An IRF-1-dependent pathway of DNA damage-induced apoptosis in mitogen-activated T lymphocytes

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.

Lipopolysaccharide and Ceramide Use Divergent Signaling Pathways to Induce Cell Death in Murine Macrophages

Ceramide is a well-known apoptotic agent that has been implicated in LPS signaling. Therefore, we examined whether LPS-induced macrophage cytotoxicity is mediated by mimicking ceramide. Both LPS and the cell-permeable ceramide analogue, C2 ceramide, induced significant cell death in IFN-γ-activated, thioglycollate-elicited peritoneal macrophages after 48 and 24 h, respectively. Ceramide-induced cell death was neither accompanied by DNA fragmentation nor phosphatidyl serine externalization, characteristics of apoptosis. In contrast, LPS induced a significant fraction of cells to undergo apoptosis, as demonstrated by DNA fragmentation and quantified by DNA analysis on FACS, yet the majority of the cells died in a necrotic fashion. C3H/HeJ Lpsd macrophages were resistant to LPS-induced cell death and less sensitive to C2 ceramide-evoked cytotoxicity, when compared with Lpsn macrophages. C2 ceramide plus IFN-γ failed to activate release of nitric oxide (NO·), whereas LPS-induced cell death, but not C2-induced cytotoxicity, was blocked by an inhibitor of inducible NO· synthase (iNOS), NG-monomethyl-l-arginine. Macrophages from IFN regulatory factor-1 (−/−) mice shown previously to respond marginally to LPS plus IFN-γ to express iNOS mRNA and NO·, were refractory to LPS plus IFN-γ-induced cytotoxicity and apoptosis. These data suggest that although LPS may mimic certain ceramide effects, signal transduction events that lead to cytotoxicity, as well as the downstream mediators, diverge.

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.

Molecular and cellular mechanism of angiotensin II-mediated apoptosis

It is well known that angiotensin II exerts growth promoting effects via the angiotensin II type 1 (AT1) receptor. We have cloned a second type of angiotensin II receptor (AT2 receptor) and demonstrated that this receptor acts as an antagonistic receptor against the AT1 receptor. Moreover, we have demonstrated that the AT2 receptor exerts growth inhibitory and proapoptotic effects by antagonizing the effects of the AT1 receptor and growth factors in several cell lines including vascular smooth muscle cells, cardiomyocytes, neuronal cell (PC12W) and fibroblasts (R3T3). We observed that the AT2 receptor activates tyrosine phosphatase(s) such as mitogen-activated protein (MAP) kinase-phosphatase-1 (MKP-1) and inactivates MAP kinase (extracellular signal-regulated kinase (ERK1 and ERK2)), resulting in Bcl-2 dephosphorylation and up-regulation of Bax. This inactivation of ERK is mediated via Gi protein coupling through its unique intracellular third loop. Moreover, we have demonstrated that interferon regulatory factor (IRF)-1 also up-regulates the AT2 receptor in apoptotic cells, suggesting that the cytokines may play an important role in angiotensin-regulated apoptosis.

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.

In vitro radiosensitization of human cervical carcinoma cells by combined use of 13-cis-retinoic acid and interferon-alpha2a

BACKGROUND

Significant antitumor activity has been reported with the combined use of 13-cis-retinoic acid (cRA) and interferon-alpha2a (IFN-alpha) in the treatment of advanced-stage cervical cancers and skin cancers. Since IFN-alpha has been shown to be a modest radiation enhancer for selected malignant tumor cells and the cytotoxic activity is more enhanced by combining cRA and IFN-alpha, we hypothesized that the exposure of selected human carcinoma cells to combined cRA and IFN-alpha would render the cells highly radiosensitive.

METHODS AND MATERIALS

Two human cervical carcinoma cell lines, ME-180 and HeLa-S3, were chosen for the present study because of the different characteristics of the retinoic acid receptor status of the cell lines. To demonstrate the effects of combined cRA and IFN-alpha treatment on radiation response, we exposed the cells to cRA, IFN-alpha, or a combination of the drugs for 72 h before radiation. Experiments were carried out at minimally cytotoxic concentrations of the drug for radiation studies. End points of the study were cell growth inhibition and clonogenic ability of the single-plated cells. Effects of cRA and IFN-alpha on radiation response were quantitatively analyzed by constructing the radiation cell survival curves of ME-180 and HeLa cells.

