Demonstration of a RNA-dependent nuclear interaction between the promyelocytic leukaemia protein and glyceraldehyde-3-phosphate dehydrogenase

Subcellular Fractionation Suitable for Studies of RNA and Protein Trafficking

The nuclear pore complex is the major conduit for trafficking between the nucleus and cytoplasm. Nuclear import and export of both proteins and RNAs represent important functional steps for many biological processes. One of the major means to study NPC activity and the nuclear and cytoplasmic distribution of proteins and RNAs is through biochemical fractionation. Here, we describe detailed methods to generate high quality nuclear and cytoplasmic fractions simultaneously capturing RNA and proteins which can be used subsequently for a wide array of biochemical characterizations including proteomics and next generation sequencings.

Apoptotic Mechanisms in Neurodegeneration: Possible Relevance to Glaucoma

Deprenyl, a monoamine oxidase inhibitor used in the treatment of Parkinson's disease, along with its primary metabolite desmethyldeprenyl (DES) have been shown to reduce neuronal apoptosis by a mechanism that requires gene transcription and involves the maintenance of mitochondrial membrane potential. This review article explores the mechanisms by which DES maintains mitochondrial membrane potential. Mediated by GAPDH binding, DES increases mitochondrial BCL-2 and BCL-xL levels and decreases BAX levels thereby preventing the permeability transition pore (PTP) form opening and preventing apoptotic degradation. The favorable effects of deprenyl on neuronal apoptosis suggests the therapeutic potential of designing compounds with the capacity to alter the configurations of pro-apoptosis or anti-apoptotic proteins.

Role of Glyceraldehyde-3-Phosphate Dehydrogenase (GAPDH) in DNA Repair

Glyceraldehyde-3-phosphate dehydrogenase (GAPDH) is widely known as a glycolytic enzyme. Nevertheless, various functions of GAPDH have been found that are unrelated to glycolysis. Some of these functions presume interaction of GAPDH with DNA, but the mechanism of its translocation to the nucleus is not fully understood. When in the nucleus, GAPDH participates in the initiation of apoptosis and transcription of genes involved in antiapoptotic pathways and cell proliferation and plays a role in the regulation of telomere length. Several authors have shown that GAPDH displays the uracil-DNA glycosylase activity and interacts with some types of DNA damages, such as apurinic/apyrimidinic sites, nucleotide analogs, and covalent DNA adducts with alkylating agents. Moreover, GAPDH can interact with proteins participating in DNA repair, such as APE1, PARP1, HMGB1, and HMGB2. In this review, the functions of GAPDH associated with DNA repair are discussed in detail.

GAPDH: a common enzyme with uncommon functions

Glyceraldehyde-3-phosphate dehydrogenase (GAPDH) has long been recognized as an important enzyme for energy metabolism and the production of ATP and pyruvate through anaerobic glycolysis in the cytoplasm. Recent studies have shown that GAPDH has multiple functions independent of its role in energy metabolism. Although increased GAPDH gene expression and enzymatic function is associated with cell proliferation and tumourigenesis, conditions such as oxidative stress impair GAPDH catalytic activity and lead to cellular aging and apoptosis. The mechanism(s) underlying the effects of GAPDH on cellular proliferation remains unclear, yet much evidence has been accrued that demonstrates a variety of interacting partners for GAPDH, including proteins, various RNA species and telomeric DNA. The present mini review summarizes recent findings relating to the extraglycolytic functions of GAPDH and highlights the significant role this enzyme plays in regulating both cell survival and apoptotic death.

The HOX gene network in hepatocellular carcinoma

Liver organogenesis and cancerogenesis share common mechanisms. HOX genes control normal development, primary cellular processes and are characterized by a unique genomic network organization. Less is known about the involvement of HOX genes with liver cancerogenesis. The comparison of the HOX gene network expression between nontumorous livers and hepatocellular carcinomas (HCCs) highlights significant differences in the locus A HOX genes, located on chromosome 7, with a consistent overexpression of HOXA13 mRNA thus validating this gene deregulation as a feature of HCC. HOXA13 is a determinant of gut primordia and posterior body structures. Transcriptome analysis of HCC/nontumorous liver mRNAs, selected on the basis of HOXA13 overexpression, recognizes a set of deregulated genes. The matching of these genes with previously reported HCC transcriptome analysis identifies cell-cycle and nuclear pore-related HCC phenotype displaying poor prognosis. HOXA13 and HOXA7 homeoproteins share a consensus sequence that physically links eIF4E nuclear bodies acting on the export of specific mRNAs (c-myc, FGF-2, vascular endothelial growth factor (VEGF), ornithine decarboxylase (ODC) and cyclin D1). We report the protein-protein interaction between HOXA13 and eIF4E in liver cancer cells and the deregulation of eIF4E mRNA and protein in cell cycle/nuclear pore HCC group phenotype and in T4 stage HCCs, respectively. Thus, transcriptional and post-transcriptional HOXA13 deregulation is involved in HCC possibly through the mRNA nuclear export of eIF4E-dependent transcripts.

