Interrelated effects of tumor necrosis factor and interleukin 1 on cell viability

DAPK-HSF1 interaction as a positive-feedback mechanism stimulating TNF-induced apoptosis in colorectal cancer cells

Death-associated protein kinase (DAPK) is a serine-threonine kinase with tumor suppressor function. Previously, we demonstrated that tumor necrosis factor (TNF) induced DAPK-mediated apoptosis in colorectal cancer. However, the protein-protein interaction network associated with TNF-DAPK signaling still remains unclear. We identified HSF1 as a new DAPK phosphorylation target in response to low concentrations of TNF and verified a physical interaction between DAPK and HSF1 both in vitro and in vivo. We show that HSF1 binds to the DAPK promoter. Transient overexpression of HSF1 protein led to an increase in DAPK mRNA level and consequently to an increase in the amount of apoptosis. By contrast, treatment with a DAPK-specific inhibitor as well as DAPK knockdown abolished the phosphorylation of HSF1 at Ser230 (pHSF1(Ser230)). Furthermore, translational studies demonstrated a positive correlation between DAPK and pHSF1(Ser230) protein expression in human colorectal carcinoma tissues. Taken together, our data define a novel link between DAPK and HSF1 and highlight a positive-feedback loop in DAPK regulation under mild inflammatory stress conditions in colorectal tumors. For the first time, we show that under TNF the pro-survival HSF1 protein can be redirected to a pro-apoptotic program.

Apoptosis-like cell death induction and aberrant fibroblast properties in human incisional hernia fascia

Incisional hernia often occurs following laparotomy and can be a source of serious problems. Although there is evidence that a biological cause may underlie its development, the mechanistic link between the local tissue microenvironment and tissue rupture is lacking. In this study, we used matched tissue-based and in vitro primary cell culture systems to examine the possible involvement of fascia fibroblasts in incisional hernia pathogenesis. Fascia biopsies were collected at surgery from incisional hernia patients and non-incisional hernia controls. Tissue samples were analyzed by histology and immunoblotting methods. Fascia primary fibroblast cultures were assessed at morphological, ultrastructural, and functional levels. We document tissue and fibroblast loss coupled to caspase-3 activation and induction of apoptosis-like cell-death mechanisms in incisional hernia fascia. Alterations in cytoskeleton organization and solubility were also observed. Incisional hernia fibroblasts showed a consistent phenotype throughout early passages in vitro, which was characterized by significantly enhanced cell proliferation and migration, reduced adhesion, and altered cytoskeleton properties, as compared to non-incisional hernia fibroblasts. Moreover, incisional hernia fibroblasts displayed morphological and ultrastructural alterations compatible with autophagic processes or lysosomal dysfunction, together with enhanced sensitivity to proapoptotic challenges. Overall, these data suggest an ongoing complex interplay of cell death induction, aberrant fibroblast function, and tissue loss in incisional hernia fascia, which may significantly contribute to altered matrix maintenance and tissue rupture in vivo.

Cytocidal effect of interleukin 1 (IL-1) on HeLa cells is mediated by both soluble and transmembrane tumor necrosis factor (TNF)

Interleukin 1 (IL-1) is a pleiotropic cytokine able to induce cytocidal effect. The aim of the presented work was to analyze the mechanism of IL-1-induced cytocidal effect in HeLa cells in the presence of cycloheximide (CHX). We found that the pattern of IL-1-induced cell death shares significant similarities with the effect of tumor necrosis factor (TNF) in these cells. Subsequently, we identified IL-1 cytotoxicity as an indirect effect. The supernatant collected from the cells treated with IL-1 and CHX showed toxic activity towards IL-1-resistant while TNF-sensitive A9 cells. Furthermore, antibodies neutralizing TNF blocked HeLa cell death induced by IL-1/CHX. TNF was then detected in HeLa cells by means of flow cytometry, fluorescence microscopy and ELISA of detergent-soluble cell extracts. In the presence of an inhibitor of TNF sheddase (TACE), the cytotoxic effect of IL-1/CHX and the amount of TNF protein in detergent-soluble cell extracts were enhanced. These results suggest that in response to interleukin 1/CHX, the amount of transmembrane TNF is increased. Taken together, we demonstrated that the mechanism of IL-1 cytotoxic activity in HeLa cells in the presence of CHX depends on the function of soluble and transmembrane TNF.

The IL-6/sIL-6R treatment of a malignant melanoma cell line enhances susceptibility to TNF-alpha-induced apoptosis

Melanoma is an intractable tumor that has shown very impressive and promising response to local administration of high dose recombinant TNF-alpha in combination with IFN-gamma in clinical studies. In this study, we investigated the effect of IL-6/sIL-6R on TNF-alpha-resistant B16/F10.9 melanoma cells. A low dose of TNF-alpha or IL-6/sIL-6R had minimal affect on the cell growth. However, the highly active fusion protein of sIL-6R and IL-6 (IL6RIL6), covalently linked by a flexible peptide, sensitized TNF-alpha-resistant F10.9 melanoma cells to TNF-alpha-induced apoptosis. Stimulation of the cells with IL6RIL6 plus TNF-alpha resulted in both the activation of caspase-3 and the reduction of bcl-2 expression. Flow cytometry analysis showed that IL6RIL6-upregulated TNF-R55 and TNF-R75 expression, suggesting an increase in TNF-alpha responsiveness by IL6RIL6 resulting from the induction of TNF receptors. Moreover, exposure of F10.9 cells to neutralizing antibody to TNF-R55 significantly inhibited IL6RIL6/TNF-alpha-induced cytotoxicity. These results suggest that the IL6/sIL6R/gp130 system, which sensitizes TNF-alpha-resistant melanoma cells to TNF-alpha-induced apoptosis, may provide a new target for immunotherapy.

