p53/p63/p73 isoforms: an orchestra of isoforms to harmonise cell differentiation and response to stress

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.

Perinatal exposure to diethylstilbestrol alters the functional differentiation of the adult rat uterus

The exposure to endocrine disrupters and female reproductive tract disorders has not been totally clarified. The present study assessed the long-term effect of perinatal (gestation+lactation) exposure to diethylstilbestrol (DES) on the rat uterus and the effect of estrogen replacement therapy. DES (5μg/kg bw/day) was administered in the drinking water from gestational day 9 until weaning and we studied the uterus of young adult (PND90) and adult (PND360) females. To investigate whether perinatal exposure to DES modified the uterine response to a long-lasting estrogen treatment, 12-month-old rats exposed to DES were ovariectomized and treated with 17β-estradiol for 3 months (PND460). In young adult rats (PND90), the DES treatment decreased both the proliferation of glandular epithelial cells and the percentage of glandular perimeter occupied by α-smooth muscle actin-positive cells. The other tissue compartments remained unchanged. Cell apoptosis was not altered in DES-exposed females. In control adult rats (PND360), there were some morphologically abnormal uterine glands. In adult rats exposed to DES, the incidence of glands with cellular anomalies increased. In response to estrogens (PND460), the incidence of cystic glands increased in the DES group. We observed glands with daughter glands and conglomerates of glands only on PND460 and in response to estrogen replacement therapy, independently of DES exposure. The p63 isoforms were expressed without changes on PND460. Estrogen receptors α and β showed no changes, while the progesterone receptor decreased in the subepithelial stroma of DES-exposed animals with estrogen treatment. The long-lasting effects of perinatal exposure to DES included the induction of abnormalities in uterine tissues of aged female rats and an altered response of the adult uterus to estradiol.

p53 as an intervention target for cancer and aging

p53 is well known for suppressing tumors but could also affect other aging processes not associated with tumor suppression. As a transcription factor, p53 responds to a variety of stresses to either induce apoptosis (cell death) or cell cycle arrest (cell preservation) to suppress tumor development. Yet, the effect p53 has on the non-cancer aspects of aging is complicated and not well understood. On one side, p53 could induce cellular senescence or apoptosis to suppress cancer but as an unintended consequence enhance the aging process especially if these responses diminish stem and progenitor cell populations. But on the flip side, p53 could reduce growth and growth-related stress to enable cell survival and ultimately delay the aging process. A better understanding of diverse functions of p53 is essential to elucidate its influences on the aging process and the possibility of targeting p53 or p53 transcriptional targets to treat cancer and ameliorate general aging.

Alternative RNA splicing and cancer

Alternative splicing of pre-messenger RNA (mRNA) is a fundamental mechanism by which a gene can give rise to multiple distinct mRNA transcripts, yielding protein isoforms with different, even opposing, functions. With the recognition that alternative splicing occurs in nearly all human genes, its relationship with cancer-associated pathways has emerged as a rapidly growing field. In this review, we summarize recent findings that have implicated the critical role of alternative splicing in cancer and discuss current understandings of the mechanisms underlying dysregulated alternative splicing in cancer cells.

Epithelio-mesenchymal transitional attributes in oral sub-mucous fibrosis

Evaluating molecular attributes in association with its epithelial and sub-epithelial changes of oral sub-mucous fibrosis is meaningful in exploring the plausibility of an epithelio-mesenchymal transition (EMT) and malignant potentiality of this pathosis. In this study histopathological and histochemical attributes for basement membrane and connective tissue in biopsies of oral sub-mucous fibrosis (n = 55) and normal oral mucosa (n = 16) were assessed and expressions of p63, E-cadherin, β-catenin, N-cadherin and TWIST were analyzed immunohistochemically. The p63 and its isoforms (TA and ∆N), PARD3, E-cadherin and β-catenin were also assessed transcriptomically by q-PCR and EMT players like TWIST1, ZEB1, MMP9 and micro-RNA 205 were searched in gene expression microarrays. Oral epithelium demonstrating impairment in progressive maturation in oral sub-mucous fibrosis concomitantly experienced an increase in basement membrane thickness and collagen deposition along with alteration in target molecular expressions. In comparison to non-dysplastic conditions dysplastic stages exhibited significant increase in p63 and p63∆N expressions whereas, E-cadherin and β-catenin exhibited loss from the membrane with concurrent increase in cytoplasm. Further the N-cadherin and TWIST were gained remarkably along with the appearance of nuclear accumulation features of β-catenin. The microarray search had noticed the up-regulation of TWIST1, ZEB1 and MMP9 along with down regulation of micro-RNA 205. The simultaneous increase in basement membrane thickness and sub-epithelial collagen deposition were the plausible indicators for increased matrix stiffness with expected impact on oral epithelial functional homoeostasis. This was corroborated with the increase in expressions of epithelial master regulator p63 and its oncogenic isoform (∆N) along with membranous loss of E-cadherin (EMT hallmark) and its associate β-catein and gain of mesenchymal markers like N-cadherin and TWIST. These also became indicative for the induction of epithelial to mesenchymal transitional mechanism in oral sub-mucous fibrosis when connoted here with the relevant modulation in expressions of EMT regulators.

