A human Cds1-related kinase that functions downstream of ATM protein in the cellular response to DNA damage

Checkpoints maintain the order and fidelity of the eukaryotic cell cycle, and defects in checkpoints contribute to genetic instability and cancer. Much of our current understanding of checkpoints comes from genetic studies conducted in yeast. In the fission yeast Schizosaccharomyces pombe (Sp), SpRad3 is an essential component of both the DNA damage and DNA replication checkpoints. The SpChk1 and SpCds1 protein kinases function downstream of SpRad3. SpChk1 is an effector of the DNA damage checkpoint and, in the absence of SpCds1, serves an essential function in the DNA replication checkpoint. SpCds1 functions in the DNA replication checkpoint and in the S phase DNA damage checkpoint. Human homologs of both SpRad3 and SpChk1 but not SpCds1 have been identified. Here we report the identification of a human cDNA encoding a protein (designated HuCds1) that shares sequence, structural, and functional similarity to SpCds1. HuCds1 was modified by phosphorylation and activated in response to ionizing radiation. It was also modified in response to hydroxyurea treatment. Functional ATM protein was required for HuCds1 modification after ionizing radiation but not after hydroxyurea treatment. Like its fission yeast counterpart, human Cds1 phosphorylated Cdc25C to promote the binding of 14-3-3 proteins. These findings suggest that the checkpoint function of HuCds1 is conserved in yeast and mammals.

Disruption of cellular translational control by a viral truncated eukaryotic translation initiation factor 2α kinase homolog

Phosphorylation of eukaryotic translation initiation factor 2α (eIF2α) is a common cellular mechanism to limit protein synthesis in stress conditions. Baculovirus PK2, which resembles the C-terminal half of a protein kinase domain, was found to inhibit both human and yeast eIF2α kinases. Insect cells infected with wild-type, but not pk2-deleted, baculovirus exhibited reduced eIF2α phosphorylation and increased translational activity. The negative regulatory effect of human protein kinase RNA-regulated (PKR), an eIF2α kinase, on virus production was counteracted by PK2, indicating that baculoviruses have evolved a unique strategy for disrupting a host stress response. PK2 was found in complex with PKR and blocked kinase autophosphorylation in vivo, suggesting a mechanism of kinase inhibition mediated by interaction between truncated and intact kinase domains.

Molecular cloning and characterization of an invertebrate cellular retinoic acid binding protein

We have cloned a cDNA and gene from the tobacco hornworm, Manduca sexta, which is related to the vertebrate cellular retinoic acid binding proteins (CRABPs). CRABPs are members of the superfamily of lipid binding proteins (LBPs) and are thought to mediate the effects of retinoic acid (RA) on morphogenesis, differentiation, and homeostasis. This discovery of a Manduca sexta CRABP (msCRABP) demonstrates the presence of a CRABP in invertebrates. Compared with bovine/murine CRABP I, the deduced amino acid sequence of msCRABP is 71% homologous overall and 88% homologous for the ligand binding pocket. The genomic organization of msCRABP is conserved with other CRABP family members and the larger LBP superfamily. Importantly, the promoter region contains a motif that resembles an RA response element characteristic of the promoter region of most CRABPs analyzed. Three-dimensional molecular modeling based on postulated structural homology with bovine/murine CRABP I shows msCRABP has a ligand binding pocket that can accommodate RA. The existence of an invertebrate CRABP has significant evolutionary implications, suggesting CRABPs appeared during the evolution of the LBP superfamily well before vertebrate/invertebrate divergence, instead of much later in evolution in selected vertebrates.

β3 integrins mediate the cellular entry of hantaviruses that cause respiratory failure

Newly emerged hantaviruses replicate primarily in the pulmonary endothelium, cause acute platelet loss, and result in hantavirus pulmonary syndrome (HPS). We now report that specific integrins expressed on platelets and endothelial cells permit the cellular entry of HPS-associated hantaviruses. Infection with HPS-associated hantaviruses, NY-1 and Sin Nombre virus (SNV), is inhibited by antibodies to β3 integrins and by the β3-integrin ligand, vitronectin. In contrast, infection with the nonpathogenic (no associated human disease) Prospect Hill virus was inhibited by fibronectin and β1-specific antibodies but not by β3-specific antibodies or vitronectin. Transfection with recombinant αIIbβ3 or αvβ3 integrins rendered cells permissive to NY-1 and SNV but not Prospect Hill virus infection, indicating that αIIbβ3 and αvβ3 integrins mediate the entry of NY-1 and SNV hantaviruses. Furthermore, entry is divalent cation independent, not blocked by arginine-glycine-aspartic acid peptides and still mediated by, ligand-binding defective, αIIbβ3-integrin mutants. Hence, NY-1 and SNV entry is independent of β3 integrin binding to physiologic ligands. These findings implicate integrins as cellular receptors for hantaviruses and indicate that hantavirus pathogenicity correlates with integrin usage.

