Post-translational modification of p53 in tumorigenesis

Nucleolin inhibits Hdm2 by multiple pathways leading to p53 stabilization

Nucleolin is a c-Myc-induced gene product with defined roles in ribosomal RNA processing and the inhibition of chromosomal DNA replication following stress. Here we find that changes in nucleolin protein levels in unstressed cells cause parallel changes in the amount of p53 protein. Alterations in p53 levels arise from nucleolin binding to the p53 antagonist Hdm2, resulting in the inhibition of both p53 ubiquitination and Hdm2 auto-ubiquitination. Nucleolin does not alter p53 ubiquitination by human papillomavirus E6, indicating that the effect is specific for Hdm2. Although the inhibition of ligase activity would be expected to stabilize Hdm2, we instead find that nucleolin also reduces Hdm2 protein levels, demonstrating that nucleolin inhibits Hdm2 using multiple mechanisms. Increases in nucleolin levels in unstressed cells led to higher expression of p21(cip1/waf1), a reduced rate of cellular proliferation, and an increase in apoptosis. Thus, nucleolin has a number of properties in common with the tumor suppressor ARF (alternate reading frame). We propose that nucleolin, like ARF, responds to hyperproliferative signals by upregulation of p53 through Hdm2 inhibition.

CARPs are ubiquitin ligases that promote MDM2-independent p53 and phospho-p53ser20 degradation

Caspase 8/10-associated RING proteins (CARPs) are a recently described family of protein ubiquitin ligases that interact with and negatively regulate death receptor-mediated apoptosis. Because CARPs are overexpressed in cancer and their silencing reduces cell viability and sensitizes tumor cells to chemotherapeutic agents, we investigated their relationship to p53 tumor suppressor signaling. p53 is a major determinant of chemosensitivity, and its levels are increased following DNA damage through N-terminal phosphorylation and inhibition of degradation. Although p53 is well known to be negatively regulated by several ubiquitin ligases including MDM2, none are known to target phosphorylated p53 for degradation. CARPs physically interact with and ubiquitinate p53, targeting it for degradation in the absence of MDM2. Serine 20-phosphorylated p53 is also ubiquitinated by CARPs. CARP silencing stimulates p53 expression and promotes downstream effects, including transcriptional activation and tumor suppression.

Arsenite inhibits p53 phosphorylation, DNA binding activity, and p53 target gene p21 expression in mouse epidermal JB6 cells

Epidemiologic investigations demonstrated that arsenite exposure increases the risk of various human cancers, including skin, lung, bladder, and kidney cancers. However, oral administration of arsenite alone has failed to induce tumors in animal models, suggesting that arsenic may act to enhance mutagenicity induced by other carcinogens. Arsenite may function as a co-carcinogen, acting by inhibiting repair of carcinogen-induced DNA damage mediated by p53 and p21, a p53 target gene. To elucidate the interaction between arsenite and p53 tumor suppressor protein, we studied the effect of arsenite on ultraviolet B (UVB)-induced p53 phosphorylation, p53 DNA binding activity, and p53-induced target gene transactivation in the JB6 Cl41 mouse epidermal skin cell model. Our results indicated that arsenite suppressed UVB-induced p53 phosphorylation and p53 DNA binding activity. Arsenite also inhibited casein kinase 2 (CK2) activity and decreased p53-regulated p21 protein expression. These data suggest that the direct inhibition of p53 functional activation is one of the mechanisms through which arsenite interferes with p53 function, and thus may be a significant mechanism for the co-carcinogenic effects of arsenite.

p53 Alterations and protein accumulation in benign breast tissue and breast cancer risk: a cohort study

Disruption of p53 gene function seems to have a pivotal role in carcinogenesis. p53 gene changes occur before the development of breast cancer and therefore might influence breast cancer risk. We investigated the association between p53 protein accumulation and p53 mutations detected in benign breast tissue and risk of subsequent breast cancer. We conducted a case-control study nested within the cohort of 4,888 women in the Canadian National Breast Screening Study who were diagnosed with biopsy-confirmed benign breast disease during active follow-up. Cases were women with benign breast disease who subsequently developed breast cancer; five controls were matched to each case. p53 protein accumulation was assessed immunohistochemically using sections of paraffin-embedded benign breast tissue from 104 cases and 385 controls; for 82 of these cases and 327 of the controls, DNA was successfully extracted from the breast tissue for p53 gene analysis using PCR-single-strand conformation polymorphism/direct sequencing. p53 protein accumulation was associated with a 2-fold increase in risk of progression to breast cancer [adjusted odds ratio (OR), 2.16; 95% confidence interval (95% CI), 1.08-4.30], whereas p53 nucleotide changes overall were not associated with altered risk (adjusted OR, 1.22; 95% CI, 0.68-2.19); those with both p53 immunopositivity and a p53 nucleotide change had an OR (95% CI) of 3.20 (1.21-8.50). Nonpolymorphic intronic changes were associated with a 2.8-fold increase in risk (OR, 2.84; 95% CI, 1.09-7.41). The results of this study suggest that p53 protein accumulation and nonpolymorphic intronic changes in p53 are associated with increased risk of progression to breast cancer in women with benign breast disease.

Inhibition of prostate cancer cell growth by human secreted PDZ domain-containing protein 2, a potential autocrine prostate tumor suppressor

A possible role of the PDZ domain-containing protein 2 (PDZD2) in prostate tumorigenesis has been suggested. Besides, PDZD2 is posttranslationally cleaved by a caspase-dependent mechanism to form a secreted PDZ domain-containing protein 2 (sPDZD2) with unknown functions in humans. In this study, we demonstrate the endogenous expression of PDZD2 and secretion of sPDZD2 in cancerous DU145, PC-3, 22Rv1, LNCaP, and immortalized RWPE-1 prostate epithelial cells. Inhibition of endogenous sPDZD2 production and secretion by DU145, PC-3, 22Rv1, and RWPE-1 cells via the caspase-3 inhibitor Z-DEVD-FMK resulted in increased cell proliferation, which was abrogated by treatment with exogenous recombinant sPDZD2. Whereas sPDZD2-induced antiproliferation in DU145, PC-3, and 22Rv1 cells, it induced apoptosis in LNCaP cells. The data suggest that endogenous sPDZD2, produced by caspase-3-mediated cleavage from PDZD2, may function as a novel autocrine growth suppressor for human prostate cancer cells. The antiproliferative effect of sPDZD2 was apparently mediated through slowing the entry of DU145, PC-3, and 22Rv1 cells into the S phase of the cell cycle. In DU145 cells, this can be attributed to stimulated p53 and p21(CIP1/WAF1) expression by sPDZD2. On the other hand, the apoptotic effect of sPDZD2 on LNCaP cells was apparently mediated via p53-independent Bad stimulation. Together our results indicate the presence of p53-dependent and p53-independent PDZD2/sPDZD2 autocrine growth suppressive signaling pathways in human prostate cancer cells and suggest a novel therapeutic approach of harnessing the latent tumor-suppressive potential of an endogenous autocrine signaling protein like sPDZD2 to inhibit prostate cancer growth.

