Increase in p202 expression during skeletal muscle differentiation: inhibition of MyoD protein expression and activity by p202

Activation and Immune Regulation Mechanisms of PYHIN Family During Microbial Infection

The innate immune system defenses against pathogen infections via patten-recognition receptors (PRRs). PRRs initiate immune responses by recognizing pathogen-associated molecular patterns (PAMPs), including peptidoglycan, lipopolysaccharide, and nucleic acids. Several nucleic acid sensors or families have been identified, such as RIG-I-like receptors (RLRs), Toll-like receptors (TLRs), cyclic GMP-AMP synthase (cGAS), and PYHIN family receptors. In recent years, the PYHIN family cytosolic DNA receptors have increased attention because of their important roles in initiating innate immune responses. The family members in humans include Absent in melanoma 2 (AIM2), IFN-γ inducible protein 16 (IFI16), interferon-inducible protein X (IFIX), and myeloid cell nuclear differentiation antigen (MNDA). The PYHIN family members are also identified in mice, including AIM2, p202, p203, p204, and p205. Herein, we summarize recent advances in understanding the activation and immune regulation mechanisms of the PYHIN family during microbial infection. Furthermore, structural characterizations of AIM2, IFI16, p202, and p204 provide more accurate insights into the signaling mechanisms of PYHIN family receptors. Overall, the molecular details will facilitate the development of reagents to defense against viral infections.

Silkworm Silk Fiber Bundles as Improved In Vitro Scaffolds for Skeletal Muscle

To mimic skeletal muscle tissues in vitro, native and transgenic spider silk/silkworm silks were seeded with C2C12 myoblasts to observe if these three-dimensional substrates are preferable to a traditional two-dimensional polystyrene cell culture surface. Silks were wound around an acrylic chassis to produce a novel, three-dimensional cell culture device with suspended muscle fibers that genetically and morphologically resemble native skeletal muscle tissue. The transgenic spider silk/silkworm silk has never before been studied for this application. Genetic expression verified skeletal muscle lineage and differentiation, while fluorescent imaging verified contractile protein synthesis. Genetic analysis also revealed an increase in expression of the Myh2 contractile protein gene on silkworm silks, particularly on the transgenic silk. Mechanical properties and protein secondary structure content of the silks indicated correlation between substrate properties and Myh2 gene expression. This increase in contractile protein gene expression suggests that biologically derived silk substrates that are suspended may be a preferable substrate for in vitro muscle modeling because of the proteinaceous character and mechanical flexibility of the silk.

The interferon-inducible protein p202 promotes osteogenesis in mouse bone marrow stromal cells

In the present study, we explored the role of the interferon-inducible protein p202 in osteoblast differentiation of mouse bone marrow stromal cells (BMSCs). Both the mRNA and protein levels of p202 increased initially and decreased afterward in the course of BMSC osteogenesis. The intracellular distribution of this protein also changed in the differentiation process. p202 knockdown inhibited, while p202 overexpression enhanced, the osteoblast differentiation of BMSCs. This was identified by evaluation of expression of osteogenic markers, Alizarin Red S staining, and determination of alkaline phosphatase activity. Further study revealed that p202 disturbs the formation of Runx2/Ids complex and frees Runx2 to induce the differentiation process. The findings demonstrated that p202 plays a positive role in BMSC osteogenesis.

The PYHIN Protein p205 Regulates the Inflammasome by Controlling Asc Expression

Members of the IFN-inducible PYHIN protein family, such as absent in melanoma-2 and IFN-γ-inducible protein (IFI)16, bind dsDNA and form caspase-1-activating inflammasomes that are important in immunity to cytosolic bacteria, DNA viruses, or HIV. IFI16 has also been shown to regulate transcription of type I IFNs during HSV infection. The role of other members of the PYHIN protein family in the regulation of immune responses is much less clear. In this study, we identified an immune-regulatory function for a member of the murine PYHIN protein family, p205 (also called Ifi205). Examination of immune responses induced by dsDNA and other microbial ligands in bone marrow-derived macrophages lacking p205 revealed that inflammasome activation by dsDNA, as well as ligands that engage the NLRP3 inflammasome, was severely compromised in these cells. Further analysis revealed that p205-knockdown cells showed reduced expression of apoptosis-associated speck-like molecule containing CARD domain (Asc) at the protein and RNA levels. p205 knockdown resulted in reduced binding of actively transcribing RNA polymerase II to the endogenous Asc gene, resulting in decreased transcription and processing of Asc pre-mRNA. Deletion of p205 in B16 melanoma cells using CRISPR/Cas9 showed a similar loss of Asc expression. Ectopic expression of p205 induced expression of an Asc promoter-luciferase reporter gene. Together, these findings suggest that p205 controls expression of Asc mRNA to regulate inflammasome responses. These findings expand on our understanding of immune-regulatory roles for the PYHIN protein family.

Interferon-inducible protein 205 (p205) plays a role in adipogenic differentiation of mouse adipose-derived stem cells

The role of p205 in the regulation of cell growth and differentiation remains poorly understood. This study aimed to determine whether p205 is involved in adipogenesis of mouse adipose-derived stem cells (mASCs). p205 was largely induced in mASCs under adipogenesis in vitro. The mRNA and protein levels of p205 reached a maximum at day 4, and decreased at days 6 and 8. p205 localized almost exclusively in the nucleus of undifferentiated cells, but also translocated to the cytoplasm in intermediately and terminally differentiated cells. Although p205 suppression impaired mASC adipogenesis, its overexpression did not enhance the differentiation process. p205 co-localized with, and bound directly to, C/EBPβ and C/EBPα at day 4. Knockdown of p205 lowered the amount of p205 interacting with C/EBPβ or C/EBPα, further downregulating the transcription activities of C/EBPα and PPARγ. This suggests the importance of these transcription factors in the role of p205 in mASC adipogenesis.

Role of interferon-inducible protein 202 (p202) in the regulation of adipogenesis in mouse adipose-derived stem cells

The interferon-inducible protein 202 (p202) has emerged as a key regulator of cell proliferation and differentiation. To explore the role of p202 in adipocyte differentiation, p202 mRNA and protein levels in differentiating mouse adipose-derived stem cells (mASCs) were examined, and were found to continuously increase during mASC adipogenesis. The nuclear and cytoplasmic distribution of p202 in the differentiation process was also determined. In addition, suppression and overexpression of p202 impaired and enhanced the differentiation process, respectively. Further, results of co-immunoprecipitation and co-immunofluorescence showed the interaction and intracellular co-localization of p202 with C/EBPβ, C/EBPα, and PPARγ at intermediate and/or late differentiation stages. Knockdown of p202 interfered with the elevated expression of C/EBPβ, C/EBPα, and PPARγ. In conclusion, the temporal and spatial profiles of p202 and the observed manner in which p202 affected the expression of these transcription factors provided evidence that p202 plays a role during mASC adipogenesis.

Enhanced functional recovery in MRL/MpJ mice after spinal cord dorsal hemisection

Adult MRL/MpJ mice have been shown to possess unique regeneration capabilities. They are able to heal an ear-punched hole or an injured heart with normal tissue architecture and without scar formation. Here we present functional and histological evidence for enhanced recovery following spinal cord injury (SCI) in MRL/MpJ mice. A control group (C57BL/6 mice) and MRL/MpJ mice underwent a dorsal hemisection at T9 (thoracic vertebra 9). Our data show that MRL/MpJ mice recovered motor function significantly faster and more completely. We observed enhanced regeneration of the corticospinal tract (CST). Furthermore, we observed a reduced astrocytic response and fewer micro-cavities at the injury site, which appear to create a more growth-permissive environment for the injured axons. Our data suggest that the reduced astrocytic response is in part due to a lower lesion-induced increase of cell proliferation post-SCI, and a reduced astrocytic differentiation of the proliferating cells. Interestingly, we also found an increased number of proliferating microglia, which could be involved in the MRL/MpJ spinal cord repair mechanisms. Finally, to evaluate the molecular basis of faster spinal cord repair, we examined the difference in gene expression changes in MRL/MpJ and C57BL/6 mice after SCI. Our microarray data support our histological findings and reveal a transcriptional profile associated with a more efficient spinal cord repair in MRL/MpJ mice.

