Protective effect of L-deprenyl against apoptosis induced by okadaic acid in cultured neuronal cells

L-Deprenyl, an irreversible MAO-B (monoamine oxidase B, EC 1.4.3.4) inhibitor, is used for the treatment of Parkinson's disease and to delay the progression of Alzheimer's disease. L-Deprenyl also exhibits protective effects against neuronal apoptosis which are independent of its ability to inhibit MAO-B. The purpose of this study was to compare the antiapoptotic efficacy of L-deprenyl against different types of apoptotic inducers in three neuronal cell culture models. The level of apoptosis was quantified by measuring the activation of caspase-3 enzyme, which is the main apoptotic executioner in neuronal cells. MTT [3-(4,5-dimethylthiazol-2-yl)-2, 5-diphenyltetrazolium bromide] and LDH (lactate dehydrogenase, EC 1. 1.1.27) assays were used to demonstrate the cytotoxic response of apoptotic treatments. Our results showed that okadaic acid, an inhibitor of protein phosphatase 1 and 2A, induced a prominent increase in caspase-3 activity both in cultured hippocampal and cerebellar granule neurons as well as in Neuro-2a neuroblastoma cells. Interestingly, L-deprenyl offered a significant protection against the apoptotic response induced by okadaic acid in all three neuronal models. The best protection appeared at the concentration level of 10(-9) M. L-Deprenyl also provided a protection against apoptosis after AraC (cytosine beta-D-arabinoside) treatment in hippocampal neurons and Neuro-2a cells and after etoposide treatment in Neuro-2a cells. However, L-deprenyl did not offer any protection against apoptosis caused by serum withdrawal or potassium deprivation. Okadaic acid treatment in vivo is known to induce an Alzheimer's type of hyperphosphorylation of tau protein, formation of beta-amyloid plaques, and a severe memory impairment. Our results show that the okadaic acid model provides a promising tool to study the molecular basis of Alzheimer's disease and to screen the neuroprotective capacity of L-deprenyl derivatives.

A differential display protocol to identify differentially expressed mRNAs in potassium-deprived cerebellar granule cells

Differential display (DD) has become a popular technique for the identification of differentially expressed genes. Here we present a DD protocol for studying mRNA expression changes during neuronal apoptosis. Neuronal apoptosis is typically dependent on macromolecular synthesis, thus suggesting that regulation of gene expression is involved in control of the activation of the cell-death machinery. In order to identify some of the genes involved, we employed the widely used cell culture model in which apoptosis is induced in rat cerebellar granule cells (CGCs) using potassium deprivation. Although DD has been applied productively in the study of various biological phenomena, the method has its drawbacks. In particular, the cloning and verification of cDNA fragments is frequently described as problematic or laborious, and often produces many "false positives". Here we report the successful use of DD including an efficient protocol for cDNA clone screening and verification. This protocol avoids some of the problems presented by heterogeneous DD bands, which may be a major cause of false-positive results. To identify the desired clones, we apply single-stranded conformational polymorphism (SSCP) and slot blot techniques.

Engineering a new magnesium binding site in the subunit contact region of Escherichia coli inorganic pyrophosphatase

Three Gln-80 residues belonging to different subunits of homohexameric Escherichia coli pyrophosphatase are separated by only one water molecule to which they are hydrogen bonded. Substitution of Glu for Gln-80 stabilizes quaternary structure of the enzyme but has only a small effect on enzyme activity. The substitution stimulates Mg2+ binding and changes the appearance of the Mg2+ concentration dependence of the rate constant for the trimer --> hexamer transition. These data suggest that a new Mg2+ binding site is formed in the intersubunit contact region as a result of the substitution. Three-dimensional modeling of the mutated protein showed that a chelate complex might form involving two of the three Glu-80 residues.

A rapid and efficient method for preparation of genomic DNA suitable for analysis of both high and low molecular weight DNA fragmentation during neuronal apoptosis

In this paper, we describe a fast (within 1 h) and efficient method for the preparation of intact genomic DNA suitable for the analysis of DNA integrity in neuronal cells undergoing apoptosis. The procedure involves the embedding of apoptotic cells into low-melting point agarose, in situ cell lysis and purification of DNA samples followed by resolution of fragmented DNA by agarose gel electrophoresis. The method has no restrictions on the number of simultaneous DNA preparations, allows detection of both high and low molecular weight DNA fragments, and is suitable for the preparation of DNA samples from cultured cells, fresh or frozen tissues, as well as from purified nuclei. This procedure is also applicable for the preparation of intact genomic DNA for the megabase-scale analysis.

