Tumor necrosis factor receptor p75 mediates cell-specific activation of nuclear factor kappa B and induction of human cytomegalovirus enhancer

Distinct modes of TNF signaling through its two receptors in health and disease

TNF is a key proinflammatory and immunoregulatory cytokine whose deregulation is associated with the development of autoimmune diseases and other pathologies. Recent studies suggest that distinct functions of TNF may be associated with differential engagement of its two receptors: TNFR1 or TNFR2. In this review, we discuss the relative contributions of these receptors to pathogenesis of several diseases, with the focus on autoimmunity and neuroinflammation. In particular, we discuss the role of TNFRs in the development of regulatory T cells during neuroinflammation and recent findings concerning targeting TNFR2 with agonistic and antagonistic reagents in various murine models of autoimmune and neuroinflammatory disorders and cancer.

Role of the type I tumor necrosis factor receptor in inflammation-associated olfactory dysfunction

BACKGROUND

To understand mechanisms of human olfactory dysfunction in chronic rhinosinusitis, an inducible olfactory inflammation (IOI) model has been utilized to chronically express inflammatory cytokines locally, resulting in neuronal loss, diminished odorant responses, and repressed olfactory regeneration. Knockout of the minor tumor necrosis factor α receptor 2 (TNFR2) was previously shown to partially rescue these olfactory changes. The purpose of current study was to investigate the role of the major TNF receptor, TNFR1, in chronic olfactory inflammation.

METHODS

Two experimental groups of mice were studied: TNFR1 knockout in IOI background and TNFR1 knockout with allergen-induced inflammation. Olfactory function was assayed by electro-olfactogram (EOG), and olfactory tissue was processed for histology and immunohistochemical staining.

RESULTS

TNF-α was dramatically induced in IOI-TNFR1 knockout mice, but the olfactory epithelium did not show inflammation. EOG responses were normal after either 2 or 8 weeks of TNF-α expression. Ovalbumin-sensitized TNFR1 knockout mice developed markedly diminished eosinophilic inflammatory infiltration.

CONCLUSION

Genetic deletion of TNFR1 completely blocks TNF-α-induced inflammation and reduces allergen-induced inflammation. Preserved EOG responses suggest a TNFR1-dependent mechanism of TNF-α-induced olfactory neuron dysfunction.

Release of nonmuscle myosin II from the cytosolic domain of tumor necrosis factor receptor 2 is required for target gene expression

Tumor necrosis factor-α (TNF-α) elicits its biological activities through activation of TNF receptor 1 (TNFR1, also known as p55) and TNFR2 (also known as p75). The activities of both receptors are required for the TNF-α-induced proinflammatory response. The adaptor protein TNFR-associated factor 2 (TRAF2) is critical for either p55- or p75-mediated activation of nuclear factor κB (NF-κB) and mitogen-activated protein kinase (MAPK) signaling, as well as for target gene expression. We identified nonmuscle myosin II (myosin) as a binding partner of p75. TNF-α-dependent signaling by p75 and induction of target gene expression persisted substantially longer in cells deficient in myosin regulatory light chain (MRLC; a component of myosin) than in cells replete in myosin. In resting endothelial cells, myosin was bound constitutively to the intracellular region of p75, a region that overlaps with the TRAF2-binding domain, and TNF-α caused the rapid dissociation of myosin from p75. At early time points after exposure to TNF-α, p75 activated Rho-associated kinase 1 (ROCK1). Inhibition of ROCK1 activity blocked TNF-α-dependent phosphorylation of MRLC and the dissociation of myosin from p75. ROCK1-dependent release of myosin was necessary for the TNF-α-dependent recruitment of TRAF2 to p75 and for p75-specific activation of NF-κB and MAPK signaling. Thus, our findings have revealed a previously uncharacterized, noncanonical regulatory function of myosin in cytokine signaling.

Regulation of inflammation-associated olfactory neuronal death and regeneration by the type II tumor necrosis factor receptor

BACKGROUND

Olfactory loss is a debilitating symptom of chronic rhinosinusitis. To study the impact of inflammation on the olfactory system, the inducible olfactory inflammation (IOI) transgenic mouse was created in which inflammation can be turned on and off within the olfactory epithelium. In this study, the type II tumor necrosis factor (TNF) receptor (TNFR2) was knocked out, and the effect on the olfactory loss phenotype was assessed.

METHODS

IOI mice were bred to TNFR2 knockout mice to yield progeny IOI mice lacking the TNFR2 receptor (TNFR2(-/-) ). TNF-α expression was induced within the olfactory epithelium for 6 weeks to generate chronic inflammation. Olfactory function was assayed by electro-olfactogram (EOG), and olfactory tissue was processed for histology and immunohistochemical staining.

RESULTS

Compared to IOI mice with wild-type TNFR2, IOI mice lacking the TNFR2 demonstrated similar levels of inflammatory infiltration and enlargement of the subepithelial layer. However, IOI-TNFR2(-/-) mice differed markedly in that the neuronal layer was largely preserved and active progenitor cell proliferation was present. Odorant responses were maintained in the IOI-TNFR2(-/-) mice, in contrast to IOI mice.

CONCLUSION

TNFR2 is the minor receptor for TNF-α, but appears to play an important role in mediating TNF-induced disruption of the olfactory system. This finding suggests that neuronal death and inhibition of proliferation in CRS may be mediated by TNFR2 on olfactory neurons and progenitor cells. Further studies are needed to elucidate the subcellular pathways involved and develop novel therapies for treating olfactory loss in the setting of CRS.

Tumor necrosis factor-α: regulation of renal function and blood pressure

Tumor necrosis factor-α (TNF-α) is a pleiotropic cytokine that becomes elevated in chronic inflammatory states such as hypertension and diabetes and has been found to mediate both increases and decreases in blood pressure. High levels of TNF-α decrease blood pressure, whereas moderate increases in TNF-α have been associated with increased NaCl retention and hypertension. The explanation for these disparate effects is not clear but could simply be due to different concentrations of TNF-α within the kidney, the physiological status of the subject, or the type of stimulus initiating the inflammatory response. TNF-α alters renal hemodynamics and nephron transport, affecting both activity and expression of transporters. It also mediates organ damage by stimulating immune cell infiltration and cell death. Here we will summarize the available findings and attempt to provide plausible explanations for such discrepancies.

A vaccinia virus deletion mutant reveals the presence of additional inhibitors of NF-kappaB

The classical nuclear factor kappa B (NF-κB) signaling pathway is an important regulator of inflammation and innate immunity that is activated by a wide variety of stimuli, including virus infection, tumor necrosis factor alpha (TNF-α), and interleukin 1β (IL-1β). Poxviruses, including vaccinia virus (VV) and ectromelia virus, encode multiple proteins that function in immune evasion. Recently, a growing number of genes encoded by poxviruses have been shown to target and disrupt the NF-κB signaling pathway. To determine if additional gene products that interfere with NF-κB signaling existed, we used a vaccinia virus deletion mutant, VV811, which is missing 55 open reading frames lacking all known inhibitors of TNF-α-induced NF-κB activation. Immunofluorescence analysis of HeLa cells treated with TNF-α and IL-1β revealed that NF-κB translocation to the nucleus was inhibited in VV811-infected cells. This was further confirmed through Western blotting of cytoplasmic and nuclear extracts for NF-κB. Additionally, VV811 infection inhibited TNF-α-induced IκBα degradation. In contrast to vaccinia virus strain Copenhagen (VVCop)-infected cells, VV811 infection resulted in the dramatic accumulation of phosphorylated IκBα. Correspondingly, coimmunoprecipitation assays demonstrated that the NF-κB-inhibitory IκBα-p65-p50 complex was intact in VV811-infected cells. Significantly, cells treated with 1-β-d-arabinofuranosylcytosine, an inhibitor of poxvirus late gene expression, demonstrated that an additional vaccinia virus late gene was involved in the stabilization of IκBα. Overall, this work indicates that unidentified inhibitors of NF-κB exist in vaccinia virus. The complex inhibition of NF-κB by vaccinia virus illustrates the importance of NF-κB activation in the antiviral response.

