongest effect exhibited against AL. Furthermore, it was non-cytotoxic against non-tumor cells and did not cause significant hemolysis. MA blocks the G2/M phase and causes cytostatic effects, acting in a similar way to dexamethasone by increasing PML expression. The compound also triggered intrinsic and extrinsic apoptosis by modulating Bax, FasR and survivin expression. This led to an extensive mitochondrial damage that resulted in AIF, cytochrome c and endonuclease G release, caspase-3 and PARP cleavage and DNA fragmentation. We have further demonstrated that MA was strongly cytotoxic against neoplastic cells collected from patients with different AL subtypes. Interestingly, MA increased the cytotoxic effect of chemotherapeutics cytarabine and vincristine. This study indicates that MA may be a new agent for AL and highlights its potential as a new source of anticancer drugs. Graphical abstract MA blocks G2/M phase with PML expression and KI67 inhibition, ROS generation and intrinsic and extrinsic apoptosis, leading to mitochondrial damage, caspase 3 and PARP cleavage and DNA fragmentation.

PML silencing inhibits cell proliferation and induces DNA damage in cultured ovarian cancer cells

The promyelocytic leukemia (PML) gene is a tumor suppressor gene. It was first identified in acute promyelocytic leukemia, in which it is fused to retinoic acid receptor α by the (15;17) chromosomal translocation. The function of the PML protein is frequently lost or aberrant in human solid tumors. In human ovarian carcinoma tissue, PML detected by immunohistochemistry was highly expressed. A PML-silencing vector, pSRG-shPml, was constructed and used to transfect human ovarian cancer cells. Cells were cultured and selected with puromycin for 10–15 days, and then the PML mRNA expression levels were detected by RT-qPCR and immunofluorescence. Proliferation and clone number of PML-depleted cells were detected using MTT assay and colony-forming assay. The protein expression associated with DNA damage and apoptosis was assessed in PML-depleted cells using western blot analysis and immunofluorescence. The results showed that PML was highly expressed in human ovarian tissue. The proliferation and colony formation of ovarian cancer cells were significantly inhibited after PML was depleted. Western blot analysis and immunofluorescence revealed that p-H2AX and cleaved caspase-3 expression significantly increased after PML silencing. PML was located in the nucleus, and it formed foci after X-ray irradiation. PML foci increased significantly with increasing irradiation doses.

Estrogen receptor alpha transcriptionally activates casein kinase 2 alpha: A pivotal regulator of promyelocytic leukaemia protein (PML) and AKT in oncogenesis

Protein kinase CK2α is frequently upregulated in different cancers. Alteration of CK2α expression and its activity is sufficient to induce dramatic changes in cell fate. It has been established that CK2α induces oncogenesis through modulation of both AKT and PML. CK2α has been found to be overexpressed in breast cancer. In contrary, statistical reports have shown low level of PML. However, the regulation of CK2α gene expression is not fully understood. In the current study, we found that CK2α and activated AKT positively correlate with ERα, whereas PML follows an inverse correlation in human breast cancer tissues. Modulation of ERα signalling leads to recruitment of activated ERα on the ERE sites of CK2α promoter, resulting in CK2α transactivation. Furthermore, the DMBA induced tumours in rat showed elevated level of active CK2α. Consequently it mediates enhancement of AKT activity and PML degradation, resulting in increased cellular proliferation, migration and metastasis. Syngeneic ERα overexpressing stable mouse 4T1 cells produce larger primary tumours and metastatic lung nodules in mice, corroborating our in vitro findings. Hence, our study provides a novel route of ERα dependent CK2α mediated oncogenesis that causes upregulation and consequent AKT activation along with degradation of tumour suppressor PML.

Role of the nucleus in apoptosis: signaling and execution

Since their establishment in the early 1970s, the nuclear changes upon apoptosis induction, such as the condensation of chromatin, disassembly of nuclear scaffold proteins and degradation of DNA, were, and still are, considered as the essential steps and hallmarks of apoptosis. These are the characteristics of the execution phase of apoptotic cell death. In addition, accumulating data clearly show that some nuclear events can lead to the induction of apoptosis. In particular, if DNA lesions resulting from deregulation during the cell cycle or DNA damage induced by chemotherapeutic drugs or viral infection cannot be efficiently eliminated, apoptotic mechanisms, which enable cellular transformation to be avoided, are activated in the nucleus. The functional heterogeneity of the nuclear organization allows the tight regulation of these signaling events that involve the movement of various nuclear proteins to other intracellular compartments (and vice versa) to initiate and govern apoptosis. Here, we discuss how these events are coordinated to execute apoptotic cell death.

