Apollon ubiquitinates SMAC and caspase-9, and has an essential cytoprotection function

Abstract

Apollon (also known as BRUCE or BIRC6) is a large protein containing baculoviral-IAP-repeat (BIR) and ubiquitin-conjugating enzyme (UBC) domains at the amino- and carboxy termini, respectively. Apollon inhibits apoptosis, but its molecular and physiological function remains unclear. Here we report that Apollon binds to, ubiquitinates and facilitates proteasomal degradation of SMAC and caspase-9, which both contain IAP-binding motifs. Targeted disruption of Apollon in mice caused embryonic and neonatal lethality. Notably, SMAC induced apoptosis in Apollon-deficient cells, but not in Apollon-expressing cells. Furthermore, the IAP-binding motif of SMAC was required to induce apoptosis in Apollon-deficient cells. These results suggest that Apollon has an essential function in preventing SMAC-induced apoptosis.

Livin promotes Smac/DIABLO degradation by ubiquitin–proteasome pathway

“…26 Apollon was reported to function as an E2-Ubc to ubiquitinate both Smac/DIABLO and caspase-9 and thus displays an essential cytoprotection function in preventing Smac/DIABLO-induced apoptosis. 27 The interacting partners of Livin include caspase-3, -7 and -9 and proteins containing an IAP-interacting motif, which currently comprise the mitochondrial protein Smac/DIABLO and Omi/HtrA2. Smac/ DIABLO is a negative regulator of Livin and is therefore a proapoptotic protein.…”

“…This is not unexpected as Smac/DIABLO is also subject to polyubiquitination by other IAP factors, such as Apollon/ Bruce, cIAP-1 and cIAP-2. 14,27 The struggle between cell survival and death is ubiquitous and this is maintained in balance under various intricate regulations including the mechanism of proteasome ubiquiti-nation. On the one hand, by catalyzing its own ubiquitination, Livin lowers the threshold for inducing apoptosis and makes cells more perceptible for cell death.…”

Oncolytic adenovirus-mediated shRNA against Apollon inhibits tumor cell growth and enhances antitumor effect of 5-fluorouracil

“…14 Several lines of evidence show that the Apollon can protect cells against apoptosis and functions as an IAP by binding the active caspases via its single BIR domain. [16][17][18] Furthermore, studies also show that Apollon can promote ubiquitination of the IAP antagonist Smac, which depends on the integrity of both the catalytically active ubiquitin-conjugating domain and the correctly folded BIR domain. [16][17][18] Apollon is upregulated in some drug-resistant cancer cells, and antisense oligonucleotides against Apollon significantly sensitizes tumor cells to apoptosis induced by chemotherapy agents.…”

“…[16][17][18] Furthermore, studies also show that Apollon can promote ubiquitination of the IAP antagonist Smac, which depends on the integrity of both the catalytically active ubiquitin-conjugating domain and the correctly folded BIR domain. [16][17][18] Apollon is upregulated in some drug-resistant cancer cells, and antisense oligonucleotides against Apollon significantly sensitizes tumor cells to apoptosis induced by chemotherapy agents. 13 These findings suggest that Apollon has an antiapoptotic role and has been considered to be an attractive target with respect to molecular therapy of cancer.…”

IAPs – the ubiquitin connection

“…[36][37][38] While all agree that deletion of BRUCE results in embryonic lethality, its exact role remains controversial, with suggested activities including preventing p53 activation, 38 directly binding to both unprocessed and mature Smac/DIABLO and both pro-and processed caspase 9, and targeting them for degradation. 36,39,40 Survivin is a BIR containing protein that acts together with INCENP and Aurora kinase B in a complex required for chromosome segregation and cytokinesis during mitosis. [41][42][43] The abundance of Survivin is coupled to the cell cycle.…”