The chemokine receptor CXCR4 enhances integrin-mediated in vitro adhesion and facilitates engraftment of leukemic precursor-B cells in the bone marrow

Involvement of BCR::ABL1 in laminin adhesion of Philadelphia chromosome-positive acute lymphoblastic leukemia through upregulation of integrin α6

BACKGROUND

Adhesion of cancer cells to extracellular matrix laminin through the integrin superfamily reportedly induces drug resistance. Heterodimers of integrin α6 (CD49f) with integrin β1 (CD29) or β4 (CD104) are major functional receptors for laminin. Higher CD49f expression is reportedly associated with a poorer response to induction therapy in childhood B-cell precursor acute lymphoblastic leukemia (BCP-ALL). Moreover, a xenograft mouse model transplanted with primary BCP-ALL cells revealed that neutralized antibody against CD49f improved survival after chemotherapy.

AIMS

Considering the poor outcomes in Philadelphia chromosome (Ph)-positive ALL treated with conventional chemotherapy without tyrosine kinase inhibitors, we sought to investigate an involvement of the laminin adhesion.

METHODS AND RESULTS

Ph-positive ALL cell lines expressed the highest levels of CD49f among the BCP-ALL cell lines with representative translocations, while CD29 and CD104 were ubiquitously expressed in BCP-ALL cell lines. The association of Ph-positive ALL with high levels of CD49f gene expression was also confirmed in two databases of childhood ALL cohorts. Ph-positive ALL cell lines attached to laminin and their laminin-binding properties were disrupted by blocking antibodies against CD49f and CD29 but not CD104. The cell surface expression of CD49f, but not CD29 and CD104, was downregulated by imatinib treatment in Ph-positive ALL cell lines, but not in their T315I-acquired sublines. Consistently, the laminin-binding properties were disrupted by the imatinib pre-treatment in the Ph-positive ALL cell line, but not in its T315I-acquired subline.

CONCLUSION

BCR::ABL1 plays an essential role in the laminin adhesion of Ph-positive ALL cells through upregulation of CD49f.

General anaesthetics reduce acute lymphoblastic leukaemia malignancies in vitro and in vivovia CXCR4 and osteopontin mediated mechanisms

Background

Acute lymphoblastic leukaemia (ALL) is a common type of cancer in children. General anaesthetics are often used on patients undergoing painful procedures during ALL treatments but their effects on ALL malignancy remain unknown. Herein, we aim to study the effect of propofol and sevoflurane on the migration, homing and chemoresistance of ALL cells.

Methods

NALM-6 and Reh cells were treated with propofol (5 and 10 μg/ml) or sevoflurane (3.6%) in vitro for six hours. Then, cells were harvested for adhesion assay and migration assay in vitro. In in vivo experiments, GFP-NALM-6 cells were pre-treated with propofol (10 μg/ml) or sevoflurane (3.6%) for six hours. Then, cells were injected intravenously to C57BL/6 female mice followed by intravital microscopy. For chemoresistance study, cells were treated with rising concentrations of Ara-c (0.05-50 nM) plus 10μg/ml of propofol or Ara-C plus 3.6% of sevoflurane for 4 hours, followed by the assessment of cell viability via CCK-8 assay and detection of autophagy via flow cytometry.

Results

Both anaesthetics reduced in vivo migration and in vivo homing as exemplified by 1) the reduction in the number of cells entering the bone marrow and 2) the disturbance in homing location in relation to endosteal surface. Our results indicated that general anaesthetics reduced the surface CXCR4 expression and the adhesion of leukaemia cells to thrombin cleaved osteopontin (OPN) was reduced. Those changes might result in the alterations in migration and homing. In addition, both anaesthetics sensitised ALL cells to Ara-c possibly through CXCR4 mediated mechanisms. Propofol but not sevoflurane enhanced chemo-related cell death via inducing cytotoxic autophagy.

Conclusion

Together, our data suggest that both propofol and sevoflurane could reduce ALL migration, and homing in vivo and in vitro via CXCR4 and OPN mediated mechanisms. Both anaesthetics could sensitise ALL cells to chemotherapy possibly via CXCR4 mediated mechanisms.

Role of CXCR4 in the progression and therapy of acute leukaemia

CXCR4 is expressed on leukaemia cells and haematopoietic stem cells (HSCs), and its ligand stromal-derived factor 1 (SDF-1) is produced abundantly by stromal cells in the bone marrow (BM). The SDF-1/CXCR4 axis plays important roles in homing to and retention in the protective BM microenvironment of malignant leukaemia cells and normal HSCs. CXCR4 expression is regulated by multiple mechanisms and the level of CXCR4 expression on leukaemia cells has prognostic indications in patients with acute leukaemia. CXCR4 antagonists can mobilize leukaemia cells from BM to circulation, which render them effectively eradicated by chemotherapeutic agents, small molecular inhibitors or hypomethylating agents. Therefore, such combinational therapies have been tested in clinical trials. However, new evidence emerged that drug-resistant leukaemia cells were not affected by CXCR4 antagonists, and the migration of certain leukaemia cells to the leukaemia niche was independent of SDF-1/CXCR4 axis. In this review, we summarize the role of CXCR4 in progression and treatment of acute leukaemia, with a focus on the potential of CXCR4 as a therapeutic target for acute leukaemia. We also discuss the potential value of using CXCR4 antagonists as chemosensitizer for conditioning regimens and immunosensitizer for graft-vs-leukaemia effects of allogeneic haematopoietic stem cell transplantation.

Pleiotropic Roles of CXCR4 in Wound Repair and Regeneration

Wound healing is a multi-step process that includes multiple cellular events such as cell proliferation, cell adhesion, and chemotactic response as well as cell apoptosis. Accumulating studies have documented the significance of stromal cell-derived factor-1 (SDF-1)/C-X-C chemokine receptor 4 (CXCR4) signaling in wound repair and regeneration. However, the molecular mechanism of regeneration is not clear. This review describes various types of tissue regeneration that CXCR4 participates in and how the efficiency of regeneration is increased by CXCR4 overexpression. It emphasizes the pleiotropic effects of CXCR4 in regeneration. By delving into the specific molecular mechanisms of CXCR4, we hope to provide a theoretical basis for tissue engineering and future regenerative medicine.

RUNX2 regulates leukemic cell metabolism and chemotaxis in high-risk T cell acute lymphoblastic leukemia

T cell acute lymphoblastic leukemia (T-ALL) is an aggressive hematologic malignancy with inferior outcome compared with that of B cell ALL. Here, we show that Runt-related transcription factor 2 (RUNX2) was upregulated in high-risk T-ALL with KMT2A rearrangements (KMT2A-R) or an immature immunophenotype. In KMT2A-R cells, we identified RUNX2 as a direct target of the KMT2A chimeras, where it reciprocally bound the KMT2A promoter, establishing a regulatory feed-forward mechanism. Notably, RUNX2 was required for survival of immature and KMT2A-R T-ALL cells in vitro and in vivo. We report direct transcriptional regulation of CXCR4 signaling by RUNX2, thereby promoting chemotaxis, adhesion, and homing to medullary and extramedullary sites. RUNX2 enabled these energy-demanding processes by increasing metabolic activity in T-ALL cells through positive regulation of both glycolysis and oxidative phosphorylation. Concurrently, RUNX2 upregulation increased mitochondrial dynamics and biogenesis in T-ALL cells. Finally, as a proof of concept, we demonstrate that immature and KMT2A-R T-ALL cells were vulnerable to pharmacological targeting of the interaction between RUNX2 and its cofactor CBFβ. In conclusion, we show that RUNX2 acts as a dependency factor in high-risk subtypes of human T-ALL through concomitant regulation of tumor metabolism and leukemic cell migration.

Toward Therapeutic Targeting of Bone Marrow Leukemic Niche Protective Signals in B-Cell Acute Lymphoblastic Leukemia

B-cell acute lymphoblastic leukemia (B-ALL) represents the malignant counterpart of bone marrow (BM) differentiating B cells and occurs most frequently in children. While new combinations of chemotherapeutic agents have dramatically improved the prognosis for young patients, disease outcome remains poor after relapse or in adult patients. This is likely due to heterogeneity of B-ALL response to treatment which relies not only on intrinsic properties of leukemic cells, but also on extrinsic protective cues transmitted by the tumor cell microenvironment. Alternatively, leukemic cells have the capacity to shape their microenvironment towards their needs. Most knowledge on the role of protective niches has emerged from the identification of mesenchymal and endothelial cells controlling hematopoietic stem cell self-renewal or B cell differentiation. In this review, we discuss the current knowledge about B-ALL protective niches and the development of therapies targeting the crosstalk between leukemic cells and their microenvironment.

