Involvement of a serine protease in tumour-necrosis-factor-mediated cytotoxicity

SERPINB3, apoptosis and autoimmunity

SERPINB3 (Squamous Cell Carcinoma Antigen, SCCA1) is a member of the ov-serpins, a serine protease inhibitors family expressed in many cell types including normal epithelium, leukocytes, tumors of epithelial origin and primary liver cancer. Several studies, carried out in vitro and in vivo, have documented an important role of SERPINB3 in the modulation of programmed cell death by different mechanisms, both in inflammatory processes and in cancer. SERPINB3 significantly attenuates apoptosis by contrasting cytochrome c release from the mitochondria and by antichemotactic effect for NK cells. Mechanisms involved in apoptosis induction and regulation play a key role in the balance between cell proliferation and death. Imbalance of this equilibrium may contribute to the development of autoimmune diseases, as defective apoptosis of immune cells leads to deregulated autoreactive cell proliferation. Since defective programmed cell death represents a critical feature of autoimmunity, the involvement of SERPINB3 in this pathological field deserves further studies.

Necrosis, a well-orchestrated form of cell demise: signalling cascades, important mediators and concomitant immune response

Necrosis has long been described as a consequence of physico-chemical stress and thus accidental and uncontrolled. Recently, it is becoming clear that necrotic cell death is as well controlled and programmed as caspase-dependent apoptosis, and that it may be an important cell death mode that is both pathologically and physiologically relevant. Necrotic cell death is not the result of one well-described signalling cascade but is the consequence of extensive crosstalk between several biochemical and molecular events at different cellular levels. Recent data indicate that serine/threonine kinase RIP1, which contains a death domain, may act as a central initiator. Calcium and reactive oxygen species (ROS) are main players during the propagation and execution phases of necrotic cell death, directly or indirectly provoking damage to proteins, lipids and DNA, which culminates in disruption of organelle and cell integrity. Necrotically dying cells initiate pro-inflammatory signalling cascades by actively releasing inflammatory cytokines and by spilling their contents when they lyse. Unravelling the signalling cascades contributing to necrotic cell death will permit us to develop tools to specifically interfere with necrosis at certain levels of signalling. Necrosis occurs in both physiological and pathophysiological processes, and is capable of killing tumour cells that have developed strategies to evade apoptosis. Thus detailed knowledge of necrosis may be exploited in therapeutic strategies.

A potential role of macrophage activation in the treatment of cancer

One of the functions of macrophages is to provide a defense mechanism against tumor cells. In the last decades the mechanism of tumor cell killing by macrophages have been studied extensively. The tumor cytotoxic function of macrophages requires stimulation either with bacterial cell wall products such as lipopolysaccharide (LPS) or muramyldipeptide (MDP) or with cytokines such as interferon-gamma (IFN-gamma) and granulocyte-macrophage colony-stimulating factor (GM-CSF). Activated macrophages secrete several substances that are directly involved in tumor cell killing i.e. tumor necrosis factor (TNF) and nitric oxide (NO). On the other hand, substances are secreted that are able to stimulate tumor cell growth, depending on the stage and the nature of the tumor. Several clinical trials have been performed aiming at the activation of macrophages or dendritic cells, a subpopulation of the macrophages. In this review we will summarize and discuss experimental studies and clinical trials based on the activation of macrophages.

Death receptor-induced apoptotic and necrotic cell death: differential role of caspases and mitochondria

In L929sAhFas cells, tumor necrosis factor (TNF) leads to necrotic cell death, whereas agonistic anti-Fas antibodies elicit apoptotic cell death. Apoptosis, but not necrosis, is correlated with a rapid externalization of phosphatidylserine and the appearance of a hypoploid population. During necrosis no cytosolic and organelle-associated active caspase-3 and -7 fragments are detectable. The necrotic process does not involve proteolytic generation of truncated Bid; moreover, no mitochondrial release of cytochrome c is observed. Bcl-2 overexpression slows down the onset of necrotic cell death. In the case of apoptosis, active caspases are released to the culture supernatant, coinciding with the release of lactate dehydrogenase. Following necrosis, mainly unprocessed forms of caspases are released. Both TNF-induced necrosis and necrosis induced by anti-Fas in the presence of the caspase inhibitor benzyloxycarbonyl-Val-Ala-Asp(OMe)-fluoromethylketone are prevented by the serine protease inhibitor N-tosyl-L-phenylalanine chloromethylketone and the oxygen radical scavenger butylated hydroxyanisole, while Fas-induced apoptosis is not affected.

