Stimulation of T cells up-regulates expression of Ifi202, an interferon-inducible lupus susceptibility gene, through activation of JNK/c-Jun pathway

Studies have revealed that increased expression of interferon (IFN)-inducible Ifi202 gene (encoding p202 protein) in splenic B and T cells from B6.Nba2 congenic (congenic for Nb2 locus derived from NZB mice) female mice is associated with lupus susceptibility. However, signaling pathways that regulate Ifi202 expression in immune cells remain to be elucidated. Here we report that stimulation of T cells up-regulates the Ifi202 expression. We found that steady-state levels of Ifi202 mRNA and protein were detectable in splenic T cells from NZB mice and stimulation of T cells with anti-CD3 and anti-CD28 up-regulated expression of the Ifi202 gene. Similarly, stimulation of cells of a mouse T cell hybridoma cell line (2B4.11) also activated transcription of the Ifi202 gene. Significantly, up-regulation of Ifi202 expression in stimulated T cells was inhibited by treatment of cells with SP600125, a specific inhibitor of c-Jun N-terminal kinase (JNK). Conversely, treatment of cells with anisomycin, a potent activator of the JNK and c-Jun, up-regulated Ifi202 expression. Consistent with the activation of JNK/c-Jun pathway by T cell stimulation, forced expression of c-Jun in 2B4 T cells and in mouse embryonic fibroblasts (MEFs) also up-regulated the Ifi202 expression. Furthermore, we found that stimulation of T cells increased association of the activated c-Jun to the 5'-regulatory region of the Ifi202 gene in chromatin immunoprecipitation assays (ChIPs). Together, our observations demonstrate that stimulation of T cells up-regulates the Ifi202 expression in part through the JNK/c-Jun pathway.

Restoration of p53 expression in human cancer cell lines upregulates the expression of Notch1: implications for cancer cell fate determination after genotoxic stress

Following genotoxic stress, transcriptional activation of target genes by p53 tumor suppressor is critical in cell fate determination. Here we report that the restoration of p53 function in human cancer cell lines that are deficient in p53 function upregulated the expression of Notch1. Interestingly, the expression of wild-type p53 in human prostate and breast cancer cell lines correlated well with increased expression of Notch1. Furthermore, knockdown of p53 expression in cancer cells that express wild-type p53 resulted in reduced expression of Notch1. Importantly, genotoxic stress to cancer cells that resulted in activation of p53 also upregulated the expression of Notch1. Moreover, p53-mediated induction of Notch1 expression was associated with stimulation of the activity of Notch-responsive reporters. Notably, p53 differentially regulated the expression of Notch family members: expression of Notch2 and Notch4 was not induced by p53. Significantly, treatment of cells with gamma secretase inhibitor, an inhibitor of Notch signaling, increased susceptibility to apoptosis in response to genotoxic stress. Together, our observations suggest that p53-mediated upregulation of Notch1 expression in human cancer cell lines contributes to cell fate determination after genotoxic stress.

IFI16 in human prostate cancer

Increased expression of IFI16 protein (encoded by the IFI16 gene) in normal human prostate epithelial cells is associated with cellular senescence-associated cell growth arrest. Consistent with a role for IFI16 protein in cellular senescence, the expression of IFI16 protein is either very low or not detectable in human prostate cancer cell lines. We now report that treatment of DU-145 and LNCaP prostate cancer cell lines with histone deacetylase inhibitor trichostatin A (TSA) or CGK1026 resulted in transcriptional activation of the IFI16 gene. The induction of IFI16 protein in LNCaP cells was dependent on the duration of TSA treatment. Furthermore, TSA treatment of LNCaP cells up-regulated the expression of Janus-activated kinase 1 protein kinase and modulated the transcription of certain IFN-activatable genes. However, overexpression of exogenous Janus-activated kinase 1 protein in LNCaP cells and treatment of cells with IFNs (alpha and gamma) did not increase the expression of IFI16. Instead, the transcriptional activation of IFI16 gene by TSA treatment of LNCaP cells was dependent on transcriptional activation by c-Jun/activator protein-1 transcription factor. Importantly, increased expression of IFI16 in LNCaP cells was associated with decreases in the expression of androgen receptor and apoptosis of cells. Conversely, knockdown of IFI16 expression in TSA-treated LNCaP cells increased androgen receptor protein levels with concomitant decreases in apoptosis. Together, our observations provide support for the idea that histone deacetylase-dependent transcriptional silencing of the IFI16 gene in prostate epithelial cells contributes to the development of prostate cancer.

Increased expression of Ifi202, an IFN-activatable gene, in B6.Nba2 lupus susceptible mice inhibits p53-mediated apoptosis

Increased expression of p202 protein (encoded by the Ifi202 gene) in splenocytes derived from B6.Nba2 mice (congenic for the Nba2 interval derived from the New Zealand Black mice) was correlated with defects in apoptosis of splenic B cells and increased susceptibility to develop systemic lupus erythematosus. We have now investigated the molecular mechanisms by which increased expression of p202 in B6.Nba2 cells contributes to defects in apoptosis. In this study, we report that increased expression of p202 in the B6.Nba2 splenocytes, as compared with cells derived from the parental C57BL/6 (B6) mice, was correlated with increased levels of p53 protein and inhibition of p53-mediated transcription of target genes that encode proapoptotic proteins. Conversely, knockdown of p202 expression in B6.Nba2 cells resulted in stimulation of p53-mediated transcription. We found that p202 bound to p53 in the N-terminal region (aa 44-83) comprising the proline-rich region that is important for p53-mediated apoptosis. Consistent with the binding of p202 to p53, increased expression of p202 in B6.Nba2 mouse embryonic fibroblasts inhibited UV-induced apoptosis. Taken together, our observations support the idea that increased expression of p202 in B6.Nba2 mice increases the susceptibility to develop lupus, in part, by inhibiting p53-mediated apoptosis.

Androgen receptor levels are increased by interferons in human prostate stromal and epithelial cells

Proliferation of normal and malignant prostate epithelium is regulated by androgen stimulation via both the androgen receptor (AR)-positive stromal and epithelial cells. However, it is not known how AR expression is regulated in human prostate cells. We report that treatment of normal human prostate stromal cells (PrSCs) with type I IFN (alpha or beta), but not type II IFN (gamma), resulted in increased levels of AR protein. The maximal increase in AR protein levels was dependent on the dose and the duration of the IFN-alpha treatment. We found that the increase in AR protein levels was independent of de novo transcription and protein synthesis. Interestingly, the IFN-alpha treatment of PrSCs resulted in considerable nuclear accumulation of AR, stimulation of AR-mediated transcription of reporter genes, and retardation of cell proliferation. Furthermore, treatment of normal human prostate epithelial cells with IFNs (alpha, beta or gamma) also resulted in increased levels of AR protein. Together, our observations identify the androgen receptor as an IFN-regulated protein in normal human prostate stromal and epithelial cells and predict that IFN-induced levels of AR in prostate cells contribute to the regulation of androgen signaling.

