p73 is transcriptionally regulated by DNA damage, p53, and p73

The Proteasome Inhibitor CEP-18770 Induces Cell Death in Medulloblastoma

Medulloblastomas (MBs) represent the most prevalent malignant solid tumors in kids. The conventional treatment regimen for MBs includes surgical removal of the tumor, followed by radiation and chemotherapy. However, this approach is associated with significant morbidity and detrimental side effects. Consequently, there is a critical demand for more precise and less harmful treatments to enhance the quality of life for survivors. CEP-18770, a novel proteasome inhibitor that targets the 20S subunit, has emerged as a promising candidate, due to its anticancer activity in metastatic solid tumors and multiple myeloma, coupled with an acceptable safety profile. In this study, we aimed to assess the anticancer efficacy of CEP-18770 by employing a variety of MB patient-derived cells and cell lines. Our preclinical investigations revealed that CEP-18770 effectively inhibits proteasome activity and induces apoptosis in MBs cells. Furthermore, we discovered that CEP-18770 and cisplatin, a current component of MB therapy, exhibit a synergistic apoptotic effect. This paper shows that CEP-18770 holds potential as an adjunctive treatment for MB tumors, thereby paving the way for more targeted and less toxic therapeutic strategies.

p73α1, a p73 C-terminal isoform, regulates tumor suppression and the inflammatory response via Notch1

Significance p73 is expressed as multiple C-terminal isoforms, but their expression and activity are largely unknown. Here, we identified p73α1 as a p73 C-terminal isoform that results from exon 12 (E12) exclusion. We showed that E12 deficiency in mice leads to systemic inflammation but not spontaneous tumors. We also showed that Notch1 is regulated by p73α1 and plays a critical role in p73-dependent tumor suppression and systemic inflammation.

Enhanced Expression of p21 Promotes Sensitivity of Melanoma Cells Towards Targeted Therapies

Metastatic melanoma patients benefit from the approved targeted BRAF inhibitor (BRAFi) therapy. Despite the great progress in the therapeutic approach to combat metastatic melanoma, fast emerging drug resistance in patients limits its long-term efficacy. In this study we aimed to unravel the role of the p53 target gene CDKN1A/p21 in the response of melanoma cells towards BRAFi. We show that p53 activation increases BRAFi sensitivity in a synergistic manner exclusively in cells with a high expression of CDKN1A/p21. In a similar way high expression of p21 was associated with a better response towards the mouse double minute 2 inhibitor (MDM2i) compared to those with low p21 expression. Indeed, p21 knockdown decreased the sensitivity towards both targeted therapies. The results indicate that the sensitivity of melanoma cells towards targeted therapies (BRAFi and MDM2i) is dependent on the p21 protein level in the cells. In addition to that, we found that p53 negatively regulates p73 expression, however, p73 seems not to have an influence on p53 expression. These findings offer new potential strategies for the treatment improvement of melanoma patients with high basal p21 levels with BRAFi by increasing treatment efficacy using combination therapies with p53 activating substances, which are able to further increase p21 expression levels. Furthermore, the data suggest that the expression and induction level of p21 could be used as a predictive biomarker in melanoma patients to forecast the outcome of a treatment with p53 activating substances and BRAFi. All in all, this manuscript shows the distinct roles and of the p53 family members and its impact on melanoma therapy. In the future, individualized treatment regimens based on p21 basal and induction levels could benefit melanoma patients with limited treatment options.

p53/p73 Protein Network in Colorectal Cancer and Other Human Malignancies

Simple Summary

The p53 family of proteins comprises p53, p63, and p73, which share high structural and functional similarity. The two distinct promoters of each locus, the alternative splicing, and the alternative translation initiation sites enable the generation of numerous isoforms with different protein-interacting domains and distinct activities. The co-expressed p53/p73 isoforms have significant but distinct roles in carcinogenesis. Their activity is frequently impaired in human tumors including colorectal carcinoma due to dysregulated expression and a dominant-negative effect accomplished by some isoforms and p53 mutants. The interactions between isoforms are particularly important to understand the onset of tumor formation, progression, and therapeutic response. The understanding of the p53/p73 network can contribute to the development of new targeted therapies.

Abstract

The p53 tumor suppressor protein is crucial for cell growth control and the maintenance of genomic stability. Later discovered, p63 and p73 share structural and functional similarity with p53. To understand the p53 pathways more profoundly, all family members should be considered. Each family member possesses two promoters and alternative translation initiation sites, and they undergo alternative splicing, generating multiple isoforms. The resulting isoforms have important roles in carcinogenesis, while their expression is dysregulated in several human tumors including colorectal carcinoma, which makes them potential targets in cancer treatment. Their activities arise, at least in part, from the ability to form tetramers that bind to specific DNA sequences and activate the transcription of target genes. In this review, we summarize the current understanding of the biological activities and regulation of the p53/p73 isoforms, highlighting their role in colorectal tumorigenesis. The analysis of the expression patterns of the p53/p73 isoforms in human cancers provides an important step in the improvement of cancer therapy. Furthermore, the interactions among the p53 family members which could modulate normal functions of the canonical p53 in tumor tissue are described. Lastly, we emphasize the importance of clinical studies to assess the significance of combining the deregulation of different members of the p53 family to define the outcome of the disease.

The Impact of Notch Signaling for Carcinogenesis and Progression of Nonmelanoma Skin Cancer: Lessons Learned from Cancer Stem Cells, Tumor Angiogenesis, and Beyond

Since many decades, nonmelanoma skin cancer (NMSCs) is the most common malignancy worldwide. Basal cell carcinomas (BCC) and squamous cell carcinomas (SCC) are the major types of NMSCs, representing approximately 70% and 25% of these neoplasias, respectively. Because of their continuously rising incidence rates, NMSCs represent a constantly increasing global challenge for healthcare, although they are in most cases nonlethal and curable (e.g., by surgery). While at present, carcinogenesis of NMSC is still not fully understood, the relevance of genetic and molecular alterations in several pathways, including evolutionary highly conserved Notch signaling, has now been shown convincingly. The Notch pathway, which was first developed during evolution in metazoans and that was first discovered in fruit flies (Drosophila melanogaster), governs cell fate decisions and many other fundamental processes that are of high relevance not only for embryonic development, but also for initiation, promotion, and progression of cancer. Choosing NMSC as a model, we give in this review a brief overview on the interaction of Notch signaling with important oncogenic and tumor suppressor pathways and on its role for several hallmarks of carcinogenesis and cancer progression, including the regulation of cancer stem cells, tumor angiogenesis, and senescence.

