tRNA biogenesis and specific aminoacyl-tRNA synthetases regulate senescence stability under the control of mTOR.

Oncogenes or chemotherapy treatments trigger the induction of suppressive pathways such as apoptosis or senescence. Senescence was initially defined as a definitive arrest of cell proliferation but recent results have shown that this mechanism is also associated with cancer progression and chemotherapy resistance. Senescence is therefore much more heterogeneous than initially thought. How this response varies is not really understood, it has been proposed that its outcome relies on the secretome of senescent cells and on the maintenance of their epigenetic marks. Using experimental models of senescence escape, we now described that the stability of this proliferative arrest relies on specific tRNAs and aminoacyl-tRNA synthetases. Following chemotherapy treatment, the DNA binding of the type III RNA polymerase was reduced to prevent tRNA transcription and induce a complete cell cycle arrest. By contrast, during senescence escape, specific tRNAs such as tRNA-Leu-CAA and tRNA-Tyr-GTA were up-regulated. Reducing tRNA transcription appears necessary to control the strength of senescence since RNA pol III inhibition through BRF1 depletion maintained senescence and blocked the generation of escaping cells. mTOR inhibition also prevented chemotherapy-induced senescence escape in association with a reduction of tRNA-Leu-CAA and tRNA-Tyr-GTA expression. Further confirming the role of the tRNA-Leu-CAA and tRNA-Tyr-GTA, results showed that their corresponding tRNA ligases, LARS and YARS, were necessary for senescence escape. This effect was specific since the CARS ligase had no effect on persistence. By contrast, the down-regulation of LARS and YARS reduced the emergence of persistent cells and this was associated with the modulation of E2F1 target genes expression. Overall, these findings highlight a new regulation of tRNA biology during senescence and suggest that specific tRNAs and ligases contribute to the strength and heterogeneity of this tumor suppressive pathway.

Sildenafil-Induced Revascularization of Rat Hindlimb Involves Arteriogenesis through PI3K/AKT and eNOS Activation.

Hypoxia and inflammation play a major role in revascularization following ischemia. Sildenafil inhibits phosphodiesterase-5, increases intracellular cGMP and induces revascularization through a pathway which remains incompletely understood. Thus, we investigated the effect of sildenafil on post-ischemic revascularization. The left femoral artery was ligated in control and sildenafil-treated (25 mg/kg per day) rats. Vascular density was evaluated and expressed as the left/right leg (L/R) ratio. In control rats, L/R ratio was 33 ± 2% and 54 ± 9%, at 7- and 21-days post-ligation, respectively, and was significantly increased in sildenafil-treated rats to 47 ± 4% and 128 ± 11%, respectively. A neutralizing anti-VEGF antibody significantly decreased vascular density (by 0.48-fold) in control without effect in sildenafil-treated animals. Blood flow and arteriolar density followed the same pattern. In the ischemic leg, HIF-1α and VEGF expression levels increased in control, but not in sildenafil-treated rats, suggesting that sildenafil did not induce angiogenesis. PI3-kinase, Akt and eNOS increased after 7 days, with down-regulation after 21 days. Sildenafil induced outward remodeling or arteriogenesis in mesenteric resistance arteries in association with eNOS protein activation. We conclude that sildenafil treatment increased tissue blood flow and arteriogenesis independently of VEGF, but in association with PI3-kinase, Akt and eNOS activation.

Dual Role of Thrombospondin-1 in Flow-Induced Remodeling.

(1) Background: Chronic increases in blood flow, as in cardiovascular diseases, induce outward arterial remodeling. Thrombospondin-1 (TSP-1) is known to interact with matrix proteins and immune cell-surface receptors, but its contribution to flow-mediated remodeling in the microcirculation remains unknown. (2) Methods: Mesenteric arteries were ligated in vivo to generate high- (HF) and normal-flow (NF) arteries in wild-type (WT) and TSP-1-deleted mice (TSP-1-/-). After 7 days, arteries were isolated and studied ex vivo. (3) Results: Chronic increases in blood flow induced outward remodeling in WT mice (increasing diameter from 221 ± 10 to 280 ± 10 µm with 75 mmHg intraluminal pressure) without significant effect in TSP-1-/- (296 ± 18 to 303 ± 14 µm), neutropenic or adoptive bone marrow transfer mice. Four days after ligature, pro inflammatory gene expression levels (CD68, Cox2, Gp91phox, p47phox and p22phox) increased in WT HF arteries but not in TSP-1-/- mice. Perivascular neutrophil accumulation at day 4 was significantly lower in TSP-1-/- than in WT mice. (4) Conclusions: TSP-1 origin is important; indeed, circulating TSP-1 participates in vasodilation, whereas both circulating and tissue TSP-1 are involved in arterial wall thickness and diameter expansion.

