CARF is a novel protein that cooperates with mouse p19ARF (human p14ARF) in activating p53

Molecular Characterization of Cancer Preventive and Therapeutic Potential of Three Antistress Compounds, Triethylene Glycol, Withanone, and Withaferin A

The molecular link between stress and carcinogenesis and the positive outcomes of stress intervention in cancer therapy have recently been well documented. Cancer stem cells (CSCs) facilitate cancer malignancy, drug resistance, and relapse and, hence, have emerged as a new therapeutic target. Here, we aimed to investigate the effect of three previously described antistress compounds (triethylene glycol, TEG; Withanone, Wi-N, and Withaferin A, Wi-A) on the stemness and differentiation characteristics of cancer cells. Breast carcinoma, glioblastoma, and neuroblastoma cells were treated with a non-toxic concentration of TEG (0.1%), Wi-N (5 µM), and Wi-A (0.1 µM) in 2D and 3D cultures. The results demonstrated that TEG, Wi-N, and Wi-A suppressed the stemness properties, which was linked with their inhibition of epithelial-mesenchymal transition (EMT) signaling. In particular, Wi-N and TEG caused a stronger reduction in the self-renewal capability of CSCs than Wi-A, as evidenced by a tumor spheroid formation assay and analyses of stemness-related genes (ALDH1, CD44, NANOG, CD133, SOX2). Furthermore, TEG and Wi-N caused the differentiation of cancer cells. Each of these was supported by (i) the upregulation of KRT18, KRT19, E-cadherin, and downregulation of vimentin in breast carcinoma; (ii) increased levels of GFAP, MAP2, and PSD-95 in astrocytoma; and (iii) increased NeuN, GAP-43, and NF200 levels in neuroblastoma. Furthermore, a reduction in cancer progression-related proteins (PI3K, N-myc) was recorded in treated cells. Our results suggest that TEG and Wi-N may be recruited to target cancer cell stemness and differentiation therapy.

Three-Way Cell-Based Screening of Antistress Compounds: Identification, Validation, and Relevance to Old-Age-Related Pathologies

A variety of environmental stress stimuli have been linked to poor quality of life, tissue dysfunctions, and ailments including metabolic disorders, cognitive impairment, and accelerated aging. Oxidative, metal, and hypoxia stresses are largely associated with these phenotypes. Whereas drug development and disease therapeutics have advanced remarkably in last 3 decades, there are still limited options for stress management. Because the latter can effectively decrease the disease burden, we performed cell-based screening of antistress compounds by recruiting 3 chemical models of oxidative (paraquat), metal (cadmium nitrate), or hypoxia (cobalt chloride) stresses. The screening of 70 compounds for their ability to offer protection against oxidative, metal, and hypoxia stresses resulted in the selection of 5 compounds: Withaferin-A (Wi-A), methoxy Withaferin-A (mWi-A), Withanone (Wi-N), triethylene glycol (TEG), and Ashwagandha (Withania somnifera) leaf M2-DMSO extract (M2DM). Molecular assays revealed that whereas stress caused increase in (a) apoptosis, (b) reactive oxygen species accumulation coupled with mitochondrial depolarization, (c) DNA double-strand breaks, and (d) protein aggregation, low nontoxic doses of the selected compounds caused considerable protection. Furthermore, Wi-N, TEG, and their mixture-treated normal human fibroblasts (at young, mature, and senescent stages representing progressively increasing accumulation of stress) showed increase in proliferation. Taken together, these results suggested 3-way (oxidative, metal, and hypoxia) antistress potential of Wi-N and TEG that may be useful for management of environmental and old-age-related pathologies.

Stress-induced changes in CARF expression serve as a quantitative predictive measure of cell proliferation fate

Discovery of CARF (Collaborator of ARF)/CDKN2AIP as an ARF-interacting protein that promotes ARF-p53-p21^WAF1 signaling and cellular senescence, initially established its role in genomic stress. Multiple reports further unraveled its role in regulation of senescence, growth arrest, apoptosis, or malignant transformation of cells in response to a variety of stress conditions in cultured human cells. It has been established as an essential protein. Whereas CARF-compromised cells undergo apoptosis, its enrichment has been recorded in a variety of cancer cells and has been associated with malignant transformation. We earlier demonstrated its role in stress-induced cell phenotypes that ranged from growth arrest, apoptosis, or malignant transformation. In the present study, we assessed the molecular mechanism of quantitative impact of change in CARF expression level on these cell fates. Stress-induced changes in CARF expression were assessed quantitatively with proteins involved in proteotoxicity, oxidative, genotoxic, and cytotoxic stress. These comparative quantitative analyses confirmed that (i) CARF responds to diverse stresses in a quantitative manner, (ii) its expression level serves as a reliable predictive measure of cell fates (iii) it correlates more with the DNA damage and MDA levels than the oxidative and proteotoxic signatures and (iv) CARF-expression based quantitative assay may be recruited for stress diagnostic applications.

Fatty Acid Excess Dysregulates CARF to Initiate the Development of Hepatic Steatosis

CARF (CDKN2AIP) regulates cellular fate in response to various stresses. However, its role in metabolic stress is unknown. We found that fatty livers from mice exhibit low CARF expression. Similarly, overloaded palmitate inhibited CARF expression in HepG2 cells, suggesting that excess fat-induced stress downregulates hepatic CARF. In agreement with this, silencing and overexpressing CARF resulted in higher and lower fat accumulation in HepG2 cells, respectively. Furthermore, CARF overexpression lowered the ectopic palmitate accumulation in HepG2 cells. We were interested in understanding the role of hepatic CARF and underlying mechanisms in the development of NAFLD. Mechanistically, transcriptome analysis revealed that endoplasmic reticulum (ER) stress and oxidative stress pathway genes significantly altered in the absence of CARF. IRE1α, GRP78, and CHOP, markers of ER stress, were increased, and the treatment with TUDCA, an ER stress inhibitor, attenuated fat accumulation in CARF-deficient cells. Moreover, silencing CARF caused a reduction of GPX3 and TRXND3, leading to oxidative stress and apoptotic cell death. Intriguingly, CARF overexpression in HFD-fed mice significantly decreased hepatic steatosis. Furthermore, overexpression of CARF ameliorated the aberrant ER function and oxidative stress caused by fat accumulation. Our results further demonstrated that overexpression of CARF alleviates HFD-induced insulin resistance assessed with ITT and GTT assay. Altogether, we conclude that excess fat-induced reduction of CARF dysregulates ER functions and lipid metabolism leading to hepatic steatosis.

A Low Dose Combination of Withaferin A and Caffeic Acid Phenethyl Ester Possesses Anti-Metastatic Potential In Vitro: Molecular Targets and Mechanisms

Simple Summary

Cancer therapy suffers from its high cost and high rate of adverse effects and relapse of the disease. Hence, the new (preferably natural), economic and safer therapeutic as well preventive measures have been on demand and have been subject of priority research. We have, earlier, demonstrated anticancer activity in the extracts of Ashwagandha leaves and propolis. A combination of Wi-A (an active anticancer ingredient in Ashwagandha extract) and CAPE (an active anticancer ingredient in propolis) was earlier shown to offer higher and cancer cell-selective cytotoxicity. In the present study, we report an anti-metastasis activity in the low dose combination of Wi-A and CAPE along with its mechanism of action and propose its use in cancer metastasis treatment.

