Exposure of human primary colon carcinoma cells to anti-Fas interactions influences the emergence of pre-existing Fas-resistant metastatic subpopulations

Interaction between p68 RNA helicase and Ca2+-calmodulin promotes cell migration and metastasis

p68 RNA helicase is a prototypical RNA helicase. Here we present evidence to show that, by interacting with Ca-calmodulin (CaM), p68 plays a role in cancer metastasis and cell migration. A peptide fragment that spans the IQ motif of p68 strongly inhibits cancer metastasis in two different animal models. The peptide interrupts p68 and CaM interaction and inhibits cell migration. Our results demonstrate that the p68-CaM interaction is essential for the formation of lamellipodia and filopodia in migrating cells. p68 interacts with microtubules in the presence of CaM. Our experiments show that interaction with microtubules stimulates p68 ATPase activity. Further, microtubule gliding assays demonstrate that p68, in the presence of CaM, can function as a microtubule motor. This motor activity may allow p68 to transport CaM to the leading edge of migrating cells.

mTOR-independent 4E-BP1 phosphorylation is associated with cancer resistance to mTOR kinase inhibitors

ATP-competitive mTOR kinase inhibitors (mTorKIs) are a new generation of mTOR-targeted agents with more potent anticancer activity than rapamycin in several tumor models. However, the sensitivity and resistance of cancer cells to mTorKIs remain poorly understood. In this study, we tested mTorKIs against a large panel of colorectal cancer (CRC) cell lines, and found that mTorKIs displayed broader anti-CRC activity than rapamycin, including CRC cells with K-Ras or B-Raf mutations, suggesting that these mTorKIs are particularly useful for CRCs resistant to EGFR inhibitors. Unexpectedly, we found that 40% CRC cell lines were intrinsically drug resistant. Moreover, we discovered an mTOR-independent 4E‑BP1 phosphorylation that was correlated with mTorKI resistance. Altogether, our findings provide compelling preclinical support for testing mTorKIs in human CRC clinical trials. They further reveal the existence of significant intrinsic mTorKI drug resistance in cancer cells and suggest that 4E-BP1 phosphorylation is a predictive biomarker for mTorKI sensitivity and resistance.

Apoptosis modulation as a promising target for treatment of systemic sclerosis

Diffuse systemic sclerosis (SSc) is a fatal autoimmune disease characterized by an excessive ECM deposition inducing a loss of function of skin and internal organs. Apoptosis is a key mechanism involved in all the stages of the disease: vascular damage, immune dysfunction, and fibrosis. The purpose of this paper is to gather new findings in apoptosis related to SSc, to highlight relations between apoptosis and fibrosis, and to identify new therapeutic targets.

The role of Fas ligand protein in the oxidative stress induced by azoxymethane on crypt colon of rats

PURPOSE: To study the protein Fas ligand (FasL) on the expression of apoptosis, using a model of oxidative stress induced by azoxymethane (AOM), in the crypt of colon in rats. METHODS: Wistar rats (n=14) were assigned into two groups: control (n=7) and AOM (n=7). A single subcutaneous administration of AOM (5mg/kg) or saline solution was performed at the beginning of third week and after three hours samples of proximal colon were collected. The expression of FasL was quantified (Software ImageLab) in percentage of areas in the top, base and all crypt. Results were expressed as mean ± sd (Shapiro-Wilks test and t Student test) (p < 0.05). RESULTS: In the animals of CG there was no significant difference between the FasL expression of the top (10.75±3.33) and basal (11.14±3.53) colon crypt (p=0.34293740). In the animals of AOM there was no significant difference between the FasL expression of the top (8.86±4.19) and basal (8.99±4.08) colon crypt (p=0.78486003). In the animals of CG (10.95±3.43) and AOM (8.92±4.13) there was a significant difference of the FasL expression (p=0.026466821). A significantly decrease on the FasL expression was observed in the animals of CG (10.75±3.33) and AOM (8.86±4.19) in the top crypt (p=0.00003755*). A significant decrease was also observed in the animals of CG (11.14±3.53) and AOM (8.99±4.08) in the basal colon crypt (p=0.00000381**). CONCLUSION: Azoxymethane induce the oxidative stress and the significantly decrease of FasL expression, although there is no significant difference between basal and top of colon crypt linked to consumption-activation of Fas ligand.

