Stimulation of the DNA-dependent protein kinase by RNA polymerase II transcriptional activator proteins

DNA-PKcs restricts Zika virus spreading and is required for effective antiviral response

Zika virus (ZIKV) is a single-strand RNA mosquito-borne flavivirus with significant public health impact. ZIKV infection induces double-strand DNA breaks (DSBs) in human neural progenitor cells that may contribute to severe neuronal manifestations in newborns. The DNA-PK complex plays a critical role in repairing DSBs and in the innate immune response to infection. It is unknown, however, whether DNA-PK regulates ZIKV infection. Here we investigated the role of DNA-PKcs, the catalytic subunit of DNA-PK, during ZIKV infection. We demonstrate that DNA-PKcs restricts the spread of ZIKV infection in human epithelial cells. Increased ZIKV replication and spread in DNA-PKcs deficient cells is related to a notable decrease in transcription of type I and III interferons as well as IFIT1, IFIT2, and IL6. This was shown to be independent of IRF1, IRF3, or p65, canonical transcription factors necessary for activation of both type I and III interferon promoters. The mechanism of DNA-PKcs to restrict ZIKV infection is independent of DSB. Thus, these data suggest a non-canonical role for DNA-PK during Zika virus infection, acting downstream of IFNs transcription factors for an efficient antiviral immune response.

PRKDC promotes hepatitis B virus transcription through enhancing the binding of RNA Pol II to cccDNA

Hepatitis B virus infection remains a major health problem worldwide due to its high risk of liver failure and hepatocellular carcinoma. Covalently closed circular DNA (cccDNA), which is present as an individual minichromosome, serves as the template for transcription of all viral RNAs and pla ays critical role in viral persistence. Therefore, there is an urgent need to gain broader insight into the transcription regulation of cccDNA. Here, we combined a modified Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR) with an engineered ascorbate peroxidase 2 (APEX2) to identify cccDNA associated proteins systematically in living cells. By functional screening, we verified that protein kinase, DNA-activated, catalytic subunit (PRKDC) was an effective activator of HBV cccDNA transcription in HBV-infected HepG2-NTCP cells and primary human hepatocytes. Mechanismly, PRKDC interacted with POLR2A and POLR2B, the two largest subunits of RNA polymerase II (Pol II) and recruited Pol II to HBV cccDNA minichromosome in a kinase-dependent manner. PRKDC knockdown or inhibitor treatment significantly decreased the enrichment of POLR2A and POLR2B on cccDNA, as well as reducing the levels of cccDNA associated Pol II Ser5 and Ser2 phosphorylation, which eventually inhibited the HBV cccDNA activity. Collectively, these findings give us new insights into cccDNA transcription regulation, thus providing new potential targets for HBV treatment in patients.

Heat shock proteins and DNA repair mechanisms: an updated overview

Heat shock proteins (HSPs), also known as molecular chaperones, participate in important cellular processes, such as protein aggregation, disaggregation, folding, and unfolding. HSPs have cytoprotective functions that are commonly explained by their antiapoptotic role. Their involvement in anticancer drug resistance has been the focus of intense research efforts, and the relationship between HSP induction and DNA repair mechanisms has been in the spotlight during the past decades. Because DNA is permanently subject to damage, many DNA repair pathways are involved in the recognition and removal of a diverse array of DNA lesions. Hence, DNA repair mechanisms are key to maintain genome stability. In addition, the interactome network of HSPs with DNA repair proteins has become an exciting research field and so their use as emerging targets for cancer therapy. This article provides a historical overview of the participation of HSPs in DNA repair mechanisms as part of their molecular chaperone capabilities.

Engagement of Components of DNA-Break Repair Complex and NFκB in Hsp70A1A Transcription Upregulation by Heat Shock

An involvement of components of DNA-break repair (DBR) complex including DNA-dependent protein kinase (DNA-PK) and poly-ADP-ribose polymerase 1 (PARP-1) in transcription regulation in response to distinct cellular signalling has been revealed by different laboratories. Here, we explored the involvement of DNA-PK and PARP-1 in the heat shock induced transcription of Hsp70A1A. We find that inhibition of both the catalytic subunit of DNA-PK (DNA-PKc), and Ku70, a regulatory subunit of DNA-PK holo-enzyme compromises transcription of Hsp70A1A under heat shock treatment. In immunoprecipitation based experiments we find that Ku70 or DNA-PK holoenzyme associates with NFκB. This NFκB associated complex also carries PARP-1. Downregulation of both NFκB and PARP-1 compromises Hsp70A1A transcription induced by heat shock treatment. Alteration of three bases by site directed mutagenesis within the consensus κB sequence motif identified on the promoter affected inducibility of Hsp70A1A transcription by heat shock treatment. These results suggest that NFκB engaged with the κB motif on the promoter cooperates in Hsp70A1A activation under heat shock in human cells as part of a DBR complex including DNA-PK and PARP-1.

TRIM28 as a novel transcriptional elongation factor

TRIM28 is a multidomain protein with versatile functions in transcription and DNA repair. Recently it was shown that this factor plays unanticipated roles in transcriptional elongation. TRIM28 was shown to stabilize the pausing of RNA polymerase II (Pol II) close to the transcriptional start site in many unactivated genes, permitting Pol II accumulation and readying genes for induction. In addition, the factor was shown to respond rapidly to signals accompanying transcriptional activation permitting the productive elongation of RNA by previously paused Pol II. We discuss here critical regulatory mechanisms of TRIM28 in transcriptional control and DNA repair that may illuminate the novel roles of this factor in pausing and elongation of Pol II.

Deregulation of DNA-dependent protein kinase catalytic subunit contributes to human hepatocarcinogenesis development and has a putative prognostic value

Background:

The DNA-repair gene DNA-dependent kinase catalytic subunit (DNA-PKcs) favours or inhibits carcinogenesis, depending on the cancer type. Its role in human hepatocellular carcinoma (HCC) is unknown.

Methods:

DNA-dependent protein kinase catalytic subuni, H2A histone family member X (H2AFX) and heat shock transcription factor-1 (HSF1) levels were assessed by immunohistochemistry and/or immunoblotting and qRT–PCR in a collection of human HCC. Rates of proliferation, apoptosis, microvessel density and genomic instability were also determined. Heat shock factor-1 cDNA or DNA-PKcs-specific siRNA were used to explore the role of both genes in HCC. Activator protein 1 (AP-1) binding to DNA-PKcs promoter was evaluated by chromatin immunoprecipitation. Kaplan–Meier curves and multivariate Cox model were used to study the impact on clinical outcome.

