c-FLIP Isoforms Research Articles

Myricetin sensitizes malignant glioma cells to TRAIL-mediated apoptosis by down-regulation of the short isoform of FLIP and bcl-2

The flavonoid Myricetin has been reported to exhibit therapeutic activity in cancer. In this study glioblastoma cells and human astrocytes were treated with Myricetin, tumour necrosis factor-related apoptosis-inducing ligand (TRAIL) or the combination of both. Treatment with subtoxic doses of Myricetin in combination with TRAIL induces rapid apoptosis in glioma cells. Notably, human astrocytes were not affected by the combined treatment consisting of Myricetin and TRAIL. Combined treatment with Myricetin and TRAIL augmented the activation of initiator caspases-8/-9 and effector caspases-3/-7. Furthermore, Myricetin down regulated the expression of the long and short isoform of c-FLIP and bcl-2 and over-expression of the short isoform of c-FLIP (S) and bcl-2 attenuated apoptosis induced by the combination of Myricetin and TRAIL. Furthermore, Myricetin did not modulate the mRNA levels of c-FLIP, suggesting that Myricetin modulates the expression of c-FLIP in a posttranscriptional manner. In summary, the short isoform of c-FLIP and bcl-2 are key regulators in TRAIL–Myricetin mediated cell death in malignant glioma.

S-Adenosylmethionine and Methylthioadenosine Inhibit Cellular FLICE Inhibitory Protein Expression and Induce Apoptosis in Colon Cancer Cells

S-Adenosylmethionine (SAMe) and its metabolite 5'-methylthioadenosine (MTA) inhibit mitogen-induced proliferative response in liver and colon cancer cells. SAMe and MTA are also proapoptotic in liver cancer cells by selectively inducing Bcl-x(S) expression. The aims of this work were to assess whether these agents are proapoptotic in colon cancer cells, and if so, to elucidate the molecular mechanisms. We found that both SAMe and MTA are proapoptotic in HT-29 and RKO cells in a dose- and time-dependent manner. Gene microarray uncovered down-regulation of cellular FLICE inhibitory protein (cFLIP). SAMe and MTA treatment led to a decrease in the mRNA and protein levels of both the long and short cFLIP isoforms. This required de novo RNA synthesis and was associated with activation of procaspase-8, Bid cleavage, and release of cytochrome c from the mitochondria. Inhibiting caspase 8 activity or overexpression of cFLIP protected against apoptosis, whereas supplementing with polyamines did not. SAMe and MTA treatment sensitized RKO cells to tumor necrosis factor alpha-related apoptosis-inducing ligand-induced apoptosis. Although SAMe and MTA are proapoptotic in colon cancer cells, they have no toxic effects in NCM460 cells, a normal colon epithelial cell line. In contrast to liver cancer cells, SAMe and MTA had no effect on Bcl-x(S) expression in colon cancer cells. In conclusion, SAMe and MTA are proapoptotic in colon cancer cells but not normal colon epithelial cells. One molecular mechanism identified is the inhibition of cFLIP expression. SAMe and MTA may be attractive agents in the chemoprevention and treatment of colon cancer.

