ARF promotes MDM2 degradation and stabilizes p53: ARF-INK4a locus deletion impairs both the Rb and p53 tumor suppression pathways

Impaired processing of DNA photoproducts and ultraviolet hypermutability with loss of p16INK4a or p19ARF

Reduced DNA repair has been linked to an increased risk of cutaneous malignant melanoma, but insights into the molecular mechanisms of that link are scarce. The INK4a/ARF (CDKN2a) locus, which codes for the p16(INK4a) and p19ARF proteins, is often mutated in sporadic and familial malignant melanoma, but it has not been directly associated with reduced DNA repair. We transfected unirradiated mouse fibroblast cells with UV-treated DNA to measure DNA repair in normal, p16INK4a mutant, p19ARF mutant, or double mutant mouse host cells. Loss of either p16(INK4a) or p19ARF reduced the ability of the cells to process UV-induced DNA damage, independent of cell cycle effects incurred by the loss. These results may further explain why INK4a/ARF mutations predispose to malignant melanoma, a UV-induced tumor.

Predictive significance of the alterations of p16INK4A, p14ARF, p53, and proliferating cell nuclear antigen expression in the progression of cervical cancer

PURPOSE

The purpose of this research was to evaluate the clinical significance of p16INK4A, p14ARF, p53, and proliferating cell nuclear antigen (PCNA) expression in tumor progression of cervical cancer.

DESIGN

Seventeen patients (40 samples) with consecutive cervical lesions from normal squamous epithelium, inflammation of the cervix to cervical intraepithelial neoplasm (CIN) and invasive cervical squamous cell cancer (SCC), or from CIN to SCC were collected for this study. Expression of p16INK4A, p14ARF, p53, and PCNA were detected by immunohistochemistry on paraffin-embedded sections. Human papillomavirus DNA was detected simultaneously with PCR and typed according to its DNA sequence.

RESULTS

p16INK4A overexpression was significantly higher in CIN (75%) and in SCC (75%) than in normal or inflammation of the cervix (12.5%; P < 0.01, P < 0.05, respectively). The positive rate of p14ARF expression was higher in SCC (83%) than in normal/inflammation of the cervix (25%; P < 0.05). PCNA expression was negative in normal or inflammation of the cervix, but an increased in expression was seen in 63.2% in CIN and 100% in SCC (P < 0.01, P < 0.05). When the time interval for disease progression from initial biopsy to CIN 3 or invasive cancer was compared with states of p16INK4A expression, cases stained positive for p16INK4A progressed within 64.2 months as compared with 122.3 months among those stained negatively (P < 0.01). Cases with increased p14ARF expression also had a short time interval for disease progression of 78.8 months as compared with 108.3 months in cases that were p14ARF negative. Cases with stable or decreased p53 expression had the shortest time interval for progression of 32.3 months in contrast to cases with no p53 expression (113.9 months). However, cases with increasing p53 expression progressed within 60.8 months.

CONCLUSIONS

Our results suggested that altered states of p16INK4A, p14ARF, p53, and PCNA may be valuable markers to predict the progression of cervical neoplasia.

Antiapoptotic signaling pathways in non-small-cell lung cancer: biology and therapeutic strategies

One of the hallmarks of lung cancer is the deregulation of apoptotic or programmed cell death mechanisms usually found in normal cells that allow for corrupted cells to undergo cellular suicide. This includes mechanisms that attenuate proapoptotic pathways and/or amplify antiapoptotic pathways. Increasing evidence suggests that lung cancer cells use multiple and perhaps redundant pathways to maintain survival. Increasing knowledge of these pathways offers a better understanding of the biology of lung cancer as well as novel therapeutic strategies that can enhance lung cancer cell death. This review discusses the apoptotic machinery and signal transduction pathways that regulate apoptosis, methods of identifying the presence of activated survival signaling pathways in human lung cancers, and the clinical significance and relevance for therapy for patients with lung cancer.

Tumor suppressor ARF inhibits HER-2/neu-mediated oncogenic growth

HER2/neu, a receptor tyrosine kinase oncogene, promotes mitogenic growth and antiapoptotic activity in cancer cells. Strong expression of HER2/neu in cancers has been associated with poor prognosis. Alternative reading frame protein (ARF), a tumor suppressor protein encoded by a gene located in the Ink4a/ARF gene locus, is frequently inactivated in human cancers. Little is known about the tumor suppressor role of ARF in HER2/neu-overexpressing cancers. Here, we applied the ARF gene as a tumor-suppressive agent for HER2/neu-overexpressing cells under the control of a tetracycline (tet)-regulated gene expression system. We found that ARF antagonized protein kinase B (PKB)/Akt-mediated p27Kip1 phosphorylation and increased p27 stability in HER2/neu-overexpressing cells. ARF expression also led to decreased levels of Cul1 and Skp2, two proteins involved in p27 degradation. We also found that ARF caused apoptosis in HER2/neu-overexpressing cells, and sensitized cells to apoptosis induced by the chemotherapeutic agents taxol and 2-methoxyestradiol. Most significantly for clinical application, we found that ARF inhibited HER2/neu-mediated cell growth, transformation, and tumorigenesis. These findings indicate that modulation of ARF activity may be a useful therapeutic intervention in HER2-overexpressing cancers.

Pathogenesis of persistent hyperplastic primary vitreous in mice lacking the arf tumor suppressor gene

PURPOSE

Persistent hyperplastic primary vitreous (PHPV) is an idiopathic developmental eye disease associated with failed involution of the hyaloid vasculature. The present work addressed the pathogenesis of PHPV in a mouse model that replicates many aspects of the human disease.

METHODS

Ophthalmoscopic and histologic analyses documented pathologic processes in eyes of mice lacking the Arf gene compared with Ink4a-deficient and wild-type control animals. Immunohistochemical staining, in situ hybridization, and RT-PCR demonstrated the expression of relevant gene products. Arf gene expression was determined by in situ hybridization using wholemounts of wild-type mouse eyes and by immunofluorescence staining for green fluorescent protein (GFP) in Arf(+/GFP) heterozygous knock-in mouse eyes.

RESULTS

Abnormalities in Arf(-/-) mice mimicked those found in patients with severe PHPV. The mice had microphthalmia; fibrovascular, retrolental tissue containing retinal pigment epithelial cells and remnants of the hyaloid vascular system; posterior lens capsule destruction with lens degeneration and opacity; and severe retinal dysplasia and detachment. Eyes of mice lacking the overlapping Ink4a gene were normal. Arf was selectively expressed in perivascular cells within the vitreous of the postnatal eye. Cells composing the retrolental mass in Arf(-/-) mice expressed the Arf promoter. The remnant hyaloid vessels expressed Flk-1. Its ligand, vascular endothelial growth factor (Vegf), was expressed in the retrolental tissue and the adjacent dysplastic neuroretina.

