Sestrin2 integrates Akt and mTOR signaling to protect cells against energetic stress-induced death

The phosphoinositide-3 kinase/Akt (PI3K/Akt) pathway has a central role in cancer cell metabolism and proliferation. More importantly, it is one of the cardinal pro-survival pathways mediating resistance to apoptosis. The role of Akt in response to an energetic stress is presently unclear. Here, we show that Sestrin2 (Sesn2), also known as Hi95, a p53 target gene that protects cells against oxidative and genotoxic stresses, participates in the protective role of Akt in response to an energetic stress induced by 2-deoxyglucose (2-DG). Sesn2 is upregulated in response to an energetic stress such as 2-DG and metformin, and mediates the inhibition of mammalian target of rapamycin (mTOR), the major cellular regulator of energy metabolism. The increase of Sesn2 is independent of p53 but requires the anti-apoptotic pathway, PI3K/Akt. Inhibition of Akt, as well as loss of Sesn2, sensitizes cells to 2-DG-induced apoptosis. In addition, the rescue of Sesn2 partially reverses the pro-apoptotic effects of 2-DG. In conclusion, we identify Sesn2 as a new energetic stress sensor, which appears to be protective against energetic stress-induced apoptosis that integrates the pro-survival function of Akt and the negative regulation of mTOR.

Anti-inflammatory activity of Mitraphylline isolated from Uncaria tomentosa bark

ETHNOPHARMACOLOGICAL RELEVANCE

Uncaria tomentosa (Willd. ex Roem. & Schult.) DC. (Rubiaceae) is widely used by populations living in South America to treat many ailments associated with inflammatory disorders. Mitraphylline was shown to be the major pentacyclic oxindolic alkaloid present in the bark chloroformic extract of this plant. Its activity against cytokines involved in inflammation process was tested in a murine model in vivo.

MATERIALS AND METHODS

Mice received mitraphylline once a day for 3 days at 30 mg/kg/day by oral route. Then, they were subjected to bacterial lipopolysaccharide (LPS) endotoxin (15 mg/kg) and the LPS-induced production of 16 different cytokines was determined by Elisa multiplex. Control group received dexamethasone orally at 2mg/kg/day. Toxicity on K565 cells and murine peritoneal macrophages, in vitro, at doses up to 100 μM was monitored by XTT-colorimetric assay.

RESULTS AND CONCLUSIONS

For the first time mitraphylline was tested in vivo against a large range of cytokines that play a crucial role in inflammation. Mitraphylline inhibited around 50% of the release of interleukins 1α, 1β, 17, and TNF-α. This activity was similar to dexamethasone. It also reduced almost 40% of the production of interleukin 4 (IL-4) while the corticoid did not. Lastly it did not show any toxicity on K565 cells nor murine macrophages at doses up to 100 μM.

Apoptosis and autophagy have opposite roles on imatinib-induced K562 leukemia cell senescence

Imatinib, the anti-Abl tyrosine kinase inhibitor used as first-line therapy in chronic myeloid leukemia (CML), eliminates CML cells mainly by apoptosis and induces autophagy. Analysis of imatinib-treated K562 cells reveals a cell population with cell cycle arrest, p27 increase and senescence-associated beta galactosidase (SA-β-Gal) staining. Preventing apoptosis by caspase inhibition decreases annexin V-positive cells, caspase-3 cleavage and increases the SA-β-Gal-positive cell population. In addition, a concomitant increase of the cell cycle inhibitors p21 and p27 is detected emphasizing the senescent phenotype. Inhibition of apoptosis by targeting Bim expression or overexpression of Bcl2 potentiates senescence. The inhibition of autophagy by silencing the expression of the proteins ATG7 or Beclin-1 prevents the increase of SA-β-Gal staining in response to imatinib plus Z-Vad. In contrast, in apoptotic-deficient cells (Bim expression or overexpression of Bcl2), the inhibition of autophagy did not significantly modify the SA-β-Gal-positive cell population. Surprisingly, targeting autophagy by inhibiting ATG5 is accompanied by a strong SA-β-Gal staining, suggesting a specific inhibitory role on senescence. These results demonstrate that in addition to apoptosis and autophagy, imatinib induced senescence in K562 CML cells. Moreover, apoptosis is limiting the senescent response to imatinib, whereas autophagy seems to have an opposite role.

Gene expression profiling of human liver transplants identifies an early transcriptional signature associated with initial poor graft function

Liver ischemia-reperfusion injury occurring in orthotopic liver transplantation (OLT) may be responsible for early graft failure. Molecular mechanisms underlying initial poor graft function (IPGF) have been poorly documented in human. The purpose of this study was to identify the major transcriptional alterations occurring in human livers during OLT. Twenty-one RNA extracts derived from liver transplant biopsies taken after graft reperfusion were compared with 7 RNA derived from normal control livers. Three hundred seventy-one genes were significantly modulated and classified in molecular pathways relevant to liver metabolism, inflammatory response, cell proliferation and liver protection. Grafts were then subdivided into two groups based on their peak levels of serum aspartate amino transferase within 72 h after OLT (group 1, non-IPGF: 14 patients; group 2, IPGF: 7 patients). The two corresponding data sets were compared using a supervised prediction method. A new set of genes able to correctly classify 71% of the patients was defined. These genes were functionally associated with oxidative stress, inflammation and inhibition of cell proliferation. This study provides a comprehensive picture of the transcriptional events associated with human OLT and IPGF. We anticipate that such alterations provide a framework for the elucidation of the molecular mechanisms leading to IPGF.

The antidiabetic drug metformin exerts an antitumoral effect in vitro and in vivo through a decrease of cyclin D1 level

Metformin is a widely used antidiabetic agent, which regulates glucose homeostasis through inhibition of liver glucose production and an increase in muscle glucose uptake. Recent studies suggest that metformin may reduce the risk of cancer, but its mode of action in cancer remains not elucidated. We investigated the effect of metformin on human prostate cancer cell proliferation in vitro and in vivo. Metformin inhibited the proliferation of DU145, PC-3 and LNCaP cancer cells with a 50% decrease of cell viability and had a modest effect on normal prostate epithelial cell line P69. Metformin did not induce apoptosis but blocked cell cycle in G(0)/G(1). This blockade was accompanied by a strong decrease of cyclin D1 protein level, pRb phosphorylation and an increase in p27(kip) protein expression. Metformin activated the AMP kinase pathway, a fuel sensor signaling pathway. However, inhibition of the AMPK pathway using siRNA against the two catalytic subunits of AMPK did not prevent the antiproliferative effect of metformin in prostate cancer cells. Importantly, oral and intraperitoneal treatment with metformin led to a 50 and 35% reduction of tumor growth, respectively, in mice bearing xenografts of LNCaP. Similar, to the in vitro study, metformin led to a strong reduction of cyclin D1 protein level in tumors providing evidence for a mechanism that may contribute to the antineoplastic effects of metformin suggested by recent epidemiological studies.