Abstract

Withaferin A (Wi-A) and Caffeic Acid Phenethyl Ester (CAPE) are the bioactive ingredients of Ashwagandha (Withania somnifera) and propolis, respectively. Both of these natural compounds have been shown to possess anticancer activity. In the present study, we recruited a low dose of each of these compounds and developed a combination that exhibited remarkably potent anti-migratory and anti-angiogenic activities. Extensive molecular analyses including a cDNA array and expression analyses of the specific gene targets demonstrated that such activities are mediated through their effect on cell adhesion/tight junction proteins (Claudins, E-cadherin), inhibition of canonical Wnt/β-catenin signaling pathways and the consequent downregulation of EMT-signaling proteins (Vimentin, MMPs, VEGF and VEGFR) that play a critical role in cancer metastasis. The data supported that this novel combination of Wi-A and CAPE (Wi-ACAPE, containing 0.5 µM of Wi-A and 10 µM of CAPE) may be recruited for the treatment of metastatic and aggressive cancers and, hence, warrant further evaluation by recruiting a variety of experimental and clinical metastatic models.

Nuclear RNA Regulation by XRN2 and XTBD Family Proteins

XRN2 is a 5'-to-3' exoribonuclease that is predominantly localized in the nucleus. By degrading or trimming various classes of RNA, XRN2 contributes to essential processes in gene expression such as transcription termination and ribosome biogenesis. Despite limited substrate specificity in vitro, XRN2 targets a specific subset of RNA by interacting with other proteins in cells. Here we review the functions of proteins that have an evolutionarily conserved XRN2-binding domain, XTBD. These proteins modulate activity of XRN2 by stabilizing it, controlling its subcellular localization or recruiting it to specific RNA targets, and thereby impact on various cellular processes.Key words: RNA regulation, XRN2, XTBD, ribosome biogenesis, subcellular localization.

LncRNA SAMMSON Mediates Adaptive Resistance to RAF Inhibition in BRAF-Mutant Melanoma Cells

The long noncoding RNA (lncRNA) SAMMSON is required for human melanoma cell growth and survival. However, whether SAMMSON regulates the response of mutant BRAF melanoma cells to RAF inhibitors remains unknown. In this work, we showed that SAMMSON is rapidly induced upon inhibition of ERK signaling, and SAMMSON overexpression conferred resistance to vemurafenib-induced cytotoxicity in melanoma cells. SOX10 mediated transcriptional induction of SAMMSON by vemurafenib, and SOX10 sumoylation at K55 was essential for this function. In addition, depletion of SAMMSON activated p53 signaling, which is dependent on the SAMMSON-interacting protein CARF. Depletion of SAMMSON sensitized mutant BRAF melanoma cells to RAF inhibitors in vitro and in vivo, while CARF knockdown reversed the enhanced sensitivity. In summary, these findings suggest that SAMMSON may function as a new mediator of adaptive resistance to RAF inhibitors in melanoma by modulating CARF-p53 signaling. SIGNIFICANCE: This study highlights the role of a SAMMSON/CARF/p53 signaling axis in modulating the adaptive resistance of mutant BRAF melanoma to RAF inhibitors.

Induction of Senescence in Cancer Cells by a Novel Combination of Cucurbitacin B and Withanone: Molecular Mechanism and Therapeutic Potential

Cancer, an uncontrolled proliferation syndrome, is treated with synthetic chemotherapeutic drugs that are associated with severe adverse effects. Development and application of new natural compounds is warranted to deal with the exponentially increasing incidence of cancer worldwide. Keeping selective toxicity to cancer cells as a priority criterion, we developed a combination of Cucurbitacin B and Withanone, and analyzed its anticancer potential using non-small cell lung cancer cells. We demonstrate that the selective cytotoxicity of the combination, called CucWi-N, to cancer cells is mediated by induction of cellular senescence that was characterized by decrease in Lamin A/C, CDK2, CDK4, Cyclin D, Cyclin E, phosphorylated RB, mortalin and increase in p53 and CARF proteins. It compromised cancer cell migration that was mediated by decrease in mortalin, hnRNP-K, vascular endothelial growth factor, matrix metalloproteinase 2, and fibronectin. We provide in silico, molecular dynamics and experimental data to support that CucWi-N (i) possesses high capability to target mortalin-p53 interaction and hnRNP-K proteins, (ii) triggers replicative senescence and inhibits metastatic potential of the cancer cells, and (iii) inhibits tumor progression and metastasis in vivo. We propose that CucWi-N is a potential natural anticancer drug that warrants further mechanistic and clinical studies.

CARF promotes spermatogonial self-renewal and proliferation through Wnt signaling pathway

Collaborator of ARF (CARF) regulates cell proliferative fate through both p53-dependent and -independent mechanisms. Recently, we reported a new function of CARF as a positive regulator of Wnt signaling. Despite these findings, the physiological function of CARF has not been well studied. Here, we generated CARF knockout mice and found that male CARF^-/- mice exhibited significantly impaired fertility and Sertoli-cell-only (SCO) syndrome phenotypes. Further studies revealed that loss of CARF in Sertoli cells led to decreased GDNF expression, which hindered spermatogonial stem cells (SSCs) self-renewal. Meanwhile, CARF loss in undifferentiated spermatogonia impaired their proliferation. These two mechanisms together led to SCO syndrome phenotypes, which could be functionally rescued by pharmacological or genetic reactivation of Wnt signaling. Finally, we identified CARF^S351F as a potential pathogenic mutation in an SCO patient. Overall, our findings reveal important roles of CARF in spermatogonial self-renewal and proliferation through the Wnt signaling pathway.

Stress-induced changes in CARF expression determine cell fate to death, survival, or malignant transformation

CARF (Collaborator of ARF) was discovered as an ARF-interacting protein that activated ARF-p53-p21WAF1 signaling involved in cellular response to a variety of stresses, including oxidative, genotoxic, oncogenic, or telomere deprotection stresses, leading to senescence, growth arrest, or apoptosis. Of note, whereas suppression of CARF was lethal, its enrichment was associated with increased proliferation and malignant transformation of cells. These reports have predicted that CARF could serve as a multi-stress marker with a predictive value for cell fates. Here, we recruited various in vitro stress models and examined their effect on CARF expression using human normal fibroblasts. We demonstrate that CARF levels in stress and post-stress conditions could predict the fate of cells towards either death or enhanced proliferation and malignant transformation. We provide extensive molecular evidence that (i) CARF expression changes in response to stress, (ii) it modulates cell death or survival signaling and determines the fate of cells, and (iii) it may serve as a predictive measure of cellular response to stress and an important marker for biosafety.

