Overexpression of the p21 sdi1 gene induces senescence-like state in human cancer cells: implication for senescence-directed molecular therapy for cancer

Repair-independent functions of DNA-PKcs protect irradiated cells from mitotic slippage and accelerated senescence

The binding of DNA-dependent protein kinase catalytic subunit (DNA-PKcs, also known as PRKDC) to Ku proteins at DNA double-strand breaks (DSBs) has long been considered essential for non-homologous end joining (NHEJ) repair, providing a rationale for use of DNA-PKcs inhibitors as cancer therapeutics. Given lagging clinical translation, we reexamined mechanisms and observed instead that DSB repair can proceed independently of DNA-PKcs. While repair of radiation-induced DSBs was blocked in cells expressing shRNAs targeting Ku proteins or other NHEJ core factors, DSBs were repaired on schedule despite targeting DNA-PKcs. Although we failed to observe a DSB repair defect, the γH2AX foci that formed at sites of DNA damage persisted indefinitely after irradiation, leading to cytokinesis failure and accumulation of binucleated cells. Following this mitotic slippage, cells with decreased DNA-PKcs underwent accelerated cellular senescence. We identified downregulation of ataxia-telangiectasia mutated kinase (ATM) as the critical role of DNA-PKcs in recovery from DNA damage, insofar as targeting ATM restored γH2AX foci resolution and cytokinesis. Considering the lack of direct impact on DSB repair and emerging links between senescence and resistance to cancer therapy, these results suggest reassessing DNA-PKcs as a target for cancer treatment.

Selective killing of human breast cancer cells by the styryl lactone (R)-goniothalamin is mediated by glutathione conjugation, induction of oxidative stress and marked reactivation of the R175H mutant p53 protein

The molecular basis of anticancer and apoptotic effects of R-goniothalamin (GON), a plant secondary metabolite was studied. We show that induction of oxidative stress and reactivation of mutant p53 underlie the strong cytotoxic effects of GON against the breast cancer cells. While GON was not toxic to the MCF10a breast epithelial cells, the SKBR3 breast cancer cells harboring an R175H mutant p53 were highly sensitive (IC50 = 7.3 µM). Flow cytometry and other pertinent assays showed that GON-induced abundant reactive oxygen species (ROS), glutathione depletion, protein glutathionylation and activation of apoptotic markers. GON was found to conjugate with glutathione both in vitro and in cells and the product was characterized by mass spectrometry. We hypothesized that the redox imbalance induced by GON may affect the structure of the R175H mutant p53 protein, and account for greater cytotoxicity. Using the SKBR3 breast cancer and p53-null H1299 lung cancer cells stably expressing the R175H p53 mutant protein, we demonstrated that GON triggers the appearance of a wild-type-like p53 protein by using conformation-specific antibodies, immunoprecipitation, DNA-binding assays and target gene expression. p53 restoration was associated with a G2/M arrest, senescence, reduced cell migration, invasion and increased cell death. GON elicited a highly synergistic cytotoxicity with cisplatin in SKBR3 cells. In SKBR3 xenografts developed in nude mice, there was a marked tumor growth delay by GON alone and GON + cisplatin combination. Our studies highlight the impact of tumor redox-stress generated by GON in activating the mutant p53 protein for greater antitumor efficacy.

Specific up-regulation of p21 by a small active RNA sequence suppresses human colorectal cancer growth

The double stranded small active RNA (saRNA)- p21-saRNA-322 inhibits tumor growth by stimulating the p21 gene expression. We focused our research of p21-saRNA-322 on colorectal cancer because 1) p21 down-regulation is a signature abnormality of the cancer, and 2) colorectal cancer might be a suitable target for in situ p21-saRNA-322 delivery. The goal of the present study is to learn the activity of p21-saRNA-322 in colorectal cancer. Three human colorectal cancer cell lines, HCT-116, HCT-116 (p53–/−) and HT-29 were transfected with the p21-saRNA-322. The expression of P21 protein and p21 mRNA were measured using the Western blot and reverse transcriptase polymerase chain reaction (RT-PCR). The effect of p21-saRNA-322 on cancer cells was evaluated in vitro; and furthermore, a xenograft colorectal tumor mode in mice was established to estimate the tumor suppressing ability of p21-saRNA-322 in vivo. The results showed that in all three colorectal cancer cell lines, the expression of p21 mRNA and P21 protein were dramatically elevated after p21-saRNA-322 transfection. Transfection of p21-saRNA-322 caused apoptosis and cell cycle arrest at the G₀/G₁. Furthermore, anti-proliferation effect, reduction of colonies formation and cell senescence were observed in p21-saRNA-322 treated cells. Animal studies showed that p21-saRNA-322 treatment significantly inhibited the HT-29 tumor growth and facilitated p21 activation in vivo. These results indicated that, p21-saRNA-322-induceded up-regulation of p21 might be a promising therapeutic option for the treatment of colorectal cancer.