Coordinate Inhibition of Cytokine-mediated Induction of Ferritin H, Manganese Superoxide Dismutase, and Interleukin-6 by the Adenovirus E1A Oncogene

Adenovirus E1A sensitizes cells to the cytotoxic action of tumor necrosis factor alpha (TNF-alpha). This effect has been attributed to direct blockade of NF-kappaB activation, as well as to increased activation of components of the apoptotic pathway and decreases in inhibitors of apoptosis. In this report we evaluated the mechanism by which E1A modulates the expression of the cytokine-inducible cytoprotective genes manganese superoxide dismutase (MnSOD), interleukin-6 (IL-6), and ferritin heavy chain (FH). We observed that E1A blocks induction of MnSOD, IL-6, and FH by TNF-alpha or IL-1alpha. Because NF-kappaB plays a role in cytokine-dependent induction of MnSOD, IL-6, and FH, we assessed the effect of E1A on NF-kappaB in cells treated with TNF. IkappaB, the inhibitor of NF-kappaB, was degraded similarly in the presence and absence of E1A. TNF induced a quantitatively and temporally equivalent activation of NF-kappaB in control and E1A-transfected cells. However, TNF-dependent acetylation of NF-kappaB was diminished in cells expressing E1A. E1A mutants unable to bind p400 or the Rb family proteins were still capable of repressing TNF-dependent induction of FH. However, mutants of E1A that abrogated binding of p300/CBP blocked the ability of E1A to repress TNF-dependent induction of FH. These results suggest that p300/CBP is a critical control point in NF-kappaB-dependent transcriptional regulation of cytoprotective genes by cytokines.

Pentoxifylline and its major oxidative metabolites exhibit different pharmacological properties

Previous investigations indicate that some of the metabolites of the hemorheological agent pentoxifylline (PTX), namely 1-(5-hydroxyhexyl)-3,7-dimethylxanthine (M1), 1-(4-carboxybutyl)-3,7-dimethylxanthine (M4) and 1-(3-carboxypropyl)-3,7-dimethylxanthine (M5), concur to some of the biological effects of the drug. However, information on the bioactivity of the major circulating oxidative metabolites of PTX (M4 and M5) is scanty. Here, we compared the effects of M4 and M5 with that of PTX and its major reductive metabolite, M1, on TNF-alpha production and cytotoxicity, endothelial cell proliferation and on the ATPase activity related to some ATP-binding cassette (ABC) transporters. Unlike PTX and M1, M4 and M5 poorly inhibited lipopolysaccaride-stimulated tumor necrosis factor-alpha (TNF-alpha) release by RAW 264.7 murine macrophages, and did not affect at all cell proliferation and upregulation of TNF-alpha-induced vascular cell adhesion molecule-1 (VCAM-1) in H5V endothelioma cells. By contrast, M4 and M5 were more effective than PTX and M1 in protecting WC/1 murine fibrosarcoma cells from TNF-alpha cytotoxicity. Moreover, results from ATP hydrolase assays indicated that neither PTX nor its tested metabolites interacted significantly with the human multidrug resistance transporters p-glycoprotein/multidrug resistance 1 (MDR1), multidrug resistance-related protein 1 (MRP1), and breast cancer resistance protein (BCRP). Based on these results and literature data, M5, retaining some of the PTX effects but lacking in significant inhibition of TNF-alpha production, may be a promising candidate drug for certain pathologic conditions.

The radical scavenger edaravone enhances the anti-tumor effects of CPT-11 in murine colon cancer by increasing apoptosis via inhibition of NF-κB

The transcription factor NF-kappaB is reportedly activated by anti-cancer chemotherapeutic compounds in many cancer cell lines and NF-kappaB activation is one mechanism by which tumors become resistant to apoptosis. Antioxidants have been reported to serve as potent NF-kB inhibitors. In this study, we investigated the ability of edaravone to enhance apoptosis induced by CPT-11 through inhibition of NF-kB. In vitro, SN38, the active metabolite of CPT-11, induced activation of NF-kB, the production of intracellular reactive oxygen species, the activation of caspase-3, and apoptosis in colon26 cells. Pretreatment with edaravone scavenged the SN38-produced reactive oxygen species, and inhibited the SN38-induced activation of NF-kB. Moreover, edaravone enhanced the activation of caspase-3, and the level of apoptosis induced by SN38. In vivo, the combination of edaravone with CPT-11 reduced subcutaneous tumor growth and number of pulmonary metastases more effectively than CPT-11 alone. These results demonstrate that the combination of edaravone with CPT-11 may constitute a new strategy for treating primary and metastatic colon cancer.

