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

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.

Human UBN1 is an ortholog of yeast Hpc2p and has an essential role in the HIRA/ASF1a chromatin-remodeling pathway in senescent cells

Cellular senescence is an irreversible proliferation arrest, tumor suppression process and likely contributor to tissue aging. Senescence is often characterized by domains of facultative heterochromatin, called senescence-associated heterochromatin foci (SAHF), which repress expression of proliferation-promoting genes. Given its likely contribution to tumor suppression and tissue aging, it is essential to identify all components of the SAHF assembly pathway. Formation of SAHF in human cells is driven by a complex of histone chaperones, namely, HIRA and ASF1a. In yeast, the complex orthologous to HIRA/ASF1a contains two additional proteins, Hpc2p and Hir3p. Using a sophisticated approach to search for remote orthologs conserved in multiple species through evolution, we identified the HIRA-associated proteins, UBN1 and UBN2, as candidate human orthologs of Hpc2p. We show that the Hpc2-related domain of UBN1, UBN2, and Hpc2p is an evolutionarily conserved HIRA/Hir-binding domain, which directly interacts with the N-terminal WD repeats of HIRA/Hir. UBN1 binds to proliferation-promoting genes that are repressed by SAHF and associates with histone methyltransferase activity that methylates lysine 9 of histone H3, a site that is methylated in SAHF. UBN1 is indispensable for formation of SAHF. We conclude that UBN1 is an ortholog of yeast Hpc2p and a novel regulator of senescence.

DNA-binding and -bending activities of SAP30L and SAP30 are mediated by a zinc-dependent module and monophosphoinositides

Deacetylation of histones is carried out by a corepressor complex in which Sin3A is an essential scaffold protein. Two proteins in this complex, the Sin3A-associated proteins SAP30L and SAP30, have previously been suggested to function as linker molecules between various corepressors. In this report, we demonstrate new functions for human SAP30L and SAP30 by showing that they can associate directly with core histones as well as naked DNA. A zinc-coordinating structure is necessary for DNA binding, one consequence of which is bending of the DNA. We provide evidence that a sequence motif previously shown to be a nuclear localization signal is also a phosphatidylinositol (PI)-binding element and that binding of specific nuclear monophosphoinositides regulates DNA binding and chromatin association of SAP30L. PI binding also decreases the repression activity of SAP30L and affects its translocation from the nucleus to the cytoplasm. Our results suggest that SAP30L and SAP30 play active roles in recruitment of deacetylating enzymes to nucleosomes, and mediate key protein-protein and protein-DNA interactions involved in chromatin remodeling and transcription.

Hmga2 promotes neural stem cell self-renewal in young but not old mice by reducing p16Ink4a and p19Arf Expression

Stem cells persist throughout life in diverse tissues by undergoing self-renewing divisions. Self-renewal capacity declines with age, partly because of increasing expression of the tumor suppressor p16(Ink4a). We discovered that the Hmga2 transcriptional regulator is highly expressed in fetal neural stem cells but that expression declines with age. This decrease is partly caused by the increasing expression of let-7b microRNA, which is known to target HMGA2. Hmga2-deficient mice show reduced stem cell numbers and self-renewal throughout the central and peripheral nervous systems of fetal and young-adult mice but not old-adult mice. Furthermore, p16(Ink4a) and p19(Arf) expression were increased in Hmga2-deficient fetal and young-adult stem cells, and deletion of p16(Ink4a) and/or p19(Arf) partially restored self-renewal capacity. let-7b overexpression reduced Hmga2 and increased p16(Ink4a)/p19(Arf) expression. Hmga2 thus promotes fetal and young-adult stem cell self-renewal by decreasing p16(Ink4a)/p19(Arf) expression. Changes in let-7 and Hmga2 expression during aging contribute to the decline in neural stem cell function.

Knockdown of MBP-1 in human foreskin fibroblasts induces p53-p21 dependent senescence

MBP-1 acts as a general transcriptional repressor. Overexpression of MBP-1 induces cell death in a number of cancer cells and regresses tumor growth. However, the function of endogenous MBP-1 in normal cell growth regulation remains unknown. To unravel the role of endogenous MBP-1, we knocked down MBP-1 expression in primary human foreskin fibroblasts (HFF) by RNA interference. Knockdown of MBP-1 in HFF (HFF-MBPsi-4) resulted in an induction of premature senescence, displayed flattened cell morphology, and increased senescence-associated beta-galactosidase activity. FACS analysis of HFF-MBPsi-4 revealed accumulation of a high number of cells in the G1-phase. A significant upregulation of cyclin D1 and reduction of cyclin A was detected in HFF-MBPsi-4 as compared to control HFF. Senescent fibroblasts exhibited enhanced expression of phosphorylated and acetylated p53, and cyclin-dependent kinase inhibitor, p21. Further analysis suggested that promyolocytic leukemia protein (PML) bodies are dramatically increased in HFF-MBPsi-4. Together, these results demonstrated that knockdown of endogenous MBP-1 is involved in cellular senescence of HFF through p53-p21 pathway.

