Translocation of the inhibitor of apoptosis protein c-IAP1 from the nucleus to the Golgi in hematopoietic cells undergoing differentiation: a nuclear export signal-mediated event

Abstract

IntroductionThe inhibitors of apoptosis proteins (IAPs) have been initially defined as natural cellular inhibitors of cell death. These proteins were identified in baculoviral genome as regulators of host-cell viability during virus infection, 1 and cellular orthologues were subsequently described in yeast, nematodes, drosophila, and mammals. The human genome encodes at least 8 IAPs (X-linked IAP [XIAP], cellular IAP1 [c-IAP1], c-IAP2, melanoma IAP [ML-IAP], neuronal apoptosis inhibitory protein [NAIP], survivin, IAP-like protein 2 [ILP-2], Apollon). 2 All of these proteins have in common the presence of 1 to 3 copies of a baculovirus IAP repeat (BIR) domain. 1 These domains are essential for the antiapoptotic properties of the IAPs, which have been attributed to the direct binding and inhibition of caspases. XIAP binds the small subunit of caspase-9 through its BIR3 domain 3 and masks the active site of caspase-3 and -7 through a distinct segment, which is immediately amino-terminal to its BIR2 domain. 4,5 c-IAP1 and c-IAP2 bind caspase-3 and -7 but their inhibitory effect on caspases is 2-to 3-log lower than that of XIAP. 6 Some of the BIR-containing proteins do not have clear links with apoptosis and several members of the family have demonstrated distinct functions including cell cycle regulation, 7 protein degradation, 8 and caspase-independent signal transduction. [9][10][11][12] In addition to the BIR domains, several IAPs, including XIAP, c-IAP1, and c-IAP2, contain a highly conserved carboxy-terminal RING finger domain that confers them an enzyme 3 (E3) function in the protein ubiquitylation process. Several proteins specifically targeted for ubiquitylation by IAPs have been identified. At least in vitro, XIAP and c-IAP2 direct the ubiquitylation of caspase-3 and caspase-7, 13,14 whereas c-IAP1 and c-IAP2 mediate ubiquitylation of second mitochondria-derived activator of caspase (Smac)/ DIABLO, an antagonist of IAPs. 15 c-IAP1 and c-IAP2 are also components of the type 2 tumor necrosis factor (TNF) receptor complex through interaction with the signaling intermediates TNF receptor-associated factor 1 (TRAF1) and TRAF2. 9 c-IAP1 could induce the ubiquitylation of TRAF2 and participated in the TNF-␣-mediated proteasomal degradation of TRAF2, 16 and c-IAP2 has been involved in the TNF-␣ signaling leading to nuclear factor-B (NF-B) activation. 17 The expression and activity of IAPs are regulated at several levels. The transcription factor NF-B enhances the expression of c-IAP1, c-IAP2, and XIAP, which may contribute to the prosurvival effect exerted in many situations by this transcription factor. 18,19 XIAP translation can be enhanced through the use of an internal ribosomal entry site in the 5Ј-untranslated region of its messenger RNA. 20 Another level of regulation of IAP functions is the modulation of their subcellular location. Such a regulation has been described for XIAP whose interaction with the protein XIAP-associated factor 1 (XAF1) induces its sequestration in the nucleus and suppresses its casp...

Interaction of heat-shock protein 90β isoform (HSP90β) with cellular inhibitor of apoptosis 1 (c-IAP1) is required for cell differentiation

“…In these THP1 cells, c-IAP1/HSP90b-interaction was observed only in the nucleus, confirming our previous observations that c-IAP1 has a nuclear localization in undifferentiated cells. 18 The c-IAP1/HSP90b-interaction was also observed in differentiated cells (Figure 1c). TPA-induced differentiation of THP1 cells was associated with an increase in c-IAP1 expression ( Figure 1c, lower panel) together with an increased amount of c-IAP1 associated with HSP90b ( Figure 1c, upper panel).…”

“…TPA-induced differentiation of THP1 cells was associated with an increase in c-IAP1 expression ( Figure 1c, lower panel) together with an increased amount of c-IAP1 associated with HSP90b ( Figure 1c, upper panel). Mutations in leucine residues that characterize the three potential c-IAP1 nuclear export sequences 18,20 did not affect the HSP90b/c-IAP1 interaction (data not shown), indicating that the protein-protein interaction involves other c-IAP1 sequences.…”

“…In all cases, the differentiation associated nuclear extrusion of both c-IAP1 and HSP90b was blocked by cell treatment with leptomycin B, a specific inhibitor of the nuclear export protein exportin 1 (Supplementary Figure 2). 18,20 The nuclear exit of c-IAP1 and HSP90b seems to be an early event in a differentiation process. Only one hour after TPA treatment of THP1 cells, the content of both proteins in the nucleus was strongly decreased (Figure 2b).…”

“…18 In a search for a function of c-IAP1 in the different cellular compartments, we have identified an interaction between c-IAP1 and heat-shock protein 90 b isoform (HSP90b), a molecular chaperone abundant in cancer cells and whose inhibition is currently being tested in cancer therapy. HSP90b migrates with c-IAP1 from the nucleus to the cytoplasm and its inhibition induces c-IAP1 degradation by the proteasomal machinery, which can be sufficient to block the differentiation process.…”

“…18 In a search for a function of c-IAP1 in the different These authors have contributed equally to this work.…”