A new pathway that regulates 53BP1 stability implicates cathepsin L and vitamin D in DNA repair

Prelamin A impairs 53BP1 nuclear entry by mislocalizing NUP153 and disrupting the Ran gradient

The nuclear lamina is essential for the proper structure and organization of the nucleus. Deregulation of A-type lamins can compromise genomic stability, alter chromatin organization and cause premature vascular aging. Here, we show that accumulation of the lamin A precursor, prelamin A, inhibits 53BP1 recruitment to sites of DNA damage and increases basal levels of DNA damage in aged vascular smooth muscle cells. We identify that this genome instability arises through defective nuclear import of 53BP1 as a consequence of abnormal topological arrangement of nucleoporin NUP153. We show for the first time that this nucleoporin is important for the nuclear localization of Ran and that the deregulated Ran gradient is likely to be compromising the nuclear import of 53BP1. Importantly, many of the defects associated with prelamin A expression were significantly reduced upon treatment with Remodelin, a small molecule recently reported to reverse deficiencies associated with abnormal nuclear lamina.

Mechanisms of oncogene-induced genomic instability

Activating mutations in oncogenes promote uncontrolled proliferation and malignant transformation. Approximately 30% of human cancers carry mutations in the RAS oncogene. Paradoxically, expression of mutant constitutively active Ras protein in primary human cells results in a premature proliferation arrest known as oncogene-induced senescence (OIS). This is more commonly observed in human pre-neoplasia than in neoplastic lesions, and is considered a tumor suppressor mechanism. Senescent cells are still metabolically active but in a status of cell cycle arrest characterized by specific morphological and physiological features that distinguish them from both proliferating cells, and cells growth-arrested by other means. Although the molecular mechanisms by which OIS is established are not totally understood, the current view is that OIS in human cells is tightly linked to persistent activation of the DNA damage response (DDR) pathway, as a consequence of replication stress. Here we will highlight recent advances in our understanding of molecular mechanisms leading to hyper-replication stress in response to oncogene activation, and of the crosstalk between replication stress and persistent activation of the DDR. We will also discuss new evidence for DNA repair deficiencies during OIS, which might increase the genomic instability that drives senescence bypass and malignant transformation.

The nuclear lamina in health and disease

The nuclear lamina (NL) is a structural component of the nuclear envelope and makes extensive contacts with integral nuclear membrane proteins and chromatin. These interactions are critical for many cellular processes, such as nuclear positioning, perception of mechanical stimuli from the cell surface, nuclear stability, 3-dimensional organization of chromatin and regulation of chromatin-binding proteins, including transcription factors. The NL is present in all nucleated metazoan cells but its composition and interactome differ between tissues. Most likely, this contributes to the broad spectrum of disease manifestations in humans with mutations in NL-related genes, ranging from muscle dystrophies to neurological disorders, lipodystrophies and progeria syndromes. We review here exciting novel insight into NL function at the cellular level, in particular in chromatin organization and mechanosensation. We also present recent observations on the relation between the NL and metabolism and the special relevance of the NL in muscle tissues. Finally, we discuss new therapeutic approaches to treat NL-related diseases.

Vitamin D/vitamin D receptor axis regulates DNA repair during oncogene-induced senescence

Oncogenic Ras expression is associated with activation of the DNA damage response (DDR) pathway, as evidenced by elevated DNA damage, primarily DNA double-strand breaks (DSBs), and activation of DNA damage checkpoints, which in primary human cells leads to entry into senescence. DDR activation is viewed as a physiological barrier against uncontrolled proliferation in oncogenic Ras-expressing cells, and arises in response to genotoxic stress due to the production of reactive oxygen species (ROS) that damage DNA, and to hyper-replication stress. Although oncogene-induced senescence (OIS) is considered a tumor suppressor mechanism, the accumulation of DNA damage in senescent cells is thought to cause genomic instability, eventually allowing secondary hits in the genome that promote tumorigenesis. To date, the molecular mechanisms behind DNA repair defects during OIS remain poorly understood. Here, we show that oncogenic Ras expression in human primary cells results in down-regulation of BRCA1 and 53BP1, two key factors in DNA DSBs repair by homologous recombination (HR) and non-homologous end joining (NHEJ), respectively. As a consequence, Ras-induced senescent cells are hindered in their ability to recruit BRCA1 and 53BP1 to DNA damage sites. While BRCA1 is down-regulated at transcripts levels, 53BP1 loss is caused by activation of cathepsin L (CTSL)-mediated degradation of 53BP1 protein. Moreover, we discovered a marked down-regulation of vitamin D receptor (VDR) during OIS, and a role for the vitamin D/VDR axis regulating the levels of these DNA repair factors during OIS. This study reveals a new functional relationship between the oncogene Ras, the vitamin D/VDR axis, and the expression of DNA repair factors, in the context of OIS. The observed deficiencies in DNA repair factors in senescent cells could contribute to the genomic instability that allows senescence bypass and tumorigenesis.