Ligustrum lucidum Ait. fruit extract induces apoptosis and cell senescence in human hepatocellular carcinoma cells through upregulation of p21

Nü-zhen-zi, the fruit of Ligustrum lucidum Ait., is one of the most frequently used liver Yin tonifying Chinese herbs for the treatment of liver cancer. However, the effect of Ligustrum lucidum fruit on hepatocarcinoma cells remains unknown. In the present study, we evaluated the effects of a Ligustrum lucidum fruit extract (LLFE) on human hepatocellular carcinoma Bel-7402 cells. The results showed that LLFE inhibited the proliferation of the Bel-7402 cells in a dose- and time-dependent manner. LLFE induced apoptosis in Bel-7402 cells accompanied by activation of caspase-3, -8 and -9. LLFE-induced apoptosis was completely abrogated by a pan caspase inhibitor, Z-VAD-FMK. LLFE treatment also caused a large and flat morphologic cellular change, positive SA-β-gal staining, and G0/G1 phase cell cycle arrest in the Bel-7402 cells, accompanied by upregulation of p21 and downregulation of RB phosphorylation. Specific knockdown of p21 expression by RNA interference partially abrogated LLFE-induced apoptosis, and significantly abrogated LLFE-induced cell senescence. These observations suggest that Nü-zhen-zi is a potential anticancer herb and support the traditional use of Nü-zhen-zi for hepatocarcinoma treatment.

Therapeutic and space radiation exposure of mouse brain causes impaired DNA repair response and premature senescence by chronic oxidant production

Despite recent epidemiological evidences linking radiation exposure and a number of human ailments including cancer, mechanistic understanding of how radiation inflicts long-term changes in cerebral cortex, which regulates important neuronal functions, remains obscure. The current study dissects molecular events relevant to pathology in cerebral cortex of 6 to 8 weeks old female C57BL/6J mice two and twelve months after exposure to a γ radiation dose (2 Gy) commonly employed in fractionated radiotherapy. For a comparative study, effects of 1.6 Gy heavy ion 56Fe radiation on cerebral cortex were also investigated, which has implications for space exploration. Radiation exposure was associated with increased chronic oxidative stress, oxidative DNA damage, lipid peroxidation, and apoptosis. These results when considered with decreased cortical thickness, activation of cell-cycle arrest pathway, and inhibition of DNA double strand break repair factors led us to conclude to our knowledge for the first time that radiation caused aging-like pathology in cerebral cortical cells and changes after heavy ion radiation were more pronounced than γ radiation.

hRAD9 functions as a tumor suppressor by inducing p21-dependent senescence and suppressing epithelial-mesenchymal transition through inhibition of Slug transcription

Senescence and epithelial-mesenchymal transition (EMT) have opposing roles in tumor progression, in that, one is a barrier against tumorigenesis, whereas the other is required for invasive malignancies. Here, we report that the DNA damage response (DDR) protein hRAD9 contributes to induction of senescence and inhibition of EMT. Our data show that hRAD9 is frequently downregulated in breast and lung cancers. Loss of hRAD9 expression is associated with tumor stage in breast and lung cancers, as well as with acquisition of an invasive phenotype. Ectopic hRAD9 expression in highly invasive cancer cell lines, H1299 and MDA-MB 231, with low endogenous hRAD9 induced senescence by upregulation of nuclear p21, independent of the p53 status. Ectopic expression of hRAD9 also significantly attenuated cellular migration and invasion in vitro and tumor growth in a xenograft mouse model in vivo. In contrast, silencing hRAD9 in lower invasive cancer cell lines, A549 and MCF7, with high endogenous hRAD9 dramatically increased their migration and invasion abilities, and simultaneously activated EMT. Knockdown of hRAD9 increased, whereas ectopic expression of hRAD9 decreased, the expression of Slug. Moreover, hRAD9 directly bound to the promoter region of slug gene and repressed its transcriptional activity. Taken together, these results suggest that hRAD9 is a potential tumor suppressor in breast and lung cancers and that it is likely to function by upregulating p21 and inhibiting Slug to regulate tumorigenesis.

Chemokine receptor CXCR2 is transactivated by p53 and induces p38-mediated cellular senescence in response to DNA damage

Mammalian cells may undergo permanent growth arrest/senescence when they incur excessive DNA damage. As a key player during DNA damage response (DDR), p53 transactivates an array of target genes that are involved in various cellular processes including the induction of cellular senescence. Chemokine receptor CXCR2 was previously reported to mediate replicative and oncogene-induced senescence in a DDR and p53-dependent manner. Here, we report that CXCR2 is upregulated in various types of cells in response to genotoxic or oxidative stress. Unexpectedly, we found that the upregulation of CXCR2 depends on the function of p53. Like other p53 target genes such as p21, CXCR2 is transactivated by p53. We identified a p53-binding site in the CXCR2 promoter that responds to changes in p53 functional status. Thus, CXCR2 may act downstream of p53. While the senescence-associated secretory phenotype (SASP) exhibits a kinetics that is distinct from that of CXCR2 expression and does not require p53, it reinforces senescence. We further showed that the cellular senescence caused by CXCR2 upregulation is mediated by p38 activation. Our results thus demonstrate CXCR2 as a critical mediator of cellular senescence downstream of p53 in response to DNA damage.

