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Previous Article | Next Article 
Molecular and Cellular Biology, June 2005, p. 4742-4751, Vol. 25, No. 11
0270-7306/05/$08.00+0 doi:10.1128/MCB.25.11.4742-4751.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.
Division of Pulmonary, Allergy, and Critical Care Medicine, Department of Medicine, University of Pittsburgh Medical Center, Pittsburgh, Pennsylvania 15213
Received 30 October 2004/ Returned for modification 13 December 2004/ Accepted 28 February 2005
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ABSTRACT |
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, 
) during hypoxia/reoxygenation and promoted an association of inactive forms of PKC with Bax. Surprisingly, FLIP expression also inhibited death-inducing signal complex (DISC) formation in the plasma membrane and promoted the accumulation of the DISC in the Golgi apparatus. FLIP expression also upregulated Bcl-XL, an antiapoptotic protein. In conclusion, FLIP decreased DISC formation in the plasma membrane by blocking its translocation from the Golgi apparatus and inhibited Bax activation through a novel PKC-dependent mechanism. The inhibitory effects of FLIP on Bax activation and plasma membrane DISC formation may play significant roles in protecting endothelial cells from the lethal effects of hypoxia/reoxygenation. |
INTRODUCTION |
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Two main apoptotic pathways have been defined in endothelial cells, an extrinsic pathway mediated by death receptor family proteins (i.e., Fas) and an intrinsic pathway mediated by the mitochondria. We have previously demonstrated that the initiation of the apoptotic process of cell death by H/R involves both pathways (40, 41). The expression of Fas ligand (FasL) during H/R may trigger Fas-dependent death pathways (37, 14), characterized by the formation of the death-inducing signaling complex (DISC). The recruitment of caspase 8 to the DISC followed by its activation leads to the subsequent activation of effector caspases. The extrinsic pathway can be amplified by the caspase 8-dependent activation of Bid, leading to mitochondrial damage and release of apoptogenic factors. Since mammalian cells depend on mitochondria for long term viability, H/R may cause cell death through irreparable mitochondrial damage (27). In human fetal alveolar type II epithelial cells, the antiapoptotic proto-oncogene Bcl-2 displayed maximum abundance in hypoxia and mild reoxygenation. With increasing partial O2 pressure, the Bcl-2 expression declined with reciprocal increase in Bax, a proapoptotic Bcl-2 family member (9). Hypoxia also induced a time-dependent mitochondrial translocation of Bax, with the subsequent release of cytochrome c and apoptotic cell death upon reoxygenation (28). Bcl-2 and Bax reciprocally control apoptosis by inhibiting or stimulating, respectively, mitochondrial cytochrome c release. Cytosolic cytochrome c and Apaf-1 cooperatively activate initiator caspase 9, which triggers a caspase cascade leading to apoptosis (2).
FLIP, also known as Fas-associated death domain (FADD) interleukin-1ß-converting enzyme (FLICE)-like inhibitory protein has been characterized as an inhibitor of apoptosis induced by death receptors such as Fas or the tumor necrosis factor-related apoptosis-inducing ligand receptors. A human cellular homolog of v-FLIP was found and termed cellular FLICE-inhibitory protein (c-FLIP; also called FLAME-1, I-FLICE, Casper, CASH, MRIT, CLARP, and usurpin) (20, 25). The c-FLIP gene localizes to chromosome 2q33-34 in a cluster of 200 kb that includes caspase 8 and caspase 10, suggesting that these genes evolved by duplication (24). Multiple splice variants of c-FLIP have been found, but so far only two, designated c-FLIPS and c-FLIPL, could be detected at the protein level (20, 24). c-FLIPL contains tandem death effector domains and a caspase-like domain which lacks amino acid residues that are critical for caspase activity. c-FLIPS resembles its viral counterparts, consisting of two death effector domains and a truncated C terminus that differs from c-FLIPL (1). The mechanisms of cell death attenuation by c-FLIP are not completely understood. c-FLIP, acting as a potential competitive inhibitor, precludes the recruitment of caspase 8 to the DISC and thereby prevents its activation (1, 18, 20, 24-25). This hypothesis is supported by the fact that overexpression of the viral homolog v-FLIP-E8 led to its association with the DISC and the inhibition of caspase 8 recruitment and activation (1).
