M2 macrophages drive leukemic transformation by imposing resistance to phagocytosis and improving mitochondrial metabolism

It is increasingly becoming clear that cancers are a symbiosis of diverse cell types and tumor clones. Combined single-cell RNA sequencing, flow cytometry, and immunohistochemistry studies of the innate immune compartment in the bone marrow of patients with acute myeloid leukemia (AML) reveal a shift toward a tumor-supportive M2-polarized macrophage landscape with an altered transcriptional program, with enhanced fatty acid oxidation and NAD⁺ generation. Functionally, these AML-associated macrophages display decreased phagocytic activity and intra-bone marrow coinjection of M2 macrophages together with leukemic blasts strongly enhances in vivo transformation potential. A 2-day in vitro exposure to M2 macrophages results in the accumulation of CALR^low leukemic blast cells, which are now protected against phagocytosis. Moreover, M2-exposed "trained" leukemic blasts display increased mitochondrial metabolism, in part mediated via mitochondrial transfer. Our study provides insight into the mechanisms by which the immune landscape contributes to aggressive leukemia development and provides alternatives for targeting strategies aimed at the tumor microenvironment.

The immune landscape in BCR-ABL negative myeloproliferative neoplasms: inflammation, infections and opportunities for immunotherapy

Breakpoint cluster region-Abelson (BCR-ABL) negative myeloproliferative neoplasms (MPNs) are chronic myeloid neoplasms initiated by the acquisition of gene mutation(s) in a haematopoietic stem cell, leading to clonal expansion and over-production of blood cells and their progenitors. MPNs encompass a spectrum of disorders with overlapping but distinct molecular, laboratory and clinical features. This includes polycythaemia vera, essential thrombocythaemia and myelofibrosis. Dysregulation of the immune system is key to the pathology of MPNs, supporting clonal evolution, mediating symptoms and resulting in varying degrees of immunocompromise. Targeting immune dysfunction is an important treatment strategy. In the present review, we focus on the immune landscape in patients with MPNs - the role of inflammation in disease pathogenesis, susceptibility to infection and emerging strategies for therapeutic immune modulation. Further detailed work is required to delineate immune perturbation more precisely in MPNs to determine how and why vulnerability to infection differs between clinical subtypes and to better understand how inflammation results in a competitive advantage for the MPN clone. These studies may help shed light on new designs for disease-modifying therapies.

Chromatin accessibility governs the differential response of cancer and T cells to arginine starvation

Depleting the microenvironment of important nutrients such as arginine is a key strategy for immune evasion by cancer cells. Many tumors overexpress arginase, but it is unclear how these cancers, but not T cells, tolerate arginine depletion. In this study, we show that tumor cells synthesize arginine from citrulline by upregulating argininosuccinate synthetase 1 (ASS1). Under arginine starvation, ASS1 transcription is induced by ATF4 and CEBPβ binding to an enhancer within ASS1. T cells cannot induce ASS1, despite the presence of active ATF4 and CEBPβ, as the gene is repressed. Arginine starvation drives global chromatin compaction and repressive histone methylation, which disrupts ATF4/CEBPβ binding and target gene transcription. We find that T cell activation is impaired in arginine-depleted conditions, with significant metabolic perturbation linked to incomplete chromatin remodeling and misregulation of key genes. Our results highlight a T cell behavior mediated by nutritional stress, exploited by cancer cells to enable pathological immune evasion.

Combination of azacitidine and enasidenib enhances leukemic cell differentiation and cooperatively hypomethylates DNA

