Post-translational modification of p53 protein in response to ionizing radiation analyzed by mass spectrometry

Hybrid-DIA: intelligent data acquisition integrates targeted and discovery proteomics to analyze phospho-signaling in single spheroids

Achieving sufficient coverage of regulatory phosphorylation sites by mass spectrometry (MS)-based phosphoproteomics for signaling pathway reconstitution is challenging, especially when analyzing tiny sample amounts. To address this, we present a hybrid data-independent acquisition (DIA) strategy (hybrid-DIA) that combines targeted and discovery proteomics through an Application Programming Interface (API) to dynamically intercalate DIA scans with accurate triggering of multiplexed tandem mass spectrometry (MSx) scans of predefined (phospho)peptide targets. By spiking-in heavy stable isotope labeled phosphopeptide standards covering seven major signaling pathways, we benchmark hybrid-DIA against state-of-the-art targeted MS methods (i.e., SureQuant) using EGF-stimulated HeLa cells and find the quantitative accuracy and sensitivity to be comparable while hybrid-DIA also profiles the global phosphoproteome. To demonstrate the robustness, sensitivity, and biomedical potential of hybrid-DIA, we profile chemotherapeutic agents in single colon carcinoma multicellular spheroids and evaluate the phospho-signaling difference of cancer cells in 2D vs 3D culture.

Irradiation potentiates p53 phosphorylation and p53 binding to the promoter and coding region of the TP53 gene

An essential factor of the DNA damage response is 53BP1, a multimeric protein that inhibits the resection-dependent double-strand break (DBS) repair. The p53 protein is a tumor suppressor known as a guardian of the genome. Although the interaction between 53BP1 and its p53 partner is well-known in regulating gene expression, a question remains whether genome injury can affect the interaction between 53BP1 and p53 proteins or p53 binding to DNA. Here, using mass spectrometry, we determine post-translational modifications and interaction properties of 53BP1 and p53 proteins in non-irradiated and γ-irradiated cells. In addition, we used Atomic Force Microscopy (AFM) and Fluorescent Lifetime Imaging Microscopy combined with Fluorescence Resonance Energy Transfer (FLIM-FRET) for studies of p53 binding to DNA. Also, we used local laser microirradiation as a tool of advanced confocal microscopy, showing selected protein accumulation at locally induced DNA lesions. We observed that 53BP1 and p53 proteins accumulate at microirradiated chromatin but with distinct kinetics. The density of 53BP1 (53BP1pS1778) phosphorylated form was lower in DNA lesions than in the non-specified form. By mass spectrometry, we found 22 phosphorylations, 4 acetylation sites, and methylation of arginine 1355 within the DNA-binding domain of the 53BP1 protein (aa1219-1711). The p53 protein was phosphorylated on 8 amino acids and acetylated on the N-terminal domain. Post-translational modifications (PTMs) of 53BP1 were not changed in cells exposed to γ-radiation, while γ-rays increased the level of S6ph and S15ph in p53. Interaction analysis showed that 53BP1 and p53 proteins have 54 identical interaction protein partners, and AFM revealed that p53 binds to both non-specific and TP53-specific sequences (AGACATGCCTA GGCATGTCT). Irradiation by γ-rays enhanced the density of the p53 protein at the AGACATGCCTAGGCATGTCT region, and the binding of p53 S15ph to the TP53 promoter was potentiated in irradiated cells. These findings show that γ-irradiation, in general, strengthens the binding of phosphorylated p53 protein to the encoding gene.

TRIM3 inhibits P53 signaling in breast cancer cells

BACKGROUND

Beast cancer is the most common women cancer worldwide, while two third of them are ER alpha positive breast cancer. Among the ER alpha positive breast cancer, about 80% are P53 wild type, indicating the potential tumor suppression role in ER alpha positive breast cancer. Since P53 is an important safeguard to inhibit cell malignant transformation, reactivating P53 signaling could a plausible approach to treat breast cancer.

METHODS

TRIM3 protein levels were measured by western blot, while the P53 classical target genes were measured by real-time PCR. WST1 assay were used to measure cell proliferation, while cleaved caspase-3 was used to detect cell apoptosis. Protein stability and ubiquitin assay were used to detect the P53 protein ubiquitin and stability. The immuno-precipitation assays were used to detect the protein interactions. Immuno-staining was used to detect the protein localization of P53 and TRIM3, while the ubiquitin-based immuno-precipitation assays were used to detect the specific ubiquitination manner of P53.

RESULTS

In our study, we identified TRIM3 as an endogenous inhibitor for P53 signaling. TRIM3 depletion inhibited breast cancer cell proliferation and promoted apoptosis. In addition, TRIM3 depletion increased P53 protein level in breast cancer cell. Further investigation showed that TRIM3 could associate with P53 and promote P53 K48-linked ubiquitination and degradation.

CONCLUSION

Our study identified a novel post-translational modification mechanism between TRIM3 and P53. TRIM3 depletion or blockage could be a promising strategy to rescue P53 signaling and inhibit breast cancer progression.

SIRT3 protects endothelial cells from high glucose-induced senescence and dysfunction via the p53 pathway

Hyperglycemia induces endothelial cells (ECs) dysfunction and vascular complications by accelerating ECs senescence. It also induces downregulation of sirtuins (SIRTs). However, the molecular mechanism involved in the regulation of ECs senescence by SIRT3 remains unclear. Here, we showed that high glucose (HG) decreased the expression level of SIRT3 in human umbilical vein endothelial cells (HUVECs), increased the proportion of cells expressing senescence-associated galactosidase (SA-gal), and HG damaged the cell's ability to form tubule networks on Matrigel. However, transfection with adenoviral construct including SIRT3 significantly inhibited HG-induced SA-gal activity, decreased p53 acetylation level at the site Lys 320 (k320), and overexpression of SIRT3 antagonized high glucose-induced angiogenic dysfunction. Our results suggested a possible molecular mechanism involving HG-SIRT3-p53 in ECs senescence.

The Global Phosphorylation Landscape of SARS-CoV-2 Infection

The causative agent of the coronavirus disease 2019 (COVID-19) pandemic, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), has infected millions and killed hundreds of thousands of people worldwide, highlighting an urgent need to develop antiviral therapies. Here we present a quantitative mass spectrometry-based phosphoproteomics survey of SARS-CoV-2 infection in Vero E6 cells, revealing dramatic rewiring of phosphorylation on host and viral proteins. SARS-CoV-2 infection promoted casein kinase II (CK2) and p38 MAPK activation, production of diverse cytokines, and shutdown of mitotic kinases, resulting in cell cycle arrest. Infection also stimulated a marked induction of CK2-containing filopodial protrusions possessing budding viral particles. Eighty-seven drugs and compounds were identified by mapping global phosphorylation profiles to dysregulated kinases and pathways. We found pharmacologic inhibition of the p38, CK2, CDK, AXL, and PIKFYVE kinases to possess antiviral efficacy, representing potential COVID-19 therapies.

