CXCR4 chemokine receptors (CD184) and alpha4beta1 integrins mediate spontaneous migration of human CD34+ progenitors and acute myeloid leukaemia cells beneath marrow stromal cells (pseudoemperipolesis)

Integrins, anchors and signal transducers of hematopoietic stem cells during development and in adulthood

Hematopoietic stem cells (HSCs), the apex of the hierarchically organized blood cell production system, are generated in the yolk sac, aorta-gonad-mesonephros region and placenta of the developing embryo. To maintain life-long hematopoiesis, HSCs emigrate from their site of origin and seed in distinct microenvironments, called niches, of fetal liver and bone marrow where they receive supportive signals for self-renewal, expansion and production of hematopoietic progenitor cells (HPCs), which in turn orchestrate the production of the hematopoietic effector cells. The interactions of hematopoietic stem and progenitor cells (HSPCs) with niche components are to a large part mediated by the integrin superfamily of adhesion molecules. Here, we summarize the current knowledge regarding the functional properties of integrins and their activators, Talin-1 and Kindlin-3, for HSPC generation, function and fate decisions during development and in adulthood. In addition, we discuss integrin-mediated mechanosensing for HSC-niche interactions, ex vivo protocols aimed at expanding HSCs for therapeutic use, and recent approaches targeting the integrin-mediated adhesion in leukemia-inducing HSCs in their protecting, malignant niches.

METTL3 mediates chemoresistance by enhancing AML homing and engraftment via ITGA4

Chemoresistant leukemia relapse is one of the most common causes of death for acute myeloid leukemia (AML) patients and the homing/engraftment in bone marrow (BM) are crucial steps for AML cells to acquire chemoresistance by interacting with stromal cell components. No crosstalk between m⁶A modification and homing/engraftment has been reported. Here, we performed comprehensive high-throughput analyses, including RNA sequencing of CR (complete remission) and relapsed AML patients, and reverse-phase protein arrays of chemoresistant cells to identify METTL3 as a key player regulating AML chemoresistance. Then, METTL3-mediated m⁶A modification was proved to induce the chemoresistance in vitro and in vivo. Furthermore, AML homing/engraftment was discovered being enhanced by upregulated-METTL3 in chemoresistant cells. And the homing/engraftment and drug-resistance associated phenotypes of chemoresistant cells could be reversed by a METTL3 inhibitor. Mechanistically, METTL3 extended the half-life of ITGA4 mRNA by m⁶A methylation, and then, increased expression of ITGA4 protein to enhance homing/engraftment of AML cells. The results provide insights into the function of m⁶A modification on the interaction between AML cells and BM niches and clarify the relationship between METTL3 and AML homing/engraftment, suggesting a therapeutic strategy for the treatment of refractory/relapsed AML with METTL3 inhibitors.

A multivalent Ara-C-prodrug nanoconjugate achieves selective ablation of leukemic cells in an acute myeloid leukemia mouse model

Current therapy in acute myeloid leukemia (AML) is based on chemotherapeutic drugs administered at high doses, lacking targeting selectivity and displaying poor therapeutic index because of severe adverse effects. Here, we develop a novel nanoconjugate that combines a self-assembled, multivalent protein nanoparticle, targeting the CXCR4 receptor, with an Oligo-Ara-C prodrug, a pentameric form of Ara-C, to highly increase the delivered payload to target cells. This 13.4 nm T22-GFP-H6-Ara-C nanoconjugate selectively eliminates CXCR4⁺ AML cells, which are protected by its anchoring to the bone marrow (BM) niche, being involved in AML progression and chemotherapy resistance. This nanoconjugate shows CXCR4-dependent internalization and antineoplastic activity in CXCR4⁺ AML cells in vitro. Moreover, repeated T22-GFP-H6-Ara-C administration selectively eliminates CXCR4⁺ leukemic cells in BM, spleen and liver. The leukemic dissemination blockage induced by T22-GFP-H6-Ara-C is significantly more potent than buffer or Oligo-Ara-C-treated mice, showing no associated on-target or off-target toxicity and, therefore, reaching a highly therapeutic window. In conclusion, T22-GFP-H6-Ara-C exploits its 11 ligands-multivalency to enhance target selectivity, while the Oligo-Ara-C prodrug multimeric form increases 5-fold its payload. This feature combination offers an alternative nanomedicine with higher activity and greater tolerability than current intensive or non-intensive chemotherapy for AML patients.

Leukemia stem cell-bone marrow microenvironment interplay in acute myeloid leukemia development

Despite the advances in intensive chemotherapy regimens and targeted therapies, overall survival (OS) of acute myeloid leukemia (AML) remains unfavorable due to inevitable chemotherapy resistance and high relapse rate, which mainly caused by the persistence existence of leukemia stem cells (LSCs). Bone marrow microenvironment (BMM), the home of hematopoiesis, has been considered to play a crucial role in both hematopoiesis and leukemogenesis. When interrupted by the AML cells, a malignant BMM formed and thus provided a refuge for LSCs and protecting them from the cytotoxic effects of chemotherapy. In this review, we summarized the alterations in the bidirectional interplay between hematopoietic cells and BMM in the normal/AML hematopoietic environment, and pointed out the key role of these alterations in pathogenesis and chemotherapy resistance of AML. Finally, we focused on the current potential BMM-targeted strategies together with future prospects and challenges. Accordingly, while further research is necessary to elucidate the underlying mechanisms behind LSC–BMM interaction, targeting the interaction is perceived as a potential therapeutic strategy to eradicate LSCs and ultimately improve the outcome of AML.

Leukemia Stem Cell Release From the Stem Cell Niche to Treat Acute Myeloid Leukemia

Acute myeloid leukemia (AML) is a heterogeneous, complex, and deadly disease, whose treatment has hardly evolved for decades and grounds on the use of intensive chemotherapy regimens. Chemotherapy helps reduce AML bulk, but promotes relapse in the long-run by selection of chemoresistant leukemia stem cells (LSC). These may diversify and result in progression to more aggressive forms of AML. In vivo models suggest that the bone marrow stem cell niche helps LSC stay dormant and protected from chemotherapy. Here, we summarize relevant changes in stem cell niche homing and adhesion of AML LSC vs. healthy hematopoietic stem cells, and provide an overview of clinical trials aiming at targeting these processes for AML treatment and future directions within this field. Promising results with various non-mutation-targeted novel therapies directed to LSC eradication via interference with their anchoring to the stem cell niche have encouraged on-going or future advanced phase III clinical trials. In the coming years, we may see a shift in the focus of AML treatment to LSC-directed therapies if the prospect of improved cure rates holds true. In the future, AML treatment should lean toward personalized therapies using combinations of these compounds plus mutation-targeted agents and/or targeted delivery of chemotherapy, aiming at LSC eradication with reduced side effects.

