The function of the bcl-x promoter in erythroid progenitor cells

PI3k/AKT signaling pathway: Erythropoiesis and beyond

Erythropoiesis is a multi-step process that involves the differentiation of hematopoietic stem cells into mature red blood cells (RBCs). This process is regulated by several signaling pathways, transcription factors and microRNAs (miRNAs). Many studies have shown that dysregulation of this process can lead to hematologic disorders. PI3K/AKT is one of the most important pathways that control many cellular processes including, cell division, autophagy, survival, and differentiation. In this review, we focus on the role of PI3K/AKT pathway in erythropoiesis and discuss the function of some of the most important genes, transcription factors, and miRNAs that regulate different stages of erythropoiesis which play roles in differentiation and maturation of RBCs, prevention of apoptosis, and autophagy induction. Understanding the role of the PI3K pathway in erythropoiesis may provide new insights into diagnosing erythrocyte disorders.

Generation and characterization of a JAK2V617F-containing erythroleukemia cell line

The JAK2V617F mutation is found in the majority of patients with myeloproliferative neoplasms (MPNs). Transgenic expression of the mutant gene causes MPN-like phenotypes in mice. We have produced JAK2V617F mice with p53 null background. Some of these mice developed acute erythroleukemia. From one of these mice, we derived a cell line designated J53Z1. J53Z1 cells were stained positive for surface markers CD71 and CD117 but negative for Sca-1, TER-119, CD11b, Gr-1, F4/80, CD11c, CD317, CD4, CD8a, CD3e, B220, CD19, CD41, CD42d, NK-1.1, and FceR1. Real time PCR analyses demonstrated expressions of erythropoietin receptor EpoR, GATA1, and GATA2 in these cells. J53Z1 cells grew rapidly in suspension culture containing fetal bovine serum with a doubling time of ∼18 hours. When transplanted into C57Bl/6 mice, J53Z1 cells induced acute erythroleukemia with massive infiltration of tumor cells in the spleen and liver. J53Z1 cells were responsive to stimulation with erythropoietin and stem cell factor and were selectively inhibited by JAK2 inhibitors which induced apoptosis of the cells. Together, J53Z1 cells belong to the erythroid lineage, and they may be useful for studying the role of JAK2V617F in proliferation and differentiation of erythroid cells and for identifying potential therapeutic drugs targeting JAK2.

Concise review: production of cultured red blood cells from stem cells

In the Western world, the volunteer-based collection system covers most transfusion needs, but transient shortages regularly develop and blood supplies are vulnerable to potentially major disruptions. The production of cultured red blood cells from stem cells is slowly emerging as a potential alternative. The various cell sources, the niche applications most likely to reach the clinic first, and some of the remaining technical issues are reviewed here.

ISG15 modulates development of the erythroid lineage

Activation of erythropoietin receptor allows erythroblasts to generate erythrocytes. In a search for genes that are up-regulated during this differentiation process, we have identified ISG15 as being induced during late erythroid differentiation. ISG15 belongs to the ubiquitin-like protein family and is covalently linked to target proteins by the enzymes of the ISGylation machinery. Using both in vivo and in vitro differentiating erythroblasts, we show that expression of ISG15 as well as the ISGylation process related enzymes Ube1L, UbcM8 and Herc6 are induced during erythroid differentiation. Loss of ISG15 in mice results in decreased number of BFU-E/CFU-E in bone marrow, concomitant with an increased number of these cells in the spleen of these animals. ISG15(-/-) bone marrow and spleen-derived erythroblasts show a less differentiated phenotype both in vivo and in vitro, and over-expression of ISG15 in erythroblasts is found to facilitate erythroid differentiation. Furthermore, we have shown that important players of erythroid development, such as STAT5, Globin, PLC γ and ERK2 are ISGylated in erythroid cells. This establishes a new role for ISG15, besides its well-characterized anti-viral functions, during erythroid differentiation.

