Senescence delay of human diploid fibroblast induced by anti-sense p16INK4a expression

CDKN2A somatic copy number amplification in normal tissues surrounding gastric carcinoma reduces cancer metastasis risk in droplet digital PCR analysis

BACKGROUND

The CDKN2A gene is frequently affected by somatic copy number variations (SCNVs, including deletions and amplifications [SCNdel and SCNamp]) in the cancer genome. Using surgical gastric margin tissue samples (SMs) as the diploid reference in SCNV analysis via CDKN2A/P16-specific real-time PCR (P16-Light), we previously reported that the CDKN2A SCNdel was associated with a high risk of metastasis of gastric carcinoma (GC). However, the status of CDKN2A SCNVs in SMs and their clinical significance have not been reported.

METHODS

Peripheral white blood cell (WBC) and frozen GC and SM tissue samples were collected from patients (n = 80). Droplet digital PCR (ddPCR) was used to determine the copy number (CN) of the CDKN2A gene in tissue samples using paired WBCs as the diploid reference.

RESULTS

A novel P16-ddPCR system was initially established with a minimal proportion (or limit, 10%) of the detection of CDKN2A CN alterations. While CDKN2A SCNamp events were detected in both SMs and GCs, fewer CDKN2A SCNdel events were detected in SMs than in GCs (15.0% vs. 41.3%, P = 4.77E-04). Notably, significantly more SCNamp and fewer SCNdel of the CDKN2A gene were detected in SMs from GC patients without metastasis than in those from patients with lymph node metastasis by P16-ddPCR (P = 0.023). The status of CDKN2A SCNVs in SM samples was significantly associated with overall survival (P = 0.032). No cancer deaths were observed among the 11 patients with CDKN2A SCNamp.

CONCLUSION

CDKN2A SCNVs in SMs identified by P16-ddPCR are prevalent and significantly associated with GC metastasis and overall survival.

Olmesartan Prevents Oligomerized Amyloid β (Aβ)-Induced Cellular Senescence in Neuronal Cells

Alzheimer's disease (AD) is a neurodegenerative disease with high morbidity. The deposition of oligomerized amyloid β (Aβ) is the pathological feature of AD. The Aβ-caused neuronal oxidative stress and cellular senescence play an important role in the development and progression of AD. Olmesartan is a novel angiotensin receptor blocker with promising antihypertensive properties and has recently been reported to exert anti-inflammatory and antioxidative stress effects. Blood pressure control using Angiotensin receptor blockers has shown multiple benefits in Alzheimer's disease models. In the present study, the effect of Olmesartan on oligomerized amyloid β (Aβ)-induced cellular senescence was investigated in cultured M17 neuronal cells. Our results show that Olmesartan treatment significantly ameliorates oligomerized Aβ-elevated ROS and MDA levels, as well as the induced senescent cells number. At the molecular level, Olmesartan inhibits the elevated expression of senescence biomarkers (p16 and p21). Furthermore, Olmesartan potently reversed the increased K382 acetylation of p53 and the downregulation of SIRT1. Moreover, we show that the effect of Olmesartan against cell senescence and deacetylation of p53 was abolished by inhibition of SIRT1, either by using nicotinamide or by transfection with SIRT1 siRNA. In conclusion, Olmesartan prevents oligomerized Aβ-induced cellular senescence in neuronal cells by downregulating p16 and p21 through a SIRT1 dependent deacetylation of p53; our finding indicates that Olmesartan has a protective effect in Aβ-induced neurotoxicity.

Role of the Wnt pathway in the formation, development, and degeneration of intervertebral discs

Intervertebral disc degeneration (IVDD) is an age-related degenerative disease that is the main cause of low back pain. It seriously affects the quality of life of patients and places a heavy economic burden on families and society. The Wnt pathway plays an important role in the growth, development, and degeneration of intervertebral discs (IVDs). In the embryonic stage, the Wnt pathway participates in the growth and development of IVD by promoting the transformation of progenitor cells into notochord cells and the extension of the notochord. However, the activation of the Wnt pathway after birth promotes IVD cell senescence, apoptosis, and degradation of the extracellular matrix and induces the production of inflammatory factors, thereby accelerating the IVDD process. This article reviews the relationship between the Wnt pathway and IVD, emphasizing its influence on IVD growth, development, and degeneration. Targeting this pathway may become an effective strategy for the treatment of IVDD.

Berberine ameliorates cellular senescence and extends the lifespan of mice via regulating p16 and cyclin protein expression

Although aging and senescence have been extensively studied in the past few decades, however, there is lack of clinical treatment available for anti‐aging. This study presents the effects of berberine (BBR) on the aging process resulting in a promising extension of lifespan in model organisms. BBR extended the replicative lifespan, improved the morphology, and boosted rejuvenation markers of replicative senescence in human fetal lung diploid fibroblasts (2BS and WI38). BBR also rescued senescent cells with late population doubling (PD). Furthermore, the senescence‐associated β‐galactosidase (SA‐β‐gal)‐positive cell rates of late PD cells grown in the BBR‐containing medium were ~72% lower than those of control cells, and its morphology resembled that of young cells. Mechanistically, BBR improved cell growth and proliferation by promoting entry of cell cycles from the G₀ or G₁ phase to S/G₂‐M phase. Most importantly, BBR extended the lifespan of chemotherapy‐treated mice and naturally aged mice by ~52% and ~16.49%, respectively. The residual lifespan of the naturally aged mice was extended by 80%, from 85.5 days to 154 days. The oral administration of BBR in mice resulted in significantly improved health span, fur density, and behavioral activity. Therefore, BBR may be an ideal candidate for the development of an anti‐aging medicine.

5-Aza-2'-deoxycytidine protects against emphysema in mice via suppressing p16Ink4a expression in lung tissue

Background

There is a growing realization that COPD, or at least emphysema, involves several processes presenting in aging and cellular senescence. Endothelial progenitor cells (EPCs) contribute to neovascularization and play an important role in the development of COPD. The gene for p16^Ink4a is a major dominant senescence one. The aim of the present study was to observe changes in lung function, histomorphology of lung tissue, and expression of p16^Ink4a in lung tissue and bone marrow-derived EPCs in emphysematous mice induced by cigarette-smoke extract (CSE), and further to search for a potential candidate agent protecting against emphysema induced by CSE.

Materials and methods

An animal emphysema model was induced by intraperitoneal injection of CSE. 5-Aza-2′-deoxycytidine (5-Aza-CdR) was administered to the emphysematous mice. Lung function and histomorphology of lung tissue were measured. The p16^Ink4a protein and mRNA in EPCs and lung tissues were detected using Western blotting and quantitative reverse-transcription polymerase chain reaction, respectively.

Results

CSE induced emphysema with increased p16^Ink4a expression in lung tissue and bone marrow-derived EPCs. 5-Aza-CdR partly protected against emphysema, especially in the lung-morphology profile, and partly protest against the overexpression of p16^Ink4a in EPCs and lung tissue induced by CSE.

Conclusion

5-Aza-CdR partly protected against emphysema in mice via suppressing p16^Ink4a expression in EPCs and lung tissue.