The interplay between Epstein-Bar virus (EBV) with the p53 and its homologs during EBV associated malignancies

p53, p63, and p73, the members of the p53 family of proteins, are structurally similar proteins that play central roles regulating cell cycle and apoptotic cell death. Alternative splicing at the carboxyl terminus and the utilization of different promoters further categorizes these proteins as having different isoforms for each. Among such isoforms, TA and ΔN versions of each protein serve as the pro and the anti-apoptotic proteins, respectively. Changes in the expression patterns of these isoforms are noted in many human cancers. Proteins of certain human herpesviruses, like Kaposi's sarcoma-associated herpesvirus (KSHV) and Epstein-Barr virus (EBV), interact with p53 family members and alter their expressions in many malignancies. Upon infections in the B cells and epithelial cells, EBV expresses different lytic or latent proteins during viral replication and latency respectively to preserve viral copy number, chromosomal integrity and viral persistence inside the host. In this review, we have surveyed and summarised the interactions of EBV gene products, known so far, with the p53 family proteins. The interactions between P53 and EBV oncoproteins are observed in stomach cancer, non-Hodgkin's lymphoma (NHL) of the head and neck, Nasopharyngeal Cancer (NPC), Gastric carcinoma (GC) and Burkitt's lymphoma (BL). EBV latent protein EBNA1, EBNA3C, LMP-1, and lytic proteins BZLF-1 can alter p53 expressions in many cancer cell lines. Interactions of p63 with EBNA-1, 2, 5, LMP-2A and BARF-1 have also been investigated in several cancers. Similarly, associations of p73 isoform with EBV latent proteins EBNA3C and LMP-1 have been reported. Methylation and single nucleotide polymorphisms in p53 have also been found to be correlated with EBV infection. Therefore, interactions and altered expression strategies of the isoforms of p53 family proteins in EBV associated cancers propose an important field for further molecular research.

APTM, a Thiophene Heterocyclic Compound, Inhibits Human Colon Cancer HCT116 Cell Proliferation Through p53-Dependent Induction of Apoptosis

To evaluate the in vitro anticancer activity and to investigate the mechanism of action of a thiophene heterocyclic compound, [3-Amino-5-[(2,6-dimethylphenyl)amino]-4-(phenylsulfonyl)-2-thienyl](4-fluorophenyl)methanone (APTM) against human colon cancer HCT116 cells. Sulforhodamine B assay and colony formation assay for cell proliferation assay; propidium iodide (PI) staining for cell cycle profile analysis; Hoechst staining; annexin V-FITC/PI double staining and Western blotting for apoptosis assay. APTM inhibits the growth of HCT116 cells dose and time dependently. The growth inhibitory effect of APTM on HCT116 cells was associated with induction of apoptosis but not cell cycle arrest. Also, the isogenic cell depletion of p53 was resistant to APTM-induced apoptosis and thus grows relatively better than the wild-type cells. The anticancer effect of APTM resulted from p53-dependent induction of apoptosis. Also, APTM is a promising lead compound for the treatment of human colon cancer.

p73 gene in dopaminergic neurons is highly susceptible to manganese neurotoxicity

Chronic exposure to elevated levels of manganese (Mn) has been linked to a Parkinsonian-like movement disorder, resulting from dysfunction of the extrapyramidal motor system within the basal ganglia. However, the exact cellular and molecular mechanisms of Mn-induced neurotoxicity remain elusive. In this study, we treated C57BL/6J mice with 30mg/kg Mn via oral gavage for 30 days. Interestingly, in nigral tissues of Mn-exposed mice, we found a significant downregulation of the truncated isoform of p73 protein at the N-terminus (ΔNp73). To further determine the functional role of Mn-induced p73 downregulation in Mn neurotoxicity, we examined the interrelationship between the effect of Mn on p73 gene expression and apoptotic cell death in an N27 dopaminergic neuronal model. Consistent with our animal study, 300μM Mn treatment significantly suppressed p73 mRNA expression in N27 dopaminergic cells. We further determined that protein levels of the ΔNp73 isoform was also reduced in Mn-treated N27 cells and primary striatal cultures. Furthermore, overexpression of ΔNp73 conferred modest cellular protection against Mn-induced neurotoxicity. Taken together, our results demonstrate that Mn exposure downregulates p73 gene expression resulting in enhanced susceptibility to apoptotic cell death. Thus, further characterization of the cellular mechanism underlying p73 gene downregulation will improve our understanding of the molecular underpinnings of Mn neurotoxicity.

The transcription factor p53: not a repressor, solely an activator

The predominant function of the tumor suppressor p53 is transcriptional regulation. It is generally accepted that p53-dependent transcriptional activation occurs by binding to a specific recognition site in promoters of target genes. Additionally, several models for p53-dependent transcriptional repression have been postulated. Here, we evaluate these models based on a computational meta-analysis of genome-wide data. Surprisingly, several major models of p53-dependent gene regulation are implausible. Meta-analysis of large-scale data is unable to confirm reports on directly repressed p53 target genes and falsifies models of direct repression. This notion is supported by experimental re-analysis of representative genes reported as directly repressed by p53. Therefore, p53 is not a direct repressor of transcription, but solely activates its target genes. Moreover, models based on interference of p53 with activating transcription factors as well as models based on the function of ncRNAs are also not supported by the meta-analysis. As an alternative to models of direct repression, the meta-analysis leads to the conclusion that p53 represses transcription indirectly by activation of the p53-p21-DREAM/RB pathway.

Interplay between TAp73 Protein and Selected Activator Protein-1 (AP-1) Family Members Promotes AP-1 Target Gene Activation and Cellular Growth

Unlike p53, which is mutated at a high rate in human cancers, its homologue p73 is not mutated but is often overexpressed, suggesting a possible context-dependent role in growth promotion. Previously, we have shown that co-expression of TAp73 with the proto-oncogene c-Jun can augment cellular growth and potentiate transactivation of activator protein (AP)-1 target genes such as cyclin D1. Here, we provide further mechanistic insights into the cooperative activity between these two transcription factors. Our data show that TAp73-mediated AP-1 target gene transactivation relies on c-Jun dimerization and requires the canonical AP-1 sites on target gene promoters. Interestingly, only selected members of the Fos family of proteins such as c-Fos and Fra1 were found to cooperate with TAp73 in a c-Jun-dependent manner to transactivate AP-1 target promoters. Inducible expression of TAp73 led to the recruitment of these Fos family members to the AP-1 target promoters on which TAp73 was found to be bound near the AP-1 site. Consistent with the binding of TAp73 and AP-1 members on the target promoters in a c-Jun-dependent manner, TAp73 was observed to physically interact with c-Jun specifically at the chromatin via its carboxyl-terminal region. Furthermore, co-expression of c-Fos or Fra1 was able to cooperate with TAp73 in potentiating cellular growth, similarly to c-Jun. These data together suggest that TAp73 plays a vital role in activation of AP-1 target genes via direct binding to c-Jun at the target promoters, leading to enhanced loading of other AP-1 family members, thereby leading to cellular growth.

Preclinical activity of combined HDAC and KDM1A inhibition in glioblastoma

BACKGROUND

Glioblastoma (GBM) is the most common and aggressive form of brain cancer. Our previous studies demonstrated that combined inhibition of HDAC and KDM1A increases apoptotic cell death in vitro. However, whether this combination also increases death of the glioma stem cell (GSC) population or has an effect in vivo is yet to be determined. Therefore, we evaluated the translational potential of combined HDAC and KDM1A inhibition on patient-derived GSCs and xenograft GBM mouse models. We also investigated the changes in transcriptional programing induced by the combination in an effort to understand the induced molecular mechanisms of GBM cell death.

METHODS

Patient-derived GSCs were treated with the combination of vorinostat, a pan-HDAC inhibitor, and tranylcypromine, a KDM1A inhibitor, and viability was measured. To characterize transcriptional profiles associated with cell death, we used RNA-Seq and validated gene changes by RT-qPCR and protein expression via Western blot. Apoptosis was measured using DNA fragmentation assays. Orthotopic xenograft studies were conducted to evaluate the effects of the combination on tumorigenesis and to validate gene changes in vivo.

