Control of erythroid cell production via caspase-mediated cleavage of transcription factor SCL/Tal-1

Transcription Factor TAL1 in Erythropoiesis

Lineage-specific transcription factors (TFs) regulate differentiation of hematopoietic stem cells (HSCs). They are decisive for the establishment and maintenance of lineage-specific gene expression programs during hematopoiesis. For this they create a regulatory network between TFs, epigenetic cofactors, and microRNAs. They activate cell-type specific genes and repress competing gene expression programs. Disturbance of this process leads to impaired lineage fidelity and diseases of the blood system. The TF T-cell acute leukemia 1 (TAL1) is central for erythroid differentiation and contributes to the formation of distinct gene regulatory complexes in progenitor cells and erythroid cells. A TAL1/E47 heterodimer binds to DNA with the TFs GATA-binding factor 1 and 2 (GATA1/2), the cofactors LIM domain only 1 and 2 (LMO1/2), and LIM domain-binding protein 1 (LDB1) to form a core TAL1 complex. Furthermore, cell-type-dependent interactions of TAL1 with other TFs such as with runt-related transcription factor 1 (RUNX1) and Kruppel-like factor 1 (KLF1) are established. Moreover, TAL1 activity is regulated by the formation of TAL1 isoforms, posttranslational modifications (PTMs), and microRNAs. Here, we describe the function of TAL1 in normal hematopoiesis with a focus on erythropoiesis.

Multi-lineage Differentiation from Hematopoietic Stem Cells

The hematopoietic stem cells (HSCs) have the ability to differentiate and give rise to all mature blood cells. Commitment to differentiation progressively limits the self-renewal potential of the original HSCs by regulating the level of lineage-specific gene expression. In this review, we will summarize the current understanding of the molecular mechanisms underlying HSC differentiation toward erythroid, myeloid, and lymphocyte lineages. Moreover, we will decipher how the single-cell technologies advance the lineage-biased HSC subpopulations and their differentiation potential.

Role of Extrinsic Apoptotic Signaling Pathway during Definitive Erythropoiesis in Normal Patients and in Patients with β-Thalassemia

Apoptosis is a process of programmed cell death which has an important role in tissue homeostasis and in the control of organism development. Here, we focus on information concerning the role of the extrinsic apoptotic pathway in the control of human erythropoiesis. We discuss the role of tumor necrosis factor α (TNFα), tumor necrosis factor ligand superfamily member 6 (FasL), tumor necrosis factor-related apoptosis-inducing (TRAIL) and caspases in normal erythroid maturation. We also attempt to initiate a discussion on the observations that mature erythrocytes contain most components of the receptor-dependent apoptotic pathway. Finally, we point to the role of the extrinsic apoptotic pathway in ineffective erythropoiesis of different types of β-thalassemia.

SCL/TAL1: a multifaceted regulator from blood development to disease

SCL/TAL1 (stem cell leukemia/T-cell acute lymphoblastic leukemia [T-ALL] 1) is an essential transcription factor in normal and malignant hematopoiesis. It is required for specification of the blood program during development, adult hematopoietic stem cell survival and quiescence, and terminal maturation of select blood lineages. Following ectopic expression, SCL contributes to oncogenesis in T-ALL. Remarkably, SCL’s activities are all mediated through nucleation of a core quaternary protein complex (SCL:E-protein:LMO1/2 [LIM domain only 1 or 2]:LDB1 [LIM domain-binding protein 1]) and dynamic recruitment of conserved combinatorial associations of additional regulators in a lineage- and stage-specific context. The finely tuned control of SCL’s regulatory functions (lineage priming, activation, and repression of gene expression programs) provides insight into fundamental developmental and transcriptional mechanisms, and highlights mechanistic parallels between normal and oncogenic processes. Importantly, recent discoveries are paving the way to the development of innovative therapeutic opportunities in SCL+ T-ALL.

Non-apoptotic functions of caspases in myeloid cell differentiation

Subtle caspase activation is associated with the differentiation of several myeloid lineages. A tightly orchestrated dance between caspase-3 activation and the chaperone HSP70 that migrates to the nucleus to protect the master regulator GATA-1 from cleavage transiently occurs in basophilic erythroblasts and may prepare nucleus and organelle expel that occurs at the terminal phase of erythroid differentiation. A spatially restricted activation of caspase-3 occurs in maturing megakaryocytes to promote proplatelet maturation and platelet shedding in the bloodstream. In a situation of acute platelet need, caspase-3 could be activated in response to IL-1α and promote megakaryocyte rupture. In peripheral blood monocytes, colony-stimulating factor-1 provokes the formation of a molecular platform in which caspase-8 is activated, which downregulates nuclear factor-kappa B (NF-κB) activity and activates downstream caspases whose target fragments such as those generated by nucleophosmin (NPM1) cleavage contribute to the generation of resting macrophages. Human monocytes secrete mature IL-1β in response to lipopolysaccharide through an alternative inflammasome activation that involves caspase-8, a pathway that does not lead to cell death. Finally, active caspase-3 is part of the proteases contained in secretory granules of mast cells. Many questions remain on how these proteases are activated in myeloid cell lineages, which target proteins are cleaved, whereas other are protected from proteolysis, the precise role of cleaved proteins in cell differentiation and functions, and the link between these non-apoptotic functions of caspases and the death of these diverse cell types. Better understanding of these functions may generate therapeutic strategies to control cytopenias or modulate myeloid cell functions in various pathological situations.