Reciprocal effects of substitutions at the subunit interfaces in hexameric pyrophosphatase of Escherichia coli. Dimeric and monomeric forms of the enzyme

A homohexameric molecule of Escherichia coli pyrophosphatase is arranged as a dimer of trimers, with an active site present in each of its six monomers. Earlier we reported that substitution of His(136) and His(140) in the intertrimeric subunit interface splits the molecule into active trimers (Velichko, I. S., Mikalahti, K., Kasho, V. N., Dudarenkov, V. Y., Hyytiä, T., Goldman, A., Cooperman, B. S., Lahti, R., and Baykov, A. A. (1998) Biochemistry 37, 734-740). Here we demonstrate that additional substitutions of Tyr(77) and Gln(80) in the intratrimeric interface give rise to moderately active dimers or virtually inactive monomers, depending on pH, temperature, and Mg(2+) concentration. Successive dissociation of the hexamer into trimers, dimers, and monomers progressively decreases the catalytic efficiency (by 10(6)-fold in total), and conversion of a trimer into dimer decreases the affinity of one of the essential Mg(2+)-binding sites/monomer. Disruptive substitutions predominantly in the intratrimeric interface stabilize the intertrimeric interface and vice versa, suggesting that the optimal intratrimeric interaction is not compatible with the optimal intertrimeric interaction. Because of the resulting "conformational strain," hexameric wild-type structure appears to be preformed to bind substrate. A hexameric triple variant substituted at Tyr(77), Gln(80), and His(136) exhibits positive cooperativity in catalysis, consistent with this model.

Regulation of PTEN expression in neuronal apoptosis

PTEN phosphatase is a tumor suppressor gene that dephosphorylates phosphatidylinositol phosphates. PTEN restrains the function of a major antiapoptotic and survival pathway involving phosphoinositide 3-kinase and Akt kinase. Our purpose was to find out whether apoptotic inducers affect the expression of PTEN in cerebellar granule neurons and neuroblastoma 2a cells (Neuro-2a). PTEN mRNA expression showed a major 5.5-kb and a lower abundance 2.5-kb transcripts. In Neuro-2a cells, serum withdrawal induced a prominent, continuous decrease both in 5.5- and 2.5-kb transcripts of PTEN mRNA. Simultaneously, the expression level of 56-kDa PTEN protein decreased in Neuro-2a cells. The decrease in PTEN expression precedes apoptotic changes observed after serum withdrawal. On the contrary, okadaic acid and etoposide only slightly affected the expression of PTEN although they induce a prominent apoptosis in Neuro-2a cells. In cerebellar granule neurons, okadaic acid treatment induced a prominent increase in PTEN mRNA expression after 6-h treatment, both at the 5.5- and 2.5-kb transcripts. The early response in PTEN mRNA expression disappeared in 5.5-kb transcripts already at 12 h and in the case of 2.5-kb transcripts it lasted up to 24 h. Potassium deprivation, known to induce apoptosis in cerebellar granule cells, did not affect PTEN mRNA expression but together with serum deprivation induced a clear decrease in the 5. 5-kb PTEN transcripts. It seems that the changes in PTEN expression level and neuronal apoptosis are not related to each other in general but the expression of PTEN phosphatase seems to regulate certain apoptotic signals affecting phosphoinositide 3-kinase function.

Evolutionary aspects of inorganic pyrophosphatase

Based on the primary structure, soluble inorganic pyrophosphatases can be divided into two families which exhibit no sequence similarity to each other. Family I, comprising most of the known pyrophosphatase sequences, can be further divided into prokaryotic, plant and animal/fungal pyrophosphatases. Interestingly, plant pyrophosphatases bear a closer similarity to prokaryotic than to animal/fungal pyrophosphatases. Only 17 residues are conserved in all 37 pyrophosphatases of family I and remarkably, 15 of these residues are located at the active site. Subunit interface residues are conserved in animal/fungal but not in prokaryotic pyrophosphatases.

