Daphnetin reduces endotoxin lethality in mice and decreases LPS-induced inflammation in Raw264.7 cells via suppressing JAK/STATs activation and ROS production

Objective

Here, we used various approaches to investigate the suppressive role of daphnetin in LPS-induced inflammatory response, with the goal to understand the underlining molecular mechanism by which daphnetin regulated these processes.

Methods

We examined the survival rate and the lung injury in the mice model of LPS-induced endotoxemia. The production of pro-inflammatory factors including tumor necrosis factor α (TNF-α), interleukin-1β (IL-1β), IL-6, nitric oxide (NO), and prostaglandin E2 (PGE2) was measured by ELISA and nitrite analysis, respectively. The expression of inducible NO synthase (iNOS), cyclooxygenase 2 (COX-2), and the activation of signaling molecules was determined by immunoblotting. The production of reactive oxygen species (ROS) was measured by the ROS assay.

Results

In vivo study showed that daphnetin enhanced the survival rate and reduced the lung injury in mice with LPS-induced endotoxemia. Both in vivo and in vitro study showed that daphnetin prevented the production of pro-inflammatory factors including TNF-α, IL-1β, IL-6, NO, and PGE2 after LPS challenge. In Raw264.7 cells, we found that daphnetin reduced LPS-induced expression of iNOS and COX-2, and suppressed LPS-induced ROS production. In addition, we found that daphnetin suppressed the activation of JAK/STATs pathway and inhibited the nucleus import of STAT1 and STAT3.

Conclusions

Here, our results indicate that daphnetin shows anti-inflammatory properties, at least in part, through suppressing LPS-induced activation of JAK/STATs cascades and ROS production.

     

Effect of anionic and cationic n-butylcyanoacrylate nanoparticles on NO and cytokine production in Raw264.7 cells

Context: Research on drug delivery carriers that use nanoparticles is currently attracting a great deal of attention. In order to evaluate the safety of these drug carriers for clinical applications, a full assessment of their toxicity and bioactivity is required. Although it is well-known that the surface charge of nanoparticles influences their bioactivity, most of the published studies on n-butylcyanoacrylate (NBCA) nanoparticles as a potential drug delivery carrier are restricted to analyzing the anionic form.

Objective: We compared biological responses of cyanoacrylate anionic nanoparticles with cationic nanoparticles in cultured murine macrophages for assessing cytotoxicity and inflammatory responses.

Materials and methods: The cytotoxicity was evaluated with the MTS and LDH leakage assays. Inflammatory responses were evaluated by measurement of nitric oxide (NO), interleukin-6 (IL-6), and tumor necrosis factor alpha (TNF-α). The interaction of the nanoparticle and the macrophage was assessed by fluorescence microscopy.

Result: Anionic and cationic NP showed no detectable cytotoxicity at a concentration of 10 µg/mL or less. At concentrations greater than 10 µg/mL, cationic NP displayed lower cytotoxicity by comparison with anionic NP. NO, IL-6, and TNF-α production were not induced by anionic or cationic nanoparticles alone. In contrast, both types of nanoparticles decreased NO, IL-6, and TNF-α productions induced by LPS. However, the anti-inflammatory effect of anionic nanoparticles was significantly greater than that of cationic nanoparticles. Nanoparticles were presumed to be either internalized or attached to the cell membranes.

Discussion and conclusion: NBCA nanoparticles are not only important as potential drug carriers but also as promising anti-inflammatory agents that may have therapeutic properties.     

Lysosomal ceramide mediates gemcitabine-induced death of glioma cells

Acid sphingomyelinase-induced ceramide release has been shown by many studies to induce apoptosis in response to various stimuli. However, the mechanisms of acid sphingomyelinase/ceramide-mediated death signaling following treatment with chemotherapeutic drugs have not been fully elucidated thus far. The present study demonstrates that treatment of glioma cells with clinically achievable doses of gemcitabine results in acid sphingomyelinase activation, lysosomal accumulation of ceramide, cathepsin D activation, Bax insertion into the mitochondria, and cell death. Pharmacological inhibition or genetic deficiency of acid sphingomyelinase prevented these events while overexpression of the enzyme sensitized cells to gemcitabine. Likewise, inhibitors of lysosomal functions also prevent gemcitabine-induced cell death. Our data indicate a critical role of the acid sphingomyelinase/ceramide system for gemcitabine-induced signaling and suggest that lysosomal ceramide accumulation mediates cell death induced by a chemotherapeutic drug.     