RESULTS

The combination of vorinostat and tranylcypromine reduced GSC viability and displayed efficacy in the U87 xenograft model. Additionally, the combination led to changes in apoptosis-related genes, particularly TP53 and TP73 in vitro and in vivo.

CONCLUSIONS

These data support targeting HDACs and KDM1A in combination as a strategy for GBM and identifies TP53 and TP73 as being altered in response to treatment.

Expression of p53 target genes in the early phase of long-term potentiation in the rat hippocampal CA1 area

Gene expression plays an important role in the mechanisms of long-term potentiation (LTP), which is a widely accepted experimental model of synaptic plasticity. We have studied the expression of at least 50 genes that are transcriptionally regulated by p53, as well as other genes that are related to p53-dependent processes, in the early phase of LTP. Within 30 min after Schaffer collaterals (SC) tetanization, increases in the mRNA and protein levels of Bax, which are upregulated by p53, and a decrease in the mRNA and protein levels of Bcl2, which are downregulated by p53, were observed. The inhibition of Mdm2 by nutlin-3 increased the basal p53 protein level and rescued its tetanization-induced depletion, which suggested the involvement of Mdm2 in the control over p53 during LTP. Furthermore, nutlin-3 caused an increase in the basal expression of Bax and a decrease in the basal expression of Bcl2, whereas tetanization-induced changes in their expression were occluded. These results support the hypothesis that p53 may be involved in transcriptional regulation during the early phase of LTP. We hope that the presented data may aid in the understanding of the contribution of p53 and related genes in the processes that are associated with synaptic plasticity.

∆N-P63α and TA-P63α exhibit intrinsic differences in transactivation specificities that depend on distinct features of DNA target sites

TP63 is a member of the TP53 gene family that encodes for up to ten different TA and ΔN isoforms through alternative promoter usage and alternative splicing. Besides being a master regulator of gene expression for squamous epithelial proliferation, differentiation and maintenance, P63, through differential expression of its isoforms, plays important roles in tumorigenesis. All P63 isoforms share an immunoglobulin-like folded DNA binding domain responsible for binding to sequence-specific response elements (REs), whose overall consensus sequence is similar to that of the canonical p53 RE. Using a defined assay in yeast, where P63 isoforms and RE sequences are the only variables, and gene expression assays in human cell lines, we demonstrated that human TA- and ΔN-P63α proteins exhibited differences in transactivation specificity not observed with the corresponding P73 or P53 protein isoforms. These differences 1) were dependent on specific features of the RE sequence, 2) could be related to intrinsic differences in their oligomeric state and cooperative DNA binding, and 3) appeared to be conserved in evolution. Since genotoxic stress can change relative ratio of TA- and ΔN-P63α protein levels, the different transactivation specificity of each P63 isoform could potentially influence cellular responses to specific stresses.

Runt-related transcription factor 2 attenuates the transcriptional activity as well as DNA damage-mediated induction of pro-apoptotic TAp73 to regulate chemosensitivity

Although runt-related transcription factor 2 (RUNX2) is known to be an essential key transcription factor for osteoblast differentiation and bone formation, RUNX2 also plays a pivotal role in the regulation of p53-dependent DNA damage response. In the present study, we report that, in addition to p53, RUNX2 downregulates pro-apoptotic TAp73 during DNA damage-dependent cell death. Upon adriamycin (ADR) exposure, human osteosarcoma-derived U2OS cells underwent cell death in association with an upregulation of TAp73 and various p53/TAp73-target gene products together with RUNX2. Small interfering RNA-mediated silencing of p73 resulted in a marked reduction in ADR-induced p53/TAp73-target gene expression, suggesting that TAp73 is responsible for the ADR-dependent DNA damage response. Immunoprecipitation and transient transfection experiments demonstrated that RUNX2 forms a complex with TAp73 and impairs its transcriptional activity. Notably, knockdown of RUNX2 stimulated ADR-induced cell death accompanied by a massive induction of TAp73 expression, indicating that RUNX2 downregulates TAp73 expression. Consistent with this notion, the overexpression of RUNX2 suppressed ADR-dependent cell death, which was associated with a remarkable downregulation of TAp73 and p53/TAp73-target gene expression. Collectively, our present findings strongly suggest that RUNX2 attenuates the transcriptional activity and ADR-mediated induction of TAp73, and may provide novel insights into understanding the molecular basis behind the development and/or maintenance of chemoresistance. Thus, we propose that the silencing of RUNX2 might be an attractive strategy for improving the chemosensitivity of malignant cancers.

Crosstalk among p53 family members in cutaneous carcinoma

Cutaneous squamous cell carcinoma (cSCC) is the second most common human cancer with a frequency increasing worldwide. The risk of developing cSCC has been strongly associated with chronic sun exposure, especially in light skin people. The aim of this viewpoint is to discuss the contribution of the tumor suppressor p53 and its homologues p63 and p73 in the formation and progression of cSCC. Mutations in the p53 gene are early and frequent events in skin carcinogenesis mainly as a consequence of UV light exposure, often followed by loss of function of the second allele. Although rarely mutated in cancer, p63 and p73 play key roles in human cancers, with their truncated isoforms lacking the N-terminal transactivating domain (∆N) being often upregulated as compared to normal tissues. ∆Np63 is abundantly expressed in cSCC, and it is likely to favour tumor initiation and progression. The function of p73 in cSCC is more enigmatic and awaits further studies. Interestingly, an intimate interplay exists between both p53 and p63, and the Notch signalling pathway, often inactivated in cSCC. Here, we summarize our current knowledge about the biological activities of p53 family members in cSCC and propose that integration of their signalling with Notch is key to cSCC formation and progression.

Identification of two novel functional p53 responsive elements in the herpes simplex virus-1 genome

Analysis of the herpes simplex virus-1 (HSV-1) genome reveals two candidate p53 responsive elements (p53RE), located in proximity to the replication origins oriL and oriS, referred to as p53RE-L and p53RE-S, respectively. The sequences of p53RE-L and p53RE-S conform to the p53 consensus site and are present in HSV-1 strains KOS, 17, and F. p53 binds to both elements in vitro and in virus-infected cells. Both p53RE-L and p53RE-S are capable of conferring p53-dependent transcriptional activation onto a heterologous reporter gene. Importantly, expression of the essential immediate early viral transactivator ICP4 and the essential DNA replication protein ICP8, that are adjacent to p53RE-S and p53RE-L, are repressed in a p53-dependent manner. Taken together, this study identifies two novel functional p53RE in the HSV-1 genome and suggests a complex mechanism of viral gene regulation by p53 which may determine progression of the lytic viral replication cycle or the establishment of latency.

Gelam honey attenuated radiation-induced cell death in human diploid fibroblasts by promoting cell cycle progression and inhibiting apoptosis

Background

The interaction between ionizing radiation and substances in cells will induce the production of free radicals. These free radicals inflict damage to important biomolecules such as chromosomes, proteins and lipids which consequently trigger the expression of genes which are involved in protecting the cells or repair the oxidative damages. Honey has been known for its antioxidant properties and was used in medical and cosmetic products. Currently, research on honey is ongoing and diversifying. The aim of this study was to elucidate the role of Gelam honey as a radioprotector in human diploid fibroblast (HDFs) which were exposed to gamma-rays by determining the expression of genes and proteins involved in cell cycle regulation and cell death.

