Hsp90α Recruited by Sp1 Is Important for Transcription of 12(S)-Lipoxygenase in A431 Cells

The downregulation of HSP90-controlled CRALBP expression is associated with age-related vision attenuation

The dysfunction of CRALBP, a key regulator of the visual cycle, is associated with retinitis punctata albescens characterized by night vision loss and retinal degeneration. In this paper, we find that the expression of CRALBP is regulated by heat shock protein 90 (HSP90). Inhibition of HSP90α or HSP90β expression by using the CRISPR-Cas9 technology downregulates CRALBP's mRNA and protein expression in ARPE-19 cells by triggering the degradation of transcription factor SP1 in the ubiquitin-proteasome pathway. SP1 can bind to CRALBP's promoter, and inhibition of SP1 by its inhibitor plicamycin or siRNA downregulates CRALBP's mRNA expression. In the zebrafish, inhibition of HSP90 by the intraperitoneal injection of IPI504 reduces the thickness of the retinal outer nuclear layer and Rlbp1b mRNA expression. Interestingly, the expression of HSP90, SP1, and CRALBP is correlatedly downregulated in the senescent ARPE-19 and Pig primary RPE cells in vitro and in the aged zebrafish and mouse retinal tissues in vivo. The aged mice exhibit the low night adaption activity. Taken together, these data indicate that the HSP90-SP1 is a novel regulatory axis of CRALBP transcriptional expression in RPE cells. The age-mediated downregulation of the HSP90-SP1-CRALBP axis is a potential etiology for the night vision reduction in senior people.

Sp1-dependent recruitment of the histone acetylase p300 to DSBs facilitates chromatin remodeling and recruitment of the NHEJ repair factor Ku70

In response to DNA damage, most factors involved in damage recognition and repair are tightly regulated to ensure proper repair pathway choice. Histone acetylation at DNA double strand breaks (DSBs) by p300 histone acetyltransferase (HAT) is critical for the recruitment of DSB repair proteins to chromatin. Here, we show that phosphorylation of Sp1 by ATM increases its interaction with p300 and that Sp1-dependent recruitment of p300 to DSBs is necessary to modify the histones associated with p300 activity and NHEJ repair factor recruitment and repair. p300 is known to acetylate multiple residues on histones H3 and H4 necessary for NHEJ. Acetylation of H3K18 by p300 is associated with the recruitment of the SWI/SNF chromatin remodeling complex and Ku70 to DSBs for NHEJ repair. Depletion of Sp1 results in decreased acetylation of lysines on histones H3 and H4. Specifically, cells depleted of Sp1 display defects in the acetylation of H3K18, resulting in defective SWI/SNF and Ku70 recruitment to DSBs. These results shed light on mechanisms by which chromatin remodelers are regulated to ensure activation of the appropriate DSB repair pathway.

Hsp90 Inhibitor STA9090 Sensitizes Hepatocellular Carcinoma to Hyperthermia-Induced DNA Damage by Suppressing DNA-PKcs Protein Stability and mRNA Transcription

As a conserved molecular chaperone, heat shock protein 90 (Hsp90) maintains the stability and homeostasis of oncoproteins and helps cancer cells survive. DNA-dependent protein kinase catalytic subunit (DNA-PKcs) plays a pivotal role in the non-homologous end joining pathway for DNA double-strand breaks (DSB) repair. Tumor cells contain higher levels of DNA-PKcs to survive by the hostile tumor microenvironment and various antitumor therapies. Here, we showed that increased levels of Hsp90α, Hsp90β, and DNA-PKcs correlated with a poor overall survival in hepatocellular carcinoma (HCC). We revealed that Hsp90 N-terminal domain and C-terminal domain have different effects on DNA-PKcs protein and mRNA levels. The stability of DNA-PKcs depended on Hsp90α N-terminal nucleotide binding domain. Transcription factor SP1 regulates the transcription of PRKDC (gene name of DNA-PKcs) and is a client protein of Hsp90. Inhibition of Hsp90 N-terminal by STA9090 decreased the location of Hsp90α in nucleus, Hsp90α-SP1 interaction, SP1 level, and the binding of Hsp90α/SP1 at the proximal promoter region of PRKDC Because hyperthermia induces DSBs with increases level of DNA-PKcs, combined STA9090 treatment with hyperthermia effectively delayed the tumor growth and significantly decreased DNA-PKcs levels in xenografts model. Consistently, inhibition of Hsp90 increased the number of heat shock-induced γ-H2AX foci and delayed the repair of DSBs. Altogether, our results suggest that Hsp90 inhibitor STA9090 decreases DNA-PKcs protein stability and PRKDC mRNA level, which provide a theoretical basis for the promising combination therapy of hyperthermia and Hsp90 inhibitor in HCC.