PKC-mediated phosphorylation regulates c-FLIP ubiquitylation and stability

Cellular FLICE-inhibitory protein (c-FLIP) proteins are crucial regulators of the death-inducing signaling complex (DISC) and caspase-8 activation. To date, three c-FLIP isoforms with distinct functions and regulation have been identified. Our previous studies have shown that the stability of c-FLIP proteins is subject to isoform-specific regulation, but the underlying molecular mechanisms have not been known. Here, we identify serine 193 as a novel in vivo phosphorylation site of all c-FLIP proteins and demonstrate that S193 phosphorylation selectively influences the stability of the short c-FLIP isoforms, as S193D mutation inhibits the ubiquitylation and selectively prolongs the half-lives of c-FLIP short (c-FLIP(S)) and c-FLIP Raji (c-FLIP(R)). S193 phosphorylation also decreases the ubiquitylation of c-FLIP long (c-FLIP(L)) but, surprisingly, does not affect its stability, indicating that S193 phosphorylation has a different function in c-FLIP(L). The phosphorylation of this residue is operated by the protein kinase C (PKC), as S193 phosphorylation is markedly increased by treatment with 12-O-tetradecanoylphorbol-13-acetate and decreased by inhibition of PKCalpha and PKCbeta. S193 mutations do not affect the ability of c-FLIP to bind to the DISC, although S193 phosphorylation is increased by death receptor stimulation. Instead, S193 phosphorylation affects the intracellular level of c-FLIP(S), which then determines the sensitivity to death-receptor-mediated apoptosis. These results reveal that the differential stability of c-FLIP proteins is regulated in an isoform-specific manner by PKC-mediated phosphorylation.

Testis atrophy and reduced sperm motility in transgenic mice overexpressing c-FLIP L

To study the effect of c-FLIP overexpression in testicular germ cells. A novel transgenic mouse model overexpressing the apoptotic modulator c-FLIP in the testis was generated. Animal facility and university research laboratory. Transgenic mice overexpressing the long isoform of c-FLIP (c-FLIP(L)) under the transcriptional control of a 400 bp long regulatory region of the Stra8 promoter. Spermatozoa motility and testis histological, immunohistochemical, and Western blot analyses were carried out in transgenic and control derived specimens. Testis morphology, sperm motility, and germ cell apoptosis were assayed. Stra8 promoter was found to activate the ectopic overexpression of c-FLIP(L) in round and elongated spermatids. As a consequence of such overexpression, a dramatic loss of germ cells was observed, resulting in testicular atrophy associated with reduced sperm motility. The data show that c-FLIP(L) forced expression in haploid male germ cells has detrimental effects on spermatogenesis and sperm quality and reveal a possible mechanism underlying the onset of testicular atrophy.

Genistein enhances proteasomal degradation of the short isoform of FLIP in malignant glioma cells and thereby augments TRAIL-mediated apoptosis

Tumour necrosis factor-related apoptosis-inducing ligand (TRAIL) is a promising cancer drug. One obstacle in TRAIL-based therapies is that many cancer cells, including gliomas, are resistant towards TRAIL. In this study one glioblastoma cell line, one human short-term glioblastoma culture and human astrocytes were treated with genistein, tumour necrosis factor-related apoptosis-inducing ligand or the combination of both. Single treatment with genistein or TRAIL does not induce cytotoxicity in malignant glioma cells. However, treatment with genistein in combination with TRAIL induces rapid apoptosis in TRAIL-resistant glioma cells. Notably, normal human astrocytes were not affected by the combination treatment consisting of genistein and TRAIL. Genistein enhanced proteasomal degradation of the short isoform of c-FLIP. Importantly, over-expression of only the short isoform of c-FLIP attenuated genistein TRAIL-mediated cytotoxicity. Taken together, we gave evidence that genistein facilitated TRAIL-mediated apoptosis at the level of the extrinsic apoptotic pathways in malignant glioma cells.

Synergistic Proapoptotic Activity of Recombinant TRAIL Plus the Akt Inhibitor Perifosine in Acute Myelogenous Leukemia Cells