Proteomics Approaches to Define Senescence Heterogeneity and Chemotherapy Response.

In primary cells, senescence induces a permanent proliferative arrest to prevent the propagation of malignant cells. However, the outcome of senescence is more complex in advanced cancer cells where senescent states are heterogeneous. Here, this heterogeneity is discussed and it is proposed that proteomic analysis should be used to identify specific signatures of cancer cells that use this pathway as an adaptive mechanism. Since senescent cells produce an inflammatory secretome, MRM approaches and quantification with internal standards might be particularly suited to follow the expression of the corresponding markers in body fluids. Used in combination with imaging medical technics, a better characterization of senescence heterogeneity should help to monitor the response to chemotherapy treatment.

Central Role of P2Y6 UDP Receptor in Arteriolar Myogenic Tone.

OBJECTIVE: Myogenic tone (MT) of resistance arteries ensures autoregulation of blood flow in organs and relies on the intrinsic property of smooth muscle to contract in response to stretch. Nucleotides released by mechanical strain on cells are responsible for pleiotropic vascular effects, including vasoconstriction. Here, we evaluated the contribution of extracellular nucleotides to MT. APPROACH AND RESULTS: We measured MT and the associated pathway in mouse mesenteric resistance arteries using arteriography for small arteries and molecular biology. Of the P2 receptors in mouse mesenteric resistance arteries, mRNA expression of P2X1 and P2Y6 was dominant. P2Y6 fully sustained UDP/UTP-induced contraction (abrogated in P2ry6(-/-) arteries). Preventing nucleotide hydrolysis with the ectonucleotidase inhibitor ARL67156 enhanced pressure-induced MT by 20%, whereas P2Y6 receptor blockade blunted MT in mouse mesenteric resistance arteries and human subcutaneous arteries. Despite normal hemodynamic parameters, P2ry6(-/-) mice were protected against MT elevation in myocardial infarction-induced heart failure. Although both P2Y6 and P2Y2 receptors contributed to calcium mobilization, P2Y6 activation was mandatory for RhoA-GTP binding, myosin light chain, P42-P44, and c-Jun N-terminal kinase phosphorylation in arterial smooth muscle cells. In accordance with the opening of a nucleotide conduit in pressurized arteries, MT was altered by hemichannel pharmacological inhibitors and impaired in Cx43(+/-) and P2rx7(-/-) mesenteric resistance arteries. CONCLUSIONS: Signaling through P2 nucleotide receptors contributes to MT. This mechanism encompasses the release of nucleotides coupled to specific autocrine/paracrine activation of the uracil nucleotide P2Y6 receptor and may contribute to impaired tissue perfusion in cardiovascular diseases.

Nucleoside triphosphate diphosphohydrolase-1 ectonucleotidase is required for normal vas deferens contraction and male fertility through maintaining P2X1 receptor function.

In this work, we report that Entpd1(-/-) mice, deficient for the ectonucleotidase nucleoside triphosphate diphosphohydrolase-1 (NTPDase1), produce smaller litters (27% reduction) compared with wild-type C57BL6 animals. This deficit is linked to reduced in vivo oocyte fertilization by Entpd1(-/-) males (61 ± 11% versus 88 ± 7% for Entpd1(+/+)). Normal epididymal sperm count, spermatozoa morphology, capacitation, and motility and reduced ejaculated sperm number (2.4 ± 0.5 versus 3.7 ± 0.4 million for Entpd1(+/+)) pointed to vas deferens dysfunction. NTPDase1 was localized by immunofluorescence in the tunica muscularis of the vas deferens. Its absence resulted in a major ATP hydrolysis deficiency, as observed in situ by histochemistry and in primary smooth muscle cell cultures. In vitro, Entpd1(-/-) vas deferens displayed an exacerbated contraction to ATP, a diminished response to its non-hydrolysable analog αßMeATP, and a reduced contraction to electrical field stimulation, suggesting altered P2X1 receptor function with a propensity to desensitize. This functional alteration was accompanied by a 3-fold decrease in P2X1 protein expression in Entpd1(-/-) vas deferens with no variation in mRNA levels. Accordingly, exogenous nucleotidase activity was required to fully preserve P2X1 receptor activation by ATP in vitro. Our study demonstrates that NTPDase1 is required to maintain normal P2X1 receptor functionality in the vas deferens and that its absence leads to impaired peristalsis, reduced spermatozoa concentration in the semen, and, eventually, reduced fertility. This suggests that alteration of NTPDase1 activity affects ejaculation efficacy and male fertility. This work may contribute to unveil a cause of infertility and open new therapeutic potentials.

