Tankyrase, a poly(ADP-ribose) polymerase at human telomeres

Cell-based chemical fingerprinting identifies telomeres and lamin A as modifiers of DNA damage response in cancer cells

Telomere maintenance by telomerase activity supports the infinite growth of cancer cells. MST-312, a synthetic telomerase inhibitor, gradually shortens telomeres at non-acute lethal doses and eventually induces senescence and apoptosis of telomerase-positive cancer cells. Here we report that MST-312 at higher doses works as a dual inhibitor of telomerase and DNA topoisomerase II and exhibits acute anti-proliferative effects on cancer cells and xenografted tumours in vivo. Our cell-based chemical fingerprinting approach revealed that cancer cells with shorter telomeres and lower expression of lamin A, a nuclear architectural protein, exhibited higher sensitivity to the acute deleterious effects of MST-312, accompanied by formation of telomere dysfunction-induced foci and DNA double-strand breaks. Telomere elongation and lamin A overexpression attenuated telomeric and non-telomeric DNA damage, respectively, and both conferred resistance to apoptosis induced by MST-312 and other DNA damaging anticancer agents. These observations suggest that sufficient pools of telomeres and a nuclear lamina component contribute to the cellular robustness against DNA damage induced by therapeutic treatment in human cancer cells.

Tankyrases maintain homeostasis of intestinal epithelium by preventing cell death

Lgr5⁺ intestinal stem cells are crucial for fast homeostatic renewal of intestinal epithelium and Wnt/β-catenin signaling plays an essential role in this process by sustaining stem cell self-renewal. The poly(ADP-ribose) polymerases tankyrases (TNKSs) mediate protein poly-ADP-ribosylation and are involved in multiple cellular processes such as Wnt signaling regulation, mitotic progression and telomere maintenance. However, little is known about the physiological function of TNKSs in epithelium homeostasis regulation. Here, using Villin-creERT2;Tnks1^-/-;Tnks2^fl/fl (DKO) mice, we observed that loss of TNKSs causes a rapid decrease of Lgr5⁺ intestinal stem cells and magnified apoptosis in small intestinal crypts, leading to intestine degeneration and increased mouse mortality. Consistently, deletion of Tnks or blockage of TNKS activity with the inhibitor XAV939 significantly inhibits the growth of intestinal organoids. We further showed that the Wnt signaling agonist CHIR99021 sustains the growth of DKO organoids, and XAV939 does not cause growth retardation of Apc^-/- organoids. Consistent with the promoting function of TNKSs in Wnt signaling, Wnt/β-catenin signaling is significantly decreased with stabilized Axin in DKO crypts. Together, our findings unravel the essential role of TNKSs-mediated protein parsylation in small intestinal homeostasis by modulating Wnt/β-catenin signaling.

Although tankyrases have been indicated to play important roles in telomere maintenance, mitosis and Wnt signaling regulation, little is known about their physiological function in intestinal epithelium. Using Villin-creERT2;Tnks1^-/-;Tnks2^fl/fl mice, which harbored conventional Tnks1 deletion and inducible intestinal epithelium-specific Tnks2 knockout, we show that tankyrases regulate adult intestinal Lgr5+ stem cells and epithelium homeostasis by preventing cell death and promoting cell proliferation.

Shelterin-Mediated Telomere Protection

For more than a decade, it has been known that mammalian cells use shelterin to protect chromosome ends. Much progress has been made on the mechanism by which shelterin prevents telomeres from inadvertently activating DNA damage signaling and double-strand break (DSB) repair pathways. Shelterin averts activation of three DNA damage response enzymes [the ataxia-telangiectasia-mutated (ATM) and ataxia telangiectasia and Rad3-related (ATR) kinases and poly(ADP-ribose) polymerase 1 (PARP1)], blocks three DSB repair pathways [classical nonhomologous end joining (c-NHEJ), alternative (alt)-NHEJ, and homology-directed repair (HDR)], and prevents hyper-resection at telomeres. For several of these functions, mechanistic insights have emerged. In addition, much has been learned about how shelterin maintains the telomeric 3' overhang, forms and protects the t-loop structure, and promotes replication through telomeres. These studies revealed that shelterin is compartmentalized, with individual subunits dedicated to distinct aspects of the end-protection problem. This review focuses on the current knowledge of shelterin-mediated telomere protection, highlights differences between human and mouse shelterin, and discusses some of the questions that remain.

Poly(ADP-Ribose) Prevents Pathological Phase Separation of TDP-43 by Promoting Liquid Demixing and Stress Granule Localization

In amyotrophic lateral sclerosis (ALS) and frontotemporal degeneration (FTD), cytoplasmic aggregates of hyperphosphorylated TDP-43 accumulate and colocalize with some stress granule components, but how pathological TDP-43 aggregation is nucleated remains unknown. In Drosophila, we establish that downregulation of tankyrase, a poly(ADP-ribose) (PAR) polymerase, reduces TDP-43 accumulation in the cytoplasm and potently mitigates neurodegeneration. We establish that TDP-43 non-covalently binds to PAR via PAR-binding motifs embedded within its nuclear localization sequence. PAR binding promotes liquid-liquid phase separation of TDP-43 in vitro and is required for TDP-43 accumulation in stress granules in mammalian cells and neurons. Stress granule localization initially protects TDP-43 from disease-associated phosphorylation, but upon long-term stress, stress granules resolve, leaving behind aggregates of phosphorylated TDP-43. Finally, small-molecule inhibition of Tankyrase-1/2 in mammalian cells inhibits formation of cytoplasmic TDP-43 foci without affecting stress granule assembly. Thus, Tankyrase inhibition antagonizes TDP-43-associated pathology and neurodegeneration and could have therapeutic utility for ALS and FTD.

Tankyrase regulates apoptosis by activating JNK signaling in Drosophila

Programmed cell death (PCD), or apoptosis, plays essential roles in various cellular and developmental processes, and dysregulation of apoptosis causes many diseases. Thus, regulation of apoptotic process is very important. Drosophila tankyrase (DTNKS) is an evolutionarily conserved protein with poly(ADP-ribose) polymerase activity. In mammalian cells, tankyrases (TNKSs) have been reported to regulate cell death. To determine whether DTNKS plays function in inducing apoptosis in in vivo development, we used Drosophila as a model system and generated transgenic flies expressing DTNKS. We show that ectopic expression of DTNKS promotes caspase-dependent apoptosis and knockdown of DTNKS by RNAi dramatically alleviates apoptotic defect caused by ectopic expression of pro-apoptotic protein hid or rpr in the adult eye. Moreover, our result shows that ectopic expression of DTNKS triggers the activation of c-Jun N-terminal kinase (JNK) signaling, which is required for DTNKS-mediated apoptosis. Taken together, our finding identifies the role of DTNKS in regulating apoptosis by activating JNK signaling in Drosophila.

What I got wrong about shelterin

The ASBMB 2018 Bert and Natalie Vallee award in Biomedical Sciences honors our work on shelterin, a protein complex that helps cells distinguish the chromosome ends from sites of DNA damage. Shelterin protects telomeres from all aspects of the DNA damage response, including ATM and ATR serine/threonine kinase signaling and several forms of double-strand break repair. Today, this six-subunit protein complex could easily be identified in one single proteomics step. But, it took us more than 15 years to piece together the entire shelterin complex, one protein at a time. Although we did a lot of things right, here I tell the story of shelterin's discovery with an emphasis on the things that I got wrong along the way.

