Domain organization of XAF1 and the identification and characterization of XIAP(RING) -binding domain of XAF1

XAF1 promotes colorectal cancer metastasis via VCP-RNF114-JUP axis

Metastasis is the main cause of colorectal cancer (CRC)-related death, and the 5-year relative survival rate for CRC patients with distant metastasis is only 14%. X-linked inhibitor of apoptosis (XIAP)-associated factor 1 (XAF1) is a zinc-rich protein belonging to the interferon (IFN)-induced gene family. Here, we report a metastasis-promoting role of XAF1 in CRC by acting as a novel adaptor of valosin-containing protein (VCP). XAF1 facilitates VCP-mediated deubiquitination of the E3 ligase RING finger protein 114 (RNF114), which promotes K48-linked ubiquitination and subsequent degradation of junction plakoglobin (JUP). The XAF1-VCP-RNF114-JUP axis is critical for the migration and metastasis of CRC cells. Moreover, we observe correlations between the protein levels of XAF1, RNF114, and JUP in clinical samples. Collectively, our findings reveal an oncogenic function of XAF1 in mCRC and suggest that the XAF1-VCP-RNF114-JUP axis is a potential therapeutic target for CRC treatment.

Molecular identification and functional analysis of X-linked inhibitor of apoptosis -associated factor-1 (XAF1) in grass carp, Ctenopharyngodon idella

X-linked inhibitor of apoptosis protein (XIAP) -associated factor 1 (XAF1) is an interferon-stimulated gene which exhibits pro-apoptosis effect. In this study, XAF1 was characterized from grass carp Ctenopharyngodon idella and its expression pattern and function were analyzed. The open reading frame (orf) of XAF1 is 789 nucleotides (nt) encoding 262 amino acids. SMART online search results showed that a C2H2-type and six C2HC-type zinc-fingers were found in XAF1, however, the XAF1 of grass carp showed high sequence identity to zebrafish (71%), low sequence identity to tetrapods (21-22%). Rt-qPCR results showed that XAF1 was constitutively expressed in all tested organs/tissues with highest expression in blood. An inductive expression of XAF1 at mRNA level was observed in peripheral blood leucocytes (PBLs) and C. idellus kidney cells (CIKs) after treatment with C. idellus recombinant interferon-γ (rIFNg). Overexpressing XAF1 in CIKs exhibited resistance against grass carp reovirus (GCRV) and more sensitivity to cisplatin. These results implied a functional homologue of XAF1 in evolution, however the mechanism may require further investigation.

Subcellular localization of X-linked inhibitor of apoptosis protein (XIAP) in cancer: does that matter?

X-linked inhibitor of apoptosis protein (XIAP) finely tunes the balance between survival and death to control homeostasis. XIAP is found aberrantly expressed in cancer, which has been shown to promote resistance to therapy-induced apoptosis and confer poor outcome. Despite its predominant cytoplasmic localization in human tissues, growing evidence implicates the expression of XIAP in other subcellular compartments in sustaining cancer hallmarks. Herein, we review our current knowledge on the prognostic role of XIAP localization and discuss molecular mechanisms underlying differential biological functions played in each compartment. The comprehension of XIAP subcellular shuttling and functional dynamics might provide the rationale for future anticancer therapeutics.

Passenger Mutations Confound Phenotypes of SARM1-Deficient Mice

The Toll/IL-1R-domain-containing adaptor protein SARM1 is expressed primarily in the brain, where it mediates axonal degeneration. Roles for SARM1 in TLR signaling, viral infection, inflammasome activation, and chemokine and Xaf1 expression have also been described. Much of the evidence for SARM1 function relies on SARM1-deficient mice generated in 129 ESCs and backcrossed to B6. The Sarm1 gene lies in a gene-rich region encompassing Xaf1 and chemokine loci, which remain 129 in sequence. We therefore generated additional knockout strains on the B6 background, confirming the role of SARM1 in axonal degeneration and WNV infection, but not in VSV or LACV infection, or in chemokine or Xaf1 expression. Sequence variation in proapoptotic Xaf1 between B6 and 129 results in coding changes and distinct splice variants, which may account for phenotypes previously attributed to SARM1. Reevaluation of phenotypes in these strains will be critical for understanding the function of SARM1.