Intestinal cell proliferation and senescence are regulated by receptor guanylyl cyclase C and p21

Guanylyl cyclase C (GC-C) is expressed in intestinal epithelial cells and serves as the receptor for bacterial heat-stable enterotoxin (ST) peptides and the guanylin family of gastrointestinal hormones. Activation of GC-C elevates intracellular cGMP, which modulates intestinal fluid-ion homeostasis and differentiation of enterocytes along the crypt-villus axis. GC-C activity can regulate colonic cell proliferation by inducing cell cycle arrest, and mice lacking GC-C display increased cell proliferation in colonic crypts. Activation of GC-C by administration of ST to wild type, but not Gucy2c(-/-), mice resulted in a reduction in carcinogen-induced aberrant crypt foci formation. In p53-deficient human colorectal carcinoma cells, ST led to a transcriptional up-regulation of p21, the cell cycle inhibitor, via activation of the cGMP-responsive kinase PKGII and p38 MAPK. Prolonged treatment of human colonic carcinoma cells with ST led to nuclear accumulation of p21, resulting in cellular senescence and reduced tumorigenic potential. Our results, therefore, identify downstream effectors for GC-C that contribute to regulating intestinal cell proliferation. Thus, genomic responses to a bacterial toxin can influence intestinal neoplasia and senescence.

Reactive oxygen species generated in different compartments induce cell death, survival, or senescence

Although reactive oxygen species (ROS) are well-established mediators of oxidative damage and cell demise, the mechanisms by which they trigger specific cell death modalities and the temporal/spatial requirements underlying this phenomenon are largely unknown. Yet, it is well established that most anticancer therapies depend on ROS production for efficient tumor eradication. Using several non-small-cell lung cancer cell lines, we have dissected how the site of ROS production and accumulation in various cell compartments affect cell fate. We demonstrate that high levels of exogenously generated H2O2 induce extensive DNA damage, ATP depletion, and severe cytotoxicity. Although these effects were independent of caspase activity, they could-at least in part-be prevented by RIP1 kinase inhibition. In contrast, low levels of exogenously produced H2O2 triggered a modest drop in ATP level, delayed toxicity, G2/M arrest, and cell senescence. Mitochondrially produced H2O2 induced a reversible ATP drop without affecting cell viability. Instead, the cells accumulated in the G1/S phase of the cell cycle and became senescent. Concomitant inhibition of glycolysis was found to markedly sensitize cells to death in the presence of otherwise nontoxic concentrations of H2O2, presumably by the inhibition of ATP-restoring mechanisms. Combined, our data provide evidence that ROS might dictate different cellular consequences depending on their overall concentration at steady-state levels and on their site of generation.

Thyroid hormone suppresses cell proliferation through endoglin-mediated promotion of p21 stability

Hypothyroidism has been associated with significantly elevated risk for hepatocellular carcinoma (HCC), although the precise underlying mechanisms remain unknown at present. Thyroid hormone (T3) and its receptor (TR) are involved in metabolism and growth. Endoglin is a T3/TR candidate target gene identified from our previous studies. Here, we demonstrated that T3 positively regulates endoglin mRNA and protein levels, both in vitro and in vivo. The thyroid hormone response elements of endoglin were identified at positions -2114/-2004 and -2032/-1973 of the promoter region using the electrophoretic mobility shift assay and chromatin immunoprecipitation assay. Endoglin was downregulated in the subgroups of HCC patients and significantly associated with histology grade (negative association, P=0.001), and this expression level was significantly associated with TRα1 in these HCC patients. Our results clearly indicate that p21 is involved in T3-mediated suppression of cell proliferation. Knock down of endoglin expression in HCC cells facilitated p21 polyubiquitination and promoted cell proliferation in the presence of T3. The data collectively suggest that T3/TR signaling suppresses cell proliferation by upregulating endoglin, in turn, affecting p21 stability. The results indicate that endoglin has a suppressor role to inhibit cell proliferation in HCC cell lines.

Cyclin-dependent kinase inhibitor p21(Waf1): contemporary view on its role in senescence and oncogenesis

p21(Waf1) was identified as a protein suppressing cyclin E/A-CDK2 activity and was originally considered as a negative regulator of the cell cycle and a tumor suppressor. It is now considered that p21(Waf1) has alternative functions, and the view of its role in cellular processes has begun to change. At present, p21(Waf1) is known to be involved in regulation of fundamental cellular programs: cell proliferation, differentiation, migration, senescence, and apoptosis. In fact, it not only exhibits antioncogenic, but also oncogenic properties. This review provides a contemporary understanding of the functions of p21(Waf1) depending on its intracellular localization. On one hand, when in the nucleus, it serves as a negative cell cycle regulator and tumor suppressor, in particular by participating in the launch of a senescence program. On the other hand, when p21(Waf1) is localized in the cytoplasm, it acts as an oncogene by regulating migration, apoptosis, and proliferation.

