Change in telomerase activity of rat organs during growth and aging.

Telomerase activity is usually undetectable in adult human tissues, but is positive in embryonic tissues and in cancers. In rodents, however, several organs of normal adult animals express substantial amounts of telomerase activity. In this study, we observed the changes in the telomerase activity in rat organs during growth and aging and found that telomerase activity showed chronological patterns which were characteristic to organs. In lung and brain, the high telomerase activity of embryonic stage decreased rapidly after birth. In lymphoid tissues, telomerase activity increased after birth and reached to its maximum at 4 to 7 week. In liver, it stayed at nearly constant level throughout life. The telomerase activity in regenerating rat liver decreased temporally immediately after partial hepatectomy, then increased to a level that is higher than normal control. In contrast, it rapidly diminished in the occluded lobes after ligation of portal vein branch.

Replication protein-A mediates the association of calf thymus DNA polymerase alpha-DNA primase complex with guanine-rich DNA sequence.

We have shown that calf thymus DNA polymerase alpha-DNA primase complex (pol alpha-primase) preferentially binds to pyrimidine-rich sequences and initiates RNA primer synthesis [Suzuki, M. et al. (1993) Biochemistry 32, 12782-12792]. Here we tested the association of pol alpha-primase with a guanine-rich DNA fragment (SVG, 30-mer) containing in vivo initiation sites of simian virus 40 DNA replication. While pyrimidine-rich fragment (CTPPS 1, 30-mer), that is a preferred sequence for calf thymus DNA primase, was well co-precipitated with pol alpha-primase using anti-pol alpha antibody, SVG was hardly precipitated under the same conditions. Competition studies in either gel-retardation assay or during de novo DNA synthesis by pol alpha-primase demonstrated that the interaction of pol alpha-primase with SVG was much weaker than that with CTPPS-1. On the other hand, replication protein-A (RP-A) could bind SVG, although less efficiently than CTPPS 1. After preincubation with RP-A, SVG could bind pol alpha-primase that was immobilized on Sepharose beads. The simian virus 40 large T antigen also enhanced association of SVG to pol alpha-primase, while Escherichia coli single-stranded DNA-binding protein did not. However, pol alpha-primase, bound to SVG in the presence of RP-A, failed to synthesize RNA primers. When SVG was extended 10 nucleotides at its 5'-end, pol alpha-primase synthesized trace amounts of RNA primers, and this activity was stimulated more than 10-fold by adding RP-A. These results suggest a new role for RP-A, i.e., as a molecular tether that allows pol alpha-primase to bind guanine-rich regions of DNA in order to initiate RNA primer synthesis.

Down-regulation of telomerase activity is an early event in the differentiation of HL60 cells.

Telomerase has been shown to be essential for unlimited cell proliferation and has been linked to immortality. However, still very little is known about the mechanism by which this enzyme is activated or inactivated. To investigate its regulation, we closely monitored telomerase activity during HL60 cell differentiation induced by either 1 alpha,25-dihydroxyvitamin D3 or all-trans retinoic acid. To that effect, we used a new combination of TRAP assay and SPA, which provides reproducible data for the quantitation and detection of variations in enzyme activity. We thereby observed that the decrease in telomerase activity after induction of differentiation by either of these agents is an early event of the differentiation process rather than its consequence, and that it is independent of the growth arrest pathway. It is neither due to a reduced expression of its RNA component nor to the appearance of a telomerase inhibitor in differentiating cells but is parallel to an increase in p21 and Rb mRNA expression.

The retinoblastoma binding factor 1 (RBF-1) site in RB gene promoter binds preferentially E4TF1, a member of the Ets transcription factors family.

The tumor suppressor retinoblastoma gene product, pRB, is a well known regulator of G1/S cell cycle progression. Moreover, mutational inactivations within the retinoblastoma gene (RB) are found in many human malignant tumors, and thus, believed to be an essential step in tumor formation. The human RB gene is considered as a housekeeping gene with no characteristic TATA or CAAT elements in its promoter region, but the sequence between 206 and 185 bases upstream of the initiation codon, essential for RB promoter activity, contains putative Sp1 and ATF recognition sites. We have previously reported that point mutations in this region, causing low penetrance retinoblastomas, completely reduced RB promoter activity, and that a nuclear factor, named RBF-1 (retinoblastoma binding factor 1), could specifically bind to this sequence, overlapping Sp1 recognition sequence. We show here, that RBF-1 can recognize a specific DNA sequence, 5'-GGCGGAAGT-3', overlapping the Sp1 and ATF sites and corresponding to the consensus DNA binding site for members of Ets transcription factors family. When RBF-1 site was used for sequence specific DNA affinity purification from erythroleukemia cells, reconstitution assays, immunoblotting analysis and peptide mapping show that the two major co-purified proteins are identical to human E4TF1-60 and -53 proteins. This reveals that E4TF1 can bind to the RBF-1 site of RB gene promoter, which, thus, constitutes a new target for this member of Ets transcription factors family.

Sulfate- and sialic acid-containing glycolipids inhibit DNA polymerase alpha activity.

