Inhibition of CaMKII-mediated c-FLIP expression sensitizes malignant melanoma cells to TRAIL-induced apoptosis

TRAIL can induce apoptosis in melanoma cells and thus may offer new hope for melanoma therapy. However, many melanoma cells are resistant to TRAIL. To examine molecular mechanisms in cell resistance, we analyzed TRAIL-induced DISC in TRAIL-sensitive melanoma cells and showed that apoptosis-initiating caspase-8 and caspase-10 were recruited to the DISC where they became activated through autocatalytical cleavage, leading to apoptosis through cleavage of downstream substrates such as caspase-3 and DFF45. In TRAIL-resistant melanoma cells, however, c-FLIP proteins were recruited to the DISC, resulting in the inhibition of caspase-8 and caspase-10 cleavage in the DISC. Both calmodulin-dependent protein kinase II (CaMKII) protein and enzymatic activity were upregulated in resistant cells and CaMKII inhibitor KN-93 downregulated expression of c-FLIP proteins, thus sensitizing resistant cells to TRAIL-induced apoptosis. Transfection of CaMKII cDNA in sensitive melanoma cells resulted in cell resistance to TRAIL, where transfection of CaMKII dominant-negative cDNA in resistant cells restored TRAIL sensitivity in cells. These results indicate that the CaMKII-mediated pathway for c-FLIP upregulation protects melanoma cells from TRAIL-induced apoptosis and targeting this pathway may provide novel therapeutic strategies in treatment of melanomas.     

Deficient tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) death receptor transport to the cell surface in human colon cancer cells selected for resistance to TRAIL-induced apoptosis

Many tumor cell types are sensitive to tumor necrosis factor-related apoptosis-inducing ligand (TRAIL)-induced apoptosis. Incubation of TRAIL-sensitive cells with TRAIL invariably leads to resistant survivors even when high doses of TRAIL are used. Because the emergence of resistance to apoptosis is a major concern in successful treatment of cancer, and TRAIL survivors may contribute to therapeutic failure, we investigated potential resistance mechanisms. We selected TRAIL-resistant SW480 human colon adenocarcinoma cells by repeatedly treating them with high and/or low doses of TRAIL. The resulting TRAIL-resistant clones were not cross-resistant to Fas or paclitaxel. Expression of modulators of apoptosis was not changed in the resistant cells, including TRAIL receptors, cFLIP, Bax, Bid, or IAP proteins. Surprisingly, we found that DISC formation was deficient in multiple selected TRAIL-resistant clones. DR4 was not recruited to the DISC upon TRAIL treatment, and caspase-8 was not activated at the DISC. Although total cellular DR4 mRNA and protein were virtually identical in TRAIL-sensitive parental and TRAIL-resistant clones, DR4 protein expression on the cell surface was essentially undetectable in the TRAIL-resistant clones. Moreover, exogenous DR4 and KILLER/DR5 were not properly transported to the cell surface in the TRAIL-resistant cells. Interestingly, TRAIL-resistant cells were resensitized to TRAIL by tunicamycin pretreatment, which increased cell surface expression of DR4 and KILLER/DR5. Our data suggest that tumor cells may become resistant to TRAIL through regulation of the death receptor cell surface transport and that resistance to TRAIL may be overcome by the glycosylation inhibitor/endoplasmic reticulum stress-inducing agent tunicamycin.     