Targeted dendrimers for antagonizing the migration and viability of NALM-6 lymphoblastic leukemia cells

Acute lymphoblastic leukemia (ALL) or white blood cell cancer is one of the major causes that kills many children worldwide. Although various therapeutic agents are available for ALL treatment, the new drug discovery and drug delivery system are needed to improve their effectiveness, to reduce the toxicity and side-effect, and to enhance their selectivity to target cancer cells. CXCR4 is a protein expressed on the surface of various types of cancer cell including ALL. In this work, the CXCR4-targeted PAMAM dendrimer was constructed by conjugating G5 PAMAM with a CXCR4 antagonist, LFC131. The results revealed that the LFC131-conjugated G5 PAMAM selectively targeted CXCR4 expressing leukemic precursor B cells (NALM-6) and the migration of NALM-6 cells induced by SDF-1α was inhibited at non-cytotoxic concentration. Further research based on this findings may contribute to potential anti-metastatic drugs for lymphoblastic leukemia.

A stem cell reporter based platform to identify and target drug resistant stem cells in myeloid leukemia

Intratumoral heterogeneity is a common feature of many myeloid leukemias and a significant reason for treatment failure and relapse. Thus, identifying the cells responsible for residual disease and leukemia re-growth is critical to better understanding how they are regulated. Here, we show that a knock-in reporter mouse for the stem cell gene Musashi 2 (Msi2) allows identification of leukemia stem cells in aggressive myeloid malignancies, and provides a strategy for defining their core dependencies. Specifically, we carry out a high throughput screen using Msi2-reporter blast crisis chronic myeloid leukemia (bcCML) and identify several adhesion molecules that are preferentially expressed in therapy resistant bcCML cells and play a key role in bcCML. In particular, we focus on syndecan-1, whose deletion triggers defects in bcCML growth and propagation and markedly improves survival of transplanted mice. Further, live imaging reveals that the spatiotemporal dynamics of leukemia cells are critically dependent on syndecan signaling, as loss of this signal impairs their localization, migration and dissemination to distant sites. Finally, at a molecular level, syndecan loss directly impairs integrin β7 function, suggesting that syndecan exerts its influence, at least in part, by coordinating integrin activity in bcCML. These data present a platform for delineating the biological underpinnings of leukemia stem cell function, and highlight the Sdc1-Itgβ7 signaling axis as a key regulatory control point for bcCML growth and dissemination.

Integrin-Mediated Adhesion and Chemoresistance of Acute Lymphoblastic Leukemia Cells Residing in the Bone Marrow or the Central Nervous System

Acute Lymphoblastic Leukemia (ALL) is the most common cancer in childhood. Despite a significantly improved prognosis over the last decade with a 5-years survival rate of ~90%, treatment-related morbidity remains substantial and relapse occurs in 10–15% of patients (1). The most common site of relapse is the bone marrow, but early colonization and subsequent reoccurrence of the disease in the central nervous system (CNS) also occurs. Integrins are a family of cell surface molecules with a longstanding history in cancer cell adherence, migration and metastasis. In chronic lymphoblastic leukemia (CLL), the VLA-4 integrin has been acknowledged as a prognostic marker and mounting evidence indicates that this and other integrins may also play a role in acute leukemia, including ALL. Importantly, integrins engage in anti-apoptotic signaling when binding extracellular molecules that are enriched in the bone marrow and CNS microenvironments. Here, we review the current evidence for a role of integrins in the adherence of ALL cells within the bone marrow and their colonization of the CNS, with particular emphasis on mechanisms adding to cancer cell survival and chemoresistance.

Notch/CXCR4 Partnership in Acute Lymphoblastic Leukemia Progression

Acute lymphoblastic leukemia (ALL) is the most common cancer among children. Recent advances in chemotherapy have made ALL a curable hematological malignancy. In children, there is 25% chance of disease relapse, typically in the central nervous system. While in adults, there is a higher chance of relapse. ALL may affect B-cell or T-cell lineages. Different genetic alterations characterize the two ALL forms. Deregulated Notch, either Notch1 or Notch3, and CXCR4 receptor signaling are involved in ALL disease development and progression. By analyzing their relevant roles in the pathogenesis of the two ALL forms, new molecular mechanisms able to modulate cancer cell invasion may be visualized. Notably, the partnership between Notch and CXCR4 may have considerable implications in understanding the complexity of T- and B-ALL. These two receptor pathways intersect other critical signals in the proliferative, differentiation, and metabolic programs of lymphocyte transformation. Also, the identification of the crosstalks in leukemia-stroma interaction within the tumor microenvironment may unveil new targetable mechanisms in disease relapse. Further studies are required to identify new challenges and opportunities to develop more selective and safer therapeutic strategies in ALL progression, possibly contributing to improve conventional hematological cancer therapy.

The role of cell adhesion in hematopoiesis and leukemogenesis

The cells of the bone marrow microenvironment are emerging as important contributors and regulators of normal hematopoiesis. This microenvironment is perturbed during leukemogenesis, and evidence points toward a bidirectional communication between leukemia cells and the normal cells of the bone marrow, mediated by direct cell-cell contact as well as soluble factors. These interactions are increasingly appreciated to play a role in leukemogenesis and possibly in resistance to chemotherapy. In fact, several compounds that specifically target the bone marrow microenvironment, including inhibitors of cell adhesion, are being tested as adjuncts to leukemia therapy.

Microglia: Brain cells on the move

In the last decade, tremendous progress has been made in understanding the biology of microglia - i.e. the fascinating immigrated resident immune cell population of the central nervous system (CNS). Recent literature reviews have largely dealt with the plentiful functions of microglia in CNS homeostasis, development and pathology, and the influences of sex and the microbiome. In this review, the intriguing aspect of their physical plasticity during CNS development will get specific attention. Microglia move around (mobility) and reshape their processes (motility). Microglial migration into and inside the CNS is most prominent throughout development and consequently most of the data described in this review concern mobility and motility in the changing environment of the developing brain. Here, we first define microglia based on their highly specialized age- and region-dependent gene expression signature and associated functional heterogeneity. Next, we describe their origin, the migration route of immature microglial cells towards the CNS, the mechanisms underlying their invasion of the CNS, and their spatiotemporal localization and surveying behaviour inside the developing CNS. These processes are dependent on microglial mobility and motility which are determined by the microenvironment of the CNS. Therefore, we further zoom in on the changing environment during CNS development. We elaborate on the extracellular matrix and the respective integrin receptors on microglia and we discuss the purinergic and molecular signalling in microglial mobility. In the last section, we discuss the physiological and pathological functions of microglia in which mobility and motility are involved to stress the importance of microglial 'movement'.

The Formin mDia1 Regulates Acute Lymphoblastic Leukemia Engraftment, Migration, and Progression in vivo

Leukemias typically arise in the bone marrow and then spread to the blood and into other tissues. To disseminate into tissues, leukemia cells migrate into the blood stream and then exit the circulation by migrating across vascular endothelial barriers. Formin proteins regulate cytoskeletal remodeling and cell migration of normal and malignant cells. The Formin mDia1 is highly expressed in transformed lymphocytes and regulates lymphocyte migration. However, the role of mDia1 in regulating leukemia progression in vivo is unknown. Here, we investigated how mDia1 mediates the ability of leukemia cells to migrate and disseminate in vivo. For these studies, we used a mouse model of Bcr-Abl pre-B cell acute lymphoblastic leukemia. Our data showed that mDia1-deficient leukemia cells have reduced chemotaxis and ability to complete transendothelial migration in vitro. In vivo, mDia1 deficiency reduced the ability of leukemia cells to engraft in recipient mice. Furthermore, leukemia dissemination to various tissues and leukemia progression were inhibited by mDia1 depletion. Finally, mDia1 depletion in leukemia cells resulted in prolonged survival of recipient mice in a leukemia transfer model. Overall, our data show that the Formin mDia1 mediates leukemia cell migration, and drives leukemia engraftment and progression in vivo, suggesting that targeting mDia1 could provide a new method for treatment of leukemia.

