Role of X-Linked Inhibitor of Apoptosis Protein in Therapeutic Resistance of Inflammatory Breast Cancer Cells

<p>Apoptotic dysregulation is a hallmark of cancer cells. The inability of cancer cells to undergo apoptosis may lead to therapeutic resistance. Inflammatory breast cancer (IBC) is a highly aggressive subtype of breast cancer that is often characterized by ErbB2 overexpression and ErbB2 activation. ErbB-targeting is clinically relevant using trastuzumab (anti-ErbB2 antibody) and lapatinib (small molecule ErbB1/2 inhibitor). However, acquired resistance is a common outcome even in IBC patients who show an initial clinical response, which limits the efficacy of these agents. Little is known about the molecular mechanisms of therapeutic resistance in IBC cells. We hypothesized that apoptotic dysregulation leads to therapeutic resistance of IBC cells to therapeutic agents, including ErbB-targeting agents. To determine whether apoptotic dysregulation and changes in anti-apoptotic proteins leads to resistance of IBC cells to therapeutic agents, we performed a variety of in vitro-based studies using agents that are used in the clinic to treat IBC patients. The sensitivity of both ErbB2 overexpressing and ErbB1 activated IBC cells to various therapeutic agents was evaluated using various cell death and apoptosis assays, and anti-apoptotic protein expression post-treatment was determined using western blot analysis. The overarching theme observed was that x-linked inhibitor of apoptosis protein (XIAP) expression inversely correlated with sensitivity of cells to therapeutic agents with various mechanisms of action, including TNF-related apoptosis inducing ligand (TRAIL), doxorubicin, cisplatin, paclitaxel, and two ErbB-targeting agents: trastuzumab and a lapatinib-analog (GW583340). Moreover, there was a specific and marked overexpression of XIAP in cells with de novo resistance to trastuzumab and with acquired resistance to GW583340. The observed overexpression was identified to be caused by IRES-mediated XIAP translation. Stable XIAP overexpression using a lentiviral system reversed sensitivity to therapeutic agents (TRAIL and GW583340) in parental IBC cells. Moreover, XIAP downregulation in cells resistant to therapeutic agents (TRAIL, trastuzumab, and GW583340) resulted in decreased viability and increased apoptosis, demonstrating that XIAP is required for survival of cells with resistance to these agents. A novel mechanism of GW583340 oxidative stress-induced mediated apoptosis was identified, and resistant cells had increased antioxidant expression and capability. Interesting, inhibition of XIAP function overcame this increase in antioxidant potential, demonstrating a new function for XIAP in oxidative stress-induced apoptosis. These studies establish the feasibility of development of an XIAP inhibitor that potentiates apoptosis for use in IBC patients with resistance to therapeutic agents.</p>Dissertatio