Results:

Total and phosphorylated DNA-PKcs and H2AFX were upregulated in HCC. Activated DNA-PKcs positively correlated with HCC proliferation, genomic instability and microvessel density, and negatively with apoptosis and patient's survival. Proliferation decline and massive apoptosis followed DNA-PKcs silencing in HCC cell lines. Total and phosphorylated HSF1 protein, mRNA and activity were upregulated in HCC. Mechanistically, we demonstrated that HSF1 induces DNA-PKcs upregulation through the activation of the MAPK/JNK/AP-1 axis.

Conclusion:

DNA-dependent protein kinase catalytic subunit transduces HSF1 effects in HCC cells, and might represent a novel target and prognostic factor in human HCC.

Identification of DNA-dependent protein kinase catalytic subunit as a novel interaction partner of lymphocyte enhancer factor 1

Lymphocyte enhancer factor 1 (LEF1), a member of the LEF/T-cell-specific factor (TCF) family of the high mobility group domain transcription factors, acts downstream in canonical Wnt signaling. Aberrant transactivation of LEF1 contributes to the tumorigenesis of colonic neoplasms, sebaceous skin tumors, and lymphoblastic leukemia. LEF1-associated proteins are crucial for regulating its transcriptional activity. In this study, glutathione-S-transferase pull-down assay and mass spectrometry enabled identification of the DNA-dependent protein kinase catalytic subunit (DNA-PKcs) as a novel interaction partner for LEF1. The interaction between LEF1 and DNA-PKcs was confirmed using in vivo co-immunoprecipitation. Furthermore, double immunofluorescence observations showed that LEF1 and DNA-PKcs colocalized in the nuclei of colon adenocarcinoma cell lines. Identification of the interaction between LEF1 and DNA-PKcs may provide clues for a novel therapy for cancer treatment as well as for understanding LEF1-mediated transcriptional regulation.

Analogues and derivatives of oncrasin-1, a novel inhibitor of the C-terminal domain of RNA polymerase II and their antitumor activities

To optimize the antitumor activity of oncrasin-1, a small molecule RNA polymerase II inhibitor, we evaluated 69 oncrasin-1 analogues for their cytotoxic activity against normal human epithelial cells and K-Ras mutant tumor cells. About 40 of those compounds were as potent as or more potent than oncrasin-1 in tumor cells and had a minimal cytotoxic effect on normal cells. Structure-activity relationship analysis revealed that most of the active compounds contained either a hydroxymethyl group or an aldehyde group as a substitute at the 3-position of the indole. Both electron-donating and electron-withdrawing groups in the benzene ring were well tolerated. The hydroxymethyl compounds ranged from equipotent with to 100 times as potent as the corresponding aldehyde compounds. We tested three active analogues' effect on RNA polymerase phosphorylation and found that they all inhibited phosphorylation of the C-terminal domain of RNA polymerase II, suggesting that the active compounds might act through the same mechanisms as oncrasin-1.

Activation of the DNA-dependent protein kinase stimulates nuclear export of the androgen receptor in vitro

The androgen receptor undergoes nuclear import in response to ligand, but the mechanism by which it undergoes nuclear export is poorly understood. We developed a permeabilized cell assay to characterize nuclear export of the androgen receptor in LNCaP prostate cancer cells. We found that nuclear export of endogenous androgen receptor can be stimulated by short double-stranded DNA oligonucleotides. This androgen receptor export pathway is dependent on ATP hydrolysis and is enhanced by phosphatase inhibition with okadaic acid. Fluorescence recovery after photobleaching in permeabilized cells, under the conditions that stimulate androgen receptor export, suggested that double-stranded DNA-dependent export does not simply reflect the relief of a nuclear retention mechanism. A radiolabeled androgen was used to show that the androgen receptor remains ligand-bound during translocation through the nuclear pore complex. A specific inhibitor to the DNA-dependent protein kinase, NU7026, inhibits androgen receptor export and phosphorylation. In living cells, NU7026 treatment increases androgen-dependent transcription from endogenous genes that are regulated by androgen receptor. We suggest that DNA-dependent protein kinase phosphorylation of the androgen receptor, or an interacting component, helps target the androgen receptor for export from the nucleus.

Functional role of Zic2 phosphorylation in transcriptional regulation

Zic2 is a transcriptional activator that plays a crucial role in mammalian forebrain development. It activates the transcription of target genes by DNA binding and recruitment of RNA helicase A (RHA). We recently reported that the Zic2-RHA interaction is decreased by phosphatase treatment in vitro. We have now identified the phosphorylation site (serine 200) in mouse Zic2. Zic2S200A was defective in RHA-binding, and its transcriptional activation ability was diminished. These data indicate that Zic2S200 is a target for phosphorylation by DNA-dependent protein kinase, regulating Zic2-mediated transcriptional activation.

The molecular causes of low ATM protein expression in breast carcinoma; promoter methylation and levels of the catalytic subunit of DNA-dependent protein kinase

AIMS

To investigate whether aberrant methylation of the ATM promoter or loss of the catalytic subunit of DNA-dependent protein kinase (DNA-PKcs) may be the underlying causes of reduced ATM protein levels often seen in breast tumours.

METHODS AND RESULTS

Methylation-specific polymerase chain reaction was used to determine the ATM promoter status and DNA-PKcs levels were measured by immunohistochemistry. None of the 74 invasive carcinomas (ICs) studied showed ATM promoter hypermethylation, whereas promoter methylation of CDKN2A/p16 (1.8%) and GSTP1 (15.8%) was detected. Of 92 ICs examined, 68 had reduced DNA-PKcs levels, supporting previous findings that alterations in double-strand break repair are associated with breast cancer pathogenesis. Although no association was found between the DNA-PKcs and ATM scores for the series of 92 tissues and 22/24 tissues with normal DNA-PKcs had reduced ATM, 29 tumours showed low expression of both DNA-PKcs and ATM compared with normal tissues.

CONCLUSIONS

No evidence was found that the reduction in ATM protein levels seen in breast carcinoma is the result of epigenetic silencing. However, cross-regulation between DNA-PKcs and ATM may be a possible cause in a subset of tumours and warrants further investigation.

ZIC2-dependent Transcriptional Regulation Is Mediated by DNA-dependent Protein Kinase, Poly(ADP-ribose) Polymerase, and RNA Helicase A

The Zic family of zinc finger proteins is essential for animal development, as demonstrated by the holoprosencephaly caused by mammalian Zic2 mutation. To determine the molecular mechanism of Zic-mediated developmental control, we characterized two types of high molecular weight complexes, including Zic2. Complex I was composed of DNA-dependent protein kinase catalytic subunit (DNA-PKcs), Ku70/80, and poly(ADP-ribose) polymerase; complex II contained Ku70/80 and RNA helicase A; all the components interacted directly with Zic2 protein. Immunoprecipitation, subnuclear localization, and in vitro phosphorylation analyses revealed that the DNA-PKcs in complex I played an essential role in the assembly of complex II. Stepwise exchange from complex I to complex II depended on phosphorylation of Zic2 by DNA-PK and poly-(ADP-ribose) polymerase. Phosphorylated Zic2 protein made a stable complex with RNA helicase A, and complex II could interact with RNA polymerase II. Phosphorylation-dependent transformation of Zic2-containing molecular complexes may occur in transcriptional regulation.

