Expression of arf tumor suppressor in spermatogonia facilitates meiotic progression in male germ cells

p53 Controls Meiotic Prophase Progression and Crossover Formation

Meiosis initiates with the formation of double strand breaks (DSBs) throughout the genome. To avoid genomic instability, these DSBs need to be correctly repaired by homologous recombination. Surveillance mechanisms involving the DNA damage response (DDR) pathway ATM-CHK2-p53 can detect the persistence of unrepaired DBSs and activate the recombination-dependent arrest at the pachytene stage. However, a complete understanding of p53 functions under normal physiological conditions remains lacking. Here, we report a detailed analysis of the p53 role during meiotic prophase in mice spermatocytes. We show that the absence of p53 regulates prophase progression by slowing down the pachytene stage when the recombination-dependent arrest occurs. Furthermore, our results show that p53 is necessary for proper crossover (CO) formation and localization. Our study contributes to a deeper understanding of p53 roles during the meiotic prophase.

Mechanisms of TP53 Pathway Inactivation in Embryonic and Somatic Cells-Relevance for Understanding (Germ Cell) Tumorigenesis

The P53 pathway is the most important cellular pathway to maintain genomic and cellular integrity, both in embryonic and non-embryonic cells. Stress signals induce its activation, initiating autophagy or cell cycle arrest to enable DNA repair. The persistence of these signals causes either senescence or apoptosis. Over 50% of all solid tumors harbor mutations in TP53 that inactivate the pathway. The remaining cancers are suggested to harbor mutations in genes that regulate the P53 pathway such as its inhibitors Mouse Double Minute 2 and 4 (MDM2 and MDM4, respectively). Many reviews have already been dedicated to P53, MDM2, and MDM4, while this review additionally focuses on the other factors that can deregulate P53 signaling. We discuss that P14^ARF (ARF) functions as a negative regulator of MDM2, explaining the frequent loss of ARF detected in cancers. The long non-coding RNA Antisense Non-coding RNA in the INK4 Locus (ANRIL) is encoded on the same locus as ARF, inhibiting ARF expression, thus contributing to the process of tumorigenesis. Mutations in tripartite motif (TRIM) proteins deregulate P53 signaling through their ubiquitin ligase activity. Several microRNAs (miRNAs) inactivate the P53 pathway through inhibition of translation. CCCTC-binding factor (CTCF) maintains an open chromatin structure at the TP53 locus, explaining its inactivation of CTCF during tumorigenesis. P21, a downstream effector of P53, has been found to be deregulated in different tumor types. This review provides a comprehensive overview of these factors that are known to deregulate the P53 pathway in both somatic and embryonic cells, as well as their malignant counterparts (i.e., somatic and germ cell tumors). It provides insights into which aspects still need to be unraveled to grasp their contribution to tumorigenesis, putatively leading to novel targets for effective cancer therapies.

Non-Canonical Functions of the ARF Tumor Suppressor in Development and Tumorigenesis

P14ARF (ARF; Alternative Reading Frame) is an extensively characterized tumor suppressor which, in response to oncogenic stimuli, mediates cell cycle arrest and apoptosis via p53-dependent and independent routes. ARF has been shown to be frequently lost through CpG island promoter methylation in a wide spectrum of human malignancies, such as colorectal, prostate, breast, and gastric cancers, while point mutations and deletions in the p14ARF locus have been linked with various forms of melanomas and glioblastomas. Although ARF has been mostly studied in the context of tumorigenesis, it has been also implicated in purely developmental processes, such as spermatogenesis, and mammary gland and ocular development, while it has been additionally involved in the regulation of angiogenesis. Moreover, ARF has been found to hold important roles in stem cell self-renewal and differentiation. As is often the case with tumor suppressors, ARF functions as a pleiotropic protein regulating a number of different mechanisms at the crossroad of development and tumorigenesis. Here, we provide an overview of the non-canonical functions of ARF in cancer and developmental biology, by dissecting the crosstalk of ARF signaling with key oncogenic and developmental pathways.

