Response to Heard et al

Recent advances in understanding inhibitor of apoptosis proteins

The inhibitor of apoptosis proteins (IAPs) are a family of proteins that were chiefly known for their ability to inhibit apoptosis by blocking caspase activation or activity. Recent research has shown that cellular IAP1 (cIAP1), cIAP2, and X-linked IAP (XIAP) also regulate signaling by receptors of the innate immune system by ubiquitylating their substrates. These IAPs thereby act at the intersection of pathways leading to cell death and inflammation. Mutation of IAP genes can impair tissue homeostasis and is linked to several human diseases. Small-molecule IAP antagonists have been developed to treat certain malignant, infectious, and inflammatory diseases. Here, we will discuss recent advances in our understanding of the functions of cIAP1, cIAP2, and XIAP; the consequences of their mutation or dysregulation; and the therapeutic potential of IAP antagonist drugs.

Regulating the balance between necroptosis, apoptosis and inflammation by inhibitors of apoptosis proteins

Understanding how inhibitor of apoptosis proteins (IAPs) regulate apoptosis and necroptosis has been fast-forwarded by the use of Smac mimetics (SMs) to deplete or inhibit the IAPs, specifically cIAP1, cIAP2 and XIAP. The loss or inhibition of cIAP1, cIAP2 and XIAP causes the majority of cells to be sensitized to death receptor induced cell death, such as with tumour necrosis factor (TNF). Mouse genetics shows that there is some functional redundancy and the use of SMs has allowed us to understand how changing the composition of proteins recruited to TNF receptor 1 on TNF ligation can alter protein complex formation and activation of apoptosis or necroptosis, particularly when caspases are inhibited. Determining when or how caspase inhibition occurs physiologically combined with the loss of IAPs will be the next challenge in understanding the ability of IAPs to prevent cell death and/or limit inflammation.