Kyung-Min Lim

Lysophosphatidic Acid Induces Thrombogenic Activity Through Phosphatidylserine Exposure and Procoagulant Microvesicle Generation in Human Erythrocytes

Objective—Although erythrocytes have been suggested to play a role in blood clotting, mediated through phosphatidylserine (PS) exposure and/or PS-bearing microvesicle generation, an endogenous substance that triggers the membrane alterations leading to a procoagulant activity in erythrocytes has not been reported. We now demonstrated that lysophosphatidic acid (LPA), an important lipid mediator in various pathophysiological processes, induces PS exposure and procoagulant microvesicle generation in erythrocytes, which represent a biological significance resulting in induction of thrombogenic activity. Methods and Results—In human erythrocytes, LPA treatment resulted in PS exposure on remnant cells and PS-bearing microvesicle generation in a concentration-dependent manner. Consistent with the microvesicle generation, scanning electron microscopic study revealed that LPA treatment induced surface changes, alteration of normal discocytic shape into echinocytes followed by spherocytes. Surprisingly, chelation of intracellular calcium did not affect LPA-induced PS exposure and microvesicle generation. On the other hand, protein kinase C (PKC) inhibitors significantly reduced PS exposure and microvesicle generation induced by LPA, reflecting the role of calcium-independent PKC. Activation of PKC was confirmed by Western blot analysis showing translocation of calcium-independent isoform, PKC&zgr;, to erythrocyte membrane. The activity of flippase, which is important in the maintenance of membrane asymmetry, was also inhibited by LPA. Furthermore, LPA-exposed erythrocytes actually potentiated the thrombin generation as determined by prothrombinase assay and accelerated the coagulation process initiated by recombinant human tissue factor in plasma. The adherence of erythrocytes to endothelial cells, another important feature of thrombogenic process, was also stimulated by LPA treatment. Conclusion—These results suggested that LPA-exposed erythrocytes could make an important contribution to thrombosis mediated through PS exposure and procoagulant microvesicle generation.

Ginkgo biloba extract enhances antiplatelet and antithrombotic effects of cilostazol without prolongation of bleeding time

Thrombosis and thromboembolic occlusions of major and minor blood vessels are a major complication in various peripheral vascular diseases. Antiplatelet agents (APA), key tools in the treatment of atherothrombosis, therefore became a mainstay medication for a wide range of vascular diseases. Cilostazol and Ginkgo biloba extract (GB), commonly used remedies for peripheral arterial disease, inhibit platelet aggregation with distinct therapeutic mechanisms. In this study, we have investigated if GB can potentiate the antiplatelet effects of cilostazol to explore the utility of combination therapy of cilostazol and GB against peripheral occlusive vascular diseases. GB or cilostazol was evaluated alone or in combination for the antiplatelet activity using in vitro and in vivo models. In addition, potential bleeding side effect of the combinative therapy was assessed by measuring bleeding time, prothrombin time (PT) and activated partial thromboplastin time (aPTT) in vivo after oral administration. In in vitro assays using freshly isolated human platelets, the combination of cilostazol and GB showed superior inhibition of both the shear and the collagen-induced platelet aggregation to those of each drug alone. In accordance with these enhanced in vitro antiplatelet activities, the combinative therapy showed enhanced anti-thrombotic effects in in vivo pulmonary embolism model and arterial thrombosis model. In particular, the increase of survival rate in pulmonary embolism model by combination treatment of cilostazol (25 mg/kg) and GB (20 mg/kg) was higher more than two-fold of those of the respective drugs. Notably, the combination of cilostazol and GB did not show a significant effect on the bleeding time, PT and aPTT increase, suggesting that GB may potentiate the antiplatelet effect of cilostazol without the prolongation of bleeding time or coagulation time. With these studies, we suggest that combinative therapy of GB and cilostazol might offer enhanced anti-thrombotic efficacies without increasing side-effects.

Doxorubicin-induced platelet cytotoxicity: a new contributory factor for doxorubicin-mediated thrombocytopenia.