Identification of Arsenic Direct-Binding Proteins in Acute Promyelocytic Leukaemia Cells

The identification of arsenic direct-binding proteins is essential for determining the mechanism by which arsenic trioxide achieves its chemotherapeutic effects. At least two cysteines close together in the amino acid sequence are crucial to the binding of arsenic and essential to the identification of arsenic-binding proteins. In the present study, arsenic binding proteins were pulled down with streptavidin and identified using a liquid chromatograph-mass spectrometer (LC-MS/MS). More than 40 arsenic-binding proteins were separated, and redox-related proteins, glutathione S-transferase P1 (GSTP1), heat shock 70 kDa protein 9 (HSPA9) and pyruvate kinase M2 (PKM2), were further studied using binding assays in vitro. Notably, PKM2 has a high affinity for arsenic. In contrast to PKM2, GSTP1and HSPA9 did not combine with arsenic directly in vitro. These observations suggest that arsenic-mediated acute promyelocytic leukaemia (APL) suppressive effects involve PKM2. In summary, we identified several arsenic binding proteins in APL cells and investigated the therapeutic mechanisms of arsenic trioxide for APL. Further investigation into specific signal pathways by which PKM2 mediates APL developments may lead to a better understanding of arsenic effects on APL.

The function, regulation and therapeutic implications of the tumor suppressor protein, PML

The tumor suppressor protein, promyelocytic leukemia protein (PML), was originally identified in acute promyelocytic leukemia due to a chromosomal translocation between chromosomes 15 and 17. PML is the core component of subnuclear structures called PML nuclear bodies (PML-NBs), which are disrupted in acute promyelocytic leukemia cells. PML plays important roles in cell cycle regulation, survival and apoptosis, and inactivation or down-regulation of PML is frequently found in cancer cells. More than 120 proteins have been experimentally identified to physically associate with PML, and most of them either transiently or constitutively co-localize with PML-NBs. These interactions are associated with many cellular processes, including cell cycle arrest, apoptosis, senescence, transcriptional regulation, DNA repair and intermediary metabolism. Importantly, PML inactivation in cancer cells can occur at the transcriptional-, translational- or post-translational- levels. However, only a few somatic mutations have been found in cancer cells. A better understanding of its regulation and its role in tumor suppression will provide potential therapeutic opportunities. In this review, we discuss the role of PML in multiple tumor suppression pathways and summarize the players and stimuli that control PML protein expression or subcellular distribution.

The implication of cancer progenitor cells and the role of epigenetics in the development of novel therapeutic strategies for chronic myeloid leukemia

SIGNIFICANCE

Chronic myeloid leukemia (CML) involves the malignant transformation of hematopoietic stem cells, defined largely by the Philadelphia chromosome and expression of the breakpoint cluster region-Abelson (BCR-ABL) oncoprotein. Pharmacological tyrosine kinase inhibitors (TKIs), including imatinib mesylate, have overcome limitations in conventional treatment for the improved clinical management of CML.

RECENT ADVANCES

Accumulated evidence has led to the identification of a subpopulation of quiescent leukemia progenitor cells with stem-like self renewal properties that may initiate leukemogenesis, which are also shown to be present in residual disease due to their insensitivity to tyrosine kinase inhibition.

CRITICAL ISSUES

The characterization of quiescent leukemia progenitor cells as a unique cell population in CML pathogenesis has become critical with the complete elucidation of mechanisms involved in their survival independent of BCR-ABL that is important in the development of novel anticancer strategies. Understanding of these functional pathways in CML progenitor cells will allow for their selective therapeutic targeting. In addition, disease pathogenesis and drug responsiveness is also thought to be modulated by epigenetic regulatory mechanisms such as DNA methylation, histone acetylation, and microRNA expression, with a capacity to control CML-associated gene transcription.

