Multiple interacting domains contribute to p14ARF mediated inhibition of MDM2

The ARF tumor suppressor targets PPM1G/PP2Cγ to counteract NF-κB transcription tuning cell survival and the inflammatory response

Inducible transcriptional programs mediate the regulation of key biological processes and organismal functions. Despite their complexity, cells have evolved mechanisms to precisely control gene programs in response to environmental cues to regulate cell fate and maintain normal homeostasis. Upon stimulation with proinflammatory cytokines such as tumor necrosis factor-α (TNF), the master transcriptional regulator nuclear factor (NF)-κB utilizes the PPM1G/PP2Cγ phosphatase as a coactivator to normally induce inflammatory and cell survival programs. However, how PPM1G activity is precisely regulated to control NF-κB transcription magnitude and kinetics remains unknown. Here, we describe a mechanism by which the ARF tumor suppressor binds PPM1G to negatively regulate its coactivator function in the NF-κB circuit thereby promoting insult resolution. ARF becomes stabilized upon binding to PPM1G and forms a ternary protein complex with PPM1G and NF-κB at target gene promoters in a stimuli-dependent manner to provide tunable control of the NF-κB transcriptional program. Consistently, loss of ARF in colon epithelial cells leads to up-regulation of NF-κB antiapoptotic genes upon TNF stimulation and renders cells partially resistant to TNF-induced apoptosis in the presence of agents blocking the antiapoptotic program. Notably, patient tumor data analysis validates these findings by revealing that loss of ARF strongly correlates with sustained expression of inflammatory and cell survival programs. Collectively, we propose that PPM1G emerges as a therapeutic target in a variety of cancers arising from ARF epigenetic silencing, to loss of ARF function, as well as tumors bearing oncogenic NF-κB activation.

Time varying causal network reconstruction of a mouse cell cycle

Background

Biochemical networks are often described through static or time-averaged measurements of the component macromolecules. Temporal variation in these components plays an important role in both describing the dynamical nature of the network as well as providing insights into causal mechanisms. Few methods exist, specifically for systems with many variables, for analyzing time series data to identify distinct temporal regimes and the corresponding time-varying causal networks and mechanisms.

Results

In this study, we use well-constructed temporal transcriptional measurements in a mammalian cell during a cell cycle, to identify dynamical networks and mechanisms describing the cell cycle. The methods we have used and developed in part deal with Granger causality, Vector Autoregression, Estimation Stability with Cross Validation and a nonparametric change point detection algorithm that enable estimating temporally evolving directed networks that provide a comprehensive picture of the crosstalk among different molecular components. We applied our approach to RNA-seq time-course data spanning nearly two cell cycles from Mouse Embryonic Fibroblast (MEF) primary cells. The change-point detection algorithm is able to extract precise information on the duration and timing of cell cycle phases. Using Least Absolute Shrinkage and Selection Operator (LASSO) and Estimation Stability with Cross Validation (ES-CV), we were able to, without any prior biological knowledge, extract information on the phase-specific causal interaction of cell cycle genes, as well as temporal interdependencies of biological mechanisms through a complete cell cycle.

Conclusions

The temporal dependence of cellular components we provide in our model goes beyond what is known in the literature. Furthermore, our inference of dynamic interplay of multiple intracellular mechanisms and their temporal dependence on one another can be used to predict time-varying cellular responses, and provide insight on the design of precise experiments for modulating the regulation of the cell cycle.

Electronic supplementary material

The online version of this article (10.1186/s12859-019-2895-1) contains supplementary material, which is available to authorized users.

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.

A redox mechanism underlying nucleolar stress sensing by nucleophosmin

The nucleolus has been recently described as a stress sensor. The nucleoplasmic translocation of nucleolar protein nucleophosmin (NPM1) is a hallmark of nucleolar stress; however, the causes of this translocation and its connection to p53 activation are unclear. Using single live-cell imaging and the redox biosensors, we demonstrate that nucleolar oxidation is a general response to various cellular stresses. During nucleolar oxidation, NPM1 undergoes S-glutathionylation on cysteine 275, which triggers the dissociation of NPM1 from nucleolar nucleic acids. The C275S mutant NPM1, unable to be glutathionylated, remains in the nucleolus under nucleolar stress. Compared with wild-type NPM1 that can disrupt the p53–HDM2 interaction, the C275S mutant greatly compromises the activation of p53, highlighting that nucleoplasmic translocation of NPM1 is a prerequisite for stress-induced activation of p53. This study elucidates a redox mechanism for the nucleolar stress sensing and may help the development of therapeutic strategies.