Synthesis and Preclinical Evaluation of a Highly Improved Anticancer Prodrug Activated by Histone Deacetylases and Cathepsin L

Lack of absolute selectivity against cancer cells is a major limitation for current cancer therapies. In the previous study, we developed a prodrug strategy for selective cancer therapy using a masked cytotoxic agent puromycin [Boc-Lys(Ac)-Puromycin], which can be sequentially activated by histone deacetylases (HDACs) and cathepsin L (CTSL) to kill cancer cells expressing high levels of both enzymes. Despite the promise as a selective cancer therapy, its requirement of relatively high dosage could be a potential issue in the clinical setting. To address this issue, we aimed to further improve the overall efficacy of our prodrug strategy. Since the proteolytic cleavage by CTSL is the rate-limiting step for the drug activation, we sought to improve the substrate structure for CTSL activity by modifying the α-amino protecting group of lysine. Here we show that protection with Fmoc [Fmoc-Lys(Ac)-Puromycin] exhibits a marked improvement in overall anticancer efficacy compared to the original Boc-Lys(Ac)-Puromycin and this is mainly due to the highly efficient cellular uptake besides its improved substrate structure. Furthermore, to address a concern that the improved drug efficacy might direct high toxicity to the normal cells, we confirmed that Fmoc-Lys(Ac)-Puromycin still retains excellent cancer selectivity in vitro and no obvious systemic off-target toxicity in vivo. Thus our preclinical evaluation data presented here demonstrate that the Fmoc-Lys(Ac)-Puromycin exhibits substantially improved anticancer efficacy, further supporting our approach for the selective cancer therapy.

Tying up loose ends: telomeres, genomic instability and lamins

On casual inspection, the eukaryotic nucleus is a deceptively simple organelle. Far from being a bag of chromatin, the nucleus is, in some ways, a structural and functional extension of the chromosomes it contains. Recently, interest has intensified in how chromosome compartmentalization and dynamics affect nuclear function. Different studies uncovered functional interactions between chromosomes and the filamentous nuclear meshwork comprised of lamin proteins. Here, we summarize recent research suggesting that telomeres, the capping structures that protect chromosome ends, are stabilized by lamin-binding and that alterations in nuclear lamins lead to defects in telomere compartmentalization, homeostasis and function. Telomere dysfunction contributes to the genomic instability that characterizes aging-related diseases, and might be an important factor in the pathophysiology of lamin-related diseases.

Methods to Monitor DNA Repair Defects and Genomic Instability in the Context of a Disrupted Nuclear Lamina

The organization of the genome within the nuclear space is viewed as an additional level of regulation of genome function, as well as a means to ensure genome integrity. Structural proteins associated with the nuclear envelope, in particular lamins (A- and B-type) and lamin-associated proteins, play an important role in genome organization. Interestingly, there is a whole body of evidence that links disruptions of the nuclear lamina with DNA repair defects and genomic instability. Here, we describe a few standard techniques that have been successfully utilized to identify mechanisms behind DNA repair defects and genomic instability in cells with an altered nuclear lamina. In particular, we describe protocols to monitor changes in the expression of DNA repair factors (Western blot) and their recruitment to sites of DNA damage (immunofluorescence); kinetics of DNA double-strand break repair after ionizing radiation (neutral comet assays); frequency of chromosomal aberrations (FISH, fluorescence in situ hybridization); and alterations in telomere homeostasis (Quantitative-FISH). These techniques have allowed us to shed some light onto molecular mechanisms by which alterations in A-type lamins induce genomic instability, which could contribute to the pathophysiology of aging and aging-related diseases.

Cathepsin L targeting in cancer treatment

Proteolytic enzymes may serve as promising targets for novel therapeutic treatment strategies seeking to impede cancer progression and metastasis. One such enzyme is cathepsin L (CTSL), a lysosomal cysteine protease. CTSL upregulation, a common occurrence in a variety of human cancers, has been widely correlated with metastatic aggressiveness and poor patient prognosis. In addition, CTSL has been implicated to contribute to cancer-associated osteolysis, a debilitating morbidity affecting both life expectancy and the quality of life. In this review, we highlight the mechanisms by which CTSL contributes to tumor progression and dissemination and discuss the therapeutic utility of CTSL intervention strategies aimed at impeding metastatic progression and bone resorption.

Chronic Alcohol Exposure Decreases 53BP1 Protein Levels Leading to a Defective DNA Repair in Cultured Primary Cortical Neurons

Chronic alcohol consumption may cause neurodevelopmental and neurodegenerative disorders. Alcohol neurotoxicity is associated with the production of acetaldehyde and reactive oxygen species that induce oxidative DNA damage. However, the molecular mechanisms by which ethanol disturbs the DNA damage response (DDR), resulting in a defective DNA repair, remain unknown. Here, we have used cultured primary cortical neurons exposed to 50 or 100 mM ethanol for 7 days to analyze the ethanol-induced DDR. Ethanol exposure produced a dose-dependent generation of double strand breaks and the formation of DNA damage foci immunoreactive for the histone γH2AX, a DNA damage marker, and for the ubiquitylated H2A, which is involved in chromatin remodeling at DNA damage sites. Importantly, these DNA damage foci failed to recruit the protein 53BP1, a crucial DNA repair factor. This effect was associated with a drop in 53BP1 mRNA and protein levels and with an inhibition of global transcription. Moreover, ethanol-exposed neurons treated with ionizing radiation (2 Gy) also failed to recruit 53BP1 at DNA damage foci and exhibited a greater vulnerability to DNA lesions than irradiated control neurons. Our results support that defective DNA repair, mediated by the deficient expression and recruitment of 53BP1 to DNA damage sites, represents a novel mechanism involved in ethanol neurotoxicity. The design of therapeutic strategies that increase or stabilize 53BP1 levels might potentially promote DNA repair and partially compensate alcohol neurotoxicity.

