Tumor surveillance via the ARF-p53 pathway

Regulation of the p53 homolog p73 by adenoviral oncogene E1A

p73 is a p53 homolog, as they are similar structurally and functionally. Unlike p53, p73 is not inactivated by the products of viral oncogenes such as SV40 T antigen and human papilloma virus E6. Here we show that the product of adenoviral oncogene E1A inhibits the transcriptional activation by both p73alpha and p73beta. Electrophoretic mobility shift assays revealed that E1A does not inhibit the sequence-specific DNA binding by p73. Transcriptional activation by a fusion protein containing the Gal4 DNA-binding domain and either of the activation domains of p73 was inhibited by wild-type (WT) E1A, but not by the N-terminal deletion mutant E1A(Delta2-36). E1A(Delta2-36), which does not bind to the p300/CBP family of coactivators, failed to inhibit p73-mediated transcription, whereas E1A(DeltaCR2), a deletion mutant that does not bind to the pRb family of proteins, inhibited p73-mediated transcription as efficiently as WT E1A. Consistent with these observations, growth arrest induced by p73 expressed from a recombinant adenovirus was abrogated by WT E1A, which correlated with inhibition of p73-mediated induction of p21(WAF1/CIP1) by E1A. However, p73 was able to induce p21(WAF1/CIP1) and to mediate growth arrest in the presence of E1A(Delta2-36). Furthermore, the expression of either wild-type E1A or E1A(Delta2-36) resulted in the stabilization of endogenous p73. However, p73 stabilized in response to the expression of E1A(Delta2-36), but not WT E1A, was able to activate the expression of p21(WAF1/CIP1). These results suggest that the transcriptional activation function of p73 is specifically targeted by E1A through a mechanism involving p300/CBP proteins during the process of transformation and that p73 may have a role to play as a tumor suppressor.

Methods for screening tumors for p53 status and therapeutic exploitation

Mutations in the p53 oncosuppressor gene occur in most human cancers and regulation of the protein is defective in a variety of others. Novel strategies are emerging for the treatment of tumors that have p53 mutations. In this context, the analysis of p53 status is useful in diagnosis and prognosis, and could serve to evaluate the effectiveness of a cancer treatment. In this review, we report an overview of major methods for screening tumors for p53 status and the major strategies suggested for restoring p53 function.

Chk2 activates E2F-1 in response to DNA damage

The E2F-1 transcription factor is regulated during cell cycle progression and induced by cellular stress, such as DNA damage. We report that checkpoint kinase 2 (Chk2) regulates E2F-1 activity in response to the DNA-damaging agent etoposide. A Chk2 consensus phosphorylation site in E2F-1 is phosphorylated in response to DNA damage, resulting in protein stabilization, increased half-life, transcriptional activation and localization of phosphorylated E2F-1 to discrete nuclear structures. Expression of a dominant-negative Chk2 mutant blocks induction of E2F-1 and prevents E2F-1-dependent apoptosis. Moreover, E2F-1 is resistant to induction by etoposide in tumour cells expressing mutant chk2. Therefore, Chk2 phosphorylates and activates E2F-1 in response to DNA damage, resulting in apoptosis. These results suggest a role for E2F-1 in checkpoint control and provide a plausible explanation for the tumour suppressor activity of E2F-1.

Improving cancer therapy by non-genotoxic activation of p53

Inactivation of p53 function is a common event in cancer. Approximately 50% of human tumours express mutant p53 and there is evidence that in others, including many childhood tumours, p53 function is impaired in other ways. These defects in p53 function may be due to the alteration of cellular factors that modulate p53 or to the expression of viral oncoproteins. Radiotherapy and many of the chemotherapeutic drugs currently used in cancer treatment are potent activators of p53. However, most of these therapies have a serious drawback, and that is the long-term consequences of their DNA damaging effects. Here, we review the discoveries in p53 research that are most significant to the development of new therapies based on the induction of the transcriptional activity of p53 in a non-genotoxic way and discuss the situations in which this type of approach may be most beneficial.

p19Arf-p53 tumor suppressor pathway regulates cell motility by suppression of phosphoinositide 3-kinase and Rac1 GTPase activities

The p19(Arf)-p53 tumor suppressor pathway plays a critical role in cell-cycle checkpoint control and apoptosis, whereas Rho family small GTPases are key regulators of actin structure and cell motility. By using primary mouse embryonic fibroblasts that lack Arf, p53, or both, we studied the involvement of the p19(Arf)-p53 pathway in the regulation of cell motility and its relationship with Rho GTPases. Deletion of Arf and/or p53 led to actin cytoskeleton reorganization and a significant increase in cell motility. The endogenous phosphoinositide (PI) 3- kinase and Rac1 activities were elevated in Arf(-/-) and p53(-/-) cells, and these activities are required for p19(Arf)- and p53-regulated migration. Reintroduction of the wild type Arf or p53 genes into Arf(-/-) or p53(-/-) cells reversed the PI 3-kinase and Rho GTPase activities as well as the migration phenotype. These results suggest a functional relationship between an established tumor suppressor pathway and a signaling module that controls actin structure and cell motility and show that p19(Arf) and p53 negatively regulate cell migration by suppression of PI 3-kinase and Rac1 activities.

Delineation of the domains required for physical and functional interaction of p14ARF with human topoisomerase I

We recently reported an interaction between the p14(ARF) protein and human topoisomerase I (Topo I) resulting in the stimulation of the relaxation activity of Topo I. Our data showed that the complex between the two proteins was located within the nucleolus. In the present work, we have investigated the regions of p14(ARF) involved in this interaction by using targeted point mutagenesis and deletion mutants. A region encompassing exon 2-encoded sequence was required for physical binding of p14(ARF) to Topo I as well as for stimulatory activity of the enzyme. Exon 1 beta-encoded segment was not implicated in the interaction. Moreover, among p14(ARF) point mutants selected for their high conservation among different mammalian species, mutant p14(ARF) (RR87, 88AA) did not stimulate Topo I in spite of its association with the enzyme, suggesting its direct implication in the functional activity of ARF. In contrast, one mutant, p14(ARF) (R71A), was more efficient than wild-type protein to activate Topo I, suggesting that this residue is a key element to modulate Topo I activity. Finally, only ARF-Topo I complexes containing p14(ARF) exon 2 segment were found to be localized in the nucleolus, suggesting that this subnuclear location is linked to the biological function of the ARF-Topo I complex.

Molecular defects in the pathogenesis of pituitary tumours

The majority of pituitary adenomas are trophically stable and change relatively little in size over many years. A comparatively small proportion behave more aggressively and come to clinical attention through inappropriate hormone secretion or adverse effects on surrounding structures. True malignant behaviour with metastatic spread is very atypical. Pituitary adenomas that come to surgery are predominantly monoclonal in origin and roughly half are aneuploid, indicating either ongoing genetic instability or transition through a period of genetic instability at some time during their development. Few are associated with the classical mechanisms of tumour formation but it is generally believed that the majority harbour quantitative if not qualitative differences in molecular composition compared to the normal pituitary. Despite their prevalence and the ready availability of biopsy material, at the present time, the precise molecular pathogenesis of the majority of pituitary adenomas remains unclear. This review summarizes current thinking.

