Cancer-associated mutations at the INK4a locus cancel cell cycle arrest by p16INK4a but not by the alternative reading frame protein p19ARF

Impact of protein and small molecule interactions on kinase conformations

Protein kinases act as central molecular switches in the control of cellular functions. Alterations in the regulation and function of protein kinases may provoke diseases including cancer. In this study we investigate the conformational states of such disease-associated kinases using the high sensitivity of the kinase conformation (KinCon) reporter system. We first track BRAF kinase activity conformational changes upon melanoma drug binding. Second, we also use the KinCon reporter technology to examine the impact of regulatory protein interactions on LKB1 kinase tumor suppressor functions. Third, we explore the conformational dynamics of RIP kinases in response to TNF pathway activation and small molecule interactions. Finally, we show that CDK4/6 interactions with regulatory proteins alter conformations which remain unaffected in the presence of clinically applied inhibitors. Apart from its predictive value, the KinCon technology helps to identify cellular factors that impact drug efficacies. The understanding of the structural dynamics of full-length protein kinases when interacting with small molecule inhibitors or regulatory proteins is crucial for designing more effective therapeutic strategies.

De Novo Purine Metabolism is a Metabolic Vulnerability of Cancers with Low p16 Expression

p16 is a tumor suppressor encoded by the CDKN2A gene whose expression is lost in approximately 50% of all human cancers. In its canonical role, p16 inhibits the G1-S-phase cell cycle progression through suppression of cyclin-dependent kinases. Interestingly, p16 also has roles in metabolic reprogramming, and we previously published that loss of p16 promotes nucleotide synthesis via the pentose phosphate pathway. However, the broader impact of p16/CDKN2A loss on other nucleotide metabolic pathways and potential therapeutic targets remains unexplored. Using CRISPR knockout libraries in isogenic human and mouse melanoma cell lines, we determined several nucleotide metabolism genes essential for the survival of cells with loss of p16/CDKN2A. Consistently, many of these genes are upregulated in melanoma cells with p16 knockdown or endogenously low CDKN2A expression. We determined that cells with low p16/CDKN2A expression are sensitive to multiple inhibitors of de novo purine synthesis, including antifolates. Finally, tumors with p16 knockdown were more sensitive to the antifolate methotrexate in vivo than control tumors. Together, our data provide evidence to reevaluate the utility of these drugs in patients with p16/CDKN2Alow tumors as loss of p16/CDKN2A may provide a therapeutic window for these agents.

SIGNIFICANCE

Antimetabolites were the first chemotherapies, yet many have failed in the clinic due to toxicity and poor patient selection. Our data suggest that p16 loss provides a therapeutic window to kill cancer cells with widely-used antifolates with relatively little toxicity.

H3K27me3 Demethylases Maintain the Transcriptional and Epigenomic Landscape of the Intestinal Epithelium

BACKGROUND & AIMS

Although trimethylation of histone H3 lysine 27 (H3K27me3) by polycomb repressive complex 2 is required for intestinal function, the role of the antagonistic process-H3K27me3 demethylation-in the intestine remains unknown. The aim of this study was to determine the contribution of H3K27me3 demethylases to intestinal homeostasis.

METHODS

An inducible mouse model was used to simultaneously ablate the 2 known H3K27me3 demethylases, lysine (K)-specific demethylase 6A (Kdm6a) and lysine (K)-specific demethylase 6B (Kdm6b), from the intestinal epithelium. Mice were analyzed at acute and prolonged time points after Kdm6a/b ablation. Cellular proliferation and differentiation were measured using immunohistochemistry, while RNA sequencing and chromatin immunoprecipitation followed by sequencing for H3K27me3 were used to identify gene expression and chromatin changes after Kdm6a/b loss. Intestinal epithelial renewal was evaluated using a radiation-induced injury model, while Paneth cell homeostasis was measured via immunohistochemistry, immunoblot, and transmission electron microscopy.

RESULTS

We did not detect any effect of Kdm6a/b ablation on intestinal cell proliferation or differentiation toward the secretory cell lineages. Acute and prolonged Kdm6a/b loss perturbed expression of gene signatures belonging to multiple cell lineages (adjusted P value < .05), and a set of 72 genes was identified as being down-regulated with an associated increase in H3K27me3 levels after Kdm6a/b ablation (false discovery rate, <0.05). After prolonged Kdm6a/b loss, dysregulation of the Paneth cell gene signature was associated with perturbed matrix metallopeptidase 7 localization (P < .0001) and expression.

CONCLUSIONS

Although KDM6A/B does not regulate intestinal cell differentiation, both enzymes are required to support the full transcriptomic and epigenomic landscape of the intestinal epithelium and the expression of key Paneth cell genes.

Loss of p16/Ink4a drives high frequency of rhabdomyosarcoma in a rat model of Duchenne muscular dystrophy

Rhabdomyosarcoma (RMS) is an aggressive type of soft tissue sarcoma, and pleomorphic RMS is a rare subtype of RMS found in adult. p16 is a tumor suppressor which inhibits cell cycle. In human RMS, p16 gene is frequently deleted, but p16-null mice do not develop RMS. We reported that genetic ablation of p16 by the crossbreeding of p16 knock-out rats (p16-KO rats) improved the dystrophic phenotype of a rat model of Duchenne muscular dystrophy (Dmd-KO rats). However, p16/Dmd double knock-out rats (dKO rats) unexpectedly developed sarcoma. In the present study, we raised p16-KO, Dmd-KO, and dKO rats until 11 months of age. Twelve out of 22 dKO rats developed pleomorphic RMS after 9 months of age, while none of p16-KO rats and Dmd-KO rats developed tumor. The neoplasms were connected to skeletal muscle tissue with indistinct borders and characterized by diffuse proliferation of pleomorphic cells which had eosinophilic cytoplasm and atypical nuclei with anisokaryosis. For almost all cases, the tumor cells immunohistochemically expressed myogenic markers including desmin, MyoD, and myogenin. The single cell cloning from tumor primary cells gained 20 individual Pax7-negative MyoD-positive RMS cell clones. Our results demonstrated that double knock-out of p16 and dystrophin in rats leads to the development of pleomorphic RMS, providing an animal model that may be useful to study the developmental mechanism of pleomorphic RMS.

