Suppression of grp78 core promoter element-mediated stress induction by the dbpA and dbpB (YB-1) cold shock domain proteins

Knockdown of DNA-binding protein A enhances the chemotherapy sensitivity of colorectal cancer via suppressing the Wnt/β-catenin/Chk1 pathway

DNA-binding protein A (dbpA) is reported to be upregulated in many cancers and associated with tumor progress. The present study aimed to investigate the role of dbpA in 5-fluorouracil (5-FU)-resistant and oxaliplatin (L-OHP)-resistant colorectal cancer (CRC) cells. We found that 5-FU and L-OPH treatment promoted the expression of dbpA. Enhanced dbpA promoted the drug resistance of SW620 cells to 5-FU and L-OHP. DbpA knockdown inhibited cell proliferation, induced cell apoptosis, and cell cycle arrested in SW620/5-FU and SW620/L-OHP cells. Besides, dbpA short hairpin RNA (shRNA) enhanced the cytotoxicity of 5-FU and L-OHP to SW620/5-FU and SW620/L-OHP cells. Meanwhile, dbpA shRNA inhibited the activation of the Wnt/β-catenin pathway that induced by 5-FU stimulation in SW620/5-FU cells. Activation of the Wnt/β-catenin pathway or overexpression of checkpoint kinase 1 (Chk1) abrogated the promoting effect of dbpA downregulation on 5-FU sensitivity of CRC cells. Importantly, downregulation of dbpA suppressed tumor growth and promoted CRC cells sensitivity to 5-FU in vivo. Our study indicated that the knockdown of dbpA enhanced the sensitivity of CRC cells to 5-FU via Wnt/β-catenin/Chk1 pathway, and DbpA may be a potential therapeutic target to sensitize drug resistance CRC to 5-FU and L-OHP.

CEMIP upregulates BiP to promote breast cancer cell survival in hypoxia

Cell migration-inducing protein (CEMIP) and binding immunoglobulin protein (BiP) are upregulated in human cancers, where they drive cancer progression and metastasis. It has been shown that CEMIP resides in the endoplasmic reticulum (ER) where it interacts with BiP to induce cell migration, but the relationship between the two proteins was previously unknown. Here we show that CEMIP mediates activation of the BiP promoter and upregulates BiP transcript and protein levels in breast cancer cell lines. Moreover, CEMIP overexpression confers protective adaptations to cancer cells under hypoxic conditions, by decreasing apoptosis, activating autophagy, and increasing glucose uptake, to facilitate tumor growth. We demonstrate that BiP signals downstream of CEMIP, modulating cellular resistance to hypoxia. Reducing BiP in CEMIP-expressing cells sensitized cells to hypoxia treatment, decreased glucose uptake, and resulted in tumor regression in vivo. Our study provides insights into the link between CEMIP and BiP expression and the pro-survival role they play in hypoxia. Better understanding of the mechanisms behind cancer cell adaptations to harsh tumor environments could lead to development of improved cancer treatments.

Expression and network analysis of YBX1 interactors for identification of new drug targets in lung adenocarcinoma

Y-Box Binding protein 1 (YBX-1) is known to be involved in various types of cancers. It's interactors also play major role in various cellular functions. Present work aimed to study the expression profile of the YBX-1 interactors during lung adenocarcinoma (LUAD). The differential expression analysis involved 57 genes from 95 lung adenocarcinoma samples, construction of gene network and topology analysis. A Total of 43 genes were found to be differentially expressed from which 17 genes were found to be down regulated and 26 genes were up-regulated. We observed that Polyadenylate-binding protein 1 (PABPC1), a protein involved in YBX1 translation, is highly correlated with YBX1. The interaction network analysis for a differentially expressed non-coding RNA Growth Arrest Specific 5 (GAS5) suggests that two proteins namely, Growth Arrest Specific 2 (GAS2) and Peripheral myelin protein 22 (PMP22) are potentially involved in LUAD progression. The network analysis and differential expression suggests that Collagen type 1 alpha 2 (COL1A2) can be potential biomarker and target for LUAD.

Cold Shock Proteins Mediate GN with Mesangioproliferation

DNA binding protein A (DbpA) is a member of the human cold shock domain-containing protein superfamily, with known functions in cell proliferation, differentiation, and stress responses. DbpA mediates tight junction-associated activities in tubular epithelial cells, but the function of DbpA in mesangial cells is unknown. Here, we found DbpA protein expression restricted to vascular smooth muscle cells in healthy human kidney tissue but profound induction of DbpA protein expression within the glomerular mesangial compartment in mesangioproliferative nephritis. In vitro, depletion or overexpression of DbpA using lentiviral constructs led to inhibition or promotion, respectively, of mesangial cell proliferation. Because platelet-derived growth factor B (PDGF-B) signaling has a pivotal role in mesangial cell proliferation, we examined the regulatory effect of PDGF-B on DbpA. In vitro studies of human and rat mesangial cells confirmed a stimulatory effect of PDGF-B on DbpA transcript numbers and protein levels. Additional in vivo investigations showed DbpA upregulation in experimental rat anti-Thy1.1 nephritis and murine mesangioproliferative nephritis models. To interfere with PDGF-B signaling, we injected nephritic rats with PDGF-B neutralizing aptamers or the MEK/ERK inhibitor U0126. Both interventions markedly decreased DbpA protein expression. Conversely, continuous PDGF-B infusion in healthy rats induced DbpA expression predominantly within the mesangial compartment. Taken together, these results indicate that DbpA is a novel target of PDGF-B signaling and a key mediator of mesangial cell proliferation.

PARP-1 and YY1 are important novel regulators of CXCL12 gene transcription in rat pancreatic beta cells

Despite significant progress, the molecular mechanisms responsible for pancreatic beta cell depletion and development of diabetes remain poorly defined. At present, there is no preventive measure against diabetes. The positive impact of CXCL12 expression on the pancreatic beta cell prosurvival phenotype initiated this study. Our aim was to provide novel insight into the regulation of rat CXCL12 gene (Cxcl12) transcription. The roles of poly(ADP-ribose) polymerase-1 (PARP-1) and transcription factor Yin Yang 1 (YY1) in Cxcl12 transcription were studied by examining their in vitro and in vivo binding affinities for the Cxcl12 promoter in a pancreatic beta cell line by the electrophoretic mobility shift assay and chromatin immunoprecipitation. The regulatory activities of PARP-1 and YY1 were assessed in transfection experiments using a reporter vector with a Cxcl12 promoter sequence driving luciferase gene expression. Experimental evidence for PARP-1 and YY1 revealed their trans-acting potential, wherein PARP-1 displayed an inhibitory, and YY1 a strong activating effect on Cxcl12 transcription. Streptozotocin (STZ)-induced general toxicity in pancreatic beta cells was followed by changes in Cxcl12 promoter regulation. PARP-1 binding to the Cxcl12 promoter during basal and in STZ-compromised conditions led us to conclude that PARP-1 regulates constitutive Cxcl12 expression. During the early stage of oxidative stress, YY1 exhibited less affinity toward the Cxcl12 promoter while PARP-1 displayed strong binding. These interactions were accompanied by Cxcl12 downregulation. In the later stages of oxidative stress and intensive pancreatic beta cell injury, YY1 was highly expressed and firmly bound to Cxcl12 promoter in contrast to PARP-1. These interactions resulted in higher Cxcl12 expression. The observed ability of PARP-1 to downregulate, and of YY1 to upregulate Cxcl12 promoter activity anticipates corresponding effects in the natural context where the functional interplay of these proteins could finely balance Cxcl12 transcription.