Oxidatively modified glyceraldehyde-3-phosphate dehydrogenase (GAPDH) and Alzheimer's disease: many pathways to neurodegeneration

Recently, the oxidoreductase, glyceraldehyde-3-phosphate dehydrogenase (GAPDH), has become a subject of interest as more and more studies reveal a surfeit of diverse GAPDH functions, extending beyond traditional aerobic metabolism of glucose. As a result of multiple isoforms and cellular locales, GAPDH is able to come in contact with a variety of small molecules, proteins, membranes, etc., that play important roles in normal and pathologic cell function. Specifically, GAPDH has been shown to interact with neurodegenerative disease-associated proteins, including the amyloid-beta protein precursor (AbetaPP). Studies from our laboratory have shown significant inhibition of GAPDH dehydrogenase activity in Alzheimer's disease (AD) brain due to oxidative modification. Although oxidative stress and damage is a common phenomenon in the AD brain, it would seem that inhibition of glycolytic enzyme activity is merely one avenue in which AD pathology affects neuronal cell development and survival, as oxidative modification can also impart a toxic gain-of-function to many proteins, including GAPDH. In this review, we examine the many functions of GAPDH with respect to AD brain; in particular, the apparent role(s) of GAPDH in AD-related apoptotic cell death is emphasized.

The diverse functions of GAPDH: views from different subcellular compartments

Multiple roles for glyceraldehyde-3-phosphate dehydrogenase (GAPDH) have been recently appreciated. In addition to the cytoplasm where the majority of GAPDH is located under the basal condition, GAPDH is also found in the particulate fractions, such as the nucleus, the mitochondria, and the small vesicular fractions. When cells are exposed to various stressors, dynamic subcellular re-distribution of GAPDH occurs. Here we review these multifunctional properties of GAPDH, especially linking them to its oligomerization, posttranslational modification, and subcellular localization. This includes mechanistic descriptions of how S-nitrosylation of GAPDH under oxidative stress may lead to cell death/dysfunction via nuclear translocation of GAPDH, which is counteracted by a cytosolic GOSPEL. GAPDH is also involved in various diseases, especially neurodegenerative disorders and cancers. Therapeutic strategies to these conditions based on molecular understanding of GAPDH are discussed.

Oxidative modifications of glyceraldehyde-3-phosphate dehydrogenase play a key role in its multiple cellular functions

Knowledge of the cellular targets of ROS (reactive oxygen species) and their regulation is an essential prerequisite for understanding ROS-mediated signalling. GAPDH (glyceraldehyde-3-phosphate dehydrogenase) is known as a major target protein in oxidative stresses and becomes thiolated in its active site. However, the molecular and functional changes of oxidized GAPDH, the inactive form, have not yet been characterized. To examine the modifications of GAPDH under oxidative stress, we separated the oxidation products by two-dimensional gel electrophoresis and identified them using nanoLC-ESI-q-TOF MS/MS (nano column liquid chromatography coupled to electrospray ionization quadrupole time-of-flight tandem MS). Intracellular GAPDH subjected to oxidative stress separated into multiple acidic spots on two-dimensional gel electrophoresis and were identified as cysteine disulfide and cysteic acids on Cys152 in the active site. We identified the interacting proteins of oxidized inactive GAPDH as p54nrb (54 kDa nuclear RNA-binding protein) and PSF (polypyrimidine tract-binding protein-associated splicing factor), both of which are known to exist as heterodimers and bind to RNA and DNA. Interaction between oxidized GAPDH and p54nrb was abolished upon expression of the GAPDH active site mutant C152S. The C-terminal of p54nrb binds to GAPDH in the cytosol in a manner dependent on the dose of hydrogen peroxide. The GAPDH–p54nrb complex enhances the intrinsic topoisomerase I activation by p54nrb–PSF binding. These results suggest that GAPDH exerts other functions beyond glycolysis, and that oxidatively modified GAPDH regulates its cellular functions by changing its interacting proteins, i.e. the RNA splicing by interacting with the p54nrb–PSF complex.

Finding one's way in proteomics: a protein species nomenclature

Our knowledge of proteins has greatly improved in recent years, driven by new technologies in the fields of molecular biology and proteome research. It has become clear that from a single gene not only one single gene product but many different ones - termed protein species - are generated, all of which may be associated with different functions. Nonetheless, an unambiguous nomenclature for describing individual protein species is still lacking. With the present paper we therefore propose a systematic nomenclature for the comprehensive description of protein species. The protein species nomenclature is flexible and adaptable to every level of knowledge and of experimental data in accordance with the exact chemical composition of individual protein species. As a minimum description the entry name (gene name + species according to the UniProt knowledgebase) can be used, if no analytical data about the target protein species are available.

Uncoupling of hTREX demonstrates that UAP56 and hTHO-complex recruitment onto herpesvirus saimiri intronless transcripts is required for replication

Herpesvirus saimiri (HVS) ORF57 nucleocytoplasmic shuttle protein binds viral RNA and interacts with the cellular nuclear export adaptor protein, Aly, to access the TAP-mediated nuclear export pathway. This enables the efficient nuclear export of HVS intronless mRNAs. Herein, we extend these studies and demonstrate that ORF57 recruits several members of hTREX, namely Aly, UAP56 and hTHO-complex proteins, onto the viral mRNAs to assemble an export-competent ribonucleoprotein particle. Moreover, using a transdominant form of Aly which inhibits UAP56 and hTHO-complex association with viral intronless mRNA, we show that complete hTREX recruitment is required for efficient HVS mRNA nuclear export and replication.

