The ability of p53 to activate downstream genes p21(WAF1/cip1) and MDM2, and cell cycle arrest following DNA damage is delayed and attenuated in scid cells deficient in the DNA-dependent protein kinase

MDM2: current research status and prospects of tumor treatment

Mousedouble minute 2 (MDM2) is one of the molecules activated by p53 and plays an important role in the regulation of p53. MDM2 is generally believed to function as a negative regulator of p53 by facilitating its ubiquitination and subsequent degradation. Consequently, blocked p53 activity often fails in damaged cells to undergo cell cycle arrest or apoptosis. Given that around 50% of human cancers involve the inactivation of p53 through genetic mutations, and directly targeting p53 through drug development has limited feasibility, targeting molecular regulation related to p53 has great potential and has become a research hotspot. For example, developing drugs that target the interaction between p53 and MDM2. Such drugs aim to reactivate p53 by targeting either MDM2 binding or p53 phosphorylation. Researchers have identified various compounds that can serve as inhibitors, either by directly binding to MDM2 or by modifying p53 through phosphorylation. Furthermore, a significant correlation exists between the expression of MDM2 in tumors and the effectiveness of immunotherapy, predominantly in the context of immune checkpoint inhibition. This review presents a comprehensive overview of the molecular characteristics of MDM2 and the current state of research on MDM2-targeting inhibitors. It includes a review of the impact of MDM2 targeting on the efficacy of immunotherapy, providing guidance and direction for the development of drugs targeting the p53-MDM2 interaction and optimization of immunotherapy.

Therapeutics Targeting p53-MDM2 Interaction to Induce Cancer Cell Death

Named as the guardian of the genome, p53 is a tumor suppressor that regulates cell function, often through many different mechanisms such as DNA repair, apoptosis, cell cycle arrest, senescence, metabolism, and autophagy. One of the genes that p53 activates is MDM2, which forms a negative feedback loop since MDM2 induces the degradation of p53. When p53 activity is inhibited, damaged cells do not undergo cell cycle arrest or apoptosis. As 50% of human cancers inactivate p53 by mutation, current research focuses on reactivating p53 by developing drugs that target the p53-MDM2 interaction, which includes the binding of MDM2 and phosphorylation of p53. The objective of this article is to provide a short list and description of p53-MDM2 antagonists that may be excellent candidates for inducing cancer cell death. Relevant articles were searched for and identified using online databases such as PubMed and ScienceDirect. Increasing p53 levels, by targeting the p53-MDM2 interaction, can help p53 play its role as a tumor suppressor and induce cancer cell death. Researchers have identified different compounds that can act as inhibitors, either by directly binding to MDM2 or by modifying p53 with phosphorylation. The results associated with the drugs demonstrate the importance of targeting such interactions to inhibit cancer cell growth, which indicates that the use of the compounds may improve cancer therapeutics.

Shift in G1-Checkpoint from ATM-Alone to a Cooperative ATM Plus ATR Regulation with Increasing Dose of Radiation

The current view of the involvement of PI3-kinases in checkpoint responses after DNA damage is that ATM is the key regulator of G₁-, S- or G₂-phase checkpoints, that ATR is only partly involved in the regulation of S- and G₂-phase checkpoints and that DNA-PKcs is not involved in checkpoint regulation. However, further analysis of the contributions of these kinases to checkpoint responses in cells exposed to ionizing radiation (IR) recently uncovered striking integrations and interplays among ATM, ATR and DNA-PKcs that adapt not only to the phase of the cell cycle in which cells are irradiated, but also to the load of DNA double-strand breaks (DSBs), presumably to optimize their processing. Specifically, we found that low IR doses in G₂-phase cells activate a G₂-checkpoint that is regulated by epistatically coupled ATM and ATR. Thus, inhibition of either kinase suppresses almost fully its activation. At high IR doses, the epistatic ATM/ATR coupling relaxes, yielding to a cooperative regulation. Thus, single-kinase inhibition suppresses partly, and only combined inhibition suppresses fully G₂-checkpoint activation. Interestingly, DNA-PKcs integrates with ATM/ATR in G₂-checkpoint control, but functions in its recovery in a dose-independent manner. Strikingly, irradiation during S-phase activates, independently of dose, an exclusively ATR-dependent G₂ checkpoint. Here, ATM couples with DNA-PKcs to regulate checkpoint recovery. In the present work, we extend these studies and investigate organization and functions of these PI3-kinases in the activation of the G₁ checkpoint in cells irradiated either in the G₀ or G₁ phase. We report that ATM is the sole regulator of the G₁ checkpoint after exposure to low IR doses. At high IR doses, ATM remains dominant, but contributions from ATR also become detectable and are associated with limited ATM/ATR-dependent end resection at DSBs. Under these conditions, only combined ATM + ATR inhibition fully abrogates checkpoint and resection. Contributions of DNA-PKcs and CHK2 to the regulation of the G₁ checkpoint are not obvious in these experiments and may be masked by the endpoint employed for checkpoint analysis and perturbations in normal progression through the cell cycle of cells exposed to DNA-PKcs inhibitors. The results broaden our understanding of organization throughout the cell cycle and adaptation with increasing IR dose of the ATM/ATR/DNA-PKcs module to regulate checkpoint responses. They emphasize notable similarities and distinct differences between G₁-, G₂- and S-phase checkpoint regulation that may guide DSB processing decisions.

A novel amplification gene PCI domain containing 2 (PCID2) promotes colorectal cancer through directly degrading a tumor suppressor promyelocytic leukemia (PML)

Using whole genome sequencing, PCI Domain Containing 2 (PCID2) was identified to be amplified in colorectal cancer (CRC). In this study, we investigated the expression, biological function, molecular mechanism, and clinical implication of PCID2 in CRC. PCID2 mRNA and protein expression were higher in CRC cells and tumor tissues compared to healthy colonic tissues. The copy number of PCID2 was positively correlated with its mRNA expression. Multivariate analysis revealed that PCID2 is an independent prognostic factor for CRC recurrence. Functional studies showed that PCID2 promoted cell growth, cell cycle progression, and cell migration/invasion, while apoptosis was suppressed. Moreover, PCID2 promoted xenograft growth and lung metastasis in nude mice. Using co-immunoprecipitation and mass spectroscopy, we showed that PCID2 binds to promyelocytic leukemia (PML), a tumor suppressor involved in non-canonical β-catenin signaling. PCID2 promoted the degradation of PML via poly-ubiquitination, which in turn, induced Wnt/β-catenin signaling while simultaneously repressing ARF-p53 pathway. Thus, these results demonstrated that PCID2 functions as an oncogene in CRC by enhancing canonical Wnt/β-catenin signaling and inhibition of CTNNB1-ARF-p53 axis. PCID2 promoted canonical Wnt/β-catenin signaling in CRC via degradation of PML. PCID2 may serve as an independent prediction marker for CRC recurrence.

Hypermethylation of heparanase 2 promotes colorectal cancer proliferation and is associated with poor prognosis

BACKGROUND

The epigenetic abnormality of tumor-associated genes contributes to the pathogenesis of colorectal carcinoma (CRC). However, methylation in colorectal cancer is still poorly characterized.

METHOD

By integration of DNA methylation data from the GEO database and gene expression data from The Cancer Genome Atlas database, the aberrantly methylated genes involved in CRC tumorigenesis were identified. Subsequent in vitro experiments further validated their role in CRC.

