Loss of Fas (CD95/APO-1) expression by antigen-specific cytotoxic T cells is reversed by inhibiting DNA methylation

Participation of the Fas/FasL signaling pathway and the lung microenvironment in the development of osteosarcoma lung metastases

The lungs are the most common site for the metastatic spread of osteosarcoma. Success in using chemotherapy to improve overall survival has reached a plateau. Understanding the biologic properties that permit osteosarcoma cells to grow in the lungs may allow the identification of novel therapeutic approaches-the goal being to alter the tumor cells' expression of cell surface proteins so that there is no longer compatibility with the metastatic niche. We have demonstrated that the Fas Ligand positive (FasL(+)) lung microenvironment eliminates Fas(+) osteosarcoma cells that metastasize to the lungs. Indeed, osteosarcoma lung metastases from patients are Fas(-), similar to what we found in several different mouse models. The Fas(+) cells are cleared from the lungs through apoptosis induced by the Fas signaling pathway following interaction of Fas on the tumor cell surface with the lung FasL. Blocking the Fas signaling pathway interferes with this process, allowing the Fas(+) cells to grow in the lungs. Our investigations show that Fas expression in osteosarcoma cells is regulated epigenetically by the micro-RNA miR-20a, encoded by the miR-17-92 cluster. Our studies support the feasibility of finding agents that can re-induce Fas expression as a novel therapeutic approach to treat osteosarcoma patients with lung metastases. We have identified two such agents, the histone deacetylase inhibitor entinostat and the chemotherapeutic agent gemcitabine (GCB). Aerosol GCB and oral entinostat induce the upregulation of Fas and the regression of established osteosarcoma lung metastases. Aerosol GCB was not effective in the FasL-deficient gld mouse confirming that the lung microenvironment was central to the success of this therapy. Our studies establish the critical role of the lung microenvironment in the metastatic process of osteosarcoma to the lungs and suggest an alternative focus for therapy, that is, incorporating the lung microenvironment as part of the treatment strategy against established osteosarcoma disease in the lungs.

Fas expression in metastatic osteosarcoma cells is not regulated by CpG island methylation

Fas expression in osteosarcoma (OS) cells is inversely correlated with the metastatic potential of OS to the lung. The purpose of this study was to determine whether loss of Fas expression in metastatic OS cells is secondary to DNA methylation of CpG islands in the Fas gene. SAOS-2 cells have high levels of Fas expression and do not form lung metastases when injected intravenously, whereas LM7 cells have low levels of Fas expression and do produce lung metastases. Using the endonucleases HpaII and MspI and a polymerase chain reaction-based methylation assay, we found that all four CpG sites in the CCGG sequence in the Fas promoter region were unmethylated in both SAOS-2 and LM7 cells. We performed detailed analysis of the 28 and 46 CpG sites in the Fas promoter and first intron region, respectively, using bisulfite-modified genomic DNA sequencing. More than 99.8% of the examined CpG sites were unmethylated and there was no difference of CpG methylation in SAOS-2 and LM7 cells as well as LM7 metastatic lung tumor tissue samples. Treatment of LM7 cells and another OS cell line, DLM8 with low levels of Fas expression, with demethylation agent, 5-azadeoxycitidine (AzadC), did not change the Fas expression and did not increase sensitivity of AzadC-treated cells to Fas ligand (FasL) treatment. In conclusion, our data indicate that decreased Fas expression in OS cells is not secondary to DNA methylation of CpG islands in the Fas gene and that Fas expression cannot be increased by using demethylation agents.

