Direct and Indirect Effects of Oxymetholone on Growth and Apoptosis of Hematopoietic Progenitor Cells In Vitro.

Blood ◽  
2004 ◽  
Vol 104 (11) ◽  
pp. 4127-4127
Author(s):  
Deog-Yeon Jo ◽  
Seong-Woo Kim ◽  
Jin-Hee Hwang ◽  
Hwan-Jung Yun ◽  
Samyong Kim
Keyword(s):  
Bone Marrow ◽  
Progenitor Cells ◽  
Indirect Effects ◽  
Clonal Growth ◽  
Hematopoietic Progenitor Cells ◽  
Direct And Indirect Effects ◽  
Hematopoietic Progenitor ◽  
Colony Forming Unit ◽  
Cd34 Cells

Abstract Despite a tarnished reputation, androgens remain a common treatment for aplastic anemia, especially in the Orient. Increased apoptosis of hematopoietic cells is characteristic of aplastic anemia; however, it has not been established whether androgens affect apoptosis of hematopoietic progenitor cells. In most previous studies regarding the in vitro effects of androgens on hematopoietic cells, whole bone marrow cells were used, rather than purified hematopoietic progenitor cells such as CD34+ cells. With these questions in mind, we investigated the direct and indirect effects of oxymetholone and other androgens on apoptosis and growth of normal hematopoietic progenitor cells (HPCs) in vitro. Oxymetholone did not rescue normal BM CD34+ cells and colony-forming cells (CFCs), other than mature erythroid CFCs, from apoptosis induced by growth factor deprivation. Unexpectedly, both testosterone and 5-dihydrotestosterone (5-DHT) at a concentration of 10−5 M, but not oxymetholone, increased the percentage of annexin-positive apoptotic cells (62.2 ± 5.9%, P < 0.05; 61.7 ± 6.4%, P < 0.05, respectively) compared with the controls (52.6 ± 5.6%). The addition of either stromal cell-derived factor-1 (SDF-1) or stem cell factor (SCF) partially relieved the increase in apoptosis induced by 5-DHT, and the addition of both SDF-1 and SCF completely reversed it. Oxymetholone did not rescue CFCs from interferon-gamma (IFN-g)-induced inhibition of clonal growth of BM CD34+ cells in methylcellulose cultures. Furthermore, oxymetholone did not mitigate IFN-g-induced suppression of CD34+ cell survival in the presence of growth factors. In a methylcellulose clonogenic assay, oxymetholone stimulated the clonal growth of colony-forming unit-erythroid at low concentrations, while not affecting colony-forming unit-granulocyte/macrophage or burst-forming unit-erythroid. Oxymetholone did not reverse the IFN-g-induced inhibition of colony formation by CD34+ cells. Interestingly, oxymetholone stimulated the production of SCF and thrombopoietin in normal human bone marrow stromal cells (BMSCs) through transcriptional regulation while inhibiting the production of interleukin-6. In agreement with this, oxymetholone-treated BMSCs better supported the survival and growth of HPCs. These results suggest that oxymetholone exerts most of its myelostimulatory effects via the regulation of cytokine production in BMSCs, rather than by direct action on hematopoietic progenitor cells.

scholarly journals Direct and Indirect Effects of Androgens on Survival of Hematopoietic Progenitor Cells In Vitro

2005 ◽  
Vol 20 (3) ◽  
pp. 409 ◽  
Author(s):  
Seong-Woo Kim ◽  
Jin-Hee Hwang ◽  
Jae-Min Cheon ◽  
Nam-Sook Park ◽  
Sang-Eun Park ◽  
...  
Keyword(s):  
Progenitor Cells ◽  
Indirect Effects ◽  
Hematopoietic Progenitor Cells ◽  
Direct And Indirect Effects ◽  
Hematopoietic Progenitor

Interferon-gamma Has Dual Potential in Inhibiting or Promoting Survival and Growth of Haematopoietic Progenitors: Interactions with Stromal Cell-Derived Factor-1.