Human GTSE-1 regulates p21(CIP1/WAF1) stability conferring resistance to paclitaxel treatment

p21(CIP1/WAF1) belongs to the CIP/KIP family of Cdk inhibitors, and its expression is tightly controlled during the cell cycle, mainly by transcriptional and post-translational mechanisms. Fine regulation of p21(CIP1/WAF1) levels is critical for cell cycle control and for cellular response to stress. In the present work, we describe a novel mechanism to modulate p21(CIP1/WAF1) levels mediated by the human GTSE-1 (G(2) and S phase-expressed-1) protein. Our results provide evidence that hGTSE-1 protects p21(CIP1/WAF1) from proteasome-dependent degradation as part of a functional complex containing the Hsp90-binding TPR protein WISp39. We further show that the hGTSE-1 N-terminal portion is sufficient for p21(CIP1/WAF1) binding and stabilization. Finally, we demonstrate that hGTSE-1 mediated-p21(CIP1/WAF1) stabilization is clearly involved in the ability of cells to counteract cytotoxicity induced by the microtubule poison paclitaxel.

Level of reactive oxygen species induced by p21Waf1/CIP1 is critical for the determination of cell fate

p21(WAF(1)/)(CIP(1)) is a well-known cell cycle regulatory protein which is overexpressed in several cancer cell lines, and known to determine cell fate. We generated three recombinant adenovirus vectors that expressed either the full-length p21 (Ad-p21F), a p21 mutant with a deletion of the C-terminal proliferative cell nuclear antigen (PCNA) binding domain (Ad-p21N), or a p21 mutant with a deletion of the N-terminal cyclin-dependent kinase binding domain (Ad-p21C). We transfected these vectors into five cancer cell lines. Premature senescence was induced in all of the lines only following transfection with Ad-p21N and Ad-p21F. In addition, apoptosis was also induced in LoVo and HCT116 cells that harbored wild-type p53 and the reactive oxygen species (ROS) level was higher than in senescent cells. Finally, the induction of apoptosis was inhibited by using siRNA to downregulate p53. This observation implies that there is a feedback signaling loop involving p21/ROS/p53 in apoptotic responses. It appears to be, at least in part, driven by high levels of p21 protein. Next, we investigated the cell death effect of endogenous p21 protein on cell fate using sodium butyrate (NaB). Treatment with 1 mM NaB or 2 to 5 mM NaB induced senescence or apoptosis, respectively. The level of intracellular ROS in 5 mM NaB treated cells was 2-fold higher, compared with that in 1 mM NaB treated cells. We also demonstrated that DNA damage response signals including ataxia telangiectasia mutated, gammaH2AX, and p38 MAPK were involved in NaB-induced cell death. The magnitude of intracellular ROS levels in response to p21 elicited either senescence or apoptosis in the cancer cell lines.

Induction of alternative lengthening of telomeres-associated PML bodies by p53/p21 requires HP1 proteins

Alternative lengthening of telomeres (ALT) is a recombination-mediated process that maintains telomeres in telomerase-negative cancer cells. In asynchronously dividing ALT-positive cell populations, a small fraction of the cells have ALT-associated promyelocytic leukemia nuclear bodies (APBs), which contain (TTAGGG)n DNA and telomere-binding proteins. We found that restoring p53 function in ALT cells caused p21 up-regulation, growth arrest/senescence, and a large increase in cells containing APBs. Knockdown of p21 significantly reduced p53-mediated induction of APBs. Moreover, we found that heterochromatin protein 1 (HP1) is present in APBs, and knockdown of HP1α and/or HP1γ prevented p53-mediated APB induction, which suggests that HP1-mediated chromatin compaction is required for APB formation. Therefore, although the presence of APBs in a cell line or tumor is an excellent qualitative marker for ALT, the association of APBs with growth arrest/senescence and with “closed” telomeric chromatin, which is likely to repress recombination, suggests there is no simple correlation between ALT activity level and the number of APBs or APB-positive cells.

Effect of dietary monosodium glutamate on trans fat-induced nonalcoholic fatty liver disease

The effects of dietary monosodium glutamate (MSG) on trans-fatty acid (TFA)-induced nonalcoholic fatty liver disease (NAFLD) are addressed in an animal model. We used Affymetrix microarray analysis to investigate hepatic gene expression and the contribution of visceral white adipose tissue (WAT) to diet-induced NAFLD. Trans-fat feeding increased serum leptin, FFA, HDL-cholesterol (HDL-C), and total cholesterol (T-CHOL) levels, while robustly elevating the expression of genes involved in hepatic lipogenesis, including the transcription factor sterol-regulatory element binding protein 1c. Histological examination revealed hepatic macrosteatosis in TFA-fed animals. Conversely, dietary MSG at doses similar to human average daily intake caused hepatic microsteatosis and the expression of beta-oxidative genes. Serum triglyceride, FFA, and insulin levels were elevated in MSG-treated animals. The abdominal cavities of TFA- or MSG-treated animals had increased WAT deposition compared with controls. Microarray analysis of WAT gene expression revealed increased lipid biosynthetic gene expression, together with a 50% decrease in the key transcription factor Ppargc1a. A combination of TFA+MSG resulted in the highest levels of serum HDL-C, T-CHOL, and leptin. Microarray analysis of TFA+MSG-treated livers showed elevated expression of markers of hepatic inflammation, lipid storage, cell damage, and cell cycle impairment. TFA+MSG mice also had a high degree of WAT deposition and lipogenic gene expression. Levels of Ppargc1a were further reduced to 25% by TFA+MSG treatment. MSG exacerbates TFA-induced NAFLD.