YY1-binding sites provide central switch functions in the PARP-1 gene expression network

Evidence is presented for the involvement of the interplay between transcription factor Yin Yang 1 (YY1) and poly(ADP-ribose) polymerase-1 (PARP-1) in the regulation of mouse PARP-1 gene (muPARP-1) promoter activity. We identified potential YY1 binding motifs (BM) at seven positions in the muPARP-1 core-promoter (-574/+200). Binding of YY1 was observed by the electrophoretic supershift assay using anti-YY1 antibody and linearized or supercoiled forms of plasmids bearing the core promoter, as well as with 30 bp oligonucleotide probes containing the individual YY1 binding motifs and four muPARP-1 promoter fragments. We detected YY1 binding to BM1 (-587/-558), BM4 (-348/-319) and a very prominent association with BM7 (+86/+115). Inspection of BM7 reveals overlap of the muPARP-1 translation start site with the Kozak sequence and YY1 and PARP-1 recognition sites. Site-directed mutagenesis of the YY1 and PARP-1 core motifs eliminated protein binding and showed that YY1 mediates PARP-1 binding next to the Kozak sequence. Transfection experiments with a reporter gene under the control of the muPARP-1 promoter revealed that YY1 binding to BM1 and BM4 independently repressed the promoter. Mutations at these sites prevented YY1 binding, allowing for increased reporter gene activity. In PARP-1 knockout cells subjected to PARP-1 overexpression, effects similar to YY1 became apparent; over expression of YY1 and PARP-1 revealed their synergistic action. Together with our previous findings these results expand the PARP-1 autoregulatory loop principle by YY1 actions, implying rigid limitation of muPARP-1 expression. The joint actions of PARP-1 and YY1 emerge as important contributions to cell homeostasis.

Y-box-binding protein 1 (YB-1) and its functions

This review describes the structure and functions of Y-box binding protein 1 (YB-1) and its homologs. Interactions of YB-1 with DNA, mRNAs, and proteins are considered. Data on the participation of YB-1 in DNA reparation and transcription, mRNA splicing and translation are systematized. Results on interactions of YB-1 with cytoskeleton components and its possible role in mRNA localization are discussed. Data on intracellular distribution of YB-1, its redistribution between the nucleus and the cytoplasm, and its secretion and extracellular functions are summarized. The effect of YB-1 on cell differentiation, its involvement in extra- and intracellular signaling pathways, and its role in early embryogenesis are described. The mechanisms of regulation of YB-1 expression in the cell are presented. Special attention is paid to the involvement of YB-1 in oncogenic cell transformation, multiple drug resistance, and dissemination of tumors. Both the oncogenic and antioncogenic activities of YB-1 are reviewed. The potential use of YB-1 in diagnostics and therapy as an early cancer marker and a molecular target is discussed.

Rat Mrp2 gene expression is regulated by an interleukin-1β-stimulated biphasic response with enhanced transcription and subcellular shuttling of YB-1

INTRODUCTION

Expression of hepatobiliary transporters is decreased during endotoxemia. Reduction of Mrp2 is mediated by IL-1β-dependent signals but underlying mechanisms are still unclear. YB-1 is a predominantly cytoplasmic protein that translocates to the nucleus in response to various stimuli. Previously we have shown that YB-1 down-regulates Mrp2 expression in vitro. Therefore we investigated the potential role of YB-1 as regulator of hepatic acute phase genes.

METHODS

Liver sections from LPS-injected rats (20 h) were stained with YB-1-specific antibodies. Real-time RT-PCR quantification was performed for Mrp2, MMP-2 and YB-1. YB-1 protein was quantified from IL-1β- or TNFα-stimulated rat hepatoma cells (FaO) and the localization of a YFP-YB-1-CFP fusion protein was visualized by confocal microscopy in HepG2 human hepatocellular carcinoma cells. ChIP-assays and EMSA were performed to analyze YB-1 binding to DNA promoter elements.

RESULTS

In endotoxemic livers Mrp2 mRNA was down-regulated by 80%, while YB-1 mRNA expression increased 2.5-fold. Immunohistochemical staining showed a marked up-regulation and predominant nuclear localization of YB-1 protein in LPS challenged rats. In FAO cells IL-1β incubation increased cytoplasmic YB-1 protein content up to 16h. IL-1β stimulation resulted in a 6-fold up-regulation of endogenous YB-1 in the nuclear compartment, which occurred within 90min. In accord with these findings nuclear fluorescence was detected with a YFP-YB-1-CFP fusion protein introduced in HepG2 cells. In addition to DNA binding studies with endotoxemic rat liver tissue, ChIP assays revealed an IL-1β-dependent increase of YB-1 binding to the Mrp2-promoter in FAO cells.

CONCLUSION

YB-1 is activated during the hepatic acute phase response. IL-1β promotes a rapid nuclear YB-1 protein shuttling in hepatoma cells within 90 min and a transcriptional induction thereafter. This biphasic response may explain the IL-1β-mediated suppression of Mrp2 expression in endotoxemic rats.

Proteomic analysis of endothelial cold-adaptation

BACKGROUND

Understanding how human cells in tissue culture adapt to hypothermia may aid in developing new clinical procedures for improved ischemic and hypothermic protection. Human coronary artery endothelial cells grown to confluence at 37°C and then transferred to 25°C become resistant over time to oxidative stress and injury induced by 0°C storage and rewarming. This protection correlates with an increase in intracellular glutathione at 25°C. To help understand the molecular basis of endothelial cold-adaptation, isolated proteins from cold-adapted (25°C/72 h) and pre-adapted cells were analyzed by quantitative proteomic methods and differentially expressed proteins were categorized using the DAVID Bioinformatics Resource.

RESULTS

Cells adapted to 25°C expressed changes in the abundance of 219 unique proteins representing a broad range of categories such as translation, glycolysis, biosynthetic (anabolic) processes, NAD, cytoskeletal organization, RNA processing, oxidoreductase activity, response-to-stress and cell redox homeostasis. The number of proteins that decreased significantly with cold-adaptation exceeded the number that increased by 2:1. Almost half of the decreases were associated with protein metabolic processes and a third were related to anabolic processes including protein, DNA and fatty acid synthesis. Changes consistent with the suppression of cytoskeletal dynamics provided further evidence that cold-adapted cells are in an energy conserving state. Among the specific changes were increases in the abundance and activity of redox proteins glutathione S-transferase, thioredoxin and thioredoxin reductase, which correlated with a decrease in oxidative stress, an increase in protein glutathionylation, and a recovery of reduced protein thiols during rewarming from 0°C. Increases in S-adenosylhomocysteine hydrolase and nicotinamide phosphoribosyltransferase implicate a central role for the methionine-cysteine transulfuration pathway in increasing glutathione levels and the NAD salvage pathway in increasing the reducing capacity of cold-adapted cells.

CONCLUSIONS

Endothelial adaptation to mild-moderate hypothermia down-regulates anabolic processes and increases the reducing capacity of cells to enhance their resistance to oxidation and injury associated with 0°C storage and rewarming. Inducing these characteristics in a clinical setting could potentially limit the damaging effects of energy insufficiency due to ischemia and prevent the disruption of integrated metabolism at low temperatures.

Role of cold shock Y-box protein-1 in inflammation, atherosclerosis and organ transplant rejection

Chemokines (chemoattractant cytokines) are crucial regulators of immune cell extravasation from the bloodstream into inflamed tissue. Dysfunctional regulation and perpetuated chemokine gene expression are linked to progressive chronic inflammatory diseases and, in respect to transplanted organs, may trigger graft rejection. RANTES (regulated upon activation, normal T cell expressed and secreted (also known as CCL5)) is a model chemokine with relevance in numerous inflammatory diseases where the innate immune response predominates. Transcription factor Y-box binding protein-1 (YB-1) serves as a trans-regulator of CCL5 gene transcription in vascular smooth muscle cells and leucocytes. This review provides an update on YB-1 as a mediator of inflammatory processes and focuses on the role of YB-1 in CCL5 expression in diseases with monocytic cell infiltrates, albeit acute or chronic. Paradigms of such diseases encompass atherosclerosis and transplant rejection where cold shock protein YB-1 takes a dominant role in transcriptional regulation.