Aging and the ubiquitinome: traditional and non-traditional functions of ubiquitin in aging cells and tissues

Ubiquitination of endogenous proteins is one of the key regulatory steps of protein degradation followed by regulation of proteasome activity. During the last years evidence has increased that proteasome activity is decreased during the aging process in various model systems and that these changes might be causally related to aging and aging associated diseases. Since in most instances ubiquitination is the primary event in target selection, the system of ubiquitination and deubiquitination might be of similar importance. Furthermore, ubiquitination and proteasomal degradation are not completely congruent, since ubiquitination also confers functions different from giving "the kiss of death" to proteins. Depending on mono- and polyubiquitination and on how ubiquitin chains are linked together, ubiquitination is involved in transcriptional regulation, receptor internalization, DNA repair, and stabilization of protein complexes. This review is therefore the first to summarize the current knowledge regarding the ubiquitinome and the underlying ubiquitin ligases and deubiquitinating enzymes in replicative senescence, tissue aging as well as in segmental progeroid syndromes and to discuss potential causes and consequences for aging.

p53—A Natural Cancer Killer: Structural Insights and Therapeutic Concepts

Every single day, the DNA of each cell in the human body is mutated thousands of times, even in absence of oncogenes or extreme radiation. Many of these mutations could lead to cancer and, finally, death. To fight this, multicellular organisms have evolved an efficient control system with the tumor-suppressor protein p53 as the central element. An intact p53 network ensures that DNA damage is detected early on. The importance of p53 for preventing cancer is highlighted by the fact that p53 is inactivated in more than 50 % of all human tumors. Thus, for good reason, p53 is one of the most intensively studied proteins. Despite the great effort that has been made to characterize this protein, the complex function and the structural properties of p53 are still only partially known. This review highlights basic concepts and recent progress in understanding the structure and regulation of p53, focusing on emerging new mechanistic and therapeutic concepts.

Ubiquitin and ubiquitin-like proteins in cancer pathogenesis

Ubiquitin and ubiquitin-like proteins (Ubls) are signalling messengers that control many cellular functions, such as cell proliferation, apoptosis, the cell cycle and DNA repair. It is becoming apparent that the deregulation of ubiquitin pathways results in the development of human diseases, including many types of tumours. Here we summarize the common principles and specific features of ubiquitin and Ubls in the regulation of cancer-relevant pathways, and discuss new strategies to target ubiquitin signalling in drug discovery.

Mechanistic insights into maintenance of high p53 acetylation by PTEN

Earlier studies have shown that PTEN regulated p53 protein stability both in a phosphatase-dependent manner through antagonizing Akt-Mdm2 pathway and in a phosphatase-independent manner through interacting with p53. In this study, we report that PTEN forms a complex with p300 in the nucleus and plays a role in maintenance of high p53 acetylation in response to DNA damage. Furthermore, p300 is required for nuclear PTEN-regulated cell cycle arrest. Interestingly, however, p53 acetylation was found to promote PTEN-p53 interaction. To investigate the molecular mechanisms, we show that acetylation promotes p53 tetramerization, which, in turn, is required for the PTEN-p53 interaction and subsequent maintenance of high p53 acetylation. Taken together, our results suggest a physiological role for the PTEN tumor suppressor in the nucleus and provide a molecular explanation for our previous observation that PTEN controls p53 protein levels independent of its phosphatase activity.

Ubiquitin and ubiquitin-like modifications of the p53 family

Regulation of p53 by the ubiquitin-proteasomal pathway has been studied considerably. Studies have also demonstrated that the ubiquitin-like proteins SUMO-1 and NEDD8 modify p53. Similarly, p63 and p73 are subject to regulation by ubiquitin and ubiquitin-like modifications, and perturbations of these pathways in the regulation of the p53 family have been implicated in tumorigenesis and developmental abnormalities. Here, we provide an overview of the current understanding of the regulation of the p53 family by covalent modification by ubiquitin, SUMO-1, and NEDD8.

Proteasome inhibitors potentiate etoposide-induced cell death in human astrocytoma cells bearing a mutated p53 isoform

Resistance to anticancer agents is often due to defects of intracellular pathways of cell death. Thus, the identification of the apoptotic pathways that can still be recruited by chemotherapeutic agents in cancerous cells can disclose new opportunities to treat malignancies. Here we show that human astrocytoma ADF cells (which are resistant to "mitochondriotropic" agents as well as to the antineoplastic drug etoposide and to proteasome inhibitors when used alone) undergo dramatic apoptotic death when exposed to a combination protocol based on the use of etoposide in the presence of proteasome inhibitors. Sensitization to cell death involved an autoamplifying loop of caspase activation, where the "executioner" phase of apoptosis was sustained by cooperation of caspase-2, -9, -8, and -3. We also show that sensitization of cells to the combination protocol involved the nuclear relocalization of p53, despite the presence of a polymorphism in its DNA-binding domain, suggesting the likely induction of p53-dependent proapoptotic genes. Conversely, p53 phosphorylation on Ser-15 did not play any role in apoptosis. In conclusion, use of etoposide in combination with proteasome inhibitors may represent an effective strategy to restore sensitivity to apoptosis in human astrocytoma cells bearing multiple defects of intracellular apoptotic pathways.

Modification of p53 with O-linked N-acetylglucosamine regulates p53 activity and stability

Post-translational addition of O-linked N-acetylglucosamine (O-GlcNAc) to p53 is known to occur, but the site of O-GlcNAcylation and its effects on p53 are not understood. Here, we show that Ser 149 of p53 is O-GlcNAcylated and that this modification is associated with decreased phosphorylation of p53 at Thr 155, which is a site that is targeted by the COP9 signalosome, resulting in decreased p53 ubiquitination. Accordingly, O-GlcNAcylation at Ser 149 stabilizes p53 by blocking ubiquitin-dependent proteolysis. Our results indicate that the dynamic interplay between O-GlcNAc and O-phosphate modifications coordinately regulate p53 stability and activity.

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

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

A novel p53-binding domain in CUL7

CUL7 is a member of the cullin RING ligase family and forms an SCF-like complex with SKP1 and FBXW8. CUL7 is required for normal mouse embryonic development and cellular proliferation, and is highly homologous to PARC, a p53-associated, parkin-like cytoplasmic protein. We determined that CUL7, in a manner similar to PARC, can bind directly to p53 but does not affect p53 expression. We identified a discrete, co-linear domain in CUL7 that is conserved in PARC and HERC2, and is necessary and sufficient for p53-binding. The presence of p53 stabilized expression of this domain and we demonstrate that this p53-binding domain of CUL7 contributes to the cytoplasmic localization of CUL7. The results support the model that p53 plays a role in regulation of CUL7 activity.

Characterization of the human PAP1 gene and its homologue possible involvement in mouse embryonic development

We have identified PAP1 gene, a novel member of the immunoglobulin superfamily (IGSF) from U251-pTet-p53 cell line, which carried a wild-type p53 transgene. The gene has been localised to chromosome 16p12-13. Alignment of the predicted protein sequence for Human, Pan troglodytes, Canis, Mus musculus and Gallus gallus revealed it was highly conserved. Its homologue, IGSF6, possible involves in mouse embryonic development. The presence of IGSF6 specific transcript was detected by Northern blot in the RNAs extracted from 11 to 14 day postconception. IGSF6 expression is different in mouse embryos of the different ages. In situ hybridization performed on mice embryos sections showed the differential presence of IGSF6 in developing lung and kidney. This structure and differential expression suggests a function involvement in embryonic development, perhaps involvement in cell proliferation.

