PLZF-mediated control on c-kit expression in CD34(+) cells and early erythropoiesis

The promyelocytic leukemia zinc-finger protein (PLZF) is a transcription factor and c-kit is a receptor tyrosine kinase associated with human disease, particularly in hematopoietic cells. MicroRNAs (miRs) are post-transcriptional regulators of gene expression, and c-kit has been described as a target of miRs-221 and -222 in erythropoiesis. In the present study, we identified c-kit as a target of PLZF in normal and leukemic cells. Particularly, in erythropoietic (E) culture of CD34(+) progenitors, PLZF is downregulated, whereas c-kit expression at both the mRNA and protein levels inversely increases during the first days of E differentiation. In functional experiments, PLZF transfection induces c-kit downregulation, inhibits E proliferation and delays differentiation, whereas PLZF knockdown induces opposite effects, independently of miRs-221 and -222 expression. The inverse correlation between PLZF and c-kit expression was found in normal CD34(+)38(+/-) hematopoietic progenitor/stem cells and in acute myeloid leukemias of M0/M1 French-American-British subtypes, suggesting that the control of PLZF on c-kit expression may be crucial at the level of the stem cell/progenitor compartment. Altogether, our data indicate a new mechanism of regulation of c-kit expression that involves a transcriptional control by PLZF in CD34(+) cells and early erythropoiesis.

Circulating haemopoietic and endothelial progenitor cells are decreased in COPD

Circulating CD34+ cells are haemopoietic progenitors that may play a role in tissue repair. No data are available on circulating progenitors in chronic obstructive pulmonary disease (COPD). Circulating CD34+ cells were studied in 18 patients with moderate-to-severe COPD (age: mean+/-sd 68+/-8 yrs; forced expiratory volume in one second: 48+/-12% predicted) and 12 controls, at rest and after endurance exercise. Plasma concentrations of haematopoietic growth factors (FMS-like tyrosine kinase 3 (Flt3) ligand, kit ligand), markers of hypoxia (vascular endothelial growth factor (VEGF)) and stimulators of angiogenesis (VEGF, hepatocyte growth factor (HGF)) and markers of systemic inflammation (tumour necrosis factor (TNF)-alpha, interleukin (IL)-6, IL-8) were measured. Compared with the controls, the COPD patients showed a three-fold reduction in CD34+ cell counts (3.3+/-2.5 versus 10.3+/-4.2 cells.microL-1), and a 50% decrease in AC133+ cells. In the COPD patients, progenitor-derived haemopoietic and endothelial cell colonies were reduced by 30-50%. However, four COPD patients showed progenitor counts in the normal range associated with lower TNF-alpha levels. In the entire sample, CD34+ cell counts correlated with exercise capacity and severity of airflow obstruction. After endurance exercise, progenitor counts were unchanged, while plasma Flt3 ligand and VEGF only increased in the COPD patients. Plasma HGF levels were higher in the COPD patients compared with the controls and correlated inversely with the number of progenitor-derived colonies. In conclusion, circulating CD34+ cells and endothelial progenitors were decreased in chronic obstructive pulmonary disease patients and could be correlated with disease severity.

Identification of a molecular signature for leukemic promyelocytes and their normal counterparts: focus on DNA repair genes

Acute promyelocytic leukemia (APL) is a clonal expansion of hematopoietic precursors blocked at the promyelocytic stage. Gene expression profiles of APL cells obtained from 16 patients were compared to eight samples of CD34+-derived normal promyelocytes. Malignant promyelocytes showed widespread changes in transcription in comparison to their normal counterpart and 1020 differentially expressed genes were identified. Discriminating genes include transcriptional regulators (FOS, JUN and HOX genes) and genes involved in cell cycle and DNA repair. The strong upregulation in APL of some transcripts (FLT3, CD33, CD44 and HGF) was also confirmed at protein level. Interestingly, a trend toward a transcriptional repression of genes involved in different DNA repair pathways was found in APL and confirmed by real-time polymerase chain reactor (PCR) in a new set of nine APLs. Our results suggest that both inefficient base excision repair and recombinational repair might play a role in APLs development. To investigate the expression pathways underlying the development of APL occurring as a second malignancy (sAPL), we included in our study eight cases of sAPL. Although both secondary and de novo APL were characterized by a strong homogeneity in expression profiling, we identified a small set of differentially expressed genes that discriminate sAPL from de novo cases.

PLZF-mediated control on VLA-4 expression in normal and leukemic myeloid cells

The promyelocytic leukemia zinc-finger protein (PLZF) is a transcriptional repressor. To investigate the role of PLZF in the regulation of cytoadhesion molecules involved in the mobilization of hemopoietic cells, we have analysed PLZF and very late antigen 4 (VLA-4) expression in normal and leukemic cells. In hematopoiesis, we found a negative correlation between PLZF and VLA-4 expression, except for the megakaryocytic lineage. In contrast, we observed a positive correlation between PLZF and VLA-4 expression in a panel of acute myeloid leukemia (AML) samples. In K562 cells expressing PLZF (K562-PLZF), we found that the expression of VLA-4 and c-kit was downmodulated. We have investigated the possibility for VLA-4 or the c-kit receptor to be direct target genes of PLZF in K562-PLZF cells and identified a PLZF DNA-binding site within the VLA-4 promoter. Furthermore, decrease in VLA-4 expression was associated with loss of adhesion on fibronectin-coated plates, which promotes drug-induced apoptosis of K562-PLZF cells. Our findings indicate that VLA-4 is a potential target gene of PLZF. However, in primary AMLs the control of PLZF on VLA-4 expression is lost. Altogether, we suggest that VLA-4 modulation by PLZF may represent an important step in the control of normal and leukemic cell mobilization.

Overexpression of Ets-1 in human hematopoietic progenitor cells blocks erythroid and promotes megakaryocytic differentiation

Ets-1 is a widely expressed transcription factor implicated in development, tumorigenesis and hematopoiesis. We analyzed Ets-1 gene expression during human erythroid and megakaryocytic (MK) differentiation in unilineage cultures of CD34+ progenitor cells. During erythroid maturation, Ets-1 is downmodulated and exported from the nucleus into the cytoplasm through an active mechanism mediated by a leucine-rich nuclear export signal. In contrast, during megakaryocytopoiesis Ets-1 increases and remains localized in the nucleus up to terminal maturation. Overexpression of Ets-1 in erythroid cells blocks maturation at the polychromatophilic stage, increases GATA-2 and decreases both GATA-1 and erythropoietin receptor expression. Conversely, Ets-1 overexpressing megakaryocytes are characterized by enhanced differentiation and maturation, coupled with upmodulation of GATA-2 and megakaryocyte-specific genes. We show that Ets-1 binds to and activates the GATA-2 promoter, in vitro and in vivo, indicating that one of the pathways through which Ets-1 blocks erythroid and promotes MK differentiation is via upmodulation of GATA-2 expression.

