N,N,N',N'-tetrakis(2-pyridylmethyl)ethylenediamine induces apoptosis through the activation of caspases-3 and -8 in human platelets. A role for endoplasmic reticulum stress

Functional role of TRPC6 and STIM2 in cytosolic and endoplasmic reticulum Ca2+ content in resting estrogen receptor-positive breast cancer cells

TRPC6 forms non-selective cation channels activated by a variety of stimuli that are involved in a wide number of cellular functions. In estrogen receptor-positive (ER+) breast cancer cells, the store-operated Ca2+ entry has been reported to be dependent on STIM1, STIM2 and Orai3, with TRPC6 playing a key role in the activation of store-operated Ca2+ entry as well as in proliferation, migration and viability of breast cancer cells. We have used a combination of biotinylation, Ca2+ imaging as well as protein knockdown and overexpression of a dominant-negative TRPC6 mutant (TRPC6dn) to show that TRPC6 and STIM2 are required for the maintenance of cytosolic and endoplasmic reticulum Ca2+ content under resting conditions in ER+ breast cancer MCF7 cells. These cells exhibit a greater plasma membrane expression of TRPC6 under resting conditions than non-tumoral breast epithelial cells. Attenuation of STIM2, TRPC6 and Orai3, alone or in combination, results in impairment of resting cytosolic and endoplasmic reticulum Ca2+ homeostasis. Similar results were observed when cells were transfected with expression plasmid for TRPC6dn. TRPC6 co-immunoprecipitates with STIM2 in resting MCF7 cells, a process that is impaired by rises in cytosolic Ca2+ concentration. Impairment of TRPC6 function leads to abnormal Ca2+ homeostasis and endoplasmic reticulum stress, thus, suggesting that TRPC6 might be a potential target for the development of anti-tumoral therapies.

NO1, a New Sigma 2 Receptor/TMEM97 Fluorescent Ligand, Downregulates SOCE and Promotes Apoptosis in the Triple Negative Breast Cancer Cell Lines

(1) Background: The structure of the Sigma 2 receptor/TMEM97 (σ2RTMEM97) has recently been reported. (2, 3) Methods and results: We used genetic and biochemical approaches to identify the molecular mechanism downstream of σ2R/TMEM97. The novel σ2R/TMEM97 fluorescent ligand, NO1, reduced the proliferation and survival of the triple negative breast cancer cell lines (TNBC: MDA-MB-231 and MDA-MB-468 cell lines), due to NO1-induced apoptosis. Greater bioaccumulation and faster uptake of NO1 in MDA-MB-231 cells compared to MCF10A or MCF7 cell lines were also shown. Accordingly, elevated σ2R/TMEM97 expression was confirmed by Western blotting. In contrast to NO1, other σ2R/TMEM97 ligands, such as SM21 and PB28, enhanced MDA-MB-231 cell proliferation and migration. Store-operated calcium entry (SOCE) is crucial for different cancer hallmarks. Here, we show that NO1, but not other σ2R/TMEM97 ligands, reduced SOCE in MDA-MB-231 cells. Similarly, TMEM97 silencing in MDA-MB-231 cells also impaired SOCE. NO1 administration downregulated STIM1-Orai1 interaction, probably by impairing the positive regulatory effect of σ2R/TMEM97 on STIM1, as we were unable to detect interaction with Orai1. (4) Conclusion: σ2R/TMEM97 is a key protein for the survival of triple negative breast cancer cells by promoting SOCE; therefore, NO1 may become a good pharmacological tool to avoid their proliferation.

