Upregulation of STAT1 protein in cells lacking or expressing mutants of the double-stranded RNA-dependent protein kinase PKR

Determination of protein turnover rates in the JAK/STAT pathway using a radioactive pulse-chase approach

The turnover rate of different protein species in a signal transduction network strongly affects the impact of the given species on the outcome of a stimulus. Whereas stable, long-lived proteins mainly account for the transmission of a signal, unstable short-lived species often comprise regulatory functions. Here, we describe a method to determine the half-lives of proteins of the JAK/STAT pathway by a pulse-chase approach in cell culture. First, radioactive labeling with (35)S-methionine is carried out to label newly synthesized proteins (pulse). Subsequently, the dynamics of the decay of these proteins is monitored in the absence of labeled amino acids over a defined time period (chase). For this purpose the protein of interest is isolated by immunoprecipitation from total cell lysates, separated on an SDS-polyacrylamide gel, and subsequently visualized by autoradiography.

Double-stranded RNA-dependent protein kinase regulates insulin-stimulated chondrogenesis in mouse clonal chondrogenic cells, ATDC-5

Double-stranded RNA-dependent protein kinase (PKR) is an interferon-induced protein that has been identified and characterized as a translational inhibitor in an interferon-regulated antiviral pathway. PKR is also reported to play important roles in the regulation of cell growth and differentiation. We have previously demonstrated that PKR inactivation suppresses osteoblast calcification and osteoclast formation. However, reports concerning the roles of PKR in chondrogenesis are limited. In this study, we have demonstrated that PKR is required for the in vitro differentiation of the mouse clonal chondrogenic cell line ATDC-5. ATDC-5 cells treated with insulin differentiated into chondrocytes and produced an alcian-blue-positive cartilage matrix. The protein expression of signal transducers and activators of transcription (STAT) peaked at day 7 of differentiation, whereas the expression of SRY-box-containing gene 9 (Sox-9), which is a transcription factor for chondrocyte differentiation, increased gradually. When the cells were treated with a PKR inhibitor (2-aminopurine), the cartilage matrix formation decreased. The protein expression of STAT1 continued to increase up to day 21, whereas the expression of Sox-9 was low and did not increase. We also demonstrated that PKR was localized to a marginal region of the mandibular condyle cartilage in mouse embryos. Our findings suggest that PKR has important functions in the differentiation of chondrocytes through the modulation of STAT1 and Sox-9 expression.

Increased expression of the dsRNA-activated protein kinase PKR in breast cancer promotes sensitivity to doxorubicin

It has been reported that the expression and activity of the interferon-inducible, dsRNA-dependent protein kinase, PKR, is increased in mammary carcinoma cell lines and primary tumor samples. To extend these findings and determine how PKR signaling may affect breast cancer cell sensitivity to chemotherapy, we measured PKR expression by immunohistochemical staining of 538 cases of primary breast cancer and normal tissues. Significantly, PKR expression was elevated in ductal, lobular and squamous cell carcinomas or lymph node metastases but not in either benign tumor specimens or cases of inflammation compared to normal tissues. Furthermore, PKR expression was increased in precancerous stages of mammary cell hyperplasia and dysplasia compared to normal tissues, indicating that PKR expression may be upregulated by the process of tumorigenesis. To test the function of PKR in breast cancer, we generated MCF7, T-47D and MDA-MB-231 breast cancer cell lines with significantly reduced PKR expression by siRNA knockdown. Importantly, while knockdown of PKR expression had no effect on cell proliferation under normal growth conditions, MCF7, T-47D or MDA-MB-231 cells with reduced PKR expression or treated with a small molecule PKR inhibitor were significantly less sensitive to doxorubicin or H₂O₂-induced toxicity compared to control cells. In addition, the rate of eIF2α phosphorylation following treatment with doxorubicin was delayed in breast cancer cell lines with decreased PKR expression. Significantly, treatment of breast cancer lines with reduced PKR expression with either interferon-α, which increases PKR expression, or salubrinal, which increases eIF2α phosphorylation, restored doxorubicin sensitivity to normal levels. Taken together these results indicate that increased PKR expression in primary breast cancer tissues may serve as a biomarker for response to doxorubicin-containing chemotherapy and that future therapeutic approaches to promote PKR expression/activation and eIF2α phosphorylation may be beneficial for the treatment of breast cancer.

