Beate Heissig

Impaired recruitment of bone-marrow-derived endothelial and hematopoietic precursor cells blocks tumor angiogenesis and growth.

The role of bone marrow (BM)-derived precursor cells in tumor angiogenesis is not known. We demonstrate here that tumor angiogenesis is associated with recruitment of hematopoietic and circulating endothelial precursor cells (CEPs). We used the angiogenic defective, tumor resistant Id-mutant mice to show that transplantation of wild-type BM or vascular endothelial growth factor (VEGF)-mobilized stem cells restore tumor angiogenesis and growth. We detected donor-derived CEPs throughout the neovessels of tumors and Matrigel-plugs in an Id1+/−Id3−/− host, which were associated with VEGF-receptor-1–positive (VEGFR1+) myeloid cells. The angiogenic defect in Id-mutant mice was due to impaired VEGF-driven mobilization of VEGFR2+ CEPs and impaired proliferation and incorporation of VEGFR1+ cells. Although targeting of either VEGFR1 or VEGFR2 alone partially blocks the growth of tumors, inhibition of both VEGFR1 and VEGFR2 was necessary to completely ablate tumor growth. These data demonstrate that recruitment of VEGF-responsive BM-derived precursors is necessary and sufficient for tumor angiogenesis and suggest new clinical strategies to block tumor growth.

Chemokine-mediated interaction of hematopoietic progenitors with the bone marrow vascular niche is required for thrombopoiesis

The molecular pathways involved in the differentiation of hematopoietic progenitors are unknown. Here we report that chemokine-mediated interactions of megakaryocyte progenitors with sinusoidal bone marrow endothelial cells (BMECs) promote thrombopoietin (TPO)-independent platelet production. Megakaryocyte-active cytokines, including interleukin-6 (IL-6) and IL-11, did not induce platelet production in thrombocytopenic, TPO-deficient (Thpo−/−) or TPO receptor–deficient (Mpl−/−) mice. In contrast, megakaryocyte-active chemokines, including stromal-derived factor-1 (SDF-1) and fibroblast growth factor-4 (FGF-4), restored thrombopoiesis in Thpo−/− and Mpl−/− mice. FGF-4 and SDF-1 enhanced vascular cell adhesion molecule-1 (VCAM-1)- and very late antigen-4 (VLA-4)-mediated localization of CXCR4+ megakaryocyte progenitors to the vascular niche, promoting survival, maturation and platelet release. Disruption of the vascular niche or interference with megakaryocyte motility inhibited thrombopoiesis under physiological conditions and after myelosuppression. SDF-1 and FGF-4 diminished thrombocytopenia after myelosuppression. These data suggest that TPO supports progenitor cell expansion, whereas chemokine-mediated interaction of progenitors with the bone marrow vascular niche allows the progenitors to relocate to a microenvironment that is permissive and instructive for megakaryocyte maturation and thrombopoiesis. Progenitor-active chemokines offer a new strategy to restore hematopoiesis in a clinical setting.

Placental growth factor reconstitutes hematopoiesis by recruiting VEGFR1 + stem cells from bone-marrow microenvironment

The mechanism by which angiogenic factors recruit bone marrow (BM)-derived quiescent endothelial and hematopoietic stem cells (HSCs) is not known. Here, we report that functional vascular endothelial growth factor receptor-1 (VEGFR1) is expressed on human CD34+ and mouse Lin−Sca-1+c-Kit+ BM-repopulating stem cells, conveying signals for recruitment of HSCs and reconstitution of hematopoiesis. Inhibition of VEGFR1, but not VEGFR2, blocked HSC cell cycling, differentiation and hematopoietic recovery after BM suppression, resulting in the demise of the treated mice. Placental growth factor (PlGF), which signals through VEGFR1, restored early and late phases of hematopoiesis following BM suppression. PlGF enhanced early phases of BM recovery directly through rapid chemotaxis of VEGFR1+ BM-repopulating and progenitor cells. The late phase of hematopoietic recovery was driven by PlGF-induced upregulation of matrix metalloproteinase-9, mediating the release of soluble Kit ligand. Thus, PlGF promotes recruitment of VEGFR1+ HSCs from a quiescent to a proliferative BM microenvironment, favoring differentiation, mobilization and reconstitution of hematopoiesis.

Vascular and haematopoietic stem cells: novel targets for anti-angiogenesis therapy?

Tumours recruit neighbouring blood vessels and vascular endothelial cells to support their own blood supply. Recent evidence has indicated, however, that tumours are also capable of mobilizing bone-marrow-derived endothelial precursor cells, inducing them to migrate to the tumour and become incorporated into the developing vasculature. Tumour-derived angiogenic factors promote the recruitment of these cells, which include circulating endothelial progenitor cells and haematopoietic stem and progenitor cells. As clinical trials with anti-angiogenic agents have been confronted with therapeutic hurdles, inhibiting the recruitment of these vascular precursors might provide a novel approach to blocking tumour angiogenesis.