Cigarette Smoke Extract Changes Expression of Endothelial Nitric Oxide Synthase (eNOS) and p16(INK4a) and is Related to Endothelial Progenitor Cell Dysfunction

BACKGROUND Endothelial dysfunction is an important pathophysiologic feature in many smoke-related diseases. Endothelial progenitor cells (EPCs) are the precursors of endothelial cells and play a fundamental role in the maintenance of endothelial integrity and function. Endothelial nitric oxide synthase (eNOS) is the dominant NOS isoform in the vasculature and plays a central role in the maintenance of endothelial homeostasis. p16(INK4a) is a cyclin-dependent kinase inhibitor and could be regarded as a major dominant senescence gene. The present study aimed to determine whether the expression of eNOS and p16(INK4a) in EPCs is related to EPCs function and the possible epigenetic mechanism, if any. MATERIAL AND METHODS We investigated EPCs capacity for proliferation, adhesion, and secretion, and the expression of eNOS and p16(INK4a) in EPCs which were altered by cigarette smoke extract (CSE) in vitro. Furthermore, Decitabine (Dec), an agent of demethylation, was used to examine whether it could alter the changes induced by CSE. RESULTS The present study demonstrated that EPCs altered by CSE in vitro displayed decreased capacities of proliferation, adhesion, and secretion, which was accompanied by decreased eNOS expression and increased p16(INK4a) expression in EPCs. Furthermore, Dec could alleviate the changes in the expression of eNOS and p16(INK4a), and protect against the EPCs dysfunction caused by CSE. CONCLUSIONS The decreased eNOS expression and increased p16(INK4a) expression was associated with dysfunction of EPCs caused by CSE. The mechanism of methylation, one of the most common epigenetic mechanism, may be involved in the EPCs dysfunction caused by CSE.

Radiosensitization by PARP Inhibition in DNA Repair Proficient and Deficient Tumor Cells: Proliferative Recovery in Senescent Cells

Radiotherapy continues to be a primary modality in the treatment of cancer. In addition to promoting apoptosis, radiation-induced DNA damage can promote autophagy and senescence, both of which can theoretically function to prolong tumor survival. In this work, we tested the hypothesis that autophagy and/or senescence could be permissive for DNA repair, thereby facilitating tumor cell recovery from radiation-induced growth arrest and/or cell death. In addition, studies were designed to elucidate the involvement of autophagy and senescence in radiosensitization by PARP inhibitors and the re-emergence of a proliferating tumor cell population. In the context of this work, the relationship between radiation-induced autophagy and senescence was also determined. Studies were performed using DNA repair-proficient HCT116 colon carcinoma cells and a repair-deficient ligase IV(-/-) isogenic cell line. Exposure to radiation promoted a parallel induction of autophagy and senescence that was strongly correlated with the extent of persistent H2AX phosphorylation in both cell lines, however, inhibition of autophagy failed to suppress senescence, indicating that the two responses were dissociable. Exposure to radiation resulted in a transient arrest in the HCT116 cells while arrest was prolonged in the ligase IV(-/-) cells, however, both cell lines ultimately recovered proliferative function, which may reflect maintenance of DNA repair capacity. The PARP inhibitors, olaparib and niraparib, increased the extent of persistent DNA damage induced by radiation exposure as well as the extent of both autophagy and senescence. Neither cell line underwent significant apoptosis by radiation exposure alone or in the presence of the PARP inhibitors. Inhibition of autophagy failed to attenuate radiosensitization, indicating that autophagy was not involved in the action of the PARP inhibitors. As with radiation alone, despite sensitization by PARP inhibition, proliferative recovery was evident within a period of 10-20 days. While inhibition of DNA repair via PARP inhibition may initially sensitize tumor cells to radiation via the promotion of senescence, this strategy does not appear to interfere with proliferative recovery, which could ultimately contribute to disease recurrence.

Docosahexaenoic acid prevented tumor necrosis factor alpha-induced endothelial dysfunction and senescence

We investigated how docosahexaenoic acid (DHA) regulated tumor necrosis factor-alpha (TNF-α)-induced senescence and dysfunction in endothelial cells (EC). We used RT-PCR to examine the expression of several genes related to senescence and dysfunction in EC. TNF-α-induced p21 protein levels were investigated by Western blot (WB) and fluorescence antibody techniques. TNF-α induced the senescence marker β-galactosidase and the expression of several senescence and endothelial dysfunction-related genes, e.g., CDKN1A, SHC1 and GLB1. DHA attenuated TNF-α-induced senescence-related gene expression and p21 protein expression. DHA attenuated TNF-α-induced gene expression related to dysfunction of EC, such as plasminogen activator inhibitor 1 (SERPINE1), lectin-like oxidized low-density lipoprotein receptor-1 (OLR1), thromboxane A2 receptor (TXA2R) and p38 MAPK (MAPK14). DHA reversed the TNF-α-mediated reduction of endothelial nitric oxide synthase (NOS3) gene expression. TNF-α-mediated upregulation of these genes was inhibited by allopurinol and apocynin. These results indicated that DHA regulated the expression of several genes that are associated with senescence and dysfunction of EC.

The effects of fucodian on senescence are controlled by the p16INK4a-pRb and p14Arf-p53 pathways in hepatocellular carcinoma and hepatic cell lines

Fucoidan is known to have various pharmacological effects, including antitumor activity. Although it has potential as a therapeutic agent for cancer cells, the anti-senescence effects and detailed mechanism of action remain poorly understood in normal hepatic cells. We investigated the anticancer functions of fucoidan using HepG2 cells as well as the mechanisms mediating the anti-senescent actions in Chang liver cells. Fucoidan effectively inhibited HepG2 cell viability and induced apoptosis. Also, fucoidan-induced G₁ phase arrest was caused by the activity of the p16(INK4a)-Rb and p14(Arf)-p53 pathways. Furthermore, upregulation of p16(INK4a) was critical to the antitumor activity of HepG2 cells treated with fucoidan and was correlated with inhibition of Cdk4 and pRb and upregulation of p21 expression. Our results suggest that fucoidan upregulates INK4a locus genes to induce apoptosis through p38 MAPK in HepG2 cells. Moreover, it prevents cellular senescence of Chang-L cells, by decreasing p14(Arf) expression as cells enter quiescence, with the reduction of p16(INK4a). Fucoidan treatment also downregulated the expression of α₂M. In conclusion, fucoidan can be considered a potential therapeutic agent against liver cancer that does not cause senescence in normal hepatic cells. Thus, it may be possible to use fucoidan therapeutically in both tumor suppression and aging.

