Long noncoding RNA Malat1 regulates differential activation of macrophages and response to lung injury

Macrophage activation, i.e., classical M1 and the alternative M2, plays a critical role in many pathophysiological processes, such as inflammation and tissue injury and repair. Although the regulation of macrophage activation has been under extensive investigation, there is little knowledge about the role of long noncoding RNAs (lncRNAs) in this event. In this study, we found that lncRNA Malat1 expression is distinctly regulated in differentially activated macrophages in that it is upregulated in LPS-treated and downregulated in IL-4-treated cells. Malat1 knockdown attenuates LPS-induced M1 macrophage activation. In contrast, Malat1 knockdown enhanced IL-4-activated M2 differentiation as well as a macrophage profibrotic phenotype. Mechanistically, Malat1 knockdown led to decreased expression of Clec16a, silencing of which phenocopied the regulatory effect of Malat1 on M1 activation. Interestingly, Malat1 knockdown promoted IL-4 induction of mitochondrial pyruvate carriers (MPCs) and their mediation of glucose-derived oxidative phosphorylation (OxPhos), which was crucial to the Malat1 regulation of M2 differentiation and profibrotic phenotype. Furthermore, mice with either global or conditional myeloid knockout of Malat1 demonstrated diminished LPS-induced systemic and pulmonary inflammation and injury. In contrast, these mice developed more severe bleomycin-induced lung fibrosis, accompanied by alveolar macrophages displaying augmented M2 and profibrotic phenotypes. In summary, we have identified what we believe is a previously unrecognized role of Malat1 in the regulation of macrophage polarization. Our data demonstrate that Malat1 is involved in pulmonary pathogeneses in association with aberrant macrophage activation.

FUSE binding protein 1 (FUBP1) expression is upregulated by T-cell acute lymphocytic leukemia protein 1 (TAL1) and required for efficient erythroid differentiation

During erythropoiesis, haematopoietic stem cells (HSCs) differentiate in successive steps of commitment and specification to mature erythrocytes. This differentiation process is controlled by transcription factors that establish stage- and cell type-specific gene expression. In this study, we demonstrate that FUSE binding protein 1 (FUBP1), a transcriptional regulator important for HSC self-renewal and survival, is regulated by T-cell acute lymphocytic leukaemia 1 (TAL1) in erythroid progenitor cells. TAL1 directly activates the FUBP1 promoter, leading to increased FUBP1 expression during erythroid differentiation. The binding of TAL1 to the FUBP1 promoter is highly dependent on an intact GATA sequence in a combined E-box/GATA motif. We found that FUBP1 expression is required for efficient erythropoiesis, as FUBP1-deficient progenitor cells were limited in their potential of erythroid differentiation. Thus, the finding of an interconnection between GATA1/TAL1 and FUBP1 reveals a molecular mechanism that is part of the switch from progenitor- to erythrocyte-specific gene expression. In summary, we identified a TAL1/FUBP1 transcriptional relationship, whose physiological function in haematopoiesis is connected to proper erythropoiesis.

The transcriptional regulator FUBP1 influences disease outcome in murine and human myeloid leukemia

The transcriptional regulator far upstream element binding protein 1 (FUBP1) acts as an oncoprotein in solid tumor entities and plays a role in the maintenance of hematopoietic stem cells. However, its potential function in leukemia is unknown. In murine models of chronic (CML) and acute myeloid leukemia (AML) induced by BCR-ABL1 and MLL-AF9, respectively, knockdown of Fubp1 resulted in prolonged survival, decreased numbers of CML progenitor cells, decreased cell cycle activity and increased apoptosis. Knockdown of FUBP1 in CML and AML cell lines recapitulated these findings and revealed enhanced DNA damage compared to leukemia cells expressing wild type FUBP1 levels. FUBP1 was more highly expressed in human CML compared to normal bone marrow cells and its expression correlated with disease progression. In AML, higher FUBP1 expression in patient leukemia cells was observed with a trend toward correlation with shorter overall survival. Treatment of mice with AML with irinotecan, known to inhibit topoisomerase I and FUBP1, significantly prolonged survival alone or in combination with cytarabine. In summary, our data suggest that FUBP1 acts as cell cycle regulator and apoptosis inhibitor in leukemia. We demonstrated that FUBP1 might play a role in DNA repair, and its inhibition may improve outcome in leukemia patients.

Interplay between transcription regulators RUNX1 and FUBP1 activates an enhancer of the oncogene c-KIT and amplifies cell proliferation

Runt-related transcription factor 1 (RUNX1) is a well-known master regulator of hematopoietic lineages but its mechanisms of action are still not fully understood. Here, we found that RUNX1 localizes on active chromatin together with Far Upstream Binding Protein 1 (FUBP1) in human B-cell precursor lymphoblasts, and that both factors interact in the same transcriptional regulatory complex. RUNX1 and FUBP1 chromatin localization identified c-KIT as a common target gene. We characterized two regulatory regions, at +700 bp and +30 kb within the first intron of c-KIT, bound by both RUNX1 and FUBP1, and that present active histone marks. Based on these regions, we proposed a novel FUBP1 FUSE-like DNA-binding sequence on the +30 kb enhancer. We demonstrated that FUBP1 and RUNX1 cooperate for the regulation of the expression of the oncogene c-KIT. Notably, upregulation of c-KIT expression by FUBP1 and RUNX1 promotes cell proliferation and renders cells more resistant to the c-KIT inhibitor imatinib mesylate, a common therapeutic drug. These results reveal a new mechanism of action of RUNX1 that implicates FUBP1, as a facilitator, to trigger transcriptional regulation of c-KIT and to regulate cell proliferation. Deregulation of this regulatory mechanism may explain some oncogenic function of RUNX1 and FUBP1.

Abstract 1500: The role of FUBP1 in the hematopoietic system and leukemia

The transcriptional regulator FUBP1 (Far Upstream Element Binding Protein 1) acts as an oncoprotein in hepatocellular carcinoma (HCC) and is important for hematopoietic stem cell (HSC) self-renewal and erythroid maturation in mice. In this study, we investigated the transcriptional network by which FUBP1 controls hematopoiesis and elucidated the relevance of FUBP1 for human erythropoiesis. Furthermore, we shed light on the role of FUBP1 in leukemia initiating cells. Searching for upstream-regulators of FUBP1, we identified E-boxes as potential TAL1 binding sites in the FUBP1 promoter. Indeed, we demonstrated the regulation of FUBP1 expression by TAL1 in human primary CD34+ donor cells. In chromatin immunoprecipitation (ChIP) experiments, the binding of TAL1 to the FUBP1 promoter increased during erythroid differentiation, correlating with up-regulated FUBP1 and TAL1 expression. Activation of the FUBP1 promoter by TAL1 binding was confirmed in luciferase assays. We observed a reduction in erythroid colony-forming units and glycophorin A positive cells derived from erythroid differentiated human CD34+ cells upon knockdown of FUBP1, supporting the hypothesis that FUBP1 is required for efficient erythropoiesis. In the transduction/transplantation leukemia mouse models for BCR-ABL1+ CML and MLL-AF9+ AML, we observed that Fubp1 knockdown resulted in reduced total cell and progenitor cell numbers. In CML, Fubp1 knockdown cells showed lower cell cycle activity and increased apoptosis. Consistently, CML and AML mice transplanted with Fubp1 knockdown cells survived longer than control mice that received transduced bone marrow expressing wildtype FUBP1 levels. Furthermore, pharmacological treatment of AML mice with the FUBP1 inhibitor irinotecan prolonged their survival significantly as a single drug or in combination with Ara-C. Analysis of FUBP1 expression in bone sections derived from CML and AML patients, and from healthy donors by immunohistochemistry showed no increased FUBP1 expression in leukemic samples, but we noticed a shorter overall survival in those AML patients with strong FUBP1 expression. In CML patients, FUBP1 levels correlate with the disease stage. Thus, elevated expression of FUBP1 might be an indicator for the aggressiveness of leukemia. Our data identify TAL1 as an FUBP1 upstream-regulator and confirm the importance of FUBP1 for HSC self-renewal and erythroid maturation, not only in murine but also in human cells. Furthermore, FUBP1 acts as an oncogenic factor in leukemia. Our findings might provide important evidence for the potential use of FUBP1 in clinical settings, e.g. as a molecular target for the treatment of leukemia patients and as a modulator for the production of red cells.