Interferon-inducible p200-family proteins as novel sensors of cytoplasmic DNA: role in inflammation and autoimmunity

Deregulated innate immune responses that result in increased levels of type I interferons (IFNs) and stimulation of IFN-inducible genes are thought to contribute to chronic inflammation and autoimmunity. One family of IFN-inducible genes is the Ifi200 family, which includes the murine (eg, Ifi202a, Ifi202b, Ifi203, Ifi204, Mndal, and Aim2) and human (eg, IFI16, MNDA, IFIX, and AIM2) genes. Genes in the family encode structurally related proteins (the p200-family proteins), which share at least one partially conserved repeat of 200-amino acid (200-AA) residues. Consistent with the presence of 2 consecutive oligonucleotide/oligosaccharide-binding folds in the repeat, the p200-family proteins can bind to DNA. Additionally, these proteins (except the p202 proteins) also contain a pyrin (PYD) domain in the N-terminus. Increased expression of p202 proteins in certain strains of female mice is associated with lupus-like disease. Interestingly, only the Aim2 protein is conserved between the mouse and humans. Several recent studies have provided evidence that the Aim2 and p202 proteins can recognize DNA in cytoplasm and the Aim2 protein upon sensing DNA can form a caspase-1-activating inflammasome. In this review, we discuss how the ability of p200-family proteins to sense cytoplasmic DNA may contribute to the development of chronic inflammation and associated diseases.

Deletion of the Isg15 gene results in up-regulation of decidual cell survival genes and down-regulation of adhesion genes: implication for regulation by IL-1beta

The ubiquitin homolog interferon stimulated gene 15 (ISG15) is up-regulated in the endometrium in response to pregnancy in primates, ruminants, pigs, and mice. ISG15 covalently attaches to intracellular proteins (isgylation) and regulates numerous intracellular responses. We hypothesized that ISG15 depletion (Isg15(-/-)) alters decidual tissue gene expression and that IL-1beta induces ISG15 expression and isgylation in cultured murine decidual explants and human uterine fibroblasts (HuFs). After studying the reproductive phenotype, contrary to earlier reports, up to 50% of the fetuses die between 7.5 and 12.5 d post coitum (dpc) in Isg15(-/-) mothers when mated to Isg15(-/-) fathers. Using microarray analysis, over 500 genes are differentially regulated in 7.5 dpc deciduas from Isg15(-/-) compared with Isg15(+/+) mice. The gene for interferon-inducible protein 202b, which functions in cell-survival mechanisms, was up-regulated (mRNA and protein) in deciduas from Isg15(-/-) mice. Culture of Isg15(+/+) mouse decidual explants (7.5 dpc) with IL-1beta decreased Isg15 mRNA but increased free and conjugated ISG15. In predecidual HuF cells, IL-1beta treatment increased ISG15 mRNA and isgylation. Additionally, IL-1beta up-regulated expression of enzymes (HERC5, UBCH8) that coordinate the covalent conjugation of ISG15 to target proteins, as well as the gene that encodes the deisglyation enzyme UBP43 in HuF cells. In conclusion, deletion of Isg15 gene results in 50% fetal loss after 7.5 dpc, which can be explained through differential decidual gene expression that is functionally tied to cell survival and adhesion pathways. This fetal death also might relate to impaired IL-1beta signaling, because ISG15 and isgylation are induced by IL-1beta in human and murine endometrial stromal cells.

Transient expression of interferon-inducible p204 in the early stage is required for adipogenesis in 3T3-L1 cells

A member of the interferon-inducible p200 family of proteins, p204, has recently been reported to function in the development of many mesoderm-derived tissues, such as bone, muscle, and cartilage. However, no published study has yet investigated the role of p204 in adipogenesis. Our preliminary experiments showed that p204 can be found in 3T3-L1 preadipocytes, and its expression was up-regulated in a differentiation-dependent manner. As such, we hypothesized that p204 is associated with adipogenesis and focused on the influence of p204 on adipogenesis. In the present study, we investigated the transient elevated expression and cytoplasm-to-nucleus translocation of p204 in the early stage of adipogenesis. To determine the effect of p204 on adipogenesis, p204-siRNA and expression vector were produced for p204 suppression and overexpression, respectively. The knockdown of p204 resulted in a significantly depressed adipocyte differentiation, whereas p204 overexpression promoted adipocyte differentiation. The mRNA expression of adipogenic markers, such as peroxisome-proliferator-activated receptor (PPAR)gamma, CCAAT/enhancer-binding-protein (C/EBP)alpha, lipoprotein lipase, and adipsin, was decreased by p204 suppression and increased by p204 overexpression. A coimmunoprecipitation assay coupled with an indirect immunofluorescence assay also indicated that p204 interacted and colocalized with C/EBPdelta in the nucleus. Furthermore, the knockdown of p204 disrupted the interaction between p204 and C/EBPdelta and partially suppressed the PPARgamma transcriptional activity by dissociating C/EBPdelta with the PPARgamma promoter element. Collectively, our data indicate that the transient expression of p204 in the early stage is indispensable for adipocyte differentiation. Disruption of p204 expression patterns at this stage leads to irreversible damage in fat formation.

p204, a p200 family protein, as a multifunctional regulator of cell proliferation and differentiation

The interferon-inducible p200 family comprises a group of homologous mouse and human proteins. Most of these have an N-terminal DAPIN domain and one or two partially conserved, 200 amino acid long C-terminal domains (designated as 200X domain). These proteins play important roles in the regulation of cell proliferation, tissue differentiation, apoptosis and senescence. p200 family proteins are involved also in autoimmunity and the control of tumor growth. These proteins function by binding to various target proteins (e.g. transcription factors, signaling proteins, oncoproteins and tumor suppressor proteins) and modulating target activity. This review concentrates on p204, a murine member of the family and its roles in regulating cell proliferation, cell and tissue differentiation (e.g. of skeletal muscle myotubes, beating cardiac myocytes, osteoblasts, chondrocytes and macrophages) and signaling by Ras proteins. The expression of p204 in various tissues as promoted by tissue-specific transcription factors, its distribution among subcellular compartments, and the controls of these features are also discussed.

Interferon-inducible Ifi200-family genes in systemic lupus erythematosus

Systemic lupus erythematosus (SLE) is the prototype of complex autoimmune diseases. Studies have suggested that genetic, hormonal, and environmental factors contribute to the development of the disease. Interestingly, several recent studies involving SLE patients and mouse models of the disease have suggested a role for interferon (IFN)-stimulated genes (ISGs) in the development of SLE. One family of ISGs is the Ifi200-family, which includes mouse (Ifi202a, Ifi202b, Ifi203, Ifi204, and Ifi205) and human (IFI16, MNDA, AIM2, and IFIX) genes. The mouse genes cluster between serum amyloid P-component (Apcs) and alpha-spectrin (Spna-1) genes on chromosome 1 and the human genes cluster in syntenic region 1q23. The Ifi200-family genes encode structurally and functionally related proteins (the p200-family proteins). Increased expression of certain p200-family proteins in cells is associated with inhibition of cell proliferation, modulation of apoptosis, and cell differentiation. Our studies involving generation of B6.Nba2 congenic mice, coupled with gene expression analyses, identified the Ifi202 as a candidate lupus-susceptibility gene. Importantly, recent studies using different mouse models of SLE have suggested that increased expression of Ifi202 gene (encoding p202 protein) in immune cells contributes to lupus susceptibility. Consistent with a functional role for the p202 protein in lupus susceptibility, increased levels of IFI16 protein in human SLE patients are associated with the diseases. This review summarizes recent findings concerning the regulation and role of p200-family proteins in the development of SLE.