Immunoaffinity purification and characterization of glyceraldehyde-3-phosphate dehydrogenase from human erythrocytes

A new procedure utilizing immunoaffinity column chromatography has been used for the purification of glyceraldehyde-3-phosphate dehydrogenase (GAPDH, EC 1.2.1.12) from human erythrocytes. The comparison between this rapid method (one step) and the traditional procedure including ammonium sulfate fractionation followed by Blue Sepharose CL-6B chromatography shows that the new method gives a highest specific activity with a highest yield in a short time. The characterization of the purified GAPDH reveals that the native enzyme is a homotetramer of ~150 kDa with an absolute specificity for the oxidized form of nicotinamide adenine dinucleotide (NAD(+)). Western blot analysis using purified monospecific polyclonal antibodies raised against the purified GAPDH showed a single 36 kDa band corresponding to the enzyme subunit. Studies on the effect of temperature and pH on enzyme activity revealed optimal values of about 43 degrees C and 8.5, respectively. The kinetic parameters were also calculated: the Vmax was 4.3 U/mg and the Km values against G3P and NAD(+) were 20.7 and 17.8 muM, respectively. The new protocol described represents a simple, economic, and reproducible tool for the purification of GAPDH and can be used for other proteins.

Recruitment of the complete hTREX complex is required for Kaposi's sarcoma-associated herpesvirus intronless mRNA nuclear export and virus replication

A cellular pre-mRNA undergoes various post-transcriptional processing events, including capping, splicing and polyadenylation prior to nuclear export. Splicing is particularly important for mRNA nuclear export as two distinct multi-protein complexes, known as human TREX (hTREX) and the exon-junction complex (EJC), are recruited to the mRNA in a splicing-dependent manner. In contrast, a number of Kaposi's sarcoma-associated herpesvirus (KSHV) lytic mRNAs lack introns and are exported by the virus-encoded ORF57 protein. Herein we show that ORF57 binds to intronless viral mRNAs and functions to recruit the complete hTREX complex, but not the EJC, in order assemble an export component viral ribonucleoprotein particle (vRNP). The formation of this vRNP is mediated by a direct interaction between ORF57 and the hTREX export adapter protein, Aly. Aly in turn interacts directly with the DEAD-box protein UAP56, which functions as a bridge to recruit the remaining hTREX proteins to the complex. Moreover, we show that a point mutation in ORF57 which disrupts the ORF57-Aly interaction leads to a failure in the ORF57-mediated recruitment of the entire hTREX complex to the intronless viral mRNA and inhibits the mRNAs subsequent nuclear export and virus replication. Furthermore, we have utilised a trans-dominant Aly mutant to prevent the assembly of the complete ORF57-hTREX complex; this results in a vRNP consisting of viral mRNA bound to ORF57, Aly and the nuclear export factor, TAP. Strikingly, although both the export adapter Aly and the export factor TAP were present on the viral mRNP, a dramatic decrease in intronless viral mRNA export and virus replication was observed in the absence of the remaining hTREX components (UAP56 and hTHO-complex). Together, these data provide the first direct evidence that the complete hTREX complex is essential for the export of KSHV intronless mRNAs and infectious virus production.

Tumor necrosis factor-alpha-induced anterior pituitary folliculostellate TtT/GF cell uncoupling is mediated by connexin 43 dephosphorylation

The anterior pituitary folliculostellate (FS) cells are key elements of the paracrine control of the pituitary function. These cells are the source and the target of growth factors and cytokines, and are connected to other pituitary cells via Cx43-mediated gap junctions. Here, we show that acute treatment of the FS TtT/GF cell line with TNF-alpha caused a transient cell uncoupling that was accompanied by the dephosphorylation of Cx43 in Ser368. These TNF-alpha-evoked effects were dependent on protein phosphatase 2A (PP2A) and protein kinase C (PKC) activities. TNF-alpha did not affect total cell Cx43-PP2A catalytic subunit interaction, but it did induce PP2A catalytic subunit recruitment to the Triton X-100 insoluble subcellular fraction, in which Cx43-gap junction plaques are recovered. This recruitment temporally coincided with Cx43 phosphorylated in Ser368-Cx43 dephosphorylation. Cx43 did not interact with the conventional PKC-alpha, but it did interact with the atypical PKC-zeta. Moreover, this interaction was weakened by TNF-alpha. Cx43 dephosphorylation in Ser368 was followed by the tyrosine phosphorylation of the protein. The temporary closure of gap junctions during acute TNF-alpha challenge may constitute a protective mechanism to limit or confine the spread of inflammatory signals among the FS cells.

