The DNA Damage Signaling Pathway Connects Oncogenic Stress to Cellular Senescence

The mechanisms of tumor suppression must be linked to the oncogenic threats that may affect a normal cell. An important cancer causing mechanism is the accidental activation of genes that stimulate cell proliferation (oncogenes) by a variety of endogenous or environmental mutagens. This event has been experimentally modelled by enforcing the expression of oncogenes in primary cells. The astonishing outcome of these manipulations is that oncogenes trigger antiproliferative responses preventing progression to malignant transformation. These responses bring to an end proliferation due to cell death or a permanent cell cycle arrest called senescence. Here we review evidence indicating that oncogene induced senescence (OIS) involves activation of p53 via the DNA damage response (DDR). These results imply mechanisms of DNA damage in cells expressing oncogenes, that may be secondary to reactive oxygen species and/or some form of “oncogenic stress” that affect normal DNA replication. Interestingly, DNA damage signals persist in cells that escape from senescence. The implications of these signals for tumorigenesis are also discussed. Given that DNA damage signals have now been observed in cells treated with any stimuli known to induce senescence, the process can be redefined as a metabolically viable but permanent cell cycle arrest with persistent DNA damage signaling.     

MYC-driven tumorigenesis is inhibited by WRN syndrome gene deficiency

MYC-induced DNA damage is exacerbated in WRN-deficient cells, leading to replication stress and accelerated cellular senescence. To determine whether WRN deficiency impairs MYC-driven tumor development, we used both xenograft and autochthonous tumor models. Conditional silencing of WRN expression in c-MYC overexpressing non-small cell lung cancer xenografts impaired both tumor establishment and tumor growth. This inhibitory effect of WRN knockdown was accompanied by increased DNA damage, decreased proliferation, and tumor necrosis. In the Eμ-Myc mouse model of B-cell lymphoma, a germline mutation in the helicase domain of Wrn (Wrn(Δhel/Δhel)) resulted in a significant delay in emergence of lethal lymphomas, extending tumor-free survival by more than 30%. Analysis of preneoplastic B cells from Eμ-Myc Wrn mutant mice revealed increased DNA damage, elevation of senescence markers, and decreased proliferation in comparison with cells from age-matched Eμ-Myc mice. Immunohistochemical and global gene expression analysis of overt Eμ-Myc Wrn(Δhel/Δhel) lymphomas showed a marked increase in expression of the CDK inhibitor, p16(Ink4a), as well as elevation of TAp63, a known mediator of senescence. Collectively, these studies show that in the context of Myc-associated tumorigenesis, loss of Wrn amplifies the DNA damage response, both in preneoplastic and neoplastic tissue, engaging activation of tumor suppressor pathways. This leads to inhibition of tumor growth and prolonged tumor-free survival. Targeting WRN or its enzymatic function could prove to be an effective strategy in the treatment of MYC-associated cancers.     

Regulation of cellular senescence by p53.

Many normal cells respond to potentially oncogenic stimuli by undergoing cellular senescence, a state of irreversibly arrested proliferation and altered differentiated function. Cellular senescence very likely evolved to suppress tumorigenesis. In support of this idea, it is regulated by several tumor suppressor genes. At the heart of this regulation is p53. p53 is essential for the senescence response to short telomeres, DNA damage, oncogenes and supraphysiological mitogenic signals, and overexpression of certain tumor suppressor genes. Despite the well-documented central role for p53 in the senescence response, many questions remain regarding how p53 senses senescence-inducing stimuli and how it elicits the senescent phenotype.     

Loss of miRNA biogenesis induces p19Arf-p53 signaling and senescence in primary cells

Dicer, an enzyme involved in microRNA (miRNA) maturation, is required for proper cell differentiation and embryogenesis in mammals. Recent evidence indicates that Dicer and miRNA may also regulate tumorigenesis. To better characterize the role of miRNA in primary cell growth, we generated Dicer-conditional mice. Ablation of Dicer and loss of mature miRNAs in embryonic fibroblasts up-regulated p19(Arf) and p53 levels, inhibited cell proliferation, and induced a premature senescence phenotype that was also observed in vivo after Dicer ablation in the developing limb and in adult skin. Furthermore, deletion of the Ink4a/Arf or p53 locus could rescue fibroblasts from premature senescence induced by Dicer ablation. Although levels of Ras and Myc oncoproteins appeared unaltered, loss of Dicer resulted in increased DNA damage and p53 activity in these cells. These results reveal that loss of miRNA biogenesis activates a DNA damage checkpoint, up-regulates p19(Arf)-p53 signaling, and induces senescence in primary cells.     

