A novel role for high-mobility group a proteins in cellular senescence and heterochromatin formation

Regulation of HMGA1 expression by microRNA-296 affects prostate cancer growth and invasion

PURPOSE

High-motility group AT-hook gene 1 (HMGA1) is a non-histone nuclear binding protein that is developmentally regulated. HMGA1 is significantly overexpressed in and associated with high grade and advance stage of prostate cancer (PC). The oncogenic role of HMGA1 is at least mediated through chromosomal instability and structural aberrations. However, regulation of HMGA1 expression is not well understood. Identification of microRNA-mediated HMGA1 regulation will provide a promising therapeutic target in treating PC.

EXPERIMENTAL DESIGN

In this study, we examined the functional relation between miR-296 and HMGA1 expression in several PC cell lines and a large PC cohort. We further examined the oncogenic property of HMGA1 regulated by miR-296.

RESULTS

Here we report that miR-296, a microRNA predicted to target HMGA1, specifically represses HMGA1 expression by promoting degradation and inhibiting HMGA1translation. Repression of HMGA1 by miR-296 is direct and sequence specific. Importantly, ectopic miR-296 expression significantly reduced PC cell proliferation and invasion, in part through the downregulation of HMGA1. Examining PC patient samples, we found an inverse correlation between HMGA1 and miR-296 expression: high levels of HMGA1 were associated with low miR-296 expression and strongly linked to more advanced tumor grade and stage.

CONCLUSIONS

Our results indicate that miR-296 regulates HMGA1 expression and is associated with PC growth and invasion.

The essence of senescence

Almost half a century after the first reports describing the limited replicative potential of primary cells in culture, there is now overwhelming evidence for the existence of "cellular senescence" in vivo. It is being recognized as a critical feature of mammalian cells to suppress tumorigenesis, acting alongside cell death programs. Here, we review the various features of cellular senescence and discuss their contribution to tumor suppression. Additionally, we highlight the power and limitations of the biomarkers currently used to identify senescent cells in vitro and in vivo.

Value and limitation of immunohistochemical expression of HMGA2 in mesenchymal tumors: about a series of 1052 cases

The high mobility group A (HMGA2) gene encodes a protein that alters chromatin structure and regulates the transcription of many genes; it is implicated in both benign and malignant neoplasias, but its rearrangements are a feature of development of several mesenchymal tumors. Given its implication in these tumors and particularly adipocytic tumors, and the availability of antibodies usable on paraffin-embedded tissues, we evaluated the immunohistochemical expression of this gene in a series of 1052 mesenchymal tumors. The objective was to define the value and limitations of HMGA2 immunohistochemical expression for histotyping, and compare with molecular data reported in the literature. We thus analyzed 880 cases on tissue microarray and 182 cases on whole sections (211 adipocytic tumors, 628 sarcomas, 213 benign mesenchymal tumors, and 10 normal adipose tissues). A nuclear immunostaining was detected in 86% of conventional and intramuscular lipomas, in 86% of well-differentiated liposarcomas and in 67% of dedifferentiated liposarcomas, as opposed to 16% of other benign adipose tumors and to 15% of non-well-differentiated liposarcoma/dedifferentiated liposarcoma sarcomas. Among benign mesenchymal tumors and lesions, it was detected in 90% of nodular fasciitis and in 88% of benign fibrous histiocytomas with respective specificities of 85 and 100%, and in 90% of aggressive angiomyxoma, contrary to other vulvovaginal tumor types, which expressed HMGA2 only rarely. The normal adipose tissue was always negative for HMGA2. Although not specific, immunohistochemical detection of the HMGA2 protein is helpful for the distinction of normal adipose tissue from well-differentiated lesions, particularly on biopsy or on re-excision. It is less sensitive than MDM2/CDK4 for dedifferentiated liposarcomas diagnosis, but it appears more specific to distinguish dedifferentiated liposarcomas from other poorly differentiated sarcomas. Finally, and may be more importantly, HMGA2 is useful for the diagnosis of benign fibrous histiocytoma, nodular fasciitis and vulvovaginal benign mesenchymal tumors.

RAS oncogene-mediated deregulation of the transcriptome: from molecular signature to function

Transcriptome analysis of cancer cells has developed into a standard procedure to elucidate multiple features of the malignant process and to link gene expression to clinical properties. Gene expression profiling based on microarrays provides essentially correlative information and needs to be transferred to the functional level in order to understand the activity and contribution of individual genes or sets of genes as elements of the gene signature. To date, there exist significant gaps in the functional understanding of gene expression profiles. Moreover, the processes that drive the profound transcriptional alterations that characterize cancer cells remain mainly elusive. We have used pathway-restricted gene expression profiles derived from RAS oncogene-transformed cells and from RAS-expressing cancer cells to identify regulators downstream of the MAPK pathway.We describe the role of epigenetic regulation exemplified by the control of several immune genes in generic cell lines and colorectal cancer cells, particularly the functional interaction between signaling and DNA methylation. Moreover, we assess the role of the architectural transcription factor high mobility AT-hook 2 (HMGA2) as a regulator of the RAS-responsive transcriptome in ovarian epithelial cells. Finally, we describe an integrated approach combining pathway interference in colorectal cancer cells, gene expression profiling and computational analysis of regulatory elements of deregulated target genes. This strategy resulted in the identification of Y-box binding protein 1 (YBX1) as a regulator of MAPK-dependent proliferation and gene expression. The implications for a therapeutic application of HMGA2 gene silencing and the role of YBX1 as a prognostic factor are discussed.

