Telomere positional effects and the regulation of cellular senescence.

Normal cells have a limited capacity to proliferate but the molecular clock that regulates the onset of cellular senescence remains unidentified. The ends of chromosomes--telomeres--have been shown to shorten progressively with age in normal cells. Here, we present a working model of how telomeric shortening may induce programmed changes in the regulation of cellular proliferation.     

Rapamycin decelerates cellular senescence

When the cell cycle is arrested but cellular growth is not, then cells senesce, permanently losing proliferative potential. Here we demonstrated that the duration of cell cycle arrest determines a progressive loss of proliferative capacity. In human and rodent cell lines, rapamycin (an inhibitor of mTOR) dramatically decelerated loss of proliferative potential caused by ectopic p21, p16 and sodium butyrate-induced p21. Thus, when the cell cycle was arrested by these factors in the presence of rapamycin, cells retained the capacity to resume proliferation, once p21, p16 or sodium butyrate were removed. While rapamycin prevented the permanent loss of proliferative potential in arrested cells, it did not force the arrested cells into proliferation. During cell cycle arrest, rapamycin transformed the irreversible arrest into a reversible condition. Our data demonstrate that senescence can be pharmacologically suppressed.     

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.     

Phospholipase D in cellular senescence

Cellular senescence appears to be an important part of organismal aging. Cellular senescence is characterized by flattened enlarged morphology, inhibition of DNA replication in response to growth factors, inability to phosphorylate the pRb tumor suppressor protein, inability to produce c-fos or AP-1 and overexpression of a variety of genes, notably p21 (CIP-1/WAF-1) and p16^INK. It is now clear that certain early mitotic signals become defective with the onset of senescence. Among these is the PLD/PKC pathway. Evidence suggests that activation of PLD and PKC is critical for mitogenesis. Recent data suggest that the defect in PLD/PKC in cellular senescence is a result of elevated cellular ceramide levels which inhibit PLD activation. It appears that the elevated ceramide is a result of neutral sphingomyelinase activation. Ceramide acts to inhibit the activation of PLD by possibly three mechanisms, inhibiting activation by Rho, translocation to the membrane and gene expression. Addition of ceramide to young cells not only inhibits PLD but also recapitulates all the standard measures of cellular senescence as described above.     

Inflammatory signaling and cellular senescence

Inflammation acts as a double-edged sword in the pathogenesis of cancer. Inflammatory responses play a key role in eliminating potentially cancerous cells; however, an inflammatory microenvironment also promotes the development of cancer. Proinflammatory cytokines, the key mediators of inflammation, also play a dual role in oncogenesis. While they can promote neoplastic progression, recent studies have revealed an unexpected function of the inflammatory pathways in inhibiting cancer development. These studies demonstrate that cells undergoing senescence, a cellular program serving as a barrier to cancer development, produce increased amount of inflammatory cytokines. These inflammatory cytokines play an essential role in the initiation and maintenance of cellular senescence, and are responsible for triggering an innate immune response that clears the senescent tumor cells in vivo. The purpose of the present review is to discuss the dual roles of the inflammatory cytokines produced by senescent cells in the pathogenesis of cancer, and the signaling pathway mediating their role in cellular senescence.     

Androgen receptor drives cellular senescence

The accepted androgen receptor (AR) role is to promote proliferation and survival of prostate epithelium and thus prostate cancer progression. While growth-inhibitory, tumor-suppressive AR effects have also been documented, the underlying mechanisms are poorly understood. Here, we for the first time link AR anti-cancer action with cell senescence in vitro and in vivo. First, AR-driven senescence was p53-independent. Instead, AR induced p21, which subsequently reduced ΔN isoform of p63. Second, AR activation increased reactive oxygen species (ROS) and thereby suppressed Rb phosphorylation. Both pathways were critical for senescence as was proven by p21 and Rb knock-down and by quenching ROS with N-Acetyl cysteine and p63 silencing also mimicked AR-induced senescence. The two pathways engaged in a cross-talk, likely via PML tumor suppressor, whose localization to senescence-associated chromatin foci was increased by AR activation. All these pathways contributed to growth arrest, which resolved in senescence due to concomitant lack of p53 and high mTOR activity. This is the first demonstration of senescence response caused by a nuclear hormone receptor.     

Calcium signaling and cellular senescence

Cellular senescence is a stable cell proliferation arrest induced by a variety of stresses including telomere shortening, oncogene activation and oxidative stress. This process plays a crucial role in many physiopathological contexts, especially during aging when cellular senescence favors development of age-related diseases, shortening lifespan. However, the molecular and cellular mechanisms controlling senescence are still a matter of active research. In the last decade, there has been emerging literature indicating a key involvement of calcium signaling in cellular senescence. In this review we will initially give an account of the direct evidence linking calcium and the regulation of senescence. We will then review our current knowledge on the role of calcium in some senescence-associated features and physiopathological conditions, which will shed light on additional ways in which calcium signaling is implicated in cellular senescence.     

