Colocalization of endogenous TNF with a functional intracellular splice form of human TNF receptor type 2

TNFR1-d2 carrying the p.(Thr79Met) pathogenic variant is a potential novel actor of TNFα/TNFR1 signalling regulation in the pathophysiology of TRAPS

Binding of tumour necrosis factor α (TNFα) to its receptor (TNFR1) is critical for both survival and death cellular pathways. TNFα/TNFR1 signalling is complex and tightly regulated at different levels to control cell fate decisions. Previously, we identified TNFR1-d2, an exon 2-spliced transcript of TNFRSF1A gene encoding TNFR1, whose splicing may be modulated by polymorphisms associated with inflammatory disorders. Here, we investigated the impact of TNFRSF1A variants involved in TNFR-associated periodic syndrome (TRAPS) on TNFR1-d2 protein expression and activity. We found that TNFR1-d2 could be translated by using an internal translation initiation codon and a de novo internal ribosome entry site (IRES), which resulted in a putative TNFR1 isoform lacking its N-terminal region. The kinetic of assembly of TNFR1-d2 clusters at the cell surface was reduced as compared with full-length TNFR1. Although co-localized with the full-length TNFR1, TNFR1-d2 neither activated nuclear factor (NF)-κB signalling, nor interfered with TNFR1-induced NF-κB activation. Translation of TNFR1-d2 carrying the severe p.(Thr79Met) pathogenic variant (also known as T50M) was initiated at the mutated codon, resulting in an elongated extracellular domain, increased speed to form preassembled clusters in absence of TNFα, and constitutive NF-κB activation. Overall, TNFR1-d2 might reflect the complexity of the TNFR1 signalling pathways and could be involved in TRAPS pathophysiology of patients carrying the p.(Thr79Met) disease-causing variant.

Characterization of putative regulatory isoforms of porcine tumor necrosis factor receptor 2 in endothelial cells

Tumor necrosis factor α (TNFα) and its receptors contribute to rejection of transplanted cells and organs. To elucidate how TNFα affects xenograft rejection, we previously cloned the cDNA of pig TNF-receptor 2 (pTNFR2) and found four isoforms: one comprising the full receptor with four cysteine-rich domains (CRD), a shorter variant (pTNFR2ΔE7-10) encoding for a soluble isoform, another lacking exon 4 (pTNFR2ΔE4) displaying only 3 CRD and poor ligand binding, and the smallest one generated by the two alternative splicings. All isoforms contained the pre-ligand assembly domain (PLAD) responsible for receptor trimerization. We now investigated their roles by structural, expression, and subcellular localization studies. Structural in silico analyses identified four amino acids potentially involved in TNFα binding and lacking in pTNFR2ΔE4. Quantitative RT-PCR determined regulated expression affecting the two pTNFR2 alternative splicings in cytokine-stimulated porcine aortic endothelial cells (PAEC). Particularly, human IL-1α and TNFα produced a strong mRNA upregulation of all isoforms, being the full receptor the predominant one. However, expression of pTNFR2 on PAEC did not correlate with mRNA and decreased after 24-hour exposure to IL-1α or TNFα. Notably, confocal microscopy confirmed the presence of pTNFR2 inside and on the plasma membrane, whereas pTNFR2ΔE4 located only intracellularly. Most interestingly, FRET analyses showed that membrane-bound isoforms pTNFR2 and pTNFR2ΔE4 colocalized intracellularly and associated through the PLAD. Our data show that pTNFR2ΔE4 bind and may retain the full receptor intracellularly. This mechanism has not been described in other species and represents a particularity that may affect the pathophysiology of pig xenografts.

Alternative splicing regulation in tumor necrosis factor-mediated inflammation

It is generally accepted that alternative splicing has an effect on disease when it leads to conspicuous changes in relevant proteins, but that the combinatorial effect of several small modifications can have marked outcomes as well. Inflammation is a complex process involving numerous signaling pathways, among which the tumor necrosis factor (TNF) pathway is one of the most studied. Signaling pathways are commonly represented as intricate cascades of molecular interactions that eventually lead to the activation of one or several genes. Alternative splicing is a common means of controlling protein expression in time and space; therefore, it can modulate the outcome of signaling pathways through small changes in their elements. Notably, the overall process is tightly regulated, which is easily overlooked when analyzing the pathway as a whole. The present review summarizes recent studies of the alternative splicing of key players of the TNF pathway leading to inflammation, and hypothesizes on the cumulative results of those modifications and the impact on cancer development.