Nucleoplasmic translocation of NPM1 is integral to nucleolar stress sensing. Here, the authors show that nucleolar oxidation is a general cellular stress response, and that oxidation-related glutathionylation of NPM1 triggers its translocation and facilitates p53 activation.

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.

MDM2-mediated degradation of p14ARF: a novel mechanism to control ARF levels in cancer cells

We here show a new relationship between the human p14ARF oncosuppressor and the MDM2 oncoprotein. MDM2 overexpression in various cancer cell lines causes p14ARF reduction inducing its degradation through the proteasome. The effect does not require the ubiquitin ligase activity of MDM2 and preferentially occurs in the cytoplasm. Interestingly, treatment with inhibitors of the PKC (Protein Kinase C) pathway and use of p14ARF phosphorylation mutants indicate that ARF phosphorylation could play a role in MDM2 mediated ARF degradation reinforcing our previous observations that ARF phosphorylation influences its stability and biological activity. Our study uncovers a new potentially important mechanism through which ARF and MDM2 can counterbalance each other during the tumorigenic process.

TBX3, a downstream target of TGF-β1, inhibits mesangial cell apoptosis

Chronic kidney disease (CKD) is an increasingly common condition characterized by progressive loss of functional nephrons leading to renal failure. TGF-β1-induced mesangial cell (MC) phenotype alterations have been linked to the genesis of CKD. Here we show that TGF-β1 regulates TBX3 gene expression in MC. This gene encodes for two main isoforms, TBX3.1 and TBX3+2α. TBX3.1 has been implicated in cell immortalization, proliferation and apoptosis by inhibiting p14(ARF)-Mdm2-p53 pathway, while TBX3+2α role has not been defined. We demonstrated that TBX3 overexpression abrogated MC apoptosis induced by serum deprivation. Moreover, we observed an enhancement in TBX3 protein expression both in glomerular and tubular regions in the model of 5/6 nephrectomy, temporally related to increased expression of TGF-β1, type IV collagen and fibronectin. Our results indicate that TBX3 acts as an anti-apoptotic factor in MC in vitro and may be involved in the mechanism by which TGF-β1 induces glomerulosclerosis and tubular fibrosis during the progression of nephropathies.

Oncomir miR-125b suppresses p14(ARF) to modulate p53-dependent and p53-independent apoptosis in prostate cancer

MicroRNAs are a class of naturally occurring small non-coding RNAs that target protein-coding mRNAs at the post-transcriptional level and regulate complex patterns of gene expression. Our previous studies demonstrated that in human prostate cancer the miRNA miR-125b is highly expressed, leading to a negative regulation of some tumor suppressor genes. In this study, we further extend our studies by showing that miR-125b represses the protein product of the ink4a/ARF locus, p14(ARF), in two prostate cancer cell lines, LNCaP (wild type-p53) and 22Rv1 (both wild type and mutant p53), as well as in the PC-346C prostate cancer xenograft model that lentivirally overexpressed miR-125b. Our results highlight that miR-125b modulates the p53 network by hindering the down-regulation of Mdm2, thereby affecting p53 and its target genes p21 and Puma to a degree sufficient to inhibit apoptosis. Conversely, treatment of prostate cancer cells with an inhibitor of miR-125b (anti-miR-125b) resulted in increased expression of p14(ARF), decreased level of Mdm2, and induction of apoptosis. In addition, overexpression of miR-125b in p53-deficient PC3 cells induced down-regulation of p14(ARF), which leads to increased cell proliferation through a p53-independent manner. Thus, we conclude that miR-125b acts as an oncogene which regulates p14(ARF)/Mdm2 signaling, stimulating proliferation of prostate cancer cells through a p53-dependent or p53-independent function. This reinforces our belief that miR-125b has potential as a therapeutic target for the management of patients with metastatic prostate cancer.