Coupling bimolecular PARylation biosensors with genetic screens to identify PARylation targets

Poly (ADP-ribose)ylation is a dynamic protein modification that regulates multiple cellular processes. Here, we describe a system for identifying and characterizing PARylation events that exploits the ability of a PBZ (PAR-binding zinc finger) protein domain to bind PAR with high-affinity. By linking PBZ domains to bimolecular fluorescent complementation biosensors, we developed fluorescent PAR biosensors that allow the detection of temporal and spatial PARylation events in live cells. Exploiting transposon-mediated recombination, we integrate the PAR biosensor en masse into thousands of protein coding genes in living cells. Using these PAR-biosensor "tagged" cells in a genetic screen we carry out a large-scale identification of PARylation targets. This identifies CTIF (CBP80/CBP20-dependent translation initiation factor) as a novel PARylation target of the tankyrase enzymes in the centrosomal region of cells, which plays a role in the distribution of the centrosomal satellites.

Structural basis for tankyrase-RNF146 interaction reveals noncanonical tankyrase-binding motifs

Poly(ADP-ribosyl)ation (PARylation) catalyzed by the tankyrase enzymes (Tankyrase-1 and -2; a.k.a. PARP-5a and -5b) is involved in mitosis, telomere length regulation, GLUT-4 vesicle transport, and cell growth and differentiation. Together with the E3 ubiquitin ligase RNF146 (a.k.a. Iduna), tankyrases regulate the cellular levels of several important proteins including Axin, 3BP2, and angiomotins, which are key regulators of Wnt, Src and Hippo signaling, respectively. These tankyrase substrates are first PARylated and then ubiquitylated by RNF146, which is allosterically activated by binding to PAR polymer. Each tankyrase substrate is recognized by a tankyrase-binding motif (TBM). Here we show that RNF146 binds directly to tankyrases via motifs in its C-terminal region. Four of these RNF146 motifs represent novel, extended TBMs, that have one or two additional amino acids between the most conserved Arg and Gly residues. The individual RNF146 motifs display weak binding, but together mediate a strong multivalent interaction with the substrate-binding region of TNKS, forming a robust one-to-one complex. A crystal structure of the first RNF146 noncanonical TBM in complex with the second ankyrin repeat domain of TNKS shows how an extended motif can be accommodated in a peptide-binding groove on tankyrases. Overall, our work demonstrates the existence of a new class of extended TBMs that exist in previously uncharacterized tankyrase-binding proteins including those of IF4A1 and NELFE.

Proteomic Analysis of the Human Tankyrase Protein Interaction Network Reveals Its Role in Pexophagy

Tankyrase 1 (TNKS) and tankyrase 2 (TNKS2) belong to the poly(ADP-ribose) polymerase family of proteins, which use nicotinamide adenine dinucleotide to modify substrate proteins with ADP-ribose modifications. Emerging evidence has revealed the pathological relevance of TNKS and TNKS2, and identified these two enzymes as potential drug targets. However, the cellular functions and regulatory mechanisms of TNKS/2 are still largely unknown. Through a proteomic analysis, we defined the protein-protein interaction network for human TNKS/2 and revealed more than 100 high-confidence interacting proteins with numerous biological functions in this network. Finally, through functional validation, we uncovered a role for TNKS/2 in peroxisome homeostasis and determined that this function is independent of TNKS enzyme activities. Our proteomic study of the TNKS/2 protein interaction network provides a rich resource for further exploration of tankyrase functions in numerous cellular processes.

Specificity of reversible ADP-ribosylation and regulation of cellular processes

Proper and timely regulation of cellular processes is fundamental to the overall health and viability of organisms across all kingdoms of life. Thus, organisms have evolved multiple highly dynamic and complex biochemical signaling cascades in order to adapt and survive diverse challenges. One such method of conferring rapid adaptation is the addition or removal of reversible modifications of different chemical groups onto macromolecules which in turn induce the appropriate downstream outcome. ADP-ribosylation, the addition of ADP-ribose (ADPr) groups, represents one of these highly conserved signaling chemicals. Herein we outline the writers, erasers and readers of ADP-ribosylation and dip into the multitude of cellular processes they have been implicated in. We also review what we currently know on how specificity of activity is ensured for this important modification.

Stretching, scrambling, piercing and entangling: Challenges for telomeres in mitotic and meiotic chromosome segregation

The consequences of telomere loss or dysfunction become most prominent when cells enter the nuclear division stage of the cell cycle. At this climactic stage when chromosome segregation occurs, telomere fusions or entanglements can lead to chromosome breakage, wreaking havoc on genome stability. Here we review recent progress in understanding the mechanisms of detangling and breaking telomere associations at mitosis, as well as the unique ways in which telomeres are processed to allow regulated sister telomere separation. Moreover, we discuss unexpected roles for telomeres in orchestrating nuclear envelope breakdown and spindle formation, crucial processes for nuclear division. Finally, we discuss the discovery that telomeres create microdomains in the nucleus that are conducive to centromere assembly, cementing the unexpectedly influential role of telomeres in mitosis.

2-Phenylquinazolinones as dual-activity tankyrase-kinase inhibitors

Tankyrases (TNKSs) are enzymes specialized in catalyzing poly-ADP-ribosylation of target proteins. Several studies have validated TNKSs as anti-cancer drug targets due to their regulatory role in Wnt/β-catenin pathway. Recently a lot of effort has been put into developing more potent and selective TNKS inhibitors and optimizing them towards anti-cancer agents. We noticed that some 2-phenylquinazolinones (2-PQs) reported as CDK9 inhibitors were similar to previously published TNKS inhibitors. In this study, we profiled this series of 2-PQs against TNKS and selected kinases that are involved in the Wnt/β-catenin pathway. We found that they were much more potent TNKS inhibitors than they were CDK9/kinase inhibitors. We evaluated the compound selectivity to tankyrases over the ARTD enzyme family and solved co-crystal structures of the compounds with TNKS2. Comparative structure-based studies of the catalytic domain of TNKS2 with selected CDK9 inhibitors and docking studies of the inhibitors with two kinases (CDK9 and Akt) revealed important structural features, which could explain the selectivity of the compounds towards either tankyrases or kinases. We also discovered a compound, which was able to inhibit tankyrases, CDK9 and Akt kinases with equal µM potency.

TRF1 participates in chromosome end protection by averting TRF2-dependent telomeric R loops

The shelterin protein TRF2 assembles protective T loops at chromosome ends by stimulating intramolecular invasion of the telomeric G-rich single-stranded DNA (ssDNA) overhang into the duplex telomeric array. The other shelterin factor, TRF1, is thought to mainly facilitate telomeric dsDNA replication without directly participating in end protection. Here we show that in vitro human TRF2 stimulates invasion of G-rich TERRA-like RNA into telomeric dsDNA, leading to formation of telomeric RNA-DNA hybrids (telR loops). The N-terminal basic domain of TRF2 binds to TERRA-like RNA and enables TRF2 to promote efficient RNA invasion. TRF1, through its N-terminal acidic domain, counteracts TRF2-mediated RNA invasion but not ssDNA invasion. In vivo, when TRF1 is depleted or replaced with a variant lacking the acidic domain, TRF2 induces formation of telR loops, which in turn cause telomere loss. Hence, uncontrolled TRF2 threatens telomere integrity, and TRF1 directly supports end protection by suppressing harmful telR loops.

