Graph Convolutional Network for predicting secondary structure of RNA

The prediction of RNA secondary structures is essential for understanding its underlying principles and applications in diverse fields, including molecular diagnostics and RNA-based therapeutic strategies. However, the complexity of the search space presents a challenge. This work proposes a Graph Convolutional Network (GCNfold) for predicting the RNA secondary structure. GCNfold considers an RNA sequence as graph-structured data and predicts posterior base-pairing probabilities given the prior base-pairing probabilities, calculated using McCaskill's partition function. The performance of GCNfold surpasses that of the state-of-the-art folding algorithms, as we have incorporated minimum free energy information into the richly parameterized network, enhancing its robustness in predicting non-homologous RNA secondary structures. A Symmetric Argmax Post-processing algorithm ensures that GCNfold formulates valid structures. To validate our algorithm, we applied it to the SARS-CoV-2 E gene and determined the secondary structure of the E-gene across the Betacoronavirus subgenera.

Aggregation determines the selectivity of membrane-active anticancer and antimicrobial peptides: The case of killerFLIP

Host defense peptides selectively kill bacterial and cancer cells (including those that are drug-resistant) by perturbing the permeability of their membranes, without being significantly toxic to the host. Coulombic interactions between these cationic and amphipathic peptides and the negatively charged membranes of pathogenic cells contribute to the selective toxicity. However, a positive charge is not sufficient for selectivity, which can be achieved only by a finely tuned balance of electrostatic and hydrophobic driving forces. A common property of amphipathic peptides is the formation of aggregated structures in solution, but the role of this phenomenon in peptide activity and selectivity has received limited attention. Our data on the anticancer peptide killerFLIP demonstrate that aggregation strongly increases peptide selectivity, by reducing the effective peptide hydrophobicity and thus the affinity towards membranes composed of neutral lipids (like the outer layer of healthy eukaryotic cell membranes). Aggregation is therefore a useful tool to modulate the selectivity of membrane active peptides and peptidomimetics.

Changes in membrane lipids drive increased endocytosis following Fas ligation

Once activated, some surface receptors promote membrane movements that open new portals of endocytosis, in part to facilitate the internalization of their activated complexes. The prototypic death receptor Fas (CD95/Apo1) promotes a wave of enhanced endocytosis that induces a transient intermixing of endosomes with mitochondria in cells that require mitochondria to amplify death signaling. This initiates a global alteration in membrane traffic that originates from changes in key membrane lipids occurring in the endoplasmic reticulum (ER). We have focused the current study on specific lipid changes occurring early after Fas ligation. We analyzed the interaction between endosomes and mitochondria in Jurkat T cells by nanospray-Time-of-flight (ToF) Mass Spectrometry. Immediately after Fas ligation, we found a transient wave of lipid changes that drives a subpopulation of early endosomes to merge with mitochondria. The earliest event appears to be a decrease of phosphatidylcholine (PC), linked to a metabolic switch enhancing phosphatidylinositol (PI) and phosphoinositides, which are crucial for the formation of vacuolar membranes and endocytosis. Lipid changes occur independently of caspase activation and appear to be exacerbated by caspase inhibition. Conversely, inhibition or compensation of PC deficiency attenuates endocytosis, endosome-mitochondria mixing and the induction of cell death. Deficiency of receptor interacting protein, RIP, also limits the specific changes in membrane lipids that are induced by Fas activation, with parallel reduction of endocytosis. Thus, Fas activation rapidly changes the interconversion of PC and PI, which then drives enhanced endocytosis, thus likely propagating death signaling from the cell surface to mitochondria and other organelles.

A Peroxidase Peroxiredoxin 1-Specific Redox Regulation of the Novel FOXO3 microRNA Target let-7

Precision in redox signaling is attained through posttranslational protein modifications such as oxidation of protein thiols. The peroxidase peroxiredoxin 1 (PRDX1) regulates signal transduction through changes in thiol oxidation of its cysteines. We demonstrate here that PRDX1 is a binding partner for the tumor suppressive transcription factor FOXO3 that directly regulates the FOXO3 stress response. Heightened oxidative stress evokes formation of disulfide-bound heterotrimers linking dimeric PRDX1 to monomeric FOXO3. Absence of PRDX1 enhances FOXO3 nuclear localization and transcription that are dependent on the presence of Cys31 or Cys150 within FOXO3. Notably, FOXO3-T32 phosphorylation is constitutively enhanced in these mutants, but nuclear translocation of mutant FOXO3 is restored with PI3K inhibition. Here we show that on H₂O₂ exposure, transcription of tumor suppressive miRNAs let-7b and let-7c is regulated by FOXO3 or PRDX1 expression levels and that let-7c is a novel target for FOXO3. Conjointly, inhibition of let-7 microRNAs increases let-7-phenotypes in PRDX1-deficient breast cancer cells. Altogether, these data ascertain the existence of an H₂O₂-sensitive PRDX1-FOXO3 signaling axis that fine tunes FOXO3 activity toward the transcription of gene targets in response to oxidative stress. Antioxid. Redox Signal. 28, 62-77.

Meta-analysis of transcriptome data identifies a novel 5-gene pancreatic adenocarcinoma classifier

PURPOSE

Pancreatic ductal adenocarcinoma (PDAC) is largely incurable due to late diagnosis. Superior early detection biomarkers are critical to improving PDAC survival and risk stratification.

EXPERIMENTAL DESIGN

Optimized meta-analysis of PDAC transcriptome datasets identified and validated key PDAC biomarkers. PDAC-specific expression of a 5-gene biomarker panel was measured by qRT-PCR in microdissected patient-derived FFPE tissues. Cell-based assays assessed impact of two of these biomarkers, TMPRSS4 and ECT2, on PDAC cells.

RESULTS

A 5-gene PDAC classifier (TMPRSS4, AHNAK2, POSTN, ECT2, SERPINB5) achieved on average 95% sensitivity and 89% specificity in discriminating PDAC from non-tumor samples in four training sets and similar performance (sensitivity = 94%, specificity = 89.6%) in five independent validation datasets. This classifier accurately discriminated PDAC from chronic pancreatitis (AUC = 0.83), other cancers (AUC = 0.89), and non-tumor from PDAC precursors (AUC = 0.92) in three independent datasets. Importantly, the classifier distinguished PanIN from healthy pancreas in the PDX1-Cre;LSL-KrasG12D PDAC mouse model. Discriminatory expression of the PDAC classifier genes was confirmed in microdissected FFPE samples of PDAC and matched surrounding non-tumor pancreas or pancreatitis. Notably, knock-down of TMPRSS4 and ECT2 reduced PDAC soft agar growth and cell viability and TMPRSS4 knockdown also blocked PDAC migration and invasion.

CONCLUSIONS

This study identified and validated a highly accurate 5-gene PDAC classifier for discriminating PDAC and early precursor lesions from non-malignant tissue that may facilitate early diagnosis and risk stratification upon validation in prospective clinical trials. Cell-based experiments of two overexpressed proteins encoded by the panel, TMPRSS4 and ECT2, suggest a causal link to PDAC development and progression, confirming them as potential therapeutic targets.

