Crosstalk between site-specific modifications on p53 and histone H3

Epigenetic regulation of TP53 is involved in prostate cancer radioresistance and DNA damage response signaling

External beam radiotherapy (RT) is a leading first-line therapy for prostate cancer (PCa), and, in recent years, significant advances have been accomplished. However, RT resistance can arise and result in long-term recurrence or disease progression in the worst-case scenario. Thus, making crucial the discovery of new targets for PCa radiosensitization. Herein, we generated a radioresistant PCa cell line, and found p53 to be highly expressed in radioresistant PCa cells, as well as in PCa patients with recurrent/disease progression submitted to RT. Mechanism dissection revealed that RT could promote p53 expression via epigenetic modulation. Specifically, a decrease of H3K27me3 occupancy at TP53 gene promoter, due to increased KDM6B activity, was observed in radioresistant PCa cells. Furthermore, p53 is essential for efficient DNA damage signaling response and cell recovery upon stress induction by prolonged fractionated irradiation. Remarkably, KDM6B inhibition by GSK-J4 significantly decreased p53 expression, consequently attenuating the radioresistant phenotype of PCa cells and hampering in vivo 3D tumor formation. Overall, this work contributes to improve the understanding of p53 as a mediator of signaling transduction in DNA damage repair, as well as the impact of epigenetic targeting for PCa radiosensitization.

BTK blocks the inhibitory effects of MDM2 on p53 activity

p53 is a tumour suppressor that is activated in response to various types of stress. It is regulated by a complex pattern of over 50 different post-translational modifications, including ubiquitination by the E3 ligase MDM2, which leads to its proteasomal degradation. We have previously reported that expression of Bruton’s Tyrosine Kinase (BTK) induces phosphorylation of p53 at the N-terminus, including Serine 15, and increases its protein levels and activity. The mechanisms involved in this process are not completely understood. Here, we show that BTK also increases MDM2 and is necessary for MDM2 upregulation after DNA damage, consistent with what we have shown for other p53 target genes. Moreover, we found that BTK binds to MDM2 on its PH domain and induces its phosphorylation. This suggested a negative regulation of MDM2 functions by BTK, supported by the fact BTK expression rescued the inhibitory effects of MDM2 on p53 transcriptional activity. Indeed, we observed that BTK mediated the loss of the ubiquitination activity of MDM2, a process that was dependent on the phosphorylation functions of BTK. Our data together shows that the kinase activity of BTK plays an important role in disrupting the MDM2-p53 negative feedback loop by acting at different levels, including binding to and inactivation of MDM2. This study provides a potential mechanism to explain how BTK modulates p53 functions.

Citrullination/Methylation Crosstalk on Histone H3 Regulates ER-Target Gene Transcription

Posttranslational modifications of histone tails are a key contributor to epigenetic regulation. Histone H3 Arg26 and Lys27 are both modified by multiple enzymes, and their modifications have profound effects on gene expression. Citrullination of H3R26 by PAD2 and methylation of H3K27 by PRC2 have opposing downstream impacts on gene regulation; H3R26 citrullination activates gene expression, and H3K27 methylation represses gene expression. Both of these modifications are drivers of a variety of cancers, and their writer enzymes, PAD2 and EZH2, are the targets of drug therapies. After biochemical and cell-based analysis of these modifications, a negative crosstalk interaction is observed. Methylation of H3K27 slows citrullination of H3R26 30-fold, whereas citrullination of H3R26 slows methylation 30,000-fold. Examination of the mechanism of this crosstalk interaction uncovered a change in structure of the histone tail upon citrullination which prevents methylation by the PRC2 complex. This mechanism of crosstalk is reiterated in cell lines using knockdowns and inhibitors of both enzymes. Based our data, we propose a model in which, after H3 Cit26 formation, H3K27 demethylases are recruited to the chromatin to activate transcription. In total, our studies support the existence of crosstalk between citrullination of H3R26 and methylation of H3K27.

