A comparison of the beta-D-xyloside, odiparcil, to warfarin in a rat model of venous thrombosis

BACKGROUND

A significant need exists for new chronic oral anticoagulation therapies to replace warfarin. Previous studies have shown that beta-D-xylosides, which prime glycosaminoglycan (GAG) synthesis, have antithrombin and antithrombotic activity. In the following report, a new orally active beta-D-xyloside (odiparcil) has been characterized in a rat model of venous thrombosis and its efficacy and bleeding liability compared to warfarin. Additionally, studies were conducted to investigate odiparcil's ex vivo antithrombin and antiplatelet activity, and also to explore the potential utility of protamine sulfate as a neutralizing agent.

METHODS AND RESULTS

In vivo thrombosis studies were conducted in a rat inferior vena cava model, and bleeding studies in a rat tail transection model. Following oral dosing, warfarin and odiparcil produced dose-related suppression of thrombus formation. A therapeutically relevant dose of warfarin in this model (international normalized ratio; INR 3.0) achieved approximately 65% inhibition of thrombus formation. Warfarin caused dose-related significant increases in bleeding indices. Odiparcil antithrombotic activity was limited by its mechanism to a maximum suppression of thrombus formation of 65-70%, and did not prolong bleeding indices. Additionally, odiparcil-induced heparin cofactor II (HCII)-dependent antithrombin activity was shown to be a function of dermatan sulfate-like GAG production. Other than thrombin-related effects, no odiparcil effects on platelet function were observed. In antidote studies, it was demonstrated that odiparcil-induced antithrombotic activity could be partially neutralized by protamine sulfate.

CONCLUSIONS

These experiments suggest that an antithrombotic approach based upon xyloside induction of circulating GAGs may have the potential to approximate the efficacy of warfarin and yet with a reduced risk to hemostasis.

P2X1 stimulation promotes thrombin receptor-mediated platelet aggregation

P2X1 receptors are ATP-gated channel demonstrated to be involved in multiple platelet responses, although in vitro analysis has been complicated by the effects of rapid desensitization. To further investigate potential roles of P2X1 receptors in platelet activation, the current study employed methods which maximally preserved P2X1 functionality. In preliminary in vivo studies, P2X1-deficiency reduced thrombus formation following the laser-induced, but not FeCl3-induced injury. Given the multiple potential mechanisms involved in thrombus formation in vivo, including tissue-factor/thrombin generation pathways, subsequent studies were designed to investigate the effects of P2X1 inhibition or stimulation on platelet activation in vitro; specifically, the interaction of P2X1 with thrombin receptor stimulation. Aggregation initiated by low/threshold levels of a protease-activated receptor (PAR)4 agonist was reduced in P2X1-deficient murine platelets, and inhibition of P2X1 in wild-type platelets similarly reduced PAR4-mediated aggregation. In human platelets, aggregation to low/threshold stimulation of PAR1 was inhibited with the P2X1 antagonist MRS2159. In addition, P2X1 stimulation primed human platelet responses, such that subsequent sub-threshold PAR1 responses were converted into significant aggregation. Selective ADP receptor inhibitors attenuated P2X1-mediated priming, suggesting that the synergy between P2X1 and sub-threshold PAR1 stimulation was in part because of enhanced granular release of ADP. Overall, the present study defines a novel interaction between platelet P2X1 and thrombin receptors, with P2X1 functioning to amplify aggregation responses at low levels of thrombin receptor stimulation.

Caspase inhibitors as neuroprotective agents

Apoptotic neuronal cell death has been demonstrated to occur in the central nervous system (CNS), following both acute injury and during chronic neurodegenerative conditions. Currently, the majority of experimental evidence for a role of caspases in CNS damage has been established following acute neuronal insults, including ischaemic stroke, traumatic brain injury and spinal cord injury. In vitro and in vivo models have been used to demonstrate caspase activation, and treatment with available caspase inhibitors can provide significant protection. Overall, acute neuronal injury represents a major unmet medical need and caspase inhibitors may be an attractive approach to preserve neuronal function by extending the therapeutic window and providing long-term neuroprotection. Currently, several inhibitors are in preclinical drug development and this review summarises recent advances in the development of novel caspase inhibitors for the treatment of acute neuronal injury.

