Mariusz Z. Ratajczak

Trafficking of normal stem cells and metastasis of cancer stem cells involve similar mechanisms: pivotal role of the SDF-1-CXCR4 axis.

The α‐chemokine stromal‐derived factor (SDF)‐1 and the G‐protein–coupled seven‐span transmembrane receptor CXCR4 axis regulates the trafficking of various cell types. In this review, we present the concept that the SDF‐1–CXCR4 axis is a master regulator of trafficking of both normal and cancer stem cells. Supporting this is growing evidence that SDF‐1 plays a pivotal role in the regulation of trafficking of normal hematopoietic stem cells (HSCs) and their homing/retention in bone marrow. Moreover, functional CXCR4 is also expressed on nonhematopoietic tissue‐committed stem/progenitor cells (TCSCs); hence, the SDF‐1–CXCR4 axis emerges as a pivotal regulator of trafficking of various types of stem cells in the body. Furthermore, because most if not all malignancies originate in the stem/progenitor cell compartment, cancer stem cells also express CXCR4 on their surface and, as a result, the SDF‐1–CXCR4 axis is also involved in directing their trafficking/metastasis to organs that highly express SDF‐1 (e.g., lymph nodes, lungs, liver, and bones). Hence, we postulate that the metastasis of cancer stem cells and trafficking of normal stem cells involve similar mechanisms, and we discuss here the common molecular mechanisms involved in these processes. Finally, the responsiveness of CXCR4+ normal and malignant stem cells to an SDF‐1 gradient may be regulated positively/primed by several small molecules related to inflammation which enhance incorporation of CXCR4 into membrane lipid rafts, or may be inhibited/blocked by small CXCR4 antagonist peptides. Consequently, strategies aimed at modulating the SDF‐1–CXCR4 axis could have important clinical applications both in regenerative medicine to deliver normal stem cells to the tissues/organs and in clinical hematology/oncology to inhibit metastasis of cancer stem cells.

CXCR4-SDF-1 signalling, locomotion, chemotaxis and adhesion.

Chemokines, small pro-inflammatory chemoattractant cytokines, that bind to specific G-protein-coupled seven-span transmembrane receptors present on plasma membranes of target cells are the major regulators of cell trafficking. In addition some chemokines have been reported to modulate cell survival and growth. Moreover, compelling evidence is accumulating that cancer cells may employ several mechanisms involving chemokine–chemokine receptor axes during their metastasis that also regulate the trafficking of normal cells. Of all the chemokines, stromal-derived factor-1 (SDF-1), an α-chemokine that binds to G-protein-coupled CXCR4, plays an important and unique role in the regulation of stem/progenitor cell trafficking. First, SDF-1 regulates the trafficking of CXCR4+ haemato/lymphopoietic cells, their homing/retention in major haemato/lymphopoietic organs and accumulation of CXCR4+ immune cells in tissues affected by inflammation. Second, CXCR4 plays an essential role in the trafficking of other tissue/organ specific stem/progenitor cells expressing CXCR4 on their surface, e.g., during embryo/organogenesis and tissue/organ regeneration. Third, since CXCR4 is expressed on several tumour cells, these CXCR4 positive tumour cells may metastasize to the organs that secrete/express SDF-1 (e.g., bones, lymph nodes, lung and liver). SDF-1 exerts pleiotropic effects regulating processes essential to tumour metastasis such as locomotion of malignant cells, their chemoattraction and adhesion, as well as plays an important role in tumour vascularization. This implies that new therapeutic strategies aimed at blocking the SDF-1–CXCR4 axis could have important applications in the clinic by modulating the trafficking of haemato/lymphopoietic cells and inhibiting the metastatic behaviour of tumour cells as well. In this review, we focus on a role of the SDF-1–CXCR4 axis in regulating the metastatic behaviour of tumour cells and discuss the molecular mechanisms that are essential to this process.

Migration of Bone Marrow and Cord Blood Mesenchymal Stem Cells In Vitro Is Regulated by Stromal‐Derived Factor‐1‐CXCR4 and Hepatocyte Growth Factor‐c‐met Axes and Involves Matrix Metalloproteinases