Determinants of flow-mediated outward remodeling in female rodents: respective roles of age, estrogens, and timing.

OBJECTIVE: Flow (shear stress)-mediated outward remodeling (FMR) of resistance arteries is a key adaptive process allowing collateral growth after arterial occlusion but declining with age. 17-ß-estradiol (E2) has a key role in this process through activation of estrogen receptor α (ERα). Thus, we investigated the impact of age and timing for estrogen efficacy on FMR. APPROACH AND RESULTS: Female rats, 3 to 18 months old, were submitted to surgery to increase blood flow locally in 1 mesenteric artery in vivo. High-flow and normal-flow arteries were collected 2 weeks later for in vitro analysis. Diameter increased by 27% in high-flow arteries compared with normal-flow arteries in 3-month-old rats. The amplitude of remodeling declined with age (12% in 18-month-old rats) in parallel with E2 blood level and E2 substitution failed restoring remodeling in 18-month-old rats. Ovariectomy of 3-, 9-, and 12-month-old rats abolished FMR, which was restored by immediate E2 replacement. Nevertheless, this effect of E2 was absent 9 months after ovariectomy. In this latter group, ERα and endothelial nitric oxide synthase expression were reduced by half compared with age-matched rats recently ovariectomized. FMR did not occur in ERα(-/-) mice, whereas it was decreased by 50% in ERα(+/-) mice, emphasizing the importance of gene dosage in high-flow remodeling. CONCLUSIONS: E2 deprivation, rather than age, leads to decline in FMR, which can be prevented by early exogenous E2. However, delayed E2 replacement was ineffective on FMR, underlining the importance of timing of this estrogen action.

AGEs breaking and antioxidant treatment improves endothelium-dependent dilation without effect on flow-mediated remodeling of resistance arteries in old Zucker diabetic rats.

BACKGROUND: A chronic increase in blood flow in resistance arteries is associated with increased lumen diameter (outward remodeling) and improved endothelium (NO)-mediated relaxation. Flow-mediated remodeling of resistance arteries is essential for revascularization in ischemic diseases. Nevertheless, it is impaired in 12 to 24-month old rats and in young Zucker Diabetic Fatty (ZDF) rats due to advanced glycation end products (AGEs) and oxidative stress. As type 2 diabetes occurs preferentially in older subjects we investigated flow-mediated remodeling and the effect of the AGEs breaker ALT-711 associated or not to the antioxidant TEMPOL in one-year old lean (LZ) and ZDF rats. METHODS: Mesenteric resistance arteries were exposed to high (HF) or normal blood flow (NF) in vivo. They were collected after 2 weeks for in vitro analysis. RESULTS: In LZ rats, diameter expansion did not occur despite a significant increase in blood flow in HF arteries. Nevertheless, endothelium-mediated relaxation was higher in HF than in NF arteries. ALT-711, alone or in combination with TEMPOL, restored outward remodeling in HF arteries in association with AGEs reduction. TEMPOL alone had no effect. ALT-711, TEMPOL or the combination of the 2 drugs did not significantly affect endothelium-mediated relaxation in HF and NF arteries.In ZDF rats, diameter did not increase despite the increase in blood flow and endothelium-mediated relaxation was further decreased in HF arteries in association with AGEs accumulation and excessive oxidative stress. In both NF and HF arteries, endothelium-mediated relaxation was lower in ZDF than in LZ rats. ALT-711, TEMPOL or their combination did not improve remodeling (diameter equivalent in HF and NF arteries). In parallel, they did not reduce AGEs level and did not improve MMPs activity. Nevertheless, ALT-711 and TEMPOL partly improved endothelium-mediated relaxation through a reduction of oxidative stress and the association of ALT-711 and TEMPOL fully restored relaxation to the level found in LZ rats. CONCLUSIONS: ALT-711 did not improve outward remodeling in mature ZDF rats but it reduced oxidative stress and consequently improved endothelium-dependent relaxation. In mature LZ rats, ALT-711 improved outward remodeling and reduced AGEs level. Consequently, AGEs breaking is differently useful in ageing whether it is associated with diabetes or not.

