Role of abnormal integrin-cytoskeletal interactions in impaired beta1 integrin function in chronic myelogenous leukemia hematopoietic progenitors

The FABD domain is critical for the oncogenicity of BCR/ABL in chronic myeloid leukaemia

BACKGROUND

Abnormally expressed BCR/ABL protein serves as the basis for the development of chronic myeloid leukaemia (CML). The F-actin binding domain (FABD), which is a crucial region of the BCR/ABL fusion protein, is also located at the carboxyl end of the c-ABL protein and regulates the kinase activity of c-ABL. However, the precise function of this domain in BCR/ABL remains uncertain.

METHODS

The FABD-deficient adenovirus vectors Ad-BCR/ABL△FABD, wild-type Ad-BCR/ABL and the control vector Adtrack were constructed, and 32D cells were infected with these adenoviruses separately. The effects of FABD deletion on the proliferation and apoptosis of 32D cells were evaluated by a CCK-8 assay, colony formation assay, flow cytometry and DAPI staining. The levels of phosphorylated BCR/ABL, p73, and their downstream signalling molecules were detected by western blot. The intracellular localization and interaction of BCR/ABL with the cytoskeleton-related protein F-actin were identified by immunofluorescence and co-IP. The effect of FABD deletion on BCR/ABL carcinogenesis in vivo was explored in CML-like mouse models. The degree of leukaemic cell infiltration was observed by Wright‒Giemsa staining and haematoxylin and eosin (HE) staining.

RESULTS

We report that the loss of FABD weakened the proliferation-promoting ability of BCR/ABL, accompanied by the downregulation of BCR/ABL downstream signals. Moreover, the deletion of FABD resulted in a change in the localization of BCR/ABL from the cytoplasm to the nucleus, accompanied by an increase in cell apoptosis due to the upregulation of p73 and its downstream proapoptotic factors. Furthermore, we discovered that the absence of FABD alleviated leukaemic cell infiltration induced by BCR/ABL in mice.

CONCLUSIONS

These findings reveal that the deletion of FABD diminished the carcinogenic potential of BCR/ABL both in vitro and in vivo. This study provides further insight into the function of the FABD domain in BCR/ABL.

Chronic Myeloid Leukemia: A Model Disease of the Past, Present and Future

Chronic myeloid leukemia (CML) has been a "model disease" with a long history. Beginning with the first discovery of leukemia and the description of the Philadelphia Chromosome and ending with the current goal of achieving treatment-free remission after targeted therapies, we describe here the journey of CML, focusing on molecular pathways relating to signaling, metabolism and the bone marrow microenvironment. We highlight current strategies for combination therapies aimed at eradicating the CML stem cell; hopefully the final destination of this long voyage.

Fibronectin and Its Receptors in Hematopoiesis

Fibronectin is a ubiquitous extracellular matrix protein that is produced by many cell types in the bone marrow and distributed throughout it. Cells of the stem cell niche produce the various isoforms of this protein. Fibronectin not only provides the cells a scaffold to bind to, but it also modulates their behavior by binding to receptors on the adjacent hematopoietic stem cells and stromal cells. These receptors, which include integrins such as α4β1, α9β1, α4β7, α5β1, αvβ3, Toll-like receptor-4 (TLR-4), and CD44, are found on the hematopoietic stem cell. Because the knockout of fibronectin is lethal during embryonal development and because fibronectin is produced by almost all cell types in mammals, the study of its role in hematopoiesis is difficult. Nevertheless, strong and direct evidence exists for its stimulation of myelopoiesis and thrombopoiesis using in vivo models. Other reviewed effects can be deduced from the study of fibronectin receptors, which showed their activation modifies the behavior of hematopoietic stem cells. Erythropoiesis was only stimulated under hemolytic stress, and mostly late stages of lymphocytic differentiation were modulated. Because fibronectin is ubiquitously expressed, these interactions in health and disease need to be taken into account whenever any molecule is evaluated in hematopoiesis.

TGF-β triggers rapid fibrillogenesis via a novel TβRII-dependent fibronectin-trafficking mechanism

There is increased recycling of soluble fibronectin from the cell surface for fibrillogenesis. This recycling is regulated by TGF-β in a transcription- and SMAD-independent manner via specific TβRII and integrin α5β1 interactions. The recycling of fibronectin is Rab11 dependent and is required for TGF-β–induced cell migration.

Fibronectin (FN) is a critical regulator of extracellular matrix (ECM) remodeling through its availability and stepwise polymerization for fibrillogenesis. Availability of FN is regulated by its synthesis and turnover, and fibrillogenesis is a multistep, integrin-dependent process essential for cell migration, proliferation, and tissue function. Transforming growth factor β (TGF-β) is an established regulator of ECM remodeling via transcriptional control of ECM proteins. Here we show that TGF-β, through increased FN trafficking in a transcription- and SMAD-independent manner, is a direct and rapid inducer of the fibrillogenesis required for TGF-β–induced cell migration. Whereas TGF-β signaling is dispensable for rapid fibrillogenesis, stable interactions between the cytoplasmic domain of the type II TGF-β receptor (TβRII) and the FN receptor (α5β1 integrin) are required. We find that, in response to TGF-β, cell surface–internalized FN is not degraded by the lysosome but instead undergoes recycling and incorporation into fibrils, a process dependent on TβRII. These findings are the first to show direct use of trafficked and recycled FN for fibrillogenesis, with a striking role for TGF-β in this process. Given the significant physiological consequences associated with FN availability and polymerization, our findings provide new insights into the regulation of fibrillogenesis for cellular homeostasis.