To potentiate the response of acute myelogenous leukemia (AML) cells to tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) cytotoxicity, we have examined the efficacy of a combination with perifosine, a novel phosphatidylinositol-3-kinase (PI3K)/Akt signaling inhibitor. The rationale for using such a combination is that perifosine was recently described to increase TRAIL-R2 receptor expression and decrease the cellular FLICE-inhibitory protein (cFLIP) in human lung cancer cell lines. Perifosine and TRAIL both induced cell death by apoptosis in the THP-1 AML cell line, which is characterized by constitutive PI3K/Akt activation, but lacks functional p53. Perifosine, at concentrations below IC(50), dephosphorylated Akt and increased TRAIL-R2 levels, as shown by Western blot, reverse transcription-PCR, and flow cytometric analysis. Perifosine also decreased the long isoform of cFLIP (cFLIP-L) and the X-linked inhibitor of apoptosis protein (XIAP) expression. Perifosine and TRAIL synergized to activate caspase-8 and induce apoptosis, which was blocked by a caspase-8-selective inhibitor. Up-regulation of TRAIL-R2 expression was dependent on a protein kinase Calpha/c-Jun-NH(2)-kinase 2/c-Jun signaling pathway activated by perifosine through reactive oxygen species production. Perifosine also synergized with TRAIL in primary AML cells displaying constitutive activation of the Akt pathway by inducing apoptosis, Akt dephosphorylation, TRAIL-R2 up-regulation, cFLIP-L and XIAP down-regulation, and c-Jun phosphorylation. The combined treatment negatively affected the clonogenic activity of CD34(+) cells from patients with AML. In contrast, CD34(+) cells from healthy donors were resistant to perifosine and TRAIL treatment. Our findings suggest that the combination of perifosine and TRAIL might offer a novel therapeutic strategy for AML.

CD95 Stimulation Results in the Formation of a Novel Death Effector Domain Protein-containing Complex

Stimulation of CD95 (APO-1/Fas) by its natural ligand CD95L (APO-1L/FasL) leads to the formation of the death-inducing signaling complex. Here we report that upon CD95 stimulation in several T and B cell lines, a novel signaling complex is formed, which we term complex II. Complex II is composed of the death effector domain proteins as follows: procaspase-8a/b, three isoforms of c-FLIP (c-FLIP(L), c-FLIP(S), c-FLIP(R)), and FADD. Notably, complex II does not contain CD95. Based on our findings we suggest that CD95 signaling includes two steps. The first step involves formation of the death-inducing signaling complex at the cell membrane. The second step involves formation of the cytosolic death effector domain protein-containing complex that may play an important role in amplification of caspase activation.

Roscovitine sensitizes breast cancer cells to TRAIL-induced apoptosis through a pleiotropic mechanism

The tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL/APO2L) is a member of the TNF gene superfamily that induces apoptosis upon engagement of cognate death receptors. While TRAIL is relatively non-toxic to normal cells, it selectively induces apoptosis in many transformed cells. Nevertheless, breast tumor cells are particularly resistant to the effects of TRAIL. Here we report that, in combination with the cyclin-dependent kinase inhibitor roscovitine, exposure to TRAIL induced marked apoptosis in the majority of TRAIL-resistant breast cancer cell lines examined. Roscovitine facilitated TRAIL death-inducing signaling complex formation and the activation of caspase-8. The cFLIP(L) and cFLIP(S) FLICE-inhibitory proteins were significantly down-regulated following exposure to roscovitine and, indeed, the knockdown of cFLIP isoforms by siRNA sensitized breast tumor cells to TRAIL-induced apoptosis. In addition, we demonstrate that roscovitine strongly suppressed Mcl-1 expression and up-regulated E2F1 protein levels in breast tumor cells. Significantly, the silencing of Mcl-1 by siRNA sensitized breast tumor cells to TRAIL-induced apoptosis. Furthermore, the knockdown of E2F1 protein by siRNA reduced the sensitizing effect of roscovitine in TRAIL-induced apoptosis. In summary, our results reveal a pleitropic mechanism for the pro-apoptotic influence of roscovitine, highlighting its potential as an antitumor agent in breast cancer in combination with TRAIL.