Caspase cleavage of iASPP potentiates its ability to inhibit p53 and NF-κB

An intriguing biological question relating to cell signaling is how the inflammatory mediator NF-kB and the tumour suppressor protein p53 can be induced by similar triggers, like DNA damage or infection, yet have seemingly opposing or sometimes cooperative biological functions. For example, the NF-κB subunit RelA/p65 has been shown to inhibit apoptosis, whereas p53 induces apoptosis. One potential explanation may be their co-regulation by common cellular factors: inhibitor of Apoptosis Stimulating p53 Protein (iASPP) is one such common regulator of both RelA/p65 and p53. Here we show that iASPP is a novel substrate of caspases in response to apoptotic stimuli. Caspase cleaves the N-terminal region of iASPP at SSLD294 resulting in a prominent 80kDa fragment of iASPP. This caspase cleavage site is conserved in various species from zebrafish to Homo sapiens. The 80kDa fragment of iASPP translocates from the cytoplasm to the nucleus via the RaDAR nuclear import pathway, independent of p53. The 80kDa iASPP fragment can bind and inhibit p53 or RelA/p65 more efficiently than full-length iASPP. Overall, these data reveal a potential novel regulation of p53 and RelA/p65 activities in response to apoptotic stimuli.

Oxidative stress in β-thalassaemia and sickle cell disease

Sickle cell disease and β-thalassaemia are inherited haemoglobinopathies resulting in structural and quantitative changes in the β-globin chain. These changes lead to instability of the generated haemoglobin or to globin chain imbalance, which in turn impact the oxidative environment both intracellularly and extracellularly. The ensuing oxidative stress and the inability of the body to adequately overcome it are, to a large extent, responsible for the pathophysiology of these diseases. This article provides an overview of the main players and control mechanisms involved in the establishment of oxidative stress in these haemoglobinopathies.

Apoptosis: role in myeloid cell development

Hematopoiesis is the process that generates blood cells in an organism from the pluripotent stem cells. Hematopoietic stem cells are characterized by their ability to undergo self-renewal and differentiation. The self-renewing ability ensures that these pluripotent cells are not depleted from the bone marrow niche. A proper balance between cell death and cell survival is necessary to maintain a homeostatic condition, hence, apoptosis, or programmed cell death, is an essential step in hematopoiesis. Recent studies, however, have introduced a new aspect to this process, citing the significance of the apoptosis mediator, caspase, in cell development and differentiation. Extensive research has been carried out to study the possible role of caspases and other apoptosis related factors in the developmental processes. This review focuses on the various apoptotic factors involved in the development and differentiation of myeloid lineage cells: erythrocytes, megakaryocytes, and macrophages.

Review: Beta-thalassemia and molecular chaperones

Thalassemia is known as a diverse single gene disorder, which is prevalent worldwide. The molecular chaperones are set of proteins that help in two important processes while protein synthesis and degradation include folding or unfolding and assembly or disassembly, thereby helping in cell homeostasis. This review recaps current knowledge regarding the role of molecular chaperones in thalassemia, with a focus on beta thalassemia.

Molecular mechanisms of luteolin-7-O-glucoside-induced growth inhibition on human liver cancer cells: G2/M cell cycle arrest and caspase-independent apoptotic signaling pathways

Luteolin-7-O-glucoside (LUT7G), a flavone subclass of flavonoids, has been found to increase anti-oxidant and anti-inflammatory activity, as well as cytotoxic effects. However, the mechanism of how LUT7G induces apoptosis and regulates cell cycles remains poorly understood. In this study, we examined the effects of LUT7G on the growth inhibition of tumors, cell cycle arrest, induction of ROS generation, and the involved signaling pathway in human hepatocarcinoma HepG2 cells. The proliferation of HepG2 cells was decreased by LUT7G in a dose-dependent manner. The growth inhibition was due primarily to the G2/M phase arrest and ROS generation. Moreover, the phosphorylation of JNK was increased by LUT7G. These results suggest that the anti-proliferative effect of LUT7G on HepG2 is associated with G2/M phase cell cycle arrest by JNK activation.