CONCLUSIONS

Arf(-/-) mice have features that accurately mimic severe PHPV. In the HVS, Arf expression in perivascular cells may block their accumulation or repress Vegf expression to promote HVS involution and prevent PHPV.

Molecular pathogenesis of astrocytic tumours

Our current knowledge of the molecular pathogenesis of the diffuse adult astrocytic tumours is vast if compared to 20 years ago, yet we are far from understanding the details of this process at the molecular level and using such an understanding to logically and specifically treat patients' tumours. In other astrocytic tumours we have little or no knowledge of the molecular processes. This article will attempt to summarise the histological classification criteria and genetic data for all the astrocytic tumours. The current World Health Organisation classification lists six entities, some with subgroups. Common problems associated with the diagnosis of these tumours are outlined. While the molecular findings are not as yet used clinically, we are approaching a time when the histological investigation will have to be supplemented with molecular data to ensure the best choice of treatment for the patient and as an accurate indicator of prognosis.

Ink4a/Arf links senescence and aging

The mammalian INK4a/ARF locus encodes two linked tumor suppressor proteins, p16INK4a and ARF, which respectively regulate the retinoblastoma (RB) and p53 pathways. Genetic data have firmly established that both proteins possess significant in vivo tumor suppressor activity. In addition to their non-overlapping roles in preventing cancer, one or both proteins are induced under certain circumstances in most cultured murine and human cell types, and thereby are critical effectors of senescence. Likewise, data from murine models have suggested that this anti-cancer growth inhibitory activity of the locus can similarly affect permanent growth arrest in vivo. When such in vivo senescence occurs in a cell possessing self-renewal potential (e.g. a tissue stem cell), there is an attendant decline in the regenerative capabilities of the organ maintained by that stem cell. In turn, the concomitant decline of this stem cell reserve is a cardinal feature of mammalian aging. Expression of the INK4a/ARF locus, therefore, appears not only to be a major suppressor of cancer, but also an effector of mammalian aging.

Correlation between p14(ARF)/p16(INK4A) expression and HPV infection in uterine cervical cancer

The CDKN2A locus on human chromosome 9p21 encodes two tumor suppressors, p14(ARF) and p16(INK4A), which enhance the growth-suppressive functions of the retinoblastoma (Rb) and the p53 proteins, respectively. Conversely, the E6 and E7 oncoproteins of the high-risk human papillomaviruses (HPVs) causally associated with carcinogenesis of the uterine cervix contributes to tumor development by inactivating p53 and Rb. Nevertheless, a correlation between expression of p14(ARF)/p16(INK4A) and HPV infection in uterine cervix is less clear. To clarify this, we examined 25 cervical cancers and 11 normal uterine cervixes. HPV was detected in 21 of 25 cervical cancers (84%) and their subtype was determined by PCR-RFLP. Quantitative real-time RT-PCR assays showed overexpression of p14(ARF) mRNA in all 21 HPV-positive cases (100%). p16(INK4A) mRNA was overexpressed in 17 cases of the HPV-positive cases (81%). In four HPV-negative cancers, reduced expression of p14(ARF) mRNA was detected in two cases (50%) and reduced p16(INK4A) mRNA in three cases (75%). Our data indicate that the overexpression of p14(ARF) and p16(INK4A) strongly associates with HPV-positive cervical cancers and that reduced expression of p14(ARF) and p16(INK4A) correlates with HPV-negative cervical cancers. These findings may indicate that impaired p14(ARF) and p16(INK4A) mRNA expression contribute to tumor development in HPV-negative cervical cancers by failure to support p53 and Rb instead of their inactivation by HPV E6 and E7.

The central acidic domain of MDM2 is critical in inhibition of retinoblastoma-mediated suppression of E2F and cell growth

Retinoblastoma (Rb) protein is a paradigm of tumor suppressors. Inactivation of Rb plays a critical role in the development of human malignancies. MDM2, an oncogene frequently found amplified and overexpressed in a variety of human tumors and cancers, directly interacts and inhibits the p53 tumor suppressor protein. In addition, MDM2 has been shown to stimulate E2F transactivation activity and promote S-phase entry independent of p53, yet the mechanism of which is still not fully understood. In this study, we demonstrate that MDM2 specifically binds to Rb C-pocket and that the central acidic domain of MDM2 is essential for Rb interaction. In addition, we show that overexpression of MDM2 reduces Rb-E2F complexes in vivo. Moreover, the ectopic expression of the wild type MDM2, but not mutant MDM2 defective in Rb interaction, stimulates E2F transactivation activity and inhibits Rb growth suppression function. Taken together, these results suggest that MDM2-mediated inhibition of Rb likely contributes to MDM2 oncogenic activity.

High-density methylation of p14ARF and p16INK4A in Epstein-Barr virus-associated gastric carcinoma

Promoter hypermethylation of various tumor-related genes is extremely frequent in gastric carcinoma (GC) associated with Epstein-Barr virus (EBV). To investigate the significance of the promoter methylation in this type of GC, we examined the methylation densities of the promoter regions of p14ARF and p16INK4A in EBV-associated (n=7) and EBV-negative (n=14) GC. Bisulfite sequencing demonstrated a high frequency of concurrent methylation of p14ARF and p16INK4A promoter regions in EBVaGC. Methylation was observed in all 29 CpG sites of p14ARF and all 16 sites of p16INK4A with equally high densities. In EBV-negative GC, the methylation profiles differed between the 2 genes. Promoter methylation was sporadic and variable in p14ARF, and only the last position of CpG in p14ARF was methylated at high frequency. High-density methylation in p16INK4A was observed in a subset of GC, but the first position of CpG was never methylated in EBV-negative GC. These findings suggest the presence of mechanisms of de novo and maintenance methylation specific to EBVaGC that might be associated with EBV infection.

Inhibition of HDM2 and activation of p53 by ribosomal protein L23

The importance of coordinating cell growth with proliferation has been recognized for a long time. The molecular basis of this relationship, however, is poorly understood. Here we show that the ribosomal protein L23 interacts with HDM2. The interaction involves the central acidic domain of HDM2 and an N-terminal domain of L23. L23 and L11, another HDM2-interacting ribosomal protein, can simultaneously yet distinctly interact with HDM2 together to form a ternary complex. We show that, when overexpressed, L23 inhibits HDM2-induced p53 polyubiquitination and degradation and causes a p53-dependent cell cycle arrest. On the other hand, knocking down L23 causes nucleolar stress and triggers translocation of B23 from the nucleolus to the nucleoplasm, leading to stabilization and activation of p53. Our data suggest that cells may maintain a steady-state level of L23 during normal growth; alternating the levels of L23 in response to changing growth conditions could impinge on the HDM2-p53 pathway by interrupting the integrity of the nucleolus.