Paradoxical epigenetic regulation of XAF1 mediates plasticity towards adaptive resistance evolution in MGMT-methylated glioblastoma

Epigenetic regulation of O⁶-alkylguanine DNA alkyltransferase (MGMT) is surrogate of intrinsic resistance to temozolomide (TMZ). However, mechanisms associated with adaptive resistance evolution of glioblastoma (GBM) relative to MGMT methylation remain unclear. We hereby report a paradoxical yet translational epigenetic regulation of plasticity towards adaptive resistance in GBM. Based on an adaptive resistance model of GBM cells with differential MGMT methylation profiles, MGMT-hypermethylation enhanced genetic and phenotypic plasticity towards adaptive resistance to TMZ while MGMT hypomethylation limited plasticity. The resulting model-associated adaptive resistance gene signature negatively correlated with GBM patient survival. XAF1, a tumor suppressor protein, paradoxically emerged as a mediator of differential plasticities towards adaptive resistance to TMZ through epigenetic regulation. XAF1 promoted resistance both in-vitro and in-vivo. Furthermore, XAF1 expression negatively correlated with XAF1 promoter methylation status, and negatively correlate with GBM patient survival. Collectively, XAF1 appears to have a pradoxical yet translational role in GBM.

A commercial Roundup® formulation induced male germ cell apoptosis by promoting the expression of XAF1 in adult mice

Roundup® is extensively used for weed control worldwide. Residues of this compound may lead to side effects of the male reproductive system. However, the toxic effects and mechanisms of Roundup® of male germ cells remain unclear. We aimed to investigate the apoptosis-inducing effects of Roundup® on mouse male germ cells and explore the role of a novel tumor suppressor XAF1 (X-linked inhibitor of apoptosis-associated factor 1) involved in this process. We demonstrated that Roundup® can impair spermatogenesis, decrease sperm motility and concentration, and increase the sperm deformity rate in mice. In addition, excessive apoptosis of germ cells accompanied by the overexpression of XAF1 occurred after Roundup® exposure both in vitro and in vivo. Furthermore, the low expression of XIAP (X-linked inhibitor of apoptosis) induced by Roundup® was inversely correlated with XAF1. Moreover, the knockdown of XAF1 attenuated germ cell apoptosis, improved XIAP expression and inhibited the activation of its downstream target proteins, caspase-3 and PARP, after Roundup® exposure. Taken together, our data indicated that XAF1 plays an important role in Roundup®-induced male germ cell apoptosis. The present study suggested that Roundup® exposure has potential negative implications on male reproductive health in mammals.

XAF1 forms a positive feedback loop with IRF-1 to drive apoptotic stress response and suppress tumorigenesis

X-linked inhibitor of apoptosis (XIAP)-associated factor 1 (XAF1) is a proapoptotic tumor suppressor that is frequently inactivated in multiple human cancers. However, the molecular basis for the XAF1-mediated growth inhibition remains largely undefined. Here, we report that XAF1 forms a positive feedback loop with interferon regulatory factor-1 (IRF-1) and functions as a transcriptional coactivator of IRF-1 to suppress tumorigenesis. Under various stressful conditions, XAF1 transcription is activated by IRF-1, and elevated XAF1 stabilizes and activates IRF-1. Mechanistically, XAF1 binds to the multifunctional domain 2 of IRF-1 via the zinc finger domain 6, thereby hindering C-terminus of Hsc70-interacting protein (CHIP) interaction with and ubiquitination of IRF-1. Activation of the IRF-1−XAF1 loop greatly increases stress-induced apoptosis and decreases the invasive capability of tumor cells. Oncogenic Ras and growth factors interfere with the IRF-1−XAF1 interplay via Erk-mediated repression of XAF1 transcription. Furthermore, XAF1 enhances IRF-1-mediated transcription of proapoptotic genes via the XAF1-IRF-1 complex formation on these target promoters. Meanwhile, XAF1 inhibits NF-κB-mediated tumor cell malignancy by reinforcing IRF-1 binding to a subset of coregulated promoters. Expression levels of IRF-1 and XAF1 correlate tightly in both cancer cell lines and primary tumors, and XAF1-induced tumor regression is markedly attenuated in IRF-1-depleted tumors. Collectively, this study identifies a novel mechanism of XAF1-mediated tumor suppression, uncovering XAF1 as a feedback coactivator of IRF-1 under stressful conditions.