RESULTS

ME-180 cells exhibited varying degrees of cytotoxicity with cRA and IFN-alpha, while HeLa cells showed no toxic effects with the same treatment. Combined treatment of cRA and IFN-alpha produced an additive cytotoxic effect in ME-180 cells. Radiosensitization was minimal when ME-180 cells were treated with either cRA or IFN-alpha before radiation. When ME-180 cells were exposed to 10 microM cRA for 48 h and 1000 U/ml IFN-alpha for 24 h prior to radiation, there was a significant enhancement in radiation-induced cell killing; the dose modification factor was 2.1 +/- 0.9 at the 1% cell-survival level. On the other hand, HeLa-S3 cells exhibited no increased cytotoxicity or radiation enhancement under the same experimental conditions.

CONCLUSION

The present data provide a radiobiological basis for using cRA and IFN-alpha as a combination radiosensitizer in selected human carcinoma cells.

Protein-DNA interactions in interferon-gamma signaling

The ability to rapidly activate new genes is essential for the biological effects mediated by IFN-gamma. Studies directed at understanding how these genes are induced by this ligand led to the identification of the STAT family of transcription factors. STATs are rapidly activated at the receptor, whereupon they translocate to the nucleus and bind to a unique enhancer found in the promoter of target genes. The ability to identify this IFN-gamma response element and the proteins that bind it was critical for the elucidation of this pathway. These techniques are the focus of this review.

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.

Inhibition of etoposide-induced apoptosis with peptide aldehyde inhibitors of proteasome

Recent investigations have indicated the involvement of proteasome in programmed cell death. The present studies show that although peptide aldehyde inhibitors of proteasome are by themselves weak inducers of apoptosis, they inhibit the apoptotic effect of the anticancer drug etoposide in rat thymocytes. Acetyl-Leu-Leu-norvalinal (LLnV-al) and other related peptide aldehydes inhibited the increase in caspase activity and DNA fragmentation that followed treatment with etoposide and their effect was related to their potency as proteasome inhibitors. To inhibit etoposide-induced apoptosis, LLnV-al must be present within 3 h of treatment with etoposide, in the same way as the inhibitor of protein synthesis cycloheximide must be. Etoposide caused a rapid accumulation of p53 protein that was not inhibited by LLnV-al, which was also a strong inducer of p53. Peptide aldehydes were also weak activators of caspase activity, suggesting that the same mechanism, i.e. the blocking of proteasome function, both triggers apoptosis and inhibits the effect of etoposide. These results are consistent with a model in which proteasome is selectively involved in the pathway used by etoposide to induce cell suicide.

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.

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.

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.

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.

Apoptosis and the cell cycle

Apoptosis is a genetically controlled response by which eukaryotic cells undergo programmed cell death. This phenomenon plays a major role in developmental pathways (1), provides a homeostatic balance of cell populations, and is deregulated in many diseases including cancer. Control of cell number is determined by an intricate balance of cell death and cell proliferation. Accumulation of cells through suppression of death can contribute to cancer and to persistent viral infections, while excessive death can result in impaired development and in degenerative diseases. Identification of genes that control cell death, and understanding of the impact of apoptosis in both development and disease has advanced our knowledge of apoptosis in the past few years. There appears to be a linkage between apoptosis and cell cycle control mechanisms. Elucidating the mechanisms that link cell cycle control with apoptosis will be of key importance in understanding tumour progression and designing new models of effective tumour therapy.

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.