Genetic interaction between Kit and Scl

SCL/TAL1, a tissue-specific transcription factor of the basic helix-loop-helix family, and c-Kit, a tyrosine kinase receptor, control hematopoietic stem cell survival and quiescence. Here we report that SCL levels are limiting for the clonal expansion of Kit⁺ multipotent and erythroid progenitors. In addition, increased SCL expression specifically enhances the sensitivity of these progenitors to steel factor (KIT ligand) without affecting interleukin-3 response, whereas a DNA-binding mutant antagonizes KIT function and induces apoptosis in progenitors. Furthermore, a twofold increase in SCL levels in mice bearing a hypomorphic Kit allele (W41/41) corrects their hematocrits and deficiencies in erythroid progenitor numbers. At the molecular level, we found that SCL and c-Kit signaling control a common gene expression signature, of which 19 genes are associated with apoptosis. Half of those were decreased in purified megakaryocyte/erythroid progenitors (MEPs) from W41/41 mice and rescued by the SCL transgene. We conclude that Scl operates downstream of Kit to support the survival of MEPs. Finally, higher SCL expression upregulates Kit in normal bone marrow cells and increases chimerism after bone marrow transplantation, indicating that Scl is also upstream of Kit. We conclude that Scl and Kit establish a positive feedback loop in multipotent and MEPs.

Anticancer activity of a synthetic peptide derived from harmoniasin, an antibacterial peptide from the ladybug Harmonia axyridis

Harmoniasin is a defensin-like antimicrobial peptide identified from the ladybug Harmonia axyridis. Among the synthetic homodimer peptide analogues derived from harmoniasin, HaA4 has been found to have antibacterial activity without hemolytic activity. In this study, we investigated whether HaA4 has anticancer activity against human leukemia cell lines such as U937 and Jurkat cells. HaA4 manifested cytotoxicity and decreased the cell viability of U937 and Jurkat cells in MTS assay and LDH release assay. We found that HaA4 induced apoptotic and necrotic cell death of the leukemia cells using flow cytometric analysis, acridine orange/ethidium bromide staining and nucleosomal fragmentation of genomic DNA. Activation of caspase-7 and -9 and fragmentation of poly (ADP-ribose) polymerase was detected in the HaA4-treated leukemia cells, suggesting induction of a caspase-dependent apoptosis pathway by HaA4. Caspase-dependent apoptosis was further confirmed by reversal of the HaA4-induced viability reduction by treatment of Z-VAD-FMK, a pan-caspase inhibitor. In conclusion, HaA4 caused necrosis and caspase-dependent apoptosis in both U937 and Jurkat leukemia cells, which suggests potential utility of HaA4 as a cancer therapeutic agent.

Erythropoiesis: development and differentiation

Through their oxygen delivery function, red blood cells are pivotal to the healthy existence of all vertebrate organisms. These cells are required during all stages of life--embryonic, fetal, neonatal, adolescent, and adult. In the adult, red blood cells are the terminally differentiated end-product cells of a complex hierarchy of hematopoietic progenitors that become progressively restricted to the erythroid lineage. During this stepwise differentiation process, erythroid progenitors undergo enormous expansion, so as to fulfill the daily requirement of ~2 × 10(11) new erythrocytes. How the erythroid lineage is made has been a topic of intense research over the last decades. Developmental studies show that there are two types of red blood cells--embryonic and adult. They develop from distinct hemogenic/hematopoietic progenitors in different anatomical sites and show distinct genetic programs. This article highlights the developmental and differentiation events necessary in the production of hemoglobin-producing red blood cells.

Ineffective erythropoiesis in β -thalassemia

In humans, β-thalassemia dyserythropoiesis is characterized by expansion of early erythroid precursors and erythroid progenitors and then ineffective erythropoiesis. This ineffective erythropoiesis is defined as a suboptimal production of mature erythrocytes originating from a proliferating pool of immature erythroblasts. It is characterized by (1) accelerated erythroid differentiation, (2) maturation blockade at the polychromatophilic stage, and (3) death of erythroid precursors. Despite extensive knowledge of molecular defects causing β-thalassemia, less is known about the mechanisms responsible for ineffective erythropoiesis. In this paper, we will focus on the underlying mechanisms leading to premature death of thalassemic erythroid precursors in the bone marrow.