More than one way to die: apoptosis, necrosis and reactive oxygen damage

Cell death is an essential phenomenon in normal development and homeostasis, but also plays a crucial role in various pathologies. Our understanding of the molecular mechanisms involved has increased exponentially, although it is still far from complete. The morphological features of a cell dying either by apoptosis or by necrosis are remarkably conserved for quite different cell types derived from lower or higher organisms. At the molecular level, several gene products play a similar, crucial role in a major cell death pathway in a worm and in man. However, one should not oversimplify. It is now evident that there are multiple pathways leading to cell death, and some cells may have the required components for one pathway, but not for another, or contain endogenous inhibitors which preclude a particular pathway. Furthermore, different pathways can co-exist in the same cell and are switched on by specific stimuli. Apoptotic cell death, reported to be non-inflammatory, and necrotic cell death, which may be inflammatory, are two extremes, while the real situation is usually more complex. We here review the distinguishing features of the various cell death pathways: caspases (cysteine proteases cleaving after particular aspartate residues), mitochondria and/or reactive oxygen species are often, but not always, key components. As these various caspase-dependent and caspase-independent cell death pathways are becoming better characterized, we may learn to differentiate them, fill in the many gaps in our understanding, and perhaps exploit the knowledge acquired for clinical benefit.

12-O-tetradecanoylphorbol-13-acetate-induced apoptosis is mediated by tumor necrosis factor alpha in human monocytic U937 cells

12-O-tetradecanoylphorbol-13-acetate (TPA), a phorbol ester that is known as a tumor promoter, induces differentiation of myeloid cells and suppresses their proliferation. We studied the regulation of apoptosis by TPA in human monocytic cell line U937 cells that lack p53. Untreated U937 cells constitutively underwent apoptosis, and TPA enhanced apoptosis in these cells. Further studies showed that TPA increased production of tumor necrosis factor-alpha (TNFalpha) in U937 cells, and exogenously added TNFalpha induced apoptosis. Moreover, the induction of apoptosis by TPA was blocked by anti-TNFalpha antibody. Similar results were obtained in the myeloblastic cell line KY821 cells. We also found that the induction of apoptosis by TPA was increased in cells overexpressed with TNF receptor 1 but not in control cells. Furthermore, TPA failed to induce the production of TNFalpha and apoptosis in cells with either their protein kinase C or mitogen-activated protein kinase pathway blocked. Our results indicate that TPA induces apoptosis, at least in part, through a pathway that requires endogenous production of TNFalpha in U937 cells. Our data also suggest that the induction of apoptosis by TPA occurs through activation of protein kinase C and mitogen-activated protein kinase and TNFalpha is an autocrine-stimulating factor for the induction of apoptosis in these cells.

Effect of cytokines on liver necrosis

AIM: To investigate the effect of cytokines on the liver necrosis.

METHODS: rIL (interleukin)-1, rIL-6, rIFN (interferon)γ, rTNF (tumor necrosis factor)α with or without D-galactosamine (D-GAL) were injected into the abdominal cavity of mice separately. ALT, TBIL (total bilirubin) and histological changes were observed.

RESULTS: There was no effect on hepatocyte of normal mice after injection of rIL-1, rIL-6, rIFN alone or together. The serum total bilirubin (TBIL) and liver necrosis of mice increased after rTNFα, rIL-6 or rIFNγ were used separately with D-GAL. The TBIL level (μmol/L)was 46.19 ± 10.62, 44.55 ± 12.9 and 41.94 ± 14.9, higher than that caused by D-GAL alone (TBIL, 26.67 μmol/L ± 11.14 μmol/L). The serum TBIL of mice and the degree of liver necrosis increased after injection of IL-1, IL-6 with D-GAL and rTNFα.

CONCLUSION: Cytokines, like IL-1, IL-6, IFNγ and TNFα joined in the process of hepatocyte necrosis. They can enhance the degree of liver necrosis induced by D-GAL.

Cytotoxic effect of cytokines on murine colon carcinoma cells involves TNF-mediated apoptosis

We have studied the cytotoxic effect of TNF-alpha on C-26 murine colon carcinoma cells in vitro. Treatment with TNF-alpha alone did not result in any demonstrable cytotoxicity. However, when combined with IFN-gamma, the cytotoxic effect of TNF-alpha was enhanced in a dose-dependent manner. An agonistic TNF-R1 specific antibody and recombinant human TNF-alpha both exerted a cytotoxic effect when combined with IFN-gamma, suggesting that the cytotoxicity was mediated through the TNF-R1. The cytotoxicity was associated with production of nitric oxide without any direct involvement in the cytotoxic effect. At the ultrastructural level, treated cells displayed a typical apoptotic morphology which was not accompanied by internucleosomal cleavage of DNA as shown by conventional electrophoresis.

