Hyperosmolarity triggers CD95 membrane trafficking and sensitizes rat hepatocytes toward CD95L-induced apoptosis

Bile salt-induced apoptosis involves NADPH oxidase isoform activation

BACKGROUND & AIMS

Hydrophobic bile salts trigger a rapid oxidative stress response as an upstream event of CD95 activation and hepatocyte apoptosis.

METHODS

The underlying mechanisms were studied by Western blot, immunocytochemistry, protein knockdown, and fluorescence resonance energy transfer microscopy in rat hepatocytes and human hepatoma cell line 7 (Huh7).

RESULTS

The rapid oxidative stress formation in response to taurolithocholate-3-sulfate (TLCS) was inhibited by diphenyleneiodonium, apocynin, and neopterin, suggestive for the involvement of nicotinamide adenine dinucleotide phosphate (NADPH) oxidases. TLCS induced a rapid serine phosphorylation of the regulatory subunit p47phox, which was sensitive to inhibition of sphingomyelinase and protein kinase Czeta (PKCzeta). Inhibitors of p47phox phosphorylation and p47phox protein knockdown abolished the TLCS-induced oxidative stress response and blunted subsequent CD95 activation. Consequences of TLCS-induced oxidative stress were c-Jun-N-terminal kinase activation and Yes-dependent activation of the epidermal growth factor receptor (EGFR), followed by EGFR-catalyzed CD95 tyrosine phosphorylation, formation of the death-inducing signaling complex, and execution of apoptosis. As shown by fluorescence resonance energy transfer experiments in Huh7 cells, TLCS induced a c-Jun-N-terminal kinase-dependent EGFR/CD95 association in the cytosol and trafficking of this protein complex to the plasma membrane. Inhibition of EGFR tyrosine kinase activity by AG1478 allowed for cytosolic EGFR/CD95 association, but prevented targeting of the EGFR/CD95 complex to the plasma membrane. Both processes, and TLCS-induced Yes and EGFR activation, were sensitive to inhibition of sphingomyelinase, PKCzeta, or NADPH oxidases.

CONCLUSIONS

The data suggest that hydrophobic bile salts activate NADPH oxidase isoforms with the resulting oxidative stress response triggering activation of the CD95 system and apoptosis.

Osmotic Regulation of MG-132-induced MAP-kinase Phosphatase MKP-1 Expression in H4IIE Rat Hepatoma Cells

BACKGROUND/AIMS

Proteasome inhibitors such as MG-132 are considered as potential therapeutical tools in different clinical settings. The dual specificity MAP-kinase phosphatase MKP-1 plays a role in balancing signals mediating cell death or survival. Here the effect of cell hydration on MG-132-induced MKP-1 expression was investigated in H4IIE rat hepatoma cells.

RESULTS

Hyperosmolarity (405mosmol/l) increased MKP-1 expression by MG-132, which was accompanied by an induction of c-Fos, c-Jun, cJun Ser73 phosphorylation, and AP-1 DNA binding. MKP-1 induction by MG-132 plus hyperosmolarity was sensitive to inhibition of p38(MAPK) and c-Jun-N-terminal kinases (JNKs) but not extracellular signal-regulated kinases Erk-1/Erk-2, and was accompanied by a decline of MAP-kinase activities. Although hyperosmolarity increased overall protein ubiquitination in presence of MG-132, ubiquitination of MKP-1 was found under normo-, but not hyperosmotic conditions. Hyperosmolarity also enabled MG-132 to induce poly-ADP-ribose polymerase (PARP) cleavage which was sensitive to inhibition of p38(MAPK) and JNKs but not Erk-1/Erk-2. PARP cleavage and caspase-3 activation in H4IIE cells treated with hyperosmolarity plus MG-132 was further increased by vanadate, consistent with a contribution of MKP-1 to counterbalance proapoptotic MAP-kinase signals.

CONCLUSION

The findings suggest that among other factors cell hydration critically determines the cellular response to proteasome inhibitors.

