Secretion of SPARC by endothelial cells transformed by polyoma middle T oncogene inhibits the growth of normal endothelial cells in vitro.

Endothelioma cells expressing the polyoma virus middle T oncogene induced hemangiomas in mice by the recruitment of nonproliferating endothelial cells from host blood vessels (Williams et al. 1989). I now report that SPARC, a Ca(2+)-binding glycoprotein that perturbs cell-matrix interactions and inhibits the endothelial cell cycle, is produced by endothelioma cells and is in part responsible for the alterations in the morphology and growth that occur when nontransformed bovine aortic endothelial cells are cocultured with endothelioma cells. Normal endothelial cells cocultured with two different middle T-positive endothelial cell lines, termed End cells, exhibited changes in shape that were accompanied by the formation of cell clusters. Media conditioned by End cells repressed proliferation of normal endothelial cells, but enhanced that of an established line of murine capillary endothelium. Radiolabeling studies revealed no apparent differences in the profile of proteins secreted by aortic or capillary cells cultured in End cell conditioned media. Characterization of proteins produced by End cells led to the identification of type IV collagen, laminin, entactin, and SPARC as major secreted products. Although SPARC did not affect the morphology of End or capillary cells, it was associated with overt changes in the shape of aortic endothelial cells. Moreover, SPARC and a synthetic peptide from SPARC domain II inhibited the incorporation of [3H]thymidine by aortic cells, but had minimal to no effect on the capillary endothelial cell line. The inhibition of growth exhibited by aortic endothelial cells cultured in End cell conditioned media could be partially reversed by antibodies specific for SPARC and SPARC peptides. These studies indicate a potential role for SPARC in the generation of hemangiomas by End cells in vivo, a process that requires normal (host) endothelial cells to disengage from the extracellular matrix, withdraw from the cell cycle, migrate, and reassociate into the disorganized cellular networks that comprise cavernous and capillary hemangiomas.