Opposing effects of engagement of integrins and stimulation of cytokine receptors on cell cycle progression of normal human hematopoietic progenitors

Complexities of modeling the bone marrow microenvironment to facilitate hematopoietic research

Hematopoiesis occurs in the bone marrow (BM), within a specialized microenvironment referred to as the stem cell niche, where the hematopoietic stem cells (HSCs) reside and are regulated for quiescence, self-renewal and differentiation through intrinsic and extrinsic mechanisms. The BM contains at least two distinctive HSC-supportive niches: an endosteal osteoblastic niche that supports quiescence and self-renewal and a more vascular/perisinusoidal niche that promotes proliferation and differentiation. Both associate with supporting mesenchymal stromal cells. Within the more hypoxic osteoblastic niche, HSCs specifically interact with the osteoblasts that line the endosteal surface, which secrete several important HSC quiescence and maintenance regulatory factors. In vivo imaging indicates that the HSCs and progenitors located further away, in the vicinity of sinusoidal endothelial cells, are more proliferative. Here, HSCs interact with endothelial cells via specific cell adhesion molecules. Endothelial cells also secrete several factors important for HSC homeostasis and proliferation. In addition, HSCs and mesenchymal stromal cells are embedded within the extracellular matrix (ECM), an important network of proteins such as collagen, elastin, laminin, proteoglycans, vitronectin, and fibronectin. The ECM provides mechanical characteristics such as stiffness and elasticity important for cell behavior regulation. ECM proteins are also able to bind, sequester, display, and distribute growth factors across the BM, thus directly affecting stem cell fate and regulation of hematopoiesis. These important physical and chemical features of the BM require careful consideration when creating three-dimensional models of the BM.

Expansion of cord blood stem cells in fibronectin-coated microfluidic bioreactor

Background

Hematopoietic stem/progenitor cell transplantation is the main treatment option for hematological malignancies and disorders. One strategy to solve the problem of low stem cell doses used in transplantation is pre-transplant expansion. We hypothesized that using fibronectin-coated microfluidic channels would expand HSPCs and keep self-renewal potential in a three-dimensional environment, compared to the conventional method. We also compared stem cell homing factors expression in microfluidic to conventional cultures.

Materials and methods

A microfluidic device was created and characterized by scanning electron microscopy. The CD133+ cells were collected from cord blood and purified. They were subsequently cultured in 24-well plates and microfluidic bioreactor systems using the StemSpan serum-free medium. Eventually, we analyzed cell surface expression levels of the CXCR4 molecule and CXCR4 mRNA expression in CD133+ cells cultured in different systems.

Results

The expansion results showed significant improvement in CD133+ cell expansion in the microfluidic system than the conventional method. The median expression of the CXCR4 in the expanded cell was lower in the conventional system than in the microfluidic system. The CXCR4 gene expression up-regulated in the microfluidic system.

Conclusion

Utilizing microfluidic systems to expand desired cells effectively is the next step in cell culture. Comparative gene expression profiling provides a glimpse of the effects of culture microenvironments on the genetic program of HSCs grown in different systems.

Hematopoietic Niche - Exploring Biomimetic Cues to Improve the Functionality of Hematopoietic Stem/Progenitor Cells

The adult bone marrow (BM) niche is a complex entity where a homeostatic hematopoietic system is maintained through a dynamic crosstalk between different cellular and non-cellular players. Signaling mechanisms triggered by cell-cell, cell-extracellular matrix (ECM), cell-cytokine interactions, and local microenvironment parameters are involved in controlling quiescence, self-renewal, differentiation, and migration of hematopoietic stem/progenitor cells (HSPC). A promising strategy to more efficiently expand HSPC numbers and tune their properties ex vivo is to mimic the hematopoietic niche through integration of adjuvant stromal cells, soluble cues, and/or biomaterial-based approaches in HSPC culture systems. Particularly, mesenchymal stem/stromal cells (MSC), through their paracrine activity or direct contact with HSPC, are thought to be a relevant niche player, positioning HSPC-MSC co-culture as a valuable platform to support the ex vivo expansion of hematopoietic progenitors. To improve the clinical outcome of hematopoietic cell transplantation (HCT), namely when the available HSPC are present in a limited number such is the case of HSPC collected from umbilical cord blood (UCB), ex vivo expansion of HSPC is required without eliminating the long-term repopulating capacity of more primitive HSC. Here, we will focus on depicting the characteristics of co-culture systems, as well as other bioengineering approaches to improve the functionality of HSPC ex vivo.

Space constraints govern fate of hematopoietic stem and progenitor cells in vitro

Deciphering exogenous cues that determine stem cell fate decisions is a persisting challenge of cell biology and bioengineering. In an effort to unravel the role of spatial constraints in the cell-instructive characteristics of bone marrow microenvironments, murine hematopoietic stem and progenitor cells (HSPC) were exposed to fibronectin-coated microcavities in vitro. Microcavity sizes were chosen to allow for the inclusion of either individual or multiple cells. Repopulation experiments using lethally irradiated mice showed that the maintenance of functional HSPC in culture critically depends on cavity dimensions. Short-term repopulating hematopoietic stem cells (ST-HSC) were found to be best supported within single-cell sized compartments while long-term repopulating HSC (LT-HSC) were maintained within both cavity sizes. In sum, the reported data reveal spatial restriction to be a simple but powerful means for directing HSPC fate ex vivo.

Poly-L-lysine increases the ex vivo expansion and erythroid differentiation of human hematopoietic stem cells, as well as erythroid enucleation efficacy

Hematopoietic stem cells (HSCs) are continuously stimulated by physical interactions with bone marrow or umbilical cord niches as well as by chemical factors found within these niches. The niche can be mimicked by modification of the cytokine composition, elasticity, topography, and/or charge. This work employed cell culture plates coated with several concentrations of poly-L-lysine (PLL), a positively charged synthetic amino-acid chain. Culture substrates that employed relatively high initial coating concentrations of PLL significantly increased the total number of HSCs during ex vivo expansion of CD34(+) cells, as well as erythroid differentiation. Furthermore, the 0.01% PLL substrate stimulated enucleation of erythroid cells, leaving behind a number of extruded nuclei at the bottom of the culture plate, followed by an increase in the number of erythrocytes. Thus, PLL will likely prove useful to enhance the expansion of HSCs and erythroid cells, in addition to the generation of red blood cells.

