Lack of the adhesion molecules P-selectin and intercellular adhesion molecule-1 accelerate the development of BCR/ABL-induced chronic myeloid leukemia-like myeloproliferative disease in mice

LSP1 promotes the progression of acute myelogenous leukemia by regulating KSR/ERK signaling pathway and cell migration

We aimed to investigate the role and mechanism of LSP1 in the progression of acute myelogenous leukemia. In this study, we established shLSP1 cell line to analyze the function of LSP1 in AML. We observed high expression of LSP1 in AML patients, whereas it showed no expression in normal adults. Furthermore, we found that LSP1 expression was associated with disease prognosis. Our results indicate that LSP1 plays a crucial role in mediating proliferation and survival of leukemia cells through the KSR/ERK signaling pathway. Additionally, LSP1 promotes cell chemotaxis and homing by enhancing cell adhesion and migration. We also discovered that LSP1 confers chemotactic ability to leukemia cells in vivo. Finally, our study identified 12 genes related to LSP1 in AML, which indicated poor survival outcome in AML patients and were enriched in Ras and cell adhesion signaling pathways. Our results revealed that the overexpression of LSP1 is related to the activation of the KSR/ERK signaling pathway, as well as cell adhesion and migration in AML patients. Reducing LSP1 expression impair AML progression, suggesting that LSP1 may serve as a potential drug therapy target for more effective treatment of AML.

Hematopoietic stem cell aging and leukemia transformation

With aging, hematopoietic stem cells (HSCs) have an impaired ability to regenerate, differentiate, and produce an entire repertoire of mature blood and immune cells. Owing to dysfunctional hematopoiesis, the incidence of hematologic malignancies increases among elderly individuals. Here, we provide an update on HSC-intrinsic and -extrinsic factors and processes that were recently discovered to contribute to the functional decline of HSCs during aging. In addition, we discuss the targets and timing of intervention approaches to maintain HSC function during aging and the extent to which these same targets may prevent or delay transformation to hematologic malignancies.

ICAM-1 Deficiency in the Bone Marrow Niche Impairs Quiescence and Repopulation of Hematopoietic Stem Cells

The bone marrow niche plays a critical role in controlling the fate of hematopoietic stem cells (HSCs) by integrating intrinsic and extrinsic signals. However, the molecular events in the HSC niche remain to be investigated. Here, we report that intercellular adhesion molecule-1 (ICAM-1) maintains HSC quiescence and repopulation capacity in the niche. ICAM-1-deficient mice (ICAM-1^−/−) displayed significant expansion of phenotypic long-term HSCs with impaired quiescence, as well as favoring myeloid cell expansion. ICAM-1-deficient HSCs presented normal reconstitution capacity during serial transplantation; however, reciprocal transplantation experiments showed that ICAM-1 deficiency in the niche impaired HSC quiescence and repopulation capacity. In addition, ICAM-1 deletion caused failure to retain HSCs in the bone marrow and changed the expression profile of stroma cell-derived factors, possibly representing the mechanism for defective HSCs in ICAM-1^−/− mice. Collectively, these observations identify ICAM-1 as a regulator in the bone marrow niche.

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ICAM-1 deficiency expands HSC^−LT with impaired quiescence and repopulation

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The defects characterizing HSC^−LT in ICAM-1^−/− mice are niche cell dependent

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ICAM-1^−/− niche brings about impaired bone marrow retention and homing of HSC^−LT

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ICAM-1 in human stroma cells might affect the progression of myelocytic leukemia

BCR/ABL oncogene-induced PI3K signaling pathway leads to chronic myeloid leukemia pathogenesis by impairing immuno-modulatory function of hemangioblasts