Ataxia telangiectasia mutated and p21CIP1 modulate cell survival of drug-induced senescent tumor cells: implications for chemotherapy

PURPOSE

Premature or stress-induced senescence is a major cellular response to chemotherapy in solid tumors and contributes to successful treatment. However, senescent tumor cells are resistant to apoptosis and may also reenter the cell cycle. We set out to find a means to specifically induce senescent tumor cells to undergo cell death and not to reenter the cell cycle that may have general application in cancer therapy.

EXPERIMENTAL DESIGN

We investigated the mechanisms regulating cell survival in drug-induced senescent tumor cells. Using immunofluorescence and flow cytometry-based techniques, we established the status of the ataxia telangiectasia mutated (ATM) signaling pathway in these cells. We assayed the requirement of ATM signaling and p21(CIP1) expression for survival in premature senescent tumor cells using pharmacologic inhibitors and antisense oligonucleotides.

RESULTS

The ATM/ATR (ATM- and Rad3-related) signaling pathway was found to be constitutively active in drug-induced senescent tumor cells. We found that blocking ATM/ATR signaling with pharmacologic inhibitors, including the novel ATM inhibitors KU55933 and CGK733, induced senescent breast, lung, and colon carcinoma cells to undergo cell death. We show that the mechanism of action of this effect is directly via p21(CIP1), which acts downstream of ATM. This is in contrast to the effects of ATM inhibitors on normal, untransformed senescent cells.

CONCLUSIONS

Blocking ATM and/or p21(CIP1) following initial treatment with a low dose of senescence-inducing chemotherapy is a potentially less toxic and highly specific treatment for carcinomas.

Triethylene tetramine, a novel ligand of G-quadruplex, induces senescence of MCF-7 cells

Interference with telomerase and telomere maintenance is emerging as an attractive target for antitumor therapies. Ligands stabilizing G-quadruplexes have the potential to interfere with telomere replication by blocking the elongation of telomeres in tumors. Here, we report that long-term treatment with triethylene tetramine (TETA), at 50 or 100 microM, induced marked cellular senescence phenotypes accompanied by increased time of population doubling of MCF-7 cells. Cyclin-dependent kinase inhibitors, including p53 and p21, were also upregulated in TETA-treated MCF-7 cells. TETA is therefore as novel ligand of G-quadruplex and can induce tumor senescence; it is a promising material for tumor treatment.

Expression of the high mobility group A family member p8 is essential to maintaining tumorigenic potential by promoting cell cycle dysregulation in LbetaT2 cells

The mechanism by which the HMGA protein p8 facilitates tumorigenesis may be cell cycle dysregulation. Control- (C) LbetaT2 cells, which express p8, form tumors at a rate five-times faster than p8-knockdown (p8-KD)-LbetaT2 cells. In association with this heightened tumorigenic potential, p8-expressing C-LbetaT2 cells avoid G(0)/G(1) arrest and become genetically unstable while p8-KD-LbetaT2 cells arrest in G(0)/G(1), become senescent upon overgrowth, and maintain a diploid population. These phenotypic changes correspond to altered cell cycle regulation at the G(1)-to-S transition that may be due to p8-mediated changes in expression of the Cip/Kip family members of cell cycle inhibitors, p21, p27, and p57.

8-chloroadenosine induced HL-60 cell growth inhibition, differentiation, and G(0)/G(1) arrest involves attenuated cyclin D1 and telomerase and up-regulated p21(WAF1/CIP1)

8-Chloroadenosine, an active dephosphorylated metabolite of the antineoplastic agent 8-chloroadenosine 3',5'-monophosphate (8-Cl-cAMP), induces growth inhibition in multiple carcinomas. Here we report that 8-chloroadenosine inhibits growth in human promyelocytic leukemia HL-60 cells by a G(0)/G(1) phase arrest and terminates cell differentiation along the granulocytic lineage. The mechanism of 8-chloroadenosine-induced G(0)/G(1) arrest is independent of apoptosis. The expressions of cyclin D1 and c-myc in HL-60 are suppressed by 8-chloroadenosine, whereas the cyclin-dependent kinases inhibitor p21(WAF1/CIP1) is up-regulated. 8-Chloroadenosine has less effect on the expressions of cyclin-dependent kinase (cdk)2 and cdk4, G(1) phase cyclin-dependent kinases, and only moderately induces the expression of transforming growth factor beta1 (TGFbeta1) and the mitotic inhibitor p27(KIP1). Telomerase activity is reduced in extracts of 8-chloroadenosine treated HL-60 cells, but 8-chloroadenosine does not directly inhibit the catalytic activity of telomerase in vitro. Therefore, anti-proliferation of HL-60 cells by 8-chloroadenosine involves coordination of cyclin D1 suppression, reduction of telomerase activity, and up-regulation of p21(WAF1/CIP1) that arrest cell-cycle progression at G(0)/G(1) phase and terminate cell differentiation.