Susceptibility to metanephric apoptosis in bradykinin B2 receptor null mice via the p53-Bax pathway

In response to gestational high salt intake, BdkrB2−/− embryos acquire an aberrant renal phenotype mimicking renal dysplasia in humans. Genetic analysis identified p53 as a mediator of the renal dysplasia in salt-stressed BdkrB2−/− mice, acting partly via repression of terminal epithelial differentiation genes. The present study tested the hypothesis that inactivation of BdkrB2 predisposes the salt-stressed embryo to p53-mediated metanephric apoptosis. Newborn BdkrB2−/− pups exhibited hyperphosphorylation of metanephric p53 on serine 20 (mouse serine 23), a modification known to increase p53 stability and apoptotic activity. As a result, there was widespread, ectopic expression of p53 in the BdkrB2−/− kidney. However, no differences were found in the apoptosis index or gene expression in BdkrB2−/− and +/+ kidneys, indicating that p53 stabilization as a result of BdkrB2 inactivation is not sufficient to induce metanephric apoptosis. On gestational salt stress, fulminant metanephric apoptosis and enhanced Bax gene expression occurred in BdkrB2−/− but not their +/− or +/+ littermates. Germline deletion of p53 from BdkrB2−/− mice prevented Bax activation and normalized the apoptosis index. Rescue of metanephric apoptosis in BdkrB2−/− mice was similarly achieved by Bax gene deletion. Aberrant apoptosis in salt-stressed BdkrB2−/− mice was triggered on embryonic day E15.5 and involved both ureteric bud (UB) and metanephric mesenchyme-derived nephron elements. Cultured E12.5 salt-stressed BdkrB2−/− metanephroi manifested stunted UB branching compared with +/− and +/+ littermates; the abnormal UB branching was corrected by p53 deletion. Our results suggest a model whereby a seemingly silent genetic mutation of BdkrB2 predisposes mice to renal dysplasia by creating a “preapoptotic” state through p53 activation.

INGN 201 (Advexin): adenoviral p53 gene therapy for cancer

Mutations in the p53 gene are the most frequent genetic alterations in human tumours, occurring in approximately 50% of all cancers. The p53 protein is pivotal in maintaining genetic integrity after DNA damage, and alterations in the p53 pathway, including mutations in the p53 gene, greatly increase the probability of tumour formation. Gene therapy using adenoviral p53 has emerged as a novel treatment option, with the potential to be safe and effective in a wide range of cancer types. INGN 201 (Ad5CMV-p53, Advexin), a replication-impaired adenoviral vector that carries the p53 gene, has been evaluated in both preclinical and clinical trials. Results show that Advexin is a well-tolerated and efficacious treatment for numerous cancers, both as monotherapy and in combination with radiation and/or chemotherapy agents. In addition, there is now data to support the use of Advexin in cancer immunotherapy.

The tumor suppressor protein p53 is required for neurite outgrowth and axon regeneration

Axon regeneration is substantially regulated by gene expression and cytoskeleton remodeling. Here we show that the tumor suppressor protein p53 is required for neurite outgrowth in cultured cells including primary neurons as well as for axonal regeneration in mice. These effects are mediated by two newly identified p53 transcriptional targets, the actin-binding protein Coronin 1b and the GTPase Rab13, both of which associate with the cytoskeleton and regulate neurite outgrowth. We also demonstrate that acetylation of lysine 320 (K320) of p53 is specifically involved in the promotion of neurite outgrowth and in the regulation of the expression of Coronin 1b and Rab13. Thus, in addition to its recognized role in neuronal apoptosis, surprisingly, p53 is required for neurite outgrowth and axonal regeneration, likely through a different post-translational pathway. These observations may suggest a novel therapeutic target for promoting regenerative responses following peripheral or central nervous system injuries.

Structure of the Tfb1/p53 complex: Insights into the interaction between the p62/Tfb1 subunit of TFIIH and the activation domain of p53

The interaction between the amino-terminal transactivation domain (TAD) of p53 and TFIIH is directly correlated with the ability of p53 to activate both transcription initiation and elongation. We have identified a region within the p53 TAD that specifically interacts with the pleckstrin homology (PH) domain of the p62 and Tfb1 subunits of human and yeast TFIIH. We have solved the 3D structure of a complex between the p53 TAD and the PH domain of Tfb1 by NMR spectroscopy. Our structure reveals that p53 forms a nine residue amphipathic alpha helix (residues 47-55) upon binding to Tfb1. In addition, we demonstrate that diphosphorylation of p53 at Ser46 and Thr55 leads to a significant enhancement in p53 binding to p62 and Tfb1. These results indicate that a phosphorylation cascade involving Ser46 and Thr55 of p53 could play an important role in the regulation of select p53 target genes.

The subcellular localization of vaccinia-related kinase-2 (VRK2) isoforms determines their different effect on p53 stability in tumour cell lines

VRK is a new kinase family of unknown function. Endogenous human vacinia-related kinase 2 (VRK2) protein is present in both the nucleus and the cytosol, which is a consequence of alternative splicing of two VRK2 messages coding for proteins of 508 and 397 amino acids, respectively. VRK2A has a C-terminal hydrophobic region that anchors the protein to membranes in the endoplasmic reticulum (ER) and mitochondria, and it colocalizes with calreticulin, calnexin and mitotracker; whereas VRK2B is detected in both the cytoplasm and the nucleus. VRK2A is expressed in all cell types, whereas VRK2B is expressed in cell lines in which VRK1 is cytoplasmic. Both VRK2 isoforms have an identical catalytic N-terminal domain and phosphorylate p53 in vitro uniquely in Thr18. Phosphorylation of the p53 protein in response to cellular stresses results in its stabilization by modulating its binding to other proteins. However, p53 phosphorylation also occurs in the absence of stress. Only overexpression of the nuclear VRK2B isoform induces p53 stabilization by post-translational modification, largely due to Thr18 phosphorylation. VRK2B may play a role in controlling the binding specificity of the N-terminal transactivation domain of p53. Indeed, the p53 phosphorylated by VRK2B shows a reduction in ubiquitination by Mdm2 and an increase in acetylation by p300. Endogenous p53 is also phosphorylated in Thr18 by VRK2B, promoting its stabilization and transcriptional activation in A549 cells. The relative phosphorylation of Thr18 by VRK2B is similar in magnitude to that induced by taxol, which might use a different signalling pathway. In this context, VRK2B kinase might functionally replace nuclear VRK1. Therefore, these kinases might be components of a new signalling pathway that is likely to play a role in normal cell proliferation.

p53-induced apoptosis occurs in the absence of p14(ARF) in malignant pleural mesothelioma