Evidence of vitamin D and interferon-β cross-talk in human osteoblasts with 1α,25-dihydroxyvitamin D3 being dominant over interferon-β in stimulating mineralization

It is well established that 1α-25-dihydroxyvitamin D3 (1,25D3) regulates osteoblast function and stimulates mineralization by human osteoblasts. The aim of this study was to identify processes underlying the 1,25D3 effects on mineralization. We started with gene expression profiling analyses of differentiating human pre-osteoblast treated with 1,25D3. Bioinformatic analyses showed interferon-related and -regulated genes (ISG) to be overrepresented in the set of 1,25D3-regulated genes. 1,25D3 down-regulated ISGs predominantly during the pre-mineralization period. This pointed to an interaction between the vitamin D and IFN signaling cascades in the regulation of osteoblast function. Separately, 1,25D3 enhances while IFNβ inhibits mineralization. Treatment of human osteoblasts with 1,25D3 and IFNβ showed that 1,25D3 completely overrules the IFNβ inhibition of mineralization. This was supported by analyses of extracellular matrix gene expression, showing a dominant effect of 1,25D3 over the inhibitory effect of IFNβ. We identified processes shared by IFNβ- and 1,25D3-mediated signaling by performing gene expression profiling during early osteoblast differentiation. Bioinformatic analyses revealed that genes being correlated or anti-correlated with interferon-induced protein with tetratricopeptide repeats 1 (IFIT1) were associated with osteoblast proliferation. In conclusion, the current study demonstrates a cross talk between 1,25D3 and IFNβ in osteoblast differentiation and bone formation/mineralization. The interaction is complex and depends on the process but importantly, 1,25D3 stimulation of mineralization is dominant over the inhibitory effect of IFNβ. These observations are of potential clinical relevance considering the impact of the immune system on bone metabolism in conditions such as rheumatoid arthritis.

ERK1 is important for Th2 differentiation and development of experimental asthma

The ERK1/2 signaling pathway regulates a variety of T-cell functions. We observed dynamic changes in the expression of ERK1/2 during T-helper cell differentiation. Specifically, the expression of ERK1/2 was decreased and increased by IL-12 and IL-4, respectively. To address this subject further, we examined the specific role of ERK1 in Th2 differentiation and development of experimental asthma using ERK1(-/-) mice. ERK1(-/-) mice were unable to mount airway inflammation and hyperreactivity in two different models of asthma, acute and chronic. ERK1(-/-) mice had reduced expression of Th2 cytokines IL-4 and IL-5 but not IL-17A or IFN-γ. They had reduced levels of allergen-specific IgE and blood eosinophils. T cells from immunized ERK1(-/-) mice manifested reduced proliferation in response to the sensitizing allergen. ERK1(-/-) T cells had reduced and short-lived expression of JunB following TCR stimulation, which likely contributed to their impaired Th2 differentiation. Immunized ERK1(-/-) mice showed reduced numbers of CD44(high) CD4 T cells in the spleen. In vitro studies demonstrated that Th2 but not Th1 cells from ERK1(-/-) mice had reduced numbers of CD44(high) cells. Finally, CD4 T cells form ERK1(-/-) mice expressed higher levels of BIM under growth factor-deprived conditions and reduced Mcl-1 on stimulation. As a result, the survival of CD4 T cells, especially CD44(high) Th2 cells, was much reduced in ERK1(-/-) mice. We conclude that ERK1 plays a nonredundant role in Th2 differentiation and development of experimental asthma. ERK1 controls Th2 differentiation and survival through its effect on JunB and BIM, respectively.

Phosphorylation-acetylation switch in the regulation of STAT1 signaling

STAT1 signaling regulates the expression of important genes controlling cell growth, differentiation, apoptosis, and immune functions. Biochemical and genetic experiments have identified how this cascade is modulated. Phosphorylation of STAT1 tyrosine and serine moieties is induced rapidly by cytokines and growth factors. Upon nuclear translocation, phosphorylated STAT1 homo- and heterodimers activate gene expression. Inactivation of phosphorylated nuclear STAT1 has to be precisely regulated in order to allow signal transduction within limited time frames. Lysine acetylation has recently been appreciated as a novel mechanism regulating signal transduction events relying on STAT proteins. Here, we review these analyses and the finding that a switch from phosphorylated to acetylated STAT1 regulates acetylation-dependent dephosphorylation of STAT1 via the T cell tyrosine phosphatase. We discuss how these observations can be integrated into our current understanding of STAT-dependent cytokine signaling and its potential relevance for endocrine functions.

