Activation of protein kinase C inhibits TRAIL-induced caspases activation, mitochondrial events and apoptosis in a human leukemic T cell line

Inflammation in atherosclerosis: pathophysiology and mechanisms

Atherosclerosis imposes a heavy burden on cardiovascular health due to its indispensable role in the pathogenesis of cardiovascular disease (CVD) such as coronary artery disease and heart failure. Ample clinical and experimental evidence has corroborated the vital role of inflammation in the pathophysiology of atherosclerosis. Hence, the demand for preclinical research into atherosclerotic inflammation is on the horizon. Indeed, the acquisition of an in-depth knowledge of the molecular and cellular mechanisms of inflammation in atherosclerosis should allow us to identify novel therapeutic targets with translational merits. In this review, we aimed to critically discuss and speculate on the recently identified molecular and cellular mechanisms of inflammation in atherosclerosis. Moreover, we delineated various signaling cascades and proinflammatory responses in macrophages and other leukocytes that promote plaque inflammation and atherosclerosis. In the end, we highlighted potential therapeutic targets, the pros and cons of current interventions, as well as anti-inflammatory and atheroprotective mechanisms.

Screening study of cancer-related cellular signals from microbial natural products

To identify bioactive natural products from various natural resources, such as plants and microorganisms, we investigated programs to screen for compounds that affect several cancer-related cellular signaling pathways, such as BMI1, TRAIL, and Wnt. This review summarizes the results of our recent studies, particularly those involving natural products isolated from microbial resources, such as actinomycetes, obtained from soil samples collected primarily around Chiba, Japan.

Pterostilbene Enhances TRAIL-Induced Apoptosis through the Induction of Death Receptors and Downregulation of Cell Survival Proteins in TRAIL-Resistance Triple Negative Breast Cancer Cells

Tumor necrosis factor-related apoptosis-induced ligand (TRAIL) is nontoxic to normal cells and preferentially cytotoxic to cancer cells. Recent data suggest that malignant breast cancer cells often become resistant to TRAIL. Pterostilbene (PTER), a naturally occurring analogue of resveratrol found in blueberries, is known to induce cancer cells to undergo apoptosis. In the present study, we examined whether PTER affects TRAIL-induced apoptosis and its mechanism in TRAIL-resistant triple negative breast cancer (TNBC) cells. Our data indicated that PTER induced apoptosis (14.68 ± 3.78% for 40 μM PTER vs 1.98 ± 0.25% for control, p < 0.01) in TNBC cells and enhanced TRAIL-induced apoptosis in TRAIL-resistant TNBC cells (18.45 ± 4.65% for 40 μM PTER vs 29.38 ± 6.35% for combination of 40 μM PTER and 100 ng/mL TRAIL, p < 0.01). We demonstrated that PTER induced death receptors DR5 and DR4 as well as decreased decoy receptor DcR-1 and DcR-2 expression. PTER also decreased the antiapoptotic proteins c-FLIPS/L, Bcl-Xl, Bcl-2, survivin, and XIAP. PTER induced the cleavage of bid protein and caused proapoptotic Bax accumulation. Moreover, we found that PTER induced the expression of DR4 and DR5 through the reactive oxygen species (ROS)/ endoplasmic reticulum (ER) stress/ERK 1/2 and p38/C/EBP-homologous protein (CHOP) signaling pathways. Overall, our results showed that PTER potentiated TRAIL-induced apoptosis via ROS-mediated CHOP activation leading to the expression of DR4 and DR5.

OSU-2S/Sorafenib Synergistic Antitumor Combination against Hepatocellular Carcinoma: The Role of PKCδ/p53

Background: Sorafenib (Nexavar^®) is an FDA-approved systemic therapy for advanced hepatocellular carcinoma (HCC). However, the low efficacy and adverse effects at high doses limit the clinical application of sorafenib and strongly recommend its combination with other agents aiming at ameliorating its drawbacks. OSU-2S, a PKCδ activator, was selected as a potential candidate anticancer agent to be combined with sorafenib to promote the anti-cancer activity through synergistic interaction. Methods: The antitumor effects of sorafenib, OSU-2S and their combination were assessed by MTT assay, caspase activation, Western blotting, migration/invasion assays in four different HCC cell lines. The synergistic interactions were determined by Calcusyn analysis. PKCδ knockdown was used to elucidate the role of PKCδ activation as a mechanism for the synergy. The knockdown/over-expression of p53 was used to explain the differential sensitivity of HCC cell lines to sorafenib and/or OSU-2S. Results: OSU-2S synergistically enhanced the anti-proliferative effects of sorafenib in the four used HCC cell lines with combination indices <1. This effect was accompanied by parallel increases in caspase 3/7 activity, PARP cleavage, PKCδ activation and inhibition of HCC cell migration/invasion. In addition, PKCδ knockdown abolished the synergy between sorafenib and OSU-2S. Furthermore, p53 restoration in Hep3B cells through the over-expression rendered them more sensitive to both agents while p53 knockdown from HepG2 cells increased their resistance to both agents. Conclusion: OSU-2S augments the anti-proliferative effect of sorafenib in HCC cell lines, in part, through the activation of PKCδ. The p53 status in HCC cells predicts their sensitivity toward both sorafenib and OSU-2S. The proposed combination represents a therapeutically relevant approach that can lead to a new HCC therapeutic protocol.

Structural basis for inhibition of the histone chaperone activity of SET/TAF-Iβ by cytochrome c

Chromatin is pivotal for regulation of the DNA damage process insofar as it influences access to DNA and serves as a DNA repair docking site. Recent works identify histone chaperones as key regulators of damaged chromatin's transcriptional activity. However, understanding how chaperones are modulated during DNA damage response is still challenging. This study reveals that the histone chaperone SET/TAF-Iβ interacts with cytochrome c following DNA damage. Specifically, cytochrome c is shown to be translocated into cell nuclei upon induction of DNA damage, but not upon stimulation of the death receptor or stress-induced pathways. Cytochrome c was found to competitively hinder binding of SET/TAF-Iβ to core histones, thereby locking its histone-binding domains and inhibiting its nucleosome assembly activity. In addition, we have used NMR spectroscopy, calorimetry, mutagenesis, and molecular docking to provide an insight into the structural features of the formation of the complex between cytochrome c and SET/TAF-Iβ. Overall, these findings establish a framework for understanding the molecular basis of cytochrome c-mediated blocking of SET/TAF-Iβ, which subsequently may facilitate the development of new drugs to silence the oncogenic effect of SET/TAF-Iβ's histone chaperone activity.

