La sénescence : Une barrière télomérique à l’immortalité ou une réponse cellulaire aux stress physiologiques ?

L’état sénescent est un arrêt irréversible du cycle cellulaire associé à des modifications morphologiques et fonctionnelles de la cellule. La sénescence est causée par le raccourcissement des télomères (sénescence réplicative) ou par l’exposition aiguë ou chronique à d’autres signaux de stress physiologique (un phénomène appelé stasis, stress or aberrant signaling-induced senescence). Cet article traite des voies qui conduisent à la sénescence cellulaire, des mécanismes impliqués et du rôle de ces voies dans la régulation du déclenchement et de la progression tumorale. La sénescence réplicative ou la perte de fonctionnalité des télomères mobilisent les protéines détectrices des cassures double brin de l’ADN, conduisant à l’activation des systèmes de réparation de l’ADN et de la protéine suppresseur de tumeur p53, qui, à son tour, induit l’inhibiteur du cycle p21WAF1. L’inactivation de p53 et de pRb permet aux cellules de continuer à proliférer, mais les fonctions télomériques se détériorent jusqu’à la catastrophe génétique, ou crise. L’immortalisation étant un prérequis essentiel pour le développement des cellules tumorales, celles-ci doivent donc contourner au moins deux barrières prolifératives, la sénescence cellulaire et la crise, pour atteindre la transformation néoplasique. Ces barrières sont régulées par le raccourcissement des télomères et par les voies suppresseurs de tumeur p16INK4a/Rb et p53. L’identification des gènes et la connaissance des mécanismes régulateurs conduisant à la sénescence et déterminant le profil d’expression génique dans les cellules sénescentes pourraient conduire à des traitements anticancéreux plus efficaces.Cells entering a state of senescence undergo a irreversible cell cycle arrest, associated by a set of functionnal and morphological changes. Senescence occurs following telomeres shortening (replicative senescence) or exposure to other acute or chronic physiologic stress signals (a phenomenon termed stasis : stress or aberrant signaling-induced senescence). In this review, I discuss the pathways of cellular senescence, the mechanisms involved and the role that these pathways have in regulating the initiation and progression of cancer. Telomere-initiated senescence or loss of telomere function trigger focal recruitement of protein sensors of the DNA double-strand breaks leading to the activation of the DNA damage checkpoint responses and the tumour suppressor gene product, p53, which in turn induces the cell-cycle inhibitor, p21WAF1. Loss of p53 and pRb function allows continued cell division despite increasing telomere dysfunction and eventually entry into telomere crisis. Immortalisation is an essential prerequisite for the formation of a tumour cell. Therefore, a developping tumour cell must circumvent at least two proliferative barriers - cellular senescence and crisis - to achieve neoplastic transformation. These barriers are regulated by telomere shortening and by the p16INK4a/Rb and p53 tumour suppressor pathways. Elucidation of the genes and emerging knowledge about the regulatory mechanisms that lead to senescence and determine the pattern of gene expression in senescent cells may lead to more effective treatments for cancer

Longitudinal follow-up of a cohort of 350 singleton infants born at less than 32 weeks of amenorrhea (neurocognitive screening, academic outcome and perinatal factors)

AIX-MARSEILLE2-BU Méd/Odontol. (130552103) / SudocPARIS-BIUM (751062103) / SudocSudocFranceF

Etude des mécanismes de la régulation de l'EMT par le suppresseur de tumeur p53 dans un modèle de cellules de carcinome du colon