Ubiquitination, phosphorylation and acetylation: the molecular basis for p53 regulation

The p53 tumor suppressor exerts anti-proliferative effects, including growth arrest, apoptosis and cell senescence, in response to various types of stress. Tight regulation of p53 activation is imperative for preventing tumorigenesis and maintaining normal cell growth; p53 stabilization and transcriptional activation are crucial early events in a cell's battle against genotoxic stress. Ubiquitination, phosphorylation and acetylation are post-translational modifications to p53 that affect its overall appearance and activity. Recent findings suggest that these modifications have a profound affect on p53 stability and function. Defining the precise roles of these modifications in p53 function may show not only that they are markers of the stress response but also that they serve as the focal point in the regulation of p53.

The mitochondrial apoptosis-inducing factor plays a role in E2F-1-induced apoptosis in human colon cancer cells

BACKGROUND

Overexpression of the transcription factor E2F-1 provokes apoptosis in cancer cells; the mechanism, however, is not completely understood. We sought to evaluate E2F-1 gene therapy in human colon cancer and to investigate the apoptotic pathway involved.

METHODS

Adenoviral vectors were used to transfect the E2F-1 gene (Ad5E2F-1) or the control gene luciferase (Ad5Luc) into four human colon carcinoma cell lines. Apoptosis was confirmed by flow cytometry and poly (ADP-ribose) polymerase cleavage. Expression of apoptotic factors was determined with Western blot analysis. Inhibitory assays were used to determine the involvement of caspases in the apoptotic pathway.

RESULTS

Overexpression of E2F-1 was evident in all cells treated with Ad5E2F-1; upregulation of Bcl-2, and activation of caspases were noted. The apoptosis-inducing factor in the cytosolic fraction was markedly upregulated after Ad5E2F-1 treatment. E2F-1 overexpression inhibited proliferation and induced significant apoptosis in all cell lines (P <.005). This apoptotic response could be only partially blocked by caspase inhibitors.

CONCLUSIONS

These findings demonstrate that E2F-1 induces apoptosis and inhibits proliferation in human colon cancer cell lines. The marked upregulation of apoptosis-inducing factor and the fact that E2F-1-induced apoptosis is incompletely blocked by caspase inhibitors suggest a caspase-independent pathway of E2F-1-mediated apoptosis, reported here for the first time.

p14ARF induces G2 arrest and apoptosis independently of p53 leading to regression of tumours established in nude mice

Until recently, the ability of ARF (human p14(ARF), murine p19(ARF)) tumour-suppressor protein, encoded by the INK4A/ARF locus, to inhibit cell growth in response to various stimuli was related to its ability to stabilize p53 through the so-called ARF/MDM2/p53 pathway. However, recent data have demonstrated that ARF is not implicated in this unique p53-dependent pathway. By use of transient and stable expression, we show here that human p14(ARF) inhibits the growth of human tumoral cells lacking functional p53 by inducing a transient G(2) arrest and subsequently apoptosis. This p14(ARF)-induced G(2) arrest was correlated with inhibition of CDC2 activity, inactivation of CDC25C phosphatase and induction of the CDK inhibitor p21(WAFI). Apoptosis was demonstrated using Hoechst 33352 staining, proteolytic activation of caspase-3 and PARP cleavage. Similar results were obtained in experiments with cells synchronized by hydroxyurea block. Importantly, we were able to reproduce these effects 'in vivo' by showing that p14(ARF) inhibits the growth of p53 nullizygous human tumours in nude mice and induces the regression of p53 -/- established tumours. In these experiments, tumoral regression was associated with inhibition of cell proliferation as well as induction of apoptosis confirming the data obtained in cell lines.

Consistent inactivation of p19(Arf) but not p15(Ink4b) in murine myeloid cells transformed in vivo by deregulated c-Myc

Cyclin-dependent kinase inhibitors p16(INK4a) and p15(INK4b), encoded by the CDKN2A and B loci, play an important role in negative regulation of the cell cycle. Furthermore, p19(ARF) also encoded by the CDKN2A locus, has been shown to regulate positively the p53 pathway leading to growth arrest and apoptosis. All three genes have been inactivated in human tumors. In myeloid cells, p15(INK4b) mRNA is upregulated during cytokine-induced differentiation and/or growth arrest, and hypermethylation of the p15(INK4b) gene promoter region is a common event in acute myeloid leukemia. In the present study, we examined murine monocyte/macrophage tumors with deregulated c-myc for evidence of Ink4 gene inactivation. p15(Ink4b) mRNA and protein were detected in the majority of leukemias, and p16(Ink4a) mRNA and protein were highly expressed in two of them. pRb was in a hypophosphorylated state in most of the neoplasms indicating that the Cdk inhibitors that were expressed in the cells were functional. The observed expression of p15(Ink4b) is inconsistent with their proliferation state, although it might be expected to be expressed owing to the maturity of the cells. These data suggest, therefore, that deregulated c-Myc bypasses the pRb restriction point and cell cycle arrest in these tumors. An examination of p19(Arf) exons revealed deletions of the gene in up to 94% of the tumors. Since this gene shares exon 2 with p16(Ink4a), it is often difficult to determine which gene is the relevant tumor suppressor. However, the loss of only the p19(Arf)-specific exon 1 beta was observed in a tumor that had normal p16(Ink4a) protein expression. In addition, the p19(Arf)-specific exon was deleted in another tumor that expressed a functional chimeric protein, p15Ex1-p16Ex2-3; it was demonstrated here that this fusion protein is capable of inducing G1 arrest. These data overall supports the hypothesis that the critical inactivation event in these hematopoietic neoplasms is elimination of p19(Arf), and not Ink4 function.

Mdm2 haplo-insufficiency profoundly inhibits Myc-induced lymphomagenesis

Mdm2 harnesses the p53 tumor suppressor, yet loss of one Mdm2 allele in Mdm2(+/-) mice has heretofore not been shown to impair tumor development. Here we report that Mdm2 haplo-insufficiency profoundly suppresses lymphomagenesis in E micro -myc transgenic mice. Mdm2(+/-)E micro -myc transgenics had greatly protracted rates of B cell lymphoma development with life spans twice that of wild-type transgenic littermates. Im paired lymphoma development was associated with drastic reductions in peripheral B cell numbers in Mdm2(+/-)E micro -myc transgenics, and primary pre-B cells from Mdm2(+/-)E micro -myc transgenics and Mdm2(+/-) littermates were extremely susceptible to spontaneous apoptosis. Loss of p53 rescued all of the effects of Mdm2 haplo-insufficiency, indicating they were p53 dependent. Furthermore, half of the lymphomas that ultimately emerged in Mdm2(+/-)E micro -myc transgenics harbored inactivating mutations in p53, and the majority overcame haplo-insufficiency by overexpressing Mdm2. These results support the concept that Mdm2 functions are rate limiting in lymphomagenesis and that targeting Mdm2 will enhance p53-mediated apoptosis, compromising tumor development and/or maintenance.

The t(8;21) fusion protein contacts co-repressors and histone deacetylases to repress the transcription of the p14ARF tumor suppressor

The t(8;21) is one of the most frequent chromosomal translocations associated with acute leukemia. The translocation fuses the DNA binding domain of AML1 to nearly all of the ETO co-repressor. ETO associates with the mSin3 and N-CoR co-repressors as well as histone deacetylases 1, 2, and 3. Although this is one of the most frequent chromosomal translocations in acute leukemia, accounting for 10-15% of the cases of acute myeloid leukemia (AML), the direct targets for transcriptional regulation that stimulate leukemogenesis are unknown. We found that AML1-ETO repressed the promoter of p14(ARF) tumor suppressor in transient transfection assays and reduced endogenous levels of p14(ARF) expression in multiple cell types. Chromatin immunoprecipitation assays demonstrated that AML1-ETO bound to the p14(ARF) promoter. In acute myeloid leukemia samples containing the t(8;21), levels of p14(ARF) mRNA were markedly lower when compared to other acute myeloid leukemias. Therefore, p14(ARF) is a direct transcriptional target of AML1-ETO.