Summary.  Background: Doxorubicin (DOX) is a widely used anticancer drug for solid tumors and hematologic malignancy, but its active use is hampered by serious adverse effects, including thrombocytopenia. Although bone marrow toxicity of DOX has been suggested to be the sole mechanism underlying the reduced platelet counts, the direct effects of DOX on platelets have never been examined. Objective: Here, we investigated the DOX‐induced platelet cytotoxicity and its underlying mechanism in an effort to elucidate the contribution of platelet cytotoxicity to DOX‐induced thrombocytopenia. Results: In freshly isolated human platelets, DOX induced platelet cytotoxicity in a time‐dependent and concentration‐dependent manner. Reactive oxygen species (ROS) generation, decreased glutathione levels and subsequent protein thiol depletion were shown to underlie the DOX‐induced platelet cytotoxicity. Conspicuously, DOX‐treated platelets displayed apoptotic features such as caspase‐3 activation, reduced mitochondrial transmembrane potential, and phosphatidylserine exposure. Decreased glutathiolation of procaspase‐3 was shown to be a link between protein thiol depletion and caspase‐3 activation. It is of note that DOX‐mediated platelet cytotoxicity was significantly enhanced by shear stress, a common complicating factor in cancer patients. These in vitro results were further confirmed by an in vivo animal model, where administration of DOX induced a platelet count decrease, ROS generation, caspase‐3 activation, protein thiol depletion, and damaged platelet integrity. Conclusion: We demonstrated that DOX can directly induce platelet cytotoxicity through ROS generation, decreased glutathione levels, and protein thiol depletion. We believe that this study provides important evidence for the role of DOX‐induced platelet cytotoxicity in the development of thrombocytopenia in DOX‐treated patients.

Low level of lead can induce phosphatidylserine-exposure and erythrophagocytosis:A new mechanism underlying lead-associated anemia

Anemia is probably one of the most well-known toxic effects of lead. Previously, lead-induced anemia was considered to be from the inhibition of δ-aminolevulinic acid dehydratase participating in the heme biosynthesis. However, little is known whether lead could affect the destruction of erythrocyte, another important factor for anemia. In the present study, we demonstrated that lead could accelerate the splenic sequestration of erythrocytes through phosphatidylserine (PS) exposure and subsequently increased erythrophagocytosis. In freshly isolated human erythrocytes, Pb(2+)- induced PS exposure at relatively low concentrations (∼0.1 μM) by inhibiting flippase, a key aminophospholipid translocase for the maintenance of PS asymmetry and adenosine triphosphate depletion appeared to underlie this phenomenon. Abnormal shape changes of erythrocytes and microvesicle generation and other triggers for the erythrophagocytosis were also observed in the Pb(2+)-exposed erythrocytes. In vitro data showed that human macrophage indeed recognized and phagocytosis PS-exposed erythrocytes. In good accordance with these in vitro results, the oral administration of Pb(2+) increased PS exposure on erythrocytes in rat in vivo. In addition, reduction of hematocrit and hemoglobin and increased spleen weight were observed along with enhanced splenic sequestration of erythrocytes in the rats exposed to Pb(2+) subchronically for 4 weeks through drinking water. In conclusion, these results suggest that Pb(2+)-induced anemia may be explained at least in part by increased PS exposure on erythrocytes, erythrophagocytosis, and splenic sequestration.

Anti-Inflammatory and Analgesic Activities of SKLJI, a Highly Purified and Injectable Herbal Extract of Lonicera japonica

The parenteral route has many merits over the oral route, including greater predictability, reproducibility of absorption, and rapid drug action, but injectable phytomedicines are uncommon due to protein precipitating tannin and hemolytic saponin components. In this study, in an effort to develop a safe injectable analgesic phytomedicine, we prepared a tannin and saponin-free Lonicera japonica extract, SKLJI, through fractionation and column purification, and evaluated its anti-inflammatory and analgesic activities in in vivo experimental models of inflammation and pain. The removal of tannin and saponin resulted in loganin and sweroside-enriched SKLJI and it showed reduced hemolysis and protein precipitation. In efficacy tests, SKLJI inhibited croton oil- and arachidonic acid-induced ear edema, acetic acid-induced writhing, and carrageenan-induced rat hind paw hyperalgesia. Inhibition of cylcooxygenase-2 (COX-2), inducible nitric oxide synthase (iNOS), and 5-lipoxyfenase (5-LO) activities by SKLJI appeared to be the mechanism underlying anti-inflammatory and analgesic efficacy. Loganin and sweroside also showed anti-inflammatory and analgesic activities, suggesting that they might be active principles in the efficacy of SKLJI. These results suggest that SKLJI is a viable candidate for a new anti-inflammatory and analgesic phytomedicine that can be administered by the parenteral route.

