Alternative reading frames of the INK4a tumor suppressor gene encode two unrelated proteins capable of inducing cell cycle arrest

Bmi1 Is a Key Epigenetic Barrier to Direct Cardiac Reprogramming

Direct reprogramming of induced cardiomyocytes (iCMs) suffers from low efficiency and requires extensive epigenetic repatterning, although the underlying mechanisms are largely unknown. To address these issues, we screened for epigenetic regulators of iCM reprogramming and found that reducing levels of the polycomb complex gene Bmi1 significantly enhanced induction of beating iCMs from neonatal and adult mouse fibroblasts. The inhibitory role of Bmi1 in iCM reprogramming is mediated through direct interactions with regulatory regions of cardiogenic genes, rather than regulation of cell proliferation. Reduced Bmi1 expression corresponded with increased levels of the active histone mark H3K4me3 and reduced levels of repressive H2AK119ub at cardiogenic loci, and de-repression of cardiogenic gene expression during iCM conversion. Furthermore, Bmi1 deletion could substitute for Gata4 during iCM reprogramming. Thus, Bmi1 acts as a critical epigenetic barrier to iCM production. Bypassing this barrier simplifies iCM generation and increases yield, potentially streamlining iCM production for therapeutic purposes.

A variant at 9p21.3 functionally implicates CDKN2B in paediatric B-cell precursor acute lymphoblastic leukaemia aetiology

Paediatric B-cell precursor acute lymphoblastic leukaemia (BCP-ALL) is the most common cancer of childhood, yet little is known about BCP-ALL predisposition. In this study, in 2,187 cases of European ancestry and 5,543 controls, we discover and replicate a locus indexed by rs77728904 at 9p21.3 associated with BCP-ALL susceptibility (Pcombined=3.32 × 10(-15), OR=1.72) and independent from rs3731217, the previously reported ALL-associated variant in this region. Of correlated SNPs tagged by this locus, only rs662463 is significant in African Americans, suggesting it is a plausible causative variant. Functional analysis shows that rs662463 is a cis-eQTL for CDKN2B, with the risk allele associated with lower expression, and suggests that rs662463 influences BCP-ALL risk by regulating CDKN2B expression through CEBPB signalling. Functional analysis of rs3731217 suggests it is associated with BCP-ALL by acting within a splicing regulatory element determining CDKN2A exon 3 usage (P=0.01). These findings provide new insights into the critical role of the CDKN2 locus in BCP-ALL aetiology.

The role of structural disorder in cell cycle regulation, related clinical proteomics, disease development and drug targeting

Understanding the molecular mechanisms of the regulation of cell cycle is a central issue in molecular cell biology, due to its fundamental role in the existence of cells. The regulatory circuits that make decisions on when a cell should divide are very complex and particularly subtly balanced in eukaryotes, in which the harmony of many different cells in an organism is essential for life. Several hundred proteins are involved in these processes, and a great deal of studies attests that most of them have functionally relevant intrinsic structural disorder. Structural disorder imparts many functional advantages on these proteins, and we discuss it in detail that it is involved in all key steps from signaling through the cell membrane to regulating transcription of proteins that execute timely responses to an ever-changing environment.

Death of a dogma: eukaryotic mRNAs can code for more than one protein

mRNAs carry the genetic information that is translated by ribosomes. The traditional view of a mature eukaryotic mRNA is a molecule with three main regions, the 5' UTR, the protein coding open reading frame (ORF) or coding sequence (CDS), and the 3' UTR. This concept assumes that ribosomes translate one ORF only, generally the longest one, and produce one protein. As a result, in the early days of genomics and bioinformatics, one CDS was associated with each protein-coding gene. This fundamental concept of a single CDS is being challenged by increasing experimental evidence indicating that annotated proteins are not the only proteins translated from mRNAs. In particular, mass spectrometry (MS)-based proteomics and ribosome profiling have detected productive translation of alternative open reading frames. In several cases, the alternative and annotated proteins interact. Thus, the expression of two or more proteins translated from the same mRNA may offer a mechanism to ensure the co-expression of proteins which have functional interactions. Translational mechanisms already described in eukaryotic cells indicate that the cellular machinery is able to translate different CDSs from a single viral or cellular mRNA. In addition to summarizing data showing that the protein coding potential of eukaryotic mRNAs has been underestimated, this review aims to challenge the single translated CDS dogma.

Nuclear trafficking of EGFR by Vps34 represses Arf expression to promote lung tumor cell survival

Epidermal growth factor receptor (EGFR) is a cell surface receptor that has an essential role in cell proliferation and survival, and overexpression of EGFR is a common feature of human cancers. In Non-small-cell lung cancer (NSCLC), activating mutations of EGFR have also been described. We recently showed that mutant EGFR-L858R inhibits the expression of the p14ARF tumor-suppressor protein to promote cell survival. In this study, we defined the molecular bases by which EGFR controls Arf expression. Using various lung tumor models, we showed that EGF stimulation inhibits Arf transcription by a mechanism involving the nuclear transport and recruitment of EGFR to the Arf promoter. We unraveled the vesicular trafficking protein Vps34 as a mediator of EGFR nuclear trafficking and showed that its neutralization prevents the accumulation of EGFR to the Arf promoter in response to ligand activation. Finally, in lung tumor cells that carry mutant EGFR-L858R, we demonstrated that inhibition of Vps34 using small interfering RNA restrains nuclear EGFR location and restores Arf expression leading to apoptosis. These findings identify the Arf tumor suppressor as a new transcriptional target of nuclear EGFR and highlight Vps34 as an important regulator of the nuclear EGFR/Arf survival pathway. As a whole, they provide a mechanistic explanation to the inverse correlation between nuclear expression of EGFR and overall survival in NSCLC patients.

Physiological Expression and Accumulation of the Products of Two Upstream Open Reading Frames mrtl and MycHex1 Along With p64 and p67 Myc From the Human c-myc Locus

In addition to the canonical c-Myc p64 and p67 proteins, the human c-myc locus encodes two distinct proteins, mrtl (myc-related translation/localization regulatory factor) and MycHex1 (Myc Human Exon 1), from the upstream open reading frames within the 5'-untranslated region of the c-myc P0 mRNA. The aim of this study is to examine simultaneously, for the first time, mrtl, MycHex1, c-Myc p64, and p67 in human tumor cell lines and pediatric brain tumor tissues. Western blot analysis demonstrated endogenous mrtl, MycHex1, c-Myc p64, and p67 simultaneously. The relative abundance of mrtl and MycHex1 were consistent among a variety of human tumor cell lines, and the relative intensities of mrtl and MycHex1 correlated positively. Confocal imaging revealed mrtl predominantly localized to the nuclear envelope, along with prominent reticular pattern in the cytoplasm. MycHex1 was observed as a series of bright foci located within the nucleus, a subset of which colocalized with fibrillarin. mrtl and MycHex1 co-immunoprecipitated with RACK1, c-Myc, fibrillarin, coilin, and with each other. These findings suggest that mrtl and MycHex1 have multiple interaction partners in both the nucleus and cytoplasm. Sequence analyses confirmed a known polymorphism of mrtl at base 1965 (G>T) and new mutations at bases 1900 (C>G) and 1798 (C>G). Evidence is presented for expression and stable accumulation of all four proteins encoded by three distinct non-overlapping open reading frames within the human c-myc locus. Additional work is warranted to further elucidate the functional or regulatory roles of these molecules in regulation of c-Myc and in oncogenesis.