Transactivation of hsp70-1/2 in geldanamycin-treated human non-small cell lung cancer H460 cells: involvement of intracellular calcium and protein kinase C

Geldanamycin is an antitumor drug that binds HSP90 and induces a wide range of heat shock proteins, including HSP70s. In this study we report that the induction of HSP70s is dose-dependent in geldanamycin-treated human non-small cell lung cancer H460 cells. Analysis of the induction of HSP70s specific isoform using LC-ESI-MS/MS analysis and Northern blotting showed that HSP70-1/2 are the major inducible forms under geldanamycin treatment. Transactivation of hsp70-1/2 was determined by electrophoretic mobility-shift assay using heat shock element (HSE) as a probe. The signaling pathway mediators involved in hsp70-1/2 transactivation were screened by the kinase inhibitor scanning technique. Pretreatment with serine/threonine protein kinase inhibitors H7 or H8 blocked geldanamycin-induced HSP70-1/2, whereas protein kinase A inhibitor HA1004, protein kinase G inhibitor KT5823, and myosin light chain kinase inhibitor ML-7 had no effect. Furthermore, the protein kinase C (PKC)-specific inhibitor Ro-31-8425 and the Ca2+-dependent PKC inhibitor Gö-6976 diminished geldanamycin-induced HSP70-1/2, suggesting an involvement of the PKC in the process. In addition, geldanamycin treatment causes a transient increase of intracellular Ca2+. Chelating intracellular Ca2+ with BAPTA-AM or depletion of intracellular Ca2+ store with A23187 or thapsigargin significantly decreased geldanamycin-transactivated HSP70-1/2 expression. Taken together, our results demonstrate that geldanamycin-induced specific HSP70-1/2 isoforms expression in H460 cells through signaling pathway mediated by Ca2+ and PKC.

Deficiency in the catalytic subunit of DNA-dependent protein kinase causes down-regulation of ATM

Previous reports have suggested a connection between reduced levels of the catalytic subunit of DNA-dependent protein kinases (DNA-PKcs), a component of the nonhomologous DNA double-strand breaks end-joining system, and a reduction in ATM. We studied this possible connection in other DNA-PKcs-deficient cell types, and following knockdown of DNA-PKcs with small interfering RNA, Chinese hamster ovary V3 cells, lacking DNA-PKcs, had reduced levels of ATM and hSMG-1, but both were restored after transfection with PRKDC. Atm levels were also reduced in murine scid cells. Reduction of ATM in a human glioma cell line lacking DNA-PKcs was accompanied by defective signaling through downstream substrates, post-irradiation. A large reduction of DNA-PKcs was achieved in normal human fibroblasts after transfection with two DNA-PKcs small interfering RNA sequences. This was accompanied by a reduction in ATM. These data were confirmed using immunocytochemical detection of the proteins. Within hours after transfection, a decline in PRKDC mRNA was seen, followed by a more gradual decline in DNA-PKcs protein beginning 1 day after transfection. No change in ATM mRNA was observed for 2 days post-transfection. Only after the DNA-PKcs reduction occurred was a reduction in ATM mRNA observed, beginning 2 days post-transfection. The amount of ATM began to decline, starting about 3 days post-treatment, then it declined to levels comparable to DNA-PKcs. Both proteins returned to normal levels at later times. These data illustrate a potentially important cross-regulation between the nonhomologous end-joining system for rejoining of DNA double-strand breaks and the ATM-dependent damage response network of pathways, both of which operate to maintain the integrity of the genome.

The life and death of DNA-PK

Double-strand breaks (DSBs) arise endogenously during normal cellular processes and exogenously by genotoxic agents such as ionizing radiation (IR). DSBs are one of the most severe types of DNA damage, which if left unrepaired are lethal to the cell. Several different DNA repair pathways combat DSBs, with nonhomologous end-joining (NHEJ) being one of the most important in mammalian cells. Competent NHEJ catalyses repair of DSBs by joining together and ligating two free DNA ends of little homology (microhomology) or DNA ends of no homology. The core components of mammalian NHEJ are the catalytic subunit of DNA protein kinase (DNA-PK(cs)), Ku subunits Ku70 and Ku80, Artemis, XRCC4 and DNA ligase IV. DNA-PK is a nuclear serine/threonine protein kinase that comprises a catalytic subunit (DNA-PK(cs)), with the Ku subunits acting as the regulatory element. It has been proposed that DNA-PK is a molecular sensor for DNA damage that enhances the signal via phosphorylation of many downstream targets. The crucial role of DNA-PK in the repair of DSBs is highlighted by the hypersensitivity of DNA-PK(-/-) mice to IR and the high levels of unrepaired DSBs after genotoxic insult. Recently, DNA-PK has emerged as a suitable genetic target for molecular therapeutics such as siRNA, antisense and novel inhibitory small molecules. This review encompasses the recent literature regarding the role of DNA-PK in the protection of genomic stability and focuses on how this knowledge has aided the development of specific DNA-PK inhibitors, via both small molecule and directed molecular targeting techniques. This review promotes the inhibition of DNA-PK as a valid approach to enhance the tumor-cell-killing effects of treatments such as IR.

Nuclear receptor coactivator thyroid hormone receptor-binding protein (TRBP) interacts with and stimulates its associated DNA-dependent protein kinase

Nuclear receptors mediate gene activation through ligand-dependent interaction with coactivators. We previously cloned and characterized thyroid hormone receptor-binding protein, TRBP (NcoA6: AIB3/ASC-2/RAP250/PRIP/TRBP/NRC), as an LXXLL-containing coactivator that associates with coactivator complexes through its C terminus. To search for protein factors involved in TRBP action, we identified a distinct set of proteins from HeLa nuclear extract that interacts with the C terminus of TRBP. Analysis by mass spectrometric protein sequencing revealed a DNA-dependent protein kinase (DNA-PK) complex including its catalytic subunit and regulatory subunits, Ku70 and Ku86. DNA-PK is a heterotrimeric nuclear phosphatidylinositol 3-kinase that functions in DNA repair, recombination, and transcriptional regulation. DNA-PK phosphorylates TRBP at its C-terminal region, which directly interacts with Ku70 but not Ku86 in vitro. In addition, in the absence of DNA, TRBP itself activates DNA-PK, and the TRBP-stimulated DNA-PK activity has an altered phosphorylation pattern from DNA-stimulated activity. An anti-TRBP antibody inhibits TRBP-induced kinase activity, suggesting that protein content of TRBP is responsible for the stimulation of DNA-independent kinase activity. Furthermore, in DNA-PK-deficient scid cells, TRBP-mediated transactivation is significantly impaired, and nuclear localization of TRBP is altered. The activation of DNA-PK in the absence of DNA ends by the coactivator TRBP suggests a novel mechanism of coactivator-stimulated DNA-PK phosphorylation in transcriptional regulation.