Distinct mechanisms underlay DNA disintegration during apoptosis induced by genotoxic and nongenotoxic agents in neuroblastoma cells

Exposure of mouse NB-2a neuroblastoma cells to genotoxic (etoposide or cytosine arabinoside) or nongenotoxic challenges (serum deprivation or okadaic acid) resulted in progressive cell death with biochemical and morphological characteristics typical of apoptosis. Apoptotic cell death induced by nongenotoxic agents was associated with the disintegration of nuclear DNA into high molecular weight (HMW) and oligonucleosomal-DNA fragments, while the formation of HMW-DNA fragments, but not oligonucleosomal-DNA ladder accompanied apoptosis induced by genotoxic agents. Combination of genotoxic and nongenotoxic insults, i.e. incubation of etoposide-treated cells in the serum-free medium, resulted in an additive effect on the profile of DNA disintegration, which involved both HMW fragmentation pattern as in etoposide alone treated cells and the oligonucleosomal-DNA ladder observed with serum-deprived cells. On the other hand, incubation of serum-deprived cells in the presence of Zn2+-ions led to the abrogation of internucleosomal DNA fragmentation but accumulation of HMW-DNA fragments. Differences in the pattern of DNA fragmentation were reproducible in a cell free apoptotic system after treatment of isolated normal nuclei with cytosolic extracts prepared from the cells treated with genotoxic or nogenotoxic apoptotic inducers. Cell free experiments also revealed that activities responsible for the formation of HMW- and oligonucleosomal-DNA fragments are separable in cytosolic extract prepared from the serum-deprived cells. Finally, DNA fragmentation induced by nongenotoxic apoptotic inducers was effectively prevented by cycloheximide and suramin, while both cycloheximide and suramin had only a slight inhibitory effect on DNA fragmentation induced by genotoxic agents. The results presented suggest that distinct pathways underlay disintegration of nuclear DNA during apoptosis induced by genotoxic and nongenotoxic inducers, and that the formation of HMW- and oligonucleosomal-DNA fragments proceeds via separate mechanisms in NB-2a neuroblastoma cells.

Contrasting effects of alendronate and clodronate on RAW 264 macrophages: the role of a bisphosphonate metabolite

Clodronate (dichloromethylidene-bisphosphonate), a halogen-containing bisphosphonate, can inhibit the release of cytokines from RAW 264 macrophages and has anti-inflammatory properties in rheumatoid arthritis, whilst amino-containing bisphosphonates such as alendronate (4-amino-1-hydroxybutylidene-bisphosphonate), have pro-inflammatory properties and can cause an acute phase response. The basis for these pharmacological properties is unclear. Recently, it was demonstrated that clodronate is metabolised by certain cell lines in vitro to an analogue of ATP, whereas amino-bisphosphonates are not. We therefore investigated whether clodronate can also be metabolised by RAW 264 macrophages and whether intracellular accumulation of the metabolite (AppCCl2p) could account for the anti-inflammatory properties of clodronate. The effect of alendronate and AppCCl2p on the release of cytokines (IL-1beta, IL-6, and TNFalpha) from RAW 264 cells was compared, and the effect of the bisphosphonates and AppCCl2p on the DNA binding activities of transcription factors, NF-kappaB and AP-1, was investigated. Pretreatment of RAW 264 macrophages with alendronate augmented the LPS-stimulated release of IL-1beta and increased the binding of NF-kappaB to DNA in an electrophoretic mobility shift assay. Without LPS-induction, alendronate did not affect cytokine release or NF-kappaB binding. Clodronate was metabolised by RAW 264 cells to AppCCl2p. Like clodronate, AppCCl2p inhibited the LPS-induced release of cytokines and NO from RAW 264 macrophages. Both clodronate and its metabolite also inhibited the LPS-stimulated binding of NF-kappaB to DNA. In conclusion, these results suggest that the metabolite of clodronate may be responsible for the anti-inflammatory properties of clodronate, and that the contrasting effects of different bisphosphonates on the release of cytokines could be mediated partly through changes in the DNA binding activity of NF-kappaB.

