A Mechanistic Investigation of Insulin Receptor Substrate 2 Function in Breast Cancer Progression

Abstract

The advancement of cancer treatment depends on understanding the biological processes that contribute to disease progression. The spread of tumor cells from the primary site to distant organs is the biggest obstacle to efficacious treatment. The insulin receptor substrate (IRS) proteins IRS1 and IRS2 are cytoplasmic adaptor proteins that organize signaling events downstream of the Insulin receptor (IR) and the Insulin-like growth factor receptor 1 (IGF1R). Both of these receptors have been implicated in cancer progression. The IRS proteins share a significant level of homology and are both capable of recruiting and activating phosphatidylinositol-3 kinase (PI3K). Despite these similarities, signaling through IRS1 and IRS2 leads to distinct tumor cell outcomes in vitro and in vivo. In vitro, IRS1 regulates cell proliferation and growth and IRS2 regulates metabolism, survival and invasion. In vivo, Irs2 is a positive regulator of tumor metastasis, whereas Irs1 does not promote metastasis. The major objective of this thesis work was to further the understanding of the mechanism by which IRS2 signaling regulates tumor progression. To investigate how IRS-1 and IRS-2 regulate distinct tumor cell outcomes, I examined the involvement of the microtubule cytoskeleton in IRS-dependent signaling. I determined that IRS2-mediated AKT activation is dependent upon an intact microtubule cytoskeleton, whereas IRS1-mediated AKT signaling occurs independently of microtubules. As a result, drugs that disrupt microtubules promote apoptosis in cells that signal through IRS2, but cells that signal through IRS1 are resistant to the effects of microtubule disruption. However, AKT inhibition sensitizes IRS1-dependent cells to apoptotic cell death upon microtubule disruption. From a clinical perspective, my studies identify IRS2 as a potential biomarker for the response of breast cancer patients to anti-microtubule drug therapy. To investigate further the mechanism of IRS2 contributions to tumor progression, I employed a mutagenesis approach to identify structural requirements of IRS2 for its function. I established that the ability of IRS2 to activate PI3K is necessary for its regulation of both invasion and tumor initiating cell (TIC) self-renewal. I also identified two independent regions within the IRS2 C-terminus that are required for invasion and self-renewal, respectively. Characterization of the invasion-promoting region identified BMP2-induced protein kinase (BMP2K) as an interacting protein. Suppression of BMP2K expression in mammary tumor cells disrupts IRS2-mediated tumor cell invasion. Taken together, my work advances the understanding of how IRS2 contributes to breast cancer progression and provides a molecular understanding for the development of novel approaches for the treatment of breast cancer and other malignancies that rely upon IRS2.