神经酰胺在γδ+T细胞活化及活化诱导的细胞死亡信号传导过程中的作用

目的 研究神经酰胺在γδ^ T细胞活化及活化诱导的细胞死亡信号传导过程中的作用。方法 用秸核杆菌耐热性多肽抗原（Mtb)刺激正常人PBMC，并用磁珠阳性分选法获得γδ^ T细胞。通过MTT试验观察Mtb，神经酰胺、鞘磷酯脂以及F B1cf γδ^ T细胞的活化作用；FACS检测其对γδ^ T细胞活化诱导的细胞死亡作用。结果 Mtb刺获得的γδ^ T细胞可达73％，磁珠分选后可高达98％；高浓度Mtb，神经酰胺及鞘磷脂酶能抑制γδ^ T细胞的活化增殖，而出现活化诱导的细胞死亡，Fumonisin B1则对γδ^ T细胞的活化增殖及活化细胞死亡无明显影响。结论 Mtb可有效地调节γδ^ T细胞活化及活化诱导的细胞死亡，水解鞘磷脂产生的神经酰胺参与了γδ^ T细胞活化及活化诱导的细胞死亡传导。     

白藜芦醇抑制脂多糖诱导破骨前体细胞Raw264.7的激活

目的探讨白藜芦醇(Res)对脂多糖(LPS)诱导的破骨前体细胞Raw 264.7细胞系释放炎性细胞因子的抑制作用。方法采用LPS刺激Raw 264.7细胞构建炎症模型,采用抗酒石酸酸性磷酸酶(TRAP)染色鉴定细胞,采用MTT检测Res对Raw 264.7细胞的毒性影响,免疫荧光双标以及反转录聚合酶链反应(RT-PCR)方法检测不同浓度Res(1μmol/L和5μmol/L)对细胞炎性因子:肿瘤坏死因子-α(TNF-α)和白细胞介素-1β(IL-1β)与细胞炎性蛋白酶:诱导型一氧化氮合酶(i NOS)和环氧合酶-2(COX-2)、炎性信号蛋白核因子-κB(NF-κB)蛋白与mRNA的表达变化。结果不同浓度的Res(1μmol/L和5μmol/L)在翻译水平和转录水平上明显抑制了LPS诱导的细胞炎性蛋白酶i NOS和COX-2表达,同时了抑制细胞炎性因子IL-1β与炎性信号蛋白NF-κB的上调。结论 Res可能通过NF-κB调控LPS诱导的Raw 264.7细胞炎性细胞因子的释放进而抑制破骨前体细胞的激活,进而具有抗骨质疏松的作用。     

肺炎支原体经ROS激活NLRP3炎性体诱导RAW264.7细胞分泌IL-1β

 目的: 观察肺炎支原体(Mycoplasma pneunoniae,Mp)促进NLRP3炎性体的活化及下游促炎性细胞因子白细胞介素-1β(IL-1β)的表达是否与活性氧簇(ROS)有关。方法: 预先用5 mmol/L ROS清除剂N-乙酰半胱氨酸(NAC)预处理RAW264.7细胞30 min,用10 MOI Mp分别感染RAW264.7细胞4、8、16和24 h。流式细胞术检测细胞内ROS水平;real-time PCR法检测细胞NLRP3、ASC和caspase-1 mRNA的表达;Western blot 法检测NLRP3、ASC和caspase-1 p20的蛋白水平;ELISA法检测细胞上清液IL-1β的分泌水平。结果: 与正常组比较,感染后4、8、16和24 h模型组ROS生成显著增加(P< 0.01);感染后8、16和24 h模型组NLRP3、ASC和caspase-1 mRNA的表达显著增加 (P<0.01),感染后16和24 h NLRP3、ASC和caspase-1 p20的蛋白水平上调(P<0.01),感染后24 h 细胞上清液IL-1β含量显著增加(P< 0.01);与模型组比较,NAC组在上述相应时点ROS生成降低,NLRP3、ASC和caspase-1 mRNA的表达下调,蛋白水平下降,细胞上清液IL-1β的含量减少。结论: Mp可能通过刺激RAW264.7细胞生成ROS而激活NLRP3炎性体。     