Inhibition of the NF-kappaB survival pathway via caspase-dependent cleavage of the IKK complex scaffold protein and NF-kappaB essential modulator NEMO

Apoptosis is mediated by cysteine-dependent, aspartate-directed proteases of the caspase family that proteolyse strategic intracellular substrates to induce cell suicide. We describe here that engagement of apoptotic processes by Fas triggering or by staurosporine stimulation leads to the caspase-dependent inactivation of the nuclear factor kappa B (NF-kappaB) pathway after cleavage of IKK1 (IkappaB kinase 1) and NEMO (NF-kappaB essential modulator), which are needed to transduce NF-kappaB activation signals. In this study, we have analyzed in more detail, the role of NEMO cleavage, as NEMO, but not IKK1, is important for the pro-survival actions of NF-kappaB. We demonstrate that NEMO is cleaved after Asp355 to remove the last 64 C-terminal amino acids. This short form was unable to rescue NF-kappaB activation by tumor necrosis factor-alpha (TNF-alpha) when transfected in NEMO-deficient cells. Consequently, inactivation of NEMO resulted in an inhibition of the expression of antiapoptotic NF-kappaB-target genes coding for caspase inhibitors (cIAP-1, cIAP-2) or adaptors of the TNF receptor family. NEMO-deficient Jurkat cells transiently expressing a non-cleavable mutant of NEMO were less sensitive to TNF-alpha-induced apoptosis. Therefore, downmodulation of NF-kappaB activation via the proteolytic cleavage of NEMO could represent an amplification loop for apoptosis.

Study on the Putative Contribution of Caspases and the Proteasome to the Degradation of Aph-1a and Pen-2

The presenilin-dependent gamma-secretase complex is mainly composed of four distinct proteins, namely presenilin 1 or presenilin 2, nicastrin, anterior pharynx defective-1 (Aph-1) and presenilin enhancer (Pen-2). The mechanisms by which the complex is assembled, how its stoichiometry is controlled and how its catalytic activity is regulated are poorly understood. Recent studies indicated that Aph-1 and Pen-2 undergo proteolysis by the proteasome. We have examined the susceptibility of endogenous and overexpressed Aph-1a and Pen-2 to proteolysis by endogenous and purified proteasome as well as by recombinant caspases. We show that endogenous Aph-1a and Pen-2 resist proteolysis by caspases and by the proteasome. Furthermore, we show that unexpected interference of proteasome inhibitors with the cmv promoter region driving expression of Aph-1a and Pen-2 led to artifactual enhancement of overexpressed Aph-1a and Pen-2-like immunoreactivities but that these proteins also resist to in vitro degradation by endogenous and purified proteasome.

Apoptosis and erythroid differentiation triggered by Bcr-Abl inhibitors in CML cell lines are fully distinguishable processes that exhibit different sensitivity to caspase inhibition

Imatinib targets the Bcr-Abl oncogene that causes chronic myelogenous leukemia (CML) in humans. Recently, we demonstrated that besides triggering apoptosis in K562 cells, imatinib also mediated their erythroid differentiation. Although both events appear to proceed concomitantly, it is not known at present whether or not imatinib-induced apoptosis and differentiation are interdependent processes. Hence, we investigated the requirements for Bcr-Abl inhibitor-mediated apoptosis and erythroid differentiation in several established and engineered CML cell lines. Imatinib triggered apoptosis and erythroid differentiation of different CML cell lines, but only apoptosis exhibited sensitivity to ZVAD-fmk inhibition. Conversely, the p38 mitogen-activated protein (MAP) kinase inhibitor, SB202190, significantly slowed down erythroid differentiation without affecting caspase activation. Furthermore, imatinib and PD166326, another Bcr-Abl inhibitory molecule, triggered erythroid differentiation of K562 cell clones, nevertheless resistant to Bcr-Abl inhibitor-induced apoptosis. Finally, short hairpin RNA inhibitor (shRNAi) silencing of caspase 3 efficiently inhibited caspase activity but had no effect on erythroid differentiation, whereas silencing of Bcr-Abl mimicked imatinib or PD166326 treatment, leading to increased apoptosis and erythroid differentiation of K562 cells. Taken together, our findings not only demonstrate that Bcr-Abl inhibitor-mediated apoptosis and differentiation are fully distinguishable events, but also that caspases are dispensable for erythroid differentiation of established CML cell lines.

A survey of the signaling pathways involved in megakaryocytic differentiation of the human K562 leukemia cell line by molecular and c-DNA array analysis

The K562 cell line serves as a model to study the molecular mechanisms associated with leukemia differentiation. We show here that cotreatment of K562 cells with PMA and low doses of SB202190 (SB), an inhibitor of the p38 MAPK pathway, induced a majority of cells to differentiate towards the megakaryocytic lineage. Electronic microscopy analysis showed that K562 cells treated with PMA+SB exhibited characteristic features of physiological megakaryocytic differentiation including the presence of vacuoles and demarcation membranes. Differentiation was also accompanied by a net increase in megakaryocytic markers and a reduction of erythroid markers, especially when both effectors were present. PMA effect was selectively mediated by new PKC isoforms. Differentiation of K562 cells by the combination of PMA and SB required Erk1/2 activation, a threshold of JNK activation and p38 MAPK inhibition. Interestingly, higher concentrations of SB, which drastically activated JNK, blocked megakaryocytic differentiation, and considerably increased cell death in the presence of PMA. c-DNA microarray membranes and PCR analysis allow us to identify a set of genes modulated during PMA-induced K562 cell differentiation. Several gene families identified in our screening, including ephrins receptors and some angiogenic factors, had never been reported so far to be regulated during megakaryocytic differentiation.

RhoA GTPase inactivation by statins induces osteosarcoma cell apoptosis by inhibiting p42/p44-MAPKs-Bcl-2 signaling independently of BMP-2 and cell differentiation

Osteosarcoma is the most common primary bone tumour in young adults. Despite improved prognosis, resistance to chemotherapy remains responsible for failure of osteosarcoma treatment. The identification of signals that promote apoptosis may provide clues to develop new therapeutic strategies for chemoresistant osteosarcoma. Here, we show that lipophilic statins (atorvastatin, simvastatin, cerivastatin) markedly induce caspases-dependent apoptosis in various human osteosarcoma cells, independently of bone morphogenetic protein (BMP)-2 signaling and cell differentiation. Although statins increased BMP-2 expression, the proapoptotic effect of statins was not prevented by the BMP antagonist noggin, and was abolished by mevalonate and geranylgeranylpyrophosphate, suggesting the involvement of defective protein geranylgeranylation. Consistently, lipophilic statins induced membrane RhoA relocalization to the cytosol and inhibited RhoA activity, which resulted in decreased phospho-p42/p44- mitogen-activated protein kinases (MAPKs) and Bcl-2 levels. Constitutively active RhoA rescued phospho-p42/p44-MAPKs and Bcl-2 and abolished statin-induced apoptosis. Thus, lipophilic statins induce caspase-dependent osteosarcoma cell apoptosis by a RhoA-p42/p44 MAPKs-Bcl-2-mediated mechanism, independently of BMP-2 signaling and cell differentiation.