Novel roles for KLF1 in erythropoiesis revealed by mRNA-seq

KLF1 (formerly known as EKLF) regulates the development of erythroid cells from bi-potent progenitor cells via the transcriptional activation of a diverse set of genes. Mice lacking Klf1 die in utero prior to E15 from severe anemia due to the inadequate expression of genes controlling hemoglobin production, cell membrane and cytoskeletal integrity, and the cell cycle. We have recently described the full repertoire of KLF1 binding sites in vivo by performing KLF1 ChIP-seq in primary erythroid tissue (E14.5 fetal liver). Here we describe the KLF1-dependent erythroid transcriptome by comparing mRNA-seq from Klf1^+/+ and Klf1^−/− erythroid tissue. This has revealed novel target genes not previously obtainable by traditional microarray technology, and provided novel insights into the function of KLF1 as a transcriptional activator. We define a cis-regulatory module bound by KLF1, GATA1, TAL1, and EP300 that coordinates a core set of erythroid genes. We also describe a novel set of erythroid-specific promoters that drive high-level expression of otherwise ubiquitously expressed genes in erythroid cells. Our study has identified two novel lncRNAs that are dynamically expressed during erythroid differentiation, and discovered a role for KLF1 in directing apoptotic gene expression to drive the terminal stages of erythroid maturation.

CopA3 peptide from Copris tripartitus induces apoptosis in human leukemia cells via a caspase-independent pathway

Our previous study demonstrated that CopA3, a disulfide dimer of the coprisin peptide analogue (LLCIALRKK), has antibacterial activity. In this study, we assessed whether CopA3 caused cellular toxicity in various mammalian cell lines. CopA3 selectively caused a marked decrease in cell viability in Jurkat T, U937, and AML-2 cells (human leukemia cells), but was not cytotoxic to Caki or Hela cells. Fragmentation of DNA, a marker of apoptosis, was also confirmed in the leukemia cell lines, but not in the other cells. CopA3-induced apoptosis in leukemia cells was mediated by apoptosis inducing factor (AIF), indicating induction of a caspase-independent signaling pathway.

Caspase substrates and cellular remodeling

The caspases are unique proteases that mediate the major morphological changes of apoptosis and various other cellular remodeling processes. As we catalog and study the myriad proteins subject to cleavage by caspases, we are beginning to appreciate the full functional repertoire of these enzymes. Here, we examine current knowledge about caspase cleavages: what kinds of proteins are cut, in what contexts, and to what end. After reviewing basic caspase biology, we describe the technologies that enable high-throughput caspase substrate discovery and the datasets they have yielded. We discuss how caspases recognize their substrates and how cleavages are conserved among different metazoan organisms. Rather than comprehensively reviewing all known substrates, we use examples to highlight some functional impacts of caspase cuts during apoptosis and differentiation. Finally, we discuss the roles caspase substrates can play in medicine. Though great progress has been made in this field, many important areas still await exploration.

Fine-tuning nucleophosmin in macrophage differentiation and activation

M-CSF-driven differentiation of peripheral blood monocytes is one of the sources of tissue macrophages. In humans and mice, the differentiation process involves the activation of caspases that cleave a limited number of proteins. One of these proteins is nucleophosmin (NPM1), a multifunctional and ubiquitous protein. Here, we show that caspases activated in monocytes exposed to M-CSF cleave NPM1 at D213 to generate a 30-kDa N-terminal fragment. The protein is further cleaved into a 20-kDa fragment, which involves cathepsin B. NPM1 fragments contribute to the limited motility, migration, and phagocytosis capabilities of resting macrophages. Their activation with lipopolysaccharides inhibits proteolytic processes and restores expression of the full-length protein that negatively regulates the transcription of genes encoding inflammatory cytokines (eg, NPM1 is recruited with NF-κB on the MCP1 gene promoter to decrease its transcription). In mice with heterozygous npm gene deletion, cytokine production in response to lipopolysaccharides, including CXCL1 (KC), MCP1, and MIP2, is dramatically enhanced. These results indicate a dual function of NPM1 in M-CSF-differentiated macrophages. Proteolysis of the protein participates in the establishment of a mature macrophage phenotype. In response to inflammatory stimuli, the full-length protein negatively regulates inflammatory cytokine production.

Role of helix-loop-helix proteins during differentiation of erythroid cells

Helix-loop-helix (HLH) proteins play a profound role in the process of development and cellular differentiation. Among the HLH proteins expressed in differentiating erythroid cells are the ubiquitous proteins Myc, USF1, USF2, and TFII-I, as well as the hematopoiesis-specific transcription factor Tal1/SCL. All of these HLH proteins exhibit distinct functions during the differentiation of erythroid cells. For example, Myc stimulates the proliferation of erythroid progenitor cells, while the USF proteins and Tal1 regulate genes that specify the differentiated phenotype. This minireview summarizes the known activities of Myc, USF, TFII-I, and Tal11/SCL and discusses how they may function sequentially, cooperatively, or antagonistically in regulating expression programs during the differentiation of erythroid cells.

