A constitutive heat shock element-binding factor is immunologically identical to the Ku autoantigen

Reactivation of heat-inactivated Ku proteins by heat shock cognate protein HSC73

PURPOSE

Mouse double-stranded DNA-dependent protein kinase (DNA-PK) activity is heat sensitive. Recovery of heat-inactivated DNA repair activity is a problem after combination therapy with radiation and heat. We investigated the mechanism of recovery of heat-inactivated DNA-PK activity.

METHODS

Hybrid cells containing a fragment of human chromosome 8 in scid cells (RD13B2) were used. DNA-PK activity was measured by an in vitro assay. Immunoprecipitation of the nuclear extract was performed with an anti-Ku80 antibody. Proteins co-precipitated with Ku80 were separated by sodium dodecyl sulfate (SDS) polyacrylamide gel electrophoresis and detected by Western blotting using anti-heat shock protein (HSP)72 and anti-heat shock cognate protein (HSC)73 antibodies. HSC73 was overexpressed with the pcDNA3.1 vector. Short hairpin (sh)RNA was used to downregulate HSC73 and HSP72.

RESULTS

The activity of heat-inactivated DNA-PK recovered to about 50% of control during an additional incubation at 37 °C after heat treatment at 44 °C for 15 min in the presence of cycloheximide (which inhibits de novo protein synthesis). Maximal recovery was observed within 3 h of incubation at 37 °C after heat treatment. Constitutively expressed HSC73, which folds newly synthesized proteins, reached maximal levels 3 h after heat treatment using a co-immunoprecipitation assay with the Ku80 protein. Inhibiting HSC73, but not HSP72, expression with shRNA decreased the recovery of DNA-PK activity after heat treatment.

CONCLUSIONS

These results suggest that de novo protein synthesis is unnecessary for recovery of some heat-inactivated DNA-PK. Rather, it might be reactivated by the molecular chaperone activity of HSC73, but not HSP72.

Azadiradione ameliorates polyglutamine expansion disease in Drosophila by potentiating DNA binding activity of heat shock factor 1

Aggregation of proteins with the expansion of polyglutamine tracts in the brain underlies progressive genetic neurodegenerative diseases (NDs) like Huntington's disease and spinocerebellar ataxias (SCA). An insensitive cellular proteotoxic stress response to non-native protein oligomers is common in such conditions. Indeed, upregulation of heat shock factor 1 (HSF1) function and its target protein chaperone expression has shown promising results in animal models of NDs. Using an HSF1 sensitive cell based reporter screening, we have isolated azadiradione (AZD) from the methanolic extract of seeds of Azadirachta indica, a plant known for its multifarious medicinal properties. We show that AZD ameliorates toxicity due to protein aggregation in cell and fly models of polyglutamine expansion diseases to a great extent. All these effects are correlated with activation of HSF1 function and expression of its target protein chaperone genes. Notably, HSF1 activation by AZD is independent of cellular HSP90 or proteasome function. Furthermore, we show that AZD directly interacts with purified human HSF1 with high specificity, and facilitates binding of HSF1 to its recognition sequence with higher affinity. These unique findings qualify AZD as an ideal lead molecule for consideration for drug development against NDs that affect millions worldwide.

Heat shock protein 70 down-regulates the production of toll-like receptor-induced pro-inflammatory cytokines by a heat shock factor-1/constitutive heat shock element-binding factor-dependent mechanism

BACKGROUND

Heat shock protein 70 (Hsp70) is an intracellular chaperone protein with regulatory and cytoprotective functions. Hsp70 can also be found in the extracellular milieu, as a result of active secretion or passive release from damaged cells. The role of extracellular Hsp70 is not fully understood. Some studies report that it activates monocytes, macrophages and dendritic cells through innate immune receptors (such as Toll-like receptors, TLRs), while others report that Hsp70 is a negative regulator of the inflammatory response. In order to address this apparent inconsistency, in this study we evaluated the response of human monocytes to a highly purified recombinant Hsp70.

METHODS

Human peripheral blood monocytes were stimulated with Hsp70, alone or in combination with TLR agonists. Cytokines were quantified in culture supernatants, their mRNAs were measured by RT-PCR, and the binding of transcription factors was evaluated by electrophoretic mobility shift assay (EMSA). Kruskal-Wallis test or one-way or two-way ANOVA were used to analyze the data.

RESULTS

The addition of Hsp70 to TLR-activated monocytes down-regulated TNF-α as well as IL-6 levels. This effect was independent of a physical interaction between Hsp70 and TLR agonists; instead it resulted of changes at the TNF-α gene expression level. The decrease in TNF-α expression correlated with the binding of HSF-1 (heat shock transcription factor 1, a transcription factor activated in response to Hsp70) and CHBF (constitutive HSE-binding factor) to the TNF-α gene promoter.

CONCLUSION

Extracellular Hsp70 negatively regulates the production of pro-inflammatory cytokines of monocytes exposed to TLR agonists and contributes to dampen the inflammatory response.

Difference in the heat sensitivity of DNA‐dependent protein kinase activity among mouse, hamster and human cells

PURPOSE

To examine the heat sensitivity of DNA-dependent protein kinase (DNA-PK) activity in a variety of cultured mouse, hamster and human cell lines.

MATERIALS AND METHODS

Eight cell lines, which have been routinely used in our laboratory, were examined. Cells were heated at 44.0 +/- 0.05 degrees C and DNA-PK activity was measured by a DNA-pull-down assay followed by gel-electrophoresis. Cellular sensitivity to hyperthermia and/or X-ray was evaluated by a colony formation assay.

RESULTS

In mouse FSA1233 and FM3A cells, DNA-PK activity dropped to 15-16% of unheated control after 20 min of heating. In Chinese hamster V79 and CHO-K1 cells, kinase activity did not change appreciably after 20 min treatment but decreased to 60-70 and 22-23% after 40 or 60 min treatment, respectively. However, even after 180 min treatment, DNA-PK activity remained almost intact in human MOLT-4, MKN45 and A7 cells, and decreased only slightly in U937 cells. Hyperthermic radiosensitization was seen even in human cells but, as a trend, it was small compared with rodent cells.