Activation of CPP32-like protease in tumor necrosis factor-induced apoptosis is dependent on mitochondrial function

Mitochondria have been implicated in apoptosis, however, the precise mechanisms whereby mitochondria exert their effect are not clear. To gain further insights, we generated a panel of cells from ML-1a cells that were rendered respiration deficient by ethidium bromide treatment. Two respiration-deficient clones were subsequently reconstituted by fusion with platelets. Respiration-deficient clones were resistant to TNF-induced apoptosis, whereas ML-1a and reconstituted clones were sensitive. In contrast, inhibition of proliferation and induction of differentiation by TNF were still observed in respiration deficient clones, suggesting a selective requirement of respiration in TNF-induced apoptosis. Furthermore the apoptosis machinery is not completely altered in respiration-deficient cells because they underwent apoptosis after staurosporine treatment. Next, we showed that apoptosis induced by TNF and staurosporine were blocked by z-DEVD-CH2F, an inhibitor of CPP32-like cysteine protease, suggesting the involvement of CPP32-like protease in both apoptosis signaling pathways. Interestingly, TNF activated CPP32-like protease in the parental and reconstituted clones but not in respiration-deficient clones, and staurosporine in all clones. Thus, the apoptosis signaling block in respiration-deficient clones is located at a step before CPP32-like protease activation, which can be bypassed by staurosporine.

Cloning of a disintegrin metalloproteinase that processes precursor tumour-necrosis factor-alpha

Tumour-necrosis factor-alpha (TNF-alpha) is a cytokine that contributes to a variety of inflammatory disease states. The protein exists as a membrane-bound precursor of relative molecular mass 26K which can be processed by a TNF-alpha-converting enzyme (TACE), to generate secreted 17K mature TNF-alpha. We have purified TACE and cloned its complementary DNA. TACE is a membrane-bound disintegrin metalloproteinase. Structural comparisons with other disintegrin-containing enzymes indicate that TACE is unique, with noteable sequence identity to MADM, an enzyme implicated in myelin degradation, and to KUZ, a Drosophila homologue of MADM important for neuronal development. The expression of recombinant TACE (rTACE) results in the production of functional enzyme that correctly processes precursor TNF-alpha to the mature form. The rTACE provides a readily available source of enzyme to help in the search for new anti-inflammatory agents that target the final processing stage of TNF-alpha production.

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.

Guinea pig Kurloff (NK-like) cells mediate TNF-dependent cytotoxic activity: analogy with NC effector cells

Kurloff cells are mononuclear cells possessing a large cytoplasmic inclusion body specific to the guinea pig. In this report, we present strong evidence that Kurloff cells can mediate NC activity against tumor cells in addition to their previously reported NK activity. Using an 18 h 51Cr-release assay we have shown that Kurloff cells were highly effective in killing the TNF-sensitive WEHI 164 target cell line. Lower but significant cytotoxic activity was also observed after only 4 h. However, our results suggest a different mechanism of lysis in the 4 h and 18 h assay. Lysis of WEHI 164 target cells by Kurloff cells in the 4 h assay could be strongly increased in the presence of TPA alone or in combination with ionomycin whereas ionomycin alone was uneffective. In contrast, stimulation of Kurloff cells for 18 h with ionomycin alone or in combination with TPA could induce the release of TNF-like factor(s) as observed by the TNF bioassay using L-929 TNF-sensitive target cells. Release of TNF-like factor(s) could also be induced by stimulation with WEHI 164 target cells. Supernatants of Kurloff cells stimulated for 18 h with TPA + ionomycin were also highly cytotoxic against WEHI 164 target cells, but not against the TNF-resistant P815 target cell line. Pretreatment of these supernatants with antimurine TNF alpha antibodies could almost completely inhibit their cytotoxic activity against WEHI 164 target cells. In contrast, supernatants of Kurloff cells stimulated for only 4 h did not show any TNF-like activity against the L-929 target cell line and were not cytotoxic against WEHI 164 target cells even after 18 h. Taken together, these results suggest that Kurloff cells can mediate NC activity against tumor cells in addition to their previously reported NK activity. By using multiple lytic pathways, these cells may play a crucial role in anti-tumor surveillance and defenses.

Degradation of nuclear matrix and DNA cleavage in apoptotic thymocytes

In dexamethasone-treated thymocyte cultures an increase in nuclear proteolytic activity paralleled chromatin fragmentation and the appearance of small apoptotic cells. The elevation of nuclear proteolytic activity was accompanied by site-specific degradation of nuclear mitotic apparatus protein and lamin B, two essential components of the nuclear matrix. Nuclear mitotic apparatus protein phosphorylation and cleavage into 200 and 48 kDa fragments occurred within 30 minutes of dexamethasone treatment. Cleavage of lamin B, which generated a fragment of 46 kDa consistent with the central rod domain of the protein, was also detected after 30 minutes of exposure to the steroid hormone. The level of lamin B phosphorylation did not change as a result of the dexamethasone treatment and the lamina did not solubilize until the later stages of apoptosis. Initial DNA breaks, detected by the terminal transferase-mediated dUTP-biotin nick end labeling assay, occurred throughout the nuclei and solubilization of lamina was not required for this process to commence. The data presented in this paper support a model of apoptotic nuclear destruction brought about by the site-specific proteolysis of key structural proteins. Both the nuclear mitotic apparatus protein and lamin B were specifically targeted by protease(s) at early stages of the cell death pathway, which possibly initiate the cascade of degradative events in apoptosis.