Molecular Signaling in Response to Charged Particle Exposures and its Importance in Particle Therapy

Energetic, charged particles elicit an orchestrated DNA damage response (DDR) during their traversal through healthy tissues and tumors. Complex DNA damage formation, after exposure to high linear energy transfer (LET) charged particles, results in DNA repair foci formation, which begins within seconds. More protein modifications occur after high-LET, compared with low-LET, irradiation. Charged-particle exposure activates several transcription factors that are cytoprotective or cytodestructive, or that upregulate cytokine and chemokine expression, and are involved in bystander signaling. Molecular signaling for a survival or death decision in different tumor types and healthy tissues should be studied as prerequisite for shaping sensitizing and protective strategies. Long-term signaling and gene expression changes were found in various tissues of animals exposed to charged particles, and elucidation of their role in chronic and late effects of charged-particle therapy will help to develop effective preventive measures.

Lysine Acetylation Goes Global: From Epigenetics to Metabolism and Therapeutics

Post-translational acetylation of lysine residues has emerged as a key regulatory mechanism in all eukaryotic organisms. Originally discovered in 1963 as a unique modification of histones, acetylation marks are now found on thousands of nonhistone proteins located in virtually every cellular compartment. Here we summarize key findings in the field of protein acetylation over the past 20 years with a focus on recent discoveries in nuclear, cytoplasmic, and mitochondrial compartments. Collectively, these findings have elevated protein acetylation as a major post-translational modification, underscoring its physiological relevance in gene regulation, cell signaling, metabolism, and disease.

Colorectal Carcinogenesis, Radiation Quality, and the Ubiquitin-Proteasome Pathway

Adult colorectal epithelium undergoes continuous renewal and maintains homeostatic balance through regulated cellular proliferation, differentiation, and migration. The canonical Wnt signaling pathway involving the transcriptional co-activator β-catenin is important for colorectal development and normal epithelial maintenance, and deregulated Wnt/β-catenin signaling has been implicated in colorectal carcinogenesis. Colorectal carcinogenesis has been linked to radiation exposure, and radiation has been demonstrated to alter Wnt/β-catenin signaling, as well as the proteasomal pathway involved in the degradation of the signaling components and thus regulation of β-catenin. The current review discusses recent progresses in our understanding of colorectal carcinogenesis in relation to different types of radiation and roles that radiation quality plays in deregulating β-catenin and ubiquitin-proteasome pathway (UPP) for colorectal cancer initiation and progression.

Resveratrol induces Sirt1-dependent apoptosis in 3T3-L1 preadipocytes by activating AMPK and suppressing AKT activity and survivin expression

Resveratrol is a natural polyphenolic compound with anti-inflammatory, antioxidant and neuroprotective properties, and it serves as a chemopreventive and chemotherapeutic agent. However, only very limited data have been obtained regarding the effects of resveratrol on preadipocytes, and the mechanisms of these effects remain largely unknown. In this study, murine 3T3-L1 preadipocytes were incubated with resveratrol, and cell apoptosis was investigated. Resveratrol caused S-phase arrest to inhibit cell proliferation and significantly increased the lactate dehydrogenase leaking ratio. Hoechst 33258 staining and transmission electron microscopy revealed the ultrastructural changes in nuclear chromatins of apoptotic cells. Furthermore, resveratrol activated the mitochondrial signaling with decreases in the mitochondrial membrane potential, cytochrome c release and the activation of caspase 9 and caspase 3. Resveratrol treatment also increased the protein level of Sirt1. By using small interfering RNAs of Sirt1, adenosine-monophosphate-activated protein kinase (AMPK) α, survivin and the AMPK agonist (5-aminoimidazole-4-carboxamide 1-β-D-ribofuranoside) and specific inhibitors for protein kinase B (AKT) or caspases, it was demonstrated that activation of Sirt1 inhibited AKT activation and further decreased the expression of survivin. It could also increase AMPK activation. Both signaling pathways activated mitochondrion-mediated pathway. Our findings clarified the apoptotic effects of resveratrol in 3T3-L1 preadipocytes and revealed the involved pathway including AMPK, AKT and survivin, suggesting its potential therapeutic application in the treatment or prevention of obesity and related metabolic symptoms.

The HDAC inhibitor depsipeptide transactivates the p53/p21 pathway by inducing DNA damage

Histone deacetylase (HDAC) inhibitors have been proven to be effective therapeutic agents to kill cancer cells through inhibiting HDAC activity or altering the structure of chromatin. As a potent HDAC inhibitor, depsipeptide not only modulates histone deacetylation but also activates non-histone protein p53 to inhibit cancer cell growth. However, the mechanism of depsipeptide-induced p53 transactivity remains unknown. Here, we show that depsipeptide causes DNA damage through induction of reactive oxygen species (ROS) generation, as demonstrated by a comet assay and by detection of the phosphorylation of H2AX. Depsipeptide induced oxidative stress was confirmed to relate to a disturbance in reduction-oxidation (redox) reactions through inhibition of the transactivation of thioredoxin reductase (TrxR) in human cancer cells. Upon treatment with depsipeptide, p53 phosphorylation at threonine 18 (Thr18) was specifically induced. Furthermore, we also demonstrated that phosphorylation of p53 at Thr18 is required for p53 acetylation at lysine 373/382 and for p21 expression in response to depsipeptide treatment. Our results demonstrate that depsipeptide plays an anti-neoplastic role by generating ROS to elicit p53/p21 pathway activation.

Arabidopsis T-DNA insertional lines for CDC25 are hypersensitive to hydroxyurea but not to zeocin or salt stress

BACKGROUND AND AIMS

In yeasts and animals, cyclin-dependent kinases are key regulators of cell cycle progression and are negatively and positively regulated by WEE1 kinase and CDC25 phosphatase, respectively. In higher plants a full-length orthologue of CDC25 has not been isolated but a shorter gene with homology only to the C-terminal catalytic domain is present. The Arabidopis thaliana;CDC25 can act as a phosphatase in vitro. Since in arabidopsis, WEE1 plays an important role in the DNA damage/DNA replication checkpoints, the role of Arath;CDC25 in conditions that induce these checkpoints or induce abiotic stress was tested. Methods arath;cdc25 T-DNA insertion lines, Arath;CDC25 over-expressing lines and wild type were challenged with hydroxyurea (HU) and zeocin, substances that stall DNA replication and damage DNA, respectively, together with an abiotic stressor, NaCl. A molecular and phenotypic assessment was made of all genotypes Key

RESULTS

There was a null phenotypic response to perturbation of Arath;CDC25 expression under control conditions. However, compared with wild type, the arath;cdc25 T-DNA insertion lines were hypersensitive to HU, whereas the Arath;CDC25 over-expressing lines were relatively insensitive. In particular, the over-expressing lines consistently outgrew the T-DNA insertion lines and wild type when challenged with HU. All genotypes were equally sensitive to zeocin and NaCl.

