Protein C, protein S and von Willebrand factor levels correlate with bleeding symptoms: a population-based study.

Although natural anticoagulant deficiencies are the established causes of thrombosis, their roles in bleeding are not fully studied. The objective is to correlate haemostatic factors with haemorrhagic symptoms quantified by a standardized questionnaire. Adult subjects were recruited from Bangkok and nearby provinces as part of routine health surveys/checkups. The validated MCMDM-1VWD form was used to assess their bleeding symptoms. At the same time, von Willebrand factor (VWF) activity, free protein S levels and protein C activity were measured. There were 5196 individuals. The mean age was 44.3 years (range 15-99) and 41% were male subjects. The mean bleeding score was -0.28 and 95% of subjects had scores between -2 and +2. The scores were lower in female subjects than in male subjects (-0.35 vs. -0.16, P < 0.001). Bleeding scores correlated negatively with age, VWF and protein C activities (Spearman's ρ-0.258, -0.091 and -0.098, respectively, all P < 0.001), but did not significantly correlate with protein S levels. Using multivariate analysis, female gender, VWF below 100 IU dL(-1), protein C below 100 IU dL(-1) and protein S over 150 IU dL(-1) significantly related to high (≥3) bleeding scores (adjusted odds ratio 1.95, 1.83, 1.56 and 2.84, P = 0.001, 0.001, 0.039 and 0.017, respectively). These findings may suggest interacting roles of VWF and natural anticoagulants in modifying bleeding symptoms.

Prognostic factors of green pit viper bites.

Clinical features of green pit viper bites vary from asymptomatic to fatal bleeding. Antivenin promptly reverses the coagulopathy but has considerable adverse side effects. In this study, potential clinical predictors of severe outcomes (wound necrosis, wound infection, and systemic bleeding) and antivenin allergy were determined in 271 moderate to severe cases of green pit viper bites by multivariate analysis. The incidences of systemic bleeding, wound necrosis, secondary infection, and antivenin allergy were 17.3%, 6.6%, 5.5%, and 20.8% respectively. The predictors of systemic bleeding were the combination of thrombocytopenia and prolonged venous clotting time and bite sites away from digits. A bite on the fingers or toes was a risk factor for skin necrosis (P = 0.03). Systemic absorption of the venom from digits may be poor, resulting in severe local but mild systemic effects. The presence of blisters often led to necrosis and secondary infections (P = 0.0037 and P = 0.0006, respectively). Although negative skin test results do not exclude the possibility of antivenin allergy, positive results indicate a high risk (P = 0.016) requiring special precautions.

The effects of green pit viper (Trimeresurus albolabris and Trimeresurus macrops) venom on the fibrinolytic system in human.

Green pit viper (Trimeresurus albolabris and Trimeresurus macrops) venom was found to have a thrombin-like effect in vitro but cause a defibrination syndrome in vivo. The effects of venom on fibrinolytic system have not been well characterized. This knowledge can help to define the roles of antifibrinolytic therapy, give insights in fibrinolytic system regulation and potentially lead to identification of a new profibrinolytic agent from this venom. Forty-six cases of green pit viper bites were studied for various coagulation and fibrinolytic parameters and correlated with serum venom levels measured by ELISA. Fibrinolytic system activation is very common as indicated by low plasminogen (50%), low antiplasmin (56.5%) and elevated fibrin-fibrinogen degradation products (FDPs, 97.4%) levels. FDP test is very sensitive and a normal level is useful for exclusion of systemic envenomation. In contrast to some other models of defibrination syndrome, such as Russell viper (Daboia russelli siamensis), elevation of plasminogen activator activity (PA) was found indicating a hyperfibrinolytic state. Definite increase in tissue-type plasminogen activator (t-PA) antigen (p = 0.00075) with a modest elevation of its inhibitor plasminogen activator inhibitor-1 (PAI-1) (p = 0.27) probably contributes to this effect. This supports the idea that the balance between plasminogen activators and inhibitors can determine fibrinolytic responses in pathologic states. Fibrinopeptide A levels were markedly elevated (68.43 +/- 51.57 ng/ml in cases and 2.83 +/- 3.80 ng/ml in control, p < 0.0001) and correlated well with clinical severity suggesting that the fibrin deposition from the thrombin-like effect is the main mechanism of fibrinolysis. Therefore, antifibrinolytic agents probably have no role in treatment. However, the components of green pit viper venom that have these profibrinolytic effects in human are interesting and should be further identified.

Actin reorganization and proplatelet formation in murine megakaryocytes: the role of protein kinase calpha.