Molecular mechanisms of glutamine action

Glutamine is the most abundant free amino acid in the body and is known to play a regulatory role in several cell specific processes including metabolism (e.g., oxidative fuel, gluconeogenic precursor, and lipogenic precursor), cell integrity (apoptosis, cell proliferation), protein synthesis, and degradation, contractile protein mass, redox potential, respiratory burst, insulin resistance, insulin secretion, and extracellular matrix (ECM) synthesis. Glutamine has been shown to regulate the expression of many genes related to metabolism, signal transduction, cell defense and repair, and to activate intracellular signaling pathways. Thus, the function of glutamine goes beyond that of a simple metabolic fuel or protein precursor as previously assumed. In this review, we have attempted to identify some of the common mechanisms underlying the regulation of glutamine dependent cellular functions.

Cell shrinkage as a signal to apoptosis in NIH 3T3 fibroblasts

Cell shrinkage is a hallmark of the apoptotic mode of programmed cell death, but it is as yet unclear whether a reduction in cell volume is a primary activation signal of apoptosis. Here we studied the effect of an acute elevation of osmolarity (NaCl or sucrose additions, final osmolarity 687 mosmol l(-1)) on NIH 3T3 fibroblasts to identify components involved in the signal transduction from shrinkage to apoptosis. After 1.5 h the activity of caspase-3 started to increase followed after 3 h by the appearance of many apoptotic-like bodies. The caspase-3 activity increase was greatly enhanced in cells expressing a constitutively active G protein, Rac (RacV12A3 cell), indicating that Rac acts upstream to caspase-3 activation. The stress-activated protein kinase, p38, was significantly activated by phosphorylation within 30 min after induction of osmotic shrinkage, the phosphorylation being accelerated in fibroblasts overexpressing Rac. Conversely, the activation of the extracellular signal-regulated kinase (Erk1/2) was initially significantly decreased. Subsequent to activation of p38, p53 was activated through serine-15 phosphorylation, and active p53 was translocated from the cytosol to the nucleus. Inhibition of p38 in Rac cells reduced the activation of both p53 and caspase-3. After 60 min in hypertonic medium the rate constants for K+ and taurine efflux were increased, particular in Rac cells. We suggest the following sequence of events in the cell shrinkage-induced apoptotic response: cellular shrinkage activates Rac, with activation of p38, followed by phosphorylation and nuclear translocation of p53, resulting in permeability increases and caspase-3 activation.

Expression of taurine transporter is regulated through the TonE (tonicity-responsive element)/TonEBP (TonE-binding protein) pathway and contributes to cytoprotection in HepG2 cells

In hypertonic environment, taurine accumulates in cells via activation of TauT (taurine transporter) as an adaptive regulation. Recent studies revealed that TonE (tonicity-responsive element)/TonEBP (TonE-binding protein) pathway regulated the expression of various molecules which protect cells against hypertonic stress. In the present study, we investigated the osmoregulatory mechanisms of TauT expression. TauT was up-regulated at both functional and transcriptional levels in HepG2 under hypertonic condition. The TonE site was identified in the promoter region of TauT gene. Reporter gene assay revealed that promoter activity was increased under hypertonic conditions, whereas deletion or mutation of TonE sequence abolished the induction of the promoter activity in response to hypertonicity. By using the reporter gene plasmids containing a TonE site of TauT promoter (p2xTonE-Luc), it was demonstrated that a TonE site was sufficient for the hypertonicity-mediated activation of TauT promoter. Importantly, co-transfection of TauT promoter gene plasmid with wild-type TonEBP expression vector enhanced promoter activity under isotonic conditions, whereas dominant-negative TonEBP abrogated the TauT promoter activity induced by hypertonicity. Finally, treatment with taurine prevented HepG2 cells from cell death induced by hypertonic medium. These findings suggested that induction of TauT by hypertonicity is mediated by the activation of the TonE/TonEBP pathway and confers resistance to hypertonic stress.

