p53 accumulates but is functionally impaired when DNA synthesis is blocked

Pathway-specific effects of ADSL deficiency on neurodevelopment

Adenylosuccinate lyase (ADSL) functions in de novo purine synthesis (DNPS) and the purine nucleotide cycle. ADSL deficiency (ADSLD) causes numerous neurodevelopmental pathologies, including microcephaly and autism spectrum disorder. ADSLD patients have normal serum purine nucleotide levels but exhibit accumulation of dephosphorylated ADSL substrates, S-Ado, and SAICAr, the latter being implicated in neurotoxic effects through unknown mechanisms. We examined the phenotypic effects of ADSL depletion in human cells and their relation to phenotypic outcomes. Using specific interventions to compensate for reduced purine levels or modulate SAICAr accumulation, we found that diminished AMP levels resulted in increased DNA damage signaling and cell cycle delays, while primary ciliogenesis was impaired specifically by loss of ADSL or administration of SAICAr. ADSL-deficient chicken and zebrafish embryos displayed impaired neurogenesis and microcephaly. Neuroprogenitor attrition in zebrafish embryos was rescued by pharmacological inhibition of DNPS, but not increased nucleotide concentration. Zebrafish also displayed phenotypes commonly linked to ciliopathies. Our results suggest that both reduced purine levels and impaired DNPS contribute to neurodevelopmental pathology in ADSLD and that defective ciliogenesis may influence the ADSLD phenotypic spectrum.

Functional relationship between p53 and RUNX proteins

RUNX genes belong to a three-membered family of transcription factors, which are well established as master regulators of development. Of them, aberrations in RUNX3 expression are frequently observed in human malignancies primarily due to epigenetic silencing, which is often overlooked. At the G1 phase of the cell cycle, RUNX3 regulates the restriction (R)-point, a mechanism that decides cell cycle entry. Deregulation at the R-point or loss of RUNX3 results in premature entry into S phase, leading to a proliferative advantage. Inactivation of Runx1 and Runx2 induce immortalization of mouse embryo fibroblast. As a consequence, RUNX loss induces pre-cancerous lesions independent of oncogene activation. p53 is the most extensively studied tumour suppressor. p53 plays an important role to prevent tumour progression but not tumour initiation. Therefore, upon oncogene activation, early inactivation of RUNX genes and subsequent mutation of p53 appear to result in tumour initiation and progression. Recently, transcription-independent DNA repairing roles of RUNX3 and p53 are emerging. Being evolutionarily old genes, it appears that the primordial function of p53 is to protect genome integrity, a function that likely extends to the RUNX gene as well. In this review, we examine the mechanism and sequence of actions of these tumour suppressors in detail.

CDK-Independent and PCNA-Dependent Functions of p21 in DNA Replication

p21^Waf/CIP1 is a small unstructured protein that binds and inactivates cyclin-dependent kinases (CDKs). To this end, p21 levels increase following the activation of the p53 tumor suppressor. CDK inhibition by p21 triggers cell-cycle arrest in the G1 and G2 phases of the cell cycle. In the absence of exogenous insults causing replication stress, only residual p21 levels are prevalent that are insufficient to inhibit CDKs. However, research from different laboratories has demonstrated that these residual p21 levels in the S phase control DNA replication speed and origin firing to preserve genomic stability. Such an S-phase function of p21 depends fully on its ability to displace partners from chromatin-bound proliferating cell nuclear antigen (PCNA). Vice versa, PCNA also regulates p21 by preventing its upregulation in the S phase, even in the context of robust p21 induction by irradiation. Such a tight regulation of p21 in the S phase unveils the potential that CDK-independent functions of p21 may have for the improvement of cancer treatments.

How the Other Half Lives: What p53 Does When It Is Not Being a Transcription Factor

It has been four decades since the discovery of p53, the designated ‘Guardian of the Genome’. P53 is primarily known as a master transcription factor and critical tumor suppressor, with countless studies detailing the mechanisms by which it regulates a host of gene targets and their consequent signaling pathways. However, transcription-independent functions of p53 also strongly define its tumor-suppressive capabilities and recent findings shed light on the molecular mechanisms hinted at by earlier efforts. This review highlights the transcription-independent mechanisms by which p53 influences the cellular response to genomic instability (in the form of replication stress, centrosome homeostasis, and transposition) and cell death. We also pinpoint areas for further investigation in order to better understand the context dependency of p53 transcription-independent functions and how these are perturbed when TP53 is mutated in human cancer.

p53 orchestrates DNA replication restart homeostasis by suppressing mutagenic RAD52 and POLθ pathways

Classically, p53 tumor suppressor acts in transcription, apoptosis, and cell cycle arrest. Yet, replication-mediated genomic instability is integral to oncogenesis, and p53 mutations promote tumor progression and drug-resistance. By delineating human and murine separation-of-function p53 alleles, we find that p53 null and gain-of-function (GOF) mutations exhibit defects in restart of stalled or damaged DNA replication forks that drive genomic instability, which isgenetically separable from transcription activation. By assaying protein-DNA fork interactions in single cells, we unveil a p53-MLL3-enabled recruitment of MRE11 DNA replication restart nuclease. Importantly, p53 defects or depletion unexpectedly allow mutagenic RAD52 and POLθ pathways to hijack stalled forks, which we find reflected in p53 defective breast-cancer patient COSMIC mutational signatures. These data uncover p53 as a keystone regulator of replication homeostasis within a DNA restart network. Mechanistically, this has important implications for development of resistance in cancer therapy. Combined, these results define an unexpected role for p53-mediated suppression of replication genome instability.

Low-burden TP53 mutations in chronic phase of myeloproliferative neoplasms: association with age, hydroxyurea administration, disease type and JAK2 mutational status

The multistep process of TP53 mutation expansion during myeloproliferative neoplasm (MPN) transformation into acute myeloid leukemia (AML) has been documented retrospectively. It is currently unknown how common TP53 mutations with low variant allele frequency (VAF) are, whether they are linked to hydroxyurea (HU) cytoreduction, and what disease progression risk they carry. Using ultra-deep next-generation sequencing, we examined 254 MPN patients treated with HU, interferon alpha-2a or anagrelide and 85 untreated patients. We found TP53 mutations in 50 cases (0.2–16.3% VAF), regardless of disease subtype, driver gene status and cytoreduction. Both therapy and TP53 mutations were strongly associated with older age. Over-time analysis showed that the mutations may be undetectable at diagnosis and slowly increase during disease course. Although three patients with TP53 mutations progressed to TP53-mutated or TP53-wild-type AML, we did not observe a significant age-independent impact on overall survival during the follow-up. Further, we showed that complete p53 inactivation alone led to neither blast transformation nor HU resistance. Altogether, we revealed patient's age as the strongest factor affecting low-burden TP53 mutation incidence in MPN and found no significant age-independent association between TP53 mutations and hydroxyurea. Mutations may persist at low levels for years without an immediate risk of progression.

Reviving the guardian of the genome: Small molecule activators of p53

The tumor suppressor p53 is one of the most important proteins for protection of genomic stability and cancer prevention. Cancers often inactivate it by either mutating its gene or disabling its function. Thus, activating p53 becomes an attractive approach for the development of molecule-based anti-cancer therapy. The past decade and half have witnessed tremendous progress in this area. This essay offers readers with a grand review on this progress with updated information about small molecule activators of p53 either still at bench work or in clinical trials.

