p51A (TAp63gamma), a p53 homolog, accumulates in response to DNA damage for cell regulation

ΔNp63 silencing, DNA methylation shifts, and epithelial-mesenchymal transition resulted from TAp63 genome editing in squamous cell carcinoma

TP63 (p63) is strongly expressed in lower-grade carcinomas of the head and neck, skin, breast, and urothelium to maintain a well-differentiated phenotype. TP63 has two transcription start sites at exons 1 and 3' that produce TAp63 and ΔNp63 isoforms, respectively. The major protein, ΔNp63α, epigenetically activates genes essential for epidermal/craniofacial differentiation, including ΔNp63 itself. To examine the specific role of weakly expressed TAp63, we disrupted exon 1 using CRISPR-Cas9 homology-directed repair in a head and neck squamous cell carcinoma (SCC) line. Surprisingly, TAp63 knockout cells having either monoallelic GFP cassette insertion paired with a frameshift deletion allele or biallelic GFP cassette insertion exhibited ΔNp63 silencing. Loss of keratinocyte-specific gene expression, switching of intermediate filament genes from KRT(s) to VIM, and suppression of cell-cell and cell-matrix adhesion components indicated the core events of epithelial-mesenchymal transition. Many of the positively and negatively affected genes, including ΔNp63, displayed local DNA methylation changes. Furthermore, ΔNp63 expression was partially rescued by transfection of the TAp63 knockout cells with TAp63α and application of DNA methyltransferase inhibitor zebularine. These results suggest that TAp63, a minor part of the TP63 gene, may be involved in the auto-activation mechanism of ΔNp63 by which the keratinocyte-specific epigenome is maintained in SCC.

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.

TP63 Functions as a Tumor Suppressor Regulated by GAS5/miR-221-3p Signaling Axis in Human Non-Small Cell Lung Cancer Cells

Background

Tumor protein p63 (TP63) has been proven to play a role as a tumor suppressor in some human cancers, including non-small cell lung cancer (NSCLC). This study aimed to investigate the mechanism of TP63 and analyze the underlying pathway dysregulating TP63 in NSCLC.

Methods

RT-qPCR and Western blotting assays were used to determine gene expression in NSCLC cells. The luciferase reporter assay was performed to explore the transcriptional regulation. Flow cytometry was used to analyze the cell cycle and cell apoptosis. Transwell and CCK-8 assays were performed to test cell invasion and cell proliferation, respectively.

Results

GAS5 interacted with miR-221-3p, and its expression was significantly reduced in NSCLC. GAS5, as a molecular sponge, upregulated the mRNA and protein levels of TP63 by inhibiting miR-221-3p in NSCLC cells. The upregulation of GAS5 inhibited cell proliferation, apoptosis, and invasion, which was partially reversed by the knockdown of TP63. Interestingly, we found that GAS5-induced TP63 upregulation promoted tumor chemotherapeutic sensitivity to cisplatin therapy in vivo and in vitro.

Conclusion

Our results revealed the mechanism by which GAS5 interacts with miR-221-3p to regulate TP63, and targeting GAS5/miR-221-3p/TP63 may be a potential therapeutic strategy for NSCLC cells.

TP53 Mutation Is a Prognostic Factor in Lower Grade Glioma and May Influence Chemotherapy Efficacy

BACKGROUND

Identification of prognostic biomarkers in cancers is a crucial step to improve overall survival (OS). Although mutations in tumour protein 53 (TP53) is prevalent in astrocytoma, the prognostic effects of TP53 mutation are unclear.

METHODS

In this retrospective study, we sequenced TP53 exons 1 to 10 in a cohort of 102 lower-grade glioma (LGG) subtypes and determined the prognostic effects of TP53 mutation in astrocytoma and oligodendroglioma. Publicly available datasets were analysed to confirm the findings.

RESULTS

In astrocytoma, mutations in TP53 codon 273 were associated with a significantly increased OS compared to the TP53 wild-type (HR (95% CI): 0.169 (0.036-0.766), p = 0.021). Public datasets confirmed these findings. TP53 codon 273 mutant astrocytomas were significantly more chemosensitive than TP53 wild-type astrocytomas (HR (95% CI): 0.344 (0.13-0.88), p = 0.0148). Post-chemotherapy, a significant correlation between TP53 and YAP1 mRNA was found (p = 0.01). In O (6)-methylguanine methyltransferase (MGMT) unmethylated chemotherapy-treated astrocytoma, both TP53 codon 273 and YAP1 mRNA were significant prognostic markers. In oligodendroglioma, TP53 mutations were associated with significantly decreased OS.

CONCLUSIONS

Based on these findings, we propose that certain TP53 mutant astrocytomas are chemosensitive through the involvement of YAP1, and we outline a potential mechanism. Thus, TP53 mutations may be key drivers of astrocytoma therapeutic efficacy and influence survival outcomes.

ECRG2, a novel transcriptional target of p53, modulates cancer cell sensitivity to DNA damage

Esophageal Cancer-Related Gene 2 (ECRG2) is a recently identified tumor suppressor, its regulation and involvement in DNA damage response are unknown. Here, we show that DNA damage-induced ECRG2 upregulation coincided with p53 activation and occurred in a p53-dependent manner. We identified two p53-binding sites within ECRG2 promoter and found the promoter activity, mRNA, and protein expression to be regulated by p53. We show that DNA damage significantly enhanced p53 binding to ECRG2 promoter at the anticipated p53-binding sites. We identified a novel natural ECRG2 promoter variant harboring a small deletion that exists in the genomes of ~38.5% of world population and showed this variant to be defective in responding to p53 and DNA-damage. ECRG2 overexpression induced cancer cell death; ECRG2 gene disruption enhanced cell survival following anticancer drug treatments even when p53 was induced. We showed that lower expression of ECRG2 in multiple human malignancies correlated with reduced disease-free survival in patients. Collectively, our novel findings indicate that ECRG2 is an important target of p53 during DNA damage-induced response and plays a critical role in influencing cancer cell sensitivity to DNA damage-inducing cancer therapeutics.

C-terminal α Domain of p63 Binds to p300 to Coactivate β-Catenin

TP63 (p63), a member of the tumor suppressor TP53 (p53) gene family, is essential for ectodermal tissue development and suppresses malignant progression of carcinomas. The most abundant isoform, ΔNp63α (referred to as p63), lacks the N-terminal transactivation (TA) domain, and was originally characterized as a dominant-negative type suppressor against p53 family proteins. It also binds to TCF/LEF to inhibit β-catenin. Nevertheless, transcriptional activation by p63 has also been observed in varied systems. To understand the puzzling results, we analyzed the structure–function relationship of p63 in the control of β-catenin-dependent transcription. p63 acted as a suppressor of moderately induced β-catenin. However, when nuclear targeted S33Y β-catenin was applied to cause the maximum enhancer activation, p63 displayed a β-catenin-coactivating function. The DNA-binding domain of p63 and the target sequence facilitated it. Importantly, we newly found that, despite the absence of TA domain, p63 was associated with p300, a general adaptor protein and chromatin modifier causing transcriptional activation. C-terminal α domain of p63 was essential for p300-binding and for the coactivator function. These results were related to endogenous p63-p300 complex formation and Wnt/β-catenin-responsive gene regulation by p63 in squamous cell carcinoma lines. The novel p63-p300 interaction may be involved in positive regulation of gene expression in tissue development and carcinogenesis.

A Decade of GWAS Results in Lung Cancer

Genome-wide association studies (GWAS) were successful to identify genetic factors robustly associated with lung cancer. This review aims to synthesize the literature in this field and accelerate the translation of GWAS discoveries into results that are closer to clinical applications. A chronologic presentation of published GWAS on lung cancer susceptibility, survival, and response to treatment is presented. The most important results are tabulated to provide a concise overview in one read. GWAS have reported 45 lung cancer susceptibility loci with varying strength of evidence and highlighted suspected causal genes at each locus. Some genetic risk loci have been refined to more homogeneous subgroups of lung cancer patients in terms of histologic subtypes, smoking status, gender, and ethnicity. Overall, these discoveries are an important step for future development of new therapeutic targets and biomarkers to personalize and improve the quality of care for patients. GWAS results are on the edge of offering new tools for targeted screening in high-risk individuals, but more research is needed if GWAS are to pay off the investment. Complementary genomic datasets and functional studies are needed to refine the underlying molecular mechanisms of lung cancer preliminarily revealed by GWAS and reach results that are medically actionable. Cancer Epidemiol Biomarkers Prev; 27(4); 363-79. ©2018 AACRSee all articles in this CEBP Focus section, "Genome-Wide Association Studies in Cancer."