Blood ◽  
2004 ◽  
Vol 104 (11) ◽  
pp. 4216-4216
Author(s):  
Deog-Yeon Jo ◽  
Jin-Hee Hwang ◽  
Seong-Woo Kim ◽  
Hwan-Jung Yun ◽  
Samyong Kim
Keyword(s):  
Bone Marrow ◽  
Growth Factor ◽  
Progenitor Cells ◽  
Clonal Growth ◽  
Hematopoietic Progenitor Cells ◽  
Hematopoietic Progenitor ◽  
Cd34 Cells ◽  
Survival And Growth ◽  
Ifn Γ ◽  
Stromal Cell Derived Factor

Abstract Despite many apparently contradictory reports, IFN-γ is well known as a hematopoiesis-inhibitory cytokine and is implicated in the pathogenesis of aplastic anemia. It has been shown that IFN-γ intrinsically and simultaneously induces specific and conflicting signalling pathways and transcriptional programs that contribute to the potential dual effects of IFN-γ in promoting or inhibiting proliferation of murine pro-B cells (Asao et al, J Biol Chem275: 867, 2000). We explored the dual roles of IFN-γ in the inhibition or promotion of the survival and growth of hematopoietic progenitors, especially with regard to interactions with stromal cell-derived factor-1 (SDF-1). IFN-γ partially rescued normal bone marrow CD34+ cells and colony-forming cells from apoptosis induced by serum- and growth factor-deprivation. SDF-1 further enhanced cell survival, in synergy with IFN-γ. Short-term IFN-γ treatment of CD34+ cells enhanced the clonal growth of the cells in synergy with SDF-1. In contrast, IFN-γ inhibited the clonal growth of hematopoietic progenitor cells in a standard methylcellulose clonogenic assay and inhibited the growth factor-mediated survival of normal CD34+ cells, CD34+ cells from patients with chronic myeloid leukaemia, and MO7e cells. The addition of SDF-1 did not alter these outcomes. IFN-γ did not activate PI3K/Akt, enhance SDF-1-induced activation of PI3K/Akt, or up-regulate the expression of CXCR4 or its chemotactic function in bone marrow CD34+ cells. IFN-γ up-regulated the expression of Socs1 mRNA in CD34+ cells and MO7e cells. Referring to a previous report describing that Socs1 binds to multiple signalling proteins and suppresses stem cell factor-dependent proliferation of bone marrow-derived mast cells (De Sepulveda et al, EMBO J 18: 904,1999), it is suggested that IFN-γ-induced inhibition of the growth factor-dependent survival of CD34+ cells might be mediated, at least in part, through the induction of Socs1. Paradoxically, IFN-γ down-regulated SDF-1 production in primary human bone marrow stromal cells. These results indicate that IFN-γ, partly in concert with SDF-1, exerts dual effects on the survival and growth of hematopoietic progenitor cells; the effects of IFN-γ on hematopoietic progenitor cells can differ depending on the particular in vitro experimental conditions, especially the presence of hematopoietic growth factors.

scholarly journals Orderly Process of Sequential Cytokine Stimulation Is Required for Activation and Maximal Proliferation of Primitive Human Bone Marrow CD34+ Hematopoietic Progenitor Cells Residing in G0

Blood ◽  
1997 ◽  
Vol 90 (2) ◽  
pp. 658-668 ◽  
Author(s):  
Amy C. Ladd ◽  
Robert Pyatt ◽  
Andre Gothot ◽  
Susan Rice ◽  
Jon McMahel ◽  
...  
Keyword(s):  
Cell Cycle ◽  
Bone Marrow ◽  
Phase I ◽  
Progenitor Cells ◽  
Phase Ii ◽  
Hematopoietic Progenitor Cells ◽  
Hematopoietic Progenitor ◽  
Cd34 Cells ◽  
Cytokine Stimulation ◽  
Phase Iv

