Control of beta-interferon expression in murine embryonal carcinoma F9 cells

Slicing and dicing viruses: antiviral RNA interference in mammals

To protect against the harmful consequences of viral infections, organisms are equipped with sophisticated antiviral mechanisms, including cell-intrinsic means to restrict viral replication and propagation. Plant and invertebrate cells utilise mostly RNA interference (RNAi), an RNA-based mechanism, for cell-intrinsic immunity to viruses while vertebrates rely on the protein-based interferon (IFN)-driven innate immune system for the same purpose. The RNAi machinery is conserved in vertebrate cells, yet whether antiviral RNAi is still active in mammals and functionally relevant to mammalian antiviral defence is intensely debated. Here, we discuss cellular and viral factors that impact on antiviral RNAi and the contexts in which this system might be at play in mammalian resistance to viral infection.

Molecular basis for an attenuated cytoplasmic dsRNA response in human embryonic stem cells

The introduction of double stranded RNA (dsRNA) into the cytoplasm of mammalian cells usually leads to a potent antiviral response resulting in the rapid induction of interferon beta (IFNβ). This response can be mediated by a number of dsRNA sensors, including TLR3, MDA5, RIG-I and PKR. We show here that pluripotent human cells (human embryonic stem (hES) cells and induced pluripotent (iPS) cells) do not induce interferon in response to cytoplasmic dsRNA, and we have used a variety of approaches to learn the underlying basis for this phenomenon. Two major cytoplasmic dsRNA sensors, TLR3 and MDA5, are not expressed in hES cells and iPS cells. PKR is expressed in hES cells, but is not activated by transfected dsRNA. In addition, RIG-I is expressed, but fails to respond to dsRNA because its signaling adapter, MITA/STING, is not expressed. Finally, the interferon-inducible RNAse L and oligoadenylate synthetase enzymes are also expressed at very low levels. Upon differentiation of hES cells into trophoblasts, cells acquire the ability to respond to dsRNA and this correlates with a significant induction of expression of TLR3 and its adaptor protein TICAM-1/TRIF. Taken together, our results reveal that the lack of an interferon response may be a general characteristic of pluripotency and that this results from the systematic downregulation of a number of genes involved in cytoplasmic dsRNA signaling.

Mice deficient in oocyte-specific oligoadenylate synthetase-like protein OAS1D display reduced fertility

The double-stranded RNA (dsRNA)-induced interferon response is a defense mechanism against viral infection. Upon interferon activation by dsRNA, 2',5'-oligoadenylate synthetase 1 (OAS1A) is induced; it binds dsRNA and converts ATP into 2',5'-linked oligomers of adenosine (called 2-5A), which activate RNase L that in turn degrades viral and cellular RNAs. In a screen to identify oocyte-specific genes, we identified a novel murine cDNA encoding an ovary-specific 2',5'-oligoadenylate synthetase-like protein, OAS1D, which displays 59% identity with OAS1A. OAS1D is predominantly cytoplasmic and is exclusively expressed in growing oocytes and early embryos. Like OAS1A, OAS1D binds the dsRNA mimetic poly(I-C), but unlike OAS1A, it lacks 2'-5' adenosine linking activity. OAS1D interacts with OAS1A and inhibits the enzymatic activity of OAS1A. Mutant mice lacking OAS1D (Oas1d(-/-)) display reduced fertility due to defects in ovarian follicle development, decreased efficiency of ovulation, and eggs that are fertilized arrest at the one-cell stage. These effects are exacerbated after activation of the interferon/OAS1A/RNase L pathway by poly(I-C). We propose that OAS1D suppresses the interferon/OAS/RNase L-mediated cellular destruction by interacting with OAS1A during oogenesis and early embryonic development.

Specific interference with gene expression induced by long, double-stranded RNA in mouse embryonal teratocarcinoma cell lines

In eukaryotes, double-stranded (ds) RNA induces sequence-specific inhibition of gene expression, referred to as RNA interference (RNAi). In invertebrates, RNAi can be triggered effectively by either long dsRNAs or 21- to 23-nt-long short interfering (si) duplex RNAs, acting as effectors of RNAi. siRNAs recently have been shown to act as potent inducers of RNAi in cultured mammalian cells. However, studies of RNAi activated by long dsRNA are impeded by its nonspecific effects, mediated by dsRNA-dependent protein kinase PKR and RNase L. Here, we report that the RNAi response can be induced effectively by long dsRNA in nondifferentiated mouse cells grown in culture. Transfection of dsRNA into embryonal carcinoma (EC) P19 and F9 cells results in a sequence-specific decrease in the level of proteins expressed from either exogenous or endogenous genes. dsRNA-mediated inhibition of the reporter gene also occurs in mouse embryonic stem cells. The RNAi effect is mediated by siRNAs, which are generated by cleavage of dsRNA by the RNaseIII-like enzyme, Dicer. We demonstrate that extracts prepared from EC cells catalyze processing of dsRNA into approximately 23-nt fragments and that Dicer localizes to the cytoplasm of EC and HeLa cells.

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.