Differential gene expression in human glioma cells: correlation with presence or absence of DNA-dependent protein kinase

The human glioma cell line M059J is deficient in DNA-dependent protein kinase (DNA-PK) due to a frame-shift mutation in PRKDC, the gene for its catalytic subunit, while cell line M059K, isolated from the same malignant tumor, has normal DNA-PK activity. DNA-PK is required for double-strand DNA break repair, and its absence is responsible for increased radiosensitivity of M059J. We show that transcripts of several melanoma antigen subfamily A (MAGE-A) genes, the expression of which is restricted to tumor and germ-line cells,are present in M059K, but that their expression is strongly downregulated in M059J. Normal levels of MAGE-A expression are restored in the PRKDC-complemented cell line M059J/Fus1, suggesting that the presence of DNA-PK is required for MAGE-A gene transcription. We also show that the MAGE-A1 promoter is methylated in M059J, while the promoter is demethylated in M059K and M059J/Fus1. Other genes, including all three major histocompatibility class I (HLA) genes, BENE, and an unnamed gene related to CNIL(CORNICHON-like), display an opposite expression profile (i.e., they are upregulated in the DNA-PK-deficient cell line, but show low levels of expression in both M059K and in the PRKDC-complemented cell line). For these genes, differential expression does not correlate with DNA methylation in upstream promoter sequences. Our results suggest that the presence of DNA-PK can exert effects on gene expression by various mechanisms and pathways, thus affecting overall cell physiology even in the absence of DNA damage.

Expression and localization of heat shock factor (Hsf) 1 in the rodent cochlea

Activation of heat shock factors (Hsfs) is one of the potential mechanisms for regulating the transcription of the heat shock proteins (Hsps) and certain other stress-responsive genes. Reverse transcription polymerase chain reaction (RT-PCR), Western blot and immunocytochemistry were used to examine the expression and localization of Hsf1, the stress-responsive member of the Hsf family, in the rat and mouse cochlea. Cerebellum was used as a positive control. Semi-quantitative RT-PCR of cochlear RNA revealed that Hsf1 was more highly expressed in a subfraction containing sensorineural epithelium and lateral wall than in a subfraction containing modiolus, with the alpha splice form predominant over the beta in both subfractions. Immunocytochemistry showed selective staining in the rodent cochlea. Hsf1 immunostaining was found in the nuclei of inner and outer hair cells in the organ of Corti, spiral ganglion cells in the modiolus, and cells in the marginal and intermediate layers of the stria vascularis. This is largely consistent with where Hsp70 induction is reported. Hsf1 activation following heat shock was examined by Western blot. Hyperthermia resulted in stress-induced Hsf1 hyperphosphorylation in cochlea as well as cerebellum. This hyperphosphorylation as well as the correlation of its localization with Hsp70 induction supports a role for Hsf1 in the cochlear stress response.

Inhibition of HTLV-1 transcription by cyclin dependent kinase inhibitors

HTLV-1 is the etiologic agent for adult T-cell leukemia/lymphoma (ATL) and HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP), where viral replication and transformation are largely dependent upon modification of regulatory and host cell cycle proteins. The mechanism of HTLV-1 transformation appears to be distinct from that of many known chronic or acute leukemia viruses and is related to the viral activator Tax. Here we show that cyclin E, can associate tightly with the coactivator p300 and Pol II complex in HTLV-1 infected cells. The cyclin E associated complex is kinase active and phosphorylates the carboxy terminal domain of RNA Pol II. More importantly, p21/Waf1, a well-known cdk inhibitor at the G1/S border, inhibits transcription of HTLV-1 in both transfections and in in vitro transcription assays. Finally, specific cdk chemical inhibitors, functionally similar to cellular cdkIs, such as p21/Waf1 which inhibits cyclin E/cdk2 activity, also inhibit transcription of the HTLV-1 promoter. In particular, Purvalanol A, with an IC50 of 0.035 microm inhibits activated, but not basal transcription, as well as HTLV-1 infected cells. Collectively, the role of cyclin E/cdk2 in HTLV-1 infected cells and its involvement in RNA Pol II phosphorylation is discussed.

DNA-PK and ATM are required for radiation-enhanced integration

Ionizing radiation is known to improve transfection of exogenous DNA, a process we have termed radiation-enhanced integration. Previous observations have demonstrated that Ku proteins are critical for radiation-enhanced integration. Since Ku proteins form the DNA-binding domain of DNA-PK and since DNA-PK is important in nonhomologous DNA end joining, it was hypothesized that DNA-PK function might be important for radiation-enhanced integration. The ATM protein has been shown to be important in the recognition of a variety of types of DNA damage and to associate with DNA-PK under certain conditions. It was thus hypothesized that ATM might also play a role in radiation-enhanced integration. To test these hypotheses, radiation-enhanced integration was measured in hamster cells that are defective in the catalytic subunit of DNA-PK and in human cells containing mutant ATM. Radiation-enhanced integration was not detected in any of the cell lines with mutant PRKDC (also known as DNA-PKcs), but it was present in cells of the same lineage with wild-type PRKDC. Radiation-enhanced integration was defective in cells lacking kinase activation. ATM-deficient cell lines also showed defective radiation-enhanced integration. These data demonstrate that DNA-PK and ATM must both be active for radiation-enhanced integration to be observed.

Effects of hyperthermia on p53 protein expression and activity

Although p53 responses after DNA damage have been investigated extensively, p53 responses after heat shock, which exerts cytotoxic action by mechanisms other than direct induction of DNA damage, are less well characterized. We investigated, therefore, the effect of hyperthermic exposures on the levels and DNA-binding activity of p53. Experiments were carried out with U2OS and ML-1 cells, known to express wild-type p53 protein. Although heating at 41 degrees C for up to 6 h had only a small effect on p53 levels or DNA binding activity, exposure to temperatures between 42.5 and 45.5 degrees C caused an immediate decrease in protein levels that was associated with a reduction in DNA binding activity. This observation is compatible with a high lability of p53 to heat shock, or heat sensitivity of the pathway regulating p53 levels in non-stressed cells. When cells were heated to 42.5 degrees C and returned to normal temperatures, a strong p53 response associated with an increase in protein levels and DNA binding activity was observed, suggesting the production of p53-inducing cellular damage. At higher temperatures, however, this response was compromised in an exposure-time-dependent manner. The increase in DNA binding activity was more heat sensitive than the increase in p53 levels and was inhibited at lower temperatures and shorter exposure times. Thus, the pathway of p53 activation is itself heat sensitive and compromised at high levels of exposure. Compared to p53 activation after exposure to ionizing radiation, heat-induced activation is rapid and short lived. When cells were exposed to combined heat and radiation, the response observed approximated that of cells exposed to heat alone. Wortmannin at 10 microM inhibited p53 activation for up to 2 h after heat shock suggesting the involvement of wortmannin-sensitive kinases, such as DNA-PK and ATM. Heat shock causes phosphorylation of p53 at Serine-15, but there is no correlation between phosphorylation at this site and activation of the protein. The results in aggregate indicate p53 activation in the absence of DNA damage by a heat-sensitive mechanism operating with faster kinetics than radiation-induced p53 activation. The former response may induce pathways preventing other stimuli from activating p53, as heat-induced activation of p53 is dominant over activation of p53 by DNA damage in combined-treatment experiments. These observations suggest means for abrogating p53 induction after DNA damage with the purpose of potentiating response and enhancing cell killing.

