Functional p85alpha gene is required for normal murine fetal erythropoiesis

Probiotic microbes: Are their anti-melanogenicity and longevity promoting activities closely linked through the major "pathogenic" kinase PAK1?

PAK1-deficient mutant of C. elegans lives 60% longer than the wild-type. Interestingly, PAK1-deficient mutant of melanocytes produces less melanin (only a half compared with the wild-type) in the presence of either serum (PDGF) or α-MSH (alpha-melanocyte stimulating hormone). These observations indicate that the major "pathogenic" kinase PAK1 is responsible for both shortening the healthy lifespan, and PDGF/α-MSH-dependent melanogenesis. For screening of PAK1-blocking probiotic bacteria or their products, their anti-melanogenic as well as longevity promoting properties were examined. Recently it was found that C. elegans fed with Lactobacillus rhamnosus in Xinjiang cheese lives 40% longer than the worm fed with the standard E. coli. Interestingly, a Chinese traditional medicine called "ChiBai" fermented with the Lactobacillus rhamnosus also inhibited the α-MSH-induced melanogenesis, and this bacteria itself produces butyric acid that blocks the oncogenic HDAC (histone deacetylase)-PAK1 signaling pathway. These findings strongly suggest, if not proven, that anti-melanogenic activity of Lactobacillus and many other probiotic bacteria might serve as a reliable indicator for their longevity promoting activity. In this context, a popular Japanese Lactobacillus-fermented milk drink called "Calpis", developed a century ago, and recently proven to inhibit the melanogenesis by suppressing the PAK1-dependent tyrosinase gene expression, may potentially prolong our healthy lifespan.

Direct targets of pSTAT5 signalling in erythropoiesis

Erythropoietin (EPO) acts through the dimeric erythropoietin receptor to stimulate proliferation, survival, differentiation and enucleation of erythroid progenitor cells. We undertook two complimentary approaches to find EPO-dependent pSTAT5 target genes in murine erythroid cells: RNA-seq of newly transcribed (4sU-labelled) RNA, and ChIP-seq for pSTAT5 30 minutes after EPO stimulation. We found 302 pSTAT5-occupied sites: ~15% of these reside in promoters while the rest reside within intronic enhancers or intergenic regions, some >100kb from the nearest TSS. The majority of pSTAT5 peaks contain a central palindromic GAS element, TTCYXRGAA. There was significant enrichment for GATA motifs and CACCC-box motifs within the neighbourhood of pSTAT5-bound peaks, and GATA1 and/or KLF1 co-occupancy at many sites. Using 4sU-RNA-seq we determined the EPO-induced transcriptome and validated differentially expressed genes using dynamic CAGE data and qRT-PCR. We identified known direct pSTAT5 target genes such as Bcl2l1, Pim1 and Cish, and many new targets likely to be involved in driving erythroid cell differentiation including those involved in mRNA splicing (Rbm25), epigenetic regulation (Suv420h2), and EpoR turnover (Clint1/EpsinR). Some of these new EpoR-JAK2-pSTAT5 target genes could be used as biomarkers for monitoring disease activity in polycythaemia vera, and for monitoring responses to JAK inhibitors.

Combined inhibition of PI3K and activation of MAPK p38 signaling pathways trigger erythroid alternative splicing switch of 4.1R pre-mRNA in DMSO-induced erythroleukemia cells

There is increasing evidence showing that many extracellular cues modulate pre-mRNA alternative splicing, through different signaling pathways. We here show that 4.1R exon 16 splicing is altered in response to specific signals. The switch from erythroblastic isoform lacking exon 16 to mature erythrocytic isoform containing this exon is tightly regulated during late erythroid differentiation, and blocage of this splicing switch in erythroleukemia cells is seen as a consequence of the deregulation of important regulatory pathways. We support that combined inhibition of PI3K and activation of p38 signaling pathways impinge on erythroid 4.1R pre-mRNA alternative splicing switch, and on cell differentiation as witnessed by hemoglobin production. By contrast, MEK/ERK signaling appeared not to affect neither cell hemoglobin production nor erythroid 4.1R pre-mRNA splicing. We also found that the signal-induced alternative splicing is not typically distinctive of EPO-non-responsive cells, but operates in EPO-responsive cells as well. Pre-mRNA splicing is a major regulatory mechanism at the crossroad between transcription and translation. We here provide evidence that inhibition of PI3K activates the splicing switch in a promoter-dependent manner, whereas p38 activation induces this event in a promoter-independent fashion. Our data further support that constitutive activation of EPO-R by the viral protein gp55 and the short form of the tyrosine kinase receptor Stk, transduces PI3K proliferation signal, but not MAPK p38 differentiation signal. Concurrently, this work lend credence to the concept that DMSO triggers transient activation of p38 signaling and irreversible inhibition of PI3K/AKT signaling pathway, hence uncovering an old conundrum regarding the mechanism by which DMSO induces erythroleukemia cell differentiation.

