Programmed anuclear cell death delimits platelet life span

Association of coagulation factor XIII-A with Golgi proteins within monocyte-macrophages: implications for subcellular trafficking and secretion

Factor XIII-A (FXIII-A) is present in the cytosol of platelets, megakaryocytes, monocytes, osteoblasts, and macrophages and may be released from cells by a nonclassical pathway. We observed that plasma FXIII-A levels were unchanged in thrombocytopenic mice (Bcl-xPlt20/Plt20 and Mpl−/−), which implicates nonclassical secretion from nucleated cells as the source of plasma FXIII-A. We, therefore, examined the intracellular targeting of FXIII-A in the THP-1 (monocyte/macrophage) cell line and in human monocyte–derived macrophages. Metabolic labeling of THP-1 cells did not show release of 35S-FXIII-A either under basal conditions or when interleukin 1-β was released in response to cell stress. However, immunofluorescence of THP-1 cells and primary macrophages showed that FXIII-A associated with podosomes and other structures adjacent to the plasma membrane, which also contain trans-Golgi network protein-46 and Golgi matrix protein-130 (GM130) but not the endoplasmic reticulum luminal protein, protein disulphide isomerase. Further, FXIII-A was present in GM130-positive intracellular vesicles that could mediate its transport, and in other contexts GM130 and its binding partner GRASP have been implicated in the delivery of nonclassically secreted proteins to the plasma membrane. Hence, this mechanism may precede FXIII-A release into the extracellular matrix from macrophages and its release into plasma from the cell type of origin.

Kv1.3 is the exclusive voltage-gated K+ channel of platelets and megakaryocytes: roles in membrane potential, Ca2+ signalling and platelet count

A delayed rectifier voltage-gated K⁺ channel (Kv) represents the largest ionic conductance of platelets and megakaryocytes, but is undefined at the molecular level. Quantitative RT-PCR of all known Kv α and ancillary subunits showed that only Kv1.3 (KCNA3) is substantially expressed in human platelets. Furthermore, megakaryocytes from Kv1.3^−/− mice or from wild-type mice exposed to the Kv1.3 blocker margatoxin completely lacked Kv currents and displayed substantially depolarised resting membrane potentials. In human platelets, margatoxin reduced the P2X₁- and thromboxaneA₂ receptor-evoked [Ca²⁺]_i increases and delayed the onset of store-operated Ca²⁺ influx. Megakaryocyte development was normal in Kv1.3^−/− mice, but the platelet count was increased, consistent with a role of Kv1.3 in apoptosis or decreased platelet activation. We conclude that Kv1.3 forms the Kv channel of the platelet and megakaryocyte, which sets the resting membrane potential, regulates agonist-evoked Ca²⁺ increases and influences circulating platelet numbers.

De novo protein synthesis in mature platelets: a consideration for transfusion medicine

Platelet function in thrombosis and haemostasis is reasonably well understood at the molecular level with respect to the proteins involved in cellular structure, signalling networks and platelet interaction with clotting factors and other cells. However, the natural history of these proteins has only recently garnered the attention of platelet researchers. De novo protein synthesis in platelets was discovered 40 years ago; however, it was generally dismissed as merely an interesting minor phenomenon until studies over the past few years renewed interest in this aspect of platelet proteins. It is now accepted that anucleate platelets not only have the potential to synthesize proteins, but this capacity seems to be required to fulfil their function. With translational control as the primary mode of regulation, platelets are able to express biologically relevant gene products in a timely and signal-dependent manner. Platelet protein synthesis during storage of platelet concentrates is a nascent area of research. Protein synthesis does occur, although not for all proteins found in the platelet protein profile. Furthermore, mRNA appears to be well preserved under standard storage conditions. Although its significance is not yet understood, the ability to replace proteins may form a type of cellular repair mechanism during storage. Disruption by inappropriate storage conditions or processes that block protein synthesis such as pathogen reduction technologies may have direct effects on the ability of platelets to synthesize proteins during storage.

Novel agents for B-cell non-Hodgkin lymphoma: science and the promise

There has been tremendous insight gained in the last two decades from basic science research. New molecular targets in neoplastic cells are emerging and provide the rationale for clinical development of novel agents in non-Hodgkin lymphoma. These novel agents can be broadly categorized into two groups. The first is by immunotherapy which includes novel monoclonal antibodies and immunomodulating drugs, which takes advantage of or optimizes immune system function. The other group of drugs target small molecules that may play an important role in tumorigenesis. The mechanisms of anti-tumor activity include targeting apoptotic pathways, inhibition of proteasomes, mammalian target of rapamycin (mTOR), cyclin-dependent kinases and histone deacetylases. The purpose of this review is to focus on these novel agents and the various treatment approaches that are currently being evaluated in non-Hodgkin lymphoma.

A phase I study of obatoclax mesylate, a Bcl-2 antagonist, plus topotecan in solid tumor malignancies

PURPOSE

To establish the safety, maximum tolerated dose (MTD), recommended phase II dose, and preliminary antitumor activity of obatoclax mesylate (GX15-070MS), a Bcl-2 antagonist, in combination with topotecan in patients with solid tumor malignancies.

PATIENTS AND METHODS

Patients with solid tumor malignancies for whom topotecan was an appropriate treatment were administered obatoclax mesylate and topotecan on a 3-week cycle in a pre-defined, standard 3 + 3 dose escalation scheme. The starting dose for obatoclax mesylate was 14 mg/m(2) by 3-h intravenous (IV) infusion. Topotecan 1.25 mg/m(2) was given concurrently as an IV infusion on days 1-5 of each cycle.