Cloning and characterisation of Ifi206: a new murine HIN-200 family member

HIN-200 proteins are interferon-inducible proteins capable of regulating cell growth, senescence, differentiation and death. Using a combination of in silico analysis of NCBI EST databases and screening of murine C57BL/6 cDNA libraries we isolated novel murine HIN-200 cDNAs designated Ifi206S and Ifi206L encoding two putative mRNA splice variants. The p206S and p206L protein isoforms have a modular domain structure consisting of an N-terminal PAAD/DAPIN/Pyrin domain, a region rich in serine, threonine and proline residues and a C-terminal 200 B domain characteristic of other HIN-200 proteins. Ifi206 mRNA was detected only in the spleen and lung of BALB/c and C57BL/6 mice and expression was up-regulated by both types I and II IFN subtypes. p206 protein was predominantly expressed in the cytoplasm and addition of LMB, a CRM1 dependent nuclear export inhibitor, caused p206 to accumulate in the nucleus. Unlike other human and mouse HIN-200 proteins that contain only a single 200 amino acid domain, overexpression of p206 impaired the clonogenic growth of tumour cell lines. Thus, p206 represents the newest HIN-200 family member discovered. It has distinct and restricted pattern of expression however maintains many of the hallmarks of HIN-200 proteins including the presence of a characteristic 200 X domain, induction by interferon and an ability to suppress tumour cell growth.

Candidate genes responsible for common and different pathology of infected muscle tissues between Trichinella spiralis and T. pseudospiralis infection

The gene expression profiles were compared between Trichinella spiralis- and T. pseudospiralis-infected muscle tissues by means of a cDNA microarray. Out of 30,000 genes, the expressions of 55 genes were up-regulated in both T. spiralis and T. pseudospiralis infections, 24 genes were down-regulated in both Trichinella infections, 30 genes were up-regulated only in T. spiralis infection, 23 genes were down-regulated only in T. spiralis infection, 25 genes were up-regulated only in T. pseudospiralis infection, and 21 genes were down-regulated only in T. pseudospiralis infection. Many of these differentially expressed genes were associated with satellite cell activation and proliferation (paired box gene 7, Pax7; Pax3; desmin; M-cadherin), myogenesis and muscle development (eyes absent 2 homolog, Eya2; myocyte enhancer factor 2C, MEF2C; pre B-cell leukemia transcription factor 1, Pbx1; chordin-like 2, Chrdl2), cell differentiation (galectin 1; insulin like growth factors, IGFs; c-ski; msh-like 1, Msx1; Numb), cell proliferation and cycle regulation (retinoblastoma 1, Rb1; granulin; p21, CDK4, cyclin A2), and apoptosis (tumor necrosis factor receptor 1, TNF-R1; programmed cell death protein 11, Pdcd11; Pdcd1; nuclear protein 1, Nuprl; clusterin, CLU). The differential expression of 17 genes was validated by quantitative real time PCR and 15 genes showed identical results with the microarray analysis. The present study listed the candidate genes that were commonly and differentially expressed between T. spiralis and/or T. pseudospiralis infection, thus suggesting that these genes need to be further investigated to reveal the mechanism of the common and/or different pathological changes induced by the two species Trichinella.

p204 Is Required for the Differentiation of P19 Murine Embryonal Carcinoma Cells to Beating Cardiac Myocytes

Among 10 adult mouse tissues tested, the p204 protein levels were highest in heart and skeletal muscle. We described previously that the MyoD-inducible p204 protein is required for the differentiation of cultured murine C2C12 skeletal muscle myoblasts to myotubes. Here we report that p204 was also required for the differentiation of cultured P19 murine embryonal carcinoma stem cells to beating cardiac myocytes. As shown by others, this process can be triggered by dimethyl sulfoxide (DMSO). We established that DMSO induced the formation of 204RNA and p204. Ectopic p204 could partially substitute for DMSO in inducing differentiation, whereas ectopic 204 antisense RNA inhibited the differentiation. Experiments with reporter constructs, including regulatory regions from the Ifi204 gene (encoding p204) in P19 cells and in cultured newborn rat cardiac myocytes, as well as chromatin coimmunoprecipitations with transcription factors, revealed that p204 expression was synergistically transactivated by the cardiac Gata4, Nkx2.5, and Tbx5 transcription factors. Furthermore, ectopic p204 triggered the expression of Gata4 and Nkx2.5 in P19 cells. p204 contains a nuclear export signal and was partially translocated to the cytoplasm during the differentiation. p204 from which the nuclear export signal was deleted was not translocated, and it did not induce differentiation. The various mechanisms by which p204 promoted the differentiation are reported in the accompanying article (Ding, B., Liu, C., Huang, Y., Yu, J., Kong, W., and Lengyel, P. (2006) J. Biol. Chem. 281, 14893-14906).

p204 protein overcomes the inhibition of the differentiation of P19 murine embryonal carcinoma cells to beating cardiac myocytes by Id proteins

We reported in the accompanying article (Ding, B., Liu, C., Huang, Y., Hickey, R. P., Yu, J., Kong, W., and Lengyel, P. (2006) J. Biol. Chem. 281, 14882-14892) that (i) the p204 protein is required for the differentiation of murine P19 embryonal carcinoma stem cells to beating cardiac myocytes, and (ii) the expression of p204 in the differentiating P19 cells is synergistically transactivated by the cardiac transcription factors Gata4, Nkx2.5, and Tbx5. Here we report that endogenous or ectopic inhibitor of differentiation (Id) proteins inhibited the differentiation of P19 cells to myocytes. This was in consequence of the binding of Id1, Id2, or Id3 protein to the Gata4 and Nkx2.5 proteins and the resulting inhibitions (i) of the binding of these transcription factors to each other and to DNA and (ii) of their synergistic transactivation of the expression of various genes, including atrial natriuretic factor and Ifi204 (encoding p204). p204 overcame this inhibition by Id proteins in consequence of (i) binding and sequestering Id proteins, (ii) accelerating their ubiquitination and degradation by proteasomes, and (iii) decreasing the level of Id proteins in the nucleus by increasing their translocation from the nucleus to the cytoplasm. Points (ii) and (iii) depended on the presence of the nuclear export signal in p204. In the course of the differentiation, Gata4, Nkx2.5, and p204 were components of a positive feedback loop. This loop arose in consequence of it that p204 overcame the inhibition of the synergistic activity of Gata4 and Nkx2.5 by the Id proteins.

p205, a potential tumor suppressor, inhibits cell proliferation via multiple pathways of cell cycle regulation

p205 is a member of the interferon-inducible p200 family of proteins that regulate cell proliferation. Over-expression of p205 inhibits cell growth, although its mechanism of action is currently unknown. Therefore, we evaluated the effect of p205 on the p53 and Rb-dependent pathways of cell cycle regulation. p205 expression results in elevated levels of p21, and activates the p21 promoter in vitro in a p53-dependent manner. In addition, p205 induces increased expression of Rb, and binds directly to Rb and p53. Interestingly, p205 also induces growth inhibition independent of p53 and Rb by delaying G2/M progression in proliferating cells, and is a substrate for Cdk2 kinase activity. Finally, we have identified other binding partners of p205 by a yeast two-hybrid screen, including the paired homeodomain protein HoxB2. Taken together, our results indicate that p205 induces growth arrest by interaction with multiple transcription factors that regulate the cell cycle, including but not entirely dependent on the Rb- and p53-mediated pathways of growth inhibition.