Hepatitis B virus protein up-regulated HLJ1 expression via the transcription factor YY1 in human hepatocarcinoma cells

The hepatitis B virus (HBV) protein plays a major role in hepatocellular carcinoma (HCC) development. However, its contribution to tumor invasion and metastasis has not been established so far. HLJ1 was recently cloned and classified as a member of the heat shock protein 40 family (Hsp40/DnaJ) which is abundantly expressed in HBV-related tumors, might be involved in tumor progression. In this study, the role of HBV in activation of HLJ1 was investigated. In HepG2 cells with transit or stable expression of HBV, HLJ1 expression was activated by HBV. The activity assay of HLJ1 promoter revealed that HBV up-regulated HLJ1 expression through the transcription factor YY1 sites within the HLJ1 promoter. YY1 expression was significantly up-regulated by HBV in a concentration-dependent manner. Knockdown of YY1 expression could partially reduce the HBV-induced HLJ1 activation which indicated that YY1 would be involved in HBV-induced HLJ1 expression. In conclusion, our data showed that HBV could promote HLJ1 expression by up-regulating the transcription factor YY1, and this provided a new insight of the mechanism of HBV induction in tumor metastasis.

CTCF and its protein partners: divide and rule?

CTCF is a ubiquitous transcription factor that is involved in numerous, seemingly unrelated functions. These functions include, but are not limited to, positive or negative regulation of transcription, enhancer-blocking activities at developmentally regulated gene clusters and at imprinted loci, and X-chromosome inactivation. Here, we review recent data acquired with state-of-the-art technologies that illuminate possible mechanisms behind the diversity of CTCF functions. CTCF interacts with numerous protein partners, including cohesin, nucleophosmin, PARP1, Yy1 and RNA polymerase II. We propose that CTCF interacts with one or two different partners according to the biological context, applying the Roman principle of governance, 'divide and rule' (divide et impera).

Transmembrane bZIP transcription factors in ER stress signaling and the unfolded protein response

Regulated intramembrane proteolysis (RIP) of the transmembrane transcription factor ATF6 represents a key step in effecting adaptive response to the presence of unfolded or malfolded protein in the endoplasmic reticulum. Recent studies have highlighted new ATF6-related transmembrane transcription factors. It is likely that current models for ER stress signaling are incomplete and that the expansion of the bZIP transmembrane family reflects selectivity in many aspects of these responses, including the type and duration of any particular stress, the cell type in which it occurs, and the integration with other aspects of cell-type-specific organization or additional intrinsic pathways, and the integration and communication between these pathways, not only in a cell-type-specific manner, but also between different tissues and organs. This review summarizes current information on the bZIP-transmembrane proteins and discusses outstanding questions on the elucidation of the stress signals, the repertoire of components involved in regulating different aspects of the forward transport, cleavage, nuclear import, transcriptional activity, and turnover of each of these factors, and dissection of the integration of the various outputs into broad coordinated responses.

GRP78 as a novel predictor of responsiveness to chemotherapy in breast cancer

The discovery of predictive factors for chemoresistance is critical for improving adjuvant therapy for cancer patients. The 78-kDa glucose-regulated protein (GRP78), widely used as an indicator of the unfolded protein response (UPR), is induced in the tumor microenvironment. In vitro studies suggest that GRP78 confers chemoresistance to topoisomerase inhibitors, such as Adriamycin (doxorubicin). Here, we report on a retrospective cohort study of 127 stage II and III breast cancer patients who were treated with Adriamycin-based chemotherapy. Archival tumor specimens were available for analysis and the relationship of GRP78 expression level to "time to recurrence" (TTR), used as a surrogate marker for drug resistance, was examined. Our data show that 67% of the study subjects expressed high level of GRP78 in their tumors before the initiation of chemotherapy and suggest an association between GRP78 positivity and shorter TTR [hazard ratio (HR), 1.78; P = 0.16]. Interestingly, subgroup analysis reveals that the HR for the GRP78-positive group increased significantly among patients who did not receive further taxane treatment (HR, 3.00; P = 0.022) and among mastectomy patients (HR, 3.33; P = 0.027). The HR was even stronger among mastectomy patients who did not receive further taxane treatment (HR, 4.82; P = 0.010). The use of GRP78 as a predictor for chemoresponsiveness and the potential interaction of GRP78 and/or the UPR pathways with taxanes warrant larger studies.

Analysis of overrepresented motifs in human core promoters reveals dual regulatory roles of YY1

A set of 723 high-quality human core promoter sequences were compiled and analyzed for overrepresented motifs. Beside the two well-characterized core promoter motifs (TATA and Inr), several known motifs (YY1, Sp1, NRF-1, NRF-2, CAAT, and CREB) and one potentially new motif (motif8) were found. Interestingly, YY1 and motif8 mostly reside immediately downstream from the TSS. In particular, the YY1 motif occurs primarily in genes with 5'-UTRs shorter than 40 base pairs (bp) and its locations coincide with the translation start site. We verified that the YY1 motif is bound by YY1 in vitro. We then performed detailed analysis on YY1 chromatin immunoprecipitation data with a whole-genome human promoter microarray (ChIP-chip) and revealed that the thus identified promoters in HeLa cells were highly enriched with the YY1 motif. Moreover, the motif overlapped with the translation start sites on the plus strand of a group of genes, many with short 5'-UTRs, and with the transcription start sites on the minus strand of another distinct group of genes; together, the two groups of genes accounted for the majority of the YY1-bound promoters in the ChIP-chip data. Furthermore, the first group of genes was highly enriched in the functional categories of ribosomal proteins and nuclear-encoded mitochondria proteins. We suggest that the YY1 motif plays a dual role in both transcription and translation initiation of these genes. We also discuss the evolutionary advantages of housing a transcriptional element inside the transcript in terms of the migration of these genes in the human genome.

Expression of endoplasmic reticulum molecular chaperone Grp78 in human lung cancer and its clinical significance

All eukaryotic cells respond to the accumulation of unfolded proteins in the endoplasmic reticulum (ER) by signaling an adaptive pathway termed the unfolded protein response (UPR). Glucose-regulated protein (Grp) 78 is a molecular chaperone involved in the UPR. The aim of this study was to detect Grp78 expression in lung cancer using immunohistochemical (IHC) staining, and also to evaluate the relationship between the Grp78 expression level and the prognosis of patients with lung cancer. We used immunohistochemistry to analyze the protein expression of Grp78 in paraffin-embedded tumor samples from 132 well-characterized lung cancer patients and compared the expression level of Grp78, clinical variables and survival outcome. A positive expression of Grp78 was detected in the cytoplasm of tumor cells in 88 of the 132 patients (66.7%) with lung cancer. No significant difference was observed between the Grp78 expression and the gender, age at operation, histological type, pathologic stage, pathologic T status, and pathologic N status. Lung cancer patients with a positive Grp78 expression tended to show a better prognosis than those with a negative Grp78 expression. In addition, a multivariate analysis of the clinicopathologic characteristics of lung cancer indicated a positive expression of Grp78 to be a significant factor for predicting a favorable prognosis (p < 0.001, risk ratio = 2.35). A positive expression of Grp78 may thus be a useful marker for predicting a favorable prognosis in patients undergoing a resection of lung cancer. The ER stress pathway mediated by Grp78 may therefore be responsible for controlling the growth of lung cancer cells.

The transcriptional factor YY1 upregulates the novel invasion suppressor HLJ1 expression and inhibits cancer cell invasion

By using microarray and an invasion/metastasis lung cell line model, we identified the DnaJ-like heat shock protein 40, HLJ1, and found that the expression of HLJ1 correlates negatively with cancer cell invasion ability. Overexpression of HLJ1 can suppress cancer cell invasion in vitro. We further characterize the putative promoter region and investigate the transcriptional regulations of human HLJ1. A serial deletion of the 1.2 kb at the 5'-flanking region of the human HLJ1 gene was subcloned into a vector containing reporter gene and transfected into human lung adenocarcinoma cell line CL1-0, followed by luciferase activity assay. The results indicated that the region from -232 to +176 could drive the basal transcriptional activity of the HLJ1 gene. Sequence analysis of the HLJ1 gene promoter region showed absence of a TATA box, but identified an inverted CCAAT box and four YY1 transcriptional factor-binding sites, which may be important in the regulation of HLJ1 expression. Co-transfection of the YY1 and HLJ1 basal promoter regions, site-directed mutagenesis, and electrophoretic mobility shift assay confirmed that YY1 could upregulate HLJ1 basal promoter activity. Furthermore, we also demonstrated that overexpression of YY1 in CL1-0 cells can increase HLJ1 expression and reduce cell invasive capability. The reduction of cancer cell invasive ability is, at least in part, through upregulation of E-cadherin expression. The increase in HLJ1 and E-cadherin expression, as well as the suppression of invasion ability, can be reversed specifically by HLJ1 siRNA.

