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

Role of the Interaction of Tumor Necrosis Factor-α and Tumor Necrosis Factor Receptors 1 and 2 in Bone-Related Cells

Tumor necrosis factor-α (TNF-α) is a pleiotropic cytokine expressed by macrophages, monocytes, and T cells, and its expression is triggered by the immune system in response to pathogens and their products, such as endotoxins. TNF-α plays an important role in host defense by inducing inflammatory reactions such as phagocytes and cytocidal systems activation. TNF-α also plays an important role in bone metabolism and is associated with inflammatory bone diseases. TNF-α binds to two cell surface receptors, the 55kDa TNF receptor-1 (TNFR1) and the 75kDa TNF receptor-2 (TNFR2). Bone is in a constant state of turnover; it is continuously degraded and built via the process of bone remodeling, which results from the regulated balance between bone-resorbing osteoclasts, bone-forming osteoblasts, and the mechanosensory cell type osteocytes. Precise interactions between these cells maintain skeletal homeostasis. Studies have shown that TNF-α affects bone-related cells via TNFRs. Signaling through either receptor results in different outcomes in different cell types as well as in the same cell type. This review summarizes and discusses current research on the TNF-α and TNFR interaction and its role in bone-related cells.

TNF-α Differentially Regulates Cell Cycle Genes in Promyelocytic and Granulocytic HL-60/S4 Cells

Tumor necrosis factor alpha (TNF-α) is a potent cytokine involved in systemic inflammation and immune modulation. Signaling responses that involve TNF-α are context dependent and capable of stimulating pathways promoting both cell death and survival. TNF-α treatment has been investigated as part of a combined therapy for acute myeloid leukemia due to its modifying effects on all-trans retinoic acid (ATRA) mediated differentiation into granulocytes. To investigate the interaction between cellular differentiation and TNF-α, we performed RNA-sequencing on two forms of the human HL-60/S4 promyelocytic leukemia cell line treated with TNF-α. The ATRA-differentiated granulocytic form of HL-60/S4 cells had an enhanced transcriptional response to TNF-α treatment compared to the undifferentiated promyelocytes. The observed TNF-α responses included differential expression of cell cycle gene sets, which were generally upregulated in TNF-α treated promyelocytes, and downregulated in TNF-α treated granulocytes. This is consistent with TNF-α induced cell cycle repression in granulocytes and cell cycle progression in promyelocytes. Moreover, we found evidence that TNF-α treatment of granulocytes shifts the transcriptome toward that of a macrophage. We conclude that TNF-α treatment promotes a divergent transcriptional program in promyelocytes and granulocytes. TNF-α promotes cell cycle associated gene expression in promyelocytes. In contrast, TNF-α stimulated granulocytes have reduced cell cycle gene expression, and a macrophage-like transcriptional program.

Redeeming an old foe: protective as well as pathophysiological roles for tumor necrosis factor in inflammatory bowel disease

Tumor necrosis factor (TNF) and its receptors TNFR1 and TNFR2 are major therapeutic targets for inflammatory bowel disease. Research advances have demonstrated that TNF produces pleiotropic responses in the gastrointestinal (GI) tract. Although in excess TNF can contribute to GI pathology, TNF is also a critical protective factor to promote GI homeostasis following injury and inflammation. Genetic studies using candidate and genome-wide association study approaches have identified variants in TNF or its receptors that are associated with Crohn's disease or ulcerative colitis in multiple populations, although the basis for these associations remains unclear. This review considers the efficacy and mechanism of anti-TNF therapies for inflammatory bowel disease to reconcile the many disparate aspects of TNF research and to consider the potential protective effects of TNF signaling in GI health.

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.

Tumor necrosis factor receptor cross-talk

Extensive research has been performed to unravel the mechanistic signaling pathways mediated by tumor necrosis factor receptor 1 (TNFR1), by contrast there is limited knowledge on cellular signaling upon activation of TNFR2. Recently published data have revealed that these two receptors not only function independently, but also can influence each other via cross-talk between the different signaling pathways initiated by TNFR1 and TNFR2 stimulation. Furthermore, the complexity of this cross-talk is also dependent on the different signaling kinetics between TNFR1 and TNFR2, by which a delicate balance between cell survival and apoptosis can be maintained. Some known signaling factors and the kinetics that are involved in the receptor cross-talk between TNFR1 and TNFR2 are the topic of this review.

Report of the ECCO workshop on anti-TNF therapy failures in inflammatory bowel diseases: biological roles and effects of TNF and TNF antagonists

This second section of the first ECCO pathogenesis workshop on anti-TNF therapy failures in inflammatory bowel diseases addresses the biological roles of TNFα and the effects and mechanisms of action of TNFα antagonists. Mechanisms underlying their failure, including induction of TNF-independent inflammatory pathways and phenomena of paradoxical inflammation are discussed.

Innate immunity mediates myocardial preconditioning through Toll-like receptor 2 and TIRAP-dependent signaling pathways

Recent studies have implicated Toll-like receptor 2 (TLR2) and TLR4 signaling in delimiting liver and brain injury following ischemia-reperfusion (I/R). To determine whether TLR2 and TLR4 conferred cytoprotection in the heart, we subjected hearts of wild-type (WT) mice and mice deficient in TLR2 (TLR2D), TLR4 (TLR4D), and TIR domain-containing adapter protein (TIRAP-D) to ischemic preconditioning (IPC). Langendorff-perfused hearts were subjected to 30 min ischemia and 60 min reperfusion with or without IPC. IPC resulted in a significant increase (P < 0.05) in the percent recovery of left ventricular developed pressure (%LVDP) in WT mouse hearts (54.4 +/- 2.7% of baseline), whereas there was no significant increase in %LVDP (P > 0.05) in TIRAP-D mouse hearts (43.8 +/- 1.9%) after I/R injury. IPC also resulted in a significant (P < 0.05) decrease in I/R-induced creatine kinase release and Evans blue dye uptake in WT but not TIRAP-D hearts. Interestingly, IPC resulted in a significant (P < 0.05) increase in %LVDP in TLR4-deficient hearts (52.7 +/- 3%) but not in TLR2D hearts (39.3 +/- 1.5%). Pretreatment with a specific TLR2 ligand (Pam3CSK) protected WT hearts against I/R-induced left ventricular dysfunction. The loss of IPC-induced cardioprotection in TIRAP-D mouse hearts was accompanied by a decreased translocation of protein kinase C-epsilon and decreased phosphorylation of GSK-3beta. Taken together, these data suggest that the cardioprotective effect of IPC is mediated, at least in part, through a TLR2-TIRAP-dependent pathway, suggesting that the modulation of this pathway represents a viable target for reducing I/R injury.