Pretreatment with PKC inhibitor triggers TNF-alpha induced apoptosis in TNF-alpha-resistant B16 melanoma BL6 cells

Tumor necrosis factor alpha (TNF-alpha) modulates various events through several different pathways. Many tumor cells are resistant to this cytokine. Pretreatment of these cells with actinomycin D enhances TNF-alpha-induced apoptosis. In the present study, we investigated the mechanism of this enhancement and whether or not the apoptosis of TNF-alpha-resistant cancer cells can be induced by the inhibition of Protein kinase C (PKC). When TNF-alpha was added after inhibition of PKC by H7, apoptosis was observed, and companied with the activation of nuclear factor kappa B (NF-kappaB). After the inhibition of protein kinase B (Akt) by LY294002 or p38 mitogen-activated protein kinase (p38MAPK) by SB203580, the addition of TNF-alpha did not cause apoptosis. However, after the inhibition of MAPK/extracellular signal-regulated kinase kinase 1/2 (MEK1/2) with U0126, apoptosis was observed when TNF-alpha was added. In the Western blotting analysis, phosphorylation of MEK1/2 occurred at 60 minutes after the addition of TNF-alpha. However, it was noted that after pretreatment with H7, a significant decrease in phosphorylated MEK1/2 was observed. The present findings suggest that MEK1/2 plays an important role in TNF-alpha-resistance in TNF-alpha-resistant B16 melanoma BL6 cells. Furthermore, it was found that MEK1/2 is more important than NF-kappaB, Akt, and p38MAPK in anti-apoptotic PKC signaling and that TNF-alpha-resistance can be overcome by inhibiting MEK1/2. These results suggest the possibility of development of a new anticancer drug treatment.

Loss of the cylindromatosis tumour suppressor inhibits apoptosis by activating NF-kappaB

Protein modification by the conjugation of ubiquitin moieties--ubiquitination--plays a major part in many biological processes, including cell cycle and apoptosis. The enzymes that mediate ubiquitin-conjugation have been well-studied, but much less is known about the ubiquitin-specific proteases that mediate de-ubiquitination of cellular substrates. To study this gene family, we designed a collection of RNA interference vectors to suppress 50 human de-ubiquitinating enzymes, and used these vectors to identify de-ubiquitinating enzymes in cancer-relevant pathways. We report here that inhibition of one of these enzymes, the familial cylindromatosis tumour suppressor gene (CYLD), having no known function, enhances activation of the transcription factor NF-kappaB. We show that CYLD binds to the NEMO (also known as IKKgamma) component of the IkappaB kinase (IKK) complex, and appears to regulate its activity through de-ubiquitination of TRAF2, as TRAF2 ubiquitination can be modulated by CYLD. Inhibition of CYLD increases resistance to apoptosis, suggesting a mechanism through which loss of CYLD contributes to oncogenesis. We show that this effect can be relieved by aspirin derivatives that inhibit NF-kappaB activity, which suggests a therapeutic intervention strategy to restore growth control in patients suffering from familial cylindromatosis.

Death receptors and apoptosis. Deadly signaling and evasive tactics

The availability of large amounts of sequence data has made it possible to identify death receptors by homology. Because the genome has not been analyzed completely, a few additional members of this family probably will be identified in the next few years. Rapid progress also has been made recently on the signaling mechanisms used by the death receptors. Considerable conservation of the intracellular signaling mechanisms is seen between different receptors suggesting that it is unlikely that new elements will be added to the molecular framework of death receptor signaling. The analysis of signaling mechanisms has exposed the complexity and multiplicity of cellular responses on death receptor activation. It is not surprising, therefore, that understanding the biological function the death receptors lags behind their characterization at the molecular level. In particular, the role of death receptors in many disease states, such as myocardial disease, remains to be elucidated. (38) This complexity in death-receptor function has constrained their potential for pharmacological manipulation. In most cases it is not sufficient to simply activate a specific death receptor. Manipulation of only one of the multiple responses induced by the receptor is desirable. Currently, no solutions to this challenge have been applied. The exception to this conundrum may be TRAIL. Injection of recombinant TRAIL has few side effects in animal studies and combination therapies that use TRAIL as a radiation sensitizer show early promise.