RESULTS

We performed integrative genomic analysis and identified HPSE2, a novel tumor suppressor gene that is frequently inactivated through promoter methylation in CRC. K-M survival analysis showed that hypermethylation-low expression of heparanase 2 (HPSE2) was related to poor patient prognosis. Overexpression of HPSE2 reduced cell proliferation in vivo and in vitro. HPSE2 could regulate the p53 signaling pathway to block the cell cycle in G1 phase.

CONCLUSION

HPSE2, a novel tumor suppressor gene that is frequently inactivated through promoter methylation in CRC. HPSE2 performs a tumor suppressive function by activating the p53/ p21 signaling cascade. The promoter hypermethylation of HPSE2 is a potential therapeutic target in patients with CRC, especially those with late-stage CRC.

The Effect of Neddylation Blockade on Slug-Dependent Cancer Cell Migration Is Regulated by p53 Mutation Status

Simple Summary

Neddylation is a process in which the small ubiquitin-like molecule NEDD8 is covalently conjugated to target proteins by sequential enzymatic reactions. Because neddylation plays critical roles in regulating cancer growth and migration, it is emerging as an effective therapeutic target. The major tumor suppressor protein p53 reduces cancer cell migration and is inhibited by neddylation. As p53 is lost or mutated in 50% of various cancer types, this study attempted to investigate how neddylation affects cancer cell migration according to p53 status. Neddylation blockade reduced or caused no change in migration of wild type or mutant p53 cancer cell lines. In contrast, neddylation blockade induced migration of p53-null cancer cell lines. These results were mediated by the differential effect of neddylation blockade on the epithelial–mesenchymal transition activator Slug according to p53 status. Thus, the p53 status of cancer cells should be considered when developing neddylation-targeted anticancer drugs.

Abstract

The tumor suppressor protein p53 is frequently inactivated in human malignancies, in which it is associated with cancer aggressiveness and metastasis. Because p53 is heavily involved in epithelial–mesenchymal transition (EMT), a primary step in cell migration, p53 regulation is important for preventing cancer metastasis. p53 function can be modulated by diverse post-translational modifications including neddylation, a reversible process that conjugates NEDD8 to target proteins and inhibits the transcriptional activity of p53. However, the role of p53 in cancer migration by neddylation has not been fully elucidated. In this study, we reported that neddylation blockade induces cell migration depending on p53 status, specifically via the EMT-promoting transcription factor Slug. In cancer cell lines expressing wild type p53, neddylation blockade increased the transcriptional activity of p53 and expression of its downstream genes p21 and MDM2, eventually promoting proteasomal degradation of Slug. In the absence of p53, neddylation blockade increased cell migration by activating the PI3K/Akt/mTOR/Slug signaling axis. Because mutant p53 was transcriptionally inactivated but maintained the ability to bind to Slug, neddylation blockade did not affect the migration of cells expressing mutant p53. Our findings highlight how the p53 expression status influences neddylation-mediated cell migration in multiple cancer cell lines via Slug.

Chrysophanol Regulates Cell Death, Metastasis, and Reactive Oxygen Species Production in Oral Cancer Cell Lines

Background

Oral cancer belongs to the class of head and neck cancers and can be life threatening if not diagnosed and treated early. Activation of cell death via apoptosis or reactive oxygen species (ROS) accumulation and inhibition of cell cycle progression, migration, and epithelial-to-mesenchymal transition (EMT) may be a good strategy to arrest the development of oral cancer. In this study, we analyzed the possible action of chrysophanol isolated from the rhizomes of Rheum palmatum on the oral cancer cell lines FaDu (human pharynx squamous cell carcinoma) and SAS (human tongue squamous carcinoma) by investigating whether chrysophanol could influence cell death.

Method

Cell viability was measured by using the MTT assay. For the detection of apoptosis, terminal deoxynucleotidyl transferase dUTP nick-end labeling (TUNEL) staining and subG1 population analysis were used. We also examined cell cycle progression and ROS levels by flow cytometry. Additionally, the expression of p53, p21, procaspase 3, cyclin D1, CDK4, cdc2, CDK2, E-cadherin, vimentin, and PCNA was evaluated by western blotting.

Conclusion

Chrysophanol has an anticancer effect on FaDu and SAS cell lines. There is an increase in subG1 accumulation, ROS production, and cell cycle G1 arrest after treatment with chrysophanol. On the other hand, chrysophanol inhibited cell migration/metastasis and EMT. We proposed that chrysophanol may be a good candidate compound on oral cancer treatment in the further.

STAT3 Regulates the Onset of Oxidant-induced Senescence in Lung Fibroblasts

Idiopathic pulmonary fibrosis (IPF) is a chronic lung disease of unknown cause with a median survival of only 3 years. Other investigators and we have shown that fibroblasts derived from IPF lungs display characteristics of senescent cells, and that dysregulated activation of the transcription factor signal transducer and activator of transcription 3 (STAT3) correlates with IPF progression. The question of whether STAT3 activation is involved in fibroblast senescence remains unanswered. We hypothesized that inhibiting STAT3 activation after oxidant-induced senescence would attenuate characteristics of the senescent phenotype. We aimed to characterize a model of oxidant-induced senescence in human lung fibroblasts and to determine the effect of inhibiting STAT3 activity on the development of senescence. Exposing human lung fibroblasts to 150 μM hydrogen peroxide (H₂O₂) resulted in increased senescence-associated β-galactosidase content and expression of p21 and IL-6, all of which are features of senescence. The shift into senescence was accompanied by an increase of STAT3 translocation to the nucleus and mitochondria. Additionally, Seahorse analysis provided evidence of increased mitochondrial respiration characterized by increased basal respiration, proton leak, and an associated increase in superoxide (O₂^-) production in senescent fibroblasts. Targeting STAT3 activity using the small-molecule inhibitor STA-21 attenuated IL-6 production, reduced p21 levels, decreased senescence-associated β-galactosidase accumulation, and restored normal mitochondrial function. The results of this study illustrate that stress-induced senescence in lung fibroblasts involves the activation of STAT3, which can be pharmacologically modulated.

Voltage-gated sodium channels β3 subunit promotes tumorigenesis in hepatocellular carcinoma by facilitating p53 degradation

The voltage-gated sodium channels (VGSCs) are aberrantly expressed in a variety of tumors and play an important role in tumor growth and metastasis. Here, we show that VGSCs auxiliary β3 subunit, encoded by the SCN3B gene, promotes proliferation and suppresses apoptosis in HepG2 cells by promoting p53 degradation. β3 significantly increases HepG2 cell proliferation, promotes tumor growth in mouse xenograft models, and suppresses senescence and apoptosis. We found that β3 knockdown stabilizes p53 protein, leading to potentiation of p53-induced cell cycle arrest, senescence, and apoptosis. Mechanistic studies revealed that β3 could bind to p53, promoting p53 ubiquitination and degradation by stabilizing the p53/MDM2 complex. Our results suggest that β3 is a novel negative regulator of p53 and a potential oncogenic factor.

The REGγ-Proteasome Regulates Spermatogenesis Partially by P53-PLZF Signaling

Development of spermatogonia and spermatocytes are the critical steps of spermatogenesis, impacting on male fertility. Investigation of the related regulators benefits the understanding of male reproduction. The proteasome system has been reported to regulate spermatogenesis, but the mechanisms and key contributing factors in vivo are poorly explored. Here we found that ablation of REGγ, a proteasome activator, resulted in male subfertility. Analysis of the mouse testes after birth showed there was a decreased number of PLZF⁺ spermatogonia and spermatocytes. Molecular analysis found that REGγ loss significantly increased the abundance of p53 protein in the testis, and directly repressed PLZF transcription in cell lines. Of note, allelic p53 haplodeficiency partially rescued the defects in spermatogenesis observed in REGγ-deficient mice. In summary, our results identify REGγ-p53-PLZF to be a critical pathway that regulates spermatogenesis and establishes a new molecular link between the proteasome system and male reproduction.