Induction of senescence markers after neo-adjuvant chemotherapy of malignant pleural mesothelioma and association with clinical outcome: an exploratory analysis

The aim of this study was to assess the induction of senescence markers versus apoptosis pathways in malignant pleural mesothelioma (MPM) tumour samples before and after neo-adjuvant platinum-based chemotherapy and to investigate their relationship with clinical outcome. Specific senescence pathways were assessed by quantifying the expression of p21 and plasminogen activator inhibitor-1 (PAI-1) for the p21-p53 pathway, IGFBP7 for the IGF pathway and ALDH1A3 for the IFN pathway. p21 and PAI-1 expression were also assessed by immunohistochemistry. In addition, beta-galactosidase activity staining at pH 6.0 was performed. Apoptosis was determined by TUNEL assay. Clinical outcome was assessed by modified RECIST criteria, progression-free and overall survival. In a training set (n=9 patients) paired comparison demonstrated a significant increase in p21 (p<0.05), PAI-1 (p<0.01) and apoptosis (p<0.01) after neo-adjuvant chemotherapy. The patients with the highest increase in PAI-1 had stable disease, whilst patients with little change in senescence markers accompanied by a high increase in apoptosis had an objective response after chemotherapy. The hypothesis that stable disease might be associated with an increase in senescence markers was confirmed in a tissue microarray (n=26 patients) using p21 and PAI-1 immunohistochemistry as readouts. For patients where survival and time to progression data were available, increased PAI-1 levels were significantly associated with a worst outcome. Our results demonstrate induction of senescence markers by neo-adjuvant chemotherapy in a proportion of patients with MPM and its potential association with a poor outcome.

At the stem of youth and health

Cellular senescence is a specialized form of growth arrest, confined to mitotic cells, induced by various stressful stimuli and characterized by a permanent growth arrest, resistance to apoptosis, an altered pattern of gene expression and the expression of some markers that are characteristic, although not exclusive, to the senescent state. Senescent cells profoundly modify neighboring and remote cells through the production of an altered secretome, eventually leading to inflammation, fibrosis and possibly growth of neoplastic cells. Mammalian aging has been defined as a reduction in the capacity to adequately maintain tissue homeostasis or to repair tissues after injury. Tissue homeostasis and regenerative capacity are nowadays considered to be related to the stem cell pool present in every tissue. For this reason, pathological and patho-physiological conditions characterized by altered tissue homeostasis and impaired regenerative capacity can be viewed as a consequence of the reduction in stem cell number and/or function. Last, cellular senescence is a double-edged sword, since it may inhibit the growth of transformed cells, preventing the occurrence of cancer, while it may facilitate growth of preneoplastic lesions in a paracrine fashion; therefore, interventions targeting this cell response to stress may have a profound impact on many age-related pathologies, ranging from cardiovascular disease to oncology. Aim of this review is to discuss both molecular mechanisms associated with stem cell senescence and interventions that may attenuate or reverse this process.

p16(INK4a) -mediated suppression of telomerase in normal and malignant human breast cells

The cyclin-dependent kinase inhibitor p16(INK4a) (CDKN2A) is an important tumor suppressor gene frequently inactivated in human tumors. p16 suppresses the development of cancer by triggering an irreversible arrest of cell proliferation termed cellular senescence. Here, we describe another anti-oncogenic function of p16 in addition to its ability to halt cell cycle progression. We show that transient expression of p16 stably represses the hTERT gene, encoding the catalytic subunit of telomerase, in both normal and malignant breast epithelial cells. Short-term p16 expression increases the amount of histone H3 trimethylated on lysine 27 (H3K27) bound to the hTERT promoter, resulting in transcriptional silencing, likely mediated by polycomb complexes. Our results indicate that transient p16 exposure may prevent malignant progression in dividing cells by irreversible repression of genes, such as hTERT, whose activity is necessary for extensive self-renewal.

Tumor suppressive functions of p53

The majority of human cancers acquire mutations that abrogate the p53 tumor suppressor network and, as a consequence, p53 is one of the most extensively studied proteins in cancer research. Because of its potent tumor suppressive activity, it is widely assumed that a molecular understanding of p53 action will produce fundamental insights into natural processes that limit tumorigenesis and may identify key molecular targets for therapeutic intervention. p53 functions largely as a transcription factor, and can trigger a variety of antiproliferative programs by activating or repressing key effector genes. Despite a significant body of literature detailing the biochemical and biological functions of p53, much remains to be elucidated. Indeed, the p53 network is as complex and enigmatic as it is relevant. It is the goal of this article, written 30 years after the discovery of p53, to present a concise review of the tumor suppressor role of the p53 network and to highlight the context-dependent nature of p53 target-gene functions.

HMGA1 down-regulation is crucial for chromatin composition and a gene expression profile permitting myogenic differentiation

Background

High mobility group A (HMGA) proteins regulate gene transcription through architectural modulation of chromatin and the formation of multi-protein complexes on promoter/enhancer regions. Differential expression of HMGA variants has been found to be important for distinct differentiation processes and deregulated expression was linked to several disorders. Here we used mouse C2C12 myoblasts and C2C12 cells stably over-expressing HMGA1a-eGFP to study the impact of deregulated HMGA1 expression levels on cellular differentiation.

Results

We found that induction of the myogenic or osteogenic program of C2C12 cells caused an immediate down-regulation of HMGA1. In contrast to wild type C2C12 cells, an engineered cell line with stable over-expression of HMGA1a-eGFP failed to differentiate into myotubes. Immunolocalization studies demonstrated that sustained HMGA1a-eGFP expression prevented myotube formation and chromatin reorganization that normally accompanies differentiation. Western Blot analyses showed that elevated HMGA1a-eGFP levels affected chromatin composition through either down-regulation of histone H1 or premature expression of MeCP2. RT-PCR analyses further revealed that sustained HMGA1a expression also affected myogenic gene expression and caused either down-regulation of genes such as MyoD, myogenin, Igf1, Igf2, Igfbp1-3 or up-regulation of the transcriptional repressor Msx1. Interestingly, siRNA experiments demonstrated that knock-down of HMGA1a was required and sufficient to reactivate the myogenic program in induced HMGA1a over-expressing cells.