Effect of N-acetyl-L-cysteine on ROS production and cell death caused by HEMA in human primary gingival fibroblasts

Previous investigations have shown that 2-hydroxyethyl methacrylate (HEMA) causes reactive oxygen species (ROS) production, which in turn affects cell survival and cell death. The purpose of this study was to evaluate the effects of the antioxidant N-acetyl-L-cysteine (NAC) on HEMA-induced toxicity in human primary gingival fibroblasts (HGF). HGF were treated with various concentrations of HEMA (0-12 mm) in the absence and presence of NAC (1, 5, and 10 mm). The 3-(4,5 dimethyiazol-2-1)-2-5-diphenyl tetrazolium bromide (MTT) assay was used to evaluate the mitochondrial dehydrogenase activity after HEMA exposure. Viability and cell death were determined by flow cytometry using Annexin V and PI staining. ROS production was detected by the increasing fluorescence of the oxidation-sensitive dye 2',7'-dichlorofluorescein diacetate (DCFH-DA) after HEMA treatment. After a 24h incubation period, HEMA concentrations higher then 10mm caused a decrease of cell viability, mitochondrial activity, and an increase of cell death. HEMA concentrations of 4-12 mm markedly increased ROS levels in a dose-dependent manner. High NAC concentrations (5 and 10 mm) significantly reduced cell death, and restored the mitochondrial activity after a 24 h co-treatment, but 1 mm NAC increased HEMA toxicity (p<0.05). All NAC concentrations significantly reduced ROS levels induced by HEMA after a 2 h exposure (p<0.05), but no such reduction was observed after a 4 h treatment. Furthermore, treatment with 10 mm HEMA and 1 mm NAC for 6h caused an increase in ROS levels compared to 10 mm HEMA alone (p<0.05). In conclusion, our results suggest that high NAC concentrations protect HGF against HEMA cytotoxicity by reducing the induced ROS levels.     

神经酰胺在γδ+T细胞活化及活化诱导的细胞死亡信号传导过程中的作用

目的研究神经酰胺在γδ+T细胞活化及活化诱导的细胞死亡信号传导过程中的作用。方法用结核杆菌耐热性多肽抗原(Mtb)刺激正常人PBMC,并用磁珠阳性分选 法获得γδ+T细胞。通过MTT试验观察Mtb、神经酰胺、鞘磷脂酶以及FumonisinB1对γδ+T细胞的活化作用;FACS检测其对γδ+T细胞活化诱导的细胞死亡作用。结果Mtb刺激获得的γδ+T细胞可达73％,磁珠分选后可高达98％;高浓度Mtb、神经酰胺及鞘磷脂酶能抑制γδ+T细胞的活化增殖,而出现活化诱导的细胞死亡,FumonisinB1则对γδ+T细胞的活化增殖及活化诱导的细胞死亡无明显影 响。结论Mtb可有效地调节γδ+T细胞活化及活化诱导的细胞死亡,水解鞘磷脂产生的神经酰胺参与了γδ+T细胞活化及活化诱导的细胞死亡信号传导。     

低强度超声联合微泡通过提高ROS水平促进甲状腺癌细胞自噬性死亡

目的:探讨低强度超声联合微泡造影剂对甲状腺癌细胞自噬性死亡的作用,并分析自噬的激活机制及其对细胞活力的影响。方法:采用频率20 k Hz、功率80 m W的低强度超声联合微泡造影剂处理人甲状腺癌TPC1细胞,处理细胞60、120和240 s后,采用Live/Dead实验和CCK-8实验分析细胞死亡和活力;Western blot分析微管相关蛋白1轻链3Ⅱ(microtubule-associated protein 1 light chain 3-Ⅱ,LC3-Ⅱ)、自噬相关蛋白5(autophagy-related protein 5,ATG5)和SQSTM1/P62的蛋白水平变化;单丹磺酰戊二胺(monodansylcadaverine,MDC)染色、绿色荧光蛋白(green fluorescent protein,GFP)-LC3转染和透射电镜观察细胞内自噬体的数量;2',7'-二氯二氢荧光素二乙酸脂(DCFDA)染色分析细胞活性氧簇(reactive oxygen species,ROS)的水平,用N-乙酰半胱氨酸(N-acetyl-L-cysteine,NAC)抑制氧化应激水平,分析ROS在自噬激活中的作用;ATG5 siRNA转染抑制自噬水平并分析自噬性死亡的作用。结果:低强度超声联合微泡显著促进TPC1细胞死亡,抑制TPC1细胞活力(P0.05),并与处理时间显著相关。相对于单纯低强度超声组和微泡组,超声联合微泡显著升高LC3-Ⅱ和ATG5蛋白水平,抑制P62蛋白水平(P0.05)。MDC染色、GFP-LC3转染和透射电镜观察发现,超声联合微泡明显增加TPC1细胞中自噬体的数量。超声联合微泡与单纯低强度超声组和微泡组相比,提高了细胞的ROS水平,而NAC显著降低超声联合微泡提高的LC3-Ⅱ蛋白水平(P0.05)。ATG5 siRNA抑制自噬并显著增加细胞活力(P0.05)。结论:本研究说明低强度超声联合微泡可能通过提高甲状腺癌细胞中的ROS水平促进细胞的自噬性死亡,从而引起甲状腺癌细胞死亡。     