Proteomic Analysis of the Downstream Signaling Network of PARP1

Poly-ADP-ribosylation (PARylation) is a protein posttranslational modification (PTM) that is critically involved in many biological processes that are linked to cell stress responses. It is catalyzed by a class of enzymes known as poly-ADP-ribose polymerases (PARPs). In particular, PARP1 is a nuclear protein that is activated upon sensing nicked DNA. Once activated, PARP1 is responsible for the synthesis of a large number of PARylated proteins and initiation of the DNA damage response mechanisms. This observation provided the rationale for developing PARP1 inhibitors for the treatment of human malignancies. Indeed, three PARP1 inhibitors (Olaparib, Rucaparib, and Niraparib) have recently been approved by the Food and Drug Administration for the treatment of ovarian cancer. Moreover, in 2017, both Olaparib and Niraparib have also been approved for the treatment of fallopian tube cancer and primary peritoneal cancer. Despite this very exciting progress in the clinic, the basic signaling mechanism that connects PARP1 to a diverse array of biological processes is still poorly understood. This is, in large part, due to the inherent technical difficulty associated with the analysis of protein PARylation, which is a low-abundance, labile, and heterogeneous PTM. The study of PARylation has been greatly facilitated by the recent advances in mass spectrometry-based proteomic technologies tailored to the analysis of this modification. In this Perspective, we discuss these breakthroughs, including their technical development, and applications that provide a global view of the many biological processes regulated by this important protein modification.

The tankyrase inhibitor G007-LK inhibits small intestine LGR5+ stem cell proliferation without altering tissue morphology

Background

The WNT pathway regulates intestinal stem cells and is frequently disrupted in intestinal adenomas. The pathway contains several potential biotargets for interference, including the poly-ADP ribosyltransferase enzymes tankyrase1 and 2. LGR5 is a known WNT pathway target gene and marker of intestinal stem cells. The LGR5⁺ stem cells are located in the crypt base and capable of regenerating all intestinal epithelial cell lineages.

Results

We treated Lgr5-EGFP-Ires-CreERT2;R26R-Confetti mice with the tankyrase inhibitor G007-LK for up to 3 weeks to assess the effect on duodenal stem cell homeostasis and on the integrity of intestinal epithelium. At the administered doses, G007-LK treatment inhibited WNT signalling in LGR5⁺ stem cells and reduced the number and distribution of cells traced from duodenal LGR5⁺ stem cells. However, the gross morphology of the duodenum remained unaltered and G007-LK-treated mice showed no signs of weight loss or any other visible morphological changes. The inhibitory effect on LGR5⁺ stem cell proliferation was reversible.

Conclusion

We show that the tankyrase inhibitor G007-LK is well tolerated by the mice, although proliferation of the LGR5⁺ intestinal stem cells was inhibited. Our observations suggest the presence of a tankyrase inhibitor-resistant cell population in the duodenum, able to rescue tissue integrity in the presence of G007-LK-mediated inhibition of the WNT signalling dependent LGR5⁺ intestinal epithelial stem cells.

Electronic supplementary material

The online version of this article (10.1186/s40659-017-0151-6) contains supplementary material, which is available to authorized users.

Pharmacological inhibition of tankyrase induces bone loss in mice by increasing osteoclastogenesis

Tankyrase is a poly (ADP-ribose) polymerase that leads to ubiquitination and degradation of target proteins. Since tankyrase inhibitors suppress the degradation of AXIN protein, a negative regulator of the canonical Wnt pathway, they effectively act as Wnt inhibitors. Small molecule tankyrase inhibitors are being investigated as drug candidates for cancer and fibrotic diseases, in which the Wnt pathways are aberrantly activated. Tankyrase is also reported to degrade the adaptor protein SH3BP2 (SH3 domain-binding protein 2). We have previously shown that SH3BP2 gain-of-function mutation enhances receptor activator of nuclear factor-κB ligand (RANKL)-induced osteoclastogenesis in murine bone marrow-derived macrophages (BMMs). Although the interaction between tankyrase and SH3BP2 has been reported, it is not clear whether and how the inhibition of tankyrase affects bone cells and bone mass. Here, we have demonstrated that tankyrase inhibitors (IWR-1, XAV939, and G007-LK) enhanced RANKL-induced osteoclast formation and function in murine BMMs and human peripheral blood mononuclear cells through the accumulation of SH3BP2, subsequent phosphorylation of SYK, and nuclear translocation of NFATc1. Tankyrase inhibitors also enhanced osteoblast differentiation and maturation, represented by increased expression of osteoblast-associated genes accompanied by the accumulation of SH3BP2 protein and enhanced nuclear translocation of ABL, TAZ, and Runx2 in primary osteoblasts. Most importantly, pharmacological inhibition of tankyrase in mice significantly decreased tibia and lumbar vertebrae bone volumes in association with increased numbers of osteoclasts. Our findings uncover the role of tankyrase inhibition in bone cells and highlight the potential adverse effects of the inhibitor on bone.

Whole proteome analysis of human tankyrase knockout cells reveals targets of tankyrase-mediated degradation

Tankyrase 1 and 2 are poly(ADP-ribose) polymerases that function in pathways critical to cancer cell growth. Tankyrase-mediated PARylation marks protein targets for proteasomal degradation. Here, we generate human knockout cell lines to examine cell function and interrogate the proteome. We show that either tankyrase 1 or 2 is sufficient to maintain telomere length, but both are required to resolve telomere cohesion and maintain mitotic spindle integrity. Quantitative analysis of the proteome of tankyrase double knockout cells using isobaric tandem mass tags reveals targets of degradation, including antagonists of the Wnt/β-catenin signaling pathway (NKD1, NKD2, and HectD1) and three (Notch 1, 2, and 3) of the four Notch receptors. We show that tankyrases are required for Notch2 to exit the plasma membrane and enter the nucleus to activate transcription. Considering that Notch signaling is commonly activated in cancer, tankyrase inhibitors may have therapeutic potential in targeting this pathway.

Tankyrase 1 and 2 are poly(ADP-ribose) polymerases that mark proteins for degradation, but there is a current lack of knowledge about their distinct functions and substrates. Here, the authors elucidate the cellular roles and substrates of these polymerases using comparative functional and proteomics analyses of tankyrase knockout cell lines.

Regulation of Wnt/β-catenin signalling by tankyrase-dependent poly(ADP-ribosyl)ation and scaffolding

The Wnt/β-catenin signalling pathway is pivotal for stem cell function and the control of cellular differentiation, both during embryonic development and tissue homeostasis in adults. Its activity is carefully controlled through the concerted interactions of concentration-limited pathway components and a wide range of post-translational modifications, including phosphorylation, ubiquitylation, sumoylation, poly(ADP-ribosyl)ation (PARylation) and acetylation. Regulation of Wnt/β-catenin signalling by PARylation was discovered relatively recently. The PARP tankyrase PARylates AXIN1/2, an essential central scaffolding protein in the β-catenin destruction complex, and targets it for degradation, thereby fine-tuning the responsiveness of cells to the Wnt signal. The past few years have not only seen much progress in our understanding of the molecular mechanisms by which PARylation controls the pathway but also witnessed the successful development of tankyrase inhibitors as tool compounds and promising agents for the therapy of Wnt-dependent dysfunctions, including colorectal cancer. Recent work has hinted at more complex roles of tankyrase in Wnt/β-catenin signalling as well as challenges and opportunities in the development of tankyrase inhibitors. Here we review some of the latest advances in our understanding of tankyrase function in the pathway and efforts to modulate tankyrase activity to re-tune Wnt/β-catenin signalling in colorectal cancer cells.