Dissecting the role of the CXCL12/CXCR4 axis in acute myeloid leukaemia

Acute myeloid leukaemia (AML) is the most common adult acute leukaemia with the lowest survival rate. It is characterised by a build-up of immature myeloid cells anchored in the protective niche of the bone marrow (BM) microenvironment. The CXCL12/CXCR4 axis is central to the pathogenesis of AML as it has fundamental control over AML cell adhesion into the protective BM niche, adaptation to the hypoxic environment, cellular migration and survival. High levels of CXCR4 expression are associated with poor relapse-free and overall survival. The CXCR4 ligand, CXCL12 (SDF-1), is expressed by multiple cells types in the BM, facilitating the adhesion and survival of the malignant clone. Blocking the CXCL12/CXCR4 axis is an attractive therapeutic strategy providing a 'multi-hit' therapy that both prevents essential survival signals and releases the AML cells from the BM into the circulation. Once out of the protective niche of the BM they would be more susceptible to destruction by conventional chemotherapeutic drugs. In this review, we disentangle the diverse roles of the CXCL12/CXCR4 axis in AML. We then describe multiple CXCR4 inhibitors, including small molecules, peptides, or monoclonal antibodies, which have been developed to date and their progress in pre-clinical and clinical trials. Finally, the review leads us to the conclusion that there is a need for further investigation into the development of a 'multi-hit' therapy that targets several signalling pathways related to AML cell adhesion and maintenance in the BM.

Predictive value of integrin α7 for acute myeloid leukemia risk and its correlation with prognosis in acute myeloid leukemia patients

Background

This study aimed to explore the predictive value of integrin α7 (ITGA7) for acute myeloid leukemia (AML) risk and subsequently investigate its correlation with risk stratification and prognosis in AML patients.

Methods

Bone marrow samples were obtained from 196 de novo AML patients prior to initiation of treatment and from 50 subjects underwent bone marrow donation or bone marrow biopsy for non‐hematologic malignant disease (as controls). ITGA7 mRNA and protein expressions were detected by real‐time quantitative polymerase chain reaction and Western blot assays, respectively. In AML patients, the risk stratification was assessed, and complete remission (CR), event‐free survival (EFS), and overall survival (OS) were evaluated.

Results

Both ITGA7 mRNA and protein expressions were increased in AML patients compared with controls, and their expressions were correlated with poorer risk stratification. For prognosis, ITGA7 mRNA expression and protein expression were declined in CR patients compared to non‐CR patients. Meanwhile, both EFS and OS were shorter in ITGA7 mRNA high expression patients compared to ITGA7 mRNA low expression patients, as well as ITGA7 protein high expression patients compared to ITGA7 protein low expression patients.

Conclusion

Integrin α7 might serve as a potential biomarker for predicting increased AML risk and worse prognosis in AML patients.

Intracranial acute promyelocytic leukemia at presentation-A case-based discussion

Intracranial disease is a very rare presentation at diagnosis in acute promyelocytic leukemia (APL). The risk associated with this particular presentation is not accounted for when the current risk stratification is based on peripheral counts. Extra medullary disease in general may challenge this risk stratification that commands initial induction treatment of this potentially fatal disease. Here we discuss a case presented at diagnosis with extensive intracranial base of the skull, clivus and sinus infiltration and heavily infiltrated bone marrow yet with low peripheral blood counts and no peripheral blood blasts. Such cases lack evidence of how to treat.

Adhesion Deregulation in Acute Myeloid Leukaemia

Cell adhesion is a process through which cells interact with and attach to neighboring cells or matrix using specialized surface cell adhesion molecules (AMs). Adhesion plays an important role in normal haematopoiesis and in acute myeloid leukaemia (AML). AML blasts express many of the AMs identified on normal haematopoietic precursors. Differential expression of AMs between normal haematopoietic cells and leukaemic blasts has been documented to a variable extent, likely reflecting the heterogeneity of the disease. AMs govern a variety of processes within the bone marrow (BM), such as migration, homing, and quiescence. AML blasts home to the BM, as the AM-mediated interaction with the niche protects them from chemotherapeutic agents. On the contrary, they detach from the niches and move from the BM into the peripheral blood to colonize other sites, i.e., the spleen and liver, possibly in a process that is reminiscent of epithelial-to-mesenchymal-transition in metastatic solid cancers. The expression of AMs has a prognostic impact and there are ongoing efforts to therapeutically target adhesion in the fight against leukaemia.

Roles of the bone marrow niche in hematopoiesis, leukemogenesis, and chemotherapy resistance in acute myeloid leukemia

OBJECTIVES

To summarize the effects of the bone marrow niche on hematopoiesis and leukemogenesis and discuss the chemotherapy resistance that can arise from interactions between the niche and leukemia stem cells.

METHODS

We review the major roles of the bone marrow niche in cell proliferation, adhesion and drug resistance. The signaling pathways and major molecular participants in the niche are discussed. We also address potential niche-targeting strategies for the treatment of acute myeloid leukemia (AML).

RESULTS

The bone marrow niche supports normal hematopoiesis and affects acute myeloid leukemia (AML) initiation, progression and chemotherapy resistance.

DISCUSSION

AML is a group of heterogeneous malignant diseases characterized by the excessive proliferation of hematopoietic stem and/or progenitor cells. Even with intensive chemotherapy regimens and stem cell transplantation, the overall survival rate for AML is poor. The bone marrow niches of malignant cells are remodeled into a leukemia-permissive environment, and these reformed niches protect AML cells from chemotherapy-induced cell death. Inhibiting the cellular and molecular interactions between the niche and leukemia cells is a promising direction for targeted therapies for AML treatment.

CONCLUSIONS

Interactions between leukemia cells and the bone marrow niche influence hematopoiesis, leukemogenesis, and chemotherapy resistance in AML and require ongoing study. Understanding the mechanisms that underlie these interactions will help identify rational niche-targeting therapies to improve treatment outcomes in AML patients.