Senescence of activated stellate cells limits liver fibrosis

Cellular senescence acts as a potent mechanism of tumor suppression; however, its functional contribution to noncancer pathologies has not been examined. Here we show that senescent cells accumulate in murine livers treated to produce fibrosis, a precursor pathology to cirrhosis. The senescent cells are derived primarily from activated hepatic stellate cells, which initially proliferate in response to liver damage and produce the extracellular matrix deposited in the fibrotic scar. In mice lacking key senescence regulators, stellate cells continue to proliferate, leading to excessive liver fibrosis. Furthermore, senescent activated stellate cells exhibit gene expression profile consistent with cell-cycle exit, reduced secretion of extracellular matrix components, enhanced secretion of extracellular matrix-degrading enzymes, and enhanced immune surveillance. Accordingly natural killer cells preferentially kill senescent activated stellate cells in vitro and in vivo, thereby facilitating the resolution of fibrosis. Therefore, the senescence program limits the fibrogenic response to acute tissue damage.

HMGA2 participates in transformation in human lung cancer

Although previous studies have established a prominent role for HMGA1 (formerly HMG-I/Y) in aggressive human cancers, the role of HMGA2 (formerly HMGI-C) in malignant transformation has not been clearly defined. The HMGA gene family includes HMGA1, which encodes the HMGA1a and HMGA1b protein isoforms, and HMGA2, which encodes HMGA2. These chromatin-binding proteins function in transcriptional regulation and recent studies also suggest a role in cellular senescence. HMGA1 proteins also appear to participate in cell cycle regulation and malignant transformation, whereas HMGA2 has been implicated primarily in the pathogenesis of benign, mesenchymal tumors. Here, we show that overexpression of HMGA2 leads to a transformed phenotype in cultured lung cells derived from normal tissue. Conversely, inhibiting HMGA2 expression blocks the transformed phenotype in metastatic human non-small cell lung cancer cells. Moreover, we show that HMGA2 mRNA and protein are overexpressed in primary human lung cancers compared with normal tissue or indolent tumors. In addition, there is a statistically significant correlation between HMGA2 protein staining by immunohistochemical analysis and tumor grade (P < 0.001). Our results indicate that HMGA2 is an oncogene important in the pathogenesis of human lung cancer. Although additional studies with animal models are needed, these findings suggest that targeting HMGA2 could be therapeutically beneficial in lung cancer and other cancers characterized by increased HMGA2 expression.

The signaling hubs at the crossroad of longevity and age-related disease networks

The established human age-related disease proteins (ARDPs) and longevity-associated proteins (LAPs) together with their first-order interacting partners form scale-free networks which significantly overlap. About half of the common proteins are involved in signal transduction. These proteins are strongly interconnected and in turn form a common signaling network which comprises over 40% of all hubs (proteins with multiple interactions) in the human interactome. Along with the insulin pathway, the common signaling network is remarkably enriched with the focal adhesion and adherens junction proteins whose relation to the control of lifespan is yet to be fully addressed. The examples of such candidate proteins include several hubs, focal adhesion kinase PTK2 and the extracellular proteins fibronectin FN1, paxillin PXN, and vinculin VCL. The results of the network-based analysis highlight the potential importance of these pathways, especially hubs, in linking the human longevity and age-related diseases.

SUMOylation of HMGA2: selective destabilization of promyelocytic leukemia protein via proteasome

The HMGA2 architectural protein functions in a variety of cellular processes, such as cell growth, transcription regulation, neoplastic transformation, and progression. Up-regulation of HMGA2 protein is observed in many tumors and is associated with advanced cancers with poor prognoses. Although the expression and biochemical properties of HMGA2 protein are regulated by microRNA and phosphorylation, it is unknown whether HMGA2 activity can also be regulated by SUMOylation, and that is what is investigated in this report. We identified HMGA2 as a SUMOylation target and showed that the expression of wild-type HMGA2, but not SUMOylation-defective HMGA2(2K/R), selectively lowered the steady-state level of PML protein. Consequently, the HMGA2-elicited PML down-regulation rendered a reduction in the average number of PML nuclear bodies per cell and the volume of PML assembled per PML nuclear body. Using small interfering RNA to suppress endogenous ubiquitin expression and proteasome inhibitor to repress ubiquitin-mediated protein degradation, we showed that HMGA2 confers PML down-regulation through ubiquitin-proteasome-dependent protein degradation. Importantly, arsenic trioxide treatment stimulated HMGA2 SUMOylation, leading to the formation of HMGA2 nuclear foci surrounding PML nuclear bodies and the stimulation of PML degradation. Collectively, our results unveil a previously unrecognized effect by HMGA2 on the modulation of PML protein level, providing a novel mechanism underlying HMGA2 function and underscoring the molecular basis for oncogenic progression by HMGA2.

Identification of genes that restrict astrocyte differentiation of midgestational neural precursor cells

During development of the mammalian CNS, neurons and glial cells (astrocytes and oligodendrocytes) are generated from common neural precursor cells (NPCs). However, neurogenesis precedes gliogenesis, which normally commences at later stages of fetal telencephalic development. Astrocyte differentiation of mouse NPCs at embryonic day (E) 14.5 (relatively late gestation) is induced by activation of the transcription factor signal transducer and activator of transcription (STAT) 3, whereas at E11.5 (mid-gestation) NPCs do not differentiate into astrocytes even when stimulated by STAT3-activating cytokines such as leukemia inhibitory factor (LIF). This can be explained in part by the fact that astrocyte-specific gene promoters are highly methylated in NPCs at E11.5, but other mechanisms are also likely to play a role. We therefore sought to identify genes involved in the inhibition of astrocyte differentiation of NPCs at midgestation. We first examined gene expression profiles in E11.5 and E14.5 NPCs, using Affymetrix GeneChip analysis, applying the Percellome method to normalize gene expression level. We then conducted in situ hybridization analysis for selected genes found to be highly expressed in NPCs at midgestation. Among these genes, we found that N-myc and high mobility group AT-hook 2 (Hmga2) were highly expressed in the E11.5 but not the E14.5 ventricular zone of mouse brain, where NPCs reside. Transduction of N-myc and Hmga2 by retroviruses into E14.5 NPCs, which normally differentiate into astrocytes in response to LIF, resulted in suppression of astrocyte differentiation. However, sustained expression of N-myc and Hmga2 in E11.5 NPCs failed to maintain the hypermethylated status of an astrocyte-specific gene promoter. Taken together, our data suggest that astrocyte differentiation of NPCs is regulated not only by DNA methylation but also by genes whose expression is controlled spatio-temporally during brain development.

