The molecular biologic study of the expression of thrombospondin in vascular smooth muscle cells and mesangial cells

Thrombospondin (TS) is a high-molecular-weight glycoprotein (MW 450,000) that is stored in alpha-granules of platelets and secreted by a wide variety of mesenchymal cells, including vascular smooth muscle cells (SMCs) and mesangial cells. TS binds to cell surfaces and to matrix macromolecules such as collagen, fibronectin, and heparin (heparan sulfate). We have isolated one of the complementary DNA (cDNA) clones of TS from human endothelial cell cDNA libraries. With the TS cDNA as a probe, we used Northern blot analysis to look at TS messenger RNA (mRNA) levels of rat aortic SMCs in a quiescent state and cultured under different stimuli. Treatment with platelet-derived growth factor (PDGF) caused a rapid increase of TS mRNA at 2 to 4 hours. This induction was enhanced by the addition of cycloheximide, suggesting that the induction of TS by PDGF does not require the synthesis of new protein species. Transforming growth factor beta (TGF-β) also induced mRNA of TS at 8 hours after stimulation and the induction was blocked by cycloheximide, suggesting that TGF-β requires the mediation of new protein synthesis to induce a TS gene. In measangial cells, we observed the same type of gene expression of TS using an in-situ hybridization study. We conclude that during SMC and mesangial cell proliferation, the induction of TS mRNA is regulated in a specific manner by PDGF and TGF-β.