Potentiation of platelet activation through the stimulation of P2X1 receptors

The platelet P2X1 purinergic receptor is a ligand-gated ion channel that responds to ATP. The precise role of P2X1 in platelet function is unknown, though stimulation with the P2X1 agonist alpha,beta-Me-ATP is known to result in platelet shape change through elevation of calcium levels. The aim of the present study was to examine further the effects of P2X1 stimulation on platelet activation. Stimulation of P2X1 with alpha,beta-Me-ATP resulted in shape change and small aggregate formation in heparin-anticoagulated platelet preparations. Given the ability of heparin to potentiate platelet activation, subsequent experiments were performed in hirudin. In these platelet preparations, aggregate formation in response to alpha,beta-Me-ATP alone was less than that observed in heparin; however, alpha,beta-Me-ATP significantly potentiated platelet aggregate formation when added in conjunction with other weak platelet agonists [epinephrine or thrombopoietin (TPO)]. Platelet aggregate formation was confirmed by single platelet loss (microaggregate formation), microscopy, and light transmittance studies. Further, the P2X1 antagonist MRS-2159 inhibited platelet shape change and aggregation responses induced by alpha,beta-Me-ATP. Overall, this study demonstrates that P2X1 stimulation can induce/potentiate platelet activation in combination with other platelet agonists. These results are the first demonstration of platelet aggregation mediated through direct P2X1 stimulation, supporting a role for this receptor in regulating platelet activation.

p38 activation is required upstream of potassium current enhancement and caspase cleavage in thiol oxidant-induced neuronal apoptosis

Oxidant-induced neuronal apoptosis has been shown to involve potassium and zinc dysregulation, energetic dysfunction, activation of stress-related kinases, and caspase cleavage. The temporal ordering and interdependence of these events was investigated in primary neuronal cultures exposed to the sulfhydryl oxidizing agent 2,2'-dithiodipyridine (DTDP), a compound that induces the intracellular release of zinc. We previously observed that tetraethylammonium (TEA), high extracellular potassium, or cysteine protease inhibitors block apoptosis induced by DTDP. We now report that both p38 and extracellular signal-regulated kinase phosphorylation are evident in neuronal cultures within 2 hr of a brief exposure to 100 microm DTDP. However, only p38 inhibition is capable of blocking oxidant-induced toxicity. Cyclohexamide or actinomycin D does not attenuate DTDP-induced cell death, suggesting that posttranslational modification of existing targets, rather than transcriptional activation, is responsible for the deleterious effects of p38. Indeed, an early robust increase in TEA-sensitive potassium channel currents induced by DTDP is attenuated by p38 inhibition but not by caspase inhibition. Moreover, we found that activation of p38 is required for caspase 3 and 9 cleavage, suggesting that potassium currents enhancement is required for caspase activation. Finally, we observed that DTDP toxicity could be blocked with niacinamide or benzamide, inhibitors of poly (ADP-ribose) synthetase. Based on these findings, we conclude that oxidation of sulfhydryl groups on intracellular targets results in intracellular zinc release, p38 phosphorylation, enhancement of potassium currents, caspase cleavage, energetic dysfunction, and translationally independent apoptotic cell death.