PI3K mediates protection against TRAIL-induced apoptosis in primary human melanocytes

Melanocytes are cells of the epidermis that synthesize melanin, which is responsible for skin pigmentation. Transformation of melanocytes leads to melanoma, a highly aggressive neoplasm, which displays resistance to apoptosis. In this report, we demonstrate that TNF-related apoptosis-inducing ligand (TRAIL), which was thought to kill only transformed cells, promotes very efficiently apoptosis of primary human melanocytes, leading to activation of caspases 8, 9 and 3, and the cleavage of vital proteins. Further, we show that stem cell factor (SCF), a physiologic melanocyte growth factor that activates both the phosphatidyl-inositol-3 kinase (PI3K) and the extracellular regulated kinase (ERK) pathways, strongly protects melanocytes from TRAIL and staurosporine killing. Interestingly, inhibition of PI3K or its downstream target AKT completely blocks the antiapoptotic effect of SCF, while inhibition of ERK has only a moderate effect. Our data indicate that protection evoked by SCF/PI3K/AKT cascade is not mediated by an increase in the intracellular level of FLIP. Further, only a sustained PI3K activity can protect melanocytes from apoptosis, thereby indicating that the PI3K/AKT pathway plays a pivotal role in melanocyte survival. The results gathered in this report bring new information on the molecular mechanisms involved in primary melanocyte apoptosis and survival that would help to better understand the process by which melanomas acquire their resistance to apoptosis.

Downregulation of caspases and Fas ligand expression, and increased lifespan of neutrophils after transmigration across intestinal epithelium

During inflammatory bowel diseases, commitment of extravased polymorphonuclear leucocytes (PMN) to apoptosis is required for the resolution of inflammation. To investigate the effect of transepithelial migration on PMN apoptotic rates, PMN transepithelial migration was reproduced in vitro using T84 intestinal monolayers. Transepithelial migration was found to delay neutrophil apoptosis, and this survival effect correlated with a downregulation of the surface expression of Fas ligand (FasL) and with a decrease in both procaspases-3, and -8 mRNA and procaspases-3, -6, -7 and -8 protein levels. Moreover, neutrophil survival and FasL shedding mediated by transepithelial migration were abrogated by a broad-spectrum metalloproteinase inhibitor, BB-94. Although Erk1/2 and p38 MAPK were activated in transmigrated PMN, inhibition of these MAP kinases did not impair transmigration-induced PMN survival. Taken together, our results show that trans-epithelial migration induces the downregulation of proapoptotic proteins expression in transmigrated PMN, which results in their increased lifespan.

T and B leukemic cell lines exhibit different requirements for cell death: correlation between caspase activation, DFF40/DFF45 expression, DNA fragmentation and apoptosis in T cell lines but not in Burkitt's lymphoma

The execution phase of apoptosis occurs through the activation and function of caspases which cleave key substrates that orchestrate the death process. Here, we have compared the sensitivity of various T and B cell lines to death receptor or staurosporine-induced apoptosis. First, we found a lack of correlation between death receptor expression and sensitivity to Fas or Trail. By contrast, a correlation between caspase activation, DNA fragmentation and cell death in T cell lines was evidenced. Among T cells, CEM underwent apoptosis in response to CH11 but were resistant to Trail in agreement with the absence of Trail receptors (DR4 and DR5) on their surface. The B cell line SKW 6.4 was sensitive to CH11 and staurosporine but resistant to Trail. As B cell lines expressed significant levels of DR4 and DR5, resistance to Trail in SKW 6.4 is likely due to the expression of the decoy receptor DcR1. Burkitt's lymphoma such as RPMI 8866 and Raji did not exhibit DNA fragmentation in response to CH11, Trail or staurosporine but showed long-term caspase-dependent loss of viability upon effector treatment. The B cell lines used in this study express very weak or undetectable levels of DFF40 and relatively high levels of DFF45. Interestingly, cytosolic extracts from RPMI 88.66 but not other B lymphoma exhibit altered levels of cytochrome c-dependent caspase activation. Taken together, our results show that: (1) death receptor expression does not correlate with sensitivity to apoptosis; (2) the very low ratio of DFF40 vs. DFF45 is unlikely to explain by itself the lack of DNA fragmentation observed in certain B cell lines; and (3) a defective cytochrome c-dependent caspase activation might account at least in part for the insensitivity of certain Burkitt's lymphoma (RPMI 88.66) to apoptosis. Thus it seems that resistance of Burkitt's lymphoma to apoptosis is not governed by a general mechanism, but is rather multifactorial and differs from one cell line to another.

Cleavage of Fyn and Lyn in their N-terminal unique regions during induction of apoptosis: a new mechanism for Src kinase regulation

The members of the Src kinase family are expressed in a wide variety of tissues, but some of them such as Blk, Hck, Fgr, Lck and Lyn are found primarily in hematopoietic cells. In the present study, we have undertaken experiments to test whether Src kinase cleavage and relocation is a general mechanism during induction of apoptosis. Our results indicate that Fyn and Lyn are efficiently cleaved in their unique region in hematopoietic cells undergoing apoptosis. Fyn cleavage occurred in Fas-stimulated Jurkat T cells but Fyn and Lyn were also processed in the SKW6.4 B cell line. Inhibition of caspases by Z-VAD-fmk or Ac-DEVD-CHO totally prevented Fyn and Lyn cleavage in both intact cells and in vitro. Fyn and Lyn but not Lck, Src and Hck were processed in vitro by human recombinant caspase 3 and by cellular extracts prepared from Fas-stimulated cells. Single mutation of Asp 19 or Asp 18 in the unique N-terminal domains of Fyn and Lyn respectively abolished their cleavage and relocation into the cytoplasm of apoptotic cells. When immunoprecipitated from COS cells N-terminal deleted Src kinases exhibited increased enzymatic kinase activity toward enolase. Thus, cleavage of Fyn and Lyn during induction of apoptosis represents a new mechanism for the regulation of Src kinases that may have important functional and physiological consequences.

Caspase inhibition protects from liver injury following ischemia and reperfusion in rats

Normothermic ischemia and reperfusion of the liver results in microcirculatory failure followed by necrosis and cell death. Recently, another type of cell death, apoptosis or programmed cell death, was found to be activated during the early phase of reperfusion after liver ischemia. Caspases are cysteine proteinases specifically involved in the initiation and execution phases of apoptosis. The aim of this study was to demonstrate that inhibition of apoptosis by a specific inhibitor of caspases might protect the liver against ischemia/reperfusion injury. Rats were divided into three groups: group 1, control, PBS administration; group 2, Z-Asp-cmk (Z-Asp-2,6-dichlorobenzoyl-oxymethylketone) treatment; group 3, sham-operated control animals. Z-Asp-cmk (0.5 mg Z-Asp-cmk dissolved in 300 microl PBS solution containing 1% DMSO) was injected intravenously, 2 min prior to induction of 120 min ischemia. Survival rates were compared and serum activities of aspartate aminotransferases and alanine aminotransferases were assessed in the blood collected from the suprahepatic vena cava. Histology of the liver was assessed 6 h after the end of ischemia. Apoptosis was detected by the terminal deoxynucleotidyl transferase-mediated dUTP-FITC nick end-labeling method (TUNEL method) and by electrophoresis for analysis of DNA fragmentation. Caspase activity was determined by measuring hydrolysis of the CPP32-like substrate Ac-DEVD-pNA and absorption of paranitroaniline. Z-Asp-cmk treatment significantly increased 7-day survival (95%) compared with that in nontreated rats (30%, P < 0.001). Serum activities of aminotransferases and the extent of liver congestion and necrosis were significantly (P < 0.001) decreased after treatment with Z-Asp-cmk. TUNEL-positive cells were detected 3-6 h after reperfusion in the control group. In Z-Asp-cmk pretreated rats, a dramatic decrease in the number of TUNEL-positive cells was observed. Analysis of DNA fragmentation of freshly isolated hepatocytes confirmed these results. Caspase activity was increased 3-6 h after reperfusion in the control group, but significantly (P < 0.001) decreased after treatment with Z-Asp-cmk. These findings demonstrate that liver injury following ischemia and reperfusion can be prevented by inhibition of caspases. Caspase inhibitors may have important implications for therapy in liver disease and after liver transplantation.

