IgG plasma cells initiate changes in the protein C system in mouse ulcerative colitis through CD14+CD64+ macrophage activation.

BACKGROUND: Inhibition of the protein C system (PCS) might be one of the mechanisms of ulcerative colitis (UC). OBJECTIVES: The aim of the study was to explore the role of IgG plasma cells in changes in the PCS in UC. MATERIAL AND METHODS: Dextran sulfate sodium (DSS) was chosen to induce mouse UC. Inflammation was assessed using hematoxylin & eosin (H&E) staining and immunofluorescence. The profiling of colonic plasma cells and macrophages from colitis mice was analyzed with flow cytometry. After stimulation of macrophages with IgG type immune complex (IgG-IC), western blot was used to determine tumor necrosis factor α (TNF-α) and interleukin 6 (IL-6) protein levels. After co-incubation of colonic mucosa microvascular endothelial cells (MVECs) with TNF-α or IL-6, mitogen-activated protein kinase (MAPK) expression was detected. RESULTS: The DSS-colitis mice showed higher inflammatory indexes (p < 0.05 or p < 0.01), accompanied by greater infiltration of CD38+IgG+ plasma cells (p < 0.01), CD14+CD64+ macrophages (p < 0.01) and IgG-IC than healthy mice. Enhancement of TNF-α and IL-6 protein expression was demonstrated in this subset of macrophages when stimulated by IgG-IC (p < 0.01). After MVECs were incubated with TNF-α or IL-6, the expression of ß-arrestin1, pP38 MAPK and pJNK MAPK exhibited an increase (p < 0.05 or p < 0.01), but downregulation of endothelial protein C receptor (EPCR) expression was observed (p < 0.05 or p < 0.01); this inhibition of EPCR expression was reversed by SB203580, SP600125 or U0126 (p < 0.05 or p < 0.01). In addition, changes in activated protein C (APC) presented results similar to those for EPCR expression (p < 0.05 or p < 0.01). CONCLUSIONS: These results reveal that the PCS is inhibited during UC processing. There is a possibility that the interaction between IgG plasma cells and CD14+CD64+ macrophages, as well as further secretion of cytokines from CD14+CD64+ macrophages by the formation and stimulation of IgG-IC, subsequently influence MVECs through the ß-arrestin-MAPK pathway. Enhancement of PCS activity may represent a novel approach for treating UC.

Acute traumatic coagulopathy: The elephant in a room of blind scientists.

Acute traumatic coagulopathy (ATC) is the failure of coagulation homeostasis that can rapidly arise following traumatic injury, hemorrhage, and shock; it is associated with higher injury severity, coagulation abnormalities, and increased blood transfusions. Acute traumatic coagulopathy has historically been defined by a prolonged prothrombin time, although newer, more informative measurements of hemostatic function have been used to improve diagnosis and support clinical decision making. The underlying biochemical mechanisms of and best practice therapeutics for ATC remain under active investigation because of its significant correlation to poor outcomes. The wide range of hypothesized mechanisms for ATC results from the large number of symptoms, phenotypes, and altered states in these patients as observed by multiple research groups. Much like the ancient fable of blind men describing an elephant from their limited perspectives, the limited nature of clinical and laboratory tools used to diagnose coagulopathy or evaluate hemostatic function has made finding causation difficult. The prolonged prothrombin time, degree of fibrinolysis, depletion of coagulation factors and inhibitors, and general failure of the blood have all been identified as being primary indicators for ATC. Therapeutic interventions including recombinant coagulation factors, antifibrinolytics, and blood products have been used with varying degrees of success as they are used to address specific symptoms. To truly understand the causes of ATC, research efforts must recognize the complexity of the hemostatic system and get to the heart of the matter by answering the question: "Is ATC a pathological condition that develops from the observed deficiencies in coagulation, fibrinolysis, and autoregulation, or is ATC an adaptive response generated as the body attempts to restore perfusion and avoid massive organ failure?" Because patient management must proceed without definitive answers regarding the entire causative chain, the current therapeutic focus should be on using what knowledge has been gained to the patient's advantage: control hemorrhage, maintain appropriate homeostatic balances of coagulation proteins, and restore oxygen perfusion.