Parole terms for a killer: directing caspase3/CAD induced DNA strand breaks to coordinate changes in gene expression

In a series of discoveries over the preceding decade, a number of laboratories have unequivocally established that apoptotic proteins and pathways are well conserved cell fate determinants, which act independent of a cell death response. Within this context, the role for apoptotic proteins in the induction of cell differentiation has been widely documented. Despite these discoveries, little information has been forthcoming regarding a conserved mechanism by which apoptotic proteins achieve this non-death outcome. In the following discussion, we will explore the premise that the penultimate step in apoptosis, genome wide DNA damage/strand breaks act as a conserved genomic reprogramming event necessary for cell differentiation (Larsen et al. Proc Natl Acad Sci USA 2010; 107:4230-5). Moreover, we hypothesis that directed DNA damage, as mediated by known apoptotic proteins, may participate in numerous forms of regulated gene expression.

Genome-wide identification of TAL1's functional targets: insights into its mechanisms of action in primary erythroid cells

Coordination of cellular processes through the establishment of tissue-specific gene expression programs is essential for lineage maturation. The basic helix-loop-helix hemopoietic transcriptional regulator TAL1 (formerly SCL) is required for terminal differentiation of red blood cells. To gain insight into TAL1 function and mechanisms of action in erythropoiesis, we performed ChIP-sequencing and gene expression analyses from primary fetal liver erythroid cells. We show that TAL1 coordinates expression of genes in most known red cell-specific processes. The majority of TAL1's genomic targets require direct DNA-binding activity. However, one-fifth of TAL1's target sequences, mainly among those showing high affinity for TAL1, can recruit the factor independently of its DNA binding activity. An unbiased DNA motif search of sequences bound by TAL1 identified CAGNTG as TAL1-preferred E-box motif in erythroid cells. Novel motifs were also characterized that may help distinguish activated from repressed genes and suggest a new mechanism by which TAL1 may be recruited to DNA. Finally, analysis of recruitment of GATA1, a protein partner of TAL1, to sequences occupied by TAL1 suggests that TAL1's binding is necessary prior or simultaneous to that of GATA1. This work provides the framework to study regulatory networks leading to erythroid terminal maturation and to model mechanisms of action of tissue-specific transcription factors.

Pro-inflammatory cytokine-mediated anemia: regarding molecular mechanisms of erythropoiesis

Anemia of cancer and chronic inflammatory diseases is a frequent complication affecting quality of life. For cancer patients it represents a particularly bad prognostic. Low level of erythropoietin is considered as one of the causes of anemia in these pathologies. The deficiency in erythropoietin production results from pro-inflammatory cytokines effect. However, few data is available concerning molecular mechanisms involved in cytokine-mediated anemia. Some recent publications have demonstrated the direct effect of pro-inflammatory cytokines on cell differentiation towards erythroid pathway, without erythropoietin defect. This suggested that pro-inflammatory cytokine-mediated signaling pathways affect erythropoietin activity. They could interfere with erythropoietin-mediated signaling pathways, inducing early apoptosis and perturbing the expression and regulation of specific transcription factors involved in the control of erythroid differentiation. In this review we summarize the effect of tumor necrosis factor (TNF)α, TNF-related apoptosis-inducing ligand (TRAIL), and interferon (IFN)-γ on erythropoiesis with a particular interest for molecular feature.

[The bHLH TAL1 protein: a key molecule in the hematopoietic and endothelial systems]

The formation of blood cells and vascular networks occurs simultaneously during development, and both lineages remain in close association in all adult tissues. The functional setting of both systems within the embryo and their renewal during adult life are highly complex processes, and require the involvement of numerous molecular actors, the activities of which are often overlapping. Here, I review the activity of TAL-1, a basic-helix-loop-helix transcription factor, which plays a key role in the formation and functioning of both blood and endothelial systems, with a particular emphasis on recent data that associate TAL-1 with angiogenesis.

Differential use of SCL/TAL-1 DNA-binding domain in developmental hematopoiesis

Dissecting the molecular mechanisms used by developmental regulators is essential to understand tissue specification/differentiation. SCL/TAL-1 is a basic helix-loop-helix transcription factor absolutely critical for hematopoietic stem/progenitor cell specification and lineage maturation. Using in vitro and forced expression experimental systems, we previously suggested that SCL might have DNA-binding-independent functions. Here, to assess the requirements for SCL DNA-binding activity in vivo, we examined hematopoietic development in mice carrying a germline DNA-binding mutation. Remarkably, in contrast to complete absence of hematopoiesis and early lethality in scl-null embryos, specification of hematopoietic cells occurred in homozygous mutant embryos, indicating that direct DNA binding is dispensable for this process. Lethality was forestalled to later in development, although some mice survived to adulthood. Anemia was documented throughout development and in adulthood. Cellular and molecular studies showed requirements for SCL direct DNA binding in red cell maturation and indicated that scl expression is positively autoregulated in terminally differentiating erythroid cells. Thus, different mechanisms of SCL's action predominate depending on the developmental/cellular context: indirect DNA binding activities and/or sequestration of other nuclear regulators are sufficient in specification processes, whereas direct DNA binding functions with transcriptional autoregulation are critically required in terminal maturation processes.