An increasing number of studies indicate that during development, endothelial and hematopoietic cells derive from common progenitors named hemangioblasts that have important roles in the pathogenesis. This is particularly true in chronic myeloid leukemia (CML). Here, we isolated fetal liver kinase-1-positive (Flk1(+)) cells from CML patients and found they expressed BCR/ABL-specific CML oncogene. We examined their biological characteristics as well as immunological functions and further detected the possible molecular mechanism involved in the leukemia genesis. We showed that CML patient-derived Flk1(+)CD31(-)CD34(-) mesenchymal stem cells (MSCs) had normal morphology, phenotype and karyotype but appeared impaired immuno-modulatory function. The capacity of Flk1(+)CD31(-)CD34(-) MSCs from CML patients to inhibit T lymphocyte activation and proliferation was impaired in vitro. CML patient-derived MSCs have dampening immuno-modulatory functions, suggesting that the dysregulation of hematopoiesis and immune response might originate from MSCs rather than hematopoietic stem cells (HSCs). These Ph(+) putative CML hemangioblast upregulated TGF-β1 and resultantly activated matrix metalloproteinase-9 (MMP-9) to enhance s-KitL and s-ICAM-1 secretion, which activated c-kit(+) HSCs from the quiescent state to the proliferative state. Further studies showed that phosphatidylinositol-3 kinase (PI3K)/Akt/nuclear factor (NF)-κB signaling pathway was involved in CML pathogenesis. Flk1(+)CD31(-)CD34(-) MSCs that express BCR/ABL leukemia oncogene are hemangioblasts and they have a critical role in the progression of CML through PI3K/Akt/NF-κB signaling pathway.

Arachidonate 15-lipoxygenase is required for chronic myeloid leukemia stem cell survival

Cancer stem cells (CSCs) are responsible for the initiation and maintenance of some types of cancer, suggesting that inhibition of these cells may limit disease progression and relapse. Unfortunately, few CSC-specific genes have been identified. Here, we determined that the gene encoding arachidonate 15-lipoxygenase (Alox15/15-LO) is essential for the survival of leukemia stem cells (LSCs) in a murine model of BCR-ABL-induced chronic myeloid leukemia (CML). In the absence of Alox15, BCR-ABL was unable to induce CML in mice. Furthermore, Alox15 deletion impaired LSC function by affecting cell division and apoptosis, leading to an eventual depletion of LSCs. Moreover, chemical inhibition of 15-LO function impaired LSC function and attenuated CML in mice. The defective CML phenotype in Alox15-deficient animals was rescued by depleting the gene encoding P-selectin, which is upregulated in Alox15-deficient animals. Both deletion and overexpression of P-selectin affected the survival of LSCs. In human CML cell lines and CD34+ cells, knockdown of Alox15 or inhibition of 15-LO dramatically reduced survival. Loss of Alox15 altered expression of PTEN, PI3K/AKT, and the transcription factor ICSBP, which are known mediators of cancer pathogenesis. These results suggest that ALOX15 has potential as a therapeutic target for eradicating LSCs in CML.

Selectins and their ligands are required for homing and engraftment of BCR-ABL1+ leukemic stem cells in the bone marrow niche

We investigated adhesion pathways that contribute to engraftment of breakpoint cluster region-Abelson murine leukemia viral oncogene homolog 1 (BCR-ABL1)-induced chronic myelogenous leukemia (CML)-like myeloproliferative neoplasia in a mouse retroviral transduction/transplantation model. Compared with normal stem/progenitor cells, BCR-ABL1(+) progenitors had similar expression of very late antigen-4 (VLA4), VLA5, leukocyte functional antigen-1, and CXCR4 but lower expression of P-selectin glycoprotein ligand-1 (PSGL-1) and of L-selectin. Whereas vascular cell adhesion molecule-1 and P-selectin were not required, deficiency of E-selectin in the recipient bone marrow endothelium significantly reduced engraftment by BCR-ABL1-expressing stem cells following intravenous injection, with leukemogenesis restored by direct intrafemoral injection. BCR-ABL1-expressing cells deficient for PSGL-1 or the selectin ligand-synthesizing enzymes core-2 β1,6-N-acetylglucosaminyltransferase or fucosyltransferases IV/VII were impaired for engraftment, and destruction of selectin ligands on leukemic progenitors by neuraminidase reduced engraftment. BCR-ABL1-expressing L-selectin-deficient progenitors were also defective in homing and engraftment, with leukemogenesis rescued by coexpression of chimeric E/L-selectin. Antibody to L-selectin decreased the engraftment of BCR-ABL1-transduced stem cells. These results establish that BCR-ABL1(+) leukemic stem cells rely to a greater extent on selectins and their ligands for homing and engraftment than do normal stem cells. Selectin blockade is a novel strategy to exploit differences between normal and leukemic stem cells that may be beneficial in autologous transplantation for CML and perhaps other leukemias.