Inhibition of RIP and c-FLIP enhances TRAIL-induced apoptosis in pancreatic cancer cells

Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) has recently emerged as a cancer therapeutic agent because it is capable of preferentially inducing apoptosis in human cancer over normal cells. The majority of human pancreatic cancers, unfortunately, are resistant to TRAIL treatment. Here, we show that the inhibition of caspase-8 cleavage is the most upstream event in TRAIL resistance in pancreatic cancers. TRAIL treatment led to the cleavage of caspase-8 and downstream caspase-9, caspase-3, and DNA fragmentation factor 45 (DFF45) in TRAIL-sensitive pancreatic cancer cell lines (BXPC-3, PACA-2). This caspase-8-initiated caspase cascade, however, was inhibited in TRAIL-resistant pancreatic cancer cell lines (PANC-1, ASPC-1, CAPAN-1, CAPAN-2). The long and short forms of cellular Fas-associated death domain-like interleukin-1beta-converting enzyme-inhibitory protein (c-FLIP(L), c-FLIP(S)) were highly expressed in the TRAIL-resistant as compared to the sensitive cells; knockdown of c-FLIP(L) and c-FLIP(S) by a short hairpin RNA (shRNA) rendered the resistant cells sensitive to TRAIL-induced apoptosis through the cleavage of caspase-8 and activation of the mitochondrial pathway. Receptor-interacting protein (RIP) has been reported in TRAIL-induced activation of NF-kappaB and we show here that knockdown of RIP sensitized the resistant cells to TRAIL-induced apoptosis. These results indicate the role of c-FLIP and RIP in caspase-8 inhibition and thus TRAIL resistance. Treatment of the resistant cells with camptothecin, celecoxib and cisplatin resulted in the downregulation of c-FLIP and caused a synergistic apoptotic effect with TRAIL. These studies therefore suggest that combination treatment with chemotherapy can overcome TRAIL resistance and enhance TRAIL therapeutic efficacy in treating pancreatic cancers.     

E1A sensitizes cancer cells to TRAIL-induced apoptosis through enhancement of caspase activation

Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) has been shown to induce apoptosis of cancer cells. Sensitization of cancer cells to TRAIL, particularly TRAIL-resistant cancer cells, could improve the effectiveness of TRAIL as an anticancer agent. The adenovirus type 5 E1A that associates with anticancer activities including sensitization to apoptosis induced by tumor necrosis factor is currently being tested in clinical trials. In this study, we investigated the sensitivity to TRAIL in the E1A transfectants ip1-E1A2 and 231-E1A cells and the parental TRAIL-resistant human ovarian cancer SKOV3.ip1 and TRAIL-sensitive human breast cancer MDA-MB-231 cells. The results indicated that the percentage of TRAIL-induced apoptotic cells was significantly higher in the E1A transfectants of both cell lines than it was in the parental cell lines. To further investigate the cellular mechanism of this effect, we found that E1A enhances TRAIL-induced activation of caspase-8, caspase-9, and caspase-3. Inhibition of caspase-3 activity by a specific inhibitor, Z-DEVD-fmk, abolished TRAIL-induced apoptosis. In addition, E1A enhanced TRAIL expression in ip1-E1A2 cells, but not in 231-E1A cells, and the anti-TRAIL neutralizing antibody N2B2 blocked the E1A-mediated bystander effect in vitro. Taken together, these results suggest that E1A sensitizes both TRAIL-sensitive and TRAIL-resistant cancer cells to TRAIL-induced apoptosis, which occurs through the enhancement of caspase activation; activation of caspase-3 is required for TRAIL-induced apoptosis; and E1A-induced TRAIL expression is involved in the E1A-mediated bystander effect. Combination of E1A and TRAIL could be an effective treatment for cancer.     

Acivicin induces apoptosis independently of gamma-glutamyltranspeptidase activity

To elucidate the molecular mechanism(s) involved in the TRAIL-induced apoptosis sensitivity, we conducted the following experiments utilizing TRAIL-sensitive and -resistant glioma cells. We examined the expression of TRAIL receptors mRNA, but no significant differences were detected in those cells. TRAIL-resistant cells were sensitized to TRAIL-induced apoptosis by staurosporine pretreatment and preferentially expressed PKCepsilon. Since several lines of evidence suggest that PKC may play a protective role for apoptosis, we analyzed the involvement of PKCepsilon in TRAIL-induced apoptosis by an adenovirus vector expression system. We found that TRAIL susceptibility was augmented by the expression of a dominant negative PKCepsilon in TRAIL-resistant cells. Conversely, PKCepsilon introduction in TRAIL-sensitive cells resulted in the reduction of TRAIL-induced apoptosis. Taken together, these data suggest that PKCepsilon may be a regulator of susceptibility to TRAIL-induced apoptosis in gliomas and probably other malignancies.     

