Bcr/Abl Increases Bcatenin Protein and Activity in An ICSBP-Dependent Manner in Myeloid Cells.

Blood ◽  
2009 ◽  
Vol 114 (22) ◽  
pp. 2958-2958
Author(s):  
Weiqi Huang ◽  
Elizabeth Horvath ◽  
Elizabeth A. Eklund
Keyword(s):  
Bone Marrow ◽  
Tumor Suppressor ◽  
Progenitor Cells ◽  
Myeloid Leukemia ◽  
Poor Prognosis ◽  
Myeloid Cells ◽  
Myeloid Cell ◽  
Dependent Manner ◽  
Myeloid Progenitor ◽  
Myeloid Progenitor Cells

Abstract Abstract 2958 Poster Board II-934 The interferon consensus sequence binding protein (ICSBP) is an interferon regulatory transcription factor (also referred to as IRF8). Similar to other IRF proteins, ICSBP regulates transcription of genes involved in the inflammatory response. However, ICSBP also functions as a myeloid leukemia tumor suppressor. Specifically, decreased ICSBP expression in myeloid progenitor cells results in cytokine hypersensitivity and resistance to apoptosis in response to Fas-activation or IL3 withdrawal. Consistent with function as a tumor suppressor, decreased ICSBP-expression is found in the bone marrow of human subjects with chronic myeloid leukemia (CML). Expression of ICSBP increases during remission and decreasing ICSBP expression is associated with progression to CML blast crisis (BC). In murine models, ICSBP-deficiency induces a myeloproliferative disorder (MPD) which resembles CML. And, ICSBP overexpression blocks MPD in mice transplanted with Bcr/abl expressing bone marrow. Increased Bcatenin activity is also associated with CML-BC. However, the impact of ICSBP on Bcatenin expression or activity has not been previously investigated. In these studies, we hypothesized that ICSBP-deficiency in CML-BC increases Bcatenin-expression. In support of this hypothesis, we found that Bcatenin protein and activity were increased in myeloid cell lines with ICSBP-knock-down and decreased in cells with ICSBP-overexpression. Bcatenin protein and activity were also increased in primary myeloid progenitors from ICSBP-/- mice in comparison to wild type or ICSBP+/- mice, in an ICSBP-dose dependent manner. Expression of Bcr/abl decreased ICSBP-expression in myeloid cells and increased Bcatenin protein and activity. Also supporting our hypothesis, the effect of Bcr/abl on Bcatenin was abolished by re-expression of ICSBP in Bcr/abl expressing myeloid cell lines or primary murine myeloid progenitor cells. In each of these situations, the increase in Bcatenin protein was not due to increased expression of Bcatenin mRNA. These results suggest ICSBP influences expression of target genes involved in Bcatenin protein stability. Therefore, these studies identify a pathway by which Bcr/abl activity impairs ICSBP expression in immature myeloid cells, thereby increasing stability of Bcatenin protein and Bcatenin activity. Poor prognosis and BC are associated with Fas resistance and increased Bcatenin activity in CML leukemia stem cells (LSC). Bcr/abl activity is also associated with decreased expression of ICSBP. Our prior studies implicated ICSBP-deficiency in Fas-resistance in CML via the ICSBP target gene PTPN13. Our current studies implicate ICSBP-deficiency in increased Bcatenin expression in CML. These results suggest that decreased ICSBP expression drives multiple aspects of the poor prognosis LSC phenotype in CML. Disclosures: No relevant conflicts of interest to declare.

scholarly journals Human bone marrow depleted of CD33-positive cells mediates delayed but durable reconstitution of hematopoiesis: clinical trial of MY9 monoclonal antibody-purged autografts for the treatment of acute myeloid leukemia

Blood ◽  
1992 ◽  
Vol 79 (9) ◽  
pp. 2229-2236 ◽  
Author(s):  
MJ Robertson ◽  
RJ Soiffer ◽  
AS Freedman ◽  
SL Rabinowe ◽  
KC Anderson ◽  
...  
Keyword(s):  
Monoclonal Antibody ◽  
Bone Marrow ◽  
Acute Myeloid Leukemia ◽  
Progenitor Cells ◽  
Myeloid Leukemia ◽  
Red Blood Cell Transfusion ◽  
Hematopoietic Stem ◽  
Myeloid Progenitor ◽  
Myeloid Progenitor Cells ◽  
Acute Myeloid