Integrative analysis of kinase networks in TRAIL-induced apoptosis provides a source of potential targets for combination therapy

Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) is an endogenous secreted peptide and, in preclinical studies, preferentially induces apoptosis in tumor cells rather than in normal cells. The acquisition of resistance in cells exposed to TRAIL or its mimics limits their clinical efficacy. Because kinases are intimately involved in the regulation of apoptosis, we systematically characterized kinases involved in TRAIL signaling. Using RNA interference (RNAi) loss-of-function and cDNA overexpression screens, we identified 169 protein kinases that influenced the dynamics of TRAIL-induced apoptosis in the colon adenocarcinoma cell line DLD-1. We classified the kinases as sensitizers or resistors or modulators, depending on the effect that knockdown and overexpression had on TRAIL-induced apoptosis. Two of these kinases that were classified as resistors were PX domain-containing serine/threonine kinase (PXK) and AP2-associated kinase 1 (AAK1), which promote receptor endocytosis and may enable cells to resist TRAIL-induced apoptosis by enhancing endocytosis of the TRAIL receptors. We assembled protein interaction maps using mass spectrometry-based protein interaction analysis and quantitative phosphoproteomics. With these protein interaction maps, we modeled information flow through the networks and identified apoptosis-modifying kinases that are highly connected to regulated substrates downstream of TRAIL. The results of this analysis provide a resource of potential targets for the development of TRAIL combination therapies to selectively kill cancer cells.

Gossypol induces death receptor-5 through activation of the ROS-ERK-CHOP pathway and sensitizes colon cancer cells to TRAIL

Development of resistance to TRAIL, an apoptosis-inducing cytokine, is one of the major problems in its development for cancer treatment. Thus, pharmacological agents that are safe and can sensitize the tumor cells to TRAIL are urgently needed. We investigated whether gossypol, a BH3 mimetic that is currently in the clinic, can potentiate TRAIL-induced apoptosis. Intracellular esterase activity, sub-G(1) cell cycle arrest, and caspase-8, -9, and -3 activity assays revealed that gossypol potentiated TRAIL-induced apoptosis in human colon cancer cells. Gossypol also down-regulated cell survival proteins (Bcl-x(L), Bcl-2, survivin, XIAP, and cFLIP) and dramatically up-regulated TRAIL death receptor (DR)-5 expression but had no effect on DR4 and decoy receptors. Gossypol-induced receptor induction was not cell type-specific, as DR5 induction was observed in other cell types. Deletion of DR5 by siRNA significantly reduced the apoptosis induced by TRAIL and gossypol. Gossypol induction of the death receptor required the induction of CHOP, and thus, gene silencing of CHOP abolished gossypol-induced DR5 expression and associated potentiation of apoptosis. ERK1/2 (but not p38 MAPK or JNK) activation was also required for gossypol-induced TRAIL receptor induction; gene silencing of ERK abolished both DR5 induction and potentiation of apoptosis by TRAIL. We also found that reactive oxygen species produced by gossypol treatment was critical for TRAIL receptor induction and apoptosis potentiation. Overall, our results show that gossypol enhances TRAIL-induced apoptosis through the down-regulation of cell survival proteins and the up-regulation of TRAIL death receptors through the ROS-ERK-CHOP-DR5 pathway.

TRAIL signaling is mediated by DR4 in pancreatic tumor cells despite the expression of functional DR5

Tumor necrosis factor related apoptosis-inducing ligand (TRAIL) and agonistic anti-DR4/TRAIL-R1 and anti-DR5/TRAIL-R2 antibodies are currently under clinical investigation for treatment of different malignancies. TRAIL activates DR4 and DR5 and thereby triggers apoptotic and non-apoptotic signaling pathways, but possible different roles of DR4 or DR5 in these responses has poorly been addressed so far. In the present work, we analyzed cell viability, DISC formation as well as IL-8 and NF-kappaB activation side by side in responses to TRAIL and agonistic antibodies against DR4 (mapatumumab) and against DR5 (lexatumumab) in pancreatic ductal adenocarcinoma cells. We found that all three reagents are able to activate cell death and pro-inflammatory signaling. Death-inducing signaling complex (DISC) analysis revealed that mapatumumab and lexatumumab induce formation of homocomplexes of either DR4 or DR5, whereas TRAIL additionally stimulated the formation of heterocomplexes of both receptors. Notably, blocking of receptors using DR4- and DR5-specific Fab fragments indicated that TRAIL exerted its function predominantly via DR4. Interestingly, inhibition of PKC by Goe6983 enabled DR5 to trigger apoptotic signaling in response to TRAIL and also strongly enhanced lexatumumab-mediated cell death. Our results suggest the existence of mechanisms that silence DR5 for TRAIL- but not for agonistic-antibody treatment.

Antitumor effects of Isatin on human neuroblastoma cell line (SH-SY5Y) and the related mechanism

The purpose of the study was to investigate the antitumor effects of Isatin and the related mechanism. Human neuroblastoma cells (SH-SY5Y) were exposed to Isatin at different concentrations for 48 h. Apoptotic features were demonstrated by means of nuclei staining with Hoechst 33258 and flow cytometry with propidium iodide (PI). Expressions of Bcl-2, Bax and vascular endothelial growth factor (VEGF) mRNA were analyzed via RT-PCR. Expressions of Bcl-2, Bax proteins and phosphorylated extracellular signal regulated protein kinases (ERKs, p42/p44) were analyzed via Western blot. Activation of caspase-3 was assayed by flow cytometry with anti-active caspase-3-McAb-PE. VEGF protein was determined by ELISA kits. And the results showed that apoptosis of SH-SY5Y cells were induced by Isatin in a dose-dependent manner. Expressions of Bcl-2, VEGF mRNA and Bcl-2, VEGF proteins were down-regulated, while expressions of Bax mRNA and Bax protein were not changed obviously. Expression of phosphorylated ERKs decreased, but the level of activated caspase-3 increased after treatment of Isatin. These results suggest that Isatin promotes the apoptosis of neuroblastoma cells, therefore, it might be a potential candidate for the treatment of neuroblastoma.