Diverse lamin-dependent mechanisms interact to control chromatin dynamics. Focus on laminopathies

Interconnected functional strategies govern chromatin dynamics in eukaryotic cells. In this context, A and B type lamins, the nuclear intermediate filaments, act on diverse platforms involved in tissue homeostasis. On the nuclear side, lamins elicit large scale or fine chromatin conformational changes, affect DNA damage response factors and transcription factor shuttling. On the cytoplasmic side, bridging-molecules, the LINC complex, associate with lamins to coordinate chromatin dynamics with cytoskeleton and extra-cellular signals.

 

Consistent with such a fine tuning, lamin mutations and/or defects in their expression or post-translational processing, as well as mutations in lamin partner genes, cause a heterogeneous group of diseases known as laminopathies. They include muscular dystrophies, cardiomyopathy, lipodystrophies, neuropathies, and progeroid syndromes. The study of chromatin dynamics under pathological conditions, which is summarized in this review, is shedding light on the complex and fascinating role of the nuclear lamina in chromatin regulation.

DNA repair defects and genome instability in Hutchinson-Gilford Progeria Syndrome

The integrity of the nuclear lamina has emerged as an important factor in the maintenance of genome stability. In particular, mutations in the LMNA gene, encoding A-type lamins (lamin A/C), alter nuclear morphology and function, and cause genomic instability. LMNA gene mutations are associated with a variety of degenerative diseases and devastating premature aging syndromes such as Hutchinson-Gilford Progeria Syndrome (HGPS) and Restrictive Dermopathy (RD). HGPS is a severe laminopathy, with patients dying in their teens from myocardial infarction or stroke. HGPS patient-derived cells exhibit nuclear shape abnormalities, changes in epigenetic regulation and gene expression, telomere shortening, genome instability, and premature senescence. This review highlights recent advances in identifying molecular mechanisms that contribute to the pathophysiology of HGPS, with a special emphasis on DNA repair defects and genome instability.

Triple-negative breast cancer in African-American women: disparities versus biology

Triple-negative breast cancer (TNBC) is an aggressive breast cancer subtype that disproportionately affects BRCA1 mutation carriers and young women of African origin. There is evidence that African-American women with TNBC have worse clinical outcomes than women of European descent. However, it is unclear whether survival differences persist after adjusting for disparities in access to health-care treatment, co-morbid disease and income. It remains controversial whether TNBC in African-American women is a molecularly distinct disease or whether African-American women have a higher incidence of aggressive biology driven by disparities: there is evidence in support of both. Understanding the relative contributions of biology and disparities is essential for improving the poor survival rate of African-American women with TNBC.

Lamin A/C-dependent interaction with 53BP1 promotes cellular responses to DNA damage

Lamins A/C have been implicated in DNA damage response pathways. We show that the DNA repair protein 53BP1 is a lamin A/C binding protein. In undamaged human dermal fibroblasts (HDF), 53BP1 is a nucleoskeleton protein. 53BP1 binds to lamins A/C via its Tudor domain, and this is abrogated by DNA damage. Lamins A/C regulate 53BP1 levels and consequently lamin A/C-null HDF display a 53BP1 null-like phenotype. Our data favour a model in which lamins A/C maintain a nucleoplasmic pool of 53BP1 in order to facilitate its rapid recruitment to sites of DNA damage and could explain why an absence of lamin A/C accelerates aging.

Nuclear envelope and chromatin, lock and key of genome integrity

More than as an inert separation between the inside and outside of the nucleus, the nuclear envelope (NE) constitutes an active toll, which controls the import and export of molecules, and also a hub for a diversity of genomic processes, such as transcription, DNA repair, and chromatin dynamics. Proteins localized at the inner surface of the NE (such as lamins, nuclear pore proteins, lamin-associated proteins) interact with chromatin in a dynamic manner, contributing to the establishment of topological domains. In this review, we address the complex interplay between chromatin and NE. We discuss the divergence of this cross talk during evolution and comment both on the current established models and the most recent findings. In particular, we focus our attention on how the NE cooperates with chromatin in protecting the genome integrity.

Lamins: nuclear intermediate filament proteins with fundamental functions in nuclear mechanics and genome regulation

Lamins are intermediate filament proteins that form a scaffold, termed nuclear lamina, at the nuclear periphery. A small fraction of lamins also localize throughout the nucleoplasm. Lamins bind to a growing number of nuclear protein complexes and are implicated in both nuclear and cytoskeletal organization, mechanical stability, chromatin organization, gene regulation, genome stability, differentiation, and tissue-specific functions. The lamin-based complexes and their specific functions also provide insights into possible disease mechanisms for human laminopathies, ranging from muscular dystrophy to accelerated aging, as observed in Hutchinson-Gilford progeria and atypical Werner syndromes.