CONCLUSIONS

Arath;CDC25 plays a role in overcoming stress imposed by HU, an agent know to induce the DNA replication checkpoint in arabidopsis. However, it could not enhance tolerance to either a zeocin treatment, known to induce DNA damage, or salinity stress.

A commentary on the G₂/M transition of the plant cell cycle

BACKGROUND

The complex events of mitosis rely on precise timing and on immaculate preparation for their success, but the G₂/M transition in the plant cell cycle is currently steeped in controversy and alternative models.

SCOPE

In this brief review, the regulation of the G₂/M transition in plants is commented on. The extent to which the G₂/M transition is phosphoregulated by WEE1 kinase and CDC25 phosphatase, as exemplified in yeasts and animals, is discussed together with an alternative model that excludes these proteins from this transition. Arabidopsis T-DNA insertional lines for WEE1 and CDC25 that develop normally prompted the latter model. An argument is then presented that environmental stress is the norm for higher plants in temperate conditions. If so, the repressive role that WEE1 has under checkpoint conditions might be part of the normal cell cycle for many proliferative plant cells. Arabidopsis CDC25 can function as either a phosphatase or an arsenate reductase and recent evidence suggests that cdc25 knockouts are hypersensitive to hydroxyurea, a drug that induces the DNA-replication checkpoint. That other data show a null response of these knockouts to hydroxyurea leads to an airing of the controversy surrounding the enigmatic plant CDC25 at the G₂/M transition.

A common origin: signaling similarities in the regulation of the circadian clock and DNA damage responses

Circadian clocks are intrinsic, time-tracking systems that endow organisms with a survival advantage. Studies of animal models and human tumor samples have revealed that the disruption of circadian rhythms is an important endogenous factor that can contribute to mammalian cancer development. The core of the circadian clock mechanism is a cell-autonomous and self-sustained oscillator system mediated by a transcription/translation-based negative feedback loop that relies on positive and negative elements. Recent studies have implicated these core circadian components in the regulation of both the cell cycle and DNA damage responses (DDR). Indeed, the circadian feedback loop controls the timing of cell proliferation by regulating the expression of key cell cycle genes. Conversely, several intracellular signaling cascades and post-translational modifications that play important roles in the cell cycle and DDR are also essential for circadian clock regulation. Importantly, alteration of a cell's reduction-oxidation (redox) state triggers the transduction of photic signals that regulate circadian clock gene transcription, suggesting that cellular responses to photo-oxidative stress may have been the evolutionary origin of the circadian clock. This review describes selected regulatory aspects of circadian machinery that are evidence of a molecular link between the circadian clock and DDR, focusing particularly on the signaling cascades involved in the light entrainment of the zebrafish circadian clock.

Folding of tetrameric p53: oligomerization and tumorigenic mutations induce misfolding and loss of function

The physiologically active form of p53 consists of a tetramer of four identical 393-amino-acid subunits associated via their tetramerization domains (TDs; residues 325-355). One in two human tumors contains a point mutation in the DNA binding domain (DBD) of p53 (residues 94-312). Most existing studies on the effects of these mutations on p53 structure and function have been carried out on the isolated DBD fragment, which is monomeric. Recent structural evidence, however, suggests that DBDs may interact with each other in full-length tetrameric forms of p53. Here, we investigate the effects of tumorigenic DBD mutations on the folding of p53 in its tetrameric form. We employ the construct consisting of DBD and TD (amino acids 94-360). We characterize the stability and conformational state of the tumorigenic DBD mutants R248Q, R249S, and R282Q using equilibrium denaturation and functional assays. Destabilizing mutations cause DBD to misfold when it is part of the p53 tetramer, but not when it is monomeric. This conformation is populated under moderately destabilizing conditions (10 degrees C in 2 M urea, and at physiological temperature in the absence of denaturant). Under those same conditions, it is not present in the isolated DBD fragment or in the presence of the TD mutation L344P, which abolishes tetramerization. Misfolding appears to involve intramolecular DBD-DBD association within a single tetrameric molecule. This association is promoted by destabilization of DBD (caused by mutation or elevated temperature) and by the high local DBD concentration enforced by tetramerization of TD. Disrupting the nonnative DBD-DBD interaction or transiently inhibiting tetramerization and allowing p53 to fold as a monomer may be potential strategies for pharmacological intervention in cancer.

The role of sirtuins in the control of metabolic homeostasis

Recently the function of the sirtuin family, named after their homology to the Saccharomyces cerevisiae gene silent information regulator 2 (Sir2), has received a lot of attention, as their beneficial impact on longevity was linked to their effects on metabolic control. All sirtuins require nicotinamide adenine dinucleotide (NAD(+)) for their deacetylase or ADP-ribosyl transferase activity, linking their function tightly to cellular energy levels. SIRT1, the founding member of the sirtuin family, modulates many aspects of glucose and lipid homeostasis in almost all key metabolic tissues. Other members including SIRT2, SIRT3, and SIRT4 are also implicated in various metabolic processes. Here, we review the recent data related to the role of sirtuins in the control of metabolic homeostasis and possible underlying molecular mechanisms.

SIRT1 confers protection against UVB- and H2O2-induced cell death via modulation of p53 and JNK in cultured skin keratinocytes

SIRT1 is a member of a highly conserved gene family (sirtuins) encoding nicotinamide adenine dinucleotide (NAD)(+)-dependent deacetylases, originally found to deacetylate histones leading to increased DNA stability and prolonged survival in yeast and higher organisms, including mammals. SIRT1 has been found to function as a deacetylase for numerous protein targets involved in various cellular pathways, including stress responses, apoptosis and axonal degeneration. However, the role of SIRT1 in ultraviolet (UV) signalling pathways remains unknown. Using cell culture and Western blot analysis in this study we found that SIRT1 is expressed in cultured human skin keratinocytes. Both UV radiation and H(2)O(2), two major inducers of skin cell damage, down-regulate SIRT1 in a time- and dose-dependent manner. We observed that reactive oxygen species-mediated JNK activation is involved in this SIRT1 down-regulation. SIRT1 activator, resveratrol, which has been considered as an important antioxidant, protects against UV- and H(2)O(2)-induced cell death, whereas SIRT inhibitors such as sirtinol and nicotinamide enhance cell death. Activation of SIRT1 negatively regulates UV- and H(2)O(2)-induced p53 acetylation, because nicotinamide and sirtinol as well as SIRT1 siRNA enhance UV- and H(2)O(2)-induced p53 acetylation, whereas SIRT1 activator resveratrol inhibits it. We also found that SIRT1 is involved in UV-induced AMP-activated protein kinase (AMPK) and downstream acetyl-CoA carboxylase (ACC), phosphofructose kinase-2 (PFK-2) phosphorylation. Collectively, our data provide new insights into understanding of the molecular mechanisms of UV-induced skin aging, suggesting that SIRT1 activators such as resveratrol could serve as new anti-skin aging agents.