Attenuation of NF-kappaB signaling response to UVB light during cellular senescence

The ability of cells to adapt to environmental stresses undergoes a progressive reduction during aging. NF-kappaB-mediated signaling is a major defensive system against various environmental challenges. The aim of this study was to find out whether replicative senescence affects the response of the NF-kappaB signaling pathway to UVB light in human WI-38 and IMR-90 fibroblasts. The exposure of early passage fibroblasts to UVB light inhibited the proliferation and induced a flat phenotype similar to that observed in replicatively senescent fibroblasts not exposed to UVB light. The UVB radiation dose used (153 mJ/cm2) did not induce apoptosis in either early or late passage WI-38 fibroblasts. UVB exposure induced a prominent activation of the NF-kappaB signaling pathway both in early and in late passage WI-38 and IMR-90 fibroblasts. Interestingly, the response to UVB light was significantly attenuated in late passage fibroblasts. This attenuation was most prominent in DNA binding activities of nuclear NF-kappaB complexes. Similar senescence-related attenuation was also observed in the DNA binding activities of nuclear AP-1 and Sp-1 factors after UVB treatment. Immunoblotting and -cytochemistry showed an increase in nuclear localization of p50 and p65 components of NF-kappaB complexes. Supershift experiments showed that the specific NF-kappaB complexes contain p50 and p65 protein components but not p52 and c-Rel proteins. Cytoplasmic IkappaBalpha showed a marked decrease at protein level but an increase in phosphorylation after UVB treatment. Transient transfection assays with TK5-CAT and TK10-CAT plasmids carrying NF-kappaB-responsive sites of the TNFalpha promoter were used to analyze the functional activity of the NF-kappaB complexes. Results showed that UVB exposure induced an increase in NF-kappaB-driven CAT expression both in early and in late passage fibroblasts though the response was significantly stronger in early passage fibroblasts. Our results show that the induction of NF-kappaB-mediated signaling by UVB light is highly attenuated in senescent fibroblasts. This attenuation may reduce the stress resistance during cellular senescence.

Directed mutagenesis studies of the metal binding site at the subunit interface of Escherichia coli inorganic pyrophosphatase

Recent crystallographic studies on Escherichia coli inorganic pyrophosphatase (E-PPase) have identified three Mg2+ ions/enzyme hexamer in water-filled cavities formed by Asn24, Ala25, and Asp26 at the trimer-trimer interface (Kankare, J., Salminen, T., Lahti, R., Cooperman, B., Baykov, A. A., and Goldman, A. (1996) Biochemistry 35, 4670-4677). Here we show that D26S and D26N substitutions decrease the stoichiometry of tight Mg2+ binding to E-PPase by approximately 0.5 mol/mol monomer and increase hexamer stability in acidic medium. Mg2+ markedly decelerates the dissociation of enzyme hexamer into trimers at pH 5.0 and accelerates hexamer formation from trimers at pH 7.2 with wild type E-PPase and the N24D variant, in contrast to the D26S and D26N variants, when little or no effect is seen. The catalytic parameters describing the dependences of enzyme activity on substrate and Mg2+ concentrations are of the same magnitude for wild type E-PPase and the three variants. The affinity of the intertrimer site for Mg2+ at pH 7.2 is intermediate between those of two Mg2+ binding sites found in the E-PPase active site. It is concluded that the metal ion binding site found at the trimer-trimer interface of E-PPase is a high affinity site whose occupancy by Mg2+ greatly stabilizes the enzyme hexamer but has little effect on catalysis.

Cloning and expression of a unique inorganic pyrophosphatase from Bacillus subtilis: evidence for a new family of enzymes

An open reading frame located in the COTF-TETB intergenic region of Bacillus subtilis was cloned and expressed in Escherichia coli and shown to encode inorganic pyrophosphatase (PPase). The isolated enzyme is Mn2+-activated, like the authentic PPase isolated from B. subtilis. Although 13 functionally important active site residues are conserved in all 31 soluble PPase sequences so far identified, only two of them are conserved in B. subtilis PPase. This suggests that B. subtilis PPase represents a new family of soluble PPases (a Bs family), putative members of which were found in Archaeoglobus fulgidus, Methanococcus jannaschii, Streptococcus mutans and Streptococcus gordonii.