Defining an EPOR- regulated transcriptome for primary progenitors, including Tnfr-sf13c as a novel mediator of EPO- dependent erythroblast formation

Certain concepts concerning EPO/EPOR action modes have been challenged by in vivo studies: Bcl-x levels are elevated in maturing erythroblasts, but not in their progenitors; truncated EPOR alleles that lack a major p85/PI3K recruitment site nonetheless promote polycythemia; and Erk1 disruption unexpectedly bolsters erythropoiesis. To discover novel EPO/EPOR action routes, global transcriptome analyses presently are applied to interrogate EPO/EPOR effects on primary bone marrow-derived CFUe-like progenitors. Overall, 160 EPO/EPOR target transcripts were significantly modulated 2-to 21.8-fold. A unique set of EPO-regulated survival factors included Lyl1, Gas5, Pim3, Pim1, Bim, Trib3 and Serpina 3g. EPO/EPOR-modulated cell cycle mediators included Cdc25a, Btg3, Cyclin-d2, p27-kip1, Cyclin-g2 and CyclinB1-IP-1. EPO regulation of signal transduction factors was also interestingly complex. For example, not only Socs3 plus Socs2 but also Spred2, Spred1 and Eaf1 were EPO-induced as negative-feedback components. Socs2, plus five additional targets, further proved to comprise new EPOR/Jak2/Stat5 response genes (which are important for erythropoiesis during anemia). Among receptors, an atypical TNF-receptor Tnfr-sf13c was up-modulated >5-fold by EPO. Functionally, Tnfr-sf13c ligation proved to both promote proerythroblast survival, and substantially enhance erythroblast formation. The EPOR therefore engages a sophisticated set of transcriptome response circuits, with Tnfr-sf13c deployed as one novel positive regulator of proerythroblast formation.

Class I phosphoinositide 3-kinases in normal and pathologic hematopoietic cells

Class I phosphoinositide 3-kinases which produce the D3-phosphoinositide second messenger phosphatidylinositol 3,4,5-trisphosphate in response to membrane receptors activation play a critical role in cell proliferation, survival, metabolism, and motility. These lipid kinases and the phosphatases regulating the level of D3-phosphoinositides have been an intense area of research these last two decades. The class I phosphoinositide 3-kinases signaling is found aberrantly activated in numerous human cancers, including in malignant hemopathies, and are important therapeutic targets for cancer therapy. Haematopoiesis is an ongoing process which generates the distinct blood cell types from a common hematopoietic stem cell through the action of a variety of cytokines. In the human adult hematopoiesis occurs primarily in the bone marrow, and defects in hematopoiesis result in diseases, such as anemia, thrombocytopenia, myeloproliferative syndromes, or leukemia. Here we give a brief overview of the role of class I phosphoinositide 3-kinases in hematopoietic stem cells, in hematopoietic lineage development and in leukemia, particularly in acute myeloid leukemia and summarize the potential therapeutic implications.

From stem cell to red cell: regulation of erythropoiesis at multiple levels by multiple proteins, RNAs, and chromatin modifications

This article reviews the regulation of production of RBCs at several levels. We focus on the regulated expansion of burst-forming unit-erythroid erythroid progenitors by glucocorticoids and other factors that occur during chronic anemia, inflammation, and other conditions of stress. We also highlight the rapid production of RBCs by the coordinated regulation of terminal proliferation and differentiation of committed erythroid colony-forming unit-erythroid progenitors by external signals, such as erythropoietin and adhesion to a fibronectin matrix. We discuss the complex intracellular networks of coordinated gene regulation by transcription factors, chromatin modifiers, and miRNAs that regulate the different stages of erythropoiesis.