Loss of p16: A Bouncer of the Immunological Surveillance?

p16INK4A (hereafter called p16) is an important tumor suppressor protein frequently suppressed in human cancer and highly upregulated in many types of senescence. Although its role as a cell cycle regulator is very well delineated, little is known about its other non-cell cycle-related roles. Importantly, recent correlative studies suggest that p16 may be a regulator of tissue immunological surveillance through the transcriptional regulation of different chemokines, interleukins and other factors secreted as part of the senescence-associated secretory phenotype (SASP). Here, we summarize the current evidence supporting the hypothesis that p16 is a regulator of tumor immunity.

A wild-derived inbred mouse strain, MSM/Ms, provides insights into novel skin tumor susceptibility genes

Cancer is one of the most catastrophic human genetic diseases. Experimental animal cancer models are essential for gaining insights into the complex interactions of different cells and genes in tumor initiation, promotion, and progression. Mouse models have been extensively used to analyze the genetic basis of cancer susceptibility. They have led to the identification of multiple loci that confer, either alone or in specific combinations, an increased susceptibility to cancer, some of which have direct translatability to human cancer. Additionally, wild-derived inbred mouse strains are an advantageous reservoir of novel genetic polymorphisms of cancer susceptibility genes, because of the evolutionary divergence between wild and classical inbred strains. Here, we review mapped Stmm (skintumor modifier of MSM) loci using a Japanese wild-derived inbred mouse strain, MSM/Ms, and describe recent advances in our knowledge of the genes responsible for Stmm loci in the 7,12-dimethylbenz(a)anthracene (DMBA)/12-O-tetradecanoylphorbol-13-acetate (TPA) two-stage skin carcinogenesis model.

Regulatory T Cells Play an Important Role in the Prevention of Murine Melanocytic Nevi and Melanomas

Melanocytic nevi are benign proliferations of pigment cells that can occasionally develop into melanomas. There is a significant correlation between increased nevus numbers and melanoma development. Our previous reports revealed that 7,12-dimethylbenz(a)anthracene (DMBA) and 12-O-tetradecanoyl-phorbol-13-acetate (TPA) induced dysplastic nevi in C3H/HeN mice, with a potential to transform into melanomas. To understand the immune mechanisms behind this transformation, we applied increasing DMBA doses followed by TPA to the skin of C3H/HeN mice. We observed that increased doses of DMBA correlated well with increased numbers of nevi. The increased DMBA dose induced diminished immune responses and promoted the expansion of regulatory T cells (Treg) that resulted in increased IL10 and reduced IFNγ levels. Mice with increased nevus numbers had loss of p16 expression. These mice had increased migration of melanocytic cells to lymph nodes (LN) and a greater percent of LNs produced immortalized melanocytic cell lines. DMBA-induced immunosuppression was lost in CD4-knockout (KO) mice. Lymphocytes in the CD4KO mice produced less IL10 than CD8KO mice. Furthermore, CD4KO mice had significantly reduced nevus numbers and size compared with wild-type and CD8KO mice. These results suggest that Tregs play a vital role in the incidence of nevi and their progression to melanoma.Prevention Relevance: There has been little progress in developing novel strategies for preventing premalignant dysplastic nevi from becoming melanomas. In this study in mice, regulatory-T cells enhanced progression of benign nevi to malignant melanomas; and by inhibiting their activity, melanomas could be retarded. The findings identify new possibilities for melanoma prevention in high risk individuals.

Survival of Lung Cancer Patients Dependent on the LOH Status for DMP1, ARF, and p53

Lung cancer is the leading cause of cancer deaths in the world, and accounts for more solid tumor deaths than any other carcinomas. The prognostic values of DMP1, ARF, and p53-loss are unknown in lung cancer. We have conducted survival analyses of non-small cell lung cancer (NSCLC) patients from the University of Minnesota VA hospital and those from the Wake Forest University Hospital. Loss of Heterozygosity (LOH) for hDMP1 was found in 26 of 70 cases (37.1%), that of the ARF/INK4a locus was found in 33 of 70 (47.1%), and that of the p53 locus in 43 cases (61.4%) in the University of Minnesota samples. LOH for hDMP1 was associated with favorable prognosis while that of p53 predicted worse prognosis. The survival was much shorter for ARF-loss than INK4a-loss, emphasizing the importance of ARF in human NSCLC. The adverse effect of p53 LOH on NSCLC patients' survival was neutralized by simultaneous loss of the hDMP1 locus in NSCLC and breast cancer, suggesting the possible therapy of epithelial cancers with metastatic ability.

Defining relative mutational difficulty to understand cancer formation

Most mutations in human cancer are low-frequency missense mutations, whose functional status remains hard to predict. Here, we show that depending on the type of nucleotide change and the surrounding sequences, the tendency to generate each type of nucleotide mutations varies greatly, even by several hundred folds. Therefore, a cancer-promoting mutation may appear only in a small number of cancer cases, if the underlying nucleotide change is too difficult to generate. We propose a method that integrates both the original mutation counts and their relative mutational difficulty. Using this method, we can accurately predict the functionality of hundreds of low-frequency missense mutations in p53, PTEN, and INK4A. Many loss-of-function p53 mutations with dominant negative effects were identified, and the functional importance of several regions in p53 structure were highlighted by this analysis. Our study not only established relative mutational difficulties for different types of mutations in human cancer, but also showed that by incorporating such a parameter, we can bring new angles to understanding cancer formation.

