DIRECT REGULATION OF APOPTOSIS BY LINEAR UBIQUTIN CHAIN ASSEMBLY COMPLEX (LUBAC) AND FEEDBACK REGULATION OF LUBAC FUNCTION BY CASPASES

Tumor Necrosis Factor-alpha (TNF-α) is a cytokine that plays a role in various cellular processes such as proliferation, differentiation (mainly through NF-κB signaling) and death (via apoptosis signaling). Recently, linear ubiquitination by LUBAC (linear ubiquitin chain assembly complex) was reported to have a regulatory function in TNF-α mediated NF-κB activation. Although LUBAC is suggested to control not only NF-kB signaling but also the apoptosis pathway, the precise mechanism of apoptosis regulation remains unknown. Moreover, NF-κB and apoptosis pathways have opposed but fundamental functions for various cellular processes. Although these two pathways actively interplay to balance the death and survival, the reciprocal regulation between these two are not completely established. Here, I report direct regulation of apoptosis by LUBAC and the novel crosstalk mechanism between these two pathways. First, I investigated the role of RNF31 in mouse development with RNF31 knockout mouse. RNF31 deletion resulted in embryonic lethality of mouse around 8.5 days. Second, I examined the direct regulation of apoptosis by linear ubiquitination and RNF31. RNF31 inhibited TNF-α induced apoptosis via regulating degradation of antiapoptotic molecule, cellular FLICE-like inhibitory protein (cFLIP). Finally, I determined the negative feedback to regulate function of LUBAC from apoptosis. Effector caspases induce RNF31 cleavage in apoptotic condition, which suppresses its role in NF-kB activation. This study is the first to demonstrate the molecular mechanism of how LUBAC governs the apoptosis pathway via linear ubiquitination and a novel regulatory loop between cell death and survival signal. Therefore, this will present the first insight into the physiological role of RNF31 and linear ubiquitination in the apoptosis pathway and may provide a novel therapeutic strategy for cancers