Enforced expression of KDR receptor promotes proliferation, survival and megakaryocytic differentiation of TF1 progenitor cell line

Vascular endothelial growth factor (VEGF) receptor-2/kinase insert domain-containing receptor (KDR) is expressed in primitive hematopoietic cells, in megakaryocytes and platelets. In primitive hematopoiesis KDR mediates cell survival via autocrine VEGF, while its effect on cell growth and differentiation has not been elucidated. We induced enforced KDR expression in the granulocyte macrophage-colony-stimulating factor (GM-CSF)-dependent TF1 progenitor cell line (TF1-KDR), treated the cells with VEGF and analyzed their response. In GM-CSF-deprived cells, VEGF induces cell proliferation and protection against apoptosis, followed by enhanced expression of megakaryocytic (MK) markers. Combined with GM-CSF, VEGF induces a mild proliferative stimulus, followed by cell adherence, accumulation in G0/G1, massive MK differentiation and Fas-mediated apoptosis. Accordingly, we observed that MK-differentiating cells, derived from hematopoietic progenitors, produce VEGF, express KDR, inhibition of which reduces MK differentiation, indicating a key role of KDR in megakaryopoiesis. In conclusion, TF1-KDR cells provide a reliable model to investigate the biochemical and molecular mechanisms underlying hematopoietic progenitor proliferation, survival and MK differentiation.

Impaired myelopoiesis in mice devoid of interferon regulatory factor 1

Interferon regulatory factor (IRF)-1 is a transcription factor controlling the expression of several genes, which are differentially induced depending on the cell type and signal. IRF-1 modulates multiple functions, including regulation of immune responses and host defence, cell growth, cytokine signalling and hematopoietic development. Here, we investigated the role of IRF-1 in granulocytic differentiation in mice with a null mutation in the IRF-1 gene. We show that IRF-1(-/-) bone marrow cells exhibit an increased number of immature granulocytic precursors, associated with a decreased number of mature granulocytic elements as compared to normal mice, suggestive of a defective maturation process. Clonogenetic analyses revealed a reduced number of CFU-G, CFU-M and CFU-GM colonies in IRF-1(-/-) mice, while the number of BFU-E/CFU-E colonies was unchanged. At the molecular level, the expression of CAAT-enhancer-binding protein (C/EBP)-epsilon, -alpha and PU.1 was substantially lower in the CD11b(+) cells from the bone marrow of IRF-1(-/-) mice as compared to cells from wild-type mice. These results, together with the fact that IRF-1 is markedly induced early during granulo-monocytic differentiation of CD34+ cells, highlight the pivotal role of IRF-1 in the early phases of myelopoiesis.

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.

Interleukin-3 receptor in acute leukemia

Recent studies indicate that abnormalities of the interleukin-3 receptor (IL-3R) are frequently observed in acute myeloid leukemias (AMLs) and may contribute to the proliferative advantage of leukemic blasts. This review analyzes the evidences indicating that the IL-3R represents one of the target molecules involved in the stimulation of proliferation of AMLs, and the overexpression of the IL-3Ralpha chain may represent one of the mechanisms contributing to the development of a highly malignant leukemic phenotype. Furthermore, there is evidence that the IL-3Ralpha is a marker of leukemic stem cells, at variance with normal stem cells that are IL-3Ralpha-. Finally, the IL-3R may represent an important target for the development of new antileukemic drugs.

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

SCL/Tal-1 is a helix-loop-helix (HLH) transcription factor required for blood cell development, whose abnormal expression is responsible for induction of T-cell acute lymphoblastic leukemia. We show here that SCL/Tal-1 is a key target of caspases in developing erythroblasts. SCL/Tal-1 degradation occurred rapidly after caspase activation and preceded the cleavage of the major erythroid transcription factor GATA-1. Expression of a caspase-resistant SCL/Tal-1 in erythroid progenitors was able to prevent amplification of caspase activation, GATA-1 degradation and impaired erythropoiesis induced by growth factor deprivation or death receptor triggering. The potent proerythropoietic activity of uncleavable SCL/Tal-1 was clearly evident in the absence of erythropoietin, a condition that did not allow survival of normal erythroid cells or expansion of erythroblasts expressing caspase-resistant GATA-1. In the absence of erythropoietin, cells expressing caspase-resistant SCL/Tal-1 maintain high levels of Bcl-X(L), which inhibits amplification of the caspase cascade and mediates protection from apoptosis. Thus, SCL/TAL-1 is a survival factor for erythroid cells, whereas caspase-mediated cleavage of SCL/Tal-1 results in amplification of caspase activation, GATA-1 degradation and impaired erythropoiesis.

C-fms expression correlates with monocytic differentiation in PML-RAR alpha+ acute promyelocytic leukemia

We have investigated the expression of the M-CSF receptor (c-fms) in 16 freshly isolated acute promyelocytic leukemias (APL) expressing the PML/RAR alpha fusion protein. In parallel, we evaluated the capacity of these cells to differentiate along the granulocytic and monocytic pathways. c-fms was constitutively and constantly expressed in all cases sensitive in vivo to all-trans retinoic acid (ATRA) and its expression was further potentiated following in vitro induction with ATRA. Furthermore, gel-shift analysis of APL cells showed elevated levels of PU.1 binding activity to the M-CSF receptor promoter, particularly after ATRA stimulation. Interestingly, the rise of PU.1 binding activity as well as of PU.1 levels after ATRA treatment was significantly higher in APL patients exhibiting monocytic maturation, as compared to those that did not undergo monocytic differentiation. A variable proportion of ATRA-induced APL cells exhibited monocyte-like morphology and immunophenotype: the proportion of monocytic cells was consistently increased by combined treatment with ATRA and diverse hematopoietic growth factors cocktails, which always comprised M-CSF. Monocytic cells originating from in vitro ATRA-induced maturation of APL cells derive from the leukemic clone as suggested by two lines of evidence: (1) monocytic cells harbor the 15;17 translocation; (2) monocytic cells possess Auer bodies. The c-fms(bright) leukemic blasts preferentially showed the capacity for monocytic differentiation as compared to the c-fms(dim/-) subset: indeed, enforced expression of c-fms into NB4, a PML/RAR alpha+ cell line, favored the onset of monocytic maturation. Finally, low c-fms expression was observed in an APL relapsing patient resistant to ATRA, as well as in an APL case with t(11;17), PLZF/RAR alpha+. These observations indicate that PML/RAR alpha+ APL blasts are bipotent for differentiation through both neutrophilic and monocytic lineages, whereby monocytic differentiation is linked to c-fms expression and stimulation.

Expression pattern of HOXB6 homeobox gene in myelomonocytic differentiation and acute myeloid leukemia

Homeobox genes encode transcription factors known to be important morphogenic regulators during embryonic development. An increasing body of work implies a role for homeobox genes in both hematopoiesis and leukemogenesis. In the present study we have analyzed the role of the homeobox gene, HOXB6, in the program of differentiation of the myeloid cell lines, NB4 and HL60. HOXB6 expression is transiently induced during normal granulocytopoiesis and monocytopoiesis, with an initial induction during the early phases of differentiation, followed by a blockade of expression at early maturation. The enforced expression of HOXB6 in promyelocytic NB4 cells or in myeloblastic HL60 cells elicited inhibition of the granulocytic or monocytic maturation, respectively. Furthermore, HOXB6 was frequently expressed (18 out of 49 cases) in AMLs lacking major translocations while it was expressed at very low frequency (two out of 47 cases) in AMLs characterized by PML/RAR-alpha, AML-1/ETO, CBFbeta/MYH11 fusion and rearrangements of the MLL gene at 11q23. According to these observations, we suggest that a regulated pattern of HOXB6 expression is required for normal granulopoiesis and monocytopoiesis. Abnormalities of the HOXB6 expression may contribute to the development of the leukemic phenotype.