CXCR4 blockade improves leukemia eradication by allogeneic lymphocyte infusion

Persistent low levels of disease in bone marrow, an immunoprivileged tissue, are responsible for relapse following allogeneic hematopoietic cell transplantation. Using mouse models carrying primary human acute lymphoblast leukemia derived from MLL-AF9-overexpressing human hematopoietic stem cells, we demonstrate that allogeneic lymphocyte infusion (ALI)-mediated graft-vs.-leukemia effects selectively spare leukemia cells in the bone marrow. The resistance of leukemia cells to ALI within bone marrow is due to the immunosuppressive status of the tissue, as ALI achieved a significantly increased complete remission rate when leukemia cells were dislodged from bone marrow by treatment with a CXCR4 antagonist AMD3100. Adoptive transfer experiments confirmed that the frequency of leukemia-initiating cells in bone marrow was significantly decreased in the recipients treated with ALI plus AMD3100 compared to those receiving ALI only. These findings indicate that the immunoprivileged nature of bone marrow is largely responsible for relapse after immunotherapies, and that treatment with AMD3100 may offer a clinically-practical approach to improving the outcome of adoptive allogeneic cell therapy.

A phase 1 study of the CXCR4 antagonist plerixafor in combination with high-dose cytarabine and etoposide in children with relapsed or refractory acute leukemias or myelodysplastic syndrome: A Pediatric Oncology Experimental Therapeutics Investigators' Consortium study (POE 10-03)

BACKGROUND

Plerixafor, a reversible CXCR4 antagonist, inhibits interactions between leukemic blasts and the bone marrow stromal microenvironment and may enhance chemosensitivity. A phase 1 trial of plerixafor in combination with intensive chemotherapy in children and young adults with relapsed or refractory acute lymphoblastic leukemia (ALL), acute myeloid leukemia (AML), and myelodysplastic syndrome (MDS) was performed to determine a tolerable and biologically active dose.

PROCEDURE

Plerixafor was administered daily for 5 days at four dose levels (6, 9, 12, and 15 mg/m² /dose) followed 4 hr later by high-dose cytarabine (every 12 hr) and etoposide (daily).

RESULTS

Nineteen patients (13 with AML, 5 with ALL, 1 with MDS) were treated. The most common grade 3 or greater nonhematologic toxicities attributable to plerixafor were febrile neutropenia and hypokalemia. There were no dose-limiting toxicities (DLTs). Plerixafor exposure increased with increasing dose levels and clearance was similar on days 1 and 5. Eighteen patients were evaluable for response. Two patients achieved complete remission (CR) and one patient achieved CR with incomplete hematologic recovery (CRi): all three had AML. No responses were seen in patients with ALL or MDS. Plerixafor mobilized leukemic blasts into the peripheral blood in 14 of 16 evaluable patients (median 3.4-fold increase), and the degree of mobilization correlated with surface CXCR4 expression.

CONCLUSIONS

Plerixafor, in combination with high-dose cytarabine and etoposide, was well tolerated in children and young adults with relapsed/refractory acute leukemias and MDS. While biologic responses were observed, clinical responses in this heavily pretreated cohort were modest.

Eviction from the sanctuary: Development of targeted therapy against cell adhesion molecules in acute lymphoblastic leukemia

Acute lymphoblastic leukemia (ALL) is a malignant hematological disease afflicting hematopoiesis in the bone marrow. While 80%-90% of patients diagnosed with ALL will achieve complete remission at some point during treatment, ALL is associated with high relapse rate, with a 5-year overall survival rate of 68%. The initial remission failure and the high rate of relapse can be attributed to intrinsic chemoprotective mechanisms that allow persistence of ALL cells despite therapy. These mechanisms are mediated, at least in part, through the engagement of cell adhesion molecules (CAMs) within the bone marrow microenvironment. This review assembles CAMs implicated in protection of leukemic cells from chemotherapy. Such studies are limited in ALL. Therefore, CAMs that are associated with poor outcomes or are overexpressed in ALL and have been shown to be involved in chemoprotection in other hematological cancers are also included. It is likely that these molecules play parallel roles in ALL because the CAMs identified to be a factor in ALL chemoresistance also work similarly in other hematological malignancies. We review the signaling mechanisms activated by the engagement of CAMs that provide protection from chemotherapy. Development of targeted therapies against CAMs could improve outcome and raise the overall cure rate in ALL.

CCR7 and CXCR4 Expression in Primary Head and Neck Squamous Cell Carcinomas and Nodal Metastases – a Clinical and Immunohistochemical Study

Background: Squamous cell carcinomas (SCCs) are common head and neck malignancies demonstrating lymph node LN involvement. Recently chemokine receptor overxpression has been reported in many cancers. Of particular interest, CCR7 appears to be a strong mediator of LN metastases, while CXCR4 may mediate distant metastases. Any relations between their expression in primary HNSCCs and metastatic lymph nodes need to be clarified. Aims: To investigate CCR7 andCXCR4 expression in primary HNSCCs of all tumor sizes, clinical stages and histological grades, as well as involved lymph nodes, then make comparisons, also with control normal oral epithelium. Materials and Methods: The sample consisted of 60 formalin-fixed, paraffin-embedded specimens of primary HNSCCs, 77 others of metastasi-positive lymph nodes, and 10 of control normal oral epithelial tissues. Sections were conventionally stained with H&E and immunohistochemically with monoclonal anti-CCR7 and monoclonal anti-CXCR4 antibodies. Positive cells were counted under microscopic assessment in four fields (X40) per case. Results: There was no variation among primary HNSCC tumors staining positive for CCR7 and CXCR4 with tumor size of for CCR7 with lymph node involvement. However, a difference was noted between primary HNSCC tumors stained by CXCR4 with a single as compared to more numerous node involvement. CXCR4 appear to vary with the clinical stagebut no links were noted with histological grades. Staining for primary HNSCC tumors and metastatic lymph nodes correlated.

Pro-inflammatory-Related Loss of CXCL12 Niche Promotes Acute Lymphoblastic Leukemic Progression at the Expense of Normal Lymphopoiesis

Pediatric oncology, notably childhood acute lymphoblastic leukemia (ALL), is currently one of the health-leading concerns worldwide and a biomedical priority. Decreasing overall leukemia mortality in children requires a comprehensive understanding of its pathobiology. It is becoming clear that malignant cell-to-niche intercommunication and microenvironmental signals that control early cell fate decisions are critical for tumor progression. We show here that the mesenchymal stromal cell component of ALL bone marrow (BM) differ from its normal counterpart in a number of functional properties and may have a key role during leukemic development. A decreased proliferation potential, contrasting with the strong ability of producing pro-inflammatory cytokines and an aberrantly loss of CXCL12 and SCF, suggest that leukemic lymphoid niches in ALL BM are unique and may exclude normal hematopoiesis. Cell competence ex vivo assays within tridimensional coculture structures indicated a growth advantage of leukemic precursor cells and their niche remodeling ability by CXCL12 reduction, resulting in leukemic cell progression at the expense of normal niche-associated lymphopoiesis.

POL5551, a novel and potent CXCR4 antagonist, enhances sensitivity to chemotherapy in pediatric ALL

The importance of the cell surface receptor CXCR4 and the chemokine stromal cell-derived factor-1 (SDF-1/CXCL12) is well-established in normal and malignant hematopoiesis. The Protein Epitope Mimetic POL5551 is a novel and potent antagonist of CXCR4. POL5551 efficiently mobilizes hematopoietic stem and progenitor cells, but its effects in acute lymphoblastic leukemia (ALL) have not been reported. Here, we demonstrate that POL5551 is a potent antagonist of CXCR4 in pre-B and T cell ALL cell lines and pediatric ALL primary samples. POL5551 has activity at nanomolar concentrations in decreasing CXCR4 antibody binding, blocking SDF-1α-mediated phosphorylation of ERK1/2, inhibiting SDF-1α-induced chemotaxis, and reversing stromal-mediated protection from chemotherapy. POL5551 is significantly more effective at inhibiting CXCR4 antibody binding than the FDA-approved CXCR4 inhibitor plerixafor in ALL cell lines and primary samples. We also show that treatment with POL5551 in vitro and cytarabine +/− POL5551 in vivo modulates surface expression of adhesion molecules, findings that may guide the optimal clinical use of POL5551. Finally, we demonstrate that POL5551 increases sensitivity to cytarabine in a xenograft model of a high-risk pediatric ALL, infant MLL-rearranged (MLL-R) ALL. Therefore, disruption of the CXCR4/SDF-1 axis with POL5551 may improve outcomes in children with high-risk ALL.