Azacitidine and enasidenib are two therapies available for treatment of acute myelogenous leukemia (AML), and the mechanisms of action of these drugs involve alteration of aberrant DNA methylation. We hypothesized that a combination of these agents could have interactive effects on DNA methylation and enhance differentiation in mIDH2 cells. Combination treatment enhanced cellular differentiation in TF-1 cells overexpressing IDJ2R140Q through increased hemoglobinization and increased hemoglobin γ RNA expression compared with the effects of single agents. Furthermore, in primary AML samples (IDH2R140Q or R172K), combination treatment reduced CD34+ cells and increased CD15+ cells to a greater extent than attained with single agents. To explore the mechanism of enhanced differentiation with combination treatment, the TF-1 epigenome was analyzed by profiling 5-hydroxymethylcytosine (5hmC) and 5-methylcytosine (5mC) DNA methylation changes. Enasidenib treatment alone increased 5hmC, consistent with reactivation of ten-eleven-translocation (TET) enzyme activity. Compared with treatment with azacitidine alone, combination treatment reduced 5mC levels at greater numbers of sites and these loci were significantly enriched in regions with increased 5hMC (25.8% vs. 7.4%). Results are consistent with a model in which enasidenib-mediated reactivation of ten-eleven-translocation enzymes cooperates with azacitidine-mediated inhibition of DNA methyltransferase enzymes, leading to greater reductions in DNA methylation and enhanced erythroid differentiation.

Effect of enasidenib (ENA) plus azacitidine (AZA) on complete remission and overall response versus AZA monotherapy in mutant-IDH2 (mIDH2) newly diagnosed acute myeloid leukemia (ND-AML)

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Background: ENA and AZA each induce overall response rates (ORR) of ~30% and complete remission (CR) rates of ~20% in ND-AML. In vitro, combining ENA + AZA enhances cell differentiation. We report results of the phase II portion of an open-label, randomized phase I/II study of ENA + AZA (“E+A”) vs. AZA monotherapy (“A”) in patients (pts) with m IDH2 ND-AML (NCT02677922). Methods: Pts age ≥ 18 years ineligible for intensive chemotherapy, with ECOG PS ≤ 2 and intermediate- or poor-risk cytogenetics, were randomized 2:1 to E+A or A in 28-day (d) cycles. All pts received SC AZA 75 mg/m2/d x 7 d/cycle; pts randomized to E+A also received ENA 100 mg QD. The primary endpoint was ORR (CR, CR with incomplete recovery, partial remission, morphologic leukemia-free state). Other endpoints include duration of response (DOR), overall and event-free survival (OS, EFS), safety, and m IDH2 VAF. Results: 101 pts received E+A (n = 68) or A (n = 33). Median age was 75 years (57–85); most pts (83%) had intermediate-risk cytogenetics. 21 pts in the E+A arm and 1 in the A arm were ongoing at data cutoff (Aug 2019). Most common reason for discontinuation was disease progression (E+A 31%, A 52%). Median number Tx cycles was 10 (1–26) in the E+A arm and 6 (1–28) in the A arm. 7 pts (21%) in the A arm received subsequent Tx with ENA. ORR, CR rate and DOR were significantly improved with E+A vs. A (Table). Median OS was 22 mo in both arms (HR 0.99 [95%CI 0.52, 1.87]; P = 0.97). Median EFS was 17.2 and 10.8 mo in the E+A and A arms, respectively (HR 0.59 [95%CI 0.30, 1.17]; P = 0.13). Maximal m IDH2 VAF change from BL was –83.4% with E+A vs. –17.7% with A ( P < 0.01). No baseline co-mutation predicted primary resistance. Common Tx-related grade 3–4 AEs in the E+A arm were thrombocytopenia (37%), neutropenia (35%), anemia (19%), and febrile neutropenia (15%); these occurred in 19%, 22%, 22%, and 16% in the A arm. Grade 3–4 infections occurred in 18% of E+A pts and 31% of A pts. IDH differentiation syndrome occurred in 12 pts (18%) in the E+A arm. 5 E+A pts (7%) and 1 A pt (3%) died in the first 60 d. Conclusions: Combining ENA + AZA resulted in significantly improved response rates and durations, and was generally well-tolerated in older patients with m IDH2 ND-AML. The impact of subsequent Tx on OS/EFS and new translational data will be presented at the meeting. Clinical trial information: NCT02677922 . [Table: see text]

TP53 mutation status divides myelodysplastic syndromes with complex karyotypes into distinct prognostic subgroups