A Novel Hydroxamate-Based Compound WMJ-J-09 Causes Head and Neck Squamous Cell Carcinoma Cell Death via LKB1-AMPK-p38MAPK-p63-Survivin Cascade

Growing evidence shows that hydroxamate-based compounds exhibit broad-spectrum pharmacological properties including anti-tumor activity. However, the precise mechanisms underlying hydroxamate derivative-induced cancer cell death remain incomplete understood. In this study, we explored the anti-tumor mechanisms of a novel aliphatic hydroxamate-based compound, WMJ-J-09, in FaDu head and neck squamous cell carcinoma (HNSCC) cells. WMJ-J-09 induced G2/M cell cycle arrest and apoptosis in FaDu cells. These actions were associated with liver kinase B1 (LKB1), AMP-activated protein kinase (AMPK) and p38 mitogen-activated protein kinase (p38MAPK) activation, transcription factor p63 phosphorylation, as well as modulation of p21 and survivin. LKB1-AMPK-p38MAPK signaling blockade reduced WMJ-J-09’s enhancing effects in p63 phosphorylation, p21 elevation and survivin reduction. Moreover, WMJ-J-09 caused an increase in α-tubulin acetylation and interfered with microtubule assembly. Furthermore, WMJ-J-09 suppressed the growth of subcutaneous FaDu xenografts in vivo. Taken together, WMJ-J-09-induced FaDu cell death may involve LKB1-AMPK-p38MAPK-p63-survivin signaling cascade. HDACs inhibition and disruption of microtubule assembly may also contribute to WMJ-J-09’s actions in FaDu cells. This study suggests that WMJ-J-09 may be a potential lead compound and warrant the clinical development in the treatment of HNSCC.

p63 and p73 repress CXCR5 chemokine receptor gene expression in p53-deficient MCF-7 breast cancer cells during genotoxic stress

Many types of chemotherapeutic agents induce of DNA-damage that is accompanied by activation of p53 tumor suppressor, a key regulator of tumor development and progression. In our previous study we demonstrated that p53 could repress CXCR5 chemokine receptor gene in MCF-7 breast cancer cells via attenuation of NFkB activity. In this work we aimed to determine individual roles of p53 family members in the regulation of CXCR5 gene expression under genotoxic stress. DNA-alkylating agent methyl methanesulfonate caused a reduction in CXCR5 expression not only in parental MCF-7 cells but also in MCF-7-p53off cells with CRISPR/Cas9-mediated inactivation of the p53 gene. Since p53 knockout was associated with elevated expression of its p63 and p73 homologues, we knocked out p63 using CRISPR/Cas9 system and knocked down p73 using specific siRNA. The CXCR5 promoter activity, CXCR5 expression and CXCL13-directed migration in MCF-7 cells with inactivation of all three p53 family genes were completely insensitive to genotoxic stress, while pairwise p53+p63 or p53+p73 inactivation resulted in partial effects. Using deletion analysis and site-directed mutagenesis, we demonstrated that effects of NFkB on the CXCR5 promoter inversely correlated with p63 and p73 levels. Thus, all three p53 family members mediate the effects of genotoxic stress on the CXCR5 promoter using the same mechanism associated with attenuation of NFkB activity. Understanding of this mechanism could facilitate prognosis of tumor responses to chemotherapy.

Improvement of gemcitabine sensitivity of p53-mutated pancreatic cancer MiaPaCa-2 cells by RUNX2 depletion-mediated augmentation of TAp73-dependent cell death

Pancreatic cancer exhibits the worst prognostic outcome among human cancers. Recently, we have described that depletion of RUNX2 enhances gemcitabine (GEM) sensitivity of p53-deficient pancreatic cancer AsPC-1 cells through the activation of TAp63-mediated cell death pathway. These findings raised a question whether RUNX2 silencing could also improve GEM efficacy on pancreatic cancer cells bearing p53 mutation. In the present study, we have extended our study to p53-mutated pancreatic cancer MiaPaCa-2 cells. Based on our current results, MiaPaCa-2 cells were much more resistant to GEM as compared with p53-proficient pancreatic cancer SW1990 cells, and there existed a clear inverse relationship between the expression levels of TAp73 and RUNX2 in response to GEM. Forced expression of TAp73α in MiaPaCa-2 cells significantly promoted cell cycle arrest and/or cell death, indicating that a large amount of TAp73 might induce cell death even in the presence of mutant p53. Consistent with this notion, overexpression of TAp73α stimulated luciferase activity driven by p53/TAp73-target gene promoters in MiaPaCa-2 cells. Similar to AsPC-1 cells, small interfering RNA-mediated knockdown of RUNX2 remarkably enhanced GEM sensitivity of MiPaCa-2 cells. Under our experimental conditions, TAp73 further accumulated in RUNX2-depleted MiaPaCa-2 cells exposed to GEM relative to GEM-treated non-silencing control cells. As expected, silencing of p73 reduced GEM sensitivity of MiPaCa-2 cells. Moreover, GEM-mediated Tyr phosphorylation level of TAp73 was much more elevated in RUNX2-depleted MiaPaCa-2 cells. Collectively, our present findings strongly suggest that knockdown of RUNX2 contributes to a prominent enhancement of GEM sensitivity of p53-mutated pancreatic cancer cells through the activation of TAp73-mediated cell death pathway, and also provides a promising strategy for the treatment of patients with pancreatic cancer bearing p53 mutation.

Lovastatin causes FaDu hypopharyngeal carcinoma cell death via AMPK-p63-survivin signaling cascade

Statins are used widely to lower serum cholesterol and the incidence of cardiovascular diseases. Growing evidence shows that statins also exhibit beneficial effects against cancers. In this study, we investigated the molecular mechanisms involved in lovastatin-induced cell death in Fadu hypopharyngeal carcinoma cells. Lovastatin caused cell cycle arrest and apoptosis in FaDu cells. Lovastatin increased p21^cip/Waf1 level while the survivin level was decreased in the presence of lovastatin. Survivin siRNA reduced cell viability and induced cell apoptosis in FaDu cells. Lovastatin induced phosphorylation of AMP-activated protein kinase (AMPK), p38 mitogen-activated protein kinase (MAPK) and transcription factor p63. Lovastatin also caused p63 acetylation and increased p63 binding to survivin promoter region in FaDu cells. AMPK-p38MAPK signaling blockade abrogated lovastatin-induced p63 phosphorylation. Lovastatin’s enhancing effect on p63 acetylation was reduced in HDAC3- or HDAC4- transfected cells. Moreover, transfection of cells with AMPK dominant negative mutant (AMPK-DN), HDAC3, HDAC4 or p63 siRNA significantly reduced lovastatin’s effects on p21^cip/Waf1 and survivin. Furthermore, lovastatin inhibited subcutaneous FaDu xenografts growth in vivo. Taken together, lovastatin may activate AMPK-p38MAPK-p63-survivin cascade to cause FaDu cell death. This study establishes, at least in part, the signaling cascade by which lovastatin induces hypopharyngeal carcinoma cell death.