Cytokine-mediated deployment of SDF-1 induces revascularization through recruitment of CXCR4+ hemangiocytes

The mechanisms through which hematopoietic cytokines accelerate revascularization are unknown. Here, we show that the magnitude of cytokine-mediated release of SDF-1 from platelets and the recruitment of nonendothelial CXCR4+VEGFR1+ hematopoietic progenitors, hemangiocytes, constitute the major determinant of revascularization. Soluble Kit-ligand (sKitL), thrombopoietin (TPO, encoded by Thpo) and, to a lesser extent, erythropoietin (EPO) and granulocyte-macrophage colony-stimulating factor (GM-CSF) induced the release of SDF-1 from platelets, enhancing neovascularization through mobilization of CXCR4+VEGFR1+ hemangiocytes. Although revascularization of ischemic hindlimbs was partially diminished in mice deficient in both GM-CSF and G-CSF (Csf2−/−Csf3−/−), profound impairment in neovascularization was detected in sKitL-deficient Mmp9−/− as well as thrombocytopenic Thpo−/− and TPO receptor–deficient (Mpl−/−) mice. SDF-1–mediated mobilization and incorporation of hemangiocytes into ischemic limbs were impaired in Thpo−/−, Mpl−/− and Mmp9−/− mice. Transplantation of CXCR4+VEGFR1+ hemangiocytes into Mmp9−/− mice restored revascularization, whereas inhibition of CXCR4 abrogated cytokine- and VEGF-A–mediated mobilization of CXCR4+VEGFR1+ cells and suppressed angiogenesis. In conclusion, hematopoietic cytokines, through graded deployment of SDF-1 from platelets, support mobilization and recruitment of CXCR4+VEGFR1+ hemangiocytes, whereas VEGFR1 is essential for their angiogenic competency for augmenting revascularization. Delivery of SDF-1 may be effective in restoring angiogenesis in individuals with vasculopathies.

Molecular pathways regulating mobilization of marrow-derived stem cells for tissue revascularization

Adult bone marrow is a rich reservoir of hematopoietic and vascular stem and progenitor cells. Mobilization and recruitment of these cells are essential for tissue revascularization. Physiological stress, secondary to tissue injury or tumor growth, results in the release of angiogenic factors, including vascular endothelial growth factor (VEGF), which promotes mobilization of stem cells to the circulation, contributing to the formation of functional vasculature. VEGF interacts with its receptors, VEGFR2 and VEGFR1, expressed on endothelial and hematopoietic stem cells, and thereby promotes recruitment of these cells to neo-angiogenic sites, accelerating the revascularization process. The mobilization of stem cells from marrow is a dynamic process, regulated by shear stress imparted by blood flow, and the activation of metalloproteinases that induce the release of Kit ligand, facilitating egress from the marrow to the circulation. Identification of the molecular pathways that support the proliferation and differentiation of vascular stem and progenitor cells will open up new avenues for the design of clinical trials to accelerate tissue vascularization and organogenesis.

Angiogenic Factors Reconstitute Hematopoiesis by Recruiting Stem Cells from Bone Marrow Microenvironment

Abstract: The mechanism by which angiogenic factors recruit bone marrow (BM)‐derived quiescent endothelial and hematopoietic stem cells (HSCs) is not known. Here, we report that functional vascular endothelial growth factor receptor‐1 (VEGFR1, Flt‐1) is expressed on a subpopulation of human CD34+ and mouse Lin−Sca‐1+c‐Kit+ BM‐repopulating stem cells, conveying signals for recruitment of HSCs and reconstitution of hematopoiesis. Inhibition of VEGFR1 signaling, but not VEGFR2 (Flk‐1, KDR), blocked HSC cell cycling, differentiation and hematopoietic recovery after BM suppression, resulting in the demise of the treated mice. Plasma elevation of placental growth factor (PlGF), which signals through VEGFR1, but not VEGFR2, restored hematopoiesis during the early and late phases following BM suppression. The mechanism whereby PlGF enhanced early phases of BM recovery was mediated directly through rapid chemotaxis of readily available VEGFR1+ BM‐repopulating and progenitor cells. The late phase of hematopoietic recovery was driven by PlGF‐induced upregulation of matrix metalloproteinase‐9 (MMP‐9) in the BM, mediating the release of soluble Kit‐ligand (sKitL). sKitL increased proliferation and motility of HSCs and progenitor cells, thereby augmenting hematopoietic recovery. PlGF promotes recruitment of VEGFR1+ HSCs from a quiescent to a proliferative microenvironment within the BM, favoring differentiation, mobilization, and reconstitution of hematopoiesis.

The regulation of hematopoietic stem cell and progenitor mobilization by chemokine SDF-1

The chemokine, stromal derived factor-1 (SDF1) has been shown to modulate the homing of stem cells to its site by mediating chemotaxis. We demontrated overexpression of SDF-1 induced mobilization of cells with stem cell potential (CFU-S, long-term reconstituting cells). Moreover, SDF-1 activated matrix metalloproteinase-9 induced in bone marrow (BM) cells, causing the release soluble Kit-ligand and permitting the transfer of hematopoietic stem cells (HSCs) from a quiescent to a proliferative niche. The role of SDF-1 in the process of HSC migration and the mechanism underlying this HSCs migration will be reviewed. These studies lay the foundation for using SDF-1 to induce mobilization of progenitor cells in a BM transplantation setting.

Matrix metalloproteinase-9 regulates TNF-α and FasL expression in neuronal, glial cells and its absence extends life in a transgenic mouse model of amyotrophic lateral sclerosis

Abstract Whether increased levels of matrix metalloproteinases (MMPs) correspond to a role in the pathogenesis of amyotrophic lateral sclerosis (ALS) needs to be determined and it is actively being pursued. Here we present evidence suggesting that MMP-9 contributes to the motor neuron cell death in ALS. We examined the role of MMP-9 in a mouse model of familial ALS and found that lack of MMP-9 increased survival (31%) in G93A SOD1 mice. Also, MMP-9 deficiency in G93A mice significantly attenuated neuronal loss, and reduced neuronal TNF-α and FasL immunoreactivities in the lumbar spinal cord. These findings suggest that MMP-9 is an important player in the pathogenesis of ALS. Our data suggest that the mechanism for MMP-9 neurotoxicity in ALS may be by upregulating neuronal TNF-α and FasL expression and activation. This study provides new mechanism and suggests that MMP inhibitors may offer a new therapeutic strategy for ALS.