ATM kinase is a master switch for the Delta Np63 alpha phosphorylation/degradation in human head and neck squamous cell carcinoma cells upon DNA damage

The dual role of p63 in cancer

The p53 family is made up of three transcription factors: p53, p63, and p73. These proteins are well-known regulators of cell function and play a crucial role in controlling various processes related to cancer progression, including cell division, proliferation, genomic stability, cell cycle arrest, senescence, and apoptosis. In response to extra- or intracellular stress or oncogenic stimulation, all members of the p53 family are mutated in structure or altered in expression levels to affect the signaling network, coordinating many other pivotal cellular processes. P63 exists as two main isoforms (TAp63 and ΔNp63) that have been contrastingly discovered; the TA and ΔN isoforms exhibit distinguished properties by promoting or inhibiting cancer progression. As such, p63 isoforms comprise a fully mysterious and challenging regulatory pathway. Recent studies have revealed the intricate role of p63 in regulating the DNA damage response (DDR) and its impact on diverse cellular processes. In this review, we will highlight the significance of how p63 isoforms respond to DNA damage and cancer stem cells, as well as the dual role of TAp63 and ΔNp63 in cancer.

DNA damage response revisited: the p53 family and its regulators provide endless cancer therapy opportunities

Antitumor therapeutic strategies that fundamentally rely on the induction of DNA damage to eradicate and inhibit the growth of cancer cells are integral approaches to cancer therapy. Although DNA-damaging therapies advance the battle with cancer, resistance, and recurrence following treatment are common. Thus, searching for vulnerabilities that facilitate the action of DNA-damaging agents by sensitizing cancer cells is an active research area. Therefore, it is crucial to decipher the detailed molecular events involved in DNA damage responses (DDRs) to DNA-damaging agents in cancer. The tumor suppressor p53 is active at the hub of the DDR. Researchers have identified an increasing number of genes regulated by p53 transcriptional functions that have been shown to be critical direct or indirect mediators of cell fate, cell cycle regulation, and DNA repair. Posttranslational modifications (PTMs) primarily orchestrate and direct the activity of p53 in response to DNA damage. Many molecules mediating PTMs on p53 have been identified. The anticancer potential realized by targeting these molecules has been shown through experiments and clinical trials to sensitize cancer cells to DNA-damaging agents. This review briefly acknowledges the complexity of DDR pathways/networks. We specifically focus on p53 regulators, protein kinases, and E3/E4 ubiquitin ligases and their anticancer potential.

CDK1 Promotes Epithelial–Mesenchymal Transition and Migration of Head and Neck Squamous Carcinoma Cells by Repressing ∆Np63α-Mediated Transcriptional Regulation

∆Np63α is a key transcription factor overexpressed in types of squamous cell carcinomas (SCCs), which represses epithelial–mesenchymal transition (EMT) and cell migration. In this study, we found that CDK1 phosphorylates ∆Np63α at the T123 site, impairing its affinity to the target promoters of its downstream genes and its regulation of them in turn. Database analysis revealed that CDK1 is overexpressed in head and neck squamous cell carcinomas (HNSCCs), especially the metastatic HNSCCs, and is negatively correlated with overall survival. We further found that CDK1 promotes the EMT and migration of HNSCC cells by inhibiting ∆Np63α. Altogether, our study identified CDK1 as a novel regulator of ΔNp63α, which can modulate EMT and cell migration in HNSCCs. Our findings will help to elucidate the migration mechanism of HNSCC cells.

Distinct interactors define the p63 transcriptional signature in epithelial development or cancer

The TP63 is an indispensable transcription factor for development and homeostasis of epithelia and its derived glandular tissue. It is also involved in female germline cell quality control, muscle and thymus development. It is expressed as multiple isoforms transcribed by two independent promoters, in addition to alternative splicing occurring at the mRNA 3′-UTR. Expression of the TP63 gene, specifically the amino-deleted p63 isoform, ΔNp63, is required to regulate numerous biological activities, including lineage specification, self-renewal capacity of epithelial stem cells, proliferation/expansion of basal keratinocytes, differentiation of stratified epithelia. In cancer, ΔNp63 is implicated in squamous cancers pathogenesis of different origin including skin, head and neck and lung and in sustaining self-renewal of cancer stem cells. How this transcription factor can control such a diverse set of biological pathways is central to the understanding of the molecular mechanisms through which p63 acquires oncogenic activity, profoundly changing its down-stream transcriptional signature. Here, we highlight how different proteins interacting with p63 allow it to regulate the transcription of several central genes. The interacting proteins include transcription factors/regulators, epigenetic modifiers, and post-transcriptional modifiers. Moreover, as p63 depends on its interactome, we discuss the hypothesis to target the protein interactors to directly affect p63 oncogenic activities and p63-related diseases.

ΔNp63α promotes Bortezomib resistance via the CYGB-ROS axis in head and neck squamous cell carcinoma

Bortezomib, a proteasome inhibitor, proved potent in the treatment of recurrent multiple myeloma or mantle cell lymphoma. However, slow progress was made when it was applied to treat solid tumors. We discovered that different head and neck squamous cell carcinoma (HNSCC) cell lines had significantly different sensitivities to bortezomib, and also demonstrated that individual relatively sensitive HNSCC cell lines had fewer ΔNp63α expressions. Based on these findings, we speculated that ΔNp63α may be a key factor in the resistance of HNSCC cells to bortezomib. ΔNp63α knockdown made HNSCC more sensitive to bortezomib, while ΔNp63α overexpression made it more resistant. RNA sequencing (RNA-seq) analysis of ΔNp63α-knockdown cells revealed clear alterations in the subset of genes that were associated with oxidative stress and antioxidant defense. The gene CYGB was downregulated significantly. CHIP-seq detection showed that CYGB was the transcriptional regulatory site of ΔNp63α. CHIP-PCR showed evidence of ΔNp63α binding. The detection of the dual-luciferase reporter gene demonstrated that ΔNp63α significantly enhanced the CYGB promoter activity. Furthermore, we confirmed that CYGB plays a role in clearing excess ROS induced by bortezomib to inhibit HNSCC apoptosis. Consequently, ΔNp63α regulated the expression of CYGB in HNSCC. CYGB was the target of transcription regulation of ΔNp63α. It reduced apoptosis by clearing excess ROS produced by bortezomib, and thus exerted drug resistance.