Deacetylation of the retinoblastoma tumour suppressor protein by SIRT1

The activity of Rb (retinoblastoma protein) is regulated by phosphorylation and acetylation events. Active Rb is hypophosphorylated and acetylated on multiple residues. Inactivation of Rb involves concerted hyper-phosphorylation by cyclin-CDK (cyclin-dependent kinase) complexes combined with deacetylation of appropriate lysine residues within Rb. In the present study, using in vivo co-immunoprecipitation experiments, we identified mammalian SIRT1 (sirtuin 1) as a binding partner for Rb and its family members p107 and p130. Formation of Rb-SIRT1 complexes required the pocket domain of Rb. p300 catalysed the acetylation of Rb, and SIRT1 was a potent deacetylase for Rb. The ability of SIRT1 to catalyse the deacetylation of Rb was dependent on NAD and was inhibited by the SIRT1 inhibitor nicotinamide. Deacetylated lysine residues within Rb formed a domain similar to the SIRT1-targeted domain of the p53 tumour suppressor protein. Cultures of arrested cells, via contact inhibition or DNA damage, exhibited decreased Rb phosphorylation and increased Rb acetylation. Overexpression of SIRT1 in either confluent or etoposide-treated cells resulted in a significant reduction in Rb acetylation, which was restored with nicotinamide. Gene knockdown of SIRT1 by siRNA (short interfering RNA) produced an accumulation of acetylated Rb. This increase was augmented further when siRNA against SIRT1 was used in conjunction with nicotinamide. In conclusion, our results demonstrate that SIRT1 is an in vitro and in vivo deacetylase for the Rb tumour suppressor protein.

Sequential and synergistic modification of human RPA stimulates chromosomal DNA repair

The activity of human replication protein A (RPA) in DNA replication and repair is regulated by phosphorylation of the middle RPA2 subunit. It has previously been shown that up to nine different N-terminal residues are modified in vivo and in response to genotoxic stress. Using a novel antibody against phospho-Ser(29), a moiety formed by cyclin-Cdk, we observed that RPA2 was phosphorylated during mitosis in nonstressed cells. Robust phosphorylation of Ser(29) was also seen in interphase cells following treatment with the DNA-damaging agent camptothecin, a rare example of stress stimulating the modification of a repair factor by cyclin-Cdk. RPA2 phosphorylation is regulated both in cis and trans. Cis-phosphorylation follows a preferred pathway. (That is, the initial modification of Ser(33) by ATR stimulates subsequent phosphorylation of Cdk sites Ser(23) and Ser(29)). These events then facilitate modification of Thr(21) and extreme N-terminal sites Ser(4) and Ser(8), probably by DNA-PK. Our data also indicate that the phosphorylation of one RPA molecule can influence the phosphorylation of other RPA molecules in trans. Cells in which endogenous RPA2 was "replaced" with a double S23A/S29A-RPA2 mutant were seen to have an abnormal cell cycle distribution both in normal and in stressed cells. Such cells also showed aberrant DNA damage-dependent RPA foci and had persistent staining of gammaH2AX following DNA damage. Our data indicate that RPA phosphorylation facilitates chromosomal DNA repair. We postulate that the RPA phosphorylation pattern provides a means to regulate the DNA repair pathway utilized.

Niacin deficiency alters p53 expression and impairs etoposide-induced cell cycle arrest and apoptosis in rat bone marrow cells

One focus of chemoprevention research is the interaction of nutrients with specific molecular targets associated with the maintenance of genomic stability. This study tested the impact of dietary niacin status on bone marrow NAD+ and poly(ADP-ribose) (pADPr) levels, p53 expression, and etoposide (ETO)-induced apoptosis and cell cycle arrest. After 3 wk on niacin-deficient (ND), pair-fed niacin-replete (PF), or nicotinic acid-supplemented (4 g/kg diet) (NA) diets, Long-Evans rats were gavaged with ETO (25 mg/kg) or vehicle. ND and NA diets caused a 72% decrease and a 240% increase in bone marrow NAD+, respectively. Basal and ETO-induced pADPr levels differed dramatically among ND, PF, and NA diets (undetectable, 42 and 216 fmol/million cells, respectively; basal and undetectable, 119 and 484 fmol/million cells, respectively, following ETO). ND diet alone caused overexpression of two distinct isoforms of p53. Levels of p53 in PF and NA marrow increased in response to ETO treatment, but this did not occur in ND bone marrow. Quantitative polymerase chain reaction of regular and alternative spliced variants of p53 mRNA revealed that niacin deficiency actually decreased both forms of p53 message, implicating protein stability in the accumulation of p53 in ND marrow. ETO-induced apoptosis (TUNEL) was suppressed during niacin deficiency and enhanced by supplementation. G1 arrest was also impaired in ND bone marrow relative to PF and NA. Despite a poor G1 arrest, p21waf1 was overexpressed in the ND bone marrow and dramatically induced following ETO treatment. In conclusion, dietary niacin deficiency causes changes in NAD+ and pADPr metabolism, alters p53 expression, and impairs cellular responses to DNA damage.

Irinotecan activates p53 with its active metabolite, resulting in human hepatocellular carcinoma apoptosis

The topoisomerase I inhibitor irinotecan is widely used in anticancer therapy, although the detailed mechanism is still unclear. We investigated the apoptotic mechanisms of irinotecan in human hepatocellular carcinoma (HCC) cell lines (Huh7). SN-38 caused a significant decrease in cell proliferation and induced apoptosis in Huh7 cells and HepG2 cells. SN-38 significantly increased the expression of p53 protein and its phosphorylation at Ser(15) in the nucleus and apoptosis-inducing proteins Bax, caspase-9, and caspase-3, while it significantly decreased the antiapoptosis protein Bcl-xL of Huh7 cells. SN-38-induced apoptosis was recovered after p53 antisense oligodeoxynucleotide (AS ODN) pretreatment, while Huh7 cells were precultured with p53 AS ODN, followed by the addition of SN-38 for 24 h. Furthermore, increases in p53 DNA-binding activity were observed in the nuclei of Huh7 cells after SN-38 treatment as shown by electrophoretic mobility shift analysis. SN-38 binding motifs were detected in the proximal promoter of p53 (bases -433 to -317 and -814 to -711). These results suggest that the p53-mediated apoptosis pathway is important in the anticancer effects of irinotecan in hepatocellular carcinoma.

