Two faces of p53: aging and tumor suppression

The p53 tumor suppressor protein, often termed guardian of the genome, integrates diverse physiological signals in mammalian cells. In response to stress signals, perhaps the best studied of which is the response to DNA damage, p53 becomes functionally active and triggers either a transient cell cycle arrest, cell death (apoptosis) or permanent cell cycle arrest (cellular senescence). Both apoptosis and cellular senescence are potent tumor suppressor mechanisms that irreversibly prevent damaged cells from undergoing neoplastic transformation. However, both processes can also deplete renewable tissues of proliferation-competent progenitor or stem cells. Such depletion, in turn, can compromise the structure and function of tissues, which is a hallmark of aging. Moreover, whereas apoptotic cells are by definition eliminated from tissues, senescent cells can persist, acquire altered functions, and thus alter tissue microenvironments in ways that can promote both cancer and aging phenotypes. Recent evidence suggests that increased p53 activity can, at least under some circumstances, promote organismal aging. Here, we discuss the role of p53 as a key regulator of the DNA damage responses, and discuss how p53 integrates the outcome of the DNA damage response to optimally balance tumor suppression and longevity.     

Dysregulation of lysophospholipid signaling by p53 in malignant cells and the tumor microenvironment

The TP53 gene has been widely studied for its roles in cell cycle control, maintaining genome stability, activating repair mechanisms upon DNA damage, and initiating apoptosis should repair mechanisms fail. Thus, it is not surprising that mutations of p53 are the most common genetic alterations found in human cancer. Emerging evidence indicates that dysregulation of lipid metabolism by p53 can have a profound impact not only on cancer cells but also cells of the tumor microenvironment (TME). In particular, intermediates of the sphingolipid and lysophospholipid pathways regulate many cellular responses common to p53 such as cell survival, migration, DNA damage repair and apoptosis. The majority of these cellular events become dysregulated in cancer as well as cell senescence. In this review, we will provide an account on the seminal contributions of Prof. Lina Obeid, who deciphered the crosstalk between p53 and the sphingolipid pathway particularly in modulating DNA damage repair and apoptosis in non-transformed as well as transformed cells. We will also provide insights on the integrative role of p53 with the lysophosphatidic acid (LPA) signaling pathway in cancer progression and TME regulation.     

Insulin‐like growth factor‐1 regulates the SIRT1‐p53 pathway in cellular senescence

Cellular senescence, which is known to halt proliferation of aged and stressed cells, plays a key role against cancer development and is also closely associated with organismal aging. While increased insulin‐like growth factor (IGF) signaling induces cell proliferation, survival and cancer progression, disrupted IGF signaling is known to enhance longevity concomitantly with delay in aging processes. The molecular mechanisms involved in the regulation of aging by IGF signaling and whether IGF regulates cellular senescence are still poorly understood. In this study, we demonstrate that IGF‐1 exerts a dual function in promoting cell proliferation as well as cellular senescence. While acute IGF‐1 exposure promotes cell proliferation and is opposed by p53, prolonged IGF‐1 treatment induces premature cellular senescence in a p53‐dependent manner. We show that prolonged IGF‐1 treatment inhibits SIRT1 deacetylase activity, resulting in increased p53 acetylation as well as p53 stabilization and activation, thus leading to premature cellular senescence. In addition, either expression of SIRT1 or inhibition of p53 prevented IGF‐1‐induced premature cellular senescence. Together, these findings suggest that p53 acts as a molecular switch in monitoring IGF‐1‐induced proliferation and premature senescence, and suggest a possible molecular connection involving IGF‐1‐SIRT1‐p53 signaling in cellular senescence and aging.     

Does p53 affect organismal aging?