Bioinformatics study of cancer-related mutations within p53 phosphorylation site motifs

p53 protein has about thirty phosphorylation sites located at the N- and C-termini and in the core domain. The phosphorylation sites are relatively less mutated than other residues in p53. To understand why and how p53 phosphorylation sites are rarely mutated in human cancer, using a bioinformatics approaches, we examined the phosphorylation site and its nearby flanking residues, focusing on the consensus phosphorylation motif pattern, amino-acid correlations within the phosphorylation motifs, the propensity of structural disorder of the phosphorylation motifs, and cancer mutations observed within the phosphorylation motifs. Many p53 phosphorylation sites are targets for several kinases. The phosphorylation sites match 17 consensus sequence motifs out of the 29 classified. In addition to proline, which is common in kinase specificity-determining sites, we found high propensity of acidic residues to be adjacent to phosphorylation sites. Analysis of human cancer mutations in the phosphorylation motifs revealed that motifs with adjacent acidic residues generally have fewer mutations, in contrast to phosphorylation sites near proline residues. p53 phosphorylation motifs are mostly disordered. However, human cancer mutations within phosphorylation motifs tend to decrease the disorder propensity. Our results suggest that combination of acidic residues Asp and Glu with phosphorylation sites provide charge redundancy which may safe guard against loss-of-function mutations, and that the natively disordered nature of p53 phosphorylation motifs may help reduce mutational damage. Our results further suggest that engineering acidic amino acids adjacent to potential phosphorylation sites could be a p53 gene therapy strategy.

Surf the post-translational modification network of p53 regulation

Among the human genome, p53 is one of the first tumor suppressor genes to be discovered. It has a wide range of functions covering cell cycle control, apoptosis, genome integrity maintenance, metabolism, fertility, cellular reprogramming and autophagy. Although different possible underlying mechanisms for p53 regulation have been proposed for decades, none of them is conclusive. While much literature focuses on the importance of individual post-translational modifications, further explorations indicate a new layer of p53 coordination through the interplay of the modifications, which builds up a complex 'network'. This review focuses on the necessity, characteristics and mechanisms of the crosstalk among post-translational modifications and its effects on the precise and selective behavior of p53.

Aurora A mediates cross-talk between N- and C-terminal post-translational modifications of p53

The serine/threonine protein kinase Aurora A is known to interact with and phosphorylate tumor suppressor p53 at Serine 215 (S215), inhibiting the transcriptional activity of p53. We show that Aurora A positively regulates human p53 protein levels and, using isogenic p53 wild-type and p53-null colorectal carcinoma cells, further show that p53 regulates human Aurora A protein expression. S215 is located in the DNA-binding core of p53 and at the center of the cryptic epitope for PAb240 antibody, which is used to detect mutant and denatured p53. Following denaturing SDS PAGE, the PAb240 epitope was detectable by immunoblotting in only two out of eight cell lines. The efficacy of novel p53-targeted anticancer therapies may be influenced by the conformational state of p53, therefore, the initial determination of p53 status may be relevant. We found no correlation between phosphorylation of p53 at S215 and PAb240 antibody recognition. However, phosphorylation at S37 was positively associated with PAb240 reactivity. More importantly, we provide the first evidence of Aurora A-mediated cross-talk between N- and C-terminal p53 post-translational modifications. As p53 and Aurora A are targets for anticancer therapy the impact of their reciprocal relationship and Aurora A-induced post-translational modification of p53 should be considered.

ATR-p53 restricts homologous recombination in response to replicative stress but does not limit DNA interstrand crosslink repair in lung cancer cells

