Platelet PMCA- and SERCA-type Ca2+ -ATPase expression in diabetes: a novel signature of abnormal megakaryocytopoiesis

Endoplasmic Reticulum Calcium Pumps and Tumor Cell Differentiation

Endoplasmic reticulum (ER) calcium homeostasis plays an essential role in cellular calcium signaling, intra-ER protein chaperoning and maturation, as well as in the interaction of the ER with other organelles. Calcium is accumulated in the ER by sarco/endoplasmic reticulum calcium ATPases (SERCA enzymes) that generate by active, ATP-dependent transport, a several thousand-fold calcium ion concentration gradient between the cytosol (low nanomolar) and the ER lumen (high micromolar). SERCA enzymes are coded by three genes that by alternative splicing give rise to several isoforms, which can display isoform-specific calcium transport characteristics. SERCA expression levels and isoenzyme composition vary according to cell type, and this constitutes a mechanism whereby ER calcium homeostasis is adapted to the signaling and metabolic needs of the cell, depending on its phenotype, its state of activation and differentiation. As reviewed here, in several normal epithelial cell types including bronchial, mammary, gastric, colonic and choroid plexus epithelium, as well as in mature cells of hematopoietic origin such as pumps are simultaneously expressed, whereas in corresponding tumors and leukemias SERCA3 expression is selectively down-regulated. SERCA3 expression is restored during the pharmacologically induced differentiation of various cancer and leukemia cell types. SERCA3 is a useful marker for the study of cell differentiation, and the loss of SERCA3 expression constitutes a previously unrecognized example of the remodeling of calcium homeostasis in tumors.

The Plasma Membrane Calcium ATPases and Their Role as Major New Players in Human Disease

The Ca²⁺ extrusion function of the four mammalian isoforms of the plasma membrane calcium ATPases (PMCAs) is well established. There is also ever-increasing detail known of their roles in global and local Ca²⁺ homeostasis and intracellular Ca²⁺ signaling in a wide variety of cell types and tissues. It is becoming clear that the spatiotemporal patterns of expression of the PMCAs and the fact that their abundances and relative expression levels vary from cell type to cell type both reflect and impact on their specific functions in these cells. Over recent years it has become increasingly apparent that these genes have potentially significant roles in human health and disease, with PMCAs1-4 being associated with cardiovascular diseases, deafness, autism, ataxia, adenoma, and malarial resistance. This review will bring together evidence of the variety of tissue-specific functions of PMCAs and will highlight the roles these genes play in regulating normal physiological functions and the considerable impact the genes have on human disease.

Platelet populations and priming in hematological diseases

In healthy subjects and patients with hematological diseases, platelet populations can be distinguished with different response spectra in hemostatic and vascular processes. These populations partly overlap, and are less distinct than those of leukocytes. The platelet heterogeneity is linked to structural properties, and is enforced by inequalities in the environment. Contributing factors are variability between megakaryocytes, platelet ageing, and positive or negative priming of platelets during their time in circulation. Within a hemostatic plug or thrombus, platelet heterogeneity is enhanced by unequal exposure to agonists, with populations of contracted platelets in the thrombus core, discoid platelets at the thrombus surface, patches of ballooned and procoagulant platelets forming thrombin, and coated platelets binding fibrin. Several pathophysiological hematological conditions can positively or negatively prime the responsiveness of platelet populations. As a consequence, in vivo and in vitro markers of platelet activation can differ in thrombotic and hematological disorders.

Altered platelet calsequestrin abundance, Na⁺/Ca²⁺ exchange and Ca²⁺ signaling responses with the progression of diabetes mellitus

INTRODUCTION

Downregulation of calsequestrin (CSQ), a major Ca(2+) storage protein, may contribute significantly to the hyperactivity of internal Ca(2+) ([Ca(2+)]i) in diabetic platelets. Here, we investigated changes in CSQ-1 abundance, Ca(2+) signaling and aggregation responses to stimulation with the progression of diabetes, especially the mechanism(s) underlying the exaggerated Ca(2+) influx in diabetic platelets.

MATERIALS AND METHODS

Type 1 diabetes was induced by streptozotocin in rats. Platelet [Ca(2+)]i and aggregation responses upon ADP stimulation were assessed by fluorescence spectrophotometry and aggregometry, respectively. CSQ-1 expression was evaluated using western blotting.