Hepatitis C Virus Exploits Death Receptor 6-mediated Signaling Pathway to Facilitate Viral Propagation

The life cycle of hepatitis C virus (HCV) is highly dependent on host proteins for virus propagation. By transcriptome sequencing analysis, we identified host genes that were highly differentially expressed in HCV-infected cells. Of these candidates, we selected Death receptor 6 (DR6) for further characterization. DR6 is an orphan member of the tumor necrosis factor receptor superfamily. In the present study, we demonstrated that both mRNA and protein levels of DR6 were increased in the context of HCV replication. We further showed that promoter activity of DR6 was increased by HCV infection. By employing promoter-linked reporter assay, we showed that HCV upregulated DR6 via ROS-mediated NF-κB pathway. Both mRNA and protein levels of DR6 were increased by NS4B or NS5A. However, NS5A but not NS4B specifically interacted with DR6. We showed that HCV modulated JNK, p38 MAPK, STAT3, and Akt signaling pathways in a DR6-dependent manner. Interestingly, Akt signaling cascade was regulated by protein interplay between DR6 and NS5A. Silencing of DR6 expression resulted in decrease of infectious HCV production without affecting viral entry, replication, and translation. Together, these data indicate that HCV modulates DR6 signaling pathway for viral propagation and may contribute to HCV-mediated pathogenesis.

Haemocytes collected from experimentally infected Pacific oysters, Crassostrea gigas: Detection of ostreid herpesvirus 1 DNA, RNA, and proteins in relation with inhibition of apoptosis

Recent transcriptomic approaches focused on anti-viral immunity in molluscs lead to the assumption that the innate immune system, such as apoptosis, plays a crucial role against ostreid herpesvirus type 1 (OsHV-1), infecting Pacific cupped oyster, Crassostrea gigas. Apoptosis constitutes a major mechanism of anti-viral response by limiting viral spread and eliminating infected cells. In this way, an OsHV-1 challenge was performed and oysters were monitored at three times post injection to investigate viral infection and host response: 2h (early after viral injection in the adductor muscle), 24h (intermediate time), and 48h (just before first oyster mortality record). Virus infection, associated with high cumulative mortality rates (74% and 100%), was demonstrated in haemocytes by combining several detection techniques such as real-time PCR, real-time RT PCR, immunofluorescence assay, and transmission electron microscopy examination. High viral DNA amounts ranged from 5.46×104 to 3.68×105 DNA copies ng-1 of total DNA, were detected in dead oysters and an increase of viral transcripts was observed from 2, 24, and 48hpi for the five targeted OsHV-1 genes encoding three putative membrane proteins (ORFs 25, 41, and 72), a putative dUTPase (ORF 75), and a putative apoptosis inhibitor (ORF 87). Apoptosis was studied at molecular and cellular levels with an early marker (phosphatidyl-serine externalisation measured by flow cytometry and epifluorescence microscopy) and a later parameter (DNA fragmentation by terminal deoxynucleotidyltransferase-mediated dUTP nick end labeling assay (TUNEL)). The down-regulation of genes encoding proteins involved in the activation of the apoptotic pathway (TNF and caspase 3) and the up-regulation of genes encoding anti-apoptotic proteins (IAP-2, and Bcl-2) suggested an important anti-apoptosis phenomenon in haemocytes from OsHV-1 infected oysters at 24 and 48hpi. Additionally, more phosphatidyl-serines were externalized and more cells with DNA fragmentation were observed in haemocytes collected from artificial seawater injected oysters than in haemocytes collected from OsHV-1 infected oysters at 24 and 48hpi, suggesting an inhibition of the apoptotic process in presence of the virus. In conclusion, this study is the first to focus on C. gigas haemocytes, cells involved in the host immune defense, during an OsHV-1 challenge in controlled conditions by combining various and original approaches to investigate apoptosis at molecular and cellular levels.