SIRT3 overexpression antagonizes high glucose accelerated cellular senescence in human diploid fibroblasts via the SIRT3-FOXO1 signaling pathway

Sirtuin 3 (SIRT3) is one of the seven mammalian sirtuins, which are homologs of the yeast Sir2 gene. SIRT3 is the only sirtuin reported to be associated with human life span. Many recent studies have indicated that SIRT3 levels are elevated by exercise and caloric restriction, but whether SIRT3 influences cell senescence under stressed conditions in human diploid fibroblasts has not been established. Our data showed that expression of SIRT3 is elevated in human diploid fibroblasts under low glucose (3.3 mM glucose) growth conditions and decreased under high glucose (25 mM glucose) growth conditions. We have demonstrated that SIRT3 interacts with forkhead box protein O1 (FOXO1). High glucose levels also increased aging phenotypes and FOXO1 acetylation level. We have demonstrated that overexpression of SIRT3 under high glucose conditions reduces FOXO1 acetylation, suggesting that deacetylation of FOXO1 by SIRT3 elevates the expression of the FOXO1 target genes, catalase, and manganese superoxide dismutase (MnSOD) while decreasing senescence phenotypes. We studied the effects of SIRT3 protein knockdown by shRNA under low glucose conditions. The data showed that shRNA-SIRT3 accelerated senescence phenotypes and acetylation of FOXO1; the expression level of catalase and MnSOD decreased compared with the control group. As a consequence, SIRT3 antagonized cellular senescence with the characteristic features of delayed SA-β-gal staining, senescence-associated heterochromatin foci (SAHF) formation, and p16(INK4A) expression. These results demonstrate for the first time that SIRT3 overexpression antagonizes high glucose-induced cellular senescence in human diploid fibroblasts via the SIRT3-FOXO1 signaling pathway.

Bisdemethoxycurcumin Increases Sirt1 to Antagonize t-BHP-Induced Premature Senescence in WI38 Fibroblast Cells

Curcuminoids are well known for their capabilities to combat risk factors that are associated with ageing and cellular senescence. Recent reports have demonstrated that curcuminoids can extend the lifespan of model organisms. However, the underlying mechanisms by which these polyphenic compounds exert these beneficial effects remain unknown. In this study, t-BHP-induced premature senescence model in human fibroblasts was chosen to explore the protective effects of a curcuminoid, bisdemethoxycurcumin (BDMC), on cellular senescence. The results demonstrated that BDMC attenuated oxidative stress-induced senescence-like features which include the induction of an enlarged cellular appearance, higher frequency of senescence-associated β-galactosidase staining activity, appearance of senescence-associated heterochromatic foci in nuclei, decrease in proliferation capability, and alteration in related molecules such as p16 and retinoblastoma protein. Notably, we found that BDMC treatment activated Sirt1/AMPK signaling pathway. Moreover, downregulating Sirt1 by the pharmacological inhibitor nicotianamine or small interfering RNA blocked BDMC-mediated protection against t-BHP-mediated decrease in proliferation. These results suggested that BDMC prevented t-BHP-induced cellular senescence, and BDMC-induced Sirt1 may be a mechanism mediating its beneficial effects.

Up-regulation of survivin during immortalization of human myofibroblasts is linked to repression of tumor suppressor p16(INK4a) protein and confers resistance to oxidative stress

Survivin is an essential component of the chromosomal passenger complex and a member of the inhibitor of apoptosis family. It is expressed at high levels in a large variety of malignancies, where it has been implicated in drug resistance. It was also shown previously that survivin is up-regulated during telomerase-mediated immortalization, which occurs at a relatively early stage during carcinogenesis. This study shows that up-regulation of survivin during immortalization of human myofibroblasts is an indirect consequence of the repression of p16(INK4a). Survivin and p16(INK4a) were functionally linked by assays that showed that either the up-regulation of survivin or repression of p16(INK4a) rendered telomerase-transduced MRC-5 myofibroblasts resistant to oxidative stress. Conversely, siRNA-mediated down-regulation of survivin activated caspases and enhanced the sensitivity of immortal MRC-5 cells to oxidative stress. The E2F1 transcription factor, which is negatively regulated by the pRB/p16(INK4a) tumor suppressor pathway, was implicated in the up-regulation of survivin. Using the ChIP assay, it was shown that E2F1 directly interacted with the survivin gene (BIRC5) promoter in cells that spontaneously silenced p16(INK4a) during telomerase-mediated immortalization. E2F1 binding to the BIRC5 was also enhanced in telomerase-transduced cells subjected to shRNA-mediated repression of p16(INK4a). Together, these data show that repression of p16(INK4a) contributes to the up-regulation of survivin and thereby provides a survival advantage to cells exposed to oxidative stress during immortalization. The up-regulation of survivin during immortalization likely contributes to the vulnerability of immortal cells to transformation by oncogenes that alter intracellular redox state.

Peroxisome proliferator-activated receptor-γ cross-regulation of signaling events implicated in liver fibrogenesis

Peroxisome proliferator-activated receptor-γ (PPARγ) is a nuclear receptor with transcriptional activity controlling multiple physical and pathological processes. Recently, PPARγ has been implicated in the pathogenesis of liver fibrosis. Its depleted expression has strong associations with the activation and transdifferentiation of hepatic stellate cells, the central event in liver fibrogenesis. Studies over the past decade demonstrate that PPARγ cross-regulates a number of signaling pathways mediated by growth factors and adipokines, and cellular events including apoptosis and senescence. These signaling and cellular events and their molecular interactions with PPARγ system are profoundly involved in liver fibrogenesis. We critically summarize these mechanistic insights into the PPARγ regulation in liver fibrogenesis based on the updated findings in this area. We conclude with a discussion of the impacts of these discoveries on the interpretation of liver fibrogenesis and their potential therapeutic implications. PPARγ activation could be a promising strategy for antifibrotic therapy.

The acetylation of transcription factor HBP1 by p300/CBP enhances p16INK4A expression

HBP1 is a sequence-specific DNA-binding transcription factor with many important biological roles. It activates or represses the expression of some specific genes during cell growth and differentiation. Previous studies have exhibited that HBP1 binds to p16^INK4A promoter and activates p16^INK4A expression. We found that trichostatin A (TSA), an inhibitor of HDAC (histone deacetylase), induces p16^INK4A expression in an HBP1-dependent manner. This result was drawn from a transactivation experiment by measuring relative luciferase activities of p16^INK4A promoter with HBP1-binding site in comparison with that of the wild-type p16^INK4A promoter by transient cotransfection with HBP1 into HEK293T cells and 2BS cells. HBP1 acetylation after TSA treatment was confirmed by immunoprecipitation assay. Our data showed that HBP1 interacted with histone acetyltransferase p300 and CREB-binding protein (CBP) and also recruited p300/CBP to p16^INK4A promoter. HBP1 was acetylated by p300/CBP in two regions: repression domain (K297/305/307) and P domain (K171/419). Acetylation of Repression domain was not required for HBP1 transactivation on p16^INK4A. However, luciferase assay and western blotting results indicate that acetylation of P domain, especially K419 acetylation is essential for HBP1 transactivation on p16^INK4A. As assayed by SA-beta-gal staining, the acetylation of HBP1 at K419 enhanced HBP1-induced premature senescence in 2BS cells. In addition, HDAC4 repressed HBP1-induced premature senescence through permanently deacetylating HBP1. We conclude that our data suggest that HBP1 acetylation at K419 plays an important role in HBP1-induced p16^INK4A expression.

