Endothelial cell injury in vitro is associated with increased secretion of an Mr 43,000 glycoprotein ligand

Abstract

A novel, serum albumin-binding glycoprotein of molecular weight (mw) 43,000 (43K protein) was initially purified from the culture medium of bovine aortic endothelial (BAE) cells (Sage, H., Johnson, C., and Bornstein, P., J. Biol. Chem. 259:3993-4007, 1984). Its secretion by normal mesenchymal cells and by transformed cells of both ectodermal and endodermal origin suggested a general role in cellular function. To examine the effect of sublethal injury in vitro on the biosynthesis of 43K protein, BAE cells were exposed to endotoxin. At concentrations which produced minimal cell detachment and lysis, the cells secreted 70-100% more protein compared to control cultures, and the relative increase in 43K protein over total protein was approximately three-fold. A second type of cellular injury, manifested by rapid cellular proliferation and migration in response to sparse plating density (a condition that we have termed 'culture shock'), was also accompanied by a significant increase in the secretion of 43K protein. Pulse-chase studies revealed that the initial product secreted within 1.5 h was of Mr 38,000, and that between 6 and 21 h this molecule was converted to the final form of Mr 43,000. The 43K protein was not associated with RNA or glycosaminoglycan, but appeared to be linked to complex oligosaccharides containing peripheral sialosyl residues. Treatment with tunicamycin produced lower mw forms that displayed reduced affinity for albumin. By immunologic criteria, peptide mapping, and amino acid analysis, the 43K protein was shown to be structurally distinct from several proteins of Mr 40,000-50,000 associated with endothelium or with serum, including tissue factor, a plasminogen anti-activator, and several apolipoproteins. In addition, the 43K protein was not present in the extracellular matrices of endothelial, fibroblastic, or smooth muscle cells, nor was it found in plasma, serum, platelet releasate, or alveolar lavage fluids. These studies identify a unique Mr 43,000 glycoprotein that is associated with cellular stress or injury in vitro. As a secreted but nonmatrix macromolecule, this protein may be part of a 'survival kit' used by the endothelium to cope with cellular injury.