Expression and genetic analysis of XIAP-associated factor 1 (XAF1) in cancer cell lines

XAF1 overexpression inhibits the malignant progression and cisplatin resistance of NSCLC by activating endoplasmic reticulum stress

BACKGROUND

XIAP-associated factor 1 (XAF1) has been found to participate in the progression of multiple human cancers. Nevertheless, its role as well as the reaction mechanism in non-small cell lung cancer (NSCLC) still remains obscure.

METHODS

In this study, the protein expression of XAF1 in NSCLC cell lines was evaluated using western blot. With the employment of CCK-8 assay, EdU staining, wound healing and transwell, capabilities of NSCLC cells to proliferate, migrate and invade were assessed. Cell apoptotic level and cell cycle were resolved utilizing flow cytometry. Western blot was applied for the estimation of apoptosis- and endoplasmic reticulum (ER) stress-related proteins.

RESULTS

It was discovered that XAF1 expression was conspicuously reduced in NSCLC cell lines. XAF1 overexpression suppressed H1299 cell proliferative, invasive and migrative capabilities, but exhibited promotive effects on cell cycle arrest. Meanwhile, XAF1 overexpression inhibited cisplatin resistance in H1299 and H1299/DDP cells by promoting cell apoptosis and enhanced the expression levels of ER stress-related proteins CHOP, GRP78 and ATF4. What's more, 4-PBA treatment reversed the impacts of XAF1 overexpression on the proliferative, invasive, migrative and apoptotic capabilities of H1299 cells, as well as cell cycle and cisplatin resistance.

CONCLUSION

In conclusion, XAF1 overexpression impeded the advancement of NSCLC and repressed cisplatin resistance of NSCLC cells through inducing ER stress, which indicated that XAF1 might be a novel targeted-therapy for NSCLC.

XAF1 promotes anti-RNA virus immune responses by regulating chromatin accessibility

A rapid induction of antiviral genes is critical for eliminating viruses, which requires activated transcription factors and opened chromatins to initiate transcription. However, it remains elusive how the accessibility of specific chromatin is regulated during infection. Here, we found that XAF1 functioned as an epigenetic regulator that liberated repressed chromatin after infection. Upon RNA virus infection, MAVS recruited XAF1 and TBK1. TBK1 phosphorylated XAF1 at serine-252 and promoted its nuclear translocation. XAF1 then interacted with TRIM28 with the guidance of IRF1 to the specific locus of antiviral genes. XAF1 de-SUMOylated TRIM28 through its PHD domain, which led to increased accessibility of the chromatin and robust induction of antiviral genes. XAF1-deficient mice were susceptible to RNA virus due to impaired induction of antiviral genes. Together, XAF1 acts as an epigenetic regulator that promotes the opening of chromatin and activation of antiviral immunity by targeting TRIM28 during infection.

Anti-microRNA-1976 as a Novel Approach to Enhance Chemosensitivity in XAF1+ Pancreatic and Liver Cancer

The current cancer treatments using chemoagents are not satisfactory in terms of outcomes and prognosis. Chemoagent treatments result in cell death or arrest, but the accompanying cellular responses are not well-studied. Exosomes, which are extracellular vesicles secreted by living cells, might mediate cellular responses through microRNAs. We found that miR-1976 was highly enriched in exosomes secreted after chemoagent treatment. We developed a novel approach for in situ mRNA target screening and discovered several miR-1976-specific mRNA targets, including the proapoptotic gene XAF1, which was targeted by miR-1976 and which suppressed chemoagent-induced cell apoptosis. Increased RPS6KA1 gene transcription was associated with the increase in its intronic pre-miR-1976 expression. Blockade of miR-1976 could enhance chemosensitivities of hepatoma and pancreatic cancer cells in an XAF1-dependent manner, as evidenced by increased levels of cell apoptosis, reduced IC50 in cell toxicity assays, and suppressed tumor growth in animal xenograft experiments in vivo. We propose that intracellular levels of miR-1976 determine chemosensitivity, and its blockade could be a novel strategy and potential therapeutic application in cancer 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.

XAF1 drives apoptotic switch of endoplasmic reticulum stress response through destabilization of GRP78 and CHIP

X-linked inhibitor of apoptosis-associated factor-1 (XAF1) is a stress-inducible tumor suppressor that is commonly inactivated in many human cancers. Despite accumulating evidence for the pro-apoptotic role for XAF1 under various stressful conditions, its involvement in endoplasmic reticulum (ER) stress response remains undefined. Here, we report that XAF1 increases cell sensitivity to ER stress and acts as a molecular switch in unfolded protein response (UPR)-mediated cell-fate decisions favoring apoptosis over adaptive autophagy. Mechanistically, XAF1 interacts with and destabilizes ER stress sensor GRP78 through the assembly of zinc finger protein 313 (ZNF313)-mediated destruction complex. Moreover, XAF1 expression is activated through PERK-Nrf2 signaling and destabilizes C-terminus of Hsc70-interacting protein (CHIP) ubiquitin E3 ligase, thereby blocking CHIP-mediated K63-linked ubiquitination and subsequent phosphorylation of inositol-required enzyme-1α (IRE1α) that is involved in in the adaptive ER stress response. In tumor xenograft assays, XAF1^-/- tumors display substantially lower regression compared to XAF1^+/+ tumors in response to cytotoxic dose of ER stress inducer. XAF1 and GRP78 expression show an inverse correlation in human cancer cell lines and primary breast carcinomas. Collectively this study uncovers an important role for XAF1 as a linchpin to govern the sensitivity to ER stress and the outcomes of UPR signaling, illuminating the mechanistic consequence of XAF1 inactivation in tumorigenesis.

PBRM1 Deficiency Sensitizes Renal Cancer Cells to DNMT Inhibitor 5-Fluoro-2'-Deoxycytidine

PBRM1 is a tumor suppressor frequently mutated in clear cell renal cell carcinoma. However, no effective targeted therapies exist for ccRCC with PBRM1 loss. To identify novel therapeutic approaches to targeting PBRM1-deficient renal cancers, we employed a synthetic lethality compound screening in isogenic PBRM1+/+ and PBRM1-/- 786-O renal tumor cells and found that a DNMT inhibitor 5-Fluoro-2’-deoxycytidine (Fdcyd) selectively inhibit PBRM1-deficient tumor growth. RCC cells lacking PBRM1 show enhanced DNA damage response, which leads to sensitivity to DNA toxic drugs. Fdcyd treatment not only induces DNA damage, but also re-activated a pro-apoptotic factor XAF1 and further promotes the genotoxic stress-induced PBRM1-deficient cell death. This study shows a novel synthetic lethality interaction between PBRM1 loss and Fdcyd treatment and indicates that DNMT inhibitor represents a novel strategy for treating ccRCC with PBRM1 loss-of-function mutations.