Tumor necrosis factor-α signaling in macrophages

Tumor necrosis factor-α (TNFα) was cloned over 2 decades ago and its identification in part led to the discovery of a super family of tumor necrosis factors (TNFs) and their receptors. TNFα signals through two transmembrane receptors, TNFR1 and TNFR2, and regulates a number of critical cell functions including cell proliferation, survival, differentiation, and apoptosis. Macrophages are the major producers of TNFα and interestingly are also highly responsive to TNFα. Aberrant TNFα production and TNF receptor signaling have been associated with the pathogenesis of several diseases, including rheumatoid arthritis, Crohn's disease, atherosclerosis, psoriasis, sepsis, diabetes, and obesity. TNFα has been shown to play a pivotal role in orchestrating the cytokine cascade in many inflammatory diseases and because of this role as a "master-regulator" of inflammatory cytokine production, it has been proposed as a therapeutic target for a number of diseases. Indeed anti-TNFα drugs are now licensed for treating certain inflammatory diseases including rheumatoid arthritis and inflammatory bowel disease. In this review we discuss the discovery of TNFα and its actions especially in regulating macrophage biology. Given its importance in several human diseases, we also briefly discuss the role of anti-TNFα therapeutics in the treatment of inflammatory diseases.

Loss of SIMPL compromises TNF-alpha-dependent survival of hematopoietic progenitors

OBJECTIVE

Emerging work has revealed an integral role of the tumor necrosis factor-alpha (TNF-alpha) nuclear factor (NF)-kappaB pathway in the regulation of hematopoiesis. TNF-alpha inhibition of hematopoietic stem/progenitor cell growth involves type I TNF-alpha receptor (TNF-RI) and type II TNF-alpha receptor (TNF-RII). However, the role of TNF-RI vs TNF-RII in mediating this response is less clear. Full induction of NF-kappaB-dependent gene expression through TNF-RI requires the transcriptional coactivator SIMPL (substrate that interacts with mouse pelle-like kinase). To address the role of SIMPL in TNF-alpha-dependent signaling in hematopoiesis, endothelial cells and hematopoietic progenitors expressing SIMPL short hairpin RNA were characterized.

MATERIAL AND METHODS

In vitro gene expression and progenitor assays employing SIMPL short hairpin RNA were used to examine the requirement for SIMPL in TNF-alpha-dependent effects upon cytokine gene expression and hematopoietic progenitor cell growth. Competitive repopulation studies were used to extend these studies in vivo.

RESULTS

SIMPL is required for full TNF-RI-dependent expression of NF-kappaB-controlled cytokines in endothelial cells. Hematopoietic progenitor cell expansion is not affected if progenitors lacked SIMPL or if progenitors are treated with human TNF-alpha, which signals through TNF-RI. In the absence of SIMPL, human TNF-alpha leads to a dramatic decrease in progenitor cell expansion that is not due to apoptosis. Loss of SIMPL does not affect the activity of transforming growth factor-beta1 and interferon-gamma, other known suppressors of hematopoiesis.

CONCLUSIONS

Suppression of myeloid progenitor cell expansion requires signaling through TNF-RI and TNF-RII. Signals transduced through the TNF-alpha-TNF-RI-SIMPL pathway support hematopoietic progenitor cell survival, growth and differentiation.

Tumor necrosis factor (TNF) receptor-2-mediated DNA synthesis and proliferation in primary cultures of adult rat hepatocytes: The involvement of endogenous transforming growth factor-alpha

We investigated the effects of tumor necrosis factor (TNF)-alpha on DNA synthesis and proliferation, and its signal transduction pathways in primary cultures of adult rat hepatocytes. TNF-alpha induced time- and dose-dependent increases in hepatocyte DNA synthesis and proliferation. The hepatocyte proliferation stimulated by 30 ng/ml TNF-alpha was significantly inhibited by anti-TNF receptor 2 antibody, but not by anti-TNF receptor 1 antibody. TNF-alpha-induced hepatocyte DNA synthesis and proliferation were blocked by AG1478 (10(-7) M), PD98059 (10(-6) M), LY 294002 (10(-7) M), and rapamycin (100 ng/ml). TNF-alpha at 30 ng/ml significantly increased phosphorylation of receptor tyrosine kinase (175 kDa) and p42 mitogen-activated protein (MAP) kinase. This data suggests that the proliferative signal for primary cultured hepatocytes induced by TNF-alpha is mediated by TNF receptor 2 and the receptor tyrosine kinase/MAP kinase pathway. In addition, TNF-alpha-induced hepatocyte mitogenesis was significantly blocked by somatostatin (10(-6) M), adenylate cyclase inhibitor dideoxyadenosine (10(-7) M), protein kinase A inhibitor H-89 (10(-7) M), and neutralizing antibody to transforming growth factor (TGF)-alpha in culture. Indeed, 30 ng/ml TNF-alpha was found to rapidly stimulate secretion of TGF-alpha, and this secretion was also blocked by anti-TNF receptor 2 antibody. Moreover, TGF-alpha secretion induced by TNF-alpha was suppressed by dideoxyadenosine, H-89, and somatostatin. Together, these results indicate that stimulation of TNF receptor 2 by 30 ng/ml TNF-alpha induces autocrine secretion of TGF-alpha via the adenylate cyclase/protein kinase A pathway, after which TGF-alpha induces hepatocyte DNA synthesis and proliferation through the TGF-alpha receptor-linked tyrosine kinase (175 kDa)/MAP kinase signaling system.

Beyond tumor necrosis factor receptor: TRADD signaling in toll-like receptors

Tumor necrosis factor receptor 1-associated death domain protein (TRADD) is the core adaptor recruited to TNF receptor 1 (TNFR1) upon TNFalpha stimulation. In cells from TRADD-deficient mice, TNFalpha-mediated apoptosis and TNFalpha-stimulated NF-kappaB, JNK, and ERK activation are defective. TRADD is also important for germinal center formation, DR3-mediated costimulation of T cells, and TNFalpha-mediated inflammatory responses in vivo. TRADD deficiency does not enhance IFNgamma-induced signaling. Importantly, TRADD has a novel role in TLR3 and TLR4 signaling. TRADD participates in the TLR4 complex formed upon LPS stimulation, and TRADD-deficient macrophages show impaired cytokine production in response to TLR ligands in vitro. Thus, TRADD is a multifunctional protein crucial both for TNFR1 signaling and other signaling pathways relevant to immune responses.

The cytokine TNFα increases the proportion of DRG neurones expressing the TRPV1 receptor via the TNFR1 receptor and ERK activation

TNFalpha is involved in the generation of hyperalgesia in pathological states such as neuropathy and inflammation. The pronociceptive action of TNFalpha may be mediated at least in part by activation of the TRPV1 receptor which transduces heat stimuli in primary nociceptive afferents and mediates thermal hyperalgesia. In the present study, we investigated in cultured dorsal root ganglion (DRG) neurones, the somata of primary afferent fibres, whether TNFalpha increases TRPV1 receptor expression. We found that long-term exposure of DRG neurones of both rat and mouse to TNFalpha significantly increased the proportion of DRG neurones expressing TRPV1 receptor-like immunoreactivity. This TNFalpha effect was abolished in mice DRG neurones when DRG cultures were obtained from tnfr1/2-/- and tnfr1-/-, but not from tnfr2-/- mice. Furthermore, we found that activation of ERK but not of p38 kinase or cyclooxygenases is critically involved in the TNFalpha-induced increase of TRPV1 receptor expression.