Mimicking p14ARF phosphorylation influences its ability to restrain cell proliferation

The INK4a/ARF locus on the short arm of chromosome 9 is one of the most frequently altered loci in human cancer. It is generally accepted that ARF is involved in oncogenic checkpoint pathways by sensitizing incipient cancer cells to undergo growth arrest or apoptosis through both p53-dependent and independent pathways. While intensive studies have been focused on ARF activation at the transcriptional level, only recently mechanisms governing ARF turnover have been identified. Here, we show for the first time that p14ARF is a PKC target. Prediction analysis showed many potential phosphorylation sites in PKC consensus sequences within ARF protein, and, among them, the threonine at position 8 was the most conserved. Substitution of this threonine influences both ARF stability and localization. Furthermore, a phosphomimetic ARF mutation reduces the ability to arrest cell growth although the ability to bind MDM2 and stabilize p53 result unaffected. Thus we propose that phosphorylation of ARF in both immortalized and tumor cell lines could be a mechanism to escape ARF surveillance following proliferative and oncogenic stress.

Apoptosis induced by 1,3,6,7-tetrahydroxyxanthone in Hepatocellular carcinoma and proteomic analysis

Gamboge is a traditional Chinese medicine and our previous study showed that gambogic acid and gambogenic acid suppress the proliferation of HCC cells. In the present study, another active component, 1,3,6,7-tetrahydroxyxanthone (TTA), was identified to effectively suppress HCC cell growth. In addition, our Hoechst-PI staining and flow cytometry analyses indicated that TTA induced apoptosis in HCC cells. In order to identify the targets of TTA in HCC cells, a two-dimensional gel electrophoresis was performed, and proteins in different expressions were identified by MALDA-TOF MS and MS/MS analyses. In summary, eighteen proteins with different expressions were identified in which twelve were up-regulated and six were down-regulated. Among them, the four most distinctively expressed proteins were further studied and validated by western blotting. The β-tubulin and translationally controlled tumor protein were decreased while the 14-3-3σ and P16 protein expressions were up-regulated. In addition, TTA suppressed tumorigenesis partially through P16-pRb signaling. 14-3-3σ silence reversed the suppressive effect of cell growth and apoptosis induced by introducing TTA. In conclusion, TTA effectively suppressed cell growth through, at least partially, up-regulation of P16 and 14-3-3σ.

Physical and functional interaction between ribosomal protein L11 and the tumor suppressor ARF

The ARF tumor suppressor protein activates p53 in response to oncogenic stress, whereas ribosomal protein L11 induces p53 following ribosomal stress. Both proteins bind to central, albeit non-overlapping, regions of MDM2 and suppress MDM2 activity toward p53. However, it is not known whether the two pathways are functionally connected. Here we show that ARF directly binds to L11 in vitro and in cells, which then forms a complex with MDM2 and p53. L11 collaboratively enhances ARF-induced p53 transcriptional activity and cell cycle arrest. Supporting these results, knocking down L11 reduces ARF-mediated p53 accumulation and alleviates ARF-induced cell cycle arrest. Interestingly, overexpression of ARF increases the levels of ribosome-free L11 and enhances the interaction of L11 with MDM2 and p53. These results demonstrate that ARF activates p53, at least partly by induction of ribosomal stress, which results in L11 suppression of MDM2, and suggest that the ARF-MDM2-p53 and the L11-MDM2-p53 pathways are functionally connected.

Evaluation of cell cycle arrest in estrogen responsive MCF-7 breast cancer cells: pitfalls of the MTS assay