Modified vaccinia virus Ankara can activate NF-kappaB transcription factors through a double-stranded RNA-activated protein kinase (PKR)-dependent pathway during the early phase of virus replication

Modified vaccinia virus Ankara (MVA), which is a promising replication-defective vaccine vector, is unusual among the orthopoxviruses in activating NF-kappaB transcription factors in cells of several types. In human embryonic kidney (HEK 293T) cells, the MVA-induced depletion of IkappaBalpha required to activate NF-kappaB is inhibited by UV-inactivation of the virus, and begins before viral DNA replication. In HEK 293T, CHO, or RK13 cells, expression of the cowpox virus CP77 early gene, or the vaccinia virus K1L early gene suppresses MVA-induced IkappaBalpha depletion. In mouse embryonic fibroblasts (MEFs), MVA induction of IkappaBalpha depletion is dependent on the expression of mouse or human double-stranded RNA-activated protein kinase (PKR). These results demonstrate that events during the early phase of MVA replication can induce PKR-mediated processes contributing both to the activation of NF-kappaB signaling, and to processes that may restrict viral replication. This property may contribute to the efficacy of this vaccine virus.

Tumor necrosis factor (TNF)-alpha persistently activates nuclear factor-kappaB signaling through the type 2 TNF receptor in chromaffin cells: implications for long-term regulation of neuropeptide gene expression in inflammation

Chromaffin cells of the adrenal medulla elaborate and secrete catecholamines and neuropeptides for hormonal and paracrine signaling in stress and during inflammation. We have recently documented the action of the cytokine TNF-alpha on neuropeptide secretion and biosynthesis in isolated bovine chromaffin cells. Here, we demonstrate that the type 2 TNF-alpha receptor (TNF-R2) mediates TNF-alpha signaling in chromaffin cells via activation of nuclear factor (NF)-kappaB. Microarray and suppression subtractive hybridization have been used to identify TNF-alpha target genes in addition to those encoding the neuropeptides galanin, vasoactive intestinal polypeptide, and secretogranin II in chromaffin cells. TNF-alpha, acting through the TNF-R2, causes an early up-regulation of NF-kappaB, long-lasting induction of the NF-kappaB target gene inhibitor kappaB (IkappaB), and persistent stimulation of other NF-kappaB-associated genes including mitogen-inducible gene-6 (MIG-6), which acts as an IkappaB signaling antagonist, and butyrate-induced transcript 1. Consistent with long-term activation of the NF-kappaB signaling pathway, delayed induction of neuropeptide gene transcription by TNF-alpha in chromaffin cells is blocked by an antagonist of NF-kappaB signaling. TNF-alpha-dependent signaling in neuroendocrine cells thus leads to a unique, persistent mode of NF-kappaB activation that features long-lasting transcription of both IkappaB and MIG-6, which may play a role in the long-lasting effects of TNF-alpha in regulating neuropeptide output from the adrenal, a potentially important feedback station for modulating long-term cytokine effects in inflammation.

Cytomegalovirus-associated allograft rejection in heart transplant patients

PURPOSE OF REVIEW

Modern antiviral strategies are effective in controlling the clinical syndromes associated with acute cytomegalovirus infection in heart transplant recipients. Despite this effectiveness, subclinical cytomegalovirus infection is a common finding in these patients and its impact on long-term graft outcome is currently underestimated.

RECENT FINDINGS

Recent studies provide evidence implicating subclinical cytomegalovirus infection in the pathogenesis of allograft rejection and cardiac allograft vasculopathy. In this process, cytomegalovirus interacts with local inflammatory pathways, and systemic immune-regulation mechanisms, which may lead to graft damage, even in the absence of cytomegalovirus replication within the graft. Consequently, in addition to pharmacologic strategies that inhibit viral replication, immune-based therapies that abrogate host immune response may provide an effective tool to prevent the indirect impact of cytomegalovirus on graft function.

SUMMARY

Current evidence suggests that subclinical cytomegalovirus infection plays an important role in the pathogenesis of long-term graft dysfunction in heart transplant recipients and in other solid organ transplant recipients. Pending the availability of definitive data from randomized trials, we propose that the use of pharmacologic and immune-based approaches, directed at complete suppression of cytomegalovirus infection, represents the best strategy for prevention of cytomegalovirus-induced rejection, cardiac allograft vasculopathy and chronic allograft damage.

Does cytomegalovirus play a causative role in the development of various inflammatory diseases and cancer?

Human cytomegalovirus (HCMV) is a herpes virus that infects and is carried by 70-100% of the world's population. During its evolution, this virus has developed mechanisms that allow it to survive in an immunocompetent host. For many years, HCMV was not considered to be a major human pathogen, as it appeared to cause only rare cases of HCMV inclusion disease in neonates. However, HCMV is poorly adapted for survival in the immunosuppressed host and has emerged as an important human pathogen in AIDS patients and in patients undergoing immunosuppressive therapy following organ or bone marrow transplantation. HCMV-mediated disease in such patients has highlighted the possible role of this virus in the development of other diseases, in particular inflammatory diseases such as vascular diseases, autoimmune diseases and, more recently, with certain forms of cancers. Current research is focused on determining whether HCMV plays a causative role in these diseases or is merely an epiphenomenon of inflammation. Inflammation plays a central role in the pathogenesis of HCMV. This virus has developed a number of mechanisms that enable it to hide from the cells of the immune system and, at the same time, reactivation of a latent infection requires immune activation. Numerous products of the HCMV genome are devoted to control central functions of the innate and adaptive immune responses. By influencing the regulation of various cellular processes including the cell cycle, apoptosis and migration as well as tumour invasiveness and angiogenesis, HCMV may participate in disease development. Thus, the various drugs now available for treatment of HCMV disease (e.g. ganciclovir, acyclovir and foscarnet), may also prove to be useful in the treatment of other, more widespread diseases.

Tumor necrosis factor receptor 2 (TNFR2) signaling is negatively regulated by a novel, carboxyl-terminal TNFR-associated factor 2 (TRAF2)-binding site

Tumor necrosis factor (TNF) superfamily receptors typically induce both NF-kappaB and JNK activation by recruiting the TRAF2 signal transduction protein to their cytoplasmic domain. The type 2 TNF receptor (TNFR2), however, is a poor activator of these signaling pathways despite its high TRAF2 binding capability. This apparent paradox is resolved here by the demonstration that TNFR2 carries a novel carboxyl-terminal TRAF2-binding site (T2bs-C) that prevents the delivery of activation signals from its conventional TRAF2-binding site (T2bs-N). T2bs-C does not conform to canonical TRAF2 binding motifs and appears to bind TRAF2 indirectly via an as yet unidentified intermediary. Specific inactivation of T2bs-N by site-directed mutagenesis eliminated most of the TRAF2 recruited to the TNFR2 cytoplasmic domain but had no effect on ligand-dependent activation of the NF-kappaB or JNK pathways. By contrast, inactivation of T2bs-C had little effect on the amount of TRAF2 recruited but greatly enhanced ligand-dependent NF-kappaB and JNK activation. In wild-type TNFR2 therefore, T2bs-C acts in a dominant fashion to attenuate signaling by the intrinsically more active T2bs-N but not by preventing TRAF2 recruitment. This unique uncoupling of TRAF2 recruitment and signaling at T2bs-N may be important in the modulation by TNFR2 of signaling through coexpressed TNFR1.