[P53 family proteins provide new insights into lung carcinogenesis and clinical treatment]

P53作为转录因子，其转录激活功能维持了基因组的稳定性，对防止肿瘤的形成起着重要作用，是目前研究得最为广泛、深入的抑癌基因，被称为“基因卫士”。P53家族的成员p63、p73与p53在DNA结合结构域上有高度的同源性，某些p53家族亚型可以与p53-反应基因相结合起着转录激活的作用，另外一些则起着负性调节作用。肺癌是世界上患病率最高的恶性肿瘤之一，p53家族成员在肺癌中的异常表达与肺癌的发生有密切联系，并导致不良的预后及对放疗、化疗的抵抗。对p53家族成员在肺癌致病机制的深入研究可有助于为临床提供合理的化疗方案及靶向治疗策略。本文着重回顾总结p53家族成员在肺癌发生、化疗敏感性以及肺癌靶向治疗中的独特的作用。

P53 family members modulate the expression of PRODH, but not PRODH2, via intronic p53 response elements

The tumor suppressor p53 was previously shown to markedly up-regulate the expression of the PRODH gene, encoding the proline dehydrogenase (PRODH) enzyme, which catalyzes the first step in proline degradation. Also PRODH2, which degrades 4-hydroxy-L-proline, a product of protein (e.g. collagen) catabolism, was recently described as a p53 target. Here, we confirmed p53-dependent induction of endogenous PRODH in response to genotoxic damage in cell lines of different histological origin. We established that over-expression of TAp73β or TAp63β is sufficient to induce PRODH expression in p53-null cells and that PRODH expression parallels the modulation of endogenous p73 by genotoxic drugs in several cell lines. The p53, p63, and p73-dependent transcriptional activation was linked to specific intronic response elements (REs), among those predicted by bioinformatics tools and experimentally validated by a yeast-based transactivation assay. p53 occupancy measurements were validated in HCT116 and MCF7 human cell lines. Conversely, PRODH2 was not responsive to p63 nor p73 and, at best, could be considered a weak p53 target. In fact, minimal levels of PRODH2 transcript induction by genotoxic stress was observed exclusively in one of four p53 wild-type cell lines tested. Consistently, all predicted p53 REs in PRODH2 were poor matches to the p53 RE consensus and showed very weak responsiveness, only to p53, in the functional assay. Taken together, our results highlight that PRODH, but not PRODH2, expression is under the control of p53 family members, specifically p53 and p73. This supports a deeper link between proteins of the p53-family and metabolic pathways, as PRODH modulates the balance of proline and glutamate levels and those of their derivative alpha-keto-glutarate (α-KG) under normal and pathological (tumor) conditions.

p53 selectively regulates developmental apoptosis of rod photoreceptors

Retinal cells become post-mitotic early during post-natal development. It is likely that p53, a well-known cell cycle regulator, is involved in regulating the genesis, differentiation and death of retinal cells. Furthermore, retinal cells are under constant oxidative stress that can result in DNA damage, due to the extremely high level of metabolic activity associated with phototransduction. If not repaired, this damage may result in p53-dependent cell death and ensuing vision loss. In this study, the role of p53 during retinal development and in the post-mitotic retina is investigated. A previously described super p53 transgenic mouse that expresses an extra copy of the mouse p53 gene driven by its endogenous promoter is utilized. Another transgenic mouse (HIP) that expresses the p53 gene in rod and cone photoreceptors driven by the human interphotoreceptor retinoid binding protein promoter was generated. The electroretinogram (ERG) of the super p53 mouse exhibited reduced rod-driven scotopic a and b wave and cone-driven photopic b wave responses. This deficit resulted from a reduced number of rod photoreceptors and inner nuclear layer cells. However, the reduced photopic signal arose only from lost inner retinal neurons, as cone numbers did not change. Furthermore, cell loss was non-progressive and resulted from increased apoptosis during retinal developmental as determined by TUNEL staining. In contrast, the continuous and specific expression of p53 in rod and cone photoreceptors in the mature retinas of HIP mice led to the selective loss of both rods and cones. These findings strongly support a role for p53 in regulating developmental apoptosis in the retina and suggest a potential role, either direct or indirect, for p53 in the degenerative photoreceptor loss associated with human blinding disorders.

Eukaryotic translation elongation factor 1-alpha 1 inhibits p53 and p73 dependent apoptosis and chemotherapy sensitivity

The p53 family of transcription factors is a key regulator of cell proliferation and death. In this report we identify the eukaryotic translation elongation factor 1-alpha 1 (eEF1A1) to be a novel p53 and p73 interacting protein. Previous studies have demonstrated that eEF1A1 has translation-independent roles in cancer. We report that overexpression of eEF1A1 specifically inhibits p53-, p73- and chemotherapy-induced apoptosis resulting in chemoresistance. Short-interfering RNA-mediated silencing of eEF1A1 increases chemosensitivity in cell lines bearing wild type p53, but not in p53 null cells. Furthermore, silencing of eEF1A1 partially rescues the chemoresistance observed in response to p53 or p73 knockdown, suggesting that eEF1A1 is a negative regulator of the pro-apoptotic function of p53 and p73. Thus, in the context of p53-family signaling, eEF1A1 has anti-apoptotic properties. These findings identify a novel mechanism of regulation of the p53 family of proteins by eEF1A1 providing additional insight into potential targets to sensitize tumors to chemotherapy.

