Mammalian beta-polymerase promoter: large-scale purification and properties of ATF/CREB palindrome binding protein from bovine testes

The role of aging and brain-derived neurotrophic factor signaling in expression of base excision repair genes in the human brain

DNA damage is a central contributor to the aging process. In the brain, a major threat to the DNA is the considerable amount of reactive oxygen species produced, which can inflict oxidative DNA damage. This type of damage is removed by the base excision repair (BER) pathway, an essential DNA repair mechanism, which contributes to genome stability in the brain. Despite the crucial role of the BER pathway, insights into how this pathway is affected by aging in the human brain and the underlying regulatory mechanisms are very limited. By microarray analysis of four cortical brain regions from humans aged 20-99 years (n = 57), we show that the expression of core BER genes is largely downregulated during aging across brain regions. Moreover, we find that expression of many BER genes correlates positively with the expression of the neurotrophin brain-derived neurotrophic factor (BDNF) in the human brain. In line with this, we identify binding sites for the BDNF-activated transcription factor, cyclic-AMP response element-binding protein (CREB), in the promoter of most BER genes and confirm the ability of BDNF to regulate several BER genes by BDNF treatment of mouse primary hippocampal neurons. Together, these findings uncover the transcriptional landscape of BER genes during aging of the brain and suggest BDNF as an important regulator of BER in the human brain.

Involvement of p38MAPK-ATF2 signaling pathway in alternariol induced DNA polymerase β expression

Base excision repair (BER) systems are important for maintaining the integrity of genomes in mammalian cells. Aberrant DNA bases or broken single strands can be repaired by BER. Consequently, DNA lesions, which may be caused by cancer and aging, have a close association with BER procedure. DNA polymerase β (polβ) is a critical BER enzyme that can excise 5'-sugar phosphate prior to adding a nucleotide in the gap by its function as a DNA polymerase in the BER process. However, DNA polβ is an error-prone DNA polymerase, and overexpressing polβ increases the cellular spontaneous mutation rate. DNA polβ overexpression has been identified in various human tumors, which implies that DNA polβ overexpression has a close association with tumorigenesis. The present study showed that alternariol (AOH), a secondary product of a fungus that is found in grains and fruits, could cause DNA damage to NIH3T3 cells in a single cell gel electrophoresis, and that 2, 10 and 20 µM AOH induced DNA polβ overexpression in a dose-dependent manner. In the process, the level of phosphorylation of mitogen-activated protein kinase 14 (p38) mitogen-activated protein kinase (MAPK) and activating transcription factor 2 (ATF2) was increased. In addition, SB203580, a p38MAPK inhibitor, resulted in decreased DNA polβ expression. Small hairpin RNA-p38MAPK had the same effect; notably, DNA polβ expression was downregulated in p38MAPK knockdown cells. These data suggest that the p38MAPK-ATF2 signaling pathway may be involved in DNA polβ expression induced by AOH.

Quantification of activated NF-kappaB/RelA complexes using ssDNA aptamer affinity-stable isotope dilution-selected reaction monitoring-mass spectrometry

Nuclear Factor-κB (NF-κB) is a family of inducible transcription factors regulated by stimulus-induced protein interactions. In the cytoplasm, the NF-κB member RelA transactivator is inactivated by binding inhibitory IκBs, whereas in its activated state, the serine-phosphorylated protein binds the p300 histone acetyltransferase. Here we describe the isolation of a ssDNA aptamer (termed P028F4) that binds to the activated (IκBα-dissociated) form of RelA with a K(D) of 6.4 × 10(-10), and its application in an enrichment-mass spectrometric quantification assay. ssDNA P028F4 competes with cognate duplex high affinity NF-κB binding sites for RelA binding in vitro, binds activated RelA in eukaryotic nuclei and reduces TNFα-stimulated endogenous NF-κB dependent gene expression. Incorporation of P028F4 as an affinity isolation step enriches for serine 536 phosphorylated and p300 coactivator complexed RelA, simultaneously depleting IκBα·RelA complexes. A stable isotope dilution (SID)-selected reaction monitoring (SRM)- mass spectrometry (MS) assay for RelA was developed that produced a linear response over 1,000 fold dilution range of input protein and had a 200 amol lower limit of quantification. This multiplex SID-SRM-MS RelA assay was used to quantify activated endogenous RelA in cytokine-stimulated eukaryotic cells isolated by single-step P028F4 enrichment. The aptamer-SID-SRM-MS assay quantified the fraction of activated RelA in subcellular extracts, detecting the presence of a cytoplasmic RelA reservoir unresponsive to TNFα stimulation. We conclude that aptamer-SID-SRM-MS is a versatile tool for quantification of activated NF-κB/RelA and its associated complexes in response to pathway activation.