CONCLUSIONS

The heat sensitivity of DNA-PK was clearly different among mouse, hamster and human cells. The results suggested a possibility that the role of DNA-PK inactivation in hyperthermic radiosensitization might be variable, depending on cells, and would reinforce the warning that the direct extrapolation of data from rodent cells might lead to overestimation of the effectiveness of hyperthermia on human cancer.

Hsp70 is required for optimal cell proliferation in mouse A6 mesoangioblast stem cells

Mouse Hsp70 (70 kDa heat shock protein) is preferentially induced by heat or stress stimuli. We previously found that Hsp70 is constitutively expressed in A6 mouse mesoangioblast stem cells, but its possible role in these cells and the control of its basal transcription remained unexplored. Here we report that in the absence of stress, Ku factor is able to bind the HSE (heat shock element) consensus sequence in vitro, and in vivo it is bound to the proximal hsp70 promoter. In addition, we show that constitutive hsp70 transcription depends on the co-operative interaction of different factors such as Sp1 (specificity protein 1) and GAGA-binding protein with Ku factor, which binds the HSE consensus sequence. We used mRNA interference assays to select knockdown cell clones. These cells were able to respond to heat stress by producing a large amount of Hsp70, and produced the same amount of Hsp70 as that synthesized by stressed A6 cells. However, severe Hsp70 knockdown cells had a longer duplication time, suggesting that constitutive Hsp70 expression has an effect on the rate of proliferation.

Purification and identification of positive regulators binding to a novel element in the c-Jun promoter

A putative response element, GAGCCTC, was observed years ago in footprinting analysis of the c-jun promoter, and here we investigate its function in regulating c-jun expression and identify a protein complex that binds there. Electrophoretic mobility shift assays demonstrate a sequence-specific binding complex with this element in HEK293 cells. Additionally, unlabeled consensus AP-1 element DNA, but not a similar NF-jun element DNA, competes with complex formation. Mutations of this element decrease c-jun promoter reporter activity by nearly 5-fold in HEK293 cells. A new, two-step oligonucleotide trapping technique was developed to purify the element binding proteins. LC-nanospray-ESI-MS/MS identification and Western blotting show that the purified complex contains Ku80 and c-jun, which was further confirmed by antibody supershift, by immunoprecipitation with Southwestern blot or with UV cross-linking analysis in vitro as well as chromatin immunoprecipitation in vivo. c-Jun promoter activity and c-jun expression were decreased by Ku80 siRNA introduction. A mutant Ku80 plasmid with normal amino acid sequence but immune to the siRNA recovers c-jun promoter activity from siRNA inhibition. Similarly, Ku70 wild type transfection can also upregulate c-jun promoter activity. Thus, Ku80-c-jun activates c-jun expression by binding to this GAGCCTC element in the c-jun promoter and Ku70 may also serve a role.

Stress response in mesoangioblast stem cells

Stem cells are presumed to survive various stresses, since they are recruited to areas of tissue damage and regeneration, where inflammatory cytokines and cytotoxic cells may result in severe cell injury. We explored the ability of mesoangioblasts to respond to different cell stresses such as heat, heavy metals and osmotic stress, by analyzing heat shock protein (HSP)70 synthesis as a stress indicator. We found that the A6 mesoangioblast stem cells constitutively synthesize HSP70 in a heat shock transcription factor (HSF)-independent way. However, A6 respond to heat shock and cadmium treatment by synthesizing HSP70 over the constitutive expression and this synthesis is HSF1 dependent. The exposure of A6 to copper or to a hypertonic medium does neither induce HSP70 synthesis nor activation of HSF1, while a constitutive binding of constitutive heat shock element binding factor was found. Together, these data suggest that mesoangioblasts constitutively express HSP70 as an 'a priori' activation mechanism, while they maintain the ability to respond to stress stimuli.

Decreased origin usage and initiation of DNA replication in haploinsufficient HCT116 Ku80+/- cells

One of the functions of the abundant heterodimeric nuclear protein, Ku (Ku70/Ku80), is its involvement in the initiation of DNA replication through its ability to bind to chromosomal replication origins in a sequence-specific and cell cycle dependent manner. Here, using HCT116 Ku80+/- cells, the effect of Ku80 deficiency on cell cycle progression and origin activation was examined. Western blot analyses revealed a 75% and 36% decrease in the nuclear expression of Ku80 and Ku70, respectively. This was concomitant with a 33% and 40% decrease in chromatin binding of both proteins, respectively. Cell cycle analysis of asynchronous and late G1 synchronized Ku80+/- cells revealed a prolonged G1 phase. Furthermore, these Ku-deficient cells had a 4.5-, 3.4- and 4.3-fold decrease in nascent strand DNA abundance at the lamin B2, beta-globin and c-myc replication origins, respectively. Chromatin immunoprecipitation (ChIP) assays showed that the association of Ku80 with the lamin B2, beta-globin and c-myc origins was decreased by 1.5-, 2.3- and 2.5-fold, respectively, whereas that of Ku70 was similarly decreased (by 2.1-, 1.5- and 1.7-fold, respectively) in Ku80+/- cells. The results indicate that a deficiency of Ku80 resulted in a prolonged G1 phase, as well as decreased Ku binding to and activation of origins of DNA replication.