Cytoplasm-localized SIRT1 enhances apoptosis

In general, SIRT1 is localized in nuclei. Here, we showed that endogenous and exogenous SIRT1 were both able to partially localize in cytoplasm in certain cell lines, and cytoplasm-localized SIRT1 was associated with apoptosis and led to increased sensitivity to apoptosis. Furthermore, we demonstrated that translocation of nucleus-localized SIRT1 from nuclei to cytoplasm was the main pathway leading to localization of SIRT1 in cytoplasm. In HeLa cells, wild type SIRT1 was completely localized in nuclei. By truncation of two predicted nuclear localization signals or fusion with an exogenous nuclear export signal, SIRT1 was partially localized in cytoplasm of HeLa cells and resulted in increased sensitivity to apoptosis. The apoptosis enhanced by cytoplasm-localized SIRT1 was independent of its deacetylase activity, but dependent on caspases. SIRT1 was distributed in cytoplasm at metaphase during mitosis, and overexpression of SIRT1 significantly augmented apoptosis for cells at metaphase. In summary, we found SIRT1 is able to localize in cytoplasm, and cytoplasm-localized SIRT1 enhances apoptosis.

Cell cycle-dependent nuclear retention of p53 by E2F1 requires phosphorylation of p53 at Ser315

We show here that the cell cycle-dependent DNA-binding and transcriptional activity of p53 correlates with E2F expression in human primary fibroblasts. E2F1 binds and stimulates DNA-binding, transactivation and apoptotic functions of p53 but not p63 and p73. E2F1 binds residues 347-370 of p53 and enhances nuclear retention of Ser315 phosphorylated p53. This regulation of p53 by E2F1 is cell cycle dependent, as the cellular distribution of Ser315 phosphorylated p53 is associated with the periodic expression of E2F and cyclin A throughout the cell cycle. This is the first demonstration that the activities of p53 are regulated during the cell cycle by E2F/p53 interactions and that phosphorylation of p53 at Ser315 is required for this regulation.

Probing lysine acetylation in proteins: strategies, limitations, and pitfalls of in vitro acetyltransferase assays

The acetylation of proteins at specific lysine residues by acetyltransferase enzymes has emerged as a posttranslational modification of high biological impact. Although lysine acetylation in histone proteins is an integral part of the histone code the acetylation of a multitude of non-histone proteins was recently recognized as a regulatory signal in many cellular processes. New substrates of acetyltransferase enzymes are continuously identified, and the analysis of acetylation sites in proteins is increasingly performed by mass spectrometry. However, the characterization of lysine acetylation in proteins using mass spectrometric techniques has some limitations and pitfalls. The non-enzymatic cysteine acetylation especially can result in false-positive identification of acetylated proteins. Here we demonstrate the application of various mass spectrometric techniques such as matrix-assisted laser desorption/ionization and electrospray ionization mass spectrometry for the analysis of protein acetylation. We describe diverse combinations of biochemical methods useful to map the acetylation sites in proteins and discuss their advantages and limitations. As an example, we present a detailed analysis of the acetylation of the HIV-1 transactivator of transcription (Tat) protein, which is known to be acetylated in vivo by the acetyltransferases p300 and p300/CBP-associated factor (PCAF).

Posttranslational modifications of p27kip1 determine its binding specificity to different cyclins and cyclin-dependent kinases in vivo

Using 2-dimensional gel electrophoresis (2D-gel) analysis, we show here that cell-cycle entry is associated with a significant increase in p27(kip1) phosphorylation in human primary B cells. A similar pattern of increase in p27(kip1) phosphorylation was also seen in 2 fast-growing tumor cell lines, Burkitt lymphoma cell line BL40 and breast carcinoma cell line Cal51, where inactive p27(kip1) is expressed at high levels. Detailed analysis revealed for the first time that different cyclins and cyclin-dependent kinases (cdk's) interact with distinct posttranslationally modified isoforms of p27(kip1) in vivo. Cyclin E but not cyclin A selectively interacts with phosphorylated p27(kip1) isoforms, while cyclin D1 and D2 favor unphosphorylated p27(kip1) isoforms in vivo. Interestingly, cyclin D3 and cdk4 selectively interact with phosphorylated p27(kip1) in BL40 cells. Among all D-type cyclin/cdk4 and cdk6 complexes, cyclin D3/cdk4 is most active in sequestering the inhibitory activity of p27(kip1) in vitro in a cyclinE/cdk2 kinase assay. This novel feature of the binding specificity of p27(kip1) to cyclins and cdk's in vivo is interpreted in the context of overexpression of cyclin D3 in the presence of high levels of p27(kip1) in human B-cell lymphomas with adverse clinical outcome.

The Sir2 family of protein deacetylases

The yeast SIR protein complex has been implicated in transcription silencing and suppression of recombination. The Sir complex represses transcription at telomeres, mating-type loci, and ribosomal DNA. Unlike SIR3 and SIR4, the SIR2 gene is highly conserved in organisms ranging from archaea to humans. Interestingly, Sir2 is active as an NAD+-dependent deacetylase, which is broadly conserved from bacteria to higher eukaryotes. In this review, we discuss the role of NAD+, the unusual products of the deacetylation reaction, the Sir2 structure, and the Sir2 chemical inhibitors and activators that were recently identified. We summarize the current knowledge of the Sir2 homologs from different organisms, and finally we discuss the role of Sir2 in caloric restriction and aging.