Role of multiple cellular proteases in the execution of programmed cell death

A family of mammalian homologues of the Caenorhabditis elegans cell death protein Ced-3 has been recently discovered. These mammalian proteins encode novel cysteine proteases with homology to the interleukin-1 beta converting enzyme (ICE). Although several studies support a role for one or more of these proteases in mediating apoptosis, their mechanism of action is far from understood. The presence of multiple mammalian ICE-like proteases, with apparently similar apoptotic function indicates that, despite its conservation during evolution, the cell death pathway is much more complex in mammals than in the worm. In addition to ICE-like proteases, several other proteases of different cleavage specificities have been implicated in apoptosis. There is now a growing body of evidence suggesting that apoptosis involves the activation of a cascade of proteases. This article summarises the presently available evidence and discusses how multiple proteases might be required in the effector phase of cell death.

Plasminogen activator inhibitor type 2 inhibits tumor necrosis factor alpha-induced apoptosis. Evidence for an alternate biological function

Plasminogen activator inhibitor type 2 (PAI-2) is a serine proteinase inhibitor or serpin that is a major product of macrophages in response to endotoxin and inflammatory cytokines. We have explored the role of PAI-2 in apoptotic cell death initiated by tumor necrosis factor alpha (TNF). HeLa cells stably transfected with PAI-2 cDNA were protected from TNF-induced apoptosis, whereas cells transfected with antisense PAI-2 cDNA, a control gene, or the plasmid vector alone remained susceptible. The level of PAI-2 expressed by different HeLa cell clones was inversely correlated with their sensitivity to TNF. Loss of TNF sensitivity was not a result of loss of TNF receptor binding. In contrast, PAI-2 expression did not confer protection against apoptosis induced by ultraviolet or ionizing radiation. The serine proteinase urokinase-type plasminogen activator was not demonstrated to be the target of PAI-2 action. The P1-Arg amino acid residue of PAI-2 was determined to be required for protection, because cells expressing PAI-2 with an Ala in this position were not protected from TNF-mediated cell death. The results suggest that intracellular PAI-2 might be an important factor in regulating cell death in TNF-mediated inflammatory processes through inhibition of a proteinase involved in TNF-induced apoptosis.

Activation of alpha-human tumour necrosis factor (TNF-alpha) by human monocytes (THP-1) exposed to 2-chloroethyl ethyl sulphide (H-MG)

Tumour necrosis factor (TNF) is a monokine produced by monocytes and macrophages in response to different stimuli. To determine whether vesicant agents such as half-mustard gas (H-MG; chemical structure: ClCH2CH2SCH2CH3) may induce the release of TNF-alpha in human monocytes (THP-1), ELISA experiments were conducted at different post exposure times. The results indicate that: (1) Significant increases in the TNF-alpha (pg mL-1) concentration were observed as a function of time when THP-1 cells were exposed to 100 microL of 2 M H-MG. A specific serine-type protease inhibitor, N alpha-p-tosyl-L-lysine chloromethyl ketone (TLCK), led to partial but significant inhibition of TNF activation. (2) Furthermore, this laboratory detected the generation of spin adducts of 2-methyl-2-nitrosopropane (MNP) having a resemblance to MNP-adducts generated from hydrogen atom abstraction of protein constituents. The EPR/Spin Trapping data indicate the trapping of by-products of protein degradation after exposure to H-MG. TNF-alpha may play a role as a biochemical marker for pathophysiological changes induced by H-MG or related agents.

Fas- and tumor necrosis factor-induced apoptosis is inhibited by the poxvirus crmA gene product

crmA is a cowpox virus gene that encodes a protease inhibitor of the serpin family. The only reported target for the CrmA protein is the cysteine protease interleukin-1 beta converting enzyme (ICE). ICE, by virtue of its homology to the Caenorhabditis elegans cell death protein Ced-3, has been suggested to play a fundamentally important role in mammalian apoptosis. We hypothesized that a function of crmA may be to inhibit cell death, since a major mechanism of viral clearance is the immune system-mediated induction of apoptosis of infected cells. The tumor necrosis factor receptor and the Fas antigen are two cytokine receptors which, by engaging and activating the death pathway, can eliminate virus-infected cells. Remarkably, crmA was found to be an exceptionally potent inhibitor of apoptosis induced by both these receptors, capable of blocking the cell death program even at pharmacological doses of the death stimulus. Therefore, an important new function for crmA is the inhibition of cytokine-induced apoptosis. Further, the data suggest that a protease, either ICE or a related crmA-inhibitable protein, is a component of the Fas- and tumor necrosis factor-induced cell death pathways.