Radiosensitization by hyperthermia in the chicken B-lymphocyte cell line DT40 and its derivatives lacking nonhomologous end joining and/or homologous recombination pathways of DNA double-strand break repair

Hyperthermia has a radiosensitizing effect, which is one of the most important biological bases for its use in cancer therapy with radiation. Although the mechanism of this effect has not been clarified in molecular terms, possible involvement of either one or both of two major DNA double-strand break (DSB) repair pathways, i.e. nonhomologous end joining (NHEJ) and homologous recombination (HR), has been speculated. To test this possibility, we examined cells of the chicken B-lymphocyte cell line DT40 and its derivatives lacking NHEJ and/or HR: KU70(-/-), DNA-PKcs(-/-/-), RAD54(-/-) and KU70(-/-)/RAD54(-/-). Radiosensitization by hyperthermia could be seen in all of the mutants, including KU70(-/-)/RAD54(-/-), which lacked both NHEJ and HR. Therefore, radiosensitization by hyperthermia cannot be explained simply by its inhibitory effects, if any, on NHEJ and/or HR alone. However, in NHEJ-defective KU70(-/-) and DNA-PKcs(-/-/-), consisting of two subpopulations with distinct radiosensitivity, the radiosensitive subpopulation, which is considered to be cells in G(1) and early S, was not sensitized. Substantial sensitization was seen only in the radioresistant subpopulation, which is considered to be cells in late S and G(2), capable of repairing DSBs through HR. This observation did not exclude possible involvement of NHEJ in G(1) and early S phase and also suggested inhibitory effects of hyperthermia on HR. Thus partial contribution of NHEJ and HR in radiosensitization by hyperthermia, especially that depending on the cell cycle stage, remains to be considered.

The Ku protein complex interacts with YY1, is up-regulated in human heart failure, and represses alpha myosin heavy-chain gene expression

Human heart failure is accompanied by repression of genes such as alpha myosin heavy chain (alphaMyHC) and SERCA2A and the induction of fetal genes such as betaMyHC and atrial natriuretic factor. It seems likely that changes in MyHC isoforms contribute to the poor contractility seen in heart failure, because small changes in isoform composition can have a major effect on the contractility of cardiac myocytes and the heart. Our laboratory has recently shown that YY1 protein levels are increased in human heart failure and that YY1 represses the activity of the human alphaMyHC promoter. We have now identified a region of the alphaMyHC promoter that binds a factor whose expression is increased sixfold in failing human hearts. Through peptide mass spectrometry, we identified this binding activity to be a heterodimer of Ku70 and Ku80. Expression of Ku represses the human alphaMyHC promoter in neonatal rat ventricular myocytes. Moreover, overexpression of Ku70/80 decreases alphaMyHC mRNA expression and increases skeletal alpha-actin. Interestingly, YY1 interacts with Ku70 and Ku80 in HeLa cells. Together, YY1, Ku70, and Ku80 repress the alphaMyHC promoter to an extent that is greater than that with YY1 or Ku70/80 alone. Our results suggest that Ku is an important factor in the repression of the human alphaMyHC promoter during heart failure.

Differential DNA binding of Ku antigen determines its involvement in DNA replication

Ku antigen (Ku70/Ku80) is a regulatory subunit of DNA-dependent protein kinase, which participates in the regulation of DNA replication and gene transcription through specific DNA sequences. In this study, we have compared the mechanism of action of Ku from A3/4, a DNA sequence that appears in mammalian origins of DNA replication, and NRE1, a transcriptional regulatory element in the long terminal repeat of mouse mammary tumor virus through which Ku antigen and its associated kinase, DNA-dependent protein kinase (DNA-PK(cs)), act to repress steroid-induced transcription. Our results indicate that replication from a minimal replication origin of ors8 is independent of DNA-PK(cs) and that Ku interacts with A3/4-like sequences and NRE1 in fundamentally different ways. UV crosslinking experiments revealed differential interactions of the Ku subunits with A3/4, NRE1, and two other proposed Ku transcriptional regulatory elements. In vitro footprinting experiments showed direct contact of Ku on A3/4 and over the region of ors8 homologous to A3/4. In vitro replication assays using ors8 templates bearing mutations in the A3/4-like sequence suggested that Ku binding to this element was necessary for replication. By contrast, in vitro replication experiments revealed that NRE1 was not involved in DNA replication. Our results establish A3/4 as a new class of Ku DNA binding site. Classification of Ku DNA binding into eight categories of interaction based on recognition and DNA crosslinking experiments is discussed.

Nuclear proteins that bind to metal response element a (MREa) in the Wilson disease gene promoter are Ku autoantigens and the Ku-80 subunit is necessary for basal transcription of the WD gene

Wilson disease (WD), an inherited disorder affecting copper metabolism, is characterized by hepatic cirrhosis and neuronal degeneration, which result from toxic levels of copper that accumulate in the liver and brain, respectively. We reported previously that the approximately 1.3-kb promoter of the WD gene contains four metal response elements (MREs). Among the four MREs, MREa plays the most important role in the transcriptional activation of the WD promoter. Electrophoretic mobility shift assays (EMSAs) using synthetic MREa and an oligonucleotide containing the binding site for transcription factor Sp1 revealed the presence of nuclear factors that bind specifically to MREa. Two MREa-binding proteins of 70 and 82 kDa were purified using avidin-biotin affinity chromatography. Amino acid sequences of peptides from each protein were found to be highly homologous to the Ku proteins. Immunoblot analysis and EMSAs showed that the MREa-binding proteins are immunologically related to the Ku proteins. To study further the functional significance of these Ku-related proteins in transcriptional regulation of the WD gene, we performed RNA interference (RNAi) assays using a Ku-80 inverted-repeat gene to inhibit expression of the Ku-80 gene in vivo. Results of the RNAi assays showed that expression of the Ku-80 protein was suppressed in transfected cells, which in turn led to the suppression of the WD gene. In addition, a truncated Ku-80 (DeltaKu-80) mutant inhibited WD promoter activity in HepG2 cells in a dominant-negative manner. We also found that WD promoter activity was decreased in Xrs5 cells, which, unlike the CHO-K1 cells, are defective in the Ku-80 protein. When Ku-80 cDNA was transfected into Xrs5 and CHO cells, WD promoter activity was recovered only in Xrs5 cells. Taken together, our findings suggest that the Ku-80 subunit is required for constitutive expression of the WD gene.