Inhibition of p38 mitogen-activated protein kinase provides neuroprotection in cerebral focal ischemia

Mitogen-activated protein kinases (MAPKs) are involved in many cellular processes. The stress-activated MAPK, p38, has been linked to inflammatory cytokine production and cell death following cellular stress. Here, we demonstrate focal ischemic stroke-induced p38 enzyme activation (i.e., phosphorylation) in the brain. The second generation p38 MAPK inhibitor SB 239063 was identified to exhibit increased kinase selectivity and improved cellular and in vivo activity profiles, and thus was selected for evaluation in two rat models of permanent focal ischemic stroke. SB 239063 was administered orally pre- and post-stroke and intravenously post-stroke. Plasma concentration levels were achieved in excess of those that effectively inhibit p38 activity. In both moderate and severe stroke, SB 239063 reduced infarct size by 28-41%, and neurological deficits by 25-35%. In addition, neuroprotective plasma concentrations of SB 239063 that reduced p38 activity following stroke also reduced the stroke-induced expression of IL-1beta and TNFalpha (i.e., cytokines known to contribute to stroke-induced brain injury). SB 239063 also provided direct protection of cultured brain tissue to in vitro ischemia. This robust SB 239063-induced neuroprotection emphasizes a significant opportunity for targeting MAPK pathways in ischemic stroke injury, and also suggests that p38 inhibition be evaluated for protective effects in other experimental models of nervous system injury and neurodegeneration.

SB 239063, a novel p38 inhibitor, attenuates early neuronal injury following ischemia

The aim of the present study was to evaluate p38 MAPK activation following focal stroke and determine whether SB 239063, a novel second generation p38 inhibitor, would directly attenuate early neuronal injury. Following permanent middle cerebral artery occlusion (MCAO), brains were dissected into ischemic and non-ischemic cortices and Western blots were employed to measure p38 MAPK activation. Neurologic deficit and MR imaging were utilized at various time points following MCAO to monitor the development and resolution of brain injury. Following MCAO, there was an early (15 min) activation of p38 MAPK (2.3-fold) which remained elevated up to 1 h (1.8-fold) post injury compared to non-ischemic and sham operated tissue. Oral SB 239063 (5, 15, 30, 60 mg/kg) administered to each animal 1 h pre- and 6 h post MCAO provided significant (P<0.05) dose-related neuroprotection reducing infarct size by 42, 48, 29 and 14%, respectively. The most effective dose (15 mg/kg) was further evaluated in detail and SB 239063 significantly (P<0.05) reduced neurologic deficit and infarct size by at least 30% from 24 h through at least 1 week. Early (i.e. observed within 2 h) reductions in diffusion weighted imaging (DWI) intensity following treatment with SB 239063 correlated (r=0.74, P<0.01) to neuroprotection seen up to 7 days post stroke. Since increased protein levels for various pro-inflammatory cytokines cannot be detected prior to 2 h in this stroke model, the early improvements due to p38 inhibition, observed using DWI, demonstrate that p38 inhibition can be neuroprotective through direct effects on ischemic brain cells, in addition to effects on inflammation.

SB 239063, a second-generation p38 mitogen-activated protein kinase inhibitor, reduces brain injury and neurological deficits in cerebral focal ischemia

The stress-activated mitogen-activated protein kinase (MAPK) p38 has been linked to the production of inflammatory cytokines/mediators/inflammation and death/apoptosis following cell stress. In these studies, a second-generation p38 MAPK inhibitor, SB 239063 (IC(50) = 44 nM), was found to exhibit improved kinase selectivity and increased cellular (3-fold) and in vivo (3- to 10-fold) activity over first-generation inhibitors. Oral SB 239063 inhibited lipopolysaccharide-induced plasma tumor necrosis factor production (IC(50) = 2.6 mg/kg) and reduced adjuvant-induced arthritis (51% at 10 mg/kg) in rats. SB 239063 reduced infarct volume (48%) and neurological deficits (42%) when administered orally (15 mg/kg, b.i.d.) before moderate stroke. Intravenous SB 239063 exhibited a clearance of 34 ml/min/kg, a volume of distribution of 3 l/kg, and a plasma half-life of 75 min. An i.v. dosing regimen that provided effective plasma concentrations of 0.38, 0.75, or 1.5 microg/ml (i.e., begun 15 min poststroke and continuing over the initial 6-h p38 activation period) was used. Significant and dose-proportional brain penetration of SB 239063 was demonstrated during these infusion periods. In both moderate and severe stroke, intravenous SB 239063 produced a maximum reduction of infarct size by 41 and 27% and neurological deficits by 35 and 33%, respectively. No effects of the drug were observed on cerebral perfusion, hemodynamics, or body temperature. Direct neuroprotective effects from oxygen and glucose deprivation were also demonstrated in organotypic cultures of rat brain tissue. This robust in vitro and in vivo SB 239063-induced neuroprotection emphasizes the potential role of MAPK pathways in ischemic stroke and also suggests that p38 inhibition warrants further study, including protection in other models of nervous system injury and neurodegeneration.