Abstract The CD33 antigen, identified by murine monoclonal antibody anti-MY9, is expressed by clonogenic leukemic cells from almost all patients with acute myeloid leukemia; it is also expressed by normal myeloid progenitor cells. Twelve consecutive patients with de novo acute myeloid leukemia received myeloablative therapy followed by infusion of autologous marrow previously treated in vitro with anti-MY9 and complement. Anti-MY9 and complement treatment eliminated virtually all committed myeloid progenitors (colony-forming unit granulocyte- macrophage) from the autografts. Nevertheless, in the absence of early relapse of leukemia, all patients showed durable trilineage engraftment. The median interval post bone marrow transplantation (BMT) required to achieve an absolute neutrophil count greater than 500/microL was 43 days (range, 16 to 75), to achieve a platelet count greater than 20,000/microL without transfusion was 92 days (range, 35 to 679), and to achieve red blood cell transfusion independence was 105 days (range, 37 to 670). At the time of BM harvest, 10 patients were in second remission, one patient was in first remission, and one patient was in third remission. Eight patients relapsed 3 to 18 months after BMT. Four patients transplanted in second remission remain disease-free 34+, 37+, 52+, and 57+ months after BMT. There was no treatment-related mortality. Early engraftment was significantly delayed in patients receiving CD33-purged autografts compared with concurrently treated patients receiving CD9/CD10-purged autografts for acute lymphoblastic leukemia or patients receiving CD6-purged allografts from HLA- compatible sibling donors. In contrast, both groups of autograft patients required a significantly longer time to achieve neutrophil counts greater than 500/microL and greater than 1,000/microL than did patients receiving normal allogeneic marrow. CD33(+)-committed myeloid progenitor cells thus appear to play an important role in the early phase of hematopoietic reconstitution after BMT. However, our results also show that human marrow depleted of CD33+ cells can sustain durable engraftment after myeloablative therapy, and provide further evidence that the CD33 antigen is absent from the human pluripotent hematopoietic stem cell.

Peripheral Blood Cytogenetic Studies in Hematological Neoplasms: Predictors of Obtaining Metaphases for Analysis.

Blood ◽  
2007 ◽  
Vol 110 (11) ◽  
pp. 3504-3504
Author(s):  
Kebede Hussein ◽  
Rhett P. Ketterling ◽  
Gordon W. Dewald ◽  
Rachael L. Hulshizer ◽  
Daniel G. Kuffel ◽  
...  
Keyword(s):  
Bone Marrow ◽  
Progenitor Cells ◽  
Clinical Diagnosis ◽  
Peripheral Blood ◽  
Myeloid Leukemia ◽  
Cytogenetic Study ◽  
Lymphocytic Leukemia ◽  
Myeloid Progenitor ◽  
Cytogenetic Studies ◽  
Myeloid Progenitor Cells

Abstract Background: Peripheral blood (PB) is sometimes used in place of bone marrow (BM) for cytogenetic studies during the evaluation of hematologic malignancies. We looked for clinical or laboratory features that predict success in obtaining analyzable metaphases during PB chromosome studies. Methods: The Mayo Clinic cytogenetics database was queried to identify adult cases (age > 18 years) with suspected or established hematologic neoplasm in whom PB cytogenetic studies were performed. Success defined as the acquisition of at least two metaphases, was correlated with clinical and laboratory information corresponding to the time of the PB cytogenetic study. Results: A total of 242 PB cytogenetic studies were performed: clinical diagnosis was a myeloid neoplasm in 169 patients (70%), lymphoid neoplasm in 50 (21%), and unexplained cytopenia or leukocytosis in 23 (9%). The 169 myeloid cases included 59 patients with either primary (n=39) or post-polycythemia vera/essential thrombocythemia (post-PV/ET MF) myelofibrosis (n=20), 42 with acute myeloid leukemia (AML), 15 with chronic myeloid leukemia, 9 with myelodysplastic syndrome (MDS), 8 with ET, 6 with PV, and 30 with other MPDs. The 50 lymphoid cases included 19 with chronic lymphocytic leukemia, 12 with lymphoma, 11 with acute lymphocytic leukemia (ALL), and 8 with plasma cell proliferative disorders. PB cytogenetic studies resulted in at least two analyzable metaphases (median 20, range 2–31) in 142 of the 242 study cases (59%); in univariate analysis, this was predicted by the specific clinical diagnosis (p<0.0001), presence and degree of circulating myeloid progenitor cells (p<0.0001), higher leukocyte count (p<0.001), lower platelet count (p=0.003), lower hemoglobin level (p=0.002), and presence of palpable splenomegaly (p=0.002). In multivariable analysis, only the presence of circulating myeloid progenitor cells sustained its significance and this was consistent with the high yield rates seen in PMF (80%), post-PV/ET MF (85%), AML (76%), and ALL (80%) as opposed to the low rates seen in ET (0%) and PV (2%). In 104 cases, BM cytogenetic studies were performed within one month of the PB cytogenetic studies; an abnormal BM cytogenetic finding was another independent predictor of a successful PB study (p=0.002). Conclusion: PB cytogenetic studies are most appropriate in diseases characterized by presence of circulating myeloid progenitors or blasts (e.g. PMF, AML, ALL); the yield otherwise is too small to be cost-effective. The current study also suggests a higher likelihood of a successful PB cytogenetic study in the presence of an abnormal bone marrow karyotype.

