Jialan Shi

Membrane Phosphatidylserine Regulates Surface Charge and Protein Localization

Electrostatic interactions with negatively charged membranes contribute to the subcellular targeting of proteins with polybasic clusters or cationic domains. Although the anionic phospholipid phosphatidylserine is comparatively abundant, its contribution to the surface charge of individual cellular membranes is unknown, partly because of the lack of reagents to analyze its distribution in intact cells. We developed a biosensor to study the subcellular distribution of phosphatidylserine and found that it binds the cytosolic leaflets of the plasma membrane, as well as endosomes and lysosomes. The negative charge associated with the presence of phosphatidylserine directed proteins with moderately positive charge to the endocytic pathway. More strongly cationic proteins, normally associated with the plasma membrane, relocalized to endocytic compartments when the plasma membrane surface charge decreased on calcium influx.

Evidence of Hepatocyte Apoptosis in Rat Liver after the Administration of Carbon Tetrachloride

In acute liver injury induced by the injection of CCl4, cell death has been attributed to the necrosis of hepatocytes in the centrilobular area. In the present study, we re-examined the hepatic injury evoked by CCl4 in rats and explored the possibility that apoptosis may also contribute to its pathogenesis. Apoptotic hepatocytes were identified and quantified by light and electron microscopy, the in situ immunohistochemical labeling of nuclear DNA fragmentation, flow cytometry, and DNA gel electrophoresis. We found that a substantial number of hepatocytes underwent apoptosis. Apoptotic changes were also observed in ballooned hepatocytes. Apoptotic hepatocytes increased in number at 3 hours and peaked at 6 hours after the CCl4 injection. Apoptotic bodies were sequestrated in the adjacent hepatocytes and sinusoidal cells. Double staining of the cells with immunostaining for phagocytes and terminal deoxynucleotidyl transferase-mediated dUTP nick end-labeling staining for labeling of DNA fragmentation showed that the majority of apoptotic hepatocytes were phagocytosed by Kupffer cells and macrophages. The results indicated that apoptosis occurs in the ballooned and injured hepatocytes of the centrilobular area. What occurs after CCl4 administration may be important in reducing inflammation, shortening the course of acute hepatic injury, and preventing the development of fibrosis.

Lactadherin Detects Early Phosphatidylserine Exposure on Immortalized Leukemia Cells Undergoing Programmed Cell Death

Phosphatidylserine (PS) appears on the outer membrane leaflet of cells undergoing programmed cell death and marks those cells for clearance by macrophages. Macrophages secrete lactadherin, a PS‐binding protein, which tethers apoptotic cells to macrophage integrins.

Lactadherin blocks thrombosis and hemostasis in vivo: correlation with platelet phosphatidylserine exposure

Summary.  Background: Platelet membrane phosphatidylserine (PS) is considered to be essential for hemostasis and thrombosis, but the in vivo topography of platelet PS has not been characterized. We hypothesized that platelet PS exposure would be identified on adherent platelets at the site of vascular injury and that blockade of PS would impede hemostasis and thrombosis. Objective: To localize and estimate the extent of platelet PS exposure and evaluate the impact of PS blockade in vivo. Methods: Lactadherin, a PS‐binding milk protein, was utilized together with annexin V to detect both partial and complete membrane PS exposure on platelets in a mouse model of thrombosis and to evaluate the functional need for PS. Preliminary experiments were performed with synthetic membranes and with purified platelets. Results: The number of lactadherin‐binding sites on synthetic membranes was proportional to PS content, whereas annexin V required a threshold of 2.5–8% PS. Approximately 95% of thrombin‐stimulated platelets exposed PS, but the quantity was below the threshold for annexin V binding at physiologic Ca2+ concentrations. In mice, most adherent and aggregated platelets on the walls of ferric chloride‐treated mesenteric veins exposed low levels of PS, rather than having complete exposure. In mice, blockade of PS with lactadherin inhibited platelet prothrombinase and factor Xase activity, and prolonged tail bleeding time and the time to carotid artery thrombosis. Conclusions: In vivo PS exposure contributes to both hemostasis and thrombosis. In this model of vascular injury, most platelets exhibit partial rather than complete PS exposure.