HMGA2 gene is a promising target for ovarian cancer silencing therapy

Ovarian cancer is one of the most lethal gynecological malignancies and the small success rate of routine therapeutic methods justifies efforts to develop new approaches. Evaluation of targets for effective inhibition of ovarian cancer cell growth should precipitate clinical application of gene silencing therapy. In our previous work, we showed upregulation of HMGA2 gene expression as a result of Ras-induced rat ovarian surface epithelial cell transformation. This gene codes the HMGA2 protein, a member of the high-mobility group AT-hook (HMGA) family of nonhistone chromatin proteins. Genome-wide studies revealed upregulation of the HMGA2 gene in human ovarian carcinomas. Herein we have evaluated over-expression of the HMGA2 gene, relevant to ovarian cancer, in subsets of human specimens and cell lines by in situ RNA hybridization and RT-PCR. Transient silencing of HMGA2 gene by means of siRNA inhibited proliferation of those ovarian cancer cells, which over-express this gene initially. Growth suppression was mediated by cell-cycle arrest. Stable silencing of highly expressed HMGA2 gene by shRNAi in A27/80, Ovcar-3 and OAW-42 ovarian cancer cell lines resulted in growth inhibition because of G1 arrest and increase of apoptosis as well. The tumor growth inhibition effect of HMGA2 silencing for Ovcar-3 cells was validated in vivo. Our findings revealed that the HMGA2 gene represents a promising target for gene silencing therapy in ovarian cancer.

Chemokine signaling via the CXCR2 receptor reinforces senescence

Cells enter senescence, a state of stable proliferative arrest, in response to a variety of cellular stresses, including telomere erosion, DNA damage, and oncogenic signaling, which acts as a barrier against malignant transformation in vivo. To identify genes controlling senescence, we conducted an unbiased screen for small hairpin RNAs that extend the life span of primary human fibroblasts. Here, we report that knocking down the chemokine receptor CXCR2 (IL8RB) alleviates both replicative and oncogene-induced senescence (OIS) and diminishes the DNA-damage response. Conversely, ectopic expression of CXCR2 results in premature senescence via a p53-dependent mechanism. Cells undergoing OIS secrete multiple CXCR2-binding chemokines in a program that is regulated by the NF-kappaB and C/EBPbeta transcription factors and coordinately induce CXCR2 expression. CXCR2 upregulation is also observed in preneoplastic lesions in vivo. These results suggest that senescent cells activate a self-amplifying secretory network in which CXCR2-binding chemokines reinforce growth arrest.

HMGA2 is the partner of MDM2 in well-differentiated and dedifferentiated liposarcomas whereas CDK4 belongs to a distinct inconsistent amplicon

Data concerning the fine structure of the 12q13-15 amplicon which contains MDM2 and CDK4 in well-differentiated and dedifferentiated liposarcomas (WDLPS/DDLPS) are scarce. We investigated a series of 38 WDLPS/DDLPS using fluorescence in situ hybridization analysis with 17 probes encompassing the 12q13-15 region. In addition, using quantitative RT-PCR we studied the expression of MDM2, CDK4, DDIT3 (CHOP/GADD153), DYRK2, HMGA2, TSPAN31 and YEATS4 (GAS41) in 11 cases. We showed that CDK4 (12q14.1) belonged to a distinct amplicon than MDM2 (12q15). There was no continuity in the amplified sequences between MDM2 and CDK4. Moreover, while MDM2 was amplified and overexpressed in all cases, CDK4 was not amplified or overexpressed in 13% of cases. The centromeric border of the CDK4 amplicon was located immediately downstream the 5' end of DDIT3, a gene known for being involved in myxoid liposarcoma translocations. DDIT3 was amplified in 3 cases and overexpressed in 9 cases. The overexpression of DDIT3 was correlated to the CDK4 amplification and not to its own amplification status. This suggested that the CDK4 amplicon, as well as the overexpression of DDIT3, might be generated by the disruption of a fragile region in 5' DDIT3. HMGA2 was always amplified and rearranged indicating that it plays a central role in WDLPS/DDLPS. HMGA2 rearrangement frequently resulted in a loss of the 3' end region that is a binding site for let-7. We also found a frequent amplification and overexpression of YEATS4, an oncogene that inactivates P53, suggesting that YEATS4 might play an important role together with MDM2 in WDLPS/DDLPS oncogenesis.