SMG1 and CDK12 Link ΔNp63α Phosphorylation to RNA Surveillance in Keratinocytes

The tumor suppressor p53-like protein p63 is required for self-renewal of epidermal tissues. Loss of p63 or exposure to ultraviolet (UV) irradiation triggers terminal differentiation in keratinocytes. However, it remains unclear how p63 diverts epidermal cells from proliferation to terminal differentiation, thereby contributing to successful tissue self-renewal. Here, we used bottom-up proteomics to identify the proteome at the chromatin in normal human epidermal keratinocytes following UV irradiation and p63 depletion. We found that loss of p63 increased DNA damage and that UV irradiation recruited the cyclin-dependent kinase CDK12 and the serine/threonine protein kinase SMG1 to chromatin only in the presence of p63. A post-translational modification analysis of ΔNp63α with mass spectrometry revealed that phosphorylation of T357/S358 and S368 was dependent on SMG1, whereas CDK12 increased the phosphorylation of ΔNp63α at S66/S68 and S301. Indirect phosphorylation of ΔNp63α in the presence of SMG1 enabled ΔNp63α to bind to the tumor suppressor p53-specific DNA recognition sequence, whereas CDK12 rendered ΔNp63α less responsive to UV irradiation and was not required for specific DNA binding. CDK12 and SMG1 are known to regulate the transcription and splicing of RNAs and the decay of nonsense RNAs, respectively, and a subset of p63-specific protein-protein interactions at the chromatin also linked p63 to RNA transcription and decay. We observed that in the absence of p63, UV irradiation resulted in more ORF1p. ORF1p is the first protein product of the intronless non-LTR retrotransposon LINE-1, indicating a derailed surveillance of RNA processing and/or translation. Our results suggest that p63 phosphorylation and transcriptional activation might correspond to altered RNA processing and/or translation to protect proliferating keratinocytes from increased genotoxic stress.

The foggy world(s) of p63 isoform regulation in normal cells and cancer

The p53 family member p63 exists as two major protein variants (TAp63 and ΔNp63) with distinct expression patterns and functional properties. Whilst downstream target genes of p63 have been studied intensively, how p63 variants are themselves controlled has been relatively neglected. Here, we review advances in understanding ΔNp63 and TAp63 regulation, highlighting their distinct pathways. TAp63 has roles in senescence and metabolism, and in germ cell genome maintenance, where it is activated post-transcriptionally by phosphorylation cascades after DNA damage. The function and regulation of TAp63 in mesenchymal and haematopoietic cells is less clear but may involve epigenetic control through DNA methylation. ΔNp63 functions to maintain stem/progenitor cells in various epithelia and is overexpressed in squamous and certain other cancers. ΔNp63 is transcriptionally regulated through multiple enhancers in concert with chromatin modifying proteins. Many signalling pathways including growth factors, morphogens, inflammation, and the extracellular matrix influence ΔNp63 levels, with inconsistent results reported. There is also evidence for reciprocal regulation, including ΔNp63 activating its own transcription. ΔNp63 is downregulated during cell differentiation through transcriptional regulation, while post-transcriptional events cause proteasomal degradation. Throughout the review, we identify knowledge gaps and highlight discordances, providing potential explanations including cell-context and cell-matrix interactions. Identifying individual p63 variants has roles in differential diagnosis and prognosis, and understanding their regulation suggests clinically approved agents for targeting p63 that may be useful combination therapies for selected cancer patients. © 2021 The Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.

Distinct p63 and p73 Protein Interactions Predict Specific Functions in mRNA Splicing and Polyploidy Control in Epithelia

Epithelial organs are the first barrier against microorganisms and genotoxic stress, in which the p53 family members p63 and p73 have both overlapping and distinct functions. Intriguingly, p73 displays a very specific localization to basal epithelial cells in human tissues, while p63 is expressed in both basal and differentiated cells. Here, we analyse systematically the literature describing p63 and p73 protein-protein interactions to reveal distinct functions underlying the aforementioned distribution. We have found that p73 and p63 cooperate in the genome stability surveillance in proliferating cells; p73 specific interactors contribute to the transcriptional repression, anaphase promoting complex and spindle assembly checkpoint, whereas p63 specific interactors play roles in the regulation of mRNA processing and splicing in both proliferating and differentiated cells. Our analysis reveals the diversification of the RNA and DNA specific functions within the p53 family.

p63-related signaling at a glance

p63 (also known as TP63) is a transcription factor of the p53 family, along with p73. Multiple isoforms of p63 have been discovered and these have diverse functions encompassing a wide array of cell biology. p63 isoforms are implicated in lineage specification, proliferative potential, differentiation, cell death and survival, DNA damage response and metabolism. Furthermore, p63 is linked to human disease states including cancer. p63 is critical to many aspects of cell signaling, and in this Cell science at a glance article and the accompanying poster, we focus on the signaling cascades regulating TAp63 and ΔNp63 isoforms and those that are regulated by TAp63 and ΔNp63, as well the role of p63 in disease.

SETDB2 promoted breast cancer stem cell maintenance by interaction with and stabilization of ΔNp63α protein

The histone H3K9 methyltransferase SETDB2 is involved in cell cycle dysregulation in acute leukemia and has oncogenic roles in gastric cancer. In our study, we found that SETDB2 plays essential roles in breast cancer stem cell maintenance. Depleted SETDB2 significantly decreased the breast cancer stem cell population and mammosphere formation in vitro and also inhibited breast tumor initiation and growth in vivo. Restoring SETDB2 expression rescued the defect in breast cancer stem cell maintenance. A mechanistic analysis showed that SETDB2 upregulated the transcription of the ΔNp63α downstream Hedgehog pathway gene. SETDB2 also interacted with and methylated ΔNp63α, and stabilized ΔNp63α protein. Restoring ΔNp63α expression rescued the breast cancer stem cell maintenance defect which mediated by SETDB2 knockdown. In conclusion, our study reveals a novel function of SETDB2 in cancer stem cell maintenance in breast cancer.