Thrombospondin-1 up-regulates expression of cell adhesion molecules and promotes monocyte binding to endothelium

Expression of cell adhesion molecules (CAM) responsible for leukocyte-endothelium interactions plays a crucial role in inflammation and atherogenesis. Up-regulation of vascular CAM-1 (VCAM-1), intracellular CAM-1 (ICAM-1), and E-selectin expression promotes monocyte recruitment to sites of injury and is considered to be a critical step in atherosclerotic plaque development. Factors that trigger this initial response are not well understood. As platelet activation not only promotes thrombosis but also early stages of atherogenesis, we considered the role of thrombospondin-1 (TSP-1), a matricellular protein released in abundance from activated platelets and accumulated in sites of vascular injury, as a regulator of CAM expression. TSP-1 induced expression of VCAM-1 and ICAM-1 on endothelium of various origins, which in turn, resulted in a significant increase of monocyte attachment. This effect could be mimicked by a peptide derived from the C-terminal domain of TSP-1 and known to interact with CD47 on the cell surface. The essential role of CD47 in the cellular responses to TSP-1 was demonstrated further using inhibitory antibodies and knockdown of CD47 with small interfering RNA. Furthermore, we demonstrated that secretion of endogenous TSP-1 and its interaction with CD47 on the cell surface mediates endothelial response to the major proinflammatory agent, tumor necrosis factor alpha (TNF-alpha). Taken together, this study identifies a novel mechanism regulating CAM expression and subsequent monocyte binding to endothelium, which might influence the development of anti-atherosclerosis therapeutic strategies.

Caveolae participate in tumor necrosis factor receptor 1 signaling and internalization in a human endothelial cell line

Caveolae are abundant in endothelial cells (ECs) in situ but markedly diminished in cultured cells, making it difficult to assess their role in cytokine signaling. We report here that the human EC line EA.hy926 retains an abundant caveolar system in culture. Tumor necrosis factor (TNF) receptor 1 (TNFR1/CD120a) was enriched in caveolae and co-immunoprecipitated with caveolin-1 from caveolae isolated from these cells. To further investigate the role(s) of caveolae in TNF signaling in ECs, cells were treated with methyl-beta-cyclodextrin to disrupt caveolae. Methyl-beta-cyclodextrin did not alter total cell surface expression of TNFR1 or TNF-induced degradation of IkappaBalpha, a measure of nuclear factor-kappaB activation, but it did inhibit TNF-induced phosphorylation of Akt, a measure of phosphatidylinositol-3 kinase activation. Serum-induced phosphorylation of AKT was unaffected. Treatment with TNF induced disappearance of TNFR1 from caveolae and dissociation from caveolin-1 within 5 minutes. In contrast to transferrin receptor, internalized TNFR1 did not co-localize with clathrin, except possibly in the Golgi, at any time point examined. By 60 minutes of treatment with TNF, TNFR1 appeared in endosomes. We conclude that caveolae function in ECs to allow TNFR1 to activate phosphatidylinositol-3 kinase and Akt, perhaps through receptor cross talk, and that ligand-induced internalization and trafficking of TNFR1 to endosomes may originate directly from this compartment.

Role of tumor necrosis factor alpha and its receptor I in preconditioning by hyperoxia

Hyperoxic pretreatment (>95% O(2)) can evoke myocardial adaptation to ischemia, a method which is potentially clinically usable. We wanted to investigate the role of tumor necrosis factor alpha (TNFalpha) and its p55 receptor (receptor I) in signaling of hyperoxic adaptation to ischemia. Mice deficient for TNFalpha (TNFalpha -/-) or the TNF receptor I (TNFRI -/-) gene and their wild types were subjected to 60 minutes of hyperoxia or sham treatment. Their lungs were then collected for immunoblotting, their hearts isolated and subjected to global ischemia and reperfusion in a Langendorff system, and aortic rings mounted in organ baths for reactivity studies. Hyperoxia increased expression of TNFalpha and TNFalpha converting enzyme in pulmonary proteins from wild type mice, in which hyperoxia increased myocardial tolerance to ischemia. Post-ischemic heart function was improved and infarct size reduced in wild type mice, but not in TNFalpha -/- or TNFRI -/-. The contractile response to TNFalpha on aortic rings was attenuated by hyperoxic pretreatment and by TNFRI -/-. Thus we conclude that TNFalpha, acting through TNFRI, appears important for the protective effects of hyperoxia.

Resistance of human ovarian cancer cells to tumor necrosis factor alpha is a consequence of nuclear factor kappaB-mediated induction of Fas-associated death domain-like interleukin-1beta-converting enzyme-like inhibitory protein

The purpose of the present studies was to examine the role and regulation of Fas-associated death domain-like IL-1- converting enzyme-like inhibitory protein [FLIP; long (FLIP(L)) and short (FLIP(S)) forms] in human ovarian epithelial cancer cells by TNFalpha and their significance in the resistance of the cells to the proapoptotic action of the cytokine. OV2008, A2780-s, and OVCAR-3 cells were cultured in serum-free media with or without cycloheximide (CHX, 10 micro g/ml) +/- TNFalpha (5, 10, 20 ng/ml) or transfected with a mammalian expression vector containing either a dominant negative inhibitor kappaB (IkappaB), FLIP(S) sense or antisense cDNA and cultured with or without TNFalpha. In the presence of CHX, TNFalpha increased caspase-8 and -3 cleavage and apoptosis. It also induced IkappaB phosphorylation, nuclear factor kappaB activation, and the expression of FLIP(S) but not of FLIP(L). Overexpression of dominant negative IkappaB attenuated TNFalpha-induced FLIP(S) expression and enhanced TNFalpha-induced apoptosis. Apoptosis induced by TNFalpha and CHX was facilitated by FLIP(S) antisense expression but attenuated by sense transfection. This study demonstrates that TNFalpha up-regulates FLIP(S) expression, and this effect is mediated by the activation of nuclear factor kappaB. The induction of FLIP(S) expression by TNFalpha might contribute to the resistance of ovarian epithelial cancer cells to the proapoptotic action of the cytokine.

Role of X-linked inhibitor of apoptosis protein in chemoresistance in ovarian cancer: possible involvement of the phosphoinositide-3 kinase/Akt pathway

Although cisplatin derivatives are first-line chemotherapeutic agents for the treatment of epithelial ovarian cancer, chemoresistance remains a major hurdle to successful therapy and the molecular mechanisms involved are poorly understood. Apoptosis is the cellular underpinning of cisplatin-induced cell death, which is associated with expression of specific "death" genes and down-regulation of "survival" counterparts. The X-linked inhibitor of apoptosis proteins (Xiap), an intracellular anti-apoptotic protein, plays a key role in cell survival by modulating death signaling pathways and is a determinant of cisplatin resistance in ovarian cancer cells in vitro. This review focuses on the role of Xiap and its interactions with the phosphoinositide-3 kinase (PI3K)/Akt cell survival pathway in conferring resistance of ovarian cancer cells to chemotherapeutic agents and discusses potential therapeutic strategies in overcoming chemoresistant ovarian cancer.

TNF receptor subtype signalling: differences and cellular consequences

Tumour necrosis factor-alpha (TNF alpha) is a multifunctional cytokine belonging to a family of ligands with an associated family of receptor proteins. The pleiotropic actions of TNF range from proliferative responses such as cell growth and differentiation, to inflammatory effects and the mediation of immune responses, to destructive cellular outcomes such as apoptotic and necrotic cell death mechanisms. Activated TNF receptors mediate the association of distinct adaptor proteins that regulate a variety of signalling processes including kinase or phosphatase activation, lipase stimulation, and protease induction. Moreover, the cytokine regulates the activities of transcription factors, heterotrimeric or monomeric G-proteins and calcium ion homeostasis in order to orchestrate its cellular functions. This review addresses the structural basis of TNF signalling, the pathways employed with their cellular consequences, and focuses on the specific role played by each of the two TNF receptor isotypes, TNFR1 and TNFR2.

