Purification from placenta, amino acid sequence, structure comparisons and cDNA cloning of human glutaredoxin

Glutaredoxin is generally a glutathione-dependent hydrogen donor for ribonucleotide reductase and also catalyses general glutathione (GSH)-disulfide-oxidoreduction reactions in the presence of NADPH and glutathione reductase. A Glutaredoxin from human placenta was purified to homogeneity, as judged by SDS/PAGE and IEF (12 kDa). Purification was monitored by the activity with hydroxyethyl disulfide as a substrate. Values of pI for glutaredoxin were obtained by IEF; the pI of the protein shifted from 7.3 in its fully reduced state to 9.0 in the oxidized state after treatment with excess hydroxyethyl disulfide. The glutaredoxin preparation showed GSH-dependent hydrogen-donor activity with recombinant mouse ribonucleotide reductase, it exhibited dehydroascorbate reductase activity as well as hydroxyethyl-disulfide-reducing activity. The amino acid sequence (residues 3-104) of glutaredoxin was determined by peptide sequencing and residues 1, 2 and 105 by cDNA sequence analysis. The glutaredoxin sequence comprised the classical active site for glutaredoxins -Cys22-Pro-Tyr-Cys25- and three additional half-cystine residues; two of these in positions 78 and 82. The sequence was similar to other known mammalian glutaredoxins (about 80% identities), with important differences such as one additional Cys residue (Cys7) and no Met residue. The sequence of human glutaredoxin was compared to that of Escherichia coli glutaredoxin with known three-dimensional structure in solution to identify conserved residues and predict a structure from alignment. In particular the GSH-binding site of glutaredoxin was conserved between all molecules. A cDNA that encodes the entire glutaredoxin gene (grx) and flanking sequences was isolated from a human spleen cDNA library. The nucleotide sequence of this cDNA (0.8 kb) was determined, including the complete grx gene.

Generation of normal T and B lymphocytes by c-jun deficient embryonic stem cells

To determine the potential roles of c-jun in lymphocyte development, we generated somatic chimeric mice by injecting homozygous c-jun mutant embryonic stem (ES) cells into blastocysts from recombination activating gene-2 (RAG-2)-deficient mice. Chimeric mice had poor restoration of thymocytes, but contained substantial numbers of mature T and B lymphocytes in the periphery. Stimulation of c-jun-/- B cells resulted in normal levels of proliferation and immunoglobulin secretion. Likewise, stimulation of c-jun-/- T cells resulted in essentially normal levels of IL-2R alpha expression, IL-2 secretion, and proliferation. We further showed that the relatively normal activation responses of the c-jun-/- T cells probably results from the fact that other members of the Jun family contribute to the bulk of the activator protein-1 (AP-1) complexes in normal T cells and, as a result, AP-1 complexes are found at relatively normal levels in c-jun-/- T cells.

Mouse JunD negatively regulates fibroblast growth and antagonizes transformation by ras

As NIH 3T3 fibroblasts become quiescent, the level of c-Jun protein decreases while JunD accumulates. When resting cells are stimulated with fresh serum, nuclear-localized JunD is rapidly degraded, followed by resynthesis of both c-Jun and JunD later in G1. Overexpression of JunD results in slower growth and an increase in the percentage of cells in G0/G1 while c-Jun overexpression produces larger S/G2 and M phase populations. In addition, JunD partially suppresses transformation by an activated ras gene whereas c-Jun cooperates with ras to transform cells. These data indicate that two closely related transcription factors can function in an opposing manner.

Cooperating nonconsensus cAMP-responsive elements are mediators of adrenocorticotropin-induced VL30 transcription in steroidogenic adrenal cells

Pituitary-derived trophic hormones regulate cell-type-specific expression of VL30 retrotransposons in tissues that are engaged in steroidogenesis. We show that adrenocorticotropic hormone and forskolin induced VL30 transcription in the steroidogenic adrenal cell line Y1 and that the transcriptional activation was cell type- and protein kinase A-dependent. Three novel cAMP-responsive elements (CREs), within the VL30 long terminal repeat, were identified and shown to activate transcription synergistically when templates bearing multiple sites were compared with templates bearing a single site. This type of regulation was evident only in forskolin-treated cells, and the response elements were found to be inactive as mediators of constitutive transcription. In vitro binding analyses indicated that a consensus CRE and the nonconsensus VL30 CREs differ with respect to binding affinity and specificity to a number of nuclear factors that were identified to be related to proteins within the CREB, Jun, and C/EBP families of transcription factors. The relatively low affinity and/or a restricted binding specificity of the VL30 CREs made it possible to detect forskolin-induced binding of CREB- and Jun-related proteins to these sequences. We suggest that cAMP-induced transcription, specific for steroidogenic cells, can be mediated by a novel type of nonconsensus CREs and that the mechanism for this type of gene regulation is distinct from that mediated through a consensus CRE. We also report the identification of a novel factor, distinct from previously characterized CRE-binding proteins, that constitutively binds to the identified CREs.