A pre-existing protease is a common effector of thymocyte apoptosis mediated by diverse stimuli

Data from a number of model systems support a role for proteolysis in apoptotic cell death. Using immature rat thymocytes, we demonstrate that the inhibitors N-tosyl-L-lysyl chloromethylketone (TLCK) and N-tosyl-L-phenylalanyl chloromethylketone (TPCK) have very different effects on apoptosis. TLCK inhibits apoptosis induced by diverse stimuli at an early stage prior to both DNA fragmentation and cytoplasmic changes. We show that the TLCK-sensitive target is pre-existing and not synthesized in response to apoptotic stimuli. The contrasting effects of TLCK and TPCK support the hypothesis that the TLCK target is a trypsin-like protease which is a common effector of thymocyte apoptosis.

Formation of 50 kbp chromatin fragments in isolated liver nuclei is mediated by protease and endonuclease activation

Isolated rat liver nuclei were incubated in the presence of divalent cations, and the mechanisms underlying the subsequent chromatin fragmentation were investigated. Either of the two cations, Ca2+ or Mg2+ was sufficient to produce chromatin fragments with sizes between 700 and 300 kbp. The formation of chromatin fragments of 50 kbp as well as the following internucleosomal DNA cleavage--which are characteristic of apoptosis--were markedly stimulated in the presence of Ca2+. Chromatin degradation to 50 kbp and smaller (oligonucleosome-size) fragments was prevented by inhibitors of endonucleases and serine proteases. We suggest a mechanism whereby the concerted activity of both proteases and endonucleases results in the widespread chromatin cleavage observed in cells undergoing apoptosis.

Molecular mechanisms of tumor necrosis factor-induced cytotoxicity. What we do understand and what we do not

Although TNF plays an important role in several physiological and pathological conditions, the hallmark of this important cytokine has been its selective cytotoxic activity on tumor cells. Since its cloning in 1984, understanding of how TNF selectively kills tumor cells has been the subject of research in many laboratories. Here we review TNF-induced post-receptor signaling mechanisms which seem to be involved in the pathway to cytotoxicity.

Protease inhibitors selectively block T cell receptor-triggered programmed cell death in a murine T cell hybridoma and activated peripheral T cells

The hypothesis that cytoplasmic proteases play a functional role in programmed cell death was tested by examining the effect of protease inhibitors on the T cell receptor-mediated death of the 2B4 murine T cell hybridoma and activated T cells. The cysteine protease inhibitors trans-epoxysuccininyl-L-leucylamido-(4-guanidino) butane (E-64) and leupeptin, the calpain selective inhibitor acetyl-leucyl-leucyl-normethional, and the serine protease inhibitors diisopropyl fluorophosphate and phenylmethylsulfonyl fluoride, all showed dose-dependent blocking of the 2B4 death response triggered by the T cell receptor complex and by anti-Thy-1. These protease inhibitors enhanced rather than inhibited IL-2 secretion triggered by T cell receptor cross-linking, showing that they did not act by preventing signal transduction. Growth inhibition induced by cross-linking the 2B4 T cell receptor, measured by inhibition of thymidine incorporation, was not generally blocked by these protease inhibitors. All five of these protease inhibitors enhanced rather than blocked 2B4 cell death triggered by dexamethasone, an agent previously shown to have a death pathway antagonistic with that of the TCR. 2B4 cytolysis by the cytotoxic agents staphylococcal alpha-toxin and dodecyl imidazole, and that caused by hypotonic conditions, was not significantly affected by the five protease inhibitors tested. The selected protease inhibitors blocked both the apoptotic nuclear morphology changes and DNA fragmentation induced by T cell receptor cross-linking, and enhanced both these properties induced by dexamethasone in 2B4 cells. The T cell receptor-induced death of activated murine lymph node T cells and human peripheral blood CD4+ T cells was blocked by both cysteine and serine protease inhibitors, showing that the protease-dependent death pathway also operates in these systems.