Procoagulant activity of erythrocytes and platelets through phosphatidylserine exposure and microparticles release in patients with nephrotic syndrome

Recent studies showed that an imbalance of prothrombotic and antithrombotic factors and impaired thrombolytic activity contribute to the thrombophilia of the nephrotic syndrome (NS). However, it is not clear whether blood cell injury and/or activation is involved in hypercoagulability in NS patients. Our objectives were to study the increase in microparticle (MP) release and phosphatidylserine (PS) exposure on the outer membrane of MP-origin cells in NS patients, and to evaluate their procoagulant activity (PCA). The subjects were patients with membranous nephropathy (MN), minimal change nephrotic syndrome (MCNS) and healthy controls. Analyses of MPs and PS exposure were performed using a flow cytometer. PCA was determined by clotting time and purified coagulation complex assays. We found that lactadherin+ MPs, which derived from red blood cells (RBC), platelet and endothelial cell, increased in NS patients. Moreover, PS exposure on RBCs and platelets in each NS group, especially in MN, are higher than that in controls. MP shedding and PS exposure of RBCs/platelets were highly procoagulant in NS patients. However, blockade of PS with lactadherin inhibited over 90% of PCA while an anti-tissue factor antibody had no significant inhibition effect. Our results demonstrate that the thrombophilic susceptibility of NS may be partly ascribed to MP release and PS exposure of RBCs, platelets and endothelial cells. Lactadherin is a sensitive probe for PS that has high anticoagulant activity.

Lactadherin functions as a probe for phosphatidylserine exposure and as an anticoagulant in the study of stored platelets

Background and Objectives  Annexin V, the long established standard method of measuring phosphatidylserine (PS) exposure, is not the most suitable probe for the study of stored platelets because of calcium dependence and low sensitivity under 8% PS exposure. The aim of this study was to show lactadherin as a sensitive probe for PS exposure and an effective anticoagulant for stored platelets.

Phosphatidylserine exposure and procoagulant activity in acute promyelocytic leukemia

Summary.  Background: Acute promyelocytic leukemia (APL) frequently causes disseminated intravascular coagulation that can worsen with cytotoxic chemotherapy but improve with the therapeutic differentiating agents, all trans retinoic acid (ATRA) and arsenic trioxide (As2O3). APL cells display tissue factor but the relationship of tissue factor and other procoagulant activity to phosphatidylserine (PS) exposure is largely unknown. Methods: Lactadherin, a milk protein with stereospecific binding to phosphatidyl‐L‐serine, was used as a probe for PS exposure on an immortalized APL cell line (NB4) and on the cells of eight patients with APL. PS exposure was evaluated with flow cytometry, confocal microscopy, coagulation assays, and purified prothrombinase and factor (F) Xase assays. Results: Plasma procoagulant activity of NB4 and APL cells increased approximately 15‐fold after exposure to etoposide or daunorubicin and decreased 80% after treatment with ATRA or As2O3. Procoagulant activity corresponded to exposed PS on viable APL cells. PS exposure decreased after treatment with ATRA or As2O3 and increased after treatment with daunorubicin or etoposide. Excess lactadherin inhibited 80–85% of intrinsic FXase, FVIIa‐tissue factor and prothrombinase activities on both NB4 cells and APL cells. Confocal microscopy identified membrane patches that stained with lactadherin, but not annexin V, demonstrating focal, low‐level PS exposure. Conclusions: PS is exposed on viable APL cells and is necessary for approximately 80% of procoagulant activity.

Daunorubicin induces procoagulant activity of cultured endothelial cells through phosphatidylserine exposure and microparticles release