The effect on quality of life of vitamin D administration for advanced cancer treatment (VIDAFACT study): protocol of a randomised controlled trial

INTRODUCTION

Vitamin D is related to resistance to chronic diseases, physiological parameters and functional measures. All of these relationships underscore the potential benefits of cholecalciferol or D3 (nutritional vitamin D) in cancer. This is the first study designed to obtain conclusive evidence on the effect of cholecalciferol in advanced patients with cancer. The main goal is to assess its effects on the patient's perceived quality of life. Cholecalciferol's impact on fatigue and physical performance, as well as its cost utility, will also be assessed.

METHODS AND ANALYSIS

A randomised triple-blind phase II/III placebo-controlled multicentre trial has been designed. Patients satisfying the inclusion and exclusion criteria will be randomly assigned to receive cholecalciferol or placebo. Eligible patients will be adults with a locally advanced or metastatic or inoperable solid cancer in palliative care, who have given signed informed consent and have matched inclusion and exclusion criteria. The randomisation will be based on a computer-generated procedure and centralised by the pharmacy service of the coordinating centre. The assigned treatment will be administered by the hospital's pharmacy to conceal group allocation for patients and healthcare providers. Cholecalciferol (4000 IU/day) or placebo, starting at day 15 and continuing up to day 42, will be added to palliative care treatment. Outpatient visits will be scheduled every 14 days.

ETHICS AND DISSEMINATION

Ethical approval was received from the Medical Ethical Commitee of the HUAV (CEIC-1169). Participants and their families will receive the research findings which will also be disseminated on local and national media, presented at national and international meetings of the specialty, and published in peer-reviewed scientific journals.

TRIAL REGISTRATION NUMBER

EudraCT: 2013-003478-29.

Novel roles of 1α,25(OH)2D3 on DNA repair provide new strategies for breast cancer treatment

Breast cancers classified as triple-negative (TNBC) and BRCA1-deficient, are particularly aggressive and difficult to treat. A major breakthrough was the finding that these tumors are exquisitely sensitive to inhibitors of poly(ADP-ribose) polymerase (PARPi). Phase II clinical trials have shown encouraging outcomes, with tolerable side effects. However, a significant fraction of these cancers acquire resistance. Elegant studies demonstrated that loss of the DNA repair protein 53BP1 contributes to the resistance of BRCA1-deficient cells and tumors to PARPi. Thus, raising the levels of 53BP1 in these aggressive tumors could potentially restore their sensitivity to PARPi and other genotoxic agents. We will review here our studies revealing that 1α,25(OH)2D3, an active form of vitamin D, stabilizes 53BP1 levels in tumor cells. Breast tumor cells that become BRCA1-deficient activate cathepsin L-mediated degradation of 53BP1 to ensure genome stability and proliferation. Importantly, 1α,25(OH)2D3 treatment restores the levels of 53BP1 as efficiently as cathepsin L inhibitors, which results in increased genomic instability in response to PARPi or radiation, and reduced proliferation. Furthermore, analysis of human breast tumors identified nuclear cathepsin L as a positive biomarker for TNBC, which correlates inversely with 53BP1 when vitamin D receptor (VDR) nuclear levels are low. The major findings of these studies are: (1) identification of a new pathway contributing to breast cancers with the poorest prognosis; (2) discovery of the ability of 1α,25(OH)2D3 to inhibit this pathway; and (3) discovery of a triple biomarker signature for identification of patients that could benefit from the treatment. This article is part of a Special Issue entitled '16th Vitamin D Workshop'.

Role of local bioactivation of vitamin D by CYP27A1 and CYP2R1 in the control of cell growth in normal endometrium and endometrial carcinoma

Vitamin D (VD) deficiency has been suggested as a risk factor for cancer. One recognized mechanism is that the low-serum 25-hydroxyvitamin D (25(OH)D) of VD deficiency reduces intratumoral 25(OH)D conversion to 1α,25-dihydroxyvitamin D (1,25D, the hormonal form of VD), compromising 1,25D-VD receptor (VDR) antitumoral actions. Reduced tumoral VDR and increased CYP24A1, the enzyme that degrades 1,25D and 25(OH)D, further worsen cancer progression. Importantly, in cells expressing CYP27A1 and/or CYP2R1, which convert inert VD into 25(OH)D, low-serum VD may reduce intratumoral 25(OH)D synthesis thereby compromising VDR antitumoral actions because 25(OH)D can activate the VDR directly and enhance 1,25D-VDR action. Therefore, this study examined whether abnormal endometrial expression of CYP27A1 and/or CYP2R1 may impair VDR-antiproliferative properties in endometrial carcinoma (EC). Immunohistochemical analysis of tissue microarrays of normal human endometrium (NE; n=60) and EC (n=157) showed the expected lower VDR expression in EC (P=0.0002). Instead, CYP24A1 expression was lower in EC compared with NE, while CYP27A1 and CYP2R1 expressions were higher (P=0.0002; P=0.03). Furthermore, in NE and EC, CYP2R1 and CYP27A1 expression correlated directly with nuclear VDR levels, an indicator of ligand-induced VDR activation, and inversely with the proliferation marker Ki67. Accordingly, in the endometrioid carcinoma cell lines IK, RL95/2 and HEC1-A, which express VDR, CYP27A1, and CYP2R1, VD efficaciously reduced cell viability and colony number, with a time course that paralleled actual increases in both intracellular 25(OH)D and nuclear VDR levels. Thus, VD may protect from EC progression in part through increased intratumoral 25(OH)D production by CYP27A1 and CYP2R1 for autocrine/paracrine enhancement of 1,25D-VDR-antiproliferative actions.