Modulation of tumor necrosis factor apoptosis-inducing ligand- induced NF-kappa B activation by inhibition of apical caspases.

Tumor necrosis factor (TNF) apoptosis-inducing ligand (TRAIL), a member of the TNF family, induces apoptosis in many transformed cells. We report TRAIL-induced NF-kappaB activation, concomitant with production of the pro-inflammatory cytokine Interleukin-8 in the relatively TRAIL-insensitive cell line, HEK293. In contrast, TRAIL-induced NF-kappaB activation occurred in HeLa cells only upon pretreatment with the caspase inhibitor, benzyloxycarbonyl-Val-Ala-Asp-(OMe) fluoromethyl ketone (z-VAD.fmk), indicating that this was due to a caspase-sensitive component of TRAIL-induced NF-kappaB activation. NF-kappaB activation was mediated by the death receptors, TRAIL-R1 and -R2, but not by TRAIL-R3 or -R4 and was only observed in HeLa cells in the presence of z-VAD.fmk. Receptor-interacting protein, an obligatory component of TNF-alpha-induced NF-kappaB activation, was cleaved during TRAIL-induced apoptosis. We show that receptor-interacting protein is recruited to the native TRAIL death-inducing signaling complex (DISC) and that recruitment is enhanced in the presence of z-VAD.fmk, thus providing an explanation for the potentiation of TRAIL-induced NF-kappaB activation by z-VAD.fmk in TRAIL-sensitive cell lines. Examination of the TRAIL DISC in sensitive and resistant cells suggests that a high ratio of c-FLIP to caspase-8 may partially explain cellular resistance to TRAIL-induced apoptosis. Sensitivity to TRAIL-induced apoptosis was also modulated by inhibition or activation of NF-kappaB. Thus, in some contexts, modulation of NF-kappaB activation possibly at the level of apical caspase activation at the DISC may be a key determinant of sensitivity to TRAIL-induced apoptosis.     

Enhanced caspase-8 recruitment to and activation at the DISC is critical for sensitisation of human hepatocellular carcinoma cells to TRAIL-induced apoptosis by chemotherapeutic drugs

Tumour necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL) exhibits potent antitumour activity upon systemic administration in mice without showing the deleterious side effects observed with other apoptosis-inducing members of the TNF family such as TNF and CD95L. TRAIL may, thus, have great potential in the treatment of human cancer. However, about 60% of tumour cell lines are not sensitive to TRAIL. To evaluate the mechanisms of tumour resistance to TRAIL, we investigated hepatocellular carcinoma (HCC) cell lines that exhibit differential sensitivity to TRAIL. Pretreatment with chemotherapeutic drugs, for example, 5-fluorouracil (5-FU), rendered the TRAIL-resistant HCC cell lines sensitive to TRAIL-induced apoptosis. Analysis of the TRAIL death-inducing signalling complex (DISC) revealed upregulation of TRAIL-R2. Caspase-8 recruitment to and its activation at the DISC were substantially increased after 5-FU sensitisation, while FADD recruitment remained essentially unchanged. 5-FU pretreatment downregulated cellular FLICE-inhibitory protein (cFLIP) and specific cFLIP downregulation by small interfering RNA was sufficient to sensitise TRAIL-resistant HCC cell lines for TRAIL-induced apoptosis. Thus, a potential mechanism for TRAIL sensitisation by 5-FU is the increased effectiveness of caspase-8 recruitment to and activation at the DISC facilitated by the downregulation of cFLIP and the consequent shift in the ratio of caspase-8 to cFLIP at the DISC.     