Conclusions

Our data demonstrate that HMGA1 down-regulation after induction is required to initiate the myogenic program in C2C12 cells. Sustained HMGA1a expression after induction prevents expression of key myogenic factors. This may be due to specific gene regulation and/or global effects on chromatin. Our data further corroborate that altered HMGA1 levels influence the expression of other chromatin proteins. Thus, HMGA1 is able to establish a specific chromatin composition. This work contributes to the understanding of how differential HMGA1 expression is involved in chromatin organization during cellular differentiation processes and it may help to comprehend effects of HMGA1 over-expression occurring in malign or benign tumours.

Histone deacetylase regulates high mobility group A2-targeting microRNAs in human cord blood-derived multipotent stem cell aging

Cellular senescence involves a reduction in adult stem cell self-renewal, and epigenetic regulation of gene expression is one of the main underlying mechanisms. Here, we observed that the cellular senescence of human umbilical cord blood-derived multipotent stem cells (hUCB-MSCs) caused by inhibition of histone deacetylase (HDAC) activity leads to down-regulation of high mobility group A2 (HMGA2) and, on the contrary, to up-regulation of p16^INK4A, p21^CIP1/WAF1 and p27^KIP1. We found that let-7a1, let-7d, let-7f1, miR-23a, miR-26a and miR-30a were increased during replicative and HDAC inhibitor-mediated senescence of hUCB-MSCs by microRNA microarray and real-time quantitative PCR. Furthermore, the configurations of chromatins beading on these miRNAs were prone to transcriptional activation during HDAC inhibitor-mediated senescence. We confirmed that miR-23a, miR-26a and miR-30a inhibit HMGA2 to accelerate the progress of senescence. These findings suggest that HDACs may play important roles in cellular senescence by regulating the expression of miRNAs that target HMGA2 through histone modification.

Identification of target genes for wild type and truncated HMGA2 in mesenchymal stem-like cells

BACKGROUND

The HMGA2 gene, coding for an architectural transcription factor involved in mesenchymal embryogenesis, is frequently deranged by translocation and/or amplification in mesenchymal tumours, generally leading to over-expression of shortened transcripts and a truncated protein.

METHODS

To identify pathways that are affected by sarcoma-associated variants of HMGA2, we have over-expressed wild type and truncated HMGA2 protein in an immortalized mesenchymal stem-like cell (MSC) line, and investigated the localisation of these proteins and their effects on differentiation and gene expression patterns.

RESULTS

Over-expression of both transgenes blocked adipogenic differentiation of these cells, and microarray analysis revealed clear changes in gene expression patterns, more pronounced for the truncated protein. Most of the genes that showed altered expression in the HMGA2-overexpressing cells fell into the group of NF-kappaB-target genes, suggesting a central role for HMGA2 in this pathway. Of particular interest was the pronounced up-regulation of SSX1, already implicated in mesenchymal oncogenesis and stem cell functions, only in cells expressing the truncated protein. Furthermore, over-expression of both HMGA2 forms was associated with a strong repression of the epithelial marker CD24, consistent with the reported low level of CD24 in cancer stem cells.

CONCLUSIONS

We conclude that the c-terminal part of HMGA2 has important functions at least in mesenchymal cells, and the changes in gene expression resulting from overexpressing a protein lacking this domain may add to the malignant potential of sarcomas.

The matricellular protein CCN1 induces fibroblast senescence and restricts fibrosis in cutaneous wound healing

Cellular senescence is a recognised mechanism of tumor suppression; however, its contribution to other pathologies is not well understood. We show that the matricellular protein CCN1/CYR61, which is dynamically expressed at sites of wound repair, can induce fibroblast senescence through its cell adhesion receptors, integrin α₆β₁ and heparan sulfate proteoglycans. CCN1 induces DNA damage response and p53 activation, and activates the RAC1-NOX1 complex to induce reactive oxygen species (ROS) generation and ROS-dependent activation of the p16^INK4a/pRb pathway, leading to senescence and concomitant expression of antifibrotic genes. Senescent fibroblasts accumulate in granulation tissues of healing cutaneous wounds and express antifibrotic genes in wild type mice. These processes are obliterated in knockin mice that express a senescence-defective CCN1 mutant, resulting in exacerbated fibrosis. Topical application of CCN1 protein to wounds reverses these defects. Thus, fibroblast senescence is a CCN1-dependent wound healing response in cutaneous injury, functioning to curb fibrosis during tissue repair.

Autophagy mediates the process of cellular senescence characterizing bile duct damages in primary biliary cirrhosis

Recent studies disclosed that autophagy is induced during and facilitates the process of senescence. Given that biliary epithelial cells (BECs) in damaged small bile ducts in primary biliary cirrhosis (PBC) show senescent features, we examined an involvement of autophagy in the process of biliary epithelial senescence in PBC. We examined immunohistochemically the expression of microtubule-associated proteins-light chain 3beta (LC3), a marker of autophagy, in livers taken from the patients with PBC (n=37) and control livers (n=75). We also examined the co-localization of LC3 with autophagy-related cathepsin D, lysosome-associated membrane protein-1 (LAMP-1), and senescent markers, p16(INK4a) and p21(WAF1/Cip1). We examined the effect of autophagy inhibitor (3-methyladenine) on the induction of cellular senescence and senescence-associated secretion (CCL2 and CX3CL1) in cultured murine BECs. The expression of LC3 was specifically seen in vesicles in BECs in the inflamed and damaged small bile ducts in PBC, when compared with non-inflamed small bile ducts in PBC and in control livers (P<0.01). The expression of LC3 was closely related to the expression of cathepsin D, LAMP-1, and senescent markers. In cultured BECs, oxidative stress, DNA damage, and serum deprivation induced cellular senescence, when compared with control and the inhibition of autophagy significantly decreased the stress-induced cellular senescence (P<0.01). Furthermore, the secretion level of CCL2 and CX3CL1 increased significantly by various stress and suppressed by the inhibition of autophagy (P<0.01). In conclusion, autophagy is specifically seen in the damaged small bile ducts along with cellular senescence in PBC. The inhibition of autophagy suppressed cellular senescence in cultured cells. These findings suggest that autophagy may mediate the process of biliary epithelial senescence and involve in the pathogenesis of bile duct lesions in PBC.