With the recent cloning and characterization of thrombopoietin, appreciation of the molecular events surrounding megakaryocyte (MK) development is growing. However, the final stages of platelet formation are less well understood. Platelet production occurs after the formation of MK proplatelet processes. In a study to explore the molecular mechanisms underlying this process, mature MKs isolated from suspension murine bone marrow cell cultures were induced to form proplatelets by exposure to plasma, and the role of various cell-signaling pathways was assessed. The results showed that (1) bis-indolylmaleimide I, which blocks protein kinase C (PKC) activation; (2) down-modulation of conventional or novel classes of PKC by phorbol myristate acetate; and (3) ribozymes specific for PKCalpha each inhibited proplatelet formation. Inhibition of several MAP kinases, PI3 kinase, or protein kinase A failed to affect MK proplatelet formation. To gain further insights into the function of PKCalpha in proplatelet formation, its subcellular localization was investigated. In cultures containing active proplatelet formation, cytoplasmic polymerized actin was highly aggregated, its subcellular distribution was reorganized, and PKCalpha colocalized with the cellular actin aggregates. A number of MK manipulations, including blockade of integrin signaling with a disintegrin or inhibition of actin polymerization with cytochalasin D, interrupted actin reorganization, PKC relocalization, and proplatelet formation. These findings suggest an important role for PKCalpha in proplatelet development and suggest that it acts by altering actin dynamics in proplatelet-forming MKs. Identification of the upstream and downstream pathways involved in proplatelet formation should provide greater insights into thrombopoiesis, potentially allowing pharmacologic manipulation of the process.

Thrombopoietin induces phosphoinositol 3-kinase activation through SHP2, Gab, and insulin receptor substrate proteins in BAF3 cells and primary murine megakaryocytes.

Thrombopoietin (TPO) is a recently characterized member of the hematopoietic growth factor family that serves as the primary regulator of megakaryocyte (MK) and platelet production. The hormone acts by binding to the Mpl receptor, the product of the cellular proto-oncogene c-mpl. Although many downstream signaling targets of TPO have been identified in cell lines, primary MKs, and platelets, the molecular mechanism(s) by which many of these molecules are activated remains uncertain. In this report we demonstrate that the TPO-induced activation of phosphoinositol 3-kinase (PI3K), a signaling intermediate vital for cellular survival and proliferation, occurs through its association with inducible signaling complexes in both BaF3 cells engineered to express Mpl (BaF3/Mpl) and in primary murine MKs. Although a direct association between PI3K and Mpl could not be demonstrated, we found that several proteins, including SHP2, Gab2, and IRS2, undergo phosphorylation and association in BaF3/Mpl cells in response to TPO stimulation, complexes that recruit and enhance the enzymatic activity of PI3K. To verify the physiological relevance of the complex, SHP2-Gab2 association was disrupted by overexpressing a dominant negative SHP2 construct. TPO-induced Akt phosphorylation was significantly decreased in transfected cells suggesting an important role of SHP2 in the complex to enhance PI3K activity. In primary murine MKs, TPO also induced phosphorylation of SHP2, its association with p85 and enhanced PI3K activity, but in contrast to the results in cell lines, neither Gab2 nor IRS2 are phosphorylated in MKs. Instead, a 100-kDa tyrosine-phosphorylated protein (pp100) co-immunoprecipitated with the regulatory subunit of PI3K. These findings support a model where PI3K activity is dependent on its recruitment into TPO-induced multiphosphoprotein complexes, implicate the existence of a scaffolding protein in primary MKs distinct from the known Gab and IRS proteins, and suggest that, in contrast to erythroid progenitor cells that employ Gab1 in PI3K signaling complexes, utilization of an alternate member of the Gab/IRS family could be responsible for specificity in TPO signaling.

A far upstream, cell type-specific enhancer of the mouse thrombospondin 3 gene is located within intron 6 of the adjacent metaxin gene.

Thrombospondin 3 (TSP3) is a secreted, pentameric glycoprotein whose regulation of expression and function are not well understood. Mouse Thbs3 is located just downstream from the divergently transcribed metaxin gene (Mtx), which encodes an outer mitochondrial membrane import protein. Although Thbs3 and Mtx share a common promoter region, previous studies showed that Mtx is regulated by proximal elements that had little effect on Thbs3 expression. In this study, transient transfection of rat chondrosarcoma cells and NIH-3T3 fibroblasts demonstrated that Thbs3 is regulated in a cell type-specific manner by a position- and orientation-independent far upstream enhancer located within intron 6 of Mtx. Despite its greater proximity to the transcription start site of Mtx, the Thbs3 enhancer did not have a significant effect on Mtx expression. Two DNA-protein complexes, which were both required for activity, were identified when nuclear extracts were assayed with a probe containing the enhancer sequence. The protein in one of these complexes was identified as Sp1, while the other DNA-protein complex remains uncharacterized. A 6-kilobase pair promoter containing the enhancer was able to direct specific expression of the E. coli lacZ gene in transgenic mice, whereas a 2-kilobase pair promoter that lacked the enhancer was inactive. Thus, despite their close proximity, the genes of the Mtx/Thbs3 gene cluster are regulated independently.