TNF alpha acting on TNFR1 promotes breast cancer growth via p42/P44 MAPK, JNK, Akt and NF-kappa B-dependent pathways

Tumor necrosis factor alpha (TNF alpha) enhances proliferation of chemically-induced mammary tumors and of T47D human cell line through not fully understood pathways. Here, we explored the intracellular signaling pathways triggered by TNF alpha, the participation of TNF alpha receptor (TNFR) 1 and TNFR2 and the molecular mechanism leading to breast cancer growth. We demonstrate that TNFalpha induced proliferation of C4HD murine mammary tumor cells and of T47D cells through the activation of p42/p44 MAPK, JNK, PI3-K/Akt pathways and nuclear factor-kappa B (NF-kappa B) transcriptional activation. A TNF alpha-specific mutein selectively binding to TNFR1 induced p42/p44 MAPK, JNK, Akt activation, NF-kappa B transcriptional activation and cell proliferation, just like wild-type TNF alpha, while a mutein selective for TNFR2 induced only p42/p44 MAPK activation. Interestingly, blockage of TNFR1 or TNFR2 with specific antibodies was enough to impair TNF alpha signaling and biological effect. Moreover, in vivo TNF alpha administration supported C4HD tumor growth. We also demonstrated, for the first time, that injection of a selective inhibitor of NF-kappa B activity, Bay 11-7082, resulted in regression of TNF alpha-promoted tumor. Bay 11-7082 blocked TNF alpha capacity to induce cell proliferation and up-regulation of cyclin D1 and of Bcl-xLin vivo and in vitro. Our results reveal evidence for TNF alpha as a breast tumor promoter, and provide novel data for a future therapeutic approach using TNF alpha antagonists and NF-kappa B pharmacological inhibitors in established breast cancer treatment.

Tumor necrosis factor alpha (TNF-alpha) receptor-II is required for TNF-alpha-induced leukocyte-endothelial interaction in vivo

Tumor necrosis factor-alpha (TNF-alpha) binds to 2 distinct cell-surface receptors: TNF-alpha receptor-I (TNFR-I: p55) and TNF-alpha receptor-II (TNFR-II: p75). TNF-alpha induces leukocyte adhesion molecules on endothelial cells (ECs), which mediate 3 defined steps of the inflammatory response; namely, leukocyte rolling, firm adhesion, and transmigration. In this study, we have investigated the role of p75 in TNF-alpha-induced leukocyte adhesion molecules using cultured ECs derived from wild-type (WT), p75-null (p75-/-), or p55-null (p55-/-) mice. We observed that p75 was essential for TNF-alpha-induced E-selectin, vascular cell adhesion molecule 1 (VCAM-1), and intercellular adhesion molecule 1 (ICAM-1) expression. We also investigated the putative role of p75 in inflammation in vivo using an intravital microscopic approach with a mouse cremaster muscle model. TNF-alpha-stimulated leukocyte rolling, firm adhesion to ECs, and transmigration were dramatically reduced in p75-/- mice. Transplanted WT cremaster in p75-/- mice showed a robust leukocyte rolling and firm adhesion upon TNF-alpha activation, suggesting that the impairment in EC-leukocyte interaction in p75-/- mice is due to EC dysfunction. These results demonstrate, for the first time, that endothelial p75 is essential for TNF-alpha-induced leukocyte-endothelial-cell interaction. Our findings may contribute to the identification of novel p75-targeted therapeutic approaches for inflammatory diseases.

LTA and LPS mediated activation of protein kinases in the regulation of inflammatory cytokines expression in macrophages

BACKGROUND

Lipoteichoic acid (LTA) and lipopolysaccharide (LPS), the toxicants from bacteria, are potent inducers of inflammatory cytokines, such as tumor necrosis factor-alpha (TNF) and interleukin-1beta (IL-1). Although LTA is much less reported than that on LPS, LTA is regarded as the gram-positive equivalent to LPS in some aspects. We investigated the LTA-induced signal transduction and biological effects, as well as to compare the effect of LTA with that of LPS.

METHODS

Kinase assay, ELISA and RT-PCR were performed to delineate LTA and LPS signaling as well as to determine the secretion and RNA expression of TNF and IL-1.

RESULTS

Src, Lyn and MAPKs are involved in LTA and LPS signaling in murine macrophages. Additionally, blockades of PKC, PI3K and p38, respectively, caused significant inhibition of both LTA- and LPS-induced proIL-1/IL-1 and TNF expression. ERK inactivation moderately reduced LTA- and LPS-induced proIL-1/IL-1, but considerably reduced TNF expression. Inhibition of JNK engendered super-induction of IL-1 secretion, but diminished TNF secretion. Strikingly, both IL-1 and TNF protein induction were declined by overexpression of dominant negative form of JNK.

CONCLUSIONS

The results clarify the similarity and difference between LTA- and LPS-mediated signal transduction and induction of inflammatory cytokines in macrophages.

Tumor necrosis factor alpha induction of NF-kappaB requires the novel coactivator SIMPL

A myriad of stimuli including proinflammatory cytokines, viruses, and chemical and mechanical insults activate a kinase complex composed of IkappaB kinase beta (IKK-beta), IKK-alpha, and IKK-gamma/N, leading to changes in NF-kappaB-dependent gene expression. However, it is not clear how the NF-kappaB response is tailored to specific cellular insults. Signaling molecule that interacts with mouse pelle-like kinase (SIMPL) is a signaling component required for tumor necrosis factor alpha (TNF-alpha)-dependent but not interleukin-1-dependent NF-kappaB activation. Herein we demonstrate that nuclear localization of SIMPL is required for type I TNF receptor-induced NF-kappaB activity. SIMPL interacts with nuclear p65 in a TNF-alpha-dependent manner to promote endogenous NF-kappaB-dependent gene expression. The interaction between SIMPL and p65 enhances p65 transactivation activity. These data support a model in which TNF-alpha activation of NF-kappaB dependent-gene expression requires nuclear relocalization of p65 as well as nuclear relocalization of SIMPL, generating a TNF-alpha-specific induction of gene expression.

Analysis of human tumour necrosis factor receptor 1 dominant-negative mutants reveals a major region controlling cell surface expression

Tumour necrosis factor receptor 1 (TNFR1) plays a critical role in host defence and inflammation. We have identified a membrane proximal region (aa 218-324) of TNFR1 that restricts surface expression. This was prompted by comparing the dominant-negative properties of a C-terminal truncation of TNFR1 with a point mutant that prevents signalling. C-terminal truncation (aa 218-426) generates a better dominant-negative TNFR1 mutant than inactivation of the death domain by point mutation. The increased dominant-negative activity correlates with increased cell surface expression. The membrane proximal region is the most important region of the receptor for restricting expression.