Evidence for protein X binding to a discontinuous epitope on the cellular prion protein during scrapie prion propagation

Studies on the transmission of human (Hu) prions to transgenic (Tg) mice suggested that another molecule provisionally designated protein X participates in the formation of nascent scrapie isoform of prion protein (PrPSc). We report the identification of the site at which protein X binds to the cellular isoform of PrP (PrPC) using scrapie-infected mouse (Mo) neuroblastoma cells transfected with chimeric Hu/MoPrP genes even though protein X has not yet been isolated. Substitution of a Hu residue at position 214 or 218 prevented PrPSc formation. The side chains of these residues protrude from the same surface of the C-terminal α-helix and form a discontinuous epitope with residues 167 and 171 in an adjacent loop. Substitution of a basic residue at positions 167, 171, or 218 also prevented PrPSc formation: at a mechanistic level, these mutant PrPs appear to act as “dominant negatives” by binding protein X and rendering it unavailable for prion propagation. Our findings seem to explain the protective effects of basic polymorphic residues in PrP of humans and sheep and suggest therapeutic and prophylactic approaches to prion diseases.

The small GTP-binding protein Rho links G protein-coupled receptors and Gα12 to the serum response element and to cellular transformation

Receptors coupled to heterotrimeric G proteins can effectively stimulate growth promoting pathways in a large variety of cell types, and if persistently activated, these receptors can also behave as dominant-acting oncoproteins. Consistently, activating mutations for G proteins of the Gαs and Gαi2 families were found in human tumors; and members of the Gαq and Gα12 families are fully transforming when expressed in murine fibroblasts. In an effort aimed to elucidate the molecular events involved in proliferative signaling through heterotrimeric G proteins we have focused recently on gene expression regulation. Using NIH 3T3 fibroblasts expressing m1 muscarinic acetylcholine receptors as a model system, we have observed that activation of this transforming G protein-coupled receptors induces the rapid expression of a variety of early responsive genes, including the c-fos protooncogene. One of the c-fos promoter elements, the serum response element (SRE), plays a central regulatory role, and activation of SRE-dependent transcription has been found to be regulated by several proteins, including the serum response factor and the ternary complex factor. With the aid of reporter plasmids for gene expression, we observed here that stimulation of m1 muscarinic acetylcholine receptors potently induced SRE-driven reporter gene activity in NIH 3T3 cells. In these cells, only the Gα12 family of heterotrimeric G protein α subunits strongly induced the SRE, while Gβ1γ2 dimers activated SRE to a more limited extent. Furthermore, our study provides strong evidence that m1, Gα12 and the small GTP-binding protein RhoA are components of a novel signal transduction pathway that leads to the ternary complex factor-independent transcriptional activation of the SRE and to cellular transformation.

Role of tyrosine phosphorylation of a cellular protein in adeno-associated virus 2-mediated transgene expression

The adeno-associated virus 2 (AAV), a single-stranded DNA-containing, nonpathogenic human parvovirus, has gained attention as a potentially useful vector for human gene therapy. However, the single-stranded nature of the viral genome significantly impacts upon the transduction efficiency, because the second-strand viral DNA synthesis is the rate-limiting step. We hypothesized that a host-cell protein interacts with the single-stranded D sequence within the inverted terminal repeat structure of the AAV genome and prevents the viral second-strand DNA synthesis. Indeed, a cellular protein has been identified that interacts specifically and preferentially with the D sequence at the 3′ end of the AAV genome. This protein, designated the single-stranded D-sequence-binding protein (ssD-BP), is phosphorylated at tyrosine residues and blocks AAV-mediated transgene expression in infected cells by inhibiting the leading strand viral DNA synthesis. Inhibition of cellular protein tyrosine kinases by genistein results in dephosphorylation of the ssD-BP, leading not only to significant augmentation of transgene expression from recombinant AAV but also to autonomous replication of the wild-type AAV genome. Dephosphorylation of the ssD-BP also correlates with adenovirus infection, or expression of the adenovirus E4orf6 protein, which is known to induce AAV DNA replication and gene expression. Thus, phosphorylation state of the ssD-BP appears to play a crucial role in the life cycle of AAV and may prove to be an important determinant in the successful use of AAV-based vectors in human gene therapy.

