Cellular commitment to oncogene-induced transformation or apoptosis is dependent on the transcription factor IRF-1

Interferon therapy lowers the rate of progression to hepatocellular carcinoma in chronic hepatitis C but not significantly in an advanced stage: a retrospective study in 1148 patients. Viral Hepatitis Therapy Study Group

BACKGROUND/AIM

Hepatocellular carcinoma frequently develops during the advanced stages of chronic hepatitis C. We examined whether interferon prevents the development of hepatocellular carcinoma in chronic hepatitis C patients.

METHODS

Japanese patients with chronic hepatitis C (n = 1.148; 117 with portal fibrous expansion (F1), 636 with bridging fibrosis (F2), 355 with bridging fibrosis and architectural distortion (F3)) and 40 cirrhotic (F4) patients were treated with interferon. These patients were followed from 1 to 7 years after interferon therapy. Blood tests and image analysis were serially performed to assess response to interferon and to detect hepatocellular carcinoma. Fifty-five cirrhotic type C patients (control F4) not receiving interferon were enrolled in this study.

RESULTS

Sustained (SR: 27.5%) and transient (TR: 23.0%) responders totaled 50.5%, while 49.5% did not respond to interferon. SR showed an improvement in disease stage reflected by increased platelet counts. Fifty-two patients (9 F2, 36 F3, and 7 F4) developed hepatocellular carcinoma in the follow-up period; 3 SR, 8 TR, and 41 non-responders (NR). The cumulative incidence of hepatocellular carcinoma in F2 was significantly lower (p = 0.019) in SR compared with NR, but not in SR in F3 and F4 patients. However, the cumulative incidence of hepatocellular carcinoma was significantly decreased in all SR (p = 0.0001) and TR (p = 0.0397) compared with all NR.

CONCLUSION

These results indicate that interferon therapy in chronic hepatitis C patients lowered the rate of progression of hepatocellular carcinoma in sensitive cases but not in patients in an advanced stage.

Oncogenic Ras triggers cell suicide through the activation of a caspase-independent cell death program in human cancer cells

To prevent neoplasia, cells of multicellular organisms activate cellular disposal programs such as apoptosis in response to deregulated oncogene expression, making the suppression of such programs an essential step for potentially neoplastic cells to become established as clinically relevant tumors. Since the mutation of ras proto-oncogenes, the most frequently mutated proto-oncogenes in human tumors, is very rare in some tumor types such as glioblastomas and gastric cancers, we hypothesized that mutated ras genes might activate a cell death program that cannot be overcome by these tumor types. Here we show that the expression of oncogenically mutated ras gene induces cellular degeneration accompanied by cytoplasmic vacuoles in human glioma and gastric cancer cell lines. Cells dying as a result of oncogenic Ras expression had relatively well-preserved nuclei that were negative for TUNEL staining. An immunocytochemical analysis demonstrated that the cytoplasmic vacuoles are derived mainly from lysosomes. This oncogenic Ras-induced cell death occurred in the absence of caspase activation, and was not inhibited by the overexpression of anti-apoptotic Bcl-2 protein. These observations suggested that oncogenic Ras-induced cell death is most consistent with a type of programmed cell death designated 'type 2 physiological cell death' or 'autophagic degeneration', and that this cell death is regulated by a molecular mechanism distinct from that of apoptosis. Our findings suggest a possible role for this non-apoptotic cell death in the prevention of neoplasia, and the activation of the non-apoptotic cell death program may become a potential cancer therapy complementing apoptosis-based therapies. In addition, the approach used in this study may be a valuable way to find genetically-regulated cell suicide programs that cannot be overcome by particular tumor types.

Characterization of AT2 receptor expression in NIH 3T3 fibroblasts

1. A high expression of angiotensin II receptors and of angiotensin-converting enzyme (ACE) activity was detected in confluent NIH 3T3 fibroblasts. 2. Characterization with selective ligands, dithiothreitol, and GTP gamma S, indicated that only the AT2 subtype was expressed. 3. AT2 receptors and ACE expression were strictly dependent on the cell density and growth phase of the cells, with AT2 receptors being expressed earlier than ACE. In contrast, high expression of AT2 receptors irrespective of their growth state was observed in NIH 3T3 cells lacking contact inhibition upon neoplastic transformation with ras. 4. Our results imply a possible relation of AT2 receptors to cell growth and cell-cell contact.

Activation and repression of the 2-5A synthetase and p21 gene promoters by IRF-1 and IRF-2

The Interferon Regulatory Factors-1 and -2 (IRF-1 and IRF-2) were originally identified as transcriptional regulators of the interferon (IFN) and IFN-stimulated genes. These factors also modulate immune response and play a role in cell growth regulation. In this study we analysed the effect of the ectopic expression of IRF-1 and IRF-2 on the regulation of two potential IRF target genes involved in cell growth regulation, 2-5A synthetase and p21 (WAF/CP1), both of which contain consensus binding sites for IRF family members within their promoters. Following ectopic expression, IRF-1 transactivated 2-5A synthetase and p21 genes, an effect that was counterbalanced by concomitant ectopic expression of IRF-2. These effects were mediated by direct binding of IRF to the gene promoters. A construct expressing an IRF-2 antisense (FRI-2) was able to revert the inhibitory effect of IRF-2 on the IRF-1 transactivation. IRF-1 also induced expression of its homologous repressor IRF-2 as indicated by EMSA analysis using an IRF-E probe from the IRF-2 promoter; and by cotransfection of IRF-1 together with an IRF-2 promoter CAT construct. Therefore, the induction of IRF-1 by IFNs or other stimuli acts as a transactivator of genes involved in cell growth regulation, as well as of its own repressor IRF-2, thus providing autoinhibitory regulation of IRF-1 activated genes.

Mutant N-ras Induces Myeloproliferative Disorders and Apoptosis in Bone Marrow Repopulated Mice

Mutations that activate the N-ras oncogene are among the most frequently detected genetic alterations in human acute myeloid leukemias (AMLs), Philadelphia chromosome-negative myeloproliferative disorders (MPDs), and myelodysplastic syndromes (MDSs). However, because N-ras has not been shown to induce these disorders in an in vivo model, the role of N-ras in the evolution of myeloid leukemia is unclear. To investigate the potential of N-ras to induce myeloid leukemia, lethally irradiated mice were reconstituted with bone marrow (BM) cells infected with a retroviral vector carrying activated N-ras. Approximately 60% of these mice developed hematopoietic disorders, including severe MPDs resembling human chronic myelogenous leukemia (CML) or AML with differentiation (French-American-British [FAB] classification M2). Other reconstituted mice succumbed to hematopoietic defects that were pathologically similar to human MDSs. The latter disorders appeared to be due to a myeloid impairment that was demonstrated by enumeration of day-12 colony-forming units-spleen (CFU-S) and by in vitro colony assays. A high level of apoptosis associated with thymic atrophy and peripheral blood (PB) lymphopenia was also evident in N-rasreconstituted mice. Our results are consistent with a model in which antiproliferative effects are a primary consequence of N-rasmutations and secondary transforming events are necessary for the development of myeloid leukemia. This is the first report of an in vivo model for N-ras induced MPD and leukemia.