RESULTS

Fourteen patients received 40 cycles of obatoclax mesylate at the following doses: 14 mg/m(2) on day 1, 14 mg/m(2) on days 1 and 3, and 20 mg/m(2) on day 1. The most common toxicities related to obatoclax were neurologic, including ataxia, mood alterations, somnolence, and cognitive dysfunction. The majority of these were grades 1 and 2 (88%). Two of five patients experienced dose-limiting grade 3 neurologic toxicity at a dose of 20 mg/m(2); no patients experienced grade 4 neurologic toxicities, and no other patients experienced grade 3 neurologic toxicity. Of the patients who experienced grade 3 neurologic events, one later developed febrile neutropenia, which was also a dose-limiting toxicity (DLT). After an additional three patients were treated without DLT at the previously tolerated dose of 14 mg/m(2) on day 1, the level was escalated to 14 mg/m(2) on days 1 and 3. Three patients were treated at this dose and, with none experiencing a DLT, 14 mg/m(2) on days 1 and 3 was defined as the recommended phase II dose. Two patients with small-cell lung cancer (SCLC) achieved partial responses and four patients had stable disease. Median time to progression (TTP) was 12 weeks.

CONCLUSION

Obatoclax mesylate administered at 14 mg/m(2) IV on days 1 and 3 is safe and well tolerated when given in combination with topotecan 1.25 mg/m(2) IV on days 1-5 of an every 3-week cycle. A phase II trial to assess the efficacy of this combination for patients with relapsed SCLC is currently accruing patients.

c-Myb promotes the survival of CD4+CD8+ double-positive thymocytes through upregulation of Bcl-xL

Mechanisms that regulate the lifespan of CD4(+)CD8(+) double-positive (DP) thymocytes help shape the peripheral T cell repertoire. However, the molecular mechanisms controlling DP thymocyte survival remain poorly understood. The Myb proto-oncogene encodes a transcription factor required during multiple stages of T cell development. We demonstrate that Myb mRNA expression is upregulated as thymocytes differentiate from the double-negative into the metabolically quiescent, small, preselection DP stage during T cell development. Using a conditional deletion mouse model, we demonstrate that Myb-deficient DP thymocytes undergo premature apoptosis, resulting in a limited Tcralpha repertoire biased toward 5' Jalpha segment usage. Premature apoptosis occurs specifically in the small preselection DP compartment in an alphabetaTCR-independent manner and is a consequence of decreased Bcl-xL expression. Forced Bcl-xL expression is able to rescue survival, and reintroduction of c-Myb restores both Bcl-xL expression and the small preselection DP compartment. We further demonstrate that c-Myb promotes transcription at the Bcl2l1 locus via a genetic pathway that is independent of the expression of T cell-specific factor-1 or RORgammat, two transcription factors that induce Bcl-xL expression in T cell development. Thus, Bcl-xL is a novel mediator of c-Myb activity during normal T cell development.

Anandamide extends platelets survival through CB(1)-dependent Akt signaling

Platelets are stored at 22 degrees C, since incubation at 37 degrees C results in loss of viability. Nonetheless, in our body (37 degrees C), platelets survive for 8-10 days. This discrepancy has been explained in terms of deprivation of viability factors or accumulation of apoptotic factors during storage. We report that the endocannabinoid anandamide (AEA) may be one of the agents allowing platelet survival. In fact, at 37 degrees C, human platelets enhance the expression of pro-apoptotic proteins (caspases, Bax, Bak) and decrease the expression of Bcl-xL, thus changing the Bcl-xL/Bak ratio, a key platelet biological clock. AEA or its non-hydrolyzable analogue, methanandamide, extend platelet life span, without reversing the changes in Bcl-xL/Bak ratio induced by heat stress. Instead, AEA binding to type-1 cannabinoid receptor activates Akt, which regulates, through phosphorylation of Bad, the interactions among different Bcl-2 family members. These findings could have implications for platelet collection and, potentially, for their clinical use.

Proteomics meets blood banking: identification of protein targets for the improvement of platelet quality

Proteomics has brought new perspectives to the fields of hematology and transfusion medicine in the last decade. The steady improvement of proteomic technology is propelling novel discoveries of molecular mechanisms by studying protein expression, post-translational modifications and protein interactions. This review article focuses on the application of proteomics to the identification of molecular mechanisms leading to the deterioration of blood platelets during storage - a critical aspect in the provision of platelet transfusion products. Several proteomic approaches have been employed to analyse changes in the platelet protein profile during storage and the obtained data now need to be translated into platelet biochemistry in order to connect the results to platelet function. Targeted biochemical applications then allow the identification of points for intervention in signal transduction pathways. Once validated and placed in a transfusion context, these data will provide further understanding of the underlying molecular mechanisms leading to platelet storage lesion. Future aspects of proteomics in blood banking will aim to make use of protein markers identified for platelet storage lesion development to monitor proteome changes when alterations such as the use of additive solutions or pathogen reduction strategies are put in place in order to improve platelet quality for patients.