Heat shock factor-4 (HSF-4a) represses basal transcription through interaction with TFIIF

The heat shock transcription factors (HSFs) regulate the expression of heat shock proteins (hsps), which are critical for normal cellular proliferation and differentiation. One of the HSFs, HSF-4, contains two alternative splice variants, one of which possesses transcriptional repressor properties in vivo. This repressor isoform inhibits basal transcription of hsps 27 and 90 in tissue culture cells. The molecular mechanisms of HSF-4a isoform-mediated transcriptional repression is unknown. Here, we present evidence that HSF-4a inhibits basal transcription in vivo when it is artificially targeted to basal promoters via the DNA-binding domain of the yeast transcription factor, GAL4. By using a highly purified, reconstituted in vitro transcription system, we show that HSF-4a represses basal transcription at an early step during preinitiation complex assembly, as pre-assembled preinitiation complexes are refractory to the inhibitory effect on transcription. This repression occurs by the HSF-4a isoform, but not by the HSF-4b isoform, which we show is capable of activating transcription from a heat shock element-driven promoter in vitro. The repression of basal transcription by HSF-4a occurs through interaction with the basal transcription factor TFIIF. TFIIF interacts with a segment of HSF-4a that is required for the trimerization of HSF-4a, and deletion of this segment no longer inhibits basal transcription. These studies suggest that HSF-4a inhibits basal transcription both in vivo and in vitro. Furthermore, this is the first report identifying an interaction between a transcriptional repressor with the basal transcription factor TFIIF.

Irradiation-Induced Rescue of Thymocyte Differentiation and V(D)J Recombination in Mice Lacking the Catalytic Subunit of DNA-Dependent Protein Kinase

Scid mice express a truncated form of the catalytic subunit of the DNA-dependent protein kinase (DNA-PKcs) and are unable to properly rearrange their Ig and TCR genes, resulting in a severe combined immunodeficiency that is characterized by arrested differentiation of B and T lymphocytes. Treatment of scid mice with low doses of gamma irradiation rescues rearrangements at several TCR loci and promotes limited thymocyte differentiation. The machinery responsible for sensing DNA damage and the mechanism by which irradiation compensates for the scid defect in TCR recombination remain unknown. Because DNA-PKcs is present in scid thymocytes, it may mediate some or all of the irradiation effects. To test this hypothesis, we examined the effects of irradiation on DNA-PKcs-deficient (slip) mice. Our data provide the first evidence that DNA-PKcs is not required for limited rescue of thymocyte differentiation or TCR rearrangements.

Transcription-independent phosphorylation of the RNA polymerase II C-terminal domain (CTD) involves ERK kinases (MEK1/2)

The largest subunit of the mammalian RNA polymerase II possesses a C-terminal domain (CTD) consisting of 52 repeats of the consensus sequence, Tyr(1)-Ser(2)-Pro(3)-Thr(4)-Ser(5)-Pro(6)-Ser(7). Phosphorylation of the CTD is known to play a key role in gene expression. We now show that treatments such as osmotic and oxidative shocks or serum stimulation generate a new type of phosphorylated subunit, the IIm form. This IIm form might be generated in vivo by ERK-type MAP kinase phosphorylation as: (i) ERK1/2 are major CTD kinases found in cell extracts; (ii) the immunoreactivity of the IIm form against a panel of monoclonal antibodies indicates that the CTD is exclusively phosphorylated on Ser-5 in the repeats, like RNA polymerase II phosphorylated in vitro by an ERK1/2; and (iii) the IIm form does not appear when ERK activation is prevented by treating cells with low concentrations of highly specific inhibitors of MEK1/2. Since the IIm subunit is not affected by inhibition of transcription and is not bound to chromatin, it does not participate in transcription.

Involvement of DNA-dependent protein kinase in UV-induced replication arrest

Cells exposed to UV irradiation are predominantly arrested at S-phase as well as at the G(1)/S boundary while repair occurs. It is not known how UV irradiation induces S-phase arrest and yet permits DNA repair; however, UV-induced inhibition of replication is efficiently reversed by the addition of replication protein A (RPA), suggesting a role for RPA in this regulatory event. Here, we show evidence that DNA-dependent protein kinase (DNA-PK), plays a role in UV-induced replication arrest. DNA synthesis of M059K (DNA-PK catalytic subunit-positive (DNA-PKcs(+))), as measured by [(3)H]thymidine incorporation, was significantly arrested by 4 h following UV irradiation, whereas M059J (DNA-PKcs(-)) cells were much less affected. Similar results were obtained with the in vitro replication reactions where immediate replication arrest occurred in DNA-PKcs(+) cells following UV irradiation, and only a gradual decrease in replication activity was observed in DNA-PKcs(-) cells. Reversal of replication arrest was observed at 8 h following UV irradiation in DNA-PKcs(+) cells but not in DNA-PKcs(-) cells. Reversal of UV-induced replication arrest was also observed in vitro by the addition of a DNA-PK inhibitor, wortmannin, or by immunodepletion of DNA-PKcs, supporting a positive role for DNA-PK in damage-induced replication arrest. The RPA-containing fraction from UV-irradiated DNA-PKcs(+) cells poorly supported DNA replication, whereas the replication activity of the RPA-containing fraction from DNA-PKcs(-) cells was not affected by UV, suggesting that DNA-PKcs may be involved in UV-induced replication arrest through modulation of RPA activity. Together, our results strongly suggest a role for DNA-PK in S-phase (replication) arrest in response to UV irradiation.