Mice lacking TNFalpha receptors 1 and 2 are resistant to death and fulminant liver injury induced by agonistic anti-Fas antibody

The liver is particularly susceptible to Fas-mediated cytotoxicity. Mice given an adequate parenteral dose of agonistic anti-Fas antibody (aFas) or of FasL are known to develop a devastating liver injury and to die in a few hours. The present work shows that mice lacking TNFR1 and TNFR2 (R(-)) both survive a single dose of aFas, otherwise rapidly lethal, and develop a mild form of hepatic damage, compared to the much more severe liver injury that in a few hours strikes wild-type mice (R(+)), eventually involving increased activity of proteases of different families (caspase 3-, 8-, and 9-like, calpains, cathepsin B). Neither the overall tissue levels of Fas and FasL nor Fas expression at the hepatocyte surface are altered in the liver of R(-) animals. The DNA-binding activity of the NF-kappaB transcription factor is enhanced after aFas treatment, but much more markedly in R(-) than in R(+) mice. Bcl2, while unchanged in untreated animals, is markedly upregulated in R(-) but not in R(+) mice challenged with aFas. The requirement of a normal TNFR1/TNFR2 phenotype for full deployment of the general and liver-specific aFas toxicity in mice most likely implies that treatment with aFas in some way results in activation of the TNFalpha-TNFRs system and that this activation synergizes with Fas-mediated signals in causing the fulminant liver injury and the animal death. The precise cellular and molecular details underlying this interplay between Fas- and TNFRs-mediated signaling systems in the general and liver-specific aFas toxicity largely remain to be clarified.

Divergent roles for p55 and p75 TNF-alpha receptors in the induction of plasminogen activator inhibitor-1

Tumor necrosis factor-alpha (TNF-alpha) is elevated in obesity and in acute inflammatory states, and contributes to the elevated plasminogen activator inhibitor-1 (PAI-1) levels associated with these conditions. Mice genetically deficient in the p55 and p75 TNF-alpha receptors were used to study the roles of these receptors in the expression of PAI-1 in obese (ob/ob) mice, and in lean mice following acute stimulation with TNF-alpha. In ob/ob mice, p55 and p75 tumor necrosis factor-alpha receptors (TNFRs) act cooperatively to induce PAI-1 mRNA in most tissues, including the adipose tissue, kidney, heart, and liver. However, in lean mice, TNF-alpha-induced PAI-1 expression is mediated primarily by the p55 TNFR. Interestingly, PAI-1 mRNA expression in all tissues of the TNF-alpha-treated p75-deficient lean mice was significantly higher than that observed in TNF-alpha-treated wild-type mice. These observations suggest that the p75 TNFR may play a role in attenuating TNF-alpha-induced PAI-1 mRNA expression in acute inflammatory conditions. Our observation that soluble p75 TNFR was elevated in the plasma of TNF-alpha-treated mice in comparison to untreated mice supports this hypothesis. These studies thus provide insights into the TNF-alpha receptors involved in mediating and modulating the expression of PAI-1 in acute and chronic (eg, obesity) inflammatory states associated with elevated TNF-alpha.

Role of TNF-alpha receptors in mice intoxicated with the parkinsonian toxin MPTP

The loss of dopaminergic neurons in Parkinson's disease is associated with a glial reaction and the overproduction of proinflammatory cytokines such as tumor necrosis factor alpha (TNF-alpha). TNF-alpha acts via two different receptors, TNFR1 and TNFR2, and is believed to have both a neuroprotective and a deleterious role for neurons. In order to analyze the putative role of TNF-alpha in parkinsonism, we compared the effect of the parkinsonian drug 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) in mice lacking TNFR1, TNFR2, or both receptors and in wild-type littermates. We show that MPTP does not affect spontaneous activity or anxiety in any of the groups and that it reduces motor activity on a rotarod in double knock out mice but not in mice lacking only one receptor. Postmortem analysis revealed no differences in the number of nigral dopaminergic neurons whatever the group. In contrast, striatal dopamine level was slightly decreased in double knock-out mice and more reduced by MPTP in this group than in the other groups of mice. In addition, dopamine turnover was significantly more increased in double knock out mice after MPTP injection. These data suggest that TNF-alpha does not participate in the death of dopaminergic neurons in parkinsonism but that it slightly alters dopamine metabolism or the survival of dopaminergic terminals by a mechanism involving both receptors.

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.

Computational modeling of the dynamics of the MAP kinase cascade activated by surface and internalized EGF receptors

We present a computational model that offers an integrated quantitative, dynamic, and topological representation of intracellular signal networks, based on known components of epidermal growth factor (EGF) receptor signal pathways. The model provides insight into signal-response relationships between the binding of EGF to its receptor at the cell surface and the activation of downstream proteins in the signaling cascade. It shows that EGF-induced responses are remarkably stable over a 100-fold range of ligand concentration and that the critical parameter in determining signal efficacy is the initial velocity of receptor activation. The predictions of the model agree well with experimental analysis of the effect of EGF on two downstream responses, phosphorylation of ERK-1/2 and expression of the target gene, c-fos.

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.

Type II tumour necrosis factor-alpha receptor (TNFR2) activates c-Jun N-terminal kinase (JNK) but not mitogen-activated protein kinase (MAPK) or p38 MAPK pathways

The pleitropic actions of tumour necrosis factor-alpha (TNF) are transmitted by the type I 55 kDa TNF receptor (TNFR1) and type II 75 kDa TNF receptor (TNFR2), but the signalling mechanisms elicited by these two receptors are not fully understood. In the present study, we report for the first time subtype-specific differential kinase activation in cell models that respond to TNF by undergoing apoptotic cell death. KYM-1 human rhabdomyosarcoma cells and HeLa human cervical epithelial cells, engineered to overexpress TNFR2, displayed c-Jun N-terminal kinase (JNK) activation by wild-type TNF, a TNFR1-specific TNF mutant and a TNFR2-specific mutant TNF in combination with an agonistic TNFR2-specific monoclonal antiserum. A combination of the TNFR2-specific mutant and agonistic antiserum elicited maximal endogenous or exogenous TNFR2 responsiveness. Moreover, alternative expression of a TNFR2 deletion mutant lacking its cytoplasmic domain rendered the cells unable to activate JNK activity through this receptor subtype. The profile of JNK activation by TNFR1 was more transient than that of TNFR2, with TNFR2-induced JNK activity also being more sensitive to the caspase inhibitor, benzyloxycarbonyl-Val-Ala-DL-Asp-fluoromethylketone. Conversely, only activation of the TNFR1 could stimulate mitogen-activated protein kinase (MAPK) or p38 MAPK activities in a time-dependent manner. The role of TNFR2 activation in enhanced apoptotic cell death was confirmed with agonistic monoclonal antisera in cells expressing high levels of TNFR2. Activation of TNFR2 alone elicited cell death, but full TNF-induced death required stimulation of both receptor types. These findings indicate that efficient activation of TNFR2 by soluble TNFs is achievable with co-stimulation by antisera, and that both receptors differentially modulate extracellular signal-regulated kinases contributing to the cytokine's cytotoxic response.