CD95 is a key mediator of invasion and accelerated outgrowth of mouse colorectal liver metastases following radiofrequency ablation

BACKGROUND & AIMS

Recently, we have shown that micro-metastases, in the hypoxic transition zone surrounding lesions generated by radiofrequency ablation (RFA), display strongly accelerated outgrowth. CD95 is best known for its ability to induce apoptosis but can also promote tumorigenesis in apoptosis-resistant tumor cells. Therefore, we tested whether CD95 signaling plays a role in accelerated outgrowth of colorectal liver metastases following RFA.

METHODS

Hypoxia-induced invasion was assessed in three-dimensional EGFP-expressing C26 tumor cell cultures by confocal microscopy. CD95 localization was tested by immunofluorescence. Invasion and outgrowth of liver metastases following RFA were analyzed by post-mortem confocal microscopy and by morphometric assessment of tumor load. Neutralization of CD95L was performed by using antibody MFL4. CD95 was suppressed by lentiviral RNA interference. The role of host CD95L was assessed using gld mice.

RESULTS

Micro-metastases in the hypoxic transition zone following RFA displayed a highly invasive phenotype and increased expression of CD95 and CD95L. Hypoxia-induced tumor cell invasion in vitro increased the expression of CD95 and CD95L and induced translocation of CD95 to the invasive front. In vitro invasion, metastasis invasion, and accelerated tumor growth in the transition zone were strongly suppressed by neutralizing CD95L or by suppressing tumor cell CD95. In contrast, metastasis invasion and outgrowth were unaffected in gld mice.

CONCLUSIONS

Hypoxia causes autocrine activation of CD95 on colorectal tumor cells, thereby promoting local invasion and accelerated metastasis outgrowth in the hypoxic transition zone following RFA. Further pre-clinical work is needed to assess the role of CD95L neutralization, either alone or in combination with chemotherapy, in limiting aggressive recurrence of liver metastases following RFA.

Role of apoptosis resistance in immune evasion and metastasis of colorectal cancer

The host immune system functions as a guardian against tumor development. It has been demonstrated that cytotoxic T lymphocyte (CTL)-mediated cytotoxic pathways function to inhibit or delay human colorectal cancer development. However, the host anti-tumor immune responses also 'edit' the tumor and select for more aggressive variants, resulting in immune evasion and tumor escape. Fas is a death receptor that mediates one of the major cytotoxic effector mechanisms of the CTLs. Fas is highly expressed in normal human colon epithelial cells but is frequently silenced in colorectal carcinoma, especially in metastatic colorectal carcinoma, suggesting that loss of Fas expression and function may be an immune evasion and tumor escape mechanism. In addition, recent studies indicated that Fas also mediates cellular proliferation signaling pathways to promote tumor development. Therefore, the death receptor Fas may not only transduce death signals to suppress tumor development but also activate cellular proliferation and the migration process to promote tumor growth and progression. Thus, understanding the mechanisms by which the Fas receptor and its associated protein complex transduces the death and survival signals may identify molecular targets for the development of therapeutic strategy to enhance the Fas-mediated death signals to increase the efficacy of cancer immunotherapy.

A multi-functional role of interferon regulatory factor-8 in solid tumor and myeloid cell biology

Understanding mechanisms of tumor escape are critically important not only to improving our knowledge of cancer biology, but also for the overall development of more effective anti-neoplastic therapies. Our laboratory focuses on mechanisms of apoptotic resistance, with emphasis on Fas loss of function as an important determinant of tumor progression. Our work in solid tumor systems has led to the identification of interferon regulatory factor-8 (IRF-8) as a differentially expressed gene important for tumor cell response to cytotoxicity, including Fas-mediated apoptosis and host-anti-tumor immunosurveillance mechanisms. Although IRF-8 was originally identified in the regulation of normal and neoplastic myeloid cell development, these findings revealed a new functional role for IRF-8 in non-hematopoietic malignancies and establish a molecular basis for its potential manipulation during cancer therapy.