Previously, we have shown that overexpression of exogenous human FLIPL significantly inhibited cell death after H/R stress in mouse lung endothelial cells (MLEC) (41). In the same model, cytoprotection against H/R stress conferred by hepatocyte growth factor was associated, in part, with the stimulation of endogenous FLIP expression (41). Since the mechanisms by which FLIP protects against H/R-induced endothelial cell apoptosis are not fully understood, the present study aimed to define these in greater detail. Here, we found that, in addition to inhibiting the recruitment of caspase 8 to the DISC, FLIPL can provide protection against both extrinsic and intrinsic apoptotic pathways. FLIPL protected against extrinsic (Fas/caspase 8-dependent) apoptosis by blocking DISC translocation from the Golgi complex to the plasma membrane and also protected against intrinsic (Bax/mitochondria-dependent) apoptosis by inhibiting Bax activation through a protein kinase C (PKC)-dependent pathway.
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MATERIALS AND METHODS |
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(H7), and anti-PKC (H1) were purchased from Santa Cruz Biotechnology, Inc. (Santa Cruz, CA). Anti-Bax 6A7 antibody was from BD PharMingen (San Diego, CA). Adenovirus-human FLIPL (Hu-FLIPL), adenovirus-lacZ, and adenovirus-crmA were supplied by the Center for Biotechnology and Bioengineering, University of Pittsburgh (www.vectorcore.pitt.edu). The protein kinase C inhibitor GF109203X, digitonin, and all other chemicals were from Sigma (St. Louis, MO). Isolation and culture of MLEC. The isolation of endothelial cells by an immunobead protocol has been reported elsewhere (41). Briefly, mouse lungs were digested in collagenase, filtered through 100-µm cell strainers, centrifuged, and washed twice with medium. Cell suspensions were incubated with a monoclonal antibody (rat anti-mouse) against platelet endothelial cell adhesion molecule 1 for 30 min at 4°C. The cells were washed twice with buffer to remove unbound antibody and were resuspended in a binding buffer containing washed magnetic beads coated with sheep anti-rat immunoglobulin G. Attached cells were washed four to five times in cell culture medium and then were digested with trypsin-EDTA to detach the beads. Bead-free cells were centrifuged and resuspended for culture. After two passages, the cells were incubated with fluorescent-labeled diacetylated low-density lipoprotein, which is taken up only by endothelial cells and macrophages and sorted to homogeneity by fluorescence-activated cell sorting.
Cell culture and treatments. The MLEC were cultured in Dulbecco's modified Eagle's medium containing 10% fetal bovine serum, 6.32 g/liter HEPES, and 3.3 ml of endothelial cell growth supplements in humidified incubators at 37°C. For adenoviral infections, the cells were grown to 30% confluence and changed to serum-free medium containing 106 PFU/ml of an adenoviral vector inserted with Hu-FLIPL, lacZ, or crmA. Infected cells were incubated for 3 h and then restored to normal Dulbecco's modified Eagle's medium containing 10% fetal calf serum for an additional 2 days of incubation. For hypoxia treatment, the MLEC were grown to 90% confluence, changed to fresh medium, and transferred to an anaerobic chamber (COY Laboratory Products, Inc., Ann Arbor, MI) with 95% N2, 5% CO2 for 24 h. After hypoxia incubation, the cells were restored to 95% air, 5% CO2 for various reoxygenation intervals.
siRNA transfection. A small interfering RNA (siRNA) transfection kit corresponding to Bax (mouse) was purchased from Santa Cruz Biotechnology (Santa Cruz, CA). MLEC were transfected with Bax siRNA or the control siRNA 36 h prior to H/R treatment according to the manufacturer's protocol.
Cell fractionation. For mitochondrial isolation, at various times following exposure to H/R, MLEC were harvested in 0.05% digitonin in an extraction buffer containing 50 mM HEPES, pH 7.5, 50 mM KCl, 5 mM EGTA, and 2 mM MgCl2 with protease inhibitors. The cell extracts were spun at 700 x g for 10 min. The supernatants were transferred to new tubes and then centrifuged again at 14,000 x g at 4°C for 20 min. The resulting supernatants were removed and the pellets retained for Western blotting.
For total membrane isolation, MLEC were harvested in MB buffer (20 mM HEPES, pH 7.5, 1.5 mM MgCI2, 10 mM KCl, 1 mM EDTA, 1 mM dichlorodiphenyltrichloroethane, 250 mM sucrose) containing protease inhibitors and homogenized in a 1.5-ml Dounce homogenizer. Nuclei and unbroken cells were removed by centrifugation for 10 min at 500 x g, and the supernatants were centrifuged at 100,000 x g for 60 min at 4°C. The resulting pellet contained total cellular membranes.