Role for PP2A in ARF signaling to p53

Activation of the ARF-p53 tumor suppressor pathway is one of the cell's major defense mechanisms against cancer induced by oncogenes. The ARF-p53 pathway is dysfunctional in a high proportion of human cancers. The regulation of the ARF-p53 signaling pathway has not yet been well characterized. In this study polyoma virus (Py) is used as a tool to better define the ARF-p53 signaling pathway. Py middle T-antigen (PyMT) induces ARF, which consequently up-regulates p53. We show that Py small T-antigen (PyST) blocks ARF-mediated activation of p53. This inhibition requires the small T-antigen PP2A-interacting domain. Our results reveal a previously unrecognized role of PP2A in the modulation of the ARF-p53 tumor suppressor pathway.

Loss of one allele of ARF rescues Mdm2 haploinsufficiency effects on apoptosis and lymphoma development

The tumor suppressor p19ARF inhibits Mdm2, which restricts the activity of p53. Complicated feedback and control mechanisms regulate ARF, Mdm2, and p53 interactions. Here we report that ARF haploinsufficiency completely rescued the p53-dependent effects of Mdm2 haploinsufficiency on B-cell development, survival, and transformation. In contrast to Mdm2+/- B cells, Mdm2+/- B cells deficient in ARF were similar to wild-type B cells in their rates of growth and apoptosis and activation of p53. Consequently, the profoundly reduced numbers of B cells in Mdm2+/-Emu-myc transgenic mice were restored to normal levels in ARF+/-Mdm2+/-Emu-myc transgenics. Additionally, ARF+/-Mdm2+/-Emu-myc transgenics developed lymphomas at rates analogous to those observed for wild-type Emu-myc transgenics, demonstrating that loss of one allele of ARF rescued the protracted lymphoma latency in Mdm2+/-Emu-myc transgenics. Importantly, in ARF+/-Mdm2+/-Emu-myc transgenic lymphomas, p53 was inactivated at the frequency observed in lymphomas of wild-type Emu-myc transgenics. Collectively, these results support a model whereby the stoichiometry of Mdm2 and ARF controls apoptosis and tumor development, which should have significant implications in the treatment of malignancies that have inactivated ARF.

Expression of p16INK4a and other cell cycle regulator and senescence associated genes in aging human kidney

BACKGROUND

Somatic cells in vitro have a finite life expectancy before entering a state of senescence. If this state has an in vivo counterpart, it could contribute to organ aging. We have previously shown that human kidney cortex displays telomere shortening with age. In the present study, we evaluated the relationship between renal age in humans and a number of phenomena associated with cellular senescence in vitro.

METHODS

Human kidney specimens were obtained at 8 weeks to 88 years of age and were assessed for changes related to aging.

RESULTS

We found that human kidneys expressed relatively constant levels of mRNAs for genes potentially related to senescence. Among the candidate genes surveyed, the cell cycle regulator p16INK4a emerged with the strongest association with renal aging for both mRNA and protein expression. Proliferation as measured by Ki-67 expression was inversely correlated with p16INK4a expression, compatible with a role for p16INK4a as an irreversible cell cycle inhibitor. Cyclooxygenase 1 and 2 (COX-1 and COX-2) mRNA expression was elevated in older kidneys, associated with increased protein expression. Comparison of gene expression with age-related histologic changes revealed that glomerulosclerosis correlated with p16INK4a and p53, whereas interstitial fibrosis and tubular atrophy were associated with p16INK4a, p53, COX-1, transforming growth factor-beta 1 (TGF-beta 1), and heat shock protein A5 (HSPA5).

CONCLUSION

We conclude that some changes observed in cellular senescence in vitro do occur in human kidney with age, particularly in the renal cortex, in some cases correlating with histologic features. P16INK4a emerged with the most consistent correlations with age and histologic changes and inversely correlated with cell replication.

Human Arf tumor suppressor specifically interacts with chromatin containing the promoter of rRNA genes

The tumor suppressor Arf (Alternative Reading Frame) protein (p14ARF in human and p19ARF in mouse) is mainly located in the nucleolus consistent with its subcellular localization, the protein has been shown to specifically interact with 5.8S rRNA and with B23/Nucleophosmin and to regulate ribosome biogenesis. Here, we show that the p14ARF protein interacts with chromatin and is recovered by chromatin immunoprecipitation (ChIP) in a fraction that contains a DNA sequence of the rRNA gene promoter. In addition, topoisomerase I (Topo I) that has been shown to interact with p14ARF coprecipitates with p14ARF containing chromatin. These data, in view of the function for Topo I in rRNA transcription, are consistent with a role for the p14ARF-Topo I complex in rRNA transcription and/or maturation.

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

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

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

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

Life and death decisions by E2F-1

Deregulation of the transcription factor E2F-1 is a common event in most human cancers. Paradoxically, E2F-1 has been shown to have the ability to induce both cell cycle progression and programmed cell death, leading potentially to both tumour-promoting as well as tumour-suppressive effects. Although the pathway to cell cycle progression seems straightforward with a number of growth-promoting E2F target genes having been described, the pathways to apoptosis are less well defined and more complex. The discovery that E2F-1 'knockout' mice are highly tumour prone has caused a recent surge in the number of reports relating to programmed cell death. This review focuses on these recent findings, highlighting the way in which they have increased our understanding of E2F-1-induced cell death, as well as indicating the questions that remain. Insight gained as to the role of this intriguing molecule in cancer and its potential for targeted therapy will also be discussed.

Herpes simplex virus type 1 infection induces the stabilization of p53 in a USP7- and ATM-independent manner

The major oncoprotein p53 regulates several cellular antiproliferation pathways that can be triggered in response to a variety of cellular stresses, including viral infection. The stabilization of p53 is a key factor in the ability of cells to initiate an efficient transcriptional response after cellular stress. Here we present data demonstrating that herpes simplex virus type 1 (HSV-1) infection of HFFF-2 cells, a low-passage-number nontransformed human primary cell line, results in the stabilization of p53. This process required viral immediate-early gene expression but occurred independently of the viral regulatory protein ICP0 and viral DNA replication. No specific viral protein could be identified as being solely responsible for the effect, which appears to be a cellular response to developing HSV-1 infections. HSV-1 infection also induced the phosphorylation of p53 at residues Ser15 and Ser20, which have previously been implicated in its stabilization in response to DNA damage. However, an HSV-1 infection of ATM(-/-) cells, which lack a kinase implicated in these phosphorylation events, did not lead to the phosphorylation of p53 at these residues, but nonetheless p53 was stabilized. We also show that the wild-type p53 expressed by osteosarcoma U2OS cells can be stabilized in response to DNA damage induced by UV irradiation, but not in response to HSV-1 infection. These data suggest that multiple cellular mechanisms are initiated to stabilize p53 during an HSV-1 infection. These mechanisms occur independently of ICP0 and its ability to sequester USP7 and may differ from those initiated in response to DNA damage.

