Antagonism of Inhibitor of Apoptosis Proteins Increases Bone Metastasis via Unexpected Osteoclast Activation

IAP (inhibitor of apoptosis) proteins play a central role in many types of cancer, and IAP antagonists are in development as anti-cancer agents. IAP antagonists cause apoptosis in many cells, but they also activate alternative NF-κB signaling through NIK, which regulates osteoclasts. In bone metastasis, a positive feedback loop between tumors and osteoclasts promotes tumor growth and osteolysis. We therefore tested the effect of IAP antagonists on the bone microenvironment for metastasis. In both drug-sensitive and drug-resistant tumors, growth in bone was favored compared to other sites during IAP antagonist treatment. These drugs also caused osteoporosis and increased osteoclastogenesis, mediated by NIK, and enhanced tumor-associated osteolysis. Co-treatment with zoledronic acid, a potent osteoclast inhibitor, reduced IAP antagonist-enhanced tumor growth in bone and osteolysis. Thus, IAP-based cancer treatment may be compromised by osteoporosis and enhanced skeletal metastasis which may be prevented by anti-resorptive agents.