Blood
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Focal adhesion kinase (FAK) plays a key role in mediating signaling downstream of integrins and growth factor receptors. In this study, we determined the roles of FAK in vivo by generating a megakaryocyte lineage-specific FAK-null mouse (Pf4-Cre/FAK-floxed). Megakaryocyte and platelet FAK expression was ablated in Pf4-Cre/FAK-floxed mice without affecting expression of the FAK homologue PYK2, although PYK2 phosphorylation was increased in FAK-/- megakaryocytes in response to fibrinogen. ⋯ In contrast, Tpo-mediated activation of positive megakaryopoiesis regulators such as ERK1/2 and AKT is increased in FAK-null megakaryocytes, providing a plausible explanation for the observed increases in megakaryopoiesis in these mice. In Pf4-Cre/FAK-floxed mice, rebleeding times are significantly increased, and FAK-null platelets exhibit diminished spreading on immobilized fibrinogen. These studies establish clear roles for FAK in megakaryocyte growth and platelet function, setting the stage for manipulation of this component of the Tpo signaling apparatus for therapeutic benefit.
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Recent studies have indicated that activated protein C (APC) may exert its cytoprotective and anti-inflammatory activities through the endothelial protein C receptor (EPCR)-dependent cleavage of protease-activated receptor 1 (PAR-1) on vascular endothelial cells. Noting that (1) the activation of protein C on endothelial cells requires thrombin, (2) relative to APC, thrombin cleaves PAR-1 with approximately 3 to 4 orders of magnitude higher catalytic efficiency, and (3) PAR-1 is a target for the proinflammatory activity of thrombin, it is not understood how APC can elicit a protective signaling response through the cleavage of PAR-1 when thrombin is present. ⋯ Thus, when EPCR is bound by protein C, the PAR-1 cleavage-dependent protective signaling responses in endothelial cells can be mediated by either thrombin or APC. These results provide a new paradigm for understanding how PAR-1 and EPCR participate in protective signaling events in endothelial cells.
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Recombinant human rhApo2L/TRAIL selectively stimulates apoptosis in various cancer cells through its receptors DR4 and DR5, and is currently in clinical trials. Preclinical studies have established antitumor activity of rhApo2L/TRAIL in models of epithelial cancers; however, efficacy in non-Hodgkin lymphoma (NHL) models is not well studied. Of 7 NHL cell lines tested in vitro, rhApo2L/TRAIL stimulated apoptosis in BJAB, Ramos RA1, and DoHH-2 cells. ⋯ Both agents exhibited greater activity against disseminated than subcutaneous BJAB xenografts, and worked together to inhibit or abolish disseminated tumors and increase survival. Moreover, rhApo2L/TRAIL helped circumvent acquired rituximab resistance of a Ramos variant. These findings provide a strong rationale for clinical investigation of rhApo2L/TRAIL in combination with rituximab as a novel strategy for NHL therapy.