Malignant pleural mesotheliomas (MPMs) are usually wild type for the p53 gene but contain homozygous deletions in the INK4A locus that encodes p14(ARF), an inhibitor of p53-MDM2 interaction. Previous findings suggest that lack of p14(ARF) expression and the presence of SV40 large T antigen (L-Tag) result in p53 inactivation in MPM. We did not detect SV40 L-Tag mRNA in either MPM cell lines or primary cultures, and treatment of p14(ARF)-deficient cells with cisplatin (CDDP) increased both total and phosphorylated p53 and enhanced p53 DNA-binding activity. On incubation with CDDP, levels of positively regulated p53 transcriptional targets p21(WAF), PIG3, MDM2, Bax, and PUMA increased in p14(ARF)-deficient cells, whereas negatively regulated survivin decreased. Significantly, p53-induced apoptosis was activated by CDDP in p14(ARF)-deficient cells, and treatment with p53-specific siRNA rendered them more CDDP-resistant. p53 was also activated by: 1) inhibition of MDM2 (using nutlin-3); 2) transient overexpression of p14(ARF); and 3) targeting of survivin using antisense oligonucleotides. However, it is noteworthy that only survivin downregulation sensitized cells to CDDP-induced apoptosis. These results suggest that p53 is functional in the absence of p14(ARF) in MPM and that targeting of the downstream apoptosis inhibitor survivin can sensitize to CDDP-induced apoptosis.

BM88 is a dual function molecule inducing cell cycle exit and neuronal differentiation of neuroblastoma cells via cyclin D1 down-regulation and retinoblastoma protein hypophosphorylation

Control of cell cycle progression/exit and differentiation of neuronal precursors is of paramount importance during brain development. BM88 is a neuronal protein associated with terminal neuron-generating divisions in vivo and is implicated in mechanisms underlying neuronal differentiation. Here we have used mouse neuroblastoma Neuro 2a cells as an in vitro model of neuronal differentiation to dissect the functional properties of BM88 by implementing gain- and loss-of-function approaches. We demonstrate that stably transfected cells overexpressing BM88 acquire a neuronal phenotype in the absence of external stimuli, as judged by enhanced expression of neuronal markers and neurite outgrowth-inducing signaling molecules. In addition, cell cycle measurements involving cell growth assays, BrdUrd incorporation, and fluorescence-activated cell sorting analysis revealed that the BM88-transfected cells have a prolonged G(1) phase, most probably corresponding to cell cycle exit at the G(0) restriction point, as compared with controls. BM88 overexpression also results in increased levels of the cell cycle regulatory protein p53, and accumulation of the hypophosphorylated form of the retinoblastoma protein leading to cell cycle arrest, with concomitant decreased levels and, in many cells, cytoplasmic localization of cyclin D1. Conversely, BM88 gene silencing using RNA interference experiments resulted in acceleration of cell proliferation accompanied by impairment of retinoic acid-induced neuronal differentiation of Neuro 2a cells. Taken together, our results suggest that BM88 plays an essential role in regulating cell cycle exit and differentiation of Neuro 2a cells toward a neuronal phenotype and further support its involvement in the proliferation/differentiation transition of neural stem/progenitor cells during embryonic development.

Molecular and cellular targets

Carcinogenesis is a multistage process consisting of initiation, promotion, and progression stages and each stage may be a possible target for chemopreventive agents. A significant outcome of these investigations on the elucidation of molecular and cellular mechanisms is the explication of signal transduction pathways induced by tumor promoters in cancer development. The current belief today is that cancer may be prevented or treated by targeting specific cancer genes, signaling proteins, and transcription factors. The molecular mechanisms explaining how normal cells undergo neoplastic transformation induced by tumor promoters are rapidly being clarified. Accumulating research evidence suggests that many of dietary factors, including tea compounds, may be used alone or in combination with traditional chemotherapeutic agents to prevent or treat cancer. The potential advantage of many natural or dietary compounds seems to focus on their potent anticancer activity combined with low toxicity and very few adverse side effects. This review summarizes some of our recent work regarding the effects of the various tea components on signal transduction pathways involved in neoplastic cell transformation and carcinogenesis.

NARF, an nemo-like kinase (NLK)-associated ring finger protein regulates the ubiquitylation and degradation of T cell factor/lymphoid enhancer factor (TCF/LEF)

beta-Catenin is a key player in the Wnt signaling pathway, and interacts with cofactor T cell factor/lymphoid enhancer factor (TCF/LEF) to generate a transcription activator complex that activates Wnt-induced genes. We previously reported that Nemo-like kinase (NLK) negatively regulates Wnt signaling via phosphorylation of TCF/LEF. To further evaluate the physiological roles of NLK, we performed yeast two-hybrid screening to identify NLK-interacting proteins. From this screen, we isolated a novel RING finger protein that we term NARF (NLK associated RING finger protein). Here, we show that NARF induces the ubiquitylation of TCF/LEF in vitro and in vivo, and functions as an E3 ubiquitin-ligase that specifically cooperates with the E2 conjugating enzyme E2-25K. We found that NLK augmented NARF binding and ubiquitylation of TCF/LEF, and this required NLK kinase activity. The ubiquitylated TCF/LEF was subsequently degraded by the proteasome. Furthermore, NARF inhibited formation of the secondary axis induced by the ectopic expression of beta-catenin in Xenopus embryos. Collectively, our findings raise the possibility that NARF functions as a novel ubiquitin-ligase to suppress the Wnt-beta-catenin signaling.

Chemotherapy resistance in osteosarcoma: current challenges and future directions

For patients with osteosarcoma, the use of chemotherapy has improved survival from 11% with surgical resection alone in the 1960s, to 70% by the mid-1980s. However, survival has since plateaued, despite advances in anticancer therapy. Elucidation of the mechanisms of chemoresistance and implementation of strategies to overcome chemoresistance will likely be pivotal to improving survival. In this review, the focus is on the current understanding of the mechanisms of resistance to the most commonly used agents in the treatment of osteosarcoma and the methods employed to overcome chemotherapy resistance.

Dual targeting of epigenetic therapy in cancer

Aberrant epigenetic silencing of tumor suppressor genes by promoter DNA hypermethylation and histone deacetylation plays an important role in the pathogenesis of cancer. The potential reversibility of epigenetic abnormalities encouraged the development of pharmacologic inhibitors of DNA methylation and histone deacetylation as anti-cancer therapeutics. (Pre)clinical studies of DNA methyltransferase (DNMT) and histone deacetylase (HDAC) inhibitors have yielded encouraging results, especially against hematologic malignancies. Recently, several studies demonstrated that DNMT and HDAC inhibitors are also potent angiostatic agents, inhibiting (tumor) endothelial cells and angiogenesis in vitro and in vivo. By reactivation of epigenetically silenced tumor suppressor genes with angiogenesis inhibiting properties, DNMT and HDAC inhibitors might indirectly - via their effects on tumor cells - decrease tumor angiogenesis in vivo. However, this does not explain the direct angiostatic effects of these agents, which can be unraveled by gene expression studies and examination of epigenetic promoter modifications in endothelial cells treated with DNMT and HDAC inhibitors. Clearly, the dual targeting of epigenetic therapy on both tumor cells and tumor vasculature makes them attractive combinatorial anti-tumor therapeutics. Here we review the therapeutic potential of DNMT and HDAC inhibitors as anti-cancer drugs, as evaluated in clinical trials, and their angiostatic activities, apart from their inhibitory effects on tumor cells.

