Identification of a Novel Anti-apoptotic E3 Ubiquitin Ligase That Ubiquitinates Antagonists of Inhibitor of Apoptosis Proteins SMAC, HtrA2, and ARTS

Identification of new anti-apoptotic genes is important for understanding the molecular mechanisms underlying apoptosis and tumorigenesis. The present study identified a novel anti-apoptotic gene named AREL1, which encodes a HECT(homologous to E6-AP carboxyl terminus) family E3 ubiquitin ligase. AREL1 interacted with and ubiquitinated IAP antagonists such as SMAC, HtrA2, and ARTS. However, AREL1 was cytosolic and did not localize to nuclei or mitochondria. The interactions between AREL1 and the IAP antagonists were specific for apoptosis-stimulated cells, in which the IAP antagonists were released into the cytosol from mitochondria. Furthermore, the ubiquitination and degradation of SMAC, HtrA2, and ARTS were significantly enhanced in AREL1-expressing cells following apoptotic stimulation, indicating that AREL1 binds to and ubiquitinates cytosolic but not mitochondria-associated forms of IAP antagonists. Furthermore, the anti-apoptotic role of AREL1-mediated degradation of SMAC, HtrA2, and ARTS was shown by simultaneous knockdown of three IAP antagonists, which caused the inhibition of caspase-3 cleavage, XIAP degradation, and induction of apoptosis. Therefore, the present study suggests that AREL1-mediated ubiquitination and degradation of cytosolic forms of three IAP antagonists plays an important role in the regulation of apoptosis.