Loss of the methylarginine reader function of SND1 confers resistance to hepatocellular carcinoma

Staphylococcal nuclease Tudor domain containing 1 (SND1) protein is an oncogene that 'reads' methylarginine marks through its Tudor domain. Specifically, it recognizes methylation marks deposited by protein arginine methyltransferase 5 (PRMT5), which is also known to promote tumorigenesis. Although SND1 can drive hepatocellular carcinoma (HCC), it is unclear whether the SND1 Tudor domain is needed to promote HCC. We sought to identify the biological role of the SND1 Tudor domain in normal and tumorigenic settings by developing two genetically engineered SND1 mouse models, an Snd1 knockout (Snd1 KO) and an Snd1 Tudor domain-mutated (Snd1 KI) mouse, whose mutant SND1 can no longer recognize PRMT5-catalyzed methylarginine marks. Quantitative PCR analysis of normal, KO, and KI liver samples revealed a role for the SND1 Tudor domain in regulating the expression of genes encoding major acute phase proteins, which could provide mechanistic insight into SND1 function in a tumor setting. Prior studies indicated that ectopic overexpression of SND1 in the mouse liver dramatically accelerates the development of diethylnitrosamine (DEN)-induced HCC. Thus, we tested the combined effects of DEN and SND1 loss or mutation on the development of HCC. We found that both Snd1 KO and Snd1 KI mice were partially protected against malignant tumor development following exposure to DEN. These results support the development of small molecule inhibitors that target the SND1 Tudor domain or the use of upstream PRMT5 inhibitors, as novel treatments for HCC.

The E2F1 transcription factor and RB tumor suppressor moonlight as DNA repair factors

The E2F1 transcription factor and RB tumor suppressor are best known for their roles in regulating the expression of genes important for cell cycle progression but, they also have transcription-independent functions that facilitate DNA repair at sites of damage. Depending on the type of DNA damage, E2F1 can recruit either the GCN5 or p300/CBP histone acetyltransferases to deposit different histone acetylation marks in flanking chromatin. At DNA double-strand breaks, E2F1 also recruits RB and the BRG1 ATPase to remodel chromatin and promote loading of the MRE11-RAD50-NBS1 complex. Knock-in mouse models demonstrate important roles for E2F1 post-translational modifications in regulating DNA repair and physiological responses to DNA damage. This review highlights how E2F1 moonlights in DNA repair, thus revealing E2F1 as a versatile protein that recruits many of the same chromatin-modifying enzymes to sites of DNA damage to promote repair that it recruits to gene promoters to regulate transcription.

Abstract 3205: Histone tyrosine phosphorylation determines glioblastoma cell survival

Histone modification is crucial for cell signaling pathways and regulates predominantly chromatin functions related with DNA damage response (DDR) that lead cancer cells to survive upon different therapies. Receptor tyrosine kinases (RTKs) have been extensively studied in cancer progression and DDR and, recently, several reports described the role of RTKs in histone tyrosine modification. We have been focusing on the role of the receptor tyrosine kinase, TIE2, in gliomas. Our study identified the expression of TIE2 in human surgical glioma specimens in relation to malignancy, specifically in brain tumor stem cells. We have revealed that the unexpected membrane-to-nuclear trafficking of TIE2 is related with radioresistance of brain tumor stem cells, functioning as a genotoxic stress sensor to activate DNA repair machinery. Interestingly, TIE2 binds, upon IR stress, to DNA/protein complexes and phosphorylates several core histones. The tandem mass spectrum of H4 and H2B demonstrates that TIE2 directly phosphorylates not previously described H4pY51 and H2BY40 respectively, which reinvigorates TIE2's role in histone code modification. We screened for pY-readers using a SH2 domain array and found that H4pY51 and H2pY40 modifications are read by the proto-oncogenes ABL1 and CRK, respectively. Our data also showed that TIE2/H4pY54/ABL1 complex binds to DNA repair proteins, activating an NHEJ DNA repair mechanism. We summarize that upon IR stress TIE2 localizes to the nucleus where it is involved in key cellular functions by directly phosphorylating core histones, and recruiting SH2 domain proteins to the DNA damage sites, complexing to the DNA repair machinery, which in turns results in radioresistance. Our data highlight the role of epigenetics in modulating the DNA damage response of glioma radiation therapy and open new avenues to find novel targetable pathways for treatment of patients with glioblastomas.

Citation Format: Mohammad Belayat Hossain, Rehnuma Shifat, Debora K. Kim, Yiesel Rivera-Molina, Francisco Puerta Martinez, Hong Jiang, Teresa Nguyen, David G. Johnson, Mark T. Bedford, Mien-Chie Hung, Erik P. Sulman, Frederick Lang, Raymond Sawaya, Juan Fueyo, Candelaria Gomez-Manzano. Histone tyrosine phosphorylation determines glioblastoma cell survival [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 3205.

The Retinoblastoma (RB) Tumor Suppressor: Pushing Back against Genome Instability on Multiple Fronts

The retinoblastoma (RB) tumor suppressor is known as a master regulator of the cell cycle. RB is mutated or functionally inactivated in the majority of human cancers. This transcriptional regulator exerts its function in cell cycle control through its interaction with the E2F family of transcription factors and with chromatin remodelers and modifiers that contribute to the repression of genes important for cell cycle progression. Over the years, studies have shown that RB participates in multiple processes in addition to cell cycle control. Indeed, RB is known to interact with over 200 different proteins and likely exists in multiple complexes. RB, in some cases, acts through its interaction with E2F1, other members of the pocket protein family (p107 and p130), and/or chromatin remodelers and modifiers. RB is a tumor suppressor with important chromatin regulatory functions that affect genomic stability. These functions include the role of RB in DNA repair, telomere maintenance, chromosome condensation and cohesion, and silencing of repetitive regions. In this review we will discuss recent advances in RB biology related to RB, partner proteins, and their non-transcriptional functions fighting back against genomic instability.