Ku represses the HIV-1 transcription: identification of a putative Ku binding site homologous to the mouse mammary tumor virus NRE1 sequence in the HIV-1 long terminal repeat

Ku has been implicated in nuclear processes, including DNA break repair, transcription, V(D)J recombination, and telomere maintenance. Its mode of action involves two distinct mechanisms: one in which a nonspecific binding occurs to DNA ends and a second that involves a specific binding to negative regulatory elements involved in transcription repression. Such elements were identified in mouse mammary tumor virus and human T cell leukemia virus retroviruses. The purpose of this study was to investigate a role for Ku in the regulation of human immunodeficiency virus (HIV)-1 transcription. First, HIV-1 LTR activity was studied in CHO-K1 cells and in CH0-derived xrs-6 cells, which are devoid of Ku80. LTR-driven expression of a reporter gene was significantly increased in xrs-6 cells. This enhancement was suppressed after re-expression of Ku80. Second, transcription of HIV-1 was followed in U1 human cells that were depleted in Ku by using a Ku80 antisense RNA. Ku depletion led to a increase of both HIV-1 mRNA synthesis and viral production compared with the parent cells. These results demonstrate that Ku acts as a transcriptional repressor of HIV-1 expression. Finally, a putative Ku-specific binding site was identified within the negative regulatory region of the HIV-1 long terminal repeat, which may account for this repression of transcription.

Analysis of gene transcription in cells lacking DNA-PK activity

The DNA-dependent protein kinase (DNA-PK), comprised of the Ku70/Ku80 (now known as G22p1/Xrcc5) heterodimer and the catalytic subunit DNA-PKcs (now known as Prkdc), is required for the nonhomologous end joining (NHEJ) pathway of DNA double-strand break repair. The mechanism of action of DNA-PK remains unclear. We have investigated whether DNA-PK regulates gene transcription in vivo after DNA damage using the subtractive hybridization technique of cDNA representational difference analysis (cDNA RDA). Differential transcription, both radiation-dependent and independent, was detected and confirmed in primary mouse embryo fibroblasts from DNA-PKcs(-/-) and DNA-PKcs(+/+) mice. We present evidence that transcription of the extracellular matrix gene laminin alpha 4 (Lama4) is regulated by DNA-PK in a radiation-independent manner. However, screening of both primary and immortalized DNA-PKcs-deficient cell lines demonstrates that the majority of differences were not consistently dependent on DNA-PK status. Similar results were obtained in experiments using KU mutant hamster cell lines, indicating heterogeneity of transcription between closely related cell lines. Our results suggest that while DNA-PK may be involved in limited gene-specific transcription, it does not play a major role in the transcriptional response to DNA damage.

Enhanced sensitivity and long-term G2 arrest in hydrogen peroxide-treated Ku80-null cells are unrelated to DNA repair defects

While the Ku complex, comprised of Ku70 and Ku80, is primarily involved in the repair of DNA double-strand breaks, it is also believed to participate in additional cellular processes. Here, treatment of embryo fibroblasts (MEFs) derived from either wild-type or Ku80-null (Ku80(-/-)) mice with various stress agents revealed that hydrogen peroxide (H(2)O(2)) was markedly more cytotoxic for Ku80(-/-) MEFs and led to their long-term accumulation in the G2 phase. This differential response was not due to differences in DNA repair, since H(2)O(2)-triggered DNA damage was repaired with comparable efficiency in both Wt and Ku80(-/-) MEFs, but was associated with differences in the expression of important cell cycle regulatory genes. Our results support the notion that Ku80-mediated cytoprotection and G2-progression are not only dependent on the cell's DNA repair but also may reflect Ku80's influence on additional cellular processes such as gene expression.

Thermotolerant desert lizards characteristically differ in terms of heat-shock system regulation

We compare the properties and activation of heat-shock transcription factor (HSF1) and the synthesis of a major family of heat-shock proteins (HSP70) in lizard species inhabiting ecological niches with strikingly different thermal parameters. Under normal non-heat-shock conditions, all desert-dwelling lizard species studied so far differ from a northern, non-desert species (Lacerta vivipara) in the electrophoretic mobility and content of proteins constitutively bound to the regulatory heat-shock elements in the heat-shock gene promoter. Under these conditions, levels of activated HSF1 and of both HSP70 mRNA and protein are higher in the desert species than in the non-desert species. Upon heat shock, HSF1 aggregates in all species studied, although in desert species HSF1 subsequently disaggregates more rapidly. Cells of the northern species have a lower thermal threshold for HSP expression than those of the desert species, which correlates with the relatively low constitutive level of HSPs and high basal content of HSF1 in their cells.

Hypermethylation of metallothionein-I promoter and suppression of its induction in cell lines overexpressing the large subunit of Ku protein

We have shown previously that the heavy metal-induced metallothionein-I (MT-I) gene expression is specifically repressed in a rat fibroblast cell line (Ku-80) overexpressing the 80-kDa subunit of Ku autoantigen but not in cell lines overexpressing the 70-kDa subunit or Ku heterodimer. Here, we explored the molecular mechanism of silencing of MT-I gene in Ku-80 cells. Genomic footprinting analysis revealed both basal and heavy metal-inducible binding at specific cis elements in the parental cell line (Rat-1). By contrast, MT-I promoter in Ku-80 cells was refractory to any transactivating factors, implying alteration of chromatin structure. Treatment of two clonal lines of Ku-80 cells with 5-azacytidine, a potent DNA demethylating agent, rendered MT-I gene inducible by heavy metals, suggesting that the gene is methylated in these cells. Bisulfite genomic sequencing revealed that all 21 CpG dinucleotides in MT-I immediate promoter were methylated in Ku-80 cells, whereas only four CpG dinucleotides were methylated in Rat-1 cells. Almost all methylated CpG dinucleotides were demethylated in Ku-80 cells after 5-azacytidine treatment. To our knowledge, this is the first report that describes hypermethylation of a specific gene promoter and its resultant silencing in response to overexpression of a cellular protein.