Sox6, jack of all trades: a versatile regulatory protein in vertebrate development

Approximately 20,000 genes are encoded in our genome, one tenth of which are thought to be transcription factors. Considering the complexity and variety of cell types generated during development, many transcription factors likely play multiple roles. Uncovering the versatile roles of Sox6 in vertebrate development sheds some light on how an organism efficiently utilizes the limited resources of transcription factors. The structure of the Sox6 gene itself may dictate its functional versatility. First, Sox6 contains no known regulatory domains; instead, it utilizes various cofactors. Second, Sox6 has a long 3'-UTR that contains multiple microRNA targets, thus its protein level is duly adjusted by cell type-specific microRNAs. Just combining these two characteristics alone makes Sox6 extremely versatile. To date, Sox6 has been reported to regulate differentiation of tissues of mesoderm, ectoderm, and endoderm origins, making Sox6 a truly multifaceted transcription factor.

Sox6 is necessary for efficient erythropoiesis in adult mice under physiological and anemia-induced stress conditions

BACKGROUND

Definitive erythropoiesis is a vital process throughout life. Both its basal activity under physiological conditions and its increased activity under anemia-induced stress conditions are highly stimulated by the hormone erythropoietin. The transcription factor Sox6 was previously shown to enhance fetal erythropoiesis together and beyond erythropoietin signaling, but its importance in adulthood and mechanisms of action remain unknown. We used here Sox6 conditional null mice and molecular assays to address these questions.

METHODOLOGY/PRINCIPAL FINDINGS

Sox6fl/flErGFPCre adult mice, which lacked Sox6 in erythroid cells, exhibited compensated anemia, erythroid cell developmental defects, and anisocytotic, short-lived red cells under physiological conditions, proving that Sox6 promotes basal erythropoiesis. Tamoxifen treatment of Sox6fl/flCaggCreER mice induced widespread inactivation of Sox6 in a timely controlled manner and resulted in erythroblast defects before reticulocytosis, demonstrating that impaired erythropoiesis is a primary cause rather than consequence of anemia in the absence of Sox6. Twenty five percent of Sox6fl/flErGFPCre mice died 4 or 5 days after induction of acute anemia with phenylhydrazine. The others recovered slowly. They promptly increased their erythropoietin level and amplified their erythroid progenitor pool, but then exhibited severe erythroblast and reticulocyte defects. Sox6 is thus essential in the maturation phase of stress erythropoiesis that follows the erythropoietin-dependent amplification phase. Sox6 inactivation resulted in upregulation of embryonic globin genes, but embryonic globin chains remained scarce and apparently inconsequential. Sox6 inactivation also resulted in downregulation of erythroid terminal markers, including the Bcl2l1 gene for the anti-apoptotic factor Bcl-xL, and in vitro assays indicated that Sox6 directly upregulates Bcl2l1 downstream of and beyond erythropoietin signaling.

CONCLUSIONS/SIGNIFICANCE

This study demonstrates that Sox6 is necessary for efficient erythropoiesis in adult mice under both basal and stress conditions. It is primarily involved in enhancing the survival rate and maturation process of erythroid cells and acts at least in part by upregulating Bcl2l1.

Intracellular IL-15 controls mast cell survival

The regulation of mast cell activities and survival is a central issue in inflammatory immune responses. Here, we have investigated the role of mouse interleukin-15, a pro-inflammatory and pleiotropic cytokine, in the control of mast cell survival and homeostasis. We report that aged IL-15-/- mice show a reduced number of peritoneal mast cells compared to WT mice. Furthermore, IL-15 deficiency in bone marrow derived mouse mast cells (BMMCs) results in increased susceptibility to apoptosis mediated by growth factor deprivation and A-SMase-treatment. IL-15-/- BMMCs show a constitutive stronger mRNA and protein expression as well as enzymatic activity of the members of the mitochondrial apoptotic pathways including acidic lysosomal aspartate protease cathepsin D (CTSD), endogenous acid sphingomyelinase (A-SMase), caspase-3 and -7 compared to wild type (WT) BMMCs. Furthermore, IL-15-/- BMMCs constitutively generate more A-SMase-derived ceramide than WT controls and display a decreased expression of pro-survival sphingosin-1-phosphate (SPP) both in cytosol and membrane cell fractions. Furthermore, pre-treatment of mast cells with imipramine or pepstatin A, inhibitors of the intracellular acid sphingomyelinase and cathepsin D pathways respectively, increases survival in IL-15-/- BMMCs. These findings suggest that intracellular IL-15 is a key regulator of pathways controlling primary mouse mast cell homeostasis.