Cytokine modulated cell-membrane bound tumour necrosis factor expression is associated with enhanced monocyte-mediated killing of human leukaemic targets

Cytokines such as interleukin-3 (IL-3) and granulocyte-macrophage colony-stimulating factor (GM-CSF) activate monocytes both in vitro and in vivo. We therefore studied whether the anti-leukaemic activity of monocytes could be augmented by IL-3 alone or in combination with GM-CSF. Using normal human monocytes stimulated with IL-3, GM-CSF, LPS or combinations of growth factor and LPS, we studied their cytotoxic activity against leukaemic cell-lines and primary AML blasts. IL-3 like GM-CSF, augmented the expression and secretion of TNF but did not prime for further expression and secretion of TNF in response to LPS. Neither GM-CSF or IL-3 increased the expression or secretion of TNF receptor p55 (TNF-Rp55), although both agents increased expression of TNF receptor p75 (TNF-Rp75). Monocyte-mediated cytotoxicity (MMC) against K562 and U937 cell-lines was increased by both GM-CSF and IL-3 stimulation, and both cytokines primed monocytes for increased killing of K562 and KG-1 cell-lines as well as primary AML blasts in response to LPS. The mechanism of action of MMC was largely confirmed to be via surface-bound TNF, although other TNF-independent mechanisms must have been involved.

CLAP, a novel caspase recruitment domain-containing protein in the tumor necrosis factor receptor pathway, regulates NF-kappaB activation and apoptosis

Molecules that regulate NF-kappaB activation play critical roles in apoptosis and inflammation. We describe the cloning of the cellular homolog of the equine herpesvirus-2 protein E10 and show that both proteins regulate apoptosis and NF-kappaB activation. These proteins were found to contain N-terminal caspase-recruitment domains (CARDs) and novel C-terminal domains (CTDs) and were therefore named CLAPs (CARD-like apoptotic proteins). The cellular and viral CLAPs induce apoptosis downstream of caspase-8 by activating the Apaf-1-caspase-9 pathway and activate NF-kappaB by acting upstream of the NF-kappaB-inducing kinase, NIK, and the IkB kinase, IKKalpha. Deletion of either the CARD or the CTD domain inhibits both activities. The CARD domain was found to be important for homo- and heterodimerization of CLAPs. Substitution of the CARD domain with an inducible FKBP12 oligomerization domain produced a molecule that can induce NF-kappaB activation, suggesting that the CARD domain functions as an oligomerization domain, whereas the CTD domain functions as the effector domain in the NF-kappaB activation pathway. Expression of the CARD domain of human CLAP abrogates tumor necrosis factor-alpha-induced NF-kappaB activation, suggesting that cellular CLAP plays an essential role in this pathway of NF-kappaB activation.

Control of inducible chemoresistance: enhanced anti-tumor therapy through increased apoptosis by inhibition of NF-kappaB

Programmed cell death (apoptosis) seems to be the principal mechanism whereby anti-oncogenic therapies such as chemotherapy and radiation effect their responses. Resistance to apoptosis, therefore, is probably a principal mechanism whereby tumors are able to overcome these cancer therapies. The transcription factor NF-kappaB is activated by chemotherapy and by irradiation in some cancer cell lines. Furthermore, inhibition of NF-kappaB in vitro leads to enhanced apoptosis in response to a variety of different stimuli. We show here that inhibition of NF-kappaB through the adenoviral delivery of a modified form of IkappaBalpha, the inhibitor of NF-kappaB, sensitizes chemoresistant tumors to the apoptotic potential of TNFalpha and of the chemotherapeutic compound CPT-11, resulting in tumor regression. These results demonstrate that the activation of NF-kappaB in response to chemotherapy is a principal mechanism of inducible tumor chemoresistance, and establish the inhibition of NF-kappaB as a new approach to adjuvant therapy in cancer treatment.

Control of inducible chemoresistance: enhanced anti-tumor therapy through increased apoptosis by inhibition of NF-kappaB

Programmed cell death (apoptosis) seems to be the principal mechanism whereby anti-oncogenic therapies such as chemotherapy and radiation effect their responses. Resistance to apoptosis, therefore, is probably a principal mechanism whereby tumors are able to overcome these cancer therapies. The transcription factor NF-kappaB is activated by chemotherapy and by irradiation in some cancer cell lines. Furthermore, inhibition of NF-kappaB in vitro leads to enhanced apoptosis in response to a variety of different stimuli. We show here that inhibition of NF-kappaB through the adenoviral delivery of a modified form of IkappaBalpha, the inhibitor of NF-kappaB, sensitizes chemoresistant tumors to the apoptotic potential of TNFalpha and of the chemotherapeutic compound CPT-11, resulting in tumor regression. These results demonstrate that the activation of NF-kappaB in response to chemotherapy is a principal mechanism of inducible tumor chemoresistance, and establish the inhibition of NF-kappaB as a new approach to adjuvant therapy in cancer treatment.

Ectopic expression of hepatitis C virus core protein differentially regulates nuclear transcription factors

The putative core protein of hepatitis C virus (HCV) regulates cellular growth and a number of cellular promoters. To further understand its effect, we investigated the role of the core protein in the endogenous regulation of two distinct transcription factors, nuclear factor-kappaB (NF-kappaB) and activating protein-1 (AP-1), and the related mitogen-activated protein kinase kinase (MAPKK) and c-Jun N-terminal kinase (JNK). Stable cell transfectants expressing the HCV core protein suppressed tumor necrosis factor (TNF)-induced NF-kappaB activation. Supershift analysis revealed that NF-kappaB consists of p50 and p65 subunits. This correlated with inhibition of the degradation of IkappaBalpha, the inhibitory subunit of NF-kappaB. The effect was not specific to TNF, as suppression in core protein-expressing cells was also observed in response to a number of other inflammatory agents known to activate NF-kappaB. In contrast to the effect on NF-kappaB, the HCV core protein constitutively activated AP-1, which correlated with the activation of JNK and MAPKK, which are known to regulate AP-1. These observations indicated that the core protein targets transcription factors known to be involved in the regulation of inflammatory responses and the immune system.