Concomitant expression of p16INK4a and p14ARF in primary breast cancer and analysis of inactivation mechanisms

The INK4a/ARF locus encodes two tumour suppressor proteins, p16INK4a and p14ARF, which act in the two main cell-cycle control pathways, p16-Rb and p14-p53 respectively. The present study examined the mRNA expression of these genes by reverse transcription-polymerase chain reaction (RT-PCR), and the inactivation mechanisms that alter these levels, in 100 primary breast carcinomas. Furthermore, the interdependence of these mechanisms was examined, since it has been reported that p14ARF is altered in most tumours in concordance with p16INK4a. The results show that promoter hypermethylation, tested by methylation-specific PCR (MSP), was the major mechanism of inactivation of these genes and was present in 31 (31%) and 50 (50%) of the tumours that showed decreased p16INK4a and p14ARF expression, respectively. Hemizygous deletion was the second cause of down-regulation. Homozygous deletion was rare and mutation was absent. In most tumours overexpressing p16INK4a or p14ARF, no detectable inactivation mechanisms were observed. Finally, the results indicate that these proteins are often co-altered in primary breast tumours and that p16INK4a and p14ARF had non-independent behaviour, since they were silenced or overexpressed concomitantly with a significant correlation (p < 0.05).

The acetylase activity of p300 is dispensable for MDM2 stabilization

It has been shown that p300 binds to MDM2 and leads to down-regulation of the p53 function. However, it remains unclear whether the acetylase activity of p300 is necessary for regulating MDM2 stability. In this study, we address this issue. First, p300 did not acetylate MDM2 in solution and in cells. Second, overexpression of p300 in cells increased the level of the MDM2 protein but not its mRNA. Similarly, the acetylase-defective p300 AT2 mutant stabilized the MDM2 protein as well. Consistently, the deacetylase inhibitor, trichostatin A, did not significantly affect the half-life of the endogenous MDM2 protein, whereas p300 enhanced the half-life of MDM2. Finally, both wild type and acetylase-defective mutant p300 proteins associated with MDM2 in nuclear body-like structures where MDM2 might be protected from proteasomal degradation. Thus, these results suggest that p300 appears to stabilize MDM2 by retaining this protein in a specific nuclear structure rather than by acetylating it.

Expression of p53R2, newly p53 target in oral normal epithelium, epithelial dysplasia and squamous cell carcinoma

Recently, the p53R2 gene has been isolated and shown to play a crucial role in DNA repair after DNA damage. The p53R2 gene encodes the p53 inducible ribonucleotide reductase small subunit 2 homologue, which is part of the p53 pathway. However, the function of p53R2 in human cancer is still unclear. We investigated p53R2 mRNA expression in human oral normal epithelium, epithelial dysplasias and squamous cell carcinomas (SCCs). Surgical or biopsy-proven specimens of 10 normal epithelium, 48 epithelial dysplasias and 63 SCCs were collected in our department. Then, p53R2 was identified by in situ hybridization to visualize and localize the expression of specific mRNAs. The authors examined the p53 gene mutation by polymerase chain reaction-single strand conformation polymorphism analysis. p53, mdm2, p21(WAF1/CIP1) and Ki-67 expression was detected by immunohistochemistry. p53R2 expression was detected in none of ten normal epithelium (0%), ten of 48 dysplasias (20.8%) and 33 of 63 SCCs (52.4%). In oral SCC, the expression of p53R2 was significantly associated with tumor size, lymph node metastasis and histological differentiation (P=0.014, 0.046 and 0.022, respectively). p53R2 expression was significantly associated with p53 abnormality in epithelial dysplasia and SCC (P=0.034 and 0.009, respectively). Of 63 patients, 37 received preoperative radiochemotherapy. p53R2 mRNA expression was significantly associated with the pathologic response to radiochemotherapy (P=0.031). This study suggested that p53R2 expression could be associated with oral carcinogenesis. The presence of p53R2 mRNA expression would be a predictive factor for tumor development, tumor cell differentiation and the sensitivity to radiochemotherapy in oral SCC.

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

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

p27 deficiency desensitizes Rb-/- cells to signals that trigger apoptosis during pituitary tumor development

Low p27 expression in many human cancers is a prognostic indicator for poor outcome. While analysing the mechanism by which p27 deficiency contributed to tumor development in the Rb+/- mouse model, we identified a role for p27 as a proapoptotic tumor suppressor. We examined the cell cycle and apoptotic response of these pituitary tumor cells to the dopamine analog bromocriptine as well as the expression of Arf and other cell cycle and apoptotic regulators in these tumors. We also examined the expression of Arf and its function in mouse embryo fibroblasts either singly or doubly deficient for Rb and p27. From these studies, we concluded that the absence of p27 disabled the trigger for an Arf-dependent apoptotic response in Rb-/- tumor cells. This suggests a novel mechanism by which the loss of p27 may impact on tumor development.

CTCF functions as a critical regulator of cell-cycle arrest and death after ligation of the B cell receptor on immature B cells

The WEHI 231 B cell lymphoma is used as a model of self-tolerance by clonal deletion because B cell receptor (BCR) ligation results in apoptosis. Two critical events precede cell death: an early rise and fall in expression of MYC and cell-cycle arrest associated with enhanced expression of p21, p27, and p53. CTCF is a transcription factor identified as a repressor of MYC recently shown to cause cell growth inhibition. The present studies demonstrate that BCR ligation of WEHI 231 as well as of normal immature B cells greatly increased expression of CTCF in association with down-regulation of MYC followed by growth arrest and cell death. Conditional expression of CTCF in WEHI 231 mimicked BCR ligation with activated cells showing repressed expression of MYC, enhanced expression of p27, p21, p53, and p19(ARF), and inhibition of cell growth and induction of apoptosis. In keeping with a central role for CTCF in control of B cell death, conditional expression of a CTCF antisense construct in WEHI 231 resulted in inhibition of p27, p21, p53, and p19(ARF) in association with enhanced expression of MYC. Activation of the endogenous CTCF locus by BCR ligation was also mimicked by three other routes to apoptotic death in WEHI 231: inhibition of the phosphoinositide 3-kinase or mTORFRAP signaling cascades and treatment with transforming growth factor (TGF)-beta. Rapid activation of CTCF by BCR ligation or treatment with TGF-beta was suppressed by ligation of CD40. These results demonstrate that CTCF is a common determinant to different pathways of death signaling in immature B cells.

Functional studies of the PI(3)-kinase signalling pathway employing synthetic and expressed siRNA

RNA interference (RNAi) is a RNA-mediated sequence-specific gene silencing mechanism. Recently, this mechanism has been used to down-regulate protein expression in mammalian cells by applying synthetic- or vector-generated small interfering RNAs (siRNAs). However, for the evaluation of this new knockdown technology, it is crucial to demonstrate biological consequences beyond protein level reduction. Here, we demonstrate that this new siRNA-based technology is suitable to analyse protein functions using the phosphatidylinositol (PI) 3-kinase signal transduction pathway as a model system. We demonstrate stable and transient siRNA-mediated knockdown of one of the PI 3-kinase catalytic subunits, p110beta, which leads to inhibition of invasive cell growth in vitro as well as in a tumour model system. Importantly, this result is consistent with loss-of-function phenotypes induced by conventional RNase H-dependent antisense molecules or treatment with the PI 3-kinase inhibitor LY294002. RNAi knockdown of the downstream kinases Akt1 and Akt2 does not reduce cell growth on extracellular matrix. Our data show that synthetic siRNAs, as well as vector-based expression of siRNAs, are a powerful new tool to interfere with signal transduction processes for the elucidation of gene function in mammalian cells.