Abstract Bone marrow (BM) CD34+ cells residing in the G0 phase of cell cycle may be the most suited candidates for the examination of cell cycle activation and proliferation of primitive hematopoietic progenitor cells (HPCs). We designed a double simultaneous labeling technique using both DNA and RNA staining with Hoechst 33342 and Pyronin Y, respectively, to isolate CD34+ cells residing in G0(G0CD34+ ). Using long-term BM cultures and limiting dilution analysis, G0CD34+ cells were found to be enriched for primitive HPCs. In vitro proliferation of G0CD34+ cells in response to sequential cytokine stimulation was examined in a two-step assay. In the first step, cells received a primary stimulation consisting of either stem cell factor (SCF), Flt3-ligand (FL), interleukin-3 (IL-3), or IL-6 for 7 days. In the second step, cells from each group were washed and split into four or more groups, each of which was cultured again for another week with one of the four primary cytokines individually, or in combination. Tracking of progeny cells was accomplished by staining cells with PKH2 on day 0 and with PKH26 on day 7. Overall examination of proliferation patterns over 2 weeks showed that cells could progress into four phases of proliferation. Phase I contained cytokine nonresponsive cells that failed to proliferate. Phase II contained cells dividing up to three times within the first 7 days. Phases III and IV consisted of cells dividing up to five divisions and greater than six divisions, respectively, by the end of the 14-day period. Regardless of the cytokine used for primary stimulation, G0CD34+ cells moved only to phase II by day 7, whereas a substantial percentage of cells incubated with SCF or FL remained in phase I. Cells cultured in SCF or FL for the entire 14-day period did not progress beyond phase III but proliferated into phase IV (with <20% of cells remaining in phases I and II) if IL-3, but not IL-6, was substituted for either cytokine on day 7. G0CD34+ cells incubated with IL-3 for 14 days proliferated the most and progressed into phase IV; however, when SCF was substituted on day 7, cells failed to proliferate into phase IV. Most intriguing was a group of cells, many of which were CD34+, detected in cultures initially stimulated with IL-3, which remained as a distinct population, mostly in G0 /G1 , unable to progress out of phase II regardless of the nature of the second stimulus received on day 7. A small percentage of these cells expressed cyclin E, suggesting that their proliferation arrest may have been mediated by a cyclin-related disruption in cell cycle. These results suggest that a programmed response to sequential cytokine stimulation may be part of a control mechanism required for maintenance of proliferation of primitive HPCs and that unscheduled stimulation of CD34+ cells residing in G0 may result in disruption of cell-cycle regulation.

Chemotactic and chemokinetic activities of stem cell factor on murine hematopoietic progenitor cells

Blood ◽  
1996 ◽  
Vol 87 (10) ◽  
pp. 4100-4108 ◽  
Author(s):  
N Okumura ◽  
K Tsuji ◽  
Y Ebihara ◽  
I Tanaka ◽  
N Sawai ◽  
...  
Keyword(s):  
Bone Marrow ◽  
Stem Cell ◽  
Progenitor Cells ◽  
Stromal Cells ◽  
Stem Cell Factor ◽  
Hematopoietic Progenitor Cells ◽  
Hematopoietic Progenitor ◽  
Murine Bone Marrow ◽  
Checkerboard Assay

We investigated the effects of stem cell factor (SCF) on the migration of murine bone marrow hematopoietic progenitor cells (HPC) in vitro using a modification of the checkerboard assay. Chemotactic and chemokinetic activities of SCF on HPC were evaluated by the numbers of HPC migrated on positive and negative gradients of SCF, respectively. On both positive and negative gradients of SCF, HPC began to migrate after 4 hours incubation, and their numbers then increased time- dependently. These results indicated that SCF functions as a chemotactic and chemokinetic agent for HPC. Analysis of types of colonies derived from the migrated HPC showed that SCF had chemotactic and chemokinetic effects on all types of HPC. When migrating activities of other cytokines were examined, interleukin (IL)-3 and IL-11 also affected the migration of HPC, but the degrees of each effect were lower than that of SCF. The results of the present study demonstrated that SCF is one of the most potent chemotactic and chemokinetic factors for HPC and suggest that SCF may play an important role in the flow of HPC into bone marrow where stromal cells constitutively produce SCF.

scholarly journals In vitro expansion of hematopoietic progenitor cells induces functional expression of Fas antigen (CD95)

Blood ◽  
1996 ◽  
Vol 88 (8) ◽  
pp. 2871-2877 ◽  
Author(s):  
K Takenaka ◽  
K Nagafuji ◽  
M Harada ◽  
S Mizuno ◽  
T Miyamoto ◽  
...  
Keyword(s):  
Growth Factors ◽  
Progenitor Cells ◽  
Calf Serum ◽  
Functional Expression ◽  
Hematopoietic Progenitor Cells ◽  
Hematopoietic Growth Factors ◽  
Hematopoietic Progenitor ◽  
Fas Antigen ◽  
Cd34 Cells