Risk stratification is critical in the care of patients with myelodysplastic syndromes (MDS). Approximately 10% have a complex karyotype (CK), defined as more than two cytogenetic abnormalities, which is a highly adverse prognostic marker. However, CK-MDS can carry a wide range of chromosomal abnormalities and somatic mutations. To refine risk stratification of CK-MDS patients, we examined data from 359 CK-MDS patients shared by the International Working Group for MDS. Mutations were underrepresented with the exception of TP53 mutations, identified in 55% of patients. TP53 mutated patients had even fewer co-mutated genes but were enriched for the del(5q) chromosomal abnormality (p < 0.005), monosomal karyotype (p < 0.001), and high complexity, defined as more than 4 cytogenetic abnormalities (p < 0.001). Monosomal karyotype, high complexity, and TP53 mutation were individually associated with shorter overall survival, but monosomal status was not significant in a multivariable model. Multivariable survival modeling identified severe anemia (hemoglobin < 8.0 g/dL), NRAS mutation, SF3B1 mutation, TP53 mutation, elevated blast percentage (>10%), abnormal 3q, abnormal 9, and monosomy 7 as having the greatest survival risk. The poor risk associated with CK-MDS is driven by its association with prognostically adverse TP53 mutations and can be refined by considering clinical and karyotype features.

Clonal heterogeneity of acute myeloid leukemia treated with the IDH2 inhibitor enasidenib

Mutations in the gene encoding isocitrate dehydrogenase 2 (IDH2) occur in several types of cancer, including acute myeloid leukemia (AML). In model systems, mutant IDH2 causes hematopoietic differentiation arrest. Enasidenib, a selective small-molecule inhibitor of mutant IDH2, produces a clinical response in 40% of treated patients with relapsed/refractory AML by promoting leukemic cell differentiation. Here, we studied the clonal basis of response and acquired resistance to enasidenib treatment. Using sequential patient samples, we determined the clonal structure of hematopoietic cell populations at different stages of differentiation. Before therapy, IDH2-mutant clones showed variable differentiation arrest. Enasidenib treatment promoted hematopoietic differentiation from either terminal or ancestral mutant clones; less frequently, treatment promoted differentiation of nonmutant cells. Analysis of paired diagnosis/relapse samples did not identify second-site mutations in IDH2 at relapse. Instead, relapse arose by clonal evolution or selection of terminal or ancestral clones, thus highlighting multiple bypass pathways that could potentially be targeted to restore differentiation arrest. These results show how mapping of clonal structure in cell populations at different stages of differentiation can reveal the response and evolution of clones during treatment response and relapse.

Single-cell analysis reveals the continuum of human lympho-myeloid progenitor cells

The hierarchy of human hemopoietic progenitor cells that produce lymphoid and granulocytic-monocytic (myeloid) lineages is unclear. Multiple progenitor populations produce lymphoid and myeloid cells, but they remain incompletely characterized. Here we demonstrated that lympho-myeloid progenitor populations in cord blood - lymphoid-primed multi-potential progenitors (LMPPs), granulocyte-macrophage progenitors (GMPs) and multi-lymphoid progenitors (MLPs) - were functionally and transcriptionally distinct and heterogeneous at the clonal level, with progenitors of many different functional potentials present. Although most progenitors had the potential to develop into only one mature cell type ('uni-lineage potential'), bi- and rarer multi-lineage progenitors were present among LMPPs, GMPs and MLPs. Those findings, coupled with single-cell expression analyses, suggest that a continuum of progenitors execute lymphoid and myeloid differentiation, rather than only uni-lineage progenitors' being present downstream of stem cells.