Novel Implications of DNA Damage Response in Drug Resistance of Malignant Cancers Obtained from the Functional Interaction between p53 Family and RUNX2

During the lifespan of cells, their genomic DNA is continuously exposed to the endogenous and exogenous DNA insults. Thus, the appropriate cellular response to DNA damage plays a pivotal role in maintaining genomic integrity and also acts as a molecular barrier towards DNA legion-mediated carcinogenesis. The tumor suppressor p53 participates in an integral part of proper regulation of DNA damage response (DDR). p53 is frequently mutated in a variety of human cancers. Since mutant p53 displays a dominant-negative behavior against wild-type p53, cancers expressing mutant p53 sometimes acquire drug-resistant phenotype, suggesting that mutant p53 prohibits the p53-dependent cell death pathway following DNA damage, and thereby contributing to the acquisition and/or maintenance of drug resistance of malignant cancers. Intriguingly, we have recently found that silencing of pro-oncogenic RUNX2 enhances drug sensitivity of aggressive cancer cells regardless of p53 status. Meanwhile, cancer stem cells (CSCs) have stem cell properties such as drug resistance. Therefore, the precise understanding of the biology of CSCs is quite important to overcome their drug resistance. In this review, we focus on molecular mechanisms behind DDR as well as the serious drug resistance of malignant cancers and discuss some attractive approaches to improving the outcomes of patients bearing drug-resistant cancers.

Common genetic variants on 3q28 contribute to non-small cell lung cancer susceptibility: evidence from 10 case-control studies

The association between common variations (rs10937405, rs4488809) on 3q28 and lung cancer has been widely evaluated in various ethnic groups, since it was first identified through genome-wide association approach. However, the results have been inconclusive. To derive a more precise estimation of the relationship and the effect of factors that might modify the risk, we performed this meta-analysis. The random-effects model was applied, addressing heterogeneity and publication bias. A total of 10 articles involving 36,221 cases and 58,108 controls were included. Overall, the summary per-allele OR of 1.19 (95 % CI 1.14-1.25, P < 10(-5)) and 1.17 (95 % CI 1.10-1.23, P < 10(-5)) was found for the rs10937405 and rs4488809 polymorphisms, respectively. Significant results were also observed in heterozygous and homozygous when compared with wild genotype for these polymorphisms. Significant results were found in East Asians when stratified by ethnicity, whereas no significant associations were found among Caucasians. After stratifying by sample size, study design, control source and sex, significant associations were also obtained. In addition, our data indicate that these polymorphisms are involved in lung cancer susceptibility and confer its effect primarily in lung adenocarcinoma when stratified by histological subtype. Furthermore, significant associations were also detected both never-smokers and smokers for these polymorphisms. In conclusion, this meta-analysis demonstrated that rs10937405 and rs4488809 are a risk factor associated with increased non-small cell lung cancer susceptibility, particularly for East Asian populations.

Rare variants of large effect in BRCA2 and CHEK2 affect risk of lung cancer

We conducted imputation to the 1000 Genomes Project of four genome-wide association studies of lung cancer in populations of European ancestry (11,348 cases and 15,861 controls) and genotyped an additional 10,246 cases and 38,295 controls for follow-up. We identified large-effect genome-wide associations for squamous lung cancer with the rare variants BRCA2 p.Lys3326X (rs11571833, odds ratio (OR) = 2.47, P = 4.74 × 10(-20)) and CHEK2 p.Ile157Thr (rs17879961, OR = 0.38, P = 1.27 × 10(-13)). We also showed an association between common variation at 3q28 (TP63, rs13314271, OR = 1.13, P = 7.22 × 10(-10)) and lung adenocarcinoma that had been previously reported only in Asians. These findings provide further evidence for inherited genetic susceptibility to lung cancer and its biological basis. Additionally, our analysis demonstrates that imputation can identify rare disease-causing variants with substantive effects on cancer risk from preexisting genome-wide association study data.

Quantitative assessment of the influence of TP63 gene polymorphisms and lung cancer risk: evidence based on 93,751 subjects

BACKGROUND

Several genome-wide association studies on lung cancer (LC) have reported similar findings of a new susceptibility locus, 3q28. After that, a number of studies reported that the rs10937405, and rs4488809 polymorphism in chromosome 3q28 has been implicated in LC risk. However, the studies have yielded contradictory results.

METHODS

PubMed, ISI web of science, EMBASE and the Chinese National Knowledge Infrastructure databases were systematically searched to identify relevant studies. Data were abstracted independently by two reviewers. A meta-analysis was performed to examine the association between rs10937405, rs4488809 polymorphism at 3q28 and susceptibility to LC. Odds ratios (ORs) and 95% confidence intervals (95% CIs) were calculated. Heterogeneity and publication bias were also tested.

RESULTS

A total of 9 studies including 35,961 LC cases and 57,790 controls were involved in this meta-analysis. An overall random-effects per-allele OR of1.19 (95% CI: 1.14-1.25; P<10(-5)) and 1.19 (95% CI: 1.13-1.25; P<10(-5)) was found for the rs10937405 and rs4488809 polymorphism respectively. Similar results were also observed using dominant or recessive genetic model. After stratified by ethnicity, significant associations were found among East Asians (per-allele OR = 1.22, 95% CI: 1.17-1.27; P<10(-5)); whereas no significant associations were found among Caucasians for rs10937405. In the sub-group analysis by sample size, significantly increased risks were found for these polymorphisms in all genetic models. When analyzed according to histological type, the effects of rs10937405, and rs4488809 at 3q28 on the risk of lung cancer were significant mostly for lung adenocarcinoma.

CONCLUSIONS

Our findings demonstrated that rs10937405-G allele and rs4488809-G allele might be risk-conferring factors for the development of lung cancer, especially for East Asian populations.

Apoptosis-Related Single Nucleotide Polymorphisms and the Risk of Non-Small Cell Lung Cancer in Women

BACKGROUND

Germline apoptosis-related single nucleotide polymorphisms (SNPs) have been shown to contribute to the risk of developing non-small cell lung cancer (NSCLC). However, very few studies have looked specifically at apoptosis-related SNPs in a racially-stratified analysis of white and African-American women.

METHODS

We examined the risk of developing NSCLC associated with 98 germline SNPs in 32 apoptosis-related genes among women in a population-based case-control study from the Detroit metropolitan area. We examined 453 cases of NSCLC and 478 control subjects. We used an unconditional logistic regression with a dominant model, stratified by race, and adjusted for age, pack-years smoked, ever/never smoking status, family history of lung cancer, history of COPD, BMI and education.

RESULTS

Our logistic regression identified 3 significant apoptosis-related SNPs in whites (APAF-1, rs1007573; CD40 rs3765459, and CD40 rs1535045), and 7 significant SNPs (ATM, rs1801516; BAK1, rs513349; TNF, rs1800629; TP63, rs6790167; TP63, rs7613791, TP63, rs35592567 and TP63, rs3856775) in African-Americans. In a downstream analysis, these SNPs were further prioritized utilizing the False Positive Report Percentage (FPRP) methodology and backwards elimination. In whites, APAF-1 (rs1007573), CD40 (rs3765459) and CD40 (rs1535045) were all found to be significant by FPRP. In African-Americans, TP63 SNPs rs6790167 and rs7613791 were found to have a significant FPRP. In parallel, a backward elimination procedure was used on the 3 significant SNPs in whites and 7 significant SNPs in African-Americans. This procedure identified APAF-1 rs1007573 (OR=1.86, 95% CI: 1.17-2.95) and CD40 rs1535045 (OR=0.58, 95% CI: 0.40-0.84) as significant independent predictors of risk among whites, and ATM rs1801516 (OR=24.15, 95% CI: 3.50-166.55), TNF rs1800629 (OR= 0.42, 95% CI: 0.18-0.99) and TP63 rs6790167 (OR: 2.85, 95% CI: 1.33-6.09) as significant, independent predictors in African-Americans.

CONCLUSION

In whites, only SNPs APAF-1 rs1007573 and CD40 rs1535045 were significant by both FPRP and backwards elimination, while in African-Americans, only TP63 rs6790167 was significant by both methodologies. Thus, we have identified three promising variants associated with increased risk of NSCLC that warrant additional investigation in future studies.

Simvastatin induced HCT116 colorectal cancer cell apoptosis through p38MAPK-p53-survivin signaling cascade

BACKGROUND

Statins, the 3-hydroxy-3-methylglutaryl coenzyme A reductase inhibitors with cholesterol-lowering properties, were recently shown to exhibit anti-cancer effects. However, the molecular mechanism underlying statin-induced cancer cell death remains to be elucidated. Elevated level of survivin is often found over-expressed in human cancers and has been implicated in the progression of tumorigenesis. Given its central role in cell division and action as an apoptosis suppressor, survivin represents a potential molecular target in cancer management.