A 54-kDa short variant of DHX33 functions in regulating mRNA translation

DEAH box protein DHX33 has been found to be necessary for cell proliferation and early development of multicellular organisms. It plays diverse roles in regulating gene transcription, ribosome RNA synthesis, and protein translation. Dysregulation of DHX33 has been observed in various human cancers. In this study, we identified a short DHX33 variant in cells. The short DHX33 (hereafter referred to as DHX33-2) has only 534 amino acids, which completely matches the C-terminal helicase domain of full-length DHX33 (DHX33-1). Different from DHX33-1, which mainly localizes to the nucleus, DHX33-2 preferentially localizes to the cytoplasm. Through protein immunoprecipitation and RNA- immunoprecipitation analysis, we found that DHX33-2 interacts with DDX3, eIF3, hnRNPs, poly (A) binding protein, and a subset of mRNAs. Further RNA sequencing analysis showed that DHX33 binds to a subset of mRNAs important in cell proliferation. DHX33-2 stimulates the translation for specific mRNAs. Our study for the first time demonstrates the function of a short DHX33 variant in protein translation.

A Polymorphic Variant in p19Arf Confers Resistance to Chemically Induced Skin Tumors by Activating the p53 Pathway

Identification of the specific genetic variants responsible for the increased susceptibility to familial or sporadic cancers is important. Using a forward genetics approach to map such loci in a mouse skin cancer model, we previously identified a strong genetic locus, Stmm3, conferring resistance to chemically induced skin papillomas on chromosome 4. Here, we report the cyclin-dependent kinase inhibitor gene Cdkn2a/p19^Arf as a major responsible gene for the Stmm3 locus. We provide evidence that the function of Stmm3 is dependent on p53 and that p19^ArfMSM confers stronger resistance to papillomas than p16^Ink4aMSMin vivo. In addition, we found that genetic polymorphism in p19^Arf between a resistant strain, MSM/Ms (Val), and a susceptible strain, FVB/N (Leu), alters the susceptibility to papilloma development, malignant conversion, and the epithelial-mesenchymal transition. Moreover, we demonstrated that the p19^ArfMSM allele more efficiently activates the p53 pathway than the p19^ArfFVB allele in vitro and in vivo. Furthermore, we found polymorphisms in CDKN2A in the vicinity of a polymorphism in mouse Cdkn2a associated with the risk of human cancers in the Japanese population. Genetic polymorphisms in Cdkn2a and CDKN2A may affect the cancer risk in both mice and humans.

Aberrant Expression of p14ARF in Human Cancers: A New Biomarker?

The ARF and INK4a genes are located on the CDKN2a locus, both showing tumor suppressive activity. ARF has been shown to monitor potentially harmful oncogenic signalings, making early stage cancer cells undergo senescence or programmed cell death to prevent cancer. Conversely, INK4a detects both aging and incipient cancer cell signals, and thus these two gene functions are different. The efficiency of detection of oncogenic signals is more efficient for the for the former than the latter in the mouse system. Both ARF and INK4a genes are inactivated by gene deletion, promoter methylation, frame shift, aberrant splicing although point mutations for the coding region affect only the latter. Recent studies show the splicing alterations that affect only ARF or both ARF and INK4a genes suggesting that ARF is inactivated in human tumors more frequently than what was previously thought. The ARF gene is activated by E2Fs and Dmp1 transcription factors while it is repressed by Bmi1, Tbx2/3, Twist1, and Pokemon nuclear proteins. It is also regulated at protein levels by Arf ubiquitin ligase named ULF, MKRN1, and Siva1. The prognostic value of ARF overexpression is controversial since it is induced in early stage cancer cells to eliminate pre-malignant cells (better prognosis); however, it may also indicate that the tumor cells have mutant p53 associated with worse prognosis. The ARF tumor suppressive protein can be used as a biomarker to detect early stage cancer cells as well as advanced stage tumors with p53 inactivation.

Aberrant expression of p16INK4a in human cancers - a new biomarker?

The ARF and INK4a genes are located in the same CDKN2a locus, both showing its tumor suppressive activity. ARF has been shown to detect potentially harmful oncogenic signals, making incipient cancer cells undergo senescence or apoptosis. INK4a, on the other hand, responds to signals from aging in a variety of tissues including islets of Langerhans, neuronal cells, and cancer stem cells in general. It also detects oncogenic signals from incipient cancer cells to induce them senescent to prevent neoplastic transformation. Both of these genes are inactivated by gene deletion, promoter methylation, frame shift, and aberrant splicing although mutations changing the amino acid sequences affect only the latter. Recent studies indicated that polycomb gene products EZH2 and BMI1 repressed p16^INK4a expression in primary cells, but not in cells deficient for pRB protein function. It was also reported that that p14^ARF inhibits the stability of the p16^INK4a protein in human cancer cell lines and mouse embryonic fibroblasts through its interaction with regenerating islet-derived protein 3γ. Overexpression of INK4a is associated with better prognosis of cancer when it is associated with human papilloma virus infection. However, it has a worse prognostic value in other tumors since it is an indicator of pRB loss. The p16^INK4a tumor suppressive protein can thus be used as a biomarker to detect early stage cancer cells as well as advanced tumor cells with pRB inactivation since it is not expressed in normal cells.

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

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

Evasion of Cell Senescence Leads to Medulloblastoma Progression

How brain tumors progress from precancerous lesions to advanced cancers is not well understood. Using Ptch1(+/-) mice to study medulloblastoma progression, we found that Ptch1 loss of heterozygosity (LOH) is an early event that is associated with high levels of cell senescence in preneoplasia. In contrast, advanced tumors have evaded senescence. Remarkably, we discovered that the majority of advanced medulloblastomas display either spontaneous, somatic p53 mutations or Cdkn2a locus inactivation. Consistent with senescence evasion, these p53 mutations are always subsequent to Ptch1 LOH. Introduction of a p53 mutation prevents senescence, accelerates tumor formation, and increases medulloblastoma incidence. Altogether, our results show that evasion of senescence associated with Ptch1 LOH allows progression to advanced tumors.