Genetic and Pharmacological Analyses of Involvement of Src-family, Syk and Btk Tyrosine Kinases in Platelet Shape Change

Platelet shape change was found to be associated with an increase in protein tyrosine phosphorylation upon stimulation of thrombin-, ADPand thromboxane A2-G-protein coupled receptors in human platelets and thromboxane A2 receptors in mouse platelets. By using PP1 and PD173956, two structurally unrelated specific inhibitors of Src-family tyrosine kinases, and mouse platelets deficient in the Src-kinase Fyn or Lyn, we show that Src-family kinases cause the increase in protein tyrosine phosphorylation. We further detected that the non-Src tyrosine kinase Syk was activated during shape change in a manner dependent on Src-family kinaseactivation. The pharmacological experiments and the studies on Fyn-, Lyn- and Syk-deficient mouse platelets showed that neither Src-family kinases nor Syk are functionally involved in shape change. Also human platelets deficient of the tyrosine kinase Btk showed a normal shape change. Binding of PAC-1 that recognizes activated integrin αIIb β3 complexes on the platelet surface was enhanced during shape change and blocked by inhibition of Src-kinases. We conclude that the activation of Src-kinases and the subsequent Syk stimulation upon activation of G-protein coupled receptors are not involved in the cytoskeletal changes underlying shape change of human and mouse platelets, but that the stimulation of this evolutionary conserved pathway leads to integrin αIIb β3 exposure during shape change.

Identifying the optimum source of mesenchymal stem cells for use in knee surgery

Single sitting procedures where the mononuclear cell fraction is extracted from bone marrow and implanted directly into cartilage and bone defects are becoming more popular as novel treatments for cartilage defects which have, until now had few treatment options. This is on the basis that the mesenchymal stem cells (MSCs) contained within will repair the damaged tissue. This study sought to determine if the femur and tibia could provide equivalent amounts of mesenchymal stem cells, with equivalent viability and proliferative capacity, to that obtained from the gold standard of the pelvis in order to potentially reduce the morbidity associated with these procedures. Bone marrow was extracted from the pelvis, femur, and tibia of human subjects. The mononuclear cell fraction was extracted and cultured in the laboratory. Mesenchymal stem cell populations were assessed using a colony forming unit count. Viability was assessed using a PrestoBlue viability assay. Population doubling number was calculated between the end of passage 0 and passage three to determine the proliferative abilities of the different populations. Finally, the cell surface phenotype of the cells was determined by flow cytometry. The results showed that the pelvis was superior to the femur and tibia in terms of the number of stem cells isolated. There was no statistically significant difference in the phenotype of the cells isolated from different locations. This work shows that when undertaking single sitting procedures, the pelvis remains the optimum source for obtaining MSCs, despite the morbidity associated with bone marrow collection from the pelvis. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 35:1868-1875, 2017.

Enasidenib induces acute myeloid leukemia cell differentiation to promote clinical response

Recurrent mutations at R140 and R172 in isocitrate dehydrogenase 2 (IDH2) occur in many cancers, including ∼12% of acute myeloid leukemia (AML). In preclinical models these mutations cause accumulation of the oncogenic metabolite R-2-hydroxyglutarate (2-HG) and induce hematopoietic differentiation block. Single-agent enasidenib (AG-221/CC-90007), a selective mutant IDH2 (mIDH2) inhibitor, produced an overall response rate of 40.3% in relapsed/refractory AML (rrAML) patients with mIDH2 in a phase 1 trial. However, its mechanism of action and biomarkers associated with response remain unclear. Here, we measured 2-HG, mIDH2 allele burden, and co-occurring somatic mutations in sequential patient samples from the clinical trial and correlated these with clinical response. Furthermore, we used flow cytometry to assess inhibition of mIDH2 on hematopoietic differentiation. We observed potent 2-HG suppression in both R140 and R172 mIDH2 AML subtypes, with different kinetics, which preceded clinical response. Suppression of 2-HG alone did not predict response, because most nonresponding patients also exhibited 2-HG suppression. Complete remission (CR) with persistence of mIDH2 and normalization of hematopoietic stem and progenitor compartments with emergence of functional mIDH2 neutrophils were observed. In a subset of CR patients, mIDH2 allele burden was reduced and remained undetectable with response. Co-occurring mutations in NRAS and other MAPK pathway effectors were enriched in nonresponding patients, consistent with RAS signaling contributing to primary therapeutic resistance. Together, these data support differentiation as the main mechanism of enasidenib efficacy in relapsed/refractory AML patients and provide insight into resistance mechanisms to inform future mechanism-based combination treatment studies.

