The insulin receptor substrate (IRS) proteins serve as essential signaling intermediates for the activation of PI3K by both the insulin-like growth factor 1 receptor (IGF-1R) and its close family member, the insulin receptor (IR). Although IRS-1 and IRS-2 share significant homology, they regulate distinct cellular responses downstream of these receptors and play divergent roles in breast cancer. To investigate the mechanism by which signaling through IRS-1 and IRS-2 results in differential outcomes, we assessed the involvement of the microtubule cytoskeleton in IRS-dependent signaling. Treatment with drugs that either stabilize or disrupt microtubules reveal that an intact microtubule cytoskeleton contributes to IRS-2-but not IRS-1-mediated activation of AKT by IGF-1. Proximal IGF-1R signaling events, including IRS tyrosine phosphorylation and recruitment of PI3K, are not inhibited by microtubule disruption, indicating that IRS-2 requires the microtubule cytoskeleton at the level of downstream effector activation. IRS-2 colocalization with tubulin is enhanced upon Taxol-mediated microtubule stabilization, which, together with the signaling data, suggests that the microtubule cytoskeleton may facilitate access of IRS-2 to downstream effectors such as AKT. Of clinical relevance is that our data reveal that expression of IRS-2 sensitizes breast carcinoma cells to apoptosis in response to treatment with microtubule-disrupting drugs, identifying IRS-2 as a potential biomarker for the response of breast cancer patients to Vinca alkaloid drug treatment.Insulin receptor substrate 1 (IRS-1) 2 and IRS-2 are cytoplasmic adaptors for the insulin receptor (IR) and insulin-like growth factor 1 receptor (IGF-1R), and they play a major role in determining the cellular response to stimulation of these receptors (1). Notably, the IRS proteins are required for the activation of PI3K downstream of the IR and IGF-1R, which activate AKT and mechanistic target of rapamycin (mTOR) to promote proliferation, survival, motility, protein synthesis, and glucose metabolism (2-5). IRS-1 and IRS-2 are expressed ubiquitously in humans, including in the normal and malignant mammary epithelium (1). Despite their considerable sequence homology, IRS-1 and IRS-2 play divergent roles in breast cancer. In vitro, studies to assess IGF-1-dependent signaling through the IRS proteins in breast carcinoma cells have revealed that IRS-1 primarily regulates proliferation and survival, whereas IRS-2 regulates motility, invasion, and glycolysis (6 -10). In vivo, overexpression of either IRS-1 or IRS-2 in the mouse mammary gland promotes mammary tumorigenesis (11). However, metastasis is diminished in the absence of Irs-2 expression and increased in the absence of Irs-1 expression (9, 12).Differential localization patterns of IRS-1 and IRS-2 in human breast tumors suggest one explanation for their divergent functions in breast cancer (13). In normal breast tissue, ductal carcinoma in situ, and invasive breast tumors, IRS-1 is primarily localized in the nucleus a...