Acetylation of the chemotaxis response regulator CheY by acetyl-CoA synthetase purified from Escherichia coli

Acetylation of CheY, the excitatory response regulator of bacterial chemotaxis, by the enzyme acetyl-CoA synthetase (Acs) is involved in Escherichia coli chemotaxis, but its function is obscure. Here, we overproduced Acs from E.coli, purified it in quantities sufficient for biochemical work, and characterized both the enzyme and the CheY acetylation reaction that it catalyzes. Such characterization is essential for revealing the function of CheY acetylation in chemotaxis. The enzyme exhibited characteristics typical of prokaryotic Acs enzymes, and it could use either acetate or AcCoA as an acetyl donor for CheY acetylation. The Acs-catalyzed acetylation of CheY was reversible, an essential property for a regulatory process, and cooperative (Hill coefficient approximately 3). By Western blotting with specific anti-acetyl-lysine antibody we demonstrated that Acs undergoes autoacetylation, that CheY is acetylated to a small extent when isolated, and that the extent is elevated following in vitro acetylation. Exposing the intact protein to matrix-assisted laser desorption ionization time-of-flight mass spectrometry and electro-spray mass spectrometry, we found that, in most cases, purified CheY is a mixture of species having zero to six acetyl groups per molecule, with non-acetylated CheY being the most abundant species. By proteolytic in-gel digestion of non-treated CheY followed by peptide fingerprinting, precursor ion scan, and tandem mass spectrometry, we found that the acetylation sites of CheY are clustered at the C terminus of the protein, with lysine residues 91, 92, 109, 119, 122 and 126 being the main acetylation sites. Following in vitro acetylation, the main change that seemed to occur was an incremental increase in the extent of acetylation of the same lysine residues. Thus, CheY is similar to many eukaryotic proteins involved in signaling, which undergo both phosphorylation and multiple acetylation, and in which the acetylation sites are restricted to a particular region.

Acetylation of the C terminus of Ku70 by CBP and PCAF controls Bax-mediated apoptosis

Apoptosis is a key tumor suppression mechanism that can be initiated by activation of the proapoptotic factor Bax. The Ku70 DNA end-joining protein has recently been shown to suppress apoptosis by sequestering Bax from mitochondria. The mechanism by which Bax is regulated remains unknown. Here, we identify eight lysines in Ku70 that are targets for acetylation in vivo. Five of these, K539, K542, K544, K533, and K556, lie in the C-terminal linker domain of Ku70 adjacent to the Bax interaction domain. We show that CBP and PCAF efficiently acetylate K542 in vitro and associate with Ku70 in vivo. Mimicking acetylation of K539 or K542 or treating cells with deacetylase inhibitors abolishes the ability of Ku70 to suppress Bax-mediated apoptosis. We demonstrate that increased acetylation of Ku70 disrupts the Ku70-Bax interaction and coincides with cytoplasmic accumulation of CBP. These results shed light on the role of acetyltransferases as tumor suppressors.

Regulating the p53 system through ubiquitination

The tumor suppressor p53 is tightly controlled at low levels in cells by constant ubiquitination and proteasomal degradation. In response to stresses, ubiquitination of p53 is inhibited through diverse pathways, depending on the nature of the stimulus and cell type. This leads to the accumulation and activation of p53, which induces cell cycle arrest and/or apoptosis to prevent cells from transformation. Many studies have indicated that defects of the p53 system are present in most, if not all, human tumor cells. Meanwhile, significant progress has been made in understanding the molecular mechanisms of p53 ubiquitination and the regulation of the p53 system. Therefore, it is possible now to consider targeting ubiquitination as a means to regulate and reactivate p53 in tumors. Emerging evidence suggests that inhibiting the E3 activity of Mdm2, blocking the interaction of p53 and Mdm2, and restoring the function of mutated p53 are potential effective strategies to kill certain tumor cells selectively. It is conceivable that new chemotherapeutic agents based on these studies will be generated in the not-so-distant future.

Prolyl isomerase Pin1: a catalyst for oncogenesis and a potential therapeutic target in cancer

Phosphorylation of proteins on serine or threonine residues preceding proline (Ser/Thr-Pro) is a major intracellular signaling mechanism. The phosphorylated Ser/Thr-Pro motifs in a certain subset of phosphoproteins are isomerized specifically by the peptidyl-prolyl cis-trans isomerase Pin1. This post-phosphorylation isomerization can lead to conformational changes in the substrate proteins and modulate their functions. Pin1 interacts with a number of mitotic phosphoproteins, and plays a critical role in mitotic regulation. Recent work indicates that Pin1 is overexpressed in many human cancers and plays an important role in oncogenesis. Pin1 regulates the expression of cyclin D1 by cooperating with Ras signaling and inhibiting the interaction of beta-catenin with the tumor suppressor APC and also directly stabilizing cyclin D1 protein. Furthermore, PIN1 is an E2F target gene essential for the Neu/Ras-induced transformation of mammary epithelial cells. Pin1 is also a critical regulator of the tumor suppressor p53 during DNA damage response. Given its role in cell growth control and oncogenesis, Pin1 could represent a new anti-cancer target.

Linking chromatin function with metabolic networks: Sir2 family of NAD(+)-dependent deacetylases

Chromatin remodeling enzymes rely on coenzymes derived from metabolic pathways, suggesting a tight synchronization among apparently diverse cellular processes. A unique example of this link is the recently described NAD(+)-dependent protein and/or histone deacetylases. The founding member of this family - yeast silent information regulator 2 (ySir2) - is involved in gene silencing, chromosomal stability and ageing. Sir2-like enzymes catalyze a reaction in which the cleavage of NAD(+)and histone and/or protein deacetylation are coupled to the formation of O-acetyl-ADP-ribose, a novel metabolite. The dependence of the reaction on both NAD(+) and the generation of this potential second messenger offers new clues to understanding the function and regulation of nuclear, cytoplasmic and mitochondrial Sir2-like enzymes.

Association of p53 and MSH2 with recombinative repair complexes during S phase

Our previous recombination and biochemical analyses have led to the hypothesis that the tumor suppressor p53 monitors homologous recombination, a function which was previously attributed to the mismatch repair protein MSH2. Here, we show that a certain fraction of p53 is concentrated within discrete nuclear foci of cells synchronized in G1 phase, a pattern which becomes even more pronounced in S phase, especially after gamma-ray treatment. p53 foci show some colocalization with MSH2 within distinct foci during G1 phase, while dots formed by BRCA1 display an independent localization pattern. In S phase nuclei, p53 foci almost completely colocalize with MSH2 foci and associate with the recombination surveillance factor BRCA1 in irradiated S phase cells. These p53 and MSH2 foci also show significant overlaps with foci of the recombination enzymes Rad50 and Rad51, which for the first time unveiled recombination-related functions of p53 in replicating cells. During S phase, p53 and MSH2 are maximally active in binding to early recombination intermediates, and coexist within the same nuclear DNA-protein complexes. Our data suggest that p53 is linked similarly to homologous recombination as MSH2 and provide further evidence for the new concept of a dual role of p53 in the regulation of growth and repair.