Fluorescence resonance energy transfer analysis of proapoptotic CD95-EGF receptor interactions in Huh7 cells

Hyperosmolarity- and CD95 ligand (CD95L)-induced interactions between CD95 (Fas/APO-1) and the epidermal growth factor receptor (EGFR) involve EGFR-catalyzed CD95 tyrosine phosphorylation. Such interactions were studied by means of fluorescence resonance energy transfer (FRET) and CD95 receptor mutagenesis in Huh7 hepatoma cells. In cells cotransfected with EGFR-cyan fluorescent protein and CD95-yellow fluorescent protein, FRET studies showed a rapid, hyperosmolarity-induced, c-Jun-N-terminal kinase-dependent CD95-EGFR association in the cytosol with subsequent microtubule-dependent translocation of the protein complex to the plasma membrane. Inhibition of EGFR tyrosine kinase activity by AG1478 and cyclic adenosine monophosphate had no effect on hyperosmotic CD95-EGFR association in the cytosol but prevented CD95 tyrosine phosphorylation, targeting of the protein complex to the plasma membrane, and formation of the death-inducing signaling complex (DISC). The requirement of EGFR-mediated CD95 tyrosine phosphorylation for hyperosmotic and CD95L-induced CD95 membrane targeting and DISC formation was also shown in CD95 mutagenesis experiments. CD95 mutants with tyrosine-phenylalanine exchanges at positions 232 and 291 failed to translocate to the plasma membrane and to recruit Fas-associated death domain and caspase 8, although these mutants still associated with the EGFR in the cytosol in response to hyperosmolarity and CD95L. Cells transfected with these mutants were also resistant to CD95L-induced apoptosis. Single mutations of tyrosine 91, 232, and 291 failed to inhibit CD95 membrane targeting, DISC formation, or CD95L-induced apoptosis. In conclusion, we identify EGFR-CD95 interaction and phosphorylation of critical CD95 tyrosine residues as important early events in hyperosmotic and CD95L-induced CD95 activation and apoptosis induction.

Neural control of hepatic osmolytes and parenchymal cell hydration

Liver cell function is strongly influenced by cell hydration and osmolyte content via osmosensing and osmosignaling pathways. Electrical stimulation of hepatic nerves increases the hepatocellular hydration state via an alpha-adrenergic mechanism, promotes taurine release from parenchymal cells and myo-inositol release from hepatic stellate cells. Although changes in liver cell hydration and osmolyte content are known regulators of liver cell function, the functional relevance and integration of nerve-stimulation-dependent alterations of liver cell volume and osmolyte content in the overall hepatic response to towards signals from the nervous system remains to be established.

Involvement of ceramide in hyperosmotic shock-induced death of erythrocytes

Erythrocytes lack nuclei and mitochondria, the organelles important for apoptosis of nucleated cells. However, following increase of cytosolic Ca(2+) activity, erythrocytes undergo cell shrinkage, cell membrane blebbing and breakdown of phosphatidylserine asymmetry, all features typical for apoptosis in nucleated cells. The same events are observed following osmotic shock, an effect mediated in part by activation of Ca(2+)-permeable cation channels. However, erythrocyte death following osmotic shock is blunted but not prevented in the absence of extracellular Ca(2+) pointing to additional mechanisms. As shown in this study, osmotic shock (950 mOsm) triggers sphingomyelin breakdown and formation of ceramide. The stimulation of annexin binding following osmotic shock is mimicked by addition of ceramide or purified sphingomyelinase and significantly blunted by genetic (aSM-deficient mice) or pharmacologic (50 microM 3,4-dichloroisocoumarin) knockout of sphingomyelinase. The effect of ceramide is blunted but not abolished in the absence of Ca(2+). Conversely, osmotic shock-induced annexin binding is potentiated in the presence of sublethal concentrations of ceramide. In conclusion, ceramide and Ca(2+) entry through cation channels concert to trigger erythrocyte death during osmotic shock.