A caspase-8-independent component in TRAIL/Apo-2L-induced cell death in human rhabdomyosarcoma cells

Tumor necrosis factor related apoptosis inducing ligand (TRAIL) belongs to the Tumor necrosis factor (TNF) family of death-inducing ligands, and signaling downstream of TRAIL ligation to its receptor(s) remains to be fully elucidated. Components of the death-inducing signaling complex (DISC) and TRAIL signaling downstream of receptor activation were examined in TRAIL - sensitive and -resistant models of human rhabdomyosarcoma (RMS). TRAIL ligation induced DISC formation in TRAIL-sensitive (RD, Rh18, Rh30) and TRAIL-resistant RMS (Rh28, Rh36, Rh41), with recruitment of FADD and procaspase-8. In RD cells, overexpression of dominant-negative FADD (DNFADD) completely abolished TRAIL-induced cell death in contrast to dominant-negative caspase- 8 (DNC8), which only partially inhibited TRAIL-induced apoptosis, growth inhibition, or loss in clonogenic survival. DNC8 did not inhibit the cleavage of Bid or the activation of Bax. Overexpression of Bcl-2 or Bcl-xL inhibited TRAIL-induced apoptosis, growth inhibition, and loss in clonogenic survival. Bcl-2 and Bcl-xL, but not DNC8, inhibited TRAIL-induced Bax activation. Bcl-xL did not inhibit the early activation of caspase-8 (<4 h) but inhibited cleavage of Bid, suggesting that Bid is cleaved downstream of the mitochondria, independent of caspase-8. Exogenous addition of sphingosine also induced activation of Bax via a caspase-8-and Bid-independent mechanism. Further, inhibition of sphingosine kinase completely protected cells from TRAIL-induced apoptosis. Data demonstrate that in RMS cells, the TRAIL signaling pathway circumvents caspase-8 activation of Bid upstream of the mitochondria and that TRAIL acts at the level of the mitochondria via a mechanism that may involve components of the sphingomyelin cycle.     

Chemotherapy overcomes TRAIL-R4-mediated TRAIL resistance at the DISC level

TNF-related apoptosis-inducing ligand or Apo2L (Apo2L/TRAIL) is a promising anti-cancer drug owing to its ability to trigger apoptosis by binding to TRAIL-R1 or TRAIL-R2, two membrane-bound receptors that are often expressed by tumor cells. TRAIL can also bind non-functional receptors such as TRAIL-R4, but controversies still exist regarding their potential to inhibit TRAIL-induced apoptosis. We show here that TRAIL-R4, expressed either endogenously or ectopically, inhibits TRAIL-induced apoptosis. Interestingly, the combination of chemotherapeutic drugs with TRAIL restores tumor cell sensitivity to apoptosis in TRAIL-R4-expressing cells. This sensitization, which mainly occurs at the death-inducing signaling complex (DISC) level, through enhanced caspase-8 recruitment and activation, is compromised by c-FLIP expression and is independent of the mitochondria. Importantly, TRAIL-R4 expression prevents TRAIL-induced tumor regression in nude mice, but tumor regression induced by TRAIL can be restored with chemotherapy. Our results clearly support a negative regulatory function for TRAIL-R4 in controlling TRAIL signaling, and unveil the ability of TRAIL-R4 to cooperate with c-FLIP to inhibit TRAIL-induced cell death.     

15-Deoxy-Δ<Superscript>12,14</Superscript>-prostaglandin J<Subscript>2</Subscript> (15d-PGJ<Subscript>2</Subscript>) sensitizes human leukemic HL-60 cells to tumor necrosis factor-related apoptosis-inducing ligand (TRAIL)-induced apoptosis through Akt downregulation

While tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL) is a promising new agent for the treatment of cancer, resistance to TRAIL remains a therapeutic challenge. Identifying agents to use in combination with TRAIL to enhance apoptosis in leukemia cells would increase the potential utility of this agent as a therapy for leukemia. Here, we show that 15-deoxy-Δ^12,14-prostaglandin J₂ (15d-PGJ₂), a natural ligand for peroxisome proliferator-activated receptor γ (PPARγ), can sensitize TRAIL-resistant leukemic HL-60 cells to TRAIL-induced apoptosis. The sensitization to TRAIL-induced apoptosis by 15d-PGJ₂ was not blocked by a PPARγ inhibitor (GW9662), suggesting a PPARγ-independent mechanism. This process was accompanied by activation of caspase-8, caspase-9, and caspase-3 and was concomitant with Bid and PARP cleavage. We observed significant decreases in XIAP, Bcl-2, and c-FLIP after cotreatment with 15d-PGJ₂ and TRAIL. We also observed the inhibition of Akt expression and phosphorylation by cotreatment with 15d-PGJ₂ and TRAIL. Furthermore, inactivation of Akt by Akt inhibitor IV sensitized human leukemic HL-60 cells to TRAIL, indicating a key role for Akt inhibition in these events. Taken together, these findings indicate that 15d-PGJ₂ may augment TRAIL-induced apoptosis in human leukemia cells by down-regulating the expression and phosphorylation of Akt.     

Identification of inhibitors of TRAIL-induced death (ITIDs) in the TRAIL-sensitive colon carcinoma cell line SW480 using a genetic approach

The tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) is a potent inducer of apoptosis in tumor cell lines, whereas normal cells appear to be protected from its cytotoxic effects. Therefore TRAIL holds promise as a potential therapeutic agent against cancer. To elucidate some of the critical factors that contribute to TRAIL resistance, we performed a genetic screen in the human colon carcinoma cell line SW480 by infecting this TRAIL-sensitive cell line with a human placental cDNA retroviral library and isolating TRAIL-resistant clones. Characterization of the resulting clones for inhibitors of TRAIL-induced death (ITIDs) led to the isolation of c-FLIP(S), Bax inhibitor 1, and Bcl-XL as candidate suppressors of TRAIL signaling. We have demonstrated that c-FLIP(S) and Bcl-XL are sufficient when overexpressed to convey resistance to TRAIL treatment in previously sensitive cell lines. Furthermore both c-FLIP(S) and Bcl-XL protected against overexpression of the TRAIL receptors DR4 and KILLER/DR5. When c-FLIP(S) and Bcl-XL were overexpressed together in SW480 and HCT 116, an additive inhibitory effect was observed after TRAIL treatment suggesting that these two molecules function in the same pathway in the cell lines tested. Furthermore, we have demonstrated for the first time that a proapoptotic member of the Bcl-2 family, Bax, is required for TRAIL-mediated apoptosis in HCT 116 cells. Surprisingly, we have found that the serine/threonine protein kinase Akt, which is an upstream regulator of both c-FLIP(S) and Bcl-XL, is not sufficient when overexpressed to protect against TRAIL in the cell lines tested. These results suggest a key role for c-FLIP(S), Bcl-XL, and Bax in determining tumor cell sensitivity to TRAIL.     

Calcium/calmodulin-dependent protein kinase II regulation of c-FLIP expression and phosphorylation in modulation of Fas-mediated signaling in malignant glioma cells

Fas, upon cross-linking with Fas ligand (FasL) or Fas agonistic antibody, transduces apoptotic yet also proliferative signals, which have been implicated in tumor pathogenesis. In this study, we investigated the molecular mechanisms that control Fas-mediated signaling in glioma cells. Fas agonistic antibody, CH-11, induced apoptosis in sensitive glioma cells through caspase-8 recruitment to the Fas-mediated death-inducing signaling complex (DISC) where caspase-8 was cleaved to initiate apoptosis through a systematic cleavage of downstream substrates. In contrast, CH-11 stimulated cell growth in resistant glioma cells through recruitment of c-FLIP (cellular Fas-associated death domain (FADD)-like interleukin-1beta-converting enzyme (FLICE)-inhibitory protein) to the Fas-mediated DISC. Three isoforms of long form c-FLIP were detected in glioma cells, but only the phosphorylated isoform was recruited to and cleaved into a p43 intermediate form in the Fas-mediated DISC in resistant cells. Calcium/calmodulin-dependent protein kinase II (CaMK II) activity was up-regulated in resistant cells. Treatment of resistant cells with the CaMK II inhibitor KN-93 inhibited CaMK II activity, reduced c-FLIP expression, inhibited c-FLIP phosphorylation, and rescued CH-11 sensitivity. Transfection of CaMK II cDNA in sensitive cells rendered them resistant to CH-11. These results indicated that CaMK II regulates c-FLIP expression and phosphorylation, thus modulating Fas-mediated signaling in glioma cells.     