Expression of PEP-19 inhibits apoptosis in PC12 cells

PEP-19 is a calmodulin-regulatory protein found specifically within neurons, though cellular functions of this protein have not been determined. In an effort to define potential effects of PEP-19, PC12 cell lines expressing this protein were generated and subjected to apoptotic stimuli. As measured by LDH release, cell death in PEP-19 expressing cells was 2- to 5-fold less following u.v. irradiation, and 2- to 4-fold less following staurosporine treatment than controls. Additionally, PEP-19-expressing cells displayed decreased DNA ladder formation, chromatin and condensation, caspase activation following staurosporine treatment. Overall, these results demonstrate that PEP-19 can inhibit apoptotic processes in PC12 cells, suggesting a potential regulatory mechanism for pathways leading to cell death.

Small proteins that modulate calmodulin-dependent signal transduction: effects of PEP-19, neuromodulin, and neurogranin on enzyme activation and cellular homeostasis

Neuromodulin (GAP-43), neurogranin (RC3), and PEP-19 are small acid-stable proteins that bind calcium-poor calmodulin through a loosely conserved IQ-motif. Even though these proteins have been known for many years, much about their function in cells is not understood. It has recently become appreciated that calmodulin activity in cells is tightly controlled and that pools of otherwise free calmodulin are sequestered so as to restrict its availability for activating calcium/calmodulin-dependent enzymes. Neuromodulin, neurogranin, and PEP-19 appear to be major participants in this type of regulation. One way in which they do this is by providing localized increases in the concentration of calmodulin in cells so that the maximal level of target activation is increased. Additionally, they can function as calmodulin antagonists by directly inhibiting the association of calcium/calmodulin with enzymes and other proteins. Although neuromodulin, neurogranin, and PEP-19 were early representatives of the small IQ-motif-containing protein family, newer examples have come to light that expand the number of cellular systems through which the IQ-peptide/calmodulin interaction could regulate biological processes including gene transcription. It is the purpose of this review to examine the behavior of neuromodulin, neurogranin, and PEP-19 in paradigms that include both in vitro and in situ systems in order to summarize possible biological consequences that are linked to the expression of this type of protein. The use of protein:protein interaction chromatography is also examined in the recovery of a new calmodulin-binding peptide, CAP-19 (ratMBF1). Consistent with earlier predictions, at least one function of small IQ-motif proteins appears to be that they lessen the extent to which calcium-calmodulin-dependent enzymes become or stay activated. It also appears that these polypeptides can function to selectively inhibit activation of intracellular targets by some agonists while simultaneously permitting activation of these same targets by other agonists. Much of the mechanism for how this occurs is unknown, and possible explanations are examined. One of the biological consequences for a cell that expresses a calmodulin-regulatory protein could be an increased resistance to calcium-mediated toxicity. This possibility is examined for cells expressing PEP-19 and both anatomical and cell-biological data is described. The study of IQ-motif-containing small proteins has stimulated considerable thought as to how calcium signaling is refined in neurons. Current evidence suggests that signaling through calmodulin is not a fulminating and homogenous process but a spatially limited and highly regulated one. Data from studies on neuromodulin, neurogranin, and PEP-19 suggest that they play an important role in establishing some of the processes by which this regulation is accomplished.