Active BR signalling adjusts the subcellular localisation of BES1/HSP90 complex formation

Heat shock proteins 90 (HSP90) are essential and play critical roles in the adaptation of organisms to diverse stimuli. In plants, HSP90 are involved in auxin, jasmonate and brassinosteroid (BR) signalling pathways. The BR-promoted activation of the BES1 transcription factor regulates BR-responsive genes. Using genetic, physiological, fluorescence live cell imaging, molecular and biochemical approaches, such as phenotypic analysis, co-immunoprecipitation assay, yeast-two hybrid and Bimolecular fluorescence complementation (BiFC), we studied complex formation between BES1 and HSP90 under control conditions and active BR signalling. Further, we determined the effect of the pharmacological inhibition of HSP90 ATPase activity on hypocotyl elongation of bes1-D mutant. We determined that HSP90 interact with BES1 in the nucleus and in the cytoplasm. During active BR signalling, nuclear complexes were absent while cytoplasmic HSP90/BES1 complexes were prominent. Our results showed that the hypocotyl length of bes1-D mutants was highly reduced when HSP90 was challenged by the geldanamycin (GDA) inhibitor of the ATPase activity of HSP90. Active BR signalling could not rescue the GDA effect on the hypocotyl elongation of bes1-D. Our results reveal that the constitutively active BES1 in the bes1-D mutant is hypersensitive to GDA. The interaction of HSP90 with BES1 argues that HSP90 facilitate the nuclear metastable conformation of BES1 to regulate BR-dependent gene expression, and our data show that HSP90 assist in the compartmentalised cycle of BES1 during active BR signalling.

Activation of thromboxane A2 receptor (TP) increases the expression of monocyte chemoattractant protein -1 (MCP-1)/chemokine (C-C motif) ligand 2 (CCL2) and recruits macrophages to promote invasion of lung cancer cells

Thromboxane synthase (TXAS) and thromboxane A₂ receptor (TP), two critical components for thromboxane A₂ (TXA₂) signaling, have been suggested to be involved in cancer invasion and metastasis. However, the mechanisms by which TXA₂ promotes these processes are still unclear. Here we show that TXA₂ mimetic, I-BOP, induced monocyte chemoattractant protein -1(MCP-1)/chemokine (C-C motif) ligand 2 (CCL2) expression at both mRNA and protein levels in human lung adenocarcinoma A549 cells stably over-expressing TP receptor α isoform (A549-TPα). The induction of MCP-1 was also found in other lung cancer cells H157 and H460 that express relatively high levels of endogenous TP. Using specific inhibitors of several signaling molecules and promoter/luciferase assay, we identified that transcription factor SP1 mediates I-BOP-induced MCP-1 expression. Furthermore, supernatants from I-BOP-treated A549-TPα cells enhanced MCP-1-dependent migration of RAW 264.7 macrophages. Moreover, co-culture of A549 cells with RAW 264.7 macrophages induced expression of MMPs, VEGF and MCP-1 genes, and increased the invasive potential in A549 cells. These findings suggest that TXA₂ may stimulate invasion of cancer cells through MCP-1-mediated macrophage recruitment.

p23 co-chaperone protects the aryl hydrocarbon receptor from degradation in mouse and human cell lines