Senescence induction in human fibroblasts and hematopoietic progenitors by leukemogenic fusion proteins

Hematologic malignancies are typically associated with leukemogenic fusion proteins, which are required to maintain the oncogenic state. Previous studies have shown that certain oncogenes that promote solid tumors, such as RAS and BRAF, can induce senescence in primary cells, which is thought to provide a barrier to tumorigenesis. In these cases, the activated oncogene elicits a DNA damage response (DDR), which is essential for the senescence program. Here we show that 3 leukemogenic fusion proteins, BCR-ABL, CBFB-MYH11, and RUNX1-ETO, can induce senescence in primary fibroblasts and hematopoietic progenitors. Unexpectedly, we find that senescence induction by BCR-ABL and CBFB-MYH11 occurs through a DDR-independent pathway, whereas RUNX1-ETO induces senescence in a DDR-dependent manner. All 3 fusion proteins activate the p38 MAPK pathway, which is required for senescence induction. Our results reveal diverse pathways for oncogene-induced senescence and further suggest that leukemias harbor genetic or epigenetic alterations that inactivate senescence induction genes.

Dissecting the unique role of the retinoblastoma tumor suppressor during cellular senescence

The RB protein family (RB, p107, and p130) has overlapping and compensatory functions in cell-cycle control. However, cancer-associated mutations are almost exclusively found in RB, implying that RB has a nonredundant role in tumor suppression. We demonstrate that RB preferentially associates with E2F target genes involved in DNA replication and is uniquely required to repress these genes during senescence but not other growth states. Consequently, RB loss leads to inappropriate DNA synthesis following a senescence trigger and, together with disruption of a p21-mediated cell-cycle checkpoint, enables extensive proliferation and rampant genomic instability. Our results identify a nonredundant RB effector function that may contribute to tumor suppression and reveal how loss of RB and p53 cooperate to bypass senescence.

Cellular senescence in usual type uterine leiomyoma

OBJECTIVE

To evaluate the role of senescence in symptomatic patients with multifibroids.

DESIGN

A cohort study.

SETTING

University research laboratory.

PATIENT(S)

Eighty-six fibroids collected from 14 patients who underwent myomectomy or hysterectomy.

INTERVENTION(S)

Senescence-associated beta-galactosidase (SA-beta-Gal) stain in fresh-frozen tissue; reverse-transcription polymerase chain reaction (RT-PCR); MicroRNA in situ hybridization (MISH); immunohistochemistry in formalin-fixed paraffin-embedded tissue.

MAIN OUTCOME MEASURE(S)

Senescence measured by percentage of SA-beta-Gal-positive cells; levels of let-7 microRNAs measured by RT-PCR and MISH; expression of p16(INK4a), Ki-67, HMGA1, and HMGA2 scaled by immunoreactivity.

RESULT(S)

About 58% (48 of 82) of tumors showed significant senescent change (SA-beta-Gal positive) in >10% of the tumor volume. The overall trend was a higher level of senescence in small fibroids and older-aged women. Senescent fibroids were additionally shown to have, high levels of let-7 c, d, and f-2 and a low Ki-67 index.

CONCLUSION(S)

Senescence is a common cellular change in a large proportion of usual type fibroids. Similarly, senescence may explain the variation in growth rates of these tumors, and may prove to be an important molecular and cellular target in prevention of fibroid growth.

CENP-A reduction induces a p53-dependent cellular senescence response to protect cells from executing defective mitoses

Cellular senescence is an irreversible growth arrest and is presumed to be a natural barrier to tumor development. Like telomere shortening, certain defects in chromosome integrity can trigger senescence; however, the roles of centromere proteins in regulating commitment to the senescent state remains to be established. We examined chromatin structure in senescent human primary fibroblasts and found that CENP-A protein levels are diminished in senescent cells. Senescence-associated reduction of CENP-A is caused by transcriptional and posttranslational control. Surprisingly, forced reduction of CENP-A by short-hairpin RNA was found to cause premature senescence in human primary fibroblasts. This premature senescence is dependent on a tumor suppressor, p53, but not on p16(INK4a)-Rb; the depletion of CENP-A in p53-deficient cells results in aberrant mitosis with chromosome missegregation. We propose that p53-dependent senescence that arises from CENP-A reduction acts as a "self-defense mechanism" to prevent centromere-defective cells from undergoing mitotic proliferation that potentially leads to massive generation of aneuploid cells.

The high mobility group A1 gene: transforming inflammatory signals into cancer?

High mobility group A1 (HMGA1) is highly expressed during embryogenesis and in poorly differentiated cancers, and high levels portend a poor prognosis in some tumors. HMGA1 induces oncogenic transformation in cultured cells and causes aggressive cancers in transgenic mice, whereas blocking it interferes with transformation in experimental models. These findings suggest a pivotal role for HMGA1 in cancer. This review focuses on two recently described HMGA1 transcriptional targets that mediate inflammatory signals and drive malignant transformation because they could serve as biomarkers or therapeutic targets. Further elucidation of HMGA1 function in transformation promises to have a major impact on our war on cancer.

HMGA2: a potential biomarker complement to P53 for detection of early-stage high-grade papillary serous carcinoma in fallopian tubes

Before high-grade papillary serous carcinoma (HG-PSC) becomes invasive, it is believed to be a poorly defined short-lived precursor lesion. A recent characterization of serous tubal intraepithelial carcinoma (STIC) and of the p53 signature suggested that HG-PSC may follow a stepwise progression on cellular and molecular levels. High-mobility group AT-hook 2 (HMGA2), an oncofetal protein, is overexpressed in ovarian cancer. To test whether HMGA2 can be another valuable marker for STIC, we examined HMGA2 expression in 3 groups of patients: (1) 24 patients with STIC and its invasive counterpart, HG-PSC of the fallopian tubes, (2) 24 patients with HG-PSC of the ovaries but without STIC (positive control), and (3) 30 patients with cancer and normal fallopian tubes (negative control). We found that HMGA2 was overexpressed in 75% of patients with STIC, was coexpressed with p53 in more than 50% of patients, and was completely negative in the secretory cells of the 30 patients with normal fallopian tubes. Among 7 patients with cells negative for p53 staining, HMGA2 was positive in 5; among 6 patients whose tumor cells were negative for HMGA2 in STIC, 3 were positive for HMGA2 in the invasive component; about 70% of invasive HG-PSC tumor cells were immunoreactive for both HMGA2 and TP53. In invasive carcinoma, HMGA2 overexpression was correlated with p53 (r=0.45), indicating the role of HMGA2 in p53 mediated tumor progression. Our findings of immunoreactivity for HMGA2 may lead to a novel, useful biomarker to complement p53 in the detection of early-stage serous carcinoma.