Abstract PR07: Transcriptome meta-analysis identifies new 5-gene classifier for early detection of pancreatic cancer

Background: Pancreatic ductal adenocarcinoma (PDAC) is a truly devastating disease, primarily due to late diagnosis when curative resection is no longer possible and the lack of accurate early detection biomarkers. Therefore, increasing PDAC survival depends heavily on improving the accuracy and effectiveness of early PDAC detection.

Methods:Key PDAC biomarkers were identified through an optimized meta-analysis strategy of raw data in 4 PDAC transcriptome datasets (training sets). Application of identical normalization and statistical testing procedures for each dataset generated a list of differentially expressed genes by empirical Bayes moderated t-statistic. The top ranking PDAC-specific genes from this list that were consistently differentially expressed in these training sets were used for developing a classifier for PDAC. The biomarker panel with the highest sensitivity and specificity in the training sets was validated for diagnostic performance in nine independent validation datasets. A subset of genes in the panel were evaluated by immunohistochemistry (IHC) in human PDAC tissue sections. Cell based assays assessed the effect of two genes on PDAC migration and invasion.

Results and Conclusions: A 5-gene classifier panel was identified that disntinguished between PDAC and normal pancreas with high sensitivity, specificity and accuracy in the four training sets. Validation of this 5-gene predictor in 9 independent test sets confirmed the diagnostic accuracy to not only discriminate accurately between PDAC and healthy controls, but also between pancreatitis and PDAC. IHC analysis of two of the proteins encoded by the panel confirmed enhanced expression in PDAC cells. Cell-based experiments demonstrated that both proteins enhance migration and invasion of PDAC cells as well as soft agar growth, suggesting a causal link to PDAC development and progression. The immediate clinical utility upon further prospective validation of this PDAC biomarker panel will be accurate PDAC diagnosis in patients with pancreatic abnormalities on imaging, risk stratification and evaluation of the risk of malignancy in pancreatic cysts.

This abstract is also presented as Poster B26.

Citation Format: Manoj Bhasin, Octavian Bucur, Ken Ndebele, Jessica Plati, Andrea Bullock, Xuesong Gu, Eduardo Castan, Robert Najarian, Jen Jen Yeh, Channing Der, Jon Cody Haines, Karl Ruping, Rebecca Miksad, Roya Khosravi-Far, Towia Aron Libermann. Transcriptome meta-analysis identifies new 5-gene classifier for early detection of pancreatic cancer. [abstract]. In: Proceedings of the AACR Special Conference on Pancreatic Cancer: Innovations in Research and Treatment; May 18-21, 2014; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2015;75(13 Suppl):Abstract nr PR07.

A novel caspase 8 selective small molecule potentiates TRAIL-induced cell death

Recombinant soluble TRAIL and agonistic antibodies against TRAIL receptors (DR4 and DR5) are currently being created for clinical cancer therapy, due to their selective killing of cancer cells and high safety characteristics. However, resistance to TRAIL and other targeted therapies is an important issue facing current cancer research field. An attractive strategy to sensitize resistant malignancies to TRAIL-induced cell death is the design of small molecules that target and promote caspase 8 activation. For the first time, we describe the discovery and characterization of a small molecule that directly binds caspase 8 and enhances its activation when combined with TRAIL, but not alone. The molecule was identified through an in silico chemical screen for compounds with affinity for the caspase 8 homodimer's interface. The compound was experimentally validated to directly bind caspase 8, and to promote caspase 8 activation and cell death in single living cells or population of cells, upon TRAIL stimulation. Our approach is a proof-of-concept strategy leading to the discovery of a novel small molecule that not only stimulates TRAIL-induced apoptosis in cancer cells, but may also provide insights into the structure-function relationship of caspase 8 homodimers as putative targets in cancer.

Abstract LB-173: Early detection of pancreatic cancer using a new 5-gene classifier

Background: Largely due to late diagnosis when curative resection is no longer possible, Pancreatic Adenocarcinoma (PDAC) is a devastating disease marked by a mortality rate nearly equivalent to its incidence rate. While serum CA19-9 is associated with PDAC, it lacks the sensitivity and specificity needed for early and accurate diagnosis. Thus, early diagnosis of PDAC is critical to improving survival for PDAC patients and for accurate PDAC risk stratification.

Methods: We have identified key PDAC biomarkers through an optimized meta-analysis strategy of raw data (training sets). From this analysis, the top ranking genes that distinguish PDAC from normal pancreas were selected through statistical significance comparison of the consistency in differential PDAC gene expression across studies. The biomarker panel with the highest sensitivity and specificity in the training sets was chosen for assessment of predictive power in nine independent validation datasets. For a subset of genes in the panel, immunohistochemistry (IHC) analysis was performed in human PDAC and pancreatitis to validate our panel.

Results and Conclusions: We have identified a 5-gene classifier panel with specificity of 88.9% and sensitivity of 100% with an overall accuracy of 93.3% and AUC of 0.94 for discriminating PDAC from non-malignant (normal or pancreatitis) tissue. IHC analysis of two of the proteins confirmed enhanced expression in PDAC cells. Cell-based experiments demonstrated that both proteins enhance migration and invasion of PDAC cells as well as soft agar growth indicating that they may be causative in PDAC. The 5-gene predictor discriminated PDAC from healthy controls, as well as pancreatitis, with highest accuracy in nine independent validation sets. Our identified PDAC panel may facilitate early diagnosis and improve risk-stratification.

Citation Format: Manoj Bhasin, Andrea Bullock, Xuesong Gu, Octavian Bucur, Robert Najarian, Jon Cody Haines, Karl Ruping, Rebecca Miksad, Towia Libermann, Roya Khosravi-Far. Early detection of pancreatic cancer using a new 5-gene classifier. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr LB-173. doi:10.1158/1538-7445.AM2014-LB-173

PLK1 is a binding partner and a negative regulator of FOXO3 tumor suppressor

FOXO family members (FOXOs: FOXO1, FOXO3, FOXO4 and FOXO6) are important transcription factors and tumor suppressors controlling cell homeostasis and cell fate. They are characterized by an extraordinary functional diversity, being involved in regulation of cell cycle, proliferation, apoptosis, DNA damage response, oxidative detoxification, cell differentiation and stem cell maintenance, cell metabolism, angiogenesis, cardiac and other organ's development, aging, and other critical cellular processes. FOXOs are tightly regulated by reversible phosphorylation, ubiquitination, acetylation and methylation. Interestingly, the known kinases phosphorylate only a small percentage of the known or predicted FOXOs phosphorylation sites, suggesting that additional kinases that phosphorylate and control FOXOs activity exist. In order to identify novel regulators of FOXO3, we have employed a proteomics screening strategy. Using HeLa cancer cell line and a Tandem Affinity Purification followed by Mass Spectrometry analysis, we identified several proteins as binding partners of FOXO3. Noteworthy, Polo Like Kinase 1 (PLK1) proto-oncogene was one of the identified FOXO3 binding partners. PLK1 plays a critical role during cell cycle (G2-M transition and all phases of mitosis) and in maintenance of genomic stability. Our experimental results presented in this manuscript demonstrate that FOXO3 and PLK1 exist in a molecular complex through most of the phases of the cell cycle, with a higher occurrence in the G2-M cell cycle phases. PLK1 induces translocation of FOXO3 from the nucleus to the cytoplasm and suppresses FOXO3 activity, measured by the decrease in the pro-apoptotic Bim protein levels and in the cell cycle inhibitor protein p27. Furthermore, PLK1 can directly phosphorylate FOXO3 in an in vitro kinase assay. These results present the discovery of PLK1 proto-oncogene as a binding partner and a negative regulator of FOXO3 tumor suppressor.