Review of cancer cell resistance mechanisms to apoptosis and actual targeted therapies

The apoptosis pathway is a programmed cell death mechanism that is crucial for cellular and tissue homeostasis and organ development. There are three major caspase-dependent pathways of apoptosis that ultimately lead to DNA fragmentation. Cancerous cells are known to highly regulate the apoptotic pathway and its role in cancer hallmark acquisition has been discussed over the past decades. Numerous mutations in cancer cell types have been reported to be implicated in chemoresistance and treatment outcome. In this review, we summarize the mutations of the caspase-dependant apoptotic pathways that are the source of cancer development and the targeted therapies currently available or in trial.

The SMAC mimetic AT-101 exhibits anti-tumor and anti-metastasis activity in lung adenocarcinoma cells by the IAPs/ caspase-dependent apoptosis and p65-NFƙB cross-talk

Objective(s):

The Inhibitors of Apoptosis (IAPs) regulate initiator and effector phases of caspase mediated apoptosis. This study evaluates the effects of SMAC mimetic AT-101 in regulation of IAPs/caspases/NFƙB-p65 in an adenocarcinoma cell line.

Materials and Methods:

MTT assay was performed in the NCI-H522 cell line. Flow cytometry was used for detecting cell cycle, apoptosis, and NFƙB-p65 regulation. Effects of AT-101 on IAPs and caspases were determined by quantitative real time-PCR and western blotting. AutoDock-VINA was used for computational analysis.

Results:

AT-101 reduced the cell proliferation of NCI-H522 with a GI50 value of 7 μM. The compound arrested adenocarcinoma cells in the G1 phase of the cell cycle and increased early and late phase apoptosis while decreasing tumor-cell trans-migration. AT-101 treatment to NCI H522 at a concentration of 0.35 μM decreased XIAP, cIAP-1, and cIAP-2 mRNA levels to 4.39±0.66, 1.93±0.26, and 2.20±0.24 folds, respectively. Increased dose of AT-101 at 0.7 μM concentration further decreased XIAP, cIAP-1, and cIAP-2 mRNA levels to 2.44±0.67, 1.46±0.93, and 0.97±0.10 folds, respectively. Similar effects of a dose-dependent decrease in the protein expressions of XIAP, cIAP-1, and cIAP-2 were observed with AT-101 treatments, while a dose-responsive increase in the mRNA and protein expression levels of caspase 6 and caspase 7 was observed in the NCI-H522 cell line. The compound exhibited binding affinity (-6.1 kcal/mol) and inhibited NFƙB-p65 in these cells.

Conclusion:

AT-101 had anti-tumor efficacy against lung adenocarcinoma cells which could be mediated through IAPs/caspase-dependent apoptosis and NFƙB-p65 cross talk. Results from this study suggests a signal cross talk between IAPs and NFkB and open new channels for further investigations in therapeutic intervention against lung cancer management.

Silencing of PGK1 Promotes Sensitivity to Paclitaxel Treatment by Upregulating XAF1-Mediated Apoptosis in Triple-Negative Breast Cancer

Objective: Triple negative breast cancer (TNBC) is known to have aggressive clinical course and a high risk of recurrence. Given the lack of effective targeted therapy options, paclitaxel-based chemotherapy is still the primary option for TNBC patients. However, patients who fail to achieve a complete response during neoadjuvant chemotherapy may be mainly due to sensitivity and resistance to chemotherapy. Thus, we concentrated the present research on the role of PGK1 in the sensitivity to paclitaxel treatment and the possible underlying mechanisms in TNBC.

Methods: After exposure to paclitaxel, a cell viability analysis was made to investigate the influence of PGK1 silencing on cell death. The effect of PGK1 on apoptosis induced by paclitaxel treatment was examined in vitro by flow cytometry cell apoptosis assays. Western blotting was performed to examine the impact of PGK1 on paclitaxel-induced apoptosis. The correlation of PGK1 with apoptosis-associated protein X-linked inhibitor of apoptosis (XIAP)-associated factor 1 (XAF1) was analyzed in 39 specimens by immunohistochemistry analysis.

Results: We observed that silencing PGK1 sensitized triple-negative breast cancer (TNBC) cell lines to paclitaxel treatment as a result of increased drug-induced apoptosis. Furthermore, mechanistic investigations suggested that XAF1 was increased in PGK1-knockdown cells along with the expression of the apoptotic proteins including cleaved caspase-3 and Bax. Immunohistochemistry analysis showed that PGK1 was negatively related to XAF1. Moreover, we found that downregulation of XAF1 reduced paclitaxel-induced apoptosis in PGK1-silenced triple-negative cell lines.

Conclusion: Our results identified PGK1 as a potential biomarker for the treatment of TNBC, and inhibition of PGK1 expression might represent a novel strategy to sensitize TNBC to paclitaxel treatment.

RING-finger protein 166 plays a novel pro-apoptotic role in neurotoxin-induced neurodegeneration via ubiquitination of XIAP

The dopaminergic neurotoxin, 6-hydroxydopamine (6-OHDA), has been widely utilized to establish experimental models of Parkinson disease and to reveal the critical molecules and pathway underlying neuronal death. The profile of gene expression changes following 6-OHDA treatment of MN9D dopaminergic neuronal cells was investigated using a TwinChip Mouse-7.4K microarray. Functional clustering of altered sets of genes identified RING-finger protein 166 (RNF166). RNF166 is composed of an N-terminal RING domain and C-terminal ubiquitin interaction motif. RNF166 localized in the cytosol and nucleus. At the tissue level, RNF166 was widely expressed in the central nervous system and peripheral organs. In the cerebral cortex, its expression decreased over time. In certain conditions, overexpression of RNF166 accelerates the naturally occurring neuronal death and 6-OHDA-induced MN9D cell death as determined by TUNEL and annexin-V staining, and caspase activation. Consequently, 6-OHDA-induced apoptotic cell death was attenuated in RNF166-knockdown cells. In an attempt to elucidate the mechanism underlying this pro-apoptotic activity, binding protein profiles were assessed using the yeast two-hybrid system. Among several potential binding candidates, RNF166 was shown to interact with the cytoplasmic X-linked inhibitor of apoptosis (XIAP), inducing ubiquitin-dependent degradation of XIAP and eventually accelerating caspase activation following 6-OHDA treatment. RNF166's interaction with and resulting inhibition of the XIAP anti-caspase activity was further enhanced by XIAP-associated factor-1 (XAF-1). Consequently, depletion of RNF166 suppressed 6-OHDA-induced caspase activation and apoptotic cell death, which was reversed by XIAP knockdown. In summary, our data suggest that RNF166, a novel E3 ligase, plays a pro-apoptotic role via caspase activation in neuronal cells.