Effects of extracellular matrix proteins on the growth of haematopoietic progenitor cells

Umbilical cord blood (UCB) transplantation and haematological recovery are currently limited by the amount of haematopoietic progenitor cells (HPCs) present in each unit. HPCs and haematopoietic stem cells (HSCs) normally interact with cells and extracellular matrix (ECM) proteins present within the endosteal and vascular niches. Hence, we investigated whether coating of culture surfaces with ECM proteins normally present in the marrow microenvironment could benefit the ex vivo expansion of HPCs. Towards this, collagen types I and IV (COL I and IV), laminin (LN) and fibronectin (FN) were tested individually or as component of two ECM-mix complexes. Individually, ECM proteins had both common and unique properties on the growth and differentiation of UCB CD34+ cells; some ECM proteins favoured the differentiation of some lineages over that of others (e.g. FN for erythroids), some the expansion of HPCs (e.g. LN and megakaryocyte (MK) progenitor) while others had less effects. Next, two ECM-mix complexes were tested; the first one contained all four ECM proteins (4ECMp), while the second 'basement membrane-like structure' was without COL I (3ECMp). Removal of COL I led to strong reductions in cell growth and HPCs expansion. Interestingly, the 4ECMp-mix complex reproducibly increased CD34+ (1.3-fold) and CD41+ (1.2-fold) cell expansions at day 6 (P < 0.05) versus control, and induced greater myeloid progenitor expansion (P < 0.05) than 3ECMp. In conclusion, these results suggest that optimization of BM ECM protein complexes could provide a better environment for the ex vivo expansion of haematopoietic progenitors than individual ECM protein.

Extracellular matrix functionalized microcavities to control hematopoietic stem and progenitor cell fate

Polymeric microcavities functionalized with extracellular matrix components were used as an experimental in vitro model to investigate principles of hematopoietic stem and progenitor cell (HSPC) fate control. Using human CD133+ HSPC we could demonstrate distinct differences in HSPC cycling and differentiation dependence on the adhesion ligand specificity (i.e., heparin, collagen I) and cytokine levels. The presented microcavity platform provides a powerful in vitro approach to explore the role of exogenous cues in HSPC fate decisions and can therefore be instrumental to progress in stem cell biology and translational research toward new therapies.

Formation of an adherent hematopoietic expansion culture using fucoidan

Expansion of transplantable cord blood (CB) progenitors using a stroma requires provision of an exogenous cell source because of the low frequency of stromal precursor cells in CB. A simpler approach from a clinical regulatory perspective would be to provide synthetic extracellular matrix. The aim of this study was to characterize the effect on hematopoietic cell culture of fucoidan. The modulation of cytokine-driven hematopoietic cell expansion by fucoidan was investigated using two-level fractional and full factorial experimental designs. Mobilized peripheral blood (PB) CD34(+) cells were grown over 10 days in various combinations of FL, SCF, TPO, G-CSF, and SDF-1. Cultures were analyzed by immunophenotype. The effect of fucoidan on the divisional recruitment of CD34(+) cells was studied by CFDA-SE division tracking. Fucoidan was adsorbed by polystyrene to tissue culture plates and promoted formation of an adherent hematopoietic culture. Factorial design experiments with mobilized PB-CD34(+) cells showed that fucoidan reduced the production of CD34(+) cells and CD34(+)CXCR4(+) ratio but did not affect the production of monocytic, granulocytic, or megakaryocytic cells. The inhibitory effect of fucoidan on expansion of CB-CD34(+) cells was greater than mobilized PB. Division tracking analysis showed that CD34(+) cell generation times were lengthened by fucoidan. Fucoidan binds growth factors via their heparin-binding domain. The formation of an adherent hematopoietic culture system by fucoidan is most likely mediated by the binding of L-selectin and integrin-αMβ2 on myeloids. Fucoidan deserves further investigation as glycan scaffold that is suitable for immobilization of other matrix molecules thought to comprise blood stem cell niche.

Integrins and bone metastasis: integrating tumor cell and stromal cell interactions

Integrins on both tumor cells and the supporting host stromal cells in bone (osteoclasts, new blood vessels, inflammatory cells, platelets and bone marrow stromal cells) play key roles in enhancing bone metastasis. Tumor cells localize to specific tissues through integrin-mediated contacts with extracellular matrix and stromal cells. Integrin expression and signaling are perturbed in cancer cells, allowing them to "escape" from cell-cell and cell-matrix tethers, invade, migrate and colonize within new tissues and matrices. Integrin signaling through αvβ3 and VLA-4 on tumor cells can promote tumor metastasis to and proliferation in the bone microenvironment. Osteoclast (OC) mediated bone resorption is a critical component of bone metastasis and can promote tumor growth in bone and αvβ3 integrins are critical to OC function and development. Tumors in the bone microenvironment can recruit new blood vessel formation, platelets, pro-tumor immune cells and bone marrow stromal cells that promote tumor growth and invasion in bone. Integrins and their ligands play critical roles in platelet aggregation (αvβ3 and αIIbβ3), hematopoietic cell mobilization (VLA-4 and osteopontin), neoangiogenesis (αvβ3, αvβ5, α6β4, and β1 integrin) and stromal function (osteopontin and VLA-4). Integrins are involved in the pathogenesis of bone metastasis at many levels and further study to define integrin dysregulation by cancer will yield new therapeutic targets for the prevention and treatment of bone metastasis.

Hematopoietic stem and progenitor cells in adhesive microcavities

The homeostasis of hematopoietic stem and progenitor cells (HSC) in the bone marrow is regulated by a complex interplay of exogenous signals, including extracellular matrix (ECM) molecules, cell-cell contacts, and cytokines. To investigate the influence of spatial restriction and adhesive interactions on HSC fate decisions, we prepared a set of fibronectin-coated micrometer-sized cavities. Analysis of human CD133+ HSCs isolated after culture on these surfaces revealed that proliferation and differentiation is decreased when HSCs are supported by substrates with small microcavities. Single cell analysis of adherent cells also revealed decreased DNA synthesis and higher levels of HSC marker expression inside the smaller cavities. Increasing the cytokine concentration highlighted the tight balance of adhesion related signals and soluble cues acting on HSC fate decisions. Our results suggest that confining human HSCs in ECM-coated microcavities is a possible method to maintain these cells in a quiescent and immature state, an important advantage for several HSC applications.

Cell adhesion induces p27Kip1-associated cell-cycle arrest through down-regulation of the SCFSkp2 ubiquitin ligase pathway in mantle-cell and other non-Hodgkin B-cell lymphomas

Mounting evidence suggests that dynamic interactions between a tumor and its microenvironment play a critical role in tumor development, cell-cycle progression, and response to therapy. In this study, we used mantle cell lymphoma (MCL) as a model to characterize the mechanisms by which stroma regulate cell-cycle progression. We demonstrated that adhesion of MCL and other non-Hodgkin lymphoma (NHL) cells to bone marrow stromal cells resulted in a reversible G(1) arrest associated with elevated p27(Kip1) and p21 (WAF1) proteins. The adhesion-mediated p27(Kip1) and p21 increases were posttranslationally regulated via the down-regulation of Skp2, a subunit of SCF(Skp2) ubiquitin ligase. Overexpression of Skp2 in MCL decreased p27(Kip1), whereas inhibition of Skp2 by siRNA increased p27(Kip1) and p21 levels. Furthermore, we found cell adhesion up-regulated Cdh1 (an activating subunit of anaphase-promoting complex [APC] ubiquitin ligase), and reduction of Cdh1 by siRNA induced Skp2 accumulation and hence p27(Kip1) degradation, thus implicating Cdh1 as an upstream effector of the Skp2/p27(Kip1) signaling pathway. Overall, this report, for the first time, demonstrates that cell-cell contact controls the tumor cell cycle via ubiquitin-proteasome proteolytic pathways in MCL and other NHLs. The understanding of this novel molecular pathway may prove valuable in designing new therapeutic approaches for modifying tumor cell growth and response to therapy.