Endocrine resistance is a major problem with anti-estrogen treatments and how to overcome resistance is a major concern in the clinic. Reliable measurement of cell viability, proliferation, growth inhibition and death is important in screening for drug treatment efficacy in vitro. This report describes and compares commonly used proliferation assays for induced estrogen-responsive MCF-7 breast cancer cell cycle arrest including: determination of cell number by direct counting of viable cells; or fluorescence SYBR®Green (SYBR) DNA labeling; determination of mitochondrial metabolic activity by 3-(4,5-dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium (MTS) assay; assessment of newly synthesized DNA using 5-ethynyl-2′-deoxyuridine (EdU) nucleoside analog binding and Alexa Fluor® azide visualization by fluorescence microscopy; cell-cycle phase measurement by flow cytometry. Treatment of MCF-7 cells with ICI 182780 (Faslodex), FTY720, serum deprivation or induction of the tumor suppressor p14ARF showed inhibition of cell proliferation determined by the Trypan Blue exclusion assay and SYBR DNA labeling assay. In contrast, the effects of treatment with ICI 182780 or p14ARF-induction were not confirmed using the MTS assay. Cell cycle inhibition by ICI 182780 and p14ARF-induction was further confirmed by flow cytometric analysis and EdU-DNA incorporation. To explore this discrepancy further, we showed that ICI 182780 and p14ARF-induction increased MCF-7 cell mitochondrial activity by MTS assay in individual cells compared to control cells thereby providing a misleading proliferation readout. Interrogation of p14ARF-induction on MCF-7 metabolic activity using TMRE assays and high content image analysis showed that increased mitochondrial activity was concomitant with increased mitochondrial biomass with no loss of mitochondrial membrane potential, or cell death. We conclude that, whilst p14ARF and ICI 182780 stop cell cycle progression, the cells are still viable and potential treatments utilizing these pathways may contribute to drug resistant cells. These experiments demonstrate how the combined measurement of metabolic activity and DNA labeling provides a more reliable interpretation of cancer cell response to treatment regimens.

ARF-dependent regulation of ATM and p53 associated KZNF (Apak) protein activity in response to oncogenic stress

The KRAB-type zinc-finger protein Apak (ATM and p53 associated KZNF protein) specifically suppresses p53-mediated apoptosis. Upon DNA damage, Apak is phosphorylated and inhibited by ATM kinase, resulting in p53 activation. However, how Apak is regulated in response to oncogenic stress remains unknown. Here we show that upon oncogene activation, Apak is inhibited in the tumor suppressor ARF-dependent but ATM-independent manner. Oncogene-induced ARF protein directly interacts with Apak and competes with p53 to bind to Apak, resulting in Apak dissociation from p53. Thus, Apak is differentially regulated in the ARF and ATM-dependent manner in response to oncogenic stress and DNA damage, respectively.

The Arf tumor suppressor protein inhibits Miz1 to suppress cell adhesion and induce apoptosis

Arf assembles a complex containing Miz1, heterochromatin, and histone H3K3 to block expression of genes involved in cell adhesion and signal transduction. The resulting blockade of cell–cell and cell–matrix interactions facilitates elimination of cells carrying oncogenic mutations.

Oncogenic stress induces expression of the alternate reading frame (Arf) tumor suppressor protein. Arf then stabilizes p53, which leads to cell cycle arrest or apoptosis. The mechanisms that distinguish both outcomes are incompletely understood. In this study, we show that Arf interacts with the Myc-associated zinc finger protein Miz1. Binding of Arf disrupts the interaction of Miz1 with its coactivator, nucleophosmin, induces the sumoylation of Miz1, and facilitates the assembly of a heterochromatic complex that contains Myc and trimethylated H3K9 in addition to Miz1. Arf-dependent assembly of this complex leads to the repression of multiple genes involved in cell adhesion and signal transduction and induces apoptosis. Our data point to a tumor-suppressive pathway that weakens cell–cell and cell–matrix interactions in response to expression of Arf and that may thereby facilitate the elimination of cells harboring an oncogenic mutation.

Role of P53-MDM2 negative-feedback in the pathopoiesis of Helicobacter pylori

Murine double minute-2 (mdm2), one of downstream genes of P53, forms a negative feedback loop with P53 to maintain P53 at a low level under normal circumstances. On one hand, P53 activates transcription of mdm2. On the other hand, MDM2 suppresses activity of P53. The negative feedback, which plays an important role in tumor development, is regulated by a variety of factors. At present, Helicobacter pylori (H. pylori) is considered as a key gastrointestinal disease pathogenic factor. Its pathogenic or carcinogenic mechanism has become a hot research issue in recent years, and there have been substantial research on the role of p53 gene networks in H. pylori pathogenic process. And the P53-MDM2 negative feedback may play an important role in this process.