Comparative assessment of plasmid and oligonucleotide DNA substrates in measurement of in vitro base excision repair activity

Mammalian base excision repair (BER) is mediated through at least two subpathways designated 'single-nucleotide' (SN) and 'long-patch' (LP) BER (2-nucleotides long/more repair patch). Two forms of DNA substrate are generally used for in vitro BER assays: oligonucleotide- and plasmid-based. For plasmid-based BER assays, the availability of large quantities of substrate DNA with a specific lesion remains the limiting factor. Using sequence-specific endonucleases that cleave only one strand of DNA on a double-stranded DNA substrate, we prepared large quantities of plasmid DNA with a specific lesion. We compared the kinetic features of BER using plasmid and oligonucleotide substrates containing the same lesion and strategic restriction sites around the lesion. The K(m) for plasmid DNA substrate was slightly higher than that for the oligonucleotide substrate, while the V(max) of BER product formation for the plasmid and oligonucleotide substrates was similar. The catalytic efficiency of BER with the oligonucleotide substrate was slightly higher than that with the plasmid substrate. We conclude that there were no significant differences in the catalytic efficiency of in vitro BER measured with plasmid and oligonucleotide substrates. Analysis of the ratio of SN BER to LP BER was addressed using cellular extracts and a novel plasmid substrate.

Age-related base excision repair activity in mouse brain and liver nuclear extracts

To assess DNA repair activity relative to age, in vitro base excision repair assays were performed using brain and liver nuclear extracts prepared from mice of various ages. An 85% decline in repair activity was observed in brain nuclear extracts and a 50% decrease in liver nuclear extracts prepared from old mice compared with 6-day-old mice. Brain nuclear extracts prepared from old mice showed a decreased abundance of DNA polymerase-beta, but the addition of purified protein did not restore base excision repair activity. Abundances of other tested base excision repair proteins did not change relative to age. The conclusion is that, during aging, a decline in DNA repair could contribute to increased levels of DNA damage and mutagenesis.

DNA polymerase beta gene expression: the promoter activator CREB-1 is upregulated in Chinese hamster ovary cells by DNA alkylating agent-induced stress

The DNA alkylating agent N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) upregulates the level of the base excision DNA repair enzyme DNA polymerase beta (beta-pol) in several mammalian cell types. Previous studies suggested that beta-pol expression is upregulated via a transcriptional mechanism that requires: the specific cAMP response element (CRE) in the beta-pol core promoter; a phosphorylated form of CRE-binding protein-1 (CREB-1); and cellular protein kinase A activity. A large family of CRE-binding proteins, ie., the ATF/CREB factors, has been identified in various cell types. This study further examines the role of CRE-binding proteins in regulating beta-pol expression through study of Chinese hamster ovary (CHO) cells. In CHO cell nuclear extract, CREB-1 and ATF-1 are the predominant CRE-binding protein family members recognizing the CRE in the beta-pol core promoter. The concentration of CREB-1 increases strongly in CHO cells after exposure to MNNG. In contrast, the level of ATF-1 does not change after MNNG treatment. Recombinant expression of CREB-1 in CHO cells is sufficient to increase expression of the endogenous beta-pol gene, even in the absence of MNNG exposure. These results indicate that beta-pol gene expression in CHO cells can be upregulated by CREB-1 and that the activation of beta-pol gene expression in response to DNA alkylating agent exposure involves a strong increase in the level of CREB-1.