Psoriatic keratinocytes show reduced IRF-1 and STAT-1alpha activation in response to gamma-IFN

Psoriasis is a chronic inflammatory dermatosis characterized by hyperproliferative keratinocytes (KC). The skin lesions are infiltrated by T cells, which secrete gamma interferon (gamma-IFN) and are believed to be necessary to maintain the psoriatic phenotype. In normal KC, gamma-IFN is a potent inhibitor of proliferation, but proliferation of KC persists in psoriatic plaques despite the presence of gamma-IFN. Immunostaining of interferon regulatory factor-1 (IRF-1) revealed that IRF-1 was localized to the basal cells of the epidermis in normal and in nonlesional psoriatic skin, but was suprabasal or completely absent in lesional psoriatic skin. This finding led to the hypothesis that abnormal signaling in the gamma-IFN pathway may occur in psoriatic KC. To test this hypothesis, we measured activation of IRF-1 and signal transducer and activator of transcription (STAT)-1alpha transcription factors in KC after stimulation with gamma-IFN. Primary cultures of KC from normal and nonlesional psoriatic skin were stimulated with gamma-IFN and subsequent transcription factor activation was measured by electrophoretic mobility shift assay. Psoriatic KC showed a reduced induction of IRF-1 and STAT-1alpha activation after stimulation with gamma-IFN, compared with normal KC. Reduced activation of IRF-1 and STAT-1alpha in response to gamma-IFN indicates a fundamental defect in the growth and differentiation control of psoriatic KC in the absence of the influence of other cell types.

Mechanisms that regulate Epstein-Barr virus EBNA-1 gene transcription during restricted latency are conserved among lymphocryptoviruses of Old World primates

Epstein-Barr virus (EBV), the only known human lymphocryptovirus (LCV), displays a remarkable degree of genetic and biologic identity to LCVs that infect Old World primates. Within their natural hosts, infection by these viruses recapitulates many key aspects of EBV infection, including the establishment of long-term latency within B lymphocytes, and is therefore a potentially valuable animal model of EBV infection. However, it is unclear whether these LCVs have adopted or maintained the same mechanisms used by EBV to express essential viral proteins, such as EBNA-1, in the face of cell-mediated repression of EBV gene expression that occurs upon establishment of the asymptomatic carrier state. To address this issue, we determined whether the endogenous LCVs of baboon (Cercopithecine herpesvirus 12) and rhesus macaque (Cercopithecine herpesvirus 15) have the functional equivalent of the EBV promoter Qp, which mediates exclusive expression of EBNA-1 during the restricted programs of EBV latency associated with the carrier state. Our results indicate that (i) both the baboon and rhesus macaque LCVs have a genomic locus that is highly homologous to the EBV Qp region, (ii) key cis-regulatory elements of Qp are conserved in these LCV genomes and compose promoters that are functionally indistinguishable from EBV Qp, and (iii) EBNA-1 transcripts identical in structure to EBV Qp-specific EBNA-1 mRNAs are present in nonhuman LCV-infected cells, demonstrating that these Qp homologs are indeed utilized as alternative EBNA-1 promoters. These observations indicate that the molecular mechanisms which regulate EBV gene expression during restricted latency have been conserved among the LCVs. The contribution of these mechanisms to viral persistence in vivo can now be experimentally tested in nonhuman primate models of LCV infection.

Insights into cancer from transgenic mouse models

The generation of mice designed to overexpress activated forms of oncogenes or carrying targeted mutations in tumour suppressor genes, has allowed scientists to causally link the function of these genes with specific tumour processes, such as proliferation, apoptosis, angiogenesis or metastasis. In addition, these mice have been interbred to assess the extent of cooperativity between different genetic lesions in disease progression, leading to a greater understanding of the multi-stage nature of tumourigenesis. The effect of genetic mutations is often influenced by the genetic background of the mouse and by analysing strain-dependent phenotypes, modifier loci have been identified. Although genetic mutations in mouse and humans do not always lead to the same tumour spectrum, the underlying molecular mechanisms are frequently relevant to both species. Furthermore, new technical approaches creating conditional mouse mutants which develop tumours in a tissue-specific manner, will allow the effect of mutation of certain genes to be studied in specific tissues, free from the fatal effects of the mutation in other clinically less relevant tissues. Several exising mouse strains have already been used to develop and test new therapies and conditional mutagenesis will undoubtedly increase the potential use of transgenic mice in understanding and treating cancer.

Interferon regulatory factors regulate interleukin-1beta-converting enzyme expression and apoptosis in vascular smooth muscle cells

-Apoptosis has been reported to play a pivotal role in vascular remodeling. However, cellular mechanisms of apoptosis in vascular smooth muscle cells (VSMCs) have not been well defined. In this study, we focused on interleukin-1beta-converting enzyme (ICE), a key protease in the induction of apoptosis in lymphocytes and fibroblasts. We observed an increase in ICE mRNA expression in rat aortic VSMCs after serum depletion, with a peak at 12 hours and then a gradual decline. This was associated with DNA fragmentation, a hallmark of apoptosis and morphological changes of apoptosis. Treatment of these VSMCs with the ICE inhibitor N-(N-acetyl-tyrosinyl-valinyl-alaninyl)-3-amino-4-oxob utanoic acid (YVAD-CHO) attenuated DNA fragmentation. The increased ICE mRNA expression was preceded by an increase in the mRNA expression of interferon regulatory factor (IRF)-1, peaking at 6 hours after serum removal, and a rapid but transient decrease in IRF-2 mRNA expression, reaching a nadir at 3 hours after serum depletion. To demonstrate that these reciprocal changes in IRF-1 and IRF-2 regulated ICE expression and induced apoptosis, we transfected antisense oligonucleotides for IRF-1 and IRF-2 into VSMCs and examined ICE mRNA expression and apoptotic changes. IRF-1 antisense pretreatment attenuated the increase in ICE expression and reduced apoptotic changes, whereas IRF-2 antisense treatment increased ICE mRNA expression and enhanced apoptotic changes. Taken together, our results suggest that serum growth factor depletion in VSMCs upregulates IRF-1 and downregulates IRF-2, thereby increasing ICE expression and inducing apoptosis.