P38及上游激酶MKK6对热损伤诱导Raw264.7细胞凋亡的调控作用

目的探讨P38及其上游激酶MKK6对热损伤诱导单核细胞系Raw264.7细胞凋亡的调控作用。方法应用脂质体介导的基因转染将P38的显性负效突变体P38(AF)及具有持续活性的组成性活性突变体MKK6b(E)导入Raw264.7细胞,观察P38对细胞凋亡的调控作用。结果荧光染色及Westernblot检测显示,外源基因转染进细胞,并在细胞中得到表达;蛋白激酶活性检测显示,MKK6b(E)转染细胞系P38激酶活性明显升高,而P38(AF)转染细胞系P38激酶未被激活;流式细胞术检测显示,MKK6b(E)转染可明显诱导Raw264.7细胞凋亡,并且MKK6b(E)基因转染促进热损伤诱导的Raw264.7细胞凋亡,而P38(AF)基因转染抑制热损伤诱导的Raw264.7细胞凋亡。结论MKK6-P38信号通路参与介导热损伤诱导的Raw264.7细胞凋亡。     

Urokinase induces ROS production in vascular smooth muscle cells

Urokinase stimulates the production of superoxide radical in cultured aortal smooth muscle cells simultaneously with activation of the expression of NAD(F)H-oxidases nox1, nox4, and phox22. Antioxidant ebselen abolishes the stimulating effect of urokinase on smooth muscle cell proliferation. The data showed that urokinase can potentiate oxidative stress in the arterial wall and can play an important role in the development of adverse arterial remodeling. __________ Translated from Byulleten’ Eksperimental’noi Biologii i Meditsiny, Vol. 142, No. 9, pp. 273–276, September, 2006     

Mechanism and characteristics of stimuli-dependent ROS generation in undifferentiated HL-60 cells

It has been widely believed that undifferentiated human promyelocytic leukemia cells (HL-60) have no ability to generate reactive oxygen species (ROS) responding to stimuli. We report here that undifferentiated HL-60 cells possess NADPH oxidase and that generation of superoxide can be measured using a highly sensitive chemiluminescence dye, L-012. Five subunits of NADPH oxidase, namely, gp91(phox), p22(phox), p67(phox), p47(phox), and Rac 2, were detected in undifferentiated HL-60 cells by immunoblotting analysis. The contents of these NADPH oxidase components in the cells were increased with the differentiation induced by phorbol myristate acetate (PMA), except for p22(phox). Messenger RNAs of these subunits were also detected by the RT-PCR method, and their expressions increased except that of p22(phox) with the differentiation induced by PMA. Kinetic analysis using L-012 revealed that HL-60 cells generated substantial amounts of ROS by various stimulants, including formylmethionyl-leucyl-phenylalanine, PMA, myristic acid, and a Ca2+ ionophore, A23187. Both diphenyleneiodonium (an inhibitor of FAD-dependent oxidase) and apocynin (a specific inhibitor of NADPH oxidase) suppressed this stimuli-dependent ROS generation. Genistein, staurosporine, uric acid, and sodium azide inhibited the ROS generation in undifferentiated HL-60 cells in a similar way to that in undifferentiated neutrophils. These results suggested that the mechanism of ROS generation in undifferentiated HL-60 cells is the same as that in primed neutrophils.     

