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EI Peerschke
Department of Pathology, SUNY, Stony Brook.
Platelets contain a pool of endogenous adhesive proteins that can be
released and may bind to surface membrane receptors under appropriate
conditions. Because the binding of exogenous fibrinogen to platelets was
shown previously to be accompanied by a time-dependent decrease in
fibrinogen accessibility to antibody and enzymes, studies were performed to
evaluate changes in the expression of endogenous fibrinogen released from
thrombin-stimulated platelets using monospecific polyclonal and monoclonal
antibody F(ab')2 fragments. Parallel studies were performed to compare the
expression of released fibronectin and von Willebrand factor (vWF). Binding
of polyclonal antibody F(ab')2 fragments directed against individual
adhesive proteins was inhibited by EDTA or the 10E5 monoclonal antibody,
suggesting that fibrinogen, fibronectin, and vWF expression was mediated,
in large part, by divalent cation-dependent interactions with the
glycoprotein IIb-IIIa complex. Interestingly, when polyclonal antibody
F(ab')2 fragments were added to platelet suspensions at discrete times
after thrombin stimulation, antifibrinogen F(ab')2 binding decreased by 72%
+/- 15% (mean +/- SD, n = 22) over a 60-minute time course, whereas
antifibronectin and anti-vWF antibody F(ab')2 fragment binding changed
minimally (6% +/- 23%, n = 22 and 3% +/- 26%, n = 14, respectively).
Similar observations were made with monoclonal antibodies. Parallel
experiments using 125I-labeled fibrinogen as a marker indicated that the
observed decrease in antifibrinogen F(ab')2 binding was not accompanied by
fibrinogen dissociation. Moreover, antibody accessibility to platelet-bound
fibrinogen could be restored after Triton X-100 platelet lysis. The data
suggest that fibrinogen, fibronectin, and vWF are not coordinately
expressed on thrombin- stimulated platelets. Rather, fibrinogen expression
appears transient compared with the expression of fibronectin and vWF. The
ability of platelets to secrete and organize adhesive proteins on their
surface is likely to have important implications for hemostasis and
thrombosis.
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