Conserved enzymatic production and biological effect of O-acetyl-ADP-ribose by silent information regulator 2-like NAD+-dependent deacetylases

Silent information regulator 2 (Sir2) family of enzymes has been implicated in many cellular processes that include histone deacetylation, gene silencing, chromosomal stability, and aging. Yeast Sir2 and several homologues have been shown to be NAD(+)-dependent histone/protein deacetylases. Previously, it was demonstrated that the yeast enzymes catalyze a unique reaction mechanism in which the cleavage of NAD(+) and the deacetylation of substrate are coupled with the formation of O-acetyl-ADP-ribose, a novel metabolite. We demonstrate that the production of O-acetyl-ADP-ribose is evolutionarily conserved among Sir2-like enzymes from yeast, Drosophila, and human. Also, endogenous yeast Sir2 complex from telomeres was shown to generate O-acetyl-ADP-ribose. By using a quantitative microinjection assay to examine the possible biological function(s) of this newly discovered metabolite, we demonstrate that O-acetyl-ADP-ribose causes a delay/block in oocyte maturation and results in a delay/block in embryo cell division in blastomeres. This effect was mimicked by injection of low nanomolar levels of active enzyme but not with a catalytically impaired mutant, indicating that the enzymatic activity is essential for the observed effects. In cell-free oocyte extracts, we demonstrate the existence of cellular enzymes that can efficiently utilize O-acetyl-ADP-ribose.

Baseline resolution of isobaric phosphorylated and sulfated peptides and nucleotides by electrospray ionization FTICR ms: another step toward mass spectrometry-based proteomics

Electrospray ionization broadband FTICR mass spectrometry at a mass resolving power, m/delta m50% > or = 400,000 has achieved the first direct mass spectral resolution of phosphorylated and sulfated peptides (or nucleotides) of the same nominal mass. The elemental composition difference in each case is PH versus S (9.5 mDa), requiring a minimum mass resolving power ((m2 - m1)/ml) of 118,000 (C terminal amidated cholecystekinin fragment 26-33 (CCK-8), DY(PO3H2)MGWMDF-NH2 versus DY(SO3H)MGWMDF-NH2) or 65,400 (adenosine triphosphate vs 3-phosphoadenosine 5'-phosphosulfate). The isobaric mass doublets were detected in broadband mode (400 < m/z <1400) in the presence of dozens of other species. It is therefore now possible to distinguish phosphorylated from sulfated peptides, even when both species are present at the same time in a protein digest.

Chemistry of gene silencing: the mechanism of NAD+-dependent deacetylation reactions

The Sir2 enzyme family is responsible for a newly classified chemical reaction, NAD(+)-dependent protein deacetylation. New peptide substrates, the reaction mechanism, and the products of the acetyl transfer to NAD(+) are described for SIR2. The final products of SIR2 reactions are the deacetylated peptide and the 2' and 3' regioisomers of O-acetyl ADP ribose (AADPR), formed through an alpha-1'-acetyl ADP ribose intermediate and intramolecular transesterification reactions (2' --> 3'). The regioisomers, their anomeric forms, the interconversion rates, and the reaction equilibria were characterized by NMR, HPLC, 18O exchange, and MS methods. The mechanism of acetyl transfer to NAD(+) includes (1) ADP ribosylation of the peptide acyl oxygen to form a high-energy O-alkyl amidate intermediate, (2) attack of the 2'-OH group on the amidate to form a 1',2'-acyloxonium species, (3) hydrolysis to 2'-AADPR by the attack of water on the carbonyl carbon, and (4) an SIR2-independent transesterification equilibrating the 2'- and 3'-AADPRs. This mechanism is unprecedented in ADP-ribosyl transferase enzymology. The 2'- and 3'-AADPR products are candidate molecules for SIR2-initiated signaling pathways.

hSIR2(SIRT1) functions as an NAD-dependent p53 deacetylase

DNA damage-induced acetylation of p53 protein leads to its activation and either growth arrest or apoptosis. We show here that the protein product of the gene hSIR2(SIRT1), the human homolog of the S. cerevisiae Sir2 protein known to be involved in cell aging and in the response to DNA damage, binds and deacetylates the p53 protein with a specificity for its C-terminal Lys382 residue, modification of which has been implicated in the activation of p53 as a transcription factor. Expression of wild-type hSir2 in human cells reduces the transcriptional activity of p53. In contrast, expression of a catalytically inactive hSir2 protein potentiates p53-dependent apoptosis and radiosensitivity. We propose that hSir2 is involved in the regulation of p53 function via deacetylation.

Site-specific phosphorylation of human p53 protein determined by mass spectrometry

Human recombinant p53 (r-p53) protein was studied by mass spectrometry (MS) to determine site-specific posttranslational differences between basal and hyperphosphorylated r-p53. Wild-type p53 was basally expressed after baculovirus infection while a parallel preparation was treated with the phosphatase inhibitor okadaic acid during the terminal stages of expression to create a hyperphosphorylated form of p53 known for its higher DNA binding and transcriptional activation. After immunoaffinity and HPLC purification, MALDI/MS measured a higher molecular mass for r-p53 from okadaic acid treatment relative to control, suggesting a higher phosphorylation state. This was supported by an acidic shift of r-p53 isoforms separated by gel isoelectric focusing. Employing a variety of mass spectrometric analyses combined with separation and affinity techniques, six specific phosphorylation sites of p53 were identified. The MS data indicated that hyperphosphorylated p53 showed a higher degree of phosphorylation than basal p53 at specific amino- and carboxy-terminal sites. In particular, ESI-MS demonstrated that Ser(315) was entirely phosphorylated after okadaic acid treatment, as confirmed biochemically by CDK2 kinase assay and by isoelectric focusing. In summary, MS analysis uniquely revealed increased, site-specific phosphorylations on p53 after phosphatase inhibition, particularly at Ser(315), which may be critical molecular events in defining p53 activity.

Stoichiometric phosphorylation of human p53 at Ser315 stimulates p53-dependent transcription

p53 protein activity as a transcription factor can be activated in vivo by antibodies that target its C-terminal negative regulatory domain suggesting that cellular enzymes that target this domain may play a role in stimulating p53-dependent gene expression. A phospho-specific monoclonal antibody to the C-terminal Ser(315) phospho-epitope was used to determine whether phosphorylation of endogenous p53 at Ser(315) can be detected in vivo, whether steady-state Ser(315) phosphorylation increases or decreases in an irradiated cell, and whether this phosphorylation event activates or inhibits p53 in vivo. A native phospho-specific IgG binding assay was developed for quantitating the extent of p53 phosphorylation at Ser(315) where one, two, three, or four phosphates/tetramer could be defined after in vitro phosphorylation by cyclin-dependent protein kinases. Using this assay, near-stoichiometric Ser(315) phosphorylation of endogenous p53 protein was detected in vivo after UV irradiation of MCF7 and A375 cells, coinciding with elevated p53-dependent transcription. Transfection of the p53 gene with an alanine mutation at the Ser(315) site into Saos-2 cells gave rise to a form of p53 protein with a substantially reduced specific activity as a transcription factor. The treatment of cells with the cyclin-dependent protein kinase inhibitor Roscovitine promoted a reduction in the specific activity of endogenous p53 or ectopically expressed p53. These results indicate that the majority of p53 protein has been phosphorylated at Ser(315) after irradiation damage and identify a cyclin-dependent kinase pathway that plays a role in stimulating p53 function.