The p53 protein plays a critical role in the prevention of cancer. It responds to a variety of cellular stresses to induce either apoptosis, a transient cell cycle arrest, or a terminal cell cycle arrest called senescence. Senescence in cultured cells is associated with augmented p53 activity and abrogation of p53 activity may delay in vitro senescence. Increasing evidence suggests that p53 may also influence aspects of organismal aging. Several mutant mouse models that display alterations in longevity and aging-related phenotypes have defects in genes that alter p53 signaling. Recently, my laboratory has developed and characterized a p53 mutant mouse line that appears to have an enhanced p53 response. These p53 mutants exhibit increased cancer resistance, yet have a shortened longevity and display a number of early aging-associated phenotypes, suggesting a role for p53 in the aging process. The nature of the aging phenotypes observed in this p53 mutant line is consistent with a model in which aging is driven in part by a gradual depletion of stem cell functional capacity.     

Influence of induced reactive oxygen species in p53-mediated cell fate decisions

The p53 tumor suppressor gene can induce either apoptosis or a permanent growth arrest (also termed senescence) phenotype in response to cellular stresses. We show that the increase in intracellular reactive oxygen species (ROS) associated with the magnitude of p53 protein expression correlated with the induction of either senescence or apoptosis in both normal and cancer cells. ROS inhibitors ameliorated both p53-dependent cell fates, implicating ROS accumulation as an effector in each case. The absence of Bax or PUMA strongly inhibited both p53-induced apoptosis and ROS increase, indicating an important role these p53 targets affecting mitochondrial function genes in p53-mediated ROS accumulation. Moreover, physiological p53 levels in combination with an exogenous ROS source were able to convert a p53 senescence response into apoptosis. All of these findings establish a critical role of ROS accumulation and mitochondrial function in p53-dependent cell fates and show that other ROS inducers can collaborate with p53 to influence these fate decisions. Thus, our studies imply that therapeutic agents that generate ROS are more likely to be toxic for normal cells than p53-negative tumor cells and provide a rationale for identifying therapeutic agents that do not complement p53 in ROS generation to ameliorate the cytotoxic side effects in normal cells.     

Repression of the human papillomavirus E6 gene initiates p53-dependent, telomerase-independent senescence and apoptosis in HeLa cervical carcinoma cells

Cervical cancer cells express high-risk human papillomavirus (HPV) E6 and E7 proteins. When both HPV oncogenes are repressed in HeLa cervical carcinoma cells, the dormant p53 and retinoblastoma (Rb) tumor suppressor pathways are activated, and the cells undergo senescence in the absence of apoptosis. When the E6 gene is repressed in cells that continue to express an E7 gene, the p53 pathway, but not the Rb pathway, is activated, and both senescence and apoptosis are triggered. To determine the role of p53 signaling in senescence or apoptosis after repression of HPV oncogenes, we introduced a dominant-negative allele of p53 into HeLa cells. Dominant-negative p53 prevented senescence and apoptosis when E6 alone was repressed but did not inhibit senescence when both E6 and E7 were repressed. To determine whether reduced telomerase activity was involved in senescence or apoptosis after E6 repression, we generated HeLa cells stably expressing an exogenous hTERT gene, which encodes the catalytic subunit of telomerase. Although these cells contained markedly elevated telomerase activity and elongated telomeres, hTERT expression did not prevent senescence and apoptosis when E6 alone was repressed. These results demonstrate that when the Rb tumor suppressor pathway is inactivated by the E7 protein, E6 repression activates p53 signaling, which in turn is required for growth inhibition, senescence, and apoptosis. Thus, sustained inactivation of the p53 pathway by the E6 protein is required for maintenance of the proliferative phenotype of HeLa cervical carcinoma cells.     