Epigenetic Inactivation of Tumor Suppressor Genes in Hematologic Malignancies

A number of genetic alterations are involved in the development of hematologic malignancies. These alterations include the activation of oncogenes by chromosomal translocation or gene amplification and the inactivation of tumor suppressor genes by gene deletion or mutations. Recently, epigenetic change has been proven to be another important means of inactivating tumor suppressor genes in tumor cells, and hypermethylation of promoter DNA is one of the most important mechanisms. In hematologic malignancies, many kinds of tumor suppressor genes and candidate suppressor genes are epigenetically inactivated. Inactivation of tumor suppressor genes usually occurs in a disease-specific manner and plays important roles in the development and progression of the disease. Some of these alterations have clinical effects on treatment results or the prognoses of the patients.

Yin Yang 1 is a negative regulator of p53

Yin Yang 1 (YY1) is a transcription factor that plays an essential role in development. However, the full spectrum of YY1's functions and mechanism of action remains unclear. We find that YY1 ablation results in p53 accumulation due to a reduction of p53 ubiquitination in vivo. Conversely, YY1 overexpression stimulates p53 ubiquitination and degradation. Significantly, recombinant YY1 is sufficient to induce Hdm2-mediated p53 polyubiquitination in vitro, suggesting that this function of YY1 is independent of its transcriptional activity. We identify direct physical interactions of YY1 with Hdm2 and p53 and show that the basis for YY1-regulating p53 ubiquitination is its ability to facilitate Hdm2-p53 interaction. Importantly, the tumor suppressor p14ARF compromises the Hdm2-YY1 interaction, which is important for YY1 regulation of p53. Taken together, these findings identify YY1 as a potential cofactor for Hdm2 in the regulation of p53 homeostasis and suggest a possible role for YY1 in tumorigenesis.

HDAC inhibitors trigger apoptosis in HPV-positive cells by inducing the E2F-p73 pathway

Histone deacetylase (HDAC) inhibitors induce an intrinsic type of apoptosis in human papillomavirus (HPV)-positive cells by disrupting the mitochondrial transmembrane potential (deltapsim). Loss of deltapsim was only detected in E7, but not in E6 oncogene-expressing cells. HDAC inhibition led to a time-dependent degradation of the pocket proteins pRb, p107 and p130, releasing 'free' E2F-1 following initial G1 arrest. Inhibition of proteasomal proteolysis, but not of caspase activity rescued pRb from degradation and functionally restored its inhibitory effect on the cyclin E gene, known to be suppressed by pRb-E2F-1 in conjunction with HDAC1. Using siRNA targeted against p53, E2F-1 still triggered apoptosis by inducing the E2F-responsive proapoptotic alpha- and beta-isoforms of p73. These data may determine future therapeutic strategies in which HDAC inhibitors can effectively eliminate HPV-positive cells by an apoptotic route that does not rely on the reactivation of the 'classical' p53 pathway through a preceding shut-off of viral gene expression.

DNA hypermethylation in gastric cancer

BACKGROUND

Transcriptional silencing of tumour suppressor genes by DNA hypermethylation plays a crucial role in the progression of gastric cancer. Many genes involved in the regulation of cell cycle, tissue invasion, DNA repair and apoptosis have been shown to be inactivated by this type of epigenetic mechanism.

RESULTS

Recent studies have demonstrated that DNA hypermethylation begins early in cancer progression, and in some cases, may precede the neoplastic process. Ageing is associated with DNA hypermethylation, and may provide a mechanistic link between ageing and cancer. Several reports have indicated that Epstein-Barr virus-related gastric cancer is associated with a high frequency of DNA hypermethylation, suggesting that viral oncogenesis might involve DNA hypermethylation with inactivation of tumour suppressor genes. Hypermethylation of hMLH1 with the resulting loss of its expression is known to cause microsatellite instability, which reflects genomic instability associated with defective DNA mismatch repair genes in the tumour.

CONCLUSIONS

In conclusion, recent studies demonstrate that DNA hypermethylation is a crucial mechanism of inactivation of tumour suppressor genes in gastric cancer. A better understanding of DNA hypermethylation will provide us with new opportunities in the diagnosis and therapy of gastric cancer.

Successful growth and characterization of mouse pancreatic ductal cells: functional properties of the Ki-RAS(G12V) oncogene

BACKGROUND & AIMS

The Ki-RAS oncogene is altered in pancreatic ductal neoplasms. Pancreatic ductal cells (PDCs) were purified from cytokeratin 19 (K19)-Ki-RAS(G12V) transgenic mice and control littermates to identify properties of Ki-Ras activation in a cell-type-specific context. Because Ki-RAS mutation has prognostic significance in patients treated with radiation, we studied the influence of Ki-RAS status on radiation survival.

METHODS

Pancreatic ductal fragments from mice with Ki-RAS(G12V) mutation or wild-type (WT)-Ki-RAS were cultured. Growth curves, electron microscopy, flow cytometry, and analysis of signaling and cell-cycle proteins were established. Farnesyltransferase inhibitor (FTI) treatment with R115777 before and after irradiation was used to determine the effect of Ki-Ras farnesylation on cell survival.

RESULTS

PDCs from WT and K19-Ki-RAS(G12V) mice had features of ductal cells with formation of 3-dimensional structures on collagen without differences in morphology, growth, and cell-cycle distribution. This may result from up-regulation of p16INK4 and p27(Kip1) and lack of hyperstimulation of the mitogen-activated protein kinase pathway in Ki-RAS(G12V) PDCs. No differences in radiation survival between Ki-RAS(G12V) PDCs and WT PDCs were observed. However, Ki-RAS(G12V) PDCs expressing mutant p53(V143A) had enhanced survival compared with WT PDCs transduced with p53(V143A). R115777 treatment sensitized Ki-RAS(G12V) PDCs and Ki-RAS(G12V)/p53(V143A) PDCs, but not WT PDCs.

CONCLUSIONS

Novel characterization of murine WT PDCs and Ki-RAS(G12V) PDCs is described. Induction of cell-cycle regulators and lack of mitogen-activated protein kinase hyperstimulation likely are responsible for constraining activated Ki-RAS(G12V)-mediated proliferation. Because its activation was required for sensitization by an FTI, R115777 may be useful against pancreatic tumors expressing oncogenic Ki-Ras.