DU-145 and PC-3 human prostate cancer cell lines express androgen receptor: Implications for the androgen receptor functions and regulation

The majority of human prostate cancer cell lines, including the two "classical" cell lines DU-145 and PC-3, are reported to be androgen receptor (AR)-negative. However, other studies have provided evidence that the DU-145 and PC-3 cell lines express AR mRNA. These contradictory observations prompted us to investigate whether DU-145 and PC-3 cell lines express the androgen receptor. Using antipeptide antibodies directed against three distinct regions of the human AR protein and an improved method to detect AR protein in immunoblotting, we report that DU-145 and PC-3 cell lines express AR protein. We found that the relative levels of the AR mRNA and protein that were detected in DU-145 and PC-3 cell lines were lower than the LNCaP, an AR-positive cell line. Moreover, the antibody directed against the non-variant region (amino acids 299-315), but not the variant N- or C-terminal region (amino acids 1-20 and 900-919, respectively) of the human AR protein, detected the expression of AR in all prostate cancer cell lines. Notably, treatment of these cell lines with dihydrotestosterone (DHT) resulted in measurable increases in the AR protein levels and considerable nuclear accumulation. Although, treatment of DU-145 and PC-3 cells with DHT did not result in stimulation of the activity of an AR-responsive reporter, knockdown of AR expression in PC-3 cells resulted in decreases in p21(CIP1) protein levels, and a measurable decrease in the activity of the p21-luc-reporter. Our observations demonstrate the expression of AR protein in DU-145 and PC-3 prostate cancer cell lines.

Androgen receptor auto-regulates its expression by a negative feedback loop through upregulation of IFI16 protein

Expression of androgen receptor (AR) in prostate epithelial cells is thought to regulate cell proliferation, differentiation, and survival. However, the molecular mechanisms remain unclear. We report that re-expression of AR in PC-3 human prostate cancer cell line resulted in upregulation of IFI16 protein, a negative regulator of cell growth. We found that the IFI16 protein bound to AR in a ligand-dependent manner and the DNA-binding domain (DBD) of the AR was sufficient to bind IFI16. Furthermore, re-expression of IFI16 protein in LNCaP prostate cancer cells, which do not express IFI16 protein, resulted in downregulation of AR expression and an inhibition of the expression of AR target genes. Our observations identify a role for IFI16 protein in AR-mediated functions.

The HIN domain of IFI-200 proteins consists of two OB folds

The interferon-inducible p200 (IFI-200/HIN-200) family of proteins regulates cell growth and differentiation, and confers resistance to the development of tumors and virus infections. IFI-200 family members are thought to exert their biological effects by modulation of the transcriptional activities of numerous factors and interaction with other proteins through the C-terminal HIN domains. However, the HIN domain structure and function have remained obscure. Therefore, we performed a comprehensive bioinformatics analysis and assembled a structure-based multiple sequence alignment of IFI-200 proteins. The application of fold recognition methods revealed that the HIN domain consists of two consecutive OB domains. Our structural models of DNA-binding HIN domains afford the long-sought interpretations for many previous experimental observations. Our results also raise the possibility of as yet unexplored functional roles of IFI-200 proteins as transcriptional regulators and as interaction partners of proteins involved in immunomodulatory and apoptotic processes.

Role of IFI 16 in cellular senescence of human fibroblasts

Defects in interferon (IFN) signaling that result in loss of expression of IFN-inducible proteins are associated with cellular immortalization, an important early event in the development of human cancer. Here we report that loss of IFN-inducible IFI 16 expression in human fibroblasts allows bypass of cellular senescence. We found that levels of IFI 16 mRNA and protein were higher in human old versus young fibroblasts and immortalization of fibroblasts with telomerase resulted in decreased expression of IFI 16. Moreover, overexpression of IFI 16 in immortalized fibroblasts strongly inhibited cell proliferation. Interestingly, knockdown of IFI 16 expression in fibroblasts inhibited p53-mediated transcription, downregulated p21(WAF1) expression, and extended the proliferation potential. Importantly, treatment of immortal cell lines with 5-aza-2'-deoxycytidine, an inhibitor of DNA methyltransferase, resulted in upregulation of IFI 16. Our observations support the idea that increased levels of IFI 16 in older populations of human fibroblasts contribute to cellular senescence.

Interleukin-6 Induces Expression of Ifi202, an Interferon-inducible Candidate Gene for Lupus Susceptibility

Systemic lupus erythematosus (SLE) is a prototype autoimmune disease. In human SLE patients, as well as in mouse models of SLE, the development of disease is associated with increased levels of pro-inflammatory cytokines, such as interleukin-6 (IL-6). However, IL-6 target genes contributing to the development of disease remain to be identified. Our previous studies of one mouse model of SLE identified an interferon-inducible gene, Ifi202, as a major contributor to the disease. We now report that IL-6 induces expression of the Ifi202 gene. We found that IL-6 treatment of mouse splenocytes increased levels of Ifi202 mRNA and p202 protein. Furthermore, IL-6 treatment of NIH 3T3 cells or expression of a constitutively active form of STAT3, a known mediator of IL-6 signaling, stimulated the activity of a 202-luc-reporter through a potential STAT3 DNA-binding site (the 202-SBS) present in the 5'-regulatory region of the Ifi202 gene. Moreover, treatment of cells with IL-6 stimulated binding of the transcription factor STAT3 to an oligonucleotide containing the 202-SBS in gel-mobility shift assays and to the 5'-regulatory region of the Ifi202 gene in chromatin immunoprecipitation assays. Importantly, site-directed mutagenesis of 202-SBS or expression of a dominant negative form of STAT3 significantly reduced constitutive as well as IL-6-stimulated activity of the 202-luc-reporter. Together, our observations support the idea that IL-6 stimulates transcription of the Ifi202 gene through STAT3 activation and predict that increased levels of IL-6 in lupus contribute to up-regulation of p202.