Ectopic expression of interferon regulatory factor-1 potentiates granulocytic differentiation

Numerous transcription factors allow haematopoietic cells to respond to lineage- and stage-specific cytokines and to act as their effectors. It is increasingly evident that the interferon regulatory factor-1 (IRF-1) transcription factor can selectively regulate different sets of genes depending on the cell type and/or the nature of cellular stimuli, evoking distinct responses in each. In the present study, we investigated mechanisms underlying the differentiation-inducing properties of granulocytic colony-stimulating factor (G-CSF) and whether IRF transcription factors are functionally relevant in myeloid differentiation. Both normal human progenitors and murine 32Dcl3 myeloblasts induced to differentiate along the granulocytic pathway showed an up-regulation of IRF-1 expression. Ectopic expression of IRF-1 did not abrogate the growth factor requirement of 32Dcl3 cells, although a small percentage of cells that survived cytokine deprivation differentiated fully to neutrophils. Moreover, in the presence of G-CSF, granulocytic differentiation of IRF-1-expressing cells was accelerated, as assessed by morphology and expression of specific differentiation markers. Down-modulation of c-Myb protein and direct stimulation of lysozyme promoter activity by IRF-1 were also observed. Conversely, constitutive expression of IRF-2, a repressor of IRF-1 transcriptional activity, completely abrogated the G-CSF-induced neutrophilic maturation. We conclude that IRF-1 exerts a pivotal role in granulocytic differentiation and that its induction by G-CSF represents a limiting step in the early events of differentiation.

Expression of P-170 glycoprotein sensitizes lymphoblastoid CEM cells to mitochondria-mediated apoptosis

Multidrug resistance caused by P-glycoprotein (P-170) is a phenomenon by which cells exposed to a single drug acquire resistance to other structurally and functionally unrelated drugs. This is a widespread phenomenon described in vivo in the management of infectious as well as non-infectious diseases. Several in vitro models have been developed in order to evaluate physiopathological properties of P-170. Among these are P-170-expressing variants of the human T-lymphoblastoid CEM cell line called VBL100. As a general rule, drug resistance normally results in resistance to apoptosis induction. By contrast, a paradoxical activity is exerted in this cell model by the cytokine tumour necrosis factor-alpha (TNF-alpha), which is capable of inducing apoptosis in P-170-expressing variants better than in wild-type (wt) cells. In the present study we partially address the mechanisms underlying this activity. In fact, the susceptibility of VBL100 cells to TNF-alpha appears to be specifically due to the depolarization of their mitochondrial membrane, a key factor for apoptotic induction. The same was observed with staurosporine, a specific mitochondrion-mediated proapoptotic chemical probe. Conversely, other proapoptotic stimuli, such as Fas/CD95 or the anti-cancer drug etoposide, did induce significant cell death in wild type cells only. Thus, schematically, mitochondrially dependent stimuli appeared to be more effective in VBL100-cell killing, while 'physiological' stimuli showed the opposite behaviour. Importantly, under steady-state conditions, VBL100 cells displayed per se a mitochondrial membrane hyperpolarization that appeared strictly related to their high susceptibility to specific apoptotic stimuli. In conclusion, the study of a well-established cell model such as that represented by the wt/VBL CEM lymphoid cell line seems to suggest that the multidrug resistance phenotype can specifically sensitize cells towards 'unphysiological', mitochondria-associated cell death cascade or, in the same fashion, it could shift cells from type I (mainly plasma membrane-associated) towards type II (mainly mitochondrial membrane-associated) phenotype.

Stromal cell-derived factor 1alpha increases polyploidization of megakaryocytes generated by human hematopoietic progenitor cells

The alpha chemokine receptor CXCR4 has been shown to be expressed on human hematopoietic progenitor cells and during the megakaryocytic differentiation pathway. Stromal cell-derived factor 1 (SDF-1) is the ligand for CXCR4. In this study, the role of SDF-1alpha in megakaryocytopoiesis was investigated. CD34(+) progenitors purified from peripheral blood were grown in serum-free liquid suspension culture supplemented with thrombopoietin to obtain a virtually pure megakaryocytic progeny. In this condition, the addition of SDF-1alpha gives rise to megakaryocytes (MKs) showing an increased DNA content and a rise of lobated nuclei, as compared with untreated cells: at day 5, approximately 20% of the cells already showed the presence of more than one nuclear lobe versus fewer than 5% in the control cells; at day 12, approximately 85% of the cells were of large size and markedly polyploid, whereas approximately 60% of the control cells were polyploid, showed fewer lobes, and were a smaller size. This effect was dose-dependent and did not affect the megakaryocytic proliferation. Experiments with the mitogen-activated protein kinase (MAPK) inhibitor PD98059 suggested a role for MAPK pathway on SDF-1alpha-induced endomitosis. Furthermore, SDF-1alpha induced a significant increase in the number of proplatelet-bearing MKs and promoted the migration of megakaryocytic cells. Treatment with SDF-1alpha caused reduction in CXCR4 abundance on the plasma membrane, seemingly owing to receptor internalization. Furthermore, the presence of SDF-1alpha did not affect the expression of megakaryocytic markers, indicating that differentiation and polyploidization are independently regulated events.

Polyclonal expansion of CD3(+)/CD4(+)/CD56(+) large granular lymphocytes and autoimmunity associated with dysregulation of Fas/FasL apoptotic pathway

Evidence is accumulating regarding CD95/CD95 ligand (Fas/FasL) pathway dysregulation in clonal diseases of the lymphohaemopoietic lineages. According to these observations, it has been proposed that this defect may represent one of the mechanisms of tumour progression. In large granular lymphocyte (LGL) leukaemia, dysregulated apoptosis may represent a key event in the development of malignancy and autoimmunity. This case report describes dysregulation of the Fas/FasL pathway in a chronic polyclonal expansion of CD3(+) LGLs associated with numerous serological immune abnormalities.

Mechanisms of differential transferrin receptor expression in normal hematopoiesis

We have investigated the expression of transferrin receptor (TfR) iron regulatory protein-1 (IRP-1) and iron regulatory protein-2 (IRP-2) in liquid suspension culture of purified hematopoietic progenitor cells (HPCs) induced by a growth factor stimulus to proliferation and unilineage differentiation/maturation through the erythroid, granulocytic, monocytic and megakaryocytic lineages. In initial HPC differentiation, TfR expression is induced in both erythroid and granulopoietic cultures. In late HPC differentiation (i.e. starting from day 5 of culture) and then differentiated precursor maturation, the TfR gene is highly expressed in the erythroid lineage, whereas it is sharply downmodulated in the granulopoietic, monocytopoietic and megakaryocytic series. The elevated TfR expression in erythroid cells is: (a) mediated through a high rate of TfR gene transcription; (b) modulated by intracellular iron levels; (c) mediated by TfR mRNA stabilization through the iron regulatory protein (IRP), in that IRP-1 activity is high in erythroid lineage as compared to the levels observed in other hemopoietic lineages; and (d) dependent on exogenous erythropoietin (Epo) (this is indicated by the marked TfR and IRP-1/IRP-2 downmodulation after Epo starvation). Interestingly, analysis of IRP-1 and IRP-2 expression during hemopoietic differentiation showed that: (a) IRP-1 expression was maintained during all steps of erythroid differentiation, while it was lost in the other hemopoietic lineages; (b) IRP-2 expression was observed during all stages of hemopoietic differentiation in all four lineages. However, IRP-1 and IRP-2 expression and activity are induced when monocytes, which express only low levels of IRP-1 and IRP-2, are induced to maturation to macrophages. These studies indicate that: (a) in normal erythropoiesis, the hyperexpression of TfR, starting from early erythroid HPC differentiation, is Epo-dependent and mediated via transcriptional and post-transcriptional mechanisms; (b) in the granulopoietic, monocytopoietic and megakaryocytic pathways, the TfR is first induced and then downmodulated (the latter phenomenon is mediated via transcriptional suppression of the TfR gene and IRP inactivation).

