Mechanistic Analysis of Differential Signal Transduction Mediated by the Insulin Receptor Substrate Proteins IRS-1 and IRS-2: A Dissertation

Abstract

The Insulin Receptor Substrate (IRS) proteins IRS-1 and IRS-2 are cytoplasmic adaptor proteins that organize and propagate intracellular signaling downstream of specific growth factor receptors, including the Insulin and Insulin-Like Growth Factor-1 Receptors (IR and IGF-1R, respectively). Despite sharing a high level of homology and the ability to stimulate Phosphotidylinositol-3-Kinase (PI3K) and Mitogen-Activated Protein Kinase (MAPK) signaling, IRS-1 and IRS-2 play distinct roles in mammary tumor progression. Specifically, IRS-1 promotes growth and proliferation, whereas IRS- 2 promotes motility, invasion, survival, aerobic glycolyis, and metastasis. To further understand the differences between IRS-1 and IRS-2, I investigated the mechanistic basis of IRS-2-mediated PI3K activation. I identified tyrosines in IRS-2 that mediate its recruitment and activation of PI3K in response to insulin and IGF-1 stimulation. Using a PI3K-binding deficient IRS-2 mutant, I demonstrated that IRS-2-dependent PI3K signaling promotes aerobic glycolysis through its ability to selectively regulate the phosphorylation of the Akt effector Glycogen Synthase Kinase-3β (Gsk-3β). I also performed a rigorous comparison of IRS-1 and IRS-2 signal transduction and their ability to regulate functions associated with tumor progression. These studies required the generation of a novel model system where IRS-1 and IRS-2 function could be compared in a genetically identical background. Using this model, I confirmed a role for IRS-1 in growth regulation and IRS-2 in tumor cell invasion, as well as expanded the understanding of differential IRS protein function by showing that IRS-2 more vi effectively promotes Akt activation. The model system I have established can be used for further characterization of IRS-1 and IRS-2-specific functions.