Oncogenic BRAF induces senescence and apoptosis through pathways mediated by the secreted protein IGFBP7

Expression of an oncogene in a primary cell can, paradoxically, block proliferation by inducing senescence or apoptosis through pathways that remain to be elucidated. Here we perform genome-wide RNA-interference screening to identify 17 genes required for an activated BRAF oncogene (BRAFV600E) to block proliferation of human primary fibroblasts and melanocytes. Surprisingly, we find a secreted protein, IGFBP7, has a central role in BRAFV600E-mediated senescence and apoptosis. Expression of BRAFV600E in primary cells leads to synthesis and secretion of IGFBP7, which acts through autocrine/paracrine pathways to inhibit BRAF-MEK-ERK signaling and induce senescence and apoptosis. Apoptosis results from IGFBP7-mediated upregulation of BNIP3L, a proapoptotic BCL2 family protein. Recombinant IGFBP7 (rIGFBP7) induces apoptosis in BRAFV600E-positive human melanoma cell lines, and systemically administered rIGFBP7 markedly suppresses growth of BRAFV600E-positive tumors in xenografted mice. Immunohistochemical analysis of human skin, nevi, and melanoma samples implicates loss of IGFBP7 expression as a critical step in melanoma genesis.

Molecular mechanisms of cellular senescence and immortalization of human cells

Cellular senescence was originally described as a phenomenon observed in cultured human cells. Accumulating lines of evidence now indicate that the same processes also take place in vivo, suggesting important implications for tumor development. Telomere shortening is the most well-established cause of cellular senescence that can be induced by many other intrinsic and extrinsic factors. The retinoblastoma susceptibility gene product is a convergent target that is downstream of these factors. p53, p38MAPK and cyclin-dependent kinase inhibitors p16INK4a (p16) and p21CIP1 (p21) are key mediators. As most stresses that induce cellular senescence are also known causes of cancer, a common strategy might be applied to the development of cancer chemopreventive agents and anti-ageing drugs.

Multiple effects of TRAIL in human carcinoma cells: induction of apoptosis, senescence, proliferation, and cytokine production

TRAIL is a death ligand that induces apoptosis in malignant but not normal cells. Recently the ability of TRAIL to induce proliferation in apoptosis-resistant normal and malignant cells was reported. In this study, we analyzed TRAIL effects in apoptosis sensitive MCF7, OVCAR3 and H460 human tumor cell lines. TRAIL at low concentrations preferentially induced cell proliferation. At 100 ng/ml, apoptotic death was readily observed, however surviving cells acquired higher proliferative capacity. TRAIL-stimulated production of several cytokines, IL-8, RANTES, MCP-1 and bFGF, and activation of caspases 1 and 8 was essential for this effect. Antibodies to IL-8, RANTES, and bFGF blocked TRAIL-induced cell proliferation and further stimulated apoptosis. For the first time, we report that high TRAIL concentrations induced cell senescence as determined by the altered morphology and expression of several senescence markers: SA-beta-gal, p21Waf1/Cip1, p16INK4a, and HMGA. Caspase 9 inhibition protected TRAIL-treated cells from senescence, whereas inhibition of caspases 1 and 8 increased the yield of SLP cells. In conclusion, in cultured human carcinoma cells, TRAIL therapy results in three functional outcomes, apoptosis, proliferation and senescence. TRAIL-induced proapoptotic and prosurvival responses correlate with the strength of signaling. TRAIL-induced cytokine production is responsible for its proliferative and prosurvival effects.

Genome-wide transcriptional analysis of the human cell cycle identifies genes differentially regulated in normal and cancer cells

Characterization of the transcriptional regulatory network of the normal cell cycle is essential for understanding the perturbations that lead to cancer. However, the complete set of cycling genes in primary cells has not yet been identified. Here, we report the results of genome-wide expression profiling experiments on synchronized primary human foreskin fibroblasts across the cell cycle. Using a combined experimental and computational approach to deconvolve measured expression values into "single-cell" expression profiles, we were able to overcome the limitations inherent in synchronizing nontransformed mammalian cells. This allowed us to identify 480 periodically expressed genes in primary human foreskin fibroblasts. Analysis of the reconstructed primary cell profiles and comparison with published expression datasets from synchronized transformed cells reveals a large number of genes that cycle exclusively in primary cells. This conclusion was supported by both bioinformatic analysis and experiments performed on other cell types. We suggest that this approach will help pinpoint genetic elements contributing to normal cell growth and cellular transformation.

Many roads lead to oncogene-induced senescence

Oncogene-induced senescence is a mechanism of tumor suppression that restricts the progression of benign tumors. Important advances have been made toward elucidating the mechanisms that regulate this response; however, there is presently no unified model that integrates all current findings. DNA damage, replicative stress, reactive oxygen species, heterochromatin formation and negative feedback signaling networks have all been proposed to play an integral role in promoting senescence in response to various oncogenic insults. In all cases, these signals have been shown to function through Rb and p53, but utilize different intermediaries. Thus, it appears that senescence is not triggered by a single, linear series of events, but instead is regulated by a complex signaling network. Accordingly, multiple proteins may cooperate to establish a senescence response, but the limiting signal(s) may be dictated by the initiating genetic alteration and/or tissue type. This review will focus on integrating current models and will highlight data that provide new insight into the signals that function to suppress human tumor development.

Roles of HMGA proteins in cancer

The high mobility group A (HMGA) non-histone chromatin proteins alter chromatin structure and thereby regulate the transcription of several genes by either enhancing or suppressing transcription factors. This protein family is implicated, through different mechanisms, in both benign and malignant neoplasias. Rearrangements of HMGA genes are a feature of most benign human mesenchymal tumours. Conversely, unrearranged HMGA overexpression is a feature of malignant tumours and is also causally related to neoplastic cell transformation. Here, we focus on the role of the HMGA proteins in human neoplastic diseases, the mechanisms by which they contribute to carcinogenesis, and therapeutic strategies based on targeting HMGA proteins.