Genetically distinct leukemic stem cells in human CD34- acute myeloid leukemia are arrested at a hemopoietic precursor-like stage

Quek and colleagues identify human leukemic stem cells (LSCs) present in CD34⁻ AML. In-depth characterization of the functional and clonal aspects of CD34⁻ LSCs indicates that most are similar to myeloid precursors.

Our understanding of the perturbation of normal cellular differentiation hierarchies to create tumor-propagating stem cell populations is incomplete. In human acute myeloid leukemia (AML), current models suggest transformation creates leukemic stem cell (LSC) populations arrested at a progenitor-like stage expressing cell surface CD34. We show that in ∼25% of AML, with a distinct genetic mutation pattern where >98% of cells are CD34⁻, there are multiple, nonhierarchically arranged CD34⁺ and CD34⁻ LSC populations. Within CD34⁻ and CD34⁺ LSC–containing populations, LSC frequencies are similar; there are shared clonal structures and near-identical transcriptional signatures. CD34⁻ LSCs have disordered global transcription profiles, but these profiles are enriched for transcriptional signatures of normal CD34⁻ mature granulocyte–macrophage precursors, downstream of progenitors. But unlike mature precursors, LSCs express multiple normal stem cell transcriptional regulators previously implicated in LSC function. This suggests a new refined model of the relationship between LSCs and normal hemopoiesis in which the nature of genetic/epigenetic changes determines the disordered transcriptional program, resulting in LSC differentiation arrest at stages that are most like either progenitor or precursor stages of hemopoiesis.

Factors influencing success of clinical genome sequencing across a broad spectrum of disorders

To assess factors influencing the success of whole-genome sequencing for mainstream clinical diagnosis, we sequenced 217 individuals from 156 independent cases or families across a broad spectrum of disorders in whom previous screening had identified no pathogenic variants. We quantified the number of candidate variants identified using different strategies for variant calling, filtering, annotation and prioritization. We found that jointly calling variants across samples, filtering against both local and external databases, deploying multiple annotation tools and using familial transmission above biological plausibility contributed to accuracy. Overall, we identified disease-causing variants in 21% of cases, with the proportion increasing to 34% (23/68) for mendelian disorders and 57% (8/14) in family trios. We also discovered 32 potentially clinically actionable variants in 18 genes unrelated to the referral disorder, although only 4 were ultimately considered reportable. Our results demonstrate the value of genome sequencing for routine clinical diagnosis but also highlight many outstanding challenges.

Azacitidine fails to eradicate leukemic stem/progenitor cell populations in patients with acute myeloid leukemia and myelodysplasia

Epigenetic therapies demonstrate significant clinical activity in acute myeloid leukemia (AML) and myelodysplasia (MDS) and constitute an important new class of therapeutic agents. However hematological responses are not durable and disease relapse appears inevitable. Experimentally, leukemic stem/progenitor cells (LSC) propagate disease in animal models of AML and it has been postulated that their relative chemo-resistance contributes to disease relapse. We serially measured LSC numbers in patients with high-risk AML and MDS treated with 5'-azacitidine and sodium valproate (VAL-AZA). Fifteen out of seventy-nine patients achieved a complete remission (CR) or complete remission with incomplete blood count recovery (CRi) with VAL-AZA therapy. There was no significant reduction in the size of the LSC-containing population in non-responders. While the LSC-containing population was substantially reduced in all patients achieving a CR/CRi it was never eradicated and expansion of this population antedated morphological relapse. Similar studies were performed in seven patients with newly diagnosed AML treated with induction chemotherapy. Eradication of the LSC-containing population was observed in three patients all of whom achieved a durable CR in contrast to patients with resistant disease where LSC persistence was observed. LSC quantitation provides a novel biomarker of disease response and relapse in patients with AML treated with epigenetic therapies. New drugs that target this cellular population in vivo are required.