Coagulopathy of Acute Sepsis.

Coagulopathy is common in acute sepsis and may range from subclinical activation of blood coagulation (hypercoagulability), which may contribute to venous thromboembolism, to acute disseminated intravascular coagulation, characterized by widespread microvascular thrombosis and consumption of platelets and coagulation proteins, eventually causing bleeding. The key event underlying this life-threatening complication is the overwhelming inflammatory host response to the pathogen leading to the overexpression of inflammatory mediators. The latter, along with the microorganism and its derivatives drive the major changes responsible for massive thrombin formation and fibrin deposition: (1) aberrant expression of tissue factor mainly by monocytes-macrophages, (2) impairment of anticoagulant pathways, orchestrated by dysfunctional endothelial cells (ECs), and (3) suppression of fibrinolysis because of the overproduction of plasminogen activator inhibitor-1 by ECs and thrombin-mediated activation of thrombin-activatable fibrinolysis inhibitor. Neutrophils and other cells, upon activation or death, release nuclear materials (neutrophil extracellular traps and/or their components such as histones, DNA, lysosomal enzymes, and High Mobility Group Box-1), which have toxic, proinflammatory and prothrombotic properties thus contributing to clotting dysregulation. The ensuing microvascular thrombosis-ischemia significantly contributes to tissue injury and multiple organ dysfunction syndromes. These insights into the pathogenesis of sepsis-associated coagulopathy may have implications for the development of new diagnostic and therapeutic tools.

Mechanisms of phagocytosis and host clearance of Pseudomonas aeruginosa.

Pseudomonas aeruginosa is an opportunistic bacterial pathogen responsible for a high incidence of acute and chronic pulmonary infection. These infections are particularly prevalent in patients with chronic obstructive pulmonary disease and cystic fibrosis: much of the morbidity and pathophysiology associated with these diseases is due to a hypersusceptibility to bacterial infection. Innate immunity, primarily through inflammatory cytokine production, cellular recruitment, and phagocytic clearance by neutrophils and macrophages, is the key to endogenous control of P. aeruginosa infection. In this review, we highlight recent advances toward understanding the innate immune response to P. aeruginosa, with a focus on the role of phagocytes in control of P. aeruginosa infection. Specifically, we summarize the cellular and molecular mechanisms of phagocytic recognition and uptake of P. aeruginosa, and how current animal models of P. aeruginosa infection reflect clinical observations in the context of phagocytic clearance of the bacteria. Several notable phenotypic changes to the bacteria are consistently observed during chronic pulmonary infections, including changes to mucoidy and flagellar motility, that likely enable or reflect their ability to persist. These traits are likewise examined in the context of how the bacteria avoid phagocytic clearance, inflammation, and sterilizing immunity.

Role of anhepatic time in endothelial-related coagulation in liver transplantation.

BACKGROUND: Disturbances in coagulation homeostasis are common in patients undergoing orthotopic liver transplantation (OLT) and anhepatic period is one of the important factors related to the coagulation abnormalities. The endothelium can regulate hemostasisby producing substances such as thrombomodulin (TM). The primary aim was to evaluate the effect of an hepatic time on the thrombomodulin-protein C system in patients undergoing OLT. METHODS: Fifty patients undergoing OLT were stratified in two groups: anhepatic time ≥ 60 min (N.=18) or anhepatic stage <60 min (N.=32). TM, protein C, activated protein C (APC) and (free) protein S plasma concentrations were measured by enzymelinked immunosorbent assays (ELISA) at the start of the surgery (To); immediately before the anhepatic period (A1); immediately before reperfusion (A2); 5 minutes; 15 minutes; 30 minutes after reperfusion of the graft (R1; R2; R3); at the end of operation (R4); the first day after operation (R5). RESULTS: Blood loss and transfusion were significantly greater in patients whose anhepatic time ≥ 60 min during the operation. TM levels increased most in patients whose anhepatic time ≥ 60 min. Protein C levels remained low throughout the surgery and decreased significantly at other points compared with To (P<0.05). There were no differences in protein C levels between groups except R5. The ratio of circulating APC activity to protein C antigen (APC/PC) increased significantly during the surgery. APC/PC ratio in the neohepatic stage increased significantly in patients whose anhepatic time ≥ 60 min (P<0.05). CONCLUSION: Patients with prolonged anhepatic time had greater changes in the thrombomodulin-protein C system.