Methods

Six groups of HDFs were studied viz. untreated control, irradiated HDFs, Gelam honey-treated HDFs and HDF treated with Gelam honey pre-, during- and post-irradiation. HDFs were treated with 6 mg/ml of sterilized Gelam honey (w/v) for 24 h and exposed to 1 Gray (Gy) of gamma-rays at the dose rate of 0.25 Gy/min.

Results

Our findings showed that, gamma-irradiation at 1 Gy up-regulated ATM, p53, p16^ink4a and cyclin D1 genes and subsequently initiated cell cycle arrest at G₀/G₁ phase and induced apoptosis (p < 0.05). Pre-treatment with Gelam honey however caused down regulation of these genes in irradiated HDFs while no significant changes was observed on the expression of GADD45 and PAK genes. The expression of ATM and p16 proteins was increased in irradiated HDFs but the p53 gene was translated into p73 protein which was also increased in irradiated HDFs. Gelam honey treatment however significantly decreased the expression of ATM, p73, and p16 proteins (p < 0.05) while the expression of cyclin D1 remained unchanged. Analysis on cell cycle profile showed that cells progressed to S phase with less percentage of cells in G₀/G₁ phase with Gelam honey treatment while apoptosis was inhibited.

Conclusion

Gelam honey acts a radioprotector against gamma-irradiation by attenuating radiation-induced cell death.

DEC1 coordinates with HDAC8 to differentially regulate TAp73 and ΔNp73 expression

P73, a member of the p53 family, plays a critical role in neural development and tumorigenesis. Due to the usage of two different promoters, p73 is expressed as two major isoforms, TAp73 and ΔNp73, often with opposing functions. Here, we reported that transcriptional factor DEC1, a target of the p53 family, exerts a distinct control of TAp73 and ΔNp73 expression. In particular, we showed that DEC1 was able to increase TAp73 expression via transcriptional activation of the TAp73 promoter. By contrast, Np73 transcription was inhibited by DEC1 via transcriptional repression of the ΔNp73 promoter. To further explore the underlying mechanism, we showed that DEC1 was unable to increase TAp73 expression in the absence of HDAC8, suggesting that HDAC8 is required for DEC1 to enhance TAp73 expression. Furthermore, we found that DEC1 was able to interact with HDAC8 and recruit HDAC8 to the TAp73, but not the ΔNp73, promoter. Together, our data provide evidence that DEC1 and HDAC8 in differentially regulate TAp73 and ΔNp73 expression, suggesting that this regulation may lay a foundation for a therapeutic strategy to enhance the chemosensitivity of tumor cells.

Epstein-Barr virus nuclear antigen 3C interact with p73: Interplay between a viral oncoprotein and cellular tumor suppressor

The p73 protein has structural and functional homology with the tumor suppressor p53, which plays an important role in cell cycle regulation, apoptosis, and DNA repair. The p73 locus encodes both a tumor suppressor (TAp73) and a putative oncogene (ΔNp73). p73 May play a significant role in p53-deficient lymphomas infected with Epstein-Barr virus (EBV). EBV produces an asymptomatic infection in the majority of the global population, but it is associated with several human B-cell malignancies. The EBV-encoded Epstein-Barr virus nuclear antigen 3C (EBNA3C) is thought to disrupt the cell cycle checkpoint by interacting directly with p53 family proteins. Doxorubicin, a commonly used chemotherapeutic agent, induces apoptosis through p53 and p73 signaling such that the lowΔNp73 level promotes the p73-mediated intrinsic pathway of apoptosis. In this report, we investigated the mechanism by which EBV infection counters p73α-induced apoptosis through EBNA3C.

PUMA Cooperates with p21 to Regulate Mammary Epithelial Morphogenesis and Epithelial-To-Mesenchymal Transition

Lumen formation is essential for mammary morphogenesis and requires proliferative suppression and apoptotic clearance of the inner cells within developing acini. Previously, we showed that knockdown of p53 or p73 leads to aberrant mammary acinus formation accompanied with decreased expression of p53 family targets PUMA and p21, suggesting that PUMA, an inducer of apoptosis, and p21, an inducer of cell cycle arrest, directly regulate mammary morphogenesis. To address this, we generated multiple MCF10A cell lines in which PUMA, p21, or both were stably knocked down. We found that morphogenesis of MCF10A cells was altered modestly by knockdown of either PUMA or p21 alone but markedly by knockdown of both PUMA and p21. Moreover, we found that knockdown of PUMA and p21 leads to loss of E-cadherin expression along with increased expression of epithelial-to-mesenchymal transition (EMT) markers. Interestingly, we found that knockdown of ΔNp73, which antagonizes the ability of wide-type p53 and TA isoform of p73 to regulate PUMA and p21, mitigates the abnormal morphogenesis and EMT induced by knockdown of PUMA or p21. Together, our data suggest that PUMA cooperates with p21 to regulate normal acinus formation and EMT.

p53 inactivation upregulates p73 expression through E2F-1 mediated transcription

While p73 overexpression has been associated with increased apoptosis in cancer tissues, p73 overexpressing tumors appear to be of high grade malignancy. Why this putative tumor suppressor is overexpressed in cancer cells and what the function of overexpressed p73 is in breast cancers are critical questions to be addressed. By investigating the effect of p53 inactivation on p73 expression, we found that both protein and mRNA levels of TAp73 were increased in MCF-7/p53siRNA cells, MCF-7/p53mt135 cells and HCT-116/p53−/− cells, as compared to wild type control, suggesting that p53 inactivation by various forms upregulates p73. We showed that p53 knockdown induced p73 was mainly regulated at the transcriptional level. However, although p53 has a putative binding site in the TAp73 promoter, deletion of this binding site did not affect p53 knockdown mediated activation of TAp73 promoter. Chromatin immuno-precipitation (ChIP) data demonstrated that loss of p53 results in enhanced occupancy of E2F-1 in the TAp73 promoter. The responsive sequence of p53 inactivation mediated p73 upregulation was mapped to the proximal promoter region of the TAp73 gene. To test the role of E2F-1 in p53 inactivation mediated regulation of p73 transcription, we found that p53 knockdown enhanced E2F-1 dependent p73 transcription, and mutations in E2F-1 binding sites in the TAp73 promoter abrogated p53 knockdown mediated activation of TAp73 promoter. Moreover, we demonstrated that p21 is a mediator of p53-E2F crosstalk in the regulation of p73 transcription. We concluded that p53 knockdown/inactivation may upregulate TAp73 expression through E2F-1 mediated transcriptional regulation. p53 inactivation mediated upregulation of p73 suggests an intrinsic rescuing mechanism in response to p53 mutation/inactivation. These findings support further analysis of the correlation between p53 status and p73 expression and its prognostic/predictive significance in human cancers.

p73 expression is regulated by RNPC1, a target of the p53 family, via mRNA stability

p73, a p53 family tumor suppressor, is expressed as TA and ΔN isoforms. Due to the role of p73 in tumor suppression and neural development, its expression and activity are tightly regulated by multiple mechanisms, including transcription and posttranslational modifications. Here, we found that p73 mRNA stability is regulated by RNPC1, an RNA binding protein and a target of the p53 family. We also showed that a CU-rich element in the 3' untranslated region of p73 is recognized by and responsive to RNPC1. To explore the physiological significance of RNPC1-regulated p73 expression, we showed that the loss of RNPC1 in p53-null mouse embryonic fibroblasts leads to reduced expression of p73, along with decreased expression of p21, p130, and γ-H2A.X, and consequently a decreased number of senescent cells. Furthermore, we observed that knockdown of TAp73 or p21, another target of RNPC1, attenuates the inhibitory effect of RNPC1 on cell proliferation and premature senescence, whereas combined knockdown of TAp73 and p21 completely abolishes it. Due to the fact that RNPC1 is a target of p73, the mutual regulation between p73 and RNPC1 constitutes a novel feed-forward loop, which might be explored as a target for tumors without a functional p53.