Expression of FAP-1 (Fas-associated phosphatase) and resistance to Fas-mediated apoptosis in T cell lines derived from human T cell leukemia virus type 1-associated myelopathy/tropical spastic paraparesis patients

Human T cell leukemia virus type 1 (HTLV-I) is the etiologic agent of HTLV-I-associated myelopathy/tropical spastic paraparesis (HAM/TSP) with an autoimmune condition. We examined the sensitivity of HTLV-I-infected T cell lines to Fas-mediated apoptosis, which plays a critical role in the elimination of self-reactive T cells. Among 13 human T-cell lines, all 4 HAM-derived T cell lines and 4 of 6 non-HAM/HTLV-I T cell lines were resistant to apoptosis induced by anti-Fas antibody, whereas only 1 of 3 uninfected cell lines was resistant to apoptosis. The cell lines resistant to apoptosis expressed the viral tax gene and/or the cellular FAP-1 (Fas-associated phosphatase) gene, both of which inhibit Fas-mediated apoptosis in T cell lines. Although Tax is a transcriptional activator of a number of cellular genes, the expression of Tax in a T cell line did not induce the expression of FAP-1, suggesting that these two antiapoptotic proteins independently function in HTLV-I-infected cells. Seven of 10 HTLV-I-infected cell lines, compared with only 1 of 3 virus-negative cell lines, expressed FAP-1. All four HAM cell lines expressed the FAP-1 gene, and its level in these cells was higher than in other T cell lines. Our results suggest that virus-infected T cells escape Fas-mediated immune surveillance by the function of Tax and FAP-1, and this escape may be involved in the autoimmune condition observed in HAM/TSP patients.

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.

Resistance to apoptosis correlates with a highly proliferative phenotype and loss of Fas and CPP32 (caspase-3) expression in human leukemia cells

Apoptosis induced by effector cells of the immune system or by cytotoxic drugs is a main mechanism mediating the prevention or elimination of tumoral cells. For instance, the human T-cell leukemia Jurkat is sensitive to Fas-induced apoptosis and to activation-induced cell death (AICD), and the promonocytic leukemia U937 is sensitive to Fas- and TNF-induced apoptosis. In this work, we have analyzed the mechanisms of resistance to physiological or pharmacological apoptosis in human leukemia by generating highly proliferative (hp) sub-lines derived from Jurkat and U937 cells. These hp sub-lines were resistant to Fas- and TNF-induced apoptosis, as well as to AICD. This was due to the complete loss of Fas and TNFR surface expression and, in the case of Jurkat-derived sub-lines, also of CD3, CD2 and CD59 molecules. The sub-lines also completely lacked the expression of the apoptotic protease CPP32, present in parental cells. Moreover, these sub-lines were no longer sensitive to doxorubicin-induced apoptosis, which was efficiently blocked by the general caspase inhibitor Z-VAD-fmk in the parental cell lines. These data suggest a molecular mechanism for the development of resistance of leukemic cells to physiological and pharmacological apoptosis inducers, giving rise to highly proliferative tumoral phenotypes. These results also indicate that Fas and CPP32 could be useful prognostic markers for the progression and/or therapy outcome of human leukemias.

Type I interferons are essential mediators of apoptotic death in virally infected cells

BACKGROUND

The interferons (IFNs) have been extensively studied in the context of host defence against viral infection. In the established model of IFN action, virally infected cells secrete type I IFNs (IFN-alpha/beta) which induce an antiviral state in uninfected cells. However, it is not clear how IFNs function on the infected cells. It has been reported that cells infected by some viruses die by apoptosis.

RESULTS

In the present study, we found that three types of viruses commonly induce apoptosis in primary cell cultures. Importantly, we observed that virus-induced apoptosis was inhibited by anti-IFN-alpha/beta antibodies, and in cells lacking either the type I IFN receptor 1 (IFNAR1) or its downstream mediator, Stat1 (Signal transducer and activator of transcription 1). IFN-alpha treatment by itself did not induce apoptosis unless it was combined with transfection by double-stranded RNA (dsRNA), which is normally generated during the course of viral infection.

CONCLUSION

These results indicate a novel antiviral function of the type I IFNs, i.e. the selective induction of apoptosis in virally infected cells. In effect, these IFNs have a bifunctional role in limiting the spread of virus; eliciting an antiviral state in uninfected cells while promoting apoptosis in infected cells. Our results may help explain why IFNs are sometimes useful in the treatment of viral diseases and will provide further insight into the mechanisms of virus-induced pathogenesis.