Absence of p53-dependent apoptosis leads to UV radiation hypersensitivity,enhanced immunosuppression and cellular senescence

Genotoxic stress triggers the p53 tumor suppressor network to activate cellular responses that lead to cell cycle arrest, DNA repair, apoptosis or senescence. This network functions mainly through transactivation of different downstream targets, including cell cycle inhibitor p21, which is required for short-term cell cycle arrest or long-term cellular senescence, or proapoptotic genes such as p53 upregulated modulator of apoptosis (PUMA) and Noxa. However, the mechanism that switches from cell cycle arrest to apoptosis is still unknown. In this study, we found that mice harboring a hypomorphic mutant p53, R172P, a mutation that abrogates p53-mediated apoptosis while keeping cell cycle control mostly intact, are more susceptible to ultraviolet-B (UVB)-induced skin damage, inflammation, and immunosuppression than wild-type mice. p53^R172P embryonic fibroblasts (MEFs) are hypersensitive to UVB and prematurely senesce after UVB exposure, in stark contrast to wild-type MEFs, which undergo apoptosis. However, these mutant cells are able to repair UV-induced DNA lesions, indicating that the UV hypersensitive phenotype results from the subsequent damage response. Mutant MEFs show an induction of p53 and p21 after UVR, while wild-type MEFs additionally induce PUMA and Noxa. Importantly, p53^R172P MEFs failed to downregulate anti-apoptotic protein Bcl-2, which has been shown to play an important role in p53-dependent apoptosis. Taken together, these data demonstrate that in the absence of p53-mediated apoptosis, cells undergo cellular senescence to prevent genomic instability. Our results also indicate that p53-dependent apoptosis may play an active role in balancing cellular growth.     

The role of p53-mediated apoptosis as a crucial anti-tumor response to genomic instability: lessons from mouse models

Genomic instability is a major force driving human cancer development. A cellular safeguard against such genetic destabilization, which can ensue from defects in telomere maintenance, DNA repair, and checkpoint function, is activation of the p53 tumor suppressor protein, which commonly responds to these DNA damage signals by inducing apoptosis. If, however, p53 becomes inactivated, as is typical of many tumors and pre-cancerous lesions, then cells with compromised genome integrity pathways survive inappropriately, and the accrual of oncogenic lesions can fuel the carcinogenic process. Studies of mouse models have been instrumental in providing support for this idea. Mouse knockouts in genes important for telomere function, DNA damage checkpoint activation and DNA repair - both non-homologous end joining and homologous recombination - are prone to the development of genomic instability. As a consequence of these DNA damage signals, p53 becomes activated in cells of these mutant mice, leading to the induction of apoptosis, sometimes at the expense of organismal viability. This apoptotic response can be rescued through crosses to p53-deficient mice, but has dire consequences: mice predisposed to genomic instability and lacking p53 are susceptible to tumorigenesis. Thus p53-mediated apoptosis provides a crucial tumor suppressive mechanism to eliminate cells succumbing to genomic instability.     

Absence of p53-dependent apoptosis leads to UV radiation hypersensitivity,enhanced immunosuppression and cellular senescence

Genotoxic stress triggers the p53 tumor suppressor network to activate cellular responses that lead to cell cycle arrest, DNA repair, apoptosis or senescence. This network functions mainly through transactivation of different downstream targets, including cell cycle inhibitor p21, which is required for short-term cell cycle arrest or long-term cellular senescence, or proapoptotic genes such as p53 upregulated modulator of apoptosis (PUMA) and Noxa. However, the mechanism that switches from cell cycle arrest to apoptosis is still unknown. In this study, we found that mice harboring a hypomorphic mutant p53, R172P, a mutation that abrogates p53-mediated apoptosis while keeping cell cycle control mostly intact, are more susceptible to ultraviolet-B (UVB)-induced skin damage, inflammation and immunosuppression than wild-type mice. p53^R172P embryonic fibroblasts (MEFs) are hypersensitive to UVB and prematurely senesce after UVB exposure, in stark contrast to wild-type MEFs, which undergo apoptosis. However, these mutant cells are able to repair UV-induced DNA lesions, indicating that the UV-hypersensitive phenotype results from the subsequent damage response. Mutant MEFs show an induction of p53 and p21 after UVR, while wild-type MEFs additionally induce PUMA and Noxa. Importantly, p53^R172P MEFs failed to downregulate anti-apoptotic protein Bcl-2, which has been shown to play an important role in p53-dependent apoptosis. Taken together, these data demonstrate that in the absence of p53-mediated apoptosis, cells undergo cellular senescence to prevent genomic instability. Our results also indicate that p53-dependent apoptosis may play an active role in balancing cellular growth.Key words: UVB irradiation, p53, DNA damage, DNA damage responses, apoptosis, senescence     

