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J. Biol. Chem., Vol. 278, Issue 19, 17438-17447, May 9, 2003
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From the Departments of Previous studies have
demonstrated that overexpression of GRP78/BiP, an endoplasmic
reticulum (ER)-resident molecular chaperone, in mammalian cells
inhibits the secretion of specific coagulation factors. However, the
effects of GRP78/BiP on activation of the coagulation cascade
leading to thrombin generation are not known. In this study, we
examined whether GRP78/BiP overexpression mediates cell surface
thrombin generation in a human bladder cancer cell line T24/83 having
prothrombotic characteristics. We report here that cells overexpressing
GRP78/BiP exhibited significant decreases in cell surface-mediated
thrombin generation, prothrombin consumption and the formation of
thrombin-inhibitor complexes, compared with wild-type or
vector-transfected cells. This effect was attributed to the ability of
GRP78/BiP to inhibit cell surface tissue factor (TF) procoagulant
activity (PCA) because conversion of factor X to Xa and factor VII to
VIIa were significantly lower on the surface of
GRP78/BiP-overexpressing cells. The additional findings that (i) cell
surface factor Xa generation was inhibited in the absence of factor
VIIa and (ii) TF PCA was inhibited by a neutralizing antibody to human
TF suggests that thrombin generation is mediated exclusively by TF.
GRP78/BiP overexpression did not decrease cell surface levels of TF,
suggesting that the inhibition in TF PCA does not result from retention
of TF in the ER by GRP78/BiP. The additional observations that both
adenovirus-mediated and stable GRP78/BiP overexpression attenuated TF
PCA stimulated by ionomycin or hydrogen peroxide suggest that GRP78/BiP
indirectly alters TF PCA through a mechanism involving cellular
Ca2+ and/or oxidative stress. Similar results were
also observed in human aortic smooth muscle cells transfected with the
GRP78/BiP adenovirus. Taken together, these findings demonstrate
that overexpression of GRP78/BiP decreases thrombin generation by
inhibiting cell surface TF PCA, thereby suppressing the prothrombotic
potential of cells.
Pathology and Molecular
Medicine, ¶ Pediatrics, and
Medicine, McMaster University
and the Henderson Research Centre, Hamilton, Ontario L8V 1C3 and the
** Division of Experimental Therapeutics, Ontario Cancer
Institute, Toronto, Ontario M5G 2M9, Canada

A Career Investigator of the Heart and Stroke Foundation of
Ontario (HSFO). To whom correspondence should be addressed: Henderson Research Centre, 711 Concession St., Hamilton, Ontario L8V 1C3, Canada.
Tel.: 905-527-2299 (ext. 42628); Fax: 905-575-2646; E-mail: raustin@thrombosis.hhscr.org.
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