Hormonal regulation of DNA polymerase beta activity and expression in rat adrenal glands and testes

We investigated whether DNA polymerase beta activity and expression in rat adrenal glands and testes are controlled by the cAMP dependent protein kinase (A-kinase) phosphorylation system in addition to anterior pituitary hormones. DNA polymerase beta mRNA expression in rat testes was decreased by hypophysectomy and recovered with administration of gonadotropic hormone, suggesting that this enzyme is controlled at the mRNA level by this pituitary hormone. In addition, DNA polymerase beta activity in the adrenal glands and testes and the amount of mRNA in the testes increased when cAMP was administered to the normal rat. This activity was decreased by administration of the cyclic AMP-dependent protein kinase inhibitor, H(8). Moreover, when alkaline phosphatase was added to the assay system in vitro, a decrease in DNA polymerase beta activity was observed. These findings indicate that changes in the activity and expression of DNA polymerase beta are mediated via cAMP and the A-kinase system, and that phosphorylation of this enzyme is also involved in this expression.

Base excision repair is limited by different proteins in male germ cell nuclear extracts prepared from young and old mice

The combined observations of elevated DNA repair gene expression, high uracil-DNA glycosylase-initiated base excision repair, and a low spontaneous mutant frequency for a lacI transgene in spermatogenic cells from young mice suggest that base excision repair activity is high in spermatogenic cell types. Notably, the spontaneous mutant frequency of the lacI transgene is greater in spermatogenic cells obtained from old mice, suggesting that germ line DNA repair activity may decline with age. A paternal age effect in spermatogenic cells is recognized for the human population as well. To determine if male germ cell base excision repair activity changes with age, uracil-DNA glycosylase-initiated base excision repair activity was measured in mixed germ cell (i.e., all spermatogenic cell types in adult testis) nuclear extracts prepared from young, middle-aged, and old mice. Base excision repair activity was also assessed in nuclear extracts from premeiotic, meiotic, and postmeiotic spermatogenic cell types obtained from young mice. Mixed germ cell nuclear extracts exhibited an age-related decrease in base excision repair activity that was restored by addition of apurinic/apyrimidinic (AP) endonuclease. Uracil-DNA glycosylase and DNA ligase were determined to be limiting in mixed germ cell nuclear extracts prepared from young animals. Base excision repair activity was only modestly elevated in pachytene spermatocytes and round spermatids relative to other spermatogenic cells. Thus, germ line short-patch base excision repair activity appears to be relatively constant throughout spermatogenesis in young animals, limited by uracil-DNA glycosylase and DNA ligase in young animals, and limited by AP endonuclease in old animals.

Low concentration N-methyl-N'-nitro-N-nitrosoguanidine activates DNA polymerase-beta expression via cyclic-AMP-protein kinase A-cAMP response element binding protein pathway

Ultraviolet light (UV), ionizing-radiation or alkylating agents are known as carcinogens, mostly because of their ability to damage DNA directly. However, they may also play a diverse role in activating the signal pathways and altering the gene expression. We have shown previously that N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) of 0.2 microM can increase the transcription of DNA polymerase-beta gene, which has a cyclic AMP response element (CRE) motif in its promoter. Using the CRE report vector, we show here, such treatment can stimulate the CRE-driven gene expression by approximately 1.5-fold compared with control. Consistent with it, the proportion of ser-133 phosphorylated CRE binding protein (CREB), the related transcription factor was 2.08-fold higher versus control in vero cells after 60 min of MNNG treatment. Although CREB has many putative kinases for its phosphorylation, such as p38 mitogen-activated protein kinase (p38 MAPK), Ca(2+)/calmodulin-dependent protein kinase Pi (CaMK Pi) and protein kinase C (PKC), we found the protein kinase A (PKA) was activated and its activation peaked when cells were treated for 60 min (with arbitrary activity unit of 11.03+/-2.80 and 0.86+/-0.43 in treatment and control, respectively), this phasic character was similar to that of the CREB phosphorylation. We also determined the intracellular cyclic AMP (cAMP) levels and it was found that the cAMP concentration was elevated after 60 min treatment (1.53-fold higher). However, to our surprise, we did not find any accompanying cAMP elevation in cells treated by MNNG for 30 min, in which PKA was activated significantly. These findings, together with other observations, suggest that cAMP-PKA-CREB signaling pathway mediates the low concentration MNNG induced pol-beta expression. In addition to elevated cAMP, there might exist a cAMP-independent PKA activation manner in this course.