Myc Induces Acute Myeloid Leukemia by Conferring Self-Renewal Activity to Committed Myeloid Progenitor Cells That Resist Apoptosis Via Endogenous Mcl-1.

Blood ◽  
2007 ◽  
Vol 110 (11) ◽  
pp. 2634-2634
Author(s):  
Hui Luo ◽  
Jennifer A. Cain ◽  
AnnaLynn Molitoris ◽  
Joseph Opferman ◽  
Michael H. Tomasson
Keyword(s):  
Bone Marrow ◽  
Progenitor Cells ◽  
Median Survival ◽  
Myeloid Leukemia ◽  
Bone Marrow Cells ◽  
Ectopic Expression ◽  
Hematopoietic Stem ◽  
Myeloid Progenitor ◽  
Myeloid Progenitor Cells ◽  
Induced Apoptosis

Abstract Ectopic expression of Myc in most primary cell types induces apoptosis, and cancer development typically requires additional, anti-apoptotic mutations. We reported previously that ectopic expression of Myc in unfractionated murine bone marrow cells induced rapid onset acute myeloid leukemia (AML) without detectable anti-apoptotic mutations. We hypothesized that AML developed in our model because a subset of normal primary bone marrow cells were inherently resistant to Myc-induced apoptosis. Consistent with this model, seven days of Myc activation in the bone marrow of mice caused the reduction of B-lineage cells while at the same time inducing the expansion of myeloid lineage cells. We sought to determine the mechanism by which myeloid progenitor cells evaded Myc-induced apoptosis, and found that Myc-induced AML cells exhibited a distinct profile of pro- and anti-apoptotic proteins, including high levels of the anti-apoptotic Bcl-2 family member Mcl-1. To prioritize apoptosis genes, we examined AML patient microarray data and found MCL1 to be uniformly expressed at high levels in human AML (94/94, 100%). We used Mcl1 heterozygous mice (Mcl1F/null) as bone marrow donors for transduction-transplantation experiments and found that, compared with Mcl1 wild-type (median survival=60 days), haploinsufficiency for Mcl1 completely protected mice from Myc-induced AML (median survival not reached). Mice transplanted with Mcl1F/null cells co-expressing Myc and Bcl2 succumbed rapidly to disease (median survival 25 days). In wild-type mice, defined hematopoietic stem and myeloid progenitor cell populations were not significantly increased by Myc activation. However, Myc transduction conferred serial replating ability to sorted hematopoietic stem and progenitor cells including lineage-committed (Lin+Kit+) progenitors cells. These data demonstrate a critical role for Mcl1 in our AML model and suggest that dysregulation of MYC in MCL1-expressing progenitor cells may mediate AML pathogenesis in humans.

scholarly journals Inhibitory effect of 2-chlorodeoxyadenosine on granulocytic, erythroid, and T-lymphocytic colony growth

Blood ◽  
1991 ◽  
Vol 78 (10) ◽  
pp. 2583-2587 ◽  
Author(s):  
AL Petzer ◽  
R Bilgeri ◽  
U Zilian ◽  
M Haun ◽  
FH Geisen ◽  
...  
Keyword(s):  
Bone Marrow ◽  
Progenitor Cells ◽  
T Lymphocyte ◽  
Human Lymphocytes ◽  
Maturation Stage ◽  
Dependent Manner ◽  
Myeloid Progenitor ◽  
Marked Inhibition ◽  
Myeloid Progenitor Cells