Citation Format: Marlene Steiner, Van T. Hoang, Jasmin Yillah, Katharina Gerlach, Jörn Lausen, Hans-Michael Kvasnicka, Thomas Oellerich, Hanibal Bohnenberger, Daniela Krause, Martin Zörnig. The role of FUBP1 in the hematopoietic system and leukemia [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 1500.

Hypoxia-induced long non-coding RNA Malat1 is dispensable for renal ischemia/reperfusion-injury

Renal ischemia-reperfusion (I/R) injury is a major cause of acute kidney injury (AKI). Non-coding RNAs are crucially involved in its pathophysiology. We identified hypoxia-induced long non-coding RNA Malat1 (Metastasis Associated Lung Adenocarcinoma Transcript 1) to be upregulated in renal I/R injury. We here elucidated the functional role of Malat1 in vitro and its potential contribution to kidney injury in vivo. Malat1 was upregulated in kidney biopsies and plasma of patients with AKI, in murine hypoxic kidney tissue as well as in cultured and ex vivo sorted hypoxic endothelial cells and tubular epithelial cells. Malat1 was transcriptionally activated by hypoxia-inducible factor 1-α. In vitro, Malat1 inhibition reduced proliferation and the number of endothelial cells in the S-phase of the cell cycle. In vivo, Malat1 knockout and wildtype mice showed similar degrees of outer medullary tubular epithelial injury, proliferation, capillary rarefaction, inflammation and fibrosis, survival and kidney function. Small-RNA sequencing and whole genome expression analysis revealed only minor changes between ischemic Malat1 knockout and wildtype mice. Contrary to previous studies, which suggested a prominent role of Malat1 in the induction of disease, we did not confirm an in vivo role of Malat1 concerning renal I/R-injury.

Camptothecin and its analog SN-38, the active metabolite of irinotecan, inhibit binding of the transcriptional regulator and oncoprotein FUBP1 to its DNA target sequence FUSE

The transcriptional regulator FUSE Binding Protein 1 (FUBP1) is overexpressed in more than 80% of all human hepatocellular carcinomas (HCCs) and other solid tumor entities including prostate and colorectal carcinoma. FUBP1 expression is required for HCC tumor cell expansion, and it functions as an important pro-proliferative and anti-apoptotic oncoprotein that binds to the single-stranded DNA sequence FUSE to regulate the transcription of a variety of target genes. In this study, we screened an FDA-approved drug library and discovered that the Topoisomerase I (TOP1) inhibitor camptothecin (CPT) and its derivative 7-ethyl-10-hydroxycamptothecin (SN-38), the active irinotecan metabolite that is used in the clinics in combination with other chemotherapeutics to treat carcinoma, inhibit FUBP1 activity. Both molecules prevent in vitro the binding of FUBP1 to its single-stranded target DNA FUSE, and they induce deregulation of FUBP1 target genes in HCC cells. Our results suggest the interference with the FUBP1/FUSE interaction as a further molecular mechanism that, in addition to the inactivation of TOP1, may contribute to the therapeutic potential of CPT/SN-38. Targeting of FUBP1 in HCC therapy with SN-38/irinotecan could be a particularly interesting option because of the high FUBP1 levels in HCC cells and their dependency on FUBP1 expression.

Dietary restriction improves repopulation but impairs lymphoid differentiation capacity of hematopoietic stem cells in early aging

Dietary restriction (DR) improves health, delays tissue aging, and elongates survival in flies and worms. However, studies on laboratory mice and nonhuman primates revealed ambiguous effects of DR on lifespan despite improvements in health parameters. In this study, we analyzed consequences of adult-onset DR (24 h to 1 yr) on hematopoietic stem cell (HSC) function. DR ameliorated HSC aging phenotypes, such as the increase in number of HSCs and the skewing toward myeloid-biased HSCs during aging. Furthermore, DR increased HSC quiescence and improved the maintenance of the repopulation capacity of HSCs during aging. In contrast to these beneficial effects, DR strongly impaired HSC differentiation into lymphoid lineages and particularly inhibited the proliferation of lymphoid progenitors, resulting in decreased production of peripheral B lymphocytes and impaired immune function. The study shows that DR-dependent suppression of growth factors and interleukins mediates these divergent effects caused by DR. Supplementation of insulin-like growth factor 1 partially reverted the DR-induced quiescence of HSCs, whereas IL-6/IL-7 substitutions rescued the impairment of B lymphopoiesis exposed to DR. Together, these findings delineate positive and negative effects of long-term DR on HSC functionality involving distinct stress and growth signaling pathways.

Abstract B39: Inhibition of the oncoprotein FUBP1 by SN-38 represents a novel therapeutic option for the treatment of hepatocellular carcinoma

Hepatocellular carcinoma (HCC) is the most common form of liver cancer. It is accounting for one million deaths per year, mostly due to the late time point of diagnosis and especially to the lack of a specified therapy. In more than 90% of HCCs the oncoprotein FUSE Binding Protein 1 (FUBP1) is overexpressed, with neglectable levels in healthy liver tissue.

FUBP1 is a single-stranded DNA-binding protein, which was originally discovered as a major upstream regulator of C-MYC gene expression. Additionally, FUBP1 is known to regulate cell cycle inhibitors like p21 and apoptosis regulating proteins like BIK. In a previous study we could show, that the knockdown of FUBP1 sensitizes HCC cell lines for apoptotic stimuli, e.g. mitomycin c treatment, and reduces tumor engraftment and growth in a Hep3B xenograft model. Consequently, FUBP1 inhibition is a very promising starting point of a targeted HCC therapy.

In a screening of FDA-approved drugs, using ALPHA-Screen technology, 1,200 substances were tested for their potential to inhibit or prevent the binding of FUBP1 to its target DNA sequence FUSE. We could demonstrate for the first time the effective inhibition of FUBP1 with small molecules: the known Topoisomerase 1 (TOP1) inhibitor camptothecin (CPT) and its clinically used analog 7-ethyl-10-hydroxycamptothecin (SN-38). Both molecules inhibited the interaction between FUBP1 and FUSE in ALPHA-Screen, Surface Plasmone Resonance (SPR) and Microscale Thermopheresis (MST) assays. Furthermore, SN-38 and CPT caused significant increase of the expression levels of FUBP1 target genes (p21, BIK) in hepatocellular carcinoma cell lines as well as in the TOP1-mutated cell line HCT116 G7. Treatment of mice injected subcutaneously with human HCC cell lines (Hep3B or HepG2) or orthotopically with murine Hepa129 cells showed high efficacy of a double treatment including irinotecan, the pro-drug of SN-38, and the apoptosis-inducing agent mitomycin c. Mice treated with either single or double treatment showed significantly prolonged tumor free survival period compared to control groups, with lower rates of tumor remission concerning irinotecan / mitomycin c drug exposure.

Most importantly, treatment of 5 patients suffering from intermediate unresectable HCCs with a combinational therapy of irinotecan and mitomycin c delivered via transarterial chemoembolization (TACE) revealed 100% response rate. Two of these patients showed complete tumor remission, with one of them being tumor free for over 1 year untill today, whereas the remaining 3 patients are in stable disease with no major side effects. Along with the in vitro and in vivo experiments, these early clinical results point to a significant breakthrough in HCC-therapy not seen before.