Distinct mode of apoptosis induced by genotoxic agent etoposide and serum withdrawal in neuroblastoma cells

Here we compared the features of apoptosis induced by DNA-damaging agent, etoposide, and by withdrawal of the growth factors in NB 2a neuroblastoma cells. We showed that serum deprivation and etoposide induced a distinct pattern of regulation of c-Fos, c-Jun and p53 protein levels, as well as the differential changes in DNA-binding activity of AP-1 and NF-kappaB transcription factors. The late phase of apoptesis induced by serum withdrawal was associated with disintegration of nuclear DNA both into high molecular weight (HMW) and oligonucleosomal DNA fragments, whereas etoposide induced the formation of HMW-DNA fragments without internucleosomal DNA cleavage. Incubation of etoposide-treated cells without serum resulted in an additive effect on the pattern of DNA fragmentation. Differences in DNA fragmentation profiles induced by serum withdrawal and etoposide in NB 2a cells were reproducible in nonproliferating cerebellar granule cells and also in a cell free system assay after treatment of isolated normal nuclei with cytosolic extracts prepared from serum-deprived or etoposide-treated cells. Both HMW and oligonucleosomal DNA fragmentation in serum-deprived cells was inhibited by aurintricarboxylic acid and was completely abrogated by cycloheximide. In contrast, DNA fragmentation in etoposide-treated cells was insensitive to the inhibitory effect of aurintricarboxylic acid, and was not prevented by cycloheximide. Our results indicate that in NB 2a neuroblastoma cells etoposide and serum withdrawal induce a distinct mode of apoptosis which is associated with a distinct pattern of regulation of immediately early response genes in the early phase, and with recruitment of different mechanisms for DNA disintegration in the late phase of apoptosis.

Expression of seizure-related PTZ-17 is induced by potassium deprivation in cerebellar granule cells

The aim of this study was to identify changes in gene expression during neuronal apoptosis using the differential display (DD) technique. Potassium deprivation was used to induce neuronal apoptosis in cultured rat cerebellar granule cells. DD analysis of about 1600 transcripts resulted in 8 cDNA clones that confirmed differential expression in a slot blot analysis. One of these clones was homologous to the 3' end of seizure-related PTZ-17 RNA. Northern blot analysis showed a marked upregulation of a 2.2 kb RNA 24 hours after potassium withdrawal. This upregulation was prevented by the RNA synthesis inhibitor actinomycin D. The increase in PTZ-17 expression was specific for potassium deprivation induced apoptosis, since the other apoptosis inducers, okadaic acid and staurosporine, did not affect PTZ-17 expression. The level of PTZ-17 RNA was not significantly affected by aging in rat cerebellum. Our data suggest that the upregulation of the PTZ-17 RNA is a part of the steps leading to apoptosis during potassium deprivation in cerebellar granule cells.

Expression of transcriptional repressor protein mSin3A but not mSin3B is induced during neuronal apoptosis

mSin3 proteins have an important role in transcriptional repression mediated by histone deacetylation. Our purpose was to find out whether apoptosis affects the expression of mSin3 proteins in neuroblastoma 2a cells. We observed that neuronal apoptosis, induced by serum withdrawal or by treatment with etoposide, okadaic acid or trichostatin A, induced a prominent increase in mSin3A protein expression but did not affect the level of mSin3B protein. Trichostatin A, an inhibitor of histone deacetylases, induced the most prominent upregulation of mSin3A protein. Metabolic labeling and immunoprecipitation of mSin3A showed a marked increase in the synthesis of mSin3A protein in agreement with the immunoblotting results. Interestingly, the expression of mSin3A preceded the activation of caspase-3 and the execution phase of neuronal apoptosis. These results suggest that the expression of mSin3A proteins may provide a regulation mechanism to enhance transcriptional repression or silencing of genes during neuronal apoptosis, as well as during degenerative diseases.

Neuronal apoptosis induced by histone deacetylase inhibitors

Histone acetylation has a key role in transcriptional activation, whereas deacetylation of histones correlates with the transcriptional repression and silencing of genes. Genetic repression may have an important role in neuronal aging, atrophy and degenerative diseases. Our aim was to study how histone deacetylase inhibitors, trichostatin A (TSA) and sodium butyrate, affect the metabolism of cultured rat cerebellar granule neurons and mouse Neuro-2a neuroblastoma cells. Cultured cells were exposed to 1-3 microM TSA and 1-10 mM butyrate for 1-2 days. Both of these inhibitors induced a prominent neuronal apoptosis characterized by morphological changes as well as by the activation of caspase-3 protease and subsequent cleavage of poly(ADP-ribose) polymerase, one of the caspase-3 targets. Caspase-3 activities reached the highest level on the second day after treatment, higher in the proliferating neuroblastoma cells than in the cerebellar granule neurons. Caspase-3 activation and morphological changes were prevented by cycloheximide treatment. Histone deacetylase inhibitors increased the DNA-binding activities of AP1, CREB and NF-kappaB transcription factors. These observations show that an excessive level of histone acetylation induces a stress response and an apoptotic cell death in neuronal cells.

