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We have previously demonstrated that endothelial cells synthesize a plasma membrane protein indistinguishable from platelet glycoprotein (GP) lla. The present study provides evidence for a further analogy between the platelet and the endothelial cell membrane by showing that cultured endothelial cells also synthesize a membrane protein complex immunologically related to the platelet GP llb/GP IIIa complex. This evidence is based on the following observations: (1) Cl7, a murine monoclonal antiplatelet GP IIIa antibody, consistently precipitates two proteins, apparent molecular weights, respectively, 115,000 and 125,000 reduced and 95,000 and 135,000 nonreduced, from metabolically (36S-methionine) as well as surface 125l-labeled cultured human endothelial cells; (2) upon crossed Immunoelectrophoresis of solubilized endothelial cells against a polyclonal rabbit antiplatelet antiserum and 125l-labeled C17 IgG, a single precipitate of the protein(s) recognized by Cl7 is observed. As judged by their mobility in 9% polyacrylamide gels, both endothelial proteins appear to have a somewhat larger molecular weight than their platelet counterparts. Patterns obtained by crossed immunoelectrophoresis are also indicative of a difference in electrophoretic behavior of the platelet GP IIb/IIIa complex and the endothelial cell protein complex.

Concentrations of plasma fibrinopeptide A (FPA) were measured by radioimmunoassay in 50 patients with venous thromboembolism or disseminated intravascular coagulation or both. A consistent discrepancy was observed in values obtained with two anti-FPA antisera. Analysis of extracts from plasma of these patients by high-perfor-mance liquid chromatography (HPLC) revealed the presence of a phosphorylated and an unphosphorylated form of the A peptide. Differences in concentrations of FPA measured with the two antisera could be accounted for by their different reactivity with phosphorylated FPA (FPA-P). The differences were abolished by treatment with alkaline phosphatase. A good correlation was observed between the FPA-P content of free A-peptide material and of fibrinogen in plasma as determined by HPLC (r= .88, P< .001, n = 11). In patients with elevated FPA levels, the mean FPA-P content of fibrinogen was significantly higher (P< .002, n = 13) than in patients with normal FPA levels (n = 8) and in healthy controls (n = 14). Phosphorus in fibrinogen did not correlate with fibrinogen degradation products or fibrinogen levels and became normal on adequate anticoagulation. Therefore, blood-clotting activation may lead to a high phosphate content of fibrinogen and of free FPA in plasma.

Glanzmann's thrombasthenia is a bleeding disorder, inherited in an autosomal recessive way and characterized by an absence or deficiency of the platelet glycoprotein (GP) Ilb/IIIa complex. Recently, we and others demonstrated that cultured human umbilical vein endothelial cells synthesize a membrane protein complex similar to the platelet GP Ilb/IIIa complex. In this article, we demonstrate that endo- thelial cells isolated from the umbilical vein of a newborn with Glanzmann's thrombasthenia, as compared with normal endothelial cells, show no difference in their ability to synthesize and express this GP Ilb/IIIa complex. Our results indicate that Glanzmann's thrombasthenia is not accompanied by an “endotheliopathy.” © 1987 by Grune & Stratton, Inc.

Endothelial cells synthesize a heterodimeric adhesion molecule, the vitronectin receptor (VnR), which is similar to the platelet glycoprotein (GP)llb/llla complex. The subunits of the endothelial VnR (VnRa and GPIIIa) have been studied for their ability to express alloantigens associated with platelet GPIIb and Ilia. We previously showed that endothelial GPIIIa can express the platelet alloantigen Zwaor PIA1, which is associated with GPIIIa. We studied the relationship between the expression of Zw* on platelets and endothelial cells in neonates (n = 13). Using immunopre-cipitation and immunofluorescence techniques, we showed that the Zwaantigen is either expressed or absent from both platelets and endothelial cells of the same individual. This finding indicates that in both cell types the same gene is expressed. We also showed that Zwa-negative endothelial cells express Zwb(PIA2), in analogy to Zwa-negative platelets. Moreover, our results strongly suggest expression on endothelial cells of Yukb, a recently described platelet alloantigen, also located on GPIIIa. However, we could not demonstrate expression on the endothelial VnRa subunit of Bakaan alloantigen located on platelet GPIIb. These findings are in agreement with the concept that the endothelial GPIIIa subunit is more closely related to its platelet counterpart than to the endothelial VnRa subunit.