Citation Format: Stefanie Hauck, Sabrina Khageh Hosseini, Josephine Wesely, Dieter Steinhilber, Jörg Schulze, Annabelle Vogt, Maria Gonzalez-Carmona, Christian Strassburg, Stefan Zeuzem, Thomas Vogl, Jörg Trojan, Stephan Zangos, Ricardo Biondi, Eugen Proschak, Martin Zörnig. Inhibition of the oncoprotein FUBP1 by SN-38 represents a novel therapeutic option for the treatment of hepatocellular carcinoma. [abstract]. In: Proceedings of the Fourth AACR International Conference on Frontiers in Basic Cancer Research; 2015 Oct 23-26; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2016;76(3 Suppl):Abstract nr B39.

Immunohistochemical Assessment of Phosphorylated mTORC1-Pathway Proteins in Human Brain Tumors

Background

Current pathological diagnostics include the analysis of (epi-)genetic alterations as well as oncogenic pathways. Deregulated mammalian target of rapamycin complex 1 (mTORC1) signaling has been implicated in a variety of cancers including malignant gliomas and is considered a promising target in cancer treatment. Monitoring of mTORC1 activity before and during inhibitor therapy is essential. The aim of our study is to provide a recommendation and report on pitfalls in the use of phospho-specific antibodies against mTORC1-targets phospho-RPS6 (Ser235/236; Ser240/244) and phospho-4EBP1 (Thr37/46) in formalin fixed, paraffin embedded material.

Methods and Findings

Primary, established cell lines and brain tumor tissue from routine diagnostics were assessed by immunocyto-, immunohistochemistry, immunofluorescent stainings and immunoblotting. For validation of results, immunoblotting experiments were performed. mTORC-pathway activation was pharmacologically inhibited by torin2 and rapamycin. Torin2 treatment led to a strong reduction of signal intensity and frequency of all tested antibodies. In contrast phospho-4EBP1 did not show considerable reduction in staining intensity after rapamycin treatment, while immunocytochemistry with both phospho-RPS6-specific antibodies showed a reduced signal compared to controls. Staining intensity of both phospho-RPS6-specific antibodies did not show considerable decrease in stability in a timeline from 0–230 minutes without tissue fixation, however we observed a strong decrease of staining intensity in phospho-4EBP1 after 30 minutes. Detection of phospho-signals was strongly dependent on tissue size and fixation gradient. mTORC1-signaling was significantly induced in glioblastomas although not restricted to cancer cells but also detectable in non-neoplastic cells.

Conclusion

Here we provide a recommendation for phospho-specific immunohistochemistry for patient-orientated therapy decisions and monitoring treatment response.

Targeting Disease Persistence in Gastrointestinal Stromal Tumors

SummaryGastrointestinal stromal tumors (GISTs) represent 20%-40% of human sarcomas. Although approximately half of GISTs are cured by surgery, prognosis of advanced disease used to be poor due to the high resistance of these tumors to conventional chemo- and radiotherapy. The introduction of molecularly targeted therapy (e.g., with imatinib mesylate) following the discovery of the role of oncogenic mutations in the receptor tyrosine kinases KIT and platelet-derived growth factor α (PDGFRA) significantly increased patient survival. However, GIST cells persist in 95%-97% of imatinib-treated patients who eventually progress and die of the disease because of the emergence of clones with drug-resistant mutations. Because these secondary mutations are highly heterogeneous, even second- and third-line drugs that are effective against certain genotypes have only moderately increased progression-free survival. Consequently, alternative strategies such as targeting molecular mechanisms underlying disease persistence should be considered. We reviewed recently discovered cell-autonomous and microenvironmental mechanisms that could promote the survival of GIST cells in the presence of tyrosine kinase inhibitor therapy. We particularly focused on the potential role of adult precursors for interstitial cells of Cajal (ICCs), the normal counterpart of GISTs. ICC precursors share phenotypic characteristics with cells that emerge in a subset of patients treated with imatinib and in young patients with GIST characterized by loss of succinate dehydrogenase complex proteins and lack of KIT or PDGFRA mutations. Eradication of residual GIST cells and cure of GIST will likely require individualized combinations of several approaches tailored to tumor genotype and phenotype.

SIGNIFICANCE

Gastrointestinal stromal tumors (GISTs) are one of the most common connective tissue cancers. Most GISTs that cannot be cured by surgery respond to molecularly targeted therapy (e.g., with imatinib); however, tumor cells persist in almost all patients and eventually acquire drug-resistant mutations. Several mechanisms contribute to the survival of GIST cells in the presence of imatinib, including the activation of "escape" mechanisms and the selection of stem-like cells that are not dependent on the expression of the drug targets for survival. Eradication of residual GIST cells and cure of GIST will likely require individualized combinations of several approaches tailored to the genetic makeup and other characteristics of the tumors.

FAM96A is a novel pro-apoptotic tumor suppressor in gastrointestinal stromal tumors

The ability to escape apoptosis is a hallmark of cancer-initiating cells and a key factor of resistance to oncolytic therapy. Here, we identify FAM96A as a ubiquitous, evolutionarily conserved apoptosome-activating protein and investigate its potential pro-apoptotic tumor suppressor function in gastrointestinal stromal tumors (GISTs). Interaction between FAM96A and apoptotic peptidase activating factor 1 (APAF1) was identified in yeast two-hybrid screen and further studied by deletion mutants, glutathione-S-transferase pull-down, co-immunoprecipitation and immunofluorescence. Effects of FAM96A overexpression and knock-down on apoptosis sensitivity were examined in cancer cells and zebrafish embryos. Expression of FAM96A in GISTs and histogenetically related cells including interstitial cells of Cajal (ICCs), "fibroblast-like cells" (FLCs) and ICC stem cells (ICC-SCs) was investigated by Northern blotting, reverse transcription-polymerase chain reaction, immunohistochemistry and Western immunoblotting. Tumorigenicity of GIST cells and transformed murine ICC-SCs stably transduced to re-express FAM96A was studied by xeno- and allografting into immunocompromised mice. FAM96A was found to bind APAF1 and to enhance the induction of mitochondrial apoptosis. FAM96A protein or mRNA was dramatically reduced or lost in 106 of 108 GIST samples representing three independent patient cohorts. Whereas ICCs, ICC-SCs and FLCs, the presumed normal counterparts of GIST, were found to robustly express FAM96A protein and mRNA, FAM96A expression was much reduced in tumorigenic ICC-SCs. Re-expression of FAM96A in GIST cells and transformed ICC-SCs increased apoptosis sensitivity and diminished tumorigenicity. Our data suggest FAM96A is a novel pro-apoptotic tumor suppressor that is lost during GIST tumorigenesis.

A pro-apoptotic function of iASPP by stabilizing p300 and CBP through inhibition of BRMS1 E3 ubiquitin ligase activity

The p53 family and its cofactors are potent inducers of apoptosis and form a barrier to cancer. Here, we investigated the impact of the supposedly inhibitory member of the apoptosis-stimulating protein of p53, iASPP, on the activity of the p53 homolog TAp73, and its cofactors p300 and CBP. We found that iASPP interacted with and stabilized the histone acetyltransferase p300 and its homolog CBP upon cisplatin treatment. Vice versa, iASPP depletion by shRNA resulted in decreased amounts of p300 and CBP, impaired binding of p300 and TAp73 to target site promoters, reduced induction of pro-apoptotic TAp73 target genes, and impaired apoptosis. Mechanistically, we observed that the p300-regulatory E3 ubiquitin ligase BRMS1 could rescue the degradation of p300 and CBP in cisplatin-treated, iASPP-depleted cells. This argues that iASPP stabilizes p300 and CBP by interfering with their BRMS1-mediated ubiquitination, thereby contributing to apoptotic susceptibility. In line, iASPP overexpression partially abolished the interaction of BRMS1 and CBP upon DNA damage. Reduced levels of iASPP mRNA and protein as well as CBP protein were observed in human melanoma compared with normal skin tissue and benign melanocytic nevi. In line with our findings, iASPP overexpression or knockdown of BRMS1 each augmented p300/CBP levels in melanoma cell lines, thereby enhancing apoptosis upon DNA damage. Taken together, destabilization of p300/CBP by downregulation of iASPP expression levels appears to represent a molecular mechanism that contributes to chemoresistance in melanoma cells.