Effectiveness of AMD3100 in treatment of leukemia and solid tumors: from original discovery to use in current clinical practice

AMD3100, also known as plerixafor, was originally developed as an anti-human immunodeficiency virus (HIV) drug, and later characterized as a C-X-C chemokine receptor type 4 (CXCR4) antagonist. Previous reviews have focused on the application of AMD3100 in the treatment of HIV, but a comprehensive evaluation of AMD3100 in the treatment of leukemia, solid tumor, and diagnosis is lacking. In this review, we broadly describe AMD3100, including the background, functional mechanism and clinical applications. Until the late 1990s, CXCR4 was known as a crucial factor for hematopoietic stem and progenitor cell (HSPC) retention in bone marrow. Subsequently, the action and synergy of plerixafor with Granulocyte-colony stimulating factor (G-CSF) led to the clinical approval of plerixafor as the first compound for mobilization of HSPCs. The amount of HSPC mobilization and the rapid kinetics promoted additional clinical uses. Recently, CXCR4/CXCL12 (C-X-C motif chemokine 12) axis was found to be involved in a variety of roles in tumors, including leukemic stem cell (LSC) homing and signaling transduction. Thus, CXCR4 targeting has been a treatment strategy against leukemia and solid tumors. Understanding this mechanism will help shed light on therapeutic potential for HIV infection, inflammatory diseases, stem-cell mobilization, leukemia, and solid tumors. Clarifying the CXCR4/CXCL12 axis and role of AMD3100 will help remove malignant cells from the bone marrow niche, rendering them more accessible to targeted therapeutic agents.

IQGAP1 promotes CXCR4 chemokine receptor function and trafficking via EEA-1+ endosomes

IQGAP1 mediates CXCR4 cell surface expression and signaling by regulating EEA-1⁺ endosome interactions with microtubules during CXCR4 trafficking and recycling.

IQ motif–containing GTPase-activating protein 1 (IQGAP1) is a cytoskeleton-interacting scaffold protein. CXCR4 is a chemokine receptor that binds stromal cell–derived factor-1 (SDF-1; also known as CXCL12). Both IQGAP1 and CXCR4 are overexpressed in cancer cell types, yet it was unclear whether these molecules functionally interact. Here, we show that depleting IQGAP1 in Jurkat T leukemic cells reduced CXCR4 expression, disrupted trafficking of endocytosed CXCR4 via EEA-1⁺ endosomes, and decreased efficiency of CXCR4 recycling. SDF-1–induced cell migration and activation of extracellular signal-regulated kinases 1 and 2 (ERK) MAPK were strongly inhibited, even when forced overexpression restored CXCR4 levels. Similar results were seen in KMBC and HEK293 cells. Exploring the mechanism, we found that SDF-1 treatment induced IQGAP1 binding to α-tubulin and localization to CXCR4-containing endosomes and that CXCR4-containing EEA-1⁺ endosomes were abnormally located distal from the microtubule (MT)-organizing center (MTOC) in IQGAP1-deficient cells. Thus, IQGAP1 critically mediates CXCR4 cell surface expression and signaling, evidently by regulating EEA-1⁺ endosome interactions with MTs during CXCR4 trafficking and recycling. IQGAP1 may similarly promote CXCR4 functions in other cancer cell types.

Expression of SOCS1 and CXCL12 Proteins in Primary Breast Cancer Are Associated with Presence of Circulating Tumor Cells in Peripheral Blood

Circulating tumor cells (CTCs) are independent prognostic factors in the primary and metastatic breast cancer patients and play crucial role in hematogenous tumor dissemination. The aim of this study was to correlate the presence of CTCs in peripheral blood with the expression of proteins in tumor tissue that have a putative role in regulation of cell growth and metastatic potential. This prospective study included 203 primary breast cancer patients treated by definitive surgery. CTCs were detected by quantitative real-time PCR for the expression of epithelial (CK19) or epithelial-to-mesenchymal transition–inducing transcription factor genes (TWIST1, SNAIL1, SLUG, and ZEB1). Expression of APC, ADAM23, CXCL12, E-cadherin, RASSF1, SYK, TIMP3, BRMS1, and SOCS1 proteins in primary breast tumor tissue was evaluated by immunohistochemistry. CTCs with epithelial markers were found in 17 (9.2%) patients. Their occurrence was associated with inhibition of SOCS1 expression (odds ratio [OR] = 0.07; 95% confidence interval [CI], 0.03-0.13; P < .001). CTCs with positive epithelial-to-mesenchymal transition markers were detected in 30 (15.8%) patients; however, no association with analyzed protein expressions was found. Overall, CTCs were detected in 44 (22.9%) patients. Presence of any CTC marker was significantly associated with positive CXCL12 expression (OR = 3.08; 95% CI, 1.15-8.26; P = .025) and lack of SOCS1 expression (OR = 0.10; 95% CI, 0.04-0.25; P < .001) in patient’s tumor tissues. As both CXCL12 and SOCS1 proteins are involved in cytokine signaling, our results provide support for the hypothesis that aberrant signaling cross talk between cytokine and chemokine responses could have an important role in hematogenous dissemination of tumor cells in breast cancer.

Myosin-IIA regulates leukemia engraftment and brain infiltration in a mouse model of acute lymphoblastic leukemia

Leukemia dissemination (the spread of leukemia cells from the bone marrow) and relapse are associated with poor prognosis. Often, relapse occurs in peripheral organs, such as the CNS, which acts as a sanctuary site for leukemia cells to escape anti-cancer treatments. Similar to normal leukocyte migration, leukemia dissemination entails migration of cells from the blood circulation into tissues by extravasation. To extravasate, leukemia cells cross through vascular endothelial walls via a process called transendothelial migration, which requires cytoskeletal remodeling. However, the specific molecular players in leukemia extravasation are not fully known. We examined the role of myosin-IIA a cytoskeletal class II myosin motor protein, in leukemia progression and dissemination into the CNS by use of a mouse model of Bcr-Abl-driven B cell acute lymphoblastic leukemia. Small hairpin RNA-mediated depletion of myosin-IIA did not affect apoptosis or the growth rate of B cell acute lymphoblastic leukemia cells. However, in an in vivo leukemia transfer model, myosin-IIA depletion slowed leukemia progression and prolonged survival, in part, by reducing the ability of B cell acute lymphoblastic leukemia cells to engraft efficiently. Finally, myosin-IIA inhibition, either by small hairpin RNA depletion or chemical inhibition by blebbistatin, drastically reduced CNS infiltration of leukemia cells. The effects on leukemia cell entry into tissues were mostly a result of the requirement for myosin-IIA to enable leukemia cells to complete the transendothelial migration process during extravasation. Overall, our data implicate myosin-IIA as a key mediator of leukemia cell migration, making it a promising target to inhibit leukemia dissemination in vivo and potentially reduce leukemia relapses.

Cell adhesion manipulation through single cell assembly for characterization of initial cell-to-cell interaction

BACKGROUND

Cell-to-cell interactions are complex processes that involve physical interactions, chemical binding, and biological signaling pathways. Identification of the functions of special signaling pathway in cell-to-cell interaction from the very first contact will help characterize the mechanism underlying the interaction and advance new drug discovery.

METHODS

This paper reported a case study of characterizing initial interaction between leukemia cancer cells and bone marrow stromal cells, through the use of an optical tweezers-based cell manipulation tool. Optical traps were used to assemble leukemia cells at different positions of the stromal cell layer and enable their interactions by applying a small trapping force to maintain the cell contact for a few minutes. Specific drug was used to inhibit the binding of molecules during receptor-ligand-mediated adhesion.