Resistance to UV-induced apoptosis in human keratinocytes during accelerated senescence is associated with functional inactivation of p53

Compared to proliferating keratinocytes (KCs), growth-arrested KCs are relatively resistant to UV-light induced apoptosis. When KCs undergo confluency, or following exposure to anti-proliferative agents such as IFN-gamma plus a phorbol ester-12-O-tetradecanoylyphorbol-13-acetate (TPA), they convert from a proliferative to a nonproliferative state resembling senescence. Since p53 regulates UV-induced apoptosis of KCs, this report further characterizes p53 half-life, post-translational modifications, and transcriptional activity using cultured human KCs and living epidermal equivalents. The half-life of p53 in KCs was longer than fibroblasts (greater than approximately 3 h vs. 30 min). Exposure of proliferating KCs to UV-light induces post-translational modifications of p53 including acetylation of lysine-382 residues. By contrast, KCs undergoing irreversible growth arrest following confluency, or exposure to IFN-gamma plus TPA, were resistant to UV-induced apoptosis, and failed to undergo the acetylation modification of p53. Exposure of KCs to IFN-gamma plus TPA reduced total cellular p53 levels and reduced the transcriptional activity of p53. Addition of Trichostatin A (TSA), an inhibitor of de-acetylation, increased acetylation of lysine-382 in confluent KCs, thereby enhancing susceptibility of confluent cultures to UV-induced apoptosis. Pre-treatment of epidermal equivalents with IFN-gamma plus TPA also blocked UV-light induced increase in p53 levels, and reduced apoptosis. In conclusion, these studies demonstrate that growth arrested KCs may resist UV-light induced apoptosis by inactivating the pro-apoptotic function of p53.

Subcellular localization and mechanisms of nucleocytoplasmic distribution of p202, an interferon-inducible candidate for lupus susceptibility

Increased expression of p202 (52 kDa), an interferon (IFN)-inducible murine protein, in splenic cells (B- and T-cells) derived from female mice of the lupus-prone strains is correlated with increased susceptibility to develop systemic lupus erythematosus. However, the molecular mechanisms remain unclear. Our previous studies have indicated that, in IFN-treated fibroblasts, p202 is detected both in the cytoplasm and in the nucleus. Moreover, in the cytoplasm, a fraction of p202 associates with a membranous organelle. Here we report that, in the cytoplasm, a fraction of p202 associated with mitochondria. Additionally, we found that the constitutive p202 is primarily detected in the cytoplasm. Remarkably, the IFN treatment of cells potentiated nuclear accumulation of p202. Our observations are consistent with the possibility that IFN signaling regulates p202 levels as well as its nucleocytoplasmic distribution. These observations will serve as a basis to elucidate the molecular mechanisms by which p202 contributes to lupus susceptibility.

Role of IFI 16, a member of the interferon-inducible p200-protein family, in prostate epithelial cellular senescence

Recent studies have implicated interferon signaling in the regulation of cellular senescence. However, the role of specific interferon-inducible proteins in cellular senescence remains to be defined. Here we report that IFI 16, an interferon-inducible transcriptional modulator from the p200-protein family, contributes to cellular senescence of prostate epithelial cells. Normal human prostate epithelial cells (PrEC) in culture expressed detectable levels of IFI 16, and the levels increased more than fourfold when cells approached cellular senescence. Consistent with a role of IFI 16 in cellular senescence, human prostate cancer cell lines either did not express IFI 16 or expressed a variant form, which was primarily detected in the cytoplasm of prostate cancer cells and not in the nucleus. Moreover, overexpression of functional IFI 16 in human prostate cancer cell lines inhibited colony formation. Additionally, ectopic expression of IFI 16 in clonal prostate cancer cell lines was associated with a senescence-like phenotype, production of senescence-associated beta-galactosidase (a biochemical marker for cellular senescence), and reduction of S-phase cells in culture. Importantly, upregulation of p21WAF1 and inhibition of E2F-stimulated transcription accompanied inhibition of cell growth by IFI 16 in prostate cancer cell lines. Collectively, our observations support the idea that increased levels of IFI 16 in PrECs contribute to senescence-associated irreversible cell growth arrest.

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.

IFN-gamma inhibits human airway smooth muscle cell proliferation by modulating the E2F-1/Rb pathway

Elucidating the factors that inhibit the increase in airway smooth muscle (ASM) mass may be of therapeutic benefit in asthma. Here, we investigated whether interferon-gamma (IFN-gamma), a potent inducer of growth arrest in various cell types, regulates mitogen-induced ASM cell proliferation. IFN-gamma (1-100 U/ml) was found to markedly decrease both DNA synthesis and ASM cell number induced by the mitogens epidermal growth factor (EGF) and thrombin. Interestingly, IFN-gamma had no effect on mitogen-induced activation of three major mitogenic signaling pathways, phosphatidylinositol 3-kinase, p70(S6k), or mitogen-activated protein kinases. Mitogen-induced expression of cell cycle regulator cyclin D1 was increased by IFN-gamma, whereas no effect was observed on degradation of p27(Kip1). Expression array analysis of 23 cell cycle-related genes showed that IFN-gamma inhibited EGF-induced increases in E2F-1 expression, whereas induction of c-myc, cyclin D2, Egr-1, and mdm2 were unaffected. Induction of E2F-1 protein and Rb hyperphosphorylation after mitogen stimulation was also suppressed by IFN-gamma. In addition, IFN-gamma decreased activation of cdk2 and expression of cyclin E, upstream signaling molecules responsible for Rb hyperphosphorylation in the late G1 phase. IFN-gamma also increased levels of IFI 16 protein, whose mouse homolog p202 has been associated with growth inhibition. Together, our data indicate that IFN-gamma is an effective inhibitor of ASM cell proliferation by blocking transition from G1-to-S phase by acting at two different levels: modulation of cdk2/cyclin E activation and inhibition of E2F-1 gene expression.

Type-I interferon receptor deficiency reduces lupus-like disease in NZB mice

Indirect evidence suggests that type-I interferons (IFN-alpha/beta) play a significant role in the pathogenesis of lupus. To directly examine the contribution of these pleiotropic molecules, we created congenic NZB mice lacking the alpha-chain of IFN-alpha/betaR, the common receptor for the multiple IFN-alpha/beta species. Compared with littermate controls, homozygous IFN-alpha/betaR-deleted NZB mice had significantly reduced anti-erythrocyte autoantibodies, erythroblastosis, hemolytic anemia, anti-DNA autoantibodies, kidney disease, and mortality. These reductions were intermediate in the heterozygous-deleted mice. The disease-ameliorating effects were accompanied by reductions in splenomegaly and in several immune cell subsets, including B-1 cells, the major producers of anti-erythrocyte autoantibodies. Decreases of B and T cell proliferation in vitro and in vivo, and of dendritic cell maturation and T cell stimulatory activity in vitro were also detected. Absence of signaling through the IFN-alpha/betaR, however, did not affect increased basal levels of the IFN-responsive p202 phosphoprotein, encoded by a polymorphic variant of the Ifi202 gene associated with the Nba2 predisposing locus in NZB mice. The data indicate that type-I IFNs are important mediators in the pathogenesis of murine lupus, and that reducing their activity in the human counterpart may be beneficial.