Therapeutic Antibodies for Myeloid Neoplasms-Current Developments and Future Directions

Therapeutic monoclonal antibodies (mAbs) such as antibody-drug conjugates, ligand-receptor antagonists, immune checkpoint inhibitors and bispecific T cell engagers have shown impressive efficacy in the treatment of multiple human cancers. Numerous therapeutic mAbs that have been developed for myeloid neoplasms, including acute myeloid leukemia (AML) and myelodysplastic syndrome (MDS), are currently investigated in clinical trials. Because AML and MDS originate from malignantly transformed hematopoietic stem/progenitor cells-the so-called leukemic stem cells (LSCs) that are highly resistant to most standard drugs-these malignancies frequently relapse and have a high disease-specific mortality. Therefore, combining standard chemotherapy with antileukemic mAbs that specifically target malignant blasts and particularly LSCs or utilizing mAbs that reinforce antileukemic host immunity holds great promise for improving patient outcomes. This review provides an overview of therapeutic mAbs for AML and MDS. Antibody targets, the molecular mechanisms of action, the efficacy in preclinical leukemia models, and the results of clinical trials are discussed. New developments and future studies of therapeutic mAbs in myeloid neoplasms will advance our understanding of the immunobiology of these diseases and enhance current therapeutic strategies.

CXCR4 antagonists suppress small cell lung cancer progression

Small cell lung cancer (SCLC) is an aggressive tumor with poor prognosis due to early metastatic spread and development of chemoresistance. Playing a key role in tumor-stroma interactions the CXCL12-CXCR4 axis may be involved in both processes and thus represent a promising therapeutic target in SCLC treatment. In this study we investigated the effect of CXCR4 inhibition on metastasis formation and chemoresistance using an orthotopic xenograft mouse model. This model demonstrates regional spread and spontaneous distant metastases closely reflecting the clinical situation in extensive SCLC. Tumor engraftment, growth, metabolism, and metastatic spread were monitored using different imaging techniques: Magnetic Resonance Imaging (MRI), Bioluminescence Imaging (BLI) and Positron Emission Tomography (PET). Treatment of mice bearing chemoresistant primary tumors with the specific CXCR4 inhibitor AMD3100 reduced the growth of the primary tumor by 61% (P<0.05) and additionally suppressed metastasis formation by 43%. In comparison to CXCR4 inhibition as a monotherapy, standard chemotherapy composed of cisplatin and etoposide reduced the growth of the primary tumor by 71% (P<0.01) but completely failed to suppress metastasis formation. Combination of chemotherapy and the CXCR4 inhibitor integrated the highest of both effects. The growth of the primary tumor was reduced to a similar extent as with chemotherapy alone and metastasis formation was reduced to a similar extent as with CXCR4 inhibitor alone. In conclusion, we demonstrate in this orthotopic mouse model that the addition of a CXCR4 inhibitor to chemotherapy significantly reduces metastasis formation. Thus, it might improve the overall therapy response and consequently the outcome of SCLC patients.

Anti-apoptotic ARC protein confers chemoresistance by controlling leukemia-microenvironment interactions through a NFκB/IL1β signaling network

To better understand how the apoptosis repressor with caspase recruitment domain (ARC) protein confers drug resistance in acute myeloid leukemia (AML), we investigated the role of ARC in regulating leukemia-mesenchymal stromal cell (MSC) interactions. In addition to the previously reported effect on AML apoptosis, we have demonstrated that ARC enhances migration and adhesion of leukemia cells to MSCs both in vitro and in a novel human extramedullary bone/bone marrow mouse model. Mechanistic studies revealed that ARC induces IL1β expression in AML cells and increases CCL2, CCL4, and CXCL12 expression in MSCs, both through ARC-mediated activation of NFκB. Expression of these chemokines in MSCs increased by AML cells in an ARC/IL1β-dependent manner; likewise, IL1β expression was elevated when leukemia cells were co-cultured with MSCs. Further, cells from AML patients expressed the receptors for and migrated toward CCL2, CCL4, and CXCL12. Inhibition of IL1β suppressed AML cell migration and sensitized the cells co-cultured with MSCs to chemotherapy. Our results suggest the existence of a complex ARC-regulated circuit that maintains intimate connection of AML with the tumor microenvironment through NFκB/IL1β-regulated chemokine receptor/ligand axes and reciprocal crosstalk resulting in cytoprotection. The data implicate ARC as a promising drug target to potentially sensitize AML cells to chemotherapy.

The good and bad faces of the CXCR4 chemokine receptor

Chemokines are chemotactic cytokines that promote cell migration and activation under homeostatic and inflammatory conditions. Chemokines bind to seven transmembrane-spanning receptors that are coupled to heterotrimeric guanine nucleotide-binding (G) proteins, which are the responsible for intracellularly transmitting the activating signals for cell migration. Hematopoiesis, vascular development, lymphoid organ morphogenesis, cardiogenesis and neural differentiation are amongst the processes involving chemokine function. In addition, immune cell trafficking from bone marrow to blood circulation, and from blood and lymph to lymphoid and inflamed tissues, is tightly regulated by chemokines both under physiological conditions and also in autoimmune diseases. Furthermore, chemokine binding to their receptors stimulate trafficking to and positioning of cancer cells into target tissues and organs during tumour dissemination. The CXCL12 chemokine (also known as stromal-cell derived factor-1α, SDF-1α) plays key roles in hematopoiesis and lymphoid tissue architecture, in cardiogenesis, vascular formation and neurogenesis, as well as in the trafficking of solid and hematological cancer cell types. CXCL12 binds to the CXCR4 receptor, a multi-facetted molecule which tightly mirrors CXCL12 functions in homeostasis and disease. This review addresses the important roles of the CXCR4-CXCL12 axis in homeostasis, specially focusing in hematopoiesis, as well as it provides a picture of CXCR4 as mediator of cancer cell spreading, and a view of the available CXCR4 antagonists in different cancer types.

Crosstalk between Stem and Progenitor Cellular Mediators with Special Emphasis on Vasculogenesis

The cellular components and molecular processes of signaling during vasculogenesis have been investigated for decades. Considerable efforts have been made to unravel regulatory mechanisms of vasculogenesis through crosstalk between vasculogenic playmakers located in the vascular niche, namely hematopoietic stem cells, endothelial progenitor cells, and mesenchymal stem and progenitor cells. Recent studies have increased the knowledge about signaling events within vascular microenvironment that leads to vasculogenesis. Findings from these recent studies indicate the impact of cellular crosstalk through signaling pathways such as vascular endothelial growth factor signaling, wingless and Notch signaling in vasculogenesis and vascular development. In this review, we highlight the signaling signature between stem and progenitor cellular mediators during vasculogenesis. We further focus on hematopoietic stem cell-endothelial progenitor cell crosstalk during vasculogenesis and discuss their potential implications and benefits for therapeutic interventions and regenerative therapy.

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.

Exosomes: The Link between GPCR Activation and Metastatic Potential?