Iron Regulatory Protein 2 Exerts its Oncogenic Activities by Suppressing TAp63 Expression

Iron regulatory protein 2 (IRP2) is a key regulator of iron homeostasis and is found to be altered in several types of human cancer. However, how IRP2 contributes to tumorigenesis remains to be elucidated. In this study, we sought to investigate the role of IRP2 in tumorigenesis and found that IRP2 promotes cell growth by repressing TAp63, a member of p53 tumor suppressor family. Specifically, we found that IRP2 overexpression decreased, whereas IRP2 deficiency increased, TAp63 expression. We also showed that the repression of TAp63 by IRP2 was independent of tumor suppressor p53. To uncover the molecular basis, we found that IRP2 stabilized TAp63 mRNA by binding to an iron response element in the 3'UTR of p63 mRNA. To determine the biological significance of this regulation, we showed that IRP2 facilitates cell proliferation, at least in part, via repressing TAp63 expression. Moreover, we found that IRP2 deficiency markedly alleviated cellular senescence in TAp63-deficient mouse embryo fibroblasts. Together, we have uncovered a novel regulation of TAp63 by IRP2 and our data suggest that IRP2 exerts its oncogenic activities at least in part by repressing TAp63 expression. IMPLICATIONS: We have revealed a novel regulation of TAp63 by IRP2 and our data suggest that IRP2 exerts its oncogenic activities, at least in part, by repressing TAp63 expression.

IL-1β Inflammatory Cytokine-Induced TP63 Isoform ∆NP63α Signaling Cascade Contributes to Cisplatin Resistance in Human Breast Cancer Cells

The mechanisms behind the induction of malignancy and chemoresistance in breast cancer cells are still not completely understood. Inflammation is associated with the induction of malignancy in different types of cancer and is highlighted as an important factor for chemoresistance. In previous work, we demonstrated that the inflammatory cytokine interleukin 1β (IL-1β)-induced upregulation of genes was associated with chemoresistance in breast cancer cells. Here, we evaluated the participation and the expression profile of TP63 in the induction of resistance to cisplatin. By loss-of-function assays, we identified that IL-1β particularly upregulates the expression of the tumor protein 63 (TP63) isoform ΔNP63α, through the activation of the IL-1β/IL-1RI/β-catenin signaling pathway. Upregulation of ΔNP63α leads to an increase in the expression of the cell survival factors epidermal growth factor receptor (EGFR) and phosphatase 1D (Wip1), and a decrease in the DNA damage sensor, ataxia-telangiectasia mutated (ATM). The participation of these processes in the increase of resistance to cisplatin was confirmed by silencing TP63 expression or by inhibition of the phosphoinositide 3-kinase (PI3K)/protein kinase B (AKT) activity in the IL-1β/IL-1RI/β-catenin signaling pathway. These data reinforced the importance of an inflammatory environment in the induction of drug resistance in cancer cells and uncovered a molecular mechanism where the IL-1β signaling pathway potentiates the acquisition of cisplatin resistance in breast cancer cells.

Human papillomavirus E7 induces p63 expression to modulate DNA damage response

Cervical cancer is the third most common malignancy diagnosed in women worldwide. The major aetiological factor underlying the malignant transformation of cervical cells is the persistent infection with high-risk human papillomaviruses (HR-HPV), with more than 99% of cases expressing viral sequences. Here, we report a previously unknown mechanism driven by high-risk human papillomavirus E7 protein to modulate response to DNA damage in cervical cancer cells. Our data shows that HR-HPV E7 oncoprotein induces the transcription of the p53-family member p63, which modulates DNA damage response pathways, to facilitate repair of DNA damage. Based on our findings, we proposed a model, where HR-HPV could interfere with the sensitivity of transformed cells to radiation therapy by modulating DNA damage repair efficiency. Importantly, we have shown for the first time a critical role for p63 in response to DNA damage in cervical cancer cells.

Structural prediction of the interaction of the tumor suppressor p27KIP1 with cyclin A/CDK2 identifies a novel catalytically relevant determinant

BACKGROUND

The cyclin-dependent kinase 2 (CDK2) together with its cyclin E and A partners is a central regulator of cell growth and division. Deregulation of CDK2 activity is associated with diseases such as cancer. The analysis of substrates identified S/T-P-X-R/K/H as the CDK2 consensus sequence. The crystal structure of cyclin A/CDK2 with a short model peptide supports this sequence and identifies key interactions. However, CDKs use additional determinants to recognize substrates, including the RXL motif that is read by the cyclin subunits. We were interested to determine whether additional amino acids beyond the minimal consensus sequence of the well-studied substrate and tumor suppressor p27^KIP1 were relevant for catalysis.

RESULTS

To address whether additional amino acids, close to the minimal consensus sequence, play a role in binding, we investigate the interaction of cyclin A/CDK2 with an in vivo cellular partner and CDK inhibitor p27^KIP1. This protein is an intrinsically unfolded protein and, in particular, the C-terminal half of the protein has not been accessible to structural analysis. This part harbors the CDK2 phosphorylation site. We used bioinformatics tools, including MODELLER, iTASSER and HADDOCK, along with partial structural information to build a model of the C-terminal region of p27^KIP1 with cyclin A/CDK2. This revealed novel interactions beyond the consensus sequence with a proline and a basic amino acid at the P + 1 and the P + 3 sites, respectively. We suggest that the lysine at P + 2 might regulate the reversible association of the second counter ion in the active site of CDK2. The arginine at P + 7 interacts with both cyclin A and CDK2 and is important for the catalytic turnover rate.

CONCLUSION

Our modeling identifies additional amino acids in p27^KIP1 beyond the consensus sequence that contribute to the efficiency of substrate phosphorylation.

The Regulation of Tumor Suppressor p63 by the Ubiquitin-Proteasome System

The protein p63 has been identified as a homolog of the tumor suppressor protein p53 and is capable of inducing apoptosis, cell cycle arrest, or senescence. p63 has at least six isoforms, which can be divided into two major groups: the TAp63 variants that contain the N-terminal transactivation domain and the ΔNp63 variants that lack the N-terminal transactivation domain. The TAp63 variants are generally considered to be tumor suppressors involved in activating apoptosis and suppressing metastasis. ΔNp63 variants cannot induce apoptosis but can act as dominant negative inhibitors to block the function of TAp53, TAp73, and TAp63. p63 is rarely mutated in human tumors and is predominately regulated at the post-translational level by phosphorylation and ubiquitination. This review focuses primarily on regulation of p63 by the ubiquitin E-3 ligase family of enzymes via ubiquitination and proteasome-mediated degradation, and introduces a new key regulator of the p63 protein.