Infection-induced bystander-apoptosis of monocytes is TNF-alpha-mediated

Phagocytosis induced cell death (PICD) is crucial for controlling phagocyte effector cells, such as monocytes, at sites of infection, and essentially contributes to termination of inflammation. Here we tested the hypothesis, that during PICD bystander apoptosis of non-phagocyting monocytes occurs, that apoptosis induction is mediated via tumor necrosis factor-alpha (TNF-α and that TNF-α secretion and -signalling is causal. Monocytes were infected with Escherichia coli (E. coli), expressing green fluorescent protein (GFP), or a pH-sensitive Eos-fluorescent protein (EOS-FP). Monocyte phenotype, phagocytic activity, apoptosis, TNF-receptor (TNFR)-1, -2-expression and TNF-α production were analyzed. Apoptosis occured in phagocyting and non-phagocyting, bystander monocytes. Bacterial transport to the phagolysosome was no prerequisite for apoptosis induction, and desensitized monocytes from PICD, as confirmed by EOS-FP expressing E. coli. Co-cultivation with non-infected carboxyfluorescein-succinimidyl-ester- (CFSE-) labelled monocytes resulted in significant apoptotic cell death of non-infected bystander monocytes. This process required protein de-novo synthesis and still occurred in a diminished way in the absence of cell-cell contact. E. coli induced a robust TNF-α production, leading to TNF-mediated apoptosis in monocytes. Neutralization with an anti-TNF-α antibody reduced monocyte bystander apoptosis significantly. In contrast to TNFR2, the pro-apoptotic TNFR1 was down-regulated on the monocyte surface, internalized 30 min. p.i. and led to apoptosis predominantly in monocytes without phagocyting bacteria by themselves. Our results suggest, that apoptosis of bystander monocytes occurs after infection with E. coli via internalization of TNFR1, and indicate a relevant role for TNF-α. Modifying monocyte apoptosis in sepsis may be a future therapeutic option.

Cytokine induction of tumor necrosis factor receptor 2 is mediated by STAT3 in colon cancer cells

The IL-6/STAT3 and TNFα/NFκB pathways are emerging as critical mediators of inflammation-associated colon cancer. TNF receptor (TNFR) 2 expression is increased in inflammatory bowel diseases, the azoxymethane/dextran sodium sulfate (AOM/DSS) model of colitis-associated cancer, and by combined interleukin (IL) 6 and TNFα. The molecular mechanisms that regulate TNFR2 remain undefined. This study used colon cancer cell lines to test the hypothesis that IL-6 and TNFα induce TNFR2 via STAT3 and/or NFκB. Basal and IL-6 + TNFα-induced TNFR2 were decreased by pharmacologic STAT3 inhibition. NFκB inhibition had little effect on IL-6 + TNFα-induced TNFR2, but did inhibit induction of endogenous IL-6 and TNFR2 in cells treated with TNFα alone. Chromatin immunoprecipitation (ChIP) revealed cooperative effects of IL-6 + TNFα to induce STAT3 binding to a -1,578 STAT response element in the TNFR2 promoter but no effect on NFκB binding to consensus sites. Constitutively active STAT3 was sufficient to induce TNFR2 expression. Overexpression of SOCS3, a cytokine-inducible STAT3 inhibitor, which reduces tumorigenesis in preclinical models of colitis-associated cancer, decreased cytokine-induced TNFR2 expression and STAT3 binding to the -1,578 STAT response element. SOCS3 overexpression also decreased proliferation of colon cancer cells and dramatically decreased anchorage-independent growth of colon cancer cells, even cells overexpressing TNFR2. Collectively, these studies show that IL-6- and TNFα-induced TNFR2 expression in colon cancer cells is mediated primarily by STAT3 and provide evidence that TNFR2 may contribute to the tumor-promoting roles of STAT3.

Characterization of TNF receptor type 2 isoform in the mouse

In this report a TNF receptor type 2 (TNFR2) isoform in the mouse, termed micp75TNFR, was compared to its human orthologue hicp75TNFR and to the known mouse TNFR2. The micp75TNFR is generated by the use of an alternative transcriptional start site within the mouse TNFR2 gene. This receptor isoform was found to be expressed in different macrophage-like and endothelial tumor cell lines and mouse spleen cells after stimulation. Lacking the leader sequence and part of the amino-terminal end of the mature TNFR2, the micp75TNFR was characterized by mainly intracellular expression. In contrast to its human paralogue, the micp75TNFR lacked the capacity to bind TNF. Therefore, micp75TNFR expression did not mediate protection from TNF-induced cytotoxicity but may interfere with TNFR2 signalling.