Tsg101 is essential for cell growth, proliferation, and cell survival of embryonic and adult tissues

Tumor susceptibility gene 101 (Tsg101) was identified in a random mutagenesis screen for potential tumor suppressors in NIH 3T3 cells. Altered transcripts of this gene have been detected in sporadic breast cancers and many other human malignancies. However, the involvement of this gene in neoplastic transformation and tumorigenesis is still elusive. Using gene targeting, we generated genetically engineered mice with a floxed allele of Tsg101. We investigated essential functions of this gene in vivo and examined whether the loss of function of Tsg101 results in tumorigenesis. Conventional knockout mice were generated through Cre-mediated excision of the first coding exon in the germ line of mouse mammary tumor virus (MMTV)-Cre transgenic mice. The complete ablation of Tsg101 in the developing embryo resulted in death around implantation. In contrast, mammary gland-specific knockout mice developed normally but were unable to nurse their young as a result of impaired mammogenesis during late pregnancy. Neither heterozygous null mutants nor somatic knockout mice developed mammary tumors after a latency of 2 years. The Cre-mediated deletion of Tsg101 in primary cells demonstrated that this gene is essential for the growth, proliferation, and survival of mammary epithelial cells. In summary, our results suggest that Tsg101 is required for normal cell function of embryonic and adult tissues but that this gene is not a tumor suppressor for sporadic forms of breast cancer.

Expression patterns of retinoblastoma protein in Parkinson disease

Cellular mechanisms implicated in Parkinson disease (PD) include oxidative stress, inflammatory response, excess dopamine, DNA damage, and loss of trophic support. These stimuli have been observed to induce changes in cell cycle proteins in several cell types. One of the key regulators of cell cycle progression is the retinoblastoma protein (pRb); therefore, we assessed the staining for pRb and its inactive hyperphosphorylated isoform, ppRb, in autopsy tissue from patients with PD. In PD we found abundant pRb staining in neuronal cytoplasm of the substantia nigra, mid-frontal cortex, and hippocampus by immunohistochemistry. In controls, pRb weakly stained nucleoli of neurons in the substantia nigra and exhibited no detectable staining in mid-frontal cortex and hippocampus. Staining for ppRb resulted in a shift from weak cytoplasmic staining in neurons from control cases to strong nuclear staining in PD cases, especially within the substantia nigra, mid-frontal cortex, and hippocampus. In the substantia nigra, ppRb also co-localized to Lewy bodies, which are a pathologic feature of PD. Lewy bodies are also found in diffuse Lewy body disease (DLBD) that do not consistently exhibit changes in pRb or ppRb. These results indicate that there are changes in pRb and its inactive phospho-isoform in neurons responding to neurodegenerative stimuli associated with PD.

Helicobacter pylori infection generated gastric cancer through p53-Rb tumor-suppressor system mutation and telomerase reactivation

AIM

To investigate the relationship between Helicobacter pylori (H.pylori) infection and the expressions of the p53, Rb, c-myc, bcl-2 and hTERT mRNA in a series of diseases from chronic gastritis (CG), intestinal metaplasia type I or II(IMI-II), intestinal metaplasia type III (IMIII), mild or modest dysplasia (DysI-II), severe dysplasia (DysIII) to gastric cancer(GC) and to elucidate the mechanism of gastric carcinogenesis relating to H.pylori infection.

METHODS

272 cases between 1998 and 2001 were available for the study including 42 cases of CG, 46 cases of IMI-II, 25 cases of IMIII, 48 cases of DysI-II, 27 cases of DysIII, 84 cases of GC. H.pylori infection and the expressions of p53, Rb, c-myc, bcl-2 were detected by means of streptavidin-peroxidase (SP) immunohistochemical method. HTERT mRNA was detected by in situ hybridization (ISH).

RESULTS

The expressions of p53, Rb, c-myc, hTERT mRNA and bcl-2 were higher in the GC than in CG, IM, Dys. The expression of c-myc was higher in IMIII with H.pylori infection (10/16) than that without infection (1/9) and the positive rate in DysI-II and DysIII with H.pylori infection was 18/30 and 13/17, respectively, higher than that without infection (4/18 and 3/10, respectively). In our experiment mutated p53 had no association with H.pylori infection, the expression of Rb was associated with H.pylori infection in GC, but the p53-Rb tumor-suppressor system abnormal in DysI-II cases, DysIII and GC cases with H.pylori infection was 21/30, 15/17 and 48/48 respectively, higher than non-infection groups (4/18, 3/10, 28/36). Furthermore the level of hTERT mRNA in GC with H.pylori infection (47/48) was higher than that without infection (30/36), however the relationship between bcl-2 and H.pylori was only in IMIII. C-myc had a close association with hTERT mRNA in DysIII and GC (P=0.0 253,0.0 305 respectively).

CONCLUSION

In the gastric carcinogenesis, H.pylori might cause the severe imbalance of proliferation and apoptosis in the precancerous lesions (IMIII and GysIII) first, leading to p53-Rb tumor-suppressor system mutation and telomerase reactivation, and finally causes gastric cancer.

ARF mutation accelerates pituitary tumor development in Rb+/- mice

Mice heterozygous for the retinoblastoma (Rb) tumor suppressor gene develop pituitary and thyroid tumors with high penetrance. We demonstrate here that loss of the ARF tumor suppressor strongly accelerates intermediate lobe pituitary tumorigenesis in Rb heterozygous mice. These effects in the pituitary are greater than those conferred by p53 loss in that Rb+-;ARF-- mice display significantly more early atypical lesions than Rb+-; p53-- mice. Also, Rb+-;ARF-- compound mutants do not develop many of the novel tumors or precancerous lesions seen in Rb+-;p53-- compound mutants. Although complete loss of ARF expression is not obligatory for pituitary tumorigenesis in Rb+- mice, alterations of the ARF locus are observed in tumors from Rb+-;ARF+- mice, consistent with a selective advantage of ARF inactivation in this context. We conclude that inactivation of ARF acts more broadly than that of p53 in connecting abrogation of the Rb pathway to tumorigenesis.

A nuclear protein tyrosine phosphatase activates p53 and induces caspase-1-dependent apoptosis

PTP-S2/TC45 is a nuclear protein tyrosine phosphatase, which induces p53-dependent apoptosis. Here we show that the p53 protein level increased in MCF-7 cells in response to PTP-S2 overexpression. PTP-S2-induced p53 protein was transcriptionally active and it could activate caspase-1 gene expression from endogenous as well as ectopic promoter. Coexpression of an active site mutant of procaspase-1 strongly inhibited PTP-S2-induced apoptosis. Mutant procaspase-1 also inhibited apoptosis induced by p53 overexpression or doxorubicin treatment, which induce caspase-1 gene expression. In contrast, apoptosis induced by staurosporine or cycloheximide, which do not increase caspase-1 gene expression, was not affected by mutant procaspase-1. These results suggest that caspase-1 may be one of the mediators of p53-dependent apoptosis in human cells.