Camptothecin induces p53-dependent and -independent apoptogenic signaling in melanoma cells

Various DNA-targeting agents may initiate p53-dependent as well as p53-independent response and subsequent apoptosis via alternative cellular systems which include for instance p73, caspase-2 or Bcl-2 family proteins. The scope of involvement of individual molecules in this process and the mechanisms governing their potential interplay are still not entirely understood, in particular in highly aggressive cancers such as in malignant melanoma. In this work we investigated the role and involvement of both p53-dependent and -independent mechanisms in selected melanoma cell lines with differing status of p53 using a model DNA topoisomerase I inhibitor camptothecin (CPT). Here we report that CPT induced in Bowes melanoma cells apoptosis which is essentially p53 and mitochondria-dependent but with some involvement of caspase-2 and p73. Conversely, in mutant p53 melanoma cells overall levels of CPT-induced apoptosis are significantly lower, with p73 and caspase-2 signaling playing important roles. In addition, in these cells the expression of micro RNAs family 34 (miR-34) were low compared to wild-type p53 cells. The ectopic expression of wild type p53 than restored apoptotic response of cells to CPT despite the fact that the expression of miR-34 and miR-155 were not influenced. These results suggest that CPT induces multivariate cellular stress responses including activation of DNA-damage response-p53 pathway as well as p53-independent signaling and their mutual crosstalk play the decisive role in the efficient triggering of apoptosis in melanoma cells.

Correlation between low-level expression of the tumor suppressor gene TAp73 and the chemoresistance of human glioma stem cells

OBJECTIVE

Glioma stem cells (GSCs) are regarded as the root of glioma growth and recurrence. Chemoresistance is one of the characteristics of GSCs that increases the difficulties in eradicating the cells by anticancer drugs.

PURPOSE

The objective of this study is to investigate the correlation between expression of the tumor suppressor gene TAp73 and the chemoresistance of human GSCs.

METHODS

MTT and tumor sphere formation assays were used to analyze the chemoresistance phenotype of GSCs derived from primary human glioma specimens under cisplatin exposure. Reverse transcription real-time PCR was applied for assaying mRNA levels of TAp73. Protein levels of TAp73, p21, Bax, and cleared caspase 3 were assayed by western blot. Cell apoptosis was analyzed by flow cytometry after the annexin V fluorescence staining.

RESULTS

GSCs exhibited increased chemoresistance compared to differentiated glioma cells (DGCs) derived from the same tumor specimen. The expression of TAp73 was lower in GSCs and was not sensitive to cisplatin treatment as compared to DGCs. Overexpression of TAp73 by transfection increased the apoptosis of GSCs in the presence of cisplatin and reduced the chemoresistance of GSC. TAp73 knockdown by siRNA in DGCs reduced cisplatin-induced apoptosis and increased the resistance to cisplatin.

CONCLUSION

These findings indicate that TAp73 silencing is hallmark of GSC to maintain their chemoresistance phenotype. Thus, targeting TAp73 may provide a novel strategy to eradicating GSCs.

p53 Family: Role of Protein Isoforms in Human Cancer

TP53, TP63, and TP73 genes comprise the p53 family. Each gene produces protein isoforms through multiple mechanisms including extensive alternative mRNA splicing. Accumulating evidence shows that these isoforms play a critical role in the regulation of many biological processes in normal cells. Their abnormal expression contributes to tumorigenesis and has a profound effect on tumor response to curative therapy. This paper is an overview of isoform diversity in the p53 family and its role in cancer.

The p73 tumor suppressor is targeted by Pirh2 RING finger E3 ubiquitin ligase for the proteasome-dependent degradation

The p73 gene, a homologue of the p53 tumor suppressor, is expressed as TA and ΔN isoforms. TAp73 has similar activity as p53 and functions as a tumor suppressor whereas ΔNp73 has both pro- and anti-survival functions. While p73 is rarely mutated in spontaneous tumors, the expression status of p73 is linked to the sensitivity of tumor cells to chemotherapy and prognosis for many types of human cancer. Thus, uncovering its regulators in tumors is of great interest. Here, we found that Pirh2, a RING finger E3 ubiquitin ligase, promotes the proteasome-dependent degradation of p73. Specifically, we showed that knockdown of Pirh2 up-regulates, whereas ectopic expression of Pirh2 down-regulates, expression of endogenous and exogenous p73. In addition, Pirh2 physically associates with and promotes TAp73 polyubiquitination both in vivo and in vitro. Moreover, we found that p73 can be degraded by both 20 S and 26 S proteasomes. Finally, we showed that Pirh2 knockdown leads to growth suppression in a TAp73-dependent manner. Taken together, our findings indicate that Pirh2 promotes the proteasomal turnover of TAp73, and thus targeting Pirh2 to restore TAp73-mediated growth suppression in p53-deficient tumors may be developed as a novel anti-cancer strategy.

Inhibition of breast cancer cell growth and invasiveness by dual silencing of HER-2 and VEGF

Overexpression of HER-2 accounts for approximately 25% of all breast cancer cases, while 87.7% of HER-2 positive breast cancers are associated with upregulated VEGF. The objective of this study is to explore the combination therapy of blocking HER-2 and VEGF expressions simultaneously using siRNA. This is the first report to examine the effect of dual silencing of HER-2 and VEGF genes on tumor growth and invasiveness. We have designed nine HER-2 siRNAs and ten VEGF siRNAs, and identified potent siRNA which can silence the target gene up to 75-83.5%. The most potent HER-2 and VEGF siRNAs were used to conduct functional studies in HER-2 positive breast cancer cells. Tumor invasiveness properties including cell morphology change, in vitro migration, cell spreading, and adhesion to ECM were evaluated. In addition, cell proliferation and apoptosis were examined after the siRNA treatment. Our data demonstrated for the first time that HER-2 siRNA could inhibit cell migration and invasion abilities. Combination of HER-2 and VEGF siRNAs exhibited synergistic silencing effect on VEGF. Both HER-2 siRNA and VEGF siRNA showed significant inhibition on cell migration and proliferation. HER-2 siRNA also demonstrated dramatic suppression on cell spreading and adhesion to ECM, as well as induction of apoptosis. Dual silencing of HER-2 and VEGF exhibited significant cell morphology change, and substantial suppression on migration, spreading, cell adhesion, and proliferation. Our observations suggested that HER-2 positive breast cancer may be more effectively treated by dual inhibition of HER-2 and VEGF gene expressions using siRNA.