Blocks to thyroid cancer cell apoptosis can be overcome by inhibition of the MAPK and PI3K/AKT pathways

Current treatment for recurrent and aggressive/anaplastic thyroid cancers is ineffective. Novel targeted therapies aimed at the inhibition of the mutated oncoprotein BRAF^V600E have shown promise in vivo and in vitro but do not result in cellular apoptosis. TNF-related apoptosis-inducing ligand (TRAIL) induces apoptosis in a tumor-selective manner by activating the extrinsic apoptotic pathway. Here, we show that a TRAIL-R2 agonist antibody, lexatumumab, induces apoptosis effectively in some thyroid cancer cell lines (HTh-7, TPC-1 and BCPAP), while more aggressive anaplastic cell lines (8505c and SW1736) show resistance. Treatment of the most resistant cell line, 8505c, using lexatumumab in combination with the BRAF^V600E inhibitor, PLX4720, and the PI3K inhibitor, LY294002, (triple-drug combination) sensitizes the cells by triggering both the extrinsic and intrinsic apoptotic pathways in vitro as well as 8505c orthotopic thyroid tumors in vivo. A decrease in anti-apoptotic proteins, pAkt, Bcl-xL, Mcl-1 and c-FLIP, coupled with an increase in the activator proteins, Bax and Bim, results in an increase in the Bax to Bcl-xL ratio that appears to be critical for sensitization and subsequent apoptosis of these resistant cells. Our results suggest that targeting the death receptor pathway in thyroid cancer can be a promising strategy for inducing apoptosis in thyroid cancer cells, although combination with other kinase inhibitors may be needed in some of the more aggressive tumors initially resistant to apoptosis.

Combination of bortezomib and mitotic inhibitors down-modulate Bcr-Abl and efficiently eliminates tyrosine-kinase inhibitor sensitive and resistant Bcr-Abl-positive leukemic cells

Emergence of resistance to Tyrosine-Kinase Inhibitors (TKIs), such as imatinib, dasatinib and nilotinib, in Chronic Myelogenous Leukemia (CML) demands new therapeutic strategies. We and others have previously established bortezomib, a selective proteasome inhibitor, as an important potential treatment in CML. Here we show that the combined regimens of bortezomib with mitotic inhibitors, such as the microtubule-stabilizing agent Paclitaxel and the PLK1 inhibitor BI2536, efficiently kill TKIs-resistant and -sensitive Bcr-Abl-positive leukemic cells. Combined treatment activates caspases 8, 9 and 3, which correlate with caspase-induced PARP cleavage. These effects are associated with a marked increase in activation of the stress-related MAP kinases p38MAPK and JNK. Interestingly, combined treatment induces a marked decrease in the total and phosphorylated Bcr-Abl protein levels, and inhibits signaling pathways downstream of Bcr-Abl: downregulation of STAT3 and STAT5 phosphorylation and/or total levels and a decrease in phosphorylation of the Bcr-Abl-associated proteins CrkL and Lyn. Moreover, we found that other mitotic inhibitors (Vincristine and Docetaxel), in combination with bortezomib, also suppress the Bcr-Abl-induced pro-survival signals and result in caspase 3 activation. These results open novel possibilities for the treatment of Bcr-Abl-positive leukemias, especially in the imatinib, dasatinib and nilotinib-resistant CML cases.

Abstract 2838: Intratumoral-regulated expression of IL-12 as a potential immunotherapeutic strategy for the treatment of breast cancer

A major obstacle for the development of immunotherapeutics is the ability of tumors to escape the immune system coupled with toxicity associated with systemic administration of cytokines. To overcome these challenges we have developed an adenoviral vector with regulated expression of IL-12, Ad-RTS-mIL-12 (Ad), was administered intratumorally under the control of the RheoSwitch Therapeutic System® (RTS®) expression platform. Previously we have shown that a single intratumoral injection of Ad combined with oral administration of the activator ligand, INXN-1001 (AL), led to significant inhibition of tumor growth and increased survival. Herein, we show that localized delivery of IL-12 is well tolerated and its mechanism of action is through an increase in tumor infiltrating leukocytes concomitant with a reduction in tumor growth. In vitro studies using HT1080 human fibrosarcoma cells transduced with Ad and incubated with AL have shown a concentration-related increase in IL-12 mRNA concomitant with an increase in expression of IL-12 protein. Removal of AL from the media resulted in return of IL-12 expression to baseline within 48 hours. In the 4T1 BALB/c mouse model, oral administration of AL elicited a dose- related increase in plasma AL levels which correlated with increasing tumor levels of AL. The increase in tumoral AL in turn activated the RheoSwitch® portion of the adenoviral vector leading to a dose-related increase in tumor IL-12 production with virtually no increase in systemic IL-12. The maximal tumoral IL-12 protein level of ∼280 pg/mg protein was achieved on Day 4 of AL dosing with a concomitant serum IL-12 level of 0.5 pg/mg protein following a single intratumoral injection of Ad-RTS-mIL-12 (1x1010 vp) on Day 1 combined with once daily oral administration of AL at 150 mg/m2 on Days 1-7. This increase in tumoral IL-12 levels correlated with a dose-related increase in tumor-infiltrating CD4-positive and CD-8 positive lymphocytes in and adjacent to the tumor, coupled with an increase in apoptotic marker caspase 3 in the tumor when compared with vehicle. No increase in IL-12 protein expression in mice treated with Ad or AL alone was observed. Moreover, this therapeutic strategy appears to be well-tolerated as no change in clinical signs or body weight was observed in the treated animals when compared with vehicle alone.

These results support our hypothesis that localized delivery of IL-12 is well tolerated and results in an increase in tumor infiltrating leukocytes concomitant with a reduction in tumor growth. These findings suggest the applicability of our immunotherapeutic strategy for the treatment of metastatic breast cancer.