A sophisticated lander for scientific exploration of Mars: scientific objectives and implementation of the Mars-96 Small Station

A mission to Mars including two Small Stations, two Penetrators and an Orbiter was launched at Baikonur, Kazakhstan, on 16 November 1996. This was called the Mars-96 mission. The Small Stations were expected to land in September 1997 (Ls approximately 178 degrees), nominally to Amazonis-Arcadia region on locations (33 N, 169.4 W) and (37.6 N, 161.9 W). The fourth stage of the Mars-96 launcher malfunctioned and hence the mission was lost. However, the state of the art concept of the Small Station can be applied to future Martian lander missions. Also, from the manufacturing and performance point of view, the Mars-96 Small Station could be built as such at low cost, and be fairly easily accommodated on almost any forthcoming Martian mission. This is primarily due to the very simple interface between the Small Station and the spacecraft. The Small Station is a sophisticated piece of equipment. With the total available power of approximately 400 mW the Station successfully supports an ambitious scientific program. The Station accommodates a panoramic camera, an alpha-proton-x-ray spectrometer, a seismometer, a magnetometer, an oxidant instrument, equipment for meteorological observations, and sensors for atmospheric measurement during the descent phase, including images taken by a descent phase camera. The total mass of the Small Station with payload on the Martian surface, including the airbags, is only 32 kg. Lander observations on the surface of Mars combined with data from Orbiter instruments will shed light on the contemporary Mars and its evolution. As in the Mars-96 mission, specific science goals could be exploration of the interior and surface of Mars, investigation of the structure and dynamics of the atmosphere, the role of water and other materials containing volatiles and in situ studies of the atmospheric boundary layer processes. To achieve the scientific goals of the mission the lander should carry a versatile set of instruments. The Small Station accommodates devices for atmospheric measurements, geophysical and geochemical studies of the Martian surface and interior, and cameras for descent phase and panoramic views. These instruments would be able to contribute remarkably to the process of solving some of the scientific puzzles of Mars.

Glutamate-induced changes in the DNA-binding complexes of transcription factor YY1 in cultured hippocampal and cerebellar granule cells

Transcription factor YY1 is a zinc finger protein which can interact and form complexes with several proteins. Depending on its binding partners, YY1 can either activate or repress the transcription of several genes. We have studied whether the regulation of YY1 is affected by glutamate-induced excitotoxic insult in cultured primary hippocampal and cerebellar granule cells. Using electrophoretic mobility shift (EMSA) technique, we observed that glutamate treatment induced a dramatic increase in the YY1-binding activity of the smallest of three complexes both in the hippocampal and cerebellar granule cells. Two larger complexes disappeared after glutamate treatment in cerebellar granule cells. Supershift assays with specific YY1 antibody showed that all three complexes contained YY1 protein. Western blot assays did not show any changes in the nuclear levels of YY1 protein. Our results show that excitotoxic treatment affects the regulation of YY1 transcription factor and suggest that YY1 is a significant nuclear target for stress-related signaling pathways in neuronal degeneration.

Down-regulation of Ku autoantigen, DNA-dependent protein kinase, and poly(ADP-ribose) polymerase during cellular senescence

During aging and cellular senescence mutations accumulate in genomic and mitochondrial DNA. Ku autoantigens, DNA-dependent protein kinase, and poly (ADP-ribose) polymerase have an essential role in DNA damage recognition. Our purpose was to find out whether cellular senescence of fibroblasts affects the protein components that recognize DNA damage and induce the repair process. We compared presenescent and replicatively senescent human WI-38 fibroblasts with each other and with SV-40 immortalized and serum-deficient quiescent WI-38 cells. Our results showed that replicative senescence significantly decreased the nuclear level of both p70 and p86 components of Ku autoantigen. SV-40 immortalization and cellular quiescence did not affect the level of the p86 component but slightly increased that of p70. Both replicative senescence and cellular quiescence decreased the activity of DNA-dependent protein kinase in WI-38 fibroblasts. On the other hand, SV-40 immortalization increased the activity of DNA-dependent protein kinase. The protein level of poly(ADP-ribose) polymerase (PARP) was strongly decreased in replicatively senescent fibroblasts. Quiescence of early-passage fibroblasts also slightly reduced the protein level of PARP. Apoptosis was not observed in replicatively senescent fibroblasts. Our results show that replicative senescence and to some extent cellular quiescence down-regulate the recognition system of DNA damage involving Ku autoantigens, DNA-dependent protein kinase, and PARP and hence could enhance the accumulation of DNA damage during aging.

