Clinical implications of AGR2 in primary prostate cancer: Results from a large-scale study

Human anterior gradient-2 (AGR2) has been implicated in carcinogenesis of various solid tumours, but the expression data in prostate cancer are contradictory regarding its prognostic value. The objective of this study is to evaluate the expression of AGR2 in a large prostate cancer cohort and to correlate it with clinicopathological data. AGR2 protein expression was analysed immunohistochemically in 1023 well-characterized prostate cancer samples with a validated antibody. AGR2 expression levels in carcinomas were compared with matched tissue samples of adjacent normal glands. AGR2 expression levels were dichotomized and tested for statistical significance. Increased AGR2 expression was found in 93.5% of prostate cancer cases. AGR2 levels were significantly higher in prostate cancer compared with normal prostate tissue. A gradual loss of AGR2 expression was associated with increasing tumour grade (ISUP), and AGR2 expression is inversely related to patient survival, however, multivariable significance is not achieved. AGR2 is clearly upregulated in the majority of prostate cancer cases, yet a true diagnostic value appears unlikely. In spite of the negative correlation of AGR2 expression with increasing tumour grade, no independent prognostic significance was found in this large-scale study.

Inhibition of double-stranded RNA- and tumor necrosis factor alpha-mediated apoptosis by tetratricopeptide repeat protein and cochaperone P58(IPK)

P58(IPK) is a tetratricopeptide repeat-containing cochaperone that is involved in stress-activated cellular pathways and that inhibits the activity of protein kinase PKR, a primary mediator of the antiviral and antiproliferative properties of interferon. To gain better insight into the molecular actions of P58(IPK), we generated NIH 3T3 cell lines expressing either wild-type P58(IPK) or a P58(IPK) deletion mutant, DeltaTPR6, that does not bind to or inhibit PKR. When treated with double-stranded RNA (dsRNA), DeltaTPR6-expressing cells exhibited a significant increase in eukaryotic initiation factor 2alpha phosphorylation and NF-kappaB activation, indicating a functional PKR. In contrast, both of these PKR-dependent events were blocked by the overexpression of wild-type P58(IPK). In addition, the P58(IPK) cell line, but not the DeltaTPR6 cell line, was resistant to dsRNA-induced apoptosis. Together, these findings demonstrate that P58(IPK) regulates dsRNA signaling pathways by inhibiting multiple PKR-dependent functions. In contrast, both the P58(IPK) and DeltaTPR6 cell lines were resistant to tumor necrosis factor alpha-induced apoptosis, suggesting that P58(IPK) may function as a more general suppressor of programmed cell death independently of its PKR-inhibitory properties. In accordance with this hypothesis, although PKR remained active in DeltaTPR6-expressing cells, the DeltaTPR6 cell line displayed a transformed phenotype and was tumorigenic in nude mice. Thus, the antiapoptotic function of P58(IPK) may be an important factor in its ability to malignantly transform cells.

The cellular inhibitor of the PKR protein kinase, P58(IPK), is an influenza virus-activated co-chaperone that modulates heat shock protein 70 activity

P58(IPK), a member of the tetratricopeptide repeat and J-domain protein families, was first recognized for its ability to inhibit the double-stranded RNA-activated protein kinase, PKR. PKR is part of the interferon-induced host defense against viral infection, and down-regulates translation initiation via phosphorylation of eukaryotic initiation factor 2 on the alpha-subunit. P58(IPK) is activated in response to infection by influenza virus, and inhibits PKR through direct protein-protein interaction. Previously, we demonstrated that the molecular chaperone heat shock protein 40 (hsp40) was a negative regulator of P58(IPK). We could now report that influenza virus activates the P58(IPK) pathway by promoting the dissociation of hsp40 from P58(IPK) during infection. We also found that the P58(IPK)-hsp40 association was disrupted during recovery from heat shock, which suggested a regulatory role for P58(IPK) in the absence of virus infection. The PKR pathway is even more complex as we show in this report that the molecular chaperone, hsp/Hsc70, was a component of a trimeric complex with hsp40 and P58(IPK). Moreover, like other J-domain proteins, P58(IPK) stimulated the ATPase activity of Hsc70. Taken together, our data suggest that P58(IPK) is a co-chaperone, possibly directing hsp/Hsc70 to refold, and thus inhibit kinase function.