Genetic deletion of the tumor necrosis factor receptor p60 or p80 abrogates ligand-mediated activation of nuclear factor-kappa B and of mitogen-activated protein kinases in macrophages

Tumor necrosis factor (TNF) is a pleiotropic cytokine known to regulate cell growth, viral replication, inflammation, immune system functioning, angiogenesis, and tumorigenesis. These effects are mediated through two different receptors, TNFR1 and TNFR2 (also called p60 and p80, respectively), with p60 receptor being expressed on all cell types and p80 receptor only on cells of the immune system and on endothelial cells. Although the role of p60 receptor in TNF signaling is well established, the role of p80 is less clear. In this report, by using macrophages derived from wild-type mice (having both receptors) and mice in which the gene for either p60 (p60(-/-)), or p80 (p80(-/-)), or both (p60(-/-) p80(-/-)) receptor have been deleted, we have redefined the role of these receptors in TNF-induced activation of nuclear factor (NF)-kappa B and of mitogen-activated protein kinases. TNF activated NF-kappa B in a dose- and time-dependent manner in wild-type macrophages but not in p60(-/-), p80(-/-), or p60(-/-) p80(-/-) macrophages. These results correlated with the I kappa B alpha degradation needed for NF-kappa B activation. We also found that TNF activated c-Jun N-terminal protein kinase in a dose- and time-dependent manner in wild-type macrophages but not in p60(-/-), p80(-/-), or p60(-/-) p80(-/-) macrophages. TNF activated p38 MAPK and p44/p42 MAPK in wild-type but not in p60(-/-), p80(-/-), or p60(-/-) p80(-/-) macrophages. TNF induced the proliferation of wild-type macrophages, but for p60(-/-) and p80(-/-) macrophages proliferation was lower, and in p60(-/-) p80(-/-) it was absent. Overall, our studies suggest that both types of TNF receptors are needed in macrophages for optimum TNF cell signaling.

Early response cytokines and innate immunity: essential roles for TNF receptor 1 and type I IL-1 receptor during Escherichia coli pneumonia in mice

The early response cytokines, TNF and IL-1, have overlapping biologic effects that may function to propagate, amplify, and coordinate host responses to microbial challenges. To determine whether signaling from these early response cytokines is essential to orchestrating innate immune responses to intrapulmonary bacteria, the early inflammatory events induced by instillation of Escherichia coli into the lungs were compared in wild-type (WT) mice and mice deficient in both TNF receptor 1 (TNFR1) and the type I IL-1 receptor (IL1R1). Neutrophil emigration and edema accumulation induced by E. coli were significantly compromised by TNFR1/IL1R1 deficiency. Neutrophil numbers in the circulation and within alveolar septae did not differ between WT and TNFR1/IL1R1 mice, suggesting that decreased neutrophil emigration did not result from decreased sequestration or delivery of intravascular neutrophils. The nuclear translocation of NF-kappa B and the expression of the chemokine macrophage inflammatory protein-2 did not differ between WT and TNFR1/IL1R1 lungs. However, the concentration of the chemokine KC was significantly decreased in the bronchoalveolar lavage fluids of TNFR1/IL1R1 mice compared with that in WT mice. Thus, while many of the molecular and cellular responses to E. coli in the lungs did not require signaling by either TNFR1 or IL1R1, early response cytokine signaling was critical to KC expression in the pulmonary air spaces and neutrophil emigration from the alveolar septae.

Tumor necrosis factor-alpha regulates secretion of the adipocyte-derived cytokine, leptin

The seminal observation that secretion of the adipocyte-derived hormone leptin was induced by inflammatory challenge has been expanded upon to demonstrate the importance of the pro-inflammatory cytokines, especially tumor necrosis factor (TNF)-alpha, in inflammatory hyperleptinemia. Initially, it was thought that cytokine-induced hyperleptinemia might somehow be involved in the anorexia and cachexia that often accompany chronic infectious, neoplastic, and autoimmune disease. While the role of leptin in disease-associated anorexia and cachexia appears tenuous in light of recent findings, there is evidence that the hyperleptinemia induced by cytokines is an integral part of the acute phase response and necessary for comprehensive immunocompetence. This hints at the existence of an integrated communication network, wherein the energy status of the animal impacts its ability to fight pathogens.

Control sites of ribosomal S6 kinase B and persistent activation through tumor necrosis factor

RSKB, a 90-kDa ribosomal S6 protein kinase family (RSK) member with two complete catalytic domains connected by a linker, is activated through p38- and ERK-mitogen-activated protein kinases. The N-terminal kinases of RSKs phosphorylate substrates; activation requires phosphorylation of linker and C-terminal kinase sites. Unlike other RSKs, the activation loop phosphorylation sites of both catalytic domains of RSKB, Ser(196) and Thr(568), were required for activity. RSKB activation depended on phosphorylation of linker Ser(343) and Ser(360) and associated with phosphorylation of nonconserved Ser(347), but Ser(347)-deficient RSKB retained partial activity. The known protein kinase A and protein kinase C inhibitors, H89 and Ro31-8220, blocked RSKB activity. Treatment of HeLa cells with tumor necrosis factor, epidermal growth factor, phorbol 12-myristate 13-acetate, and ionomycin but not with insulin resulted in strong activation of endogenous RSKB. High RSKB activity and Ser(347)/Ser(360) phosphorylation persisted for 3 h in tumor necrosis factor-treated cells, in contrast to the short bursts of p38, ERK, and RSK1-3 activities. In conclusion, a variety of stimuli induced phosphorylation and activation of RSKB through both p38 and ERK pathways; the persistence of activation indicated that RSKB selectively escaped cell mechanisms causing rapid deactivation of upstream p38 and ERK and other RSKs.

Tumor necrosis factor-alpha regulates secretion of the adipocyte-derived cytokine, leptin

The seminal observation that secretion of the adipocyte-derived hormone leptin was induced by inflammatory challenge has been expanded upon to demonstrate the importance of the pro-inflammatory cytokines, especially tumor necrosis factor (TNF)-alpha, in inflammatory hyperleptinemia. Initially, it was thought that cytokine-induced hyperleptinemia might somehow be involved in the anorexia and cachexia that often accompany chronic infectious, neoplastic, and autoimmune disease. While the role of leptin in disease-associated anorexia and cachexia appears tenuous in light of recent findings, there is evidence that the hyperleptinemia induced by cytokines is an integral part of the acute phase response and necessary for comprehensive immunocompetence. This hints at the existence of an integrated communication network, wherein the energy status of the animal impacts its ability to fight pathogens.