Clinicopathological and Targeted Exome Gene Features of a Patient with Metastatic Acinic Cell Carcinoma of the Parotid Gland Harboring an ARID2 Nonsense Mutation and CDKN2A/B Deletion

We describe the presentation, treatment, clinical outcome, and targeted genome analysis of a metastatic salivary acinic cell carcinoma (AciCC). A 71-year-old male presented with a 3 cm right tail of a parotid lesion, first detected as a nodule by the patient seven months earlier. He had a right total parotidectomy with cranial nerve VII resection, right facial nerve resection and grafting, resection of the right conchal cartilage, and right modified radical neck dissection. The primary tumor revealed AciCC with two distinct areas: a well-differentiated component with glandular architecture and a dedifferentiated component with infiltrative growth pattern associated with prominent stromal response, necrosis, perineural invasion, and cellular pleomorphism. Tumor staging was pT4 N0 MX. Immunohistochemistry staining showed pankeratin (+), CD56 (−), and a Ki67 proliferation index of 15%. Upon microscopic inspection, 49 local lymph nodes resected during parotidectomy were negative for cancer cells. Targeted sequencing of the primary tumor revealed deletions of CDKN2A and CDKN2B, a nonsense mutation in ARID2, and single missense mutations of unknown significance in nine other genes. Despite postoperative localized radiation treatment, follow-up whole body PET/CT scan showed lung, soft tissue, bone, and liver metastases. The patient expired 9 months after resection of the primary tumor.

Liver Sinusoidal Endothelial Cells Escape Senescence by Loss of p19ARF

Liver sinusoidal endothelial cells (LSECs) represent a highly differentiated cell type that lines hepatic sinusoids. LSECs form a discontinuous endothelium due to fenestrations under physiological conditions, which are reduced upon chronic liver injury. Cultivation of rodent LSECs associates with a rapid onset of stress-induced senescence a few days post isolation, which limits genetic and biochemical studies ex vivo. Here we show the establishment of LSECs isolated from p19^ARF-/- mice which undergo more than 50 cell doublings in the absence of senescence. Isolated p19^ARF-/- LSECs display a cobblestone-like morphology and show the ability of tube formation. Analysis of DNA content revealed a stable diploid phenotype after long-term passaging without a gain of aneuploidy. Notably, p19^ARF-/- LSECs express the endothelial markers CD31, vascular endothelial growth factor receptor (VEGFR)-2, VE-cadherin, von Willebrand factor, stabilin-2 and CD146 suggesting that these cells harbor and maintain an endothelial phenotype. In line, treatment with small molecule inhibitors against VEGFR-2 caused cell death, demonstrating the sustained ability of p19^ARF-/- LSECs to respond to anti-angiogenic therapeutics. From these data we conclude that loss of p19^ARF overcomes senescence of LSECs, allowing immortalization of cells without losing endothelial characteristics. Thus, p19^ARF-/- LSECs provide a novel cellular model to study endothelial cell biology.

Post-translational Down-regulation of Melanoma Antigen-A11 (MAGE-A11) by Human p14-ARF Tumor Suppressor

X-linked primate-specific melanoma antigen-A11 (MAGE-A11) is a human androgen receptor (AR) coactivator and proto-oncogene expressed at low levels in normal human reproductive tract tissues and at higher levels in castration-resistant prostate cancer where it is required for androgen-dependent cell growth. In this report, we show that MAGE-A11 is targeted for degradation by human p14-ARF, a tumor suppressor expressed from an alternative reading frame of the p16 cyclin-dependent kinase inhibitor INK4a/ARF gene. MAGE-A11 degradation by the proteasome was mediated by an interaction with p14-ARF and was independent of lysine ubiquitination. A dose-dependent inverse relationship between MAGE-A11 and p14-ARF correlated with p14-ARF inhibition of the MAGE-A11-induced increase in androgen-dependent AR transcriptional activity and constitutive activity of a splice variant-like AR. Reciprocal stabilization between MAGE-A11 and AR did not protect against degradation promoted by p14-ARF. p14-ARF prevented MAGE-A11 interaction with the E2F1 oncoprotein and inhibited the MAGE-A11-induced increase in E2F1 transcriptional activity. Post-translational down-regulation of MAGE-A11 promoted by p14-ARF was independent of HDM2, the human homologue of mouse double minute 2, an E3 ubiquitin ligase inhibited by p14-ARF. However, MAGE-A11 had a stabilizing effect on HDM2 in the absence or presence of p14-ARF and cooperated with HDM2 to increase E2F1 transcriptional activity in the absence of p14-ARF. We conclude that degradation of MAGE-A11 promoted by the human p14-ARF tumor suppressor contributes to low levels of MAGE-A11 in nontransformed cells and that higher levels of MAGE-A11 associated with low p14-ARF increase AR and E2F1 transcriptional activity and promote the development of castration-resistant prostate cancer.

Nucleolar stress with and without p53

A veritable explosion of primary research papers within the past 10 years focuses on nucleolar and ribosomal stress, and for good reason: with ribosome biosynthesis consuming ~80% of a cell’s energy, nearly all metabolic and signaling pathways lead ultimately to or from the nucleolus. We begin by describing p53 activation upon nucleolar stress resulting in cell cycle arrest or apoptosis. The significance of this mechanism cannot be understated, as oncologists are now inducing nucleolar stress strategically in cancer cells as a potential anti-cancer therapy. We also summarize the human ribosomopathies, syndromes in which ribosome biogenesis or function are impaired leading to birth defects or bone narrow failures; the perplexing problem in the ribosomopathies is why only certain cells are affected despite the fact that the causative mutation is systemic. We then describe p53-independent nucleolar stress, first in yeast which lacks p53, and then in other model metazoans that lack MDM2, the critical E3 ubiquitin ligase that normally inactivates p53. Do these presumably ancient p53-independent nucleolar stress pathways remain latent in human cells? If they still exist, can we use them to target >50% of known human cancers that lack functional p53?

Intratumor heterogeneity and transcriptional profiling in glioblastoma: translational opportunities

The study of phenotypic and genetic intratumor heterogeneity in glioblastoma is attracting a lot of attention. Recent studies have demonstrated that transcriptional profiling analysis can help interpret the complexity of this disease. Previously proposed molecular classifiers have been recently challenged due to the unexpected degree of intratumor heterogeneity that has been described spatially and at single-cell level. Different computational methods have been employed to analyze this huge amount of data, but new experimental designs including multisampling from individual patients and single-cell experiments require new specific approaches. In light of these results, there is hope that integration of genetic, phenotypic and transcriptional data coupled with functional experiments might help define new therapeutic strategies and classify patients according to key pathways and molecular targets that can be further investigated to develop personalized and combinatorial treatment strategies.