Molecular signatures of chronic myeloid leukemia stem cells

BCR-ABL tyrosine kinase inhibitors (TKIs) are effective in controlling Philadelphia-positive (Ph⁺) chronic myeloid leukemia (CML) are unlikely to cure the disease because TKIs are unable to eradicate leukemia stem cells (LSCs) responsible for the disease relapse even after tyrosine kinase inhibition. In addition, the TKI resistance of LSCs is not associated with the BCR-ABL kinase domain mutations. These observations indicate that TKI-insensitive LSCs and TKI-sensitive leukemic progenitor cells are biologically different, which leads us to believe that LSCs and more differentiated leukemic cells have different genetic mechanisms. Further study of LSCs to identify the novel gene signatures and mechanisms that control the function and molecular phenotype of LSCs is critical. In this mini-review, we will discuss our current understanding of the biology of LSCs and novel genes that could serve as a molecular signature of LSCs in CML. These novel genes could also serve as potential targets for eradicating LSCs in CML.

Impaired immunomodulatory function of chronic myeloid leukemia cancer stem cells and the possible mechanism involved in it

BACKGROUND

Cancer stem cells (CSCs) are proposed to persist in tumors as a distinct population and cause relapse and metastasis by giving rise to new tumors. Development of specific therapies targeted at CSCs holds hope for the improvement of survival and quality of life of cancer patients, especially for sufferers of metastatic disease. This is particularly true in chronic myeloid leukemia (CML).

METHODS

In this study, we isolated fetal liver kinase-1-positive (Flk1(+)) cells carrying the BCR/ABL fusion gene from the bone marrow of Philadelphia chromosome-positive (Ph(+)) patients with stem cells property. We examined their biological characteristics as well as immunological function and further detected the possible molecular mechanism involved in the leukemia genesis.

RESULTS

We showed that CML patient-derived Flk1(+)CD31(-)CD34(-) MSCs had normal morphology, phenotype and karyotype but appeared impaired immunomodulatory function. The capacity of Flk1(+)CD31(-)CD34(-) MSCs from CML patients to inhibit T lymphocyte activation and proliferation was impaired in vitro. CML patient-derived MSCs have dampening immunomodulatory functions, suggesting that the dysregulation of hematopoiesis and immune response might originate from MSCs rather than HSCs. These Ph(+) putative CML hemangioblast upregulated TGF-β1 and resultantly activated matrix metalloproteinase-9 (MMP-9) to enhance s-KitL and s-ICAM-1 secretion, which activated c-kit(+) HSCs from the quiescent state to proliferative state. Further studies showed that phosphatidylinositol-3 kinase (PI3K)/Akt/nuclear factor (NF)-κB signaling pathway was involved in CML pathogenesis.

CONCLUSIONS

Flk1(+)CD31(-)CD34(-) MSCs that express BCR/ABL leukemia oncogene are CSCs of CML and they play a critical role in the progression of CML through PI3K/Akt/NF-κB/MMP-9/s-ICAM-1/s-KitL signaling pathway beyond HSCs.

Functional ramifications for the loss of P-selectin expression on hematopoietic and leukemic stem cells

Hematopoiesis is a tightly regulated biological process that relies upon complicated interactions between blood cells and their microenvironment to preserve the homeostatic balance of long-term hematopoietic stem cells (LT-HSCs), short-term HSCs (ST-HSCs), multipotent progenitors (MPPs), and differentiated cells. Adhesion molecules like P-selectin (encoded by the Selp gene) are essential to hematopoiesis, and their dysregulation has been linked to leukemogenesis. Like HSCs, leukemic stem cells (LSCs) depend upon their microenvironments for survival and propagation. P-selectin plays a crucial role in Philadelphia chromosome -positive (Ph⁺) chronic myeloid leukemia (CML). In this paper, we show that cells deficient in P-selectin expression can repopulate the marrow more efficiently than wild type controls. This results from an increase in HSC self-renewal rather than alternative possibilities like increased homing velocity or cell cycle defects. We also show that P-selectin expression on LT-HSCs, but not ST-HSCs and MPPs, increases with aging. In the absence of P-selectin expression, mice at 6 months of age possess increased levels of short-term HSCs and multipotent progenitors. By 11 months of age, there is a shift towards increased levels of long-term HSCs. Recipients of BCR-ABL-transduced bone marrow cells from P-selectin-deficient donors develop a more aggressive CML, with increased percentages of LSCs and progenitors. Taken together, our data reveal that P-selectin expression on HSCs and LSCs has important functional ramifications for both hematopoiesis and leukemogenesis, which is most likely attributable to an intrinsic effect on stem cell self-renewal.