Regulated phosphorylation of the RNA polymerase II C-terminal domain (CTD)

The largest subunit of RNA polymerase II has an intriguing feature in its carboxyl-terminal domain (CTD) that consists of multiple repeats of an evolutionary conserved motif of seven amino acids. CTD phosphorylation plays a pivotal role in controlling mRNA synthesis and maturation. In exponentially growing cells, the phosphate turnover on the CTD is fast; it is blocked by common inhibitors of transcription, such as 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole and actinomycin D. Transcription-independent changes in CTD phosphorylation are observed at critical developmental stages, such as meiosis and early development.Key words: RNA polymerase II, phosphorylation, transcription inhibitors, cyclin-dependent kinases, development.

DNA-dependent protein kinase protects against heat-induced apoptosis

Purified heat shock transcription factor 1 (HSF1) binds to both the regulatory and catalytic components of the DNA-dependent protein kinase (DNA-PK). This observation suggests that DNA-PK may have a physiological role in the heat shock response. To investigate this possibility, we performed a comparison of cell lines that were deficient in either the Ku protein or the DNA-PK catalytic subunit versus the same cell lines that had been rescued by the introduction of a functional gene. DNA-PK-negative cell lines were up to 10-fold more sensitive to heat-induced apoptosis than matched DNA-PK-positive cell lines. There may be a regulatory interaction between DNA-PK and HSF1 in vivo, because constitutive overexpression of HSF1 sensitized the DNA-PK-positive cells to heat but had no effect in DNA-PK-negative cells. The initial burst of hsp70 mRNA expression was similar in DNA-PK-negative and -positive cell lines, but the DNA-PK-negative cells showed an attenuated rate of mRNA synthesis at later times and, in some cases, lower heat shock protein expression. These findings provide evidence for an antiapoptotic function of DNA-PK that is experimentally separable from its mechanical role in DNA double strand break repair.

Ku, a DNA repair protein with multiple cellular functions?

The Ku protein binds to DNA ends and other types of discontinuity in double-stranded DNA. It is a tightly associated heterodimer of approximately 70 kDa and approximately 80 kDa subunits that together with the approximately 470 kDa catalytic subunit, DNA-PKcs, form the DNA-dependent protein kinase. This enzyme is involved in repairing DNA double-strand breaks (DSBs) caused, for example, by physiological oxidation reactions, V(D)J recombination, ionizing radiation and certain chemotherapeutic drugs. The Ku-dependent repair process, called illegitimate recombination or nonhomologous end joining (NHEJ), appears to be the main DNA DSB repair mechanism in mammalian cells. Ku itself is probably involved in stabilizing broken DNA ends, bringing them together and preparing them for ligation. Ku also recruits DNA-PKcs to the DSB, activating its kinase function. Targeted disruption of the genes encoding Ku70 and Ku80 has identified significant differences between Ku-deficient mice and DNA-PKcs-deficient mice. Although all three gene products are clearly involved in repairing ionizing radiation-induced damage and in V(D)J recombination, Ku-knockout mice are small, and their cells fail to proliferate in culture and show signs of premature senescence. Recent findings have implicated yeast Ku in telomeric structure in addition to NHEJ. Some of the phenotypes of the Ku-knockout mice may indicate a similar role for Ku at mammalian telomeres.

Catalytic subunit of DNA-dependent protein kinase: impact on lymphocyte development and tumorigenesis

The DNA-dependent protein kinase (DNA-PK) consists of a heterodimer DNA-binding complex, Ku70 and Ku80, and a large catalytic subunit, DNA-PKcs. To examine the role of DNA-PKcs in lymphocyte development, radiation sensitivity, and tumorigenesis, we disrupted the mouse DNA-PKcs by homologous recombination. DNA-PKcs-null mice exhibit neither growth retardation nor a high frequency of T cell lymphoma development, but show severe immunodeficiency and radiation hypersensitivity. In contrast to the Ku70-/- and Ku80-/- phenotype, DNA-PKcs-null mice are blocked for V(D)J coding but not for signal-end joint formation. Furthermore, inactivation of DNA-PKcs leads to hyperplasia and dysplasia of the intestinal mucosa and production of aberrant crypt foci, suggesting a novel role of DNA-PKcs in tumor suppression.

Regulation of the DNA-dependent protein kinase (DNA-PK) activity in eukaryotic cells

The DNA-dependent protein kinase (DNA-PK) is a trimeric nuclear serine/threonine protein kinase consisting of a large catalytic sub-unit and the Ku heterodimer that regulates kinase activity by its association with DNA. DNA-PK is a major component of the DNA double strand break repair apparatus, and cells deficient in one of its component are hypersensitive to ionizing radiation. DNA-PK is also required to lymphoid V(D)J recombination and its absence confers in mice a severe combined immunodeficiency phenotype. The purpose of this review is to summarize the current knowledge on the mechanisms that contribute to regulate DNA-PK activity in vivo or in vitro and relates them to the role of DNA-PK in cellular functions. Finally, the studies devoted to drug-inhibition of DNA-PK in order to enhance cancer therapy by DNA-damaging agents are presented.

Nuclear extracts lacking DNA-dependent protein kinase are deficient in multiple round transcription

We have compared levels of in vitro transcription in nuclear extracts from DNA-dependent protein kinase (DNA-PK)-deficient and DNA-PK-containing Chinese hamster ovary cell lines. DNA-PK-deficient cell lines are radiosensitive mutants lacking either the catalytic subunit or the 80-kDa subunit of the Ku protein regulatory component. Extracts from DNA-PK-deficient cell lines had a 2-7-fold decrease in the level of in vitro transcription when compared with matched controls. This decrease was observed with several promoters. Transcription could be restored to either of the deficient extracts by addition of small amounts of extract from the DNA-PK-containing cell lines. Transcription was not restored by addition of purified DNA-PK catalytic subunit, Ku protein, or individually purified general transcription factors. We conclude that extracts from DNA-PK-deficient cells lack a positively acting regulatory factor or a complex of factors not readily reconstituted with individual proteins. We have also investigated the mechanistic defect in the deficient extracts and have found that the observed differences in transcription levels between Ku-positive and Ku-negative cell lines can be attributed solely to a greater ability of the Ku-positive nuclear extracts to carry out secondary initiation events subsequent to the first round of transcription.

Interaction of Ku protein and DNA-dependent protein kinase catalytic subunit with nucleic acids

The Ku protein-DNA-dependent protein kinase system is one of the major pathways by which cells of higher eukaryotes respond to double-strand DNA breaks. The components of the system are evolutionarily conserved and homologs are known from a number of organisms. The Ku protein component binds directly to DNA ends and may help align them for ligation. Binding of Ku protein to DNA also nucleates formation of an active enzyme complex containing the DNA-dependent protein kinase catalytic subunit (DNA-PKcs). The interaction between Ku protein, DNA-PKcs and nucleic acids has been extensively investigated. This review summarizes the results of these biochemical investigations and relates them to recent molecular genetic studies that reveal highly characteristic repair and recombination defects in mutant cells lacking Ku protein or DNA-PKcs.