Expression of stress response protein glucose regulated protein-78 mediated by c-Myb

Glucose regulated protein-78, GRP78 has been implicated in the protection of tumor cells from cytotoxic damage and apoptosis. When protein profiles of colon cell lines were investigated we found remarkably high GRP78 expression in two cell lines. These cell lines express elevated levels of the transcription factor c-Myb due to genomic amplification of the c-myb locus and we hypothesized that c-Myb regulates GRP78 expression in colon cancer cells. The promoters of human and murine GRP78 and the related family member GRP94 were examined and potential c-Myb binding sites were identified and characterized. DNA binding studies with recombinant c-Myb and nuclear extracts together with ChIP assays on colon cell lines validated these sites. Endogenous GRP78 expression was further induced in these colon cells in response to Thapsigargin treatment, a potent inducer of the unfolded protein response. Transactivation studies with the human GRP78 promoter in colon cell lines showed reporter activity was dependent upon the presence of a conserved c-Myb binding site independent of sequences associated with the unfolded protein response. Finally, over-expression of c-Myb induced the endogenous GRP78 gene. These data suggest that amplification of c-myb in tumor cells may lead to robust GRP78 gene induction, which may in turn assist cells in survival under conditions of oxygen deprivation and nutrient stress.

The pleiotropic functions of the Y-box-binding protein, YB-1

The Y-box-binding protein (YB-1) represents the most evolutionary conserved nucleic-acid-binding protein currently known. YB-1 is a member of the cold-shock domain (CSD) protein superfamily. It performs a wide variety of cellular functions, including transcriptional regulation, translational regulation, DNA repair, drug resistance and stress responses to extracellular signals. As a result, YB-1 expression is closely associated with cell proliferation. In this review, we will begin by briefly describing the characteristics of YB-1 and will then summarize the pleiotropic functions brought about via DNA-RNA transaction and protein-protein interactions. In addition, we will discuss the diverse range of potential physiological and pathological functions of YB-1.

Poly(A)-binding protein positively affects YB-1 mRNA translation through specific interaction with YB-1 mRNA

The major protein of cytoplasmic mRNPs from rabbit reticulocytes, YB-1, is a member of an ancient family of proteins containing a common structural feature, cold-shock domain. In eukaryotes, this family is represented by multifunctional mRNA/Y-box DNA-binding proteins that control gene expression at different stages. To address possible post-transcriptional regulation of YB-1 gene expression, we examined effects of exogenous 5'- and 3'-untranslatable region-containing fragments of YB-1 mRNA on its translation and stability in a cell-free system. The addition of the 3' mRNA fragment as well as its subfragment I shut off protein synthesis at the initiation stage without affecting mRNA stability. UV cross-linking revealed four proteins (69, 50, 46, and 44 kDa) that specifically interacted with the 3' mRNA fragment; the inhibitory subfragment I bound two of them, 69- and 50-kDa proteins. We have identified these proteins as PABP (poly(A)-binding protein) (69 kDa) and YB-1 (50 kDa) and demonstrated that titrating out of PABP by poly(A) strongly and specifically inhibits YB-1 mRNA cap(+)poly(A)(-) translation in a cell-free system. Thus, PABP is capable of positively affecting YB-1 mRNA translation in a poly(A) tail-independent manner.

Mitochondrial calcium-mediated reactive oxygen species are essential for the rapid induction of the grp78 gene in 9L rat brain tumour cells

The glucose-regulated protein grp78 gene is rapidly transactivated in 9L rat brain tumour (RBT) cells treated with okadaic acid (OA) followed by heat shock (HS) (termed OA-->HS treatment). By Northern blotting analyses and transient transfection assays, we herein show that transactivation of grp78 by OA-->HS is abolished by an intracellular calcium chelator, bis(aminophenoxy)ethane N,N'-tetraacetic acid (BAPTA), and an inhibitor of mitochondrial Ca(2+) uniporter, ruthenium red (RR), while unaffected by cyclosporin A (CsA), an inhibitor of mitochondrial permeability transition pore (MTP). The inhibitory effects of BAPTA and RR also present in OA-->HS induction of transient elevation of intracellular hydrogen peroxide. The requirement of reactive oxygen intermediates (ROIs) is confirmed by substitutional addition of antioxidants, N-acetyl cysteine (NAC) and pyrrolidinedithiocarbamate (PDTC) during OA-->HS treatment, mimicking these inhibitory effects of BAPTA and RR. Western blotting analyses show that phosphorylation of transcription factor CREB is diminished only by BAPTA but not by RR, while phosphorylation of ATF-2 is unaffected by either agent. Conclusively, we present that both the disturbances of mitochondrial calcium homeostasis and reactive oxygen intermediates are essential for rapid transactivation of grp78, and this pathway is separate from protein kinase A (PKA)-dependent CREB activation or p38 mitogen-activated protein kinase (p38(MAPK))-dependent ATF-2 activation and signalling.

A novel mechanism of repression of the vascular endothelial growth factor promoter, by single strand DNA binding cold shock domain (Y-box) proteins in normoxic fibroblasts

Overexpression of vascular endothelial growth factor (VEGF) is implicated in a number of diseases. It is therefore critical that mechanisms exist to strictly regulate VEGF expression. A hypoxia-responsive (HR) region of the VEGF promoter which binds the HIF-1 transcription factor is a target for many signals that up-regulate VEGF transcription. Repressors targeting the HIF-1 transcription factor have been identified but no repressors directly binding the HR promoter region had been reported. We now report a novel mechanism of repression of the VEGF HR region involving DNA binding. We find that single strand DNA-specific cold shock domain (CSD or Y-box) proteins repress the HR region via a binding site downstream of the HIF-1 site. The repressor site is functional in unstimulated, normoxic fibroblasts and represents a novel means to prevent expression of VEGF in the absence of appropriate stimuli. We characterized complexes forming on the VEGF repressor site and identified a previously unreported nuclear CSD protein complex containing dbpA. Nuclear dbpA appears to bind as a dimer and we determined a means by which nuclear CSD proteins may enter double strand DNA to bind to their single strand sites to bring about repression of the VEGF HR region.

Novel translational control through an iron-responsive element by interaction of multifunctional protein YB-1 and IRP2

The eukaryotic Y-box-binding protein YB-1 functions in various biological processes, including DNA repair, cell proliferation, and transcriptional and translational controls. To gain further insight into how human YB-1 plays its role in pleiotropic functions, we here used two-hybrid screenings to identify partners of this protein; the results showed that YB-1 itself, iron-regulatory protein 2 (IRP2), and five ribosomal proteins each served as partners to YB-1. We then examined the biological effect of the interaction of YB-1 and IRP2 on translational regulation. Both in vitro binding and coimmunoprecipitation assays showed the direct interaction of YB-1 and IRP2 in the presence of a high concentration of iron. RNA gel shift assays showed that YB-1 reduced the formation of the IRP2-mRNA complex when the iron-responsive element of the ferritin mRNA 5' untranslated region (UTR) was used as a probe. By using an in vitro translation assay using luciferase mRNA ligated to the ferritin mRNA 5'UTR as a reporter construct, we showed that both YB-1 and IRP2 inhibited the translation of the mRNA. However, coadministration of YB-1 and IRP2 proteins abrogated the inhibition of protein synthesis by each protein. An In vivo coimmunoprecipitation assay showed that IRP2 bound to YB-1 in the presence of iron and a proteasome inhibitor. The direct interaction of YB-1 and IRP2 provides the first evidence of the involvement of YB-1 in the translational regulation of an iron-related protein.