Biochemical and molecular mechanisms regulating apoptosis

In eukaryotes, the regulation of tissue cell numbers is a critical homeostatic objective that is achieved through tight control of apoptosis, mitosis and differentiation. While much is known about the genetic regulation of cell growth and differentiation, the molecular basis of apoptosis is less well understood. Genes involved in both cell proliferation and apoptosis reflect the role of some stimuli in both of these processes, the cell response depending on the overall cellular milieu. Recent research has given fascinating insights into the complex genetic and molecular mechanisms regulating apoptosis. A picture is emerging of the initiation in certain cells, after an apoptotic trigger, of sequential gene expression and specific signal transduction cascades that guide cells along the cell death pathway. Changes in gene expression precede the better known biochemical and morphological changes of apoptosis. It seems possible that, as a result of increased understanding of the cellular events preceding cell death, apoptosis may become more amenable to manipulation by appropriate drug- and gene-based therapies.

Respiratory syncytial virus infection of neonatal monocytes stimulates synthesis of interferon regulatory factor 1 and interleukin-1beta (IL-1beta)-converting enzyme and secretion of IL-1beta

Interleukin-1beta (IL-1beta) production in response to respiratory syncytial virus (RSV) was investigated in normal neonate monocytes. Intracellular or culture supernatant IL-1beta protein levels were measured by enzyme immunoassay. The expression of mRNAs for interferon regulatory factor 1 (IRF-1), IL-1beta-converting enzyme (ICE), and IL-1beta in the cells was analyzed semiquantitatively by reverse transcriptase-PCR. Before RSV exposure, some IRF-1, ICE, and IL-1beta transcripts were already expressed in the monocytes. The levels of these transcripts increased significantly 2 h after RSV exposure compared with those in mock-infected cells. At that time, significantly higher intracellular IL-1beta protein levels were observed in RSV-exposed cells. After 20 h of RSV exposure, quantities of soluble IL-1beta secreted from RSV-exposed cells were moderately higher than those from noninfected cells. These observations suggest that RSV infection of neonatal monocytes triggers enhanced transcription and increased translation of the IL-1beta gene and increased secretion of the soluble protein. The later phase of these processes may be promoted by ICE activity, which was upregulated by increased IRF-1. The increase in IRF-1 activity may also result from RSV infection.

Inhibition of Interferon Regulatory Factor-1 Expression Results in Predominance of Cell Growth Stimulatory Effects of Interferon-γ Due to Phosphorylation of Stat1 and Stat3

Interferon-γ (IFN-γ) is a potent inhibitor of hematopoiesis in vitro and has been implicated in the pathophysiology of human bone marrow failure syndromes. IFN-γ both inhibits cell cycling and induces expression of the Fas-receptor, resulting in subsequent apoptosis of hematopoietic progenitor cells. IFN regulatory factor-1 (IRF-1) mediates some of these suppressive effects by activation of downstream inducible genes, such as double-stranded RNA-activatable protein kinase and inducible nitric oxide synthase. However, under certain experimental conditions, IFN-γ appears to stimulate proliferation of hematopoietic cells. Based on the hypothesis that IFN-γ–receptor triggering may activate diverse signaling cascades, we designed experiments to determine which intracellular mechanisms (in addition to the IRF-1 transduction pathway) influence the biologic effects of IFN-γ. Using antisense technique, we inhibited the IRF-1–mediated pathway in KG1a cells stimulated with IFN-γ. In contrast to the suppressive effects of IFN-γ observed in control cells, untreated and IFN-γ–treated KG-1a cells that were transduced with retroviral vectors expressing IRF-1 antisense mRNA showed enhanced proliferation. The increased growth rate was associated with decreased levels of IRF-1 mRNA and protein but unchanged levels of IRF-2. We inferred that IFN-γ could also activate a stimulatory transduction pathway that, under specific conditions, may control the cellular response to this cytokine. The family of Stat proteins is involved in signal transduction of hematopoietic growth factors. We showed that, in KG-1a cells, IFN-γ also induced phosphorylation of Stat1 and Stat3, whereas p42 MAP kinase was phosphorylated regardless of the presence of IFN-γ. Using electrophoresis mobility shift assays, IFN-γ enhanced Stat1-Stat1 homodimer and Stat1-Stat3 heterodimer formation, suggesting that, in addition to inhibitory signals mediated by IRF-1, IFN-γ may activate proliferative signals by phosphorylation of Stat1 and Stat3 proteins. The observations made in experiments with KG-1a cells were confirmed in primary hematopoietic cells. After inhibition of the IRF-1 pathway by transduction of an antisense IRF-1 retrovirus into human CD34+ cells, IFN-γ produced an aberrant stimulatory effect on hematopoietic colony formation. Conversely, in control vector-transduced CD34+ cells, the typical inhibitory response to IFN-γ was seen. Our results indicate that inhibitory cytokines such as IFN-γ may exhibit diverse biologic effects depending on the intracellular balance of transcriptional regulators, in turn influenced by the activation and differentiation status of the target cells.