Multiple myeloma-related deregulation of bone marrow-derived CD34(+) hematopoietic stem and progenitor cells

Multiple myeloma (MM) is a clonal plasma cell disorder frequently accompanied by hematopoietic impairment. We show that hematopoietic stem and progenitor cells (HSPCs), in particular megakaryocyte-erythrocyte progenitors, are diminished in the BM of MM patients. Genomic profiling of HSPC subsets revealed deregulations of signaling cascades, most notably TGFβ signaling, and pathways involved in cytoskeletal organization, migration, adhesion, and cell-cycle regulation in the patients. Functionally, proliferation, colony formation, and long-term self-renewal were impaired as a consequence of activated TGFβ signaling. In accordance, TGFβ levels in the BM extracellular fluid were elevated and mesenchymal stromal cells (MSCs) had a reduced capacity to support long-term hematopoiesis of HSPCs that completely recovered on blockade of TGFβ signaling. Furthermore, we found defective actin assembly and down-regulation of the adhesion receptor CD44 in MM HSPCs functionally reflected by impaired migration and adhesion. Still, transplantation into myeloma-free NOG mice revealed even enhanced engraftment and normal differentiation capacities of MM HSPCs, which underlines that functional impairment of HSPCs depends on MM-related microenvironmental cues and is reversible. Taken together, these data implicate that hematopoietic suppression in MM emerges from the HSPCs as a result of MM-related microenvironmental alterations.

CCN3 suppresses mitogenic signalling and reinstates growth control mechanisms in Chronic Myeloid Leukaemia

CCN3, a tumour suppressor gene, is down-regulated as a result of BCR-ABL tyrosine kinase activity in Chronic Myeloid Leukaemia (CML). We have established a stable CCN3 expression model in the human K562 CML cell line and have further validated the role for CCN3 in the leukaemogenic process. K562 cells stably transfected with CCN3 (K562/CCN3; 2.25 × 10(6) copies per 50 ng cDNA) demonstrated over 50% reduction in cell growth in comparison to cells stably transfected with empty vector (K562/control; p = 0.005). K562/CCN3 cells had reduced colony formation capacity (reduced by 29.7%, p = 0.03) and reduced mitogenic signalling in comparison to K562/control cells (reduced by 29.5% (p = 0.002) and 37.4% (p = 0.017) for phosphorylation levels of ERK and AKT respectively). K562/CCN3 cells showed an accumulation of events within the subG(0) phase of the cell cycle and increased apoptosis was confirmed by a three-fold increase in annexin V binding (p < 0.05). K562/CCN3 cells exposed to Imatinib (1 μM and 5 μM) showed an increase in events within the subG(0) phase of cell cycle over 96 h and mirrored the enhanced cell kill demonstrated by Annexin staining. Wild type K562 cells treated with recombinant human Ccn3 (10 nM) in combination with Imatinib (5 μM) also displayed enhanced cell kill (p = 0.008). K562/CCN3 cells displayed increased adhesion to matrigel™ (2.92 ± 0.52 fold increase compared to K562/control) which was commensurate with increased expression of the alpha 6 and beta 4 integrins (6.53 ± 0.47 and 1.94 ± 0.07 fold increase in gene expression respectively (n = 3, p < 0.05)). CCN3 restores cellular growth regulatory properties that are absent in CML and sensitises CML cells to imatinib induced apoptosis. CCN3 may provide novel avenues for the development of alternate therapeutic strategies.

BCR-ABL1 kinase-dependent alteration of mRNA metabolism: potential alternatives for therapeutic intervention

The use of first- and second-generation tyrosine kinase inhibitors (TKIs) significantly improves prognosis for patients with early chronic phase chronic myeloid leukemia (CML) and efficiently counteracts leukemia in most patients with CML bearing a disease characterized by the expression of BCR-ABL1 mutants. However, the so-called 'tinib' TKIs (e.g. imatinib, nilotinib, dasatinib, and bosutinib) are both ineffective in patients who undergo blastic transformation and unable to eradicate CML at the stem cell level. This raises a few important questions. Is BCR-ABL1 expression and/or activity essential for blastic transformation? Is blastic transformation the result of genetic or epigenetic events that occur at the stem cell level which only become apparent in the granulocyte-macrophage progenitor (GMP) cell pool, or does it arise directly at the GMP level? As altered mRNA metabolism contributes to the phenotype of blast crisis CML progenitors (decreased translation of tumor suppressor genes and transcription factors essential for terminal differentiation and increased translation of anti-apoptotic genes), one attractive concept is to restore levels of these essential molecules to their normal levels. In this review, we discuss the mechanisms by which mRNA processing, translation, and degradation are deregulated in BCR-ABL1 myeloid blast crisis CML progenitors, and present encouraging results from studies with pharmacologic inhibitors which support their inclusion in the clinic.

Endogenous cAbl regulates receptor endocytosis

There are two key processes underlying ligand-induced receptor endocytosis: receptor ubiquitylation and remodeling of the actin cytoskeleton. Tyrosine kinases play critical roles in both receptor endocytosis and actin reorganization. Interestingly, members of the Abl family are the only known tyrosine kinases that possess an actin-binding domain and thus have the potential to directly regulate the actin cytoskeleton. However, the role of non-transforming cAbl in receptor endocytosis remains undefined. We report that cAbl promotes ligand-induced antigen receptor endocytosis in B lymphocytes. We show that pharmacologic inhibition or genetic deletion of cAbl causes a defect in tyrosine phosphorylation of the cytoskeletal adapter CrkII. cAbl inhibition or ablation also impairs Rac activation downstream of CrkII, as well as antigen receptor capping and endocytosis. Although phosphorylation of CrkII has been suggested to maintain it in a closed inactive conformation, we demonstrate that it is in fact essential for the activation of Rac. On the other hand, association of CrkII with cCbl, a key mediator of receptor ubiquitylation, does not require CrkII phosphorylation and is cAbl-independent. Phosphorylation of cCbl itself is also cAbl-independent. Our results thus indicate that CrkII links receptor engagement to cytoskeletal remodeling by coupling cCbl- and cAbl-mediated signaling pathways that cooperatively regulate ligand-induced receptor endocytosis.