细胞衰老与肿瘤治疗

人口老龄化是全世界都面临的重大挑战,随着老年人口的增加,肿瘤等衰老相关疾病发病率随之升高.流行病学调查结果显示,大约2/3的新增肿瘤患者为65岁以上的老年人,并且这一比例在不断攀升.细胞衰老是指在DNA损伤或癌基因失调等一系列条件下引起的稳定的细胞周期阻滞,并伴有形态、生化及表观遗传的改变.大量研究证明细胞衰老对抑制潜在癌细胞增殖具有重要作用.然而,目前研究认为除了抑制肿瘤发生,细胞衰老也可能促进肿瘤的演进,细胞衰老对肿瘤发挥了双刃剑作用.因此,深入了解细胞衰老与肿瘤之间的联系,充分利用细胞衰老对肿瘤抑制功能,规避其对肿瘤的促进作用可为肿瘤的治疗提供更多可能的选择.     

ID1, inhibitor of differentiation/DNA binding, is an effector of the p53-dependent DNA damage response pathway

ID1, inhibitor of differentiation/DNA binding, plays an important role in cell proliferation, differentiation, and tumorigenesis. It has been shown that ID1 is de-regulated in multiple cancers and up-regulation of ID1 is correlated with high grades and poor prognosis of human cancers. In contrast, the p53 tumor suppressor was found to be mutated or inactivated in most human cancers and loss of p53 results in early onset of multiple cancers. Although the biological functions of the ID1 oncogene and the p53 tumor suppressor have been intensively investigated, little is known about the upstream regulators of ID1 and the cross-talk between ID1 and p53. Here, we showed that ID1 is down-regulated in cells treated with various DNA damage agents in a p53-dependent manner. Interestingly, we found that DEC1, which was recently identified as a p53 target and mediates p53-dependent cell cycle arrest and senescence, is capable of inhibiting ID1 expression. Conversely, we found that knockdown of DEC1 attenuates DNA damage-induced ID1 repression. In addition, we identified several potential DEC1 responsive elements in the proximal promoter region of the ID1 gene. Moreover, we showed that overexpression of ID1 or ID1', an isoform of ID1, promotes cell proliferation potentially through inhibition of p21 expression. Finally, we found that the extent of DNA damage-induced premature senescence was substantially decreased by overexpression of ID1 or ID1'. Taken together, our study suggests that p53 trans-repressional activity can be mediated by its own target DEC1 and ID1 is an effector of the p53-dependent DNA damage response pathway.     

Senescence-associated heterochromatin foci are dispensable for cellular senescence,occur in a cell type- and insult-dependent manner and follow expression of p16ink4a

Cellular senescence, an irreversible proliferation arrest evoked by stresses such as oncogene activation, telomere dysfunction, or diverse genotoxic insults, has been implicated in tumor suppression and aging. Primary human fibroblasts undergoing oncogene-induced or replicative senescence are known to form senescence-associated heterochromatin foci (SAHF), nuclear DNA domains stained densely by DAPI and enriched for histone modifications including lysine9-trimethylated histone H3. While cellular senescence occurs also in premalignant human lesions, it is unclear how universal is SAHF formation among various cell types, under diverse stresses, and whether SAHF occur in vivo. Here, we report that human primary fibroblasts (BJ and MRC-5) and primary keratinocytes undergoing replicative senescence, or premature senescence induced by oncogenic H-Ras, diverse chemotherapeutics and bacterial cytolethal distending toxin, show differential capacity to form SAHF. Whereas all tested cell types formed SAHF in response to activated H-Ras, only MRC-5, but not BJ fibroblasts or keratinocytes, formed SAHF under senescence induced by etoposide, doxorubicin, hydroxyurea, bacterial intoxication or telomere attrition. In addition, DAPI-defined SAHF were detected on paraffin sections of Ras-transformed cultured fibroblasts, but not human lesions at various stages of tumorigenesis. Overall, our results indicate that unlike the widely present DNA damage response marker γH2AX, SAHF is not a common feature of cellular senescence. Whereas SAHF formation is shared by diverse cultured cell types under oncogenic stress, SAHF are cell-type-restricted under genotoxin-induced and replicative senescence. Furthermore, while the DNA/DAPI-defined SAHF formation in cultured cells parallels enhanced expression of p16^ink4a, such ‘prototypic’ SAHF are not observed in tissues, including premalignant lesions, irrespective of enhanced p16^ink4a and other features of cellular senescence.     