Mutational analyses of c-FLIPR, the only murine short FLIP isoform, reveal requirements for DISC recruitment

Cellular FLICE-inhibitory protein (c-FLIP) proteins are known as potent inhibitors of death receptor-mediated apoptosis by interfering with caspase-8 activation at the death-inducing signaling complex (DISC). Among the three human isoforms, c-FLIP(long), c-FLIP(short) and c-FLIP(R), the latter isoform is poorly characterized. We report here the characterization of murine c-FLIP(R) and show that it is the only short c-FLIP isoform expressed in mice. By generating several mutants, we demonstrate that both death effector domains (DEDs) are required for DISC binding and the antiapoptotic function of c-FLIP(R). Surprisingly, the C-terminal tail is important for both protein stability and DISC recruitment. Three-dimensional modeling of c-FLIP(R) revealed a substantial similarity of the overall structures and potential interaction motifs with the viral FLIP MC159. We found, however, that c-FLIP(R) uses different structural motifs for its DISC recruitment. Whereas MC159 interferes with interaction and self-oligomerization of the DISC component FADD by its extensive hydrophilic surface, a narrow hydrophobic patch of c-FLIP(R) on the surface of DED2 is crucial for DISC association. Thus, despite the presence of similar tandem DEDs, viral and cellular FLIPs inhibit apoptosis by remarkably divergent mechanisms.

Environmental Mediated-Immune Resistance (EM-IR) to APO2L/TRAIL Mediated Apoptosis.

Resistance to chemotherapy arises in a complex microenvironment and it is essential to study how it influences apoptosis in order to identify novel therapeutic targets. APO2L/TRAIL anti-tumor activity is attractive since it spares normal tissue. Previous studies in our laboratory have shown that cancer:stroma microenvironment interactions confer resistance to immune control via APO2L/TRAIL, a process referred to as environmental mediated-immune resistance (EM-IR). We studied EM-IR in multiple myeloma because it is a disease subject to bone marrow environmental influences and expresses the specific APO2L/TRAIL receptor(s). We have shown in a transwell assay, with myeloma (RPMI-8226, U266 and MM1s) in the upper well and HS5 stromal cells in the lower well, that APO2L/TRAIL (5 to 50ng/mL for 5 hours) apoptosis is reduced when compared to cells exposed to medium in the lower well. Additional studies using conditioned medium from HS5 and myeloma patient's stroma (n=3) grown for 14 days, confirmed that soluble factor(s) produced by tumor bone marrow microenvironment protected cells from APO2/TRAIL apoptosis. APO2/TRAIL resistance was reversible as sensitivity was restored after HS5 stroma cells were removed (20 min). APO2/TRAIL signaling pathway studies showed that baseline levels of c-FLIP, an anti-apoptotic factor, increased in RPMI-8226 (2.86 fold), in U266 (9.5 fold increase) and in MM1s (1.34 fold increase) myeloma cell lines. Inhibition of c-FLIP by means of RNA interference using siRNA duplex that targeted both c-FLIP isoforms, c-FLIP long and short, increased APO2/TRAIL sensitivity of RPMI-8226 treated in the presence of HS5 stroma. Sub-cellular fractionation showed that c-FLIP is maintained in the membrane fraction and is not significantly increased in the cytosol when exposed to soluble factor(s). Pre-treatment (19 hours) with cyclohexamide, a protein synthesis inhibitor, restored APO2L/TRAIL sensitivity in association with down regulation of c-FLIP while maintained anti-apoptotic c-IAP, Bcl2 and Mcl-1 levels suggesting that c-FLIP synthesis, not intracellular traffic, is essential for soluble factors to regulate c-FLIP. To dissect which soluble factor(s) are involved in apoptosis resistance we have tested the effects of HS27a stroma cells. HS27a lack IL-6 and/or IL-1 production compared to HS5 stroma and do not confer APO2/TRAIL resistance suggesting that these cytokines may be involved in apoptosis resistance. Treatment with neutralizing IL-6 specific antibody of HS5 transwell decreased IL-6 levels and partially restored sensitivity to APO2L/TRAIL in myeloma. Treatment with rh-IL-6 (0.001ng/mL to 10ng/mL) with APO2L/TRAIL conferred a dose-dependent resistance to APO2L/TRAIL in myeloma cells associated with increased c-FLIP levels. IL-1 receptor antagonist (IL-1ra) either by pre-treatment (1hour–18 hours) or co-administration with APO2L/TRAIL did not reverse APO2L/TRAIL resistance. These findings suggest that IL-6 in part mediates APO2L/TRAIL EM-IR by increasing c-FLIP levels among other unknown mechanisms. We conclude that c-FLIP up-regulation inhibitors and/or targeting hematopoietic soluble factor(s) may contribute to enhanced APO2/TRAIL function.

