Title: Role of ?-catenin and Wnt signaling in regulating lipid homeostasis Project Summary: Wnt signaling, normally limited to embryogenesis, stem cell renewal and wound healing, is inappropriately re- employed in a variety of human cancers, such as hepatocellular carcinoma and colorectal cancer, as well as other diseases. Aberrant Wnt signaling and altered lipid metabolism are both signs of oncogenesis, and recent data suggest that Wnt control of adipogenesis and lipid metabolism may occur through separate mechanisms. Currently, the mechanisms remain poorly understood, and so remain outside of our ability to monitor, mitigate, prevent, or correct. It has been impossible to clearly delineate separate functions of Wnt in adipogenesis, lipid anabolism, and lipid catabolism, because these processes are inextricably interconnected in mammals. To circumvent this limitation, we use Drosophila as a primary experimental system, which provides unparalleled sophistication in manipulating Wnt (Wingless in Drosophila) activity in vivo. More importantly, the unique temporal separation of adipogenesis, lipogenesis, lipolysis, and fatty acid ?-oxidation during the Drosophila life cycle allows us to precisely monitor and manipulate these fundamental processes. Our genetic analyses of Axin and ?-catenin, two components of the Wnt signaling pathway, have revealed that Wnt signaling regulates lipid homeostasis during the late larval stage, separately from adipogenesis completed during embryogenesis. We have confirmed that the phenotypes of Axin mutants are caused by a gain of the canonical Wnt activity, elevated expression of ?-catenin target genes, and altered expression of genes encoding enzymes involved in lipid catabolism. By screening a library of diverse FDA-approved drugs, we discovered that both the defective lipid homeostasis and the hyperactive Wnt signaling are potently suppressed by peptide boronic acids, a class of proteasome inhibitors. The suppressive effects of these inhibitors are dependent on ?-catenin. Despite the important role of ?-catenin in Wnt signaling, the precise mechanisms that normally regulate the stability of ?- catenin remain unclear. Thus the objective of this proposal is to determine how ?-catenin stability in particular, and Wnt signaling in general, regulates lipid catabolism. We will identify the molecular and cellular mechanisms that control the stability of ?-catenin in Drosophila by analyzing fat deposition and lipid accumulation. Our investigations will define the molecular mechanism(s) that control the stability of ?-catenin and reveal how Wnt signaling regulates lipid mobilization and lipid catabolism, thereby advancing our understanding of the tumor suppressive effects of ?-catenin and how Wnt signaling regulates lipid homeostasis.
Wnt cell signaling is associated with obesity, diabetes, and multiple types of cancer, potentially explaining the association between tumorigenesis and aberrant lipid metabolism. Wnt cell signaling is modulated by ?-catenin, a known tumor suppressor. To effectively treat these cancers, it is crucial to understand how ?-catenin and Wnt cell signaling regulate lipid homeostasis. This project will address these two critical gaps in our knowledge by uniquely leveraging both the powerful model organism Drosophila and cultured mammalian cells as experimental systems.
