The Role of SPARC in Neuroblastoma Angiogenesis
The pediatric cancer neuroblastoma (NB) is biologically and clinically heterogeneous. NB tumors contain both neuroblastic/ganglionic cells and Schwannian stroma. The presence of abundant Schwannian stroma favorably impacts prognosis and is inversely correlated with tumor vascularity. Schwann cells as well as differentiated NB tumor cells produce a spectrum of angiogenesis inhibitors including tissue inhibitor of metalloproteinase-2 (TIMP-2) and pigment epithelium-derived factor (PEDF). Chromatography studies have recently led to the identification of another Schwann cell-derived secreted inhibitor of angiogenesis, Secreted Protein Acidic and Rich in Cysteine (SPARC). In vitro and in vivo studies show that SPARC potently inhibits angiogenesis, and that SPARC induces endothelial cell apoptosis. We hypothesize that the low level of vascularity and more benign clinical behavior of NB tumors with abundant Schwannian stroma results from the Schwann cell production of a spectrum of angiogenesis inhibitors, and that SPARC is a key contributor to the anti-angiogenic activity of factors secreted by the Schwann cells. In the current proposal, the role SPARC plays in the regulation of NB angiogenesis will be investigated. The first specific aim is to examine the effects of SPARC on NB tumor vascularity and growth in vivo. We plan to expand and extend our preliminary preclinical studies that suggest that SPARC inhibits NB angiogenesis. SPARC will be administered to animals with subcutaneous NB xenografts as well as an orthotopic NB nude mouse model using ALZET osmotic pumps. Additional experiments will be performed testing the effects of exogenously expressed SPARC on tumor growth. The second specific aim is to produce and characterize recombinant SPARC, and SPARC domains and peptides that confer angiogenesis inhibitory activity. Recombinant proteins and peptides found to have anti-angiogenesis activity will be further evaluated in the preclinical models. The third specific aim of this grant is to delineate the mechanisms by which SPARC induces apoptosis and analyze its effects on endothelial cell function. The studies outlined in this grant will lead to a further understanding of the complex regulation of angiogenesis in NB, and provide insight into the role that SPARC plays in determining NB phenotype. The long-term goal of this research grant is to generate potent anti-angiogenic SPARC-derived molecules that will prove to be effective in the treatment of children with highly vascular, clinically aggressive NB.