Inhibition of Jak1-dependent signal transduction in airway epithelial cells infected with adenovirus

Adenoviral evolution has generated mechanisms to resist host cell defense systems, but the biochemical basis for evasion of multiple antiviral pathways in the airway by adenoviruses is incompletely understood. We hypothesized that adenoviruses modulate airway epithelial responses to type I interferons by altering the levels and activation of specific Janus family kinase-signal transducer and activator of transcription (JAK-STAT) signaling components. In this study, specific effects of adenovirus type 5 (AdV) on selected JAK-STAT signal transduction pathways were identified in human tracheobronchial epithelial cells, with focus on type I interferon-dependent signaling and gene expression. We found that wild-type AdV infection inhibited IFN-alpha-induced expression of antiviral proteins in epithelial cells by blocking phosphorylation of the Stat1 and Stat2 transcription factors that are required for activation of type I interferon-dependent genes. These effects correlated with AdV-induced down-regulation of expression of the receptor-associated tyrosine kinase Jak1 through a decrease in Jak1 mRNA levels. Phosphorylation of Stat3 in response to IL-6 and oncostatin M was also lost in AdV-infected cells, indicating loss of epithelial cell responses to other cytokines that depend on Jak1. In contrast, IL-4- and IL-13-dependent phosphorylation of Stat6 was not affected during AdV infection, indicating that the virus modulates specific signaling pathways, as these Stat6-activating pathways can function independent of Jak1. Taken together, the results indicate that AdV down-regulates host epithelial cell Jak1 to assure inhibition of the antiviral effects of multiple mediators to subvert airway defense responses and establish a productive infection.

Adenoviral infection induces a multi-faceted innate cellular immune response that is mediated by the toll-like receptor pathway in A549 cells

Adenovirus vectors are known to induce certain genes and impact innate response networks, but a broad understanding of this process and its mechanisms is currently lacking. For this reason, we chose to investigate and characterize Ad innate immunity using homogeneous, primary MEF cells replete with all the elements of the pathogen-sensing Toll-Like Receptor (TLR) pathway. By using an array-based approach to maximally define transcriptome changes induced upon Ad vector infection, we discovered that Ad infection induces a potent gene and transcription factor network response. This response is characterized by significant changes in the expression of genes involved in focal adhesion, tight junction, and RNA regulation, in addition to TLR pathway and other innate sensing genes. Further investigation using human A549 cells knocked down for various TLR pathway adaptors, revealed significant impacts on the Ad initiation of NF-kB and interferon responses, thus confirming TLR involvement in Ad-mediated immunity across diverse species.

IFN-gamma-induced upregulation of Fcgamma-receptor-I during activation of monocytic cells requires the PKR and NFkappaB pathways

Interferon (IFN)-gamma is a potent activator of macrophages, increasing the cells capacity to perform specific functions during inflammation and immune response. In this report we use IFN-gamma-induced upregulation of the high affinity receptor for IgG (FcgammaRI/CD64) in the human monocytic cell line U-937 as a model for monocytic activation. We show that upregulation of FcgammaRI is dependent on signals mediated by the dsRNA-dependent kinase PKR, and the transcription factor NFkappaB. Silencing of PKR expression by siRNA or inhibition of PKR by 2-aminopurine (2-AP) potently blocks the IFN-gamma-induced transcriptional activation of the FcgammaRI promoter. We find that the serine 727 phosphorylation of Stat1, required for full IFN-gamma-induced FcgammaRI promoter activity, is dependent on PKR. We further show that IFN-gamma induction of FcgammaRI upregulation is dependent on the NFkappaB pathway, as evidenced by inhibition of NFkappaB using a phosphorylation defective IkappaBalpha (S32A/S36A) mutant, or inhibiting the IkappaB-kinase (IKK) by treatment with BMS345541. Our results suggest that IFN-gamma-induced increase of FcgammaRI expression requires the integration of two signalling events: PKR-dependent Stat1 serine 727 phosphorylation, and activation of NFkappaB.

Double-stranded RNA-dependent protein kinase is required for bone calcification in MC3T3-E1 cells in vitro

In this study, we demonstrated that double-stranded RNA-dependent protein kinase (PKR) is required for the calcification of osteoblasts via the signal transducers and activators of transcription 1alpha (STAT1alpha) signaling in vitro. A dominant-negative mutant PKR cDNA, in which the amino acid lysine at 296 was replaced with arginine and which does not have catalytic activity, was transfected into mouse osteoblastic MC3T3-E1 cells; thereby, we established cells that stably expressed the PKR mutant gene (PKR-K/R). Phosphorylation of PKR was not stimulated by polyinosic-polycytidylic acid in the mutant cells. The PKR-K/R mutant cells exhibited up-regulated cell growth and had low alkaline phosphatase (ALP) activity. The PKR-K/R mutant cells were not able to form bone nodules in vitro. In the PKR-K/R mutant cells, runt-related gene 2 (Runx2)-mediated transcription decreased compared with the levels in the control cells. The expression of STAT1alpha protein increased and the protein was translocated to the nucleus in the PKR-K/R mutant cells. When the expression of STAT1alpha protein in PKR mutant cells was suppressed using RNAi, the activity of Runx2-mediated transcription recovered to the control level. Our results indicate that PKR is a stimulator of Runx2 transcription and is a negative modulator of STAT1alpha expression. Our findings also suggest that PKR plays important roles in the differentiation and calcification of osteoblasts by modulating STAT1alpha and/or Runx2 expression.