The activation of G-Protein Coupled Receptors (GPCRs) by their respective ligands initiates a cascade of multiple signaling processes within the cell, regulating growth, metabolism and other essential cellular functions. Dysregulation and aberrant expression of these GPCRs and their subsequent signaling cascades are associated with many different types of pathologies, including cancer. The main life threatening complication in patients diagnosed with cancer is the dissemination of cells from the primary tumor to distant vital organs within the body, metastasis. Communication between the primary tumor, immune system, and the site of future metastasis are some of the key events in the early stages of metastasis. It has been postulated that the communication is mediated by nanovesicles that, under non-pathological conditions, are released by normal cells to relay signals to other cells in the body. These nanovesicles are called exosomes, and are utilized by the tumor cell to influence changes within the recipient cell, such as bone marrow progenitor cells, and cells within the site of future metastatic growth, in order to prepare the site for colonization. Tumor cells have been shown to release an increased number of exosomes when compared to their normal cell counterpart. Exosome production and release are regulated by proteins involved in localization, degradation and size of the multivesicular body, whose function may be altered within cancer cells, resulting in the release of an increased number of these vesicles. This review investigates the possibility of GPCR signaling cascades acting as the upstream activator of proteins involved in exosome production and release, linking a commonly targeted trans-membrane protein class with cellular communication utilized by tumor cells in early stages of metastasis.

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.

CXCR4 expression on pathogenic T cells facilitates their bone marrow infiltration in a mouse model of aplastic anemia

Aplastic anemia (AA) is a disease characterized by T-cell-mediated destruction of bone marrow (BM) hematopoietic stem and progenitor cells. Physiologically, T cells migrate to the BM in response to chemokines, such as SDF-1α, the ligand for CXCR4. However, how T cells traffic to the BM in AA is poorly understood. CXCR4 is aberrantly expressed in immune-mediated diseases and its regulation by nuclear factor-κB (NF-κB) in cancer models is well documented. In this study, we show that CXCR4 is highly expressed on BM-infiltrating CD4(+) and CD8(+) T cells in a mouse model of AA. Inhibiting CXCR4 in AA mice, using CXCR4(-/-) splenocytes or AMD3100, significantly reduced BM infiltration of T cells. We also report that NF-κB occupancy at the CXCR4 promoter is enhanced in BM-infiltrating CD8(+) T cells of AA mice. Moreover, inhibiting NF-κB signaling in AA mice using Bay11 or dehydroxymethylepoxyquinomicin, or transferring p50(-/-) splenocytes, decreased CXCR4 expression on CD8(+) T cells, significantly reduced BM infiltration of T cells, and strongly attenuated disease symptoms. Remarkably, therapeutic administration of Bay11 significantly extended survival of AA mice. Overall, we demonstrate that CXCR4 mediates migration of pathogenic T cells to the BM in AA mice, and inhibiting NF-κB signaling may represent a novel therapeutic approach to treating AA.

The N-terminal CEBPA mutant in acute myeloid leukemia impairs CXCR4 expression

CXC chemokine receptor 4 (CXCR4) is an essential regulator for homing and maintenance of hematopoietic stem cells within the bone marrow niches. Analysis of clinical implications of bone marrow CXCR4 expression in patients with acute myeloid leukemia showed not only higher CXCR4 expression was an independent poor prognostic factor, irrespective of age, white blood cell counts, cytogenetics, and mutation status of NPM1/FLT3-ITD and CEBPA, but also showed CXCR4 expression was inversely associated with mutations of CEBPA, a gene encoding transcription factor C/EBPα. Patients with wild-type CEBPA had significantly higher CXCR4 expression than those with mutated CEBPA. We hypothesized that CEBPA might influence the expression of CXCR4. To test this hypothesis, we first examined endogenous CXCR4 expression in 293T and K562 cells over-expressing wild-type C/EBPα p42 and demonstrated that CXCR4 levels were increased in these cells, whilst the expression of the N-terminal mutant, C/EBPα p30, diminished CXCR4 transcription. We further showed p42 was bound to the CXCR4 promoter by the chromatin immunoprecipitation assays. Induction of p42 in the inducible K562-C/EBPα cell lines increased the chemotactic migration. Moreover, decreased expression of C/EBPα by RNA interference decreased levels of CXCR4 protein expression in U937 cells, thereby abrogating CXCR4-mediated chemotaxis. Our results provide, for the first time, evidence that C/EBPα indeed regulates the activation of CXCR4, which is critical for the homing and engraftment of acute myeloid leukemia cells, while p30 mutant impairs CXCR4 expression.

Combination of imatinib with CXCR4 antagonist BKT140 overcomes the protective effect of stroma and targets CML in vitro and in vivo

Functional role of CXCR4 in chronic myelogenous leukemia (CML) progression was evaluated. Elevated CXCR4 significantly increased the in vitro survival and proliferation in response to CXCL12. CXCR4 stimulation resulted in activation of extracellular signal-regulated kinase (Erk)-1/2, Akt, S6K, STAT3, and STAT5 prosurvival signaling pathways. In accordance, we found that in vitro treatment with CXCR4 antagonist BKT140 directly inhibited the cell growth and induced cell death of CML cells. Combination of BKT140 with suboptimal concentrations of imatinib significantly increased the anti-CML effect. BKT140 induced apoptotic cell death, decreasing the levels of HSP70 and HSP90 chaperones and antiapoptotic proteins BCL-2 and BCL-XL, subsequently promoting the release of mitochondrial factors cytochrome c and SMAC/Diablo. Bone marrow (BM) stromal cells (BMSC) markedly increased the proliferation of CML cells and protected them from imatinib-induced apoptosis. Furthermore, BMSCs elevated proto-oncogene BCL6 expression in the CML cells in response to imatinib treatment, suggesting the possible role of BCL6 in stroma-mediated TKI resistance. BKT140 reversed the protective effect of the stroma, effectively promoted apoptosis, and decreased BCL6 levels in CML cells cocultured with BMSCs. BKT140 administration in vivo effectively reduced the growth of subcutaneous K562-produced xenografts. Moreover, the combination of BKT140 with low-dose imatinib markedly inhibited tumor growth, achieving 95% suppression. Taken together, our data indicate the importance of CXCR4/CXCL12 axis in CML growth and CML-BM stroma interaction. CXCR4 inhibition with BKT140 antagonist efficiently cooperated with imatinib in vitro and in vivo. These results provide the rational basis for CXCR4-targeted therapy in combination with TKI to override drug resistance and suppress residual disease.