Citation Format: John A. Barrett, Lei Sun, Maria Grigoriadis, Christan Fulan-Freguia, Tim Chan, Bill Fogler, Laurent Humeau, Roya Khosravi-Far, Robert A. Morgan, Hagop Youssoufian. Intratumoral-regulated expression of IL-12 as a potential immunotherapeutic strategy for the treatment of breast cancer. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 2838. doi:10.1158/1538-7445.AM2013-2838

Abstract 3058: miRNA networks mediate the tumor suppressive function of FOXO proteins

FOXO transcription factors, as regulators of a variety of cellular functions, including promotion of apoptotic cell death, are negatively regulated by various oncogenic signaling cascades and have emerged as bona fide tumor suppressors. Indeed, studies by others and us have shown that the suppression of FOXO function is critical in promoting evasion of apoptosis and is a significant occurrence in several hematological malignancies. Consequently, FOXO proteins signify an important therapeutic target in hematopoietic malignancies as well as other types of cancer, where repression of FOXO plays a key role in malignancy. Since transcription factors are not yet easily “druggable," it is vital to identify mediators of FOXO function that can be readily targeted. It is now apparent that aberrant miRNA expression plays a major role in cell and tissue-specific manner in the pathogenesis of various types of cancers including hematological malignancies. Both miRNA loss of function and miRNA overexpression have been shown to target key cellular proteins and contribute to cancer pathogenesis. Conversely, miRNA genes themselves have been shown to be direct targets of transcription factors that are known as important tumor suppressors or oncogenic proteins. Through a systematic screening approach, we have identified members of three miRNA clusters that are dependent on FOXO transcription factors for expression. We have observed that the level of these miRNAs is dependent on BCR-ABL status of hematopoeitc cells and they their levels are changed in specimens from Leukemia patients. Our studies indicates a mechanism of action for FOXO tumor suppressors whereby members of two miRNA families are direct transcriptional targets of FOXO3. Moreover, these miRNAs target and reduce the expression of two key oncogenic factors, the serine/threonine kinase mTOR, and proto-onocogene c-myc, to keep cell growth at bay and prevent tumorigenesis. Our investigations have unraveled the miRNA arm of the FOXO tumor suppression network.

Citation Format: Monica Nadler, Robin Zabolian, Kenneth Ndebele, thierry bertomeu, Octavian Bucur, Bodvael Pennarun, Roya Khosravi-Far. miRNA networks mediate the tumor suppressive function of FOXO proteins. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 3058. doi:10.1158/1538-7445.AM2013-3058

Note: This abstract was not presented at the AACR Annual Meeting 2013 because the presenter was unable to attend.

Harnessing the tumor suppressor function of FOXO as an alternative therapeutic approach in cancer

The promotion of cellular survival, dedifferentiation, and uncontrolled proliferation via the suppression of apoptotic effectors is a fundamental characteristic of tumor cells. As substrates that are negatively regulated by oncogenic signaling cascades driven by AKT, SGK (serum- and glucocorticoid-inducible kinase), IkB kinase (IKK), ERK, and cyclin-dependent kinases (CDK), forkhead box-class O (FOXO) transcription factors have emerged as bona fide tumor suppressors. These transcription factors indeed regulate a variety of cellular responses and themselves are regulated by reversible phosphorylation, acetylation, ubiquitination and miRNAs. This review will discuss our current understanding of mechanisms for FOXO regulation and the potential implications for therapeutically restoring FOXO transcriptional activity.

Bortezomib treatment causes remission in a Ph+ALL patient and reveals FoxO as a theranostic marker

BCR-ABL is a key mediator in the pathogenesis of all cases of chronic myelogenous leukemia (CML) and a subset of precursor B-acute lymphoblastic leukemia (Ph+ALL). Previous animal and cell-based studies have shown that the expression of members of the Forkhead family of tumor suppressors, including FoxO3, is suppressed in BCR-ABL-expressing cells. Furthermore, it has been reported that the proteasomal degradation pathway plays an important role in suppression of FoxO expression in BCR-ABL-transformed cells. In this study, a patient diagnosed with Ph+ALL and refractory to standard therapies was treated with a proteasome inhibitor (bortezomib)-based chemotherapy regimen. This treatment resulted in complete hematologic, cytogenetic and molecular remission with excellent performance status for > 4 years since her initial diagnosis. FoxO3 was not detectable within the blasts of this patient at diagnosis and was 'rescued' following treatment with bortezomib therapy, leading to her recovery. As a next step, in the attempt to propose FoXO3 as a therapeutic target and a theranostic marker, we further validated FoxO3 expression in human bone marrow biopsy samples. Human core biopsy samples of Ph+ALL and Ph-negative-negative ALL, along with uninvolved controls, revealed that FoxO3 down-regulation was specific to Ph+ALL. This study provides support that FoxO3 is a good biomarker for BCR-ABL-mediated leukemogenesis. Additionally, proteasomal inhibition by bortezomib may be a promising therapeutic option in Philadelphia-positive ALL, where FoxO3 is downregulated.

Apoptotic cell signaling in cancer progression and therapy

Apoptosis is a tightly regulated cell suicide program that plays an essential role in the development and maintenance of tissue homeostasis by eliminating unnecessary or harmful cells. Impairment of this native defense mechanism promotes aberrant cellular proliferation and the accumulation of genetic defects, ultimately resulting in tumorigenesis, and frequently confers drug resistance to cancer cells. The regulation of apoptosis at several levels is essential to maintain the delicate balance between cellular survival and death signaling that is required to prevent disease. Complex networks of signaling pathways act to promote or inhibit apoptosis in response to various cues. Apoptosis can be triggered by signals from within the cell, such as genotoxic stress, or by extrinsic signals, such as the binding of ligands to cell surface death receptors. Various upstream signaling pathways can modulate apoptosis by converging on, and thereby altering the activity of, common central control points within the apoptotic signaling pathways, which involve the BCL-2 family proteins, inhibitor of apoptosis (IAP) proteins, and FLICE-inhibitory protein (c-FLIP). This review highlights the role of these fundamental regulators of apoptosis in the context of both normal apoptotic signaling mechanisms and dysregulated apoptotic pathways that can render cancer cells resistant to cell death. In addition, therapeutic strategies aimed at modulating the activity of BCL-2 family proteins, IAPs, and c-FLIP for the targeted induction of apoptosis are briefly discussed.

Abstract 4514: Proteasome inhibition causes regression of leukemia in part through regulation of forkhead tumor suppressors

BCR-ABL plays an essential role in the pathogenesis of chronic myeloid leukemia (CML) and some cases of acute lymphocytic leukemia (ALL). Although ABL kinase inhibitors have shown great promise in the treatment of CML, the persistence of residual disease and the occurrence of resistance have prompted investigations into the molecular effectors of BCR-ABL. Here, we show that BCR-ABL stimulates the proteasome-dependent degradation of members of the forkhead family of tumor suppressors in vitro, in an in vivo animal model, and in samples from patients with BCR-ABL-positive CML or ALL. As several downstream mediators of BCR-ABL are regulated by the proteasome degradation pathway, we also show that inhibition of this pathway, using bortezomib, causes regression of CML-like disease. Bortezomib treatment led to inhibition of BCR-ABL-induced suppression of FoxO proteins and their proapoptotic targets, tumor necrosis factor-related apoptosis-inducing ligand and BIM, thereby providing novel insights into the molecular effects of proteasome inhibitor therapy. We additionally show sensitivity of imatinib-resistant BCR-ABL T315I cells to bortezomib. Our data delineate the involvement of FoxO proteins in BCR-ABL-induced evasion of apoptosis and provide evidence that bortezomib is a candidate therapeutic in the treatment of BCR-ABL-induced leukemia.

Note: This abstract was not presented at the AACR 101st Annual Meeting 2010 because the presenter was unable to attend.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 4514.