Dysfunctional telomeres induce p53‐dependent and independent apoptosis to compromise cellular proliferation and inhibit tumor formation

Aging is associated with progressive telomere shortening, resulting in the formation of dysfunctional telomeres that compromise tissue proliferation. However, dysfunctional telomeres can limit tumorigenesis by activating p53‐dependent cellular senescence and apoptosis. While activation of both senescence and apoptosis is required for repress tumor formation, it is not clear which pathway is the major tumor suppressive pathway in vivo. In this study, we generated Eμ‐myc; Pot1b ^?/? mouse to directly compare tumor formation under conditions in which either p53‐dependent apoptosis or senescence is activated by telomeres devoid of the shelterin component Pot1b. We found that activation of p53‐dependent apoptosis plays a more critical role in suppressing lymphoma formation than p53‐dependent senescence. In addition, we found that telomeres in Pot1b^?/?; p53^?/? mice activate an ATR‐Chk1‐dependent DNA damage response to initiate a robust p53‐independent, p73‐dependent apoptotic pathway that limited stem cell proliferation but suppressed B‐cell lymphomagenesis. Our results demonstrate that in mouse models, both p53‐dependent and p53‐independent apoptosis are important to suppressing tumor formation.     

Dysregulation of microRNA biogenesis machinery in cancer

MicroRNAs (miRNAs) are integral to the gene regulatory network. A single miRNA is capable of controlling the expression of hundreds of protein coding genes and modulate a wide spectrum of biological functions, such as proliferation, differentiation, stress responses, DNA repair, cell adhesion, motility, inflammation, cell survival, senescence and apoptosis, all of which are fundamental to tumorigenesis. Overexpression, genetic amplification, and gain-of-function mutation of oncogenic miRNAs (“onco-miRs”) as well as genetic deletion and loss-of-function mutation of tumor suppressor miRNAs (“suppressor-miRs”) are linked to human cancer. In addition to the dysregulation of a specific onco-miR or suppressor-miRs, changes in global miRNA levels resulting from a defective miRNA biogenesis pathway play a role in tumorigenesis. The function of individual onco-miRs and suppressor-miRs and their target genes in cancer has been described in many different articles elsewhere. In this review, we primarily focus on the recent development regarding the dysregulation of the miRNA biogenesis pathway and its contribution to cancer.     

Aberrant expression of proteins involved in signal transduction and DNA repair pathways in lung cancer and their association with clinical parameters

BACKGROUND: Because cell signaling and cell metabolic pathways are executed through proteins, protein signatures in primary tumors are useful for identifying key nodes in signaling networks whose alteration is associated with malignancy and/or clinical outcomes. This study aimed to determine protein signatures in primary lung cancer tissues. METHODOLOGY/ PRINCIPAL FINDINGS: We analyzed 126 proteins and/or protein phosphorylation sites in case-matched normal and tumor samples from 101 lung cancer patients with reverse-phase protein array (RPPA) assay. The results showed that 18 molecules were significantly different (p<0.05) by at least 30% between normal and tumor tissues. Most of those molecules play roles in cell proliferation, DNA repair, signal transduction and lipid metabolism, or function as cell surface/matrix proteins. We also validated RPPA results by Western blot and/or immunohistochemical analyses for some of those molecules. Statistical analyses showed that Ku80 levels were significantly higher in tumors of nonsmokers than in those of smokers. Cyclin B1 levels were significantly overexpressed in poorly differentiated tumors while Cox2 levels were significantly overexpressed in neuroendocrinal tumors. A high level of Stat5 is associated with favorable survival outcome for patients treated with surgery. CONCLUSIONS/ SIGNIFICANCE: Our results revealed that some molecules involved in DNA damage/repair, signal transductions, lipid metabolism, and cell proliferation were drastically aberrant in lung cancer tissues, and Stat5 may serve a molecular marker for prognosis of lung cancers.     