p38 isoforms have opposite effects on AP-1-dependent transcription through regulation of c-Jun. The determinant roles of the isoforms in the p38 MAPK signal specificity

p38 MAPK pathway signaling is known to participate in cell proliferation, apoptosis, and differentiation, in a manner dependent on the cellular context. The factors that determine the specific biological response in a given cell type, however, remain largely unknown. We report opposite effects of the p38 isoforms on regulation of AP-1-dependent activities by p38 activators MAPK kinase 6 (MKK6) and/or arsenite in human breast cancer cells. The p38beta isoform increases the activation of AP-1 transcriptional activities by MKK6 and/or arsenite, whereas p38gamma/p38delta inhibits or has no effect on the stimulation. The p38beta does so by increasing the levels of phosphorylated c-Jun, whereas the p38gamma and -delta isoforms may act by regulating the c-jun transcription. AP-1-dependent processes such as vitamin D receptor gene promoter activation and cellular proliferation were similarly activated by the p38beta or inhibited by the p38gamma and/or -delta isoforms. Whereas the human breast cancer cells express all four isoforms, mouse NIH 3T3 and EMT-6 cells express only some of the p38 family members, with p38beta higher in 3T3 cells but p38delta only detected in the EMT-6 line. Consistent with the positive and negative roles of p38beta and p38delta in AP-1 regulation, MKK6 stimulates AP-1-dependent transcription in NIH 3T3 but not EMT-6 cells. In support of a role of c-Jun regulation by p38 isoforms in determining AP-1 activity, the levels of endogenous c-Jun and its phosphorylated form on p38 activation are higher in NIH 3T3 cells. These results demonstrate the contrasting activities of the different p38 isoforms in transmitting the upstream signal to AP-1 and show that the expression profile of p38 isoforms determines whether the p38 signal pathway activates or inhibits AP-1-dependent processes.

Role of INK4a/Arf locus-encoded senescent checkpoints activated in normal and psoriatic keratinocytes

During malignant transformation in skin, epidermal keratinocytes (KCs) frequently acquire the capacity to by-pass cellular senescence, a response that normally limits their unrestricted proliferation. Despite growing interest in the role for senescence during aging of skin and cutaneous carcinogenesis, little is known regarding regulation of three proteins encoded by the INK4a/ARF locus (p12, p14(ARF), p16) in KCs. In this study, several molecular pathways are explored using cultured KCs and KCs freshly isolated from psoriatic plaques. p16 and p14(ARF) are predominantly expressed spontaneously when foreskin-derived early-passage KCs undergo confluency-induced premature senescence. Induction of p14(ARF) on confluency occurred with low E2F-1 levels. Suspension of KCs in methylcellulose induced p12 expression. Addition of various cytokines (interferon-gamma, tumor necrosis factor-alpha) or a phorbol ester [12-O-tetradecanoylphorbol-13-acetate (TPA)] only induced p16, but not p14(ARF). Confluent KCs up-regulated Ras activity and the downstream signaling involving ERK. Addition of MAPK inhibitor blocked cytokine and TPA-induced p16 expression. Confluency and interferon-gamma induced premature senescence and p16 expression was linked to induction of the transcription factor Egr-1. KCs derived from chronic psoriatic plaques were characterized by enhanced p16, p14(ARF), and p12 expression accompanied by elevated Egr-1 levels. These results demonstrate that multiple and highly divergent stimuli can trigger the senescent checkpoint in human KCs with differential regulation of p16, p14(ARF), and p12. Although abnormal mitogenic signaling by oncogenic Ras is generally cited as being responsible for induction of premature senescence, our findings indicate that a broader perspective is warranted, to include confluency and cytokine-/TPA-induced pathways for KCs.

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.

Regulation of apoptosis by p53 in UV-irradiated human epidermis, psoriatic plaques and senescent keratinocytes

The carcinogenic effects of sunlight in human epidermis may be thwarted by either: transient growth arrest and repair of DNA photodamage in keratinocytes (KCs); elimination of KCs with damaged DNA via apoptosis; or by stimulating a senescence switch whereby KCs become irreversibly growth arrested. Using normal human skin organ cultures and living epidermal equivalents, we demonstrate that in the proliferative basal layer, removal of KCs via apoptosis had a rapid onset (beginning within 2 h) following UV-light exposure generating progressively greater numbers of KCs with thymine dimers as the dose of UV-light was increased; involved induction of Apaf-1, activation of caspase-3, and was dependent on p53 activation as addition of a p53 chemical inhibitor blocked the apoptotic response. Suprabasal layer KCs underwent apoptosis at much later time points (>8 h). KCs in the basal layer repaired DNA damage more rapidly than KCs in suprabasal layers. Steady state levels of p53 increased in irradiated cells, and the increase was accompanied by phosphorylation of serine 9 and serine 15, but not serine 6 residues. By contrast, cultured KCs undergoing spontaneous replicative senescence were resistant to UV-induced apoptosis. Senescent KCs constitutively contained low levels of p53, which were neither increased nor phosphorylated or acetylated after UV-exposure and possessed minimal DNA binding activity, indicative of functional inactivation. Furthermore, treatment of senescent KCs with DNA damaging agent adriamycin did not result in activation of latent p53 or apoptosis. When KCs within psoriatic plaques were examined, they resembled senescent KCs in that they expressed p53, which was not phosphorylated or acetylated. Thus, UV-light induces DNA damage in human epidermal KCs triggering p53 activation, and subsequent apoptosis involving distinct cell layers and kinetics. However, the lack of p53 activation as seen in senescent KCs and psoriatic plaques, is associated with a relative resistance of KCs to UV-induced apoptosis. In conclusion, the sensitivity and resistance of KCs to apoptosis depends not only on the location within various layers of epidermis and levels of p53, but may also involve p53 activation via post-translational modifications.

Interferon-inducible p202 in the susceptibility to systemic lupus

Systemic lupus erythematosus (SLE) is the prototype systemic autoimmune disease, which has potential to involve multiple organ systems. Studies in human SLE patients and murine models of lupus have indicated that genetic predisposition plays a crucial role in the development of this disease. To identify the genetic basis of human lupus and to understand the molecular mechanisms, mouse models of SLE have been studied. Generation of mice congenic for the Nba2 locus on the C57BL/6 genetic background, coupled with gene expression profiling, recently identified the interferon-activatable Ifi202 gene (encodes the protein p202) as a candidate lupus-susceptibility gene. The protein p202 is a member of the 200-protein family. The family includes structurally and functionally related mouse and human interferon-inducible proteins. The protein p202 (52-kDa) is a relatively well-characterized phosphoprotein in the family with demonstrated ability to control cell-signaling pathways regulating cell proliferation, survival, and differentiation. Here, we review what is known about the gene Ifi202 and the protein it encodes. Moreover, we discuss how an understanding of the role of p202 in cell growth regulation, particularly in cells of the immune system, will help elucidate the molecular mechanisms contributing to the development of lupus.