Noncanonical functions of BCL-2 proteins in the nervous system

BCL-2 family proteins form heterodimers or homo-oligomers to inhibit or induce apoptotic cell death, respectively. They often relocalize from the cytoplasm to mitochondria to carry out these functions. The traditional model is that in healthy cells, anti-death family members hold pro-death BCL-2 family members in check. Upon receiving a death stimulus, another set of proteins (BH3-only proteins) inactivate the protective BCL-2 proteins, forcing them to release their pro-death partners that are subsequently triggered to oligomerize and porate the mitochondrial outer membrane leading to cell death. In support of this traditional view, there is a preponderance of supporting evidence derived from the study of events that occur following treatment of cells with a death stimulus. Knockout and mutant mice also exhibit many developmental and treatment-induced phenotypes consistent with this model of antagonism between BCL-2 family proteins. Emphasis is logically placed on those phenotypes that support the model. However, this working model of BCL-2 family interactions has become so engrained that alternative, potentially valid interpretations are sometimes dismissed. Therefore, it is useful to consider the evidence that seems contrary to accepted models. In particular, the analysis of BCL-2 family functions in the nervous system has revealed unexpected outcomes that can serve to further stimulate critical probing of the yet unknown biochemical functions of BCL-2 proteins.

BH3-only protein bid participates in the Bcl-2 network in healthy liver cells

Bcl-2 homology domain 3 (BH3)-only protein Bid is posttranslationally cleaved by caspase-8 into its truncated form (tBid) and couples with stress signals to the mitochondrial cell death pathway. However, the physiological relevance of Bid is not clearly understood. Hepatocyte-specific knockout (KO) of Bcl-xL leads to naturally-occurring apoptosis despite co-expression of Mcl-1, which shares a similar anti-apoptotic function. We generated Bcl-xL KO, Bcl-xL/Bid double KO, Bcl-xL/Bak double KO, Bcl-xL/Bax double KO, and Bcl-xL/Bak/Bax triple KO mice and found that hepatocyte apoptosis caused by Bcl-xL deficiency was completely dependent on Bak and Bax, and surprisingly on Bid. This indicated that, in the absence of Bid, Bcl-xL is not required for the integrity of differentiated hepatocytes, suggesting a complicated interaction between core Bcl-2 family proteins and BH3-only proteins even in a physiological setting. Indeed, a small but significant level of tBid was present in wild-type liver under physiological conditions. tBid was capable of binding to Bcl-xL and displacing Bak and Bax from Bcl-xL, leading to release of cytochrome c from wild-type mitochondria. Bcl-xL-deficient mitochondria were more susceptible to tBid-induced cytochrome c release. Finally, administration of ABT-737, a pharmacological inhibitor of Bcl-2/Bcl-xL, caused Bak/Bax-dependent liver injury, but this was clearly ameliorated with a Bid KO background.

CONCLUSION

Bid, originally considered to be a sensor for apoptotic stimuli, is constitutively active in healthy liver cells and is involved in the Bak/Bax-dependent mitochondrial cell death pathway. Healthy liver cells are addicted to a single Bcl-2-like molecule because of BH3 stresses, and therefore special caution may be required for the use of the Bcl-2 inhibitor for cancer therapy.

p38 mitogen-activated protein kinase activation during platelet storage: consequences for platelet recovery and hemostatic function in vivo

Platelets undergo several modifications during storage that reduce their posttransfusion survival and functionality. One important feature of these changes, which are known as platelet storage lesion, is the shedding of the surface glycoproteins GPIb-alpha and GPV. We recently demonstrated that tumor necrosis factor-alpha converting enzyme (TACE/ADAM17) mediates mitochondrial injury-induced shedding of adhesion receptors and that TACE activity correlates with reduced posttransfusion survival of these cells. We now confirm that TACE mediates receptor shedding and clearance of platelets stored for 16 hours at 37 degrees C or 22 degrees C. We further demonstrate that both storage and mitochondrial injury lead to the phosphorylation of p38 mitogen-activated kinase (MAPK) in platelets and that TACE-mediated receptor shedding from mouse and human platelets requires p38 MAP kinase signaling. Protein kinase C, extracellular regulated-signal kinase MAPK, and caspases were not involved in TACE activation. Both inhibition of p38 MAPK and inactivation of TACE during platelet storage led to a markedly improved posttransfusion recovery and hemostatic function of platelets in mice. p38 MAPK inhibitors had only minor effects on the aggregation of fresh platelets under static or flow conditions in vitro. In summary, our data suggest that inhibition of p38 MAPK or TACE during storage may significantly improve the quality of stored platelets.

A genome-wide meta-analysis identifies 22 loci associated with eight hematological parameters in the HaemGen consortium

The number and volume of cells in the blood affect a wide range of disorders including cancer and cardiovascular, metabolic, infectious and immune conditions. We consider here the genetic variation in eight clinically relevant hematological parameters, including hemoglobin levels, red and white blood cell counts and platelet counts and volume. We describe common variants within 22 genetic loci reproducibly associated with these hematological parameters in 13,943 samples from six European population-based studies, including 6 associated with red blood cell parameters, 15 associated with platelet parameters and 1 associated with total white blood cell count. We further identified a long-range haplotype at 12q24 associated with coronary artery disease and myocardial infarction in 9,479 cases and 10,527 controls. We show that this haplotype demonstrates extensive disease pleiotropy, as it contains known risk loci for type 1 diabetes, hypertension and celiac disease and has been spread by a selective sweep specific to European and geographically nearby populations.

Dual roles for hepatic lectin receptors in the clearance of chilled platelets

Chilling rapidly (<4 h) clusters Glycoprotein - (GP)Ib receptors on blood platelets, and ß2-integrins of hepatic macrophages bind ßGlcNAc residues in the clusters leading to rapid clearance of acutely chilled platelets following transfusion. Although capping the ßGlcNAc moieties by galactosylation prevents clearance, this strategy is ineffective after prolonged (>24 h) refrigeration. We report here that prolonged refrigeration increases the density/concentration of exposed galactose residues such that hepatocytes become increasingly involved in the removal of platelets using their Ashwell-Morell receptors. Macrophages always rapidly remove a large fraction of transfused platelets (~40%). With platelet cooling, hepatocyte-dependent clearance further diminishes their recoveries following transfusion.