Forging a signature of in vivo senescence

'Cellular senescence', a term originally defining the characteristics of cultured cells that exceed their replicative limit, has been broadened to describe durable states of proliferative arrest induced by disparate stress factors. Proposed relationships between cellular senescence, tumour suppression, loss of tissue regenerative capacity and ageing suffer from lack of uniform definition and consistently applied criteria. Here, we highlight caveats in interpreting the importance of suboptimal senescence-associated biomarkers, expressed either alone or in combination. We advocate that more-specific descriptors be substituted for the now broadly applied umbrella term 'senescence' in defining the suite of diverse physiological responses to cellular stress.

Establishment and mutation analysis of a novel malignant peritoneal mesothelioma cell line, TU-MM-1, using whole genome sequencing

A new cell line of human malignant peritoneal mesothelioma (MPM), TU-MM-1, was established and characterized. The cells showed polygonal morphology, grew in monolayers without contact inhibition and were arranged like a jigsaw puzzle. The chromosome numbers ranged from 41 to 44. A low rate of proliferation was observed and the doubling time was 67.9 h. Genomic DNA sequencing revealed that TU-MM-1 cells harbored missense mutations in APC, LATS2, BRCA1/2, and TP53, and mutation of a splice donor site in BAP1 and loss of CDKN2A gene. We observed the absence of BAP1 and p16(INK4a) proteins, underexpression of LATS2 protein, and overexpression of p53 protein in TU-MM-1 cells in western blot analysis. Heterotransplantation to nude mice produced tumors that had the characteristics of the original tumor. This cell line may be useful for studying biological properties and contribute to novel treatment strategies.

EZH2 expression in gliomas: Correlation with CDKN2A gene deletion/ p16 loss and MIB-1 proliferation index

Enhancer of zeste homolog 2 (EZH2) mediated down-regulation of CDKN2A/p16 has been observed in cell lines as well as in a few carcinomas. However, there is no study correlating EZH2 expression with CDKN2A/p16 status in gliomas. Hence, the present study was conducted to evaluate EZH2 expression in astrocytic and oligodendroglial tumors and correlate with CDKN2A/p16 status as well as MIB-1 labeling index (LI). Gliomas of all grades (n = 118) were studied using immunohistochemistry to assess EZH2, p16 and MIB-1 LI and fluorescence in situ hybrization to evaluate CDKN2A gene status. EZH2 expression and CDKN2A homozygous deletion (HD) were both significantly more frequent in high-grade gliomas (HGG). Further, strong EZH2 expression (LI ≥ 25%) was significantly more common in HGGs without CDKN2A HD (48.7%; 19/39) as compared to cases with deletion (15.8%; 3/19). Loss of p16 expression was noted in 100% and 51.3% of CDKN2A deleted and non-deleted tumors, respectively. Notably, 80% (16/20) of the CDKN2A non-deleted HGGs with p16 loss had strong EZH2 expression, in contrast to only 15.8% (3/19) in the deleted group. Loss of p16 expression significantly correlated with MIB-1 LI, irrespective of EZH2 status. Thus, this study shows that EZH2 expression correlates with tumor grade in both astrocytic and oligodendroglial tumors and hence can be used as a diagnostic marker to differentiate between low and HGGs. Further, this is the first report demonstrating an inverse correlation of strong EZH2 expression with CDKN2A HD in HGGs. Loss of p16 protein expression is mostly attributable to CDKN2A HD and correlates significantly with MIB-1 LI. Notably, our study for the first time suggests a possible epigenetic mechanism of p16 loss in CDKN2A non-deleted HGGs mediated by strong EZH2 expression. A hypothetical model for control of proliferative activity in low versus HGGs is therefore proposed.

Tumoral TP53 and/or CDKN2A alterations are not reliable prognostic biomarkers in patients with localized Ewing sarcoma: a report from the Children's Oncology Group

BACKGROUND

A growing collection of retrospective studies have suggested that TP53 mutations and/or CDKN2A deletions have prognostic significance in Ewing sarcoma. We sought to evaluate these variables in patients with localized disease treated prospectively on a single Children's Oncology Group protocol.

PROCEDURE

Of the 568 patients enrolled on Children's Oncology Group protocol AEWS0031 (NCT00006734), 112 had tumor specimens of sufficient quality and quantity to allow for analysis of TP53 mutations status by DNA sequencing, and CDKN2A deletion by dual color fluorescent in situ hybridization.

RESULTS

Eight of 93 cases (8.6%) were found to have TP53 point mutations and 12 of 107 cases (11.2%) demonstrated homozygous CDKN2A deletion. Two cases were found to have an alteration in both genes. There was no significant difference in event-free survival of patients with TP53 mutations and/or CDKN2A deletions compared to patients with normal TP53/CDKN2A gene status, as demonstrated by log rank test (p = 0.58).

CONCLUSIONS

Although previous retrospective studies suggest their significance, TP53 mutation and/or CDKN2A deletion are not reliable prognostic biomarkers in localized Ewing sarcoma.

DNA Methylation Levels of Melanoma Risk Genes Are Associated with Clinical Characteristics of Melanoma Patients

In melanoma development, oncogenic process is mediated by genetic and epigenetic mutations, and few studies have so far explored the role of DNA methylation either as predisposition factor or biomarker. We tested patient samples for germline CDKN2A methylation status and found no evidence of inactivation by promoter hypermethylation. We have also investigated the association of clinical characteristics of samples with the DNA methylation pattern of twelve genes relevant for melanomagenesis. Five genes (BAP1, MGMT, MITF, PALB2, and POT1) presented statistical association between blood DNA methylation levels and either CDKN2A-mutation status, number of lesions, or Breslow thickness. In tumors, five genes (KIT, MGMT, MITF, TERT, and TNF) exhibited methylation levels significantly different between tumor groups including acral compared to nonacral melanomas and matched primary lesions and metastases. Our data pinpoint that the methylation level of eight melanoma-associated genes could potentially represent markers for this disease both in peripheral blood and in tumor samples.

Murine double minute 2 predicts response of advanced esophageal squamous cell carcinoma to definitive chemoradiotherapy

BACKGROUND

Definitive chemoradiotherapy (dCRT) has recently become one of the most effective therapies for the treatment of esophageal squamous cell carcinoma (ESCC). However, it is also true this treatment has not been effective in all patients. Therefore, it is very important to evaluate the surrogate marker of dCRT in order to improve clinical outcomes of patients with ESCC. On the other hand, our previous study had suggested that murine double minute 2 (MDM2) and p16 were associated with chemoradioresistance in ESCC.