Differential motility of p190bcr-abl- and p210bcr-abl-expressing cells: respective roles of Vav and Bcr-Abl GEFs

The chimeric oncogene Bcr-Abl is known to induce autonomous motility of leukemic cells. We show here that p210(bcr-abl) responsible for chronic myelogenous leukemia induces an amoeboid type of motility while p190(bcr-abl), associated with acute lymphoid leukemia, induces a rolling type of motility. We previously reported that p210(bcr-abl) activates RhoA and Rac1, while p190(bcr-abl) although devoid of a Dbl-homology (DH) domain activates Rac1, but not RhoA. We investigated the regulation of GDP/GTP exchange factor (GEF) activities in the Bcr-Abl complex. For that purpose, different GEF activity mutants of Vav and of Bcr-Abl were constructed and stably transfected in Ba/F3 cells. Using these mutants, we demonstrate that RhoA is exclusively activated by the DH domain of p210(bcr-abl), while Rac1 activation is mostly due to Vav. Inhibition of Rac1 by Vav GEF mutant leads to immobilization of cells. Vav depletion using shRNA also induces immobilization of cells and suppression of GTP-bound Rac1. RhoA inactivation induces the specific loss of amoeboid movements. These results suggest that Rac1 activation by Vav triggers the motility of Bcr-Abl-expressing Ba/F3 cells, while the specific amoeboid mode of motility induced by p210(bcr-abl) is a consequence of RhoA activation.

The rho-kinase inhibitors Y-27632 and fasudil act synergistically with imatinib to inhibit the expansion of ex vivo CD34(+) CML progenitor cells

Evidence from cell line-based studies indicates that rho-kinase may play a role in the leukaemic transformation of human cells mediated by the BCR/ABL tyrosine kinase, manifest clinically as chronic myeloid leukaemia (CML). We therefore employed two separate inhibitors, Y-27632 and fasudil, to inhibit the activity of rho-kinase against ex vivo CD34(+) cells collected from patients with CML. We compared the effects of rho-kinase inhibition in those cells with the effects of direct inhibition of BCR/ABL using the specific inhibitor imatinib. We found that inhibition of rho-kinase inhibited the effective proliferation, and reduced survival of CML progenitor cells. When combined with imatinib, rho-kinase inhibition added to the anti-proliferative and pro-apoptotic effects of the BCR/ABL inhibitor. Our studies may indicate therapeutic benefit in some cases for the combination of rho-kinase inhibitors with imatinib.

Bcr-Abl induces abnormal cytoskeleton remodeling, beta1 integrin clustering and increased cell adhesion to fibronectin through the Abl interactor 1 pathway

Hematopoietic cells isolated from patients with Bcr-Abl-positive leukemia exhibit multiple abnormalities of cytoskeletal and integrin function. These abnormalities are thought to play a role in the pathogenesis of leukemia; however, the molecular events leading to these abnormalities are not fully understood. We show here that the Abi1 pathway is required for Bcr-Abl to stimulate actin cytoskeleton remodeling, integrin clustering and cell adhesion. Expression of Bcr-Abl induces tyrosine phosphorylation of Abi1. This is accompanied by a subcellular translocation of Abi1/WAVE2 to a site adjacent to membrane, where an F-actin-enriched structure containing the adhesion molecules such as beta1-integrin, paxillin and vinculin is assembled. Bcr-Abl-induced membrane translocation of Abi1/WAVE2 requires direct interaction between Abi1 and Bcr-Abl, but is independent of the phosphoinositide 3-kinase pathway. Formation of the F-actin-rich complex correlates with an increased cell adhesion to fibronectin. More importantly, disruption of the interaction between Bcr-Abl and Abi1 by mutations either in Bcr-Abl or Abi1 not only abolished tyrosine phosphorylation of Abi1 and membrane translocation of Abi1/WAVE2, but also inhibited Bcr-Abl-stimulated actin cytoskeleton remodeling, integrin clustering and cell adhesion to fibronectin. Together, these data define Abi1/WAVE2 as a downstream pathway that contributes to Bcr-Abl-induced abnormalities of cytoskeletal and integrin function.

Role of integrin cross-regulation in parvovirus B19 targeting

Most viral vectors used for gene therapy lack the ability to target a defined cell population. Parvovirus B19 has a restricted tropism for human erythroid progenitor cells and uses activated alpha5beta1 integrins as coreceptors for entry [Weigel-Kelley, K.A., Yoder, M.C., and Srivastava, A. (2003). Blood 102, 3927-3933]. In this study we examined the role of coexpressed integrins in alpha5beta1 integrin coreceptor function. Antibody-mediated cross-linking of beta1, beta2, and beta3 integrins and the integrin-associated protein (IAP) increased parvovirus B19 entry into nontarget K562 cells. Functional silencing of one integrin group, however, reduced the virus uptake- promoting function of a subsequently activated integrin group, indicating that the three integrins did not operate in isolation but through shared signaling pathways. This was further corroborated by direct competition between simultaneously clustered beta2 and beta1 integrins that could be overcome by stabilizing clustered beta1 integrins in a high-affinity conformation. In contrast, parvovirus B19 entry into primary erythroid progenitor cells was characterized by strong clustering-induced beta1 integrin coreceptor activity that was not abolished by subsequent beta2 and beta3 integrin activation and was, in fact, substantially increased in the presence of preclustered beta2 and beta3 integrins. Thus, integrin function is regulated in a cell type-specific manner through coexpressed integrins and preferential parvovirus B19 entry into erythroid progenitor cells is promoted by a robust beta1 integrin response that is enhanced through stable preclustering of coexpressed integrins. These results have implications for other viral vectors that use integrins as receptors/coreceptors and for gene therapy of hematopoietic progenitor cells using parvovirus B19 vectors.