TNF-alpha and IL-1beta cross-desensitization of astrocytes and astrocytoma cell lines

Exposure of human astrocytes and astrocytoma cell lines to TNF-alpha, IL-1beta and gammaIFN induce expression of a specific member of the intercrine/chemokine family of cytokines, RANTES. Pre-incubation with non-stimulatory concentrations of TNF-alpha inhibit IL-1beta-stimulated RANTES expression and similarly, non-stimulatory concentrations of IL-1beta inhibits TNF-alpha induced RANTES expression. The lowered responsiveness of these cells is stably maintained for at least 24 h. The inhibitory effect of TNF-alpha on IL-1beta-induced responses was mediated by TNF receptor-1 since low concentrations of a specific anti-TNF receptor-1 antiserum mimicked the inhibitory effect. These results indicate that TNF and IL-1 receptors mediate pro- and antiinflammatory responses in a concentration dependent manner, suggesting that at low receptor occupancy, TNF and IL-1 receptors may share a common signaling pathway and behave as endogenous antiinflammatory cytokines.

Temporal effect of tumor necrosis factor alpha on murine macrophages infected with Mycobacterium avium

Members of the Mycobacterium avium complex are a family of bacteria that persist within macrophages in the face of an immune response. Elimination of these organisms is likely due to cytokine-induced macrophage activation. Because macrophage activation by tumor necrosis factor alpha (TNF-alpha) appears critical for killing of intracellular M. avium, early downregulation of TNF-alpha levels in infected macrophages has been suggested as a survival mechanism for virulent strains of M. avium. We examined the relationship between TNF-alpha and growth of M. avium strains of differing virulence, as measured by their ability to grow in murine bone marrow-derived macrophages. When exogenous TNF-alpha was added immediately following macrophage infection, significant growth inhibition of virulent M. avium strains was observed. If TNF-alpha addition was delayed by 24 h or more, growth inhibition was abrogated. To determine if early downregulation of TNF-alpha levels could explain the differential growth of virulent and avirulent strains, levels of TNF-alpha and prostaglandin E2 (PGE2), which has been shown to suppress TNF-alpha production in uninfected macrophages, were quantified over time. Upregulation of both TNF-alpha and PGE2, as measured by enzyme-linked immunosorbent assay, was evident by 6 h postinfection, indicating that the ability of M. avium to replicate in macrophages was not directly correlated with early downregulation of TNF-alpha production. However, TNF-alpha bioactivity, as measured by cytotoxicity, was significantly decreased in virulent M. avium strains at all time periods examined. Treatment of infected macrophages with gamma interferon immediately after infection resulted in significantly increased levels of nitric oxide but did not affect the growth of virulent M. avium strains. These results suggest that while significant levels of TNF-alpha are present in supernatants from all M. avium strains, levels of biologically active TNF-alpha are significantly reduced in supernatants from virulent M. avium strains. Preliminary results suggest that upregulation of the soluble p75 TNF receptor may be one mechanism by which TNF-alpha bioactivity reduction occurs.

MAP3K-related kinase involved in NF-kappaB induction by TNF, CD95 and IL-1

Several members of the tumour-necrosis/nerve-growth factor (TNF/NGF) receptor family activate the transcription factor NF-kappaB through a common adaptor protein, Traf2 (refs 1-5), whereas the interleukin 1 type-I receptor activates NF-kappaB independently of Traf2 (ref. 4). We have now cloned a new protein kinase, NIK, which binds to Traf2 and stimulates NF-kappaB activity. This kinase shares sequence similarity with several MAPKK kinases. Expression in cells of kinase-deficient NIK mutants fails to stimulate NF-kappaB and blocks its induction by TNF, by either of the two TNF receptors or by the receptor CD95 (Fas/Apo-1), and by TRADD, RIP and MORT1/FADD, which are adaptor proteins that bind to these receptors. It also blocked NF-kappaB induction by interleukin-1. Our findings indicate that NIK participates in an NF-kappaB-inducing signalling cascade common to receptors of the TNF/NGF family and to the interleukin-1 type-I receptor.

TNF- and cancer therapy-induced apoptosis: potentiation by inhibition of NF-kappaB

Many cells are resistant to stimuli that can induce apoptosis, but the mechanisms involved are not fully understood. The activation of the transcription factor nuclear factor-kappa B (NF-kappaB) by tumor necrosis factor (TNF), ionizing radiation, or daunorubicin (a cancer chemotherapeutic compound), was found to protect from cell killing. Inhibition of NF-kappaB nuclear translocation enhanced apoptotic killing by these reagents but not by apoptotic stimuli that do not activate NF-kappaB. These results provide a mechanism of cellular resistance to killing by some apoptotic reagents, offer insight into a new role for NF-kappaB, and have potential for improvement of the efficacy of cancer therapies.

