Type I collagen is a molecular target for inhibition of angiogenesis by endogenous thrombospondin-1

Three-dimensional explant cultures of muscle tissue were used to characterize secreted proteins regulated by endogenous levels of the angiogenesis modulator thrombospondin (TSP)-1. Explants from TSP1 null mice exhibit enhanced neovascularization associated with increased endothelial outgrowth but decreased outgrowth of perivascular smooth muscle cells . The absence of endogenous TSP1 did not diminish activation of latent transforming growth factor- β and moderately decreased matrix metalloproteinase levels. However, significant changes in other secreted proteins were observed. Endogenous TSP1 decreased mRNA levels for collagens I α 1, I α 2, and III α 1 and laminin α 4 and increased collagen IV α 1 mRNA expression. Endogenous TSP1 also decreased the level of type I collagen protein produced by the vascular outgrowths. Collagens I α 1, I α 2, and III α 1 are known tumor endothelial markers, suggesting that TSP1 coordinately regulates a set of extracellular matrix genes that reverse the angiogenic switch. Suppression of collagen I α 1 or I α 2 mRNAs using antisense morpholinos inhibited outgrowth in TSP1 null explants and proliferation of TSP1 null endothelial cells, indicating that type I collagen synthesis is limiting for this neovascularization response.