Preimplantation development of the mammalian embryo and its regulation by growth factors

Preimplantation development in mammals involves both the development of the embryo and the preparation of the uterus in anticipation of blastocysts implantation. Preparation of the uterus for implantation is primarily under the control of the ovarian sex steroids, estrogen and progesterone. Increasing evidence is revealing that their effects on cell proliferation and differentiation in the uterus are medicated by locally produced growth factors and cytokines. In contrast, preimplantation development of the embryo to the blastocyst stage appears to be independent of exogenous growth factors. Implantation, the point at which the blastocyst forms a more intimate association with the maternal tissues, is regulated by the uterine expression of leukemia inhibitory factor (LIF). LIF is required both to promote embryo attachment and for decidualization of the uterus. In the absence of LIF, neither of these events occur. Uterine expression of LIF at the time of implantation has been described in many species, suggesting that LIF may be of general significance in regulating embryo implantation in mammals.

Phosphorylation of the membrane proximal region of tumor necrosis factor receptor CD120a (p55) at ERK consensus sites

The interaction of tumor necrosis factor-alpha with its receptor CD120a (p55) initiates downstream signaling cascades that include the activation of the mitogen-activated protein kinase (MAPK), p42(mapk/erk2). The membrane proximal region of CD120a (p55) is Ser-, Thr-, and Pro-rich and contains four mitogen-activated protein kinase consensus phosphorylation sites. In recent work, we showed that CD120a (p55) itself is a target of phosphorylation by p42(mapk/erk2), and after phosphorylation, the receptor is redistributed from the cell surface and Golgi complex to intracellular tubular structures associated with elements of the endoplasmic reticulum. The goal of this study was to define the specific amino acid residues that are phosphorylated. Deletional mutagenesis of the cytoplasmic domain of CD120a (p55) indicated that two sites located between residues 207-254 and 250-300 were phosphorylated predominantly on Thr and Ser residues, respectively. Site-directed mutagenesis of Ser and Thr residues contained within the extracellular signal-regulated kinase (ERK) consensus sequences indicated that the preferred residues were Thr-236 and Ser-270. Primary phosphorylation at these sites appeared to enable subsequent phosphorylation at Ser-240 and Ser-244, although the level of phosphorylation of these latter two sites was less than the preferred sites. Through the use of specific ligation of CD120a (p55) alone and mice deficient in CD120a (p55), CD120b (p75), or both receptors, CD120a (p55) was shown to be necessary and sufficient for the induction of kinase activity. These findings thus suggest that the phosphorylation of Thr-236 and Ser-270 within the membrane proximal region of CD120a (p55) are the preferred sites of phosphorylation by p42(mapk/erk2) and may set in motion phosphorylation at other sites.

Tumor necrosis factor (TNF)-alpha induces leptin production through the p55 TNF receptor

Tumor necrosis factor (TNF)-alpha acts directly on adipocytes to increase production of the lipostatic factor, leptin. However, which TNF receptor (TNFR) mediates this response is not known. To answer this question, leptin was measured in plasma of wild-type (WT), p55, and p75 TNFR knockout (KO) mice injected intraperitoneally with murine TNF-alpha and in supernatants from cultured WT, p55, and p75 TNFR KO adipocytes incubated with TNF-alpha. Leptin also was measured in supernatants from C3H/HeOuJ mouse adipocytes cultured with blocking antibodies to each TNFR and TNF-alpha as well as in supernatants from adipocytes incubated with either human or murine TNF-alpha, which activate either one or both TNFR, respectively. The results using all four strategies show that the induction of leptin production by TNF-alpha requires activation of the p55 TNFR and that although activation of the p75 TNFR alone cannot cause leptin production, its presence affects the capability of TNF-alpha to induce leptin production through the p55 TNFR. These results provide new information on the interplay between cells of the immune system and adipocytes.

Involvement of protein kinase C-beta and ceramide in tumor necrosis factor-alpha-induced but not Fas-induced apoptosis of human myeloid leukemia cells

The role of protein kinase C-beta (PKC-beta) in apoptosis induced by tumor necrosis factor (TNF)-alpha and anti-Fas monoclonal antibody (mAb) in the human myeloid HL-60 leukemia cell line was studied by using its variant HL-525, which is deficient in PKC-beta. In contrast to the parental HL-60 cells, HL-525 is resistant to TNF-alpha-induced apoptosis but sensitive to anti-Fas mAb-induced apoptosis. Both cell types expressed similar levels of the TNF-receptor I, whereas the Fas receptor was detected only in HL-525 cells. Transfecting the HL-525 cells with an expression vector containing PKC-beta reestablished their susceptibility to TNF-alpha-induced apoptosis. The apoptotic effect of TNF-alpha in HL-60 and the transfectants was abrogated by fumonisin, an inhibitor of ceramide generation, and by the peptide Ac-YVAD-BoMK, an inhibitor of caspase-1 and -4. Supplementing HL-525 cells with exogenous ceramides bypassed the PKC-beta deficiency and induced apoptosis, which was also restrained by the caspase-1 and -4 inhibitor. The apoptotic effect of anti-Fas mAb in HL-525 cells was abrogated by the antioxidants N-acetylcysteine and glutathione and by the peptide z-DEVD-FMK, an inhibitor of caspase-3 and -7. We suggest that TNF-alpha-induced apoptosis involves PKC-beta and then ceramide and, in turn, caspase-1 and/or -4, whereas anti-Fas mAb-induced apoptosis utilizes reactive oxygen intermediates and, in turn, caspase-3 and/or -7.

Tumor necrosis factor receptor and Fas signaling mechanisms

Four members of the tumor necrosis factor (TNF) ligand family, TNF-alpha, LT-alpha, LT-beta, and LIGHT, interact with four receptors of the TNF/nerve growth factor family, the p55 TNF receptor (CD120a), the p75 TNF receptor (CD120b), the lymphotoxin beta receptor (LT beta R), and herpes virus entry mediator (HVEM) to control a wide range of innate and adaptive immune response functions. Of these, the most thoroughly studied are cell death induction and regulation of the inflammatory process. Fas/Apo1 (CD95), a receptor of the TNF receptor family activated by a distinct ligand, induces death in cells through mechanisms shared with CD120a. The last four years have seen a proliferation in knowledge of the proteins participating in the signaling by the TNF system and CD95. The downstream signaling molecules identified so far--caspases, phospholipases, the three known mitogen activated protein (MAP) kinase pathways, and the NF-kappa B activation cascade--mediate the effects of other inducers as well. However, the molecules that initiate these signaling events, including the death domain- and TNF receptor associated factor (TRAF) domain-containing adapter proteins and the signaling enzymes associated with them, are largely unique to the TNF/nerve growth factor receptor family.