Identification of genes differentially regulated by interferon alpha, beta, or gamma using oligonucleotide arrays

The pleiotropic activities of interferons (IFNs) are mediated primarily through the transcriptional regulation of many downstream effector genes. The mRNA profiles from IFN-alpha, -beta, or -gamma treatments of the human fibrosarcoma cell line, HT1080, were determined by using oligonucleotide arrays with probe sets corresponding to more than 6,800 human genes. Among these were transcripts for known IFN-stimulated genes (ISGs), the expression of which were consistent with previous studies in which the particular ISG was characterized as responsive to either Type I (alpha, beta) or Type II (gamma) IFNs, or both. Importantly, many novel IFN-stimulated genes were identified that were diverse in their known biological functions. For instance, several novel ISGs were identified that are implicated in apoptosis (including RAP46/Bag-1, phospholipid scramblase, and hypoxia inducible factor-1alpha). Furthermore, several IFN-repressed genes also were identified. These results demonstrate the usefulness of oligonucleotide arrays in monitoring mammalian gene expression on a broad and unprecedented scale. In particular, these findings provide insights into the basic mechanisms of IFN actions and ultimately may contribute to better therapeutic uses for IFNs.

Involvement of Fas/Fas ligand system-mediated apoptosis in the development of concanavalin A-induced hepatitis

Concanavalin A (Con A)-induced hepatitis is an experimental hepatitis model in which hepatic injury is caused by the action of cytokines produced by T cells. Using IFN-gamma-deficient mice, we previously demonstrated that IFN-gamma plays a central role in Con A-induced hepatitis. Here, we show that development of the disease is completely suppressed in gld/gld mice, in which Fas ligand is defective. In contrast, suppression of the disease in Ipr/Ipr mice was incomplete, since a small amount of the fas mRNA was produced in these mice. The data indicate that activation of the Fas/Fas ligand system is a necessary step in the development of Con A-induced hepatitis. Furthermore, we found that not only fas but also caspase-1 expression was reduced in IFN-gamma-deficient mice. Since caspase-1 is an integral component of Fas signal transduction, these observations suggest that IFN-gamma-induced activation of both fas and caspase-1 expression causes enhancement of hepatocyte apoptosis resulting in the development of hepatitis.

Signaling Pathways Mediated by the TNF- and Cytokine-Receptor Families Target a Common cis-Element of the IFN Regulatory Factor 1 Promoter

CD40 activation of B cells is strongly influenced by the presence of cytokines. However, the molecular basis for the interplay between these distinct stimuli is not clearly delineated. IFN regulatory factor 1 (IRF-1) is a transcription factor activated by either CD40 or cytokines. We have found that these different sets of signals target a common cis-acting element in the promoter of this gene, the IRF-1 gamma-activated site (GAS). Targeting of the IRF-1 GAS is not confined to activation via CD40 but extends to other stimuli that mimic the CD40 signaling cascade, like TNF-α and EBV. In contrast to induction of STATs by cytokines, the IRF-1 GAS-binding complex activated by CD40, TNF-α, or EBV contains Rel proteins, specifically p50 and p65. In this system, simultaneous exposure to CD40L together with either IL-4 or IFN-γ does not lead to the activation of novel Rel/STAT complexes. Given the importance of IRF-1 in a variety of biologic functions from proliferation to apoptosis, our findings support the notion that modulation of IRF-1 levels may be a critical control point in B cell activation.

Human T-cell lymphotropic/leukemia virus type 1 Tax abrogates p53-induced cell cycle arrest and apoptosis through its CREB/ATF functional domain

Human T-cell lymphotropic/leukemia virus type 1 (HTLV-1) transforms human T cells in vitro, and Tax, a potent transactivator of viral and cellular genes, plays a key role in cell immortalization. Tax activity is mediated by interaction with cellular transcription factors including members of the CREB/ATF family, the NF-kappaB/c-Rel family, serum response factor, and the coactivators CREB binding protein-p300. Although p53 is usually not mutated in HTLV-1-infected T cells, its half-life is increased and its function is impaired. Here we report that transient coexpression of p53 and Tax results in the suppression of p53 transcriptional activity. Expression of Tax abrogates p53-induced G1 arrest in the Calu-6 cell line and prevents the apoptosis induced by overexpressing p53 in the HeLa/Tat cell line. The Tax mutants M22 and G148V, which selectively activate the CREB/ATF pathway, exert these same biological effects on p53 function. In contrast, the NF-kappaB-active Tax mutant M47 has no effect on p53 activity in any of these systems. Consistent with the negative effect of Tax on p53, no activity on a p53-responsive promoter was observed upon transfection of HTLV-1-infected T-cell lines. The p53 protein is expressed at high levels in the nucleus, and nuclear extracts of HTLV-1-infected T cells bind constitutively to a DNA oligonucleotide containing the p53 response element, indicating that Tax does not interfere with p53 binding to DNA. Tax is able to suppress the transactivation function of p53 in three different cell lines, and this suppression required Tax-mediated activation of the CREB/ATF, but not the NF-kappaB/c-Rel, pathway. Tax and the active Tax mutants were able to abrogate the G1 arrest and apoptosis induced by p53, and this effect does not correlate with an altered localization of nuclear p53 or with the disruption of p53-DNA complexes. The suppression of p53 activity by Tax could be important in T-cell immortalization induced by HTLV-1.

Molecular mechanisms of programmed cell death

Programmed cell death and apoptosis have now been recognised as biological phenomena which are of fundamental importance to the integrity of organisms. What may have evolved as an altruistic defence against pathogen invasion in simple organisms is now a major regulatory mechanism in the development and maintenance of multi-cellular organisms. The classically defined morphological characteristics of apoptosis are now accompanied by a plethora of information regarding common biochemical and genetic mediators of programmed cell death. It is apparent that life and death decisions are taken by individual cells based on their interpretation of physiological signals, or their own self-assessment of internal damage. The knowledge that cell death is a genetically regulated process has highlighted an inherent potential for manipulation and offered new avenues for research into several diseases, and also productivity improvements in the biotechnology industry. This relatively "new frontier" in cell science has undoubtedly widened our perspectives and may provide novel strategies to expedite both medical and biotechnological research.

Interferon yield and MHC antigen expression of human medulloblastoma cells and its suppression during dibutyryl cyclic AMP-induced differentiation: do medulloblastoma cells derive from bipotent neuronal and glial progenitors?