Overexpression of far upstream element (FUSE) binding protein (FBP)-interacting repressor (FIR) supports growth of hepatocellular carcinoma

The far upstream element binding protein (FBP) and the FBP-interacting repressor (FIR) represent molecular tools for transcriptional fine tuning of target genes. Strong overexpression of FBP in human hepatocellular carcinoma (HCC) supports tumor growth and correlates with poor patient prognosis. However, the role of the transcriptional repressor FIR in hepatocarcinogenesis remains poorly delineated. We show that overexpression of FIR correlates with tumor dedifferentiation and tumor cell proliferation in about 60% of primary HCCs. Elevated FIR levels are associated with genomic gains of the FIR gene locus at chromosome 8q24.3 in human HCC specimens. In vitro, nuclear enrichment of FIR supports HCC cell proliferation and migration. Expression profiling of HCC cells after small interfering RNA (siRNA)-mediated silencing of FIR identified the transcription factor DP-1 (TFDP1) as a transcriptional target of FIR. Surprisingly, FIR stimulates the expression of FBP in a TFDP1/E2F1-dependent manner. FIR splice variants lacking or containing exon 2 and/or exon 5 are expressed in the majority of HCCs but not in normal hepatocytes. Specific inhibition of FIR isoforms with and without exon 2 revealed that both groups of FIR splice variants facilitate tumor-supporting effects. This finding was confirmed in xenograft transplantation experiments with lentiviral-infected short hairpin RNA (shRNA) targeting all FIR variants as well as FIR with and without exon 2.

CONCLUSION

High-level nuclear FIR does not facilitate repressor properties but supports tumor growth in HCC cells. Thus, the pharmacological inhibition of FIR might represent a promising therapeutic strategy for HCC patients with elevated FIR expression.

Loss of FUBP1 expression in gliomas predicts FUBP1 mutation and is associated with oligodendroglial differentiation, IDH1 mutation and 1p/19q loss of heterozygosity

AIMS

The Far Upstream Element [FUSE] Binding Protein 1 (FUBP1) regulates target genes, such as the cell cycle regulators MYC and p21. FUBP1 is up-regulated in many tumours and acts as an oncoprotein by stimulating proliferation and inhibiting apoptosis. Recently, FUBP1 mutations were identified in approximately 15% of oligodendrogliomas. To date, all reported FUBP1 mutations have been predicted to inactivate FUBP1, which suggests that in contrast to most other tumours FUBP1 may act as a tumour suppressor in oligodendrogliomas.

METHODS

As no data are currently available concerning FUBP1 protein levels in gliomas, we examined the FUBP1 expression profiles of human glial tumours by immunohistochemistry and immunofluorescence. We analysed FUBP1 expression related to morphological differentiation, IDH1 and FUBP1 mutation status, 1p/19q loss of heterozygosity (LOH) as well as proliferation rate.

RESULTS

Our findings demonstrate that FUBP1 expression levels are increased in all glioma subtypes as compared with normal central nervous system (CNS) control tissue and are associated with increased proliferation. In contrast, FUBP1 immunonegativity predicted FUBP1 mutation with a sensitivity of 100% and a specificity of 90% in our cohort and was associated with oligodendroglial differentiation, IDH1 mutation and 1p/19q loss of heterozygosity (LOH). Using this approach, we detected a to-date undescribed FUBP1 mutation in an oligodendroglioma.

CONCLUSION

In summary, our data indicate an association between of FUBP1 expression and proliferation in gliomas. Furthermore, our findings present FUBP1 immunohistochemical analysis as a helpful additional tool for neuropathological glioma diagnostics predicting FUBP1 mutation.

EEF1A2 inactivates p53 by way of PI3K/AKT/mTOR-dependent stabilization of MDM4 in hepatocellular carcinoma

Mouse Double Minute homolog 4 (MDM4) gene up-regulation often occurs in human hepatocellular carcinoma (HCC), but the molecular mechanisms responsible for its induction remain poorly understood. Here we investigated the role of the phosphoinositide-3-kinase/v-akt murine thymoma viral oncogene homolog/mammalian target of rapamycin (PI3K/AKT/mTOR) axis in the regulation of MDM4 levels in HCC. The activity of MDM4 and the PI3K/AKT/mTOR pathway was modulated in human HCC cell lines by way of silencing and overexpression experiments. Expression of main pathway components was analyzed in an AKT mouse model and human HCCs. MDM4 inhibition resulted in growth restraint of HCC cell lines both in vitro and in vivo. Inhibition of the PI3K-AKT and/or mTOR pathways lowered MDM4 protein levels in HCC cells and reactivated p53-dependent transcription. Deubiquitination by ubiquitin-specific protease 2a and AKT-mediated phosphorylation protected MDM4 from proteasomal degradation and increased its protein stability. The eukaryotic elongation factor 1A2 (EEF1A2) was identified as an upstream inducer of PI3K supporting MDM4 stabilization. Also, we detected MDM4 protein up-regulation in an AKT mouse model and a strong correlation between the expression of EEF1A2, activated/phosphorylated AKT, and MDM4 in human HCC (each rho > 0.8, P < 0.001). Noticeably, a strong activation of this cascade was associated with shorter patient survival.

CONCLUSION

The EEF1A2/PI3K/AKT/mTOR axis promotes the protumorigenic stabilization of the MDM4 protooncogene in human HCC by way of a posttranscriptional mechanism. The activation level of the EEF1A2/PI3K/AKT/mTOR/MDM4 axis significantly influences the survival probability of HCC patients in vivo and may thus represent a promising molecular target.

Long noncoding RNA MALAT1 regulates endothelial cell function and vessel growth

RATIONALE

The human genome harbors a large number of sequences encoding for RNAs that are not translated but control cellular functions by distinct mechanisms. The expression and function of the longer transcripts namely the long noncoding RNAs in the vasculature are largely unknown.

OBJECTIVE

Here, we characterized the expression of long noncoding RNAs in human endothelial cells and elucidated the function of the highly expressed metastasis-associated lung adenocarcinoma transcript 1 (MALAT1).

METHODS AND RESULTS

Endothelial cells of different origin express relative high levels of the conserved long noncoding RNAs MALAT1, taurine upregulated gene 1 (TUG1), maternally expressed 3 (MEG3), linc00657, and linc00493. MALAT1 was significantly increased by hypoxia and controls a phenotypic switch in endothelial cells. Silencing of MALAT1 by small interfering RNAs or GapmeRs induced a promigratory response and increased basal sprouting and migration, whereas proliferation of endothelial cells was inhibited. When angiogenesis was further stimulated by vascular endothelial growth factor, MALAT1 small interfering RNAs induced discontinuous sprouts indicative of defective proliferation of stalk cells. In vivo studies confirmed that genetic ablation of MALAT1 inhibited proliferation of endothelial cells and reduced neonatal retina vascularization. Pharmacological inhibition of MALAT1 by GapmeRs reduced blood flow recovery and capillary density after hindlimb ischemia. Gene expression profiling followed by confirmatory quantitative reverse transcriptase-polymerase chain reaction demonstrated that silencing of MALAT1 impaired the expression of various cell cycle regulators.

CONCLUSIONS

Silencing of MALAT1 tips the balance from a proliferative to a migratory endothelial cell phenotype in vitro, and its genetic deletion or pharmacological inhibition reduces vascular growth in vivo.