XAF1 as a modifier of p53 function and cancer susceptibility

Cancer risk is highly variable in carriers of the common TP53-R337H founder allele, possibly due to the influence of modifier genes. Whole-genome sequencing identified a variant in the tumor suppressor XAF1 (E134*/Glu134Ter/rs146752602) in a subset of R337H carriers. Haplotype-defining variants were verified in 203 patients with cancer, 582 relatives, and 42,438 newborns. The compound mutant haplotype was enriched in patients with cancer, conferring risk for sarcoma (P = 0.003) and subsequent malignancies (P = 0.006). Functional analyses demonstrated that wild-type XAF1 enhances transactivation of wild-type and hypomorphic TP53 variants, whereas XAF1-E134* is markedly attenuated in this activity. We propose that cosegregation of XAF1-E134* and TP53-R337H mutations leads to a more aggressive cancer phenotype than TP53-R337H alone, with implications for genetic counseling and clinical management of hypomorphic TP53 mutant carriers.

A tumour-selective cascade activatable self-detained system for drug delivery and cancer imaging

Achieving the activation of drugs within cellular systems may provide targeted therapies. Here we construct a tumour-selective cascade activatable self-detained system (TCASS) and incorporate imaging probes and therapeutics. We show in different mouse models that the TCASS system accumulates in solid tumours. The molecules show enhanced accumulation in tumour regions via the effect of recognition induced self-assembly. Analysis of the molecular penetration in tumour tissue shows that in vivo self-assembly increases the penetration capability compared to typical soft or hard nanomaterials. Importantly, the in vivo self-assembled molecules exhibit a comparable clearance pathway to that of small molecules, which are excreted from organs of the reticuloendothelial system (liver and kidney), while are relatively slowly eliminated from tumour tissues. Finally, this system, combined with the NIR probe, shows high specificity and sensitivity for detecting bladder cancer in isolated intact patient bladders.

The activation of drugs within cellular systems may provide targeted therapies for cancer. Here, the authors make a drug delivery system that is activated within the cell and exploits XIAP expression to cleave a linker region, resulting in the self-assembly of the system and drug release within cancer cells.

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.

Combined effects of XAF1 and TRAIL on the apoptosis of lung adenocarcinoma cells

This study aimed to investigate the effects and mechanisms of X-linked inhibitor of apoptosis protein (XIAP)-associated factor 1 (XAF1) and tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) on the apoptosis of A549 lung adenocarcinoma cell lines. Recombinant lentiviral vector of Ad5/F35-XAF1 and controlled lentiviral vector of Ad5/F35-Null were transfected into A549 cells at same multiplicity of infection (MOI), respectively. Based on whether recombinant human TRAIL (rhTRAIL) was added or not, cells were divided into different groups as follows: XAF1 group, XAF1 + TRAIL group, XAF1-Null group, and XAF1-Null + TRAIL group. Following culturing for 48 h, the mRNA and protein expression levels of related genes were determined by reverse transcription-quantitative polymerase chain reaction and western blotting analyses, respectively. Cell proliferationand cell apoptosis were detected by MTT assay and Annexin V-FITC/PI double staining, respectively. Xenograft mice models were established with A549 lung adenocarcinoma cells and treated with recombinant virus Ad5/F35-XAF1 and controlled virus Ad5/F35-Null for immunohistochemical analysis. Expression levels of XAFl at the mRNA and protein levels were significantly higher in the XAF1 group and XAF1 + TRAIL groups when compared with the levels in the other groups (P<0.05). Cleavage of apoptosis-associated proteins, poly ADP-ribose polymerase and caspase-3, was noted in the XAF1 + TRAIL group, whereas they were not detected in other groups. Apoptosis rates of A549 cells in the XAF1, Null + TRAIL and XAFl + TRAIL groups were significantly higher than those in the NOR and Null groups (P<0.05). Apoptotic rates were highest in the XAF1 + TRAIL group. In conclusion, these findings suggest that combined use of XAF1 and TRAIL may synergistically induce the apoptosis of A549 lung adenocarcinoma cells.

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.

XIAP RING domain mediates miR-4295 expression and subsequently inhibiting p63α protein translation and promoting transformation of bladder epithelial cells

The X-linked inhibitor of apoptosis protein (XIAP) contains three N-terminal BIR domains that mediate anti-apoptosis and one C-terminal RING finger domain whose function(s) are not fully defined. Here we show that the RING domain of XIAP strongly inhibits the expression of p63α, a known tumor suppressor. XIAP knockdown in urothelial cells or RING deletion in knockin mice markedly upregulates p63α expression. This RING-mediated p63α downregulation is critical for the malignant transformation of normal urothelial cells following EGF treatment. We further show that the RING domain promotes Sp1-mediated transcription of miR-4295 which targets the 3′UTR of p63α mRNA and consequently inhibits p63α translation. Our results reveal a previously unknown function of the RING of XIAP in promoting miR-4295 transcription, thereby reducing p63α translation and enhancing urothelial transformation. Our data offer novel insights into the multifunctional effects of the XIAP RING domain on urothelial tumorigenesis and the potential for targeting this frequently overexpressed protein as a therapeutic alternative.

Ampelopsin-induced reactive oxygen species enhance the apoptosis of colon cancer cells by activating endoplasmic reticulum stress-mediated AMPK/MAPK/XAF1 signaling

Ampelopsin (Amp) is bioactive natural product and exerts anti-cancer effects against several cancer types. The present study investigated the anti-colon cancer activity of Amp and explored its mechanism of action. The treatment of colon cancer cells with Amp resulted in the dose- and time-dependent induction of apoptosis via the activation of endoplasmic reticulum (ER) stress, 5' adenosine monophosphate-activated protein kinase (AMPK), and c-Jun N-terminal protein kinase (JNK)/p38 mitogen-activated protein kinases (MAPKs). Salubrinal, an ER stress inhibitor, prevented the upregulation of ER stress-associated proteins, including phosphorylated protein kinase RNA-like ER kinase, phosphorylated eukaryotic translation initiation factor 2α, glucose-regulated protein 78, and CCAAT/enhancer-binding protein homologous protein, as well as suppressing AMPK activation and the MAPK signaling pathway. Knockdown of AMPK by RNA interference failed to block ER stress. Additionally, SP600125 (a JNK inhibitor) and SB203580 (a p38-MAPK inhibitor) effectively inhibited apoptosis and attenuated the expression of X-linked IAP-associated factor 1 (XAF1) and apoptotic Bcl-2 family proteins (BCL2 antagonist/killer 1 and BCL2-associated X protein) in Amp-treated colon cancer cells. Furthermore, reactive oxygen species (ROS)-mediated ER stress/AMPK apoptotic signaling pathway in Amp-treated colon cancer cells were markedly inhibited by treatment with N-acetyl-L-cysteine, a ROS scavenger. These results demonstrate that treatment with Amp induces the apoptotic death of colon cancer cells through ER stress-initiated AMPK/MAPK/XAF1 signaling. These results also provide experimental information for developing Amp as therapeutic drug against colon cancer.