Tumor promoting effects of CD95 signaling in chemoresistant cells

Background

CD95 is a death receptor controlling not only apoptotic pathways but also activating mechanisms promoting tumor growth. During the acquisition of chemoresistance to oxaliplatin there is a progressive loss of CD95 expression in colon cancer cells and a decreased ability of this receptor to induce cell death. The aim of this study was to characterize some key cellular responses controlled by CD95 signaling in oxaliplatin-resistant colon cancer cells.

Results

We show that CD95 triggering results in an increased metastatic ability in resistant cells. Moreover, oxaliplatin treatment itself stimulates cell migration and decreases cell adhesion through CD95 activation, since CD95 expression inhibition by siRNA blocks the promigratory effects of oxaliplatin. These promigratory effects are related to the epithelia-to-mesenchymal transition (EMT) phenomenon, as evidenced by the up-regulation of some transcription factors and mesenchymal markers both in vitro and in vivo.

Conclusions

We conclude that oxaliplatin treatment in cells that have acquired resistance to oxaliplatin-induced apoptosis results in tumor-promoting effects through the activation of CD95 signaling and by inducing EMT, all these events jointly contributing to a metastatic phenotype.

Oncogenic K-Ras turns death receptors into metastasis-promoting receptors in human and mouse colorectal cancer cells

BACKGROUND & AIMS

Death receptors expressed on tumor cells can prevent metastasis formation by inducing apoptosis, but they also can promote migration and invasion. The determinants of death receptor signaling output are poorly defined. Here we investigated the role of oncogenic K-Ras in determining death receptor function and metastatic potential.

METHODS

Isogenic human and mouse colorectal cancer cell lines differing only in the presence or absence of the K-Ras oncogene were tested in apoptosis and invasion assays using CD95 ligand and tumor necrois factor-related apoptosis-inducing ligand (TRAIL) as stimuli. Metastatic potential was assessed by intrasplenic injections of green fluorescent protein- or luciferase-expressing tumor cells, followed by intravital fluorescence microscopy or bioluminescence imaging, and confocal microscopy and immunohistochemistry. Ras-effector pathway control of CD95 output was assessed by an RNA-interference and inhibitor-based approach.

RESULTS

CD95 ligand and TRAIL stimulated invasion of colorectal tumor cells and liver metastases in a K-Ras-dependent fashion. Loss of mutant K-Ras switched CD95 and TRAIL receptors back into apoptosis mode and abrogated metastatic potential. Raf1 was essential for the switch in CD95 function, for tumor cell survival in the liver, and for K-Ras-driven formation of liver metastases. K-Ras and Raf1 suppressed Rho kinase (ROCK)/LIM kinase-mediated phosphorylation of the actin-severing protein cofilin. Overexpression of ROCK or LIM kinase allowed CD95L to induce apoptosis in K-Ras-proficient cells and prevented metastasis formation, whereas their suppression protected K-Ras-deficient cells against apoptosis.

CONCLUSIONS

Oncogenic K-Ras and its effector Raf1 convert death receptors into invasion-inducing receptors by suppressing the ROCK/LIM kinase pathway, and this is essential for K-Ras/Raf1-driven metastasis formation.

Evaluation of metastatic and angiogenic potentials of human colon carcinoma cells in chick embryo model systems

Increased metastatic and angiogenic potentials of aggressive human colon carcinoma cells were verified in independent chick embryo models by comparing in vivo highly metastatic SW620 colon carcinoma cell line with its isogenic, non-metastatic SW480 cell variant. In the experimental metastasis model, both cell types rapidly arrested in the chorioallantoic membrane (CAM) vasculature as demonstrated by quantitative PCR and immunohistochemistry. Live cell imaging also indicated that both SW620 and SW480 cells efficiently extravasated from the CAM capillary system. However, only few SW480 cells were present in the CAM tissue after 24-48 h. In contrast, the numbers of SW620 cells increased exponentially, indicating proliferative and survival advantages of metastatic colon carcinoma cells in vivo. Multicellular SW620 foci were identified in close proximity to CAM blood vessels. A positive correlation between increased metastatic ability and VEGF-expression of colon carcinoma SW620 cells was demonstrated by the substantial inhibitory effects of anti-VEGF treatment on the levels of metastatic colonization and density of blood vessels adjacent to tumor cell foci. Furthermore, the chick embryo angiogenesis model confirmed high levels of VEGF-dependent angiogenesis induced by SW620 cells, but not SW480 cells. Thus, chick embryo experimental metastasis and CAM angiogenesis models appear to coordinately reflect critical features of advanced colon carcinomas, i.e., the acquisition of enhanced survival and increased angiogenic potentials, both constituting critical determinants of colon cancer progression. The use of rapid and quantitative chick embryo models might provide alternative approaches to conventional mammalian model systems for screening anti-cancer agents.