Binding to nucleophosmin determines the localization of human and chicken ARF but not its impact on p53

The ARF tumour suppressor gene encodes a small highly basic protein whose known functions are largely determined by the amino acids encoded within the first exon. In mammals, the protein incorporates additional residues specified by an alternative reading frame in the second exon of INK4a, but this arrangement does not apply to the chicken homologue. In exploring the intracellular localization of chicken p7(ARF), we found that while the FLAG- and HA-tagged versions localize in the nucleolus, in line with mammalian ARF, the GFP-tagged version is excluded from the nucleolus. Here we show that irrespective of the source or composition of the ARF fusion proteins, versions that accumulate in the nucleolus share the ability to bind to nucleophosmin (NPM). Depletion of NPM with siRNA results in the re-location and destabilization of nucleolar forms of ARF but has little effect on the location or stability of a nucleoplasmic form of ARF. Importantly, knockdown of endogenous NPM does not impair the ability of ARF to bind to MDM2 and stabilize p53. These findings support the view that nucleolar localization determines the stability of ARF but not its primary function.

Differences in global gene expression in melanoma cell lines with and without homozygous deletion of the CDKN2A locus genes

We studied differential global gene expression in four melanoma cell lines with three cell lines without homozygous deletion of the CDKN2A locus using HG-U133A microarrays with 22 277 transcripts. None of the cell lines carried mutations in the B-RAF and N-RAS genes. Data analysis using stringent criteria showed specific upregulation of 70 genes and downregulation of 86 genes in cell lines with homozygous deletion of the CDKN2A gene. A comparison with previous expression data showed overlapping of upregulation and downregulation of seven and 23 genes, respectively, in melanoma cell lines with homozygous deletion of the CDKN2A locus or mutations in the B-RAF and N-RAS genes. Microarray data for eight selected genes were validated with an extended number of cell lines using quantitative real-time polymerase chain reaction. The upregulated genes in cell lines with the deletion besides others included MAGE A2 [fold change 128, 95% confidence interval (CI) 82.8-172.2; t-test P=0.004], MAGE A6 (fold change 623, 95% CI 473.4-772.1; t-test P=0.001), MAGE A12 (fold change 90, 95% CI 65.1-115.5; t-test P=0.001) and dopachrome tautomerase (fold change 42, 95% CI 32.5-51.8; t-test P=0.001). Downregulated genes included interleukin 18 (fold change 489, 95% CI 146.4-831.2; t-test P=0.04), ID2 (fold change 3, 95% CI 2.2-4.9; t-test P=0.001), KLF4 (fold change 9, 95% CI 4.3-14.7; P=0.01) and CD24 antigen (fold change 1308, 95% CI 766.0-1850.8; t-test P=0.01). The upregulated genes common to cell lines with homozygous deletion of the CDKN2A gene and mutations in B-RAF and N-RAS gene included those that are involved in RAS/RAF/MEK/ERK pathways. Our results highlight effects of homozygous deletion of the CDKN2A locus on global gene expression.

Regulation of the MDM2-p53 pathway by ribosomal protein L11 involves a post-ubiquitination mechanism

Inhibition of the MDM2-p53 feedback loop is critical for p53 activation in response to cellular stresses. The ribosomal proteins L5, L11, and L23 can block this loop by inhibiting MDM2-mediated p53 ubiquitination and degradation in response to ribosomal stress. Here, we show that L11, but not L5 and L23, leads to a drastic accumulation of ubiquitinated and native MDM2. This effect is dependent on the ubiquitin ligase activity of MDM2, but not p53, and requires the central MDM2 binding domain (residues 51-108) of L11. We further show that L11 inhibited 26 S proteasome-mediated degradation of ubiquitinated MDM2 in vitro and consistently prolonged the half-life of MDM2 in cells. These results suggest that L11, unlike L5 and L23, differentially regulates the levels of ubiquitinated p53 and MDM2 and inhibits the turnover and activity of MDM2 through a post-ubiquitination mechanism.

Comprehensive analysis of CDKN2A (p16INK4A/p14ARF) and CDKN2B genes in 53 melanoma index cases considered to be at heightened risk of melanoma

OBJECTIVE

Comprehensive analysis of the 9p21 locus including the CDKN2A, ARF, and CDKN2B genes in 53 individuals from melanoma index cases considered to be at heightened risk of melanoma.