High CXCR4 and low VLA-4 expression predicts poor survival in adults with acute lymphoblastic leukemia

Data regarding the prognostic significance of CXCR4 and VLA-4 in ALL are limited. Especially, VLA-4 has not been evaluated at the time of diagnosis in both adult and childhood ALL patients. We prospectively analyzed the expression of VLA-4 and CXCR4 in 54 patients (VLA-4 in 29 adults and 25 children and CXCR4 in 22 adults and 24 children) newly diagnosed with ALL by flow cytometry. Expression levels of VLA-4 and CXCR4 were not different between adults and children with ALL. High CXCR4 and low VLA-4 expression each correlated with worse prognosis in adults; patients with high CXCR4 expression had shorter disease-free survival (p=0.01) and overall survival (p=0.04) and patients with low VLA-4 expression had shorter disease-free survival (p=0.02). Expression levels of CXCR4 and VLA-4 did not predict patient prognosis in children. Analysis of CXCR4 and VLA-4 expression at diagnosis in adults with ALL can provide useful information on patient prognosis.

Proteomic profiling identifies distinct protein patterns in acute myelogenous leukemia CD34+CD38- stem-like cells

Acute myeloid leukemia (AML) is believed to arise from leukemic stem-like cells (LSC) making understanding the biological differences between LSC and normal stem cells (HSC) or common myeloid progenitors (CMP) crucial to understanding AML biology. To determine if protein expression patterns were different in LSC compared to other AML and CD34+ populations, we measured the expression of 121 proteins by Reverse Phase Protein Arrays (RPPA) in 5 purified fractions from AML marrow and blood samples: Bulk (CD3/CD19 depleted), CD34-, CD34+(CMP), CD34+CD38+ and CD34+CD38-(LSC). LSC protein expression differed markedly from Bulk (n=31 cases, 93/121 proteins) and CD34+ cells (n= 30 cases, 88/121 proteins) with 54 proteins being significantly different (31 higher, 23 lower) in LSC than in either Bulk or CD34+ cells. Sixty-seven proteins differed significantly between CD34+ and Bulk blasts (n=69 cases). Protein expression patterns in LSC and CD34+ differed markedly from normal CD34+ cells. LSC were distinct from CD34+ and Bulk cells by principal component and by protein signaling network analysis which confirmed individual protein analysis. Potential targetable submodules in LSC included the proteins PU.1(SP1), P27, Mcl1, HIF1α, cMET, P53, Yap, and phospho-Stats 1, 5 and 6. Protein expression and activation in LSC differs markedly from other blast populations suggesting that studies of AML biology should be performed in LSC.

Dynamic chemotherapy-induced upregulation of CXCR4 expression: a mechanism of therapeutic resistance in pediatric AML

Cure rates in pediatric acute leukemias remain suboptimal. Overexpression of the cell-surface chemokine receptor CXCR4 is associated with poor outcome in acute lymphoblastic leukemia (ALL) and acute myeloid leukemia (AML). Certain nonchemotherapeutic agents have been shown to modulate CXCR4 expression and alter leukemia interactions with stromal cells in the bone marrow microenvironment. Because chemotherapy is the mainstay of AML treatment, it was hypothesized that standard cytotoxic chemotherapeutic agents induce dynamic changes in leukemia surface CXCR4 expression, and that chemotherapy-induced upregulation of CXCR4 represents a mechanism of acquired therapeutic resistance. Here, it was shown that cell lines variably upregulate CXCR4 with chemotherapy treatment. Those that showed upregulation were differentially protected from chemotherapy-induced apoptosis when cocultured with stroma. The functional effects of chemotherapy-induced CXCR4 upregulation in an AML cell line (MOLM-14, which harbors consistent upregulated CXCR4) and clinical specimens were explored. Importantly, enhanced stromal-cell derived factor-1α (SDF1A/CXCL12)-mediated chemotaxis and stromal protection from additional chemotherapy-induced apoptosis was found. Furthermore, treatment with plerixafor, a CXCR4 inhibitor, preferentially decreased stromal protection with higher chemotherapy-induced upregulation of surface CXCR4. Thus, increased chemokine receptor CXCR4 expression after treatment with conventional chemotherapy may represent a mechanism of therapeutic resistance in pediatric AML.

IMPLICATIONS

CXCR4 may be a biomarker for the stratification and optimal treatment of patients using CXCR4 inhibitors.

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.

Dependence of acute myeloid leukemia on adhesion within the bone marrow microenvironment

Acute myeloid leukemia (AML) cells home to the endosteal region of the bone marrow. They interact with bone marrow stromal components including extracellular matrix proteins, glycosaminoglycans, and stromal cells, by which they derive proliferative and growth inhibitory signals. Furthermore, adhesion to marrow stroma confers chemotherapy drug resistance and thereby promotes leukemia survival. A subpopulation of the leukemic blasts, known as leukemia stem cells, that are capable of propagating the leukemia, remain sheltered in the bone marrow microenvironment, exhibit resistance to chemotherapy, and serve as the origin of relapse after a variable period of remission. Detachment of these cells from the bone marrow in combination with chemotherapy may improve the outcome of therapy for AML.

Oxygen tension plays a critical role in the hematopoietic microenvironment in vitro

BACKGROUND

In the bone marrow mesenchymal stromal cells and osteoblasts form functional niches for hematopoietic stem and progenitor cells. This microenvironment can be partially mimicked using in vitro co-culture systems. In this study, we examined the oxygen tension in three distinct compartments in a co-culture system of purified CD34(+) cells and mesenchymal stromal cells with regard to different spatial localizations.

DESIGN AND METHODS

Hypoxic cells in the co-culture were visualized by pimonidazole staining. Hematopoietic cell distribution, and functional and phenotypic characteristics were analyzed by flow cytometry. The secretion of vascular endothelial growth factor and stromal-derived factor-1 by mesenchymal stromal cells in low oxygen co-cultures was determined by an enzyme-linked immunosorbent assay. The effect of co-culture medium on the hematopoietic cell migration potential was tested in a transwell assay.

RESULTS

In co-cultures under atmospheric oxygen tension, regions of low oxygen tension could be detected beneath the feeder layer in which a reservoir of phenotypically more primitive hematopoietic cells is located in vitro. In low oxygen co-culture, the adhesion of hematopoietic cells to the feeder layer was decreased, whereas hematopoietic cell transmigration beneath mesenchymal stromal cells was favored. Increased vascular endothelial growth factor-A secretion by mesenchymal stromal cells under low oxygen conditions, which increased the permeability of the monolayer, was responsible for this effect. Furthermore, vascular endothelial growth factor-A expression in low oxygen mesenchymal stromal cells was induced via hypoxia-inducible factor signaling. However, stromal cell-derived factor-1 secretion by mesenchymal stromal cells was down-regulated under low oxygen conditions in a hypoxia-inducible factor-independent manner.