Cooperation of DLC1 and CDK6 affects breast cancer clinical outcome

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

ARF tumor suppression in the nucleolus

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

p14ARF induces apoptosis via an entirely caspase-3-dependent mitochondrial amplification loop

The p14(ARF) tumor suppressor triggers cell death or cell cycle arrest upon oncogenic stress. In MCF-7 breast carcinoma cells, expression of the tumor suppressor gene p14(ARF) fails to trigger apoptosis but induces an arrest in the G1 and, to a lesser extent, in the G2 phase in the cell division cycle. Here, inhibition of cell cycle arrest resulted in apoptosis induction in caspase-3 proficient MCF-7 cells upon expression of p14(ARF) . This occurred in the absence of S-phase progression or mitotic entry. In contrast, syngeneic, caspase-3-deficient MCF-7 cells remained entirely resistant to p14(ARF) -induced apoptosis. Thus, cell cycle checkpoint abrogation overcomes resistance to p14(ARF) -induced cell death and promotes cell death via a caspase-3-dependent pathway. Cell death coincided with dissipation of the mitochondrial membrane potential, release of cytochrome c, and was inhibitable by pan-caspase inhibitors and the caspase-3/7 inhibitor zDEVD-fmk. Of note, mitochondrial events of apoptosis execution depended entirely on caspase-3 proficiency indicating that caspase-3 either acts "up-stream" of the mitochondria in a "non-canonical" pathway or mediates a mitochondrial feedback loop to amplify the apoptotic caspase signal in p14(ARF) -induced stress signaling.

Loss of ARF sensitizes transgenic BRAFV600E mice to UV-induced melanoma via suppression of XPC

Both genetic mutations and UV irradiation (UVR) can predispose individuals to melanoma. Although BRAF(V600E) is the most prevalent oncogene in melanoma, the BRAF(V600E) mutant is not sufficient to induce tumors in vivo. Mutation at the CDKN2A locus is another melanoma-predisposing event that can disrupt the function of both p16(INK4a) and ARF. Numerous studies have focused on the role of p16(INK4a) in melanoma, but the involvement of ARF, a well-known p53 activator, is still controversial. Using a transgenic BRAF(V600E) mouse model previously generated in our laboratory, we report that loss of ARF is able to enhance spontaneous melanoma formation and cause profound sensitivity to neonatal UVB exposure. Mechanistically, BRAF(V600E) and ARF deletion synergize to inhibit nucleotide excision repair by epigenetically repressing XPC and inhibiting the E2F4/DP1 complex. We suggest that the deletion of ARF promotes melanomagenesis not by abrogating p53 activation but by acting in concert with BRAF(V600E) to increase the load of DNA damage caused by UVR.

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

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

Cancer Stem Cells, EMT, and Developmental Pathway Activation in Pancreatic Tumors

Pancreatic cancer is a disease with remarkably poor patient survival rates. The frequent presence of metastases and profound chemoresistance pose a severe problem for the treatment of these tumors. Moreover, cross-talk between the tumor and the local micro-environment contributes to tumorigenicity, metastasis and chemoresistance. Compared to bulk tumor cells, cancer stem cells (CSC) have reduced sensitivity to chemotherapy. CSC are tumor cells with stem-like features that possess the ability to self-renew, but can also give rise to more differentiated progeny. CSC can be identified based on increased in vitro spheroid- or colony formation, enhanced in vivo tumor initiating potential, or expression of cell surface markers. Since CSC are thought to be required for the maintenance of a tumor cell population, these cells could possibly serve as a therapeutic target. There appears to be a causal relationship between CSC and epithelial-to-mesenchymal transition (EMT) in pancreatic tumors. The occurrence of EMT in pancreatic cancer cells is often accompanied by re-activation of developmental pathways, such as the Hedgehog, WNT, NOTCH, and Nodal/Activin pathways. Therapeutics based on CSC markers, EMT, developmental pathways, or tumor micro-environment could potentially be used to target pancreatic CSC. This may lead to a reduction of tumor growth, metastatic events, and chemoresistance in pancreatic cancer.

Transforming growth factor β1 (TGF-β1) suppresses growth of B-cell lymphoma cells by p14(ARF)-dependent regulation of mutant p53

Previously we reported that TGF-β1-induced growth suppression was associated with a decrease in mutant p53 levels in B-cell lymphoma cells. The goal of the present study was to understand the mechanism involved in TGF-β1-mediated down-regulation of mutant p53. In RL and CA46, two B-cell lymphoma cell lines, TGF-β1 treatment caused down-regulation of E2F-1 transcription factor resulting in the down-regulation of both p14(ARF) and mutant p53, leading to growth arrest. Experimental overexpression of E2F-1 increased p14(ARF) level and blocked TGF-β1-induced down-regulation of p14(ARF). Overexpression of p14(ARF) blocked the down-regulation of mutant p53 and prevented growth arrest. p14(ARF) also attenuated TGF-β1-induced p21(Cip1/WAF1) induction, which was reversible by p53 siRNA, indicating the involvement of mutant p53 in controlling the TGF-β1-induced expression of p21(Cip1/WAF1). The interaction observed between phospho-Smad2 and mutant p53 in the nucleus could be the mechanism responsible for blocking the growth-suppressive effects of TGF-β1. In RL cells, p14(ARF) is present in a trimer consisting of mutant p53-Mdm2-p14(ARF) and in a dimer consisting of Mdm2-p14(ARF). Because it is known that Mdm2 can degrade p53, it is possible that, in its trimeric form, p14(ARF) is able to stabilize mutant p53 by inhibiting Mdm2. In its dimeric form, p14(ARF) may be sequestering Mdm2, limiting its ability to degrade p53. Collectively, these data demonstrate a unique mechanism in which the inhibition of TGF-β1-mediated growth suppression by mutant p53 can be reversed by the down-regulation of its stabilizing protein p14(ARF). This work suggests that the high levels of p14(ARF) often found in tumor cells could be a potential therapeutic target.

The blues of P(16)INK(4a): aberrant promoter methylation and association with colorectal cancer in the Kashmir valley

Hypermethylation of the promoter region of the p16INK4a (p16) gene plays a significant role in the development and progression of colorectal cancer (CRC). The aim of the present study was to establish the role of the methylation status of the p16 gene in 114 CRC cases and to correlate it with the various clinicopathological parameters. Analysis of p16 promoter methylation was performed by methylation-specific PCR. Forty-eight (42.1%) of the CRC cases were found to be methylated for the p16 gene in our population. The methylation status was found to be associated with the gender, lymph node status, tumour stage, smoking status and tumour grade of the CRC patients. p16 plays a pivotal role in tumour development and progression to advanced stages.

