Upregulation of CHOP participates in caspase activation and virus release in human astrovirus-infected cells.

Human astroviruses (HAstVs), non-enveloped RNA viruses with positive-sense RNA genomes, are an important cause of acute gastroenteritis in young children, although the processes that produce infectious virions are not clearly defined. To track the viral replication complex (RC) upon HAstV1 infection, the subcellular distribution of double-stranded (ds) RNA and of ORF1b, a viral RNA polymerase, was examined by immunocytochemistry. Foci that were positive for dsRNA and for ORF1b were co-localized, and both foci were also co-localized with resident proteins of the endoplasmic reticulum (ER). Focusing on the association between the HAstV RC and ER, we examined the expression of unfolded protein response (UPR) markers and found that targets of eukaryotic translation initiation factor 2α (eIF2α)-activating transcription factor 4 (ATF4), including CCAAT/enhancer-binding protein homologous protein (CHOP), a proapoptotic transcription factor, were upregulated at the late phase in HAstV-infected cells. Consistently, eIF2α was phosphorylated at the late phase of HAstV infection. The formation of foci resembling stress granules, another known downstream response to eIF2α phosphorylation, was also observed at the same period. Phosphorylation of eIF2α was attenuated in protein kinase R (PKR)-knockdown cells, suggesting that, unlike the canonical ER stress response, PKR was involved in eIF2α phosphorylation in response to HAstV infection. Studies have indicated that immature HAstV capsid protein is processed by caspases, and caspase cleavage is integral to particle release. Inhibition of CHOP upregulation reduced caspase activation and the release of HAstV RNA from cells during HAstV infection. Our results suggest that the eIF2α-ATF4-CHOP pathway participates in HAstV propagation.

Prediction and experimental validation of a putative non-consensus binding site for transcription factor STAT3 in serum amyloid A gene promoter.

We previously demonstrated that though the human SAA1 gene shows no typical STAT3 response element (STAT3-RE) in its promoter region, STAT3 and the nuclear factor (NF-κB) p65 first form a complex following interleukin IL-1 and IL-6 (IL-1+6) stimulation, after which STAT3 interacts with a region downstream of the NF-κB RE in the SAA1 promoter. In this study, we employed a computational approach based on indirect read outs of protein-DNA contacts to identify a set of candidates for non-consensus STAT3 transcription factor binding sites (TFBSs). The binding of STAT3 to one of the predicted non-consensus TFBSs was experimentally confirmed through a dual luciferase assay and DNA affinity chromatography. The present study defines a novel STAT3 non-consensus TFBS at nt -75/-66 downstream of the NF-κB RE in the SAA1 promoter region that is required for NF-κB p65 and STAT3 to activate SAA1 transcription in human HepG2 liver cells. Our analysis builds upon the current understanding of STAT3 function, suggesting a wider array of mechanisms of STAT3 function in inflammatory response, and provides a useful framework for investigating novel TF-target associations with potential therapeutic implications.

Adenovirus E1A inhibits SCF(Fbw7) ubiquitin ligase.

The SCF(Fbw7) ubiquitin ligase complex plays important roles in cell growth, survival, and differentiation via the ubiquitin-proteasome-mediated regulation of protein stability. Fbw7 (also known as Fbxw7, Sel-10, hCdc4, or hAgo), a substrate recognition subunit of SCF(Fbw7) ubiquitin ligase, facilitates the degradation of several proto-oncogene products by the proteasome. Given that mutations in Fbw7 are found in various types of human cancers, Fbw7 is considered to be a potent tumor suppressor. In the present study, we show that E1A, an oncogene product derived from adenovirus, interferes with the activity of the SCF(Fbw7) ubiquitin ligase. E1A interacted with SCF(Fbw7) and attenuated the ubiquitylation of its target proteins in vivo. Furthermore, using in vitro purified SCF(Fbw7) component proteins, we found that E1A directly bound to Roc1/Rbx1 and CUL1 and that E1A inhibited the ubiquitin ligase activity of the Roc1/Rbx1-CUL1 complex but not that of another RING-type ubiquitin ligase, Mdm2. Ectopically expressed E1A interacted with cellular endogenous Roc1/Rbx1 and CUL1 and decelerated the degradation of several protooncogene products that were degraded by SCF(Fbw7) ubiquitin ligase. Moreover, after wild-type adenovirus infection, adenovirus-derived E1A interacted with endogenous Roc1/Rbx1 and decelerated degradation of the endogenous target protein of SCF(Fbw7). These observations demonstrated that E1A perturbs protein turnover regulated by SCF(Fbw7) through the inhibition of SCF(Fbw7) ubiquitin ligase. Our findings may help to explain the mechanism whereby adenovirus infection induces unregulated proliferation.

