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SummaryHemostatic molecular markers were serially monitored in a prospective fashion during remission induction therapy with all-trans retinoic acid (ATRA) in sixteen patients with acute promyelocytic leukemia (APL). One patient with leukocytosis before treatment and three patients who later developed hyperleukocytosis also received chemotherapy with behenoyl Ara-C and daunorubicin. Plasma levels of E-fragment of fibrin and fibrinogen degradation product (FDP-E), FDP-D dimer (D-D), thrombin-antithrombin complex (TAT), and plasmin-α2 plasmin inhibitor complex (PIC) were markedly elevated in all but one patient before treatment, and these parameters decreased to normal or near normal ranges in most patients within the first 7 days of treatment. Interestingly, we have found that these parameters were again elevated during the later course of ATRA therapy (after day +7) in eleven patients for various reasons including cytotoxic chemotherapy (3 cases), fever (5 cases; 2 cases with apparent infection, 3 cases without known etiology), Caesarean section (1 case), and no apparent etiology (2 cases). Three patients showed bleeding complications during reelevation of molecular markers, but none developed thrombosis. Plasma elastase-α1 proteinase inhibitor complex (E-α1PI) was markedly elevated in all patients at diagnosis and did not decrease significantly during ATRA therapy. Plasma tissue factor antigen was mildly elevated in one out of four patients studied, and thrombomodulin was elevated in two out of ten patients tested. These results confirmed the rapid normalization of coagulopathy during the early phase of remission induction therapy with ATRA but suggest that re-elevation of molecular markers occurs frequently during the later course of ATRA therapy.

SummaryWe characterized a murine monoclonal antibody, PT25-2 (IgG1), raised against washed human platelets. The antibody and its Fab fragments were both capable of inducing platelet aggregation in a fibrinogen-dependent manner and induced 125I-fibrinogen binding to unstimulated platelets (120,000 molecules/platelet at a 100 nM IgG concentration). The antibody immunoprecipitated the αIIbβ3 complex from lysates of iodinated platelets but did not react with the respective subunits when complex formation was disrupted by treatment with 5 mM EDTA at 37°C for 30 min. However, simply removing the extracellular divalent cation with EDTA had no effect on antibody binding indicating that the antibody’s epitope depends upon a conformational structure maintained by αβ subunit association. Antibody binding to unstimulated, washed platelets yielded binding parameters (Kd = 40 nM, Bmax = 100,000 molecules/platelet), which were found to be virtually unchanged when binding was performed using thrombin or RGDS-peptide-stimulated platelets. Thus, the PT25-2 antibody defines a novel regulatory epitope expressed by the αIIbβ3 integrin on unstimulated, quiescent platelets.

SummaryShear stress-induced platelet aggregation (SIPA) may be essential in thrombus formation in pathologically stenotic arteries. Intracellular events during SIPA are, however, poorly understood. Washed platelets were exposed to shear stress (108 dyne/cm2) in the presence of von Willebrand factor (vWf, 10 μg/ml) and 1 mM CaCl2 for various time intervals, and then lyzed in SDS. Platelet proteins were separated by 10% SDS-PAGE and tyrosine phosphorylated proteins were detected by immunoblotting with an anti-phosphotyrosine monoclonal antibody. Increased tyrosine phosphorylation of proteins of 130, 100, 85, 74, 70, 64, 58, and 40 kDa was observed within 30 s after the beginning of exposure of platelets to high shear force and the degree of tyrosine phosphorylation continued to increase up to approximately 2 min after the exposure. A monoclonal antibody (MoAb) against vWf-binding domain of glycoprotein (GP) Ibα (GUR83-35), anti-vWf MoAb that inhibits binding of vWf to GPIbα (NMC-4), or a MoAb against GP IIb/IIIa complex (AP-2) inhibited SIPA as well as tyrosine phosphorylation of these proteins. Apyrase (an ADP scavenger, 2 U/ml), EDTA (5 mM), or RGDS peptide (200 μg/ml) also had inhibitory effects on both SIPA and tyrosine phosphorylation. However, Cytochalasin D (2 μM) or staurosporin (1 μM) did not affect SIPA, while they inhibited SIPA-associated tyrosine phosphorylation of those proteins. SIPA-associated tyrosine phosphorylation is a novel post-aggregatory pathway in signal transduction, which is dependent on the binding of vWf to GP Ib/IX and GP IIb/IIIa, endogenous ADP, and intact cytoskeleton.