Remodeling of vascular microenvironments during normal and tumor-induced angiogenesis is an important, yet poorly understood mechanism by which endothelial cells (ECs) contribute to the activation or resolution of angiogenesis. We used microarray analyses to monitor changes in the transcriptome of ECs undergoing angiogenesis when cultured onto Matrigel matrices. This strategy identified 308 genes whose expression in ECs was altered at least 3-fold by angiogenesis, of which 63 genes were found to encode for secretory proteins. In vitro assays that modeled key steps in the angiogenic process showed that several identified genes possessed pro- or anti-angiogenic activities (e.g., SMOC-2, secreted modular calcium-binding protein-2; CRELD-2, cysteine-rich with EGF-like domains-1; MAGP-2, microfibril-associated glycoprotein-2; lumican; and ECM-1, extracellular matrix protein-1). In particular, MAGP-2 expression potentiated EC proliferation and p38 MAPK activation stimulated by the pro-angiogenic factors, basic fibroblast growth factor (bFGF), epidermal growth factor (EGF), and vascular endothelial growth factor (VEGF); it also stimulated EC invasion and angiogenic sprouting, and more importantly, promoted the development and infiltration of vessels into Matrigel plugs implanted into genetically normal mice. Conversely, lumican inhibited EC activation of p38 MAPK, as well as their invasion, angiogenic sprouting, and vessel formation in mice. Collectively, our findings provide new insights into how EC stromal remodeling regulates angiogenesis activation and resolution, as well as identify two novel EC-secreted stromal proteins that modulate angiogenesis both in vitro and in vivo.

Angiogenesis is highly sensitive to the composition of the vascular microenvironment, however, our understanding of the structural and matricellular components of the vascular microenvironment that regulate angiogenesis and the molecular mechanisms by which these molecules function remains incomplete. Our previous results described a novel pro-angiogenic activity for Microfibril-Associated Glycoprotein-2 (MAGP-2), but did not address the molecular mechanism(s) by which this is accomplished. We now demonstrate that MAGP-2 promotes angiogenic cell sprouting by antagonizing Notch signaling pathways in endothelial cells. MAGP-2 decreased basal and Jagged1 induced expression from the Notch sensitive Hes-1 promoter in ECs, and blocked Jagged1 stimulated Notch1 receptor processing in transiently transfected 293T cells. Interestingly, inhibition of Notch signaling by MAGP-2 seems to be restricted to ECs since MAGP-2 increased Hes-1 promoter activity and Notch1 receptor processing in heterologous cell types. Importantly, constitutive activation of the Notch signaling pathway blocked the ability of MAGP-2 to promote angiogenic cell sprouting, as well as morphological changes associated with angiogenesis. Collectively, these observations indicate that MAGP-2 promotes angiogenic cell spouting in vitro by antagonizing Notch signaling pathways in ECs.