The novel beta-blocker, carvedilol, provides neuroprotection in transient focal stroke

Increasing evidence supports a role for oxidative stress, proinflammatory cytokines, and apoptosis in the pathophysiology of focal ischemic stroke. Previous studies have found that the multi-action drug, carvedilol, is a mixed adrenergic antagonist, and that it behaves as an antioxidant and inhibits apoptosis. In the current study, the authors investigated whether carvedilol provides protection in focal cerebral ischemia and whether this protection is associated with reduced apoptosis and the downregulation of the inflammatory cytokines, tumor necrosis factor-alpha (TNF-alpha) and interleukin- 1beta (IL-1beta). Male Sprague-Dawley rats were subjected to transient middle cerebral artery occlusion (MCAO) by an intraluminal filament technique. Carvedilol (1, 3, and 10 mg/kg) was injected daily subcutaneously 2 or 4 days before the induction of ischemia. Neurologic scores, infarct volumes, TUNEL staining, and mRNA levels of TNF-alpha and IL-1beta were assessed at 24 hours reperfusion. The effect of carvedilol on microvascular cortical perfusion was studied with continuous laser-Doppler flowmetry. Twenty-four hours after MCAO, carvedilol at all three doses reduced infarct volumes by at least 40% and reduced neurologic deficits on average by 40% compared with vehicle-treated controls when given 2 or 4 days before the induction of ischemia. This protection was not mediated by changes in temperature or blood flow. Treatment with all three dose regimens resulted in fewer TUNEL positive cells compared with controls. At 24 hours reperfusion, carvedilol decreased TNF-alpha and IL-1beta expression by 40% to 50% in the ipsilateral ischemic cortex compared with the contralateral controls. The results of the current study indicate that carvedilol is neuroprotective in focal cerebral ischemia and may protect the ischemic brain by inhibiting apoptosis and attenuating the expression of TNF-alpha and IL-1beta.

Quantitative changes in interleukin proteins following focal stroke in the rat

The aim of the present study was to quantitate the temporal changes in protein concentration for interleukin (IL)-1alpha, IL-1beta, IL-1ra, and IL-6 from 1 h to 15 days following focal ischemia. Protein expression was evaluated by enzyme-linked immunosorbent assay utilizing newly available rat antibodies. There were no detectable basal levels of IL-1alpha, 1L-1beta, or IL-6 in the sham-operated or non-ischemic control cortex. IL-1beta (increased significantly (P<0.05) as early as 4 h and peaked at 3 to 5 days. IL-1alpha (increased significantly (P<0.05) at 3 days. IL-6 increased early and peaked at 24 h (P<0.05). IL-1ra increased significantly (P<0.05) over basal levels from 12 h to 5 days. The present study provides the first quantitative determination of interleukin protein concentrations in the rat brain following focal stroke and demonstrates that this technology is now available for mechanistic studies in neuroprotection.

Detection of tumor necrosis factor-alpha mRNA induction in ischemic brain tolerance by means of real-time polymerase chain reaction

A short duration of ischemia (i.e., ischemic preconditioning) results in significant brain protection to subsequent severe ischemic insult. Because previous studies suggest that tumor necrosis factor-alpha (TNF-alpha) plays a role in both promoting ischemic damage and neuroprotection, the present work aimed to evaluate the expression of TNF-alpha mRNA in an established model of ischemic preconditioning using a transient 10-minute occlusion of the middle cerebral artery. Because the level of TNF-alpha mRNA expression in the brain was too low to be consistently detected by Northern technique, a real-time polymerase chain reaction method was applied to quantitate the absolute copy number of TNF-alpha transcript in rat brain after the preconditioning procedure. TNF-alpha mRNA was induced in the ipsilateral cortex as early as 1 hour (27 +/- 1 copies of mRNA per microgram of tissue compared to 11 +/- 3 copies in sham-operated samples) after preconditioning, reached a peak level at 6 hours (49 +/- 10 copies of transcript, n = 4, P < 0.01), and persisted up to 2 days. These data not only demonstrate the utility of real-time polymerase chain reaction for sensitive and accurate measurement of mRNA expression in normal and injured tissues but also suggest a potential role of TNF-alpha in the phenomenon of ischemic preconditioning.