Pathophysiology of anemia and erythrocytosis

An increasing understanding of the process of erythropoiesis raises some interesting questions about the pathophysiology, diagnosis and treatment of anemia and erythrocytosis. The mechanisms underlying the development of many of the erythrocytoses, previously characterised as idiopathic, have been elucidated leading to an increased understanding of oxygen homeostasis. Characterisation of anemia and erythrocytosis in relation to serum erythropoietin levels can be a useful addition to clinical diagnostic criteria and provide a rationale for treatment with erythropoiesis stimulating agents (ESAs). Recombinant human erythropoietin as well as other ESAs are now widely used to treat anemias associated with a range of conditions, including chronic kidney disease, chronic inflammatory disorders and cancer. There is also heightened awareness of the potential abuse of ESAs to boost athletic performance in competitive sport. The discovery of erythropoietin receptors outside of the erythropoietic compartment may herald future applications for ESAs in the management of neurological and cardiac diseases. The current controversy concerning optimal hemoglobin levels in chronic kidney disease patients treated with ESAs and the potential negative clinical outcomes of ESA treatment in cancer reinforces the need for cautious evaluation of the pleiotropic effects of ESAs in non-erythroid tissues.

A jekyll and hyde role of cyclin E in the genotoxic stress response: switching from cell cycle control to apoptosis regulation

Cyclin E protein levels and associated kinase activity rise in late G(1) phase, reach a peak at the G(1)/S transition, and quickly decline during S phase. The cyclin E/Cdk2 complex has a well-established function in regulating two fundamental biological processes: cell cycle progression and DNA replication. However, cyclin E expression is deregulated in a wide range of tumors. Our recent reports have uncovered a critical role for cyclin E, independent of Cdk2, in the cell death of hematopoietic tumor cells exposed to genotoxic stress. An 18-kD C-terminal fragment of cyclin E, p18-cyclin E, which is generated by caspase-mediated cleavage in hematopoietic cells during genotoxic stress-induced apoptosis has a critical role in the amplification of the intrinsic apoptotic pathway. By interacting with Ku70, p18-cyclin E liberates Bax, which participates in the amplification of apoptosis by sustaining a positive feedback loop targeting mitochondria. This process is independent of p53 function and new RNA or protein synthesis. Therefore, cyclin E emerges as an arbiter of the genotoxic stress response by regulating a finite physiological balance between cell proliferation and death in hematopoietic cells.

Interaction of a cyclin E fragment with Ku70 regulates Bax-mediated apoptosis

The cyclin E/Cdk2 complex plays an essential role in the G(1)/S cell cycle transition and DNA replication. Earlier we showed that in hematopoietic tumor cells, caspase-mediated cleavage of cyclin E generates p18-cyclin E, which is unable to interact with Cdk2 and therefore plays a role independent of the cell cycle. The expression of a cleavage-resistant cyclin E mutant greatly diminishes apoptosis, indicating the critical role of cyclin E cleavage. p18-cyclin E expression can induce apoptosis or sensitization to apoptotic stimuli in many cell types. Here we identify Ku70 as a specific p18-cyclin E-interacting partner. In hematopoietic tumor cell lines, the association of p18-cyclin E with Ku70 induces the dissociation of Bax from Ku70, followed by Bax activation. This mechanism of Bax activation leads to the amplification of the apoptosis signal in all tumor cell lines examined. N-terminal Ku70 deletion mutants are unable to bind to p18-cyclin E to regulate its apoptotic effect. p18-cyclin E-mediated amplification of apoptosis is dependent on Bax and Ku70 being greatly diminished in Ku70(-/-) and Bax(-/-) mouse embryo fibroblasts and in hematopoietic cells where Bax knockdown was achieved by short interfering RNA. The p18-cyclin E/Ku70 and Bax/Ku70 interactions provide a balance between apoptosis and the survival of cells exposed to genotoxic stress.

Twist is substrate for caspase cleavage and proteasome-mediated degradation

Twist is a member of the basic helix-loop-helix family of transcription factors. An aberrant Twist expression has been found in diverse types of cancer, including sarcomas, carcinomas and lymphomas, supporting a role for Twist in tumor progression. Twist is known to be essential for mesodermal development. However, since a prolonged Twist expression results in a block of muscle, cartilage and bone differentiation, Twist has to be excluded from somites during late embryogenesis for terminal differentiation to occur. This implies that Twist expression must be target of a tight control. Here we provide evidence that Twist undergoes post-transcriptional regulation. Twist is substrate for cleavage by caspases during apoptosis and its cleavage results in ubiquitin-mediated proteasome degradation. Our findings suggest that Twist post-transcriptional regulation may play an important role in tissue determination and raise the possibility that alterations in the protein turnover may account for Twist overexpression observed in tumors.