Diverse roles for CDK-associated activity during spermatogenesis

The primary function of cyclin-dependent kinases (CDKs) in complex with their activating cyclin partners is to promote mitotic division in somatic cells. This canonical cell cycle-associated activity is also crucial for fertility as it allows the proliferation and differentiation of stem cells within the reproductive organs to generate meiotically competent cells. Intriguingly, several CDKs exhibit meiosis-specific functions and are essential for the completion of the two reductional meiotic divisions required to generate haploid gametes. These meiosis-specific functions are mediated by both known CDK/cyclin complexes and meiosis-specific CDK-regulators and are important for a variety of processes during meiotic prophase. The majority of meiotic defects observed upon deletion of these proteins occur during the extended prophase I of the first meiotic division. Importantly a lack of redundancy is seen within the meiotic arrest phenotypes described for many of these proteins, suggesting intricate layers of cell cycle control are required for normal meiotic progression. Using the process of male germ cell development (spermatogenesis) as a reference, this review seeks to highlight the diverse roles of selected CDKs their activators, and their regulators during gametogenesis.

Testis-specific Arf promoter expression in a transposase-aided BAC transgenic mouse model

CDKN2A is an evolutionarily conserved gene encoding proteins implicated in tumor suppression, ocular development, aging, and metabolic diseases. Like the human form, mouse Cdkn2a encodes two distinct proteins-p16^Ink4a, which blocks cyclin-dependent kinase activity, and p19^Arf, which is best known as a positive regulator of the p53 tumor suppressor-and their functions have been well-studied in genetically engineered mouse models. Relatively little is known about how expression of the two transcripts is controlled in normal development and in certain disease states. To better understand their coordinate and transcript-specific expression in situ, we used a transposase-aided approach to generate a new BAC transgenic mouse model in which the first exons encoding Arf and Ink4a are replaced by fluorescent reporters. We show that mouse embryo fibroblasts generated from the transgenic lines faithfully display induction of each transgenic reporter in cell culture models, and we demonstrate the expected expression of the Arf reporter in the normal testis, one of the few places where that promoter is normally expressed. Interestingly, the TGFβ-2-dependent induction of the Arf reporter in the eye-a process essential for normal eye development-does not occur. Our findings illustrate the value of BAC transgenesis in mapping key regulatory elements in the mouse by revealing the genomic DNA required for Cdkn2a induction in cultured cells and the developing testis, and the apparent lack of elements driving expression in the developing eye.

Aging of spermatogonial stem cells by Jnk-mediated glycolysis activation

Because spermatogonial stem cells (SSCs) are immortal by serial transplantation, SSC aging in intact testes is considered to be caused by a deteriorated microenvironment. Here, we report a cell-intrinsic mode of SSC aging by glycolysis activation. Using cultured SSCs, we found that aged SSCs proliferated more actively than young SSCs and showed enhanced glycolytic activity. Moreover, they remained euploid and exhibited stable androgenetic imprinting patterns with robust SSC activity despite having shortened telomeres. Aged SSCs showed increased Wnt7b expression, which was associated with decreased Polycomb complex 2 activity. Our results suggest that aberrant Wnt7b expression activated c-jun N-terminal kinase (JNK), which down-regulated mitochondria numbers by suppressing Ppargc1a Down-regulation of Ppargc1a probably decreased reactive oxygen species and enhanced glycolysis. Analyses of the Klotho-deficient aging mouse model and 2-y-old aged rats confirmed JNK hyperactivation and increased glycolysis. Therefore, not only microenvironment but also intrinsic activation of JNK-mediated glycolysis contributes to SSC aging.

Dual Role of the Alternative Reading Frame ARF Protein in Cancer

The CDKN2a/ARF locus expresses two partially overlapping transcripts that encode two distinct proteins, namely p14ARF (p19Arf in mouse) and p16INK4a, which present no sequence identity. Initial data obtained in mice showed that both proteins are potent tumor suppressors. In line with a tumor-suppressive role, ARF-deficient mice develop lymphomas, sarcomas, and adenocarcinomas, with a median survival rate of one year of age. In humans, the importance of ARF inactivation in cancer is less clear whereas a more obvious role has been documented for p16INK4a. Indeed, many alterations in human tumors result in the elimination of the entire locus, while the majority of point mutations affect p16INK4a. Nevertheless, specific mutations of p14ARF have been described in different types of human cancers such as colorectal and gastric carcinomas, melanoma and glioblastoma. The activity of the tumor suppressor ARF has been shown to rely on both p53-dependent and independent functions. However, novel data collected in the last years has challenged the traditional and established role of this protein as a tumor suppressor. In particular, tumors retaining ARF expression evolve to metastatic and invasive phenotypes and in humans are associated with a poor prognosis. In this review, the recent evidence and the molecular mechanisms of a novel role played by ARF will be presented and discussed, both in pathological and physiological contexts.