Proteasome inhibitors lactacystin and MG132 inhibit the dephosphorylation of HSF1 after heat shock and suppress thermal induction of heat shock gene expression

Recently, we have shown that two proteasome inhibitors, MG132 and lactacystin, induce hyperphosphorylation and trimerization of HSF1, and transactivate heat shock genes at 37 degrees C. Here, we examined the effects of these proteasome inhibitors and, in addition, a phosphatase inhibitor calyculin A (CCA) on the activation of HSF1 upon heat shock and during post-heat-shock recovery, with emphasis on HSF1 hyperphosphorylation and the ability of HSF1 to transactivate heat shock genes. When lactacystin, MG132, or CCA was present after heat shock, HSF1 remained hyperphosphorylated during post-heat-shock recovery at 37 degrees C. Failure of HSF1 to recover to its preheated dephosphorylated state correlated well with the suppression of the heat-induced hsp70 expression. In vitro, HSF1 from heat-shocked cells, when dephosphorylated, showed an increase in HSE-binding affinity. Taken together, these data suggest that phosphorylation of HSF1 plays an important role in the negative regulation of heat-shock response. Specifically, during post-heat-shock recovery phase, prolonged hyperphosphorylation of HSF1 suppresses heat-induced expression of heat shock genes.

The short 5' untranslated region of the betaA3/A1-crystallin mRNA is responsible for leaky ribosomal scanning

Leaky ribosomal scanning allows the expression of multiple proteins from a single mRNA by occasionally skipping the first start codon, and initiating translation at a subsequent one. BetaA3- and betaA1-crystallin, two members of the beta-crystallin family of vertebrate eye lens proteins, are produced via this mechanism, of which, until now, only very few examples have been found in eukaryotic genes. Since the two start codons on the betaA3/A1 messenger lie in the same reading frame, the two translated proteins are identical, except for the 17 residues shorter N-terminal extension of betaA1-crystallin. It has been suggested that the very short leader (5-7 nucleotides) of the betaA3/A1 messenger might cause slippage at the first start codon, although the unfavorable context of this start codon might also be responsible. Using transient transfections, we now demonstrate that increasing the length of the leader sequence to 67 nucleotides indeed completely abolishes translation initiation at the second start codon, and thus expression of the betaA1-crystallin protein. Messengers having a leader of 5, 7 or 14 nucleotides all express both betaA3- and betaA1-crystallin at very similar relative levels.

The nuclear localization signal of the human Ku70 is a variant bipartite type recognized by the two components of nuclear pore-targeting complex

Ku protein is a complex of two subunits, Ku70 and Ku80. Ku is suspected to participate in both DNA double-strand break repair and transcription. Since both of these processes take place in the cell nucleus, we have been investigating the subcellular localization and nuclear transport of Ku proteins. In the present study, we analyzed the subcellular localization and nuclear localization signal (NLS) of Ku70. Fusion proteins of Ku70 and green fluorescent protein (GFP) transiently expressed in cells were clearly localized in the nuclei of interphase cells. Ku70 staining was distributed throughout both the nucleus and the cytoplasm in late telophase to early G1 phase cells. The NLS of Ku70 was located at the region composed of 18 amino acid residues (positions 539 to 556). This region overlapped with the Ku80-independent DNA-binding domain reported previously. The Ku70 NLS consisted of two basic subregions and a nonbasic intervening region. All the subregions were necessary for complete NLS activity. The amino acids in the nonbasic intervening region of Ku70 might be important for full NLS activity not only to provide sufficient length between the two separated clusters of basic amino acids but also to have an adequate amino acid sequence. All of the basic amino acid residues in the basic subregions were conserved among mammalian and avian homologues, confirming their importance in the nuclear translocation of Ku70. The structure of the Ku70 NLS resembled the consensus of a bipartite-type NLS. The Ku70 NLS was mediated to target to the nuclear rim by two components of the nuclear pore-targeting complex, PTAC58 and PTAC97.

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.

Proteasome inhibitors MG132 and lactacystin hyperphosphorylate HSF1 and induce hsp70 and hsp27 expression

MG132 and lactacystin, two 26S proteasome-specific protease inhibitors, can upregulate heat-shock gene transcription without heat shock. In this study, we showed that both of these inhibitors induce hyperphosphorylation and DNA-binding activity of HSF1 in the absence of heat shock (at 37 degreesC). Since trimerization of HSF1 is known to precede the acquisition of HSF1-DNA binding activity, it seems that MG132- and lactacystin-induced hyperphosphorylation of HSF1 causes conformational changes of HSF1 molecules at 37 degreesC and subsequently triggers its trimerization. Inhibition of protein synthesis by cycloheximide abolished the MG132- or lactacystin-induced hyperphosphorylation and DNA-binding activity of HSF1. These data suggest that the activity of a putative kinase(s) targeting HSF1 is upregulated in the presence of MG132 or lactacystin. The upregulation of the kinase activity requires de novo protein synthesis and is likely due to the inhibition of protein degradation of a short-lived, kinase(s) targeting HSF1 and/or the cofactor(s) for the kinases, through the ubiquitin-proteasome pathway.

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.

Subcellular localization and protein-protein interaction regions of Ku proteins

The Ku protein is a complex of Ku70 and Ku80 subunits and is capable of binding promoters in a sequence-specific manner, although it remains unclear whether Ku is involved in transcriptional regulation. We examined the subcellular localization and determined the interaction regions of Ku. Our results indicate that heterodimers of Ku70 and Ku80 are localized in the nucleus, and that the stretches from amino acid (aa) 378 to 482 of Ku70 and from aa 374 to 502 of Ku80 are necessary for heterodimerization. These interaction regions do not contain any previously recognized protein-protein interaction motifs. To determine whether Ku contains a potential transcriptional activation domain, we examined N- and C-terminal deletion mutants of Ku70 and Ku80 for their ability to activate transcription in the GAL4-based one-hybrid system. We found that the whole Ku protein had no transcriptional activity, although the N-terminal peptide fragment of Ku70 was capable of activating transcription of the HIS3 and lacZ reporter genes in yeast cells.

