Heat stress downregulates FLIP and sensitizes cells to Fas receptor-mediated apoptosis.

The heat shock response and death receptor-mediated apoptosis are both key physiological determinants of cell survival. We found that exposure to a mild heat stress rapidly sensitized Jurkat and HeLa cells to Fas-mediated apoptosis. We further demonstrate that Hsp70 and the mitogen-activated protein kinases, critical molecules involved in both stress-associated and apoptotic responses, are not responsible for the sensitization. Instead, heat stress on its own induced downregulation of FLIP and promoted caspase-8 cleavage without triggering cell death, which might be the cause of the observed sensitization. Since caspase-9 and -3 were not cleaved after heat shock, caspase-8 seemed to be the initial caspase activated in the process. These findings could help understanding the regulation of death receptor signaling during stress, fever, or inflammation.

MAPK/ERK overrides the apoptotic signaling from Fas, TNF, and TRAIL receptors.

The tumor necrosis factor (TNF), Fas, and TNF-related apoptosis-inducing ligand (TRAIL) receptors (R) are highly specific physiological mediators of apoptotic signaling. We observed earlier that a number of FasR-insensitive cell lines could redirect the proapoptotic signal to an anti-apoptotic ERK1/2 signal resulting in inhibition of caspase activation. Here we determine that similar mechanisms are operational in regulating the apoptotic signaling of other death receptors. Activation of the FasR, TNF-R1, and TRAIL-R, respectively, rapidly induced subsequent ERK1/2 activation, an event independent from caspase activity. Whereas inhibition of the death receptor-mediated ERK1/2 activation was sufficient to sensitize the cells to apoptotic signaling from FasR and TRAIL-R, cells were still protected from apoptotic TNF-R1 signaling. The latter seemed to be due to the strong activation of the anti-apoptotic factor NF-kappaB, which remained inactive in FasR or TRAIL-R signaling. However, when the cells were sensitized with cycloheximide, which is sufficient to sensitize the cells also to apoptosis by TNF-R1 stimulation, we noticed that adenovirus-mediated expression of constitutively active MKK1 could rescue the cells from apoptosis induced by the respective receptors by preventing caspase-8 activation. Taken together, our results show that ERK1/2 has a dominant protecting effect over apoptotic signaling from the death receptors. This protection, which is independent of newly synthesized proteins, acts in all cases by suppressing activation of the caspase effector machinery.

MAPK/ERK signaling in activated T cells inhibits CD95/Fas-mediated apoptosis downstream of DISC assembly.

When T cells are activated, the expression of the CD95 ligand is elevated, with the purpose of inducing apoptosis in target cells and to later eliminate the activated T cells. We have shown previously that mitogen-activated protein kinase (MAPK or ERK) signaling suppresses CD95-mediated apoptosis in different cellular systems. In this study we examined whether MAPK signaling controls the persistence and CD95-mediated termination of an immune response in activated T cells. Our results show that activation of Jurkat T cells through the T cell receptor immediately suppresses CD95-mediated apoptosis, and that this suppression is mediated by MAPK activation. During the phase of elevated MAPK activity, the activation of caspase-8 and Bid is inhibited, whereas the assembly of a functional death-inducing signaling complex (DISC) is not affected. These results explain the resistance to CD95 responses observed during the early phase of T cell activation and suggest that MAPK-activation deflects DISC signaling from activating caspase-8 and Bid. The physiological relevance of the results was confirmed in activated primary peripheral T cells, in which inhibition of MAPK signaling markedly sensitized the cells to CD95-mediated apoptosis.

Effector caspases are dispensable for the early nuclear morphological changes during chemical-induced apoptosis.