FUTURE DIRECTIONS

A number of compounds in combination with TKIs are under preclinical and clinical investigation to assess their synergistic potential in targeting leukemic progenitor cells and/or the epigenome in CML. Despite the collective promise, further research is required in order to refine understanding, and, ultimately, advance antileukemic therapeutic strategies.

Structural and functional characterization of the phosphorylation-dependent interaction between PML and SUMO1

PML and several other proteins localizing in PML-nuclear bodies (PML-NB) contain phosphoSIMs (SUMO-interacting motifs), and phosphorylation of this motif plays a key role in their interaction with SUMO family proteins. We examined the role that phosphorylation plays in the binding of the phosphoSIMs of PML and Daxx to SUMO1 at the atomic level. The crystal structures of SUMO1 bound to unphosphorylated and tetraphosphorylated PML-SIM peptides indicate that three phosphoserines directly contact specific positively charged residues of SUMO1. Surprisingly, the crystal structure of SUMO1 bound to a diphosphorylated Daxx-SIM peptide indicate that the hydrophobic residues of the phosphoSIM bind in a manner similar to that seen with PML, but important differences are observed when comparing the phosphorylated residues. Together, the results provide an atomic level description of how specific acetylation patterns within different SUMO family proteins can work together with phosphorylation of phosphoSIM's regions of target proteins to regulate binding specificity.

Polymorphisms in microRNA target sites modulate risk of lymphoblastic and myeloid leukemias and affect microRNA binding

BACKGROUND

MicroRNA dysregulation is a common event in leukemia. Polymorphisms in microRNA-binding sites (miRSNPs) in target genes may alter the strength of microRNA interaction with target transcripts thereby affecting protein levels. In this study we aimed at identifying miRSNPs associated with leukemia risk and assessing impact of these miRSNPs on miRNA binding to target transcripts.

METHODS

We analyzed with specialized algorithms the 3' untranslated regions of 137 leukemia-associated genes and identified 111 putative miRSNPs, of which 10 were chosen for further investigation. We genotyped patients with acute myeloid leukemia (AML, n = 87), chronic myeloid leukemia (CML, n = 140), childhood acute lymphoblastic leukemia (ALL, n = 101) and healthy controls (n = 471). Association between SNPs and leukemia risk was calculated by estimating odds ratios in the multivariate logistic regression analysis. For miRSNPs that were associated with leukemia risk we performed luciferase reporter assays to examine whether they influence miRNA binding.

RESULTS

Here we show that variant alleles of TLX1_rs2742038 and ETV6_rs1573613 were associated with increased risk of childhood ALL (OR (95% CI) = 3.97 (1.43-11.02) and 1.9 (1.16-3.11), respectively), while PML_rs9479 was associated with decreased ALL risk (OR = 0.55 (0.36-0.86). In adult myeloid leukemias we found significant associations between the variant allele of PML_rs9479 and decreased AML risk (OR = 0.61 (0.38-0.97), and between variant alleles of IRF8_ rs10514611 and ARHGAP26_rs187729 and increased CML risk (OR = 2.4 (1.12-5.15) and 1.63 (1.07-2.47), respectively). Moreover, we observed a significant trend for an increasing ALL and CML risk with the growing number of risk genotypes with OR = 13.91 (4.38-44.11) for carriers of ≥3 risk genotypes in ALL and OR = 4.9 (1.27-18.85) for carriers of 2 risk genotypes in CML. Luciferase reporter assays revealed that the C allele of ARHGAP26_rs187729 creates an illegitimate binding site for miR-18a-3p, while the A allele of PML_rs9479 enhances binding of miR-510-5p and the C allele of ETV6_rs1573613 weakens binding of miR-34c-5p and miR-449b-5p.

CONCLUSIONS

Our study implicates that microRNA-binding site polymorphisms modulate leukemia risk by interfering with the miRNA-mediated regulation. Our findings underscore the significance of variability in 3' untranslated regions in leukemia.