B-MYB delays cell aging by repressing p16 (INK4α) transcription

p16 ( INK4α ), an inhibitor of cyclin-dependent kinase 4 and 6, has been proposed to play an important role in cellular aging and in premature senescence. The expression of the p16 ( INK4α ) is primarily under transcriptional control. Our previous data showed that a negative regulation element lies in its promoter. In that element, a MYB-binding site (MBS) was uncovered by transcription analysis. Here, we report that MBS is a negative regulation element and B-MYB binds to this site in vivo. In human embryonic lung fibroblast cells, B-MYB downregulated p16 ( INK4α ) expression, whereas knocking down of B-MYB upregulated it. Evidence also showed that overexpression of B-MYB in cells could increase the number of utmost passage and decrease G1 block, whereas knocking down of B-MYB could impair their replicative ability. This study provides evidence of the capacity of B-MYB not only to regulate p16 ( INK4α ) expression but also the phenotypic consequence on cellular senescence.

High-affinity Na(+)-dependent dicarboxylate cotransporter promotes cellular senescence by inhibiting SIRT1

High-affinity Na⁺-dependent dicarboxylate cotransporter (NaDC3) can transport Krebs cycle intermediates into cells. Our previous study has shown that NaDC3 promotes cellular senescence, but its mechanism is not clear. It is known that when the concentration of intermediates in Krebs cycle is increased, NAD⁺/NADH ratio will be decreased. NAD⁺-dependent histone deacetylase sirtuin1 (SIRT1) prolongs mammalian cellular lifespan. Therefore, we propose that NaDC3 accelerates cellular aging by inhibiting SIRT1. After NaDC3 was overexpressed in two human embryo lung fibroblastic cell lines, WI38 and MRC-5, we found that the cells displayed aging-related phenotypes in advance. Meanwhile, the level of SIRT1 activity was down-regulated. In WI38/hNaDC3 cells treated with the activators of SIRT1, aging-related phenotypes induced by NaDC3 were obviously improved. The NAD⁺/NADH ratio in WI38/hNaDC3 cells was also decreased. Further study found that enhanced intracellular NAD⁺ level could attenuate the aging phenotypes induced by NaDC3. Thus, NaDC3 promotes cellular senescence probably by inhibiting NAD⁺-dependent SIRT1.

Stress-induced responses of human skin fibroblasts in vitro reflect human longevity

Unlike various model organisms, cellular responses to stress have not been related to human longevity. We investigated cellular responses to stress in skin fibroblasts that were isolated from young and very old subjects, and from offspring of nonagenarian siblings and their partners, representatives of the general population. Fibroblasts were exposed to rotenone and hyperglycemia and assessed for senescence-associated beta-galactosidase (SA-beta-gal) activity by flow cytometry. Apoptosis/cell death was measured with the Annexin-V/PI assay and cell-cycle analysis (Sub-G1 content) and growth potential was determined by the colony formation assay. Compared with fibroblasts from young subjects, baseline SA-beta-gal activity was higher in fibroblasts from old subjects (P = 0.004) as were stress-induced increases (rotenone: P < 0.001, hyperglycemia: P = 0.027). For measures of apoptosis/cell death, fibroblasts from old subjects showed higher baseline levels (Annexin V+/PI+ cells: P = 0.040, Sub-G1: P = 0.014) and lower stress-induced increases (Sub-G1: P = 0.018) than fibroblasts from young subjects. Numbers and total size of colonies under nonstressed conditions were higher for fibroblasts from young subjects (P = 0.017 and 0.006, respectively). Baseline levels of SA-beta-gal activity and apoptosis/cell death were not different between fibroblasts from offspring and partner. Stress-induced increases were lower for SA-beta-gal activity (rotenone: P = 0.064, hyperglycemia: P < 0.001) and higher for apoptosis/cell death (Annexin V+/PI- cells: P = 0.041, Annexin V+/PI+ cells: P = 0.008). Numbers and total size of colonies under nonstressed conditions were higher for fibroblasts from offspring (P = 0.001 and 0.024, respectively) whereas rotenone-induced decreases were lower (P = 0.008 and 0.004, respectively). These data provide strong support for the hypothesis that in vitro cellular responses to stress reflect the propensity for human longevity.

Senescence delay and repression of p16INK4a by Lsh via recruitment of histone deacetylases in human diploid fibroblasts

Lymphoid specific helicase (Lsh) belongs to the family of SNF2/helicases. Disruption of Lsh leads to developmental growth retardation and premature aging in mice. However, the specific effect of Lsh on human cellular senescence remains unknown. Herein, we report that Lsh overexpression delays cell senescence by silencing p16^INK4a in human fibroblasts. The patterns of p16^INK4a and Lsh expression during cell senescence present the inverse correlation. We also find that Lsh requires histone deacetylase (HDAC) activity to repress p16^INK4a and treatment with trichostatin A (TSA) is sufficient to block the repressor effect of Lsh. Moreover, overexpression of Lsh is correlated with deacetylation of histone H3 at the p16 promoter, and TSA treatment in Lsh-expressing cells reverses the acetylation status of histones. Additionally, we demonstrate an interaction between Lsh, histone deacetylase 1 (HDAC1) and HDAC2 in vivo. Furthermore, we demonstrate that Lsh interacts in vivo with the p16 promoter and recruits HDAC1. Our data suggest that Lsh represses endogenous p16^INK4a expression by recruiting HDAC to establish a repressive chromatin structure at the p16^INK4a promoter, which in turn delays cell senescence.

CSIG inhibits PTEN translation in replicative senescence

Using a suppressive subtractive hybridization system, we identified CSIG (cellular senescence-inhibited gene protein; RSL1D1) that was abundant in young human diploid fibroblast cells but declined upon replicative senescence. Overexpression or knockdown of CSIG did not influence p21(Cip1) and p16(INK4a) expressions. Instead, CSIG negatively regulated PTEN and p27(Kip1) expressions, in turn promoting cell proliferation. In PTEN-silenced HEK 293 cells and PTEN-deficient human glioblastoma U87MG cells, the effect of CSIG on p27(Kip1) expression and cell division was abolished, suggesting that PTEN was required for the role of CSIG on p27(Kip1) regulation and cell cycle progression. Investigation into the underlying mechanism revealed that the regulation of PTEN by CSIG was achieved through a translational suppression mechanism. Further study showed that CSIG interacted with PTEN mRNA in the 5' untranslated region (UTR) and that knockdown of CSIG led to increased luciferase activity of a PTEN 5' UTR-luciferase reporter. Moreover, overexpression of CSIG significantly delayed the progression of replicative senescence, while knockdown of CSIG expression accelerated replicative senescence. Knockdown of PTEN diminished the effect of CSIG on cellular senescence. Our findings indicate that CSIG acts as a novel regulatory component of replicative senescence, which requires PTEN as a mediator and involves in a translational regulatory mechanism.

