cIAP1/TRAF2 interplay promotes tumor growth through the activation of STAT3.

Cellular inhibitor of apoptosis-1 (cIAP1) is a signaling regulator with oncogenic properties. It is involved in the regulation of signaling pathways controlling inflammation, cell survival, proliferation, differentiation and motility. It is recruited into membrane-receptor-associated signaling complexes thanks to the molecular adaptor TRAF2. However, the cIAP1/TRAF2 complex exists, independently of receptor engagement, in several subcellular compartments. The present work strengthens the importance of TRAF2 in the oncogenic properties of cIAP1. cIAPs-deficient mouse embryonic fibroblasts (MEFs) were transformed using the HRas-V12 oncogene. Re-expression of cIAP1 enhanced tumor growth in a nude mice xenograft model, and promoted lung tumor nodes formation. Deletion or mutation of the TRAF2-binding site completely abolished the oncogenic properties of cIAP1. Further, cIAP1 mediated the clustering of TRAF2, which was sufficient to stimulate tumor growth. Our TRAF2 interactome analysis showed that cIAP1 was critical for TRAF2 to bind to its protein partners. Thus, cIAP1 and TRAF2 would be two essential subunits of a signaling complex promoting a pro-tumoral signal. cIAP1/TRAF2 promoted the activation of the canonical NF-κB and ERK1/2 signaling pathways. NF-κB-dependent production of IL-6 triggered the activation of the JAK/STAT3 axis in an autocrine manner. Inhibition or downregulation of STAT3 specifically compromised the growth of cIAP1-restored MEFs but not that of MEFs expressing a cIAP1-mutant and treating mice with the STAT3 inhibitor niclosamide completely abrogated cIAP1/TRAF2-mediated tumor growth. Altogether, we demonstrate that cIAP1/TRAF2 binding is essential to promote tumor growth via the activation of the JAK/STAT3 signaling pathway.

Repeat Element Activation-Driven Inflammation: Role of NFκB and Implications in Normal Development and Cancer?

The human genome is composed of unique DNA sequences that encode proteins and unique sequence noncoding RNAs that are essential for normal development and cellular differentiation. The human genome also contains over 50% of genome sequences that are repeat in nature (tandem and interspersed repeats) that are now known to contribute dynamically to genetic diversity in populations, to be transcriptionally active under certain physiological conditions, and to be aberrantly active in disease states including cancer, where consequences are pleiotropic with impact on cancer cell phenotypes and on the tumor immune microenvironment. Repeat element-derived RNAs play unique roles in exogenous and endogenous cell signaling under normal and disease conditions. A key component of repeat element-derived transcript-dependent signaling occurs via triggering of innate immune receptor signaling that then feeds forward to inflammatory responses through interferon and NFκB signaling. It has recently been shown that cancer cells display abnormal transcriptional activity of repeat elements and that this is linked to either aggressive disease and treatment failure or to improved prognosis/treatment response, depending on cell context and the amplitude of the so-called 'viral mimicry' response that is engaged. 'Viral mimicry' refers to a cellular state of active antiviral response triggered by endogenous nucleic acids often derived from aberrantly transcribed endogenous retrotransposons and other repeat elements. In this paper, the literature regarding transcriptional activation of repeat elements and engagement of inflammatory signaling in normal (focusing on hematopoiesis) and cancer is reviewed with an emphasis on the role of innate immune receptor signaling, in particular by dsRNA receptors of the RIG-1 like receptor family and interferons/NFκB. How repeat element-derived RNA reprograms cell identity through RNA-guided chromatin state modulation is also discussed.

Protein Arginine Methyltransferases as Therapeutic Targets in Hematological Malignancies.

