Expression of the sis gene by endothelial cells in culture and in vivo.

Recognition that the sis gene codes for a protein homologous with at least one of the two chains of platelet-derived growth factor has made it possible to directly assess transcriptional expression of platelet-derived growth factor both in cultured cells and in tissue obtained in vivo. We have found that a 3.7-kilobase RNA homologous to the sis gene is expressed at moderate levels in cultured human and bovine endothelial cells, at low levels in in vivo endothelium from human umbilical vein, and at very low levels in bovine aortic endothelium in vivo. This RNA migrates at the same rate as the previously reported sis band in the HUT 102 human T-cell lymphoma line. This band is not found in RNA extracted from freshly obtained bovine aortic media or from human foreskin fibroblasts or cultured fetal human aortic smooth muscle cells. Our in vitro results suggest that the sis gene is responsible for at least part of the platelet-derived growth factor-like mitogenic activity secreted by cultured endothelial cells and indicate that the sis gene is readily activated in endothelial cells during the transition from in vivo conditions to in vitro growth as a monolayer on plastic. Expression of the sis gene by endothelium in vivo raises the possibility that platelet-derived growth factor has a role in the development of the vascular system in the young animal and in the maintenance of the normal vascular system in the adult.