Coexistence of LMPP-like and GMP-like leukemia stem cells in acute myeloid leukemia

The relationships between normal and leukemic stem/progenitor cells are unclear. We show that in ∼80% of primary human CD34+ acute myeloid leukemia (AML), two expanded populations with hemopoietic progenitor immunophenotype coexist in most patients. Both populations have leukemic stem cell (LSC) activity and are hierarchically ordered; one LSC population gives rise to the other. Global gene expression profiling shows the LSC populations are molecularly distinct and resemble normal progenitors but not stem cells. The more mature LSC population most closely mirrors normal granulocyte-macrophage progenitors (GMP) and the immature LSC population a previously uncharacterized progenitor functionally similar to lymphoid-primed multipotential progenitors (LMPPs). This suggests that in most cases primary CD34+ AML is a progenitor disease where LSCs acquire abnormal self-renewal potential.

Molecular therapies in beta-thalassaemia

The beta-thalassaemias have a major global impact on health and mortality. Allogeneic haemopoietic stem cell transplantation is the only approach that may lead to a cure but this approach is not available to most patients. The mainstay treatment for the majority remains life-long blood transfusion in combination with a rigorous regime of iron chelation. Improved understanding of the pathophysiology and molecular basis of the disease has provided clues for more effective strategies that aim to correct the defect in beta-globin chain synthesis at the primary level or redress the alpha/beta-globin chain imbalance at the secondary level. Improved understanding of the molecular basis of the disease complications, such as iron overloading, has also provided clues for potential molecular targets at the tertiary level.

Genetic and pharmacological analyses of involvement of Src-family, Syk and Btk tyrosine kinases in platelet shape change. Src-kinases mediate integrin alphaIIb beta3 inside-out signalling during shape change

Platelet shape change was found to be associated with an increase in protein tyrosine phosphorylation upon stimulation of thrombin-, ADP- and thromboxane A2-G-protein coupled receptors in human platelets and thromboxane A2 receptors in mouse platelets. By using PP1 and PD173956, two structurally unrelated specific inhibitors of Src-family tyrosine kinases, and mouse platelets deficient in the Src-kinase Fyn or Lyn, we show that Src-family kinases cause the increase in protein tyrosine phosphorylation. We further detected that the non-Src tyrosine kinase Syk was activated during shape change in a manner dependent on Src-family kinaseactivation. The pharmacological experiments and the studies on Fyn-, Lyn- and Syk-deficient mouse platelets showed that neither Src-family kinases nor Syk are functionally involved in shape change. Also human platelets deficient of the tyrosine kinase Btk showed a normal shape change. Binding of PAC-1 that recognizes activated integrin alphaIIb beta3 complexes on the platelet surface was enhanced during shape change and blocked by inhibition of Src-kinases. We conclude that the activation of Src-kinases and the subsequent Syk stimulation upon activation of G-protein coupled receptors are not involved in the cytoskeletal changes underlying shape change of human and mouse platelets, but that the stimulation of this evolutionary conserved pathway leads to integrin alphaIIb beta3 exposure during shape change.

Evidence of a role for SHP-1 in platelet activation by the collagen receptor glycoprotein VI

The Src homology (SH)2 domain-containing protein-tyrosine phosphatase SHP-1 is tyrosine phosphorylated in platelets in response to the glycoprotein VI (GPVI)-selective agonist collagen-related peptide (CRP), collagen, and thrombin. Two major unidentified tyrosine-phosphorylated bands of 28 and 32 kDa and a minor band of 130 kDa coprecipitate with SHP-1 in response to all three agonists. Additionally, tyrosine-phosphorylated proteins of 50-55 and 70 kDa specifically associate with SHP-1 following stimulation by CRP and collagen. The tyrosine kinases Lyn, which exists as a 53 and 56-kDa doublet, and Syk were identified as major components of these bands, respectively. Kinase assays on SHP-1 immunoprecipitates performed in the presence of the Src family kinase inhibitor PP1 confirmed the presence of a Src kinase in CRP- but not thrombin-stimulated cells. Lyn, Syk, and SLP-76, along with tyrosine-phosphorylated 28-, 32-, and 130-kDa proteins, bound selectively to a glutathione S-transferase protein encoding the SH2 domains of SHP-1, suggesting that this is the major site of interaction. Platelets isolated from motheaten viable mice (mev/mev) revealed the presence of a heavily tyrosine-phosphorylated 26-kDa protein that was not found in wild-type platelets. CRP-stimulated mev/mev platelets manifested hypophosphorylation of Syk and Lyn and reduced P-selectin expression relative to controls. These observations provide evidence of a functional role for SHP-1 in platelet activation by GPVI.