Marie R, Wang TC, Lyle S, Kurt-Jones E, Houghton J. Human and mouse colon cancer utilizes CD95 signaling for local growth and metastatic spread to liver

BACKGROUND & AIMS

Analysis of clinical colon cancer specimens show alterations in the CD95 (Fas Ag/Fas L) pathway as tumors progress from local to metastatic disease, suggesting that this pathway may play a role in invasive behavior of colon cancer. However, direct causality between these alterations and clinical disease progression has not been shown.

METHODS

Surgically resected metastatic colon cancer samples were evaluated for Fas Ag/L and apoptosis. Alterations in the Fas-signaling pathway found in human samples were recreated through a series of staged transfection experiments in the MC38 mouse colon cancer cell line and the effects on growth tested in vitro and in vivo.

RESULTS

Expression of FLICE-like inhibitory protein confers apoptosis resistance, increasing the incidence of primary tumors through a survival advantage by avoiding apoptosis and inducing Fas-mediated proliferation. Coexpression of Fas L enables colon cancer cells to metastasize to the liver from local tumors as well as from intravenous injection of cells. MC38-FasL/FLICE-like inhibitory protein colon cancer cells induce apoptosis in hepatocytes via activation of type II Fas Ag signaling, thus creating a niche conducive to tumor growth and fueling their own growth via Fas proliferative signaling.

CONCLUSIONS

Alterations in the Fas Ag pathway which inhibit apoptosis and increase Fas-mediated proliferation directly increase local colon cancer growth, and enhance metastasis to the liver. Delineating points in the pathway responsible for growth and metastasis will offer targets that may be exploited for therapy.

The role of MMP7 and its cross-talk with the FAS/FASL system during the acquisition of chemoresistance to oxaliplatin

Background

The efficacy of oxaliplatin in cancer chemotherapy is limited by the development of drug resistance. MMP7 has been related to the loss of tumor cell response to cytotoxic agents although the exact mechanism is not fully understood. Moreover, MMP7 is an independent prognosis factor for survival in patients with colorectal cancer. The aim of the present study was to analyze the role of MMP7 and its cross-talk with the Fas/FasL system during the acquisition of oxaliplatin resistance in colon cancer cells.

Principal Findings

For this purpose we have developed three different oxaliplatin-resistant cell lines (RHT29, RHCT116 p53^+/+, RHCT116 p53^−/−) from the parental HT29, HCT116 p53^+/+ and HCT116 p53^−/− colon cancer cells. MMP7 basal expression was higher in the resistant compared to the parental cell lines. MMP7 was also upregulated by oxaliplatin in both HT29 (p53 mutant) and RHCT116 p53^−/− but not in the RHCT116 p53^+/+. Inhibition of MMP by 1,10-phenantroline monohydrate or siRNA of MMP7 restores cell sensitivity to oxaliplatin-induced apoptosis in both HT29 and RHCT116 p53^−/− but not in the RHCT116 p53^+/+. Some of these effects are caused by alterations in Fas receptor. Fas is upregulated by oxaliplatin in colon cancer cells, however the RHT29 cells treated with oxaliplatin showed a 3.8-fold lower Fas expression at the cell surface than the HT29 cells. Decrease of Fas at the plasma membrane seems to be caused by MMP7 since its inhibition restores Fas levels. Moreover, functional analysis of Fas demonstrates that this receptor was less potent in inducing apoptosis in RHT29 cells and that its activation induces MAPK signaling in resistant cells.

Conclusions

Taking together, these results suggest that MMP7 is related to the acquisition of oxaliplatin-resistance and that its inhibition restores drug sensitivity by increasing Fas receptor. Furthermore, Fas undergoes a change in its functionality in oxaliplatin-resistant cells inducing survival pathways instead of apoptotic signals.