Regulation of the resistance to TRAIL-induced apoptosis in human primary T lymphocytes: Role of NF-κB inhibition

Several combined strategies have been recently proposed to overcome the resistance to tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL) showed by some tumor cells, thus improving the use of this death ligand in antitumor therapy. However, the molecular mechanisms of the tumor selective activity of TRAIL are not completely understood and hence the effects of the combined therapy on normal cells are unknown. Here, we have studied the resistance of primary T lymphocytes to TRAIL-mediated apoptosis. No significant differences were found in the expression of proteins involved in TRAIL-mediated apoptosis between resting and activated T cells. The low expression of death receptors TRAIL-R1/-R2 as well as the high levels of the antiapoptotic proteins TRAIL-R4 and cellular Fas-associated death domain-like IL-1beta-converting enzyme-inhibitory protein (c-FLIP) may explain the lack of caspase-8 activation observed upon TRAIL treatment in both cell types. We have also analyzed the effect of different sensitizing agents such as genotoxic drugs, phosphatidylinositol-3 kinase (PI3K) inhibitors, proteasome inhibitors, microtubule depolymerizing agents, histone deacetylase inhibitors (HDACi), and NF-kappaB inhibitors. Although some of them induced T cell death, only NF-kappaB inhibitors sensitized activated T cells to TRAIL-induced apoptosis, maybe through the regulation of the antiapoptotic proteins TRAIL-R4, c-FLIP(S) and members of the inhibitors of apoptosis proteins (IAP) family. These results question the safety of the combined treatments with TRAIL and NF-kappaB inhibitors against tumors.

Geldanamycin, a heat-shock protein 90-binding agent, induces thymocyte apoptosis through destabilization of Lck in presence of 12-O-tetradecanoylphorbol 13-acetate

Geldanamycin, a heat-shock protein 90 (Hsp90)-binding agent, modulates various cellular activities. The present study found that, although geldanamycin by itself had no effect on thymocyte viability, it induced apoptosis in thymocytes with a reduction of the mitochondrial transmembrane potential (DeltaPsim) in the presence of 12-O-tetradecanoylphorbol 13-acetate (TPA), an activator of protein kinase C (PKC). This apoptosis depended on transcription and translation, and on activation of caspase-8 and -3. Geldanamycin treatment in the presence of TPA also enhanced destabilization of Lck. This destabilization was independent of transcription and translation. It was inhibited, however, by conventional PKC inhibitors, preventing apoptosis. Proteasome inhibitor affected neither the degradation of Lck nor DNA fragmentation, although they inhibited reduction of DeltaPsim. These results suggest that the ubiquitin-proteasome system is not involved in Lck destabilization, and that DeltaPsim reduction is not directly related to the progression of apoptosis. Furthermore, inhibition of Lck in the presence of TPA induced apoptosis in thymocytes. Our findings suggest that Hsp90 modulates thymocyte apoptosis in concert with PKC through the destabilization of Lck and in a caspase-8- and -3-dependent manner.

Apoptosis resistance in ulcerative colitis: high expression of decoy receptors by lamina propria T cells

Intestinal mucosa is constantly exposed to normal environmental antigens. A significant number of intestinal mucosal T cells are being deleted through apoptosis. In contrast, T cells from inflamed mucosa of ulcerative colitis patients did not undergo apoptosis. In this study, we determined whether the apoptosis of normal mucosal T cells was induced by antigen receptor stimulation and further determined pathways that mediated the apoptosis. Freshly isolated lamina propria T cells were stimulated with CD3 mAb and apoptosis was determined by Annexin V staining. Normal mucosal T cells underwent apoptosis upon CD3 mAb stimulation whereas the T cells from inflamed mucosa did not. The apoptosis in normal T cells was blocked by TRAIL-R1:Fc and an inhibiting CD95 antibody. Interestingly, decoy receptor (DcR)1, DcR2, and DcR3 that compete with death receptor (DR)4/5 and CD95 were highly expressed by the T cells from inflamed mucosa, but much lower by T cells from normal mucosa. Our data suggest that normal mucosal T cells are constantly deleted in response to environmental antigens mediated through DR4/5 and CD95 pathways and mucosal T cells from ulcerative colitis resist to undergoing apoptosis due to highly expression of DcR1, DcR2, and DcR3.

The mitogen-activated protein kinase pathway can inhibit TRAIL-induced apoptosis by prohibiting association of truncated Bid with mitochondria

Breast cancer cells often show increased activity of the mitogen-activated protein kinase (MAPK) pathway. We report here that this pathway reduces their sensitivity to death ligand tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) and present the underlying mechanism. Activation of protein kinase C (PKC) inhibited TRAIL-induced apoptosis in a protein synthesis-independent manner. Deliberate activation of MAPK was also inhibitory. In digitonin-permeabilized cells, PKC activation interfered with the capacity of recombinant truncated (t)Bid to release cytochrome c from mitochondria. MAPK activation did not affect TRAIL or tumor necrosis factor (TNF)alpha-induced Bid cleavage. However, it did inhibit translocation of (t)Bid to mitochondria as determined both by subcellular fractionation analysis and confocal microscopy. Steady state tBid mitochondrial localization was prohibited by activation of the MAPK pathway, also when the Bcl-2 homology domain 3 (BH3) domain of tBid was disrupted. We conclude that the MAPK pathway inhibits TRAIL-induced apoptosis in MCF-7 cells by prohibiting anchoring of tBid to the mitochondrial membrane. This anchoring is independent of its interaction with resident Bcl-2 family members.