Hypertonic preconditioning prevents hepatocellular injury following ischemia/reperfusion in mice: a role for interleukin 10

Ischemia/reperfusion (I/R) of the liver occurs in many clinical scenarios including trauma, elective surgery, and transplantation. Events initiated by this process can lead to inflammation in the liver, culminating in local injury as well as distant organ dysfunction. Recent studies have suggested that hypertonic saline exerts anti-inflammatory effects, which may be beneficial in preventing organ injury. In the present study, we examine the effect of hypertonic saline on the development of liver inflammation following I/R in both rat and mouse models. Hypertonic pretreatment was shown to prevent liver enzyme release concomitant with a reduction in liver neutrophil sequestration. Hypertonic saline appeared to exert this effect by inhibiting liver tumor necrosis factor alpha (TNF-alpha) generation, an effect that culminated in reduced liver adhesion molecule expression. Hypertonic saline pretreatment was shown to augment liver interleukin 10 (IL-10) expression following I/R, as a potential mechanism underlying its anti-inflammatory effect. To examine the role of IL-10 in the protective effect of hypertonic saline on liver I/R injury, we used a murine model of I/R. In wild type mice, hypertonic pretreatment similarly prevented liver injury induced by I/R. However, in IL-10 knockout animals, hypertonic pretreatment was unable to prevent the liver enzyme release, TNF-alpha generation, or neutrophil sequestration induced by I/R. In conclusion, these findings define a novel mechanism responsible for the anti-inflammatory effects of hypertonic saline and also suggest a potential clinical role for hyperosmolar solutions in the prevention of liver injury associated with I/R.

Suramin inhibits death receptor–induced apoptosis in vitro and fulminant apoptotic liver damage in mice

Suramin is a polysulfonated derivative of urea and has been widely used both to treat infections and as a chemotherapeutic drug. Suramin has been shown to inhibit growth factor signaling pathways; however, its effect on apoptosis is unknown. Here we show that suramin inhibits apoptosis induced through death receptors in hepatoma and lymphoma cells. It also inhibits the proapoptotic effect of chemotherapeutic drugs. The antiapoptotic mechanism is specific to cell type and is caused by reduced activation, but not altered composition, of the death-inducing signaling complex (DISC), and by inhibition of the initiator caspases 8, 9 and 10. Suramin also shows similar effects in in vivo models: apoptotic liver damage induced by CD95 stimulation and endotoxic shock mediated by tumor-necrosis factor (TNF) are inhibited in mice, but necrotic liver damage is not inhibited in a rat model of liver transplantation. Thus, the antiapoptotic property of suramin in the liver may be therapeutically exploited.

The Src family kinase Yes triggers hyperosmotic activation of the epidermal growth factor receptor and CD95

Hyperosmotic exposure of rat hepatocytes triggers epidermal growth factor receptor (EGFR) activation, which results in an activation of the CD95 system and sensitizes the cells toward apoptosis (Reinehr, R., Schliess, F., and Haüssinger, D. (2003) FASEB J. 17, 731-733). The mechanisms underlying the hyperosmotic EGFR activation were studied. Hyperosmotic exposure (405 mosm) resulted in a rapid activation of the Src kinase family members Yes, Fyn, and Lck. Hyperosmotic Yes, but not Fyn activation, was antioxidant-sensitive and was followed by a rapid Yes/EGFR association. PP-2 abolished the hyperosmotic activation of Fyn and Lck but not activation of Yes and EGFR and their association. However, these latter processes were prevented in the presence of SU6656. SU6656 and antioxidants, but not PP-2 and AG1478, also inhibited the hyperosmotic JNK activation. Cyclic AMP had no effect on hyperosmotic Yes and JNK activation but prevented EGFR/Yes association and EGFR activation in an H89-sensitive way. When the hyperosmolarity-induced Yes-EGFR protein complex started to disappear after 30 min, an association between EGFR and CD95 became apparent, which was followed by CD95 tyrosine phosphorylation and activation. SU6656 but not PP-2 also inhibited EGFR/CD95 association, CD95 tyrosine phosphorylation, CD95 membrane trafficking, and death-inducing signaling complex (DISC) formation. EGFR knockdown had no effect on hyperosmotic Yes activation but prevented CD95 tyrosine phosphorylation, membrane targeting, and DISC formation. Hyperosmotic EGFR and CD95 activation was also largely blunted following Yes knockdown. The data suggest that hyperosmotic signaling triggers an oxidative stress-dependent Yes activation, which is followed by JNK and EGFR activation and subsequent activation of the CD95 system. However, the functional relevance of hyperosmolarity-induced Fyn and Lck activation remains to be elucidated.