Expression of interleukin 3 and granulocyte-macrophage colony-stimulating factor receptor common chain betac, betaIT in normal haematopoiesis: lineage specificity and proliferation-independent induction

Interleukin 3 (IL-3), granulocyte-macrophage colony-stimulating factor (GM-CSF) and interleukin 5 (IL-5) exert their biological activities through interaction with cell-surface receptors that consist of two subunits, a specific alpha subunit and a common beta transducing subunit (betac). We have evaluated the expression of betac on purified haematopoietic progenitor cells (HPCs) induced to unilineage differentiation/maturation through the erythroid (E), granulocytic (G), megakaryocytic (Mk) or monocytic (Mo) lineage. HPCs displayed low betac expression, which increased during the initial stages of HPC differentiation along the E, G, Mo or Mk lineages. At later stages of differentiation, betac chain expression increased in both G and Mo lineages, was expressed at low levels in the Mk lineage and declined to undetectable levels in the E lineage. Analysis of the full-length betac and intracytoplasmically truncated betac (betaIT) mRNAs showed that the former was predominant in the G and Mo lineages, whereas the latter was prevalent in the E and Mk lineages. The betac induction takes place even in the absence of cell cycling. Thus, incubation of HPCs with graded amounts of IL-3 showed that the initial induction of betac expression is unrelated to cell proliferation. Furthermore, circulating monocytes and granulocytes exhibit a low level of betac expression that is greatly stimulated following incubation with either IL-3 or GM-CSF.

Unilineage hematopoietic differentiation in bulk and single cell culture

The rarity of hematopoietic stem and progenitor cells (HSCs, HPCs) has hampered the analysis of cellular and molecular mechanisms underlying early hematopoiesis. Methodology for HPC purification has partially offset this limitation. A further hurdle has been represented by the heterogeneity of the analyzed HPC/precursor populations: recently, development of unilineage HPC differentiation cultures has provided homogeneous populations of hematopoietic cells, particularly in the early differentiation state, i.e., populations pertaining to a single lineage and a restricted stage of differentiation/maturation, but sufficiently large for cellular/molecular analysis. This report focuses on the development and characterization of the unilineage HPC differentiation culture systems. A section is devoted to selected cellular and molecular mechanisms underlying hematopoiesis, which have been investigated by the HPC unilineage culture approach. Finally, recent advances in the development of HPC unilineage cultures at single cell level are discussed.

Hemoglobin switching in unicellular erythroid culture of sibling erythroid burst-forming units: kit ligand induces a dose-dependent fetal hemoglobin reactivation potentiated by sodium butyrate

Mechanisms underlying fetal hemoglobin (HbF) reactivation in adult life have not been elucidated; particularly, the role of growth factors (GFs) is controversial. Interestingly, histone deacetylase (HD) inhibitors (sodium butyrate, NaB, trichostatin A, TSA) reactivate HbF. We developed a novel model system to investigate HbF reactivation: (1) single hematopoietic progenitor cells (HPCs) were seeded in serum-free unilineage erythroid culture; (2) the 4 daughter cells (erythroid burst-forming units, [BFU-Es]), endowed with equivalent proliferation/differentiation and HbF synthesis potential, were seeded in 4 unicellular erythroid cultures differentially treated with graded dosages of GFs and/or HD inhibitors; and (3) HbF levels were evaluated in terminal erythroblasts by assay of F cells and gamma-globin content (control levels, 2.4% and 1.8%, respectively, were close to physiologic values). HbF was moderately enhanced by interleukin-3 (IL-3) and granulocyte-macrophage colony-stimulating factor treatment (up to 5%-8% gamma-globin content), while sharply reactivated in a dose-dependent fashion by c-kit ligand (KL) and NaB (20%-23%). The stimulatory effects of KL on HbF production and erythroid cell proliferation were strictly correlated. A striking increase of HbF was induced by combined addition of KL and NaB or TSA (40%-43%). This positive interaction is seemingly mediated via different mechanisms: NaB and TSA may modify the chromatin structure of the beta-globin gene cluster; KL may activate the gamma-globin promoter via up-modulation of tal-1 and possibly FLKF transcription factors. These studies indicate that KL plays a key role in HbF reactivation in adult life. Furthermore, combined KL and NaB administration may be considered for sickle cell anemia and beta-thalassemia therapy.

IL-6, M-CSF and IAP cytokines in ovarian cancer: simultaneous assessment of serum levels

The aim of this study was to simultaneously determine IL-6, M-CSF and IAP levels in 61 serum samples of previously untreated ovarian cancer patients. A direct correlation between IL-6 and M-CSF has been found in our patient population (r = +0.41, p = 0.013), while IAP serum levels failed to correlate with M-CSF (r = +0.15, p = 0. 24) and IL-6 (r = +0.17, p = 0.18) levels. Since IL-6 and M-CSF have been demonstrated to be both induced in response to the same agents, it is conceivable that a mechanism of coregulation in the production of these cytokines by tumor cells and macrophages might occur. The direct correlation between IL-6 and M-CSF also suggests that tumor-derived cytokines can potentially lead to a self-maintaining cytokine network by recruiting cytokine-producing host cells and by perpetuating cytokine production.

Negative regulation of erythropoiesis by caspase-mediated cleavage of GATA-1

The production of red blood cells follows the sequential formation of proerythroblasts and basophilic, polychromatophilic and orthochromatic erythroblasts, and is promoted by the hormone erythropoietin (Epo) in response to tissue hypoxia. However, little is known about the negative regulation of this process. Death receptors are a family of surface molecules that trigger caspase activation and apoptosis in a variety of cell types. Here we show that immature erythroid cells express several death receptors whose ligands are produced by mature erythroblasts. Exposure of erythroid progenitors to mature erythroblasts or death-receptor ligands resulted in caspase-mediated degradation of the transcription factor GATA-1, which is associated with impaired erythroblast development. Expression of a caspase-resistant GATA-1 mutant, but not of the wild-type gene, completely restored erythroid expansion and differentiation following the triggering of death receptors, indicating that there is regulatory feedback between mature and immature erythroblasts through caspase-mediated cleavage of GATA-1. Similarly, erythropoiesis blockade following Epo deprivation was largely prevented by the expression of caspase-inhibitory proteins or caspase-resistant GATA-1 in erythroid progenitors. Caspase-mediated cleavage of GATA-1 may therefore represent an important negative control mechanism in erythropoiesis.