Soyasapogenol-A targets CARF and results in suppression of tumor growth and metastasis in p53 compromised cancer cells

We screened some phytochemicals for cytotoxic activity to human cancer cells and identified Soyasapogenol-A (Snol-A) as a potent candidate anti-cancer compound. Interestingly, Soyasapogenin-I (Snin-I) was ineffective. Viability assays endorsed toxicity of Snol-A to a wide variety of cancer cells. Of note, wild type p53 deficient cancer cells (SKOV-3 and Saos-2) also showed potent growth inhibitory effect. Molecular analyses demonstrated that it targets CARF yielding transcriptional upregulation of p21^WAF1 (an inhibitor of cyclin-dependent kinases) and downregulation of its effector proteins, CDK2, CDK-4, Cyclin A and Cyclin D1. Targeting of CARF by Snol-A also caused (i) downregulation of pATR-Chk1 signaling leading to caspase-mediated apoptosis and (ii) inactivation of β-catenin/Vimentin/hnRNPK-mediated EMT signaling resulting in decrease in migration and invasion of cancer cells. In in vivo assays, Snol-A caused suppression of tumor growth in subcutaneous xenograft model and inhibited lung metastasis in tail vein injection model. Taken together, we demonstrate that Snol-A is a natural inhibitor of CARF and may be recruited as a potent anti-tumor and anti-metastasis compound for treatment of p53-deficient aggressive malignancies.

Id1 expression in kidney endothelial cells protects against diabetes-induced microvascular injury

The inhibitor of differentiation (Id) transcription regulators, which are induced in response to oxidative stress, promote cell proliferation and inhibit senescence. Inhibitor of differentiation 1 (Id1) expression is limited to endothelial cells (EC) in the normal mouse kidney and is required for a normal response to injury. Endothelial dysfunction leads to the development of diabetic nephropathy, and so, we hypothesized that endothelial Id1 may help protect against hyperglycemia-induced microvascular injury and nephropathy. Here, we tested this hypothesis by using streptozotocin to induce diabetes in Id1 knockout (KO) mice and WT B6;129 littermates and examining the mice at 3 months. Expression of Id1 was observed to be increased 15-fold in WT kidney EC, and Id1 KO mice exhibited increased mesangial and myofibroblast proliferation, matrix deposition, and albuminuria compared with WT mice. Electron microscopy demonstrated peritubular capillary EC injury and lumen narrowing, and fluorescence microangiography showed a 45% reduction in capillary perfusion area with no reduction in CD31-stained areas in Id1 KO mice. Microarray analysis of EC isolated from WT and KO control and diabetic mice demonstrated activation of senescence pathways in KO cells. Kidneys from KO diabetic mice showed increased histological expression of senescence markers. In addition, premature senescence in cultured KO EC was also seen in response to oxidative stress. In conclusion, endothelial Id1 upregulation with hyperglycemia protects against microvascular injury and senescence and subsequent nephropathy.

Modulation of Diacylglycerol-Induced Melanogenesis in Human Melanoma and Primary Melanocytes: Role of Stress Chaperone Mortalin

Skin color/pigmentation is regulated through melanogenesis process in specialized melanin-producing cells, melanocytes, involving multiple signaling pathways. It is highly influenced by intrinsic and extrinsic factors such as oxidative, ultraviolet radiations and other environmental stress conditions. Besides determining the color, it governs response and tolerance of skin to a variety of environmental stresses and pathological conditions including photodamage, hyperpigmentation, and skin cancer. Depigmenting reagents have been deemed useful not only for cosmetics but also for pigmentation-related pathologies. In the present study, we attempted modulation of 1-oleoyl-2-acetyl-glycerol- (OAG-) induced melanogenesis in human melanoma and primary melanocytes. In both cell types, OAG-induced melanogenesis was associated with increase in enhanced expression of melanin, tyrosinase, as well as stress chaperones (mortalin and HSP60) and Reactive Oxygen Species (ROS). Treatment with TXC (trans-4-(Aminomethyl) cyclohexanecarboxylic acid hexadecyl ester hydrochloride) and 5/40 natural compounds resulted in their reduction. The data proposed an important role of mortalin and oxidative stress in skin pigmentation and the use of TXC and natural extracts for modulation of pigmentation pathways in normal and pathological conditions.

CARF enrichment promotes epithelial-mesenchymal transition via Wnt/β-catenin signaling: its clinical relevance and potential as a therapeutic target

CARF (Collaborator of ARF)/CDKN2AIP was discovered as a novel ARF-binding protein. It has been established as an essential cell survival, p53-, and cell proliferation-regulatory protein. Although a moderate upregulation of CARF caused growth arrest and senescence, its excessively enriched levels were shown to facilitate aggressive proliferation and malignant transformation of cancer cells. Here, we examined the relevance of CARF levels in clinical tumors and found its amplification (both at gene and transcript levels) in a variety of invasive and metastatic malignancies. Consistent with the clinical readouts, enrichment of CARF in cancer cells promoted epithelial–mesenchymal transition (EMT). Cancer database and molecular analyses revealed that it activates Wnt/β-catenin signaling axis, as evident by enhanced nuclear localization and function of β-catenin marked by increased level of SNAIL1, SNAIL2, ZEB1, and TWIST1 and its downstream gene targets. Of note, targeted knockdown of CARF led to decrease in nuclear β-catenin and its key downstream effectors, involved in EMT progression. Consistent with this, CARF targeting in vivo either by naked siRNA or CARF shRNA harboring adeno-oncolytic virus caused suppression of tumor progression and lung metastasis. Taken together, we report clinical and therapeutic relevance of CARF in EMT and cancer invasiveness/metastasis, and propose it as a potent therapeutic target of aggressive cancers.

Tumor suppressor activity of miR-451: Identification of CARF as a new target

microRNAs (miRs) have recently emerged as small non-coding regulators of gene expression. We performed a loss-of-function screening by recruiting retrovirus mediated arbitrary manipulation of genome coupled with escape of cells from 5-Aza-2′-deoxycytidine (5-Aza-dC)-induced senescence. miRNA pool from cells that emerged from 5-Aza-dC-induced senescence was subjected to miR-microarray analysis with respect to the untreated control. We identified miR-451 as one of the upregulated miRs and characterized its functional relevance to drug resistance, cell growth, tumor suppressor proteins p53 and pRb, and stress response. We report that miR-451 caused growth arrest in cells leading to their resistance to 5-Aza-dC-induced senescence. Decrease in cyclin D1, CDK4 and phosphorylated pRB supported the growth arrest in miR-451 transfected cells. We demonstrate that Collaborator of ARF (CARF) protein is a new target of miR-451 that intermediates its function in tumor suppressor and stress signaling.

Chemical biology reveals CARF as a positive regulator of canonical Wnt signaling by promoting TCF/β-catenin transcriptional activity

Wnt/β-catenin signaling regulates multiple biological processes and aberration of this pathway is frequently observed in human cancers. Previously, we uncovered NC043 as a small-molecule inhibitor of Wnt/β-catenin signaling. Here, we identified CARF as the cellular target of NC043. We found that NC043 binds directly to CARF through forming a covalent bond with the Cys-516 residue of CARF. Further study revealed that CARF interacts with Dvl, which potentiates the Dvl-c-Jun-β-catenin-TCF transcriptional complex and thus promotes Wnt signaling activation. NC043 could disrupt the interaction between CARF and Dvl, thereby impairing Wnt signal transduction. In line with this, knockdown of CARF in zebrafish leads to impairment of embryonic development, hematopoietic stem cell generation and caudal fin regeneration. Collectively, we identified CARF as the cellular target of NC043 and revealed CARF as a positive regulator of Wnt/β-catenin signal transduction.