Cellular senescence and protein degradation: breaking down cancer

Autophagy and the ubiquitin-proteasome pathway (UPP) are the major protein degradation systems in eukaryotic cells. Whereas the former mediate a bulk nonspecific degradation, the UPP allows a rapid degradation of specific proteins. Both systems have been shown to play a role in tumorigenesis, and the interest in developing therapeutic agents inhibiting protein degradation is steadily growing. However, emerging data point to a critical role for autophagy in cellular senescence, an established tumor suppressor mechanism. Recently, a selective protein degradation process mediated by the UPP was also shown to contribute to the senescence phenotype. This process is tightly regulated by E3 ubiquitin ligases, deubiquitinases, and several post-translational modifications of target proteins. Illustrating the complexity of UPP, more than 600 human genes have been shown to encode E3 ubiquitin ligases, a number which exceeds that of the protein kinases. Nevertheless, our knowledge of proteasome-dependent protein degradation as a regulated process in cellular contexts such as cancer and senescence remains very limited. Here we discuss the implications of protein degradation in senescence and attempt to relate this function to the protein degradation pattern observed in cancer cells.

Inositol 1,4,5-trisphosphate receptor-isoform diversity in cell death and survival

Cell-death and -survival decisions are critically controlled by intracellular Ca(2+) homeostasis and dynamics at the level of the endoplasmic reticulum (ER). Inositol 1,4,5-trisphosphate (IP3) receptors (IP3Rs) play a pivotal role in these processes by mediating Ca(2+) flux from the ER into the cytosol and mitochondria. Hence, it is clear that many pro-survival and pro-death signaling pathways and proteins affect Ca(2+) signaling by directly targeting IP3R channels, which can happen in an IP3R-isoform-dependent manner. In this review, we will focus on how the different IP3R isoforms (IP3R1, IP3R2 and IP3R3) control cell death and survival. First, we will present an overview of the isoform-specific regulation of IP3Rs by cellular factors like IP3, Ca(2+), Ca(2+)-binding proteins, adenosine triphosphate (ATP), thiol modification, phosphorylation and interacting proteins, and of IP3R-isoform specific expression patterns. Second, we will discuss the role of the ER as a Ca(2+) store in cell death and survival and how IP3Rs and pro-survival/pro-death proteins can modulate the basal ER Ca(2+) leak. Third, we will review the regulation of the Ca(2+)-flux properties of the IP3R isoforms by the ER-resident and by the cytoplasmic proteins involved in cell death and survival as well as by redox regulation. Hence, we aim to highlight the specific roles of the various IP3R isoforms in cell-death and -survival signaling. This article is part of a Special Issue entitled: Calcium signaling in health and disease. Guest Editors: Geert Bultynck, Jacques Haiech, Claus W. Heizmann, Joachim Krebs, and Marc Moreau.

How the TP53 family proteins TP63 and TP73 contribute to tumorigenesis: regulators and effectors

In mammals, the p53 family comprises two additional members, p63 and p73 (hereafter referred to as TP53, TP63, and TP73, respectively). The usage of two alternative promoters produces protein variants either with (transactivating [TA] isoforms) or without (ΔN isoforms) the N-terminal transactivation domain (TAD). In general, the TA proteins exert TP53-like tumor-suppressive activities through their ability to activate a common set of target genes. The ΔN proteins can act as dominant-negative inhibitors of the transcriptionally active family members. Additionally, they possess intrinsic-specific biological activities due to the presence of alternative TADs, and as a result of engaging a different set of regulators. This review summarizes the current understanding of upstream regulators and downstream effectors of the TP53 family proteins, with particular emphasis on those that are relevant for their role in tumorigenesis. Furthermore, we highlight the existence of networks and cross-talks among the TP53 family members, their modulators, as well as the transcriptional targets.

Targeting survival pathways in chronic myeloid leukaemia stem cells

Chronic myeloid leukaemia (CML) is a clonal myeloproliferative disorder characterized by the presence of a fusion oncogene BCR-ABL, which encodes a protein with constitutive TK activity. The implementation of tyrosine kinase inhibitors (TKIs) marked a major advance in CML therapy; however, there are problems with current treatment. For example, relapse occurs when these drugs are discontinued in the majority of patients who have achieved a complete molecular response on TKI and these agents are less effective in patients with mutations in the BCR-ABL kinase domain. Importantly, TKI can effectively target proliferating mature cells, but do not eradicate quiescent leukaemic stem cells (LSCs), therefore allowing disease persistence despite treatment. It is essential that alternative strategies are used to target the LSC population. BCR-ABL activation is responsible for the modulation of different signalling pathways, which allows the LSC fraction to evade cell death. Several pathways have been shown to be modulated by BCR-ABL, including PI3K/AKT/mTOR, JAK-STAT and autophagy signalling pathways. Targeting components of these survival pathways, alone or in combination with TKI, therefore represents an attractive potential therapeutic approach for targeting the LSC. However, many pathways are also active in normal stem cells. Therefore, potential targets must be validated to effectively eradicate CML stem cells while sparing normal counterparts. This review summarizes the main pathways modulated in CML stem cells, the recent developments and the use of novel drugs to target components in these pathways which may be used to target the LSC population.