Administration of various chemotherapeutic agents is associated with an increased risk of thrombotic events. Although vascular endothelium plays a predominant role in blood coagulation and fibrinolysis, the effect of cytotoxic drugs on the procoagulant activity (PCA) of endothelial cells has not been well evaluated. Our study aims to investigate the possibility that daunorubicin, a chemotherapeutic agent, exerts prothrombotic effect on endothelial cells. We tested the impact of daunorubicin on phosphatidylserine (PS) exposure, endothelial microparticles (EMPs) release and consequent PCA. Human umbilical vein endothelial cells (HUVECs) were treated with daunorubicin (0.1, 0.2, 0.5, 1, 2 mM) for 24 hours. PCA of HUVECs was measured using clotting time and purified coagulation complex assays. Counts and PCA of EMPs were evaluated by flow cytometry and clotting time assay, respectively. Lactadherin was used as a novel probe for detection of PS exposure and EMPs release. We found that daunorubicin dose-dependently increased the PS exposure and consequent PCA of HUVECs. Moreover, daunorubicin treatment also enhanced the release of EMPs which were highly procoagulant. This increment was especially significant at 0.2 mM of daunorubicin or more. Blockade of PS with lactadherin inhibited over 90% of HUVECs and EMPs PCA. However, anti-TF antibody had no significant inhibition effect. Our results demonstrate that daunorubicin treatment enhanced PCA of HUVECs through PS exposure and shedding of procoagulant EMPs. Lactadherin acts as an efficient anticoagulant in this process.

Role of erythrocytes and platelets in the hypercoagulable status in polycythemia vera through phosphatidylserine exposure and microparticle generation

The development of thrombosis in polycythaemia vera (PV) involves multifactorial processes including pathological activation of blood cells. Release of microparticles (MPs) by activated cells in diseases is associated with thrombotic risk, but relatively few data are available in PV. The aim of the present study was to investigate the increase in MP release and exposure of phosphatidylserine (PS) on the outer membrane of MP-origin cells in patients with PV, and to analyse their procoagulant activity (PCA). PS-positive MPs and cells were detected by flow cytometry, while PCA was assessed with clotting time and purified coagulation complex assays. We found that PV patients had elevated circulating lactadherin+ MPs, which mostly originating from erythrocytes, platelets, granulocytes, and endothelial cells, as well as increased PS exposing erythrocytes/platelets as compared to secondary polycythaemia patients or healthy controls. These PS-bearing MPs and cells were highly procoagulant. Moreover, lactadherin competed factor V and VIII to PS and inhibited about 90% of the detected PCA in a dose-response manner while anti-TF antibody did no significant inhibition. Treatment with hydroxyurea is associated with a decrease in PS exposure and lactadherin+ MP release of erythrocytes/platelets. Our data demonstrate that PV patients are characterised by increased circulating procoagulant MPs and PS exposing erythrocytes/platelets, which could contribute to the hypercoagulable state in these patients.

Targeting homologous recombination and telomerase in Barrett's adenocarcinoma: Impact on telomere maintenance, genomic instability and tumor growth

Homologous recombination (HR), a mechanism to accurately repair DNA in normal cells, is deregulated in cancer. Elevated/deregulated HR is implicated in genomic instability and telomere maintenance, which are critical lifelines of cancer cells. We have previously shown that HR activity is elevated and significantly contributes to genomic instability in Barrett’s esophageal adenocarcinoma (BAC). The purpose of this study was to evaluate therapeutic potential of HR inhibition, alone and in combination with telomerase inhibition, in BAC. We demonstrate that telomerase inhibition in BAC cells increases HR activity, RAD51 expression, and association of RAD51 to telomeres. Suppression of HR leads to shorter telomeres as well as markedly reduced genomic instability in BAC cells over time. Combination of HR suppression (whether transgenic or chemical) with telomerase inhibition, causes a significant increase in telomere attrition and apoptotic death in all BAC cell lines tested, relative to either treatment alone. A subset of treated cells also stain positive for β-galactosidase, indicating senescence. The combined treatment is also associated with decline in S-phase and a strong G2/M arrest, indicating massive telomere attrition. In a subcutaneous tumor model, the combined treatment resulted in the smallest tumors, which were even smaller (P=0.001) than those that resulted from either treatment alone. Even the tumors removed from these mice had significantly reduced telomeres and evidence of apoptosis. We therefore conclude that although telomeres are elongated by telomerase, elevated RAD51/HR assist in their maintenance/stabilization in BAC cells. Telomerase inhibitor prevents telomere elongation but induces RAD51/HR, which contributes to telomere maintenance/stabilization and prevention of apoptosis, reducing the efficacy of treatment. Combining HR inhibition with telomerase renders telomeres more vulnerable to degradation and significantly increases/expedites their attrition, leading to apoptosis. We therefore demonstrate that a therapy targeting HR and telomerase has the potential to prevent both tumor growth and genomic evolution in BAC.