CONCLUSIONS

We demonstrate for the first time that differences in oxygen tension cause selective modification of hematopoietic cell and mesenchymal stromal cell interactions in a co-culture system, thus confirming that oxygen tension plays a critical role in the interaction between hematopoietic cells and the niche environment.

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.

High VLA-4 expression is associated with adverse outcome and distinct gene expression changes in childhood B-cell precursor acute lymphoblastic leukemia at first relapse

BACKGROUND

Resistance to therapy and subsequent relapse remain major challenges in the clinical management of relapsed childhood acute lymphoblastic leukemia. As the bone marrow environment plays an important role in survival and chemotherapy resistance of leukemia cells by activating different signaling pathways, such as the VLA-4 and PI3K/Akt pathways, we studied the prognostic and biological impact of VLA-4 expression in leukemia cells from children with relapsed B-cell precursor acute lymphoblastic leukemia and its influence on the sensitivity of the leukemia cells to drugs.

DESIGN AND METHODS

VLA-4 expression was quantified by real-time polymerase chain reaction in leukemia cells from 56 patients with relapsed acute lymphoblastic leukemia enrolled in the ALL-REZ BFM 2002 trial of the Berlin-Frankfurt-Münster study group. Gene expression changes related to VLA-4 expression were investigated by microarray-based mRNA profiling. The effect of VLA-4 signaling on proliferation and drug resistance was studied in co-cultures of leukemia and stromal cells.

RESULTS

High expression of VLA-4 at first relapse was associated with adverse prognostic factors, poor molecular response to therapy and significantly worse probabilities of event-free and overall survival. VLA-4 expression was an independent prognostic parameter. Comparing gene expression profiles of leukemia cells with high versus low VLA-4 expression, we identified 27 differentially expressed genes primarily involved in the PI3K/Akt, ephrin and Rho GTPase pathways. Blocking of VLA-4 signaling in combination with cytarabine treatment abolished the growth supportive effect of stromal cells.

CONCLUSIONS

Our results show that high VLA-4 expression is a marker of poor prognosis and a potential therapeutic target in children with relapsed acute lymphoblastic leukemia and confirm that cellular interactions and biological effects related to VLA-4 play a decisive role in the survival of leukemia cells and response to therapy. (ClinicalTrials.gov identifier: NCT00114348).

Homing of cancer cells to the bone

A variety of tumor cells preferentially home to the bone. The homing of cancer cells to the bone represents a multi-step process that involves malignant progression of the tumor, invasion of the tumor through the extracellular matrix and the blood vessels and settling of the tumor cells in the bone. Gaining a greater understanding as to the mechanisms used by cancer cells in these processes will facilitate the design of drugs which could specifically target the homing process. In this review we will discuss the properties of tumor cells and the bone microenvironment which promote homing of a cancer cell to the bone. We will highlight the different steps and the molecular pathways involved when a cancer cell metastasize to the bone. Since bone is the major home for hematopoietic stem cells (HSCs), we will also highlight the similarities between the homing of cancer and HSC to the bone. Finally we will conclude with therapeutic and early detection strategies which can prevent homing of a cancer cell to the bone.

Pan-histone deacetylase inhibitor panobinostat depletes CXCR4 levels and signaling and exerts synergistic antimyeloid activity in combination with CXCR4 antagonists

Stromal cell derived factor-1 (SDF-1 or CXCL12) and its receptor CXCR4 are involved in the directional homing to the bone marrow niches and in peripheral mobilization of normal and transformed hematopoietic stem and myeloid progenitor cells. Elevated CXCR4 expression confers poor prognosis, whereas inhibition of CXCR4 signaling overcomes stroma-mediated chemoresistance in acute myeloid leukemia (AML). Here, we demonstrate that treatment with the pan-histone deacetylase inhibitor panobinostat (PS) depleted the mRNA and protein levels of CXCR4 in the cultured and primary AML cells. PS-induced acetylation of the heat shock protein (hsp) 90 reduced the chaperone association between CXCR4 and hsp90, directing CXCR4 to degradation by the 20S proteasome. PS treatment also depleted G protein–coupled receptor kinase 3, as well as attenuated the phosphorylation of AKT and ERK1/2 in AML cells, which was not affected by cotreatment with CXCL12. Compared with each agent alone, cotreatment with PS and CXCR4 antagonist AMD3100 or FC-131 synergistically induced apoptosis of cultured and primary AML cells. PS and FC-131 exerted more lethal effects on primary AML versus normal CD34+ bone marrow progenitor cells. These findings support the rationale to test the in vivo efficacy of PS in enhancing the lethal effects of CXCR4 antagonists against AML cells.

Communication of substance P, calcitonin-gene-related neuropeptides and chemokine receptor 4 (CXCR4) in cord blood hematopoietic stem cells

BACKGROUND

Modulation of the expression of CXCR4 as a critical adhesion molecule on cord blood (CB) CD34+ cells could overcome delay following cord blood transplantation. Identification of beneficial effects of growth factors including cytokines and neuropeptides on CXCR4 expression would enable our understanding of this complicated network. Therefore, we aimed to assess the role of substance P (SP) and Calcitonin gene related peptide (CGRP) on CXCR4 levels.

MATERIAL AND METHODS

CD34+cells purified from CB were cultured in a serum-free liquid culture system. Different concentrations of SP and CGRP were used in combination with cytokine cocktail. Expression of CXCR4 at protein and genomic levels was assessed by flow cytometry and real time RT-PCR.

RESULTS

Our results indicate increased CXCR4+ CD34+ cells after 7 days cultivation with SP and/or CGRP. Increased gene expression of the CXCR4 molecule was observed at 10(-9) M either SP or CGRP individually, by day 11 as compared to control group.

CONCLUSIONS

Our study indicates that SP and CGRP induce CXCR4 protein expression in short term culture, and stimulate its expression. Consequently, the increased expression of CXCR4 could improve engraftment of CB CD34+ cells.

The role of molecular tests in acute myelogenous leukemia treatment decisions

The prognosis for patients with acute myelogenous leukemia (AML) is dependent on age, karyotype, and the genetics of the neoplastic cell. The molecular markers with prognostic impact include mutations in FLT3, NPM1, MLL, WT1, c-KIT, and expression levels of BAALC, NM1, ERG, and CXCR4. Gene expression profiles and microRNA expression patterns in AML may prove highly useful in defining the prognosis of AML. Cytogenetic and, increasingly, molecular findings are used in determining the best therapy for AML patients, especially the choice of whether to perform allogeneic stem cell transplantation.

