Inhibitor of apoptosis proteins as a therapeutic target in breast cancer

Background

Apoptosis is the process of programmed cell death by which damaged or unhealthy cells are normally destroyed. Cancer cells are able to avoid apoptosis and thereby survive inappropriately. Inhibitor of apoptosis proteins (IAPs) are a family of proteins that block apoptosis in normal cells, by binding to active caspases, the proteases that mediate cell death. There are eight human IAPs, including NAIP, XIAP, cIAP1, cIAP2, livin, survivin and apollon. An upregulation of IAPs could cause resistance to apoptosis. Targeting IAPs in cancer therapy may therefore improve the clinical effectiveness of apoptosis-inducing chemotherapeutics. A number of studies have shown that XIAP and survivin are up-regulated in cancer, and inhibiting these IAPs increased the apoptotic response induced by some chemotherapeutics. We aim, first, to examine the expression profile of all IAPs in breast cancer and, second, to determine whether inhibiting IAPs will enhance the apoptotic response to traditional chemotherapeutics and newly developed targeted therapies, such as Herceptin.

Methods

IAP levels were detected in patient and cell line samples by immunoblotting with validated antibodies using the Li-Cor Odyssey system (Li-Cor Biosciences, Lincoln, NE, USA). IAPs were inhibited using siRNA or cell-permeable mimics of endogenous inhibitors. Control cells and cells with XIAP knocked down or inhibited were exposed to TNF-related apoptosis inducing ligand (10 ng/ml), Herceptin (100 μg/ml), Iressa (10 μM), or Lapatinib (100 nM) for 48 hours. Apoptosis was scored by examining nuclear morphology (DAPI) or active caspase 3 staining. Proliferation was examined by Ki67 staining.

Results

We have found that IAPs are widely upregulated in breast cancer. In particular cIAP2, XIAP and survivin were more prevalent in breast cancer cells than normal breast epithelium. Knock down of XIAP or inhibition with small molecule inhibitors resulted in an increased apoptotic response to TNF-related apoptosis inducing ligand, in both sensitive and resistant cell lines. Knocking down XIAP also increased the apoptotic response to a number of growth factor receptor-targeted therapies such as Herceptin, Iressa and Lapatinib.

Conclusion

Inhibiting IAPs in combination with both chemotherapeutic agents and targeted therapies, such as Herceptin and Lapatinib, which act as receptor antagonists, will improve clinical outcome.