The research on the immuno-modulatory defect of mesenchymal stem cell from Chronic Myeloid Leukemia patients

Overwhelming evidence from leukemia research has shown that the clonal population of neoplastic cells exhibits marked heterogeneity with respect to proliferation and differentiation. There are rare stem cells within the leukemic population that possess extensive proliferation and self-renewal capacity not found in the majority of the leukemic cells. These leukemic stem cells are necessary and sufficient to maintain the leukemia. While the hematopoietic stem cell (HSC) origin of CML was first suggested over 30 years ago, recently CML-initiating cells beyond HSCs are also being investigated. We have previously isolated fetal liver kinase-1-positive (Flk1⁺) cells carrying the BCR/ABL fusion gene from the bone marrow of Philadelphia chromosome-positive (Ph⁺) patients with hemangioblast property. Here, we showed that CML patient-derived Flk1⁺CD31^-CD34^-MSCs had normal morphology, phenotype and karyotype but appeared impaired in immuno-modulatory function. The capacity of patient Flk1⁺CD31^-CD34^- MSCs to inhibit T lymphocyte activation and proliferation was impaired in vitro. CML patient-derived MSCs have impaired immuno-modulatory functions, suggesting that the dysregulation of hematopoiesis and immune response may originate from MSCs rather than HSCs. MSCs might be a potential target for developing efficacious cures for CML.

Targeted therapy of chronic myeloid leukemia

Inhibition of BCR-ABL with kinase inhibitors has become a well-accepted strategy for targeted therapy of Philadelphia-positive (Ph(+)) chronic myeloid leukemia (CML) and has been shown to be highly effective in controlling the disease. However, BCR-ABL kinase inhibitors do not efficiently kill leukemic stem cells (LSCs), indicating that this therapeutic strategy does not lead to a cure of CML. Development of curative therapies of CML require the identification of genes/pathways that play critical roles in survival and self-renewal of LSCs. Targeting of these key BCR-ABL downstream genes provides an opportunity to eradicate LSCs, as shown in our work that identifies the Alox5 gene as a key regulator of the function of CML LSCs. Immediate clinical trials are necessary to test the effectiveness of targeting a key BCR-ABL downstream gene in eradicating LSCs in CML patients. In this review, we will discuss current targeted therapies of CML using BCR-ABL kinase inhibitors, with a focus on the importance of developing a targeted therapy of CML through identification of target genes in CML LSCs.

Shear stress-mediated adhesion of acute myeloid leukemia and KG-1 cells to endothelial cells involves functional P-selectin

Acute myeloid leukemia (AML) shows malignant behavior through the ability of immature cells to circulate in blood and to invade peripheral tissues. Whereas binding of human AML cells to endothelial cells (ECs) through E-selectin has been shown to occur using classical adhesion assays, little is known about the ability of endothelial P-selectin to support this process. We therefore characterized the ability of AML blasts and KG-1 cells to bind to endothelial selectin type ligands. Flow cytometry revealed that, in addition to various integrin adhesion receptors, AML cells regularly express the P-selectin glycoprotein ligand (PSGL)-1, a ligand for P- and E-selectin on ECs. In parallel flow chambers, AML cells both rolled and adhered to TNF-alpha pretreated human umbilical vein endothelial cells (HUVECs). Pretreatment of HUVECs with anti-P- or anti-E-selectin function blocking antibodies significantly reduced both, rolling and subsequent arrest of primary AML cells. Intravital microscopy of i.v. injected fluorescence-labeled KG-1 cells into P-selectin deficient or wild type mice confirmed a significant role of endothelial P-selectin in the binding of human primary AML cells to ECs also in vivo. Thus, the currently available data suggest a role of P- and E-selectin in coordinated circulation of AML cells. Thus, P- or E-selectin mediated adhesion of AML cells may provide a target for the development anti-leukemic therapies.

Immunophenotypic profile and role of adhesion molecules in splenic marginal zone lymphoma with bone marrow involvement