Suicide by order: some open questions about the cell-killing activities of the TNF ligand and receptor families

That leukocytes can produce proteins with cell-killing activities has been known for almost 30 years (Granger GA, Kolb EP. J Immunol 1968, 101; Ruddle NH, Waksman BH. J Exp Med 1968, 128, 1267-1279; Carswell EA, Old LJ, Kassel S, Green S, Fiore N, Williamson B. Proc Natl Acad Sci USA 1975, 72, 3666-3670). However, it is only recently that the nature of this cell killing activity has become clear. What appeared initially to be merely a toxic effect of the leukocyte-produced proteins (hence their initial name, 'lymphotoxins') has turned out to represent a new kind of biological mechanism, whose understanding required a radical change of concepts concerning the ways in which the life and death of the cell are controlled. The leukocyte-produced 'toxic' proteins turned out to act, not through any toxic feature of the proteins themselves, but by activating destructive mechanisms that pre-exist within the target cell. Their action thus represents a way by which one cell can dictate suicide to another. Within the last few years more has been learnt about their mechanisms of action than was learnt throughout the two preceding decades. Nevertheless, many questions still remain unresolved. The purpose of this exposition is to spell out some of these open questions.

Dual role of the p75 tumor necrosis factor (TNF) receptor in TNF cytotoxicity

Whereas there is ample evidence for involvement of the p55 tumor necrosis factor (TNF) receptor (p55-R) in the cytocidal effect of TNF, the role of the p75 TNF receptor (p75-R) in this effect is a matter of debate. In this study, we probed the function of p75-R in cells sensitive to the cytotoxicity of TNF using a wide panel of antibodies (Abs) against the receptor's extracellular domain. Two distinct Ab effects were observed. The Abs triggered signaling for cytotoxicity. This effect: (a) was correlated with the extent of p75-R expression by the cells; (b) was dependent on receptor cross-linking by the Abs; (c) occurred in HeLa cells, but not in A9 cells transfected with human p75-R or in HeLa cells expressing cytoplasmically truncated p75-R mutants, indicating that it involves cell-specific activities of the intracellular domain of the receptor; (d) was synergistic with the cytocidal effect of Abs against p55-R. Moreover, it seemed to reverse induced desensitization to the cytocidal effect of anti p55-R Abs, suggesting that it involves mechanisms different from those of the signaling by the p55 TNF-R. In addition, the Abs affected the response to TNF in a way that does not involve the signaling activity of p75-R. These effects: (a) could be observed also in cells in which only p55-R signaled for the cytocidal effect; (b) were not dependent on receptor cross-linking by the Abs; (c) varied according to the site at which the Abs bound to the receptor; and (d) were correlated inversely with the effects of the Abs on TNF binding to p75-R. That is, Abs binding to the membrane-distal part of the receptor's extracellular domain displaced TNF from the p75 receptor and enhanced cytocidal effect, whereas Abs that bind to the membrane-proximal part of the extracellular domain--a region at which a conformational change seems to take place upon TNF binding--decreased the dissociation of TNF from p75-R and inhibited its cytocidal effect. The above findings suggest that p75-R contributes to the cytocidal effect of TNF both by its own signaling and by regulating the access of TNF to p55-R.

Roles of tumor necrosis factor alpha, granulocyte-macrophage colony-stimulating factor, platelet-activating factor, and arachidonic acid metabolites in interleukin-1-induced resistance to infection in neutropenic mice

Treatment with a single low dose (80 to 800 ng) of interleukin-1 (IL-1) 24 h before a lethal bacterial challenge in granulocytopenic and in normal mice enhances nonspecific resistance. The mechanism behind this protection has only partially been elucidated. Since IL-1 induces production of tumor necrosis factor alpha (TNF-alpha), granulocyte-macrophage colony-stimulating factor (GM-CSF), platelet-activating factor (PAF), and arachidonic acid metabolites, we investigated the potential role of these substances in IL-1-induced protection. Low doses of murine TNF-alpha but not of human TNF-alpha enhanced survival, suggesting an effect via the type II TNF receptor rather than the type I TNF receptor, which has little species specificity. In line with this TNF-alpha-induced protection from infection, pretreatment with a low dose of a rat anti-murine TNF-alpha monoclonal antibody tended to inhibit IL-1-induced protection, suggesting a role of TNF-alpha as a mediator of IL-1-induced enhanced resistance to infection. Pretreatment with higher doses of anti-TNF-alpha, however, showed a dose-related protective effect per se, which could be further enhanced by a suboptimal dose of IL-1. A combination of optimal doses of anti-TNF-alpha and IL-1 produced an increase in survival similar to that produced by separate pretreatments. This lack of further enhancement of survival by combined optimal pretreatments suggests a similar mechanism of protection, most likely attenuation of deleterious effects of overproduced proinflammatory cytokines like TNF-alpha during lethal infection. Pretreatment with different doses of GM-CSF before a lethal Pseudomonas aeruginosa challenge in neutropenic mice did not enhance survival. Different doses of WEB 2170, a selective PAF receptor antagonist, of MK-886, a selective inhibitor of leukotriene biosynthesis, or of several cyclooxygenase inhibitors did not reduce the protective effect of IL-1 pretreatment. We conclude that IL-1-induced nonspecific resistance is partially mediated by induction of TNF-alpha and not by GM-CSF, PAF, and arachidonic acid metabolites. The mechanism of action of IL-1 seems to be similar to that of anti-TNF-alpha.