Developmental disorder associated with increased cellular nucleotidase activity

Four unrelated patients are described with a syndrome that included developmental delay, seizures, ataxia, recurrent infections, severe language deficit, and an unusual behavioral phenotype characterized by hyperactivity, short attention span, and poor social interaction. These manifestations appeared within the first few years of life. Each patient displayed abnormalities on EEG. No unusual metabolites were found in plasma or urine, and metabolic testing was normal except for persistent hypouricosuria. Investigation of purine and pyrimidine metabolism in cultured fibroblasts derived from these patients showed normal incorporation of purine bases into nucleotides but decreased incorporation of uridine. De novo synthesis of purines and cellular phosphoribosyl pyrophosphate content also were moderately decreased. The distribution of incorporated purines and pyrimidines did not reveal a pattern suggestive of a deficient enzyme activity. Assay of individual enzymes in fibroblast lysates showed no deficiencies. However, the activity of cytosolic 5′-nucleotidase was elevated 6- to 10-fold. Based on the possibility that the observed increased catabolic activity and decreased pyrimidine salvage might be causing a deficiency of pyrimidine nucleotides, the patients were treated with oral pyrimidine nucleoside or nucleotide compounds. All patients showed remarkable improvement in speech and behavior as well as decreased seizure activity and frequency of infections. A double-blind placebo trial was undertaken to ascertain the efficacy of this supplementation regimen. Upon replacement of the supplements with placebo, all patients showed rapid regression to their pretreatment states. These observations suggest that increased nucleotide catabolism is related to the symptoms of these patients, and that the effects of this increased catabolism are reversed by administration of uridine.

Identification of a cellular receptor for subgroup E avian leukosis virus

Genetic studies in chickens and receptor interference experiments have indicated that avian leukosis virus (ALV)-E may utilize a cellular receptor related to the receptor for ALV-B and ALV-D. Recently, we cloned CAR1, a tumor necrosis factor receptor (TNFR)-related protein, that serves as a cellular receptor for ALV-B and ALV-D. To determine whether the cellular receptor for ALV-E is a CAR1-like protein, a cDNA library was made from turkey embryo fibroblasts (TEFs), which are susceptible to ALV-E infection, but not to infection by ALV-B and ALV-D. The cDNA library was screened with a radioactively labeled CAR1 cDNA probe, and clones that hybridized with the probe were isolated. A 2.3-kb cDNA clone was identified that conferred susceptibility to ALV-E infection, but not to ALV-B infection, when expressed in transfected human 293 cells. The functional cDNA clone is predicted to encode a 368 amino acid protein with significant amino acid similarity to CAR1. Like CAR1, the TEF protein is predicted to have two extracellular TNFR-like cysteine-rich domains and a putative death domain similar to those of TNFR I and Fas. Flow cytometric analysis and immunoprecipitation experiments demonstrated specific binding between the TEF CAR1-related protein and an immunoadhesin composed of the surface (SU) envelope protein of subgroup E (RAV-0) virus fused to the constant region of a rabbit immunoglobulin. These two activities of the TEF CAR1-related protein, specific binding to ALV-E SU and permitting entry only of ALV-E, have unambiguously identified this protein as a cellular receptor specific for subgroup E ALV.

A deletion within the murine Werner syndrome helicase induces sensitivity to inhibitors of topoisomerase and loss of cellular proliferative capacity

Werner syndrome (WS) is an autosomal recessive disorder characterized by genomic instability and the premature onset of a number of age-related diseases. The gene responsible for WS encodes a member of the RecQ-like subfamily of DNA helicases. Here we show that its murine homologue maps to murine chromosome 8 in a region syntenic with the human WRN gene. We have deleted a segment of this gene and created Wrn-deficient embryonic stem (ES) cells and WS mice. While displaying reduced embryonic survival, live-born WS mice otherwise appear normal during their first year of life. Nonetheless, although several DNA repair systems are apparently intact in homozygous WS ES cells, such cells display a higher mutation rate and are significantly more sensitive to topoisomerase inhibitors (especially camptothecin) than are wild-type ES cells. Furthermore, mouse embryo fibroblasts derived from homozygous WS embryos show premature loss of proliferative capacity. At the molecular level, wild-type, but not mutant, WS protein copurifies through a series of centrifugation and chromatography steps with a multiprotein DNA replication complex.

An essential role for p300/CBP in the cellular response to hypoxia

p300 and CBP are homologous transcription adapters targeted by the E1A oncoprotein. They participate in numerous biological processes, including cell cycle arrest, differentiation, and transcription activation. p300 and/or CBP (p300/CBP) also coactivate CREB. How they participate in these processes is not yet known. In a search for specific p300 binding proteins, we have cloned the intact cDNA for HIF-1α. This transcription factor mediates hypoxic induction of genes encoding certain glycolytic enzymes, erythropoietin (Epo), and vascular endothelial growth factor. Hypoxic conditions lead to the formation of a DNA binding complex containing both HIF-1α and p300/CBP. Hypoxia-induced transcription from the Epo promoter was specifically enhanced by ectopic p300 and inhibited by E1A binding to p300/CBP. Hypoxia-induced VEGF and Epo mRNA synthesis were similarly inhibited by E1A. Hence, p300/CBP–HIF complexes participate in the induction of hypoxia-responsive genes, including one (vascular endothelial growth factor) that plays a major role in tumor angiogenesis. Paradoxically, these data, to our knowledge for the first time, suggest that p300/CBP are active in both transformation suppression and tumor development.

