Membrane-derived vesicles (MV) are released from the surface of activated eucaryotic cells and exert pleiotropic effects on surrounding cells. Since the maintenance of pluripotency and undifferentiated propagation of embryonic stem (ES) cells in vitro requires tight cell to cell contacts and effective intercellular signaling, we hypothesize that MV derived from ES cells (ES-MV) express stem cell-specific molecules that may also support self-renewal and expansion of adult stem cells. To address this hypothesis, we employed expansion of hematopoietic progenitor cells (HPC) as a model. We found that ES-MV (10 μg/ml) isolated from murine ES cells (ES-D3) in serum-free cultures significantly (i) enhanced survival and improved expansion of murine HPC, (ii) upregulated the expression of early pluripotent (Oct-4, Nanog and Rex-1) and early hematopoietic stem cells (Scl, HoxB4 and GATA 2) markers in these cells, and (iii) induced phosphorylation of MAPK p42/44 and serine-threonine kinase AKT. Furthermore, molecular analysis revealed that ES-MV express Wnt-3 protein and are selectively highly enriched in mRNA for several pluripotent transcription factors as compared to parental ES cells. More important, this mRNA could be delivered by ES-MV to target cells and translated into the corresponding proteins. The biological effects of ES-MV were inhibited after heat inactivation or pretreatment with RNAse, indicating a major involvement of protein and mRNA components of ES-MV in the observed phenomena. We postulate that ES-MV may efficiently expand HPC by stimulating them with ES-MV expressed ligands (e.g., Wnt-3) as well as increase their pluripotency after horizontal transfer of ES-derived mRNA.

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.

The alpha-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.

By employing multiparameter sorting, we identified in murine bone marrow (BM) a homogenous population of rare ( ∼ 0.02% of BMMNC) Sca-1⁺lin⁻CD45⁻ cells that express by RQ-PCR and immunohistochemistry markers of pluripotent stem cells (PSC) such as SSEA-1, Oct-4, Nanog and Rex-1. The direct electronmicroscopical analysis revealed that these cells are small ( ∼ 2–4 μm), posses large nuclei surrounded by a narrow rim of cytoplasm, and contain open-type chromatin (euchromatin) that is typical for embryonic stem cells. In vitro cultures these cells are able to differentiate into all three germ-layer lineages. The number of these cells is highest in BM from young ( ∼ 1-month-old) mice and decreases with age. It is also significantly diminished in short living DBA/2J mice as compared to long living B6 animals. These cells in vitro respond strongly to SDF-1, HGF/SF and LIF and express CXCR4, c-met and LIF-R, respectively, and since they adhere to fibroblasts they may be coisolated with BM adherent cells. We hypothesize that this population of Sca-1⁺lin⁻CD45⁻ very small embryonic-like (VSEL) stem cells is deposited early during development in BM and could be a source of pluripotent stem cells for tissue/organ regeneration.