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Blood, Vol. 93 No. 8 (April 15), 1999:
pp. 2605-2616
Identification and Characterization of Endothelial Glycoprotein Ib
Using Viper Venom Proteins Modulating Cell Adhesion
Li Tan,
M. Anna Kowalska,
Gabriel M. Romo,
Jose A. Lopez,
Zbigniew Darzynkiewicz, and
Stefan Niewiarowski
From the Department of Physiology, and Sol Sherry Thrombosis Research
Center, Philadelphia, PA; the Division of Hematology and Oncology,
Baylor College of Medicine and Veterans Administration Medical Center,
Houston, TX; and The Cancer Research Institute, New York Medical
College, Valhalla, NY.
The expression and function of a glycoprotein Ib (GPIb) complex on
human umbilical vein endothelial cells (HUVECs) is still a matter of
controversy. We characterized HUVEC GPIb using viper venom proteins:
alboaggregins A and B, echicetin, botrocetin, and echistatin. Echicetin
is an antagonist, and alboaggregins act as agonists of the platelet
GPIb complex. Botrocetin is a venom protein that alters von Willebrand
factor (vWF) conformation and increases its binding affinity for the
GPIb complex. Echistatin is a disintegrin that blocks  v 3.
Echistatin, but not echicetin, inhibited the adhesion to vWF of Chinese
hamster ovary (CHO) cells transfected with v3. We found the
following: (1) Binding of monoclonal antibodies against GPIb to
HUVECs was moderately increased after stimulation with cytokines and
phorbol ester. Echicetin demonstrated an inhibitory effect. (2) Both
echicetin and echistatin, an v3 antagonist, inhibited the
adhesion of HUVECs to immobilized vWF in a dose-dependent manner. The
inhibitory effect was additive when both proteins were used together.
(3) Botrocetin potentiated the adhesion of HUVECs to vWF, and this
effect was completely abolished by echicetin, but not by echistatin.
(4) CHO cells expressing GPIb/IX adhered to vWF (in the presence
of botrocetin) and to alboaggregins; GPIb was required for this
reaction. Echicetin, but not echistatin, inhibited the adhesion of
cells transfected with GPIb/IX to immobilized vWF. (5) HUVECs
adhered strongly to immobilized vWF and alboaggregins with extensive
spreading, which was inhibited by LJ1b1, a monoclonal antibody against
GPIb. The purified v3 receptor did not interact with the
alboaggregins, thereby excluding the contribution of v3 in
inducing HUVEC spreading on alboaggregins. In conclusion, our data
confirm the presence of a functional GPIb complex expressed on HUVECs
in low density. This complex may mediate HUVEC adhesion and spreading
on immobilized vWF and alboaggregins.
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