Elevated levels of PtdIns5P in NPM-ALK transformed cells: implication of PIKfyve.

Phosphatidylinositol 5-monophosphate (PtdIns5P), one of the latest phosphoinositides discovered, has been suggested to play important cellular functions. Here, we report the presence of higher levels of this lipid in cells expressing the oncogenic tyrosine kinase nucleophosmin anaplastic lymphoma kinase (NPM-ALK), a chimeric protein found in the large majority of anaplastic large cell lymphomas (ALCLs). In addition, we describe that a pool of PtdIns5P is located in the membrane extensions characteristic of NPM-ALK-transformed cells. Finally, we show that the increase of PtdIns5P is controlled by the kinase PIKfyve, which is known for its role in vesicular trafficking. These data suggest for the first time a role of PtdIns5P and PIKfyve in oncogenesis, potentially linking intracellular trafficking to cancer.

A crosstalk between the Wnt and the adhesion-dependent signaling pathways governs the chemosensitivity of acute myeloid leukemia.

Relapses following chemotherapy are a major hindrance to patients' survival in acute myeloid leukemia (AML). To investigate the role of the hematopoietic niche in the chemoresistance of leukemic cells, we examined two pathways: one mediated by adhesion molecules/integrins, and the other by soluble factors of the morphogen Wnt pathway. In our study, both the adhesion of leukemic blasts to fibronectin and the addition of Wnt antagonists induced, independently, resistance of AML cells to daunorubicin in a cell survival assay. Using pharmacological inhibitors and siRNA, we showed that both resistance pathways required the activity of the glycogen synthase kinase 3beta (GSK3beta). Moreover, the AML cell protection downstream of GSK3beta was mediated by NF-kappaB. A link between the adhesion and the Wnt pathway was found, as adhesion of U937 on human osteoblasts, a component of the hematopoietic niche, triggered the secretion of the Wnt antagonist sFRP-1 and supported resistance to daunorubicin. The osteoblast-conditioned medium could also confer chemoresistance to U937 cells cultured in suspension, and this cell protective effect was abrogated after depletion of sFRP-1. In the context of this potential double in vivo resistance, modulators of the common signal GSK3beta and of its target NF-kappaB could represent important novel therapeutic tools.

Expression of beta-catenin by acute myeloid leukemia cells predicts enhanced clonogenic capacities and poor prognosis.

Activation of the Wnt/beta-catenin pathway has recently been shown to be crucial to the establishment of leukemic stem cells in chronic myeloid leukemia. We sought to determine whether beta-catenin was correlated to clonogenic capacity also in the acute myeloid leukemia (AML) setting. Eighty-two patients were retrospectively evaluated for beta-catenin expression by Western blot. beta-Catenin was expressed (although at various protein levels) in 61% of patients, and was undetectable in the remaining cases. In our cohort, beta-catenin expression was correlated with the clonogenic proliferation of AML-colony forming cells (AML-CFC or CFU-L) in methylcellulose in the presence of 5637-conditioned medium, and more strikingly with self-renewing of leukemic cells, as assessed in vitro by a re-plating assay. In survival analyses, beta-catenin appeared as a new independent prognostic factor predicting poor event-free survival and shortened overall survival (both with P<0.05). Furthermore, variations in beta-catenin protein levels were dependent on post-transcriptional mechanisms involving the Wnt/beta-catenin pathway only in leukemic cells. Indeed, beta-catenin negative leukemic cells were found to increase beta-catenin in response to Wnt3a agonist in contrast to normal counterparts. Altogether, our data pave the way to the evaluation of Wnt pathway inhibition as a new rationale for eradicating the clonogenic pool of AML cells.

Mass Assays to Quantify Bioactive PtdIns3P and PtdIns5P During Autophagic Responses.

Autophagy is a cellular process whereby cytoplasmic substrates are targeted for degradation in the lysosome via the membrane structures autophagosomes. This process is initiated by specific phosphoinositides, PtdIns3P and PtdIns5P, which play a key role in autophagy by recruiting effectors such as Atg18/WIPI2. Therefore, quantifying those lipids is important to better understand the assembly of the complex autophagic machinery. Herein, we describe in detail methods to quantify PtdIns3P and PtdIns5P by specific mass assays feasible in most laboratories.

Cdc42-dependent F-actin dynamics drive structuration of the demarcation membrane system in megakaryocytes.

UNLABELLED: Essentials Information about the formation of the demarcation membrane system (DMS) is still lacking. We investigated the role of the cytoskeleton in DMS structuration in megakaryocytes. Cdc42/Pak-dependent F-actin remodeling regulates DMS organization for proper megakaryopoiesis. These data highlight the mandatory role of F-actin in platelet biogenesis. SUMMARY: Background Blood platelet biogenesis results from the maturation of megakaryocytes (MKs), which involves the development of a complex demarcation membrane system (DMS). Therefore, MK differentiation is an attractive model for studying membrane remodeling. Objectives We sought to investigate the mechanism of DMS structuration in relationship to the cytoskeleton. Results Using three-dimensional (3D) confocal imaging, we have identified consecutive stages of DMS organization that rely on F-actin dynamics to polarize membranes and nuclei territories. Interestingly, microtubules are not involved in this process. We found that the mechanism underlying F-actin-dependent DMS formation required the activation of the guanosine triphosphate hydrolase Cdc42 and its p21-activated kinase effectors (Pak1/2/3). Förster resonance energy transfer demonstrated that active Cdc42 was associated with endomembrane dynamics throughout terminal maturation. Inhibition of Cdc42 or Pak1/2/3 severely destructured the DMS and blocked proplatelet formation. Even though this process does not require containment within the hematopoietic niche, because DMS structuration was observed upon thrombopoietin-treatment in suspension, integrin outside-in signaling was required for Pak activation and probably resulted from secretion of extracellular matrix by MKs. Conclusions These data indicate a functional link, mandatory for MK differentiation, between actin dynamics, regulated by Cdc42/Pak1/2/3, and DMS maturation.