Mixed spermatogenic germ cell nuclear extracts exhibit high base excision repair activity

Spermatogenic cells exhibit a lower spontaneous mutation frequency than somatic tissues in a lacI transgene and many base excision repair (BER) genes display the highest observed level of expression in the testis. In this study, uracil-DNA glycosylase-initiated BER activity was measured in nuclear extracts prepared from tissues obtained from each of three mouse strains. Extracts from mixed spermatogenic germ cells displayed the greatest activity followed by liver then brain for all three strains, and the activity for a given tissue was consistent among the three strains. Levels of various BER proteins were examined by western blot analyses and found to be consistent with activity levels. Nuclear extracts prepared from purified Sertoli cells, a somatic component of the seminiferous epithelium, exhibited significantly lower activity than mixed spermatogenic cell-type nuclear extracts, thereby suggesting that the high BER activity observed in mixed germ cell nuclear extracts was not a characteristic of all testicular cell types. Nuclear extracts from thymocytes and small intestines were assayed to assess activity in a mitotically active cell type and tissue. Overall, the order of tissues/cells exhibiting the greatest to lowest activity was mixed germ cells > Sertoli cells > thymocytes > small intestine > liver > brain.

Ca2+-induced p38/SAPK signalling inhibited by the immunosuppressant cyclosporin A in human peripheral blood mononuclear cells

To understand the effects of the immunosuppressant cyclosporin A (CsA) on Ca2+-mediated intracellular signalling pathways in human peripheral blood mononuclear cells (PBMCs), we investigated its effects on the activity profiles of mitogen-activated protein kinase (MAPK) cascades. PBMCs, or subpopulations thereof, were simultaneously stimulated with a phorbol ester and the calcium ionophore ionomycin, in the presence or absence of therapeutic concentrations of CsA. In these primary human cells, CsA significantly inhibited PMA/ionomycin-mediated and ionomycin-mediated activation of the MAPK kinase MKK6, as well as its downstream kinases SAPK2a (p38alpha) and MAPKAP-K2. PMA/ionomycin treatment also mediated activation of SAPK1 (JNKs) which was inhibited by CsA. Treatment with ionomycin alone also resulted in CsA-sensitive activation of SAPK1. With regard to transcription factors targeted by the Ca2+-induced MAPK signalling network, we found CsA to inhibit the ionomycin-mediated phosphorylation of ATF2 at Thr71. We identified the heterodimeric transcription factor ATF2/CREB as constitutively binding to the essential cAMP response element (CRE) site within the Ca2+-regulated DNA polymerase beta promoter and contributing to the activation of this promoter. Our data implicate ATF2 phosphorylation status as a nuclear sensor within PBMCs that monitors converging intracellular Ca2+-signalling pathways.

Functional mutation of DNA polymerase beta found in human gastric cancer--inability of the base excision repair in vitro

DNA polymerase beta (polbeta) is one of mammalian DNA polymerases and is known to be involved in a G:T/G:U mismatch repair. In order to investigate an involvement of this enzyme in a base excision repair, we searched a mutation of human polbeta in human gastric cancer and studied a function of the mutation. We observed cancer-specific missense mutations in 6 of 20 samples. All of these mutations were, however, heterozygous. We further analyzed the base excision repair activity of these mutants to know whether these mutants cause an error of mismatch repair. One of these mutants, which resulted in an amino acid substitution of Glu for Lys at codon 295, showed an inhibitory effect by in vitro base excision repair assay, suggesting that this mutation might play some role in carcinogenesis of the gastric mucosa.

Promoter activation via a cyclic AMP response element in vitro

Transcription activation via activating transcription factor cyclic AMP response element binding (ATF/CREB) sites in vitro was explored using transcription and permanganate assay for open complex formation. These sites were used to drive transcription from an adenovirus major late core sequence. Under conditions where activation is strong, 20-50-fold, ATF/CREB is required for preinitiation complexes to reach the open complex stage. Complete opening requires activator, ATP, and initiating nucleotides. In exploration of postinitiation steps, no stimulation of promoter clearance was observed but a modest stimulation of the rate of continuous transcription occurred. High amounts of DNA template, commonly used in in vitro studies, allows some templates to open without activator, but leaves the nucleotide requirements intact. This leads to a drastic lowering of the dependence on ATF/CREB. Taken together, the data indicate that ATF/CREB activates this system primarily by stimulating the formation of functional preinitiation complexes.