METHODS

We selected pretreatment biopsy specimens of ESCC patients from our prospective clinical study on dCRT. Seventy-nine cases histologically diagnosed as ESCC were used. We immunohistochemically investigated these specimens using antibodies against MDM2, p53, p16, and Ki-67.

RESULTS

The patients included 68 males and 11 females with a mean age of 63.3 years. The number of patients in each clinical stage was as follows: 22 in c-Stage I; 17 in c-Stage II; and 40 in c-Stage III. cT, cN, and cStage were significantly more advanced in the Failure group (including patients with persistent and recurrent disease after dCRT) than in the complete response (CR) group (patients with persistent CR after dCRT). The clinical stage inversely correlated with the CR rate and the rescue rate after failure. The overall survival rate was significantly worse in the patients with advanced cT, cN, and cStage levels, and in the Failure group. MDM2 positivity was significantly higher in the Failure group than in the CR group in cStageIII (P = 0.014). The number of patients with an absence of p16 immunoreactivity was significantly higher in the Failure group than in the CR group in cStageIII (P = 0.010) but not in cStageI or cStageII. Moreover, the overall survival with a Ki-67 ≥ 33.7% was significantly better than that with <33.7% for patients in cStageIII (P = 0.024).

CONCLUSIONS

The results of this study suggested that MDM2 and p16 are predictive markers for chemoradioresistance in cStageIII ESCC and Ki-67 is a prognostic marker following dCRT in cStageIII ESCC. These issues could contribute to the formulation of treatment strategy for patients with advanced ESCC.

The BRAF kinase domain promotes the development of gliomas in vivo

In-frame BRAF fusions have been observed in over 80% of sporadic pilocytic astrocytomas. In each fusion, the N-terminal autoinhibitory domain of BRAF is lost, which results in constitutive activation via the retained C-terminal kinase domain (BRAF-KD). We set out to determine if the BRAF-KD is sufficient to induce gliomas alone or in combination with Ink4a/Arf loss. Syngeneic cell lines demonstrated the transforming ability of the BRAF-KD following Ink4a/Arf loss. In vivo, somatic cell gene transfer of the BRAF-KD did not cause tumors to develop; however, gliomas were detected in 21% of the mice following Ink4a/Arf loss. Interestingly, these mice demonstrated no obvious symptoms. Histologically the tumors were highly cellular and atypical, similar to BRAF^V600E tumors reported previously, but with less invasive borders. They also lacked the necrosis and vascular proliferation seen in BRAFV600E-driven tumors. The BRAF-KD-expressing astrocytes showed elevated MAPK signaling, albeit at reduced levels compared to the BRAF^V600E mutant. Pharmacologic inhibition of MEK and PI3K inhibited cell growth and induced apoptosis in astrocytes expressing BRAF-KD. Our findings demonstrate that the BRAF-KD can cooperate with Ink4a/Arf loss to drive the development of gliomas and suggest that glioma development is determined by the level of MAPK signaling.

Transposable element-driven transcript diversification and its relevance to genetic disorders

The human genome project and subsequent gene annotation projects have shown that the human genome contains 22,000-25,000 functional genes. Therefore, it is believed that the diversity of protein repertoire is achieved by the alternative splicing (AS) mechanism. Transposable elements (TEs) are mobile in nature and can therefore alter their position in the genome. The insertion of TEs into a new gene region can result in AS of a particular transcript through various mechanisms, including intron retention, and alternative donor or acceptor splice sites. TE-derived AS is thought to have played a part in primate evolution and in hominid radiation. However, TE-derived AS or genetic instability may sometimes result in genetic disorders. For the past two decades, numerous studies have been performed on TEs and their role in genomes. Accumulating evidence shows that the term 'junk DNA', previously used for TEs is a misnomer. Recent research has indicated that TEs may have clinical potential. However, to explore the feasibility of using TEs in clinical practice, additional studies are required. This review summarizes the available literature on TE-derived AS, alternative promoter, and alternative polyadenylation. The review covers the effects of TEs on coding genes and their clinical implications, and provides our perspectives and directions for future research.

Intracellular CD24 disrupts the ARF-NPM interaction and enables mutational and viral oncogene-mediated p53 inactivation

CD24 is overexpressed in nearly 70% human cancers, whereas TP53 is the most frequently mutated tumour-suppressor gene that functions in a context-dependent manner. Here we show that both targeted mutation and short hairpin RNA (shRNA) silencing of CD24 retard the growth, progression and metastasis of prostate cancer. CD24 competitively inhibits ARF binding to NPM, resulting in decreased ARF, increase MDM2 and decrease levels of p53 and the p53 target p21/CDKN1A. CD24 silencing prevents functional inactivation of p53 by both somatic mutation and viral oncogenes, including the SV40 large T antigen and human papilloma virus 16 E6-antigen. In support of the functional interaction between CD24 and p53, in silico analyses reveal that TP53 mutates at a higher rate among glioma and prostate cancer samples with higher CD24 mRNA levels. These data provide a general mechanism for functional inactivation of ARF and reveal an important cellular context for genetic and viral inactivation of TP53.

P53 is a tumour suppressor that is frequently mutated or downregulated in cancer. Here, Wang et al. show that CD24, a molecule frequently overexpressed in cancer, promotes p53 degradation by disrupting a regulatory ARF–MDM2 interaction, and silencing CD24 prevents the downregulation of p53.

INK4 locus of the tumor-resistant rodent, the naked mole rat, expresses a functional p15/p16 hybrid isoform

The naked mole rat (Heterocephalus glaber) is a long-lived and tumor-resistant rodent. Tumor resistance in the naked mole rat is mediated by the extracellular matrix component hyaluronan of very high molecular weight (HMW-HA). HMW-HA triggers hypersensitivity of naked mole rat cells to contact inhibition, which is associated with induction of the INK4 (inhibitors of cyclin dependent kinase 4) locus leading to cell-cycle arrest. The INK4a/b locus is among the most frequently mutated in human cancer. This locus encodes three distinct tumor suppressors: p15(INK4b), p16(INK4a), and ARF (alternate reading frame). Although p15(INK4b) has its own ORF, p16(INK4a) and ARF share common second and third exons with alternative reading frames. Here, we show that, in the naked mole rat, the INK4a/b locus encodes an additional product that consists of p15(INK4b) exon 1 joined to p16(INK4a) exons 2 and 3. We have named this isoform pALT(INK4a/b) (for alternative splicing). We show that pALT(INK4a/b) is present in both cultured cells and naked mole rat tissues but is absent in human and mouse cells. Additionally, we demonstrate that pALT(INK4a/b) expression is induced during early contact inhibition and upon a variety of stresses such as UV, gamma irradiation-induced senescence, loss of substrate attachment, and expression of oncogenes. When overexpressed in naked mole rat or human cells, pALT(INK4a/b) has stronger ability to induce cell-cycle arrest than either p15(INK4b) or p16(INK4a). We hypothesize that the presence of the fourth product, pALT(INK4a/b) of the INK4a/b locus in the naked mole rat, contributes to the increased resistance to tumorigenesis of this species.