Lineage-specific expression of human immunodeficiency virus (HIV) receptor/coreceptors in differentiating hematopoietic precursors: correlation with susceptibility to T- and M-tropic HIV and chemokine-mediated HIV resistance

Human immunodeficiency virus (HIV) entry is mediated not only by the CD4 receptor, but also by interaction with closely related molecules that act as membrane coreceptors. We have analyzed mRNA expression and/or cell membrane exposition of the coreceptors most widely used by diverse HIV-1 strains (CXCR4, CCR5, and CCR3) on purified hematopoietic progenitor cells (HPCs) induced in liquid suspension culture to unilineage differentiation/maturation through the erythroid (E), granulocytic (G), megakaryocytic (Mk), and monocytic (Mo) lineages. Reverse transcriptase-polymerase chain reaction (RT-PCR) and cytofluorimetric analysis showed the presence of both CXCR4 and CCR5 in quiescent HPCs, but failed to detect CCR3-specific transcripts. Chemokine expression in HPC progenies showed that CXCR4 receptor is detected on the majority of MKs from early to late stages of maturation, whereas it is moderately decreased in the Mo lineage. In the G pathway, two distinct cell populations, CXCR4(+) and CXCR4(-), were observed: morphological analysis of the sorted populations showed that the CXCR4(+) cells were largely eosinophils and the CXCR4(-) were granulocytes of the neutrophilic series. Furthermore, in the E pathway, CXCR4 was almost completely absent. CCR5 expression is restricted to Mo cultures, ie, approximately 30% to 80% cells throughout all monocytopoietic differentiation/maturation stages. Finally, CCR3 mRNA is always absent in all the unilineage cultures. Evaluation of CD4 expression by flow cytometry on both quiescent HPCs and differentiating unilineage precursors showed that the CD4 receptor is present on approximately 15% of the starting CD34(+) HPC population, highly expressed in the Mo lineage up to 80% at terminal maturation, present on 20% to 30% of maturing Mks, and not detectable in either the E or G lineage. Expression of CD4 receptor together with CXCR4 and/or CCR5 coreceptor in the four lineages correlates with hematopoietic precursor susceptibility to T-lymphotropic and macrophage (M)-tropic HIV strains infection: (1) CD4(-) G and E cells were resistant to both M-tropic and T-lymphotropic strains; (2) HPC-derived Mks were susceptible to T-tropic, but resistant to M-tropic, infection; (3) Mo differentiating cells efficiently replicate both HIV strains. Furthermore, we showed that the CXCR4 and CCR5 ligands (stromal-derived factor 1 and macrophage-inflammatory protein-1alpha [MIP-1alpha], MIP-1beta and RANTES, respectively) inhibit HIV replication in both maturing Mo and Mk cells. Taken together, our data show a lineage-specific modulation of chemokine receptor/coreceptor during hematopoietic cell differentiation and extend previous observations on the relationship between the expression of HIV receptor/coreceptors, susceptibility, and chemokine-mediated resistance to HIV infection.

STAT1 activation during monocyte to macrophage maturation: role of adhesion molecules

Human monocytes isolated from peripheral blood of healthy donors show a time-dependent differentiation into macrophages upon in vitro cultivation, closely mimicking their in vivo migration and maturation into extravascular tissues. The mediator(s) of this maturation process has not been yet defined. We investigated the involvement of signal transducers and activators of transcription (STAT) factors in this phenomenon and reported the specific, time-dependent, activation of STAT1 protein starting at day 0/1 of cultivation and maximally expressed at day 5. STAT1 activity was evident on the STAT binding sequences (SBE) present in the promoters of genes which are up-regulated during monocyte to macrophage maturation such as FcgammaRI and ICAM-1, and in the promoter of the transcription factor IFN regulatory factor-1. Moreover, the effect of cell adhesion to fibronectin or laminin was studied to investigate mechanisms involved in STAT1 activation. Compared with monocytes adherent on plastic surfaces, freshly isolated cells allowed to adhere either to fibronectin- or laminin-coated flasks exhibited an increased STAT1 binding activity both in control and in IFN-gamma-treated cells. The molecular events leading to enhanced STAT1 activation and cytokine responsiveness concerned both Y701 and S727 STAT1 phosphorylation. Exogenous addition of transforming growth factor-beta, which exerts an inhibitory effect on some monocytic differentiation markers, inhibited macrophage maturation, integrin expression and STAT1 binding activity. Taken together these results indicate that STAT1 plays a pivotal role in the differentiation/maturation process of monocytes as an early transcription factor initially activated by adherence and then able to modulate the expression of functional genes, such as ICAM-1 and FcgammaRI.

Expression and role of PML gene in normal adult hematopoiesis: functional interaction between PML and Rb proteins in erythropoiesis

The expression of the PML gene was investigated in purified early hematopoietic progenitor cells (HPCs) induced to unilineage erythroid or granulocytic differentiation. PML mRNA and protein, while barely detectable in quiescent HPCs, are consistently induced by growth factor stimulation through the erythroid or granulocytic lineage. Thereafter, PML is downmodulated in late granulocytic maturation, whereas it is sustainably expressed through the erythroid pathway. In functional studies, PML expression was inhibited by addition of antisense oligomers targeting PML mRNA (alpha-PML). Interestingly, early treatment (day 0 HPCs) with alpha-PML reduced the number of both erythroid and granulocytic colonies, whereas late treatment (day 5 culture) reduced erythroid, but not granulocytic, clonogenesis. These findings suggest that PML is required for early hematopoiesis and erythroid, but not granulocytic maturation. The pattern of PML expression in normal hematopoiesis mimics that of retinoblastoma pRb 105. Combined treatment of HPCs with alpha-PML and alpha-Rb oligomers inhibited both PML and Rb protein expression and completely blocked erythroid colony development. Furthermore, PML and pRb 105 were co-immunoprecipitated in cellular lysates derived from erythroid precursors indicating that this functional interaction may have a biochemical basis. These results suggest a key functional role of PML in early hematopoiesis and late erythropoiesis: the latter phenomenon may be related to the molecular and functional interaction of PML with pRb 105.

Apoptotic role of Fas/Fas ligand system in the regulation of erythropoiesis

The possible involvement of Fas and Fas ligand (FasL) in the regulation of erythropoiesis was evaluated. Immunohistochemistry of normal bone marrow specimens revealed that several immature erythroblasts undergo apoptosis in vivo. Analysis of bone marrow erythroblasts and purified progenitors undergoing unilineage erythroid differentiation showed that Fas is rapidly upregulated in early erythroblasts and expressed at high levels through terminal maturation. However, Fas crosslinking was effective only in less mature erythroblasts, particularly at basophilic level, where it induced apoptosis antagonized by high levels of erythropoietin (Epo). In contrast, FasL was selectively induced in late differentiating Fas-insensitive erythroblasts, mostly at the orthochromatic stage. FasL is functional in mature erythroblasts, as it was able to kill Fas-sensitive lymphoblast targets in a Fas-dependent manner. Importantly, FasL-bearing mature erythroblasts displayed a Fas-based cytotoxicity against immature erythroblasts, which was abrogated by high levels of Epo. These findings suggest the existence of a negative regulatory feedback between mature and immature erythroid cells, whereby the former cell population might exert a cytotoxic effect on the latter one in the erythroblastic island. Hypothetically, this negative feedback operates at low Epo levels to moderate the erythropoietic rate; however, it is gradually inhibited at increasing Epo concentrations coupled with enhanced erythrocyte production. Thus, the interaction of Fas and FasL may represent an apoptotic control mechanism for erythropoiesis, contributing to the regulation of red blood cell homeostasis.

