Casitas b-lineage lymphoma (c-Cbl) is a multiadaptor protein with E3-ubiquitin ligase activity involved in regulating the degradation of receptor tyrosine kinases. We have recently reported that c-CblϪ/Ϫ mice exhibit a lean phenotype and enhanced peripheral insulin action likely due to elevated energy expenditure. In the study reported here, we examined the effect of a high-fat diet on energy homeostasis and glucose metabolism in these animals. When c-Cbl Ϫ/Ϫ mice were fed a high-fat diet for 4 weeks, they maintained hyperphagia, higher whole-body oxygen consumption (27%), and greater activity (threefold) compared with wild-type animals fed the same diet. In addition, the activity of several enzymes involved in mitochondrial fat oxidation and the phosphorylation of acetyl CoA carboxylase was significantly increased in muscle of high-fat-fed c-Cbl-deficient mice, indicating a greater capacity for fat oxidation in these animals. As a result of these differences, fat-fed c-Cbl Ϫ/Ϫ mice were 30% leaner than wild-type animals and were protected against high-fat diet-induced insulin resistance. These studies are consistent with a role for c-Cbl in regulating nutrient partitioning in skeletal muscle and emphasize the potential of c-Cbl as a therapeutic target in the treatment of obesity and type 2 diabetes. Diabetes 55:708 -715, 2006 T he incidence of obesity and type 2 diabetes is increasing throughout the world. This has been ascribed to changes in food intake combined with a more sedentary lifestyle. Regardless of the cause, this emerging health care problem has sparked renewed interest in the study of insulin action and fuel metabolism. In particular, the identification of genes that regulate energy homeostasis in mammals has become a major research interest. Over the past decade, largely through the use of genetically manipulated animal models, a number of genes that result in lean phenotypes have been described. These genes include those that regulate appetite, food absorption, and increased energy expenditure in either muscle or adipose tissue (1). A major advantage of manipulations that increase energy expenditure is that this depletes fat stores not only in adipose tissue but possibly in other cells that are susceptible to lipotoxic damage, thus providing a protective mechanism against the development of insulin resistance and diabetes (2,3).Genes that are known to regulate whole-body energy expenditure include mitochondrial uncoupling proteins that divert energy stores into heat production (4,5) and lipid handling enzymes, such as acetyl CoA carboxylase (ACC), which regulates the entry of long-chain acyl CoAs into mitochondria (6), and DGAT, a key enzyme in triacylglyceride synthesis (7). Unexpectedly, reduced expression of several molecules that negatively regulate insulin signaling, like the tyrosine phosphatase PTP1b (8), have also been shown to cause a significant increase in whole-body energy expenditure. This provides further evidence for an intimate link between insulin action and energy homeostasis.We...