METHODS

In this study, we explored the underlying mechanisms in simvastatin-induced HCT116 colorectal cancer cell apoptosis.

RESULTS

Simvastatin decreased cell viability and induced cell apoptosis in HCT116 cells. These results are associated with the modulation of p21(cip/Waf1) and survivin. Survivin knockdown using survivin siRNAs also decreased cell viability and induced cell apoptosis. Simvastatin's actions on p21(cip/Waf1), survivin and apoptosis were reduced in p53 null HCT116 cells. Simvastatin caused an increase in p53 phosphorylation and acetylation. In addition, simvastatin activated p38 mitogen-activated protein kinase (p38MAPK), whereas an inhibitor of p38MAPK signaling abrogated simvastatin's effects of increasing p53 and p21(cip/Waf1) promoter luciferase activity. Cell viability and survivin promoter luciferase activity in the presence of simvastatin were also restored by p38MAPK inhibitor. Furthermore, Sp1 binding to the survivin promoter region decreased while p53 and p63 binding to the promoter region increased after simvastatin exposure.

CONCLUSIONS

Simvastatin activates the p38MAPK-p53-survivin cascade to cause HCT116 colorectal cancer cell apoptosis.

GENERAL SIGNIFICANCE

This study delineates, in part, the underlying mechanisms of simvastatin in decreasing survivin and subsequent colorectal cancer cell apoptosis.

p63 is an alternative p53 repressor in melanoma that confers chemoresistance and a poor prognosis

p63 is up-regulated in melanoma and prevents nuclear accumulation of p53.

The role of apoptosis in melanoma pathogenesis and chemoresistance is poorly characterized. Mutations in TP53 occur infrequently, yet the TP53 apoptotic pathway is often abrogated. This may result from alterations in TP53 family members, including the TP53 homologue TP63. Here we demonstrate that TP63 has an antiapoptotic role in melanoma and is responsible for mediating chemoresistance. Although p63 was not expressed in primary melanocytes, up-regulation of p63 mRNA and protein was observed in melanoma cell lines and clinical samples, providing the first evidence of significant p63 expression in this lineage. Upon genotoxic stress, endogenous p63 isoforms were stabilized in both nuclear and mitochondrial subcellular compartments. Our data provide evidence of a physiological interaction between p63 with p53 whereby translocation of p63 to the mitochondria occurred through a codependent process with p53, whereas accumulation of p53 in the nucleus was prevented by p63. Using RNA interference technology, both isoforms of p63 (TA and ΔNp63) were demonstrated to confer chemoresistance, revealing a novel oncogenic role for p63 in melanoma cells. Furthermore, expression of p63 in both primary and metastatic melanoma clinical samples significantly correlated with melanoma-specific deaths in these patients. Ultimately, these observations provide a possible explanation for abrogation of the p53-mediated apoptotic pathway in melanoma, implicating novel approaches aimed at sensitizing melanoma to therapeutic agents.

Role of p63 in Development, Tumorigenesis and Cancer Progression

The p53-related protein p63 has pleiotropic functions, including cell proliferation, survival, apoptosis, differentiation, senescence, and aging. The p63 gene is expressed as multiple isoforms that either contain an N-terminal p53-homologous transactivation domain (TAp63) or that lack this domain (ΔNp63). Multiple studies have demonstrated that p63 plays a crucial role in stratified epithelial development, and have shown the importance of p63 for maintaining proliferation potential, inducing differentiation, and preventing senescence. Additionally, much research focuses on the role of p63 in cancer progression. Clinical evidence suggests that p63 may play a role in inhibiting metastasis. Similarly, genetic mice models together with cell culture data strongly indicate that p63 deficiency may be a causative factor for metastatic spread. Moreover, the role of p63 in cancer metastasis has been shown to be greatly related to the ability of mutant p53 to promote cancer malignancy. However, there is still much confusion as to what the role of each specific isoform is. In this review, we highlight some of the major findings in the current literature regarding the role of specific p63 isoforms in development, tumorigenesis, and particularly in cancer metastasis.

TAp63γ enhances nucleotide excision repair through transcriptional regulation of DNA repair genes

p63 and p73, two p53 family members, play crucial roles in development and tumor suppression. p63 and p73 have multiple isoforms, which have similar or distinct biological functions. Transactivation (TA) isoforms of p63 and p73 have high similarity with p53 and often have biological functions similar to p53. p53 plays an important role in nucleotide excision repair (NER) through transcriptional regulation of target genes involved in NER, including DDB2, XPC and GADD45. To investigate whether TAp63 and TAp73 play a similar role in NER, Saos2 cells with inducible expression of specific isoforms of TAp63 and TAp73, including TAp63α/β/γ and TAp73α/β/γ isoforms, were employed. Overexpression of TAp63γ significantly enhances NER of ultraviolet (UV)-induced DNA damage, including cyclobutane pyrimidine dimers (CPDs) and 6-4 photoproducts, and enhances cell survival after UV irradiation in Soas2 cells. The enhancement of NER of UV-induced DNA damage by TAp63γ was also confirmed in H1299 cells with overexpression of TAp63γ. Consistently, knockdown of endogenous TAp63 decreases NER of UV-induced DNA damage in H1299 cells. TAp63α/β and TAp73α/β/γ isoforms do not have a clear effect on NER in Saos2 or H1299 cells. TAp63γ overexpression clearly induces the expression of DDB2, XPC and GADD45 at both RNA and protein levels. Furthermore, luciferase reporter assays show that TAp63γ transcriptionally activates DDB2, XPC and GADD45 genes through the regulation of the p53 binding elements in these genes. These results demonstrate that TAp63γ enhances NER to remove UV-induced DNA damage and maintain genomic stability through transcriptional induction of a set of NER proteins, which provides an additional important mechanism that contributes to the function of TAp63 in tumor suppression.

Trichostatin A and sirtinol suppressed survivin expression through AMPK and p38MAPK in HT29 colon cancer cells

BACKGROUND

Elevated levels of survivin and histone deacetylases (HDACs) are often found over-expressed in human cancers, including colorectal cancer, and have been implicated in tumorigenesis. HDAC inhibition induces growth arrest and cell death in various transformed cell; however, the mechanisms by which this reduces cell viability in colorectal cancer cells remain unexplained.

METHODS

We explored the actions of two HDAC inhibitors, trichostatin A (TSA) and sirtinol, in HT29 colon cancer cells.

RESULTS

TSA and sirtinol induced apoptosis and inhibited cell proliferation in HT29 cells. These results are associated with the modulation of survivin. Survivin promoter luciferase activity and Sp1, a transcription factor that contributes to survivin expression, were suppressed in cells exposed to TSA or sirtinol. TSA and sirtinol also activated p38 mitogen-activated protein kinase (p38MAPK) and AMP-activated protein kinase (AMPK). Inhibitors of p38MAPK or AMPK signaling abrogated TSA and sirtinol's effects of decreasing cell viability. Survivin promoter luciferase activity in the presence of TSA or sirtinol was restored by AMPK dominant negative mutant or p38MAPK inhibitor. Furthermore, Sp1 binding to the survivin promoter region decreased while p63 binding to the promoter region increased after TSA or sirtinol exposure.

CONCLUSIONS

We report a p38MAPK- and AMPK-mediated downregulation of survivin, and its functional correlation with decreased colon cancer cell viability in the presence of HDAC inhibitor. p63 and Sp1 may also contribute to TSA and sirtinol actions.

GENERAL SIGNIFICANCE

This study delineates, in part, the underlying mechanisms of TSA and sirtinol in decreasing survivin expression and subsequent colon cancer cell viability.

p63 is a suppressor of tumorigenesis and metastasis interacting with mutant p53

p53 mutations, occurring in two-thirds of all human cancers, confer a gain of function phenotype, including the ability to form metastasis, the determining feature in the prognosis of most human cancer. This effect seems mediated at least partially by its ability to physically interact with p63, thus affecting a cell invasion pathway, and accordingly, p63 is deregulated in human cancers. In addition, p63, as an 'epithelial organizer', directly impinges on epidermal mesenchimal transition, stemness, senescence, cell death and cell cycle arrest, all determinant in cancer, and thus p63 affects chemosensitivity and chemoresistance. This demonstrates an important role for p63 in cancer development and its progression, and the aim of this review is to set this new evidence that links p63 to metastasis within the context of the long conserved other functions of p63.