Impact of chemotherapy on activated protein C-dependent thrombin generation--association with VTE occurrence.

Chemotherapy has been associated with an increased risk of venous thromboembolism (VTE). However, the prevalence of coagulation abnormalities or VTE occurrence as a consequence of different anti-cancer agents or treatment schemes is largely uncharacterized. Thus, this study was aimed at analyzing the impact of different anticancer drugs on the prothrombotic status of cancer out-patients scheduled for chemotherapy. To this purpose, a mono-institutional study was prospectively conducted to monitor serial changes of activated protein C (APC) function in 505 consecutive cancer out-patients with primary or relapsing solid cancer at the start of a new chemotherapy regimen. The results obtained showed that age >65 years (p = 0.01), ECOG performance status (p = 0.01), platinum-based (p = 0.035) and fluoropyrimidine-based regimens (p = 0.008) were independent predictors of an acquired APC resistance during the first chemotherapy cycle. Multivariate model of Cox proportional hazards survival analysis demonstrated that a decline in APC functionality (HR = 2.4; p = 0.013) and platinum-based regimens (HR = 2.2; p = 0.042) were both capable of predicting the occurrence of a first VTE episode during chemotherapy. Indeed, 14% of patients with platinum-associated APC impairment had VTE over a 1-year follow-up, compared to 3% of patients treated with other regimens and in whom APC functionality remained stable (HR = 1.5; p = 0.003). We may, thus, conclude that use of platinum-based regimens is responsible for induction of an acquired thrombophilic condition and represents a predictor for VTE even after adjustment for other risk factors.

Effect of cardiopulmonary bypass on thrombin generation and protein C pathway.

OBJECTIVE: The purpose of this study was to evaluate the mechanisms of cardiopulmonary bypass (CPB)-induced dysregulation between thrombin and its regulatory anticoagulant activated protein C (APC). DESIGN: A prospective observational cohort study. SETTING: A tertiary care university hospital and associated research laboratory. PATIENTS: Twenty patients undergoing elective coronary artery bypass surgery with (n = 10) or without CPB (n = 10). INTERVENTIONS: Blood samples were collected at 7 time points: preinduction; after heparin; 1 hour after the institution of CPB (or the completion of distal anastomoses in off-CPB group); after protamine; and at 0, 4, and 18 hours in the Intensive care unit (ICU). Samples were analyzed for prothrombin fragments (F1+2), thrombin-antithrombin complexes, protein C (PC), APC, soluble thrombomodulin (sTM), and soluble endothelial protein C receptor (sEPCR). MEASUREMENTS AND MAIN RESULTS: F1+2 levels increased significantly 1 hour after the initiation of CPB in comparison with baseline (2.7 ± 0.5 v 0.5 ± 0.2 nmol/L, p < 0.001) (mean ± standard deviation) and remained elevated until 4 hours after ICU admission (p < 0.001). In contrast, APC levels did not show any significant changes over time in either group. sEPCR, sTM, and PC levels did not change during CPB although sEPCR decreased significantly after the termination of CPB compared with baseline in the CPB group. CONCLUSIONS: Exposure to CPB is associated with a distinct thrombin surge that continues postoperatively for 4 hours. The impaired ability to generate APC reflects a complex process that is not associated with increased levels of sEPCR and thrombomodulin during CPB. Further studies are required to evaluate the regulation of the host APC response in cardiac surgery.