Up-regulation of the interferon-inducible IFI16 gene by oxidative stress triggers p53 transcriptional activity in endothelial cells

Reactive oxygen species (ROS), including hydrogen peroxide (H2O2), induces injury of endothelium in a variety of pathophysiological conditions, such as inflammation, aging, and cancer. In our study, we characterized the signaling pathway linking oxidative stress induced by sublethal concentrations of H2O2 to p53 in primary human endothelial cells through the interferon (IFN)-inducible gene IFI16. Induction of IFI16 by H2O2 was concentration- and time-dependent (maximum at 50 microM, 6 h after treatment) and down-regulated by pretreatment with N-acetyl-L-cysteine, which acts as an antioxidant. This pathway is a general response to ROS and not specific to H2O2 treatment, as two other ROS-generating compounds, i.e., S-nitroso-N-acetylpenicillamine and tert-butyl hydroperoxide, were equally capable to induce IFI16. Moreover, IFI16 up-regulation is a result of protein accumulation, as expression of corresponding mRNA, assessed by real-time polymerase chain reaction, was not affected. To investigate the mechanism of IFI16 accumulation, cells were incubated for 6 h in the presence of H2O2 or IFN-beta, and then cycloheximide was added to inhibit further protein synthesis. The half-life of IFI16 protein was found to be significantly increased in H2O2-treated cells compared with IFN-beta-treated cells (t1/2 = 120 min vs. > 30 min in H2O2- vs. IFN-beta-treated cells, respectively). An increase of IFI16 was accompanied by interaction with p53 phosphorylated at its N terminus, as shown by immunoprecipitation experiments. Moreover, binding to IFI16 resulted in its transcriptional activation as shown by an increase in the activity of a reporter gene driven by p53-responsive sequences derived from the p21(WAF1) promoter, along with an increase in the p21 mRNA and protein levels. Altogether, these results demonstrate a novel role of IFI16 in the signal transduction pathway that leads to p53 activation by oxidative stress in endothelial cells.

Novel functions of proteins encoded by viral stress-inducible genes

The interferon (IFN) system is the first line of defense against viral infection in vertebrates. It is well known that IFN synthesis is induced by viral infection and secreted IFN act upon as yet uninfected neighboring cells to prepare them for combating oncoming virus infection. The products of IFN-stimulated genes (ISG), which number in hundreds, mediate this antiviral action of IFN. Recent evidence suggests that many of these genes are also induced directly by double-stranded RNA (dsRNA), a common byproduct of virus infection, or by other viral products. We refer to this family of genes, on which this article is focused, as viral stress-inducible genes (VSIG). First, we will discuss the different signaling pathways that lead to induce transcription of these genes in response to different agents. Second, we will review the available information about the inducibility of different VSIG by IFN, dsRNA, and viruses. In this article, we will review the functions of proteins encoded by selected members of the VSIG family. Because most of these proteins affect many aspects of cellular physiology, the information presented here is important for understanding not only the nature of host response to virus infection but also cellular responses to cytokines, such as IFN and exogenous dsRNA, which is known to signal through Toll-like receptor 3 (TLR3). Finally, we will present a future perspective and point out the main gaps of our knowledge in the field.

Biochemical and growth regulatory activities of the HIN-200 family member and putative tumor suppressor protein, AIM2

The human HIN-200 family member AIM2 was originally identified in a screen for suppressors of melanoma tumorigenicity following introduction of chromosome 6 into the UACC903 human melanoma cell line. Although the AIM2 protein contained many of the conserved structural motifs common to other HIN-200 proteins, the biochemical characteristics of AIM2 and the ability of overexpressed AIM2 to phenocopy the effect of introduction of chromosome 6 in the UACC903 cells had not been assessed. Herein we demonstrated that AIM2 was localised within the nucleus of transfected or interferon-treated human cells. In addition, AIM2 could homodimerise via the amino-terminal (PAAD/DAPIN) region and heterodimerise with the related IFI 16 protein. However, overexpressed AIM2 did not significantly affect the growth or survival of UACC903 cells or another human melanoma cell line. These data indicate that AIM2 has many of the biochemical and structural characteristics of HIN-200 proteins, however, its expression is not sufficient to induce a tumor-suppressor-like phenotype.

Variable expression of human myeloid specific nuclear antigen MNDA in monocyte lineage cells in atherosclerosis

MNDA (human myeloid nuclear differentiation antigen) is expressed in specific lineages of hematopoietic cells and most notably at high levels in macrophages at sites of inflammation. MNDA and related proteins appear to modulate the activity of transcription factors and in some cases have a role in mediating cell death. The expression of MNDA was characterized in normal and diseased human aorta. MNDA positive cells double labeled for CD68 in all tissue examined. Twenty percent of normal aortas were negative or contained rare MNDA positive cells while other normal aorta contained more frequent positive cells. In atherosclerotic aorta, the number of MNDA positive cells increased with progression of disease. In normal and early lesions, MNDA positive cells adjacent to the endothelium generally displayed a strong MNDA reactivity associated with small amount of CD68 reactive cytoplasm. In the same sections, MNDA positive cells at increasing distances from the endothelium displayed lower MNDA reactivity and were associated with larger amounts of CD68 reactive cytoplasm. Foam cells in fatty streaks exhibited MNDA reactivity that ranged from strong to weak or negative. In advanced lesions, cells in the shoulder and those in fibrous tissue surrounding an atheroma were highly reactive for MNDA. However, only a fraction of the CD68 positive foam cells near the lipid core under the cap and shoulder contained MNDA reactivity. The variation in MNDA expression appeared to change with phenotypic specialization of monocytes in atherosclerosis consistent with its association with inflammation and suspected roles in regulating gene expression or in mediating cell death.

The Interferon-inducible p204 Protein Acts as a Transcriptional Coactivator of Cbfa1 and Enhances Osteoblast Differentiation

The differentiation of uncommitted mesenchymal cells into osteoblasts is a fundamental molecular event governing both embryonic development and bone repair. The bone morphogenetic proteins (BMPs) are important regulators of this process; they function by binding to cell surface receptors and signaling by means of Smad proteins. Core binding factor alpha-1 (Cbfa1), a member of the runt family of transcription factors, is an essential transcriptional regulator of osteoblast differentiation and bone formation, and this process is positively or negatively regulated by a variety of coactivators and corepressors. We report that p204, an interferon-inducible protein that was previously shown to inhibit cell proliferation and promote the differentiation of myoblasts to myotubes, is a novel regulator in the course of osteogenesis. p204 is expressed in embryonic osteoblasts and hypertrophic chondrocytes in the growth plate as well as in the calvaria osteoblasts of neonatal mice. Its level is increased in the course of the BMP-2-triggered osteoblast differentiation of pluripotent C2C12 cells. This increase is probably due to the activation of the gene encoding 204 (Ifi204) by Smad transcription factor, including Smad1, -4, and -5. Overexpression of p204 enhances the BMP-2-induced osteoblast differentiation in vitro, as revealed by elevated alkaline phosphatase activity and osteocalcin production. p204 acts as a cofactor of Cbfa1: 1) high levels of p204 augment, whereas the lowering of p204 level decreases, the Cbfa1-dependent transcription, and 2) p204 associates with Cbfa1 both in vitro and in vivo. Two nonoverlapping segments in p204 bind to Cbfa1, and the N-terminal 88-amino acid segment of Cbfa1 is required for binding to p204. p204, which is the first interferon-inducible protein found to associate with Cbfa1, functions as a novel regulator of osteoblast differentiation.

