RESUMO
The t(10;11) chromosomal translocation gives rise to the CALM-AF10 fusion gene and is found in patients with aggressive and difficult-to-treat hematopoietic malignancies. CALM-AF10-driven leukemias are characterized by HOXA gene up-regulation and a global reduction in H3K79 methylation. DOT1L, the H3K79 methyltransferase, interacts with the octapeptide/leucine zipper domain of AF10, and this region has been shown to be necessary and sufficient for CALM-AF10-mediated transformation. However, the precise role of CALM in leukemogenesis remains unclear. Here, we show that CALM contains a nuclear export signal (NES) that mediates cytoplasmic localization of CALM-AF10 and is necessary for CALM-AF10-dependent transformation. Fusions of the CALM NES (NES(CALM)-AF10) or NES motifs from heterologous proteins (ABL1, Rev, PKIA, APC) in-frame with AF10 are sufficient to immortalize murine hematopoietic progenitors in vitro. The CALM NES is essential for CALM-AF10-dependent Hoxa gene up-regulation and aberrant H3K79 methylation, possibly by mislocalization of DOT1L. Finally, we observed that CALM-AF10 leukemia cells are selectively sensitive to inhibition of nuclear export by Leptomycin B. These findings uncover a novel mechanism of leukemogenesis mediated by the nuclear export pathway and support further investigation of the utility of nuclear export inhibitors as therapeutic agents for patients with CALM-AF10 leukemias.
Assuntos
Transporte Ativo do Núcleo Celular/fisiologia , Transformação Celular Neoplásica/patologia , Regulação da Expressão Gênica , Leucemia Experimental/etiologia , Proteínas Monoméricas de Montagem de Clatrina/fisiologia , Sinais de Exportação Nuclear/genética , Proteínas de Fusão Oncogênica/metabolismo , Sequência de Aminoácidos , Animais , Antibióticos Antineoplásicos/farmacologia , Transplante de Medula Óssea , Células Cultivadas , Ácidos Graxos Insaturados/farmacologia , Citometria de Fluxo , Células-Tronco Hematopoéticas/citologia , Células-Tronco Hematopoéticas/metabolismo , Proteínas de Homeodomínio/metabolismo , Humanos , Leucemia Experimental/metabolismo , Leucemia Experimental/patologia , Camundongos , Camundongos Endogâmicos C57BL , Dados de Sequência Molecular , Proteínas de Fusão Oncogênica/genética , Transporte Proteico , Homologia de Sequência de Aminoácidos , Taxa de SobrevidaRESUMO
The ubiquitously expressed phosphatidylinositol binding clathrin assembly (PICALM) protein associates with the plasma membrane, binds clathrin, and plays a role in clathrin-mediated endocytosis. Alterations of the human PICALM gene are present in aggressive hematopoietic malignancies, and genome-wide association studies have recently linked the PICALM locus to late-onset Alzheimer's disease. Inactivating and hypomorphic Picalm mutations in mice cause different degrees of severity of anemia, abnormal iron metabolism, growth retardation and shortened lifespan. To understand PICALM's function, we studied the consequences of PICALM overexpression and characterized PICALM-deficient cells derived from mutant fit1 mice. Our results identify a role for PICALM in transferrin receptor (TfR) internalization and demonstrate that the C-terminal PICALM residues are critical for its association with clathrin and for the inhibitory effect of PICALM overexpression on TfR internalization. Murine embryonic fibroblasts (MEFs) that are deficient in PICALM display several characteristics of iron deficiency (increased surface TfR expression, decreased intracellular iron levels, and reduced cellular proliferation), all of which are rescued by retroviral PICALM expression. The proliferation defect of cells that lack PICALM results, at least in part, from insufficient iron uptake, since it can be corrected by iron supplementation. Moreover, PICALM-deficient cells are particularly sensitive to iron chelation. Taken together, these data reveal that PICALM plays a critical role in iron homeostasis, and offer new perspectives into the pathogenesis of PICALM-associated diseases.
Assuntos
Homeostase , Ferro/metabolismo , Proteínas Monoméricas de Montagem de Clatrina/metabolismo , Aminoácidos/metabolismo , Animais , Membrana Celular/efeitos dos fármacos , Membrana Celular/metabolismo , Proliferação de Células/efeitos dos fármacos , Clatrina/metabolismo , Embrião de Mamíferos/citologia , Endocitose/efeitos dos fármacos , Fibroblastos/citologia , Fibroblastos/efeitos dos fármacos , Fibroblastos/metabolismo , Regulação da Expressão Gênica/efeitos dos fármacos , Células HEK293 , Homeostase/efeitos dos fármacos , Humanos , Espaço Intracelular/efeitos dos fármacos , Espaço Intracelular/metabolismo , Quelantes de Ferro/farmacologia , Deficiências de Ferro , Camundongos , Proteínas Monoméricas de Montagem de Clatrina/química , Proteínas Monoméricas de Montagem de Clatrina/deficiência , Fenótipo , Ligação Proteica/efeitos dos fármacos , Estrutura Terciária de Proteína , RNA Mensageiro/genética , RNA Mensageiro/metabolismo , Receptores da Transferrina/genética , Receptores da Transferrina/metabolismoRESUMO
beta-dystroglycan (DG) and the dystrophin-glycoprotein complex (DGC) are localized at costameres and neuromuscular junctions in the sarcolemma of skeletal muscle. We present evidence for an ankyrin-based mechanism for sarcolemmal localization of dystrophin and beta-DG. Dystrophin binds ankyrin-B and ankyrin-G, while beta-DG binds ankyrin-G. Dystrophin and beta-DG require ankyrin-G for retention at costameres but not delivery to the sarcolemma. Dystrophin and beta-DG remain intracellular in ankyrin-B-depleted muscle, where beta-DG accumulates in a juxta-TGN compartment. The neuromuscular junction requires ankyrin-B for localization of dystrophin/utrophin and beta-DG and for maintenance of its postnatal morphology. A Becker muscular dystrophy mutation reduces ankyrin binding and impairs sarcolemmal localization of dystrophin-Dp71. Ankyrin-B also binds to dynactin-4, a dynactin subunit. Dynactin-4 and a subset of microtubules disappear from sarcolemmal sites in ankyrin-B-depleted muscle. Ankyrin-B thus is an adaptor required for sarcolemmal localization of dystrophin, as well as dynactin-4.