Regulation of Pdha-2 expression is mediated by proximal promoter sequences and CpG methylation

Spermatogenesis is a complex process requiring the coordinate expression of a number of testis-specific genes. One of these, Pdha-2, codes for the murine spermatogenesis-specific isoform of the E1a subunit of the pyruvate dehydrogenase complex. To begin to delineate the mechanisms regulating its expression in vivo, we have generated transgenic mice lines carrying Pdha-2 promoter deletion constructs. Here we report that transgenic mice harboring a construct containing only 187 bp of promoter and upstream sequences (core promoter) is sufficient for directing the testis-specific expression of a chloramphenicol acetyltransferase (CAT) reporter gene. Like the endogenous Pdha-2, the CAT gene is expressed in testis in a stage-specific manner. Our studies also show a correlation between CpG methylation within the core promoter and its capacity to regulate transcription. In NIH 3T3 cell lines stably transfected with the Pdha-2 core promoter-CAT construct, high levels of CAT reporter expression are observed, whereas the endogenous Pdha-2 gene is repressed. In these cells, the CpG dinucleotides residing within the transfected promoter are hypomethylated whereas those residing in the endogenous promoter are methylated. Furthermore, promoter activity can be abated by the in vitro methylation of its CpG dinucleotides. DNase I footprint analysis indicates that at least one site for the methylation-mediated repression may occur through the ATF/cyclic AMP response element binding element located within the core promoter. Mutations within this element reduces activity to approximately 50% of the wild-type promoter activity. These results suggest that tissue-specific gene expression may be modulated by other mechanisms in addition to specific transcription factor availability and cooperativity. We propose that methylation may be a mechanism by which repression of the testis-specific Pdha-2 gene is established in somatic tissue.

Specific interaction of DNA polymerase beta and DNA ligase I in a multiprotein base excision repair complex from bovine testis

Base excision repair (BER) is a cellular defense mechanism repairing modified bases in DNA. Recently, a G:U repair reaction has been reconstituted with several purified enzymes from Escherichia coli (Dianov, G., and Lindahl, T.(1994) Curr. Biol. 4, 1069-1076). Using bovine testis crude nuclear extract, we have shown that G:U is repaired efficiently in vitro, and DNA polymerase beta (beta-pol) is responsible for the single nucleotide gap-filling synthesis (Singhal, R. K., Prasad, R., and Wilson, S. H.(1995) J. Biol. Chem. 270, 949-957). To investigate potential interaction of beta-pol with other BER protein(s), we developed affinity chromatography matrices by cross-linking purified rat beta-pol or antibody against beta-pol to solid supports. Crude nuclear extract from bovine testis was applied to these affinity columns, which were then extensively washed. Proteins that bound specifically to the affinity columns were co-eluted in a complex with beta-pol. This complex had a molecular mass of approximately 180 kDa and was able to conduct the complete uracil-initiated BER reaction. The BER complex contained both beta-pol and DNA ligase I. An antibody to beta-pol was able to shift the complex in sucrose gradients to a much larger molecular mass (>300 kDa) that again contained both beta-pol and DNA ligase I. Furthermore, DNA ligase I and beta-pol were co-immunoprecipitated from the testis nuclear extract with anti beta-pol IgG. Thus, we conclude that beta-pol and DNA ligase I are components of a multiprotein complex that performs BER.

The bovine DNA polymerase beta promoter: cloning, characterization and comparison with the human core promoter