Variation in TP63 is associated with lung adenocarcinoma in the UK population

BACKGROUND

Variation at TP63 has recently been shown to be associated with lung adenocarcinoma in the Asian population.

METHODS

To investigate how this finding translates to the European population we compared the genotypes of SNPs annotating the TP63 locus at 3q28 in 4,462 lung cancer patients, including 911 with adenocarcinoma, and 8,235 controls from the United Kingdom.

RESULTS

A statistically significant association between adenocarcinoma risk and SNP genotype was shown: rs10937405, OR = 1.21, P = 1.82 × 10(-4); rs17429138, OR = 1.23, P = 7.49 × 10(-5); and rs4396880, OR = 1.21, P = 2.03 × 10(-4). Haplotype analysis was consistent with a single TP63 risk locus defined by SNPs rs10937405, rs17429138, and rs4396880. While no association between SNPs and small cell lung cancer was shown, the rs10937405 and rs439680 associations were significant for squamous cancer (respective P-values, 0.0022 and 0.02).

CONCLUSIONS

These findings show TP63 variation is a risk factor for the development of lung adenocarcinoma in the UK population. Furthermore, they provide additional insight into the subtype-specificity of the 3q28 lung cancer association.

IMPACT

Our data confirm the association of 3q28 with lung adenocarcinoma and that this association is not confined to the Asian population. Elucidating the functional basis of this association will be contingent on future fine mapping of the TP63 loci.

TAp63 plays compensatory roles in p53-deficient cancer cells under genotoxic stress

p53, p63, and p73 belong to the p53 family of proteins, which mediate development, differentiation, and various other cellular responses. p53 is involved in many anti-cancer mechanisms, such as cell cycle regulation, apoptosis, and the maintenance of genomic integrity. The p63 gene is controlled by two promoters that direct the expression of two isoforms, one with and one without transactivating properties, known as TAp63 and ΔNp63. In this study, p53-deficient cells (Hep3B and PC-3) and p53-expressing cells (A549 and HepG2) were treated with doxorubicin to examine the possible roles of TAp63 in these cells under genotoxic stress; TAp63 expression was induced in p53-deficient cell lines, but not in p53-expressing cell lines. The ectopic expression of p53 in p53-deficient cells (Hep3B) reduced TAp63 promoter activity, and knockdown of TAp63 attenuated doxorubicin-induced cell growth arrest by promoting cell cycle progression, leading to an increase in the percentage of G(2)/M cells. Moreover, knockdown of TAp63 increased cell sensitivity to doxorubicin-induced genomic damage. Our results suggest that TAp63 may play a compensatory role in cell cycle regulation and DNA damage repair in p53-deficient cancer cells.

Variation in TP63 is associated with lung adenocarcinoma susceptibility in Japanese and Korean populations

Lung cancer is the most common cause of death from cancer worldwide, and its incidence is increasing in East Asian and Western countries. To identify genetic factors that modify the risk of lung adenocarcinoma, we conducted a genome-wide association study in a Japanese cohort, with replication in two independent studies in Japanese and Korean individuals, in a total of 2,098 lung adenocarcinoma cases and 11,048 controls. The combined analyses identified two susceptibility loci for lung adenocarcinoma: TERT (rs2736100, combined P = 2.91 × 10⁻¹¹), odds ratio (OR) = 1.27) and TP63 (rs10937405, combined P = 7.26 × 10⁻¹²), OR = 1.31). Fine mapping of the region containing TP63 showed that a SNP (rs4488809) in intron 1 of TP63 showed the most significant association. Our results suggest that genetic variation in TP63 may influence susceptibility to lung adenocarcinoma in East Asian populations.

A case of prostatic adenocarcinoma with aberrant p63 expression: presentation with detailed immunohistochemical study and FISH analysis

Prostate carcinomas showing aberrant diffuse-nuclear p63 expression are extremely rare, and there is only 1 article in the literature reporting a series of 21 such cases. We document an additional case of p63-positive prostatic adenocarcinoma in a 60-year-old man, whose diagnosis was difficult. The patient was found to have an elevated prostate-specific antigen (PSA) level at a general health check-up and was referred to the hospital. His serum PSA was 4.2 ng/mL. Digital rectal examination and transrectal ultrasonography did not reveal a lesion. Transrectal needle biopsy of the prostate detected atypical, small prostatic glands suspected for adenocarcinoma at 2 cores. However immunohistochemistry showed nuclear p63 expression in the suspicious glands. Repeat biopsy revealed only high-grade prostatic intraepithelial neoplasia. In the third transrectal biopsy, finding of the same atypical glands showing perineural invasion facilitated the diagnosis of malignancy. The patient underwent a radical prostatectomy. Five different small tumor foci were seen in the prostate after pathological evaluation, one of which was p63 positive and the others p63 negative. The largest of the classic p63-negative tumors showed a TMPRSS2-ERG translocation by fluorescent in situ hybridization while the p63-positive tumor did not. The authors submit that this subtype (p63-positive prostate adenocarcinoma) should be listed among the recognized rare variants of prostatic adenocarcinoma.

TAp63 is a transcriptional target of NF-kappaB

The p53 homologue p63 encodes multiple protein isoforms either with (TA) or without (DeltaN) the N-terminal transactivation domain. Accumulating evidence indicates that TAp63 plays an important role in various biological processes, including cell proliferation, differentiation, and apoptosis. However, how TAp63 is regulated remains largely unclear. In this study, we demonstrate that NF-kappaB induces TAp63 gene expression. The responsible elements for NF-kappaB-mediated TAp63 induction are located within the region from -784 to -296 bp in the TAp63 promoter, which contains two NF-kappaB binding sites. Ectopic expression of RelA stimulates TAp63 promoter-driven reporter activity and increases endogenous TAp63 mRNA levels. Inhibition of NF-kappaB by IkappaBalpha super-repressor or with a chemical inhibitor leads to down regulation of TAp63 mRNA expression and activity. In addition, mutations in the critical NF-kappaB-binding sites significantly abolish the effects of NF-kappaB on TAp63. Activation of NF-kappaB by TNFalpha enhances p50/RelA binding to the NF-kappaB binding sites. Furthermore, we show that an Sp1 site adjacent to the NF-kappaB sites plays a role in NF-kappaB-mediated upregulation of TAp63. Taken together, these data reveal that TAp63 is a transcriptional target of NF-kappaB, which may play a role in cell proliferation, differentiation and survival upon NF-kappaB activation by various stimuli.

MicroRNA 203 expression in keratinocytes is dependent on regulation of p53 levels by E6

A screen of microRNA (miRNA) expression following differentiation in human foreskin keratinocytes (HFKs) identified changes in several miRNAs, including miRNA 203 (miR-203), which has previously been shown to play an important role in epithelial cell biology by regulating p63 levels. We investigated how expression of human papillomavirus type 16 (HPV16) oncoproteins E6 and E7 affected miR-203 expression during proliferation and differentiation of HFKs. We demonstrated that miR-203 expression is reduced in HFKs where p53 function is compromised, either by the viral oncoprotein E6 or by knockout of p53 using short hairpin RNAs (p53i). We show that the induction of miR-203 observed during calcium-induced differentiation of HFKs is significantly reduced in HFKs expressing E6 and in p53i HFKs. Induction of miR-203 in response to DNA damage is also reduced in the absence of p53. We report that proliferation of HFKs is dependent on the level of miR-203 expression and that overexpression of miR-203 can reduce overproliferation in E6/E7-expressing and p53i HFKs. In summary, these results indicate that expression of miR-203 is dependent on p53, which may explain how expression of HPV16 E6 can disrupt the balance between proliferation and differentiation, as well as the response to DNA damage, in keratinocytes.