Overexpression of cFLIP in head and neck squamous cell carcinoma and its clinicopathologic correlations

The aim of this study was to determine the expression of cellular FLICE-like inhibitory protein (cFLIP) in head and neck squamous cell carcinoma (HNSCC) and revealed its possible correlation to Fas protein and tumour clinical parameters. The expression of cFLIP was analysed in 58 HNSCC samples and 30 morphologically normal tissues adjacent to the carcinomas using immunohistochemistry, reverse transcription-polymerase chain reaction (RT-PCR) and western blot analysis. Furthermore, its possible correlation to the expression of Fas protein and tumour clinicopathologic parameters were discussed. Cellular FLICE-like inhibitory protein was demonstrated to be up regulated in most HNSCC than in normal tissues by immunohistochemistry (p<0.01). Although the mRNA levels of both isoforms of cFLIP, long form (cFLIP(L)) and short form (cFLIP(S)), in HNSCC were higher than those in normal tissues (p<0.01), only cFLIP(L) protein could be detected by western blot. Furthermore, the expression of cFLIP(L) protein was significantly associated with tumour clinical stage (p<0.01) and lymph node metastasis (p=0.01). Since all of the tumours with Fas immunostaining also express cFLIP protein, there was no significant correlation between them (p>0.05). Overexpression of cFLIP(L) is a frequent event in HNSCC and HNSCC cells in vivo may need it to evade apoptosis mediated by Fas or other receptors, which might contribute to tumour development and progression.

Up-regulation of c-FLIPS+R upon CD40 stimulation is associated with inhibition of CD95-induced apoptosis in primary precursor B-ALL

Previous studies on apoptosis defects in acute lymphoblastic leukemia (ALL) have focused on chemotherapy-induced, primarily mitochondrial death pathways. Yet, immunologic surveillance mechanisms including sensitization to apoptotic signals mediated via the death receptor CD95 might contribute to leukemic control. Here, we show that primary B-cell precursor ALL cells from children escape from receptor-dependent cell death in 2 ways: Resting ALL blasts are protected from receptor-mediated apoptosis due to the absence of CD95 surface expression. However, even though CD40 ligation results in up-regulation of CD95, ALL blasts, unlike normal B cells, remain resistant to apoptosis. We show that this apoptosis resistance involves the selective up-regulation of the short isoforms of the caspase-8 inhibitor c-FLIP acting directly at the CD95 receptor level. Treatment with cycloheximide during CD40 activation prevents up-regulation of those c-FLIP isoforms and sensitizes ALL cells toward CD95-mediated apoptosis. We therefore propose that induction of the short c-FLIP isoforms inhibits the onset of CD95-induced apoptosis in primary CD40-stimulated ALL cells despite high CD95 expression.

Unique resistance of breast carcinoma cell line T47D to TRAIL but not anti-Fas is linked to p43cFLIPL