For plasma membrane isolation, the homogenates of the cells in MBS [25 mM 2-(N-morpholino)-ethanesulfonic acid (MES), pH 6.5, 0.15 M NaCl] containing 1% Triton X-100 were adjusted to 40% sucrose by the addition of 2 ml of 80% sucrose prepared in MBS and placed at the bottom of an ultracentrifuge tube. A 5 to 30% discontinuous sucrose gradient was formed above (4 ml of 5% sucrose/4 ml of 30% sucrose, both in MBS lacking detergent) and centrifuged at 39,000 rpm for 18 h in an SW41 rotor (Beckman Instruments, Palo Alto, CA). A band at the interface of 5% and 30% sucrose was collected and used for immunoprecipitation and Western blotting. After direct addition of substrate solution (p-nitrophenyl phosphate; R&D Systems, Minneapolis, MN), the absorbance at 405 nm was measured.
The Golgi complex was isolated using sucrose density gradient centrifugation, as described elsewhere (12). After washing with phosphate-buffered saline, the cells were harvested in G buffer (10 mM Tris-HCl, 0.25 M sucrose, 2 mM MgCl2, pH 7.4) containing 10 mM CaCl2 and proteinase inhibitors. The cells were disrupted with 20 strokes in a Potter-type homogenizer. The homogenate was centrifuged at 2,500 x g for 10 min, and the pellet was discarded. The resulting postnuclear supernatant was harvested, and the sucrose concentration was adjusted to a 1.4 M final concentration. This suspension was loaded onto the bottom of an ultracentrifuge tube and overlaid in succession with 1.2 M, 1.0 M, and 0.8 M sucrose in G buffer. Samples were then centrifuged at 95,000 x g for 2.5 h. Two bands from the top, representing 0.8/1.0 and 1.0/1.2 M interfaces, were carefully removed, diluted with G buffer without sucrose, collected by centrifugation at 80,000 x g for 30 min, and used for the experiments. Golgi fraction purity was assessed by enzymatic activity and immunoblotting methods as previously described (40).
Western blot analysis. Proteins were isolated from the culture of MLEC with radioimmunoprecipitation assay buffer (1x phosphate-buffered saline, 1% [vol/vol] Nonidet P-40, 0.5% [wt/vol] sodium deoxycholate, 0.1% [wt/vol] sodium dodecyl sulfate [SDS], 0.1 mg/ml phenylmethylsulfonyl fluoride, 30 µl/ml aprotinin, and 1 mM sodium orthovanadate). For immunoprecipitation, 1 µg of anti-Fas antibody was added to 500 µg of total protein in 500 µl, rotated for 2 h at 4°C, incubated with 20 µl of beads (protein A-sucrose; Santa Cruz Biotechnology, Inc., Santa Cruz, CA) for another 2 h, spun down at 500 x g, and washed three times with radioimmunoprecipitation assay buffer. Then, 20 µl of loading buffer (100 mM Tris-HCl, 200 mM dithiothreitol, 4% SDS, 0.2% bromophenol blue, and 20% glycerol) was added. For SDS-polyacrylamide gel electrophoresis, samples containing equal amounts of protein were boiled in the loading buffer and separated by SDS-polyacrylamide gel electrophoresis, followed by transfer to polyvinylidene difluoride membranes. The membranes were blocked with 5% nonfat milk and stained with the primary antibodies for 2 h at the optimal concentrations. After five washes in phosphate-buffered saline with 0.2% Tween 20, the horseradish peroxidase-conjugated secondary antibody was applied and the blot was developed with enhanced chemiluminescence reagents (Amersham Biosciences, Piscataway, NJ).
LDH release assay. Lactate dehydrogenase (LDH) release was measured using a commercially available assay (cytotoxicity detection kit; Roche Molecular Biochemicals, Indianapolis, IN). After gentle agitation, 200 µl of medium was removed at different times to be used for the assay. The samples were incubated (30 min) with buffer containing NAD+, lactate, and tetrazolium. LDH converts lactate to pyruvate, generating NADH. The NADH then reduces tetrazolium (yellow) to formazan (red), which was detected by absorption at 490 nm.
Statistical analysis. All values are expressed as means ± standard errors. Statistical significance was determined by the Student t test, and a value of P < 0.05 was considered significant.
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RESULTS |
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-specific antibody (data not shown). A series of additional PKC isoforms were tested, including ß, 
, 
, 
, 
, , and 
. Of these, only PKC was strongly activated by H/R treatment (Fig. 4B). FLIP expression suppressed the activation and membrane translocation of PKC (Fig. 4B). Under normal conditions, PKC associates with Bax in MLEC. A reduced level of PKC associated with Bax was detected after 24 h of hypoxia, and the level declined further during the first 6 h of reoxygenation (Fig. 4C). The activation of Bax reciprocally increased during the first 3 h of reoxygenation, suggesting that PKC binding to Bax might block Bax apoptotic activity (18).