The aryl hydrocarbon receptor (AhR) is a ligand-sensitive transcription factor which is responsible for most 2,3,7,8-tetrachlorodibenzo-p-dioxin toxicities. Without ligand, the AhR complex is cytoplasmic and contains p23. Our objective was to investigate whether the wild type p23 levels are important for the AhR function. We generated eight p23-specific knockdown stable cell lines via either electroporation or lentiviral infection. Five of these stable cell lines were generated from a mouse hepatoma cell line (Hepa1c1c7) and three were from human hepatoma and cervical cell lines (Hep3B and HeLa). All of them expressed lower AhR protein levels, leading to reduced ligand-induced, DRE-driven downstream activity. The AhR protein levels in p23-specific knockdown stable cells were reversed back to wild type levels after exogenous p23 was introduced. Reduction of the AhR protein levels in these stable cells was caused by a decrease in the AhR message levels and an increase of the AhR protein degradation in the absence of ligand. This ligand-independent degradation of AhR was not reversed by MG132, suggesting that the 26S proteasome was not responsible for the degradation. In addition, MG132 could not protect AhR from the ligand-induced degradation in both mouse and human p23-knockdown stable cells.

Transcription factor proteomics: identification by a novel gel mobility shift-three-dimensional electrophoresis method coupled with southwestern blot and high-performance liquid chromatography-electrospray-mass spectrometry analysis

Transcription factor (TF) purification and identification is an important step in elucidating gene regulatory mechanisms. In this study, we present two new electrophoretic mobility shift assay (EMSA)-based multi-dimensional electrophoresis approaches to isolate and characterize TFs, using detection with either southwestern or western blotting and HPLC-nanoESI-MS/MS analysis for identification. These new techniques involve several major steps. First, EMSA is performed with agents that diminish non-specific DNA-binding and the DNA-protein complex is separated by native PAGE gel. The gel is then electrotransferred to PVDF membrane and visualized by autoradiography. Next, the DNA-protein complex, which has been transferred onto the blot, is extracted using a detergent-containing elution buffer. Following detergent removal, concentrated extract is separated by SDS-PAGE (EMSA-2DE), followed by in-gel trypsin digestion and HPLC-nanoESI-MS/MS analysis, or the concentrated extract is separated by two-dimensional gel electrophoresis (EMSA-3DE), followed by southwestern or western blot analysis to localize DNA binding proteins on blot which are further identified by on-blot trypsin digestion and HPLC-nanoESI-MS/MS analysis. Finally, the identified DNA binding proteins are further validated by EMSA-immunoblotting or EMSA antibody supershift assay. This approach is used to purify and identify GFP-C/EBP fusion protein from bacterial crude extract, as well as purifying AP1 and CEBP DNA binding proteins from a human embryonic kidney cell line (HEK293) nuclear extract. AP1 components, c-Jun, Jun-D, c-Fos, CREB, ATF1 and ATF2 were successfully identified from 1.5 mg of nuclear extract (equivalent to 3×10(7) HEK293 cells) with AP1 binding activity of 750 fmol. In conclusion, this new strategy of combining EMSA with additional dimensions of electrophoresis and using southwestern blotting for detection proves to be a valuable approach in the identification of transcriptional complexes by proteomic methods.

Novel therapeutic strategies in multiple myeloma: role of the heat shock protein inhibitors

Despite advances in understanding the molecular pathogenesis of multiple myeloma and promising new therapies, almost all patients eventually relapse with resistant disease. There is therefore a strong rationale for combining novel therapies that target intrinsic molecular pathways mediating multiple myeloma cell resistance. One such protein family is the heat shock proteins (HSP), especially the HSP90 family. Heat shock protein inhibitors have been identified as promising cancer treatments as, while they only inhibit a single biologic function, the chaperone-protein association, their effect is widespread as it results in the destruction of numerous client proteins. This article reviews the preclinical and clinical data, which support the testing of HSP90 inhibitors as cancer drugs and update the reader on the current status of the ongoing clinical trials of HSP90 inhibitors in multiple myeloma.