Differential requirements for ERK1/2 and P38 MAPK activation by thrombin in T cells. Role of P59Fyn and PKCε

Activation of the mitogen-activated protein kinase (MAPK) cascade is a well documented mechanism for the G-protein-coupled receptors. Here, we have analysed the requirements for ERKs and p38 MAPK activation by thrombin in Jurkat T cells. We show that thrombin-mediated ERKs activation requires both PTK and PKC activities, whereas p38 MAPK activation is dependent only on PTKs. Thrombin-induced ERK and p38 MAPK activation was more pronounced in p56Lck deficient cells indicating that this PTK exerts a negative control on MAPK activity. Accordingly, overexpression of p50 Csk a kinase that inactivates p56Lck induced constitutive activation of ERKs. Requirement for a Src kinase was evidenced by expression of a constitutively active form of p59Fyn in Jurkat cells. Besides its effect on tyrosine phosphorylation events, thrombin also triggered a rapid and robust redistribution of PKCepsilon and delta from the cytosol to the membrane. Expression of constitutively active and dominant negative PKCepsilon demonstrates the pivotal role of this PKC isoform in ERKs activation by thrombin. These data are consistent with a model where thrombin induces ERK activation via both PKC-dependent and independent pathways, whereas p38 MAPK activation requires only PTKs. The PKC-independent pathway requires Src kinases other than p56Lck more likely p59Fyn, while the PKC-dependent mechanism depends on PKCepsilon

A Jurkat T cell variant resistant to death receptor-induced apoptosis. Correlation with heat shock protein (Hsp) 27 and 70 levels

Ligation of Fas induces an apoptotic program in Jurkat cells (Jd). We describe a Jurkat T cell variant (Jr) which shows total resistance to Fas-mediated apoptosis but which exhibits sensitivity to non-death-receptor pro-apoptotic stimuli such as staurosporine. Resistance to Fas-induced apoptosis in Jr cells is correlated with high expression of Hsps. A prior heat-shock increases Hsp27 and 70 expression and protects Jd and Jr cells from Fas- and staurosporine-induced apoptosis. Staurosporine, but not the anti-Fas antibody CH11, abrogates constitutive Hsp70 expression at 37 degrees C and staurosporine also inhibit Hsp27 expression in Jd and Jr cells at 42 degrees C. These data suggest that constitutive expression of Hsp27 inhibits Fas-mediated apoptosis, but only induced expression of Hsp70 can protect T cells from staurosporine-induced apoptosis. Thus, Hsp27 could play a role in the regulation of death receptor-mediated apoptosis, while Hsp70 could regulate mitochondrial-dependent cell death.

Helicobacter pylori lipopolysaccharide hinders polymorphonuclear leucocyte apoptosis

A prominent histologic feature of Helicobacter pylori infection is a dense infiltration of polymorphonuclear leukocytes (PMNL) in gastric mucosa. H. pylori lipopolysaccharide (LPS) has been recognized as a primary virulence factor evoking acute mucosal inflammatory reaction. Previous works have shown that H. pylori LPS immunologic activities are lower than those of enterobacterial LPS. However, the effect of H. pylori LPS on spontaneous PMNL apoptosis, and mechanisms by which this H. pylori LPS may promote PMNL survival remain to be established. In this study, we investigated, by both morphologic and biochemical approaches, the action of H. pylori LPS on PMNL apoptosis in vitro, using broth culture filtrates (BCF) of H. pylori strains with different genotypes. We found that BCF from H. pylori caused a significant delay in spontaneous PMNL apoptosis and this delay was independent of the VacA, cag pathogenicity island and urease status. We demonstrated that LPS in BCF is responsible for this effect because it was abrogated by the LPS antagonist B287 (a synthetic analog of Rhodobactersphaeroides lipid A). Moreover, BCF from H. pylori induced P42/44MAP kinase activation in PMNL. Similar results were obtained with BCF of an Escherichia coli strain. Taken together these data suggest that longer survival of PMNL induced by H. pylori LPS may increase gastric epithelium injury in H. pylori-associated diseases.

Protein kinase C theta and epsilon promote T-cell survival by a rsk-dependent phosphorylation and inactivation of BAD

Both MAPK and protein kinase C (PKC) signaling pathways promote cell survival and protect against cell death. Here, we show that 12-O-tetradecanoylphorbol-13-acetate (TPA) prevents Fas-induced apoptosis in T lymphocytes. The effect of TPA was specifically abolished by the PKC inhibitor GF109203X and by dominant negative PKCtheta, PKCepsilon, and PKCalpha, suggesting that novel and conventional PKC isoforms mediate phorbol ester action. Moreover, TPA stimulated phosphorylation of BAD at serine 112, an effect abrogated by GF109203X but not by the MEK inhibitor PD98059. Expression of constitutively active PKC increased the phosphorylation of BAD at serine 112 but not at serine 136. Additionally, Fas-mediated cell death was enhanced by overexpression of a catalytically inactive form of p90Rsk (Rsk2-KN). Finally, Rsk2-KN abolished the protective effect of constitutively active PKC and totally blocked phosphorylation of BAD on serine 112. Thus, novel PKCtheta and PKCepsilon rescue T lymphocytes from Fas-mediated apoptosis via a p90Rsk-dependent phosphorylation and inactivation of BAD.