Fas antigen (Fas Ag; CD95) is a cell surface molecule that can mediate apoptosis. Bcl-2 is a cytoplasmic molecule that prolongs cellular survival by inhibiting apoptosis. To investigate the role of both molecules in hematopoiesis, we evaluated the expression of Fas Ag and Bcl-2 on CD34+ hematopoietic progenitor cells expanded in vitro. CD34+ cells isolated from bone marrow were cultured in iscove's modified Dulbecco's medium supplemented with 10% fetal calf serum, 1% bovine serum albumin, 50 ng/mL stem cell factor, 50 ng/mL interleukin-3 (IL-3), 50 ng/mL IL-6, 100 ng/mL granulocyte colony-stimulating factor, and 3 U/mL erythropoietin for 7 days. Colony-forming unit of granulocytes/macrophages (CFU-GM) and burst-forming unit of erythroids (BFU-E) were expanded 6.9-fold and 8.8-fold in number at day 5 of culture, respectively. Freshly isolated CD34+ cells did not express Fas Ag, whereas approximately half of them expressed Bcl-2. CD34+ cells cultured with hematopoietic growth factors gradually became positive for Fas Ag and rapidly lost Bcl-2 expression. Furthermore, apoptosis was induced in the cultured CD34+ population when anti-Fan antibody (IgM; 1 microgram/mL) was added, as shown by significant decrease in the number of viable cells, morphologic changes, induction of DNA fragmentation, and significant decrease in the number of clonogenic progenitor cells including CFU. GM and BFU-E. These results indicate that functional expression of Fas Ag is induced on CD34+ cells expanded in vitro in the presence of hematopoietic growth factors. Induction of Fas Ag and downregulation of Bcl-2 may be expressed as part of the differentiation program of hematopoietic cells and may be involved in the regulation of hematopoiesis.

scholarly journals Intracellular Immunization of Rhesus CD34+ Hematopoietic Progenitor Cells With a Hairpin Ribozyme Protects T Cells and Macrophages From Simian Immunodeficiency Virus Infection

Blood ◽  
1997 ◽  
Vol 90 (12) ◽  
pp. 4822-4831 ◽  
Author(s):  
Michael Rosenzweig ◽  
Douglas F. Marks ◽  
Donna Hempel ◽  
Marina Heusch ◽  
Günter Kraus ◽  
...  
Keyword(s):  
T Cells ◽  
Progenitor Cells ◽  
Hematopoietic Progenitor Cells ◽  
Hairpin Ribozyme ◽  
Hematopoietic Progenitor ◽  
Cd34 Cells ◽  
Immunodeficiency Virus ◽  
Cell Gene ◽  
Stem Cell Gene Therapy

Abstract Evaluation of candidate genes for stem cell gene therapy for acquired immunodeficiency syndrome (AIDS) has been limited by the difficulty of supporting in vitro T-cell differentiation of genetically modified hematopoietic progenitor cells. Using a novel thymic stromal culture technique, we evaluated the ability of a hairpin ribozyme specific for simian immunodeficiency virus (SIV) and human immunodeficiency virus type 2 (HIV-2) to inhibit viral replication in T lymphocytes derived from transduced CD34+ progenitor cells. Retroviral transduction of rhesus macaque CD34+ progenitor cells with a retroviral vector (p9456t) encoding the SIV-specific ribozyme and the selectable marker neomycin phosphotransferase in the presence of bone marrow stroma and in the absence of exogenous cytokines resulted in efficient transduction of both colony-forming units and long-term culture-initiating cells, with transduction efficiencies ranging between 21% and 56%. After transduction, CD34+ cells were cultured on rhesus thymic stromal culture (to support in vitro differentiation of T cells) or in the presence of cytokines (to support differentiation of macrophage-like cells). After expansion and selection with the neomycin analog G418, cells derived from transduced progenitor cells were challenged with SIV. CD4+ T cells derived from CD34+ hematopoietic cells transduced with the ribozyme vector p9456t were highly resistant to challenge with SIV, exhibiting up to a 500-fold decrease in SIV replication, even after high multiplicities of infection. Macrophages derived from CD34+ cells transduced with the 9456 ribozyme exhibited a comparable level of inhibition of SIV replication. These results show that a hairpin ribozyme introduced into CD34+ hematopoietic progenitor cells can retain the ability to inhibit AIDS virus replication after T-cell differentiation and support the feasibility of intracellular immunization of hematopoietic stem cells against infection with HIV and SIV. Protection of multiple hematopoietic lineages with the SIV-specific ribozyme should permit analysis of stem cell gene therapy for AIDS in the SIV/macaque model.

scholarly journals Long-term generation and expansion of human primitive hematopoietic progenitor cells in vitro

Blood ◽  
1993 ◽  
Vol 81 (3) ◽  
pp. 661-669 ◽  
Author(s):  
EF Srour ◽  
JE Brandt ◽  
RA Briddell ◽  
S Grigsby ◽  
T Leemhuis ◽  
...  
Keyword(s):  
Bone Marrow ◽  
Progenitor Cells ◽  
Bone Marrow Cells ◽  
Fold Increase ◽  
Hematopoietic Progenitor Cells ◽  
Proliferative Potential ◽  
Hematopoietic Progenitor ◽  
Hla Dr