Cellular gene expression altered by human cytomegalovirus: Global monitoring with oligonucleotide arrays

Mechanistic insights to viral replication and pathogenesis generally have come from the analysis of viral gene products, either by studying their biochemical activities and interactions individually or by creating mutant viruses and analyzing their phenotype. Now it is possible to identify and catalog the host cell genes whose mRNA levels change in response to a pathogen. We have used DNA array technology to monitor the level of ≈6,600 human mRNAs in uninfected as compared with human cytomegalovirus-infected cells. The level of 258 mRNAs changed by a factor of 4 or more before the onset of viral DNA replication. Several of these mRNAs encode gene products that might play key roles in virus-induced pathogenesis, identifying them as intriguing targets for further study.

Altered states: Involvement of phosphorylated CagA in the induction of host cellular growth changes by Helicobacter pylori

Helicobacter pylori, present in half of the world’s population, is a very successful pathogen. It can survive for decades in the human stomach with few obvious consequences to the host. However, it is also the cause of gastric diseases ranging from gastritis to ulcers to gastric cancer and has been classified a type 1 carcinogen by the World Health Organization. We have previously shown that phosphorylation of a 145-kDa protein and activation of signal transduction pathways are associated with the attachment of H. pylori to gastric cells. Here we identify the 145-kDa protein as the H. pylori CagA protein. We also show that CagA is necessary to induce a growth-factor-like phenotype (hummingbird) in host gastric cells similar to that induced by hepatocyte growth factor (HGF). Additionally, we identify a second cellular phenotype induced after attachment by H. pylori, which we call SFA (stress fiber associated). SFA is CagA independent and is produced by type I and type II H. pylori.

Dissociation among in vitro telomerase activity, telomere maintenance, and cellular immortalization

The immortalization of human cells is a critical step during tumorigenesis. In vitro, normal human somatic cells must overcome two proliferative blockades, senescence and crisis, to become immortal. Transformation with viral oncogenes extends the life span of human cells beyond senescence. Such transformed cells eventually succumb to crisis, a period of widespread cellular death that has been proposed to be the result of telomeric shortening. We now show that ectopic expression of the telomerase catalytic subunit (human telomerase reverse transcriptase or hTERT) and subsequent activation of telomerase can allow postsenescent cells to proliferate beyond crisis, the last known proliferative blockade to cellular immortality. Moreover, we demonstrate that alteration of the carboxyl terminus of human telomerase reverse transcriptase does not affect telomerase enzymatic activity but impedes the ability of this enzyme to maintain telomeres. Telomerase-positive cells expressing this mutant enzyme fail to undergo immortalization, further tightening the connection between telomere maintenance and immortalization.

Retina-specific nuclear receptor: A potential regulator of cellular retinaldehyde-binding protein expressed in retinal pigment epithelium and Müller glial cells

In an effort to identify nuclear receptors important in retinal disease, we screened a retina cDNA library for nuclear receptors. Here we describe the identification of a retina-specific nuclear receptor (RNR) from both human and mouse. Human RNR is a splice variant of the recently published photoreceptor cell-specific nuclear receptor [Kobayashi, M., Takezawa, S., Hara, K., Yu, R. T., Umesono, Y., Agata, K., Taniwaki, M., Yasuda, K. & Umesono, K. (1999) Proc. Natl. Acad. Sci. USA 96, 4814–4819] whereas the mouse RNR is a mouse ortholog. Northern blot and reverse transcription–PCR analyses of human mRNA samples demonstrate that RNR is expressed exclusively in the retina, with transcripts of ≈7.5 kb, ≈3.0 kb, and ≈2.3 kb by Northern blot analysis. In situ hybridization with multiple probes on both primate and mouse eye sections demonstrates that RNR is expressed in the retinal pigment epithelium and in Müller glial cells. By using the Gal4 chimeric receptor/reporter cotransfection system, the ligand binding domain of RNR was found to repress transcriptional activity in the absence of exogenous ligand. Gel mobility shift assays revealed that RNR can interact with the promoter of the cellular retinaldehyde binding protein gene in the presence of retinoic acid receptor (RAR) and/or retinoid X receptor (RXR). These data raise the possibility that RNR acts to regulate the visual cycle through its interaction with cellular retinaldehyde binding protein and therefore may be a target for retinal diseases such as retinitis pigmentosa and age-related macular degeneration.