An in vitro workflow to create and modify infectious clones using replication cycle reaction

Reverse genetics systems are critical tools in combating emerging viruses which enable a better understanding of the genetic mechanisms by which viruses cause disease. Traditional cloning approaches using bacteria are fraught with difficulties due to the bacterial toxicity of many viral sequences, resulting in unwanted mutations within the viral genome. Here, we describe a novel in vitro workflow that leverages gene synthesis and replication cycle reaction to produce a supercoiled infectious clone plasmid that is easy to distribute and manipulate. We developed two infectious clones as proof of concept: a low passage dengue virus serotype 2 isolate (PUO-218) and the USA-WA1/2020 strain of SARS-CoV-2, which replicated similarly to their respective parental viruses. Furthermore, we generated a medically relevant mutant of SARS-CoV-2, Spike D614G. Results indicate that our workflow is a viable method to generate and manipulate infectious clones for viruses that are notoriously difficult for traditional bacterial-based cloning methods.

Subsewershed SARS-CoV-2 Wastewater Surveillance and COVID-19 Epidemiology Using Building-Specific Occupancy and Case Data

To evaluate the use of wastewater-based surveillance and epidemiology to monitor and predict SARS-CoV-2 virus trends, over the 2020-2021 academic year we collected wastewater samples twice weekly from 17 manholes across Virginia Tech's main campus. We used data from external door swipe card readers and student isolation/quarantine status to estimate building-specific occupancy and COVID-19 case counts at a daily resolution. After analyzing 673 wastewater samples using reverse transcription quantitative polymerase chain reaction (RT-qPCR), we reanalyzed 329 samples from isolation and nonisolation dormitories and the campus sewage outflow using reverse transcription digital droplet polymerase chain reaction (RT-ddPCR). Population-adjusted viral copy means from isolation dormitory wastewater were 48% and 66% higher than unadjusted viral copy means for N and E genes (1846/100 mL to 2733/100 mL/100 people and 2312/100 mL to 3828/100 mL/100 people, respectively; n = 46). Prespecified analyses with random-effects Poisson regression and dormitory/cluster-robust standard errors showed that the detection of N and E genes were associated with increases of 85% and 99% in the likelihood of COVID-19 cases 8 days later (incident-rate ratio (IRR) = 1.845, p = 0.013 and IRR = 1.994, p = 0.007, respectively; n = 215), and one-log increases in swipe card normalized viral copies (copies/100 mL/100 people) for N and E were associated with increases of 21% and 27% in the likelihood of observing COVID-19 cases 8 days following sample collection (IRR = 1.206, p < 0.001, n = 211 for N; IRR = 1.265, p < 0.001, n = 211 for E). One-log increases in swipe normalized copies were also associated with 40% and 43% increases in the likelihood of observing COVID-19 cases 5 days after sample collection (IRR = 1.403, p = 0.002, n = 212 for N; IRR = 1.426, p < 0.001, n = 212 for E). Our findings highlight the use of building-specific occupancy data and add to the evidence for the potential of wastewater-based epidemiology to predict COVID-19 trends at subsewershed scales.

Vaccine Effectiveness during Outbreak of COVID-19 Alpha (B.1.1.7) Variant in Men's Correctional Facility, United States

In April 2021, a COVID-19 outbreak occurred at a correctional facility in rural Virginia, USA. Eighty-four infections were identified among 854 incarcerated persons by facilitywide testing with reverse transcription quantitative PCR (qRT-PCR). We used whole-genome sequencing to link all infections to 2 employees infected with the B.1.1.7α (UK) variant. The relative risk comparing unvaccinated to fully vaccinated persons (mRNA-1273 [Moderna, https://www.modernatx.com]) was 7.8 (95% CI 4.8–12.7), corresponding to a vaccine effectiveness of 87.1% (95% CI 79.0%–92.1%). Average qRT-PCR cycle threshold values were lower, suggesting higher viral loads, among unvaccinated infected than vaccinated cases for the nucleocapsid, envelope, and spike genes. Vaccination was highly effective at preventing SARS-CoV-2 infection in this high-risk setting. This approach can be applied to similar settings to estimate vaccine effectiveness as variants emerge to guide public health strategies during the ongoing pandemic.

Development and implementation of a scalable and versatile test for COVID-19 diagnostics in rural communities

Rapid and widespread testing of severe acute respiratory coronavirus 2 (SARS-CoV-2) is essential for an effective public health response aimed at containing and mitigating the coronavirus disease 2019 (COVID-19) pandemic. Successful health policy implementation relies on early identification of infected individuals and extensive contact tracing. However, rural communities, where resources for testing are sparse or simply absent, face distinctive challenges to achieving this success. Accordingly, we report the development of an academic, public land grant University laboratory-based detection assay for the identification of SARS-CoV-2 in samples from various clinical specimens that can be readily deployed in areas where access to testing is limited. The test, which is a quantitative reverse transcription polymerase chain reaction (RT-qPCR)-based procedure, was validated on samples provided by the state laboratory and submitted for FDA Emergency Use Authorization. Our test exhibits comparable sensitivity and exceeds specificity and inclusivity values compared to other molecular assays. Additionally, this test can be re-configured to meet supply chain shortages, modified for scale up demands, and is amenable to several clinical specimens. Test development also involved 3D engineering critical supplies and formulating a stable collection media that allowed samples to be transported for hours over a dispersed rural region without the need for a cold-chain. These two elements that were critical when shortages impacted testing and when personnel needed to reach areas that were geographically isolated from the testing center. Overall, using a robust, easy-to-adapt methodology, we show that an academic laboratory can supplement COVID-19 testing needs and help local health departments assess and manage outbreaks. This additional testing capacity is particularly germane for smaller cities and rural regions that would otherwise be unable to meet the testing demand.

The Pro-Inflammatory Chemokines CXCL9, CXCL10 and CXCL11 Are Upregulated Following SARS-CoV-2 Infection in an AKT-Dependent Manner

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is a highly transmissible RNA virus that is the causative agent of the Coronavirus disease 2019 (COVID-19) pandemic. Patients with severe COVID-19 may develop acute lung injury (ALI) or acute respiratory distress syndrome (ARDS) and require mechanical ventilation. Key features of SARS-CoV-2 induced pulmonary complications include an overexpression of pro-inflammatory chemokines and cytokines that contribute to a 'cytokine storm.' In the current study an inflammatory state in Calu-3 human lung epithelial cells was characterized in which significantly elevated transcripts of the immunostimulatory chemokines CXCL9, CXCL10, and CXCL11 were present. Additionally, an increase in gene expression of the cytokines IL-6, TNFα, and IFN-γ was observed. The transcription of CXCL9, CXCL10, IL-6, and IFN-γ was also induced in the lungs of human transgenic angiotensin converting enzyme 2 (ACE2) mice infected with SARS-CoV-2. To elucidate cell signaling pathways responsible for chemokine upregulation in SARS-CoV-2 infected cells, small molecule inhibitors targeting key signaling kinases were used. The induction of CXCL9, CXCL10, and CXCL11 gene expression in response to SARS-CoV-2 infection was markedly reduced by treatment with the AKT inhibitor GSK690693. Samples from COVID-19 positive individuals also displayed marked increases in CXCL9, CXCL10, and CXCL11 transcripts as well as transcripts in the AKT pathway. The current study elucidates potential pathway specific targets for reducing the induction of chemokines that may be contributing to SARS-CoV-2 pathogenesis via hyperinflammation.