Heat shock protein 90 is important for Sp1 stability during mitosis

Our previous study has revealed that heat shock protein (Hsp) 90 can interact with Sp1 to regulate the transcriptional activity of 12(S)-lipoxygenase. Herein, we further found that the interaction between Hsp90 and Sp1 occurred during mitosis. By geldanamycin (GA) treatment and knockdown of Hsp90, we found that this interaction during mitosis was involved in the maintenance of Sp1 stability, and that the phospho-c-Jun N-terminal kinase (JNK)-1 level also decreased. As the JNK-1 was knocked down by the shRNA of JNK-1, Sp1 was degraded through a ubiquitin-dependent proteasome pathway. In addition, for mutation of the JNK-1 phosphorylated residues of Sp1, namely, Sp1(T278/739A) and Sp1(T278/739D), the effect of GA on Sp1 stability was reversed. Finally, based on the involvement of Hsp90 in Sp1 stability, the transcriptional activities of p21(WAF1/CIP1) and 12(S)-lipoxygenase under GA treatment were observed to have decreased. Taken together, Hsp90 is important for maintaining Sp1 stability during mitosis by the JNK-1-mediated phosphorylation of Sp1 to enable division into daughter cells and to regulate the expression of related genes in the interphase.

Trithorax requires Hsp90 for maintenance of active chromatin at sites of gene expression

Molecular chaperone heat-shock protein 90 kDa (Hsp90) is known to facilitate the conformational maturation of a diverse range of proteins involved in different signal transduction pathways during development. Recent studies have implicated Hsp90 in transcriptional regulation and an important role for Hsp90 in epigenetic processes has been proposed. Importantly, genetic and pharmacological perturbation of Hsp90 was shown to reveal heritable phenotypic variation and Hsp90 was found to play an important role in buffering genetic and epigenetic variation whose expression led to altered phenotypes. The underlying molecular mechanism remains elusive, however. Here, we show a direct molecular interaction between Hsp90 and Trithorax (Trx). Trx is a member of the TrxG chromatin proteins controlling, together with the members of the Polycomb group, the developmental fate of cells by modulating epigenetic signals. Hsp90 cooperates with Trx at chromatin for maintaining the active expression state of targets like the Hox genes. Pharmacological inhibition of Hsp90 results in degradation of Trx and a concomitant down-regulation of homeotic gene expression. A similar effect is observed with the human orthologue mixed-lineage leukemia. Connecting an epigenetic network controlling major developmental and cellular pathways with a system sensing external cues may explain the rapid fixation and epigenetic inheritance of phenotypic variation as a result of impaired Hsp90.

Association of hsp90 to the hTERT promoter is necessary for hTERT expression in human oral cancer cells

Enhanced expression of human telomerase reverse transcriptase (hTERT) occurs frequently during cellular immortalization. The current study was undertaken to determine the mechanism regulating the hTERT promoter activity during cellular immortalization of human oral keratinocytes. Normal human oral keratinocytes (NHOKs) were immortalized with Bmi-1 and the E6 oncoprotein of human papillomavirus type 16 to establish the telomerase-positive HOK-Bmi-1/E6 cell line. Using DNA-protein-binding assay, we found that heat shock protein 90 (hsp90) physically interacts with the hTERT promoter in vitro. The hsp90 interaction with the promoter was detected more strongly in the telomerase-positive HOK-Bmi-1/E6 cells compared with that in senescing NHOK. Chromatin immunoprecipitation confirmed the in vivo interaction between hsp90 and the hTERT promoter in SCC4 cells, a telomerase-positive oral cancer cell line, but not in the NHOK. To determine the physiological significance of this interaction, SCC4 cells were exposed to geldanamycin (GA), a competitive inhibitor of hsp90. GA exposure led to decrease in telomerase activity, hTERT promoter activity and hTERT messenger RNA expression in SCC4 cells, even in the absence of de novo protein synthesis. Also, it abolished the in vivo interaction of the hTERT promoter region with hsp90 but not with Sp1 or c-Myc. These results indicate that physical interaction between hsp90 and the hTERT promoter occurs in telomerase-positive cells but not in normal human cells and is necessary for the enhanced hTERT expression and telomerase activity in cancer cells.