Bcl-2 modulation to activate apoptosis in prostate cancer

Apoptosis resistance is a hallmark of cancer linked to disease progression and treatment resistance, which has led to the development of anticancer therapeutics that restore apoptotic function. Antiapoptotic Bcl-2 is frequently overexpressed in refractory prostate cancer and increased following standard hormonal therapy and chemotherapy; however, the rationally designed Bcl-2 antagonist, ABT-737, has not shown single agent apoptosis-promoting activity against human prostate cancer cell lines. This is likely due to the coordinate expression of antiapoptotic, Bcl-2-related Mcl-1 that is not targeted by ABT-737. We developed a mouse model for prostate cancer in which apoptosis resistance and tumorigenesis were conferred by Bcl-2 expression. Combining ABT-737 with agents that target Mcl-1 sensitized prostate cancer cell lines with an apoptotic block to cell death in vitro. In mice in vivo, ABT-737 showed single agent efficacy in prostate tumor allografts in which tumor cells are under hypoxic stress. In human prostate cancer tissue, examined using a novel tumor explant system designated Tumor Tissue Assessment for Response to Chemotherapy, combination chemotherapy promoted efficient apoptosis. Thus, rational targeting of both the Bcl-2 and Mcl-1 mechanisms of apoptosis resistance may be therapeutically advantageous for advanced prostate cancer.

"Message in the platelet"--more than just vestigial mRNA!

Although mammalian platelets are anucleated cells, a number of studies have shown that they retain a pool of messenger RNA (mRNA) carried over from the megakaryocyte during thrombopoiesis. Platelet mRNA was originally thought to be relatively unstable and short-lived within the youngest cells and has been used as a potential marker of platelet turnover. In this article we will discuss both theoretical and methodological issues related to the measurement of these younger, "reticulated platelets". A key question relating to platelet mRNA is also whether it has any functional relevance other than a marker of platelet immaturity. Evidence going back more than 30 years suggests that platelets can biosynthesize proteins. However, it is only very recently that the nature and specificity of platelet mRNA has been examined in any detail. Difficulties in obtaining pure platelet mRNA, free of contamination from other cells has added to the complexity of unravelling this story. However, there is now clear evidence that platelets contain small but significant levels of message for a variety of proteins. The platelet mRNA pool is much richer and more diverse than previously thought and recent data suggests that regulated synthesis of a selected number of proteins can be induced on platelet activation. The full complexity of the platelet genome is now just being revealed and may open the possibility for improved diagnosis and therapy of many haemostatic and thrombotic disorders.

Dual requirement for the ETS transcription factors Fli-1 and Erg in hematopoietic stem cells and the megakaryocyte lineage

Fli-1 and Erg are closely related members of the Ets family of transcription factors. Both genes are translocated in human cancers, including Ewing's sarcoma, leukemia, and in the case of Erg, more than half of all prostate cancers. Although evidence from mice and humans suggests that Fli-1 is required for megakaryopoiesis, and that Erg is required for normal adult hematopoietic stem cell (HSC) regulation, their precise physiological roles remain to be defined. To elucidate the relationship between Fli-1 and Erg in hematopoiesis, we conducted an analysis of mice carrying mutations in both genes. Our results demonstrate that there is a profound genetic interaction between Fli-1 and Erg. Double heterozygotes displayed phenotypes more dramatic than single heterozygotes: severe thrombocytopenia, with a significant deficit in megakaryocyte numbers and evidence of megakaryocyte dysmorphogenesis, and loss of HSCs accompanied by a reduction in the number of committed hematopoietic progenitor cells. These results illustrate an indispensable requirement for both Fli-1 and Erg in normal HSC and megakaryocyte homeostasis, and suggest these transcription factors may coregulate common target genes.

Two distinct pathways regulate platelet phosphatidylserine exposure and procoagulant function

Procoagulant platelets exhibit hallmark features of apoptotic cells, including membrane blebbing, microvesiculation, and phosphatidylserine (PS) exposure. Although platelets possess many well-known apoptotic regulators, their role in regulating the procoagulant function of platelets is unclear. To clarify this, we investigated the consequence of removing the essential mediators of apoptosis, Bak and Bax, or directly inducing apoptosis with the BH3 mimetic compound ABT-737. Treatment of platelets with ABT-737 triggered PS exposure and a marked increase in thrombin generation in vitro. This increase in procoagulant function was Bak/Bax- and caspase-dependent, but it was unaffected by inhibitors of platelet activation or by chelating extracellular calcium. In contrast, agonist-induced platelet procoagulant function was unchanged in Bak−/−Bax−/− or caspase inhibitor–treated platelets, but it was completely eliminated by extracellular calcium chelators or inhibitors of platelet activation. These studies show the existence of 2 distinct pathways regulating the procoagulant function of platelets.

The role of the intrinsic apoptosis pathway in platelet life and death

In recent years, it has become increasingly apparent that the production of platelets and their subsequent life span in the circulation are regulated, at least in part, by apoptotic mechanisms. There is also evidence implicating the apoptotic machinery in the regulation of platelet functional responses. This review examines the role of the intrinsic apoptosis pathway, regulated by the Bcl-2 family of proteins, in platelet biology.