EWS/FLI-1 oncoprotein subtypes impose different requirements for transformation and metastatic activity in a murine model

Ewing sarcoma/primitive neuroectodermal tumors (EWS/PNET) are characterized by specific chromosomal translocations most often generating a chimeric EWS/FLI-1 gene. Depending on the number of juxtaposed exons assembled, several fusion types have been described with different incidences and prognoses. To assess the impact of each fusion type on the specific phenotypic, tumorigenic, and metastatic features of EWS/PNET, we developed an amenable system using a murine mesenchymal multipotent C3H10T1/2 cell line. Upon transduction of EWS/FLI-1, cells acquired dramatic morphological changes in vitro, including a smaller size and "neurite-like" membrane elongations. Chimeric fusion proteins conferred oncogenic properties in vitro, including anchorage-independent growth and an increased rate of proliferation. Furthermore, EWS/FLI-1 expression blocked mineralization, with concomitant repression of osteoblastic genes, and induced a dramatic repression of the adipocytic differentiation program. Moreover, EWS/FLI-1 promoted an aberrant neural phenotype by the de novo expression of specific neural genes. The intramuscular injection of transduced cells led to tumor development and the induction of overt osteolytic lesions. Analogously, to what was observed in human tumors, type 2 EWS/FLI-1 cells formed primary tumors in immunodeficient mice with a higher incidence and a lower latency than cells bearing types 1 and 3 fusions. By contrast, cells expressing types 2 and 3 fusions showed specific metastatic activity with a higher number of macroscopic metastases in soft tissues and osteolytic lesions in the limbs as compared to type-1-expressing cells. Therefore, the structure of each oncoprotein strongly influenced its tumorigenicity and metastagenicity. Thus, this model provides a basis for understanding the genetic determinants involved in Ewing tumor development and metastatic activity and represents a cellular system to analyze other oncoproteins involved in human sarcomagenesis.

Polarization of human hematopoietic progenitors during contact with multipotent mesenchymal stromal cells: effects on proliferation and clonogenicity

Establishment of a defined cell culture system that facilitates ex vivo expansion of isolated hematopoietic stem and progenitor cells (HSPCs) is a crucial issue in hematology and stem cell transplantation. Here we have evaluated the capacity of primary human multipotent mesenchymal stromal cells (MSCs) to support the ex vivo expansion of peripheral CD34(+)-enriched HSPCs. We observed that HSPCs co-cultured on MSCs showed a substantially higher total expansion rate compared to those growing without. Moreover, in addition to the expansion of CD34(+)CD133(+) and CD34(+)CD133(-) cells, a third population of CD133(+)CD34(-) stem cells became detectable after expansion. Direct contact between HSPCs and the feeder layer appears beneficial for the expansion of HSPCs harboring CD133(+) phenotype, i.e., CD34(+)CD133(+) and CD133(+)CD34(-), in contrast to CD34(+)CD133(-) cells. Interestingly, electron microscopy and immunofluorescence analyses revealed that adherent HSPCs display various morphologies; they are either round with, in some cases, the appearance of a microvillar pole or exhibit several distinct types of plasma membrane protrusions such as lamellipodium and magnupodium. CD133 is selectively concentrated therein, whereas CD34 is randomly distributed over the entire surface of HSPCs. Together, this co-culture offers a unique experimental system to further characterize the biology and role of markers of rare stem cell populations.

Ex vivo expansion of umbilical cord blood stem cells for transplantation: growing knowledge from the hematopoietic niche

Umbilical cord blood transplantation (UCBT) in adults is limited by the small number of primitive hematopoietic stem cells (HSC) in each graft, resulting in delayed engraftment post transplant, and both short- and long-term infectious complications. Initial efforts to expand UCB progenitors ex vivo have resulted in expansion of mature rather than immature HSC, confounded by the inability to accurately and reliably measure long-term reconstituting cells. Ex vivo expansion of UCB HSC has failed to improve engraftment because of resulting defects that promote apoptosis, disrupt marrow homing and initiate cell cycling. Here we discuss the future of ex vivo expansion, which we suggest will include the isolation of immature hematopoietic progenitors on the basis of function rather than surface phenotype and will employ both cytokines and stroma to maintain and expand the stem cell niche. We suggest that ex vivo expansion could be enhanced by manipulating newly discovered signaling pathways (Notch, Wnt, bone morphogenetic protein 4 and Tie2/angiopoietin-1) and intracellular mediators (phosphatase and tensin homolog and glycogen synthase kinase-3) in a manner that promotes HSC expansion with less differentiation. Improved methods for ex vivo expansion will make UCBT available to more patients, decrease engraftment times and allow more rapid immune reconstitution post transplant.

Comparison of various kinds of bone marrow stem cells for the repair of infarcted myocardium: single clonally purified non-hematopoietic mesenchymal stem cells serve as a superior source

A variety of adult stem cells have been used to transplant into the infarcted (MI) heart, however, comparative studies are lacking to show more suitable source of cells for transplantation. We have identified a single non-hematopoietic mesenchymal stem cell subpopulation (snMSCs) isolated from human bone marrow and clonally purified, that over 99% of them expressed MSC marker proteins and cardiomyocyte marker proteins when induction in vitro. We also compared the effects of the snMSCs with unpurified MSC (uMSCs), mononuclear cells (BMMNCs), or peripheral blood mononuclear cells (PBMNCs) on myocardial repair after induction of MI in rats. Ninety days later, we observed a better cardiac function assessed by ejection fraction, fraction of shortening and lung wet/dry weight ratios, less remodeling of left ventricle (LV), lower collagen density in the LV, and more vessels in the ischemic wall in the snMSCs transplantation group than in other cell-transplanted groups. Furthermore, the transplanted cells expressing cardiomyocyte specific proteins or vascular endothelial cell marker proteins were more in the snMSCs group than in other ones. We conclude that transplantation with single clonally purified MSCs seems to be more beneficial to the cardiac repair than with other stem cells after MI.

Cytokine and integrin stimulation synergize to promote higher levels of GATA-2, c-myb, and CD34 protein in primary human hematopoietic progenitors from bone marrow

We have previously shown that engagement of the integrins VLA-4 and VLA-5 to the fibronectin fragment CH-296 in combination with cytokines sustained the capacity of cultured human CD34(+) cells to undergo hematopoiesis in immunodeficient mice for 7 to 12 months, whereas this capacity was rapidly lost in cells cultured in suspension with the same cytokines. In the current study, we assessed the molecular pathways that might explain the loss of long-term engraftment capacity in cells cultured in suspension. Although the cell cycle profile was similar between cells cultured in suspension versus on fibronectin, levels of cell death were higher in the suspended cultures. While the CDK inhibitors p27Kip1 and p57Kip2 were present at equal levels in cells from both cultures, low levels of p21Cip1 were detectable only in the cytoplasmic compartment of cells cultured in suspension. Cytoplasmic location of p21Cip1 has been linked to monocytic differentiation. The levels of c-myb and GATA-2, transcription factors associated with stem cell maintenance, were higher in cells cultured on fibronectin as compared with suspension. In contrast, the levels of PU.1, which is induced during myeloid differentiation, were higher in cells cultured in suspension. There were no significant differences in surface expression of CD34 on the cells after culture, but total CD34 protein, assessed by immunoblotting, was significantly higher in cells cultured on fibronectin. Our data suggest that, in the presence of cytokines, the engagement of VLA-4 and VLA-5 integrins to the fibronectin fragment CH-296 preserves the expression of specific transcription factors associated with primitive stem cell maintenance. In contrast, a lack of integrin engagement leads to the induction of cellular markers associated with myeloid differentiation.