Inhibition of Interferon Regulatory Factor-1 Expression Results in Predominance of Cell Growth Stimulatory Effects of Interferon-γ Due to Phosphorylation of Stat1 and Stat3

Interferon-γ (IFN-γ) is a potent inhibitor of hematopoiesis in vitro and has been implicated in the pathophysiology of human bone marrow failure syndromes. IFN-γ both inhibits cell cycling and induces expression of the Fas-receptor, resulting in subsequent apoptosis of hematopoietic progenitor cells. IFN regulatory factor-1 (IRF-1) mediates some of these suppressive effects by activation of downstream inducible genes, such as double-stranded RNA-activatable protein kinase and inducible nitric oxide synthase. However, under certain experimental conditions, IFN-γ appears to stimulate proliferation of hematopoietic cells. Based on the hypothesis that IFN-γ–receptor triggering may activate diverse signaling cascades, we designed experiments to determine which intracellular mechanisms (in addition to the IRF-1 transduction pathway) influence the biologic effects of IFN-γ. Using antisense technique, we inhibited the IRF-1–mediated pathway in KG1a cells stimulated with IFN-γ. In contrast to the suppressive effects of IFN-γ observed in control cells, untreated and IFN-γ–treated KG-1a cells that were transduced with retroviral vectors expressing IRF-1 antisense mRNA showed enhanced proliferation. The increased growth rate was associated with decreased levels of IRF-1 mRNA and protein but unchanged levels of IRF-2. We inferred that IFN-γ could also activate a stimulatory transduction pathway that, under specific conditions, may control the cellular response to this cytokine. The family of Stat proteins is involved in signal transduction of hematopoietic growth factors. We showed that, in KG-1a cells, IFN-γ also induced phosphorylation of Stat1 and Stat3, whereas p42 MAP kinase was phosphorylated regardless of the presence of IFN-γ. Using electrophoresis mobility shift assays, IFN-γ enhanced Stat1-Stat1 homodimer and Stat1-Stat3 heterodimer formation, suggesting that, in addition to inhibitory signals mediated by IRF-1, IFN-γ may activate proliferative signals by phosphorylation of Stat1 and Stat3 proteins. The observations made in experiments with KG-1a cells were confirmed in primary hematopoietic cells. After inhibition of the IRF-1 pathway by transduction of an antisense IRF-1 retrovirus into human CD34+ cells, IFN-γ produced an aberrant stimulatory effect on hematopoietic colony formation. Conversely, in control vector-transduced CD34+ cells, the typical inhibitory response to IFN-γ was seen. Our results indicate that inhibitory cytokines such as IFN-γ may exhibit diverse biologic effects depending on the intracellular balance of transcriptional regulators, in turn influenced by the activation and differentiation status of the target cells.

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.

Defective TNF-alpha-induced apoptosis in STAT1-null cells due to low constitutive levels of caspases

Signal transducers and activators of transcription (STATs) enhance transcription of specific genes in response to cytokines and growth factors. STAT1 is also required for efficient constitutive expression of the caspases Ice, Cpp32, and Ich-1 in human fibroblasts. As a consequence, STAT1-null cells are resistant to apoptosis by tumor necrosis factor alpha (TNF-alpha). Reintroduction of STAT1alpha restored both TNF-alpha-induced apoptosis and the expression of Ice, Cpp32, and Ich-1. Variant STAT1 proteins carrying point mutations that inactivate domains required for STAT dimer formation nevertheless restored protease expression and sensitivity to apoptosis, indicating that the functions of STAT1 required for these activities are different from those that mediate induced gene expression.