Induction of macrophage-inflammatory protein-3alpha gene expression by TNF-dependent NF-kappaB activation

Macrophage-inflammatory protein-3alpha (MIP-3alpha), also designated as liver and activation-regulated chemokine (LARC), Exodus, or CCL20, is a recently identified CC chemokine that is expected to play a crucial role in the initiation of immune responses. In this study, we describe that MIP-3alpha expression is under the direct control of NF-kappaB, a key transcription factor of immune and inflammatory responses. Overexpression of the p65/RelA subunit of NF-kappaB significantly increased the MIP-3alpha mRNA level. MIP-3alpha transcription was stimulated by TNF, and this stimulation was inhibited by an NF-kappaB inhibitor, I-kappaBalpha superrepressor. Analysis of the human MIP-3alpha promoter demonstrated a functional NF-kappaB site responsible for its expression. We also show that MIP-3alpha expression is induced in LPS-treated mouse livers that were primed with Propionibacterium acnes, which developed massive liver injury with infiltration of inflammatory cells. This induction was fully dependent on the TNF signaling cascade, because it was not observed in the livers of TNFR1-deficient mice. Furthermore, pretreatment with gliotoxin, an inhibitor of NF-kappaB activity, abrogated the P. acnes/LPS-induced MIP-3alpha expression of wild-type mice. These results clearly demonstrate that MIP-3alpha gene expression is dependent on NF-kappaB activity in vitro, and indicate that the TNFR1-mediated TNF signaling cascade that leads to NF-kappaB activation plays an essential role in MIP-3alpha expression in the murine liver injury model.

African swine fever virus IAP-like protein induces the activation of nuclear factor kappa B

African swine fever virus (ASFV) encodes a homologue of the inhibitor of apoptosis (IAP) that promotes cell survival by controlling the activity of caspase-3. Here we show that ASFV IAP is also able to activate the transcription factor NF-kappaB. Thus, transient transfection of the viral IAP increases the activity of an NF-kappaB reporter gene in a dose-responsive manner in Jurkat cells. Similarly, stably transfected cells expressing ASFV IAP have elevated basal levels of c-rel, an NF-kappaB-dependent gene. NF-kappaB complexes in the nucleus were increased in A224L-expressing cells compared with control cells upon stimulation with phorbol myristate acetate (PMA) plus ionomycin. This resulted in greater NF-kappaB-dependent promoter activity in ASFV IAP-expressing than in control cells, both in basal conditions and after PMA plus ionophore stimulation. The elevated NF-kappaB activity seems to be the consequence of higher IkappaB kinase (IKK) basal activity in these cells. The NF-kappaB-inducing activity of ASFV IAP was abrogated by an IKK-2 dominant negative mutant and enhanced by expression of tumor necrosis factor receptor-associated factor 2.

Differential activation of nuclear factor-kappaB by tumour necrosis factor receptor subtypes. TNFR1 predominates whereas TNFR2 activates transcription poorly

Tumour necrosis factor-alpha (TNF-alpha) signals though two receptors, TNFR1 and TNFR2. TNFR1 has a role in cytotoxicity, whereas TNFR2 regulates death responses or proliferation. TNF activates pro-inflammatory transcription factor nuclear factor-kappaB (NF-kappaB) by uncertain signalling mechanisms. Here we report the contribution of each TNFR towards the NF-kappaB activation processes. In human cells expressing endogenous or exogenous TNFR2, in addition to TNFR1, we found both TNFRs capable of activating NF-kappaB, as measured by IkappaBalpha (inhibitor of NF-kappaB) degradation, electrophoretic mobility shift assay and NF-kappaB gene reporter assays. TNFR2 activation did not degrade IkappaBbeta. However, TNF-effects on NF-kappaB activation occurred predominantly through TNFR1, with TNFR2 activating the transcription factor poorly.

Molecular mechanisms involved in gastrin-mediated regulation of cAMP-responsive promoter elements

In the present study, we explore the role of cAMP-responsive (CRE) promoter elements in gastrin-mediated gene activation. By using the minimal CRE promoter reporter plasmid, pCRELuc, we show that gastrin can activate CRE. This activation is blocked by H-89 and GF 109203x, which inhibit protein kinases A and C, respectively. Moreover, Ca(2+)-activated pathways seem to be involved, because the calmodulin inhibitor W-7 reduced gastrin-mediated activation of pCRELuc. Deletion of CRE from the c-fos promoter rendered this promoter completely unresponsive to gastrin, indicating that CRE plays a central role in c-fos transactivation. Interestingly, gastrin-induced expression of the inducible cAMP early repressor (ICER), a gene that is known to be regulated by CRE promoter elements, was not reduced by H-89, W-7, or GF 109203x. Furthermore, bandshift analyses indicated that the region of the ICER promoter containing the CRE-like elements CARE 3-4 binds transcription factors that are not members of the CRE-binding protein-CRE modulator protein-activating transcription factor, or CREB/CREM/ATF-1, family. Our results underline the significance of the CRE promoter element in gastrin-mediated gene regulation and indicate that a variety of signaling mechanisms are involved, depending on the CRE promoter context.

Increased persistence of lung gene expression using plasmids containing the ubiquitin C or elongation factor 1alpha promoter

For effective gene therapy of chronic disease, persistent transgene expression at therapeutic levels is required. Clinical studies of airway gene transfer in patients with cystic fibrosis (CF) have resulted in short-lived transgene expression. We used intra-nasal dosing of naked plasmid DNA to the murine lung as a model for investigating the duration of airway gene transfer from a series of reporter expression plasmids. Transgene expression was transient when mediated by the viral promoters CMV, RSV and SV40, falling to less than 10% of peak expression after 2 weeks, although the presence of the adenoviral E4ORF3 gene in cis, resulted in extended duration of reporter activity from the CMV promoter. Transient expression from these promoters was not due to loss of the vector as determined by quantitative TaqMan PCR analysis. However, use of the promoters from the human polybiquitin C (UbC) and the elongation factor 1alpha (EF1alpha) genes resulted in persistent gene expression in the mouse lung. The UbC promoter directed high-level reporter activity which was maintained for up to 8 weeks and was still detectable 6 months after a single administration. Such persistent airway transgene expression from a nonviral vector without the concomitant expression of a potential antigen has not been reported previously. Thus, despite the persistence of vector DNA in vivo, attenuation of promoter function may lead to silencing of transgene expression and careful selection of promoter sequences is recommended for in vivo gene transfer.

Cowpox virus and other members of the orthopoxvirus genus interfere with the regulation of NF-kappaB activation

NF-kappaB comprises a family of transcription factors that regulate key immune processes. In this study, the effects of orthopoxvirus infection upon the activation of NF-kappaB were examined. During the early phase of infection, cowpox virus can inhibit the induction of NF-kappaB-regulated gene expression by interfering with the process of IkappaBalpha degradation. Although either okadaic acid or tumor necrosis factor (TNF) treatment of infected cells can induce IkappaBalpha phosphorylation, further processing of IkappaBalpha is inhibited. These results suggest that cowpox virus is capable of inhibiting the activation of NF-kappaB at a point where multiple signal transduction pathways converge. Other orthopoxviruses affect NF-kappaB activity, but in a type-specific manner. Raccoonpox virus and vaccinia virus (Copenhagen strain) negatively affect NF-kappaB induction by TNF. In contrast, the modified vaccinia virus Ankara strain induces NF-kappaB activation, even in the absence of other stimuli. These findings suggest that orthopoxviruses may affect a broad range of virus-host interactions through their effects upon NF-kappaB activation. Moreover, because of the central role for NF-kappaB in immune processes and disease, these type-specific effects may contribute significantly to the immunogenic and pathogenic properties of poxviruses.

HIV type 1 Tat inhibits tumor necrosis factor alpha-induced repression of tumor necrosis factor receptor p55 and amplifies tumor necrosis factor alpha activity in stably tat-transfected HeLa Cells

The human immunodeficiency virus type 1 (HIV-1) Tat protein is a key regulatory protein in the HIV-1 replication cycle. Tat interacts with cellular transcriptional factors and cytokines, such as tumor necrosis factor (TNF-alpha), and alters the expression of a variety of genes in HIV-1-infected and noninfected cells. To further elucidate the mechanisms by which HIV-1 Tat amplifies the activity of TNF-alpha, we transfected the HIV-1 tat gene into an epithelial (HeLa) cell line. We observed that Tat-expressing cells had increased NF-kappa B-dependent trans-activational activity due to enhanced NF-kappa B--DNA binding in response to TNF-alpha treatment. Tumor necrosis factor receptor (TNFR) p55 was the prominent receptor, as neutralizing antibodies to TNFR p55, but not to TNFR p75, blocked TNF-alpha-mediated NF-kappa B activation. Furthermore, tat-transfected cells were more sensitive to TNF-alpha-induced cytotoxicity and only the neutralizing antibodies to TNFR p55 completely protected the cells. To determine whether TNFR p55 was involved in amplification of cellular response to TNF-alpha by HIV-1 Tat, we investigated the effect of TNF-alpha on TNFR p55 expression in the tat-transfected cells. TNF-alpha treatment resulted in a reduction in both TNFR p55 mRNA and protein levels in the control cells but not in the tat-transfected cells as determined with Northern blot and Western blot analyses, respectively. Our results indicate that HIV-1 Tat may inhibit TNF-alpha-induced repression of TNFR p55 and thereby amplify TNF-alpha activity in these stably transfected cells.