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.

Evidence for an interferon-inducible gene, Ifi202, in the susceptibility to systemic lupus

The Nba2 locus is a major genetic contribution to disease susceptibility in the (NZB x NZW)F(1) mouse model of systemic lupus. We generated C57BL/6 mice congenic for this NZB locus, and these mice produced antinuclear autoantibodies characteristic of lupus. F(1) offspring of congenic and NZW mice developed high autoantibody levels and severe lupus nephritis similar to (NZB x NZW)F(1) mice. Expression profiling with oligonucleotide microarrays revealed only two differentially expressed genes, interferon-inducible genes Ifi202 and Ifi203, in congenic versus control mice, and both were within the Nba2 interval. Quantitative PCR localized increased Ifi202 expression to splenic B cells and non-T/non-B cells. These results, together with analyses of promoter region polymorphisms, strain distribution of expression, and effects on cell proliferation and apoptosis, implicate Ifi202 as a candidate gene for lupus.

Abnormal NF-kappaB signaling pathway with enhanced susceptibility to apoptosis in immortalized keratinocytes

The transcriptional activation and proper regulation of NF-kappaB is known to be important to the apoptotic resistant phenotype of epidermal-derived keratinocytes. By comparing and contrasting the responses of normal foreskin-derived keratinocytes versus an immortalized skin-derived keratinocyte cell line (i.e. HaCaT cells), several molecular defects involving NF-kappaB signaling pathway were delineated in the immortalized keratinocytes. While exposure to IFN-gamma plus TPA produces growth arrest in both normal and immortalized keratinocytes, with rapid phosphorylation of MEKKI and recruitment of distinctive protein kinase C isoforms into the signalosome complex, subsequent molecular events necessary for NF-kappaB activation were abnormal in HaCaT cells. This disrupted NF-kappaB activation in HaCaT cells was accompanied by enhanced susceptibility to UV-light induced apoptosis, which was associated with elevated levels of E2F-1 and decreased TRAF1/TRAF2 levels. Additional defects in HaCaT cells included markedly diminished levels of IKKbeta (and lack of induction of kinase activity) in response to inflammatory stimuli, a failure of p21(WAF1/CIP1) to associate with CDK2, and a decreased association between p65 and p300. These studies suggest caution in using HaCaT cells as a substitute for normal keratinocytes to study apoptosis in the skin. Thus, it appears that while the immortalized cells can escape cell cycle checkpoints by elevated levels of E2F-1, an adverse biological consequence of such dysregulated cell cycle control is the inability to activate the anti-apoptotic NF-kappaB signaling pathway. Therefore, exploiting this apoptosis vulnerability in pre-malignant, or immortalized cells, prior to acquiring a death-defying phenotype characteristic of more advanced malignant cell types, provides the basis for an early interventional therapeutic strategy for cutaneous oncologists.

The gene encoding p202, an interferon-inducible negative regulator of the p53 tumor suppressor, is a target of p53-mediated transcriptional repression

The p53 tumor suppressor protein regulates the transcription of regulatory genes involved in cell cycle arrest and apoptosis. We reported previously that overexpression of p202, an interferon-inducible negative regulator of cell growth, negatively regulates the transcriptional activity of p53. Now we identify the gene encoding p202 as one whose mRNA and protein expression decrease in cells following the expression of wild-type, but not mutant, p53. Furthermore, the levels of p202 also decrease after exposure of cells to ultra violet light, which correlate with increase in the levels of p53. We report that the sequence-specific DNA binding of p53 to the 5'-regulatory region of the 202 gene contributes to the transcriptional repression of the 202 gene. Interestingly, overexpression of p202 in cells induced to undergo p53-dependent apoptosis significantly delays this process, indicating that the negative regulation of the 202 gene by wild-type p53 is important to potentiate apoptosis.

Differential induction of the 200-family proteins in Daudi Burkitt's lymphoma cells by interferon-alpha

Interferon (IFN)-inducible "effector" proteins mediate the biological activities of the IFNs. Therefore, the identification of the functional role(s) of IFN inducible proteins in IFN action is important to elucidate the molecular mechanisms by which IFNs inhibit cell growth. One family (the "200-family") of IFN-inducible proteins includes structurally related murine (p202a, p202b, p203, p204 and D3) and human (MNDA, IFI-16 and AIM2) proteins. However, their role in IFN action remains to be established. Here we report that IFN-alpha treatment of Daudi Burkitt's lymphoma cells resulted in differential induction of MNDA, IFI 16, and a p202-related protein (p202RP). Interestingly, IFN induction of p202RP preceded the induction of MNDA and IFI 16 proteins and the growth inhibition by IFN. Additionally, the induction of these proteins by IFN was accompanied by: (i) a transient increase in p21(WAF1/CIP1) levels; (ii) an increase in the functional form of pRb and p130; (iii) an inhibition of the sequence-specific DNA binding activity of E2F complexes; and (iv) a marked decrease in c-Myc levels. Our observations reported herein provide support to the hypothesis that IFN-inducible p202RP and MNDA proteins from the 200-family contribute to the growth inhibitory activities of the IFNs.

p202 levels are negatively regulated by serum growth factors

p202 is an IFN-inducible phosphoprotein (Mr 52,000) whose expression in transfected cells retards proliferation. Interestingly, the reduced levels of p202 in fibroblasts (in consequence of the expression of antisense to 202 RNA), under reduced serum conditions, increase the susceptibility of cells to apoptosis. To identify the functional role of p202 in cell growth regulation, we tested whether serum growth factor levels in the culture medium affect p202 levels. Here we report that, under reduced serum conditions, the p202 levels were increased in fibroblasts, and the increase was seen at both the mRNA and protein levels. Moreover, an increase in p202 levels was correlated with cell growth arrest in the G1 phase of the cell cycle. Interestingly, the presence of platelet-derived growth factor AB, basic fibroblast growth factor, or transforming growth factor beta1 in the culture medium abrogated the increase in p202 levels seen under reduced serum conditions. We found that the increase in p202 levels was accompanied by an increase in JunD/activation protein 1(AP-1) levels, and transfection of a JunD-encoding plasmid along with a reporter plasmid in which transcription of the reporter gene (luciferase) was driven by the 5'-regulatory region of the 202 gene resulted in an increase in the activity of luciferase. Additionally, stable overexpression of JunD in cells, under reduced serum conditions, also resulted in an increase in p202 levels. Interestingly, one of the AP-1-like DNA-binding sequences present in the 5'-regulatory region of the 202 gene could selectively bind to the JunD/AP-1 transcription factor. Taken together, our observations reported herein suggest that in fibroblasts, under reduced serum conditions, the increased levels of JunD/AP-1 contribute to the transcriptional up-regulation of p202 levels, which may be important for the regulation of apoptosis.