CD95-tyrosine Nitration Inhibits Hyperosmotic and CD95 Ligand-induced CD95 Activation in Rat Hepatocytes

Epidermal growth factor receptor-dependent CD95-tyrosine phosphorylation was recently identified as an early step in apoptosis induction via the CD95 system (Reinehr, R., Schliess, F., and Häussinger, D. (2003) FASEB J. 17, 731-733). The effect of peroxynitrite (ONOO(-)) on modulation of the hyperosmotic and CD95 ligand (CD95L)-induced CD95 activation process was studied. Pretreatment of hepatocytes with ONOO(-) inhibited CD95L- and hyperosmolarity-induced CD95 membrane trafficking and formation of the death-inducing signaling complex, but not epidermal growth factor receptor activation and its association with CD95. Under these conditions, however, no tyrosine phosphorylation of CD95 occurred; instead, CD95 was tyrosine-nitrated. When ONOO(-) was added after induction of CD95-tyrosine phosphorylation by CD95L or hyperosmolarity, tyrosine nitration of CD95 was largely prevented and death-inducing signaling complex formation occurred. CD95-tyrosine nitration abolished the hyperosmotic sensitization of hepatocytes toward CD95L-induced apoptosis. Additionally, in CD95-yellow fluorescent protein-transfected Huh7-hepatoma cells, ONOO(-) induced CD95 Tyr nitration and prevented CD95L-induced Tyr phosphorylation and apoptosis. Tyrosine-nitrated CD95 was also found in rat livers derived from an in vivo model of endotoxinemia. The data suggest that CD95-tyrosine nitration prevents CD95 activation by inhibiting CD95-tyrosine phosphorylation. Apparently, CD95-tyrosine phosphorylation and nitration are mutually exclusive. The data identify critical tyrosine residues of CD95 as another target of the anti-apoptotic action of NO.

Inhibition of bile salt-induced apoptosis by cyclic AMP involves serine/threonine phosphorylation of CD95

BACKGROUND AND AIMS

Cyclic AMP (cAMP) inhibits bile salt-induced hepatocyte apoptosis; the underlying mechanisms are unclear.

METHODS

The effects of cAMP on taurolithocholate-3-sulfate-(TLCS)- or glycochenodesoxycholate (GCDC)-induced CD95 (Fas/APO-1) activation and apoptosis were studied in 24-hour cultured rat hepatocytes and in perfused rat liver.

RESULTS

TLCS induced a rapid oxidative stress response, c-Jun-N-terminal kinase (JNK) and epidermal growth factor (EGF) receptor (EGF-R) activation, subsequent EGF-R/CD95 association and CD95 tyrosine phosphorylation, CD95 membrane targeting, death-inducing signal complex (DISC) formation and hepatocyte apoptosis. None of these responses was triggered by cAMP; however, cAMP induced H89-sensitive serine/threonine phosphorylation of CD95. Similar data were obtained with GCDC, another proapoptotic bile acid. cAMP did not prevent the TLCS-induced oxidative stress response, JNK activation and EGF-R/CD95 association, however abolished EGF-R activation and subsequent CD95 tyrosine phosphorylation, CD95 membrane trafficking, and DISC formation in a H89-sensitive way. Also in presence of TLCS, cAMP induced rapid Ser/Thr phosphorylation of CD95 within 10 min. The effects of cAMP on the various steps of CD95 activation were also found in the intact perfused rat liver. Evidence is given that a cAMP-induced Ser/Thr phosphorylation favors internalization of CD95.

CONCLUSIONS

Inhibition of bile salt-induced apoptosis by cAMP involves both PKA-dependent Ser/Thr phosphorylation of the CD95 and inhibition of EGF-R activation, which results in an inhibition of CD95 tyrosine phosphorylation, CD95 membrane targeting, and DISC formation. CD95 regulation involves complex phosphorylations with CD95-tyrosine phosphorylation favoring CD95 membrane trafficking and DISC formation, whereas CD95 Ser/Thr phosphorylation inhibits these processes.