Analysis of a yeast SNARE complex reveals remarkable similarity to the neuronal SNARE complex and a novel function for the C terminus of the SNAP-25 homolog, Sec9

SNARE proteins represent a family of related proteins that are thought to have a central role in vesicle targeting and fusion in all eukaryotic cells. The binding properties of the neuronal proteins synaptobrevin 1 (VAMP1), syntaxin 1, SNAP-25, and soluble N-ethylmaleimide-sensitive factor attachment protein (alpha-SNAP), have been extensively studied. We report here the first biochemical characterization of a nonneuronal SNARE complex using recombinant forms of the yeast exocytic SNARE proteins Snc1, Sso1, and Sec9 and the yeast alpha-SNAP homolog, Sec17. Despite the low level of sequence identity, the association properties of the yeast and neuronal complexes are remarkably similar. The most striking difference we have found between the yeast and neuronal proteins is that individually neither of the target membrane SNAREs (t-SNAREs), Sso1 nor Sec9, show any detectable binding to the synaptobrevin homolog, Snc1. However, as a hetero-oligomeric complex, Sec9 and Sso1 show strong binding to Snc1. The clear dependence on the Sso1-Sec9 complex for t-SNARE function suggests that regulating the formation of this complex may be a key step in determining the site of vesicle fusion. In addition, we have used this in vitro assay to examine the biochemical effects of several mutations in Sec9 that result in pronounced growth defects in vivo. As expected, a temperature-sensitive mutation in the region most highly conserved between Sec9 and SNAP-25 is severely diminished in its ability to bind Sso1 and Snc1 in vitro. In contrast, a temperature-sensitive mutation near the C terminus of Sec9 shows no defect in SNARE binding in vitro. Similarly, a deletion of the C-terminal 17 residues, which is lethal in vivo, also binds Sso1 and Snc1 normally in vitro. Interestingly, we find that these same two C-terminal mutants, but not mutants that show SNARE assembly defects in vitro, act as potent dominant negative alleles when expressed behind a strong regulated promoter. Taken together these results suggest that the C-terminal domain of Sec9 is specifically required for a novel interaction that is required at a step following SNARE assembly.

Age-related changes in the regulation of transcription factor NF-kappa B in rat brain

Aging process involves an increase in stress at cellular level. We studied whether aging affects the regulation of stress responsive transcription factor NF-kappa B in brain samples of Wistar rats. Hippocampus, cerebellum, and temporal and frontal lobes of cortex were studied. We observed a significant up-regulation in the constitutive, nucleus-located NF-kappa B binding activity in 30-month-old Wistar rats compared to young and 18-month-old rats. The increase was most prominent in cerebellum and in frontal cortex, but age-related changes did not occur in hippocampus. Inducible, cytoplasmic NF-kappa B binding activity was not affected by aging in any of the samples studied. Western blot assays did not show any age-related changes in the nuclear level of p50, p52, and p65 protein components of NF-kappa B complex. Cytoplasmic level of inhibitory I kappa B-alpha was also unaffected. The increase in nuclear constitutive NF-kappa B binding activity during aging may be related to the NF-kappa B driven cellular response to adapt neurons against apoptotic pressure, as observed recently in several apoptotic conditions.

Down-regulation of transcription factors AP-1, Sp-1, and NF-kappa B precedes myocyte differentiation