Identification of XAF1-MT2A mutual antagonism as a molecular switch in cell-fate decisions under stressful conditions

XIAP-associated factor 1 (XAF1) is a tumor suppressor that is commonly inactivated in multiple human neoplasms. However, the molecular mechanism underlying its proapoptotic function remains largely undefined. Here, we report that XAF1 induction by heavy metals triggers an apoptotic switch of stress response by destabilizing metallothionein 2A (MT2A). XAF1 directly interacts with MT2A and facilitates its lysosomal degradation, resulting in the elevation of the free intercellular zinc level and subsequent activation of p53 and inactivation of XIAP. Intriguingly, XAF1 is activated as a unique transcription target of metal-regulatory transcription factor-1 (MTF-1) in signaling apoptosis, and its protein is destabilized via the lysosomal pathway by MTF-1-induced MT2A under cytostatic stress conditions, indicating the presence of mutual antagonism between XAF1 and MT2A. The antagonistic interplay between XAF1 and MT2A acts as a key molecular switch in MTF-1-mediated cell-fate decisions and also plays an important role in cell response to various apoptotic and survival factors. Wild-type (WT) XAF1 but not MT2A binding-deficient mutant XAF1 increases the free intracellular zinc level and accelerates WT folding of p53 and degradation of XIAP. Consistently, XAF1 evokes a more drastic apoptotic effect in p53+/+ versus isogenic p53-/- cells. Clinically, expression levels of XAF1 and MT2A are inversely correlated in primary colon tumors and multiple cancer cell lines. XAF1-depleted xenograft tumors display an increased growth rate and a decreased apoptotic response to cytotoxic heavy metals with strong MT2A expression. Collectively, this study uncovers an important role for XAF1-MT2A antagonism as a linchpin to govern cell fate under various stressful conditions including heavy metal exposure.

XIAP Interacts with and Regulates the Activity of FAF1

Cell death depends on the balance between the activities of pro- and anti-apoptotic factors. X-linked inhibitor of apoptosis protein (XIAP) plays an important role in the cytoprotective process by inhibiting the caspase cascade and regulating pro-survival signaling pathways. While searching for novel interacting partners of XIAP, we identified Fas-associated factor 1 (FAF1). Contrary to XIAP, FAF1 is a pro-apoptotic factor that also regulates several signaling pathways in which XIAP is involved. However, the functional relationship between FAF1 and XIAP is unknown. Here, we describe a new interaction between XIAP and FAF1 and describe the functional implications of their opposing roles in cell death and NF-κB signaling. Our results clearly demonstrate the interaction of XIAP with FAF1 and define the specific region of the interaction. We observed that XIAP is able to block FAF1-mediated cell death by interfering with the caspase cascade and directly interferes in NF-κB pathway inhibition by FAF1. Furthermore, we show that XIAP promotes ubiquitination of FAF1. Conversely, FAF1 does not interfere with the anti-apoptotic activity of XIAP, despite binding to the BIR domains of XIAP; however, FAF1 does attenuate XIAP-mediated NF-κB activation. Altered expression of both factors has been implicated in degenerative and cancerous processes; therefore, studying the balance between XIAP and FAF1 in these pathologies will aid in the development of novel therapies.

Human basonuclin 2 up-regulates a cascade set of interferon-stimulated genes with anti-cancerous properties in a lung cancer model

BACKGROUND

Human basonuclin 2 (BNC2) acts as a tumor suppressor in multiple cancers in an as yet unidentified manner. The role and expression of the BNC2 gene in lung cancer has not yet been investigated.

METHODS

BNC2 expression was studied in the A549 and BEAS-2B cell lines, as well as in lung cancer tissue. Illumina array analysis and a viability assay were used to study the effects of transient transfection of BNC2 in A549 cells. Ingenuity pathway analysis and g:Profiler were applied to identify affected pathways and networks. RT-qPCR was used to validate the array results.

RESULTS

We showed the reduced mRNA expression of BNC2 in non-small cell lung cancer tissue and lung cancer cell line A549 compared to non-cancerous lung tissue and BEAS-2B cells, respectively. Further array analysis demonstrated that the transfection of BNC2 into A549 cells resulted in the increased expression of 139 genes and the down-regulation of 13 genes. Pathway analysis revealed that half of the up-regulated genes were from the interferon/signal transducer and activator of transcription signaling pathways. The differential expression of selected sets of genes, including interferon-stimulated and tumor suppressor genes of the XAF1 and OAS families, was confirmed by RT-qPCR. In addition, we showed that the over-expression of BNC2 inhibited the proliferation of A549 cells.

CONCLUSION

Our data suggest that human BNC2 is an activator of a subset of IFN-regulated genes and might thereby act as a tumor suppressor.

XIAP-associating factor 1, a transcriptional target of BRD7, contributes to endothelial cell senescence

X-linked inhibitor of apoptosis (XIAP)-associated factor 1 (XAF1) is well known as an antagonist of XIAP-mediated caspase inhibition. Although XAF1 serves as a tumor-suppressor gene, the role of XAF1 in cellular senescence remains unclear. We found that XAF1 expression was increased by genotoxic agents, such as doxorubicin and ionizing radiation in pulmonary microvascular endothelial cells, consequently leading to premature senescence. Conversely, downregulation of XAF1 in premature senescent cells partially overcame endothelial cell senescence. p53 knockdown, but not p16 knockdown, abolished senescence phenotypes caused by XAF1 induction. XAF1 expression was transcriptionally regulated by Bromodomain 7 (BRD7). XAF1 induction with interferon-gamma (IFN-γ) treatment was abrogated by BRD7 knockdown, which resulted in blocking interferon-induced senescence. In lung cancer cells, XAF1 tumor suppressor activity was decreased by BRD7 knockdown, and inhibition of tumor growth by IFN-γ did not appear in BRD7-depleted xenograft tumors. These data suggest that XAF1 is involved in BRD7-associated senescence and plays an important role in the regulation of endothelial senescence through a p53-dependent pathway. Furthermore, regulation of the BRD7/XAF1 system might contribute to tissue or organismal aging and protection against cellular transformation.