The prognostic value of p73 overexpression in colorectal carcinoma: a clinicopathologic, immunohistochemical, and statistical study of 204 patients

INTRODUCTION

The protein p73 is the first identified homolog of the tumor suppressor gene p53, but its function in tumor development has not been established. Indeed, the results regarding the p73 implication in colorectal cancers is still controversial.

AIM

We investigated whether the p73 is implicated in colorectal cancer, whether the p73 expression is related to prognosis and whether the p73 expression is correlated with p21-ras or p53.

MATERIALS AND METHODS

We performed a comparative immunohistochemical analysis of p73, p53, and p21ras proteins in primary colorectal tumor with matched normal mucosa and metastasis from 204 patients with colorectal cancer. We correlated these expressions with clinicopathologic variables and we compared the different profiles between nonmucinous carcinoma and mucinous carcinoma.

RESULTS

In this study, we did not find any correlation between p73 expression, sex, age, site, differentiation and stage. Overexpression of p73 was significantly correlated with infiltrating growth pattern (P<0.0001) and nonmucinous carcinoma (P<0.0001). Furthermore, frequency and intensity of p73 expression were marquedly increased from normal mucosa (26%), to primary tumors (75%) and to metastasis (97%). Furthermore, expression of p73 was also correlated with shorter survival period. The prognostic significance of p73 expression remained, even after adjustment for the clinical and pathologic variables. The p73 expression was positively correlated only with p21ras expression (P<0.0001).

CONCLUSIONS

All these findings prove that p73 expression should be considered as a valuable poor prognostic marker. Our data also suggest that TP73 gene may play a role in colorectal carcinoma development.

Securinine induces p73-dependent apoptosis preferentially in p53-deficient colon cancer cells

The identification of agents that preferentially kill cancer cells while protecting normal cells offers the potential to overcome toxicities found in many existing chemotherapeutic agents. Because p53 is frequently inactivated in cancer, agents that preferentially kill p53-null cells and protect wild-type p53-expressing cells are highly desirable chemotherapeutic agents. By using pairs of isogenic colon cancer cell lines that differ only in p53 expression (RKO and HCT116), securinine was found to exhibit these properties. Securinine (30 microM) induces apoptosis in 73% of p53-null HCT116 cells (LD(50) 17.5 microM) as opposed to 17.6% of HCT116 parental cells (LD(50) 50 microM) at 72 h after treatment. The mechanism of securinine-mediated death in p53-deficient cells involves the induction of the p53 family member, p73. Interestingly, the proapoptotic protein p73 is down-regulated in colon cancer cells expressing p53. This differential regulation of p73 in a p53-dependent fashion reveals a novel pathway for preferentially targeting cancer cells. In contrast to p53-deficient cells, cells expressing p53 are protected from cell death through the p53-mediated up-regulation of p21. These studies reveal a novel approach to specifically target colon cancer cells lacking p53 as well as identify a novel clinically relevant pathway to selectively induce p73 in p53-null cells.

p73 is essential for vitamin D-mediated osteoblastic differentiation

The secosteroid hormone vitamin D3 (VD3) exerts its biological actions through its cognate receptor, the vitamin D receptor (VDR). Vitamin D3 and VDR have a key function in bone formation and keratinocyte differentiation, exert antiproliferative actions in human cancer, and is widely used as a chemotherapeutic agent for cancer. In addition, VD3 promotes differentiation of human osteosarcoma cells by up-regulating genes involved in cell cycle arrest and osteoblastic differentiation. Although considerable work has been carried out in understanding the molecular mechanisms underlying the VD3-mediated differentiation of human osteosarcoma cells, the upstream regulation of VD3 signaling pathway is still unclear. In this study, we show that p73 acts as an upstream regulator of VD3-mediated osteoblastic differentiation. Transcription factor p73, a p53 homolog, has been shown to have a function in development and recently been termed as a tumor suppressor. Silencing p73 results in a significant reduction of VD3-mediated osteoblastic differentiation; although DNA damage induced p73 leads to an increase in VD3-mediated differentiation of osteosarcoma cells. Together, our data implicate a novel function for p73 in vitamin D-mediated differentiation of human osteosarcoma cells.

PML involvement in the p73-mediated E1A-induced suppression of EGFR and induction of apoptosis in head and neck cancers

Epidermal growth factor receptor (EGFR) tyrosine kinase is commonly overexpressed in human cancers; however, the cellular mechanisms regulating EGFR expression remain unclear. p53, p63 and p73 are transcription factors regulating many cellular targets involved in controlling the cell cycle and apoptosis. p53 activates EGFR expression, whereas TAp63 represses EGFR transcription. The involvement of p73 in the regulation of EGFR has not been reported. Here, a strong correlation between EGFR overexpression and increased levels of the oncogenic DeltaNp73 isoform in head and neck squamous cell carcinoma (HNSCC) cell lines was observed. Ectopic expression of TAp73, particularly TAp73beta, resulted in suppression of the EGFR promoter, significant downregulation of EGFR protein and efficient induction of cell death in all six EGFR-overexpressing HNSCC cell lines. EGFR overexpression from a heterologous LTR promoter protected lung cancer cells from TAp73beta-induced EGFR suppression and apoptosis. Expression of TAp73beta efficiently induced promyelocytic leukaemia (PML) protein expression and PML knockdown by shRNA attenuated the downregulation of EGFR and induction of apoptosis by p73 in HNSCC cells. Furthermore, PML was found to be important for E1A-induced suppression of EGFR and subsequent killing of HNSCC cells. Our data therefore suggest a novel pathway involving PML and p73 in the regulation of EGFR expression.

Acetylation status of E2F-1 has an important role in the regulation of E2F-1-mediated transactivation of tumor suppressor p73

Tumor suppressor p73 plays an important role in the regulation of DNA damage response. E2F-1 acts as a transcriptional regulator for p73. In the present study, we have found that acetylation of E2F-1 has a critical role in the E2F-1-mediated transactivation of p73. In response to adriamycin (ADR), p73 was stabilized in HeLa cells and the expression levels of its target genes increased in association with an induction of apoptosis. Of note, E2F-1 and several its target genes were transactivated in response to ADR, whereas p73 mRNA level remained unchanged. Immunoprecipitation analysis revealed that ADR has a marginal effect on acetylation status of E2F-1. Intriguingly, acetylation level of E2F-1 remarkably increased in the presence of trichostatin A (TSA) and thereby inducing the expression level of p73 mRNA. Taken together, our present findings suggest that acetylation status of E2F-1 contributes to the selective activation of its target genes.

Protein kinase C-dependent phosphorylation regulates the cell cycle-inhibitory function of the p73 carboxy terminus transactivation domain

The transcription factor p73, a member of the p53 family of proteins, is involved in the regulation of cell cycle progression and apoptosis. However, the regulatory mechanisms controlling the distinct roles for p73 in these two processes have remained unclear. Here, we report that p73 is able to induce cell cycle arrest independently of its amino-terminal transactivation domain, whereas this domain is crucial for p73 proapoptotic functions. We also characterized a second transactivation domain in the carboxy terminus of p73 within amino acid residues 381 to 399. This carboxy terminus transactivation domain was found to preferentially regulate genes involved in cell cycle progression. Moreover, its activity is regulated throughout the cell cycle and modified by protein kinase C-dependent phosphorylation at serine residue 388. Our results suggest that this novel posttranslational modification within the p73 carboxy terminus transactivation domain is involved in the context-specific guidance of p73 toward the selective induction of cell cycle arrest.