Interplay between p53 and Ink4c in spermatogenesis and fertility

The tumor suppressor p53, and the cyclin-dependent kinase inhibitor Ink4c, have been both implicated in spermatogenesis control. Both p53-/- and Ink4c-/- single knockout male mice are fertile, despite testicular hypertrophy, Leydig cell differentiation defect, and increased sperm count in Ink4c-/- males. To investigate their collaborative roles, we studied p53-/- Ink4c-/- dual knockout animals, and found that male p53-/- Ink4c-/- mice have profoundly reduced fertility. Dual knockout male mice show a marked decrease in sperm count, abnormal sperm morphology and motility, prolongation of spermatozoa proliferation and delay of meiosis entry, and accumulation of DNA damage. Genetic studies showed that the effects of p53 loss on fertility are independent of its downstream effector Cdkn1a. Absence of p53 also partially reverses the hyperplasia seen upon Ink4c loss, and normalizes the Leydig cell differentiation defect. These results implicate p53 in mitigating both the delayed entry into meiosis and the secondary apoptotic response that occur in the absence of Ink4c. We conclude that the cell cycle genes p53 and Ink4c collaborate in sperm cell development and differentiation, and may be important candidates to investigate in human male infertility conditions.

Small mitochondrial Arf (smArf) protein corrects p53-independent developmental defects of Arf tumor suppressor-deficient mice

The mouse p19^Arf (human p14^ARF) tumor suppressor protein, encoded in part from an alternative reading frame of the Ink4a (Cdkn2a) gene, inhibits the Mdm2 E3 ubiquitin ligase to activate p53. Arf is not expressed in most normal tissues of young mice but is induced by high thresholds of aberrant hyperproliferative signals, thereby activating p53 in incipient tumor cells that have experienced oncogene activation. The single Arf mRNA encodes two distinct polypeptides, including full-length p19^Arf and N-terminally truncated and unstable p15^smArf ("small mitochondrial Arf") initiated from an internal in-frame AUG codon specifying methionine-45. Interactions of p19^Arf with Mdm2, or separately with nucleophosmin (NPM, B23) that localizes and stabilizes p19^Arf within the nucleolus, require p19^Arf N-terminal amino acids that are not present within p15^smArf We have generated mice that produce either smARF alone or M45A-mutated (smArf-deficient) full-length p19^Arf proteins. BCR-ABL-expressing pro/pre-B cells producing smArf alone are as oncogenic as their Arf-null counterparts in generating acute lymphoblastic leukemia when infused into unconditioned syngeneic mice. In contrast, smArf-deficient cells from mice of the Arf^M45A strain are as resistant as wild-type Arf^+/+ cells to comparable oncogenic challenge and do not produce tumors. Apart from being prone to tumor development, Arf-null mice are blind, and their male germ cells exhibit defects in meiotic maturation and sperm production. Although Arf^M45A mice manifest the latter defects, smArf alone remarkably rescues both of these p53-independent developmental phenotypes.

p53 and TAp63 participate in the recombination-dependent pachytene arrest in mouse spermatocytes

To protect germ cells from genomic instability, surveillance mechanisms ensure meiosis occurs properly. In mammals, spermatocytes that display recombination defects experience a so-called recombination-dependent arrest at the pachytene stage, which relies on the MRE11 complex—ATM—CHK2 pathway responding to unrepaired DNA double-strand breaks (DSBs). Here, we asked if p53 family members—targets of ATM and CHK2—participate in this arrest. We bred double-mutant mice combining a mutation of a member of the p53 family (p53, TAp63, or p73) with a Trip13 mutation. Trip13 deficiency triggers a recombination-dependent response that arrests spermatocytes in pachynema before they have incorporated the testis-specific histone variant H1t into their chromatin. We find that deficiency for either p53 or TAp63, but not p73, allowed spermatocytes to progress further into meiotic prophase despite the presence of numerous unrepaired DSBs. Even so, the double mutant spermatocytes apoptosed at late pachynema because of sex body deficiency; thus p53 and TAp63 are dispensable for arrest caused by sex body defects. These data affirm that recombination-dependent and sex body-deficient arrests occur via genetically separable mechanisms.