Silencing HS6ST3 inhibits growth and progression of breast cancer cells through suppressing IGF1R and inducing XAF1

Heparan sulfate 6-O-sulfation is biologically edited by 6-O-sulfotransferases (HS6STs) within heparan sulfate chains. Three isoforms of HS6ST have been identified. These enzymes are found to be differentially expressed in a variety of tissues. Recently, several studies have shown that dysregulation of 6-O-sulfotransferases could be involved in tumorigenesis of several cancers. This study aimed to analyze the expression and function of HS6ST3 in breast cancer. HS6ST3 was found up-regulated in T47D, MCF7 and MDA-MB231 breast cancer cell lines. HS6ST3 was then silenced in T47D and MCF7 using siRNA. Silencing HS6ST3 diminished tumor cell growth, migration and invasion, but enhanced cell adhesion and apoptosis in breast cancer. Gene microarray analysis revealed that silencing HS6ST3 significantly changed the expression of IGF1R and XAF1 in breast cancer cells. Further functional studies showed that the cellular processes were mediated by IGF1R and XAF1 after silencing HS6ST3 in breast cancer cells. Together these results indicate that HS6ST3 might be involved in the tumorigenesis of breast cancer and it could be a promising target in breast cancer therapy.

The DNA damage-induced cell death response: a roadmap to kill cancer cells

Upon massive DNA damage cells fail to undergo productive DNA repair and trigger the cell death response. Resistance to cell death is linked to cellular transformation and carcinogenesis as well as radio- and chemoresistance, making the underlying signaling pathways a promising target for therapeutic intervention. Diverse DNA damage-induced cell death pathways are operative in mammalian cells and finally culminate in the induction of programmed cell death via activation of apoptosis or necroptosis. These signaling routes affect nuclear, mitochondria- and plasma membrane-associated key molecules to activate the apoptotic or necroptotic response. In this review, we highlight the main signaling pathways, molecular players and mechanisms guiding the DNA damage-induced cell death response.

RIP kinase-mediated ROS production triggers XAF1 expression through activation of TAp73 in casticin-treated bladder cancer cells

The p53 family protein p73 plays an important role in apoptosis induced by chemotherapeutic drugs. Transcriptionally active (TA) p73 (TAp73) substitutes for p53 in the response to stress. XIAP associated factor 1 (XAF1) is a novel predictive and prognostic factor in patients with bladder cancer, but the association between TAp73 and XAF1 expression in bladder cancer cells is poorly understood. Here, we investigated the status of TAp73 and XAF1 in T24 bladder cancer cells to identify molecular mechanisms in casticin‑exposed T24 cells. Casticin induced activation of JNK/p38 MAPK that preceded activation of the caspase cascade and disruption of the mitochondria membrane potential (∆ψm). Expression of XAF1 and TAp73 was also upregulated in casticin-treated T24 cells. Casticin treatment of T24 cells induced receptor-interacting protein (RIP) kinase expression and increased intracellular production of reactive oxygen species (ROS). Casticin-mediated ROS induced an increase in phosphorylated JNK/p38 MAPK, resulting in progressive upregulation of TAp73, which in turn led to XAF1 expression. Our data suggest that the apoptotic activity of casticin in T24 cells is mediated by activation of the TAp73-XAF1 signaling pathway through RIP kinase-mediated ROS production.

Tumor suppression in mice lacking GABARAP, an Atg8/LC3 family member implicated in autophagy, is associated with alterations in cytokine secretion and cell death

GABARAP belongs to an evolutionary highly conserved gene family that has a fundamental role in autophagy. There is ample evidence for a crosstalk between autophagy and apoptosis as well as the immune response. However, the molecular details for these interactions are not fully characterized. Here, we report that the ablation of murine GABARAP, a member of the Atg8/LC3 family that is central to autophagosome formation, suppresses the incidence of tumor formation mediated by the carcinogen DMBA and results in an enhancement of the immune response through increased secretion of IL-1β, IL-6, IL-2 and IFN-γ from stimulated macrophages and lymphocytes. In contrast, TGF-β1 was significantly reduced in the serum of these knockout mice. Further, DMBA treatment of these GABARAP knockout mice reduced the cellularity of the spleen and the growth of mammary glands through the induction of apoptosis. Gene expression profiling of mammary glands revealed significantly elevated levels of Xaf1, an apoptotic inducer and tumor-suppressor gene, in knockout mice. Furthermore, DMBA treatment triggered the upregulation of pro-apoptotic (Bid, Apaf1, Bax), cell death (Tnfrsf10b, Ripk1) and cell cycle inhibitor (Cdkn1a, Cdkn2c) genes in the mammary glands. Finally, tumor growth of B16 melanoma cells after subcutaneous inoculation was inhibited in GABARAP-deficient mice. Together, these data provide strong evidence for the involvement of GABARAP in tumorigenesis in vivo by delaying cell death and its associated immune-related response.

Survivin and Tumorigenesis: Molecular Mechanisms and Therapeutic Strategies

Survivin is the smallest member of the inhibitor of apoptosis protein family, which has key roles in regulating cell division and inhibiting apoptosis by blocking caspase activation. Survivin is highly expressed in most human cancers, such as lung, pancreatic and breast cancers, relative to normal tissues. Aberrant survivin expression is associated with tumor cell proliferation, progression, angiogenesis, therapeutic resistance, and poor prognosis. Studies on the underlying molecular mechanisms indicate that survivin is involved in the regulation of cytokinesis and cell cycle progression, as well as participates in a variety of signaling pathways such as the p53, Wnt, hypoxia, transforming growth factor, and Notch signaling pathways. In this review, recent progress in understanding the molecular basis of survivin is discussed. Therapeutic strategies targeting survivin in preclinical studies are also briefly summarized.

Stimulation of the hypoxia pathway modulates chemotherapy resistance in Hodgkin's lymphoma cells

Hodgkin's lymphoma (HL) is a malignant disease of the lymphatic system. The therapy has been improved during the last decades but there are still patients who cannot be cured, and the therapy is associated with several adverse late effects. Therefore, we asked which genes might be involved in the chemotherapy resistance of HL cells. We observed that HL cells became more resistant against cisplatin after treatment with cobalt chloride. Therefore, we analyzed which genes were differentially expressed between cells incubated in medium with or without cobalt chloride. We found several genes which were up- or downregulated in the presence of cobalt chloride and might be involved in the modulation of chemotherapy resistance. Cobalt chloride is a hypoxia-mimetic agent. Therefore, we tested chemo-resistance and gene expression of HL cells under hypoxic conditions and confirmed the results from the cobalt chloride experiments. Taken together, activation of the hypoxia pathway led to altered gene expression and drug resistance of HL cells. Differentially expressed genes might be interesting targets for the development of future treatment strategies against drug-resistant HL.