Current strategies in overcoming resistance of cancer cells to apoptosis melanoma as a model

Most anticancer agents mediate their effects through common pathways which induce apoptosis or in some cases necrosis of cancer cells. The apoptotic pathways are regulated by Bcl-2 family proteins, which include both pro- and anti-apoptotic members. Much is known about the interactions of these proteins involved in apoptosis and this information is being utilized in the development of new reagents that may be used to treat patients with cancers. The inhibitor of apoptosis family of proteins constitute a second group of proteins which inhibit the effector caspases. Reagents that inhibit their activity are also under development. Resistance of cancer cells to treatment can in many instances be attributed to activation of intracellular signal pathways involved in survival, such as the Ras-Raf-MEK-ERK1/2 or the P13K-Akt pathway. Again, much has been learned about the control of these pathways and their activation of resistance mechanisms. Inhibitors of such pathways are being evaluated in preclinical and clinical studies and are showing promise as a new class of anticancer agents. Much of the progress in future studies will likely depend on the ability to target these new treatments to particular subgroups of patients with tumor characteristics that make them responsive to the agents in question.

Instant decisions: transcription-independent control of death-receptor-mediated apoptosis

Transcription-independent modulation of signaling mediated by death receptors (DRs) has emerged as an important determinant of cell survival during both development and cellular homeostasis. Frequently, a given DR signal must be redirected rapidly either to inhibit or to potentiate the apoptotic response. This process requires immediate, protein-synthesis-independent modifications of the regulatory molecules involved. Numerous mechanisms have been shown to regulate DR responses without engaging the apoptosis-directing transcription machinery. These mechanisms involve key posttranslational modifications such as phosphorylation, ubiquitination and proteolytic degradation, all of which affect the activities of proteins at different levels in the DR signaling pathways. Changes in the organization of regulatory molecules and in their interactions with other factors also affect the DR signaling pathways. The balance between these modulatory signals rapidly decides the fate of a cell.

Casein kinase II (CK2) enhances death-inducing signaling complex (DISC) activity in TRAIL-induced apoptosis in human colon carcinoma cell lines

Protein kinase casein kinase II (CK2) is increased in response to diverse growth stimuli, as well as being elevated in many human cancers examined. We have demonstrated that CK2 is a key survival factor that protects human colon carcinoma cells from TNF-related apoptosis-inducing ligand (TRAIL)-induced apoptosis. We determined that inhibition of CK2 phosphorylation events by DRB (5,6-dichlorobenzimidazole) resulted in dramatic sensitization of tumor cells to TRAIL-induced apoptosis, in the absence of effects in normal cells. Sensitization was caspase dependent, and independent of regulation via NF-kappaB. Further, inhibition of phosphorylation by CK2 did not modify the expression level of antiapoptotic proteins. Analysis of TRAIL-induced death-inducing signaling complex (DISC) formation demonstrated enhanced formation of the DISC, enhanced cleavage of caspase-8 and cleavage of Bid in the presence of DRB, thereby facilitating the release of proapoptotic factors from the mitochondria with subsequent downregulation of the expression of XIAP and c-IAP1. Further, silencing of CK2alpha in HT29 cells following transfection of CK2alpha shRNA abrogated CK2 kinase activity while simultaneously increasing TRAIL sensitivity. These findings demonstrate that CK2 plays a critical antiapoptotic role by conferring resistance to TRAIL at the level of the DISC.

Variable expression of protein kinase Cε in human melanoma cells regulates sensitivity to TRAIL-induced apoptosis

Protein kinase C (PKC) activation is believed to protect against apoptosis induced by death receptors. We have found however that the effect of activation of PKC on tumor necrosis factor-related apoptosis-inducing ligand (TRAIL)-induced apoptosis of melanoma differs between cell lines. Pretreatment with phorbol 12-myristate 13-acetate (PMA) led to inhibition of apoptosis in the majority of the melanoma cell lines, but those with relatively low PKC epsilon expression were sensitized to TRAIL-induced apoptosis. Introduction of PKC epsilon into PKC epsilon-low cell lines reversed sensitization of the cells to TRAIL-induced apoptosis by PMA. In contrast, a dominant-negative form of PKC epsilon caused an increase in sensitivity. The changes in sensitivity to TRAIL-induced apoptosis were reflected in similar changes in conformation of Bax and its relocation from the cytosol to mitochondria. Similarly, there were concordant increases or decreases in mitochondrial release of second mitochondria-derived activator of caspase/DIABLO, activation of caspase-3, and processing of its substrates. Activation of PKC seemed to mediate its effects upstream of mitochondria but downstream of caspase-8 and Bid in that pretreatment with PMA did not cause significant changes in the expression levels of TRAIL death receptors, alterations in the levels of caspase-8 activation, or cleavage of Bid. PKC activated the anti-apoptotic extracellular signal-regulated kinase 1/2 pathway, but inhibitors of this pathway only partially reversed the protective effect of PKC against TRAIL-induced apoptosis. These results provide further insights into the variable responses of melanoma to TRAIL-induced apoptosis and may help define responsive phenotypes to treatment of melanoma with TRAIL.

Ingenol 3-angelate induces dual modes of cell death and differentially regulates tumor necrosis factor–related apoptosis-inducing ligand–induced apoptosis in melanoma cells

Ingenol 3-angelate (PEP005), one of the active ingredients in an extract from Euphorbia peplus, was shown in preclinical studies to have activity against human melanoma xenografts in nude mice. In the present study, we have tested its ability to induce the apoptosis of melanoma cells in vitro in the absence or presence of tumor necrosis factor-related apoptosis inducing ligand (TRAIL). The results showed that at relatively high concentrations (100 μg/mL), PEP005 killed melanoma cells mainly by induction of necrosis. In 20% of cell lines, evidence of apoptosis was observed. Apoptosis was caspase-dependent and associated with changes in mitochondrial membrane potential that were not inhibitable by overexpression of Bcl-2 or inhibition of caspases but were blocked by inhibition of protein kinase C (PKC). Low concentrations (1 or 10 μg/mL) of PEP005 either increased or decreased TRAIL-induced apoptosis in a cell line–dependent manner. These changes in TRAIL-induced apoptosis seemed to be due to activation of PKC and varying levels of PKC isoenzymes in different melanoma cell lines. PEP005-mediated enhancement of apoptosis seemed to be associated with low expression of the PKCε isoform. These results indicate that PEP005 may enhance or inhibit sensitivity of melanoma to treatments associated with TRAIL-induced apoptosis depending on the PKC isoform content of melanoma cells.