Paraquat modulates alternative pre-mRNA splicing by modifying the intracellular distribution of SRPK2

Paraquat (PQ) is a neurotoxic herbicide that induces superoxide formation. Although it is known that its toxic properties are linked to ROS production, the cellular response to PQ is still poorly understood. We reported previously that treatment with PQ induced genome-wide changes in pre-mRNA splicing. Here, we investigated the molecular mechanism underlying PQ-induced pre-mRNA splicing alterations. We show that PQ treatment leads to the phosphorylation and nuclear accumulation of SRPK2, a member of the family of serine/arginine (SR) protein-specific kinases. Concomitantly, we observed increased phosphorylation of SR proteins. Site-specific mutagenesis identified a single serine residue that is necessary and sufficient for nuclear localization of SRPK2. Transfection of a phosphomimetic mutant modified splice site selection of the E1A minigene splicing reporter similar to PQ-treatment. Finally, we found that PQ induces DNA damage and vice versa that genotoxic treatments are also able to promote SRPK2 phosphorylation and nuclear localization. Consistent with these observations, treatment with PQ, cisplatin or γ-radiation promote changes in the splicing pattern of genes involved in DNA repair, cell cycle control, and apoptosis. Altogether, our findings reveal a novel regulatory mechanism that connects PQ to the DNA damage response and to the modulation of alternative splicing via SRPK2 phosphorylation.

p53 and translation attenuation regulate distinct cell cycle checkpoints during endoplasmic reticulum (ER) stress

Cell cycle checkpoints ensure that proliferation occurs only under permissive conditions, but their role in linking nutrient availability to cell division is incompletely understood. Protein folding within the endoplasmic reticulum (ER) is exquisitely sensitive to energy supply and amino acid sources because deficiencies impair luminal protein folding and consequently trigger ER stress signaling. Following ER stress, many cell types arrest within the G(1) phase, although recent studies have identified a novel ER stress G(2) checkpoint. Here, we report that ER stress affects cell cycle progression via two classes of signal: an early inhibition of protein synthesis leading to G(2) delay involving CHK1 and a later induction of G(1) arrest associated both with the induction of p53 target genes and loss of cyclin D(1). We show that substitution of p53/47 for p53 impairs the ER stress G(1) checkpoint, attenuates the recovery of protein translation, and impairs induction of NOXA, a mediator of cell death. We propose that cell cycle regulation in response to ER stress comprises redundant pathways invoked sequentially first to impair G(2) progression prior to ultimate G(1) arrest.

TAp73 protein stability is controlled by histone deacetylase 1 via regulation of Hsp90 chaperone function

Histone deacetylases (HDACs) play important roles in fundamental cellular processes, and HDAC inhibitors are emerging as promising cancer therapeutics. p73, a member of the p53 family, plays a critical role in tumor suppression and neural development. Interestingly, p73 produces two classes of proteins with opposing functions: the full-length TAp73 and the N-terminally truncated ΔNp73. In the current study, we sought to characterize the potential regulation of p73 by HDACs and found that histone deacetylase 1 (HDAC1) is a key regulator of TAp73 protein stability. Specifically, we showed that HDAC1 inhibition by HDAC inhibitors or by siRNA shortened the half-life of TAp73 protein and subsequently decreased TAp73 expression under normal and DNA damage-induced conditions. Mechanistically, we found that HDAC1 knockdown resulted in hyperacetylation and inactivation of heat shock protein 90, which disrupted the interaction between heat shock protein 90 and TAp73 and thus promoted the proteasomal degradation of TAp73. Functionally, we found that down-regulation of TAp73 was required for the enhanced cell migration mediated by HDAC1 knockdown. Together, we uncover a novel regulation of TAp73 protein stability by HDAC1-heat shock protein 90 chaperone complex, and our data suggest that TAp73 is a critical downstream mediator of HDAC1-regulated cell migration.

Role of p73 in Alzheimer disease: lack of association in mouse models or in human cohorts

Background

P73 belongs to the p53 family of cell survival regulators with the corresponding locus Trp73 producing the N-terminally distinct isoforms, TAp73 and DeltaNp73. Recently, two studies have implicated the murine Trp73 in the modulation in phospho-tau accumulation in aged wild type mice and in young mice modeling Alzheimer’s disease (AD) suggesting that Trp73, particularly the DeltaNp73 isoform, links the accumulation of amyloid peptides to the creation of neurofibrillary tangles (NFTs). Here, we reevaluated tau pathologies in the same TgCRND8 mouse model as the previous studies.

Results

Despite the use of the same animal models, our in vivo studies failed to demonstrate biochemical or histological evidence for misprocessing of tau in young compound Trp73^+/- + TgCRND8 mice or in aged Trp73^+/- mice analyzed at the ages reported previously, or older. Secondly, we analyzed an additional mouse model where the DeltaNp73 was specifically deleted and confirmed a lack of impact of the DeltaNp73 allele, either in heterozygous or homozygous form, upon tau pathology in aged mice. Lastly, we also examined human TP73 for single nucleotide polymorphisms (SNPs) and/or copy number variants in a meta-analysis of 10 AD genome-wide association datasets. No SNPs reached significance after correction for multiple testing and no duplications/deletions in TP73 were found in 549 cases of AD and 544 non-demented controls.

Conclusion

Our results fail to support P73 as a contributor to AD pathogenesis.

Ratio disruption of the ∆133p53 and TAp53 isoform equilibrium correlates with poor clinical outcome in intrahepatic cholangiocarcinoma

All p53 family members are expressed in several isoforms through alternative promoters and alternative splicing. However, the significance of these isoforms is not yet well understood in cholangiocarcinoma (CCA). In this study, we investigated the expression of p53, p63, p73 and their isoforms at the mRNA and protein levels in CCA. The overexpression of ∆133p53 was observed in the CCA cell lines and clinical specimens. Moreover, the high expression of ∆133p53/TAp53 correlated with short overall survival (p<0.001). Defective p53, including mutant and ∆Np53, was associated with poor prognosis (p<0.024). Multivariate analysis demonstrated that ∆133p53/TAp53 and mutant p53 protein may be used as independent prognostic factors for CCA. To our knowledge, this is the first report of the use of ∆133p53/TAp53 as a potential biomarker in CCA.