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.

Role of activating transcription factor 3 on TAp73 stability and apoptosis in paclitaxel-treated cervical cancer cells

Taxol (paclitaxel) is a potent anticancer drug that has been found to be effective against several tumor types, including cervical cancer. However, the exact mechanism underlying the antitumor effects of paclitaxel is poorly understood. Here, paclitaxel induced the apoptosis of cervical cancer HeLa cells and correlated with the enhanced activation of caspase-3 and TAp73, which was strongly inhibited by TAp73beta small interfering RNA (siRNA). In wild-type activating transcription factor 3 (ATF3)-overexpressed cells, paclitaxel enhanced apoptosis through increased alpha and beta isoform expression of TAp73; however, these events were attenuated in cells containing inactive COOH-terminal-deleted ATF3 [ATF3(DeltaC)] or ATF3 siRNA. In contrast, paclitaxel-induced ATF3 expression did not change in TAp73beta-overexpressed or TAp73beta siRNA-cotransfected cells. Furthermore, paclitaxel-induced ATF3 translocated into the nucleus where TAp73beta is expressed, but not in ATF3(DeltaC) or TAp73beta siRNA-transfected cells. As confirmed by the GST pull-down assay, ATF3 bound to the DNA-binding domain of p73, resulting in the activation of p21 or Bax transcription, a downstream target of p73. Overexpression of ATF3 prolonged the half-life of TAp73beta by inhibiting its ubiquitination and thereby enhancing its transactivation and proapoptotic activities. Additionally, ATF3 induced by paclitaxel potentiated the stability of TAp73beta, not its transcriptional level. Chromatin immunoprecipitation analyses show that TAp73beta and ATF3 are recruited directly to the p21 and Bax promoter. Collectively, these results reveal that overexpression of ATF3 potentiates paclitaxel-induced apoptosis of HeLa cells, at least in part, by enhancing TAp73beta's stability and its transcriptional activity. The investigation shows that ATF3 may function as a tumor-inhibiting factor through direct regulatory effects on TAp73beta, suggesting a functional link between ATF3 and TAp73beta.

Identification of flotillin-2, a major protein on lipid rafts, as a novel target of p53 family members

p73 and p63 are members of the p53 gene family and have been shown to play an important role in development and homeostasis mainly by regulating the transcription of a variety of genes. A subset of these genes encodes secreted proteins and receptors that may be involved in the communication between adjacent cells. We report here that flotillin-2, a major hydrophobic protein on biomembrane microdomain lipid rafts, is a direct transcriptional target of the p53 family member genes. It has been suggested that such rafts could play an important role in many cellular processes including signal transduction, membrane trafficking, cytoskeletal organization, and pathogen entry. We found that the expression of flotillin-2 was specifically up-regulated by either TAp73beta or TAp63gamma, but not significantly by p53. In addition, flotillin-2 transcription is activated in response to cisplatin in a manner dependent on endogenous p73. By using small interference RNA designed to target p73, we showed that silencing endogenous p73 abolishes the induction of flotillin-2 transcription following cisplatin treatment. Furthermore, we identified a p73/p63-binding site located upstream of the flotillin-2 gene that is responsive to the p53 family members. This response element is highly conserved between humans and rodents. We also found that ectopic expression of TAp73 as well as TAp63 enhances signal transduction by assessing the interleukin-6-mediated phosphorylation of signal transducers and activators of transcription 3. Thus, in addition to direct transactivation, p53 family member genes enhance a set of cellular processes via lipid rafts.

DNp73 a matter of cancer: mechanisms and clinical implications

The p53 family proteins carry on a wide spectrum of biological functions from differentiation, cell cycle arrest, apoptosis, and chemosensitivity of tumors. NH2-terminally truncated p73 (referred to as DNp73) acts as a potent inhibitor of all these tumor suppressor properties, implying that it has oncogenic functions in human tumorigenesis. This was favored by the observation that high DNp73 expression levels in a variety of cancers are associated with adverse clinico-pathological characteristics and the response failure to chemotherapy. The actual challenge is the deciphering of the molecular mechanisms by which DNp73 promotes malignancy and to unravel the regulatory pathways for controlling TP73 isoform expression. This review is focused on recent findings leaving no doubt that N-terminally truncated p73 proteins are operative during oncogenesis, thus underscoring its significance as a marker for disease severity in patients and as target for cancer therapy.

Transcriptional regulation of the p73 gene, a member of the p53 family, by early growth response-1 (Egr-1)

To elucidate the regulatory mechanism of p73 gene expression, we analyzed the human p73 promoter and found three putative Egr-1-binding sites located upstream of exon 1 (-1728, -321, and -38). The Egr-1 responsiveness of these sites was analyzed by transient transfection assays using 5'- and 3'-serial truncations of the p73 promoter, subcloned in a CAT reporter vector. The functional significance of the region was further confirmed by an electrophoretic mobility shift assay using the Egr-1 protein synthesized in vitro and a [32P]-labeled middle site sequence, followed by competition with unlabeled wild-type or mutant oligonucleotides and supershift assays using an anti-Egr-1 antibody. When induced by either the nitric oxide donor NOC-18 or the PPARgamma agonist troglitazone, Egr-1 bound to the p73 promoter, as assessed by chromatin immunoprecipitation assays, accompanied by increased expression of p73. MTT assays revealed that cell growth was significantly inhibited on treating the cells with troglitazone. Overall, our results provide direct evidence that Egr-1 positively regulated p73 expression by binding to its promoter in vivo, consistent with Egr-1 and p73 being involved in p53-independent tumor suppression.

Differential regulation of vitamin D receptor (VDR) by the p53 Family: p73-dependent induction of VDR upon DNA damage

p63 and p73, members of the p53 family, have been shown to be functionally distinct from p53. Vitamin D receptor (VDR) is a ligand (vitamin D(3))-dependent transcription factor, which is shown to play a major role in calcium homeostasis and keratinocyte differentiation. Vitamin D and its analogues in combination with DNA-damaging agents are extensively used for cancer chemotherapy. In this report, we examined whether p53 affects p63-mediated induction of VDR and studied the effect of DNA damage on VDR induction in p53 null cell lines. Our results demonstrate that p53 itself does not induce VDR expression, nor does it affect p63-mediated VDR induction in the cell lines tested in this study. Furthermore, we observed p53-independent activation of VDR upon DNA damage and associated the induction of VDR to p73. We have demonstrated that ectopic expression of various p73 isoforms can induce VDR expression. Inhibition of p73 in cells treated with DNA-damaging agents exhibited decreased VDR expression. Finally, we show that upon DNA damage, induction of VDR sensitizes the cells to vitamin D treatment. In conclusion, our results indicate that VDR is regulated by p63 and p73 and that the induction of VDR expression upon DNA damage is p73-dependent.