Mdm2 inhibition confers protection of p53-proficient cells from the cytotoxic effects of Wee1 inhibitors

Pharmacological inhibition of the cell cycle regulatory kinase Wee1 represents a promising strategy to eliminate cancer cells. Wee1 inhibitors cooperate with chemotherapeutics, e. g. nucleoside analogues, pushing malignant cells from S phase towards premature mitosis and death. However, considerable toxicities are observed in preclinical and clinical trials. A high proportion of tumor cells can be distinguished from all other cells of a patient's body by inactivating mutations in the tumor suppressor p53. Here we set out to develop an approach for the selective protection of p53-proficient cells against the cytotoxic effects of Wee1 inhibitors. We pretreated such cells with Nutlin-3a, a prototype inhibitor of the p53-antagonist Mdm2. The resulting transient cell cycle arrest effectively increased the survival of cells that were subsequently treated with combinations of the Wee1 inhibitor MK-1775 and/or the nucleoside analogue gemcitabine. In this constellation, Nutlin-3a reduced caspase activation and diminished the phosphorylation of Histone 2AX, an indicator of the DNA damage response. Both effects were strictly dependent on the presence of p53. Moreover, Nutlin pre-treatment reduced the fraction of cells that were undergoing premature mitosis in response to Wee1 inhibition. We conclude that the pre-activation of p53 through Mdm2 antagonists serves as a viable option to selectively protect p53-proficient cells against the cytotoxic effects of Wee1 inhibitors, especially when combined with a nucleoside analogue. Thus, Mdm2 antagonists might prove useful to avoid unwanted side effects of Wee1 inhibitors. On the other hand, when a tumor contains wild type p53, care should be taken not to induce its activity before applying Wee1 inhibitors.

AMP-activated Protein Kinase (AMPK) Control of mTORC1 Is p53- and TSC2-independent in Pemetrexed-treated Carcinoma Cells

The key sensor of energy status in mammalian cells, AMP-activated protein kinase (AMPK), can also be activated by the AMP analog aminoimidazolecarboxamide nucleoside monophosphate (ZMP) generated directly from aminoimidazolecarboxamide ribonucleoside (AICAR) or from inhibition of purine synthesis by the antifolate pemetrexed (PTX), a drug used extensively in the treatment of lung cancers. Despite this common mechanism, signaling downstream of AMPK activated by PTX or AICAR differed. AICAR-activated AMPK inhibited mTORC1 both directly by phosphorylation of the mTORC1 subunit Raptor and indirectly by phosphorylation of the regulator TSC2. In contrast, PTX-activated AMPK inhibited mTORC1 solely through Raptor phosphorylation. This dichotomy was due to p53 function. Transcription of p53 target genes, including TSC2, was activated by AICAR but not by PTX. Although both PTX and AICAR stabilized p53, only AICAR activated Chk2 phosphorylation, stimulating p53-dependent transcription. However, Raptor phosphorylation by AMPK was independent of p53 and was sufficient, after PTX treatment, to inhibit mTORC1. We concluded that PTX effects on mTORC1 were independent of TSC2 and p53 and that the activation of a p53 transcriptional response by AICAR was due to an activation of Chk2 that was not elicited by PTX.

Homozygous mdm2 SNP309 cancer cells with compromised transcriptional elongation at p53 target genes are sensitive to induction of p53-independent cell death

A single nucleotide polymorphism (T to G) in the mdm2 P2 promoter, mdm2 SNP309, leads to MDM2 overexpression promoting chemotherapy resistant cancers. Two mdm2 G/G SNP309 cancer cell lines, MANCA and A875, have compromised wild-type p53 that co-localizes with MDM2 on chromatin. We hypothesized that MDM2 in these cells inhibited transcription initiation at the p53 target genes p21 and puma. Surprisingly, following etoposide treatment transcription initiation occurred at the compromised target genes in MANCA and A875 cells similar to the T/T ML-1 cell line. In all cell lines tested there was equally robust recruitment of total and initiated RNA polymerase II (Pol II). We found that knockdown of MDM2 in G/G cells moderately increased expression of subsets of p53 target genes without increasing p53 stability. Importantly, etoposide and actinomycin D treatments increased histone H3K36 trimethylation in T/T, but not G/G cells, suggesting a G/G correlated inhibition of transcription elongation. We therefore tested a chemotherapeutic agent (8-amino-adenosine) that induces p53-independent cell death for higher clinically relevant cytotoxicity. We demonstrated that T/T and G/G mdm2 SNP309 cells were equally sensitive to 8-amino-adenosine induced cell death. In conclusion for cancer cells overexpressing MDM2, targeting MDM2 may be less effective than inducing p53-independent cell death.

A physical association between the human mutY homolog (hMYH) and DNA topoisomerase II-binding protein 1 (hTopBP1) regulates Chk1-induced cell cycle arrest in HEK293 cells

Background

Human DNA topoisomerase II-binding protein 1 (hTopBP1) plays an important role in DNA replication and the DNA damage checkpoint pathway. The human mutY homolog (hMYH) is a base excision repair DNA glycosylase that excises adenines or 2-hydroxyadenines that are mispaired with guanine or 7,8-dihydro-8-oxoguanine (8-oxoG). hTopBP1 and hMYH were involved in ATR-mediated Chk1 activation, moreover, both of them were associated with ATR and hRad9 which known as checkpoint-involved proteins. Therefore, we investigated whether hTopBP1 interacted with hMYH, and what the function of their interaction is.

Results

We documented the interaction between hTopBP1 and hMYH and showed that this interaction increased in a hydroxyurea-dependent manner. We also mapped the hMYH-interacting region of hTopBP1 (residues 444–991). In addition, we investigated several cell cycle-related proteins and found that co-knockdown of hTopBP1 and hMYH significantly diminished cell cycle arrest due to compromised checkpoint kinase 1 (Chk1) activation. Moreover, we observed that hMYH was essential for the accumulation of hTopBP1 on damaged DNA, where hTopBP1 interacts with hRad9, a component of the Rad9-Hus1-Rad1 complex. The accumulation of hTopBP1 on chromatin and its subsequent interaction with hRad9 lead to cell cycle arrest, a process mediated by Chk1 phosphorylation and ataxia telangiectasia and Rad3-related protein (ATR) activation.

Conclusions

Our results suggested that hMYH is necessary for the accumulation of hTopBP1 to DNA damage lesion to induce the association of hTopBP1 with 9-1-1 and that the interaction between hMYH and hTopBP1 is essential for Chk1 activation. Therefore, we suggest that the interaction between hMYH and hTopBP1 is crucial for activation of the ATR-mediated cell cycle checkpoint.

Electronic supplementary material

The online version of this article (doi:10.1186/s13578-015-0042-x) contains supplementary material, which is available to authorized users.

Direct relationship between the level of p53 stabilization induced by rRNA synthesis-inhibiting drugs and the cell ribosome biogenesis rate

Many drugs currently used in chemotherapy work by hindering the process of ribosome biogenesis. In tumors with functional p53, the inhibition of ribosome biogenesis may contribute to the efficacy of this treatment by inducing p53 stabilization. As the level of stabilized p53 is critical for the induction of cytotoxic effects, it seems useful to highlight those cancer cell characteristics that can predict the degree of p53 stabilization following the treatment with inhibitors of ribosome biogenesis. In the present study we exposed a series of p53 wild-type human cancer cell lines to drugs such as actinomycin D (ActD), doxorubicin, 5-fluorouracil and CX-5461, which hinder ribosomal RNA (rRNA) synthesis. We found that the amount of stabilized p53 was directly related to the level of ribosome biogenesis in cells before the drug treatment. This was due to different levels of inactivation of the ribosomal proteins-MDM2 pathway of p53 digestion. Inhibition of rRNA synthesis always caused cell cycle arrest, independent of the ribosome biogenesis rate of the cells, whereas apoptosis occurred only in cells with a high rDNA transcription rate. The level of p53 stabilization induced by drugs acting in different ways from the inhibition of ribosome biogenesis, such as hydroxyurea (HU) and nutlin-3, was independent of the level of ribosome biogenesis in cells and always lower than that occurring after the inhibition of rRNA synthesis. Interestingly, in cells with a low ribosome biogenesis rate, the combined treatment with ActD and HU exerted an additive effect on p53 stabilization. These results indicated that (i) drugs inhibiting ribosome biogenesis may be highly effective in p53 wild-type cancers with a high ribosome biogenesis rate, as they induce apoptotic cell death, and (ii) the combination of drugs capable of stabilizing p53 through different mechanisms may be useful for treating cancers with a low ribosome biogenesis rate.