The majority of breast cancer cell lines are resistant to tumor necrosis factor -related apoptosis inducing ligand (TRAIL) induced apoptosis. TRAIL and Fas receptor death-inducing signaling complex (DISCs) formation are similar and involve ligand-dependent recruitment of FADD and caspase-8. We have found that the breast carcinoma cell line T47D is an unusual example of selective sensitivity to anti-Fas mAb treatment but resistant to TRAIL. Therefore, a detailed comparison of these two signaling pathways in one cell line should provide insight into the mechanism of TRAIL resistance. We observed that only anti-Fas mAb induces caspase activation and cell death in T47D. Further, FADD and caspase-8 interact with both TRAIL-R1 and TRAIL-R2, and that the amount of caspase-8 recruited by Fas-, TRAIL-R1 and TRAIL-R2 are the same. cFLIP(S) and cFLIP(R )isoforms block death receptor-induced apoptosis by inhibiting caspase-8 activation at the DISC; the role of cFLIP(L )at the DISC is still controversial. It has been suggested that the presence of the cleaved form of FLIP(L)-p43 at the DISC prevents caspase-8 cleavage. We found that both TRAIL and anti-Fas mAb-induced DISCs contain the cleaved form of p43 cFLIP(L) and its amount at the Fas DISC was higher compared to the TRAIL DISC. We also found that inhibition of cFLIP(L) expression in T47D cells decreased Fas-mediated caspase-8 activation and activation of effector caspases. We propose that in T47D p43 cFLIP(L) in the Fas-DISC may promote caspase-8 activation. The mechanism by which different amounts of p43cFLIP(L) regulates caspase-8 activation remains to be investigated.

Targeting the short form of cFLIP by RNA interference is sufficient to enhance TRAIL sensitivity in PC3 prostate carcinoma cells

The pro-apoptotic death receptor ligand tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) has received significant attention as a novel cancer therapeutic, since it selectively induces apoptosis in malignant and not normal cells. Unfortunately, prostate cancer cells display little if any susceptibility to TRAIL-induced apoptosis. However, sensitivity to TRAIL is enhanced by doxorubicin, which correlated with a decrease in expression of the caspase-8 inhibitor cFLIP (Kelly et. al. Cancer Biology & Therapy 1: 520). In this study we show that doxorubicin induces a time- and dose-dependent loss of cFLIP protein, but does not affect steady-state mRNA levels. Proteasome inhibition stabilized cFLIPL in the presence of doxorubicin. Although proteasome inhibitors increased basal levels of short cFLIP isoforms, cFLIPS declined at a similar rate in the absense or presence of proteasome inhibition during doxorubicin treatment. Ectopic expression of a cFLIPSGFP fusion protein protected PC3 cells from TRAIL-induced apoptosis in the absence or presence of doxorubicin, whereas downregulation of cFLIPS by RNA interference resulted in sensitization to TRAIL-induced apoptosis. We conclude that doxorubicin-mediated downregulation of cFLIPS, which occurs at the post-transcriptional level independent of proteasome-mediated pathways, is sufficient to enhance TRAIL sensitivity in PC3 prostate carcinoma cells.

The CD40-induced protection against CD95-mediated apoptosis is associated with a rapid upregulation of anti-apoptotic c-FLIP

CD95/Fas and CD40 receptors are important regulators of cell survival during germinal center reaction. In this study we used a human follicular lymphoma cell line, HF1A3, to study molecular mechanisms of CD95-mediated apoptosis and CD40-induced rescue from apoptosis. CD95 stimulation induced activation of caspase-8 and -3, collapse of mitochondrial membrane potential (Δ Ψm), release of cytochrome c and fragmentation of nuclear DNA. All these apoptotic events were abrogated, when cells were pretreated with CD40 antibodies before CD95 stimulation. CD40 induced a rapid up regulation of both short and long isoforms of c-FLIP, as these proteins were detectable 4 h after receptor stimulation. The induction of c-FLIP as well as the anti-apoptotic function of CD40 was completely abolished when NF-κB activity was inhibited by a selective inhibitor PDTC. We conclude that the anti-apoptotic signaling of CD40 involves NF-κB-mediated induction of c-FLIP proteins which can interfere with caspase-8 activation. However, it remains to be seen whether c-FLIP proteins are the only one ones involved in CD40-mediated protection.