Mouse transient receptor potential channel type 6 selectively regulates agonist-induced platelet function

While changes in intracellular calcium levels is a central step in platelet activation and thrombus formation, the contribution and mechanism of receptor-operated calcium entry (ROCE) via transient receptor potential channels (TRPCs) in platelets remains poorly defined. In previous studies, we have shown that TRPC6 regulates hemostasis and thrombosis, in mice. In the present studies, we employed a knockout mouse model system to characterize the role of TRPC6 in ROCE and platelet activation. It was observed that the TRPC6 deletion (Trpc6^−/−) platelets displayed impaired elevation of intracellular calcium, i.e., defective ROCE. Moreover, these platelets also exhibited defects in a host of functional responses, namely aggregation, granule secretion, and integrin αIIbβ3. Interestingly, the aforementioned defects were specific to the thromboxane receptor (TPR), as no impaired responses were observed in response to ADP or the thrombin receptor-activating peptide 4 (TRAP4). The defect in ROCE in the Trpc6^−/− was also observed with 1-oleoyl-2-acetyl-sn-glycerol (OAG). Finally, our studies also revealed that TRPC6 regulates clot retraction. Taken together, our findings demonstrate that TRPC6 directly regulates TPR-dependent ROCE and platelet function. Thus, TRPC6 may serve as a novel target for the therapeutic management of thrombotic diseases.

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TRPC6 regulates TPR-mediated/receptor-operated calcium entry.

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TRPC6 regulates TPR-dependent platelet aggregation, secretion and integrin activation.

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TRPC6 regulates clot retraction.

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TRPC6 expression levels are age-dependent in platelets.

βγ-CAT, a non-lens betagamma-crystallin and trefoil factor complex, induces calcium-dependent platelet apoptosis

In recent years, it has been reported that apoptosis may occur in platelets and play a role in the clearance of effete platelets. βγ-CAT, a newly identified non-lens βγ-crystallin and trefoil factor complex from frog Bombina maxima skin secretions, caused several in vivo toxic effects on mammals. Through determined haematological parameters of rabbits, it has been found that βγ-CAT significantly reduced the number of platelets in a time-dependent manner. Here, in order to explore the effect of βγ-CAT on platelets, washed platelets were incubated with various concentrations of βγ-CAT for 30 minutes. We found that βγ-CAT induced several apoptosis events in human platelets, including caspase-3 activation, phosphatidylserine (PS) exposure, depolarisation of mitochondrial inner transmembrane potential (ΔΨm), cytochrome c re-lease and strong expression of pro-apoptotic Bax and Bak proteins. However, βγ-CAT did not significantly induce platelet activation as detected by P-selectin surface expression, GPIIb/IIIa activation and platelet aggregation. In addition, we observed that βγ-CAT-induced PS exposure and ΔΨm depolarisation in platelets are Ca2+-dependent. Taken together, βγ-CAT can induce Ca2+-dependent platelet apoptosis but does not cause platelet activation.

Age-related cataracts: Role of unfolded protein response, Ca2+ mobilization, epigenetic DNA modifications, and loss of Nrf2/Keap1 dependent cytoprotection

Age-related cataracts are closely associated with lens chronological aging, oxidation, calcium imbalance, hydration and crystallin modifications. Accumulating evidence indicates that misfolded proteins are generated in the endoplasmic reticulum (ER) by most cataractogenic stresses. To eliminate misfolded proteins from cells before they can induce senescence, the cells activate a clean-up machinery called the ER stress/unfolded protein response (UPR). The UPR also activates the nuclear factor-erythroid-2-related factor 2 (Nrf2), a central transcriptional factor for cytoprotection against stress. Nrf2 activates nearly 600 cytoprotective target genes. However, if ER stress reaches critically high levels, the UPR activates destructive outputs to trigger programmed cell death. The UPR activates mobilization of ER-Ca2+ to the cytoplasm and results in activation of Ca2+-dependent proteases to cleave various enzymes and proteins which cause the loss of normal lens function. The UPR also enhances the overproduction of reactive oxygen species (ROS), which damage lens constituents and induce failure of the Nrf2 dependent cytoprotection. Kelch-like ECH-associated protein 1 (Keap1) is an oxygen sensor protein and regulates the levels of Nrf2 by the proteasomal degradation. A significant loss of DNA methylation in diabetic cataracts was found in the Keap1 promoter, which overexpresses the Keap1 protein. Overexpressed Keap1 significantly decreases the levels of Nrf2. Lower levels of Nrf2 induces loss of the redox balance toward to oxidative stress thereby leading to failure of lens cytoprotection. Here, this review summarizes the overall view of ER stress, increases in Ca2+ levels, protein cleavage, and loss of the well-established stress protection in somatic lens cells.