Regulation of oncogenic transcription factor hTAF(II)68-TEC activity by human glyceraldehyde-3-phosphate dehydrogenase (GAPDH)

Tumour-specific chromosomal rearrangements are known to create chimaeric products with the ability to generate many human cancers. hTAF(II)68-TEC (where hTAF(II)68 is human TATA-binding protein-associated factor II 68 and TEC is translocated in extraskeletal chondrosarcoma) is such a fusion product, resulting from a t(9;17) chromosomal translocation found in extraskeletal myxoid chondrosarcomas, where the hTAF(II)68 NTD (N-terminal domain) is fused to TEC protein. To identify proteins that control hTAF(II)68-TEC function, we used affinity chromatography on immobilized hTAF(II)68 (NTD) and MALDI-TOF (matrix-assisted laser-desorption ionization-time-of-flight) MS and isolated a novel hTAF(II)68-TEC-interacting protein, GAPDH (glyceraldehyde-3-phosphate dehydrogenase). GAPDH is a glycolytic enzyme that is also involved in the early steps of apoptosis, nuclear tRNA export, DNA replication, DNA repair and transcription. hTAF(II)68-TEC and GAPDH were co-immunoprecipitated from cell extracts, and glutathione S-transferase pull-down assays revealed that the C-terminus of hTAF(II)68 (NTD) was required for interaction with GAPDH. In addition, three independent regions of GAPDH (amino acids 1-66, 67-160 and 160-248) were involved in binding to hTAF(II)68 (NTD). hTAF(II)68-TEC-dependent transcription was enhanced by GAPDH, but not by a GAPDH mutant defective in hTAF(II)68-TEC binding. Moreover, a fusion of GAPDH with the GAL4 DNA-binding domain increased the promoter activity of a reporter containing GAL4 DNA-binding sites, demonstrating the presence of a transactivation domain(s) in GAPDH. The results of the present study suggest that the transactivation potential of the hTAF(II)68-TEC oncogene product is positively modulated by GAPDH.

Biological targets for isatin and its analogues: Implications for therapy

Isatin and its metabolites are constituents of many natural substances. They are also components of many synthetic compounds exhibiting a wide range of effects, including antiviral activity, antitumor and antiangiogenic activity, antibacterial, antitubercular, antifungal, antiaptotic, anticonvulsant and anxyolytic activities. Isatin itself is an endogenous oxidized indole with a wide spectrum of behavioral and metabolic effects. It has a distinct and discontinuous distribution in the brain, peripheral tissues and body fluids and isatin binding sites are widely distributed also. Its output is increased during stress. Its most potent known in vitro actions are as an antagonist of atrial natriuretic peptide (ANP) function and NO signaling. As we understand more about its function and sites of action we may be able to develop new pharmacological agents to mimic or counteract its activity. We consider here the most promising biological targets for various isatin analogues and/or metabolites, which are employed for the development of various groups of therapeutics. It is also possible that the level of endogenous isatin may influence the in vivo pharmacological activity of compounds possessing the isatin moiety.

Isatin interaction with glyceraldehyde-3-phosphate dehydrogenase, a putative target of neuroprotective drugs: partial agonism with deprenyl

There is evidence that the binding of deprenyl, a monoamine oxidase (MAO) B inhibitor, and other propargylamines to glyceraldehyde-3-phosphate dehydrogenase (GAPDH) is primarily responsible for their neuroprotective and antiapoptotic effects. Thus, GAPDH may be a target for other neuroprotective drugs. Using two independent approaches, radioligand analysis and an optical biosensor technique, we demonstrate here that GAPDH also interacts with the endogenous, reversible MAO B inhibitor, isatin. Deprenyl inhibited both [3H]isatin binding to GAPDH, and the binding of this enzyme to an isatin analogue, 5-aminoisatin, immobilized on to an optical biosensor cell. Another MAO inhibitor, tranylcypromine, was ineffective. Both deprenyl and isatin inhibited GAPDH-mediated cleavage of E. coli tRNA, and their effects were not additive. We suggest that isatin may be an endogenous partial functional agonist of deprenyl in its effect on GAPDH and GAPDH-mediated RNA cleavage. Changes in level of endogenous isatin may influence the neuroprotective effect of deprenyl in vivo.

GAPDH as a sensor of NO stress

Glyceraldehyde-3-phosphate dehydrogenase (GAPDH) is a classic glycolytic enzyme, and accumulating evidence has suggested that GAPDH is a multi-functional protein. In particular, its role as a mediator for cell death has been highlighted. For the last decade, many groups reported that a pool of GAPDH translocates to the nucleus under a variety of stressors, most of which are associated with oxidative stress. At the molecular level, sequential steps lead to nuclear translocation of GAPDH during cell death as follows: first, a catalytic cysteine in GAPDH (C150 in rat GAPDH) is S-nitrosylated by nitric oxide (NO) that is generated from inducible nitric oxide synthase (iNOS) and/or neuronal NOS (nNOS); second, the modified GAPDH becomes capable of binding with Siah1, an E3 ubiquitin ligase, and stabilizes it; third, the GAPDH-Siah protein complex translocates to the nucleus, dependent on Siah1's nuclear localization signal, and degrades Siah1's substrates in the nucleus, which results in cytotoxicity. A recent report suggests that GAPDH may be genetically associated with late-onset of Alzheimer's disease. (-)-deprenyl, which has originally been used as a monoamine oxidase inhibitor for Parkinson's disease, binds to GAPDH and displays neuroprotective actions, but its molecular mechanism is still unclear. The NO/GAPDH/Siah1 death cascade will contribute to the molecular understanding of a role of GAPDH in neurodegenerative disorders and help to establish novel therapeutic strategies.