High expression of the very late antigen-4 integrin independently predicts reduced risk of relapse and improved outcome in pediatric acute myeloid leukemia: a report from the children's oncology group

PURPOSE

To evaluate the prognostic significance of the integrin cell adhesion molecule very late antigen-4 (VLA-4) in acute myeloid leukemia (AML).

PATIENTS AND METHODS

We prospectively quantified VLA-4 expression in 216 patients enrolled onto COG-AAML03P1 by flow cytometry and correlated expression levels with disease characteristics and clinical outcome.

RESULTS

VLA-4 mean fluorescence intensity (MFI) varied 35-fold (range, 30 to 1,110; median, 219.5). High VLA-4 expression (> median MFI), compared with low expression, was associated with younger age (7.1 v 12.1 years, respectively; P < .001), lower FLT3 internal tandem duplication prevalence (4% v 21%, respectively; P < .001), and higher likelihood of extramedullary disease (16% v 5%, respectively; P = .013). In low- and high-expression groups, rates of remission (89% v 80%, respectively; P = .137) and minimal residual disease (29% v 25%, respectively; P = .700) were similar. Patients with low VLA-4 expression, compared with high expression, had a higher relapse rate (RR; 44% +/- 10% v 24% +/- 9%, respectively; P = .011) and lower disease-free survival (DFS; 48% +/- 11% v 67% +/- 10%, respectively; P = .023) after 3 years. Multivariate analyses showed that low VLA-4 expression was an independent adverse prognostic factor for DFS (hazard ratio [HR] = 1.98; P = .038) and RR (HR = 2.77; P = .009). Subgroup analyses indicated that the prognostic role of VLA-4 expression was most prominent in patients with standard-risk AML, in whom low VLA-4 expression was associated with inferior DFS (34% +/- 16% v 69% +/- 14% for high expression; P = .011) and higher RR (61% +/- 16% v 26% +/- 14% for high expression; P = .009). A similar trend was seen in low-risk but not high-risk patients.

CONCLUSION

High VLA-4 expression is associated with better clinical outcome in pediatric AML and is an independent predictor of relapse that may refine our abilities to stratify patients without identifiable cytogenetic or molecular risk factors.

Stromal cell-derived factor-1alpha-directed chemoattraction of transiently CXCR4-overexpressing bone marrow stromal cells into functionalized three-dimensional biomimetic scaffolds

Three-dimensional (3D) bone substitute material should not only serve as scaffold in large bone defects but also attract mesenchymal stem cells, a subset of bone marrow stromal cells (BMSCs) that are able to form new bone tissue. An additional crucial step is to attract BMSCs from the surface into deeper structures of 3D porous bone substitute scaffolds. Here we show that transient overexpression of CXCR4 in human BMSCs induced by mRNA transfection enhances stromal cell-derived factor-1alpha (SDF-1alpha)-directed chemotactic capacity to invade internal compartments of porous 3D bone substitute scaffolds in vitro and in vivo. In vitro native BMCSs invaded up to 500 mum into SDF-1alpha-releasing 3D scaffolds, whereas CXCR4-overexpressing BMSCs invaded up to 800 mum within 5 days. In addition, 60% downregulation of endogenous SDF-1 transcription in BMSCs by endoribonuclease-prepared siRNA before CXCR4 mRNA transfection enhanced SDF-1alpha-directed migration of human BMSCs by 50%. Implantation of SDF-1alpha-releasing scaffolds seeded with transiently CXCR4-overexpressing BMSCs resulted in an increase of invasion into internal compartments of the scaffolds in a mouse model. In vivo native BMCS invaded up to 250 mum into SDF-1alpha-releasing 3D scaffolds, whereas CXCR4-overexpressing BMSC invaded up to 500 mum within 5 days. Thus, the SDF-1alpha/CXCR4 chemoattraction system can be used to efficiently recruit BMSCs into SDF-1alpha-releasing 3D scaffolds in vitro and in vivo.

The chemokine network in acute myelogenous leukemia: molecular mechanisms involved in leukemogenesis and therapeutic implications

Acute myelogenous leukemia (AML) is a bone marrow disease in which the leukemic cells show constitutive release of a wide range of CCL and CXCL chemokines and express several chemokine receptors. The AML cell release of various chemokines is often correlated and three release clusters have been identified: CCL2-4/CXCL1/8, CCL5/CXCL9-11, and CCL13/17/22/24/CXCL5. CXCL8 is the chemokine usually released at highest levels. Based on their overall constitutive release profile, patients can be classified into distinct subsets that differ in their T cell chemotaxis towards the leukemic cells. The release profile is modified by hypoxia, differentiation status, pharmacological interventions, and T cell cytokine responses. The best investigated single chemokine in AML is CXCL12 that binds to CXCR4. CXCL12/CXCR4 is important in leukemogenesis through regulation of AML cell migration, and CXCR4 expression is an adverse prognostic factor for patient survival after chemotherapy. Even though AML cells usually release high levels of several chemokines, there is no general increase of serum chemokine levels in these patients and the levels are also influenced by patient age, disease status, chemotherapy regimen, and complicating infections. However, serum CXCL8 levels seem to partly reflect the leukemic cell burden in AML. Specific chemokine inhibitors are currently being developed, although redundancy and pleiotropy of the chemokine system are obstacles in drug development.

CXCR4 antagonists: targeting the microenvironment in leukemia and other cancers

Hematopoietic and epithelial cancer cells express CXCR4, a seven-transmembrane G-protein-coupled chemokine receptor. Stromal cells within the bone marrow microenvironment constitutively secrete stromal cell-derived factor-1 (SDF-1/CXCL12), the ligand for CXCR4. Activation of CXCR4 induces leukemia cell trafficking and homing to the marrow microenvironment, where CXCL12 retains leukemia cells in close contact with marrow stromal cells that provide growth and drug resistance signals. CXCR4 antagonists, such as Plerixafor (AMD3100) and T140 analogs, can disrupt adhesive tumor-stroma interactions and mobilize leukemia cells from their protective stromal microenvironment, making them more accessible to conventional drugs. Therefore, targeting the CXCR4-CXCL12 axis is a novel, attractive therapeutic approach that is explored in ongoing clinical trials in leukemia patients. Initially, CXCR4 antagonists were developed for the treatment of HIV, where CXCR4 functions as a co-receptor for virus entry into T cells. Subsequently, CXCR4 antagonists were noticed to induce leukocytosis, and are currently used clinically for mobilization of hematopoietic stem cells. However, because CXCR4 plays a key role in cross-talk between leukemia cells (and a variety of other tumor cells) and their microenvironment, cancer treatment may become the ultimate application of CXCR4 antagonists. Here, we summarize the development of CXCR4 antagonists and their preclinical and clinical activities, focusing on leukemia and other cancers.