A novel F box protein, NFB42, is highly enriched in neurons and induces growth arrest

NFB42 (neural F Box 42 kDa) is a novel gene product that is highly enriched in the nervous system. Its predicted protein contains an F box, a motif recently shown to couple cell cycle regulation to the proteasome pathway (Bai, C., Sen, P., Hofmann, K., Ma, L., Goebl, M., Harper, J. W., and Elledge, S. (1996) Cell 86, 263-274). NFB42 mRNA and protein are expressed in all major areas of the adult rat brain but are not detected in non-neural tissues. NFB42 protein is localized primarily to the cytoplasm of neurons and does not appear to be present in glia. The presence of an F box in NFB42 suggests that it may be involved in cell cycle regulation; however, its expression in postmitotic neurons indicates that it is not involved in regulating typical cell cycle events. In an initial attempt to characterize the function of this protein, NFB42 was transfected into N1E-115 neuroblastoma and Chinese hamster ovary cells. The expression of full-length NFB42, but not an F box deletion mutant, inhibits proliferation in both cell lines. Additional experiments demonstrate that NFB42 interacts with Skp1p, a component of the proteasome pathway, and deletion of the F box also inhibits this interaction. Overall, the expression pattern of NFB42, along with the presence of an F box domain and the ability to inhibit growth, suggests that it may play a role in maintaining neurons in a postmitotic state.

Ectopic p21(WAF1) expression induces differentiation-specific cell cycle changes in PC12 cells characteristic of nerve growth factor treatment

Nerve growth factor treatment of PC12 cells results in neuronal differentiation, a process accompanied by induction of the Cdk inhibitor p21(WAF1). To determine the role of p21 in differentiation, PC12 clones containing an inducible p21 construct were utilized to induce growth arrest. Expression of p21 led to accumulation of cyclins D1 and E and to a decrease in cyclins A and B. Levels of Cdc2 and Cdk4 also decreased after p21 induction. Initially, thymidine incorporation into DNA was dramatically inhibited; however, low levels of incorporation were observed during prolonged p21 expression. Fluorescence-activated cell sorter analysis revealed that this low level of DNA synthesis resulted in the generation of polyploid cells. Results from Western blots were consistent with phosphorylation of p21 protein coincident with the resumption of DNA synthesis. Finally, treatment of p21-arrested populations with epidermal growth factor, a known PC12 mitogen, resulted in neurite extension, a key feature of neuronal differentiation. Overall, cell cycle changes following p21 overexpression in PC12 cells closely mimic distinctive events previously shown to occur during differentiation. These results suggest that the mechanism by which nerve growth factor induces the many cellular changes associated with growth arrest during differentiation is through p21(WAF1) induction.

p21WAF1 induces permanent growth arrest and enhances differentiation, but does not alter apoptosis in PC12 cells

p21WAF1 cyclin-dependent kinase inhibitor has been implicated in the control of proliferation, differentiation, and death in various cell lines. To further examine p21 regulation of the transitions between these cellular processes, an inducible p21 vector (lac operon system) was transfected into the rat pheochromocytoma (PC12) neural cell line. Induction of p21 led to permanent growth arrest, as evidenced by cell counts, FACS analysis, and thymidine incorporation. This arrest was maintained, even after removal of the inducing signal (IPTG). Northern analysis revealed that endogenous p21 mRNA increased following IPTG removal, which may be responsible for the continued growth arrest despite the decrease in ectopic p21 expression. p21 overexpression did not directly lead to a differentiated phenotype; however, differentiation in response to nerve growth factor (NGF) was greatly accelerated. To examine effects on cell death, and specifically test the hypothesis that apoptosis caused by withdrawal of trophic support results from inappropriate entry into cell cycle, serum was removed from proliferating and p21-arrested PC12 cells. The rate of apoptotic death was not affected by p21, nor was it effective in altering the extent of death following other apoptotic stimuli.