Kaposi's sarcoma-associated herpesvirus-encoded LANA positively affects on ubiquitylation of p53.

We established a series of stable transfectants expressing wild-type and three mutant LANA; amino terminus, carboxyl terminus and amino terminus plus DNA binding domain, as a new strategy to assess systematically the interactions and binding domains with cellular proteins. Using the system, we reported that LANA specifically bound to p53 via DNA binding domain. As for LANA function in the regulation of p53 through the interaction, we showed that polyubiquitylation of p53 in the presence of LANA was obviously increased. LANA also associated with Cullin 5 and Rbx1, active subunit of E3 ubiquitin ligase complex. Taken together, the present study suggests that LANA induce enhancement of p53 ubiquitylation and degradation into proteasome, consequently contributing to latent persistence.

High expression of Pirh2, an E3 ligase for p27, is associated with low expression of p27 and poor prognosis in head and neck cancers.

Downregulation of the cyclin-dependent kinase inhibitory protein p27 is frequently observed in various cancers due to enhancement of its degradation. We recently reported that p53-inducible protein with RING-H2 domain (Pirh2) is a novel ubiquitin ligase for p27, required for the ubiquitylation and consequent degradation of p27 protein. However, there is no reports about the involvement of Pirh2 in both p27 downregulation and pathogenesis in human cancers. In the present study, we investigated them using cultured cell lines and surgical specimens derived from human head and neck squamous cell carcinoma (HNSCC). Depletion of Pirh2 by short interfering RNA induced accumulation of p27 and inhibited the growth of cultured HNSCC cells. By immunohistochemical analysis in 57 cases of HNSCC specimens, higher levels of Pirh2 expression (labeling index > or = 60%) were found in 61.4% of HNSCC in comparison with 0% of normal mucosa. In addition, 83.3% of HNSCC with lower p27 expression (labeling index < 20%) displayed high Pirh2 levels. Therefore, Pirh2 expression was inversely correlated with p27 expression. Finally, Pirh2 expression was well correlated with poor prognosis. These findings suggest that Pirh2 overexpression may have an important role in the development and maintenance of HNSCC at least partially through p27 degradation, and that Pirh2 may be a potential molecular target for human HNSCC.

Up-regulation of GPR48 induced by down-regulation of p27Kip1 enhances carcinoma cell invasiveness and metastasis.

A reduced expression level of the cyclin-dependent kinase inhibitor p27(Kip1) is associated with increased tumor malignancy and poor prognosis in individuals with various types of cancer. To investigate the basis for this relation, we applied microarray analysis to screen for genes differentially expressed between p27(+/-) and parental (p27(+/+)) HCT116 human colon carcinoma cells. Expression of the gene for G protein-coupled receptor 48 (GPR48) was increased in the p27(+/-) cells. Forced expression of GPR48 increased both in vitro invasive activity and lung metastasis potency of HCT116 cells. In contrast, depletion of endogenous GPR48 by RNA interference reduced the invasive potential of HeLa and Lewis lung carcinoma cells not only in vitro but also in vivo. Moreover, GPR48 expression was significantly associated with lymph node metastasis and inversely correlated with p27 expression in human colon carcinomas. GPR48 may thus play an important role in invasiveness and metastasis of carcinoma and might therefore represent a potential prognostic marker or therapeutic target.

Effects of MdmX on Mdm2-mediated downregulation of pRB.

