ABSTRACT
Casitas b-lineage lymphoma (c-Cbl) is an E3 ubiquitin ligase that has an important role in regulating the degradation of cell surface receptors. In the present study we have examined the role of c-Cbl in whole-body energy homeostasis. c-Cbl-/- mice exhibited a profound increase in whole-body energy expenditure as determined by increased core temperature and whole-body oxygen consumption. As a consequence, these mice displayed a decrease in adiposity, primarily due to a reduction in cell size despite an increase in food intake. These changes were accompanied by a significant increase in activity (2- to 3-fold). In addition, c-Cbl-/- mice displayed a marked improvement in whole-body insulin action, primarily due to changes in muscle metabolism. We observed increased protein levels of the insulin receptor (4-fold) and uncoupling protein-3 (2-fold) in skeletal muscle and a significant increase in the phosphorylation of AMP-activated protein kinase and acetyl-CoA carboxylase. These findings suggest that c-Cbl plays an integral role in whole-body fuel homeostasis by regulating whole-body energy expenditure and insulin action.
Subject(s)
Adipose Tissue/metabolism , Energy Metabolism , Insulin/metabolism , Proto-Oncogene Proteins/genetics , Proto-Oncogene Proteins/physiology , Ubiquitin-Protein Ligases/genetics , Ubiquitin-Protein Ligases/physiology , AMP-Activated Protein Kinases , Acetyl-CoA Carboxylase/metabolism , Adipocytes/metabolism , Animals , Body Composition , Body Temperature , Body Weight , Carrier Proteins/metabolism , Cell Membrane/metabolism , Female , Glucose/metabolism , In Situ Hybridization , Ion Channels , Liver/metabolism , Male , Mice , Mice, Inbred C57BL , Mice, Transgenic , Microscopy, Electron , Mitochondria/metabolism , Mitochondrial Proteins , Multienzyme Complexes/metabolism , Muscle, Skeletal/metabolism , Muscles/metabolism , Oxygen Consumption , Phosphorylation , Protein Serine-Threonine Kinases/metabolism , Proto-Oncogene Proteins c-cbl , Receptor, Insulin/metabolism , Time Factors , Uncoupling Protein 3ABSTRACT
The unique tyrosine kinase binding (TKB) domain of Cbl targets phosphorylated tyrosines on activated protein tyrosine kinases (PTKs); this targeting is considered essential for Cbl proteins to negatively regulate PTKs. Here, a loss-of-function mutation (G304E) in the c-Cbl TKB domain, first identified in Caenorhabditis elegans, was introduced into a mouse and its effects in thymocytes and T cells were studied. In marked contrast to the c-Cbl knockout mouse, we found no evidence of enhanced activity of the ZAP-70 PTK in thymocytes from the TKB domain mutant mouse. This finding contradicts the accepted mechanism of c-Cbl-mediated negative regulation, which requires TKB domain targeting of phosphotyrosine 292 in ZAP-70. However, the TKB domain mutant mouse does show aspects of enhanced signaling that parallel those of the c-Cbl knockout mouse, but these involve the constitutive activation of Rac and not enhanced PTK activity. Furthermore, the enhanced signaling in CD4(+)CD8(+) double positive thymocytes appears to be compensated by the selective down-regulation of CD3 on mature thymocytes and peripheral T cells from both strains of mutant c-Cbl mice.