The PEA-15 Protein Regulates Autophagy via Activation of JNK

PEA-15/PED (phosphoprotein enriched in astrocytes 15 kDa/phosphoprotein enriched in diabetes) is a death effector domain-containing protein which is known to modulate apoptotic cell death. The mechanism by which PEA-15 inhibits caspase activation and increases ERK (extracellular-regulated kinase) activity is well characterized. Here, we demonstrate that PEA-15 is not only pivotal in the activation of the ERK pathway but also modulates JNK (c-Jun N-terminal kinase) signaling. Upon overexpression of PEA-15 in malignant glioma cells, JNK is potently activated. The PEA-15-induced JNK activation depends on the phosphorylation of PEA-15 at both phosphorylation sites (serine 104 and serine 116). The activation of JNK is substantially inhibited by siRNA-mediated down-regulation of endogenous PEA-15. Moreover, we demonstrate that glioma cells overexpressing PEA-15 show increased signs of autophagy in response to classical autophagic stimuli such as ionizing irradiation, serum deprivation, or rapamycin treatment. In contrast, the non-phosphorylatable mutants of PEA-15 are not capable of promoting autophagy. The inhibition of JNK abrogates the PEA-15-mediated increase in autophagy. In conclusion, our data show that PEA-15 promotes autophagy in glioma cells in a JNK-dependent manner. This might render glioma cells more resistant to adverse stimuli such as starvation or ionizing irradiation.     

PED/PEA-15 interacts with the 67 kD laminin receptor and regulates cell adhesion, migration, proliferation and apoptosis

Phosphoprotein enriched in diabetes/phosphoprotein enriched in astrocytes-15 kD (PED/PEA-15) is an anti-apoptotic protein whose expression is increased in several human cancers. In addition to apoptosis, PED/PEA-15 is involved in the regulation of other major cellular functions, including cell adhesion, migration, proliferation and glucose metabolism. To further understand the functions of this protein, we performed a yeast two-hybrid screening using PED/PEA-15 as a bait and identified the 67 kD high-affinity laminin receptor (67LR) as an interacting partner. 67 kD laminin receptor is a non-integrin cell-surface receptor for the extracellular matrix (ECM), derived from the dimerization of a 37 kD cytosolic precursor (37LRP). The 67LR is highly expressed in human cancers and widely recognized as a molecular marker of metastatic aggressiveness. The molecular interaction of PED/PEA-15 with 67LR was confirmed by pull-down experiments with recombinant His-tagged 37LRP on lysates of PED/PEA-15 transfected HEK-293 cells. Further, overexpressed or endogenous PED/PEA-15 was co-immunoprecipitated with 67LR in PED/PEA-15-transfected HEK-293 cells and in U-373 glioblastoma cells, respectively. PED/PEA-15 overexpression significantly increased 67LR-mediated HEK-293 cell adhesion and migration to laminin that, in turn, determined PED/PEA-15 phosphorylation both in Ser-104 and Ser-116, thus enabling cell proliferation and resistance to apoptosis. PED/PEA-15 ability to induce cell responses to ECM-derived signals through interaction with 67LR may be of crucial importance for tumour cell survival in a poor microenvironment, thus favouring the metastatic spread and colonization.     