RESULTS

During the 12-week course of diabetes, the abundance of CSQ-1, basal [Ca(2+)]i and ADP-induced Ca(2+) release were progressively altered in diabetic platelets, while the elevated Ca(2+) influx and platelet aggregation were not correlated with diabetes development. 2-Aminoethoxydiphenyl borate, the store-operated Ca(2+) channel blocker, almost completely abolished ADP-induced Ca(2+) influx in normal and diabetic platelets, whereas nifedipine, an inhibitor of the nicotinic acid adenine dinucleotide phosphate receptor, showed no effect. Additionally, inhibition of Na(+)/Ca(2+) exchange induced much slower Ca(2+) extrusion and more Ca(2+) influx in normal platelets than in diabetic platelets. Furthermore, under the condition of Ca(2+)-ATPase inhibition, ionomycin caused greater Ca(2+) mobilization and Ca(2+) influx in diabetic platelets than in normal platelets.

CONCLUSIONS

These data demonstrate that platelet hyperactivity in diabetes is caused by several integrated factors. Besides the downregulation of CSQ-1 that mainly disrupts basal Ca(2+) homeostasis, insufficient Na(+)/Ca(2+) exchange also contributes, at least in part, to the hyperactive Ca(2+) response to stimulation in diabetic platelets.

Effects of acute and chronic psychological stress on platelet aggregation in mice

Although psychological stress has long been known to alter cardiovascular function, there have been few studies on the effect of psychological stress on platelets, which play a pivotal role in cardiovascular disease. In the present study, we investigated the effects of acute and chronic psychological stress on the aggregation of platelets and platelet cytosolic free calcium concentration ([Ca(2+)]i). Mice were subjected to both transportation stress (exposure to novel environment, psychological stress) and restraint stress (psychological stress) for 2 h (acute stress) or 3 weeks (2 h/day) (chronic stress). In addition, adrenalectomized mice were subjected to similar chronic stress (both transportation and restraint stress for 3 weeks). The aggregation of platelets from mice and [Ca(2+)]i was determined by light transmission assay and fura-2 fluorescence assay, respectively. Although acute stress had no effect on agonist-induced platelet aggregation, chronic stress enhanced the ability of the platelet agonists thrombin and ADP to stimulate platelet aggregation. However, chronic stress failed to enhance agonist-induced increase in [Ca(2+)]i. Adrenalectomy blocked chronic stress-induced enhancement of platelet aggregation. These results suggest that chronic, but not acute, psychological stress enhances agonist-stimulated platelet aggregation independently of [Ca(2+)]i increase, and the enhancement may be mediated by stress hormones secreted from the adrenal glands.

The recovery of platelet cyclooxygenase activity explains interindividual variability in responsiveness to low-dose aspirin in patients with and without diabetes

BACKGROUND

Interindividual variability in response to aspirin has been popularized as 'resistance'. We hypothesized that faster recovery of platelet cyclooxygenase-1 activity may explain incomplete thromboxane (TX) inhibition during the 24-h dosing interval.

OBJECTIVE

To characterize the kinetics and determinants of platelet cyclooxygenase-1 recovery in aspirin-treated diabetic and non-diabetic patients.

PATIENTS/METHODS

One hundred type 2 diabetic and 73 non-diabetic patients on chronic aspirin 100 mg daily were studied. Serum TXB(2) was measured every 3 h, between 12 and 24 h after a witnessed aspirin intake, to characterize the kinetics of platelet cyclooxygenase-1 recovery. Patients with the fastest TXB(2) recovery were randomized to aspirin 100 mg once daily, 200 mg once daily or 100 mg twice daily, for 28 days and TXB(2) recovery was reassessed.

RESULTS AND CONCLUSIONS

Platelet TXB(2) production was profoundly suppressed at 12 h in both groups. Serum TXB(2) recovered linearly, with a large interindividual variability in slope. Diabetic patients in the third tertile of recovery slopes (≥ 0.10 ng mL(-1) h(-1) ) showed significantly higher mean platelet volume and body mass index, and younger age. Higher body weight was the only independent predictor of a faster recovery in non-diabetics. Aspirin 100 mg twice daily completely reversed the abnormal TXB(2) recovery in both groups. Interindividual variability in the recovery of platelet cyclooxygenase activity during the dosing interval may limit the duration of the antiplatelet effect of low-dose aspirin in patients with and without diabetes. Inadequate thromboxane inhibition can be easily measured and corrected by a twice daily regimen.