Modern Approaches to Treating Acute Promyelocytic Leukemia

The advent of all-trans-retinoic acid (ATRA) and its combination with anthracycline-containing chemotherapy have contributed in the past 2 decades to optimize the antileukemic efficacy in acute promyelocytic leukemia (APL), leading to complete remission rates greater than 90%, virtual absence of resistance, and cure rates of nearly 80%. Recently reported studies from large cooperative trials have also shown that more rational delivery of treatment and improved outcomes may derive from the use of risk-adapted protocols. In particular, patients at higher risk of relapse (ie, those presenting with WBC > 10 × 109/L) seem to benefit from treatments that include cytarabine in the ATRA-plus-chemotherapy scheme, whereas patients with standard-risk disease can be successfully managed with less-intensive regimens that contain ATRA and anthracycline-based chemotherapy. After the outstanding results with arsenic trioxide (ATO) in the treatment of APL relapse, several experimental trials have been designed to explore the role of ATO in front-line therapy with the aim not only of minimizing the use of chemotherapy but also to reinforce standard ATRA-plus-chemotherapy regimens and additionally improve therapeutic efficacy. In this review article, we discuss most recent advances in the treatment of patients with newly diagnosed and relapsed APL.

Regulatory subunits of class IA PI3K

All class I PI3K enzymes are obligate heterodimers, consisting of a catalytic subunit tightly bound to a regulatory subunit. The regulatory subunit influences the subcellular location, binding partners, and activity of the catalytic subunit. Regulatory subunits also possess adaptor functions in cellular signaling, which are largely independent of their role in regulating PI3K activity. This chapter reviews the structure and function of PI3K regulatory subunits, focusing on the class IA subgroup.

Erythropoietin receptor response circuits

PURPOSE OF REVIEW

In 1985-1989, erythropoietin (EPO), its receptor (EPOR), and janus kinase 2 were cloned; established to be essential for definitive erythropoiesis; and initially intensely studied. Recently, new impetus, tools, and model systems have emerged to re-examine EPO/EPOR actions, and are addressed in this review. Impetus includes indications that EPO affects significantly more than standard erythroblast survival pathways, the development of novel erythropoiesis-stimulating agents, increasing evidence for EPO/EPOR cytoprotection of ischemically injured tissues, and potential EPO-mediated worsening of tumorigenesis.

RECENT FINDINGS

New findings are reviewed in four functional contexts: (pro)erythroblast survival mechanisms, new candidate EPO/EPOR effects on erythroid cell development and new EPOR responses, EPOR downmodulation and trafficking, and novel erythropoiesis-stimulating agents.

SUMMARY

As Current Opinion, this monograph seeks to summarize, and provoke, new EPO/EPOR action concepts. Specific problems addressed include: beyond (and before) BCL-XL, what key survival factors are deployed in early-stage proerythroblasts? Are distinct EPO/EPOR signals transduced in stage-selective fashions? Is erythroblast proliferation also modulated by EPO/EPOR signals? What functions are subserved by new noncanonical EPO/EPOR response factors (e.g. podocalyxin like-1, tribbles 3, reactive oxygen species, and nuclear factor kappa B)? What key regulators mediate EPOR inhibition and trafficking? And for emerging erythropoiesis-stimulating agents, to what extent do activities parallel EPOs (or differ in advantageous, potentially complicating ways, or both)?