Modulation of tumor necrosis factor and interleukin-1-dependent NF-kappaB activity by mPLK/IRAK

The innate immune response is an important defense against pathogenic agents. A component of this response is the NF-kappaB-dependent activation of genes encoding inflammatory cytokines such as interleukin-8 (IL-8) and cell adhesion molecules like E-selectin. Members of the serine/threonine innate immune kinase family of proteins have been proposed to mediate the innate immune response. One serine/threonine innate immune kinase family member, the mouse Pelle-like kinase/human interleukin-1 receptor-associated kinase (mPLK/IRAK), has been proposed to play an obligate role in promoting IL-1-mediated inflammation. However, it is currently unknown whether mPLK/IRAK catalytic activity is required for IL-1-dependent NF-kappaB activation. The present study demonstrates that mPLK/IRAK catalytic activity is not required for IL-1-mediated activation of an NF-kappaB-dependent signal. Intriguingly, catalytically inactive mPLK/IRAK inhibits type 1 tumor necrosis factor (TNF) receptor-dependent NF-kappaB activation. The pathway through which mPLK/IRAK mediates this TNF response is TRADD- and TRAF2-independent. Our data suggest that in addition to its role in IL-1 signaling, mPLK/IRAK is a component of a novel signal transduction pathway through which TNF R1 activates NF-kappaB-dependent gene expression.

Activation of p42/p44 mitogen-activated protein kinases (MAPK) and p38 MAPK by tumor necrosis factor (TNF) is mediated through the death domain of the 55-kDa TNF receptor

In the mouse fibrosarcoma cell line L929sA, tumor necrosis factor (TNF) stimulates activation of the stress-responsive p38 mitogen-activated protein kinase (MAPK), as well as the classical p42 and p44 MAPK. TNF signaling can be mediated by p55 or p75 TNF receptors. Here, we demonstrate that TNF-R55 is sufficient to activate p42/p44 MAPK and p38 MAPK. Moreover, by expressing different membrane-bound or purely cytoplasmic truncations of TNF-R55, we show that the intracellular death domain of TNF-R55 is the crucial domain involved. The cytoplasmic membrane-proximal region of TNF-R55, known to induce neutral sphingomyelinase activation, is not required for activation of p38 MAPK or p421p44 MAPK.

RSK-B, a novel ribosomal S6 kinase family member, is a CREB kinase under dominant control of p38alpha mitogen-activated protein kinase (p38alphaMAPK)

A novel ribosomal S6 kinase (RSK) family member, RSK-B, was identified in a p38alphaMAPK-baited intracellular interaction screen. RSK-B presents two catalytic domains typical for the RSK family. The protein kinase C-like N-terminal and the calcium/calmodulin kinase-like C-terminal domains both contain conserved ATP-binding and activation consensus sequences. RSK-B is a p38alphaMAPK substrate, and activated by p38alphaMAPK and, more weakly, by ERK1. RSK-B phosphorylates the cAMP response element-binding protein (CREB) and c-Fos peptides. In intracellular assays, RSK-B drives cAMP response element- and AP1-dependent reporter expression. RSK-B locates to the cell nucleus and co-translocates p38alphaMAPK. In conclusion, RSK-B is a novel CREB kinase under dominant p38alphaMAPK control, also phosphorylating additional substrates.

Immune suppression by recombinant interleukin (rIL)-12 involves interferon gamma induction of nitric oxide synthase 2 (iNOS) activity: inhibitors of NO generation reveal the extent of rIL-12 vaccine adjuvant effect

Recombinant interleukin 12 (IL-12) can profoundly suppress cellular immune responses in mice. To define the underlying mechanism, recombinant murine (rm)IL-12 was given to C57BL/6 mice undergoing alloimmunization and found to transiently but profoundly suppress in vivo and in vitro allogeneic responses and in vitro splenocyte mitogenic responses. Use of neutralizing antibodies and genetically deficient mice showed that IFN-gamma (but not TNF-alpha) mediated rmIL-12-induced immune suppression. Splenocyte fractionation studies revealed that adherent cells from rmIL-12-treated mice suppressed the mitogenic response of normal nonadherent cells to concanavalin A and IL-2. Addition of an inhibitor of nitric oxide synthase (NOS) restored mitogenic responses, and inducible (i)NOS-/- mice were not immunosuppressed by rmIL-12. These results support the view that suppression of T cell responses is due to NO produced by macrophages responding to the high levels of IFN-gamma induced by rmIL-12. When a NOS inhibitor was given with rmIL-12 during vaccination of A/J mice with irradiated SCK tumor cells, immunosuppression was averted and the extent of rmIL-12's ability to enhance induction of protective antitumor immunity was revealed. This demonstrates that rmIL-12 is an effective vaccine adjuvant whose efficacy may be masked by its transient immunosuppressive effect.

Analysis of liver regeneration in mice lacking type 1 or type 2 tumor necrosis factor receptor: requirement for type 1 but not type 2 receptor

We used KO mice lacking either TNF receptor 1 (TNFR-1) or receptor 2 (TNFR-2) to determine whether signaling at the start of liver regeneration after partial hepatectomy (PH) involves only one or both TNF receptors and to analyze in more detail the abnormalities caused by lack of TNFR-1 receptor, which is required for the initiation of liver regeneration. Lack of TNFR-2 had little effect on NF-kappaB and STAT3 binding, and no effect in interleukin-6 production after PH, but caused a delay in AP-1 and C/EBP binding and in the expression of c-jun and c-myc messenger RNA (mRNA). In contrast to mice lacking TNFR-1, which had deficient hepatocyte DNA synthesis and massive lipid accumulation in hepatocytes, TNFR-2 KO mice had normal liver structure and similar levels of hepatocyte DNA replication as those of wild type mice. We conclude that TNFR-1, but not TNFR-2, is necessary for liver regeneration, and that NF-kappaB and STAT3 binding are activated by signals transduced by TNFR-1. Inhibition of AP-1 and C/EBP binding and in the expression of c-jun and c-myc mRNA in the first 4 hours after PH, as well as the apparent lack of Fos in AP-1 complexes, had no effect on the timing or extent of DNA replication.

Cooperative signaling by tumor necrosis factor receptors CD120a (p55) and CD120b (p75) in the expression of nitric oxide and inducible nitric oxide synthase by mouse macrophages

Tumor necrosis factor-alpha (TNFalpha) is recognized by the cell-surface receptors CD120a (p55) and CD120b (p75). In the present study, we have investigated the role of these receptors in the expression of NO2-, a stable metabolite of nitric oxide, and inducible nitric oxide synthase (iNOS) by mouse macrophages. Specific antibody-mediated aggregation of CD120a (p55) induced NO2- accumulation in culture supernatants and iNOS mRNA expression in macrophage lysates, whereas cross-linking of CD120b (p75) had a minimal effect. In contrast, simultaneous cross-linking of both receptors led to a marked augmentation in NO2- and iNOS mRNA expression. Antibody-mediated blockade of CD120a (p55) completely inhibited NO2- expression in response to TNFalpha, whereas blockade of CD120b (p75) reduced NO2- accumulation by approximately 50%. Specific ligation of CD120a (p55) with either (i) human TNFalpha or (ii) by incubation with mouse TNFalpha following pretreatment of macrophages with blocking concentrations of anti-CD120b (p75) antibody resulted in a similar reduction in NO2- production in response to TNFalpha. Quantification of iNOS mRNA, protein, and NO2- expression during independent and co-ligation of CD120a (p55) and CD120b (p75) indicated that iNOS mRNA and protein expression was transient in nature when CD120a (p55) was cross-linked alone but was prolonged when both receptors were simultaneously cross-linked. In addition, cross-linking both receptors also led to a potentiation of NO2- accumulation in culture supernatants that was more pronounced at later time points. These findings suggest that while cross-linking of CD120a (p55) is necessary and sufficient for iNOS mRNA and NO2- expression, CD120b (p75) participates by (i) increasing the sensitivity of the cells to TNFalpha, probably by "passing" ligand to CD120a (p55), and (ii) initiating a signaling event that results in a more sustained induction of iNOS mRNA and protein and thereby augments the production of nitric oxide.