Human mesenchymal stem cells (MSCs) are increasingly being considered in cell‐based therapeutic strategies for regeneration of various organs/tissues. However, the signals required for their homing and recruitment to injured sites are not yet fully understood. Because stromal‐derived factor (SDF)‐1 and hepatocyte growth factor (HGF) become up‐regulated during tissue/organ damage, in this study we examined whether these factors chemoattract ex vivo‐expanded MSCs derived from bone marrow (BM) and umbilical cord blood (CB). Specifically, we investigated the expression by MSCs of CXCR4 and c‐met, the cognate receptors of SDF‐1 and HGF, and their functionality after early and late passages of MSCs. We also determined whether MSCs express matrix metalloproteinases (MMPs), including membrane type 1 (MT1)‐MMP, matrix‐degrading enzymes that facilitate the trafficking of hematopoietic stem cells. We maintained expanded BM‐ or CB‐derived MSCs for up to 15–18 passages with monitoring of the expression of 1) various tissue markers (cardiac and skeletal muscle, neural, liver, and endothelial cells), 2) functional CXCR4 and c‐met, and 3) MMPs. We found that for up to 15–18 passages, both BM‐ and CB‐derived MSCs 1) express mRNA for cardiac, muscle, neural, and liver markers, as well as the vascular endothelial (VE) marker VE‐cadherin; 2) express CXCR4 and c‐met receptors and are strongly attracted by SDF‐1 and HGF gradients; 3) express MMP‐2 and MT1‐MMP transcripts and proteins; and 4) are chemo‐invasive across the reconstituted basement membrane Matrigel. These in vitro results suggest that the SDF‐1‐CXCR4 and HGF‐c‐met axes, along with MMPs, may be involved in recruitment of expanded MSCs to damaged tissues.

Microvesicles derived from activated platelets induce metastasis and angiogenesis in lung cancer.

The role of platelets in tumor progression and metastasis has been recognized but the mechanism of their action remains unclear. Five human lung cancer cell lines (A549, CRL 2066, CRL 2062, HTB 183, HTB 177) and a murine Lewis lung carcinoma (LCC) cell line (for an in vivo model of metastasis) were used to investigate how platelet‐derived microvesicles (PMV), which are circular fragments shed from the surface membranes of activated platelets, and exosomes released from platelet α‐granules, could contribute to metastatic spread. We found that PMV transferred the platelet‐derived integrin CD41 to most of the lung cancer cell lines tested and stimulated the phosphorylation of mitogen‐activated protein kinase p42/44 and serine/threonine kinase as well as the expression of membrane type 1‐matrix metalloproteinase (MT1‐MMP). PMV chemoattracted 4 of the 5 cell lines, with the highly metastatic A549 cells exhibiting the strongest response. In A549 cells, PMV were shown to stimulate proliferation, upregulate cyclin D2 expression and increase trans‐Matrigel chemoinvasion. Furthermore, in these cells, PMV stimulated mRNA expression for angiogenic factors such as MMP‐9, vascular endothelial growth factor, interleukin‐8 and hepatocyte growth factor, as well as adhesion to fibrinogen and human umbilical vein endothelial cells. Intravenous injection of murine PMV‐covered LLC cells into syngeneic mice resulted in significantly more metastatic foci in their lungs and LLC cells in bone marrow than in control animals injected with LCC cells not covered with PMV. Based on these findings, we suggest that PMV play an important role in tumor progression/metastasis and angiogenesis in lung cancer.

Mobilization of CD34/CXCR4+, CD34/CD117+, c-met+ Stem Cells, and Mononuclear Cells Expressing Early Cardiac, Muscle, and Endothelial Markers Into Peripheral Blood in Patients With Acute Myocardial Infarction

Background—Adult stem cells can contribute to myocardial regeneration after ischemic injury. Bone marrow and skeletal muscles contain a population of CXCR4+ cells expressing genes specific for muscle progenitor cells that can be mobilized into the peripheral blood. The aims of the study were (1) to confirm the presence of early tissue-committed cells expressing cardiac, muscle, and endothelial markers in populations of mononuclear cells in peripheral blood and (2) to assess the dynamics and magnitude of the mobilization of CD34+, CD117+, CXCR4+, c-met+, CD34/CD117+, and CD34/CXCR4+ stem cells into peripheral blood in relation to inflammatory and hematopoietic cytokines in patients with ST-segment–elevation acute myocardial infarction (STEMI). Methods and Results—Fifty-six patients with STEMI (<12 hours), 39 with stable angina, and 20 healthy control subjects were enrolled. Real-time reverse transcription–polymerase chain reaction (RT-PCR) was used for detection of tissue-specific markers. The number of the cells was assessed by use of a flow cytometer on admission, after 24 hours, and after 7 days. RT-PCR revealed increased expression of mRNA (up to 3.5-fold increase) for specific cardiac (GATA4, MEF2C, Nkx2.5/Csx), muscle (Myf5, Myogenin, MyoD), and endothelial (VE-cadherin, von Willebrand factor) markers in peripheral blood mononuclear cells. The number of CD34/CXCR4+ and CD34/CD117+ and c-met+ stem cells in peripheral blood was significantly higher in STEMI patients than in stable angina and healthy subjects, peaking on admission, without further significant increase after 24 hours and 7 days. Conclusions—The study demonstrates in the setting of STEMI a marked mobilization of mononuclear cells expressing specific cardiac, muscle, and endothelial markers as well as CD34/CXCR4+ and CD34/CD117+ and c-met+ stem cells and shows that stromal cell–derived factor-1 is an important factor influencing the mobilization.