Inactivation of the p16 gene by hypermethylation and loss of heterozygosity in adenocarcinoma of the lung

We investigated the aberrant promoter hypermethylation of p16, p15 and p14 genes and loss of heterozygosity (LOH) at 9p21-22 in 48 cases of adenocarcinoma of the lung. The frequencies of hypermethylation of genes were as follows: p16, 25.0%; p15, 22.9%; and p14, 18.8%. The frequency of LOH at chromosome 9p21-22 was 60.9%. The frequency of two-hit inactivation of the p16 gene by hypermethylation and LOH was 21.7%. Two-hit inactivation of the p16 gene showed loss of protein expression and was significantly correlated with tumor size, tumor grade and the Ki-67 labeling index. Hypermethylation of the p16 gene was not significantly correlated with hypermethylation of the p15 and p14 genes, both of which are close to the p16 gene locus, suggesting that hypermethylation of these genes occurs selectivity. In conclusion, biallelic inactivation of the p16 gene by hypermethylation and LOH might cause loss of p16 expression and play an important role in the development of adenocarcinoma of the lung. Therefore, controlling and monitoring for hypermethylation of the p16 gene may be partially useful for treatment and early diagnosis of adenocarcinoma of the lung.

Repression of the Arf tumor suppressor by E2F3 is required for normal cell cycle kinetics

Tumor development is dependent upon the inactivation of two key tumor-suppressor networks, p16(Ink4a)-cycD/cdk4-pRB-E2F and p19(Arf)-mdm2-p53, that regulate cellular proliferation and the tumor surveillance response. These networks are known to intersect with one another, but the mechanisms are poorly understood. Here, we show that E2F directly participates in the transcriptional control of Arf in both normal and transformed cells. This occurs in a manner that is significantly different from the regulation of classic E2F-responsive targets. In wild-type mouse embryonic fibroblasts (MEFs), the Arf promoter is occupied by E2F3 and not other E2F family members. In quiescent cells, this role is largely fulfilled by E2F3b, an E2F3 isoform whose function was previously undetermined. E2f3 loss is sufficient to derepress Arf, triggering activation of p53 and expression of p21(Cip1). Thus, E2F3 is a key repressor of the p19(Arf)-p53 pathway in normal cells. Consistent with this notion, Arf mutation suppresses the activation of p53 and p21(Cip1) in E2f3-deficient MEFs. Arf loss also rescues the known cell cycle re-entry defect of E2f3(-/-) cells, and this correlates with restoration of appropriate activation of classic E2F-responsive genes. Our data also demonstrate a direct role for E2F in the oncogenic activation of Arf. Specifically, we observe recruitment of the endogenous activating E2Fs, E2F1, and E2F3a, to the Arf promoter. Thus, distinct E2F complexes directly contribute to the normal repression and oncogenic activation of Arf. We propose that monitoring of E2F levels and/or activity is a key component of Arf's ability to respond to inappropriate, but not normal cellular proliferation.

Methylation of p14(ARF) gene in meningiomas and its correlation to the p53 expression and mutation

We have previously reported the statistically significant correlation of immunohistochemical expression of MIB-1 and p53 proteins among benign, atypical, and anaplastic meningiomas and p53 protein expression was high in atypical and anaplastic meningiomas. In the present study, we analyzed 22 cases of meningiomas for mutation of p53 gene in its spectrum of exon 5 to 8 using automated genetic analyzer. We did not find any mutation of p53 in any of these cases, thus suggesting the p53 protein expression is wild type. We analyzed 72 cases of meningiomas for determining the methylation status of p14(ARF) gene and the immunohistochemical expression of MDM2 protein to explain p53 protein expression in these meningiomas. We found methylation of p14(ARF) gene in five of 58 cases of benign meningiomas (8.6%), two of 10 cases of atypical meningiomas (20%), and two of four cases of anaplastic meningiomas (50%). In absence of p53 gene mutation, the high percentage of p14(ARF) gene methylation in high-grade meningioma may have been responsible for accumulation of wild-type p53 protein. In addition, we also found the loss of MDM2 protein in high-grade meningiomas. These deregulations of p14-MDM2-p53 pathway may contribute to the malignant progression of meningioma.

Essential role of ribosomal protein L11 in mediating growth inhibition-induced p53 activation

The ribosomal protein L11 binds to and suppresses the E3 ligase function of HDM2, thus activating p53. Despite being abundant as a component of the 60S large ribosomal subunit, L11 does not induce p53 under normal growth conditions. In search of mechanisms controlling L11-HDM2 interaction, we found that the induction of p53 under growth inhibitory conditions, such as low dose of actinomycin D or serum depletion, can be significantly attenuated by knocking down L11, indicating the importance of L11 in mediating these growth inhibitory signals to p53. We show that L11 is not regulated by transcription or protein stability and its level remains relatively constant during serum starvation. However, serum starvation induces translocation of L11 from the nucleolus to the nucleoplasm, where it participates in a complex with HDM2. We propose that the nucleolus acts as a barrier to prevent L11 interacting with HDM2 during normal growth. Growth inhibition, presumably through suppression of rRNA production in the nucleolus, facilitates translocation of L11 to the nucleoplasm, thus activating p53 through inhibiting HDM2.

Identification of five chromosomal regions involved in predisposition to melanoma by genome-wide scan in the MeLiM swine model

In human familial melanoma, 3 risk susceptibility genes are already known, CDKN2A, CDK4 and MC1R. However, various observations suggest that other melanoma susceptibility genes have not yet been identified. To search for new susceptibility loci, we used the MeLiM swine as an animal model of hereditary melanoma to perform a genome scan for linkage to melanoma. Founders of the affected MeLiM stock were crossed with each other and with healthy Duroc pigs, generating MeLiM, F1 and backcross families. As we had previously excluded the MeLiM CDKN2A gene, we paid special attention to CDK4 and MC1R, as well as to other candidates such as BRAF and the SLA complex, mapping them on the swine radiation hybrid map and/or isolating close microsatellite markers to introduce them into the genome scan. The results revealed, first, that swine melanoma was inherited as an autosomal dominant trait with incomplete penetrance, preferably in black animals. Second, 4 chromosomal regions potentially involved in melanoma susceptibility were identified on Sus Scrofa chromosomes (SSC) 1, 2, 7 and 8, respectively, in intervals 44-103, 1.9-18, 59-73 and 47-62 cM. A fifth region close to MC1R was revealed on SSC 6 by analyzing an individual marker located at position 7.5 cM. Lastly, CDK4 and BRAF were unlikely to be melanoma susceptibility genes in the MeLiM swine model. The 3 regions on SSC 1, 6 and 7, respectively, have counterparts on human chromosomes (HSA) 9p, 16q and 6p, harboring melanoma candidate loci. The 2 others, on SSC 2 and 8, have counterparts on HSA 11 and 4, which might therefore be of interest for human studies.

Rb Inhibits E2F-1-induced Cell Death in a LXCXE-dependent Manner by Active Repression

Rb (retinoblastoma protein) inhibits E2F-1-induced cell death. We now show that the ability of Rb to inhibit E2F-1-induced cell death is dependent on a functional LXCXE-binding site in Rb, thereby suggesting that proteins that bind the LXCXE-binding site in Rb may regulate the anti-apoptotic activity of Rb. HDAC1, an LXCXE protein that plays a critical role in Rb-mediated transcription repression, abrogates the effect of Rb on E2F-1-induced cell death. In contrast, RF-Cp145, another LXCXE protein, cooperates with Rb to inhibit E2F-1-induced cell death. Both proteins exert their effect in an LXCXE-dependent manner. Rb regulates E2F-induced cell death by acting upstream of p73. Rb represses the p73 promoter. Our results further suggest a model in which Rb-E2F-1 complexes mediate the anti-apoptotic activity of Rb through active repression of target genes without recruiting HDAC1.