Splenic Marginal Zone Lymphoma (SMZL), with or without villous lymphocytes (VL+/-), is a low-grade lymphoproliferative disorder with constant involvement of the bone marrow (BM). Different BM infiltration patterns, mainly intra-sinusoidal, interstitial and nodular, have been described. Adhesion molecules (AMs) constitute a heterogeneous group of antigenic receptors playing a major role in leukocyte recruitment, in lymphocyte homing and in cellular-mediated immune response. Evolution and pattern of the BM infiltrate could be influenced by a variable expression of AM on SMZL lymphocytes. The degree and pattern of BM infiltration and the immunohistochemical expression of AM (H-CAM, BL-CAM, L-selectin, PSGL-1, E-selectin, ICAM-1, VCAM-1 and Beta-1 integrin) among the different infiltration patterns were evaluated in BM biopsies of 38 patients with SMZL and graded according to a semi-quantitative score ranging from 0-4 and based on the percentage of positive cells. An intra-sinusoidal infiltration was constantly observed, alone or in conjunction with other patterns. H-CAM and BL-CAM showed a moderate-to-high degree of positivity in the intra-sinusoidal infiltrate (median expression grade-3) and were expressed in the neoplastic lymphocytes independently from the pattern. PSGL-1 was mostly expressed in the perisinusoidal region and in case of interstitial infiltration (grade-2). ICAM-1 and VCAM-1 were selectively expressed in the nodules as a reticular meshwork located in the core region (grade-2); VCAM-1 was also expressed in the perinodular endothelia. E-selectin, L-selectin and beta-1 integrin proved constantly negative. These data suggest that different expression of AM can influence the modality of BM infiltration in SMZL.

Role of ROS and MAPK in TPA-induced ICAM-1 expression in the myeloid ML-1 cell line

Intercellular adhesion molecule 1 (ICAM-1) has been implicated in playing a key role in the mechanism of inflammatory process initiated in response to environmental agents, and during normal hematopoietic cell differentiation. Though induction of ICAM-1 by 12-O-tetradecanoyl-phorbol-13-acetate (TPA) in myeloid cells has been reported, the molecular mechanism by which TPA upregulates ICAM-1 expression remains unclear. In the present study, we investigated the signaling mechanism associated with TPA-induced ICAM-1 expression in ML-1 cells. Herein, our microarray, flow cytometry, and Western blot analysis indicated that ICAM-1 was constitutively expressed at a low level in ML-1 cells, but its expression was further upregulated at both the mRNA and protein levels in response to TPA. ICAM-1 expression in response to TPA was inhibited by pretreatment with GF109203X [a specific inhibitor of protein kinase C (PKC)], or with PD98059 and U0126 (specific inhibitors of MEK), suggesting the importance of PKC, and Erk1/2 signaling cascades in this response. Interestingly, ICAM-1 expression in response to TPA-induced PKC activation was linked to the generation of reactive oxygen species (ROS), as pretreatment with NAC (an ROS scavenger) blocked both ErK1/2 activation and ICAM-1 expression induced by TPA. In addition, TPA-induced ICAM-1 expression was blocked by inhibition of nuclear factor-kappaB (NF-kappaB) activation following pretreatment with BAY11-7085 (a specific inhibitor of NF-kappaB activation). TPA-induced NF-kappaB activation was shown by increased degradation of IkB (NF-kappaB specific inhibitory protein). Together, these observations demonstrated that TPA, a potent activator of PKC, induces ICAM-1 expression via a ROS- and ERK1/2-dependent signaling mechanism in ML-1 cells.

BCR and its mutants, the reciprocal t(9;22)-associated ABL/BCR fusion proteins, differentially regulate the cytoskeleton and cell motility

Background

The reciprocal (9;22) translocation fuses the bcr (breakpoint cluster region) gene on chromosome 22 to the abl (Abelson-leukemia-virus) gene on chromosome 9. Depending on the breakpoint on chromosome 22 (the Philadelphia chromosome – Ph+) the derivative 9+ encodes either the p40^(ABL/BCR) fusion transcript, detectable in about 65% patients suffering from chronic myeloid leukemia, or the p96^(ABL/BCR) fusion transcript, detectable in 100% of Ph+ acute lymphatic leukemia patients. The ABL/BCRs are N-terminally truncated BCR mutants. The fact that BCR contains Rho-GEF and Rac-GAP functions strongly suggest an important role in cytoskeleton modeling by regulating the activity of Rho-like GTPases, such as Rho, Rac and cdc42. We, therefore, compared the function of the ABL/BCR proteins with that of wild-type BCR.

Methods

We investigated the effects of BCR and ABL/BCRs i.) on the activation status of Rho, Rac and cdc42 in GTPase-activation assays; ii.) on the actin cytoskeleton by direct immunofluorescence; and iii) on cell motility by studying migration into a three-dimensional stroma spheroid model, adhesion on an endothelial cell layer under shear stress in a flow chamber model, and chemotaxis and endothelial transmigration in a transwell model with an SDF-1α gradient.