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FLIP inhibits DISC formation in the plasma membrane. The relative contribution of extrinsic apoptotic pathways to H/R-induced cell death has been examined previously (40, 41). Since FLIP is a known inhibitor of caspase 8, we examined the involvement of caspase 8 processing in H/R-induced cell death. MLEC were infected with adenovirus containing lacZ or the caspase 8 inhibitor crmA for 2 days, exposed to hypoxia for 24 h, and then restored to normoxic culture conditions for various reoxygenation intervals. Inhibition of caspase 8 by CrmA expression caused a significant decrease in H/R-induced cell death during 3 to 12 h of reoxygenation compared to lacZ-infected controls (Fig. 5A).
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To examine the effect of FLIP expression on the subcellular localization of the DISC, MLEC infected with FLIP or lacZ were subjected to H/R treatment and then further separated into Golgi or plasma membrane fractions. MLEC infected with FLIP displayed a greater accumulation of the DISC in the Golgi fraction following H/R treatment relative to lacZ-infected controls (Fig. 5B). In contrast, MLEC infected with FLIP displayed a diminished accumulation of the DISC in the plasma membrane fraction following H/R treatment relative to lacZ-infected controls (Fig. 5C).
FLIP stabilizes Bcl-XL protein after reoxygenation. Bcl-XL can block mitochondrial release of cytochrome c induced by Bax or Bid. Bcl-XL was expressed in normal MLEC after 24 h of hypoxia and decayed to background levels during the first 6 h of reoxygenation (Fig. 6). Infection of MLEC with FLIP stabilized Bcl-XL protein expression for at least 12 h postreoxygenation relative to the lacZ-infected controls (Fig. 6). These results suggest that another mechanism of FLIP-mediated cytoprotection in MLEC may involve the enhanced expression of Bcl-XL, which inhibits Bax- and Bid-induced apoptotic cell death despite the mitochondrial translocation of these species.
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DISCUSSION |
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Previously, we presented evidence that H/R stress activated the Fas-mediated apoptotic pathway in MLEC, especially at early time points (40, 41). In the present model of the extrinsic apoptotic pathway, FADD binds to the receptor Fas, leading to the recruitment of procaspase 8 to form the DISC. The association of caspase 8 with the DISC leads to its autocleavage and activation. Caspase 8 in turn cleaves effector caspases as well as Bid, which stimulates loss of cytochrome c from the mitochondria. Recent studies show that the Bid-mediated mitochondrial pathway is critical to ischemic neuronal apoptosis and focal cerebral ischemia in a murine model (23, 43). Interestingly, the FLIP proteins, which effectively resemble catalytically inactive procaspase 8 molecules, act as important regulators of death receptor-induced apoptosis. Increased levels of FLIP can confer protection against Fas-induced apoptosis. Both FLIPL and FLIPS can be recruited to the DISC, but they function differently. FLIPS prevents the initial cleavage step of caspase 8 activation between the p20 and p10 subunits of the caspase homology domain, whereas FLIPL inhibits the final cleavage between the prodomain and the p20 subunit of the p43/41 intermediate (16). However, recent reports show that FLIPL, when recruited to the DISC, can also potentially promote caspase 8 activation by dimerizing with caspase 8 (33).
Cells can express Fas both intracellularly and at the cell surface. Cytoplasmic Fas localizes in the Golgi apparatus (14), a series of functionally distinct processing compartments that plays a critical role in the biosynthesis and secretion of macromolecules in eukaryotic cells (44). Other DISC components, such as FADD and caspase 8 proteins may also localize within the Golgi complex for processing and then transfer to the plasma membrane (29, 38, 13). After treatment of MLECs with H/R, we have shown that the DISC can form within the Golgi complex (Fig. 5B) and translocate to the plasma membrane. Here, we also demonstrate that FLIPL may detain the DISC in the Golgi complex (Fig. 5B). We and others have previously shown that other apoptosis-related molecules, such as FAP-1 (13) or Bcl-XL (40), also block DISC translocation.
H/R treatment enhanced Bax translocation in MLEC without significantly changing total Bax protein levels (41). The modulation of Bax translocation during H/R is not understood. Bax is subject to inhibition by Bcl-2 and/or Bcl-XL, which block cytochrome c release from the mitochondria (28). On the other hand, Bid promotes Bax-dependent apoptosis by facilitating a conformational change in Bax associated with its activation (8, 39). Here, the results indicate that FLIP may block Bax activation and its translocation (Fig. 2). The mechanism for this inhibition apparently occurs independently of cross-regulation from Bid, since the effect was sustained in Bid–/– MLEC (Fig. 3B).