Abstract 205: Bortezomib and paclitaxel synergistically induce apoptosis in chronic myelogenous leukemia cells

Bcr-Abl fusion protein plays a critical role in the pathogenesis and progression of Chronic Myelogenous Leukemia (CML) and in some of the Acute Lymphocytic Leukemia (ALL) cases. Current inhibitors of Abl kinases, such as Imatinib, have shown great promise in the treatment of CML, however, the emergence of the resistance and the persistence of the residual disease have underlined the importance of finding new ways of treatment CML and especially of the resistant forms of CML. We and others have previously shown that Bortezomib (FDA approved for the treatment of Multiple Myeloma), a selective proteasome inhibitor, efficiently induces apoptosis in Imatinib resistant Bcr-Abl T315I cells, and established Bortezomib as a potential important treatment in Bcr-Abl positive leukemias, determining the initiation of at least five different clinical trials for the treatment of CML, alone or in combination.

We are further showing here that Bortezomib in combination with the mitotic inhibitor Paclitaxel act synergistically in inducing apoptosis in Bcr-Abl positive cells. Paclitaxel, a mitotic inhibitor drug (FDA approved drug for the treatment of lung, ovarian, breast, head and neck cancers, and advanced forms of Kaposi's sarcoma) is also now in clinical trials for the treatment of CML. However, to our knowledge, there are no clinical trials or other studies regarding the combined treatment of Bortezomib and Paclitaxel in Bcr-Abl positive CML.

RESULTS: Combined treatment of Bortezomib and Paclitaxel synergistically induces cell death in human Bcr-Abl positive K562 cell line, by activating the initiator caspase 9 and effector caspase 3, which leads to the cleavage of a number of substrates, such as Poly ADP Ribose Polymerase (PARP), and results in apoptosis. Additionally, the exposure of the Bcr-Abl positive leukemia cells to the Bortezomib/Paclitaxel regimen results in an increase in the activation of the stress-related MAP kinases (such as p38 MAPK and JNK) versus the cytoprotective kinases (ERK1, ERK2), suggesting a possible implication of JNK and p38 MAPK in mediating the effect of the combined treatment. Moreover, we also show that the combined treatment is effective in inducing apoptosis in the murine Baf3 Bcr-Abl and Imatinib resistant Baf3 Bcr-Abl T315I cell lines, suggesting a potential use of the regimen in Imatinib resistant forms of CML.

CONCLUSION: Taken together, these findings underline that the combined treatment with Bortezomib and Paclitaxel represents a potentially promising strategy for the treatment of Chronic Myelogenous Leukemia in general, and of Imatinib resistant CML in particular.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 205.

Abstract 2909: ApoFLIP: A bioactive peptide that restarts the apoptotic machinery in cancer cells

The cellular factors inhibiting apoptosis are often employed by cancer cells to resist therapy. c-FLIP is an endogenous inhibitor of apoptosis upregulated in many drug resistant tumors. Herein, we show that a peptide derived from c-FLIP is a potent inducer of apoptosis in both in vitro and in vivo cancer models. Mechanistic studies suggest that the apoptogenic peptide of c-FLIP (ApoFLIP) removes the inhibition of pro-apoptotic signaling at the level of activated death receptors by competing c-FLIPL off receptor complexes and clearing the way for activation of the caspase cascade. At the same time, ApoFLIP impairs the pro-survival signaling along the PI3K/Akt pathway by blocking the interaction between the p85 subunit of PI3K and c-FLIPL. The selectivity and apoptogenic potency of ApoFLIP in killing cancer cells suggests that simultaneous targeting of two or more hallmark features of cancer may prove to be a winning therapeutic strategy.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 2909.

Protein phosphatase 2A reactivates FOXO3a through a dynamic interplay with 14-3-3 and AKT

This article describes a functional interaction between PP2A and FOXO3a in which PP2A promotes rapid dephosphorylation of FOXO3a at its conserved AKT phosphorylation sites, leading to FOXO3a dissociation from 14-3-3, nuclear translocation, and transcriptional activation in response to inhibition of PI3K signaling.

Forkhead box transcription factor FOXO3a, a key regulator of cell survival, is regulated by reversible phosphorylation and subcellular localization. Although the kinases regulating FOXO3a activity have been characterized, the role of protein phosphatases (PP) in the control of FOXO3a subcellular localization and function is unknown. In this study, we detected a robust interaction between FOXO3a and PP2A. We further demonstrate that 14-3-3, while not impeding the interaction between PP2A and FOXO3a, restrains its activity toward AKT phosphorylation sites T32/S253. Disruption of PP2A function revealed that after AKT inhibition, PP2A-mediated dephosphorylation of T32/S253 is required for dissociation of 14-3-3, nuclear translocation, and transcriptional activation of FOXO3a. Our findings reveal that distinct phosphatases dephosphorylate conserved AKT motifs within the FOXO family and that PP2A is entwined in a dynamic interplay with AKT and 14-3-3 to directly regulate FOXO3a subcellular localization and transcriptional activation.

Proteasome inhibition causes regression of leukemia and abrogates BCR-ABL-induced evasion of apoptosis in part through regulation of forkhead tumor suppressors

BCR-ABL plays an essential role in the pathogenesis of chronic myeloid leukemia (CML) and some cases of acute lymphocytic leukemia (ALL). Although ABL kinase inhibitors have shown great promise in the treatment of CML, the persistence of residual disease and the occurrence of resistance have prompted investigations into the molecular effectors of BCR-ABL. Here, we show that BCR-ABL stimulates the proteasome-dependent degradation of members of the forkhead family of tumor suppressors in vitro, in an in vivo animal model, and in samples from patients with BCR-ABL-positive CML or ALL. As several downstream mediators of BCR-ABL are regulated by the proteasome degradation pathway, we also show that inhibition of this pathway, using bortezomib, causes regression of CML-like disease. Bortezomib treatment led to inhibition of BCR-ABL-induced suppression of FoxO proteins and their proapoptotic targets, tumor necrosis factor-related apoptosis-inducing ligand and BIM, thereby providing novel insights into the molecular effects of proteasome inhibitor therapy. We additionally show sensitivity of imatinib-resistant BCR-ABL T315I cells to bortezomib. Our data delineate the involvement of FoxO proteins in BCR-ABL-induced evasion of apoptosis and provide evidence that bortezomib is a candidate therapeutic in the treatment of BCR-ABL-induced leukemia.

A double hit to kill tumor and endothelial cells by TRAIL and antiangiogenic 3TSR

As tumor development relies on a coordination of angiogenesis and tumor growth, an efficient antitumor strategy should target both the tumor and its associated vessels. Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) induces apoptosis in a tumor-selective manner. Additionally, thrombospondin-1, a naturally occurring inhibitor of angiogenesis, and a recombinant protein containing functional domains of thrombospondin-1, 3TSR, have been shown to be necessary and sufficient to inhibit tumor angiogenesis. Here, we show that a combination of a TRAIL receptor 2 agonist antibody, Lexatumumab, and 3TSR results in a significantly enhanced and durable tumor inhibition. We further observed that 3TSR induces apoptosis in primary endothelial cells by up-regulating the expression of TRAIL receptors 1 and 2 in a CD36 and Jun NH(2)-terminal kinase-dependent manner leading to the activation of both intrinsic and extrinsic apoptotic machineries. The modulation of these pathways is critical for 3TSR-induced apoptosis as disrupting either via specific inhibitors reduced apoptosis. Moreover, 3TSR attenuates the Akt survival pathway. These studies indicate that 3TSR plays a critical role in regulating the proapoptotic signaling pathways that control growth and death in endothelial cells and that a combination of TRAIL and 3TSR acts as a double hit against tumor and tumor-associated vessels.