Role of sialic acid for platelet life span: exposure of beta-galactose results in the rapid clearance of platelets from the circulation by asialoglycoprotein receptor-expressing liver macrophages and hepatocytes

Although surface sialic acid is considered a key determinant for the survival of circulating blood cells and glycoproteins, its role in platelet circulation lifetime is not fully clarified. We show that thrombocytopenia in mice deficient in the St3gal4 sialyltransferase gene (St3Gal-IV(-/-) mice) is caused by the recognition of terminal galactose residues exposed on the platelet surface in the absence of sialylation. This results in accelerated platelet clearance by asialoglycoprotein receptor-expressing scavenger cells, a mechanism that was recently shown to induce thrombocytopenia during Streptococcus pneumoniae sepsis. We now identify platelet GPIbalpha as a major counterreceptor on ST3Gal-IV(-/-) platelets for asialoglycoprotein receptors. Moreover, we report data that establish the importance of sialylation of the von Willebrand factor in its function.

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

BACKGROUND

Apoptosis or programmed cell death involves a number of biochemical events, including the activation of caspases, which lead to specific cell morphology changes and ultimately cell death. Traditionally, two apoptotic pathways have been described: the cell-surface death receptor-dependent extrinsic pathway and the mitochondria-dependent intrinsic pathway. Alternatively, apoptosis has been reported to be induced by endoplasmic reticulum (ER) stress, which is mainly induced by a reduction in intraluminal free Ca(2+) concentration ([Ca(2+)](ER)).

OBJECTIVES

The present study aimed to investigate the development of apoptotic events after ER stress induced by N,N,N',N'-tetrakis(2-pyridylmethyl)ethylenediamine (TPEN), an ER Ca(2+) chelator, in human platelets.

METHODS

Changes in cytosolic free Ca(2+) concentration, caspase activity and phosphatidylserine externalization were determined by fluorimetric techniques.

RESULTS

Our results indicate that TPEN reduces the amount of free Ca(2+) releasable by the Ca(2+)-mobilizing agonist thrombin. TPEN induced activation of caspase-3, -8 and -9 and subsequent phosphatidylserine externalization. The ability of TPEN to induce phosphatidylserine externalization was smaller than that of thrombin. In addition, TPEN was able to induce phosphorylation of the eukaryotic initiation factor 2 alpha (eIF2 alpha). TPEN-mediated caspase-3 activation requires functional caspase-8, but is independent of H(2)O(2) generation. Activation of caspase-3 and -8 by TPEN was prevented by salubrinal, an agent that prevents ER stress-induced apoptosis.

CONCLUSION

These findings provide experimental evidence for the existence of ER stress-mediated apoptosis in human platelets, a process that might limit platelet life span upon prolonged stimulation with agonists.

Association of genetic polymorphisms with interferon-induced haematologic adverse effects in chronic hepatitis C patients

Interferon (IFN)-based combination therapy with ribavirin has become the gold standard for the treatment of chronic hepatitis C virus infection. Haematologic toxicities, such as neutropenia, thrombocytopenia, and anaemia, however, frequently cause poor treatment tolerance, resulting in poor therapeutic efficacy. The aim of this study was to identify host genetic polymorphisms associated with the efficacy or haematologic toxicity of IFN-based combination therapy in chronic hepatitis C patients. We performed comprehensive single nucleotide polymorphism detection in all exonic regions of the 12 genes involved in the IFN signalling pathway in 32 healthy Japanese volunteers. Of 167 identified polymorphisms, 35 were genotyped and tested for an association with the efficacy or toxicity of IFN plus ribavirin therapy in 240 chronic hepatitis C patients. Multiple logistic regression analysis revealed that low viral load, viral genotypes 2 and 3, and a lower degree of liver fibrosis, but none of the genetic polymorphisms, were significantly associated with a sustained virologic response. In contrast to efficacy, multiple linear regression analyses demonstrated that two polymorphisms (IFNAR1 10848-A/G and STAT2 4757-G/T) were significantly associated with IFN-induced neutropenia (P = 0.013 and P = 0.011, respectively). Thrombocytopenia was associated with the IRF7 789-G/A (P = 0.031). In conclusion, genetic polymorphisms in IFN signalling pathway-related genes were associated with IFN-induced neutropenia and thrombocytopenia in chronic hepatitis C patients. In contrast to toxicity, the efficacy of IFN-based therapy was largely dependent on viral factors and degree of liver fibrosis.

Pathogenesis of chronic immune thrombocytopenia: increased platelet destruction and/or decreased platelet production

Chronic immune thrombocytopenia (ITP) is a haematological disorder in which patients predominantly develop skin and mucosal bleeding. Early studies suggested ITP was primarily due to immune-mediated peripheral platelet destruction. However, increasing evidence indicates that an additional component of this disorder is immune-mediated decreased platelet production that cannot keep pace with platelet destruction. Evidence for increased platelet destruction is thrombocytopenia following ITP plasma infusions in normal subjects, in vitro platelet phagocytosis, and decreased platelet survivals in ITP patients that respond to therapies that prevent in vivo platelet phagocytosis; e.g., intravenous immunoglobulin G, anti-D, corticosteroids, and splenectomy. The cause of platelet destruction in most ITP patients appears to be autoantibody-mediated. However, cytotoxic T lymphocyte-mediated platelet (and possibly megakaryocyte) lysis, may also be important. Studies supporting suppressed platelet production include: reduced platelet turnover in over 80% of ITP patients, morphological evidence of megakaryocyte damage, autoantibody-induced suppression of in vitro megakaryocytopoiesis, and increased platelet counts in most ITP patients following treatment with thrombopoietin receptor agonists. This review summarizes data that indicates that the pathogenesis of chronic ITP may be due to both immune-mediated platelet destruction and/or suppressed platelet production. The relative importance of these two mechanisms undoubtedly varies among patients.