Evidence for a hsp25-specific mechanism involved in transcriptional activation by heat shock

Transcriptional stimulation of heat shock genes is generally due to the activation of heat shock transcription factor (HSF) 1. We demonstrate that in the murine leucemic cell line, P388, trimerization of HSF1, transcriptional activation of the hsp70 gene, and expression of Hsp70 are achieved as a result of heat shock. In contrast, the small heat shock proteins Hsp25 and alpha B-crystallin are not expressed in these cells and cannot be induced upon heat shock. Furthermore, no hsp25 transcript can be detected, indicating that there is a defect in the hsp25 gene or a block in its expression. Comparison of the hsp25 gene structure between P388 and Hsp25-expressing Ehrlich ascites tumor (EAT) cells by Southern blot analysis revealed no differences in the structural organization of the gene and no changes in its localization in the genome. However, sequence analysis of the hsp25 promoter region in P388 cells demonstrates minor differences. Despite these differences, the hsp25 promoter from P388 cells mediates heat shock-induced activation of a reporter gene when transfected into human HeLa cells which is comparable to that of the hsp25 promoter from EAT cells. Furthermore, the hsp25 gene isolated from EAT or P388 cells can both be expressed in HeLa cells and lead to a similar heat shock-stimulated accumulation of Hsp25. Silencing of the hsp25 and alpha B-crystallin genes in P388 cells by DNA-methylation could also be excluded since 5-azacytidine treatment does not influence expression of these genes. Interestingly, when expressed in P388 cells the hsp25 promoter from EAT cells is not activated upon heat shock, whereas the human hsp70 promoter is activated. Taken together, the data suggest cell line-specific differences in a mechanism of regulation of hsp25 transcription, which interferes with the activation of the promoter by HSF1 and which may also affect the alpha B-crystallin gene.

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.

Analysis of DNA Binding Proteins Associated With Hemin-Induced Transcriptional Inhibition. The Hemin Response Element Binding Protein Is a Heterogeneous Complex That Includes the Ku Protein

Hemin inhibits transcription of the tartrate resistant acid phosphatase (TRAP) gene. Using deletion mutagenesis of the mouse TRAP 5′-flanking region, we previously identified a 27-bp DNA segment containing a central GAGGC tandem repeat sequence (the hemin response element [HRE]), which bound nuclear proteins (hemin response element binding proteins [HREBPs]) from hemin-treated cells and appeared to be responsible for mediating transcriptional inhibition in response to hemin. We now have used affinity binding to HRE-derivatized beads to identify four HREBP components with apparent molecular masses of 133-, 90-, 80-, and 37-kD, respectively. The 80- and 90-kD components correspond to the p70 and p80/86 subunits of Ku antigen (KuAg) as documented by partial amino acid microsequencing of tryptic digests and immunologic reactivity. Based on reactivity of the HREBP gel shift band with antibodies to the redox factor protein (ref1) in shift Western experiments, it is shown that the 37-kD component represents ref1. The 133-kD component appeared to be a unique protein. KuAg participation in HREBP complexes was specific as it was present in HREBPs bound to HRE microcircles. Results of depletion/reconstitution experiments suggested that KuAg does not bind alone or directly to HRE DNA, but does so only in conjunction with the 133- and/or 37-kD proteins. We conclude that HREBP is a heterogeneous complex composed of KuAg, ref1, and a unique 133-kD protein. We speculate that the role of heme may be to promote interactions among these components, thereby facilitating HRE binding and downregulation of hemin responsive genes.

Double-strand break repair mediated by DNA end-joining

DNA double-strand breaks formed by ionizing irradiation or other stresses are repaired by homologous recombination or DNA end-joining. This review focuses on the mechanism of double-strand break repair mediated by DNA end-joining, in which many factors have recently been identified. After DNA double-strand breakage, DNA end-joining takes place between the DNA ends that have nonhomologous sequences or very short regions ofhomology. The broken DNA is repaired if the DNA end-joining occurs in the same molecule, while it causes chromosome aberrations such as deletions, insertions, translocations and inversions if it occurs between different molecules. Rad50 and its relatives, Ku-proteins, DNA ligase VI and silencing factors, are involved in DNA end-joining in yeast and mammalian cells. These findings led us to propose a model in which the formation of a heterochromatin-like complex at broken ends is an important element in DNA end-joining.

Analysis of DNA Binding Proteins Associated With Hemin-Induced Transcriptional Inhibition. The Hemin Response Element Binding Protein Is a Heterogeneous Complex That Includes the Ku Protein

Hemin inhibits transcription of the tartrate resistant acid phosphatase (TRAP) gene. Using deletion mutagenesis of the mouse TRAP 5′-flanking region, we previously identified a 27-bp DNA segment containing a central GAGGC tandem repeat sequence (the hemin response element [HRE]), which bound nuclear proteins (hemin response element binding proteins [HREBPs]) from hemin-treated cells and appeared to be responsible for mediating transcriptional inhibition in response to hemin. We now have used affinity binding to HRE-derivatized beads to identify four HREBP components with apparent molecular masses of 133-, 90-, 80-, and 37-kD, respectively. The 80- and 90-kD components correspond to the p70 and p80/86 subunits of Ku antigen (KuAg) as documented by partial amino acid microsequencing of tryptic digests and immunologic reactivity. Based on reactivity of the HREBP gel shift band with antibodies to the redox factor protein (ref1) in shift Western experiments, it is shown that the 37-kD component represents ref1. The 133-kD component appeared to be a unique protein. KuAg participation in HREBP complexes was specific as it was present in HREBPs bound to HRE microcircles. Results of depletion/reconstitution experiments suggested that KuAg does not bind alone or directly to HRE DNA, but does so only in conjunction with the 133- and/or 37-kD proteins. We conclude that HREBP is a heterogeneous complex composed of KuAg, ref1, and a unique 133-kD protein. We speculate that the role of heme may be to promote interactions among these components, thereby facilitating HRE binding and downregulation of hemin responsive genes.

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.

Ku70 is required for DNA repair but not for T cell antigen receptor gene recombination In vivo

Ku is a complex of two proteins, Ku70 and Ku80, and functions as a heterodimer to bind DNA double-strand breaks (DSB) and activate DNA-dependent protein kinase. The role of the Ku70 subunit in DNA DSB repair, hypersensitivity to ionizing radiation, and V(D)J recombination was examined in mice that lack Ku70 (Ku70(-/-)). Like Ku80(-/-) mice, Ku70(-/-) mice showed a profound deficiency in DNA DSB repair and were proportional dwarfs. Surprisingly, in contrast to Ku80(-/-) mice in which both T and B lymphocyte development were arrested at an early stage, lack of Ku70 was compatible with T cell receptor gene recombination and the development of mature CD4+CD8- and CD4-CD8+ T cells. Our data shows, for the first time, that Ku70 plays an essential role in DNA DSB repair, but is not required for TCR V(D)J recombination. These results suggest that distinct but overlapping repair pathways may mediate DNA DSB repair and V(D)J recombination.