Accelerated senescence: an emerging role in tumor cell response to chemotherapy and radiation

Treatment of malignancies with chemotherapeutic drugs and/or radiotherapy is designed to eliminate the disease by depriving the tumor cell of its reproductive potential. Frequently, the desired effect of cell killing is achieved through the promotion of apoptosis; however, accumulating evidence suggests that apoptosis may not be the exclusive or even primary mechanism whereby tumor cells lose their self-renewal capacity after radiation or drug treatment, particularly in the case of solid tumors. While failure to undergo apoptosis in response to chemotherapeutic drugs or radiation may represent a mechanism of drug and radiation resistance, particularly in the case of leukemias and lymphomas, it is gradually being recognized that in the case of solid tumors, loss of reproductive capacity can occur through alternative pathways including reproductive cell death or mitotic catastrophe, through autophagic cell death, and as described below, through a terminally arrested state similar to replicative senescence. Studies building upon the phenomenon of replicative senescence in normal cells approaching the limit of their reproductive potential have identified a comparable senescence-like arrest as a component of the tumor cell response to chemotherapeutic drugs and radiation. This response, which has been termed "premature senescence", "senescence-like growth arrest", "stress-induced premature senescence", and "accelerated senescence", can also result from supraphysiological mitogenic signaling, sub-optimal culture conditions, and ectopic expression of oncogenes. Here, we will use the term "accelerated senescence" in our consideration of the morphological, biochemical, and molecular aspects of treatment-induced senescence, its relationship to classical replicative senescence, its prevalence in clinical specimens and the implications of accelerated senescence for the outcome of cancer therapy.

PPAR{gamma} accelerates cellular senescence by inducing p16INK4{alpha} expression in human diploid fibroblasts

Peroxisome proliferator-activated receptor gamma (PPARgamma) plays an important role in the inhibition of cell growth by promoting cell-cycle arrest, and PPARgamma activation induces the expression of p16(INK4alpha) (CDKN2A), an important cell-cycle inhibitor that can induce senescence. However, the role of PPARgamma in cellular senescence is unknown. Here, we show that PPARgamma promotes cellular senescence by inducing p16(INK4alpha) expression. We found several indications that PPARgamma accelerates cellular senescence, including enhanced senescence-associated (SA)-beta-galactosidase staining, increased G1 arrest and delayed cell growth in human fibroblasts. Western blotting studies demonstrated that PPARgamma activation can upregulate the expression of p16(INK4alpha). PPARgamma can bind to the p16 promoter and induce its transcription, and, after treatment with a selective PPARgamma agonist, we observed more-robust expression of p16(INK4alpha) in senescent cells than in young cells. In addition, our data indicate that phosphorylation of PPARgamma decreased with increased cell passage. Our results provide a possible molecular mechanism underlying the regulation of cellular senescence.

Ginsenoside Rg1 delays tert-butyl hydroperoxide-induced premature senescence in human WI-38 diploid fibroblast cells

Tert-butyl hydroperoxide (t-BHP), an analog of hydroperoxide, induced characteristic changes of senescence in human diploid fibroblasts WI-38 cells. It was reported that ginsenoside Rg1, an active ingredient of ginseng, ameliorated learning deficits in aged rats. The present study was aimed to investigate whether ginsenoside Rg1 can delay the premature senescence of WI-38 cells induced by t-BHP and to explore the underlying molecular mechanisms. First, Rg1 pretreatment markedly reversed senescent morphological changes in WI-38 cells induced by t-BHP. Second, t-BHP treatment alone resulted in an increase in the protein levels of P16 and P21, and a decline in intracellular adenosine 5'-triphosphate (ATP) level and mitochondrial complex IV activity. Ginsenoside Rg1 pretreatment had significant effects of attenuating these changes. These data indicate that ginsenoside Rg1 has an anti-aging effect on t-BHP-induced premature senescence in WI-38 cells. This effect may be mediated by regulating cell cycle proteins and enhancing mitochondrial functioning.

SIRT1 overexpression antagonizes cellular senescence with activated ERK/S6k1 signaling in human diploid fibroblasts

Sir2, a NAD-dependent deacetylase, modulates lifespan in yeasts, worms and flies. The SIRT1, mammalian homologue of Sir2, regulates signaling for favoring survival in stress. But whether SIRT1 has the function to influence cell viability and senescence under non-stressed conditions in human diploid fibroblasts is far from unknown. Our data showed that enforced SIRT1 expression promoted cell proliferation and antagonized cellular senescence with the characteristic features of delayed Senescence-Associated beta-galactosidase (SA-beta-gal) staining, reduced Senescence-Associated Heterochromatic Foci (SAHF) formation and G1 phase arrest, increased cell growth rate and extended cellular lifespan in human fibroblasts, while dominant-negative SIRT1 allele (H363Y) did not significantly affect cell growth and senescence but displayed a bit decreased lifespan. Western blot results showed that SIRT1 reduced the expression of p16(INK4A) and promoted phosphorylation of Rb. Our data also exposed that overexpression of SIRT1 was accompanied by enhanced activation of ERK and S6K1 signaling. These effects were mimicked in both WI38 cells and 2BS cells by concentration-dependent resveratrol, a SIRT1 activator. It was noted that treatment of SIRT1-.transfected cells with Rapamycin, a mTOR inhibitor, reduced the phosphorylation of S6K1 and the expression of Id1, implying that SIRT1-induced phosphorylation of S6K1 may be partly for the decreased expression of p16(INK4A) and promoted phosphorylation of Rb in 2BS. It was also observed that the expression of SIRT1 and phosphorylation of ERK and S6K1 was declined in senescent 2BS. These findings suggested that SIRT1-promoted cell proliferation and antagonized cellular senescence in human diploid fibroblasts may be, in part, via the activation of ERK/ S6K1 signaling.

Stress chaperones, mortalin, and pex19p mediate 5-aza-2' deoxycytidine-induced senescence of cancer cells by DNA methylation-independent pathway

DNA demethylating agents are used to reverse epigenetic silencing of tumor suppressors in cancer therapeutics. Understanding of the molecular and cellular factors involved in DNA demethylation-induced gene desilencing and senescence is still limited. We have tested the involvement of two stress chaperones, Pex19p and mortalin, in 5-Aza-2' deoxycytidine (5AZA-dC; DNA demethylating agent)-induced senescence. We found that the cells overexpressing these chaperones were highly sensitive to 5AZA-dC, and their partial silencing eliminated 5AZA-dC-induced senescence in human osteosarcoma cells. We demonstrate that these chaperones modulate the demethylation and chromatin remodeling-dependent (as accessed by p16(INK4A) expression) and remodeling-independent (such as activation of tumor suppressor p53 pathway) senescence response of cells. Furthermore, we found the direct interactions of 5AZA-dC with these chaperones that may alter their functions. We conclude that both mortalin and Pex19p are important mediators, prognostic indicators, and tailoring tools for 5AZA-dC-induced senescence in cancer cells.

Effect of cell cycle inhibitor p19ARF on senescence of human diploid cell

To investigate the effect of cell cycle inhibitor p14ARF on replicative senescence of human diploid cell, recombinant p19ARF eukaryotic expression vector was constructed and p19ARF gene was transfected into human diploid fibroblasts (WI-38 cells) by liposome-mediated transfection for overexpression. Then, the effects of p19ARF on replicative senescence of WI-38 cells were observed. The results revealed that, compared with control cells, the WI-38 cells in which p19ARF gene was introduced showed significant up-regulation of p53 and p21 expression level, decrease of cell generation by 10-12 generations, decline of cell growth rate with cell cycle being arrested at G1 phase, increase of positive rate of senescent marker SA-beta-gal staining, and decrease of mitochondrial membrane potential. The morphology of the transfected fibroblasts presented the characteristics changes similar to senescent cells. These results indicated that high expression of p19ARF may promote the senescent process of human diploid cells.