New nuclear functions of the glycolytic protein, glyceraldehyde-3-phosphate dehydrogenase, in mammalian cells

Recent studies establish that the glycolytic protein, glyceraldehyde-3-phosphate dehydrogenase (GAPDH), is not simply a classical metabolic protein involved in energy production. Instead, it is a multifunctional protein with defined functions in numerous subcellular processes. New investigations establish a primary role for GAPDH in a variety of critical nuclear pathways apart from its already recognized role in apoptosis. These new roles include its requirement for transcriptional control of histone gene expression, its essential function in nuclear membrane fusion, its necessity for the recognition of fraudulently incorporated nucleotides in DNA, and its mandatory participation in the maintenance of telomere structure. Each of these new functions requires GAPDH association into a series of multienzyme complexes. Although other proteins in those complexes are variable, GAPDH remains the single constant protein in each structure. To undertake these new functions, GAPDH is recruited to the nucleus in S phase or its intracellular distribution is regulated as a function of drug exposure. Other investigations relate a substantial role for nuclear GAPDH in hyperglycemic stress and the development of metabolic syndrome. Considerations of future directions as well as the role of GAPDH post-translational modification as a basis for its multifunctional activities is suggested.

Eukaryotic translation initiation factor 4E activity is modulated by HOXA9 at multiple levels

The eukaryotic translation initiation factor 4E (eIF4E) alters gene expression on multiple levels. In the cytoplasm, eIF4E acts in the rate-limiting step of translation initiation. In the nucleus, eIF4E facilitates nuclear export of a subset of mRNAs. Both of these functions contribute to eIF4E's ability to oncogenically transform cells. We report here that the homeodomain protein, HOXA9, is a positive regulator of eIF4E. HOXA9 stimulates eIF4E-dependent export of cyclin D1 and ornithine decarboxylase (ODC) mRNAs in the nucleus, as well as increases the translation efficiency of ODC mRNA in the cytoplasm. These activities depend on direct interactions of HOXA9 with eIF4E and are independent of the role of HOXA9 in transcription. At the biochemical level, HOXA9 mediates these effects by competing with factors that repress eIF4E function, in particular the proline-rich homeodomain PRH/Hex. This competitive mechanism of eIF4E regulation is disrupted in a subset of leukemias, where HOXA9 displaces PRH from eIF4E, thereby contributing to eIF4E's dysregulation. In regard to these results and our previous finding that approximately 200 homeodomain proteins contain eIF4E binding sites, we propose that homeodomain modulation of eIF4E activity is a novel means through which this family of proteins implements their effects on growth and development.

Aberrant eukaryotic translation initiation factor 4E-dependent mRNA transport impedes hematopoietic differentiation and contributes to leukemogenesis

The eukaryotic translation initiation factor 4E (eIF4E) acts as both a key translation factor and as a promoter of nucleocytoplasmic transport of specific transcripts. Traditionally, its transformation capacity in vivo is attributed to its role in translation initiation in the cytoplasm. Here, we demonstrate that elevated eIF4E impedes granulocytic and monocytic differentiation. Our subsequent mutagenesis studies indicate that this block is a result of dysregulated eIF4E-dependent mRNA transport. These studies indicate that the RNA transport function of eIF4E could contribute to leukemogenesis. We extended our studies to provide the first evidence that the nuclear transport function of eIF4E contributes to human malignancy, specifically in a subset of acute and chronic myelogenous leukemia patients. We observe an increase in eIF4E-dependent cyclin D1 mRNA transport and a concomitant increase in cyclin D1 protein levels. The aberrant nuclear function of eIF4E is due to abnormally large eIF4E bodies and the loss of regulation by the proline-rich homeodomain PRH. We developed a novel tool to modulate this transport activity. The introduction of IkappaB, the repressor of NF-kappaB, leads to suppression of eIF4E, elevation of PRH, reorganization of eIF4E nuclear bodies, and subsequent downregulation of eIF4E-dependent mRNA transport. Thus, our findings indicate that this nuclear function of eIF4E can contribute to leukemogenesis by promoting growth and by impeding differentiation.

Proapoptotic protein glyceraldehyde-3-phosphate dehydrogenase: a possible site of action of antiapoptotic drugs

Glyceraldehyde-3-phosphate dehydrogenase (GAPDH) has long been recognized as a classical glycolytic protein and has been used as a "housekeeping" gene in studies of genetic expression and regulation. However, recent advances reveal that GAPDH displays diverse nonglycolytic functions depending on its subcellular localization. Among those functions, one of the most intriguing is likely to be the induction of apoptosis. Previous works by our group and others have demonstrated that the overexpression of GAPDH and its subsequent nuclear translocation appear to be implicated in the initiation of one or more apoptotic cascades and also in the etiology of some neurological diseases. This review addresses new data demonstrating that a protein, referred to as proapoptotic protein GAPDH, with a quite mundane function in healthy cells behaves very differently during cell suicide, and summarizes emphatically the importance of this protein as a putative molecular target in developing antiapoptotic therapeutic agents for the treatment of certain neurodegenerative disorders.