MDM2 Mediates p300/CREB-binding Protein-associated Factor Ubiquitination and Degradation

We recently reported that MDM2, a negative feedback regulator of the tumor suppressor p53, inhibits p300/CREB-binding protein-associated factor (PCAF)-mediated p53 acetylation. Our further study showed that MDM2 also regulates the stability of PCAF. MDM2 ubiquitinated PCAF in vitro and in cells. PCAF ubiquitination occurred at the N terminus and in the nucleus, as the nuclear localization signal sequence-deletion mutant of MDM2, which localized in the cytoplasm and degraded p53, was unable to degrade nuclear PCAF. Restriction of PCAF in the nucleus by leptomycin B did not affect MDM2-mediated PCAF degradation. Consistently, overexpression of MDM2 in p53 null cells caused the reduction of the protein level of PCAF, but not the mRNA level. Conversely, PCAF levels were higher in MDM2-deficient mouse p53(-/-)/mdm2(-/-) embryonic fibroblast (MEF) cells than that in MDM2-containing MEF cells. Furthermore, MDM2 reduced the half-life of PCAF by 50%. These results demonstrate that MDM2 regulates the stability of PCAF by ubiquitinating and degrading this protein.

The importance of p53 location: nuclear or cytoplasmic zip code?

The regulation of p53 functions is tightly controlled through several mechanisms including p53 transcription and translation, protein stability, post-translational modifications, and subcellular localization. Despite intensive study of p53, the regulation of p53 subcellular localization although important for its function is still poorly understood. The regulation of p53 localization depends on factors that influence its nuclear import and export, subnuclear localization and cytoplasmic tethering and sequestration. In this review, we will focus on various proteins and modifications that regulate the location and therefore the activity of p53. For example, MDM2 is the most important regulator of p53 nuclear export and degradation. Cytoplasmic p53 associates with the microtubule cytoskeleton and the dynein family of motor proteins; while Parc and mot2 are involved in its cytoplasmic sequestration. Finally, a portion of p53 is localized to the mitochondria as part of the non-transcriptional apoptotic response. In this review we strive to present the most recent data on how the activity of p53 is regulated by its location.

Overexpression of leukocyte marker CD43 causes activation of the tumor suppressor proteins p53 and ARF

CD43 or leukosialin is a transmembrane sialoglycoprotein, whose extracellular domain participates in cell adhesiveness and the cytoplasmic tail regulates a variety of intracellular signal transduction pathways involved in cell proliferation. CD43 is abundantly expressed on the surface of hematopoietic cells, but CD43 expression is also frequently found in the tumor cells of nonhematopoietic origin. In the early stages of some tumors, the accumulation of tumor suppressor protein p53 has been described. Here, we show that the expression of CD43 causes the induction of functionally active p53 protein. Moreover, we found that the activation of p53 by CD43 is mediated by tumor suppressor protein ARF. The coexpression of CD43 and ARF in ARF-null mouse embryonic fibroblasts resulted in programmed cell death, but that was not the case when CD43 alone was expressed in these cells. These data provide the first evidence of the connection between p53- and CD43-dependent pathways.

Regulating the p53 system through ubiquitination

The tumor suppressor p53 is tightly controlled at low levels in cells by constant ubiquitination and proteasomal degradation. In response to stresses, ubiquitination of p53 is inhibited through diverse pathways, depending on the nature of the stimulus and cell type. This leads to the accumulation and activation of p53, which induces cell cycle arrest and/or apoptosis to prevent cells from transformation. Many studies have indicated that defects of the p53 system are present in most, if not all, human tumor cells. Meanwhile, significant progress has been made in understanding the molecular mechanisms of p53 ubiquitination and the regulation of the p53 system. Therefore, it is possible now to consider targeting ubiquitination as a means to regulate and reactivate p53 in tumors. Emerging evidence suggests that inhibiting the E3 activity of Mdm2, blocking the interaction of p53 and Mdm2, and restoring the function of mutated p53 are potential effective strategies to kill certain tumor cells selectively. It is conceivable that new chemotherapeutic agents based on these studies will be generated in the not-so-distant future.

Limited role of N-terminal phosphoserine residues in the activation of transcription by p53

The p53 tumor suppressor is phosphorylated in response to various cellular stress signals, such as DNA damage, leading to its release from MDM2 and consequent stabilization and activation as a transcription factor. In human U2OS cells, treatment with adriamycin causes p53 to be phosphorylated on all six serine residues tested, leading to the dissociation of p53 from MDM2 and transcription of the p21 and mdm2 genes. In contrast, in these cells, IPTG-dependent induction of p14ARF, which sequesters MDM2 away from p53, does not lead to detectable phosphorylation of any of the five N-terminal serine residues tested (6, 9, 15, 20, 37). Only C-terminal serine 392 is phosphorylated. However, the increase of p21 and mdm2 mRNAs was indistinguishable following treatment with adriamycin or induction of p14ARF. By using cDNA arrays to examine global p53-dependent gene expression in response to adriamycin or p14ARF, we found that most genes were regulated similarly by the two treatments. However, a subset of p53-regulated genes whose products have proliferative roles or regulate VEGF activity, newly described here, are repressed by p14ARF much more than by adriamycin. We conclude that the phosphorylation of p53 on N-terminal serine residues is not required for increased transcription of the great majority of p53-responsive genes and that the induction of p53 by p14ARF, with little phosphorylation, leads to substantial repression of genes whose products have roles in proliferation.

p53 activation in chronic radiation-treated breast cancer cells: regulation of MDM2/p14ARF

Mammalian cells chronically exposed to ionizing radiation (IR) induce stress response with a tolerance to the subsequent cytotoxicity of IR. Although p53 is well documented in IR response, the signaling network causing p53 activation in chronic IR remains to be identified. Using breast carcinoma MCF+FIR cells that showed a transient radioresistance after exposure chronically to fractionated IR (FIR), the present study shows that the basal DNA binding and transcriptional activity of p53 was elevated by FIR. p53-controlled luciferase activity was strikingly induced ( approximately 7.9-fold) with little enhancement of p53/DNA binding activity ( approximately 1.3-fold). The phosphorylated p53 (Thr 55) was increased in the cytoplasm and nucleus of MCF+FIR but not in the sham-FIR control cells. On the contrary, the sham-FIR control MCF-7 cells showed a low p53 luciferase transcription ( approximately 3-fold) but a striking enhancement of p53/DNA binding (12-fold) after 5 Gy of IR. To determine the signaling elements regulating p53 activity, DNA microarray of MCF+FIR using sham-FIR MCF-7 cells as a reference demonstrated that the mRNA of p21, MDM2, and p14ARF was up-regulated. Time course Western blot analysis, however, showed no difference in p21 induction. In contrast, MDM2 that was absent in control cells and was predominantly induced by IR was not induced in MCF+FIR cells. In agreement with MDM2 inhibition, MDM2-inhibitory protein p14ARF was increased in MCF+FIR cells. In summary, these results demonstrate that up-regulation of p14ARF paralleled with MDM2 inhibition contributes to p53 accumulation in the nucleus and causes a high responsiveness of p53 in chronic IR-treated breast cancer cells.