Call volume and insulin signaling

Perturbations of cell hydration as provoked by changes in ambient osmolarity or under isoosmotic conditions by hormones, second messengers, intracellular substrate accumulation, or reactive oxygen intermediates critically contribute to the physiological regulation of cell function. In general an increase in cell hydration stimulates anabolic metabolism and proliferation and provides cytoprotection, whereas cellular dehydration leads to a catabolic situation and sensitizes cells to apoptotic stimuli. Insulin produces cell swelling by inducing a net K+ and Na+ accumulation inside the cell, which results from a concerted activation of Na+/H+ exchange, Na+/K+/2Cl- symport, and the Na+/K(+)-ATPase. In the liver, insulin-induced cell swelling is critical for stimulation of glycogen and protein synthesis as well as inhibition of autophagic proteolysis. These insulin effects can largely be mimicked by hypoosmotic cell swelling, pointing to a role of cell swelling as a trigger of signal transduction. This article discusses insulin-induced signal transduction upstream of swelling and introduces the hypothesis that cell swelling as a signal amplifyer represents an essential component in insulin signaling, which contributes to the full response to insulin at the level of signal transduction and function. Cellular dehydration impairs insulin signaling and may be a major cause of insulin resistance, which develops in systemic hyperosmolarity, nutrient deprivation, uremia, oxidative challenges, and unbalanced production of insulin-counteracting hormones. Hydration changes affect cell functions at multiple levels (such as transcriptom, proteom, phosphoproteom, and the metabolom) and a system biological approach may allow us to develop a more holistic view on the hydration dependence of insulin signaling in the future.

Induction of apoptosis and cell cycle arrest by polyvinylpyrrolidone K-30 and protective effect of alpha-tocopherol

Polyvinylpyrrolidone is a macromolecular polymer with widespread use in industry as well as in medicine for various purposes. Its effect on cells cultured in vitro, however, has not been fully investigated. To elucidate this issue, we studied the influence of PVP K-30 on cultured HeLa cells. PVP K-30 treatment produced a dose- and time-dependent toxicity to HeLa cells. Cells exposed to PVP K-30 exhibited several morphological features of apoptosis. Gel electrophoresis of DNA from PVP K-30-treated cells showed typical apoptotic ladder. And flow cytometric analysis demonstrated that PVP K-30 induced cell cycle arrest at G2/M phase and the subsequent appearance of sub-G1 population. In addition, it was shown that procaspase-3 was activated in response to PVP K-30 treatment. We also found that alpha-tocopherol efficiently protected HeLa cells from PVP K-30 cytotoxicity. This is the first demonstration that PVP K-30 could induce apoptosis in HeLa cells and cell cycle arrest at G2/M phase, and that PVP K-30 toxicity could be attenuated by alpha-tocopherol.

Bile salt-induced hepatocyte apoptosis involves epidermal growth factor receptor-dependent CD95 tyrosine phosphorylation

BACKGROUND & AIMS

Hydrophobic bile acids induce CD95-dependent hepatocyte apoptosis.

METHODS

The mechanisms of bile acid-induced CD95 activation were studied in 24-hour cultured rat hepatocytes, in situ-perfused rat livers, and livers from bile duct-ligated rats.

RESULTS

Within 1 minute, the proapoptotic bile salts taurolithocholate-3-sulfate and glycochenodeoxycholate induced oxidative stress and EGF receptor (EGF-R) tyrosine phosphorylation followed by rapid c-Jun-N-terminal kinase (JNK) activation. Thereafter, EGF-R associated with CD95 with subsequent CD95 tyrosine phosphorylation, CD95 membrane targeting, and death-inducing signal complex (DISC) formation. All of these responses were also triggered by taurochenodeoxycholate except that DISC formation only occurred in the presence of phosphatidylinositol 3-kinase inhibitors. No activation of EGF-R or CD95 was observed with tauroursodeoxycholate or taurocholate. Taurolithocholate-3-sulfate-induced EGF-R phosphorylation was sensitive to N-acetylcysteine (NAC) and genistein, whereas CD95/EGF-R association was inhibited by NAC, JNK, or protein kinase C inhibition but not by AG1478. However, the latter compound as well as NAC, genistein, inhibition of JNK, or protein kinase C inhibited CD95 tyrosine phosphorylation, membrane trafficking, and DISC formation.