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.

Interferon action and apoptosis are defective in mice devoid of 2',5'-oligoadenylate-dependent RNase L

2',5'-Oligoadenylate-dependent RNase L functions in the interferon-inducible, RNA decay pathway known as the 2-5A system. To determine the physiological roles of the 2-5A system, mice were generated with a targeted disruption of the RNase L gene. The antiviral effect of interferon alpha was impaired in RNase L-/- mice providing the first evidence that the 2-5A system functions as an antiviral pathway in animals. In addition, remarkably enlarged thymuses in the RNase L-/- mice resulted from a suppression of apoptosis. There was a 2-fold decrease in apoptosis in vivo in the thymuses and spleens of RNase L-/- mice. Furthermore, apoptosis was substantially suppressed in RNase L-/- thymocytes and fibroblasts treated with different apoptotic agents. These results suggest that both interferon action and apoptosis can be controlled at the level of RNA stability by RNase L. Another implication is that the 2-5A system is likely to contribute to the antiviral activity of interferon by inducing apoptosis of infected cells.

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.

Convergence of TNFalpha and IFNgamma signalling pathways through synergistic induction of IRF-1/ISGF-2 is mediated by a composite GAS/kappaB promoter element

The molecular basis for the well known synergistic biological effects of tumor necrosis factor alpha (TNFalpha) and interferon gamma (IFNgamma) is still poorly understood. This report demonstrates that expression of interferon-regulatory factor 1 (IRF-1), also known as interferon-stimulated-gene factor 2 (ISGF-2), is synergistically induced by these cytokines. The induction is a primary transcriptional response that occurs rapidly without a requirement for new protein synthesis. Synergism is mediated by a novel composite element in the IRF-1 promoter that includes an IFNgamma-activation site (GAS) overlapped by a non-consensus site for nuclear factor kappa B (NFkappaB). These sequences are bound strongly by signal transducer and activator of transcription 1 (STAT-1) and weakly by the p50/p65 heterodimer form of NFkappaB, respectively. However, the binding of STAT-1 and NFkappaB to the GAS/kappaB element in vitro seems to be mutually exclusive and independent. Synergistic induction of IRF-1 is likely to be an important early step in regulatory networks critical to the synergism of TNFalpha and IFNgamma. The GAS/kappaB element may mediate synergistic transcriptional induction of IRF-1 by other pairs of ligands that together activate NFkappaB and STAT family members. Other genes are likely to contain this motif and be regulated similarly.

The role of the bcl-2/ced-9 gene family in cancer and general implications of defects in cell death control for tumourigenesis and resistance to chemotherapy

Cell production within an organ is determined by the rate of immigration, proliferation, differentiation, emigration and death of cells. Abnormalities in any one of these processes will disturb normal control of cell production, thereby eliciting hyperplasia can be an early event in neoplasia. Cell death, apoptosis, is a physiological process responsible for removing unwanted cells. It is used in multi-cellular organisms for tissue remodelling during embryogenesis, regulation of cell turnover and as a defence strategy against invading pathogens. In this review article we describe the role of the bcl-2/ced-9 gene family in cancer and discuss the general implications of defects in the apoptosis program for tumourigenesis and resistance of cancer cells to chemotherapy in light of current knowledge of the molecular mechanisms of cell death.

Role for CD40-mediated activation of c-Rel and maintenance of c-myc RNA levels in mitigating anti-IgM-induced growth arrest

CD40 crosslinking on B cells activates NF-kappaB and stress-activated protein kinase (SAPK) pathways. Since CD40 crosslinking rescues WEHI 231 B cells from anti-IgM-induced apoptosis, those pathways were likely candidates to be involved. Indeed, both signaling cascades predominated in anti-IgM-treated WEHI 231 cells, treated concurrently with anti-CD40 to rescue them from apoptosis. Crosslinking of CD40 activated the NF-kappaB proteins c-Rel and p50, but had no influence on their cytoplasmic steady state level. However, in contrast to-and even in the presence of-anti-IgM-mediated signals, engagement of CD40 resulted in a prolonged nuclear translocation of c-Rel, thereby allowing the formation of active NF-kappaB complexes. Consistent with this, the upstream regulatory element of the c-myc promoter, known to be regulated by NF-kappaB, was differently regulated after BCR ligation vs BCR plus CD40 crosslinking. The level of c-myc RNA was rapidly downregulated after BCR engagement, but persistent in the presence of CD40 signaling.