Abstract 5854: TIE2-mediated epigenetic marks regulate therapeutic resistance of glioblastoma

Glioblastomas (GBs), the most common subtype of primary brain tumors in adults, are resistant to current strategies of surgery, irradiation and chemotherapy with a median survival that ranges from 9 to 15 months. GBs invariably recur after therapy due to the presence of cells exhibiting a multidrug-resistance phenotype in the core of the tumor. There is an urgent need of developing the new therapeutic strategies for brain tumor treatment including the identification of novel molecular pathways regulating this resistant phenotype. One of the key phenomena of GB is that tyrosine kinase receptors (TKRs) are abnormally regulated and related to poor treatment outcomes. We have previously reported the expression of TIE2 in human surgical glioma specimens in relation to malignancy, and the role of TIE2 in endothelial-glioma adhesion, tumor invasion and multi-drug resistance of gliomas. Recently, we have identified that the unexpected membrane-to-nuclear trafficking of TIE2 is related to radioresistance of brain tumor stem cells. Interestingly, TIE2 binds, upon IR stress, to DNA/protein complexes and directly phosphorylates core histones. Specifically we discovered a new histone H4 mark (H4pY51) that is read by ABL1. TIE2/H4pY54/ABL1 complex binds to DNA repair proteins, such as ATM, DNA-PK, and pChk2, activating a NHEJ DNA repair mechanism. In an effort to find the molecular mechanisms underlying TIE2 nuclear translocation, we identified that TIE2 binds and directly phosphorylates caveolin-1 (CAV1) at Tyr14 residue in vitro and in vivo. Importantly, CAV1-pTyr14 is necessary for the translocation of both TIE2 and CAV1 to the nucleus. We also uncovered new TIE2-mediated epigenetic marks. Thus, TIE2 binds and phosphorylates histone H2B at Tyr37 and this modification enhances the recruitment of DNA repair proteins to the DNA damage site. Our results summarize that upon IR stress TIE2 localizes to the nucleus where it is involved in key cellular functions by directly phosphorylating core histones, and recruiting SH2 domain proteins to the DNA damage sites, that are complexing to the DNA repair machinery. Our discovery related to tyrosine modification of core histones might be of high significance to understand the resistance of cancer to DNA-damage inducers, which eventually might result in the design of TIE2-targeting combinational therapies for patients with GBs.

Citation Format: Mohammad Belayat Hossain, Rehnuma Shifat, Jingyi Li, Yisel Rivera-Mokina, Francisco Puerta Martinez, David G. Johnson, Mark T. Bedford, Mien-Chie Hung, Erik P. Sulman, Frederick Lang, Raymond Sawaya, Juan Fueyo, Candelaria Gomez-Manzano. TIE2-mediated epigenetic marks regulate therapeutic resistance of glioblastoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 5854. doi:10.1158/1538-7445.AM2017-5854

RB localizes to DNA double-strand breaks and promotes DNA end resection and homologous recombination through the recruitment of BRG1

The retinoblastoma (RB) tumor suppressor is recognized as a master regulator that controls entry into the S phase of the cell cycle. Its loss leads to uncontrolled cell proliferation and is a hallmark of cancer. RB works by binding to members of the E2F family of transcription factors and recruiting chromatin modifiers to the promoters of E2F target genes. Here we show that RB also localizes to DNA double-strand breaks (DSBs) dependent on E2F1 and ATM kinase activity and promotes DSB repair through homologous recombination (HR), and its loss results in genome instability. RB is necessary for the recruitment of the BRG1 ATPase to DSBs, which stimulates DNA end resection and HR. A knock-in mutation of the ATM phosphorylation site on E2F1 (S29A) prevents the interaction between E2F1 and TopBP1 and recruitment of RB, E2F1, and BRG1 to DSBs. This knock-in mutation also impairs DNA repair, increases genomic instability, and renders mice hypersensitive to IR. Importantly, depletion of RB in osteosarcoma and breast cancer cell lines results in sensitivity to DNA-damaging drugs, which is further exacerbated by poly-ADP ribose polymerase (PARP) inhibitors. We uncovered a novel, nontranscriptional function for RB in HR, which could contribute to genome instability associated with RB loss.

The p53 R72P polymorphism does not affect the physiological response to ionizing radiation in a mouse model

Tissue culture and mouse model studies show that the presence of the arginine (R) or proline (P) coding single nucleotide polymorphism (SNP) of the tumor suppressor gene p53 at codon 72 (p53 R72P) differentially affects the responses to genotoxic insult. Compared to the P variant, the R variant shows increased apoptosis in most cell cultures and mouse model tissues in response to genotoxins, and epidemiological studies suggest that the R variant may enhance cancer survival and reduce the risks of adverse reactions to genotoxic cancer treatment. As ionizing radiation (IR) treatment is often used in cancer therapy, we sought to test the physiological effects of IR in mouse models of the p53 R72P polymorphism. By performing blood counts, immunohistochemical (IHC) staining and survival studies in mouse populations rigorously controlled for strain background, sex and age, we discovered that p53 R72P polymorphism did not differentially affect the physiological response to IR. Our findings suggest that genotyping for this polymorphism to personalize IR therapy may have little clinical utility.

Abstract A11: RB localizes to DNA double strand breaks and promotes DNA end resection and homologous recombination through the recruitment of BRG1

The retinoblastoma (RB) tumor suppressor is widely recognized as a master regulator of the transcriptional program that controls entry into the S phase of the cell cycle. Its loss, which is a hallmark of cancer, leads to uncontrolled cell proliferation. RB works by binding to members of the E2F family of transcription factors and recruiting chromatin modifiers and remodelers to the promoters of E2F target genes. Here we show that RB also localizes to DNA double strand breaks (DSBs) dependent on E2F1 and ATM kinase activity. RB promotes DNA end resection and DSB repair through homologous recombination (HR), and its loss results in genome instability. RB is necessary for the recruitment of the BRG1 ATPase to DSBs, which stimulates DNA end resection and HR. A knock-in mutation of the ATM phosphorylation site on E2F1 (S29A) prevents the interaction between E2F1 and TopBP1 and recruitment of RB, E2F1 and BRG1 to DSBs. This knock-in mutation also impairs DNA repair, increases genomic instability and renders mice hypersensitive to IR. Importantly, depletion of either RB or BRG1 in osteosarcoma and breast cancer cell lines results in sensitivity to DNA damaging agents, thus underscoring the importance of these factors in DNA repair. We uncover a novel, non-transcriptional function for RB in HR, which could contribute to genome instability associated with RB loss.

Citation Format: Renier Vélez-Cruz, Swarnalatha Manickavinayaham, Anup K. Biswas, Regina W. Clary, David G. Johnson. RB localizes to DNA double strand breaks and promotes DNA end resection and homologous recombination through the recruitment of BRG1 [abstract]. In: Proceedings of the AACR Special Conference on DNA Repair: Tumor Development and Therapeutic Response; 2016 Nov 2-5; Montreal, QC, Canada. Philadelphia (PA): AACR; Mol Cancer Res 2017;15(4_Suppl):Abstract nr A11.