LINKED ARTICLES

This article is part of a themed section on WNT Signalling: Mechanisms and Therapeutic Opportunities. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v174.24/issuetoc.

ADP-Ribosylation, a Multifaceted Posttranslational Modification Involved in the Control of Cell Physiology in Health and Disease

Posttranslational modifications (PTMs) regulate protein functions and interactions. ADP-ribosylation is a PTM, in which ADP-ribosyltransferases use nicotinamide adenine dinucleotide (NAD⁺) to modify target proteins with ADP-ribose. This modification can occur as mono- or poly-ADP-ribosylation. The latter involves the synthesis of long ADP-ribose chains that have specific properties due to the nature of the polymer. ADP-Ribosylation is reversed by hydrolases that cleave the glycosidic bonds either between ADP-ribose units or between the protein proximal ADP-ribose and a given amino acid side chain. Here we discuss the properties of the different enzymes associated with ADP-ribosylation and the consequences of this PTM on substrates. Furthermore, the different domains that interpret either mono- or poly-ADP-ribosylation and the implications for cellular processes are described.

Modulation of telomere protection by the PI3K/AKT pathway

Telomeres and the insulin/PI3K pathway are considered hallmarks of aging and cancer. Here, we describe a role for PI3K/AKT in the regulation of TRF1, an essential component of the shelterin complex. PI3K and AKT chemical inhibitors reduce TRF1 telomeric foci and lead to increased telomeric DNA damage and fragility. We identify the PI3Kα isoform as responsible for this TRF1 inhibition. TRF1 is phosphorylated at different residues by AKT and these modifications regulate TRF1 protein stability and TRF1 binding to telomeric DNA in vitro and are important for in vivo TRF1 telomere location and cell viability. Patient-derived breast cancer PDX mouse models that effectively respond to a PI3Kα specific inhibitor, BYL719, show decreased TRF1 levels and increased DNA damage. These findings functionally connect two of the major pathways for cancer and aging, telomeres and the PI3K pathway, and pinpoint PI3K and AKT as novel targets for chemical modulation of telomere protection.

Regulation of telomeres and the insulin/PI3K pathway both have roles in aging and cancer development but have not been functionally linked. Here the authors demonstrate that PI3K, via downstream targets, regulates TRF1 via phosphorylation.

BRD4 inhibitors block telomere elongation

Cancer cells maintain telomere length equilibrium to avoid senescence and apoptosis induced by short telomeres, which trigger the DNA damage response. Limiting the potential for telomere maintenance in cancer cells has been long been proposed as a therapeutic target. Using an unbiased shRNA screen targeting known kinases, we identified bromodomain-containing protein 4 (BRD4) as a telomere length regulator. Four independent BRD4 inhibitors blocked telomere elongation, in a dose-dependent manner, in mouse cells overexpressing telomerase. Long-term treatment with BRD4 inhibitors caused telomere shortening in both mouse and human cells, suggesting BRD4 plays a role in telomere maintenance in vivo. Telomerase enzymatic activity was not directly affected by BRD4 inhibition. BRD4 is in clinical trials for a number of cancers, but its effects on telomere maintenance have not been previously investigated.

Tankyrase1-mediated poly(ADP-ribosyl)ation of TRF1 maintains cell survival after telomeric DNA damage

Oxidative DNA damage triggers telomere erosion and cellular senescence. However, how repair is initiated at telomeres is largely unknown. Here, we found unlike PARP1-mediated Poly-ADP-Ribosylation (PARylation) at genomic damage sites, PARylation at telomeres is mainly dependent on tankyrase1 (TNKS1). TNKS1 is recruited to damaged telomeres via its interaction with TRF1, which subsequently facilitates the PARylation of TRF1 after damage. TNKS inhibition abolishes the recruitment of the repair proteins XRCC1 and polymerase β at damaged telomeres, while the PARP1/2 inhibitor only has such an effect at non-telomeric damage sites. The ANK domain of TNKS1 is essential for the telomeric damage response and TRF1 interaction. Mutation of the tankyrase-binding motif (TBM) on TRF1 (13R/18G to AA) disrupts its interaction with TNKS1 concomitant recruitment of TNKS1 and repair proteins after damage. Either TNKS1 inhibition or TBM mutated TRF1 expression markedly sensitizes cells to telomere oxidative damage as well as XRCC1 inhibition. Together, our data reveal a novel role of TNKS1 in facilitating SSBR at damaged telomeres through PARylation of TRF1, thereby protecting genome stability and cell viability.

ADP-ribosylation: new facets of an ancient modification

Rapid response to environmental changes is achieved by uni- and multicellular organisms through a series of molecular events, often involving modification of macromolecules, including proteins, nucleic acids and lipids. Amongst these, ADP-ribosylation is of emerging interest because of its ability to modify different macromolecules in the cells, and its association with many key biological processes, such as DNA-damage repair, DNA replication, transcription, cell division, signal transduction, stress and infection responses, microbial pathogenicity and aging. In this review, we provide an update on novel pathways and mechanisms regulated by ADP-ribosylation in organisms coming from all kingdoms of life.

The ADP-ribose polymerase Tankyrase regulates adult intestinal stem cell proliferation during homeostasis in Drosophila

Wnt/β-catenin signaling controls intestinal stem cell (ISC) proliferation, and is aberrantly activated in colorectal cancer. Inhibitors of the ADP-ribose polymerase Tankyrase (Tnks) have become lead therapeutic candidates for Wnt-driven cancers, following the recent discovery that Tnks targets Axin, a negative regulator of Wnt signaling, for proteolysis. Initial reports indicated that Tnks is important for Wnt pathway activation in cultured human cell lines. However, the requirement for Tnks in physiological settings has been less clear, as subsequent studies in mice, fish and flies suggested that Tnks was either entirely dispensable for Wnt-dependent processes in vivo, or alternatively, had tissue-specific roles. Here, using null alleles, we demonstrate that the regulation of Axin by the highly conserved Drosophila Tnks homolog is essential for the control of ISC proliferation. Furthermore, in the adult intestine, where activity of the Wingless pathway is graded and peaks at each compartmental boundary, Tnks is dispensable for signaling in regions where pathway activity is high, but essential where pathway activity is relatively low. Finally, as observed previously for Wingless pathway components, Tnks activity in absorptive enterocytes controls the proliferation of neighboring ISCs non-autonomously by regulating JAK/STAT signaling. These findings reveal the requirement for Tnks in the control of ISC proliferation and suggest an essential role in the amplification of Wnt signaling, with relevance for development, homeostasis and cancer.

Telomere Biology of Human Hematopoietic Stem Cells

BACKGROUND

Telomeres are protein DNA structures present at the ends of chromosomes and are essential for genetic stability and cell replication. Telomerase is the enzyme complex that maintains telomere integrity. Hematopoietic stem cells express telomerase and contain long telomeres, which become shorter as cells differentiate and mature. The extent of telomere shortening and the level of telomerase activity often correlate with the presence and severity of some hematopoietic diseases.