Structural basis and function of XRN2 binding by XTB domains

The ribonuclease XRN2 is an essential player in RNA metabolism. In Caenorhabditis elegans, XRN2 functions with PAXT-1, which shares a putative XRN2-binding domain (XTBD) with otherwise unrelated mammalian proteins. Here, we characterize structure and function of an XTBD – XRN2 complex. Although XTBD stably interconnects two XRN2 domains through numerous interacting residues, mutation of a single critical residue suffices to disrupt XTBD – XRN2 complexes in vitro, and recapitulates paxt-1 null mutant phenotypes in vivo. Demonstrating conservation of function, vertebrate XTBD-containing proteins bind XRN2 in vitro, and human CDKN2AIPNL (C2AIL) can substitute for PAXT-1 in vivo. In vertebrates, where three distinct XTBD-containing proteins exist, XRN2 may partition to distinct stable heterodimeric complexes, likely differing in subcellular localization or function. In C. elegans, complex formation with the unique PAXT-1 serves to preserve the stability of XRN2 in the absence of substrate.

Collaborator of alternative reading frame protein (CARF) regulates early processing of pre-ribosomal RNA by retaining XRN2 (5'-3' exoribonuclease) in the nucleoplasm

Collaborator of alternative reading frame protein (CARF) associates directly with ARF, p53, and/or human double minute 2 protein (HDM2), a ubiquitin-protein ligase, without cofactors and regulates cell proliferation by forming a negative feedback loop. Although ARF, p53, and HDM2 also participate in the regulation of ribosome biogenesis, the involvement of CARF in this process remains unexplored. In this study, we demonstrate that CARF associates with 5′-3′ exoribonuclease 2 (XRN2), which plays a major role in both the maturation of rRNA and the degradation of a variety of discarded pre-rRNA species. We show that overexpression of CARF increases the localization of XRN2 in the nucleoplasm and a concomitant suppression of pre-rRNA processing that leads to accumulation of the 5′ extended from of 45S/47S pre-rRNA and 5′-01, A0-1 and E-2 fragments of pre-rRNA transcript in the nucleolus. This was also observed upon XRN2 knockdown. Knockdown of CARF increased the amount of XRN2 in the nucleolar fraction as determined by cell fractionation and by immnocytochemical analysis. These observations suggest that CARF regulates early steps of pre-rRNA processing during ribosome biogenesis by controlling spatial distribution of XRN2 between the nucleoplasm and nucleolus.

Collaborator of ARF (CARF) regulates proliferative fate of human cells by dose-dependent regulation of DNA damage signaling

Collaborator of ARF (CARF) has been shown to directly bind to and regulate p53, a central protein that controls tumor suppression via cellular senescence and apoptosis. However, the cellular functions of CARF and the mechanisms governing its effect on senescence, apoptosis, or proliferation are still unknown. Our previous studies have shown that (i) CARF is up-regulated during replicative and stress-induced senescence, and its exogenous overexpression caused senescence-like growth arrest of cells, and (ii) suppression of CARF induces aneuploidy, DNA damage, and mitotic catastrophe, resulting in apoptosis via the ATR/CHK1 pathway. In the present study, we dissected the cellular role of CARF by investigating the molecular pathways triggered by its overexpression in vitro and in vivo. We found that the dosage of CARF is a critical factor in determining the proliferation potential of cancer cells. Most surprisingly, although a moderate level of CARF overexpression induced senescence, a very high level of CARF resulted in increased cell proliferation. We demonstrate that the level of CARF is crucial for DNA damage and checkpoint response of cells through ATM/CHK1/CHK2, p53, and ERK pathways that in turn determine the proliferative fate of cancer cells toward growth arrest or proproliferative and malignant phenotypes. To the best of our knowledge, this is the first report that demonstrates the capability of a fundamental protein, CARF, in controlling cell proliferation in two opposite directions and hence may play a key role in tumor biology and cancer therapeutics.

Molecular characterization of collaborator of ARF (CARF) as a DNA damage response and cell cycle checkpoint regulatory protein

CARF is an ARF-binding protein that has been shown to regulate the p53-p21-HDM2 pathway. CARF overexpression was shown to cause growth arrest of human cancer cells and premature senescence of normal cells through activation of the p53 pathway. Because replicative senescence involves permanent withdrawal from the cell cycle in response to DNA damage response-mediated signaling, in the present study we investigated the relationship between CARF and the cell cycle and whether it is involved in the DNA damage response. We demonstrate that the half-life of CARF protein is less than 60 min, and that in cycling cells CARF levels are highest in G2 and early prophase. Serially passaged normal human skin and stromal fibroblasts showed upregulation of CARF during replicative senescence. Induction of G1 growth arrest and senescence by a variety of drugs was associated with increase in CARF expression at the transcriptional and translational level and was seen to correlate with increase in DNA damage response and checkpoint proteins, ATM, ATR, CHK1, CHK2, γH2AX, p53 and p21. Induction of growth arrest by oncogenic RAS and shRNA-mediated knockdown of TRF2 in cancer cells also caused upregulation of CARF. We conclude that CARF is associated with DNA damage response and checkpoint signaling pathways.

BCL6-mediated repression of p53 is critical for leukemia stem cell survival in chronic myeloid leukemia

Chronic myeloid leukemia (CML) is induced by the oncogenic BCR-ABL1 tyrosine kinase and can be effectively treated for many years with tyrosine kinase inhibitors (TKIs). However, unless CML patients receive life-long TKI treatment, leukemia will eventually recur; this is attributed to the failure of TKI treatment to eradicate leukemia-initiating cells (LICs). Recent work demonstrated that FoxO factors are critical for maintenance of CML-initiating cells; however, the mechanism of FoxO-dependent leukemia initiation remained elusive. Here, we identified the BCL6 protooncogene as a critical effector downstream of FoxO in self-renewal signaling of CML-initiating cells. BCL6 represses Arf and p53 in CML cells and is required for colony formation and initiation of leukemia. Importantly, peptide inhibition of BCL6 in human CML cells compromises colony formation and leukemia initiation in transplant recipients and selectively eradicates CD34⁺ CD38⁻ LICs in patient-derived CML samples. These findings suggest that pharmacological inhibition of BCL6 may represent a novel strategy to eradicate LICs in CML. Clinical validation of this concept could limit the duration of TKI treatment in CML patients, which is currently life-long, and substantially decrease the risk of blast crisis transformation.

p14(ARF) inhibits the functions of adenovirus E1A oncoprotein

The tumour suppressor ARF (alternative reading frame) is one of the most important oncogenic stress sensors. ARF provides an 'oncogenic checkpoint' function through both p53-dependent and p53-independent mechanisms. In the present study, we demonstrate a novel p53-independent interaction between p14(ARF) and the adenovirus oncoprotein E1A. p14(ARF) inhibits E1A transcriptional function and promotes ubiquitination-dependent degradation of E1A. p14(ARF) overexpression relocalizes E1A into the nucleolus and inhibits E1A-induced cellular DNA replication independent of p53. Knockdown of endogenous p14(ARF) increases E1A transactivation. In addition, E1A can competitively inhibit ARF-Mdm2 (murine double minute 2) complex formation. These results identify a novel binding partner of p14(ARF) and reveal a mutually inhibitory interaction between p14(ARF) and E1A. We speculate that the ARF-E1A interaction may represent an additional host defence mechanism to limit viral replication. Alternatively, the interaction may allow adenovirus to sense the functional state of p53 in host cells, and fine-tune its own replication activity to prevent the triggering of a detrimental host response.