Abstract PR05: RB localizes to DNA double strand breaks and promotes DNA end resection and homologous recombination through the recruitment of SWI/SNF complex

The retinoblastoma (RB) tumor suppressor is widely recognized as a master regulator of the transcriptional program that controls entry into the S phase of the cell cycle. RB works by binding to members of the E2F family of transcription factors and recruiting chromatin-remodelers and modifiers to the promoters of E2F target genes. Here we show that RB localizes to DNA double strand breaks (DSBs) dependent on E2F1 and ATM kinase activity. Moreover, RB promotes DNA end resection and homologous recombination, and its loss results in genome instability. RB mediates this important role in repair by recruiting the SWI/SNF chromatin remodeler BRG1 to DSBs. In agreement with these results, loss of BRG1 also results in impaired DNA end resection and HR. RB is recruited to DSBs through a DNA damage-inducible phospho-specific interaction between E2F1 and TopBP1. A knock-in mutation of the ATM phosphorylation site on E2F1 (S29A) prevents the interaction between E2F1 and TopBP1 and recruitment of RB, E2F1 and BRG1 to DSBs. This E2F1 mutation also impairs DNA repair, increases genomic instability and renders knock-in mice hypersensitive to IR. We uncover a novel, non-transcriptional function for RB in HR, which could contribute to genome instability associated with RB loss in human cancers and explain their sensitivity to DSB-inducing agents.

This abstract is also being presented as Poster B09.

Citation Format: Renier Velez-Cruz, Swarnalatha Manickavinayaham, Anup K. Biswas, Regina Weaks Clary, David G. Johnson. RB localizes to DNA double strand breaks and promotes DNA end resection and homologous recombination through the recruitment of SWI/SNF complex. [abstract]. In: Proceedings of the AACR Precision Medicine Series: Cancer Cell Cycle - Tumor Progression and Therapeutic Response; Feb 28-Mar 2, 2016; Orlando, FL. Philadelphia (PA): AACR; Mol Cancer Res 2016;14(11_Suppl):Abstract nr PR05.

TIE2-mediated tyrosine phosphorylation of H4 regulates DNA damage response by recruiting ABL1

DNA repair pathways enable cancer cells to survive DNA damage induced after genotoxic therapies. Tyrosine kinase receptors (TKRs) have been reported as regulators of the DNA repair machinery. TIE2 is a TKR overexpressed in human gliomas at levels that correlate with the degree of increasing malignancy. Following ionizing radiation, TIE2 translocates to the nucleus, conferring cells with an enhanced nonhomologous end-joining mechanism of DNA repair that results in a radioresistant phenotype. Nuclear TIE2 binds to key components of DNA repair and phosphorylates H4 at tyrosine 51, which, in turn, is recognized by the proto-oncogene ABL1, indicating a role for nuclear TIE2 as a sensor for genotoxic stress by action as a histone modifier. H4Y51 constitutes the first tyrosine phosphorylation of core histones recognized by ABL1, defining this histone modification as a direct signal to couple genotoxic stress with the DNA repair machinery.

Structure of Amido Pyridinium Betaines: Persistent Intermolecular C-H⋅⋅⋅N Hydrogen Bonding in Solution

A hydrogen bond of the type C-H⋅⋅⋅X (X=O or N) is known to influence the structure and function of chemical and biological systems in solution. C-H⋅⋅⋅O hydrogen bonding in solution has been extensively studied, both experimentally and computationally, whereas the equivalent thermodynamic parameters have not been enumerated experimentally for C-H⋅⋅⋅N hydrogen bonds. This is, in part, due to the lack of systems that exhibit persistent C-H⋅⋅⋅N hydrogen bonds in solution. Herein, a class of molecule based on a biologically active norharman motif that exhibits unsupported intermolecular C-H⋅⋅⋅N hydrogen bonds in solution has been described. A pairwise interaction leads to dimerisation to give bond strengths of about 7 kJ mol^-1 per hydrogen bond, which is similar to chemically and biologically relevant C-H⋅⋅⋅O hydrogen bonding. The experimental data is supported by computational work, which provides additional insight into the hydrogen bonding by consideration of electrostatic and orbital interactions and allowed a comparison between calculated and extrapolated NMR chemical shifts.

Chirality Transfer in Gold(I)-Catalysed Direct Allylic Etherifications of Unactivated Alcohols: Experimental and Computational Study

Gold(I)-catalysed direct allylic etherifications have been successfully carried out with chirality transfer to yield enantioenriched, γ-substituted secondary allylic ethers. Our investigations include a full substrate-scope screen to ascertain substituent effects on the regioselectivity, stereoselectivity and efficiency of chirality transfer, as well as control experiments to elucidate the mechanistic subtleties of the chirality-transfer process. Crucially, addition of molecular sieves was found to be necessary to ensure efficient and general chirality transfer. Computational studies suggest that the efficiency of chirality transfer is linked to the aggregation of the alcohol nucleophile around the reactive π-bound Au-allylic ether complex. With a single alcohol nucleophile, a high degree of chirality transfer is predicted. However, if three alcohols are present, alternative proton transfer chain mechanisms that erode the efficiency of chirality transfer become competitive.

Abstract 3298: ABL1 is required for Tie2-mediated DNA repair in brain tumor stem cells

Glioblastoma is the most aggressive primary brain tumor and, in spite of surgery and chemoradiotherapy, invariably recurs. The poor prognosis associated with this disease, with a median survival of 15 months, is largely caused by the striking radioresistance of these tumors. The development of new therapeutic strategies for patient with brain tumors requires the identification of key molecular pathways regulating their resistant phenotype. The abnormal function of tyrosine kinase receptors (TKRs) is a hallmark of malignant gliomas. We previously reported the expression of the TKR Tie2 in brain tumor stem cells (BTSCs) and in human surgical glioma specimens in relation to malignancy. In in vivo experiments, consisting of ionizing irradiation (IR) of mice bearing intracranial BTSCs-derived xenografts showed unexpected Tie2 nuclear localization. These results were confirmed by using immunofluorescence studies using confocal microscope and subcellular fractionation followed by Western blots. Of clinical interest, the presence of Tie2 in the nucleus is associated with radioresistance, as observed after mutagenesis of a newly discovered nuclear localization signal. In addition, upon IR, we detected increased levels of Tie2 natural ligand, Angiopoietin1 (Ang1). The blocking of the Ang1/Tie2 interaction, by the use of a soluble receptor, modulated the IR-mediated Tie2 nuclear translocation, indicating Tie2 intracellular trafficking was ligand dependent. Additionally we also found that after IR treatment, Tie2 localized in the DNA-repair foci and complexed with the H2AX, the key DNA repair protein. The data presented here clearly suggested a role of Tie2 in the DNA damage repair machinery. To test our hypothesis, we used a fluorescent reporter construct in which a functional GFP gene was reconstituted following a non-homologous end joining (NHEJ) event (gift from Dr. Gorbunova, University of Rochester), and we observed that Tie2-expressing cells displayed a more efficient NHEJ repair than Tie2 negative counterparts. Based on the recently reported role of ABL1 (cAbl) in the ATM and KAT5 mediated DNA damage repair, we explored the relationship between ABL1 and the Tie2-mediated radioresistance. Our data clearly showed that DNA repair efficiency significantly and specifically decreased by using ABL1 inhibitor but not by knocking down ABL2 expression. We further analyzed the interactions between Tie2 and chromatin and, interestingly, observed that Tie2 complexes with core histones. Collectively, our results should propel the development of preclinical studies on the combination of nuclear Tie2-targeting strategies with radiotherapy for patients with glioblastomas.