Histone deacetylase controls adult stem cell aging by balancing the expression of polycomb genes and jumonji domain containing 3

Aging is linked to loss of the self-renewal capacity of adult stem cells. Here, we observed that human multipotent stem cells (MSCs) underwent cellular senescence in vitro. Decreased expression of histone deacetylases (HDACs), followed by downregulation of polycomb group genes (PcGs), such as BMI1, EZH2 and SUZ12, and by upregulation of jumonji domain containing 3 (JMJD3), was observed in senescent MSCs. Similarly, HDAC inhibitors induced cellular senescence through downregulation of PcGs and upregulation of JMJD3. Regulation of PcGs was associated with HDAC inhibitor-induced hypophosphorylation of RB, which causes RB to bind to and decrease the transcriptional activity of E2F. JMJD3 expression regulation was dependant on histone acetylation status at its promoter regions. A histone acetyltransferase (HAT) inhibitor prevented replicative senescence of MSCs. These results suggest that HDAC activity might be important for MSC self-renewal by balancing PcGs and JMJD3 expression, which govern cellular senescence by p16^INK4A regulation.

Oncogene-induced cellular senescence

Oncogene-induced senescence (OIS) is a robust and sustained antiproliferative response brought about by oncogenic signaling resulting from an activating mutation of an oncogene, or the inactivation of a tumor-suppressor gene. The pathways mediating OIS are complex and incompletely elucidated but, the proliferative arrest involves activation of both the RB and p53 pathways. In addition, whereas there are indications that at least in some situations, negative feedback loops abolish the increased mitogenic signaling resulting from the oncogenic mutations, also an unexpected contribution of interleukin-mediated signaling has recently been found. OIS brings about cessation of growth of some benign tumors, including melanocytic nevi and several other lesions, including pituitary and thyroid adenomas. It protects against progression to cancer, and in this way complements oncogene-induced apoptosis. Perhaps, OIS has evolved as an alternative to apoptosis especially regarding long-lived cell types that are not replaceable in large numbers. Contrary to the earlier belief, OIS is not entirely irreversible, at least in some well documented in vitro systems. This means that its induction does not entirely eliminate the oncogenic threat resulting from the mutated cell. It also means that OIS, or related phenomena that may affect a proportion of the tumor cells of some cancers, may have an influence on responsiveness to cytotoxic cancer therapies, because OIS is associated with an antiapoptosis phenotype.

Healing and hurting: molecular mechanisms, functions, and pathologies of cellular senescence

Cellular senescence is a proliferation arrest that is typically irreversible and caused by various cellular stresses, including excess rounds of cell division and cancer-causing genetic alterations. Senescence actively contributes to a tissue-level response to tissue wounding and incipient cancer, healing the tissue and suppressing tumor formation. However, in the long term, the same senescence program may hurt the tissue, thereby contributing to tissue aging. Tumor suppression, wound healing, and aging are each associated with inflammation, and here it is proposed that cellular senescence contributes to a "nonimmune cell" component of the tissue inflammatory response.

Could Sirt1-mediated epigenetic effects contribute to the longevity response to dietary restriction and be mimicked by other dietary interventions?

Dietary restriction (DR) increases lifespan in a range of evolutionarily distinct species. The polyphenol resveratrol may be a dietary mimetic of some effects of DR. The pivotal role of the mammalian histone deacetylase (HDAC) Sirt1, and its homologue in other organisms, in mediating the effects of both DR and resveratrol on lifespan/ageing suggests it may be the common conduit through which these dietary interventions influence ageing. We propose the novel hypothesis that effects of DR relevant to lifespan extension include maintenance of DNA methylation patterns through Sirt1-mediated epigenetic effects, and proffer the view that dietary components, including resveratrol, may mimic these actions.

Senescence: an antiviral defense that is tumor suppressive?

Normal mammalian somatic cells proliferate a finite number of times in vitro before permanently withdrawing from the cell cycle into a cellular state referred to as senescence. Senescence may be triggered by excessive mitogenic stimulation or by various forms of cellular damage including excessive telomere shortening. Over the past decade, there has been continuing accumulation of evidence that senescence occurs in vivo, that it is relevant to aging and that it has a tumor suppressor function. However, the phenotype of senescence has also been found to include a number of puzzling features, including the secretion of proinflammatory factors that may foster tumorigenesis as well as the senescence of neighboring cells. On the basis of these antagonistic pro- and antitumorigenic effects, and of the observation that many viruses have developed proteins that prevent senescence of the cells they infect, it is argued that the primary function of senescence may have been as an antiviral defense mechanism. Recent progress in understanding how tumor cells evade senescence is also reviewed here.

Proteinopathy-induced neuronal senescence: a hypothesis for brain failure in Alzheimer's and other neurodegenerative diseases

Background

Alzheimer's disease (AD) and a host of other neurodegenerative central nervous system (CNS) proteinopathies are characterized by the accumulation of misfolded protein aggregates. Simplistically, these aggregates can be divided into smaller, soluble, oligomeric and larger, less-soluble or insoluble, fibrillar forms. Perhaps the major ongoing debate in the neurodegenerative disease field is whether the smaller oligomeric or larger fibrillar aggregates are the primary neurotoxin. Herein, we propose an integrative hypothesis that provides new insights into how a variety of misfolded protein aggregates can result in neurodegeneration.