Inhibition of Growth by p205: A Nuclear Protein and Putative Tumor Suppressor Expressed during Myeloid Cell Differentiation

p205 belongs to a family of interferon-inducible proteins called the IFI-200 family, which have been implicated in the regulation of cell growth and differentiation. While p205 is induced in hematopoietic stem cells during myeloid cell differentiation, its function is not known. Therefore, the aim of this study was to determine the role of p205 in regulating proliferation in hematopoietic progenitor cells and in nonhematopoietic cell lines. We found that p205 localizes to the nucleus in hematopoietic and nonhematopoietic cell lines. Transient expression of p205 in murine IL-3-dependent BaF3 and 32D-C123 progenitor cell lines inhibited IL-3-induced growth and proliferation. The closely related IFI-200 family members, p204 and p202, similarly inhibited IL-3-dependent progenitor cell proliferation. p205 also inhibited the proliferation and growth of normal hematopoietic progenitor cells. In nonhematopoietic cell lines, p205 and p204 expression inhibited NIH3T3 cell colony formation in vitro, and microinjection of p205 expression vectors into NIH3T3 fibroblasts inhibited serum-induced proliferation. We have determined the functional domains of p205 necessary for activity, which were identified as the N-terminal domain in apoptosis and interferon response (DAPIN)/PYRIN domain, and the C-terminal retinoblastoma protein (Rb)-binding motif. In addition, we have demonstrated that a putative ataxia telangiectasia, mutated (ATM) kinase phosphorylation site specifically regulates the activity of p205. Taken together, these data suggest that p205 is a potent cell growth regulator whose activity is mediated by its protein-binding domains. We propose that during myelomonocytic cell differentiation, induction of p205 expression contributes to cell growth arrest, thus allowing progenitor cells to differentiate.

Retinoblastoma (Rb) protein upregulates expression of the Ifi202 gene encoding an interferon-inducible negative regulator of cell growth

Studies have indicated that ectopic expression of p202, an interferon (IFN)-inducible retinoblastoma (Rb)-binding protein, in cultured cells retards cell proliferation and modulates cell survival. Consistent with a role of p202 in cell cycle regulation, levels of p202 increase in cells arrested in the G0/G1 phase of cell cycle after withdrawal of serum growth factors. However, a role for p202 in cell growth arrest remains to be defined. Moreover, it remains unclear how levels of p202 are upregulated during the cell growth arrest. Here, we report that Rb upregulates expression of Ifi202 gene. We found that basal as well as IFN-induced levels of p202 were significantly higher in wild-type (Rb(+/+)) mouse embryonic fibroblasts (MEFs) than isogenic Rb(-/-) MEFs. Consistent with the regulation of Ifi202 gene by Rb, expression of functional Rb, but not a pocket mutant of it, stimulated the activity of a reporter whose expression was driven by the 5'-regulatory region of Ifi202 gene. Importantly, the stimulation by Rb was dependent, in part, on a JunD/AP-1 DNA-binding site present in the 5'-regulatory region of the Ifi202 gene. Moreover, basal levels of p202 were significantly higher in wild-type (JunD(+/+)) than isogenic JunD(-/-) MEFs. Additionally, we found that increased expression of p202 potentiated the Rb-mediated inhibition of cell growth and mutations in the Rb-binding motif (LxCxE) of p202 significantly reduced cell survival. Together, our observations support the idea that the transcriptional activation of Ifi202 gene by Rb/JunD may be important for the regulation of cell growth and survival.

The interferon-inducible p202a protein modulates NF-kappaB activity by inhibiting the binding to DNA of p50/p65 heterodimers and p65 homodimers while enhancing the binding of p50 homodimers

p202a is a member of the interferon-inducible murine p200 family of proteins. These proteins share 1 or 2 partially conserved 200 amino acid segments of the a or the b type. The known biological activities of p202a include among others the regulation of muscle differentiation, cell proliferation, and apoptosis. These biological activities of p202a can be correlated with the inhibition of the activity of several transcription factors. Thus, the binding of p202a results in the inhibition of the sequence-specific binding to DNA of the c-Fos, c-Jun, E2F1, E2F4, MyoD, myogenin, and c-Myc transcription factors. This study concerns the mechanisms by which p202a inhibits the activity of NF-kappaB, a transcription factor involved among others in host defense, inflammation, immunity, and the apoptotic response. NF-kappaB consists of p50 and p65 subunits. We demonstrate that p202a can inhibit in vitro and in vivo the binding to DNA of p65 homodimers and p50/65 heterodimers, whereas it increases the binding of p50 homodimers. Thus p202a can impair NF-kappaB activity both by inhibiting the binding to DNA of the transcriptionally active p65 homodimers and p50/p65 heterodimers and by boosting the binding of the repressive p50 homodimers. p202a can bind p50 and p65 in vitro and in vivo, and p202a can be part of the p50 homodimer complex bound to DNA. p50 binds in p202a to the a type segment, whereas p65 binds to the b type segment. Transfected ectopic p202a increases the apoptotic effect of tumor necrosis factor (at least in part) by inhibiting NF-kappaB and its antiapoptotic activity.

Differential signaling through NFkappaB does not ameliorate skeletal myoblast apoptosis during differentiation

During 23A2 skeletal myoblast differentiation, roughly 30% of the population undergoes apoptosis. Further, constitutive signaling by G12V:H-Ras or Raf:CAAX abrogates this apoptosis. In this study, we demonstrate an increase in NFkappaB activity in myoblasts that have survived and are expressing muscle-specific genes. NFkappaB activity is also elevated in myoblasts expressing constitutively active G12V:H-Ras but not Raf:CAAX. Expression of a dominant negative IkappaB (IkappaB-SR) sufficient to eliminate this elevated level of NFkappaB activity, in either the 23A2 myoblasts or their G12V:H-Ras-expressing counterparts, however, does not affect survival. Furthermore, expression of a constitutively active IkappaB kinase in 23A2 myoblasts does not protect these cells from the apoptosis associated with differentiation. Since signaling by IkappaB kinase can abrogate differentiation, this result demonstrates that abrogated differentiation and abrogated apoptosis are separable phenotypes.

Mutation at the acidic residue-rich domain of eukaryotic initiation factor 2 (eIF2alpha)-associated glycoprotein p67 increases the protection of eIF2alpha phosphorylation during heat shock

Eukaryotic initiation factor 2 (eIF2)-associated glycoprotein p67 protects eIF2alpha phosphorylation from kinases. The N-terminal lysine-rich domains increase this activity and the acidic residue-rich domain inhibits it. Conserved amino acid residues D251, D262, E364, and E459 are involved in this inhibition. During heat shock, the overall protein synthesis rate decreases due to the increased levels of eIF2alpha phosphorylation. In this study, we examined whether the above inhibition is also found during heat shock. Indeed, the acidic residue-rich domain mutant (D6/2) showed a decreased level of eIF2alpha phosphorylation, and its second-site alanine substitutions at D251, D262, and E459 reversed this effect, whereas second-site alanine substitution at H331 and E364 residues further augmented it. A high-molecular-weight phosphoprotein and at least two faster-migrating phosphoproteins were detected by the monospecific polyclonal antibody against eIF2alpha(P) form in rat tumor hepatoma cells constitutively expressing the double mutant D6/2+D251A. Although the levels of p67 mutants were unaffected during heat shock, those of p67 and p67-deactivating enzyme varied. Furthermore, the overall rate of protein synthesis correlated with the level of eIF2alpha phosphorylation. Taken together, these results suggest that the lysine-rich domains and conserved amino acid residues of p67 are involved in the regulation of eIF2alpha phosphorylation during heat shock.

