Mechanisms by which TRA-8 anti-DR5 antibody and chemotherapy enhance cytotoxicity in breast cancer.

摘要

Breast cancer is the second leading cause of cancer-related death in American women and metastatic breast cancer has a 5-year survival rate of only 26%. Current targeted treatments for this disease include anti-estrogen strategies for estrogen receptor positive tumors (∼60%) and anti-Her2/Neu strategies for tumors overexpressing this receptor (20-25%). A percentage of breast cancer patients, however, are resistant to these therapies and are left without any effective treatment options. One of the agents currently being investigated to improve breast cancer survival is TRA-8, an agonistic monoclonal antibody to death receptor 5 (DR5), which induces apoptosis in various types of cancer cells; however, the degree of sensitivity varies from highly sensitive to resistant. Our laboratory has previously shown resistance to TRA-8 can be reversed using chemotherapeutic agents, but the mechanism underlying this sensitization was not fully understood. In the current dissertation studies, the main objective was to characterize various components of the apoptotic and intracellular signaling pathways to determine the mechanistic basis of the synergistic interaction between TRA-8 and chemotherapy drugs (doxorubicin and bortezomib). Both drugs synergistically sensitized TRA-8 resistant breast cancer cell lines to the antibody. This effect was accompanied by enhanced activation of apoptosis (demonstrated by caspase and PARP cleavage, reduced Bid, and increased mitochondrial membrane depolarization). Doxorubicin was shown to impact the cell cycle and modulate the Akt and NFkappaB signaling pathways, but each was shown not to contribute to the TRA-8 sensitivity of BT-474 cells. In examining the apoptotic modulatory proteins, doxorubicin or bortezomib were shown to reduce Bcl-XL and XIAP protein levels in TRA-8 treated cells. To examine the Bcl-2 and IAP inhibitory pathways further, we combined TRA-8 with AT-101 or BH3I-2', inhibitors of anti-apoptotic Bcl-2 proteins, produced synergistic cytotoxicity in breast cancer cell lines. The IAP targeting compound, AT-406, was also synergistic with TRA-8 in some but not all cell lines examined. Collectively, these studies demonstrate that the Bcl-2 and IAP families of proteins are involved in TRA-8 sensitization via their modulation of the intrinsic apoptotic pathway. Targeting these proteins may represent a new therapeutic strategy in the treatment of breast cancer.;Keywords: breast cancer, TRAIL, death receptor antibody, apoptosis