Immortalization of human mammary epithelial cells is associated with inactivation of the p14ARF-p53 pathway

Inactivation of the ARF-p53 tumor suppressor pathway leads to immortalization of murine fibroblasts. The role of this pathway in immortalization of human epithelial cells is not clear. We analyzed the functionality of the p14(ARF)-p53 pathway in human mammary epithelial cells (MEC) immortalized by human papillomavirus 16 (HPV16) E6, the p53 degradation-defective E6 mutant Y54D, or hTERT. E6-MEC or E6Y54D-MEC maintains high-level expression of p14(ARF). Late-passage hTERT-immortalized MEC express p53 but down-regulate p14(ARF). Enforced expression of p14(ARF) induces p53-dependent senescence in hTERT-MEC, while both E6-MEC and E6Y54D-MEC are resistant. We show that E6Y54D inhibits p14(ARF)-induced activation of p53 without inactivation of the p53-dependent DNA damage response. Hence, p53 degradation and inhibition of p14(ARF) signaling to p53 are independent functions of HPV16 E6. Our observations imply that long-term proliferation of MEC requires inactivation of the p14(ARF)-p53 pathway.

Endogenous oncogenic K-ras(G12D) stimulates proliferation and widespread neoplastic and developmental defects

Activating mutations in the ras oncogene are not considered sufficient to induce abnormal cellular proliferation in the absence of cooperating oncogenes. We demonstrate that the conditional expression of an endogenous K-ras(G12D) allele in murine embryonic fibroblasts causes enhanced proliferation and partial transformation in the absence of further genetic abnormalities. Interestingly, K-ras(G12D)-expressing fibroblasts demonstrate attenuation and altered regulation of canonical Ras effector signaling pathways. Widespread expression of endogenous K-ras(G12D) is not tolerated during embryonic development, and directed expression in the lung and GI tract induces preneoplastic epithelial hyperplasias. Our results suggest that endogenous oncogenic ras is sufficient to initiate transformation by stimulating proliferation, while further genetic lesions may be necessary for progression to frank malignancy.

Inactivation of the ARF-MDM-2-p53 pathway in sporadic Burkitt's lymphoma in children

Burkitt's lymphomas (BLs) are characterized by an activated MYC gene that provides a constitutive proliferative signal. However, activated myc can initiate ARF-dependent activation of p53 and apoptosis as well. Data derived from cell culture and animal models suggest that the inactivation of the ARF-MDM-2-p53 apoptotic signaling pathway may be a necessary secondary event for the development of BL. This has not been tested in freshly excised BL tissue. We investigated the ARF-MDM-2-p53 pathway in tumor specimen from 24 children with sporadic BL/B-ALL. Direct sequencing revealed a point mutation in the p53 gene in four BL. Overexpression of MDM-2 was evident in 10 of the BL samples analyzed by real-time quantitative PCR. Deletion of the CDKN2A locus that encodes ARF or reduced expression of ARF could not be detected in any BL by fluorescence in situ hybridization analysis or real-time quantitative PCR, respectively. Our results indicate that the ARF-MDM-2-p53 apoptotic pathway is disrupted in about 55% of the cases of childhood sporadic BL. We suggest that in addition to the inactivation of the ARF-MDM-2-p53 protective checkpoint function other antiapoptotic mutations may occur in a substantial part of children with sporadic BL.

Molecular pathways executing the "trophic sentinel" response in HPV-16 E7-expressing normal human diploid fibroblasts upon growth factor deprivation

In response to oncogenic insults, normal human cells execute a defense response that culminates in cellular suicide, apoptosis. Normal human diploid fibroblasts expressing the human papillomavirus type 16 (HPV-16) E7 oncoprotein are predisposed to apoptosis when they are deprived of growth factors. Even though a dominant negative p53 mutant abrogates the cell death response, it is not accompanied by p53 phosphorylation, the DNA binding capacity of p53 remains unaltered, and no activation of common p53-dependent transcriptional targets is observed. Expression of two insulin-like growth factor-1 binding proteins, IGFBP-2 and -5, is increased presumably in response to enhanced NF-kappaB activity in HPV-16 E7-expressing serum-starved cells. Phosphorylation of AKT, an important modulator of IGF-1 survival signaling, is lower in serum-starved E7-expressing cells, and exogenously added IGF-1 can partially inhibit the cell death response. This suggests that IGFBP-2 and -5 may limit IGF-1 availability thus decreasing survival signaling. Caspase 3 but not caspase 8 is activated in serum-starved HPV-16 E7-expressing cells. Caspase inhibition affects nuclear DNA fragmentation, but cell death is not inhibited. Although mitochondria play important roles in caspase-dependent as well as -independent forms of cell death, there is no evidence for cytochrome c release and thus for mitochondrial permeabilization in growth factor deprived HPV-16 E7-expressing cells.

The cell cycle and its relevance to the urologist

PURPOSE

We surveyed fundamental concepts of the cell cycle to help the average urologist better understand the molecular basis for specific aspects of urological disease.

MATERIALS AND METHODS

Important publications that have shaped our current understanding of the cell cycle were selected for review. Definitions of key terms are provided in a glossary.

RESULTS

Cell proliferation, survival and programmed cell death (apoptosis) are the net result of a complex interaction of molecular signals that regulate DNA and protein synthesis. Many of the abnormal patterns of cell behavior that contribute to the pathology of malignant urological disease arise from disruptions in the molecular controls that normally regulate the cell cycle. Benign urological conditions, including cystic diseases and hypertrophy, also reflect abnormal growth that results from the disruption of cell cycle controls.

CONCLUSIONS

This review is designed for the clinician and for the nonspecialist who is interested in the science of the cell cycle and its regulation as it broadly pertains to urological disease. Recent advances in the understanding of cell cycle regulation are presented with clinical correlations illustrating how these processes are involved in coordinating cell growth and cell death at the molecular level.

p53, p21, pRB, and p16 Expression Predict Clinical Outcome in Cystectomy With Bladder Cancer

Purpose

To determine whether p53, p21, pRB, and/or p16 expression is associated with bladder cancer stage, progression, and prognosis.