Tumor suppressive pathways in the control of neurogenesis

The generation of specialized neural cells in the developing and postnatal central nervous system is a highly regulated process, whereby neural stem cells divide to generate committed neuronal progenitors, which then withdraw from the cell cycle and start to differentiate. Cell cycle checkpoints play a major role in regulating the balance between neural stem cell expansion and differentiation. Loss of tumor suppressors involved in checkpoint control can lead to dramatic alterations of neurogenesis, thus contributing to neoplastic transformation. Here we summarize and critically discuss the existing literature on the role of tumor suppressive pathways and their regulatory networks in the control of neurogenesis and transformation.

Blockage of melatonin receptors impairs p53-mediated prevention of DNA damage accumulation

Melatonin has been known to be a chemopreventive agent since its levels inversely correlate with the risk of developing cancer. We have recently shown that melatonin induces p38-dependent phosphorylation of both p53 and histone H2AX. This is associated with a p53-mediated increase in repair of both endogenous and chemotherapy-induced DNA damage. In addition, the inhibition of p38 activities impairs melatonin's capability to induce a p53-dependent DNA damage response and thus its ability to maintain genome integrity. Since melatonin-induced p53 phosphorylation requires an intact p38 phosphorylation cascade and p38 can be activated by G proteins, we supposed that melatonin's activities could be mediated by its G-protein-coupled membrane receptors, MT1 and MT2. Here, we show that the activation of the p53-dependent DNA damage response by melatonin is indeed mediated by MT1 and MT2. As a result, the absence of either receptor impairs melatonin's ability to reduce both cell proliferation and clonogenic potential of cancer cells. In addition, this causes an impairment of the p53-dependent DNA damage response. By providing molecular insight, our findings might have translational impact, suggesting the involvement of melatonin receptors in tumorigenesis.

Intrinsically disordered regions of p53 family are highly diversified in evolution

Proteins of the p53 family are expressed in vertebrates and in some invertebrate species. The main function of these proteins is to control and regulate cell cycle in response to various cellular signals, and therefore to control the organism's development. The regulatory functions of the p53 family members originate mostly from their highly-conserved and well-structured DNA-binding domains. Many human diseases (including various types of cancer) are related to the missense mutations within this domain. The ordered DNA-binding domains of the p53 family members are surrounded by functionally important intrinsically disordered regions. In this study, substitution rates and propensities in different regions of p53 were analyzed. The analyses revealed that the ordered DNA-binding domain is conserved, whereas disordered regions are characterized by high sequence diversity. This diversity was reflected both in the number of substitutions and in the types of substitutions to which each amino acid was prone. These results support the existence of a positive correlation between protein intrinsic disorder and sequence divergence during the evolutionary process. This higher sequence divergence provides strong support for the existence of disordered regions in p53 in vivo for if they were structured, they would evolve at similar rates as the rest of the protein.

Detecting p53 isoforms at protein level

The human p53 protein isoforms are expressed in several cell lines and modulate p53 tumor suppressor -activity, mainly through modulation of gene expression (1-4). Thus, identifying the pattern of p53 isoforms expression in cell lines is a key step for future studies of the p53 network (5). At the moment, the detection of p53 protein isoforms is based on the use of a panel of antibodies allowing their identification by comparing their molecular weights and their detection pattern by different antibodies (6). Here, classical protocols supplemented with technical know-how are described to detect p53 protein isoforms at protein level by Western blotting and immunoprecipitation. Furthermore, a simple method to study the impact of p53 protein isoforms on p53 transcriptional activity through luciferase reporter gene assays is provided.

p63 the guardian of human reproduction

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

p63 regulates cell proliferation and cell cycle progression‑associated genes in stromal cells of giant cell tumor of the bone

Giant cell tumor of bone (GCT) is a destructive neoplasm of uncertain etiology that affects the epiphyseal ends of long bones in young adults. GCT stromal cells (GCTSCs) are the primary neoplastic cells of this tumor and are the only proliferating cell component in long-term culture, which recruits osteoclast-like giant cells that eventually mediate bone destruction. The oncogenesis of GCT and factors driving the neoplastic stromal cells to proliferate extensively and pause at an early differentiation stage of pre-osteoblast lineage remain unknown. Overexpression of p63 was observed in GCTSCs and there is growing evidence that p63 is involved in oncogenesis through different mechanisms. This study aimed to understand the specific role of p63 in cell proliferation and oncogenesis of GCTSCs. We confirmed p63 expression in the mononuclear cells in GCT by immunohistochemical staining. By real-time PCR analysis, we showed a higher level (>15-fold) of TAp63 expression in GCTSCs compared to that in mesenchymal stem cells. Furthermore, we observed that knockdown of the p63 gene by siRNA transfection greatly reduced cell proliferation and induced cell cycle arrest at S phase in GCTSCs. We found that the mRNA expression of CDC2 and CDC25C was substantially suppressed by p63 knockdown at 24–72 h. Moreover, p63 was found to be recruited on the regulatory regions of CDC2 and CDC25C, which contain p53-responsive elements. In summary, our data suggest that p63 regulates GCTSC proliferation by binding to the CDC2 and CDC25C p53-REs, which may inhibit the p53 tumor suppressor activity and contribute to GCT tumorigenesis.

The large tumor antigen: a "Swiss Army knife" protein possessing the functions required for the polyomavirus life cycle

The SV40 large tumor antigen (L-Tag) is involved in the replication and cell transformation processes that take place during the polyomavirus life cycle. The ability of the L-Tag to interact with and to inactivate the tumor suppressor proteins p53 and pRb, makes this polyfunctional protein an interesting target in the search for compounds with antiviral and/or antiproliferative activities designed for the management of polyomavirus-associated diseases. The severe diseases caused by polyomaviruses, mainly in immunocompromised hosts, and the absence of licensed treatments, make the discovery of new antipolyomavirus drugs urgent. Parallels can be made between the SV40 L-Tag and the human papillomavirus (HPV) oncoproteins (E6 and E7) as they are also able to deregulate the cell cycle in order to promote cell transformation and its maintenance. In this review, a presentation of the SV40 L-Tag characteristics, regarding viral replication and cellular transformation, will show how similar these two processes are between the polyoma- and papillomavirus families. Insights at the molecular level will highlight similarities in the binding of polyoma- and papillomavirus replicative helicases to the viral DNA and in their disruptions of the p53 and pRb tumor suppressor proteins.