Abstract Previous studies have shown that 2-chloro-2′-deoxyadenosine (CdA) is markedly toxic to normal and malignant human lymphocytes in vitro and in vivo. Recent clinical trials have shown that CdA is a very promising drug for the treatment of lymphoid malignancies. The present investigations were designed to test the effect of CdA on the in vitro clonal growth of both myeloid progenitors and T-lymphocyte colony- forming cells (CFU-TL) obtained from normal human bone marrow and peripheral blood. Cells were exposed to CdA in doses up to 1280 nmol/L. To reduce indirect effects of CdA mediated by accessory cells, monocyte- and T-lymphocyte-depleted bone marrow cells were used for our investigations. The results show a marked inhibition of myeloid progenitor and lymphocyte colony-forming cells in a dose-dependent manner, correlating with maturation stage in that the immature progenitor cells are more sensitive to this drug. Furthermore, our studies suggest that a sequence of metabolic events previously described for lymphocytes may be operative in myeloid progenitor cells because a minimal exposure time of 48 hours is required to obtain a marked inhibition. CdA toxicity was proposed to be linked with phosphorylation by deoxycytidine-kinase (E.C. 2.7.1.74), the levels of which have been found to be high in lymphocytes, but low in granulocytes. However, the marked inhibition of myeloid progenitor cells shown in these studies suggests that other factors such as modulation of the effect of CdA by the ambient levels of other deoxynucleosides might influence the apparent sensitivity of myeloid cells.

scholarly journals ANTIGENIC DIFFERENCES BETWEEN HEMOPOIETIC STEM CELLS AND MYELOID PROGENITORS

1974 ◽  
Vol 139 (6) ◽  
pp. 1621-1627 ◽  
Author(s):  
Gerrit J. Van den Engh ◽  
Edward S. Golub
Keyword(s):  
Stem Cells ◽  
Bone Marrow ◽  
Stem Cell ◽  
Progenitor Cells ◽  
Bone Marrow Cells ◽  
Myeloid Cell ◽  
Myeloid Progenitor ◽  
Myeloid Progenitor Cells ◽  
Myeloid Progenitors

Bone marrow contains pluripotent stem cells which give rise to colonies when injected into irradiated syngenic hosts as well as more differentiated progenitor cells of the myeloid cell which are able to form colonies in vitro. Antisera against brain is known to contain antistem cell antibody. The present experiments were designed to determine if the myeloid progenitor cell still expresses the stem cell antigen. Bone marrow cells were treated with antibrain antiserum plus complement and then survival of stem cells was determined by injection into irradiated hosts. Survival of myeloid progenitor cells was determined by culturing the cells in vitro. It was found that stem cells were eliminated by the antiserum but that myeloid progenitors were not. Inefficient in vitro lysis was ruled out as the reason for this difference since in vitro colonies were not reduced when the cells were treated with anti-immunoglobulin or after passage through an irradiated host. In the differentiation from stem cell to myeloid progenitor there is an associated surface antigen change.

scholarly journals Human bone marrow depleted of CD33-positive cells mediates delayed but durable reconstitution of hematopoiesis: clinical trial of MY9 monoclonal antibody-purged autografts for the treatment of acute myeloid leukemia

Blood ◽  
1992 ◽  
Vol 79 (9) ◽  
pp. 2229-2236 ◽  
Author(s):  
MJ Robertson ◽  
RJ Soiffer ◽  
AS Freedman ◽  
SL Rabinowe ◽  
KC Anderson ◽  
...  
Keyword(s):  
Monoclonal Antibody ◽  
Bone Marrow ◽  
Acute Myeloid Leukemia ◽  
Progenitor Cells ◽  
Myeloid Leukemia ◽  
Red Blood Cell Transfusion ◽  
Hematopoietic Stem ◽  
Myeloid Progenitor ◽  
Myeloid Progenitor Cells ◽  
Acute Myeloid