Caffeine potentiates the lethality of tumour necrosis factor in cancer cells

In this study we have investigated the interaction of caffeine, a prototypic methylxanthine, and TNF on the induction of cell death in mouse and human cell lines during progression from G1 to successive phases of the cell cycle. Exposure of cells to TNF (0.1-100 ng ml-1) as single agent for 48 h caused low or no lethality. The rates of cell death increased significantly when cells cultured with TNF for 24 h were exposed to caffeine (2.5-20 mM). The magnitude of the enhancement by caffeine was TNF and caffeine dose-dependent. The most effective response to this combination was observed in the mouse cell lines, WEHI and L929, followed by the human cell lines, HeLa, A375 and MCF-7, respectively. In L929 cells, TNF treatment did not inhibit DNA synthesis during the first S phase of the cell cycle (20-24 h), but it did block the progress toward a second S phase, indicating the cells were arrested at G2 phase or mitosis. Caffeine had great enhancer effect on L929 cells exposed to TNF for 24 h, but the effect was reduced in cells with either less than 24 h or greater than 28 h of exposure. L929 cells stimulated with TNF died via apoptosis, as judged by both morphological criteria and the occurrence of internucleosomal DNA cleavage. Exposure of TNF-treated cells to caffeine caused a greater increase in the proportion of apoptotic cells as well as the extent of internucleosomal DNA fragmentation.

Tumor necrosis factor cytotoxicity is associated with phospholipase D activation

The activation of phospholipase D in different cell lines treated with recombinant human tumor necrosis factor (TNF) has been investigated. When the murine fibrosarcoma cell lines L929 and WEHI164c113, as well as the human promonocytic cell line U937, were prelabeled with [14C]palmitic acid or [3H]arachidonic acid, and treated with TNF in the presence of ethanol, TNF induced the synthesis of [14C]phosphatidylethanol or [3H]phosphatidylethanol, respectively, as the result of a phospholipase-D-catalyzed transphosphatidylation. TNF-induced phospholipase D activity was observed 1 h before the onset of cell killing and gradually increased thereafter. Subclones selected for resistance to TNF cytotoxicity did not show phospholipase D activation in response to TNF. In contrast, when these subclones were treated with TNF in the presence of actinomycin D, TNF cytotoxicity as well as TNF-induced phospholipase D activity could be restored. TNF cytotoxicity and TNF-induced phospholipase D activity were equally modulated by various drugs known to interfere with different steps in the TNF-signaling pathway. Phospholipase D activation was found not to be the result of cell killing per se, as a number of other cytotoxic reagents were unable to activate phospholipase D. Prelabeling of cells with [14C]lysophosphatidylcholine indicated phosphatidylcholine as one of the substrates for TNF-activated phospholipase D. In conclusion, this study demonstrates that TNF-induced cytotoxicity is associated with activation of phospholipase D.

Tumor necrosis factor activities and cancer therapy--a perspective

Tumor necrosis factor (TNF) is a multifunctional cytokine which has excited and fascinated numerous investigators and commercial entities due to its promise as a therapeutic agent against cancer and as a target for drugs treating septic shock. TNF is a protein having cytotoxic, cytostatic, immunomodulatory as well as several other activities and is also involved in septic shock. This review covers the structure of TNF and its receptors, various in vitro activities and in vivo activities based on studies in animal model systems. The role of TNF as an anticancer therapeutic agent, based on various phase I and phase II clinical studies, has also been considered. The review concludes with several considerations for increasing the therapeutic utility of TNF in terms of targeting, toxicity and half-life.

Induction of HLA-DR by interferon-gamma requires a trypsin-like protease

Induction of HLA-DR antigen expression by interferon-gamma (IFN-gamma) is inhibited by trypsin inhibitors and an anti-trypsin monoclonal antibody, but not by chymotrypsin inhibitors, suggesting a requirement for trypsin-like protease (TLP) activity in IFN-gamma-induced HLA-DR expression. Using p-nitroanilide and thioester substrates, TLP activity was demonstrated in cellular extracts of a hybrid epidermal cell line and judged to be essential for HLA-DR expression. TLP activity was inhibited by the trypsin inhibitors soybean trypsin inhibitor, ovomucoid trypsin inhibitor, and tosyl-lysyl-chloromethyl ketone and by an anti-trypsin monoclonal antibody, closely paralleling inhibition of HLA-DR expression by such agents. TLP activity was enhanced by exposure to trypsin-linked agarose, indicating that the protease normally exists in an inactive form, perhaps in an enzyme-inhibitor complex or as an activatable proenzyme. Finding glucocorticoids (GC) to also inhibit IFN-gamma-induced HLA-DR expression and to regulate serine protease, especially urokinase plasminogen activator (uPA), activity raised the possibility of GC regulation of TLP activity. However, TLP activity was found to be constitutively expressed, regulated by neither GC nor IFN-gamma, nor was uPA activity involved in HLA-DR regulation. Trypsin inhibitors and GC also inhibited induction of intracellular 2',5'-oligoadenylate (2-5A) synthetase by IFN-gamma. Thus, TLP activity is required for IFN-gamma induction of HLA-DR and 2-5A synthetase.