Erythropoietin activates caspase-3 and downregulates CAD during erythroid differentiation in TF-1 cells - A protection mechanism against DNA fragmentation

The involvement of caspase-3 and its failure in the induction of DNA fragmentation during erythropoiesis were investigated with TF-1 cells. During erythroid differentiation, caspase-3 activation and cleavage of caspase-3 substrates such as ICAD (inhibitor of caspase-activated DNase) were detected without concomitant phosphatidyl-serine (PS) externalization and DNA fragmentation. These observations are in contrast to our understanding that DNA is degraded by CAD (caspase-activated DNase) when ICAD is cleaved by caspase-3. Our study demonstrates that CAD is downregulated at the mRNA and protein level during the erythroid differentiation in TF-1 cells. This provides a mechanism for the first time how cells avoid DNA fragmentation with activated caspase-3.

In vitro dual effect of arsenic trioxide on hemopoiesis: inhibition of erythropoiesis and stimulation of megakaryocytic maturation

Although the arsenic compounds are now widely utilized in clinics in the treatment of various tumors, their effects on normal hematopoiesis do not seem to have been explored. In the present study, we provide evidence that arsenic trioxide (As(2)O(3)) exerts in vitro a potent inhibitory effect on normal erythropoiesis and a stimulatory action on megakaryocytic differentiation. The effect of As(2)O(3) on erythroid and megakaryocytic differentiation was evaluated on both erythroleukemic cell lines K562 and HEL and on normal hemopoietic progenitor cells (HPCs) induced to selective erythroid or megakaryocytic differentiation. The inhibitory effect of As(2)O(3) on erythropoiesis is related to: (a) the inhibition of Stat5 activation with consequent reduced expression of the target genes Bcl-X(L) and glycophorin-A; (b) the activation of an apoptotic mechanism that leads to the cleavage of the erythroid transcription factors Tal-1 and GATA-1, whose integrity is required for erythroid cell survival and differentiation; (c) the reduced expression of heat shock protein 70, required for GATA-1 integrity. The stimulatory effect of As(2)O(3) on normal megakaryocytopoiesis is seemingly related to upmodulation of GATA-2 expression and to stimulation of MAPK activity. These observations may have implications for the patients undergoing anti-leukemic treatment with this compound.

Vital functions for lethal caspases

Caspases are a family of cysteine proteases expressed as inactive zymogens in virtually all animal cells. These enzymes play a central role in most cell death pathways leading to apoptosis but growing evidences implicate caspases also in nonapoptotic functions. Several of these enzymes, activated in molecular platforms referred to as inflammasomes, play a role in innate immune response by processing some of the cytokines involved in inflammatory response. Caspases are requested for terminal differentiation of specific cell types, whether this differentiation process leads to enucleation or not. These enzymes play also a role in T and B lymphocyte proliferation and, in some circumstances, appear to be cytoprotective rather than cytotoxic. These pleiotropic functions implicate caspases in the control of life and death but the fine regulation of their dual effect remains poorly understood. The nonapoptotic functions of caspases implicate that cells can restrict the proteolytic activity of these enzymes to selected substrates. Deregulation of the pathways in which caspases exert these nonapoptotic functions is suspected to play a role in the pathophysiology of several human diseases.

Multiple Members of the TNF Superfamily Contribute to IFN-γ-Mediated Inhibition of Erythropoiesis

IFN-gamma inhibits the growth and differentiation of erythroid precursor cells and mediates hemopoietic suppression through mechanisms that are not completely understood. We found that treatment of human erythroid precursor cells with IFN-gamma up-regulates the expression of multiple members of the TNF family, including TRAIL and the recently characterized protein TWEAK. TWEAK and its receptor fibroblast growth factor-inducible 14 (Fn14) were expressed by purified erythroblasts at all the stages of maturation. Exposure to recombinant TWEAK or agonist anti-Fn14 Abs was able to inhibit erythroid cell growth and differentiation through caspase activation. Because other members of the TNF family such as TRAIL and CD95 ligand (CD95L) are known to interfere with erythroblast growth and differentiation, we investigated the role of different TNF/TNFR family proteins as potential effectors of IFN-gamma in the immature hemopoietic compartment. Treatment of erythroid precursor cells with agents that blocked either TRAIL, CD95L, or TWEAK activity was partially able to revert the effect of IFN-gamma on erythroid proliferation and differentiation. However, the simultaneous inhibition of TRAIL, TWEAK, and CD95L resulted in a complete abrogation of IFN-gamma inhibitory effects, indicating the requirement of different receptor-mediated signals in IFN-gamma-mediated hemopoietic suppression. These results establish a new role for TWEAK and its receptor in normal and IFN-gamma-mediated regulation of hematopoiesis and show that the effects of IFN-gamma on immature erythroid cells depend on multiple interactions between TNF family members and their receptors.