Temperature compensation of the circadian period length--a special case among general homeostatic mechanisms of gene expression?

In Neurospora crassa, as well as in other organisms, the expression of housekeeping genes is transiently suppressed after exposure to higher temperatures (30-45 degrees C); expression is then reactivated and adapts after a few-hours to values closer to the initial rates. Adaptive mechanisms apparently exist in the processes of transcription, RNA processing, and translation and render protein synthesis rates temperature compensated. Heat shock proteins (HSPs) play an important role within these mechanisms ("acquired thermotolerance of protein synthesis"), but their function is as yet not exactly known. Adaptive mechanisms seem also to involve intracellular ion changes after exposure to moderate temperature elevation. The expression of heat shock genes is transiently enhanced after exposure to higher temperatures and also adapts after a few hours. The adaptation mechanism includes inactivation of the heat shock transcription factor (HSF) by means of phosphorylation changes and possibly by binding of a gene product (HSP70)-a mechanism representing a negative feedback control. These examples demonstrate the existence of general adaptive mechanisms at different levels of gene expression that may also be at work in the temperature compensation of clock gene expression. Apart from such adaptation processes, antagonistic reactions within the processes of gene expression and protein modification might be equally enhanced or suppressed by temperature changes, leaving the equilibrium unaffected or balanced (antagonistic balance, see Ruoff et al., this issue of Chronobiology International). This principle is shown to apply to the effect of temperature elevation on total protein synthesis and degradation. It may also apply to other antagonistic processes such as phosphorylation-dephosphorylation or monomer-dimer formation. The circadian clock mechanism is assumed to consist of several processes that can either adapt or produce a balance. Single amino acid changes in a clock protein are assumed to partially upset this adaptation or balance.

DNA looping by Ku and the DNA-dependent protein kinase

The DNA-dependent protein kinase (DNA-PK) is required for DNA double-strand break (DSB) repair and immunoglobulin gene rearrangement and may play a role in the regulation of transcription. The DNA-PK holoenzyme is composed of three polypeptide subunits: the DNA binding Ku70/86 heterodimer and an approximately 460-kDa catalytic subunit (DNA-PKcs). DNA-PK has been hypothesized to assemble at DNA DSBs and play structural as well as signal transduction roles in DSB repair. Recent advances in atomic force microscopy (AFM) have resulted in a technology capable of producing high resolution images of native protein and protein-nucleic acid complexes without staining or metal coating. The AFM provides a rapid and direct means of probing the protein-nucleic acid interactions responsible for DNA repair and genetic regulation. Here we have employed AFM as well as electron microscopy to visualize Ku and DNA-PK in association with DNA. A significant number of DNA molecules formed loops in the presence of Ku. DNA looping appeared to be sequence-independent and unaffected by the presence of DNA-PKcs. Gel filtration of Ku in the absence and the presence of DNA indicates that Ku does not form nonspecific aggregates. We conclude that, when bound to DNA, Ku is capable of self-association. These findings suggest that Ku binding at DNA DSBs will result in Ku self-association and a physical tethering of the broken DNA strands.

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.

Hyperphosphorylation of heat shock transcription factor 1 is correlated with transcriptional competence and slow dissociation of active factor trimers

In the course of its activation by heat and other stresses, the inactive monomer of human heat shock transcription factor 1 (HSF1) is converted to a DNA-binding homotrimer and is hyperphosphorylated. At least four Ser/Thr residues in HSF1 appeared to be inducibly phosphorylated during heat shock. Ser/Thr protein kinase inhibitors inhibited, and protein phosphatase inhibitor calyculin A and phorbol ester enhanced, hsp70-CAT reporter gene expression but not heat shock element DNA binding activity in HeLa cells undergoing a moderate heat shock. Calyculin A (5-20 nM) caused hyperphosphorylation of HSF1, the extent of which was comparable to that produced by moderate to severe heat shock. Upon recovery from a 42 degrees C/30 min-heat shock, HSF1 trimers disassembled quantitatively within 2 h. Calyculin A interfered with the dissociation of HSF1 trimers. Thus, hyperphosphorylation increases the effective half-life of the HSF1 trimer, which may prolong factor activity subsequent to heat shock. Hyperphosphorylation also dramatically stimulated the transactivation function of HSF1: exposure to calyculin A of cells induced to form inactive HSF1 trimers resulted in the conversion of the inactive to active trimers. Given that deletion of certain sequences renders HSF1 constitutively active, these results suggested that the activation of HSF1 trimers by calyculin A was a consequence of hyperphosphorylation of HSF1 rather than of a downstream factor.

Evidence for the involvement of mouse heat shock factor 1 in the atypical expression of the HSP70.1 heat shock gene during mouse zygotic genome activation

The mouse HSP70.1 gene, which codes for a heat shock protein (hsp70), is highly transcribed at the onset of zygotic genome activation (ZGA). This expression, which occurs in the absence of stress, is then repressed. It has been claimed that this gene does not exhibit a stress response until the blastocyst stage. The promoter of HSP70.1 contains four heat shock element (HSE) boxes which are the binding sites of heat shock transcription factors (HSF). We have been studying the presence and localization of the mouse HSFs, mHSF1 and mHSF2, at different stages of embryo development. We show that mHSF1 is already present at the one-cell stage and concentrated in the nucleus. Moreover, by mutagenizing HSE sequences and performing competition experiments (in transgenic embryos with the HSP70.1 promoter inserted before a reporter gene), we show that, in contrast with previous findings, HSE boxes are involved in this spontaneous activation. Therefore, we suggest that HSF1 and HSE are important in this transient expression at the two-cell stage and that the absence of typical inducibility at this early stage of development results mainly from the high level of spontaneous transcription of this gene during the ZGA.

