Frédéric Barabé

Comment on "Tumor Growth Need Not Be Driven by Rare Cancer Stem Cells"

Kelly et al. (Brevia, 20 July 2007, p. 337) questioned xenotransplant experiments supporting the cancer stem cell (CSC) hypothesis because they found a high frequency of leukemia-initiating cells (L-IC) in some transgenic mouse models. However, the CSC hypothesis depends on prospective purification of cells with tumor-initiating capacity, irrespective of frequency. Moreover, we found similar L-IC frequencies in genetically comparable leukemias using syngeneic or xenogeneic models.

Crystal-induced neutrophil activation VI. Involvement of FcγRIIIB (CD16) and CD11b in response to inflammatory microcrystals

The inflammatory reaction associated with the deposition of monosodium urate (MSU) crystals in synovial spaces is known to be due to interactions with polymorphonuclear neutrophils mediated by presently unidentified surface structures. In this study, we have observed that antibodies directed against CD16 (VIFcRIII) and CD11b (VIM12) selectively and potently inhibit the activation of neutrophils by MSU crystals. The responses affected include the stimulation of tyrosine phosphorylation, activation of the tyrosine kinase syk, tyrosine phosphorylation of the proto‐oncogene Cbl, mobilization of calcium, and stimulation of the activity of phospholipase D and of the production of superoxide anions. Tyrosine phosphorylation responses to MSU crystals develop during the Me2SO4‐induced differentiation of HL‐60 cells in parallel with the surface expression of CD16. These data strongly support the hypothesis that inflammatory microcrystals interact opportunistically with CD16 initially, and that the signal transduction pathways activated thereby depend on CD11b. An examination of the relevance of the hypothesis that an uncontrolled activation of CD16/CD11b may play a role in inflammatory reactions associated with a dysregulation of neutrophil function (other than crystal arthropathies) appears warranted on the basis of the present results.—Barabé, F., Gilbert, C., Liao, N., Bourgoin, S. G., Naccache, P. H. Crystal‐induced neutrophil activation: involvement of FcγRIIIB (CD16) in response to inflammatory microcrystals. FASEB J. 12, 209–220 (1998)

The transcriptomic landscape and directed chemical interrogation of MLL-rearranged acute myeloid leukemias.

Using next-generation sequencing of primary acute myeloid leukemia (AML) specimens, we identified to our knowledge the first unifying genetic network common to the two subgroups of KMT2A (MLL)-rearranged leukemia, namely having MLL fusions or partial tandem duplications. Within this network, we experimentally confirmed upregulation of the gene with the most subtype-specific increase in expression, LOC100289656, and identified cryptic MLL fusions, including a new MLL-ENAH fusion. We also identified a subset of MLL fusion specimens carrying mutations in SPI1 accompanied by inactivation of its transcriptional network, as well as frequent RAS pathway mutations, which sensitized the leukemias to synthetic lethal interactions between MEK and receptor tyrosine kinase inhibitors. This transcriptomics-based characterization and chemical interrogation of human MLL-rearranged AML was a valuable approach for identifying complementary features that define this disease.

DCIR-mediated enhancement of HIV-1 infection requires the ITIM-associated signal transduction pathway

Dendritic cell immunoreceptor (DCIR) is a C-type lectin receptor expressed at high levels on dendritic cells (DCs). This surface molecule acts as an attachment factor for HIV-1 on DCs and contributes to trans- and cis-infection pathways. Moreover, DICR is induced by HIV-1 in CD4(+) T cells and promotes virus replication in this cell type. Nothing is known hitherto about the DCIR-dependent signaling, which is induced following HIV-1 ligation. First, specific pharmacologic inhibitors were tested on HIV-1 binding/entry and, second, specific antisense oligonucleotides targeted, more specifically kinases and phosphatases, were used. Our results show that SHP-1, SHP-2, Syk, and Src kinases (ie, Src, Fyn, and Hck) as well as PKC-α and MAP kinases (ie, Erk1/2 and p38) are all involved in the DCIR-mediated signal transduction pathway triggered by HIV-1. By mutagenesis and through the use of intracellular phosphorylated peptides, we show as well a pivotal role for the tyrosine and threonine residues of the DCIR immunoreceptor tyrosine-based inhibitory motif (ITIM). Our data suggest for the first time an involvement of ITIM domain in HIV-1-mediated signaling events and a relationship between phosphorylation events and DCIR function with respect to HIV-1 biology.

Preservation of the pattern of tyrosine phosphorylation in human neutrophil lysates

Activation of various cell types by different agonists is known to stimulate a transient increase in the level of tyrosine phosphorylation of certain cellular proteins. Such phosphorylation is essential for mediating signalling by these agonists. The preservation of the tyrosine phosphorylation of proteins in lysates has proven to be a difficult task in neutrophils because of their large arsenal of proteases and phosphatases. Here we describe a technique that we found useful for preserving the tyrosine phosphorylation of cellular proteins. The technique depends on the denaturing lysis of neutrophils followed by the removal of the denaturing agents using Sephadex columns. Preparing neutrophil lysates by this technique has proven to be reliable in terms of maintaining the stability of the tyrosine phosphorylated proteins of various molecular weights and their subsequent immunoprecipitation and identification.