Potential regulatory role of calsequestrin in platelet Ca(2+) homeostasis and its association with platelet hyperactivity in diabetes mellitus

BACKGROUND

Altered Ca(2+) homeostasis contributes significantly to platelet hyperactivity in diabetes mellitus. Calsequestrin (CSQ), as a Ca(2+) buffer protein in the sarcoplasmic reticulum, also regulates the Ca(2+) release process in muscles. We hypothesized that CSQ may be expressed in platelets, but is altered and involved in diabetic platelet Ca(2+) abnormalities and hyperaggregability.

METHODS

CSQ expression in platelets from streptozotocin-induced type 1 diabetes rats, type 2 diabetes volunteers and Goto-Kakizaki rats were analyzed by western blotting and RT-qPCR. Platelet Ca(2+) and aggregation were evaluated with Fura2 and an aggregometer, respectively.

RESULTS

Platelets from diabetic patients and rats exhibited increased resting Ca(2+) levels, and hyperactive Ca(2+) and aggregation responses to agonists. This enhanced basal Ca(2+) was largely dependent on intracellular Ca(2+) and insensitive to inositol 1,4,5-trisphosphate receptor (IP(3)R) antagonism. Additionally, the expression of the skeletal CSQ isotype (CSQ-1) was detected in both rat and human platelets, but its levels were significantly lowered in diabetic platelets as compared with normal platelets. Impairment of CSQ by trifluoperazine caused concentration-dependent Ca(2+) release in normal platelets and HEK293 cells. Knocking down CSQ-1 in HEK293 cells resulted in increased leakage of Ca(2+), which was also insensitive to IP(3)R inhibition, and exaggerated Ca(2+) release following carbachol treatment.

CONCLUSIONS

Downregulation of CSQ-1 in diabetic platelets and impairment of CSQ-1 in normal cells leads to disturbed Ca(2+) release, demonstrating a potential role for CSQ-1 in the regulation of the platelet Ca(2+) release process and a possible causal contribution to diabetic platelet hyperactivity.

Endoplasmic-reticulum calcium depletion and disease

The endoplasmic reticulum (ER) as an intracellular Ca(2+) store not only sets up cytosolic Ca(2+) signals, but, among other functions, also assembles and folds newly synthesized proteins. Alterations in ER homeostasis, including severe Ca(2+) depletion, are an upstream event in the pathophysiology of many diseases. On the one hand, insufficient release of activator Ca(2+) may no longer sustain essential cell functions. On the other hand, loss of luminal Ca(2+) causes ER stress and activates an unfolded protein response, which, depending on the duration and severity of the stress, can reestablish normal ER function or lead to cell death. We will review these various diseases by mainly focusing on the mechanisms that cause ER Ca(2+) depletion.

Two distinct calcium pools in the endoplasmic reticulum of HEK-293T cells

Agonist-sensitive intracellular Ca2+ stores may be heterogeneous and exhibit distinct functional features. We have studied the properties of intracellular Ca2+ stores using targeted aequorins for selective measurements in different subcellular compartments. Both, HEK-293T [HEK (human embryonic kidney)-293 cells expressing the large T-antigen of SV40 (simian virus 40)] and HeLa cells accumulated Ca2+ into the ER (endoplasmic reticulum) to near millimolar concentrations and the IP3-generating agonists, carbachol and ATP, mobilized this Ca2+ pool. We find in HEK-293T, but not in HeLa cells, a distinct agonist-releasable Ca2+ pool insensitive to the SERCA (sarco/endoplasmic reticulum Ca2+ ATPase) inhibitor TBH [2,5-di-(t-butyl)-benzohydroquinone]. TG (thapsigargin) and CPA (cyclopiazonic acid) completely emptied this pool, whereas lysosomal disruption or manoeuvres collapsing endomembrane pH gradients did not. Our results indicate that SERCA3d is important for filling the TBH-resistant store as: (i) SERCA3d is more abundant in HEK-293T than in HeLa cells; (ii) the SERCA 3 ATPase activity of HEK-293T cells is not fully blocked by TBH; and (iii) the expression of SERCA3d in HeLa cells generated a TBH-resistant agonist-mobilizable compartment in the ER. Therefore the distribution of SERCA isoforms may originate the heterogeneity of the ER Ca2+ stores and this may be the basis for store specialization in diverse functions. This adds to recent evidence indicating that SERCA3 isoforms may subserve important physiological and pathophysiological mechanisms.