Casein Kinase I Attenuates Tumor Necrosis Factor–Related Apoptosis-Inducing Ligand–Induced Apoptosis by Regulating the Recruitment of Fas-Associated Death Domain and Procaspase-8 to the Death-Inducing Signaling Complex

Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) induces apoptosis in a wide variety of malignant cell lines, in contrast to normal cells, but with considerable heterogeneity in response. Death receptor-mediated apoptosis may be attenuated by a variety of different mechanisms, including phosphorylation-based signaling pathways. We have demonstrated that casein kinase I can attenuate TRAIL-induced apoptosis in human cell lines derived from colon adenocarcinoma (HT29 and HCT8) and pediatric rhabdomyosarcoma (JR1). Inhibition of casein kinase I (CKI) phosphorylation events in HT29, HCT8, and JR1 cells by CKI-7 dramatically increased apoptosis after exposure to TRAIL, in the absence of apoptosis induced by TRAIL treatment alone. CKI inhibition enhanced the recruitment of Fas-associated death domain and procaspase-8 to the death-inducing signaling complex after TRAIL treatment and enhanced cleavage of procaspase-8 at the death-inducing signaling complex. In HT29 cells studied further, rapid cleavage of caspase-8, caspase-3, Bid, and the caspase substrate poly(ADP-ribose) polymerase occurred when CKI-7 and TRAIL were combined. Overexpression of Bcl-2, Bcl-xL, or mutant DN-Fas-associated death domain protected HT29 cells from TRAIL-induced apoptosis in the presence of the CKI inhibitor. In addition, TRAIL combined with CKI-7 promoted the release of cytochrome c, Smac/DIABLO, HtrA2/Omi, and AIF from the mitochondria and down-regulated the expression of XIAP and c-IAP1. Small hairpin RNAs directed against CKI revealed that the CKIalpha isoform contributed significantly to the inhibition of TRAIL-induced apoptosis. These findings suggest that CKIalpha plays an antiapoptotic role through the generation of phosphorylated sites at the level of the death-inducing signaling complex, thereby conferring resistance to caspase cleavage mediated by TRAIL.

Influence of Casein Kinase II in Tumor Necrosis Factor-Related Apoptosis-Inducing Ligand-Induced Apoptosis in Human Rhabdomyosarcoma Cells

Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) induces apoptosis via the death receptors DR4 and DR5 in transformed cells in vitro and exhibits potent antitumor activity in vivo with minor side effects. Protein kinase casein kinase II (CK2) is increased in response to diverse growth stimuli and is aberrantly elevated in a variety of human cancers. Rhabdomyosarcoma tumors are the most common soft-tissue sarcoma in childhood. In this investigation, we demonstrate that CK2 is a key survival factor that protects tumor cells from TRAIL-induced apoptosis. We have demonstrated that inhibition of CK2 phosphorylation events by 5,6-dichlorobenzimidazole (DRB) resulted in dramatic sensitization of tumor cells to TRAIL-induced apoptosis. CK2 inhibition also induced rapid cleavage of caspase-8, -9, and -3, as well as the caspase substrate poly(ADP-ribose) polymerase after TRAIL treatment. Overexpression of Bcl-2 protected cells from TRAIL-induced apoptosis in the presence of the CK2 inhibitor. Death signaling by TRAIL in these cells was Fas-associated death domain and caspase dependent because dominant negative Fas-associated death domain or the cowpox interleukin 1beta-converting enzyme inhibitor protein cytokine response modifier A prevented apoptosis in the presence of DRB. Analysis of death-inducing signaling complex (DISC) formation demonstrated that inhibition of CK2 by DRB increased the level of recruitment of procaspase-8 to the DISC and enhanced caspase-8-mediated cleavage of Bid, thereby increasing the release of the proapoptotic factors cytochrome c, HtrA2/Omi, Smac/DIABLO, and apoptosis inducing factor (AIF) from the mitochondria, with subsequent degradation of X-linked inhibitor of apoptosis protein (XIAP). To further interfere with CK2 function, JR1 and Rh30 cells were transfected with either short hairpin RNA targeted to CK2alpha or kinase-inactive CK2alpha (K68M) or CK2alpha' (K69M). Data show that the CK2 kinase activity was abrogated and that TRAIL sensitivity in both cell lines was increased. Silencing of CK2alpha expression with short hairpin RNA was also associated with degradation of XIAP. These findings suggest that CK2 regulates TRAIL signaling in rhabdomyosarcoma by modulating TRAIL-induced DISC formation and XIAP expression.

Interferon-Gamma and TRAIL in Human Breast Tumor Cells

Induction of apoptosis in tumor cells by death receptor activation is a novel therapeutic strategy. However, in systemic antitumor treatments, severe toxic effects have been observed with tumor necrosis factor-alpha (TNF-alpha) and CD95 ligand. TNF-alpha causes a lethal inflammatory response and CD95L produces lethal liver damage. Preclinical studies in mice and nonhuman primates showed no systemic cytotoxicity upon injection of recombinant TNF-related apoptosis-inducing ligand (TRAIL) at doses that effectively suppressed solid tumors such as colon and mammary carcinomas. Although unwanted effects of some TRAIL preparations have been reported in normal cells, these data suggest that TRAIL could be a suitable approach in cancer therapy. However, several mechanisms of resistance to TRAIL-mediated apoptosis have been described in tumor cells such as lack of TRAIL apoptotic receptors, enhanced expression of TRAIL-decoy receptors, and expression of apoptosis inhibitors. In combination regimes, interferon-gamma (IFN-gamma) could provide a promising antitumor therapeutic approach as it has been described to enhance cellular susceptibility to apoptosis in a variety of tumor cells. The mechanism by which IFN-gamma promotes cell death seems to be via the regulation of the expression of different proteins involved in apoptosis. Altogether, these data suggest a combination strategy to selectively kill tumor cells that need to be further explored.