Circadian disruption promotes tumor-immune microenvironment remodeling favoring tumor cell proliferation

Circadian disruption negatively affects physiology, posing a global health threat that manifests in proliferative, metabolic, and immune diseases, among others. Because outputs of the circadian clock regulate daily fluctuations in the immune response, we determined whether circadian disruption results in tumor-associated immune cell remodeling, facilitating tumor growth. Our findings show that tumor growth rate increased and latency decreased under circadian disruption conditions compared to normal light-dark (LD) schedules in a murine melanoma model. Circadian disruption induced the loss or inversion of daily patterns of M1 (proinflammatory) and M2 (anti-inflammatory) macrophages and cytokine levels in spleen and tumor tissues. Circadian disruption also induced (i) deregulation of rhythmic expression of clock genes and (ii) of cyclin genes in the liver, (iii) increased CcnA2 levels in the tumor, and (iv) dampened expression of the cell cycle inhibitor p21^WAF/CIP1 , all of which contribute to a proliferative phenotype.

Structural, in silico, and functional analysis of a Disabled-2-derived peptide for recognition of sulfatides

Disabled-2 (Dab2) is an adaptor protein that regulates the extent of platelet aggregation by two mechanisms. In the first mechanism, Dab2 intracellularly downregulates the integrin α_IIbβ₃ receptor, converting it to a low affinity state for adhesion and aggregation processes. In the second mechanism, Dab2 is released extracellularly and interacts with the pro-aggregatory mediators, the integrin α_IIbβ₃ receptor and sulfatides, blocking their association to fibrinogen and P-selectin, respectively. Our previous research indicated that a 35-amino acid region within Dab2, which we refer to as the sulfatide-binding peptide (SBP), contains two potential sulfatide-binding motifs represented by two consecutive polybasic regions. Using molecular docking, nuclear magnetic resonance, lipid-binding assays, and surface plasmon resonance, this work identifies the critical Dab2 residues within SBP that are responsible for sulfatide binding. Molecular docking suggested that a hydrophilic region, primarily mediated by R42, is responsible for interaction with the sulfatide headgroup, whereas the C-terminal polybasic region contributes to interactions with acyl chains. Furthermore, we demonstrated that, in Dab2 SBP, R42 significantly contributes to the inhibition of platelet P-selectin surface expression. The Dab2 SBP residues that interact with sulfatides resemble those described for sphingolipid-binding in other proteins, suggesting that sulfatide-binding proteins share common binding mechanisms.

A Systems Biology Approach Identifies Hidden Regulatory Connections Between the Circadian and Cell-Cycle Checkpoints

Circadian rhythms form a self-sustaining, endogenous, time-keeping system that allows organisms to anticipate daily environmental changes. The core of the clock network consists of interlocking transcriptional-translational feedback loops that ensures that metabolic, behavioral, and physiological processes run on a 24 h timescale. The hierarchical nature of the clock manifests itself in multiple points of control on the daily cell division cycle, which relies on synthesis, degradation, and post-translational modification for progression. This relationship is particularly important for understanding the role of clock components in sensing stress conditions and triggering checkpoint signals that stop cell cycle progression. A case in point is the interplay among the circadian factor PERIOD2 (PER2), the tumor suppressor p53, and the oncogenic mouse double minute-2 homolog protein (MDM2), which is the p53's negative regulator. Under unstressed conditions, PER2 and p53 form a stable complex in the cytosol and, along with MDM2, a trimeric complex in the nucleus. Association of PER2 to the C-terminus end of p53 prevents MDM2-mediated ubiquitylation and degradation of p53 as well as p53's transcriptional activation. Remarkably, when not bound to p53, PER2 acts as substrate for the E3-ligase activity of MDM2; thus, PER2 is degraded in a phosphorylation-independent fashion. Unexpectedly, the phase relationship between PER2 and p53 are opposite; however, a systematic modeling approach, inferred from the oscillatory time course data of PER2 and p53, aided in identifying additional regulatory scenarios that explained, a priori, seemingly conflicting experimental data. Therefore, we advocate for a combined experimental/mathematical approach to elucidating multilevel regulatory cellular processes.

Abstract 3456: Distinct control of PERIOD2 degradation and circadian rhythms by the oncoprotein MDM2

The circadian clock relies on post-translational modifications to set the timing for degradation of core regulatory components and, thus, sets clock progression. Ubiquitin-modifying enzymes targeting clock components for degradation are known to mostly recognize phosphorylated substrates. A case in point is the circadian factor PERIOD 2 (PER2) whose phospho-specific turnover involves its recognition by β-transducin repeat containing proteins (β-TrCPs). Yet, the existence of this unique mode of regulation of PER2’s stability falls short of explaining persistent oscillatory phenotypes reported in biological systems lacking functional elements of the phospho-dependent PER2 degradation machinery.

In this study, we challenge the phosphorylation-centric view that PER2 degradation enhances circadian rhythm robustness by i) identifying the PER2:MDM2 endogenous complex, ii) establishing PER2 as a previously uncharacterized substrate for MDM2, iii) revealing an alternative phosphorylation-independent mechanism for PER2 ubiquitin-mediated degradation, iv) pinpointing residues for ubiquitin modification, and v) establishing the importance of MDM2-mediated PER2 turnover for defining the circadian period length. Our results not only expand MDM2’s suite of specific substrates beyond the cell cycle to include circadian components but also uncover novel regulatory players that likely impact our view of how other mechanisms crosstalk and modulate the clock itself.

Citation Format: Xianlin Zou, Jingjing Liu, Tetsuya Gotoh, Anne M. Brown, Liang Jiang, Esther L. Wisdom, Jae Kyoung Kim, Carla V. Finkielstein. Distinct control of PERIOD2 degradation and circadian rhythms by the oncoprotein MDM2 [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 3456.

Distinct control of PERIOD2 degradation and circadian rhythms by the oncoprotein and ubiquitin ligase MDM2

The circadian clock relies on posttranslational modifications to set the timing for degradation of core regulatory components, which drives clock progression. Ubiquitin-modifying enzymes that target clock components for degradation mainly recognize phosphorylated substrates. Degradation of the circadian clock component PERIOD 2 (PER2) is mediated by its phospho-specific recognition by β-transducin repeat-containing proteins (β-TrCPs), which are F-box-containing proteins that function as substrate recognition subunits of the SCF^β-TRCP ubiquitin ligase complex. However, this mode of regulating PER2 stability falls short of explaining the persistent oscillatory phenotypes reported in biological systems lacking functional elements of the phospho-dependent PER2 degradation machinery. We identified PER2 as a previously uncharacterized substrate for the ubiquitin ligase mouse double minute 2 homolog (MDM2) and found that MDM2 targeted PER2 for degradation in a manner independent of PER2 phosphorylation. Deregulation of MDM2 plays a major role in oncogenesis by contributing to the accumulation of genomic and epigenomic alterations that favor tumor development. MDM2-mediated PER2 turnover was important for defining the circadian period length in mammalian cells, a finding that emphasizes the connection between the circadian clock and cancer. Our results not only broaden the range of specific substrates of MDM2 beyond the cell cycle to include circadian components but also identify a previously unknown regulator of the clock as a druggable node that is often found to be deregulated during tumorigenesis.