High mobility group protein HMGA1 inhibits retinoblastoma protein-mediated cellular G0 arrest

Retinoblastoma protein (RB) acts as a tumor suppressor in many tissue types, by promoting cell arrest via E2F-mediated transcriptional repression. In addition to the aberrant forms of the RB gene found in different types of cancers, many viral oncoproteins including the simian virus 40 large T antigen target RB. However, cellular factors that inhibit RB function remain to be elucidated. Here, we report that RB interacts with the high mobility group protein A1 (HMGA1), a-non-histone architectural chromatin factor that is frequently overexpressed in cancer cells. HMGA1 binds the small pocket domain of RB, and competes with HDAC1. Subsequently, overexpression of HMGA1 abolishes the inhibitory effect of RB on E2F-activated transcription from the cyclin E promoter. Under serum starvation, T98G cells had been previously shown to be arrested in the G0 phase in an RB-mediated manner. The G0 phase was characterized by growth arrest and low levels of transcription, together with the hypophosphorylation of RB and the downregulation of HMGA1. In contrast, such serum-depleted G0 arrest was abrogated in T98G cells overexpressing HMGA1. The overexpressed HMGA1 was found to form complexes with cellular RB, suggesting that downregulation of HMGA1 is required for G0 arrest. There were no phenotypic changes in HMGA1-expressing T98G cells in the presence of serum, but the persistent expression of HMGA1 under serum starvation caused various nuclear abnormalities, which were similarly induced in T antigen-expressing T98G cells. Our present findings indicate that overexpression of HMGA1 disturbs RB-mediated cell arrest, suggesting a negative control of RB by HMGA1.

BS69 is involved in cellular senescence through the p53-p21Cip1 pathway

The multidomain-containing cellular protein BS69 interacts with adenovirus E1A and several other viral and cellular factors, and acts as a transcription repressor. Here, we show that BS69 is involved in the p53-p21Cip1-mediated senescence pathway. Knockdown of BS69 by RNA interference in human primary fibroblasts results in elevated levels of p21Cip1 and the appearance of several senescent markers, including enhanced senescence-associated beta-galactosidase activity and formation of senescence-associated heterochromatin foci. Importantly, knockdown of either p53 or p21Cip1, but not p16(INK4a) or Rb, allows cells to bypass premature senescence that is induced by BS69 knockdown. Furthermore, we show that BS69 forms complexes with both p53 and p400, and that BS69 associates with the p21Cip1 promoter through p53. Together, our data indicate that BS69 is involved in cellular senescence mainly through the p53-p21Cip1 pathway.

Downregulation of Wnt signaling is a trigger for formation of facultative heterochromatin and onset of cell senescence in primary human cells

Cellular senescence is an irreversible proliferation arrest of primary cells and an important tumor suppression process. Senescence is often characterized by domains of facultative heterochromatin, called senescence-associated heterochromatin foci (SAHF), which repress expression of proliferation-promoting genes. Formation of SAHF is driven by a complex of histone chaperones, HIRA and ASF1a, and depends upon prior localization of HIRA to PML nuclear bodies. However, how the SAHF assembly pathway is activated in senescent cells is not known. Here we show that expression of the canonical Wnt2 ligand and downstream canonical Wnt signals are repressed in senescent human cells. Repression of Wnt2 occurs early in senescence and independently of the pRB and p53 tumor suppressor proteins and drives relocalization of HIRA to PML bodies, formation of SAHF and senescence, likely through GSK3beta-mediated phosphorylation of HIRA. These results have major implications for our understanding of both Wnt signaling and senescence in tissue homeostasis and cancer progression.

Remodeling chromatin for senescence

Cellular senescence is an irreversible proliferation arrest that contributes to tumor suppression and, perhaps, tissue aging. Senescence is frequently accompanied by an increase in nuclear heterochromatin, which is thought to promote proliferation arrest. In this issue, Medrano and co-workers describe new insights into the protein complexes that regulate these changes in chromatin structure.

DNA architectural factor and proto-oncogene HMGA2 regulates key developmental genes in pluripotent human embryonic stem cells

The high-mobility group (HMG) protein A2 has been studied mostly in the mouse where its function seems critical for embryonic cell growth and adipogenesis, leading to a pygmy phenotype with greatly reduced fat tissue in homozygous knock out mice. We showed recently that among the major HMG proteins, HMGA2 is highly expressed in two human embryonic stem (hES) cell lines. Here, we employed siRNA technology in combination with quantitative reverse transcriptase polymerase chain reaction, stem cell-specific microarray analyses, and cell proliferation assays in order to probe into HMGA2's role(s) in pluripotent hES cells. Our results establish HMGA2 as a regulator of human genes linked to mesenchymal cell differentiation, adipogenesis, and hES cell growth.

Cellular senescence and chromatin structure

Cellular senescence is characterized by stable cell cycle arrest that is triggered by various forms of stress stimuli. Senescent cells show a series of morphological and physiological alterations including a flat and enlarged morphology, an increase in acidic beta-galactosidase activity, chromatin condensation, and changes in gene expression pattern. These features are not observed in proliferating cells or quiescent cells in vitro. Using these senescence markers, cellular senescence has been shown to occur in benign or premalignant lesions but not in malignant lesions and to act as a tumor-suppressing mechanism in vivo. The onset and maintenance of the senescent state are regulated by two tumor suppressor proteins, p53 and Rb, which mediate senescence signals through p38 mitogen-activated protein kinase and cyclin-dependent kinase inhibitors. Alterations of chromatin structure are believed to contribute to the irreversible nature of the senescent state. Senescent cells form characteristic heterochromatin structure called senescence-associated heterochromatic foci (SAHFs), which may repress the expression of proliferation-promoting genes, such as E2F target genes. Recent studies have provided molecular insights into the structure and the mechanism of SAHF formation. In this paper, we review the role of cellular senescence in tumor suppression in vivo and the molecular mechanism of stable growth arrest in senescent cells, focusing on the special form of heterochromatin, SAHFs.