Mdm2, a RING-finger type ubiquitin ligase, is overexpressed in a variety of human cancers. It promotes ubiquitination of the tumor suppressor p53 and can function as an oncogene by largely downregulating p53. Recently, we reported that Mdm2 degrades retinoblastoma tumor suppressor protein (pRB) via the ubiquitin-proteasome system. In the present study, we assessed the effects of MdmX, a structural homolog of Mdm2, on the Mdm2-mediated ubiquitination of pRB. MdmX is known to negatively regulate p53 function by enhancing the Mdm2-mediated ubiquitination and degradation of p53. Interestingly, MdmX inhibited the Mdm2-mediated pRB ubiquitination. Furthermore, an MdmX siRNA decreased the endogenous pRB level, while MdmX overexpression stimulated pRB functions in cultured cells. Therefore, MdmX may have different roles in the regulation of Mdm2 activity for ubiquitination of pRB and p53.

Characterization of the Human Transcription Elongation Factor Rtf1: Evidence for Nonoverlapping Functions of Rtf1 and the Paf1 Complex.

Restores TBP function 1 (Rtf1) is generally considered to be a subunit of the Paf1 complex (PAF1C), a multifunctional protein complex involved in histone modification and transcriptional or posttranscriptional regulation. Rtf1, however, is not stably associated with the PAF1C in most species except Saccharomyces cerevisiae, and its biochemical functions are not well understood. Here, we show that human Rtf1 is a transcription elongation factor that may function independently of the PAF1C. Rtf1 requires "Rtf1 coactivator" activity, which is most likely unrelated to the PAF1C or DSIF, for transcriptional activation in vitro. A mutational study revealed that the Plus3 domain of human Rtf1 is critical for its coactivator-dependent function. Transcriptome sequencing (RNA-seq) and chromatin immunoprecipitation studies in HeLa cells showed that Rtf1 and the PAF1C play distinct roles in regulating the expression of a subset of genes. Moreover, contrary to the finding in S. cerevisiae, the PAF1C was apparently recruited to the genes examined in an Rtf1-independent manner. The present study establishes a role for human Rtf1 as a transcription elongation factor and highlights the similarities and differences between the S. cerevisiae and human Rtf1 proteins.

DSIF and NELF interact with Integrator to specify the correct post-transcriptional fate of snRNA genes.

The elongation factors DSIF and NELF are responsible for promoter-proximal RNA polymerase II (Pol II) pausing. NELF is also involved in 3' processing of replication-dependent histone genes, which produce non-polyadenylated mRNAs. Here we show that DSIF and NELF contribute to the synthesis of small nuclear RNAs (snRNAs) through their association with Integrator, the large multisubunit complex responsible for 3' processing of pre-snRNAs. In HeLa cells, Pol II, Integrator, DSIF and NELF accumulate at the 3' end of the U1 snRNA gene. Knockdown of NELF results in misprocessing of U1, U2, U4 and U5 snRNAs, while DSIF is required for proper transcription of these genes. Knocking down NELF also disrupts transcription termination and induces the production of polyadenylated U1 transcripts caused by an enhanced recruitment of cleavage stimulation factor. Our results indicate that NELF plays a key role in determining the post-transcriptional fate of Pol II-transcribed genes.

Activation-induced cytidine deaminase auto-activates and triggers aberrant gene expression.

DNA methylation is a well-characterized epigenetic landmark involved in transcriptional regulation; however, mechanisms underlying its regulation remain poorly characterized. Recent studies demonstrate that activation-induced cytidine deaminase (AID) is involved in active DNA demethylation. AID is aberrantly expressed in inflammation-associated cancers and generates point mutations; however, cellular disorders attributed to its demethylation function are largely unexplored. Here we demonstrate that ectopic AID expression perturbs tumor-related gene expression. AID (with Gadd45) activated a methylated paired box gene 5 (Pax5) reporter construct, and induced expression and association of endogenous Pax5 with the AID promoter, suggesting that aberrant AID expression triggers an auto-activation circuit to consolidate self-expression.

Comparative evaluation of the effects of treatment with tocilizumab and TNF-α inhibitors on serum hepcidin, anemia response and disease activity in rheumatoid arthritis patients.