Protein lysine acetylation in normal and leukaemic haematopoiesis: HDACs as possible therapeutic targets in adult AML

Several new therapeutic strategies are now considered for acute myelogenous leukaemia (AML), including modulation of protein lysine acetylation through inhibition of histone deacetylases (HDACs): a large group of enzymes that alters the acetylation and, thereby, the function of a wide range of nuclear and cytoplasmic proteins. Firstly, HDACs can deacetylate histones as well as transcription factors, and can modulate gene expression through both these mechanisms. Secondly, acetylation is an important post-translational modulation of several proteins involved in the regulation of cell proliferation, differentiation and apoptosis (e.g., p53, tubulin, heat-shock protein 90). The only HDAC inhibitors that have been investigated in clinical studies of AML are butyrate derivatives, valproic acid and depsipeptide. In the first studies, the drugs have usually been used as continuous therapy for several weeks or months, and in most studies the drugs were used alone or in combination with all-trans retinoic acid for treatment of patients with relapsed or primary resistant AML. Neurological toxicity and gastrointestinal side effects seem to be common for all three drugs. Complete haematological remission lasting for several months has been reported for a few patients (< 5% of included patients), whereas increased peripheral blood platelet counts seem more common and have been described both for patients with AML and myelodysplastic syndromes. Taken together, these studies suggest that HDAC inhibition can mediate antileukaemic effects in AML, but for most patients the clinical benefit seems limited and further studies of combination therapy are required.

MAGE-A tumor antigens target p53 transactivation function through histone deacetylase recruitment and confer resistance to chemotherapeutic agents

The MAGE gene family is characterized by a conserved domain (MAGE Homology Domain). A subset of highly homologous MAGE genes (group A; MAGE-A) belong to the chromosome X-clustered cancer/testis antigens. MAGE-A genes are normally expressed in the human germ line and overexpressed in various tumor types; however, their biological function is largely unknown. Here we present evidence indicating that MageA2 protein, belonging to the MAGE-A subfamily, confers wild-type-p53-sensitive resistance to etoposide (ET) by inducing a novel p53 inhibitory loop involving recruitment of histone deacetylase 3 (HDAC3) to MageA2/p53 complex, thus strongly down-regulating p53 transactivation function. In fact, enhanced MageA2 protein levels, in addition to ET resistance, correlate with impaired acetylation of both p53 and histones surrounding p53-binding sites. Association between MAGE-A expression levels and resistance to ET treatment is clearly shown in short-term cell lines obtained from melanoma biopsies harboring wild-type-p53, whereas cells naturally, or siRNA-mediated expressing low MAGE-A levels, correlate with enhanced p53-dependent sensitivity to ET. In addition, combined trichostatin A/ET treatment in melanoma cells expressing high MAGE-A levels reestablishes p53 response and reverts the chemoresistance.

Distinct p53 acetylation cassettes differentially influence gene-expression patterns and cell fate

The activity of the p53 gene product is regulated by a plethora of posttranslational modifications. An open question is whether such posttranslational changes act redundantly or dependently upon one another. We show that a functional interference between specific acetylated and phosphorylated residues of p53 influences cell fate. Acetylation of lysine 320 (K320) prevents phosphorylation of crucial serines in the NH₂-terminal region of p53; only allows activation of genes containing high-affinity p53 binding sites, such as p21/WAF; and promotes cell survival after DNA damage. In contrast, acetylation of K373 leads to hyperphosphorylation of p53 NH₂-terminal residues and enhances the interaction with promoters for which p53 possesses low DNA binding affinity, such as those contained in proapoptotic genes, leading to cell death. Further, acetylation of each of these two lysine clusters differentially regulates the interaction of p53 with coactivators and corepressors and produces distinct gene-expression profiles. By analogy with the “histone code” hypothesis, we propose that the multiple biological activities of p53 are orchestrated and deciphered by different “p53 cassettes,” each containing combination patterns of posttranslational modifications and protein–protein interactions.

Protein serine/threonine phosphatase-1 dephosphorylates p53 at Ser-15 and Ser-37 to modulate its transcriptional and apoptotic activities

We have previously demonstrated that the serine/threonine protein phosphatase-1 (PP-1) plays an important role in promoting cell survival. However, the molecular mechanisms by which PP-1 promotes survival remain largely unknown. In the present study, we provide evidence to show that PP-1 can directly dephosphorylate a master regulator of apoptosis, p53, to negatively modulate its transcriptional and apoptotic activities, and thus to promote cell survival. As a transcriptional factor, the function of p53 can be greatly regulated by phosphorylation and dephosphorylation. While the kinases responsible for phosphorylation of the 17 serine/threonine sites have been identified, the dephosphorylation of these sites remains largely unknown. In the present study, we demonstrate that PP-1 can dephosphorylate p53 at Ser-15 and Ser-37 through co-immunoprecipitation, in vitro and in vivo dephosphorylation assays, overexpression and silence of the gene encoding the catalytic subunit for PP-1. We further show that mutations mimicking constitutive dephosphorylation or phosphorylation of p53 at these sites attenuate or enhance its transcriptional activity, respectively. As a result of the changed p53 activity, expression of the downstream apoptosis-related genes such as bcl-2 and bax is accordingly altered and the apoptotic events are either largely abrogated or enhanced. Thus, our results demonstrate that PP-1 directly dephosphorylates p53, and dephosphorylation of p53 has as important impact on its functions as phosphorylation does. In addition, our results reveal that one of the molecular mechanisms by which PP-1 promotes cell survival is to dephosphorylate p53, and thus negatively regulate p53-dependent death pathway.

Small molecule-based reversible reprogramming of cellular lifespan

Most somatic cells encounter an inevitable destiny, senescence. Little progress has been made in identifying small molecules that extend the finite lifespan of normal human cells. Here we show that the intrinsic 'senescence clock' can be reset in a reversible manner by selective modulation of the ataxia telangiectasia-mutated (ATM) protein and ATM- and Rad3-related (ATR) protein with a small molecule, CGK733. This compound was identified by a high-throughput phenotypic screen with automated imaging. Employing a magnetic nanoprobe technology, magnetism-based interaction capture (MAGIC), we identified ATM as the molecular target of CGK733 from a genome-wide screen. CGK733 inhibits ATM and ATR kinase activities and blocks their checkpoint signaling pathways with great selectivity. Consistently, siRNA-mediated knockdown of ATM and ATR induced the proliferation of senescent cells, although with lesser efficiency than CGK733. These results might reflect the specific targeting of the kinase activities of ATM and ATR by CGK733 without affecting any other domains required for cell proliferation.