Strategies for manipulating the p53 pathway in the treatment of human cancer

Human cancer progression is driven in part by the mutation of oncogenes and tumour-suppressor genes which, under selective environmental pressures, give rise to evolving populations of biochemically altered cells with enhanced tumorigenic and metastatic potential. Given that human cancers are biologically and pathologically quite distinct, it has been quite surprising that a common event, perturbation of the p53 pathway, occurs in most if not all types of human cancers. The central role of p53 as a tumour-suppressor protein has fuelled interest in defining its mechanism of function and regulation, determining how its inactivation facilitates cancer progression, and exploring the possibility of restoring p53 function for therapeutic benefit. This review will highlight the key biochemical properties of p53 protein that affect its tumour-suppressor function and the experimental strategies that have been developed for the re-activation of the p53 pathway in cancers.

The human histone deacetylase family

Since the identification of the first histone deacetylase (Taunton et al., Science 272, 408-411), several new members have been isolated. They can loosely be separated into entities on the basis of their similarity to various yeast histone deacetylases. The first class is represented by its closeness to the yeast Rpd3-like proteins, and the second most recently discovered class has similarities to yeast Hda1-like proteins. However, due to the fact that several different research groups isolated the Hda1-like histone deacetylases independently, there have been various different nomenclatures used to describe the various members, which can lead to confusion in the interpretation of this family's functions and interactions. With the discovery of another novel murine histone deacetylase, homologous to yeast Sir2, the number of members of this family is set to increase, as 7 human homologues of this gene have been isolated. In the light of these recent discoveries, we have examined the literature data and conducted a database analysis of the isolated histone deacetylases and potential candidates. The results obtained suggest that the number of histone deacetylases within the human genome may be as high as 17 and are discussed in relation to their homology to the yeast histone deacetylases.

Increased p53 phosphorylation after microtubule disruption is mediated in a microtubule inhibitor- and cell-specific manner

p53 is present at low levels in unstressed cells. Numerous cellular insults, including DNA damage and microtubule disruption, elevate p53 protein levels. Phosphorylation of p53 is proposed to be important for p53 stabilization and activation after genotoxic stress; however, p53 phosphorylation after microtubule disruption has not been analysed. The goal of the current study was to determine if p53 phosphorylation increases after microtubule disruption, and if so, to identify specific p53 residues necessary for microtubule inhibitor-induced phosphorylation. Two dimensional gel analyses demonstrated that the number of p53 phospho-forms in cells increased after treatment with microtubule inhibitors (MTIs) and that the pattern of p53 phosphorylation was distinct from that observed after DNA damage. p53 phosphorylation also varied in a MTI-dependent manner, as Taxol and Vincristine induced more p53 phospho-forms than nocodazole. Further, MTI treatment increased phosphorylation of p53 on serine-15 in epithelial tumor cells. In contrast, serine-15 phosphorylation of p53 did not increase in MTI-treated primary cultures of human fibroblasts. Analysis of ectopically expressed p53 phospho-mutant proteins from Taxol- and nocodazole-treated cells indicated that multiple p53 amino terminal residues, including serine-15 and threonine-18, were required for Taxol-mediated phosphorylation of p53. Taken together, the results of this study demonstrate that distinct p53 phospho-forms are induced by MTI treatment as compared to DNA damage and that p53 phosphorylation is mediated in a MTI- and cell-specific manner. Oncogene (2001) 20, 113 - 124.

Regulation of transcription of the human presenilin-1 gene by ets transcription factors and the p53 protooncogene

The expression of the human presenilin-1 cellular gene is suppressed by the p53 protooncogene. The rapid kinetic of the down-regulation has suggested that it may result from a primary mechanism. We show here that p53 also suppresses the transcription of a presenilin-1 promoter-chloramphenicol acetyltransferase reporter synthetic gene in transient infection assays in neuroblastoma (SK-N-SH) and hepatoma (HepG2) cell lines. Only a minimum promoter including sequences from -35 to + 6 from the transcription initiation is sufficient to confer down-regulation. We have previously defined a crucial DNA element controlling 90% of the expression of the gene within the same short area, and the identification of the transcription factors involved should also provide insights into the regulation of PS1 by p53. This region contains an Ets transcription factor binding motif, and a 2-base pair alteration within the core sequence (GGAA to TTAA) of the Ets consensus also reduced transcription by more than 90%. We now show that Ets1 and Ets2 indeed transactivate a PS1 promoter-chloramphenicol acetyltransferase reporter including the (-35 to +6) fragment. Furthermore, in vitro translated Ets2 binds specifically to the -10 Ets motif in electrophoretic mobility shift assays. Therefore, Ets1/2 factors bind specifically to the -10 Ets element and activate PS1 transcription. We also show that the coactivator p300 enhances the activation by Ets1 and Ets2 as well as the repression by p53. p300 is known to interact with p53 as well as with Ets1 and Ets2. We show that p53 does not bind directly to the PS1 promoter. Hence the repression of PS1 transcription by p53 is likely to be mediated through protein-protein interactions.

Synergistic activation of p53-dependent transcription by two cooperating damage recognition pathways

High level activation of p53-dependent transcription occurs following cellular exposure to genotoxic damaging agents such as UV-C, while ionizing radiation damage does not induce a similarly potent induction of p53-dependent gene expression. Reasoning that one of the major differences between UV-C and ionizing radiation damage is that the latter does not inhibit general transcription, we attempted to reconstitute p53-dependent gene expression in ionizing irradiated cells by co-treatment with selected transcription inhibitors that alone do not activate p53. p53-dependent transcription can be dramatically enhanced by the treatment of ionizing irradiated cells with low doses of DRB, which on its own does not induce p53 activity. The mechanism of ionizing radiation-dependent activation of p53-dependent transcription using DRB is more likely due to inhibition of gene transcription rather than prolonged DNA damage, as the non-genotoxic and general transcription inhibitor Roscovitine also synergistically activates p53 function in ionizing irradiated cells. These results identify two distinct signal transduction pathways that cooperate to fully activate p53-dependent gene expression: one responding to lesions induced by ionizing radiation and the second being a kinase pathway that regulates general RNA Polymerase II activity.