Le suppresseur de tumeur p53 est un facteur de transcription impliqué dans la progression du cycle cellulaire et dans l'apoptose. Outre ses fonctions majeures, p53 régule également la migration et l'adhérence cellulaire qui sont deux évènements impliqués dans le processus métastatique. L'évolution maligne d'un carcinome peut aussi impliquer la répression transcriptionnelle de CDH1, qui code pour la E-cadhérine, protéine constitutive des jonctions adhérentes. Nous avons recherche si et comment p53 régule certains évènements moléculaires qui contrôle le processus métastatique. Nous montrons que la forme sauvage de p53 réprime directement la transcription de CDH1 dans des lignées cellulaires humaines de carcinome du colon (HCT116). Cette répression est associée à une expression de novo de la vimentine et à l'acquisition d'une morphologie plus fibroblastique. L'une des cibles transcriptionnelle majeure de p53, p21WAF1 court-circuite l'effet répresseur de p53 sur la transcription de CDH1. Trois mutants dominant-négatifs de p53 (R273H, R175H et V143A) répriment également la transcription de CDH1. De plus, l'expression stable du mutant V143A dans les cellules HCT116 p53-/- mime partiellement le phénotype observé suite l'accumulation aberrante de p53. De façon surprenante, ce phénotype mésenchymateux n'est pas associé à une augmentation des propriétés invasives. Ce travail implique p53 dans la régulation d'évènements moléculaires qui peuvent conduire à l'acquisition d'un phénotype mésenchymateuxThe p53 tumour suppressor gene encodes a transcriptional regulator that monitor proliferation signals to prevent cells from uncontrolled growth. However, p53 has also alternative functions. Notably, loss of p53 favours cell migration and invasion, processes involved in tumour metastasis. Given that epithelial to mesenchymal transition (EMT) also increases cell migration by altering the cell phenotype and morphology, we hypothesized that p53 controls molecular alterations that mediate EMT during cancer progression. Analysis of E-cadherin promoter activity and chromatin immunoprecipitation identified p53 as a direct transcriptional repressor of E-cadherin in human colon carcinoma cells, HCT116. Aberrant levels of p53 disrupted E-Cadherin-based cell-cell contacts and induced a more mesenchymal phenotype with downregulation of E-Cadherin and induction of the mesenchymal gene, vimentin. In addition, p21Waf-1 impeded p53 transcriptional repression and restored in part cell to cell adhesion. Furthermore, HCT116p53-/- cells overexpressing dominant-negative form of p53 also displayed the EMT-like phenotype. Neither p53 nor mutant p53 mediated shift toward mesenchymal morphology led to an increase of cell invasiveness. This work and our previous finding of mutant p53-mediated cell invasion identify p53 as a novel regulator of EMT and offer new perspectives in the comprehension of metastasisMONTPELLIER-BU Sciences (341722106) / SudocSudocFranceF

Mechanisms and Regulation of Cellular Senescence

International audienceCellular senescence entails a state of an essentially irreversible proliferative arrest in which cells remain metabolically active and secrete a range of pro-inflammatory and proteolytic factors as part of the senescence-associated secretory phenotype. There are different types of senescent cells, and senescence can be induced in response to many DNA damage signals. Senescent cells accumulate in different tissues and organs where they have distinct physiological and pathological functions. Despite this diversity, all senescent cells must be able to survive in a nondividing state while protecting themselves from positive feedback loops linked to the constant activation of the DNA damage response. This capacity requires changes in core cellular programs. Understanding how different cell types can undergo extensive changes in their transcriptional programs, metabolism, heterochromatin patterns, and cellular structures to induce a common cellular state is crucial to preventing cancer development/progression and to improving health during aging. In this review, we discuss how senescent cells continuously evolve after their initial proliferative arrest and highlight the unifying features that define the senescent state

Multiple connexin expression in peripheral nerve, Schwann cells, and Schwannoma cells