Cooperation of DLC1 and CDK6 affects breast cancer clinical outcome

Low DLC1 expression is found to frequently co-occur with aberrant expression of cell cycle genes including CDK6 in human lung and colon cancer. Here, we explore the influence of the synergistic effect of DLC1 and CDK6 on human breast cancer survival at the genetic, transcriptional, and translational levels. We found that high DLC1 and low CDK6 expression are associated with good prognosis. The DLC1 intronic SNP rs561681 is found to fit a recessive model, complying with the tumor suppressive role of DLC1. The heterozygote of the DLC1 SNP is found to increase the hazard when the CDK6 intronic SNP rs3731343 is rare homozygous, and it becomes protective when rs3731343 is common homozygous. We propose that DLC1 expression is the lowest in patients harboring the rare homozygote of rs561681 and functional DLC1 is the lowest when rs561681 is heterozygous and rs3731343 is rare homozygous. We are the first to report such synergistic effects of DLC1 and CDK6 on breast cancer survival at the transcriptional level, the overdominant model fitted by the SNP pair, and the dominant negative effect at the translational level. These findings link the germline genetic polymorphisms and synergistic effect of DLC1 and CDK6 with breast cancer progression, which provide the basis for experimentally elucidating the mechanisms driving differential tumor progression and avail in tailoring the clinical treatments for such patients based on their genetic susceptibility.

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.

p53 Family and Cellular Stress Responses in Cancer

p53 is an important tumor suppressor gene, which is stimulated by cellular stress like ionizing radiation, hypoxia, carcinogens, and oxidative stress. Upon activation, p53 leads to cell-cycle arrest and promotes DNA repair or induces apoptosis via several pathways. p63 and p73 are structural homologs of p53 that can act similarly to the protein and also hold functions distinct from p53. Today more than 40 different isoforms of the p53 family members are known. They result from transcription via different promoters and alternative splicing. Some isoforms have carcinogenic properties and mediate resistance to chemotherapy. Therefore, expression patterns of the p53 family genes can offer prognostic information in several malignant tumors. Furthermore, the p53 family constitutes a potential target for cancer therapy. Small molecules (e.g., Nutlins, RITA, PRIMA-1, and MIRA-1 among others) have been objects of intense research interest in recent years. They restore pro-apoptotic wild-type p53 function and were shown to break chemotherapeutic resistance. Due to p53 family interactions small molecules also influence p63 and p73 activity. Thus, the members of the p53 family are key players in the cellular stress response in cancer and are expected to grow in importance as therapeutic targets.

RABL6A promotes G1-S phase progression and pancreatic neuroendocrine tumor cell proliferation in an Rb1-dependent manner

Mechanisms of neuroendocrine tumor (NET) proliferation are poorly understood, and therapies that effectively control NET progression and metastatic disease are limited. We found amplification of a putative oncogene, RABL6A, in primary human pancreatic NETs (PNET) that correlated with high-level RABL6A protein expression. Consistent with those results, stable silencing of RABL6A in cultured BON-1 PNET cells revealed that it is essential for their proliferation and survival. Cells lacking RABL6A predominantly arrested in G1 phase with a moderate mitotic block. Pathway analysis of microarray data suggested activation of the p53 and retinoblastoma (Rb1) tumor-suppressor pathways in the arrested cells. Loss of p53 had no effect on the RABL6A knockdown phenotype, indicating that RABL6A functions independent of p53 in this setting. By comparison, Rb1 inactivation partially restored G1 to S phase progression in RABL6A-knockdown cells, although it was insufficient to override the mitotic arrest and cell death caused by RABL6A loss. Thus, RABL6A promotes G1 progression in PNET cells by inactivating Rb1, an established suppressor of PNET proliferation and development. This work identifies RABL6A as a novel negative regulator of Rb1 that is essential for PNET proliferation and survival. We suggest RABL6A is a new potential biomarker and target for anticancer therapy in PNET patients.

Human papillomavirus induced transformation in cervical and head and neck cancers

Human papillomavirus (HPV) is one of the most widely publicized and researched pathogenic DNA viruses. For decades, HPV research has focused on transforming viral activities in cervical cancer. During the past 15 years, however, HPV has also emerged as a major etiological agent in cancers of the head and neck, in particular squamous cell carcinoma. Even with significant strides achieved towards the screening and treatment of cervical cancer, and preventive vaccines, cervical cancer remains the leading cause of cancer-associated deaths for women in developing countries. Furthermore, routine screens are not available for those at risk of head and neck cancer. The current expectation is that HPV vaccination will prevent not only cervical, but also head and neck cancers. In order to determine if previous cervical cancer models for HPV infection and transformation are directly applicable to head and neck cancer, clinical and molecular disease aspects must be carefully compared. In this review, we briefly discuss the cervical and head and neck cancer literature to highlight clinical and genomic commonalities. Differences in prognosis, staging and treatment, as well as comparisons of mutational profiles, viral integration patterns, and alterations in gene expression will be addressed.

Krüppel-like factor 6 interferes with cellular transformation induced by the H-ras oncogene

KLF6 is a member of the Krüppel-like factor family of transcription factors, with diverse roles in the regulation of cell physiology, including proliferation, signal transduction, and apoptosis. Mutations or down-regulation of KLF6 have been described in several human cancers. In this work, we found that KLF6-knockdown resulted in the formation of transformed foci and allowed the spontaneous conversion of NIH3T3 cells to a tumorigenic state. We further assessed the role of KLF6 in the context of oncogenic Ras. We showed that KLF6 was up-regulated by H-Ras(G12V) expression in a Jun N-terminal kinase (JNK)-dependent manner, correlated with enhanced klf6 promoter activity. We found that ectopic KLF6 expression induced a G1-phase cell cycle arrest, thereby decreasing the cell proliferation rate. In addition, constitutive KLF6 expression impaired H-Ras(G12V)-mediated loss of density-dependent growth inhibition and anchorage-independent growth. Moreover, growth of H-Ras(G12V)-driven tumors was reduced in mice challenged with cells stably expressing KLF6. KLF6 expression correlated with the up-regulation of p21, whereas neither p53 induction nor apoptotic cell death was detected. Further, p21 knockdown impaired KLF6-induced cell cycle arrest. These findings provide novel evidence highlighting KLF6 function in response to malignant transformation, suggesting the relevance of KLF6 in controlling cell proliferation and hindering tumorigenesis.

