BASIC FIBROBLAST GROWTH-FACTOR MODULATES THE AMINOPHOSPHOLIPID TRANSLOCASE ACTIVITY PRESENT IN THE PLASMA-MEMBRANE OF BOVINE AORTIC ENDOTHELIAL-CELLS

Vascular endothelial cells form the inner nonthrombogenic lining of the large blood vessel. Through back-exchange and fluorescence recovery after photobleaching experiments and using the two fluorescent lipids 1-acyl-2-[6-[N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)amino]hexanoyl]glycerophosphocholine and 1-acyl-2[6-N-(7-nitrobenz-2-oxa- 1,3-diazol-4-yl)amino]hexanoyl]glycerophosphoethanolamine, we have recently shown that an energy-dependent and protein-dependent aminophospholipid translocase activity is present in the plasma membrane of cultured bovine aortic endothelial cells, which specifically transports phosphatidylethanolamine from the outer leaflet toward the inner leaflet of the membrane lipid bilayer. In the present study, using the same approach and 1-acyl-2-[6-[N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)amino]hexanoyl]glycerophosphoserine as the probe, it is shown that this conclusion is also valid for phosphatidylserine. Furthermore, evidence is presented indicating that this aminophospholipid translocase activity can be maintained, suppressed, and restored at will, depending on the conditions of cell incubation. Thus, the translocase activity is detected for cells maintained in their normal culture medium or in a serum-free incubation medium [Dulbecco's modified Eagle's medium (DMEM)] supplemented with the basic fibroblast growth factor, whereas inhibition is observed for cells exposed for at least 2 h to DMEM. The translocase activity is restored when these pretreated cells are further incubated at least for 1 h in the presence of serum or of basic fibroblast growth factor. In view of the importance of basic fibroblast growth factor as a mitogenic and differentiating agent for vascular endothelial cells, various growth factors were tested (acidic fibroblast growth factor, epidermal growth factor, platelet-derived growth factor, transforming growth factors alpha and beta, vascular endothelial growth factor, interferon gamma, tumor-necrosis factor, insulin, and interleukin 4). Only basic fibroblast growth factor was active in the maintenance and restoration of the translocase activity. With respect to the effects of serum, evidence is presented showing that high-density lipoproteins might play a role in the control of the translocase activity. However, the positive effects of basic fibroblast growth factor, serum and high-density lipoproteins on the translocase activity were suppressed when experiments were carried out in the presence of an anti-(basic fibroblast growth factor) IgG, thus indicating that in all cases, basic fibroblast growth factor was directly involved in the modulation of the aminophospholipid translocase activity present in the plasma membrane of bovine aortic endothelial cells.