Cytoprotective effects of hypoxia against cisplatin-induced tubular cell apoptosis: involvement of mitochondrial inhibition and p53 suppression

Hypoxia that is caused by vascular defects or disruption is commonly associated with renal diseases. During cisplatin nephrotoxicity, hypoxic regions are identified in the outer medulla and the renal cortex. However, the regulation of cisplatin injury by hypoxia is unclear. Previous work has demonstrated the cytoprotective effects of hypoxia against apoptotic injury. This study further examines the cytoprotective mechanisms in models of cisplatin-induced tubular cell apoptosis. In cultured renal tubular cells, 20 microM cisplatin induced approximately 60% apoptosis within 16 h. The rate of apoptosis was suppressed to < 20%, when the incubation was conducted under hypoxia (2% O2). Mitochondrial events of apoptosis, namely Bax accumulation and cytochrome c release, also were ameliorated. During cisplatin treatment, cell ATP was maintained in both normoxic and hypoxic cells. Hypoxic incubation lowered extracellular pH, but prevention of the pH decrease did not restore cisplatin-induced apoptosis. The cytoprotective effects of hypoxia also were independent of hypoxia-inducible factor 1 (HIF-1). Cobalt, as hypoxia, activated HIF-1 yet did not suppress cisplatin-induced apoptosis. Moreover, hypoxia suppressed cisplatin-induced apoptosis in HIF-1-deficient mouse embryonic stem cells and renal proximal tubular cells. Conversely, mitochondrial inhibitors, particularly inhibitors of respiration complex III (antimycin A and myxothiazol), mimicked hypoxia in apoptosis suppression. The effects of hypoxia and mitochondrial inhibitors were not additive. It is interesting that both hypoxia and complex III inhibitors ameliorated cisplatin-induced p53 activation. Therefore, the cytoprotective effects of hypoxia are independent of changes in cell ATP, pH, or HIF but may involve mitochondrial inhibition and the suppression of p53.

VRK1 signaling pathway in the context of the proliferation phenotype in head and neck squamous cell carcinoma

The vaccinia-related kinase (VRK) proteins are a new family with three members in the human kinome. The VRK1 protein phosphorylates several transcription factors and has been postulated to be involved in regulation of cell proliferation. In normal squamous epithelium, VRK1 is expressed in the proliferation area. Because VRK1 can stabilize p53, the expression of the VRK1 protein was analyzed in the context of the p53 pathway and the proliferation phenotype in a series of 73 head and neck squamous cell carcinomas. VRK1 protein level positively correlated with p53 response proteins, particularly hdm2 and p21. The VRK1 protein also correlated positively with several proteins associated with proliferation, such as cyclin-dependent kinase 2 (CDK2), CDK6, cdc2, cyclins B1 and A, topoisomerase II, survivin, and Ki67. The level of VRK1 protein behaves like a proliferation marker in this series of head and neck squamous cell carcinomas. To identify a possible regulatory role for VRK1 and because it regulates gene transcription, the promoters of two genes were studied, CDK2 and SURVIVIN, whose proteins correlated positively with VRK1. VRK1 increases the activity of both the CDK2 and SURVIVIN gene promoters. The expression of VRK1 was analyzed in the context of regulators of the G1-S transition. VRK1 protein levels increase in response to E2F1 and are reduced by retinoblastoma and p16. These data suggest that VRK1 might play a role in cell cycle regulation and is likely to represent the beginning of a new control mechanism of cell cycle, particularly late in the G1-S phase.

Levels of HdmX expression dictate the sensitivity of normal and transformed cells to Nutlin-3

Hdm2 and HdmX coordinately regulate the stability and function of p53. Each is overexpressed in subsets of many different types of malignancy, and most of these subsets maintain wild-type p53. Nutlins, newly discovered small-molecule inhibitors of the Hdm2-p53 interaction, offer a novel strategy for therapy of tumors with wild-type p53. We now show that Nutlin-3 efficiently induces apoptosis and diminishes long-term survival of human fibroblasts transformed in vitro by Hdm2 but not HdmX. The resistance of cells overexpressing HdmX to Nutlin-3 is due to its inability to disrupt the p53-HdmX interaction, resulting in continued suppression of p53 activity. Although HdmX overexpression yielded cells resistant to Nutlin-3, ablation of HdmX expression by short hairpin RNA sensitized tumor cells to Nutlin-3-mediated cell death or arrest. Furthermore, deletion of the COOH-terminal RING finger domain of HdmX completely reversed the resistance to Nutlin-3, probably reflecting the requirement of the RING finger for interaction with Hdm2. Thus, the relative abundance of Hdm2 and HdmX and the specificity of Nutlin-3 for Hdm2 influence the sensitivity of cells to p53-dependent apoptosis or arrest in response to Nutlin-3. Our findings establish Hdm2 and HdmX as independent therapeutic targets with respect to reactivating wild-type p53 as a means for cancer therapy.

Tp53-associated growth arrest and DNA damage repair gene expression is attenuated in mammary epithelial cells of rats fed whey proteins

Dietary protection from mammary cancer is likely coordinated through multiple signaling pathways, based on the known heterogeneity of the disease and the distinct origins of mammary tumor cells. The present study examined the modulatory effects of dietary intake of whey protein hydrolysate (WPH) relative to casein (CAS), on mammary epithelial cell resistance to endogenous DNA damage using Tp53 gene expression and signaling as a read-out, and on systemic proapoptotic and immune surveillance activity, in young adult female Sprague-Dawley rats. Rats were fed AIN-93G diets made with CAS or WPH as the sole protein source beginning at gestation d 4. At postnatal day (PND) 50, mammary glands of rats fed WPH had lower levels of activated Tp53 and p38 mitogen-activated protein kinase proteins, and reduced transcript levels for Tp53-associated DNA damage repair, growth arrest, and proapoptotic genes than those of CAS-fed rats. Serum from WPH-fed rats had greater apoptotic activity in MCF-7 tumor cells than that from rats fed CAS. Serum levels of monocyte chemoattractant protein (MCP)-1 were higher in WPH- than in CAS-fed rats. MCF-7 cells treated with CAS serum + recombinant rat MCP-1 had apoptotic activity and Tp53 and p21 gene expression levels comparable to those treated with WPH serum or recombinant MCP-1. Results indicate that mammary glands of rats fed a WPH diet are more protected from endogenous DNA damage than are those of CAS-fed rats, and identify MCP-1 as a potential serum biomarker for the positive effects of healthy diets.