Sp1 is essential for p16 expression in human diploid fibroblasts during senescence

BACKGROUND

p16(INK4a) tumor suppressor protein has been widely proposed to mediate entrance of the cells into the senescent stage. Promoter of p16(INK4a) gene contains at least five putative GC boxes, named GC-I to V, respectively. Our previous data showed that a potential Sp1 binding site, within the promoter region from -466 to -451, acts as a positive transcription regulatory element. These results led us to examine how Sp1 and/or Sp3 act on these GC boxes during aging in cultured human diploid fibroblasts.

METHODOLOGY/PRINCIPAL FINDINGS

Mutagenesis studies revealed that GC-I, II and IV, especially GC-II, are essential for p16(INK4a) gene expression in senescent cells. Electrophoretic mobility shift assays (EMSA) and ChIP assays demonstrated that both Sp1 and Sp3 bind to these elements and the binding activity is enhanced in senescent cells. Ectopic overexpression of Sp1, but not Sp3, induced the transcription of p16(INK4a). Both Sp1 RNAi and Mithramycin, a DNA intercalating agent that interferes with Sp1 and Sp3 binding activities, reduced p16(INK4a) gene expression. In addition, the enhanced binding of Sp1 to p16(INK4a) promoter during cellular senescence appeared to be the result of increased Sp1 binding affinity, not an alteration in Sp1 protein level.

CONCLUSIONS/SIGNIFICANCE

All these results suggest that GC- II is the key site for Sp1 binding and increase of Sp1 binding activity rather than protein levels contributes to the induction of p16(INK4a) expression during cell aging.

p21Waf1/Cip1 plays a critical role in modulating senescence through changes of DNA methylation

It has been reported that genomic DNA methylation decreases gradually during cell culture and an organism's aging. However, less is known about the methylation changes of age-related specific genes in aging. p21(Waf1/Cip1) and p16(INK4a) are cyclin-dependent kinase (Cdk) inhibitors that are critical for the replicative senescence of normal cells. In this study, we show that p21(Waf1/Cip1) and p16(INK4a) have different methylation patterns during the aging process of normal human 2BS and WI-38 fibroblasts. p21(Waf1/Cip1) promoter is gradually methylated up into middle-aged fibroblasts but not with senescent fibroblasts, whereas p16(INK4a) is always unmethylated in the aging process. Correspondently, the protein levels of DNA methyltransferase 1 (DNMT1) and DNMT3a increase from young to middle-aged fibroblasts but decrease in the senescent fibroblasts, while DNMT3b decreases stably from young to senescent fibroblasts. p21(Waf1/Cip1) promoter methylation directly represses its expression and blocks the radiation-induced DNA damage-signaling pathway by p53 in middle-aged fibroblasts. More importantly, demethylation by 5-aza-CdR or DNMT1 RNA interference (RNAi) resulted in an increased p21(Waf1/Cip1) level and premature senescence of middle-aged fibroblasts demonstrated by cell growth arrest and high beta-Galactosidase expression. Our results suggest that p21(Waf1/Cip1) but not p16(INK4a) is involved in the DNA methylation mediated aging process. p21(Waf1/Cip1) promoter methylation may be a critical biological barrier to postpone the aging process.

Inactivation of p16INK4a (inhibitor of cyclin-dependent kinase 4A) immortalizes primary human keratinocytes by maintaining cells in the stem cell compartment

Replicative senescence of human keratinocytes is determined by a progressive decline of clonogenic and dividing cells, and its timing is controlled by clonal evolution (i.e., the transition from stem cells to transient amplifying and postmitotic cells). Progressive increase of p16INK4a (inhibitor of cyclin-dependent kinase 4A) expression has been shown to correlate with keratinocyte clonal evolution. Thus, the aim of our study is to understand whether p16INK4a accumulation is a triggering mechanism of epidermal clonal evolution or a secondary event. We show that inactivation of p16INK4a, by an antisense strategy, allows primary human keratinocytes to escape replicative senescence. Specifically, p16INK4a inactivation alone blocks clonal evolution and maintains keratinocytes in the stem cell compartment. Antisense excision is followed by keratinocyte senescence, confirming that persistent p16INK4a inactivation is required for maintenance of clonal evolution block. Immortalization is accompanied by resumption of B-Cell Specific Moloney murine leukemia virus site 1 (Bmi-1) expression and telomerase activity, hallmarks of tissue regenerative capacity. In turn, Bmi-1 expression is necessary to maintain the impairment of clonal evolution induced by p16INK4a inactivation. Finally, p16INK4a down-regulation in transient amplifying keratinocytes does not affect clonal evolution, and cells undergo senescence. Thus, p16INK4a inactivation appears to selectively prevent clonal conversion in cells endowed with a high proliferative potential. These data indicate that p16INK4a regulates keratinocyte clonal evolution and that inactivation of p16INK4a in epidermal stem cells is necessary for maintaining stemness.

Characterization of the activities of p21Cip1/Waf1 promoter-driven reporter systems during camptothecin-induced senescence-like state of BHK-21 cells

It was attempted in this work to establish a cell line in which senescent cells can be readily and directly identified in situ in live culture. Transcriptional activation of p21(Cip1/Waf1) gene is known to be one of the key steps in the development of cellular senescence, whereas the elements within the p21(Cip1/Waf1) promoter that regulate the transcriptional activation of p21(Cip1/Waf1) during cellular senescence have not been clearly defined. Thus, several reporter plasmids were constructed in each of which the gene of green fluorescent protein was placed under the control of a selected fragment of p21(Cip1/Waf1) promoter, and stably transfected into BHK-21 cells. The transfected cells were induced to become senescence-like by camptothecin and assayed for fluorescence intensity. It was shown that the reporter system constructed with bases -2504 to +406 of the p21(Cip1/Waf1) promoter was very efficient in reflecting the senescence of BHK-21 cells by increased cytosolic fluorescence, and the fluorescence intensity of senescent cells was easily distinguished from that of quiescent cells.

Irreversible cellular senescence induced by prolonged exposure to H2O2 involves DNA-damage-and-repair genes and telomere shortening

H2O2 has been the most commonly used inducer for stress-induced premature senescence (SIPS), which shares features of replicative senescence. However, there is still uncertainty whether SIPS and replicative senescence differ or utilize different pathways. 'Young' human diploid fibroblasts (HDFs), treated with prolonged low doses of hydrogen peroxide, led to irreversible cellular senescence. Cells exhibited senescent-morphological features, irreversible G1 cell cycle arrest and irreversible senescence-associated beta-galactosidase positivity. The appearance of these cellular senescence markers was accompanied by significant increases of p21, gadd45 expression and p53 binding activity, as well as a significant decline in DNA repair capability and accelerated telomere shortening. Our results suggest that multiple pathways might be involved in oxidative SIPS, including genes related to DNA-damage-and-repair and telomere shortening, and that SIPS shares the same mechanisms with replicative senescence in vivo. Our findings indicate that several aging theories can be merged together by a common mechanism of oxidative damage, and that the level of oxidative DNA-damage-and-repair capacity may be exploited as reliable markers of cell senescence.