Differential effects of diverse p53 isoforms on TAp73 transcriptional activity and apoptosis

The p53 activities are due, at least in part, to its ability to form oligomers that bind to specific DNA sequences and activate transcription. Since some mutant p53 proteins and ΔNp73 isoforms form heterocomplexes with TAp73, we asked whether p53 isoforms can do the same and potentially act as dominant-negative inhibitors of TAp73. Moreover, it has already been found that some isoforms form complex with wtp53 and some of them inhibit p53 tumor-suppressor functions. Therefore, we studied the complex formation and co-immunoprecipitation assays show that all six p53 isoforms examined can form complexes with TAp73β, whereas only Δ133p53α/β/γ isoforms form complex with TAp73α. All p53 isoforms counteract TAp73β transactivation function but with different efficiency and in a promoter-dependent manner. Furthermore, apoptotic activity of TAp73β was augmented by coexpression of p53β, whereas Δ133p53α and β inhibit its apoptotic activity most efficiently. We have determined the half-life of different p53 isoforms: p53γ isoform has the shortest half-life, whereas Δ133p53γ has the longest half-life. Inhibitory interactions of two proteins in complex often lead to their stabilization. However, only three isoforms (Δ133p53α, Δ133p53β and Δ40p53α) stabilize TAp73β. We are convinced that defining the interactions between p53/p73 would give a new insight into how the p53 isoforms modulate the p73 functions in tumorigenesis.

The significance of p53 isoform expression in serous ovarian cancer

Epithelial ovarian cancer still carries a high mortality rate and remains the leading cause of gynecologic cancer death in the USA, despite decades of research. Hence, there is considerable interest in developing biomarkers that could be used to stratify patients for subsequent treatment. Mutation of the p53 tumor suppressor gene occurs very frequently (∼96%) in high-grade serous ovarian cancer. However, loss of p53 has not proven to be a reliable prognostic marker. Recent evidence indicates that the truncated p53 protein isoforms can regulate activated p53 and thus may play a role in tumorigenesis. In the article by Hofstetter et al., the authors examined the relationship between the expression of two p53 isoforms (Δ133p53 and Δ40p53) and prognosis in patients with serous ovarian cancer. Their findings indicate that Δ133p53 constitutes an independent prognostic marker for improved recurrence-free and overall survival. Intriguingly, this relationship was observed in patients whose tumors expressed a mutant p53, suggesting that Δ133p53 might suppress the actions of mutant p53. The mutational status of p53 alone did not have prognostic significance. These studies suggest that mutant p53 activity may be counteracted by Δ133p53, which leads to a more favorable prognosis in advanced serous ovarian carcinomas. Novel therapeutic approaches could be built upon these findings.

Structural basis for ASPP2 recognition by the tumor suppressor p73

Tumor suppressors p53, p63 and p73 comprise a family of stress-responsive transcription factors with distinct functions in development and tumor suppression. Most human cancers lose p53 function, yet all three proteins are capable of inducing apoptosis or cellular senescence. Mechanisms are therefore under investigation to activate p73-dependent apoptosis in p53-deficient cancer cells. Significantly, the DNA-binding domain (DBD) of p73 escapes viral oncoproteins and displays an enhanced thermal stability. To further understand the variant features of p73, we solved the high-resolution crystal structure of the p73 DBD as well as its complex with the ankyrin repeat and SH3 domains of the pro-apoptotic factor ASPP2. The p73 structure exhibits the same conserved architecture as p53 but displays a divergent L2 loop, a known site of protein-protein interaction. The loop in p73 is changed by a two-residue insertion that also induces repacking around the site of the p53 mutational hotspot R175. Importantly, the binding of ASPP2 is preserved by conformational changes in both the ankyrin repeat and SH3 domains. These results further highlight the structural variation that impacts p53 family interactions within the p53 interactome.

Methyl methanesulfonate induces apoptosis in p53-deficient H1299 and Hep3B cells through a caspase 2- and mitochondria-associated pathway

Methyl methanesulfonate (MMS) has been shown to induce apoptosis in various cell types through p53-dependent pathways. Nevertheless, pharmacological and genetic blockade of p53 functions results in similar or delayed sensitivity to MMS treatment, suggesting the presence of p53-independent apoptotic mechanisms. To understand the p53-independent mechanisms that are engaged during MMS-induced apoptosis, we established MMS-induced apoptotic cell models using p53-deficient H1299 and Hep3B cells. Our results demonstrated that MMS at concentrations of 50, 100, 200, 400 and 800 μM induced the formation of gammaH2AX foci, and that at higher concentrations, 400 and 800 μM, MMS treatment led to apoptosis in the two cell lines. This apoptotic cell death was concurrent with the loss of mitochondrial membrane potential, nuclear-cytosolic translocation of active caspase 2, release of cytochrome c from mitochondria, and the cleavage of caspase 9, caspase 3 and PARP. However, MMS-induced DNA damage failed to stabilize the p53 family members TAp73 and DNp73. These results demonstrated a p53- and p73-independent mechanism for MMS-induced apoptosis that involves the nuclear-cytosolic translocation of active caspase 2 as well as the mitochondria-mediated pathway.