Role of phosphatidylinositol 3-kinase in friend spleen focus-forming virus-induced erythroid disease

Infection of erythroid cells by Friend spleen focus-forming virus (SFFV) leads to acute erythroid hyperplasia in mice due to expression of its unique envelope glycoprotein, gp55. Erythroid cells expressing SFFV gp55 proliferate in the absence of their normal regulator, erythropoietin (Epo), because of interaction of the viral envelope protein with the erythropoietin receptor and a short form of the receptor tyrosine kinase Stk (sf-Stk), leading to constitutive activation of several signal transduction pathways. Our previous in vitro studies showed that phosphatidylinositol 3-kinase (PI3-kinase) is activated in SFFV-infected cells and is important in mediating the biological effects of the virus. To determine the role of PI3-kinase in SFFV-induced disease, mice deficient in the p85alpha regulatory subunit of class IA PI3-kinase were inoculated with different strains of SFFV. We observed that p85alpha status determined the extent of erythroid hyperplasia induced by the sf-Stk-dependent viruses SFFV-P (polycythemia-inducing strain of SFFV) and SFFV-A (anemia-inducing strain of SFFV) but not by the sf-Stk-independent SFFV variant BB6. Our data also indicate that p85alpha status determines the response of mice to stress erythropoiesis, consistent with a previous report showing that SFFV uses a stress erythropoiesis pathway to induce erythroleukemia. We further showed that sf-Stk interacts with p85alpha and that this interaction depends upon sf-Stk kinase activity and tyrosine 436 in the multifunctional docking site. Pharmacological inhibition of PI3-kinase blocked proliferation of primary erythroleukemia cells from SFFV-infected mice and the erythroleukemia cell lines derived from them. These results indicate that p85alpha may regulate sf-Stk-dependent erythroid proliferation induced by SFFV as well as stress-induced erythroid hyperplasia.

beta-Thalassemia: HiJAKing Ineffective Erythropoiesis and Iron Overload

β-thalassemia encompasses a group of monogenic diseases that have in common defective synthesis of β-globin. The defects involved are extremely heterogeneous and give rise to a large phenotypic spectrum, with patients that are almost asymptomatic to cases in which regular blood transfusions are required to sustain life. As a result of the inefficient synthesis of β-globin, the patients suffer from chronic anemia due to a process called ineffective erythropoiesis (IE). The sequelae of IE lead to extramedullary hematopoiesis (EMH) with massive splenomegaly and dramatic iron overload, which in turn is responsible for many of the secondary pathologies observed in thalassemic patients. The processes are intimately linked such that an ideal therapeutic approach should address all of the complications. Although β-thalassemia is one of the first monogenic diseases to be described and represents a global health problem, only recently has the scientific community started to focus on the real molecular mechanisms that underlie this disease, opening new and exciting therapeutic perspectives for thalassemic patients worldwide.

Nuclear translocation of active AKT is required for erythroid differentiation in erythropoietin treated K562 erythroleukemia cells

Erythroid differentiation of human erythroleukemia cell line K562 induced by erythropoietin is a complex process that involves modifications at nuclear level, including nuclear translocation of phosphatidyl-inositol 3-kinase. In this work we show that erythropoietin stimulation of K562 cells can induce nuclear translocation of active Akt, a downstream molecule of the phosphatidyl-inositol 3-kinase signaling pathway. Akt shows a peak of activity in whole cell homogenates at earlier stage when compared to the nucleus, which shows a peak delayed of 10 min. Akt increases its intranuclear amount and activity rapidly and transiently in response to EPO. Almost all Akt kinase that translocates to the nucleus shows a marked phosphorylation on serine 473. Nuclear enzyme translocation is blocked by the phosphatidyl-inositol 3-kinase inhibitor Ly294002 or Wortmannin. The specific Akt pharmacological inhibitor VI, VII and VIII that act as blocking enzyme activation inhibited translocation as well, whereas Akt inhibitor IX, that inhibits Akt activity, did not block Akt nuclear translocation. When cells were treated by means of siRNA sequences or with the Akt inhibitors the differentiation process was arrested, thus showing the requirement of the nuclear translocation of the active enzyme to differentiate. These findings strongly suggest that the intranuclear translocation of active Akt kinase represents an important step in the signaling pathway that mediates erythropoietin-induced erythroid differentiation.

