| Atherosclerotic plaque rupture and subsequent thrombosis are the key events in acute coronary syndromes. The thrombogenicity of atherosclerotic plaques has been attributed to the initiator of coagulation, tissue factor (TF), and specifically to TF procoagulant activity (PCA). Although TF is a critical component of hemostatic and thrombotic responses, the cellular factors that regulate its PCA are relatively unknown. I have demonstrated that TF PCA is regulated by the 78 kDa glucose-regulated protein (GRP78), an endoplasmic reticulum (ER)-resident chaperone. Overexpression of GRP78 inhibits TF PCA on the surface of cell types with prothrombotic properties, including T24/83 cells (human bladder carcinoma cells) and human aortic smooth muscle cells (HASMC). Overexpression of GRP78 inhibits TF PCA independent of a decrease in cell surface TF levels or formation of a stable intracellular complex between GRP78 and TF. An indirect mechanism involving calcium (Ca2+), reactive oxygen species (ROS) and/or apoptotic cell death is suggested by findings that GRP78 attenuates the induction of TF PCA by ionomycin, hydrogen peroxide or apoptotic agents. Overexpression of GRP78 attenuates apoptosis- and Ca2+ ionophore-induced TF PCA by blocking caspase activation and increases in intracellular Ca2+, but not ROS generation. Although conventionally believed to be an ER-resident chaperone, I have demonstrated that GRP78 is expressed on the surface of T24/83 cells. Furthermore, antibodies or peptides directed against GRP78 block its inhibition of TF PCA, suggesting that GRP78 interacts with TF on the cell surface resulting in attenuation of TF PCA. Together these findings demonstrate that GRP78 inhibits TF PCA through Ca2+ sequestration and attenuation of caspase activation which may affect TF directly, or may prevent the dissociation of a TF-GRP78 complex on the cell surface. These findings enhance our understanding of how TF PCA is regulated and provide the basis to develop GRP78-based therapies with the potential to attenuate thromboembolic disease. |