Role of interferon alpha (IFN-alpha) and interferon gamma (IFN-gamma) in the control of the infection of monocyte-like cells with human cytomegalovirus (HCMV)

The role of cytokines in the control of HCMV infection has been studied in THP-1 cells, a macrophage-like cell model and in MRC-5 cells. HCMV replication was studied by immune detection of viral immediate-early antigens (IEA) and virus yield was evaluated in MRC-5 cells by immunoperoxidase staining. Pretreatment of MRC-5 and phorbol 12-myristate 13-acetate (PMA)-treated THP-1 cells with IFN-alpha or IFN-gamma for 24 hr prior to the infection reduced the number of infected cells and virus yield. A synergistic anti-CMV activity in synthesis of early proteins was obtained with these cytokines in combination with TNF-alpha in differentiated THP-1 cells only. Treatment of HCMV-infected differentiated THP-1 cells or MRC-5 cells with IFN-alpha or IFN-gamma alone had no inhibitory effect on virus replication, however the virus yield was reduced with ganciclovir. A synergistic anti-CMV activity in virus yield was obtained only when infected differentiated THP-1 cells were treated with ganciclovir in combination with IFN-gamma. The current study shows that IFN-alpha and IFN-gamma can play a role in the reduction of HCMV replication in macrophage-like cells and in the efficiency of therapies with ganciclovir in this cell type and that the anti-CMV effect of cytokines may be different in fibroblasts and in macrophage-like cells.

Regulation of inducible cAMP early repressor expression by gastrin and cholecystokinin in the pancreatic cell line AR42J

The CREM gene encodes both activators and repressors of cAMP-induced transcription. Inducible cAMP early repressor (ICER) isoforms are generated upon activation of an alternative, intronic promoter within the CREM gene. ICER is proposed to down-regulate both its own expression and the expression of other genes that contain cAMP-responsive elements such as a number of growth factors. Thus, ICER has been postulated to play a role in proliferation and differentiation. Here we show that ICER gene expression is induced by gastrin, cholecystokinin (CCK), and epidermal growth factor in AR42J cells. The time course of gastrin- and CCK-mediated ICER induction is rapid and transient, similar to forskolin- and phorbol 12-myristate 13-acetate-induced ICER expression. The specific CCK-B receptor antagonist L740,093 blocks the gastrin but not the CCK response, indicating that both the CCK-B and the CCK-A receptor can mediate ICER gene activation. Noteworthy, CREB is constitutively phosphorylated at Ser-133 in AR42J cells, and ICER induction proceeds in the absence of increased CREB Ser(P)-133. Gastrin-mediated ICER induction was not reduced in the presence of the protein kinase A inhibitor H-89, indicating a protein kinase A-independent mechanism. This is the first report on ICER inducibility via G(q)/G(11) protein-coupled receptors.

Roles of tumor necrosis factor receptor signaling during murine Escherichia coli pneumonia

We hypothesized that tumor necrosis factor (TNF)-alpha signaling is essential to inflammation and host defense during Escherichia coli pneumonia. We tested this hypothesis by instilling E. coli into the lungs of wild-type (WT) mice and gene-targeted mice that lack both p55 and p75 receptors for TNF-alpha. The emigration of neutrophils 6 h after instillation of E. coli was not decreased, but rather was significantly increased (167% of WT), in TNF receptor (TNFR)-deficient mice. This increased neutrophil emigration did not result from peripheral blood neutrophilia or enhanced neutrophil sequestration, inasmuch as the numbers of neutrophils in the circulating blood and in the pulmonary capillaries did not differ between TNFR-deficient and WT mice. The accumulation of pulmonary edema fluid was not inhibited in TNFR-deficient compared with WT mice. Nuclear factor-kappaB (NF-kappaB) translocation in the lungs was not prevented in TNFR-deficient mice. Thus, signaling pathways independent of TNFRs can mediate the acute inflammatory response during E. coli pneumonia. However, despite this inflammatory response, bacterial clearance was impaired in TNFR-deficient mice (109 +/- 8% versus 51 +/- 14% of the original inoculum viable after 6 h in TNFR-deficient and WT mice, respectively). Increased neutrophil emigration during E. coli pneumonia in TNFR-deficient mice may thus result from an increased bacterial burden in the lungs. During acute E. coli pneumonia, the absence of TNFR signaling compromised bacterial killing, but did not prevent inflammation, as measured by the accumulation of edema fluid and neutrophils.

LPS mediated production of IL-1, PGE2 and PGF2alpha from term decidua involves tumour necrosis factor and tumour necrosis factor receptor p55

Prostaglandins, with cytokines involved as intermediate factors, may have an essential role in premature labour when infection is present. We therefore wanted to study tumour necrosis factor (TNF), in cytokine and prostaglandin production in reproductive tissue. Decidual cell cultures were established and cells were stimulated with lipopolysaccharides (LPS). Media concentrations of TNF, interleukin-1 (IL-1), IL-6 and prostaglandin E2 and F2alpha were analysed, and involvement of LPS receptor CD14, TNF and TNF receptors (p55 and p75) were analysed, by studying effects after administration of specific antibodies. LPS induced an early peak elevation of TNF, with a subsequent release of IL-1, IL-6 and prostaglandins. Antibodies against CD14 inhibited these LPS effects. TNF antibodies reduced production of IL-1 and prostaglandins, whereas no significant influence on IL-6 production was observed. Antibodies against the TNF receptor p55 reduced all observed TNF effects. In contrast, p75 antibodies did not influence cytokine or prostaglandin production in this system. Our results suggest that increased TNF production is a prerequisite for LPS stimulated production of IL-1 and prostaglandins from decidual cells. LPS may directly stimulate IL-6 production. Of the two TNF receptors studied, only p55 seemed to be involved in the TNF signal transduction.

Apoptosis, proliferation and NF-kappaB activation induced by agonistic Fas antibodies in the human myeloma cell line OH-2: amplification of Fas-mediated apoptosis by tumor necrosis factor

Tumor necrosis factor (TNF) is known to be a growth factor for several myeloma cell lines. However, in the presence of the agonistic Fas antibody CH 11, TNF enhanced the level of apoptosis in cultures of the human myeloma cell line OH-2. This pro-apoptotic effect of TNF was explained at least in part by a TNF-mediated enhancement of Fas expression. TNF induces proliferation of OH-2 by activating nuclear transcription factor kappa-B (NF-kappaB). The proliferative effect of TNF on OH-2 cells was abrogated by CH11, but this was not caused by an inhibition of the translocation of NF-kappaB. On the contrary, CH11 could by itself activate NF-kappaB in OH-2 cells, and in the presence of an inhibitor of caspase-1 induce proliferation of the cells. The relationship between stimulation of TNF receptors and Fas and the level of NF-kappaB activation was also examined in three other myeloma cell lines. RPMI-8226 cells showed NF-kappaB activation by TNF, but contrary to OH-2, not by CH11. Unstimulated U-266 and JJN-3 cells had high levels of activated NF-kappaB. This shows that NFkappa-B is either constitutively activated or inducible in myeloma cells. Modulation of Fas expression and inhibition of NF-kappaB activation can potentially be of therapeutic importance in multiple myeloma.