RESULTS AND CONCLUSIONS

Our results showed that the amount of adhesion molecule could affect cell adhesion during the first few minutes contact. We also found that leukemia cancer cells could migrate on the stromal cell layer, which was dependent on the adhesion state and activation triggered by specific chemokine. The reported approaches provided a new opportunity to investigate cell-to-cell interaction through single cell adhesion manipulation.

Advances in understanding the acute lymphoblastic leukemia bone marrow microenvironment: From biology to therapeutic targeting

The bone marrow (BM) microenvironment regulates the properties of healthy hematopoietic stem cells (HSCs) localized in specific niches. Two distinct microenvironmental niches have been identified in the BM, the "osteoblastic (endosteal)" and "vascular" niches. Nevertheless, these niches provide sanctuaries where subsets of leukemic cells escape chemotherapy-induced death and acquire a drug-resistant phenotype. Moreover, it is emerging that leukemia cells are able to remodel the BM niches into malignant niches which better support neoplastic cell survival and proliferation. This review focuses on the cellular and molecular biology of microenvironment/leukemia interactions in acute lymphoblastic leukemia (ALL) of both B- and T-cell lineage. We shall also highlight the emerging role of exosomes/microvesicles as efficient messengers for cell-to-cell communication in leukemia settings. Studies on the interactions between the BM microenvironment and ALL cells have led to the discovery of potential therapeutic targets which include cytokines/chemokines and their receptors, adhesion molecules, signal transduction pathways, and hypoxia-related proteins. The complex interplays between leukemic cells and BM microenvironment components provide a rationale for innovative, molecularly targeted therapies, designed to improve ALL patient outcome. A better understanding of the contribution of the BM microenvironment to the process of leukemogenesis and leukemia persistence after initial remission, may provide new targets that will allow destruction of leukemia cells without adversely affecting healthy HSCs. This article is part of a Special Issue entitled: Tumor Microenvironment Regulation of Cancer Cell Survival, Metastasis,Inflammation, and Immune Surveillance edited by Peter Ruvolo and Gregg L. Semenza.

CD9, a key actor in the dissemination of lymphoblastic leukemia, modulating CXCR4-mediated migration via RAC1 signaling

CD9, a member of the tetraspanin family, has been implicated in hematopoietic and leukemic stem cell homing. We investigated the role of CD9 in the dissemination of B acute lymphoblastic leukemia (B-ALL) cells, by stably downregulating CD9 in REH and NALM6 cells. CD9 expression was associated with higher levels of REH cell adhesion to fibronectin and C-X-C motif chemokine receptor 4 (CXCR4)-mediated migration. Death occurred later in NOD/SCID mice receiving REH cells depleted of CD9 for transplantation than in mice receiving control cells. After C-X-C motif chemokine ligand 12 (CXCL12) stimulation, CD9 promoted the formation of long cytoplasmic actin-rich protrusions. We demonstrated that CD9 enhanced RAC1 activation, in both REH cells and blasts from patients. Conversely, the overexpression of a competing CD9 C-terminal tail peptide in REH cytoplasm decreased RAC1 activation and cytoplasmic extension formation in response to CXCL12. Finally, the inhibition of RAC1 activation decreased migration in vitro, and the depletion of RAC1 protein from transplanted REH cells increased mouse survival. Furthermore, a testis-conditioned medium induced the migration of REH and NALM6 cells, and this migration was impeded by an anti-CD9 antibody. The level of CD9 expression also influenced the homing of these cells in mouse testes. These findings demonstrate, for the first time, that CD9 plays a key role in the CXCR4-mediated migration and engraftment of B-ALL cells in the bone marrow or testis, through RAC1 activation.

Plerixafor as a chemosensitizing agent in pediatric acute lymphoblastic leukemia: efficacy and potential mechanisms of resistance to CXCR4 inhibition

In spite of advances in the treatment of pediatric acute lymphoblastic leukemia (ALL), a significant number of children with ALL are not cured of their disease. We and others have shown that signaling from the bone marrow microenvironment confers therapeutic resistance, and that the interaction between CXCR4 and stromal cell-derived factor-1 (SDF-1 or CXCL12) is a key mediator of this effect. We demonstrate that ALL cells that upregulate surface CXCR4 in response to chemotherapy treatment are protected from chemotherapy-induced apoptosis when co-cultured with bone marrow stroma. Treatment with the CXCR4 inhibitor plerixafor diminishes stromal protection and confers chemosensitivity. Using xenograft models of high-risk pediatric ALL, plerixafor plus chemotherapy induces significantly decreased leukemic burden, compared to chemotherapy alone. Further, treatment with plerixafor and chemotherapy influences surface expression of CXCR4, VLA-4, and CXCR7 in surviving ALL blasts. Finally, prolonged exposure of ALL blasts to plerixafor leads to a persistent increase in surface CXCR4 expression, along with modulation of surface expression of additional adhesion molecules, and enhanced SDF-1α-induced chemotaxis, findings that may have implications for therapeutic resistance. Our results suggest that while CXCR4 inhibition may prove useful in ALL, further study is needed to understand the full effects of targeting the leukemic microenvironment.

CXCL12/CXCR4 axis in the pathogenesis of acute lymphoblastic leukemia (ALL): a possible therapeutic target

Acute lymphoblastic leukemia (ALL) is the commonest childhood malignancy, accounting for approximately 80 % of leukemia in the pediatric group, and its etiology is unknown. This neoplasia is characterized by male predominance, high-risk features and poor outcome, mainly in recurrence patients and adults. In recent years, advances in the success of childhood ALL treatment were verified, and the rate of cure is over 80 % of individuals. However, there is a considerable scope for improving therapeutic outcome in this neoplasia. Improvements in ALL therapy might readily be achieved by developing additional biomarkers that can predict and refine prognosis in patients with ALL. In normal hematopoietic cells, cytokines provide the stimulus for proliferation, survival, self-renewal, differentiation and functional activation. Abnormalities of cytokines are characteristic in all forms of leukemia, including ALL. The stromal cell-derived factor-1 (SDF-1 or CXCL12) is a member of the CXC chemokine family that binds to CXC chemokine receptor 4 (CXCR4). The CXCL12/CXCR4 axis appears to play a role in dissemination of solid tumors and hematopoietic diseases. Understanding the mechanisms by which ALL cells are disseminated will provide additional information to expand therapeutic approach. Therefore, this review summarizes information relating to ALL cell biology, focusing specifically in a cytokine receptor important axis, CXCL12/CXCR4, that may have implications for novel treatment strategies to improve life expectancy of patients with this neoplasia.

Integrated analysis of gene expression and microRNA regulation in three leukemia-related lymphoblastic cell lines

RNA-sequencing technology is progressively being applied in various fields since high-throughput data analysis provides deeper mining on both the genomic and transcriptomic level. Lymphoblastic leukemogenesis is a complex process caused by abnormalities occurring during lymphocyte differentiation, and can be initiated by various triggers. Each leukemia subtype has distinct characteristics that can be identified in the corresponding cell lines; the detection of the exclusive genetic signatures of these varying cell lines is critical. Our analysis revealed that approximately 8000 human genes were differentially expressed between samples. Signaling pathways such as the NOD-like signaling pathway, cell surface receptor signaling pathways, and leukemia-related pathways were significantly regulated, as determined by KEGG annotation. Furthermore, several oncogenes and differentiation-related genes were differentially expressed between leukemia cell lines and lymphocyte types, respectively. Our miRNA analysis demonstrated that the expression of approximately one-third of all expressed miRNAs appeared to be significantly different between the leukemia cell lines studied. We also analyzed the mRNA-miRNA regulatory networks of both lymphocyte differentiation and leukemogenesis to determine key regulators of interest. We combined the results of the mRNA and miRNA analyses in order to investigate the regulatory relationship between them. This study not only identifies differences in the pathways and networks of acute lymphocytic leukemia (ALL) relative to normal lymphocytes, but also identifies unique functional characteristics of lymphoid cells and distinct gene expression patterns during lymphoid development. The discovery of leukemia-related miRNAs may provide meaningful insights into the biology of the disease.