Citation Format: Mohammad B. Hossain, Rehnuma Shifat, David G. Johnson, Mark T. Bedford, Mien-Chie Hung, Nahir Cortes-Santiago, Konrad Gabrusiewicz, Joy Gumin, Ravesanker Ezhilarasan, Erik P. Sulman, Frederick Lang, Raymond Sawaya, W.K. Alfred Yung, Juan Fueyo, Candelaria Gomez-Manzano. ABL1 is required for Tie2-mediated DNA repair in brain tumor stem cells. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 3298. doi:10.1158/1538-7445.AM2015-3298

Gold-Catalyzed Proto- and Deuterodeboronation

A mild gold-catalyzed protodeboronation reaction, which does not require acid or base additives and can be carried out in "green" solvents, is described. As a result, the reaction is very functional-group-tolerant, even to acid- and base-sensitive functional groups, and should allow for the boronic acid group to be used as an effective traceless directing or blocking group. The reaction has also been extended to deuterodeboronations for regiospecific ipso-deuterations of aryls and heteroaryls from the corresponding organoboronic acid. Based on density functional theory calculations, a mechanism is proposed that involves nucleophilic attack of water at boron followed by rate-limiting B-C bond cleavage and facile protonolysis of a Au-σ-phenyl intermediate.

Palladium-catalyzed direct C-H functionalization of benzoquinone

A direct Pd-catalyzed C-H functionalization of benzoquinone (BQ) can be controlled to give either mono- or disubstituted BQ, including the installation of two different groups in a one-pot procedure. BQ can now be directly functionalized with aryl, heteroaryl, cycloalkyl, and cycloalkene groups and, moreover, the reaction is conducted in environmentally benign water or acetone as solvents.

Gold(I)-catalysed direct thioetherifications using allylic alcohols: an experimental and computational study

A gold(I)-catalysed direct thioetherification reaction between allylic alcohols and thiols is presented. The reaction is generally highly regioselective (S(N)2'). This dehydrative allylation procedure is very mild and atom economical, producing only water as the by-product and avoiding any unnecessary waste/steps associated with installing a leaving or activating group on the substrate. Computational studies are presented to gain insight into the mechanism of the reaction. Calculations indicate that the regioselectivity is under equilibrium control and is ultimately dictated by the thermodynamic stability of the products.

Far-infrared spectroscopy of the troposphere: calibration with a cold background

The far-infrared spectroscopy of the troposphere (FIRST) instrument is a Fourier-transform spectrometer developed to measure the Earth's thermal emission spectrum with a particular emphasis on the far-infrared. FIRST has observed the atmosphere from both the ground looking up and from a high-altitude balloon looking down. A recent absolute laboratory calibration of FIRST under ground-like operating conditions showed accuracy to better than 0.3 K at near-ambient temperatures (270-325 K) but reduced accuracy at lower temperatures. This paper presents calibration results for balloon-flight conditions using a cold blackbody to simulate the space view used for on-board calibration. An unusual detector nonlinearity was discovered and corrected, and stray light was measured and removed. Over most of the range of Earth scene temperatures (205-300 K), the accuracy of FIRST is 0.35-0.15 K (one sigma).

E2F1 responds to ultraviolet radiation by directly stimulating DNA repair and suppressing carcinogenesis

In response to DNA damage, the E2F1 transcription factor is phosphorylated at serine 31 (serine 29 in mouse) by the ATM or ATR kinases, which promotes E2F1 protein stabilization. Phosphorylation of E2F1 also leads to the recruitment of E2F1 to sites of DNA damage, where it functions to enhance DNA repair. To study the role of this E2F1 phosphorylation event in vivo, a knock-in mouse model was generated, in which serine 29 was mutated to alanine. The S29A mutation impairs E2F1 stabilization in response to ultraviolet (UV) radiation and doxorubicin treatment, but has little effect on the expression of E2F target genes. The apoptotic and proliferative responses to acute UV radiation exposure are also similar between wild-type and E2f1(S29A/) (S29A) mice. As expected, the S29A mutation prevents E2F1 association with damaged DNA and reduces DNA repair efficiency. Moreover, E2f1(S29A/) (S29A) mice display increased sensitivity to UV-induced skin carcinogenesis. This knock-in mouse model thus links the ability of E2F1 to directly promote DNA repair with the suppression of tumor development.

Identification of prohibitin and prohibiton as novel factors binding to the p53 induced gene 3 (PIG3) promoter (TGYCC)(15) motif

The promoter of p53 induced gene 3 (PIG3) contains a variable number of tandem repeats (VNTRs) of pentanucleotides (TGYCC)n that is known as a p53 binding site. In this study, we investigated whether other potential molecules could bind to this PIG3 promoter (TGYCC)n motif. Ligand-chromatography combined with liquid chromatography-tandem mass spectrometry analyses indicated direct interactions of prohibitin and/or prohibiton with the (TGYCC)15 motif, which was confirmed by electrophoretic mobility shift assay and super-gel shift analysis with anti-prohibitin and anti-prohibiton antibodies. Using the chromatin immunopercipipation assay, we further demonstrated that prohibitin and prohibiton associated with the (TGYCC)15 motif in vivo regardless of the p53 status and apoptotic stress. We also found that prohibitin and prohibiton up-regulated PIG3 transcription independent of p53, although p53 obviously enhanced this process, and that the knock-down of prohibitin and prohibiton inhibited camptothecin-induced apoptosis. Taken together, our findings suggest that prohibitin and prohibiton contribute to PIG3-mediated apoptosis by binding to the PIG3 promoter (TGYCC)15 motif.