BCR/ABL-mediated downregulation of genes implicated in cell adhesion and motility leads to impaired migration toward CCR7 ligands CCL19 and CCL21 in primary BCR/ABL-positive cells

The mechanism underlying p210(BCR/ABL) oncoprotein-mediated transformation in chronic myelogenous leukemia (CML) is not fully understood. We hypothesized that p210(BCR/ABL) suppresses expression of genes which may explain at least some of the pathogenetic features of CML. A subtractive cDNA library was created between BCR/ABL-enhanced-green-fluorescent-protein (GFP)-transduced umbilical cord blood (UCB) CD34+ cells and GFP-transduced UCB CD34+ cells to identify genes whose expression is downregulated by p210(BCR/ABL). At least 100 genes were identified. We have confirmed for eight of these genes that expression was suppressed by quantitative real-time-RT-PCR (Q-RT-PCR) of additional p210(BCR/ABL)-transduced CD34+ UCB cells as well as primary early chronic phase (CP) bone marrow (BM) CML CD34+ cells. Imatinib mesylate reversed downregulation of some genes, to approximately normal levels. Several of the genes are implicated in cell adhesion and motility, including L-selectin, intercellular adhesion molecule-1 (ICAM-1), and the chemokine receptor, CCR7, consistent with the known defect in adhesion and migration of CML cells. Compared with GFP UCB or normal (NL) BM CD34+ cells, p210 UCB and CML CD34+ cells migrated poorly towards the CCR7 ligands, CCL19 and CCL21, suggesting a possible role for CCR7 in the abnormal migratory behavior of CML CD34+ cells.

CD34+ marrow progenitors from MDS patients with high levels of intramedullary apoptosis have reduced expression of α4β1 and α5β1 integrins

Excessive intramedullary apoptosis is central in the pathogenesis of myelodysplastic syndromes (MDS). Growth-inhibiting cytokines, the Fas/FasLigand pathway, and autoreactive cytotoxic T-lymphocytes have been identified to be important proapoptotic factors in MDS. In normal hematopoiesis, alpha4beta1 and alpha5beta1 integrin-mediated interactions between progenitors and fibronectin are critical for progenitor cell survival. In this study, we have used flow cytometry to quantify the expression levels of members of the beta1 integrin family on CD34(+) marrow progenitors in 27 untreated patients with MDS, three with s-AML, and 25 control subjects. In MDS, we observed that nonapoptotic progenitors significantly downregulate cell surface expression levels of alpha4 and beta1 integrin chains compared with healthy controls. Downregulation of alpha4, beta1, and also alpha5 was present in MDS patients with > or =25% apoptotic progenitors, irrespective of their French, American, British subcategory. Reduced cell surface expression levels of alpha4, alpha5, and beta1 did also correlate with decreased in vitro adhesiveness to fibronectin fragments. Therefore, our observations suggest that downregulation of alpha4beta1 and alpha5beta1 integrins on CD34(+) progenitors could be a newly identified proapoptotic mechanism in MDS.

Integrin engagement-induced inhibition of human myelopoiesis is mediated by proline-rich tyrosine kinase 2 gene products

OBJECTIVE

Hematopoietic progenitor proliferation and differentiation are inhibited by integrin engagement of fibronectin (FN). Focal adhesion kinases have been shown to mediate intracellular signaling from integrins, and we recently demonstrated that gene expression and pre-mRNA splicing of the focal adhesion kinase, PYK2, is abnormal in CD34(+) cells from chronic myelogenous leukemia (CML) patients. Here we investigated whether PYK2 gene products mediate integrin signaling in hematopoietic stem and progenitor cells.

METHODS

Cord blood CD34(+) cells were retrovirally transduced with vectors encoding Pyk2H, Pyk2, or the dominant negative-acting, kinase-deficient, C-terminal PYK2 fragment, PRNK, and myeloid proliferation and differentiation was assessed using colony-forming cell (CFC), long-term culture-initiating cell (LTC-IC), and liquid culture assays.

RESULTS

CD34(+) cells overexpressing Pyk2H or Pyk2 generated 50% less colony-forming unit granulocyte/macrophage (CFU-GM) than eGFP-transduced controls. Although the number of CFC generated by PRNK-expressing cells was unchanged, LTC-IC were significantly reduced. Culture of CD34(+) cells on FN significantly reduced the generation of mature myeloid cells vs those cultured on BSA-coated wells, and could be overcome by addition of SCF. As is observed when integrins are engaged, overexpression of either Pyk2H or Pyk2 decreased committed myeloid progenitor proliferation and differentiation; however, SCF could not override this inhibition. Finally, as is observed when integrins are not engaged, PRNK-mediated inhibition of endogenous Pyk2H resulted in integrin-nonresponsive proliferation and differentiation of myeloid precursors and accelerated differentiation of primitive hematopoietic progenitors.