Differential regulation of E2F1, DP1, and the E2F1/DP1 complex by ARF

The tumor suppressor protein ARF inhibits MDM2 to activate and stabilize p53. Recent studies provided evidence for p53-independent tumor suppression functions of ARF. For example, it has been shown that ARF induces proteolysis of certain E2F species, including E2F1. In addition, ARF relocalizes E2F1 from the nucleoplasm to nucleolus and inhibits E2F1-activated transcription. Because DP1 is a functional partner of the E2F family of factors, we investigated whether DP1 is also regulated by ARF. Here we show that DP1 associates with ARF. Coexpression of ARF relocalizes DP1 from the cytoplasm to the nucleolus, suggesting that DP1 is also a target of the ARF regulatory pathways. Surprisingly, however, the E2F1/DP1 complex is refractory to ARF regulation. Coexpression of E2F1 and DP1 blocks ARF-induced relocalization of either subunit to the nucleolus. The E2F1/DP1 complex localizes in the nucleoplasm, whereas ARF is detected in the nucleolus, suggesting that ARF does not interact with the E2F1/DP1 complex. Moreover, we show that E2F1 is more stable in the presence of ARF when coexpressed with DP1. These results suggest that ARF differentially regulates the free and heterodimeric forms of E2F1 and DP1. DP1 is a constitutively expressed protein, whereas E2F1 is mainly expressed at the G(1)/S boundary of the cell cycle. Therefore, the E2F1/DP1 complex is abundant only between late G(1) and early S phase. Our results on the differential regulation E2F1, DP1, and the E2F1/DP1 complex suggest the possibility that ARF regulates the function of these cell cycle factors by altering the dynamics of their heterodimerization during progression from G(1) to S phase.

Apoptosis: biochemical aspects and clinical implications

Apoptosis and necrosis represent two distinct types of cell death. Apoptosis possesses unique morphologic and biochemical features which distinguish this mechanism of programmed cell death from necrosis. Extrinsic apoptotic cell death is receptor-linked and initiates apoptosis by activating caspase 8. Intrinsic apoptotic cell death is mediated by the release of cytochrome c from mitochondrial and initiates apoptosis by activating caspase 3. Cancer chemotherapy utilizes apoptosis to eliminate tumor cells. Agents which bind to the minor groove of DNA, like camptothecin and Hoechst 33342, inhibit topoisomerase I, RNA polymerase II, DNA polymerase and initiate intrinsic apoptotic cell death. Hoechst 33342-induced apoptosis is associated with disruption of TATA box binding protein/TATA box complexes, replication protein A/single-stranded DNA complexes, topoisomerase I/DNA cleavable complexes and with an increased intracellular concentration of E2F-1 transcription factor and nitric oxide concentration. Nitric oxide and transcription factor activation or respression also regulate the two apoptotic pathways. Some human diseases are associated with excess or deficient rates of apoptosis, and therapeutic strategies to regulate the rate of apoptosis include inhibition or activation of caspases, mRNA antisense to reduce anti-apoptotic factors like Bcl-2 and survivin and recombinant TRAIL to activate pro-apoptotic receptors, DR4 and DR5.

"Super p53" mice exhibit enhanced DNA damage response, are tumor resistant and age normally

The tumor suppressor p53 is critical in preventing cancer due to its ability to trigger proliferation arrest and cell death upon the occurrence of a variety of stresses, most notably, DNA damage and oncogenic stress. Here, we report the generation and characterization of mice carrying supernumerary copies of the p53 gene in the form of large genomic transgenes. Prior to this, we demonstrate that the p53 transgenic allele (p53-tg), when present in a p53-null genetic background, behaves as a functional replica of the endogenous gene. "Super p53" mice, carrying p53-tg alleles in addition to the two endogenous alleles, exhibit an enhanced response to DNA damage. Importantly, "super p53" mice are significantly protected from cancer when compared with normal mice. Finally, in contrast to previously reported mice with constitutively active p53, "super p53" mice do not show any indication of premature aging, probably reflecting the fact that p53 is under normal regulatory control. Together, our results prove that cancer resistance can be enhanced by a simple genetic modification and in the absence of undesirable effects.

Cdk4 disruption renders primary mouse cells resistant to oncogenic transformation, leading to Arf/p53-independent senescence

A large number of human cancers display alterations in the Ink4a/cyclin D/Cdk4 genetic pathway, suggesting that activation of Cdk4 plays an important role in oncogenesis. Here we report that Cdk4-null mouse embryonic fibroblasts are resistant to transformation in response to Ras activation with dominant-negative (DN) p53 expression or in the Ink4a/Arf-null background, judged by foci formation, anchorage-independent growth, and tumorigenesis in athymic mice. Cdk4-null fibroblasts proliferate at normal rates during early passages. Whereas Cdk4(+/+)Ink4a/Arf(-/-) cells are immortal in culture, Cdk4(-/-)Ink4a/Arf(-/-) cells undergo senescence during continuous culture, as do wild-type cells. Activated Ras also induces premature senescence in Cdk4(-/-)Ink4a/Arf(-/-) cells and Cdk4(-/-) cells with DNp53 expression. Thus, Cdk4 deficiency causes senescence in a unique Arf/p53-independent manner, which accounts for the loss of transformation potential. Cdk4-null cells express high levels of p21(Cip1/Waf1) with increased protein stability. Suppression of p21(Cip1/Waf1) by small interfering RNA (siRNA), as well as expression of HPV-E7 oncoprotein, restores immortalization and Ras-mediated transformation in Cdk4(-/-)Ink4a/Arf(-/-) cells and Cdk4(-/-) cells with DNp53 expression. Therefore, Cdk4 is essential for immortalization, and suppression of Cdk4 could be a prospective strategy to recruit cells with inactive Arf/p53 pathway to senescence.

Targeted deletion of the Tsg101 gene results in cell cycle arrest at G1/S and p53-independent cell death

The tumor susceptibility gene 101 (Tsg101) was originally discovered in a screen for potential tumor suppressors using insertional mutagenesis in immortalized fibroblasts. To investigate essential functions of this gene in cell growth and neoplastic transformation, we derived primary mouse embryonic fibroblasts from Tsg101 conditional knockout mice. Expression of Cre recombinase from a retroviral vector efficiently down-regulated Tsg101. The deletion of Tsg101 caused growth arrest and cell death but did not result in increased proliferation and cellular transformation. Inactivation of p53 had no influence on the deleterious phenotype, but Tsg101(-/-) cells were rescued through expression of exogenous Tsg101. Fluorescence-activated cell sorting, proliferation assays, and Western blot analysis of crucial regulators of the cell cycle revealed that Tsg101 deficiency resulted in growth arrest at the G(1)/S transition through inactivation of cyclin-dependent kinase 2. As a consequence, DNA replication was not initiated in Tsg101-deficient cells. Our results clearly demonstrate that Tsg101 is not a primary tumor suppressor in mouse embryonic fibroblasts. However, the protein is crucial for cell proliferation and cell survival.