Trps1 deficiency enlarges the proliferative zone of growth plate cartilage by upregulation of Pthrp

We have reported that elongation of the columnar proliferative zone of long bone growth plates in Trps1-/- mice during the late fetal stage in the previous study [1]. Since expression of Trps1 protein was found to overlap with that of mRNAs for Indian hedgehog (Ihh), PTH/PTHrP receptor (PPR), and PTHrP, we hypothesized that Trps1 may inhibit the hypertrophic differentiation of chondrocytes by interacting with the Ihh/PTHrP feedback loop. To investigate whether Trps1 has a role in this Ihh/PTHrP feedback loop, we compared the growth plates of Trps1-/- mice and wild-type (Trps1+/+) mice. Immunohistochemistry showed that Trps1 protein was strongly expressed in the periarticular and prehypertrophic zones of the fetal growth plate in wild-type mice on embryonic day 18.5 (E18.5). On the other hand, Ihh, PPR, and PTHrP mRNAs were predominantly expressed in the prehypertrophic zone at this stage of development. While expression of Ihh and PPR by prehypertrophic chondrocytes was unaffected in the growth plates of Trps1-/- mice, the range of PTHrP expression was expanded toward the proliferating zone in these mice. Quantitative real-time PCR analysis demonstrated upregulation of PTHrP in the epiphyseal growth plates of Trps1-/- mice. Furthermore, promoter analysis combined with the chromatin immunoprecipitation (ChIP) assay demonstrated that direct binding of Trps1 to the PTHrP promoter suppressed the transcription of PTHrP. Finally, organ culture of E14.5 tibiae in the absence or the presence of Pthrp revealed that the proliferative zone of the tibial growth plate was elongated by culture with Pthrp compared to that of control tibiae. Taken together, these data provide the first genetic evidence that lack of Trps1 leads to overexpression of PTHrP, and that Trps1 is required to maintain the normal organization of chondrocytes in the growth plate.

Bacillus subtilis glutamine synthetase regulates its own synthesis by acting as a chaperone to stabilize GlnR-DNA complexes

The Bacillus subtilis GlnR repressor controls gene expression in response to nitrogen availability. Because all GlnR-regulated genes are expressed constitutively in mutants lacking glutamine synthetase (GS), GS is required for repression by GlnR. Feedback-inhibited GS (FBI-GS) was shown to activate GlnR DNA binding with an in vitro electophoretic mobility shift assay (EMSA). The activation of GlnR DNA binding by GS in these experiments depended on the feedback inhibitor glutamine and did not occur with mutant GS proteins defective in regulating GlnR activity in vivo. Although stable GS-GlnR-DNA ternary complexes were not observed in the EMSA experiments, cross-linking experiments showed that a protein-protein interaction occurs between GlnR and FBI-GS. This interaction was reduced in the absence of the feedback inhibitor glutamine and with mutant GS proteins. Because FBI-GS significantly reduced the dissociation rate of the GlnR-DNA complexes, the stability of these complexes is enhanced by FBI-GS. These results argue that FBI-GS acts as a chaperone that activates GlnR DNA binding through a transient protein-protein interaction that stabilizes GlnR-DNA complexes. GS was shown to control the activity of the B. subtilis nitrogen transcription factor TnrA by forming a stable complex between FBI-GS and TnrA that inhibits TnrA DNA binding. Thus, B. subtilis GS is an enzyme with dual catalytic and regulatory functions that uses distinct mechanisms to control the activity of two different transcription factors.