Disabled-2: A modular scaffold protein with multifaceted functions in signaling

Disabled-2 (Dab2) is a multimodular scaffold protein with signaling roles in the domains of cell growth, trafficking, differentiation, and homeostasis. Emerging evidences place Dab2 as a novel modulator of cell-cell interaction; however, its mode of action has remained largely elusive. In this review, we highlight the relevance of Dab2 function in cell signaling and development and provide the most recent and comprehensive analysis of Dab2's action as a mediator of homotypical and heterotypical interactions. Accordingly, Dab-2 controls the extent of platelet aggregation through various motifs within its N-terminus. Dab2 interacts with the cytosolic tail of the integrin receptor blocking inside-out signaling, whereas extracellular Dab2 competes with fibrinogen for integrin αIIb β3 receptor binding and, thus, modulates outside-in signaling. An additional level of regulation results from Dab2's association with cell surface lipids, an event that defines the extent of cell-cell interactions. As a multifaceted regulator, Dab2 acts as a mediator of endocytosis through its association with the [FY]xNPx[YF] motifs of internalized cell surface receptors, phosphoinositides, and clathrin. Other emerging roles of Dab2 include its participation in developmental mechanisms required for tissue formation and in modulation of immune responses. This review highlights the various novel mechanisms by which Dab2 mediates an array of signaling events with vast physiological consequences.

Chronotherapy: Intuitive, Sound, Founded…But Not Broadly Applied

Circadian rhythms are a collection of endogenously driven biochemical, physiological, and behavioral processes that oscillate in a 24-h cycle and can be entrained by external cues. Circadian clock molecules are responsible for the expression of regulatory components that modulate, among others, the cell’s metabolism and energy consumption. In clinical practice, the regulation of clock mechanisms is relevant to biotransformation of therapeutics. Accordingly, xenobiotic metabolism and detoxification, the two processes that directly influence drug effectiveness and toxicity, are direct manifestations of the daily oscillations of the cellular and biochemical processes taking place within the gastrointestinal, hepatic/biliary, and renal/urologic systems. Consequently, the impact of circadian timing should be factored in when developing therapeutic regimens aimed at achieving maximum efficacy, minimum toxicity, and decreased adverse effects in a patient. However, and despite a strong mechanistic foundation, only 0.16 % of ongoing clinical trials worldwide exploit the concept of ‘time-of-day’ administration to develop safer and more effective therapies. In this article, we (1) emphasize points of control at which circadian biology intersects critical processes governing treatment interventions; (2) explore the extent to which chronotherapeutics are incorporated into clinical trials; (3) recognize roadblocks; and (4) recommend approaches to precipitate the integration of chronobiological concepts into clinical practice.

H2S-Releasing Polymer Micelles for Studying Selective Cell Toxicity

We report the preparation of S-aroylthiooxime (SATO) functionalized amphiphilic block copolymer micelles that release hydrogen sulfide (H₂S), a gaseous signaling molecule of relevance to various physiological and pathological conditions. The micelles release H₂S in response to cysteine with a half-life of 3.3 h, which is substantially slower than a related small molecule SATO. Exogenous administration of H₂S impacts growth and proliferation of cancer cells; however, the limited control over H₂S generation from inorganic sulfide sources results in conflicting reports. Therefore, we compare the cellular cytotoxicity of SATO-functionalized micelles, which release H₂S in a sustained manner, to Na₂S, which releases H₂S in a single dose. Our results show that H₂S-releasing micelles significantly reduce the survival of HCT116 colon cancer cells relative to Na₂S, GYY4137, and a small molecule SATO, indicating that release kinetics may play an important role in determining toxicity of H₂S toward cancer cells. Furthermore, H₂S-releasing micelles are well tolerated by immortalized fibroblasts (NIH/3T3 cells), suggesting a selective toxicity of H₂S toward cancer cells.

Identification of Lipid Binding Modulators Using the Protein-Lipid Overlay Assay

The protein-lipid overlay assay is an inexpensive, easy-to-implement, and high-throughput methodology that employs nitrocellulose membranes to immobilize lipids in order to rapid screen and identify protein-lipid interactions. In this chapter, we show how this methodology can identify potential modulators of protein-lipid interactions by screening water-soluble lipid competitors or even the introduction of pH changes during the binding assay to identify pH-dependent lipid binding events.

Role of CREB on heme oxygenase-1 induction in adrenal cells: involvement of the PI3K pathway

In addition to the well-known function of ACTH as the main regulator of adrenal steroidogenesis, we have previously demonstrated its effect on the transcriptional stimulation of HO-1 expression, a component of the cellular antioxidant defense system. In agreement, we hereby demonstrate that, in adrenocortical Y1 cells, HO-1 induction correlates with a significant prevention of the generation of reactive oxygen species induced by H2O2/Fe(2+) ACTH/cAMP-dependent activation of redox-imbalanced related factors such as NRF2 or NFκB and the participation of MAPKs in this mechanism was, however, discarded based on results with specific inhibitors and reporter plasmids. We suggest the involvement of CREB in HO-1 induction by ACTH/cAMP, as transfection of cells with a dominant-negative isoform of CREB (DN-CREB-M1) decreased, while overexpression of CREB increased HO-1 protein levels. Sequence screening of the murine HO-1 promoter revealed CRE-like sites located at -146 and -37 of the transcription start site and ChIP studies indicated that this region recruits phosphorylated CREB (pCREB) upon cAMP stimulation in Y1 cells. In agreement, H89 (PKA inhibitor) or cotransfection with DN-CREB-M1 prevented the 8Br-cAMP-dependent increase in luciferase activity in cells transfected with pHO-1[-295/+74].LUC. ACTH and cAMP treatment induced the activation of the PI3K/Akt signaling pathway in a PKA-independent mechanism. Inhibition of this pathway prevented the cAMP-dependent increase in HO-1 protein levels and luciferase activity in cells transfected with pHO-1[-295/+74].LUC. Finally, here we show a crosstalk between the cAMP/PKA and PI3K pathways that affects the binding of p-CREB to its cognate element in the murine promoter of the Hmox1 gene.

Moderate Exercise Prevents Functional Remodeling of the Anterior Pituitary Gland in Diet-Induced Insulin Resistance in Rats: Role of Oxidative Stress and Autophagy

A sustained elevation of glucocorticoid production, associated with the establishment of insulin resistance (IR) could add to the deleterious effects of the IR state. The aim of this study is to analyze the consequences of long-term feeding with a sucrose-rich diet (SRD) on Pomc/ACTH production, define the underlying cellular processes, and determine the effects of moderate exercise (ME) on these parameters. Animals fed a standard chow with or without 30% sucrose in the drinking water were subjected to ME. Circulating hormone levels were determined, and pituitary tissues were processed and analyzed by immunobloting and quantitative real-time PCR. Parameters of oxidative stress (OxS), endoplasmic reticulum stress, and autophagy were also determined. Rats fed SRD developed a decrease in pituitary Pomc/ACTH expression levels, increased expression of antioxidant enzymes, and induction of endoplasmic reticulum stress and autophagy. ME prevented pituitary dysfunction as well as induction of antioxidant enzymes and autophagy. Reporter assays were performed in AtT-20 corticotroph cells incubated in the presence of palmitic acid. Pomc transcription was inhibited by palmitic acid-dependent induction of OxS and autophagy, as judged by the effect of activators and inhibitors of both processes. Long-term feeding with SRD triggers the generation of OxS and autophagy in the pituitary gland, which could lead to a decline in Pomc/ACTH/glucocorticoid production. These effects could be attributed to an increase in fatty acids availability to the pituitary gland. ME was able to prevent these alterations, suggesting additional beneficial effects of ME as a therapeutic strategy in the management of IR.