METHODS AND RESULTS

Using a combination of DNA sequencing, gene copy number by real time quantitative PCR, linkage analysis, and transcript analysis in haploid somatic cell hybrids, we found no evidence for germline alteration in either coding or non-coding domains of CDKN2A and CDKN2B. However, we identified a p14ARF exon 1beta missense germline mutation (G16D) in a melanoma-neural system tumour syndrome (CMM+NST) family and a 8474 bp germline deletion from 196 bp upstream of p14ARF exon 1beta initiation codon to 11233 bp upstream of exon 1alpha of p16(INK4A) in a family with five melanoma cases. For three out of 10 families with at least three melanoma cases, the disease gene was unlinked to the 9p21 region, while linkage analysis was not fully conclusive for seven families.

CONCLUSIONS

These data reinforce the hypothesis that ARF is a melanoma susceptibility gene and suggest that germline deletions specifically affecting p14ARF may not be solely responsible for NST susceptibility. Predisposition to CMM+NST could either be due to complete disruption of the CDKN2A locus or be the result of more complex genetic inheritance. In addition, the absence of any genetic alteration in 50 melanoma prone families or patients suggests the presence of additional tumour suppressor genes possibly in the 9p21 region, and on other chromosomes.

Biological evaluation of penetration domain and killing domain peptides

BACKGROUND

Cancer gene therapy must impact the majority of cells to be effective. Current gene delivery systems are unable to achieve sufficient transfer efficiency to the tumor cells. Cell killing can be dramatically increased through a bystander effect. Modeling the gene product with synthetic peptides can identify key elements for creating cell killing through a bystander effect.

METHODS

Fluorescent labeled peptides were used for uptake kinetic studies and determination of intracellular localization in human glioblastoma cell lines, rat glioma cells lines and pressurized rat cerebral arteries. The degree of cell killing was assayed using propidium iodide coupled with fluorescence-activated cell sorting (FACS) analysis.

RESULTS

Peptides derived from HIV Tat and Drosophila antennapedia homeodomain were taken up by all tumor and primary cells. Attachment of an Mdm-2-binding domain derived from P14(ARF) resulted in cell killing and was independent of domain orientation. Uptake kinetics showed rapid uptake for both tumor and primary cells equilibrating with the external media within 10 min. Intraluminal or extraluminal administration of peptides into pressurized cerebral arteries showed a lack of extravasation across the subbasement lamina. Assay of biological activity following intraluminal administration showed selective suppression of response to vasodilation with no effect on response by smooth muscle cells.

CONCLUSIONS

The results from these studies identified: (1) a cell trafficking domain and a cytotoxic domain for killing brain tumor cells; (2) that cell killing was independent of the domain orientations with regard to the cell trafficking domain being at the C-terminus or N-terminus; and (3) that the dual domain peptide can also be taken up by endothelial cells as shown by the cerebral artery studies. Hence, localized expression of the cytotoxic gene has the potential to not only kill brain tumor cells, but also tumor endothelium, thus further increasing the effectiveness of the therapy.

A novel methionine-53-valine mutation of p16 in a hereditary melanoma kindred

We report a novel germline Met53Val mutation in CDKN2A from a large melanoma-prone family; this mutation occurs in exon 2 of CDKN2A where p16 and alternative reading frame (ARF) both share transcript sequences. The previously reported Met53Ile and the current Met53Val mutations are coupled to distinct Asp68His and Asp67Gly alterations in ARF, respectively. The coincidence of second, independent p16 Met53 alteration that differentially alters ARF suggests that there may be selectivity for targeting the p16 transcript over the ARF transcript.