TNF, apoptosis and autoimmunity: a common thread?

A subset of cytokine mediators belonging to the tumor necrosis factor (TNF) family cause apoptosis, acting through receptors and signaling pathways that have recently come to light. Further, at least one autoimmune disease results from a defined defect of apoptosis (mutations of the Fas ligand or its receptor). It is offered that many, and perhaps most autoimmune diseases may result from primary defects of apoptosis. Such defects may cause reflexive overproduction of TNF and other pro-apoptotic cytokines. The collateral damage produced by these mediators may be of pathogenetic importance in complex autoimmune disorders such as rheumatoid arthritis and Crohn disease, wherein TNF blockade is known to have ameliorative effects.

Differential regulation of extracellular signal-regulated kinase and nuclear factor-kappa B signal transduction pathways by hydrogen peroxide and tumor necrosis factor

Reactive oxygen metabolites are increasingly recognized for their ability to stimulate signal transduction pathways. This is important because these oxidants are frequently generated at sites of inflammation. However, little is known about the manner in which reactive oxygen species may selectively stimulate distinct signaling pathways. We have examined this question by stimulating mesothelial cells with hydrogen peroxide (H2O2) as a model oxidant stimulus. The response to H2O2 was examined by measuring the activation of the extracellular signal-regulated kinase (ERK1/2) and the nuclear factor-kappa B (NF-kappa B) signal transduction pathways. We found that H2O2 stimulated activity of the ERK1/2 pathway in a dose- and time-dependent manner. The ability of H2O2 to activate ERK1/2 was similar to that found with tumor necrosis factor (TNF) stimulation. The oxidant effect was inhibited by various reactive oxygen scavengers. An inhibitor of mitogen-activated protein kinase/extracellular signal-regulated kinase, the upstream kinase that activates ERK1/2, inhibited the oxidant effect. The superoxide anion (O2-) also stimulated ERK1/2 activity. In contrast, H2O2 did not stimulate proteolysis of I kappa B-alpha and induced only a small degree of NF-kappa B nuclear translocation. Stimulation of the cells with O2- also induced a minimal degree of NF-kappa B activation. TNF was a potent stimulus for I kappa B-alpha proteolysis and NF-kappa B activation, demonstrating that the cells did have a functional NF-kappa B pathway. These results suggest that oxidants may selectively stimulate certain pathways, thereby preserving some specificity of the signaling process. Furthermore, different cell types and distinct signaling pathways within cells may demonstrate unique profiles in the manner in which they respond to oxidant stimulation.

Regulation of Raf-1 kinase by TNF via its second messenger ceramide and cross-talk with mitogenic signalling

Raf-1 kinase is a central regulator of mitogenic signal pathways, whereas its general role in signal transduction of tumour necrosis factor (TNF) is less well defined. We have investigated mechanisms of Raf-1 regulation by TNF and its messenger ceramide in cell-free assays, insect and mammalian cell lines. In vitro, ceramide specifically bound to the purified catalytic domain and enhanced association with activated Ras proteins, but did not affect the kinase activity of Raf-1. Cell-permeable ceramides induced a marked increase of Ras-Raf-1 complexes in cells co-expressing Raf-1 and activated Ras. Likewise, a fast elevation of the endogeneous ceramide level, induced by TNF treatment of human Kym-1 rhabdomyosarcoma cells, was followed by stimulation of Ras-Raf-1 association without significant Raf-1 kinase activation. Failure of TNF or ceramide to induce Raf-1 kinase was observed in several TNF-responsive cell lines. Both TNF and exogeneous C6-ceramide interfered with the mitogenic activation of Raf-1 and ERK by epidermal growth factor and down-regulated v-Src-induced Raf-1 kinase activity. TNF also induced the translocation of Raf-1 from the cytosolic to the particulate fraction, indicating that this negative regulatory cross-talk occurs at the cell membrane. Interference with mitogenic signals at the level of Raf-1 could be an important initial step in TNF's cytostatic action.

Tumor necrosis factor (TNF) receptor type 2 mediates thymocyte proliferation independently of TNF receptor type 1

Tumor necrosis factor (TNF) mediates its biological effects by binding to two distinct but homologous receptor molecules. The type 1 receptor (TNF-R1) has been shown to be essential and sufficient for most cellular responses to soluble TNF. In contrast, only limited data exist concerning the role of the type 2 receptor (TNF-R2) in TNF responses, both in vitro and in vivo. Here, we demonstrate by the use of thymocytes from TNF-R-deficient mice that the TNF-R2-dependent enhancement of proliferation and secretion of granulocyte-macrophage colony-stimulating factor is in fact mediated by TNF-R2 on its own, independent of co-expression and/or stimulation of TNF-R1.

Differential Activities of Secreted Lymphotoxin-α3 and Membrane Lymphotoxin-α1β2 in Lymphotoxin-Induced Inflammation: Critical Role of TNF Receptor 1 Signaling

Lymphotoxin (LT, LTα, TNFβ) is a member of the immediate TNF family that also includes TNF-α and lymphotoxin-β (LTβ). LT is produced by activated lymphocytes and functions as either a secreted homotrimer or a membrane-associated heterotrimer that includes the transmembrane protein LTβ. Secreted LTα3 can bind to two cell surface receptors, TNFR1 and TNFR2, while the membrane-bound heterotrimer LTα1β2 has been shown to interact with a distinct receptor, LTβR. LTα induces inflammation at the sites of expression of a rat insulin promoter-driven lymphotoxin (RIPLT) transgene in the pancreas and kidney. To determine the role of the various ligands and their receptors in LT-induced inflammation, mice deficient in either TNFR1, TNFR2, or LTβ were crossed to RIPLT-transgenic mice. Our results indicate that LTα-induced inflammation is dependent on the interaction of LTα3 with TNFR1, and there is no obvious role for TNFR2, since in its absence, LTα-induced inflammation is quantitatively and qualitatively similar to that seen in the wild type. However, the absence of LTβ results in accentuated infiltration of the kidney with an increase in the proportion of memory cells in the infiltrate. These data show a crucial role for the secreted LTα3 signaling via TNFR1 in LTα-induced inflammation, and a separate and distinct role for the membrane LTα1β2 form in this inflammatory process.