BMI1 attenuates etoposide-induced G2/M checkpoints via reducing ATM activation

The BMI1 protein contributes to stem cell pluripotency and oncogenesis via multiple functions, including its newly identified role in DNA damage response (DDR). Although evidence clearly demonstrates that BMI1 facilitates the repair of double-stranded breaks via homologous recombination (HR), it remains unclear how BMI1 regulates checkpoint activation during DDR. We report here that BMI1 has a role in G2/M checkpoint activation in response to etoposide (ETOP) treatment. Ectopic expression of BMI1 in MCF7 breast cancer and DU145 prostate cancer cells significantly reduced ETOP-induced G2/M arrest. Conversely, knockdown of BMI1 in both lines enhanced the arrest. Consistent with ETOP-induced activation of the G2/M checkpoints via the ATM pathway, overexpression and knockdown of BMI1, respectively, reduced and enhanced ETOP-induced phosphorylation of ATM at serine 1981 (ATM pS1981). Furthermore, the phosphorylation of ATM targets, including γH2AX, threonine 68 (T68) on CHK2 (CHK2 pT68) and serine 15 (S15) on p53 were decreased in overexpression and increased in knockdown BMI1 cells in response to ETOP. In line with the requirement of NBS1 in ATM activation, we were able to show that BMI1 associates with NBS1 and that this interaction altered the binding of NBS1 with ATM. BMI1 consists of a ring finger (RF), helix-turn-helix-turn-helix-turn (HT), proline/serine (PS) domain and two nuclear localization signals (NLS). Although deletion of either RF or HT did not affect the association of BMI1 with NBS1, the individual deletions of PS and one NLS (KRMK) robustly reduced the interaction. Stable expression of these BMI1 mutants decreased ETOP-induced ATM pS1981 and CHK2 pT68, but not ETOP-elicited γH2AX in MCF7 cells. Furthermore, ectopic expression of BMI1 in non-transformed breast epithelial MCF10A cells also compromised ETOP-initiated ATM pS1981 and γH2AX. Taken together, we provide compelling evidence that BMI1 decreases ETOP-induced G2/M checkpoint activation via reducing NBS1-mediated ATM activation.

The ARF tumor suppressor prevents chromosomal instability and ensures mitotic checkpoint fidelity through regulation of Aurora B

The ARF tumor suppressor is best known for its role in stabilizing p53. This study identifies p53-independent functions of ARF in chromosome segregation and the mitotic checkpoint. Mitotic defects caused by loss of ARF are recapitulated by Aurora B overexpression and rescued by partial depletion of Aurora B.

The ARF tumor suppressor is part of the CDKN2A locus and is mutated or undetectable in numerous cancers. The best-characterized role for ARF is in stabilizing p53 in response to cellular stress. However, ARF has tumor suppressive functions outside this pathway that have not been fully defined. Primary mouse embryonic fibroblasts (MEFs) lacking the ARF tumor suppressor contain abnormal numbers of chromosomes. However, no role for ARF in cell division has previously been proposed. Here we demonstrate a novel, p53-independent role for ARF in the mitotic checkpoint. Consistent with this, loss of ARF results in aneuploidy in vitro and in vivo. ARF^−/− MEFs exhibit mitotic defects including misaligned and lagging chromosomes, multipolar spindles, and increased tetraploidy. ARF^−/− cells exhibit overexpression of Mad2, BubR1, and Aurora B, but only overexpression of Aurora B phenocopies mitotic defects observed in ARF^−/− MEFs. Restoring Aurora B to near-normal levels rescues mitotic phenotypes in cells lacking ARF. Our results define an unexpected role for ARF in chromosome segregation and mitotic checkpoint function. They further establish maintenance of chromosomal stability as one of the additional tumor-suppressive functions of ARF and offer a molecular explanation for the common up-regulation of Aurora B in human cancers.

Restoration of INK4a/ARF gene inhibits cell growth and cooperates with imatinib mesylate in Philadelphia chromosome-positive leukemias

VSV-G-pseudotyped lentiviral vectors expressing p16(INK4a) or p14(ARF) were used to infect at high-efficiency Philadelphia chromosome (Ph)-positive leukemia cell lines lacking endogenous transcripts. Restoration of p16(INK4a) accumulated cells in the G0/G1 phase of cell cycle and restoration of p14(ARF) induced their apoptosis, followed by significant growth inhibition. Transduction of primary blast cells from chronic myeloid leukemia in blast crisis (CML-BC) and Ph-positive acute lymphoblastic leukemia (ALL) with p16(INK4a) or p14(ARF) virus also resulted in cell growth inhibition and/or apoptosis with a patient-to-patient variation, whereas clonal growth and differentiation of cord blood progenitor cells were not affected by enforced expression of INK4a/ARF. Furthermore, upon viral transduction at low multiplicity of infection, INK4a/ARF potentiated the effect of imatinib mesylate on Ph-positive leukemia cell lines in an additive but not synergistic manner. These results suggest that INK4a/ARF protein-mimetic agents may be promising options for Ph-positive leukemias in combination with imatinib mesylate.

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.

Anti-proliferative and anti-tumour effects of lymphocyte-derived microparticles are neither species- nor tumour-type specific

BACKGROUND

Unregulated cell proliferation or growth is a prominent characteristic of cancer. We have previously demonstrated that LMPs (cell membrane microparticles derived from apoptotic human CEM T lymphoma cells stimulated with actinomycin D) strongly suppress the proliferation of not only human endothelial cells but also mouse Lewis lung carcinoma cells.

METHODS

LMPs were generated either from CEM T cells using different stimuli or from 3 different types of lymphocytes. The effects of LMPs on cancer cell proliferation were examined using cell lines from different species and tissues. The cell cycle kinetics was evaluated by FACS and the expression of cell cycle-related genes was determined using quantitative RT-PCR. The in vivo anti-tumor effect of LMPs was investigated using xenografts and allografts.

RESULTS

LMPs at doses far above physiological levels dramatically suppressed the proliferation of cancer cells in a non species-specific manner. LMPs selectively target high proliferating cells and their anti-proliferative effect is not dependent on parental cell origin or stimuli. The anti-proliferative effect of LMPs was due to induction of cell-cycle arrest in G0/G1, with associated increases in expression of the cyclin-dependent kinase inhibitors p15(INK4b), p16(INK4a), and p21(Cip1). In vivo, LMPs significantly suppressed tumor growth in animal tumor models.

CONCLUSION

These results highlight the potential role of LMPs in modulating the growth of high proliferating cells. Given that cell-based therapies are considered less toxic than pharmacologic approaches and have the potential to target multiple pathways in a synergistic manner, LMPs may serve as a veritable option for cancer treatment.

Nucleostemin stabilizes ARF by inhibiting the ubiquitin ligase ULF

Up-regulated expression of nucleolar GTPase Nucleostemin (NS) has been associated with increased cellular proliferation potential and tumor malignancy during cancer development. Recent reports attribute the growth regulatory effects of NS protein to its role in facilitating ribosome production. However, the oncogenic potential of NS remains unclear since imbalanced levels of NS have been reported to exert growth inhibitory effect by modulating p53 tumor suppressor activity. It also remains in questions if aberrant NS levels might play a p53-independent role in regulation of cell proliferation and growth. In this study, we performed affinity purification and mass spectrometry analysis to explore protein-protein interactions influencing NS growth regulatory properties independently of p53 tumor suppressor. We identified the Alternative Reading Frame (ARF) protein as a key protein associating with NS and further verified the interaction through in vitro and in vivo assays. We demonstrated that NS is able to regulate cell cycle progression by regulating the stability of the ARF tumor suppressor. Furthermore, overexpression of NS suppressed ARF polyubiquitination by its E3 ligase ULF and elongated its half-life, while knockdown of NS led to the decrease of ARF levels. Also, we found that NS can enhance NPM stabilization of ARF. Thus, we propose that in the absence of p53, ARF can be stabilized by NS and NPM to serve as an alternative tumor suppressor surveillance, preventing potential cellular transformation resulting from the growth inducing effects of NS overexpression.