Molecular characterization of apoptosis induced by CARF silencing in human cancer cells

Collaborator of ARF (CARF) was cloned as an ARF-interacting protein and shown to regulate the p53-p21(WAF1)-HDM2 pathway, which is central to tumor suppression via senescence and apoptosis. We had previously reported that CARF inhibition in cancer cells led to polyploidy and caspase-dependent apoptosis, however, the mechanisms governing this phenomenon remained unknown. Thus, we examined various cell death and survival pathways including the mitochondrial stress, ataxia telangiectasia mutated (ATM)-ATR, Ras-MAP kinase and retinoblastoma cascades. We found that CARF is a pleiotropic regulator with widespread effects; its suppression affected all investigated pathways. Most remarkably, it protected the cells against genotoxicity; CARF knockdown elicited DNA damage response as evidenced by increased levels of phosphorylated ATM and γH2AX, leading to induction of mitotic arrest and eventual apoptosis. We also show that the CARF-silencing-induced apoptosis in vitro translates to in vivo. In a human tumor xenograft mouse model, treatment of developing tumors with short hairpin RNA (shRNA) against CARF via an adenovirus carrier induced complete suppression of tumor growth, suggesting that CARF shRNA is a strong candidate for an anticancer reagent. We demonstrate that CARF has a vital role in genome preservation and tumor suppression and CARF siRNA is an effective novel cancer therapeutic agent.

Malignant pleural mesothelioma: genome-wide expression patterns reflecting general resistance mechanisms and a proposal of novel targets

Malignant pleural mesothelioma is an asbestos-related multi-resistant tumour with increasing incidence worldwide. Well-characterized snap-frozen normal parietal, visceral pleura and mesothelioma samples were analysed with Affymetrix Human Genome U133 Plus 2.0 GeneChip oligoarray of 38500 genes. We discovered a close relation between gene profile and resistance towards topoisomerase poisons, alkylating agents, antitubulines, antifolates, platinum compounds and radiation therapy. Target genes of chemo- (e.g. TOP2A, BIRC5/Survivin and proteasome) and radiotherapy (e.g. BRCA2, FANCA, FANCD2, CCNB1 and RAD50) were significantly overexpressed. The Fanconi anemia/BRCA2 pathway, responsible for homologous recombination DNA repair appears as a key pathway in both chemo- and radio-resistance of mesothelioma. Leukocyte trans-endothelial migration gene down-regulation could partly explain resistance against immunological therapies. Gene expression features found in other resistant cancer types related to DNA repair and replication are shared by mesothelioma and could represent general features of tumour resistance. Targeted suppression of some of those key genes and pathways combined with chemotherapy or radiation could improve the outcome of mesothelioma therapy. We propose CHEK1, RAD21, FANCD2 and RAN as new co-targets for mesothelioma treatment. The pro-angiogenic AGGF1 mRNA and protein was highly overexpressed in all tumours and may serve as a target for anti-angiogenic treatment. Overexpression of NQO1 may render mesothelioma sensitive to the novel compound beta-Lapachone.

Ischemic and non-ischemic acute kidney injury cause hepatic damage

Recent studies have documented that remote organs are affected by ischemic injury to the kidney. Here we studied whether the liver also suffers damage during induction of renal ischemia-reperfusion in rats and compared this to bilateral nephrectomy. Hepatic levels of tumor necrosis factor-alpha increased significantly after 6 and 24 h of renal ischemia or nephrectomy. Malondialdehyde, an index of lipid peroxidation, increased while total glutathione was decreased in the liver in both the renal ischemia and nephrectomy groups, suggesting activation of oxidative stress. Expression of liver spermine-spermidine acetyl transferase, an enzyme upregulated in early phases of hepatic injury was significantly increased 6 h after either kidney ischemia or nephrectomy. Apoptosis was increased in hepatocytes 24 h after nephrectomy. We also found histological evidence of hepatocyte injury following both ischemia and bilateral nephrectomy. Infusion of reduced glutathione, before the induction of renal ischemia, significantly improved liver architecture and was associated with a reduction in hepatic malondialdehyde and serum alanine transaminase levels. Our study shows that acute kidney ischemia or renal failure activates oxidative stress and promotes inflammation, apoptosis, and tissue damage in hepatocytes.

CARF Is a vital dual regulator of cellular senescence and apoptosis

The tumor suppressor protein, p53, is central to the pathways that monitor the stress, DNA damage repair, cell cycle, aging, and cancer. Highly complex p53 networks involving its upstream sensors and regulators, downstream effectors and regulatory feedback loops have been identified. CARF (Collaborator of ARF) was shown to enhance ARF-dependent and -independent wild-type p53 function. Here we report that (i) CARF overexpression causes premature senescence of human fibroblasts, (ii) it is vital for replicative and stress-induced senescence, and (iii) the lack of CARF function causes aneuploidy and apoptosis. We provide evidence that CARF plays a dual role in regulating p53-mediated senescence and apoptosis, the two major tumor suppressor mechanisms.

Frequent deletion of ING2 locus at 4q35.1 associates with advanced tumor stage in head and neck squamous cell carcinoma

BACKGROUND

Loss of heterozygosity (LOH) in the ING family members has been shown in head and neck squamous cell carcinoma (HNSCC) except for ING2. Like all the other members of ING family, ING2, which is located at chromosome 4q35.1, is a promising tumor suppressor gene (TSG). In this study, we performed LOH analysis of ING2 in HNSCC and compared it with clinicopathological variables.

MATERIALS AND METHODS

We performed LOH analysis in DNAs from 80 paired of normal and HNSCC tissues, using a specifically designed microsatellite marker on chromosome 4q35.1, which detects allelic loss of ING2. TP53 mutation analysis and its relationship with ING2 chromosomal deletion were also performed in available 68 of the samples. The correlation between LOH status and clinicopathological characteristics was evaluated by using statistical methods. The overall survival (OS) and disease free survival (DFS) were also determined.

RESULTS

LOH was detected in 54.6% (30/55) of the informative samples. Statistical significance was obtained between LOH and tumor (T) stage (P = 0.02), application of radiotherapy and chemotherapy. Positive node status (N) appeared to be the only independent prognostic factor for both OS (P = 0.031) and DFS (P = 0.044).

CONCLUSIONS

Our study showed allelic loss of 4q35.1 in HNSCC. The high percentage of LOH suggests ING2 as a candidate TSG in HNSCC. High LOH frequency was statistically associated with advanced T stage, suggesting that ING2 LOH might occur in late stages during HNSCC progression.