Regulation of interferon-induced protein kinase PKR: modulation of P58IPK inhibitory function by a novel protein, P52rIPK

The cellular response to environmental signals is largely dependent upon the induction of responsive protein kinase signaling pathways. Within these pathways, distinct protein-protein interactions play a role in determining the specificity of the response through regulation of kinase function. The interferon-induced serine/threonine protein kinase, PKR, is activated in response to various environmental stimuli. Like many protein kinases, PKR is regulated through direct interactions with activator and inhibitory molecules, including P58IPK, a cellular PKR inhibitor. P58IPK functions to represses PKR-mediated phosphorylation of the eukaryotic initiation factor 2alpha subunit (eIF-2alpha) through a direct interaction, thereby relieving the PKR-imposed block on mRNA translation and cell growth. To further define the molecular mechanism underlying regulation of PKR, we have utilized an interaction cloning strategy to identify a novel cDNA encoding a P58IPK-interacting protein. This protein, designated P52rIPK, possesses limited homology to the charged domain of Hsp90 and is expressed in a wide range of cell lines. P52rIPK and P58IPK interacted in a yeast two-hybrid assay and were recovered as a complex from mammalian cell extracts. When coexpressed with PKR in yeast, P58IPK repressed PKR-mediated eIF-2alpha phosphorylation, inhibiting the normally toxic and growth-suppressive effects associated with PKR function. Conversely, introduction of P52rIPK into these strains resulted in restoration of both PKR activity and eIF-2alpha phosphorylation, concomitant with growth suppression due to inhibition of P58IPK function. Furthermore, P52rIPK inhibited P58IPK function in a reconstituted in vitro PKR-regulatory assay. Our results demonstrate that P58IPK is inhibited through a direct interaction with P52rIPK which, in turn, results in upregulation of PKR activity. Taken together, our data describe a novel protein kinase-regulatory system which encompasses an intersection of interferon-, stress-, and growth-regulatory pathways.

Interaction of the interferon-induced PKR protein kinase with inhibitory proteins P58IPK and vaccinia virus K3L is mediated by unique domains: implications for kinase regulation

Expression of the double-stranded RNA-activated protein kinase (PKR) is induced by interferons, with PKR activity playing a pivotal role in establishing the interferon-induced antiviral and antiproliferative states. PKR is directly regulated by physical association with the specific inhibitor, P58IPK, a cellular protein of the tetratricopeptide repeat (TPR) family, and K3L, the product of the corresponding vaccinia virus gene. P58IPK and K3L repress PKR activation and activity. To investigate the mechanism of P58IPK- and K3L-mediated PKR inhibition, we have used a combination of in vitro and in vivo binding assays to identify the interactive regions of these proteins. The P58IPK-interacting site of PKR was mapped to a 52-amino-acid aa segment (aa 244 to 296) spanning the ATP-binding region of the protein kinase catalytic domain. The interaction with PKR did not require the C-terminal DNA-J homology region of P58IPK but was dependent on the presence of the eukaryotic initiation factor 2-alpha homology region, mapping to the 34 aa within the sixth P58IPK TPR motif. Consistent with other TPR proteins, P58IPK formed multimers in vivo: the N-terminal 166 aa were both necessary and sufficient for complex formation. A parallel in vivo analysis to map the K3L-binding region of PKR revealed that like P58IPK , K3L interacted exclusively with the PKR protein kinase catalytic domain. In contrast, however, the K3L-binding region of PKR was localized to within aa 367 to 551, demonstrating that each inhibitor bound PKR in unique, nonoverlapping domains. These data, taken together, suggest that P58IPK and K3L may mediate PKR inhibition by distinct mechanisms. Finally, we will propose a model of PKR inhibition in which P58IPK or a P58IPK complex binds PKR and interferes with nucleotide binding and autoregulation, while formation of a PKR-K3L complex interferes with active-site function and/or substrate association.