INTRODUCTION: Anemia of inflammation (AI) is a common complication of rheumatoid arthritis (RA) and has a negative impact on RA symptoms and quality of life. Upregulation of hepcidin by inflammatory cytokines has been implicated in AI. In this study, we evaluated and compared the effects of IL-6 and TNF-α blocking therapies on anemia, disease activity, and iron-related parameters including serum hepcidin in RA patients. METHODS: Patients (n = 93) were treated with an anti-IL-6 receptor antibody (tocilizumab) or TNF-α inhibitors for 16 weeks. Major disease activity indicators and iron-related parameters including serum hepcidin-25 were monitored before and 2, 4, 8, and 16 weeks after the initiation of treatment. Effects of tocilizumab and infliximab (anti-TNF-α antibody) on cytokine-induced hepcidin expression in hepatoma cells were analyzed by quantitative real-time PCR. RESULTS: Anemia at base line was present in 66% of patients. Baseline serum hepcidin-25 levels were correlated positively with serum ferritin, C-reactive protein (CRP), vascular endothelial growth factor (VEGF) levels and Disease Activity Score 28 (DAS28). Significant improvements in anemia and disease activity, and reductions in serum hepcidin-25 levels were observed within 2 weeks in both groups, and these effects were more pronounced in the tocilizumab group than in the TNF-α inhibitors group. Serum hepcidin-25 reduction by the TNF-α inhibitor therapy was accompanied by a decrease in serum IL-6, suggesting that the effect of TNF-α on the induction of hepcidin-25 was indirect. In in vitro experiments, stimulation with the cytokine combination of IL-6+TNF-α induced weaker hepcidin expression than did with IL-6 alone, and this induction was completely suppressed by tocilizumab but not by infliximab. CONCLUSIONS: Hepcidin-mediated iron metabolism may contribute to the pathogenesis of RA-related anemia. In our cohort, tocilizumab was more effective than TNF-α inhibitors for improving anemia and normalizing iron metabolism in RA patients by inhibiting hepcidin production.

Pirh2 interacts with and ubiquitylates signal recognition particle receptor beta subunit.

Pirh2 is a RING finger type ubiquitin ligase which ubiquitylates various proteins including p53, p27(Kip1), HDAC1, and epsilon-COP. In this study, we identified signal recognition particle receptor beta subunit (SRbeta), an integral membrane protein of the endoplasmic reticulum (ER), as a novel Pirh2-interacting protein by yeast two-hybrid screening. We confirmed that Pirh2 interacted with SRbeta in mammalian cells. An immunofluorescent staining revealed that Pirh2 colocalized with SRbeta in the ER. Pirh2 poly-ubiquitylated SRbeta in an intact RING finger domain-dependent manner in vivo and in vitro. Unexpectedly, different from other Pirh2 substrates, neither overexpression of Pirh2 nor depletion of cellular Pirh2 affected SRbeta protein stability. Pirh2 preferentially utilized lysine residues 6 and 29 of the ubiquitin to mediate the formation of polyubiquitin chains on SRbeta. These results suggest that Pirh2 may regulate SRbeta function by mediating poly-ubiquitylation of SRbeta without affecting the stability of SRbeta protein per se.

Transcriptional complex formation of c-Fos, STAT3, and hepatocyte NF-1 alpha is essential for cytokine-driven C-reactive protein gene expression.

C-reactive protein (CRP) is a sensitive marker and mediator of inflammation, whereas IL-6 blocking therapy can normalize serum levels of CRP in chronic inflammatory diseases. We investigated the precise synergistic induction mechanism of CRP gene expression by IL-1 and IL-6 in Hep3B cells. In the early induction phase, IL-1 inhibited IL-6-mediated CRP gene expression, and NF-kappaB p65 inhibited the luciferase activity of pGL3-CRP by IL-1 plus IL-6 even in the presence of overexpressed STAT3. In the late induction phase, we focused on JNK and p38 activated by IL-1. SP600125 reduced the expression of the CRP gene induced by IL-1 plus IL-6. Unexpectedly, overexpression of c-Fos dramatically enhanced the luciferase activity by IL-1 and IL-6 even though the CRP gene has no AP-1 response element (RE) in its promoter. The augmentative effect of c-Fos required the presence of STAT3 and 3'-hepatocyte NF-1 (HNF-1) RE, which were eliminated by dominant negative STAT3 and HNF-1alpha, respectively. SB203580 inhibited the phosphorylation of c-Fos enhanced by IL-1 plus IL-6, and diminished expression of the CRP gene. Immunoprecipitation, Western blot analysis, the Supershift assay using a CRP oligonucleotide containing STAT3 and 3'-HNF-1 RE, and the chromatin immunoprecipitation assay demonstrated that c-Fos/STAT3/HNF-1alpha forms a complex on the CRP gene promoter. Because human fetus liver cells failed to express c-Fos/STAT3/HNF-1alpha showed no CRP production, transcriptional complex formation of c-Fos/STAT3/HNF-1alpha is essential for the synergistic induction of CRP gene expression by IL-1 plus IL-6. Our findings fully explain the clinical results of IL-6 blocking therapy and are expected to contribute to the development of a therapeutic strategy for chronic inflammatory diseases.