1. Human medulloblastoma (ONS-76), a central nervous system (CNS)-derived undifferentiated cell line, was found to possess glial characteristics as defined by responses in the interferon (IFN) system; ONS-76 cells produced as much IFN-beta as human fibroblast and glioma cells by viral infection and poly(I):poly(C) induction. 2. Major histocompatibility complex (MHC) class I antigens were also induced under IFN-beta stimulation. ONS-76 cells expressed neurofilament protein, as shown by Northern blot analysis, and morphological differentiation was induced by dibutyryl cyclic AMP (dcAMP). 3. Expression of IFN-beta and MHC class I antigens was suppressed in ONS-76 cells during the dcAMP-induced differentiation. 4. These results showed that ONS-76 cells possessed a glial property in IFN system responses and a neuronal property in cytoskeleton protein, suggesting that the precursors of medulloblastoma may be characterized as bipotent neuronal and glial progenitors in CNS.

Functional domains of interferon regulatory factor I (IRF-1)

Interferon (IFN) regulatory factors (IRFs) are a family of transcription factors among which are IRF-1, IRF-2, and IFN consensus sequence binding protein (ICSBP). These factors share sequence homology in the N-terminal DNA-binding domain. IRF-1 and IRF-2 are further related and have additional homologous sequences within their C-termini. Whereas IRF-2 and ICSBP are identified as transcriptional repressors, IRF-1 is an activator. In the present work, the identification of functional domains in murine IRF-1 with regard to DNA-binding, nuclear translocation, heterodimerization with ICSBP and transcriptional activation are demonstrated. The minimal DNA-binding domain requires the N-terminal 124 amino acids plus an arbitrary C-terminal extension. By using mutants of IRF-1 fusion proteins with green fluorescent protein and monitoring their distribution in living cells, a nuclear location signal (NLS) was identified and found to be sufficient for nuclear translocation. Heterodimerization was confirmed by a two-hybrid system adapted to mammalian cells. The heterodimerization domain in IRF-1 was defined by studies in vitro and was shown to be homologous with a sequence in IRF-2, suggesting that IRF-2 also heterodimerizes with ICSBP through this sequence. An acidic domain in IRF-1 was found to be required and to be sufficient for transactivation. Epitope mapping of IRF-1 showed that regions within the NLS, the heterodimerization domain and the transcriptional activation domain are exposed for possible contacts with interacting proteins.

Thymocyte selection in Vav and IRF-1 gene-deficient mice

T cells undergo a defined program of phenotypic and genetic changes during differentiation within the thymus. These changes define commitment of T-cell receptor (TCR) gamma delta and TCR alpha beta cells and lineage differentiation into CD4+ T helper and CD8+ cytotoxic T cells. T-cell differentiation and selection in the thymus constitute a tightly co-ordinated multistep journey through a network that can be envisaged as a three-dimensional informational highway made up of stromal cells and extracellular matrix molecules. This intrathymic journey is controlled by information exchange, with thymocytes depending on two-way cellular interactions with thymic stromal cells in order to receive essential signals for maturation and selection. Genetic inactivation of surface receptors, signal transduction molecules, and transcription factors using homologous recombination has provided novel insight into the signaling cascades that relay surface receptor engagement to gene transcription and subsequent progression of the developmental program. In this review we discuss molecular mechanisms of T lymphocyte development in mice that harbour genetic mutations in the guanine nucleotide exchange factor Vav and the interferon regulatory transcription factor 1 (IRF-1). We also propose a novel model of T-cell selection based on TCR alpha chain-directed signals for allelic exclusion and TCR alpha-based selection for single receptor usage.

Premature senescence involving p53 and p16 is activated in response to constitutive MEK/MAPK mitogenic signaling

Oncogenic Ras transforms immortal rodent cells to a tumorigenic state, in part, by constitutively transmitting mitogenic signals through the mitogen-activated protein kinase (MAPK) cascade. In primary cells, Ras is initially mitogenic but eventually induces premature senescence involving the p53 and p16(INK4a) tumor suppressors. Constitutive activation of MEK (a component of the MAPK cascade) induces both p53 and p16, and is required for Ras-induced senescence of normal human fibroblasts. Furthermore, activated MEK permanently arrests primary murine fibroblasts but forces uncontrolled mitogenesis and transformation in cells lacking either p53 or INK4a. The precisely opposite response of normal and immortalized cells to constitutive activation of the MAPK cascade implies that premature senescence acts as a fail-safe mechanism to limit the transforming potential of excessive Ras mitogenic signaling. Consequently, constitutive MAPK signaling activates p53 and p16 as tumor suppressors.

Linking extracellular survival signals and the apoptotic machinery

The survival of cells in multicellular organisms requires continuous stimulation from the extracellular environment. The phosphatidylinositol-3' kinase/Akt signaling cascade has been identified as a critical pathway for the transduction of extracellular survival signals. The finding that the pro-apoptotic protein BAD is a substrate of Akt/PKB has provided the first link between extracellular survival signals and the apoptotic machinery.

Infrequent somatic mutation of the adenomatous polyposis coli gene in aberrant crypt foci of human colon tissue

BACKGROUND

The authors examined somatic mutations of the adenomatous polyposis coli (APC) gene in 84 human aberrant crypt foci (ACF) to determine whether APC gene mutations were involved in the histologic progression of ACF.

METHODS

Mutation cluster regions of the APC gene were subjected to polymerase chain reaction single-strand conformation polymorphism analysis and direct sequencing.

RESULTS

Four kinds of deletion were detected in the mutation cluster regions of APC gene in five ACF. APC mutation was detected in 1 of 18 ACF with Stage I abnormalities (6%). Four of 10 adenomatous ACF (40%) harbored the mutation. There were no mutations in 56 hyperplastic ACF.

CONCLUSIONS

These results suggest that APC mutations may be involved initially in only a limited number of adenomas in ACF.

Crystallographic Characterization of the DNA-Binding Domain of Interferon Regulatory Factor-2 Complexed with DNA

Interferon regulatory factors (IRFs) are transcription factors for interferon-related genes, which manifest both antiviral and tumor-suppressor activities and regulate cell growth in response to DNA damage. For the transcription initiation of the interferon-beta gene, IRFs form a macromolecular assembly bound to the promoter DNA, referred to as an enhancesome, together with several other transcription factors and DNA-binding proteins. The three-dimensional structure of IRF-DNA complex would provide insights into the structure and function of the enhancesome. In this study, we crystallized the DNA-binding domain of interferon regulatory factor-2 complexed with a DNA fragment. The crystals reproducibly grew by the vapor diffusion technique with 2-methyl-pentanediol from solutions containing small detergents, such as n-octyl-beta-d-glucoside. Cryocrystallographic experiments showed that crystals belong to space group P212121 with a = 90.66 Å, b = 101.01 Å, c = 171.58 Å and diffract up to 2.8 Å resolution. The absorption measurements of a solution in which the crystals were dissolved indicate that the DNA-binding domain binds to the DNA as a dimer. The calculated values of the solvent contents suggest that the protein-DNA complexes form a multimer in the crystal. These features may reflect the association of the complexes in the enhancesome. Copyright 1998 Academic Press.