Early Events in the Activation of FcγRIIA in Human Neutrophils: Stimulated Insolubilization, Translocation to Detergent-Resistant Domains, and Degradation of FcγRIIA

The signal transduction mechanisms associated with the ligation of FcγRIIA in human neutrophils are as yet only incompletely characterized. In the present study, we have investigated the distribution and fate of FcγRIIA following its cross-linking. The results obtained indicate that cross-linking of FcγRIIA led, within a few seconds, to its translocation into a nonionic detergent-insoluble fraction. This was followed, within a couple of minutes, by a substantial loss of immunoreactive FcγRIIA in the cells. The stimulated degradation of FcγRIIA was blocked by the Src kinase inhibitor PP1 but not by wortmannin, ST-638, piceatannol, or cytochalasin B. Cross-linked FcγRIIA could be solubilized by saponin (in the presence of Nonidet P-40) and by β-octylglucoside. Sucrose gradient analysis of the distribution of FcγRIIA revealed that its cross-linking led to its translocation into the pellets and not the light buoyant density fractions classically associated with lipid rafts. Disruption of cholesterol-containing membrane microdomains with filipin prevented the degradation of FcγRIIA but did not inhibit the stimulation of the pattern of tyrosine phosphorylation or the mobilization of calcium that followed FcγRIIA cross-linking. These data suggest that both cholesterol-rich domains and Src kinases are required for the degradation of the activated FcγRIIA and provide new insights into the early events following FcγRIIA cross-linking.

Investigating human leukemogenesis: from cell lines to in vivo models of human leukemia

The hematopoietic system produces appropriate levels of blood cells over an individuals lifetime through a careful balance of differentiation, proliferation and self-renewal. The acquisition of genetic and epigenetic alterations leads to deregulation of these processes and the development of acute leukemias. A prerequisite to targeted therapies directed against these malignancies is a thorough understanding of the processes that subvert the normal developmental program of the hematopoietic system. This involves identifying the molecular lesions responsible for malignant transformation, their mechanisms of action and the cell type(s) in which they occur. Over the last 3 decades, significant progress has been made through the identification of recurrent genetic alterations and translocations in leukemic blast populations, and their subsequent functional characterization in cell lines and/or mouse models. Recently, primary human hematopoietic cells have emerged as a complementary means to characterize leukemic oncogenes. This approach enables the process of leukemogenesis to be precisely modeled in the appropriate cellular context: from primary human hematopoietic cells to leukemic stem cells capable of initiating disease in vivo. Here we review the model systems used to study leukemogenesis, and focus particularly on recent advances provided by in vitro and in vivo studies with primary human hematopoietic cells.

GPR56 identifies primary human acute myeloid leukemia cells with high repopulating potential in vivo

Acute myeloid leukemia (AML) is a genetically heterogeneous hematologic malignancy, which is initiated and driven by a rare fraction of leukemia stem cells (LSCs). Despite the difficulties of identifying a common LSC phenotype, there is increasing evidence that high expression of stem cell gene signatures is associated with poor clinical outcome. Identification of functionally distinct subpopulations in this disease is therefore crucial to dissecting the molecular machinery underlying LSC self-renewal. Here, we combined next-generation sequencing technology with in vivo assessment of LSC frequencies and identified the adhesion G protein-coupled receptor 56 (GPR56) as a novel and stable marker for human LSCs for the majority of AML samples. High GPR56 expression was significantly associated with high-risk genetic subgroups and poor outcome. Analysis of GPR56 in combination with CD34 expression revealed engraftment potential of GPR56(+)cells in both the CD34(-)and CD34(+)fractions, thus defining a novel LSC compartment independent of the CD34(+)CD38(-)LSC phenotype.

Evidence for a Role for SAM68 in the Responses of Human Neutrophils to Ligation of CD32 and to Monosodium Urate Crystals

SAM68 (Src-associated in mitosis 68 kDa) is a member of the signal transduction of activator RNA novel gene family coding for proteins postulated to be involved in signal transduction and activation of RNA. It has been implicated through its phosphorylation status in the control of the transition from the G1 to the S phases during mitosis. However, the implication and role of SAM68 in nonproliferative cells are unknown. The present study was initiated to examine the role of SAM68 in the phagocytic responses of the terminally differentiated human neutrophils. The results obtained show that SAM68 is present in human neutrophils and that it is tyrosine phosphorylated in response to stimulation by monosodium urate crystals or by ligation of CD32. Stimulation of neutrophils by these agonists decreases the association of SAM68 with Sepharose-conjugated poly-U beads. Additionally, the amount of immunoprecipitable SAM68 was modulated differentially after stimulation by monosodium urate crystals or by CD32 engagement indicating that the posttranslational modifications and/or protein associations of SAM68 induced by these two agonists differed. The results of this study provide evidence for an involvement of SAM68 in signal transduction by phagocytic agonists in human neutrophils and indicate that SAM68 may play a role in linking the early events of signal transduction to the posttranscriptional modulation of RNA.

UBAP2L is a novel BMI1-interacting protein essential for hematopoietic stem cell activity

Multipotent long-term repopulating hematopoietic stem cells (LT-HSCs) can self-renew or differentiate into the less primitive short-term repopulating stem cells (ST-HSCs), which themselves produce progenitors that ensure the daily supply of all essential blood components. The Polycomb group (PcG) protein BMI1 is essential for the activity of both HSCs and progenitor cells. Although BMI1 operates by suppressing the Ink4a/Arf locus in progenitors and ST-HSCs, the mechanisms through which this gene regulates the activity of LT-HSCs remain poorly understood. Toward this goal, we isolated BMI1-containing protein complexes and identified UBAP2L as a novel BMI1-interacting protein. We also showed that UBAP2L is preferentially expressed in mouse and human HSC-enriched populations when compared with more mature cell types, and that this gene is essential for the activity of LT-HSCs. In contrast to what is observed for Bmi1 knockdown, we found that UBAP2L depletion does not affect the Ink4a/Arf locus. Given that we demonstrated that BMI1 overexpression is able to rescue the deleterious effects of Ubap2l downregulation on LT-HSC activity and that UBAP2L is part of a PcG subcomplex comprising BMI1, we propose a model in which at least 2 different BMI1-containing PcG complexes regulate HSC activity, which are distinguishable by the presence of UBAP2L.