Abstract 53: Interplay among E3 ubiquitin ligases regulate timely degradation of the circadian factor Period 2

Circadian rhythms are mechanisms that measure time on a scale of about 24 h and that adjusts our body to external environmental signals. Core circadian clock genes are defined as genes whose protein products are necessary components for the generation and regulation of circadian rhythms.

Oscillatory rhythms are generated as result of the activation of positive and negative transcriptional feedback loops in which post-translational modifications, shuttling, and degradation are common themes. In mammals, the positive feedback loop involves regulation of bmal1 transcription. BMAL1/CLOCK selectively binds to E-box enhancers and drives the expression of per, cry and rev-erbα genes. REV-ERBa protein then represses bmal1 transcription through Rev-Erba/ROR response elements in its promoter. Then, the level of bmal1 RNA falls, as per and cry RNA levels peak. Period proteins (i.e., PER1, 2, and 3) accumulate in the cytoplasm, becomes phosphorylated by CK1e/d, ubiquitylated and degraded. Later in the day, cryptochrome accumulates (i.e., CRY2), associates with PER2/CK1e/d and this complex translocates to the nucleus where CRY disrupts the CLOCK/BMAL1-associated transcriptional complex, resulting in the inhibition of cry, per and rev-erba and de-repression of bmal1 transcription.

In addition to the transcriptional loops, ubiquitin E3 ligases-mediated protein degradation is critical to sustain physiological rhythms. The F-box proteins b-TrCP1/2 target PER1 and PER2 for degradation via ubiquitination. Whereas knocking down β-trcp results in PER stabilization and period lengthening in synchronized culture cells; knock-out b-trcp1 mice do not display an abnormal circadian rhythmicity. Our findings indicate that PER2 ubiquitination engages two different types of E3 ligases, a process that directly influences the timely control of PER2 accumulation in the nucleus. Our data show that the mouse double-minute 2 homolog (Mdm2) RING finger E3 ligase binds to PER2 in multiple regions including an overlapping site with b-TrCP. We determined that PER2 is a specific substrate for Mdm2 and that PER2's polyubiquitination occurs both in vitro and in cells. Accordingly, overexpression Mdm2 results in the decrease of PER2 half-life, whereas, down-regulation of Mdm2 by siRNA increases PER2 stability. Furthermore, we found Mdm2-dependent post-translational modification of PER2 directly influences the amplitude of the circadian oscillatory response in synchronized cells and that both, b-TrCP and Mdm2 target PER2 at different circadian times. Overall, our findings support a model in which, the rate of nuclear degradation of PER2 leads to slow accumulation of the protein in the nucleus and, later in the day, to a temporal switch in which b-TrCP takes control over PER2 degradation. Thus, the interplaying between these two E3 ligases is indispensible for sustaining robust circadian oscillation.

Citation Format: Carla V. Finkielstein, jingjing liu. Interplay among E3 ubiquitin ligases regulate timely degradation of the circadian factor Period 2. [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 53. doi:10.1158/1538-7445.AM2015-53

Association of the circadian factor Period 2 to p53 influences p53's function in DNA-damage signaling

Association of the circadian Per2 factor to p53 results in cytosol–nuclear shuttling of the complex and further association to Mdm2. The trimeric complex remains in the nucleus until a genotoxic signal frees p53, allowing for a transcriptional checkpoint response.

Circadian period proteins influence cell division and death by associating with checkpoint components, although their mode of regulation has not been firmly established. hPer2 forms a trimeric complex with hp53 and its negative regulator Mdm2. In unstressed cells, this association leads to increased hp53 stability by blocking Mdm2-dependent ubiquitination and transcription of hp53 target genes. Because of the relevance of hp53 in checkpoint signaling, we hypothesize that hPer2 association with hp53 acts as a regulatory module that influences hp53's downstream response to genotoxic stress. Unlike the trimeric complex, whose distribution was confined to the nuclear compartment, hPer2/hp53 was identified in both cytosol and nucleus. At the transcriptional level, a reporter containing the hp21^WAF1/CIP1 promoter, a target of hp53, remained inactive in cells expressing a stable form of the hPer2/hp53 complex even when treated with γ-radiation. Finally, we established that hPer2 directly acts on the hp53 node, as checkpoint components upstream of hp53 remained active in response to DNA damage. Quantitative transcriptional analyses of hp53 target genes demonstrated that unbound hp53 was absolutely required for activation of the DNA-damage response. Our results provide evidence of the mode by which the circadian tumor suppressor hPer2 modulates hp53 signaling in response to genotoxic stress.

The circadian factor Period 2 modulates p53 stability and transcriptional activity in unstressed cells

Period 2 forms a trimeric complex with p53 and Mdm2. As a result, p53’s transcriptional activity and stability are modulated in unstressed cells, ensuring that basal levels are present if a p53-mediated response is needed. These data provide evidence of cross-talk between circadian and checkpoint components, adding a level of regulation to the checkpoint.

Human Period 2 (hPer2) is a transcriptional regulator at the core of the circadian clock mechanism that is responsible for generating the negative feedback loop that sustains the clock. Its relevance to human disease is underlined by alterations in its function that affect numerous biochemical and physiological processes. When absent, it results in the development of various cancers and an increase in the cell's susceptibility to genotoxic stress. Thus we sought to define a yet-uncharacterized checkpoint node in which circadian components integrate environmental stress signals to the DNA-damage response. We found that hPer2 binds the C-terminal half of human p53 (hp53) and forms a stable trimeric complex with hp53’s negative regulator, Mdm2. We determined that hPer2 binding to hp53 prevents Mdm2 from being ubiquitinated and targeting hp53 by the proteasome. Down-regulation of hPer2 expression directly affects hp53 levels, whereas its overexpression influences both hp53 protein stability and transcription of targeted genes. Overall our findings place hPer2 directly at the heart of the hp53-mediated response by ensuring that basal levels of hp53 are available to precondition the cell when a rapid, hp53-mediated, transcriptional response is needed.

Biophysical and molecular-dynamics studies of phosphatidic acid binding by the Dvl-2 DEP domain

The Wnt-dependent, β-catenin-independent pathway modulates cell movement and behavior. A downstream regulator of this signaling pathway is Dishevelled (Dvl), which, among other multiple interactions, binds to the Frizzled receptor and the plasma membrane via phosphatidic acid (PA) in a mechanism proposed to be pH-dependent. While the Dvl DEP domain is central to the β-catenin-independent Wnt signaling function, the mechanism underlying its physical interaction with the membrane remains elusive. In this report, we elucidate the structural and functional basis of PA association to the Dvl2 DEP domain. Nuclear magnetic resonance, molecular-dynamics simulations, and mutagenesis data indicated that the domain interacted with the phospholipid through the basic helix 3 and a contiguous loop with moderate affinity. The association suggested that PA binding promoted local conformational changes in helix 2 and β-strand 4, both of which are compromised to maintain a stable hydrophobic core in the DEP domain. We also show that the Dvl2 DEP domain bound PA in a pH-dependent manner in a mechanism that resembles deprotonation of PA. Collectively, our results structurally define the PA-binding properties of the Dvl2 DEP domain, which can be exploited for the investigation of binding mechanisms of other DEP domain-interacting proteins.