Intracoronary infusion of bone marrow-derived selected CD34+CXCR4+ cells and non-selected mononuclear cells in patients with acute STEMI and reduced left ventricular ejection fraction: results of randomized, multicentre Myocardial Regeneration by Intracoronary Infusion of Selected Population of Stem Cells in Acute Myocardial Infarction (REGENT) Trial

AIMS Comparison of intracoronary infusion of bone marrow (BM)-derived unselected mononuclear cells (UNSEL) and selected CD34(+)CXCR4(+) cells (SEL) in patients with acute myocardial infarction (AMI) and reduced <40% left ventricular ejection fraction (LVEF). METHODS AND RESULTS Two hundred patients were randomized to intracoronary infusion of UNSEL (n = 80) or SEL (n = 80) BM cells or to the control (CTRL) group without BM cell treatment. Primary endpoint: change of LVEF and volumes measured by magnetic resonance imaging before and 6 months after the procedure. After 6 months, LVEF increased by 3% (P = 0.01) in patients treated with UNSEL, 3% in patients receiving SEL (P = 0.04) and remained unchanged in CTRL group (P = 0.73). There were no significant differences in absolute changes of LVEF between the groups. Absolute changes of left ventricular end-systolic volume and left ventricular end-diastolic volume were not significantly different in all groups. Significant increase of LVEF was observed only in patients treated with BM cells who had baseline LVEF < median (37%). Baseline LVEF < median and time from the onset of symptoms to primary percutaneous coronary intervention > or = median were predictors of LVEF improvement in patients receiving BM cells. There were no differences in major cardiovascular event (death, re-infarction, stroke, target vessel revascularization) between groups. CONCLUSION In patients with AMI and impaired LVEF, treatment with BM cells does not lead to a significant improvement of LVEF or volumes. There was however a trend in favour of cell therapy in patients with most severely impaired LVEF and longer delay between the symptoms and revascularization.

Cells Expressing Early Cardiac Markers Reside in the Bone Marrow and Are Mobilized Into the Peripheral Blood After Myocardial Infarction

The concept that bone marrow (BM)–derived cells participate in cardiac regeneration remains highly controversial and the identity of the specific cell type(s) involved remains unknown. In this study, we report that the postnatal BM contains a mobile pool of cells that express early cardiac lineage markers (Nkx2.5/Csx, GATA-4, and MEF2C). These cells are present in significant amounts in BM harvested from young mice but their abundance decreases with age; in addition, the responsiveness of these cells to gradients of motomorphogens SDF-1, HGF, and LIF changes with age. FACS analysis, combined with analysis of early cardiac markers at the mRNA and protein levels, revealed that cells expressing these markers reside in the nonadherent, nonhematopoietic CXCR4+/Sca-1+/lin−/CD45− mononuclear cell (MNC) fraction in mice and in the CXCR4+/CD34+/AC133+/CD45− BMMNC fraction in humans. These cells are mobilized into the peripheral blood after myocardial infarction and chemoattracted to the infarcted myocardium in an SDF-1-CXCR4–, HGF-c-Met–, and LIF-LIF-R–dependent manner. To our knowledge, this is the first demonstration that the postnatal BM harbors a nonhematopoietic population of cells that express markers for cardiac differentiation. We propose that these potential cardiac progenitors may account for the myocardial regenerative effects of BM. The present findings provide a novel paradigm that could reconcile current controversies and a rationale for investigating the use of BM-derived cardiac progenitors for myocardial regeneration.

Platelet-derived microparticles stimulate proliferation, survival, adhesion, and chemotaxis of hematopoietic cells.

OBJECTIVE Peripheral blood platelet-derived microparticles (PMPs) circulate in blood and may interact directly with target cells affecting their various biological functions. METHODS To investigate the effect of human PMPs on hematopoiesis, we first phenotyped them for expression of various surface molecules and subsequently studied various biological responses of normal stem/progenitor (CD34(+)) and more differentiated precursor cells as well as several leukemic cell lines to PMPs. RESULTS We found that, in addition to platelet-endothelium attachment receptors (CD41, CD61 and CD62), PMPs express G-protein-coupled seven transmembrane-span receptors such as CXCR4 and PAR-1; cytokine receptors including TNF-RI, TNF-RII, and CD95; and ligands such as CD40L and PF-4. Moreover, we found that several of these receptors could be transferred by PMPs to the membranes of normal as well as malignant cells and observed that PMPs: 1) chemoattract these cells, 2) increase their adhesion, proliferation, and survival, and 3) activate in these cells various intracellular signaling cascades including MAPK p42/44, PI-3K-AKT, and STAT proteins. The biological effects of PMPs were only partly reduced by heat inactivation or trypsin digest, indicating that, in addition to the protein components of PMPs, lipid components are also responsible for their biological activity. CONCLUSIONS We conclude that PMPs modulate biological functions of hematopoietic cells and postulate that they play an important but as yet not fully understood role in intercellular cross-talk in hematopoiesis. Further studies, however, are needed to identify the PMP components that exert specific biological effects.