Functionally inactivating point mutation in the tumor-suppressor IRF-1 gene identified in human gastric cancer

Loss of heterozygosity (LOH) observed in human tumors strongly suggests the existence of (a) tumor-suppressor gene(s) at the concerned locus. A series of studies has revealed that LOH on the long arm of chromosome 5 (5q) frequently occurs in differentiated gastric adenocarcinomas. Furthermore, it has been shown that the interferon regulatory factor-1 (IRF-1) locus on chromosome 5q31.1 is one of the common minimal regions of LOH in these cancers. IRF-1 is a transcriptional activator that shows tumor-suppressor activity in the mouse. In the present study, we examined the sequence of the IRF-1 gene in 9 cases of histologically differentiated gastric adenocarcinomas, all of which exhibited LOH at the IRF-1 locus. We identified a mis-sense mutation in the residual allele in one case. This mutated form of IRF-1 showed markedly reduced transcriptional activity. In addition, overexpression of wild-type IRF-1 induced cell-cycle arrest, whereas such activity was attenuated in the mutant IRF-1. These results suggest that the loss of functional IRF-1 is critical for the development of human gastric cancers.

In vivo formation of IRF-1 homodimers

Interferon regulatory factor 1 (IRF-1) is a transcriptional activator which exerts different biological activities. IRF-1 activates interferon induced genes as well as genes which are not directly linked to the interferon system, such as the ICE protease gene. IRF-1 activity is post-transcriptionally regulated in addition to transcriptional regulation by interferons, cytokines, hormones and many other factors. This includes heterodimerisation with activators and repressors of transcription. These protein interactions modulate the transactivating capacity of IRF-1. By using a two-hybrid system, we demonstrate that IRF-1 forms homodimers in vivo. The homodimerization domain was determined to be located in the N-terminal part of IRF-1 which belongs to the DNA-binding domain. Since this sequence is highly conserved between members of the IRF-family, our observation raises the question of homodimerization of other IRFs through this domain.

The role of interferon regulatory factors in the interferon system and cell growth control

Complex cellular responses are often coordinated by a genetic regulatory network in which a given transcription factor controls the expression of a diverse set of target genes. Interferon regulatory factor (IRF)-1 and IRF-2 have originally been identified as a transcriptional activator and repressor, respectively, of the interferon-beta (IFN-beta) as well as of IFN-inducible genes. However, these factors have since been shown to modulate not only the cellular response to IFNs, but also cell growth, susceptibility to transformation by oncogenes, induction of apoptosis, and development of the T cell immune response. Furthermore, the evidence suggests that deletion and/or inactivation of the IRF-1 gene may be a critical step in the development of some human hematopoietic neoplasms. Subsequently, these factors have been shown to constitute a family of transcription factors, termed the IRF-family. Recent studies indicate that other IRF family members also involve the regulation of the IFN system and cell transformation. The IRF-family may be examples of transcription factors which can selectively modulate several sets of genes depending on the cell type and/or nature of the cellular stimuli, so as to evoke host defense mechanisms against infection and oncogenesis.

Role of the interferon regulatory factors (IRFs) in virus-mediated signaling and regulation of cell growth

As a response to viral infection, cells express the early inflammatory genes that encode small proteins generally called cytokines or chemokines. These protein can activate immune responses to viral infection as well as to modulate directly the outcome of viral infection. The group of proteins with the direct antiviral effects have been called interferons. The stimulation of interferon synthesis in infected cells is regulated on a transcriptional level and two families of cellular transcriptional factors seem to play a critical role in the transcriptional activation of interferon genes. The first one are the proteins of NF-kappaB family and the second is the family of the interferon responsive factors. While both of the types of the transcriptional factors are important for the induction of interferon beta gene, the NF-kappaB factor do not seems to participate in the induction of interferon alpha genes. The present review is focused on the recently identified new members of cellular IRF family and their role in virus mediated response, responses and cell growth. In addition the HHV-8 encoded vIRFs are described.

Human papillomavirus 16 E6 oncoprotein binds to interferon regulatory factor-3 and inhibits its transcriptional activity

Interferon regulatory factor-3 (IRF-3) was found to specifically interact with HPV16 E6 in a yeast two-hybrid screen. IRF-3 is activated by the presence of double-stranded RNA or by virus infection to form a stable complex with other transcriptional regulators that bind to the regulatory elements of the IFNbeta promoter. We show that IRF-3 is a potent transcriptional activator and demonstrate that HPV16 E6 can inhibit its transactivation function. The expression of HPV16 E6 in primary human keratinocytes inhibits the induction of IFNbeta mRNA following Sendai virus infection. The binding of HPV16 E6 to IRF-3 does not result in its ubiquitination or degradation. We propose that the interaction of E6 with IRF-3 and the inhibition of IRF-3's transcriptional activity may provide the virus a means to circumvent the normal antiviral response of an HPV16-infected cell.

Suppression of steady-state, but not stimulus-induced NF-kappaB activity inhibits alphavirus-induced apoptosis

Recent studies have established cell type- specific, proapoptotic, or antiapoptotic functions for the transcription factor NF-kappaB. In each of these studies, inhibitors of NF-kappaB activity have been present before the apoptotic stimulus, and so the role of stimulus- induced NF-kappaB activation in enhancing or inhibiting survival could not be directly assessed. Sindbis virus, an alphavirus, induces NF-kappaB activation and apoptosis in cultured cell lines. To address whether Sindbis virus- induced NF-kappaB activation is required for apoptosis, we used a chimeric Sindbis virus that expresses a superrepressor of NF-kappaB activity. Complete suppression of virus-induced NF-kappaB activity neither prevents nor potentiates Sindbis virus-induced apoptosis. In contrast, inhibition of NF-kappaB activity before infection inhibits Sindbis virus-induced apoptosis. Our results demonstrate that suppression of steady-state, but not stimulus-induced NF-kappaB activity, regulates expression of gene products required for Sindbis virus-induced death. Furthermore, we show that in the same cell line, NF-kappaB can be proapoptotic or antiapoptotic depending on the death stimulus. We propose that the role of NF-kappaB in regulating apoptosis is determined by the death stimulus and by the timing of modulating NF-kappaB activity relative to the death stimulus.