Ca2+ leakage rate from agonist-sensitive intracellular pools is altered in platelets from patients with type 2 diabetes

Platelets from patients with type 2 diabetes show abnormalities in intracellular Ca(2+) homeostasis that are involved in platelet hyperaggregability and the development of thrombotic complications. Different Ca(2+) transport mechanisms have been reported to be altered in platelets from patients with type 2 diabetes, including the sarcoendoplasmic and plasma membrane Ca(2+)-ATPases, plasma membrane Ca(2+) channels, or the Na(+)/Ca(2+) exchanger. Here, we have investigated whether passive Ca(2+) leak from the stores is altered in platelets from patients with type 2 diabetes. Resting cytosolic Ca(2+) concentration ([Ca(2+)](i)) was found to be greater in platelets from patients with type 2 diabetes than in healthy controls. In a Ca(2+)-free medium, platelet stimulation with thrombin or ADP evokes a rapid and transient increase in [Ca(2+)](i) that was found to be greater in patients with diabetes than in healthy controls. Sequential or combined inhibition of Ca(2+) extrusion and Ca(2+) sequestration into the stores reduced the difference between the responses to agonists in patients with diabetes and healthy controls, although agonist-induced Ca(2+) efflux from the stores was still significantly greater in patients with diabetes. Ca(2+) leak from the dense tubular system or the acidic stores, induced by a low concentration of thapsigargin or 2,5-di-(t-butyl)-1,4-hydroquinone (TBHQ), respectively, was clearly greater in patients with diabetes than in controls, and was not significantly modified by treatment with 2-APB. These findings indicate that passive Ca(2+) leakage rate from the intracellular stores in platelets is significantly enhanced in patients with type 2 diabetes mellitus and this might explain the increased resting [Ca(2+)](i).

Allosteric inhibitors of plasma membrane Ca2+ pumps: Invention and applications of caloxins

Plasma membrane Ca²⁺ pumps (PMCA) play a major role in Ca²⁺ homeostasis and signaling by extruding cellular Ca²⁺ with high affinity. PMCA isoforms are encoded by four genes which are expressed differentially in various cell types in normal and disease states. Therefore, PMCA isoform selective inhibitors would aid in delineating their role in physiology and pathophysiology. We are testing the hypothesis that extracellular domains of PMCA can be used as allosteric targets to obtain a novel class of PMCA-specific inhibitors termed caloxins. This review presents the concepts behind the invention of caloxins and our progress in this area. A section is also devoted to the applications of caloxins in literature. We anticipate that isoform-selective caloxins will aid in understanding PMCA physiology in health and disease. With strategies to develop therapeutics from bioactive peptides, caloxins may become clinically useful in cardiovascular diseases, neurological disorders, retinopathy, cancer and contraception.

The plasma membrane calcium ATPase modulates calcium homeostasis, intracellular signaling events and function in platelets

BACKGROUND

The plasma membrane calcium ATPase (PMCA) regulates localized signaling events in a variety of cell types, although its functional role in platelets remains undefined.

OBJECTIVES

To investigate the role of PMCA in determining platelet intracellular calcium concentration ([Ca²(+) ](i) ) at rest and following agonist stimulation, and to define the corresponding effects upon different stages of platelet activation.

METHODS

[Ca²(+) ](i) was continuously measured in Fura-2-loaded platelets and in vitro and in vivo functional analyses performed in the presence of the PMCA inhibitor carboxyeosin (CE).

RESULTS

Concentrations of CE that selectively inhibited Ca²(+) extrusion through PMCA were established in human platelets. [Ca²(+) ](i) was elevated by CE in resting platelets, although collagen-stimulated Ca²(+) release was reduced. Impaired Ca²(+) mobilization upon agonist stimulation was accompanied by reduced dense granule secretion and impaired platelet aggregation. Platelet aggregation responses were also reduced in PMCA4(-/-) mice and in an in vivo mouse model of platelet thromboembolism. Conversely, inhibition of PMCA promoted the early and later stages of platelet activation, observed as enhanced adhesion to fibrinogen, and accelerated clot retraction. Investigations into the signaling mechanisms underlying CE-mediated inhibition of platelet aggregation implicated cGMP-independent vasodilator-stimulated phosphoprotein phosphorylation.