NAD(P)H quinone oxidoreductase protects TAp63gamma from proteasomal degradation and regulates TAp63gamma-dependent growth arrest

Background

p63 is a member of the p53 transcription factor family. p63 is expressed from two promoters resulting in proteins with opposite functions: the transcriptionally active TAp63 and the dominant-negative ΔNp63. Similar to p53, the TAp63 isoforms induce cell cycle arrest and apoptosis. The ΔNp63 isoforms are dominant-negative variants opposing the activities of p53, TAp63 and TAp73. To avoid unnecessary cell death accompanied by proper response to stress, the expression of the p53 family members must be tightly regulated. NAD(P)H quinone oxidoreductase (NQO1) has recently been shown to interact with and inhibit the degradation of p53. Due to the structural similarities between p53 and p63, we were interested in studying the ability of wild-type and polymorphic, inactive NQO1 to interact with and stabilize p63. We focused on TAp63γ, as it is the most potent transcription activator and it is expected to have a role in tumor suppression.

Principal Findings

We show that TAp63γ can be degraded by the 20S proteasomes. Wild-type but not polymorphic, inactive NQO1 physically interacts with TAp63γ, stabilizes it and protects it from this degradation. NQO1-mediated TAp63γ stabilization was especially prominent under stress. Accordingly, we found that downregulation of NQO1 inhibits TAp63γ-dependant p21 upregulation and TAp63γ-induced growth arrest stimulated by doxorubicin.

Conclusions/Significance

Our report is the first to identify this new mechanism demonstrating a physical and functional relationship between NQO1 and the most potent p63 isoform, TAp63γ. These findings appoint a direct role for NQO1 in the regulation of TAp63γ expression, especially following stress and may therefore have clinical implications for tumor development and therapy.

Involvement of p63 in the herpes simplex virus-1-induced demise of corneal cells

Background

The transcription factor p63 plays a pivotal role in the development and maintenance of epithelial tissues, including the ocular surface. In an effort to gain insight into the pathogenesis of keratitis caused by HSV-1, we determined the expression patterns of the p63 and Bax proteins in the Staatens Seruminstitute Rabbit Cornea cell line (SIRC).

Methods

SIRC cells were infected with HSV-1 at various multiplicities and maintained for different periods of time. Virus replication was measured by indirect immunofluorescence assay and Western blot analysis. Cell viability was determined by MTT assay. The apoptotic response of the infected cells was quantified by ELISA detecting the enrichment of nucleosomes in the cytoplasm. Western blot analysis was used to determine the levels of p63 and Bax proteins.

Results

Indirect immunofluorescence assays and Western blot analyses demonstrated the presence of HSV-1 glycoprotein D (gD) in the infected SIRC cell line, and the pattern of gD expression was consistent with efficient viral replication. The results of MTT and ELISA assays showed that HSV-1 elicited a strong cytopathic effect, and apoptosis played an important role in the demise of the infected cells. Mock-infected SIRC cells displayed the constitutive expression of ΔNp63α. The expressions of the Bax-β and TAp63γ isoforms were considerably increased, whereas the level of ΔNp63α was decreased in the HSV-1-infected SIRC cells. Experiments involving the use of acyclovir showed that viral DNA replication was necessary for the accumulation of TAp63γ.

Conclusion

These data suggest that a direct, virus-mediated cytopathic effect may play an important role in the pathogenic mechanism of herpetic keratitis. By disturbing the delicate balance between the pro-survival ΔN and the pro-apoptotic TA isoforms, HSV-1 may cause profound alterations in the viability of the ocular cells and in the tissue homeostasis of the ocular surface.

The herpes simplex virus-induced demise of keratinocytes is associated with a dysregulated pattern of p63 expression

p63 plays a pivotal role in the development and maintenance of stratified epithelial tissues. In an effort to gain insight into the pathogenic mechanisms of skin infections caused by HSV-1 and HSV-2, we determined the patterns of p63 expression in primary keratinocytes and in the HaCaT cell line. The levels of DeltaNp63alpha and a 50kDa p73 isoform were decreased, Bax-alpha remained unaffected, while the expressions of the Bax-beta, TAp63gamma and a 44.5kDa p73 isoform were highly increased in both HSV-1-infected HaCaT cells and primary keratinocytes. In contrast, in response to HSV-2 infection the levels of DeltaNp63alpha, a 50kDa p73 isoform and a 44.5kDa p73 protein were decreased, Bax-alpha and TAp63gamma remained unaffected, while the expression of Bax-beta was slightly increased. The knockdown of TAp63 expression enhanced the viability of HSV-1-infected cells. Thus, HSV-1 and HSV-2 modulate the patterns of p63 and Bax expression in a serotype-specific manner. The dysregulated pattern of p63 expression observed in HSV-infected keratinocytes may comprise part of a mechanism by which these viruses perturb the functions of keratinocytes and lead to their demise.

Apaf-1-deficient fog mouse cell apoptosis involves hypo-polarization of the mitochondrial inner membrane, ATP depletion and citrate accumulation

To explore how the intrinsic apoptosis pathway is controlled in the spontaneous fog (forebrain overgrowth) mutant mice with an Apaf1 splicing deficiency, we examined spleen and bone marrow cells from Apaf1(+/+) (+/+) and Apaf1(fog/fog) (fog/fog) mice for initiator caspase-9 activation by cellular stresses. When the mitochondrial inner membrane potential (Deltapsim) was disrupted by staurosporine, +/+ cells but not fog/fog cells activated caspase-9 to cause apoptosis, indicating the lack of apoptosome (apoptosis protease activating factor 1 (Apaf-1)/cytochrome c/(d)ATP/procaspase-9) function in fog/fog cells. However, when a marginal ( approximately 20%) decrease in Deltapsim was caused by hydrogen peroxide (0.1 mM), peroxynitritedonor 3-morpholinosydnonimine (0.1 mM) and UV-C irradiation (20 J/m(2)), both +/+ and fog/fog cells triggered procaspase-9 auto-processing and its downstream cascade activation. Supporting our previous results, procaspase-9 pre-existing in the mitochondria induced its auto-processing before the cytosolic caspase activation regardless of the genotypes. Cellular ATP concentration significantly decreased under the hypoactive Deltapsim condition. Furthermore, we detected accumulation of citrate, a kosmotrope known to facilitate procaspase-9 dimerization, probably due to a feedback control of the Krebs cycle by the electron transfer system. Thus, mitochondrial in situ caspase-9 activation may be caused by the major metabolic reactions in response to physiological stresses, which may represent a mode of Apaf-1-independent apoptosis hypothesized from recent genetic studies.

Differential effects of p63 mutants on transactivation of p53 and/or p63 responsive genes

p63, known to play a role in development, has more recently also been implicated in cancer progression. Mutations in p63 have been shown to be responsible for several human developmental diseases. Differential splicing of the p63 gene gives rise to p63 isoforms, which can act either as tumor suppressors or as oncogene. In this report, we studied the effects of naturally occurring TAp63gamma mutants on the regulation of p53/p63 and p63 specific target genes. We observed significant differences among p63 mutants to regulate the p53/p63 and p63 specific target genes. Additionally, we observed a differential effect of p63 mutants on wildtype-p63-mediated induction of p53/p63 and p63 specific target genes. We also demonstrated that these mutants differentially regulate the binding of wildtype p63 to the promoter of target genes. Furthermore, the effects of these mutants on cell death and survival were consistent with their ability to regulate the downstream targets when compared to wildtype TAp63gamma. In summary, our data demonstrate that p63 mutants exhibit differential effects on p63 and p53/p63 specific target genes and on the induction of apoptosis, and provide further insight into the function of p63.

The p53 family and programmed cell death

The p53 tumor suppressor continues to hold distinction as the most frequently mutated gene in human cancer. The ability of p53 to induce programmed cell death, or apoptosis, of cells exposed to environmental or oncogenic stress constitutes a major pathway whereby p53 exerts its tumor suppressor function. In the past decade, we have discovered that p53 is not alone in its mission to destroy damaged or aberrantly proliferating cells: it has two homologs, p63 and p73, that in various cellular contexts and stresses contribute to this process. In this review, the mechanisms whereby p53, and in some cases p63 and p73, induce apoptosis are discussed. Other reviews have focused more extensively on the contribution of individual p53-regulated genes to apoptosis induction by this protein, whereas in this review, we focus more on those factors that mediate the decision between growth arrest and apoptosis by p53, p63 and p73, and on the post-translational modifications and protein-protein interactions that influence this decision.