Modeling gene-environment interactions in oral cavity and esophageal cancers demonstrates a role for the p53 R72P polymorphism in modulating susceptibility

A large number of epidemiological studies have linked a common single-nucleotide polymorphism (SNP) in the human p53 gene to risk for developing a variety of cancers. This SNP encodes either an arginine or proline at position 72 (R72P) of the p53 protein, which can alter the apoptotic activity of p53 via transcriptional and non-transcriptional mechanisms. This SNP has also been reported to modulate the development of human papilloma virus (HPV)-driven cancers through differential targeting of the p53 variant proteins by the E6 viral oncoprotein. Mouse models for the p53 R72P polymorphism have recently been developed but a role for this SNP in modifying cancer risk in response to viral and chemical carcinogens has yet to be established experimentally. Here, we demonstrate that the p53 R72P polymorphism modulates the hyperprolferative, apoptotic and inflammatory phenotypes caused by expression of the HPV16 E6 and E7 oncoproteins. Moreover, the R72P SNP also modifies the carcinogenic response to the chemical carcinogen 4NQO, in the presence and absence of the HPV16 transgene. Our findings confirm several human epidemiological studies associating the codon 72 proline variant with increased risk for certain cancers but also suggest that there are tissue-specific differences in how the R72P polymorphism influences the response to environmental carcinogens.

Chromatin: receiver and quarterback for cellular signals

Signal transduction pathways converge upon sequence-specific DNA binding factors to reprogram gene expression. Transcription factors, in turn, team up with chromatin modifying activities. However, chromatin is not simply an endpoint for signaling pathways. Histone modifications relay signals to other proteins to trigger more immediate responses than can be achieved through altered gene transcription, which might be especially important to time-urgent processes such as the execution of cell-cycle check points, chromosome segregation, or exit from mitosis. In addition, histone-modifying enzymes often have multiple nonhistone substrates, and coordination of activity toward different targets might direct signals both to and from chromatin.

Far-infrared spectroscopy of the troposphere: instrument description and calibration performance

The far-infrared spectroscopy of the troposphere (FIRST) instrument is a Fourier transform spectrometer developed to measure the Earth's thermal emission spectrum with a particular emphasis on far-infrared (far-IR) wavelengths greater than 15 μm. FIRST was developed under NASA's Instrument Incubator Program to demonstrate technology for providing measurements from 10 to 100 μm (1000 to 100 cm(-1)) on a single focal plane with a spectral resolution finer than 1 cm(-1). Presently no spectrometers in orbit are capable of directly observing the Earth's far-IR spectrum. This fact, coupled with the fundamental importance of the far-IR to Earth's climate system, provided the impetus for the development of FIRST. In this paper the FIRST instrument is described and results of a detailed absolute laboratory calibration are presented. Specific channels in FIRST are shown to be accurate in the far-IR to better than 0.3 K at 270 K scene temperature, 0.5 K at 247 K, and 1 K at 225 K.

Absolute linearity measurement of photodetectors using sinusoidal modulated radiation

A method is presented for characterizing the linearity of photodetectors based on time-domain analysis of response to sinusoidal excitation. Nonlinearity is quantified solely from the output distortion. Relative response is converted to absolute response by including two calibration points. For low signal level, one calibration point is required, while using dark current as the second point. The response is mapped over a wider range using a series of overlapping sinusoids for calibration transfer. The method is demonstrated with a relatively linear photodiode and a nonlinear phototransistor. A Michelson interferometer is used to generate sinusoidal modulation of a laser source. Results demonstrate the potential of the proposed technique.

Combined effects of E2F1 and E2F2 polymorphisms on risk and early onset of squamous cell carcinoma of the head and neck

Deregulated expression of most members of the E2F family has been detected in many human cancers. We examined the association of common single nucleotide polymorphisms (SNPs) of E2F transcription factors 1 and 2 (E2F1 and E2F2) with risk of squamous cell carcinoma of the head and neck (SCCHN) in 1,096 SCCHN patients and 1,090 cancer-free controls. We genotyped 10 selected SNPs in E2F1 and E2F2, including those at the near 5'-untranslated region (UTR), microRNA (miRNA)-binding sites at the near 3'-UTR and tagSNPs according to bioinformatics analysis. Although none of the selected SNPs alone was significantly associated with risk of SCCHN, there was a statistically significantly increased risk of SCCHN associated with the combined risk genotypes (i.e., rs3213182 AA, rs3213183 GG, rs3213180 GG, rs321318121 GG, rs2742976 GT+TT, rs6667575 GA+AA, rs3218203 CC, rs3218148 AA, rs3218211 CC, and rs3218123 GT+TT). Compared with those with 0-4 risk genotypes, an increased risk was observed for those who carried 5-8 risk genotypes (adjusted OR = 1.04; 95% CI = 0.86-1.26) and 9-10 risk genotypes (adjusted OR = 1.62; 95% CI = 1.14-2.30) in a dose-response manner (P = 0.045). Furthermore, the joint effect was more pronounced among patients with oropharyngeal cancer, younger adults (≤57 yr old), men, non-smokers, non-drinkers, and individuals with family history of cancer in first-degree relatives. Additionally, we also observed that those with 5-10 risk genotypes had an earlier SCCHN onset than those with 0-4 risk genotypes, particularly for non-smokers and/or non-drinkers. We concluded that E2F1 and E2F2 genetic variants may jointly play important roles in head and neck carcinogenesis.

Transcriptional and nontranscriptional functions of E2F1 in response to DNA damage

E2F is a family of transcription factors that regulate the expression of genes involved in a wide range of cellular processes, including cell-cycle progression, DNA replication, DNA repair, differentiation, and apoptosis. E2F1, the founding member of the family, undergoes posttranslational modifications in response to DNA damage, resulting in E2F1 stabilization. In some cases, E2F1 is important for DNA damage-induced apoptosis through the transcriptional activation of p73 and perhaps other proapoptotic target genes. However, in other contexts, E2F1 can stimulate DNA repair and promote survival in response to DNA damage. The E2F1 protein accumulates at sites of both DNA double-strand breaks and UV radiation-induced damage, indicating that E2F1 has a nontranscriptional function at sites of damage. This review summarizes recent progress made in understanding the role of E2F1 in the DNA damage response, including transcription-independent activities that facilitate DNA repair in the context of chromatin.