Protein kinase activation by warm and cold hypoxia- reoxygenation in primary-cultured rat hepatocytes-JNK(1)/SAPK(1) involvement in apoptosis

Ischemia-reperfusion procedures induced severe hepatic damages owing to different processes related to hypoxia and reoxygenation (H/R) phases, including the consecutive oxygen free radical (OFR) release. Stress-activated protein kinases (SAPKs) could be activated by extracellular stimuli. The aim of this study was to show whether H/R stress conditions could stimulate these kinases, and especially c-jun-N-terminal kinase (JNK(1)/SAPK(1)), to reveal a potential role of JNK(1)/SAPK(1) in the control of hepatocyte apoptosis. Primary cultured rat hepatocytes, isolated from other liver cells and blood flow, were subjected to warm and cold hypoxia-reoxygenation phases mimicking surgical and transplant conditions. The activation status of SAPKs was evaluated by immunoprecipitation or Western-blotting experiments, whereas apoptosis was assessed by measuring caspase activation and internucleosomal DNA fragmentation in vitro and by TUNEL reaction, in vivo. Hypoxia, and especially hypoxia-reoxygenation, significantly increased JNK(1)/SAPK(1) activation in cultured hepatocytes. Either in warm or cold conditions, OFR scavengers (N-Acetylcystein, Di-Phenyleneiodonium, Deferoxamine) decreased this stimulation. Warm ischemia-reperfusion also led to JNK activation. Hypoxia and especially hypoxia-reoxygenation induced programmed cell death in vivo and in vitro. This last phenomenon was inhibited when hepatocytes were treated with SB 202190, which was described as a potent inhibitor of p38 and JNK activities. Altogether, these results confirmed that JNK(1)/SAPK(1) was activated during the hypoxia-reoxygenation process, and that this activity participated in the onset of the apoptosis program.

Sustained polymorphonuclear leukocyte transmigration induces apoptosis in T84 intestinal epithelial cells

Acute colitis is characterized by a large number of polymorphonuclear leukocytes (PMNLs) migrating across the columnar epithelium in response to inflammatory stimuli. Several of these inflammatory factors have been characterized as proapoptotic inducers for intestinal epithelial cells. Our aim was to elucidate the role of PMNL transmigration in the onset of intestinal epithelial cell apoptosis. We found that PMNL migration, in response to N-formyl-methionyl-leucyl-phenylalanine across monolayers of intestinal epithelial cells (T84), was associated with activation of caspase-2, -3, and -9 and poly(ADP-ribose) polymerase cleavage within epithelial cells. Moreover, dihydrocytochalasin B treatment of T84 cells induced apoptosis with similar characteristics. Although Fas and Fas ligand were expressed on T84 cells and PMNLs, treatment of epithelial cells with an antagonistic anti-Fas antibody failed to prevent apoptosis induced by migrating PMNLs. Owing to the F-actin reorganization accompanying PMNL transmigration, these findings indicate a direct relationship between PMNL migration and induction of apoptosis in epithelial cells. This apoptotic process appears to involve remodeling of the actin cytoskeleton of enterocytes independent of the Fas/Fas ligand pathway.

Neprilysin, a novel target for ultraviolet B regulation of melanogenesis via melanocortins

Compelling evidence suggest a role for melanocortins in the regulation of melanogenesis by ultraviolet radiation. Within the epidermis, melanocytes and keratinocytes produce alpha-melanocyte-stimulating hormone and adrenocorticotropic hormone. The persistence and the strength of the biologic signal delivered by these peptides depend on their local concentration, which is controlled by the rate of peptide production and by the rate of its degradation. In this study, we investigated the mechanism of melanocortin degradation by melanocytes and the effect of ultraviolet on this process. We have focused our attention on a neutral endopeptidase, neprilysin, which has been implicated in the ending of numerous peptidergic signals. We have shown that this enzyme is expressed at the surface of human melanocytes. Interestingly, its activity and its expression are dramatically downregulated by ultraviolet B treatment. Moreover, in the presence of phosphoramidon, a stable inhibitor of neprilysin, we observed an increased efficiency of alpha-melanocyte-stimulating hormone and adrenocorticotropic hormone to stimulate both tyrosinase activity and microphthalmia expression. Taken together, these data indicate that neprilysin expressed by melanocytes has a physiologic role in the regulation of melanogenesis by proopiomelanocortin peptide. Further, its downregulation by ultraviolet B irradiation shed light on a new and appealing mechanism of ultraviolet B induced melanogenesis via the control of melanocortins degradation.

Effect of Helicobacter pylori on polymorphonuclear leukocyte migration across polarized T84 epithelial cell monolayers: role of vacuolating toxin VacA and cag pathogenicity island

Helicobacter pylori infection can induce polymorphonuclear leukocyte (PMNL) infiltration of the gastric mucosa, which characterizes acute chronic gastritis. The mechanisms underlying this process are poorly documented. The lack of an in vitro model has considerably impaired the study of transepithelial migration of PMNL induced by H. pylori. In the present work, we used confluent polarized monolayers of the human intestinal cell line T84 grown on permeable filters to analyze the epithelial PMNL response induced by broth culture filtrates (BCFs) and bacterial suspensions from different strains of H. pylori. We have evaluated the role of the vacuolating cytotoxin VacA and of the cag pathogenicity island (PAI) of H. pylori in PMNL migration via their effects on T84 epithelial cells. We noted no difference in the rates of PMNL transepithelial migration after epithelial preincubation with bacterial suspensions or with BCFs of VacA-negative or VacA-positive H. pylori strains. In contrast, PMNL transepithelial migration was induced after incubation of the T84 cells with cag PAI-positive and cagE-positive H. pylori strains. Finally, PMNL migration was correlated with a basolateral secretion of interleukin-8 by T84 cells, thus creating a subepithelial chemotactic gradient for PMNL. These data provide evidence that the vacuolating cytotoxin VacA is not involved in PMNL transepithelial migration and that the cag PAI, with a pivotal role for the cagE gene, provokes a transcellular signal across T84 monolayers, inducing a subepithelial PMNL response.

Cleavage of the serum response factor during death receptor-induced apoptosis results in an inhibition of the c-FOS promoter transcriptional activity

The c-FOS protooncogene is rapidly induced by a wide variety of extracellular stimuli including mitogenic signals. Regulation of c-FOS expression is tightly dependent on the serum response element localized within its promoter. Two transcription factors, the serum response factor (SRF) and the ternary complex factor, bind to the serum response element and play a key role in the regulation of the c-FOS promoter activity. In the present study, we show that two death effectors (CH11 and TRAIL) severely impaired the transcriptional activity of the c-FOS promoter in Jurkat T cells. This inhibition can be accounted for by the specific cleavage by caspase 3 of the SRF both in vitro and in vivo, since acetyl-DEVD-aldehyde prevented SRF cleavage and abolished the inhibitory effect of CH11 and TRAIL on the c-FOS promoter activity. Moreover, phorbol myristate acetate, a potent anti-apoptotic effector, was found to protect SRF completely from cleavage by caspase 3 and also to prevent the inhibition of the c-FOS promoter activity by death effectors. Survival factors play an essential function in the regulation of cell growth mainly by regulating the expression of immediate early gene such as c-FOS. In this line, cleavage of SRF at the onset of apoptosis could abrogate the ability of the cell to induce inappropriate survival pathways. All together, our results are consistent with a role of SRF at the interface between cell survival and death pathways.