Extracellular histone release in response to traumatic injury: implications for a compensatory role of activated protein C.

BACKGROUND: Tissue injury leads to the release of DAMPs (damage-associated molecular patterns) that may drive a sterile inflammatory response; however, the role of extracellular histone levels after traumatic injury remains unexplored. We hypothesized that extracellular histone levels would be increased and associated with poor outcomes after traumatic injury. METHODS: In this prognostic study, plasma was prospectively collected from 132 critically injured trauma patients on arrival and 6 hours after admission to an urban Level I trauma intensive care unit. Circulating extracellular histone levels and plasma clotting factors were assayed and linked to resuscitation and outcome data. RESULTS: Of 132 patients, histone levels were elevated to a median of 14.0 absorbance units (AU) on arrival, declining to 6.4 AU by 6 hours. Patients with elevated admission histone levels had higher ISS (Injury Severity Score), lower admission GCS (Glasgow Coma Scale) score, more days of mechanical ventilation, and higher incidences of multiorgan failure, acute lung injury, and mortality (all p ≤ 0.05). Histone levels correlated with prolonged international normalized ratio and partial thromboplastin time, fibrinolytic markers D-dimer and tissue-type plasminogen activator, and anticoagulants tissue factor pathway inhibitor and activated protein C (aPC; all p < 0.03). Increasing histone level from admission to 6 hours was a multivariate predictor of mortality (hazard ratio, 1.005; p = 0.013). When aPC level trends were included, the impact of histone level increase on mortality was abrogated (p = 0.206) by a protective effect of increasing aPC levels (hazard ratio, 0.900; p = 0.020). CONCLUSION: Extracellular histone levels are elevated in response to traumatic injury and correlate with fibrinolysis and activation of anticoagulants. An increase in histone levels from admission to 6 hours is predictive of mortality, representing evidence of ongoing release of intracellular antigens similar to that seen in sepsis. Concomitant elevation of aPC abrogates this effect, suggesting a possible role for aPC in mitigating the sterile inflammatory response after trauma through the proteolysis of circulating histones. LEVEL OF EVIDENCE: Prognostic study, level III.

Activated protein C has a protective effect against myocardial I/R injury by improvement of endothelial function and activation of AKT1.

OBJECTIVES: Activated protein C (APC) has a protective efficacy against ischemia-reperfusion (I/R) injury in several organs. The objective of this study was to investigate effect of APC in myocardium with possible mechanism. METHODS: We used regional and global myocardial I/R injury models of rats. They consisted of I/R injuries (1) by ligation of left coronary artery, or (2) using Langendorff apparatus. Langendorff was used to focus the mechanism of APC excluding coagulation cascade in a working heart. Each experiment had an APC group (n=10) and a control group with normal saline (n=10). Injections of these solutions into rats were performed 30 minutes before the planned-I/R injury. Cardiac performance after the procedure was evaluated by echocardiography or indices with Langendorff apparatus. Coronary flow (CF) was measured in the global I/R injury model. Western blotting was performed to detect the change of AKT1 signal in myocardium after global I/R injury. RESULTS: LV FUNCTION IMPROVED SIGNIFICANTLY IN THE APC GROUP: %EF at 2 weeks after procedure, 70.8%± 4.5% vs. 56.5%± 0.7%; APC vs. control; p<0.01. Percent LV development pressure (LVDP) also improved in the APC group significantly, 88.8%± 45.3% vs. 28.1%± 15.4%; APC vs. control; p<0.01. In APC group, %CF improved significantly, 88.5%± 15.8% vs. 65.0%± 13.4%; APC vs. control; p<0.01. It was enhanced significantly when acetylcholine was administered; % CF: 103.5%± 9.9% vs. 87.0%± 12.1%; APC vs. control; p<0.05. Western blotting revealed that APC significantly induced activation of phosphorylated AKT1 in myocardium (p<0.05). CONCLUSIONS: APC has a novel effect to protect myocardium and cardiac performance against I/R injury through improvement of endothelial function and activation of AKT1.