The activity of the CCAAT-box binding factor NF-Y is modulated through the regulated expression of its A subunit during monocyte to macrophage differentiation: regulation of tissue-specific genes through a ubiquitous transcription factor

In this study, we analyzed the regulation of NF-Y expression during human monocyte to macrophage maturation. NF-Y is a ubiquitous and evolutionarily conserved transcription factor that binds specifically to the CCAAT motif present in the 5' promoter region of a wide variety of genes. We show here that in circulating monocytes, NF-Y binding activity is not detected on the CCAAT motif present in the promoters of genes such as major histocompatibility complex (MHC) class II, gp91-phox, mig, and fibronectin, whereas during macrophage differentiation, a progressive increase in NF-Y binding activity is observed on these promoters. Analysis of NF-Y subunit expression indicates that the absence of NF-Y activity in circulating monocytes is caused by a lack of the A subunit. Furthermore, addition of the recombinant NF-YA subunit restores NF-Y binding. We show that the lack of NF-YA protein is due to posttranscriptional regulation and not to a specific proteolytic activity. In fact, NF-YA mRNA is present at the same level at all days of monocyte cultivation, whereas the protein is absent in freshly isolated monocytes but is progressively synthesized during the maturation process. We thus conclude that the NF-YA subunit plays a relevant role in activating transcription of genes highly expressed in mature monocytes. In line with this conclusion, we show that the cut/CDP protein, a transcriptional repressor that inhibits gpc91-phox gene expression by preventing NF-Y binding to the CAAT box, is absent in monocytes.

Oxidative stress leads to a rapid alteration of transferrin receptor intravesicular trafficking

Several studies have demonstrated that perturbations of intracellular oxidative balance play a key role in numerous physiological as well as pathological conditions leading to various morbidity states. In previous studies we have shown that the free radical inducer menadione rapidly and specifically downmodulates the membrane transferrin receptor (TfR) by blocking receptor recycling. This modulation is due to receptor redistribution and not to receptor loss. Here we show that other oxidant compounds, such as hydrogen peroxide, also induce a rapid downmodulation of membrane TfR and that pretreatment of cells with the antioxidant, thiol supplier, N-acetylcysteine inhibits the downmodulation of these receptors elicited by either menadione or hydrogen peroxide. This observation suggests that intracellular thiol redox status may be a critical determinant of TfR downmodulation induced by oxidative stress. Furthermore, immunocytochemical results show that, in menadione-treated cells, TfRs are associated with the Golgi complex, where normally only 20% of total cellular TfRs is found and is mainly detected in the cytoplasm as scattered punctuations. Accordingly, menadione and hydrogen peroxide also elicited a downmodulation of low density lipoprotein receptor (LDLR) which mediates, like TfR, the transport of nutrients to the cell and is endocytosed through clathrin-coated pits. Finally, experiments carried out using okadaic acid, an inhibitor of phosphatases, suggest that H2O2 and menadione downmodulate surface TfR via different biochemical pathways. Taken together these results suggest the existence of a potentially important protective mechanism through which iron uptake is prevented in oxidatively imbalanced cells. Iron uptake can in fact give rise to the formation of highly toxic hydroxyl radicals reacting with hydrogen peroxide and leading to cytotoxicity. Downmodulation of surface TfR may thus represent the physiological control mechanism for reducing iron uptake in diverse pathological conditions including hypoxia-reperfusion injury, acquired immunodeficiency syndrome, and aging.

The PML/RARalpha fusion protein inhibits tumor necrosis factor-alpha-induced apoptosis in U937 cells and acute promyelocytic leukemia blasts

We investigated the effect of the acute promyelocytic leukemia (APL) specific PML/RARalpha fusion protein on the sensitivity to TNF-alpha-mediated apoptosis. The U937 leukemia cell line was transduced with PML/RARalpha cDNA. PML/RARalpha expression caused a markedly reduced sensitivity to TNF-alpha, even if apoptosis was triggered by agonistic antibodies to TNF-alpha receptors I and II (TNF-alphaRI, II). PML/RARalpha induced a 10-20-fold decrease of the TNF-alpha-binding capacity via downmodulation of both TNF-alphaRI and TNF-alphaRII: this may mediate at least in part the reduced sensitivity to TNF-alpha. Furthermore, the fusion protein did not modify Fas expression (CD95) or sensitivity to Fas-mediated apoptosis. The pathophysiological significance of these findings is supported by two series of observations. (a) Fresh APL blasts exhibit no TNF-alpha binding and are resistant to TNF-alpha-mediated apoptosis. Conversely, normal myeloblasts-promyelocytes show marked TNF-alphaR expression and are moderately sensitive to TNF-alpha-mediated cytotoxicity. Similarly, blasts from other types of acute myeloid leukemia (AML M1, M2, and M4 FAB types) show an elevated TNF-alpha binding. (b) The NB4 APL cell line, which is PML/RARalpha+, shows low TNF-alphaR expression capacity and is resistant to TNF-alpha-triggered apoptosis; conversely a PML/RARalpha- NB4 subclone (NB4.306) exhibits detectable TNF-alpha-binding capacity and is sensitive to TNF-alpha-mediated cytotoxicity. These studies indicate that the PML/RARalpha fusion protein protects against TNF-alpha-induced apoptosis, at least in part via downmodulation of TNF-alphaRI/II: this phenomenon may play a significant role in APL, which is characterized by prolonged survival of leukemic blasts.

Terminal megakaryocytic differentiation of TF-1 cells is induced by phorbol esters and thrombopoietin and is blocked by expression of PML/RARalpha fusion protein

We have analyzed the differentiation program of growth factor-dependent TF-1 erythroleukemia cells as well as clones with inducible expression of the APL-specific PML/RARalpha protein. We have shown that TF-1 cells may be induced to megakaryocytic differentiation by phorbol ester (phorbol dibutyrate, PDB) addition, particularly when combined with thrombopoietin (Tpo). RT-PCR studies showed that Tpo induces Tpo receptor (TpoR or c-mpl), whose expression was further potentiated by PDB addition. When the cells are induced with both PDB and Tpo erythropoietin receptor (EpoR) expression was inhibited. In the absence of Zn2+-induced PML/RARalpha expression, PDB and Tpo induced megakaryocytic differentiation of TF-1 MTPR clones as observed in 'wild-type' TF-1 cells. Conversely, when PML/RARalpha expression was induced by Zn2+, PDB and Tpo treatment of these clones caused only a reduced level of megakaryocytic differentiation. These observations indicate that: (1) TF-1 cells as well as other erythroleukemic cells, possess the capacity to differentiate to megakaryocytic cells when grown in the presence of protein kinase (PKC) activators and more efficiently when combined with Tpo; (2) the PML/RARalpha gene has a wide capacity to interfere with the program of hematopoietic differentiation, including megakaryocytic differentiation. Finally, we also observed that PML/RARalpha expression in TF-1 cells induces an up-modulation of interleukin-3 receptor, c-kit and c-mpl, a phenomenon which may offer these cells a growth advantage.