Human p14(Arf): an exquisite sensor of morphological changes and of short-lived perturbations in cell cycle and in nucleolar function

The human Ink4a/Arf tumor suppressor locus encodes two distinct products: p16(Ink4a) which prevents phosphorylation and inactivation of the retinoblastoma protein and, p14(Arf), a nucleolar protein which activates the function of the tumor suppressor p53 protein in the nucleoplasm in response to oncogenic stimulation through an as yet ill-defined mechanism. Here we show that the level of endogenous p14(Arf) and its balance between the nucleolus and the nucleoplasm in HeLa cells are exquisitely sensitive to changes in cell morphology and to short-lived perturbations in cell cycle and in nucleolar function such as those induced by the cyclin-dependent kinase inhibitor, roscovitine, and the casein kinase II and RNA synthesis inhibitor, DRB. Most remarkably, whereas p14(Arf) predominantly concentrates in the nucleolus of interphase cells and transiently disappears between metaphase and early G1 under normal growth conditions, it massively and reversibly accumulates in the nucleoplasm of postmitotic and S-phase cells upon short-term treatment with roscovitine and, at a lesser extent, DRB. In line with the fact that the nuclear level of p53 reaches a peak between mid-G1 and the G1/S border in p53-expressor cells which lack Arf expression, these results provide a clue that, in p53+/Arf+ cells, Arf proteins might serve both to speed and to amplify p53-mediated responses in conditions and cell cycle periods in which the mechanisms involved in p53 stabilization and activation are not fully operational. They further suggest that human endogenous p14(Arf) might activate p53 pathways in physiologic situations by acting inside the nucleoplasm, especially when normal cell cycle progression and nucleolar function are compromised.

Chromosomal translocations and their role in the pathogenesis of non-Hodgkin's lymphomas

The discovery that non-Hodgkin's lymphomas are monoclonal and that recurrent chromosomal translocations are involved in their pathogenesis has greatly revolutionised their diagnosis and improved our understanding of these diseases. In the last decades, many genes deregulated by such recurrent chromosomal translocations have been identified. However, we have also learned that these genetic alterations are apparently insufficient, in themselves, to cause neoplastic cell transformation and that more complex genetic events must be involved. This review examines the involved genes in chromosomal translocations and current evidence and postulated mechanisms for their role in the pathogenesis of non-Hodgkin's lymphomas.

Role of genetic and epigenetic changes in Burkitt lymphoma

All Burkitt lymphomas (BLs) carry reciprocal chromosomal translocations that activate the c-myc oncogene through juxtaposition to one of the immunoglobulin (Ig) loci. Many BL carry point mutation in the p53 tumor suppressor gene or other defects in the p14ARF-MDM2-p53 pathway, and inactivation of the p16INK4a gene by promoter methylation or homozygous deletion. This indicates that disruption of both the pRb and p53 tumor suppressor pathways is critical for BL development. Alterations of other genes, including Bax, p73, and BCL-6, may provide further growth stimulation and apoptosis protection. Thus, BL development involves multiple genetic and epigenetic changes that drive cell cycle progression and avert cell death by apoptosis.

Modulation of the expression of p16INK4a and p14ARF by hnRNP A1 and A2 RNA binding proteins: implications for cellular senescence

Cellular senescence is a terminal growth phase characteristic of normal human diploid fibroblasts. Altered gene expression during cellular senescence is numerous compared to that of younger proliferative cells in culture. We have previously reported that the levels and activities of hnRNP A1 and A2 RNA binding proteins are decreased in senescent human fibroblasts. Both proteins are multifunctional and may influence the expression of mRNA isoforms during development. In this study, we tested whether overexpression of either protein could modulate the mRNA isoforms of the INK4a locus, specifically p14(ARF) and p16(INK4a). Both INK4a mRNA isoforms have been shown to be growth suppressors and deletions of this locus allow cells to escape cellular senescence. We have found that increasing the ratio of either hnRNP A1 or A2 over that of splicing factor SF2/ASF results in the preferential generation of the p14(ARF) isoform. Overexpression of A1 or A2 RNA binding proteins also appear to increase the steady state mRNA levels of both isoforms, suggesting that in addition to alternative splicing, A1 and A2 may effect p14(ARF) and p16(INK4a) mRNA stability. A constitutive decrease in the ratio of hnRNP A1 or A2 to SF2/ASF in senescent fibroblasts is typically accompanied by an increase in the level of p16(INK4a) isoform. Our studies suggest that hnRNP A1 and A2 may exert an important role during replicative senescence by altering expression of cell cycle regulatory proteins through mRNA metabolism.

p16INK4a promoter mutations are frequent in primary sclerosing cholangitis (PSC) and PSC-associated cholangiocarcinoma

BACKGROUND & AIMS

Primary sclerosing cholangitis (PSC) predisposes individuals to cholangiocarcinoma; however, the molecular mechanisms involved in the carcinogenesis process remain unclear. Because p16(INK4a) inactivation has been implicated in cholangiocarcinoma, our aims were to examine PSC cholangiocytes for p16(INK4a) gene mutations.

METHODS

We studied 4 patient groups: PSC patients without cholangiocarcinoma (n = 10), patients with PSC-associated cholangiocarcinoma (n = 10), non-PSC controls (n = 10), and disease controls with primary biliary cirrhosis (n = 10). Cholangiocytes and hepatocytes were isolated from tissue sections using laser capture microdissection. Genomic DNA was extracted, and the promoter region and the 3 exons for p16(INK4a) were amplified by PCR and directly sequenced.

RESULTS

In the promoter region, 8-point mutations in 5 PSC cases and 14 mutations in 8 cholangiocarcinoma cases were observed. In exon 1, 1 PSC patient and 3 cholangiocarcinoma patients had point mutations. In contrast, no case had a mutation in exon 2 or 3. Mutations were not detected in cholangiocytes from control patients or primary biliary cirrhosis patients nor in hepatocytes from any of the groups; these data indicate that the observed base changes were disease specific and not genetic polymorphisms. Several of the promoter mutations (4 of 8) dramatically decreased promoter activity (>50% reduction in luciferase activity) in a reporter gene assay.

CONCLUSIONS

The results show that functional point mutations in the p16(INK4a) promoter region likely contribute to the initiation/progression of cholangiocarcinoma in PSC. Promoter mutations in CpG islands may function as a methylation equivalent phenomenon resulting in gene inactivation.

DeltaNp73, a dominant-negative inhibitor of wild-type p53 and TAp73, is up-regulated in human tumors

p73 has significant homology to p53. However, tumor-associated up-regulation of p73 and genetic data from human tumors and p73-deficient mice exclude a classical Knudson-type tumor suppressor role. We report that the human TP73 gene generates an NH(2) terminally truncated isoform. DeltaNp73 derives from an alternative promoter in intron 3 and lacks the transactivation domain of full-length TAp73. DeltaNp73 is frequently overexpressed in a variety of human cancers, but not in normal tissues. DeltaNp73 acts as a potent transdominant inhibitor of wild-type p53 and transactivation-competent TAp73. DeltaNp73 efficiently counteracts transactivation function, apoptosis, and growth suppression mediated by wild-type p53 and TAp73, and confers drug resistance to wild-type p53 harboring tumor cells. Conversely, down-regulation of endogenous DeltaNp73 levels by antisense methods alleviates its suppressive action and enhances p53- and TAp73-mediated apoptosis. DeltaNp73 is complexed with wild-type p53, as demonstrated by coimmunoprecipitation from cultured cells and primary tumors. Thus, DeltaNp73 mediates a novel inactivation mechanism of p53 and TAp73 via a dominant-negative family network. Deregulated expression of DeltaNp73 can bestow oncogenic activity upon the TP73 gene by functionally inactivating the suppressor action of p53 and TAp73. This trait might be selected for in human cancers.