METHODS

The fundamentals of telomeres and telomerase are reviewed, and the telomere biology of human hematopoietic cells is discussed.

RESULTS

Telomere length and telomerase activity are important in the self-renewal of hematopoietic stem cells. Changes within these compartments affect both normal hematopoietic cells and the generation of hematopoietic disease. Telomere length provides information pertaining to the proliferative history and potential of a hematopoietic cell.

CONCLUSIONS

The role of telomerase and telomeres within the hematopoietic compartment needs further clarification. Advances in our knowledge in this field may improve clinical outcomes for the treatment of hematologic disease.

Molecular regulation and pharmacological targeting of the β-catenin destruction complex

The β‐catenin destruction complex is a dynamic cytosolic multiprotein assembly that provides a key node in Wnt signalling regulation. The core components of the destruction complex comprise the scaffold proteins axin and adenomatous polyposis coli and the Ser/Thr kinases casein kinase 1 and glycogen synthase kinase 3. In unstimulated cells, the destruction complex efficiently drives degradation of the transcriptional coactivator β‐catenin, thereby preventing the activation of the Wnt/β‐catenin pathway. Mutational inactivation of the destruction complex is a major pathway in the pathogenesis of cancer. Here, we review recent insights in the regulation of the β‐catenin destruction complex, including newly identified interaction interfaces, regulatory elements and post‐translationally controlled mechanisms. In addition, we discuss how mutations in core destruction complex components deregulate Wnt signalling via distinct mechanisms and how these findings open up potential therapeutic approaches to restore destruction complex activity in cancer cells.

Linked Articles

This article is part of a themed section on WNT Signalling: Mechanisms and Therapeutic Opportunities. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v174.24/issuetoc

Inhibitors of telomerase and poly(ADP-ribose) polymerases synergize to limit the lifespan of pancreatic cancer cells

Imetelstat (GRN163L) is a potent and selective inhibitor of telomerase. We have previously reported that GRN163L could shorten telomeres and limit the lifespan of CD18/HPAF and CAPAN1 pancreatic cancer cells. Here, we examined the effects of GRN163L on two other pancreatic cancer cell lines: AsPC1 and L3.6pl. In both lines, chronic exposure to GRN163L led to an initial shortening of telomeres followed by a stabilization of extremely short telomeres. In AsPC1 cells, telomere attrition eventually led to the induction of crisis and the loss of the treated population. In L3.6pl cells, crisis was transient and followed by the emergence of GRN163L-resistant cells, which could grow at increasing concentrations of GRN163L. The Shelterin complex is a telomere-associated complex that limits the access of telomerase to telomeres. The telomerase inhibitory function of this complex can be enhanced by drugs that block the poly(ADP-ribosyl)ation of its TRF1 and/or TRF2 subunits. Combined treatment of the GRN163L-resistant L3.6pl cells with GRN163L and 3-aminobenzamide (3AB), a general inhibitor of poly(ADP-ribose) polymerases, led to additional telomere shortening and limited the lifespan of the resistant cells. Results from this work suggest that inhibitors of telomerase and poly(ADP-ribose) polymerases can cooperate to limit the lifespan of pancreatic cancer cells.

mTOR signaling mediates resistance to tankyrase inhibitors in Wnt-driven colorectal cancer

Activation of Wnt/β-catenin signaling is essential for colorectal carcinogenesis. Tankyrase, a member of the poly(ADP-ribose) polymerase (PARP) family, is a positive regulator of the Wnt/β-catenin signaling. Accordingly, tankyrase inhibitors are under preclinical development for colorectal cancer (CRC) therapy. However, Wnt-driven colorectal cancer cells are not equally sensitive to tankyrase inhibitors, and cellular factors that affect tankyrase inhibitor sensitivity remain elusive. Here, we established a tankyrase inhibitor-resistant cell line, 320-IWR, from Wnt/β-catenin-dependent CRC COLO-320DM cells. 320-IWR cells exhibited resistance to tankyrase inhibitors, IWR-1 and G007-LK, but remained sensitive to a PARP-1/2 inhibitor, olaparib, and several anti-CRC agents. In 320-IWR cells, nuclear localization of active β-catenin was decreased and expression of β-catenin target genes was constitutively repressed, suggesting that these cells repressed the Wnt/β-catenin signaling and were dependent on alternative proliferation pathways. 320-IWR cells exhibited upregulated mTOR signaling and were more sensitive to mTOR inhibition than the parental cells. Importantly, mTOR inhibition reversed resistance to tankyrase inhibitors and potentiated their anti-proliferative effects in 320-IWR cells as well as in CRC cell lines in which the mTOR pathway was intrinsically activated. These results indicate that mTOR signaling confers resistance to tankyrase inhibitors in CRC cells and suggest that the combination of tankyrase and mTOR inhibitors would be a useful therapeutic approach for a subset of CRCs.

Tankyrase inhibition regulates corpus luteum development and luteal function in gonadotropin-treated rats

Tankyrases are physiological regulators of Axin, a protein involved in several cellular processes, including Wnt signaling. Here, we investigated the effect of a specific Tankyrase inhibitor (XAV939) in follicular-luteal dynamics, and its possible relationship with ovarian vascular development. Studies were designed to analyze the effect of intrabursa administration of XAV939 in gonadotropin-treated prepubertal rats. In particular, we examined follicle and corpus luteum development, steroidogenesis, angiogenic markers, and apoptotic parameters. We found that in vivo inhibition of Wnt signaling impaired corpus luteum development, with a decrease in the number of corpora lutea balanced by a high number of cysts; decreased circulating progesterone levels, likely due to a decrease in Steroidogenic acute regulatory protein content in the corpus luteum; and increased pro-apoptotic parameters. In addition, Extracellular signal-regulated kinase phosphorylation, Vascular endothelium growth factor 120 content, and endothelial cell area were diminished in corpora lutea of inhibitor-treated ovaries. Thus, Wnt/β-catenin signaling appears to participate in the regulation of corpus luteum development and luteal cell function.

Interaction of tankyrase and peroxiredoxin II is indispensable for the survival of colorectal cancer cells

Mammalian 2-Cys peroxiredoxin (Prx) enzymes are overexpressed in most cancer tissues, but their specific signaling role in cancer progression is poorly understood. Here we demonstrate that Prx type II (PrxII) plays a tumor-promoting role in colorectal cancer by interacting with a poly(ADP-ribose) polymerase (PARP) tankyrase. PrxII deletion in mice with inactivating mutation of adenomatous polyposis coli (APC) gene reduces intestinal adenomatous polyposis via Axin/β-catenin axis and thereby promotes survival. In human colorectal cancer cells with APC mutations, PrxII depletion consistently reduces the β-catenin levels and the expression of β-catenin target genes. Essentially, PrxII depletion hampers the PARP-dependent Axin1 degradation through tankyrase inactivation. Direct binding of PrxII to tankyrase ARC4/5 domains seems to be crucial for protecting tankyrase from oxidative inactivation. Furthermore, a chemical compound targeting PrxII inhibits the expansion of APC-mutant colorectal cancer cells in vitro and in vivo tumor xenografts. Collectively, this study reveals a redox mechanism for regulating tankyrase activity and implicates PrxII as a targetable antioxidant enzyme in APC-mutation-positive colorectal cancer.

2-Cys peroxiredoxin (Prx) enzymes are highly expressed in most cancers but how they promote cancer progression is unclear. Here the authors show that in colorectal cancers with APC mutation, PrxII binds to tankyrase and prevents its oxidative inactivation, thereby preventing Axin1-dependent degradation of ²b-catenin.