Involvement of p53 in the repair of DNA double strand breaks: multifaceted Roles of p53 in homologous recombination repair (HRR) and non-homologous end joining (NHEJ)

p53 is a tumor suppressor protein that prevents oncogenic transformation and maintains genomic stability by blocking proliferation of cells harboring unrepaired or misrepaired DNA. A wide range of genotoxic stresses such as DNA damaging anti-cancer drugs and ionizing radiation promote nuclear accumulation of p53 and trigger its ability to activate or repress a number of downstream target genes involved in various signaling pathways. This cascade leads to the activation of multiple cell cycle checkpoints and subsequent cell cycle arrest, allowing the cells to either repair the DNA or undergo apoptosis, depending on the intensity of DNA damage. In addition, p53 has many transcription-independent functions, including modulatory roles in DNA repair and recombination. This chapter will focus on the role of p53 in regulating or influencing the repair of DNA double-strand breaks that mainly includes homologous recombination repair (HRR) and non-homologous end joining (NHEJ). Through this discussion, we will try to establish that p53 acts as an important linchpin between upstream DNA damage signaling cues and downstream cellular events that include repair, recombination, and apoptosis.

Chk1 inhibits E2F6 repressor function in response to replication stress to maintain cell-cycle transcription

BACKGROUND

In eukaryotic cells, detection of replication stress results in the activation of the DNA replication checkpoint, a signaling cascade whose central players are the kinases ATR and Chk1. The checkpoint response prevents the accumulation of DNA damage and ensures cell viability by delaying progression into mitosis. However, the role and mechanism of the replication checkpoint transcriptional response in human cells, which is p53 independent, is largely unknown.

RESULTS

We show that, in response to DNA replication stress, the regular E2F-dependent cell-cycle transcriptional program is maintained at high levels, and we establish the mechanisms governing such transcriptional upregulation. E2F6, a repressor of E2F-dependent G1/S transcription, replaces the activating E2Fs at promoters to repress transcription in cells progressing into S phase in unperturbed conditions. After replication stress, the checkpoint kinase Chk1 phosphorylates E2F6, leading to its dissociation from promoters. This promotes E2F-dependent transcription, which mediates cell survival by preventing DNA damage and cell death.

CONCLUSIONS

This work reveals, for the first time, that the regular cell-cycle transcriptional program is part of the DNA replication checkpoint response in human cells and establishes the molecular mechanism involved. We show that maintaining high levels of G1/S cell-cycle transcription in response to replication stress contributes to two key functions of the DNA replication checkpoint response, namely, preventing genomic instability and cell death. Given the critical role of replication stress in oncogene transformation, a detailed understanding of the molecular mechanisms involved in the checkpoint response will contribute to a better insight into cancer development.

p53 activation by Ni(II) is a HIF-1α independent response causing caspases 9/3-mediated apoptosis in human lung cells

Hypoxia mimic nickel(II) is a human respiratory carcinogen with a suspected epigenetic mode of action. We examined whether Ni(II) elicits a toxicologically significant activation of the tumor suppressor p53, which is typically associated with genotoxic responses. We found that treatments of H460 human lung epithelial cells with NiCl2 caused activating phosphorylation at p53-Ser15, accumulation of p53 protein and depletion of its inhibitor MDM4 (HDMX). Confirming the activation of p53, its knockdown suppressed the ability of Ni(II) to upregulate MDM2 and p21 (CDKN1A). Unlike DNA damage, induction of GADD45A by Ni(II) was p53-independent. Ni(II) also increased p53-Ser15 phosphorylation and p21 expression in normal human lung fibroblasts. Although Ni(II)-induced stabilization of HIF-1α occurred earlier, it had no effect on p53 accumulation and Ser15 phosphorylation. Ni(II)-treated H460 cells showed no evidence of necrosis and their apoptosis and clonogenic death were suppressed by p53 knockdown. The apoptotic role of p53 involved a transcription-dependent program triggering the initiator caspase 9 and its downstream executioner caspase 3. Two most prominently upregulated proapoptotic genes by Ni(II) were PUMA and NOXA but only PUMA induction required p53. Knockdown of p53 also led to derepression of antiapoptotic MCL1 in Ni(II)-treated cells. Overall, our results indicate that p53 plays a major role in apoptotic death of human lung cells by Ni(II). Chronic exposure to Ni(II) may promote selection of resistant cells with inactivated p53, providing an explanation for the origin of p53 mutations by this epigenetic carcinogen.

The DNA binding and accumulation of p53 from breast cancer cell lines and the link with serine 15 phosphorylation

Stress treatment generally causes the post-translational modification and accumulation of the p53 protein, although the role of these aspects has not been always understood in relation to this protein's tumor suppressor activity. We analyzed these attributes of p53 in eight different breast cancer cell lines, with either wild-type or mutant p53 protein, in response to oxidative stress. We found that the wild-type p53 protein from MCF-7 and ZR-75-1 cells binds with different affinity to 12 gene sequences covering several pathways regulated by p53. Treatment of MCF-7 cells with H2O2 caused an increase in this binding affinity while this same treatment of ZR-75-1 cells caused the p53 protein to lose binding affinity to several genes. The mutant p53 proteins from all cell lines had minimal to weak binding to these sequences even after treatment with H2O2. The p53 protein from the ZR-75-1 cells and three cell lines with mutant p53 showed serine 15 phosphorylated protein, but we found no correlation between that modification and the levels or localization of this protein although DNA binding affinity of wild-type protein might be affected by this modification. From this and other work, it appears that the mutation status of the TP53 gene alone cannot predict the activity of this tumor suppressor since cell lines with the same genetic information do not show the same properties of this protein.

S-phase sensing of DNA-protein crosslinks triggers TopBP1-independent ATR activation and p53-mediated cell death by formaldehyde

We examined genotoxic signaling and cell fate decisions in response to a potent DNA-protein crosslinker formaldehyde (FA). DNA-protein crosslinks (DPC) are poorly understood lesions produced by bifunctional carcinogens and several cancer drugs. FA-treated human cells showed a rapid activation of ATR kinase that preferentially targeted the p53 transcription factor at low doses and CHK1 kinase at more severe damage, producing bell-shaped and sublinear responses, respectively. CHK1 phosphorylation was transient, and its loss was accompanied by increased p53 accumulation and Ser15 phosphorylation. Activation of p53 was insensitive to inhibition of mismatch repair and nucleotide and base excision repair, excluding the role of small DNA adducts in this response. The p53-targeted signaling was transcription-independent, absent in quiescent cells and specific to S-phase in cycling populations. Unlike other S-phase stressors, FA-activated p53 was functional transcriptionally, promoted apoptosis in lung epithelial cells and caused senescence in normal lung fibroblasts. FA did not induce ATR, RAD1 or RPA foci, and p53 phosphorylation was TopBP1-independent, indicating a noncanonical mode of ATR activation. Replication arrest by FA caused a dissociation of ATR from a chromatin-loaded MCM helicase but no PCNA monoubiquitination associated with stalled polymerases. These results suggest that unlike typical DNA adducts that stall DNA polymerases, replication inhibition by bulkier DPC largely results from blocking upstream MCM helicase, which prevents accumulation of ssDNA. Overall, our findings indicate that S-phase-specific, TopBP1-independent activation of the ATR-p53 axis is a critical stress response to FA-DPC, which has implications for understanding of FA carcinogenesis.