Cloning, expression, and cellular localization of the oncogenic 58-kDa inhibitor of the RNA-activated human and mouse protein kinase

The 58-kDa inhibitor of the interferon-induced double-stranded RNA-activated protein kinase (PKR) is a cellular protein that is activated during influenza virus infection to down-regulate the activity of PKR. This study was initiated to further our understanding of the inhibitor which, when overproduced, has the capacity to malignantly transform cells. We report here the isolation and characterization of cDNA clones encoding the inhibitor, designated p58, from human HeLa and mouse NIH 3T3 cells. The human and mouse p58 cDNAs were 6.5 and 1.6 kb in length, respectively. Surprisingly, the deduced amino acid sequences of the human and mouse p58 were 96% identical, indicating a remarkably high degree of conservation between species. An examination of p58 mRNA expression in human tissues revealed a 6.5-kb transcript in all tissues examined, with a particularly high level of expression present in the pancreas and liver, and also in certain leukemic cell lines. Similarly, p58 expression was detected in all mouse tissues examined, with the highest level of expression found in liver. In contrast to human tissues, three p58 transcripts of approximately 1.7, 3.3 and 5.4 kb were observed in mouse tissues, suggesting that p58 expression may be regulated differently in human and mouse cells. Western blot analysis of subcellular fractions and indirect immunofluorescence analysis of intact cells revealed that p58 was found predominantly in the cytoplasm, consistent with its function as an inhibitor of PKR, which is also a predominantly cytoplasmic protein.

The P58 cellular inhibitor complexes with the interferon-induced, double-stranded RNA-dependent protein kinase, PKR, to regulate its autophosphorylation and activity

The 58-kDa protein, referred to as P58, is a cellular inhibitor of the interferon-induced, double-stranded RNA-activated protein kinase, PKR. The P58 protein inhibits both the autophosphorylation of PKR and the phosphorylation of the PKR natural substrate, the alpha subunit of eukaryotic initiation factor eIF-2. Sequence analysis revealed that P58 is a member of the tetratricopeptide family of proteins. Utilizing experimental approaches, which included coprecipitation or coselection of native and recombinant wild-type and mutant proteins, we found that P58 can efficiently complex with the PKR protein kinase. Attempts to map the P58 interactive sites revealed a correlation between the ability of P58 to inhibit PKR in vitro and bind to PKR. The DnaJ sequences, present at the carboxyl terminus of P58, were dispensable for binding in vitro, while sequences containing the eIF-2 alpha similarity region were essential for efficient complex formation. Furthermore, not all tetratricopeptide motifs were necessary for PKR-P58 interactions. Initial experiments to map the binding domains present in PKR showed that P58 complexed with PKR molecules that lacked the first RNA binding domain but did not bind to a PKR mutant containing only the amino terminus. These data, taken together, demonstrate that P58 inhibits PKR through a direct interaction, which is likely independent of the binding of double-stranded RNA to the protein kinase.

Binding of the influenza virus NS1 protein to double-stranded RNA inhibits the activation of the protein kinase that phosphorylates the elF-2 translation initiation factor

The NS1 protein of influenza A virus binds not only to poly(A) and a stem-bulge region in U6 small nuclear RNA (snRNA), but also to double-stranded (ds) RNA. Binding assays with NS1 protein mutants established that the previously identified RNA-binding domain of the NS1 protein is required for binding to ds RNA as well as for binding to poly(A) and U6 snRNA. In addition, dsRNA competed with U6 snRNA for binding to the NS1 protein, consistent with both RNAs sharing the same binding site on the protein. As a consequence of its binding to dsRNA, the NS1 protein blocks the activation of the dsRNA-activated protein kinase (PKR) in vitro. This kinase phosphorylates the alpha subunit of eukaryotic translation initiation factor 2 (elF-2 alpha), leading to a decrease in the rate of initiation of translation. Assays using purified PKR and purified elF2 demonstrated that the NS1 protein blocks the dsRNA activation of PKR, and experiments using reticulocyte extracts showed that the NS1 protein blocks the inhibition of translation caused by dsRNA activation of PKR. The implications of these results for control mechanisms occurring in influenza virus-infected cells are discussed.