The synergistic effect of simultaneous addition of retinoic acid and vitamin D3 on the in-vitro differentiation of human promyelocytic leukemia cell lines could be efficiently transposed in vivo

Both human cell lines HL-60 and AML-193 exhibit a myeloblastic and promyelocytic morphology, respectively, but may be regarded as bipotent leukemic precursors. They can be triggered to differentiate to either granulocytes or monocytes upon retinoic acid (RA) or 1,25-dihydroxyvitamin D (D3) addition, respectively. We have investigated the effect of combined addition of these chemical inducers on the in-vitro differentiation of both cell lines. RA and D3 added together exert synergistic effects on the in-vitro maturation of these myeloid cell lines. Interestingly, the additive effects were lost if the cells were incubated with the inducers added at sequential times. The synergistic effect could be transposed in vivo and could be clinically significant in the treatment of the promyelocytic leukemia. This clinical strategy may help to prevent retinoic acid resistance or to overcome it in patients relapsed after RA therapy and usually unresponsive to a reinduction therapy with RA alone.

Productive human immunodeficiency virus-1 infection of purified megakaryocytic progenitors/precursors and maturing megakaryocytes

We have evaluated the susceptibility to human immunodeficiency virus (HIV)-1 infection of in vitro grown megakaryopoietic progenitors/precursors and maturing megakaryocytes (MKs), based on the following approach: (1) human hematopoietic progenitor cells (HPCs), stringently purified from peripheral blood and grown in serum-free liquid suspension culture supplemented with thrombopoietin (Tpo), generated a relatively large number of >/= 98% to 99% pure megakaryocytic precursors and then mature-terminal MKs; (2) at different days of culture (ie, 0, 5, 8, 10) the cells were inoculated with 0.1 to 1.0 multiplicity of infection (m.o.i.) of the lymphotropic NL4-3 or 0.1 m.o.i. of the monocytotropic BaL-1 HIV-1 strain; (3) finally, the presence of viral mRNA and proteins was analyzed by reverse transcriptase-polymerase chain reaction (RT-PCR)/in situ hybridization and antigen capture assays, respectively, on day 2 to 12 of culture. MKs derived from day 0 and day 5 BaL-1-challenged cells do not support viral replication as assessed by p24 enzyme-linked immunosorbent assay (ELISA) and RT-PCR. On the contrary, HIV transcripts and proteins were clearly detected in all NL4-3 infection experiments by RT-PCR and p24 assay, respectively, with the highest viral expression in day 5 to 8 challenged MKs. In situ hibridization studies indicate that the percentage of HIV+ MKs varies from at least 1% and 5% for day 0 and day 5 infected cells, respectively. Production of an infectious viral progeny, evaluated by the capability of culture supernatants from day 5 NL4-3-challenged MKs to infect C8166 T-lymphoblastoid cell line, was consistently observed (viral titer, approximately 5 x 10(3) tissue culture infectious dose50/mL/10(6) cells). Exposure of MKs to saturating concentration of anti-CD4 OKT4A monoclonal antibody (MoAb), which recognizes the CD4 region binding with the gp120 envelope glycoprotein, markedly inhibited HIV infection, as indicated by a reduction of p24 content in the supernatants: because the inhibitory effect was incomplete, it is apparent that the infection is only partially CD4-dependent, suggesting that an alternative mechanism of viral entry may exist. Morphologic analysis of day 12 MKs derived from HPCs infected at day 0 showed an impaired megakaryocytic differentiation/maturation: the percentage of mature MKs was markedly reduced, in that approximately 80% of cells showed only one nuclear lobe and a pale cytoplasm with few granules. Conversely, megakaryocytic precursors challenged at day 5 to 8 generated fully mature day 10 to 12 MKs showing multiple nuclear segmentation. Thus, the inhibitory effect of HIV on the megakaryopoietic gene program relates to the differentiation stage of cells subjected to the viral challenge. Finally, HPCs treated with 20 or 200 ng/mL of recombinant Tat protein, analyzed at different days of culture, showed an impaired megakaryocytopoiesis comparable to that observed in HIV-infected cells, thus suggesting that Tat is a major mediator in the above described phenomena. These results shed light on the pathogenesis of HIV-related thrombocytopenia; furthermore, they provide a model to investigate the effects of HIV on megakaryocytic differentiation and function.

Enforced TAL-1 expression stimulates primitive, erythroid and megakaryocytic progenitors but blocks the granulopoietic differentiation program

In human adult hematopoiesis, the TAL-1 gene is up- and down-modulated in erythropoiesis and granulopoiesis, respectively [G. L. Condorelli et al., Blood, 86: 164-175, 19951. Here, it is shown that, in a hematopoietic progenitor cell (HPC) unilineage differentiation culture, tal-1 is induced and then expressed, in a sustained manner, in the megakaryopoietic lineage, whereas it is barely or not detected in the monocytopoietic series. We have investigated the role of enforced tal-1 expression by retroviral transfer into HPCs [erythroid burst-forming units and megakaryocytic and granulomonocytic colony-forming units (CFUs)], primitive HPCs (high proliferative potential colony-forming cells), and putative hematopoietic stem cells (HSCs), assayed as long-term culture initiating cells. TAL-1 overexpression induces an increase of erythroid burst-forming unit colony number and size and megakaryocytic CFU colony number and an inhibition of granulomonocytic CFU and granulocytic CFU (CFU-G) but not monocytic CFU colony number; conversely, TAL-1 mutants with defective heterodimerizing or DNA-binding domains do not exert these effects at a significant level. Although it does not affect long-term culture initiating cells, exogenous TAL-1 causes a significant proliferative stimulus on primary and secondary high proliferative potential colony-forming cells. In conclusion, exogenous tal-1 exerts differential and stage- and lineage-specific effects on the HPC/HSC differentiation/proliferation gene programs. Thus, it induces a stimulatory effect at the level of erythroid and megakaryocytic HPCs, while exerting a selective proliferative action on downstream erythropoiesis. Furthermore, it induces differential effects on the myeloid series: the partial blockade of CFU-G differentiation is possibly linked to the sharp down-modulation of endogenous TAL-1 expression at the level of the CFU-G-to-granulopoietic precursor differentiation step; in contrast, no significant effect is observed on monocytic CFU colony formation. Finally, the stimulatory effect on primitive HPCs but not putative stem cells suggests subtle differences in the effects exerted by tal-1 overexpression on primitive HPC/HSC subsets in adult life.

Arsenic trioxide as an inducer of apoptosis and loss of PML/RAR alpha protein in acute promyelocytic leukemia cells

BACKGROUND

Retinoids, which are derivatives of vitamin A, induce differentiation of acute promyelocytic leukemia (APL) cells in vitro and in patients. However, APL cells develop resistance to retinoic acid treatment. Arsenic trioxide (As2O3) can induce clinical remission in patients with APL, including those who have relapsed after retinoic acid treatment, by inducing apoptosis (programmed cell death) of the leukemia cells. In this study, we investigated the molecular mechanisms by which As2O3 induces apoptosis in retinoic acid-sensitive NB4 APL cells, in retinoic acid-resistant derivatives of these cells, and in fresh leukemia cells from patients.

METHODS

Apoptosis was assessed by means of DNA fragmentation analyses, TUNEL assays (i.e., deoxyuridine triphosphate labeling of DNA nicks with terminal deoxynucleotidyl transferase), and flow cytometry. Expression of the PML/RAR alpha fusion protein in leukemia cells was assessed by means of western blotting, ligand binding, and immunohistochemistry. Northern blotting and ribonuclease protection assays were used to evaluate changes in gene expression in response to retinoic acid and As2O3 treatment.