Increased ROS generation in subsets of OGG1 knockout fibroblast cells

Oxoguanine DNA glycosylase (OGG1) is a major base excision repair protein responsible for excision of the mutagenic 8-oxoguanosine (8-oxoG) lesions from the genome. Despite OGG1's importance, the moderate phenotype of Ogg1-null (Ogg1(-/-)) mice is not well understood. This study addresses a mechanism by which Ogg1(-/-) cells limit accumulation of 8-oxoG in their genome. Our data reveal that a subset of Ogg1(-/-) cells shows higher ROS levels ((H)ROS cells), while approximately 85% of Ogg1(-/-) cells exhibit physiological levels of ROS ((L)ROS cells). Ogg1(-/-) cells were sorted based on their DCF fluorescence intensity to obtain (L)ROS and (H)ROS cell cultures. (L)ROS cultures proliferated at a rate comparable to Ogg1(+/+) and gradually accumulated cells exhibiting increased ROS and 8-oxoG levels. (L)ROS cells show a 2.8-fold increase in 8-oxoG level vs. (H)ROS cells (7-27-fold). Mitochondria of (H)ROS cells released more H(2)O(2) than (L)ROS and Ogg1(+/+) cells and were eliminated by apoptotic-like processes. These findings suggest that in the absence of OGG1, a surveillance system is activated that removes cells with extreme 8-oxoG levels from Ogg1(-/-) cultures. Whether similar mechanisms exists in tissues of Ogg1(-/-) mice is the focus of future investigations.     

Life and death decisions: ceramide generation and EGF receptor trafficking are modulated by oxidative stress

Reactive oxidants are associated with the pathogenesis of pulmonary diseases and affect various cell functions, from proliferation to apoptosis. We have shown that oxidants exert growth control on airway epithelial cells by modulating upstream receptor function. Additionally, hydrogen peroxide-mediated oxidative stress modulates ceramide levels to induce apoptosis in lung epithelium. Depletion of glutathione in lung epithelial cells results in ceramide accumulation, suggesting that ceramide elevation, coupled to oxidative stress, initiates apoptosis. While it is desirable to prevent cell death and tissue injury induced by oxidants in diseases such as asthma or acute respiratory distress syndrome, the opposite is sought in cancer. But oxidants may also activate growth factor receptors, enhancing cell proliferation and facilitating tumor promotion. Under oxidative stress, phosphorylation of the epidermal growth factor receptor (EGFR) is abrogated at tyrosine 1,045, the docking site for the ubiquitin ligase c-Cbl, rendering EGFR unable to recruit c-Cbl and be ubiquitylated and degraded. We thus proposed that this deficiency, which confers prolonged receptor signaling at the plasma membrane, links oxidative stress, EGFR, and tumorigenesis. Decoding the molecular interactions between oxidative stress and ceramide pathways and characterizing ubiquitylation control of receptor desensitization should provide new strategies for intervention in diverse pulmonary diseases and in diagnosing and eradicating cancer.     

LPS影响p38蛋白在单核细胞内定位

目的： 探讨 LPS作用下p38蛋白激酶激活的动力学特点及其在细胞内超微结构中的定位。方法： 应用激酶活性测定、胶体金标记的免疫电镜技术观察LPS刺激前后p38蛋白激酶的动力学特点及在单核细胞株Raw264.7中的分布特征。结果： 动力学检测结果显示，LPS作用后15 min，p38磷酸化活性明显升高，30 min达到高峰，2 h达基线水平；p38在LPS浓度为100 μg/L时达最大激活效应。超微定位结果显示，未受刺激的及EGF刺激的细胞，p38在胞浆和胞核中金颗粒呈弥散性分布，金颗粒弥散在细胞的各个部分，如细胞质中线粒体、内质网、溶酶体；单核细胞株受到LPS刺激后，细胞核区的金颗粒明显增多，而胞浆区域的金颗粒显著减少。结论： 单核细胞株Raw264.7受LPS刺激后，其p38蛋白激酶由胞浆移位到胞核。     

