The C terminus of p73 is essential for hippocampal development.

The p53 family member p73 has a complex gene structure, including alternative promoters and alternative splicing of the 3' UTR. This results in a complex range of isoforms whose biological relevance largely remains to be determined. By deleting exon 13 (which encodes a sterile α motif) from the Trp73 gene, we selectively engineered mice to replace the most abundantly expressed C-terminal isoform, p73α, with a shorter product of alternative splicing, p73ß. These mice (Trp73Δ13/Δ13 ) display severe neurodevelopmental defects with significant functional and morphological abnormalities. Replacement of p73α with p73ß results in the depletion of Cajal-Retzius (CR) cells in embryonic stages, thus depriving the developing hippocampus of the pool of neurons necessary for correct hippocampal architecture. Consequently, Trp73Δ13/Δ13 mice display severe hippocampal dysgenesis, reduced synaptic functionality and impaired learning and memory capabilities. Our data shed light on the relevance of p73 alternative splicing and show that the full-length C terminus of p73 is essential for hippocampal development.

ZNF281 inhibits neuronal differentiation and is a prognostic marker for neuroblastoma.

Derangement of cellular differentiation because of mutation or inappropriate expression of specific genes is a common feature in tumors. Here, we show that the expression of ZNF281, a zinc finger factor involved in several cellular processes, decreases during terminal differentiation of murine cortical neurons and in retinoic acid-induced differentiation of neuroblastoma (NB) cells. The ectopic expression of ZNF281 inhibits the neuronal differentiation of murine cortical neurons and NB cells, whereas its silencing causes the opposite effect. Furthermore, TAp73 inhibits the expression of ZNF281 through miR34a. Conversely, MYCN promotes the expression of ZNF281 at least in part by inhibiting miR34a. These findings imply a functional network that includes p73, MYCN, and ZNF281 in NB cells, where ZNF281 acts by negatively affecting neuronal differentiation. Array analysis of NB cells silenced for ZNF281 expression identified GDNF and NRP2 as two transcriptional targets inhibited by ZNF281. Binding of ZNF281 to the promoters of these genes suggests a direct mechanism of repression. Bioinformatic analysis of NB datasets indicates that ZNF281 expression is higher in aggressive, undifferentiated stage 4 than in localized stage 1 tumors supporting a central role of ZNF281 in affecting the differentiation of NB. Furthermore, patients with NB with high expression of ZNF281 have a poor clinical outcome compared with low-expressors. These observations suggest that ZNF281 is a controller of neuronal differentiation that should be evaluated as a prognostic marker in NB.

TAp73 contributes to the oxidative stress response by regulating protein synthesis.

TAp73 is a transcription factor that plays key roles in brain development, aging, and cancer. At the cellular level, TAp73 is a critical homeostasis-maintaining factor, particularly following oxidative stress. Although major studies focused on TAp73 transcriptional activities have indicated a contribution of TAp73 to cellular metabolism, the mechanisms underlying its role in redox homeostasis have not been completely elucidated. Here we show that TAp73 contributes to the oxidative stress response by participating in the control of protein synthesis. Regulation of mRNA translation occupies a central position in cellular homeostasis during the stress response, often by reducing global rates of protein synthesis and promoting translation of specific mRNAs. TAp73 depletion results in aberrant ribosomal RNA (rRNA) processing and impaired protein synthesis. In particular, polysomal profiles show that TAp73 promotes the integration of mRNAs that encode rRNA-processing factors in polysomes, supporting their translation. Concurrently, TAp73 depletion causes increased sensitivity to oxidative stress that correlates with reduced ATP levels, hyperactivation of AMPK, and translational defects. TAp73 is important for maintaining active translation of mitochondrial transcripts in response to oxidative stress, thus promoting mitochondrial activity. Our results indicate that TAp73 contributes to redox homeostasis by affecting the translational machinery, facilitating the translation of specific mitochondrial transcripts. This study identifies a mechanism by which TAp73 contributes to the oxidative stress response and describes a completely unexpected role for TAp73 in regulating protein synthesis.

p53 mutants cooperate with HIF-1 in transcriptional regulation of extracellular matrix components to promote tumor progression.