Prep1 directly regulates the intrinsic apoptotic pathway by controlling Bcl-XL levels

The Prep1 homeodomain transcription factor is essential in embryonic development. Prep1 hypomorphic mutant mouse (Prep1(i/i)) embryos (embryonic day 9.5) display an increased terminal deoxynucleotidyltransferase-mediated dUTP-biotin nick end labeling reaction compared to wild-type (WT) littermates. Prep1(i/i) mouse embryo fibroblasts (MEFs) show an increased basal level of annexin V binding activity, reduction of the mitochondrial-membrane potential, and increased caspase 9 and 3 activation, indicating increased apoptosis. Prep1(i/i) MEFs also respond faster than WT MEFs to genotoxic stress, indicating increased activation of the intrinsic apoptotic pathways. We did not observe an increase in p53 or an abnormal p53 response to apoptotic stimuli. However, hypomorphic MEFs have decreased endogenous levels of antiapoptotic Bcl-X(L) mRNA and protein, and Bcl-x overexpression rescues the defect of Prep1(i/i) MEFs. Using transient transfections and chromatin immunoprecipitation, we identified the Bcl-x promoter as a novel target of Prep1. Thus, Prep1 directly controls mitochondrial homeostasis (and the apoptotic potential) by modulating Bcl-x gene expression.

Pseudomonas aeruginosa-Induced Human Mast Cell Apoptosis Is Associated with Up-Regulation of Endogenous Bcl-xSand Down-Regulation of Bcl-xL

Mast cells play a critical role in the host defense against bacterial infection. Recently, apoptosis has been demonstrated to be essential in the regulation of host response to Pseudomonas aeruginosa. In this study we show that human mast cell line HMC-1 and human cord blood-derived mast cells undergo apoptosis as determined by the ssDNA formation after infection with P. aeruginosa. P. aeruginosa induced activation of caspase-3 in mast cells as evidenced by the cleavage of D4-GDI, an endogenous caspase-3 substrate and the generation of an active form of caspase-3. Interestingly, P. aeruginosa treatment induced up-regulation of Bcl-x(S) and down-regulation of Bcl-x(L). Bcl-x(S), and Bcl-x(L) are alternative variants produced from the same Bcl-x pre-mRNA. The former is proapoptotic and the latter is antiapoptotic likely through regulating mitochondrial membrane integrity. Treatment of mast cells with P. aeruginosa induced release of cytochrome c from mitochondria and loss of mitochondrial membrane potentials. Moreover, P. aeruginosa treatment reduced levels of Fas-associated death domain protein-like IL-1beta-converting enzyme-inhibitory proteins (FLIPs) that are endogenous apoptosis inhibitors through counteraction with caspase-8. Thus, human mast cells undergo apoptosis after encountering P. aeruginosa through a mechanism that likely involves both the Bcl family protein mitochondrial-dependent and the FLIP-associated caspase-8 pathways.

Distinct promoters mediate constitutive and inducible Bcl-XL expression in malignant lymphocytes