Differential regulation of Foxo3a target genes in erythropoiesis

The cooperation of stem cell factor (SCF) and erythropoietin (Epo) is required to induce renewal divisions in erythroid progenitors, whereas differentiation to mature erythrocytes requires the presence of Epo only. Epo and SCF activate common signaling pathways such as the activation of protein kinase B (PKB) and the subsequent phosphorylation and inactivation of Foxo3a. In contrast, only Epo activates Stat5. Both Foxo3a and Stat5 promote erythroid differentiation. To understand the interplay of SCF and Epo in maintaining the balance between renewal and differentiation during erythroid development, we investigated differential Foxo3a target regulation by Epo and SCF. Expression profiling revealed that a subset of Foxo3a targets was not inhibited but was activated by Epo. One of these genes was Cited2. Transcriptional control of Epo/Foxo3a-induced Cited2 was studied and compared with that of the Epo-repressed Foxo3a target Btg1. We show that in response to Epo, the allegedly growth-inhibitory factor Foxo3a associates with the allegedly growth-stimulatory factor Stat5 in the nucleus, which is required for Epo-induced Cited2 expression. In contrast, Btg1 expression is controlled by the cooperation of Foxo3a with cyclic AMP- and Jun kinase-dependent Creb family members. Thus, Foxo3a not only is an effector of PKB but also integrates distinct signals to regulate gene expression in erythropoiesis.

Phosphoinositide 3-kinase signalling regulates early development and developmental haemopoiesis

Phosphoinositide 3-kinase (PI3K)-dependent signalling regulates a wide variety of cellular functions including proliferation and differentiation. Disruption of class I(A) PI3K isoforms has implicated PI3K-mediated signalling in development of the early embryo and lymphohaemopoietic system. We have used embryonic stem (ES) cells as an in vitro model to study the involvement of PI3K-dependent signalling during early development and haemopoiesis. Both pharmacological inhibition and genetic manipulation of PI3K-dependent signalling demonstrate that PI3K-mediated signals, most likely via 3-phosphoinositide-dependent protein kinase 1 (PDK1), are required for proliferation of cells within developing embryoid bodies (EBs). Surprisingly, the haemopoietic potential of EB-derived cells was not blocked upon PI3K inhibition but rather enhanced, correlating with modest increases in expression of haemopoietic marker genes. By contrast, PDK1-deficient EB-derived progeny failed to generate terminally differentiated haemopoietic lineages. This deficiency appeared to be due to a requirement for PI3K signalling during the proliferative phase of blast-colony-forming cell (BL-CFC) expansion, rather than as a result of effects on differentiation per se. We also demonstrate that PI3K-dependent signalling is required for optimal generation of erythroid and myeloid progenitors and their differentiation into mature haemopoietic colony types. These data demonstrate that PI3K-dependent signals play important roles at different stages of haemopoietic development.

Abnormal hematopoiesis in Gab2 mutant mice

Gab2 is an important adapter molecule for cytokine signaling. Despite its major role in signaling by receptors associated with hematopoiesis, the role of Gab2 in hematopoiesis has not been addressed. We report that despite normal numbers of peripheral blood cells, bone marrow cells, and c-Kit(+)Lin(-)Sca-1(+) (KLS) cells, Gab2-deficient hematopoietic cells are deficient in cytokine responsiveness. Significant reductions in the number of colony-forming units in culture (CFU-C) in the presence of limiting cytokine concentrations were observed, and these defects could be completely corrected by retroviral complementation. In earlier hematopoiesis, Gab2-deficient KLS cells isolated in vitro responded poorly to hematopoietic growth factors, resulting in an up to 11-fold reduction in response to a cocktail of stem cell factor, flt3 ligand, and thrombopoietin. Gab2-deficient c-Kit(+)Lin(-) cells also demonstrate impaired activation of extracellular signal-regulated kinase (ERK) and S6 in response to IL-3, which supports defects in activating the phosphatidylinositol-3 kinase (PI-3K) and mitogen-associated protein kinase (MAPK) signaling cascades. Associated with the early defects in cytokine response, competitive transplantation of Gab2(-/-) bone marrow cells resulted in defective long-term multilineage repopulation. Therefore, we demonstrate that Gab2 adapter function is intrinsically required for hematopoietic cell response to early-acting cytokines, resulting in defective hematopoiesis in Gab2-deficient mice.