Plumbagin (5-Hydroxy-2-methyl-1,4-naphthoquinone) Induces Apoptosis and Cell Cycle Arrest in A549 Cells through p53 Accumulation via c-Jun NH2-Terminal Kinase-Mediated Phosphorylation at Serine 15 in Vitro and in Vivo

This study first investigates the anticancer effect of plumbagin (5-hydroxy-2-methyl-1,4-naphthoquinone) in human nonsmall cell lung cancer cells, A549. Plumbagin has exhibited effective cell growth inhibition by inducing cancer cells to undergo G2/M phase arrest and apoptosis. Blockade of cell cycle was associated with increased levels of p21 and reduced amounts of cyclinB1, Cdc2, and Cdc25C. Plumbagin treatment also enhanced the levels of inactivated phosphorylated Cdc2 and Cdc25C. Blockade of p53 activity by dominant-negative p53 transfection partially decreased plumbagin-induced apoptosis and G2/M arrest, suggesting it might be operated by p53-dependent and independent pathway. Plumbagin treatment triggered the mitochondrial apoptotic pathway indicated by a change in Bax/Bcl-2 ratios, resulting in mitochondrial membrane potential loss, cytochrome c release, and caspase-9 activation. We also found that c-Jun NH2-terminal kinase (JNK) is a critical mediator in plumbagin-induced cell growth inhibition. Activation of JNK by plumbagin phosphorylated p53 at serine 15, resulting in increased stability of p53 by decreasing p53 and MDM2 interaction. SP600125 (anthra [1,9-cd]pyrazol-6(2H)-one-1,9-pyrazoloanthrone), a specific inhibitor of JNK, significantly decreased apoptosis by inhibiting the phosphorylation of p53 (serine 15) and subsequently increased the interaction of p53 and MDM2. SP6000125 also inhibited the phosphorylation of Bcl-2 (Ser70) induced by plumbagin. Further investigation revealed that plumbagin's inhibition of cell growth effect was also evident in a nude mice model. Taken together, these results suggest a critical role for JNK and p53 in plumbagin-induced G2/M arrest and apoptosis of human nonsmall cell lung cancer cells.

AMPK and cell proliferation--AMPK as a therapeutic target for atherosclerosis and cancer

AMPK is a serine/threonine protein kinase, which serves as an energy sensor in all eukaryotic cell types. Published studies indicate that AMPK activation strongly suppresses cell proliferation in non-malignant cells as well as in tumour cells. These actions of AMPK appear to be mediated through multiple mechanisms including regulation of the cell cycle and inhibition of protein synthesis, de novo fatty acid synthesis, specifically the generation of mevalonate as well as other products downstream of mevalonate in the cholesterol synthesis pathway. Cell cycle regulation by AMPK is mediated by up-regulation of the p53-p21 axis as well as regulation of TSC2-mTOR (mammalian target of rapamycin) pathway. The AMPK signalling network contains a number of tumour suppressor genes including LKB1, p53, TSC1 and TSC2, and overcomes growth factor signalling from a variety of stimuli (via growth factors and by abnormal regulation of cellular proto-oncogenes including PI3K, Akt and ERK). These observations suggest that AMPK activation is a logical therapeutic target for diseases rooted in cellular proliferation, including atherosclerosis and cancer. In this review, we discuss about exciting recent advances indicating that AMPK functions as a suppressor of cell proliferation by controlling a variety of cellular events in normal cells as well as in tumour cells.

Correlation analysis of two-dimensional gel electrophoretic protein patterns and biological variables

BACKGROUND

Two-dimensional gel electrophoresis (2DE) is a powerful technique to examine post-translational modifications of complexly modulated proteins. Currently, spot detection is a necessary step to assess relations between spots and biological variables. This often proves time consuming and difficult when working with non-perfect gels. We developed an analysis technique to measure correlation between 2DE images and biological variables on a pixel by pixel basis. After image alignment and normalization, the biological parameters and pixel values are replaced by their specific rank. These rank adjusted images and parameters are then put into a standard linear Pearson correlation and further tested for significance and variance.

RESULTS

We validated this technique on a set of simulated 2DE images, which revealed also correct working under the presence of normalization factors. This was followed by an analysis of p53 2DE immunoblots from cancer cells, known to have unique signaling networks. Since p53 is altered through these signaling networks, we expected to find correlations between the cancer type (acute lymphoblastic leukemia and acute myeloid leukemia) and the p53 profiles. A second correlation analysis revealed a more complex relation between the differentiation stage in acute myeloid leukemia and p53 protein isoforms.

CONCLUSION

The presented analysis method measures relations between 2DE images and external variables without requiring spot detection, thereby enabling the exploration of biosignatures of complex signaling networks in biological systems.

Casein kinase 2, circadian clocks, and the flight from mutagenic light

Circadian clocks play a fundamental role in biology and disease. Much has been learned about the molecular underpinnings of these biological clocks from genetic studies in model organisms, such as the fruit fly, Drosophila melanogaster. Here we review the literature from our lab and others that establish a role for the protein kinase CK2 in Drosophila clock timing. Among the clock genes described thus far, CK2 is unique in its involvement in plant, fungal, as well as animal circadian clocks. We propose that this reflects an ancient, conserved function for CK2 in circadian clocks. CK2 and other clock genes have been implicated in cellular responses to DNA damage, particularly those induced by ultraviolet (UV) light. The finding of a dual function of CK2 in clocks and in UV responses supports the notion that clocks evolved to assist organisms in avoiding the mutagenic effects of daily sunlight.

Coordination and communication between the p53 and IGF-1-AKT-TOR signal transduction pathways

Over the past 10 years the signal transduction networks for p53, IGF-1-AKT, and TOR pathways have been assembled in worms, flies, and mammals, and their functions elucidated. In the past 1-2 years a number of genes and their proteins have been identified that permit extensive communication and coordination between these pathways. These three pathways are involved in sensing and integrating signals arising from nutrient and growth factor availability, signals from sensory and sexual organs, and intrinsic and extrinsic stress signals. In turn these pathways regulate cell growth, proliferation, and death. These networks are central to our understanding of a variety of physiological and pathological conditions, including cancer, diabetes, and longevity.

Acetylation of p53 at lysine 373/382 by the histone deacetylase inhibitor depsipeptide induces expression of p21(Waf1/Cip1)

Generally, histone deacetylase (HDAC) inhibitor-induced p21(Waf1/Cip1) expression is thought to be p53 independent. Here we found that an inhibitor of HDAC, depsipeptide (FR901228), but not trichostatin A (TSA), induces p21(Waf1/Cip1) expression through both p53 and Sp1/Sp3 pathways in A549 cells (which retain wild-type p53). This is demonstrated by measuring relative luciferase activities of p21 promoter constructs with p53 or Sp1 binding site mutagenesis and was further confirmed by transfection of wild-type p53 into H1299 cells (p53 null). That p53 was acetylated after depsipeptide treatment was tested by sequential immunoprecipitation/Western immunoblot analysis with anti-acetylated lysines and anti-p53 antibodies. The acetylated p53 has a longer half-life due to a significant decrease in p53 ubiquitination. Further study using site-specific antiacetyllysine antibodies and transfection of mutated p53 vectors (K319/K320/K321R mutated and K373R/K382R mutations) into H1299 cells revealed that depsipeptide specifically induces p53 acetylation at K373/K382, but not at K320. As assayed by coimmunoprecipitation, the K373/K382 acetylation is accompanied by a recruitment of p300, but neither CREB-binding protein (CBP) nor p300/CBP-associated factor (PCAF), to the p53 C terminus. Furthermore, activity associated with the binding of the acetylated p53 at K373/K382 to the p21 promoter as well as p21(Waf1/Cip1) expression is significantly increased after depsipeptide treatment, as tested by chromatin immunoprecipitations and Western blotting, respectively. In addition, p53 acetylation at K373/K382 is confirmed to be required for recruitment of p300 to the p21 promoter, and the depsipeptide-induced p53 acetylation at K373/K382 is unlikely to be dependent on p53 phosphorylation at Ser15, Ser20, and Ser392 sites. Our data suggest that p53 acetylation at K373/K382 plays an important role in depsipeptide-induced p21(Waf1/Cip1) expression.