12-O-Tetradecanoylphorbol-13-acetate May Both Potentiate and Decrease the Generation of Apoptosis by the Antileukemic Agent Arsenic Trioxide in Human Promonocytic Cells

Arsenic trioxide (As(2)O(3)) caused apoptosis in U-937 human promonocytic cells. This effect was potentiated by the simultaneous addition of the glutathione (GSH) synthesis inhibitor DL-buthionine-(R,S)-sulfoximine or the protein kinase C activators 12-O-tetradecanoylphorbol-13-acetate (TPA) and bryostatin 1. In addition TPA decreased the intracellular GSH content, caused ERK activation, and potentiated the As(2)O(3)-provoked activation of p38 and JNK. The addition of N-acetyl-L-cysteine, the PKC inhibitor GF109203X, and the MEK/ERK inhibitors PD98059 and U0126 attenuated both apoptosis induction and GSH decrease, whereas the p38 inhibitor SB203580 and the JNK inhibitor SP600125 were ineffective. TPA also potentiated ERK activation and GSH depletion when added simultaneously to cadmium chloride (CdCl(2)) and doxorubicin. However, TPA only enhanced apoptosis in the case of CdCl(2), which is a GSH-sensitive agent, whereas it reduced the toxicity of doxorubicin and other DNA-specific drugs. Finally, preincubation for 14-24 h with TPA did not potentiate but, instead, attenuated the As(2)O(3)- and CdCl(2)-provoked apoptosis. The same result was obtained by preincubation with bryostatin 1 and other differentiation inducers. It is concluded that TPA increases the apoptotic action of As(2)O(3), an effect mediated by ERK activation and GSH depletion. However, the increase in apoptosis is only effective in non-differentiated cells.

Protein kinase C modulates tumor necrosis factor-related apoptosis-inducing ligand-induced apoptosis by targeting the apical events of death receptor signaling

We have further examined the mechanism by which phorbol ester-mediated protein kinase C (PKC) activation protects against tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL)-induced cytotoxicity. We now report that activation of PKC targets death receptor signaling complex formation. Pre-treatment with 12-O-tetradecanoylphorbol-13-acetate (PMA) led to inhibition of TRAIL-induced apoptosis in HeLa cells, which was characterized by a reduction in phosphatidylserine (PS) externalization, decreased caspase-8 processing, and incomplete maturation and activation of caspase-3. These effects of PMA were completely abrogated by the PKC inhibitor, bisindolylmaleimide I (Bis I), clearly implicating PKC in the protective effect of PMA. TRAIL-induced mitochondrial release of the apoptosis mediators cytochrome c and Smac was blocked by PMA. This, together with the observed decrease in Bid cleavage, suggested that PKC activation modulates apical events in TRAIL signaling upstream of mitochondria. This was confirmed by analysis of TRAIL death-inducing signaling complex formation, which was disrupted in PMA-treated cells as evidenced by a marked reduction in Fas-associated death domain protein (FADD) recruitment, an effect that could not be explained by any change in FADD phosphorylation state. In an in vitro binding assay, the intracellular domains of both TRAIL-R1 and TRAIL-R2 bound FADD: activation of PKC significantly inhibited this interaction suggesting that PKC may be targeting key apical components of death receptor signaling. Significantly, this effect was not confined to TRAIL, because isolation of the native TNF receptor signaling complex revealed that PKC activation also inhibited TNF receptor-associated death domain protein recruitment to TNF-R1 and TNF-induced phosphorylation of IkappaB-alpha. Taken together, these results show that PKC activation specifically inhibits the recruitment of key obligatory death domain-containing adaptor proteins to their respective membrane-associated signaling complexes, thereby modulating TRAIL-induced apoptosis and TNF-induced NF-kappaB activation, respectively.

Emerging applications of the tumor necrosis factor family of ligands and receptors in cancer therapy

Abnormalities of the tumor necrosis factor (TNF) family members have been linked to several human diseases, including cancer. Novel treatment strategies for cancer are emerging based on an understanding of the function of TNF family members. The advantage of these strategies is their potential to selectively target cancer cells, while sparing normal cells. Combining these new strategies with currently available treatments such as chemotherapy and radiation therapy is under investigation, with promising results. However, because some TNF family members are toxic to normal mammalian cells when administered systemically, only a few TNF family members have potential therapeutic value. This concise review focuses on the clinical implications of four TNF family members for cancer treatment: CD30/CD30 ligand, CD40/CD40 ligand, receptor activator of nuclear factor-kappaB (RANK)/RANK ligand, and TNF-related apoptosis-inducing ligand (TRAIL) Apo-2L/TRAIL receptors.

Clinical implications of the tumor necrosis factor family in benign and malignant hematologic disorders

Tumor necrosis factor (TNF), originally identified as a factor produced in the serum of endotoxin-injected animals, is a cytokine that mediates tumor necrosis. To date, 20 different members of the TNF superfamily and 21 different receptors have been identified. All ligands of the TNF superfamily have been found to activate transcription factor NF-kappaB and c-Jun kinase. Members of this family have diverse biologic effects, including induction of apoptosis, promotion of cell survival, and regulation of the immune system. The current review focuses on four members that play important roles in regulating hematopoietic cells and are involved in the pathogenesis of several hematologic malignancies. The potential therapeutic use of these members also is discussed.

