Rosario Yerbes

Flavopiridol Induces Cellular FLICE-Inhibitory Protein Degradation by the Proteasome and Promotes TRAIL–Induced Early Signaling and Apoptosis in Breast Tumor Cells

The cyclin-dependent kinase inhibitor flavopiridol is undergoing clinical trials as an antitumor drug. We show here that pretreatment of different human breast cancer cell lines with flavopiridol facilitates tumor necrosis factor-related apoptosis-inducing ligand (TRAIL)-induced apoptosis. In breast tumor cells, apoptosis induction by TRAIL is blocked at the level of apical caspase-8 activation. Flavopiridol treatment enhances TRAIL-induced formation of death-inducing signaling complex and early processing of procaspase-8. Subsequently, a TRAIL-induced, mitochondria-operated pathway of apoptosis is activated in cells treated with flavopiridol. Down-regulation of cellular FLICE-inhibitory proteins (c-FLIP; c-FLIP(L) and c-FLIP(S)) is observed on flavopiridol treatment. c-FLIP loss and apoptosis sensitization by flavopiridol are both prevented in cells treated with an inhibitor of the ubiquitin-proteasome system. Furthermore, targeting c-FLIP directly with small interfering RNA oligonucleotides also sensitizes various human breast tumor cell lines to TRAIL-induced apoptosis. Our results indicate that flavopiridol sensitizes breast cancer cells to TRAIL-induced apoptosis by facilitating early events in the apoptotic pathway, and this combination treatment could be regarded as a potential therapeutic tool against breast tumors.

The therapeutic potential of TRAIL receptor signalling in cancer cells

In tumour cells, activation of the apoptotic machinery by death receptor ligands of the tumour necrosis factor (TNF) receptor superfamily of cytokines represents a novel therapeutic strategy. However, systemic treatment of tumours with TNF-α and CD95 ligand may produce severe toxic effects. The tumour necrosis-related apoptosis-inducing ligand (TRAIL) is a member of the TNF family capable of inducing apoptosis in a wide variety of cancer cells upon binding to pro-apoptotic receptors, while having no effect on the majority of normal human cells tested. Interestingly, preclinical studies in mice and nonhuman primates showed no systemic cytotoxicity upon injection of either recombinant TRAIL or agonistic TRAIL-receptor antibodies. Furthermore, these treatments have been shown to effectively suppress the growth of a range of tumour xenografts. Although unwanted effects of some TRAIL preparations have been reported in normal cells, the use of TRAIL receptor agonists could represent a suitable approach in cancer therapy. Here, we shall review our current understanding of apoptotic and non-apoptotic TRAIL signalling, the therapeutic potential of TRAIL-based approaches in cancer treatment, and the results of phase 1 and 2 clinical trials with recombinant TRAIL or agonistic TRAIL receptor antibodies, either as monotherapy or in combination with other chemotherapeutic agents.

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 cFLIPL and cFLIPS 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.

Activated ERBB2/HER2 licenses sensitivity to apoptosis upon endoplasmic reticulum stress through a PERK-Dependent pathway

HER2/Neu/ERBB2 is a receptor tyrosine kinase overexpressed in approximately 20% of human breast tumors. Truncated or mutant isoforms that show increased oncogenicity compared with the wild-type receptor are found in many breast tumors. Here, we report that constitutively active ERBB2 sensitizes human breast epithelial cells to agents that induce endoplasmic reticulum stress, altering the unfolded protein response (UPR) of these cells. Deregulation of the ERK, AKT, and mTOR activities elicited by mutant ERBB2 was involved in mediating this differential UPR response, elevating the response to endoplasmic reticulum stress, and apoptotic cell death. Mechanistic investigations revealed that the increased sensitivity of mutant ERBB2-expressing cells to endoplasmic reticulum stress relied upon a UPR effector signaling involving the PERK-ATF4-CHOP pathway, upregulation of the proapoptotic cell surface receptor TRAIL-R2, and activation of proapoptotic caspase-8. Collectively, our results offer a rationale for the therapeutic exploration of treatments inducing endoplasmic reticulum stress against mutant ERBB2-expressing breast tumor cells.