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.

A novel splice variant of XIAP-associated factor 1 (XAF1) is expressed in peripheral blood containing gastric cancer-derived circulating tumor cells

BACKGROUND

XIAP-associated factor 1 (XAF1) is ubiquitously expressed in normal tissues, but its suppression in cancer cells is strongly associated with tumor progression. Although downregulation of XAF1 is observed in tumors, its expression profile in the peripheral blood of cancer patients has not yet been investigated. Here, we identified a novel XAF1 splice variant in cancer cells and then investigated the expression level of this variant in peripheral blood containing gastric cancer-derived circulating tumor cells (CTCs).

METHODS

To identify splice variants, RT-PCR and DNA sequencing were performed in mRNAs extracted from many cancer cells. We then carried out quantitative RT-PCR to investigate expression in peripheral blood from all 96 gastric cancer patients and 22 healthy volunteers.

RESULTS

The XAF1 variant harbored a premature termination codon (PTC) and was differentially expressed in highly metastatic cancer cells versus the parental cells, and that nonsense-mediated mRNA decay (NMD) was suppressed in the variant-expressing cells. Furthermore, splice variants of XAF1 were upregulated in peripheral blood containing CTCs. In XAF1 variant-expressing patients, the expression levels of other NMD-targeted genes also increased, suggesting that the NMD pathway was suppressed in CTCs.

CONCLUSIONS

Our study identified a novel splice variant of XAF1 in cancer cells. This variant was regulated through the NMD pathway and accumulated in NMD-suppressed metastatic cancer cells and peripheral blood containing CTCs. The presence of XAF1 transcripts harboring the PTC in the peripheral blood may be useful as an indicator of NMD inhibition in CTCs.

[XAF1 inhibits cell proliferation and induces apoptosis in human lung adenocarcinoma cell line A549 in vitro]

BACKGROUND AND OBJECTIVE

XAF1 is a factor necessary to inhibit tumor cell growth. Low XAF1 expression is associated with various tumor cells. The aim of this study is to investigate the effect and the mechanism of adenovirus vector Ad5/F35 mediated X-linked inhibitor of apoptosis protein associated factor-1 (XAF1) on the inhibition of cell proliferation and the induction of apoptosis of human lung adenocarcinoma cell A549.

METHODS

Recombinant virus Ad5/F35-XAF1 and controlled virus Ad5/F35-NULL exhibited different multiplicities of infection (MOI) at the same time. mRNA and protein expressions of XAF1 were determined by reverse transcriptase polymerase chain reaction (RT-PCR) and Western blot, respectively. Cell proliferation was observed by methyl thiazolyl tetrazolium (MTT) assay, and cell apoptosis was analyzed by FACS with Annexin V-FITC/PI double staining. The expressions of apoptosis-associated proteins, such as PARP, Caspase-3, and Caspase-8, were also determined by Western blot.

RESULTS

mRNA and protein expressions of XAF1 were significantly increased in human lung adenocarcinoma cell A549 after this cell was transfected with Ad5/F35-XAF1 for 48 h; these expressions were higher than those of the controlled group Ad5/F35-NULL. Cell proliferation was inhibited and apoptosis was induced in a dose-dependent manner in the Ad5/F35-XAF1 group. After Ad5/F35-XAF1 transfection was performed, the cleavage of apoptosis-associated proteins, such as PARP, Caspase-3, and Caspase-8, was activated.

CONCLUSIONS

Restored XAF1 expression inhibits cell proliferation and induces cell apoptosis in human lung adenocarcinoma cell line A549. Furthermore, XAF1 may activate associated apoptotic signaling pathways in A549 cell line.

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.

Tumor suppressor XAF1 induces apoptosis, inhibits angiogenesis and inhibits tumor growth in hepatocellular carcinoma

X-linked inhibitor of apoptosis (XIAP)-associated factor 1 (XAF1), a XIAP-binding protein, is a tumor suppressor gene. XAF1 was silent or expressed lowly in most human malignant tumors. However, the role of XAF1 in hepatocellular carcinoma (HCC) remains unknown. In this study, we investigated the effect of XAF1 on tumor growth and angiogenesis in hepatocellular cancer cells. Our results showed that XAF1 expression was lower in HCC cell lines SMMC-7721, Hep G2 and BEL-7404 and liver cancer tissues than that in paired non-cancer liver tissues. Adenovirus-mediated XAF1 expression (Ad5/F35-XAF1) significantly inhibited cell proliferation and induced apoptosis in HCC cells in dose- and time- dependent manners. Infection of Ad5/F35-XAF1 induced cleavage of caspase -3, -8, -9 and PARP in HCC cells. Furthermore, Ad5/F35-XAF1 treatment significantly suppressed tumor growth in a xenograft model of liver cancer cells. Western Blot and immunohistochemistry staining showed that Ad5/F35-XAF1 treatment suppressed expression of vascular endothelial growth factor (VEGF), which is associated with tumor angiogenesis, in cancer cells and xenograft tumor tissues. Moreover, Ad5/F35-XAF1 treatment prolonged the survival of tumor-bearing mice. Our results demonstrate that XAF1 inhibits tumor growth by inducing apoptosis and inhibiting tumor angiogenesis. XAF1 may be a promising target for liver cancer treatment.

XAF1 directs apoptotic switch of p53 signaling through activation of HIPK2 and ZNF313

X-linked inhibitor of apoptosis (XIAP)-associated factor 1 (XAF1) is a tumor suppressor that is frequently inactivated in many human cancers. However, the molecular mechanism underlying its growth-inhibitory function remains largely unknown. Here, we report that XAF1 forms a positive feedback loop with p53 and acts as a molecular switch in p53-mediated cell-fate decisions favoring apoptosis over cell-cycle arrest. XAF1 binds directly to the N-terminal proline-rich domain of p53 and thus interferes with E3 ubiquitin ligase MDM2 binding and ubiquitination of p53. XAF1 stimulates homeodomain-interacting protein kinase 2 (HIPK2)-mediated Ser-46 phosphorylation of p53 by blocking E3 ubiquitin ligase Siah2 interaction with and ubiquitination of HIPK2. XAF1 also steps up the termination of p53-mediated cell-cycle arrest by activating zinc finger protein 313 (ZNF313), a p21(WAF1)-targeting ubiquitin E3 ligase. XAF1 interacts with p53, Siah2, and ZNF313 through the zinc finger domains 5, 6, and 7, respectively, and truncated XAF1 isoforms preferentially expressed in cancer cells fail to form a feedback loop with p53. Together, this study uncovers a novel role for XAF1 in p53 stress response, adding a new layer of complexity to the mechanisms by which p53 determines cell-fate decisions.