Mutants of the RNA-dependent protein kinase (PKR) lacking double-stranded RNA binding domain I can act as transdominant inhibitors and induce malignant transformation

Recently we reported that introduction of catalytically inactive PKR molecules into NIH 3T3 cells causes malignant transformation and the development of tumors in nude mice. We have proposed that PKR may be a tumor suppressor gene possibly because of its translational inhibitory properties. We have now designed and characterized a number of PKR mutants encoding proteins that retain their catalytic competence but are mutated in their regulatory double-stranded RNA (dsRNA) binding domains (RBDs). RNA binding analysis revealed that PKR proteins either lacking or with point mutations in the first RBD (RBD-1) bound negligible amounts of dsRNA activator or adenovirus VAI RNA inhibitor. Despite the lack of binding, such variants remained functionally competent but were much less active than wild-type PKR. PKR variants completely lacking RBD-1 were largely unresponsive to dsRNA in activation assays but could be activated by heparin. To complement these studies, we evaluated the effects of point mutations in RBD-1 or the removal of either RBD-1 or RBD-2 on the proliferation rate of mouse 3T3 cells. We were unsuccessful at isolating stably transformed cells expressing RBD-1 point mutants or RBD-2-minus mutants. In contrast, NIH 3T3 cells, which constitutively expressed PKR proteins that lacked RBD-1, were selected. These cells displayed a transformed phenotype and caused tumors after inoculation in nude mice. Further, levels of endogenous eIF-2 alpha phosphorylation in RBD-1-minus cell lines were reduced, suggesting that such mutants act in a dominant negative manner to inhibit the function of endogenous PKR. These results emphasize the importance of RBD-1 in PKR control of cell growth and provide additional evidence for the critical role played by PKR in the regulation of malignant transformation.

Translational regulation by the interferon-induced double-stranded-RNA-activated 68-kDa protein kinase

Activation of the interferon-inducible 68-kDa protein kinase (referred to as P68) by double-stranded RNA catalyzes phosphorylation of the alpha subunit of eukaryotic protein synthesis initiation factor 2. We have analyzed the transient expression of mutant and wild-type kinase molecules in transfected COS cells to examine the effects of the kinase on gene expression in the absence of other interferon-induced gene products. The wild-type P68 kinase was expressed inefficiently whereas a catalytically inactive P68 was expressed at 30- to 40-fold higher levels. Protein stability measurements and primer-extension analysis of human kinase-specific mRNA levels provided evidence that kinase expression was regulated at the level of mRNA translation. Further, cotransfection experiments revealed that the domain II catalytically inactive mutant could stimulate reporter gene protein synthesis in a transdominant manner. We also examined the expression of mutants with deletions in the N-terminal double-stranded RNA binding domains and found that a kinase construct lacking aa 156-243 was expressed at levels comparable to the wild type whereas a P68 construct lacking aa 91-243 was expressed at levels 70-fold higher. Both the inactive domain II P68 mutant and the deletion mutant lacking aa 91-243 were less inhibitory to growth in yeast due to the reduced ability to phosphorylate initiation factor 2 alpha in vivo. In conclusion we have demonstrated that the P68 kinase can regulate mRNA translation primarily of its own mRNA and to a lesser extent of a heterologous mRNA and that this regulation is notably affected by mutations in either the catalytic or N-terminal regulatory domains.

Functional expression and RNA binding analysis of the interferon-induced, double-stranded RNA-activated, 68,000-Mr protein kinase in a cell-free system

Eukaryotic viruses have devised numerous strategies to downregulate activity of the interferon-induced, double-stranded (dsRNA)-activated protein kinase (referred to as p68 on the basis of its Mr of 68,000 in human cells). Viruses must exert this control to avoid extensive phosphorylation of the alpha subunit of eukaryotic initiation factor 2 (eIF-2) by p68 and the resultant negative effects on protein synthesis initiation. To begin to define the molecular mechanisms underlying this regulation, we optimized expression of p68 in an in vitro transcription-translation system utilizing the full-length cDNA clone. The in vitro-expressed kinase was autophosphorylated in response to dsRNAs and heparin in a manner similar to that for the native p68 provided that the kinase inhibitor, 2-aminopurine, was present during the in vitro translation reaction. Further, the activated kinase efficiently phosphorylated its natural substrate, the alpha subunit of eIF-2. Binding experiments revealed that the expressed kinase complexed with the dsRNA activator, reovirus dsRNA, as well as the adenovirus-encoded inhibitor, VAI RNA. Interestingly, both the reovirus RNAs and VAI RNA also complexed with protein kinase molecules that lacked the carboxyl terminus and all catalytic domains. Deletion analysis confirmed that the p68 amino terminus contained critical determinants for reovirus dsRNA and VAI RNA binding. Further, reovirus dsRNA efficiently bound to, but failed to activate, p68 kinase molecules containing a single amino acid substitution in the invariant lysine 295 present in catalytic domain II. Taken together, these data demonstrate that this expression system permits a detailed mutagenic analysis of the regions of p68 required for interaction with virus-encoded activators and repressors.