Two AP1 sites binding JunB are essential for human papillomavirus type 18 transcription in keratinocytes

The activity and epithelial tropism of the human papillomavirus type 18 P105 early promoter, which directs the synthesis of the E6 and E7 transforming genes, are controlled by cis elements included in the viral long control region. To identify potential cellular regulators of this promoter, we mutagenized one or both of the 5'-TGACTAA-3' cis elements capable of interacting with the AP1 transcription factor, which is composed either of homodimers or heterodimers of the Jun products or of heterodimers of Jun and Fos. Mutation of both elements completely abolished P105 promoter activity in human keratinocytes. We show that either AP1 site can interact efficiently in vitro with any of the three different Jun products as heterodimers with c-Fos. However, in nuclear extracts prepared from human keratinocytes, JunB was the predominant Jun component bound to the DNA probe containing this cis element. These results implicate JunB as an important factor in human papillomavirus type 18 transcription in keratinocytes and strongly suggest a potential role of this Jun gene product in the tissue-specific transcription of the genital papillomaviruses.

Overexpression of c-jun, junB, or junD affects cell growth differently

The coding sequences of murine c-jun, junB, or junD, which code for proteins with practically identical dimerization and DNA binding properties, were introduced into a nondefective retroviral vector, and the phenotype of primary avian fibroblasts chronically infected with each of these viruses was studied. Cells expressing c-jun grew in low-serum medium and developed into colonies in agar, two properties characteristic of in vitro transformation. Cells expressing junB grew in agar, with a reduced efficiency as compared to c-jun, but did not grow in low-serum medium. Finally, no effect of junD expression on cell growth was observed. These different phenotypes suggest that these three closely related transcription factors play distinct roles during normal cell growth. Analysis of c-jun deletion mutants and of c-jun/junB and c-jun/junD chimeric genes showed that the N-terminal portion (amino acids 2-168) of the c-Jun protein that is involved in transcriptional activation is required for efficient transformation. On the contrary, cells expressing a truncated mouse c-Jun lacking this N-terminal domain grew slower than normal embryo fibroblasts. The reduced growth rate may be related to the finding that expression of the intact or the truncated mouse c-jun repressed the endogenous avian c-Jun homologue, suggesting that functional c-Jun product is required for normal cell growth.

Unregulated expression of c-Jun or c-Fos proteins but not Jun D inhibits oestrogen receptor activity in human breast cancer derived cells

We present evidence that oestrogen receptor activity in human MCF-7 breast cancer cells is reduced by over-expression of c-Jun or c-Fos proteins and to a lesser extent by Jun B overexpression. In contrast, overexpression of Jun D protein does not affect the activity of the oestrogen receptor. A region of c-Jun found to be required for repression of oestrogen receptor activity is located outside the DNA binding domain and is not conserved among the three Jun proteins. Finally, we suggest that c-Jun and c-Fos act independently to inactivate the oestrogen receptor.

Characterization of the flavin reductase gene (fre) of Escherichia coli and construction of a plasmid for overproduction of the enzyme

The enzyme NAD(P)H:flavin oxidoreductase (flavin reductase) catalyzes the reduction of soluble flavins by reduced pyridine nucleotides. In Escherichia coli it is part of a multienzyme system that reduces the Fe(III) center of ribonucleotide reductase to Fe(II) and thereby sets the stage for the generation by dioxygen of a free tyrosyl radical required for enzyme activity. Similar enzymes are known in other organisms and may more generally be involved in iron metabolism. We have now isolated the gene for the E. coli flavin reductase from a lambda gt11 library. After DNA sequencing we found an open reading frame coding for a polypeptide of 233 amino acids, with a molecular weight of 26,212 and with an N-terminal segment identical to that determined by direct Edman degradation. The coding sequence is preceded by a weak ribosome binding site centered 8 nucleotides from the start codon and by a promoterlike sequence centered at a distance of 83 nucleotides. In a Kohara library the gene hybridized to position 3680 on the physical map of E. coli. A bacterial strain that overproduced the enzyme approximately 100-fold was constructed. The translated amino acid sequence contained a potential pyridine nucleotide-binding site and showed 25% identity with the C-terminal part of one subunit (protein C) of methane monooxygenase from methanotropic bacteria that reduces the iron center of a second subunit (protein A) of the oxygenase by pyridine nucleotides.

