Editors' ChoiceAngiogenesis

Rosettes Mark the Branch Point

Sci. Signal.  28 Oct 2014:
Vol. 7, Issue 349, pp. ec302
DOI: 10.1126/scisignal.aaa1570

During the process of angiogenesis, endothelial cells must break down the vascular basement membrane. Podosomes are F-actin- and cortactin-rich structures that require integrins for their formation, that can degrade extracellular matrix through matrix metalloproteinases (MMPs), and that are found in endothelial cells as individual structures or in clusters called podosome rosettes. Seano et al. (see also Warren and Iruela-Arispe) investigated the role of these podosome rosettes in angiogenesis. VEGF-A (vascular endothelial growth factor A) stimulation increased (i) the number of podosome rosettes in cultured endothelial cells (when combined with exposure to the phorbol ester PMA) and mouse aortic explants, (ii) the amount of active membrane type 1 (MT1) MMP in endothelial cells, and (iii) the ability of endothelial cells to degrade gelatin. Structures that resembled podosome rosettes and were enriched for F-actin, cortactin, and other podosome markers were detected in tumors from two different mouse models of tumor angiogenesis, mouse hindlimbs subjected to ischemia (an injury that triggers angiogenesis), and human lung tumor biopsies. Although function-blocking antibodies directed against various integrins impaired podosome rosette formation, only those targeting α6β1 integrin completely blocked the formation of podosome rosettes in VEGF-stimulated endothelial cells, the membrane localization of MT1-MMP in podosome rosettes, and the ability of these cells to degrade gelatin. Podosomes with higher amounts of overexpressed α6 integrin had longer lifespans. The number of podosome rosettes formed in response to VEGF-A was decreased in endothelial cells plated on laminin, the ligand for α6β1 integrin, and in aortic explants from mice lacking the α4 laminin subunit, a component of one of the main laminins in vascular basement membrane. Imaging analysis of cells treated with various inhibitors suggested that binding of laminin to α6β1 integrin stabilized the localization of this integrin in focal adhesions rather than podosome rosettes and prevented the trafficking of this integrin from focal adhesions to podosome rosettes. Analysis of mouse aortic rings in an ex vivo assay for angiogenesis indicated that the formation of podosome rosettes preceded the formation of lateral branches, which was reduced in aortic rings from mice with an endothelial cell-specific deficiency of α6 integrin and increased in those from mice lacking Lama4-/-. Treatment with the function-blocking α6β1 integrin antibody or endothelial cell-specific deficiency of α6 integrin reduced the formation of endothelial podosome rosettes in the mouse models of tumor angiogenesis and the hindlimb ischemia model, and also impaired tumor branching in the mouse models of tumor angiogenesis. Thus, blocking α6β1 integrin function to prevent endothelial podosome rosette formation could be used as a supplement to existing anti-angiogenic therapies.

G. Seano, G. Chiaverina, P. A. Gagliardi, L. di Blasio, A. Puliafito, C. Bouvard, R. Sessa, G. Tarone, L. Sorokin, D. Helley, R. K.. Jain, G. Serini, F. Bussolino, L. Primo, Endothelial podosome rosettes regulate vascular branching in tumour angiogenesis. Nat. Cell Biol. 16, 931–941 (2014). [PubMed]

C. M. Warren, M. L. Iruela-Arispe, Podosome rosettes precede vascular sprouts in tumour angiogenesis. Nat. Cell Biol. 16, 928–930 (2014). [PubMed]