Circadian rhythms in acute intermittent porphyria--a pilot study

BACKGROUND

Acute intermittent porphyria (AIP) is an inherited disorder of haem synthesis wherein a partial deficiency of porphobilinogen (PBG) deaminase (PBGD) with other factors may give rise to biochemical and clinical manifestations of disease. The biochemical hallmarks of active AIP are relative hepatic haem deficiency and uncontrolled up-regulation of hepatic 5-aminolevulinic acid (ALA) synthase-1 (ALAS1) with over-production of ALA and PBG. The treatment of choice is intravenous haem, which restores the deficient regulatory haem pool of the liver and represses ALAS1. Recently, haem has been shown to influence circadian rhythms by controlling their negative feedback loops. We evaluated whether subjects with AIP exhibited an altered circadian profile.

MATERIALS AND METHODS

Over a 21-h period, we measured levels of serum cortisol, melatonin, ALA, PBG and mRNA levels (in peripheral blood mononuclear cells) of selected clock-controlled genes and genes involved in haem synthesis in 10 Caucasian (European-American) women who were either postmenopausal or had been receiving female hormone therapy, six of whom have AIP and four do not and are considered controls.

RESULTS

Four AIP subjects with biochemical activity exhibited higher levels of PBG and lower levels and dampened oscillation of serum cortisol, and a trend for lower levels of serum melatonin, than controls or AIP subjects without biochemical activity. Levels of clock-controlled gene mRNAs showed significant increases over baseline in all subjects at 5 a.m. and 11 p.m., whereas mRNA levels of ALAS1, ALAS2 and PBGD were increased only at 11 p.m. in subjects with active AIP.

CONCLUSIONS

This pilot study provides evidence for disturbances of circadian markers in women with active AIP that may trigger or sustain some common clinical features of AIP.

Abstract 5173: The circadian factor period 2 modulates p53 stability and function in downstream signaling

The human Period 2 (hPer2) factor is a transcriptional regulator placed at the core of the circadian clock mechanism responsible for generating the negative feedback loop that sustains the clock. Its relevance to human diseases is underlined by alterations in its function that impacts many biochemical and physiological processes and, when absent, results in the development of various cancers. The tumor suppressor role of Per2 is speculated to involve transcriptional activation of p53, altered expression of cell cycle components, and regulation of the DNA-damage response pathway. However, it is entirely unclear how Per2 operates mechanistically. First, we identified hPer2 binds the C-terminus half of human p53 (hp53) and forms a stable trimeric complex with hp53’s negative regulator, the oncogenic protein Mdm2. Second, we determined that hPer2 binding to hp53 prevents Mdm2 from ubiquitinating and targeting hp53 by the proteasome. Accordingly, downregulation of hPer2 expression directly impacts hp53 levels whereas its overexpression influences both hp53 protein stability and transcription. Furthermore, we spatially define the distribution of the trimeric complex and determine the site for processing to be located in the nucleus. Third, we establish that hp53-mediated gene transcription is influenced by the presence of hPer2. Target genes such as 14-3-3σ, hp21WAF1/CIP1, and gadd45α show a synergistic increase in expression when hPer2 and hp53 are co-expressed in a hp53-deficient background. This result is the direct consequence of hPer2 dissociation of hp53 as result of checkpoint activation as shown by studies performed using a constitutively bound form of the hPer2/hp53 complex. Overall, our findings directly place hPer2 at the heart of the hp53-mediated response by modulating its stability and controlling its function.

Citation Format: Carla V. Finkielstein, Tetsuya Gotoh, Marian Vila-Caballer, carlo santos, jingjing liu, jianhua yang. The circadian factor period 2 modulates p53 stability and function in downstream signaling. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 5173. doi:10.1158/1538-7445.AM2013-5173

Note: This abstract was not presented at the AACR Annual Meeting 2013 because the presenter was unable to attend.

Abstract P6-06-05: Down-regulation of the circadian factor Period 2 by the oncogenic E3 ligase Mdm2: Relevance of circadian components for cell cycle progression

Circadian rhythms are mechanisms that measure time on a scale of about 24 h and that adjusts our body to external environmental signals. Core circadian clock genes are defined as genes whose protein products are necessary components for the generation and regulation of circadian rhythms. Circadian proteins also regulate genes involved in either cell division or death; and a perturbation of the balance among these processes leads to cancer development and progression.

A key aspect of cancer research is identifying new regulatory pathways involved in proliferation and differentiation of cell. Disruption of circadian rhythm has recently emerged as a new potential risk factor in the development of cancer, pointing to the core gene period 2 (per2) as a tumor suppressor. However, it remains unclear how the circadian network regulates tumor suppression, nor which, if any, of its components is either the ultimate effector that influences the fate of the cell.

Initial experiments were devoted to identifying new interacting partners for Per2 using a two-hybrid system. Interestingly, among the positive clones analyzed was the oncogenic protein Mdm2. This result was validated by immunoprecipitation of recombinant and endogenous Per2/Mdm2 complexes from unstressed cells. Pull-down assays using tagged-expressed proteins fragments and labeled proteins were later used to map the interacting regions between Per2 and Mdm2. Our results show Mdm2 binds to the central flexible region of Per2 known to interact with various protein partners. Thus, we hypothesized that binding of Mdm2 to Per2 might act by mediating its ubiquitination and therefore altering Per2 stability. We next examined the formation of the Mdm2/p53/Per2 complex by immunoprecipitation. Our data show anti-p53 antibody is able to co-immunoprecipitate Per2 and Mdm2. Moreover, in vitro and in vivo ubiquitination assays show that binding of Per2 to p53 prevented ubiquitination of p53 by Mdm2 without altering their binding. Immunofluorescence studies using H1299 cells (p53-) confirmed Per2 role in p53 stabilization and for localization. Overall our results suggest that Mdm2 modulates the stability of Per2 and p53 in unstressed cells, and might be responsible for the oscillatory levels of these proteins observed in a 24 h cycle.

Citation Information: Cancer Res 2012;72(24 Suppl):Abstract nr P6-06-05.

Structure, sulfatide binding properties, and inhibition of platelet aggregation by a disabled-2 protein-derived peptide

Disabled-2 (Dab2) targets membranes and triggers a wide range of biological events, including endocytosis and platelet aggregation. Dab2, through its phosphotyrosine-binding (PTB) domain, inhibits platelet aggregation by competing with fibrinogen for α(IIb)β(3) integrin receptor binding. We have recently shown that the N-terminal region, including the PTB domain (N-PTB), drives Dab2 to the platelet membrane surface by binding to sulfatides through two sulfatide-binding motifs, modulating the extent of platelet aggregation. The three-dimensional structure of a Dab2-derived peptide encompassing the sulfatide-binding motifs has been determined in dodecylphosphocholine micelles using NMR spectroscopy. Dab2 sulfatide-binding motif contains two helices when embedded in micelles, reversibly binds to sulfatides with moderate affinity, lies parallel to the micelle surface, and when added to a platelet mixture, reduces the number and size of sulfatide-induced aggregates. Overall, our findings identify and structurally characterize a minimal region in Dab2 that modulates platelet homotypic interactions, all of which provide the foundation for rational design of a new generation of anti-aggregatory low-molecular mass molecules for therapeutic purposes.

