ABSTRACT
The phosphatidylinositol (PI)-3 kinase (PI3K) pathway plays a central role in regulating many biological processes via the generation of the key second messenger PI-3,4,5-trisphosphate (PI-3,4,5-P3). This membrane-associated phospholipid, which is rapidly, albeit transiently, synthesized from PI-4,5-P2 by PI3K in response to a diverse array of extracellular stimuli, attracts pleckstrin homology (PH) domain-containing proteins to membranes to mediate its many effects. To ensure that the activation of this pathway is appropriately suppressed/terminated, the ubiquitously expressed tumor suppressor PTEN hydrolyzes PI-3,4,5-P3 back to PI-4,5-P2 while the 145-kDa hemopoietic-restricted SH2-containing inositol 5'- phosphatase, SHIP (also known as SHIP1), the 104-kDa stem cell-restricted SHIP (sSHIP) and the more widely expressed 150-kDa SHIP2 hydrolyze PI-3,4,5-P3 to PI-3,4-P2. In this review we will concentrate on the properties of the three SHIPs, with special emphasis being placed on the role that SHIP plays in cytokine-induced signaling.
Subject(s)
Cytokines/physiology , Phosphoric Monoester Hydrolases/physiology , Animals , Humans , Mice , Mice, Knockout , Models, Biological , Phenotype , Phosphatidylinositol 3-Kinases/metabolism , Phosphatidylinositol Phosphates/metabolism , Phosphatidylinositol-3,4,5-Trisphosphate 5-Phosphatases , Phosphoric Monoester Hydrolases/chemistry , Phosphoric Monoester Hydrolases/deficiency , Phosphoric Monoester Hydrolases/genetics , Signal Transduction , src Homology DomainsABSTRACT
The transcription factor nuclear factor kappa B (NF-kappaB) has been implicated in the regulation of genes mainly involved in inflammation and immune response. We analysed the role of NF-kappaB in signalling pathways induced by the hematopoietic growth factor erythropoietin (EPO). Our data, obtained by electrophoretic mobility shift assays (EMSA) and reporter gene assays, show that the intracellular domain of the EPO receptor (EPOR) transmits signals leading to the activation of NF-kappaB. Studies employing an inhibitor specific for the EPOR-associated tyrosine kinase JAK2 suggest that JAK2-dependent pathways are not involved. The induction of an NF-kappaB-triggered reporter gene construct was inhibited by cotransfection of dominant negative forms of the src kinase Lyn, but not by dominant negative JAK2. Using epidermal growth factor (EGF)/EPOR hybrids containing mutant forms of the EPOR intracellular domain, we were able to further define the critical structures for the induction of NF-kappaB. The data show that although the activity of JAK2 seems to be dispensable, its association to the receptor, as well as the phosphorylation of membrane proximal tyrosine residues, are essential. Furthermore, the functional analysis of different receptor forms revealed a correlation of the abilities to induce NF-kappaB activity and to generate antiapoptotic signals.
Subject(s)
NF-kappa B/metabolism , Proto-Oncogene Proteins , Receptors, Erythropoietin/chemistry , Receptors, Erythropoietin/physiology , Signal Transduction , Animals , Apoptosis , Cell Line , Enhancer Elements, Genetic , Enzyme Inhibitors/pharmacology , Erythropoietin/pharmacology , Genes, Reporter , Janus Kinase 2 , Mice , Mutation , Phosphotyrosine/physiology , Protein Structure, Tertiary , Protein-Tyrosine Kinases/antagonists & inhibitors , Protein-Tyrosine Kinases/genetics , Receptors, Erythropoietin/genetics , Tyrphostins/pharmacology , src-Family Kinases/geneticsABSTRACT
The ligand-dependent activation of the JAK/STAT (Januskinase/Signal Transducer and Activator of Transcription) pathway has been implicated in the explanation of cytokine-specific regulation of gene expression. Previous studies have reported conflicting results on the role of the transcription factor STAT5 in erythropoietin (EPO)-induced cellular responses. In this study we focused on the functional importance of STAT5 docking sites in the intracellular EPO receptor (EPOR) domain for the mediation of antiapoptotic activities. We demonstrate that EPO-dependent survival of erythroleukemic cell lines is accompanied by sustained STAT5 DNA-binding activity. The role of single tyrosine residues was dissected by the analysis of myeloid FDCP-1 cells stably expressing mutant EPOR proteins. The data show that receptors having a high potential to mediate antiapoptotic signals also effectively activate STAT5, whereas receptors lacking STAT5 docking sites are diminished in both activities. We conclude that the transcription factor STAT5 is functionally implicated in the EPO-dependent survival of cells.