Nuclear morphological changes during apoptosis are very distinct and effector caspases have been implicated to play a central role in these processes. To investigate this in greater detail we examined the effect of blocking caspase activity and its activation on the nuclear morphological change in Jurkat T cells undergoing apoptosis after staurosporine treatment. In the presence of caspase inhibitors, like benzyloxycarbonyl-Val-Ala-Asp fluoro-methylketone (z-VAD-FMK), N-acetyl Tyr-Val-Ala-Asp chloromethylketone (Ac-YVAD-CMK) and benzyloxy-carbonyl-Asp-Glu-Val-Asp (OMe) fluoromethylketone (z-DEVD-FMK), staurosporine-treated Jurkat cells displayed a nuclear morphological change distinct from that of normal and apoptotic cells. This nuclear morphological change is an early event, characterised by convoluted nuclei with cavitations, and clumps of chromatin abutting to inner regions of the nuclear envelope between the nuclear pores. Both the nuclear envelope and endoplasmic reticulum were grossly dilated. This pre-apoptotic nuclear change precedes the externalisation of phosphatidylserine, chromatin condensation and DNA laddering, and can be dissociated from the formation of high molecular weight DNA fragments and cell shrinkage. Although cytochrome c efflux from the mitochondria and the processing of caspase-3 were observed in Jurkat cells with pre-apoptotic nuclear morphology, caspase-2, -6, -7 and -8 were not activated. In the presence of z-DEVD-FMK or Ac-YVAD-CMK, caspase-3 was processed to both the p17 and p20 fragments in staurosporine-treated cells, but only to p20 fragment in the presence of z-VAD-FMK. However, the caspase-3 substrate, poly(ADP ribose) polymerase was not cleaved in the presence of z-VAD-FMK, despite >70% of the cells have pre-apoptotic nuclei. In addition, caspase-3 null MCF-7 cells also undergo pre-apoptotic nuclear change when treated with staurosporine in the presence of caspase inhibitors, indicating that caspase-3 is not required for the early nuclear morphological change in cells undergoing apoptosis. Although cell death in staurosporine-treated Jurkat cells was markedly delayed, they eventually die without discernible downstream apoptotic features. Other apoptotic stimuli like etoposide and the heavy metal chelator, N,N,N',N'-tetrakis (2-pyridylmethyl) ethylenediamine also induced this nuclear morphological change in Jurkat cells in the presence of z-VAD-FMK. In summary, the effector caspases are not involved in early nuclear morphological change, which precedes the conventional hallmark morphological changes associated with chemical-induced apoptosis.

Phosphorylation-Based signaling in Fas receptor-mediated apoptosis.

Apoptosis or programmed cell death plays an essential role during development of the immune system, in immune responses, and in the control of tissue homeostasis in the adult. An important physiological mediator of apoptosis is the Fas/APO-1/CD95 receptor (FasR), a surface receptor belonging to the tumor necrosis factor receptor family. Apoptosis consists of a series of characteristic features that occur following activation of caspases, a collective term for apoptosis-specific proteases. The focus in FasR research has been on determining the mechanisms resulting in caspase activation. However, the role of phosphorylation-based signaling has received increasing attention both as an outcome of FasR activation and as a factor regulating FasR responses. Tyrosine-directed phosphorylation has been implicated to be induced and required during FasR stimulation. The FasR also activates all major signaling pathways that belong to the family of mitogen-activated protein kinase (MAPK) pathways, by either caspase-independent or -dependent mechanisms. Furthermore, phosphorylation-based signaling serves as a potent modifier of FasR responses. In this respect, especially the extracellular signal-regulated kinase and the phosphoinositide 3-kinase signaling pathways have been established as important regulators. This type of control seems to be directly phosphorylation-mediated without the requirement of newly synthesized proteins. Signaling through phosphorylation also regulates the expression of the Fas ligand (FasL), the FasR, as well as various other proteins that affect the outcome of receptor stimulation. While the involvement of phosphorylation has been established in FasR responses, the targets, molecular mechanisms, and biological significance of this aspect of the FasR signaling machinery still require further elucidation.

Inhibition of mitogen-activated kinase signaling sensitizes HeLa cells to Fas receptor-mediated apoptosis.

The Fas receptor (FasR) is an important physiological mediator of apoptosis in various tissues and cells. However, there are also many FasR-expressing cell types that are normally resistant to apoptotic signaling through this receptor. The mitogen-activated protein kinase (MAPK) signaling cascade has, apart from being a growth-stimulating factor, lately received attention as an inhibitory factor in apoptosis. In this study, we examined whether MAPK signaling could be involved in protecting FasR-insensitive cells. To this end, we used different approaches to inhibit MAPK signaling in HeLa cells, including treatment with the MAPK kinase inhibitor PD 98059, serum withdrawal, and expression of dominant-interfering MAPK kinase mutant protein. All of these treatments were effective in sensitizing the cells to FasR-induced apoptosis, demonstrating that MAPK indeed is involved in the control of FasR responses. The MAPK-mediated control seemed to occur at or upstream of caspase 8, the initiator caspase in apoptotic FasR responses. Transfection with the constitutively active MAPK kinase abrogated FasR-induced apoptosis also in the presence of cycloheximide, indicating that the MAPK-generated suppression of FasR-mediated apoptotic signaling is protein synthesis independent. In cells insensitive to FasR-induced apoptosis, stimulation of the FasR with an agonistic antibody resulted in significant MAPK activation, which was inhibited by PD 98059. When different cell types were compared, the FasR-mediated MAPK activation seemed proportional to the degree of FasR insensitivity. These results suggest that the FasR insensitivity is likely to be a consequence of FasR-induced MAPK activation, which in turn interferes with caspase activation.