Elevated transcription of the p53 gene in early S-phase leads to a rapid DNA-damage response during S-phase of the cell cycle

p53 induces the transcription of genes that negatively regulate progression of the cell cycle in response to DNA damage or other cellular stressors, and thus participates in maintaining genome stability. Under stress conditions, p53 must be activated to prohibit the replication of cells containing damaged DNA. However, in normal, non-stressed cells, p53 activity must be inhibited. Previous studies have demonstrated that p53 transcription is activated before or during early S-phase in cells progressing from G(0)/G(1) into S-phase. Since this is not what would be predicted from a gene involved in growth arrest and apoptosis, in this study, we provide evidence that this induction occurs to provide sufficient p53 mRNA to ensure a rapid response to DNA damage before exiting S-phase. When comparing exponentially growing Swiss3T3 cells to those synchronized to enter S-phase simultaneously and treated with the DNA damaging agent camptothecin, we found that with cells in S-phase, p53 protein levels increased earlier, Bax and p21 transcription was activated earlier and to a greater extent and apoptosis occurred earlier and to a greater extent. These findings are consistent with p53 transcription being induced in S-phase to provide for a rapid DNA-damage response during S-phase of the cell cycle.

Hdm2- and proteasome-dependent turnover limits p21 accumulation during S phase

Double-strand DNA breaks detected in different phases of the cell cycle induce molecularly distinct checkpoints downstream of the ATM kinase. p53 is known to induce arrest of cells in G 1 and occasionally G 2 phase but not S phase following ionizing radiation, a time at which the MRN complex and cdc25-dependent mechanisms induce arrest. Our understanding of how cell cycle phase modulates pathway choice and the reasons certain pathways might be favored at different times is limited. In this report, we examined how cell cycle phase affects the activation of the p53 checkpoint and its ability to induce accumulation of the cdk2 inhibitor p21. Using flow cytometric tools and centrifugal elutriation, we found that the p53 response to ionizing radiation is largely intact in all phases of the cell cycle; however, the accumulation of p21 protein is limited to the G 1 and G 2 phase of the cell cycle because of the activity of a proteasome-dependent p21 turnover pathway in S-phase cells. We found that the turnover of p21 was independent of the SCF (skp2) E3 ligase but could be inhibited, at least in part, by reducing hdm2, although this depended on the cell type studied. Our results suggest that there are several redundant pathways active in S-phase cells that can prevent the accumulation of p21.

ATR-p53 restricts homologous recombination in response to replicative stress but does not limit DNA interstrand crosslink repair in lung cancer cells

Homologous recombination (HR) is required for the restart of collapsed DNA replication forks and error-free repair of DNA double-strand breaks (DSB). However, unscheduled or hyperactive HR may lead to genomic instability and promote cancer development. The cellular factors that restrict HR processes in mammalian cells are only beginning to be elucidated. The tumor suppressor p53 has been implicated in the suppression of HR though it has remained unclear why p53, as the guardian of the genome, would impair an error-free repair process. Here, we show for the first time that p53 downregulates foci formation of the RAD51 recombinase in response to replicative stress in H1299 lung cancer cells in a manner that is independent of its role as a transcription factor. We find that this downregulation of HR is not only completely dependent on the binding site of p53 with replication protein A but also the ATR/ATM serine 15 phosphorylation site. Genetic analysis suggests that ATR but not ATM kinase modulates p53's function in HR. The suppression of HR by p53 can be bypassed under experimental conditions that cause DSB either directly or indirectly, in line with p53's role as a guardian of the genome. As a result, transactivation-inactive p53 does not compromise the resistance of H1299 cells to the interstrand crosslinking agent mitomycin C. Altogether, our data support a model in which p53 plays an anti-recombinogenic role in the ATR-dependent mammalian replication checkpoint but does not impair a cell's ability to use HR for the removal of DSB induced by cytotoxic agents.

Genistein, isoflavonoids in soybeans, prevents the formation of excess radiation-induced centrosomes via p21 up-regulation

The centrosome is a cytoplasmic organelle which duplicates once during each cell cycle, and the presence of excess centrosomes promote chromosome instability through chromosome missegregation following cytokinesis. Ionizing radiation (IR) can induce extra centrosomes by permitting the continuation of CDK2/Cyclin-A/E-mediated centrosome duplication when cells are arrested in the cell cycle after irradiation. The work described here shows that, in addition to IR, extra centrosomes were induced in human U2OS and mouse NIH3T3 cells after treatment with agents which include DNA adduct-forming chemicals: benzopyrene (BP), 4-nitroquinoline 1-oxide (4NQO), a DNA cross linker: cis-diamminedichloro-platinum (cisplatin), topoisomerase inhibitors: camptothecin, etoposide, genistein, and ultra-violet light (UV). These agents were divided into two categories with respect to the regulation of p21, which is an inhibitor of CDK2/Cyclin-A/E: specifically, p21 was up-regulated by an IR exposure and treatment with topoisomerase inhibitors. However, UV, BP, 4NQO and cisplatin down-regulated p21 below basal levels. When cells were irradiated with IR in combination with all of these agents, except genistein, enhanced induction of extra centrosomes was observed, regardless of the nature of p21 expression. Genistein significantly suppressed the frequency of IR-induced extra centrosomes in a dose-dependent manner, and 20μg/ml of genistein reduced this frequency to 66%. Consistent with this, genistein substantially up-regulated p21 expression over the induction caused by IR alone, while other agents down-regulated or marginally affected this. This suggests the inhibitory effect of genistein on the induction of extra centrosomes occurs through the inactivation of CDK2/Cyclin-A/E via p21 up-regulation. This hypothesis is supported by the observation that p21 knockdown with siRNA reduced the activity of CDK2/Cyclin-A/E and restored the enhanced effect of a combined treatment with genistein and IR. These results demonstrate the preventive effect of genistein and a crucial role for p21 in IR-induced excess centrosomes.

Hydroxycarbamide: a user's guide for chronic myeloproliferative disorders

Hydroxycarbamide is a nonalkylating antiproliferative and antiviral agent that has been used for over 40 years to treat a variety of neoplastic and non-neoplastic conditions. Hydroxycarbamide is readily absorbed and widely distributed throughout the body. It acts primarily to inhibit DNA synthesis, which underpins its use in solid tumors, viral infections and chronic myeloproliferative disorders. Hydroxycarbamide is an effective treatment for preventing transient ischemic attacks associated with thrombocytosis in chronic myeloproliferative disorders because it is a nitric oxide donor. While its mechanism of action and side-effect profile are well defined, its potential for leukemic transformation as a single agent is still a matter of controversy. Based on a search of the Medline database, this article encompasses the pharmacokinetics, pharmacodynamics, clinical use and tolerability of hydroxycarbamide, plus its potential for mutagenicity with special reference to the chronic myeloproliferative disorders. The toxicity profile of hydroxycarbamide is also discussed to enable clinicians to balance potential risks with therapeutic benefits.

A reduction of licensed origins reveals strain-specific replication dynamics in mice

Replication origin licensing builds a fundamental basis for DNA replication in all eukaryotes. This occurs during the late M to early G1 phases in which chromatin is licensed by loading of the MCM2-7 complex, an essential component of the replicative helicase. In the following S phase, only a minor fraction of chromatin-bound MCM2-7 complexes are activated to unwind the DNA. Therefore, it is proposed that the vast majority of MCM2-7 complexes license dormant origins that can be used as backups. Consistent with this idea, it has been repeatedly demonstrated that a reduction (~60%) in chromatin-bound MCM2-7 complexes has little effect on the density of active origins. In this study, however, we describe the first exception to this observation. A reduction of licensed origins due to Mcm4 ( chaos3 ) homozygosity reduces active origin density in primary embryonic fibroblasts (MEFs) in a C57BL/6J (B6) background. We found that this is associated with an intrinsically lower level of active origins in this background compared to others. B6 Mcm4 ( chaos3/chaos3 ) cells proliferate slowly due to p53-dependent upregulation of p21. In fact, the development of B6 Mcm4 ( chaos3/chaos3 ) mice is impaired and a significant fraction of them die at birth. While inactivation of p53 restores proliferation in B6 Mcm4 ( chaos3/chaos3 ) MEFs, it paradoxically does not rescue animal lethality. These findings indicate that a reduction of licensed origins may cause a more profound effect on cell types with lower densities of active origins. Moreover, p53 is required for the development of mice that suffer from intrinsic replication stress.