Basal cell carcinomas arising within multiple trichoepitheliomas

Although trichoepitheliomas (TEs) are commonly regarded as benign tumors of follicular origin, the natural history of multiple familial trichoepitheliomas (MFT) and their risk for malignancy has been unclear. We describe a 57-year-old male with numerous skin-colored firm papules and plaques present on the central face since 6 years of age. Recently, some lesions had enlarged and ulcerated. Other family members were similarly affected. Biopsies from multiple lesions showed TEs both alone and associated with basal cell carcinoma (BCC) in the same section, suggesting the secondary development of BCCs within TEs. Many prior reports of BCCs arising within TEs in patients with presumed MFT were likely misdiagnosed cases of nevoid BCC. This report is a compelling example of MFT in which BCCs evolved secondarily. Awareness of the potential for the evolution of carcinoma in patients with MFT is important in the management of these patients.

CARF: an emerging regulator of p53 tumor suppressor and senescence pathway

Replicative senescence, a major outcome of normal cells with finite lifespan, is a widely accepted in vitro model for ageing studies. Limited repair and defense mechanisms of normal cells, in addition to DNA alterations and oncogene inductions under stress, are believed to result in senescence as a protective mechanism to prevent undesirable proliferation of cells. The ARF/p53/p21(cip1/waf1) tumor suppression pathway acts as a molecular sensor and regulator of cellular stress, senescence, and immortalization. Understanding the molecular regulation of this pathway by intrinsic and extrinsic signals is extremely important to address unsolved questions in senescence and cancer. CARF was first discovered as a binding partner of ARF and has since been shown to have both ARF-dependent and -independent functions that converge to regulate p53 pathway. CARF directly binds to p53 and HDM2, and functions in a negative feedback pathway. Whereas CARF transcriptionally represses HDM2 to increase p53 activity, HDM2 in return degrades CARF. Thus, CARF may act as a novel key regulator of the p53 pathway at multiple checkpoints. The aim of this article is to discuss the current knowledge about functions of CARF and its impact on p53 pathway in regulation of senescence and carcinogenesis.

CARF binds to three members (ARF, p53, and HDM2) of the p53 tumor-suppressor pathway

The INK4a locus (chromosome 9p21) encodes two structurally distinct tumor-suppressor proteins, p16(INK4a) and the alternative reading frame protein, ARF (p19(ARF) in mouse and p14(ARF) in human). Each of these proteins has a major role in cell cycle control and senescence pathways. We originally identified a novel collaborator of ARF, CARF, from a two-hybrid interactive screen using p19(ARF) as bait and found that CARF interacts with ARF in the perinucleolar region and activates p53 function. In the absence of ARF, it interacts with p53 directly leading to ARF-independent enhancement of p53 function and in turn undergoes a negative feedback regulation. Very recently, we found that CARF interacts with HDM2 and undergoes degradation by an HDM2-dependent proteasome pathway. CARF may exert a vital control on p53-HDM2-p21(WAF1) pathway that is central to the cell cycle control, senescence, and DNA damage response of human cells.

A novel nuclear interactor of ARF and MDM2 (NIAM) that maintains chromosomal stability

The ARF tumor suppressor signals through p53 and other poorly defined anti-proliferative pathways to block carcinogenesis. In a search for new regulators of ARF signaling, we discovered a novel nuclear protein that we named NIAM (nuclear interactor of ARF and MDM2) for its ability to bind both ARF and the p53 antagonist MDM2. NIAM protein is normally expressed at low to undetectable levels in cells because of, at least in part, MDM2-mediated ubiquitination and proteasomal degradation. When reintroduced into cells, NIAM activated p53, caused a G1 phase cell cycle arrest, and collaborated with ARF in an additive fashion to suppress proliferation. Notably, NIAM retains growth inhibitory activity in cells lacking ARF and/or p53, and knockdown experiments revealed that it is not essential for ARF-mediated growth inhibition. Thus, NIAM and ARF act in separate anti-proliferative pathways that intersect mechanistically and suppress growth more effectively when jointly activated. Intriguingly, silencing of NIAM accelerated chromosomal instability, and microarray analyses showed reduced NIAM mRNA expression in numerous primary human tumors. This study identifies a novel protein with tumor suppressor-like behaviors and functional links to ARF-MDM2-p53 signaling.

Circumvention and reactivation of the p53 oncogene checkpoint in mouse colon tumors

The p53 tumor suppressor protein is sequence-normal in azoxymethane (AOM)-induced mouse colon tumors, making them a good model for human colon cancers that retain a wild type p53 gene. Cellular localization and co-immunoprecipitation experiments using a cell line derived from an AOM-induced colon tumor (AJ02-NM(0) cells) pointed to constitutively expressed Mdm2 as being an important negative regulator of p53 in these cells. Although the Mdm2 inhibitory protein p19/ARF was expressed in AJ02-NM(0) cells, its level of expression was not sufficient for p53 activation. We tested the response of AJ02-NM(0) cells to the recently developed Mdm2 inhibitor, Nutlin-3. Nutlin-3 was found to activate p53 DNA binding in AJ02-NM(0) cells, to a level comparable to doxorubicin and 5-fluorouracil (5-FU). In addition, Nutlin-3 increased expression of the p53 target genes Bax and PERP to a greater extent than doxorubicin or 5-FU, and triggered a G2/M phase arrest in these cells, compared to a G1 arrest triggered by doxorubicin and 5-FU. The differences in the cellular response may be related to differences in the kinetics of p53 activation and/or its post-translational modification status. In an ex vivo experiment, Nutlin-3 was found to activate p53 target gene expression and apoptosis in AOM-induced tumor tissue, but not in normal adjacent mucosa. Our data indicate that Mdm2 inhibitors may be an effective means of selectively targeting colon cancers that retain a sequence-normal p53 gene while sparing normal tissue and that the AOM model is an appropriate model for the preclinical development of these drugs.

Divorcing ARF and p53: an unsettled case

Mammalian cells that sustain oncogenic insults can invoke defensive programmes that either halt their division or trigger their apoptosis, but these countermeasures must be finely tuned to discriminate between physiological and potentially harmful growth-promoting states. By functioning specifically to oppose abnormally prolonged and sustained proliferative signals produced by activated oncogenes, the ARF tumour suppressor antagonizes functions of MDM2 to induce protective responses that depend on the p53 transcription factor and its many target genes. However, ARF has been reported to physically associate with proteins other than MDM2 and to have p53-independent activities, most of which remain controversial and poorly understood.

Mutational and LOH analyses of the chromosome 4q region in esophageal adenocarcinoma

OBJECTIVE

Mortality due to esophageal adenocarcinoma has risen markedly, but the molecular mechanisms underlying this carcinogenesis are still incompletely understood. Findings from loss of heterozygosity (LOH) studies have suggested that the long arm of chromosome 4 might harbor tumor suppressor genes relevant to esophageal adenocarcinoma.

METHODS

We performed LOH analysis of 4q in esophageal adenocarcinomas. Regions of LOH were further evaluated by studying two candidate tumor suppressor genes, hCDC4 and CARF, located within them.