Expression of IFI 16 in epithelial cells and lymphoid tissues

IFI 16 is a member of the HIN-200 protein family named for their haemopoietic expression, interferon-inducibility and nuclear localisation. These proteins have been characterised as transcriptional regulators that modulate the cell cycle. IFI 16 is expressed in some haemopoietic lineages including CD34+ progenitor cells, mature lymphocytes and monocytes, but is absent from granulocytes, erythrocytes and megakaryocytes. We present a wider study of IFI 16 expression in normal human tissues using a monoclonal antibody specifically recognising the C-terminus of IFI 16. As expected, IFI 16 was detected in the nuclei of lymphocytes in the spleen, thymus, lymph node and palatine tonsil, but was also found in epithelial cells in these tissues. Interestingly, IFI 16 protein was also expressed in non-lymphoid tissues including trachea, gastrointestinal tract, skin and testis, but was absent from others including heart and brain. In each tissue, IFI 16 was predominantly expressed in surface epithelial cells and staining was strongest in basal epithelial layers. Therefore, IFI 16 expression is not restricted to cells of the immune system, but is also expressed in epithelial cells. In contrast to the perceived role of HIN-200 proteins as suppressors of cell growth, maximal expression of IFI 16 was in cells with high proliferative potential.

Induction of p202, a modulator of apoptosis, during oncogenic transformation of NIH 3T3 cells by activated H-Ras (Q61L) contributes to cell survival

Previous studies have revealed that p202 (52 kDa), an interferon (IFN) and differentiation-inducible protein, negatively regulates cell proliferation and modulates cell survival. However, the role of p202 in transformed cells remains to be investigated. Here we report that constitutive expression of oncogenic H-Ras (Q61L) in NIH 3T3 cells, which resulted in cell transformation, was associated with increases in the steady-state levels of 202 RNA and protein. Interestingly, the increase in p202 levels in transformed cells correlated with increases in the activity of the transcription factor c-Jun/AP-1, which bound to the two potential AP-1 DNA binding sites (the AP-1CS1 and AP-1CS2) in the 5'-regulatory region of the 202 gene in gel mobility shift assays. Furthermore, the site-directed mutagenesis, coupled with promoter-reporter analyses, revealed that these two AP-1 DNA binding sites contribute to the regulation of the 202 gene in Ras transformed cells. Because treatment of transformed cells with a specific inhibitor of MEK (PD 98059) resulted in significant decreases in the levels of p202, these observations raise the possibility that in transformed cells Ras/Raf/MEK pathway regulates the transcriptional activation of the 202 gene. Significantly, decreases in the levels of p202 in Ras transformed NIH 3T3 cells under reduced serum conditions increased the susceptibility to apoptosis. Collectively, our observations support the idea that the transcriptional increases in the levels of p202 by oncogenic H-Ras in NIH 3T3 cells are needed for cell survival.

Retroviral mediated expression of the human myeloid nuclear antigen in a null cell line upregulates Dlk1 expression

The human myeloid nuclear differentiation antigen (MNDA) is a hematopoietic cell specific nuclear protein. MNDA and other related gene products interact with and alter the activity of a large number of proteins involved in regulating specific gene transcription. MNDA and related genes exhibit expression characteristics, which suggest functions unique to specific lineages of cells, in addition to mediating the effects of interferons. Cells of the human K562 myeloid line do not express MNDA and are relatively immature compared to lines that express MNDA (HL-60, U937, and THP1). The hypothesis that MNDA influences the expression of specific genes was tested by creating MNDA expressing K562 cells using stable retroviral mediated gene transfer followed by evaluation of transcription profiles. Two macroarrays containing a total of 2,350 cDNAs of known genes showed a specific up-regulation of Dlk1 expression in MNDA expressing K562 cell clones. Real time quantitative RT-PCR analysis confirmed an average of over 3- and 7-fold upregulation of Dlk1 in two clones of MNDA expressing K562 cells. The effects on Dlk1 were also confirmed by Northern blotting. Dlk1 is essential for normal hematopoiesis and abnormal expression is a proposed marker of myelodysplastic syndrome. Additional screening of transcription profiles after induced erythroid and megakaryoblastic differentiation showed no additional gene transcripts altered by the presence of MNDA. These results indicate that MNDA alters expression of a gene essential for normal hematopoiesis.

The increase in levels of interferon-inducible proteins p202a and p202b and RNA-dependent protein kinase (PKR) during myoblast differentiation is due to transactivation by MyoD: their tissue distribution in uninfected mice does not depend on interferons

The murine 200 family proteins p202a, p202b, and p204, and also RNA-dependent protein kinase (PKR) are inducible by interferons (IFNs). p202a, p202b, and p204 modulate the activity of a large variety of transcription factors and also are involved in muscle differentiation. PKR is a multifunctional serine/threonine kinase, which is involved in antiviral defense and cell growth control and in the response to various stress signals. We reported earlier that the level of p204 increases during cultured C2C12 myoblast differentiation to myotubes in consequence of transactivation by the skeletal muscle-specific MyoD protein. The levels of p202a, p202b, and PKR also increase during the differentiation. We report here that these increased protein levels also are due to the transactivation of their genes by MyoD. This is made possible by the occurrence in each of these genes of at least six E boxes, which are recognition sites for MyoD. We also show that the distribution of the p204, p202a, p202b, and PKR proteins in five tissues of adult C129 mice is the same in wild-type mice and mice lacking the IFN-alpha, IFN-beta, and IFN-gamma receptors. This indicates that the synthesis and distribution of these proteins in uninfected adult mice are not affected by endogenous IFNs.

The MyoD-inducible p204 protein overcomes the inhibition of myoblast differentiation by Id proteins

The murine p204 protein level is highest in heart and skeletal muscle. During the fusion of cultured myoblasts to myotubes, the p204 level increases due to transcription dependent on the muscle-specific MyoD protein, and p204 is phosphorylated and translocated from the nucleus to the cytoplasm. p204 overexpression accelerates myoblast fusion in differentiation medium and triggers this process even in growth medium. Here we report that p204 is required for the differentiation of C2C12 myoblasts. We propose that it enables the differentiation, at least in part, by overcoming the inhibition of the activities of the MyoD and E47 proteins by the Id proteins: Id1, Id2, and Id3. These are known to inhibit skeletal muscle differentiation by binding and blocking the activity of MyoD, E12/E47, and other myogenic basic helix-loop-helix (bHLH) proteins. Our hypothesis is based on the following findings. (i) A decrease in the p204 level in C2C12 myoblasts by antisense RNA (a) increased the level of the Id2; (b) inhibited the MyoD-, E12/E47-, and other bHLH protein-dependent accumulation of the muscle-specific myosin heavy-chain protein; and (c) inhibited the fusion of myoblasts to myotubes in differentiation medium. (ii) p204 bound to the Id proteins in vitro and in vivo. (iii) In the binding of p204 to Id2, the b segment of p204 and the HLH segment of Id2 were involved. (iv) Addition of p204 overcame the inhibition by the Id proteins of the binding of MyoD and E47 to DNA in vitro. (v) Overexpression of p204 in myoblasts (a) decreased the level of the Id proteins, even in a culture in growth medium, and (b) overcame the inhibition by the Id proteins of MyoD- and E47 dependent transcription and also overcame the inhibition by Id2 of the fusion of myoblasts to myotubes.