LINKED ARTICLES

This article is part of a themed section on Emerging Therapeutic Aspects in Oncology. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2013.169.issue-8.

PML overexpression inhibits proliferation and promotes the osteogenic differentiation of human mesenchymal stem cells

The promyelocytic leukemia (PML) gene, as an important tumor-suppressor, has been proven to regulate stem cell function in multiple tissues; however its role in human mesenchymal stem cells (hMSCs) remains unclear. In the present study, the effect of PML on regulating the proliferation and osteogenic differentiation of hMSCs was explored. New downstream genes that may be responsible for the regulation of PML were found, and possible mechanisms were analyzed. The lentiviral vector which encodes full-length human PML cDNA or shRNA against PML was transfected into hMSCs. RT-PCR and western blotting were used to detect mRNA and protein expression. Flow cytometry was used to analyze apoptosis and the cell cycle distribution. Osteogenic differentiation of hMSCs was induced by osteo-inductive medium for 7 to 14 days. cDNA microarray was used to scan the gene expression profile and to identify significant changes in gene expression. In the present study, we found that PML was stably expressed in hMSCs, and the expression was increased time-dependently along with cell osteogenic differentiation. Overexpression of PML inhibited hMSC proliferation by inducing apoptosis and arresting the cell cycle. However, PML enhanced the osteoblast differentiation potential of hMSCs. PML-overexpressing hMSCs had a significant increase in mineralized matrix production and ALP activity on day 7 under osteogenic or non-osteogenic differentiation conditions. Upregulation of integrin-binding sialoprotein (IBSP, bone sialoprotein) induced by PML overexpression was found. Our data indicate that PML regulates hMSCs as an inhibitor of cell proliferation but a promoter of osteogenic differentiation.

Metabolic profiling of human CD4+ cells following treatment with methotrexate and anti-TNF-α infliximab

The autoimmune process in rheumatoid arthritis depends on activation of immune cells, which utilize intracellular kinases to respond to external stimuli such as cytokines, immune complexes, and antigens. CD4+ T cells comprise a large proportion of the inflammatory cells that invade the synovial tissue and may therefore be a cell type of pathogenic importance. Both methotrexate and infliximab are effective in the treatment of inflammatory arthritis; however, the biological effects triggered by these treatments and the biochemical mechanisms underlining the cell response are still not fully understood. Thus, in this study the global metabolic changes associated with methotrexate or infliximab treatment of isolated human CD4+ T cells were examined using gas chromatography/mass spectrometry or liquid chromatography/mass spectrometry. In total 148 metabolites involved in selective pathways were found to be significantly altered. Overall, the changes observed are likely to reflect the effort of CD4+ cells to increase the production of cellular reducing power to offset the cellular stress exerted by treatment. Importantly, analysis of the global metabolic changes associated with MTX or infliximab treatment of isolated human CD4+ T cells suggested that the toxicity associated with these agents is minimal when used at clinically relevant concentrations.

New Strategies to Direct Therapeutic Targeting of PML to Treat Cancers

The tumor suppressor function of the promyelocytic leukemia (PML) protein was first identified as a result of its dysregulation in acute promyelocytic leukemia, however, its importance is now emerging far beyond hematological neoplasms, to an extensive range of malignancies, including solid tumors. In response to stress signals, PML coordinates the regulation of numerous proteins, which activate fundamental cellular processes that suppress tumorigenesis. Importantly, PML itself is the subject of specific post-translational modifications, including ubiquitination, phosphorylation, acetylation, and SUMOylation, which in turn control PML activity and stability and ultimately dictate cellular fate. Improved understanding of the regulation of this key tumor suppressor is uncovering potential opportunities for therapeutic intervention. Targeting the key negative regulators of PML in cancer cells such as casein kinase 2, big MAP kinase 1, and E6-associated protein, with specific inhibitors that are becoming available, provides unique and exciting avenues for restoring tumor suppression through the induction of apoptosis and senescence. These approaches could be combined with DNA damaging drugs and cytokines that are known to activate PML. Depending on the cellular context, reactivation or enhancement of tumor suppressive PML functions, or targeted elimination of aberrantly functioning PML, may provide clinical benefit.