RESULTS AND CONCLUSIONS

As2O3 induces apoptosis without differentiation in retinoic acid-sensitive and retinoic acid-resistant APL cells at concentrations that are achievable in patients. As2O3 induces loss of the PML/RAR alpha fusion protein in NB4 cells, in retinoic-acid resistant cells derived from them, in fresh APL cells from patients, and in non-APL cells transfected to express this protein. As2O3 and retinoic acid induce different patterns of gene regulation, and they inhibit the phenotypes induced by each other. Understanding the molecular basis of these differences in the effects of As2O3 and retinoic acid may guide the clinical use of arsenic compounds and provide insights into the management of leukemias that do not respond to retinoic acid.

Regulation of expression of ferritin H-chain and transferrin receptor by protoporphyrin IX

The effect of protoporphyrin IX (hemin without iron) on the expression of transferrin receptor and ferritin was investigated in Friend leukemia cells. Cells treated with protoporphyrin IX exhibit enhanced transferrin-receptor expression and markedly reduced ferritin synthesis. Stimulation of transferrin-receptor expression is observed at both the mRNA and protein level. The effect on ferritin synthesis is mediated by translational inhibition of the mRNA, which, in contrast, is transcriptionally stimulated by protoporphyrin IX treatment. The regulation of transferrin receptor and ferritin in response to iron perturbations has been studied extensively and is mediated by the binding of iron-regulatory proteins (IRP) to the iron-responsive elements (IRE) present in the 3' and 5' untranslated regions of the transferrin-receptor and ferritin mRNA, respectively. To elucidate the molecular mechanisms underlying the effects of protoporphyrin IX on ferritin and transferrin-receptor expression, the role of the IRE sequence was investigated both in vivo by transfection experiments, with a construct containing the coding region for the chloramphenicol acetyltransferase (CAT) reporter gene under the translational control of the ferritin IRE, and in vitro by RNA band-shift assays. Whereas, examination of IRP binding to the IRE by in vitro assays suggests an apparent inactivation of IRP by protoporphyrin IX treatment, CAT assays indicate that protoporphyrin IX is able to induce in vivo a translational inhibition similar to that obtained by treatment with the iron chelator Desferal. This observation raises the possibility of different effects on the IRP activity exerted by porphyrin treatment in intact tissue-culture cells and in vitro. We conclude that translation of ferritin mRNA and degradation of transferrin-receptor mRNA are inhibited in intact tissue-culture cells by protoporphyrin IX through a mechanism similar to that exerted by iron chelation, thus involving depletion of the intracellular iron pool. These results can improve the understanding of the regulation of ferritin gene expression in some pathological conditions associated with disturbed heme synthesis.

Inhibition of lymphocyte blastogenic response in healthy donors treated with recombinant human granulocyte colony-stimulating factor (rhG-CSF): possible role of lactoferrin and interleukin-1 receptor antagonist

The effects of rhG-CSF on lymphocyte blastogenesis were evaluated in six healthy donors, submitted to progenitor cell mobilization for allogeneic transplantation. Neutrophil, monocyte and lymphocyte count increased 6.7-fold, 5.3-fold and 2.0-fold on day +4 of rhG-CSF as compared with baseline. The DNA stimulation index (DNA SI) of 72 h phytohemagglutinin (PHA)-treated cultures decreased from 20% (15-35.5) prior to rhG-CSF to 6.7% (1.5-11.9; P = 0.0026), 8% (4-12; P = 0.0091) and 15% (9-22; P = 0.0091) on days +2, +4 and +6; similarly, reactivity to concanavalin A decreased from 18% (12-20) to 1.8% (0.5-7; P < 0.01), 3% (2-8; P < 0.01) and 5% (2-11; P = 0.009). No changes of lymphocyte response to pokeweed mitogen were observed. DNA SI of PHA-treated cultures inversely correlated with neutrophil and monocyte count. IL-1 receptor antagonist (IL-1ra) and lactoferrin (Lf) plasma levels sharply increased and correlated with neutrophil and monocyte count. IL-10 increased five-fold on day +2, returned to pretreatment values thereafter and did not show any correlation with DNA SI, suggesting that it was not responsible for the observed phenomena. Interestingly, DNA SI of PHA-treated cultures inversely correlated with IL-1ra and Lf levels. CD3+ and CD19+ lymphocyte activation status, ie CD23, CD25, CD30 and HLA-DR coexpression, was not affected by rhG-CSF administration. Pharmacological doses of rhG-CSF in healthy donors inhibit lymphocyte blastogenesis via an increased production and/or release of immunoregulatory soluble mediators, ie IL-1ra and Lf, by primed neutrophils and monocytes.

Interleukins 6 and 8 as mediators of acute phase response in acute myocardial infarction

The present study provides evidence that interleukin (IL)-6 and IL-8 are the main endogenous mediators of acute phase response in patients with myocardial infarction. This conclusion was supported by the observation of a strict relation between IL-6 elevation and the extent of myocardial tissue damage and rise in body temperature.

Opposite effects of the acute promyelocytic leukemia PML-retinoic acid receptor alpha (RAR alpha) and PLZF-RAR alpha fusion proteins on retinoic acid signalling

Fusion proteins involving the retinoic acid receptor alpha (RAR alpha) and the PML or PLZF nuclear protein are the genetic markers of acute promyelocytic leukemias (APLs). APLs with the PML-RAR alpha or the PLZF-RAR alpha fusion protein are phenotypically indistinguishable except that they differ in their sensitivity to retinoic acid (RA)-induced differentiation: PML-RAR alpha blasts are sensitive to RA and patients enter disease remission after RA treatment, while patients with PLZF-RAR alpha do not. We here report that (i) like PML-RAR alpha expression, PLZF-RAR alpha expression blocks terminal differentiation of hematopoietic precursor cell lines (U937 and HL-60) in response to different stimuli (vitamin D3, transforming growth factor beta1, and dimethyl sulfoxide); (ii) PML-RAR alpha, but not PLZF-RAR alpha, increases RA sensitivity of hematopoietic precursor cells and restores RA sensitivity of RA-resistant hematopoietic cells; (iii) PML-RAR alpha and PLZF-RAR alpha have similar RA binding affinities; and (iv) PML-RAR alpha enhances the RA response of RA target genes (those for RAR beta, RAR gamma, and transglutaminase type II [TGase]) in vivo, while PLZF-RAR alpha expression has either no effect (RAR beta) or an inhibitory activity (RAR gamma and type II TGase). These data demonstrate that PML-RAR alpha and PLZF-RAR alpha have similar (inhibitory) effects on RA-independent differentiation and opposite (stimulatory or inhibitory) effects on RA-dependent differentiation and that they behave in vivo as RA-dependent enhancers or inhibitors of RA-responsive genes, respectively. Their different activities on the RA signalling pathway might underlie the different responses of PML-RAR alpha and PLZF-RAR alpha APLs to RA treatment. The PLZF-RAR alpha fusion protein contains an approximately 120-amino-acid N-terminal motif (called the POZ domain), which is also found in a variety of zinc finger proteins and a group of poxvirus proteins and which mediates protein-protein interactions. Deletion of the PLZF POZ domain partially abrogated the inhibitory effect of PLZF-RAR alpha on RA-induced differentiation and on RA-mediated type II TGase up-regulation, suggesting that POZ-mediated protein interactions might be responsible for the inhibitory transcriptional activities of PLZF-RAR alpha.