U-shaped Dose Response in Vasomotor Tone: A Mixed Result of Heterogenic Response of Multiple Cells to Xenobiotics

U-shaped response has been frequently encountered in various biological areas including epidemiology, toxicology, and oncology. Despite its frequent observation, the theory of U-shaped response has been crippled by the lack of a robust mechanism underlying and incomplete in vitro and in vivo correlation. In the present study, a novel mechanism is provided for a U-shaped response, based on the findings of agonist-induced vasomotor tone change affected by menadione (MEN) (synthetic vitamin K(3)), a reactive oxygen species generator, and arsenic, an environmental pollutant, which showed typical U-shaped responses in both in vitro aortic contractile response and in vivo blood pressure. U-shaped responses by MEN and arsenic were a combined result from heterogenic susceptibilities and responses of multiple target cells composing blood vessels, that is, endothelium and smooth muscle. Notably, endothelium, a regulator of vasomotor tone, was primarily affected by low-dose stimuli, whereas smooth muscle, an effector of vascular contraction, was affected later by high-dose. The dysfunction of smooth muscle was produced by high-dose MEN-induced hydrogen peroxide, resulting in the attenuation of vascular contractile reactivity, whereas low-dose MEN-induced superoxide led to the quenching of vasodilatory nitric oxide in endothelial cells, resulting in the enhancement of vasoconstriction. This mechanistic theory, the difference in susceptibilities and responses to a common stimulus between regulator and effector components of a system, could give a new insight into the explanation of various U-shaped responses and provide a new evidence for the need of the risk assessment of toxicants with a wider dose range.

Doxorubicin-induced platelet procoagulant activities: an important clue for chemotherapy-associated thrombosis.

Thrombotic risk associated with chemotherapy including doxorubicin (DOX) has been frequently reported; yet, the exact mechanism is not fully understood. Here, we report that DOX can induce procoagulant activity in platelets, an important contributor to thrombus formation. In human platelets, DOX increased phosphatidylserine (PS) exposure and PS-bearing microparticle (MP) generation. Consistently, DOX-treated platelets and generated MPs induced thrombin generation, a representative marker for procoagulant activity. DOX-induced PS exposure appeared to be from intracellular Ca²⁺ increase and ATP depletion, which resulted in the activation of scramblase and inhibition of flippase. Along with this, apoptosis was induced by DOX as determined by the dissipation of mitochondrial membrane potential (Δψ), cytochrome c release, Bax translocation, and caspase-3 activation. A Ca²⁺ chelator ethylene glycol tetraacetic acid, caspase inhibitor Q-VD-OPh, and antioxidants (vitamin C and trolox) can attenuate DOX-induced PS exposure and procoagulant activity significantly, suggesting that Ca²⁺, apoptosis, and reactive oxygen species (ROS) were involved in DOX-enhanced procoagulant activity. Importantly, rat in vivo thrombosis model demonstrated that DOX could manifest prothrombotic effects through the mediation of platelet procoagulant activity, which was accompanied by increased PS exposure and Δψ dissipation in platelets.

Novel Antiplatelet Activity of Protocatechuic Acid through the Inhibition of High Shear Stress-Induced Platelet Aggregation