Peripheral tolerance limits CNS accumulation of CD8 T cells specific for an antigen shared by tumor cells and normal astrocytes

T cell mediated immunotherapies are proposed for many cancers including malignant astrocytoma. As such therapies become more potent, but not necessarily more tumor-specific, the risk of collateral autoimmune damage to normal tissue increases. Tumors of the brain present significant challenges in this respect, as autoimmune destruction of brain tissue could have severe consequences. To investigate local immune reactivity toward a tumor-associated antigen in the brain, transgenic mice were generated that express a defined antigen (CW3 170-179) in astroglial cells. The resulting six transgenic mouse lines expressed the transgenic self-antigen in cells of the gastrointestinal tract and CNS compartments, or in the CNS alone. By challenging transgenic mice with tumor cells that express CW3, self/tumor-specific immune responses were visualized within a normal polyclonal T cell repertoire. A large expansion of the endogenous CW3 170-179-specific CD8 T cell population was observed in nontransgenic mice after both subcutaneous and intracerebral implantation of tumor cells. In contrast, CW3 170-179-specific immune responses were not observed in transgenic mice that exhibited extracerebral transgene expression. Importantly, in certain groups of mice in which transgene expression was restricted to the CNS, antigen-specific immune responses occurred when tumor was implanted subcutaneously, but not intracerebrally. This local immune tolerance in the brain was induced via peripheral (extrathymic) rather than central (thymic) tolerance mechanisms. Thus, this study highlights the role of regional immune regulation in the prevention of autoimmunity in the brain, and the potential impact of these mechanisms for brain tumor immunotherapy.

Human embryo immune escape mechanisms rediscovered by the tumor

Towards the end of the 1990s, the two opposing theories on immunosurveillance and immunostimulation were extensively studied by researchers in an attempt to understand the complex mechanisms that regulate the relation between tumors and the host's immune system. Both theories probably have elements that would help us to comprehend how the host can induce anti-tumor clinical responses through stimulation of the immune system and which could also give us a deeper insight into the mechanisms of tumor immunosuppression. The model that most resembles the behavior of tumor cells in terms of growth, infiltration and suppression of the immune system of the environment in which they live is undoubtedly that of the embryonic cell. The fetus behaves like an allogenic transplant within the mother's body, using every means it has to escape from and defend itself against the mother's immune system. The majority of these mechanisms are the same as those found in tumor cells: antigenic loss, lack of expression of classic HLA-I molecules, production of immunosuppressive cytokines, induction of lack of expression of co-stimulatory molecules in antigen presenting cells, and induction of apoptosis in infiltrating lymphocytes, with activation of a type Th2 regulatory lymphocyte response. A careful and comparative study of key mechanisms capable of triggering tolerance or cytotoxicity in both embryonic and tumor cells could prove immensely valuable in designing new strategies for anti-tumor immunotherapy.

Dissecting lipid raft facilitated cell signaling pathways in cancer

Cancer is one of the most devastating disorders in our lives. Higher rate of proliferation than death of cells is one of the essential factors for development of cancer. The dynamicity of cell membrane plays some vital roles in cell survival and cell death, including protection, endocytosis, signaling, and increases in mechanical stability during cell division, as well as decrease of shear forces during separation of two cells after division, and cell separation from tissues for cancer metastasis. Within the membrane, there are specialized domains, known as lipid rafts. A raft can coordinate various signaling pathways. Recent data on the proteomics of lipid rafts/caveolae have highlighted the enigmatic role of various signaling proteins in cancer development. Analysis of these data of raft proteome from various tumors, cancer tissues, and cell lines cultured without and with therapeutic agents, as well as from model rafts revealed that there may be two subsets of raft assemblage in cell membrane. One subset of raft is enriched with cholesterol-sphingomyeline-ganglioside-cav-1/Src/EGFR (hereafter, "chol-raft") that is involved in normal cell signaling, and when dysregulated promotes cell transformation and tumor progression; another subset of raft is enriched with ceramide-sphingomyeline-ganglioside-FAS/Ezrin (hereafter, "cer-raft") that generally promotes apoptosis. In view of this, and to focus insight into the cancer cell physiology caused by the lipid rafts mediated signals and their receptors, and the downstream transmitters, either proliferative (for example, EGF and EGFR) or death-inducing (for example, FASL and FAS), and the precise roles of some therapeutic drugs and endogenous acid sphingomylenase in this scenario in in situ transformation of "chol-raft" into "cer-raft" are summarized and discussed in this contribution.