Defective thymocyte maturation in p44 MAP kinase (Erk 1) knockout mice

The p42 and p44 mitogen-activated protein kinases (MAPKs), also called Erk2 and Erk1, respectively, have been implicated in proliferation as well as in differentiation programs. The specific role of the p44 MAPK isoform in the whole animal was evaluated by generation of p44 MAPK-deficient mice by homologous recombination in embryonic stem cells. The p44 MAPK-/- mice were viable, fertile, and of normal size. Thus, p44 MAPK is apparently dispensable and p42 MAPK (Erk2) may compensate for its loss. However, in p44 MAPK-/- mice, thymocyte maturation beyond the CD4+CD8+ stage was reduced by half, with a similar diminution in the thymocyte subpopulation expressing high levels of T cell receptor (CD3high). In p44 MAPK-/- thymocytes, proliferation in response to activation with a monoclonal antibody to the T cell receptor in the presence of phorbol myristate acetate was severely reduced even though activation of p42 MAPK was more sustained in these cells. The p44 MAPK apparently has a specific role in thymocyte development.

Cleavage and relocation of the tyrosine kinase P59FYN during Fas-mediated apoptosis in T lymphocytes

Ligation of Fas with its natural ligand or with anti-Fas antibodies induces an apoptotic program in Fas sensitive cells. We report here the identification of the tyrosine kinase p59Fyn as a substrate for CPP32-like proteinases and more particularly caspase 3 during Fas-mediated apoptosis in Jurkat T cells. Inhibition of CPP32-like proteinases by Ac-Asp-Glu-Val-Asp-aldehyde but not by Ac-Tyr-Val-Ala-Asp-aldehyde prevents CPP32, PARP and p59Fyn cleavage indicating that CPP32 or CPP32-like proteinases are responsible for the cleavage of p59Fyn. Cleavage occurs in the N-terminal domain of p59Fyn between Asp19 and Gly20 and is accompanied by relocation of an active p57Fyn kinase to cytoplasm of Fas-stimulated Jurkat cells as judged by both biochemical and confocal microscopy experiments. Thus, p59Fyn relocation and activity may play an important role during Fas-mediated cell death in human T lymphocytes.

Evidence for a p23 caspase-cleaved form of p27[KIP1] involved in G1 growth arrest

p27[KIP1] (p27) is a cyclin dependent kinase inhibitor, involved in the negative regulation of G1 progression in response to a number of anti-proliferative signals. In this study we show, in growing mouse hybridoma (7TD1) and human myeloma (U266) cell lines, that p27 is highly expressed but slightly upregulated when cells are arrested, regardless to the phases of the cell cycle. In contrast, the specific blockade of these cells in early G1 phase reveals the induction of a protein of 23 kDa (p23) specifically recognized by polyclonal anti-p27 antibodies raised against the NH2 terminal part of p27 but not by anti-p21[CIP1] antibodies. Experiments using caspase inhibitors strongly suggest that p23 results from the proteolysis of p27 by a 'caspase-3-like' protease. This cleavage leads to the cytosolic sequestration of p23 but does not alter its binding properties to CDK2 and CDK4 kinases. Indeed, p23 associated in vivo with high molecular weight complexes and coprecipitated with CDK2 and CDK4. We demonstrate by transfection experiments in SaOS-2 cells that p23 induces a G1 phase growth arrest by inhibition of cyclin/CDK2 activity. In summary we describe here a caspase-cleaved form of p27, induced in absence of detectable apoptosis and likely involved in cell cycle regulation.

SP220K is a novel matrix serine proteinase

Matrix proteinases play a critical role in extracellular matrix remodeling, which is particularly involved in cancer invasion and metastasis. We have previously characterized and purified a new tetrameric serine proteinase (SP220K) from human kidney clear cell carcinoma plasma membranes. Here, we report that SP220K exhibits gelatinase activity as assessed both in solution and by zymography. Optimum gelatinase activity ranges between pH 7.5 to pH 9.0. Fibronectin and type I collagen were hydrolyzed by SP220K, at variance with laminin and type IV collagen. Like other trypsin-like fibronectin degrading proteinases, SP220K released the 29-kDa N-terminal heparin-binding domain of fibronectin. By using a panel of proteinase inhibitors, we found that the inhibition profile of SP220K was different from that of other known serine proteinases such as thrombin, trypsin, plasmin, plasminogen activators and tryptase. Altogether, our results indicate that SP220K corresponds to a novel matrix proteinase that exhibits a marked specificity for fibronectin and type I collagen.

T-Cell receptor signaling pathway exerts a negative control on thrombin-mediated increase in [Ca2+]i and p38 MAPK activation in Jurkat T cells: implication of the tyrosine kinase p56Lck

Activation of the mitogen-activated protein kinase (Erk) and c-Jun terminal kinase is a well-documented mechanism for the seven transmembrane spanning receptors. We have previously shown that thrombin stimulation of the T-leukemic cell line Jurkat induced a transient increase in [Ca2+]i and tyrosine phosphorylation of several cellular proteins. Here, we have analyzed p42-44 MAPK, JNK and p38 MAPK activation using Jurkat T-cell lines deficient in either the tyrosine kinase p56Lck (JCaM1) or the tyrosine phosphatase CD45 (J45.01). Our results demonstrate that p56Lck and CD45 exert a negative control on thrombin-induced p38 MAPK activation and [Ca2+]i release in Jurkat cells. Thrombin receptor expression was identical on the different cell lines as assessed by FACS analysis. Tyrosine phosphorylation of p38 MAPK was drastically increased after thrombin stimulation of JCaM1 or J45.01 cells, as compared with parental cells (JE6.1). P42-44 MAPK and JNK activity also enhanced after thrombin treatment of JE6.1 and JCaM1 cell lines, whereas basal kinase activity was higher in J45.01 cells and was not further stimulated by thrombin. Thrombin and thrombin receptor agonist peptide-induced [Ca2+]i mobilization paralleled p38 MAPK activation in JCaM1 and J45.01 cells. Moreover, reconstitution of J45.01 and JCaM1 cell lines with either CD45 or Lck is accompanied by restoration of a normal thrombin-induced [Ca2+]i response and p38MAPK phosphorylation. These data show that a component of the T-cell receptor signaling pathway exerts a negative control on thrombin-induced responses in Jurkat T cells. Accordingly, we found that thrombin enhanced tyrosine phosphorylation of p56Lck and decreased p56Lck kinase activity in J45.01 cells. Our results are consistent with a negative role for p56Lck on thrombin-induced [Ca2+]i release and p38 MAPK activation in Jurkat T-cell lines.

Differential expression of the Kell blood group and CD10 antigens: two related membrane metallopeptidases during differentiation of K562 cells by phorbol ester and hemin

The erythroleukemic cell line K562 can undergo further differentiation in erythroid or megakaryocytic lineage depending on the nature of the stimulus. Phorbol ester (PMA) stimulates megakaryocytic development whereas hemin promotes erythroid differentiation of these cells. We have examined the effect of PMA and hemin on the expression of the Kell blood group and CD10 antigens, two related proteins that belong to a family of membrane-bound neutral metalloendopeptidases. We show here that differentiation of K562 cells by PMA in the megakaryocytic lineage results in abolishment of Kell mRNA accumulation and protein expression and, in parallel, the induction of CD10 mRNA accumulation, protein expression, and enzymatic activity. Conversely, differentiation of these cells by hemin in the erythroid lineage is accompanied by an up-regulation of Kell mRNA and protein expression, with no changes in CD10 mRNA and protein expression. Thus, CD10 and Kell can be regarded as specific markers of the differentiation of K562 cells in the megakaryocytic and erythroid lineages, respectively.