The contribution of zinc to platelet behaviour during haemostasis and thrombosis

Platelets are the primary cellular determinants of haemostasis and pathological thrombus formation leading to myocardial infarction and stroke. Following vascular injury or atherosclerotic plaque rupture, platelets are recruited to sites of damage and undergo activation induced by a variety of soluble and/or insoluble agonists. Platelet activation is a multi-step process culminating in the formation of thrombi, which contribute to the haemostatic process. Zinc (Zn(2+)) is acknowledged as an important signalling molecule in a diverse range of cellular systems, however there is limited understanding of the influence of Zn(2+) on platelet behaviour during thrombus formation. This review evaluates the contributions of exogenous and intracellular Zn(2+) to platelet function and assesses the potential pathophysiological implications of Zn(2+) signalling. We also provide a speculative assessment of the mechanisms by which platelets could respond to changes in extracellular and intracellular Zn(2+) concentration.

Prostate-specific membrane antigen-targeted liposomes specifically deliver the Zn(2+) chelator TPEN inducing oxidative stress in prostate cancer cells

AIM

To evaluate the potential use of zinc chelation for prostate cancer therapy using a new liposomal formulation of the zinc chelator, N,N,N',N'-tetrakis(2-pyridylmethyl)-ethylenediamine (TPEN).

MATERIALS & METHODS

TPEN was encapsulated in nontargeted liposomes or liposomes displaying an aptamer to target prostate cancer cells overexpression prostate-specific membrane antigen. The prostate cancer selectivity and therapeutic efficacy of liposomal (targeted and nontargeted) and free TPEN were evaluated in vitro and in tumor-bearing mice.

RESULTS & CONCLUSION

TPEN chelates zinc and results in reactive oxygen species imbalance leading to cell death. Delivery of TPEN using aptamer-targeted liposomes results in specific delivery to targeted cells. In vivo experiments show that TPEN-loaded, aptamer-targeted liposomes reduce tumor growth in a human prostate cancer xenograft model.

Zinc Chelation Mediates the Lysosomal Disruption without Intracellular ROS Generation

We report the molecular mechanism for zinc depletion caused by TPEN (N,N,N′,N′-Tetrakis(2-pyridylmethyl)ethylenediamine) in neuroblastoma cells. The activation of p38 MAP kinase and subsequently caspase 3 is not due to or followed by redox imbalance or ROS generation, though these are commonly observed in literature. We found that TPEN is not responsible for ROS generation and the mechanism involves essentially lysosomal disruption caused by intracellular zinc depletion. We also observed a modest activation of Bax and no changes in the Bcl-2 proteins. As a result, we suggest that TPEN causes intracellular zinc depletion which can influence the breakdown of lysosomes and cell death without ROS generation.

Zinc deficiency mediates alcohol-induced apoptotic cell death in the liver of rats through activating ER and mitochondrial cell death pathways

Hepatic zinc deficiency has been well documented in alcoholic patients, but the mechanisms by which zinc deficiency mediates cell death have not been well defined. The objectives of this study were to determine whether alcohol perturbs subcellular zinc homeostasis and how organelle zinc depletion may link with cell death pathways. Wistar rats were pair-fed with the Lieber-DeCarli control or ethanol diet for 5 mo. Chronic alcohol exposure significantly reduced zinc level in isolated hepatic endoplasmic reticulum (ER) and mitochondria. Among the detected zinc transporters, ER Zrt/Irt-like protein (ZIP)13 and mitochondrial ZIP8, which transport zinc from ER and mitochondria to cytosol, were significantly increased. Mitochondrial zinc transporter (ZnT) 4, which transports zinc from cytosol to mitochondria, was also increased. ER phosphorylated eukaryotic initiation factor 2α, activating transcription factor 4, and C/EBP homologous protein were significantly upregulated, and mitochondrial cytochrome c release and Bax insertion were detected in association with caspase-3 activation and apoptotic cell death. To define the role of zinc deficiency in ER and mitochondrial stress, H4IIEC3 cells were treated with 3 μM N,N,N',N'-tetrakis (2-pyridylmethyl) ethylenediamine for 6 h with or without supplementation with zinc or N-acetylcysteine (NAC). The results demonstrated that zinc deprivation induced caspase-3 activation and apoptosis in association with ER and mitochondria dysfunction, which were inhibited by zinc as low as 10 μM but not by 2 mM NAC. These results suggest that chronic ethanol exposure induced in ER and mitochondrial zinc deficiency might activate intrinsic cell death signaling pathway, which could not be effectively rescued by antioxidant treatment.