E2F1 promotes the recruitment of DNA repair factors to sites of DNA double-strand breaks

The E2F1 transcription factor is post-translationally modified and stabilized in response to various forms of DNA damage to regulate the expression of cell cycle and pro-apoptotic genes. E2F1 also forms foci at DNA double-strand breaks (DSBs) but the function of E2F1 at sites of damage is unknown. Here we demonstrate that the absence of E2F1 leads to spontaneous DNA breaks and impaired recovery following exposure to ionizing radiation. E2F1 deficiency results in defective NBS1 phosphorylation and foci formation in response to DSBs but does not affect NBS1 expression levels. Moreover, an increased association between NBS1 and E2F1 is observed in response to DNA damage, suggesting that E2F1 may promote NBS1 foci formation through a direct or indirect interaction at sites of DNA breaks. E2F1 deficiency also impairs RPA and Rad51 foci formation indicating that E2F1 is important for DNA end resection and the formation of single-stranded DNA at DSBs. These findings establish new roles for E2F1 in the DNA damage response, which may directly contribute to DNA repair and genome maintenance.

Effects of MDM2 promoter polymorphisms and p53 codon 72 polymorphism on risk and age at onset of squamous cell carcinoma of the head and neck

Both p53 tumor suppressor and murine double minute 2 (MDM2) oncoprotein are crucial in carcinogenesis. We hypothesized that MDM2 promoter single nucleotide polymorphisms (SNPs) SNP309 T > G, A2164G, and p53 codon 72 are associated with risk and age at onset of squamous cell carcinoma of head and neck (SCCHN). We genotyped these SNPs in a study of 1,083 Caucasian SCCHN cases and 1,090 cancer-free controls. Although none of these SNPs individually had a significant effect on risk of SCCHN, nor did their combined putative risk genotypes (i.e., MDM2 SNP309 GT + GG, 2164 AA, and p53 codon 72 CC), we found that individuals with two to three risk genotypes had significantly increased risk of non-oropharyngeal cancer (OR = 1.42; 95% CI = 1.07-1.88). This increased risk was more pronounced among young subjects, men, smokers, and drinkers. In addition, female patients carrying the MDM2 SNP309 GT and GG genotypes showed a 3-yr (56.7 yr) and 9-yr (51.2 yr) earlier age at onset of non-oropharyngeal cancer (P(trend) = 0.007), respectively, compared with those carrying the TT genotype (60.1 yr). The youngest age (42.5 yr) at onset of non-oropharyngeal cancer was observed in female patients with the combined MDM2 SNP309 GG and p53 codon 72 CC genotypes. The findings suggest that MDM2 SNP309, A2164G, and p53 codon 72 SNPs may collectively contribute to non-oropharyngeal cancer risk and that MDM2 SNP309 individually or in combination with p53 codon 72 may accelerate the development of non-oropharyngeal cancer in women. Further studies with large sample sizes are warranted to validate these results.

Implantable micropump technologies for murine intracochlear infusions

Due to the very small size of the mouse inner ear, 600 nL volume, developing effective, controlled infusion systems is quite challenging. Key technologies have been created to minimize both size and power for an implantable pump for murine intracochlear infusions. A method for coupling fine capillary tubing to microfluidic channels is presented which provides low volume, biocompatible interconnects withstanding pressures as high as 827 kPa (120 psi) and consuming less than 20 nL of volume exiting in-plane with the pump. Surface micromachined resistive bridges integrated into the flow channel for anemometry based flow rate measurement have been optimized for low power operation in the ultra-low flow rate regime. A process for creation of deformable diaphragms over pump chambers with simultaneous coating of the microfluidic channels has been developed allowing integration of a biocompatible fluid flow path. These advances represent enabling capabilities for a drug delivery system suitable for space constrained applications such as subcutaneous implantation in mice.

GCN5 and E2F1 stimulate nucleotide excision repair by promoting H3K9 acetylation at sites of damage

Chromatin structure is known to be a barrier to DNA repair and a large number of studies have now identified various factors that modify histones and remodel nucleosomes to facilitate repair. In response to ultraviolet (UV) radiation several histones are acetylated and this enhances the repair of DNA photoproducts by the nucleotide excision repair (NER) pathway. However, the molecular mechanism by which UV radiation induces histone acetylation to allow for efficient NER is not completely understood. We recently discovered that the E2F1 transcription factor accumulates at sites of UV-induced DNA damage and directly stimulates NER through a non-transcriptional mechanism. Here we demonstrate that E2F1 associates with the GCN5 acetyltransferase in response to UV radiation and recruits GCN5 to sites of damage. UV radiation induces the acetylation of histone H3 lysine 9 (H3K9) and this requires both GCN5 and E2F1. Moreover, as previously observed for E2F1, knock down of GCN5 results in impaired recruitment of NER factors to sites of damage and inefficient DNA repair. These findings demonstrate a direct role for GCN5 and E2F1 in NER involving H3K9 acetylation and increased accessibility to the NER machinery.

Birth traits of pure Holstein calves versus Montbeliarde-sired crossbred calves

Pure Holstein calves and Montbeliarde-sired crossbred calves from multiparous Holstein dams were compared for gestation length, calf weight at birth, calving difficulty, and stillbirth in 2 research herds of the University of Minnesota. The Montbeliarde-sired calves from multiparous Holstein dams had significantly longer gestation lengths (283.2 d) than Holstein-sired calves from Holstein dams (278.4 d), and Montbeliarde-sired calves from multiparous Holstein dams had significantly greater calf weight at birth (48.3kg) compared with Holstein-sired calves from Holstein dams (43.3kg). However, calves sired by Montbeliarde bulls were not significantly different from calves sired by Holstein bulls for calving difficulty and stillbirth. In addition, Jersey x Holstein crossbred cows mated to Montbeliarde artificial insemination (AI) bulls were compared with pure Holstein cows mated to Holstein AI bulls for gestation length, calf weight at birth, calving difficulty, and stillbirth at their first 3 calvings. Gestation length was significantly longer for Jersey x Holstein cows bred to Montbeliarde bulls than for pure Holstein cows bred to Holstein bulls at first calving (280.3 versus 277.7 d) and second and third calving (282.2 versus 278.6 d); however, Jeresy x Holstein cows bred to Montbeliarde AI bulls were not significantly different from pure Holstein cows bred to Holstein AI bulls for calf weight at birth, calving difficulty, and stillbirth at the first 3 calvings.

A high-frequency regulatory polymorphism in the p53 pathway accelerates tumor development

MDM2, a negative regulator of p53, is elevated in many cancers that retain wild-type p53. A single nucleotide polymorphism (SNP) in the human MDM2 promoter increases the affinity of Sp1 resulting in elevated MDM2 levels. We generated mice carrying either the MDM2(SNP309T) or the MDM2(SNP309G) allele to address the impact of MDM2(SNP309G) on tumorigenesis. Mdm2(SNP309G/G) cells exhibit elevated Mdm2 levels, reduced p53 levels, and decreased apoptosis. Importantly, some Mdm2(SNP309G/G) mice succumbed to tumors before 1 year of age, suggesting that this allele increases tumor risk. Additionally, the Mdm2(SNP309G) allele potentiates the tumor phenotype and alters tumor spectrum in mice inheriting a p53 hot-spot mutation. These data provide causal evidence for increased cancer risk in carriers of the Mdm2(SNP309G) allele.