Abstract 1184: The circadian factor Period2 modulates p53 function in unstressed cells

Circadian rhythms are mechanisms that measure time on a scale of about 24 h and that adjusts our body to external environmental signals. Core circadian clock genes are defined as genes whose protein products are necessary components for the generation and regulation of circadian rhythms. Circadian proteins also regulate genes involved in either cell division or death; and a perturbation of the balance among these processes leads to cancer development and progression. A key aspect of cancer research is identifying new regulatory pathways involved in proliferation and differentiation of cell. Disruption of circadian rhythm has recently emerged as a new potential risk factor in the development of cancer, pointing to the core gene period 2 (per2) as a tumor suppressor. However, it remains unclear how the circadian network regulates tumor suppression, nor which, if any, of its components is either the ultimate effector that influences the fate of the cell. Initial experiments were devoted to identify new interacting partners for Per2 using a two-hybrid system. Interestingly, among the positive clones analyzed was the tumor suppressor protein p53. This result was validated by immunoprecipitation of recombinant and endogenous Per2/p53 complexes from unstressed cells. Pull-down assays using tagged-expressed proteins fragments and labeled proteins were later used to map the interacting regions between Per2 and p53. Our results show Per2 binds to the C-terminus of p53 in a region that includes the DNA binding and tetramerization domains and that includes the ubiquitination sites. Thus, we hypothesized that binding of Per2 to p53 might act by blocking Mdm2-mediated ubiquitination of p53 target residues in the C-terminus and therefore altering its stability. We next examined the formation of the Mdm2/p53/Per2 complex by immunoprecipitation. Our data show anti-p53 antibody is able to co-immunoprecipitate Per2 and Mdm2. Moreover, in vitro and in vivo ubiquitination assays show that binding of Per2 to p53 prevented ubiquitination of p53 by Mdm2 without altering their binding. Immunofluorescence studies using H1299 cells (p53-) confirmed Per2 role in p53 stabilization and for localization. Overall our results suggest that Per2 modulates the stability of p53 in unstressed cells, and might be responsible for the oscillatory levels of p53 observed in a 24 h cycle.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 1184. doi:1538-7445.AM2012-1184

P5-03-04: Circadian Modulation of Estrogen Receptor alpha Promoter Activity

The circadian clock is a molecular mechanism that synchronizes environment variations with physiological changes. Disruption of the circadian clock has been linked to increased risk in diseases and a number of disorders (e.g. jet lag, insomnia, coronary heart disease and cancer). Period 2 (PER2), a circadian protein, is at the center of the clock's function. The complete loss of per2 has been shown to be common in several types of cancer including breast and ovarian. Epidemiological studies established a correlation between circadian disruption and the development of estrogen dependent tumors. The expression of estrogen receptor alpha (ERa) mRNA oscillates in a 24-hour period and, unlike PER2, ERa peaks during the light phase of the day. Because up regulation of ER relates to tumor development, defining the mechanisms of ERa expression will contribute to our comprehension of cellular proliferation and regulation of normal developmental processes. The overall goal of this project is to investigate the molecular basis for circadian control of estrogen receptor alpha (ERa) transcription.

Transcriptional activation of ERa was measured using a reporter system in Chinese Hamster Ovary and MCF-7 cell lines. Data show that PER2 influence ERa transcription through a non-canonical mechanism independent of its circadian counterparts. Promoter analysis using the Alibaba 2.0 suite revealed the presence of various putative novel partners. Among them, OCT-1 was found to directly interact with Per2, in vitro, and with corresponding response element within the ERa promoter. The predicted binding site of OCT-1 is 267 bps upstream of the promoter. Pull-down assays were used to map direct interaction of various PER2 recombinant proteins and the DNA-binding domain of OCT-1. Radiolabeled OCT-1 was found to bind to the C-terminal end of PER2. Electrophoretic mobility shift assays were used to determine direct binding of OCT-1 to the promoter. ChIP assay confirmed the recruitment of PER2 to the estrogen promoter by OCT-1. Fluorescence anisotropy and surface plasmon resonance were employed to establish the binding affinity of OCT-1 to the ERa promoter and PER2. OCT-1 was found to have a high affinity for the ERa promoter (∼10 nM). Furthermore, OCT-1 recruits PER2 to the estrogen promoter. Preliminary data suggests that the transcription factor PER2 might play a role in the regulation of the estrogen promoter.

Citation Information: Cancer Res 2011;71(24 Suppl):Abstract nr P5-03-04.

Regulatory pathways coordinating cell cycle progression in early Xenopus development

The African clawed frog, Xenopus laevis, is used extensively as a model organism for studying both cell development and cell cycle regulation. For over 20 years now, this model organism has contributed to answering fundamental questions concerning the mechanisms that underlie cell cycle transitions--the cellular components that synthesize, modify, repair, and degrade nucleic acids and proteins, the signaling pathways that allow cells to communicate, and the regulatory pathways that lead to selective expression of subsets of genes. In addition, the remarkable simplicity of the Xenopus early cell cycle allows for tractable manipulation and dissection of the basic components driving each transition. In this organism, early cell divisions are characterized by rapid cycles alternating phases of DNA synthesis and division. The post-blastula stages incorporate gap phases, lengthening progression, and allowing more time for DNA repair. Various cyclin/Cdk complexes are differentially expressed during the early cycles with orderly progression being driven by both the combined action of cyclin synthesis and degradation and the appropriate selection of specific substrates by their Cdk components. Like other multicellular organisms, chief developmental events in early Xenopus embryogenesis coincide with profound remodeling of the cell cycle, suggesting that cell proliferation and differentiation events are linked and coordinated through crosstalk mechanisms acting on signaling pathways involving the expression of cell cycle control genes.