USP25 regulates Wnt signaling by controlling the stability of tankyrases

Here, Xu et al. identified USP25, a ubiquitin-specific protease, as a positive regulator of Wnt–β-catenin signaling. They found that USP25 directly interacted with tankyrases to promote their deubiquitination and stabilization, and USP25 deficiency could promote the degradation of tankyrases and consequent stabilization of Axin to antagonize Wnt signaling. Their findings provide new insights into the molecular mechanism that regulates the turnover of tankyrases and the Wnt–β catenin pathway.

Aberrant activation of the Wnt signaling pathway plays an important role in human cancer development. Wnt signaling is negatively regulated by Axin, a scaffolding protein that controls a rate-limiting step in the destruction of β-catenin, the central activator of the Wnt pathway. In Wnt-stimulated cells, Axin is rapidly modified by tankyrase-mediated poly(ADP-ribosyl)ation, which promotes the proteolysis of Axin and consequent stabilization of β-catenin. Thus, regulation of the levels and activity of tankyrases is mechanistically important in controlling Wnt signaling. Here, we identify ubiquitin-specific protease 25 (USP25) as a positive regulator of Wnt/β-catenin signaling. We found that USP25 directly interacted with tankyrases to promote their deubiquitination and stabilization. We demonstrated that USP25 deficiency could promote the degradation of tankyrases and consequent stabilization of Axin to antagonize Wnt signaling. We further characterized the interaction between TNKS1 and USP25 by X-ray crystal structure determination. Our results provide important new insights into the molecular mechanism that regulates the turnover of tankyrases and the possibility of targeting the stability of tankyrases by antagonizing their interaction with USP25 to modulate the Wnt/β-catenin pathway.

FH535 Suppresses Osteosarcoma Growth In Vitro and Inhibits Wnt Signaling through Tankyrases

Osteosarcoma (OS) is an aggressive primary bone tumor which exhibits aberrantly activated Wnt signaling. The canonical Wnt signaling cascade has been shown to drive cancer progression and metastasis through the activation of β-catenin. Hence, small molecule inhibitors of Wnt targets are being explored as primary or adjuvant chemotherapy. In this study, we have investigated the ability of FH535, an antagonist of Wnt signaling, to inhibit the growth of OS cells. We found that FH535 was cytotoxic in all OS cell lines which were tested (143b, U2OS, SaOS-2, HOS, K7M2) but well tolerated by normal human osteoblast cells. Additionally, we have developed an in vitro model of doxorubicin-resistant OS and found that these cells were highly responsive to FH535 treatment. Our analysis provided evidence that FH535 strongly inhibited markers of canonical Wnt signaling. In addition, our findings demonstrate a reduction in PAR-modification of Axin2 indicating inhibition of the tankyrase 1/2 enzymes. Moreover, we observed inhibition of auto-modification of PARP1 in the presence of FH535, indicating inhibition of PARP1 enzymatic activity. These data provide evidence that FH535 acts through the tankyrase 1/2 enzymes to suppress Wnt signaling and could be explored as a potent chemotherapeutic agent for the control of OS.

Ubiquitination and SUMOylation in Telomere Maintenance and Dysfunction

Telomeres are essential nucleoprotein structures at linear chromosomes that maintain genome integrity by protecting chromosome ends from being recognized and processed as damaged DNA. In addition, they limit the cell’s proliferative capacity, as progressive loss of telomeric DNA during successive rounds of cell division eventually causes a state of telomere dysfunction that prevents further cell division. When telomeres become critically short, the cell elicits a DNA damage response resulting in senescence, apoptosis or genomic instability, thereby impacting on aging and tumorigenesis. Over the past years substantial progress has been made in understanding the role of post-translational modifications in telomere-related processes, including telomere maintenance, replication and dysfunction. This review will focus on recent findings that establish an essential role for ubiquitination and SUMOylation at telomeres.

Mammalian telomeres and their partnership with lamins

Chromosome ends are complex structures, which require a panel of factors for their elongation, replication, and protection. We describe here the mechanics of mammalian telomeres, dynamics and maintainance in relation to lamins. Multiple biochemical connections, including association of telomeres to the nuclear envelope and matrix, of telomeric proteins to lamins, and of lamin-associated proteins to chromosome ends, underline the interplay between lamins and telomeres. Paths toward senescence, such as defective telomere replication, altered heterochromatin organization, and impaired DNA repair, are common to lamins' and telomeres' dysfunction. The convergence of phenotypes can be interpreted through a model of dynamic, lamin-controlled functional platforms dedicated to the function of telomeres as fragile sites. The features of telomeropathies and laminopathies, and of animal models underline further overlapping aspects, including the alteration of stem cell compartments. We expect that future studies of basic biology and on aging will benefit from the analysis of this telomere-lamina interplay.

Nucleophosmin Interacts with PIN2/TERF1-interacting Telomerase Inhibitor 1 (PinX1) and Attenuates the PinX1 Inhibition on Telomerase Activity

Telomerase activation and telomere maintenance are critical for cellular immortalization and transformation. PIN2/TERF1-interacting telomerase inhibitor 1 (PinX1) is a telomerase regulator and the aberrant expression of PinX1 causes telomere shortening. Identifying PinX1-interacting proteins is important for understanding telomere maintenance. We found that PinX1 directly interacts with nucleophosmin (NPM), a protein that has been shown to positively correlate with telomerase activity. We further showed that PinX1 acts as a linker in the association between NPM and hTERT, the catalytic subunit of telomerase. Additionally, the recruitment of NPM by PinX1 to the telomerase complex could partially attenuate the PinX1-mediated inhibition on telomerase activity. Taken together, our data reveal a novel mechanism that regulates telomerase activation through the interaction between NPM, PinX1 and the telomerase complex.

Tankyrase-Binding Protein TNKS1BP1 Regulates Actin Cytoskeleton Rearrangement and Cancer Cell Invasion

Tankyrase, a PARP that promotes telomere elongation and Wnt/β-catenin signaling, has various binding partners, suggesting that it has as-yet unidentified functions. Here, we report that the tankyrase-binding protein TNKS1BP1 regulates actin cytoskeleton and cancer cell invasion, which is closely associated with cancer progression. TNKS1BP1 colocalized with actin filaments and negatively regulated cell invasion. In TNKS1BP1-depleted cells, actin filament dynamics, focal adhesion, and lamellipodia ruffling were increased with activation of the ROCK/LIMK/cofilin pathway. TNKS1BP1 bound the actin-capping protein CapZA2. TNKS1BP1 depletion dissociated CapZA2 from the cytoskeleton, leading to cofilin phosphorylation and enhanced cell invasion. Tankyrase overexpression increased cofilin phosphorylation, dissociated CapZA2 from cytoskeleton, and enhanced cell invasion in a PARP activity-dependent manner. In clinical samples of pancreatic cancer, TNKS1BP1 expression was reduced in invasive regions. We propose that the tankyrase-TNKS1BP1 axis constitutes a posttranslational modulator of cell invasion whose aberration promotes cancer malignancy. Cancer Res; 77(9); 2328-38. ©2017 AACR.