Selective involvement of serum response factor in pressure-induced myogenic tone in resistance arteries.

OBJECTIVE: In resistance arteries, diameter adjustment in response to pressure changes depends on the vascular cytoskeleton integrity. Serum response factor (SRF) is a dispensable transcription factor for cellular growth, but its role remains unknown in resistance arteries. We hypothesized that SRF is required for appropriate microvascular contraction. METHODS AND RESULTS: We used mice in which SRF was specifically deleted in smooth muscle or endothelial cells, and their control. Myogenic tone and pharmacological contraction was determined in resistance arteries. mRNA and protein expression were assessed by quantitative real-time PCR (qRT-PCR) and Western blot. Actin polymerization was determined by confocal microscopy. Stress-activated channel activity was measured by patch clamp. Myogenic tone developing in response to pressure was dramatically decreased by SRF deletion (5.9±2.3%) compared with control (16.3±3.2%). This defect was accompanied by decreases in actin polymerization, filamin A, myosin light chain kinase and myosin light chain expression level, and stress-activated channel activity and sensitivity in response to pressure. Contractions induced by phenylephrine or U46619 were not modified, despite a higher sensitivity to p38 blockade; this highlights a compensatory pathway, allowing normal receptor-dependent contraction. CONCLUSIONS: This study shows for the first time that SRF has a major part to play in the control of local blood flow via its central role in pressure-induced myogenic tone in resistance arteries.

Reactive oxygen species are necessary for high flow (shear stress)-induced diameter enlargement of rat resistance arteries.

OBJECTIVES: Chronic increases in blood flow induce remodeling associated with increases in diameter and endothelium-mediated dilation. Remodeling requires cell growth and migration, which may involve reactive oxygen species (ROS). Nevertheless, the role of ROS in flow-mediated remodeling in resistance arteries is not known. MATERIALS AND METHODS: Rat mesenteric resistance arteries (MRAs) were exposed to high flow (HF) by sequentially ligating second-order MRAs in vivo. After three weeks, arteries were collected for structural, pharmacological, and biochemical analysis. RESULTS: In HF arteries, luminal diameter (431+/-12 to 553+/-14 microm; n=10), endothelium (acetylcholine)-mediated vasodilatation (61+/-6 to 77+/-6% relaxation) and NAD(P)H subunit (gp91phox and p67phox) expression levels, and ROS (dihydroethydine microphotography) and peroxynitrite (3-nitro-tyrosine) production were higher than in normal flow arteries. Acute ROS scavenging with tempol improved acetylcholine-dependent relaxation (92+/-4% relaxation), confirming that ROS are produced in HF arteries. Chronic treatment with tempol prevented the increase in diameter, reduced ROS and peroxynitrite production, and improved endothelium-mediated relaxation in HF arteries. Thus, ROS and NO were involved in HF-induced diameter enlargement, possibly through the formation of peroxynitrite, while ROS reduced the increase in endothelium-dependent relaxation. CONCLUSIONS: ROS production is necessary for flow-mediated diameter enlargement of resistance arteries. However, ROS counteract, in part, the associated improvement in endothelium-mediated relaxation.

Chemotherapy-induced senescence, an adaptive mechanism driving resistance and tumor heterogeneity.

Senescence is activated in response to chemotherapy to prevent the propagation of cancer cells. In transformed cells, recent studies have shown that this response is not always definitive and that persistent populations can use senescence as an adaptive pathway to restart proliferation and become more aggressive. Here we discuss the results showing that an incomplete and heterogeneous senescence response plays a key role in chemotherapy resistance. Surviving to successive chemotherapy regimens, chronically existing senescent cells can create a survival niche through paracrine cooperations with neighboring cells. This favors chemotherapy escape of premalignant clones but might also allow the survival of adjacent clones presenting a lower fitness. A better characterization of senescence heterogeneity in transformed cells is therefore necessary. This will help us to understand this incomplete response to therapy and how it could generate clones with increased tumor capacity leading to disease relapse.