Selective Inhibitors of Cytosolic or Secretory Phospholipase A2 Block TNF-Induced Activation of Transcription Factor Nuclear Factor-κB and Expression of ICAM-1

TNF signaling mechanisms involved in activation of transcription factor NF-κB were studied in the human keratinocyte cell line HaCaT. We show that TNF-induced activation of NF-κB was inhibited by the well-known selective inhibitors of cytosolic phospholipase A2 (cPLA2): the trifluoromethyl ketone analogue of arachidonic acid (AACOCF3) and methyl arachidonyl fluorophosphate. The trifluoromethyl ketone analogue of eicosapentaenoic acid (EPACOCF3) also suppressed TNF-induced NF-κB activation and inhibited in vitro cPLA2 enzyme activity with a similar potency as AACOCF3. The arachidonyl methyl ketone analogue (AACOCH3) and the eicosapentanoyl analogue (EPACHOHCF3), which both failed to inhibit cPLA2 enzyme activity in vitro, had no effect on TNF-induced NF-κB activation. TNF-induced NF-κB activation was also strongly reduced in cells stimulated in the presence of the secretory PLA2 (sPLA2) inhibitors 12-epi-scalaradial and LY311727. Addition of excess arachidonic acid suppressed the inhibitory effect of 12-epi-scalaradial and LY311727. Moreover, both methyl arachidonyl fluorophosphate and 12-epi-scalaradial blocked TNF-mediated enhancement of expression of ICAM-1. Activation of NF-κB by IL-1β was markedly less sensitive to both cPLA2 and sPLA2 inhibitors. The results indicate that both cPLA2 and sPLA2 may be involved in the TNF signal transduction pathway leading to nuclear translocation of NF-κB and to NF-κB-activated gene expression in HaCaT cells.

Overexpression of the p80 TNF Receptor Leads to TNF-Dependent Apoptosis, Nuclear Factor-κB Activation, and c-Jun Kinase Activation

Because they have distinct intracellular domains, it has been proposed that the p60 and p80 forms of the TNF receptor mediate different signals. Several signaling proteins have been isolated that associate with either the p60 or the p80 receptor. By using TNF muteins specific to the p60 and p80 receptors, we have previously shown that cytotoxicity and nuclear factor-κB (NF-κB) activation are mediated through the p60 form of the endogenous receptor. What signals are mediated through the p80 receptor is less clear. This study was an effort to answer that question. HeLa cells, which express only p60 receptors, were transfected with p80 receptor cDNA and then examined for apoptosis, NF-κB activation, and c-Jun kinase activation induced by TNF and by p60 or p80 receptor-specific muteins. The p80 mutein, like TNF and the p60 mutein, induced apoptosis and activation of NF-κB and c-Jun kinase in cells overexpressing recombinant p80 receptor but had no effect on cells expressing a high level of endogenous p80 receptor. The apoptosis mediated through the p60 receptor was also potentiated after overexpression of the p80 receptor, suggesting a synergistic relationship between the two receptors. Interestingly, Abs to the p80 receptor blocked apoptosis induced by all ligands but by itself activated NF-κB in the p80-transfected cells. Overall, our results show that the p80 receptor, which lacks the death domain, mediated apoptosis, NF-κB activation, and c-Jun kinase activation, but only when it was overexpressed, whereas endogenous p60 receptor mediated similar signals without overexpression.

Evidence for a synergistic role of two types of human tumor necrosis factor receptors for the ligand-dependent activation of the nuclear transcription factor NF-kappaB

Tumor necrosis factor (TNF) is a multipotential cytokine that interacts with a wide variety of cells through two distinct receptors, referred to as the p60 and p80 receptors. Why there are two distinct receptors for the same ligand and whether these receptors mediate their signal independently or synergistically is not known. We examined the role of these two receptors in the ligand-dependent activation of a transcriptional factor, NF-kappaB, an early response (5-15 min) to TNF in human myeloid ML-1a cells. By using receptor type-specific antibodies, these cells were found to express almost equal amounts of both receptors. TNF-dependent activation of NF-kappaB could be blocked partially by both anti-p60 and anti-p80, suggesting that TNF mediates its effect independently through the p60 and p80 receptors. In comparison, the activation of NF-kappaB by lymphotoxin (LT), which shares receptors with TNF, was completely blocked by anti-p60, whereas anti-p80 had no effect. Anti-p60 but not anti-p80 by itself was found to activate NF-kappaB in a dose-dependent manner, but on a molar basis anti-p60 was found to be 100 times less potent than TNF. Interestingly, even though anti-p80 by itself was inactive, it potentiated the effect of anti-p60 synergistically, suggesting an interaction between the two types of TNF receptor. Thus, overall these results demonstrate that the two forms of TNF receptors could mediate their signal in both an independent and synergistic manner and that TNF mediates its signal through both forms of receptors, whereas LT mediates its signal through the p60 receptor.

Synergistic activation of the human cytomegalovirus major immediate early promoter by prostaglandin E2 and cytokines

Human cytomegalovirus (HCMV) is a common cause of morbidity and mortality in immunosuppressed patients, especially transplant recipients. In this population, infection is frequently due to reactivation of latent virus. The major immediate early promoter of HCMV controls production of immediate early gene products, which are both trans- and cis-active and are responsible for reactivation. Activation of this promoter is therefore a crucial step in regulation of reactivation infection. It is known that there are cAMP-response elements in the HCMV major immediate early promoter. We hypothesized that prostaglandins (PG), like PGE2, which are known to increase cAMP, as well as cytokines known to be released during acute inflammation, may be important in the regulation of this promoter and thus in reactivation of HCMV. To examine this, we transfected pCAT760, a plasmid containing the major immediate early promoter of HCMV upstream of a chloramphenicol acetyltransferase (CAT) gene, into THP-1 cells. These cells were subsequently stimulated with PGE2 and/or one of a variety of cytokines. We found that PGE2, tumor necrosis factor (TNF)-alpha, and interleukin (IL)-1 beta each upregulated the HCMV major immediate early promoter. TNF-alpha, IL-1 beta, IL-6, and IL-10 were each synergistic or additive with PGE2 in upregulating the promoter. Since PGE2 and the cytokines are all products of activated macrophages, we suggest that acute inflammation and macrophage activation may predispose to reactivation of latent HCMV.

Promoter attenuation in gene therapy: interferon-gamma and tumor necrosis factor-alpha inhibit transgene expression

One of the major limitations to current gene therapy is the low-level and transient vector gene expression due to poorly defined mechanisms, possibly including promoter attenuation or extinction. Because the application of gene therapy vectors in vivo induces cytokine production through specific or nonspecific immune responses, we hypothesized that cytokine-mediated signals may alter vector gene expression. Our data indicate that the cytokines interferon-gamma (IFN-gamma) and tumor necrosis factor-alpha (TNF-alpha) inhibit transgene expression from certain widely used viral promoters/enhancers (cytomegalovirus, Rous sarcoma virus, simian virus 40, Moloney murine leukemia virus long terminal repeat) delivered by adenoviral, retroviral or plasmid vectors in vitro. A constitutive cellular promoter (beta-actin) is less sensitive to these cytokine effects. Inhibition is at the mRNA level and cytokines do not cause vector DNA degradation, inhibit total cellular protein synthesis, or kill infected/transfected cells. Administration of neutralizing anti-IFN-gamma monoclonal antibody results in enhanced transgene expression in vivo. Thus, standard gene therapy vectors in current use may be improved by altering cytokine-responsive regulatory elements. Determination of the mechanisms involved in cytokine-regulated vector gene expression may improve the understanding of the cellular disposition of vectors for gene transfer and gene therapy.