ARF and p53 coordinate tumor suppression of an oncogenic IFN-β-STAT1-ISG15 signaling axis

The ARF and p53 tumor suppressors are thought to act in a linear pathway to prevent cellular transformation in response to various oncogenic signals. Here, we show that loss of p53 leads to an increase in ARF protein levels, which function to limit the proliferation and tumorigenicity of p53-deficient cells by inhibiting an IFN-β-STAT1-ISG15 signaling axis. Human triple-negative breast cancer (TNBC) tumor samples with coinactivation of p53 and ARF exhibit high expression of both STAT1 and ISG15, and TNBC cell lines are sensitive to STAT1 depletion. We propose that loss of p53 function and subsequent ARF induction creates a selective pressure to inactivate ARF and propose that tumors harboring coinactivation of ARF and p53 would benefit from therapies targeted against STAT1 and ISG15 activation.

Loss of function tp53 mutations do not accelerate the onset of myc-induced T-cell acute lymphoblastic leukaemia in the zebrafish

The TP53 tumour suppressor is activated in response to distinct stimuli, including an ARF-dependent response to oncogene stress and an ATM/ATR-dependent response to DNA damage. In human T-cell acute lymphoblastic leukaemia (T-ALL), TP53-dependent tumour suppression is typically disabled via biallelic ARF deletions. In murine models, loss of Arf (Cdkn2a) or Tp53 markedly accelerates the onset of Myc-induced lymphoblastic malignancies. In zebrafish, no ARF ortholog has been identified, but the sequence of ARF is very poorly conserved evolutionarily, making it difficult to exclude the presence of a zebrafish ARF ortholog without functional studies. Here we show that tp53 mutations have no significant influence on the onset of myc-induced T-ALL in zebrafish, consistent with the lack of additional effects of Tp53 loss on lymphomagenesis in Arf-deficient mice. By contrast, irradiation leads to complete T-ALL regression in tp53 wild-type but not homozygous mutant zebrafish, indicating that the tp53-dependent DNA damage response is intact. We conclude that tp53 inactivation has no impact on the onset of myc-induced T-ALL in the zebrafish, consistent with the lack of a functional ARF ortholog linking myc-induced oncogene stress to tp53-dependent tumour suppression. Thus, the zebrafish model is well suited to the study of ARF-independent pathways in T-ALL pathobiology.

Genetic alterations and personalized medicine in melanoma: progress and future prospects

High-throughput sequencing technologies are providing new insights into the genetic alterations involved in melanomagenesis. It appears likely that most genetic events important in the pathogenesis of melanoma will be discovered over the next few years. Genetic analysis is also increasingly being used to direct patient care. In parallel with the discovery of new genes and the elucidation of molecular pathways important in the development of melanoma, therapies targeting these pathways are becoming available. In other words, the age of personalized medicine has arrived, characterized by molecular profiling of melanoma to identify the relevant genetic alterations and the abnormal signaling mechanisms involved, followed by selection of optimal, individualized therapies. In this review, we summarize the key genetic alterations in melanoma and the development of targeted agents against melanomas bearing specific mutations. These developments in melanoma serve as a model for the implementation of personalized medicine for patients with all cancers.

MKK7 and ARF: new players in the DNA damage response scenery

Sensing, integrating, and processing of stressogenic signals must be followed by accurate differential response(s) for a cell to survive and avoid malignant transformation. The DNA damage response (DDR) pathway is vital in this process, as it deals with genotoxic/oncogenic insults, having p53 as a nodal effector that performs most of the above tasks. Accumulating data reveal that other pathways are also involved in the same or similar processes, conveying also to p53. Emerging questions are if, how, and when these additional pathways communicate with the DDR axis. Two such stress response pathways, involving the MKK7 stress-activated protein kinase (SAPK) and ARF, have been shown to be interlocked with the ATM/ATR-regulated DDR axis in a highly ordered manner. This creates a new landscape in the DDR orchestrated response to genotoxic/oncogenic insults that is currently discussed.

The Mdm network and its regulation of p53 activities: a rheostat of cancer risk

The potent transcriptional activity of p53 (Trp53, TP53) must be kept in check for normal cell growth and survival. Tumors, which drastically deviate from these parameters, have evolved multiple mechanisms to inactivate TP53, the most prevalent of which is the emergence of TP53 missense mutations, some of which have gain-of-function activities. Another important mechanism by which tumors bypass TP53 functions is via increased levels of two TP53 inhibitors, MDM2, and MDM4. Studies in humans and in mice reveal the complexity of TP53 regulation and the exquisite sensitivity of this pathway to small changes in regulation. Here, we summarize the factors that impinge on TP53 activity and thus cell death/arrest or tumor development.

Nuclear interactor of ARF and Mdm2 regulates multiple pathways to activate p53

The p53 tumor suppressor is controlled by an interactive network of factors that stimulate or inhibit its transcriptional activity. Within that network, Mdm2 functions as the major antagonist of p53 by promoting its ubiquitylation and degradation. Conversely, Tip60 activates p53 through direct association on target promoters as well as acetylation of p53 at lysine 120 (K120). This study examines the functional relationship between Mdm2 and Tip60 with a novel p53 regulator, NIAM (nuclear interactor of ARF and Mdm2). Previous work showed NIAM can suppress proliferation and activate p53 independently of ARF, indicating that other factors mediate those activities. Here, we demonstrate that NIAM is a chromatin-associated protein that binds Tip60. NIAM can promote p53 K120 acetylation, although that modification is not required for NIAM to inhibit proliferation or induce p53 transactivation of the p21 promoter. Notably, Tip60 silencing showed it contributes to but is not sufficient for NIAM-mediated p53 activation, suggesting other mechanisms are involved. Indeed, growth-inhibitory forms of NIAM also bind to Mdm2, and increased NIAM expression levels disrupt p53-Mdm2 association, inhibit p53 polyubiquitylation, and prevent Mdm2-mediated inhibition of p53 transcriptional activity. Importantly, loss of NIAM significantly impairs p53 activation. Together, these results show that NIAM activates p53 through multiple mechanisms involving Tip60 association and Mdm2 inhibition. Thus, NIAM regulates 2 critical pathways that control p53 function and are altered in human cancers, implying an important role for NIAM in tumorigenesis.