Results

Here we show that both ABL/BCRs lost fundamental functional features of BCR regarding the regulation of small Rho-like GTPases with negative consequences on cell motility, in particular on the capacity to adhere to endothelial cells.

Conclusion

Our data presented here describe for the first time an analysis of the biological function of the reciprocal t(9;22) ABL/BCR fusion proteins in comparison to their physiological counterpart BCR.

Identification of genes involved in imatinib resistance in CML: a gene-expression profiling approach

The use of the tyrosine kinase inhibitor imatinib, which blocks the enzymatic action of the BCR-ABL fusion protein, has represented a critical advance in chronic myeloid leukemia (CML) treatment. However, a subset of patients initially fails to respond to this treatment. Use of complementary DNA (cDNA) microarray expression profiling allows the identification of genes whose expression is associated with imatinib resistance. Thirty-two CML bone marrow samples, collected before imatinib treatment, were hybridized to a cDNA microarray containing 6500 cancer genes, and analyzed using bootstrap statistics. Patients refractory to interferon-alpha treatment were evaluated for cytogenetic and molecular responses for a minimum of 12 months. A set of 46 genes was differentially expressed in imatinib responders and non-responders. This set includes genes involved in cell adhesion (TNC and SCAM-1), drug metabolism (cyclooxygenase 1), protein tyrosine kinases and phosphatases (BTK and PTPN22). A six-gene prediction model was constructed, which was capable of distinguishing cytogenetic response with an accuracy of 80%. This study identifies a set of genes that may be involved in primary resistance to imatinib, suggesting BCR-ABL-independent mechanisms.

Id1 transcription inhibitor-matrix metalloproteinase 9 axis enhances invasiveness of the breakpoint cluster region/abelson tyrosine kinase-transformed leukemia cells

Breakpoint cluster region/Abelson (BCR/ABL) tyrosine kinase enhances the ability of leukemia cells to infiltrate various organs. We show here that expression of the helix-loop-helix transcription factor Id1 is enhanced by BCR/ABL in a signal transducer and activator of transcription 5 (STAT5)-dependent manner. Enhanced expression of Id1 plays a key role in BCR/ABL-mediated cell invasion. Down-regulation of Id1 in BCR/ABL leukemia cells by the antisense cDNA significantly reduced their invasive capability through the Matrigel membrane and their ability to infiltrate hematopoietic and nonhematopoietic organs resulting in delayed leukemogenesis in mice. The Id1-promoted cell invasiveness was seemingly mediated by matrix metalloproteinase 9 (MMP9). Transactivation of MMP9 promoter in BCR/ABL cells was dependent on Id1 and abrogation of the MMP9 catalytic activity by a metalloproteinase inhibitor or blocking antibody decreased invasive capacity of leukemia cells. These data suggest that BCR/ABL-STAT5-Id1-MMP9 pathway may play a critical role in BCR/ABL-mediated leukemogenesis by enhancing invasiveness of leukemia cells.

Differential effect of leukaemogenic tyrosine kinases on cell motility is governed by subcellular localisation

The chemokine, stromal cell-derived factor-1 (SDF-1) is a crucial regulator of stem cell homing and tethering, and potentiation of this pathway in leukaemias may contribute to the pathogenesis of the disease. A key second messenger in SDF-1 signal/response coupling is phosphatidylinositol 3,4,5-triphosphate [PtdIns(3,4,5)P3]. SDF-1 elevated PtdIns(3,4,5)P3 levels markedly in the multipotent FDCP-mix stem cell line. Similarly, transfection with BCR/ABL or TEL/PDGFRbeta leukaemogenic tyrosine kinases chronically elevated PtdIns(3,4,5)P3 levels. However, whilst an SDF-1 chemotactic response was observed in TEL/PDGFRbeta-transfected cells, in BCR/ABL cells this was markedly decreased, which was not due to Ras-pathway activation. Thus, multipotent cells can respond to SDF-1, despite chronic increases in this second messenger indicating that a discrete pool of SDF-1-stimulated PtdIns(3,4,5)P3 production drives the chemotactic response. To discern the mechanism for the differential effects of these oncogenes we considered subcellular localisation. As TEL/PDGFRbeta has a cytosolic location whilst BCR/ABL associates with actin, we removed the actin-binding domain from BCR/ABL. We observed relocation of BCR/ABL to the cytosol and increased SDF-1 responses. We conclude that the localisation of BCR/ABL to the cytoskeleton is essential for effects on motility and moderating SDF-1 responses is not essential in tyrosine kinase-mediated leukaemic transformation.