Thrombopoietin-induced activation of the mitogen-activated protein kinase (MAPK) pathway in normal megakaryocytes: role in endomitosis.

Thrombopoietin (TPO) plays a critical role in megakaryocyte proliferation and differentiation. Using various cultured cell lines, several recent studies have implicated the mitogen-activated protein kinase (MAPK) pathway in megakaryocyte differentiation. In the study reported here, we examined the role played by thrombopoietin-induced MAPK activity in a cytokine-dependent cell line (BAF3/Mpl) and in primary murine megakaryocytes. In both systems, extracellular signal-regulated protein kinase (ERK) 1 and 2 MAPK phosphorylation was rapidly induced by TPO stimulation. To identify the Mpl domain responsible for MAPK activation, BAF3 cells expressing truncated forms of the Mpl receptor were studied. Phosphorylation of ERKs did not require elements of the cytoplasmic signaling domain distal to Box 2 and was not dependent on phosphorylation of the adapter protein Shc. ERK activation in murine megakaryocytes was maximal at 10 minutes and was markedly decreased over the subsequent 3 hours. Next, the physiologic consequences of MAPK inhibition were studied. Using the MAPK kinase (MEK) inhibitor, PD 98059, blockade of MAPK activity substantially reduced TPO-dependent proliferation in BAF3/Mpl cells and markedly decreased mean megakaryocyte ploidy in cultures. To exclude an indirect effect of MAPK inhibition on stromal cells in whole bone marrow, CD41(+) cells were selected and then cultured in TPO. The number of polyploid megakaryocytes derived from the CD41-selected cells was also significantly reduced by MEK inhibition, as was their geometric mean ploidy. These studies show an important role for MAPK in TPO-induced endomitosis and underscore the value of primary cells when studying the physiologic effects of signaling pathways.

Thrombopoietin signal transduction: studies from cell lines and primary cells.

Thrombopoietin (TPO) and its receptor Mpl support all of the developmental step necessary for megakaryocytopoiesis. In the past few years, the signaling pathways utilized by this member of the cytokine receptor family have been extensively studied, especially JAK/STAT, Ras/MAP kinase, Shc, and other adapter molecules. Many if not most of the secondary signaling pathways activated by thrombopoietin have also been identified upon binding of other hematopoietic growth factors to their cognate receptors, making the study of Mpl signaling representative of the field in general. However, identifying unique molecules or combinations of signals that direct megakaryocyte development has been an elusive goal and has led some investigators to conclude that there is little specificity during Mpl signal transduction. In this article we review the data regarding Mpl signaling with particular attention to the methods employed and critical interpretation of the data generated. Future studies will have to focus on primary bone marrow cells and intact animal models rather than transformed cell lines. Furthermore, it is likely that a comprehensive, integrative analysis of the many pathways activated by ligand binding will be necessary to understand the physiology of cytokine signaling.

The roles of phosphatidylinositol 3-kinase and protein kinase Czeta for thrombopoietin-induced mitogen-activated protein kinase activation in primary murine megakaryocytes.

Thrombopoietin (TPO) stimulates a network of intracellular signaling pathways that displays extensive cross-talk. We have demonstrated previously that the ERK/mitogen-activated protein kinase pathway is important for TPO-induced endomitosis in primary megakaryocytes (MKs). One known pathway by which TPO induces ERK activation is through the association of Shc with the penultimate phosphotyrosine within the TPO receptor, Mpl. However, several investigators found that the membrane-proximal half of the cytoplasmic domain of Mpl is sufficient to activate ERK in vitro and support base-line megakaryopoiesis in vivo. Using BaF3 cells expressing a truncated Mpl (T69Mpl) as a tool to identify non-Shc/Ras-dependent signaling pathways, we describe here novel mechanisms of TPO-induced ERK activation mediated, in part, by phosphoinositide 3-kinase (PI3K). Similar to cells expressing full-length receptor, PI3K was activated by its incorporation into a complex with IRS2 or Gab2. Furthermore, the MEK-phosphorylating activity of protein kinase Czeta (PKCzeta) was also enhanced after TPO stimulation of T69Mpl, contributing to ERK activity. PKCzeta and PI3K also contribute to TPO-induced ERK activation in MKs, confirming their physiological relevance. Like in BaF3 cells, a TPO-induced signaling complex containing p85PI3K is detectable in MKs expressing T61Mpl and is probably responsible for PI3K activation. These data demonstrate a novel role of PI3K and PKCzeta in steady-state megakaryopoiesis.