Pirh2 promotes ubiquitin-dependent degradation of the cyclin-dependent kinase inhibitor p27Kip1.

The cyclin-dependent kinase inhibitor p27(Kip1) is degraded in late G(1) phase by the ubiquitin-proteasome pathway, allowing cells to enter S phase. Due to accelerated degradation of p27(Kip1), various human cancers express low levels of p27(Kip1) associated with poor prognosis. S-phase kinase-associated protein 2, the F-box protein component of an SCF ubiquitin ligase complex, is implicated in degradation of p27(Kip1) during S-G(2) phases. Recently, Kip1 ubiquitination-promoting complex has been reported as another ubiquitin ligase that targets cytoplasmic p27(Kip1) exported from the nucleus in G(0)-G(1) phases. Here, we identified a RING-H2-type ubiquitin ligase, Pirh2, as a p27(Kip1)-interacting protein. Endogenous Pirh2 physically interacted with endogenous p27(Kip1) in mammalian cells. Pirh2 directly ubiquitinated p27(Kip1) in an intact RING finger domain-dependent manner in vivo, as well as in vitro. Ablation of endogenous Pirh2 by small interfering RNA increased the steady-state level of p27(Kip1) and decelerated p27(Kip1) turnover. Depletion of Pirh2 induced accumulation of p27(Kip1) in both the nucleus and cytoplasm. Pirh2 expression was induced from late G(1)-S phase, whereas p27(Kip1) was decreased in synchronization with accumulation of Pirh2. Furthermore, reduction of Pirh2 resulted in an impairment of p27(Kip1) degradation and an inhibition of cell cycle progression at G(1)-S transition in a p53-independent manner. Overall, the results indicate that Pirh2 acts as a negative regulator of p27(Kip1) function by promoting ubiquitin-dependent proteasomal degradation.

ARA54 is involved in transcriptional regulation of the cyclin D1 gene in human cancer cells.

Cyclin D1 is one of the major enhancers of cell cycle progression and its expression is regulated in several growth stimulatory signaling pathways. ARA54 is an androgen receptor (AR) co-activator that enhances AR-dependent transcriptional activation. Although expression of ARA54 mRNA is observed in a variety of human tissues at low levels, the AR- or androgen-independent function of ARA54 in those tissues remains unclear. In this study, we identified a novel role for ARA54 in the regulation of cyclin D1 expression in the absence of AR stimulation in human cancer cells. Depletion of endogenous ARA54 by small interfering RNA decreased both the protein and mRNA levels of cyclin D1. These changes did not result from a reduction in the half-life of either the protein or the mRNA, but from suppression of cyclin D1 gene transcription. In T98G cells, depletion of ARA54 increased the population of cells in G(1) phase, but reduced the population of cells in S phase, leading to a significant increase in the G(1)/S ratio and impaired cell growth. Furthermore, the amount of ARA54 mRNA appeared to positively correlate with cyclin D1 mRNA levels in specimens of clinical colon carcinomas, indicating that ARA54 is not only involved in the regulation of cyclin D1 expression in cultured cell lines but also in clinical cancer specimens. These results suggest that ARA54 might participate in enhancing cell cycle progression and cell proliferation via induction of cyclin D1.

Renal damage in obstructive nephropathy is decreased in Skp2-deficient mice.