Macrocyclized Extended Peptides: Inhibiting the Substrate-Recognition Domain of Tankyrase

We report a double-click macrocyclization approach for the design of constrained peptide inhibitors having non-helical or extended conformations. Our targets are the tankyrase proteins (TNKS), poly(ADP-ribose) polymerases (PARP) that regulate Wnt signaling by targeting Axin for degradation. TNKS are deregulated in many different cancer types, and inhibition of TNKS therefore represents an attractive therapeutic strategy. However, clinical development of TNKS-specific PARP catalytic inhibitors is challenging due to off-target effects and cellular toxicity. We instead targeted the substrate-recognition domain of TNKS, as it is unique among PARP family members. We employed a two-component strategy, allowing peptide and linker to be separately engineered and then assembled in a combinatorial fashion via click chemistry. Using the consensus substrate-peptide sequence as a starting point, we optimized the length and rigidity of the linker and its position along the peptide. Optimization was further guided by high-resolution crystal structures of two of the macrocyclized peptides in complex with TNKS. This approach led to macrocyclized peptides with submicromolar affinities for TNKS and high proteolytic stability that are able to disrupt the interaction between TNKS and Axin substrate and to inhibit Wnt signaling in a dose-dependent manner. The peptides therefore represent a promising starting point for a new class of substrate-competitive inhibitors of TNKS with potential for suppressing Wnt signaling in cancer. Moreover, by demonstrating the application of the double-click macrocyclization approach to non-helical, extended, or irregularly structured peptides, we greatly extend its potential and scope, especially given the frequency with which such motifs mediate protein-protein interactions.

APC Mutations as a Potential Biomarker for Sensitivity to Tankyrase Inhibitors in Colorectal Cancer

In most colorectal cancers, Wnt/β-catenin signaling is activated by loss-of-function mutations in the adenomatous polyposis coli (APC) gene and plays a critical role in tumorigenesis. Tankyrases poly(ADP-ribosyl)ate and destabilize Axins, a negative regulator of β-catenin, and upregulate β-catenin signaling. Tankyrase inhibitors downregulate β-catenin and are expected to be promising therapeutics for colorectal cancer. However, colorectal cancer cells are not always sensitive to tankyrase inhibitors, and predictive biomarkers for the drug sensitivity remain elusive. Here we demonstrate that the short-form APC mutations predict the sensitivity of colorectal cancer cells to tankyrase inhibitors. By using well-established colorectal cancer cell lines, we found that tankyrase inhibitors downregulated β-catenin in the drug-sensitive, but not resistant, colorectal cancer cells. The drug-sensitive cells showed higher Tcf/LEF transcriptional activity than the resistant cells and possessed "short" truncated APCs lacking all seven β-catenin-binding 20-amino acid repeats (20-AARs). In contrast, the drug-resistant cells possessed "long" APC retaining two or more 20-AARs. Knockdown of the long APCs with two 20-AARs increased β-catenin, Tcf/LEF transcriptional activity and its target gene AXIN2 expression. Under these conditions, tankyrase inhibitors were able to downregulate β-catenin in the resistant cells. These results indicate that the long APCs are hypomorphic mutants, whereas they exert a dominant-negative effect on Axin-dependent β-catenin degradation caused by tankyrase inhibitors. Finally, we established 16 patient-derived colorectal cancer cells and confirmed that the tankyrase inhibitor-responsive cells harbor the short-form APC mutations. These observations exemplify the predictive importance of APC mutations, the most common genetic alteration in colorectal cancers, for molecular targeted therapeutics. Mol Cancer Ther; 16(4); 752-62. ©2017 AACR.

Tankyrase Activity in Organs and Tissues of Mice

Tankyrase, one of the NAD+ ADP-ribosyltransferases, is a target for drugs developed for their anticancer and other pharmacological activities. We designed an assay for estimation of the inhibition or activation of the enzyme in preclinical studies. In mice, the highest specific activity of tankyrase was observed in thymus, spleen, pancreas, and bone marrow. In murine liver, tankyrase is active in ontogenesis and during reparative regeneration; however, the basal activity is hardly detectable in normal liver and most of other organs of adult animals. We suggest that tankyrase is a part of the tissue growth and repair machinery, while its age-dependent inhibition, when an organism stops growing, turns on phenoptosis.

Highly Potent and Isoform Selective Dual Site Binding Tankyrase/Wnt Signaling Inhibitors That Increase Cellular Glucose Uptake and Have Antiproliferative Activity

Compounds 13 and 14 were evaluated against 11 PARP isoforms to reveal that both 13 and 14 were more potent and isoform selective toward inhibiting tankyrases (TNKSs) than the "standard" inhibitor 1 (XAV939)⁵, i.e., IC₅₀ = 100 pM vs TNKS2 and IC₅₀ = 6.5 μM vs PARP1 for 14. In cellular assays, 13 and 14 inhibited Wnt-signaling, enhanced insulin-stimulated glucose uptake, and inhibited the proliferation of DLD-1 colorectal adenocarcinoma cells to a greater extent than 1.

Identifying and Validating Tankyrase Binders and Substrates: A Candidate Approach

The poly(ADP-ribose)polymerase (PARP) enzyme tankyrase (TNKS/ARTD5, TNKS2/ARTD6) uses its ankyrin repeat clusters (ARCs) to recognize degenerate peptide motifs in a wide range of proteins, thereby recruiting such proteins and their complexes for scaffolding and/or poly(ADP-ribosyl)ation. Here, we provide guidance for predicting putative tankyrase-binding motifs, based on the previously delineated peptide sequence rules and existing structural information. We present a general method for the expression and purification of tankyrase ARCs from Escherichia coli and outline a fluorescence polarization assay to quantitatively assess direct ARC-TBM peptide interactions. We provide a basic protocol for evaluating binding and poly(ADP-ribosyl)ation of full-length candidate interacting proteins by full-length tankyrase in mammalian cells.

A comprehensive look of poly(ADP-ribose) polymerase inhibition strategies and future directions for cancer therapy

The finding of promising drugs represents a huge challenge in cancer therapeutics, therefore it is important to seek out novel approaches and elucidate essential cellular processes in order to identify potential drug targets. Studies on DNA repair pathway suggested that an enzyme, PARP, which plays a significant role in DNA repair responses, could be targeted in cancer therapy. Hence, the efficacy of PARP inhibitors in cancer therapy has been investigated and has progressed from the laboratory to clinics, with olaparib having already been approved by the US FDA for ovarian cancer treatment. Here, we have discussed the development of PARP inhibitors, strategies to improve their selectivity and efficacy, including innovative combinational and synthetic lethality approaches to identify effective PARP inhibitors in cancer treatment.

Cell cycle-regulated ubiquitination of tankyrase 1 by RNF8 and ABRO1/BRCC36 controls the timing of sister telomere resolution

Timely resolution of sister chromatid cohesion in G2/M is essential for genome integrity. Resolution at telomeres requires the poly(ADP-ribose) polymerase tankyrase 1, but the mechanism that times its action is unknown. Here, we show that tankyrase 1 activity at telomeres is controlled by a ubiquitination/deubiquitination cycle depending on opposing ubiquitin ligase and deubiquitinase activities. In late S/G2 phase, the DNA damage-responsive E3 ligase RNF8 conjugates K63-linked ubiquitin chains to tankyrase 1, while in G1 phase such ubiquitin chains are removed by BRISC, an ABRO1/BRCC36-containing deubiquitinase complex. We show that K63-linked ubiquitin chains accumulate on tankyrase 1 in late S/G2 to promote its stabilization, association with telomeres, and resolution of cohesion. Timing of this posttranslational modification coincides with the ATM-mediated DNA damage response that occurs on functional telomeres following replication in G2. Removal of ubiquitin chains is controlled by ABRO1/BRCC36 and occurs as cells exit mitosis and enter G1, ensuring that telomere cohesion is not resolved prematurely in S phase. Our studies suggest that a cell cycle-regulated posttranslational mechanism couples resolution of telomere cohesion with completion of telomere replication to ensure genome integrity.