Dysregulation of Membrane-Bound Tumor Necrosis Factor-α and Tumor Necrosis Factor Receptors on Mononuclear Cells in Human Immunodeficiency Virus Type 1 Infection: Low Percentage of p75-Tumor Necrosis Factor Receptor Positive Cells in Patients With Advanced Disease and High Viral Load

The correlation of persistent tumor necrosis factor-α (TNF-α) activation with disease progression in patients infected with human immunodeficiency virus type 1 (HIV-1), suggests a role for TNF-α in the pathogenesis of HIV-1 infection. In the present study, we examined by flow cytometry the expression of membrane-bound (m) components of the TNF system in 33 HIV-1–infected patients and 12 healthy controls. While peripheral blood mononuclear cells (PBMC) from asymptomatic and symptomatic non-acquired immune deficiency syndrome (AIDS) patients showed a significantly increased percentage of mTNF-α+ and mTNF receptor (TNFR)+ cells compared with controls, this was not found in the AIDS group. Compared with healthy controls, AIDS patients had a significantly decreased percentage of both monocytes and lymphocytes expressing p75-TNFR. PBMC from AIDS patients showed a higher p75-TNFR mRNA level and a higher spontaneous release of soluble p75-TNFR than healthy individuals, suggesting enhanced cell surface turnover of this TNFR. The low expression of TNFRs on both lymphocytes and monocytes in the AIDS group was associated with high numbers of HIV-1 RNA copies in plasma, low numbers of CD4+ lymphocytes, and high serum levels of soluble TNFRs. AIDS patients had a decreased percentage of CD8+ lymphocytes expressing TNFRs compared with healthy controls. In contrast, these patients, as well as symptomatic non-AIDS patients, had an increased percentage of TNF-α+ and TNFRs+ cells among remaining CD4+ lymphocytes. The pattern of abnormalities seen in AIDS patients suggests a role for persistent activation of the TNF system in the accelerated CD4+ lymphocyte destruction, the enhanced HIV-1 replication, and the markedly impaired antimicrobial defense in advanced HIV-1-related disease.

Dysregulation of Membrane-Bound Tumor Necrosis Factor-α and Tumor Necrosis Factor Receptors on Mononuclear Cells in Human Immunodeficiency Virus Type 1 Infection: Low Percentage of p75-Tumor Necrosis Factor Receptor Positive Cells in Patients With Advanced Disease and High Viral Load

The correlation of persistent tumor necrosis factor-α (TNF-α) activation with disease progression in patients infected with human immunodeficiency virus type 1 (HIV-1), suggests a role for TNF-α in the pathogenesis of HIV-1 infection. In the present study, we examined by flow cytometry the expression of membrane-bound (m) components of the TNF system in 33 HIV-1–infected patients and 12 healthy controls. While peripheral blood mononuclear cells (PBMC) from asymptomatic and symptomatic non-acquired immune deficiency syndrome (AIDS) patients showed a significantly increased percentage of mTNF-α+ and mTNF receptor (TNFR)+ cells compared with controls, this was not found in the AIDS group. Compared with healthy controls, AIDS patients had a significantly decreased percentage of both monocytes and lymphocytes expressing p75-TNFR. PBMC from AIDS patients showed a higher p75-TNFR mRNA level and a higher spontaneous release of soluble p75-TNFR than healthy individuals, suggesting enhanced cell surface turnover of this TNFR. The low expression of TNFRs on both lymphocytes and monocytes in the AIDS group was associated with high numbers of HIV-1 RNA copies in plasma, low numbers of CD4+ lymphocytes, and high serum levels of soluble TNFRs. AIDS patients had a decreased percentage of CD8+ lymphocytes expressing TNFRs compared with healthy controls. In contrast, these patients, as well as symptomatic non-AIDS patients, had an increased percentage of TNF-α+ and TNFRs+ cells among remaining CD4+ lymphocytes. The pattern of abnormalities seen in AIDS patients suggests a role for persistent activation of the TNF system in the accelerated CD4+ lymphocyte destruction, the enhanced HIV-1 replication, and the markedly impaired antimicrobial defense in advanced HIV-1-related disease.

Suppression of tumor necrosis factor-induced cell death by inhibitor of apoptosis c-IAP2 is under NF-kappaB control

Members of the NF-kappaB/Rel and inhibitor of apoptosis (IAP) protein families have been implicated in signal transduction programs that prevent cell death elicited by the cytokine tumor necrosis factor alpha (TNF). Although NF-kappaB appears to stimulate the expression of specific protective genes, neither the identities of these genes nor the precise role of IAP proteins in this anti-apoptotic process are known. We demonstrate here that NF-kappaB is required for TNF-mediated induction of the gene encoding human c-IAP2. When overexpressed in mammalian cells, c-IAP2 activates NF-kappaB and suppresses TNF cytotoxicity. Both of these c-IAP2 activities are blocked in vivo by coexpressing a dominant form of IkappaB that is resistant to TNF-induced degradation. In contrast to wild-type c-IAP2, a mutant lacking the C-terminal RING domain inhibits NF-kappaB induction by TNF and enhances TNF killing. These findings suggest that c-IAP2 is critically involved in TNF signaling and exerts positive feedback control on NF-kappaB via an IkappaB targeting mechanism. Functional coupling of NF-kappaB and c-IAP2 during the TNF response may provide a signal amplification loop that promotes cell survival rather than death.

Interference with c-myc expression and RB phosphorylation during TNF-mediated growth arrest in human endothelial cells

Incorporation of [3H]-thymidine into DNA in non-synchronized cultures of human endothelial cells was blocked by a 24 h exposure to TNF in a dose dependent manner that resulted in accumulation of cells in G1, as assayed by flow cytometry analysis of DNA content. Proliferation restarted when cells were replated in the absence of TNF. Northern analysis of c-myc mRNA in synchronized untreated cultures showed a transient increase previous to DNA synthesis that was decreased with TNF treatment. Western analysis of the retinoblastoma gene product RB in untreated synchronized cultures showed reduced electrophoretic mobility during the transition from G1 to S, congruent with RB inactivation by phosphorylation. TNF treatment prevented RB retardation and reduced total levels of RB protein. Taken together our results show that the TNF-mediated block of endothelial proliferation correlates with deficient activation of the G1 events necessary for entry into S, despite the presence of serum and endothelial mitogens.

Ligand-induced formation of p55 and p75 tumor necrosis factor receptor heterocomplexes on intact cells

The p55 and p75 tumor necrosis factor receptors are known to mediate their effects on cells through distinct signaling pathways. Under certain circumstances, the two classes of TNF receptors cooperate with each another to produce enhanced cellular responses. The only molecular mechanism proposed thus far to explain this effect is the process of "ligand passing," whereby TNF is concentrated at cell surfaces by binding to p75 and then following dissociation from this receptor class binds with high efficiency to p55. Using the in vivo model of TNF-induced TNF receptor shedding we have uncovered a novel ligand-dependent interaction of the two TNF receptors that occurs upon exposure of cells to TNF. Using TNF receptor-specific monoclonal antibodies that bind TNF receptors in the presence or absence of ligand, we report that TNF induces the formation of heterocomplexes consisting of both p55 and p75 TNF receptors. Whereas immunoprecipitates from untreated or human TNF-treated cells formed with either p55 or p75 TNF receptor-specific monoclonal antibodies contained only the relevant TNF receptor class, anti-p55 or anti-p75 precipitated both receptor types from murine TNF-treated cells. Ligand-induced complex formation was transient, occurred at physiologically relevant concentrations of TNF, and occurred with receptors lacking intracellular domains or that contained irrelevant transmembrane domains. Formation of TNF receptor heterocomplexes may therefore 1) define a novel molecular mechanism of ligand passing and/or 2) contribute to cooperative TNF receptor signaling via the juxtaposition of the intracellular domains of the two receptor classes and the signaling proteins that they recruit.