DNA repair and aging: the impact of the p53 family

Cells are constantly exposed to endogenous and exogenous factors that threaten the integrity of their DNA. The maintenance of genome stability is of paramount importance in the prevention of both cancer and aging processes. To deal with DNA damage, cells put into operation a sophisticated and coordinated mechanism, collectively known as DNA damage response (DDR). The DDR orchestrates different cellular processes, such as DNA repair, senescence and apoptosis. Among the key factors of the DDR, the related proteins p53, p63 and p73, all belonging to the same family of transcription factors, play multiple relevant roles. Indeed, the members of this family are directly involved in the induction of cell cycle arrest that is necessary to allow the cells to repair. Alternatively, they can promote cell death in case of prolonged or irreparable DNA damage. They also take part in a more direct task by modulating the expression of core factors involved in the process of DNA repair or by directly interacting with them. In this review we will analyze the fundamental roles of the p53 family in the aging process through their multifaceted function in DDR.

Photodynamic treatment with hexyl-aminolevulinate mediates reversible thiol oxidation in core oxidative stress signaling proteins

HAL-PDT mediates reversible cysteine oxidation in core proteins involved in oxidative stress and apoptotic signalling.

JNK1 inhibits transcriptional and pro-apoptotic activity of TAp63γ

TAp63γ is a homologue of tumor suppressor p53 and functions as a transcriptional factor playing key roles in cell cycle and cell apoptosis. In the present work, we find that JNK1 can physically interact with N-terminal transactivation domain (TAD) of TAp63. Overexpression of JNK1 inhibits TAp63γ-mediated transcription, while knockdown or inhibition of endogenous JNK1 increases transactivity of TAp63γ. Further study reveals that Ser12 site in TAD is critical for JNK1-mediated inhibition of TAp63γ. This JNK1-mediated inhibition can impair pro-apoptotic activity of TAp63γ. Together, we report a novel regulation of TAp63γ transactivity and pro-apoptotic activity mediated by JNK1.

PMS2 expression in epithelial ovarian cancer is posttranslationally regulated by Akt and essential for platinum-induced apoptosis

Epithelial ovarian cancer (EOC) is the most lethal of the gynecologic malignancies, mainly due to the advanced stage at diagnosis and development of cisplatin resistance. The sensitivity of tumor cells to cisplatin is frequently affected by defect in DNA mismatch repair (MMR), which repairs mispaired DNA sequences and regulates DNA-damage-induced apoptosis. However, the role of postmeiotic segregation increased 2 (PMS2), a member of MMR protein family, in cisplatin resistance remains elusive. In the present study, we demonstrated the frequent deficiency of PMS2 and phosphorylation of Akt in EOC cell lines and tissues. Results of complex immunoprecipitation (co-IP) and protein stability assay indicated that activated Akt could directly bind to PMS2 and cause degradation of PMS2 in EOC cells. In addition, functional experiments revealed that PMS2 was required for cisplatin-induced apoptosis and cell cycle arrest in G2/M phase. These findings provide a novel insight into molecular mechanisms linking MMR with chemoresistance and suggest that stabilization of PMS2 expression may be useful in overcoming the cisplatin resistance in EOC.

PKCδ stabilizes TAp63 to promote cell apoptosis

PKCδ and p63 are respectively reported to play important roles in cell apoptosis. But there is no report on interaction between them in regulation of apoptosis. In the present study, we found that PKCδ can directly associate and up-regulate TA isoforms of p63 (TAp63) proteins via increasing their stability. PKCδ kinase activity and Thr157 site in TAp63 are crucial for this PKCδ-induced accumulation of TAp63. PKCδ can also enhance TAp63-mediated transcription and cell apoptosis. Taken together, our data indicate that PKCδ phosphorylates TAp63 proteins at Thr157 to stabilize them and promote cell apoptosis.

Phospho-ΔNp63α-responsive microRNAs contribute to the regulation of necroptosis in squamous cell carcinoma upon cisplatin exposure

This study shows that specific microRNAs differentially regulated by ΔNp63α in cisplatin-sensitive and resistant squamous cell carcinoma (SSC) cells of larynx and tongue affect the expression of members of the necroptotic pathway CYLD, RIPK1, and MLKL. Different degrees of protein interaction between necroptotic signaling intermediates were also observed in SCC cells sensitive or resistant to cisplatin. Modulation of RIPK1 with miR-101-3p mimic or inhibitor, as well as with siRNA, or chemical inhibitors was shown to affect sensitivity of SCC cells to cisplatin. This is the first report showing the modulatory effect of ΔNp63α-responsive microRNAs on the specific members of necroptotic pathway in SCC tumor cells variably responding to platinum chemotherapy.

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.

Genome-wide characterization reveals complex interplay between TP53 and TP63 in response to genotoxic stress

In response to genotoxic stress the TP53 tumour suppressor activates target gene expression to induce cell cycle arrest or apoptosis depending on the extent of DNA damage. These canonical activities can be repressed by TP63 in normal stratifying epithelia to maintain proliferative capacity or drive proliferation of squamous cell carcinomas, where TP63 is frequently overexpressed/amplified. Here we use ChIP-sequencing, integrated with microarray analysis, to define the genome-wide interplay between TP53 and TP63 in response to genotoxic stress in normal cells. We reveal that TP53 and TP63 bind to overlapping, but distinct cistromes of sites through utilization of distinctive consensus motifs and that TP53 is constitutively bound to a number of sites. We demonstrate that cisplatin and adriamycin elicit distinct effects on TP53 and TP63 binding events, through which TP53 can induce or repress transcription of an extensive network of genes by direct binding and/or modulation of TP63 activity. Collectively, this results in a global TP53-dependent repression of cell cycle progression, mitosis and DNA damage repair concomitant with activation of anti-proliferative and pro-apoptotic canonical target genes. Further analyses reveal that in the absence of genotoxic stress TP63 plays an important role in maintaining expression of DNA repair genes, loss of which results in defective repair.

Regulation of p63 protein stability via ubiquitin-proteasome pathway

The p53-related p63 gene encodes multiple protein isoforms, which are involved in a variety of biological activities. p63 protein stability is mainly regulated by the ubiquitin-dependent proteasomal degradation pathway. Several ubiquitin E3 ligases have been identified and some protein kinases as well as other kinds of proteins are involved in regulation of p63 protein stability. These regulators are responsive to diverse extracellular signaling, resulting in changes of the p63 protein levels and impacting different biological processes.