Homologous recombination (HR) is required for the restart of collapsed DNA replication forks and error-free repair of DNA double-strand breaks (DSB). However, unscheduled or hyperactive HR may lead to genomic instability and promote cancer development. The cellular factors that restrict HR processes in mammalian cells are only beginning to be elucidated. The tumor suppressor p53 has been implicated in the suppression of HR though it has remained unclear why p53, as the guardian of the genome, would impair an error-free repair process. Here, we show for the first time that p53 downregulates foci formation of the RAD51 recombinase in response to replicative stress in H1299 lung cancer cells in a manner that is independent of its role as a transcription factor. We find that this downregulation of HR is not only completely dependent on the binding site of p53 with replication protein A but also the ATR/ATM serine 15 phosphorylation site. Genetic analysis suggests that ATR but not ATM kinase modulates p53's function in HR. The suppression of HR by p53 can be bypassed under experimental conditions that cause DSB either directly or indirectly, in line with p53's role as a guardian of the genome. As a result, transactivation-inactive p53 does not compromise the resistance of H1299 cells to the interstrand crosslinking agent mitomycin C. Altogether, our data support a model in which p53 plays an anti-recombinogenic role in the ATR-dependent mammalian replication checkpoint but does not impair a cell's ability to use HR for the removal of DSB induced by cytotoxic agents.

Formation of stress-specific p53 binding patterns is influenced by chromatin but not by modulation of p53 binding affinity to response elements

The p53 protein is crucial for adapting programs of gene expression in response to stress. Recently, we revealed that this occurs partly through the formation of stress-specific p53 binding patterns. However, the mechanisms that generate these binding patterns remain largely unknown. It is not established whether the selective binding of p53 is achieved through modulation of its binding affinity to certain response elements (REs) or via a chromatin-dependent mechanism. To shed light on this issue, we used a microsphere assay for protein-DNA binding to measure p53 binding patterns on naked DNA. In parallel, we measured p53 binding patterns within chromatin using chromatin immunoprecipitation and DNase I coupled to ligation-mediated polymerase chain reaction footprinting. Through this experimental approach, we revealed that UVB and Nutlin-3 doses, which lead to different cellular outcomes, induce similar p53 binding patterns on naked DNA. Conversely, the same treatments lead to stress-specific p53 binding patterns on chromatin. We show further that altering chromatin remodeling using an histone acetyltransferase inhibitor reduces p53 binding to REs. Altogether, our results reveal that the formation of p53 binding patterns is not due to the modulation of sequence-specific p53 binding affinity. Rather, we propose that chromatin and chromatin remodeling are required in this process.

Nordihydroguaiaretic acid inhibits growth of cervical cancer SiHa cells by up-regulating p21

Nordihydroguaiaretic acid (NDGA) and its derivatives possess anti-cancer effects on various types of cancer via the induction of apoptosis or cell cycle arrest. This study proved that NDGA inhibited cervical cancer SiHa cell growth and induced cell cycle arrest at the G(1) phase, which may be a consequence of cell cycle kinase inhibitor p21 induction. NDGA promoted acetylation of histone H3 in total and p21 gene-associated chromatin. This effect is gene selective, since NDGA has no impact on the p27 gene. NDGA also inhibited HPV-16 E6 gene transcription, which in turn resulted in the restoration of p53 protein levels. The silencing mediator for retinoid and thyroid hormone receptors (SMRT) is a key component of the HDAC3-HDAC4-N-CoR/SMRT complex. We found that NDGA significantly inhibited the transcription of SMRT, which, together with p53, may aid in the detection of the increase of histone H3 acetylation within the p21 gene. Our results suggest that NDGA induces p21 transcription by selectively elevating histone H3 acetylation associated with p21 gene and p53 protein levels via the inhibition of HPV-16 E6 expression.

SIRT1 undergoes alternative splicing in a novel auto-regulatory loop with p53

BACKGROUND

The NAD-dependent deacetylase SIRT1 is a nutrient-sensitive coordinator of stress-tolerance, multiple homeostatic processes and healthspan, while p53 is a stress-responsive transcription factor and our paramount tumour suppressor. Thus, SIRT1-mediated inhibition of p53 has been identified as a key node in the common biology of cancer, metabolism, development and ageing. However, precisely how SIRT1 integrates such diverse processes remains to be elucidated.