DNA methylation represses IFN-gamma-induced and signal transducer and activator of transcription 1-mediated IFN regulatory factor 8 activation in colon carcinoma cells

IFN regulatory factor 8 (IRF8) is both constitutively expressed and IFN-gamma inducible in hematopoietic and nonhematopoietic cells. We have shown that IRF8 expression is silenced by DNA methylation in human colon carcinoma cells, but the molecular mechanism underlying methylation-dependent IRF8 silencing remains elusive. In this study, we observed that IRF8 protein level is inversely correlated with the methylation status of the IRF8 promoter and the metastatic phenotype in human colorectal carcinoma specimens in vivo. Demethylation treatment or knocking down DNMT1 and DNMT3b expression rendered the tumor cells responsive to IFN-gamma to activate IRF8 transcription in vitro. Bisulfite genomic DNA sequencing revealed that the entire CpG island of the IRF8 promoter is methylated. Electrophoresis mobility shift assay revealed that DNA methylation does not directly inhibit IFN-gamma-activated phosphorylated signal transducer and activator of transcription 1 (pSTAT1) binding to the IFN-gamma activation site element in the IRF8 promoter in vitro. Chromatin immunoprecipitation assay revealed that pSTAT1 is associated with the IFN-gamma activation site element of the IRF8 promoter in vivo regardless of the methylation status of the IRF8 promoter. However, DNA methylation results in preferential association of PIAS1, a potent inhibitor of pSTAT1, with pSTAT1 in the methylated IRF8 promoter region. Silencing methyl-CpG binding domain protein 1 (MBD1) expression resulted in IRF8 activation by IFN-gamma in human colon carcinoma cells with methylated IRF8 promoter. Our data thus suggest that human colon carcinoma cells silence IFN-gamma-activated IRF8 expression through MBD1-dependent and PIAS1-mediated inhibition of pSTAT1 function at the methylated IRF8 promoter.

Transformation, translation and TRAIL: an unexpected intersection

Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) is a cytokine with roles in tumor surveillance and tolerance. TRAIL selectively induces apoptosis in many malignant but not normal cells but the underlying cause for spontaneous TRAIL sensitivity remains elusive. We propose a novel hypothesis that links TRAIL sensitivity to translational arrest following stresses that inactivate eukaryotic elongation factor 2 (EF2). Affected cells experience a reduction in apoptotic threshold because, due to their short half-lives, levels of anti-apoptotic proteins quickly drop off once translation elongation is inhibited leaving pro-apoptotic proteins unchallenged. This change in protein profile renders affected cells sensitive to TRAIL-mediated apoptosis and places EF2 into the role of a sensor for cellular damage.

Dissection of spontaneous cytotoxicity by human intestinal intraepithelial lymphocytes: MIC on colon cancer triggers NKG2D-mediated lysis through Fas ligand

Human intestinal intraepithelial lymphocytes (IELs), which are T-cell receptor alphabeta+ CD8+ T cells located between epithelial cells (ECs), are likely to participate in the innate immune response against colon cancer. IELs demonstrate spontaneous cytotoxic (SC) activity specifically directed against EC tumours but not against other solid tumour types. The aim of this study was to dissect out the mechanism of SC activity, focusing on the interaction of NKG2D on IELs with its ligands [major histocompatibility complex (MHC) class I chain-related protein (MIC) and UL16 binding protein (ULBP)] found mainly on EC tumours. A novel series of events occurred. The NKG2D-MIC/ULBP interaction induced Fas ligand (FasL) production and FasL-mediated SC activity against HT-29 cells and MIC-transfectants. Tumour necrosis factor-alpha and interferon-gamma, produced independently of this interaction, promoted SC activity. The immune synapse was strengthened by the interaction of CD103 on IELs with E-cadherin on HT-29 cells. Neither T-cell receptor nor MHC class I was involved. While the HT-29 cells were destroyed by soluble FasL, tumour necrosis factor-alpha and interferon-gamma, the IELs were resistant to the effects of these mediators and to FasL expressed by the HT-29 cells. This unidirectional FasL-mediated cytotoxicity of IELs against HT-29 cells, triggered through NKG2D, is unique and is likely to be a property of those CD8+ tumour-infiltrating lymphocytes that phenotypically resemble IELs.