Involvement of both extracellular signal-regulated kinase and c-jun N-terminal kinase pathways in the 12-O-tetradecanoylphorbol-13-acetate-induced upregulation of p21(Cip1) in colon cancer cells

Protein kinase C (PKC), a family of serine-threonine kinases, has been implicated in the regulation of colon tumorigenesis. However, the specific isoform of PKC involved in this process is not clear. In the present study, we found that treatment of the cultured human colon cancer cell line COLO-205 with a PKC agonist, 12-O-tetradecanoylphorbol-13-acetate (TPA), resulted in cell-cycle arrest at the G(0)/G(1) phase, decrease in cell number, PKCgamma isoform translocation, and upregulation of p21(Cip1) protein. Pretreatment of the cells with a PKC inhibitor, staurosporine, prevented the TPA-induced upregulation of p21(Cip1) protein. Based on the findings of the present study including that (a) both extracellular signal-regulated kinase (ERK) and c-jun N-terminal kinase (JNK) were activated in the TPA-treated COLO-205 cells, (b) pretreatment with the mitogen-activated protein kinase kinase inhibitor PD98059 but not with the p38 mitogen-activated protein kinase inhibitor SB203580 blocked the TPA-induced p21(Cip1) in COLO-205 cells, and (c) transient transfection of the COLO-205 cells with dominant negative ERK or JNK plasmid significantly suppressed the TPA-induced p21(Cip1) protein induction, we conclude that both the ERK and JNK pathways are involved in the TPA-induced upregulation of p21(Cip1) protein in the COLO-205 cells.

Selective chemosensitization of rhabdomyosarcoma cell lines following wild-type p53 adenoviral transduction

Rhabdomyosarcoma (RMS) cell lines were transduced with an adenoviral vector containing the wild-type p53 (wtp53) cDNA (Ad-p53) and then exposed to four cytotoxic agents: actinomycin D, vincristine, 5-fluorouracil and bleomycin. Potentiation of cytotoxicity following wild-type p53 expression varied from 0- to 20-fold for different drugs and between cell lines. It appeared that alveolar RMS cells (n = 2) were more susceptible to p53-mediated chemosensitization than embryonal RMS cells (n = 3), although this was independent of pax3-FKHR expression. Overall, cells that were most chemosensitive prior to Ad-p53 exposure were those that were most susceptible to p53 potentiation of cytotoxicity. The different results obtained with these RMS cell lines does not appear to be related to expression of pax3-FKHR, p21, Bax or Bcl-2 but may in part be due to differential regulation of p53 target genes, such as MDM2. In conclusion, exogenous wild-type expression selectively chemosensitizes RMS cells to cytotoxic agents. However, expression of transcriptionally active wtp53 does not predict a chemosensitive phenotype.

Recombinant adenovirus-mediated p14(ARF) overexpression sensitizes human breast cancer cells to cisplatin

p14(ARF), the alternative product from the human INK4a/ARF locus, is one of the major targets for alterations in the development of human cancers. Overexpression of p14(ARF) results in cell cycle arrest and apoptosis. To examine the potential therapeutic role of re-expressing p14(ARF) gene product in human breast cancer, a recombinant adenovirus expressing the human p14(ARF) cDNA (Adp14(ARF)) was constructed and used to infect breast cancer cells. Five days after infection, Adp14(ARF) had considerable cytotoxicity on p53-wild-type MCF-7 cells. A time-course study showed that Adp14(ARF) infection of MCF-7 cells at 100pfu/cell increased the number of cells in G0/G1 phase and decreased that in S and G2/M phases. The presence of apoptotic cells was confirmed using the TUNEL assay. Adp14(ARF)-mediated expression of p14(ARF) also resulted in a considerable increase in the amounts of p53 and its target proteins, p21(WAF1) and MDM2. Furthermore, the combination treatment of MCF-7 cells with Adp14(ARF) and cisplatin resulted in a significantly greater cell death. Together, we conclude that p14(ARF) plays an important role in the induction of cell cycle arrest and apoptosis in breast cancer cells and recombinant adenovirus-mediated p14(ARF) expression greatly increases the sensitivity of these cells to cisplatin. These results demonstrate that the proper combination of Adp14(ARF) with conventional chemotherapeutic drug(s) could have potential benefits in treating breast cancer that carries wild-type p53 gene.

TIP49, but not TRRAP, modulates c-Myc and E2F1 dependent apoptosis

We previously described two nuclear cofactors, TRRAP and TIP49, that have functional roles in Myc-mediated oncogenesis. We have now expanded the analysis of these Myc-associated cofactors to investigate their roles in apoptosis and cell proliferation. Although TRRAP and TIP49 are both essential for transformation, TIP49 modulates c-Myc-mediated apoptosis whereas disruption of TRRAP activity has no apparent effect on apoptosis. We extended our analysis of TIP49 to show that it also binds to the E2F1 transactivation domain and modulates both transforming and apoptotic activities. These results indicate that individual cofactors differentially potentiate c-Myc and E2F1 functions.

A strong candidate gene for the Papg1 locus on mouse chromosome 4 affecting lung tumor progression

Lung cancer is the leading cause of cancer death among both men and women, accounting for more than 28% of all cancer deaths. In fact, more people die of lung cancer than of colon, breast, and prostate cancers combined. Although lung cancer is largely induced by smoking, there is strong evidence for genetic susceptibility and gene-environment interactions in the development of lung cancer. Inbred mouse models offer an effective means of identifying candidate lung cancer susceptibility loci since genetic heterogeneity and enormous variation in exposure levels to environmental agents make it difficult to identify lung cancer susceptibility loci in humans. Papg-1 (pulmonary adenoma progression 1) was previously mapped to a region on mouse chromosome 4. This locus contains a candidate gene, Cdkn2a also referred to as Ink4a/Arf, which dually encodes two established tumor suppressors p16(INK4a) and ARF. Cdkn2a became a primary candidate for Papg-1 for two reasons: (1) two haplotypes of mouse Cdkn2a were found to segregate with differential genetic susceptibility to lung tumor progression in mice; and (2) in vitro studies showed that the p16(INK4a) allele from the BALB/cJ mouse had a significantly decreased ability to bind and inhibit CDK6 and to suppress cell growth when compared with the p16(INK4a) allele from the A/J mouse. Here, we report that mice with a heterozygous deficiency for the A/J Cdkn2a allele were significantly more susceptible to lung tumor progression than mice with a heterozygous deficiency for a BALB/cJ Cdkn2a allele, when compared to their respective wild type mice. These results offer strong evidence that naturally occurring variation of p16(INK4a) influences susceptibility to enhance lung tumor progression making it a strong candidate for the lung tumor progression locus, Papg-1.

Death-associated protein kinase as a potential therapeutic target

Death associated protein kinase (DAPK) is a calmodulin (CaM)-regulated serine/threonine protein kinase implicated in diverse apoptosis pathways, including those involved in neuronal cell death and tumour suppression. The requirement of DAPK catalytic activity for its proposed cell functions and the validation of protein kinases as therapeutic targets demand that DAPK be examined as a potential therapeutic target in human disease. The relevant placement of DAPK activity in apoptosis pathways is at an early stage of investigation, making its study as a therapeutic target tenuous. However, the current body of knowledge raises the possibility of DAPK as a therapeutic target for diseases characterised by rapid neurodegeneration, such as stroke or traumatic brain injury. The unmet need in these diseases is for an acute treatment schedule that might reduce neuronal loss. Bioavailable inhibitors of DAPK catalytic activity that target the central nervous system have a potential to fill this need. The development of such DAPK inhibitors is now feasible based on the recent emergence of enabling technology and knowledge. These include a quantitative and selective enzyme assay, a high resolution structure of the active catalytic domain and discovery of cell-permeable, low molecular weight inhibitors of CaM kinases that cross the blood-brain barrier. DAPK as a potential therapeutic target for cancer is less attractive due to the incomplete state of knowledge about DAPK and inherent limitations in drug development for the discovery of specific activators of genes downregulated by promoter hypermethylation. This article provides a brief summary of relevant research and the rationale that is at the foundation of this opinion.