Phosphatidylinositol 3,4,5-trisphosphate regulates Ca(2+) entry via btk in platelets and megakaryocytes without increasing phospholipase C activity

The role of phosphatidylinositol 3,4,5-trisphosphate (PI3,4,5P(3)) and Btk in signalling by the collagen receptor glycoprotein VI was investigated. PI3,4,5P(3) was increased in platelets from mice deficient in the SH2 domain-containing inositol 5-phosphatase (SHIP), in response to collagen related peptide (CRP). Tyrosine phosphorylation and activation of phospholipase Cgamma2 (PLCgamma2) were unaltered in SHIP(-/-) platelets, whereas Btk was heavily tyrosine phosphorylated under basal conditions and maximally phosphorylated by low concentrations of CRP. There was an increase in basal Ca(2+), maximal expression of P-selectin, and potentiation of Ca(2+) and aminophospholipid exposure to CRP in SHIP(-/-) platelets in the presence of Ca(2+) (1 mM). Microinjection of PI3,4, 5P(3) into megakaryocytes caused a 3-fold increase in Ca(2+) in response to CRP, which was absent in X-linked immunodeficiency (Xid) mice, which have a mutation in the PH domain of Btk. There was a corresponding partial reduction in the sustained level of intracellular Ca(2+) in response to CRP in Xid mice but no change in PLC activity. These results demonstrate a novel pathway of Ca(2+) entry that involves PI3,4,5P(3) and Btk, and which is independent of increased PLC activity.

Thrombopoietin potentiates collagen receptor signaling in platelets through a phosphatidylinositol 3-kinase-dependent pathway

Collagen activates platelets through a tyrosine kinase-dependent pathway, involving phospholipase Cgamma2. Functional responses such as aggregation and secretion induced by collagen are potentiated by preincubation with thrombopoietin (TPO). In this study, we show that collagen and thrombopoietin activate the phosphatidylinositol 3-kinase (PI 3-kinase) pathway and that this contributes to their respective actions. The structurally distinct inhibitors of PI 3-kinase, wortmannin, and LY294002, completely inhibit formation of phosphatidylinositol 3,4,5-trisphosphate by collagen. This leads to a substantial reduction in the formation of inositol phosphates and phosphatidic acid, 2 indices of PLC activity, and the consequent inhibition of intracellular Ca(++) [Ca(++)](i), aggregation and secretion. Potentiation of the collagen response by TPO is prevented in the presence of wortmannin and LY294002. However, when the 2 PI 3-kinase inhibitors are given after the addition of TPO but before the collagen, recovery of potentiation is observed. This suggests that potentiation is mediated through activation of PI 3-kinase. TPO stimulates aggregation of platelets from a low percentage of donors and this is also blocked by wortmannin. These results suggest that the PI 3-kinase pathway plays an important role in signaling by collagen and in the priming action of TPO.

Regulation and function of WASp in platelets by the collagen receptor, glycoprotein VI

Wiskott Aldrich syndrome (WAS) is an X-linked recessive disorder associated with abnormalities in platelets and lymphocytes giving rise to thrombocytopenia and immunodeficiency. WAS is caused by a mutation in the gene encoding the cytoskeletal protein (WASp). Despite its importance, the role of WASp in platelet function is not established. WASp was recently shown to undergo tyrosine phosphorylation in platelets after activation by collagen, suggesting that it may play a selective role in activation by the adhesion molecule. In the present study, we show that WASp is heavily tyrosine phosphorylated by a collagen-related peptide (CRP) that binds to the collagen receptor glycoprotein (GP) VI, but not to the integrin alpha2beta1. Tyrosine phosphorylation of WASp was blocked by Src family kinase inhibitors and reduced by treatment with wortmannin and in patients with X-linked agammaglobulinemia (XLA), a condition caused by a lack of functional expression of Btk. This indicates that Src kinases, phosphatidylinositol 3-kinase (PI 3-kinase), and Btk all contribute to the regulation of tyrosine phosphorylation of WASp. The functional importance of WASp was investigated in 2 WAS brothers who show no detectable expression of WASp. Platelet aggregation and secretion from dense granules induced by CRP and thrombin was slightly enhanced in the WAS platelets relative to controls. Furthermore, there was no apparent difference in morphology in WAS platelets after stimulation by these agonists. These observations suggest that WASp does not play a critical role in intracellular signaling downstream of tyrosine kinase-linked and G protein-coupled receptors in platelets.