Structural basis for recognition of SMRT/N-CoR by the MYND domain and its contribution to AML1/ETO's activity

AML1/ETO results from the t(8;21) associated with 12%-15% of acute myeloid leukemia. The AML1/ETO MYND domain mediates interactions with the corepressors SMRT and N-CoR and contributes to AML1/ETO's ability to repress proliferation and differentiation of primary bone marrow cells as well as to enhance their self renewal in vitro. We solved the solution structure of the MYND domain and show it to be structurally homologous to the PHD and RING finger families of proteins. We also determined the solution structure of an MYND-SMRT peptide complex. We demonstrated that a single amino acid substitution that disrupts the interaction between the MYND domain and the SMRT peptide attenuated AML1/ETO's effects on proliferation, differentiation, and gene expression.

Remodeling of chromatin structure in senescent cells and its potential impact on tumor suppression and aging

Cellular senescence is an important tumor suppression process, and a possible contributor to tissue aging. Senescence is accompanied by extensive changes in chromatin structure. In particular, many senescent cells accumulate specialized domains of facultative heterochromatin, called Senescence-Associated Heterochromatin Foci (SAHF), which are thought to repress expression of proliferation-promoting genes, thereby contributing to senescence-associated proliferation arrest. This article reviews our current understanding of the structure, assembly and function of these SAHF at a cellular level. The possible contribution of SAHF to tumor suppression and tissue aging is also critically discussed.

Dual Role for SUMO E2 Conjugase Ubc9 in Modulating the Transforming and Growth-promoting Properties of the HMGA1b Architectural Transcription Factor

Members of the HMGA1 (high mobility group A1) family of architectural transcription factors, HMGA1a and HMGA1b, play important roles in many normal cellular processes and in tumorigenesis. We performed a yeast two-hybrid screen for HMGA1-interacting proteins and identified the SUMO E2 conjugase Ubc9 as one such partner. The Ubc9-interacting domain of HMGA1 is bipartite, consisting of a proline-rich region near the N terminus and an acidic domain at the extreme C terminus, whereas the HMGA1-interacting domain of Ubc9 comprises a single region previously shown to associate with and SUMOylate other transcription factors. Consistent with these findings, endogenous HMGA1 proteins and Ubc9 could be co-immunoprecipitated from several human cell lines. Studies with HMGA1b proteins containing mutations of either or both Ubc9-interacting domains and with Ubc9-depleted cell lines indicated that the proline-rich domain of HMGA1b positively influences transformation and growth, whereas the acidic domain negatively influences these properties. None of the changes in HMGA1 protein functions mediated by Ubc9 appears to require SUMOylation. These findings are consistent with the idea that Ubc9 can act as both a positive and negative regulator of proliferation and transformation via its non-SUMO-dependent interaction with HMGA1 proteins.

Epigenetic control of cellular senescence in disease: opportunities for therapeutic intervention

Understanding how senescence is established and maintained is an important area of study both for normal cell physiology and in tumourigenesis. Modifications to N-terminal tails of histone proteins, which can lead to chromatin remodelling, appear to be key to the regulation of the senescence phenotype. Epigenetic mechanisms such as modification of histone proteins have been shown to be sufficient to regulate gene expression levels and specific gene promoters can become epigenetically altered at senescence. This suggests that epigenetic mechanisms are important in senescence and further suggests epigenetic deregulation could play an important role in the bypass of senescence and the acquisition of a tumourigenic phenotype. Tumour suppressor proteins and cellular senescence are intimately linked and such proteins are now known to regulate gene expression through chromatin remodelling, again suggesting a link between chromatin modification and cellular senescence. Telomere dynamics and the expression of the telomerase genes are also both implicitly linked to senescence and tumourigenesis, and epigenetic deregulation of the telomerase gene promoters has been identified as a possible mechanism for the activation of telomere maintenance mechanisms in cancer. Recent studies have also suggested that epigenetic deregulation in stem cells could play an important role in carcinogenesis, and new models have been suggested for the attainment of tumourigenesis and bypass of senescence. Overall, proper regulation of the chromatin environment is suggested to have an important role in the senescence pathway, such that its deregulation could lead to tumourigenesis.

Irreversibility of cellular senescence: dual roles of p16INK4a/Rb-pathway in cell cycle control

The retinoblastoma (Rb) tumor suppressor gene product, pRb, has an established role in the implementation of cellular senescence, the state of irreversible G1 cell cycle arrest provoked by diverse oncogenic stresses. In murine cells, senescence cell cycle arrest can be reversed by subsequent inactivation of pRb, indicating that pRb is required not only for the onset of cellular senescence, but also for the maintenance of senescence program in murine cells. However, in human cells, once pRb is fully activated by p16INK4a, senescence cell cycle arrest becomes irreversible and is no longer revoked by subsequent inactivation of pRb, suggesting that p16INK4a/Rb-pathway activates an alternative mechanism to irreversibly block the cell cycle in human senescent cells. Here, we discuss the molecular mechanism underlying the irreversibility of senescence cell cycle arrest and its potential towards tumor suppression.