Bleeding is the most common and serious adverse effect of currently available antiplatelet drugs. Many efforts are being made to develop novel antithrombotic agents without bleeding risks. Shear stress-induced platelet aggregation (SIPA), which occurs under abnormally high shear stress, plays a crucial role in the development of arterial thrombotic diseases. Here, we demonstrate that protocatechuic acid (PCA), a bioactive phytochemical from Lonicera (honeysuckle) flowers, selectively and potently inhibits high shear (>10,000 s−1)-induced platelet aggregation. In isolated human platelets, PCA decreased SIPA and attenuated accompanying platelet activation, including intracellular calcium mobilization, granule secretion, and adhesion receptor expression. The anti-SIPA effect of PCA was mediated through blockade of von Willebrand factor binding to activated glycoprotein Ib, a primary and initial event for the accomplishment of SIPA. Conspicuously, PCA did not inhibit platelet aggregation induced by other endogenous agonists like collagen, thrombin, or ADP that are important in both pathological thrombosis and normal hemostasis. Antithrombotic effects of PCA were confirmed in vivo in a rat arterial thrombosis model, where PCA significantly delayed the arterial occlusion induced by FeCl3. Of particular note, PCA did not increase bleeding times in a rat tail transection model, whereas conventional antiplatelet drugs, aspirin, and clopidogrel substantially prolonged it. Collectively, these results suggest that PCA may be a novel antiplatelet agent that can prevent thrombosis without increasing bleeding risks.

Vascular Smooth Muscle Dysfunction and Remodeling Induced by Ginsenoside Rg3, a Bioactive Component of Ginseng

Ginseng, one of most well-known herbal medicines, is widely and indiscreetly used among the patients with cardiovascular disorders, raising concern over abuse of this medicine and unwanted effects. In this study, we investigated the effects of ginsenoside Rg3 (Rg3), an active ingredient of ginseng, on vascular contractility and structural integrity to explore its potential vascular toxicity. In isolated rat aorta, Rg3 suppressed the normal agonist-induced contractile response. This suppression persisted even after a rigorous washout. In the endothelium-denuded aortic ring, impairment of vascular contractility by Rg3 was retained, suggesting that vascular smooth muscle was affected. In primary vascular smooth muscle cells, Rg3 abolished agonist-induced Ca(2+) increase, indicating that Ca(2+) regulation was disrupted. Rg3 suppressed the contraction induced by Bay K8644, an L-type Ca(2+) channel activator, whereas store-operated Ca(2+) channel or intracellular Ca(2+) store-mediated contraction was not affected, suggesting that the L-type Ca(2+) channel was selectively impaired by Rg3. These in vitro results were further confirmed in vivo where Rg3 treatment significantly attenuated the agonist-induced pressor response. More importantly, 4-week repeated treatment with Rg3 in normal animals induced eutrophic outward remodeling in the thoracic aorta, that is, it brought about an increased luminal area without changes in the wall area. These results suggest that Rg3 can induce the vascular smooth muscle dysfunction by disturbing Ca(2+) influx from the L-type Ca(2+) channel, ultimately leading to impaired vascular contractility and structural remodeling.

Trivalent methylated arsenical-induced phosphatidylserine exposure and apoptosis in platelets may lead to increased thrombus formation

Trivalent methylated metabolites of arsenic, monomethylarsonous acid (MMA(III)) and dimethylarsinous acid (DMA(III)), have been found highly reactive and toxic in various cells and in vivo animal models, suggesting their roles in the arsenic-associated toxicity. However, their effects on cardiovascular system including blood cells, one of the most important targets for arsenic toxicity, remain poorly understood. Here we found that MMA(III) and DMA(III) could induce procoagulant activity and apoptosis in platelets, which play key roles in the development of various cardiovascular diseases (CVDs) through excessive thrombus formation. In freshly isolated human platelets, treatment of MMA(III) resulted in phosphatidylserine (PS) exposure, a hallmark of procoagulant activation, accompanied by distinctive apoptotic features including mitochondrial membrane potential disruption, cytochrome c release, and caspase-3 activation. These procoagulant activation and apoptotic features were found to be mediated by the depletion of protein thiol and intracellular ATP, and flippase inhibition by MMA(III), while the intracellular calcium increase or reactive oxygen species generation was not involved. Importantly, increased platelet procoagulant activity by MMA(III) resulted in enhanced blood coagulation and excessive thrombus formation in a rat in vivo venous thrombosis model. DMA(III) also induced PS-exposure with apoptotic features mediated by protein thiol depletion, which resulted in enhanced thrombin generation. In summary, we believe that this study provides an important evidence for the role of trivalent methylated arsenic metabolites in arsenic-associated CVDs, giving a novel insight into the role of platelet apoptosis in toxicant-induced cardiovascular toxicity.