Differential expression of EDF-1 and endothelial nitric oxide synthase by proliferating, quiescent and senescent microvascular endothelial cells

Endothelial Differentiation-related Factor (EDF)-1 is a low molecular weight polypeptide downregulated in endothelial cells exposed to HIV-1-Tat or the phorbol ester TPA. EDF-1 acts in the cytosol as a calmodulin binding protein, and in the nucleus as a transcriptional coactivator. Here, we show that EDF-1 is downregulated in non-proliferating microvascular endothelial cells. Indeed, both quiescence and senescence reduce the levels of EDF-1 and this is due to protein degradation through the proteasome. We also describe a different subcellular localization of EDF-1 which is mainly nuclear in senescent 1G11 cells. Since (i) endothelial nitric oxide (NO) seems to play a role in endothelial proliferation and (ii) NO is an important mediator involved in the control of vascular tone, inflammatory responses and angiogenesis, it is noteworthy that senescence downregulates the expression and the activity of endothelial nitric oxide synthase (eNOS) in microvascular endothelial cells. On the contrary, quiescence does not affect NOS expression and activity. The modulation of EDF-1 in microvascular endothelial cells might offer new insights into the molecular events involved in angiogenesis and in microvascular dysfunctions in the elderly.

Transition to an irreversible state of senescence in HeLa cells arrested by repression of HPV E6 and E7 genes

Inhibition of human papillomavirus (HPV) E6 and E7 transcription by means of the E2 protein of bovine papillomavirus 1 (BPV1) has been shown to induce acute growth arrest in HPV-positive cervical carcinoma cells. This state of arrest is marked by the expression of senescence phenotypes including SA beta-Gal activity and lipofuscin accumulation. In this study, we examined the reversibility of these phenotypes by exogenously expressing the E6 and E7 genes into HeLa cells growth-arrested by the depletion of E6/E7. Re-expression of E7 (but not E6) in 2 days following E2 transduction induced the cells to resume growth. The proliferating cells manifested the phenotype of untreated HeLa cells, suggesting that E7 is the major factor responsible for the continued proliferation and the suppression of the senescence phenotype in cervical carcinoma cells. However, E7 in 5 days following E2 transduction did not prevent HeLa cells from entering the senescent state, indicating that the arrested state becomes irreversible. Our results suggest that, upon depletion of the viral oncoproteins, a senescent state is irreversibly induced in HeLa cells after a period of commitment. The status and cellular location of certain factors involved in signal transduction and cell cycle control was altered as well along with this irreversibility transition.

Senescent phenotype can be reversed by reduction of caveolin status

Hyporesponsiveness to growth factors is one of the fundamental characteristics of senescent cells. We previously reported that the up-regulation of caveolin attenuates the growth factor response and the subsequent downstream signal cascades in senescent human diploid fibroblasts. Therefore, in the present experiment, we investigated the modulation of caveolin status in senescent cells to determine the effect of caveolin on mitogenic signaling efficiency and cell cycling. We reduced the level of caveolin-1 in senescent human diploid fibroblasts using its antisense oligonucleotides and small interfering RNA, and this resulted in the restoration of normal growth factor responses such as the increased phosphorylation of Erk, the nuclear translocation of p-Erk, and the subsequent activation of p-Elk upon epidermal growth factor stimulation. Moreover, DNA synthesis and the re-entry of senescent cells into cell cycle were resumed upon epidermal growth factor stimulation concomitantly with decreases in p53 and p21. Taken together, we conclude that the loss of mitogenic signaling in senescent cells is strongly related to their elevated levels of caveolin-1 and that the functional recovery of senescent cells at least in the terms of growth factor responsiveness and cell cycle entry might be achieved simply by lowering the caveolin level.

Cell cycle arrest by human cytomegalovirus 86-kDa IE2 protein resembles premature senescence

Primary human embryo lung fibroblasts and adult diploid fibroblasts infected by the human cytomegalovirus (HCMV) display beta-galactosidase (beta-Gal) activity at neutral pH (senescence-associated beta-Gal [SA-beta-Gal] activity) and overexpression of the plasminogen activator inhibitor type 1 (PAI-1) gene, two widely recognized markers of the process designated premature cell senescence. This activity is higher when cells are serum starved for 48 h before infection, a process that speeds and facilitates HCMV infection but that is insufficient by itself to induce senescence. Fibroblasts infected by HCMV do not incorporate bromodeoxyuridine, a prerequisite for the formal definition of senescence. At the molecular level, cells infected by HCMV, beside the accumulation of large amounts of the cell cycle regulators p53 and pRb, the latter in its hyperphosphorylated form, display a strong induction of the cyclin-dependent kinase inhibitor (cdki) p16(INK4a), a direct effector of the senescence phenotype in fibroblasts, and a decrease of the cdki p21(CIP1/WAF). Finally, a replicative senescence state in the early phases of infection significantly increased the number of cells permissive to virus infection and enhanced HCMV replication. HCMV infection assays carried out in the presence of phosphonoformic acid, which inhibits the virus DNA polymerase and the expression of downstream genes, indicated that immediate-early and/or early (alpha) genes are sufficient for the induction of SA-beta-Gal activity. When baculovirus vectors expressing HCMV IE1-72 or IE2-86 proteins were inoculated into fibroblasts, the increase of p16(INK4a) (observed predominantly with IE2-86) was similar to that observed with the whole virus, as was the induction of SA-beta-Gal activity, suggesting that the viral IE2 gene leads infected cells into senescence. Altogether our results demonstrate for the first time that HCMV, after arresting the cell cycle and inhibiting apoptosis, triggers the cellular senescence program, probably through the p16(INK4a) and p53 pathways.