A cellular timetable of autumn senescence

We have studied autumn leaf senescence in a free-growing aspen (Populus tremula) by following changes in pigment, metabolite and nutrient content, photosynthesis, and cell and organelle integrity. The senescence process started on September 11, 2003, apparently initiated solely by the photoperiod, and progressed steadily without any obvious influence of other environmental signals. For example, after this date, senescing leaves accumulated anthocyanins in response to conditions inducing photooxidative stress, but at the beginning of September the leaves did not. Degradation of leaf constituents took place over an 18-d period, and, although the cells in each leaf did not all senesce in parallel, senescence in the tree as a whole was synchronous. Lutein and beta-carotene were degraded in parallel with chlorophyll, whereas neoxanthin and the xanthophyll cycle pigments were retained longer. Chloroplasts in each cell were rapidly converted to gerontoplasts and many, although not all, cells died. From September 19, when chlorophyll levels had dropped by 50%, mitochondrial respiration provided the energy for nutrient remobilization. Remobilization seemed to stop on September 29, probably due to the cessation of phloem transport, but, up to abscission of the last leaves (over 1 week later), some cells were metabolically active and had chlorophyll-containing gerontoplasts. About 80% of the nitrogen and phosphorus was remobilized, and on September 29 a sudden change occurred in the delta15N of the cellular content, indicating that volatile compounds may have been released.     

On the evolution of cellular senescence

The idea that senescent cells are causally involved in aging has gained strong support from findings that the removal of such cells alleviates many age‐related diseases and extends the life span of mice. While efforts proceed to make therapeutic use of such discoveries, it is important to ask what evolutionary forces might have been behind the emergence of cellular senescence, in order better to understand the biology that we might seek to alter. Cellular senescence is often regarded as an anti‐cancer mechanism, since it limits the division potential of cells. However, many studies have shown that senescent cells often also have carcinogenic properties. This is difficult to reconcile with the simple idea of an anti‐cancer mechanism. Furthermore, other studies have shown that cellular senescence is involved in wound healing and tissue repair. Here, we bring these findings and ideas together and discuss the possibility that these functions might be the main reason for the evolution of cellular senescence. Furthermore, we discuss the idea that senescent cells might accumulate with age because the immune system had to strike a balance between false negatives (overlooking some senescent cells) and false positives (destroying healthy body cells).     

Implication of p53-dependent cellular senescence related gene, TARSH in tumor suppression

A novel target of NESH-SH3 (TARSH) was identified as a cellular senescence related gene in mouse embryonic fibroblasts (MEFs) replicative senescence, the expression of which has been suppressed in primary clinical lung cancer specimens. However, the molecular mechanism underlying the regulation of TARSH involved in pulmonary tumorigenesis remains unclear. Here we demonstrate that the reduction of TARSH gene expression by short hairpin RNA (shRNA) system robustly inhibited the MEFs proliferation with increase in senescence-associated β-galactosidase (SA-β-gal) activity. Using p53^−/− MEFs, we further suggest that this growth arrest by loss of TARSH is evoked by p53-dependent p21^Cip1 accumulation. Moreover, we also reveal that TARSH reduction induces multicentrosome in MEFs, which is linked in chromosome instability and tumor development. These results suggest that TARSH plays an important role in proliferation of replicative senescence and may serve as a trigger of tumor development.     

端粒、端粒酶与细胞衰老

端粒和端粒酶是现代生物学研究的热点,端粒的缺失与细胞的衰老,端粒酶的活性与细胞的老化及癌化均有密切的关系。章综述了端粒和端粒酶的结构和功能,及其与细胞衰老的关系,并在此基础之上展望了端粒酶在抗衰老、抑制肿瘤等方面的应用。     

Phospholipase D in cellular senescence.

Cellular senescence appears to be an important part of organismal aging. Cellular senescence is characterized by flattened enlarged morphology, inhibition of DNA replication in response to growth factors, inability to phosphorylate the pRb tumor suppressor protein, inability to produce c-fos or AP-1 and overexpression of a variety of genes, notably p21 (CIP-1/WAF-1) and p16(INK). It is now clear that certain early mitotic signals become defective with the onset of senescence. Among these is the PLD/PKC pathway. Evidence suggests that activation of PLD and PKC is critical for mitogenesis. Recent data suggest that the defect in PLD/PKC in cellular senescence is a result of elevated cellular ceramide levels which inhibit PLD activation. It appears that the elevated ceramide is a result of neutral sphingomyelinase activation. Ceramide acts to inhibit the activation of PLD by possibly three mechanisms, inhibiting activation by Rho, translocation to the membrane and gene expression. Addition of ceramide to young cells not only inhibits PLD but also recapitulates all the standard measures of cellular senescence as described above.     

Prevalent intron retention fine‐tunes gene expression and contributes to cellular senescence

Intron retention (IR) is the least well‐understood alternative splicing type in animals, and its prevalence and function in physiological and pathological processes have long been underestimated. Cellular senescence contributes to individual aging and age‐related diseases and can also serve as an important cancer prevention mechanism. Dynamic IR events have been observed in senescence models and aged tissues; however, whether and how IR impacts senescence remain unclear. Through analyzing polyA⁺ RNA‐seq data from human replicative senescence models, we found IR was prevalent and dynamically regulated during senescence and IR changes negatively correlated with expression alteration of corresponding genes. We discovered that knocking down (KD) splicing factor U2AF1, which showed higher binding density to retained introns and decreased expression during senescence, led to senescence‐associated phenotypes and global IR changes. Intriguingly, U2AF1‐KD‐induced IR changes also negatively correlated with gene expression. Furthermore, we demonstrated that U2AF1‐mediated IR of specific gene (CPNE1 as an example) contributed to cellular senescence. Decreased expression of U2AF1, higher IR of CPNE1, and reduced expression of CPNE1 were also discovered in dermal fibroblasts with age. We discovered prevalent IR could fine‐tune gene expression and contribute to senescence‐associated phenotypes, largely extending the biological significance of IR.