Chelerythrin activates caspase-8, downregulates FLIP long and short, and overcomes resistance to tumour necrosis factor-related apoptosis-inducing ligand in KG1a cells

KG1a cells (CD34+/38-) express FAS and TRAIL (tumour-necrosis-factor-related apoptosis-inducing ligand) receptors but are resistant to FAS-ligand and TRAIL/APO2-L (apoptosis antigen-2 ligand)-induced apoptosis. KG1a cells are sensitized to FAS-induced apoptosis by chelerythrin, an inhibitor of protein kinase C (PKC). As cytoplasmatic adaptor molecules of FAS, e.g. FLIP [Fas-associated death domain protein (FADD)-like interleukin 1 beta-converting enzyme [FLICE (caspase-8)-inhibitory protein]], also modulate TRAIL signals, we determined whether chelerythrin affected TRAIL-mediated apoptosis. Chelerythrin by itself induced apoptosis in KG1a cells, and apoptosis was associated with activation of caspase-8. While TRAIL alone failed to activate caspase-8 or induce apoptosis, the addition of TRAIL to chelerythrin-treated cells significantly enhanced cleavage of caspase-8 and apoptosis. Chelerythrin-pretreated KG1a cells showed decreased phosphorylation of protein kinase C (PKC)-zeta and downregulation of both FLIP long and FLIP short proteins. Downregulation of FLIP and induction of apoptosis were partially abrogated by pretreatment with the specific caspase-8 inhibitor, Z-IETD-FMK. The decrease in FLIP protein expression induced by chelerythrin was accompanied by a progressive increase in mRNA levels of both FLIP long and FLIP short. CD34+ precursors from normal human marrow were also sensitive to chelerythrin but, in contrast to KG1a cells, were not sensitized to TRAIL-mediated apoptosis. Thus, resistance to TRAIL-induced apoptosis in leukaemic KG1a cells but not in normal CD34+ precursors was overcome in the presence of chelerythrin. The mechanism appeared to involve inhibition of PKC. Central targets were FLIP long and FLIP short, and their interactions with caspase-8. Whether such a pathway can be exploited to selectively target leukaemic progenitor cells remains to be determined.

Apo2L/TRAIL: apoptosis signaling, biology, and potential for cancer therapy

Apo2 ligand or tumor necrosis factor (TNF)-related apoptosis-inducing ligand (Apo2L/TRAIL) is one of several members of the TNF gene superfamily that induce apoptosis through engagement of death receptors. Apo2L/TRAIL is unusual as compared to any other cytokine as it interacts with a complex system of receptors: two pro-apoptotic death receptors and three anti-apoptotic decoys. This protein has generated tremendous excitement as a potential tumor-specific cancer therapeutic because, as a stable soluble trimer, it selectively induces apoptosis in many transformed cells but not in normal cells. Transcriptional activation of Apo2L/TRAIL by interferons (IFNs) through specific regulatory elements in its promoter, and possibly by a number of other cytokines, reveals its possible involvement in the activation of natural killer cells, cytotoxic T lymphocytes, and dendritic cells. In this review, we focus on the apoptosis signaling pathways stimulated by Apo2L/TRAIL, summarize what is known to date about the physiological role of this ligand and the potential for its application to cancer therapy.

Apo2L/TRAIL and its death and decoy receptors

Apo2 ligand or tumour necrosis factor-related apoptosis-inducing ligand (Apo2L/TRAIL) is one of the several members of the tumour necrosis factor (TNF) gene superfamily that induce apoptosis through engagement of death receptors (DRs). Apo2L/TRAIL interacts with an unusually complex receptor system of two DRs and three decoys. This protein has garnered intense interest as a potential candidate for cancer therapy because as a trimer it selectively induces apoptosis in many transformed cells but not in normal cells. While much of the early characterisation of Apo2L/TRAIL and its receptors relied on overexpression studies, recent work using untransfected cells has clarified how endogenous proteins transmit apoptotic signals from this ligand. In this review, we focus on the apoptotic signalling pathways stimulated by Apo2L/TRAIL and summarise what is known about its physiological role.

Mitogen-activated protein kinase/extracellular signal-regulated kinase signaling in activated T cells abrogates TRAIL-induced apoptosis upstream of the mitochondrial amplification loop and caspase-8

Fas ligand and TNF-related apoptosis-inducing ligand (TRAIL) induce apoptosis in many different cell types. Jurkat T cells die rapidly by apoptosis after treatment with either ligand. We have previously shown that mitogen-activated protein kinase (MAPK)/extracellular signal-regulated kinase (ERK) can act as a negative regulator of apoptosis mediated by the Fas receptor. In this study we examined whether MAPK/ERK can also act as a negative regulator of apoptosis induced by TRAIL. Activated Jurkat T cells were efficiently protected from TRAIL-induced apoptosis. The protection was shown to be MAPK/ERK dependent and independent of protein synthesis. MAPK/ERK suppressed TRAIL-induced apoptosis upstream of the mitochondrial amplification loop because mitochondrial depolarization and release of cytochrome c were inhibited. Furthermore, caspase-8-mediated relocalization and activation of Bid, a proapoptotic member of the Bcl family, was also inhibited by the MAPK/ERK signaling. The protection occurred at the level of the apoptotic initiator caspase-8, as the cleavage of caspase-8 was inhibited but the assembly of the death-inducing signaling complex was unaffected. Both TRAIL and Fas ligand have been suggested to regulate the clonal size and persistence of different T cell populations. Our previous results indicate that MAPK/ERK protects recently activated T cells from Fas receptor-mediated apoptosis during the initial phase of an immune response before the activation-induced cell death takes place. The results of this study show clearly that MAPK/ERK also participates in the inhibition of TRAIL-induced apoptosis after T cell activation.

Mitochondria-dependent and -independent mechanisms in tumour necrosis factor-related apoptosis-inducing ligand (TRAIL)-induced apoptosis are both regulated by interferon-gamma in human breast tumour cells

Tumour necrosis factor-related apoptosis-inducing ligand (TRAIL/APO-2L) induces apoptosis in a variety of tumour cells upon binding to death receptors TRAIL-R1 and TRAIL-R2. Here we describe the sensitization by interferon (IFN)-gamma to TRAIL-induced apoptosis in the breast tumour cell lines MCF-7 and MDA-MB231. IFN-gamma promoted TRAIL-mediated activation of caspase-8, Bcl-2 interacting domain death agonist (Bid) degradation, Bcl-2-associated X protein (Bax) translocation to mitochondria, cytochrome c release to the cytosol and activation of caspase-9 in these cell lines. No changes in the expression of TRAIL receptors were observed upon IFN-gamma treatment. Overexpression of Bcl-2 in MCF-7 cells completely inhibited IFN-gamma-induced sensitization to TRAIL-mediated cell death. Interestingly, TRAIL-induced apoptosis was also clearly enhanced by IFN-gamma in caspase-3-overexpressing MCF-7 cells, in the absence of Bax translocation to mitochondria and cytochrome c release to the cytosol. In summary, our results suggest that IFN-gamma facilitates TRAIL-induced activation of mitochondria-regulated as well as mitochondria-independent apoptotic pathways in breast tumour cells.