Prognostic value of CXCR4 and FAK expression in acute myelogenous leukemia

We analysed, by flow cytometry, the "adhesive" phenotype of acute myelogenous leukemia (AML) cells from 36 patients treated in our institution. In univariate analysis, the main prognostic factor for CR achievement was lower CXCR4 expression (p=0.03). Overall survival (OS) was negatively influenced by higher CXC chemokine receptor 4 (CXCR4) (p=0.01), very late antigen-4 (VLA-4) (p=0.01), and focal adhesion kinase (FAK) expression (p=0.04). Combination of these markers allowed to distinguish two prognostic groups: patients overexpressing 2 or 3 factors had a significantly shorter OS (p=0.015). CXCR4, VLA-4 and FAK are new phenotypic markers which could be helpful to establish risk-stratified therapeutic strategies.

Very late antigen-4 function of myeloblasts correlates with improved overall survival for patients with acute myeloid leukemia

Adhesion of acute myeloid leukemia (AML) blasts in the bone marrow microenvironment confers protection from chemotherapy-induced apoptosis. One mechanism for retention of blasts within the bone marrow is adhesion via very late antigen-4 (VLA-4), the alpha(4)beta(1) integrin heterodimer that binds to its main ligands, fibronectin, and vascular cell adhesion molecule-1 (VCAM-1). To examine the relationship of functional expression of VLA-4 to prognosis in AML, we studied marrow samples from 175 adult AML patients who underwent induction chemotherapy with anthracycline and cytarabine on Southwest Oncology Group trials. The studies included flow cytometry and functional in vitro assays for ligand binding and maximal beta(1) activation. VLA-4 expression varied widely, with mean expression 60.6% for alpha(4), and was not significantly associated with response to chemotherapy, relapse-free, or overall survival (OS). However, increased binding of soluble VCAM-1 via VLA-4 was significantly associated with longer OS, corrected for age (P = .033). Estimated 5-year OS was 31% (95% confidence interval, 14%-48%) in 30 patients with soluble VCAM-1 binding greater than or equal to 40%, compared with 10% (confidence interval, 3%-17%) in 72 patients with lower binding. Adhesion and migratory properties of AML blasts thus appear to influence chemosensitivity and therefore may be therapeutic targets.

The critical role of SDF-1/CXCR4 axis in cancer and cancer stem cells metastasis

Chemokines exert their multifunctional role in several physiologic and pathologic processes through interaction with their specific receptors. Much evidence have revealed that metastatic spread tumor cells may use chemokine-mediated mechanisms. In particular, an involvement of stromal cell-derived factor-1 (SDF-1) in growth of primary tumors and in metastatic process has been demonstrated. Indeed, it has been suggested that CXCR4 expression by tumor cells, plays a critical role in cell metastasis by a chemotactic gradient to organs expressing the ligand SDF-1. Moreover, CXCR4 overexpression correlated with poor prognosis in many types of cancer. In physiologic condition, SDF-1 also plays an essential role modulating stem cell proliferation, survival, and homing through its canonical receptor CXCR4. Recently, several studies have demonstrated the existence of a small subset of cancer cells which share many characteristics with stem cells and named cancer stem cells (CSC). They constitute a reservoir of self-sustaining cells with the ability to maintain the tumor growth. In particular, most of them express CXCR4 receptor and respond to a chemotactic gradient of its specific ligand SDF-1, suggesting that CSC probably represent a subpopulation capable of initiating metastasis. This review focuses on the role of SDF-1/CXCR4 axis in cancer and in the metastatic progression by tumoral cells, as well as the role of CSC in tumor pathogenesis and in metastatic process. A better understanding of migratory mechanism involving cancer cells and CSC provides a powerful tool for developing novel therapies reducing both local and distant recurrences.

Stem cells: a revolution in therapeutics-recent advances in stem cell biology and their therapeutic applications in regenerative medicine and cancer therapies

Basic and clinical research accomplished during the last few years on embryonic, fetal, amniotic, umbilical cord blood, and adult stem cells has constituted a revolution in regenerative medicine and cancer therapies by providing the possibility of generating multiple therapeutically useful cell types. These new cells could be used for treating numerous genetic and degenerative disorders. Among them, age-related functional defects, hematopoietic and immune system disorders, heart failures, chronic liver injuries, diabetes, Parkinson's and Alzheimer's diseases, arthritis, and muscular, skin, lung, eye, and digestive disorders as well as aggressive and recurrent cancers could be successfully treated by stem cell-based therapies. This review focuses on the recent advancements in adult stem cell biology in normal and pathological conditions. We describe how these results have improved our understanding on critical and unique functions of these rare sub-populations of multipotent and undifferentiated cells with an unlimited self-renewal capacity and high plasticity. Finally, we discuss some major advances to translate the experimental models on ex vivo and in vivo expanded and/or differentiated stem cells into clinical applications for the development of novel cellular therapies aimed at repairing genetically altered or damaged tissues/organs in humans. A particular emphasis is made on the therapeutic potential of different tissue-resident adult stem cell types and their in vivo modulation for treating and curing specific pathological disorders.

The CXCR4 chemokine receptor in acute and chronic leukaemia: a marrow homing receptor and potential therapeutic target

Chemokine (C-X-C motif) receptor 4 (CXCR4) is essential for homing and maintenance of haematopoietic stem cells in distinct stromal cell niches within the marrow. Chemotactic responsiveness of haematopoietic stem cells is restricted to the ligand for CXCR4, stromal cell-derived factor-1 (SDF-1/CXCL12), which is constitutively secreted by marrow stromal cells. Myeloid and lymphoid leukaemia cells also express CXCR4 that induces leukaemia cell chemotaxis and migration beneath marrow stromal cells. CXCR4 expression levels have a major prognostic impact in acute myeloid leukaemia. There is growing in vitro and in vivo evidence that CXCR4 expression by leukaemia cells allows for homing and their retention within the marrow. As such, leukaemia cells appear to utilise CXCR4 to access niches that are normally restricted to progenitor cells, and thereby reside in a microenvironment that favours their growth and survival. CXCR4- and integrin-mediated contact between leukaemia cells and stromal cells protects leukaemia cells from spontaneous and chemotherapy-induced cell death and therefore may represent a mechanism to explain minimal residual disease and subsequent relapses commonly seen in the treatment of these diseases. This review summarises our current knowledge regarding the importance of CXCR4 in acute and chronic leukaemia, discusses the importance of CXCR4 detection by flow cytometry in the diagnostic workup of leukaemia patients, and introduces the potential role of CXCR4-targeting compounds for the treatment of leukaemia patients.