Myelinating Schwann cells express the gap junction protein, connexin (Cx)32, which is present at the nodes of Ranvier and Schmidt-Lantermann incisures (Bergoffen et al. [1993] Science (Wash. ) 262:2039-2042). Following peripheral nerve injury, other members of the connexin gene family are also expressed (Chandross et al. [1996a] Mol. Cell. Neurosci. 7:501-518). This study surveys the connexin(s) expressed by rat sciatic nerve, cultured Schwann cells, and a mouse Schwannoma (TR6 Bc1) cell line. Reverse transcriptase-polymerase chain reaction (RT-PCR) amplification revealed a constitutive expression of mRNA encoding Cx32 and 43 but not Cx26, 37, 40, 45, and 46 in sciatic nerve. Mitogenic stimulation of cultured Schwann cells expressing Cx32 also resulted in the appearance of Cx43 mRNA. Schwannoma cells expressed exclusively Cx43 mRNA. These results were confirmed by Northern blot analysis. Functional gap junctions in cultured Schwann and Schwannoma cells were shown by analysis of the intercellular transfer of Lucifer yellow, although the coupling between primary Schwann cells was weak or undetectable. Treatment of primary Schwann cells with mitogens resulted in extensive dye coupling. An immunohistochemical study of adult sciatic nerve sections demonstrated Cx32 immunoreactivity at the nodes of Ranvier and in Schwann cell bodies. Lower intensity staining of Cx43 along the myelin sheath and Schwann cell bodies was also observed. Indirect immunofluorescent studies of Schwann cells treated with mitogens showed characteristic punctate cell surface staining of Cx43; Cx32 staining was detected mainly intracellularly. These results lead to the conclusion that in addition to the expression of Cx32 by normal adult sciatic nerve, low amounts of Cx43 protein are also present. The implications of the expression of two connexins by Schwann cells in Charcot-Marie-Tooth X-linked disease, a demyelinating peripheral neuropathy, are discussed

Gain of oncogenic function of p53 mutants regulates E-cadherin expression uncoupled from cell invasion in colon cancer cells.

International audienceMutations in the p53 tumour suppressor gene are associated clinically with tumour progression and metastasis. Downregulation of the E-cadherin cell-cell adhesion molecule is a key event for epithelial to mesenchymal transition (EMT) in tumour progression. Here, we show that wild-type p53 induced to adopt a mutant conformation, and hot-spot p53 mutants, which are both transcriptionally inactive, downregulate E-cadherin expression in the colon carcinoma cell line HCT116. Downregulation of E-cadherin occurred concomitantly with the upregulation of Slug and Zeb-1, transcriptional factors known to repress E-cadherin gene expression. In addition, knockdown of Slug and Zeb-1 expression diminished p53-mediated E-cadherin repression. Knocking down endogenous mutant p53 in MDA-MB-231 and SW620 cancer cell lines lacking E-cadherin protein restored the expression of E-cadherin. Complete loss of E-cadherin expression in HCT116 cells induced morphological alterations along with upregulation of vimentin, a mesenchymal marker. These changes characteristic of the EMT phenotype were, however, not sufficient to confer invasiveness in a three-dimensional matrix. Downregulation of E-cadherin by mutant p53 was not required to promote the invasive phenotype induced by inactivation of p53. These findings indicate that independent control of E-cadherin expression and cell motility could be essential molecular events in p53 mutant-induced invasive phenotypes

A case of Norman-Roberts syndrome identified from postnatal diagnosis of microlissencephaly.

Place: EnglandLissencephaly is a rare brain malformation. What differentiates microlissencephaly from classical lissencephaly and other variants is the presence of severe microcephaly. Very few postnatal cases of Norman-Roberts syndrome are described in the literature. We report a case of microlissencephaly with a polymalformative syndrome that prompted postnatal diagnosis of Norman-Roberts syndrome

DNA damage checkpoint kinase Chk2 triggers replicative senescence

Telomere shortening in normal human cells causes replicative senescence, a p53-dependent growth arrest state, which is thought to represent an innate defence against tumour progression. However, although it has been postulated that critical telomere loss generates a ‘DNA damage' signal, the signalling pathway(s) that alerts cells to short dysfunctional telomeres remains only partially defined. We show that senescence in human fibroblasts is associated with focal accumulation of γ-H2AX and phosphorylation of Chk2, known mediators of the ataxia-telangiectasia mutated regulated signalling pathway activated by DNA double-strand breaks. Both these responses increased in cells grown beyond senescence through inactivation of p53 and pRb, indicating that they are driven by continued cell division and not a consequence of senescence. γ-H2AX (though not Chk2) was shown to associate directly with telomeric DNA. Furthermore, inactivation of Chk2 in human fibroblasts led to a fall in p21(waf1) expression and an extension of proliferative lifespan, consistent with failure to activate p53. Thus, Chk2 forms an essential component of a common pathway signalling cell cycle arrest in response to both telomere erosion and DNA damage