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REGγ loss results in male subfertility

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REGγ loss results in a decrease of spermatocytes and PLZF⁺ spermatogonial cells

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p53 protein, increased in REGγ^−/− mouse testes, represses PLZF expression

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Allelic p53 haplodeficiency partially rescues defects in REGγ^−/− mouse spermatogenesis

Mad1 destabilizes p53 by preventing PML from sequestering MDM2

Mitotic arrest deficient 1 (Mad1) plays a well-characterized role in the mitotic checkpoint. However, interphase roles of Mad1 that do not impact mitotic checkpoint function remain largely uncharacterized. Here we show that upregulation of Mad1, which is common in human breast cancer, prevents stress-induced stabilization of the tumor suppressor p53 in multiple cell types. Upregulated Mad1 localizes to ProMyelocytic Leukemia (PML) nuclear bodies in breast cancer and cultured cells. The C-terminus of Mad1 directly interacts with PML, and this interaction is enhanced by sumoylation. PML stabilizes p53 by sequestering MDM2, an E3 ubiquitin ligase that targets p53 for degradation, to the nucleolus. Upregulated Mad1 displaces MDM2 from PML, freeing it to ubiquitinate p53. Upregulation of Mad1 accelerates growth of orthotopic mammary tumors, which show decreased levels of p53 and its downstream effector p21. These results demonstrate an unexpected interphase role for Mad1 in tumor promotion via p53 destabilization.

Tannic acid-stabilized gold nano-particles are superior to native tannic acid in inducing ROS-dependent mitochondrial apoptosis in colorectal carcinoma cells via the p53/AKT axis

Tannic acid and AuNP-TA lead to death of colon cancer cells via the ROS/p53/Akt pathway, and AuNP-TA is more potent.

ER stress regulating protein phosphatase 2A-B56γ, targeted by hepatitis B virus X protein, induces cell cycle arrest and apoptosis of hepatocytes

Hepatitis B virus X (HBx) protein contributes to the progression of hepatitis B virus (HBV)-related hepatic injury and diseases, but the exact mechanism remains unclear. Protein phosphatase 2 A (PP2A) is a major serine/threonine phosphatase involved in regulating many cellular phosphorylation signals that are important for regulation of cell cycle and apoptosis. Does HBx target to PP2A-B56γ and therefore affect HBx-induced hepatotoxicity? In the present study, the expression of B56γ positively correlated with the level of HBx in HBV-infected primary human hepatocytes in human-liver-chimeric mice, HBx-transgenic mice, HBV-infected cells, and HBx-expressing hepatic cells. B56γ promoted p53/p21-dependent cell cycle arrest and apoptosis. Mechanistically, B56γ was transactivated by AP-1, which was under the regulation of endoplasmic reticulum (ER) stress induced CREBH signaling in HBx-expressing hepatic cells. B56γ dephosphorylated p-Thr55-p53 to trigger p53/p21 pathway-dependent cell cycle G1 phase arrest, resulting in apoptosis of hepatic cells. In conclusion, this study provides a novel insight into a mechanism of B56γ mediating cell cycle arrest and apoptosis of HBx-expressing hepatic cells and a basis for B56γ being a potential therapeutic target for HBV-infected hepatic cells.

MDGA2 is a novel tumour suppressor cooperating with DMAP1 in gastric cancer and is associated with disease outcome

BACKGROUND

Using the promoter methylation assay, we have shown that MDGA2 (MAM domain containing glycosylphosphatidylinositol anchor 2) is preferentially methylated in gastric cancer. We analysed its biological effects and prognostic significance in gastric cancer.

METHODS

MDGA2 methylation status was evaluated by combined bisulfite restriction analysis and bisulfite genomic sequencing. The effects of MDGA2 re-expression or knockdown on cell proliferation, apoptosis and the cell cycle were determined. MDGA2 interacting protein was identified by mass spectrometry and MDGA2-related cancer pathways by reporter activity and PCR array analyses. The clinical impact of MDGA2 was assessed in 218 patients with gastric cancer.

RESULTS

MDGA2 was commonly silenced in gastric cancer cells (10/11) and primary gastric cancers due to promoter hypermethylation. MDGA2 significantly inhibited cell proliferation by causing G1-S cell cycle arrest and inducing cell apoptosis in vitro, and suppressed xenograft tumour growth in both subcutaneous and orthotopic xenograft mouse models (both p<0.001). The anti-tumorigenic effect of MDGA2 was mediated through direct stabilising of DNA methyltransferase 1 associated protein 1 (DMAP1), which played a tumour suppressive role in gastric cancer. This interaction activated their downstream key elements of p53/p21 signalling cascades. Moreover, promoter methylation of MDGA2 was detected in 62.4% (136/218) of gastric cancers. Multivariate analysis showed that patients with MDGA2 hypermethylation had a significantly decreased survival (p=0.005). Kaplan-Meier survival curves showed that MDGA2 hypermethylation was significantly associated with shortened survival in patients with early gastric cancer.

CONCLUSIONS

MDGA2 is a critical tumour suppressor in gastric carcinogenesis; its hypermethylation is an independent prognostic factor in patients with gastric cancer.

Novel Pactamycin Analogs Induce p53 Dependent Cell-Cycle Arrest at S-Phase in Human Head and Neck Squamous Cell Carcinoma (HNSCC) Cells

Pactamycin, although putatively touted as a potent antitumor agent, has never been used as an anticancer drug due to its high cytotoxicity. In this study, we characterized the effects of two novel biosynthetically engineered analogs of pactamycin, de-6MSA-7-demethyl-7-deoxypactamycin (TM-025) and 7-demethyl-7-deoxypactamycin (TM-026), in head and neck squamous cell carcinoma (HNSCC) cell lines SCC25 and SCC104. Both TM-025 and TM-026 exert growth inhibitory effects on HNSCC cells by inhibiting cell proliferation. Interestingly, unlike their parent compound pactamycin, the analogs do not inhibit synthesis of nascent protein in a cell-based assay. Furthermore, they do not induce apoptosis or autophagy in a dose- or a time-dependent manner, but induce mild senescence in the tested cell lines. Cell cycle analysis demonstrated that both analogs significantly induce cell cycle arrest of the HNSCC cells at S-phase resulting in reduced accumulation of G₂/M-phase cells. The pactamycin analogs induce expression of cell cycle regulatory proteins including master regulator p53, its downstream target p21^Cip1/WAF1, p27^kip21, p19, cyclin E, total and phospho Cdc2 (Tyr15) and Cdc25C. Besides, the analogs mildly reduce cyclin D1 expression without affecting expression of cyclin B, Cdk2 and Cdk4. Specific inhibition of p53 by pifithrin-α reduces the percentage of cells accumulated in S-phase, suggesting contribution of p53 to S-phase increase. Altogether, our results demonstrate that Pactamycin analogs TM-025 and TM-026 induce senescence and inhibit proliferation of HNSCC cells via accumulation in S-phase through possible contribution of p53. The two PCT analogs can be widely used as research tools for cell cycle inhibition studies in proliferating cancer cells with specific mechanisms of action.