A functional yeast survival screen of tumor-derived cDNA libraries designed to identify anti-apoptotic mammalian oncogenes

Yeast cells can be killed upon expression of pro-apoptotic mammalian proteins. We have established a functional yeast survival screen that was used to isolate novel human anti-apoptotic genes overexpressed in treatment-resistant tumors. The screening of three different cDNA libraries prepared from metastatic melanoma, glioblastomas and leukemic blasts allowed for the identification of many yeast cell death-repressing cDNAs, including 28% of genes that are already known to inhibit apoptosis, 35% of genes upregulated in at least one tumor entity and 16% of genes described as both anti-apoptotic in function and upregulated in tumors. These results confirm the great potential of this screening tool to identify novel anti-apoptotic and tumor-relevant molecules. Three of the isolated candidate genes were further analyzed regarding their anti-apoptotic function in cell culture and their potential as a therapeutic target for molecular therapy. PAICS, an enzyme required for de novo purine biosynthesis, the long non-coding RNA MALAT1 and the MAST2 kinase are overexpressed in certain tumor entities and capable of suppressing apoptosis in human cells. Using a subcutaneous xenograft mouse model, we also demonstrated that glioblastoma tumor growth requires MAST2 expression. An additional advantage of the yeast survival screen is its universal applicability. By using various inducible pro-apoptotic killer proteins and screening the appropriate cDNA library prepared from normal or pathologic tissue of interest, the survival screen can be used to identify apoptosis inhibitors in many different systems.

The noncoding RNA MALAT1 is a critical regulator of the metastasis phenotype of lung cancer cells

The long noncoding RNA MALAT1 (metastasis-associated lung adenocarcinoma transcript 1), also known as MALAT-1 or NEAT2 (nuclear-enriched abundant transcript 2), is a highly conserved nuclear noncoding RNA (ncRNA) and a predictive marker for metastasis development in lung cancer. To uncover its functional importance, we developed a MALAT1 knockout model in human lung tumor cells by genomically integrating RNA destabilizing elements using zinc finger nucleases. The achieved 1,000-fold MALAT1 silencing provides a unique loss-of-function model. Proposed mechanisms of action include regulation of splicing or gene expression. In lung cancer, MALAT1 does not alter alternative splicing but actively regulates gene expression including a set of metastasis-associated genes. Consequently, MALAT1-deficient cells are impaired in migration and form fewer tumor nodules in a mouse xenograft. Antisense oligonucleotides (ASO) blocking MALAT1 prevent metastasis formation after tumor implantation. Thus, targeting MALAT1 with ASOs provides a potential therapeutic approach to prevent lung cancer metastasis with this ncRNA serving as both predictive marker and therapeutic target. Finally, regulating gene expression, but not alternative splicing, is the critical function of MALAT1 in lung cancer metastasis. In summary, 10 years after the discovery of the lncRNA MALAT1 as a biomarker for lung cancer metastasis, our loss-of-function model unravels the active function of MALAT1 as a regulator of gene expression governing hallmarks of lung cancer metastasis.

Complexome profiling identifies TMEM126B as a component of the mitochondrial complex I assembly complex

Macromolecular complexes are essential players in numerous biological processes. They are often large, dynamic, and rather labile; approaches to study them are scarce. Covering masses up to ∼30 MDa, we separated the native complexome of rat heart mitochondria by blue-native and large-pore blue-native gel electrophoresis to analyze its constituents by mass spectrometry. Similarities in migration patterns allowed hierarchical clustering into interaction profiles representing a comprehensive analysis of soluble and membrane-bound complexes of an entire organelle. The power of this bottom-up approach was validated with well-characterized mitochondrial multiprotein complexes. TMEM126B was found to comigrate with known assembly factors of mitochondrial complex I, namely CIA30, Ecsit, and Acad9. We propose terming this complex mitochondrial complex I assembly (MCIA) complex. Furthermore, we demonstrate that TMEM126B is required for assembly of complex I. In summary, complexome profiling is a powerful and unbiased technique allowing the identification of previously overlooked components of large multiprotein complexes.

Loss of the abundant nuclear non-coding RNA MALAT1 is compatible with life and development

The metastasis-associated lung adenocarcinoma transcript 1, MALAT1, is a long non-coding RNA (lncRNA) that has been discovered as a marker for lung cancer metastasis. It is highly abundant, its expression is strongly regulated in many tumor entities including lung adenocarcinoma and hepatocellular carcinoma as well as physiological processes, and it is associated with many RNA binding proteins and highly conserved throughout evolution. The nuclear transcript MALAT-1 has been functionally associated with gene regulation and alternative splicing and its regulation has been shown to impact proliferation, apoptosis, migration and invasion.   Here, we have developed a human and a mouse knockout system to study the loss-of-function phenotypes of this important ncRNA. In human tumor cells, MALAT1 expression was abrogated using Zinc Finger Nucleases. Unexpectedly, the quantitative loss of MALAT1 did neither affect proliferation nor cell cycle progression nor nuclear architecture in human lung or liver cancer cells. Moreover, genetic loss of Malat1 in a knockout mouse model did not give rise to any obvious phenotype or histological abnormalities in Malat1-null compared with wild-type animals. Thus, loss of the abundant nuclear long ncRNA MALAT1 is compatible with cell viability and normal development.

Overexpression of the anti-apoptotic protein AVEN contributes to increased malignancy in hematopoietic neoplasms

AVEN has been identified as an inhibitor of apoptosis, which binds to the adaptor protein, APAF-1, and thereby prevents apoptosome formation and mitochondrial apoptosis. Recent data have demonstrated high expression levels of AVEN messenger RNA in acute leukemias as well as a positive correlation between AVEN mRNA overexpression and poor prognosis in childhood acute lymphoblastic leukemia. On the basis of these data, we investigated the potential involvement of AVEN in tumorigenesis. First, we confirmed the overexpression of AVEN in T-cell acute lymphoblastic leukemia/lymphoma (T-ALL) patient samples. We then established a transgenic mouse model with T-cell-specific overexpression of AVEN, with which we demonstrated the oncogenic cooperation of AVEN with heterozygous loss of p53. Finally, we used a subcutaneous xenograft mouse model to show that AVEN knockdown in the T-ALL cell lines, MOLT-4 and CCRF-CEM, and in the acute myeloblastic leukemia cell line, Kasumi-1, leads to a halt in tumor growth owing to the increased apoptosis and decreased proliferation of tumor cells. Collectively, our data demonstrate that the anti-apoptotic molecule, AVEN, functions as an oncoprotein in hematopoietic neoplasms.

Abstract A21: MALAT-1 is essential for lung cancer metastasis in a novel human knockout model

The highly conserved long non-coding RNA MALAT-1 (Metastasis-Associated in Lung Adenocarcinoma Transcript 1) had been discovered as a prognostic marker associated with poor survival and development of distant metastasis in lung adenocarcinoma. Since then, it has been found to be deregulated in numerous tumor entities and has been linked to splicing. However, its functional relevance in tumor cells remains to be elucidated. Knockdown models for MALAT1 have been described but suffer from insufficient silencing efficiency of the highly abundant, nuclear non-coding RNA (ncRNA).

In this project, we have developed a novel strategy to create ncRNA knockouts in human cancer cell lines. We have successfully used a synthetic Zinc Finger Nuclease engineered to target the 5′-region of MALAT1 to stably and biallelically integrate RNA-destabilizing elements into the genome of human lung cancer cells (A549). This approach resulted in a specific and more than 1000-fold silencing of MALAT1 in individual clones compared to a less than 5-fold silencing using siRNAs. Thus, this approach can be used to create functional knockouts of coding as well as non-coding genes also in human tumor cell lines allowing loss-of-function studies also of non-conserved ncRNAs in the future.