Disparate chromatin landscapes and kinetics of inactivation impact differential regulation of p53 target genes

The p53 transcription factor regulates the expression of genes involved in cellular responses to stress, including cell cycle arrest and apoptosis. The p53 transcriptional program is extremely malleable, with target gene expression varying in a stress- and cell type-specific fashion. The molecular mechanisms underlying differential p53 target gene expression remain elusive. Here we provide evidence for gene-specific mechanisms affecting expression of three important p53 target genes. First we show that transcription of the apoptotic gene PUMA is regulated through intragenic chromatin boundaries, as revealed by distinct histone modification territories that correlate with binding of the insulator factors CTCF, Cohesins and USF1/2. Interestingly, this mode of regulation produces an evolutionary conserved long non-coding RNA of unknown function. Second, we demonstrate that the kinetics of transcriptional competence of the cell cycle arrest gene p21 and the apoptotic gene FAS are markedly different in vivo, as predicted by recent biochemical dissection of their core promoter elements in vitro. After a pulse of p53 activity in cells, assembly of the transcriptional apparatus on p21 is rapidly reversed, while FAS transcriptional activation is more sustained. Collectively these data add to a growing list of p53-autonomous mechanisms that impact differential regulation of p53 target genes.

Effects of hydroxyurea on monoclonal antibody production induced by anti-mIgG and LPS stimulation on murine B cell hybridomas

Chemical treatment with hydroxyurea (HU) has been selected as a simple and low cost strategy to generate a cell population enriched for the G1 phase. After the chemical treatment with HU, cells were stimulated with anti-mIgG to test if the positive effects of anti-mIgG on CD40 expression and specific IgG2a production rate were improved upon a cell population with a higher percentage of cells in G1 phase at the beginning of the cell culture. In addition, other treatments assayed in this work were the cell stimulation with Lipopolysaccharide (LPS) both before and after the HU treatment. It has been observed that the use of HU under conditions able to maintain the cells in viable state (0.1 mM for 20 h), has a negative effect on CD40 expression and specific IgG2a production rate induced by anti-mIgG. The positive effect of LPS on cell stimulation induced by anti-mIgG is reduced on cells treated with HU.

Loss of DNA polymerase zeta enhances spontaneous tumorigenesis

Mammalian genomes encode at least 15 distinct DNA polymerases, functioning as specialists in DNA replication, DNA repair, recombination, or bypass of DNA damage. Although the DNA polymerase zeta (polzeta) catalytic subunit REV3L is important in defense against genotoxins, little is known of its biological function. This is because REV3L is essential during embryogenesis, unlike other translesion DNA polymerases. Outstanding questions include whether any adult cells are viable in the absence of polzeta and whether polzeta status influences tumorigenesis. REV3L-deficient cells have properties that could influence the development of neoplasia in opposing ways: markedly reduced damage-induced point mutagenesis and extensive chromosome instability. To answer these questions, Rev3L was conditionally deleted from tissues of adult mice using MMTV-Cre. Loss of REV3L was tolerated in epithelial tissues but not in the hematopoietic lineage. Thymic lymphomas in Tp53(-/-) Rev3L conditional mice occurred with decreased latency and higher incidence. The lymphomas were populated predominantly by Rev3L-null T cells, showing that loss of Rev3L can promote tumorigenesis. Remarkably, the tumors were frequently oligoclonal, consistent with accelerated genetic changes in the absence of Rev3L. Mammary tumors could also arise from Rev3L-deleted cells in both Tp53(+/+) and Tp53(+/-) backgrounds. Mammary tumors in Tp53(+/-) mice deleting Rev3L formed months earlier than mammary tumors in Tp53(+/-) control mice. Prominent preneoplastic changes in glandular tissue adjacent to these tumors occurred only in mice deleting Rev3L and were associated with increased tumor multiplicity. Polzeta is the only specialized DNA polymerase yet identified that inhibits spontaneous tumor development.

PCNA-coupled p21 degradation after DNA damage: The exception that confirms the rule?

While many are the examples of DNA damaging treatments that induce p21 accumulation, the conception of p21 upregulation as the universal response to genotoxic stress has come to an end. Compelling evidences have demonstrated the existence of converging signals that negatively regulate p21 bellow basal levels when replication forks are blocked. Moreover, conclusive reports identified the E3-ligase CRL4(CDT2) (CUL4-DDB1-CDT2) as the enzymatic complex that promotes p21 proteolysis when treatments such as UV irradiation trigger replication fork stress. A pre-requisite for CRL4(CDT2)-driven proteolysis is the interaction of p21 with PCNA. Interestingly as well, CRL4(CDT2)-dependent proteolysis is not limited to p21 and affects other PCNA partners, including the specialized DNA polymerase eta (pol eta). These recent discoveries are particularly intriguing since the UV-induced degradation of p21 has been shown to be required for efficient pol eta recruitment to DNA lesions. Herein we review the findings that lead to the identification of the molecular mechanism that triggers damage-induced PCNA-coupled protein proteolysis. We propose a novel model in which CRL4(CDT2)-dependent protein degradation facilitates a sequential and dynamic exchange between PIP box bearing proteins at stall forks during Translesion DNA synthesis (TLS). Moreover, given the tight spatiotemporal control that CRL4(CDT2)-driven proteolysis is able to confer to PCNA-regulated processes, we discuss the impact that this degradation mechanism might have in other molecular switches associated with the repair of damaged DNA.

Awakening guardian angels: drugging the p53 pathway

Currently, around 11 million people are living with a tumour that contains an inactivating mutation of TP53 (the human gene that encodes p53) and another 11 million have tumours in which the p53 pathway is partially abrogated through the inactivation of other signalling or effector components. The p53 pathway is therefore a prime target for new cancer drug development, and several original approaches to drug discovery that could have wide applications to drug development are being used. In one approach, molecules that activate p53 by blocking protein-protein interactions with MDM2 are in early clinical development. Remarkable progress has also been made in the development of p53-binding molecules that can rescue the function of certain p53 mutants. Finally, cell-based assays are being used to discover compounds that exploit the p53 pathway by either seeking targets and compounds that show synthetic lethality with TP53 mutations or by looking for non-genotoxic activators of the p53 response.

A role for Chk1 in blocking transcriptional elongation of p21 RNA during the S-phase checkpoint

We reported previously that when cells are arrested in S phase, a subset of p53 target genes fails to be strongly induced despite the presence of high levels of p53. When DNA replication is inhibited, reduced p21 mRNA accumulation is correlated with a marked reduction in transcription elongation. Here we show that ablation of the protein kinase Chk1 rescues the p21 transcription elongation defect when cells are blocked in S phase, as measured by increases in both p21 mRNA levels and the presence of the elongating form of RNA polymerase II (RNAPII) toward the 3' end of the p21 gene. Recruitment of specific elongation and 3' processing factors (DSIF, CstF-64, and CPSF-100) is also restored. While additional components of the RNAPII transcriptional machinery, such as TFIIB and CDK7, are recruited more extensively to the p21 locus after DNA damage than after replication stress, their recruitment is not enhanced by ablation of Chk1. Significantly, ablating Chk2, a kinase closely related in substrate specificity to Chk1, does not rescue p21 mRNA levels during S-phase arrest. Thus, Chk1 has a direct and selective role in the elongation block to p21 observed during S-phase arrest. These findings demonstrate for the first time a link between the replication checkpoint mediated by ATR/Chk1 and the transcription elongation/3' processing machinery.