Fas death receptor enhances endocytic membrane traffic converging into the Golgi region

The death receptor Fas/CD95 initiates apoptosis by engaging diverse cellular organelles including endosomes. The link between Fas signaling and membrane traffic has remained unclear, in part because it may differ in diverse cell types. After a systematic investigation of all known pathways of endocytosis, we have clarified that Fas activation opens clathrin-independent portals in mature T cells. These portals drive rapid internalization of surface proteins such as CD59 and depend upon actin-regulating Rho GTPases, especially CDC42. Fas-enhanced membrane traffic invariably produces an accumulation of endocytic membranes around the Golgi apparatus, in which recycling endosomes concentrate. This peri-Golgi polarization has been documented by colocalization analysis of various membrane markers and applies also to active caspases associated with internalized receptor complexes. Hence, T lymphocytes show a diversion in the traffic of endocytic membranes after Fas stimulation that seems to resemble the polarization of membrane traffic after their activation.

Tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL) induced mitochondrial pathway to apoptosis and caspase activation is potentiated by phospholipid scramblase-3

Tumor Necrosis Factor (TNF)-Related Apoptosis-Inducing Ligand (TRAIL) initiate pathways of cell death in which caspase activation is mediated either directly (without mitochondrial amplification), or indirectly via the release of apoptogenic factors from mitochondria. Phospholipid scramblases (PLS) are enzymes that play a key role in cellular function by inducing bidirectional movement of membrane lipids. Changes in mitochondrial membrane lipids, cardiolipin, are critical for mediating apoptotic response in many cell-types. PLS3 is a phospholipid scramblase that is localized to mitochondria and is thought to be involved in the regulation of apoptotic signals. Here we report that exogenous-expression of PLS3 enhances apoptotic death induced by TRAIL. This is acheived by potentiating the mitochondrial arm of the death pathway. Thereby, PLS3 expression facilitates changes in mitochondrial membrane lipids that promote the release of apoptogenic factors and consequent full activation and processing of the caspase-9 and effector caspase-3. Moreover, we show that knock-down of endogenous PLS3 suppresses TRAIL-induced changes in cardiolipin. Finally, we demonstrate that TRAIL-induced activation of PKC-delta mediates regulation of the PLS3-induced changes in cardiolipin.

Dysregulation of apoptotic signaling in cancer: molecular mechanisms and therapeutic opportunities

Apoptosis is a tightly regulated cell suicide program that plays an essential role in the maintenance of tissue homeostasis by eliminating unnecessary or harmful cells. Defects in this native defense mechanism promote malignant transformation and frequently confer chemoresistance to transformed cells. Indeed, the evasion of apoptosis has been recognized as a hallmark of cancer. Given that multiple mechanisms function at many levels to orchestrate the regulation of apoptosis, a multitude of opportunities for apoptotic dysregulation are present within the intricate signaling network of cell. Several of the molecular mechanisms by which cancer cells are protected from apoptosis have been elucidated. These advances have facilitated the development of novel apoptosis-inducing agents that have demonstrated single-agent activity against various types of cancers cells and/or sensitized resistant cancer cells to conventional cytotoxic therapies. Herein, we will highlight several of the central modes of apoptotic dysregulation found in cancer. We will also discuss several therapeutic strategies that aim to reestablish the apoptotic response, and thereby eradicate cancer cells, including those that demonstrate resistance to traditional therapies.

Analysis of TNF-related apoptosis-inducing ligand in vivo through bone marrow transduction and transplantation

Tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL) is a member of the TNF family of cytokines. TRAIL has gained much attention because of its ability to preferentially kill tumor cells with no apparent toxic side effects. Recently, different TRAIL receptor agonists, including TRAIL itself and various agonistic monoclonal antibodies against the two apoptosis-inducing human TRAIL receptors, have been developed as novel cancer therapeutics and are currently under investigation in clinical trials. However, the mechanisms by which TRAIL mediates its selective antineoplastic activity are still not well understood. In addition to playing a role in cancer immune surveillance and tumor suppression, TRAIL has been associated with immune homeostasis, inflammatory diseases, and autoimmunity. In light of the multifunctional role of TRAIL in mediating various pathologic conditions and the potential benefits of TRAIL-based therapies, the study of the physiologic significance of TRAIL is of great importance. Here, we describe a syngeneic system for the characterization of the in vivo function of TRAIL. By use of this model, in which the full-length murine TRAIL protein is overexpressed in the hematopoietic cells of wild-type mice, the in vivo tumoricidal activity of TRAIL overexpression can be studied on syngeneic murine tumor cell challenge, and the potential toxicity of TRAIL protein to normal tissues can also be analyzed.

Neuropilin-1 modulates p53/caspases axis to promote endothelial cell survival

Vascular permeability factor/vascular endothelial growth factor (VPF/VEGF), one of the crucial pro-angiogenic factors, functions as a potent inhibitor of endothelial cell (EC) apoptosis. Previous progress has been made towards delineating the VPF/VEGF survival signaling downstream of the activation of VEGFR-2. Here, we seek to define the function of NRP-1 in VPF/VEGF-induced survival signaling in EC and to elucidate the concomitant molecular signaling events that are pivotal for our understanding of the signaling of VPF/VEGF. Utilizing two different in vitro cell culture systems and an in vivo zebrafish model, we demonstrate that NRP-1 mediates VPF/VEGF-induced EC survival independent of VEGFR-2. Furthermore, we show here a novel mechanism for NRP-1-specific control of the anti-apoptotic pathway in EC through involvement of the NRP-1-interacting protein (NIP/GIPC) in the activation of PI-3K/Akt and subsequent inactivation of p53 pathways and FoxOs, as well as activation of p21. This study, by elucidating the mechanisms that govern VPF/VEGF-induced EC survival signaling via NRP-1, contributes to a better understanding of molecular mechanisms of cardiovascular development and disease and widens the possibilities for better therapeutic targets.

c-Fos as a proapoptotic agent in TRAIL-induced apoptosis in prostate cancer cells

Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL)/Apo-2L promotes apoptosis in cancer cells while sparing normal cells. Although many cancers are sensitive to TRAIL-induced apoptosis, some evade the proapoptotic effects of TRAIL. Therefore, differentiating molecular mechanisms that distinguish between TRAIL-sensitive and TRAIL-resistant tumors are essential for effective cancer therapies. Here, we show that c-Fos functions as a proapoptotic agent by repressing the antiapoptotic molecule c-FLIP(L). c-Fos binds the c-FLIP(L) promoter, represses its transcriptional activity, and reduces c-FLIP(L) mRNA and protein levels. Therefore, c-Fos is a key regulator of c-FLIP(L), and activation of c-Fos determines whether a cancer cell will undergo cell death after TRAIL treatment. Strategies to activate c-Fos or inhibit c-FLIP(L) may potentiate TRAIL-based proapoptotic therapies.