The p53 tumor suppressor protein is a critical regulator of hematopoietic stem cell behavior

In response to diverse stresses, the tumor suppressor p53 differentially regulates its target genes, variably inducing cell-cycle arrest, apoptosis or senescence. Emerging evidence indicates that p53 plays an important role in regulating hematopoietic stem cell (HSC) quiescence, self-renewal, apoptosis and aging. The p53 pathway is activated by DNA damage, defects in ribosome biogenesis, oxidative stress and oncogene induced p19ARF upregulation. We present an overview of the current state of knowledge about p53 (and its target genes) in regulating HSC behavior, with the hope that understanding the molecular mechanisms that control p53 activity in HSCs and how p53 mutations affect its role in these events may facilitate the development of therapeutic strategies for eliminating leukemia (and cancer) propagating cells.     

Signaling pathway requirements for induction of senescence by telomere homolog oligonucleotides

Cellular senescence is a major defense against cancer. In human fibroblasts, suppressing both the p53 and pRb pathways is necessary to bypass replicative senescence as well as senescence induced by ectopic expression of a dominant negative form of the telomere repeat binding factor 2, TRF2(DN). We recently reported that exposure to oligonucleotides homologous to the telomere 3' overhang (T-oligos) activates both the p53 and pRb pathways and leads to senescence in primary human fibroblasts. To further characterize T-oligo-induced senescence, we compared established isogenic fibroblast cell lines lacking functional p53 and/or pRb pathways to the normal parental line. Here, we report that, as in physiologic senescence, inactivation of both the p53 and pRb pathways is necessary to suppress T-oligo-induced senescence. Moreover, T-oligo rapidly induces senescence in a malignant fibroblast-derived cell line, demonstrating the potential of using T-oligo as a novel anticancer therapeutic. Our data support the hypothesis that exposure of the TTAGGG tandem repeat telomere 3' overhang sequence is the event that initiates signaling through DNA damage response pathways after experimental telomere disruption, serial passage, or acute genomic damage of normal cells.     

Autophagy-independent senescence and genome instability driven by targeted telomere dysfunction

Telomere dysfunction plays a complex role in tumorigenesis. While dysfunctional telomeres can block the proliferation of incipient cancer clones by inducing replicative senescence, fusion of dysfunctional telomeres can drive genome instability and oncogenic genomic rearrangements. Therefore, it is important to define the regulatory pathways that guide these opposing effects. Recent work has shown that the autophagy pathway regulates both senescence and genome instability in various contexts. Here, we apply models of acute telomere dysfunction to determine whether autophagy modulates the resulting genome instability and senescence responses. While telomere dysfunction rapidly induces autophagic flux in human fibroblast cell lines, inhibition of the autophagy pathway does not have a significant impact upon the transition to senescence, in contrast to what has previously been reported for oncogene-induced senescence. Our results suggest that this difference may be explained by disparities in the development of the senescence-associated secretory phenotype. We also show that chromosome fusions induced by telomere dysfunction are comparable in autophagy-proficient and autophagy-deficient cells. Altogether, our results highlight the complexity of the senescence-autophagy interface and indicate that autophagy induction is unlikely to play a significant role in telomere dysfunction-driven senescence and chromosome fusions.     