Inhibition of Hsp90: a multitarget approach to radiosensitization

Hsp90, the 90 kDa heat shock protein, is a highly expressed molecular chaperone that modulates the stability and/or transport of a diverse set of critical cellular regulatory proteins. Among Hsp90 clients are a number of proteins, which in a cell type-dependent manner, contribute to tumor cell radioresistance. Exposure of a variety of solid tumor cell lines to clinically relevant Hsp90 inhibitors results in the simultaneous loss of these radioresponse-associated proteins, which is accompanied by an increase in radiosensitivity. This radiosensitization has been linked to a compromise in the DNA damage response to radiation including the inhibition of cell cycle checkpoint activation and DNA double-strand break repair. With respect to potential clinical application, the expression of ErbB3 seems to predict tumor cells that are resistant to the effects of Hsp90 inhibition on radiosensitivity. Moreover, whereas an increase in tumor cell radiosensitivity was consistently reported, the radiosensitivity of normal fibroblasts was not affected by Hsp90 inhibition, suggesting the potential for tumor-selective radiosensitization. This review summarizes the preclinical data available on Hsp90 inhibition and cellular radiosensitivity. Results generated to date suggest that Hsp90 inhibition can provide a multitarget approach to tumor radiosensitization.

The Herpes simplex virus gene Pol expressed in herpes-associated erythema multiforme lesions upregulates/activates SP1 and inflammatory cytokines

BACKGROUND/AIMS

Herpes-simplex-virus-associated erythema multiforme (HAEM) is characterized by lesional skin expression of the viral protein Pol and localized inflammation. The objective of this study is to examine the mechanism whereby Pol induces localized inflammation.

METHODS

A431 cells transfected with Pol or an empty vector and lesional skin from HAEM or drug-induced erythema multiforme patients were examined for expression of the transcription factor SP1 and SP1-regulated genes by immunoblotting, immunohistochemistry and immunofluorescence.

RESULTS

SP1, TGF-beta, p21(waf1) and Hsp27 were upregulated in A431 cells transfected with Pol but not the empty vector. Expression was further increased by exposure to IFN-gamma. Pol+ HAEM lesional skin expressed SP1, Hsp27, TGF-beta and p21(waf1). Normal skin and drug-induced erythema multiforme lesional skin were negative.

CONCLUSION

The data indicate that Pol activates SP1, causing upregulation of SP1 target genes (notably TGF-beta) involved in localized inflammation. Upregulation is potentiated by IFN-gamma.

IL-4 and IL-13 negatively regulate TNF-alpha- and IFN-gamma-induced beta-defensin expression through STAT-6, suppressor of cytokine signaling (SOCS)-1, and SOCS-3

Human beta-defensins (HBDs) are a major class of antimicrobial peptides that play an important role in the innate immune response, however, the induction and regulation of these antimicrobial peptides is not well understood. We demonstrate here that stimulation of keratinocytes with TNF-alpha/IFN-gamma induces HBD-2 and HBD-3 by activating STAT-1 and NF-kappaB signaling. We further demonstrate that IL-4 and IL-13 activate STAT-6 and induce the suppressors of cytokine signaling (SOCS)-1 and -3. This interferes with STAT-1 and NF-kappaB signaling, thereby inhibiting TNF-alpha/IFN-gamma-mediated induction of HBD-2 and HBD-3. These data suggest that targeting the STAT-1-signaling pathway or suppressor of cytokine signaling expression enhances beta-defensin expression and represents a new therapeutic strategy for reduction of infection in human diseases associated with beta-defensin deficiency.

Proteomic identification of proteins associated with the osmoregulatory transcription factor TonEBP/OREBP: functional effects of Hsp90 and PARP-1

Hypertonicity (e.g., high NaCl) activates the transcription factor tonicity-responsive enhancer/osmotic response element-binding protein (TonEBP/OREBP), increasing transcription of protective genes. In the present studies, by stably expressing amino acids 1-547 of TonEBP/OREBP in HEK 293 cells and immunoprecipitating it plus associated proteins from the nuclei of cells exposed to high NaCl, we identify 14 proteins that are physically associated with TonEBP/OREBP. The associated proteins fall into several classes: 1) DNA-dependent protein kinase, both its catalytic subunit and regulatory subunit, Ku86; 2) RNA helicases, namely RNA helicase A, nucleolar RNA helicase II/Gu, and DEAD-box RNA helicase p72; 3) small or heterogeneous nuclear ribonucleoproteins (snRNPs or hnRNPs), namely U5 snRNP-specific 116 kDa protein, U5 snRNP-specific 200 kDa protein, hnRNP U, hnRNP M, hnRNP K, and hnRNP F; 4) heat shock proteins, namely Hsp90beta and Hsc70; and 5) poly(ADP-ribose) polymerase-1 (PARP-1). We confirm identification of most of the proteins by Western analysis and also demonstrate by electrophoretic mobility-shift assay that they are present in the large complex that binds specifically along with TonEBP/OREBP to its cognate DNA element. In addition, we find that PARP-1 and Hsp90 modulate TonEBP/OREBP activity. PARP-1 expression reduces TonEBP/OREBP transcriptional activity and the activity of its transactivating domain. Hsp90 enhances those activities and sustains the increased abundance of TonEBP/OREBP protein in cells exposed to high NaCl.