Cellular FLICE-like inhibitory protein (c-FLIP): A novel target for Taxol-induced apoptosis

It is known that by binding to the FAS-associated death domain (FADD) protein and/or caspases-8 and -10 at the level of the death-inducing signaling complex (DISC), cellular FLICE-like inhibitory protein (c-FLIP) can prevent apoptosis triggered by death-inducing ligands. We investigated whether the c-FLIP splice variants, c-FLIP long [c-FLIP(L)] and c-FLIP short [c-FLIP(S)], play a role in Taxol-induced apoptosis. Our results showed that low Taxol concentrations triggered caspase-8- and caspase-10-dependent apoptosis in the CCRF-HSB-2 human lymphoblastic leukemia cell line, and induced the down-regulation of c-FLIP(S) and c-FLIP(L). Taxol decreased the expression of c-FLIP by a post-transcriptional and caspase-independent mechanism. To explore the distinct functions of the c-FLIP variants in Taxol-induced apoptosis, we transfected the cells with expression vectors carrying c-FLIP(L) and c-FLIP(S) in the sense orientation or c-FLIP(S) in the antisense orientation [c-FLIP(S)-AS]. Caspases-8 and -10 were more efficiently activated in the c-FLIP(S)-AS strain treated with 5–50 nM Taxol, which revealed that c-FLIP regulates Taxol-induced apoptosis by interacting with these caspases. Furthermore, our data showed that increased expression of c-FLIP(L) or c-FLIP(S) reduced apoptosis at 5–50 nM Taxol concentrations suggesting that both isoforms of c-FLIP prevent Taxol-induced apoptosis. These results revealed that Taxol induces apoptosis by down-regulating c-FLIP(S) and c-FLIP(L) expression.

Expression of Cellular FLICE Inhibitory Proteins (cFLIP) in Normal and Traumatic Murine and Human Cerebral Cortex

Cellular Fas-associated death domain-like interleukin-1-beta converting enzyme (FLICE) inhibitory proteins (cFLIPs) are endogenous caspase homologues that inhibit programmed cell death. We hypothesized that cFLIPs are differentially expressed in response to traumatic brain injury (TBI). cFLIP-alpha and cFLIP-delta mRNA were expressed in normal mouse brain-specifically cFLIP-delta (but not cFLIP-alpha) protein was robustly expressed. After controlled cortical impact (CCI), cFLIP-alpha expression increased initially then decreased to control levels at 12 h, increasing again at 24-72 h (P<0.05). cFLIP-delta expression was decreased in brain homogenates by 12 h after CCI, then increased again at 24 to 72 h (P<0.05). cFLIP-delta immunostaining was markedly reduced in injured cortex, but not hippocampus, at 3 to 72 h after CCI. In cortex, reduced cFLIP-delta staining was found in TUNEL-positive cells, but in hippocampus TUNEL-positive cells expressed cFLIP-delta immunoreactivity. cFLIP-delta was increased in a subset of reactive astrocytes in pericontusional cortex and hippocampus at 48 to 72 h. Low levels of both cFLIP isoforms were detected in human cortical tissue with no TBI, from four patients undergoing brain surgery for epilepsy and <24 h post mortem from three patients without CNS pathologic assessment. In cortical tissue surgically removed <18 h after severe TBI (n=3), cFLIP-alpha expression was increased relative to epilepsy controls (P<0.05) but not relative to post-mortem controls. The data suggest differential spatial and temporal regulation of cFLIP-alpha and cFLIP-delta expression that may influence the magnitude of cell death and further implicate programmed mechanisms of cell death after TBI.