Biologicals, platelet apoptosis and human diseases: An outlook

Platelets, once considered mediators of hemostasis and thrombosis, are now known to be involved in wound healing, inflammation, cardiovascular diseases, diabetes, arthritis, and cancer. Recent reports attest that platelets possess the cellular machinery to undergo apoptosis and that platelet apoptosis can be triggered by myriad stimuli including chemical and physical agonists, and pathophysiological conditions. Augmented rate of platelet apoptosis leads to thrombocytopenia, bleeding disorders and microparticle generation. Despite knowing the significant role of platelets in health and disease, and that any alterations in platelet functions can wreak havoc to the health, the offshoot reactions of therapeutic drugs on platelets and the far-reaching consequences are often neglected. The present review focuses on the impact of platelet apoptosis and the role of platelet-derived microparticles on different pathophysiological conditions. It also touches upon the effects of biologicals on platelets, and discusses the need to overcome the adverse effects of pro-apoptotic drugs through auxiliary therapy.

Therapeutic drug-induced platelet apoptosis: an overlooked issue in pharmacotoxicology

The surfacing of the applied fields of biology such as, biotechnology, pharmacology and drug discovery was a boon to the modern man. However, it had its share of disadvantages too. The indiscriminate use of antibiotics and other biological drugs resulted in numerous adverse reactions including thrombocytopenia. One of the reasons for drug-induced thrombocytopenia could be attributed to an enhanced rate of platelet apoptosis, which is a less investigated aspect. The present essay sheds light on the adverse (pro-apoptotic) effects of some of the commonly used drugs and antibiotics on platelets viz. cisplatin, aspirin, vancomycin and balhimycin. Furthermore, the undesirable reactions resulting from chemotherapy could be attributed at least to some extent to the systemic stress induced by microparticles, which in turn are the byproducts of platelet apoptosis. Thereby, the essay aims to highlight the challenges in the emerging trend of cross-disciplinary implications, i.e., drug-induced platelet apoptosis, which is a nascent field. Thus, the different mechanisms through which drugs induce platelet apoptosis are discussed, which also opens up a new perspective through which the adverse effects of commonly used drugs could be dealt. The drug-associated platelet toxicity is of grave concern and demands immediate attention. Besides, it would also be appealing to examine the platelet pro-apoptotic effects of other commonly used therapeutic drugs.

Mechanisms of andrographolide-induced platelet apoptosis in human platelets: regulatory roles of the extrinsic apoptotic pathway

Andrographolide, a novel nuclear factor-κB (NF-κB) inhibitor, is isolated from the leaves of Andrographis paniculata. Platelet activation is relevant to a variety of coronary heart diseases. Our recent studies revealed that andrographolide possesses potent antiplatelet activity by inhibition of the p38 MAPK/(●) HO-NF-κB-ERK2 cascade. Although platelets are anucleated cells, apoptotic machinery apparatus recently has been found to regulate platelet activation and limit platelet lifespan. Therefore, we further investigated the regulatory effects of andrographolide on platelet apoptotic events. In this study, apoptotic signaling events for caspase-3, -8, and Bid were time (10-60 min)- and dose (25-100 μΜ)-dependently activated by andrographolide in human platelets. Andrographolide could also disrupt mitrochondrial membrane potential. In addition, caspase-8 inhibitor (z-IETD-fmk, 50 μΜ) was found to reverse andrographolide-induced caspase-8 activation, whereas the antagonistic anti-Fas receptor (ZB4, 500 ng/mL) and anti-tumor necrosis factor-R1 (H398, 10 µg/mL) monoclonal antibodies did not. In conclusion, this study for the first time demonstrated that andrographolide might limit platelet lifespan by initiating the caspase-8-dependent extrinsic apoptotic pathway, in spite of no direct evidence that death receptors are involved in this process proved. Overall, the various medicinal properties of andrographolide suggest its potential value in treating patients with thromboembolic disorders.