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.

Role of NF-kappaB in the apoptotic-resistant phenotype of keratinocytes

Several studies point to a role for NF-kappaB in modulating epidermal thickness and apoptotic susceptibility of keratinocytes. When phorbol esters such as 12-O-tetradecanoylphorbol-13-acetate (TPA) are topically applied, prominent epidermal thickening occurs, and exposure to interferon (IFN)-gamma promotes increased epidermal thickness producing psoriatic lesions. While keratinocytes derived from psoriatic plaque resist apoptosis, and combination of TPA and IFN-gamma activates NF-kappaB, the molecular mechanism linking NF-kappaB activation and keratinocyte apoptosis resistance was unknown. Therefore, we examined the ability of IFN-gamma plus TPA to influence NF-kappaB activity, gene expression, and response to UV light-induced apoptosis. These responses in normal keratinocytes were compared with immortalized keratinocytes (HaCaT cells). Exposure of normal keratinocytes to IFN-gamma plus TPA produced a synergistic activation of NF-kappaB, compared with when each reagent was used individually. Normal keratinocytes when exposed to IFN-gamma plus TPA acquired a resistance to UV light-induced apoptosis, which was dependent on NF-kappaB because expression of a dominant negative form of IkappaBalpha overcame the resistance. Compared with normal keratinocytes, HaCaT cells have a dysfunctional constitutive NF-kappaB signaling pathway not induced by IFN-gamma and TPA, rendering HaCaT cells highly susceptible to UV-induced apoptosis. Thus, immortalized HaCaT cells have an abnormal constitutive and dysfunctional NF-kappaB signaling system. These results provide evidence that activation and proper regulation of NF-kappaB is essential for acquisition of an apoptotic-resistant phenotype for epidermal-derived keratinocytes.

Apoptosis in proliferating, senescent, and immortalized keratinocytes

Skin provides an attractive organ system for exploring coordinated regulation of keratinocyte (KC) proliferation, differentiation, senescence, and apoptosis. Our main objective was to determine whether various types of cell cycle arrest confer resistance to apoptosis. We postulated that KC cell cycle and cell death programs are tightly regulated to ensure epidermal homeostasis. In this report, simultaneous expression of cyclin-dependent kinase inhibitors (p15, p16, p21, and p27), a marker of early differentiation (keratin 1), mediators of apoptosis (caspases 3 and 8), and NF-kappaB were analyzed in three types of KCs. By comparing the response of proliferating, senescent, and immortalized KCs (HaCaT cells) to antiproliferative agents followed by UV exposure, we observed: 1) Normal KCs follow different pathways to abrupt cell cycle arrest; 2) KCs undergoing spontaneous replicative senescence or confluency predominantly express p16; 3) Abruptly induced growth arrest, confluency, and senescence pathways are associated with resistance to apoptosis; 4) The death-defying phenotype of KCs does not require early differentiation; 5) NF-kappaB is one regulator of resistance to apoptosis; and 6) HaCaT cells have undetectable p16 protein (hypermethylation of the promoter), dysfunctional NF-kappaB, and diminished capacity to respond to antiproliferative treatments, and they remain highly sensitive to apoptosis with cleavage of caspases 3 and 8. These data indicate that KCs (but not HaCaT cells) undergoing abruptly induced cell cycle arrest or senescence become resistant to apoptosis requiring properly regulated activation of NF-kappaB but not early differentiation.

Retardation of cell proliferation after expression of p202 accompanies an increase in p21(WAF1/CIP1)

p202 is an IFN-inducible, primarily nuclear, phosphoprotein (52-kDa) whose constitutive overexpression in transfected cells inhibits colony formation. To investigate the molecular mechanism(s) by which expression of p202 protein impairs colony formation, we established stable cell lines that inducibly express p202. Using this cell model, we demonstrate that the induced expression of p202 in asynchronous cultures of these cells was accompanied by: (a) an increase in steady-state levels of p21(WAF1/CIP1/SDI1) (p21) mRNA and protein; (b) a decrease in Cdk2 protein kinase activity; and (c) an increase in the functional form of retinoblastoma protein (pRb). Transient transfection of a p202-encoding plasmid in Saos-2 cells, which do not harbor a wild-type p53 protein, resulted in an increase in p21 protein, which indicated that p202 could regulate expression of p21 protein independent of p53 protein. Moreover, we demonstrate that expression of p202 in these cells increased cell doubling time without accumulation of cells in a particular phase of the cell cycle. Taken together, these results are consistent with the possibility that p202 protein contributes to the cell growth retardation activity of the IFNs, at least in part, by modulating p21 protein levels.

Reduced growth rate and transformation phenotype of the prostate cancer cells by an interferon-inducible protein, p202

Interferons (IFNs) can exert cytostatic and immunomodulatory effects on carcinoma cells. In particular, growth inhibition of human prostate carcinoma by IFNs has been demonstrated both in vitro and in vivo. p202 is a 52 kd nuclear phosphoprotein known to be induced by IFNs. In this report, we showed that the expression of p202 was associated with an anti-proliferative effect on human prostate cancer cells. More importantly, cells that expressed p202 showed reduced ability to grow in soft-agar, indicating a loss of transformation phenotype. Our data suggest that p202 is a growth inhibitor gene in prostate cancer cells and its expression may also suppress transformation phenotype of prostate cancer cells.

p202 self-associates through a sequence conserved among the members of the 200-family proteins

Murine p202 is an interferon-inducible primarily nuclear phosphoprotein (52 kDa) whose expression in transfected cells inhibits colony formation. p202-binding proteins include the pocket proteins (pRb, p107 and p130), a p53-binding protein (sm53BP1), and transcription factors (e.g. NF-kappaB (p50 and p65), AP-1 (c-Fos and c-Jun), E2F-1, E2F-4, MyoD, and myogenin). p202 modulates the transcriptional activity of these factors in transfected cells. Here we demonstrate that p202 self-associates directly and a sequence in p202, which is conserved among the members of the 200-family proteins, was sufficient for self-association in vitro. Our observations reported herein raise the possibility that self-association of p202 may provide a mechanism for the regulation of its activity.