Phenotypically, the MALAT1-Knockout cells (KO) greatly differ from their parental cell line and wildtype clones (WT): Next to morphological changes, the migration of the KO cells is largely impaired as shown in scratch assays. In xenograft assays after i.v. injection, the KO cells form significantly fewer and smaller lung metastases than their WT counterparts. Since no large difference was observed after subcutaneous injection of the WT and the KO cells, this indicates a specific, active and essential function of MALAT1 in metastasis.

Exon microarrays of the WT and KO cell lines have revealed multiple migration- or metastasis-associated transcripts deregulated by loss of MALAT1 and hence potential target genes. These analyses also uncovered, that splicing is likely not the only functional mechanism of MALAT1 in the nucleus.

Taken together, we have developed a novel, highly effective approach for the knockout of genes that can be used for non-coding as well as coding RNAs in human tumor cells as well as cells from other species. Knockout of MALAT1 in human lung cancer cells revealed its essential function in metastasis as well as genes directly and indirectly targeted by MALAT1.

Citation Format: Tony Gutschner, Moritz Eißmann, Monika Hämmerle, Marion Stentrup, Catherina Hildenbrandt, Matthias Groß, Martin Zörnig, Sven Diederichs. MALAT-1 is essential for lung cancer metastasis in a novel human knockout model [abstract]. In: Proceedings of the AACR Special Conference on Noncoding RNAs and Cancer; 2012 Jan 8-11; Miami Beach, FL. Philadelphia (PA): AACR; Cancer Res 2012;72(2 Suppl):Abstract nr A21.

Palmitoylation of human FasL modulates its cell death-inducing function

Fas ligand (FasL) is a transmembrane protein that regulates cell death in Fas-bearing cells. FasL-mediated cell death is essential for immune system homeostasis and the elimination of viral or transformed cells. Because of its potent cytotoxic activity, FasL expression at the cell surface is tightly regulated, for example, via processing by ADAM10 and SPPL2a generating soluble FasL and the intracellular fragments APL (ADAM10-processed FasL form) and SPA (SPPL2a-processed APL). In this study, we report that FasL processing by ADAM10 counteracts Fas-mediated cell death and is strictly regulated by membrane localization, interactions and modifications of FasL. According to our observations, FasL processing occurs preferentially within cholesterol and sphingolipid-rich nanodomains (rafts) where efficient Fas–FasL contact occurs, Fas receptor and FasL interaction is also required for efficient FasL processing, and FasL palmitoylation, which occurs within its transmembrane domain, is critical for efficient FasL-mediated killing and FasL processing.

Mcl-1-mediated impairment of the intrinsic apoptosis pathway in circulating neutrophils from critically ill patients can be overcome by Fas stimulation

The systemic inflammatory response syndrome and subsequent organ failure are mainly driven by activated neutrophils with prolonged life span, which is believed to be due to apoptosis resistance. However, detailed underlying mechanisms leading to neutrophil apoptosis resistance are largely unknown, and possible therapeutic options to overcome this resistance do not exist. Here we report that activated neutrophils from severely injured patients exhibit cell death resistance due to impaired activation of the intrinsic apoptosis pathway, as evidenced by limited staurosporine-induced mitochondrial membrane depolarization and decreased caspase-9 activity. Moreover, we found that these neutrophils express high levels of antiapoptotic Mcl-1 and low levels of proapoptotic Bax protein. Mcl-1 up-regulation was dependent on elevated concentrations of GM-CSF in patient serum. Accordingly, increased Mcl-1 protein stability and GM-CSF serum concentrations were shown to correlate with staurosporine-induced apoptosis resistance. However, cross-linking of neutrophil Fas by immobilized agonistic anti-Fas IgM resulted in caspase-dependent mitochondrial membrane depolarization and apoptosis induction. In conclusion, the observed impairment of the intrinsic pathway and the resulting apoptosis resistance may be overcome by immobilized agonistic anti-Fas IgM. Targeting of neutrophil Fas by immobilized agonistic effector molecules may represent a new therapeutic tool to limit neutrophil hyperactivation and its sequelae in patients with severe immune disorders.

Overexpression of the far upstream element binding protein 1 in hepatocellular carcinoma is required for tumor growth

We identified the far upstream element binding protein 1 (FBP1), an activator of transcription of the proto-oncogene c-myc, in a functional yeast survival screen for tumor-related antiapoptotic proteins and demonstrated strong overexpression of FBP1 in human hepatocellular carcinoma (HCC). Knockdown of the protein in HCC cells resulted in increased sensitivity to apoptotic stimuli, reduced cell proliferation, and impaired tumor formation in a mouse xenograft transplantation model. Interestingly, analysis of gene regulation in these cells revealed that c-myc levels were not influenced by FBP1 in HCC cells. Instead, we identified the cell cycle inhibitor p21 as a direct target gene repressed by FBP1, and in addition, expression levels of the proapoptotic genes tumor necrosis factor alpha, tumor necrosis factor-related apoptosis-inducing ligand, Noxa, and Bik were elevated in the absence of FBP1.

CONCLUSION

Our data establish FBP1 as an important oncoprotein overexpressed in HCC that induces tumor propagation through direct or indirect repression of cell cycle inhibitors and proapoptotic target genes.

The Fas ligand intracellular domain is released by ADAM10 and SPPL2a cleavage in T-cells

Fas ligand (FasL) is a type II transmembrane protein belonging to the tumor necrosis factor family. Its binding to the cognate Fas receptor triggers the apoptosis that plays a pivotal role in the maintenance of immune system homeostasis. The cell death-inducing property of FasL has been associated with its extracellular domain, which can be cleaved off by metalloprotease activity to produce soluble FasL. The fate of the remaining membrane-anchored N-terminal part of the FasL molecule has not been determined. Here we show that post-translational processing of overexpressed and endogenous FasL in T-cells by the disintegrin and metalloprotease ADAM10 generates a 17-kDa N-terminal fragment, which lacks the receptor-binding extracellular domain. This FasL remnant is membrane anchored and further processed by SPPL2a, a member of the signal peptide peptidase-like family of intramembrane-cleaving proteases. SPPL2a cleavage liberates a smaller and highly unstable fragment mainly containing the intracellular FasL domain (FasL ICD). We show that this fragment translocates to the nucleus and is capable of inhibiting gene transcription. With ADAM10 and SPPL2a we have identified two proteases implicated in FasL processing and release of the FasL ICD, which has been shown to be important for retrograde FasL signaling.

Targeting the Oligomerization Domain of ETO Interferes with RUNX1/ETO Oncogenic Activity in t(8;21)-Positive Leukemic Cells

About 12% of all de novo acute myeloid leukemias are characterized by the translocation t(8;21), which generates the oncogenic fusion protein RUNX1/ETO. RUNX1/ETO has a modular structure and contains several docking sites for heterologous proteins, including transcriptional co-repressors like N-CoR, SMART, and mSIN3A. RUNX1/ETO is found in high molecular weight complexes, which are crucial for the block in myeloid differentiation observed in RUNX1/ETO-transformed cells. Essential for high molecular weight complex formation is the nervy homology region 2 (NHR2) within ETO, which serves as interacting surface for oligomerization as well as association with members of the ETO protein family. Here, we show that the expression of a fusion peptide consisting of 128 amino acids (NC128), including the entire NHR2 domain of ETO, disrupts the stability of the RUNX1/ETO high molecular weight complexes, restores transcription of RUNX1/ETO target genes, and reverts the differentiation block induced by RUNX1/ETO in myeloid cells. In the presence of NC128, RUNX1/ETO-transformed cells lose their progenitor cell characteristics, are arrested in cell cycle progression, and undergo cell death. Our results indicate that selective interference with the oligomerization domain of ETO could provide a promising strategy to inhibit the oncogenic properties of the leukemia-associated fusion protein RUNX1/ETO.