Role of the G Protein-Coupled Receptor, mGlu1, in Melanoma Development

Melanoma remains one of the cancers for which a decline in morbidity has not been achieved with current scientific and medical advances. Mono-therapies targeting melanoma have been largely ineffective, increasing the need for identification of new drugable targets. Multiple tumor suppressors and oncogenes that impart genetic predisposition to melanoma have been identified and are being studied in an attempt to provide insight on the development of anti-melanoma therapies. Metabotropic Glutamate Receptor I (GRM1) has recently been implicated as a novel oncogene involved in melanomagenesis. GRM1 (mGlu₁, protein) belongs to the G protein coupled receptor (GPCR) super family and is normally functional in the central nervous system. Our group showed in a transgenic mouse model system that ectopic expression of Grm1 in melanocytes is sufficient to induce spontaneous melanoma development in vivo. GPCRs are some of the most important therapeutic drug targets discovered to date and they make up a significant proportion of existing therapies. This super family of transmembrane receptors has wide spread expression and interacts with a diverse array of ligands. Diverse physiological responses can be induced by stimulator(s) or suppressor(s) of GPCRs, which contributes to their attractiveness in existing and emerging therapies. GPCR targeting therapies are employed against a variety of human disorders including those of the central nervous system, cardiovascular, metabolic, urogenital and respiratory systems. In the current review, we will discuss how the identification of the oncogenic properties of GRM1 opens up new strategies for the design of potential novel therapies for the treatment of melanoma.

ARF suppresses tumor angiogenesis through translational control of VEGFA mRNA

Vascular endothelial growth factor A (VEGFA) is a specific mitogen for vascular endothelial cells that plays a critical role in cancer neoangiogenesis. Here, we report that the nucleolar tumor suppressor p19(ARF) suppresses VEGFA expression, acting at the level of mRNA translation without affecting the transcription of the VEGFA gene. Translational repression of VEGFA mRNA by p19(ARF) does not require p53, a major target of the ARF tumor suppressor pathway, but instead correlates with binding to nucleophosmin/B23. Maintaining VEGFA expression relies on nucleophosmin/B23, and downregulating this protein by RNAi or p19(ARF) leads to translational repression of VEGFA. p19(ARF) inhibits VEGFA-dependent tumor angiogenesis in nude mice. Additionally, p14(ARF) expression and microvessel density are inversely correlated in human colon carcinomas. Taken together, our results define a mechanism by which the ARF tumor suppressor targets the translational repression of specific oncogenes during neoplastic transformation.

Inactivation of tumor suppressor genes and the progression of adult T-cell leukemia-lymphoma

Almost three decades have passed since adult T-cell leukemia-lymphoma (ATLL) was proposed as a new disease entity. During this period, its causative agent, human T-cell leukemia virus type-1 (HTLV-1), was found and a crucial role of the viral product Tax in the development of ATLL was disclosed. However, the long latent period after infection with HTLV-1 indicates the need for additional factors for full-blown ATLL, most of which are supposed to be provided by somatic mutations of cellular genes. Recent progress in cell-cycle research has revealed that the uncontrolled and superior proliferative activity of malignant cells is mainly caused by the breakdown of cell-cycle regulation and that most malignancies carry aberrations in p16-pRB and/or p53 pathways. ATLL is not an exception, despite the consistent association of HTLV-1 in primary leukemia cells, and accumulating evidence indicates that the breakdown of these pathways is indeed involved in the leukemogenesis of ATLL, especially in its later steps, which serve as the key events for promotion of indolent ATLL to aggressive ATLL.

The H3K27me3 demethylase JMJD3 contributes to the activation of the INK4A-ARF locus in response to oncogene- and stress-induced senescence

The tumor suppressor proteins p16INK4A and p14ARF, encoded by the INK4A-ARF locus, are key regulators of cellular senescence. The locus is epigenetically silenced by the repressive H3K27me3 mark in normally growing cells, but becomes activated in response to oncogenic stress. Here, we show that expression of the histone H3 Lys 27 (H3K27) demethylase JMJD3 is induced upon activation of the RAS-RAF signaling pathway. JMJD3 is recruited to the INK4A-ARF locus and contributes to the transcriptional activation of p16INK4A in human diploid fibroblasts. Additionally, inhibition of Jmjd3 expression in mouse embryonic fibroblasts results in suppression of p16Ink4a and p19Arf expression and in their immortalization.

Oxidative stress is linked to ERK1/2-p16 signaling-mediated growth defect in ATM-deficient astrocytes

The gene that encodes the ATM protein kinase is mutated in ataxia-telangiectasia (A-T). One of the prominent features of A-T is progressive neurodegeneration. We have previously reported that primary astrocytes isolated from Atm(-/-) mice grow slowly and die earlier than control cells in culture. However, the mechanisms for this remain unclear. We show here that intrinsic elevated intracellular levels of reactive oxygen species (ROS) are associated with the senescence-like growth defect of Atm(-/-) astrocytes. This condition is accompanied by constitutively higher levels of ERK1/2 phosphorylation and p16(Ink4a) in Atm(-/-) astrocytes. We also observe that ROS-induced up-regulation of p16(Ink4a) occurs correlatively with ERK1/2-dependent down-regulation and subsequent dissociation from chromatin of Bmi-1. Furthermore, both mitogen-activated protein kinase (MAPK)/ERK inhibitor PD98059 and antioxidant N-acetyl-l-cysteine restored normal proliferation of Atm(-/-) astrocytes. These results suggest that ATM is required for normal astrocyte growth through its ability to stabilize intracellular redox status and that the inability to control ROS is the molecular basis of limited cell growth of Atm(-/-) astrocytes. This defect may be mediated by a mechanism involving ERK1/2 activation and Bmi-1 derepression of p16(Ink4a). These data identify new potential targets for therapeutic intervention in A-T neurodegeneration.