CD10 inhibitors increase f-Met-Leu-Phe-induced neutrophil transmigration

A variety of bacterial enterocolitis in their active stages are characterized by the migration of polymorphonuclear leukocytes (PMNs) across epithelial surfaces. These mechanisms could explain some effects of enterotoxins observed in the intestinal mucosae. Here, using specific inhibitors, we investigated the potential role of CD10 (E.C. 3.4.24.11), present at the surface of human neutrophils, on formyl-Met-Leu-Phe (fMLP)-induced PMN migration across cultured monolayers of the human intestinal cell line T84. Transmigration of human neutrophils across T84 epithelial cells was observed for concentrations of fMLP as low as 10(-9) M, whereas maximal effect was achieved at 10(-7) M as determined by transepithelial resistances and PMN myeloperoxidase assays. RB25, a CD10 inhibitor, reduced by two orders of magnitude the concentration of fMLP required to obtain full neutrophil transmigration across T84 epithelial cell line. RB25 response was concentration dependent with half-maximal and maximal effect occurring at 10(-9) and 10(-7) M, respectively. These concentrations of RB25 corresponded exactly to the half-maximal and maximal inhibition of endopeptidase 24.11 at the neutrophil cell surface. However, the effect of CD10 inhibitors on PMN transmigration cannot be accounted for by a direct action on T84 epithelial cells, since these cells fail to express any detectable endopeptidase 24.11 activity. Moreover, blocking of CD10 enzymatic activity by various and selective inhibitors potentiated the effect of low concentrations of fMLP on PMN transmigration. Finally, RB25 failed to affect interleukin-8 (IL-8)-induced PMN transmigration across T84 epithelial cells, in agreement with the preference of CD10 for small peptidic substrates. Taken together, these results demonstrate that inhibition of CD10 significantly reduced the concentration of fMLP needed for eliciting transmigration of PMN across intestinal epithelia.

Regulation of thymic development by neprilysin inhibition

Development of T lymphocyte is regulated by both thymocyte-stromal cell interactions and production of soluble factors such as cytokines, peptides and hormones. The local concentration of active biological peptides is regulated by a specialized family of enzymes expressed at the cell surface, the ectopeptidases. We found that treatment of fetal thymic organ cultures (FTOC) with the specific CD10 (endopeptidase 24.11) inhibitor, (N-(3-[(hydroaxyamino)carbonyl]-2-benzylidene-1-oxopropyl]-N-glyci ne), RB25 results in a marked delay in thymocyte differentiation. RB25 causes a significant decrease in the number of DP (CD4+CD8+) cells in favor of the TN (TcR alpha beta-CD4-CD8-) population. RB25 also blocks T lymphocyte differentiation in FTOC when preinjected into pregnant mice. Finally, RB25 was found to essentially affect the CD44+CD25- and CD44-CD25- thymocytes in "in vitro" and "in vivo" experiments after 2 days FTOC. Thus, a selective and stable endopeptidase 24.11 inhibitor impairs T cell development, an observation in agreement with the involvement of the CD10 antigen in early T cell development.

CD10 is expressed on human thymic epithelial cell lines and modulates thymopentin-induced cell proliferation

Thymic hormones such as thymopoietin (TP) have been shown to regulate thymocyte differentiation and lymphocyte activation. However, it is not known whether thymopoietin affects thymic epithelial cell (TEC) functions. In this study we have examined the effect of a five amino acid active peptide (TP5), corresponding to amino acids 32-36 of TP, on the proliferation of nontransformed clones of human TEC. Our results indicate that TP5 induced reinitiation of DNA synthesis and potentiated fetal calf serum (FCS)-induced cell growth in postnatal and fetal-derived human TEC. We also found that TEC lines express high levels of endopeptidase 24.11, a cell-surface metallopeptidase also known as the CD10 antigen. We show that TP5 is cleaved by CD10 at the surface of TEC lines, indicating that this endopeptidase may regulate TP5-induced TEC proliferation. Phosphoramidon, a specific endopeptidase 24.11 inhibitor, consistently acts in synergy with TP5 to enhance FCS-induced TEC growth. Hence, we conclude that 1) TP5 alone or in combination with FCS supports the growth of TEC lines, and 2) TEC lines express high levels of CD10, which regulates TP5-induced TEC proliferation by acting as a thymic peptide degrading enzyme.

Differential SP220K expression in renal carcinoma and oncocytoma cells

SP220K is a newly described serine proteinase which displays guanidinobenzoatase activity in its inactive form and gelatinolytic activity in its active form. SP220K expression was studied in 20 renal clear-cell carcinomas and in a series of renal oncocytomas, a rare benign tumor derived from the kidney tubule epithelium. We provide evidence that SP220K expression, as assessed by guanidinobenzoatase activity, gelatin zymography and Western blot immunodetection, was increased markedly in cancer basolateral membranes compared to kidney cortex controls, whereas no signal was detectable in basolateral membranes from the 5 renal oncocytomas studied. Cytoplasms of carcinoma cells were immunodetected consistently, whereas no expression was seen in oncocytic cells from any of the oncocytomas studied (12/12). Endothelial cells were immunodetected in all 3 tissue types. Our data favor a potential mechanistic relationship between expression of the matrix proteinase SP220K and invasive phenotype in kidney epithelium proliferative processes.

Endopeptidase 24.11 (CD10/NEP) is required for phorbol ester-induced growth arrest in Jurkat T cells

Jurkat T cells express a functional endopeptidase 24.11 that is involved in the regulation of T cell activation. We have analyzed the effect of ectopic CD10 expression in mutant Jurkat cell clones that fail to express CD10 and, unlike wild-type cells, are resistant to the growth-inhibitory effects of the protein kinase C activator, PMA. No differences in the expression of the mRNA encoding the alpha, beta, gamma, delta, epsilon, and zeta isoforms of PKC were found in parental vs. PMA-resistant Jurkat cells, ruling out the possibility that the defect could be accounted for by an altered expression of one of these isoforms. Phorbol ester-induced growth arrest was not due to apoptosis since PMA failed to trigger DNA fragmentation in parental and mutant Jurkat T cells. CD10 mRNA expression and activity were abrogated in four independent PMA-resistant Jurkat T cell clones compared to parental cells, whereas the activities of several other peptidases were unaffected. Transfection of one mutant clone with a functional endopeptidase 24.11 restored in a significant manner PMA-induced growth arrest in all the clones selected and tested, whereas transfection of an inactive form of endopeptidase 24.11 had no effect, demonstrating that the enzymatic activity of CD10 is critical in the mediation of the PMA growth arrest. The data presented here demonstrate that a functional CD10 is required for PMA-induced growth arrest in Jurkat cells and provide further evidence for a role of endopeptidase 24.11 in the regulation of tumor cell proliferation.