TPEN induces apoptosis independently of zinc chelator activity in a model of acute lymphoblastic leukemia and ex vivo acute leukemia cells through oxidative stress and mitochondria caspase-3- and AIF-dependent pathways

Acute lymphoblastic leukemia is still an incurable disease with resistance to therapy developing in the majority of patients. We investigated the effect of TPEN, an intracellular zinc chelator, in Jurkat and in ex vivo acute lymphoblastic leukemia (ALL) cells resistant to chemotherapy. Changes of nuclei morphology, reactive oxygen species generation, presence of hypodiploid cells, phosphatidylserine translocation, mitochondrial membrane depolarization, immunohistochemical identification of cell death signalling molecules, and pharmacological inhibition were assayed to detect the apoptotic cell death pathways. We found that TPEN induces apoptosis in both types of cells by a molecular oxidative stress pathway involving O(2)(•-) > H(2)O(2) >> NF-κB (JNK/c-Jun) >p53> loss ΔΨ(m)> caspase-3, AIF > chromatin condensation/DNA fragmentation. Interestingly, TPEN induced apoptosis independently of glucose; leukemic cells are therefore devoid of survival capacity by metabolic resistance to treatment. Most importantly, TPEN cytotoxic effect can eventually be regulated by the antioxidant N-acetyl-cysteine and zinc ions. Our data suggest that TPEN can be used as a potential therapeutic prooxidant agent against refractory leukemia. These data contribute to understanding the importance of oxidative stress in the treatment of ALL.

Diabetes-induced hepatic pathogenic damage, inflammation, oxidative stress, and insulin resistance was exacerbated in zinc deficient mouse model

Objectives

Zinc (Zn) deficiency often occurs in the patients with diabetes. Effects of Zn deficiency on diabetes-induced hepatic injury were investigated.

Methods

Type 1 diabetes was induced in FVB mice with multiple low-dose streptozotocin. Hyperglycemic and age-matched control mice were treated with and without Zn chelator, N,N,N′,N′-tetrakis (2-pyridylemethyl) ethylenediamine (TPEN), at 5 mg/kg body-weight daily for 4 months. Hepatic injury was examined by serum alanine aminotransferase (ALT) level and liver histopathological and biochemical changes.

Results

Hepatic Zn deficiency (lower than control level, p<0.05) was seen in the mice with either diabetes or TPEN treatment and more evident in the mice with both diabetes and TPEN. Zn deficiency exacerbated hepatic injuries, shown by further increased serum ALT, hepatic lipid accumulation, inflammation, oxidative damage, and endoplasmic reticulum stress-related cell death in Diabetes/TPEN group compared to Diabetes alone. Diabetes/TPEN group also showed a significant decrease in nuclear factor-erythroid 2-related factor 2 (Nrf2) expression and transcription action along with significant increases in Akt negative regulators, decrease in Akt and GSK-3β phosphorylation, and increase in nuclear accumulation of Fyn (a Nrf2 negative regulator). In vitro study with HepG2 cells showed that apoptotic effect of TPEN at 0.5–1.0 µM could be completely prevented by simultaneous Zn supplementation at the dose range of 30–50 µM.

Conclusions

Zn is required for maintaining Akt activation by inhibiting the expression of Akt negative regulators; Akt activation can inhibit Fyn nuclear translocation to export nuclear Nrf2 to cytoplasm for degradation. Zn deficiency significantly enhanced diabetes-induced hepatic injury likely through down-regulation of Nrf2 function.