Amphiphile-induced phosphatidylserine exposure in human erythrocytes.

Nonionic and anionic water-soluble amphiphiles were shown to increase strongly the binding of fluorescein isothiocyanate-conjugated annexin V (FITC-annexin V) in human erythrocytes pretreated with the aminophospholipid translocase (APLT) inhibitor n-ethylmaleimide (NEM). At high sublytic amphiphile-concentrations the binding of FITC-annexin V, monitored in a flow cytometer, was time- and temperature-dependent and occurred heterogeneously in the cell population, with 43-81% of cells being stained above background following incubation for 60 minutes at 37 degrees C. The increased FITC-annexin V binding apparently indicates an increased flop rate of phosphatidylserine (PS) to the outer membrane leaflet. When the NEM-pretreatment was omitted, the FITC-annexin V binding was markedly, but not completely, reduced. In erythrocytes incubated with a zwitter-ionic amphiphile, a small increase in FITC-annexin V binding was detected, while cationic amphiphiles did not induce an increased FITC-annexin V binding. The potency of amphiphiles to induce PS exposure was not related to the type of shape alteration or vesiculation induced. Our results indicate a significant role of the charge status of a membrane intercalated amphiphile for its capability to induce PS exposure.

Suppression of Fas/APO-1-mediated apoptosis by mitogen-activated kinase signaling.

Jurkat T cells undergo rapid apoptosis upon stimulation of the Fas/APO-1 (CD95) receptor. We examined the role of the mitogen-activated protein kinase (MAPK) cascade as a negative regulator of Fas-mediated apoptosis. To this end, we used both physiologic and artificial activators of MAPK, all of which activate MAPK by distinct routes. MAPK activity could be efficiently elevated by two T cell mitogens, the lectin PHA and an agonistic Ab to the T cell receptor complex as well as by the type 1 and 2A phosphatase inhibitor, calyculin A, and the protein kinase C-activating phorbol ester, tetradecanoyl phorbol acetate. All these treatments were effective in preventing the characteristic early and late features of Fas-mediated apoptosis, including activation of caspases. Our results indicate that the elevated MAPK activities intervene upstream of caspase activation. The degree of MAPK activation by the different stimuli used in our study corresponds well to their potency to inhibit apoptosis, indicating that MAPK activation serves as an efficient modulator of Fas-mediated apoptosis. The role of MAPK in modulation of Fas-mediated apoptosis was further corroborated by transient transfection with constitutively active MAPK kinase, resulting in complete inhibition of the Fas response, whereas transfection with a dominant negative form of MAPK kinase had no effect. Furthermore, the apoptosis inhibitory effect of the MAPK activators could be abolished by the specific MAPK kinase inhibitor PD 098059. Modulation of Fas responses by MAPK signaling may determine the persistence of an immune response and may explain the insensitivity of recently activated T cells to Fas receptor stimulation.

Enhancement of fibroblast collagenase (matrix metalloproteinase-1) gene expression by ceramide is mediated by extracellular signal-regulated and stress-activated protein kinase pathways.

Inflammatory cytokines tumor necrosis factor-alpha and interleukin-1 trigger the ceramide signaling pathway, initiated by neutral sphingomyelinase-elicited hydrolysis of cell membrane phospholipid sphingomyelin to ceramide, a new lipid second messenger. Here, we show that triggering the ceramide pathway by sphingomyelinase or C2- and C6-ceramide enhances collagenase-1 (matrix metalloproteinase-1; MMP-1) gene expression by fibroblasts. C2-ceramide activates three distinct mitogen-activated protein kinases (MAPKs) in dermal fibroblasts, i.e. extracellular signal-regulated kinase 1/2 (ERK1/2), stress-activated protein kinase/Jun N-terminal-kinase (SAPK/JNK), and p38. Stimulation of MMP-1 promoter activity by C2-ceramide is dependent on the presence of a functional AP-1 cis-element and is entirely inhibited by overexpression of MAPK inhibitor, dual specificity phosphatase CL100 (MAPK phosphatase-1). Activation of MMP-1 promoter by C2-ceramide is also effectively inhibited by kinase-deficient forms of ERK1/2 kinase (MEK1/2) activator Raf-1, ERK1 and ERK2, SAPK/JNK activator SEK1, or SAPKbeta. In addition, ceramide-dependent induction of MMP-1 expression is potently prevented by PD 98059, a selective inhibitor of MEK1 activation, and by specific p38 inhibitor SB 203580. These results show that triggering the ceramide signaling pathway activates MMP-1 gene expression via three distinct MAPK pathways, i.e. ERK1/2, SAPK/JNK, and p38, and suggest that targeted modulation of the ceramide signaling pathway may offer a novel therapeutic approach for inhibiting collagenolytic activity, e.g. in inflammatory disorders.