Melatonin attenuates methamphetamine-induced inhibition of proliferation of adult rat hippocampal progenitor cells in vitro

Methamphetamine (METH) is an extremely addictive stimulatory drug. A recent study suggested that METH may cause an impairment in the proliferation of hippocampal neural progenitor cells, but the underlying mechanism of this effect remains unknown. Blood and cerebrospinal levels of melatonin derive primarily from the pineal gland, and that performs many biological functions. Our previous study demonstrated that melatonin promotes the proliferation of progenitor cells originating from the hippocampus. In this study, hippocampal progenitor cells from adult Wistar rats were used to determine the effects of METH on cell proliferation and the mechanisms underlying these effects. We investigated the effects of melatonin on the METH-induced alteration in cell proliferation. The results demonstrated that 500 μm METH induced a decrease (63.0%) in neurosphere cell proliferation and altered the expression of neuronal phenotype markers in the neurosphere cell population. Moreover, METH induced an increase in the protein expression of the tumor suppressor p53 (124.4%) and the cell cycle inhibitor p21(CIP) (1) (p21) (128.1%), resulting in the accumulation of p21 in the nucleus. We also found that METH altered the expression of the N-methyl-d-aspartate (NMDA) receptor subunits NR2A (79.6%) and NR2B (126.7%) and Ca(2+) /calmodulin-dependent protein kinase II (CAMKII) (74.0%). In addition, pretreatment with 1 μm melatonin attenuated the effects induced by METH treatment. According to these results, we concluded that METH induces a reduction in cell proliferation by upregulating the cell cycle regulators p53/p21 and promoting the accumulation of p21 in the nucleus and that melatonin ameliorates these negative effects of METH.

Dinuclear Cu(I) complexes of pyridyl-diazadiphosphetidines and aminobis(phosphonite) ligands: synthesis, structural studies and antiproliferative activity towards human cervical, colon carcinoma and breast cancer cells

The copper(i) complexes containing phosphorus donor ligands such as diazadiphosphetidine, cis-{(o-OCH2C5H4N)P(μ-N(t)Bu)}2 (1) and aminobis(phosphonite), C6H5N{P(OC6H3(OMe-o)(C3H5-p))2}2 (2, PNP), have been synthesized. Treatment of 1 with copper iodide afforded the 1D coordination polymer [{Cu(μ-I)}2{(o-OCH2C5H4N)P(μ-N(t)Bu)}2]n (3). Treatment of 3 with 2,2'-bipyridine (bpy) and 1,10-phenanthroline (phen) produced mixed-ligand complexes [(L)2Cu2{(o-OCH2C5H4N)P(μ-N(t)Bu)}2][I]2 (4 L = bpy; 5 L = phen) in good yields. The reaction of 2 with copper iodide yielded a rare tetranuclear copper complex [(CuI)2C6H5N(PR2)2]2 (6), which on subsequent treatment with various pyridyl ligands produced binuclear complexes [{Cu(μ-I)(py)}2(μ-PNP)] (7), [Cu2(μ-I)(bpy)2(μ-PNP)]I (8), [Cu2(μ-I)I(bpy)(μ-PNP)] (9), [Cu2(phen)(bpy)(μ-PNP)](OTf)2 (10), [Cu2(μ-I)I(phen)(μ-PNP)] (11) and [Cu2(μ-I)(phen)2(μ-PNP)]I (12), in an almost quantitative yield. The new copper(i) complexes (4, 5 and 7-12) were tested for anti-cancer activity against three human tumor cell lines. Compounds 5, 10 and 12 showed in vitro antitumor activity 5-7 fold higher than cisplatin, the most used anticancer drug. These three most potent compounds (5, 10 and 12) were chosen for detailed study to understand their mechanism of action. The copper(i) compounds studied in the present investigation were found to inhibit tumor cell growth by arresting cells at the S-phase of the cell cycle. The characteristic nuclear morphology of treated cells showed signs of DNA damage. The experimental evidence clearly indicated that these compounds initiated apoptosis, which is mediated through the p53 pathway.

Nek4 regulates entry into replicative senescence and the response to DNA damage in human fibroblasts

When explanted into culture, normal human cells exhibit a finite number of cell divisions before entering a proliferative arrest termed replicative senescence. To identify genes essential for entry into replicative senescence, we performed an RNA interference (RNAi)-based loss-of-function screen and found that suppression of the Never in Mitosis Gene A (NIMA)-related protein kinase gene NEK4 disrupted timely entry into senescence. NEK4 suppression extended the number of population doublings required to reach replicative senescence in several human fibroblast strains and resulted in decreased transcription of the cyclin-dependent kinase inhibitor p21. NEK4-suppressed cells displayed impaired cell cycle arrest in response to double-stranded DNA damage, and mass spectrometric analysis of Nek4 immune complexes identified a complex containing DNA-dependent protein kinase catalytic subunit [DNA-PK(cs)], Ku70, and Ku80. NEK4 suppression causes defects in the recruitment of DNA-PK(cs) to DNA upon induction of double-stranded DNA damage, resulting in reduced p53 activation and H2AX phosphorylation. Together, these observations implicate Nek4 as a novel regulator of replicative senescence and the response to double-stranded DNA damage.

Transient dephosphorylation of p53 serine 376 as an early response to ionizing radiation

In a previous paper we reported that the cytoplasmic sequestered p53 in cells of the SK-N-SH neuroblastoma cell line could be induced to translocate to the nucleus by exposure to ionizing radiation. We have extended these studies to determine the fate of p53 in HCT116 colorectal carcinoma cells where constitutive p53 protein resides in the nucleus. A continuous increase in the nuclear p53 protein was observed in irradiated cells beginning 1 h after irradiation that persisted for 8 h. Surprisingly, immunofluorescence microscopy revealed a transient, rapid and sensitive increase in a radiation-induced nuclear dephosphorylated p53 using antibody PAb421, which detects p53 when serine 376 is dephosphorylated. The PAb421 epitope was detectable after exposure to radiation doses as low as 0.5 cGy and was 10 to 20 times more sensitive compared to detection of p53 protein levels. The results are consistent with a radiation-induced, sensitive and rapid dephosphorylation of p53 at serine 376. The rapid increase in the nuclear PAb421 epitope was blocked by the protein serine phosphatase inhibitor calyculin A but was not blocked by the protein synthesis inhibitor cycloheximide, suggesting that serine 376 was dephosphorylated by protein serine phosphatase 1 or 2A acting on pre-existing p53 protein. The data suggest that dephosphorylation of serine 376 on constitutive nuclear p53 is a sensitive and early signaling event in the response of cells to DNA damage induced by ionizing radiation.

Involvement of Oct-1 in the regulation of CDKN1A in response to clinically relevant doses of ionizing radiation

CDKN1A is a cyclin-dependent kinase inhibitor that plays a critical role in cell cycle checkpoint regulation. It is transcriptionally induced by TP53 (p53) following exposure to ionizing radiation (IR). Induction of CDKN1A after irradiation is closely related to IR-sensitivity of tumor cells, but the underlying mechanisms remain obscure because conventional reporter gene systems respond poorly to IR unless hyperlethal doses are used. Here, we performed a promoter analysis of the CDKN1A gene following irradiation with clinically relevant doses of IR using the adeno-associated virus-mediated reporter system which we have recently shown to be highly responsive to IR. We demonstrate that there are regulatory elements at -1.1 kb, -1.4 kb, and -1.8 kb, and deletion of these elements attenuate induction of the CDKN1A gene promoter in response to 0.2-2.0 Gy of IR. EMSA and ChIP assays showed that Oct-1 binds constitutively to the elements at -1.1 kb and -1.8 kb. Functional involvement of Oct-1 was confirmed by RNA interference targeting the Oct-1 gene, which suppressed both the basal and IR-inducible components of the CDKN1A expression. Thus, our results reveal that Oct-1 is crucial to the TP53-mediated regulation of the CDKN1A gene promoter following exposure to clinically relevant doses of IR.