The second AT-hook of the architectural transcription factor HMGA2 is determinant for nuclear localization and function

High Mobility Group A (HMGA) is a family of architectural nuclear factors which play an important role in neoplastic transformation. HMGA proteins are multifunctional factors that associate both with DNA and nuclear proteins that have been involved in several nuclear processes including transcription. HMGA localization is exclusively nuclear but, to date, the mechanism of nuclear import for these proteins remains unknown. Here, we report the identification and characterization of a nuclear localization signal (NLS) for HMGA2, a member of the HMGA family. The NLS overlaps with the second of the three AT-hooks, the DNA-binding domains characteristic for this group of proteins. The functionality of this NLS was demonstrated by its ability to target a heterologous β-galactosidase/green fluorescent protein fusion protein to the nucleus. Mutations to alanine of basic residues within the second AT-hook resulted in inhibition of HMGA2 nuclear localization and impairment of its function in activating the cyclin A promoter. In addition, HMGA2 was shown to directly interact with the nuclear import receptor importin-α2 via the second AT-hook. HMGA proteins are overexpressed and rearranged in a variety of tumors; our findings can thus help elucidating their role in neoplastic transformation.

Cellular senescence and chromatin organisation

Despite the potential importance of senescence in tumour suppression, its effector mechanism is poorly understood. Recent studies suggest that alterations in the chromatin environment might add an additional layer of stability to the phenotype. In this review, recent discoveries on the interplay between senescence and chromatin biology are overviewed.

Vascular cell senescence: contribution to atherosclerosis

Cardiologists and most physicians believe that aging is an independent risk factor for human atherosclerosis, whereas atherosclerosis is thought to be a characteristic feature of aging in humans by many gerontologists. Because atherosclerosis is among the age-associated changes that almost always escape the influence of natural selection in humans, it might be reasonable to regard atherosclerosis as a feature of aging. Accordingly, when we investigate the pathogenesis of human atherosclerosis, it may be more important to answer the question of how we age than what specifically promotes atherosclerosis. Recently, genetic analyses using various animal models have identified molecules that are crucial for aging. These include components of the DNA-repair system, the tumor suppressor pathway, the telomere maintenance system, the insulin/Akt pathway, and other metabolic pathways. Interestingly, most of the molecules that influence the phenotypic changes of aging also regulate cellular senescence, suggesting a causative link between cellular senescence and aging. For example, DNA-repair defects can cause phenotypic changes that resemble premature aging, and senescent cells that show DNA damage accumulate in the elderly. Excessive calorie intake can cause diabetes and hyperinsulinemia, whereas dysregulation of the insulin pathway has been shown to induce cellular senescence in vitro. Calorie restriction or a reduction of insulin signals extends the lifespan of various species and decreases biomarkers of cellular senescence in vivo. There is emerging evidence that cellular senescence contributes to the pathogenesis of human atherosclerosis. Senescent vascular cells accumulate in human atheroma tissues and exhibit various features of dysfunction. In this review, we examine the hypothesis that cellular senescence might contribute to atherosclerosis, which is a characteristic of aging in humans.

High-level expression of DNA architectural factor HMGA2 and its association with nucleosomes in human embryonic stem cells

The state of chromatin in human embryonic stem (hES) cells is a key factor determining stem cell identity. The non-histone chromatin-associated factor HMGA2 has been studied mostly in the mouse where its function seems critical for embryonic cell growth and adipocytic cell differentiation. Here we show that HMGA2 is highly expressed in two undifferentiated human embryonic stem cell lines at a level of at least 10(5) copies per individual stem cell. Interestingly, expression is further upregulated by a factor of three at day 7 of embryoid body formation, before it quickly drops to or below the level found in undifferentiated cells. We also show that HMGA2 is stably associated with inter- and metaphase hES cell chromatin, and that up to 12 HMGA2 protomers stably associate in vitro with a single nucleosome core particle of known atomic structure. Our data lend support to the possibility that HMGA2 interacts with nucleosomes in a way that imposes a global effect on the state of ES cell chromatin, which may contribute to the establishment of both ES cell identity and the initiation of specific differentiation programs.

Molecular dissection of formation of senescence-associated heterochromatin foci

Senescence is characterized by an irreversible cell proliferation arrest. Specialized domains of facultative heterochromatin, called senescence-associated heterochromatin foci (SAHF), are thought to contribute to the irreversible cell cycle exit in many senescent cells by repressing the expression of proliferation-promoting genes such as cyclin A. SAHF contain known heterochromatin-forming proteins, such as heterochromatin protein 1 (HP1) and the histone H2A variant macroH2A, and other specialized chromatin proteins, such as HMGA proteins. Previously, we showed that a complex of histone chaperones, histone repressor A (HIRA) and antisilencing function 1a (ASF1a), plays a key role in the formation of SAHF. Here we have further dissected the series of events that contribute to SAHF formation. We show that each chromosome condenses into a single SAHF focus. Chromosome condensation depends on the ability of ASF1a to physically interact with its deposition substrate, histone H3, in addition to its cochaperone, HIRA. In cells entering senescence, HP1gamma, but not the related proteins HP1alpha and HP1beta, becomes phosphorylated on serine 93. This phosphorylation is required for efficient incorporation of HP1gamma into SAHF. Remarkably, however, a dramatic reduction in the amount of chromatin-bound HP1 proteins does not detectably affect chromosome condensation into SAHF. Moreover, abundant HP1 proteins are not required for the accumulation in SAHF of histone H3 methylated on lysine 9, the recruitment of macroH2A proteins, nor other hallmarks of senescence, such as the expression of senescence-associated beta-galactosidase activity and senescence-associated cell cycle exit. Based on our results, we propose a stepwise model for the formation of SAHF.

