Shp-2 tyrosine phosphatase functions as a negative regulator of the interferon-stimulated Jak/STAT pathway.
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Shp-2 is an SH2 domain-containing protein tyrosine phosphatase. Although the mechanism remains to be defined, substantial experimental data suggest that Shp-2 is primarily a positive regulator in cell growth and development. We present evidence here that Shp-2, while acting to promote mitogenic signals, also functions as a negative effector in interferon (IFN)-induced growth-inhibitory and apoptotic pathways. Treatment of mouse fibroblast cells lacking a functional Shp-2 with IFN-alpha or IFN-gamma resulted in an augmented suppression of cell viability compared to that of wild-type cells. To dissect the molecular mechanism, we examined IFN-induced activation of signal transducers and activators of transcription (STATs) by electrophoretic mobility shift assay, using a specific DNA probe (hSIE). The amounts of STAT proteins bound to hSIE upon IFN-alpha or IFN-gamma stimulation were significantly increased in Shp-2(-/-) cells. Consistently, tyrosine phosphorylation levels of Stat1 upon IFN-gamma treatment and, to a lesser extent, upon IFN-alpha stimulation were markedly elevated in mutant cells. Furthermore, IFN-gamma induced a higher level of caspase 1 expression in Shp-2(-/-) cells than in wild-type cells. Reintroduction of wild-type Shp-2 protein reversed the hypersensitivity of Shp-2(-/-) fibroblasts to the cytotoxic effect of IFN-alpha and IFN-gamma. Excessive activation of STATs by IFNs was also diminished in mutant cells in which Shp-2 had been reintroduced. Together, these results establish that Shp-2 functions as a negative regulator of the Jak/STAT pathway. We propose that Shp-2 acts to promote cell growth and survival through two mechanisms, i.e., the stimulation of growth factor-initiated mitogenic pathways and the suppression of cytotoxic effect elicited by cytokines, such as IFNs. (+info)
The Salmonella invasin SipB induces macrophage apoptosis by binding to caspase-1.
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Recently, Salmonella spp. were shown to induce apoptosis in infected macrophages. The mechanism responsible for this process is unknown. In this report, we establish that the Inv-Spa type III secretion apparatus target invasin SipB is necessary and sufficient for the induction of apoptosis. Purified SipB microinjected into macrophages led to cell death. Binding studies show that SipB associates with the proapoptotic protease caspase-1. This interaction results in the activation of caspase-1, as seen in its proteolytic maturation and the processing of its substrate interleukin-1beta. Caspase-1 activity is essential for the cytotoxicity. Functional inhibition of caspase-1 activity by acetyl-Tyr-Val-Ala-Asp-chloromethyl ketone blocks macrophage cytotoxicity, and macrophages lacking caspase-1 are not susceptible to Salmonella-induced apoptosis. Taken together, the data demonstrate that SipB functions as an analog of the Shigella invasin IpaB. (+info)
Caspase-1 is not involved in experimental hepatitis in mouse.
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Experimental hepatitis induced by tumor necrosis factor in D-(+)-galactosamine-sensitized mice or by an agonistic anti-Fas antibody in normal mice is accompanied by dramatic apoptosis of hepatocytes. Apoptosis is the final result of activation of a cascade of caspases. We used caspase-1-/- mice, generated by gene targeting, to study the role of this protease in TNF- and anti-Fas-induced lethal hepatitis. We found that mutant mice exhibited the typical caspase-1-/- phenotype, since they resisted to a lethal injection of LPS and released no interleukin-1beta in the circulation, in contrast to wild-type littermates. When caspase-1-/- mice were challenged with different doses of tumor necrosis factor/D-(+)-galactosamine or with anti-Fas, no increased survival was observed compared with control mice. Furthermore, apoptosis in the livers of these mice and serum levels of alanine aminotransferase were not reduced. These data indicate that caspase-1 deficiency does not lead to reduced apoptosis in these models, either because caspase-1 is irrelevant in this model or because of functional redundancy. (+info)
Alkaline conditions accelerate polymorphonuclear leukocyte apoptosis in vitro.