Bcl-X(L) is a Bcl-2-related survival protein that is essential for normal development. Bcl-X(L) expression is rapidly induced by a wide range of survival signals and many cancer cells constitutively express high levels. The Bcl-X gene has a complex organization with multiple promoters giving rise to RNAs with alternate 5' non-coding exons. Here we have investigated the mechanisms that control basal and induced expression of Bcl-X(L) in B-lymphoma cells. Antisense experiments demonstrated that Bcl-X(L) was essential for survival of Akata6 B-lymphoma cells. The levels of RNAs containing the IB Bcl-X non-coding exon, derived from the distal 1B promoter, correlated with basal expression of Bcl-X(L) in primary malignant B cells and this promoter was highly active in B-cell lines. The activity of this promoter was largely dependent on a single Ets binding site and Ets family proteins were bound at this promoter in intact cells. CD40 ligand (CD40L)-induced cell survival was associated with increased Bcl-X(L) expression and accumulation of exon IA-containing RNAs, derived from the proximal 1A promoter. Nuclear factor-kappaB (NF-kappaB) inhibition prevented induction of Bcl-X(L) protein and exon IA-containing RNAs by CD40L. Therefore, the distal Bcl-X 1B promoter plays a critical role in driving constitutive expression-mediated via Ets family proteins in malignant B cells, whereas NF-kappaB plays a central role in the induction of Bcl-X(L) in response to CD40 signalling via the proximal 1A promoter.

Enhancement of growth and survival and alterations in Bcl-family proteins in beta-thalassemic erythroid progenitors by novel short-chain fatty acid derivatives

Accelerated apoptosis of erythroid progenitors is a characteristic of beta-thalassemia which presents a significant barrier to definitive therapeutic approaches utilizing induction of endogenous fetal globin gene expression. gamma-globin gene expression may not be inducible in, or may not be able to rescue, erythroid cells in which programmed cell death is initiated early in erythroblast development. In this report, short-chain fatty acid derivatives (SCFADs) which induce fetal globin gene expression were tested for their ability to promote proliferation and survival of erythroid progenitors cultured from beta-thalassemic subjects, and of cytokine-dependent erythroid cell lines. Certain SCFADs promoted thalassemic Bfu-e growth and cytokine-independent growth and survival of erythroid cell lines. A 40-80% increase in erythroid Bfu-e colony number was observed in cultures established with any of five mitogenic SCFADs, compared to control or butyrate-treated cultures from the same subjects. Immunoblot analysis demonstrated that these same SCFADs also regulated the expression of specific protein inhibitors of apoptosis. Anti-apoptotic ratios of the proteins Bcl-xL/Bcl-xS in thalassemic Bfu-e were increased by 30-120% with exposure to the SCFDs, compared to the ratios in the same cells cultured under control conditions. Similar anti-apoptotic increases in Mcl-1L/Mcl-1S ratios were induced by the SCFADs. These findings suggest that select fetal globin-inducing SCFADs which enhance proliferation of beta-thalassemia progenitors may enhance survival of these progenitors by altering levels of Bcl-family protein members. This combination of effects should enhance erythroid cell survival in the beta-thalassemia syndromes, allowing fetal globin gene expression to be induced more effectively than currently available, growth-suppressing, fetal globin-inducing agents, such as the butyrates or chemotherapeutic agents.

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.

Regulation of Bcl-xL expression by H2O2 in cardiac myocytes

Oxidative stress promotes cardiac myocyte apoptosis through the mitochondrial death pathway. Since Bcl-2 family proteins are key regulators of apoptosis, we examined the effects of H2O2 on the expression of principal Bcl-2 family proteins (Bcl-2, Bcl-xL, Bax, Bad) in neonatal rat cardiac myocytes. Protein expression was assessed by immunoblotting. Bcl-2, Bax, and Bad were all down-regulated in myocytes exposed to 0.2 mm H2O2, a concentration that induces apoptosis. In contrast, although Bcl-xL levels initially declined, the protein was re-expressed from 4-6 h. Bcl-xL mRNA was up-regulated from 2 to 4 h in neonatal rat or mouse cardiac myocytes exposed to H2O2, consistent with the re-expression of protein. Four different untranslated first exons have been identified for the Bcl-x gene (exons 1, 1B, 1C, and 1D, where exon 1 is the most proximal and exon 1D the most distal to the coding region). All were detected in mouse or rat neonatal cardiac myocytes, but exon 1D was not expressed in adult mouse hearts. In neonatal mouse or rat cardiac myocytes, H2O2 induced the expression of exons 1B, 1C, and 1D, but not exon 1. These data demonstrate that the Bcl-x gene is selectively responsive to oxidative stress, and the response is mediated through distal promoter regions.