Itch/AIP4-independent proteasomal degradation of cFLIP induced by the histone deacetylase inhibitor SAHA sensitizes breast tumour cells to TRAIL

SummaryThe histone deacetylase inhibitor suberoylanilide hydroxamic acid (SAHA, vorinostat) is undergoing clinical trials as an antitumor drug and has received regulatory approval for cancer treatment. Here, we show that pre-treatment of human breast cancer cells with SAHA makes them susceptible to apoptosis induced by TRAIL (tumour necrosis factor-related apoptosis-inducing ligand). The apoptosis of breast tumour cells induced by TRAIL is blocked at the level of apical activation of caspase-8 and SAHA enhances the TRAIL-induced processing of procaspase-8. Consequently, a TRAIL associated pathway of apoptosis operated via mitochondria is activated in cells treated with SAHA. Interestingly, degradation of cellular FLICE-inhibitory proteins (cFLIPL and cFLIPS) by an ubiquitin/proteasome-dependent Itch/AIP4-independent mechanism is observed upon exposure to SAHA. Targeting cFLIPL directly with siRNA oligonucleotides also sensitizes human breast tumour cells to TRAIL-induced apoptosis. Furthermore, cFLIPL over-expression significantly inhibits the apoptosis elicited through the combined effects of SAHA and TRAIL. Together, these results indicate that SAHA sensitizes breast cancer cells to TRAIL-induced apoptosis by facilitating the activation of early events in the apoptotic TRAIL pathway. Therefore, the combination of TRAIL and SAHA may represent a therapeutic tool to combat breast tumours.

Itch/AIP4-independent proteasomal degradation of cFLIP mediates sensitization of breast tumor cells to TRAIL by the histone deacetylase inhibitor SAHA

SummaryThe histone deacetylase inhibitor suberoylanilide hydroxamic acid (SAHA, vorinostat) is undergoing clinical trials as an antitumor drug and has received regulatory approval for cancer treatment. Here, we show that pre-treatment of human breast cancer cells with SAHA makes them susceptible to apoptosis induced by TRAIL (tumour necrosis factor-related apoptosis-inducing ligand). The apoptosis of breast tumour cells induced by TRAIL is blocked at the level of apical activation of caspase-8 and SAHA enhances the TRAIL-induced processing of procaspase-8. Consequently, a TRAIL associated pathway of apoptosis operated via mitochondria is activated in cells treated with SAHA. Interestingly, degradation of cellular FLICE-inhibitory proteins (cFLIPL and cFLIPS) by an ubiquitin/proteasome-dependent Itch/AIP4-independent mechanism is observed upon exposure to SAHA. Targeting cFLIPL directly with siRNA oligonucleotides also sensitizes human breast tumour cells to TRAIL-induced apoptosis. Furthermore, cFLIPL over-expression significantly inhibits the apoptosis elicited through the combined effects of SAHA and TRAIL. Together, these results indicate that SAHA sensitizes breast cancer cells to TRAIL-induced apoptosis by facilitating the activation of early events in the apoptotic TRAIL pathway. Therefore, the combination of TRAIL and SAHA may represent a therapeutic tool to combat breast tumours.

Cellular FLIPL plays a survival role and regulates morphogenesis in breast epithelial cells

Strong evidences support the inhibitory activity of cellular FLICE-inhibitory protein (FLIP) in the apoptotic signalling by death receptors in tumor cells. However, little is known about the role of FLIP in the regulation of apoptosis in non-transformed cells. In this report, we demonstrate that FLIP(L) plays an important role as a survival protein in non-transformed breast epithelial cells. Silencing of FLIP(L) by siRNA methodology enhances TRAIL-R2 expression and activates a caspase-dependent cell death process in breast epithelial cells. This cell death requires the expression of TRAIL, TRAIL-R2, FADD and procaspase-8 proteins. A mitochondria-operated apoptotic pathway is partially required for FLIP(L) siRNA-induced apoptosis. Interestingly, FLIP(L) silencing markedly abrogates formation of acinus-like structures in a three-dimensional basement membrane culture model (3D) of the human mammary MCF-10A cell line through a caspase-8 dependent process. Furthermore, over-expression of FLIP(L) in MCF-10A cells delayed lumen formation in 3D cultures. Our results highlight the central role of FLIP in maintaining breast epithelial cell viability and suggest that the mechanisms regulating FLIP levels should be finely controlled to prevent unwanted cell demise.