DNA damage and lamins

The spatial and temporal organization of the genome has emerged as an additional level of regulation of nuclear functions. Structural proteins associated with the nuclear envelope play important roles in the organization of the genome. The nuclear lamina, a polymeric meshwork formed by lamins (A- and B-type) and lamin-associated proteins, is viewed as a scaffold for tethering chromatin and protein complexes regulating a variety of nuclear functions. Alterations in lamins function impact DNA transactions such as transcription, replication, and repair, as well as epigenetic modifications that change chromatin structure. These data, and the association of defective lamins with a whole variety of degenerative disorders, premature aging syndromes, and cancer, provide evidence for these proteins operating as caretakers of the genome. In this chapter, we summarize current knowledge about the function of lamins in the maintenance of genome integrity, with special emphasis on the role of A-type lamins in the maintenance of telomere homeostasis and mechanisms of DNA damage repair. These findings have begun to shed some light onto molecular mechanisms by which alterations in A-type lamins induce genomic instability and contribute to the pathophysiology of aging and aging-related diseases, especially cancer.

Lamins are rapamycin targets that impact human longevity: a study in centenarians

The dynamic organisation of the cell nucleus is profoundly modified during growth, development and senescence as a result of changes in chromatin arrangement and gene transcription. A plethora of data suggests that the nuclear lamina is a key player in chromatin dynamics and argues in favour of a major involvement of prelamin A in fundamental mechanisms regulating cellular senescence and organism ageing. As the best model to analyse the role of prelamin A in normal ageing, we used cells from centenarian subjects. We show that prelamin A is accumulated in fibroblasts from centenarians owing to downregulation of its specific endoprotease ZMPSTE24, whereas other nuclear envelope constituents are mostly unaffected and cells do not enter senescence. Accumulation of prelamin A in nuclei of cells from centenarians elicits loss of heterochromatin, as well as recruitment of the inactive form of 53BP1, associated with rapid response to oxidative stress. These effects, including the prelamin-A-mediated increase of nuclear 53BP1, can be reproduced by rapamycin treatment of cells from younger individuals. These data identify prelamin A and 53BP1 as new targets of rapamycin that are associated with human longevity. We propose that the reported mechanisms safeguard healthy ageing in humans through adaptation of the nuclear environment to stress stimuli.

Lamin A Δexon9 mutation leads to telomere and chromatin defects but not genomic instability

Over 300 mutations in the LMNA gene, encoding A-type lamins, are associated with 15 human degenerative disorders and premature aging syndromes. Although genomic instability seems to contribute to the pathophysiology of some laminopathies, there is limited information about what mutations cause genomic instability and by which molecular mechanisms. Mouse embryonic fibroblasts depleted of A-type lamins or expressing mutants lacking exons 8-11 (Lmna(Δ8-11/Δ8-11)) exhibit alterations in telomere biology and DNA repair caused by cathepsin L-mediated degradation of 53BP1 and reduced expression of BRCA1 and RAD51. Thus, a region encompassing exons 8-11 seems essential for genome integrity. Given that deletion of lamin A exon 9 in the mouse (Lmna(Δ9/Δ9)) results in a progeria phenotype, we tested if this domain is important for genome integrity. Lmna(Δ9/Δ9) MEFs exhibit telomere shortening and heterochromatin alterations but do not activate cathepsin L-mediated degradation of 53BP1 and maintain expression of BRCA1 and RAD51. Accordingly, Lmna(Δ9/Δ9) MEFs do not present genomic instability, and expression of mutant lamin A Δexon9 in lamin-depleted cells restores DNA repair factors levels and partially rescues nuclear abnormalities. These data reveal that the domain encoded by exon 9 is important to maintain telomere homeostasis and heterochromatin structure but does not play a role in DNA repair, thus pointing to other exons in the lamin A tail as responsible for the genomic instability phenotype in Lmna(Δ8-11/Δ8-11) mice. Our study also suggests that the levels of DNA repair factors 53BP1, BRCA1 and RAD51 could potentially serve as biomarkers to identify laminopathies that present with genomic instability.