RESULTS

54% of the adenocarcinomas examined showed allelic deletion. LOH was observed in 53, 40, 32, 38, and 27% of tumors at positions D4S1554 (the locus of CARF), D4S1572, D4S1548, D4S2934, and D4S3021, respectively. An area of allelic deletion (spanning 3 million bases) was identified at 4q31.1-3 in 37% of tumors. This region harbors a candidate tumor suppressor gene: hCDC4. However, sequencing of the coding regions of CARF and hCDC4 at 4q35 and 4q31, respectively, did not identify mutations.

CONCLUSIONS

Our findings demonstrate frequent LOH in esophageal adenocarcinoma at several loci including a novel area of allelic deletion at 4q31.1-3. The results imply that mutational or other alterations at these loci may be involved in the pathogenesis of esophageal adenocarcinoma. Candidate tumor suppressor genes located within these regions merit further study.

CARF Regulates p19ARF-p53-p21WAF1 Senescence Pathway by Multiple Checkpoints

CARF was first cloned as a novel binding partner of ARF from a yeast-interactive screen. CARF and ARF colocalize in the perinucleolar region and have a collaborative function. In the nucleoplasm, CARF interacts with p53 and enhances its function. We demonstrate that p53 downregulates CARF in a negative feedback regulatory loop and may also involve p53 antagonist HDM2.

Innate immune responses in NF-kappaB-repressing factor-deficient mice

NF-kappaB-repressing factor (NRF) is a transcriptional silencer protein that specifically counteracts the basal activity of several NF-kappaB-dependent promoters by direct binding to specific neighboring DNA sequences. In cell culture experiments, the reduction of NRF mRNA leads to a derepression of beta interferon, interleukin-8, and inducible nitric oxide synthase transcription. The X chromosome-located single-copy NRF gene is ubiquitously expressed and encodes a protein of 690 amino acids. The N-terminal part contains a nuclear localization signal, the DNA-binding domain, and the NF-kappaB-repressing domain, while the C-terminal part is responsible for double-stranded RNA binding and nucleolar localization. To study the function of NRF in a systemic context, transgenic mice lacking the NRF gene were created. Against predictions from in vitro experiments, mice with a deletion of the NRF gene are viable and have a phenotype that is indistinguishable from wild-type mice, even after challenge with different pathogens. The data hint towards an unexpected functional redundancy of NRF.

How the Nucleolar Sequestration of p53 Protein or Its Interplayers Contributes to Its (Re)-Activation

The tumor suppressor p53 is a short-lived protein that under normal conditions is reduced to a barely detectable level. The stability of p53 protein is primarily regulated in normal non-transformed cells by two interplayers: Mdm2 and p14(ARF). Relocation of p53, Mdm2, and p14(ARF) to the nucleolus seems to regulate, at least partially, the steady-state of p53. Moreover, there are alternative pathways of the regulation of p53 stability in unstressed cells. Jun-N(amino)-terminal kinase (JNK) and poly(ADP-ribose) polymerase-1 (PARP-1) are involved in the regulation of the steady-state of wild-type (wt) p53 protein. However, in most human cervical carcinomas, which express the high-risk human papilloma viruses (HPVs) E6 protein, a complete switch from Mdm2 to HPV E6-mediated degradation of p53 occurs. Virally encoded E6 protein utilizes the cellular ubiquitin-protein ligase termed E6-associated protein (E6-AP) to target p53 protein for proteolytic degradation. We recently addressed the question of whether p53 protein can be generally reactivated by chemotherapy in HeLa cells despite the E6 activity. We observed an increase of cellular p53 after cisplatin (CP) treatment. p53 protein accumulated preferentially in the nucleoli. We checked the cellular level of E6 during CP therapy. Six hours after application of CP the expression of E6 protein was markedly reduced. This coincided with the increase of cellular p53 level and preceded the nucleolar accumulation of p53 protein, thereby indicating that repression of virally coded E6 protein by CP contributes to the restoration of p53 expression.

A novel ARF-binding protein (LZAP) alters ARF regulation of HDM2

The tumour suppressor ARF (alternative reading frame) is encoded by the INK4a (inhibitor of cyclin-dependent kinase 4)/ARF locus, which is frequently altered in human tumours. ARF binds MDM2 (murine double minute 2) and releases p53 from inhibition by MDM2, resulting in stabilization, accumulation and activation of p53. Recently, ARF has been found to associate with other proteins, but, to date, little is known about ARF-associated proteins that are implicated in post-translational regulation of ARF activity. Using a yeast two-hybrid screen, we have identified a novel protein, LZAP (LXXLL/leucine-zipper-containing ARF-binding protein), that interacts with endogenous ARF in mammalian cells. In the present study, we show that LZAP reversed the ability of ARF to inhibit HDM2's ubiquitin ligase activity towards p53, but simultaneously co-operated with ARF, maintaining p53 stability and increasing p53 transcriptional activity. Expression of LZAP, in addition to ARF, increased the percentage of cells in the G1 phase of the cell cycle. Expression of LZAP also caused activation of p53 and a p53-dependent G1 cell-cycle arrest in the absence of ARF. Taken together, our data suggest that LZAP can regulate ARF biochemical and biological activity. Additionally, LZAP has p53-dependent cell-cycle effects that are independent of ARF.

The co-transfection of p16(INK4a) and p14(ARF) genes into human lung cancer cell line A549 and the effects on cell growth and chemosensitivity

Two functionally and structurally different proteins, p16(INK4a) and p14(ARF), encoded by the gene INK4a/ARF located at 9p21 are cyclin-dependent kinase (cdk) inhibitors and important cell cycle regulators. More and more evidences have been accumulated to show that the exogenous p16(INK4a) or p14(ARF) can inhibit the cell growth and/or induce the apoptosis. But it is still unclear if they can play positive role when combine with the conventional chemotherapy in cancer treatment. Here we show that cationic liposome-mediated gene transfection of INK4a/ARF into lung cancer cell line A549, in which the INK4a/ARF locus was lost, suppressed the growth and induced apoptosis. When treated with five different chemotherapy drugs with different mechanism after the transfection, A549 got an increased chemosensitivity for adriamycin and cisplatin and an unchanged result for topotecan, taxol or vinorelbine. The results indicated that cell cycle redistribution and increased apoptosis index after transfection might be the main explanation for the enhanced chemosensitivity. The combination of gene therapy with conventional chemotherapy is not always better than single chemotherapy. This trial will be of benefit to the treatment of lung cancer when combine the conventional chemotherapy and gene therapy in the future.

Activation of wild type p53 function by its mortalin-binding, cytoplasmically localizing carboxyl terminus peptides

The Hsp70 family member mortalin (mot-2/mthsp70/GRP75) binds to a carboxyl terminus region of the tumor suppressor protein p53. By in vivo co-immunoprecipitation of mot-2 with p53 and its deletion mutants, we earlier mapped the mot-2-binding site of p53 to its carboxyl terminus 312-352 amino acid residues. In the present study we attempted to disrupt mot-2-p53 interactions by overexpression of short p53 carboxyl-terminal peptides. We report that p53 carboxyl-terminal peptides (amino acid residues 312-390, 312-352, 323-390, and 323-352) localize in the cytoplasm, whereas 312-322, 337-390, 337-352, and 352-390 locate mostly in the nucleus. Most interestingly, the cytoplasmically localizing p53 peptides harboring the residues 323-337 activated the endogenous p53 function by displacing it from p53-mortalin complexes and relocating it to the nucleus. Such activation of p53 function was sufficient to cause growth arrest of human osteosarcoma and breast carcinoma cells.