Cyclin-dependent kinases as therapeutic targets in melanoma

Decades of scientific insights have led to a recent expansion of the therapeutic menu for melanoma. Despite these advances, the current targeted therapies and immune checkpoint agents continue to yield suboptimal response and cure rates. Hitherto, the most effective targeted therapy strategies have centered on effectors in the mitogen-activated protein kinase (MAPK) pathway. This review focuses on the emerging evidence of combinatorial approaches targeting both MAPK signaling and dysregulations in cell-cycle checkpoints. We discuss the prospects and limitations of utilizing strategies that promote cellular senescence, such as inhibition of the interphase cyclin-dependent kinases (CDKs) and highlight the current state of CDK drug discovery in melanoma.

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.

High- and low-penetrance cutaneous melanoma susceptibility genes

The aim of this review is to report the current understanding of the molecular genetics of melanoma predisposition. To date, two high-penetrance melanoma susceptibility genes, cyclin-dependent kinas inhibitor (CDKN)2A on chromosome 9p21 and cyclin-dependent kinase (CDK4) on 12q13, have been identified. Germline inactivating mutations of the CDKN2A gene are the most common cause of inherited susceptibility to melanoma. Worldwide, a few families have been found to harbor CDK4 mutations. However, predisposing alterations to familial melanoma are still unknown in a large proportion of kindreds. Other melanoma susceptibility loci have been mapped through genome-wide linkage analysis, although the putative causal genes at these loci have yet to be identified. Much ongoing research is being focused on the identification of low-penetrance melanoma susceptibility genes that confer a lower melanoma risk with more frequent variations. Specific variants of the MC1R gene have been demonstrated to confer an increase in melanoma risk. In addition, conflicting data are available on other potential low-penetrance genes encoding proteins involved in pigmentation, cell growth and differentiation, DNA repair or detoxifying of metabolites.

miR-34a is essential for p19(Arf)-driven cell cycle arrest

The Arf tumor suppressor gene product, p19(Arf), regulates cell proliferation in incipient cancer cells and during embryo development. Beyond its commonly accepted p53-dependent actions, p19(Arf) also acts independently of p53 in both contexts. One such p53-independent effect with in vivo relevance includes its repression of Pdgfrβ, a process that is essential for vision in the mouse. We have utilized cell culture-based and mouse models to define a new role for miR-34a in this process. Ectopic expression of Arf in cultured cells enhanced the expression of several microRNAs predicted to target Pdgfrß synthesis, including the miR-34 family. Because miR-34a has been implicated as a p53-dependent effector, we investigated whether it also contributed to p53-independent effects of p19(Arf). Indeed, in mouse embryo fibroblasts (MEFs) lacking p53, Arf-driven repression of Pdgfrβ and its blockade of Pdgf-B stimulated DNA synthesis were both completely interrupted by anti-microRNA against miR-34a. Ectopic miR-34a directly targeted Pdgfrβ and a plasmid reporter containing wild-type Pdgfrβ 3'UTR sequence, but not one in which the miR-34a target sequence was mutated. Although miR-34a expression has been linked to p53-a well-known effector of p19(Arf)-Arf expression and its knockdown correlated with miR-34a level in MEFs lacking p53. Finally, analysis of the mouse embryonic eye demonstrated that Arf controlled expression of miR-34a, and the related miR-34b and c, in vivo during normal mouse development. Our findings indicate that miR-34a provides an essential link between p19(Arf) and its p53-independent capacity to block cell proliferation driven by Pdgfrβ. This has ramifications for developmental and tumor suppressor roles of Arf.

P14ARF suppresses tumor-induced thrombosis by regulating the tissue factor pathway

How necrotic areas develop in tumors is incompletely understood but can impact progression. Recent findings suggest that the formation of vascular microthrombi contributes to tumor necrosis, prompting investigation of coagulation cascades. Here, we report that loss of tumor suppressor P14ARF can contribute to activating the clotting cascade in glioblastoma. P14ARF transcriptionally upregulated TFPI2, a Kunitz-type serine protease in the tissue factor pathway that inhibits the initiation of thrombosis reactions. P14ARF activation in tumor cells delayed their ability to activate plasma clotting. Mechanistically, P14ARF activated the TFPI2 promoter in a p53-independent manner that relied upon c-JUN, SP1, and JNK activity. Taken together, our results identify the critical signaling pathways activated by P14ARF to prevent vascular microthrombosis triggered by glioma cells. Stimulation of this pathway might be used as a therapeutic strategy to reduce aggressive phenotypes associated with necrotic tumors, including glioblastoma.

Genetic progression of pancreatic cancer

The progression from normal cells to invasive pancreatic ductal adenocarcinoma (PDAC) requires the accumulation of multiple inherited or acquired mutations. Activating point mutations in the KRAS oncogene are prevalent in pancreatic cancer and result in the stimulation of several pathways including the RAF-mitogen-activated protein kinase pathway and the phosphoinositide 3-kinase pathway. Other genetic alterations, including telomere shortening and the inactivation of tumor suppressor genes such as CDKN2A, TP53, and SMAD4, which encode p16, p53, and SMAD4, respectively, also contribute to the progression of pancreatic cancer. These, and other genetic events, can present at different stages in the development of PDAC at histologically defined precursor lesions known as pancreatic intraepithelial neoplasia, intraductal papillary mucinous neoplasms, or mucinous cystic neoplasms. Each precursor lesion represents alternate routes to PDAC formation and has a unique presentation and somewhat distinct genetic events controlling its development. Despite the advances in the understanding of the genetics of PDAC, the prognosis for this cancer remains poor, and several important aspects of its pathogenesis must be clarified to improve therapeutics, including the timing and method of metastases, as well as the relationship of the tumor cells with the desmoplastic stroma, which is a characteristic feature of the cancer. This review discusses the principal genetic alterations in PDAC and its precursor lesions, including their effects on promoting carcinogenesis.

RABL6A, a novel RAB-like protein, controls centrosome amplification and chromosome instability in primary fibroblasts

RABL6A (RAB-like 6 isoform A) is a novel protein that was originally identified based on its association with the Alternative Reading Frame (ARF) tumor suppressor. ARF acts through multiple p53-dependent and p53-independent pathways to prevent cancer. How RABL6A functions, to what extent it depends on ARF and p53 activity, and its importance in normal cell biology are entirely unknown. We examined the biological consequences of RABL6A silencing in primary mouse embryo fibroblasts (MEFs) that express or lack ARF, p53 or both proteins. We found that RABL6A depletion caused centrosome amplification, aneuploidy and multinucleation in MEFs regardless of ARF and p53 status. The centrosome amplification in RABL6A depleted p53−/− MEFs resulted from centrosome reduplication via Cdk2-mediated hyperphosphorylation of nucleophosmin (NPM) at threonine-199. Thus, RABL6A prevents centrosome amplification through an ARF/p53-independent mechanism that restricts NPM-T199 phosphorylation. These findings demonstrate an essential role for RABL6A in centrosome regulation and maintenance of chromosome stability in non-transformed cells, key processes that ensure genomic integrity and prevent tumorigenesis.