Adenovirus-mediated p14ARF gene transfer cooperates with Ad5CMV-p53 to induce apoptosis in human cancer cells

p14(ARF), a product of the INK4A/ARF locus, induces p53 upregulation by neutralizing the effects of MDM2, a transcriptional target of p53 that antagonizes its function. Here we report that adenovirus-mediated p14(ARF) gene transfer leads to the accumulation of ectopically transduced p53 and to apoptosis in human cancer cells. We constructed an adenoviral vector expressing p14(ARF) (Ad-ARF) and examined its synergistic effect with p53-expressing adenovirus (Ad5CMV-p53 or Ad-p53) in human lung and esophageal cancer cells. Simultaneous Ad-ARF and Ad-p53 infection increased p53 protein levels not only in a wild-type p53-expressing cell line, but also in cell lines with deleted p53. This resulted in a significant in vitro cytotoxicity compared with Ad-p53 infection alone. Coinfection of Ad-ARF and Ad-p53 also resulted in an increase in expression of p53-inducible genes, including p21(WAF-1/Cip1), p53R2, and Noxa. In addition, the growth of human lung cancer tumors subcutaneously implanted into nu/nu mice was inhibited significantly by intratumoral injection with Ad-ARF and Ad-p53. Our data demonstrate that overexpression of ectopic p14(ARF) may render cells more sensitive to p53-mediated apoptosis, an outcome that has important implications for the treatment of human cancers.

The T-box repressors TBX2 and TBX3 specifically regulate the tumor suppressor gene p14ARF via a variant T-site in the initiator

The murine tumor suppressor p19(ARF) (p14(ARF) in humans) is thought to fulfill an important protective role in preventing primary cells from oncogenic transformation via its action in the p53 pathway. Several disease-implicated regulators of p19(ARF) are known to date, among which are the T-box genes TBX2, which resides on an amplicon in primary breast tumors, and TBX3, which is mutated in the human developmental disorder Ulnar-Mammary syndrome. Here we identify a variant T-site, matching 13 of 20 nucleotides of a consensus T-site, as the essential TBX2/TBX3-binding element in the human p14(ARF) promoter. Mutant analysis indicates that both the consensus T-box and a C-terminal conserved repression domain are essential for p14(ARF) repression. Whereas the core nucleotides required for interaction of the archetypal T-box protein Brachyury with a consensus T-site are conserved in the variant site, additional flanking nucleotides contribute to the specificity of TBX2 binding. This is illustrated by the inability of TBX1A or Xbra to activate via the variant p14(ARF) T-site. Importantly, this suggests a hitherto unsuspected level of specificity associated with T-box factors and corresponding recognition sites in regulating their target genes in vivo.

Multiple interacting domains contribute to p14ARF mediated inhibition of MDM2

The small basic protein p14ARF, encoded by one of the alternative transcripts from the human INK4A/ARF locus, interferes with MDM2-mediated ubiquitination of the p53 tumour suppressor protein. The resultant stabilization of p53 leads to increased expression of p53-regulated genes, such as MDM2 itself and the cyclin-dependent kinase inhibitor p21(CIP1). Here we relate physical interactions between p14ARF and MDM2, as determined using synthetic peptides and systematic deletions of p14ARF, with consequential effects on p53 stabilization and transcriptional activity. The data imply that the amino terminal half of p14ARF, encoded by the alternative first exon (exon 1beta) contacts MDM2 through multiple domains that can independently impede MDM2-mediated degradation of p53, provided that they are localized in the cell nucleus. As well as identifying previously unrecognized functional domains, our findings offer an explanation for the relative paucity of missense mutations in exon 1beta in human tumours.

Alternative and aberrant splicing of MDM2 mRNA in human cancer

MDM2 has been characterized as a protein that binds to and facilitates degradation of the tumor suppressor p53. Interestingly, more than 40 different splice variants of MDM2 transcripts have been identified both in tumors and normal tissues, and the majority of these variants do not contain sequence encoding the p53 binding site. This review describes the different splice forms, the tissues in which they have been identified, and their association with tumor progression and prognosis. In addition, we discuss the potential functions of these variants and how they interact with full-length MDM2 protein.

E2F transcriptional repressor complexes are critical downstream targets of p19(ARF)/p53-induced proliferative arrest

The p16(INK4a)/pRB/E2F and p19(ARF)/p53 tumor suppressor pathways are disrupted in most human cancers. Both p19(ARF) and p53 are required for the induction of senescence in primary mouse embryonic fibroblasts (MEFs), but little is known about their downstream targets. Disruption of E2F-mediated transcriptional repression in MEFs caused a general increase in the expression of E2F target genes, including p19ARF. We detected no contribution of E2F-mediated transactivation in this setting, indicating that a predominant role of endogenous E2F in asynchronously growing primary MEFs is to repress its target genes. Moreover, relief of transcriptional repression by E2F rendered MEFs resistant to senescence induced by either p19(ARF), p53, or RAS(V12). Thus, E2F transcriptional repressor complexes are critical downstream targets of antiproliferative p19(ARF)/p53 signaling.

Effect of nifedipine on the expression of p53 protein in rat gingiva

The aim of the present study was to determine the profile of p53 protein expression in gingival tissues after treatment with nifedipine in rats. Rats were treated daily by gastric intubation with or without DMSO alone or DMSO-dissolved nifedipine at concentrations of 15, 30 or 60 mg/kg body weight for 1, 3 or 6 week(s). Gingival width and height were measured macroscopically. Monoclonal antibodies recognizing both wild-type and mutant p53 protein were applied on paraffin-embedded gingival sections using microwave pretreatment and immunohistochemical methods. The gingival width and height were increased in the animals treated with nifedipine at concentrations of 30 and 60 mg/kg body weight. Increased gingival width and height were already seen in the animals treated with 60 mg of nifedipine for 1 week, whereas treatments with 30 mg of nifedipine resulted in increased gingival width and height after treatment for at least 3 weeks. The expression of p53 protein was elevated in the animals treated with 30 or 60 mg of nifedipine. Treatments with nifedipine at the concentration of 60 mg/kg body weight for 1 week induced the expression of p53 protein in the gingival tissues. Treatment with nifedipine in rats led to the inducement of gingival hyperplasia and increase in the numbers of p53-positive gingival epithelial cells by a dose and frequency dependent mechanism, suggesting that p53 protein may play a crucial role in the regulation of nifedipine-induced gingival hyperplasia.

Does p53 affect organismal aging?

The p53 protein plays a critical role in the prevention of cancer. It responds to a variety of cellular stresses to induce either apoptosis, a transient cell cycle arrest, or a terminal cell cycle arrest called senescence. Senescence in cultured cells is associated with augmented p53 activity and abrogation of p53 activity may delay in vitro senescence. Increasing evidence suggests that p53 may also influence aspects of organismal aging. Several mutant mouse models that display alterations in longevity and aging-related phenotypes have defects in genes that alter p53 signaling. Recently, my laboratory has developed and characterized a p53 mutant mouse line that appears to have an enhanced p53 response. These p53 mutants exhibit increased cancer resistance, yet have a shortened longevity and display a number of early aging-associated phenotypes, suggesting a role for p53 in the aging process. The nature of the aging phenotypes observed in this p53 mutant line is consistent with a model in which aging is driven in part by a gradual depletion of stem cell functional capacity.