The Roles of ROS and Caspases in TRAIL-Induced Apoptosis and Necroptosis in Human Pancreatic Cancer Cells

Death signaling provided by tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL) can induce death in cancer cells with little cytotoxicity to normal cells; this cell death has been thought to involve caspase-dependent apoptosis. Reactive oxygen species (ROS) are also mediators that induce cell death, but their roles in TRAIL-induced apoptosis have not been elucidated fully. In the current study, we investigated ROS and caspases in human pancreatic cancer cells undergoing two different types of TRAIL-induced cell death, apoptosis and necroptosis. TRAIL treatment increased ROS in two TRAIL-sensitive pancreatic cancer cell lines, MiaPaCa-2 and BxPC-3, but ROS were involved in TRAIL-induced apoptosis only in MiaPaCa-2 cells. Unexpectedly, inhibition of ROS by either N-acetyl-L-cysteine (NAC), a peroxide inhibitor, or Tempol, a superoxide inhibitor, increased the annexin V-/propidium iodide (PI)⁺ early necrotic population in TRAIL-treated cells. Additionally, both necrostatin-1, an inhibitor of receptor-interacting protein kinase 1 (RIP1), and siRNA-mediated knockdown of RIP3 decreased the annexin V^-/PI⁺ early necrotic population after TRAIL treatment. Furthermore, an increase in early apoptosis was induced in TRAIL-treated cancer cells under inhibition of either caspase-2 or -9. Caspase-2 worked upstream of caspase-9, and no crosstalk was observed between ROS and caspase-2/-9 in TRAIL-treated cells. Together, these results indicate that ROS contribute to TRAIL-induced apoptosis in MiaPaCa-2 cells, and that ROS play an inhibitory role in TRAIL-induced necroptosis of MiaPaCa-2 and BxPC-3 cells, with caspase-2 and -9 playing regulatory roles in this process.     

Progesterone induces apoptosis in TRAIL-resistant ovarian cancer cells by circumventing c-FLIPL overexpression

Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) holds great potential as an anticancer drug, since it induces selective cell death in cancer cells but not in normal ones. However, cancer cells often acquire resistance to TRAIL, which hinders its clinical efficacy. We previously demonstrated that progesterone triggers apoptosis in human ovarian cancer (OCa) cells. In the present study, we evaluated the prospect of utilizing progestins in combination with TRAIL to enhance cell death in TRAIL-sensitive (OVCA 420, OVCA 429, and OVCA 433) and -resistant (OVCA 432) OCa cell lines. TRAIL sensitivity (60-80% cell kill) bore no correlation with expression of the TRAIL receptors (DR4, DR5) or their decoys (DcR1 and DcR2), but was associated with activation of caspase-8 and -3, and downregulation of the long isoform of FLICE-like inhibitory protein (c-FLIP(L)), an anti-apoptosis mediator. Small interfering RNA-mediated knockdown of c-FLIP(L) expression restored TRAIL sensitivity in OVCA 432 cells. Induction of c-FLIP(L) overexpression increased TRAIL resistance in TRAIL-sensitive lines. Thus, persistent high level of c-FLIP(L) expression likely mediates TRAIL resistance in OCa cells. Treatment of OCa cells with progesterone enhanced TRAIL-induced cell death (>85%), but only in TRAIL-sensitive cell lines. Combined treatment with two progestins was superior to single progestin treatment, with progesterone plus medroxyprogesterone acetate (MPA) achieving over 85% cell kill in both TRAIL-sensitive and -resistant OCa cell lines. Significantly, unlike TRAIL, progestin-induced cell death did not involve c-FLIP(L) downregulation. Hence, combined progestin regimens, with or without TRAIL, may serve as an effective therapy for OCa by circumventing the anti-apoptotic action of c-FLIP(L).     

hPEBP4 resists TRAIL-induced apoptosis of human prostate cancer cells by activating Akt and deactivating ERK1/2 pathways