Conversion of Normal To Malignant Phenotype: Telomere Shortening, Telomerase Activation, and Genomic Instability During Immortalization of Human Oral Keratinocytes

Normal somatic cells terminate their replicative life span through a pathway leading to cellular senescence, which is triggered by activation of p53 and/or pRb in response to critically shortened telomere DNA. Potentially neoplastic cells must first overcome the senescence checkpoint mechanisms and subsequently activate telomerase to propagate indefinitely. Although telomerase activation is closely associated with cellular immortality, telomerase alone is not sufficient to warrant tumorigenicity. Environmental factors, including chemical carcinogens and viral infection, often contribute to aberrant changes leading to tumorigenic conversion of normal cells. Of particular importance in oral cancer development are tobacco-related chemical carcinogens and human papillomavirus (HPV) infection. To describe the molecular mechanisms by which these environmental factors facilitate the genesis of oral cancer, we first established an in vitro multistep oral carcinogenesis model by sequential exposure of normal human oral keratinocytes (NHOK) to "high risk" HPV and chemical carcinogens. Upon introduction of the HPV genome, the cells bypassed the senescence checkpoint and entered into an extended, but not immortal, life span during which telomere DNA continued to shorten. In a few immortal clones surviving beyond the crisis, we found a marked elevation of telomerase activity and stabilization of telomere length. Furthermore, the E6 and E7 oncoproteins of "high risk" HPV disrupted the cell cycle control and DNA repair in immortalized HOK, and enhanced mutation frequency resulting from genomic instability. However, HPV infection alone failed to give rise to a tumorigenic cell population, which required further exposure to chemical carcinogens in addition to HPV infection. Analysis of the data presented suggests that oral carcinogenesis is a series of discrete genetic alterations that result from a continued genotoxic challenge by environmental risk factors. Our in vitro model may be useful for investigators with interest in furthering our understanding of oral carcinogenesis.

PARP, transcription and chromatin modeling

Compaction mode of chromatin and chromatin highly organised structures regulate gene expression. Posttranslational modifications, histone variants and chromatin remodelers modulate the compaction, structure and therefore function of specific regions of chromatin. The generation of poly(ADP-ribose) (PAR) is emerging as one of the key signalling events on sites undergoing chromatin structure modulation. PAR is generated locally in response to stresses. These include genotoxic stress but also differentiation signals, metabolic and hormonal cues. A pictures emerges in which transient PAR formation is essential to orchestrate chromatin remodelling and transcription factors allowing the cell to adapt to alteration in its environment. This review summarizes the diverse factors of ADP-ribosylation in the adaptive regulation of chromatin structure and transcription.

Expanding functions of ADP-ribosylation in the maintenance of genome integrity

Cell response to genotoxic stress requires a complex network of sensors and effectors from numerous signaling and repair pathways, among them the nuclear poly(ADP-ribose) polymerase 1 (PARP1) plays a central role. PARP1 is catalytically activated in the setting of DNA breaks. It uses NAD⁺ as a donor and catalyses the synthesis and subsequent covalent attachment of branched ADP-ribose polymers onto itself and various acceptor proteins to promote repair. Its inhibition is now considered as an efficient therapeutic strategy to potentiate the cytotoxic effect of chemotherapy and radiation or to exploit synthetic lethality in tumours with defective homologous recombination mediated repair. Still, efforts made on understanding the role of PARylation in DNA repair continues to yield novel discoveries. Over the last years, our knowledge in this field has been particularly advanced by the discovery of novel biochemical and functional properties featuring PARP1, by the characterization of the other PARP family members and by the identification of a panel of enzymes capable of erasing poly(ADP-ribose). The aim of this review is to provide an overview of these newest findings and their relevance in genome surveillance.

Tankyrase inhibition promotes a stable human naïve pluripotent state with improved functionality

The derivation and maintenance of human pluripotent stem cells (hPSCs) in stable naïve pluripotent states has a wide impact in human developmental biology. However, hPSCs are unstable in classical naïve mouse embryonic stem cell (ESC) WNT and MEK/ERK signal inhibition (2i) culture. We show that a broad repertoire of conventional hESC and transgene-independent human induced pluripotent stem cell (hiPSC) lines could be reverted to stable human preimplantation inner cell mass (ICM)-like naïve states with only WNT, MEK/ERK, and tankyrase inhibition (LIF-3i). LIF-3i-reverted hPSCs retained normal karyotypes and genomic imprints, and attained defining mouse ESC-like functional features, including high clonal self-renewal, independence from MEK/ERK signaling, dependence on JAK/STAT3 and BMP4 signaling, and naïve-specific transcriptional and epigenetic configurations. Tankyrase inhibition promoted a stable acquisition of a human preimplantation ICM-like ground state via modulation of WNT signaling, and was most efficacious in efficiently reprogrammed conventional hiPSCs. Importantly, naïve reversion of a broad repertoire of conventional hiPSCs reduced lineage-primed gene expression and significantly improved their multilineage differentiation capacities. Stable naïve hPSCs with reduced genetic variability and improved functional pluripotency will have great utility in regenerative medicine and human disease modeling.

Heregulin, a new regulator of telomere length in human cells

The growth factor heregulin (HRG) promotes breast cancer (BC) tumorigenesis and metastasis and differentially modulates BC cell responses to DNA-damaging agents via its dual extracellular and nuclear localization. Given the central role of telomere dysfunction to drive carcinogenesis and to alter the chemotherapeutic profile of transformed cells, we hypothesized that an unanticipated nuclear function of HRG might be to regulate telomere length. Engineered overexpression of the HRGβ2 isoform in non-aggressive, HRG-negative MCF-7 BC cells resulted in a significant shortening of telomeres (up to 1.3 kb) as measured by Southern blotting of telomere terminal restriction fragments. Conversely, antisense-mediated suppression of HRGβ2 in highly aggressive, HRG-overexpressing MDA-MB-231 and Hs578T cells increased telomere length up to 3.0 kb. HRGβ2 overexpression promoted a marked upregulation of telomere-binding protein 2 (TRF2) protein expression, whereas its knockdown profoundly decreased TRF2 expression. Double staining of endogenous HRGβ2 with telomere-specific peptide nucleic acid probe/fluorescence in situ hybridization (PNA/FISH) revealed the partial localization of HRG at the chromosome ends. Moreover, a predominantly nucleoplasmic staining pattern of endogenous HRGβ2 appeared to co-localize with TRF2 and, concomitantly with RAP1, a telomere regulator that specifically interacts with TRF2. Small interfering RNA-mediated knockdown of HRG decreased the expression of TRF2 and RAP1, decreased their presence at chromosome ends, and coincidentally resulted in the formation of longer telomeres. This study uncovers a new function for HRGβ2 in controlling telomere length, in part due to its ability to regulate and interact with the telomere-associated proteins TRF2 and RAP1.