A designed peptide targeting CXCR4 displays anti-acute myelocytic leukemia activity in vitro and in vivo

Leukemia cells highly expressing chemokine receptor CXCR4 can actively response to stroma derived factor 1α (CXCL12), trafficking and homing to the marrow microenvironment, which causes poor prognosis and relapse. Here we demonstrate that a novel designed peptide (E5) targeting CXCR4 inhibits CXCL12- and stroma-induced activation in multiple acute myelocytic leukemia (AML) cell lines and displays anti-AML activity. We show that E5 has high affinity to multiple AML cells with high CXCR4 level in a concentration dependent manner. E5 significantly inhibits CXCL12- or murine stromal cell (MS-5)-induced migration of leukemia cells and prevents the cells from adhering to stromal cells. Mechanistic studies demonstrate that E5 down-regulates CXCL12-induced phosphorylation of Akt, Erk, and p38, which affects the cytoskeleton F-actin organization and ultimately results in the inhibition of CXCL12- and stroma-mediated leukemia cell responses. E5 can induce concentration-dependent apoptosis in the four AML cell lines tested while did not affect the viability of MS-5 or human umbilical vein cell (ea.hy926) even at 80 µM, both of which have a low level of CXCR4. In vivo experimental results show that immunocompromised mice transplanted with HL-60 cells survived longer when treated with E5 twice a week in comparison to those treated with cyclophosphamide.

Regulation of hematopoietic and leukemic stem cells by the immune system

Hematopoietic stem cells (HSCs) are rare, multipotent cells that generate via progenitor and precursor cells of all blood lineages. Similar to normal hematopoiesis, leukemia is also hierarchically organized and a subpopulation of leukemic cells, the leukemic stem cells (LSCs), is responsible for disease initiation and maintenance and gives rise to more differentiated malignant cells. Although genetically abnormal, LSCs share many characteristics with normal HSCs, including quiescence, multipotency and self-renewal. Normal HSCs reside in a specialized microenvironment in the bone marrow (BM), the so-called HSC niche that crucially regulates HSC survival and function. Many cell types including osteoblastic, perivascular, endothelial and mesenchymal cells contribute to the HSC niche. In addition, the BM functions as primary and secondary lymphoid organ and hosts various mature immune cell types, including T and B cells, dendritic cells and macrophages that contribute to the HSC niche. Signals derived from the HSC niche are necessary to regulate demand-adapted responses of HSCs and progenitor cells after BM stress or during infection. LSCs occupy similar niches and depend on signals from the BM microenvironment. However, in addition to the cell types that constitute the HSC niche during homeostasis, in leukemia the BM is infiltrated by activated leukemia-specific immune cells. Leukemic cells express different antigens that are able to activate CD4⁺ and CD8⁺ T cells. It is well documented that activated T cells can contribute to the control of leukemic cells and it was hoped that these cells may be able to target and eliminate the therapy-resistant LSCs. However, the actual interaction of leukemia-specific T cells with LSCs remains ill-defined. Paradoxically, many immune mechanisms that evolved to activate emergency hematopoiesis during infection may actually contribute to the expansion and differentiation of LSCs, promoting leukemia progression. In this review, we summarize mechanisms by which the immune system regulates HSCs and LSCs.

Mouse xenograft modeling of human adult acute lymphoblastic leukemia provides mechanistic insights into adult LIC biology

The distinct nature of acute lymphoblastic leukemia (ALL) in adults, evidenced by inferior treatment outcome and different genetic landscape, mandates specific studies of disease-initiating mechanisms. In this study, we used NOD/LtSz-scid IL2Rγ null(c) (NSG) mouse xenotransplantation approaches to elucidate leukemia-initiating cell (LIC) biology in primary adult precursor B (pre-B) ALL to optimize disease modeling. In contrast with xenografting studies of pediatric ALL, we found that modification of the NSG host environment using preconditioning total body irradiation (TBI) was indispensable for efficient engraftment of adult non-t(4;11) pre-B ALL, whereas t(4;11) pre-B ALL was successfully reconstituted without this adaptation. Furthermore, TBI-based xenotransplantation of non-t(4;11) pre-B ALL enabled detection of a high frequency of LICs (<1:6900) and permitted frank leukemic engraftment from a remission sample containing drug-resistant minimal residual disease. Investigation of TBI-sensitive stromal-derived factor-1/chemokine receptor type 4 signaling revealed greater functional dependence of non-t(4;11) pre-B ALL on this niche-based interaction, providing a possible basis for the differential engraftment behavior. Thus, our studies establish the optimal conditions for experimental modeling of human adult pre-B ALL and demonstrate the critical protumorogenic role of microenvironment-derived SDF-1 in regulating adult pre-B LIC activity that may present a therapeutic opportunity.

Disturbed CXCR4/CXCL12 axis in paediatric precursor B-cell acute lymphoblastic leukaemia

Malignant cells infiltrating the bone marrow (BM) interfere with normal cellular behaviour of supporting cells, thereby creating a malignant niche. We found that CXCR4-receptor expression was increased in paediatric precursor B-cell acute lymphoblastic leukaemia (BCP-ALL) cells compared with normal mononuclear haematopoietic cells (P < 0·0001). Furthermore, high CXCR4-expression correlated with an unfavourable outcome in BCP-ALL (5-year cumulative incidence of relapse ± standard error: 38·4% ± 6·9% in CXCR4-high versus 12% ± 4·6% in CXCR4-low expressing cases, P < 0·0001). Interestingly, BM levels of the CXCR4-ligand (CXCL12) were 2·7-fold lower (P = 0·005) in diagnostic BCP-ALL samples compared with non-leukaemic controls. Induction chemotherapy restored CXCL12 levels to normal. Blocking the CXCR4-receptor with Plerixafor showed that the lower CXCL12 serum levels at diagnosis could not be explained by consumption by the leukaemic cells, nor did we observe an altered CXCL12-production capacity of BM-mesenchymal stromal cells (BM-MSC) at this time-point. We rather observed that a very high density of leukaemic cells negatively affected CXCL12-production by the BM-MSC while stimulating the secretion levels of granulocyte colony-stimulating factor (G-CSF). These results suggest that highly proliferative leukaemic cells are able to down-regulate secretion of cytokines involved in homing (CXCL12), while simultaneously up-regulating those involved in haematopoietic mobilization (G-CSF). Therefore, interference with the CXCR4/CXCL12 axis may be an effective way to mobilize BCP-ALL cells.

GSK-3β inhibition promotes early engraftment of ex vivo-expanded haematopoietic stem cells

OBJECTIVES

Umbilical cord blood (UCB) is a source of stem cells used for allogeneic transplantation, in addition to bone marrow and peripheral blood. Limited numbers of stem cells in a single UCB unit is associated with slow haematopoietic recovery and high risk of graft failure, particularly in adult patients. UCB stem cells can be expanded ex vivo; however, rapid differentiation reduces their regenerative potential. We have recently shown that Wnt/β-catenin signalling is down-regulated in ex vivo-expanded stem cells; therefore, we propose that re-activation of Wnt signalling using GSK-3β inhibition may act to improve regenerative potential of these ex vivo-expanded stem cells.

MATERIALS AND METHODS

Immunocompromised mice were employed in transplantation studies to determine stem-cell engraftment. Flow cytometry was used to phenotype the engrafted human cells. Retroviral gene transfer was used to examine the role of Myc gene up-regulated by GSK-3β inhibition, in ex vivo-expanded stem cells.

RESULTS

Treatment with GSK-3β inhibitor, 6-bromoindirubin 3'-oxime (BIO) improved early human cell engraftment in the mice and elevated the numbers of myeloid progenitor cells in cytokine-stimulated culture. BIO up-regulated β-catenin and c-myc in ex vivo-expanded stem cells. Ectopic expression of Myc acted to increase clonogenic potential and to delay differentiation of haematopoietic progenitor cells, suggesting the potential mechanism to improve regenerative potential of ex vivo-expanded grafts.

CONCLUSIONS

Pharmacological inhibition of GSK-3β provided a novel approach to improve early engraftment of ex vivo-expanded haematopoietic progenitor cells.