K-ras is critical for modulating multiple c-kit-mediated cellular functions in wild-type and Nf1+/- mast cells

p21(ras) (Ras) proteins and GTPase-activating proteins (GAPs) tightly modulate extracellular growth factor signals and control multiple cellular functions. The specific function of each Ras isoform (H, N, and K) in regulating distinct effector pathways, and the role of each GAP in negatively modulating the activity of each Ras isoform in myeloid cells and, particularly, mast cells is incompletely understood. In this study, we use murine models of K-ras- and Nf1-deficient mice to examine the role of K-ras in modulating mast cell functions and to identify the role of neurofibromin as a GAP for K-ras in this lineage. We find that K-ras is required for c-kit-mediated mast cell proliferation, survival, migration, and degranulation in vitro and in vivo. Furthermore, the hyperactivation of these cellular functions in Nf1(+/-) mast cells is decreased in a K-ras gene dose-dependent fashion in cells containing mutations in both loci. These findings identify K-ras as a key effector in multiple mast cell functions and identify neurofibromin as a GAP for K-ras in mast cells.

AKT induces erythroid-cell maturation of JAK2-deficient fetal liver progenitor cells and is required for Epo regulation of erythroid-cell differentiation

AKT serine threonine kinase of the protein kinase B (PKB) family plays essential roles in cell survival, growth, metabolism, and differentiation. In the erythroid system, AKT is known to be rapidly phosphorylated and activated in response to erythropoietin (Epo) engagement of Epo receptor (EpoR) and to sustain survival signals in cultured erythroid cells. Here we demonstrate that activated AKT complements EpoR signaling and supports erythroid-cell differentiation in wild-type and JAK2-deficient fetal liver cells. We show that erythroid maturation of AKT-transduced cells is not solely dependent on AKT-induced cell survival or proliferation signals, suggesting that AKT transduces also a differentiation-specific signal downstream of EpoR in erythroid cells. Down-regulation of expression of AKT kinase by RNA interference, or AKT activity by expression of dominant negative forms, inhibits significantly fetal liver-derived erythroid-cell colony formation and gene expression, demonstrating that AKT is required for Epo regulation of erythroid-cell maturation.

Genetic reduction of class IA PI-3 kinase activity alters fetal hematopoiesis and competitive repopulating ability of hematopoietic stem cells in vivo

Class I(A) phosphatidylinositol-3 kinase (PI-3K) is a lipid kinase, which is activated in blood cells by hematopoietic growth factors. In vitro experiments using chemical inhibitors of PI-3K suggest that this kinase is potentially important for hematopoietic stem and progenitor cell (HSC/P) function, and recent studies identify PI-3K as a therapeutic target in treating different leukemias and lymphomas. However, the role of PI-3K in regulating fetal liver or adult hematopoiesis in vivo is unknown. Therefore, we examined PI-3K-deficient embryos generated by a targeted deletion of the p85alpha and p85beta regulatory subunits of PI-3K (p85alpha-/-p85beta+/-). The absolute frequency and number of hematopoietic progenitor cells were reduced in p85alpha-/- p85beta+/- fetal livers compared with wild-type (WT) controls. Further, p85alpha-/-p85beta+/- fetal liver hematopoietic stem cells (HSCs) had decreased multilineage repopulating ability in vivo compared with WT controls in competitive repopulation assays. Finally, purified p85alpha-/-p85beta+/- c-kit+ cells had a decrease in proliferation in response to kit ligand (kitL), a growth factor important for controlling HSC function in vivo. Collectively, these data identify PI-3K as an important regulator of HSC function and potential therapeutic target in treating leukemic stem cells.

Erythropoietin stimulates phosphorylation and activation of GATA-1 via the PI3-kinase/AKT signaling pathway

Erythropoietin (Epo) stimulation of its receptor's downstream signaling pathways and optimum function of GATA-1 transcription factor are both essential for normal erythroid cell development. Epo-receptor (EpoR) signaling and GATA-1 regulate proliferation, survival, differentiation, and maturation of erythroid cells. Whether any signal that is generated by EpoR targets GATA-1 or affects GATA-1 transcriptional activity is not known. Here, we demonstrate that stimulation of EpoR results in phosphorylation of GATA-1 at serine 310 (S310) in primary fetal liver erythroid progenitors and in cultured erythroid cells. We show that phosphorylation of GATA-1 is important for Epo-induced maturation of fetal liver erythroid progenitor cells. The PI3-kinase/AKT signaling pathway is identified as a mediator of Epo-induced phosphorylation of GATA-1. AKT serine threonine kinase phosphorylates GATA-1S310 in vitro and in erythroid cells and enhances GATA-1 transcriptional activity. These data demonstrate that EpoR signaling phosphorylates GATA-1 and modulates its activity via the PI3-kinase/AKT signaling pathway.