细胞衰老与肿瘤发生

细胞衰老（cell senescence）是指细胞在信号转导作用下不可逆地脱离细胞周期并丧失增殖能力后进入的一种相对稳定的状态。细胞衰老有增殖衰老与早熟衰老两种形式：增殖衰老由端粒缩短激发的信号转导激发,与TP53/CDKN1a（p21^WAF-1/Cip1）/pRB/E2F信号通路密切相关;早熟衰老由细胞内在或外在急慢性应激信号引发,与TP53/CDKN1a（p21^WAF-1/Cip1）/pRB/E2F或CDKN2a（p16^ink4A）/pRB/E2F信号通路相关。目前研究已经证实早熟衰老是细胞在癌变过程中的天然屏障,是继DNA修复、细胞凋亡后的第三大细胞内在抗癌机制,在机体防止肿瘤形成中起重要作用。     

Lysyl oxidase activity regulates oncogenic stress response and tumorigenesis

Cellular senescence, a stable proliferation arrest, is induced in response to various stresses. Oncogenic stress-induced senescence (OIS) results in blocked proliferation and constitutes a fail-safe program counteracting tumorigenesis. The events that enable a tumor in a benign senescent state to escape from OIS and become malignant are largely unknown. We show that lysyl oxidase activity contributes to the decision to maintain senescence. Indeed, in human epithelial cell the constitutive expression of the LOX or LOXL2 protein favored OIS escape, whereas inhibition of lysyl oxidase activity was found to stabilize OIS. The relevance of these in vitro observations is supported by in vivo findings: in a transgenic mouse model of aggressive pancreatic ductal adenocarcinoma (PDAC), increasing lysyl oxidase activity accelerates senescence escape, whereas inhibition of lysyl oxidase activity was found to stabilize senescence, delay tumorigenesis, and increase survival. Mechanistically, we show that lysyl oxidase activity favors the escape of senescence by regulating the focal-adhesion kinase. Altogether, our results demonstrate that lysyl oxidase activity participates in primary tumor growth by directly impacting the senescence stability.     

Residual Cdk1/2 activity after DNA damage promotes senescence

In response to DNA damage, a cell can be forced to permanently exit the cell cycle and become senescent. Senescence provides an early barrier against tumor development by preventing proliferation of cells with damaged DNA. By studying single cells, we show that Cdk activity persists after DNA damage until terminal cell cycle exit. This low level of Cdk activity not only allows cell cycle progression, but also promotes cell cycle exit at a decision point in G2 phase. We find that residual Cdk1/2 activity is required for efficient p21 production, allowing for nuclear sequestration of Cyclin B1, subsequent APC/C^Cdh1‐dependent degradation of mitotic inducers and induction of senescence. We suggest that the same activity that triggers mitosis in an unperturbed cell cycle enforces senescence in the presence of DNA damage, ensuring a robust response when most needed.     

Hedgehog信号通路的作用机制及研究进展

HH(Hedgehog)信号通路参与多种生物学过程,包括细胞分化、细胞增殖、细胞衰老、肿瘤的发生、肿瘤恶性转化以及肿瘤耐药,HH信号通路相关基因的异常表达或突变会在生物发生发展的不同阶段引起各种疾病的发生.而HH信号通路通过复杂的机制调控诸多信号通路,进一步影响生物体的功能.所以深入了解HH信号通路在各种遗传疾病、肿瘤...     

Four faces of cellular senescence

Cellular senescence is an important mechanism for preventing the proliferation of potential cancer cells. Recently, however, it has become apparent that this process entails more than a simple cessation of cell growth. In addition to suppressing tumorigenesis, cellular senescence might also promote tissue repair and fuel inflammation associated with aging and cancer progression. Thus, cellular senescence might participate in four complex biological processes (tumor suppression, tumor promotion, aging, and tissue repair), some of which have apparently opposing effects. The challenge now is to understand the senescence response well enough to harness its benefits while suppressing its drawbacks.