Mutations in the TP53 gene and microenvironmentally driven activation of hypoxia-inducible factor-1 (HIF-1) typically occur in later stages of tumorigenesis. An ongoing challenge is the identification of molecular determinants of advanced cancer pathogenesis to design alternative last-line therapeutic options. Here, we report that p53 mutants influence the tumor microenvironment by cooperating with HIF-1 to promote cancer progression. We demonstrate that in non-small cell lung cancer (NSCLC), p53 mutants exert a gain-of-function (GOF) effect on HIF-1, thus regulating a selective gene expression signature involved in protumorigenic functions. Hypoxia-mediated activation of HIF-1 leads to the formation of a p53 mutant/HIF-1 complex that physically binds the SWI/SNF chromatin remodeling complex, promoting expression of a selective subset of hypoxia-responsive genes. Depletion of p53 mutants impairs the HIF-mediated up-regulation of extracellular matrix (ECM) components, including type VIIa1 collagen and laminin-γ2, thus affecting tumorigenic potential of NSCLC cells in vitro and in mouse models in vivo. Analysis of surgically resected human NSCLC revealed that expression of this ECM gene signature was highly correlated with hypoxic tumors exclusively in patients carrying p53 mutations and was associated with poor prognosis. Our data reveal a GOF effect of p53 mutants in hypoxic tumors and suggest synergistic activities of p53 and HIF-1. These findings have important implications for cancer progression and might provide innovative last-line treatment options for advanced NSCLC.

TAp73 depletion accelerates aging through metabolic dysregulation.

Aging is associated with impaired scavenging of reactive oxygen species (ROS). Here, we show that TAp73, a p53 family member, protects against aging by regulating mitochondrial activity and preventing ROS accumulation. TAp73-null mice show more pronounced aging with increased oxidative damage and senescence. TAp73 deletion reduces cellular ATP levels, oxygen consumption, and mitochondrial complex IV activity, with increased ROS production and oxidative stress sensitivity. We show that the mitochondrial complex IV subunit cytochrome C oxidase subunit 4 (Cox4i1) is a direct TAp73 target and that Cox4i1 knockdown phenocopies the cellular senescence of TAp73-null cells. Results indicate that TAp73 affects mitochondrial respiration and ROS homeostasis, thus regulating aging.

TAp73 opposes tumor angiogenesis by promoting hypoxia-inducible factor 1α degradation.

Tumor hypoxia and hypoxia-inducible factor 1 (HIF-1) activation are associated with cancer progression. Here, we demonstrate that the transcription factor TAp73 opposes HIF-1 activity through a nontranscriptional mechanism, thus affecting tumor angiogenesis. TAp73-deficient mice have an increased incidence of spontaneous and chemically induced tumors that also display enhanced vascularization. Mechanistically, TAp73 interacts with the regulatory subunit (α) of HIF-1 and recruits mouse double minute 2 homolog into the protein complex, thus promoting HIF-1α polyubiquitination and consequent proteasomal degradation in an oxygen-independent manner. In human lung cancer datasets, TAp73 strongly predicts good patient prognosis, and its expression is associated with low HIF-1 activation and angiogenesis. Our findings, supported by in vivo and clinical evidence, demonstrate a mechanism for oxygen-independent HIF-1 regulation, which has important implications for individualizing therapies in patients with cancer.

TAp73 upregulates IL-1ß in cancer cells: Potential biomarker in lung and breast cancer?

p73 is a transcription factor belonging to the p53 tumour suppressor family. p73-/- mice exhibit a range of phenotypes including neurological, reproductive and inflammatory defects. Although the role of p73 in the control of genomic stability explains part of these phenotypes, a clear mechanism of how p73 participates in the inflammatory response is still elusive. Interleukin-1ß (IL-1ß) has a crucial role in mediating the inflammatory response. Because of its high potency to induce inflammation, the activation and secretion of IL-1ß is tightly regulated by large protein complexes, named inflammasomes. Inflammasomes regulate activation of proinflammatory caspase-1, which in turn proteolytically processes its substrates, including pro-IL-1ß. Caspase-1 gene transcription is strongly activated by p53 protein family members including p73. Here, we have addressed whether p73 might be directly involved in IL-1ß regulation and therefore in the control of the inflammatory response. Our results show that TAp73ß upregulates pro-IL-1ß mRNA and processed IL-1ß protein. In addition, analysis of breast and lung cancer patient cohorts demonstrated that interaction between p73 and IL-1ß predicts a negative survival outcome in these human cancers.

Role of p63 and the Notch pathway in cochlea development and sensorineural deafness.