Expression of cellular genes in CD4 positive lymphoid cells infected by the human immunodeficiency virus, HIV-1: evidence for a host protein synthesis shut-off induced by cellular mRNA degradation

We have investigated the effects of HIV-1 infection on cellular gene expression in two different human CD4 positive lymphoid cell lines: CEM and C8166 cells. As a prerequisite for this study it was necessary to develop virus-cell culture systems in which greater than 90% of the cells could be near synchronously infected by HIV-1. Further, since HIV-1 is a cytopathic virus, it was essential that cellular gene expression be examined in virus-infected cells which remained viable. After meeting these requirements, we measured cellular RNA and protein levels in virus-infected lymphocytes. In the cell lines examined the levels of cellular protein synthesis markedly decreased at times when viral-specific protein synthesis was increasing. Both Northern and slot blot analysis revealed that the declines in host protein synthesis were due, at least in part, to declines in steady state levels of cellular mRNAs. Runoff assays with nuclei isolated from infected cells demonstrated that the decreases in cellular mRNA levels were not due to declines in cellular RNA polymerase II transcription rates. To determine if the decreases in cellular protein synthesis also might be due to specific translational controls exerted by HIV-1, we compared the polysome association of cellular RNAs in infected and uninfected C8166 cells. The polysome distribution of cellular mRNAs was virtually identical in mock- and HIV-1-infected cells although, as expected, the total amount of cellular mRNAs were significantly lower in virus-infected cells. Taken together, these results suggest that HIV-1 may encode mechanisms to inhibit cellular protein synthesis, likely as a result of cellular mRNA degradation, rather than specific blocks in cellular mRNA translation.

Nontranslated cellular mRNAs are associated with the cytoskeletal framework in influenza virus or adenovirus infected cells

In an effort to understand the molecular mechanisms underlying the selective shutoff of host protein synthesis in influenza virus and adenovirus infected cells, we analyzed the subcellular location of representative cellular and viral mRNAs. Earlier work has shown that the majority of cellular mRNAs remain polysome associated after infection by either virus and that both the initiation and elongation steps of host protein synthesis were blocked in infected cells (M. G. Katze, D. DeCorato, and R. M. Krug, J. Virol., 60, 1027-1039, 1986). The present study was undertaken to test whether these cellular mRNAs were rendered nontranslatable during infection as a result of their dissociation from the cytoskeleton framework. HeLa cells were fractionated into subcellular components by first gently disrupting the cells with Triton X-100 yielding the soluble fraction (SOL); the cytoskeleton (CSK) fraction was obtained from the Triton insoluble material by the double detergent treatment of Tween-40 and sodium deoxycholate. In uninfected cells the majority of host mRNAs were associated with polysomes which were exclusively bound to the CSK as would be expected of actively translated mRNAs. The cellular mRNAs also remained almost totally associated with the cytoskeleton in adenovirus and influenza virus infected cells despite the fact that these mRNAs are not translated during infection. Indeed, the host mRNAs and the efficiently translated viral mRNAs were CSK associated to the same extent. In contrast to the adenovirus and influenza systems, significant amounts of cellular mRNAs were dissociated from the CSK and found in the SOL fraction of poliovirus infected cells as others have reported. In accordance with the biochemical analysis, morphological studies utilizing electron microscopy revealed that the cytoskeleton remained relatively intact during adenovirus and influenza infection but was substantially reorganized in poliovirus infected cells. We conclude that translational regulatory events are likely different in the poliovirus system and that cytoskeletal association of mRNAs may be required but is not sufficient for efficient mRNA translation during adenovirus or influenza virus infection.