The direct binding of the catalytic subunit of protein phosphatase 1 to the PKR protein kinase is necessary but not sufficient for inactivation and disruption of enzyme dimer formation

The PKR protein kinase is among the best-studied effectors of the host interferon (IFN)-induced antiviral and antiproliferative response system. In response to stress signals, including virus infection, the normally latent PKR becomes activated through autophosphorylation and dimerization and phosphorylates the eIF2alpha translation initiation factor subunit, leading to an inhibition of mRNA translation initiation. While numerous virally encoded or modulated proteins that bind and inhibit PKR during virus infection have been studied, little is known about the cellular proteins that counteract PKR activity in uninfected cells. Overexpression of PKR in yeast also leads to an inhibition of eIF2alpha-dependent protein synthesis, resulting in severe growth suppression. Screening of a human cDNA library for clones capable of counteracting the PKR-mediated growth defect in yeast led to the identification of the catalytic subunit (PP1(C)) of protein phosphatase 1alpha. PP1(C) reduced double-stranded RNA-mediated auto-activation of PKR and inhibited PKR transphosphorylation activities. A specific and direct interaction between PP1(C) and PKR was detected, with PP1(C) binding to the N-terminal regulatory region regardless of the double-stranded RNA-binding activity of PKR. Importantly, a consensus motif shared by many PP1(C)-interacting proteins was necessary for PKR binding to PP1(C). The PKR-interactive site was mapped to a C-terminal non-catalytic region that is conserved in the PP1(C)2 isoform. Indeed, co-expression of PP1(C) or PP1(C)2 inhibited PKR dimer formation in Escherichia coli. Interestingly, co-expression of a PP1(C) mutant lacking the catalytic domain, despite retaining its ability to bind PKR, did not prevent PKR dimerization. Our findings suggest that PP1(C) modulates PKR activity via protein dephosphorylation and subsequent disruption of PKR dimers.

Initiation factor eIF2 alpha phosphorylation in stress responses and apoptosis

The alpha subunit of polypeptide chain initiation factor eIF2 can be phosphorylated by a number of related protein kinases which are activated in response to cellular stresses. Physiological conditions which result in eIF2 alpha phosphorylation include virus infection, heat shock, iron deficiency, nutrient deprivation, changes in intracellular calcium, accumulation of unfolded or denatured proteins and the induction of apoptosis. Phosphorylated eIF2 acts as a dominant inhibitor of the guanine nucleotide exchange factor eIF2B and prevents the recycling of eIF2 between successive rounds of protein synthesis. Extensive phosphorylation of eIF2 alpha and strong inhibition of eIF2B activity can result in the downregulation of the overall rate of protein synthesis; less marked changes may lead to alterations in the selective translation of alternative open reading frames in polycistronic mRNAs, as demonstrated in yeast. These mechanisms can provide a signal transduction pathway linking eukaryotic cellular stress responses to alterations in the control of gene expression at the translational level.

Enhanced antiviral and antiproliferative properties of a STAT1 mutant unable to interact with the protein kinase PKR

We have previously reported a physical association between STAT1 and the protein kinase double-stranded RNA-activated protein kinase (PKR). PKR inhibited STAT1 function in a manner independent of PKR kinase activity. In this report, we have further characterized the properties of both molecules by mapping the sites of their interaction. A STAT1 mutant unable to interact with PKR displays enhanced interferon gamma (IFN-gamma)-induced transactivation capacity compared with STAT1. This effect appears to be mediated by the higher capacity of STAT1 mutant to heterodimerize with STAT3. Furthermore, expression of STAT1 mutant in STAT1(-/-) cells enhances both the antiviral and antiproliferative effects of IFNs as opposed to STAT1. We also provide evidence that STAT1 functions as an inhibitor of PKR in vitro and in vivo. That is, phosphorylation of eIF-2alpha is enhanced in STAT1(-/-) than STAT1(+/+) cells in vivo, and this correlates with higher activation capacity of PKR in STAT1(-/-) cells. Genetic experiments in yeast demonstrate the inhibition of PKR activation and eIF-2alpha phosphorylation by STAT1 but not by STAT1 mutant. These data substantiate our previous findings on the inhibitory effects of PKR on STAT1 and implicate STAT1 in translational control through the modulation of PKR activation and eIF-2alpha phosphorylation.