Inhibition of hsp90 compromises the DNA damage response to radiation

Inhibitors of the molecular chaperone Hsp90 have been shown to enhance tumor cell radiosensitivity. To begin to address the mechanism responsible, we have determined the effect of the Hsp90 inhibitor 17-(dimethylaminoethylamino)-17-demethoxygeldanamycin (17DMAG) on the DNA damage response to radiation. Exposure of MiaPaCa tumor cells to 17DMAG, which results in radiosensitization, inhibited the repair of DNA double-strand breaks according to gammaH2AX foci dispersal and the neutral comet assay. This repair inhibition was associated with reduced DNA-PK catalytic subunit (DNA-PKcs) phosphorylation after irradiation and a disruption of DNA-PKcs/ErbB1 interaction. These data suggest that the previously established 17DMAG-mediated reduction in ErbB1 activity reduces its interaction with DNA-PKcs and thus accounts for the attenuation of radiation-induced DNA-PK activation. 17DMAG was also found to abrogate the activation of the G(2)- and S-phase cell cycle checkpoints. Associated with these events was a reduction in radiation-induced ataxia-telangiectasia mutated (ATM) activation and foci formation in 17DMAG-treated cells. Although no interaction between ATM and Hsp90 was detected, Hsp90 was found to interact with the MRE11/Rad50/NBS1 (MRN) complex. 17DMAG exposure reduced the ability of the MRN components to form nuclear foci after irradiation. Moreover, 17DMAG exposure reduced the interaction between NBS1 and ATM, although no degradation of the MRN complex was detected. These results suggest that the diminished radiation-induced activation of ATM in 17DMAG-treated cells was the result of a compromise in the function of the MRN complex. These data indicate that Hsp90 can contribute to the DNA damage response to radiation affecting both DNA repair and cell cycle checkpoint activation.

NF-Y and Sp1/Sp3 are involved in the transcriptional regulation of the peptidylarginine deiminase type III gene (PADI3) in human keratinocytes

Human peptidylarginine deiminase type III gene (PADI3) encodes a crucial post-translational modification enzyme that converts protein-bound arginine residues into citrulline residues. Its expression is restricted to a few cell types, including keratinocytes in the granular layer of the epidermis and in the inner root sheath of hair follicles. In these cells, the enzyme is involved in terminal processing of intermediate filament-binding proteins such as filaggrin and trichohyalin. To study the molecular mechanisms that control the expression of PADI3 in human keratinocytes at the transcriptional level, we characterized its promoter region using human keratinocytes transfected with variously deleted fragments of the 5'-upstream region of PADI3 coupled to the luciferase gene. We found that as few as 129 bp upstream from the transcription initiation site were sufficient to direct transcription of the reporter gene. Electrophoretic mobility-shift and chromatin immunoprecipitation assays revealed that NF-Y (nuclear factor Y) and Sp1/Sp3 (specificity protein 1/3) bind to this region in vitro and in vivo. Moreover, mutation of the Sp1- or NF-Y-binding motif markedly reduced PADI3 promoter activity. Furthermore, Sp1 or NF-YA (NF-Y subunit) small interfering RNAs effectively diminished PADI3 expression in keratinocytes cultured in both low- and high-calcium medium. These data indicate that PADI3 expression is driven by Sp1/Sp3 and NF-Y binding to the promoter region.