Functional interaction between JC virus late regulatory agnoprotein and cellular Y-box binding transcription factor, YB-1

Human polyomavirus JC virus (JCV) is a causative agent of progressive multifocal leukoencephalopathy which results from lytic infection of glial cells. Although significant progress has been made in understanding the regulation of JCV gene transcription, the mechanism(s) underlying the viral lytic cycle remains largely unknown. We recently reported that the JCV late auxiliary Agnoprotein may have a regulatory role in JCV gene transcription and replication. Here, we investigated its regulatory function in viral gene transcription through its physical and functional interaction with YB-1, a cellular transcription factor which contributes to JCV gene expression in glial cells. Time course studies revealed that Agnoprotein is first detected at day 3 postinfection and that its level increased during the late stage of the infection cycle. Agnoprotein is mainly localized to the cytoplasmic compartment of the infected cell, with high concentrations found in the perinuclear region. While the position of Agnoprotein throughout the infection cycle remained relatively unaltered, the subcellular distribution of YB-1 between the cytoplasm and nucleus changed. Results from coimmunoprecipitation and glutathione S-transferase pull-down experiments revealed that Agnoprotein physically interacts with YB-1 and that the amino-terminal region of Agnoprotein, between residues 1 and 36, is critical for this association. Further investigation of this interaction by functional assays demonstrated that Agnoprotein negatively regulates YB-1-mediated gene transcription and that the region corresponding to residues 1 to 36 of Agnoprotein is important for the observed regulatory event. Taken together, these data demonstrate that the interaction of the viral late regulatory Agnoprotein and cellular Y-box binding factor YB-1 modulates transcriptional activity of JCV promoters.

A member of the Y-box protein family interacts with an upstream element in the alpha1(I) collagen gene

Transforming growth factor beta (TGF-beta) stimulates protein complex formation on a TGF-beta response element (TAE) found in the distal portion (-1624) of the collagen alpha 1(I) promoter. To identify the fibroblast proteins in this complex, an expression library constructed from human embryonic lung fibroblasts mRNA was screened using a tetramer of TAE. Y-box binding protein (YB-1), was identified as a protein in the TAE-protein complex. The protein expressed by phage clones formed a specific complex with labeled TAE but not mutated TAE (mTAE) similar to the complex formed with nuclear protein. Nuclear protein-TAE complexes isolated from native gels contained YB-1 by Western analysis. TGF-beta treatment increased the amount of YB-1 protein in nuclear extracts, decreased its amount in cytoplasm, but did not alter the steady state levels of YB-1 mRNA. A full-length YB-1 protein expressed in human lung fibroblasts was primarily located in the nucleus with punctate staining in cytoplasmic regions. The expression of YB-1 decreased in the cytoplasm after 2 h of TGF-beta treatment. Therefore, the increased binding activity seen in TGF-beta-stimulated nuclear extracts was due primarily to relocalization of YB-1 from the cytoplasm to the nuclear compartment. Co-transfection of YB-1 cDNA with a collagen promoter-reporter construct caused a dose-dependent activation of collagen promoter activity in rat fibroblasts whereas the promoter with a mutation in the TAE element was not sensitive to YB-1 co-expression. In conclusion, we have identified YB-1 as a protein that interacts with a TGF-beta response element in the distal region of the collagen alpha 1(I) gene. YB-1 protein activates the collagen promoter and translocates into the nucleus during TGF-beta addition to fibroblasts, suggesting a role for this protein in TGF-beta signaling.

YY1 is a positive regulator of transcription of the Col1a1 gene

Both cell-specific and ubiquitous transcription factors in fibroblasts have been identified as critical for expression of the Col1a1 gene, which encodes the alpha1 chain of type I collagen. Here, we report that Yin Yang 1 (YY1) binds to the Col1a1 promoter immediately upstream of the TATA box, and we examine the functional implications of YY1 binding for regulation of Col1a1 gene expression in BALBc/3T3 fibroblasts. The Col1a1 promoter region spanning base pairs (bp) -56 to -9 bound purified recombinant YY1 and the corresponding binding activity in nuclear extracts was supershifted using a YY1-specific antibody. Mutation of the TATA box to TgTA enhanced YY1 complex formation. Mutation analysis revealed two YY1 core binding sites at -40/-37 bp (YY1A) and, on the reverse strand, at -32/-29 bp (YY1B) immediately adjacent to the TATA box. In transfections using Col1a1-luciferase constructs, mutation of YY1A decreased activity completely (wild-type p350 (p350wt), -222/+113 bp) or partially (p130wt, -84 bp/+13 bp), whereas mutation of YY1B blocked the expression of both promoter constructs. Cotransfection with pCMV-YY1 increased p350wt and p130wt activities by as much as 10-fold, whereas antisense YY1 decreased constitutive expression and blocked the increased activity due to pCMV-YY1 overexpression. The mTgTA constructs were devoid of activity, arguing for a requirement for cognate binding of the TATA box-binding protein (TBP). Electrophoretic mobility shift assays performed under conditions permitting TBP binding showed that recombinant TBP/TFIID and YY1 could bind to the -56/-9 bp fragment and that YY1B was the preferred site for YY1 binding. Our results indicate that YY1 binds to the Col1a1 proximal promoter and functions as a positive regulator of constitutive activity in fibroblasts. Although YY1 is not sufficient for transcriptional initiation, it is a required component of the transcription machinery in this promoter.

The glucose-regulated proteins: stress induction and clinical applications

A protective mechanism used by cells to adapt to stress of the endoplasmic reticulum (ER) is the induction of members of the glucose-regulated protein (Grp) family. The induction of mammalian Grp proteins in response to ER stress involves a complex network of regulators and novel mechanisms. The elucidation of Grp function and regulation opens up new therapeutic approaches to diseases associated with ER stress and cancer.

The transcriptional regulator Yin Yang 1 activates the myelin PLP gene

Inauguration of the myelin program in developing oligodendrocytes requires the activation of those genes that encode the myelin proteins and the enzymes responsible for the synthesis and degradation of myelin lipids. An activator of the most abundantly expressed myelin protein, proteolipid protein (PLP), has been identified in a yeast one-hybrid system. The ubiquitously expressed zinc finger protein Yin Yang 1 (YY1) recognizes the myelin PLP promoter in vitro and in vivo. When overexpressed in an oligodendrocyte cell line, YY1 enhances transcription of the PLP promoter. A truncated version of YY1 that includes only the four zinc finger domains has little effect. The binding site for YY1 in the PLP promoter (site 3) fits the YY1 consensus site and DNA-protein complexes containing site 3 can be supershifted with an antibody directed against YY1 protein. Moreover, oligonucleotides with a mutated version of the PLP promoter site 3 are unable to bind to nuclear proteins or to compete for binding in a gel shift system. Finally, mutation of this site greatly reduces the activity of a 1-kb PLP promoter region in transfected glial cells. Our results suggest that PLP is a target gene for the transcriptional regulator YY1. This versatile transcription factor and nuclear matrix protein may boost transcription of the PLP gene to meet the demands of actively myelinating oligodendrocytes.

Cold shock domain factors activate the granulocyte-macrophage colony-stimulating factor promoter in stimulated Jurkat T cells

Cold shock domain (CSD) family members have been shown to play roles in either transcriptional activation or repression of many genes in various cell types. We have previously shown that CSD proteins dbpAv and dbpB (also known as YB-1) act to repress granulocyte-macrophage colony-stimulating factor transcription in human embryonic lung (HEL) fibroblasts via binding to single-stranded DNA regions across the promoter. Here we show that the same CSD factors are involved in granulocyte-macrophage colony-stimulating factor transcriptional activation in Jurkat T cells. Unlike the mechanisms of CSD repression in HEL fibroblasts, CSD-mediated activation in Jurkat T cells is not mediated through DNA binding but presumably through protein-protein interactions via the C terminus of the CSD protein with transcription factors such as RelA/NF-kappaB p65. We demonstrate that Jurkat T cells lack truncated CSD factor subtypes present in HEL fibroblasts, which raises the possibility that the cellular content of CSD proteins may determine their final role as activators or repressors of transcription.