Receptor of activated protein C promotes metastasis and correlates with clinical outcome in lung adenocarcinoma.

RATIONALE: Efficient metastasis requires survival and adaptation of tumor cells to stringent conditions imposed by the extracellular milieu. Identification of critical survival signaling pathways in tumor cells might unveil novel targets relevant in disease progression. OBJECTIVES: To investigate the contribution of activated protein C (APC) and its receptor (endothelial protein C receptor [EPCR]) in animal models of lung cancer metastasis and in patients with lung adenocarcinoma. METHODS: Signaling pathway triggered by APC/EPCR and its relevance in apoptosis was studied in vitro. Functional significance was assessed by silencing and blocking antibodies in several in vivo models of lung cancer metastasis in athymic nude Foxn1(nu) mice. We examined EPCR levels using a microarray dataset of 107 patients. Immunohistochemical analysis was performed in an independent cohort of 295 patients with lung adenocarcinoma. MEASUREMENTS AND MAIN RESULTS: The effects of APC binding to EPCR rapidly triggered Akt and extracellular signal-regulated kinase signaling pathways, leading to attenuated in vitro apoptosis. In vivo, silencing of EPCR expression or blocking APC/EPCR interaction reduced infiltration in the target organ, resulting in impaired prometastatic activity. Moreover, overexpression of EPCR induced an increased metastatic activity to target organs. Analysis of clinical samples showed a robust association between high EPCR levels and poor prognosis, particularly in stage I patients. CONCLUSIONS: EPCR and its ligand APC promote cell survival that contributes to tumor cell endurance to stress favoring prometastatic activity of lung adenocarcinoma. EPCR/APC is a novel target of relevance in the clinical outcome of early-stage lung cancer.

Unexpected role of anticoagulant protein C in controlling epithelial barrier integrity and intestinal inflammation.

The protein C (PC) pathway is a well-characterized coagulation system. Endothelial PC receptors and thrombomodulin mediate the conversion of PC to its activated form, a potent anticoagulant and anti-inflammatory molecule. Here we show that the PC pathway is expressed on intestinal epithelial cells. The epithelial expression of PC and endothelial PC receptor is down-regulated In patients with inflammatory bowel disease. PC(-/-)/PC(Tg) mice, expressing only 3% of WT PC, developed spontaneous intestinal inflammation and were prone to severe experimental colitis. These mice also demonstrated spontaneous elevated production of inflammatory cytokines and increased intestinal permeability. Structural analysis of epithelial tight junction molecules revealed that lack of PC leads to decreased JAM-A and claudin-3 expression and an altered pattern of ZO-1 expression. In vitro, treatment of epithelial cells with activated PC led to protection of tight junction disruption induced by TNF-α, and in vivo, topical treatment with activated PC led to mucosal healing and amelioration of colitis. Taken together, these findings demonstrate that the PC pathway is a unique system involved in controlling intestinal homeostasis and inflammation by regulating epithelial barrier function.

Coagulation and inflammation. Molecular insights and diagnostic implications.

Overwhelming evidence has linked inflammatory disorders to a hypercoagulable state. In fact, thromboembolic complications are among the leading causes of disability and death in many acute and chronic inflammatory diseases. Despite this clinical knowledge, coagulation and immunity were long regarded as separate entities. Recent studies have unveiled molecular underpinnings of the intimate interconnection between both systems. The studies have clearly shown that distinct pro-inflammatory stimuli also activate the clotting cascade and that coagulation in turn modulates inflammatory signaling pathways. In this review, we use evidence from sepsis and inflammatory bowel diseases as a paradigm for acute and chronic inflammatory states in general and rise hypotheses how a systematic molecular understanding of the "inflammation-coagulation" crosstalk may result in novel diagnostic and therapeutic strategies that target the inflammation-induced hypercoagulable state.

