A model of partnership co-opted by the homeodomain protein TGIF and the Itch/AIP4 ubiquitin ligase for effective execution of TNF-alpha cytotoxicity.

The homeodomain protein TGIF functions as a negative regulator of multiple classes of transcription factors. Here we report on the characterization of TGIF as an essential component of the tumor necrosis factor alpha (TNF-alpha) cytotoxic program. This proapoptotic role of TGIF does not appear to rely on transcriptional modulation but instead is executed in conjunction with Itch/AIP4, an E3 ubiquitin ligase operating in TNF-alpha-induced apoptosis through its ability to target the caspase antagonist cFlip(L) for degradation. Notably, we found that activation of TNF-alpha signaling induced the association of TGIF with Itch/AIP4, resulting in increased accessibility of cFlip(L) for association and ubiquitination by Itch/AIP4. Moreover, we show that Itch/AIP4 can also stabilize the TGIF protein in response to TNF-alpha by triggering its monoubiquitination at lysine 259, thereby revealing the existence of a functional network that can evolve into a positive feedback loop for ensuring effective execution of the TNF-alpha apoptotic signaling.