FoxO tumor suppressors and BCR-ABL-induced leukemia: a matter of evasion of apoptosis

Numerous studies have revealed that the BCR-ABL oncoprotein abnormally engages a multitude of signaling pathways, some of which may be important for its leukemogenic properties. Central to this has been the determination that the tyrosine kinase function of BCR-ABL is mainly responsible for its transforming potential, and can be targeted with small molecule inhibitors, such as imatinib mesylate (Gleevec, STI-571). Despite this apparent success, the development of clinical resistance to imatinib therapy, and the inability of imatinib to eradicate BCR-ABL-positive malignant hematopoietic progenitors demand detailed investigations of additional effector pathways that can be targeted for CML treatment. The promotion of cellular survival via the suppression of apoptotic pathways is a fundamental characteristic of tumor cells that enables resistance to anti-cancer therapies. As substrates of survival kinases such as Akt, the FoxO family of transcription factors, particularly FoxO3a, has emerged as playing an important role in the cell cycle arrest and apoptosis of hematopoietic cells. This review will discuss our current understanding of BCR-ABL signaling with a focus on apoptotic suppressive mechanisms and alternative approaches to CML therapy, as well as the potential for FoxO transcription factors as novel therapeutic targets.

CDIP, a novel pro-apoptotic gene, regulates TNFalpha-mediated apoptosis in a p53-dependent manner

We have identified a novel pro-apoptotic p53 target gene named CDIP (Cell Death Involved p53-target). Inhibition of CDIP abrogates p53-mediated apoptotic responses, demonstrating that CDIP is an important p53 apoptotic effector. CDIP itself potently induces apoptosis that is associated with caspase-8 cleavage, implicating the extrinsic cell death pathway in apoptosis mediated by CDIP. siRNA-directed knockdown of caspase-8 results in a severe impairment of CDIP-dependent cell death. In investigating the potential involvement of extrinsic cell death pathway in CDIP-mediated apoptosis, we found that TNF-alpha expression tightly correlates with CDIP expression, and that inhibition of TNF-alpha signaling attenuates CDIP-dependent apoptosis. We also demonstrate that TNF-alpha is upregulated in response to p53 and p53 inducing genotoxic stress, in a CDIP-dependent manner. Consistently, knockdown of TNF-alpha impairs p53-mediated stress-induced apoptosis. Together, these findings support a novel p53 --> CDIP --> TNF-alpha apoptotic pathway that directs apoptosis after exposure of cells to genotoxic stress. Thus, CDIP provides a new link between p53-mediated intrinsic and death receptor-mediated extrinsic apoptotic signaling, providing a novel target for cancer therapeutics aimed at maximizing the p53 apoptotic response of cancer cells to drug therapy.

Death receptor ligation triggers membrane scrambling between Golgi and mitochondria

Subcellular organelles such as mitochondria, endoplasmic reticulum (ER) and the Golgi complex are involved in the progression of the cell death programme. We report here that soon after ligation of Fas (CD95/Apo1) in type II cells, elements of the Golgi complex intermix with mitochondria. This mixing follows centrifugal dispersal of secretory membranes and reflects a global alteration of membrane traffic. Activation of apical caspases is instrumental for promoting the dispersal of secretory organelles, since caspase inhibition blocks the outward movement of Golgi-related endomembranes and reduces their mixing with mitochondria. Caspase inhibition also blocks the FasL-induced secretion of intracellular proteases from lysosomal compartments, outlining a novel aspect of death receptor signalling via apical caspases. Thus, our work unveils that Fas ligand-mediated apoptosis induces scrambling of mitochondrial and secretory organelles via a global alteration of membrane traffic that is modulated by apical caspases.

Transduction of tumor necrosis factor-related apoptosis-inducing ligand into hematopoietic cells leads to inhibition of syngeneic tumor growth in vivo

Tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL) is a member of the TNF family of cytokines and has been shown to induce cell death in many types of tumor and transformed cells but not in normal cells. This tumor-selective property has made TRAIL a promising candidate for the development of cancer therapy. However, safety issues are a concern because certain preparations of recombinant TRAIL protein were reported to induce toxicity in normal human hepatocytes in culture. In addition, previous studies on tumor selectivity of exogenous TRAIL protein were carried out in xenograft models, which do not directly address the tumor selectivity issue. It was not known whether exogenous or overexpression of TRAIL in a syngeneic system could induce tumor cell death while leaving normal tissue cells unharmed. Thus, the tumor selectivity of TRAIL-induced apoptosis remains to be further characterized. In our study, we established mice that overexpress TRAIL by retroviral-mediated gene transfer in bone marrow cells followed by bone marrow transplantation. Our results show that TRAIL overexpression is not toxic to normal tissues, as analyzed by hematologic and histologic analyses of tissue samples from TRAIL-transduced mice. We show for the first time that TRAIL overexpression in hematopoietic cells leads to significant inhibition of syngeneic tumor growth in certain tumor lines. This approach may be used further to identify important molecules that regulate the sensitivity of tumor cells to TRAIL-induced cell death in vivo.

Activation of extracellular signal-regulated protein kinase 5 downregulates FasL upon osmotic stress

Extracellular signal-regulated protein kinase (ERK) 5 is a mitogen-activated protein kinase (MAPK) that is activated by dual phosphorylation via a unique MAPK/ERK kinase 5, MEK5. The physiological importance of this signaling cascade is underscored by the early embryonic death caused by the targeted deletion of the erk5 or the mek5 genes in mice. Here, we have found that ERK5 is required for mediating the survival of fibroblasts under basal conditions and in response to sorbitol treatment. Increased Fas ligand (FasL) expression acts as a positive feedback loop to enhance apoptosis of ERK5- or MEK5-deficient cells under conditions of osmotic stress. Compared to wild-type cells, erk5-/- and mek5-/- fibroblasts treated with sorbitol display a reduced protein kinase B (PKB) activity associated with increased Forkhead box O3a (Foxo3a) activity. Based on these results, we conclude that the ERK5 signaling pathway promotes cell survival by downregulating FasL expression via a mechanism that implicates PKB-dependent inhibition of Foxo3a downstream of phosphoinositide 3 kinase.

Regulation of tumor angiogenesis by thrombospondin-1

Angiogenesis plays a critical role in the growth and metastasis of tumors. Thrombospondin-1 (TSP-1) is a potent angiogenesis inhibitor, and down-regulation of TSP-1 has been suggested to alter tumor growth by modulating angiogenesis in a variety of tumor types. Expression of TSP-1 is up-regulated by the tumor suppressor gene, p53, and down-regulated by oncogenes such as Myc and Ras. TSP-1 inhibits angiogenesis by inhibiting endothelial cell migration and proliferation and by inducing apoptosis. In addition, activation of transforming growth factor beta (TGF-beta) by TSP-1 plays a crucial role in the regulation of tumor progression. An understanding of the molecular basis of TSP-1-mediated inhibition of angiogenesis and tumor progression will aid in the development of novel therapeutics for the treatment of cancer.

