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ML-IAP [melanoma IAP (inhibitor of apoptosis)] is an anti-apoptotic protein that is expressed highly in melanomas where it contributes to resistance to apoptotic stimuli. The anti-apoptotic activity and elevated expression of IAP family proteins in many human cancers makes IAP proteins attractive targets for inhibition by cancer therapeutics. Small-molecule IAP antagonists that bind with high affinities to select BIR (baculovirus IAP repeat) domains have been shown to stimulate auto-ubiquitination and rapid proteasomal degradation of c-IAP1 (cellular IAP1) and c-IAP2 (cellular IAP2). In the present paper, we report ML-IAP proteasomal degradation in response to bivalent, but not monovalent, IAP antagonists. This degradation required ML-IAP ubiquitin ligase activity and was independent of c-IAP1 or c-IAP2. Although ML-IAP is best characterized in melanoma cells, we show that ML-IAP expression in normal mammalian tissues is restricted largely to the eye, being most abundant in ciliary body epithelium and retinal pigment epithelium. Surprisingly, given this pattern of expression, gene-targeted mice lacking ML-IAP exhibited normal intraocular pressure as well as normal retinal structure and function. The results of the present study indicate that ML-IAP is dispensable for both normal mouse development and ocular homoeostasis.

Jumping Translocation Breakpoint (JTB) is an orphan receptor that is conserved from nematodes to humans and whose gene expression in humans is strikingly upregulated in diverse types of cancers. Translocations occur frequently at the hJTB genomic locus, leading to multiple copies of a truncated JTB gene, which potentially encodes a soluble secreted ectodomain. In addition, JTB and its orthologs likely represent a unique and ancient protein family since homologs could not be identified by direct sequence comparison. In the present study, we have determined the NMR solution structure of the N-terminal ectodomain of human JTB, showing that its fold architecture is a new variant of a three-β-strand antiparallel β-meander. The JTB structure has a distant relationship to the midkine/pleiotrophin fold, particularly in the conservation of distinctive disulfide bridge patterns. The structure of this newly characterized small cysteine-rich domain suggests potential involvement of JTB in interactions with proteins or extracellular matrix and may help to uncover the elusive biological functions of this protein., (Copyright © 2011 Elsevier Ltd. All rights reserved.)

Melanoma inhibitor of apoptosis (ML-IAP) is a potent inhibitor of apoptosis, which is highly expressed in melanomas and likely contributes to their resistance to chemotherapeutic treatments. Herein, we show that the lineage survival oncogene microphthalmia-associated transcription factor (MITF) is a critical regulator of ML-IAP transcription in melanoma cells. The ML-IAP promoter contains two MITF consensus sites, and analysis of MITF and ML-IAP mRNA levels revealed a high correlation in melanoma tumor samples and cell lines. In reporter assays, MITF promoted a strong stimulation of transcriptional activity from the ML-IAP promoter, and MITF bound the endogenous ML-IAP promoter in melanoma cells by chromatin immunoprecipitation and electrophoretic mobility shift assay. Strikingly, small interfering RNA (siRNA)-mediated knockdown of MITF in melanoma cells led to a dramatic decrease in ML-IAP mRNA and protein levels, establishing that ML-IAP expression in melanoma cells is MITF dependent. Additionally, cyclic AMP-mediated induction of MITF expression in melanocytes resulted in increased ML-IAP expression, suggesting that melanocytes can express ML-IAP when MITF levels are heightened. Disruption of MITF by siRNA led to a decrease in melanoma cell viability, which could be rescued by ectopic expression of ML-IAP. Collectively, these findings implicate MITF as a major transcriptional regulator of ML-IAP expression in melanomas, and suggest that ML-IAP contributes to the prosurvival activity of MITF in melanoma progression.

The inhibitors of apoptosis (IAPs) are a family of cell death inhibitors found in viruses and metazoans. All members of the IAP family have at least one baculovirus IAP repeat (BIR) motif that is essential for their anti-apoptotic activity. The t(11, 18)(q21;q21) translocation fuses the BIR domains of c-IAP2 with the paracaspase/MALT1 (mucosa-associated lymphoid tissue) protein, a critical mediator of T cell receptor-stimulated activation of NF-kappaB. The c-IAP2.MALT1 fusion protein constitutively activates the NF-kappaB pathway, and this is considered critical to malignant B cell transformation and lymphoma progression. The BIR domains of c-IAP1 and c-IAP2 interact with tumor necrosis factor receptor-associated factors 1 and 2 (TRAF1 and TRAF2). Here we investigated the importance of TRAF1 and TRAF2 for c-IAP2.MALT1-stimulated NF-kappaB activation. We identified a novel epitope within the BIR1 domains of c-IAP1 and c-IAP2 that is crucial for their physical interaction with TRAF1 and TRAF2. The c-IAP2.MALT1 fusion protein associates with TRAF1 and TRAF2 using the same binding site. We explored the functional relevance of this interaction and established that binding to TRAF1 and TRAF2 is not required for c-IAP2.MALT1-stimulated NF-kappaB activation. Furthermore, gene ablation of TRAF2 or combined down-regulation of TRAF1 and TRAF2 did not affect c-IAP2.MALT1-stimulated signaling. However, TRAF1/2-binding mutants of c-IAP2.MALT1 still oligomerize and activate NF-kappaB, suggesting that oligomerization might be important for signaling of the fusion protein. Therefore, the t(11, 18)(q21;q21) translocation creating the c-IAP2.MALT1 fusion protein activates NF-kappaB and contributes to human malignancy in the absence of signaling adaptors that might otherwise regulate its activity.