Regulation of senescence escape by TSP1 and CD47 following chemotherapy treatment.

Senescence is a tumor-suppressive mechanism induced by telomere shortening, oncogenes, or chemotherapy treatment. Although it is clear that this suppressive pathway leads to a permanent arrest in primary cells, this might not be the case in cancer cells that have inactivated their suppressive pathways. We have recently shown that subpopulations of cells can escape chemotherapy-mediated senescence and emerge as more transformed cells that induce tumor formation, resist anoikis, and are more invasive. In this study, we characterized this emergence and showed that senescent cells favor tumor growth and metastasis, in vitro and in vivo. Senescence escape was regulated by secreted proteins produced during emergence. Among these, we identified thrombospondin-1 (TSP1), a protein produced by senescent cells that prevented senescence escape. Using SWATH quantitative proteomic analysis, we found that TSP1 can be detected in the serum of patients suffering from triple-negative breast cancer and that its low expression was associated with treatment failure. The results also indicate that senescence escape is explained by the emergence of CD47low cells that express a reduced level of CD47, the TSP1 receptor. The results show that CD47 expression is regulated by p21waf1. The cell cycle inhibitor was sufficient to maintain senescence since its downregulation in senescent cells increased cell emergence. This leads to the upregulation of Myc, which then binds to the CD47 promoter to repress its expression, allowing the generation of CD47low cells that escape the suppressive arrest. Altogether, these results uncovered a new function for TSP1 and CD47 in the control of chemotherapy-mediated senescence.

Regulation of senescence escape by the cdk4-EZH2-AP2M1 pathway in response to chemotherapy.

Senescence is a tumor suppressive mechanism that induces a permanent proliferative arrest in response to an oncogenic insult or to the genotoxic stress induced by chemotherapy. We have recently described that some cells can escape this arrest, either because senescence was incomplete or as a consequence of a phenotypic adaptation. Malignant cells which resisted senescence emerged as more transformed cells that resist anoikis and rely on survival pathways activated by Akt and Mcl-1. In this study, we further characterize senescence escape, investigating how emergent cells could reproliferate. During the initial step of chemotherapy-induced senescence (CIS), we found that cyclin D1 was upregulated and that cell emergence was prevented when its main partner cdk4 was inactivated. Results indicate that this kinase induced the upregulation of the EZH2 methylase, a component of the polycomb PRC2 complex. Downregulated during the early step of treatment, the methylase was reactivated in clones that escaped senescence. The inactivation of EZH2, either by siRNA or by specific inhibitors, led to a specific inhibition of cell emergence. We used quantitative proteomic analysis to identify new targets of the methylase involved in senescence escape. We identified proteins involved in receptor endocytosis and described new functions for the AP2M1 protein in the control of chemotherapy-mediated senescence. Our results indicate that AP2M1 is involved in the transmission of secreted signals produced by senescent cells, suggesting that this pathway might regulate specific receptors involved in the control of CIS escape. In light of these results, we therefore propose that the cdk4-EZH2-AP2M1 pathway plays an important role during chemotherapy resistance and senescence escape. Since targeted therapies are available against these proteins, we propose that they should be tested in the treatment of colorectal or breast cancers that become resistant to first-line genotoxic therapies.

Iron-related transcriptomic variations in CaCo-2 cells, an in vitro model of intestinal absorptive cells.

Regulation of iron absorption by duodenal enterocytes is essential for the maintenance of homeostasis by preventing iron deficiency or overload. Despite the identification of a number of genes implicated in iron absorption and its regulation, it is likely that further factors remain to be identified. For that purpose, we used a global transcriptomic approach, using the CaCo-2 cell line as an in vitro model of intestinal absorptive cells. Pangenomic screening for variations in gene expression correlating with intracellular iron content allowed us to identify 171 genes. One hundred nine of these genes are clustered into five types of expression profile. This is the first time that most of these genes have been associated with iron metabolism. Functional annotation of these five clusters indicates potential links between the immune response, proteolysis processes, and iron depletion. In contrast, iron overload is associated with cellular metabolism, especially that of lipids and glutathione involving redox function and electron transfer.