Fludarabine nucleoside induces accumulations of p53, p63 and p73 in the nuclei of human B-lymphoid cell lines, with cytosolic and mitochondrial increases in p53

PURPOSE

Human Raji cells treated with fludarabine nucleoside (2-FaraA, 3 μM) undergo apoptosis with accumulation of p53 in the nuclei as multiple phosphorylated isoforms and C-terminal truncated derivatives. Changes induced by 2-FaraA in the levels of p53, p63 and p73 in the nuclear, cytosolic and mitochondrial fractions have been determined in four human B-lymphoid cell lines that are TP53-functional (Raji and IM9) and TP53-mutated (MEC1 and U266).

EXPERIMENTAL DESIGN

The B-lymphoid cell lines were treated with 2-FaraA (3 μM, 24 h, 48 h) and viability determined. Protein extracts of subcellular fractions from 2-FaraA-treated cells were analysed by 1D and 2D electrophoresis; multiple phosphorylated isoforms and truncated derivatives were identified by Western blots for p53, p63 and p73.

RESULTS

p53 and p63 were present in all three fractions, while p73 was only detected in nuclei. After treatment with 2-FaraA, nuclear p53, p63 and p73 accumulated as multiple phosphorylated isoforms and truncated derivatives. The association of p63 with mitochondria in human cells is novel.

CONCLUSIONS AND CLINICAL RELEVANCE

Comprehensive information on the subcellular distributions and responses of p53, p63 and p73 to 2-FaraA provides additional insight into mechanisms for induction of apoptosis in the treatment of B-lymphoproliferative disorders with fludarabine.

Glucocorticoid-induced fetal brain growth restriction is associated with p73 gene activation

Fetal exposure to excessive amounts of glucocorticoids (GCs) hampers proper brain development. The molecular mechanism(s) underlying these GCs effects are not well understood. We explored the impact of fetal exposure to maternal GCs on fetal brain expression of p63 and p73 transactivation (TA) and dominant negative (ΔN) gene variants that promote neural cell death (TA) and cell survival programs (ΔN). The fetoplacental enzyme 11β-hydroxysteroid dehydrogenase 2, which shields fetuses from maternal glucocorticoids, was inhibited throughout pregnancy by daily injection of carbenoxolone to pregnant dams. The expression of p63 and p73 gene variants and proteins was monitored by real-time rtPCR and Western blot in the brains of male and female fetuses. Carbenoxolone administration led to an overall enhanced level of corticosterone in the amniotic fluid of both male and female fetuses at late pregnancy. These enhanced corticosterone levels were associated with a significant reduction in fetal brain weights and a significant increase in TAp73 mRNA and p73 protein levels. However, the expression levels of TAp63 mRNA and p63 proteins were either suppressed or unaffected. The pro-neural survival gene variant ΔNp73 was significantly reduced in female and enhanced in male fetal brains, whereas ΔNp63 was significantly reduced in the brains of both genders. These data suggest that the GCs-induced negative impact on fetal brain development likely is due, at least in part, to their action of the pro-neural cell death gene variant TAp73 and to the modulation of the pro-survival ΔNp63 and ΔNp73 gene variants in a gender-dependent fashion.

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.

TAp73-mediated the activation of c-Jun N-terminal kinase enhances cellular chemosensitivity to cisplatin in ovarian cancer cells

P73, one member of the tumor suppressor p53 family, shares highly structural and functional similarity to p53. Like p53, the transcriptionally active TAp73 can mediate cellular response to chemotherapeutic agents in human cancer cells by up-regulating the expressions of its pro-apoptotic target genes such as PUMA, Bax, NOXA. Here, we demonstrated a novel molecular mechanism for TAp73-mediated apoptosis in response to cisplatin in ovarian cancer cells, and that was irrespective of p53 status. We found that TAp73 acted as an activator of the c-Jun N-terminal kinase (JNK) signaling pathway by up-regulating the expression of its target growth arrest and DNA-damage-inducible protein GADD45 alpha (GADD45α) and subsequently activating mitogen-activated protein kinase kinase-4 (MKK4). Inhibition of JNK activity by a specific inhibitor or small interfering RNA (siRNA) significantly abrogated TAp73-mediated apoptosis induced by cisplatin. Furthermore, inhibition of GADD45α by siRNA inactivated MKK4/JNK activities and also blocked TAp73-mediated apoptosis induction by cisplatin. Our study has demonstrated that TAp73 activated the JNK apoptotic signaling pathway in response to cisplatin in ovarian cancer cells.

p53 isoform profiling in glioblastoma and injured brain

The tumor suppressor p53 has been found to be the most commonly mutated gene in human cancers; however, the frequency of p53 mutations varies from 10 to 70% across different cancer types. This variability can partly be explained by inactivating mechanisms aside from direct genomic polymorphisms. The p53 gene encodes 12 isoforms, some of which can modulate full-length p53 activity in cancer. In this study, we characterized p53 isoform expression patterns in glioblastoma, gliosis, non-tumor brain and neural progenitor cells by SDS-PAGE, immunoblot, mass spectrometry and reverse transcription-PCR. We found that the most consistently expressed isoform in glioblastoma, Δ40p53, was uniquely expressed in regenerative processes, such as those involving neural progenitor cells and gliosis compared with tumor samples. Isoform profiling of glioblastoma tissues revealed the presence of both Δ40p53 and full-length p53, neither of which were detected in non-tumor cerebral cortex. Upon xenograft propagation of tumors, p53 levels increased. The variability of overall p53 expression and relative levels of isoforms suggest fluctuations in subpopulations of cells with greater or lesser capacity for proliferation, which can change as the tumor evolves under different growth conditions.