Downregulation of IFN-gammaR in association with loss of Fas function is linked to tumor progression

The host immune system functions as an intrinsic surveillance network in the recognition and destruction of tumor cells, and it has been demonstrated that lymphocytes and IFN-gamma are the primary tumor suppressors of the immune system. However, the immune system can concurrently select for tumor variants with reduced immunogenicity and aggressive phenotypes. We report here that tumor escape variants that have survived CTL adoptive immunotherapy exhibited decreased expression levels of both Fas and IFN-gammaR in vitro. Furthermore, examination of spontaneously arising mouse primary mammary carcinoma and lung metastases revealed that both Fas and IFN-gammaR protein levels were dramatically lower in lung metastases than in primary tumors in vivo. Functional disruption of either the Fas- or the IFN-gamma signaling pathway enhanced the colonization efficiency of preexisting metastatic tumor cells, whereas disruption of both Fas and IFN-gammaR pathways resulted in synergistic augmentation of the colonization efficiency of the preexisting metastatic tumor cells, as determined by experimental lung metastases assay. Gene expression profiling revealed that altered expression of genes involved in immediate IFN-gammaR signaling, the interferon primary response, apoptosis and tumor colonization is associated with loss of IFN-gammaR function and enhanced metastatic potential. Interestingly, disruption of IFN-gammaR function did not alter tumor cell susceptibility to CTL-mediated cytotoxicity, but is linked to enhanced infiltration of endogenous T cells in the tumor microenvironment in vivo. These findings suggest that coordinate downregulation of Fas and IFN-gammaR, 2 key components of cancer immunosurveillance system on tumor cells, leads to a more aggressive metastatic phenotype.

Fas-negative osteosarcoma tumor cells are selected during metastasis to the lungs: the role of the Fas pathway in the metastatic process of osteosarcoma

Low expression of Fas by different tumors including osteosarcoma, correlates with poor prognosis. We found that osteosarcoma lung metastases from patients expressed negligible amounts of Fas, but primary tumors often expressed high Fas levels. The reason for this discrepancy is unknown. We hypothesized that because FasL is constitutively expressed in the lungs, Fas-positive (Fas(+)) tumor cells entering the lungs would bind with FasL and die from Fas-induced apoptosis, resulting in the "selection" of Fas-negative (Fas(-)) cells, which would eventually form metastases. To test this hypothesis, we injected K7 osteosarcoma cells, which express functional Fas in vitro, into mice and confirmed that its bone tumors were Fas(+), but lung metastases were Fas(-). Next, to inhibit Fas signaling without affecting Fas expression, we transfected these cells with a FADD-dominant negative (FDN) plasmid and developed K7/FDN cells. Metastases formed by K7/FDN cells contained Fas(+) tumor cells. Moreover, K7/FDN cells were retained in the lungs longer and formed more lung metastases than K7 cells. In addition, the incidence of lung metastases in FasL-deficient mice injected with K7 cells was higher than that in wild-type mice. Metastases from FasL-deficient mice but not from wild-type mice contained Fas(+) tumor cells. Based on that, we conclude that Fas(-) osteosarcoma cells are selected during lung metastases formation and that inhibition of Fas signaling in tumors or lack of FasL in the host environment allows the proliferation of Fas(+) osteosarcoma cells in the lungs and promotes metastases growth. Therefore, Fas may be considered as a new therapeutic target for osteosarcoma treatment.

Is there a genetic signature for liver metastasis in colorectal cancer?

Even though liver metastasis accounts for the vast majority of cancer deaths in patients with colorectal cancer (CRC), fundamental questions about the molecular and cellular mechanisms of liver metastasis still remain unanswered. Determination of gene expression profiles by microarray technology has improved our knowledge of CRC molecular pathways. However, defined gene signatures are highly variable among studies. Expression profiles and molecular markers have been specifically linked to liver metastases mechanistic paths in CRC. However, to date, none of the identified signatures or molecular markers has been successfully validated as a diagnostic or prognostic tool applicable to routine clinical practice. To obtain a genetic signature for liver metastasis in CRC, measures to improve reproducibility, to increase consistency, and to validate results need to be implemented. Alternatives to expression profiling with microarray technology are continuing to be used. In the recent past, many genes codifying for proteins that are directly or indirectly involved in adhesion, invasion, angiogenesis, survival and cell growth have been linked to mechanisms of liver metastases in CRC.