Ku-related antigens are associated with transcriptionally active loci in Chironomus polytene chromosomes

Antigens of Chironomus reactive with human sera containing anti-Ku antibodies and also with specific antibodies to each Ku subunit were characterized by immunoblot analysis. Three main antigen species were identified in nuclear-enriched extracts from salivary gland cells of Chironomus thummi, ranging in Mr from 55000 to 67000. The nuclear localization of Ku-related antigens in the dipteran Chironomus was studied by immunofluorescent labeling in polytene chromosomes of the salivary glands. Balbiani rings, loci highly active in transcription, were found to be strongly labeled by anti-Ku antibodies. Sugar-induced changes in the activity of the Balbiani ring genes were accompanied by the redistribution of Ku-related antigens as visualized by their absence in regressed Balbiani ring loci, and continued presence only in those that were transcriptionally active. A drastic change in the distribution of Ku-related antigens was also observed when C. thummi larvae underwent heat treatment as the immunofluorescent staining was restricted to previously described heat shock puffs. Anti-Ku sera reacted in addition with several chromosomal bands in which the presence of RNA polymerase II was also immunologically detected. The results show that Chironomus antigens reactive with anti-Ku antibodies are related to transcription in polytene chromosomes.

Ku86-deficient mice exhibit severe combined immunodeficiency and defective processing of V(D)J recombination intermediates

Ku is a heterodimeric DNA end binding complex composed of 70 and 86 kDa subunits. Here, we show that Ku86 is essential for normal V(D)J recombination in vivo, as Ku86-deficient mice are severely defective for formation of coding joints. Unlike severe combined immunodeficient (scid) mice, Ku86-deficient mice are also defective for signal joint formation. Both hairpin coding ends and blunt full-length signal ends accumulate. Contrary to expectation, Ku86 is evidently not required for protection of either type of V(D)J recombination intermediate. Instead, V(D)J recombination appears to be arrested after the cleavage step in Ku86-deficient mice. We suggest that Ku86 may be required to remodel or disassemble DNA-protein complexes containing broken ends, making them available for further processing and joining.

Modulation of thermal induction of hsp70 expression by Ku autoantigen or its individual subunits

Previously, we proposed a dual control mechanism for the regulation of the heat shock response in mammalian cells: a positive control mediated by the heat shock transcription factor HSF1 and a negative control mediated by the constitutive heat shock element-binding factor (CHBF). To study the physiological role of CHBF in the regulation of heat shock response, we purified CHBF to apparent homogeneity and showed it to be identical to the Ku autoantigen, a heterodimer consisting of 70-kDa (Ku-70) and 86-kDa (Ku-80) polypeptides. To study further the functional significance of Ku/CHBF in the cellular response to heat shock, we established rodent cell lines that stably and constitutively overexpressed one or both subunits of the human Ku protein, and examined the thermal induction of hsp70 and other heat shock proteins in these Ku-overexpressing ing cells. We show that expression of the human Ku-70 and Ku-80 subunits jointly or of the Ku-70 subunit alone specifically inhibits heat-induced hsp70 expression. Conversely, expression of human Ku-80 alone does not have this effect. Thermal induction of other heat shock proteins in all of the Ku-overexpressing cell lines appears not to be significantly affected, nor is the state of phosphorylation or the DNA-binding ability of HSF1 affected. These findings support a model in which hsp70 expression is controlled by a second regulatory factor in addition to the positive activation of HSF1. The Ku protein, specifically the Ku-70 subunit, is involved in the regulation of hsp70 gene expression.

Cloning and characterization of the promoter for murine 84-kDa heat-shock protein

The 90-kDa heat-shock (HS) proteins (HSP90) are members of the HSP family. Their synthesis is inducible by HS and a variety of stress signals. HSP90 is also abundant under normal physiological conditions and its synthesis can be regulated during growth and differentiation. Therefore, HSP90 is speculated to have important biological functions, in addition to its role in mediating stress responses. However, the mechanism(s) regulating hsp90 gene expression in nonstressed cells is poorly understood. As a prerequisite towards understanding the basis for hsp90 regulation, we have cloned and characterized the 5' flanking region of murine hsp84, one of two genes which code for HSP90 proteins. Full basal promoter activity of hsp84 was found to be associated with a 627-bp region immediately upstream from the transcription start point (tsp). Sequence analysis revealed several putative regulatory elements, including a HS element (HSE), an AP1-binding site (AP1), a cyclic AMP response element (CRE), and four stimulatory protein-1-binding sites (SP1). HS inducibility required the HSE which was bound by HS transcription factor-1(HSF-1) present in extracts prepared from cells exposed to HS. The HSE was not required for basal (non-HS) expression, but, interestingly, two protein-HSE complexes, devoid of HSF-1 and HSF-2, were formed under these conditions. The potential significance of these findings to the expression of hsp84 under normal physiological conditions is discussed.

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 interaction between Ku antigen and REF1 protein mediates negative gene regulation by extracellular calcium

Through the specific binding of a negative calcium-responsive element to its binding protein in response to extracellular Ca (Ca2+e), negative calcium-responsive element-bearing genes, such as the human parathyroid hormone gene, are negatively regulated by Ca2+e. The Ku antigen mediated negative gene regulation by Ca2+e by interacting with a redox factor protein, REF1. Although sequence-nonspecific DNA binding activity of the Ku antigen has been well characterized, the mechanism of its sequence-specific DNA binding remained obscure. Here, we report that the specific binding of the Ku antigen to another protein, REF1, leads to DNA-protein complex formation with a novel sequence specificity and thereby regulates gene expression.

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.

Thermotolerance and heat shock proteins: possible involvement of Ku autoantigen in regulating Hsp70 expression

Here we characterize and compare the phenomenon of thermotolerance and permanent heat resistance in mammalian cells. The biochemical and molecular mechanisms underlying the induction of thermotolerance, and the role that heat shock proteins play in its development and decay are discussed. Finally, we describe a novel constitutive HSE-binding factor (CHBF/Ku) that appears to be involved in the regulation of the heat shock response.