Upon intracellular processing, the C-terminal death domain-containing fragment of the p53-inducible PIDD/LRDD protein translocates to the nucleoli and interacts with nucleolin

The p53-inducible and death domain-containing PIDD/LRDD protein has been described as an adaptor protein, which forms large protein complexes with RAIDD, another death domain-containing protein, leading to recruitment, and activation of the initiator caspase-2, and p53-mediated apoptosis. Here, we describe in further detail the proteolytic processing of PIDD/LRDD that occurs in healthy cells before induction of apoptosis. We could demonstrate that the C-terminal fragment containing the PIDD death domain shuttles into the nucleoli. This translocation is mediated by or leads to the interaction of the PIDD death domain with nucleolin, a protein important for rRNA processing within nucleoli and possibly involved in the DNA damage response. Ectopically expressed LRDD and endogenous nucleolin co-localized within the nucleoli, and overexpression of both full-length LRDD and the LRDD death domain sensitized cells for UV-induced apoptosis. When expressed alone, the PIDD/LRDD death domain tended to form large filamentous structures resembling so-called death filaments. The functional consequences of the identified PIDD/nucleolin interaction remain to be elucidated, but may be related to a recently discovered new role for PIDD in the activation of NF-kappaB upon genotoxic stress.

Regulation of FasL expression: A SH3 domain containing protein family involved in the lysosomal association of FasL

As a death factor of T cells and Natural Killer (NK) cells, Fas Ligand (FasL) is stored in association with secretory lysosomes. Upon stimulation, these cytotoxic granules are transported to the cell membrane where FasL is exposed on the cell surface, shed or secreted. It has been noted before that the proline-rich domain within the cytosolic part of FasL is required for its vesicular association. However, the molecular interactions involved in targeting FasL to secretory lysosomes or to the plasma membrane have not been elucidated. We now identified a family of structurally related proteins that upon co-expression with FasL reallocate the death factor from a membrane to an intracellular localization. Members of this protein family are characterized by a similar domain structure and include FBP17, PACSIN1-3, CD2BP1, CIP4, Rho-GAP C1 and several hypothetical proteins. We show that all tested members of this "FCH/SH3-family" co-precipitate FasL from transfectants. The interactions strictly depend on functional SH3 domains within the FCH/SH3 proteins. Since co-expression of FasL with individual FCH/SH3 proteins dramatically alters the intracellular localization of FasL especially in non-hematopoietic cells, our data suggest that FCH/SH3 proteins might play an important role for the subcellular distribution and lysosomal association of FasL.

Agonists of an ecdysone-inducible mammalian expression system inhibit Fas Ligand- and TRAIL-induced apoptosis in the human colon carcinoma cell line RKO

The ecdysone-inducible mammalian expression system is frequently used for inducible transgene expression in vitro and in vivo. Here, we describe a strong antiapoptotic effect of ecdysone analogs in the human colon carcinoma cell line RKO, which is in contrast to published data that ecdysteroids do not influence mammalian cell physiology. Inhibition of Fas ligand- and TNF-related apoptosis-inducing ligand-induced apoptosis by muristerone A occurs at the level of caspase-8 activation and is neutralized by phosphatidylinositol-3-kinase/Akt, protein kinase C and mitogen-activated protein kinase inhibitors. Microarray, Northern and Western blot analysis revealed that incubation of RKO cells with muristerone A leads to changes in gene expression levels, including an upregulation of bcl-x(L) mRNA and protein levels. Our data imply that ecdysteroids and ecdysone mimics can induce and/or repress gene transcription in RKO and other mammalian cells, thereby influencing the apoptotic behavior. Therefore, the ecdysone-inducible mammalian expression system may not be suitable for the analysis of apoptosis-related genes.

Fas ligand is localized to membrane rafts, where it displays increased cell death–inducing activity

Fas ligand (FasL), a member of the TNF protein family, potently induces cell death by activating its matching receptor Fas. Fas-mediated killing plays a critical role in naturally and pathologically occurring cell death, including development and homeostasis of the immune system. In addition to its receptor-interacting and cell death–inducing extracellular domain, FasL has a well-conserved intracellular portion with a proline-rich SH3 domain–binding site probably involved in non-apoptotic functions. We report here that, as with the Fas receptor, a fraction of FasL is constitutively localized in rafts. These dynamic membrane microdomains, enriched in sphingolipids and cholesterol, are important for cell signaling and trafficking processes. We show that FasL is partially localized in rafts and that increased amounts of FasL are found in rafts after efficient FasL/Fas receptor interactions. Raft disorganization after cholesterol oxidase treatment and deletions within the intracellular FasL domain diminish raft partitioning and, most important, lead to decreased FasL killing. We conclude that FasL is recruited into lipid rafts for maximum Fas receptor contact and cell death–inducing potency. These findings raise the possibility that certain pathologic conditions may be treated by altering the cell death–inducing capability of FasL with drugs affecting its raft localization.

Increased expression of high mobility group box 1 (HMGB1) is associated with an elevated level of the antiapoptotic c-IAP2 protein in human colon carcinomas

BACKGROUND

High mobility group box 1 (HMGB1) is a non-histone chromosomal protein implicated in a variety of biologically important processes, including transcription, DNA repair, V(D)J recombination, differentiation, and development. Overexpression of HMGB1 inhibits apoptosis, arguing that the molecule may act as an antiapoptotic oncoprotein. Indeed, increased expression of HMGB1 has been reported for several different tumour types. In this study, we analysed human colon carcinoma for HMGB1 as well as for c-IAP2 expression levels. c-IAP2 is an antiapoptotic protein which may be upregulated as a consequence of nuclear factor kappaB (NFkappaB) activation via HMGB1.

METHODS

A comparative genomic hybridisation (CGH) database comprising 1645 cases from different human tumour types was screened to detect cytogenetic changes at the HMGB1 locus. Immunohistochemical staining of human colon tissue microarrays and tumour biopsies, as well as western blot analysis of tumour lysates, were performed to detect elevated HMGB1 and c-IAP2 expression in colon carcinomas. The antiapoptotic potential of HMGB1 was analysed by measuring caspase activities, and luciferase reporter assays and quantitative polymerase chain reaction analysis were employed to confirm NFkappaB activation and c-IAP2 mRNA upregulation on HMGB1 overexpression.

RESULTS

According to CGH analysis, the genomic locus containing the HMGB1 gene was overrepresented in one third (35/96) of colon cancers. Correspondingly, HMGB1 protein levels were significantly elevated in 90% of the 60 colon carcinomas tested compared with corresponding normal tissues evaluable from the same patients. HMGB1 increased NFkappaB activity and led to co-overexpression of the antiapoptotic NFkappaB target gene product c-IAP2 in vitro. Furthermore, increased HMGB1 levels correlated with enhanced amounts of c-IAP2 in colon tumours analysed by us. Finally, we demonstrated that HMGB1 overexpression suppressed caspase-9 and caspase-3 activity, suggesting that HMGB1 interferes with the apoptotic machinery at the level of apoptosomal caspase-9 activation.

CONCLUSIONS

We identified in vitro a molecular pathway triggered by HMGB1 to inhibit apoptosis via c-IAP2 induction. Our data indicate a strong correlation between upregulation of the apoptosis repressing HMGB1 and c-IAP2 proteins in the pathogenesis of colon carcinoma.

Binding of the Intracellular Fas Ligand (FasL) Domain to the Adaptor Protein PSTPIP Results in a Cytoplasmic Localization of FasL

The tumor necrosis factor family member Fas ligand (FasL) induces apoptosis in Fas receptor-expressing target cells and is an important cytotoxic effector molecule used by CTL- and NK-cells. In these hematopoietic cells, newly synthesized FasL is stored in specialized secretory lysosomes and only delivered to the cell surface upon activation and target cell recognition. FasL contains an 80-amino acid-long cytoplasmic tail, which includes a proline-rich domain as a bona fide Src homology 3 domain-binding site. This proline-rich domain has been implicated in FasL sorting to secretory lysosomes, and it may also be important for reverse signaling via FasL, which has been described to influence T-cell activation. Here we report the identification of the Src homology 3 domain-containing adaptor protein PSTPIP as a FasL-interacting partner, which binds to the proline-rich domain. PSTPIP co-expression leads to an increased intracellular localization of Fas ligand, thereby regulating extracellular availability and cytotoxic activity of the molecule. In addition, we demonstrate recruitment of the tyrosine phosphatase PTP-PEST by PSTPIP into FasL.PSTPIP.PTP-PEST complexes which may contribute to FasL reverse signaling.