Selective role of a distinct tyrosine residue on Tie2 in heart development and early hematopoiesis

The development of the cardiovascular system and the development of the early hematopoietic systems are closely related, and both require signaling through the Tie2 receptor tyrosine kinase. Although endothelial cells and hematopoietic cells as well as their precursors share common gene expression patterns during development, it remains completely unknown how Tie2 signaling coordinately regulates cardiovascular development and early hematopoiesis in vivo. We show here that mice with a targeted mutation in tyrosine residue 1100 in the carboxyl-terminal tail of Tie2 display defective cardiac development and impaired hematopoietic and endothelial cell development in the paraaortic splanchnopleural mesoderm similar to that seen in Tie2-null mutant mice. Surprisingly, however, unlike Tie2-null mutant mice, mice deficient in signaling through this tyrosine residue show a normal association of perivascular cells with nascent blood vessels. These studies are the first to demonstrate the physiological importance of a single tyrosine residue in Tie2, and they suggest that multiple tyrosine residues in the receptor may coordinate cardiovascular development and early hematopoietic development.

Turning cells red: signal transduction mediated by erythropoietin

Erythropoietin (EPO) is the crucial cytokine regulator of red blood-cell production. Since the discovery of EPO in 1985 and the isolation of its cognate receptor four years later, there has been significant interest in understanding the unique ability of this ligand-receptor pair to promote erythroid mitogenesis, survival and differentiation. The development of knockout mice has elucidated the precise role of the ligand, receptor and downstream players in murine erythroid development. In this review, we summarize EPO-mediated signaling pathways and examine their significance in vivo.

K-Ras is essential for normal fetal liver erythropoiesis

In vitro studies suggest that Ras activation is necessary for erythroid cell development. However, genetic inactivation of the Ras isoforms H-Ras, N-Ras, and K-Ras in mice reportedly did not affect adult or fetal erythropoiesis, though K-Ras-/- embryos were anemic. Given these discrepancies, we performed a more detailed analysis of fetal erythropoiesis in K-Ras-/- embryos. Day-13.5 K-Ras-/- embryos were pale with a marked reduction of mature erythrocytes in their fetal livers. The frequency and number of both early (erythroid burst-forming unit [BFU-E]) and late erythroid progenitors (erythroid colony-forming unit [CFU-E]) were reduced in K-Ras-/- fetal livers compared with wild-type controls and displayed a delay in terminal erythroid cell maturation. Further, K-Ras-/- hematopoietic progenitors had reduced proliferation in response to erythropoietin and Kit ligand compared with control cells. Thus, these studies identify K-Ras as a unique Ras isoform that is essential for regulating fetal erythropoiesis in vivo.

Role of c-Kit and erythropoietin receptor in erythropoiesis

Erythropoiesis is regulated by a number of growth factors, among which stem cell factor (SCF) and erythropoietin (Epo) play a non-redundant function. Viable mice with mutations in the SCF gene (encoded by the Steel (Sl) locus), or its receptor gene c-Kit (encoded by the White spotting (W) locus) develop a hypoplastic macrocytic anemia. Mutants of W or Sl that are completely devoid of c-Kit or SCF expression die in utero of anemia between days 14 and 16 of gestation and contain reduced numbers of erythroid progenitors in the fetal liver. Likewise, Epo and Epo receptor (Epo-R)-deficient mice die in utero due to a marked reduction in the number of committed fetal liver derived erythroid progenitors. Thus, committed erythroid progenitors require both c-Kit and Epo-R signal transduction pathways for their survival, proliferation and differentiation. In vitro, Epo alone is capable of generating mature erythroid progenitors; however, a combined treatment of Epo and SCF results in synergistic proliferation and expansion of developing erythroid progenitors. This review summarizes recent advances made towards understanding the signaling mechanisms by which Epo-R and c-Kit regulate growth, survival, and differentiation of erythroid progenitors alone and cooperatively.