Regulation of IRF and STAT gene expression by retinoic acid

Retinoic acid has antiproliferative and differentiative effects on many cell types. However, the molecular mechanisms involved in ATRA (all-trans retinoic acid) -dependent growth inhibition and cell differentiation are poorly understood. On the other hand, several different cytokine specific transcription factors such as signal transducers and activators of transcription (STAT) and interferon regulatory factors (IRF) are known to be instrumental in mediating differentiative, growth regulatory and antiproliferative effects in cells. The IRF family consists of six different proteins, of which IRF-1 has been demonstrated to have antiproliferative and tumor suppressive functions. We have shown that ATRA activates IRF-1 gene expression in several myeloid leukemia cell lines (HL-60, NB4, THP-1, U937), all of which respond to ATRA by growth inhibition. In addition, during ATRA-induced myeloid differentiation, gene expression of STAT1, STAT2, and p48 was upregulated. These proteins are involved in IFN-alpha specific signaling. ATRA-induced expression of IRF and/or STAT transcription factors may be one of the molecular mechanisms mediating growth inhibition by ATRA.

Selective activation of oncogenic Ha-ras-induced apoptosis in NIH/3T3 cells

A Ha-ras transformant '7-4', derived from mouse NIH/3T3 fibroblasts, was used to study the relationship between overexpression of activated Ha-ras and cell apoptosis. This cell line contains an inducible Ha-rasVal12 oncogene, which was under the regulation of the Escherichia coli (E. coli) lac operator/repressor system. We demonstrate that overexpression of activated Ha-ras oncogene by exogenous isopropyl-beta-D-thiogalactoside (IPTG) under serum-depleted conditions can stimulate cell apoptosis. Cell cycle analysis showed that most of the 7-4 cells with Ha-ras overexpression accumulated at S-phase and that the expression level of p34cdc2 kinase was decreased, suggesting that p34cdc2 may be involved in 7-4 cell apoptosis. Overexpression of bcl-2 transgene in these cells blocked Ha-ras-induced apoptosis, and this blockage was confirmed downstream of Ha-ras gene expression. Cycloheximide blocked the apoptosis of 7-4 cells in a dose-dependent manner, indicating that specific protein regulating apoptosis may be synthesized through Ha-ras overexpression. Ha-ras overexpression-triggered apoptosis was also prevented in the 7-4 derivatives that express either dominant-negative rasAsn17 or dominant-negative raf-1C4B to suppress Ha-ras signal transduction at different stages, indicating that overexpression of activated Ha-ras can induce cell apoptosis and that raf-1 pathway activity is required for this process.

Regulation of interferon responses in medulloblastoma cells by interferon regulatory factor-1 and -2

Transcriptional activator interferon regulatory factor (IRF)-1 and repressor IRF-2 are known to play a critical role in the regulation of interferon (IFN) responses and oncogenesis in fibroblasts. Although these two factors are expressed in many tissues, including the brain, the role of IRFs in the central nervous system (CNS) has not been elucidated. We analysed a medulloblastoma cell line, ONS-76, as a CNS-derived model system and generated its derivatives, R1 and R2 cells, which constitutively expressed each mouse IRF-1 and IRF-2 cDNA at high levels. By viral infection, R1 and R2 cells showed IFN-beta gene expression 3 h earlier than the control ONS-76 (C-76) cells, with 2.46- and 2.24-fold increase in IFN-beta production respectively. In the presence of cycloheximide, virally induced IFN-beta gene expression of C-76 cells was suppressed, whereas R1 and R2 cells produced IFN-beta 7.5- and 2.2-fold higher than C-76 cells respectively. On the other hand, induction of IFN-inducible genes was enhanced in R1 cells but was suppressed in R2 cells compared with C-76 cells. These results demonstrate that IRF-1 and IRF-2 may play an important role in the regulation of IFN-beta and IFN-inducible genes and that IRF-2 may have dual functions as an activator and repressor in CNS-derived cells.

Acute promyelocytic leukemia as a model for cross-talk between interferon and retinoic acid pathways: from molecular biology to clinical applications

Acute promyelocytic leukemia (APL) has been regarded as the paradigm for therapeutic approaches utilizing differentiating agents, due to the fact that almost 95% of patients undergo complete remission when treated with all-trans retinoic acid (ATRA). However, complete clinical remission with ATRA alone is always transient, and relapse in APL is almost invariably associated with the acquisition of resistance to ATRA. Acquired resistance to ATRA in APL cell lines and in some APL clinical cases can be partially overcome by interferons (IFNs), cytokines which have well established tumor-growth suppressive activities. APL is associated in 99% of cases with a 15;17 translocation that fuses the PML and Retinoic Acid Receptor alpha (RARalpha) genes. RARalpha is one of the Retinoic Acid (RA) nuclear receptors which mediates, at the transcriptional level, ATRA differentiating and growth suppressive activity. PML is a tumor-growth suppressor whose expression is directly regulated by IFNs. Here we review the molecular mechanisms by which IFNs and RA can cooperate in controlling cell growth and differentiation of normal hemopoietic cells and leukemic cells, focusing on APL as a model system.

Identification and regulation of testicular interferon-gamma (IFNgamma) receptor subunits: IFNgamma enhances interferon regulatory factor-1 and interleukin-1beta converting enzyme expression

Interferon-gamma (IFNgamma) transmits its signal through a specific cell surface receptor (IFNgammaR), which consists of a primary ligand binding alpha-chain (IFNgammaR alpha) and a signaling beta-chain (IFNgammaR beta). Recent studies identified the cytokines IFNgamma, interleukin-6 (IL-6), IL-1alpha, and tumor necrosis factor-alpha in testicular cells. Therefore, we: 1) examined the expression of IFNgammaR alpha and IFNgammaR beta subunits in freshly isolated and purified rat testicular cells; 2) examined the differential regulation of receptor components by cytokines using primary cultures of Sertoli cells; 3) identified the cell signaling pathway components of testicular IFNgammaR; and 4) characterized the functional role of testicular IFNgamma using primary Sertoli cells. We demonstrated the messenger RNAs for both chains of IFNgammaR in rat testicular cells using Northern hybridization analysis. Western blot analysis and immunocytochemistry showed that both specific IFNgammaR protein subunits were present in cultured primary Leydig and Sertoli cells prepared from the testes of immature rats. The expression of both IFNgammaR component messenger RNAs in cultured Sertoli cells was increased by its specific ligand (IFNgamma), as well as IL-1alpha and tumor necrosis factor-alpha, in both a time- and dose-dependent manner. IFNgamma-activation of the Janus (JAK) tyrosine kinases, JAK1 and JAK2 proteins, indicate that IFNgammaR, expressed in the Sertoli cell, is functional. Moreover, IFNgamma modulates the expression of interferon regulatory factor (IRF)-1 and IL-1beta converting enzyme genes in Sertoli cells. Thus, our data are suggestive of a role(s) for IFN-gamma in the regulation of distinct gene expression and cell-specific sensitivity to apoptosis in the testis.