NF-kappa B-independent activation of beta-interferon expression in mouse F9 embryonal carcinoma cells

We have examined the behaviour of the beta-interferon promoter in mouse F9 embryonal carcinoma cells. In undifferentiated cells, the beta-interferon promoter is not responsive to dsRNA or to Sendai virus. In cells stimulated to differentiate into parietal endoderm by treatment with retinoic acid, the beta-interferon promoter responds to both inducers, but only Sendai virus can activate the transcription factor NF-kappa B previously thought to be essential for beta-interferon induction. Differentiated F9 cells therefore present an unprecedented situation in which induction of the beta-interferon gene does not require NF-kappa B. In addition to these differences, induction by dsRNA, but not by Sendai virus, is significantly enhanced by a pretreatment with interferon (priming). These observations suggest that paramyxo-viruses can participate in beta-interferon induction in a manner that is distinct from a simple generator of dsRNA. Analysis of the promoter requirements for induction in differentiated F9 cells suggests that induction is brought about by a novel mechanism using the currently identified regulatory domains.

Differentiation-dependent activation of interferon-stimulated gene factors and transcription factor NF-kappa B in mouse embryonal carcinoma cells

We have recently shown that the adenovirus E1A gene products block interferon-alpha-induced signal transduction and transcription factor NF-kappa B-mediated gene induction. Here we report that the same responses are also blocked in undifferentiated F9 teratocarcinoma cells. The block was removed upon cellular differentiation and regained upon the introduction of viral E1A into the differentiated cells. In undifferentiated cells, interferon-beta failed to induce the transcription of interferon-responsive genes because of a lack of activation of the cognate trans-acting factors. As a result, in these cells, virus replication was not inhibited by interferon. Similarly, in undifferentiated but not in differentiated F9 cells, tumor necrosis factor alpha failed to stimulate NF-kappa B-mediated transcription of a reporter gene because of a failure in the activation of NF-kappa B trans-acting factor. These results suggest that a cellular E1A-like activity, present in undifferentiated F9 cells, and adenoviral E1A use similar mechanisms for repressing the expression of specific cellular genes.

Interferon consensus sequence-binding protein, a member of the interferon regulatory factor family, suppresses interferon-induced gene transcription

We previously isolated a cDNA clone encoding interferon consensus sequence-binding protein (ICSBP), a member of the interferon regulatory factor (IRF) family, that binds to the interferon (IFN)-stimulated response element (ISRE) of many IFN-regulated genes. In this investigation, we studied the functional role of ICSBP by transient cotransfection of ICSBP cDNA with IFN-responsive reporter genes into the human embryonal carcinoma cell line N-Tera2. These cells were shown not to express ICSBP or IRF-2, thus allowing functional analysis of transfected cDNAs. Cotransfection of ICSBP into cells treated with retinoic acid or any of the IFNs (alpha, beta, or gamma) repressed expression of a chloramphenicol acetyltransferase reporter driven by the major histocompatibility complex class I gene promoter. Similarly, ICSBP repressed expression of chloramphenicol acetyltransferase reporters driven by the ISREs of the 2'-5' oligoadenylate synthetase, guanylate-binding protein, and ISG-15 genes in IFN-treated cells. The repression was dependent on the presence of the ISRE in the reporter. Deletion analysis showed that the putative N-terminal DNA binding domain of ICSBP by itself is capable of mediating the repression. Using the same cotransfection conditions as for ICSBP, a similar repression of these reporters was observed with IRF-2. Finally, ICSBP repressed the IRF-1-mediated induction of major histocompatibility complex class I and IFN-beta reporters in the absence of IFN or retinoic acid. Taken together, these results suggest that ICSBP is a negative regulatory factor capable of repressing transcription of target genes induced by IFN, retinoic acid, or IRF-1.

Interferon consensus sequence-binding protein, a member of the interferon regulatory factor family, suppresses interferon-induced gene transcription

We previously isolated a cDNA clone encoding interferon consensus sequence-binding protein (ICSBP), a member of the interferon regulatory factor (IRF) family, that binds to the interferon (IFN)-stimulated response element (ISRE) of many IFN-regulated genes. In this investigation, we studied the functional role of ICSBP by transient cotransfection of ICSBP cDNA with IFN-responsive reporter genes into the human embryonal carcinoma cell line N-Tera2. These cells were shown not to express ICSBP or IRF-2, thus allowing functional analysis of transfected cDNAs. Cotransfection of ICSBP into cells treated with retinoic acid or any of the IFNs (alpha, beta, or gamma) repressed expression of a chloramphenicol acetyltransferase reporter driven by the major histocompatibility complex class I gene promoter. Similarly, ICSBP repressed expression of chloramphenicol acetyltransferase reporters driven by the ISREs of the 2'-5' oligoadenylate synthetase, guanylate-binding protein, and ISG-15 genes in IFN-treated cells. The repression was dependent on the presence of the ISRE in the reporter. Deletion analysis showed that the putative N-terminal DNA binding domain of ICSBP by itself is capable of mediating the repression. Using the same cotransfection conditions as for ICSBP, a similar repression of these reporters was observed with IRF-2. Finally, ICSBP repressed the IRF-1-mediated induction of major histocompatibility complex class I and IFN-beta reporters in the absence of IFN or retinoic acid. Taken together, these results suggest that ICSBP is a negative regulatory factor capable of repressing transcription of target genes induced by IFN, retinoic acid, or IRF-1.