lncRNA在细胞衰老中的作用

衰老是身体器官系统功能逐渐衰退的复杂的生物学过程,能诱发多种老年病。 长链非编码RNA（long non-coding RNA, lncRNA）是长度大于200个核苷酸的非编码RNA,在多种生理学和病理学过程中发挥重要作用。 细胞衰老是重要的衰老生物学过程之一,已经发现大量的lncRNA参与了细胞周期、端粒长度和表观遗传学等的调控,并影响关键的细胞周期过程,如细胞的衰老、增殖、分化、静止等;同时,lncRNA还参与了衰老相关重要信号通路的调控,如p53/p21、哺乳动物雷帕霉素靶蛋白（mTOR）、视网膜母细胞瘤蛋白（Rb）/p16和磷脂酰肌醇3激酶/苏氨酸蛋白激酶（PI3K/Akt）信号通路,它们均与许多衰老相关重大疾病密切相关。 本文综述了最近发现的与细胞衰老相关的lncRNA的功能和作用机制,并总结了lncRNA参与调控的细胞衰老信号通路,最后讨论了lncRNA未来的研究方向。     

A switch-like dynamic mechanism for the initiation of replicative senescence

Telomeres are specialized structures protecting chromosomes against genome instability. Telomeres shorten with cell division, and replicative senescence is induced when telomeres are badly eroded. Whereas TRF2 (telomeric-repeat binding factor 2), ATM (ataxia telangiectasia mutated) and p53 have been identified involved in senescence induction, how it is triggered remains unclear. Here, we propose an integrated model associating telomere loss with senescence trigger. We characterize the dynamics of telomere shorting and the p53-centered regulatory network. We show that senescence is initiated in a switch-like manner when both the shortest telomere becomes uncapped and the TRF2-ATM-p53-Siah1 positive feedback loop is switched on. This work provides a coherent picture of senescence induction in terms of telomere shortening and p53 activation.     

mTORC1 and p53

A balance must be struck between cell growth and stress responses to ensure that cells proliferate without accumulating damaged DNA. This balance means that optimal cell proliferation requires the integration of pro-growth and stress-response pathways. mTOR (mechanistic target of rapamycin) is a pleiotropic kinase found in complex 1 (mTORC1). The mTORC1 pathway governs a response to mitogenic signals with high energy levels to promote protein synthesis and cell growth. In contrast, the p53 DNA damage response pathway is the arbiter of cell proliferation, restraining mTORC1 under conditions of genotoxic stress. Recent studies suggest a complicated integration of these pathways to ensure successful cell growth and proliferation without compromising genome maintenance. Deciphering this integration could be key to understanding the potential clinical usefulness of mTORC1 inhibitors like rapamycin. Here we discuss how these p53-mTORC1 interactions might play a role in the suppression of cancer and perhaps the development of cellular senescence and organismal aging.     

The yin and yang of the Cdkn2a locus in senescence and aging

Senescence of cultured cells involves activation of the p19Arf-p53 and the p16Ink4a-Rb tumor suppressor pathways. This, together with the observation that p19Arf and p16Ink4a expression increases with age in many tissues of humans and rodents, led to the speculation that these pathways drive in vivo senescence and natural aging. However, it has been difficult to test this hypothesis using a mammalian model system because inactivation of either of these pathways results in early death from tumors. One approach to bypass this problem would be to inactivate these pathways in a murine segmental progeria model such as mice that express low amounts of the mitotic checkpoint protein BubR1 (BubR1 hypomorphic mice). These mice have a five-fold reduced lifespan and develop a variety of early-aging associated phenotypes including cachetic dwarfism, skeletal muscle degeneration, cataracts, arterial stiffening, (subcutaneous) fat loss, reduced stress tolerance and impaired wound healing. Importantly, BubR1 hypomorphism elevates both p16Ink4a and p19Arf expression in skeletal muscle and fat. Inactivation of p16Ink4a in BubR1 mutant mice delays both cellular senescence and aging specifically in these tissues. Surprisingly, however, inactivation of p19Arf has the opposite effect; it exacerbates in vivo senescence and aging in skeletal muscle and fat. These mouse studies suggest that p16Ink4a is indeed an effector of aging and in vivo senescence, but p19Arf an attenuator. Thus, the role of the p19Arf-p53 pathway in aging and in vivo senescence seems far more complex than previously anticipated.