Results

We introduce the concept that a wide range of highly stable misfolded protein aggregates in AD and other neurodegenerative proteinopathies are recognized as non-self and chronically activate the innate immune system. This pro-inflammatory state leads to physiological senescence of CNS cells. Once CNS cells undergo physiological senescence, they secrete a variety of pro-inflammatory molecules. Thus, the senescence of cells, which was initially triggered by inflammatory stimuli, becomes a self-reinforcing stimulus for further inflammation and senescence. Ultimately, senescent CNS cells become functionally impaired and eventually die, and this neurodegeneration leads to brain organ failure.

Conclusion

This integrative hypothesis, which we will refer to as the proteinopathy-induced senescent cell hypothesis of AD and other neurodegenerative diseases, links CNS proteinopathies to inflammation, physiological senescence, cellular dysfunction, and ultimately neurodegeneration. Future studies characterizing the senescent phenotype of CNS cells in AD and other neurodegenerative diseases will test the validity of this hypothesis. The implications of CNS senescence as a contributing factor to the neurodegenerative cascade and its implications for therapy are discussed.

Nuclear functions of the HMG proteins

Although the three families of mammalian HMG proteins (HMGA, HMGB and HMGN) participate in many of the same nuclear processes, each family plays its own unique role in modulating chromatin structure and regulating genomic function. This review focuses on the similarities and differences in the mechanisms by which the different HMG families impact chromatin structure and influence cellular phenotype. The biological implications of having three architectural transcription factor families with complementary, but partially overlapping, nuclear functions are discussed.

Large cell change of hepatocytes in chronic viral hepatitis represents a senescent-related lesion

Large cell change involves the clustering of hepatocytes with hyperchromatism and cellular enlargement without an increase in the nuclear/cytoplasmic ratio. This study investigated whether large cell change in chronic viral hepatitis reflects cellular senescence because of morphological similarities between the 2 conditions. The expression of markers of senescence such as senescence-associated beta-galactosidase, senescence-associated heterochromatic foci, and p21, as well as markers of cell kinetics such as Ki-67, was examined in 26 frozen and 82 formalin-fixed liver specimens. Large cell change was frequently detected in chronic hepatitis B cases with advanced histologic staging, particularly those with hepatocellular carcinoma. Senescence-associated beta-galactosidase activity, senescence-associated heterochromatic foci, and p21 were frequently detected in areas of large cell change. Hepatocytes with large cell change showed no proliferative or apoptotic activity. The frequent expression of senescent features and the absence of proliferative or apoptotic activity suggest that large cell change represents senescence. The parallel increase in large cell change and hepatocellular carcinoma in chronic hepatitis B raises the possibility that cellular senescence develops as a safeguard against malignant transformation rather than as a precursor of hepatocellular carcinoma.

pRb, a local chromatin organizer with global possibilities

The retinoblastoma (pRb) family of proteins are well known for their tumor suppressor properties and for their ability to regulate transcription. The action of pRb family members correlates with the appearance of repressive chromatin marks at promoter regions of genes encoding key regulators of cell proliferation. Recent studies raise the possibility that pRb family members do not simply act by controlling the activity of individual promoters but that they may also function by promoting the more general organization of chromatin. In several contexts, pRb family members stimulate the compaction or condensation of chromatin and promote the formation of heterochromatin. In this review, we summarize studies that link pRb family members to the condensation or compaction of DNA.

Senescence and p130/Rbl2: a new beginning to the end

Senescence is the process of cellular aging dependent on the normal physiological functions of non-immortalized cells. With increasing data being uncovered in this field, the complex molecular web regulating senescence is gradually being unraveled. Recent studies have suggested two main phases of senescence, the triggering of senescence and the maintenance of senescence. Each has been supported by data implying precise roles for DNA methyltransferases, reactive oxygen species and other factors. We will first summarize the data supporting these claims and then highlight the specific role that we hypothesize that p130/Rbl2 plays in the modulation of the senescence process.

A versatile viral system for expression and depletion of proteins in mammalian cells

The ability to express or deplete proteins in living cells is crucial for the study of biological processes. Viral vectors are often useful to deliver DNA constructs to cells that are difficult to transfect by other methods. Lentiviruses have the additional advantage of being able to integrate into the genomes of non-dividing mammalian cells. However, existing viral expression systems generally require different vector backbones for expression of cDNA, small hairpin RNA (shRNA) or microRNA (miRNA) and provide limited drug selection markers. Furthermore, viral backbones are often recombinogenic in bacteria, complicating the generation and maintenance of desired clones. Here, we describe a collection of 59 vectors that comprise an integrated system for constitutive or inducible expression of cDNAs, shRNAs or miRNAs, and use a wide variety of drug selection markers. These vectors are based on the Gateway technology (Invitrogen) whereby the cDNA, shRNA or miRNA of interest is cloned into an Entry vector and then recombined into a Destination vector that carries the chosen viral backbone and drug selection marker. This recombination reaction generates the desired product with >95% efficiency and greatly reduces the frequency of unwanted recombination in bacteria. We generated Destination vectors for the production of both retroviruses and lentiviruses. Further, we characterized each vector for its viral titer production as well as its efficiency in expressing or depleting proteins of interest. We also generated multiple types of vectors for the production of fusion proteins and confirmed expression of each. We demonstrated the utility of these vectors in a variety of functional studies. First, we show that the FKBP12 Destabilization Domain system can be used to either express or deplete the protein of interest in mitotically-arrested cells. Also, we generate primary fibroblasts that can be induced to senesce in the presence or absence of DNA damage. Finally, we determined that both isoforms of the AT-Rich Interacting Domain 4B (ARID4B) protein could induce G1 arrest when overexpressed. As new technologies emerge, the vectors in this collection can be easily modified and adapted without the need for extensive recloning.