The proline-rich homeodomain protein, PRH, is a tissue-specific inhibitor of eIF4E-dependent cyclin D1 mRNA transport and growth

The translation initiation factor eIF4E is involved in the modulation of cellular growth. In the nucleus, where eIF4E is associated with PML nuclear bodies, eIF4E mediates nucleocytoplasmic transport of specific transcripts, and this contributes to its transformation activity. Surprisingly, we found that a trans cription factor, the proline-rich homeodomain protein PRH, is a negative regulator of eIF4E in myeloid cells, interacting with eIF4E through a conserved binding site typically found in translational regulators. Through this interaction, PRH inhibits eIF4E-dependent mRNA transport and subsequent transformation. These activities of PRH are independent of its transcriptional functions. Further, we found that 199 homeodomain proteins contain potential eIF4E-binding sites. Thus, there could be many tissue-specific regulators of eIF4E. These findings provide a model for regulation of a general factor, eIF4E, in tissue- specific contexts, and suggest that its regulation is important in differentiation and development.

Subcellular alteration of glyceraldehyde-3-phosphate dehydrogenase in Alzheimer's disease fibroblasts

The regulation of glyceraldehyde-3-phosphate dehydrogenase (GAPDH) has been implicated both in age-related neurodegenerative disease and in apoptosis. Previous in vitro studies suggest an interaction between GAPDH and the beta-amyloid precursor protein (beta-APP), a protein directly involved in Alzheimer's disease (AD). New studies indicate that GAPDH is a multidimensional protein with diverse membrane, cytoplasmic, and nuclear functions; each is distinct from its role in glycolysis. The nuclear functions of GAPDH include a role in apoptosis that requires its translocation to the nucleus. Accordingly, beta-APP-GAPDH interactions, altering GAPDH structure in vivo, may affect energy generation, inducing hypometabolism, a characteristic AD phenotype. Because GAPDH is a multifunctional protein, pleiotropic effects may also occur in a variety of fundamental cellular pathways in AD cells. This may include unique GAPDH-RNA interactions. We report here the identification of a high-molecular-weight (HMW) GAPDH species present exclusively in the postnuclear fraction of AD cells. The latter is characterized by reduced GAPDH activity. The HMW GAPDH species was not detected in postnuclear age-matched control (AMC) fractions nor in AD whole-cell preparations. Each is characterized by normal GAPDH activity. By definition, the preparation of whole-cell extracts entails the destruction of subcellular structure. The latter findings indicate that the dissociation of the GAPDH protein from the HMW species restores its enzymatic activity. Thus, these results reveal a new, unique intracellular phenotype in AD cells. The functional consequences of subcellular alteration in GAPDH structure in AD cells are considered.

Finding a role for PML in APL pathogenesis: a critical assessment of potential PML activities

In normal mammalian cells the promyelocytic leukemia protein (PML) is primarily localized in multiprotein nuclear complexes called PML nuclear bodies. However, both PML and PML nuclear bodies are disrupted in acute promyelocytic leukemia (APL). The treatment of APL patients with all-trans retinoic acid (ATRA) results in clinical remission associated with blast cell differentiation and reformation of the PML nuclear bodies. These observations imply that the structural integrity of the PML nuclear body is critically important for normal cellular functions. Indeed, PML protein is a negative growth regulator capable of causing growth arrest in the G(1) phase of the cell cycle, transformation suppression, senescence and apoptosis. These PML-mediated, physiological effects can be readily demonstrated. However, a discrete biochemical and molecular model of PML function has yet to be defined. Upon first assessment of the current PML literature there appears to be a seemingly endless list of potential PML partner proteins implicating PML in a variety of regulatory mechanisms at every level of gene expression. The purpose of this review is to simplify this confusing field of research by using strict criteria to deduce which models of PML body function are well supported.

The acute promyelocytic leukemia-associated protein, promyelocytic leukemia zinc finger, regulates 1,25-dihydroxyvitamin D(3)-induced monocytic differentiation of U937 cells through a physical interaction with vitamin D(3) receptor

Monocyte differentiation induced by 1,25-dihydroxyvitamin D(3) (1,25(OH)(2)D(3)) is interrupted during the course of acute promyelocytic leukemia (APL). One form of APL is associated with the translocation t(11;17), which joins the promyelocytic leukemia zinc finger (PLZF) and retinoic acid receptor alpha (RARalpha) genes. Because PLZF is coexpressed in the myeloid lineage with the vitamin D(3) receptor (VDR), the interplay between PLZF and VDR was examined. It was found that PLZF interacts directly with VDR. This occurred at least partly through contacts in the DNA-binding domain of VDR and the broad complex, tram-trak, bric-a-brac/pox virus zinc finger (BTB/POZ) domain of PLZF. Moreover, PLZF altered the mobility of VDR derived from nuclear extracts when bound to its cognate binding site, forming a slowly migrating DNA-protein complex. Overexpression of PLZF in a monocytic cell line abrogated 1,25(OH)(2)D(3) activation from both a minimal VDR responsive reporter and the promoter of p21(WAF1/CIP1), a target gene of VDR. Deletion of the BTB/POZ domain significantly relieved PLZF-mediated repression of 1,25(OH)(2)D(3)-dependent activation. In addition, stable, inducible expression of PLZF in U937 cells inhibited the ability of 1,25(OH)(2)D(3) to induce surface expression of the monocytic marker CD14 and morphologic changes associated with differentiation. These results suggest that PLZF may play an important role in regulating the process by which 1,25(OH)(2)D(3) induces monocytic differentiation in hematopoietic cells.