Therapeutic prospects for p73 and p63: rising from the shadow of p53

The p53 protein family consists of three transcription factors: p53, p63, and p73. These proteins share significant structural and functional similarities and each has unique biological functions as well. Although the role of p53 in cellular stress is extensively studied, many questions remain about p63 and p73. In this review we summarize current data on functional interactions within the p53 family, their regulation and roles in response to genotoxic stress. We also discuss the significance of p73 and p63 for cancer therapy and outline novel approaches in development of therapeutic drugs that specifically target the p53 family.

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

We previously found that the pro-apoptotic DNA damaging agent, cisplatin, mediated the proteasome-dependent degradation of Delta Np63 alpha associated with its increased phosphorylated status. Since Delta Np63 alpha usually plays an opposite role to p53 and TAp63 in human cancers, we tested the notion that phosphorylation events induced by DNA damage would affect the protein degradation of Delta Np63 alpha in HNSCC cells upon cisplatin exposure. We found that Delta Np63 alpha is phosphorylated in the time-dependent fashion at the following positions: S385, T397 and S466, which were surrounded by recognition motifs for ATM, CDK2 and p70s6K kinases, respectively. We showed that chemical agents or siRNA inhibiting the activity of ATM, CDK2 and p70s6K kinases blocked degradation of Delta Np63 alpha in HNSCC cells after cisplatin exposure. Site-specific mutagenesis of Delta Np63 alpha residues targeted for phosphorylation by ATM, CDK2 or p70s6k led to dramatic modulation of Delta Np63 alpha degradation. Finally, we demonstrated that the Delta Np63 alpha protein is a target for direct in vitro phosphorylation by ATM, CDK2 or p70s6K. Our results implicate specific kinases, and target phosphorylation sites in the degradation of Delta Np63 alpha following DNA damage.

p63 in prostate biology and pathology

The identification of stem cells and differentiation programs regulating the development and maintenance of the normal prostate epithelium is essential for the identification of the cell type(s) and molecular alterations involved in the development and propagation of prostate cancer (CaP). The p53-homologue p63 is highly expressed in normal prostate basal cells and is a clinically useful biomarker for the diagnosis of CaP. Importantly, p63 has been shown to play a critical role in prostate development. Recent experimental evidence also suggests that this gene is essential for normal stem cell function in the prostate as well as other epithelial organs. Future studies aimed at better defining the role of p63 in the renewal of the adult prostate epithelium are likely to shed new light on the mechanisms involved in prostate carcinogenesis.

Properties of the six isoforms of p63: p53-like regulation in response to genotoxic stress and cross talk with DeltaNp73

TP63, a member of the TP53 gene family, encodes two groups of three isoforms (alpha, beta and gamma). The TAp63 isoforms act as transcription factors. The DeltaNp63 isoforms lack the main transcription activation domain and act as dominant-negative inhibitors of transactivation (TA) isoforms. To clarify the role of these isoforms and to better understand their functional overlap with p53, we ectopically expressed each p63 isoform in the p53-null hepatocellular carcinoma cell line Hep3B. All TA isoforms, as well as DeltaNp63alpha, had a half-life of <1 h when transiently expressed and were degraded by the proteasome pathway. The most stable form was DeltaNp63gamma, with a half-life of >8 h. As expected, TA isoforms differed in their transcriptional activities toward genes regulated by p53, TAp63gamma being the most active form. In contrast, DeltaNp63 isoforms were transcriptionally inactive on genes studied and inhibited TA isoforms in a dose-dependent manner. When stably expressed in polyclonal cell populations, TAp63beta and gamma isoforms were undetectable. However, when treated with doxorubicin (DOX), p63 proteins rapidly accumulated in the cells. This stabilization was associated with an increase in phosphorylation. Strikingly, in DOX-treated polyclonal populations, increase in TAp63 levels was accompanied by overexpression of DeltaNp73. This observation suggests complex regulatory cross talks between the different isoforms of the p53 family. In conclusion, p63 exhibits several transcriptional and stress-response properties similar to those of p53, suggesting that p63 activities should be taken into consideration in approaches to improve cancer therapies based on genotoxic agents.

Impaired repair of cyclobutane pyrimidine dimers in human keratinocytes deficient in p53 and p63

While many p53-deficient cell types are impaired in global genomic nucleotide excision repair of cyclobutane pyrimidine dimers (CPDs), human epidermal keratinocytes expressing human papillomavirus type 16 E6 and E7 are p53 deficient and yet maintain repair of CPD. We hypothesized that the p53 homolog, p63, may participate in governing global repair instead of p53 in keratinocytes. Following ultraviolet radiation (UVR) of E6/E7 keratinocytes, depletion of p63 but not of p73 impaired global genomic repair of CPD relative to control cells. In all cases, repair of pyrimidine(6-4)pyrimidone photoproducts, the other major UVR-induced DNA lesions, was unaffected. In E6/E7 keratinocytes treated with p63 small interfering RNA, reduced global repair of CPD was associated not with reduced levels of messenger RNA-encoding DNA damage recognition proteins but rather with decreased levels of DDB2 and XPC proteins, suggesting that p63 posttranscriptionally regulates levels of these proteins. These results indicate that global repair may be regulated at multiple levels and suggest a novel role for p63 in modulating repair of DNA damage in human keratinocytes. The results may provide insight into mechanisms of genomic stability in epithelia infected with oncogenic human papilloma viruses and may further explain the lack of increased skin cancer incidence in Li-Fraumeni syndrome.

p63: defining roles in morphogenesis, homeostasis, and neoplasia of the epidermis

p63 is a member of a gene family also including the p53 tumor suppressor and p73. In contrast to p53, p63 is rarely mutated in human cancers. Rather, gene amplification and dysregulated expression of p63 protein have been observed, particularly in squamous cell carcinomas. p63 is essential for development of stratified squamous epithelium, including the epidermis. The p63 gene is expressed as multiple protein isoforms with different functional capacities, and the balance of these isoforms, along with the presence or absence of the other family members, p53 and p73, can impact biological outcome. Both gene silencing and overexpression approaches have been utilized to elucidate the contributions of specific p63 isoforms to normal epidermal morphogenesis and tissue maintenance. While numerous studies have established the essential nature of p63 in the epidermis, the basis of this requirement, and the unique, as well as, overlapping functions of the individual isoforms, remain controversial. In this review, we summarize the current understanding of roles played by specific p63 isoforms within the context of epidermal morphogenesis and homeostasis of the established epidermis, and the potential impact of p63 dysregulation on cancer development.

p51/p63, a novel p53 homologue, potentiates p53 activity and is a human cancer gene therapy candidate

BACKGROUND

p51 (p73L/p63/p40/KET), a recently isolated novel p53 homologue, binds to p53-responsive elements to upregulate some p53 target genes and has been suggested to share partially overlapping functions with p53. p51 may be a promising candidate target molecule for anti-cancer therapy.

METHODS

In this study, we adenovirally transduced p51A cDNA into human lung, gastric and pancreatic cancer cells and analyzed the intracellular function of p51 in anti-oncogenesis in vitro and in vivo.

RESULTS

Overexpression of p51A revealed an anti-proliferative effect in vitro in all the cancer cells examined in this study. The anchorage-dependent and -independent cell growth of EBC1 cells carrying mutations in both p51 and p53 was suppressed and significant apoptosis following adenoviral transduction with p51 and/or p53 was seen. This growth suppression was cooperatively enhanced by the combined infection with adenoviral vectors encoding both p51 and p53. Furthermore, p51 activated several, but not all, p53-inducible genes, indicating that the mechanisms controlling p51- and p53-mediated tumor suppression differed.