Association of a high mobility group gene (HMGA2) variant with bone mineral density

High mobility group (HMG) proteins regulate chromatin architecture and gene expression. Constitutional rearrangement of an HMG family member, HMGA2, in an 8-year old boy resulted in extreme overgrowth and advanced bone development. Moreover, a recent genome-wide association study documented an association between a variant in the 3' untranslated region of HMGA2 (rs1042725) and height in otherwise healthy individuals. We attempted to extend these findings by testing if this HMGA2 polymorphism is associated with other skeletal measures in two large population cohorts of diverse race/ethnicity. Genotyping was completed in 1680 Afro-Caribbean men aged > or = 40 years and 1548 Caucasian American men aged > or = 69 years. Bone mineral density (BMD) was assessed with peripheral quantitative computed tomography. The minor allele frequency of rs1042725 was 32% among Afro-Caribbeans and 48% among Caucasians (p<0.0001). No association was observed with height in either study cohort. However, presence of the minor allele of this SNP was associated with decreased tibia trabecular volumetric BMD in both populations (p=0.007 Afro-Caribbean; p=0.0007 Caucasian). Real-time quantitative RT-PCR and Western blot analysis demonstrated HMGA2 mRNA and protein expression in the human fetal osteoblast cell line, hFOB. Our analyses suggest a novel association between a common genetic variant in HMGA2 and trabecular BMD in ethnically diverse older men. Additional research is needed to better understand the role of HMGA2 in the regulation of bone metabolism.

A comparative analysis of the cell biology of senescence and aging

Various intracellular organelles, such as lysosomes, mitochondria, nuclei, and cytoskeletons, change during replicative senescence, but the utility of these changes as general markers of senescence and their significance with respect to functional alterations have not been comprehensively reviewed. Furthermore, the relevance of these alterations to cellular and functional changes in aging animals is poorly understood. In this paper, we review the studies that report these senescence-associated changes in various aging cells and their underlying mechanisms. Changes associated with lysosomes and mitochondria are found not only in cells undergoing replicative or induced senescence but also in postmitotic cells isolated from aged organisms. In contrast, other changes occur mainly in cells undergoing in vitro senescence. Comparison of age-related changes and their underlying mechanisms in in vitro senescent cells and aged postmitotic cells would reveal the relevance of replicative senescence to the physiological processes occurring in postmitotic cells as individuals age.

The ageing epigenome: damaged beyond repair?

Of all the proposed causes of ageing, DNA damage remains a leading, though still debated theory. Unlike most other types of age-related cellular damage, which can hypothetically be reversed, mutations in DNA are permanent. Such errors result in the accumulation of changes to RNA and protein sequences with age, and are tightly linked to cellular senescence and overall organ dysfunction. Over the past few years, an additional, more global role has emerged for the contribution of DNA damage and genomic instability to the ageing process. We, and others have found that DNA damage and the concomitant repair process can induce genome-wide epigenetic changes, which may promote a variety of age-related transcriptional and functional changes. Here, we discuss the link between DNA damage, chromatin alterations and ageing, an interplay that explains how seemingly random DNA damage could manifest in predictable phenotypic changes that define ageing, changes that may ultimately be reversible.

The high mobility group protein HMGA2: a co-regulator of chromatin structure and pluripotency in stem cells?

The small, chromatin-associated HMGA proteins contain three separate DNA binding domains, so-called AT hooks, which bind preferentially to short AT-rich sequences. These proteins are abundant in pluripotent embryonic stem (ES) cells and most malignant human tumors, but are not detectable in normal somatic cells. They act both as activator and repressor of gene expression, and most likely facilitate DNA architectural changes during formation of specialized nucleoprotein structures at selected promoter regions. For example, HMGA2 is involved in transcriptional activation of certain cell proliferation genes, which likely contributes to its well-established oncogenic potential during tumor formation. However, surprisingly little is known about how HMGA proteins bind DNA packaged in chromatin and how this affects the chromatin structure at a larger scale. Experimental evidence suggests that HMGA2 competes with binding of histone H1 in the chromatin fiber. This could substantially alter chromatin domain structures in ES cells and contribute to the activation of certain transcription networks. HMGA2 also seems capable of recruiting enzymes directly involved in histone modifications to trigger gene expression. Furthermore, it was shown that multiple HMGA2 molecules bind stably to a single nucleosome core particle whose structure is known. How these features of HMGA2 impinge on chromatin organization inside a living cell is unknown. In this commentary, we propose that HMGA2, through the action of three independent DNA binding domains, substantially contributes to the plasticity of ES cell chromatin and is involved in the maintenance of a un-differentiated cell state.

Let-7 and miR-200 microRNAs: guardians against pluripotency and cancer progression

Micro (mi)RNAs are emerging as important regulators of cellular differentiation, their importance underscored by the fact that they are often dysregulated during carcinogenesis. Two evolutionary conserved families, let-7 and miR-200, regulate key differentiation processes during development. Loss of let-7 in cancer results in reverse embryogenesis and dedifferentiation, and miR-200 has been identified as a powerful regulator of epithelial-to-mesenchymal transition (EMT). Recent findings have connected let-7 with stem cell maintenance and point at a connection between EMT and stem cell formation. A part of tumor progression can be viewed as a continuum of progressive dedifferentiation (EMT) with a cell at the endpoint that has stem cell-like properties. I propose that steps of this process are driven by specific changes in the expression of let-7 and miR-200 family members.

Autophagy mediates the mitotic senescence transition

As a stress response, senescence is a dynamic process involving multiple effector mechanisms whose combination determines the phenotypic quality. Here we identify autophagy as a new effector mechanism of senescence. Autophagy is activated during senescence and its activation is correlated with negative feedback in the PI3K-mammalian target of rapamycin (mTOR) pathway. A subset of autophagy-related genes are up-regulated during senescence: Overexpression of one of those genes, ULK3, induces autophagy and senescence. Furthermore, inhibition of autophagy delays the senescence phenotype, including senescence-associated secretion. Our data suggest that autophagy, and its consequent protein turnover, mediate the acquisition of the senescence phenotype.

Chromatin modifications: the driving force of senescence and aging?