Direct interaction of p53 with the Y-box binding protein, YB-1: a mechanism for regulation of human gene expression

The Y-box binding protein, YB-1, belongs to a family of multifunctional proteins which regulate gene expression on both transcriptional and translational levels. The tumor suppressor gene p53 displays growth suppressive properties by regulating gene expression through transcriptional regulation. We now demonstrate that YB-1 directly interacts with p53 using an in vitro pull-down assay. Using immunochemical co-precipitation methods, we also found that the two proteins are bound in vivo. Deletion analysis showed that three independent domains of YB-1, one at the N-terminal and two at the C-terminal, interact with p53. Conversely, a 14 amino acid sequence at the C-terminal of p53 was required for its interaction with YB-1. Gel mobility shift assays showed that the interaction of YB-1 with p53 stimulated the sequence-specific DNA binding of p53 to its consensus sequence. By contrast, this interaction inhibited the binding of YB-1. Using a p53-responsive p21 promoter linked to a reporter gene, it can be shown that antisense expression of YB-1 inhibits the induction of this promoter by p53 in transient transfection assays. These findings delineate a straightforward mechanism for gene expression through p53-YB-1 interaction.

Physical and functional interaction between two pluripotent proteins, the Y-box DNA/RNA-binding factor, YB-1, and the multivalent zinc finger factor, CTCF

CTCF is a unique, highly conserved, and ubiquitously expressed 11 zinc finger (ZF) transcriptional factor with multiple DNA site specificities. It is able to bind to varying target sequences to perform different regulatory roles, including promoter activation or repression, creating hormone-responsive gene silencing elements, and functional block of enhancer-promoter interactions. Because different sets of ZFs are utilized to recognize different CTCF target DNA sites, each of the diverse DNA.CTCF complexes might engage different essential protein partners to define distinct functional readouts. To identify such proteins, we developed an affinity chromatography method based on matrix-immobilized purified recombinant CTCF. This approach resulted in isolation of several CTCF protein partners. One of these was identified as the multifunctional Y-box DNA/RNA-binding factor, YB-1, known to be involved in transcription, replication, and RNA processing. We examined CTCF/YB-1 interaction by reciprocal immunoprecipitation experiments with anti-CTCF and anti-YB-1 antibodies, and found that CTCF and YB-1 form complexes in vivo. We show that the bacterially expressed ZF domain of CTCF is fully sufficient to retain YB-1 in vitro. To assess possible functional significance of CTCF/YB-1 binding, we employed the very first identified by us, negatively regulated, target for CTCF (c-myc oncogene promoter) as a model in co-transfection assays with both CTCF and YB-1 expression vectors. Although expression of YB-1 alone had no effect, co-expression with CTCF resulted in a marked enhancement of CTCF-driven c-myc transcriptional repression. Thus our findings demonstrate, for the first time, the biological relevance of the CTCF/YB-1 interaction.

Thrombin activates a Y box-binding protein (DNA-binding protein B) in endothelial cells

Thrombin stimulates the expression of multiple genes in endothelial cells (ECs), but the trans-acting factors responsible for this induction remain undefined. We have previously described a thrombin-inducible nuclear factor (TINF), which binds to an element in the PDGF B promoter and is responsible for the thrombin inducibility of this gene. Inactive cytoplasmic TINF is rapidly activated and translocated to nuclei of ECs upon stimulation with thrombin. We have now purified TINF from thrombin-treated ECs. Amino acid sequencing revealed it to be a member of the Y-box protein family, and the sole Y-box protein-encoding cDNA we detected in human or bovine ECs corresponded to DNA-binding protein B (dbpB). DbpB translocated to the nucleus after thrombin stimulation of ECs as shown by FACS analysis of nuclei from ECs expressing GFP-dbpB fusion proteins. During thrombin activation, dbpB was found to be cleaved, yielding a 30-kDa NH(2)-terminal fragment that recognized the thrombin-response element sequence, but not the Y-box consensus sequence. Preincubation of ECs with protein tyrosine phosphatase inhibitors completely blocked dbpB activation by thrombin and blocked induction of endogenous PDGF B-chain mRNA and promoter activation by thrombin. Y-box proteins are known to act constitutively to regulate the expression of several genes. Activation of this class of transcription factors in response to thrombin or any other agonist represents a novel signaling pathway.

ATF6 as a transcription activator of the endoplasmic reticulum stress element: thapsigargin stress-induced changes and synergistic interactions with NF-Y and YY1

ATF6, a member of the leucine zipper protein family, can constitutively induce the promoter of glucose-regulated protein (grp) genes through activation of the endoplasmic reticulum (ER) stress element (ERSE). To understand the mechanism of grp78 induction by ATF6 in cells subjected to ER calcium depletion stress mediated by thapsigargin (Tg) treatment, we discovered that ATF6 itself undergoes Tg stress-induced changes. In nonstressed cells, ATF6, which contains a putative short transmembrane domain, is primarily associated with the perinuclear region. Upon Tg stress, the ATF6 protein level dropped initially but quickly recovered with the additional appearance of a faster-migrating form. This new form of ATF6 was recovered as soluble nuclear protein by biochemical fractionation, correlating with enhanced nuclear localization of ATF6 as revealed by immunofluorescence. Optimal ATF6 stimulation requires at least two copies of the ERSE and the integrity of the tripartite structure of the ERSE. Of primary importance is a functional NF-Y complex and a high-affinity NF-Y binding site that confers selectivity among different ERSEs for ATF6 inducibility. In addition, we showed that YY1 interacts with ATF6 and in Tg-treated cells can enhance ATF6 activity. The ERSE stimulatory activity of ATF6 exhibits properties distinct from those of human Ire1p, an upstream regulator of the mammalian unfolded protein response. The requirement for a high-affinity NF-Y site for ATF6 but not human Ire1p activity suggests that they stimulate the ERSE through diverse pathways.

Nucleolin and YB-1 are required for JNK-mediated interleukin-2 mRNA stabilization during T-cell activation

Regulated mRNA turnover is a highly important process, but its mechanism is poorly understood. Using interleukin-2 (IL-2) mRNA as a model, we described a role for the JNK-signaling pathway in stabilization of IL-2 mRNA during T-cell activation, acting via a JNK response element (JRE) in the 5′ untranslated region (UTR). We have now identified two major RNA-binding proteins, nucleolin and YB-1, that specifically bind to the JRE. Binding of both proteins is required for IL-2 mRNA stabilization induced by T-cell activation signals and for JNK-induced stabilization in a cell-free system that duplicates essential features of regulated mRNA decay. Nucleolin and YB-1 are required for formation of an IL-2 mRNP complex that responds to specific mRNA stabilizing signals.

Nucleolin and YB-1 are required for JNK-mediated interleukin-2 mRNA stabilization during T-cell activation

Regulated mRNA turnover is a highly important process, but its mechanism is poorly understood. Using interleukin-2 (IL-2) mRNA as a model, we described a role for the JNK-signaling pathway in stabilization of IL-2 mRNA during T-cell activation, acting via a JNK response element (JRE) in the 5' untranslated region (UTR). We have now identified two major RNA-binding proteins, nucleolin and YB-1, that specifically bind to the JRE. Binding of both proteins is required for IL-2 mRNA stabilization induced by T-cell activation signals and for JNK-induced stabilization in a cell-free system that duplicates essential features of regulated mRNA decay. Nucleolin and YB-1 are required for formation of an IL-2 mRNP complex that responds to specific mRNA stabilizing signals.