Respiratory syncytial virus infection of human alveolar epithelial cells enhances interferon regulatory factor 1 and interleukin-1beta-converting enzyme gene expression but does not cause apoptosis

The induction kinetics of the transcriptional activities of interferon regulatory factor 1 (IRF-1), interleukin-1beta-converting enzyme (ICE), and CPP32 by respiratory syncytial virus (RSV) infection of human type II alveolar epithelial cells (A549 cells) were analyzed semiquantitatively by reverse transcriptase PCR. The appearance of ICE and CPP32 protein in cell lysate was examined by Western blotting analysis. The induction of apoptosis by RSV infection was examined by the appearance of DNA fragmentation detected by terminal deoxynucleotidyltransferase-mediated dUTP-biotin nick end labeling. RSV moderately enhanced IRF-1 mRNA as early as 4 h after infection, and this enhancement lasted several hours. Following induction of the IRF-1 gene, ICE gene expression increased significantly, and an increase of ICE protein was observed in the RSV-infected cell lysate. These increments were observed in cells treated with live RSV but not in cells treated with inactivated RSV or control antigen. However, no infection-specific increase of CPP32 gene expression or the protein was observed. No nucleosomal fragmentation was observed in RSV-infected cells during the whole course of infection, despite the appearance of extensive cytopathic change and cell death. These observations suggest that RSV infection of human alveolar epithelial cells induces the ICE gene and its protein as a result of increased IRF-1 induction but that the increased ICE was insufficient to cause apoptosis in the RSV-infected cells. ICE might not be able to activate CPP32, which is thought to be the more important protease for apoptosis.

Tumorigenic conversion of p53-deficient colon epithelial cells by an activated Ki-ras gene

Distinct genetic abnormalities (loss-of-function mutations of APC and p53 and oncogenic activation of Ki-ras) are associated with specific stages of the sporadic, most common types of colorectal tumors. However, the inability to maintain primary colon epithelial cells in culture has hindered the analysis of the pathogenetic role of these abnormalities in colorectal tumorigenesis. We have now established primary cultures of epithelial cells from the colon crypts of p53-deficient mice; these cells are nontumorigenic as indicated by their failure to form colonies in soft agar and to grow as tumors in immunodeficient SCID mice and in immunocompetent syngeneic hosts. Upon ectopic expression of an activated Ki-ras gene, p53-deficient colon epithelial cells form colonies in soft agar and highly invasive subcutaneous tumors in both immunodeficient and immunocompetent mice. Ectopic expression of wild-type p53, but not of a DNA-binding-deficient mutant, markedly suppressed the colony-forming ability of the Ki-ras-transformed p53-deficient epithelial cells. Together, these findings establish a functional synergism in colorectal tumorigenesis dependent on the effects of an oncogenic Ki-ras in a p53-deficient background. This model of tumorigenic conversion of colon epithelial cells might be useful to identify genetic changes associated with disease progression and to evaluate the therapeutic response to conventional and novel anticancer drugs.

Dual role of Ras and Rho proteins: at the cutting edge of life and death

Small GTP-binding proteins of the Ras superfamily are master controllers of the cell physiology. The range of processes in which these proteins are involved include cell cycle progression, cell division, regulation of cell morphology and motility and intracellular trafficking of molecules and organelles. The study of apoptosis, the physiological form of cell suicide, is progressively linking the functions of small G proteins to the control of the mechanisms that trigger the genetic programmes of cell death. To date, isoforms of the Ras and Rho groups have been related to both promotion and suppression of apoptosis. Further, signalling pathways driven by these proteins have been associated with the function and/or expression of molecules that regulate apoptotic responses. Thus, all available evidence points to a critical role for Ras and Rho proteins as major gatekeepers of the decision between cellular life and death.

Induction of apoptosis in a macrophage cell line RAW 264.7 by acemannan, a beta-(1,4)-acetylated mannan

Acemannan is a polydispersed beta-(1,4)-linked acetylated mannan with antiviral properties. It is an immunomodulator, and studies in our laboratory have shown that it causes activation of macrophages. In the presence of IFNgamma, acemannan induced apoptosis in RAW 264. 7 cells. These cells exhibited chromatin condensation, DNA fragmentation, and laddering characteristic of apoptosis. The induction of apoptosis by acemannan and IFNgamma does not seem to be mediated by nitric oxide, since N-nitro-L-arginine methyl ester, the nitric oxide inhibitor, had no effect. Acemannan in the presence of IFNgamma also inhibited the expression of bcl-2. These results suggest that acemannan in the presence of IFNgamma induces apoptosis in RAW 264.7 cells through a mechanism involving the inhibition of bcl-2 expression.

DNA damaging agents induce expression of Fas ligand and subsequent apoptosis in T lymphocytes via the activation of NF-kappa B and AP-1

Apoptosis induced by DNA damage and other stresses can proceed via expression of Fas ligand (FasL) and ligation of its receptor, Fas (CD95). We report that activation of the two transcription factors NF-kappa B and AP-1 is crucially involved in FasL expression induced by etoposide, teniposide, and UV irradiation. A nondegradable mutant of I kappa B blocked both FasL expression and apoptosis induced by DNA damage but not Fas ligation. These stimuli also induced the stress-activated kinase pathway (SAPK/JNK), which was required for the maximal induction of apoptosis. A 1.2 kb FasL promoter responded to DNA damage, as well as coexpression with p65 Rel or Fos/Jun. Mutations in the relevant NF-kappa B and AP-1 binding sites eliminated these responses. Thus, activation of NF-kappa B and AP-1 contributes to stress-induced apoptosis via the expression of FasL.

Ras versus cyclin-dependent kinase inhibitors

In the past year, complex interactions between Ras and the cell cycle have been identified. In primary cells, activated Ras induces a cell-cycle arrest via the induction of cyclin-dependent kinase inhibitors (CDKIs). Oncogenic changes that cooperate with Ras act by neutralising CDKIs by various mechanisms. In the absence of a negative growth signal from Ras, such as in most immortalised cell lines, Ras acts positively on the cell cycle. Insights have been made into the mechanisms by which Ras abrogates remaining cell-cycle controls.

