Obesity is a major risk factor for chronic disease and new pathways for understanding and targeting obesity are needed. Recently, the identification of functional brown fat in adults has revived the idea that increased heat production in the mitochondria of brown fat could help prevent obesity. Furthermore, the distinction between white fat and brown fat may not be absolute. White fat depots can be induced to express substantial proportions of brown adipocytes. Both stimulation of heat production by existing brown fat, and "browning" of the white fat, are controlled by ?3-adrenergic stimulation. The ?-arrestins are major regulators of cellular ?-adrenergic signaling. In response to agonist, receptor signaling rapidly diminishes with time due to binding of the ?-arrestins. ?-arrestins bind to active receptor and cause desensitization by receptor ubiquitination and endosomal recycling. However, the beta-arrestins are part of a larger family of proteins that contain an arrestin-like structure. The other subfamily of arrestin-like proteins known as the "alpha arrestins". In humans, the alpha-arrestins include the best studied member, thioredoxin- interacting protein (TXNIP), and five other proteins with as yet unclear functions. TXNIP has important metabolic actions: it is a glucose sensor that inhibits glucose uptake but its mechanism of action is unclear. This ancient family of proteins may be fundamentally important for receptor signaling and metabolic functions. The Lee lab has identified one of these alpha-arrestins, ARRDC3, as a factor resulting resistance to obesity through increased energy expenditure when knocked down in mice. Very recent data from Nabhan et al. have revealed that ARRDC3, much like the ?-arrestins, binds to ?-adrenergic receptors and marks them for ubiquitination and recycling. Our most recent preliminary data further suggest that ARRDC3 regulates obesity specifically through its actions in fat since isolated white adipose tissue from ARRDC3-null mice has increased lipolysis and increased expression of Ucp1 relative to wild-type littermates. This proposal aims to define the mechanisms by which ARRDC3 regulates obesity. I will first investigating the role that ARRDC3 plays in regulating ?3-adrenergic receptor signaling. Specifically, I will use both overexpression and loss of expression and a mutagenesis study to examine the effect of ARRDC3 on cAMP and p38 MAPK signaling in response to ?3-specific agonist. I will then carry out an in vivo study to investigate if ARRDC3 is a novel regulator of adiposity and energy expenditure through ?-adrenergic activation of white adipose tissue. Specifically, I will use mice with tissue-specific gene deletion of Arrdc3 in white adipose tissue, and measure adiposity and energy expenditure in a diet-induced obesity model. The insights from the proposed research are critically important to the design of successful treatments and preventative measures against obesity.
Development of therapies to treat and prevent obesity are currently impeded by a lack of understanding of mechanisms involved in development of the disease. The proposed research uses rigorous approached to investigate these mechanisms and may lead to the design of successful obesity therapies.