Replicative senescence and senescence-like state induced in cancer-derived cells

Studies on the replicative senescence and premature senescence induced by various stresses in normal somatic cells have provided important clues on the role of telomere shortening and mechanisms involved in aging processes and carcinogenesis. Recent work revealed that cancer cells also are induced to undergo replicative senescence state via telomere shortening as well as to enter a senescence-like state by the activation of cell cycle inhibitory pathways. Although less relevant in terms of aging physiology, studies on these phenomena in cancer cells have yielded important information on telomerase regulation and the roles of tumor suppressors in senescence and immortalization, and are expected to generate valuable anti-cancer strategies. Several features of the phenotypes specific for the senescent and senescence-like states induced in cancer cells are discussed.

Induction of senescence-like state and suppression of telomerase activity through inhibition of HPV E6/E7 gene expression in cells immortalized by HPV16 DNA

The E6 and E7 oncoproteins of human papillomavirus (HPV) play a major role in the development of cervical carcinoma. In this study, a recombinant adenovirus that expresses the bovine papillomavirus (BPV) E2, which has been shown to inhibit HPV early gene expression, was delivered to two HPV-immortalized cell lines as well as CaSki, a cervical carcinoma cell line. We tested whether the carcinoma and the immortal cells were equally affected by the expression of BPV E2. In all cell lines, BPV E2-mediated inhibition of HPV E6/E7 expression caused a dramatic suppression of cell growth, being preceded by the activation of the p53-Rb growth-inhibitory pathway, and a decrease in hTERT mRNA expression and telomerase activity. This suggests that the HPV E6 and E7 proteins are required not only for induction of the proliferative phenotype and telomerase activity, but also for their maintenance. In both the carcinoma and the immortal lines, the number of cells with enlarged cytoplasm and senescence-associated beta-galactosidase activity, which are markers for cellular senescence, was significantly increased. These results suggest that a senescence program exists in cells immortalized by HPV DNA as well as in cervical carcinoma cells.

The effects of natural ligands of hormone receptors and their antagonists on telomerase activity in the androgen sensitive prostatic cancer cell line LNCaP

The effects of the 17beta-estradiol, dihydrotestosterone and hormone antagonists tamoxifen and bicalutamide on telomerase activity and expression of cell cycle related proteins in the androgen-sensitive prostatic cancer cell line LNCaP were studied. The cell line was grown in RPMI supplemented with 2.5% charcoal-stripped FBS for 72 hr. The IC(50) of tamoxifen and bicalutamide and the optimal stimulatory concentrations of 17beta-estradiol and dihydrotestosterone were determined by means of the cell-viability assay, the activity of telomerase was measured by the telomere repeat amplification protocol (TRAP) and the expression of proteins was analysed by the Western blot technique. 17beta-estradiol stimulated cell growth more effectively than dihydrotestosterone whereas hormone antagonists tamoxifen and bicalutamide caused a significant decrease in cell viability. The treatment of cells by a combination of low doses of 17 beta-estradiol and dihydrotestosterone stimulated cells stronger than treatment by a single hormone. Only 17beta-estradiol, in concentration of 10nM, increased strongly the expression of p21(Waf1/Cip1) and increased slightly telomerase activity in the LNCaP cells. 50 microM of bicalutamide down-regulated the levels of the androgen receptor, the proliferating cell nuclear antigen and telomerase activity, and up-regulated the expression of p27(Kip1). We hereby describe the first observation of the influence of bicalutamide on telomerase activity and a positive correlation between the effect of 17beta-estradiol and the induction of both the endogenous cyclin-dependent kinase inhibitor, p21(Waf1/Cip1), and telomerase activity in a prostatic cancer cell line LNCaP. These findings can shed a new light on the steroid-signaling pathway in prostate cancer cells.

Complex regulatory mechanisms of telomerase activity in normal and cancer cells: how can we apply them for cancer therapy?

Telomerase activation is observed in almost 90% of human cancers but not in normal tissues of somatic origin and thus is a critical step for multistep carcinogenesis. A more thorough understanding of telomerase regulation may provide not only a molecular basis of cancer progression but also as a way to manipulate telomerase activity as a potential therapeutic modality. Recent progress in studies on telomerase regulation has shown that telomerase activation is achieved at various steps, including transcriptional and post-transcriptional levels of the telomerase reverse transcriptase (hTERT) gene. Although a number of potentially important mechanisms of telomerase activation have been proposed, none of the current models can fully explain tumor-specific activation of telomerase, suggesting a need for further extensive analysis. This review includes a summary of recent works on telomerase regulation and a discussion of how we can overcome this situation.

