ABSTRACT
The acquisition of terminal cell fate and onset of differentiation are instructed by cell type-specific master control genes. Loss of differentiation is frequently observed during cancer progression, but the underlying causes and mechanisms remain poorly understood. We tested the hypothesis that master regulators of differentiation may be key regulators of tumor formation. Using loss- and gain-of-function analyses in Drosophila, we describe a critical anti-oncogenic function for the atonal transcription factor in the fly retina, where atonal instructs tissue differentiation. In the tumor context, atonal acts by regulating cell proliferation and death via the JNK stress response pathway. Combined with evidence that atonal's mammalian homolog, ATOH1, is a tumor suppressor gene, our data support a critical, evolutionarily conserved, function for ato in oncogenesis.
Subject(s)
Basic Helix-Loop-Helix Transcription Factors/genetics , Cell Transformation, Neoplastic/genetics , Drosophila , Eye Neoplasms/genetics , Gene Expression Regulation, Developmental , Gene Expression Regulation, Neoplastic , Nerve Tissue Proteins/genetics , Animals , Apoptosis/genetics , Basic Helix-Loop-Helix Transcription Factors/metabolism , Body Patterning/genetics , Cell Differentiation/genetics , Drosophila/embryology , Drosophila/genetics , Drosophila Proteins/genetics , Drosophila Proteins/metabolism , Eye Neoplasms/metabolism , Gene Silencing , Nerve Tissue Proteins/metabolism , Organisms, Genetically Modified , Retina/cytology , Retina/embryologyABSTRACT
Syndecans are heparan sulfate proteoglycans that modulate the activity of several growth factors and cell adhesion molecules. PDZ domains in the adaptor protein syntenin interact with syndecans and with the phosphoinositide PIP(2), which is involved in the regulation of the actin cytoskeleton and membrane trafficking. Here, we show that the syntenin PDZ domain-PIP(2) interaction controls Arf6-mediated syndecan recycling through endosomal compartments. FGF receptor accompanies syndecan along the syntenin-mediated recycling pathway, in a heparan sulfate- and FGF-dependent manner. Syndecans that cannot recycle via this pathway become trapped intracellularly and inhibit cell spreading. This syntenin-mediated syndecan recycling pathway may regulate the surface availability of a number of cell adhesion and signaling molecules.
Subject(s)
ADP-Ribosylation Factors/metabolism , Carrier Proteins/metabolism , Intracellular Signaling Peptides and Proteins/metabolism , Membrane Glycoproteins/metabolism , Membrane Proteins/metabolism , Phosphatidylinositol 4,5-Diphosphate/metabolism , Proteoglycans/metabolism , ADP-Ribosylation Factor 6 , Cell Adhesion/physiology , Cell Line, Tumor , Cell Membrane/metabolism , Cell Membrane/ultrastructure , Endosomes/metabolism , Endosomes/ultrastructure , Humans , Models, Biological , Phosphatidylinositol 4,5-Diphosphate/chemistry , Syndecan-2 , Syndecans , SynteninsABSTRACT
PDZ (Postsynaptic density protein, Disc large, Zona occludens) domains are protein-protein interaction modules that predominate in submembranous scaffolding proteins. Recently, we showed that the PDZ domains of syntenin-1 also interact with phosphatidylinositol 4,5-bisphosphate (PIP2) and that this interaction controls the recruitment of the protein to the plasma membrane. Here we evaluate the general importance of PIP2-PDZ domain interactions. We report that most PDZ proteins bind weakly to PIP2, but that syntenin-2, the closest homolog of syntenin-1, binds with high affinity to PIP2 via its PDZ domains. Surprisingly, these domains target syntenin-2 to nuclear PIP2 pools, in nuclear speckles and nucleoli. Targeting to these sites is abolished by treatments known to affect these PIP2 pools. Mutational and domain-swapping experiments indicate that high-affinity binding to PIP2 requires both PDZ domains of syntenin-2, but that its first PDZ domain contains the nuclear PIP2 targeting determinants. Depletion of syntenin-2 disrupts the nuclear speckles-PIP2 pattern and affects cell survival and cell division. These findings show that PIP2-PDZ domain interactions can directly contribute to subnuclear assembly processes.
Subject(s)
Cell Nucleolus/metabolism , Cell Nucleus/metabolism , Nerve Tissue Proteins/metabolism , Phosphatidylinositol 4,5-Diphosphate/metabolism , Carrier Proteins/metabolism , Cell Division/physiology , Cell Line, Tumor , Cell Membrane/metabolism , Cell Survival/physiology , Cells, Cultured , Humans , Intracellular Signaling Peptides and Proteins/metabolism , Lipid Bilayers/metabolism , Membrane Proteins/metabolism , Protein Interaction Mapping , Protein Structure, Tertiary , Syntenins , Time FactorsABSTRACT
PDZ proteins organize multiprotein signaling complexes. According to current views, PDZ domains engage in protein-protein interactions. Here we show that the PDZ domains of several proteins bind phosphatidylinositol 4,5-bisphosphate (PIP(2)). High-affinity binding of syntenin to PIP(2)-containing lipid layers requires both PDZ domains of this protein. Competition and mutagenesis experiments reveal that the protein and the PIP(2) binding sites in the PDZ domains overlap. Overlay assays indicate that the two PDZ domains of syntenin cooperate in binding to cognate peptides and PIP(2). Experiments on living cells demonstrate PIP(2)-dependent and peptide-dependent modes of plasma membrane association of the PDZ domains of syntenin. These observations suggest that local changes in phosphoinositide concentration control the association of PDZ proteins with their target receptors at the plasma membrane.