Role of integrin cross-regulation in parvovirus B19 targeting

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

Tumor microenvironment and drug resistance in hematologic malignancies

Increasing evidence supports the role of the tumor microenvironment in conferring drug resistance as a major cause of relapse and incurability of cancers. The tumor microenvironment consists of normal stromal cells, extracellular matrix, and soluble factors such as cytokines and growth factors. Tumor-tumor cell interaction, tumor-stromal cell interaction, as well as tumor-ECM interaction, all contribute to direct cell contact mediated drug resistance. In addition, soluble factors produced in the tumor microenvironment provide further signals for tumor cell growth and survival. Environment mediated-drug resistance (EM-DR) could be considered as the totality of cell adhesion mediated drug resistance (CAM-DR) and soluble factor mediated drug resistance (SM-DR) produced by the tumor-host interaction. This review focuses on the EM-DR model system and signaling pathways involved in cell survival of hematological malignancies.

Cell Adhesion Regulates CDC25A Expression and Proliferation in Acute Myeloid Leukemia

The effects of cell adhesion on leukemia cell proliferation remain poorly documented and somehow controversial. In this work, we investigated the effect of adhesion to fibronectin on the proliferation of acute myeloid leukemia (AML) cell lines (U937 and KG1a) and CD34+ normal or leukemic primary cells. We observed an increased rate of proliferation of AML cells when adhered to fibronectin, concomitant with accelerated S-phase entry and accumulation of CDC25A. Conversely, normal CD34+ cell proliferation was decreased by adhesion to fibronectin with a concomitant drop in CDC25A expression. Importantly, we showed that both small interfering RNA (siRNA)-mediated CDC25A down-regulation and a recently developed CDC25 pharmacologic inhibitor impaired this adhesion-dependent proliferation, establishing a functional link between CDC25A accumulation and adhesion-dependent proliferation in leukemic cells. CDC25A accumulation was found only slightly dependent on transcriptional regulation and essentially due to modifications of the proteasomal degradation of the protein as shown using proteasome inhibitors and reverse transcription-PCR. Interestingly, CDC25A regulation was Chk1 dependent in these cells as suggested by siRNA-mediated down-regulation of this protein. Finally, we identified activation of the phosphatidylinositol 3-kinase/Akt pathway as an adhesion-dependent regulation mechanism of CDC25A protein expression. Altogether, our data show that in leukemic cells adhesion to fibronectin increases CDC25A expression through proteasome- and Chk1-dependent mechanisms, resulting in enhanced proliferation. They also suggest that these adhesion-dependent proliferation properties of hematopoietic cells may be modified during leukemogenesis.

A novel role for fibronectin type I domain in the regulation of human hematopoietic cell adhesiveness through binding to follistatin domains of FLRG and follistatin

FLRG and follistatin belong to the family of follistatin proteins involved in the regulation of various biological effects, such as hematopoiesis, mediated by their binding to activin and BMP, both members of the TGFbeta family. To further characterize the function of FLRG, we searched for other possible functional partners using a yeast two-hybrid screen. We identified human fibronectin as a new partner for both FLRG and follistatin. We also demonstrated that their physical interaction is mediated by type I motifs of fibronectin and follistatin domains. We then analyzed the biological consequences of these protein interactions on the regulation of hematopoiesis. For the first time, we associated a biological effect with the regulation of human hematopoietic cell adhesiveness of both the type I motifs of fibronectin and the follistatin domains of FLRG and follistatin. Indeed, we observed a significant and specific dose-dependent increase of cell adhesion to fibronectin in the presence of FLRG or follistatin, using either a human hematopoietic cell line or primary cells. In particular, we observed a significantly increased adhesion of immature hematopoietic precursors (CFC, LTC-IC). Altogether these results highlight a new mechanism by which FLRG and follistatin regulate human hematopoiesis.

The role of mesenchymal stem cells in haemopoiesis

The ontogeny of haemopoiesis during fetal life and the differentiation of blood cells in adult life depend upon a fully competent microenvironment to provide appropriate signals via production of soluble factors and cell contact interactions. The cellular constituents of the microenvironment, also defined as the haemopoietic niche, largely derive from a common progenitor of mesenchymal origin. Mesenchymal stem cells (MSC), initially identified in adult bone marrow, have also been described in fetal haemopoietic tissues where they accompany the migration of haemopoietic development. Their precise identity remains ill-defined because of the lack of specific markers. Their ability to self-renew and differentiate into tissues of mesodermal origin (osteocytes, adipocytes, chondrocytes) and their lack of expression of haemopoietic molecules are currently the main criteria for isolation. In the bone marrow the most important elements of the niche appear to be osteoblasts, whilst a less defined population of fibroblasts regulates the maturation of immature T cells in the thymus. Recently, MSC have been shown to exert a profound immunosuppressive effect on polyclonal as well as antigen-specific T cell responses by inducing a state of division arrest anergy. Thus, the multipotent capacity of MSC, their role in supporting haemopoiesis, and their immunoregulatory activity make MSC particularly attractive for therapeutic exploitation.

Gene-expression profiling of CD34+ hematopoietic cells expanded in a collagen I matrix

CD34+ hematopoietic stem/progenitor cells (HSCs) reside in the bone marrow in close proximity to the endosteal bone surface, surrounded by osteoblasts, stromal cells, and various extracellular matrix molecules. We used a bioartificial matrix of fibrillar collagen I, the major matrix component of bone, as a scaffold for ex vivo expansion of HSCs. CD34+ HSCs were isolated from umbilical cord blood and cultivated within reconstituted collagen I fibrils in the presence of fms-like tyrosine kinase-3 ligand, stem cell factor, and interleukin (IL)-3. After 7 days of culture, the cell number, number of colony-forming units (CFU-C), and gene-expression profile of the cultured cells were assessed. Although the total expansion factor of CD34+ cells was slightly lower when cells were cultivated in the collagen I gel, the frequency of CFU-C was greater than in control suspension cultures. Gene-expression analysis with microarray chip technology revealed the upregulation of more than 50 genes in the presence of collagen I. Among these, genes for several growth factors, cytokines, and chemokines (e.g., IL-8 and macrophage inhibitory protein 1alpha) could be confirmed using quantitative polymerase chain reaction. Furthermore, greater expression levels of the negative cell-cycle regulator BTG2/TIS21 and an inhibitor of the mitogen-activated protein kinase pathway, DUSP2, underline the regulatory role of the extracellular matrix. Together, these data show that the expansion of CD34+ cord blood cells in a culture system containing a three-dimensional collagen I matrix induces a qualitative change in the gene-expression profile of cultivated HSCs.