Abstract 2125: Circadian crosstalk modulation of estrogen receptor alpha promoter activity

The proper timing of cell division is a major factor contributing to the regulation of normal growth and emerges as a fundamental process in the development of most cancers. The existence in most, if not all, living organisms of many behavioral and physiological events that occur at well-defined and controlled times argues for the presence of time-keeping systems that allow them to adapt to cyclic changes in environmental conditions. Disruption of circadian rhythm has recently emerged as a new potential risk factor in the development of cancer, pointing to the core gene period 2 (per2) as a tumor suppressor. Circadian disruption has been epidemiologically linked to a raising number of estrogen dependent tumors identified in night-shift workers. In the present study, we investigate the signaling molecules that modulate the circadian expression of estrogen receptor alpha (ERa). ERa mRNA oscillates in a 24 h period and, unlike the circadian factor Per2, ERa peaks during the light phase of the day. Because up regulation of ER relates to tumor development, defining the mechanisms of ERa expression will contribute to our comprehension of cellular proliferation and regulation of normal developmental processes. Transcriptional activation of ERa was measured using a reporter system in Chinese Hamster Ovary and MCF-7 cell lines. Data show that Per2 influence ERa transcription through a non-canonical mechanism independent of its circadian counterparts. Promoter analysis using the Alibaba 2.0 suite revealed the presence of various putative novel partners. Among them, OCT-1 was found to directly interact with Per2 and with corresponding response element within the ERa promoter bit in vitro and endogenously in CHO and MCF-7 cells. The predicted binding site of OCT-1 is 867 bps downstream of the promoter. Pull-down assays were used to map direct interaction of various Per2 recombinant proteins and the DNA-binding domain of OCT-1. Radiolabeled OCT-1 was found to bind to the C-terminal end of Per2. Electrophoretic mobility shift assays were used to determine direct binding of OCT-1 to the promoter and chromatine immunoprecipitation validate the interactions in cells. Fluorescence anisotropy and surface plasmon resonance were employed to establish the binding affinity of OCT-1 to the ERa promoter and Per2. OCT-1 was found to have a high affinity for the ERa promoter (∼10 nM). Overall, our data points towards a direct regulation of estrogen receptor signaling by circadian modulators.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 2125. doi:10.1158/1538-7445.AM2011-2125

Modeling the relationship between the p53 C-terminal domain and its binding partners using molecular dynamics

Fifty percent of all cancer cases result from mutations of the TP53 gene, which encodes the tumor suppressor p53, and it is hypothesized that the p53-mediated checkpoint pathway is compromised in most of the remaining cases. The p53 C-terminal domain (CTD) is an important site of p53 regulation but by nature is difficult to study, as it is intrinsically disordered. In this study, we performed molecular dynamics simulations on the p53 CTD and five known regulatory binding partners. We identified distinct trends in fluctuation within and around the p53 CTD binding site on each partner demonstrating a behavior that facilitates association. Further, we present evidence that the size of the hydrophobic pocket in each p53 CTD binding site governs the secondary structure of the p53 CTD when in the bound state. This information will be useful for predicting new binding partners for the p53 CTD, identifying interacting regions within other known partners, and discovering inhibitors that provide additional points of control over p53 activity.

Backbone (1)H, (15)N, and (13)C resonance assignments and secondary structure of the Tollip CUE domain

The Toll-interacting protein (Tollip) is a negative regulator of the Toll-like receptor (TLR)-mediated inflammation response. Tollip is a modular protein that contains an Nterminal Tom1-binding domain (TBD), a central conserved domain 2 (C2), and a C-terminal coupling of ubiquitin to endoplasmic reticulum degradation (CUE) domain. Here, we report the sequence-specific backbone (1)H, (15)N, and (13)C assignments of the human Tollip CUE domain. The CUE domain was found to be a stable dimer as determined by size-exclusion chromatography and molecular crosslinking studies. Analysis of the backbone chemical shift data indicated that the CUE domain exhibits three helical elements corresponding to 52% of the protein backbone. Circular dichroism spectrum analysis confirmed the helical nature of this domain. Comparison of the location of these helical regions with those reported for yeast CUE domains suggest differences in length for all helical elements. We expect the structural analysis presented here will be the foundation for future studies on the biological significance of the Tollip CUE domain, its molecular interactions, and the mechanisms that modulate its function during the inflammatory response.

Fast proteomic protocol for biomarker fingerprinting in cancerous cells

The advance of novel technologies that will enable the detection of large sets of biomarker proteins, to greatly improve the sensitivity and specificity of an assay, represents a major objective in biomedical research. To demonstrate the power of mass spectrometry (MS) detection for large-scale biomarker screening in cancer research, a simple, one-step approach for fast biomarker fingerprinting in complex cellular extracts is described. MCF-7 breast cancer cells were used as a model system. Fast proteomic profiling of whole cellular extracts was achieved on a linear trap quadrupole (LTQ) mass spectrometer by one of the following techniques: (a) data-dependent liquid chromatography (LC)-MS/MS of un-labeled cell extracts, (b) data-dependent LC-MS/MS with pulsed Q dissociation (PQD) detection of iTRAQ labeled samples, and (c) multiple reaction monitoring (MRM)-MS of low abundant proteins that could not be detected with data-dependent MS/MS. The data-dependent LC-MS/MS analysis of MCF-7 cells enabled the identification of 796 proteins (p<0.001) and the simultaneous detection of 156 previously reported putative cancer biomarkers. PQD detection of iTRAQ labeled cells resulted in the detection of 389 proteins and 64 putative biomarkers. MRM-MS analysis enabled the successful monitoring of a panel of low-abundance proteins in one single experiment, highlighting the utility of this technique for targeted analysis in cancer investigations. These results demonstrate that MS-based technologies relying on a one-step separation protocol have the potential to revolutionize biomarker research and screening applications by enabling fast, sensitive and reliable detection of large panels of putative biomarkers. To further stimulate the exploration of proteins that have been previously reported in the literature to be differentially expressed in a variety of cancers, an extensive list of approximately 1100 candidate biomarkers has been compiled and included in the manuscript.

Abstract 1070: Circadian transcriptional factor Period 2 modulates cyclin B1 expression

The proper timing of cell division is a major factor contributing to the regulation of normal growth and emerges as a fundamental process in the development of most cancers. The existence in most, if not all, living organisms of many behavioral and physiological events that occur at well-defined and controlled times argues for the presence of time-keeping systems that allow them to adapt to cyclic changes in environmental conditions. Disruption of circadian rhythm has recently emerged as a new potential risk factor in the development of cancer, pointing to the core gene period 2 (per2) as a tumor suppressor. However, it remains unclear how the circadian network regulates tumor suppression. In the present study, we use cDNA microarray analysis to identify Per2 target genes following siRNA knock-down expression of Per2 in mouse hepatocytes AML12 cells. We identified cyclin B1, a key G2/M regulator of the cell cycle, as a downstream target gene that is negatively regulated by Per2 under normal conditions. Quantitative real-time PCR confirmed our observations by showing cyclin B1 expression is enhanced under Per2 knock-down condition. Accordingly, Bmal1 expression, a target gene of Per2, was specifically augmented under Per2 over-expression conditions. Further experiments focused in characterizing the regulation of the human cyclin B1 promoter by Per2 using luciferase reporter assays. Results show cyclin B1 reporter activity is enhanced in siPer2 transfected cells compared to control scrambled siRNA. Promoter analysis defined the region comprising −484 to −284 of human cyclin B1 promoter containing response elements for gene trans-activation while the region between −284-1 is mainly responsible for Per2 regulation. Our results show the canonical E-box between −284-1 is not required for Per2 regulation of cyclin B1, suggesting non-canonical E-boxes might be directly responsible for Per2-mediated expression. FACS analysis revealed that siPer2 treated cells exhibit a reduced G2/M phase with correspondingly higher cyclin B1 protein levels and reduced Tyr-15-cdc2 phosphorylation compared to scrambled siRNA treated cells. Our previous results show Per2 stability is directly controlled by the endogenous levels of heme whose endogenous levels oscillate in a 24 h cycle. Here, we evaluated whether cyclin B1 expression is altered in heme-treated cells. In contrast to its up-regulation in siPer2 treated cells, luciferase reporter assay showed a decrease in cyclin B1 promoter activity in response to heme treatment. These results may reflect a complex pattern of regulation in which Per2 acts as a sensor molecule to monitor changes in both environmental and cellular conditions that will impact cell progression.