CD10 plays a specific role in early thymic development

Development of T lymphocyte is a complex process that depends on both thymocytestromal cell interactions and the production of soluble factors such as cytokines, peptides, and hormones. In many tissues, the concentration of active biological peptides is regulated locally by a specialized family of enzymes: the ectopeptidases. We show here that treatment of fetal thymic organ cultures (FTOC) with the specific CD10 (endopeptidase 24.11) inhibitors SCH 32615: (N-[L-(1-carboxy-2-phenyl)ethyl]-L-phenylalanyl-beta-alanine), RB25: (N-(3-[(hydroaxyamino)carbonyl]-2-benzylidene-1-oxopropyl]-N-glyci ne), and thymopentin (TP5) results in the inhibition of thymocyte differentiation. Each agent induces a significant decrease in the number of double positive (CD4+CD8+) cells in favor of the TN (TcR alpha beta-CD4-CD8-) population. RB25 also blocks T lymphocyte differentiation in FTOC when preinjected into pregnant mice. Finally, RB25 and TP5 were also shown to reduce the number of CD44+CD25- and CD44-CD25- thymocytes both in vitro and after preinjection in vivo in day 2 FTOC. Thus, agents that affect endopeptidase 24.11 activity impair T cell development both in vitro and in vivo. Our results show that the CD10 molecule plays a specific role in promoting early T cell development.

CD10 (endopeptidase 24.11) is a thymic peptide-degrading enzyme possibly involved in the regulation of thymocyte functions

Human immature thymocytes express significant levels of the CD10 (endopeptidase 24.11) cell surface antigen. We report here that IOB5, an anti-CD10 mAb, as well as the phorbol ester PMA down-regulate CD10 activity at the surface of human thymocytes. The kinetics of CD10 modulation were drastically different for both effectors, indicating different regulatory mechanisms. We also demonstrated that intact human thymocytes hydrolyze thymopentin and that CD10 significantly participates in this process. Finally, we found that thymopentin and to a lesser extent phosphoramidon, a specific endopeptidase 24.11 inhibitor, induced up-regulation of CD4 and CD8 molecules at the thymocyte cell surface. In view of these results, we suggest that down-regulation of endopeptidase 24.11 at the thymocyte cell surface might reduce its activity toward thymic factors possibly involved in the regulation of thymocyte functions.

Tyrosine phosphorylation of I kappa B-alpha activates NF-kappa B without proteolytic degradation of I kappa B-alpha

The transcription factor NF-kappa B regulates genes participating in immune and inflammatory responses. In T lymphocytes, NF-kappa B is sequestered in the cytosol by the inhibitor I kappa B-alpha and released after serine phosphorylation of I kappa B-alpha that regulates its ubiquitin-dependent degradation. We report an alternative mechanism of NF-kappa B activation. Stimulation of Jurkat T cells with the protein tyrosine phosphatase inhibitor and T cell activator pervanadate led to NF-kappa B activation through tyrosine phosphorylation but not degradation of I kappa B-alpha. Pervanadate-induced I kappa B-alpha phosphorylation and NF-kappa B activation required expression of the T cell tyrosine kinase p56ick. Reoxygenation of hypoxic cells appeared as a physiological effector of I kappa B-alpha tyrosine phosphorylation. Tyrosine phosphorylation of I kappa B-alpha represents a proteolysis-independent mechanism of NF-kappa B activation that directly couples NF-kappa B to cellular tyrosine kinase.

Thrombin and trypsin-induced Ca(2+) mobilization in human T cell lines through interaction with different protease-activated receptors

This study was conducted to determine whether serine proteinases may induce [Ca(2+)]i mobilization in different hematopoietic cell lines and to analyze their mechanisms of action. We show that in addition to thrombin and thrombin receptor agonist peptide (TRP, SFLLRN), trypsin induced [Ca(2+)]i mobilization in a highly thrombin-sensitive Jurkat T cell clone. Thrombin, TRP, and trypsin were found to induce [Ca(2+)]i release in three different Jurkat T cell clones differing in the level of T cell receptor expression. Similar results were obtained with a prothymocytic leukemic cell line, HPB.ALL, although these cells were much more responsive to trypsin than to thrombin and TRP. Other cell types such as THP1, a myelomonocytic cell line, or CEM, a CD4(+) positive leukemic cell line, were unresponsive to thrombin, TRP, and trypsin. The effect of trypsin was mimicked by SLIGRL, a peptide corresponding to the cleaved amino-terminal sequence of the recently characterized murine trypsin-activated receptor (PAR2). At suboptimal concentrations, the effects of SFLLRN and SLIGRL were additive, whereas saturating doses of peptides did not further increase [Ca(2+)]i mobilization in Jurkat T cells, indicating that both peptides were able to mobilize the same pool of calcium. Northern blot analysis of mRNAs from different leukemic cell lines indicated a remarkable correlation between PAR2 expression in different cell lines and SLIGRL or trypsin responses in the same cells. The expression of the "trypsin receptor" was also confirmed by polymerase chain reaction analysis. Moreover, a 24 h treatment of Jurkat cells by an anti-CD3 monoclonal antibody, a condition known to down-regulate thrombin receptor expression, induced loss of thrombin and TRP responses but only partially affected trypsin stimulation of [Ca(2+)]i release. Finally, after a first stimulation with either thrombin or trypsin, Jurkat cells were still able to respond to trypsin or thrombin, respectively, demonstrating that thrombin and trypsin essentially activated their own receptors. Our data provided evidence that 1) the human T leukemic cell line Jurkat and other T cell lines express at least two different functional protease-activated receptors, the thrombin receptor and a highly sensitive trypsin receptor, likely the human counterpart of the murine PAR2, and 2) at variance with the commonly accepted model, trypsin exerts most of its effect in T leukemic cell lines by thrombin receptor-independent mechanisms.

Distinct mechanisms regulate 5-HT2 and thrombin receptor desensitization

We have compared the desensitization of two receptors, the thrombin receptor which displays dual coupling to both pertussis toxin-sensitive (Gi) and -insensitive (Gq) proteins and the serotonin type 2 (5-HT2) receptor which selectively couples to Gq. In the case of the thrombin receptor, cleavage induces activation and irreversible receptor modification followed by rapid (T1/2 = 3 min) and extensive desensitization of the receptor's ability to modulate phospholipase C (Gq). 5-HT-induced desensitization of its receptor is markedly slower (T1/2 = 10 min) and by 60 min only 50% of the phospholipase C response is lost. This effect occurs with a parallel disappearance of 5-HT receptors from the cell surface. Whole cell phosphorylation studies showed that the thrombin receptor is rapidly phosphorylated upon activation. In contrast, the 5-HT2 receptor displays a low basal level of phosphorylation which is not increased upon agonist treatment. The cytoplasmic tail of the 5-HT2 receptor which contains several protein kinase consensus sequences was found not to be involved in receptor activation or desensitization. However, a chimeric receptor having the core of the 5-HT2 receptor and the cytoplasmic tail of the thrombin receptor was able to undergo 5-HT-induced desensitization and phosphorylation. These results indicate that (i) both 5-HT2 and thrombin receptors have unique shut-off mechanisms, and (ii) that sequences in the carboxyl terminus of the thrombin receptor are sufficient to trigger rapid uncoupling of the receptor from its G protein(s) and downstream effector(s).