The alpha4beta1 and alpha5beta1 integrins mediate engraftment of granulocyte-colony-stimulating factor-mobilized human hematopoietic progenitor cells

BACKGROUND

The alpha4beta1 and alpha5beta1 integrins are major adhesion molecules of murine and human hematopoietic progenitor cells. Granulocyte-colony-stimulating factor (G-CSF)-mobilized peripheral blood progenitor cells (PBPCs) are the most important source for clinical hematopoietic cell transplantation today. The contribution of alpha4beta1 and alpha5beta1 integrins to homing of PBPCs has not been studied yet.

STUDY DESIGN AND METHODS

The expression of alpha4beta1 and alpha5beta1 integrins on purified human PBPCs was analyzed. Integrin function in adhesion to recombinant fibronectin and migration on fibronectin-coated transwells was assessed with fragments combining different adhesion domains and function-blocking antibodies. Finally, the function of those integrins in a transplantation model was investigated with repopulating cells of nonobese diabetic/severe combined immune-deficient (NOD/SCID) mice.

RESULTS

More than 90 percent of all purified peripheral blood CD34+ cells express alpha4beta1 integrins, whereas only 10 to 15 percent express alpha5beta1. The alpha4beta1 integrin alone influences adhesion whereas alpha4beta1 and alpha5beta1 both mediate chemotaxis of clonogenic CD34+ progenitor cells on recombinant fibronectin. Importantly, antibodies against the integrins alpha4beta1 and alpha5beta1 independently reduce the repopulation of NOD/SCID mouse marrow after transplantation of human peripheral blood CD34+ cells.

CONCLUSIONS

Alpha4beta1 and alpha5beta1 integrins are functional and critical adhesion receptors expressed on G-CSF-mobilized CD34+ hematopoietic blood progenitor cells with repopulating capacity mediating engraftment after transplantation.

BCR-ABL induces the expression of Skp2 through the PI3K pathway to promote p27Kip1 degradation and proliferation of chronic myelogenous leukemia cells

Chronic myelogenous leukemia (CML) is characterized by the expression of the BCR-ABL tyrosine kinase, which results in increased cell proliferation and inhibition of apoptosis. In this study, we show in both BCR-ABL cells (Mo7e-p210 and BaF/3-p210) and primary CML CD34+ cells that STI571 inhibition of BCR-ABL tyrosine kinase activity results in a G(1) cell cycle arrest mediated by the PI3K pathway. This arrest is associated with a nuclear accumulation of p27(Kip1) and down-regulation of cyclins D and E. As a result, there is a reduction of the cyclin E/Cdk2 kinase activity and of the retinoblastoma protein phosphorylation. By quantitative reverse transcription-PCR we show that BCR-ABL/PI3K regulates the expression of p27(Kip1) at the level of transcription. We further show that BCR-ABL also regulates p27(Kip1) protein levels by increasing its degradation by the proteasome. This degradation depends on the ubiquitinylation of p27(Kip1) by Skp2-containing SFC complexes: silencing the expression of Skp2 with a small interfering RNA results in the accumulation of p27(Kip1). We also demonstrate that BCR-ABL cells show transcriptional up-regulation of Skp2. Finally, expression of a p27(Kip1) mutant unable of being recognized by Skp2 results in inhibition of proliferation of BCR-ABL cells, indicating that the degradation of p27(Kip1) contributes to the pathogenesis of CML. In conclusion, these results suggest that BCR-ABL regulates cell cycle in CML cells at least in part by inducing proteasome-mediated degradation of the cell cycle inhibitor p27(Kip1) and provide a rationale for the use of inhibitors of the proteasome in patients with BCR-ABL leukemias.

Phosphatidylinositol-3-kinase activation mediates proline-rich tyrosine kinase 2 phosphorylation and recruitment to β1-integrins in human CD34+ cells

OBJECTIVE

beta1-integrins mediate hematopoietic stem and progenitor cell homing and retention in the bone marrow (BM) and inhibit hematopoietic proliferation and differentiation. Having no intrinsic kinase activity, integrins recruit intracellular kinases, such as the focal adhesion kinase (FAK) or the related proline-rich tyrosine kinase 2 (PYK2), to initiate signal transduction. Phosphatidylinositol-3-kinase (PI3K), which is involved in beta1-integrin signaling in many cell types, is physically and functionally associated with FAK in anchorage-dependent cells. Because PYK2 is the principal focal adhesion kinase expressed in primary human CD34+ cells, we assessed its functional relationship with PI3K in CD34+ cells in response to integrin engagement.

METHODS

beta1-integrins on primary mobilized peripheral blood CD34+ cells and CD34+ KG1A cells were engaged by adhesion to fibronectin (FN) or by cross-linking with an anti-beta1 integrin antibody, respectively. PI3K activity and PYK2 phosphorylation were then assessed in the presence or absence of the PI3K inhibitor, wortmannin. Association between PI3K, PYK2, and the beta1-integrin subunit were also evaluated in co-immunoprecipitation experiments.

RESULTS

beta1-integrin engagement induced PI3K activation, which was required for, and temporally preceded, PYK2 phosphorylation, indicating that PI3K lies upstream of PYK2 in CD34+ cells. Furthermore, although PYK2 and PI3K were constitutively associated, interaction of the PYK2/PI3K complex with beta1-integrins required prior integrin engagement and PI3K activation.

CONCLUSION

Activation of PI3K following beta1-integrin engagement on human CD34+ cells results in subsequent phosphorylation of PYK2, and is required for the recruitment of the PI3K/PYK2 complex to beta1-integrins at the cell surface.

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

OBJECTIVE

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

METHODS

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

RESULTS

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

CONCLUSION

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

The presence of the carboxy-terminal fragment of fibronectin allows maintenance of non-human primate long-term hematopoietic repopulating cells during extended ex vivo culture and transduction

OBJECTIVE

Ex vivo expansion of primitive hematopoietic cells remains of interest for gene therapy and transplantation. Previous studies reported loss of repopulating activity following culture of cells for more than 4-7 days in the presence of cytokines or stromal cells. In the current study, we investigated whether prolonged culture and transduction in the presence of the carboxy-terminal portion of fibronectin (FN) could maintain or expand retrovirally transduced repopulating hematopoietic stem cells (HSCs).

METHODS

The impact of culture and transduction on rhesus macaque CD34+ peripheral blood stem cells (PBSCs) was assessed in the presence of FN and stimulatory cytokines. A competitive repopulation design using up to three retroviral vectors allowed direct comparison of repopulating activity between cells transduced and cultured for 4 days vs 10 days.

RESULTS

In the first animal, all cells were cultured and transduced for 10 days, with one vector used on days 0-4 and a second on days 4-10. There was stable long-term marking from both vectors, indicating that cells cycling both early and late could engraft. In three animals, we compared cells that were cryopreserved following a 4-day transduction to cells that were continued in culture for an additional 6 days. Total marking derived from the 10-day expanded cells was significantly higher than marking from the 4-day cultured cells.

CONCLUSIONS

These results suggest that culture on FN support allows prolonged ex vivo maintenance and even expansion of transduced repopulating stem cells.