Altered DNA repair and dysregulation of p53 in IRF-1 null hepatocytes

The tumor suppressor proteins IRF-1 and p53 are involved in response pathways after DNA damage. In different cell types, IRF-1 and p53 can cooperate to produce cell cycle arrest (embryo fibroblasts) or can independently trigger apoptosis (lymphoid cells). p53 may also regulate DNA repair, but there is no information on IRF-1 and repair. The cell lineage dependency of these effects precludes extrapolation of findings to other tissues of relevance to human cancer. Here, we report the consequences of IRF-1 deficiency for apoptosis, cell cycle arrest, and DNA repair in primary hepatocytes after DNA damage and extend previous work on the role of p53 in hepatocytes. IRF-1-deficient hepatocytes showed reduced DNA repair activity compared with wild-type, as assessed by unscheduled DNA synthesis after UV irradiation (10J/m2) and by host reactivation of a UV-damaged reporter construct. p53-deficient hepatocytes also showed reduced unscheduled DNA synthesis after UV, but there was no impairment of specific repair in host reactivation assays. IRF-1 deficiency did not affect the p53-dependent G1/S arrest after UV irradiation. Hepatocyte apoptosis after UV treatment, previously reported to be independent of p53, was also independent of IRF-1. However, IRF-1 deficiency produced dysregulation of p53, manifested as increased transactivation of a p53-reporter plasmid in undamaged hepatocytes, and accelerated p53 stabilization after DNA damage. Hence, in hepatocytes, IRF-1 is not required for growth arrest or apoptosis after DNA damage, but the results suggest for the first time a role in DNA repair regulation.

The growing family of interferon regulatory factors

Interferons (IFN) exert their multiple biological effects through the induction of expression of over 30 genes encoding proteins with antiviral, antiproliferative and immunomodulatory functions. Among the many IFN-inducible proteins are the Interferon Regulatory Factors (IRFs), a family of transcription regulators, originally consisting of the well-characterized IRF-1 and IRF-2 proteins; the family has now expanded to over 10 members and is still growing. The present review provides a detailed description of recently characterized IRF family members. Studies analyzing IRF-expressing cell lines and IRF knockout mice reveal that each member of the IRF family exerts distinct roles in biological processes such as pathogen response, cytokine signalling, cell growth regulation and hematopoietic development. Understanding the molecular mechanisms by which the IRFs affect these important cellular events and IFN expression will contribute to a greater understanding of events leading to various viral, immune and malignant disease states and will suggest novel strategies for antiviral and immune modulatory therapy.

Identification of a novel transcriptional regulatory element common to the p53 and interferon regulatory factor 1 genes

The promoter regions of both the interferon regulatory factor (IRF1) and p53 antioncogenes contain a previously unidentified sequence denoted IRF1 p53 common sequence (IPCS), which markedly increases the transcriptional activity of a reporter gene placed under the control of an heterologous promoter in transfected U937 cells. In contrast, transfection of U937 cells with reporter vectors containing p53 and IRF1 promoters with mutated IPCS sites resulted in a 4-fold reduction in the constitutive expression of those two genes. The transcriptional activity of IPCS is strictly correlated with the binding of a novel nuclear factor, IPCS-binding factor (IPCS-BF). IPCS-BF, which is composed of a single polypeptide of 26 kDa, is present constitutively in nuclear extracts of both U937 cells and peripheral blood mononuclear cells from healthy donors. The finding that the pattern of binding of IPCS-BF to the IPCS is unlike that of any known transcription factor and that the IPCS sequence does not exhibit any significant homology with any known binding site present in the data base, strongly suggest that IPCS-BF is a novel transcription factor which, by virtue of this ability to regulate the expression of the p53 and IRF1 genes, could play a central role in the control of cell proliferation and/or apoptosis.

Dexamethasone inhibits lung epithelial cell apoptosis induced by IFN-gamma and Fas

Lung epithelium plays a central role in modulation of the inflammatory response and in lung repair. Airway epithelial cells are targets in asthma, viral infection, acute lung injury, and fibrotic lung disease. Activated T lymphocytes release cytokines such as interferon-gamma (IFN-gamma) that can cooperate with apoptotic signaling pathways such as the Fas-APO-1 pathway to induce apoptosis of damaged epithelial cells. We report that IFN-gamma alone and in combination with activation of the Fas pathway induced apoptosis in A549 lung epithelial cells. Interestingly, the corticosteroid dexamethasone was the most potent inhibitor of IFN-gamma- and IFN-gamma plus anti-Fas-induced apoptosis. IFN-gamma induced expression of an effector of apoptosis, the cysteine protease interleukin-1 beta-converting enzyme, in A549 cells. Dexamethasone, in contrast, induced expression of an inhibitor of apoptosis, human inhibitor of apoptosis (hIAP-1), also known as cIAP2. We suggest that the inhibition of epithelial cell apoptosis by corticosteroids may be one mechanism by which they suppress the inflammatory response.

Deregulation of MUM1/IRF4 by chromosomal translocation in multiple myeloma

The pathogenesis of multiple myeloma (MM), an incurable tumour causing the deregulated proliferation of terminally differentiated B cells, is unknown. Chromosomal translocations (14q1) affecting band 14q32 and unidentified partner chromosomes are common in this tumour, suggesting that they may cause the activation of novel oncogenes. By cloning the chromosomal breakpoints in an MM cell line, we show that the 14q+ translocation represents a t(6;14)x(p25;q32) and that this aberration is recurrent in MM, as it was found in two of eleven MM cell lines. The translocation juxtaposes the immunoglobulin heavy-chain (IgH) locus to MUM1 (multiple myeloma oncogene 1)/IRF4 gene, a member of the interferon regulatory factor (IRF) family known to be active in the control of B-cell proliferation and differentiation. As a result, the MUM1/IRF4 gene is overexpressed--an event that may contribute to tumorigenesis, a MUM1/IRF4 has oncogenic activity in vitro. These findings identify a novel genetic alteration associated with MM, with implications for the pathogenesis and diagnostics of this tumour.

IRF-7, a new interferon regulatory factor associated with Epstein-Barr virus latency

The Epstein-Barr virus (EBV) BamHI Q promoter (Qp) is the only promoter used for the transcription of Epstein-Barr virus nuclear antigen 1 (EBNA-1) mRNA in cells in the most restricted (type I) latent infection state. However, Qp is inactive in type III latency. With the use of the yeast one-hybrid system, a new cellular gene has been identified that encodes proteins which bind to sequence in Qp. The deduced amino acid sequence of the gene has significant homology to the interferon regulatory factors (IRFs). This new gene and products including two splicing variants are designated IRF-7A, IRF-7B, and IRF-7C. The expression of IRF-7 is predominantly in spleen, thymus, and peripheral blood leukocytes (PBL). IRF-7 proteins were identified in primary PBL with specific antiserum against IRF-7B protein. IRF-7s can bind to interferon-stimulated response element (ISRE) sequence and repress transcriptional activation by both interferon and IRF-1. Additionally, a functional viral ISRE sequence, 5'-GCGAAAACGAAAGT-3', has been identified in Qp. Finally, the expression of IRF-7 is consistently high in type III latency cells and almost undetectable in type I latency, corresponding to the activity of endogenous Qp in these latency states and the ability of the IRF-7 proteins to repress Qp-reporter constructs. The identification of a functional viral ISRE and association of IRF-7 with type III latency may be relevant to the mechanism of regulation of Qp.