Expression of Functional CXCR4 by Muscle Satellite Cells and Secretion of SDF-1 by Muscle-Derived Fibroblasts is Associated with the Presence of Both Muscle Progenitors in Bone Marrow and Hematopoietic Stem/Progenitor Cells in Muscles

We found that the murine cell lines C2C12 and G7 derived from muscle satellite cells, which are essential for muscle regeneration, express the functional CXCR4 receptor on their surface and that the specific ligand for this receptor, α‐chemokine stromal‐derived factor 1 (SDF‐1), is secreted in muscle tissue. These cell lines responded to SDF‐1 stimulation by chemotaxis, phosphorylation of mitogen‐activated protein kinase (MAPK) p42/44 and AKT serine‐threonine kinase, and calcium flux, confirming the functionality of the CXCR4 receptor. Moreover, supernatants derived from muscle fibroblasts chemoattracted both satellite cells and human CD34+ hematopoietic stem/progenitor cells. In a similar set of experiments, supernatants from bone marrow fibroblasts were found to chemoattract CXCR4+ satellite cells just as they chemoattract CD34+ cells. Moreover, preincubation of both muscle satellite cells and hematopoietic stem/progenitor CD34+ cells before chemotaxis with T140, a specific CXCR4 inhibitor, resulted in a significantly lower chemotaxis to media conditioned by either muscle‐ or bone marrow‐derived fibroblasts. Based on these observations, we postulate that the SDF‐1‐CXCR4 axis is involved in chemoattracting circulating CXCR4+ muscle stem/progenitor and circulating CXCR4+ hematopoietic CD34+ cells to both muscle and bone marrow tissues. Thus, it appears that tissue‐specific stem cells circulating in peripheral blood could compete for SDF‐1+ niches, and this would explain, without invoking the concept of stem cell plasticity, why hematopoietic colonies can be cultured from muscles and early muscle progenitors can be cultured from bone marrow.

Mobilization of bone marrow-derived Oct-4+ SSEA-4+ very small embryonic-like stem cells in patients with acute myocardial infarction.

OBJECTIVES This study sought to assess of the mobilization of nonhematopoietic very small embryonic-like stem cells (VSELs) in acute myocardial infarction (MI). BACKGROUND Acute MI induces mobilization of bone marrow stem cells. Recently, a rare population of VSELs, expressing markers of embryonic pluripotent stem cells (PSCs), was identified in adult murine bone marrow and human umbilical cord blood. METHODS Thirty-one patients with acute MI and 30 healthy subjects were enrolled. Blood was sampled on admission, after 24 h, and 5 days later. Erythrocytes were lysed and lin(-)CD45(-) VSELs were isolated using a live cell sorting system (FACSAria, Beckton Dickinson, San Jose, California). RESULTS In healthy subjects the median number of circulating VSELs was very low (median 0.8 [range 0 to 1.3]) cells/microl. In acute MI, VSELs were mobilized early (median 2.7 [range 0.2 to 3.9] cells/microl; p < 0.001) and remained elevated after 24 h and 5 days (median 4.7 [range 0.2 to 6.4] cells/microl; p < 0.003, and median 2.6 [range 0.3 to 3.6] cells/microl; p < 0.03, respectively). The mobilization of VSEL was significantly reduced in patients older than 50 years and with diabetes in comparison with younger and nondiabetic patients. Circulating VSELs were small (7 to 8 microm) and enriched in the messenger ribonucleic acid of PSC markers (Oct-4, Nanog), cardiac lineage (GATA-4, Nkx2.5/Csx, MEF2C), and endothelial (VE-cadherin) markers. The presence of PSC markers (Oct-4, SSEA-4) and the chemokine receptor CXCR4 in circulating VSELs was confirmed at the protein level by immunofluorescent staining and ImageStream system (Amnis Corporation, Seattle, Washington) analysis. CONCLUSIONS Acute MI induced mobilization of VSELs expressing pluripotent markers, early cardiac and endothelial markers, and chemokine receptor CXCR4.