ABSTRACT
Inhibitor of Apoptosis Proteins (IAPs) are frequently overexpressed in tumors and have become promising targets for developing anti-cancer drugs. IAPs can be inhibited by natural antagonists, but a physiological requirement of mammalian IAP antagonists remains to be established. Here we show that deletion of the mouse Sept4 gene, which encodes the IAP antagonist ARTS, promotes tumor development. Sept4-null mice have increased numbers of hematopoietic stem and progenitor cells, elevated XIAP protein, increased resistance to cell death, and accelerated tumor development in an Eµ-Myc background. These phenotypes are partially suppressed by inactivation of XIAP. Our results suggest that apoptosis plays an important role as a frontline defense against cancer by restricting the number of normal stem cells.
Subject(s)
Apoptosis , Cytoskeletal Proteins/metabolism , GTP-Binding Proteins/metabolism , Stem Cells/metabolism , Animals , Bone Marrow Cells/cytology , Bone Marrow Cells/metabolism , Cell Count , Cells, Cultured , Cytoskeletal Proteins/genetics , Female , Flow Cytometry , GTP-Binding Proteins/genetics , Hematologic Neoplasms/genetics , Hematologic Neoplasms/metabolism , Hematologic Neoplasms/pathology , Hematopoietic Stem Cells/cytology , Hematopoietic Stem Cells/metabolism , Humans , Immunoblotting , Kaplan-Meier Estimate , Lymphoid Tissue/metabolism , Lymphoid Tissue/pathology , Lymphoma/genetics , Lymphoma/metabolism , Lymphoma/pathology , Male , Mice , Mice, Inbred C57BL , Mice, Knockout , Septins , Stem Cells/cytology , Suppression, Genetic , X-Linked Inhibitor of Apoptosis Protein/genetics , X-Linked Inhibitor of Apoptosis Protein/metabolismABSTRACT
Inhibitor of Apoptosis Proteins (IAPs) can bind to and inhibit caspases, the key executioners of apoptosis. Because IAPs are frequently overexpressed in human tumors, they have become major pharmacological targets for developing new cancer therapeutics. However, the precise physiological function of individual mammalian IAPs and their role as E3 ubiquitin-ligases in situ remain largely obscure. Here, we investigated the function of XIAP ubiquitin-ligase activity by inactivating the RING motif via gene targeting in the mouse. Removing the RING stabilized XIAP in apoptotic thymocytes, demonstrating that XIAP ubiquitin-ligase activity is a major determinant of XIAP protein stability. Surprisingly, the increased amounts of "XIAP-BIR-only" protein did not lead to attenuated but rather increased caspase activity and apoptosis. DeltaRING embryonic stem cells and fibroblasts had elevated caspase-3 enzyme activity, and XIAP DeltaRING embryonic fibroblasts were strongly sensitized to TNF-alpha-induced apoptosis. Similar results were obtained with XIAP deficient mice. Furthermore, deletion of the RING also improved the survival of mice in the Emu-Myc lymphoma model. This demonstrates a physiological requirement of XIAP ubiquitin-ligase activity for the inhibition of caspases and for tumor suppression in vivo.