CONCLUSIONS

Disruption of PMCA activity perturbs platelet Ca²(+) homeostasis and function in a time-dependent manner, demonstrating that PMCA differentially regulates Ca²(+) -dependent signaling events, and hence function, throughout the platelet activation process.

Role of platelet plasma membrane Ca2+-ATPase in health and disease

Platelets have essential roles in both health and disease. Normal platelet function is required for hemostasis. Inhibition of platelet function in disease or by pharmacological treatment results in bleeding disorders. On the other hand, hyperactive platelets lead to heart attack and stroke. Calcium is a major second messenger in platelet activation, and elevated intracellular calcium leads to hyperactive platelets. Elevated platelet calcium has been documented in hypertension and diabetes; both conditions increase the likelihood of heart attack and stroke. Thus, proper regulation of calcium metabolism in the platelet is extremely important. Plasma membrane Ca²⁺-ATPase (PMCA) is a major player in platelet calcium metabolism since it provides the only significant route for calcium efflux. In keeping with the important role of calcium in platelet function, PMCA is a highly regulated transporter. In human platelets, PMCA is activated by Ca²⁺/calmodulin, by cAMP-dependent phosphorylation and by calpain-dependent removal of the inhibitory peptide. It is inhibited by tyrosine phosphorylation and calpain-dependent proteolysis. In addition, the cellular location of PMCA is regulated by a PDZ-domain-dependent interaction with the cytoskeleton during platelet activation. Rapid regulation by phosphorylation results in changes in the rate of platelet activation, whereas calpain-dependent proteolysis and interaction with the cytoskeleton appears to regulate later events such as clot retraction. In hypertension and diabetes, PMCA expression is upregulated while activity is decreased, presumably due to tyrosine phosphorylation. Clearly, a more complete understanding of PMCA function in human platelets could result in the identification of new ways to control platelet function in disease states.

Effect of homocysteine on calcium mobilization and platelet function in type 2 diabetes mellitus

Type 2 diabetes mellitus induces a characteristic platelet hyperactivity that might be due to several factors including oxidative stress and abnormal intracellular Ca2+ homeostasis. Hyperhomocysteinaemia is considered a risk factor in the development of thrombosis although its effect on platelet function and the mechanisms involved are still poorly understood. Here we show that homocysteine (Hcy) induce a concentration-dependent increase in endogenous production of reactive oxygen species (ROS), which was significantly greater in platelets from diabetic patients than in controls. Platelet treatment with Hcy resulted in Ca2+ release from the dense tubular system and the acidic stores. Ca2+ mobilisation-induced by Hcy consisted in two components, an initial slow increase in intracellular free Ca2+ concentration ([Ca2+]i) and a rapid and marked increase in [Ca2+]i, the second leading to the activation of platelet aggregation. As well as ROS generation, Ca2+ mobilization and platelet aggregation were significantly greater in platelets from diabetic donors than in controls, which indicate that platelets from diabetic donors are more sensitive to Hcy. These findings, together with the hyperhomocysteinaemia reported in diabetic patients, strongly suggest that Hcy might be considered a risk factor in the development of cardiovascular complications associated to type 2 diabetes mellitus.

Steady-state kinetic modeling constrains cellular resting states and dynamic behavior

A defining characteristic of living cells is the ability to respond dynamically to external stimuli while maintaining homeostasis under resting conditions. Capturing both of these features in a single kinetic model is difficult because the model must be able to reproduce both behaviors using the same set of molecular components. Here, we show how combining small, well-defined steady-state networks provides an efficient means of constructing large-scale kinetic models that exhibit realistic resting and dynamic behaviors. By requiring each kinetic module to be homeostatic (at steady state under resting conditions), the method proceeds by (i) computing steady-state solutions to a system of ordinary differential equations for each module, (ii) applying principal component analysis to each set of solutions to capture the steady-state solution space of each module network, and (iii) combining optimal search directions from all modules to form a global steady-state space that is searched for accurate simulation of the time-dependent behavior of the whole system upon perturbation. Importantly, this stepwise approach retains the nonlinear rate expressions that govern each reaction in the system and enforces constraints on the range of allowable concentration states for the full-scale model. These constraints not only reduce the computational cost of fitting experimental time-series data but can also provide insight into limitations on system concentrations and architecture. To demonstrate application of the method, we show how small kinetic perturbations in a modular model of platelet P2Y₁ signaling can cause widespread compensatory effects on cellular resting states.