Viruses and interferons

The interferon system is the first line of defense against viral infection in mammals. This system is designed to block the spread of virus infection in the body, sometimes at the expense of accelerating the death of the infected cells. As expected of potent cytokines, in addition to their antiviral effects, interferons have profound effects on many aspects of cell physiology. All these actions of interferons are mediated by hundreds of interferon-induced proteins that are usually not synthesized in resting cells. Interferons induce their synthesis by activating the Jak-STAT pathways, a paradigm of cell signaling used by many cytokines and growth factors. Surprisingly, some of the same genes can also be induced directly by viruses and double-stranded RNA, a common viral by-product. Some of the interferon-induced proteins have novel biochemical properties and some are inactive as such but can be activated by double-stranded RNA produced during virus infection. Finally, almost all viruses have evolved mechanisms to evade the interferon system by partially blocking interferon synthesis or interferon action. Thus, in nature interferons and viruses maintain an equilibrium that allows regulated viral replication.

p202, an interferon-inducible negative regulator of cell growth, is a target of the adenovirus E1A protein

Studies have revealed that human adenovirus-encoded E1A protein promotes cell proliferation through the targeted interaction with cellular proteins that act as key negative regulators of cell growth. The targets of E1A protein include the retinoblastoma tumor suppressor protein (pRb). Because p202, an interferon (IFN)-inducible murine protein (52-kDa), negatively regulates cell growth in part through the pRb/E2F pathway, we tested whether the p202 is a target of the adenovirus-encoded E1A protein for functional inactivation. Here we report that the expression of E1A protein overcame p202-mediated inhibition of cell growth and this correlated with an alleviation of p202-mediated inhibition of the transcriptional activity of E2F. Furthermore, E1A protein relieved p202-mediated inhibition of the specific DNA-binding activity of E2F complexes, including those containing the pocket proteins. Additionally, the E1A protein bound to p202 both in vitro and in vivo and a deletion of four amino acids in the conserved region 2 (CR2) of E1A protein significantly reduced the binding of E1A to p202. Interestingly, ectopic expression of p202 under reduced serum conditions significantly reduced E1A-mediated apoptosis. Taken together, our observations provide support to the idea that the p202 and adenovirus E1A protein functionally counteract each other and E1A protein targets p202 to promote cell proliferation.

Antiviral actions of interferons

Tremendous progress has been made in understanding the molecular basis of the antiviral actions of interferons (IFNs), as well as strategies evolved by viruses to antagonize the actions of IFNs. Furthermore, advances made while elucidating the IFN system have contributed significantly to our understanding in multiple areas of virology and molecular cell biology, ranging from pathways of signal transduction to the biochemical mechanisms of transcriptional and translational control to the molecular basis of viral pathogenesis. IFNs are approved therapeutics and have moved from the basic research laboratory to the clinic. Among the IFN-induced proteins important in the antiviral actions of IFNs are the RNA-dependent protein kinase (PKR), the 2',5'-oligoadenylate synthetase (OAS) and RNase L, and the Mx protein GTPases. Double-stranded RNA plays a central role in modulating protein phosphorylation and RNA degradation catalyzed by the IFN-inducible PKR kinase and the 2'-5'-oligoadenylate-dependent RNase L, respectively, and also in RNA editing by the IFN-inducible RNA-specific adenosine deaminase (ADAR1). IFN also induces a form of inducible nitric oxide synthase (iNOS2) and the major histocompatibility complex class I and II proteins, all of which play important roles in immune response to infections. Several additional genes whose expression profiles are altered in response to IFN treatment and virus infection have been identified by microarray analyses. The availability of cDNA and genomic clones for many of the components of the IFN system, including IFN-alpha, IFN-beta, and IFN-gamma, their receptors, Jak and Stat and IRF signal transduction components, and proteins such as PKR, 2',5'-OAS, Mx, and ADAR, whose expression is regulated by IFNs, has permitted the generation of mutant proteins, cells that overexpress different forms of the proteins, and animals in which their expression has been disrupted by targeted gene disruption. The use of these IFN system reagents, both in cell culture and in whole animals, continues to provide important contributions to our understanding of the virus-host interaction and cellular antiviral response.

The murine p202 protein, an IFN-inducible modulator of transcription, is activated by the mitogen platelet-derived growth factor

p202 is a murine interferon (IFN)-inducible protein belonging to a cluster of IFN-inducible genes (the 200 family) located in a segment of chromosome 1. It is a nuclear DNA-binding protein that is able to modulate transcription by interacting with a heterogeneous set of transcription factors, including NF-kappaB, (p50/p65), AP-1, c-fos, c-jun, and RB-1. The p202 protein is believed to attenuate cell growth/proliferation, mainly through the activation of IFN-stimulated of gene factor 3 (ISGF3), which binds IFN-stimulated response elements (ISRE) located in the promoters of type I IFN genes. In this report, we show that the p202 gene can also be induced by platelet-derived growth factor (PDGF), a mitogen known to drive G(0)-arrested cells toward reentry into the cell cycle. PDGF transiently enhances the steady-state mRNA level of p202 and increases the p202 protein level independently from IFN signaling, by acting at the transcriptional level on its promoter. The kinetics of p202 induction by PDGF are faster and more transient than those of IFN. These data identify p202 as a member of the IFN-inducible gene family that can be directly regulated by mitogenic stimuli.

Functional interaction between p53 and the interferon-inducible nucleoprotein IFI 16

Interferons are important in regulating cell growth and differentiation, immune function and initiating anti-viral responses. While the pleotrophic actions of interferons have been well documented, the molecular mechanisms underpinning their biological effects have not been fully characterized. IFI 16 is a member of the interferon-inducible HIN-200 family of nuclear proteins, which we have recently shown can function as a potent transcriptional repressor. A murine member of the HIN-200 family, p202, can indirectly interact with p53 via the p53 binding protein (p53bp) and inhibit p53-mediated transcriptional activation. The binding activity of p202 to p53bp was shown to require the conserved MFHATVAT motif present in all 200 amino acid repeat regions of HIN-200 proteins. Given that IFI 16 contains two MFHATVAT motifs, we sought to determine whether IFI 16 may form a complex with p53 and if so to ascertain the functional significance of this interaction. We demonstrate that IFI 16 can directly bind to the C-terminal region of p53 and augment p53-mediated transcriptional activation without altering the steady state levels of p53. Thus, in addition to its ability to directly regulate gene expression, IFI 16 can also modulate the transcription function of other cellular transcription factors. These findings demonstrate a possible link between gene induction following interferon stimulation and p53-mediated cellular events.

Regulation of muscle regulatory factors by DNA-binding, interacting proteins, and post-transcriptional modifications

Skeletal muscle differentiation is influenced by multiple pathways, which regulate the activity of myogenic regulatory factors (MRFs)-the myogenic basic helix-loop-helix proteins and the MEF2-family members-in positive or negative ways. Here we will review and discuss the network of signals that regulate MRF function during myocyte proliferation, differentiation, and post-mitotic growth. Elucidating the mechanisms governing muscle-specific transcription will provide important insight in better understanding the embryonic development of muscle at the molecular level and will have important implications in setting out strategies aimed at muscle regeneration. Since the activity of MRFs are compromised in tumors of myogenic derivation-the rhabdomyosarcomas-the studies summarized in this review can provide a useful tool to uncover the molecular basis underlying the formation of these tumors.