Synergistic suppression effect on tumor growth of hepatocellular carcinoma by combining oncolytic adenovirus carrying XAF1 with cisplatin

PURPOSE

The potent anticancer efficacy of oncolytic viruses has been verified in Clinic in recent years. Cisplatin (DDP) is one of most common chemotherapeutic drugs, but is accompanied by side effects and drug resistance. Our previous studies have shown the strategy of cancer -targeting gene-viro-therapy (CTGVT) mediated by the oncolytic virus ZD55 containing the XAF1 cDNA (ZD55-XAF1), which exhibited potent antitumor effects in various tumor cells and no apparent toxicities on normal cells. In the study, the CTGVT strategy is broadened by combining DDP with ZD55-XAF1 for growth inhibition of hepatocellular carcinoma (HCC) cells.

METHODS

The transgenic expression was evaluated by both in vitro and in vivo experiments, and the enhanced inhibitory effect of ZD55-XAF1 combined with cisplatin was assessed in HCC cells. The cytotoxicity on normal liver cells was evaluated by MTT assay and apoptotic cell staining. Activation of caspase-9 and PARP for apoptosis was further detected by Western blot analysis. The in vivo antitumor efficacy of combination treatment with cisplatin and ZD55-XAF1 was estimated in an HCC xenograft mouse model.

RESULTS

We found that the combination of ZD55-XAF1 and cisplatin showed enhanced inhibitory effects on the proliferation of HCC cells in vitro and tumor growth in mice. Furthermore, the combined treatment of ZD55-XAF1 and DDP decreases the chemotherapy dose needed to achieve the same inhibitory effect without overlapping toxicities on normal liver cells and induces tumor cell apoptosis via the activation of caspase-9/PARP pathway.

CONCLUSION

Thus, these data suggest that the chemo-gene-viro-therapeutic strategy by combining ZD55-XAF1 and DDP reveals a novel therapeutic strategy for hepatocellular carcinoma.

A molecular signaling map and its application

Cancer research over the past decades has revealed a number of molecular, biochemical, and cellular events that reflect progressive transformation of normal human cells into their malignant derivatives. These findings help to better understand the complexity of human tumorigenesis. In our study, molecular information is organized to chart a comprehensive map of the signaling network for human cancer. It includes transcriptional and translational regulation and diverse feedback-control loops. It is demonstrated that applying this signaling network map allows predicting the effect of targeted therapy before it can be applied into practice to reduce clinical trial risks. Hence, the proposed map with prognosticating potential effect might become part of drug discovery programs for targeted therapy. Applied in individual patient care it helps to reduce the current reliance of cancer treatment on chemotherapies with low therapeutic indices. This study also demonstrates that continuing elucidation of tumorigenesis will not only need heterotypic organ culture systems in vitro and increasingly refined animal models in vivo, but also computationally calculable virtual cell models in silico.

Combination of gefitinib and DNA methylation inhibitor decitabine exerts synergistic anti-cancer activity in colon cancer cells

Despite recent advances in the treatment of human colon cancer, the chemotherapy efficacy against colon cancer is still unsatisfactory. In the present study, effects of concomitant inhibition of the epidermal growth factor receptor (EGFR) and DNA methyltransferase were examined in human colon cancer cells. We demonstrated that decitabine (a DNA methyltransferase inhibitor) synergized with gefitinib (an EGFR inhibitor) to reduce cell viability and colony formation in SW1116 and LOVO cells. However, the combination of the two compounds displayed minimal toxicity to NCM460 cells, a normal human colon mucosal epithelial cell line. The combination was also more effective at inhibiting the AKT/mTOR/S6 kinase pathway. In addition, the combination of decitabine with gefitinib markedly inhibited colon cancer cell migration. Furthermore, gefitinib synergistically enhanced decitabine-induced cytotoxicity was primarily due to apoptosis as shown by Annexin V labeling that was attenuated by z-VAD-fmk, a pan caspase inhibitor. Concomitantly, cell apoptosis resulting from the co-treatment of gefitinib and decitabine was accompanied by induction of BAX, cleaved caspase 3 and cleaved PARP, along with reduction of Bcl-2 compared to treatment with either drug alone. Interestingly, combined treatment with these two drugs increased the expression of XIAP-associated factor 1 (XAF1) which play an important role in cell apoptosis. Moreover, small interfering RNA (siRNA) depletion of XAF1 significantly attenuated colon cancer cells apoptosis induced by the combination of the two drugs. Our findings suggested that gefitinib in combination with decitabine exerted enhanced cell apoptosis in colon cancer cells were involved in mitochondrial-mediated pathway and induction of XAF1 expression. In conclusion, based on the observations from our study, we suggested that the combined administration of these two drugs might be considered as a novel therapeutic regimen for treating colon cancer.

Inhibitor of apoptosis proteins as targets for anticancer therapy

Cell death by apoptosis plays a critical role in regulating the subtle balance between cell death and proliferation to maintain tissue homeostasis. Accordingly, tipping the balance in either direction may cause human disease. Too little cell death may promote tumor formation and progression. In addition, killing of cancer cells by current therapies is largely due to induction of apoptosis in tumor cells. Since a hallmark of human cancers is their resistance to apoptosis, there is a demand to develop novel strategies that restore the apoptotic machinery in order to overcome cancer resistance. Inhibitor of apoptosis proteins (IAPs) block apoptosis at the core of the apoptotic machinery by inhibiting caspases. Elevated levels of IAPs are found in many human cancers and have been associated with poor prognosis. Recent insights into the role of IAPs have provided the basis for various exciting developments that aim to modulate the expression or function of IAPs in human cancers. Targeting IAPs (e.g., by antisense approaches or small-molecule inhibitors) presents a promising novel approach to either directly trigger apoptosis or to potentiate the efficacy of cytotoxic therapies in cancer cells. Thus, inhibition of IAPs such as X chromosome-linked IAP may prove to be a successful strategy to overcome apoptosis resistance of human cancers that deserves further exploitation.