Cells respond to extracellular signals through a complex coordination of interacting molecular components. Computational models can serve as powerful tools for prediction and analysis of signaling systems, but constructing large models typically requires extensive experimental datasets and computation. To facilitate the construction of complex signaling models, we present a strategy in which the models are built in a stepwise fashion, beginning with small “resting” networks that are combined to form larger models with complex time-dependent behaviors. Interestingly, we found that only a minor fraction of potential model configurations were compatible with resting behavior in an example signaling system. These reduced sets of configurations were used to limit the search for more complicated solutions that also captured the dynamic behavior of the system. Using an example model constructed by this approach, we show how a cell's resting behavior adjusts to changes in the kinetic rate processes of the system. This strategy offers a general and biologically intuitive framework for building large-scale kinetic models of steady-state cellular systems and their dynamics.

Effect of homocysteine on calcium mobilization and platelet function in type 2 diabetes mellitus

Type 2 diabetes mellitus induces a characteristic platelet hyperactivity that might be due to several factors including oxidativ stress and abnormal intracellular Ca²⁺ homeostasis. Hyperhomocysteinaemia is considered a risk factor in the development of thrombosis although its effect on platelet function and the mechanisms involved are still poorly understood. Here we show tha homocysteine induce a concentration-dependent increase in endogenous production of reactive oxygen species (ROS), which was significantly greater in platelets from diabetic patients than in controls. Platelet treatment with homocysteine resulted in Ca²⁺ release from the dense tubular system and the acidic stores. Ca²⁺ mobilization-induced by homocysteine consisted in two components, an initial slow increase in intracellular free Ca ⁺ concentration ([Ca ⁺]i) and a rapid and marked increase in [Ca²⁺]i, th second leading to the activation of platelet aggregation. As well as ROS generation, Ca²⁺ mobilization and platelet aggregation were significantly greater in platelets from diabetic donors than in controls, which indicate that platelets from diabetic donors are more sensitive to homocysteine. These findings, together with the hyperhomocysteinaemia reported in diabetic patients, strongly suggest that homocysteine might be considered a risk factor in the development of cardiovascular complications associated to type 2 diabetes mellitus.

Platelet signalling abnormalities in patients with type 2 diabetes mellitus: a review

The hyperactivation of platelets is involved in the cardiovascular complications associated with type 2 diabetes mellitus. Altered platelet behavior contributes to the angiopathies associated with diabetes. A number of mechanisms involved in platelet activation are altered in diabetes. Platelets from type 2 diabetic patients show an enhanced endogenous reactive oxygen species production and a reduced antioxidant capability, which increase the activity of several tyrosine kinases, such as the Bruton's tyrosine kinase, MAP kinases or proteins of the SRC family. Oxidative stress is also involved in the abnormal intracellular calcium homeostasis observed in platelets from type 2 diabetics, including an enhanced resting cytosolic calcium concentration and calcium release and entry in response to agonists. Moreover, diabetes alters the bioavailability of nitric oxide in platelets. Basal nitric oxide synthase activity is reduced in homogenates of platelets obtained from patients with type 2 diabetes mellitus. The study of these abnormalities might be helpful in the development of new pharmacological strategies to reduce platelet activation in type 2 diabetes mellitus.

Cinnamtannin B-1 as an antioxidant and platelet aggregation inhibitor

Cinnamtannin B-1 is a naturally occurring trimeric A-type proanthocyanidin, present in a limited number of plants, which exhibits a large number of cellular actions mostly derived from its antioxidant properties. Cinnamtannin B-1 modulates several biological processes such as changes in cytosolic free Ca(2+) concentration, endogenous reactive oxygen species generation, protein tyrosine phosphorylation and platelet aggregation. Proanthocyanidins, such as cinnamtannin B-1, have been reported to exert antitumoral activity mediated by a selective proapoptotic action in a number of tumoral cell lines associated with antiapoptotic activity in normal cells. The opposite effects of proanthocyanidins in normal and tumoral cells suggest that these compounds might be the base for therapeutic strategies directed selectively against tumoral cells. In addition, cinnamtannin B-1 shows antithrombotic actions through inhibition, in platelets, of endogenous ROS generation, Ca(2+) mobilization and, subsequently, aggregation. This has been reported to be especially relevant in platelets from diabetic patients, where cinnamtannin B-1 reverses both platelet hypersensitivity and hyperactivity. Considering the large number of cellular effects of cinnamtannin B-1 the development of therapeutic strategies for thrombotic disorders or certain types of cancer deserves further studies. This review summarizes the current knowledge on the actions and relevance of the signalling pathways modulated by cinnamtannin B-1.