MyoD-dependent induction during myoblast differentiation of p204, a protein also inducible by interferon

p204, an interferon-inducible p200 family protein, inhibits rRNA synthesis in fibroblasts by blocking the binding of the upstream binding factor transcription factor to DNA. Here we report that among 10 adult mouse tissues tested, the level of p204 was highest in heart and skeletal muscles. In cultured C2C12 skeletal muscle myoblasts, p204 was nucleoplasmic and its level was low. During myoblast fusion this level strongly increased, p204 became phosphorylated, and the bulk of p204 appeared in the cytoplasm of the myotubes. Leptomycin B, an inhibitor of nuclear export that blocked myoblast fusion, inhibited the nuclear export signal-dependent translocation of p204 to the cytoplasm. The increase in the p204 level during myoblast fusion was a consequence of MyoD transcription factor binding to several MyoD-specific sequences in the gene encoding p204, followed by transcription. Overexpression of p204 (in C2C12 myoblasts carrying an inducible p204 expression plasmid) accelerated the fusion of myoblasts to myotubes in differentiation medium and induced the fusion even in growth medium. The level of p204 in mouse heart muscle strongly increased during differentiation; it was barely detectable in 10. 5-day-old embryos, reached the peak level in 16.5-day-old embryos, and remained high thereafter. p204 is the second p200 family protein (after p202a) found to be involved in muscle differentiation. (p202a was formerly designated p202. The new designation is due to the identification of a highly similar protein-p202b [H. Wang, G. Chatterjee, J. J. Meyer, C. J. Liu, N. A. Manjunath, P. Bray-Ward, and P. Lengyel, Genomics 60:281-294, 1999].) These results reveal that p204 and p202a function in both muscle differentiation and interferon action.

The retinoblastoma protein is an essential mediator that links the interferon-inducible 204 gene to cell-cycle regulation

We have previously demonstrated that overexpression of p204, a member of the Ifi 200 gene family, inhibits growth, delays G0/G1 progression into S phase, and impairs E2F-mediated transcriptional activity. In this study, we show that p204 directly binds the retinoblastoma protein (pRb) in vivo to exert its activity. Transient p204 overexpression in Rb+/+ mouse embryo fibroblasts (MEF) inhibits cell proliferation, but does not affect cell growth in MEF derived from Rb-/- mice. Two human cell lines, Saos2 and C33A, bearing an inactive pRb, but not primary human embryo fibroblasts, are resistant to the p204 antiproliferative activity. p204 contains two 200 amino acid motifs, designated as type a or b domains, each containing a canonical Rb binding motif (LXCXE). When dominant-negative mutants at the Rb binding motif were transfected in Rb+/+ MEF, p204 lost its ability to inhibit cell growth, delay cell transition from G1 to S phase, and impair DNA synthesis. Moreover p204 overexpression in Rb+/+ MEF led to a significant decrease of both DHFR and PCNA proteins, two S phase markers. By contrast, this effect was not observed when Rb+/+ MEF were transfected with a p204 mutated at both Rb binding sites. Finally, overexpression of the LXCXE p204 mutant rendered Rb+/+ MEF resistant to the IFN-alpha antiproliferative activity, in comparison to the untransfected Rb+/+ MEF. As expected, Rb-/- cells were unsensitive to the IFN-alpha induced growth inhibition. Taken as a whole, these results suggest that (i) p204 contributes to the IFN-alpha antiproliferative activity and (ii) the primary target of p204 leading to efficient G1 arrest as well as to blockade of DNA replication from G1 phase is the pRb regulatory system.

The 2'-5' oligoadenylate/RNase L/RNase L inhibitor pathway regulates both MyoD mRNA stability and muscle cell differentiation

The 2'-5' oligoadenylate (2-5A)/RNase L pathway is one of the enzymatic pathways induced by interferon. RNase L is a latent endoribonuclease which is activated by 2-5A and inhibited by a specific protein known as RLI (RNase L inhibitor). This system has an important role in regulating viral infection. Additionally, variations in RNase L activity have been observed during cell growth and differentiation but the significance of the 2-5A/RNase L/RLI pathway in these latter processes is not known. To determine the roles of RNase L and RLI in muscle differentiation, C2 mouse myoblasts were transfected with sense and antisense RLI cDNA constructs. Importantly, the overexpression of RLI in C2 cells was associated with diminished RNase L activity, an increased level of MyoD mRNA, and accelerated kinetics of muscle differentiation. Inversely, transfection of the RLI antisense construct was associated with increased RNase L activity, a diminished level of MyoD mRNA, and delayed differentiation. In agreement with these data, MyoD mRNA levels were also decreased in C2 cells transfected with an inducible RNase L construct. The effect of RNase L activity on MyoD mRNA levels was relatively specific because expression of several other mRNAs was not altered in C2 transfectants. Therefore, RNase L is directly involved in myoblast differentiation, probably through its role in regulating MyoD stability. This is the first identification of a potential mRNA target for RNase L.

Cytoplasmic localization of the interferon-inducible protein that is encoded by the AIM2 (absent in melanoma) gene from the 200-gene family

While interferons (IFNs) (alpha, beta and gamma), a family of cytokines, have the ability to exert the growth-inhibitory effect on target cells, the molecular mechanism(s) by which IFNs inhibit cell growth remains to be identified. Because IFN-inducible 'effector' proteins mediate the biological activities of IFNs, characterization of IFN-inducible proteins is critical to identify their functional role in IFN action. One family (the 200-family) of IFN-inducible proteins is encoded by structurally related murine (Ifi202a, Ifi202b, Ifi203, Ifi204 and D3) and human (IFI16, MNDA and AIM2) genes. The proteins encoded by genes in the family share a unique repeat of 200-amino acids and are primarily nuclear. The AIM2 gene is a newly identified gene that is not expressed in a human melanoma cell line. Here we report that AIM2 is estimated to be a 39 kDa protein and, unlike other proteins in the family, is localized primarily in the cytoplasm. Interestingly, overexpression of AIM2 in transfected cells retards proliferation and, under reduced serum conditions, increases the susceptibility to cell death. Moreover, AIM2 can heterodimerize with p202 in vitro. Together, these observations provide support to the idea that AIM2 may be an important mediator of IFN action.

The retinoblastoma-like protein p130 is involved in the determination of reserve cells in differentiating myoblasts

During skeletal muscle differentiation, a subset of myoblasts remains quiescent and undifferentiated but retains the capacity to self-renew and give rise to differentiating myoblasts [1] [2] [3]: this sub-population of muscle cells was recently termed 'reserve cells' [3]. In order to characterise genes that can regulate the ratio between reserve cells and differentiating myoblasts, we examined members of the retinoblastoma tumor suppressor family - Rb, p107 and p130 - an important family of negative regulators of E2F transcription factors and cell cycle progression [4]. Although pRb and p107 positively regulate muscle cell differentiation [5] [6] [7], the role of p130 in muscle cells remains unknown. We show here that p130 (protein and mRNA), but neither pRb nor p107, preferentially accumulates during muscle differentiation in reserve cells. Also, p130 is the major Rb-family protein present in E2F complexes in this sub-population of cells. Although forced expression of either p130 or pRb in mouse C2 myoblasts efficiently blocked cell cycle progression, only p130 inhibited the differentiation program. Furthermore, muscle cells overexpressing p130 had reduced levels of the muscle-promoting factor MyoD. In addition, p130 repressed the transactivation capacity of MyoD, an effect abolished by co-transfection of pRb. Thus, we propose that p130, by blocking cell cycle progression and differentiation, could be part of a specific pathway that defines a pool of reserve cells during terminal differentiation.

Novel functions of interferon-induced proteins

Interferons are important cytokines which regulate antiviral, cell growth, immune modulatory and anti-tumor functions. These pleiotropic effects of interferons are brought about by a large number of cellular proteins, the interferon-inducible proteins. Investigation of the biochemical and cellular activities of some of these proteins have revealed new pathways of regulation of cellular RNA and protein metabolism, growth and differentiation, apoptosis and signal transduction. In this article we discuss recent findings on the novel activities of a selected number of interferon-induced proteins.