Fibronectin promotes VEGF-induced CD34 cell differentiation into endothelial cells

BACKGROUND

Adult endothelial progenitor cells (EPC) may be a useful source for engineering the endothelialization of vascular grafts. However, the optimal factors that promote differentiation of EPCs into endothelium remain to be elucidated. The goal of this current report was to determine which extracellular matrix (ECM) protein might modulate or enhance the effects of EPCs on differentiation into mature endothelium.

METHODS

Human EPCs (CD34(+) cells) were cultured in ECM-coated six-well plates in MCDB-131 medium containing vascular endothelial growth factor (VEGF), insulin-like growth factor-1, and basic fibroblast growth factor. After 21 days, differentiated endothelial colonies were confirmed by immunofluorescence for von Willebrand factor (vWF) and vascular-endothelial (VE)-cadherin and mRNA expression of the endothelial markers Flk-1, vWF, and VE-cadherin. Cell migration toward the VEGF-matrix protein combinations was also measured.

RESULTS

As judged by positive staining for endothelial markers vWF and VE-cadherin, the combination of VEGF with fibronectin (FN) produced significantly more endothelial colonies (P <.05) than did collagens I or IV or vitronectin. Defined fragments of FN did not enhance VEGF-mediated effects. Fibrinogen produced intermediate stimulation of differentiation. FN also enhanced VEGF-mediated CD34(+) cell migration. Blockade of alpha5beta1, but not alphavbeta3 or alphavbeta5, inhibited both VEGF-mediated CD34(+) cell differentiation and migration.

CONCLUSIONS

VEGF and FN together significantly promote the migration and differentiation of CD34(+) cells. This synergism is specific to FN and the alpha5beta1 integrin. Combinations of VEGF and FN may be useful in promoting differentiation of circulating endothelial progenitors into endothelial cells for tissue engineering. Clinical relevance Treatment of injured or diseased tissues with adult stem cells is a promising approach. In particular, bone marrow derived circulating endothelial progenitors (CEP's) have been shown to differentiate into endothelial cells in vitro and promote tissue revascularization of ischemic limbs and myocardium in vivo. Because of the relative ease of obtaining CEP's and as well as its high proliferative rate, CEP's may have clinical potential for endothelialization of prosthetic vascular grafts and revascularization of injured myocardium. However, there is a need to better understand the molecular pathways involved in the proliferation and differentiation of CEP's to take full advantage of its clinical potential.

Fibronectin and laminin increase resistance to ionizing radiation and the cytotoxic drug Ukrain in human tumour and normal cells in vitro

PURPOSE

Cell-extracellular matrix (ECM) interactions are thought to mediate drug and radiation resistance. Dependence of cell survival, beta1-integrin expression and cell cycling on the ECM proteins and beta1-integrin ligands fibronectin (FN) and laminin (LA) were examined in malignant and normal cells exposed to the cytotoxic drug Ukrain plus/minus irradiation.

MATERIALS AND METHODS

Human A549 lung cancer and MDAMB231 (MDA231) breast cancer cells and normal fibroblasts (HSF1) grown on FN, LA, bovine serum albumin (BSA) or polystyrene were treated with Ukrain (1 microg ml(-1), 24 h) plus/minus irradiation (2-8 Gy) and the effects studied using colony formation assays, flow cytometry (beta1-integrin, DNA analysis) and adhesion assays.

RESULTS

FN and LA reduced the cytotoxic effect of single Ukrain treatment compared with polystyrene and BSA. FN and LA also abolished Ukrain-dependent radiosensitization in A549 cells and decreased the radiosensitivity of MDA231 and HSF1 cells. Single Ukrain exposure on polystyrene significantly reduced beta1-integrin expression and promoted G2-phase accumulation of A549 cells. In contrast, Ukrain-treated MDA231 and HSF1 cells showed elevated beta1-integrin expression and no Ukrain-specific cell cycle effect. Under Ukrain-radiation exposure, irradiation, FN or LA abolished Ukrain-mediated reduction of beta1-integrin expression and G2-phase accumulation in A549 cells, whereas in MDA231 cells and fibroblasts beta1-integrin expression and cell cycle distribution were stabilized. Cell adhesion to FN or LA was significantly impaired (A549) or improved (MDA231, HSF1) upon Ukrain treatment.

CONCLUSIONS

The data corroborate the findings of other groups that cell adhesion-mediated resistance to either single or combined drug and radiation exposure is tightly correlated to specific ECM proteins. By demonstrating a strong modulatory impact of FN and LA on the radiosensitivity-modifying activity of the drug Ukrain, the set findings are also highly important for the assessment of drug and radiation effects within in vitro cytotoxicity studies. The data give the first mechanistic insights into specific FN- and LA-modulated cellular resistance mechanisms as well as into the important role for beta1-integrins using the unique cytotoxic and radiosensitivity-modifying drug Ukrain.

Transplantable stem cells: home to specific niches

PURPOSE OF REVIEW

Although the concept of engraftment and clinical reconstitution of the bone marrow was described several decades ago, the analysis of individual steps within this process remains a major focus of much current research in stem cell biology. In particular, this extends to the identification and characterization of the specific stem cell niche first proposed by Schofield in 1978. It is appropriate, therefore, that on the 25th anniversary of this publication, that we review recent progress in our understanding of the location and composition of the bone marrow stem cell niche and of the mechanisms involved in the initial phases of hematopoietic stem cell engraftment.

RECENT FINDINGS

During the past 12 months there have been significant advancements in our understanding of the interplay of molecules involved in the homing of hematopoietic stem cells to the bone marrow. In addition, innovative methodologies have become available for the visualization of hematopoietic stem cells within the bone marrow in situ. In an important development in this area, studies our now focusing on events after transendothelial migration into the marrow cords, including mechanisms involved in hematopoietic stem cell migration to and lodgment within the hematopoietic stem cell niche. Furthermore, there have been numerous new reports analyzing the molecular regulation of hematopoietic stem cells within the bone marrow niche in situ.

SUMMARY

Overall, recent advancements in our understanding of hematopoietic stem cell biology and, in particular, the interaction of hematopoietic stem cells with the hematopoietic microenvironment paves the way for expanded use in regenerative medicine.

Role of the tumor microenvironment in mediating de novo resistance to drugs and physiological mediators of cell death

The emergence of clinical drug resistance continues to be an obstacle for the successful treatment of cancer. Our current understanding of mechanisms associated with drug resistance has been ascertained by investigating drug-resistant models created by exposing a parental population to increasing concentrations of a cytotoxic. These unicellular drug-resistant models have been critical in elucidating drug-resistant mechanism and in some cases have aided in the identification of drug targets. However, these models do not address resistance mechanisms that contribute to de novo drug resistance. We propose that specific niches within the tumor microenvironment may provide a sanctuary for subpopulations of tumors cells that affords a survival advantage following initial drug exposure and may facilitate the acquisition of acquired drug resistance. More specifically, we propose that the bone marrow microenvironment is a sanctuary for hema-topoietic cancers. This review will focus on the bone marrow microenvironment and its role in conferring resistance to cytotoxics and physiological mediators of cell death.