An ordered array of cold shock domain repressor elements across tumor necrosis factor-responsive elements of the granulocyte-macrophage colony-stimulating factor promoter

The tumor necrosis factor-alpha-responsive region of the human granulocyte-macrophage colony-stimulating factor (GM-CSF) promoter (-114 to -31) encompasses binding sites for NF-kappaB, CBF, AP-1, ETS, and NFAT families of transcription factors. We show both here and previously that mutation of any one of these binding sites greatly reduces tumor necrosis factor-alpha induction of the GM-CSF promoter. Interspersed between these elements are sequences that when mutated lead to an increase in GM-CSF promoter activity. We have previously shown that two of these repressor elements bind proteins known as cold shock domain (CSD) factors and that overexpression of CSD proteins leads to repression of GM-CSF promoter activity in fibroblasts. CSD proteins are single strand DNA- and RNA-binding proteins that contact 5'-CCTG-3' sequences in the GM-CSF repressor elements. We show here that two newly identified repressor sequences in the proximal promoter can also bind CSD proteins. We have characterized the CSD-containing protein complexes that bind to the GM-CSF promoter and identified a novel protein related to mitochondrial single strand binding protein that forms part of one of these complexes. The four CSD-binding sites on the promoter occur in pairs on opposite strands of the DNA and appear to form an ordered array of binding elements. A similar ordered array of CSD sites are present in the promoters of the granulocyte colony-stimulating factor and interleukin-3 genes, implying a common mechanism for negative regulation of these myeloid growth factors.

Induction of glucose-regulated protein 78 in rat uterine glandular epithelium during uterine sensitization for the decidual cell reaction

Endometrial receptivity for implantation and sensitization for decidualization in rodents is a transient state under the control of the ovarian steroids estrogen and progesterone. It is unclear, however, what molecular events mediate the onset of uterine receptivity. Messenger RNA differential display was performed on endometrial RNA from ovariectomized rats differentially sensitized for decidualization. Maximally sensitized uteri were at the equivalent of Day 5 of pseudopregnancy, and temporally nonsensitized uteri at Day 4 or 6; hormonally nonsensitized uteri were from animals on Day 5 treated with low or high doses of estradiol on Day 4. A cDNA with endometrial expression restricted to maximally sensitized uteri was isolated, cloned, and sequenced. The cDNA matched the sequence for glucose-regulated protein 78 (GRP78), a heat shock 70-related protein that resides in the lumen of the endoplasmic reticulum (ER) and has roles in several cellular processes including multimeric protein assembly, the degradation of proteins, and the storage and regulation of ER luminal calcium. Northern blot analysis indicated a dramatic increase in GRP78 mRNA levels restricted to the sensitized, Day 5 endometrium, suggesting a role in the onset of the sensitized phase. In situ hybridization and immunohistochemistry experiments localized the up-regulation of GRP78 within the receptive endometrium to the glandular epithelium.

Transcriptional activation of the MDR1 gene by UV irradiation. Role of NF-Y and Sp1

The MDR1 promoter is subject to control by various internal and external stimuli. We have previously shown that the CCAAT box-binding protein, NF-Y, mediates MDR1 activation by the histone deacetylase inhibitors, trichostatin A and sodium butyrate, through the recruitment of the co-activator, P/CAF. We have now extended our investigation to the activation of MDR1 by genotoxic stress. We show that activation of the MDR1 promoter by UV irradiation is also dependent on the CCAAT box (-82 to -73) as well as on a proximal GC element (-56 to -42). Gel shift and supershift analyses with nuclear extracts prepared from human KB-3-1 cells identified NF-Y as the transcription factor interacting with the CCAAT box, while Sp1 was the predominant factor binding to the GC element. Mutations that abrogated binding of either of these factors reduced or abolished activation by ultraviolet irradiation; moreover, co-expression of a dominant-negative NF-Y protein (NF-YA29) reduced UV-activated transcription. Interestingly, YB-1, a transcription factor that also recognizes the CCAAT motif and had been reported to mediate induction of the MDR1 promoter by ultraviolet light, was incapable of interacting with the double-stranded MDR1 CCAAT box oligonucleotide in nuclear extracts, although it did interact with a single-stranded oligonucleotide. Furthermore, a mutation that abolished activation of MDR1 by UV-irradiation had no effect on YB-1 binding and co-transfection of a YB-1 expression plasmid had a repressive effect on UV-inducible transcription. Taken together, these results indicate a role for both NF-Y and Sp1 in the transcriptional activation of the MDR1 gene by genotoxic stress, and indicate that YB-1, if involved, is not sufficient to mediate this activation.

The single-stranded DNA-binding proteins, Puralpha, Purbeta, and MSY1 specifically interact with an exon 3-derived mouse vascular smooth muscle alpha-actin messenger RNA sequence

Amino acids 44-53 of mouse vascular smooth muscle alpha-actin are encoded by a region of exon 3 that bears structural similarity to an essential MCAT enhancer element in the 5' promoter of the gene. The single-stranded DNA-binding proteins, Puralpha, Purbeta, and MSY1, interact with each other and with opposite strands of the enhancer to repress transcription in fibroblasts (Sun, S., Stoflet, E. S., Cogan, J. G., Strauch, A. R., and Getz, M. J. (1995) Mol. Cell. Biol. 15, 2429-2436; Kelm, R. J., Jr., Cogan, J. G., Elder, P. K., Strauch, A. R., and Getz, M. J. (1999) J. Biol. Chem. 274, 14238-14245). In this study, we employed both recombinant and fibroblast-derived proteins to demonstrate that all three proteins specifically interact with the mRNA counterpart of the exon 3 sequence in cell-free binding assays. When placed in the 5'-untranslated region of a reporter mRNA, the exon 3-derived sequence suppressed mRNA translation in transfected fibroblasts. Translational efficiency was restored by mutations that impaired mRNA binding of Puralpha, Purbeta, and MSY1, implying that these proteins can also participate in messenger ribonucleoprotein formation in living cells. Additionally, primary structure determinants required for interaction of Purbeta with single-stranded DNA, mRNA, and protein ligands were mapped by deletion mutagenesis. These experiments reveal highly specific protein-mRNA interactions that are potentially important in regulating expression of the vascular smooth muscle alpha-actin gene in fibroblasts.

Physical and functional interaction between the Y-box binding protein YB-1 and human polyomavirus JC virus large T antigen

Y-box binding protein YB-1 is a member of a family of DNA and RNA binding proteins which have been shown to affect gene expression at both the transcriptional and translational levels. We have previously shown that YB-1 modulates transcription from the promoters of the ubiquitous human polyomavirus JC virus (JCV). Here we investigate the physical and functional interplay between YB-1 and the viral regulatory protein large T antigen (T-antigen), using JCV as a model system. Results of mobility band shift assays demonstrated that the efficiency of binding of YB-1 to a 23-bp single-stranded viral target sequence was significantly increased when T-antigen was included in the binding reaction mixture. Affinity chromatography and coimmunoprecipitation assays demonstrated that YB-1 and T-antigen physically interact with each other. Additionally, results of transcription studies demonstrated that these two proteins interact functionally on the JCV early and late gene promoters. Whereas ectopic expression of YB-1 and T-antigen results in synergistic transactivation of the viral late promoter, YB-1 alleviates T-antigen-mediated transcriptional suppression of the viral early promoter activity. Furthermore, we have localized, through the use of a series of deletion mutants, the sequences of these proteins which are important for their interaction. The T-antigen-interacting region of YB-1 is located in the cold shock domain of YB-1 and its immediate flanking sequences, and the YB-1-interacting domain of T-antigen maps to the carboxy-terminal half of T-antigen. Results of transient transfection assays with various YB-1 mutants and T-antigen expression constructs confirm the specificity of the functional interaction between YB-1 and T-antigen. Taken together, these data demonstrate that the cellular factor YB-1 and the viral regulatory protein T-antigen interact both physically and functionally and that this interaction modulates transcription from the JCV promoters.

Conservation and divergence of the yeast and mammalian unfolded protein response. Activation of specific mammalian endoplasmic reticulum stress element of the grp78/BiP promoter by yeast Hac1

Yeast Hac1 (yHac1), the transcription factor that binds and activates the unfolded protein response element of endoplasmic reticulum (ER)-chaperone gene promoters, only accumulates in stressed cells after an unconventional splicesosome-free mRNA processing step and escape from translation block. In determining whether the novel regulatory mechanisms for yHac1 are conserved in mammalian cells, we discovered a unique unfolded protein response element-like sequence within the endoplasmic reticulum stress element 163, one of the three ER stress elements recently identified in the rat grp78 promoter. The unspliced form of yHac1 is stably expressed in nonstressed mammalian cells and is as active as the spliced form in stimulating the promoter activities of grp genes. Further, the yHac1 mRNA is not processed in response to ER stress in mammalian cells. We identified a CCAGC motif as the yHac1 binding site, which is contained within a YY1 binding site previously shown to be important for mammalian UPR. Dissection of the yHac1 and the YY1 binding sites uncovered specific contact points for an activator protein predicted to be the mammalian homolog of yHac1, the activity of which can be stimulated by YY1. A model of the conserved and unique features of the yeast and mammalian unfolded protein response transcription machinery is proposed.