Differences in 53BP1 and BRCA1 regulation between cycling and non-cycling cells

BRCA1 and 53BP1 play decisive roles in the choice of DNA double-strand break repair mechanisms. BRCA1 promotes DNA end resection and homologous recombination (HR) during S/G 2 phases of the cell cycle, while 53BP1 inhibits end resection and facilitates non-homologous end-joining (NHEJ), primarily during G 1. This competitive relationship is critical for genome integrity during cell division. However, their relationship in the many cells in our body that are not cycling is unknown. We discovered profound differences in 53BP1 and BRCA1 regulation between cycling and non-cycling cells. Cellular growth arrest results in transcriptional downregulation of BRCA1 and activation of cathepsin-L (CTSL)-mediated degradation of 53BP1. Accordingly, growth-arrested cells do not form BRCA1 or 53BP1 ionizing radiation-induced foci (IRIF). Interestingly, cell cycle re-entry reverts this scenario, with upregulation of BRCA1, downregulation of CTSL, stabilization of 53BP1, and 53BP1 IRIF formation throughout the cycle, indicating that BRCA1 and 53BP1 are important in replicating cells and dispensable in non-cycling cells. We show that CTSL-mediated degradation of 53BP1, previously associated with aggressive breast cancers, is an endogenous mechanism of non-cycling cells to balance NHEJ (53BP1) and HR (BRCA1). Breast cancer cells exploit this mechanism to ensure genome stability and viability, providing an opportunity for targeted therapy.

Cathepsin L plays a role in quinolinic acid-induced NF-Κb activation and excitotoxicity in rat striatal neurons

The present study seeks to investigate the role of cathepsin L in glutamate receptor-induced transcription factor nuclear factor-kappa B (NF-κB) activation and excitotoxicity in rats striatal neurons. Stereotaxic administration of the N-methyl-d-aspartate (NMDA) receptor agonist Quinolinic acid (QA) into the unilateral striatum was used to produce the in vivo excitotoxic model. Co-administration of QA and the cathepsin L inhibitor Z-FF-FMK or 1-Naphthalenesulfonyl-IW-CHO (NaphthaCHO) was used to assess the contribution of cathepsin L to QA-induced striatal neuron death. Western blot analysis and cathepsin L activity assay were used to assess the changes in the levels of cathepsin L after QA treatment. Western blot analysis was used to assess the changes in the protein levels of inhibitor of NF-κB alpha isoform (IκB-α) and phospho-IκB alpha (p-IκBα) after QA treatment. Immunohistochemical analysis was used to detect the effects of Z-FF-FMK or NaphthaCHO on QA-induced NF-κB. Western blot analysis was used to detect the effects of Z-FF-FMK or NaphthaCHO on QA-induced IκB-α phosphorylation and degradation, changes in the levels of IKKα, p-IKKα, TP53, caspase-3, beclin1, p62, and LC3II/LC3I. The results show that QA-induced loss of striatal neurons were strongly inhibited by Z-FF-FMK or NaphthaCHO. QA-induced degradation of IκB-α, NF-κB nuclear translocation, up-regulation of NF-κB responsive gene TP53, and activation of caspase-3 was strongly inhibited by Z-FF-FMK or NaphthaCHO. QA-induced increases in beclin 1, LC3II/LC3I, and down-regulation of p62 were reduced by Z-FF-FMK or NaphthaCHO. These results suggest that cathepsin L is involved in glutamate receptor-induced NF-κB activation. Cathepsin L inhibitors have neuroprotective effects by inhibiting glutamate receptor-induced IκB-α degradation and NF-κB activation.

Hutchinson-Gilford progeria syndrome through the lens of transcription

Lamins are nuclear intermediate filaments. In addition to their structural roles, they are implicated in basic nuclear functions such as chromatin organization, DNA replication, transcription, DNA repair, and cell-cycle progression. Mutations in human LMNA gene cause several diseases termed laminopathies. One of the laminopathic diseases is Hutchinson-Gilford progeria syndrome (HGPS), which is caused by a spontaneous mutation and characterized by premature aging. HGPS phenotypes share certain similarities with several apparently comparable medical conditions, such as aging and atherosclerosis, with the conspicuous absence of neuronal degeneration and cancer rarity during the short lifespan of the patients. Cell lines from HGPS patients are characterized by multiple nuclear defects, which include abnormal morphology, altered histone modification patterns, and increased DNA damage. These cell lines provide insight into the molecular pathways including senescence that require lamins A and B1. Here, we review recent data on HGPS phenotypes through the lens of transcriptional deregulation caused by lack of functional lamin A, progerin accumulation, and lamin B1 silencing.

A-type lamins maintain the positional stability of DNA damage repair foci in mammalian nuclei

A-type lamins encoded by LMNA form a structural fibrillar meshwork within the mammalian nucleus. How this nuclear organization may influence the execution of biological processes involving DNA transactions remains unclear. Here, we characterize changes in the dynamics and biochemical interactions of lamin A/C after DNA damage. We find that DNA breakage reduces the mobility of nucleoplasmic GFP-lamin A throughout the nucleus as measured by dynamic fluorescence imaging and spectroscopy in living cells, suggestive of incorporation into stable macromolecular complexes, but does not induce the focal accumulation of GFP-lamin A at damage sites. Using a proximity ligation assay and biochemical analyses, we show that lamin A engages chromatin via histone H2AX and its phosphorylated form (γH2AX) induced by DNA damage, and that these interactions are enhanced after DNA damage. Finally, we use three-dimensional time-lapse imaging to show that LMNA inactivation significantly reduces the positional stability of DNA repair foci in living cells. This defect is partially rescued by the stable expression of GFP-lamin A. Thus collectively, our findings suggest that the dynamic structural meshwork formed by A-type lamins anchors sites of DNA repair in mammalian nuclei, providing fresh insight into the control of DNA transactions by nuclear structural organization.