Apigenin causes G(2)/M arrest associated with the modulation of p21(Cip1) and Cdc2 and activates p53-dependent apoptosis pathway in human breast cancer SK-BR-3 cells

We studied the effects of apigenin on the cell cycle distribution and apoptosis of human breast cancer cells and explored the mechanisms underlying these effects. We first investigated the antiproliferative effects in SK-BR-3 cells exposed to between 1 and 100 microM apigenin for 24, 48 and 72 h. Apigenin significantly inhibited cell proliferation at concentrations over 50 microM, regardless of exposure time (P<.05), and resulted in significant cell cycle arrest in the G(2)/M phase after 48 h of treatment at high concentrations (50 and 100 microM; P<.05). To investigate the regulatory proteins of cell cycle arrest affected by apigenin, we treated cells with 50 and 100 microM apigenin for 72 h. Apigenin caused a slight decrease in cyclin D and cyclin E expression, with no change in CDK2 and CDK4. In addition, the apigenin-induced accumulation of the cell population in the G(2)/M phase resulted in a decrease in CDK1 together with cyclin A and cyclin B. In an additional study, apigenin also increased the accumulation of p53 and further enhanced the level of p21(Cip1), with no change in p27(Kip1). The expression of Bax and cytochrome c of p53 downstream target was increased markedly at high concentration treatment over 50 microM apigenin. Based on our findings, the mechanism by which apigenin causes cell cycle arrest via the regulation of CDK1 and p21(Cip1) and induction of apoptosis seems to be involved in the p53-dependent pathway.

PKBalpha/Akt1 acts downstream of DNA-PK in the DNA double-strand break response and promotes survival

Protein kinase B (PKB/Akt) is a well-established regulator of several essential cellular processes. Here, we report a route by which activated PKB promotes survival in response to DNA insults in vivo. PKB activation following DNA damage requires 3-phosphoinositide-dependent kinase 1 (PDK1) and DNA-dependent protein kinase (DNA-PK). Active PKB localizes in the nucleus of gamma-irradiated cells adjacent to DNA double-strand breaks, where it colocalizes and interacts with DNA-PK. Levels of active PKB inversely correlate with DNA damage-induced apoptosis. A significant portion of p53- and DNA damage-regulated genes are misregulated in cells lacking PKBalpha. PKBalpha knockout mice show impaired DNA damage-dependent induction of p21 and increased tissue apoptosis after single-dose whole-body irradiation. Our findings place PKB downstream of DNA-PK in the DNA damage response signaling cascade, where it provides a prosurvival signal, in particular by affecting transcriptional p21 regulation. Furthermore, this function is apparently restricted to the PKBalpha isoform.

Apoptosis of murine melanoma cells induced by heavy-ion radiation combined with Tp53 gene transfer

PURPOSE

To investigate the effect of exogenous wild type p53 (Tp53) on murine melanoma B16 cell apoptosis induced by carbon-ion beam (C-beam) irradiation.

MATERIALS AND METHODS

The murine cell line B16, which has wild-type Tp53 gene status was studied, as well as B16 cells transfected with an adenoviral vector containing the wild-type Tp53 gene (B16/Tp53). Cells were irradiated with C-beam and assayed for cell survival (colony-forming assay), cellular morphology (acridine orange assay), the frequency of apoptotic cells (fluorescence microscopy) and protein expression (Western blot analysis).

RESULTS

The radiosensitivity of B16/Tp53 cells was significantly higher than that of B16 cells. In contrast with Tp53 transfer alone, the combination of C-beam with Tp53 transfer induced a higher proportion of apoptotic cells and micronuclei. After C-beam irradiation, there was no significant increased expression of the cyclin-dependent kinase inhibitor p21 and Tp53 in B16/Tp53 cells compared to B16 cells, but a decreased expression of murine double minute-2 (Mdm2) was observed.

CONCLUSION

The results of this study suggest the potential application of C-beam combined with Tp53 in the treatment of melanoma in human patients.

G alpha 12/13 basally regulates p53 through Mdm4 expression

G alpha(12/13), which belongs to the G alpha(12) family, participates in the regulation of diverse physiologic processes. In view of the control of G alpha(12/13) in cell proliferation, this study investigated the role of G alpha(12/13) in the regulation of p53 and mdm4. Immunoblotting and immunocytochemistry revealed that p53 was expressed in control embryonic fibroblasts and was largely localized in the nuclei. G alpha(12) deficiency decreased p53 levels and its DNA binding activity, accompanying p21 repression with Bcl(2) induction, whereas G alpha(13) deficiency exerted weak effects. G alpha(12) or G alpha(13) deficiency did not change p53 mRNA expression. ERK1/2 or Akt was not responsible for p53 repression due to G alpha(12) deficiency. Mdm4, a p53-stabilizing protein, was repressed by G alpha(12) deficiency and to a lesser extent by G alpha(13) deficiency, whereas mdm2, PTEN, beta-catenin, ATM, and Chk2 were unaffected. p53 accumulation by proteasomal inhibition during G alpha(12) deficiency suggested the role of G alpha(12) in p53 stabilization. Constitutively active G alpha(12) (G alpha(12)QL) or G alpha(13) (G alpha(13)QL) promoted p53 accumulation with mdm4 induction in MCF10A cells. p53 accumulation by mdm4 overexpression, but no mdm4 induction by p53 overexpression, and small interfering RNA knockdown verified the regulatory role of mdm4 for p53 downstream of G alpha(12/13). In control or G alpha(12)/G alpha(13)-deficient cells, genotoxic stress led to p53 accumulation. At concentrations increasing the flow cytometric pre-G(1) phase, doxorubicin or etoposide treatment caused serine phosphorylations in G alpha(12)-/- or G alpha(12/13)-/- cells, but did not induce mdm4. G alpha(12/13)QL transfection failed to phosphorylate p53 at serines. Our results indicate that G alpha(12/13) regulate basal p53 levels via mdm4, which constitutes a cell signaling pathway distinct from p53 phosphorylations elicited by genotoxic stress.

Cancer therapy-induced residual bone marrow injury-Mechanisms of induction and implication for therapy

Bone marrow (BM) suppression is the important dose-limiting side effect of chemotherapy and radiotherapy for cancer. Although acute myelosuppression is an immediate concern for patients undergoing cancer therapy, its management has been improved significantly in recent years by the use of various hematopoietic growth factors. However, many patients receiving chemotherapy and/or ionizing radiation (IR) also develop residual (or long-term) BM injury (a sustained decrease in HSC reserves due to an impairment in HSC self-renewal) after the recovery from acute myelosuppression. Unlike acute myelosuppression, residual BM injury is latent and long lasting and shows little tendency for recovery. Following additional hematopoietic stress such as subsequent cycles of consolidation cancer treatment or autologous BM transplantation, residual BM injury can deteriorate to become a hypoplastic or myelodysplastic syndrome. This article review some of the new developments in elucidating the cellular and molecular mechanisms whereby chemotherapy and radiotherapy cause residual BM injury. Particularly, we discuss the role of induction of hematopoietic stem cell (HSC) senescence via the p53-p21(Cip1/Waf1) and/or p16(Ink4a)-RB pathways in the induction of the injury and the therapeutic potential of molecularly targeting these pathways for amelioration of chemotherapy- and radiotherapy-induced long-term BM toxicity.

p21CDKN1A allows the repair of replication-mediated DNA double-strand breaks induced by topoisomerase I and is inactivated by the checkpoint kinase inhibitor 7-hydroxystaurosporine