IFN regulatory factor 8 mediates apoptosis in nonhemopoietic tumor cells via regulation of Fas expression

IFN regulatory factor 8 (IRF8) is a transcription factor that was originally identified in myeloid cells and has been shown to be essential for differentiation and function of hemopoietic cells. Mice with a null mutation of IRF8 exhibit uncontrolled expansion of the granulocytic and monocytic lineages that progress into a phenotype resembling human chronic myelogenous leukemia. In human patients with chronic myelogenous leukemia, IRF8 transcript levels are frequently diminished. Therefore, IRF8 is a key regulator of myeloid tumor development. In this study, we report that IRF8 is a critical regulator of apoptosis in nonhemopoietic tumor cells. Disruption of IRF8 function with IRF8 dominant-negative mutants diminished Fas-mediated apoptosis in sarcoma tumor cells. Both constitutively expressed and IFN-gamma-activated IRF8 were involved in regulation of apoptosis. Furthermore, it was found that constitutively expressed IRF8 is associated with the Fas promoter to activate Fas transcription. In addition, disruption of constitutively expressed IRF8 function diminished JAK1 expression and thereby inhibited IFN-gamma-initiated induction of STAT1 phosphorylation, which in turn, blocked IFN-gamma-induced Fas up-regulation. Interestingly, the constitutively expressed IRF8 was also essential for TNF-alpha sensitization of Fas-mediated apoptosis because disruption of IRF8 function also inhibited TNF-alpha-sensitized and Fas-mediated apoptosis. Taken together, our data suggest that IRF8 is an essential mediator of Fas-mediated apoptosis and that IRF8 mediates apoptosis through regulation of Fas expression in nonhemopoietic tumor cells.

Repression of IFN Regulatory Factor 8 by DNA Methylation Is a Molecular Determinant of Apoptotic Resistance and Metastatic Phenotype in Metastatic Tumor Cells

Apoptotic resistance is often associated with metastatic phenotype in tumor cells and is considered a hallmark of tumor progression. In this study, IFN regulatory factor 8 (IRF8) expression was found to be inversely correlated with an apoptotic-resistant and metastatic phenotype in human colon carcinoma cell lines in vitro. This inverse correlation was further extended to spontaneously arising primary mammary carcinoma and lung metastases in a mouse tumor model in vivo. Exogenous expression of IRF8 in the metastatic tumor cell line restored, at least partially, the sensitivity of the tumor cells to Fas-mediated apoptosis, and disruption of IRF8 function conferred the poorly metastatic tumors with enhanced apoptotic resistance and metastatic capability. DNA demethylation restored IRF8 expression and sensitized the metastatic tumor cells to Fas-mediated apoptosis. Analysis of genomic DNA isolated from both primary and metastatic tumor cells with methylation-sensitive PCR revealed hypermethylation of the IRF8 promoter in metastatic tumor cells but not in primary tumor cells. Taken together, our data suggest that IRF8 is both an essential regulator in Fas-mediated apoptosis pathway and a metastasis suppressor in solid tumors and that metastatic tumor cells use DNA hypermethylation to repress IRF8 expression to evade apoptotic cell death and to acquire a metastatic phenotype.

CTL adoptive immunotherapy concurrently mediates tumor regression and tumor escape