Advances in understanding the leukaemia microenvironment

Dynamic interactions between leukaemic cells and cells of the bone marrow are a feature of haematological malignancies. Two distinct microenvironmental niches in the bone marrow, the 'osteoblastic (endosteal)' and 'vascular' niches, provide a sanctuary for subpopulations of leukaemic cells to evade chemotherapy-induced death and allow acquisition of drug resistance. Key components of the bone marrow microenvironment as a home for normal haematopoietic stem cells and the leukaemia stem cell niches, and the molecular pathways critical for microenvironment/leukaemia interactions via cytokines, chemokines and adhesion molecules as well as hypoxic conditions, are described in this review. Finally, the genetic abnormalities of leukaemia-associated stroma are discussed. Further understanding of the contribution of the bone marrow niche to the process of leukaemogenesis may provide new targets that allow destruction of leukaemia stem cells without adversely affecting normal stem cell self-renewal.

Emerging targets in cancer management: role of the CXCL12/CXCR4 axis

The chemokine CXCL12 (SDF-1) and its cell surface receptor CXCR4 were first identified as regulators of lymphocyte trafficking to the bone marrow. Soon after, the CXCL12/CXCR4 axis was proposed to regulate the trafficking of breast cancer cells to sites of metastasis. More recently, it was established that CXCR4 plays a central role in cancer cell proliferation, invasion, and dissemination in the majority of malignant diseases. The stem cell concept of cancer has revolutionized the understanding of tumorigenesis and cancer treatment. A growing body of evidence indicates that a subset of cancer cells, referred to as cancer stem cells (CSCs), plays a critical role in tumor initiation, metastatic colonization, and resistance to therapy. Although the signals generated by the metastatic niche that regulate CSCs are not yet fully understood, accumulating evidence suggests a key role of the CXCL12/CXCR4 axis. In this review we focus on physiological functions of the CXCL12/CXCR4 signaling pathway and its role in cancer and CSCs, and we discuss the potential for targeting this pathway in cancer management.

Pharmacogenomic considerations of xenograft mouse models of acute leukemia

The use of combination chemotherapy to cure acute lymphoblastic leukemia in children and acute myeloid leukemia in adults emerged for acute myeloid leukemia in the 1960s and for acute lymphoblastic leukemia in the 1980s as a paradigm for curing any disseminated cancer. This article summarizes recent developments and considerations in the use of acute leukemia xenografts established in immunodeficient mice to elucidate the genetic and genomic basis of acute leukemia pathogenesis and treatment response.

Targeting the CD20 and CXCR4 pathways in non-hodgkin lymphoma with rituximab and high-affinity CXCR4 antagonist BKT140

PURPOSE

Chemokine axis CXCR4/CXCL12 is critically involved in the survival and trafficking of normal and malignant B lymphocytes. Here, we investigated the effect of high-affinity CXCR4 antagonist BKT140 on lymphoma cell growth and rituximab-induced cytotoxicity in vitro and in vivo.

EXPERIMENTAL DESIGN

In vitro efficacy of BKT140 alone or in combination with rituximab was determined in non-Hodgkin lymphoma (NHL) cell lines and primary samples from bone marrow aspirates of patients with NHL. In vivo efficacy was evaluated in xenograft models of localized and disseminated NHL with bone marrow involvement.

RESULTS

Antagonizing CXCR4 with BKT140 resulted in significant inhibition of CD20+ lymphoma cell growth and in the induction of cell death, respectively. Combination of BKT140 with rituximab significantly enhanced the apoptosis against the lymphoma cells in a dose-dependent manner. Moreover, rituximab induced CXCR4 expression in lymphoma cell lines and primary lymphoma cells, suggesting the possible interaction between CD20 and CXCR4 pathways in NHL. Primary bone marrow stromal cells (BMSC) further increased CXCR4 expression and protected NHL cells from rituximab-induced apoptosis, whereas BKT140 abrogated this protective effect. Furthermore, BKT140 showed efficient antilymphoma activity in vivo in the xenograft model of disseminated NHL with bone marrow involvement. BKT140 treatment inhibited the local tumor progression and significantly reduced the number of NHL cells in the bone marrow. Combined treatment of BKT140 with rituximab further decreased the number of viable lymphoma cells in the bone marrow, achieving 93% reduction.

CONCLUSIONS

These findings suggest the possible role of CXCR4 in NHL progression and response to rituximab and provide the scientific basis for the development of novel CXCR4-targeted therapies for refractory NHL.

MLL-rearranged acute lymphoblastic leukaemia stem cell interactions with bone marrow stroma promote survival and therapeutic resistance that can be overcome with CXCR4 antagonism

Infants with MLL-rearranged (MLL-R) acute lymphoblastic leukaemia (ALL) have a dismal prognosis. While most patients achieve remission, approximately half of patients recur with a short latency to relapse. This suggests that chemotherapy-resistant leukaemia stem cells (LSCs) survive and can recapitulate the leukaemia. We hypothesized that interactions between LSCs and the bone marrow microenvironment mediate survival and chemotherapy resistance in MLL-R ALL. Using primary samples of infant MLL-R ALL, we studied the influence of bone marrow stroma on apoptosis, proliferation, and cytotoxicity induced by the FLT3 inhibitor lestaurtinib. MLL-R ALL were differentially protected by stroma from spontaneous apoptosis compared to non-MLL-R ALL. Co-culture of bulk MLL-R ALL in direct contact with stroma or with stroma-produced soluble factors promoted proliferation and cell cycle entry. Stroma also protected bulk MLL-R ALL cells and MLL-R ALL LSCs from lestaurtinib-mediated cytotoxicity. Previous studies have demonstrated that CXCR4 mediates bone marrow microenvironment signalling. Using a xenograft model of MLL-R ALL, we demonstrated that CXCR4 inhibition with AMD3100 (plerixafor) led to markedly enhanced efficacy of lestaurtinib. Therefore, the bone marrow microenvironment is a mediator of chemotherapy resistance in MLL-R ALL and targeting leukaemia-stroma interactions with CXCR4 inhibitors may prove useful in this high-risk subtype of paediatric ALL.

Treatment of acute lymphoblastic leukemia with an rGel/BLyS fusion toxin

Acute lymphoblastic leukemia (ALL) is the most common malignancy affecting children and a major cause of mortality from hematopoietic malignancies in adults. A substantial number of patients become drug resistant during chemotherapy, necessitating the development of alternative modes of treatment. rGel (recombinant Gelonin)/BlyS (B-lymphocyte stimulator) is a toxin-cytokine fusion protein used for selective killing of malignant B-cells expressing receptors for B-cell-activating factor (BAFF/BLyS) by receptor-targeted delivery of the toxin, Gelonin. Here, we demonstrate that rGel/BLyS binds to ALL cells expressing BAFF receptor (BAFF-R) and upon internalization, it induces apoptosis of these cells and causes downregulation of survival genes even in the presence of stromal protection. Using an immunodeficient transplant model for human ALL, we show that rGel/BLyS prolongs survival of both Philadelphia chromosome-positive and negative ALL-bearing mice. Furthermore, we used AMD3100, a CXCR4 antagonist, to mobilize the leukemic cells protected in the bone marrow (BM) microenvironment and the combination with rGel/BLyS resulted in a significant reduction of the tumor load in the BM and complete eradication of ALL cells from the circulation. Thus, a combination treatment with the B-cell-specific fusion toxin rGel/BLyS and the mobilizing agent AMD3100 could be an effective alternative approach to chemotherapy for the treatment of primary and relapsed ALL.

Mobilization of hematopoietic stem and leukemia cells

HSC mobilization is an essential homeostatic process during inflammation and for the maintenance of hematopoietic progenitors. It has been exploited for the therapeutic application of HSC transplantation. Recent evidence suggests that leukemic cells share surface molecules in common with stem cells and may be mobilized under similar conditions. This effect could be used for therapeutic interventions. In this review, we will provide evidence showing that leukemia cells and stem cells traffic similarly and may share a common niche. Studies are discussed comparing and contrasting the mechanism of normal stem cells and leukemic cell mobilization through the CXCR4/CXCL12 axis and other key intermediaries.

The bone marrow microenvironment and leukemia: biology and therapeutic targeting

Multiple studies have demonstrated that interaction with the bone marrow stromal microenvironment contributes to the survival of leukemia cells. One explanation for this phenomenon is the interaction between the cell surface receptors CXCR4 and CXCL12. Through CXCL12/CXCR4-mediated chemotaxis, leukemia cells migrate to microscopic niches within the bone marrow, which leads to increased proliferation and survival. Several studies have suggested that increased CXCR4 expression may portend a poor prognosis in various types of leukemia, possibly due to increased protection of leukemia cells by bone marrow stroma. A potential therapeutic strategy to overcome this stromal-mediated survival advantage is to target CXCR4. Inhibition of CXCR4 may allow leukemia cells to be released from bone marrow niches that confer resistance to chemotherapy and negate the survival benefit imparted by bone marrow stroma.