CONCLUSION

These studies indicate that PYK2 gene products mediate integrin-induced signals that regulate myelopoiesis.

p210BCR/ABL-induced alteration of pre-mRNA splicing in primary human CD34+ hematopoietic progenitor cells

Chronic myelogenous leukemia (CML) is a malignancy of the human hematopoietic stem cell (HSC) caused by the p210BCR/ABL oncoprotein. Although alternative splicing of pre-mRNA is a critical determinant of a cell's protein repertoire, it has not been associated with CML pathogenesis. We identified a BCR/ABL-dependent increase in expression of multiple genes involved in pre-mRNA splicing (eg SRPK1, RNA Helicase II/Gu, and hnRNPA2/B1) by subtractive hybridization of cDNA from p210BCR/ABL-eGFP vs eGFP-transduced umbilical cord blood CD34+ cells. beta1-integrin signaling is important to HSC maintenance and proliferation/differentiation, and is abnormal in CML. As an example of how changes in pre-mRNA processing might contribute to CML pathogenesis, we observed alternative splicing of a gene for a beta1-integrin-responsive nonreceptor tyrosine kinase (PYK2), resulting in increased expression of full-length Pyk2 in BCR/ABL-containing cells. Treatment of p210BCR/ABL-positive cells with the Abl-specific tyrosine kinase inhibitor STI571 reverted PYK2 splicing to a configuration more consistent with normal cells, and correlated with decreased expression of BCR/ABL-induced proteins involved in pre-mRNA processing. Whether altered PYK2 splicing contributes to CML pathogenesis remains undetermined; however, we propose that generic changes in pre-mRNA splicing as a result of p210BCR/ABL kinase activity may contribute to CML pathogenesis.

Alpha5beta1 integrin as a cellular coreceptor for human parvovirus B19: requirement of functional activation of beta1 integrin for viral entry

Replication of the pathogenic human parvovirus B19 is restricted to erythroid progenitor cells. Although blood group P antigen has been reported to be the cell surface receptor for parvovirus B19, a number of nonerythroid cells, which express P antigen, are not permissive for parvovirus B19 infection. We have documented that P antigen is necessary for parvovirus B19 binding but not sufficient for virus entry into cells. To test whether parvovirus B19 utilizes a cell surface coreceptor for entry, we used human erythroleukemia cells (K562), which allow parvovirus B19 binding but not entry. We report here that upon treatment with phorbol esters, K562 cells become adherent and permissive for parvovirus B19 entry, which is mediated by alpha 5 beta 1 integrins, but only in their high-affinity conformation. Mature human red blood cells (RBCs), which express high levels of P antigen, but not alpha 5 beta 1 integrins, bind parvovirus B19 but do not allow viral entry. In contrast, primary human erythroid progenitor cells express high levels of both P antigen and alpha 5 beta 1 integrins and allow beta1 integrin-mediated entry of parvovirus B19. Thus, in a natural course of infection, RBCs are likely exploited for a highly efficient systemic dissemination of parvovirus B19.

CD7 expression by CD34+ cells in CML patients, of prognostic significance?

The purpose of the study was to identify a unique immunophenotype of normal or Philadelphia chromosome positive (Ph+) CD34+ cells that might be used to purify normal CD34+ cells from chronic myelogenous leukemia (CML) patients. An immunophenotypical study of CD34+ bone marrow cells of 20 patients with CML at diagnosis and during hydroxyurea treatment, and 39 controls were performed. All patients were Ph+, two patients had variant translocations and three patients displayed cytogenetic signs of clonal evolution. The immature progenitor cell compartment (CD34+ HLA-DR- and CD34+ CD38- cells) was comparable. The CD34+ AC133+ progenitor cell compartment was decreased in CML patients. We found no difference for any of the adhesion molecules examined except for CD62L, where the percentage of CD34+ CD62L+ cells was decreased in CML patients. The number of myeloid progenitors (CD34+ CD33+) was increased at the expense of B-lymphoid progenitors (CD34+ CD10+ and CD34+ CD19+) in CML patients indicating that B-lymphopoiesis is inhibited in CML. The megakaryocytic (CD34+ CD61+) and erythroid (CD34+ CD71+) progenitors were increased in CML patients. The number of CD34+ CD7+ cells was also significantly increased (mean 25.3% vs. 4.9%). However, the level of CD7 expression was quite heterogeneous, and the patients could be separated into two populations according to CD7 expression (more or less than 20% CD7+ CD34+ cells). The Sokal and Hasford risk scores did not differ between CD34+ CD7- CML and CD34+ CD7+ CML, but all patients with signs of disease progression clustered in the CD34+ CD7+ population indicating that the level of CD7 expression on CD34+ cells may be of prognostic importance in CML.

Localization of BCR-ABL to F-actin regulates cell adhesion but does not attenuate CML development

We have previously found that P210BCR-ABL increases the adhesion of hematopoietic cell lines to fibronectin by a mechanism that is independent of tyrosine kinase activity. To investigate the pathway(s) by which P210BCR-ABL influences cell adhesion, we used a quantitative cell adhesion device that can discern small changes in cell adhesion to assay P210BCR-ABL with mutations in several critical domains. We expressed P210BCR-ABL mutants in 32D myeloblast cells and found that binding to fibronectin is mediated primarily by the alpha5beta1 integrin. We performed a structure/function analysis to map domains important for cell adhesion. Increased adhesion was mediated by 3 domains: (1) the N-terminal coiled-coil domain that facilitates oligomerization and F-actin localization; (2) bcr sequences between aa 163 to 210; and (3) F-actin localization through the C-terminal actin-binding domain of c-abl. We compared our adhesion results with the ability of these mutants to cause a chronic myelogenous leukemia (CML)-like disease in a murine bone marrow transplantation assay and found that adhesion to fibronectin did not correlate with the ability of these mutants to cause CML. Together, our results suggest that F-actin localization may play a pivotal role in modulating adhesion but that it is dispensable for the development of CML.