Sodium butyrate induces growth arrest and senescence-like phenotypes in gynecologic cancer cells

We demonstrated here the growth-suppressing effects of sodium butyrate (NaB) on human endometrial and ovarian cancer cells. The arrest of cells at the G1 checkpoint accounted for this effect. NaB-mediated p21 might arrest endometrial and ovarian cancer cells at the G0/G1 phase by eliciting pRb unphosphorylation. To demonstrate the role of pRb regulation by p21, we measured the sensitivity to NaB of cervical cancer cells in which pRb had been inactivated by HPV E7. The cervical cancer cells displayed a sensitivity in NaB-mediated G2/M arrest in addition to their sensitivity in G0/G1 arrest. Arrest at G0/G1 and G2/M accompanied induction of senescence-like phenotypes (SLPs). Most importantly, the effect of NaB on senescence induction was not coupled with the predominance of hypophosphorylated pRb forms in the cervical cancer cells. This suggested that NaB had the potential to elicit SLPs through p21-mediated withdrawal from cell cycle progression. The consequences of p21 induction were manifold. The effects of NaB on gynecologic cancer cell growth indicated its potential use in cancer treatment. NaB was effective even in the cancer cells with mutant p53 and/or Rb genes by eliciting cell senescence.

Upregulation of p21(WAF1/CIP1) leads to morphologic changes and esterase activity in TPA-mediated differentiation of human prostate cancer cell line TSU-Pr1

We reported previously that human prostate cancer cell line TSU-Pr1 can differentiate into microglia-like cells by 12-O-tetra-decanoylphorbol-13-acetate (TPA) treatment. In this study, we identified a signal transduction pathway involved in TPA-induced TSU-Pr1 cell differentiation and investigated the mechanism of growth arrest that accompanies this differentiation. TPA-induced differentiation and growth arrest of TSU-Pr1 cells were inhibited by treatment with Protein kinase C (PKC) inhibitor GF109203X and mitogen-activated protein (MAP) kinase inhibitor PD98059. Treatment of TSU-Pr1 cells with TPA for 15 min or longer resulted in translocation of PKCalpha, PKCgamma, and PKCepsilon from cytosolic to membrane fraction. Our results suggest that TPA-induced TSU-Pr1 cell differentiation is associated with activation of MAP kinase and PKCalpha, PKCgamma, and PKCepsilon. The mechanism of growth arrest in TSU-Pr1 cells that underwent TPA-induced differentiation were examined for factors in the signaling pathway downstream of MAP kinase that control the cell cycle. Upregulation of p21(WAF1/CIP1) cyclin-dependent kinase inhibitor protein was observed in a manner dependent on PKC or MAP kinase. Moreover, adenovirus-mediated overexpression of recombinant p21(WAF1/CIP1) in TSU-Pr1 cells result in growth arrest, morphological change to microglia-like cells, and increased alpha-naphthyl acetate esterase activity, all of which are associated with cellular differentiation. Thus, our results indicate that p21(WAF1/CIP1) mediates TPA-induced growth arrest and differentiation of TSU-Pr1 cells.

Papillomavirus E2 induces senescence in HPV-positive cells via pRB- and p21(CIP)-dependent pathways

A hallmark of human papillomavirus (HPV) associated carcinogenesis is the integration of the viral DNA into the cellular genome, usually accompanied by the loss of expression of the viral E2 gene. E2 binds to and represses the viral promoter directing expression of the E6 and E7 oncogenes. The re-introduction and expression of exogenous E2 in HPV-positive cancer cells results in cellular growth arrest, while growth in the context of exogenous E2 can be restored through the expression of exogenous E6 and E7. Here we examine the individual contributions of the viral E6 and E7 genes to this phenotype. E6 alone displays moderate activity, whereas both E7 and adenovirus E1A display high activity in reversing E2-mediated cellular growth suppression. Using defined mutants of E7 and E1A, we show that an intact retinoblastoma interaction domain is required for this function. In addition, we show that the E2-mediated growth arrest of HPV-positive cells results in cellular senescence, and implicate the cyclin/cdk inhibitor p21(CIP) as a downstream E2 effector in this phenotype.

Telomerase as a potential anticancer target: growth inhibition and genomic instability

Stabilization of telomere length in chromosomes by an RNA-dependent DNA polymerase (telomerase) appears to be responsible for the replicative immortality of cancer cells. These findings provide the rational basis for generating experimental models to develop anti-telomerase drugs. However, there is conflicting evidence in the literature about the outcome of telomerase inhibition. While tumor cytostatic and cytotoxic effects associated with telomerase inhibition have been described, absence of telomerase has been associated with genetic instability and tumor development. Therefore, a therapeutic strategy based on telomerase inhibition will likely have to cope with problems related to innate or acquired mechanisms of drug resistance and possibly to therapy-related tumors. Copyright 2000 Harcourt Publishers Ltd.