Therapeutic doses of hydroxyurea cause telomere dysfunction and reduce TRF2 binding to telomeres

Hydroxyurea (HU) is a chemotherapeutic agent commonly used for various malignancies and hematological disorders, including chronic myelogenous leukemia and sickle cell anemia. We show here that chronic, low-level treatment with HU induces a variety of defects in telomere replication and maintenance. HU treatment preferentially decreased the rate of telomere DNA synthesis and altered the cell cycle timing of telomere replication. HU reduced the expression levels of telomere repeat RNA (TERRA). In some cells, HU caused a rapid loss of telomere restriction fragment length. Chromatin immunoprecipitation (ChIP) assay indicated that telomere repeat binding factors TRF1 and TRF2 dissociate from telomere DNA after HU treatment. TRF2 protein purified from HU treated cells showed a modest reduction in DNA binding activity and a change in isoelectric point as measured by 2D gel electrophoresis. However, chronic low level HU treatment did not evoke a DNA replication checkpoint response, suggesting that the mechanism of action is distinct from the well-characterized S-phase checkpoint pathway. We conclude that therapeutic doses of HU preferentially effects telomere replication and maintenance, through a mechanism that may involve the direct modification of TRF2. These findings provide new insight into the potential mechanisms of action of HU at telomeres and in cancer chemotherapies.

Analysis of p53 dependent damage response in sperm-irradiated mouse embryos

Ionizing radiation activates a series of DNA damage response, cell cycle checkpoints to arrest cells at G1/S, S and G2/M, DNA repair, and apoptosis. The DNA damage response is thought to be the major determinant of cellular radiosensitivity and thought to operate in all higher eukaryotic cells. However, the radiosensitivity is known to differ considerably during ontogeny of mammals and early embryos of mouse for example are much more sensitive to radiation than adults. We have focused on the radiation-induced damage response during pre-implantation stage of mouse embryo. Our study demonstrates a hierarchy of damage responses to assure the genomic integrity in early embryonic development. In the sperm-irradiated zygotes, p53 dependent S-phase checkpoint functions to suppress erroneous replication of damaged DNA. The transcription-dependent function is not required and the DNA-binging domain of the protein is essential for this p53 dependent S-phase checkpoint. p21 mediated cleavage arrest comes next during early embryogenesis to prevent delayed chromosome damage at morula/ blastocyst stages. Apoptosis operates even later only in the cells of ICM at the blastocyst stage to eliminate deleterious cells. Thus, early development of sperm-irradiated embryos is protected at least by three mechanisms regulated by p53 and by p21.

Triphala inhibits both in vitro and in vivo xenograft growth of pancreatic tumor cells by inducing apoptosis

BACKGROUND

Triphala is commonly used in Ayurvedic medicine to treat variety of diseases; however its mechanism of action remains unexplored. This study elucidates the molecular mechanism of Triphala against human pancreatic cancer in the cellular and in vivo model.

METHODS

Growth-inhibitory effects of Triphala were evaluated in Capan-2, BxPC-3 and HPDE-6 cells by Sulphoradamine-B assay. Apoptosis was determined by cell death assay and western blotting. Triphala was administered orally to nude mice implanted with Capan-2 xenograft. Tumors were analyzed by immunohistochemistry and western blotting.

RESULTS

Exposure of Capan-2 cells to the aqueous extract of Triphala for 24 h resulted in the significant decrease in the survival of cells in a dose-dependent manner with an IC50 of about 50 microg/ml. Triphala-mediated reduced cell survival correlated with induction of apoptosis, which was associated with reactive oxygen species (ROS) generation. Triphala-induced apoptosis was linked with phosphorylation of p53 at Ser-15 and ERK at Thr-202/Tyr-204 in Capan-2 cells. Above mentioned effects were significantly blocked when the cells were pretreated with an antioxidant N-acetylcysteine (NAC), suggesting the involvement of ROS generation. Pretreatment of cells with pifithrin-alpha or U0126, specific inhibitors of p53 or MEK-1/2, significantly attenuated Triphala-induced apoptosis. Moreover, NAC or U0126 pretreatment significantly attenuated Triphala-induced p53 transcriptional activity. Similarly, Triphala induced apoptosis in another pancreatic cancer cell line BxPC-3 by activating ERK. On the other hand, Triphala failed to induce apoptosis or activate ERK or p53 in normal human pancreatic ductal epithelial (HPDE-6) cells. Further, oral administration of 50 mg/kg or 100 mg/kg Triphala in PBS, 5 days/week significantly suppressed the growth of Capan-2 pancreatic tumor-xenograft. Reduced tumor-growth in Triphala fed mice was due to increased apoptosis in the tumors cells, which was associated with increased activation of p53 and ERK.

CONCLUSION

Our preclinical studies demonstrate that Triphala is effective in inhibiting the growth of human pancreatic cancer cells in both cellular and in vivo model. Our data also suggests that the growth inhibitory effects of Triphala is mediated by the activation of ERK and p53 and shows potential for the treatment and/or prevention of human pancreatic cancer.

Dissecting the role of p53 phosphorylation in homologous recombination provides new clues for gain-of-function mutants

Regulation of homologous recombination (HR) represents the best-characterized DNA repair function of p53. The role of p53 phosphorylation in DNA repair is largely unknown. Here, we show that wild-type p53 repressed repair of DNA double-strand breaks (DSBs) by HR in a manner partially requiring the ATM/ATR phosphorylation site, serine 15. Cdk-mediated phosphorylation of serine 315 was dispensable for this anti-recombinogenic effect. However, without targeted cleavage of the HR substrate, serine 315 phosphorylation was necessary for the activation of topoisomerase I-dependent HR by p53. Moreover, overexpression of cyclin A1, which mimics the situation in tumors, inappropriately stimulated DSB-induced HR in the presence of oncogenic p53 mutants (not Wtp53). This effect required cyclin A1/cdk-mediated phosphorylation for stable complex formation with topoisomerase I. We conclude that p53 mutants have lost the balance between activation and repression of HR, which results in a net increase of potentially mutagenic DNA rearrangements. Our data provide new insight into the mechanism underlying gain-of-function of mutant p53 in genomic instability.

DNA synthesis from unbalanced nucleotide pools causes limited DNA damage that triggers ATR-CHK1-dependent p53 activation

p53-dependent G(1) and G(2) cell cycle checkpoints are activated in response DNA damage that help to maintain genomic stability. p53 also helps to protect cells from damage that occurs during S phase, for example, when the cells are starved for DNA precursors or irradiated with a low dose of UV. p53 is activated in normal cells starved for pyrimidine nucleotides by treatment with N-(phosphonacetyl)-l-aspartate (PALA). The treated cells progress through a first S phase with kinetics similar to those of untreated cells. However, the DNA of the treated cells begins to become damaged rapidly, within 12 h, as revealed by a comet assay, which detects broken DNA, and by staining for phosphorylated histone H2AX, which accumulates at sites of DNA damage. Because the cells survive, the damage must be reversible, suggesting single-strand breaks or gaps as the most likely possibility. The transiently damaged DNA stimulates activation of ATR and CHK1, which in turn catalyze the phosphorylation and accumulation of p53. Although PALA-induced DNA damage occurs only in dividing cells, the p53 that is activated is only competent to transcribe genes such as p21 and macrophage inhibitory cytokine 1 (whose products regulate G(2) and G(1) or S phase checkpoints, respectively) after the cells have exited the S phase during which damage occurs. We propose that p53 is activated by stimulation of mismatch repair in response to the misincorporation of deoxynucleotides into newly synthesized DNA, long before the lack of pyrimidine nucleoside triphosphates causes the rate of DNA synthesis to slow appreciably.