Quantitative PCR identifies a minimal deleted region of 120 kb extending from the Philadelphia chromosome ABL translocation breakpoint in chronic myeloid leukemia with poor outcome

Fluorescence in situ hybridization (FISH) analysis has shown previously that 10-15% of chronic myeloid leukemias (CML) have hemizygous deletions of variable sizes affecting regions that flank the ABL and BCR translocation breakpoints on the derivative chromosome 9, and these patients have a poor outcome. FISH studies using large commercial genomic probes have previously suggested that haploinsufficiency of sequences flanking either ABL or BCR modify the disease process of CML and lead to an unfavorable prognosis. In this present study, real-time quantitative PCR (Q-PCR) analysis was used to identify and map much smaller hemizygous microdeletions in a subset of CML patients that were not deleted using large genomic FISH probes. Microdeletions were identified by Q-PCR in 25 of 71 patients selected based on less favorable outcome (chronic phase duration of less than 96 months and a survival time of less than 84 months). In contrast, no microdeletion was detected in any of 18 CML samples selected from a group with a more favorable outcome. Detailed mapping of the 25 Q-PCR microdeletions showed that the minimal deleted region extended approximately 120 kb from the 5' end of the ABL gene in the centromeric direction on the derivative chromosome 9, and the region 3' to BCR on chromosome 22 was excluded. Of the four ESTs and/or genes that map to the 120 kb region, the putative tumor suppressor PRDM12 is the strongest candidate gene. The potential role for each sequence in modifying the clinical behavior of CML is presented.

The biology of chronic myelogenous leukemia:mouse models and cell adhesion

Chronic myelogenous leukemia (CML) is a biphasic neoplasm of the bone marrow that is precipitated by the Philadelphia chromosome, a t(9;22) balanced translocation that encodes a constitutively activated nonreceptor tyrosine kinase termed P210(BCR-ABL). This oncoprotein has several intracellular functions; however, the most important effect of P210(BCR-ABL) leading to cell transformation is phosphorylation of signaling molecules through a constitutively active tyrosine kinase domain. Despite extensive knowledge of the structure and functional domains of BCR-ABL, its precise function in transformation is not known. Progress has been hampered, in part, by the lack of relevant CML models, as cell culture and in vitro assays do not mimic the pathogenesis of CML. Recently, there has been significant progress toward improving murine models that closely resemble human CML. This has allowed researchers to evaluate critical functions of BCR-ABL and has provided a model to test the efficacy of therapeutic medications that block these pathways. Our laboratory has developed two intersecting research programs to better understand the functioning of P210(BCR-ABL) in leukemogenesis. In one approach, we have developed a murine CML model by transferring HSCs that express BCR-ABL from a retroviral vector. All recipients develop a rapidly fatal MPD that shares several important features with CML. This model has been extremely useful for studying the function of BCR-ABL in the pathogenesis of CML. A second approach utilizes a quantitative cell detachment apparatus capable of measuring small changes in cell adhesion to investigate the mechanism by which P210(BCR-ABL) causes abnormal cell binding. Altered cell adhesion may contribute to the imbalance between proliferation and self-renewal in the hematopoietic progenitor compartment. To better understand the role abnormal adhesion may play in the development of leukemia, we have attempted to correlate the effects of functional P210(BCR-ABL) mutants in regulating adhesion and oncogenicity.

Immature leukemic CD34+CXCR4+ cells from CML patients have lower integrin-dependent migration and adhesion in response to the chemokine SDF-1

Chronic myelogenous leukemia (CML), a malignant myeloproliferative disorder originating from a pluripotent stem cell expressing the bcr-abl oncogene, is characterized by abnormal release of the expanded, malignant stem cell clone from the bone marrow (BM) into the circulation. Moreover, immature CD34+ CML cells have lower adhesion to stromal cells and fibronectin as well as lower engraftment potential in severe combined immunedeficient (SCID) and nonobese diabetic (NOD)/SCID mice than normal CD34+ cells. We report in this study that leukemic Philadelphia chromosome-positive (Ph+)CD34+ cells from newly diagnosed CML patients that express the chemokine receptor CXCR4 migrate in response to stromal-derived factor-1 (SDF-1). However, normal Ph-CD34+CXCR4+ cells derived from the same patient have significantly higher migration levels toward SDF-1. In contrast to their transwell migration potential, the SDF-1-mediated integrin-dependent polarization and migration of the Ph+CD34+CXCR4+ cells through extracellular matrix-like gels were significantly lower than for normal cells. Concomitantly, binding of these cells to vascular cell adhesion molecule-1 or fibronectin, in the presence of SDF-1, was also substantially lower. These findings suggest a major role for SDF-1-mediated, integrin-dependent BM retention of Ph+CD34+ cells.

Reduced focal adhesion kinase and paxillin phosphorylation in BCR-ABL-transfected cells

BACKGROUND

BCR-ABL formation is critical to oncogenic transformation in chronic myelogenous leukemia and has been implicated as a key event leading to alterations in cytoskeletal structures and adhesion in the leukemic cells. The authors therefore investigated the effect of p210(BCR-ABL) on actin polymerization as well as on the expression and phosphorylation state of the adhesion proteins paxillin and focal adhesion kinase (FAK).

METHODS

Transfection with BCR-ABL constructs abrogated the ability of NIH 3T3 fibroblasts to adhere and the cells underwent striking morphologic changes.

RESULTS

Scanning electron microscopy revealed that the cells lost their elongated appearance and became rounded. This alteration was associated with significantly reduced actin polymerization. In addition, steady-state levels of paxillin and FAK protein were increased. However, while the overall level of phosphotyrosines was also increased, the amount of tyrosine phosphorylated paxillin and FAK was reduced in the BCR-ABL-transfected cells as compared to the parental cells. Culture on extracellular fibronectin matrix partially reversed the morphologic changes and resulted in a return, albeit incomplete, of filamentous actin in BCR-ABL-transfected 3T3 fibroblasts. In addition, phosphorylation of paxillin and FAK in the BCR-ABL-transfected NIH 3T3 cells was restored.