Intracellular compartmentation of deoxycytidine nucleotide pools in S phase mouse 3T3 fibroblasts

We labeled mouse 3T3 fibroblasts, synchronized in G0 or S phase, from [3H]cytidine or [3H]deoxycytidine and measured the flow of isotope into and through deoxycytidine nucleotide pools, including the two deoxyliponucleotides dCDP choline and dCDP ethanolamine. Compared to G0 cells, S phase cells had much larger pools with a 20-40-fold faster turnover. The dCTP pool of S phase cells during steady state conditions attained a 6-fold higher specific activity than the pool of G0 cells when labeled from cytidine but a 10-fold lower specific activity when labeled from deoxycytidine. The dCTP pool of G0 cells showed a slow but measurable turnover indicating a limited amount of de novo synthesis also in resting cells. The labeling pattern of dCTP and deoxyliponucleotides of G0 cells was compatible with a simple precursor-product relationship. In S phase cells, however, dCDP choline had a 4-6 times higher specific activity during steady state conditions than dCTP and dCMP when the cells were labeled with [3H]deoxycytidine. We suggest that 3T3 cells contain two distinct intracellular dCTP pools, one labeled preferentially from cytidine and used for DNA replication, the other labeled from deoxycytidine and used for deoxyliponucleotide synthesis. We speculate that the latter pool during S phase may be temporarily sequestered in the cell's membrane fraction before equilibration with the much larger dCTP pool originating in S phase cells from the reduction of CDP.

Deoxycytidine is salvaged not only into DNA but also into phospholipid precursors

Deoxycytidine metabolism was investigated in light density human tonsillar lymphocytes using 5-3H-deoxycytidine as extracellular precursor. A significant portion of the deoxycytidine (more than 50% of ethanol soluble pool) was found to incorporate into dCDP-choline and dCDP-ethanolamine beside the well-known pathway i.e. incorporation into DNA in form of dCMP and dTMP. Hydroxyurea increased the labeling of the deoxyliponucleotides from 5-3H-deoxycytidine in spite of its inhibition of DNA synthesis.

Dynamics of the thymidine triphosphate pool during the cell cycle of synchronized 3T3 mouse fibroblasts

To investigate whether resting cells of 3T3 mouse fibroblasts carry out de novo synthesis of deoxyribonucleoside triphosphates, we determined the turnover of the thymidine triphosphate pool of G0 cells obtained by starvation of cultures for platelet-derived growth factor. These cells were contaminated by less than 1% S-phase cells. In the absence of deoxyribonucleosides in the medium one million G0 cells contained 5 pmole of dTTP with a turnover of 0.09 pmole/min. S-phase cells in comparison contained a 20 times larger dTTP pool with a more than 200-fold faster turnover. Our results suggest that G0 cells carry out a slow but finite de novo synthesis of deoxyribonucleoside triphosphates to satisfy the cells' requirement for DNA repair and mitochondrial DNA synthesis.

Compartmentation of dCTP pools. Evidence from deoxyliponucleotide synthesis

The nucleotide fraction of cultured 3T6 and 3T3 mouse fibroblasts contains deoxy-CDP choline and deoxy-CDP ethanolamine as well as the corresponding riboliponucleotides. In permeabilized cells both deoxyliponucleotides were formed from dCTP. In intact cells they could be labeled from [5-3H] deoxycytidine or cytidine via transformation of the nucleosides to dCTP. Their turnover was slow compared to that of dCTP. When rapidly growing 3T3 cells were labeled during 90 min from deoxycytidine the specific activity of dCDP choline was 2.4 times higher than that of dCTP while after labeling from cytidine both nucleotides (and CTP) reached the same specific activity under steady state conditions. Also dCDP ethanolamine was labeled more rapidly from deoxycytidine than from cytidine. Our results suggest that the deoxyliponucleotides were synthesized from a dCTP pool that was labeled preferentially from deoxycytidine. Earlier work (Nicander, B., and Reichard, P. (1983) Proc. Natl. Acad. Sci. U. S. A. 80, 1347-1351) had demonstrated synthesis of DNA from a dCTP pool labeled preferentially from cytidine. Taken together our results suggest that deoxyliponucleotides and DNA are synthesized from separate dCTP pools.

Ribonucleotides are not channeled into DNA in permeabilized mammalian cells

Reddy and Pardee earlier reported that permeabilized CHEF/18 cells (J. Biol. Chem 257, 12526) or soluble extracts from such cells (Cell 32, 443) under the proper conditions preferentially incorporated a labeled ribonucleotide (CDP) into DNA without dilution by an excess of non-labeled dCTP, suggesting channeling of ribonucleotides into DNA by a multiprotein complex. Here we present results suggesting that in these experiments CDP was incorporated into RNA and not into DNA.