Arginine methylation is a common post-translational modification affecting protein activity and the transcription of target genes when methylation occurs on histone tails. There are nine protein arginine methyltransferases (PRMTs) in mammals, divided into subgroups depending on the methylation they form on a molecule of arginine. During the formation and maturation of the different types of blood cells, PRMTs play a central role by controlling cell differentiation at the transcriptional level. PRMT enzymatic activity is necessary for many cellular processes in hematological malignancies, such as the activation of cell cycle and proliferation, inhibition of apoptosis, DNA repair processes, RNA splicing, and transcription by methylating histone tails' arginine. Chemical tools have been developed to inhibit the activity of PRMTs and have been tested in several models of hematological malignancies, including primary samples from patients, xenografts into immunodeficient mice, mouse models, and human cell lines. They show a significant effect by reducing cell viability and increasing the overall survival of mice. PRMT5 inhibitors have a strong therapeutic potential, as phase I clinical trials in hematological malignancies that use these molecules show promising results, thus, underlining PRMT inhibitors as useful therapeutic tools for cancer treatment in the future.

IAP-Mediated Protein Ubiquitination in Regulating Cell Signaling.

Over the last decade, the E3-ubiquitine ligases from IAP (Inhibitor of Apoptosis) family have emerged as potent regulators of immune response. In immune cells, they control signaling pathways driving differentiation and inflammation in response to stimulation of tumor necrosis factor receptor (TNFR) family, pattern-recognition receptors (PRRs), and some cytokine receptors. They are able to control the activity, the cellular fate, or the stability of actors of signaling pathways, acting at different levels from components of receptor-associated multiprotein complexes to signaling effectors and transcription factors, as well as cytoskeleton regulators. Much less is known about ubiquitination substrates involved in non-immune signaling pathways. This review aimed to present IAP ubiquitination substrates and the role of IAP-mediated ubiquitination in regulating signaling pathways.

Heat-shock proteins: chaperoning DNA repair.

Cells are repeatedly exposed to environmental or endogenous stresses that can alter normal cell behavior and increase cell vulnerability. In order to ensure tissue integrity and function, cells cope with cellular injuries by adapting their metabolism, protecting essential intracellular constituents, inhibiting cell death signaling pathways and activating those devoted to damage repair. The molecular chaperones of the heat-shock protein (HSP) family are critical effectors of this adaptive response. They protect intracellular proteins from misfolding or aggregation, inhibit cell death signaling cascades and preserve the intracellular signaling pathways that are essential for cell survival. Most HSPs are rapidly overexpressed in response to cellular injuries including genotoxic stress. DNA damage can dramatically alter cell behavior and contribute to a number of diseases including developmental defects, neurodegenerative disorders, and cancer. Thus, the ability of cells to repair DNA damage is essential for preserving cell integrity. DNA damage activates a coordinated response that includes detecting DNA lesions before their transmission to daughter cells, blocking cell cycle progression and DNA replication and repairing the damage. Although the role of HSPs in proteins homeostasis and cell death, especially apoptosis has been widely reported, much less is known about their function in DNA repair. This review aims to present the role of HSPs in DNA repair signaling pathways.

HSP110 translocates to the nucleus upon genotoxic chemotherapy and promotes DNA repair in colorectal cancer cells.

A multicenter clinical study demonstrated the presence of a loss-of-function HSP110 mutation in about 15% of colorectal cancers, which resulted from an alternative splicing and was produced at the detriment of wild-type HSP110. Patients expressing low levels of wild-type HSP110 had excellent outcomes (i.e. response to an oxaliplatin-based chemotherapy). Here, we show in vitro, in vivo, and in patients' biopsies that HSP110 co-localizes with DNA damage (γ-H2AX). In colorectal cancer cells, HSP110 translocates into the nucleus upon treatment with genotoxic chemotherapy such as oxaliplatin. Furthermore, we show that HSP110 interacts with the Ku70/Ku80 heterodimer, an essential element of the non-homologous end joining (NHEJ) repair machinery. We also demonstrate by evaluating the resolved 53BP1 foci that depletion in HSP110 impairs repair steps of the NHEJ pathway, which is associated with an increase in DNA double-strand breaks and in the cells' sensitivity to oxaliplatin. HSP110-depleted cells sensitization to oxaliplatin-induced DNA damage is abolished upon re-expression of HSP110. Confirming a role for HSP110 in DNA non-homologous repair, SCR7 and NU7026, two inhibitors of the NHEJ pathway, circumvents HSP110-induced resistance to chemotherapy. In conclusion, HSP110 through its interaction with the Ku70/80 heterodimer may participate in DNA repair, thereby inducing a protection against genotoxic therapy.