Tumor escape and recurrence are major impediments for successful immunotherapy. It is well-documented that the emergence of Ag-loss variants, as well as regulatory mechanisms suppressing T cell function, have been linked to inadequate antitumor activity. However, little is known regarding the role of Fas-mediated cytotoxicity by tumor-specific CD8(+) CTL in causing immune evasion of Fas resistant variants during adoptive immunotherapy. In this study, we made use of an adoptive transfer model of experimental lung metastasis using tumor-specific CTL as a relevant immune-based selective pressure, and wherein the Fas ligand pathway was involved in the antitumor response. Surviving tumor cells were recovered and examined for alterations in antigenic, functional, and biologic properties. We showed that diminished susceptibility to Fas-mediated cytotoxicity in vivo was an important determinant of tumor escape following CTL-based immunotherapy. Tumor escape variants (TEV) recovered from the lungs of CTL-treated mice exhibited more aggressive behavior in vivo. However, these TEV retained relevant MHC class I and tumor Ag expression and sensitivity to CTL via the perforin pathway but reduced susceptibility to Fas-mediated lysis. Moreover, TEV were significantly less responsive to eradication by CTL adoptive immunotherapy paradigms as a consequence of increased Fas resistance. Overall, we identified that Fas(low)-TEV emerged as a direct consequence of CTL-tumor interactions in vivo, and that such an altered neoplastic Fas phenotype compromised immunotherapy efficacy. Together, these findings may have important implications for both tumor progression and the design of immunotherapeutic interventions to confront these selective pressures or escape mechanisms.

Immune Selection and Emergence of Aggressive Tumor Variants as Negative Consequences of Fas-Mediated Cytotoxicity and Altered IFN-γ-Regulated Gene Expression

Antitumor responses can be induced in patients via active or adoptive immunotherapy, yet complete tumor eradication occurs infrequently. This paradox in tumor immunology led us to address two questions: (a) Does an antitumor response, which is intended to destroy the aberrant target population, also at the same time select for aggressive tumor variants (ATV) in vivo? (b) If this process does occur, what is the contribution of the perforin- or Fas-mediated effector mechanism in the immune selection of such ATV? Here, in an experimental mouse lung metastasis model, we showed that ATV generated either naturally in vivo or in vitro by anti-Fas selection resembled each other biologically and genetically as judged by enhanced tumor growth and genome-scale gene expression profiling, respectively. Furthermore, ATV that survived CTL adoptive immunotherapy displayed an even more profound loss of Fas expression and function as well as enhanced malignant proficiency in vivo. ATV, however, retained sensitivity to perforin-mediated lysis in vitro. Lastly, such ATV displayed a diminished responsiveness in their expression of IFN-gamma-regulated genes, including those mechanistically linked to Fas-mediated death (i.e., Fas and caspase-1). Overall, we showed that (a) immune selection did occur in vivo and played an important role in the emergence of ATV, (b) ATV bearing a Fas-resistant phenotype was a chief consequence of immune selection, and (c) an overall diminished responsiveness of IFN-gamma-regulated gene expression was characteristic of ATV. Thus, in this model, Fas-mediated cytotoxicity, in concert with IFN-gamma-regulated gene expression, mechanistically constituted significant determinants of immune selection of ATV in vivo.

Cooperative Disengagement of Fas and Intercellular Adhesion Molecule-1 Function in Neoplastic Cells Confers Enhanced Colonization Efficiency

Understanding the mechanisms of tumor progression is crucial toward the development of therapeutic interventions. Although the loss of sensitivity to cell death is a hallmark of neoplastic progression, it is likely one of several essential features that underlie a malignantly proficient or aggressive tumorigenic phenotype. Here, we identified intercellular adhesion molecule-1 (ICAM-1) as a molecule with expression coordinately regulated with Fas and inversely correlated with malignant phenotype between matched pairs of differentially aggressive malignant subpopulations in three mouse models. To determine whether coordinate expression of Fas and ICAM-1 regulated malignant behavior, tumor sublines were produced that expressed either lower levels of both Fas and ICAM-1, lower levels of Fas, or lower levels of ICAM-1 and then assessed for metastatic lung tumor growth. Tumor sublines rendered both Fas incompetent and ICAM-1 incompetent displayed significantly higher numbers of tumor nodules compared with tumor sublines separately expressing low levels of Fas or ICAM-1. However, all tumor sublines regardless of their Fas and ICAM-1 levels comparably infiltrated the lung, suggesting that Fas- and ICAM-1–based interactions ultimately influenced lung colonization efficiency. Overall, these data suggested that both Fas and ICAM-1 pathways cooperated to regulate tumor progression and that the coordinate down-regulation of Fas and ICAM-1 intensified malignant progression at the level of colonization. Thus, a FasloICAM-1lo phenotype may be characteristic of at least certain advancing, immune-resistant neoplastic subpopulations.

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