[Contribution to tumor escape and chemotherapy response: A choice between senescence and apoptosis in heterogeneous tumors]. / Échappement tumoral lors de la chimiothérapie: un choix entre sénescence et apoptose dans des tumeurs hétérogènes.

Understanding adaptive signaling pathways in response to chemotherapy is one of the main challenges of cancer treatment. Activated in response to DNA damage, cell cycle and mitotic checkpoints activate the p53-p21 and p16-Rb pathways and induce apoptosis or senescence. Since senescent cells survive and produce a secretome that influences neighbouring cells, it is not particularly clear whether these responses are equivalent and if tumor cells escape these two suppressive pathways to the same extent. Predicting escape is also complicated by the fact that cancer cells adapt to treatments by activating the epithelial-mesenchymal transition and by producing clones with cancer-initiating cells features. Dedifferentiation pathways used in stressful conditions reconstitute dividing and sometimes more aggressive populations in response to chemotherapy. These observations illustrate the importance of tumor heterogeneity and the adaptation capacities of different intra-tumoral subclones. Depending on their oncogenic profile, on their localisation within the tumor and on their interaction with stromal cells, these subclones are expected to have different responses and adaptation capacities to chemotherapy. A complete eradication will certainly rely on combination therapies that can kill at the same time the bulk of the sensitive tumor but can also prevent plasticity and the generation of persistent clones.

Resveratrol Improved Flow-Mediated Outward Arterial Remodeling in Ovariectomized Rats with Hypertrophic Effect at High Dose.

OBJECTIVES: Chronic increases in blood flow in resistance arteries induce outward remodeling associated with increased wall thickness and endothelium-mediated dilatation. This remodeling is essential for collateral arteries growth following occlusion of a large artery. As estrogens have a major role in this remodeling, we hypothesized that resveratrol, described as possessing phytoestrogen properties, could improve remodeling in ovariectomized rats. METHODS: Blood flow was increased in vivo in mesenteric arteries after ligation of adjacent arteries in 3-month old ovariectomized rats treated with resveratrol (5 or 37.5 mg/kg per day: RESV5 or RESV37.5) or vehicle. After 2 weeks arterial structure and function were measured in vitro in high flow (HF) and normal flow (NF) arteries isolated from each rat. RESULTS: Arterial diameter was greater in HF than in NF arteries in ovariectomized rats treated with RESV5 or RESV37.5, not in vehicle-treated rats. In mice lacking estrogen receptor alpha diameter was equivalent in HF and NF arteries whereas in mice treated with RESV5 diameter was greater in HF than in NF vessels. A compensatory increase in wall thickness and a greater phenylephrine-mediated contraction were observed in HF arteries. This was more pronounced in HF arteries from RESV37.5-treated rats. ERK1/2 phosphorylation, involved in hypertrophy and contraction, were higher in RESV37.5-treated rats than in RESV5- and vehicle-treated rats. Endothelium-dependent relaxation was greater in HF than in NF arteries in RESV5-treated rats only. In HF arteries from RESV37.5-treated rats relaxation was increased by superoxide reduction and markers of oxidative stress (p67phox, GP91phox) were higher than in the 2 other groups. CONCLUSION: Resveratrol improved flow-mediated outward remodeling in ovariectomized rats thus providing a potential therapeutic tool in menopause-associated ischemic disorders. This effect seems independent of the estrogen receptor alpha. Nevertheless, caution should be taken with high doses inducing excessive contractility and hypertrophy in association with oxidative stress in HF arteries.

Akt inhibition improves irinotecan treatment and prevents cell emergence by switching the senescence response to apoptosis.