The CD33 antigen, identified by murine monoclonal antibody anti-MY9, is expressed by clonogenic leukemic cells from almost all patients with acute myeloid leukemia; it is also expressed by normal myeloid progenitor cells. Twelve consecutive patients with de novo acute myeloid leukemia received myeloablative therapy followed by infusion of autologous marrow previously treated in vitro with anti-MY9 and complement. Anti-MY9 and complement treatment eliminated virtually all committed myeloid progenitors (colony-forming unit granulocyte- macrophage) from the autografts. Nevertheless, in the absence of early relapse of leukemia, all patients showed durable trilineage engraftment. The median interval post bone marrow transplantation (BMT) required to achieve an absolute neutrophil count greater than 500/microL was 43 days (range, 16 to 75), to achieve a platelet count greater than 20,000/microL without transfusion was 92 days (range, 35 to 679), and to achieve red blood cell transfusion independence was 105 days (range, 37 to 670). At the time of BM harvest, 10 patients were in second remission, one patient was in first remission, and one patient was in third remission. Eight patients relapsed 3 to 18 months after BMT. Four patients transplanted in second remission remain disease-free 34+, 37+, 52+, and 57+ months after BMT. There was no treatment-related mortality. Early engraftment was significantly delayed in patients receiving CD33-purged autografts compared with concurrently treated patients receiving CD9/CD10-purged autografts for acute lymphoblastic leukemia or patients receiving CD6-purged allografts from HLA- compatible sibling donors. In contrast, both groups of autograft patients required a significantly longer time to achieve neutrophil counts greater than 500/microL and greater than 1,000/microL than did patients receiving normal allogeneic marrow. CD33(+)-committed myeloid progenitor cells thus appear to play an important role in the early phase of hematopoietic reconstitution after BMT. However, our results also show that human marrow depleted of CD33+ cells can sustain durable engraftment after myeloablative therapy, and provide further evidence that the CD33 antigen is absent from the human pluripotent hematopoietic stem cell.

scholarly journals Inhibitory effect of 2-chlorodeoxyadenosine on granulocytic, erythroid, and T-lymphocytic colony growth

Blood ◽  
1991 ◽  
Vol 78 (10) ◽  
pp. 2583-2587 ◽  
Author(s):  
AL Petzer ◽  
R Bilgeri ◽  
U Zilian ◽  
M Haun ◽  
FH Geisen ◽  
...  
Keyword(s):  
Bone Marrow ◽  
Progenitor Cells ◽  
T Lymphocyte ◽  
Human Lymphocytes ◽  
Maturation Stage ◽  
Dependent Manner ◽  
Myeloid Progenitor ◽  
Marked Inhibition ◽  
Myeloid Progenitor Cells

Previous studies have shown that 2-chloro-2′-deoxyadenosine (CdA) is markedly toxic to normal and malignant human lymphocytes in vitro and in vivo. Recent clinical trials have shown that CdA is a very promising drug for the treatment of lymphoid malignancies. The present investigations were designed to test the effect of CdA on the in vitro clonal growth of both myeloid progenitors and T-lymphocyte colony- forming cells (CFU-TL) obtained from normal human bone marrow and peripheral blood. Cells were exposed to CdA in doses up to 1280 nmol/L. To reduce indirect effects of CdA mediated by accessory cells, monocyte- and T-lymphocyte-depleted bone marrow cells were used for our investigations. The results show a marked inhibition of myeloid progenitor and lymphocyte colony-forming cells in a dose-dependent manner, correlating with maturation stage in that the immature progenitor cells are more sensitive to this drug. Furthermore, our studies suggest that a sequence of metabolic events previously described for lymphocytes may be operative in myeloid progenitor cells because a minimal exposure time of 48 hours is required to obtain a marked inhibition. CdA toxicity was proposed to be linked with phosphorylation by deoxycytidine-kinase (E.C. 2.7.1.74), the levels of which have been found to be high in lymphocytes, but low in granulocytes. However, the marked inhibition of myeloid progenitor cells shown in these studies suggests that other factors such as modulation of the effect of CdA by the ambient levels of other deoxynucleosides might influence the apparent sensitivity of myeloid cells.

scholarly journals Bone marrow lymphoid and myeloid progenitor cells are suppressed in 7,12-dimethylbenz(a)anthracene (DMBA) treated mice

Toxicology ◽  
2010 ◽  
Vol 271 (1-2) ◽  
pp. 27-35 ◽  
Author(s):  
A.U. N’jai ◽  
M. Larsen ◽  
L. Shi ◽  
C.R. Jefcoate ◽  
C.J. Czuprynski
Keyword(s):  
Bone Marrow ◽  
Progenitor Cells ◽  
Myeloid Progenitor ◽  
Myeloid Progenitor Cells

scholarly journals Expression of Ia antigens on myeloid progenitor cells in chronic myeloid leukemia: direct analysis using partially purified colony- forming cells