CONCLUSIONS

Our observations indicate that, although p51 exhibited reduced anti-oncogenetic effects compared with p53, it cooperatively enhanced the anti-tumor effects of p53. Our results suggest that p51 functions as a tumor suppressor in human cancer cells in vitro and in vivo and may be useful as a potential tool for cancer gene therapy.

p53 downregulates expression of the G1/S cell cycle phosphatase Cdc25A

Overexpression of Cdc25A phosphatase is often observed in cancer and results in poor prognosis. Cdc25A mainly dephosphorylates and thereby activates Cyclin-dependent kinase 2 and thus induces progression in the cell cycle from G(1) to S phase. Here, we demonstrate that the tumor suppressor p53 downregulates expression from the Cdc25A gene. In a p53-inducible cell system, Cdc25A expression on the mRNA and protein level is downregulated upon p53 expression. Promoter-reporter assays show that this regulation is dependent on the Cdc25A promoter. Mutant p53 fails to reduce Cdc25A transcription. In contrast to p53, neither p63 nor p73 can repress Cdc25A transcription. The Cdc25A promoter displays no p53 binding site, and p53 does not bind directly to the promoter DNA as shown by chromatin immunoprecipitation assays. Previously, the contribution of p53 to G(1)/S arrest has been mostly linked to activating the expression of the Cdk inhibitor p21(WAF1/CIP1). By downregulating Cdc25A expression, p53 may impair transition from G(1) to S phase independently of p21(WAF1/CIP1). Therefore, the data suggest that, as long as p53 is intact, Cdc25A transcriptional downregulation might play a role in cancer prevention.

p63 and epithelial biology

The transcription factor p63 is a homologue of the tumor suppressor p53. Unlike p53, which is dispensable for normal development, p63 is critical for the development of stratified epithelial tissues such as epidermis, breast, and prostate. p63 encodes multiple protein isoforms with both transactivating and transcriptional repressor activities that can regulate a wide spectrum of target genes. p63 is also implicated in tumor formation and progression in stratified epithelia, with evidence for both tumor suppressive and oncogenic properties. This review will examine current data and hypotheses regarding the role of p63 in the development, maintenance, and tumorigenesis of stratified epithelium.

The role of p63 in germ cell apoptosis in the developing testis

The fetal and neonatal development of male germ cells (gonocytes) is a poorly understood but crucial process for establishing fertility. In rodents, gonocytes go through two phases of proliferation accompanied by apoptosis and separated by a quiescent period during the end of fetal development. P63 is a member of the P53 gene family that yields six isoforms. We detected only the p63 protein and no p53 and p73 in the nucleus of the gonocytes of mouse testes. We report for the first time the ontogeny of each p63 mRNA isoform during testis development. We observed a strong expression of p63gamma mRNA and protein when gonocytes are in the quiescent period. In vitro treatment with retinoic acid prevented gonocytes from entering the quiescent period and was correlated with a reduced production of p63gamma isoform mRNA. We investigated the function of p63 by studying the testicular phenotype of P63-null mice. P63 invalidation slightly, but significantly increased the number of gonocytes counted during the quiescent period. As P63-null animals die at birth we used an original organ culture that mimicked neonatal in vivo development to study further the testicular development. P63 invalidation resulted in a sharply increased number of gonocytes during the culture period due to a decrease in spontaneous apoptosis with no change in proliferation. P63 invalidation also caused abnormal morphologies in the germ cells that were also found in P63(+/-) adult male mice. Thus, p63 appears as an important regulator of germ cell development.

Dysregulation of TAp63 mRNA and Protein Levels in Psoriasis

Psoriasis is a chronic and excessive inflammation of the skin and is currently incurable. The cause of psoriasis remains poorly understood and a central and cooperative role for keratinocytes and T-cells in triggering the disease is highlighted. The p63 gene encodes six different proteins with homology to the tumor suppressor protein p53 that are crucial for normal development of ectodermally derived structures such as skin and oral mucosa. In this study, we have analyzed levels of the different p63 isoforms using quantitative reverse transcription-polymerase chain reaction (RT-PCR) and immunohistochemistry in 15 patients diagnosed with psoriasis. Quantitative RT-PCR results showed downregulation of the full-length TAp63 in psoriatic lesions compared to both clinically normal skin from patients (P<0.001) and matched healthy controls (P<0.001); however, p63 protein levels detected by immunohistochemistry were similar. All psoriasis lesions also had detectable levels of activated Stat3, a protein indicated in development of the disease, whereas control tissue lacked this protein. The present data show a different regulation of TAp63 in psoriasis, where the discrepancy between mRNA levels and protein expression indicates a post-transcriptional regulation analogous to that seen in p53.

p63: oncogene or tumor suppressor?

p53, the original member of the family of genes now known to include p63 and p73, was first heralded as an oncogene because of its potent transformation capabilities and its robust expression in human tumors. However, it was later discovered that only mutant p53 was oncogenic, and that wild type p53 functioned as a tumor suppressor. Decades later, p63, the newest member of this gene family, is involved in a similar controversy: is p63 an oncogene or a tumor suppressor? Recent progress on understanding the in vivo role of p63 in cancer has focused primarily on investigating its involvement in the tumor-suppressive mechanism of apoptosis, by analyzing mouse models to assess its tumor-suppressive capabilities, and by assessing its expression in human cancers.

The expression of TA and DeltaNp63 are regulated by different mechanisms in liver cells

The TP63 gene, a member of the TP53 gene family, encodes several isoforms with (TAp63) or without (DeltaNp63) transactivating properties. Whereas the role of p63 in the normal development of squamous epithelia is well established, its function in other cell types remains to be elucidated. Here, we have analysed the expression of TA and DeltaNp63 isoforms in liver cells, by using both primary hepatocytes from wild type and p53-null mice and three human hepatocellular carcinoma (HCC) cell lines, according to the transformation state and the TP53 status of the cells. We observed the expression of DeltaNp63 isoforms only in a p53-null context. On the other hand, the expression of TAp63 isoforms was restricted to the HCC cell lines, whatever the TP53 status. We then studied the expression of TP63 upon genotoxic treatment. When treated with UVB or H(2)O(2), hepatocytes did not exhibit any change in p63 mRNA level. At the opposite, upon treatment with topoisomerase II inhibitors (doxorubicin or etoposide), the expression of TAp63 isoforms was clearly induced, independently of the TP53 status of cells. The same treatment did not induce any variation in the expression of DeltaNp63 isoforms, both at mRNA and protein levels. In HCC cell lines, doxorubicin or etoposide treatment also resulted in an increase of TAp63 transcripts only. This increase was accompanied by an increase in the intracellular level of TAp63 alpha protein. In parallel, we observed an upregulation of some p53-target genes related to cell cycle regulation, such as WAF1/CIP1, PIG3, 14-3-3sigma or GADD45, independently of the TP53 status of cells. In conclusion, we report for the first time that TA and DeltaNp63 alpha proteins are present in liver cells. Furthermore, our results suggest that p63 may partially substitute for wild-type p53, in counteracting uncontrolled liver cell proliferation in response to certain forms of DNA-damage.

p51/p63 Controls Subunit α3 of the Major Epidermis Integrin Anchoring the Stem Cells to the Niche

p51/p63, a member of the tumor suppressor p53 gene family, is crucial for skin development. We describe here identification of ITGA3 encoding integrin alpha(3) as a target of its trans-activating function, proposing that p51/p63 allows epidermal stem cells to express laminin receptor alpha(3)beta(1) for anchorage to the basement membrane. When activated by genotoxic stress or overexpressed ectopically in non-adherent cells, p51/p63 transduced a phenotype to attach to extracellular matrices, which was accompanied by expression of ITGA3. Motifs matching the p53-binding consensus sequence were located in a scattered form in intron 1 of human ITGA3, and served as p51/p63-responsive elements in reporter assays. In addition to the trans-activating ability of the TA isoform, we detected a positive effect of the DeltaN isoform on ITGA3. The high level alpha(3) production in human keratinocyte stem cells diminished upon elimination of p51/p63 by small interfering RNA or by Ca(2+)-induced differentiation. Furthermore, a chromatin immunoprecipitation experiment indicated a physical interaction of p51/p63 with intron 1 of ITGA3. This study provides a molecular basis for the standing hypothesis that p51/p63 is essential for epidermal-mesenchymal interactions.