Cytokines involved in monocyte mediated tumor cell death and growth inhibition in serum-free medium

In a serum-free culture system, the release of TNF, lI-1, lI-6, IFN-alpha, and IFN-beta during interaction of elutriated human monocytes (MO) with human tumor cells (TC) was studied by ELISA-technique. Contributions of these cytokines to inhibition of TC-growth and to induction of TC-death by supernatants (SU) gained from such MO/TC-interaction cultures were investigated using affinity chromatography for removal of individual cytokines. Although the TC used are relatively insensitive to recombinant human TNF, withdrawal of TNF causes 50% to 75% reduction of SU-induced TC-death rates, suggesting that susceptibility to TNF is raised during MO/TC-interaction by the other cytokines. Individual removal of other cytokines does not cause reduction of SU-mediated TC-death. However, combined withdrawal of lI-1 and IFN-alpha/beta causes in 2 of 4 TC-lines significant reduction of TC-death. Combined removal of TNF, IFN-alpha/beta, lI-1, and lI-6 leads to complete prevention of SU-mediated growth inhibitory and lytic effects, suggesting that besides these cytokines other signals are not involved significantly. SU-effects can be mimicked by appropriate combinations of authentic cytokines. The response of TC to SU- or cytokine-exposure is strikingly dependent on TC-density, leading at subconfluent TC-density exclusively to inhibition of growth and at postconfluent TC-density to induction of cell death. The principal effect of SU or cytokine combinations in this context seems to be the activation of growth inhibitory signal transduction pathways leading to TC-death in postconfluent TC-populations exclusively if growth stimulatory pathways are activated at the same time. Mouse L cells do not follow this reaction pattern: Their death is exclusively dependent on the presence of TNF in SU and they die upon SU-exposure at postconfluent as well as at subconfluent cell density.

Extracorporeal photochemotherapy induces the production of tumor necrosis factor-alpha by monocytes: implications for the treatment of cutaneous T-cell lymphoma and systemic sclerosis

Extracorporeal photopheresis (ExP) has been shown to be an efficacious and well-tolerated therapy in the treatment of cutaneous T-cell lymphoma (CTCL) and systemic sclerosis. However, the precise mechanisms of its action have not been defined. Because of a correlation between the development of fever in the early phase of treatment of CTCL and subsequent anti-tumor responses, we examined the production of the proinflammatory, pyrogenic cytokines tumor necrosis factor-alpha (TNF), IL-6, IL-1 alpha, and IL-1 beta before and after ExP. Monocytes were purified from peripheral blood specimens of normal volunteers (n = 4) or from peripheral blood specimens of CTCL (n = 6) or systemic sclerosis (n = 3) patients that were obtained immediately prior to ExP and also directly from the photopheresis unit after ExP, just prior to reinfusion into the patient. Monocytes were then cultured under various conditions for 16 h, after which the culture supernatants were collected and assayed for specific cytokine production. ExP induced a significant increase in the production of TNF (p less than 0.008) and IL-6 (p less than 0.05) as compared to non-ExP-treated cells, whereas no significant differences were observed in IL-1 alpha (p less than 0.5) and IL-1 beta (p less than 0.2) production following ExP. Exposure of monocyte cultures to IFN-gamma (100 U/mL) either before or after ExP further enhanced TNF production by 4 to 28 times. In contrast, incubation with IFN-alpha (100 U/mL) had no significant effect on TNF production. Addition of TNF (500 U/ml) to monocyte cultures obtained prior to ExP resulted in a slight but insignificant increase in TNF production in 2 of 10 cases. However, when monocytes obtained prior to ExP were incubated with 8-methoxypsoralen (8-MOP, 100 ng/ml), exposed to ultraviolet light A (UVA, 2J/cm2), washed, and then incubated with TNF, a significant increase (p less than 0.01) in TNF production was observed in 8 of 10 cases, suggesting that the combination of 8-MOP and UVA may sensitize cells to TNF. Based on studies of endotoxin (LPS)-stimulated production of TNF by monocytes, levels of endotoxin in culture reagents or photopheresis equipment could not account for the increased production of TNF following treatment by ExP. Increased TNF production as a result of ExP may have important implications for treating both CTCL and systemic sclerosis because, in the case of CTCL, it could mediate numerous anti-tumor effects, whereas, in the case of systemic sclerosis, it could suppress collagen synthesis and induce collagenase production.

