Platelet-derived growth factor (PDGF) is a polypeptide growth factor first discovered in, and purified from, human blood platelets. As assayed by its ability to stimulate proliferation of cultured vascular smooth muscle cells, PDGF is the major mitogen in human whole blood serum. PDGF has also been reported to be chemotactic for fibroblasts, vascular smooth muscle cells, and leukocytes, and to be able to stimulate contraction of arterial smooth muscle. This, spectrum of activities suggests that PDGF could play a significant role in several vascular processes, including wound repair and the formation of atherosclerotic lesions (reviewed in Ross et al., 1986 Cell 46:155). Several cell types in addition to the platelet have now been shown to be capable of secreting PDGF-like molecules. In culture, vascular endothelial cells from many sources secrete significant levels of PDGF (DiCorleto and Bowen-Pope, 1983 PNAS 80:1919). Rates of secretion can be increased four fold and more bythe activated procoagulants thrombin (Harlan et al 1986 J. Cell Biol. 103:1129) and factor Xa (Gajdusek et al 1986 J. Cell Biol. 103:419). Thrombin stimulates secretion by the earliest times measurable (about 1.5hr) and this early response is not diminished by inhibitors of protein and RNA synthesis. Nevertheless, unlike secretion from the platelet, stimulated secretion does not represent release of sequestered active PDGF since no reservoir of active PDGF can be detected within the cells prior to stimulation. It is likely therefore that stimulation of secrtion involves the activation or unmasking of an inactive form of PDGF. The proteolytic activities of thrombin and Xa are necessary for activation of secretion but the mechanism does not seem to to involve direct proteolytic activation by thrombin of a precursor since thrombin treatment does not generate active PDGF in freeze-thawed preparations of endothelial cells. We have recently found that tumor necrosis factor alpha (TNF) and gamma interferon (IFN) can stimulate increased rates of secretion of PDGF by cultured human saphenous vein and umbilical vein endothelial cells. Stimulation by a combination of the two is more than additive. In contrast to the rapid kinetics of stimulation by thrombin and Xa, TNF and IFN do not measurably increase secretion for at lease four hrs. This delayed kinetics is paralleled by increases in mRNA encoding the two subunit chains of PDGF ("A" and "B") and it seems likely that in this case stimulation of secretion results from increased rates of mRNA and protein synthesis. Since evidence is accumulating that TNF and IFN are both present in human atherosclerotic lesions, it is possible that they help stimulate production of endothelial cell-derived mitogens, including PDGF and thus contribute to the development of the lesion.