Modulation of Sp1 phosphorylation by human immunodeficiency virus type 1 Tat

We previously reported (K. T. Jeang, R. Chun, N. H. Lin, A. Gatignol, C. G. Glabe, and H. Fan, J. Virol. 67: 6224-6233, 1993) that human immunodeficiency virus type 1 (HIV-1) Tat and Sp1 form a protein-protein complex. Here, we have characterized the physical interaction and a functional consequence of Tat-Sp1 contact. Using in vitro protein chromatography, we mapped the region in Tat that contacts Sp1 to amino acids 30 to 55. We found that in cell-free reactions, Tat augmented double-stranded DNA-dependent protein kinase (DNA-PK)-mediated Sp1 phosphorylation in a contact-dependent manner. Tat mutants that do not bind Sp1 failed to influence phosphorylation of the latter. In complementary experiments, we also found that Tat forms protein-protein contacts with DNA-PK. We confirmed that in HeLa and Jurkat cells, Tat expression indeed increased the intracellular amount of phosphorylated Sp1 in a manner consistent with the results of cell-free assays. Furthermore, using two phosphatase inhibitors and a kinase inhibitor, we demonstrated a modulation of reporter gene expression as a consequence of changes in Sp1 phosphorylation. Taken together, these findings suggest that activity at the HIV-1 promoter is influenced by phosphorylation of Sp1 which is affected by Tat and DNA-PK.

Characterization of the residues phosphorylated in vitro by different C-terminal domain kinases

The C-terminal part of the largest subunit of eukaryotic RNA polymerase II is composed solely of the highly repeated consensus sequence Tyr1-Ser2-Pro3-Thr4-Ser5-Pro6-Ser7. This domain, called the C-terminal domain (CTD), is phosphorylated mostly at serine residues during transcription initiation, but the precise role of this phosphorylation remains controversial. Several protein kinases are able to phosphorylate this sequence in vitro. The aim of this work was to define the positions of the amino acids phosphorylated by four of these CTD kinases (transcription factor (TF) IIH-kinase, DNA-dependent protein kinase, and the mitogen-activated protein kinases ERK1 and ERK2) and to compare the specificity of these different protein kinases. We show that TFIIH kinase and the mitogen-activated protein kinases phosphorylate only serine 5 of the CTD sequence, whereas DNA-dependent protein kinase phosphorylates serines 2 and 7. Among the different CTD kinases, only TFIIH kinase is appreciably more active on two repeats of the consensus sequence than on one motif. These in vitro results can provide some clues to the nature of the protein kinases responsible for the in vivo phosphorylation of the RNA polymerase CTD. In particular, the ratio of phosphorylated serine to threonine observed in vivo cannot be explained if TFIIH kinase is the only protein kinase involved in the phosphorylation of the CTD.

mRNA encoding the catalytic subunit of DNA-dependent protein kinase is widely expressed in Xenopus cells

Here, the sequence of a Xenopus laevis cDNA encoding the 640 carboxy-terminal amino acids of the catalytic subunit of DNA-dependent protein kinase (DNA-PKcs) is reported. The predicted Xenopus protein segment is 65% identical to the human counterpart. Northern blot analysis indicates that Xenopus DNA-PKcs is encoded by an approx. 13000 nt transcript. DNA-PKcs mRNA is widely expressed in adult tissues as well as in oocytes and embryos. It is also shown that outside the conserved kinase domain, Xenopus DNA-PKcs bears significant similarities to hypothetical 420.8 and 433.2 kDa proteins in yeast species.

Heat shock transcription factor 1 binds selectively in vitro to Ku protein and the catalytic subunit of the DNA-dependent protein kinase

Heat shock transcription factor 1 (HSF1) functions as the master regulator of the heat shock response in eukaryotes. We have previously shown that, in addition to its role as a transcription factor, HSF1 stimulates the activity of the DNA-dependent protein kinase (DNA-PK). DNA-PK is composed of two components: a 460-kDa catalytic subunit and a 70- and 86-kDa heterodimeric regulatory component, also known as the Ku protein. We report here that HSF1 binds specifically to each of the two components of DNA-PK. Binding occurs in the absence of DNA. The complex with the Ku protein is stable and forms at a stoichiometry close to unity between the Ku protein heterodimer and the active HSF1 trimer. The binding is blocked by antibodies against HSF1. Our results show that HSF1 also binds directly, but more weakly, to the catalytic subunit of DNA-PK. Both interactions are dependent on a specific region within the HSF1 regulatory domain. This sequence is necessary but not sufficient for HSF1 stimulation of DNA-PK activity. The ability of HSF1 to interact with both components of DNA-PK provides a potential mechanism for the activation of DNA-PK in response to heat and other forms of stress.

Sequence-specific DNA binding and transcription factor phosphorylation by Ku Autoantigen/DNA-dependent protein kinase. Phosphorylation of Ser-527 of the rat glucocorticoid receptor

NRE1 is a DNA sequence element through which Ku antigen/DNA-dependent protein kinase (DNA-PK) catalytic subunit represses the induction of mouse mammary tumor virus transcription by glucocorticoids. Although Ku is an avid binder of DNA ends and has the ability to translocate along DNA, we report that direct sequence-specific Ku binding occurs with higher affinity (Kd = 0.84 +/- 0.24 nM) than DNA end binding. Comparison of Ku binding to several sequences over which Ku can accumulate revealed two classes of sequence. Sequences with similarity to NRE1 competed efficiently for NRE1 binding. Conversely, sequences lacking similarity to NRE1 competed poorly for Ku and were not recognized in the absence of DNA ends. Phosphorylation of glucocorticoid receptor (GR) fusion proteins by DNA-PK reflected Ku DNA-binding preferences and demonstrated that co-localization of GR with DNA-PK on DNA in cis was critical for efficient phosphorylation. Phosphorylation of the GR fusion protein by DNA-PK mapped to a single site, Ser-527. This site occurs adjacent the GR nuclear localization sequence between the DNA and ligand binding domains of GR, and thus its phosphorylation, if confirmed, has the potential to affect receptor function in vivo.

Attenuation of DNA-dependent protein kinase activity and its catalytic subunit by the herpes simplex virus type 1 transactivator ICP0

The DNA-dependent protein kinase (DNA-PK) is involved in several fundamental nuclear processes, including DNA double-strand break repair, V(D)J recombination, and transcription by RNA polymerases I and II. In this study, we show that infection of mammalian cells with herpes simplex virus type 1 attenuates DNA-PK activity by specifically depleting the p350/DNA-PKcs catalytic subunit. The half-life of the p350/DNA-PKcs protein decreases from greater than 24 h to less than 4 h following infection. The depletion of DNA-PK activity and p350/DNA-PKcs abundance is dependent on expression of the viral immediate-early protein ICP0. As ICP0 acts as a promoter-independent transactivator of gene expression, these data suggest that ICP0 may function by directly or indirectly targeting the p350/DNA-PKcs subunit of DNA-PK, thereby altering the inhibitory effects of DNA-PK on RNA polymerase II transcription.