Talaromyces marneffei Mp1p Is a Virulence Factor that Binds and Sequesters a Key Proinflammatory Lipid to Dampen Host Innate Immune Response

Talaromyces (Penicillium) marneffei is one of the leading causes of systemic mycosis in immunosuppressed or AIDS patients in Southeast Asia. How this intracellular pathogen evades the host immune defense remains unclear. We provide evidence that T. marneffei depletes levels of a key proinflammatory lipid mediator arachidonic acid (AA) to evade the host innate immune defense. Mechanistically, an abundant secretory mannoprotein Mp1p, shown previously to be a virulence factor, does so by binding AA with high affinity via a long hydrophobic central cavity found in the LBD2 domain. This sequesters a critical proinflammatory signaling lipid, and we see evidence that AA, AA's downstream metabolites, and the cytokines interleukin-6 and tumor necrosis factor α are downregulated in T. marneffei-infected J774 macrophages. Given that Mp1p-LBD2 homologs are identified in other fungal pathogens, we expect that this novel class of fatty-acid-binding proteins sequestering key proinflammatory lipid mediators represents a general virulence mechanism of pathogenic fungi.

Aberrant splicing and drug resistance in AML

The advent of next-generation sequencing technologies has unveiled a new window into the heterogeneity of acute myeloid leukemia (AML). In particular, recurrent mutations in spliceosome machinery and genome-wide aberrant splicing events have been recognized as a prominent component of this disease. This review will focus on how these factors influence drug resistance through altered splicing of tumor suppressor and oncogenes and dysregulation of the apoptotic signaling network. A better understanding of these factors in disease progression is necessary to design appropriate therapeutic strategies recognizing specific alternatively spliced or mutated oncogenic targets.

Epigenetic silencing of the XAF1 gene is mediated by the loss of CTCF binding

XAF1 is a tumour suppressor gene that compromises cell viability by modulating different cellular events such as mitosis, cell cycle progression and apoptosis. In cancer, the XAF1 gene is commonly silenced by CpG-dinucleotide hypermethylation of its promoter. DNA demethylating agents induce transcriptional reactivation of XAF1, sensitizing cancer cells to therapy. The molecular mechanisms that mediate promoter CpG methylation have not been previously studied. Here, we demonstrate that CTCF interacts with the XAF1 promoter in vivo in a methylation-sensitive manner. By transgene assays, we demonstrate that CTCF mediates the open-chromatin configuration of the XAF1 promoter, inhibiting both CpG-dinucleotide methylation and repressive histone posttranslational modifications. In addition, the absence of CTCF in the XAF1 promoter inhibits transcriptional activation induced by well-known apoptosis activators. We report for the first time that epigenetic silencing of the XAF1 gene is a consequence of the loss of CTCF binding.

Enhancement of cellular radiation sensitivity through degradation of Chk1 by the XIAP-XAF1 complex

X-linked inhibitor of apoptosis (XIAP) and Chk1 are potential molecular targets in radiotherapy. However, their molecular association in the regulation of radiation sensitivity has been rarely studied. Here, we show that XIAP modulates radiation sensitivity by regulating stability of Chk1 in lung cancer cells. Both Chk1 and XIAP are highly expressed in various lung cancer cells. Overexpression of XIAP increased cell survival following genotoxic treatments by preventing downregulation of Chk1. However, XIAP reversed Chk1-protective activity in the presence of XIAP-associated factor 1 (XAF1) by degrading Chk1 via ubiquitination-dependent proteasomal proteolysis. The XIAP-XAF1 complex-mediated Chk1 degradation also required CUL4A and DDB1. Chk1 or XIAP was associated with DDB1 and CUL4A. Depletion of CUL4A or DDB1 prevented the XIAP-XAF1-mediated Chk1 degradation suggesting involvement of a CUL4A/DDB1-based E3 ubiquitin ligase in the process or its collaboration with XIAP E3 ligase activity. Taken together, our findings show that XIAP plays a dual role in modulation of Chk1 stability and cell viability following IR. In the absence of XAF1, XIAP stabilizes Chk1 under IR with corresponding increase of cell viability. By contrast, when XAF1 is overexpressed, XIAP facilitates Chk1 degradation, which leads to enhancement of radiation sensitivity. This selective regulation of Chk1 stability by XIAP and XAF1 could be harnessed to devise a strategy to modulate radiation sensitivity in lung cancer cells.

p53 mediated apoptosis in osteosarcoma MG-63 cells by inhibition of FANCD2 gene expression

PURPOSE

The aim of this study was to investigate the association between osteosarcoma (OS) and Fanconi anemia (FA) related pathways and the molecular mechanisms.