p202 prevents apoptosis in murine AKR-2B fibroblasts

p202 is an interferon (IFN)-inducible, primarily nuclear, phosphoprotein (52-kDa) whose overexpression in transfected cells inhibits colony formation. p202 binds to the retinoblastoma tumor suppressor protein and two other members of the pocket family proteins (p107 and p130). Moreover, overexpression of p202 in transfected cells inhibits the transcriptional activity of E2Fs (E2F-1/DP-1 and E2F-4/DP-1), p53, AP-1 c-Fos and c-Jun, NF-kappaB p50 and p65. Here we demonstrate that inhibition of endogenous p202 production in murine AKR-2B fibroblasts did not result in an increase in cell proliferation. Instead, these cells exhibited increased susceptibility to apoptosis in response to decrease in serum concentrations in the growth medium. These observations are consistent with the notion that normal levels of p202 may be needed for the regulation of cell proliferation.

Inhibition of E2F-4/DP-1-stimulated transcription by p202

The interferon (IFN)-inducible proteins mediate activities of the interferons including the cell growth-regulatory activity. We have shown that p202, an IFN-inducible 52kDa primarily nuclear phosphoprotein whose expression in transfected cells inhibits cell proliferation, interacts with the retinoblastoma tumor suppressor protein (pRb) and the transcription factor E2F (E2F-1/ DP-1) in vitro and in vivo. p202 was shown to inhibit E2F-1/DP-1-stimulated transcription of a reporter gene and of endogenous genes. Here we report that expression of p202 inhibited E2F-4/DP-1-stimulated transcription of a reporter gene in transfected cells. Furthermore, this inhibition was associated with the inhibition of the sequence-specific DNA-binding of E2F-4 both in complex with the pocket proteins p107 or p130 and in its 'free' form in vitro. p202 bound to p107 and p130 in vitro and in vivo and also associated with E2F-4, supporting the notion that complexes containing p107/E2F-4 or p130/ E2F-4 and p202 exist in vivo. Moreover, cotransfection of E2F-4-encoding plasmid in AKR-2B cells overcame p202-mediated inhibition of cell growth, raising the possibility that p202 contributes to cell growth inhibition by the interferons, at least in part, by modulating E2F-4-mediated transcription.

p202, an interferon-inducible modulator of transcription, inhibits transcriptional activation by the p53 tumor suppressor protein, and a segment from the p53-binding protein 1 that binds to p202 overcomes this inhibition

p202, an interferon-inducible murine protein, is a member of the "200 family" of proteins and is primarily nuclear. p202 is a modulator of transcription; it binds several transcription factors, including NF-kappaB p50 and p65, AP-1 c-Fos and c-Jun, and E2F1, and inhibits their transcriptional activity. p202 also binds pRb, the retinoblastoma protein, and if overexpressed it retards cell proliferation. Here we report that using the yeast two-hybrid assay we found that p202 bound the murine homolog of the human p53-binding protein 1 (53BP1), a protein shown to interact with the DNA binding domain of the p53 tumor suppressor protein. p202 bound a 98amino acid segment from 53BP1. This binding was inhibited by the replacement in p202 of a histidine (from the M(F/L)HATVA(T/S) sequence that is conserved among all of the 200 family proteins) by phenylalanine. We also report that overexpression of p202 inhibited the p53-dependent expression of reporter genes containing p53-activable segments from the mdm2 and p21 genes, whereas a decrease in the p202 level (in consequence of the expression of 202 antisense RNA) resulted in an increase in the p53-dependent expression of these reporters. Expression of the 53BP1 segment binding to p202 overcame the inhibition by overexpressed p202 of the transcription of reporters mediated by the p53 or the AP-1 transcription factors and of the proliferation of yeast.

Inhibition of E2F-mediated transcription by p202

Many of the antimicrobial, immunomodulatory and cell growth inhibitory activities of the interferons are mediated by interferon-inducible proteins. Earlier we characterized an interferon-inducible murine protein, p202, whose expression in transfected cells inhibits cell proliferation and which can form a complex with retinoblastoma protein (pRb). Here we report that in transfected cells expression of p202 inhibits E2F-stimulated transcription of a reporter gene and of endogenous genes. Inhibition of the transcriptional activity of E2F by p202 does not depend on fully functional pRb and is correlated with inhibition of the sequence-specific DNA binding of E2F. p202 interacts with the transcription factor E2F (E2F-1/DP-1) in vitro and in vivo. Inhibition of E2F activity by p202 may contribute to growth inhibition by the interferons.

The interferon-inducible growth-inhibitory p202 protein: DNA binding properties and identification of a DNA binding domain

p202 is an interferon-inducible protein whose expression in transfected cells inhibits proliferation. p202 binds to the retinoblastoma tumor suppressor protein in vitro and in vivo and the transcription factors AP-1 c-Fos and c-Jun, NF-kappaB p50 and p65, and inhibits the transcriptional activity of these factors in vivo. Here we report that p202 nonspecifically binds to double-stranded DNA and to single-stranded DNA in vitro. Analysis with recombinant p202 revealed that DNA binding activity is intrinsic to p202. A C-terminal deletion mutant of p202 exhibited DNA-binding properties, indicating that the C-terminus is dispensable for DNA binding. We also found that underphosphorylated p202 efficiently binds to DNA. Our data suggest that DNA binding activity of p202 may contribute to its functions.

Binding of an interferon-inducible protein (p202) to the retinoblastoma protein

Many of the antimicrobial, immunomodulatory, and cell growth regulatory activities of the interferons are mediated by interferon-inducible proteins. One family of such murine proteins is encoded by six or more adjacent and structurally related genes (gene 200 cluster). Two homologous human genes have also been reported. p202, encoded by the Ifi202 gene in the gene 200 cluster, is a 52-kDa nuclear phosphoprotein. Constitutive overexpression of p202 in transfected cells is growth-inhibitory. We report here that p202 binds the cell growth regulatory retinoblastoma protein (pRb) in vitro and in vivo. The binding is due to direct interaction between the two proteins. p202 has two nonoverlapping segments for binding pRb, and pRb has two nonoverlapping segments (one of them including the pocket region) for binding p202. The hypophosphorylated form of pRb binds to p202, p202 is the first interferon-inducible protein found to bind pRb.