Residues in the alternative reading frame tumor suppressor that influence its stability and p53-independent activities

The Alternative Reading Frame (ARF) protein suppresses tumorigenesis through p53-dependent and p53-independent pathways. Most of ARF's anti-proliferative activity is conferred by sequences in its first exon. Previous work showed specific amino acid changes occurred in that region during primate evolution, so we programmed those changes into human p14ARF to assay their functional impact. Two human p14ARF residues (Ala(14) and Thr(31)) were found to destabilize the protein while two others (Val(24) and Ala(41)) promoted more efficient p53 stabilization and activation. Despite those effects, all modified p14ARF forms displayed robust p53-dependent anti-proliferative activity demonstrating there are no significant biological differences in p53-mediated growth suppression associated with simian versus human p14ARF residues. In contrast, p53-independent p14ARF function was considerably altered by several residue changes. Val(24) was required for p53-independent growth suppression whereas multiple residues (Val(24), Thr(31), Ala(41) and His(60)) enabled p14ARF to block or reverse the inherent chromosomal instability of p53-null MEFs. Together, these data pinpoint specific residues outside of established p14ARF functional domains that influence its expression and signaling activities. Most intriguingly, this work reveals a novel and direct role for p14ARF in the p53-independent maintenance of genomic stability.

Establishment of ponasterone A-inducible the wild-type p53 protein-expressing clones from HSC-1 cells, cell growth suppression by p53 expression and the suppression mechanism

Gene therapy for a variety of human cancers containing the mutant p53 (mt-p53) gene has been performed by direct injection of a retroviral or adenoviral vector containing the wild-type p53 (wt-p53) gene. Because many individuals with skin squamous cell carcinoma (SCC) have been shown to carry the p53 gene mutation, these patients are candidates for p53 gene therapy. For this reason, we established ponasterone A-inducible the wild-type p53 (wt-p53) protein-expressing clones by transfecting a ponasterone-inducible vector containing the wt-p53 gene into HSC-1 cells, which harbor the mutated p53 (m/w) at codon 173 (GTG --> TTG in one allele). Upon the induction of the wt-p53 protein, severe growth suppression was observed. Based on the results of the expression patterns of the p21, p16, RB, BAX and Bcl-2 proteins, as well as on the results of senescence-associated beta-galactosidase staining, the suppression was caused by senescence-like growth arrest of the cells. Although it is generally accepted that the suppression of tumor cell growth is caused by p53-induced apoptosis, permanent G1 arrest induced by p53 is also an important part of the growth-suppression mechanism in p53 gene therapy. The present results should expand the possibilities for p53 gene therapy for human skin SCCs containing the mutant p53 gene.

Population-based prevalence of CDKN2A and CDK4 mutations in patients with multiple primary melanomas

The presence of multiple primary cutaneous melanomas (MPM) has been advocated as guidance to identifying melanoma families. Frequencies of CDKN2A mutations in materials of sporadic MPM cases from pigmented lesion clinics vary between 8 and 15%. Patients with MPM have therefore been regarded as good candidates for CDKN2A mutational screening. We describe a population-based study where all persons in Norway diagnosed with MPM between 1953 and 2004 (n = 738 alive per April 2004) were invited to participate. Three-hundred-and-ninety patients (52.8%) responded confidentially. Mutations in CDKN2A were found in 6.9% of the respondents. Eighty-one MPM patients (20.8%) reported that they belonged to melanoma families, and 17 (21.0%) of these harboured a CDKN2A mutation, compared to 3.2% of the nonfamilial cases. The probability of finding a CDKN2A mutation increased when the patients had three or more melanomas, or a young age of onset of first melanoma. We identified five novel CDKN2A variants (Ala57Gly, Pro81Arg, Ala118Val, Leu130Val, and Arg131Pro) and four that previously have been reported in melanoma families (Glu27X, Met53Ile, Arg87Trp, and Ala127Pro). A large deletion (g.13623_23772del10150) encompassing exon 1alpha and the 5' part of exon 2 was detected in six patients with a family history of melanoma. Three patients, belonging to the same family, had the CDK4 Arg24His mutation. The frequency of CDKN2A mutations was lower than previously reported in other studies, an observation which probably is due to the population-based design of our study.

Changes in p19Arf localization accompany apoptotic crisis during pre-B-cell transformation by Abelson murine leukemia virus

Transformation by Abelson murine leukemia virus (Ab-MLV) is a multistep process in which growth-stimulatory signals from the v-Abl oncoprotein and growth-suppressive signals from the p19(Arf)-p53 tumor suppressor pathway oppose each other and influence the outcome of infection. The process involves a proliferative phase during which highly viable primary transformants expand, followed by a period of marked apoptosis (called "crisis") that is dependent on the presence of p19(Arf) and p53; rare cells that survive this phase emerge as fully transformed and malignant. To understand the way in which v-Abl expression affects p19(Arf) expression, we examined changes in expression of Arf during all stages of Ab-MLV transformation process. As is consistent with the ability of v-Abl to stimulate Myc, a transcription factor known to induce p19(Arf), Myc and Arf are induced soon after infection and p19(Arf) is expressed. At these early time points, the infected cells remain highly viable. The onset of crisis is marked by an increase in p19(Arf) expression and a change in localization of the protein from the nucleoplasm to the nucleolus. These data together suggest that the localization and expression levels of p19(Arf) modulate the effects of the protein during oncogenesis and reveal that the p19(Arf)-mediated response is subject to multiple layers of regulation that influence its function during Ab-MLV-mediated transformation.

Mitochondrial p32 is a critical mediator of ARF-induced apoptosis

The shared exon 2 of the p14ARF-p16INK4a locus is frequently mutated in human cancers. However, in contrast to the exon 1beta-encoded N-terminal half of ARF, the function of the exon 2-encoded C-terminal half of ARF has been elusive. Here, we report that the mitochondrial protein p32/C1QBP binds the ARF C terminus. We show that p32 is required for ARF to localize to mitochondria and induce apoptosis, and that ARF mutations specifically disrupting p32 binding can impair both of these functions. Wild-type ARF, but not a p32-binding-deficient ARF mutant, localizes to mitochondria, reduces mitochondrial membrane potential, and sensitizes cells to p53-induced apoptosis. These findings provide a potential explanation for the frequent human cancer mutations targeting the ARF C terminus.