New-generation drugs that stimulate platelet production in chronic immune thrombocytopenic purpura

Idiopathic thrombocytopenic purpura is an acquired disease characterised by a low platelet count. Development of autoantibodies is a main cause of the disease. Although many patients have few symptoms, life-threatening bleeding can arise and hence, when platelet counts fall to unacceptable values treatment should be initiated. However, conventional immunosuppressive approaches can fail, perhaps because of the heterogeneous nature of the disease. Newly developed agents that increase platelet production by stimulating megakaryocytes-such as drugs that bind to the thrombopoietin receptor c-MPL-offer an alternative treatment strategy. Although initial thrombopoietin analogues caused adverse immune reactions, second-generation thrombopoietin-receptor agonists that are in late-stage clinical development seem promising. In particular, eltrombopag and romiplostim safely increase and maintain platelet production in patients with refractory disease. However, long-term side-effects are being assessed and the exact role of these agents in the overall treatment strategy of chronic idiopathic thrombocytopenic purpura remains to be established.

Different forms of cell death induced by putative BCL2 inhibitors

Several inhibitors of BCL2 proteins have been identified that induce apoptosis in a variety of tumor cells, indicating their potential in cancer therapy. We investigated the specificity of six putative BCL2 inhibitors (obatoclax, gossypol, apogossypol, EM20-25, chelerythrine and ABT-737). Using cells deficient either for Bax/Bak or caspase-9, we found that only ABT-737 specifically targeted BCL2 proteins and induced apoptosis by activation of caspase-9, as only ABT-737 induced apoptosis was completely inhibited in cells deficient for Bax/Bak or caspase-9. Our data show that only ABT-737 is a specific BCL2 inhibitor and all other compounds investigated were not specific for BCL2 proteins. Furthermore, investigations of the effects of these compounds in primary chronic lymphocytic leukemic cells showed that all compounds induced certain biochemical hallmarks of apoptosis, such as release of cytochrome c and caspase cleavage. However, they all caused strikingly different ultrastructural changes. ABT-737 induced all the characteristic ultrastructural changes of apoptosis together with early rupture of the outer mitochondrial membrane, whereas obatoclax, chlelerythrine and gossypol induced pronounced mitochondrial swelling with formation of phospholipid inclusions. Therefore, we conclude that biochemical measurements used earlier to define apoptosis like mitochondrial release of cytochrome c and caspase cleavage, are insufficient to distinguish between classic apoptosis and other forms of cell death.

Removing the cloak of invisibility: phenotyping the mouse

If you study a human disease, it is likely that you have tried to generate a mouse model. Sometimes, these models are excellent; others are disappointing. Or, so we think. How often does our mouse mutant not model the human disease because of limitations in how we may look at it? In any living organism, many factors work together to produce the phenotype. Here, new phenotyping paradigms for assessing mouse biology and physiology are described and proposed. Advances in mouse phenotype assessments have paralleled human clinical diagnostics. The future brings a multitude of mouse strains that might be exposed to a variety of conditions. To assess health will require the ability to perform a broad-based phenotype assessment of every animal until we can understand how the perturbation of one system affects others.

Mitochondrial control of platelet apoptosis: effect of cyclosporin A, an inhibitor of the mitochondrial permeability transition pore

The role of the mitochondrial permeability transition pore (MPTP) in apoptosis of nucleated cells is well documented. In contrast, the role of MPTP in apoptosis of anucleated platelets is largely unknown. The aim of this study was to elucidate the contribution of MPTP in the control of different manifestations of platelet apoptosis by analyzing the effect of cyclosporin A (CsA), a potent inhibitor of MPTP formation. Using flow cytometry, we studied the effect of pretreatment of platelets with CsA on apoptotic responses in human platelets stimulated with calcium ionophore A23187. We found that CsA inhibited A23187-stimulated platelet apoptosis, completely preventing (i) depolarization of mitochondrial inner membrane potential (DeltaPsim), (ii) activation of cytosolic apoptosis executioner caspase-3, (iii) platelet shrinkage, and (iv) fragmentation of platelets to microparticles, but (v) only partially (approximately 25%), inhibiting phosphatidylserine (PS) exposure on the platelet surface. This study shows that MPTP formation is upstream of DeltaPsim depolarization, caspase-3 activation, platelet shrinkage and microparticle formation, and stringently controls these apoptotic events in A23187-stimulated platelets but is less involved in PS externalization. These data also indicate that CsA may rescue platelets from apoptosis, preventing caspase-3 activation and inhibiting the terminal cellular manifestations of platelet apoptosis, such as platelet shrinkage and degradation to microparticles. Furthermore, the results suggest a novel potentially useful application of CsA as an inhibitor of platelet demise through apoptosis in thrombocytopenias associated with enhanced platelet apoptosis.