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.

Requirement of the caspase-3/CPP32 protease cascade for apoptotic death following cytokine deprivation in hematopoietic cells

Hematopoietic cytokines transduce cell survival signals, which are distinct from the signals necessary for the stimulation of DNA synthesis. Recently, the Ras and phosphatidylinositol 3-kinase pathways have been shown to play important roles in preventing apoptosis in various cell types, e.g. hematopoietic cells and neuronal cells. Withdrawal of cytokine(s), in turn, results in rapid inactivation of these survival pathways and eventually leads to cell death accompanied by the hallmarks of apoptosis. However, the mechanism of cell death caused by cytokine deprivation has not been fully elucidated. In this study, we demonstrate that caspase-3/CPP32, a member of the caspase/interleukin-1beta-converting enzyme family, is activated upon interleukin (IL)-3 deprivation in IL-3-dependent cells as well as IL-2 deprivation in IL-2-dependent cells. In addition, poly(ADP-ribose) polymerase, a cellular substrate for the caspase family proteases, was degraded into apoptotic fragments in both cell lines after cytokine removal. Furthermore, inhibition of a caspase family protease by synthetic peptides suppressed apoptotic death. These results indicate that the activation of a caspase-like protease(s) is required for the progression of apoptosis following cytokine deprivation. However, readdition of IL-3 did not restore the proliferative potential of the cells that survived in the presence of the peptide inhibitor after IL-3 depletion. Therefore, cellular commitment to apoptosis appears to precede the activation of a caspase-like protease(s).

How does interferon exert its cell growth inhibitory effect?

The interferons (IFNs) have become accepted therapy in a range of haematological and non-haematological malignancies. The mechanism behind IFN's antitumour action is, however, unclear. Interferons (IFNs) are capable of modulating a variety of cellular responses. One prominent effect of IFNs is their cell growth inhibitory activity, which has also been suggested to be of major importance in their antitumour action. In the present review we will discuss the cellular events leading to a decreased number of cells following IFN treatment, the molecular mechanisms underlying these phenomena, and the importance of these effects in a clinical perspective.

Interferon regulatory factor 1 is induced by interferon-gamma equally in neurons and glial cells

Class I MHC protein is induced in glia but not mature neurons by IFN-gamma. We have compared IFN-gamma signal transduction in these populations. There were identical levels of STAT1 homodimers and IRF-1 by gel-shift and IRF-1 mRNA was induced equally. However class I MHC, beta2-microglobulin and interleukin 1-beta converting enzyme mRNA levels were greatly reduced in neurons. These experiments show that there is no defect in expression of IRF-1 in response to IFN-gamma in mature mouse neurons but that insufficient class I MHC gene expression is induced for detectable cell surface protein expression.

Activation of the STAT signaling pathway can cause expression of caspase 1 and apoptosis

Protein tyrosine kinases activate the STAT (signal transducer and activator of transcription) signaling pathway, which can play essential roles in cell differentiation, cell cycle control, and development. However, the potential role of the STAT signaling pathway in the induction of apoptosis remains unexplored. Here we show that gamma interferon (IFN-gamma) activated STAT1 and induced apoptosis in both A431 and HeLa cells, whereas epidermal growth factor (EGF) activated STAT proteins and induced apoptosis in A431 but not in HeLa cells. EGF receptor autophosphorylation and mitogen-activated protein kinase activation in response to EGF were similar in both cell lines. The breast cancer cell line MDA-MB-468 exhibited a similar response to A431 cells, i.e., STAT activation and apoptosis correlatively resulted from EGF or IFN-gamma treatment. In addition, in a mutant A431 cell line in which STAT activation was abolished, no apoptosis was induced by either EGF or IFN-gamma. We further demonstrated that both EGF and IFN-gamma induced caspase 1 (interleukin-1beta converting enzyme [ICE]) gene expression in a STAT-dependent manner. IFN-gamma was unable to induce ICE gene expression and apoptosis in either JAK1-deficient HeLa cells (E2A4) or STAT1-deficient cells (U3A). However, ICE gene expression and apoptosis were induced by IFN-gamma in U3A cells into which STAT1 had been reintroduced. Moreover, both EGF-induced apoptosis and IFN-gamma-induced apoptosis were effectively blocked by Z-Val-Ala-Asp-fluoromethylketone (ZVAD) in all the cells tested, and studies from ICE-deficient cells indicated that ICE gene expression was necessary for IFN-gamma-induced apoptosis. We conclude that activation of the STAT signaling pathway can induce apoptosis through the induction of ICE gene expression.