ROS-PIASγ cross talk channelizes ATM signaling from resistance to apoptosis during chemosensitization of resistant tumors

With the existing knowledge of ATM's role in therapeutic resistance, the present study aimed at identifying the molecular mechanisms that influence ATM to oscillate between chemoresistance and chemosensitivity. We observed that the redox status of tumors functions as a major determinant of ATM-dependent ‘resistance-to-apoptosis' molecular switch. At a low reactive oxygen species (ROS) condition during genotoxic insult, the ATM/sumoylated-IKKγ interaction induced NFκB activation that resisted JNK-mediated apoptosis, whereas increasing cellular ROS restored ATM/JNK apoptotic signaling. A search for the upstream missing link revealed that high ROS induces oxidation and ubiquitin-mediated degradation of PIASγ, thereby disrupting PIASγ-IKKγ cross talk, a pre-requisite for IKKγ sumoylation and subsequent NFκB activation. Interruption in the PIASγ-mediated resistance pathway channels ATM signaling toward ATM/JNK pro-death circuitry. These in vitro results also translated to sensitive and resistant tumor allograft mouse models in which low ROS-induced resistance was over-ruled in PIASγ knockout tumors, while its overexpression inhibited high ROS-dependent apoptotic cues. Cumulatively, our findings identified an unappreciated yet critical combinatorial function of cellular ROS and PIASγ in regulating ATM-mediated chemosensitization of resistant tumors. Thus, therapeutic strategies employing ROS upregulation to inhibit PIASγ during genotoxic therapy may, in future, help to eliminate the problems of NFκB-mediated tumor drug resistance.

Phospho-ΔNp63α/microRNA network modulates epigenetic regulatory enzymes in squamous cell carcinomas

The tumor protein (TP) p63/microRNAs functional network may play a key role in supporting the response of squamous cell carcinomas (SCC) to chemotherapy. We show that the cisplatin exposure of SCC-11 cells led to upregulation of miR-297, miR-92b-3p, and miR-485-5p through a phosphorylated ΔNp63α-dependent mechanism that subsequently modulated the expression of the protein targets implicated in DNA methylation (DNMT3A), histone deacetylation (HDAC9), and demethylation (KDM4C). Further studies showed that mimics for miR-297, miR-92b-3p, or miR-485-5p, along with siRNA against and inhibitors of DNMT3A, HDAC9, and KDM4C modulated the expression of DAPK1, SMARCA2, and MDM2 genes assessed by the quantitative PCR, promoter luciferase reporter, and chromatin immunoprecipitation assays. Finally, the above-mentioned treatments affecting epigenetic enzymes also modulated the response of SCC cells to chemotherapeutic drugs, rendering the resistant SCC cells more sensitive to cisplatin exposure, thereby providing the groundwork for novel chemotherapeutic venues in treating patients with SCC.

Tumor Protein p63/microRNA Network in Epithelial Cancer Cells

Non-coding microRNAs are involved in multiple regulatory mechanisms underlying response of cancer cells to stress leading to apoptosis, cell cycle arrest and autophagy. Many molecular layers are implicated in such cellular response including epigenetic regulation of transcription, RNA processing, metabolism, signaling. The molecular interrelationship between tumor protein (TP)-p53 family members and specific microRNAs is a key functional network supporting tumor cell response to chemotherapy and potentially playing a decisive role in chemoresistance of human epithelial cancers. TP63 was shown to modulate the expression of numerous microRNAs involved in regulation of epithelial cell proliferation, differentiation, senescence, "stemness" and skin maintenance, epithelial/ mesenchymal transition, and tumorigenesis in several types of epithelial cancers (e.g. squamous cell carcinoma, ovarian carcinoma, prostate carcinoma, gastric cancer, bladder cancer, and breast tumors), as well as in chemoresistance of cancer cells. TP63/microRNA network was shown to be involved in cell cycle arrest, apoptosis, autophagy, metabolism and epigenetic transcriptional regulation, thereby providing the groundwork for novel chemotherapeutic venues.

Phospho-ΔNp63α-dependent microRNAs modulate chemoresistance of squamous cell carcinoma cells to cisplatin: at the crossroads of cell life and death

The tumor protein p63/microRNA functional network appears to play a decisive role in chemoresistance of human epithelial cancers. The cisplatin- and phosphorylated-ΔNp63α-dependent microRNAs, whose expression was varied in sensitive and resistant squamous cell carcinoma cells (SCC, which were derived from larynx and tongue tumors), were shown to modulate the expression of multiple members of cell cycle arrest, apoptosis and autophagy pathways. The specific microRNAs were further shown to modulate the resistant phenotype of SCC cells in vitro, thereby providing groundwork for novel chemotherapeutic venues for head and neck cancer.

Phospho-ΔNp63α/microRNA feedback regulation in squamous carcinoma cells upon cisplatin exposure

Our previous reports showed that the cisplatin exposure induced the ATM-dependent phosphorylation of ΔNp63a, which is subsequently involved in transcriptional regulation of gene promoters encoding mRNAs and microRNAs in squamous cell carcinoma (SCC) cells upon cisplatin-induced cell death. We showed that phosphorylated (p)-ΔNp63a plays a role in upregulation of pro-apoptotic proteins, while non-p-ΔNp63a is implicated in pro-survival signaling. In contrast to non-p-ΔNp63a, p-ΔNp63a modulated expression of specific microRNAs in SCC cells exposed to cisplatin. These microRNAs were shown to attenuate the expression of several proteins involved in cell death/survival, suggesting the critical role for p-ΔNp63a in regulation of tumor cell resistance to cisplatin. Here, we studied the function of ΔNp63a in transcriptional activation and repression of the specific microRNA promoters whose expression is affected by cisplatin treatment of SCC cells. We quantitatively studied chromatin-associated proteins bound to tumor protein (TP) p63-responsive element, we found that p-ΔNp63a along with certain transcription coactivators (e.g., CARM1, KAT2B, TFAP2A, etc.) necessary to induce gene promoters for microRNAs (630 and 885-3p) or with transcription corepressors (e.g., EZH2, CTBP1, HDACs, etc.) needed to repress promoters for microRNAs (181a-5p, 374a-5p and 519a-3p) in SCC cells exposed to cisplatin.

p63 the guardian of human reproduction

p63 is a transcriptional factor implicated in cancer and development. The presence in TP63 gene of alternative promoters allows expression of one isoform containing the N-terminal transactivation domain (TA isoform) and one N-terminal truncated isoform (ΔN isoform). Complete ablation of all p63 isoforms produced mice with fatal developmental abnormalities, including lack of epidermal barrier, limbs and other epidermal appendages. Specific TAp63-null mice, although they developed normally, failed to undergo in DNA damage-induced apoptosis during primordial follicle meiotic arrest, suggesting a p63 involvement in maternal reproduction. Recent findings have elucidated the role in DNA damage response of a novel Hominidae p63 isoform, GTAp63, specifically expressed in human spermatic precursors. Thus, these findings suggest a unique strategy of p63 gene, to evolve in order to preserve the species as a guardian of reproduction. Elucidation of the biological basis of p63 function in reproduction may provide novel approaches to the control of human fertility.