The ectodermal dysplasias are a group of inherited autosomal dominant syndromes associated with heterozygous mutations in the Tumor Protein p63 (TRP63) gene. Here we show that, in addition to their epidermal pathology, a proportion of these patients have distinct levels of deafness. Accordingly, p63 null mouse embryos show marked cochlea abnormalities, and the transactivating isoform of p63 (TAp63) protein is normally found in the organ of Corti. TAp63 transactivates hairy and enhancer of split 5 (Hes5) and atonal homolog 1 (Atoh1), components of the Notch pathway, known to be involved in cochlear neuroepithelial development. Strikingly, p63 null mice show morphological defects of the organ of Corti, with supernumerary hair cells, as also reported for Hes5 null mice. This phenotype is related to loss of a differentiation property of TAp63 and not to loss of its proapoptotic function, because cochleas in mice lacking the critical Bcl-2 homology domain (BH-3) inducers of p53- and p63-mediated apoptosis--Puma, Noxa, or both--are normal. Collectively, these data demonstrate that TAp63, acting via the Notch pathway, is crucial for the development of the organ of Corti, providing a molecular explanation for the sensorineural deafness in ectodermal dysplasia patients with TRP63 mutations.

TAp73 knockout mice show morphological and functional nervous system defects associated with loss of p75 neurotrophin receptor.

Total and N-terminal isoform selective p73 knockout mice show a variety of central nervous system defects. Here we show that TAp73 is a transcriptional activator of p75 neurotrophin receptor (p75(NTR)) and that p75(NTR) mRNA and protein levels are strongly reduced in the central and peripheral nervous systems of p73 knockout mice. In parallel, primary cortical neurons from p73 knockout mice showed a reduction in neurite outgrowth and in nerve growth factor-mediated neuronal differentiation, together with reduced miniature excitatory postsynaptic current frequencies and behavioral defects. p73 null mice also have impairments in the peripheral nervous system with reduced thermal sensitivity, axon number, and myelin thickness. At least some of these morphological and functional impairments in p73 null cells can be rescued by p75(NTR) re-expression. Together, these data demonstrate that loss of p75(NTR) contributes to the neurological phenotype of p73 knockout mice.

Loss of p63 and its microRNA-205 target results in enhanced cell migration and metastasis in prostate cancer.

p63 inhibits metastasis. Here, we show that p63 (both TAp63 and ΔNp63 isoforms) regulates expression of miR-205 in prostate cancer (PCa) cells, and miR-205 is essential for the inhibitory effects of p63 on markers of epithelial-mesenchymal transition (EMT), such as ZEB1 and vimentin. Correspondingly, the inhibitory effect of p63 on EMT markers and cell migration is reverted by anti-miR-205. p53 mutants inhibit expression of both p63 and miR-205, and the cell migration, in a cell line expressing endogenous mutated p53, can be abrogated by pre-miR-205 or silencing of mutated p53. In accordance with this in vitro data, ΔNp63 or miR-205 significantly inhibits the incidence of lung metastasis in vivo in a mouse tail vein model. Similarly, one or both components of the p63/miR-205 axis were absent in metastases or colonized lymph nodes in a set of 218 human prostate cancer samples. This was confirmed in an independent clinical data set of 281 patients. Loss of this axis was associated with higher Gleason scores, an increased likelihood of metastatic and infiltration events, and worse prognosis. These data suggest that p63/miR-205 may be a useful clinical predictor of metastatic behavior in prostate cancer.

Neuronal differentiation by TAp73 is mediated by microRNA-34a regulation of synaptic protein targets.

The p53-family member TAp73 is a transcription factor that plays a key role in many biological processes. Here, we show that p73 drives the expression of microRNA (miR)-34a, but not miR-34b and -c, by acting on specific binding sites on the miR-34a promoter. Expression of miR-34a is modulated in parallel with that of TAp73 during in vitro differentiation of neuroblastoma cells and cortical neurons. Retinoid-driven neuroblastoma differentiation is inhibited by knockdown of either p73 or miR-34a. Transcript expression of miR-34a is significantly reduced in vivo both in the cortex and hippocampus of p73(-/-) mice; miR-34a and TAp73 expression also increase during postnatal development of the brain and cerebellum when synaptogenesis occurs. Accordingly, overexpression or silencing of miR-34a inversely modulates expression of synaptic targets, including synaptotagmin-1 and syntaxin-1A. Notably, the axis TAp73/miR-34a/synaptotagmin-1 is conserved in brains from Alzheimer's patients. These data reinforce a role for TAp73 in neuronal development.

microRNA-34a regulates neurite outgrowth, spinal morphology, and function.