Activation of caspases-9, -3 and -8 in human platelets triggered by BH3-only mimetic ABT-737 and calcium ionophore A23187: caspase-8 is activated via bypass of the death receptors

Platelet apoptosis and activation have been studied in human platelets treated with BH3-only mimetic ABT-737 and calcium ionophore A23187, agents triggering apoptosis through the intrinsic mitochondrial pathway. Platelet apoptosis was determined as activation of crucial apoptosis-associated caspases, initiator caspase-9 of intrinsic apoptosis pathway, executioner caspase-3 and initiator caspase-8 of extrinsic death receptor pathway, and platelet activation was detected by P-selectin (CD62) exposure on the platelet surface. We found that ABT-737 predominantly induced activation of caspases-9, -3 and -8 rather than CD62 exposure, whereas A23187 induces both caspases activation and CD62 exposure. Caspase-8 activation was stimulated independently of the extrinsic apoptosis pathway via mitochondrial membrane permeabilization and depolarization. These data suggest that (i) caspase-8 activation is triggered in ABT-737- and A23187-treated anucleate platelets through the mitochondria-initiated caspase activation cascade bypassing the death receptors, and (ii) ABT-737-treated platelets are a useful experimental tool for discerning the role of platelet apoptosis in platelet function and survival.

Zinc enhances adiponectin oligomerization to octadecamers but decreases the rate of disulfide bond formation

Adiponectin, a hormone secreted from adipocytes, has been shown to protect against development of insulin resistance, ischemia-reperfusion injury, and inflammation. Adiponectin assembles into multiple oligomeric isoforms: trimers, hexamers and several higher molecular weight (HMW) species. Of these, the HMW species are selectively decreased during the onset of type 2 diabetes. Despite the critical role of HMW adiponectin in insulin responsiveness, its assembly process is poorly understood. In this report, we investigated the role of divalent cations in adiponectin assembly. Purified adiponectin 18mers, the largest HMW species, did not collapse to smaller oligomers after treatment with high concentrations of EDTA. However, treatment with EDTA or another chelator DTPA inhibited the oligomerization of 18mers from trimers in vitro. Zn(2+) specifically increased the formation of 18mers when compared with Cu(2+), Mg(2+), and Ca(2+). Distribution of adiponectin oligomers secreted from zinc chelator TPEN-treated rat adipocytes skewed toward increased proportions of hexamers and trimers. While we observed presence of zinc in adiponectin purified from calf serum, the role of zinc in disulfide bonding between oligomers was examined because the process is critical for 18mer assembly. Surprisingly, Zn(2+) inhibited disulfide bond formation early in the oligomerization process. We hypothesize that initial decreases in disulfide formation rates could allow adiponectin subunits to associate before becoming locked in fully oxidized conformations incapable of further oligomerization. These data demonstrate that zinc stimulates oligomerization of HMW adiponectin and possibly other disulfide-dependent protein assembly processes.

Apoptosis in the anucleate platelet

For many years, programmed cell death, known as apoptosis, was attributed exclusively to nucleated cells. Currently, however, apoptosis is also well-documented in anucleate platelets. This review describes extrinsic and intrinsic pathways of apoptosis in nucleated cells and in platelets, platelet apoptosis induced by multiple chemical stimuli and shear stresses, markers of platelet apoptosis, mitochodrial control of platelet apoptosis, and apoptosis mediated by platelet surface receptors PAR-1, GPIIbIIIa and GPIbα. In addition, this review presents data on platelet apoptosis provoked by aging of platelets in vitro during platelet storage, platelet apoptosis in pathological settings in humans and animal models, and inhibition of platelet apoptosis by cyclosporin A, intravenous immunoglobulin and GPIIbIIIa antagonist drugs.

Lipid rafts are essential for the regulation of SOCE by plasma membrane resident STIM1 in human platelets