The RB-E2F1 pathway regulates autophagy

Autophagy is a protective mechanism that renders cells viable in stressful conditions. Emerging evidence suggests that this cellular process is also a tumor suppressor pathway. Previous studies showed that cyclin-dependent kinase inhibitors (CDKI) induce autophagy. Whether retinoblastoma protein (RB), a key tumor suppressor and downstream target of CDKIs, induces autophagy is not clear. Here, we show that RB triggers autophagy and that the RB activators p16INK4a and p27/kip1 induce autophagy in an RB-dependent manner. RB binding to E2 transcription factor (E2F) is required for autophagy induction and E2F1 antagonizes RB-induced autophagy, leading to apoptosis. Downregulation of E2F1 in cells results in high levels of autophagy. Our findings indicate that RB induces autophagy by repressing E2F1 activity. We speculate that this newly discovered aspect of RB function is relevant to cancer development and therapy.

Determination of the primary charge separation rate in isolated photosystem II reaction centers with 500-fs time resolution

We have measured directly the rate of formation of the oxidized chlorophyll a electron donor (P680(+)) and the reduced electron acceptor pheophytin a(-) (Pheoa(-)) following excitation of isolated spinach photosystem II reaction centers at 4 degrees C. The reaction-center complex consists of D(1), D(2), and cytochrome b-559 proteins and was prepared by a procedure that stabilizes the protein complex. Transient absorption difference spectra were measured from 440 to 850 nm as a function of time with 500-fs resolution following 610-nm laser excitation. The formation of P680(+)-Pheoa(-) is indicated by the appearance of a band due to P680(+) at 820 nm and corresponding absorbance changes at 505 and 540 nm due to formation of Pheoa(-). The appearance of the 820-nm band is monoexponential with tau = 3.0 +/- 0.6 ps. The time constant for decay of (1*)P680, the lowest excited singlet state of P680, monitored at 650 nm, is tau = 2.6 +/- 0.6 ps and agrees with that of the appearance of P680(+) within experimental error. Treatment of the photosystem II reaction centers with sodium dithionite and methyl viologen followed by exposure to laser excitation, conditions known to result in accumulation of Pheoa(-), results in formation of a transient absorption spectrum due to (1*)P680. We find no evidence for an electron acceptor that precedes the formation of Pheoa(-).

Mouse models for the p53 R72P polymorphism mimic human phenotypes

The p53 tumor suppressor gene contains a common single nucleotide polymorphism (SNP) that results in either an arginine or proline at position 72 of the p53 protein. This polymorphism affects the apoptotic activity of p53 but the mechanistic basis and physiologic relevance of this phenotypic difference remain unclear. Here, we describe the development of mouse models for the p53 R72P SNP using two different approaches. In both sets of models, the human or humanized p53 proteins are functional as evidenced by the transcriptional induction of p53 target genes in response to DNA damage and the suppression of early lymphomagenesis. Consistent with in vitro studies, mice expressing the 72R variant protein (p53R) have a greater apoptotic response to several stimuli compared with mice expressing the p53P variant. Molecular studies suggest that both transcriptional and nontranscriptional mechanisms may contribute to the differential abilities of the p53 variants to induce apoptosis. Despite a difference in the acute response to UV radiation, no difference in the tumorigenic response to chronic UV exposure was observed between the polymorphic mouse models. These findings suggest that under at least some conditions, the modulation of apoptosis by the R72P polymorphism does not affect the process of carcinogenesis.

E2F1 localizes to sites of UV-induced DNA damage to enhance nucleotide excision repair

The E2F1 transcription factor is a well known regulator of cell proliferation and apoptosis, but its role in the DNA damage response is less clear. Using a local UV irradiation technique and immunofluorescence staining, E2F1 is shown to accumulate at sites of DNA damage. Localization of E2F1 to UV-damaged DNA requires the ATM and Rad3-related (ATR) kinase and serine 31 of E2F1 but not an intact DNA binding domain. E2F1 deficiency does not appear to affect the expression of nucleotide excision repair (NER) factors, such as XPC and XPA. However, E2F1 depletion does impair the recruitment of NER factors to sites of damage and reduces the efficiency of DNA repair. E2F1 mutants unable to bind DNA or activate transcription retain the ability to stimulate NER. These findings demonstrate that E2F1 has a direct, non-transcriptional role in DNA repair involving increased recruitment of NER factors to sites of damage.

Abstract 4836: RB/E2F1 in the crossroad of autophagy and apoptosis

Autophagy is a protective mechanism that renders cells viable in stressful conditions. Mounting evidence suggests that this cellular process is also a tumor suppressor pathway. We hypothesized that retinoblastoma protein (RB), a key tumor suppressor, induces autophagy. To test this hypothesis, we first transduced RB into RB-defective human cancer cells: sarcoma osteogenic (Saos-2) cells, hepatoma (Hep3B) cells, and brain tumor stem cells (MDNSC23). The ectopic RB induced autophagy as demonstrated by significantly increased percentage of cells with acidic vesicular organelles (from less than 10% to more than 30%, P < 0.05), and the lipidation of LC3-I leading to the formation of autophagosomes/autolysosomes that were visualized by EGFP-LC3 punctation (from less than 20% to more than 80%, P = 0.003). These double-membraned vesicles were clearly shown by ultrastructural study via transmission electron microscopy. However, when Beclin 1 was silenced with siRNA in Saos-2 cells, the RB-mediated autophagy was blocked. In addition, study of autophagy flux with Saos-2 cells expressing double-tagged mRFP-EGFP-LC3 fusion protein revealed that RB stimulated the formation and maturation of autophagosomes. Consistently, RB activators p16INK4a and p27/kip1 caused autophagy in an RB-dependent manner. Importantly, we found RB mutants Delta-22 and R661W deficient for binding E2F failed to induce autophagy. Moreover, E2F1 overexpression antagonized RB-mediated autophagy, leading to apoptosis in Saos-2 cells. In agreement with the above observations, silencing E2F1 with siRNA resulted in autophagy in U-87 MG cells; and autophagy levels increased from 4% in wild-type MEFs to 40% in E2F1 knockout MEFs (P = 0.002). Collectively, our data reveal that RB/E2F1 plays a key role in the decision of a cell to undergo autophagy or apoptosis.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 4836.