The core promoter of the human DNA polymerase beta (beta Pol)-encoding gene (POL beta) is regulated through cis-elements for the ATF/CREB protein(s), and GC box-binding and initiation-site-binding proteins. The mechanism of promoter regulation has been studied using a nuclear extract transcription system from HeLa cells [Narayan et al., J. Biol. Chem. 269 (1994) 12755-12763]. To study the homologous promoter (ppol beta) in a bovine system, we cloned and characterized the 5'-flanking region of the bovine gene (pol beta). A 15.3-kb fragment of bovine genomic DNA containing the first two exons and 11 kb of 5'-flanking region was isolated from a testis library in bacteriophage lambda EMBL3. S1 nuclease mapping and primer extension analysis of the 5'-end of the pol beta mRNA identified the major transcription start point (tsp), which is located 142-bp 5' of the translational start codon. In transient expression assays using a bovine cell line, analysis of various 5'-deletion mutants demonstrated that a fragment of only 91-bp 5' of the tsp had promoter activity similar to that of a 1.37-kb fragment, so that cis-elements for basal transcription are located within this approx. 100-bp core promoter, as in the human promoter (pPOL beta). Comparison of the core promoters from the bovine and human genes revealed striking similarity, including an almost precise match of the tsp, the ATF/CREB-binding and Sp1-binding sites, and the spacing separating them.

Phorbol ester abrogates up-regulation of DNA polymerase beta by DNA-alkylating agents in Chinese hamster ovary cells

Mammalian DNA polymerase beta (beta-pol), a DNA repair polymerase, is known to be constitutively expressed in cultured cells, but treatment of cells with the DNA-alkylating agents MNNG or methyl methanesulfonate has been shown to up-regulate beta-pol mRNA level. To further characterize this response, we prepared a panel of monoclonal antibodies and used one of them to quantify beta-pol in whole cell extracts by immunoblotting. We found that treatment of Chinese hamster ovary cells with either DNA-alkylating agent up-regulated the beta-pol protein level 5-10-fold. This induction appeared to be secondary to DNA alkylation, as induction was not observed with a genetically altered cell line overexpressing the DNA repair enzyme O6-methylguanine-methyltransferase. We also found that 12-O-tetradecanoylphorbol-13-acetate (TPA) treatment of wild type Chinese hamster ovary cells increased expression of beta-pol protein (approximately 10-fold). Any interrelationship between this TPA response and the DNA-alkylation response was studied by treatment with combinations of MNNG and TPA. The beta-pol up-regulation observed with MNNG treatment was abrogated by TPA, and conversely the up-regulation observed with TPA treatment was abrogated by MNNG.

DNA polymerase beta conducts the gap-filling step in uracil-initiated base excision repair in a bovine testis nuclear extract

The G:U mismatch in genomic DNA mainly arises from deamination of cytosine residues and is repaired by the base excision repair pathway. We found that a bovine testis crude nuclear extract conducts uracil-initiated base excision repair in vitro. A 51-base pair synthetic DNA substrate containing a single G:U mismatch was used, and incorporation of dCMP during repair was exclusively to replace uracil. A neutralizing polyclonal antibody against DNA polymerase beta (beta-pol) inhibited the repair reaction. ddCTP also inhibited the repair reaction, whereas aphidicolin had no significant effect, suggesting that activity of beta-pol was required. Next, the base excision repair system was reconstituted using partially purified components. Several of the enzymatic activities required were resolved, such that DNA ligase and the uracil-DNA glycosylase/apurinic/apyrimidinic endonuclease activities were separated from the DNA polymerase requirement. We found that purified beta-pol could restore full DNA repair activity to the DNA polymerase-depleted fraction, whereas purified DNA polymerases alpha, delta, and epsilon could not. These results with purified proteins corroborated results obtained with the crude extract and indicate that beta-pol is responsible for the single-nucleotide gap filling reaction involved in this in vitro base excision repair system.

The human DNA polymerase beta gene structure. Evidence of alternative splicing in gene expression

DNA polymerase beta (beta-pol) is a single-copy gene that is considered to be part of the DNA repair machinery in mammalian cells. Using two human genomic libraries we have cloned the complete human beta-pol gene and determined the organization of the beta-pol coding sequence within the gene. The human beta-pol gene spans 33 kb and contains 14 exons that range from 50 to 233 bp. The 13 introns vary from 96 bp to 6.5 kb. Information derived from this study was used in defining the location of a deletion/insertion type restriction fragment length polymorphism (RFLP) 5' to exon I of the human beta-pol gene. This RFLP was utilized in linkage analysis of DNAs from CEPH families and the results confirm the previous assignment of the human beta-pol gene to chromosome 8 (p12-p11). Analysis of mRNA from six human cell lines using the polymerase chain reaction showed the expression of two beta-pol transcripts. Sequence analysis revealed that the size difference in these transcripts was due to deletion of the 58 bp sequence encoded by exon II, suggesting that the smaller transcript results from an alternative splicing of the exon II sequence during processing of the beta-pol precursor RNA.