Effects of Human Na+/Dicarboxylate Cotransporter 3 on the Replicative Senescence of Human Embryonic Lung Diploid Fibroblasts

To investigate the role of human Na(+)/dicarboxylate cotransporter 3 (hNaDC3) in the replicative senescence of normal human embryonic lung diploid fibroblasts (WI-38), a retroviral vector containing hNaDC3 was constructed. hNaDC3 was introduced into normal WI-38 cells through infection with the retroviral virus. Monoclones were selected with G418. The integration and expression of exotic genes were confirmed by Northern blot and Western blot. When compared with the control cells, WI-38 cells transfected with hNaDC3 cDNA showed significant suppression of growth rate (by 40%), increase of positive rate of SA-beta-gal staining, decrease of mitochondrial membrane potential, shortening of telomere length, and increase of P16 and P21 expression. The morphology characteristics of senescent fibroblasts appeared earlier. Our results have, for the first time, demonstrated that high expression of hNaDC3 may be able to, at least partly, promote the cellular senescence of human diploid fibroblasts.

Expression of the Leo1‐like domain of replicative senescence down‐regulated Leo1‐like (RDL) protein promotes senescence of 2BS fibroblasts

Replicative senescence is thought to relate to aging in vivo and tumor suppression. In this report, we isolated a gene and designated it as RDL (replicative senescence down-regulated Leo1-like gene). RDL's expression decreased upon replicative senescence of human diploid 2BS fibroblasts. Overexpression of RDL slightly delayed 2BS fibroblast senescence, whereas suppression of RDL expression imposed no obvious effects on senescence. However, introduction of cDNA fragment encoding the Leo1-like domain of RDLp (Leo) alone shortened the replicative life span of 2BS fibroblasts and promoted several senescent features; the introduction of truncated RDL cDNA fragment resulting from deletion of Leo (RDL-Leo-) significantly prolonged 2BS life span and caused a noticeable delay of these senescent features. We demonstrated that introduction of Leo obviously increased the expression of p16INK4a, p21WAF1, and PTEN, whereas introduction of RDL-Leo- distinctly decreased p16INK4a expression. Taken together, our results suggest that the Leo1-like domain of RDLp is a senescence-associated domain that accelerates the senescence of 2BS fibroblasts and that there should be another counteractive domain in the remaining part of RDLp.

Down-regulation of p21WAF1 promotes apoptosis in senescent human fibroblasts: involvement of retinoblastoma protein phosphorylation and delay of cellular aging

It has been suggested that genes which exercise checkpoint control during cell cycle traverse are equally important to the process of apoptotic cell death. In this study, we show that the key cell cycle regulatory gene p21(WAF1) is also involved in the execution of apoptosis. p21(WAF1) expression was down-regulated during NaBu-induced apoptosis of senescent normal diploid human 2BS fibroblasts. Conversely, when p21(WAF1) expression was actively suppressed in 2BS cells by a stably transfected antisense p21(WAF1) construct, apoptosis was accelerated and senescence was delayed, as shown by several markers of cell aging. Down-regulation of p21(WAF1) by antisense caused an increase in the phosphorylation and inactivation of pRb. Phosphorylation of pRb was further enhanced upon induction of apoptosis by NaBu. Our results suggest that p21(WAF1), acting through the phosphorylation of pRb, regulates whether 2BS cells cease to proliferate and become senescent but resistant to apoptosis, or whether they accelerate proliferation while becoming more susceptible to apoptotic stimuli.

Cloning and characterization of cellular senescence-associated genes in human fibroblasts by suppression subtractive hybridization

Cellular senescence marks the end of the proliferative life span of normal cells in tissue culture and occurs after cells have undergone a certain number of population doublings (PDLs). It is accompanied by alterations in the pattern of gene expression. A specific human embryonic lung diploid fibroblast cell line, 2BS, has been studied as a model of senescence in our laboratory. Here, we report a set of cellular senescence-associated genes identified from suppression subtractive cDNA libraries from senescent and young 2BS cells. They include three novel genes and six previously identified genes of unknown function. The genes whose functions are known belong to various functional pathways that have been reported to change with the onset of senescence. These include three pre-mRNA splicing factors with reduced expression in senescent cells, indicating that the regulation of mRNA splicing is altered during cell senescence. In addition, the expression of the gene TOM1 (target of Myb 1), which has not previously been associated with cellular senescence, is shown to increase in senescent cells, and we demonstrate that the expression of antisense TOM1 gene in 2BS cells can delay the progress of senescence.

Contribution of p16INK4a to replicative senescence of human fibroblasts

In standard conditions of tissue culture, human fibroblasts undergo a limited number of population doublings before entering a state of irreversible growth arrest termed replicative senescence or M1. The arrest is triggered by a combination of telomere dysfunction and the stresses inflicted by culture conditions and is implemented, at least in part, by the cyclin-dependent kinase inhibitors p21(CIP1) and p16(INK4a). To investigate the role of p16(INK4a), we have studied fibroblasts from members of melanoma prone kindreds with mutations in one or both copies of the CDKN2A locus. The mutations affect the function of p16(INK4a) but not of the alternative product, p14(ARF). The p16(INK4a)-defective fibroblasts have an above average life span, compared to the heterozygous and normal age-matched controls, but they arrest with characteristics typical of senescence. Using agents that are known to bypass M1, such as DNA tumor virus oncoproteins or the Bmi1 transcriptional repressor, we provide evidence that p16(INK4a) defective cells arrest at a stage that is operationally between M1 and M2 (crisis). As well as indicating that p16(INK4a) contributes to but is not essential for replicative senescence of human fibroblasts, our data reveal considerable heterogeneity in the levels and accumulation of p16(INK4a) in different strains.

mRNA level of alpha-2-macroglobulin as an aging biomarker of human fibroblasts in culture

Cellular senescence is a well-established model system for studying the molecular basis of aging. To identify a reliable biomarker for cellular age and further study the gene expression of aging, we profiled the gene expression difference between aged and young cultured human embryonic lung fibroblasts by high-density complementary deoxyribonucleic acid (cDNA) arrays. Among the differentially expressed genes, alpha-2-macroglobulin (alpha(2)M) was selected for further study. Its gene expression level as a function of population doubling level (PDL) in cultured fibroblasts was determined by RT-PCR and northern hybridization. mRNA level of alpha(2)M showed a positive linear-correlation with cumulative PDL. Additional assays revealed that the levels of alpha(2)M increased in irreversible growth arrest induced by sublethal H(2)O(2), but not in quiescent state of cultured fibroblasts induced by serum-deprivation, and remained stable in Hela cells. These results suggest that mRNA level of alpha(2)M can be used as a biomarker of aging in cultured fibroblasts. mRNA level of alpha(2)M showed significant difference between newborn and old human leucocytes, which suggest that the mRNA level of alpha(2)M may be used as a biomarker of aging in vivo.

Mitogen stimulation cooperates with telomere shortening to activate DNA damage responses and senescence signaling

Replicative senescence is induced by critical telomere shortening and limits the proliferation of primary cells to a finite number of divisions. To characterize the activity status of the replicative senescence program in the context of cell cycle activity, we analyzed the senescence phenotypes and signaling pathways in quiescent and growth-stimulated primary human fibroblasts in vitro and liver cells in vivo. This study shows that replicative senescence signaling operates at a low level in cells with shortened telomeres but becomes fully activated when cells are stimulated to enter the cell cycle. This study also shows that the dysfunctional telomeres and nontelomeric DNA lesions in senescent cells do not elicit a DNA damage signal unless the cells are induced to enter the cell cycle by mitogen stimulation. The amplification of senescence signaling and DNA damage responses by mitogen stimulation in cells with shortened telomeres is mediated in part through the MEK/mitogen-activated protein kinase pathway. These findings have implications for the further understanding of replicative senescence and analysis of its role in vivo.