This study provides evidence for the importance of p21(CDKN1A) for the repair of replication-mediated DNA double-strand breaks (DSBs) induced by topoisomerase I. We report that defects of p21(CDKN1A) and p53 enhance camptothecin-induced histone H2AX phosphorylation (gammaH2AX), a marker for DNA DSBs. In human colon carcinoma HCT116 cells with wild-type (wt) p53, gammaH2AX reverses after camptothecin removal. By contrast, gammaH2AX increases after camptothecin removal in HCT116 cells deficient for p53 (p53-/-) or p21(CDKN1A) (p21-/-) as the cells reach the late-S and G2 phases. Since p21-/- cells exhibit similar S-phase arrest as wt cells in response to camptothecin and aphidicolin does not abrogate the enhanced gammaH2AX formation in p21-/- cells, we conclude that enhanced gammaH2AX formation in p21-/- cells is not due to re-replication. The cell cycle checkpoint abrogator and Chk1/Chk2 inhibitor 7-hydroxystaurosporine (UCN-01) also increases camptothecin-induced gammaH2AX formation and inhibits camptothecin-induced p21(CDKN1A) upregulation in HCT116 wt cells. TUNEL (terminal deoxynucleotidyl transferase (TdT)-mediated dUTP-biotin nick end labeling) assays demonstrate that gammaH2AX formation in late S and G2 cells following CPT treatment corresponds to DNA breaks. However, these breaks are not related to apoptotic DNA fragmentation. We propose that p21(CDKN1A) prevents the collapse of replication forks damaged by stabilized topoisomerase I cleavage complexes.

Tachpyridine, a metal chelator, induces G2 cell-cycle arrest, activates checkpoint kinases, and sensitizes cells to ionizing radiation

Iron is critical for cell growth and proliferation. Iron chelators are being explored for a number of clinical applications, including the treatment of neurodegenerative disorders, heart disease, and cancer. To uncover mechanisms of action of tachpyridine, a chelator currently undergoing preclinical evaluation as an anticancer agent, cell-cycle analysis was performed. Tachpyridine arrested cells at G2, a radiosensitive phase of the cell cycle, and enhanced the sensitivity of cancer cells but not nontransformed cells to ionizing radiation. G2 arrest was p53 independent and was accompanied by activation of the checkpoint kinases CHK1 and CHK2. G2 arrest was blocked by UCN-01, a CHK1 inhibitor, but proceeded in CHK2 knock-out cells, indicating a critical role for CHK1 in G2 arrest. Tachpyridine-induced cell-cycle arrest was abrogated in cells treated with caffeine, an inhibitor of the ataxia-telangiectasia mutated/ataxia-telangiectasia-mutated and Rad3-related (ATM/ATR) kinases. Further, G2 arrest proceeded in ATM-deficient cells but was blocked in ATR-deficient cells, implicating ATR as the proximal kinase in tachpyridine-mediated G2 arrest. Collectively, our results suggest that iron chelators may function as antitumor and radioenhancing agents and uncover a previously unexplored activity of iron chelators in activation of ATR and checkpoint kinases.

Glioblastoma cells deficient in DNA-dependent protein kinase are resistant to cell death

DNA-dependent protein kinase (DNA-PK), a nuclear serine/threonine kinase, is responsible for the DNA double-strand break repair. Cells lacking or with dysfunctional DNA-PK are often associated with mis-repair, chromosome aberrations, and complex exchanges, all of which are known to contribute to the development of human cancers including glioblastoma. Two human glioblastoma cell lines were used in the experiment, M059J cells lacking the catalytic subunit of DNA-PK, and their isogenic but DNA-PK proficient counterpart, M059K. We found that M059K cells were much more sensitive to staurosporine (STS) treatment than M059J cells, as demonstrated by MTT assay, TUNEL detection, and annexin-V and propidium iodide (PI) staining. A possible mechanism responsible for the different sensitivity in these two cell lines was explored by the examination of Bcl-2, Bax, Bak, and Fas. The cell death stimulus increased anti-apoptotic Bcl-2 and decreased pro-apoptotic Bcl-2 members (Bak and Bax) and Fas in glioblastoma cells deficient in DNA-PK. Activation of DNA-PK is known to promote cell death of human tumor cells via modulation of p53, which can down-regulate the anti-apoptotic Bcl-2 member proteins, induce pro-apoptotic Bcl-2 family members and promote a Bax-Bak interaction. Our experiment also demonstrated that the mode of glioblastoma cell death induced by STS consisted of both apoptosis and necrosis and the percentage of cell death in both modes was similar in glioblastoma cell lines either lacking DNA-PK or containing intact DNA-PK. Taken together, our findings suggest that DNA-PK has a positive role in the regulation of apoptosis in human glioblastomas. The aberrant expression of Bcl-2 family members and Fas was, at least in part, responsible for decreased sensitivity of DNA-PK deficient glioblastoma cells to cell death stimuli.

Genetic and epigenetic features in radiation sensitivity

Recent progress especially in the field of gene identification and expression has attracted greater attention to genetic and epigenetic susceptibility to cancer, possibly enhanced by ionising radiation. It has been proposed that the occurrence and severity of the adverse reactions to radiation therapy are also influenced by such genetic susceptibility. This issue is especially important for radiation therapists since hypersensitive patients may suffer from adverse effects in normal tissues following standard radiation therapy, while normally sensitive patients could receive higher doses of radiation offering a better likelihood of cure for malignant tumours. This paper, the first of two parts, reviews the main mechanisms involved in cell response to ionising radiation. DNA repair machinery and cell signalling pathways are considered and their role in radiosensitivity is analysed. The implication of non-targeted and delayed effects in radiosensitivity is also discussed.

Role of DNA–PK in the cellular response to DNA double-strand breaks

The DNA-dependent protein kinase (DNA-PK) plays a critical role in DNA double-strand break (DSB) repair and in V(D)J recombination. DNA-PK also plays a very important role in triggering apoptosis in response to severe DNA damage or critically shortened telomeres. Paradoxically, components of the DNA-PK complex are present at the mammalian telomere where they function in capping chromosome ends to prevent them from being mistaken for double-strand breaks. In addition, DNA-PK appears to be involved in mounting an innate immune response to bacterial DNA and to viral infection. As DNA-PK localizes very rapidly to DNA breaks and phosphorylates itself and other damage-responsive proteins, it appears that DNA-PK serves as both a sensor and a transducer of DNA-damage signals. The many roles of DNA-PK in the mammalian cell are discussed in this review with particular emphasis on recent advances in our understanding of the phosphorylation events that take place during the activation of DNA-PK at DNA breaks.

P53 and radiation responses

Cells have evolved elaborate mechanisms (checkpoints) to monitor genomic integrity in order to ensure the high-fidelity transmission of genetic information. Cells harboring defects in checkpoint pathways respond to DNA damage improperly, which in turn may enhance the rate of cancer development. Ionizing radiation (IR) primarily leads to double-strand DNA breaks (DSBs), which activate DNA damage checkpoints to initiate signals ultimately leading to a binary decision between cell death and cell survival. TP53 has been recognized as an important checkpoint protein, functioning mainly through transcriptional control of target genes that influence multiple response pathways and leading to the diversity of responses to IR in mammalian cells. We review how the tumor suppressor P53 is involved in the complex response to IR to enforce the cell's fate to live by inducing the growth arrest coupled to DNA damage repair or to die by inducing irreversible growth arrest or apoptosis. Moreover, recent insights have emerged in our understanding of how P53 modulates radiosensitivity in tissues following IR as well as its role in sensitizing cells to chemo- and radiotherapy. The P53 pathway remains an attractive target for exploitation in the war on cancer.