Mutational inhibition of c-Myb or p300 ameliorates treatment-induced thrombocytopenia

The transcription factor c-Myb and coregulator p300 have a key role in maintaining production of controlled numbers of megakaryocytes and platelets. In mice, mutations in c-Myb or p300 cause thrombocytosis in otherwise wild-type animals and can ameliorate the thrombocytopenia in mice lacking the thrombopoietin receptor, c-Mpl, a model for human congenital amegakaryocytic thrombocytopenia. To examine whether inhibition of c-Myb/p300 is effective in other models of thrombocytopenia, the effect of the c-Myb(Plt4) mutation on thrombocytopenia associated with reduced platelet life span in Bcl-X(Plt20/Plt20) mice was assessed, as were responses in c-Myb(Plt4) and/or p300(Plt6) mutant mice to thrombocytopenia associated with antiplatelet antibodies, chemotherapy, or bone marrow transplantation. Homozygosity of the c-Myb(Plt4) allele ameliorated thrombocytopenia associated with reduced platelet life span, and c-Myb(Plt4/+) mice exhibited more rapid than normal recovery from thrombocytopenia caused by antiplatelet serum or bone marrow transplantation. Recovery to pretreatment platelet levels was unaltered in 5-fluorouracil-treated c-Myb(Plt4/+) mice relative to wild-type controls, but enhanced platelet production during subsequent thrombocytosis was evident. More modest enhancement of platelet recovery after 5-fluorouracil or bone marrow transplantation was also evident in p300(Plt6/+) animals. The data suggest potential utility of c-Myb/p300 as a target for therapeutic intervention in thrombocytopenia of diverse origins.

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.

Thrombocytosis

Major progress in understanding the pathogenesis in patients with thrombocytosis has been made by identifying mutations in the key regulators of thrombopoietin: the thrombopoietin receptor MPL and JAK2. Together, these mutations can be found in 50% to 60% of patients with essential thrombocythemia or primary myelofibrosis and in 10% to 20% of hereditary thrombocytosis. A decrease in expression of the Mpl protein can cause thrombocytosis even in the absence of mutations in the coding sequence, due to a shift in the balance between stimulation of signaling in megakaryopoiesis and removal of thrombopoietin by receptor mediated internalization in platelets. When present in a heterozygous state the JAK2-V617F mutation preferentially stimulates megakaryopoiesis and in most cases manifests as essential thrombocythemia (ET), whereas homozygous JAK2-V617F reduces megakaryopoiesis in favor of increased erythropoiesis, resulting in polycythemia vera and/or myelofibrosis. In 30% to 40% of patients with ET or primary myelofibrosis (PMF) and in 80% to 90% of pedigrees with hereditary thrombocytosis the disease-causing gene remains unknown. Ongoing genetic and genomic screens have identified genes that, when mutated, can cause thrombocytosis in mouse models. A more complete picture of the pathways that regulate megakaryopoisis and platelet production will be important for finding new ways of controlling platelet production in patients with thrombocytosis.

New drugs for the treatment of lymphoma

Historically, most drugs developed for treatment of leukemias, lymphomas, and myeloma had already been studied in the solid tumor setting. Nearly 10 years ago, chronic myelogenous leukemia (CML) forever changed this paradigm. Imatinib showed that it was possible to nullify the pathognomic genetic lesion in a hematologic malignancy. Since the approval of imatinib for CML, a host of new drugs active in blood cancers have emerged. This article highlights some areas of innovative drug development in lymphoma where possible; it emphasizes the biologic basis for the approach, linking this essential biology to the biochemical pharmacology. The article focuses on the many new targets including Syk, Bcl-2, CD-40, and the phosphoinositide-3 kinase/AKT/mammalian target of rapamycin pathway.

Platelet storage lesion: a new understanding from a proteomic perspective

Platelet storage and availability for the purposes of transfusion are currently restricted by a markedly short shelf life of 5 to 7 days owing to an increased risk of bacterial growth and storage-related deterioration called the platelet storage lesion. Because most bacteria grow to confluence within 5 days during storage at room temperature, there is little increased risk of bacterial overgrowth with testing in place, and the only remaining issue is the quality of platelets during the extended storage. Although the manifestations of the storage lesion have been well studied using a variety of in vitro measures, the precise biochemical pathways involved in the initiation and progression of this process have yet to be identified. Proteomics has emerged as a powerful tool to identify and monitor changes during platelet storage and, in combination with biochemical and physiologic studies, facilitates the development of a sophisticated mechanistic view. In this review, we summarize recent experimental work that has led to a detailed overview of protein changes linked to platelet functions and signaling pathways, providing potential targets for inhibitors to ameliorate the storage lesion.

An ENU-induced recessive mutation in Mpl leads to thrombocytopenia with overdominance

OBJECTIVE

The aim of this study was to identify and characterize the causative mutation in the thrombocytopenic mouse strain HLB219 that was generated at the Jackson Laboratory as part of a large-scale N-ethyl-N-nitrosourea mutagenesis screen.

MATERIALS AND METHODS

The HLB219 mutation was identified by interval mapping of F2 mice generated from intercross breeding of HLB219 to both BALB/cByJ (BALB) and 129/SvImJ (129/Sv). Mpl was identified as a candidate gene and sequenced. The mutation was characterized in vivo in mouse hematopoietic stem/progenitor cell assays and in cell culture by expression in Ba/F3 cells.

RESULTS

A novel mutation in the thrombopoietin (TPO) receptor Mpl in HLB219 mice caused a Cys-->Arg substitution at codon 40 in the extracellular region of the receptor. Mice homozygous for the Mpl(hlb219) mutation had an 80% decrease in the number of platelets in comparison to the wild-type C57BL/6J strain, low numbers of bone marrow megakaryocytes, high TPO levels, and decreased competitive repopulating ability, consistent with a loss-of-function mutation in the receptor. Mice heterozygous for Mpl(hlb219) however, showed an overdominance effect with a significant increase in platelet number. Functional analysis in vitro demonstrated that Ba/F3 cells expressing the mutant MPL(hlb219) protein failed to activate extracellular signal-regulated kinase and signal transducers and activators of transcription 5, but proliferated in the absence of TPO and required constitutive phosphorylation of RAC-alpha serine/threonine protein kinase (AKT) for cytokine-independent growth.