The anaemia of cancer: death by a thousand cuts

Cancer has a negative systemic impact on its host in addition to its local or metastatic effects, and no cancer complication is more ubiquitous than anaemia, a condition for which there is now a specific remedy, the recombinant growth factor erythropoietin. This is not a trivial therapeutic consideration, because cancer-associated anaemia has an adverse influence on survival regardless of tumour type. However, the pharmacological correction of anaemia with recombinant erythropoietin could promote tumour growth, whereas the use of tumour-necrosis factor-alpha (TNFalpha) and TNF-related apoptosis-inducing ligand as antitumour agents could exacerbate anaemia, thereby perpetuating tissue hypoxia and tumour progression.

Life's smile, death's grin: vital functions of apoptosis-executing proteins

Apoptosis is executed by caspases as well as caspase-independent death effectors. Caspases are expressed as inactive zymogens in virtually all animal cells and are activated in cells destined to undergo apoptosis. However, there are many examples where caspase activation is actually required for cellular processes not related to cell death, namely terminal differentiation, activation, proliferation, and cytoprotection. Several caspase-independent death effectors including apoptosis-inducing factor, endonuclease G and a serine protease (Omi/HtrA2) are released from the mitochondrial intermembrane space upon permeabilization of the outer membrane. Such proteins also have important roles in cellular redox metabolism and/or mitochondrial biogenesis. As a general rule, it thus appears that cell-death-relevant proteins, especially those involved in the core of the executing machinery, have a dual function in life and death. This has important implications for pathophysiology. The fact that the building blocks of the apoptotic machinery have normal functions not related to cell death may mean that essential parts of the apoptotic executioner cannot be lost and thus reduces the possibility of oncogenic mutations that block the apoptotic program. Moreover, therapeutic suppression of unwarranted cell death must be designed to target only the lethal (and not the vital) role of death effectors.

Transferrin receptor 2 protein is not expressed in normal erythroid cells

Human TFR2 (transferrin receptor 2) is a membrane-bound protein homologous with TFR1. High levels of TFR2 mRNA were found mainly in the liver and, to a lesser extent, in erythroid precursors. However, although the presence of the TFR2 protein in hepatic cells has been confirmed in several studies, evidence is lacking about the presence of the TFR2 protein in normal erythroid cells. Using two anti-TFR2 monoclonal antibodies, G/14C2 and G/14E8, we have provided evidence that TFR2 protein is not expressed in normal erythroid cells at any stage of differentiation, from undifferentiated CD34+ cells to mature orthochromatic erythroblasts. In contrast, erythroleukaemic cells (K562 cells) exhibited a high level of expression of TFR2 at both the mRNA and the protein level. We can therefore conclude that an elevated expression of TFR2 protein is observed in leukaemic cells, but not in normal erythroblasts. The implications of this observation for the understanding of the phenotypic features of haemochromatosis due to mutation of the TFR2 gene are discussed.

Apoptotic mechanisms in the control of erythropoiesis

Erythropoiesis is a complex multistep process encompassing the differentiation of hemopoietic stem cells to mature erythrocytes. The steps involved in this complex differentiation process are numerous and involve first the differentiation to early erythoid progenitors (burst-forming units-erythroid, BFU-E), then to late erythroid progenitors (colony-forming units-erythroid) and finally to morphologically recognizable erythroid precursors. A key event of late stages of erythropoiesis is nuclear condensation, followed by extrusion of the nucleus to produce enucleated reticulocytes and finally mature erythrocytes. During the differentiation process, the cells became progressively sensitive to erythropoietin that controls both the survival and proliferation of erythroid cells. A normal homeostasis of the erythropoietic system requires an appropriate balance between the rate of erythroid cell production and red blood cell destruction. Growing evidences outlined in the present review indicate that apoptotic mechanism play a relevant role in the control of erythropoiesis under physiologic and pathologic conditions. Withdrawal of erythropoietin or stimulation of death receptors such as Fas or TRAIL-Rs leads to activation of a subset of caspase-3, -7 and -8, which then cleave the transcription factors GATA-1 and TAL-1 and trigger apoptosis. In addition, there is evidence that a number of caspases are physiologically activated during erythroid differentiation and are functionally required for erythroid maturation. Several caspase substrates are cleaved in differentiating cells, including the protein acinus whose activation by cleavage is required for chromatin condensation. The studies on normal erythropoiesis have clearly indicated that immature erythroid precursors are sensitive to apoptotic triggering mediated by activation of the intrinsic and extrinsic apoptotic pathways. These apoptotic mechanisms are frequently exacerbated in some pathologic conditions, associated with the development of anemia (ie, thalassemias, multiple myeloma, myelodysplasia, aplastic anemia). The considerable progress in our understanding of the apoptotic mechanisms underlying normal and pathologic erythropoiesis may offer the way to improve the treatment of several pathologic conditions associated with the development of anemia.