Meiosis is a specialized cell division that generates haploid gametes by halving chromosome content through two consecutive rounds of chromosome segregation. At the onset of the first meiotic division, SPO11 protein introduces double-strand breaks (DSBs) throughout the genome. These DSBs are repaired through homologous recombination, which promotes pairing and synapsis of the homologous chromosomes. Some DSBs will become repaired as crossovers, providing a physical connection between the homologous chromosomes which promotes correct chromosome segregation. In fact, recombination defects can lead to formation of aneuploid gametes, one of the major causes of miscarriages and chromosome abnormalities in humans. To protect germ cells from genomic instability and to produce balanced gametes, surveillance mechanisms ensure that meiosis occurs properly. It is known that in the presence of unrepaired DSBs a control mechanism promotes a spermatogenic block at the pachytene stage. Here we describe that, downstream MRE11-ATM-CHK2 pathway, p53 and TAp63 are the effectors responsible for activating recombination-dependent arrest in mouse spermatocytes.

p14ARF interacts with the focal adhesion kinase and protects cells from anoikis

The ARF protein functions as an important sensor of hyper-proliferative stimuli restricting cell proliferation through both p53-dependent and -independent pathways. Although to date the majority of studies on ARF have focused on its anti-proliferative role, few studies have addressed whether ARF may also have pro-survival functions. Here we show for the first time that during the process of adhesion and spreading ARF re-localizes to sites of active actin polymerization and to focal adhesion points where it interacts with the phosphorylated focal adhesion kinase. In line with its recruitment to focal adhesions, we observe that hampering ARF function in cancer cells leads to gross defects in cytoskeleton organization resulting in apoptosis through a mechanism dependent on the Death-Associated Protein Kinase. Our data uncover a novel function for p14ARF in protecting cells from anoikis that may reflect its role in anchorage independence, a hallmark of malignant tumor cells.

Tumor Promoting Aspects of Senescence in Cancer Progression

Cancers induced by gene mutation, deletion, and genome instability might be related to aging. With similar pathways of aging but distinct functions, senescence at the cellular level is an irreversible arrest of cell cycle. Senescence has long been believed as a barrier to restrict tumor expansion. However, more and more evidence has been shown that senescence inducers regulate epithelial-mesenchymal transition, stem cell self-renewal, inflammatory response, crosstalk with the oncogenic bypass signaling, and conversion of oncogene to tumor suppressor. Here we will discuss the most recent findings of the oncogenic aspects of senescence which crosstalk with multiple pathways in cancer progression.

Elimination of p19ARF-expressing cells enhances pulmonary function in mice

Senescent cells accumulate in many tissues as animals age and are considered to underlie several aging-associated pathologies. The tumor suppressors p19^ARF and p16^INK4a, both of which are encoded in the CDKN2A locus, play critical roles in inducing and maintaining permanent cell cycle arrest during cellular senescence. Although the elimination of p16^INK4a-expressing cells extends the life span of the mouse, it is unclear whether tissue function is restored by the elimination of senescent cells in aged animals and whether and how p19^ARF contributes to tissue aging. The aging-associated decline in lung function is characterized by an increase in compliance as well as pathogenic susceptibility to pulmonary diseases. We herein demonstrated that pulmonary function in 12-month-old mice was reversibly restored by the elimination of p19^ARF-expressing cells. The ablation of p19^ARF-expressing cells using a toxin receptor-mediated cell knockout system ameliorated aging-associated lung hypofunction. Furthermore, the aging-associated gene expression profile was reversed after the elimination of p19^ARF. Our results indicate that the aging-associated decline in lung function was, at least partly, attributed to p19^ARF and was recovered by eliminating p19^ARF-expressing cells.

Expressions of Sox9, Sox5, and Sox13 transcription factors in mice testis during postnatal development