The link between vascular features and thrombosis.

The mechanisms of vascular control of thrombotic events remain unclear. The vasculature possesses essential anticoagulant factors that regulate coagulation. Because the endothelium-to-blood ratios are much higher in the microcirculation, it is likely that stasis contributes to thrombotic risk, due in large part to failure to rapidly access the microcirculation and to gain access to this highly anticoagulant environment. Inflammation can potentiate thrombosis in part through downregulation of the vascular anticoagulants, a process that appears to be exacerbated in aging, a well-known risk factor for thrombosis. Surgery and trauma, two major risk factors for thrombosis, result in the release of a variety of cellular components that trigger coagulation through separate mechanisms.

The effect of homocysteine on the clinical outcomes of ventilated patients with severe sepsis.

BACKGROUND: There is considerable evidence that elevated plasma homocysteine levels are associated with a prothrombotic milieu, whereas activation of the coagulation cascade is an important component of the pathogenesis of sepsis. The protein C pathway has been reported to play a central role both in the propagation of sepsis and a hyperhomocysteinemia-induced hypercoagulable state. Our primary aim was to measure plasma homocysteine levels in mechanically ventilated patients with severe sepsis/septic shock and to assess the association of these levels with relevant clinical outcomes. METHODS: The study cohort included 102 mechanically ventilated patients with severe sepsis or septic shock. Demographics, comorbidities, clinical data and severity scores were recorded. Plasma homocysteine, vitamin B12, folate, creatinine, and protein C levels were measured in all study subjects upon enrollment, and genotyping for the C677T and A1298C polymorphisisms of the methylenetetrahydrofolate reductase (MTHFR) gene and for factor V Leiden (FVL) mutations was performed as well. The primary outcomes were mortality at 28 and 90 days; secondary outcomes included the number of days without renal or cardiovascular failure and the ventilator-free days during the study period. RESULTS: Homocysteine levels were not significantly associated with any primary or secondary outcomes in the multivariable analysis. In addition, a synergistic effect of homocysteine with protein C levels was not detected. CONCLUSION: Our data suggest that plasma homocysteine levels may not inform the prognosis of mechanically ventilated patients with severe sepsis/septic shock.

Activated protein C targets CD8+ dendritic cells to reduce the mortality of endotoxemia in mice.

Activated protein C (aPC) therapy reduces mortality in adult patients with severe sepsis. In mouse endotoxemia and sepsis models, mortality reduction requires the cell signaling function of aPC, mediated through protease-activated receptor-1 (PAR1) and endothelial protein C receptor (EPCR; also known as Procr). Candidate cellular targets of aPC include vascular endothelial cells and leukocytes. Here, we show that expression of EPCR and PAR1 on hematopoietic cells is required in mice for an aPC variant that mediates full cell signaling activity but only minimal anticoagulant function (5A-aPC) to reduce the mortality of endotoxemia. Expression of EPCR in mature murine immune cells was limited to a subset of CD8+ conventional dendritic cells. Adoptive transfer of splenic CD11chiPDCA-1- dendritic cells from wild-type mice into animals with hematopoietic EPCR deficiency restored the therapeutic efficacy of aPC, whereas transfer of EPCR-deficient CD11chi dendritic cells or wild-type CD11chi dendritic cells depleted of EPCR+ cells did not. In addition, 5A-aPC inhibited the inflammatory response of conventional dendritic cells independent of EPCR and suppressed IFN-gamma production by natural killer-like dendritic cells. These data reveal an essential role for EPCR and PAR1 on hematopoietic cells, identify EPCR-expressing dendritic immune cells as a critical target of aPC therapy, and document EPCR-independent antiinflammatory effects of aPC on innate immune cells.

Contributions of extravascular and intravascular cells to fibrin network formation, structure, and stability.