Stimulation of the mitogen-activated protein kinase pathway antagonizes TRAIL-induced apoptosis downstream of BID cleavage in human breast cancer MCF-7 cells

We studied the role of the mitogen-activated protein kinase (MAPK) pathway in the regulation of tumor necrosis factor-related apoptosis-inducing ligand (TRAIL)-mediated apoptosis in breast tumor MCF-7 cells. We found that addition of a protein kinase C (PKC) activator to MCF-7 cultures prevented TRAIL-induced apoptosis, by inhibiting a step downstream of both caspase-8 activation and BID cleavage. TRAIL-induced translocation of Bax from cytosol to mitochondria, release of cytochrome c from mitochondria and activation of caspase-9 were all inhibited by PKC activation. PKC-mediated prevention of mitochondrial apoptotic events and apoptosis was found to be dependent on the MAPK pathway. Since TRAIL is a ligand of potential use in antineoplastic clinical trials, our findings may provide relevant information in cancer therapy.

Blocking NF-kappaB activation in Jurkat leukemic T cells converts the survival agent and tumor promoter PMA into an apoptotic effector

The transcription factor NF-kappaB promotes cell survival. Using a variant of Jurkat leukemic T cells expressing IkappaB-alphaDeltaN, a super-repressor of NF-kappaB activation we first show that the tumor promoter PMA could prevent Fas-induced apoptosis via activation of NF-kappaB. Moreover, we demonstrate that in the absence of NF-kappaB activation, PMA became a strong inducer of apoptosis through stimulation of the upstream caspases 8 and 9 as well as of the effector caspase 3. A RNase-protection analysis showed that PMA stimulated the expression of several known anti-apoptotic genes (TRAF1, TRAF4, c-IAP-1, c-IAP-2, Bfl-1, Bcl-xl). In the absence of NF-kappaB activation, these survival influences were strongly lowered revealing the apoptotic effect of PMA. These results suggest that NF-kappaB activation could be an important step in the tumor promoting effect of PMA.

Phorbol 12-myristate 13-acetate inhibits death receptor-mediated apoptosis in Jurkat cells by disrupting recruitment of Fas-associated polypeptide with death domain

Regulation of death receptor-mediated apoptosis is incompletely understood. Previous studies have demonstrated that phorbol 12-myristate 13-acetate (PMA), a protein kinase C activator, inhibits Fas (CD95)-mediated apoptosis in Jurkat (type II) cells but not SKW6.4 (type I) cells. In this study, we demonstrated that PMA also protects Jurkat cells from apoptosis induced by tumor necrosis factor-alpha and the tumor necrosis factor-alpha-related apoptosis-inducing ligand (TRAIL). Interestingly, PMA failed to protect Jurkat cells from apoptosis induced by other agents, including etoposide, camptothecin, and gamma-irradiation. Analysis of the initial events induced by agonistic anti-Fas antibodies revealed that PMA inhibited Fas binding to Fas-associated polypeptide with death domain (FADD) in Jurkat cells but not in SKW6.4 cells. Although the protein kinase inhibitor bisindoylmaleimide VIII increased apoptosis induced by agonistic anti-Fas antibody, tumor necrosis factor-alpha, and TRAIL, these effects were not observed with the protein kinase C inhibitor H7 and were not associated with increased FADD recruitment to Fas. These results indicate that PMA inhibits death signaling induced by a number of discrete receptors and suggest that the effects are mediated at the level of receptor-mediated adaptor molecule recruitment.

Apoptosis and the response to anticancer therapy

Apoptosis is a distinctive form of cell death that reflects cleavage of a subset of intracellular polypeptides by proteases known as caspases. Two major intracellular caspase cascades, one activated predominantly by death receptor ligands and the other triggered by various cellular stresses, including DNA damage and microtubule disruption, have been delineated. Activation of these protease cascades is tightly regulated by a number of polypeptides, including Bcl-2 family members, inhibitor of apoptosis proteins, and several protein kinases. The demonstration that many antineoplastic agents induce apoptosis in susceptible cells raises the possibility that factors affecting caspase activation and activity might be important determinants of anticancer drug sensitivity. Here, we review recent studies describing the regulation of apoptotic pathways and identify potential implications of these findings for resistance to antineoplastic agents.

Potential and caveats of TRAIL in cancer therapy

Induction of apoptosis in tumor cells is a major goal for chemotherapy and radiation treatment strategies. However, disordered gene expression often leads to apoptosis resistance rendering tumor cells insensitive to various conventional treatments. TNF-related apoptosis-inducing ligand (TRAIL) is a recently identified cytokine of the TNF superfamily that induces apoptosis in tumor cells upon binding to different receptors. Remarkably, the majority of tumor cell lines are sensitive to TRAIL-induced apoptosis, while most nontransformed cell types are TRAIL-resistant. Furthermore, a combination treatment of TRAIL with ionizing irradiation or chemotherapeutic agents induces apoptosis in a highly synergistic manner, particularly in those cells that are otherwise resistant to a sole treatment. In contrast to other TNF members, TRAIL apparently does not exert overt systemic toxicity in murine and primate models, although unexpected concerns about a potential hepatotoxicity of TRAIL have been recently raised. While the molecular mechanisms of TRAIL sensitivity and resistance are poorly understood, TRAIL seems to be a promising biological agent for combination therapy with chemotherapeutic drugs or irradiation.