SRY-related box (Sox) transcription factors are conserved among vertebrate species. These proteins regulate multiple processes including sex determination and testis differentiation of the male embryo. Although members of the Sox family have been identified in pre- and postnatal Sertoli cells, they have never been characterized in adult Leydig cells. The objectives of this research were to identify expressions of Sox9, Sox5, and Sox13 in mice Leydig cell cultures and to establish their expression profiles in postnatal mice testes at different developmental stages. Methods used include Western blots and qPCR of stimulated MA-10 cell cultures and whole mice testes. Sox9, Sox5, and Sox13 proteins were detected in MA-10 cells as well as whole mouse testis. Although Sox9, Sox5, and Sox13 mRNA levels from whole mice testes tended to increase according to postnatal development, these results were not significant. Sox members were also detected in whole mice testis by Western Blot. However, Sox9, Sox5, and Sox13 protein expressions remained relatively constant during postnatal development from postnatal (P) day 60 to P365. Being newly characterized in the mouse testis, Sox13 was mainly localized by immunofluorescence within the nuclei of cells from seminiferous tubules, possibly spermatocytes and Sertoli cells. In addition, Sox9, Sox5, and Sox13 proteins were characterized in the nuclei of MA-10 Leydig cell cultures. Their expressions and transcriptional activities remained unaffected by activators of the cAMP/PKA pathway. Thus, Sox9, Sox5, and Sox13 transcription factors are expressed in postnatal testis and may regulate multiple functions such as steroidogenesis and spermatogenesis.

The ATM signaling cascade promotes recombination-dependent pachytene arrest in mouse spermatocytes

Most mutations that compromise meiotic recombination or synapsis in mouse spermatocytes result in arrest and apoptosis at the pachytene stage of the first meiotic prophase. Two main mechanisms are thought to trigger arrest: one independent of the double-strand breaks (DSBs) that initiate meiotic recombination, and another activated by persistent recombination intermediates. Mechanisms underlying the recombination-dependent arrest response are not well understood, so we sought to identify factors involved by examining mutants deficient for TRIP13, a conserved AAA+ ATPase required for the completion of meiotic DSB repair. We find that spermatocytes with a hypomorphic Trip13 mutation (Trip13^mod/mod) arrest with features characteristic of early pachynema in wild type, namely, fully synapsed chromosomes without incorporation of the histone variant H1t into chromatin. These cells then undergo apoptosis, possibly in response to the arrest or in response to a defect in sex body formation. However, TRIP13-deficient cells that additionally lack the DSB-responsive kinase ATM progress further, reaching an H1t-positive stage (i.e., similar to mid/late pachynema in wild type) despite the presence of unrepaired DSBs. TRIP13-deficient spermatocytes also progress to an H1t-positive stage if ATM activity is attenuated by hypomorphic mutations in Mre11 or Nbs1 or by elimination of the ATM-effector kinase CHK2. These mutant backgrounds nonetheless experience an apoptotic block to further spermatogenic progression, most likely caused by failure to form a sex body. DSB numbers are elevated in Mre11 and Nbs1 hypomorphs but not Chk2 mutants, thus delineating genetic requirements for the ATM-dependent negative feedback loop that regulates DSB numbers. The findings demonstrate for the first time that ATM-dependent signaling enforces the normal pachytene response to persistent recombination intermediates. Our work supports the conclusion that recombination defects trigger spermatocyte arrest via pathways than are genetically distinct from sex body failure-promoted apoptosis and confirm that the latter can function even when recombination-dependent arrest is inoperative. Implications of these findings for understanding the complex relationships between spermatocyte arrest and apoptosis are discussed.

Meiosis is the specialized cell division by which haploid cells are produced. As germ cells enter the first meiotic prophase, programmed double-stranded breaks (DSBs) are formed throughout the genome. Repair of these DSBs by homologous recombination is crucial for proper segregation of homologous chromosomes at the end of the first meiotic division, and thus, for the production of haploid gametes. Moreover, failure to correctly repair these DSBs can have deleterious effects on the genomic integrity of offspring. To ensure that meiocytes that fail to repair meiotic DSBs do not complete meiosis, recombination is tightly controlled. However, the signaling pathway(s) tying meiotic recombination to meiotic progression in mouse spermatocytes is not known. We report here that the ATM-signaling pathway, composed of the MRE11 complex, ATM and CHK2, is responsible for activation of the recombination-dependent arrest that occurs in Trip13 mutant mouse spermatocytes, which accumulate unrepaired DSBs during meiotic prophase.