CONCLUSIONS

Induction of apoptosis by hydrophobic bile salts involves EGF-R activation and EGF-R-dependent CD95 tyrosine phosphorylation, which triggers CD95 membrane targeting and Fas-associated death domain/caspase-8 recruitment. The latter step is apparently also controlled by phosphatidylinositol 3-kinase.

Inhibition of taurolithocholate 3-sulfate-induced apoptosis by cyclic AMP in rat hepatocytes involves protein kinase A-dependent and -independent mechanisms

The mechanisms underlying the inhibition of bile acid-induced apoptosis by cyclic AMP (cAMP) were studied in 24-h-cultured rat hepatocytes. Taurolithocholate 3-sulfate (TLCS, 100 micromol/l) led to a sustained activation of mitogen activated protein (MAP) kinases (JNK, p38(MAPK), and ERKs), dephosphorylation of protein kinase B (PKB), activation of caspases 3 and 8, and hepatocyte apoptosis. cAMP prevented TLCS-induced apoptosis, shifted the persistent TLCS-induced MAP kinase response to a transient pattern, and prevented PKB dephosphorylation. TLCS-induced CD95 and TRAIL receptor-2 trafficking to the plasma membrane were significantly inhibited. Blockade of protein kinase A (PKA) abolished the inhibitory effect of cAMP on TLCS-induced CD95 membrane targeting, but not TRAIL receptor-2 membrane targeting, PKB and MAP kinase responses. H89, an inhibitor of PKA, had no effect on cAMP-induced inhibition of TLCS-triggered poly(ADP) ribose polymerase (PARP) cleavage and caspase activation, but abolished the cAMP-induced inhibition of TLCS-triggered TUNEL- and Annexin V staining. It is concluded that cAMP inhibits bile acid-induced apoptosis via PKA-dependent and -independent mechanisms.

Hyperosmolarity and CD95L trigger CD95/EGF receptor association and tyrosine phosphorylation of CD95 as prerequisites for CD95 membrane trafficking and DISC formation

The mechanisms underlying CD95 ligand (CD95L)- and hyperosmolarity-induced activation of the CD95 system [Reinehr, R., Graf, D., Fischer, R., Schliess, F., and Haussinger, D. (2002) Hepatology 36, 602-614] as initial steps of apoptosis were studied. Hyperosmotic exposure (405 mosmol/l) of rat hepatocytes induced within 1 min oxidative stress and antioxidant-sensitive activation of the epidermal growth factor receptor (EGFR) and c-Jun-N-terminal-kinase (JNK). After 30 min of hyperosmotic exposure EGFR associated with CD95 and CD95 became tyrosine phosphorylated. Inhibition of JNK or protein kinase C (PKC) had no effect on EGFR phosphorylation but abolished CD95/EGFR association, CD95-tyrosine phosphorylation, membrane targeting, and Fas-associated death domain/caspase 8 recruitment to CD95 [death-inducing signaling complex (DISC) formation]. Inhibition of EGFR tyrosine kinase activity prevented CD95 tyrosine phosphorylation and DISC formation but not hyperosmolarity-induced EGFR phosphorylation and EGFR association with CD95. Tyrosine-phosphorylated CD95 was enriched in the plasma membrane. All maneuvers preventing CD95 tyrosine phosphorylation inhibited CD95 membrane trafficking and DISC formation. Stimulation of EGFR by EGF induced EGFR phosphorylation but no association with CD95 or CD95 phosphorylation. Addition of CD95L also induced EGFR and JNK activation, EGFR/CD95 association, CD95 tyrosine phosphorylation, DISC formation, and CD95 membrane targeting with an inhibitor sensitivity profile similar to that of hyperosmotic CD95 activation, except that inhibition of PKC was ineffective. The data suggest that moderate hyperosmolarity or CD95L trigger oxidative stress and EGFR activation followed by a JNK-dependent EGFR/CD95association and CD95 tyrosine phosphorylation, probably through EGFR tyrosine kinase activity. This provides a signal for CD95 membrane trafficking and DISC formation.