Control of FLIP L expression and TRAIL resistance by the extracellular signal-regulated kinase1/2 pathway in breast epithelial cells

Increased activation of the epidermal growth factor receptor (EGFR) is frequently observed in tumors, and inhibition of the signaling pathways originated in the EGFR normally renders tumor cells more sensitive to apoptotic stimuli. However, we show that inhibition of EGFR signaling in non-transformed breast epithelial cells by EGF deprivation or gefitinib, an inhibitor of EGFR tyrosine kinase, causes the upregulation of the long isoform of caspase-8 inhibitor FLICE-inhibitory protein (FLIPL) and makes these cells more resistant to the tumor necrosis factor-related apoptosis-inducing ligand (TRAIL). We demonstrate that the extracellular signal-regulated kinase (ERK)1/2 pathway plays a pivotal role in the regulation of FLIPL levels and sensitivity to TRAIL-induced apoptosis by EGF. Upregulation of FLIPL upon EGF deprivation correlates with a decrease in c-Myc levels and c-Myc knockdown by siRNA induces FLIPL expression. FLIPL upregulation and resistance to TRAIL in EGF-deprived cells are reversed following activation of an estrogen activatable form of c-Myc (c-Myc-ER). Finally, constitutive activation of the ERK1/2 pathway in HER2/ERBB2-transformed cells prevents EGF deprivation-induced FLIPL upregulation and TRAIL resistance. Collectively, our results suggest that a regulated ERK1/2 pathway is crucial to control FLIPL levels and sensitivity to TRAIL in non-transformed cells, and this mechanism may explain the increased sensitivity of tumor cells to TRAIL, in which the ERK1/2 pathway is frequently deregulated.

The Long and Winding Road to Cancer Treatment: The Trail System

Activation of cell surface death receptors of the tumor necrosis factor (TNF) receptor superfamily by the appropriate ligands represents an attractive therapeutic strategy to induce cell death by apoptosis in cancer cells. However, the toxic effects of TNF-alpha and CD95/Fas ligand (FasL) in normal tissues have significantly hampered the clinical application of these ligands in cancer treatment. TNF-related apoptosis-inducing ligand (TRAIL/APO-2L), another member of the TNF family, has been shown to induce apoptosis selectively in many tumor cell lines. Interestingly, TRAIL treatment also results in significant growth suppression of TRAIL-sensitive human cancer xenografts in mice and nonhuman primates. At the same time, recombinant TRAIL and agonistic TRAIL receptor antibodies show no significant cytotoxicity in these studies. Despite some adverse effects of certain TRAIL preparations, activation of proapoptotic TRAIL receptors represents a promising approach in cancer therapy. Herein we review what is known about proapoptotic TRAIL signaling, the role of intracellular survival pathways in the regulation of resistance to TRAIL and the activation of non-apoptotic signaling by TRAIL. We also discuss the role of the TRAIL system in tumorigenesis and the results of clinical trials with recombinant TRAIL and various TRAIL receptor agonistic antibodies, either as monotherapy or in combination with targeted or conventional chemotherapy.

Oncogenic p95HER2/611CTF primes human breast epithelial cells for metabolic stress-induced down-regulation of FLIP and activation of TRAIL-R/Caspase-8-dependent apoptosis

Oncogenic transformation triggers reprogramming of cell metabolism, as part of the tumorigenic process. However, metabolic reprogramming may also increase the sensitivity of transformed cells to microenvironmental stress, at the early stages of tumor development. Herein, we show that transformation of human breast epithelial cells by the p95HER2/611CTF oncogene markedly sensitizes these cells to metabolic stress induced by the simultaneous inhibition of glucose and glutamine metabolism. In p95HER2/611CTF-transformed cells, metabolic stress activates a TNF related apoptosis-inducing ligand (TRAIL)-R and caspase-8-dependent apoptotic process that requires prior down-regulation of cellular FLICE-like inhibitor protein (c-FLIP) levels. Importantly, sustained mTOR activation is involved in FLIP down-regulation and apoptosis induced by metabolic stress. In vivo experiments in immunodeficient mice demonstrate a requirement for caspase-8 in restraining primary tumor growth of xenografts with p95HER2/611CTF-transformed cells. Collectively, these data define a critical role of the extrinsic pathway of apoptosis in the control of tumor initiation by microenvironmental cues.