Notch signaling regulates tumor-induced angiogenesis in SPARC-overexpressed neuroblastoma

Despite existing aggressive treatment modalities, the prognosis for advanced stage neuroblastoma remains poor with significant long-term illness in disease survivors. Advance stage disease features are associated with tumor vascularity, and as such, angiogenesis inhibitors may prove useful along with current therapies. The matricellular protein, secreted protein acidic and rich in cysteine (SPARC), is known to inhibit proliferation and migration of endothelial cells stimulated by growth factors. Here, we sought to determine the effect of SPARC on neuroblastoma tumor cell-induced angiogenesis and to decipher the molecular mechanisms involved in angiogenesis inhibition. Conditioned medium from SPARC-overexpressed neuroblastoma cells (pSPARC-CM) inhibited endothelial tube formation, cell proliferation, induced programmed cell death and suppressed expression of pro-angiogenic molecules such as VEGF, FGF, PDGF, and MMP-9 in endothelial cells. Further analyses revealed that pSPARC-CM-suppressed expression of growth factors was mediated by inhibition of the Notch signaling pathway, and cells cultured on conditioned medium from tumor cells that overexpress both Notch intracellular domain (NICD-CM) and SPARC resumed the pSPARC-CM-suppressed capillary tube formation and growth factor expression in vitro. Further, SPARC overexpression in neuroblastoma cells inhibited neo-vascularization in vivo in a mouse dorsal air sac model. Furthermore, SPARC overexpression-induced endothelial cell death was observed by co-localization studies with TUNEL assay and an endothelial marker, CD31, in xenograft tumor sections from SPARC-overexpressed mice. Our data collectively suggest that SPARC overexpression induces endothelial cell apoptosis and inhibits angiogenesis both in vitro and in vivo.