Abstract Although sustained production of committed human hematopoietic progenitor cells in long-term bone marrow cultures (LTBMC) is well documented, evidence for the generation and expansion of human primitive hematopoietic progenitor cells (PHPC) in such cultures is lacking. For that purpose, we attempted to determine if the human high proliferative potential colony-forming cell (HPP-CFC), a primitive hematopoietic marrow progenitor cell, is capable of generation and expansion in vitro. To that effect, stromal cell-free LTBMC were initiated with CD34+ HLA-DR-CD15- rhodamine 123dull bone marrow cells and were maintained with repeated addition of c-kit ligand and a synthetic interleukin-3/granulocyte-macrophage colony-stimulating factor fusion protein. By day 21 of LTBMC, a greater than twofold increase in the number of assayable HPP-CFC was detected. Furthermore, the production of HPP-CFC in LTBMC continued for up to 4 weeks, resulting in a 5.5-fold increase in HPP-CFC numbers. Weekly phenotypic analyses of cells harvested from LTBMC showed that the number of CD34+ HLA-DR- cells increased from 10(4) on day 0 to 56 CD34+ HLA-DR- cells increased from 10(4) on day 0 to 56 x 10(4) by day 21. To examine further the nature of the in vitro HPP-CFC expansion, individual HPP- CFC colonies were serially cloned. Secondary cloning of individual, day 28 primary HPP-CFC indicated that 46% of these colonies formed an average of nine secondary colony-forming unit--granulocyte-macrophage (CFU-GM)--derived colonies, whereas 43% of primary HPP-CFC gave rise to between one and six secondary HPP-CFC colonies and 6 to 26 CFU-GM. These data show that CD34+ HLA-DR- CD15- rhodamine 123dull cells represent a fraction of human bone marrow highly enriched for HPP-CFC and that based on their regeneration and proliferative capacities, a hierarchy of HPP-CFC exists. Furthermore, these studies indicate that in the presence of appropriate cytokine stimulation, it is possible to expand the number of PHPC in vitro.

scholarly journals Myeloperoxidase expression in CD34+ normal human hematopoietic cells

Blood ◽  
1993 ◽  
Vol 82 (7) ◽  
pp. 2069-2078 ◽  
Author(s):  
H Strobl ◽  
M Takimoto ◽  
O Majdic ◽  
G Fritsch ◽  
C Scheinecker ◽  
...  
Keyword(s):  
Progenitor Cells ◽  
Transferrin Receptor ◽  
Staining Pattern ◽  
Hematopoietic Progenitor ◽  
Colony Forming Unit ◽  
Cd34 Cells ◽  
Normal Human ◽  
Colony Forming Capacity ◽  
Adult Peripheral Blood

Abstract Bone marrow (BM), adult peripheral blood (aPB), and umbilical cord blood (CB) samples contain small proportions of CD34+ cells that include virtually all hematopoietic progenitor cells. Myeloperoxidase (MPO) is considered to be selectively expressed in cells committed to granulomonocytic differentiation. Using flow cytometry and an antibody against MPO, we studied at which stage of normal hematopoietic differentiation CD34+ cells being to express MPO. We consistently observed a characteristic MPO/CD34 staining pattern and found that 35% +/- 9% of CD34+ BM cells express MPO. The MPO+ CD34+ subset and the CD33+ CD34+ subset were of similar size and overlapped considerably. MPO+ CD34+ cells expressed high levels of HLA-D molecules, were weakly CD71/transferrin receptor positive to negative, were CD45RA+ and lacked the CD45RO isoform of the leukocyte common antigen. Additionally, MPO+ CD34+ cells were on average larger in size than MPO- CD34+ cells. Virtually identical phenotypic features have previously been described for in vitro colony-forming granulomonocytic progenitor cells. In vitro clonogenic assays performed with MPO-enriched and MPO-depleted fractions of CD34+ BM cells performed by us also suggest, but do not formally prove, that at least a portion of MPO+ CD34+ cells have in vitro cluster (10 to 50 cells/colony) or colony-forming unit granulocyte-macrophage (> or = 50 cells/colony) forming capacity. CD34+ cells from CB and aPB resembled CD34+ BM cells in that considerable proportions of them coexpressed CD33. However, in contrast to BM, CD34+ cells from CB and aPB samples lacked significant MPO expression and, in line with this, the majority of them (CB, 59% +/- 7%; aPB, 66% +/- 5%) coexpressed CD45RO.