BRCA1 loss activates cathepsin L-mediated degradation of 53BP1 in breast cancer cells

Cathepsin L degrades 53BP1 to overcome genomic instability and growth arrest in BRCA1-deficient and triple-negative breast cancers.

Loss of 53BP1 rescues BRCA1 deficiency and is associated with BRCA1-deficient and triple-negative breast cancers (TNBC) and with resistance to genotoxic drugs. The mechanisms responsible for decreased 53BP1 transcript and protein levels in tumors remain unknown. Here, we demonstrate that BRCA1 loss activates cathepsin L (CTSL)–mediated degradation of 53BP1. Activation of this pathway rescued homologous recombination repair and allowed BRCA1-deficient cells to bypass growth arrest. Importantly, depletion or inhibition of CTSL with vitamin D or specific inhibitors stabilized 53BP1 and increased genomic instability in response to radiation and poly(adenosine diphosphate–ribose) polymerase inhibitors, compromising proliferation. Analysis of human breast tumors identified nuclear CTSL as a positive biomarker for TNBC, which correlated inversely with 53BP1. Importantly, nuclear levels of CTSL, vitamin D receptor, and 53BP1 emerged as a novel triple biomarker signature for stratification of patients with BRCA1-mutated tumors and TNBC, with potential predictive value for drug response. We identify here a novel pathway with prospective relevance for diagnosis and customization of breast cancer therapy.

How to live without BRCA1

Enzyme helps cancer cells reactivate repair mechanism and overcome loss of tumor suppressor.

Age-related disease association of endogenous γ-H2AX foci in mononuclear cells derived from leukapheresis

The phosphorylated form of histone H2AX (γ-H2AX) forms immunohistochemically detectable foci at DNA double strand breaks. In peripheral blood mononuclear cells (PBMCs) derived from leukapheresis from patients enrolled in the Baltimore Longitudinal Study of Aging, γ-H2AX foci increased in a linear fashion with regards to age, peaking at ∼57 years. The relationship between the frequency of γ-H2AX foci and age-related pathologies was assessed. We found a statistically significant (p = 0.023) 50% increase in foci in PBMCs derived from patients with a known history of vitamin D deficiency. In addition, there were trends toward increased γ-H2AX foci in patients with cataracts (34% increase, p<0.10) and in sleep apnea patients (44%, p<0.10). Among patients ≥57 y/o, we found a significant (p = 0.037) 36% increase in the number of γ-H2AX foci/cell for patients with hypertension compared to non-hypertensive patients. Our results support a role for increased DNA damage in the morbidity of age-related diseases. γ -H2AX may be a biomarker for human morbidity in age-related diseases.

The problems of vitamin d insufficiency in older people

This report reviews evidence on disorders related to inadequate vitamin D repletion in older people. Vitamin D is as essential for bone health in adults as in children, preventing osteomalacia and muscle weakness and protecting against falls and low-impact fractures. Vitamin D is provided by skin synthesis by UVB-irradiation from summer sunshine and to a small extent by absorption from food. However, these processes become less efficient with age. Loss of mobility or residential care restricts solar exposure. Reduced appetite and financial problems often add to these problems. Thus, hypovitaminosis D is common world-wide, but is more common and more severe in older people. Non-classical effects of vitamin D, depending on serum circulating 25-hydroxyvitamin D concentrations, are present in most non-bony tissues; disorders associated with hypovitaminosis D include increased risks of sepsis [bacterial, mycobacterial and viral], cardiovascular and metabolic disorders [e.g. hyperlipidemia, type 2 diabetes mellitus, acute vascular events, dementia, stroke and heart failure]. Many cancer risks are associated with vitamin D inadequacy, though causality is accepted only for colo-rectal cancer. Maintenance of repletion in healthy older people requires intakes of ≥800IU/day [20μg], as advised by the Institute of Medicine [IOM], but achieving such intakes usually requires supplementation. Excessive intakes are dangerous, especially in undiagnosed primary hyperparathyroidism or sarcoidosis, but the IOM finds doses <4000 IU/day are safe. Many experts suggest that ≥1000-2000 IU [25-50μg] of vitamin D daily is necessary for older people, especially when independence is lost, or hypovitaminosis D could add to the clinical problem[s]. Much higher doses than these are needed for treatment of established deficiency or insufficiency.