Activated in response to chemotherapy, senescence is a tumor suppressive mechanism that induces a permanent loss of proliferation. However, in response to treatment, it is not really known how cells can escape senescence and how irreversible or incomplete this pathway is. We have recently described that cells that escape senescence are more transformed than non-treated parental cells, they resist anoikis and rely on Mcl-1. In this study, we further characterize this emergence in response to irinotecan, a first line treatment used in colorectal cancer. Our results indicate that Akt was activated as a feedback pathway during the early step of senescence. The inhibition of the kinase prevented cell emergence and improved treatment efficacy, both in vitro and in vivo. This improvement was correlated with senescence inhibition, p21waf1 downregulation and a concomitant activation of apoptosis due to Noxa upregulation and Mcl-1 inactivation. The inactivation of Noxa prevented apoptosis and increased the number of emergent cells. Using either RNA interference or p21waf1-deficient cells, we further confirmed that an intact p53-p21-senescence pathway favored cell emergence and that its downregulation improved treatment efficacy through apoptosis induction. Therefore, although senescence is an efficient suppressive mechanism, it also generates more aggressive cells as a consequence of apoptosis inhibition. We therefore propose that senescence-inducing therapies should be used sequentially with drugs favoring cell death such as Akt inhibitors. This should reduce cell emergence and tumor relapse through a combined induction of senescence and apoptosis.

Irinotecan treatment and senescence failure promote the emergence of more transformed and invasive cells that depend on anti-apoptotic Mcl-1.

Induction of senescence by chemotherapy was initially characterized as a suppressive response that prevents tumor cell proliferation. However, in response to treatment, it is not really known how cells can survive senescence and how irreversible this pathway is. In this study, we analyzed cell escape in response to irinotecan, a first line treatment used in colorectal cancer that induced senescence. We detected subpopulations of cells that adapted to chemotherapy and resumed proliferation. Survival led to the emergence of more transformed cells that induced tumor formation in mice and grew in low adhesion conditions. A significant amount of viable polyploid cells was also generated following irinotecan failure. Markers such as lgr5, CD44, CD133 and ALDH were downregulated in persistent clones, indicating that survival was not associated with an increase in cancer initiating cells. Importantly, malignant cells which resisted senescence relied on survival pathways induced by Mcl-1 signaling and to a lesser extent by Bcl-xL. Depletion of Mcl-1 increased irinotecan efficiency, induced the death of polyploid cells, prevented cell emergence and inhibited growth in low-adhesion conditions. We therefore propose that Mcl-1 targeting should be considered in the future to reduce senescence escape and to improve the treatment of irinotecan-refractory colorectal cancers.

Promoter analysis of the human translation termination factor 1 gene.

The human translation termination factor 1 (ETF1) gene encodes a class-1 release factor, eRF1, which catalyses termination of protein synthesis at all three stop codons. In this report, we describe the functional organization of the 5'-region of the gene. Primer extension and ribonuclease protection mapping revealed three transcription start sites clustered within approximately 10 bp. DNase I-hypersensitive site analysis identified five hypersensitive sites, one of which was located downstream of the initiation start sites. We used transient expression assays to define the 5'-regulating regions and in vivo and in vitro footprinting analysis to identify potential cis-acting regulatory elements. A basal promoter, spanning nucleotides -210/+117, contained no TATA box but a putative initiator element (Inr) and multiple potential Sp1/Sp3 binding sites, and thus displayed some of the features of a housekeeping gene. An additional upstream promoter containing positive and negative regulatory elements also regulated ETF1 gene expression. Real-time quantitative RT-PCR analysis showed tissue-specific expression of ETF1 transcripts in mouse tissues. Our results are suggestive of a constitutive expression of the human ETF1 gene but with possible cell- and tissue-specific regulation.

Evaluation of ETF1/eRF1, mapping to 5q31, as a candidate myeloid tumor suppressor gene.

Interstitial deletion of the long arm of chromosome 5 is a recurrent abnormality, mainly associated with myelodysplastic syndromes (MDS) and acute myeloid leukemia (AML), and it has been proposed therefore that the deleted region may contain a myeloid tumor suppressor gene. We have recently mapped a human translation termination factor gene, ETF1, to band 5q31 at D5S500, and thus to the smallest commonly deleted segment. We have evaluated ETF1 as a candidate myeloid tumor suppressor gene by analysis of the human acute myeloid leukemia cell line HL60, and of patients suffering from malignant myeloid diseases with cytogenetically-defined abnormalities of chromosome 5. Fluorescence in situ hybridization analysis revealed hemizygous loss of the ETF1 locus in HL60 cells and in four of five leukemic samples, but no inactivating mutations were identified by sequencing of the remaining ETF1 allele.