The state of the art in the pathogenesis of ATL and new potential targets associated with HTLV-1 and ATL

Almost 30 years have passed since adult T-cell leukemia (ATL) was identified as a new disease entity in Japan. During this period, its causative agent, human T-cell leukemia virus (HTLV-1), was discovered, and a crucial role of the viral product Tax in ATL leukemogenesis was demonstrated. Recently, another HTLV-1 product, HBZ, which is encoded on the negative strand, was found, and it has now become a subject of intensive research because of its possible activity in cell proliferation. It is, however, impossible to elucidate the whole process of ATL leukemogenesis by studying only HTLV-1, and aberrations of cellular genes such as tumor suppressor genes are also profoundly involved in the later stages of ATL development. In contrast with the progress in the understanding of ATL pathogenesis, more progress in developing therapy for ATL is needed, and there has been only slight improvement in the prognosis. Recently, unique therapeutic approaches targeting molecules and/or mechanisms involved in the pathogenesis have been explored, and some of them produced encouraging results that might lead to breakthrough therapies. One of these approaches, the use of monoclonal antibody against chemokine receptor CCR4, is now ongoing as a multicenter clinical trial in Japan. Here we review the state of the art regarding our understanding of ATL leukemogenesis and new potential molecular targets in ATL therapy.

The clinical importance of Ki-67, p16, p14, and p57 expression in patients with advanced ovarian carcinoma

The present study addressed the impact of p14, p16, p57, and Ki-67 in a large cohort of uniformly treated patients with stage III ovarian cancer in relation to other clinicopathologic variables and prognosis. We immunohistochemically studied 171 primary tumors from previously untreated patients with advanced ovarian carcinomas for expression of Ki-67, p16, p14, and p57. High protein levels of Ki-67 (>10% positive nuclei) were found in 144 cases (84%), p16 (>50% positive nuclei) in 53 cases (31%), p57 (>10% positive nuclei) in 41 cases (24%), and p14 (any positive nuclei) in 19 cases (11%). A correlation between high Ki-67 expression and presence of residual disease after primary surgery (P = 0.019), ascites (P = 0.006), higher International Federation of Gynecology and Obstetrics substage (P < 0.001), poor differentiation (P < 0.001), and higher Silverberg histopathologic grade (P < 0.0001) was seen. High expression of p16 correlated to poor differentiation (P = 0.033) and higher Silverberg histopathologic grade (P = 0.018). In univariate analysis, high expression of Ki-67 (P = 0.0001) and p16 (P = 0.005) was associated with poor survival. However, in multivariate analysis, only high expression of Ki-67 was significantly associated with shorter survival (P = 0.025). No correlations were seen between expression of p14 and p57 and clinicopathologic parameters. None of the factors studied was able to predict response to chemotherapy. Our results showed that Ki-67 represents an independent prognostic predictor in stage III ovarian cancer. We did not find p16, p14, and p57 to be useful as prognostic markers.

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.

Gene expression profiling in melanoma identifies novel downstream effectors of p14ARF

p14ARF is inactivated by deletions/mutations in many cancer types and can suppress cell growth by both p53-dependent and p53-independent mechanisms. To identify novel downstream effectors of p14ARF, we used gene expression profiling as a primary screening tool to select candidates for follow up validation studies using in vitro cell-based assays. Gene expression profiles of a panel of 35 melanoma cell lines with either wild-type (n = 12) or mutant (n = 23) p14ARF were compared to identify genes associated with inactivation of p14ARF. Analysis of the microarray data identified 1,316 probe sets that were significantly (p < 0.01) differentially expressed between the p14ARF wild-type and mutant cell lines. Pathway analysis of these genes showed an overrepresentation of many receptor-mediated signal transduction pathways, e.g. TGFbeta, EGF, HGF, PDGF, MAPK, Wnt and integrin pathways. A number of components of these pathways, including FLRT3, RUNX2, MIG-6 and SMURF2 were confirmed as downstream targets of p14ARF using p14ARF-inducible cell lines and RNAi. We propose that regulation of these genes may contribute to melanoma development when p14ARF function is lost.

Relationship between expression and methylation status of p16INK4a and the proliferative activity of different areas' tumour cells in human colorectal cancer

The p16(INK4a) gene is a cell cycle inhibitor and a major tumour suppressor protein, but the regulation and effects on tumour cells' invasion process of p16(INK4a) is poorly known. A role for p16(INK4a) in basal cell carcinoma is suggested by the observation that p16(INK4a) was upregulated at the invasive front of the majority of basal cell carcinomas with infiltrative growth patterns, accompanied by cessation of proliferation. In this paper, we explore whether there is a difference of tumour cells' proliferative activity between the centre and the invasion front tissues of human colorectal cancer and its relationship with the expression and methylation status of p16(INK4a) gene. We obtained the centre and the invasion front tissues of colorectal cancer respectively by the technology of laser mircodissection. The expressions of the proliferating cell nuclear antigen ki67 and p16(INK4a) were assessed by immunohistochemistry, methylation-specific polymerase chain reaction (MS-PCR) and reverse-transcription polymerase chain reaction (RT-PCR) in the different areas. There was a significant difference in the expressions of ki67 between the centre and the invasion front tissues (p < 0.05). The difference did not correlate with age, sex, Dukes stage but did correlate with expression of p16(INK4a) gene (chi(2) = 25.37, p < 0.01). Furthermore, hypermethylation of the promoter was the major mechanism of inactivation of p16(INK4a) in the centre areas. Demethylation of the p16(INK4a) promoter, the elevated expression of p16(INK4a) protein and mRNA level was proved in the invasion front. Our finding suggested that enhanced invasion in tumours was accompanied by ceased proliferation in the invasion fronts of human colorectal cancer. This interesting phenomenon may be due to not only the microenvironment, but also the molecular changes such as p16(INK4a) status.