Immunoepigenetics: the unseen side of cancer immunoediting

Cancer immunosurveillance representing, till recently, the explanatory framework relating cancer and the immune system, does not convincingly explain tumor escape. At the beginning of the decade, a new theory emerged, namely the immunoediting theory, and it comprehensively defines the role of the immune system in carcinogenesis. The core of this theory embraces the concept that the immune system on the one hand protects the body from cancer and on the other it shapes the immunogenicity of these cancers, thus presents a persuasive rationalization of the resistance of tumors against the immune response. With the immune system playing, in this context, such a pivotal role in shaping the tumor immune profile and in subsequent oncogenesis, it seems rather paradoxical to accept the immunocompetent host's immune system as a constant moiety. While DNA mutations of immune genes create a rather polymorphic condition, their frequency is much lower than that of other genetic events. Of these, epigenetic alterations give rise to new epialleles, which can reach up to 100% per locus. Bearing in mind that cancer is characterized by a tremendous amount of epigenetic aberrations, in both gene and global level, it is reasonable to postulate that, for the same unknown causes, analogous aberrations could affect the immune genes. Should this be the case, the relation between oncogenesis and the immune system appears much more dynamic and complex. Such an immunoepigenetic approach to carcinogenesis could improve our understanding of a series of common cancer-related aspects, such as environmental risk factors, effectiveness of demethylating agents, failure of current immunotherapies, etc. Moreover, this immunoepigenetic paradigm will take the current perception of the immune system and cancer interrelation further and beyond, constituting that the immunoresistant cancer cell phenotype is not shaped by the immune system acting as a steady and rigid evolutionary pressure, but rather as an extremely dynamic variable.

A self-defence mechanism of astrocytes against Fas-mediated death involving interleukin-8 and CXCR2

Astrocytes play multiple roles from passive support to the regulation of inflammation during brain injury. This latter function is in part achieved by responses induced by the triggering of Fas expressed on astrocytes both and. It was previously shown that astrocytes are resistant to Fas-mediated death, responding to Fas triggering by interleukin-8 production. However, the cellular mechanisms by which astrocytes protect themselves from Fas-mediated death are unclear. Here, we show that survival of cultured astrocytes after Fas triggering is governed by the interaction of interleukin-8 with one of its receptors, CXCR2. Furthermore, interleukin-8 secretion and CXCR2 expression are both induced in human astrocytes after Fas stimulation, suggesting a new mechanism of self-defence against Fas-mediated death.

IFN-gammaupregulates apoptosis-related molecules and enhances Fas-mediated apoptosis in human cholangiocarcinoma

Human cholangiocarcinoma is a malignancy with no effective therapy and a poor prognosis. Previously, we demonstrated that cultured human cholangiocarcinoma cell lines heterogeneously express Fas on their surface, resulting in 2 subpopulations, Fas-high and Fas-low cells. Fas-low cells are resistant to apoptosis induced by Fas antibody and the calmodulin antagonists tamoxifen and trifluoperazine and are tumorigenic in nude mice (Pan et al., Am J Pathol 1999;155:193-203). Here, we show that IFN-gamma enhances apoptosis in both Fas-high and Fas-low cells. IFN-gamma upregulates many apoptosis-related molecules, including Fas, caspase-3, caspase-4, caspase-7, caspase-8 and Bak, in both cell lines. Pretreatment with IFN-gamma facilitated Fas-mediated caspase cleavage, cytochrome c release and Bax translocation. The ability of IFN-gamma to inhibit tumorigenesis of Fas-low cells was demonstrated in nude mice. Intratumoral injection of IFN-gamma decreased tumor volumes by 78%. These findings indicate that IFN-gamma modulates the apoptotic pathway by upregulating apoptosis-related genes. This renders tumorigenic Fas-low cholangiocarcinoma cells nontumorigenic and sensitive to Fas apoptosis, thus representing a possible therapeutic modality.

Exposure to exogenous DNA can modify the sensitivity of the Fas apoptotic pathway

BACKGROUND

Gene-transfer techniques are commonly employed for both in vitro and in vivo studies. However, modifications of the target cell following the introduction of the gene of interest are not often examined. These modifications can alter the immunogenicity and/or the susceptibility of the target cell to apoptosis and may produce unwanted consequences in vivo.