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Apoptosis was monitored in polymorphonuclear leukocytes (PMNs) cultured under mildly acidic, neutral, and alkaline conditions. Within 3 h, 9.0% of the PMNs underwent apoptosis at pH 6.7, as did 12% at pH 7.2, 38% at pH 7.7, and 60% at pH 8.2. Inhibitors of serine proteases, caspase-1, or caspase-3 significantly inhibited PMN apoptosis at pH 8.2, suggesting an involvement by these enzymes. (+info)
Restoration of transforming growth factor beta signaling pathway in human prostate cancer cells suppresses tumorigenicity via induction of caspase-1-mediated apoptosis.
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Previous studies (Y. Guo and N. Kyprianou, Cell Growth Diff., 9: 185-193, 1998) have demonstrated that overexpression of transforming growth factor (TGF) beta type II receptor (TbetaRII) gene in human prostate cancer cells LNCaP, which are refractory to TGF-beta1 and lack TbetaRII receptor expression, can restore TGF-beta1 sensitivity and suppress in vitro tumorigenic growth by inhibiting cell proliferation. In the present study, we investigated the effect of TbetaRII receptor overexpression in LNCaP cells on apoptosis induction and tumorigenicity. The ability of LNCaP cells that overexpress TbetaRII to undergo apoptosis in response to TGF-beta1 was examined by DNA fragmentation and terminal transferase-mediated dUTP-biotin end labeling analysis. To explore the potential apoptotic nature of TGF-beta1-mediated antitumor effect against human prostate cancer cells, the expression of apoptotic proteins bcl-2 and bax was examined by Western blot analyses. The significance of caspase 1 in TGF-beta1-mediated apoptosis was also determined by examining the expression and activation of caspase 1 by reverse transcription-PCR and Western blot analyses, respectively. Comparative analysis of tumorigenicity of the parental LNCaP and TbetaRII-overexpressing clones in severely combined immunodeficient mice revealed a significant suppression of tumor growth in TbetaRII transfectant clones compared with parental LNCaP cells and neomycin-control clones (P < 0.05). A significantly higher incidence of endogenous apoptosis was observed in TbetaRII clone-61-derived tumor compared with the parental LNCaP tumors. This induction of apoptosis in the LNCaP tumors with restored TGF-beta1 signaling was associated with decreased bcl-2 expression, increased bax, and caspase-1 immunoreactivty. Moreover, an increased expression of the cyclin-dependent kinase inhibitor p27Kip1 was detected in TbetaRII-overexpressing tumors compared with the parental tumors. LNCaP TbetaRII transfectant cells exhibited a marked induction of apoptosis, paralleled with a decreased bcl-2 expression in response to TGF-beta1 treatment in vitro. This TGF-beta1-mediated apoptosis induction in TbetaRII transfectant cells was significantly protected by the caspase-1 inhibitor (zVAD-fmk) in a dose-dependent manner. Furthermore, a significant temporal induction of caspase-1 mRNA and protein expression was detected in TbetaRII cells in response to TGF-beta1 treatment. Our findings suggest that restoration of TGF-beta1 signaling suppresses tumorigenicity of human prostate cancer cells by inducing apoptosis, potentially via a caspase-1-mediated pathway. (+info)
The p42 variant of ETS1 protein rescues defective Fas-induced apoptosis in colon carcinoma cells.
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ETS1 is a cellular homologue of the product of the viral ets oncogene of the E26 virus, and it functions as a tissue-specific transcription factor. It plays an important role in cell proliferation, differentiation, lymphoid cell development, transformation, angiogenesis, and apoptosis. ETS1 controls the expression of critical genes involved in these processes by binding to ets binding sites present in the transcriptional regulatory regions. The ETS1 gene generates two proteins, p51 and a spliced variant, p42, lacking exon VII. In this paper we show that p42-ETS1 expression bypasses the damaged Fas-induced apoptotic pathway in DLD1 colon carcinoma cells by up-regulating interleukin 1beta-converting enzyme (ICE)/caspase-1 and causes these cancer cells to become susceptible to the effects of the normal apoptosis activation system. ICE/caspase-1 is a redundant system in many cells and tissues, and here we demonstrate that it is important in activating apoptosis in cells where the normal apoptosis pathway is blocked. Blocking ICE/caspase-1 activity by using specific inhibitors of this protease prevents the p42-ETS1-induced apoptosis from occurring, indicating that the induced ICE/caspase-1 enzyme is responsible for killing the cancer cells. p42-ETS1 activates a critical alternative apoptosis pathway in cancer cells that are resistant to normal immune attack, and thus it may be useful as an anticancer therapeutic. (+info)
Inhibition of caspases inhibits the release of apoptotic bodies: Bcl-2 inhibits the initiation of formation of apoptotic bodies in chemotherapeutic agent-induced apoptosis.