Phospho-ΔNp63α/SREBF1 protein interactions: bridging cell metabolism and cisplatin chemoresistance

Tumor protein (TP)-p53 family members (TP63, TP63 and TP73) are guardians of the genome and key players in orchestrating the cellular response to cisplatin treatment. Cisplatin-induced phosphorylation of ΔNp63α was shown to have a role in regulating intracellular ΔNp63α protein levels. We previously found that squamous cell carcinoma (SCC) cells exposed to cisplatin displayed the ATM-dependent phosphorylation of ΔNp63α (p-ΔNp63α), which is critical for the transcriptional regulation of specific downstream mRNAs and microRNAs and is likely to underlie the chemoresistance of SCC cells. However, SCC cells expressing non-p-ΔNp63α became more cisplatin-resistant. We also found that p-ΔNp63α forms complexes with a number of proteins involved in cell death response through regulation of cell cycle arrest, apoptosis, autophagy, RNA splicing and chromatin modifications. Here, we showed that p-ΔNp63α induced ARG1, GAPDH, and CPT2 gene transcription in cisplatin-sensitive SCC cells, while non-p-ΔNp63α increased a transcription of CAD, G6PD and FASN genes in cisplatin-resistant SCC cells. We report that the p-ΔNp63α-dependent regulatory mechanisms implicated in the modulation of plethora of pathways, including amino acid, carbohydrate, lipid and nucleotide metabolisms, thereby affect tumor cell response to cisplatin-induced cell death, suggesting that the ATM-dependent ΔNp63α pathway plays a role in the resistance of tumor cells to platinum therapy.

Global tumor protein p53/p63 interactome: making a case for cisplatin chemoresistance

Cisplatin chemoresistance is a clinical problem that leads to treatment failure in various human epithelial cancers. Members of tumor protein (TP) p53 family play various critical roles in the multiple molecular mechanisms underlying the chemoresistance of tumor cells. However, the in-depth mechanisms of the cellular response to cisplatin-induced cell death are still under thorough investigation. We previously showed that squamous cell carcinoma (SCC) cells exposed to cisplatin display an ATM-dependent phosphorylation of ΔNp63α, leading to a specific function of the phosphorylated (p)-ΔNp63α transcription factor in cisplatin-sensitive tumor cells. We further found that SCC cells expressing non-p-ΔNp63α-S385G became cisplatin-resistant. Using quantitative mass-spectrometry of protein complexes labeled with isobaric tags, we showed that TP53 and ΔNp63α are involved in numerous protein-protein interactions, which are likely to be implicated in the response of tumor cells to cisplatin exposure. We found that p-ΔNp63α binds to the splicing complex, leading to repression of mRNA splicing and activation of ACIN1-mediated cell death pathway. In contrast to p-ΔNp63α, non-p-ΔNp63α fails to bind the critical members of the splicing complex, thereby leading to activation of RNA splicing and reduction of cell death pathway. Overall, our studies provide an integrated proteomic platform in making a case for the role of the p53/p63 interactome in cisplatin chemoresistance.

Phospho-ΔNp63α-dependent regulation of autophagic signaling through transcription and micro-RNA modulation

Cisplatin was shown to induce the ataxia telangiectasia mutated (ATM)-dependent phosphorylation of tumor protein p63 isoform, (ΔNp63α), leading to a transcriptional regulation of specific genes implicated in the control of cell death of squamous cell carcinoma (SCC) cells. We previously observed that the cisplatin-induced phosphorylated (p)-ΔNp63α transcriptionally regulates the expression of specific microRNAs (miRNAs) in SCC cells. We found here that cisplatin exposure of SCC cells led to modulation of the members of the autophagic pathway, such as Atg1/Ulk1, Atg3, Atg4A, Atg5, Atg6/Becn1, Atg7, Atg9A and Atg10, by a direct p-ΔNp63α-dependent transcriptional regulation. We further found that specific miRNAs (miR-181a, miR-519a, miR-374a and miR-630), which are critical downstream targets of the p-ΔNp63α, modulated the protein levels of ATG5, ATG6/BECN1, ATG10, ATG12, ATG16L1 and UVRAG, adding another level of expression control for autophagic pathways in SCC cells upon cisplatin exposure. Our data support the notion that the cisplatin-induced p-ΔNp63α could regulate key pathways implicated in response of cancer cells to chemotherapeutics.

Phospho-ΔNp63α/miR-885-3p axis in tumor cell life and cell death upon cisplatin exposure

The cisplatin-induced ATM-dependent phosphorylated (p)-ΔNp63α plays an important role in transcriptional regulation of specific genes encoding mRNAs and microRNAs (miRs) implicated in cell death, cell survival, and chemoresistance. The p-ΔNp63α-induced miR-885-3p functions as a critical regulator of MDM4, ATK1, BCL2, ATG16L2, ULK2, CASP2, and CASP3 mRNAs via pairing with their respective 'recognition' sequences. Cisplatin exposure modulated the levels of target proteins (reduced BCL2, AKT1, ATG16L2, and ULK2, while activated MDM4) in cisplatin-sensitive wild type ΔNp63α cells leading to distinct changes in cell viability. Finally, miR-885-3p modulated the cisplatin-induced TP53-dependent mitochondrial apoptosis by up regulation of MDM4 levels and down regulation of BCL2 levels in mitochondria. Altogether, our results support the notion that miR-885-3p might contribute in regulation of cell viability, apoptosis and/or autophagy in squamous cell carcinoma cells upon cisplatin exposure.