The effects of various glycolipids on the activity of immunoaffinity-purified calf thymus DNA polymerase alpha were studied in vitro. Preincubation with sialic acid-containing glycolipids, such as sialosylparagloboside (SPG), GM3, GM1, and GD1a, and sulfatide (cerebroside sulfate ester, CSE) dose-dependently inhibited the activity of DNA polymerase alpha, while other glycolipids, as well as free sphingosine and ceramide did not. About 50% inhibition was achieved by preincubating the enzyme with 2.5 microM of CSE, 50 microM of SPG or GM3, and 80 microM of GM1. Inhibition was noncompetitive with both the DNA template and the substrate dTTP, as well as with the other dNTPs. Since the inhibition was largely reversed by the addition of 0.05% Nonidet P40, these glycolipids may interact with the hydrophobic region of the enzyme protein. Apparently, the sulfate moiety in CSE and the sialic acid moiety in gangliosides were essential for the inhibition since neither neutral glycolipids (i.e., glucosylceramide, galactosylceramide, lactosylceramide) nor asialo-gangliosides (GA1 and GA2) showed any inhibitory effect. Furthermore, the ceramide backbone was also found to be necessary for maximal inhibition since the inhibition was largely abolished by substituting the lipid backbone with cholesterol. Increasing the number of sialic acid moieties per molecule further enhanced the inhibition, while elongating the sugar chain diminished it. It was clearly shown that the N-acetyl residue of the sialic acid moiety is particularly essential for inhibition by both SPG and GM3 because the loss of this residue or substitution with a glycolyl residue completely negated their inhibitory effect on DNA polymerase alpha activity.

RNA priming coupled with DNA synthesis on natural template by calf thymus DNA polymerase alpha-primase.

A bovine genomic DNA library was surveyed with respect to the template activity for RNA-primed DNA synthesis by calf thymus DNA polymerase alpha-primase complex. About 7% of the single-stranded DNA clones contained distinct initiation sites consisting of pyrimidine clusters of pyrimidine-rich sequences. The initiation sites were located at or near the 3'-end of the pyrimidine clusters. One of these sequences, containing a 10-mer pyrimidine cluster with major initiation sites, was analyzed in detail. By the successive substitutions of pyrimidines in the cluster with oligodeoxydenylate [(dA)n] in the 5' to 3' direction, the minimum length of the initiation sequence was estimated to be as long as the 7-mer. In contrast, when one or three pyrimidines at the 3'-end of the cluster were replaced with (dA)n, the priming activity was largely lost, indicating that these pyrimidine residues were indispensable for priming. Furthermore, base substitutions of upstream or downstream sequences outside the pyrimidine cluster also decreased the total priming frequencies. Interestingly, the base substitutions inside or outside of the pyrimidine cluster sometimes caused a shift in the major priming sites. These results indicate that the minimum priming unit of the CTPPS1 template for RNA-primed DNA synthesis consists of a pyrimidine cluster (6-mer) with one purine at its 3'-border and that both the 3'-downstream 6-bases and the 5'-upstream 17-bases modulate the priming by enhancing the priming frequency and/or slightly shifting the sites of initiation of primer synthesis. It was also revealed that the lengths of the product RNA primers became shorter as the length of pyrimidine cluster was shortened by substitution with (dA)n. The gel retardation assay further showed that the complex formation between DNA polymerase alpha-primase and the DNA templates was strongly in competition with poly(dC), poly(dG), and poly(dT) but not with poly(dA). Furthermore, template activities as well as the pyrimidine contents of a series of base-substituted DNA correlated well with their affinities to the enzyme, as measured by both gel retardation assay and their Km values for the priming reaction. Apparently, DNA polymerase alpha-primase primarily recognizes the minimum priming unit consisting of a pyrimidine cluster with a purine at the 3'-boundary of purine, but the initiation of primer RNA synthesis can be modified by pyrimidine residues outside of the minimum priming unit.

Deoxypyrimidine cluster mediates the priming by calf thymus DNA primase subunit.

Homogeneously purified DNA polymerase alpha subunit-free primase was used to analyze primer RNA synthesis. On a chemically synthesized 36 mer DNA template, a part of upstream region of human c-myc gene, the primer synthesis started from a doublet of deoxythymidine (TT) in the deoxypyrimidine-rich sequence. The primase in DNA polymerase alpha-primase complex synthesized 21-mer reaction product, while DNA polymerase alpha-free primase gave the similar products, 21- and 22-mer, indicating that the site recognition was carried out by primase itself and DNA polymerase alpha subunit has an auxiliary role on it. Product analysis using DNA fragments carrying base substitutions further revealed that the existence of deoxypyrimidine residues around the starting sites was important for priming frequencies. Competition analysis showed that the priming was strongly competed by poly(dC), and to a much lesser extent by poly(dA). Gel-shift analysis showed that the primase could bind to the DNA template, and this complex formation was also competed by poly(dC), but not by poly(dA). These results indicate that primase subunit interacts with the starting site by binding directly with deoxypyrimidine residues.

Immunopurified Rb protein inhibits SV40 T antigen-dependent stimulation of DNA polymerase alpha.