Allotype imbalance or microsatellite mutation in low-grade soft tissue sarcomas of the extremities in adults

The ability to repair DNA double-strand breaks is essential to maintain chromosomal stability. Virtually all soft tissue sarcomas contain chromosomal instabilities, including clonal aberrations and cytogenetic aberrations. However, the relevance of DNA-dependent protein kinase (DNA-PK) in the pathogenesis of soft tissue sarcoma has not been clarified. The main aim of this work is to compare the prognostic impact of genotypic imbalance in low-grade soft tissue sarcomas of the extremities, and to correlate this with the translational level of DNA-PK. This study investigated 28 adult low-grade malignant spindle cell tumours of the extremities, predominantly fibrosarcomas, for loss of heterozygosity (LOH) and microsatellite mutation on flanking regions of each DNA-PK subunit, with identical immunophenotypes. Twelve different polymorphic markers flanking the specific loci of three subunits comprise the genetic map of DNA-PK, at 22q13, 2q35, and 8q11. Translational activity was also analysed by western blot and conventional immunohistochemistry. The overall sarcoma 5-year survival rate was 61.7%. LOH was identified in the specific coding region of DNA-PK in 39.29% for the DNA-PK catalytic subunit (cs), 17.86% for Ku70, and only 7.14% for Ku80. A positive LOH for DNA-PKcs was shown to be a significant factor for poor survival (log rank test p = 0.0160). Immunoreactivity and immunoblot results correlated with the loss of DNA-PKcs allotype in soft tissue sarcoma (Fisher's exact test p = 0.0037). Ku70 and DNA-PKcs were almost identical in terms of immunoreactivity. In conclusion, whereas microsatellite mutation seems an uncommon event during the evolution of low-grade fibrosarcoma of the extremities in adults, the loss of DNA-PKcs defines a biologically more aggressive subset.

p53 expression in human rectal tissue after radiotherapy: upregulation in normal mucosa versus functional loss in rectal carcinomas

PURPOSE

In vitro, ionizing radiation of epithelial cells leads to upregulation of wild-type p53 and subsequent induction of p21(waf1). The effect of radiotherapy (RT) on the expression of these proteins in patients is unknown. We assessed the influence of RT on the expression of p53 and p21(waf1) in normal mucosa and rectal carcinomas in vivo.

METHODS

Tumor and normal tissue samples were derived from rectal cancer patients randomized in a clinical trial in which the value of preoperative RT was evaluated. p53 and p21(waf1) expression was determined in 51 irradiated and 52 nonirradiated patients using immunohistochemistry.

RESULTS

In normal mucosa, both p53 and p21(waf1) were strongly upregulated after RT compared with the expression in unirradiated normal tissue (p <0.001). In tumor cells, no significant difference in the expression of p53 or p21(waf1) was found in the irradiated vs. nonirradiated group. In the few rectal tumors with wild-type p53, induction of p53 after RT did not necessarily lead to upregulation of p21(waf1).

CONCLUSION

These findings demonstrate that in normal mucosa, a functional p53-p21(waf1) pathway is present, whereas in tumor cells it is defective in almost all cases because of either p53 mutation or down- or upstream disruption in tumors with wild-type p53. Therefore, we believe that the role of p53 expression as a single prognostic marker in rectal cancer needs reconsideration.

Studies on p53 and Bax protein expression in Cockayne syndrome cells after UV irradiation and interferon-beta treatment

Human interferon (HuIFN) has a protective effect against ultraviolet (UV)-induced killing of Cockayne syndrome (CS) and xeroderma pigmentosum (XP) cells. Irradiation with ultraviolet (UV) resulted in nuclear accumulation of p53 in normal human fibroblast cells, and this accumulation was suppressed by treatment with HuIFN-beta. On the other hand, a large amount of p53 was found in both nuclear and cytoplasmic fractions of one SV40-transformed XP and two SV40-transformed CS cell strains irrespective of UV irradiation. Treatment with HuIFN-beta reduced the level of pro-apoptotic Bax protein without suppression of nuclear accumulation of p53 in the CS cells but not in the XP cells. These findings suggest that there are different mechanisms of UV-refractoriness caused by HuIFN-beta in UV-sensitive CS and XP cells.

Role of DNA-dependent protein kinase in neuronal survival

DNA-dependent protein kinase (DNA-PK) is a DNA repair enzyme composed of a DNA-binding component called Ku70/80 and a catalytic subunit called DNA-PKcs. Many investigators have utilized DNA-PKcs-deficient cells and cell lines derived from severe combined immunodeficiency (scid) mice to study DNA repair and apoptosis. However, little is known about the CNS of these mice. This study was carried out using primary neuronal cultures derived from the cerebral hemispheres of new-born wild-type and scid mice to investigate the effects of loss of DNA-PK function on neuronal maturation and survival. Purified neuronal cultures developed comparably in terms of neurite formation and expression of neuronal markers, but scid cultures showed a significant increase in the percentage of dying cells. Furthermore, when apoptosis was induced by staurosporine, scid neurons died more rapidly and in higher numbers. Apoptotic scid neurons exhibited nuclear condensation, DNA fragmentation and caspase-3 activation, but treatment with the general caspase inhibitor, N-benzyloxycarbonyl-Val-Ala-Asp-(O-methyl) fluoromethyl ketone did not prevent staurosporine-induced apoptosis. We conclude that a DNA-PK deficiency in cultured scid neurons may cause an accumulation of DNA damage and increased susceptibility to caspase-independent forms of programmed cell death.

High-intensity p38 kinase activity is critical for p21(cip1) induction and the antiproliferative function of G(i) protein-coupled receptors

G protein-coupled receptors can stimulate the p38 kinase cascade, but the effect this has on cell growth remains poorly characterized. Here we show human somatostatin sst(2) and sst(4) receptors inhibit basic fibroblast growth factor (bFGF)-induced proliferation, via a mechanism that was blocked by the p38 inhibitor PD 169316. The sst(4) receptor could also induce a proliferative activity in the absence of bFGF, which was unaffected by PD 169316. In contrast, the sst(3) receptor had no effect on basal cell growth or on the proliferation evoked by bFGF. The extracellular signal-regulated kinase activity stimulated by the sst(3) receptor was transient in duration compared with a sustained activity induced by the sst(2) and sst(4) receptors and which was critical for the proliferative response of the latter receptor. In addition, activated sst(2) and sst(4) but not sst(3) receptors evoked a prolonged phosphorylation of p38 that was amplified by bFGF. The accumulation of the cell cycle inhibitor p21(cip1) was only apparent after sst(2) and sst(4) receptor activation in the presence of bFGF, which was sensitive to PD 169316 or pertussis toxin. Thus, the contrasting antiproliferative effects evoked by the human sst(2), sst(3), and sst(4) receptors can be accounted for by their differential abilities to activate p38. This activity is critical for p21(cip1) induction, blockade of entry into S phase, as indicated by the lack of retinoblastoma protein phosphorylation, and the associated antiproliferative activity of somatostatin. Furthermore, by changing the intracellular signaling threshold of p38 through cooperative effects of somatostatin and bFGF, the sst(4) receptor can mediate opposing effects on cell proliferation.

Hippocampal neurons of mice deficient in DNA-dependent protein kinase exhibit increased vulnerability to DNA damage, oxidative stress and excitotoxicity

DNA damage has been documented in neurodegenerative conditions ranging from Alzheimer's disease to stroke. DNA-dependent protein kinase (DNA-PK) is involved in V(D)J recombination and DNA double strand break repair, and may play a role in cell death induced by DNA damage. We now report that cultured hippocampal neurons from severe combined immunodeficient (scid) mice which lack DNA-PK activity are hypersensitive to apoptosis induced by exposure to topoisomerase inhibitors, amyloid beta peptide (A beta) and glutamate. A similar increased vulnerability of hippocampal CA1 and CA3 neurons was observed in adult scid mice after kainate-induced seizures. Our results suggest that DNA-PK activity is important for neuron survival under conditions that may occur in neurological disorders.