Rapid Turnover of c-FLIPshort Is Determined by Its Unique C-terminal Tail

The caspase-8 inhibitor c-FLIP exists as two splice variants, c-FLIP(L) and c-FLIP(S), with distinct roles in death receptor signaling. The mechanisms determining their turnover have not been established. We found that in differentiating K562 erythroleukemia cells both c-FLIP isoforms were inducibly degraded by the proteasome, but c-FLIP(S) was more prone to ubiquitylation and had a considerably shorter half-life. Analysis of the c-FLIP(S)-specific ubiquitylation revealed two lysines, 192 and 195, C-terminal to the death effector domains, as principal ubiquitin acceptors in c-FLIP(S) but not in c-FLIP(L). Furthermore the c-FLIP(S)-specific tail of 19 amino acids, adjacent to the two target lysines, was demonstrated to be the key element determining the isoform-specific instability of c-FLIP(S). Molecular modeling in combination with site-directed mutagenesis demonstrated that the C-terminal tail is required for correct positioning and subsequent ubiquitylation of the target lysines. Because the antiapoptotic operation of c-FLIP(S) was not affected by the tail deletion, the antiapoptotic activity and ubiquitin-mediated degradation of c-FLIP(S) are functionally and structurally independent processes. The presence of a small destabilizing sequence in c-FLIP(S) constitutes an important determinant of c-FLIP(S)/c-FLIP(L) ratios by allowing differential degradation of c-FLIP isoforms. The conformation-based predisposition of c-FLIP(S) to ubiquitin-mediated degradation introduces a novel concept to the regulation of the death-inducing signaling complex.

C-FLIPR, a New Regulator of Death Receptor-induced Apoptosis

c-FLIPs (c-FLICE inhibitory proteins) play an essential role in regulation of death receptor-induced apoptosis. Multiple splice variants of c-FLIP have been described on the mRNA level; so far only two of them, c-FLIP(L) and c-FLIP(S,) had been found to be expressed at the protein level. In this report, we reveal the endogenous expression of a third isoform of c-FLIP. We demonstrate its presence in a number of T and B cell lines as well as in primary human T cells. We identified this isoform as c-FLIP(R), a death effector domain-only splice variant previously identified on the mRNA level. Impor-/tantly, c-FLIP(R) is recruited to the CD95 (Fas/APO-1) death-inducing signaling complex upon CD95 stimulation. Several properties of c-FLIP(R) are similar to c-FLIP(S): both isoforms have a short half-life, a similar pattern of expression during activation of primary human T cells, and are strongly induced in T cells upon CD3/CD28 costimulation. Taken together, our data demonstrate endogenous expression of c-FLIP(R) and similar roles of c-FLIP(R) and c-FLIP(S) isoforms in death receptor-mediated apoptosis.

202 The proteasome inhibitor bortezomib (VELCADE™) sensitizes human tumor cells to TRAIL-mediated apoptosis by reduction of c-FLIP

It is known that by binding to the FAS-associated death domain (FADD) protein and/or caspases-8 and -10 at the level of the death-inducing signaling complex (DISC), cellular FLICE-like inhibitory protein (c-FLIP) can prevent apoptosis triggered by death-inducing ligands. We investigated whether the c-FLIP splice variants, c-FLIP long [c-FLIP(L)] and c-FLIP short [c-FLIP(S)], play a role in Taxol-induced apoptosis. Our results showed that low Taxol concentrations triggered caspase-8- and caspase-10-dependent apoptosis in the CCRF-HSB-2 human lymphoblastic leukemia cell line, and induced the down-regulation of c-FLIP(S) and c-FLIP(L). Taxol decreased the expression of c-FLIP by a post-transcriptional and caspase-independent mechanism. To explore the distinct functions of the c-FLIP variants in Taxol-induced apoptosis, we transfected the cells with expression vectors carrying c-FLIP(L) and c-FLIP(S) in the sense orientation or c-FLIP(S) in the antisense orientation [c-FLIP(S)-AS]. Caspases-8 and -10 were more efficiently activated in the c-FLIP(S)-AS strain treated with 5–50 nM Taxol, which revealed that c-FLIP regulates Taxol-induced apoptosis by interacting with these caspases. Furthermore, our data showed that increased expression of c-FLIP(L) or c-FLIP(S) reduced apoptosis at 5–50 nM Taxol concentrations suggesting that both isoforms of c-FLIP prevent Taxol-induced apoptosis. These results revealed that Taxol induces apoptosis by down-regulating c-FLIP(S) and c-FLIP(L) expression.