The p53 family member TAp73 is a transcription factor that plays a key role in many biological processes, including neuronal development. In particular, we have shown that p73 drives the expression of miR-34a, but not miR-34b and c, in mouse cortical neurons. miR-34a in turn modulates the expression of synaptic targets including synaptotagmin-1 and syntaxin-1A. Here we show that this axis is retained in mouse ES cells committed to differentiate toward a neurological phenotype. Moreover, overexpression of miR-34a alters hippocampal spinal morphology, and results in electrophysiological changes consistent with a reduction in spinal function. Therefore, the TAp73/miR-34a axis has functional relevance in primary neurons. These data reinforce a role for miR-34a in neuronal development.

Differential altered stability and transcriptional activity of ΔNp63 mutants in distinct ectodermal dysplasias.

Heterozygous mutations of p63, a key transcription factor in epithelial development, are causative in a variety of human ectodermal dysplasia disorders. Although the mutation spectrum of these disorders displays a striking genotype-phenotype association, the molecular basis for this association is only superficially known. Here, we characterize the transcriptional activity and protein stability of ΔNp63 mutants (that is, mutants of a p63 isoform that lacks the N-terminal transactivation domain) that are found in ectrodactyly-ectodermal dysplasia-cleft syndrome (EEC), ankyloblepharon-ectodermal dysplasia-clefting syndrome (AEC) and nonsyndromic split-hand/split-foot malformation (SHFM). DNA-binding and sterile alpha motif (SAM) domain mutants accumulate in the skin of EEC and AEC syndrome patients, respectively, and show extended half lives in vitro. By contrast, C-terminal mutations found in SHFM patients have half-lives similar to that of the wild-type protein. The increased half-life of EEC and AEC mutant proteins was reverted by overexpression of wild-type ΔNp63. Interestingly, the mutant proteins exhibit normal binding to and degradation by the E3 ubiquitin ligase Itch. Finally, EEC and AEC mutant proteins have reduced transcriptional activity on several skin-specific gene promoters, whereas SHFM mutant proteins are transcriptionally active. Our results, therefore, provide evidence for a regulatory feedback mechanism for p63 that links transcriptional activity to regulation of protein homeostasis by an unknown mechanism. Disruption of this regulatory mechanism might contribute to the pathology of p63-related developmental disorders.

Differential control of TAp73 and DeltaNp73 protein stability by the ring finger ubiquitin ligase PIR2.

p73 is a p53-related transcription factor with fundamental roles in development and tumor suppression. Transcription from two different promoters on the p73 gene results in generation of transcriptionally active TAp73 isoforms and dominant negative DeltaNp73 isoforms with opposing pro- and anti-apoptotic functions. Therefore, the relative ratio of each isoform is an important determinant of the cell fate. Proteasomal degradation of p73 is mediated by polyubiquitination-dependent and -independent processes both of which appear, thus far, to lack selectivity for the TAp73 and DeltaNp73 isoforms. Here, we describe the characterization of another transcriptional target of TAp73; a ring finger domain ubiquitin ligase p73 Induced RING 2 protein (PIR2). Although PIR2 was initially identified a p53-induced gene (p53RFP), low abundance of PIR2 transcript in mouse embryonic fibroblasts of TAp73 KO mice compared with WT mice and comparison of PIR2 mRNA and protein levels following TAp73 or p53 overexpression substantiate TAp73 isoforms as strong inducers of PIR2. Although PIR2 expression was induced by DNA damage, its expression did not alter apoptotic response or cell cycle profile per se. However, coexpression of PIR2 with TAp73 or DeltaNp73 resulted in an increase of the TA/DeltaNp73 ratio, due to preferential degradation of DeltaNp73. Finally, PIR2 was able to relieve the inhibitory effect of DeltaNp73 on TAp73 induced apoptosis following DNA damage. These results suggest that PIR2, by being induced by TAp73 and degrading DeltaNp73, differentially regulates TAp73/DeltaNp73 stability, and, hence, it may offer a therapeutic approach to enhance the chemosensitivity of tumor cells.

miRNAs, 'stemness' and skin.