Cellular proteins localized at and interacting within ND10/PML nuclear bodies/PODs suggest functions of a nuclear depot

ND10, PML bodies or PODs have become the defining nuclear structure for a highly complex protein complement involved in cell activities such as aging, apoptosis, the cell cycle, stress response, hormone signaling, transcriptional regulation and development. ND10 are present in many but not all cell types and are not essential for cell survival. Here, we review the cellular proteins found in ND10, their few known interactions and their contribution to the ND10 structure per se and to functions elsewhere in the nucleus. The discrepancy between the functions of the ND10 proteins and the nonessential nature of the structure in which they are aggregated at their highest concentrations leads to the conclusion that the proteins function elsewhere. The regulated recruitment of specific proteins into ND10 as well as their controlled release upon external induced stress points to a regulated nuclear depot function for ND10. These nuclear depot functions seem important as nuclear defense against viral attack and other external insults.

Glyceraldehyde-3-phosphate dehydrogenase in neurodegeneration and apoptosis signaling

Glyceraldehyde-3-phosphate dehydrogenase (GAPDH) is a well-studied glycolytic enzyme that plays a key role in energy metabolism. GAPDH catalyzes the conversion of glyceraldehyde-3-phosphate to 1,3-bisphosphoglycerate in the glycolytic pathway. As part of the conversion, GAPDH converts NAD+ to the high-energy electron carrier NADH. GAPDH has been referred to as a "housekeeping" protein and based on the view that GAPDH gene expression remains constant under changing cellular conditions, the levels of GAPDH mRNA have frequently been used to normalize northern blots. In recent years, that view has changed since GAPDH is now known to contribute to a number of diverse cellular functions unrelated to glycolysis. Normative functions of GAPDH now include nuclear RNA export, DNA replication, DNA repair, exocytotic membrane fusion, cytoskeletal organization and phosphotransferase activity. Pathologically, GAPDH has been implicated in apoptosis, neurodegenerative disease, prostate cancer and viral pathogenesis (see Sirover (1999) for a recent review of GAPDH functions). Most recently, it has been shown that GAPDH is a target for deprenyl related compounds (Carlile et al., 2000; Kragten et al., 1998) and may contribute to the neuroprotection offered by those compounds.

The Rossmann fold of glyceraldehyde-3-phosphate dehydrogenase (GAPDH) is a nuclear docking site for antisense oligonucleotides containing a TAAAT motif

The subcellular localisation of oligodeoxynucleotides (ODN) is a major limitation for their use against nuclear targets. In this study we demonstrate that an antisense ODN directed against cytosolic phospholipase A(2) (cPLA2) mRNA is efficiently taken up and accumulates in the nuclei of endothelial cells (HUVEC), human monocytes and HeLa cells. Gel shift experiments and incubation of cells with oligonucleotide derivatives show that the anti-cPLA2 oligo binds a 37 kDa protein in nuclear extracts. The TAAAT sequence was identified as the major binding motif for the nuclear protein in competition experiments with mutated ODNs. Modification of the AAA triplet resulted in an ODN which failed to localise in the nucleus. Moreover, inserting a TAAAT motif into an ODN localising in the cytosol did not modify its localisation. The 37 kDa protein was purified and identified after peptide sequencing as glyceraldehyde-3-phosphate dehydrogenase (GAPDH). It was shown by confocal microscopy that GAPDH co-localises with anti-cPLA2 ODN in the nucleus and commercial GAPDH effectively binds the oligo. Competition experiments with increasing concentration of NAD(+) co-factor indicate that the GAPDH Rossmann fold is a docking site for antisense oligonucleotides containing a TAAAT motif.

Glyceraldehyde-3-phosphate dehydrogenase and apoptosis

Glyceraldehyde-3-phosphate dehydrogenase (GAPDH) has long been recognized as playing an integral role in glycolysis. During the past 20 years, however, a number of novel, additional functions for GAPDH have been described. These include acting as an uracil DNA glycosylase, activator of transcription, binding to RNA, and an involvement in tubulin assembly. One of the most intriguing functions which has recently been recognized is an involvement in the initiation of apoptosis. Further, GAPDH associates with proteins implicated in human neurodegenerative disorders. This review summarizes the evidence implicating GAPDH in the initiation of one or more apoptotic cascades. The possible functions of GAPDH in the nucleus which could result in the initiation of apoptosis are also discussed.

