Both the high affinity thrombin receptor (GPIb-IX-V) and GPIIb/IIIa are implicated in expression of thrombin-induced platelet procoagulant activity

Platelets activated by alpha-thrombin express surface procoagulant activity (PCA) that accelerates the conversion of prothrombin to alpha-thrombin. Following activation with 10 nM alpha-thrombin, the PCA of normal platelets was approximately five-fold higher than that of Bernard-Soulier platelets (lacking GPIb). Normal platelet PCA was inhibited approximately 50% by activation in the presence of the anti-GPIb MoAbs LJIb10 or TM60. Moreover, normal platelet PCA was completely abrogated in the presence of a combination of both LJIb10 and c7E3, a MoAb directed against alphaIIbbeta3 (GPIIb/IIIa). In contrast. PCA expressed by Bernard Soulier or Glanzmann platelets was not inhibited by either LJIb10 or c7E3 MoAb. The platelet activating peptide SFLLRN at 10 microM, a concentration which fully activates platelet aggregation and Ca2+ mobilization, generated PCA activity one fifth of that generated by alpha-thrombin at 10 nM but anti-PAR1 antibodies did not affect thrombin-induced PCA expression. These results demonstrate that GPIb mediates, at least in part, the thrombin-induced activation of platelets that leads to PCA, and that alphaIIbbeta3 is also involved in PCA generation, but these results do not support a major role for PAR1 in this activation.

Human carbon catabolite repressor protein (CCR4)-associative factor 1: cloning, expression and characterization of its interaction with the B-cell translocation protein BTG1

The human BTG1 protein is thought to be a potential tumour suppressor because its overexpression inhibits NIH 3T3 cell proliferation. However, little is known about how BTG1 exerts its anti-proliferative activity. In this study, we used the yeast 'two-hybrid' system to screen for interacting protein partners and identified human carbon catabolite repressor protein (CCR4)-associative factor 1 (hCAF-1), a homologue of mouse CAF-1 (mCAF-1) and Saccharomyces cerevisiae yCAF-1/POP2. In vitro the hCAF-1/BTG1 complex formation was dependent on the phosphorylation of a putative p34cdc2 kinase site on BTG1 (Ser-159). In yeast, the Ala-159 mutant did not interact with hCAF-1. In addition, phosphorylation of Ser-159 in vitro showed specificity for the cell cycle kinases p34CDK2/cyclin E and p34CDK2/cyclin A, but not for p34CDK4/cyclin D1 or p34cdc2/cyclin B. Cell synchrony experiments with primary cultures of rat aortic smooth-muscle cells (RSMCs) demonstrated that message and protein levels of rat CAF-1 (rCAF-1) were up-regulated under conditions of cell contact, as previously reported for BTG1 [Wilcox, Scott, Subramanian, Ross, Adams-Burton, Stoltenborg and Corjay (1995) Circulation 92, I34-I35]. Western blot and immunohistochemical analysis showed that rCAF-1 localizes to the nucleus of contact-inhibited RSMCs, where it was physically associated with BTG1, as determined by co-immunoprecipitation with anti-hCAF-1 antisera. Overexpression of hCAF-1 in NIH 3T3 and osteosarcoma (U-2-OS) cells was itself anti-proliferative with colony formation reduced by 67% and 90% respectively. Taken together, these results indicate that formation of the hCAF-1/BTG1 complex is driven by phosphorylation at BTG1 (Ser-159) and implicates this complex in the signalling events of cell division that lead to changes in cellular proliferation associated with cell-cell contact.

Glycoprotein IIb/IIIa receptor antagonists inhibit the development of platelet procoagulant activity

We examined the effects of glycoprotein IIb/IIIa (GPIIb/IIIa) antagonists c7E3 Fab and DMP728 on the development of platelet prothrombinase (PT) activity. c7E3 Fab dose-dependently inhibited the rate of thrombin-stimulated thrombin generation over a 1-minute reaction time. The IC50 was 11 nM with an IC90 of 1000 nM. DMP728 inhibited PT activity maximally by 60% at 100 nM. A similar profile was observed for the inhibition of platelet tenase activity. Inhibition was platelet specific up to approximately 200 nM c7E3 Fab. Above 200 nM, inhibition was platelet independent, as shown by the inhibition of activity assembled on PS/PC vesicles. c7E3 Fab and DMP728 did not inhibit calcium ionophore-induced activity. DMP728 potency diminished with reaction time (over 6 minutes) whereas c7E3 Fab potency did not. Inhibition by 2 microM DMP728 was not further increased by 20 nM c7E3 Fab. Heparin inhibition of platelet PT activity was additive to that of c7E3 Fab. Studies with added von Willebrand factor (vWf) indicate that in the context of thrombin activation vWf activates platelets through mechanisms independent of GPIIb/IIIa to promote PT activity. Thrombin activation induced binding of FITC-Annexin V to a subpopulation of platelets which was reduced by approximately 50% by pretreatment with either c7E3 Fab or DMP728. Together, these data indicate that c7E3 Fab and DMP728 inhibit the development of GPIIb/IIIa-mediated platelet PT activity at events during platelet activation. The inhibitory activities are not additive, suggesting these agents compete for the same site or inhibit via the same mechanism. Inhibition accompanies a reduction in the number of phosphatidylserine binding sites, implying that GPIIb/IIIa receptor antagonists reduce platelet membrane scrambling induced by thrombin. The additivity of inhibition with heparin by c7E3 Fab suggests a combination of these agents might have a greater bleeding liability than the use of either agent alone.