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During apoptosis, the cell actively dismantles itself and reduces cell size by the formation and pinching off of portions of cytoplasm and nucleus as "apoptotic bodies." We have combined our previously established quantitative assay relating the amount of release of [3H]-membrane lipid to the degree of apoptosis with electron microscopy (EM) at a series of timepoints to study apoptosis of lymphoid cells exposed to vincristine or etoposide. We find that the [3H]-membrane lipid release assay correlates well with EM studies showing the formation and release of apoptotic bodies and cell death, and both processes are regulated in parallel by inducers or inhibitors of apoptosis. Overexpression of Bcl-2 or inhibition of caspases by DEVD inhibited equally well the activation of caspases as indicated by PARP cleavage. They also inhibited [3H]-membrane lipid release and release of apoptotic bodies. EM showed that cells overexpressing Bcl-2 displayed near-normal morphology and viability in response to vincristine or etoposide. In contrast, DEVD did not prevent cell death. Although DEVD inhibited the chromatin condensation, PARP cleavage, release of apoptotic bodies, and release of labeled lipid, DEVD-treated cells showed accumulation of heterogeneous vesicles trapped in the condensed cytoplasm. These results suggest that inhibition of caspases arrested the maturation and release of apoptotic bodies. Our results also imply that Bcl-2 regulates processes in addition to caspase activation. (+info)
High and low molecular weight DNA cleavage in ovarian granulosa cells: characterization and protease modulation in intact cells and in cell-free nuclear autodigestion assays.
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To continue elucidation of the biochemical and molecular pathways involved in the induction of apoptosis in granulosa cells (GC) of ovarian follicles destined for atresia, we characterized the occurrence and protease modulation of high and low molecular weight (MW) DNA fragmentation during rat GC death. Atresia of ovarian follicles, occurring either spontaneously in vivo or induced in vitro, was associated with both high MW and internucleosomal (low MW) DNA cleavage. Incubation of follicles in the presence of a putative irreversible and non-competitive inhibitor of caspase-1 (interleukin-1beta-converting enzyme or ICE), sodium aurothiomalate (SAM), completely prevented internucleosomal, but not high MW, DNA cleavage. As reported previously, morphological features of apoptosis (pyknosis, cellular condensation) and atresia (granulosa cell disorganization, oocyte pseudomaturation) remained detectable in SAM-treated follicles. The potential involvement of proteases in endonuclease activation was further analyzed in cell-free assays using nuclei from both GC (which autodigest their DNA) and HeLa cells (HC, which do not autodigest their DNA unless incubated with extracts prepared from other cell types). Crude cytoplasmic extracts prepared from GC induced both high MW and internucleosomal DNA cleavage in HC nuclei. The induction of low, but not high, MW DNA cleavage in HC nuclei by GC extracts was suppressed by pretreatment of the extracts with SAM or with any one of the serine protease inhibitors, dichloroisocoumarin (DCI), N-tosyl-L-leucylchloromethylketone (TLCK) or N-tosyl-L-phenylchloromethylketone (TPCK). Interestingly, SAM and DCI also prevented cation-induced low MW DNA fragmentation in GC nuclei; however, TLCK and TPCK were without effect. Our results support a role for cytoplasmic and nuclear serine proteases in the activation of the endonuclease(s) responsible for internucleosomal DNA cleavage during apoptosis. (+info)