Ubiquitin-dependent degradation of the cyclin-dependent kinase inhibitor p27 mediated by SCF-Skp2 ubiquitin ligase is involved in cell cycle regulation. Proliferation of tubular cells is a characteristic feature in obstructed kidneys of unilateral ureteral obstruction. Comparing Skp2(+/+) mice with Skp2(-/-) mice, we investigated the involvement of Skp2, a component of SCF-Skp2 ubiquitin ligase for p27, in the progression of renal lesions in unilateral ureteral obstructed kidneys. mRNA expression of Skp2 was markedly increased in the obstructed kidneys from Skp2(+/+) mice and peaked 3 days after unilateral ureteral obstruction. Renal atrophy, tubular dilatation, tubulointerstitial fibrosis, and increases in alpha-smooth muscle actin expression, the number of tubular cells, and proliferating tubular cells positive for Ki67 were observed in the obstructed kidneys from Skp2(+/+) mice; however, these findings were significantly attenuated in Skp2(-/-) mice. The p27 protein level was increased in the obstructed kidneys but was significantly greater in Skp2(-/-) mice. The number of Ki67-positive p27-negative cells was lower in obstructed kidneys from Skp2(-/-) mice than Skp2(+/+) mice, whereas that of Ki67-negative p27-positive cells was greater in Skp2(-/-) mice. These findings suggest that p27 accumulation, which results from SCF-Skp2 ubiquitin ligase deficiency in Skp2(-/-) mice, is involved in the amelioration of renal damage induced by obstructive nephropathy.

Ubiquitin-dependent degradation of adenovirus E1A protein is inhibited by BS69.

Adenovirus E1A protein perturbs the cell cycle and promotes cell transformation. Although E1A is relatively unstable, regulation of E1A stability has not been fully elucidated. Here, we showed that E1A was ubiquitinated and degraded using a proteasome in vivo system. Interestingly, we found that BS69, one of the E1A-binding proteins, inhibited ubiquitination of E1A. BS69 mutants lacking the MYND domain could not bind to E1A and did not inhibit ubiquitination of E1A. Moreover, we demonstrated that overexpression of BS69 stabilized E1A in vivo. These results suggest that BS69 controls E1A stability via inhibition of ubiquitination.

Essential role of STAT3 in cytokine-driven NF-kappaB-mediated serum amyloid A gene expression.

Serum amyloid A (SAA) is a sensitive marker of acute-phase responses and known as a precursor protein of amyloid fibril in amyloid A (AA) (secondary) amyloidosis. Since the serum SAA level is also closely related to activity of chronic inflammatory disease and coronary artery disease, it is important to clarify the exact induction mechanism of SAA from the clinical point of view. Here we provide evidence that STAT3 plays an essential role in cytokine-driven SAA expression, although the human SAA gene shows no typical STAT3 response element (RE) in its promoters. STAT3 and nuclear factor kappaB (NF-kappaB) p65 first form a complex following interleukin (IL)-1 and IL-6 (IL-1+6) stimulation, after which STAT3 interacts with nonconsensus sequences at a 3' site of the SAA gene promoter's NF-kappaB RE. Moreover, co-expression of p300 with STAT3 dramatically enhances the transcriptional activity of SAA. The formation of a complex with STAT3, NF-kappaB p65, and p300 is thus essential for the synergistic induction of the SAA gene by IL-1+6 stimulation. Our findings are expected to aid the understanding of the inflammatory status of AA amyloidosis to aid development of a therapeutic strategy for this disease by means of normalization of serum SAA levels.

Enhanced Mdm2 activity inhibits pRB function via ubiquitin-dependent degradation.

Retinoblastoma gene product (pRB) plays critical roles in regulation of the cell cycle and tumor suppression. It is known that downregulation of pRB can stimulate carcinogenesis via abrogation of the pRB pathway, although the mechanism has not been elucidated. In this study, we found that Mdm2, a ubiquitin ligase for p53, promoted ubiquitin-dependent degradation of pRB. pRB was efficiently ubiquitinated by wild-type Mdm2 in vivo as well as in vitro, but other RB family proteins were not. Mutant Mdm2 with a substitution in the RING finger domain showed dominant-negative stabilization of pRB. Both knockout and knockdown of Mdm2 caused accumulation of pRB. Moreover, Mdm2 inhibited pRB-mediated flat formation of Saos-2 cells. Downregulation of pRB expression was correlated with a high level of expression of Mdm2 in human lung cancers. These results suggest that Mdm2 regulates function of pRB via ubiquitin-dependent degradation of pRB.