METHODS

Gene transfer into the murine fibroblastic Psi-CRIP packaging cell line was performed using calcium phosphate precipitation, cationic liposome-DNA complexes or a retroviral RNA-mediated method. After gene transfer, Fas expression, cytokine production, and sensitivity to Fas ligand (FasL)-mediated death were assessed.

RESULTS

Following transfection of a FasL expression vector by calcium phosphate precipitation, an unexpected increase was observed in apoptotic cell death in previously Fas-resistant Psi-CRIP cells. This apoptosis was due to Fas upregulation and an increase of sensitivity to FasL-mediated death. Other plasmids coding non-cytotoxic factors also modulated this apoptotic pathway. The co-stimulatory molecule CD80 was also upregulated. Exposure to naked DNA alone elicited the same response. The effect was not dependent on the methylation status of exogenous DNA, but was found to be dependent on the target cell type and might be avoided by the use of an RNA-mediated retroviral system.

CONCLUSIONS

Plasmid transfection or simple exposure to naked DNA can increase sensitivity to apoptosis. The generation of FasL packaging cell lines is therefore limited by an increase in FasL/Fas-mediated apoptosis. These findings should be considered when using genetically modified transplantable cells in order to prevent elimination by host cytotoxic cells and in particular when cells are engineered using FasL.

How do cultured CD8(+) murine T cell clones survive repeated ligation of the TCR?

Many murine T cell clones grow continuously in culture despite weekly ligation of their TCR by antigen. To learn how the cultured cells avoid or minimize antigen-induced cell death (AICD), we compared Fas and tumor necrosis factor (TNF) receptors (TNFR) on several long-term cultured CD8(+) T cell clones with those on naive and activated naive cells expressing the same TCR (2C). In contrast to the naive cells, Fas was absent on the cultured clones and the TNFR-II receptor, present initially at high levels on the cultured cells, was rapidly down-modulated in response to TCR ligation and had virtually disappeared by 2 h, when only approximately 10% of the cloned cells had been induced to express TNF-alpha. The extent of AICD of the cultured clones in response to cognate peptide-MHC on the presenting cells used for routine stimulation of the cultures was also considerably less than the massive cell death of the clones following exposure to anti-CD3 antibody plate-bound at high density.

Hypermethylation of the tumor necrosis factor receptor superfamily 6 (APT1, Fas, CD95/Apo-1) gene promoter at rel/nuclear factor kappaB sites in prostatic carcinoma

DNA hypermethylation of CpG-rich promoter sequences is associated with tumor suppressor gene inactivation in many human cancers, notably in carcinoma of the prostate and the urinary bladder. Recently, the mouse homologue of the tumor necrosis factor receptor superfamily 6 (TNFRSF6) gene was reported to be inactivated by DNA methylation in various cell types. The Fas (CD95, Apo-1) protein encoded by the TNFRSF6 gene is an important mediator of apoptosis, which also is downregulated in different types of human carcinoma. We therefore investigated the methylation of the TNFRSF6 promoter in prostatic and bladder carcinomas and cell lines. In a restriction enzyme polymerase chain reaction assay, four of 32 prostatic carcinomas and three of 15 advanced bladder carcinomas showed evidence of hypermethylation at the rel/nuclear factor kappaB (NFkappaB) binding sites essential for promoter activity. The DU145 cell line derived from a metastasis of a prostate carcinoma also displayed hypermethylation in this assay, which was confirmed by bisulfite sequencing. Treatment of DU145 cells with the methylation inhibitor deoxyazacytidine slightly increased Fas protein expression, as detected by flow cytometry analysis. In vitro methylation of the TNFRSF6 promoter at the rel/NFkappaB sites completely abolished its activity. Thus, although the TNFRSF6 gene can be inactivated efficiently by DNA methylation, hypermethylation occurs neither frequently nor extensively in human carcinomas and appears to play a limited role in downregulation of Fas expression.