Regulation of cellular senescence and p16(INK4a) expression by Id1 and E47 proteins in human diploid fibroblast

Id1, a member of Id family of helix-loop-helix transcriptional regulatory proteins, is implicated in cellular senescence by repressing p16(INK4a) expression, but the mechanisms and cellular effects in human diploid fibroblasts remain unknown. Here we analyzed the patterns of p16(INK4a) and Id1 expression during the lifespan of 2BS cells and presented the inverse correlation between these two proteins. Immunoprecipitation assays demonstrated the presence of endogenous interaction of Id1 and E47 proteins that was strong in young 2BS cells and weakened during replicative senescence and, thereby, influenced the transcription activation of p16(INK4a) by E47. Furthermore, we found that E47 protein could bind to the E-box-containing region in p16(INK4a) promoter in senescent cells by chromatin immunoprecipitation analyses, suggesting that E47 is indeed ultimately involved in the regulation of p16(INK4a) transcription in vivo. Silencing Id1 expression in young cells by RNA interference induced an increased p16(INK4a) level and premature cellular senescence, whereas silencing E47 expression inhibited the expression of p16(INK4a) and delayed the onset of senescent phenotype. The present study demonstrated not only the capacity of Id1 to regulate p16(INK4a) gene expression by E47, but also the phenotypic consequence of the regulation on cellular senescence, moreover, raised the possibility of Id1-specific gene silencing for human cancer therapy.

HDTIC-1 and HDTIC-2, two compounds extracted from Astragali Radix, delay replicative senescence of human diploid fibroblasts

Astragalus membranceus (Fish) Bunge Var. mongholicus (Bge) Hsiao is a Chinese herb considered as an effective traditional anti-ageing material. The two isomers of 4-hydroxy-5-hydroxymethyl-[1,3]dioxolan-2,6'-spirane-5',6',7',8'-tetrahydro-indolizine-3'-carbaldehyde (HDTIC), HDTIC-1 and HDTIC-2, were extracted from the herb. We chose them to investigate their effects on replicative senescence in vitro. In this study, we observed the effects of HDTIC-1 and HDTIC-2 on morphology, replicative lifespan, and specific markers related to replicative senescence in human fetal lung diploid fibroblast (2BS cell). Results have shown that both the HDTIC-1 and HDTIC-2 maintain non-senescent phenotype of 2BS cells even at late population doubling (PD) and increase cumulative population doublings (CPDs) by at least 15-20PDs. The senescence-associated-galactosidase (SA-beta-gal) positive cell rates of late PD cells grown from early PD in medium containing HDTIC, were much lower than that of late PD control cells, and similar to that of young cells. HDTIC also improved cell growth and proliferation and promoted the entry of 2BS cells from G0 or G1 phase to S-phase. In addition, the advanced glycation end product (AGE) levels of late PD cells grown from early PD in DMEM containing HDTIC decreased significantly compared with those of late PD control cells. Taken together, the results strongly suggest that both the HDTIC-1 and HDTIC-2 delay replicative senescence of 2BS cells, and indicate that the senescence-delaying effect of HDTIC appears to be due to its many biological properties including its potentials of proliferation improvement, inhibitory effect of AGE formation, and its antioxidant activity. The differences of optimum concentrations of HDTIC-1 (0.1 microM) and HDTIC-2 (1.0 microM) for delaying senescence also indicate that the structure of HDTIC may be very sensitive to its activity.

Acquired cellular resistance to flavopiridol in a human colon carcinoma cell line involves up-regulation of the telomerase catalytic subunit and telomere elongation. Sensitivity of resistant cells to combination treatment with a telomerase inhibitor

Flavopiridol is a broad-spectrum inhibitor of cyclin-dependent kinases and of global transcription via the inhibition of positive transcription elongation factor b (P-TEFb). Although flavopiridol is currently undergoing phase II clinical trials, acquired cellular resistance to the compound during treatment is a potential problem, as it is with almost all current anticancer agents. A HCT116 human colon carcinoma cell line with an acquired 8-fold resistance to flavopiridol has been established. We report here that there are changes in these resistant cells in terms of telomere length and telomerase activity, whereas no change in the expression of the P-TEFb subunits CDK9, cyclin T1, cyclin T2a, or cyclin T2b was observed. The level of mRNA expression for the telomerase catalytic subunit hTERT was increased over 2-fold in the resistant cells, and mean telomere length was found to be 2 kb longer than the parental length, although telomerase activity was unchanged. The level of mRNA expression for the telomeric binding protein Pot1 was also increased. We also report that treatment of HCT116 cells with a combination of the G-quadruplex interacting telomerase inhibitor BRACO-19 and flavopiridol results in a 3-fold decrease in population doubling and prevents recovery from treatment with either compound alone. Treatment of flavopiridol-resistant cells with BRACO-19 alone also led to rapid inhibition of cell growth, which is not observed in the parental line. The finding that only the resistant line, with up-regulated telomerase, responds to this G-quadruplex inhibitor is consistent with the hypothesis that the mechanism of BRACO-19 down-regulation of cell growth directly involves the targeting of telomeres and telomerase.

Immortalization of bovine capillary endothelial cells by hTERT alone involves inactivation of endogenous p16INK4A/pRb

Recent studies show that stable expression of the human telomerase catalytic subunit, hTERT, alone can lead several types of normal human somatic cells to bypass replicative senescence and become immortal. The molecular mechanisms by which telomerase immortalizes cells are not fully understood, although a key function of telomerase is to maintain a critical length of telomeres in order to preserve the stability and integrity of the genome. Here we report that stable transfection of hTERT alone was sufficient to allow bovine capillary endothelial (BCE) cells to bypass senescence and acquire immortality. Surprisingly, telomere lengths in these stable transfectants were progressively shortened during an increasing number of population doublings (PDLs), despite high telomerase activity. The expression of the cyclin-dependent kinase inhibitors (CDKIs) p16INK4A and p21CIP1/WAF1 was concomitantly repressed, and the retinoblastoma protein (pRb) was maintained in a hyperphosphorylated state in the telomerase-expressing cells. Re-expression of p16INK4A in these cells by either treatment with a demethylating agent or by adenovirus-mediated expression reinduced a senescence-like phenotype, suggesting that the inactivation of p16INK4A was due to DNA methylation and was crucial for the immortalization process. In agreement with this finding, the expression levels of the prototypic DNA methyltransferase DNMT1 were elevated in the hTERT-positive cells.

Replicative senescence revisited

Forty years after its discovery, replicative senescence remains a rich source of information about cell-cycle regulation and the progression from a normal to a transformed phenotype. Effectors of this growth-arrested state are being discovered at a great pace. This review discusses the latest findings on the players responsible for establishing replicative senescence, as well as the associated telomere shortening.