ML-IAP (melanoma inhibitor of apoptosis) is a potent anti-apoptotic protein that is strongly up-regulated in melanoma and confers protection against a variety of pro-apoptotic stimuli. The mechanism by which ML-IAP regulates apoptosis is unclear, although weak inhibition of caspases 3 and 9 has been reported. Here, the binding to and inhibition of caspase 9 by the single BIR (baculovirus IAP repeat) domain of ML-IAP has been investigated and found to be significantly less potent than the ubiquitously expressed XIAP (X-linked IAP). Engineering of the ML-IAP-BIR domain, based on comparisons with the third BIR domain of XIAP, resulted in a chimeric BIR domain that binds to and inhibits caspase 9 significantly better than either ML-IAP-BIR or XIAP-BIR3. Mutational analysis of the ML-IAP-BIR domain demonstrated that similar enhancements in caspase 9 affinity can be achieved with only three amino acid substitutions. However, none of these modifications affected binding of the ML-IAP-BIR domain to the IAP antagonist Smac (second mitochondrial activator of caspases). ML-IAP-BIR was found to bind mature Smac with low nanomolar affinity, similar to that of XIAP-BIR2-BIR3. Correspondingly, increased expression of ML-IAP results in formation of a ML-IAP-Smac complex and disruption of the endogenous interaction between XIAP and mature Smac. These results suggest that ML-IAP might regulate apoptosis by sequestering Smac and preventing it from antagonizing XIAP-mediated inhibition of caspases, rather than by direct inhibition of caspases.

Melanoma inhibitor of apoptosis (ML-IAP) is a potent anti-apoptotic protein that is upregulated in a number of melanoma cell lines but not expressed in most normal adult tissues. Overexpression of IAP proteins, such as ML-IAP or the ubiquitously expressed X-chromosome-linked IAP (XIAP), in human cancers has been shown to suppress apoptosis induced by a variety of stimuli. Peptides based on the processed N-terminus of Smac/DIABLO can negate the ability of overexpressed ML-IAP or XIAP to suppress drug-induced apoptosis. Such peptides have been demonstrated to bind to the single baculovirus IAP repeat (BIR) of ML-IAP and the third BIR of XIAP with similar high affinities (approximately 0.5 microM). Herein, we use phage-display of naïve peptide libraries and synthetic peptides to investigate the peptide-binding properties of ML-IAP-BIR and XIAP-BIR3. X-ray crystal structures of ML-IAP-BIR in complex with Smac- and phage-derived peptides, together with peptide structure-activity-relationship data, indicate that the peptides can be modified to provide increased binding affinity and selectivity for ML-IAP-BIR relative to XIAP-BIR3. For instance, substitution of Pro3' in the Smac-based peptide (AVPIAQKSE) with (2S,3S)-3-methylpyrrolidine-2-carboxylic acid [(3S)-methyl-proline] results in a peptide with 7-fold greater affinity for ML-IAP-BIR and about 100-fold specificity for ML-IAP-BIR relative to XIAP-BIR3.

Inhibitors of apoptosis (IAPs) physically interact with a variety of pro-apoptotic proteins and inhibit apoptosis induced by diverse stimuli. X-linked IAP (X-IAP) is a prototype IAP family member that inhibits several caspases, the effector proteases of apoptosis. The inhibitory activity of X-IAP is regulated by SMAC, a protein that is processed to its active form upon receipt of a death stimulus. Cleaved SMAC binds X-IAP and antagonizes its anti-apoptotic activity. Here we show that melanoma IAP (ML-IAP), a potent anti-cell death protein and caspase inhibitor, physically interacts with SMAC through its BIR (baculovirus IAP repeat) domain. In addition to binding full-length SMAC, ML-IAP BIR associates with SMAC peptides that are derived from the amino terminus of active, processed SMAC. This high affinity interaction is very specific and can be completely abolished by single amino acid mutations either in the amino terminus of active SMAC or in the BIR domain of ML-IAP. In cells expressing ML-IAP and X-IAP, SMAC coexpression or addition of SMAC peptides abrogates the ability of the IAPs to inhibit cell death. These results demonstrate the feasibility of using SMAC peptides as a way to sensitize IAP-expressing cells to pro-apoptotic stimuli such as chemotherapeutic agents.

The solution structure of melanoma growth stimulating activity (MGSA) has been investigated using proton NMR spectroscopy. Sequential resonance assignments have been carried out, and elements of secondary structure have been identified on the basis of NOE, coupling constant, chemical shift, and amide proton exchange data. Long-range NOEs have established that MGSA is a dimer in solution. The secondary structure and dimer interface of MGSA appear to be similar to those found previously for the homologous chemokine interleukin-8 [Clore et al. (1990) Biochemistry 29, 1689-1696]. The MGSA monomer contains a three stranded anti-parallel beta-sheet arranged in a 'Greek-key' conformation, and a C-terminal alpha-helix (residues 58-69).