Roles of ubiquitination and SUMOylation on prostate cancer: mechanisms and clinical implications

The initiation and progression of human prostate cancer are highly associated with aberrant dysregulations of tumor suppressors and proto-oncogenes. Despite that deletions and mutations of tumor suppressors and aberrant elevations of oncogenes at the genetic level are reported to cause cancers, emerging evidence has revealed that cancer progression is largely regulated by posttranslational modifications (PTMs) and epigenetic alterations. PTMs play critical roles in gene regulation, cellular functions, tissue development, diseases, malignant progression and drug resistance. Recent discoveries demonstrate that ubiquitination and SUMOylation are complicated but highly-regulated PTMs, and make essential contributions to diseases and cancers by regulation of key factors and signaling pathways. Ubiquitination and SUMOylation pathways can be differentially modulated under various stimuli or stresses in order to produce the sustained oncogenic potentials. In this review, we discuss some new insights about molecular mechanisms on ubiquitination and SUMOylation, their associations with diseases, oncogenic impact on prostate cancer (PCa) and clinical implications for PCa treatment.

Repression of the soma-specific transcriptome by Polycomb-repressive complex 2 promotes male germ cell development

Using conditional mutagenesis to remove the core PRC2 subunits EED and SUZ12 during male germ cell development, Mu et al. identified a requirement for PRC2 in both mitotic and meiotic germ cells. Mutant spermatocytes exhibited ectopic expression of somatic lamins and an abnormal distribution of SUN1 proteins on the nuclear envelope. These defects were coincident with abnormal chromosome dynamics, affecting homologous chromosome pairing and synapsis.

Polycomb-repressive complex 2 (PRC2) catalyzes the methylation of histone H3 Lys27 (H3K27) and functions as a critical epigenetic regulator of both stem cell pluripotency and somatic differentiation, but its role in male germ cell development is unknown. Using conditional mutagenesis to remove the core PRC2 subunits EED and SUZ12 during male germ cell development, we identified a requirement for PRC2 in both mitotic and meiotic germ cells. We observed a paucity of mutant spermatogonial stem cells (SSCs), which appears independent of repression of the known cell cycle inhibitors Ink4a/Ink4b/Arf. Moreover, mutant spermatocytes exhibited ectopic expression of somatic lamins and an abnormal distribution of SUN1 proteins on the nuclear envelope. These defects were coincident with abnormal chromosome dynamics, affecting homologous chromosome pairing and synapsis. We observed acquisition of H3K27me3 on stage-specific genes during meiotic progression, indicating a requirement for PRC2 in regulating the meiotic transcriptional program. Together, these data demonstrate that transcriptional repression of soma-specific genes by PRC2 facilitates homeostasis and differentiation during mammalian spermatogenesis.

A short acidic motif in ARF guards against mitochondrial dysfunction and melanoma susceptibility

ARF is a small, highly basic protein that can be induced by oncogenic stimuli and exerts growth-inhibitory and tumour-suppressive activities through the activation of p53. Here we show that, in human melanocytes, ARF is cytoplasmic, constitutively expressed, and required for maintaining low steady-state levels of superoxide under conditions of mitochondrial dysfunction. This mitochondrial activity of ARF is independent of its known autophagic and p53-dependent functions, and involves the evolutionarily conserved acidic motif GHDDGQ, which exhibits weak homology to BCL-2 homology 3 (BH3) domains and mediates interaction with BCL-xL--an important regulator of mitochondrial redox homeostasis. Melanoma-predisposing CDKN2A germline mutations, which affect conserved glycine and aspartate residues within the GHDDGQ motif, impair the ability of ARF to control superoxide production and suppress growth of melanoma cells in vivo. These results reveal an important cell-protective function of ARF that links mitochondrial dysfunction and susceptibility to melanoma.

ARF: a versatile DNA damage response ally at the crossroads of development and tumorigenesis

Alternative reading frame (ARF) is a tumor suppressor protein that senses oncogenic and other stressogenic signals. It can trigger p53-dependent and -independent responses with cell cycle arrest and apoptosis induction being the most prominent ones. Other ARF activities, particularly p53-independent ones, that could help in understanding cancer development and provide potential therapeutic exploitation are underrated. Although ARF is generally not expressed in normal tissues, it is essential for ocular and male germ cells development. The underlying mechanism(s) in these processes, while not clearly defined, point toward a functional link between ARF, DNA damage and angiogenesis. Based on a recent study from our group demonstrating a functional interplay between ataxia-telangiectasia mutated (ATM) and ARF during carcinogenesis, we discuss the role of ARF at the crossroads of cancer and developmental processes.