Golgi-SNARE GS28 potentiates cisplatin-induced apoptosis by forming GS28-MDM2-p53 complexes and by preventing the ubiquitination and degradation of p53

In the present study, we observed that the Golgi-SNARE (soluble N-ethylmaleimide-sensitive fusion protein-attachment protein receptor) GS28 forms a complex with p53 in HEK (human embryonic kidney)-293 cells. Given that p53 represents a tumour suppressor that affects the sensitivity of cancer cells to various chemotherapeutic drugs, we examined whether GS28 may influence the level of sensitivity to the DNA-damaging drug cisplatin. Indeed, knockdown of GS28 using short-hairpin RNA (shGS28) induced resistance to cisplatin in HEK-293 cells. On the other hand, overexpression of GS28 sensitized HEK-293 cells to cisplatin, whereas no sensitization effect was noted for the mitotic spindle-damaging drugs vincristine and taxol. Accordingly, we observed that knockdown of GS28 reduced the accumulation of p53 and its pro-apoptotic target Bax. Conversely, GS28 overexpression induced the accumulation of p53 and Bax as well as the pro-apoptotic phosphorylation of p53 on Ser(46). Further experiments showed that these cellular responses could be abrogated by the p53 inhibitor PFT-α (pifithrin-α), indicating that GS28 may affect the stability and activity of p53. The modulatory effects of GS28 on cisplatin sensitivity and p53 stability were absent in lung cancer H1299 cells which are p53-null. As expected, ectopic expression of p53 in H1299 cells restored the modulatory effects of GS28 on sensitivity to cisplatin. In addition, GS28 was found to form a complex with the p53 E3 ligase MDM2 (murine double minute 2) in H1299 cells. Furthermore, the ubiquitination of p53 was reduced by overexpression of GS28 in cells, confirming that GS28 enhances the stability of the p53 protein. Taken together, these results suggest that GS28 may potentiate cells to DNA-damage-induced apoptosis by inhibiting the ubiquitination and degradation of p53.

The role of Dlc1 isoform 2 in K-Ras2(G12D) induced thymic cancer

The Deleted in liver cancer one (Dlc1) tumor suppressor gene encodes a RhoGTPase activating protein (RhoGAP). The Dlc1 gene has multiple transcriptional isoforms and we have previously established a mouse strain containing a gene trap (gt) insertion, which specifically reduces the expression of the 6.1 kb isoform (isoform 2). This gene trapped allele when homozygous results in embryonic lethality and the heterozygous gene trapped mice do not show an increased incidence of cancers, suggesting that cooperating oncogenic changes may be required for transformation. In the present work, we have studied the in vivo cooperation between oncogenic K-Ras2 and Dlc1 genes in tumourigenesis. We have observed an increase in invasive thymic cancers, including both thymomas and lymphomas, resulting in significantly shortened life spans in mice heterozygous for the gt Dlc1 allele and an inducible LSL-K-Ras2^G12D allele compared with the LSL-K-Ras2^G12D only mice. The heterozygous mice showed a high degree of metastasis in the lung. We have found tumour specific selective hypermethylation of the Dlc1 isoform 2 promoter and reduction of the corresponding protein expression in thymic lymphoma (TL) and thymic epithelial carcinoma (TEC) derived from the thymic tumours. The Dlc1 deficient thymic lymphoma cell lines exhibited increased trans-endothelial cell migration. TEC cell lines also exhibited increased stress fiber formation and Rho activity. Introduction of the three Dlc1 isoforms tagged with GFP into these cells resulted in different morphological changes. These results suggest that loss of expression of only isoform 2 may be sufficient for the development of thymic tumors and metastasis.

Low dose proteasome inhibition affects alternative splicing

Protein degradation by the ubiquitin proteasome system ensures controlled degradation of structural proteins, signaling mediators, and transcription factors. Inhibition of proteasome function by specific proteasome inhibitors results in dose-dependent cellular effects ranging from induction of apoptosis to protective stress responses. The present study seeks to identify nuclear regulators mediating the protective stress response to low dose proteasome inhibition. Primary human endothelial cells were treated with low doses of the proteasome inhibitor MG132 for 2 h, and proteomic analysis of nuclear extracts was performed. Using a 2-D differential in gel electrophoresis (DIGE) approach, we identified more than 24 splice factors to be differentially regulated by low dose proteasome inhibition. In particular, several isoforms of hnRNPA1 were shown to be increased, pointing toward altered posttranslational modification of hnRNPA1 upon proteasome inhibition. Elevated levels of splice factors were associated with a different alternative splicing pattern in response to proteasome inhibition as determined by Affymetrix exon array profiling. Of note, we observed alternative RNA processing for stress associated genes such as caspases and heat shock proteins. Our study provides first evidence that low dose proteasome inhibition affects posttranscriptional regulation of splice factors and early alternative splicing events.

A novel dual signaling axis for NSP 5a3a induced apoptosis in head and neck carcinoma

NSP 5a3a is a novel structural protein found to be over-expressed in certain cancer cell lines in-vitro such as Hela, Saos-2, and MCF-7 while barely detectable levels in normal body tissues except for Testis. This particular isoform has been known to interact with cyto- nuclear proteins B23, known to be involved in multi-faceted cellular processes such as cell division, apoptosis, ribosome biogenesis, and rRNA processing, as well as with hnRNP-L, known to be involved with RNA metabolism and rRNA processing. A previous preliminary investigation of NSP 5a3a as a potential target in Head and Neck Carcinoma revealed a novel p73 dependent mechanism through which NSP 5a3a induced apoptosis in Head and Neck cell lines when over-expressed in-vitro. Our present investigation further elucidated a novel dual axis signaling point by which NSP 5a3a induces apoptosis in Head and Neck cell line HN30 through p73-DAXX and TRAF2-TRADD. Interestingly, this novel mechanism appears independent of canonical caspases involved in the intrinsic mitochondrial pathway as well as those in the death receptor pathway thru TRAF2 and TRADD.