Ataxia-telangiectasia: chronic activation of damage-responsive functions is reduced by alpha-lipoic acid

Cells from patients with the genetic disorder ataxia-telangiectasia (A-T) are hypersensitive to ionizing radiation and radiomimetic agents, both of which generate reactive oxygen species capable of causing oxidative damage to DNA and other macromolecules. We describe in A-T cells constitutive activation of pathways that normally respond to genotoxic stress. Basal levels of p53 and p21(WAF1/CIP1), phosphorylation on serine 15 of p53, and the Tyr15-phosphorylated form of cdc2 are chronically elevated in these cells. Treatment of A-T cells with the antioxidant alpha-lipoic acid significantly reduced the levels of these proteins, pointing to the involvement of reactive oxygen species in their chronic activation. These findings suggest that the absence of functional ATM results in a mild but continuous state of oxidative stress, which could account for several features of the pleiotropic phenotype of A-T.

Human DNA repair genes

DNA repair systems are essential for the maintenance of genome integrity. Consequently, the disregulation of repair genes can be expected to be associated with significant, detrimental health effects, which can include an increased prevalence of birth defects, an enhancement of cancer risk, and an accelerated rate of aging. Although original insights into DNA repair and the genes responsible were largely derived from studies in bacteria and yeast, well over 125 genes directly involved in DNA repair have now been identified in humans, and their cDNA sequence established. These genes function in a diverse set of pathways that involve the recognition and removal of DNA lesions, tolerance to DNA damage, and protection from errors of incorporation made during DNA replication or DNA repair. Additional genes indirectly affect DNA repair, by regulating the cell cycle, ostensibly to provide an opportunity for repair or to direct the cell to apoptosis. For about 70 of the DNA repair genes listed in Table I, both the genomic DNA sequence and the cDNA sequence and chromosomal location have been elucidated. In 45 cases single-nucleotide polymorphisms have been identified and, in some cases, genetic variants have been associated with specific disorders. With the accelerating rate of gene discovery, the number of identified DNA repair genes and sequence variants is quickly rising. This report tabulates the current status of what is known about these genes. The report is limited to genes whose function is directly related to DNA repair.

Receptor isoforms mediate opposing proliferative effects through gbetagamma-activated p38 or Akt pathways

The opposing effects on proliferation mediated by G-protein-coupled receptor isoforms differing in their COOH termini could be correlated with the abilities of the receptors to differentially activate p38, implicated in apoptotic events, or phosphatidylinositol 3-kinase (PI 3-K), which provides a source of survival signals. These contrasting growth responses of the somatostatin sst(2) receptor isoforms, which couple to identical Galpha subunit pools (Galpha(i3) > Galpha(i2) >> Galpha(0)), were both inhibited following betagamma sequestration. The sst(2(a)) receptor-mediated ATF-2 activation and inhibition of proliferation induced by basic fibroblast growth factor (bFGF) were dependent on prolonged phosphorylation of p38. In contrast, cell proliferation and the associated transient phosphorylation of Akt and p70(rsk) induced by sst(2(b)) receptors were blocked by the PI 3-K inhibitor LY 294002. Stimulation with bFGF alone had no effect on the activity of either p38 or Akt but markedly enhanced p38 phosphorylation mediated by sst(2(a)) receptors, suggesting that a complex interplay exists between the transduction cascades activated by these distinct receptor types. In addition, although all receptors mediated a sustained activation of extracellular signal-regulated kinases (ERK1 and ERK2), induction of the tumor suppressor p21(cip1) was detected only following amplification of ERK and p38 phosphorylation by concomitant bFGF and sst(2(a)) receptor activation. Expression of constitutively active Akt in the presence of a p38 inhibitor enabled a proliferative response to be detected in sst(2(a)) receptor-expressing cells. These findings demonstrate that the duration of activation and a critical balance between the mitogen-activated protein kinase and PI 3-K pathways are important for controlling cell proliferation and that the COOH termini of the sst(2) receptor isoforms may determine the selection of appropriate betagamma-pairings necessary for interaction with distinct kinase cascades.

Dial 9-1-1 for p53: mechanisms of p53 activation by cellular stress

The tumor suppressor protein, p53, is part of the cell's emergency team that is called upon following cellular insult. How do cells sense DNA damage and other cellular stresses and what signal transduction pathways are used to alert p53? How is the resulting nuclear accumulation of p53 accomplished and what determines the outcome of p53 induction? Many posttranslational modifications of p53, such as phosphorylation, dephosphorylation, acetylation and ribosylation, have been shown to occur following cellular stress. Some of these modifications may activate the p53 protein, interfere with MDM2 binding and/or dictate cellular localization of p53. This review will focus on recent findings about how the p53 response may be activated following cellular stress.

Regulation of p53 stability and activity in response to genotoxic stress

The p53 tumor suppressor is a universal sensor of genotoxic stress that regulates the transcription of genes required for cell-cycle arrest and apoptosis. In response to DNA damage, the p53 protein is phosphorylated at its amino-terminus and becomes stabilized upon disruption of an interaction with its negative regulator, MDM2. Subsequent phosphorylation and acetylation of p53 promote different interactions with other proteins and with target gene regulatory elements to facilitate cell-cycle arrest, apoptosis, or adaptation in response to DNA damage. Downstream of p53, p21 is responsible for growth arrest in G1, but other p53 target genes are responsible for G2 cell-cycle arrest. In response to genotoxic insult, p53-induced apoptosis results from overlapping downstream pathways that both suppress mitogenic and survival signaling and promote pro-apoptotic signaling. Adaptation to DNA damage is manifested by p53-mediated expression of its negative regulator, MDM2. The frequency of observed mutations in p53 predicts that its inactivation is a requisite step in tumorigenesis, as p53 is mutated in approximately 50% of human tumors. Thus, it is likely that in the remaining tumors, genetic aberrations will occur in pathways that regulate p53 or in pathways directly downstream of p53. The advances in the understanding of p53 signaling over the past few years point to many potential overlapping signaling pathways, where mutations may occur as alternative modes to p53 mutation.

Dissociation of p53-mediated suppression of homologous recombination from G1/S cell cycle checkpoint control

The tumor suppressor p53 is considered as the guardian of the genome which is activated following genotoxic stress. In many cell types, p53 mediates G1 cell cycle arrest as the predominant cellular response. Inactivation of wild-type p53 leads to loss of G1/S checkpoint control and to genomic instability, including increased spontaneous homologous recombination (HR). To determine whether regulation of the G1/S checkpoint is required for suppression of HR, we assessed recombination events using a plasmid substrate that stably integrated into the genome of p53-null mouse fibroblasts. Exogenous expression of a temperature-sensitive p53 protein (Ala135 to Val), which had lost trans-activation function and could not regulate G1/S transition when in mutant conformation, reduced HR rates to the same extent as wild-type p53. Furthermore, a p53 construct with an alternatively-spliced carboxy terminus also retained this ability in the absence of both activities, G1/S control and non-sequence specific DNA binding as mediated by the carboxy terminus. Our data dissociate regulation of HR by p53 from its role as a cell cycle checkpoint protein. The results support a model which extends p53's role as a guardian of the genome to include transactivation-independent regulatory functions in DNA repair, replication and recombination.