Slug regulates E-cadherin repression via p19Arf in prostate tumorigenesis

SLUG represses E-cadherin to promote epithelial-mesenchymal transition (EMT) in various cancers. Mechanisms that regulate SLUG/E-cadherin pathway remain poorly understood, especially during tumorigenesis in vivo. Here we report that p19(Arf) (p14(ARF) in human) stabilizes Slug to inhibit E-cadherin in prostate cancer mouse models. Inactivation of p19(Arf) reduces Slug levels, resulting in increased E-cadherin expression and delaying the onset and progression of prostate cancer in Pten/Trp53 double null mice. Mechanistically, p14(ARF) stabilizes SLUG through increased sumoylation at lysine residue 192. Importantly, levels of SLUG and p14(ARF) are positively correlated in human prostate cancer specimens. These data demonstrated that ARF modulates the SLUG/E-cadherin signaling axis for augmenting prostate tumorigenesis in vivo, revealing a novel paradigm where the oncogenic functions of SLUG require ARF to target E-cadherin in prostate cancer. Collectively, our findings further support that ARF has dual tumor suppressive/oncogenic roles in cancers in a context-dependent manner.

ARF tumor suppression in the nucleolus

Since its discovery close to twenty years ago, the ARF tumor suppressor has played a pivotal role in the field of cancer biology. Elucidating ARF's basal physiological function in the cell has been the focal interest of numerous laboratories throughout the world for many years. Our current understanding of ARF is constantly evolving to include novel frameworks for conceptualizing the regulation of this critical tumor suppressor. As a result of this complexity, there is great need to broaden our understanding of the intricacies governing the biology of the ARF tumor suppressor. The ARF tumor suppressor is a key sensor of signals that instruct a cell to grow and proliferate and is appropriately localized in nucleoli to limit these processes. This article is part of a Special Issue entitled: Role of the Nucleolus in Human Disease.

Arf tumor suppressor and miR-205 regulate cell adhesion and formation of extraembryonic endoderm from pluripotent stem cells

Induction of the Arf tumor suppressor (encoded by the alternate reading frame of the Cdkn2a locus) following oncogene activation engages a p53-dependent transcriptional program that limits the expansion of incipient cancer cells. Although the p19(Arf) protein is not detected in most tissues of fetal or young adult mice, it is physiologically expressed in the fetal yolk sac, a tissue derived from the extraembryonic endoderm (ExEn). Expression of the mouse p19(Arf) protein marks late stages of ExEn differentiation in cultured embryoid bodies (EBs) derived from either embryonic stem cells or induced pluripotent stem cells. Arf inactivation delays differentiation of the ExEn lineage within EBs, but not the formation of other germ cell lineages from pluripotent progenitors. Arf is required for the timely induction of ExEn cells in response to Ras/Erk signaling and, in turn, acts through p53 to ensure the development, but not maintenance, of the ExEn lineage. Remarkably, a significant temporal delay in ExEn differentiation detected during the maturation of Arf-null EBs is rescued by enforced expression of mouse microRNA-205 (miR-205), a microRNA up-regulated by p19(Arf) and p53 that controls ExEn cell migration and adhesion. The noncanonical and canonical roles of Arf in ExEn development and tumor suppression, respectively, may be conceptually linked through mechanisms that govern cell attachment and migration.

Cooperation between Rb and Arf in suppressing mouse retinoblastoma

Retinoblastoma is a pediatric cancer that has served as a paradigm for tumor suppressor gene function. Retinoblastoma is initiated by RB gene mutations, but the subsequent cooperating mutational events leading to tumorigenesis are poorly characterized. We investigated what these additional genomic alterations might be using human retinoblastoma samples and mouse models. Array-based comparative genomic hybridization studies revealed deletions in the CDKN2A locus that include ARF and P16INK4A, both of which encode tumor suppressor proteins, in both human and mouse retinoblastoma. Through mouse genetic analyses, we found that Arf was the critical tumor suppressor gene in the deleted region. In mice, inactivation of one allele of Arf cooperated with Rb and p107 loss to rapidly accelerate retinoblastoma, with frequent loss of heterozygosity (LOH) at the Arf locus. Arf has been reported to exhibit p53-independent tumor suppressor roles in other systems; however, our results showed no additive effect of p53 and Arf coinactivation in promoting retinoblastoma. Moreover, p53 inactivation completely eliminated any selection for Arf LOH. Thus, our data reveal important insights into the p53 pathway in retinoblastoma and show that Arf is a key collaborator with Rb in retinoblastoma suppression.