Clustering of integrins into focal adhesions and focal complexes is regulated by the actin cytoskeleton. In turn, actin dynamics are governed by Rho family GTPases. Integrin-mediated adhesion activates these GTPases, triggering assembly of filopodia, lamellipodia and stress fibers. In the past few years, signaling pathways have begun to be identified that promote focal adhesion disassembly and integrin dispersal. Many of these pathways result in decreased myosin-mediated cell contractility.

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Thrombosis with associated inflammation (thromboinflammation) occurs commonly in a broad range of human disorders. It is well recognized clinically in the context of superficial thrombophlebitis (thrombosis and inflammation of superficial veins); however, it is more dangerous when it develops in the microvasculature of injured tissues and organs. Microvascular thrombosis with associated inflammation is well recognized in the context of sepsis and ischemia-reperfusion injury; however, it also occurs in organ transplant rejection, major trauma, severe burns, the antiphospholipid syndrome, preeclampsia, sickle cell disease, and biomaterial-induced thromboinflammation. Central to thromboinflammation is the loss of the normal antithrombotic and anti-inflammatory functions of endothelial cells, leading to dysregulation of coagulation, complement, platelet activation, and leukocyte recruitment in the microvasculature. α-Thrombin plays a critical role in coordinating thrombotic and inflammatory responses and has long been considered an attractive therapeutic target to reduce thromboinflammatory complications. This review focuses on the role of basic aspects of coagulation and α-thrombin in promoting thromboinflammatory responses and discusses insights gained from clinical trials on the effects of various inhibitors of coagulation on thromboinflammatory disorders. Studies in sepsis patients have been particularly informative because, despite using anticoagulant approaches with different pharmacological profiles, which act at distinct points in the coagulation cascade, bleeding complications continue to undermine clinical benefit. Future advances may require the development of therapeutics with primary anti-inflammatory and cytoprotective properties, which have less impact on hemostasis. This may be possible with the growing recognition that components of blood coagulation and platelets have prothrombotic and proinflammatory functions independent of their hemostatic effects.

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BH3 mimetics are a new class of proapoptotic anticancer agents that have shown considerable promise in preclinical animal models and early-stage human trials. These agents act by inhibiting the prosurvival function of one or more Bcl-2-related proteins. Agents that inhibit Bcl-x L induce rapid platelet death that leads to thrombocytopenia; however, their impact on the function of residual circulating platelets remains unclear. In this study, we demonstrate that the BH3 mimetics, ABT-737 or ABT-263, induce a time-and dose-dependent decrease in platelet adhesive function that correlates with ectodomain shedding of the major platelet adhesion receptors, glycoprotein Ib␣ and glycoprotein VI, and functional downregulation of integrin ␣ IIb ␤ 3 . Analysis of platelets from mice treated with higher doses of BH3 mimetics revealed the presence of a subpopulation of circulating platelets undergoing cell death that have impaired activation responses to soluble agonists. Functional analysis of platelets by intravital microscopy revealed a timedependent defect in platelet aggregation at sites of vascular injury that correlated with an increase in tail bleeding time.Overall, these studies demonstrate that Bcl-x L -inhibitory BH3 mimetics not only induce thrombocytopenia but also a transient thrombocytopathy that can undermine the hemostatic function of platelets. (Blood. 2011;118(6):1663-1674) IntroductionApoptosis is an evolutionarily conserved process important for mammalian development, tissue homeostasis, and immune tolerance. Apoptosis also acts as an important barrier against malignant transformation. Resistance to apoptosis is a hallmark feature of cancer, and several pathways regulating apoptosis are commonly altered in human malignancies. 1,2 For example, Ͼ 50% of all human cancers contain mutations in the p53 tumor suppressor gene, with nearly all of these mutations preventing p53 from triggering apoptosis. 3 Furthermore, the overexpression or enhanced function of certain prosurvival proteins, such as Bcl-2, 2 Bcl-x L , 2 PI3K/Akt/mTOR, 4 and nuclear factor-B, 5 is found in many types of human cancers. As a consequence, strategies targeting specific prosurvival pathways have gained prominence as novel anticancer therapies.Progress unraveling the complex pathways underlying apoptosis and cell survival, and elucidation of their roles in human cancers, has led to the development of several potential anticancer drugs, such as inhibitors of the Bcl-2 protein family, 6 inhibitors of apoptosis (IAPs), 7 MDM2, 8 PI3K/Akt/mTOR, 9 as well as TNFrelated apoptosis-inducing ligand. 10 A particularly promising approach is the ability to inhibit tumor cell survival through the use of agents that mimic the proapoptotic Bcl-2 homology 3 (BH3) domain proteins. BH3 proteins play an important role in inducing apoptosis by antagonizing the function of prosurvival Bcl-2 family proteins. Preclinical studies with the BH3 mimetic ABT-737, which selectively antagonizes Bcl-2, Bcl-x L , and Bcl-w, have demonstrated potent cytotoxic activity a...

Rap1b, an abundant small GTPase in platelets, becomes rapidly activated upon stimulation with agonists. Though it has been implicated to act downstream from G protein-coupled receptors (GPCRs) and upstream of integrin α IIb β 3 , the precise role of Rap1b in platelet function has been elusive. Here we report the generation of a murine rap1b knockout and show that Rap1b deficiency results in a bleeding defect due to defective platelet function. Aggregation of Rap1b-null platelets is reduced in response to stimulation with both GPCR-linked and GPCR-independent agonists. Underlying the defective Rap1b-null platelet function is decreased activation of integrin α IIb β 3 in response to stimulation with agonists and signaling downstream from the integrin α IIb β 3 . In vivo, Rap1b-null mice are protected from arterial thrombosis. These data provide genetic evidence that Rap1b is involved in a common pathway of integrin activation, is required for normal hemostasis in vivo, and may be a clinically relevant antithrombotic therapy target.

Toll-like receptor 4 (TLR4) is activated by bacterial lipopolysaccharide (LPS) to mount innate immune responses. The TLR4-induced release of pro-and anti-inflammatory cytokines generates robust inflammatory responses, which must then be restrained to avoid disease. New mechanisms for the critical regulation of TLR-induced cytokine responses are still emerging. Here we find TLR4 complexes localized in LPS-induced dorsal ruffles on the surface of macrophages. We discover that the small GTPase Rab8a is enriched in these ruffles and recruits phosphatidylinositol 3-kinase (PI3Kg) as an effector by interacting directly through its Ras-binding domain. Rab8a and PI3Kg function to regulate Akt signalling generated by surface TLR4. Rab8a and PI3Kg do not affect TLR4 endocytosis, but instead regulate mammalian target of rapamycin signalling as a mechanism for biasing the cytokine profile to constrain inflammation in innate immunity.

The central importance of platelets in the development of arterial thrombosis and cardiovascular disease is well established. No other single cell type is responsible for as much morbidity and mortality as the platelet and, as a consequence, it represents a major target for therapeutic intervention. The growing awareness of the importance of platelets is reflected in the increasing number of patients receiving antiplatelet therapy, a trend that is likely to continue in the future. There are, however, significant drawbacks with existing therapies, including issues related to limited efficacy and safety. The discovery of a 'magic bullet' that selectively targets pathological thrombus formation without undermining haemostasis remains elusive, although recent progress in unravelling the molecular events regulating thrombosis has provided promising new avenues to solve this long-standing problem.