A Lentiviral CXCR4 overexpression and knockdown model in colorectal cancer cell lines reveals plerixafor-dependent suppression of SDF-1α-induced migration and invasion

BACKGROUND

The development of distant metastasis is associated with poor outcome in patients with colorectal cancer (CRC). The stromal cell-derived factor-1 (SDF-1) and its receptor CXC chemokine receptor 4 (CXCR4) have pivotal roles in the chemotaxis of migrating tumor cells during metastasis. Thus, hampering the SDF-1/CXCR4 cross-talk is a promising strategy to suppress metastasis.

METHODS

We investigated the invasive behavior of the lentivirally CXCR4 overexpressing CRC cell lines SW480, SW620 and RKO in chemotaxis and invasion assays toward an SDF-1α gradient. Low endogenous CXCR4 expression levels were determined by quantitative realtime polymerase chain reaction (PCR) and fluorescence-activated cell sorting (FACS) analyses.

RESULTS

A lentiviral CXCR4 overexpression and knockdown model was established in these CRC cells. In transwell migration assays, CXCR4 overexpression favored chemotaxis and invasion of cells in all 3 lines depending on an SDF-1α gradient (p < 0.001 vs. untransduced cells). Functional CXCR4 knockdown using lentiviral short hairpin RNA (shRNA) vectors significantly decreased the migration behavior in CRC cell lines (p < 0.001), confirming a CXCR4-specific effect. Pharmacologic inhibition of the SDF-1α/CXCR4 interaction by the bicyclam Plerixafor(TM) at 100 μM significantly abrogated CXCR4-dependent migration and invasion through Matrigel(TM) (SW480, SW620, RKO; p < 0.05).

CONCLUSION

Our results indicate that a CXCR4-antagonistic therapy might prevent tumor cell dissemination and metastasis in CRC patients, consequently improving survival.

Extranodal dissemination of non-Hodgkin lymphoma requires CD47 and is inhibited by anti-CD47 antibody therapy

Non-Hodgkin lymphoma (NHL) presents as both localized and disseminated disease with spread to secondary sites carrying a worse prognosis. Although pathways driving NHL dissemination have been identified, there are few therapies capable of inhibiting them. Here, we report a novel role for the immunomodulatory protein CD47 in NHL dissemination, and we demonstrate that therapeutic targeting of CD47 can prevent such spread. We developed 2 in vivo lymphoma metastasis models using Raji cells, a human NHL cell line, and primary cells from a lymphoma patient. CD47 expression was required for Raji cell dissemination to the liver in mouse xenotransplants. Targeting of CD47 with a blocking antibody inhibited Raji cell dissemination to major organs, including the central nervous system, and inhibited hematogenous dissemination of primary lymphoma cells. We hypothesized that anti-CD47 antibody-mediated elimination of circulating tumor cells occurred through phagocytosis, a previously described mechanism for blocking anti-CD47 antibodies. As predicted, inhibition of dissemination by anti-CD47 antibodies was dependent on blockade of phagocyte SIRPα and required macrophage effector cells. These results demonstrate that CD47 is required for NHL dissemination, which can be therapeutically targeted with a blocking anti-CD47 antibody. Ultimately, these findings are potentially applicable to the dissemination and metastasis of other solid tumors.

Cellular elements of the subarachnoid space promote ALL survival during chemotherapy

CNS infiltration by leukemic cells remains a problematic disease manifestation of acute lymphoblastic leukemia (ALL). Prophylactic regimens for CNS leukemia including intrathecal chemotherapeutics have decreased CNS involvement in ALL, but are not without toxicities. Using co-culture models, we show that astrocytes, choroid plexus epithelial cells, and meningeal cells protect ALL cells from chemotherapy-induced cell death using drugs included in prophylactic regimens-cytarabine, dexamethasone, and methotrexate. Understanding how ALL cells survive in the CNS remains invaluable for designing strategies to prevent CNS leukemia and minimizing the need for treatment in this sensitive anatomical site where treatment-induced toxicity is of significant concern.

Can inhibition of the SDF-1/CXCR4 axis eradicate acute leukemia?

Poor prognosis of acute leukemia with current treatments is mainly due to the relapse of the disease following chemotherapy. In the last decade, an emerging concept has proposed that the leukemia stem cells (LSCs) and their interactions with the BM microenvironment are the major cause of the acute leukemia relapse. Adhesion to the stromal niche is crucial for LSCs as it directly supports self-renewal, proliferation, arrest of differentiation and protects from damaging chemo-agents. One of the key players in this crosstalk between leukemic cells and the BM stroma niche is the chemokine SDF-1. SDF-1 regulates the process of homing and engraftment of LSCs into the BM and inhibition of its receptor CXCR4 induces leukemic cell mobilization into the circulation. However, besides its chemotactic and adhesive functions, SDF-1 is also a pleiotropic cytokine that regulates leukemic cell proliferation as well as their program of differentiation. CXCR4 antagonists are used in combination with chemotherapy in preclinical and clinical studies, which demonstrate that blocking CXCR4 is a novel promising approach of therapy. In this review, we focus on the multifaceted SDF-1/CXCR4 axis in acute leukemia and discuss how targeting this pathway could provide potential interest to eradicate the LSCs.

CD38 increases CXCL12-mediated signals and homing of chronic lymphocytic leukemia cells

Homing of chronic lymphocytic leukemia (CLL) cells to sites favoring growth, a critical step in disease progression, is principally coordinated by the CXCL12/CXCR4 axis. A cohort of 62 CLL patients was divided into migrating and nonmigrating subsets according to chemotaxis toward CXCL12. Migrating patients phosphorylated extracellular signal-regulated kinase 1/2 (ERK1/2) proteins more than nonmigrating patients (P<0.0002). CD38 expression was the parameter most strongly associated with heightened CXCL12 signaling (P<0.0001), confirmed by independent statistical approaches. Consistent with this observation, CD38(-) CLL cells in samples with bimodal CD38 expression responded less to CXCL12 than the intact clone (P=0.003). Furthermore, lentivirus-induced de novo expression of CD38 was paralleled by increased responses to CXCL12, as compared with cells infected with a control virus. CD38 ligation with agonistic monoclonal antibodies (mAbs) enhanced CXCL12 signaling, whereas blocking anti-CD38 mAbs inhibited chemokine effects in vitro. This is attributed to physical proximity on the membrane between CD38 and CXCR4 (the CXCL12 receptor), as shown by (i) coimmunoprecipitation and (ii) confocal microscopy experiments. Blocking anti-CD38 mAbs significantly compromised homing of CLL cells from blood to lymphoid organs in a mouse model. These results indicate that CD38 synergizes with the CXCR4 pathway and support the working hypothesis that migration is a central step in disease progression.

CXCR4 in clinical hematology

Pharmacological manipulation of CXCR4 has proven clinically useful for mobilization of stem and progenitor cells and in several preclinical models of disease. It is a key component in the localization of leukocytes and stem cells. For patients with multiple myeloma and non-Hodgkin's Lymphoma, treatment with plerixafor, an inhibitor of CXCL12 binding to CXCR4, plus G-CSF mobilizes stem cells for autologous transplantation to a greater degree than the treatment with G-CSF alone, and in some cases when patients could not be mobilized with cytokines, chemotherapy, or the combination. Stem cells from healthy donors mobilized with single agent plerixafor have been used for allogeneic transplantation in acute myelogenous leukemia (AML) patients, although this is still in the early phase of clinical development. Plerixafor is also undergoing evaluation to mobilize tumor cells in patients with AML and chronic lymphocytic leukemia (CLL) to enhance the effectiveness of chemotherapy regimens. Plerixafor's effect on neutrophils may also restore circulating neutrophil counts to normal levels in patients with chronic neutropenias such as in WHIMs syndrome. Other areas where inhibition of CXCR4 may be useful based upon preclinical or clinical data include peripheral vascular disease, autoimmune diseases such as rheumatoid arthritis, pulmonary inflammation, and HIV.