Nucleophosmin (B23) targets ARF to nucleoli and inhibits its function

The ARF tumor suppressor is a nucleolar protein that activates p53-dependent checkpoints by binding Mdm2, a p53 antagonist. Despite persuasive evidence that ARF can bind and inactivate Mdm2 in the nucleoplasm, the prevailing view is that ARF exerts its growth-inhibitory activities from within the nucleolus. We suggest ARF primarily functions outside the nucleolus and provide evidence that it is sequestered and held inactive in that compartment by a nucleolar phosphoprotein, nucleophosmin (NPM). Most cellular ARF is bound to NPM regardless of whether cells are proliferating or growth arrested, indicating that ARF-NPM association does not correlate with growth suppression. Notably, ARF binds NPM through the same domains that mediate nucleolar localization and Mdm2 binding, suggesting that NPM could control ARF localization and compete with Mdm2 for ARF association. Indeed, NPM knockdown markedly enhanced ARF-Mdm2 association and diminished ARF nucleolar localization. Those events correlated with greater ARF-mediated growth suppression and p53 activation. Conversely, NPM overexpression antagonized ARF function while increasing its nucleolar localization. These data suggest that NPM inhibits ARF's p53-dependent activity by targeting it to nucleoli and impairing ARF-Mdm2 association.

DNA damage uncouples the mitogenic response to IGF-I in MCF-7 malignant breast cancer cells by switching the roles of PI3 kinase and p21WAF1/Cip1

In addition to its mitogenic and survival actions, recent evidence indicates that IGF-I can enhance DNA repair, implying IGF activity may limit the efficacy of many therapeutic strategies that rely on induction of DNA damage. Although the individual pathways by which DNA damage and IGF-I act are well understood, the cross-talk between these signaling events is not well defined. We examined the effects of DNA damage on the IGF-I response of MCF-7 breast cancer cells. Cells were exposed to the UV mimetic, 4-NQO, or the gamma-irradiation mimetic and chemotherapeutic drug, bleomycin; cellular proliferation was assessed by cell counting, tritiated thymidine incorporation and FACS cell cycle analysis. Although IGF-I acutely suppressed the p53 response to both agents, it subsequently enhanced the chronic increase in p53 and p21(WAF1/Cip1), resulting in cell cycle arrest; however, no apoptosis was observed. Use of specific inhibitors demonstrated that PI3 kinase was utilized with p38 MAPK to induce the p53 response to DNA damage, but was also utilized by IGF-I to diminish the acute p53 response. In addition, p21WAF1/Cip1 was increased by IGF-I, which has previously been shown to contribute to the mitogenic response. Here we demonstrate that with DNA damage IGF-I can also enhance the chronic p53-dependent increase in p21WAF1/Cip1, culminating in growth arrest. Overall, we have shown that PI3 kinase and p21WAF1/Cip1 play dual roles in mediating the mitogenic response to IGF-I, but these are both switched by cellular DNA damage to mediate a growth arrest.

Alternative reading frame protein (ARF)-independent function of CARF (collaborator of ARF) involves its interactions with p53: evidence for a novel p53-activation pathway and its negative feedback control

CARF, a collaborator of ARF (alternative reading frame protein), was cloned as a novel ARF-binding protein from a yeast-interaction screen. It potentiated ARF-mediated p53 function, and also caused a moderate increase in p53 activity in the absence of ARF. We herein report the molecular mechanism of ARF-independent function of CARF. By employing a variety of approaches, including overexpression of CARF, its suppression by small interfering RNA and use of protease inhibitors, we demonstrate that: (i) CARF directly interacts with wild-type p53, causing its stabilization and functional activation; and (ii) CARF and p53 levels show an inverse relationship that is instigated by a negative-feedback control via a proteasome-mediated degradation pathway.

Strategic attack on host cell gene expression during adenovirus infection

To understand the interaction between the virus and its host, we used three sources of cDNA microarrays to examine the expression of 12,309 unique genes at 6 h postinfection of HeLa cells with high multiplicities of adenovirus type 2. Seventy-six genes with significantly changed expression ratios were identified, suggesting that adenovirus only modulates expression of a limited set of cellular genes. Quantitative real-time PCR analyses on selected genes were performed to confirm the microarray results. Significantly, a pronounced transcriptional activation by the promiscuous E1A-289R transcriptional activator was not apparent. Instead, promoter sequences in 45% of the upregulated genes harbored a potential E2F binding site, suggesting that the ability of the amino-terminal domain of E1A to regulate E2F-dependent transcription may be a major pathway for regulation of cellular gene expression. CDC25A was the only upregulated gene directly involved in cell cycle control. In contrast, several genes implicated in cell growth arrest were repressed. The transforming growth factor beta superfamily was specifically affected in the expression of both the upstream ligand and an intracellular regulator. In agreement with previous reports, adenovirus also targeted the innate immune response by downregulating several cytokines, including CLL2, CXCL1, and interleukin-6. Finally, stress response genes encoding GADD45B, ATF3, and TP53AP1 were upregulated. Importantly, we also found a novel countermeasure-activation of the apoptosis inhibitor survivin.

Phosphorylation regulates the interaction and complex formation between wt p53 protein and PARP-1

We recently characterized the interaction between poly(ADP-ribose) polymerase-1 (PARP-1) and the product of the tumor suppressor gene p53. We investigated which domains of human PARP-1 and of human wild-type (wt) p53 were involved in this protein-protein interaction. We generated baculoviral constructs encoding full length or distinct functional domains of both proteins. Full length PARP-1 was simultaneously coexpressed in insect cells with full length wt p53 protein or its distinct truncated fragments and vice versa. Reciprocal immunoprecipitation of Sf9 cell lysates revealed that the central and carboxy-terminal fragments of p53 were sufficient to confer binding to PARP-1, whereas the amino-terminal part harboring the transactivation functional domain was dispensable. On the other hand, the amino-terminal and central fragments of PARP-1 were necessary for complex formation with p53 protein. As the most important features of p53 protein are regulated by phosphorylation, we addressed the question of whether its phosphorylation is essential for binding between the two proteins. Baculovirally expressed wt p53 was post-translationally modified. At least six distinct p53 isomeres were resolved by immunoblotting following two-dimensional separation of baculovirally expressed wt p53 protein. Using specific phospho-serine antibodies, we identified phosphorylation of baculovirally expressed p53 protein at five distinct sites. To define the role of p53 phosphorylation, pull-down assays using untreated and dephosphorylated p53 protein were performed. Dephosphorylated p53 failed to bind PARP-1 indicating that complex formation between both proteins is regulated by phosphorylation of p53. The marked phosphorylation of p53 at Ser392 observed in unstressed cells suggests that the phosphorylated carboxy-terminal part of p53 undergoes complex formation with PARP-1 resulting in masking of the NES and thereby preventing its export. The functional significance of the interaction between both proteins was investigated at two different conditions: inactivation of PARP-1 and overexpression of PARP-1. Our results unequivocally show that the presence of PARP-1 regulates the basal expression of wt p53 in unstressed cells.