DNA damage response of cloned DNA beta-polymerase promoter is blocked in mutant cell lines deficient in protein kinase A

DNA beta-polymerase (beta-pol), one of the recognized DNA polymerizing enzymes in vertebrates, has a role in 'very short patch' gap-filling synthesis during nucleotide excision DNA repair. In human and mouse, the enzyme is encoded by a single-copy gene located on the short arm of chromosome 8 near the centromere. In a series of studies, we have found that the cloned human beta-pol promoter is regulated by signals acting through the single ATF/CRE palindrome in the core promoter. These signals include transactivation by: adenovirus E1a/E1b proteins; activated p21ras; and in CHO cells, treatment with the DNA damaging agent MNNG. Hence, several types of stimulatory signals are mediated through the single ATF/CRE site, including DNA damage induction. To understand the mechanism of beta-pol promoter activation by MNNG in CHO cells, we asked whether induction of the cAMP/protein kinase A pathway can increase transcription of the cloned promoter in this system. Agents that increase cellular cAMP levels (8-BrcAMP; forskolin and IBMx) activated the beta-pol promoter fusion gene in transient expression experiments, and a mutation in the ATF/CRE palindrome blocked this response. Thus, the ATF/CRE site appears to be cAMP responsive in the CHO cell system. We found that the activation of the cloned beta-pol promoter by MNNG does not occur with two mutant CHO cell lines that are deficient in protein kinase A activity. Further, simultaneous treatment of wild-type CHO cells, with MNNG and to elevate cAMP, failed to result in an additive effect for activation of the beta-pol promoter. Thus, these effectors may act through a common pathway. These results suggest that the activation of the cloned beta-pol promoter in CHO cells following MNNG treatment is mediated through the cAMP/protein kinase A signal transduction pathway.

The CREB family of transcription activators

A diverse family of transcription factors bind to the cAMP-response elements found in a variety of mammalian and viral gene promoters. One of the members of this family, CREB, is being intensively studied so as to elucidate the mechanisms by which second messenger signal transduction pathways act to positively and negatively regulate transcription.

The cloned promoter of the human DNA beta-polymerase gene contains a cAMP response element functional in HeLa cells

The mammalian DNA beta-polymerase (beta-pol) gene is constitutively expressed in cultured cells as a function of growth stage and DNA replication, but is expressed in rodents in a tissue-specific fashion. As revealed by transient expression experiments with wild-type and mutated beta-pol promoter fusion genes, the cloned human beta-pol promoter is transcriptionally regulated by signals acting through the single palindromic sequence (GT-GACGTCAC) known as an ATF/CRE-binding site centered at position -45 in the core promoter. Although the mere presence of the ATF/CRE palindromic sequence in a promoter does not always confer cAMP responsiveness or protein binding over and around the ATF/CRE sequence, we find that agents that increase cAMP levels (forskolin and IBMX) in HeLa cells activate the beta-pol promoter; activation also can be observed by coexpression of the protein kinase A catalytic subunit. Experiments with mutagenized beta-pol promoters indicate that the ATF/CRE-binding site mediates these effects. Thus, the ATF/CRE-binding site in the context of this TATA-less constitutive promoter is able to respond to the kinase A signal transduction pathway.

Cisplatin resistance in human cancers

Cancer chemotherapeutic agents primarily act by damaging cellular DNA directly or indirectly. Tumor cells, in contrast to normal cells, respond to cisplatin with transient gene expression to protect and/or repair their chromosomes. Repeated cisplatin treatments results in a stable resistant cell line with enhanced gene expression but lacking gene amplification for the proteins that will limit cisplatin cytotoxicity. Recently, several new human cell lines have been characterized for cisplatin resistance. These cell lines have led to a better understanding of the molecular and biochemical basis of cisplatin resistance. The c-fos proto-oncogene, a master switch for turning on other genes in response to a wide range of stimuli, has been shown to play an important role in cisplatin resistance both in vitro and in patients. Based on these studies, new strategies have been developed to circumvent and/or exploit clinical cisplatin resistance.