STIM1 is a transmembrane protein essential for the activation of store-operated Ca²+ entry (SOCE), a major Ca²+ influx mechanism. STIM1 is either located in the endoplasmic reticulum, communicating the Ca²+ concentration in the stores to plasma membrane channels or in the plasma membrane, where it might sense the extracellular Ca²+ concentration. Plasma membrane-located STIM1 has been reported to mediate the SOCE sensitivity to extracellular Ca²+ through its interaction with Orai1. Here we show that plasma membrane lipid raft domains are essential for the regulation of SOCE by extracellular Ca²+. Treatment of platelets with the SERCA inhibitor thapsigargin (TG) induced Mn²+ entry, which was inhibited by increasing concentrations of extracellular Ca²+. Platelet treatment with methyl-β-cyclodextrin, which removes cholesterol and disrupts the lipid raft domains, impaired the inactivation of Ca²+ entry induced by extracellular Ca²+. Methyl-β-cyclodextrin also abolished translocation of STIM1 to the plasma membrane stimulated by treatment with TG and prevented TG-evoked co-immunoprecipitation between plasma membrane-located STIM1 and the Ca²+ permeable channel Orai1. These findings suggest that lipid raft domains are essential for the inactivation of SOCE by extracellular Ca²+ mediated by the interaction between plasma membrane-located STIM1 and Orai1.

Comparison of the relative activities of inducing platelet apoptosis stimulated by various platelet-activating agents

Apoptosis-like events are known to occur in anuclear platelets. Although the mechanisms responsible for these events are still not completely understood, studies suggested that some platelet agonists can activate platelet apoptosis. However, the relative activities of various platelet agonists in inducing apoptosis have not yet been investigated. In the present study we explored this issue, and attempted to identify the correlation between platelet activation and apoptosis. In a platelet aggregation study, there were no significant differences respectively stimulated by arachidonic acid (AA; 100 microM), ADP (20 microM), collagen (10 microg/mL), thrombin (0.1 U/mL), U46619 (10 microM), and A23187 (5 microM). In a subsequent study, we fixed these concentrations of agonists to further compare their relative activities in inducing platelet apoptosis. Our results found that thrombin, U46619, and A23187 possess stronger activities than the other agonists in inducing platelet apoptosis (i.e., phosphatidylserine exposure, mitochondrial membrane potential depolarization, eukaryotic initiation factor (eIF)2alpha, and caspase activation). On the other hand, AA induced no apoptotic events in platelets. Based on this approach, we demonstrated for the first time that thrombin, U46619, and A23187, but not AA, possess stronger activity in inducing platelet apoptosis. In addition, we also found that platelet activation might not necessarily be associated with the occurrence of platelet apoptosis. The in vivo physiological function of the apoptotic machinery in platelets is not yet clearly understood, and needs to be further investigated in the future.

The glycoprotein Ibalpha-von Willebrand factor interaction induces platelet apoptosis

BACKGROUND

The interaction of glycoprotein (GP) Ibalpha with von Willebrand factor (VWF) initiates platelet adhesion, and simultaneously triggers intracellular signaling cascades leading to platelet aggregation and thrombus formation. Some of the signaling events are similar to those occurring during apoptosis, however, it is still unclear whether platelet apoptosis is induced by the GPIbalpha-VWF interaction.

OBJECTIVES

To investigate whether the GPIbalpha-VWF interaction induces platelet apoptosis and the role of 14-3-3zeta in apoptotic signaling.

METHODS

Apoptotic events were assessed in platelets or Chinese hamster ovary (CHO) cells expressing wild-type (1b9) or mutant GPIb-IX interacting with VWF by flow cytometry or western blotting.

RESULTS

Ristocetin-induced GPIbalpha-VWF interaction elicited apoptotic events in platelets, including phosphatidylserine exposure, elevations of Bax and Bak, gelsolin cleavage, and depolarization of mitochondrial inner transmembrane potential. Apoptotic events were also elicited in platelets exposed to pathologic shear stresses in the presence of VWF; however, the shear-induced apoptosis was eliminated by the anti-GPIbalpha antibody AK2. Furthermore, apoptotic events occurred in 1b9 cells stimulated with VWF and ristocetin, but were significantly diminished in two CHO cell lines expressing mutant GPIb-IX with GPIbalpha truncated at residue 551 or a serine-to-alanine mutation at the 14-3-3zeta-binding site in GPIbalpha.

CONCLUSIONS

This study demonstrates that the GPIbalpha-VWF interaction induces apoptotic events in platelets, and that the association of 14-3-3zeta with the cytoplasmic domain of GPIbalpha is essential for apoptotic signaling. This finding may suggest a novel mechanism for platelet clearance or some thrombocytopenic diseases.