Stability of nucleolar versus non-nucleolar forms of human p14(ARF)

Fusion proteins containing the amino-terminal domain of human p14(ARF) linked to green fluorescent protein are able to bind MDM2 and stabilize p53 without localization in the nucleolus. However, these fusion proteins are inherently unstable, with half-lives considerably shorter than either authentic ARF or chimaeras containing the entire coding domain, both of which are predominantly nucleolar. We present evidence that the unstable fusion proteins are significantly stabilized if redirected to the nucleolus by addition of a basic motif based on the nuclear localization signal of SV40 T-antigen. Moreover, the stability of these proteins can be enhanced by modulating the functions of MDM2 and p53. These data are consistent with a model in which ARF interacts with MDM2 in the nucleoplasm but is consequently subject to proteasomal degradation. Nucleolar localization may serve to store or stabilize ARF.

p14 Arf promotes small ubiquitin-like modifier conjugation of Werners helicase

Here we demonstrate a novel p53-independent interaction between the nucleolar tumor suppressors, p14 Arf and Werners helicase (WRN). Binding of p14 Arf to WRN is multivalent and resembles the binding of p14 Arf to Mdm2. Residues 2-14 and 82-101 of p14 Arf and residues in the central region and C terminus of WRN have particular importance for binding. p14 Arf promotes small ubiquitin-like modifier (SUMO) modification of WRN in a synergistic manner with the SUMO-conjugating enzyme, UBCH9. p14 Arf causes redistribution of WRN within the nucleus, and this effect is reversed by expression of a SUMO-specific protease, thus implicating the SUMO conjugation pathway in WRN re-localization. We establish that the ability to promote SUMO conjugation is a general property of the p14 Arf tumor suppressor.

Distinct MDM2 and P14ARF expression and centrosome amplification in well-differentiated liposarcomas

Well-differentiated liposarcomas (WDLs) are common soft-tissue tumors in adults. They are characterized by large marker chromosomes and/or ring chromosomes containing 12q-derived sequences in which MDM2 is consistently amplified. WDLs are subdivided into two subtypes according to their karyotype. Type D cells exhibit a near-diploid karyotype, with very few or no chromosome changes. Type H cells exhibit a near-tetraploid karyotype and many structural changes. Expression of P14ARF, MDM2, and TP53 proteins was assayed in the two WDL subtypes to establish whether distinct expression profiles correlated with cell ploidy. Although a transcriptionally functional TP53 was present in most tumors independent of their karyotype, type H cells were characterized by high levels of P14ARF and MDM2 proteins. Although amplified within similar chromosome markers in type D tumors, MDM2 did not appear to be overexpressed. In addition, it was present as a C-terminal truncated protein, indicative of alternatively spliced variants of MDM2 mRNA. As the existence of karyotypically distinct tumors could result from alterations of the mitotic machinery, we investigated the centrosome behavior in the two WDL subtypes. Centrosome amplification occurred in WDL tumors types H and D independent of their ploidy status. Moreover, no functional centrosome difference was found between the two tumor subtypes.

MDM2 promotes p21waf1/cip1 proteasomal turnover independently of ubiquitylation

The CDK inhibitor p21waf1/cip1 is degraded by a ubiquitin-independent proteolytic pathway. Here, we show that MDM2 mediates this degradation process. Overexpression of wild-type or ring finger-deleted, but not nuclear localization signal (NLS)-deleted, MDM2 decreased p21waf1/cip1 levels without ubiquitylating this protein and affecting its mRNA level in p53(-/-) cells. This decrease was reversed by the proteasome inhibitors MG132 and lactacystin, by p19(arf), and by small interfering RNA (siRNA) against MDM2. p21waf1/cip1 bound to MDM2 in vitro and in cells. The p21waf1/cip1-binding-defective mutant of MDM2 was unable to degrade p21waf1/cip1. MDM2 shortened the half-life of both exogenous and endogenous p21waf1/cip1 by 50% and led to the degradation of its lysine-free mutant. Consequently, MDM2 suppressed p21waf1/cip1-induced cell growth arrest of human p53(-/-) and p53(-/-)/Rb(-/-)cells. These results demonstrate that MDM2 directly inhibits p21waf1/cip1 function by reducing p21waf1/cip1 stability in a ubiquitin-independent fashion.