The tumour suppressor p53 is frequently nonfunctional in Sézary syndrome

BACKGROUND

Primary cutaneous T-cell lymphomas (CTCLs) are a heterogeneous group with Sézary syndrome (SS) as one of the most aggressive variants. Recently, we identified a loss of E2A as a recurrent event in SS, which enhanced proliferation via upregulation of the proto-oncogene MYC. MYC-induced transformation usually requires deleterious alterations of key apoptotic genes including p53; however, p53 functionality and mutation status in SS are unclear.

OBJECTIVES

We investigated functionality of p53 signalling by pharmacological treatment with the MDM2 antagonist nutlin-3, which might result in p53 activation. Furthermore, we analysed the TP53 mutation status in CTCL cell lines and highly purified tumour cells from patients with SS by mRNA and DNA sequencing.

METHODS

We analysed the apoptosis induction due to nutlin-3 treatment in various SS cell lines and primary patient samples by annexin V/propidium iodide staining. Induction of p53 target genes was analysed by immunoblotting, and TP53 was sequenced at the mRNA and DNA level.

RESULTS

We identified various TP53 mutations and an impaired p53 signalling in the vast majority of the investigated cell lines and primary SS cells.

CONCLUSIONS

In accordance with the importance of MYC deregulation in SS, p53 signalling is frequently nonfunctional in SS. However, although most likely ineffective as exclusive treatment in SS, it remains possible that pharmacological p53 activation could be beneficial in combination with other approaches including classical chemotherapeutics.

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.

Structure of p73 DNA-binding domain tetramer modulates p73 transactivation

The transcription factor p73 triggers developmental pathways and overlaps stress-induced p53 transcriptional pathways. How p53-family response elements determine and regulate transcriptional specificity remains an unsolved problem. In this work, we have determined the first crystal structures of p73 DNA-binding domain tetramer bound to response elements with spacers of different length. The structure and function of the adaptable tetramer are determined by the distance between two half-sites. The structures with zero and one base-pair spacers show compact p73 DNA-binding domain tetramers with large tetramerization interfaces; a two base-pair spacer results in DNA unwinding and a smaller tetramerization interface, whereas a four base-pair spacer hinders tetramerization. Functionally, p73 is more sensitive to spacer length than p53, with one base-pair spacer reducing 90% of transactivation activity and longer spacers reducing transactivation to basal levels. Our results establish the quaternary structure of the p73 DNA-binding domain required as a scaffold to promote transactivation.

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

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

The p53 circuit board

The p53 tumor suppressor is embedded in a large gene network controlling diverse cellular and organismal phenotypes. Multiple signaling pathways converge onto p53 activation, mostly by relieving the inhibitory effects of its repressors, MDM2 and MDM4. In turn, signals originating from increased p53 activity diverge into distinct effector pathways to deliver a specific cellular response to the activating stimuli. Much attention has been devoted to dissecting how the various input pathways trigger p53 activation and how the activity of the p53 protein itself can be modulated by a plethora of co-factors and post-translational modifications. In this review we will focus instead on the multiple configurations of the effector pathways. We will discuss how p53-generated signals are transmitted, amplified, resisted and eventually integrated by downstream gene circuits operating at the transcriptional, post-transcriptional and post-translational levels. We will also discuss how context-dependent variations in these gene circuits define the cellular response to p53 activation and how they may impact the clinical efficacy of p53-based targeted therapies.

TNF-α response of vascular endothelial and vascular smooth muscle cells involve differential utilization of ASK1 kinase and p73

Atherosclerosis involves a specialized inflammatory process regulated by an intricate network of cytokine and chemokine signaling. Atherosclerotic lesions lead to the release of cytokines that can have multiple affects on various vascular cell functions either promoting lesion expansion or alternatively retard progression. Tumor necrosis factor-α (TNF-α) is one such cytokine that can activate both cell survival and cell death mechanisms simultaneously. Here we show that TNF-α induces apoptosis in human aortic endothelial cells (HAECs), while it promotes the proliferation of vascular smooth muscle cells (VSMCs). Both events involved the activation of the Rb-E2F1 transcriptional regulatory pathway. Stimulation of HAECs with TNF-α led to an increased expression of p73 protein and a reduction in the levels of p53. This involved apoptosis signal-regulating kinase 1 (ASK1)- mediated inactivation of Rb and its dissociation from the p73 promoter. In contrast, TNF-α stimulation of VSMCs enhanced the association of E2F1 with proliferative promoters like thymidylate synthase and cdc25A, while Rb was dissociated. ASK1 kinase has a critical role in the apoptotic process, as its depletion or dissociation from Rb reduced TNF-α-induced apoptosis. These results show that the cytokine TNF-α can elicit diametrically opposite responses in vascular endothelial cells and VSMCs, utilizing the Rb-E2F pathway.

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.

MicroRNA: a third dimension in autophagy

Autophagy is a catabolic process that allows cellular macromolecules to be broken down and recycled into metabolic precursors. It is a highly conserved, critical process, allowing cells to gain survival advantages under various stress situations due to growth and environmental changes. In the past few years, mounting evidence indicates that the post-transcriptional and translational controls mediated by non-coding miRNAs contribute significantly to autophagy in cancer. Such acute modulation of protein synthesis mediated by miRNAs provides cells with advantages in response to starvation, genotoxic stress and hypoxia. In this review, we highlight some of the important discoveries and molecular insights of miRNAs in regulating autophagy based on various cancer models.