The treatment options available for prostate cancer are limited because of its resistance to therapeutic agents. Thus, a better understanding of the underlying mechanisms of the resistance of prostate cancer will facilitate the discovery of more efficient treatment protocols. Human phosphatidylethanolamine-binding protein 4 (hPEBP4) is recently identified by us as an anti-apoptotic molecule and a potential candidate target for breast cancer treatment. Here we found the expression levels of hPEBP4 were positively correlated with the severity of clinical prostate cancer. Furthermore, hPEBP4 was not expressed in TRAIL-sensitive DU145 prostate cancer cells, but was highly expressed in TRAIL-resistant LNCaP cells, which show highly activated Akt. Interestingly, hPEBP4 overexpression in TRAIL-sensitive DU145 cells promoted Akt activation but inhibited ERK1/2 activation. The hPEBP4-overexpressing DU145 cells became resistant to TRAIL-induced apoptosis consequently, which could be reversed by PI3K inhibitors. In contrast, silencing of hPEBP4 in TRAIL-resistant LNCaP cells inhibited Akt activation but increased ERK1/2 activation, resulting in their sensitivity to TRAIL-induced apoptosis that was restored by the MEK1 inhibitor. Therefore, hPEBP4 expression in prostate cancer can activate Akt and deactivate ERK1/2 signaling, leading to TRAIL resistance. We also demonstrated that hPEBP4-mediated resistance to TRAIL-induced apoptosis occurred downstream of caspase-8 and at the level of BID cleavage via the regulation of Akt and ERK pathways, and that hPEBP4-regulated ERK deactivation was upstream of Akt activation in prostate cancer cells. Considering that hPEBP4 confers cellular resistance to TRAIL-induced apoptosis and is abundantly expressed in poorly differentiated prostate cancer, silencing of hPEBP4 suggests a promising approach for prostate cancer treatment.     

Phorbol esters induce intracellular accumulation of the anti-apoptotic protein PED/PEA-15 by preventing ubiquitinylation and proteasomal degradation

Phosphoprotein enriched in diabetes/phosphoprotein enriched in astrocytes (PED/PEA)-15 is an anti-apoptotic protein whose expression is increased in several cancer cells and following experimental skin carcinogenesis. Exposure of untransfected C5N keratinocytes and transfected HEK293 cells to phorbol esters (12-O-tetradecanoylphorbol-13-acetate (TPA)) increased PED/PEA-15 cellular content and enhanced its phosphorylation at serine 116 in a time-dependent fashion. Ser-116 --> Gly (PED(S116G)) but not Ser-104 --> Gly (PED(S104G)) substitution almost completely abolished TPA regulation of PED/PEA-15 expression. TPA effect was also prevented by antisense inhibition of protein kinase C (PKC)-zeta and by the expression of a dominant-negative PKC-zeta mutant cDNA in HEK293 cells. Similar to long term TPA treatment, overexpression of wild-type PKC-zeta increased cellular content and phosphorylation of WT-PED/PEA-15 and PED(S104G) but not of PED(S116G). These events were accompanied by the activation of Ca2+-calmodulin kinase (CaMK) II and prevented by the CaMK blocker, KN-93. At variance, the proteasome inhibitor lactacystin mimicked TPA action on PED/PEA-15 intracellular accumulation and reverted the effects of PKC-zeta and CaMK inhibition. Moreover, we show that PED/PEA-15 bound ubiquitin in intact cells. PED/PEA-15 ubiquitinylation was reduced by TPA and PKC-zeta overexpression and increased by KN-93 and PKC-zeta block. Furthermore, in HEK293 cells expressing PED(S116G), TPA failed to prevent ubiquitin-dependent degradation of the protein. Accordingly, in the same cells, TPA-mediated protection from apoptosis was blunted. Taken together, our results indicate that TPA increases PED/PEA-15 expression at the post-translational level by inducing phosphorylation at serine 116 and preventing ubiquitinylation and proteosomal degradation.