The DNA-dependent protein kinase, DNA-PK: 10 years and no ends in sight

The phenomenon of DNA-dependent protein phosphorylation was first described little over 10 years ago. Since then a DNA-dependent protein kinase, DNA-PK, has been purified from human cells and many of its biochemical properties have been characterized. DNA-PK is composed of a large catalytic subunit, DNA-PKcs, and a DNA-targeting protein, Ku. When assembled on a suitable DNA molecule, the DNA-PK holoenzyme acts as a serine/threonine protein kinase that in vitro phosphorylates many DNA binding and non-binding proteins and transcription factors. Recent genetic studies point strongly to functions in DNA double-strand break repair and V(D)J recombination. In addition, biochemical studies suggest a role in the regulation of transcription. Here we discuss, from a historical perspective, the events leading up to our current understanding of the function of DNA-PK, including recent results from our own studies suggesting the involvement of DNA-PK in apoptosis and in viral infection of human cells.

The DNA-dependent protein kinase: a matter of life and (cell) death

The catalytic subunit of the DNA-dependent protein kinase (DNA-PK) is a member of the phosphatidylinositol 3-kinase family. Recent genetic and biochemical studies indicate the involvement of DNA-PK in immunoglobulin/T-cell-receptor gene recombination, double-strand DNA break repair, the stress response and autoimmunity. A role in the suppression of apoptosis could link some of the enzyme's diverse functions.

The DNA-dependent protein kinase is inactivated by autophosphorylation of the catalytic subunit

The DNA-dependent protein kinase (DNA-PK) requires for activity free ends or other discontinuities in the structure of double strand DNA. In vitro, DNA-PK phosphorylates several transcription factors and other DNA-binding proteins and is thought to function in DNA damage recognition or repair and/or transcription. Here we show that in vitro DNA-PK undergoes autophosphorylation of all three protein subunits (DNA-PKcs, Ku p70 and Ku p80) and that phosphorylation correlates with inactivation of the serine/threonine kinase activity of DNA-PK. Significantly, activity is restored by the addition of purified native DNA-PKcs but not Ku, suggesting that inactivation is due to autophosphorylation of DNA-PKcs. Our data also suggest that autophosphorylation results in dissociation of DNA-PKcs from the Ku-DNA complex. We suggest that autophosphorylation is an important mechanism for the regulation of DNA-PK activity.

Autoantibodies to DNA-dependent protein kinase. Probes for the catalytic subunit

DNA-dependent protein kinase (DNA-PK) is an important nuclear enzyme which consists of a catalytic subunit known as DNA-PKcs and a regulatory component identified as the Ku autoantigen. In the present study, we surveyed 312 patients in a search for this specificity. 10 sera immunoprecipitated a large polypeptide which exactly comigrated with DNA-PKcs in SDS-PAGE. Immunoblot analysis demonstrated that this polypeptide was recognizable by a rabbit antiserum specific for DNA-PKcs. Although the patient sera did not bind to biochemically purified DNA-PKcs in immunoblots or ELISA, they were able to deplete DNA-PK catalytic activity from extracts of HeLa cells in a dose-dependent manner. We conclude that these antibodies should be useful probes for studies which aim to define the role of DNA-PK in cells. Since six sera simultaneously contained antibodies to the Ku protein, these studies suggest that relatively intact forms of DNA-PK complex act as autoantigenic particles in selected patients.

Identification of human autoantibodies to transcription factor IIB

We have characterized the ability of various human autoimmune sera to react with RNA polymerase II transcription factors. One serum, which strongly inhibited transcription in a cell-free system, was shown to contain antibodies directed against human TFIIB. The serum did not show reactivity against the other general transcription factors, including human TBP, TFIIE and TFIIF. The inhibition of transcription was directly attributable to depletion of TFIIB activity, as demonstrated by reconstitution of activity with recombinant TFIIB. It has long been recognized that components of the RNA processing machinery are major human autoantigens. The present results show that at least one general transcription factor required for messenger RNA synthesis is an autoantigen as well.

Loss of the catalytic subunit of the DNA-dependent protein kinase in DNA double-strand-break-repair mutant mammalian cells

The DNA-dependent protein kinase (DNA-PK) consists of three polypeptide components: Ku-70, Ku-80, and an approximately 350-kDa catalytic subunit (p350). The gene encoding the Ku-80 subunit is identical to the x-ray-sensitive group 5 complementing gene XRCC5. Expression of the Ku-80 cDNA rescues both DNA double-strand break (DSB) repair and V(D)J recombination in group 5 mutant cells. The involvement of Ku-80 in these processes suggests that the underlying defect in these mutant cells may be disruption of the DNA-PK holoenzyme. In this report we show that the p350 kinase subunit is deleted in cells derived from the severe combined immunodeficiency mouse and in the Chinese hamster ovary cell line V-3, both of which are defective in DSB repair and V(D)J recombination. A centromeric fragment of human chromosome 8 that complements the scid defect also restores p350 protein expression and rescues in vitro DNA-PK activity. These data suggest the scid gene may encode the p350 protein or regulate its expression and are consistent with a model whereby DNA-PK is a critical component of the DSB-repair pathway.

The two faces of a steroid antagonist: when an antagonist isn't

Activation of protein kinase A potentiates the transcriptional response mediated by the glucocorticoid receptor in responsive fibroblasts and in mammary carcinoma cells. This potentiation is ligand-dependent and occurs in responsive fibroblasts and in mammary carcinoma cells. This potentiation is ligand-dependent and occurs without detectable change in the phosphorylation of receptor. The transcriptional response to glucocorticoid or progestin agonists can be blocked by potent antagonists like RU 486. However, upon activation of protein kinase A, the antagonist action of RU 486 on both receptors is blunted. Indeed, RU 486 can itself activate transcription of a hormone-responsive promoter. The conditional agonist activity is observed with type II antagonists, those which recapitulate many of the early steps of ligand-dependent receptor activation, but not type I antagonists, which do not. These studies have now been extended to antimineralocorticoids. In COS-1 cells transfected with a mineralocorticoid receptor expression vector, treatment with 8-BromocAMP potentiates the response to the agonist aldosterone and elicits additional agonist activity in mineralocorticoid antagonists. A model is proposed wherein type II antagonist-receptor complexes occupy receptor binding sites on the genome. The antagonist, however, fails to promote a receptor conformation that can interact productively with a coactivator mediating the communication between receptor and the basal transcription apparatus. Activation of protein kinase A results in the recruitment or activation of a coactivator that permits recovery of receptor-mediated activation function.(ABSTRACT TRUNCATED AT 250 WORDS)