Terminal differentiation of myocytes involves withdrawal from the cell cycle, induction of myogenin expression, and finally formation of myotubes. To study the factors that regulate the initial phase of muscle differentiation, we analyzed the binding activities of transcription factors AP-1, Sp-1, and NF-kappa B in L6, C2C12, and rhabdomyosarcoma BA-Han-1C cells. Temporal changes in transcription factor binding activities were compared to the activation of myogenin promoter-driven CAT reporter gene and the expression level of myogenin, a master gene of myogenic differentiation. We observed a prominent decrease in the nuclear binding activities of AP-1, Sp-1, and NF-kappa B already 12 to 24 h after the transfer of cells to differentiation medium. The response was very similar in L6 and C2C12 myocytes and in BA-Han-1C rhabdomyosarcoma cells. The down-regulation clearly preceded the activation of myogenin promoter and the induction of myogenin and retinoblastoma expression, as well as the initiation of myocyte fusion. Cholera toxin and okadaic acid, established inhibitors of myogenin expression and muscle differentiation, strongly up-regulated the binding activities of AP-1, Sp-1, and NF-kappa B in differentiation medium. Myogenin expression and myocyte fusion were also inhibited. Levels of nuclear c-Fos and c-Jun proteins, components of the AP-1 complex, showed a prominent decrease already after 12 h in differentiation medium. These results show that the down-regulation of the proliferation-promoting transcription factors is a prerequisite to the initiation of myocyte differentiation.

Down-regulation of nuclear binding activities of OXBOX-REBOX transcription factors during cellular senescence

Functional capacity of mitochondria declines during aging and this impairment may have a major role in aging process. Several observations indicate that transcriptional efficiency is reduced during aging. Our purpose was to find out whether aging and cellular senescence affect the nuclear binding activities of transcription factors which bind to OXBOX-REBOX sequence present in promoter regions of numerous nuclear genes encoding mitochondrial proteins. These factors regulate and coordinate the expression of mitochondrial proteins. We observed a strong down-regulation in the nuclear binding activities of OXBOX-REBOX factors in replicatively senesced human WI-38 and IMR-90 fibroblasts. On the contrary, SV-40 immortalization highly increased the nuclear binding activities. A considerable down-regulation of OXBOX-REBOX factors was also observed in UVB-irradiated WI-38 fibroblasts. Irradiation induced photoaging in fibroblasts which involved cell cycle arrest and senescent morphology. Interestingly, the nuclear binding activities of OXBOX-REBOX factors were also prominently decreased in the liver of Wistar rats at the age of 30 months but not yet at the age of 18 months. Our results suggest that the down-regulation of OXBOX-REBOX factors could affect the expression level of mitochondrial proteins encoded in nucleus and hence induce disturbances in mitochondrial function and promote the cellular aging process.

Changes associated with aging and replicative senescence in the regulation of transcription factor nuclear factor-kappa B

Both the aging of animals and the senescence of cultured cells involve an altered pattern of gene expression, suggesting changes in transcription factor regulation. We studied age-related changes in transcription factors nuclear factor (NF)-kappa B, activator protein factor-1 (AP-1) and Sp-1 by using electrophoretic mobility shift binding assays; we also analysed changes in the protein components of NF-kappa B complex with Western blot assays. Nuclear and cytoplasmic extracts were prepared from heart, liver, kidney and brain of young adult and old NMRI mice and Wistar rats as well as from presenescent, senescent and simian virus 40-immortalized human WI-38 fibroblasts. Aging of both mice and rats induced a strong and consistent increase in the nuclear binding activity of NF-kappa B factor in all tissues studied, whereas those of AP-1 and Sp-1 decreased, e.g. in liver. Protein levels of p50, p52 and p65 components of the NF-kappa B complex did not show any age-associated changes in the cytoplasmic fraction but in the nuclear fraction the level of p52 strongly increased in heart and liver during aging. The protein levels of inhibitory I kappa B-alpha and Bcl-3 components were not affected by aging in any of the tissues studied. Replicative cellular senescence of human WI-38 fibroblasts induced a strong decrease in nuclear NF-kappa B, AP-1 and Sp-1 binding activities. Protein levels of p50 and p52 components of NF-kappa B complex were decreased in the nuclear fraction of senescent WI-38 fibroblasts but in the cytoplasm of senescent fibroblasts the level of p65 protein was increased. Cellular senescence also slightly decreased the protein levels of I kappa B-alpha and Bcl-3. Transfection assays with NF-kappa B-enhancer-driven chloramphenicol acetyltransferase reporter gene showed a significant down-regulation of NF-kappa B promoter activity in senescent WI-38 fibroblasts. Results suggest that the aging process might be regulated differently in tissues and cultured fibroblasts, perhaps reflecting differences between mitotic and post-mitotic cells. In tissues, aging seems to involve specific changes in the regulation of NF-kappa B components and perhaps also changes in the DNA-binding affinities of the NF-kappa B complex.