CONCLUSIONS

The authors conclude that, in the current system, transfection of BCR-ABL attenuates FAK and paxillin phosphorylation and reduces actin polymerization, events accompanied by significant alterations in cellular morphology. The observation that exposure of the cells to fibronectin partially reverses all these changes suggests that the focal adhesion proteins and actin structures nevertheless remain responsive to signaling from the outside.

Imatinib mesylate (STI571) inhibits growth of primitive malignant progenitors in chronic myelogenous leukemia through reversal of abnormally increased proliferation

Imatinib mesylate (STI571) is a promising new treatment for chronic myelogenous leukemia (CML). The effect of imatinib mesylate on primitive malignant progenitors in CML has not been evaluated, and it is not clear whether suppression of progenitor growth represents inhibition of increased proliferation, induction of apoptosis, or both. We demonstrated here that in vitro exposure to concentrations of imatinib mesylate usually achieved in patients (1-2 microM) for 96 hours inhibited BCR/ABL-positive primitive progenitors (6-week long-term culture-initiating cells [LTCICs]) as well as committed progenitors (colony-forming cells [CFCs]). No suppression of normal LTCICs and significantly less suppression of normal CFCs were observed. A higher concentration of imatinib mesylate (5 microM) did not significantly increase suppression of CML or normal LTCICs but did increase suppression of CML CFCs, and to a lesser extent, normal CFCs. Analysis of cell division using the fluorescent dye carboxyfluorescein diacetate succinimidyl ester indicated that imatinib mesylate (1-2 microM) inhibits cycling of CML primitive (CD34(+)CD38(-)) and committed (CD34(+)CD38(+)) progenitors to a much greater extent than normal cells. Conversely, treatment with 1 to 2 microM imatinib mesylate did not significantly increase the percentage of cells undergoing apoptosis. Although a higher concentration of imatinib mesylate (5 microM) led to an increase in apoptosis of CML cells, apoptosis also increased in normal samples. In summary, at clinically relevant concentrations, imatinib mesylate selectively suppresses CML primitive progenitors by reversing abnormally increased proliferation but does not significantly increase apoptosis. These results suggest that inhibition of Bcr-Abl tyrosine kinase by imatinib mesylate restores normal hematopoiesis by removing the proliferative advantage of CML progenitors but that elimination of all CML progenitors may not occur.

Adhesion to fibronectin selectively protects Bcr-Abl+ cells from DNA damage-induced apoptosis

The phenotype of Bcr-Abl-transformed cells is characterized by a growth factor-independent survival and a reduced susceptibility to apoptosis. Furthermore, Bcr-Abl kinase alters adhesion features by phosphorylating cytoskeletal and/or signaling proteins important for integrin function. Integrin-mediated adhesion to extracellular matrix molecules is critical for the regulation of growth and apoptosis. However, effects of integrin signaling on regulation of apoptosis in cells expressing Bcr-Abl are largely unknown. The influence of adhesion on survival and apoptosis in Bcr-Abl+ and Bcr-Abl- BaF3 cells was investigated. p185bcr-abl-transfected BaF3 cells preadhered to immobilized fibronectin had a significant survival advantage and reduced susceptibility to apoptosis following gamma-irradiation when compared with the same cells grown on laminin, on polylysin, or in suspension. Both inhibition of Bcr-Abl kinase by STI571 and inhibition of specific adhesion reversed the fibronectin-mediated antiapoptotic effect in BaF3p185. The DNA damage response of Bcr-Abl- BaF3 cells was not affected by adhesion to fibronectin. In contrast to parental BaF3 cells, BaF3p185 adherent to fibronectin did not release cytochrome c to the cytosol following irradiation. The fibronectin-mediated antiapoptotic mechanism in Bcr-Abl-active cells was not mediated by overexpression of Bcl-XL or Bcl-2 but required an active phosphatidylinositol 3-kinase (PI-3K). Kinase-active Bcr-Abl in combination with fibronectin-induced integrin signaling led to a hyperphosphorylation of AKT. Thus, cooperative activation of PI-3K/AKT by Bcr-Abl and integrins causes synergistic protection of Bcr-Abl+ cells from DNA damage-induced apoptosis.

Primary chromosomal rearrangements of leukemia are frequently accompanied by extensive submicroscopic deletions and may lead to altered prognosis

BCR/ABL fluorescent in situ hybridization study of chronic myeloid leukemia (CML) and Philadelphia(+) (Ph(+)) acute lymphoid leukemia (ALL) indicated that approximately 9% of patients exhibited an atypical hybridization pattern consistent with a submicroscopic deletion of the 5' region of ABL and the 3' region of the BCR genes on the 9q(+) chromosome. The CML patients with deletions had a shorter survival time and a high relapse rate following bone marrow transplant. Since deletions are associated with both Ph(+) CML and ALL, it seemed probable that other leukemia-associated genomic rearrangements may also have submicroscopic deletions. This hypothesis was confirmed by the detection of deletions of the 3' regions of the CBFB and the MLL genes in AML M4 patients with inv(16) and in patients with ALL and AML associated with MLL gene translocations, respectively. In contrast, analysis of the AML M3 group of patients and AML M2 showed that similar large deletions were not frequently associated with the t(15;17) or t(8;21) translocations. Analysis of sequence data from each of the breakpoint regions suggested that large submicroscopic deletions occur in regions with a high overall density of Alu sequence repeats. These findings are the first to show that the process of deletion formation is not disease specific in leukemia and also implicate that the presence of repetitive DNA in the vicinity of breakpoint regions may facilitate the generation of submicroscopic deletions. Such deletions could lead to the loss of one or more genes, and the associated haploinsufficiency may result in the observed differences in clinical behavior. (Blood. 2001;97:3581-3588)

Novel therapies for chronic myelogenous leukemia

The BCR-ABL oncogene is essential to the pathogenesis of chronic myelogenous leukemia, and immune mechanisms play an important role in control of this disease. Understanding of the molecular pathogenesis of chronic myelogenous leukemia has led to the development of several novel therapies, which can be broadly divided into therapies based on 1) inhibition of the BCR-ABL oncogene expression, 2) inhibition of other genes important to the pathogenesis of chronic myelogenous leukemia, 3) inhibition of BCR-ABL protein function, and 4) immunomodulation. We have systematically reviewed each of these novel therapeutic approaches in this article.