ATP-dependent activation of p21WAF1/CIP1-associated Cdk2 by Cdc6

When cells progressing in mid-S phase are damaged with a base-modifying chemical, they arrest in S phase long after the CHK1 checkpoint signal fades out, partly because of p53-mediated long-lasting induction of the cyclin-dependent kinase inhibitor p21(WAF1/CIP1). We have recently found that enforced expression of Cdc6, the assembler of prereplicative complexes, markedly advances recovery from the prolonged S-phase arrest and reactivation of Cdk2 despite the presence of a high level of induced p21. Here, we report that Cdc6 protein can activate p21-associated Cdk2 in an ATP-dependent manner in vitro. Consistently, Cdc6 mutated for ATPase or a putative cyclin binding motif is no longer able to activate the Cdk2 in vitro or promote reinitiation of S-phase progression and reactivation of Cdk2 in vivo. These results reveal the never anticipated function of Cdc6 and redefine its role in the control of S-phase progression in mammalian cells.

Hbo1 Links p53-dependent stress signaling to DNA replication licensing

Hbo1 is a histone acetyltransferase (HAT) that is required for global histone H4 acetylation, steroid-dependent transcription, and chromatin loading of MCM2-7 during DNA replication licensing. It is the catalytic subunit of protein complexes that include ING and JADE proteins, growth regulatory factors and candidate tumor suppressors. These complexes are thought to act via tumor suppressor p53, but the molecular mechanisms and links between stress signaling and chromatin, are currently unknown. Here, we show that p53 physically interacts with Hbo1 and negatively regulates its HAT activity in vitro and in cells. Two physiological stresses that stabilize p53, hyperosmotic shock and DNA replication fork arrest, also inhibit Hbo1 HAT activity in a p53-dependent manner. Hyperosmotic stress during G(1) phase specifically inhibits the loading of the MCM2-7 complex, providing an example of the chromatin output of this pathway. These results reveal a direct regulatory connection between p53-responsive stress signaling and Hbo1-dependent chromatin pathways.

Replicational stress selects for p53 mutation

p53 is a critical mediator of cellular responses to a variety of stresses. Given the frequency of p53 mutations in human malignancies and that disruption of p53 has been implicated in chemoresistance, understanding the factors that select for p53 disruption is important both for understanding tumor evolution and for designing cancer therapies. While it is widely believed that genotoxic stress selects for p53 mutations, the effects of DNA damaging agents on long-term proliferative potential are usually not affected by p53 status. Previous reports have demonstrated that despite being activated, p53 loss does not prevent cell cycle arrest and senescence in response to high levels of acute replicational stress. In contrast, we recently reported that chronic exposure of non-transformed cells to low, clinically relevant levels of replicational stress induces p53-dependent senescence-like arrest. Disruption of p53 or its target gene p21(CIP1) antagonizes this arrest, leading to a long-term proliferative advantage. However, when replicational stress is associated with substantial DNA strand breaks, the ability of p53 disruption to up-regulate RAD51 dependent homologous recombination becomes important. Replicational stress is induced by many chemotherapeutic treatments and perhaps by some dietary deficiencies, and may be an important factor that selects for p53 mutations during cancer initiation and progression.

p53 mediates senescence-like arrest induced by chronic replicational stress

Previous studies have shown that exposure of cells to high levels of replicational stress leads to permanent proliferation arrest that does not require p53. We have examined cellular responses to therapeutically relevant low levels of replicational stress that allow limited proliferation. Chronic exposure to low concentrations of hydroxyurea, aphidicolin, or etoposide induced irreversible cell cycle arrest after several population doublings. Inhibition of p53 activity antagonized this arrest and enhanced the long-term proliferation of p53 mutant cells. p21CIP1 was found to be a critical p53 target for arrest induced by hydroxyurea or aphidicolin, but not etoposide, as judged by the ability of p21CIP1 suppression to mimic the effects of p53 disruption. Suppression of Rad51 expression, required for homologous recombination repair, blocked the ability of mutant p53 to antagonize arrest induced by etoposide, but not aphidicolin. Thus, the ability of mutant p53 to prevent arrest induced by replicational stress per se is primarily dependent on preventing p21CIP1 up-regulation. However, when replication stress is associated with DNA strand breaks (such as with etoposide), up-regulation of homologous recombination repair in response to p53 disruption becomes important. Since replicational stress leads to clonal selection of cells with p53 mutations, our results highlight the potential importance of chronic replicational stress in promoting cancer development.

Loss of FANCC function is associated with failure to inhibit late firing replication origins after DNA cross-linking

Fanconi anemia (FA) cells are abnormally sensitive to DNA cross-linking agents with increased levels of apoptosis and chromosomal instability. Defects in eight FA complementation groups inhibit monoubiquitination of FANCD2, and subsequent recruitment of FANCD2 to DNA damage and S-phase-associated nuclear foci. The specific functional defect in repair or response to DNA damage in FA cells remains unknown. Damage-resistant DNA synthesis is present 2.5-5 h after cross-linker treatment of FANCC, FANCA and FANCD2-deficient cells. Analysis of the size distribution of labeled DNA replication strands revealed that diepoxybutane treatment suppressed labeling of early but not late-firing replicons in FANCC-deficient cells. In contrast, normal responses to ionizing radiation were observed in FANCC-deficient cells. Absence of this late S-phase response in FANCC-deficient cells leads to activation of secondary checkpoint responses.

p53-Dependent p21 mRNA elongation is impaired when DNA replication is stalled

We have previously reported that when DNA replication is blocked in some human cell lines, p53 is impaired in its ability to induce a subset of its key target genes, including p21(WAF1/CIP1). Here, we investigated the reason for this impairment by comparing the effects of two agents, hydroxyurea (HU), which arrests cells in early S phase and impairs induction of p21, and daunorubicin, which causes a G(2) block and leads to robust activation of p21 by p53. HU treatment was shown to inhibit p21 mRNA transcription rather than alter its mRNA stability. Nevertheless, chromatin immunoprecipitation assays revealed that HU impacts neither p53 binding nor acetylation of histones H3 and H4 within the p21 promoter. Furthermore, recruitment of the TFIID/TATA-binding protein complex and the large subunit of RNA polymerase II (RNA Pol II) are equivalent after HU and daunorubicin treatments. Relative to daunorubicin treatment, however, transcription elongation of the p21 gene is significantly impaired in cells treated with HU, as evidenced by reduced occupancy of RNA Pol II at regions downstream of the start site. Likewise, in the p21 downstream region after administration of HU, there is less of a specifically phosphorylated form of RNA Pol II (Pol II-C-terminal domain serine 2P) which occurs only when the polymerase is elongating RNA. We propose that while the DNA replication checkpoint is unlikely to regulate the assembly of a p21 promoter initiation complex, it signals to one or more factors involved in the process of transcriptional elongation.

Abrogation of the transactivation activity of p53 by BCCIP down-regulation

The tumor suppression function of p53 is mostly conferred by its transactivation activity, which is inactivated by p53 mutations in approximately 50% of human cancers. In cancers harboring wild type p53, the p53 transactivation activity may be compromised by other mechanisms. Identifying the mechanisms by which wild type p53 transactivation activity can be abrogated may provide insights into the molecular etiology of cancers harboring wild type p53. In this report, we show that BCCIP, a BRCA2 and CDKN1A-interacting protein, is required for the transactivation activity of wild type p53. In p53 wild type cells, BCCIP knock down by RNA interference diminishes the transactivation activity of p53 without reducing the p53 protein level, inhibits the binding of p53 to the promoters of p53 target genes p21 and HDM2, and reduces the tetrameric formation of p53. These data demonstrate a critical role of BCCIP in maintaining the transactivation activity of wild type p53 and further suggest down-regulation of BCCIP as a novel mechanism to impair the p53 function in cells harboring wild type p53.

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.