Fibrin is essential for hemostasis; however, abnormal fibrin formation is hypothesized to increase thrombotic risk. We previously showed that in situ thrombin generation on a cell's surface modulates the 3-dimensional structure and stability of the fibrin network. Currently, we compared the abilities of extravascular and intravascular cells to support fibrin formation, structure, and stability. Extravascular cells (fibroblasts, smooth muscle) supported formation of dense fibrin networks that resisted fibrinolysis, whereas unstimulated intravascular (endothelial) cells produced coarse networks that were susceptible to fibrinolysis. All 3 cell types produced a fibrin structural gradient, with a denser network near, versus distal to, the cell surface. Although fibrin structure depended on cellular procoagulant activity, it did not reflect interactions between integrins and fibrin. These findings contrasted with those on platelets, which influenced fibrin structure via interactions between beta3 integrins and fibrin. Inflammatory cytokines that induced prothrombotic activity on endothelial cells caused the production of abnormally dense fibrin networks that resisted fibrinolysis. Blocking tissue factor activity significantly reduced the density and stability of fibrin networks produced by cytokine-stimulated endothelial cells. Together, these findings indicate fibrin structure and stability reflect the procoagulant phenotype of the endogenous cells, and suggest abnormal fibrin structure is a novel link between inflammation and thrombosis.

Proteasome inhibitors enhance endothelial thrombomodulin expression via induction of Krüppel-like transcription factors.

OBJECTIVE: Impairment of the thrombomodulin-protein C anticoagulant pathway has been implicated in pathological thrombosis associated with malignancy. Patients who receive proteasome inhibitors as part of their chemotherapeutic regimen appear to be at decreased risk for thromboembolic events. We investigated the effects of proteasome inhibitors on endothelial thrombomodulin expression and function. METHODS AND RESULTS: Proteasome inhibitors as a class markedly induced the expression of thrombomodulin and enhanced the protein C activating capacity of endothelial cells. Thrombomodulin upregulation was independent of NF-kappaB signaling, a principal target of proteasome inhibitors, but was instead a direct consequence of increased expression of the Krüppel-like transcription factors, KLF2 and KLF4. These effects were confirmed in vivo, where systemic administration of a proteasome inhibitor enhanced thrombomodulin expression that was paralleled by changes in the expression of KLF2 and KLF4. CONCLUSIONS: These findings identify a novel mechanism of action of proteasome inhibitors that may help to explain their clinically observed thromboprotective effects.

Protamine sulfate down-regulates thrombin generation by inhibiting factor V activation.

Protamine sulfate is a positively charged polypeptide widely used to reverse heparin-induced anticoagulation. Paradoxically, prospective randomized trials have shown that protamine administration for heparin neutralization is associated with increased bleeding, particularly after cardiothoracic surgery with cardiopulmonary bypass. The molecular mechanism(s) through which protamine mediates this anticoagulant effect has not been defined. In vivo administration of pharmacologic doses of protamine to BALB/c mice significantly reduced plasma thrombin generation and prolonged tail-bleeding time (from 120 to 199 seconds). Similarly, in pooled normal human plasma, protamine caused significant dose-dependent prolongations of both prothrombin time and activated partial thromboplastin time. Protamine also markedly attenuated tissue factor-initiated thrombin generation in human plasma, causing a significant decrease in endogenous thrombin potential (41% +/- 7%). As expected, low-dose protamine effectively reversed the anticoagulant activity of unfractionated heparin in plasma. However, elevated protamine concentrations were associated with progressive dose-dependent reduction in thrombin generation. To assess the mechanism by which protamine mediates down-regulation of thrombin generation, the effect of protamine on factor V activation was assessed. Protamine was found to significantly reduce the rate of factor V activation by both thrombin and factor Xa. Protamine mediates its anticoagulant activity in plasma by down-regulation of thrombin generation via a novel mechanism, specifically inhibition of factor V activation.