Differential STAT3, STAT5, and NF-kappaB activation in human hematopoietic progenitors by endogenous interleukin-15: implications in the expression of functional molecules

Different forms of interleukin-15 (IL-15) have been identified and shown to elicit different transduction pathways whose impact on hematopoiesis is poorly understood. We demonstrated herein that hematopoietic CD34+ cells constitutively produced endogenous secreted IL-15 (ES-IL-15) that activated different transcription factors and controlled the expression of several functional proteins, depending on the progenitor source. Thus, nuclear factor-kappa B (NF-kappa B) was activated in bone marrow (BM) and cord blood (CB) progenitors, whereas signal transducer and activator of transcription 3 (STAT3) and STAT5 activation was restricted to peripheral granulocyte-colony-stimulating factor (G-CSF)-mobilized and BM progenitors, respectively. ES-IL-15 acts through autocrine/paracrine loops controlled by high-affinity receptors involving IL-15 receptor alpha (IL-15Ralpha). Furthermore, ES-IL-15 was found to differentially control the expression of several functional molecules important for hematopoietic differentiation. Indeed, in BM precursors, neutralizing anti-IL-15 monoclonal antibody (mAb) inhibits the expression of the gamma c chain and of the chemokine stromal derived factor-1 (SDF-1) but had no effect on vascular cell adhesion molecule 1 (VCAM-1) and beta1 integrin adhesion molecule expression. Conversely, in CB progenitors, anti-IL-15 mAb inhibited VCAM-1 and beta1 integrin expression without affecting gammac chain expression and, most important, up-regulated SDF-1 expression. In conclusion, unprimed human hematopoietic CD34+ cells secrete cell-unbound IL-15, which activates through autocrine/paracrine loop distinct signaling pathways, depending on the progenitor source, thereby influencing the expression of several molecules important in the control of hematopoiesis.

The symmetry of initial divisions of human hematopoietic progenitors is altered only by the cellular microenvironment

OBJECTIVE

We examined if cellular elements or adhesive ligands were able to alter asymmetric divisions of CD34(+)/CD38(-) cells in contrast to soluble factors at a single cell level.

MATERIALS AND METHODS

After single cell deposition onto 96-well plates, cells were cocultured for 10 days with the stem cell supporting cell line AFT024, fibronectin (FN), or bovine serum albumin (BSA). The divisional history was monitored with time-lapse microscopy. Subsequent function for the most primitive cells was assessed using the myeloid-lymphoid-initiating cell (ML-IC) assay. Committed progenitors were measured using colony-forming cells (CFC).

RESULTS

Only contact with AFT024 recruited significant numbers of CD34(+)/CD38(-) cells into cell cycle and increased asymmetric divisions. Although most ML-IC were still identified among cells that have divided fewer than 3 times, a significant number of ML-IC shifted into the fast-dividing fraction after exposure to AFT024. The increase in ML-IC frequency was predominantly due to recruitment of quiescent and slow-dividing cells from the starting population. Increase in CFC activity induced by AFT024 was found only among rapidly dividing cells.

CONCLUSIONS

For the first time, we have demonstrated that asymmetric divisions can be altered upon exposure with a stem cell-supporting microenvironment. For the primitive subset of cells (ML-IC), this was predominantly due to recruitment into cell cycle and increased rounds of cycling without loss of function. Exposure to AFT024 cells also increased proliferation and asymmetric divisions of committed CFC. Hence direct communication between hematopoietic progenitors with stroma cells is required for maintaining self-renewal potential.

Flt3-ligand induces adhesion of haematopoietic progenitor cells via a very late antigen (VLA)-4- and VLA-5-dependent mechanism

The adhesion of haematopoietic progenitor cells (HPC) to the bone marrow microenvironment is a process regulated by cytokines. In this study, we have shown that flt3-ligand (FL), a growth factor that controls early haematopoiesis, regulated the function and expression of the beta-1 integrins, very late antigen (VLA)-4 and VLA-5 on HPC. The modulation of the adhesiveness of HPC by FL was studied by adhesion assays on umbilical vein endothelial cells (HUVEC). Stimulation by FL induced two peaks of increased adhesiveness of HPC. The first peak was at around 30 min and was mechanistically related to an activation of the beta-1 integrins, mainly VLA-4 and VLA-5. The second peak was at around 12 h and was related to increased expression of VLA-4 and VLA-5. The control of HPC adhesiveness by FL is a previously unreported property of FL that may be important for the homing and the retention of flt3-expressing HPC within the bone marrow microenvironment.

Activation of Rac and tyrosine phosphorylation of cytokine receptors induced by cross-linking of integrin alpha4beta1 and cell adhesion in hematopoietic cells

Adhesion of hematopoietic cells, mainly through alpha4beta1 and alpha5beta1 integrins, to the bone marrow microenvironment may play important roles in regulation of hematopoiesis. However, the mechanisms for signaling, outside-in signaling, have largely remained to be established. We demonstrate here that cross-linking of alpha4beta1 by anti-alpha4 antibody induces tyrosine phosphorylation of Pyk2, Shc, and Cbl as well as binding of the adaptor protein CrkL with Cbl in a murine hematopoietic cell line, 32D/EpoR-Wt. Furthermore, cross-linking of alpha4beta1 induced activation of the Rho family small GTPase Rac, which was enhanced by induced overexpression of CrkL and was inhibited by the phosphatidylinositol 3(')-kinase (PI3K) inhibitor LY294002. In addition, adhesion of 32D/EpoR-Wt cells to immobilized H-296, a recombinant fibronectin peptide specific for alpha4beta1, induced tyrosine phosphorylation of Jak2, the erythropoietin receptor (EpoR), and the IL-3 receptor beta subunit as well as Pyk2, Shc, and Cbl. Tyrosine phosphorylation of Jak2 and EpoR was also induced in a human leukemic cell line, UT-7, by adhesion to immobilized H-296. However, adhesion of 32D/EpoR-PM4 cells, expressing the W282R mutant EpoR defective in coupling with Jak2, to immobilized H-296 failed to induce tyrosine phosphorylation of the mutant EpoR. These results implicate CrkL in PI3K-dependent activation of Rac by outside-in signaling from alpha4beta1 and suggest that adhesion through alpha4beta1 further activates cytokine receptor-associated Jak2 to induce phosphorylation of these receptors.

Adhesion of synchronized human hematopoietic progenitor cells to fibronectin and vascular cell adhesion molecule-1 fluctuates reversibly during cell cycle transit in ex vivo culture

Ex vivo expansion of hematopoietic stem/progenitor cells may result in defective engraftment. Human cord blood CD34(+) progenitor cells were synchronized and assayed for adhesion and migration onto fibronectin (Fn) and vascular cell adhesion molecule-1 (VCAM-1) at different stages of a first cell cycle executed ex vivo. During S phase transit, adhesion to Fn was transiently increased while binding to VCAM-1 was reversibly decreased, after which adhesion to both ligands returned to baseline levels with cell cycle completion. Transmigration across Fn and VCAM-1 decreased irreversibly during S phase progression. The function of alpha4 and alpha5 integrins was assessed with specific neutralizing antibodies. In uncultured CD34(+) cells and long-term culture-initiating cells (LTC-ICs), both adhesion and migration on Fn were inhibited by anti-alpha4 but not by anti-alpha5 antibodies. In mitotically activated CD34(+) cells and LTC-ICs, adhesion and migration on Fn were mainly dependent on alpha5 integrin and to a lesser extent on alpha4 integrin. Changes in integrin function were not dependent on parallel modulation of integrin expression. In conclusion, Fn and VCAM-1 binding of progenitor cells fluctuates reversibly during cell cycle transit ex vivo. In addition, our data show that mitogenic activation induces a shift from a dominant alpha4 to a preferential alpha5 integrin-dependent interaction with Fn.