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 1070.

Shorter exposures to harder X-rays trigger early apoptotic events in Xenopus laevis embryos

Background

A long-standing conventional view of radiation-induced apoptosis is that increased exposure results in augmented apoptosis in a biological system, with a threshold below which radiation doses do not cause any significant increase in cell death. The consequences of this belief impact the extent to which malignant diseases and non-malignant conditions are therapeutically treated and how radiation is used in combination with other therapies. Our research challenges the current dogma of dose-dependent induction of apoptosis and establishes a new parallel paradigm to the photoelectric effect in biological systems.

Methodology/Principal Findings

We explored how the energy of individual X-ray photons and exposure time, both factors that determine the total dose, influence the occurrence of cell death in early Xenopus embryo. Three different experimental scenarios were analyzed and morphological and biochemical hallmarks of apoptosis were evaluated. Initially, we examined cell death events in embryos exposed to increasing incident energies when the exposure time was preset. Then, we evaluated the embryo's response when the exposure time was augmented while the energy value remained constant. Lastly, we studied the incidence of apoptosis in embryos exposed to an equal total dose of radiation that resulted from increasing the incoming energy while lowering the exposure time.

Conclusions/Significance

Overall, our data establish that the energy of the incident photon is a major contributor to the outcome of the biological system. In particular, for embryos exposed under identical conditions and delivered the same absorbed dose of radiation, the response is significantly increased when shorter bursts of more energetic photons are used. These results suggest that biological organisms display properties similar to the photoelectric effect in physical systems and provide new insights into how radiation-mediated apoptosis should be understood and utilized for therapeutic purposes.

Structural and membrane binding properties of the prickle PET domain

The planar cell polarity (PCP) pathway is required for fetal tissue morphogenesis as well as for maintenance of adult tissues in animals as diverse as fruit flies and mice. One of the key members of this pathway is Prickle (Pk), a protein that regulates cell movement through its association with the Dishevelled (Dsh) protein. Pk presents three LIM domains and a PET domain of unknown structure and function. Both the PET and LIM domains control membrane targeting of Dsh, which is necessary for Dsh function in the PCP pathway. Here, we show that the PET domain is monomeric and presents a nonglobular conformation with some properties of intrinsically disordered proteins. The PET domain adopts a helical conformation in the presence of 2,2,2-trifluoroethanol (TFE), a solvent known to stabilize hydrogen bonds within the polypeptide backbone, as analyzed by circular dichroism (CD) and NMR spectroscopy. Furthermore, we found that the conserved and single tryptophan residue in PET, Trp 536, moves to a more hydrophobic environment when accompanied with membrane penetration and that the protein becomes more helical in the presence of lipid micelles. The presence of LIM domains, downstream of PET, increases protein folding, thermostability, and tolerance to limited proteolysis. In addition, pull-down and tryptophan fluorescence analyses suggest that the LIM domains physically interact to regulate membrane penetration of the PET domain. The findings reported here favor a model where the PET domain is engaged in Pk membrane insertion, whereas the LIM domains modulate this function.

Wee1 kinase alters cyclin E/Cdk2 and promotes apoptosis during the early embryonic development of Xenopus laevis

Background

The cell cycles of the Xenopus laevis embryo undergo extensive remodeling beginning at the midblastula transition (MBT) of early development. Cell divisions 2–12 consist of rapid cleavages without gap phases or cell cycle checkpoints. Some remodeling events depend upon a critical nucleo-cytoplasmic ratio, whereas others rely on a maternal timer controlled by cyclin E/Cdk2 activity. One key event that occurs at the MBT is the degradation of maternal Wee1, a negative regulator of cyclin-dependent kinase (Cdk) activity.

Results

In order to assess the effect of Wee1 on embryonic cell cycle remodeling, Wee1 mRNA was injected into one-cell stage embryos. Overexpression of Wee1 caused cell cycle delay and tyrosine phosphorylation of Cdks prior to the MBT. Furthermore, overexpression of Wee1 disrupted key developmental events that normally occur at the MBT such as the degradation of Cdc25A, cyclin E, and Wee1. Overexpression of Wee1 also resulted in post-MBT apoptosis, tyrosine phosphorylation of Cdks and persistence of cyclin E/Cdk2 activity. To determine whether Cdk2 was required specifically for the survival of the embryo, the cyclin E/Cdk2 inhibitor, Δ34-Xic1, was injected in embryos and also shown to induce apoptosis.

Conclusion

Taken together, these data suggest that Wee1 triggers apoptosis through the disruption of the cyclin E/Cdk2 timer. In contrast to Wee1 and Δ34-Xic1, altering Cdks by expression of Chk1 and Chk2 kinases blocks rather than promotes apoptosis and causes premature degradation of Cdc25A. Collectively, these data implicate Cdc25A as a key player in the developmentally regulated program of apoptosis in X. laevis embryos.

Cell migration and signaling specificity is determined by the phosphatidylserine recognition motif of Rac1

The Rho guanosine triphosphatases (GTPases) control cell shape and motility and are frequently overexpressed during malignant growth. These proteins act as molecular switches cycling between active GTP- and inactive GDP-bound forms. Despite being membrane anchored via their isoprenylated C termini, Rho GTPases rapidly translocate between membrane and cytosolic compartments. Here, we show that the Rho GTPase Rac1 preferentially interacts with phosphatidylserine (PS)-containing bilayers through its polybasic motif (PBM). Rac1 isoprenylation contributes to membrane avidity but is not critical for PS recognition. The similar protein Cdc42 (cell division cycle 42), however, only associates with PS when prenylated. Conversely, other Rho GTPases such as Rac2, Rac3, and RhoA do not bind to PS even when they are prenylated. Cell stimulation with PS induces translocation of Rac1 toward the plasma membrane and stimulates GTP loading, membrane ruffling, and filopodia formation. This stimulation also promotes Cdc42 activation and phosphorylation of mitogen-activated protein kinase through Rac1/PS signaling. Consequently, the PBM specifically directs Rac1 to effect cytoskeletal rearrangement and cell migration by selective membrane phospholipid targeting.

Mechanisms of conformational change for a replicative hexameric helicase of SV40 large tumor antigen

The large tumor antigen (LTag) of simian virus 40, an AAA(+) protein, is a hexameric helicase essential for viral DNA replication in eukaryotic cells. LTag functions as an efficient molecular machine powered by ATP binding and hydrolysis for origin DNA melting and replication fork unwinding. To understand how ATP binding and hydrolysis are coupled to conformational changes, we have determined high-resolution structures ( approximately 1.9 A) of LTag hexamers in distinct nucleotide binding states. The structural differences of LTag in various nucleotide states detail the molecular mechanisms of conformational changes triggered by ATP binding/hydrolysis and reveal a potential mechanism of concerted nucleotide binding and hydrolysis. During these conformational changes, the angles and orientations between domains of a monomer alter, creating an "iris"-like motion in the hexamer. Additionally, six unique beta hairpins on the channel surface move longitudinally along the central channel, possibly serving as a motor for pulling DNA into the LTag double hexamer for unwinding.