Significance of tumor necrosis factor (TNF) and interleukin-1 (IL-1) in the pathogenesis of fulminant hepatitis: possible involvement of serine protease in TNF-mediated liver injury

The effect of tumor necrosis factor (TNF)-alpha and interleukin-1 (IL-1) alpha, which are thought to be principal mediators inducing homeostatic abnormalities during endotoxemia, were investigated on cultured hepatocytes in an attempt to understand their role in the pathogenesis of fulminant hepatitis. Both TNF and IL-1 had no direct cytotoxicity on cultured adult rat hepatocytes as assessed by their effects on protein synthesis and also cytosolic enzyme activity released into the culture medium in the presence of 5 mM D-galactosamine (Ga1N). However, IL-1 caused a dose-dependent inhibition of DNA synthesis in cultured adult rat hepatocytes. Moreover, the serum from TNF-treated rats, prepared after intravenous administration of TNF (5 X 10(4) U per rat), caused a significant increase of enzyme release into culture medium in contrast to control rat serum. The cytotoxicity disappeared when the serum from TNF-treated rats was pretreated by heating at 56 degrees C for 30 min, and was decreased by the addition of the protease inhibitor, aprotinin. In vivo, gabexate mesilate, a serine-type protease inhibitor, prevented GalN/TNF-induced fulminant hepatitis, whereas MX-1, an anti-complement agent, had no such effect. These results strongly suggest that IL-1 has a inhibitory effect on hepatocytes in terms of DNA synthesis and that TNF indirectly induces hepatocellular damage through the serine proteases which are possibly activated by the cytokine in vivo.

Tumor necrosis factor. Characterization at the molecular, cellular and in vivo level

TNF was originally characterized as an antitumor agent and a factor cytotoxic for many malignant cells. It is now clear that it plays an important role in the defense against viral, bacterial and parasitic infections, - and in (auto-)immune responses. Natural induction of TNF is protective, but its overproduction may be detrimental and even lethal to the host. The structure of TNF and its interaction with the two types of cellular receptor are becoming better understood. TNF elicits a variety of events in different cell types. It subverts the electron transport system or the mitochondria into production of oxygen radicals, which can kill the (malignant) cells when these do not contain or produce protective enzymes. Furthermore, TNF induces a set of genes and at least part of this transcriptional activation is mediated by NF kappa B. The prospects of TNF as an antitumor drug can be improved on the one hand by agents such as LI+, which synergizes, and on the other hand by inhibitors of the systemic toxicity which do not interfere with the antitumor efficacy. Also, in tumor-bearing animals which have been rendered tolerant by administration of small doses of TNF, an effective and complete elimination of the tumors can be obtained by the combined action of TNF plus interferon.

Cell death mechanisms and the immune system

The immune system provides good models for cell death, a phenomenon now recognized to be of fundamental importance in many fields of biology. Cell death is strikingly polymorphic: it can proceed via necrosis (as in complement-mediated cell death) or apoptosis, but the latter displays different patterns (in the receptor-mediated death of some thymocytes, in cell death mediated by TNF alpha or by cytotoxic T cells), perhaps reflecting different pathways of control of a common core mechanism. Even though there are differences in the morphological and metabolic changes associated with the different patterns of apoptosis, some recurrent sequences of events are observed in almost all dying cells. The metabolic state of a cell often seems to play a major role in determining if and how this cell will die in given external circumstances. The nature of molecules causally involved in the dying cell can now be approached in some systems.

Tumour-necrosis-factor-mediated cytotoxicity is correlated with phospholipase-A2 activity, but not with arachidonic acid release per se