Cellular events in Fas/APO-1-mediated apoptosis in JURKAT T lymphocytes.

In the present study we investigated the Fas-mediated cellular events using the human leukemic T cell line, JURKAT. Ligation of the Fas receptor with a monoclonal antibody (IgM) resulted in the rapid (within 3 h) induction of apoptosis and was characterized by a sequence of distinct morphological and biochemical events. Thus, plasma membrane blebbing, condensation of the chromatin, and formation of high-molecular-weight (HMW) DNA fragments were the earliest events observed (by 45 min). They were followed by cleavage of DNA into oligonucleosomal-length fragments (laddering pattern) and the formation of apoptotic bodies, and finally, rounding of the apoptotic cells and complete cleavage of DNA into oligonucleosomal-length fragments occurred. The mitochondria remained structurally intact up to the stage of oligonucleosomal-length DNA cleavage, and the ability of the cells to exclude trypan blue was not compromised throughout the time course of the experiments. In contrast to many other model systems, apoptosis in JURKAT cells after anti-Fas treatment did not require the presence of extracellular Ca2+ or Mg2+ and was only partially inhibited by Zn2+. In addition, Fas-mediated apoptosis was unaffected by the presence of free radical scavengers or inhibitors of protein phosphatases, protein kinases, and nitric oxide synthesis. However, the serine protease inhibitors, N-tosyl-L-phenylalanine chloromethyl ketone (TPCK) and 3,4-dichloroisocoumarin (DCI) prevented anti-Fas-induced apoptosis in JURKAT cells. Low concentrations of these inhibitors blocked oligonucleosomal-length, but not HMW, DNA fragmentation. The latter required a higher concentration of TPCK or DCI to block. In addition, low concentrations of DCI also prevented Fas-mediated plasma membrane blebbing. In summary, our results suggest that proteolysis plays a central role in Fas-mediated apoptosis and that distinct proteolytic enzymes are involved in HMW DNA fragmentation, and oligonucleosomal-length DNA fragmentation, as well as in plasma membrane blebbing.

The toxicity of the mutagen 'MX' and its analogue, mucochloric acid, to rainbow trout hepatocytes and gill epithelial cells and to Daphnia magna.

The cytotoxicity of the, in Salmonella, potent mutagenic compound, 3-chloro-4-(dichloromethyl)-5-hydroxy-2(5H)-furanone (MX) and its structural analogue 3,4-dichloro-5-hydroxy-2[5H]-furanone (mucochloric acid, MCA), was studied in freshly isolated rainbow trout hepatocytes and gill epithelial cells by determining 86Rb-leakage and decrease in fluorescence intensity in calcein AM-loaded cells. The acute toxicity of the compounds to Daphnia magna was studied by determining the concentration causing immobilization of the organism. MX proved to be more toxic than MCA both in the cellular assays and in the acute toxicity test with D. magna. MX was more toxic to hepatocytes than to gill epithelial cells. The uptake of [14C]MX was also much more efficient in hepatocytes than in gill epithelial cells. The uptake of [14C]MX in hepatocytes was not inhibited by taurocholic acid in excess, indicating that MX is not taken up by the carrier complex responsible for the uptake of taurocholate in the hepatocytes. Both the acute toxicity to D. magna and cytotoxicity of MX and MCA was rather low (EC50 values > 0.1 mM) and we conclude that it is very unlikely that MX and MCA at concentrations occurring in recipients receiving chlorination effluents from pulp mills or chlorinated domestic sewage, as regards their acute toxicity, implies a risk for aquatic animals.

Involvement of multiple proteases during Fas-mediated apoptosis in T lymphocytes.

The mechanism of Fas antigen-mediated apoptosis is at present unclear. We show here that the 100,000 x g supernatant from cell lysates prepared from anti-Fas-stimulated JUR-KAT T cells, induces chromatin fragmentation in isolated nuclei with concomitant morphological changes typically seen in apoptosis. The formation of this apoptotic nuclei promoting activity (ANPA) in JURKAT T cells after Fas antigen ligation was blocked by the serine protease inhibitors, TPCK and DCI, and by the interleukin 1-beta-converting enzyme inhibitor, VAD-FMK. In addition, chromatin degradation and morphological changes mediated by the ANPA in isolated nuclei were inhibited by TPCK, but not by DCI or VAD-FMK. These results suggest that Fas-mediated apoptosis in T cells involves the activation of a cascade of proteases.