Peroxide resistance in human and mouse lens epithelial cell lines is related to long-term changes in cell biology and architecture

It is well established that the response of the cell to environmental stress is a major basis for cell modification. Such modification is believed to adapt the cell to better survive its environment. Oxidative stress, a major and ubiquitous stressing factor, was selected for investigating the cellular response to stress. Most studies investigating such cellular response have employed examination of the cell either during or shortly after exposure to stress. We have employed a different approach arguing that the short-term response to stress obscures the biological changes that allow the cell to continue to thrive in its new environment. Reflecting this concept, murine and human cell lines capable of surviving regular exposure to toxic levels of H(2)O(2) or TBOOH have been developed. It was found that certain fundamental long-term changes in cell biology had occurred. The peroxide-resistant cells are diploid rather than aneuploid, show fundamental changes in the cytoskeletal cellular structure, suggesting less rigid more flexible cells, express a new lower molecular mass of p53, a key stress protein responder involved in adaptation, and finally have an immunochemical modification in alphaA-crystallin, a small heat-shock protein. Previously, it was found that there is a dramatic increase in catalase and gluthathione S-transferase activity and a remarkable limited change in expression in other antioxidative genes in these cells. The impact of these changes is discussed. It is apparent that evolutionary cell modifications can occur in response to relatively rapid changes in environment over periods ranging from days to months rather than the thousands of years considered in most evolutionary modifications.

Loss of rereplication control in Saccharomyces cerevisiae results in extensive DNA damage

To maintain genome stability, the entire genome of a eukaryotic cell must be replicated once and only once per cell cycle. In many organisms, multiple overlapping mechanisms block rereplication, but the consequences of deregulating these mechanisms are poorly understood. Here, we show that disrupting these controls in the budding yeast Saccharomyces cerevisiae rapidly blocks cell proliferation. Rereplicating cells activate the classical DNA damage-induced checkpoint response, which depends on the BRCA1 C-terminus checkpoint protein Rad9. In contrast, Mrc1, a checkpoint protein required for recognition of replication stress, does not play a role in the response to rereplication. Strikingly, rereplicating cells accumulate subchromosomal DNA breakage products. These rapid and severe consequences suggest that even limited and sporadic rereplication could threaten the genome with significant damage. Hence, even subtle disruptions in the cell cycle regulation of DNA replication may predispose cells to the genomic instability associated with tumorigenesis.

p53 elevation in relation to levels and cytotoxicity of mono- and bifunctional melphalan-DNA adducts

We tested the hypothesis that bifunctional DNA adducts formed by a nitrogen mustard-based anticancer drug were more efficient than monofunctional adducts at causing elevation of p53, consistent with the difference in cytotoxicity. Human leukemia cell line ML-1 was exposed for 1 h to melphalan or its monofunctional derivative monohydroxymelphalan. Levels of DNA adducts, measured by specific immunoassay, were linearly related to the concentration of alkylating agent. Monohydroxymelphalan formed twice as many adducts as did equal concentrations of melphalan. After the removal of the alkylating agent, adduct levels were maintained or increased slightly up to 8 h and then decreased by 27 to 44% by 24 h. Alkaline elution analyses confirmed the absence of detectable DNA interstrand cross-links in cells exposed to monohydroxymelphalan. DNA single-strand breaks were detected after monohydroxymelphalan but not after melphalan. Levels of p53 were quantified by sensitive fluorogenic enzyme-linked immunosorbent assay at intervals up to 24 h after exposure of cells to various concentrations of melphalan and monohydroxymelphalan. The level of initially formed DNA adducts needed to cause elevation of p53 from a baseline level of 0.5 ng/mg total protein to 2 ng/mg was 5- to 8-fold higher for monohydroxymelphalan than melphalan. The concentrations of melphalan and monohydroxymelphalan (+/-S.D.) causing 50% growth inhibition were 1.2 +/- 0.4 and 28.1 +/- 1.6 microg/ml, respectively, a 23-fold difference. The adduct levels induced by these exposures were 9.3 and 420 nmol/g DNA for melphalan and monohydroxymelphalan, respectively, a 45-fold difference, which is considerably greater than the difference in efficacy at elevating p53.

p53 differentially inhibits cell growth depending on the mechanism of telomere maintenance

Telomere stabilization is critical for tumorigenesis. A number of tumors and cell lines use a recombination-based mechanism, alternative lengthening of telomeres (ALT), to maintain telomere repeat arrays. Current data suggest that the mutation of p53 facilitates the activation of this pathway. In addition to its functions in response to DNA damage, p53 also acts to suppress recombination, independent of transactivation activity, raising the possibility that p53 might regulate the ALT mechanism via its role as a regulator of recombination. To test the role of p53 in ALT we utilized inducible alleles of human p53. We show that expression of transactivation-incompetent p53 inhibits DNA synthesis in ALT cell lines but does not affect telomerase-positive cell lines. The expression of temperature-sensitive p53 in clonal cell lines results in ALT-specific, transactivation-independent growth inhibition, due in part to the perturbation of S phase. Utilizing chromatin immunoprecipitation assays, we demonstrate that p53 is associated with the telomeric complex in ALT cells. Furthermore, the inhibition of DNA synthesis in ALT cells by p53 requires intact specific DNA binding and suppression of recombination functions. We propose that p53 causes transactivation-independent growth inhibition of ALT cells by perturbing telomeric recombination.

Homologous recombination and cell cycle checkpoints: Rad51 in tumour progression and therapy resistance

We provide an overview of the functional interrelationship between genes and proteins related to DNA repair by homologous recombination and cell cycle regulation in relation to the progression and therapy resistance of human tumours. To ensure the high-fidelity transmission of genetic information from one generation to the next, cells have evolved mechanisms to monitor genome integrity. Upon DNA damage, cells initiate complex response pathways including cell cycle arrest, activation of genes and gene products involved in DNA repair, and under some circumstances, the triggering of programmed cell death. Deregulation of this co-ordinated response leads to genetic instability and is fundamental to the aetiology of human cancer. Homologous recombination involved in DNA repair is induced by environmental damage as well as misreplication during the normal cell cycle. However, when not regulated properly, it can result in the loss of heterozygocity or genetic rearrangements, central to the process of carcinogenesis. The central step of homologous recombination is the DNA strand exchange reaction catalysed by the eukaryotic Rad51 protein. Here, we describe the recent progress in our understanding of how Rad51 is involved in the signalling and repair of DNA damage and how tumour suppressors, such as p53, ATM, BRCA1, BRCA2, BLM and FANCD2 are linked to Rad51-dependent pathways. An increased knowledge of the role of Rad51 in DNA repair by homologous recombination and its effects on cell cycle progression, tumour development and tumour resistance may provide opportunities for identifying improved diagnostic markers and developing more effective treatments for cancer.

Bloom syndrome cells undergo p53-dependent apoptosis and delayed assembly of BRCA1 and NBS1 repair complexes at stalled replication forks

Bloom syndrome (BS) is a hereditary disorder characterized by pre- and postnatal growth retardation, genomic instability, and cancer. BLM, the gene defective in BS, encodes a DNA helicase thought to participate in genomic maintenance. We show that BS human fibroblasts undergo extensive apoptosis after DNA damage specifically when DNA replication forks are stalled. Damage during S, but not G1, caused BLM to rapidly form foci with gammaH2AX at replication forks that develop DNA breaks. These BLM foci recruited BRCA1 and NBS1. Damaged BS cells formed BRCA1/NBS1 foci with markedly delayed kinetics. Helicase-defective BLM showed dominant-negative activity with respect to apoptosis, but not BRCA1/NBS1 recruitment, suggesting catalytic and structural roles for BLM. Strikingly, inactivation of p53 prevented the death of damaged BS cells and delayed recruitment of BRCA1/NBS1. These findings suggest that BLM is an early responder to damaged replication forks. Moreover, p53 eliminates cells that rapidly assemble BRCA1/NBS1 without BLM, suggesting that BLM is essential for timely BRCA1/NBS1 function.