Acetylation of UBF changes during the cell cycle and regulates the interaction of UBF with RNA polymerase I

The upstream binding factor UBF, an activator of RNA polymerase I transcription, is posttranslationally modified by phosphorylation and acetylation. We found that in NIH3T3 cells, UBF is acetylated in S-phase but not in G₁-phase. To assess the role of acetylation in regulation of UBF activity, we have established an NIH3T3 cell line that inducibly overexpresses HDAC1. Both in vivo and in vitro, HDAC1 efficiently hypoacetylates UBF. Immunoprecipitation with antibodies against the Pol I-associated factor PAF53 co-precipitated UBF in mock cells but not in cells overexpressing HDAC1. Pull-down experiments showed that acetylation of UBF augments the interaction with Pol I. Consistent with acetylation of UBF being important for association of PAF53 and recruitment of Pol I, the level of Pol I associated with rDNA and pre-rRNA synthesis were reduced in cells overexpressing HDAC1. The results suggest that acetylation and deacetylation of UBF regulate rRNA synthesis during cell cycle progression.

Grg1 acts as a lung-specific oncogene in a transgenic mouse model

Groucho proteins are transcriptional corepressors that are recruited to gene regulatory regions by numerous transcription factors. Long isoforms, such as Grg1, have all the domains of the prototype Drosophila Groucho. Short Groucho proteins, such as Grg5, have only the amino-terminal Q and G/P domains. We generated Grg1 and Grg5 transgenic mice and found that Grg1 overexpression induces lung adenocarcinoma, whereas Grg5 overexpression does not. Coexpression of Grg5 with Grg1 reduces tumor burden. Grg1 and Grg5 both diminish p53 protein levels; however, only Grg1 overexpression induces elevated levels of ErbB1 and ErbB2 receptor tyrosine kinases. The molecular and biological changes that accompany tumor progression in Grg1 transgenic mice closely reiterate events seen in human lung cancer. We also found that within a human lung tumor tissue array, a significant number of carcinomas overexpress Grg1/TLE1. Our data suggest that Grg1 overexpression contributes to malignancy in human lung cancers.

p53 suppresses the self-renewal of adult neural stem cells

There is increasing evidence that tumors are heterogeneous and that a subset of cells act as cancer stem cells. Several proto-oncogenes and tumor suppressors control key aspects of stem cell function, suggesting that similar mechanisms control normal and cancer stem cell properties. We show here that the prototypical tumor suppressor p53, which plays an important role in brain tumor initiation and growth, is expressed in the neural stem cell lineage in the adult brain. p53 negatively regulates proliferation and survival, and thereby self-renewal, of neural stem cells. Analysis of the neural stem cell transcriptome identified the dysregulation of several cell cycle regulators in the absence of p53, most notably a pronounced downregulation of p21 expression. These data implicate p53 as a suppressor of tissue and cancer stem cell self-renewal.

Mdm2 inhibitor Nutlin-3a induces p53-mediated apoptosis by transcription-dependent and transcription-independent mechanisms and may overcome Atm-mediated resistance to fludarabine in chronic lymphocytic leukemia

Although TP53 mutations are rare in B-cell chronic lymphocytic leukemia (CLL), Mdm2 overexpression has been reported as an alternative cause of p53 dysfunction. We investigated the potential therapeutic use of nongenotoxic p53 activation by a small-molecule antagonist of Mdm2, Nutlin-3a, in CLL. Nutlin-3a induced significant apoptosis in 30 (91%) of 33 samples from previously untreated patients with CLL; all resistant samples had TP53 mutations. Low levels of Atm (ataxia telangiectasia mutated) or high levels of Mdm2 (murine double minute 2) did not prevent Nutlin-3a from inducing apoptosis. Nutlin-3a used transcription-dependent and transcription-independent pathways to induce p53-mediated apoptosis. Predominant activation of the transcription-independent pathway induced more pronounced apoptosis than that of the transcription-dependent pathway, suggesting that activation of the transcription-independent pathway is sufficient to initiate p53-mediated apoptosis in CLL. Combination treatment of Nutlin-3a and fludarabine synergistically increased p53 levels, and induced conformational change of Bax and apoptosis in wild-type p53 cells but not in cells with mutant p53. The synergistic apoptotic effect was maintained in samples with low Atm that were fludarabine resistant. Results suggest that the nongenotoxic activation of p53 by targeting the Mdm2-p53 interaction provides a novel therapeutic strategy for CLL.

The N-terminal domain of tumor suppressor p53 is involved in the molecular interaction with the anti-apoptotic protein Bcl-Xl

Emerging evidences suggest that transcription-independent mechanism of p53 appears to make an important contribution to the overall p53-dependent apoptosis. Recently, it has been postulated that the DNA-binding domain of p53 can interact with Bcl-Xl, and subsequently the proposed molecular interaction has been shown by NMR studies. Interestingly, Chipuk et al. [Cancer Cell 4 (2003) 371] reported that the N-terminal domain of p53 (p53NTD) alone is necessary and sufficient to induce transcription-independent apoptosis. To further define and understand the nature of the molecular recognition between p53 and Bcl-Xl, our current study focuses on p53NTD. We first demonstrated the molecular interaction between p53NTD and Bcl-Xl by co-expressing and purifying the complex. Second, to define the binding interface of the molecular interaction, which is not previously characterized, in the current we employed a NMR-based binding study, showing that the binding site on Bcl-Xl is located in the region including alpha4, the N- and C-termini of alpha3, the N-terminus of alpha5, and the central part of alpha2. To further probe this observation, we then performed fluorescence resonance energy transfer (FRET) assay in cells. The FRET efficiency detected between the donor and acceptor molecules appears to suggest the presence of molecular interaction of p53NTD with Bcl-Xl in cells. Taken together, our data suggest that p53NTD interacts with Bcl-Xl but the characteristic of the molecular interaction appears to be different from that of the DNA-binding domain of p53.

The biological impact of the human master regulator p53 can be altered by mutations that change the spectrum and expression of its target genes

Human tumor suppressor p53 is a sequence-specific master regulatory transcription factor that targets response elements (REs) in many genes. p53 missense mutations in the DNA-binding domain are often cancer associated. As shown with systems based on the yeast Saccharomyces cerevisiae, p53 mutants can alter the spectra and intensities of transactivation from individual REs. We address directly in human cells the relationship between changes in the p53 master regulatory network and biological outcomes. Expression of integrated, tightly regulated DNA-binding domain p53 mutants resulted in many patterns of apoptosis and survival following UV or ionizing radiation, or spontaneously. These patterns reflected changes in the spectra and activities of target genes, as demonstrated for P21, MDM2, BAX, and MSH2. Thus, as originally proposed for "master genes of diversity," p53 mutations in human cells can differentially influence target gene transactivation, resulting in a variety of biological consequences which, in turn, might be expected to influence tumor development and therapeutic efficacy.