The lymphocytic choriomeningitis virus RING protein Z associates with eukaryotic initiation factor 4E and selectively represses translation in a RING-dependent manner

Only a few host cell proteins that associate with arenaviruses have been identified. To date, the arenavirus Z protein associates with the promyelocytic leukemia protein PML and the ribosomal P proteins. The majority of PML is present in nuclear bodies which are translocated to the cytoplasm by infection with the arenavirus, lymphocytic choriomeningitis virus (LCMV). The Z protein is a small zinc-binding RING protein with an unknown function which is required for the viral life cycle. Here, we demonstrate an association between Z and the host cell translation factor, eukaryotic initiation factor 4E (eIF-4E) in infected and transfected cells. Z's association with both ribosomal proteins and this translation factor led us to investigate whether Z could modulate host cell translation. In cell culture, Z selectively represses protein production in an eIF-4E-dependent manner. Specifically, we see reduction in cyclin D1 protein production with no effect on glyceraldehyde-3-phosphate dehydrogenase (GAPDH) in cells transfected with Z. Previous reports indicate that cyclin D1 is sensitive to eIF-4E levels, whereas GAPDH is not. Consistent with this, we observe preferential downregulation of cyclin D1 during infection and no effect on GAPDH. Further, no changes in RNA levels were observed for cyclin D1 or GAPDH transcripts. The interaction between eIF-4E and Z may provide a mechanism for slower growth observed in infected cells and a viral strategy for establishing chronic infection.

The promyelocytic leukemia (PML) protein suppresses cyclin D1 protein production by altering the nuclear cytoplasmic distribution of cyclin D1 mRNA

The majority of the promyelocytic leukemia (PML) protein is present in nuclear bodies which are altered in several pathogenic conditions including acute promyelocytic leukemia. PML nuclear bodies are found in nearly all cells yet their function remains unknown. Here, we demonstrate that PML and the eukaryotic initiation factor 4E (elF-4E) co-localize and co-immunopurify. eIF-4E is involved in nucleocytoplasmic transport of specific mRNAs including cyclin D1. eIF-4E overexpression leads to increased cyclin D1 protein levels; whereas, overexpression of PML leads to decreased cyclin D1 levels. Neither PML nor eIF-4E cause significant changes in cyclin D1 mRNA levels. The association with eIF-4E led us to investigate if PML could alter mRNA distribution as a possible post-transcriptional mechanism for suppressing cyclin D1 production. We show that overexpression of PML results in nuclear retention of cyclin D1 mRNA and that intact PML nuclear bodies are required. Addition of eIF-4E overcomes PML induced retention and alters the morphology of PML bodies suggesting a mechanism by which eIF-4E can modulate PML function. These results raise the possibility that PML nuclear bodies may participate in the regulation of nucleocytoplasmic transport of specific mRNAs.

The promyelocytic leukemia protein PML interacts with the proline-rich homeodomain protein PRH: a RING may link hematopoiesis and growth control

Acute promyelocytic leukemia (APL) is characterized by a block in myeloid cell differentiation. As a result of a chromosomal translocation in these patients, the promyelocytic leukemia protein PML is disrupted as are the nuclear bodies it forms. Disruption of PML and PML nuclear bodies in APL is linked to a loss of growth control and subsequent leukemogenesis. PML contains a zinc-binding domain known as the RING which is required for formation of these bodies. Using yeast 2-hybrid techniques, we found that PML and a related RING protein, Z, bind the proline rich homeodomain protein (PRH) through their RING domains. Previous reports indicate that PRH functions in hematopoiesis and may act as a transcriptional repressor. Our data indicate that PML and Z both bind the repressor domain of PRH and are the first protein partners reported for PRH. We observe that PRH has a punctate pattern in both the nucleus and cytoplasm of chronic myelogenous leukemia K562 cells and in the APL cell line, NB4. Immunoprecipitation and co-localization studies indicate that PML and PRH interact in both cell lines. The effect on cell growth by PML and the hematopoietic actions of PRH raises the possibility that the interaction between PML and PRH represents a link between growth control and hematopoiesis.

New insights into an old protein: the functional diversity of mammalian glyceraldehyde-3-phosphate dehydrogenase

Glyceraldehyde-3-phosphate dehydrogenase (GAPDH) was considered a classical glycolytic protein examined for its pivotal role in energy production. It was also used as a model protein for analysis of protein structure and enzyme mechanisms. The GAPDH gene was utilized as a prototype for studies of genetic organization, expression and regulation. However, recent evidence demonstrates that mammalian GAPDH displays a number of diverse activities unrelated to its glycolytic function. These include its role in membrane fusion, microtubule bundling, phosphotransferase activity, nuclear RNA export, DNA replication and DNA repair. These new activities may be related to the subcellular localization and oligomeric structure of GAPDH in vivo. Furthermore, other investigations suggest that GAPDH is involved in apoptosis, age-related neurodegenerative disease, prostate cancer and viral pathogenesis. Intriguingly, GAPDH is also a unique target of nitric oxide. This review discusses the functional diversity of GAPDH in relation to its protein structure. The mechanisms through which mammalian cells may utilize GAPDH amino acid sequences to provide these new functions and to determine its intracellular localization are considered. The interrelationship between new GAPDH activities and its role in cell pathologies is addressed.

The PML nuclear bodies: actors or extras?

The PML and SP100-containing nuclear bodies (NBs) represent the best-studied example of a defined nuclear substructure the integrity of which is compromised in certain human diseases, including leukemia, neurodegenerative disorders and viral infection. Although recent progress has underscored the unexpectedly broad involvement of NB constituents in the control of cell growth, gene regulation and apoptosis in both pathological and normal contexts, evidence for a specific physiological activity within the NBs remains scant, thus precluding a unifying hypothesis for NB function.