Caspases: the executioners of apoptosis

Apoptosis is a major form of cell death, characterized initially by a series of stereotypic morphological changes. In the nematode Caenorhabditis elegans, the gene ced-3 encodes a protein required for developmental cell death. Since the recognition that CED-3 has sequence identity with the mammalian cysteine protease interleukin-1 beta-converting enzyme (ICE), a family of at least 10 related cysteine proteases has been identified. These proteins are characterized by almost absolute specificity for aspartic acid in the P1 position. All the caspases (ICE-like proteases) contain a conserved QACXG (where X is R, Q or G) pentapeptide active-site motif. Capases are synthesized as inactive proenzymes comprising an N-terminal peptide (prodomain) together with one large and one small subunit. The crystal structures of both caspase-1 and caspase-3 show that the active enzyme is a heterotetramer, containing two small and two large subunits. Activation of caspases during apoptosis results in the cleavage of critical cellular substrates, including poly(ADP-ribose) polymerase and lamins, so precipitating the dramatic morphological changes of apoptosis. Apoptosis induced by CD95 (Fas/APO-1) and tumour necrosis factor activates caspase-8 (MACH/FLICE/Mch5), which contains an N-terminus with FADD (Fas-associating protein with death domain)-like death effector domains, so providing a direct link between cell death receptors and the caspases. The importance of caspase prodomains in the regulation of apoptosis is further highlighted by the recognition of adapter molecules, such as RAIDD [receptor-interacting protein (RIP)-associated ICH-1/CED-3-homologous protein with a death domain]/CRADD (caspase and RIP adapter with death domain), which binds to the prodomain of caspase-2 and recruits it to the signalling complex. Cells undergoing apoptosis following triggering of death receptors execute the death programme by activating a hierarchy of caspases, with caspase-8 and possibly caspase-10 being at or near the apex of this apoptotic cascade.

Interferon-regulatory factors during development of CD4 and CD8 thymocytes

Selection events in the thymus occur at the double-positive CD4+ CD8+ (DP) developmental stage leading either to further differentiation of the CD4+ and CD8+ lineages or to deletion. The interferon-regulatory factor IRF-1 has been implicated in signalling for T-cell death and also in CD8+ thymic differentiation. IRF-1 is an activator and IRF-2 a repressor of gene transcription regulated by type 1 interferons (IFN). To evaluate the role of IRF-1 and IRF-2 in the differentiation of CD4 and CD8 thymocytes, we analysed their DNA-binding activity before and after antigenic stimulation at different stages of thymic development and in peripheral T cells. Unseparated, double-positive and single-positive thymocytes as well as peripheral T lymphocytes from mice transgenic (tg) for a T-cell receptor (TCR), restricted either by major histocompatibility complex class I or class II, were stimulated by their nominal antigen. Our results demonstrate that the DNA-binding activity of IRF-2 and, weakly, that of IRF-1 are inducible in total thymocytes in response to antigen. There is no induction of IRF-1/IRF-2 binding activity at the double-positive stage of thymic development in the MHC class II-restricted model whereas in the MHC class I-restricted model IRF-1/IRF-2 activity is induced weakly. At the single-positive stage, antigen induces the IRF-1/IRF-2 DNA binding in both CD4+ and CD8+ thymocytes, but not in mature lymphocytes from the periphery. This pattern of expression suggests that IRF-1/IRF-2 binding activities resulting from antigen stimulation are developmentally regulated. No evidence for a selective role of IRF-1 in the development of the CD8+ lineage was found, however.