ARF promotes accumulation of retinoblastoma protein through inhibition of MDM2

The INK4a/ARF locus, encoding two tumor suppressor proteins, p16(INK4a) and p14(ARF) (ARF), plays key roles in many cellular processes including cell proliferation, apoptosis, cellular senescence and differentiation. Inactivation of INK4a/ARF is one of the most frequent events during human cancer development. Although p16(INK4a) is a critical component in retinoblastoma protein (Rb)-mediated growth regulatory pathway, p14(ARF) plays a pivotal role in the activation of p53 upon oncogenic stress signals. A body of evidence indicates that ARF also possesses growth suppression functions independent of p53, the mechanism of which is not well understood. We have recently shown that MDM2 interacts with Rb and promotes proteasome-dependent Rb degradation. In this study, we show that ARF disrupts MDM2-Rb interaction resulting in Rb accumulation. Wild-type ARF, but not ARF mutant defective in MDM2 interaction, stabilizes Rb and inhibits colony foci formation independent of p53. In addition, ablation of Rb impairs ARF function in growth suppression. Thus, this study demonstrates that ARF plays a direct role in regulation of Rb and suggests that inactivation of ARF may lead to defects in both p53 and Rb pathways in human cancer development.

Regulation of the INK4b-ARF-INK4a tumour suppressor locus: all for one or one for all

The INK4b-ARF-INK4a locus encodes two members of the INK4 family of cyclin-dependent kinase inhibitors, p15(INK4b) and p16(INK4a), and a completely unrelated protein, known as ARF. All three products participate in major tumour suppressor networks that are disabled in human cancer and influence key physiological processes such as replicative senescence, apoptosis and stem-cell self-renewal. Transcription from the locus is therefore kept under strict control. Mounting evidence suggests that although the individual genes can respond independently to positive and negative signals in different contexts, the entire locus might be coordinately suppressed by a cis-acting regulatory domain or by the action of Polycomb group repressor complexes.

p53-Dependent and -independent functions of the Arf tumor suppressor

The Ink4a-Arf locus encodes two closely wedded tumor suppressor proteins (p16(Ink4a) and p19(Arf)) that inhibit cell proliferation by activating Rb and p53, respectively. With few exceptions, the Arf gene is repressed during mouse embryonic development, thereby helping to limit p53 expression during organogenesis. However, in adult mice, sustained hyperproliferative signals conveyed by somatically activated oncogenes can induce Arf gene expression and trigger a p53 response that eliminates incipient cancer cells. Disruption of this tumor surveillance pathway predisposes to cancer, and inactivation of INK4a- ARF by deletion, silencing, or mutation has been frequently observed in many forms of human cancer. Although it is accepted that much of Arf's tumor-suppressive activity is mediated by p53, more recent genetic evidence has pointed to additional p53- independent functions of Arf, including its ability to inhibit gene expression by a number of other transcription factors. Surprisingly, the enforced expression of Arf in mammalian cells promotes the sumoylation of several Arf-interacting proteins, implying that Arf has an associated catalytic activity. We speculate that transcriptional down-regulation in response to Arf-induced sumoylation may account for Arf's p53-independent functions.

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.

Frequent co-alterations of TP53, p16/CDKN2A, p14ARF, PTEN tumor suppressor genes in human glioma cell lines

In this study we established the simultaneous status of TP53, p16, p14ARF and PTEN tumor suppressor genes in 34 randomly chosen human glioma cell lines. Nine cell lines (26.4%) harbored mutations or deletions in all four tumor suppressor genes and 22 cell lines (64%) had alterations in at least three. Mutations/deletions were found at the following frequencies: TP53 (76.5%), p14ARF (64.7%), p16 (64.7%), PTEN (73.5%). Thus, there was a high incidence of alterations in the cellular pathways involving the p53 transcription factor (94.1%), the retinoblastoma protein (64.7%) and the PTEN phosphatase (73.5%) and 91% of cell lines carried mutations in two or more pathways. This provides the first clear genetic evidence that these tumor suppressors participate in biological pathways which are functioning separately/independently in glioma cells. The status of the gene alterations did not correlate with tumorigenicity in immunocompromized mice or any clinical parameters. Although the mutation rate was higher in glioma cell lines than that reported for glioma tissues, the alterations were molecularly representative of those found in adult de novo glioblastoma. This study highlights the importance of developing therapeutic approaches applicable to tumors with a broad range of genetic alterations and also provides an invaluable panel of glioma cell lines to make this possible.

Molecular biology of malignant mesothelioma: a review

Malignant mesothelioma (MM) is an uncommon tumor with high mortality and morbidity rates. It arises from mesothelial cells that line the pleural, pericardial, peritoneal, and testicular cavities. This is a disease with an indolent course because tumors arise 20 to 40 years after exposure to an inciting agent. Extensive research has shown that mesothelial cells are transformed into MM cells through various chromosomal and cellular pathway defects. These changes alter the normal cells' ability to survive, proliferate, and metastasize. This article discusses the alterations that occur in transforming normal mesothelial cells into MM. It also details some of the signal transduction pathways that seem to be important in MM with the potential for novel targeted therapeutics.

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.

Dual action of the inhibitors of cyclin-dependent kinases: targeting of the cell-cycle progression and activation of wild-type p53 protein

The inhibition of cyclin-dependent kinases (CDKs) represents a novel approach to the therapy of human malignancies. Already in clinical trials, recently developed CDK inhibitors very efficiently target the rapidly proliferating cancer cells and inhibit their cell-cycle progression. Interestingly, some CDK inhibitors additionally affect the stability and activity of the tumour-suppressor protein p53, thereby enhancing their antiproliferative action towards cancer cells. Considering the fact that the p53 protein is mutated or inactivated in approximately 50% of all human cancers, the efficacy of CDK inhibitor therapy could differ between cancer cells depending on their p53 status. Moreover, recent reports demonstrating that some cancer cells can proliferate despite CDK2 inhibition questioned the central role of CDK2 in the cell-cycle control and suitability of CDK2 as a therapeutic target; however, the p53 activation that is mediated by CDK inhibitors could be essential for the efficacy of CDK inhibitors in therapy of CDK2-independent cancers. Furthermore, there is also reason to believe that CDK2 inhibitors could be used for another purpose, to protect normal cells from the effects of chemotherapy.