L929, a murine fibrosarcoma cell line highly sensitive to the anti-proliferative and cytotoxic action of tumour necrosis factor (TNF), was used as a target cell in our studies. We [Suffys et al. (1987) Biochem. Biophys. Res. Commun. 149, 735-743], as well as others, have previously provided evidence that a phospholipase (PL), most probably a PL-A2-type enzyme, is likely to be involved in TNF-mediated cell killing. We now further document this conclusion and provide suggestive evidence that the enzyme activity specifically involved in TNF cytotoxicity differs from activities associated with the eventual cell death process itself or with non-toxic serum treatment. We also show that the 5,8,11,14-icosatetraenoic acid (arachidonic acid, delta 4 Ach) released by PL, and possibly metabolized, is unlikely to be a key mediator of the TNF-mediated cytotoxicity. These conclusions are based on the following experimental findings. 1. TNF treatment of cells, prelabelled for 24 h with [3H] delta 4Ach or [14C] delta 3Ach (delta 3Ach identical to 5,8,11-icosatrienoic acid) resulted in an early, time-dependent and concentration-dependent release of radioactivity in the supernatant preceding actual cell death. The extent of this response was moderate, albeit reproducible and significant. Analysis of the total lipid fraction from cells plus supernatant revealed that only release of arachidonic acid from phospholipids, but not its metabolization was induced by TNF. However, the release of less unsaturated fatty acids, such as linoleic acid (Lin) or palmitic acid (Pam), was not affected during the first hours after TNF addition. 2. An L929 subclone, selected for resistance to TNF toxicity, was found to be defective in TNF-induced delta 4Ach libration. 3. Interleukin-1 (IL1) was not cytotoxic for L929 and did not induce release of delta 4Ach. 4. Release of delta 4Ach was not restricted to TNF; the addition of serum to the cells also induced release of fatty acids into the medium. In this case, however, there was no specificity, as all fatty acids tested, including Lin and Pam, were released. 5. Inhibition of PL-A2 activity by appropriate drugs markedly diminished TNF-induced delta 4Ach release and resulted also in a strong decrease in TNF-induced cytotoxicity. 6. Other drugs, including serine protease inhibitors, which strongly inhibit TNF-induced cytotoxicity, also decreased the TNF-induced delta 4Ach release, whereas LiCl potentiated both TNF-mediated effects. 7. Protection of cells against TNF toxicity by means of various inhibitors was not counteracted by addition of exogenous fatty acids, including delta 4Ach.(ABSTRACT TRUNCATED AT 400 WORDS)

Agents which modify colonial morphology of tumor cells also affect acid vesicle function and fibronectin deposition in the extracellular matrix

The tumor cell line U937A is motile, weakly plastic-adherent and forms large, loosely packed colonies in vitro and is invasive and metastatic in vivo. U937A/R, a mutant selected for resistance to killing by tumor necrosis factor (TNF), is less motile, more adherent and forms small, tightly packed colonies and is not invasive or metastatic. U937A and U937A/R also have differing cytoplasmic distributions of acid vesicles, and unlike U937A, U937A/R fails to deposit fibronectin into its extracellular matrix. In this study we have sought reagents that could convert "loose" U937A cells into the nonmetastatic, "tight" colonial phenotype. Six effective reagents were found: wheat germ agglutinin, phytohemagglutinin-L, dexamethasone, chloroquine, promethazine, and monensin. All 6 reagents caused swelling and/or redistribution of acid vesicles but phytohemagglutinin-L, dexamethasone, and monensin also reduced fibronectin deposition in the extracellular matrix. Therefore, these agents probably reduce motility by interference with recycling of cell surface receptors through acid vesicles and also in some cases by altering the extracellular matrix.

Effects of human and ovine pancreatic secretory trypsin inhibitors on the proliferation of normal human fibroblasts

Human pancreatic secretory trypsin inhibitor inhibited cell-surface proteolytic activity in human fibroblasts. In the range of concentrations which caused proteinase inhibition, fibroblast proliferation was also inhibited by this reagent and by the ovine equivalent. At lower concentrations, there was some evidence for a mitogenic effect, and this was confirmed by obvious stimulation of DNA synthesis at these concentrations. Human alpha 1-proteinase inhibitor, previously demonstrated to be an inhibitor of fibroblast proliferation, was also mitogenic at concentrations lower than those which inhibited proteolytic activity and cell proliferation. Human pancreatic secretory trypsin inhibitor and epidermal growth factor apparently work through independent mechanisms, since their mitogenic effects are additive.

Lithium chloride potentiates tumor necrosis factor-mediated cytotoxicity in vitro and in vivo

Tumor necrosis factor (TNF) is cytotoxic for several transformed cell lines in vitro. In the presence of LiCl, the murine fibrosarcoma cell lines L929 and WEHI 164 clone 13 became greater than 10 times more sensitive to TNF-mediated cytotoxicity. The human tumor cell lines BT20 and HeLa D98/AH2 were also responsive to the cytotoxicity-enhancing effect of LiCl. Other monovalent or divalent cations did not affect TNF-mediated cytotoxicity. The potentiating effect of LiCl on TNF cytotoxicity was largely independent of transcription, and LiCl could be added to the cells as early as 2 hr before or as late as 4 hr after TNF without loss of effectiveness. The mechanism by which LiCl increases the cytotoxic response seems to differ from the sensitizing effect of actinomycin D or interferon gamma, since the latter treatments overcame TNF resistance of several cell lines, whereas LiCl did not. Evidence is presented that LiCl acts, either directly or indirectly, via the TNF-activated phospholipase A2 pathway. In nude mice, a combination of TNF and LiCl led to hemorrhagic necrosis and growth inhibition of L929 tumors, whereas little effect was observed when TNF was administered alone. HeLa D98/AH2 tumors also were sensitive to the potentiating effect of LiCl in vivo. We conclude that LiCl enhances the effectiveness of TNF in vitro and in vivo, results that may have therapeutic implications.