METHODOLOGY/PRINCIPAL FINDINGS

Here we report that SIRT1 is alternatively spliced in mammals, generating a novel SIRT1 isoform: SIRT1-ΔExon8. We show that SIRT1-ΔExon8 is expressed widely throughout normal human and mouse tissues, suggesting evolutionary conservation and critical function. Further studies demonstrate that the SIRT1-ΔExon8 isoform retains minimal deacetylase activity and exhibits distinct stress sensitivity, RNA/protein stability, and protein-protein interactions compared to classical SIRT1-Full-Length (SIRT1-FL). We also identify an auto-regulatory loop whereby SIRT1-ΔExon8 can regulate p53, while in reciprocal p53 can influence SIRT1 splice variation.

CONCLUSIONS/SIGNIFICANCE

We characterize the first alternative isoform of SIRT1 and demonstrate its evolutionary conservation in mammalian tissues. The results also reveal a new level of inter-dependency between p53 and SIRT1, two master regulators of multiple phenomena. Thus, previously-attributed SIRT1 functions may in fact be distributed between SIRT1 isoforms, with important implications for SIRT1 functional studies and the current search for SIRT1-activating therapeutics to combat age-related decline.

Drosophila p53 is required to increase the levels of the dKDM4B demethylase after UV-induced DNA damage to demethylate histone H3 lysine 9

Chromatin undergoes a variety of changes in response to UV-induced DNA damage, including histone acetylation. In human and Drosophila cells, this response is affected by mutations in the tumor suppressor p53. In this work, we report that there is a global decrease in trimethylated Lys-9 in histone H3 (H3K9me3) in salivary gland cells in wild type flies in response to UV irradiation. In contrast, flies with mutations in the Dmp53 gene have reduced basal levels of H3K9me3, which are then increased after UV irradiation. The reduction of H3K9me3 in response to DNA damage occurs preferentially in heterochromatin. Our experiments demonstrate that UV irradiation enhances the levels of Lys-9 demethylase (dKDM4B) transcript and protein in wild type flies, but not in Dmp53 mutant flies. Dmp53 binds to a DNA element in the dKdm4B gene as a response to UV irradiation. Furthermore, heterozygous mutants for the dKdm4B gene are more sensitive to UV irradiation; they are deficient in the removal of cyclobutane-pyrimidine dimers, and the decrease of H3K9me3 levels following DNA damage is not observed in dKdm4B mutant flies. We propose that in response to UV irradiation, Dmp53 enhances the expression of the dKDM4B histone demethylase, which demethylates H3K9me3 preferentially in heterochromatin regions. This mechanism appears to be essential for the proper function of the nucleotide excision repair system.

Oncogenic viral protein HPV E7 up-regulates the SIRT1 longevity protein in human cervical cancer cells

Senescence is blocked in human cervical keratinocytes infected with high risk human papillomavirus (e.g. HPV type16). Viral oncoproteins HPV E6 and HPV E7 access the cell cycle via cellular p53 and retinoblastoma proteins respectively. Previously we have shown that HPV E7, not HPV E6, is also responsible for cervical cancer cell survival (SiHa cells; HPV type16). We now present evidence that SIRT1, an aging-related NAD-dependent deacetylase, mediates HPV E7 survival function in SiHa cervical cancer cells. Moreover, HPV E7 up-regulates SIRT1 protein when expressed in primary human keratinocytes. Conversely, SIRT1 levels decrease following RNAi-mediated silencing of HPV E7 in SiHa cells. Silencing HPV E6 has no effect on SIRT1 but, as expected, causes marked accumulation of p53 protein accompanied by p53-mediated up-regulation of p21. However, p53 acetylation (K382Ac) was barely detectable. Since p53 is a known SIRT1 substrate we propose that elevated SIRT1 levels (induced by HPV E7) attenuate p53 pro-apoptotic capacity via its de-acetylation. Our discovery that HPV E7 up-regulates SIRT1 links a clinically important oncogenic virus with the multi-functional SIRT1 protein. This link may open the way for a more in-depth understanding of the process of HPV-induced malignant transformation and also of the inter-relationships between aging and cancer.