Interferon action: cytoplasmic and nuclear localization of the interferon-inducible 52-kD protein that is encoded by the Ifi 200 gene from the gene 200 cluster

Recently, we reported that an interferon (IFN)-inducible, murine 72-kD phosphoprotein (the 204 protein) that is encoded by the Ifi 204 gene from the gene 200 cluster is localized in the nucleolus and the nucleoplasm. We have now raised a polyclonal antiserum against the 202 protein that is encoded by the Ifi 202 gene from the same gene cluster and regions of which are homologous to those from the 204 protein. Using the antiserum, we established that the 202 protein is a 52-kD phosphoprotein whose level in cells from various murine lines can be increased up to 16-fold upon treatment with IFN-alpha. Experiments involving fractionation of cell lysates and indirect immunofluorescence microscopy of cultured cells revealed that the 202 protein was localized in the cytoplasm and the nucleus. Upon treatment of cells with IFN, the 202 protein first accumulated on the surface of a cytoplasmic, membranous fraction and after prolonged treatment with IFN it was localized mainly in the nucleus. In IFN-treated mitotic AKR cells, the 202 protein was colocalized with chromosomes. 202 protein extracted from IFN-treated AKR cells bound double-stranded DNA in vitro. Studies on 202 protein function should be facilitated by the availability of complete cDNA clones and the finding of cell lines and an inbred strain of mice in which the expression of this protein was impaired.

Interferon action: nucleolar and nucleoplasmic localization of the interferon-inducible 72-kD protein that is encoded by the Ifi 204 gene from the gene 200 cluster

The interferons are cytokines with antiviral, cell growth regulatory, and immunomodulatory activities. These activities are mediated by the proteins induced by the interferons. Earlier we described a gene cluster (the 200 cluster) consisting of at least six adjacent, interferon-activatable genes located next to the erythroid alpha-spectrin locus on murine chromosome 1. The genes of the cluster arose by repeated gene duplication and they specify proteins with pronounced sequence similarity. We have now raised polyclonal antibodies against a segment from one of these proteins (the 204 protein of 72 kD). Using these, we established that the 204 protein is a phosphoprotein whose level in cells from various murine lines can be increased up to 75-fold upon treatment with alpha interferon. Experiments involving fractionation of cell lysates and indirect immunofluorescence microscopy of control and interferon-treated cells revealed that the 204 protein is nucleolar and nucleoplasmic. This conclusion was confirmed by co-localization with B23, a known nucleolar protein. The 204 protein is the first interferon-induced protein found to be located in the nucleoli, the subcellular organelles of ribosomal RNA production and ribosome assembly. It remains to be seen whether the 204 protein affects any of these processes. Studies on 204 protein function should be facilitated by the availability of complete cDNA clones and the finding of cell lines in which the expression of this protein is impaired.

Interferons as gene activators. Indications for repeated gene duplication during the evolution of a cluster of interferon-activatable genes on murine chromosome 1

We have described earlier a gene cluster, including at least six interferon-activatable genes closely linked to the erythroid alpha spectrin locus and the serum amyloid P-component locus on murine chromosome 1. Here, we report that sequences of three genes from the cluster (the 201, 202, and 204 genes) are very similar in a segment extending from at least 550 nucleotides upstream of the 3' end of the transcription initiation region to beyond the first exon intron border (96% similarity between the 202 and 204 genes and 89% similarity between the 201 and 204 genes). This region contains the following two types of interferon-responsive enhancers: a GA box and a Friedman Stark sequence. The proteins coded for by the 202 gene (51 kDa) and the 204 gene (72 kDa) are hydrophilic. The amino acids have been conserved in the two proteins in 47% of the sequence. Each protein includes two apparently contiguous, approximately 200-amino acid long segments with much sequence similarity (27% in the 202 protein and 34% in the 204 protein). These segments are preceded in the 204 protein only by a segment including four perfect and three imperfect repeats of a 7 amino acid sequence. These and other data suggest that the evolution of the gene cluster involved the duplication of a DNA segment generating a double length transcription unit and subsequent divergence and duplication of this unit giving rise to at least two interferon-activatable genes (the 202 and 204 genes).

Physical mapping of a family of interferon-activated genes, serum amyloid P-component, and alpha-spectrin on mouse chromosome 1

This report defines genetic and physical relationships among alpha-spectrin (Spna-1), serum amyloid P-component (Sap), and a family of interferon-activated genes, provisionally designated Ifi202, Ifi203, and Ifi204. By linkage analysis using a large panel of interspecific backcross mice, Sap, Ifi202, and Ifi204 were shown to be tightly linked to Spna-1 on distal mouse chromosome 1. By pulsed field electrophoresis, a genomic restriction map of 6400 kb of distal mouse chromosome 1 was generated, linking genes encoding Sap, (Ifi202, Ifi203, Ifi204), and Spna-1 in that order within 450-1000 kb (where the genes within brackets were not ordered). The interferon-activated genes Ifi202, Ifi203, and Ifi204 were linked within 75-150 kb. Furthermore, genes transcriptionally activated by cytokines, namely Sap, Ifi202, Ifi203, and Ifi204, were located within 450 kb. These studies suggest the possibility that selective pressure may maintain in physical proximity gene clusters which are under coordinate transcriptional control.

Interferons as gene activators: a cluster of six interferon-activatable genes is linked to the erythroid alpha-spectrin locus on murine chromosome 1

Several interferon-activatable murine genes were mapped to murine chromosomes by hybridizing cDNA probes to Southern blots of genomic DNA samples from a panel of mouse-hamster somatic cell hybrid lines. The 12 gene is located on chromosome 12 and it specifies a 3.6-kb mRNA. The 204 gene (specifying a 2.5-kb mRNA), and three genes of the 203 gene family (hybridizing to five mRNAs of sizes between 2 and 4.5 kb), together with the 202 gene (specifying a 2-kb mRNA) are located on murine chromosome 1. By restriction fragment length polymorphism analysis of DNA samples prepared from a panel of recombinant inbred mouse lines (C57BL/6J D DBA/2J) and from 85 [C3H/HeJ-gld/gld x Mus spretus) F1 X C3H/HeJ-gld/gld] backcross mice we established a close linkage of the 202, 203, and 204 genes to the erythroid alpha-spectrin gene (Spna-1) on distal murine chromosome 1. Cosmids containing the 202, 203, and 204 genes were isolated from a library derived from AKR mouse DNA. Southern blot analysis of such cosmids revealed: (a) hybridization of a partial 203 cDNA to three genes of the 203 gene family; (b) cross-hybridization of the 202 and 204 genes with one another and with a third gene (designated as 201 gene), and (c) a close linkage of genes of the 203 family with the 201, 202, and 204 genes. These results indicate the existence of a cluster of at least six closely linked, interferon-activatable genes on distal murine chromosome 1 in the vicinity of the Spna-1 locus and also of the Minor lymphocyte stimulating locus (Mlsa).