The epidermis and its appendages provide organisms with protection from the environment, keeping pathogens out and preventing the loss of essential body fluids. To perform both functions, the skin has elaborated a complex differentiation process known as cornification. The renewal capacity of the skin, which is responsible for maintaining tissue homeostasis, regenerating hair and repairing the epidermis after injury, resides in the basal proliferating compartment in which epidermal stem cells are located. These cells possess the remarkable capacity to both self-perpetuate and give rise to the differentiating cells that form mature tissues. Recent findings indicate that microRNAs have an essential role in orchestrating the formation of epidermis and skin appendages, in particular, at the interface between stemness and differentiation.

ZFP750 affects the cutaneous barrier through regulating lipid metabolism.

An essential function of the epidermis is to provide a physical barrier that prevents the loss of water. Essential mediators of this barrier function include ceramides, cholesterol, and very long chain fatty acids, and their alteration causes human pathologies, including psoriasis and atopic dermatitis. A frameshift mutation in the human ZNF750 gene, which encodes a zinc finger transcription factor, has been shown to cause a seborrhea-like dermatitis. Here, we show that genetic deletion of the mouse homolog ZFP750 results in loss of epidermal barrier function, which is associated with a substantial reduction of ceramides, nonpolar lipids. The alteration of epidermal lipid homeostasis is directly linked to the transcriptional activity of ZFP750. ZFP750 directly and/or indirectly regulates the expression of crucial enzymes primarily involved in the biosynthesis of ceramides. Overall, our study identifies the transcription factor ZFP750 as a master regulator epidermal homeostasis through lipid biosynthesis and thus contributing to our understanding of the pathogenesis of several human skin diseases.

Metabolic regulation by p53 prevents R-loop-associated genomic instability.

Gene-environment interactions can perturb the epigenome, triggering network alterations that participate in cancer pathogenesis. Integrating epigenomics, transcriptomics, and metabolic analyses with functional perturbation, we show that the tumor suppressor p53 preserves genomic integrity by empowering adequate levels of the universal methyl donor S-adenosylmethionine (SAM). In p53-deficient cells, perturbation of DNA methylation promotes derepression of heterochromatin, massive loss of histone H3-lysine 9 methylation, and consequent upregulation of satellite RNAs that triggers R-loop-associated replication stress and chromosomal aberrations. In p53-deficient cells, the inadequate SAM level underlies the inability to respond to perturbation because exogenous reintroduction of SAM represses satellite elements and restores the ability to cope with stress. Mechanistically, p53 transcriptionally controls genes involved in one-carbon metabolism, including Slc43a2, the methionine uptake transporter that is critical for SAM synthesis. Supported by clinical data, our findings shed light on the role of p53-mediated metabolism in preventing unscheduled R-loop-associated genomic instability.

Desmethylclomipramine induces the accumulation of autophagy markers by blocking autophagic flux.

Alterations in the autophagic pathway are associated with the onset and progression of various diseases. However, despite the therapeutic potential for pharmacological modulators of autophagic flux, few such compounds have been characterised. Here we show that clomipramine, an FDA-approved drug long used for the treatment of psychiatric disorders, and its active metabolite desmethylclomipramine (DCMI) interfere with autophagic flux. Treating cells with DCMI caused a significant and specific increase in autophagosomal markers and a concomitant blockage of the degradation of autophagic cargo. This observation might be relevant in therapy in which malignant cells exploit autophagy to survive stress conditions, rendering them more susceptible to the action of cytotoxic agents. In accordance, DCMI-mediated obstruction of autophagic flux increased the cytotoxic effect of chemotherapeutic agents. Collectively, our studies describe a new function of DCMI that can be exploited for the treatment of pathological conditions in which manipulation of autophagic flux is thought to be beneficial.

Cell death pathology: cross-talk with autophagy and its clinical implications.

Autophagy is a self-digesting mechanism that cells adopt to respond to stressful stimuli. Morphologically, cells dying by autophagy show multiple cytoplasmic double-membraned vacuoles, and, if prolonged, autophagy can lead to cell death, "autophagic cell death". Thus, autophagy can act both as a temporary protective mechanism during a brief stressful episode and be a mode of cell death in its own right. In this mini-review we focus on recent knowledge concerning the connection between autophagy and programmed cell death, evaluating their possible implications for therapy in pathologies like cancer and neurodegeneration.