Mcl-1短发夹RNA在Raw264.7细胞内特异性的筛选

目的:将Mcl-1shRNA转染到Raw264.7细胞内,针对shRNA对小鼠Raw264.7巨噬细胞系中Mcl-1表达的影响,筛选出沉默Mcl-1基因效果最明显的特异性shRNA真核表达质粒。方法:将特异性shRNA经脂质体介导转染小鼠巨噬细胞系Raw264.7;半定量RT-PCR和Western blot分别检测转染24、48 h后Mcl-1 mRNA水平变化和Mcl-1蛋白表达情况,分析对应不同位点的三对特异性shRNA片段对Mcl-1的沉默效果。结果:特异性shRNA片段在24、48 h均能有效降低Mcl-1 mRNA和蛋白水平,沉默效率高于正常组、脂质体组和阴性对照组,差异具有统计学意义(P0.05);对应不同位点的三对shRNA真核表达质粒,其中Mcl-1 shRNA3对Mcl-1 mRNA和蛋白的抑制作用均最强。结论:RNA干扰技术可有效下调小鼠Raw264.7巨噬细胞系中Mcl-1 mRNA水平,明显下调Mcl-1蛋白表达。成功筛选出了沉默Mcl-1基因效果最明显的特异性shRNA真核表达质粒。     

Stimulation of glutathione depletion, ROS production and cell cycle arrest of dental pulp cells and gingival epithelial cells by HEMA

2-Hydroxy-ethyl methacrylate (HEMA) is the major component released from resin-modified glass ionomer cements and dental adhesives. Human tissues mainly affected by HEMA are oral epithelium and dental pulp. We treated human gingival epithelial S-G cells and pulp fibroblasts (HPF) with various concentrations of HEMA, to evaluate its effects on cell growth, cell cycle progression, intracellular glutathione (GSH) level and reactive oxygen species (ROS) production. HEMA-induced growth inhibition in HPF and S-G cells in a dose-dependent manner, which may be partially explained by induction of cell cycle perturbation. G(2)/M phase arrest was noted after exposure of HPF to 5 and 10mm of HEMA, concomitant with glutathione depletion and ROS production. S-phase arrest occurred in S-G cells when treated with 2.5 and 5mm, while at 10mm a sub-G(0)/G(1) peak was noted, indicating the potential induction of apoptosis. GSH depletion was marked in S-G cells only at concentrations of 5 and 10mm, but excessive ROS production was noted at concentration of 1mm and rose with dose increase between 1 and 5mm, then lessened at 10mm. This suggested that the increase of ROS in S-G cells was not mainly caused by GSH depletion. These results helped to define the mechanism of the cytotoxicity caused by HEMA.     

Redox-sensitive events in Fas-induced apoptosis in human NK cells include ceramide generation and protein tyrosine dephosphorylation

We previously reported that intracellular oxidation-reduction (redox) regulates NK cell functions and that IL-2-activated NK cells undergo apoptosis upon contact with NK-sensitive target cells. We now report that apoptosis in activated human NK cells is also regulated by redox. Thiol deprivation increased apoptosis in NK cells induced by anti-Fas mAb or Fas ligand-transfected cells, and pretreatment of cells with N- acetyl cysteine, which increased intracellular glutathione, partially inhibited the apoptosis and reversed the effect of thiol-deficient medium, suggesting that Fas-induced apoptosis in NK cells is also redox sensitive. Thiol deprivation did not alter cell surface Fas expression, but did increase ceramide generation following Fas engagement. Although exogenous ceramides induced apoptosis of NK cells, thiol depletion had no effect on this apoptosis. Thiol deprivation increased CPP32 activation induced by Fas engagement, but not by ceramides. These findings suggest that, if ceramide is required for Fas-induced apoptosis, thiol deprivation affects the Fas-mediated signaling pathway at the generation of ceramide and/or upstream thereof. Though tyrosine phosphorylation following Fas engagement was not significantly affected by thiol deprivation, tyrosine dephosphorylation was delayed, suggesting that tyrosine phosphatases may also be redox sensitive. The notion that dephosphorylation is important in the Fas signaling pathway is supported by the finding that tyrosine phosphatase inhibitors significantly enhanced both CPP32 activity and apoptosis following Fas ligation. We conclude that events downstream of tyrosine phosphorylation and upstream of CPP32 activation, including tyrosine dephosphorylation and possibly ceramide generation, are sensitive to regulation by redox in human NK cells, requiring a reducing environment for optimal protection from apoptosis induced by Fas ligation.