Interaction of the universal mRNA-binding protein, p50, with actin: a possible link between mRNA and microfilaments

We have shown previously that p50 is the most abundant protein associated with a variety of eukaryotic mRNAs and exhibits about 98% amino acid sequence identity to mammalian Y-box binding transcription factors. The dual function of p50 in the cell as a regulator of both transcription and translation has been suggested. To gain insight into the role of p50 in these processes, we performed the yeast two-hybrid screen to identify p50 molecular partners. Here we report the identification of actin as a p50-interacting protein. Coimmunoprecipitation of p50 and actin from HeLa extracts as well as in vitro binding studies indicate specificity and a high affinity for the interaction between p50 and actin. Interestingly, p50 binding to actin is affected by mRNA; binding was observed at a low p50/mRNA ratio and was greatly reduced at higher ratios. Since the p50/mRNA ratio appears to be important for mRNA translatability, we speculate that p50 can regulate the attachment of mRNA to the actin network depending on its translational activity. Using immunofluorescence, we show that p50 binds to actin filaments in permeabilized cells and causes actin fibers to bundle in vitro. Together, these findings support the view that p50 may play an important role in mRNA transport, anchoring, and localization on actin filaments in the cell.

Signal transduction from the endoplasmic reticulum to the cell nucleus

The endoplasmic reticulum (ER) serves several important functions. Cholesterol, an essential component of cellular membranes, is synthesized on the ER surface. Inside the organelle, proteins destined for secretion or transport to the cell surface are folded and become glycosylated. Because these processes are essential for cell viability, a disturbance in ER function presents significant stress to the cell. In response to ER stress, three distinct signal transduction pathways can be activated. Two of these, the unfolded protein response and the ER-overload response, respond to disturbances in protein processing. The third, the sterol regulatory cascade, is activated by depletion of cholesterol. This review summarizes the recent advances in our understanding of these ER-nuclear signal transduction pathways. In addition, it points to novel regulatory mechanisms discovered in these pathways, which may be widely used in other systems.

Molecular interactions between single-stranded DNA-binding proteins associated with an essential MCAT element in the mouse smooth muscle alpha-actin promoter

Transcriptional activity of the mouse vascular smooth muscle alpha-actin gene in fibroblasts is regulated, in part, by a 30-base pair asymmetric polypurine-polypyrimidine tract containing an essential MCAT enhancer motif. The double-stranded form of this sequence serves as a binding site for a transcription enhancer factor 1-related protein while the separated single strands interact with two distinct DNA binding activities termed VACssBF1 and 2 (Cogan, J. G., Sun, S., Stoflet, E. S., Schmidt, L. J., Getz, M. J., and Strauch, A. R. (1995) J. Biol. Chem. 270, 11310-11321; Sun, S., Stoflet, E. S., Cogan, J. G., Strauch, A. R., and Getz, M. J. (1995) Mol. Cell. Biol. 15, 2429-2936). VACssBF2 has been recently cloned and shown to consist of two closely related proteins, Puralpha and Purbeta (Kelm, R. J., Elder, P. K., Strauch, A. R., and Getz, M. J. (1997) J. Biol. Chem. 272, 26727-26733). In this study, we demonstrate that Puralpha and Purbeta interact with each other via highly specific protein-protein interactions and bind to the purine-rich strand of the MCAT enhancer in the form of both homo- and heteromeric complexes. Moreover, both Pur proteins interact with MSY1, a VACssBF1-like protein cloned by virtue of its affinity for the pyrimidine-rich strand of the enhancer. Interactions between Puralpha, Purbeta, and MSY1 do not require the participation of DNA. Combinatorial interactions between these three single-stranded DNA-binding proteins may be important in regulating activity of the smooth muscle alpha-actin MCAT enhancer in fibroblasts.

Reciprocal interaction between two cellular proteins, Puralpha and YB-1, modulates transcriptional activity of JCVCY in glial cells

Cross communication between regulatory proteins is an important event in the control of eukaryotic gene transcription. Here we have examined the structural and functional interaction between two cellular regulatory proteins, YB-1 and Puralpha, on the 23-bp sequence element derived from the enhancer-promoter of the human polyomavirus JCV. YB-1 and Puralpha are single-stranded DNA binding proteins which recognize C/T- and GC/GA-rich sequences, respectively. Results from band shift studies demonstrated that while both proteins interact directly with their DNA target sequences within the 23-bp motif, each protein can regulate the association of the other one with the DNA. Affinity chromatography and coimmunoprecipitation provide evidence for a direct interaction between Puralpha and YB-1 in the absence of the DNA sequence. Ectopic expression of YB-1 and Puralpha in glial cells synergistically stimulated viral promoter activity via the 23-bp sequence element. Results from mutational studies revealed that residues between amino acids 75 and 203 of YB-1 and between amino acids 85 and 215 of Puralpha are important for the interaction between these two proteins. Functional studies with glial cells indicated that the region within Puralpha which mediates its association with YB-1 and binding to the 23-bp sequence is important for the observed activation of the JCV promoter by the Puralpha and YB-1 proteins. The results of this study suggest that the cooperative interaction between YB-1 and Puralpha mediates the synergistic activation of the human polyomavirus JCV genome by these cellular proteins. The importance of these findings for cellular and viral genes which are regulated by Puralpha and YB-1 is discussed.

DNA binding protein dbpA binds Cdk5 and inhibits its activity

Progress in the cell cycle is governed by the activity of cyclin dependent kinases (Cdks). Unlike other Cdks, the Cdk5 catalytic subunit is found mostly in differentiated neurons. Interestingly, the only known protein that activates Cdk5 (i.e. p35) is expressed solely in the brain. It has been suggested that, besides its requirement in neuronal differentiation, Cdk5 activity is induced during myogenesis. However, it is not clear how this activity is regulated in the pathway that leads proliferative cells to differentiation. In order to find if there exists any Cdk5-interacting protein, the yeast two-hybrid system was used to screen a HeLa cDNA library. We have determined that a C-terminal 172 amino acid domain of the DNA binding protein, dbpA, binds to Cdk5. Biochemical analyses reveal that this fragment (dbpA(Cdelta)) strongly inhibits p35-activated Cdk5 kinase. The protein also interacts with Cdk4 and inhibits the Cdk4/cyclin D1 enzyme. Surprisingly, dbpA(Cdelta) does not bind Cdk2 in the two-hybrid assay nor does it inhibit Cdk2 activated by cyclin A. It could be that dbpA's ability to inhibit Cdk5 and Cdk4 reflects an apparent cross-talk between distinct signal transduction pathways controlled by dbpA on the one hand and Cdk5 or Cdk4 on the other.

Translational regulation by Y-box transcription factor: involvement of the major mRNA-associated protein, p50

p50, the major core protein of messenger ribonucleoprotein particles (mRNPs), is a universal protein found exclusively in association with different mRNA species in the cytoplasm of somatic mammalian cells. Furthermore, p50 is the most abundant and tightly bound protein within both inactive mRNPs and active mRNPs derived from polysomes, although the latter contain a lower level of p50. Recent experiments have revealed that, depending on the p50 to mRNA ratio, p50 may either act as a repressor or an activator of protein synthesis. On the other hand, p50 exhibits about 98% amino acid sequence identity to mammalian transcription factors that bind specifically to Y-box containing DNA. Thus, it is a counterpart of the Y-box binding proteins which are found in bacteria, plants and animals, exhibiting multiple biological activities ranging from transcriptional regulation of a wide variety of genes to 'masking' mRNA activity in germinal cells. This review summarizes our current knowledge of p50 structure and function. It also discusses the biological roles of p50 and related proteins in gene expression and describes the likely mechanisms of their action.