STIM1 regulates acidic Ca2+ store refilling by interaction with SERCA3 in human platelets

Ca(2+) mobilization regulates a wide variety of cellular functions. Platelets possess agonist-releasable Ca(2+) stores in acidic organelles where sarcoendoplasmic reticulum Ca(2+)-ATPase-3 (SERCA) pump is involved in store refilling. Stromal interaction molecule 1 (STIM1), which has been presented as a central regulator of platelet function, is a Ca(2+) sensor of the intracellular Ca(2+) stores. Here we present that STIM1 is required for acidic store refilling. Electrotransjection of cells with anti-STIM1 (Y(231)-K(243)) antibody, directed towards a cytoplasmic sequence of STIM1, significantly reduced acidic store refilling, which was tested by remobilizing Ca(2+) from the acidic stores using 2,5-di-(t-butyl)-1,4-hydroquinone (TBHQ) after a brief refilling period that followed thrombin stimulation. Platelet treatment with thrombin or thapsigargin in combination with ionomycin, to induce extensive Ca(2+) store depletion, resulted in a transient increase in the interaction between STIM1 and SERCA3, reaching a maximum 30 s after stimulation. The coupling between STIM1 and SERCA3 was abolished by electrotransjection with anti-STIM1 antibody. The interaction between STIM1 and SERCA3 induced by thrombin or by treatment with thapsigargin plus ionomycin is reduced in platelets from type 2 diabetic patients, as well as Ca(2+) reuptake into the acidic Ca(2+) stores. These findings provide evidence for a role of STIM1 in acidic store refilling in platelets probably acting as a Ca(2+) sensor and regulating the activity of SERCA3. This action is impaired in platelets from type 2 diabetics, which might lead to the enhanced cytosolic Ca(2+) concentration observed and, therefore, in platelet hyperactivity.

Intracellular Ca2+ store depletion induces the formation of macromolecular complexes involving hTRPC1, hTRPC6, the type II IP3 receptor and SERCA3 in human platelets

Endogenously expressed human canonical transient receptor potential 1 (hTRPC1) and human canonical transient receptor potential 6 (hTRPC6) have been shown to play a role in store-operated Ca2+ entry (SOCE) in human platelets, where two mechanisms for SOCE, regulated by the dense tubular system (DTS) or the acidic granules, have been identified. In cells preincubated for 1 min with 100 microM flufenamic acid we show that hTRPC6 is involved in SOCE activated by both mechanisms, as demonstrated by selective depletion of the DTS or the acidic stores, using thapsigargin (TG) (10 nM) or 2,5-di-(tert-butyl)-1,4-hydroquinone (TBHQ) (20 microM), respectively, although it is more relevant after acidic store depletion. Co-immunoprecipitation experiments indicated that depletion of both stores separately results in time-dependent interaction between hTRPC1 and hTRPC6, and also between both hTRPCs and the type II IP3 receptor (IP3RII). The latter was greater after treatment with TG. TBHQ-induced coupling between hTRPC1 and 6 was transient and decreased after 30s of treatment, while that induced by TG increased for at least 3 min. TBHQ induced association between SERCA3, located in the acidic stores, hTRPC1, hTRPC6 and Orai1. TBHQ also evoked coupling between SERCA3 and IP3RII, presumably located in the DTS, thus suggesting interplay between both Ca2+ stores. Similarly, TG induces the interaction of SERCA2b with hTRPC1 and 6 and the IP3RII. The interactions between hTRPC1, hTRPC6, IP3RII and SERCA3 were impaired by disruption of the microtubules, supporting a role for microtubules in Ca2+ homeostasis. In conclusion, the present data demonstrate for the first time that hTRPC1, hTRPC6, IP3RII and SERCA3 are parts of a macromolecular protein complex activated by depletion of the intracellular Ca2+ stores in human platelets.