E2F1 binds to the peptide-binding groove within the BIR3 domain of cIAP1 and requires cIAP1 for chromatin binding.

The cellular inhibitor of apoptosis 1 (cIAP1) is an E3-ubiquitin ligase that regulates cell signaling pathways involved in fundamental cellular processes including cell death, cell proliferation, cell differentiation and inflammation. It recruits ubiquitination substrates thanks to the presence of three baculoviral IAP repeat (BIR) domains at its N-terminal extremity. We previously demonstrated that cIAP1 promoted the ubiquitination of the E2 factor 1 (E2F1) transcription factor. Moreover, we showed that cIAP1 was required for E2F1 stabilization during the S phase of cell cycle and in response to DNA damage. Here, we report that E2F1 binds within the cIAP1 BIR3 domain. The BIR3 contains a surface hydrophobic groove that specifically anchors a conserved IAP binding motif (IBM) found in a number of intracellular proteins including Smac. The Smac N-7 peptide that includes the IBM, as well as a Smac mimetic, competed with E2F1 for interaction with cIAP1 demonstrating the importance of the BIR surface hydrophobic groove. We demonstrated that the first alpha-helix of BIR3 was required for E2F1 binding, as well as for the binding of Smac and Smac mimetics. Overexpression of cIAP1 modified the ubiquitination profile of E2F1, increasing the ratio of E2F1 conjugated with K11- and K63-linked ubiquitin chains, and decreasing the proportion of E2F1 modified by K48-linked ubiquitin chains. ChIP-seq analysis demonstrated that cIAP1 was required for the recruitment of E2F1 onto chromatin. Lastly, we identified an E2F-binding site on the cIAP1-encoding birc2 gene promoter, suggesting a retro-control regulation loop.

Correction: DNA damage and S phase-dependent E2F1 stabilization requires the cIAP1 E3-ubiquitin ligase and is associated with K63-poly-ubiquitination on lysine 161/164 residues.

Correction to:Cell Death & Disease8, e2816 (2017); https://doi.org/10.1038/cddis.2017.222 ; published online 25 May 2017.

DNA damage and S phase-dependent E2F1 stabilization requires the cIAP1 E3-ubiquitin ligase and is associated with K63-poly-ubiquitination on lysine 161/164 residues.

The E2F transcription factor 1 is subtly regulated along the cell cycle progression and in response to DNA damage by post-translational modifications. Here, we demonstrated that the E3-ubiquitin ligase cellular inhibitor of apoptosis 1 (cIAP1) increases E2F1 K63-poly-ubiquitination on the lysine residue 161/164 cluster, which is associated with the transcriptional factor stability and activity. Mutation of these lysine residues completely abrogates the binding of E2F1 to CCNE, TP73 and APAF1 promoters, thus inhibiting transcriptional activation of these genes and E2F1-mediated cell proliferation control. Importantly, E2F1 stabilization in response to etoposide-induced DNA damage or during the S phase of cell cycle, as revealed by cyclin A silencing, is associated with K63-poly-ubiquitinylation of E2F1 on lysine 161/164 residues and involves cIAP1. Our results reveal an additional level of regulation of the stability and the activity of E2F1 by a non-degradative K63-poly-ubiquitination and uncover a novel function for the E3-ubiquitin ligase cIAP1.