PKC comprises a family of Ser/Thr kinases which promote cell survival and protect against cell death. The PKC family consists of 12 closely related isozymes that can be divided into three groups determined by their requirements for activation, the conventional PKCs (, ß1, ß2, and ), the novel PKCs (, , 
, and ), and the atypical PKCs (, , and 
) (21). Various studies have implicated the involvement of PKC in the activation of phosphatidylinositol 3-kinase (35) and the mitogen-activated protein kinase pathway (10, 11, 26, 35) as well as inactivation of the Bcl-XL/Bcl-2-associated death promoter during the inhibition of FAS-mediated signaling (36). PKC upregulates FLIP (34), downregulates caspase 8, and regulates FADD recruitment and DISC formation (7). An investigation into the biological significance of the association of PKC with Bax provided the first evidence of an inverse relationship between the endogenous levels of PKC and the apoptotic effects of phorbol esters (19).
PKC associates with Bax in MLEC under normal culture conditions, which may inhibit the Bax-dependent apoptotic pathway (Fig. 4C and 7).
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and isoforms (Fig. 4A and B). The overexpression of FLIP inhibits the expression and plasma membrane translocation of PKC/ under H/R. The inactive forms of PKC, which potentially include PKC and (Fig. 4A and B) but not the other isoforms tested (ß, , , , , ) (data not shown), bind to Bax to suppress Bax activity induced by H/R. Currently, we do not understand the mechanism by which FLIP inhibits PKC translocation, and this warrants further investigation. Overexpression of FLIP also stabilizes Bcl-XL (Fig. 6), which is downregulated by H/R. In our laboratory, we have previously found that Bcl-XL inhibits both apoptotic pathways (40). Bcl-XL inhibits Bax activation and Bax redistribution. Furthermore, Bcl-XL inactivates caspase 8 by disrupting DISC formation in the plasma membrane, Golgi apparatus, and nucleus, while diverting the DISC to the mitochondria where caspase 8 is inactivated. Bcl-XL breaks the physical association of Fas and caspase 8 with GRASP65, a Golgi apparatus-related protein, suggesting that Bcl-XL decreases the transfer of Fas and caspase 8 to the plasma membrane by the Golgi component, while promoting its accumulation in the mitochondria. We currently do not understand the mechanism by which FLIP may regulate Bcl-XL.
The extracellular signal-regulated kinases ERK1/2 respond to stimulation by growth-related signals and play a key role in the regulation of many cellular processes, such as cell growth and proliferation, differentiation, and apoptosis. Hypoxia can induce PKC-dependent ERK1/2 activation (6). The ERK1/2 signaling pathway can induce Bcl-2 and Bcl-XL expression and inhibit caspase 3 activation (4). This pathway is also involved in modulating the activity of antiapoptotic molecules, including the phosphorylation and inactivation of Bcl-2 (31). In activated human T lymphocytes, Fas-recruited FLIP activated the ERK1/2 signaling pathway and the NF-
B pathway through interactions with tumor necrosis factor receptor-associated factors 1 and 2 as well as with the kinases RIP and Raf-1 (15). In our experimental model, FLIP activated the ERK1/2 signaling pathway during H/R (data not shown), which is consistent with previous studies. However, FLIP also inhibits PKC activation, suggesting that the activation of ERK1/2 by FLIP does not depend on PKC activities. Furthermore, FLIP expression did not have an apparent effect on cell growth over 3 days (data not shown).
In summary, H/R induces mouse lung endothelial cell death by activating both extrinsic (Fas/caspase 8/Bid-dependent) and intrinsic (Bax/mitochondrion-dependent) apoptotic signaling pathways (Fig. 7). FLIP, an inhibitor of caspase 8, can inhibit both apoptotic pathways in the context of H/R stress. In addition to inhibiting the recruitment of caspase 8 into the DISC by decreasing DISC formation in the plasma membrane, FLIP blocks the transfer of the DISC formed in the Golgi apparatus to the plasma membrane. FLIP expression also inhibits Bax activation and Bax-induced apoptotic cell death by promoting the association of the inactive form of PKC to Bax, which inactivates Bax.
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ACKNOWLEDGMENTS |
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This work was supported by an award from the American Heart Association (AHA no. 0335035N) to S.W.R. and by NIH grants R01-HL60234, R01-AI42365, and R01-HL55330 awarded to A.M.K.C.
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FOOTNOTES |
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