METHODS

siRNA for Fanconi anemia complementation group D2 (FANCD2) was constructed and transfected into the osteosarcoma cell line MG-63 cells. Expression of TP53INP1, p53, p21, caspase-9, and caspase-3 mRNA in MG-63 cells were examined by real-time fluorescence quantitative PCR, and the protein levels were also determined by western blot.

RESULTS

After silence of the FANCD2 gene in MG-63 cells, cell proliferation was inhibited, cell cycle was arrested and cell apoptosis was induced. The apoptosis was mediated by the p53 signaling pathway. After FANCD2 expression was inhibited, TP53INP1 gene expression was up-regulated, phosphorylation of p53 was promoted and the p21 protein was activated, leading to cell cycle arrested in G1, finally resulted in caspase-dependent cell apoptosis.

CONCLUSIONS

Inhibition of FANCD2 gene expression can induce apoptosis of osteosarcoma cells, which indicated that FANCD2 played an important role in the development of osteosarcoma and it might be a potential target for treatment of osteosarcoma.

On-line coupling of continuous-flow gel electrophoresis with inductively coupled plasma-mass spectrometry to quantitatively evaluate intracellular metal binding properties of metallochaperones HpHypA and HpHspA in E. coli cells

On-line coupling of gel electrophoresis with inductively coupled plasma-mass spectrometry (GE-ICP-MS) offers a strategy to monitor intracellular metals and their associated proteins simultaneously. Herein, we examine the feasibility of the GE-ICP-MS system in the quantitative analysis of intracellular metal binding properties using two Helicobacter pylori metallochaperones HypA and HspA overexpressed in E. coli cells as showcases. We show that parallel detection of metal and sulfur signals allows accurate quantification of intracellular metal-protein stoichiometries, even for metalloproteins that bind metal ions with micromolar affinities. Using this approach, we demonstrate that only a trace amount of Ni(2+) is associated with HpHypA in cells, distinct from the in vitro observation of stoichiometric binding, while HpHypA exhibits high fidelity towards its structural metal Zn(2+) with stoichiometric Zn(2+) binding. In contrast, HpHspA associates with Zn(2+), Ni(2+), Cu(2+) and Co(2+) from an essential metal pool with ca. 0.5 molar equivalents of total metals bound per HpHspA monomer. The metal binding properties of both HpHypA and HpHspA were altered by Bi(3+). The binding of both Zn(2+) and Ni(2+) to HpHypA was suppressed under the stress of Bi(3+) in cells, different from in vitro studies that showed that Bi(3+) interfered with Zn(2+) but not Ni(2+) binding. This study provides an analytical approach to investigate the intracellular metal selectivity of overexpressed metalloproteins.

Regulation of ubiquitin transfer by XIAP, a dimeric RING E3 ligase

RING domains of E3 ligases promote transfer of Ub (ubiquitin) from the E2~Ub conjugate to target proteins. In many cases interaction of the E2~Ub conjugate with the RING domain requires its prior dimerization. Using cross-linking experiments we show that E2 conjugated ubiquitin contacts the RING homodimer interface of the IAP (inhibitor of apoptosis) proteins, XIAP (X-linked IAP) and cIAP (cellular IAP) 2. Structural and biochemical analysis of the XIAP RING dimer shows that an aromatic residue at the dimer interface is required for E2~Ub binding and Ub transfer. Mutation of the aromatic residue abolishes Ub transfer, but not interaction with Ub. This indicates that nuleophilic attack on the thioester bond depends on precise contacts between Ub and the RING domain. RING dimerization is a critical activating step for the cIAP proteins; however, our analysis shows that the RING domain of XIAP forms a stable dimer and its E3 ligase activity does not require an activation step.