HMG chromosomal proteins in development and disease

The high mobility group (HMG) proteins are a superfamily of abundant and ubiquitous nuclear proteins that bind to DNA and nucleosomes and induce structural changes in the chromatin fiber. They are important in chromatin dynamics and influence DNA processing in the context of chromatin. Results emerging from studies of human disease, genetically modified mice and cells with altered HMG expression indicate that the expression of the HMG proteins is developmentally regulated and that changes in HMG protein levels alter the cellular phenotype and can lead to developmental abnormalities and disease. Here, we focus on the biological function of HMG proteins and highlight their possible roles in cellular differentiation and in the etiology of various diseases.

Loss of linker histone H1 in cellular senescence

Cellular senescence is a tumor-suppressing mechanism that is accompanied by characteristic chromatin condensation called senescence-associated heterochromatic foci (SAHFs). We found that individual SAHFs originate from individual chromosomes. SAHFs do not show alterations of posttranslational modifications of core histones that mark condensed chromatin in mitotic chromosomes, apoptotic chromatin, or transcriptionally inactive heterochromatin. Remarkably, SAHF-positive senescent cells lose linker histone H1 and exhibit increased levels of chromatin-bound high mobility group A2 (HMGA2). The expression of N-terminally enhanced green fluorescent protein (EGFP)–tagged histone H1 induces premature senescence phenotypes, including increased levels of phosphorylated p53, p21, and hypophosphorylated Rb, and a decrease in the chromatin-bound endogenous histone H1 level but not in p16 level accumulation or SAHF formation. However, the simultaneous ectopic expression of hemagglutinin-tagged HMGA2 and N-terminally EGFP-tagged histone H1 leads to significant SAHF formation (P < 0.001). It is known that histone H1 and HMG proteins compete for a common binding site, the linker DNA. These results suggest that SAHFs are a novel type of chromatin condensation involving alterations in linker DNA–binding proteins.

Dissecting the senescence-like program in tumor cells activated by Ras signaling

Activated Ras signaling can induce a permanent growth arrest in osteosarcoma cells. Here, we report that a senescence-like growth inhibition is also achieved in human carcinoma cells upon the transduction of H-Ras(V12). Ras-induced tumor senescence can be recapitulated by the transduction of activated, but not wild-type, MEK. The ability for H-Ras(V12) to suppress tumor cell growth is drastically compromised in cells that harbor endogenous activating ras mutations. Notably, growth inhibition of tumor cells containing ras mutations can be achieved through the introduction of activated MEK. Tumor senescence induced by Ras signaling can occur in the absence of p16 or Rb and is not interrupted by the inactivation of Rb, p107, or p130 via short hairpin RNA or the transduction with HPV16 E7. In contrast, inactivation of p21 via short hairpin RNA disrupts Ras-induced tumor senescence. In summary, this study uncovers a senescence-like program activated by Ras signaling to inhibit cancer cell growth. This program appears to be intact in cancer cells that do not harbor ras mutations. Moreover, cancer cells that carry ras mutations remain susceptible to tumor senescence induced by activated MEK. These novel findings can potentially lead to the development of innovative cancer intervention.

Stem cells, senescence, neosis and self-renewal in cancer

We describe the basic tenets of the current concepts of cancer biology, and review the recent advances on the suppressor role of senescence in tumor growth and the breakdown of this barrier during the origin of tumor growth. Senescence phenotype can be induced by (1) telomere attrition-induced senescence at the end of the cellular mitotic life span (MLS*) and (2) also by replication history-independent, accelerated senescence due to inadvertent activation of oncogenes or by exposure of cells to genotoxins. Tumor suppressor genes p53/pRB/p16INK4A and related senescence checkpoints are involved in effecting the onset of senescence. However, senescence as a tumor suppressor mechanism is a leaky process and senescent cells with mutations or epimutations in these genes escape mitotic catastrophe-induced cell death by becoming polyploid cells. These polyploid giant cells, before they die, give rise to several cells with viable genomes via nuclear budding and asymmetric cytokinesis. This mode of cell division has been termed neosis and the immediate neotic offspring the Raju cells. The latter inherit genomic instability and transiently display stem cell properties in that they differentiate into tumor cells and display extended, but, limited MLS, at the end of which they enter senescent phase and can undergo secondary/tertiary neosis to produce the next generation of Raju cells. Neosis is repeated several times during tumor growth in a non-synchronized fashion, is the mode of origin of resistant tumor growth and contributes to tumor cell heterogeneity and continuity. The main event during neosis appears to be the production of mitotically viable daughter genome after epigenetic modulation from the non-viable polyploid genome of neosis mother cell (NMC). This leads to the growth of resistant tumor cells. Since during neosis, spindle checkpoint is not activated, this may give rise to aneuploidy. Thus, tumor cells also are destined to die due to senescence, but may escape senescence due to mutations or epimutations in the senescent checkpoint pathway. A historical review of neosis-like events is presented and implications of neosis in relation to the current dogmas of cancer biology are discussed. Genesis and repetitive re-genesis of Raju cells with transient "stemness" via neosis are of vital importance to the origin and continuous growth of tumors, a process that appears to be common to all types of tumors. We suggest that unlike current anti-mitotic therapy of cancers, anti-neotic therapy would not cause undesirable side effects. We propose a rational hypothesis for the origin and progression of tumors in which neosis plays a major role in the multistep carcinogenesis in different types of cancers. We define cancers as a single disease of uncontrolled neosis due to failure of senescent checkpoint controls.