Bone marrow–derived endothelial progenitor cells (EPCs) circulate to nascent blood vessels during normal physiological and tumor vasculogenesis. Vascular endothelial growth factor (VEGF) and placental growth factor (PlGF) attract EPCs and initiate differentiation by activation of receptor tyrosine kinase signaling through the VEGF receptor 1 [VEGFR1, also known as fms-related tyrosine kinase 1 (Flt1)]. Flt1 transcripts are alternatively spliced to produce the transmembrane signaling competent receptor (mFlt1) and a soluble extracellular portion (sFlt1). Qi et al. showed that the secreted cytokine tumor necrosis factor superfamily 15 (TNFSF15) inhibited VEGF-Flt1 signaling in EPCs. Using freshly isolated EPCs in an in vivo Matrigel plug assay, the authors found that mice injected with recombinant TNFSF15 showed reduced neovascularization in response to VEGF. Moreover, TNFSF15-exposed EPCs isolated from Matrigel plugs or grown in culture had reduced abundance of mFlt1 and increased abundance of sFlt1. Because sFlt1 consists of only the extracellular domain, it does not participate in VEGF signal transduction and functions as a decoy receptor. TNFSF15 triggered the ubiquitin- and proteasome-dependent degradation of mFlt1. The degradation of mFlt1 was associated with dephosphorylation of Akt but required PI3K activity. Direct pharmacological inhibition of Akt did not affect the TNFSF15-stimulated increase in sFlt1 mRNA. In contrast, drugs that inhibit protein kinase C or mitogen-activated protein kinase kinase (MEK) prevented the TNFSF15-mediated increase in sFlt1 and the TNFSF15-mediated reduction in the abundance of Jumonji domain–containing protein 6, a histone arginine demethylase that suppresses splicing of sFlt1. TNFSF15 inhibited VEGF- or PlGF-mediated signaling in EPCs, including the ability of VEGF to induce endothelial nitric oxide synthase and the ability of VEGF or PlGF to stimulate phosphorylation of p38 mitogen-activated protein kinase. Thus, TNFSF15 shifts EPCs from responding to provasculogenic stimuli to producing an inhibitor of these stimuli, which may suggest new avenues for VEGFR-targeted therapies.
J.-W. Qi, T.-T. Qin, L.-X. Xu, K. Zhang, G.-L. Yang, J. Li, H.-Y. Xiao, Z.-S. Zhang, L.-Y. Li, TNFSF15 inhibits vasculogenesis by regulating relative levels of membrane-bound and soluble isoforms of VEGF receptor 1. Proc. Natl. Acad. Sci. U.S.A. 110, 13863–13868 (2013). [Abstract] [Full]