The p80 TNF receptor-associated kinase (p80TRAK) associates with residues 354-397 of the p80 cytoplasmic domain: similarity to casein kinase

The cytoplasmic domain of the p80 TNF receptor associates with a protein kinase, termed p80TRAK, that phosphorylates both the p60 and p80 TNF receptors. To determine the region of the cytoplasmic domain that is necessary for binding of p80TRAK and the region that it phosphorylates, a series of deletions of the p80 cytoplasmic domain were constructed and expressed as glutathione-S-transferase fusion proteins. These fusions were then used to examine the binding of p80TRAK derived from cellular extracts. We found that out of 174 residues (266-439) in the cytoplasmic domain of p80 receptor, 44 residues (354-397) were sufficient for binding of p80TRAK. Interestingly, this was also the region that contained the phosphorylation site for p80TRAK. Phosphoamino acid analysis of this region revealed phosphorylation primarily on serine residues. Furthermore, we found that, like p80TRAK, purified casein kinase 1 (CK1) also binds to residues 354-397 of the p80 TNF receptor and causes its phosphorylation. Additionally, the activity of p80TRAK was inhibited by CK1-7, the CK1-specific inhibitor. Thus, our results indicate that p80TRAK associates with a short stretch of approximately 44 residues located in the cytoplasmic domain of the p80 TNF receptor and that this kinase is similar to CK1.

IkappaB alpha overexpression in human breast carcinoma MCF7 cells inhibits nuclear factor-kappaB activation but not tumor necrosis factor-alpha-induced apoptosis

Nuclear factor-kappaB (NF-kappaB) is one of major component induced by tumor necrosis factor-alpha (TNF), and its role in the signaling of TNF-induced cell death remains controversial. In order to delineate whether the involvement of NF-kappaB activation is required for triggering of the apoptotic signal of TNF, we inhibited the nuclear translocation of this transcription factor in TNF-sensitive MCF7 cells by introducing a human MAD-3 mutant cDNA coding for a mutated IkappaB alpha that is resistant to both phosphorylation and proteolytic degradation and that behaves as a potent dominant negative IkappaB alpha protein. Our results demonstrated that the mutated IkappaB alpha was stably expressed in the transfected MCF7 cells and blocked the TNF-induced NF-kappaB nuclear translocation. Indeed, TNF treatment of these cells induced the proteolysis of only the endogenous IkappaB alpha but not the mutated IkappaB alpha. The nuclear NF-kappaB released from the endogenous IkappaB alpha within 30 min of TNF treatment was rapidly inhibited by the mutated IkappaB alpha. There was no significant difference either in cell viability or in the kinetics of cell death between control cells and the mutated IkappaB alpha transfected cells. Furthermore, electron microscopic analysis showed that the cell death induced by TNF in both control and mutated IkappaB alpha transfected cells was apoptotic. The inhibition of NF-kappaB translocation in mutated IkappaB alpha-transfected cells persisted throughout the same time course that apoptosis was occurring. Our data provide direct evidence that the inhibition of NF-kappaB did not alter TNF-induced apoptosis in MCF7 cells and support the view that TNF-mediated apoptosis is NF-kappaB independent.

The tumor necrosis factor receptor 2 signal transducers TRAF2 and c-IAP1 are components of the tumor necrosis factor receptor 1 signaling complex

The two cell surface receptors for tumor necrosis factor (TNF) interact with a number of intracellular signal transducing proteins. The association of TRADD, a 34-kDa cytoplasmic protein containing a C-terminal death domain, with aggregated TNF receptor 1 (TNF-R1) through their respective death domains leads to NF-kappa B activation and programmed cell death. In contrast, TNF receptor 2 (TNF-R2) interacts with the TNF receptor associated factors 2/1 (TRAF2/TRAF1) heterocomplex, which mediates the recruitment of two cellular inhibitor of apoptosis proteins (c-IAP1 and c-IAP2) to TNF-R2. Here we show that the TNF-R2 signal transducers TRAF2 and c-IAP1 are a part of the TNF-R1 signaling complex. The recruitment of TRAF2 and c-IAP1 to TNF-R1 is TNF-dependent, is mediated by TRADD, and is independent of TNF-R2. These data establish the physiological involvement of TRAF2 and c-IAP1 in TNF-R1 signaling and help provide a molecular explanation for both the overlapping and distinct signals generated by the two TNF receptors.

Tumor necrosis factor receptors (Tnfr) in mouse fibroblasts deficient in Tnfr1 or Tnfr2 are signaling competent and activate the mitogen-activated protein kinase pathway with differential kinetics

To dissect tumor necrosis factor receptor (Tnfr)-1 (CD120a) and Tnfr2 (CD120b)-dependent signal transduction pathways, primary fibroblasts isolated from inguinal adipose tissue of wild type (wt), tnfr1(o), tnfr2(o), and tnfr1(o)/tnfr2(o) mice were studied. The mitogen-activated protein kinases Erk1 and Erk2 were found to be tyrosine-phosphorylated and activated by Tnf treatment in all wt, tnfr1(o), and tnfr2(o) fibroblasts; the activation was down-regulated 60 min after the start of steady state Tnf treatment. Distinct kinetics of Erk1 and Erk2 activation were detected; the Tnfr1-mediated activation of Erk1 and Erk2 started more slowly and persisted for more prolonged times as compared with Tnfr2 activation. Raf-1, Raf-B, Mek-1, Mek kinase, and p90(rsk) kinases were also shown to be activated independently in a distinct time-dependent pattern through the two Tnf receptors. In addition, both Tnfr1 and Tnfr2 mediated independently the activation of the transcription factor Ap-1 albeit with parallel activation kinetics. In contrast, Tnfr1 exclusively mediated activation of NF-kappaB and fibroblast proliferation; however, Tnfr2 enhanced proliferation triggered through Tnfr1. These findings indicate distinct but also overlapping roles of Tnfr1 and Tnfr2 in primary mouse fibroblasts and suggest different regulation mechanisms of signal transduction pathways under the control of both Tnf receptors.

Lymphotoxin beta receptor triggering induces activation of the nuclear factor kappaB transcription factor in some cell types

NFkappaB is a pleiotropic transcription factor capable of activating the expression of a great variety of genes critical for the immunoinflammatory response. Tumor necrosis factor alpha (TNFalpha) and lymphotoxin alpha (LTalpha, originally TNFbeta) are potent nuclear factor kappaB (NFkappaB) activators in various cell types. The LTalpha molecule, in addition to being secreted as a soluble trimer, can also form membrane-anchored heterotrimers with the LTbeta chain, another member of the TNF family. The LTalpha1beta2 heterotrimer binds a specific receptor, called the LTbeta receptor (LTbeta-R), which is also a member of the TNF receptor family. Here, we show that engagement of LTbeta-R with a soluble form of LTalpha1beta2 or with a specific anti-LTbeta-R agonistic monoclonal antibody CBE11 quickly induces activation of NFkappaB in HT-29 and WiDr human adenocarcinomas. LTbeta-R triggering activates NFkappaB and induces proliferation in WI-38 human lung fibroblasts. No NFkappaB activation is observed in human umbilical vein endothelial cells, correlating with the inability of LTbeta-R activation to induce expression of NFkappaB-dependent cell surface adhesion molecules. Thus, like several other members of the TNF receptor family, the LTbeta-R can activate NFkappaB following receptor ligation in some but not all LTbeta-R-positive cells.

Evidence for exclusive role of the p55 tumor necrosis factor (TNF) receptor in mediating the TNF-induced collagenase expression by human dermal fibroblasts

The aim of this study was to examine the roles of the TNF receptors p55 and p75 in the TNF-enhanced expression of collagenase by human dermal fibroblasts. The agonistic p55 monoclonal antibody Htr9 and TNF induced production of similar amounts of collagenase. Polyclonal or monoclonal agonistic p75 antibodies failed to enhance collagenase production, and the antagonistic p75 antibody 5E12 did not inhibit TNF-enhanced expression of collagenase. This strongly suggests that p55, but not p75, is involved in TNF-induced production of collagenase. Cells continued to produce an elevated level of collagenase after the removal of TNF or Htr9. These data suggest that it may be useful to use specific inhibitors of collagenase rather than to block cytokine action directly in the treatment of diseases with chronic enhanced collagenolytic activity. A peptide of residues 36-62 of TNF previously reported to be chemotactic to leukocytes was also able to enhance the expression of collagenase activity by dermal fibroblasts. Thus, design of peptides with specific TNF effects may offer a novel approach for treatment of fibrotic disorders.