The microenvironment of AFT024 cells maintains primitive human hematopoiesis by counteracting contact mediated inhibition of proliferation

We have previously shown that maintenance of primitive human hematopoietic stem cells is poor when cultured in contact with marrow stromal feeders. However, when separated from stromal contact, human progenitors can be maintained because adhesion mediated proliferation inhibition does not occur. In this study we demonstrate how the murine fetal liver cell line, AFT024, supports primitive human hematopoiesis better in contact cultures compared to primary feeders. We evaluated if better progenitor maintenance in contact with AFT024 cells can be explained by decreased adhesion itself or decreased adhesion mediated inhibition of proliferation. We show that primitive human hematopoietic cells adhered equally well to AFT024 and primary feeders, such as M2-10B4. Further, contact with metabolically inactive AFT024 cells prevented cell cycle progression and decreased maintenance of primitive progenitors to the same extent as contact with M2-10B4 feeders. However, contact with viable AFT024 feeders did not inhibit proliferation, suggesting that AFT024-factors counteract contact mediated inhibition of proliferation. Cytokine production by M2-10B4 and AFT024 cells was similar. Large-size O-sulfated heparan sulfate glycosaminoglycans, known to be important for hematopoietic support, were found only in AFT024-matrix. We hypothesize that these factors may explain, in part, our observations. Finally, we show that more than 100% of primitive myeloid progenitors could be maintained for at least five weeks when cultured in contact with AFT024 feeders in the presence of Interleukin-3 and Macrophage Inflammatory Protein-1alpha. In conclusion, AFT024 cells produce factor(s), that counteract contact induced growth inhibition of primitive human hematopoietic progenitors, leading to expansion of these cells in contact with the microenvironment.

The v-erbA oncogene blocks expression of alpha2/beta1 integrin a normal inhibitor of erythroid progenitor proliferation

T2EC are chicken erythrocytic progenitors that balance between self-renewal and differentiation as a function of response to specific growth factors. Their transformation by the v-erbA oncogene locks them into the self-renewal program. We show here that the expression of the VLA-2 integrin alpha2 subunit mRNA is downregulated by v-erbA and that VLA-2 engagement and clustering, brought about by treatment with an alpha2-specific antibody or by culture on the VLA-2 ligand collagen I, inhibits T2EC proliferation. From competition studies using antibodies, VLA-2 was shown to be involved in the collagen-induced response. While engagement of VLA-2 inhibited proliferation, it was not sufficient to induce differentiation. The transformation of T2EC by v-erbA decreased their interaction with collagen I and the VLA-2 brake on cell proliferation, which may account for the increased proliferation potential of transformed erythrocytic progenitors and for their shedding into the blood of infected chickens. Our data suggest that the interaction between erythroid progenitors and collagen, mediated by VLA-2, play a major role in the control of erythropoiesis in vitro and that this pathway is a target of the v-erbA oncogene.

Signal transduction by cell adhesion receptors and the cytoskeleton: functions of integrins, cadherins, selectins, and immunoglobulin-superfamily members

Cellular interactions with the extracellular matrix and with neighboring cells profoundly influence a variety of signaling events including those involved in mitogenesis, survival, and differentiation. Recent advances have provided insights into mechanisms underlying the ability of integrins, cadherins, selectins, and other cell adhesion molecules to regulate signal transduction cascades. These mechanisms often involve the ability of cell adhesion molecules to initiate the formation of organized structures or scaffolds that permit the efficient flow of information in signaling pathways.

Purification and ex vivo expansion of postnatal human marrow mesodermal progenitor cells

It is here reported that mesenchymal stem cells known to give rise to limb-bud mesoderm can, at the single-cell level, also differentiate into cells of visceral mesoderm and can be expanded extensively by means of clinically applicable methods. These cells were named mesodermal progenitor cells (MPCs). MPCs were selected by depleting bone marrow mononuclear cells from more than 30 healthy human donors of CD45(+)/glycophorin-A (GlyA)(+) cells. Cells were cultured on fibronectin with epidermal growth factor and platelet-derived growth factor BB and 2% or less fetal calf serum. It was found that 1/5 x 10(3) CD45(-)GlyA(-) cells, or 1/10(6) bone marrow mononuclear cells, gave rise to clusters of small adherent cells. Cell-doubling time was 48 to 72 hours, and cells have been expanded in culture for more than 60 cell doublings. MPCs are CD34(-), CD44(low), CD45(-), CD117 (cKit)(-), class I-HLA(-), and HLA-DR(-). MPCs differentiated into cells of limb-bud mesoderm (osteoblasts, chondrocytes, adipocytes, stroma cells, and skeletal myoblasts) as well as visceral mesoderm (endothelial cells). Retroviral marking was used to definitively prove that single MPCs can differentiate into cells of limb bud and visceral mesoderm. Thus, MPCs that proliferate without obvious senescence under clinically applicable conditions and differentiate at the single-cell level not only into mesenchymal cells but also cells of visceral mesoderm may be an ideal source of stem cells for treatment of genetic or degenerative disorders affecting cells of mesodermal origin.

Decorin inhibits macrophage colony-stimulating factor proliferation of macrophages and enhances cell survival through induction of p27(Kip1) and p21(Waf1)

Decorin is a small proteoglycan that is ubiquitous in the extracellular matrix of mammalian tissues. It has been extensively demonstrated that decorin inhibits tumor cell growth; however, no data have been reported on the effects of decorin in normal cells. Using nontransformed macrophages from bone marrow, results of this study showed that decorin inhibits macrophage colony-stimulating factor (M-CSF)-dependent proliferation by inducing blockage at the G(1) phase of the cell cycle without affecting cell viability. In addition, decorin rescues macrophages from the induction of apoptosis after growth factor withdrawal. Decorin induces the expression of the cdk inhibitors p21(Waf1) and p27(Kip1). Using macrophages from mice where these genes have been disrupted, inhibition of proliferation mediated by decorin is related to p27(Kip1) expression, whereas p21(Waf1) expression is necessary to protect macrophages from apoptosis. Decorin also inhibits M-CSF-dependent expression of MKP-1 and extends the kinetics of ERK activity, which is characteristic when macrophages become activated instead of proliferating. The effect of decorin on macrophages is not due to its interaction with epidermal growth factor or interferon-gamma receptors. Furthermore, decorin increases macrophage adhesion to the extracellular matrix, and this may be partially responsible for the expression of p27(Kip1) and the modification of ERK activity, but not for the increased cell survival.