Telomeres and telomerase dance to the rhythm of the cell cycle

The stability of the ends of linear eukaryotic chromosomes is ensured by functional telomeres, which are composed of short, species-specific direct repeat sequences. The maintenance of telomeres depends on a specialized ribonucleoprotein (RNP) called telomerase. Both telomeres and telomerase are dynamic entities with different physical behaviors and, given their substrate-enzyme relation, they must establish a productive interaction. Regulatory mechanisms controlling this interaction are key missing elements in our understanding of telomere functions. Here, we review the dynamic properties of telomeres and the maturing telomerase RNPs, and summarize how tracking the timing of their dance during the cell cycle will yield insights into chromosome stability mechanisms. Cancer cells often display loss of genome integrity; therefore, these issues are of particular interest for our understanding of cancer initiation or progression.

Lamin misexpression upregulates three distinct ubiquitin ligase systems that degrade ATR kinase in HeLa cells

Lamins are the major structural components of the nucleus and mutations in the human lamin A gene cause a number of genetic diseases collectively termed laminopathies. At the cellular level, lamin A mutations cause aberrant nuclear morphology and defects in nuclear functions such as the response to DNA damage. We have investigated the mechanism of depletion of a key damage sensor, ATR (Ataxia-telangiectasia-mutated-and-Rad3-related) kinase, in HeLa cells expressing lamin A mutants or lamin A shRNA. The degradation of ATR kinase in these cells was through the proteasomal pathway as it was reversed by the proteasomal inhibitor MG132. Expression of lamin A mutants or shRNA led to transcriptional activation of three ubiquitin ligase components, namely, RNF123 (ring finger protein 123), HECW2 (HECT domain ligase W2) and the F-box protein FBXW10. Ectopic expression of RNF123, HECW2 or FBXW10 directly resulted in proteasomal degradation of ATR kinase and the ring domain of RNF123 was required for this degradation. However, these ligases did not alter the stability of DNA-dependent protein kinase, which is not depleted upon lamin misexpression. Although degradation of ATR kinase was reversed by MG132, it was not affected by the nuclear export inhibitor, leptomycin B, suggesting that ATR kinase is degraded within the nucleus. Our findings indicate that lamin misexpression can lead to deleterious effects on the stability of the key DNA damage sensor, ATR kinase by upregulation of specific components of the ubiquitination pathway.

Effects of 22-oxa-calcitriol on podocyte injury in adriamycin-induced nephrosis

BACKGROUND

In various animal studies, vitamin D has been shown to have a significant effect on reduction of proteinuria and the progression of kidney disease. However, little is known on its renoprotective effect in adriamycin (ADR)-induced nephrosis mice. The present study was intended to determine the therapeutic benefit of 22-oxa-calcitriol (OCT), a vitamin D analog, in reducing proteinuria and its renoprotective effect, i.e. preventing podocyte injury on ADR-induced nephrosis mice.

METHODS

Three experimental groups were used as follows: (1) nephrosis mice, established by a single intravenous injection of ADR; (2) ADR+OCT mice, nephrosis mice treated with OCT, and (3) mice treated only with OCT as the control group. Podocyte injury was assessed by podocyte apoptosis using the TUNEL assay, podocyte counting, podocyte-specific expressed protein by immunofluorescence and Western blot analysis, and foot process effacement using electron microscopy.

RESULTS

Lower proteinuria was observed in ADR+OCT mice. Improvement in glomerulosclerosis and interstitial fibrosis, and prevention of glomerular hyperfiltration were observed in ADR+OCT mice. Immunofluorescence and Western blot analyses showed restoration of downregulated expression of nephrin, CD2AP and podocin. Nevertheless, dendrin expression was not restored. An insignificant reduction in podocyte numbers was found in ADR+OCT mice. Complete foot process effacement was partially prevented in ADR+OCT mice.

CONCLUSIONS

The results indicate that OCT reduces podocyte injury and has renoprotective effects in ADR nephrosis mice.

Regulating the levels of key factors in cell cycle and DNA repair: new pathways revealed by lamins

Spatial and temporal organization of the genome represents an additional step in the regulation of nuclear functions. The nuclear lamina, a polymeric meshwork formed by lamins (A/C and B type) and lamin-associated proteins, plays a key role in the maintenance of genome localization, structure and function. Specifically, mutations in the LMNA gene encoding lamins A/C or changes in its expression, either upregulation or silencing, are associated with defects in DNA replication, transcription and repair, as well as alterations in epigenetic modifications of chromatin. These data, together with the fact that defects in A-type lamins are associated with a whole variety of degenerative disorders, premature aging syndromes and cancer, support the notion that these proteins operate as caretakers of the genome. However, our understanding of their functions is limited due to the lack of well-defined mechanisms behind the genomic instability observed in lamin-related diseases. Here, we summarize our recent discovery of new pathways that are affected by the loss of A-type lamins. In particular, we found that A-type lamins control transcription and degradation of proteins with key roles in cell cycle regulation and DNA double-strand breaks (DSBs) repair by non-homologous end-joining (NHEJ) and homologous-recombination (HR). Importantly, the proteins regulated by A-type lamins--Rb family members, 53BP1, BRCA1 and RAD51--exert tumor suppressor functions, with their loss being associated with cancer susceptibility. Moreover, our studies revealed novel pathways that contribute to genomic instability and that can be activated in disease states independent of the status of A-type lamins.