Valproic acid induces extracellular signal-regulated kinase 1/2 activation and inhibits apoptosis in endothelial cells

The histone deacetylase (HDAC) inhibitor valproic acid (VPA) was recently shown to inhibit angiogenesis, but displays no toxicity in endothelial cells. Here, we demonstrate that VPA increases extracellular signal-regulated kinase 1/2 (ERK 1/2) phosphorylation in human umbilical vein endothelial cells (HUVEC). The investigation of structurally modified VPA derivatives revealed that the induction of ERK 1/2 phosphorylation is not correlated to HDAC inhibition. PD98059, a pharmacological inhibitor of the mitogen-activated protein kinase kinase 1/2, prevented the VPA-induced ERK 1/2 phosphorylation. In endothelial cells, ERK 1/2 phosphorylation is known to promote cell survival and angiogenesis. Our results showed that VPA-induced ERK 1/2 phosphorylation in turn causes phosphorylation of the antiapoptotic protein Bcl-2 and inhibits serum starvation-induced HUVEC apoptosis and cytochrome c release from the mitochondria. Moreover, the combination of VPA with PD98059 synergistically inhibited angiogenesis in vitro and in vivo.

Transgenic overexpression of the Caspase-8 inhibitor FLIPshortleads to impaired T cell proliferation and an increased memory T cell pool after staphylococcal enterotoxin B injection

The cellular homologues of the viral anti-apoptotic v-FLIP proteins exist as a long (c-FLIP(L)) and a short (c-FLIP(S)) splice variant. While c-FLIP(S) and v-FLIP are composed solely of two death effector domains, c-FLIP(L) contains an (inactive) caspase-like domain in addition to these two death effector domains, thereby structurally resembling pro-Caspase-8. Both c-FLIP(L) and c-FLIP(S) suppress apoptosis by inhibiting Caspase-8 activation, although at different levels of pro-Caspase-8 processing. To analyze the consequences of deregulated c-FLIP(S) expression in vivo, we established lck FLIP(S)-transgenic mice overexpressing the transgene in thymocytes and in mature T cells. As expected, CD95L-induced apoptosis was impaired in lck FLIP(S)-transgenic T cells, indicating the functionality of the FLIP(S) transgene. Remarkably, activation-induced cell death of transgenic T cells was unaffected, despite the observed inhibition of CD95-induced T cell death. Thymic and splenic cell numbers as well as CD4/CD8 cellularity were normal in lck FLIP(S)-transgenic animals, which in contrast to CD95-deficient mice do not accumulate Thy1(+) B220(+) CD4(-) CD8(-) peripheral T cells. c-FLIP(S) overexpression leads to a significant decrease in activation-induced T cell proliferation in vitro. Despite the capacity of FLIP(S) to inhibit CD95-induced apoptosis, T cell lymphomagenesis is not observed in lck FLIP(S)-transgenic mice. Interestingly, the Vbeta8(+) memory T cell pool is enlarged upon staphylococcal enterotoxin B injections, suggesting a specific in vivo function for FLIP(S) in the maintenance of restimulated T cells.

Transgenic overexpression of a dominant negative mutant of FADD that, although counterselected during tumor progression, cooperates in L-myc-induced tumorigenesis

Activation of the so-called death receptors, e.g., CD95/Fas/Apo-1, is a potent stimulus to trigger apoptosis. Overexpression of the C-terminal FADD deletion mutant FADD-DN blocks death receptor-induced apoptosis, but despite this antiapoptotic activity, lck FADD-DN transgenic mice do not develop lymphomas. To analyze whether functional inactivation of FADD cooperates with Myc overexpression in tumorigenesis, lck FADD-DN transgenic mice were crossed with Emicro L-myc transoncogenic animals. While no tumors were detected in single transgenic FADD-DN or L-myc mice within 15 months, 5 of 17 (29%) FADD-DN/L-myc double transgenic animals developed lymphomas with an average latency period of 47 weeks. Protein analysis of FADD-DN/L-myc tumors showed, however, undetectable levels of FADD-DN protein. FADD-DN protein expression was again lost in 16 of 17 FADD-DN/p53 k.o. T-cell lymphomas, though no significant acceleration of tumorigenesis in P53-deficient lck FADD-DN mice compared to p53 k.o. animals was observed. These data suggest a strong counterselection against the FADD-DN protein during tumor progression, which could be explained by the cell cycle inhibitory activity of FADD-DN. Such counterselection would have to be compensated for by other antiapoptotic mutations, and indeed, strong upregulation of the antiapoptotic Bcl-2 family member Bcl-xL was found in one of the tumors. This in vivo mouse model demonstrates that an antiapoptotic protein involved in the onset of tumorigenesis is selected against and consequently lost during tumor progression because of its additional antiproliferative activity.

The role of Bcl-2 family members in tumorigenesis

The Bcl-2 family consists of about 20 homologues of important pro- and anti-apoptotic regulators of programmed cell death. The established mode of function of the individual members is to either preserve or disturb mitochondrial integrity, thereby inducing or preventing release of apoptogenic factors like Cytochrome c (Cyt c) from mitochondria. Recent findings also indicate further Bcl-2-controlled mitochondria-independent apoptosis pathways. Bcl-2 represents the founding member of the new and growing class of cell death inhibiting oncoproteins. In this review, we try to briefly summarize current models of Bcl-2 family function and to outline the work demonstrating the influence of deregulated Bcl-2 family member expression on tumorigenesis and cancer therapy. Since several Bcl-2 homologues, in addition to influencing apoptotic behaviour, also impinge on cell cycle progression, we discuss possible implications of this additional role for the expression of Bcl-2 family members in tumor cells.

HMGB1 inhibits cell death in yeast and mammalian cells and is abundantly expressed in human breast carcinoma

Apoptosis is a fundamental biological process used to eliminate unwanted cells in a multicellular organism. An increasing number of regulatory proteins have been identified that either promote or inhibit apoptosis. For tumors to arise, apoptosis must be blocked in the transformed cells, for example by mutational overexpression of anti-apoptotic proteins, which represent attractive target proteins for molecular therapy strategies. In a functional yeast survival screen designed to select new anti-apoptotic mammalian genes, we have identified the chromosomal high-mobility group box-1 protein (HMGB1) as an inhibitor of yeast cell death induced by the pro-apoptotic Bcl-2 family member Bak. The C-terminal 33 amino acids of HMGB1 are dispensable for this inhibitory function. HMGB1 is also able to protect mammalian cells against different death stimuli including ultraviolet radiation, CD95-, TRAIL-, Casp-8-, and Bax-induced apoptosis. We found high HMGB1 protein levels in human primary breast carcinoma. Hmgb1 RNA levels are changing during different stages of mouse mammary gland development and are particularly low during lactation and involution. These data suggest that HMGB1 may participate in the regulation of mammary gland apoptosis and that its high expression level promotes tumor growth because of its anti-apoptotic properties.

Apoptosis regulators and their role in tumorigenesis

It has become clear that, together with deregulated growth, inhibition of programmed cell death (PCD) plays a pivotal role in tumorigenesis. In this review, we present an overview of the genes and mechanisms involved in PCD. We then summarize the evidence that impaired PCD is a prerequisite for tumorigenesis, as indicated by the fact that more and more neoplastic mutations appear to act by interfering with PCD. This has made the idea of restoration of corrupted 'death programs' an intriguing new area for potential cancer therapy.