Flt3 Ligand Negatively Regulates In Vitro Migration, Intracellular Signaling Activated by SDF-1α/CXCR4 and In Vivo Homing of Hematopoietic Progenitor Cells.

Blood ◽  
2004 ◽  
Vol 104 (11) ◽  
pp. 2674-2674
Author(s):  
Seiji Fukuda ◽  
Hal E. Broxmeyer ◽  
Louis M. Pelus
Keyword(s):  
Bone Marrow ◽  
Progenitor Cells ◽  
Hematopoietic Progenitor Cells ◽  
Leukemia Cells ◽  
Flt3 Ligand ◽  
Hematopoietic Progenitor ◽  
Short Term ◽  
Hematopoietic Stem

Abstract The Flt3 receptor tyrosine kinase (Flt3) is expressed on primitive normal and transformed hematopoietic cells and Flt3 ligand (FL) facilitates hematopoietic stem cell mobilization in vivo. The CXC chemokine SDF-1α(CXCL12) attracts primitive hematopoietic cells to the bone marrow microenvironment while disruption of interaction between SDF-1α and its receptor CXCR4 within bone marrow may facilitate their mobilization to the peripheral circulation. We have previously shown that Flt3 ligand has chemokinetic activity and synergistically increases migration of CD34+ cells and Ba/F3-Flt3 cells to SDF-1α in short-term migration assays; this was associated with synergistic phosphorylation of MAPKp42/p44, CREB and Akt. Consistent with these findings, over-expression of constitutively active ITD (internal tandem duplication) Flt3 found in patients with AML dramatically increased migration to SDF-1α in Ba/F3 cells. Since FL can induce mobilization of hematopoietic stem cells, we examined if FL could antagonize SDF-1α/CXCR4 function and evaluated the effect of FL on in vivo homing of normal hematopoietic progenitor cells. FL synergistically increased migration of human RS4;11 acute leukemia cells, which co-express wild-type Flt3 and CXCR4, to SDF-1α in short term migration assay. Exogenous FL had no effect on SDF-1α induced migration of MV4-11 cells that express ITD-Flt3 and CXCR4 however migration to SDF-1α was partially blocked by treatment with the tyrosine kinase inhibitor AG1296, which inhibits Flt3 kinase activity. These results suggest that FL/Flt3 signaling positively regulates SDF-1α mediated chemotaxis of human acute leukemia cells in short-term assays in vitro, similar to that seen with normal CD34+ cells. In contrast to the enhancing effect of FL on SDF-1α, prolonged incubation of RS4;11 and THP-1 acute myeloid leukemia cells, which also express Flt3 and CXCR4, with FL for 48hr, significantly inhibited migration to SDF-1α, coincident with reduction of cell surface CXCR4. Similarly, prolonged exposure of CD34+ or Ba/F3-Flt3 cells to FL down-regulates CXCR4 expression, inhibits SDF-1α-mediated phosphorylation of MAPKp42/p44, CREB and Akt and impairs migration to SDF-1α. Despite reduction of surface CXCR4, CXCR4 mRNA and intracellular CXCR4 in Ba/F3-Flt3 cells were equivalent in cells incubated with or without FL, determined by RT-PCR and flow cytometry after cell permeabilization, suggesting that the reduction of cell surface CXCR4 expression is due to accelerated internalization of CXCR4. Furthermore, incubation of Ba/F3-Flt3 cells with FL for 48hr or over-expression of ITD-Flt3 in Ba/F3 cells significantly reduced adhesion to VCAM1. Consistent with the negative effect of FL on in vitro migration and adhesion to VCAM1, pretreatment of mouse bone marrow cells with 100ng/ml of FL decreased in vivo homing of CFU-GM to recipient marrow by 36±7% (P<0.01), indicating that FL can negatively regulate in vivo homing of hematopoietic progenitor cells. These findings indicate that short term effect of FL can provide stimulatory signals whereas prolonged exposure has negative effects on SDF-1α/CXCR4-mediated signaling and migration and suggest that the FL/Flt3 axis regulates hematopoietic cell trafficking in vivo. Manipulation of SDF-1α/CXCR4 and FL/Flt3 interaction could be clinically useful for hematopoietic cell transplantation and for treatment of hematopoietic malignancies in which both Flt3 and CXCR4 are expressed.

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