Phase II study of paclitaxel and estramustine in patients with recurrent and refractory non-Hodgkin lymphoma

BACKGROUND

The current study was conducted to evaluate the efficacy of paclitaxel, administered weekly or once every 3 weeks, in combination with oral estramustine phosphate (EMP) in patients with recurrent or refractory aggressive non-Hodgkin lymphoma (NHL).

METHODS

Between February 1996 and February 2001, 23 patients with recurrent NHL were enrolled onto this Phase II trial. The median age for all patients was 65 years (range, 27-80 years). The initial 12 patients (who received a mean number of 2.4 prior treatments, including 1 patient who received a prior peripheral blood stem cell transplant) received paclitaxel at a dose of 175 mg/m2 given as a 3-hour intravenous infusion every 21 days. The next 11 patients (who received a mean number of 2.8 prior treatments, including 1 patient who received prior peripheral blood stem cell transplant) were registered (1 patient refused treatment) to receive paclitaxel at a dose of 80 mg/m2 as a 1-hour intravenous infusion weekly for 6 weeks of an 8-week cycle. All patients received EMP at a dose of 600 mg/m2 orally per day beginning the day prior to each dose of paclitaxel for a total of 3 days.

RESULTS

When paclitaxel was administered every 21 days, 4 partial responses were observed in 12 evaluable patients (33.3%). The median survival was 147 days. The median duration of response was 102 days (range, 42-127 days) and the median time to disease progression was 66 days. Grade 3 and Grade 4 neutropenia (according to the revised version of the Common Toxicity Criteria of the National Cancer Institute) were observed in 5 patients (42%) in this group. In an attempt to reduce the incidence of myelosuppression, paclitaxel dosing was changed to weekly dosing. In the cohort of patients receiving weekly paclitaxel, an objective response was reported to occur in 3 (1 complete response and 2 partial responses) of 11 evaluable patients (27%). The median survival was 132 days (range, 33-462 days). The median duration of response was 64 days and the median time to disease progression was 57 days. There was no significant difference noted between the cohort receiving paclitaxel three times weekly and those receiving paclitaxel weekly with regard to overall survival and time to disease progression (P = 0.7 and P = 0.8, respectively by the log-rank test). Grade 3 or 4 neutropenia was observed in only 2 of 11 patients (18%) in the weekly paclitaxel group. There were no significant differences noted in terms of thrombocytopenia, anemia, nausea, anorexia, or fatigue between the treatment groups.

CONCLUSIONS

Paclitaxel given once weekly or three times weekly, in combination with oral EMP, was found to have comparable efficacy in patients with recurrent NHL, with an overall response rate of 30%. The response rate was found to be higher than that reported in prior studies of paclitaxel as a single agent in the treatment of NHL, suggesting that EMP may enhance paclitaxel efficacy in patients with NHL. Hematologic toxicity was diminished when paclitaxel was administered on a weekly schedule. The minimal myelotoxicity of weekly paclitaxel makes this a potentially attractive agent for combination regimens for patients with recurrent/refractory NHL.

New Insights Into Erythropoietin and Epoetin Alfa: Mechanisms of Action, Target Tissues, and Clinical Applications

Recombinant human erythropoietin (epoetin alfa) has proven beneficial for the treatment of various anemias. The mechanism of action of endogenous erythropoietin and the therapeutic use of epoetin alfa to stimulate red blood cell production and improve the quality of life in cancer patients are reviewed here. Epoetin alfa may also attenuate the cognitive dysfunction associated with cancer therapy. Interestingly, functional endogenous erythropoietin receptor signaling pathways have been demonstrated in numerous nonerythropoietic tissues. Of particular importance, epoetin alfa confers neurotrophic and neuroprotective effects in cultured neurons and in several animal models for neurologic disease. In one clinical trial, epoetin alfa appeared to limit functional and histologic damage in patients with stroke. Therefore, in cancer patients receiving chemotherapy, the beneficial effects of epoetin alfa could be mediated not only through enhanced erythrocyte production but also via direct effects on the nervous system. Further investigation into the nonerythropoietic effects of epoetin alfa could broaden its clinical utility for patients with cancer and also provide new therapies for various neurologic disorders.