TAp73 is a downstream target of p53 in controlling the cellular defense against stress

TAp73 is a p53 tumor suppressor gene homologue that is known to be mainly involved in apoptosis. We report here that TAp73 is necessary for the cellular response to oxidative stress and that TAp73 functions as a downstream target of p53 in this process. We show that p53 physically interacts with the TAp73 promoter under stress conditions that lead to cell death. Particularly, p53 binds to a palindromic site in the TAp73 promoter, activates the promoter of TAp73, and selectively induces TAp73 transcription. TAp73 expression is highly increased under oxidative stress in a p53-dependent manner. Furthermore, knock-down of TAp73 expression inhibits the cellular apoptotic response to oxidative damage. In contrast, the ectopic expression of TAp73 in p53(-/-) mouse embryonic fibroblasts induces oxidative cell death. Our findings demonstrate that p53 is a direct transcriptional regulator of TAp73. Our data reveal a new pathway for cellular protection against oxidative damage and provide evidence that TAp73 is a stress-response gene and a downstream effector in the p53 pathway.

Recombinant adenoviral vectors can induce expression of p73 via the E4-orf6/7 protein

Despite the utility of recombinant adenoviral vectors in basic research, their therapeutic promise remains unfulfilled. Most engineered adenoviral vectors use a heterologous promoter to transcribe a foreign gene. We show that adenoviruses containing the cytomegalovirus immediate-early promoter induce the expression of the proapoptotic cellular protein TAp73 via the cyclin-dependent kinase-retinoblastoma protein-E2F pathway in murine embryonic fibroblasts. Cells transduced with these vectors also expressed high levels of the adenoviral E4-orf6/7 and E2A proteins. By contrast, adenoviruses containing the ubiquitin C promoter failed to elicit these effects. E4-orf6/7 is necessary and sufficient for increased TAp73 expression, as shown by using retrovirus-mediated E4-orf6/7 expression and adenovirus with the E4-orf6/7 gene deleted. Activation of TAp73 likely occurs via E4-orf6/7-induced dimerization of E2F and subsequent binding to the inverted E2F-responsive elements within the TAp73 promoter. In addition, adenoviral vectors containing the cytomegalovirus immediate-early promoter, but not the ubiquitin C promoter, cooperated with chemotherapeutic agents to decrease cellularity in vitro. In contrast to murine embryonic fibroblasts, adenoviruses containing the ubiquitin C promoter, but not the cytomegalovirus immediate-early promoter, induced both E4-orf6/7 and TAp73 in human foreskin fibroblasts, emphasizing the importance of cellular context for promoter-dependent effects. Because TAp73 is important for the efficacy of chemotherapy, adenoviruses that increase TAp73 expression may enhance cancer therapies by promoting apoptosis. However, such adenoviruses may impair the long-term survival of transduced cells during gene replacement therapies. Our findings reveal previously unknown effects of foreign promoters in recombinant adenoviral vectors and suggest means to improve the utility of engineered adenoviruses by better controlling their impact on viral and cellular gene expression.

p53 dependent and independent sensitivity to oxaliplatin of colon cancer cells

Oxaliplatin is an efficient chemotherapeutic agent used for the treatment of metastatic human colon cancer, but cancer cells are frequently resistant. The aim of this study was to analyse the underlying mechanisms in a panel of 10 human colorectal cancer cell lines submitted to a short (2h) oxaliplatin treatment period, accordingly to the usual therapeutic procedure in humans. Sensitivity to oxaliplatin was a characteristic of p53 wild-type colon cancer cells. In contrast, all p53-mutated cell lines had a high IC50 to oxaliplatin, with the exception of the V9P cell line. Exposure to oxaliplatin resulted in G0/G1 arrest in p53 wild-type cell lines, and in S phase in p53-mutated cell lines. In our treatment conditions, no DNA accumulation in sub G0/G1 phase, no caspase-3 activation nor PARP cleavage were detected after oxaliplatin treatment, except for the V9P cell line. The major role of the p53-p21 pathway in oxaliplatin sensitivity was confirmed in the p53 wild-type HCT116 cell line, using siRNA duplex, and knockdown of the TAp73 protein also enhanced resistance to oxaliplatin in this cell line. Surprisingly, siRNA duplex invalidation revealed a residual effect of the mutant p53 protein in p53-mutated cell lines. Persistent sensitivity to oxaliplatin of the p53-mutated V9P cell line was associated with oxalipatin-induced apoptosis but TAp73 was not the responsible alternative pathway.

p73: a chiaroscuro gene in cancer

p73 is a member of the p53 family which is gaining increasing importance in the field of cancer. Its structural homology with p53 led to the assumption that it could act as a new tumour suppressor gene. Increasing knowledge of its function, however, has cast doubts on this role. A particularly interesting characteristic of p73 is that the cell contains different isoforms with distinct and sometimes opposite functions. Evidence in the last few years clearly indicates that p73 does share some activities with p53 but also that it has some distinct functions. This review focuses on p73's role in the development and progression of cancer, analysing the gene structure and regulation and discussing similarities with p53 and differences. Recent results obtained with specific detection methods on the levels and functions of the different isoforms in tumours are also discussed.

A network of p73, p53 and Egr1 is required for efficient apoptosis in tumor cells

p73, a transcription factor rarely mutated in cancer, regulates a subset of p53 target genes that cause cells to respond to genotoxic stress by growth arrest and apoptosis. p73 is produced in two main forms; only TAp73 reiterates the roles of p53, while DeltaNp73 can be oncogenic in character. We show that the TAp73 form produced by TP73 P1 promoter has five distinct Egr1-binding sites, each contributing to the transcriptional upregulation of TAp73 by Egr1 in several cell types. In contrast, TP73 P2 promoter transcribes DeltaNp73, is not induced by Egr1, but is induced by TAp73 and p53. Induction of TAp73 by genotoxic stress requires Egr1 in mouse in vivo. Newly discovered non-consensus p53-binding sites in p73, p53 and Egr1 promoters reveal inter-regulating networks and sustained expression by feedback loops in response to stress, resulting in prolonged expression of the p53 family of genes and efficient apoptosis.

Adenovirus-mediated TA-p73β gene transfer increases chemosensitivity of human malignant melanomas

Malignant melanoma is the most aggressive form of skin cancer and has proven to be highly resistant to conventional chemotherapy. Intriguingly, the p53 tumor suppressor, a main mediator of chemoresistance in other tumor types, is rarely mutated in melanoma. However, we have previously shown that anti-apoptotic isoforms of p73 (deltaTA-p73), another member of the p53 family, are overexpressed in metastatic melanomas. DeltaTA-p73 can oppose the pro-apoptotic functions of p53 and full length p73, and thus it could contribute to melanoma chemoresistance. In this study, we use an efficient adenoviral-based gene transfer approach to introduce a transcriptionally active form of p73 (TA-p73beta) in melanoma cells, with the objective of overcoming drug resistance. Interestingly, TA-p73beta significantly sensitized 5 out of 7 aggressive melanoma cell lines to the standard therapeutic agents adriamycin and cisplatin. More importantly, TA-p73beta displayed a synergistic effect in vivo allowing adriamycin or cisplatin to block melanoma cell growth in mouse xenograft models (p < 0.05). In summary, our data show that Ad-mediated TA-p73beta gene expression can markedly sensitize a subset of melanoma cell lines to adriamycin and cisplatin in vitro and in vivo, suggesting a new chemosensitization strategy for malignant melanomas.