It has been shown that purified SV40 large T antigen (Tag) forms a complex with both human and calf thymus DNA polymerase alpha and stimulates its activity. Furthermore, Tag has also been found to complex with purified human Rb protein. Here, we show the effect of Rb protein on the stimulation of DNA polymerase alpha by Tag, in an in vitro system using either purified human or calf thymus DNA polymerase alpha and either primed single-stranded M13 DNA or calf thymus-activated DNA. Both human and calf thymus enzymes were dose-dependently stimulated several fold by Tag. The stimulation was also observed in the coupled reaction of primase and polymerase alpha, using unprimed single-stranded M13 DNA. These stimulatory effects were, however, completely abolished by preincubating Tag with an equimolar amount of Rb protein. Primase activity of DNA polymerase alpha-primase complex was also stimulated by Tag, and this stimulation was abolished by the presence of Rb protein. In contrast, free primase was not affected by either Tag or Rb protein. Kinetic analysis revealed that in the presence of Tag the apparent Km for the template of either human or calf DNA polymerase alpha was decreased by approximately 2.5-fold and the Vmax was increased twofold, whereas Tag complexed with Rb protein did not affect the Km or the Vmax. These results suggest a competition between Rb protein and DNA polymerase alpha for binding to Tag, which may be a key step for the initiation of SV40 DNA replication.

Poly(ADP-ribose) polymerase stimulates DNA polymerase alpha by physical association.

The direct effect of the eukaryotic nuclear DNA-binding protein poly(ADP-ribose) polymerase on the activity of DNA polymerase alpha was investigated. Homogenously purified poly(ADP-ribose) polymerase (5 to 10 micrograms/ml) stimulated the activity of immunoaffinity-purified calf or human DNA polymerase alpha by about 6 to 60-fold in a dose-dependent manner. It had no effect on the activities of DNA polymerase beta, DNA polymerase gamma, and primase, indicating that its effect is specific for DNA polymerase alpha. Apparently, poly(ADP-ribosyl)ation of DNA polymerase alpha was not necessary for the stimulation. The stimulatory activity is due to poly(ADP-ribose) polymerase itself since it was immunoprecipitated with a monoclonal antibody directed against poly(ADP-ribose) polymerase. Kinetic analysis showed that, in the presence of poly(ADP-ribose) polymerase, the saturation curve for DNA template primer became sigmoidal; at very low concentrations of DNA, it rather inhibited the reaction in competition with template DNA, while, at higher DNA doses, it greatly stimulated the reaction by increasing the Vmax of the reaction. By the automodification of poly(ADP-ribose) polymerase, however, both the inhibition at low DNA concentration and the stimulation at high DNA doses were largely lost. Furthermore, stimulation by poly(ADP-ribose) polymerase could not be attributed to its DNA-binding function alone since its fragment, containing only the DNA-binding domain, could not exert full stimulatory effect on DNA polymerase, as of the intact enzyme. Poly(ADP-ribose) polymerase is co-immunoprecipitated with DNA polymerase alpha, using anti-DNA polymerase alpha antibody, clearly showing that poly(ADP-ribose) polymerase may be physically associated with DNA polymerase alpha. In a crude extract of calf thymus, a part of poly(ADP-ribose) polymerase activity existed in a 400-kDa, as well as, a larger 700-kDa complex containing DNA polymerase alpha, suggesting the existence in vivo of a complex of these two enzymes.

Efficient purification of a full length and biochemically active p110Rb, the retinoblastoma gene product.

Rb protein was purified from recombinant baculovirus-infected Sf9 cells to apparent homogeneity by a simple solubilization and by immunoaffinity column chromatography. It was mainly obtained as p110Rb, the underphosphorylated form of the Rb protein family. This p110Rb was shown to form a specific complex in vitro with SV40 Tag and to bind to double or single stranded DNA. It could also affect Tag helicase activity in a biphasic-dose dependent manner, due to these two biochemical functions. This purification procedure provides sufficient amounts of native Rb protein to further elucidate its role as an anti-oncogene protein and a negative regulator of the cell cycle.

Sequence and analysis of BECV F15 matrix protein.

Clones from the bovine enteric coronavirus (F15) cDNA library were cloned in pBR322 and sequenced by the method of Sanger and Coulson. This led to the identification of a sequence of 1,300 bases which contained a single open reading frame of 690 bases yielding a protein having properties of the matrix protein (M). It was comprised of 230 amino acids with a molecular weight of 26,376 Da. It was hydrophobic and had a net charge of +8 at neutral pH. Analysis of its secondary structure could not establish a simple transmembrane arrangement of the amino acids. Comparison of its nucleotide sequence with that of BECV Mebus strain showed only a two-base change resulting in a 100% homology between the two amino acid sequences. Furthermore, a very conserved structure of M appeared on comparison with the Dayoff optimal alignment of MHV-A59, MHV-JHM, TGEV, IBV Beaudette and IBV 6/82M amino acid sequences. As the two strains of BECV, F15 and Mebus present some antigenic differences, this led us to reconsider the role of M in viral antigen specificity. A hypothesis is that, as it seems to possess the necessary information on its transmembrane region, it is an ideal candidate for the viral budding process.