Tumor protein p63/nuclear factor κB feedback loop in regulation of cell death

Tumor protein (TP)-p53 family members often play proapoptotic roles, whereas nuclear factor κB (NF-κB) functions as a proapoptotic and antiapoptotic regulator depending on the cellular environment. We previously showed that the NF-κB activation leads to the reduction of the TP63 isoform, ΔNp63α, thereby rendering the cells susceptible to cell death upon DNA damage. However, the functional relationship between TP63 isotypes and NF-κB is poorly understood. Here, we report that the TAp63 regulates NF-κB transcription and protein stability subsequently leading to the cell death phenotype. We found that TAp63α induced the expression of the p65 subunit of NF-κB (RELA) and target genes involved in cell cycle arrest or apoptosis, thereby triggering cell death pathways in MCF10A cells. RELA was shown to concomitantly modulate specific cell survival pathways, making it indispensable for the TAp63α-dependent regulation of cell death. We showed that TAp63α and RELA formed protein complexes resulted in their mutual stabilization and inhibition of the RELA ubiquitination. Finally, we showed that TAp63α directly induced RelA transcription by binding to and activating of its promoter and, in turn, leading to activation of the NF-κB-dependent cell death genes. Overall, our data defined the regulatory feedback loop between TAp63α and NF-κB involved in the activation of cell death process of cancer cells.

HIPK2 phosphorylates ΔNp63α and promotes its degradation in response to DNA damage

Homeodomain-interacting protein kinase 2 (HIPK2) is an emerging player in cell response to genotoxic agents that senses damage intensity and contributes to the cell's choice between cell cycle arrest and apoptosis. Phosphorylation of p53 at S46, an apoptosis-specific p53 posttranslational modification, is the most characterized HIPK2 function in response to lethal doses of ultraviolet (UV), ionizing radiation or different anticancer drugs, such as cisplatin, roscovitine and doxorubicin (DOX). Indeed, like p53, HIPK2 has been shown to contribute to the effectiveness of these treatments. Interestingly, p53-independent mechanisms of HIPK2-induced apoptosis were described for UV and tumor growth factor-β treatments; however, it is unknown whether these mechanisms are relevant for the responses to anticancer drugs. Because of the importance of the so-called 'p53-independent apoptosis and drug response' in human cancer chemotherapy, we asked whether p53-independent factor(s) might be involved in HIPK2-mediated chemosensitivity. Here, we show that HIPK2 depletion by RNA interference induces resistance to different anticancer drugs even in p53-null cells, suggesting the involvement of HIPK2 targets other than p53 in response to chemotherapy. In particular, we found that HIPK2 phosphorylates and promotes proteasomal degradation of ΔNp63α, a prosurvival ΔN isoform of the p53 family member, p63. Indeed, effective cell response to different genotoxic agents was shown to require phosphorylation-induced proteasomal degradation of ΔNp63α. In DOX-treated cells, we show that HIPK2 depletion interferes with ΔNp63α degradation, and expression of a HIPK2-resistant ΔNp63α-Δ390 mutant induces chemoresistance. We identify T397 as the ΔNp63α residue phosphorylated by HIPK2, and show that the non-phosphorylatable ΔNp63α-T397A mutant is not degraded in the face of either HIPK2 overexpression or DOX treatment. These results indicate ΔNp63α as a novel target of HIPK2 in response to genotoxic drugs.

Phospho-ΔNp63α is a key regulator of the cisplatin-induced microRNAome in cancer cells

Head and neck squamous cell carcinoma (HNSCC) cells exposed to cisplatin (CIS) displayed a dramatic ATM-dependent phosphorylation of ΔNp63α that leads to the transcriptional regulation of downstream mRNAs. Here, we report that phospho (p)-ΔNp63α transcriptionally deregulates miRNA expression after CIS treatment. Several p-ΔNp63α-dependent microRNA species (miRNAs) were deregulated in HNSCC cells upon CIS exposure, including miR-181a, miR-519a, and miR-374a (downregulated) and miR-630 (upregulated). Deregulation of miRNA expression led to subsequent modulation of mRNA expression of several targets (TP53-S46, HIPK2, ATM, CDKN1A and 1B, CASP3, PARP1 and 2, DDIT1 and 4, BCL2 and BCL2L2, TP73, YES1, and YAP1) that are involved in the apoptotic process. Our data support the notion that miRNAs are critical downstream targets of p-ΔNp63α and mediate key pathways implicated in the response of cancer cells to chemotherapeutic drugs.

DeltaNp63alpha confers tumor cell resistance to cisplatin through the AKT1 transcriptional regulation

Strategies to address resistance to platin drugs are greatly needed in human epithelial cancers (e.g., ovarian, head/neck, and lung) where platins are used widely and resistance occurs commonly. We found that upon ΔNp63α overexpression, AKT1 and phospho-AKT1 levels are upregulated in cancer cells. Investigations using gel-shift, chromatin immunoprecipitation and functional reporter assays implicated ΔNp63α in positive regulation of AKT1 transcription. Importantly, we found that ΔNp63α, AKT1, and phospho-AKT levels are greater in 2008CI3 CDDP-resistant ovarian cancer cells than in 2008 CDDP-sensitive cells. siRNA-mediated knockdown of ΔNp63α expression dramatically decreased AKT1 expression, whereas knockdown of either ΔNp63α or AKT1 decreased cell proliferation and increased death of ovarian and head/neck cancer cells. Conversely, enforced expression of ΔNp63α increased cancer cell proliferation and reduced apoptosis. Together, our findings define a novel ΔNp63α-dependent regulatory mechanism for AKT1 expression and its role in chemotherapeutic resistance of ovarian and head/neck cancer cells.

Phosphorylated TP63 induces transcription of RPN13, leading to NOS2 protein degradation

Head and neck squamous cell carcinoma cells exposed to cisplatin display ATM-dependent phosphorylation of the most predominant TP63 isoform (ΔNp63α), leading to its activation as a transcription factor. Here, we found that the phospho-ΔNp63α protein binds to the genomic promoter of RPN13 through the TP63-responsive element. We further found that the phospho-ΔNp63α protein associates with other transcription factors (DDIT3 (also known as CHOP), NF-Y, and NF-κB), activating RPN13 gene transcription. Furthermore, cisplatin-induced and phospho-ΔNp63α-dependent RPN13 gene transcription leads to NOS2 degradation. Finally, we show that RPN13 knockdown by siRNA essentially rescues NOS2 from cisplatin-dependent inactivation. These data provide a novel mechanism for the phospho-ΔNp63α-dependent regulation of NOS2 function in cells upon cisplatin treatment, contributing to the cell death pathway of tumor cells.