An emerging field of investigation in the search for treatment of human disease is the modulation of chromatin modifications. Chromatin modifications impart virtually all processes occurring in the mammalian nucleus, from regulation of transcription to genomic stability and nuclear high order organization. It has been well recognized that, as the mammalian cell ages, its chromatin structure evolves, both at a global level and at specific loci. While these observations are mostly correlative, recent technical developments allowing loss-of-function experiments and genome-wide approaches have permitted the identification of a causal relationship between specific changes in chromatin structure and the aging phenotype. Here we review the evidence pointing to the modulation of chromatin structure as a potential driving force of cellular aging in mammals.

Senescence-messaging secretome: SMS-ing cellular stress

Oncogene-induced cellular senescence constitutes a strong anti-proliferative response, which can be set in motion following either oncogene activation or loss of tumour suppressor signalling. It serves to limit the expansion of early neoplastic cells and as such is a potent cancer-protective response to oncogenic events. Recently emerging evidence points to a crucial role in oncogene-induced cellular senescence for the 'senescence-messaging secretome' or SMS, setting the stage for cross-talk between senescent cells and their environment. How are such signals integrated into a coordinated response and what are the implications of this unexpected finding?

Epigenetics and the nervous system

We are in the midst of a revolution in the genomic sciences that will forever change the way we view biology and medicine, particularly with respect to brain form, function, development, evolution, plasticity, neurological disease pathogenesis and neural regenerative potential. The application of epigenetic principles has already begun to identify and characterize previously unrecognized molecular signatures of disease latency, onset and progression, mechanisms underlying disease pathogenesis, and responses to new and evolving therapeutic modalities. Moreover, epigenomic medicine promises to usher in a new era of neurological therapeutics designed to promote disease prevention and recovery of seemingly lost neurological function via reprogramming of stem cells, redirecting cell fate decisions and dynamically modulating neural network plasticity and connectivity.

HMGA2, microRNAs, and stem cell aging

Mammalian aging results from a replicative decline in the function of somatic stem cells and other self-renewing cells. Recent studies (Monzen et al., 2008; Nishino et al., 2008; Sanna et al., 2008; Weedon et al., 2008) link a chromatin-associated protein, HMGA2, to development, height, and mouse stem cell aging during late fetal development and young adulthood.

SIRT1 redistribution on chromatin promotes genomic stability but alters gene expression during aging

Genomic instability and alterations in gene expression are hallmarks of eukaryotic aging. The yeast histone deacetylase Sir2 silences transcription and stabilizes repetitive DNA, but during aging or in response to a DNA break, the Sir complex relocalizes to sites of genomic instability, resulting in the desilencing of genes that cause sterility, a characteristic of yeast aging. Using embryonic stem cells, we show that mammalian Sir2, SIRT1, represses repetitive DNA and a functionally diverse set of genes across the mouse genome. In response to DNA damage, SIRT1 dissociates from these loci and relocalizes to DNA breaks to promote repair, resulting in transcriptional changes that parallel those in the aging mouse brain. Increased SIRT1 expression promotes survival in a mouse model of genomic instability and suppresses age-dependent transcriptional changes. Thus, DNA damage-induced redistribution of SIRT1 and other chromatin-modifying proteins may be a conserved mechanism of aging in eukaryotes.

Senescence and immortality in hepatocellular carcinoma

Cellular senescence is a process leading to terminal growth arrest with characteristic morphological features. This process is mediated by telomere-dependent, oncogene-induced and ROS-induced pathways, but persistent DNA damage is the most common cause. Senescence arrest is mediated by p16(INK4a)- and p21(Cip1)-dependent pathways both leading to retinoblastoma protein (pRb) activation. p53 plays a relay role between DNA damage sensing and p21(Cip1) activation. pRb arrests the cell cycle by recruiting proliferation genes to facultative heterochromatin for permanent silencing. Replicative senescence that occurs in hepatocytes in culture and in liver cirrhosis is associated with lack of telomerase activity and results in telomere shortening. Hepatocellular carcinoma (HCC) cells display inactivating mutations of p53 and epigenetic silencing of p16(INK4a). Moreover, they re-express telomerase reverse transcriptase required for telomere maintenance. Thus, senescence bypass and cellular immortality is likely to contribute significantly to HCC development. Oncogene-induced senescence in premalignant lesions and reversible immortality of cancer cells including HCC offer new potentials for tumor prevention and treatment.

ING1a expression increases during replicative senescence and induces a senescent phenotype

The ING family of tumor suppressor proteins affects cell growth, apoptosis and response to DNA damage by modulating chromatin structure through association with different HAT and HDAC complexes. The major splicing isoforms of the ING1 locus are ING1a and INGlb. While INGlb plays a role in inducing apoptosis, the function of ING1a is currently unknown. Here we show that alternative splicing of the ING1 message alters the INGla:INGlb ratio by approximately 30-fold in senescent compared to low passage primary fibroblasts. INGla antagonizes INGlb function in apoptosis, induces the formation of structures resembling senescence-associated heterochromatic foci containing heterochromatin protein 1 gamma, the accumulation of senescence-associated beta-galactosidase activity and promotes senescent cell morphology and cell cycle arrest. Phenotypic effects may result from differential effects on gene expression since ING1a increases levels of both retinoblastoma and the p16 cyclin-dependent kinase inhibitor and ING1a and ING1b have opposite effects on the expression of proliferating nuclear cell antigen (PCNA), which is required for cell growth. Gene expression appears to be altered by targeting of HDAC complexes to gene promoters since INGla associates with several-fold higher levels of HDAC1 in senescent, compared to replication-competent cells and ING1 is found on the PCNA promoter by chromatin immunoprecipitation analysis. These data demonstrate a novel role for the ING1 proteins in differentially regulating senescence-associated chromatin remodeling vs. apoptosis and support the idea that altered ratios of the ING1 splicing isoforms may contribute to establishing the senescent phenotype through HDAC and HAT complex-mediated effects on chromatin structure.