14-3-3 sigma positively regulates p53 and suppresses tumor growth

The 14-3-3 sigma (sigma) protein, a negative regulator of the cell cycle, is a human mammary epithelium-specific marker that is downregulated in transformed mammary carcinoma cells. It has also been identified as a p53-inducible gene product involved in cell cycle checkpoint control after DNA damage. Although 14-3-3 sigma is linked to p53-regulated cell cycle checkpoint control, detailed mechanisms of how cell cycle regulation occurs remain unclear. Decreased expression of 14-3-3 sigma was recently reported in several types of carcinomas, further suggesting that the negative regulatory role of 14-3-3 sigma in the cell cycle is compromised during tumorigenesis. However, this possible tumor-suppressive role of 14-3-3 sigma has not yet been characterized. Here, we studied the link between 14-3-3 sigma activities and p53 regulation. We found that 14-3-3 sigma interacted with p53 in response to the DNA-damaging agent adriamycin. Importantly, 14-3-3 sigma expression led to stabilized expression of p53. In studying the molecular mechanism of this increased stabilization of p53, we found that 14-3-3 sigma antagonized the biological functions of Mdm2 by blocking Mdm2-mediated p53 ubiquitination and nuclear export. In addition, we found that 14-3-3 sigma facilitated the oligomerization of p53 and enhanced p53's transcriptional activity. As a target gene of p53, 14-3-3 sigma appears to have a positive feedback effect on p53 activity. Significantly, we also showed that overexpression of 14-3-3 sigma inhibited oncogene-activated tumorigenicity in a tetracycline-regulated 14-3-3 sigma system. These results defined an important p53 regulatory loop and suggested that 14-3-3 sigma expression can be considered for therapeutic intervention in cancers.

Oligomerization of the human ARF tumor suppressor and its response to oxidative stress

The tumor suppressor ARF plays an important role as an inhibitor of the Mdm2-mediated degradation of p53. Here we demonstrate that human ARF (p14ARF) can form homo-oligomers. The stability of the oligomers is favored by oxidizing agents in a reversible fashion and involves all three cysteine residues in p14ARF. Furthermore, the effect of p14ARF in clonogenic assays is moderately but reproducibly increased by the mutation of its cysteine residues. We also observed that altering the amino terminus of p14ARF resulted in the appearance of remarkably stable oligomers. This indicates that the amino terminus of p14ARF interferes with the ability of the protein to form multimeric complexes. These observations suggest that p14ARF activity may be linked to its oligomerization status and sensitive to the redox status of the cell.

Association of p14ARF with the p120E4F transcriptional repressor enhances cell cycle inhibition

The p14(ARF) tumor suppressor is a key regulator of cellular proliferation and is frequently inactivated in human cancer. This tumor suppressor functions in the p53 and pRb cell cycle regulatory pathways and can effectively activate both pathways to induce growth arrest or cell death. We now report that p14(ARF) forms a complex with the E1A-regulated transcriptional repressor, p120(E4F). p120(E4F) contacts p14(ARF) and p53 to form a ternary complex in vivo and enhances p14(ARF)-induced G(2) cell cycle arrest in a p53-dependent manner. We suggest that the interaction of p14(ARF) and p120(E4F) forms an important link between the p14(ARF) and p53 tumor suppressor proteins, both of which exhibit enhanced cell cycle inhibitory activity in the presence of this transcriptional repressor.

Absence of p16INK4a and truncation of ARF tumor suppressors in chickens

The INK4b-ARF-INK4a locus on human chromosome 9p21 (Human Genome Organization designation CDKN2B-CDKN2A), and the corresponding locus on mouse chromosome 4, encodes three distinct products: two members of the INK4 cyclin-dependent kinase inhibitor family and a completely unrelated protein, ARF, whose carboxyl-terminal half is specified by the second exon of INK4a but in an alternative reading frame. As INK4 proteins block the phosphorylation of the retinoblastoma gene product and ARF protects p53 from degradation, the locus plays a key role in tumor suppression and the control of cell proliferation. To gain further insights into the relative importance of INK4a and ARF in different settings, we have isolated and characterized the equivalent locus in chickens. Surprisingly, although we identified orthologues of INK4b and ARF, chickens do not encode an equivalent of INK4a. Moreover, the reading frame for chicken ARF does not extend into exon 2, because splicing occurs in a different register to that used in mammals. The resultant 60-aa product nevertheless shares functional attributes with its mammalian counterparts. As well as indicating that the locus has been subject to dynamic evolutionary pressures, these unexpected findings suggest that in chickens, the tumor-suppressor functions of INK4a have been compensated for by other genes.