Blood ◽  
1985 ◽  
Vol 65 (2) ◽  
pp. 414-422
Author(s):  
SA Cannistra ◽  
JF Daley ◽  
P Larcom ◽  
JD Griffin
Keyword(s):  
Cell Cycle ◽  
Chronic Myeloid Leukemia ◽  
Progenitor Cells ◽  
Myeloid Leukemia ◽  
Antigen Expression ◽  
Colony Stimulating Factors ◽  
Myeloid Progenitor ◽  
Hla Dr ◽  
Ia Antigens ◽  
Myeloid Progenitor Cells

The regulation of Ia (HLA-DR) antigen expression on myeloid progenitor cells may be closely related to the control of myelopoiesis in both normal individuals and chronic myeloid leukemia (CML) patients. In an effort to study directly the expression and behavior of Ia surface molecules on myeloid progenitor cells, we used an immunologic purification technique to enrich these cells approximately 100-fold from the peripheral blood of CML patients. The majority of cells in this blast population expressed HLA-DR antigens. Thirty percent to 40% of cells could form a granulocyte or monocyte colony in agar, and these cells tended to express the highest levels of HLA-DR. The number of HLA- DR molecules per cell increased about twofold as the cells tranversed the cell cycle from G0/G1 to G2/M. This was true for unstimulated cells or cells exposed to colony-stimulating factors. Some of this increase was related to a corresponding increase in cell size and is also seen with other cell surface antigens such as beta-2-microglobulin. Ia antigen expression was not modified by culture with colony-stimulating factors, fetal calf serum, or serum-free, prostaglandin-free medium for periods of up to 24 hours. These results demonstrate that Ia antigens are expressed on the myeloid progenitor cells of CML, are increased in the S and G2/M phases of the cell cycle, and are stable under most in vitro culture conditions for at least 24 hours of culture.

scholarly journals Involvement of p21cip-1 and p27kip-1 in the molecular mechanisms of steel factor-induced proliferative synergy in vitro and of p21cip-1 in the maintenance of stem/progenitor cells in vivo

Blood ◽  
1996 ◽  
Vol 88 (10) ◽  
pp. 3710-3719 ◽  
Author(s):  
C Mantel ◽  
Z Luo ◽  
J Canfield ◽  
S Braun ◽  
C Deng ◽  
...  
Keyword(s):  
Progenitor Cells ◽  
Proliferative Response ◽  
Myeloid Cell ◽  
Cyclin Dependent Kinase ◽  
Hematopoietic Progenitor ◽  
Gm Csf ◽  
Myeloid Progenitor ◽  
Myeloid Progenitor Cells

Steel factor (SLF) is a hematopoietic cytokine that synergizes with other growth factors to induce a greatly enhanced proliferative state of hematopoietic progenitor cells and factor-dependent cell lines. Even though the in vivo importance of SLF in the maintenance and responsiveness of stem and progenitor cells is well documented, the molecular mechanism involved in its synergistic effects are mainly unknown. Some factor-dependent myeloid cell lines respond to the synergistic proliferative effects of SLF plus other cytokines in a manner similar to that of normal myeloid progenitor cells from bone marrow and cord blood. We show here that SLF can synergize with granulocyte-macrophage colony-stimulating factor (GM-CSF) to induce an enhanced phosphorylation of the retinoblastoma gene product and a synergistic increase in the total intracellular protein level of the cyclin-dependent kinase inhibitor, p21cip-1, which is correlated with a simultaneous decrease in p27kip-1 in the human factor-dependent myeloid cell line, M07e. Moreover, these cytokines synergize to increase p21cip- 1 binding and decrease p27kip-1 binding to cyclin-dependent kinase-2 (cdk2), an enzyme required for normal cell cycle progression; these inverse events correlated with increased cdk2 kinase activity. It is also shown that exogenous purified p21cip-1 can displace p27kip-1 already bound to cdk2 in vitro. These data implicate increased p21cip-1 and decreased p27kip-1 intracellular concentrations and their stoichiometric interplay in the enhanced proliferative status of cells stimulated by the combination of SLF and GM-CSF. In support of these findings, it is shown that hematopoietic progenitor cells from mice lacking p21cip-1 are defective in SLF synergistic proliferative response in vitro. Moreover, the cycling status of marrow and spleen progenitors and absolute numbers of marrow progenitors were significantly decreased in the p21cip-1 -/-, compared with the +/+ mice. We conclude that the cdk threshold regulators p21cip-1 and p27kip- 1 play a critical role in the normal mitogenic response of M07e cells and murine myeloid progenitor cells to these cytokines and particularly in the SLF synergistic proliferative response that is important to the normal maintenance of the stem/progenitor cell compartment.

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