LAT is required for tyrosine phosphorylation of phospholipase cgamma2 and platelet activation by the collagen receptor GPVI

In the present study, we have addressed the role of the linker for activation of T cells (LAT) in the regulation of phospholipase Cgamma2 (PLCgamma2) by the platelet collagen receptor glycoprotein VI (GPVI). LAT is tyrosine phosphorylated in human platelets heavily in response to collagen, collagen-related peptide (CRP), and FcgammaRIIA cross-linking but only weakly in response to the G-protein-receptor-coupled agonist thrombin. LAT tyrosine phosphorylation is abolished in CRP-stimulated Syk-deficient mouse platelets, whereas it is not altered in SLP-76-deficient mice or Btk-deficient X-linked agammaglobulinemia (XLA) human platelets. Using mice engineered to lack the adapter LAT, we showed that tyrosine phosphorylation of Syk and Btk in response to CRP was maintained in LAT-deficient platelets whereas phosphorylation of SLP-76 was slightly impaired. In contrast, tyrosine phosphorylation of PLCgamma2 was substantially reduced in LAT-deficient platelets but was not completely inhibited. The reduction in phosphorylation of PLCgamma2 was associated with marked inhibition of formation of phosphatidic acid, a metabolite of 1,2-diacylglycerol, phosphorylation of pleckstrin, a substrate of protein kinase C, and expression of P-selectin in response to CRP, whereas these parameters were not altered in response to thrombin. Activation of the fibrinogen receptor integrin alpha(IIb)beta(3) in response to CRP was also reduced in LAT-deficient platelets but was not completely inhibited. These results demonstrate that LAT tyrosine phosphorylation occurs downstream of Syk and is independent of the adapter SLP-76, and they establish a major role for LAT in the phosphorylation and activation of PLCgamma2, leading to downstream responses such as alpha-granule secretion and activation of integrin alpha(IIb)beta(3). The results further demonstrate that the major pathway of tyrosine phosphorylation of SLP-76 is independent of LAT and that there is a minor, LAT-independent pathway of tyrosine phosphorylation of PLCgamma2. We propose a model in which LAT and SLP-76 are required for PLCgamma2 phosphorylation but are regulated through independent pathways downstream of Syk.

A novel inhibitory action of wheat germ agglutinin on phospholipase C in HEL and MEG-01 cell lines

Stimulation of HEL megakaryocytic cells by Fc gammaRIIA crosslinking is associated with tyrosine phosphorylation of syk and phospholipase C gamma2 (PLCgamma2) and is accompanied by formation of inositol phosphates and release of intracellular Ca2+. These responses are inhibited by the kinase inhibitors, staurosporine and ST271. In contrast, the G-protein receptor agonist, thrombin induces formation of inositol phosphates and release of intracellular calcium without an increase in tyrosine phosphorylation. The plant lectin wheat germ agglutinin (WGA) stimulates tyrosine phosphorylation of syk and PLCgamma2 but surprisingly does not stimulate formation of inositol phosphates and induce release of intracellular Ca2+. WGA also inhibited formation of inositol phosphates and release of intracellular Ca2+ by Fc gammaRIIA crosslinking and thrombin-stimulation. A similar inhibitory effect of WGA was observed against elevation of Ca2+ by the same two stimuli in MEG-01 megakaryotic cells. The results demonstrate a novel pathway of inhibition of PLC on crosslinking of cell surface proteins that is not present in platelets.