Melphalan-induced apoptosis of EBV-transformed B cells through upregulation of TAp73 and XAF1 and nuclear import of XPA

Melphalan (Mel) is widely used to treat patients with hematologic cancer, including multiple myeloma, but its mechanism of action in EBV-transformed B cells is poorly described. In this study, we demonstrate a novel mechanism by which transcriptionally active p73 (TAp73) induces translocation of X-linked inhibitor of apoptosis protein-associated factor 1 (XAF1) and xeroderma pigmentosum group A (XPA) during apoptosis caused by Mel treatment. We observed that Mel induced significant generation of reactive oxygen species (ROS) and subsequent apoptosis, as well as an early phosphorylation of p38 MAPK that preceded expression of the mitochondria membrane potential disruption-related molecules and the cleavage of caspases. In particular, Mel led to upregulation of TAp73, XAF1, and Puma and induced XPA nuclear import and translocation of Bax into mitochondria. Mel-induced apoptosis was inhibited by pretreatment with the ROS scavenger 4-amino-2,4-pyrrolidine-dicarboxylic acid (APDC) and the p38 MAPK inhibitor SB203580. We supposed that ROS generation might be the first event in Mel-induced apoptosis, because APDC blocked the increase in ROS, p38 MAPK, and TAp73, but SB203580 did not block ROS generation. Moreover, Mel elicited activation of ATR, and APDC inhibited phosphorylation of ATR but not SB203580. APDC and SB203580 completely blocked XPA and Bax translocation. We conclude that Mel promotes TAp73-mediated XAF1 and Puma expression via ROS generation and ATR/p38 MAPK pathway activation, thereby triggering apoptosis. Our results provide evidence of a novel alternate regulatory mechanism of TAp73 and reveal that Mel may be a therapeutic drug for curing EBV-related malignancies.

Phase I trial of AEG35156 an antisense oligonucleotide to XIAP plus gemcitabine in patients with metastatic pancreatic ductal adenocarcinoma

OBJECTIVES

AEG35156 is an antisense oligonucleotide (ASO) that targets the X-linked inhibitor of apoptosis mRNA. Preclinical studies showed potent activity of AEG35156 in combination with gemcitabine in pancreatic ductal adenocarcinoma (PDA). A phase I study was conducted to establish the maximum-tolerated dose, safety, and antitumor activity of AEG35156 plus gemcitabine in metastatic PDA.

METHODS

Fourteen patients with metastatic PDA were enrolled. Nine patients were treated at 350 mg IV and 5 patients at 500 mg IV of AEG35156, 3 weeks on/1 week off of a 28-day cycle. Gemcitabine was administered at 1000 mg/m(2) IV over 30 minutes immediately after AEG35156 in both groups. Because of perceived neurotoxicity dose deescalation to 350 mg was recommended.

RESULTS

All 14 patients were evaluable for tolerability and toxicity. Toxicities include neutropenia (grade 3/4, 6 patients), thrombocytopenia (grade 3, 2 patients), peripheral neuropathy (grade 3, 2 patients), fatigue (grade 3, 4 patients), ascites (grade 3, 2 patients), and nausea/vomiting (grade 4, 2 patients). Five patients (45%) experienced stable disease with a median progression-free survival of 58 days (95% CI, 52-107 d).

CONCLUSIONS

The maximum-tolerated dose is AEG35156 500 mg plus gemcitabine 1000 mg/m(2) given on days 1, 8, and 15 every 28 days. AEG35156 plus gemcitabine failed to show significant clinical activity in advanced PDA.

X-linked inhibitor of apoptosis protein (XIAP) regulation of cyclin D1 protein expression and cancer cell anchorage-independent growth via its E3 ligase-mediated protein phosphatase 2A/c-Jun axis

The X-linked inhibitor of apoptosis protein (XIAP) is a well known potent inhibitor of apoptosis; however, it is also involved in other cancer cell biological behavior. In the current study, we discovered that XIAP and its E3 ligase played a crucial role in regulation of cyclin D1 expression in cancer cells. We found that deficiency of XIAP expression resulted in a marked reduction in cyclin D1 expression. Consistently, cell cycle transition and anchorage-independent cell growth were also attenuated in XIAP-deficient cancer cells compared with those of the parental wild-type cells. Subsequent studies demonstrated that E3 ligase activity within the RING domain of XIAP is crucial for its ability to regulate cyclin D1 transcription, cell cycle transition, and anchorage-independent cell growth by up-regulating transactivation of c-Jun/AP-1. Moreover, we found that E3 ligase within RING domain was required for XIAP inhibition of phosphatase PP2A activity by up-regulation of PP2A phosphorylation at Tyr-307 in its catalytic subunit. Such PP2A phosphorylation and inactivation resulted in phosphorylation and activation of its downstream target c-Jun in turn leading to cyclin D1 expression. Collectively, our studies uncovered a novel function of E3 ligase activity of XIAP in the up-regulation of cyclin D1 expression, providing significant insight into the understanding of the biomedical significance of overexpressed XIAP in cancer development, further offering a new molecular basis for utilizing XIAP E3 ligase as a cancer therapeutic target.

ROS and ERK1/2-mediated caspase-9 activation increases XAF1 expression in dexamethasone-induced apoptosis of EBV-transformed B cells

Dexamethasone (Dex) inhibits the growth of diverse types of cancer cells and is utilized clinically for the therapy of hematological malignancies. In this study, we investigated the molecular mechanisms of Dex action in the apoptosis of Epstein-Barr virus (EBV)-transformed B cells. We showed that Dex inhibited the proliferation of EBV-transformed B cells and induced apoptosis by activating caspase-9, -3 and -8. While activation of caspase-9 was triggered as early as 2 h after Dex treatment, cleavage of caspase-8 was deferred and was found 8 h after the exposure. Dex-dependent activation of caspase-8 was blocked by the specific caspase-9 inhibitor, z-LEHD-fmk. Moreover, Dex significantly increased the expression of X-linked inhibitor of apoptosis (XIAP)-associated factor 1 (XAF1) and induced the translocation of XAF1 into the cytosol. Cytosolic XAF1 with Puma induced the translocation of Bax into mitochondria. Dex led to up-regulation of reactive oxygen species (ROS) generation and the phosphorylation of ERK1/2 after the exposure. We speculated that ROS generation might be the first event of Dex-induced apoptosis because ROS inhibitor NAC abrogated ROS production and ERK1/2 activation, but PD98059 did not block ROS production. NAC and PD98059 also suppressed the translocation of XAF1, Puma and Bax into mitochondria. These results demonstrated that Dex-mediated activation of caspase-9 via ROS generation and ERK1/2 pathway activation resulted in the activation of caspase-8 and the increment of XAF1, thereby induced apoptosis of EBV-transformed B cells. These findings suggest that Dex constitutes a probable therapy for EBV-associated hematological malignancies.