Senescence and aging: the critical roles of p53

p53 functions as a transcription factor involved in cell-cycle control, DNA repair, apoptosis and cellular stress responses. However, besides inducing cell growth arrest and apoptosis, p53 activation also modulates cellular senescence and organismal aging. Senescence is an irreversible cell-cycle arrest that has a crucial role both in aging and as a robust physiological antitumor response, which counteracts oncogenic insults. Therefore, via the regulation of senescence, p53 contributes to tumor growth suppression, in a manner strictly dependent by its expression and cellular context. In this review, we focus on the recent advances on the contribution of p53 to cellular senescence and its implication for cancer therapy, and we will discuss p53's impact on animal lifespan. Moreover, we describe p53-mediated regulation of several physiological pathways that could mediate its role in both senescence and aging.

Alternative approaches to eradicating the malignant clone in chronic myeloid leukemia: tyrosine-kinase inhibitor combinations and beyond

In patients with chronic myeloid leukemia (CML) in chronic phase who have achieved complete molecular remission on imatinib therapy, clinical trials from France and Australia have demonstrated that the majority experience prompt molecular relapse of their leukemia upon discontinuation of the drug, showing that long-term monotherapy with tyrosine kinase inhibitors is not curative in the majority of patients with CML. This has focused attention on strategies to eradicate residual disease in CML that is presumed to arise from malignant Ph+ stem cells, which should result in permanent cure and long-term leukemia-free survival. Here, we review the evidence that targeting CML stem cells will be of clinical benefit and discuss pharmacological and immunological approaches to accomplish this goal. Where possible, we link preclinical studies of CML stem cell biology to emerging results from clinical trials of agents that may target these cells.

Regulation of the tumor suppressor PML by sequential post-translational modifications

Post-translational modifications (PTMs) regulate multiple biological functions of the promyelocytic leukemia (PML) protein and also the fission, disassembly, and rebuilding of PML nuclear bodies (PML-NBs) during the cell cycle. Pathway-specific PML modification patterns ensure proper signal output from PML-NBs that suit the specific functional requirements. Here we comprehensively review the signaling pathways and enzymes that modify PML and also the oncogenic PML-RARα fusion protein. Many PTMs occur in a hierarchical and timely organized fashion. Phosphorylation or acetylation constitutes typical starting points for many PML modifying events, while degradative ubiquitination is an irreversible end point of the modification cascade. As this hierarchical organization of PTMs frequently turns phosphorylation events as primordial events, kinases or phosphatases regulating PML phosphorylation may be interesting drug targets to manipulate the downstream modifications and thus the stability and function of PML or PML-RARα.

Targeting virulence mechanisms for the prevention and therapy of arenaviral hemorrhagic fever

A number of arenaviruses are pathogenic for humans, but they differ significantly in virulence. Lassa virus, found in West Africa, causes severe hemorrhagic fever (HF), while the other principal Old World arenavirus, lymphocytic choriomeningitis virus, causes mild illness in persons with normal immune function, and poses a threat only to immunocompromised individuals. The New World agents, including Junin, Machupo and Sabia virus, are highly pathogenic for humans. Arenaviral HF is characterized by high viremia and general immune suppression, the mechanism of which is unknown. Studies using viral reverse genetics, cell-based assays, animal models and human genome-wide association analysis have revealed potential mechanisms by which arenaviruses cause severe disease in humans. Each of the four viral gene products (GPC, L polymerase, NP, and Z matrix protein) and several host-cell factors (e.g., α-dystroglycan) are responsible for mediating viral entry, genome replication, and the inhibition of apoptosis, translation and interferon-beta (IFNβ) production. This review summarizes current knowledge of the role of each viral protein and host factor in the pathogenesis of arenaviral HF. Insights from recent studies are being exploited for the development of novel therapies.