Comparison of the cytostatic and cytolytic activity of tumor necrosis factor-alpha and interleukin 1 alpha in human malignant cell lines

The cytokines tumor necrosis factor-alpha (TNF-alpha) and interleukin 1 alpha (IL-1 alpha) share many properties, including in-vitro cytotoxicity. Because cytotoxicity can result from either cytolytic or cytostatic activity, and because differentiating between these activities may have clinical relevance, we determined the cytostatic and cytolytic activity of TNF-alpha and IL-1 alpha for the human cell lines ME-180, SiHa (cervical carcinomas) and A375 (melanoma). Results of these analyses showed that IL-1 alpha mediated cytostatic activity only for A375 cells. IL-1 alpha was not cytolytic in the presence or absence of protein synthesis inhibitors. TNF-alpha was cytostatic for A375 and ME-180 cells, and although TNF-alpha was not cytolytic in the absence of protein synthesis inhibitors, it was cytolytic in the presence of protein synthesis inhibitors. These results suggest that the difference between the cytolytic and cytostatic activities of IL-1 alpha and TNF-alpha may have therapeutic implications for the use of these biological response modifiers in the treatment of gynecological malignancies.

Induction of hyporesponsiveness to an early post-binding effect of tumor necrosis factor by tumor necrosis factor itself and interleukin 1

Modulation of cellular responsiveness to tumor necrosis factor (TNF) was studied in the human SV-80 cells. A marked cytocidal effect is exhibited by these cells at about 4 to 8 h after application of TNF together with protein synthesis inhibitors. Sensitivity of the cells to TNF toxicity was shown to be markedly decreased following their pretreatment with TNF itself or with interleukin (IL) 1 in the absence of protein synthesis inhibitors. The SV-80 cells respond to TNF also with enhanced phosphorylation of the small heat-shock protein, HSP27. This TNF effect is much more rapid than the cytocidal effect; it is observed within minutes of TNF application. The response to this effect, just like the response to the cytocidal effect, is markedly decreased following preexposure of the cells to either TNF or IL 1. Responsiveness to both effects of TNF is regained at the same time, about 15 to 20 h following removal of TNF or IL 1. The decrease in responsiveness after pretreatment with TNF or IL 1 does not reflect an inability of the pretreated cells to bind TNF. Although there is an initial decrease in TNF binding after such pretreatment, it is fully reversed already about 5 h following removal of the cytokines. The rate of uptake of TNF by the pretreated cells is also normal. In view of the rapidity of the effect of TNF on the phosphorylation of HSP27, it seems likely that the observed hyporesponsiveness reflects impairment of an early step in a signaling pathway, perhaps common to both the stimulation of phosphorylation and the induction of cell death by TNF. By restricting the duration of the effects of TNF this desensitization mechanism may safeguard against harmful consequences of these effects.

Density-dependent tumor cell death and reversible cell cycle arrest: mutually exclusive modes of monocyte-mediated growth control

The population development of five human tumor cell lines is examined under the influence of elutriator-prepared human monocytes in a serum-free hormone- and growth factor-supplemented medium. Analysis was performed by electronic counting and sizing of tumor cell nuclei and flow cytometric detection of cell cycle phases. Tumor cell death is triggered at rather low monocyte:tumor cell ratios (1:2 to 1:4) whereas it is strongly reduced at high monocyte densities. Furthermore, it is shown that confluence of the target cell population is a necessary prerequisite for lysis. The data suggest that in monocyte/tumor cell cocultures the decision on target cell lysis is not made by the effector cell, but rather by the target cell and that the criterion for this decision is the target cell's ability or inability to respond to a monocyte challenge by arresting the cell cycle in G1. Interactions between target cells play an important role in determining the result of this decision process. A common basis is suggested for this kind of density-dependent monocyte-triggered lysis and density-dependent cell death in 3T3 cell cultures as described previously.

Expression of tumor necrosis factor-alpha and transforming growth factor-beta 1 in acute liver injury

Tumor necrosis factor-alpha (TNF-alpha) and transforming growth factor-beta 1 (TGF-beta 1) have a number of in vitro functions that could be important in vivo in acute liver injury and repair. Therefore, we investigated these two cytokines in acute liver damage. Northern blots of RNA isolated from rats sacrificed at various time intervals after a single oral dose of CCl4 revealed that TNF-alpha mRNA levels were elevated within 6 hr of CCl4 administration and returned to control values by 24-32 hr. In contrast, TGF-beta 1 mRNA levels started to rise significantly at 24 hr, peaked at 48 hr, and approached baseline levels by 72 hr. Identical changes in TNF-alpha and TGF-beta 1 mRNA levels were also seen with D-galactosamine-induced hepatotoxicity. Immunohistochemical analysis using a TGF-beta 1 antibody demonstrated increased hepatic staining in CCl4-treated rats, at times corresponding to the increases in TGF-beta 1 gene expression. Therefore, there is a differential expression of these cytokines in acute CCl4 and galactosamine hepatotoxicity with an early rise in TNF-alpha, suggesting that this cytokine may affect inflammation and cell toxicity, while TGF-beta 1 peaks later, when it may regulate hepatocyte proliferation and extracellular matrix repair.