Phospho-ΔNp63α/Rpn13-dependent regulation of LKB1 degradation modulates autophagy in cancer cells

Oxidative stress was shown to promote the translocation of Ataxia-telangiectasia mutated (ATM) to cytoplasm and trigger the LKB1-AMPK-tuberin pathway leading to a down-regulation of mTOR and subsequently inducing the programmed cell death II (autophagy). Cisplatin was previously found to induce the ATM-dependent phosphorylation of ΔNp63α in squamous cell carcinoma (SCC) cells. In this study, phosphorylated (p)-ΔNp63α was shown to bind the ATM promoter, to increase the ATM promoter activity and to enhance the ATM cytoplasmic accumulation. P-ΔNp63α protein was further shown to interact with the Rpn13 protein leading to a proteasome-dependent degradation of p-ΔNp63α and thereby protecting LKB1 from the degradation. In SCC cells (with an altered ability to support the ATM-dependent ΔNp63α phosphorylation), the non-phosphorylated ΔNp63α protein failed to form protein complexes with the Rpn13 protein and thereby allowing the latter to bind and target LKB1 into a proteasome-dependent degradation pathway thereby modulating a cisplatin-induced autophagy. We thus suggest that SCC cells sensitive to cisplatin-induced cell death are likely to display a greater ratio of p-ΔNp63α/non-phosphorylated ΔNp63α than cells with the innate resistant/impaired response to a cisplatin-induced cell death. Our data also suggest that the choice made by Rpn13 between p-ΔNp63α or LKB1 to be targeted for degradation is critical for cell death decision made by cancer cells in response to chemotherapy.

DeltaNp63 transcriptionally regulates ATM to control p53 Serine-15 phosphorylation

BACKGROUND

DeltaNp63alpha is an epithelial progenitor cell marker that maintains epidermal stem cell self-renewal capacity. Previous studies revealed that UV-damage induced p53 phosphorylation is confined to DeltaNp63alpha-positive cells in the basal layer of human epithelium.

RESULTS

We now report that phosphorylation of the p53 tumour suppressor is positively regulated by DeltaNp63alpha in immortalised human keratinocytes. DeltaNp63alpha depletion by RNAi reduces steady-state ATM mRNA and protein levels, and attenuates p53 Serine-15 phosphorylation. Conversely, ectopic expression of DeltaNp63alpha in p63-null tumour cells stimulates ATM transcription and p53 Serine-15 phosphorylation. We show that ATM is a direct DeltaNp63alpha transcriptional target and that the DeltaNp63alpha response element localizes to the ATM promoter CCAAT sequence. Structure-function analysis revealed that the DeltaNp63-specific TA2 transactivation domain mediates ATM transcription in coordination with the DNA binding and SAM domains.

CONCLUSIONS

Germline p63 point mutations are associated with a range of ectodermal developmental disorders, and targeted p63 deletion in the skin causes premature ageing. The DeltaNp63alpha-ATM-p53 damage-response pathway may therefore function in epithelial development, carcinogenesis and the ageing processes.

Ataxia telangiectasia mutated nuclear localization in head and neck cancer cells is PPP2R2B-dependent

Background: Protein phosphatase 2A (PP2A) has been implicated in radiation-induced activation of cellular responses, likely by its ability to regulate the autophosphorylation of the ataxia telangiectasia mutated (ATM) protein, a key molecule involved in the DNA damage response initiated by double-stranded DNA breaks. Interestingly, a hereditary defect in the PPP2R2B gene, which encodes the beta isoform of PP2A regulatory subunit B, causes autosomal dominant spinocerebellar ataxia 12, a clinical condition resembling that of ataxia telangiectasia patients. Moreover, PPP2R2B is significantly down-regulated in many human cancers, including head and neck squamous cell carcinomas (HNSCCs). Objective: Examine whether PPP2R2B regulates ATM function, thereby contributing to tumor progression due to the resulting defective DNA repair. Methods: The roles of PPP2R2B were evaluated in irradiated HNSCC cell lines, siRNAPPP2R2B cells and okadaic acid treated cells. Expression of PPP2R2B was measured by microarray, Western blot analysis and real time quantitative rtPCR. ATM quantity and localization, ATM phosphorylation and γ-H2AX were determined by Western blot analysis and/or immunofluorescence assay. Clonogenic cell survival assay was performed to determine ionizing radiation sensitivity. Results: PPP2R2B expression is reduced in multiple tumor types, including HNSCCs. Indeed, HNSCC cell lines that have lower PPP2R2B mRNA expression and siRNAPPP2R2B cells lower basal and radiation-induced levels of phosphorylated ATM and the consequent reduction in the levels of phosphorylation of the downstream ATM target, γ-H2AX. Depletion of PPP2R2B and inhibition of PP2A with okadaic acid resulted in limited ATM nuclear localization. Finally, siRNAPPP2R2B cells displayed enhanced sensitivity to death after radiation. Conclusion: In HNSCCs, ATM nuclear localization is PPP2R2B dependent, and decreased PPP2R2B expression may result in limited ATM activation by preventing its nuclear accumulation and ATM-chromatin interaction. Therefore, decreased PPP2R2B expression in HNSCCs may contribute to genomic instability, cancer development and radiation sensitivity by limiting ATM functions.

p53-family proteins and their regulators: hubs and spokes in tumor suppression

The tumor suppressor p53 is a central hub in a molecular network controlling cell proliferation and death in response to potentially oncogenic conditions, and a wide array of covalent modifications and protein interactions modulate the nuclear and cytoplasmic activities of p53. The p53 relatives, p73 and p63, are entangled in the same regulatory network, being subject at least in part to the same modifications and interactions that convey signals on p53, and actively contributing to the resulting cellular output. The emerging picture is that of an interconnected pathway, in which all p53-family proteins are involved in the response to oncogenic stress and physiological inputs. Therefore, common and specific interactors of p53-family proteins can have a wide effect on function and dysfunction of this pathway. Many years of research have uncovered an impressive number of p53-interacting proteins, but much less is known about protein interactions of p63 and p73. Yet, many interactors may be shared by multiple p53-family proteins, with similar or different effects. In this study we review shared interactors of p53-family proteins with the aim to encourage research into this field; this knowledge promises to unveil regulatory elements that could be targeted by a new generation of molecules, and allow more efficient use of currently available drugs for cancer treatment.