CONCLUSION

Thrombocytopenia in HLB219 mice is caused by a recessive mutation in Mpl that abrogates mitogen-activated protein kinase-extracellular signal regulated kinase and janus kinase-signal transducers and activators of transcription signaling.

Defining the target specificity of ABT-737 and synergistic antitumor activities in combination with histone deacetylase inhibitors

The apoptotic and therapeutic activities of the histone deacetylase inhibitor (HDACi) vorinostat are blocked by overexpression of Bcl-2 or Bcl-X(L). Herein, we used the small molecule inhibitor ABT-737 to restore sensitivity of Emu-myc lymphomas overexpressing Bcl-2 or Bcl-X(L) to vorinostat and valproic acid (VPA). Combining low-dose ABT-737 with vorinostat or VPA resulted in synergistic apoptosis of these cells. ABT-737 was ineffective against Emu-myc/Mcl-1 and Emu-myc/A1 cells either as a single agent or in combination with HDACi. However, in contrast to the reported binding specificity data, Emu-myc/Bcl-w lymphomas were insensitive to ABT-737 used alone or in combination with HDACi, indicating that the regulatory activity of ABT-737 is restricted to Bcl-2 and Bcl-X(L). Emu-myc lymphomas that expressed Bcl-2 throughout the tumorigenesis process were especially sensitive to ABT-737, while those forced to overexpress Mcl-1 were not. This supports the notion that tumor cells "addicted" to ABT-737 target proteins (ie, Bcl-2 or Bcl-X(L)) are likely to be the most sensitive target cell population. Our studies provide important preclinical data on the binding specificity of ABT-737 and its usefulness against primary hematologic malignancies when used as a single agent and in combination with HDACi.

Involvement of BH3-only proteins in hematologic malignancies

The interaction between anti-apoptotic and pro-apoptotic members of the Bcl-2 family proteins determines life or death for cancer cells. In this context, BH3-only proteins (such as Bim), members of the pro-apoptotic Bcl-2 family proteins, act as key initiators of apoptosis by activating Bax and Bak through liberating them from anti-apoptotic Bcl-2 members. This then leads to the disruption of mitochondrial outer membrane, and eventually promotes proteolytic cascades for cellular dismantling. We here review the growing evidence of how BH3-only proteins are involved in tumorigenesis and in apoptosis induced by anti-cancer agents in hematologic malignancies. A deeper understanding of the roles of BH3-only proteins in cell death regulation may yield crucial insights for the further development of more effective and rational cell killing strategies. Recent developments in the direct therapeutic manipulation of Bcl-2 proteins using BH3-mimicking agents, such as ABT-737 or GX15-070, for hematologic malignancies are also summarized.

Apoptosis is triggered when prosurvival Bcl-2 proteins cannot restrain Bax

A central issue in the control of apoptosis is whether its essential mediators Bax and Bak must be restrained by Bcl-2-like prosurvival relatives to prevent their damaging mitochondria and unleashing apoptosis. The issue is particularly vexed for Bax, which is largely a cytosolic monomer in unstressed cells. To determine whether Bax regulation requires its binding by prosurvival relatives, we replaced a conserved aspartate in its BH3 interaction domain with arginine. Bax D68R functioned and behaved like wild-type Bax in localization and activation but had greatly impaired binding to the prosurvival family members. Nevertheless, Bcl-x(L) remained able to block apoptosis induced by Bax D68R. Whereas cells with sufficient Bcl-x(L) tolerated expression of Bax D68R, it provoked apoptosis when Bcl-x(L) was absent, downregulated, or inactivated. Moreover, Bax D68R rendered membrane bound by a C-terminal anchor mutation overwhelmed endogenous Bcl-x(L) and killed cells. These unexpected results suggest that engagement of Bax by its prosurvival relatives is a major barrier to its full activation. We propose that the Bcl-2-like proteins must capture the small proportion of Bax molecules with an exposed BH3 domain, probably on the mitochondrial membrane, to prevent Bax-imposed cell death, but that Bcl-x(L) also controls Bax by other mechanisms.

Treatment of B-RAF mutant human tumor cells with a MEK inhibitor requires Bim and is enhanced by a BH3 mimetic

B-RAF is frequently mutated in solid tumors, resulting in activation of the MEK/ERK signaling pathway and ultimately tumor cell growth and survival. MEK inhibition in these cells results in cell cycle arrest and cytostasis. Here, we have shown that MEK inhibition also triggers limited apoptosis of human tumor cell lines with B-RAF mutations and that this effect was dependent on upregulation and dephosphorylation of the proapoptotic, Bcl-2 homology 3-only (BH3-only) Bcl-2 family member Bim. However, upregulation of Bim was insufficient for extensive apoptosis and was countered by overexpression of Bcl-2. To overcome apoptotic resistance, we treated the B-RAF mutant cells both with MEK inhibitors and with the BH3 mimetic ABT-737, resulting in profound synergism and extensive tumor cell death. This treatment was successful because of both efficient antagonism of the prosurvival Bcl-2 family member Mcl-1 by Bim and inhibition of Bcl-2 and Bcl-x(L) by ABT-737. Critically, addition of ABT-737 converted the predominantly cytostatic effect of MEK inhibition to a cytotoxic effect, causing long-term tumor regression in mice xenografted with human tumor cell lines. Thus, the therapeutic efficacy of MEK inhibition requires concurrent unleashing of apoptosis by a BH3 mimetic and represents a potent combination treatment for tumors harboring B-RAF mutations.