Human brain glial cells synthesize trombospondin
Thrombospondin, a 450-kDa multinodular glycoprotein with lectin-type activity, is found in human platelets, endothelial cells, fibroblasts, smooth muscle cells, monocytes, and granular pneumocytes. Thrombospondin interacts with heparin, fibrinogen, fibronectin, collagen, histidine-rich glycoprotein, and plasminogen. Recently, thrombospondin synthesis by smooth muscle cells has been reported to be augmented by platelet-derived growth factor. We present evidence that thrombospondin is present within and synthesized by astrocytic neuroglial cells. Heparin-Sepharose affinity chromatography of material derived from a human brain homogenate yielded a protein that, when reduced, had an apparent size of 180 kDa and comigrated with reduced platelet thrombospondin on NaDodSO4/PAGE. Immunoblot analysis with monospecific anti-thrombospondin confirmed the presence of immunoreactive thrombospondin. Indirect immunofluorescence of cultured human glial cells indicated the presence of thrombospondin. Metabolic labeling of glial cell cultures with [35S]methionine followed by immunoprecipitation with monospecific anti-thrombospondin revealed synthesis of a 180-kDa polypeptide that comigrated with platelet thrombospondin on NaDodSO4/PAGE. Cultured human glial cells were incubated for 48 hr in serum-free medium with purified platelet-derived growth factor at concentrations up to 50 ng/ml. Aliquots taken at intervals were analyzed by a quantitative double-antibody ELISA. The growth factor stimulated the release of thrombospondin into the culture medium by as much as 10-fold over control cultures. The presence of thrombospondin within glial cells of the central nervous system and the augmentation of its synthesis by platelet-derived growth factor suggest that thrombospondin may play an important role in regulating cell-cell and cell-matrix interactions during periods of cell division and growth.