Patients and Methods

Immunohistochemical staining for p53, p21, pRB, and p16 was carried out on serial sections from archival specimens of 80 patients who underwent bilateral pelvic lymphadenectomy and radical cystectomy for bladder cancer (median follow-up, 101 months).

Results

p53, p21, and pRB or p16 expression was altered in 45 (56%), 39 (49%), and 43 (54%) tumors, respectively. Sixty-six patients (83%) had at least one marker altered, and 21 patients (26%) had all three altered. Abnormal expressions of p53, p21, and pRB/p16 expression were associated with muscle-invasive disease (P = .007, P = .003, and P = .003, respectively). The alteration of each marker was independently associated with disease progression (P ≤ .038) and disease-specific survival (P ≤ .039). In multivariable models that included standard pathologic features and p53 with p21 or p53 with pRB/p16, only p53 and lymph node metastases were associated with bladder cancer progression (P ≤ .026) and death (P ≤ .028). In models that included p21 and pRB/p16, only p21 and lymph node metastases were associated with bladder cancer progression (P ≤ .022) and death (P ≤ .028). In a model that included the combined variables p53/p21 and pRB/p16, only p53/p21 and lymph node status were associated with bladder cancer progression (P ≤ .047) and death (P ≤ .036). The incremental number of altered markers was independently associated with an increased risk of bladder cancer progression (P = .005) and mortality (P = .007).

Conclusion

Although altered expression of each of the four cell cycle regulators is associated with bladder cancer outcome in patients undergoing radical cystectomy, p53 is the strongest predictor, followed by p21, suggesting a more pivotal role of the p53/p21 pathway in bladder cancer progression.

The Bmi-1 oncoprotein is overexpressed in human colorectal cancer and correlates with the reduced p16INK4a/p14ARF proteins

To clarify the roles of Bmi-1 in colorectal carcinoma, we examined the expression of Bmi-1 in 41 samples out of 46 colorectal carcinomas by reverse transcription-PCR, whereas all 46 were analyzed by immunostaining. In addition, we analyzed the expression patterns of Bmi-1 in association with p16INK4a and p14ARF (in mouse p19ARF) in a series of colorectal carcinomas. The level of Bmi-1 mRNA in the carcinoma tissues was significantly higher than those of the adjacent non-neoplastic colonic mucosal tissues. Immunohistochemistry for Bmi-1 showed moderate or strong expression levels in 65% (30/46) of colorectal carcinomas. Colorectal carcinomas with moderate or strong Bmi-1 expression were more likely to have low levels of the INK4 locus proteins (p16INK4a/p14ARF) (P<0.07). These results suggested that modulation of Bmi-1 protein might be involved in human colorectal carcinogenesis by repressing the INK4a/ARF proteins.

Loss of p53 transcriptional activity in hepatocellular carcinoma evaluated by yeast-based functional assay: comparison with p53 immunohistochemistry

We studied the transcriptional activity of p53 protein in 50 tissues of hepatocellular carcinoma (HCC) using a yeast functional assay. In this assay, red yeast colonies indicate that p53 protein cannot bind to its specific domain and has lost its transcriptional activity. We also clarified whether mutant p53 protein could inactivate wild-type p53 protein in a transdominant manner using a modified yeast assay. In addition, we examined whether immunohistochemically detectable p53 protein was functionally inactive. The incidence of p53 inactivation was significantly higher in tumors with capsular invasion. Out of 21 tumors diagnosed with p53 mutations, 11 exhibited >75% red colonies, and all contained missense mutations. In these tumors, p53 function was lost because there was supposedly no intact p53 gene on either allele. One missense mutant produced <60% red colonies, but it was also considered inactive as a p53 protein heterotetramer because of its transdominant activity. In 7 of the remaining 9 tumors, p53 was considered to be mutated on one allele and intact on the other. All of these 7 tumors contained nonsense or frameshift mutations and had no transdominant activity, which suggested that p53 function remained intact. Alternately, immunohistochemical analysis demonstrated that all of the tumors with missense mutations were positively immunostained, whereas those that contained nonsense or frameshift mutations were negatively stained. Consequently, positively immunostaining tumors mostly coincided with p53-inactive tumors. These yeast-based assays suggested that p53 function was retained in some mutant cases. Immunohistochemistry was helpful in screening functionally inactive p53 protein in HCCs.

MDM2 promotes p21waf1/cip1 proteasomal turnover independently of ubiquitylation

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

p53 alterations in myeloid leukemia

Although most solid tumors contain inactivating mutations of the p53 tumor suppressor, hematological malignancies do not contain frequent alterations in the p53 gene (<20%). How these tumors arise in the presence of a super tumor suppressor like p53 remains to be elucidated. Given the number of downstream effectors of p53, it is likely that critical targets of p53 are inactivated in leukemia, bypassing the requirement for p53 gene mutations in these tumors. This review describes new biochemical and transcriptional activities of p53 as well as the status of p53 in acute myelogenous leukemia and chronic myelogenous leukemia.

Mdm2 regulates p53 independently of p19(ARF) in homeostatic tissues

Tumor suppressor proteins must be exquisitely regulated since they can induce cell death while preventing cancer. For example, the p19(ARF) tumor suppressor (p14(ARF) in humans) appears to stimulate the apoptotic function of the p53 tumor suppressor to prevent lymphomagenesis and carcinogenesis induced by oncogene overexpression. Here we present a genetic approach to defining the role of p19(ARF) in regulating the apoptotic function of p53 in highly proliferating, homeostatic tissues. In contrast to our expectation, p19(ARF) did not activate the apoptotic function of p53 in lymphocytes or epithelial cells. These results demonstrate that the mechanisms that control p53 function during homeostasis differ from those that are critical for tumor suppression. Moreover, the Mdm2/p53/p19(ARF) pathway appears to exist only under very restricted conditions.

Tumor suppressor ARF degrades B23, a nucleolar protein involved in ribosome biogenesis and cell proliferation

The tumor suppressor ARF induces a p53-dependent and -independent cell cycle arrest. Unlike the nucleoplasmic MDM2 and p53, ARF localizes in the nucleolus. The role of ARF in the nucleolus, the molecular target, and the mechanism of its p53-independent function remains unclear. Here we show that ARF interacts with B23, a multifunctional nucleolar protein involved in ribosome biogenesis, and promotes its polyubiquitination and degradation. Overexpression of B23 induces a cell cycle arrest in normal fibroblasts, whereas in cells lacking p53 it promotes S phase entry. Conversely, knocking down B23 inhibits the processing of preribosomal RNA and induces cell death. Further, oncogenic Ras induces B23 only in ARF null cells, but not in cells that retain wild-type ARF. Together, our results reveal a molecular mechanism of ARF in regulating ribosome biogenesis and cell proliferation via inhibiting B23, and suggest a nucleolar role of ARF in surveillance of oncogenic insults.