Tumor necrosis factor-related apoptosis-inducing ligand alters mitochondrial membrane lipids

Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) has been shown to have selective antitumor activity. TRAIL induces ubiquitous pathways of cell death in which caspase activation is mediated either directly or via the release of apoptogenic factors from mitochondria; however, the precise components of the mitochondrial signaling pathway have not been well defined. Notably, mitochondria constitute an important target in overcoming resistance to TRAIL in many types of tumors. Bid is considered to be fundamental in engaging mitochondria during death receptor-mediated apoptosis, but this action is dependent on mitochondrial lipids. Here, we report that TRAIL signaling induces an alteration in mitochondrial membrane lipids, particularly cardiolipin. This occurs independently of caspase activation and primes mitochondrial membranes to the proapoptotic action of Bid. We unveil a link between TRAIL signaling and alteration of membrane lipid homeostasis that occurs in parallel to apical caspase activation but does not take over the mode of cell death because of the concurrent activation of caspase-8. In particular, TRAIL-induced alteration of mitochondrial lipids follows an imbalance in the cellular homeostasis of phosphatidylcholine, which results in an elevation in diacylglycerol (DAG). Elevated DAG in turn activates the delta isoform of phospholipid-dependent serine/threonine protein kinase C, which then accelerates the cleavage of caspase-8. We also show that preservation of phosphatidylcholine homeostasis by inhibition of lipid-degrading enzymes almost completely impedes the activation of pro-caspase-9 while scarcely changing the activation of caspase-8.

Pro-apoptotic Bid induces membrane perturbation by inserting selected lysolipids into the bilayer

Bid is a BH3-only member of the Bcl-2 family that regulates cell death at the level of mitochondrial membranes. Bid appears to link the mitochondrial pathway with the death receptor-mediated pathway of cell death. It is generally assumed that the f.l. (full-length) protein becomes activated after proteolytic cleavage, especially by apical caspases like caspase 8. The cleaved protein then relocates to mitochondria and promotes membrane permeabilization, presumably by interaction with mitochondrial lipids and other Bcl-2 proteins that facilitate the release of apoptogenic proteins like cytochrome c. Although the major action may reside in the C-terminus part, tBid (cleaved Bid), un-cleaved Bid also has pro-apoptotic potential when ectopically expressed in cells or in vitro. This pro-apoptotic action of f.l. Bid has remained unexplained, especially at the biochemical level. In the present study, we show that f.l. (full-length) Bid can insert specific lysolipids into the membrane surface, thereby priming mitochondria for the release of apoptogenic factors. This is most effective for lysophosphatidylcholine species that we report to accumulate in mitochondria during apoptosis induction. A Bid mutant that is not pro-apoptotic in vivo is defective in lysophosphatidylcholine-mediated membrane perturbation in vitro. Our results thus provide a biochemical explanation for the pro-apoptotic action of f.l. Bid.

Death receptor signals to mitochondria

Apoptosis is the best-characterized form of programmed cell death (PCD) and is of fundamental importance in tissue homeostasis. In mammalian systems, there are two major pathways that are involved in the initiation of apoptosis: the "extrinsic" death receptor pathway and the "intrinsic" mitochondrial pathway. Although these pathways act independently to initiate the death machinery in some cellular systems, in many cell types, including numerous tumor cells, there is delicate coordination and cross talk between the extrinsic and intrinsic pathways, which leads to the activation of the executioner caspase cascade. Additionally, there appears to be a fine balance between the caspase-mediated arm of death receptor signaling that engages mitochondria and the caspase-independent arm that promotes vacuole proliferation in many cells. Here, we review our current knowledge about the layers of complexity that are posed by the interactions between death receptor-induced pathways and how they influence mitochondria to regulate cellular life and death decisions.

Persistent c-FLIP(L) expression is necessary and sufficient to maintain resistance to tumor necrosis factor-related apoptosis-inducing ligand-mediated apoptosis in prostate cancer

Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) has been shown to induce apoptosis in a variety of tumorigenic and transformed cell lines but not in many normal cells. Hence, TRAIL has the potential to be an ideal cancer therapeutic agent with minimal cytotoxicity. FLICE inhibitory protein (c-FLIP) is an important regulator of TRAIL-induced apoptosis. Here, we show that persistent expression of c-FLIP(Long) [c-FLIP(L)] is inversely correlated with the ability of TRAIL to induce apoptosis in prostate cancer cells. In contrast to TRAIL-sensitive cells, TRAIL-resistant LNCaP and PC3-TR (a TRAIL-resistant subpopulation of PC3) cells showed increased c-FLIP(L) mRNA levels and maintained steady protein expression of c-FLIP(L) after treatment with TRAIL. Ectopic expression of c-FLIP(L) in TRAIL-sensitive PC3 cells changed their phenotype from TRAIL sensitive to TRAIL resistant. Conversely, silencing of c-FLIP(L) expression by small interfering RNA in PC3-TR cells reversed their phenotype from TRAIL resistant to TRAIL sensitive. Therefore, persistent expression of c-FLIP(L) is necessary and sufficient to regulate sensitivity to TRAIL-mediated apoptosis in prostate cancer cells.

Fas-associated protein with death domain (FADD)-independent recruitment of c-FLIPL to death receptor 5

Here we show a novel mechanism by which FLICE-like inhibitory protein (c-FLIP) regulates apoptosis induced by tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL) and one of its receptors, DR5. c-FLIP is a critical regulator of the TNF family of cytokine receptor signaling. c-FLIP has been postulated to prevent formation of the competent death-inducing signaling complex (DISC) in a ligand-dependent manner, through its interaction with FADD and/or caspase-8. In order to identify regulators of TRAIL function, we used the intracellular death domain (DD) of DR5 as a target to screen a phage-displayed combinatorial peptide library. The DD of DR5 selected from the library a peptide that showed sequence similarity to a stretch of amino acids in the C terminus of c-FLIP(L). The phage-displayed peptide selectively interacted with the DD of DR5 in in vitro binding assays. Similarly, full-length c-FLIP (c-FLIP(L)) and the C-terminal p12 domain of c-FLIP interacted with DR5 both in in vitro pull-down assays and in mammalian cells. This interaction was independent of TRAIL. To the contrary, TRAIL treatment released c-FLIP(L) from DR5, permitting the recruitment of FADD to the active DR5 signaling complex. By employing FADD-deficient Jurkat cells, we demonstrate that DR5 and c-FLIP(L) interact in a FADD-independent manner. Moreover, we show that a cellular membrane permeable version of the peptide corresponding to the DR5 binding domain of c-FLIP induces apoptosis in mammalian cells. Taken together, these findings indicate that c-FLIP interacts with the DD of DR5, thus preventing death (L)signaling by DR5 prior to the formation of an active DISC. Because TRAIL and DR5 are ubiquitously expressed, the interaction of c-FLIP(L) and DR5 indicates a mechanism by which tumor selective apoptosis can be achieved through protecting normal cells from undergoing death receptor-induced apoptosis.