VLA-4 mediated adhesion to bone marrow stromal cells confers chemoresistance to adherent lymphoma cells

The mechanisms of maintenance of residual lymphoma in bone marrow during chemotherapy are currently not well understood. Previous studies have shown that primary lymphoma cells obtained from histologically negative bone marrow of non-Hodgkin's lymphoma (NHL) patients grew in long-term bone marrow cultures primarily in association with bone marrow stromal cells. Furthermore, the interaction of NHL patient cells with bone marrow stromal cells inhibited their spontaneous apoptosis. The current studies were designed to characterize the components of the heterotypic interaction between lymphoma cells and bone marrow stromal cells as well as to probe the consequences of this interaction as it pertains to the potential survival of minimal numbers of lymphoma cells during chemotherapy. Cellular adhesion assays performed in the presence of either neutralizing antibodies to VCAM- or the alpha and beta subunit of VLA-4 resulted in >95%, 82% and 35% inhibition of lymphoma cell line adhesion to the bone marrow stromal line MS-5, respectively. Modulation of VLA-4 affinity by the 8A2 antibody resulted in enhanced secondary adhesion at 24 and 72 hours to either cellular fibronectin (65% and 65%) or MS-5 cells (60% and 55%), superceding levels obtained using untreated lymphoma cells (<20%). The bone marrow stromal cells induced a chemoprotective effect for adherent lymphoma cells over a 3-log dose range of vincristine, resulting in a 2-log increase in the ED50 at day 6 of culture. The failure of glutaraldehyde fixed stromal cells to induce a chemoprotective effect demonstrated that viable bone marrow stromal cells were necessary. Similarly, lymphoma/stromal cell conditioned medium also failed to provide a survival advantage. These data demonstrated that viable bone marrow stromal cells possessed the ability to actively inhibit the apoptotic pathways of intimately adherent lymphoma cells and this potentially contributes to their survival during chemotherapy.

Biological effects of stroma-derived factor-1 alpha on normal and CML CD34+ haemopoietic cells

We compared the biological effects of the CXC chemokine SDF-1alpha on immunomagnetically purified CD34+ cells isolated from human normal bone marrow (NBM), leukapheresis products (LP) and patients with chronic myeloid leukaemia (CML). LP CD34+ cells showed a significantly stronger migration response to SDF-1alpha (100 ng/ml) than CD34+ cells isolated from the peripheral blood (PB) of CML patients (P < 0.05). The chemotactic response to SDF-1alpha was also reduced in CML BM CD34+ cells in comparison to NBM CD34+ cells but the observed differences were not statistically significant. In analogy to normal CD34+ cells circulating CML PB CD34+ cells were less responsive to SDF-1alpha than their BM counterparts (P < 0.05). Furthermore, SDF-1alpha elicited similar concentration-dependent growth suppressive effects on normal and CML CD34+ cells (P > 0.05) in colony-forming cell assays. We then demonstrated that SDF-1alpha triggers intracellular calcium increases in CD34+ cells and there were no differences in the time course and dose response characteristics of normal and CML CD34+ cells. The reduced migration response to SDF-1alpha in CML CD34+ cells was not due to a down-regulation of the SDF-1alpha receptor CXCR-4 as flow cytometric analysis revealed similar CXCR-4 expression levels on NBM, LP, CML PB and CML BM CD34+ cells (P > 0.05). Finally, no differences in the modulation of CXCR-4 levels in response to SDF-1alpha and serum were observed in CML and normal CD34+ cells. Our data suggest that the impaired chemotactic response of CML CD34+ cells to SDF-1alpha is not caused by a lack or complete uncoupling of CXCR-4, but may be due to an intracellular signalling defect downstream of the receptor.

Stem cell factor and chronic myeloid leukemia CD34+ cells

Normal hematopoiesis is a tightly regulated process involving a balance between signals that stimulate and those that inhibit the proliferation and differentiation of hematopoietic progenitors. In chronic myeloid leukemia (CML) there is a perturbation of these controlling elements, resulting in overgrowth of leukemic cells in the bone marrow and spleen. In part, the proliferation of CML CD34+ cells may result from an abnormal response to the cytokine Stem Cell Factor (SCF). SCF induced proliferation and adhesion to the extracellular matrix via fibronectin are not coupled in CML as they are in normal cells and this may contribute to the accumulation of leukemic progenitors. We have previously shown that CD34+ CML cells and the more primitive CD34+ CD38- CML cells do not require the addition of synergistic cytokines to cultures, but are capable of proliferation in SCF alone, and that leukemic CFU-GM are selectively supported in these cultures. In the presence of other cytokines the response of CML cells to SCF is no greater than that of cells from normal donors, suggesting that the leukemic cells are not more sensitive to SCF, but that accessory pathways are already activated in these cells. Cells from patients with myeloproliferative disorders show variable proliferative response to SCF as the sole mitogenic stimulus, suggesting that expression of bcr-abl is essential for proliferation in this cytokine. Further studies to identify the key determinants of the abnormal response to SCF in CML may lead to a better understanding of the proliferative abnormality that underlies CML.