E2F2 suppresses Myc-induced proliferation and tumorigenesis

Deregulation of E2F transcriptional activity as a result of alterations in the p16-cyclin D-Rb pathway is a hallmark of cancer. However, the roles of the different E2F family members in the process of tumorigenesis are still being elucidated. Studies in mice and humans suggest that E2F2 functions as a tumor suppressor. Here we demonstrate that E2f2 inactivation cooperates with transgenic expression of Myc to enhance tumor development in the skin and oral cavity. In fact, hemizygosity at the E2f2 locus was sufficient to increase tumor incidence in this model. Loss of E2F2 enhanced proliferation in Myc transgenic tissue but did not affect Myc-induced apoptosis. E2F2 did not behave as a simple activator of transcription in epidermal keratinocytes but instead appeared to differentially regulate gene expression dependent on the individual target. E2f2 inactivation also altered the changes in gene expression in Myc transgenic cells by enhancing the increase of some genes, such as cyclin E, and reversing the repression of other genes. These findings demonstrate that E2F2 can function as a tumor suppressor in epithelial tissues, perhaps by limiting proliferation in response to Myc.

Enhanced skin carcinogenesis and lack of thymus hyperplasia in transgenic mice expressing human cyclin D1b (CCND1b)

Cyclin D1b is an alternative transcript of the cyclin D1 gene (CCND1) expressed in human tumors. Its abundance is regulated by a single base pair polymorphism at the exon 4/intron 4 boundary (nucleotide 870). Epidemiological studies have shown a correlation between the presence of the G870A allele (that favors the splicing for cyclin D1b) with increased risk and less favorable outcome in several forms of cancer. More recently, it has been shown that, unlike cyclin D1a, the alternative transcript D1b by itself has the capacity to transform fibroblasts in vitro. In order to study the oncogenic potential of cyclin D1b, we developed transgenic mice expressing human cyclin D1b under the control of the bovine K5 promoter (K5D1b mice). Seven founders were obtained and none of them presented any significant phenotype or developed spontaneous tumors. Interestingly, K5D1b mice do not develop the fatal thymic hyperplasia, which is characteristic of the cyclin D1a transgenic mice (K5D1a). Susceptibility to skin carcinogenesis was tested in K5D1b mice using two-stage carcinogenesis protocols. In two independent experiments, K5D1b mice developed higher papilloma multiplicity as compared with wild-type littermates. However, when K5D1b mice were crossed with cyclin D1KO mice, the expression of cyclin D1b was unable to rescue the carcinogenesis-resistant phenotype of the cyclin D1 KO mice. To further explore the role of cyclin D1b in mouse models of carcinogenesis we carried out in silico analysis and in vitro experiments to evaluate the existence of a mouse homologous of the human cyclin D1b transcript. We were unable to find any evidence of an alternatively spliced transcript in mouse Ccnd1. These results show that human cyclin D1b has different biological functions than cyclin D1a and confirm its oncogenic properties.

Transgenic expression of E2F3a causes DNA damage leading to ATM-dependent apoptosis

Many early stage human tumors display markers of a DNA-damage response (DDR), including ataxia telangiectasia mutated (ATM) kinase activation. This suggests that DNA damage accumulates during the process of carcinogenesis and that the ATM-dependent response to this damage may function to suppress cancer progression. The E2F3a transcription factor plays an important role in regulating cell proliferation and is amplified in a subset of human cancers. Similar to human premalignant lesions, we find activated ATM and other markers of the DDR in the hyperplastic epidermis of transgenic mice expressing E2F3a through a keratin 5 (K5) promoter. Primary keratinocytes from K5 E2F3a transgenic mice contain increased levels of DNA breaks compared to wild-type cells. E2F3a overexpression also induced DNA damage in primary human fibroblasts that was inhibited by blocking DNA replication. The absence of ATM impaired apoptosis induced by E2F3a and treating K5 E2F3a transgenic mice with caffeine, an inhibitor of ATM and Rad3-related (ATR), promoted skin tumor development. These findings demonstrate that the deregulated expression of E2F3a causes DNA damage under physiological conditions and indicate that the ATM-dependent response to this damage is important for the induction of apoptosis and tumor suppression.

E2F1 and E2F3 activate ATM through distinct mechanisms to promote E1A-induced apoptosis

Deregulation of the Rb-E2F pathway occurs in many cancers and results in aberrant cell proliferation as well as an increased propensity to undergo apoptosis. In most cases, apoptosis in response to Rb inactivation involves the activation of p53 but the molecular details of the signaling pathway connecting Rb loss to p53 are poorly understood. Here we demonstrate that the E1A oncoprotein, which binds and inhibits Rb family members, induces the accumulation and phosphorylation of p53 through the DNA damage-responsive ATM kinase. As a result, E1A-induced apoptosis is significantly impaired in cells lacking ATM. In contrast, inactivation of ARF, which is widely believed to activate p53 in response to oncogenic stress, has no effect on p53 induction and only a modest effect on apoptosis in response to E1A. Both E2F1 and E2F3 contribute to ATM-dependent phosphorylation of p53 and apoptosis in cells expressing E1A. However, deregulated E2F3 activity is implicated in the DNA damage caused by E1A while E2F1 stimulates ATM- and NBS1-dependent p53 phosphorylation and apoptosis through a mechanism that does not involve DNA damage.

Transgenic E2F1 expression in the mouse brain induces a human-like bimodal pattern of tumors

The Rb/E2F pathway is deregulated in most human brain tumors, and the finding that loss of E2F1 reduced pituitary tumorigenesis in Rb(+/-) mice suggests that loss of pRb induces brain tumors by activating E2F1. We therefore investigated the role of E2F1 in the development and maintenance of brain cancer using a transgenic mouse model engineered to express E2F1 specifically within glial cells (GFAP-tgE2F1). GFAP-tgE2F1 mice developed a highly penetrant phenotype characterized by neurologic defects, and examination of the brains revealed the presence of brain tumors in 20% of these animals. Importantly, the distribution of tumors according to mouse age suggests the existence of a bimodal pattern of tumor development, forcing a comparison with the human disease. Mice, at an early age, with deregulated E2F1 show the formation of embryonal brain tumors such as medulloblastoma, choroid plexus carcinoma, and primary neuroectodermal tumor. Conversely, at an older age, mice escaping embryonal tumor formation present with malignant gliomas, which are typically identified in the human adult population. Thus, this study offers the first evidence for a global role of E2F1 in the formation and maintenance of multilineage brain tumors, irrefutably establishing E2F1 as an oncogene in the brain.